Priority is claimed on Japanese Patent Application No. 2004-011139, filed January 19,2004, the contents of which are incorporated herein by reference.

BACKGROUND ART For steel used in containers such as drink or food containers, developments to reduce the thickness of the material have proceeded in order to reduce the manufacturing cost of the containers, to the point that materials as thin as 0.2 mm or less are being used.

Examples of problems that can exist when manufacturing containers from such thin materials include a reduction in the color tone as a result of the difficulty in controlling the surface state, in the adhesiveness of the surface coating and in the weldability of the material.

It is known that the state of the surface of the steel has a large effect on color tone, surface coating adhesiveness and weldability. For example, this has been described in Japanese Unexamined Patent Application, First Publication Nos.

H11-197704, H08-3781 and H06-57488. Furthermore, a method for controlling the surface roughness has been described in Japanese Unexamined Patent Application, First

Publication No. H07-9005. In these examples, it is likely not possible to avoid a deterioration in the productivity for these materials as it is necessary to precisely control the manufacturing conditions in order to control the surface state. Furthermore, in the control methods in these examples, it has not been possible to sufficiently improve the color tone of the container, the surface coating adhesiveness and the weldability of the ultra-thin material used, which is addressed by the exemplary embodiments of the present invention.

DISCLOSURE OF INVENTION One of the objects of the present invention is to provide a steel sheet and a manufacturing method therefor in which it is possible to not only reform the surface state of the material by controlling the nitride configuration, but also to avoid the use of special treatments which could adversely affect productivity, and which solves the problems in using ultra-thin materials in the manufacture of containers, such as color tone of the container resulting from the surface state of the steel, surface coating adhesiveness and weldability.

It is possible to accurately control the nitride configuration in the thickness direction of a steel sheet when nitriding during the processes post-annealing with the goal of significantly increasing the deformation resistance of the container without degrading the ductility of the steel sheet, as described in Japanese Unexamined Patent Application, First Publication Nos. 2003-119381 and 2003-100720. When evaluating the weldability of this material, as in conventional methods for the same material, the surface state of the steel should be good, without needing to perform special pressure rolling to precisely control the rolling density, to precisely control the formation of chromium oxide materials, to surfactants or to employ cathodic-electrolysis to control the surface state.

Therefore, certain conditions can be obtained in which it is possible to significantly increase the color tone of the container, the surface coating adhesiveness and the weldability of the material which results from the surface state of the steel, and which had been a problem in containers using ultra-thin steel sheet material.

In other words, when increasing the amount of nitrogen within the steel through nitriding treatment following the cold-rolling process, the color tone, surface coating adhesiveness and weldability of the can are not greatly improved by simply constructing the surface hardness, but in the present invention, by limiting the constituent amounts, in particular, the amount of nitrogen, to be within a specific range, and further, by optimizing the nitriding conditions, it is possible to favorably control the surface layers of the material, in particular, the formation of nitrogen compounds on the uppermost layer. Furthermore, as it is possible to also modify the roughness etc. of the steel surface, as a result, it is also possible to improve the color tone, surface coating adhesiveness and weldability of the can which applies the ultra-thin material.

According to certain exemplary embodiments of the present invention, such conditions can be provided, as well as the manufacturing method using these conditions.

A first aspect of the steel sheet according to the present invention is a steel sheet for use in containers can be provided with a thickness of less than or equal to 0.400 mm.

This material contains less than or equal to 0.0800% C, 0.600% N, 2.0% Si, 2.0% Mn, 0.10% P, 0.05% S and 2. 0% Al. Furthermore, the ratio of the area of the nitrogen compounds on the surface is greater than or equal to 1.0%.

According to a second aspect of the steel sheet according to the present invention, (ratio of the area of nitrogen compounds on the surface position of the steel)/ (ratio of the area of nitrogen compounds on the cross-sectional position at a depth of 1/4 the thickness of the steel sheet) is greater than or equal to 1.5.

According to a third aspect of the steel sheet according to the present invention, the density of range of individual nitrogen compounds on the surface of the diameter may be greater than or equal to 0.1 urn and the density of the range of the individual steel, the density which is greater is greater than or equal to 0. 001 units/. m2.

According to a further aspect of the present invention, it is possible for the surface roughness to be less than or equal to 0. 90 am Ra, or for the number of irregular peaks in a region of length 1 inch to be greater than or equal to 250 PPI. Furthermore, the material may contain one or two or greater constituents where the amount is less than or equal to 0.08% Ti, 0.08% Nb, 0.015% B, 5.0% Ni, 2.0% Cu and 2.0% Cr. In addition, the material may contain a sum of less than or equal to 0.1% of Sn, Sb, Mo, Ta, V, and W.

Exemplary embodiment of the manufacturing method for a steel sheet according to the present invention may include a manufacturing method for a steel sheet for use in containers with a thickness of less than or equal to 0.400 mm. This material contains less than or equal to 0.0800% C, 0. 0300% N, 2. 0% Si, 2.0% Mn, 0.10% P, 0.05% S and 2.0% Al, along with residual Fe and other unavoidable impurities. After cold-rolling this steel, a nitriding treatment is performed at the same time as the recrystallization annealing process, or after this process, and increasing the amount of N to be greater than or equal to 0.0002%, the ratio of the area of nitrogen compounds on the surface of the steel is greater than or equal to 1.0%. Furthermore, the amount of N within the steel sheet is less than or equal to 0.600%.

According to another exemplary embodiment of the manufacturing method of the present invention, the material of the steel sheet may contain less than or equal to 0.0800% C, 0.0300% N, 2. 0% Si, 2.0% Mn, 0.10% P, 0.05% S and 2.0% Al, along with residual Fe and other unavoidable impurities. After cold-rolling this steel, a nitriding

treatment is performed at the same time as the recrystallization annealing process, or after this process, and increasing the amount of N to be greater than or equal to 0.0002%, the ratio of the area of nitrogen compounds on the surface of the steel is greater than or equal to 1.0%. Furthermore, the amount of N within the steel sheet is less than or equal to 0.600%.

In a further exemplary embodiment of the manufacturing method according to the present invention, after cold-rolling this steel, a nitriding treatment may be performed at the same time as the recrystallization annealing process, or after this process, and increasing the amount of N to be greater than or equal to 0.0002%, the (ratio of the area of nitrogen compounds on the surface position of the steel) / (ratio of the area of nitrogen compounds on the cross-sectional position at a depth of 1/4 the thickness of the steel sheet) is greater than or equal to 1.5. Furthermore, the amount of N within the steel sheet is less than or equal to 0.600%.

According to still another exemplary embodiment of the manufacturing method of the present invention, after cold-rolling this steel, a nitriding treatment may be performed at the same time as the recrystallization annealing process, or after this process, and increasing the amount of N to be greater than or equal to 0. 0002% ; of the density of range of individual nitrogen compounds on the surface of diameter greater than or equal to 0. 1 um and the density of the range of the individual steel, the density which is greater of these two must be greater than or equal to 0.001 units/. ma.

Furthermore, the amount of N within the steel sheet is less than or equal to 0.600%.

According to yet another exemplary embodiment of the present invention, after cold-rolling this steel, a nitriding treatment is performed at the same time as the recrystallization annealing process, or after this process, and at this time, when the sheet temperature is within a range of 550 to 800°C, it is kept for longer than 0.1 seconds and

less than 360 seconds within an environment containing greater than or equal to 0.02% ammonia gas. In another variant of the present invention, after cold-rolling this steel, a nitriding treatment is performed at the same time as the recrystallization annealing process, or after this process, and at this time, (sheet temperature (°C)-550 when <BR> <BR> beginning nitriding) / (concentration (%) of the ammonia gas when beginning nitriding) must be less than 150.

According to still another exemplary embodiment of the steel sheet for use in containers and the manufacturing method thereof of the present invention, it is possible to improve the color tone of the container, the surface coating adhesiveness and the weldability without needing to perform complex treatments after the nitriding treatment which could adversely affect productivity. In this manner, it is possible to increase the productivity of the manufacture of steel sheets for ultra-thin containers as well as to achieve extraordinary efficacy in the manufacturing process.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing each component in the thickness direction of the steel sheet for use in containers according to the present invention.

FIG. 2 is a diagram showing the nitride components region of the steel sheet for use in containers according to the present invention.

FIG. 3 is a diagram showing the steel range of the steel sheet for use in containers according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION First, the exemplary constituents of the steel material are described below. All constituents of the steel are represented in % thereof as contained therein.

An upper limit of the amount of C is preferable to avoid degradation of workability of the material, and should be less than or equal to 0.0800% C. It is preferable for this to be less than or equal to 0.0600%, and it is even more preferable for this to be less than or equal to 0.040% C.

In the steel according to an exemplary embodiment of the present invention where the amount of N (which has the similar properties as C) is increased through nitriding treatment after annealing, it is possible for the amount of C contained therein and required from the viewpoint of ensuring strength to be low. It is possible to obtain the required strength with less than or equal to 0.0050% C, and can even be less than or equal to 0. 0020%. If the amount is less than or equal to 0.0015%, there may be some overlap with the amount of nitriding, and it is still possible to manufacture ultra-soft materials which would fall outside of the standards using normal container materials.

From the fact that increasing the r value will keep the press-forming properties of the material high, it is preferable to keep the amount of C prior to nitriding treatment as low as possible.

It may also be preferable to have an upper limit for the amount of N in the final product, and as, similar to C, it is necessary to avoid degradation of processability of the material, it is good to keep the amount of N to be less than or equal to 0.600%. It is preferable for the amount of N to be less than or equal to 0.200%, and it is even more preferable for the amount of N to be less than or equal to 0.150%. Even more preferable may be to set the amount of N to be less than or equal to 0.100%. However, in order to obtain the efficacy of an exemplary embodiment of the present invention, a certain amount of N to form the appropriate amount of nitrogen compounds on the surface of the steel sheet is required. In regards to this amount of N, it may depend on the distribution of N in the thickness direction, but when using normal nitriding treatment,

it is of course necessary to increase the amount of N in comparison to the amount prior to the nitriding treatment, and in particular, in the present invention, this has been increased to greater than or equal to 0.0002%. This may appear to be an extremely small increase in the amount of N, but from the view of the increase in the amount of N on the surface of the steel sheet, it is quite large.

In other words, if, for instance, the thickness corresponding to the surface constituents of the steel sheet is assumed to be 1/100,0. 20 mm sheet thickness, then the portion of 2 um thickness will be determined to be the surface layer, and by increasing the amount of N by 0.0002% as an average sheet thickness, the amount of N increased in this surface layer will be 0.0100%. As this increase in the amount of N grows larger, the amount of nitrides on the surface which is required in the present invention will increase, and therefore, it is preferable for this to be greater than or equal to 0.0005%. It is more preferable for this to be greater than or equal to 0. 0010% ; it is even more preferable for this to be greater than or equal to 0.0020%, and it is even more preferable for this to be greater than or equal to 0.0050%. Furthermore, a value of greater than or equal to 0.0100% is preferable to these lower values, and similarly, a value of greater than or equal to 0. 0200% and 0.0400% is even more preferable. In particular, when the value is greater than or equal to 0. 0100%, the amount of N on the surface of the steel sheet is very high, resulting in more than sufficient required nitrides, and the efficacy of the present invention will be stable. However, if the amount of N increase is excessive, large and rough nitrified components will form not only on the surface of the steel sheet but also within the plate, leading to not only a degradation in the processing properties of the material but also to instances of surface defects, so care must be taken in the amount of N increase. Based on this type of reason, it is necessary to ensure that the upper limit of the amount of N increased in the steel sheet on average is not exceeded.

It may also be preferable to have an upper limit for the amount of N prior to nitriding treatment, and as, similar to C, it is necessary to avoid degradation of the processing properties of the material, it is good to keep the amount of N to be less than or equal to 0.0300%. It may be preferable for the amount of N to be less than or equal to 0.0200%, and it is even more preferable for the amount of N to be less than or equal to 0. 0150%. It may also be preferable for the amount of N to be less than or equal to 0. 0100%, and even more preferable for the amount of N to be less than or equal to 0.0050%. Yet even more preferable may be to set the amount of N to be less than or equal to 0.0030%. From the fact that increasing the r value will likely keep the press-forming properties of the material high, it is preferable to keep the amount of N prior to nitriding treatment as low as possible. Care should be taken here, as shall be described below, and the N contained in the material through the nitriding treatment after annealing will exist in order to provide good color tone, surface coating adhesiveness and weldability on the can, and therefore, this N likely shows different efficacy from the N which exists prior to the annealing process.

Si can be added in order to adjust the strength of the material, but if the amount of Si is too high, the processing and plating properties of the material may degrade.

Therefore, it may be preferable to keep Si to be equal to or below 2.0%. In the exemplary embodiment of the steel according to the present invention, nitrides may be formed at the grain boundary N embedded within the steel during the nitriding treatment, and not only can these lead to brittle cracking, but they can also lead to a loss in the efficacy of the present invention, and therefore, there may be instances where the amount of Si needs to be kept to less than or equal to 1.5%, or even less than or equal to 1.0%.

In particular, from the view that it ensures high forming characteristics in the material, it is preferable for the amount of Si to be as low as possible, and by setting it to be less than

or equal to 0.5%, 0. 1% or even 0.07%, it is possible to increase the forming characteristics of the material as well as to control the formation of Si-nitrides.

Mn can be added in order to adjust the strength of the material, but as too much Mn can result in a degradation of workability of the material, it is proper to keep this amount to less than or equal to 2.0%. From the view that it ensures high forming characteristics in the material, it is preferable for the amount of Mn to be as low as possible, and by setting it to be less than or equal to 0.6%, or even 0.2%, it is possible to increase the forming characteristics of the material.

P can be added in order to adjust the strength of the material, but as too much P may result in not only a degradation of workability of the material but also in a reduction of the nitriding treatment of the steel sheet ; it is preferable to keep this amount to less than or equal to 0.10%. From the view that it ensures high forming characteristics in the material, it is preferable for the amount of P to be as low as possible, and by setting it to be less than or equal to 0.05%, or even 0.01%, it is possible to increase the forming characteristics of the material.

S may degrade the hot ductility characteristics of the material as well as to provide an obstacle in casting or hot ductility the material, and therefore, it is preferable to keep the amount of S to be less than or equal to 0.05%. Frorn the view that it ensures high forming characteristics in the material, it is preferable for the amount of S to be as low as possible, and by setting it to be less than or equal to 0. 02Al0, or even 0. 01%, it is possible to increase the forming characteristics of the material.

Al may be an element added to deoxidize the material, but if the amount of Al is high, casting the material may become difficult, and as damages such as flaws in the surface may increase, it is proper to keep the amount of Al to be less than or equal to 2.0%. Furthermore, when the amount of Al is high, at greater than or equal to 0.2%, a

large amount of AlN can be formed within the steel as it bonds with the N embedded within the steel sheet through nitriding, resulting in a hardening of the nitrified areas.

However, as the formation of large AIN can result in embrittlement, care should be taken in the adjustment of the amount of Al. From the view that it likely ensures high forming characteristics in the central layers of the steel sheet where the level of being nitrified is low, it is preferable for the amount of Al to be as low as possible, and by setting it to be less than or equal to 0.2%, or even 0. 1%, it is possible to increase the forming characteristics of the parts of the material where the level of being nitrified is low.

The efficacy and control of constituents other than the basic elements described above that can be used in normal steel sheet for containers is described below.

Ti can increase the recrystallization temperature of the steel sheet, and as it may also greatly degrade the annealing threading performance of the ultra-thin steel sheet which is the subject of the present invention ; it is appropriate to keep the amount of Ti to be less than or equal to 0.080%. In particular, there may not be a need to add Ti in normal applications where a high r value is not required, and therefore, it is good to keep the amount of Ti to be less than or equal to 0.04% or even 0. 01%. Furthermore, the Ti which is set within the steel sheet during nitriding treatment can form microscopic TiN within the steel as it bonds with the N embedded in the sheet through nitriding treatment, and it may have high efficacy in the hardening of the nitrified area. Therefore, as there are instances where material hardening can occur even in the mid-thickness layers of the steel sheet where the level of being nitrified is low, when there is a need to obtain a soft steel sheet, it is preferable to keep the amount of Ti to be as low as possible, and by setting it to be less than or equal to 0.005% or even 0.003%, it is possible to control the unintentional hardening of the steel sheet. The TiN formed on the steel sheet surface

during nitriding treatment is very microscopic, and as the surface reformation efficacy as intended by the present invention is small, it is possible to use these molecules to increase the surface reformation efficacy. In this manner, it is possible to consciously add it such that it falls within the specific addition range of the present invention.

Nb shows the same effects as Ti. As Nb can increase the recrystallization temperature of the steel sheet (as well as significantly degrade the annealing threading performance of the ultra-thin steel sheet which is the subject of the present invention), it may be preferable to keep the amount of Nb to be less than or equal to 0.080%. In particular, there is no need to add Nb in normal applications where a high r value is not required, and therefore, it is good to keep the amount of Nb to be less than or equal to 0.04% or even 0. 01%. Furthermore, the Nb which is set within the steel sheet during nitriding treatment can form microscopic NbN within the steel as it bonds with the N embedded in the sheet through nitriding treatment, and it has strong efficacy in the hardening of the nitrified area. Therefore, as there are instances where material hardening can occur even in the mid-thickness layers of the steel sheet where the level of nitriding is low, when there is a need to obtain a soft steel sheet, it is preferable to keep the amount of Nb to be as low as possible, and by setting it to be less than or equal to 0.005% or even 0.003%, it is possible to control the unintentional hardening of the steel sheet. The NbN formed on the steel sheet surface during nitriding treatment can be very microscopic, and as the surface reformation efficacy as intended by the present invention is small, it is possible to use these molecules to increase the surface reformation efficacy.

In this way, it is possible to consciously add it such that it falls within the specific addition range of the present invention.

When B is added to a steel sheet which contains greater than or equal to 0. 01% of Ti and Nb, it can increase the recrystallization temperature of the steel sheet, and can

greatly degrade the annealing threading performance of the ultra-thin steel sheet which is the scope of the present invention. When the amount of Ti and Nb in the sheet is low, there are few adverse effects of B, and as it actually can decrease the recrystallization temperature of the steel sheet, it is possible to perform recrystallization annealing at a lower temperature. Furthermore, as it also has the efficacy of increasing the annealing properties of the material, it is possible to add B proactively. If excessive B is added, breakages of the cast form during casting may become prominent. Therefore, it may be preferable to keep the upper limit of the added B at 0.015%. Since the goal is to decrease the recrystallization temperature while increasing the annealing properties of the material, it is possible to set the relationship between B and the amount of N prior to the nitriding treatment to be B/N=0.6-1. 5.

Furthermore, B which is set within the steel prior to nitriding treatment can combine with the N embedded within the steel sheet during nitriding treatment to form microscopic BN, which is effective in hardening the nitrified area. When utilizing this surface hardening due to the BN, it is preferable to set the ratio between the amount of B and the amount of N contained prior to nitriding treatment to be B/N>0.8. By making this ratio greater than or equal to 1.5, or to even greater than or equal to 2.5, hardening due to BN formation will become prominent. As there may be instances where the formation of BN may result in more hardening of the material than is required, care must be taken in the amount of B added. For the exemplary embodiment of the steel according to the present invention, if hardening due to BN formation is not to be utilized, then it is acceptable to set the ratio of B contained and B contained prior to nitriding treatment to be B/N<0.8, or even more strictly, to set this ratio to be B/N<0. 1. The BN formed on the steel sheet surface during nitriding treatment is very microscopic, and as the surface reformation efficacy as intended according to the exemplary embodiment of

the present invention is small, it is possible to use these molecules to increase the surface reformation efficacy. In this manner, it is possible to consciously add it such that it falls within the specific addition range according to the exemplary embodiment of the present invention.

Furthermore, in order to obtain characteristics which are not explicitly specified in this description such as increasing the corrosion resistivity, it is possible to add less than or equal to 20% Cr, 10% Ni and 5% Cu. However, if excessive amounts are added, the result can be a reduction in the nitriding function which is required in the steel in the present invention, so it is preferable to set the amount of Cr to be less than or equal to 30%, of Ni to be less than or equal to 15% and of Cu to be less than or equal to 5%. It is even more preferable to set the amount of Cr to be less than or equal to 15%, of Ni to be less than or equal to 5% and of Cu to be less than or equal to 2%. In particular, the Cr which is set within the steel prior to nitriding treatment can combine with the N embedded within the steel sheet during nitriding treatment, with the effect of forming microscopic Cr-nitrides on the surface of the steel sheet, and utilizing these nitrides, it is possible to increase the efficacy of the present invention. With this goal in mind, it is preferable to add greater than or equal to 0. 01% Cr.

Cr can increase the recrystallization temperature of the steel sheet. If excess amounts are added, the annealing threading performance of the ultra-thin steel sheet which is within the scope of the present invention may be significantly degraded. In order to avoid a reduction in the annealing threading performance as a result of the increase in the recrystallization temperature, it is preferable to add less than or equal to 2.0% Cr, and if less than or equal to 0.6% is added, then it is possible to control the increase in recrystallization temperature to a level which would not become a problem.

Furthermore, if Sn, Sb, Mo, Ta, V and W can be added for a total of less than or

equal to 0.1% in order to obtain characteristics not specified in the present invention, there will be no adverse affects to the efficacy of the present invention.

Of the above elements, depending on the conditions, P, B, Sn and Sb can reduce the nitriding efficacy which is an important requirement in the present invention, and therefore it is necessary to pay attention to the upper limit of the amounts added in conjunction with the nitriding conditions. In particular, in order to prevent a significant reduction in the nitriding efficacy, it is preferable to set the amounts to Sn and Sb to be less than or equal to 0.06%, and it is even more preferable to set these amount to be less than or equal to 0.02% each.

Above, the classification of the parts of the steel sheet in the thickness directions used in the explanation of the specifications of the present invention have been explained with reference to FIG. 1.

In particular, "Sheet thickness 1/4 depth"illustrates the corresponding position within FIG. 1. The location corresponding to this"sheet thickness 1/4 depth"exists on both surfaces of the steel sheet, but in the present invention, it corresponds to either surface that is within the scope of the present invention. It is comparatively easy to modify the distribution of nitrides on the top and bottom surfaces through the nitriding method, surface treatment prior to nitriding treatment and furthermore, to some sort of treatment after nitriding treatment. An exemplary embodiment of the present invention may have this type of steel sheet with different surfaces on the top and bottom. This is due to the fact that it is possible to obtain an increase in efficacy in color tone, surface coating adhesiveness and weldability, which is the goal of the present invention, even on only one side of the sheet.

According to another exemplary embodiment of the present invention, the area ratio and the density relating to the nitride components in a specific position on the steel

sheet surface or in the thickness direction of the steel sheet is regulated. It is possible to identify the existing deposits through the diffraction pattern in an electron microscope or through an attached X-Ray analyzer. Of course, it is also possible to identify them in other methods such as chemical analysis. It is possible to quantify the diameter, the area ratio and the density, as shall be below, for example, an electron microscope for observation. In order to control the nitride size, the area ratio and the density, it is effective to appropriately control the temperature, the time and the cooling speed immediately prior to entering this temperature region to the range of 450 to 700°C to be explained later. The effect of this is, in an approximately the same way as the general deposit formation under normal conditions, that nitride size formed will be finer and the density will be greater as the speed is increased and the temperature lowered, and if the duration is increased, nitride size becomes larger and coarser.

The exemplary embodiments of the present invention do not concern only deposits from nitrides, but also in complex deposits along with oxides, carbides and sulfides. For example, when complex deposits are formed, it may be difficult to specify the size of each compound along with the type of a single deposit, so excluding the instances where it is possible to clearly distinguish that a single deposit is a nitride, it can be determined to be a single nitride component.

The observation method for the nitrides is not limited in particular, and it is equally acceptable to directly observe them using an EDX scanning electron microscope or some other surface observation device, or to observe them using extraction replicas obtained through a"Speed"method. However, with extraction replicas, it is usual to take certain care when creating the replicas such that the replicas are formed of only the information of the observed surface of the steel sheet. The reason is that, in the Speed method, if the amount extracted through electrolysis is too large, the information in the

thickness direction can overlap, and the number of deposits observed may be larger than when directly observing the steel sheet. Therefore, the amount extracted through electrolysis should be kept to within 2 um in terms of the steel sheet thickness. When performing analysis through EDX, when the main observed non-metallic elements are N, they can be treated as sulfides. Furthermore, as the size is small, even if the characteristic spectrum of the N is not distinguishable, Fe, Ti, Nb, B and Cr can be detected. Moreover, even if the distinguishable spectrums of O and S cannot be observed, deposits which can be approximately determined to be nitrides from a comparison of the configuration of nitrides and other deposits will be treated in the present invention as nitrides. Furthermore, it is acceptable to use an electron diffraction pattern in the quantization of the deposits.

An identification of nitrides is not limited to methods using EDX or electron diffraction patterns, and it is acceptable to use other analytical devices. In particular, the determination of the deposit type, size and density can be performed in any method which is accepted as valid. Depending on the deposit, there may be instances where it is difficult to differentiate between carbides and nitrides, but it is possible to exclude those items for which the type of deposit cannot be validly determined using normal analytical devices from the present invention. In other words, those items which are exceedingly small, and where it is not possible to identify them using an EDX spectrum or some other normal type of analytical device, will be excluded from the nitrides which are the subject of the present invention. According to certain exemplary embodiments of the present invention, the minimum size of deposits may be provided which can be identified using analytical devices in common use to be approximately 0.02 um. Of course, if a more precise analytical device is used and even finer nitrides can be observed, then the area ratio, etc. , will likely increase. Furthermore, in the case where the

individual atomic arrangement is displayed, there may be difficulties in determining what of the ultra-fine atomic unit of the metal atom and the N is to be classified as a nitride, but considering the current state of analytical levels, it is possible to exclude items that are smaller than those having the above listed size.

Furthermore, there may be instances where nitrides may be seen for which the form has been elongated, but for those items where the shape is not isotropic, the average value between the long diameter and the short diameter will be taken as the diameter of that deposit.

The steel sheet according to the exemplary embodiment of the present invention should preferably be ground in order to observe the cross-sectional position at 1/4 depth of the steel sheet. However, in order to clean the surface to observe the surface of the steel sheet, and to clarify any deposits to perform an accurate observation, it is possible to perform some type of grinding or ageing process. When performing grinding or ageing of the surface of the steel sheet, as, strictly speaking, the observed surface is preferably not the surface of the steel sheet, it is preferable to avoid any machining process. Therefore, it is preferable to select a particular type of method which may not require grinding. Furthermore, in the case of machining the surface, it is preferable to limit the amount of sheet thickness reduced through the machining process to be less than 2 pm.

Furthermore, as it is not the observation from the surface direction of the sheet (a two-dimensional observation) but one from the cross-sectional direction (the observation relating to the surface and the 1/4 depth position is not two-dimensional but one-dimensional), it is possible to calculate the area ratio, density and diameter. It is also possible to use image analysis to determine the number and diameter of the nitrides.

Described below is the state of the steel sheet surface which is provided for a

preferred embodiment of the present invention.

This exemplary embodiment of the present invention can control the configuration of nitrides on the surface of a steel sheet. For example, by increasing the non-uniformity of the surface by blanketing the surface of the steel sheet with materials other than steel, it is possible to improve the characteristics relating to the desired surface state. Furthermore, the characteristics relating to the surface state may be dependent on < the state of the nitrides formed on the surface, and the present invention makes it possible to control the configuration of the nitrides through the thermal history after nitriding treatment and through the cooling conditions. In this way, the uniqueness of the present invention lies in the state of the nitrides on the surface of the steel sheet. According to an exemplary embodiment of the present invention, the area ratio of the nitrides can be used as the method to regulate this, and the upper limit for this is greater than or equal to 1.0%. It is preferable to set this to be greater than or equal to 2.0%, or to 5. 0%, 10%, 20%, or even more preferably, to set this to be greater than or equal to 40%. It is not a problem even if the entire surface of the steel sheet is blanketed in nitrides.

However, nitrides which are formed in a membrane shape may be easily broken, and in the transport of the sheet during the manufacturing process. Indeed, some will likely be broken. It is important to pay attention to when the film of nitrides on the surface is particularly thick, as this can become the origin for fractures of the sheet.

There can be instances where it may be determined to be some type of surface defect.

Therefore, it is important to avoid excessive concentrations of N on the surface.

Furthermore, it is also possible to regulate the ratio of (nitrides area ratio on the surface position of the steel sheet)/ (nitrides area ratio of the cross-sectional position at 1/4 depth of the steel sheet). It is preferable that this ratio be greater than or equal to 1. 5 ; it may be more preferable for it to be greater than or equal to 3,6, 10,30, and for it

to be even greater than or equal to 100. If this ratio is small, the efficacy of the present invention may be reduced, and it is not possible to obtain the desired steel sheet.

Furthermore, in cases where nitrides are used as a method of increasing the density of nitrides on the surface part in this way, it is also possible to regulate the ratio (nitrified area ratio on the surface position of the steel sheet after nitriding treatment) / (nitrified area ratio on the surface position of the steel sheet prior to nitriding treatment). In this case as well, it is preferable that this ratio be greater than or equal to 1.5, it is more preferable for it to be greater than or equal to 3,6, 10, 30, and for it to be even greater than or equal to 100. It goes without saying that, the greater this ratio is, the greater the basic efficacy of the present invention.

Furthermore, it is preferable for the configuration of the nitrides on the surface to be one where relatively small items are distributed uniformly rather than one where coarse items are distributed roughly. However, as exceedingly small items will contribute little to the surface reformation efficacy which is the purpose of the present invention, it is preferable that the diameter of these particles be greater than or equal to 0. 10 um. In the present invention, the density is limited in relation to the individual nitrified area or the individual steel region on the surface of the steel sheet. According to an exemplary embodiment of the present invention, in the deposit region and the steel region, it is possible to use the value for which the density is higher. The higher this value is, the more finely distributed the abovementioned region will be on the surface of the steel sheet. In the present invention, it is preferable for this density to be greater than or equal to 0.001 units/llm2. It is more preferable for it to be greater than or equal to 0.003 units/, m2, even more preferable for it to be greater than or equal to 0. 010 units/, um2, as it is more preferable for it to be greater than or equal to 0.030 units/pLm2, 0.10 units/llm2, 0.30 units/llm2, 1.0 units/, um2, and it is even more preferable for it to be

greater than or equal to 3.0 units/, um2. FIGS. 2 and 3 illustrate diagrammatically the above area ratio and the density.

Next, the control of the surface roughness of the steel sheet according to an exemplary embodiment of the present invention is described below. Various notations may come to mind in terms of surface roughness, but in the present invention, it is possible to use the notation of Ra for surface roughness, and PPI which illustrates the number of irregular peaks (concavo-convex like peaks) in a 1-inch long section. This measurement method is not particularly limited, but it is common to use the tracer stylus method or the laser method, and to perform two-dimensional and three-dimensional measurements.

In the present invention, Ra should be less than or equal to 0.90 um, and PPI should be greater than or equal to 250. If Ra is too high or if PPI is too low, the characteristics of color tone, surface coating adhesiveness and weldability as desired in the present invention degrades due to surface irregularities (concavo-convex like peaks).

It is preferable for Ra to be less than or equal to 0. 80 um ; it is more preferable for it to be less than or equal to 0.70 u, m, or 0.60 u. m, and it is even more preferable for it to be less than or equal to 0. 50 am. Furthermore, it is preferable for PPI to be greater than or equal to 300, it is more preferable for it to be greater than or equal to 350 or 400, and it is even more preferable for it to be greater than or equal to 450 or even 500. Qualitatively, it is preferable for irregularities (concavo-convex like peaks) of equal height to exist at high density. The lower limit for Ra is not particularly specified, but it can be controlled to the desired value according to the nitriding conditions and the temper rolling conditions. However, the lower limit of Ra is does not include 0, and in practice, it is greater than or equal to 0. 02 um.

The upper limit of PPI is not explicitly specified as well, and can be controlled

to the desired value according to the nitriding conditions and the temper rolling conditions. Basically, in order to segregate the N such that the concentration of N in the vicinity of the surface can be high, Ra should be low and PPI should be high. One method of segregating N on the surface is to perform nitriding treatment in a comparatively short period of time within an ammonia atmosphere. Of course, the state of the surface may be affected by the grains diameter and steel constituents prior to that, as well as by the annealing temperature and the cold-rolling conditions, the reduction and pass number during temper rolling after nitriding treatment, the roll rouglmess and, if metal plating is performed, by the plating conditions. Therefore, it may be difficult to limit the state of the surface to a specific range, but the basic control is performed in the same manner as is conventionally performed, and it is possible to control this with no problems after a few trials.

Conventionally, in order to control roughness, it has been common to transfer the irregularities of the roller in the temper rolling process after annealing, or to perform configuration control through surface coating metals such as plating and in special electrolysis treatments. Furthermore, as the roughness is strongly dependent on the adhesiveness of the metal plating on the surface of the steel sheet, it has also been common to exhaustively perform configuration control on the coating materials used in plating. However, according to the present invention, as these conditions are rarely used, it is possible to obtain many advantages in production. For instance, conventionally, in terms of the irregularity of the roller, as the irregularities of the roller will gall through rolling, in order to set the irregularity of the surface of the steel sheet to a desirable range, it is not only necessary to perform regular exchanges of the roller or to machine the irregularities, production should be halted for these types of exercises, leading to an excessive burden in terms of productivity and labor cost.

In contrast, according to exemplary embodiments of the present invention, the surface state of the steel sheet is hardly affected by the temper rolling method, and therefore, it is possible to perform processing of a large number without needing to manage the galling of the irregularities on the roller. Furthermore, in regards to the state of the metal plating, it is possible to uniformly disperse the metal plating with exceedingly fine and uniform particles without needing to exhaustively control the plating conditions. As a result, it is possible to significantly increase the productivity in the manufacture of steel sheets. In general, it is common to perform temper rolling after annealing, but in the steel in the present invention, as fine cracking and irregularities (concavo-convex like peaks) can be formed even due to bending of the hearth roll when passing the sheet through a conventional continuous annealing line, there is no need for temper rolling.

The reason why the roughness of the surface of the steel sheet is not greatly affected by the method of generating that roughness or by those conditions is thought to be the fact that the cause of the roughness is the steel sheet itself, in other words, it lies in the dispersion state of the nitrides on the surface of the steel. Below, this mechanism is described. The steel according to the exemplary embodiment of the present invention is blanketed to an extremely large extent by nitrides in comparison to conventional steel, and is in a state that is different from the conventional steel sheets which are formed from basically homogeneous Fe. When the surface of the steel sheet is blanketed in places by nitrides, it is natural to imagine that there will be different surface characteristics between the portions where the nitrides are exposed and the portions where the steel base material itself is exposed. As the nitrides which cover the surface differ greatly in terms of deformation characteristics from the steel, the deformations due to the bending process corresponding to the skin pass rolling or to the transport of the plate during the

manufacture of the steel sheet are thought to differ in the micro-region between the nitride portion and the exposed steel sheet portion. As a result, the effects of the machining conditions such as the skin pass rolling will be moderated in terms of the surface rouglmess, and so the surface roughness is strongly dependent on only the state of the nitrides on the surface of the metal sheet. Furthermore, in the case where almost the entire surface is blanketed in nitrides, these nitrides will break microscopically due to even minor deformations, and to it is possible to form a uniform surface roughness.

Furthermore, in regards to this kind of nitride, its wettability and reactivity with coating materials which coat the surface of the steel sheet formed through surface treatments such as plating is different from that of the steel sheet itself, and as a result, it has the effect of changing the characteristics towards the desired direction. In other words, in conventional steel sheets (wherein nitrides do not exist), where the coating material coats the surface of the steel sheet relatively uniformly, if there are nitrides on one part of the surface of the steel sheet, the areas where the nitrides are exposed will differ in configuration (thickness etc. ) of the coating materials from the areas where the steel base material is exposed. As a result, the coating material during the surface treatment can be unevenly distributed, dependent on the existence of nitrides on the surface of the steel sheet. Therefore, by either finely dispersing the nitrides or the exposed steel on the surface of the steel sheet during the manufacture of the steel sheet or by blanketing the entire surface of the steel sheet with finely broken nitrides, it is possible to finely disperse the coating material during surface treatment regardless of the surface treatment conditions. Then, the coating material which has been unevenly distributed in this way (in other words, micro-scale non-uniformities of the coating material) may increase the color tone, adhesiveness and weldability of the material.

Next, the nitriding conditions are described below. From the viewpoint of

productivity, it is preferable to perform the nitriding conditions in the present invention at the same time as the recrystallization annealing process after the cold-rolling, or after this process, in succession with the recrystallization annealing process, but it is not limited to this. For the method of annealing, either the batch method or the continuous annealing method can be used. However, from the view point of uniformity of the materials within the coil of the nitrified material and from the view point of the productivity during the nitriding treatment, it is likely advantageous to use the continuous annealing method.

Furthermore, in order to obtain the great efficacy in controlling the materials within the inner layer as prescribed by the exemplary embodiment of the present invention, from the point that it is disadvantageous to increase the duration of the nitriding treatment and of the following thermal history, it may be preferable to perform at least the nitriding treatment in a continuous annealing apparatus. If there is no particular reason, then it can be used in continuous annealing. There may be many merits in partially controlling the atmosphere within the furnace during the continuous annealing process and in performed recrystallization in the first half, with nitriding in the second half such as increasing the ease of controlling the nitride configuration, the productivity and the uniformity of the material.

Furthermore, if nitriding treatment is performed prior to completing the recrystallization, the recrystallization can be significantly inhibited, and un-recrystallized structures may remain, leading to prominent degradation of the processing characteristics of the material, and therefore, sufficient care is required. This boundary can be determined by the steel constituents, the nitriding conditions and the recrystallization conditions, but for one skilled in the prior art, it should be easy to determine the conditions where there can be no un-recrystallized structures remaining after the appropriate trials. In the nitriding treatment, it is important to consider not only the

increase in the amount of N in the steel sheet, but to also consider the steel constituents and the recrystallization annealing conditions as well as the thermal history after nitriding treatment, and to consider the hardening of the sheet cross-section and the proliferation of N from the surface of the steel sheet to the inner part of the sheet when determining the conditions. Simply selecting a material based on its Rockwell hardness can not result in the preferable color tone, surface coating adhesiveness and weldability as desired in the present invention. In practical operation, these conditions should be decided after performing the appropriate number of trials, but the basic theory as is follows, and the present invention is prescribed based on that theory.

In other words, it is preferable to perform nitriding treatment where the plate temperature is within a range of 550 to 800°C. As in conventional annealing, the nitriding atmosphere is set to this temperature, and by passing the steel sheet through that atmosphere, it is possible to perform nitriding treatment in this range simultaneously.

Or, it is also acceptable to set the nitriding atmosphere to a lower temperature, and, by embedding the steel sheet which was heated to this temperature range into that atmosphere, to perform the nitriding treatment. When increasing the nitriding atmosphere to this temperature, there may be instances where the nitriding efficacy of the steel sheet may decrease due to deterioration of the atmosphere or degradation unrelated to the nitriding treatment of the steel sheet, so the range should be within a range of 550 to 750°C. It is preferable for the range to be within a range of 600 to 700°C, and it is even more preferable for the range to be within a range of 630 to 680°C.

In terms of the nitriding atmosphere, it is preferable for the ratio of nitriding gas to be greater than or equal to 10%. It is more preferable for it to be greater than or equal to 20%, and it is even more preferable for it to be greater than or equal to 40%. It is even better for it to be greater than or equal to 60%. As necessary, it is acceptable to set

the amount of hydrogen gas to be less than or equal to 90%, less than or equal to 80%, 60% or even 20%. Furthermore, as necessary, it is possible to set the ammonia gas contained to be greater than or equal to 0.02%. It is possible to include various inactive gases such as oxygen gas, hydrogen gas, or carbon dioxide gas for the remainder.

In particular, ammonia gas shows high efficacy in increasing the nitriding efficiency, and as it is possible to obtain the specific nitriding amount in a short period of time, it is possible to inhibit the proliferation of N into the center of the steel sheet, and to obtain favorable effects in terms of the present invention. Less than or equal to 0. 02% for this effect is sufficient, but it is preferable for this effect to be greater than or equal to 0. 1%, 0.2%, 1.0%, or even 5%. If the effect is greater than or equal to 10%, it is possible to obtain sufficient effects in less than 5 seconds of nitriding treatment.

Furthermore, from the point of nitriding efficiency, it is good when the ratio of the main gas components other than ammonia gas, in particular, of nitrogen gas and hydrogen gas, is greater than or equal to 1 (nitrogen gas) / (hydrogen gas), and it is possible to obtain even more efficient nitriding if this ratio is greater than or equal to 2. Furthermore, in conventional annealing processes, the annealing can be performed at conditions such as within an atmosphere of mainly nitrogen and hydrogen gas, where the material may not undergo nitriding, but for one experienced in the prior art, it should be possible to change the conditions under which nitriding will occur through modifications of the gas ratio, of the dew point, of the mixing of minute amount of gas regardless of the incorporation of the above ammonia gas after performing the appropriate trials. At the very least, the exemplary embodiment of the present invention has as its subject those items which have undergone nitriding treatment through a heat treatment including annealing, and which can be detected through current analytical methods.

The duration of time spent under the nitriding atmosphere is not particularly

specified, but considering the temperature conditions of the exemplary embodiment of the present invention (greater than or equal to 550°C), and the thickness of the steel sheet, which is at most 0.400 mm, the upper limit for this duration is set to 360 seconds, since likely it would not be possible to obtain the hardness distribution or the N distribution as desired in the present invention if the duration were longer, as the N embedded within the surface of the steel sheet due to nitriding treatment would reach the center layers of the steel sheet through dispersion of the N within the steel during the holding period.

Furthermore, 1 second is required to obtain the hardness distribution and the nitrogen in the thickness direction of the steel sheet as well as the nitriding amount required in the present invention, even through improving the nitriding efficiency. It is preferable to take within a range of 2 to 120 seconds, it is more preferable to take within a range of 3 to 60 seconds or within a range of 4 to 30 seconds, and it is even better to take within a range of 5 to 15 seconds. When controlling the duration to be a short period of time, it may be important to increase the nitriding efficiency through such method as increasing the ammonia concentration.

According to the exemplary embodiment of the present invention, it is preferable to control the dispersion state of the nitrides on the surface of the steel sheet, and it is effective to control the conditions during nitriding treatment as a method of controlling this dispersion effectively. Below, the gas nitride can be noted when using the most preferable ammonia gas.

The technical standpoint which follows in regards to this control method should be understood by those have ordinary skill in the art, and it should be relatively easy to apply different nitriding methods to this control theory. According to the exemplary embodiment of the present invention, in order to preferably control the dispersion state of nitrides on the surface of the steel sheet, when performing the nitriding treatment, the

ratio of (nitriding initiation sheet temperature (°C)-550)/ (concentration (%) of ammonia gas at nitriding initiation) < 150. The value which illustrates this equation (nitriding initiation sheet temperature (°C)-550)/ (concentration (%) of ammonia gas at nitriding initiation) should be less than or equal to 100, and it is preferable for it to be less than or equal to 50. When the sheet temperature at the beginning of the nitriding treatment is less than 550°C, the result of the above equation will be negative, but this case is also included within the present invention. When the denominator in the above equation is 0, no reasonable value can be obtained, but a value of 0 in the denominator means that no nitriding has occurred, and as the"nitriding initiation"part of the above equation will no longer apply, this case is automatically excluded. The meaning of the above equation, in other words, the technical meaning of this control, is as follows.

The dispersion state of the nitrides is dependent on the initial state of nitride formation, or to rephrase, it is greatly dependent on the state of nucleus formation of the nitrides, and therefore, the conditions at the initiation of nitriding affect the final dispersion state of the nitrides. Then, the nucleus formation of the nitrides is, in approximately the same manner as the deposits within conventional steel, a phenomena wherein high density and fine nucleus formation occurs in a state where the deposited elements are supersaturated at low temperatures. In other words, while the supersaturated elements may try to form some kind of deposit, if the temperature at that time is low, no dispersion may occur and the dispersion distance can grow small, resulting in a fine deposit configuration.

According to the exemplary embodiment of the present invention, as the process is controlled to a supersaturated state, it is preferable for the concentration of ammonia gas to be high. However, if the temperature it too low, insufficient nitriding can occur, and if the ammonia gas concentration is high, it may not be possible to sufficiently

control the super-saturation state. It may be difficult to accurately describe the optimal conditions of this state in numeral equation, but basically, according to the above description, it can be represented in the kind of shape as in the control equation of the present invention. The optimal state is where sufficient nitriding occurs, and in a temperature range where there is no excessive dispersion (for instance, when using ammonia nitriding, the temperature range is within a range of 550 to 700°C), and it is preferable to begin nitriding treatment at a relatively high gas concentration, and to perform nucleus formation of the nitrides at the surface of the steel sheet. This does not contain any temporal factors, but by performing nucleus formation over a period of time at a low temperature which inhibits dispersion, it should be possible to perform fine nucleus dispersion. In this type of case, the value of the above equation can be in the negative range, but inclusion in the present invention is as previously discussed.

The above exemplary control procedure is not limited to ammonia gas nitriding treatment, and therefore, the nitriding gas is not limited to ammonia. Furthermore, the nitriding treatment method is not limited to the gas nitride. In other words, the deposit dispersion is controlled using well-known metallurgy of nucleus formation, and if one is experienced in conventional steel materials, it should be easy to establish favorable conditions according to the specific nitriding treatment method.

Above, the case of gas nitriding treatment of the steel sheet has been described.

However, the procedure to obtain the steel sheet in the present invention is not limited to gas nitriding treatment, and it is also possible to perform this through liquid nitriding treatment, plasma nitriding treatment or ion injection. The present invention requires the blanketing of at least a certain region of the surface with nitrides, and therefore, as long as it results in a concentration of N on the surface, it is possible to use any other applicable method. In particular, if the process is one wherein the N distribution in the

thickness direction of the steel sheet is changed without increasing the total amount of N contained in the steel sheet and where N is concentrated on the surface only, modifications to the processing of the steel sheet can be minimized, which is optimal.

Furthermore, regardless of the process of nitriding the steel sheet, it is possible to realize the present invention in processes to reform the surface of the steel sheet through attaching some sort of N-containing material on the surface of the steel sheet.

In particular, if some kind of nitride is adhered to the surface which has low reactivity with the steel, then the effects of it on the characteristics such as processing of the steel sheet mother material itself may be reduced, which is also optimal.

In the manufacture of thin steel sheets for use in containers, there are situations where a second cold-rolling process is performed after the recrystallization annealing in order to adjust the roughness or the thickness of the sheet. This rolling reduction can be on the scale of 1% in the skin pass rolling performed to rectify the shape, or as much as 50%, the same as in cold-rolling. In the exemplary embodiment of the steel sheet according to the present invention, as the result of performing tests, changing the range of the rolling reduction of this second cold-rolling after nitriding treatment according to application within a range of 0 to 90%, there may be the same characteristic modifications through the increase of the rolling reduction as are seen conventionally, such as an increase in strength, a decrease in ductility and in deformation resistance after forming the can. However, the level of color tone, surface coating adhesiveness and weldability resulting from the surface deposits which is the characteristic of the present invention may be kept at levels equal to or greater that of conventional materials. In other words, as there is no disappearance of the characteristics of the present invention through performing a second cold-rolling, the second cold-rolling conditions can be determined according to the customer's needs within the range of prior art, and it is

possible to employ the same second cold-rolling process in the present invention as is used in conventional steels. When no rectification of the form is required, it is possible to avoid the second cold-rolling process entirely. Furthermore, when the goal is rectification of the shape, rolling can be performed at a rolling reduction in the range of 0.5 to 2.5%, but the steel in the present invention can also undergo a similar rolling treatment. If the second cold-rolling reduction is high, the steel sheet itself will be sufficiently hardened. Therefore, as it can be possible to obtain a sufficiently can strength without controlling the material distribution in the thickness direction as provided for the exemplary embodiment of the present invention, the significance of increasing the second cold-rolling reduction greatly beyond the conventionally used range diminishes.

Furthermore, as the processing characteristics of the material will fall if the second cold-rolling reduction grows, any unintentional use of a high second cold-rolling reduction should be avoided. Based on the above, when using a second cold-rolling operation on the steel in the present invention, it is preferable that the reduction be on the scale of 70%. This limit can be determined after considering the can strength and ductility, but, for instance, when using a second cold-rolling reduction which is greater than 70%, there would be no loss in the increased efficacy of the control of the surface deposits on the surface characteristics or weldability, which is the unique characteristic of the present invention.

If a second cold-rolling operation is performed in order to manufacture a hard material, it goes without saying that it is preferable to have a high second cold-rolling reduction. It is appropriate for this second cold-rolling reduction to be greater than or equal to 6%, 10%, 20%, or 30%, and it is even more preferable for it to be greater than or equal to 40% to increase the hardness. If maintaining ductility is preferred, then it goes

without saying that it is preferable to have a low rolling reduction in the second cold-rolling. It is good for this second cold-rolling reduction to be less than or equal to 50%, 40%, 30%, 20% or 10%, and it is even more preferable for it to be 5%, in order to secure the ductility of the steel sheet. From the view point of productivity, it is appropriate for the timing of the second cold-rolling to be after the specific heat treatment in processes where the recrystallization annealing and the specific heat treatment are performed continuously. However, if the recrystallization annealing and the specific heat treatment are performed in separate processes, it is possible to perform the second cold-rolling prior to the specific heat treatment.

Furthermore, when considering the weld location, in conventional materials, the material may undergo localized softening due to the weld heat, and there have been problems where the process strain during formation of the flange may accumulate and degrade the formation characteristics of the material. In the steel in the present invention which contains a large amount of N on the surface part, as this softening due to weld heat can be controlled, it is possible to obtain merits even in terms of formation characteristics of the weld location.

The exemplary embodiment of the present invention is applicable to steel sheets of thickness less than or equal to 0.400 mm. The reason for this is that, in steel sheets of thickness greater than this, it is difficult to have problems in color tone, surface coating adhesiveness and weldability of the formed material. It is preferable for the steel sheet in question to be less than 0.300 mm thick, and even more preferable for it to be less than or equal to 0.240 mm thick. In steel sheets of less than 0.190 mm of even 0.160 mm thickness, it is possible to obtain extremely prominent efficacy. In this way, by controlling the state of the nitrides on the surface of the steel sheet after nitriding treatment, it is possible to obtain the material properties unique to the steel in the present

invention, and which do not exist in steel which is nitrified with the purpose of only creating the surface hardness or of forming a steel which contains N. In other words, by performing configuration control of the nitrides on the surface of the steel sheet based on the nitriding conditions prescribed by the present invention, it is possible to obtain extremely favorable color tone, surface coating adhesiveness and weldability.

The effects of the exemplary embodiment of the present invention do not depend on the thermal history or manufacturing history after adjustment of the constituents or prior to annealing. The slab when performing hot-rolling is not limited to manufacturing methods such as the ingot method or the continuous casting method, and as it does not depend on the thermal history leading up to the hot-rolling process, it is possible to obtain the efficacy of the present invention even in thin cast slabs where the rough rolling has been omitted, or in the CC-DR method which directly hot-rolls the material without reheating the cast slab through the slab reheating method. Furthermore, regardless of the hot-rolling conditions, it is possible to obtain the efficacy of the present invention even through dual stage rolling where the finish temperature is separated into a + y phase, or through continuous hot-rolling where rough bar is joined and rolled.

Furthermore, when using the steel according to the exemplary embodiment of the present invention as a material in a container which has a weld location, by inhibiting the softening of the heat-affected part, the surface layer with a high concentration of N will undergo rapid quenching and will harden, so it is also possible to improve the strength of the weld location. This affect will become even more prominent when elements which inhibit softening of the heat-affected part even in conventional materials, such as B or Nb, are added.

The steel sheet according to the exemplary embodiment of the present invention includes those which have undergone some sort of surface treatment. In other words, in

steel sheets which are employed by the user after undergoing surface treatment, these sheets can require color tone or welding after the surface treatment, and so the favorable state of the surface of the steel sheet which is required in these characteristics may not be lost due to the surface treatment in the steel sheet manufactured as described above. Of course, there can be some change to the absolute values of Ra and PPI due to the surface treatment, but it is possible to adequately detect the function which optimizes the surface state of the steel sheet generated through controlling the nitride configuration on the surface, in other words, the state where multiple short irregularities have been formed even on the steel sheet which have undergone surface treatment. Based on this effect, it is possible to provide extremely favorable color tone and weldability even in the steel sheet which have undergone surface treatment.

The surface state of the steel sheet prior to performing the surface treatment is important in terms of the adhesiveness of surface coatings such as metal plating, paint, or organic films (laminate). In regards to these characteristics, by controlling the hardness in the thickness direction of the steel sheet according to the exemplary embodiment of the present invention, and by optimizing the surface state of the steel sheet, in other words, by making the state one wherein multiple short irregularities are formed, it is possible to provide an extremely favorable adhesiveness. A commonly used method in surface treatment is metal plating, where tin, chrome (tin-free), nickel, zinc or aluminum is used. Not only can the adhesiveness of these coatings be improved, but the color tone and weldability after formation of the coating can also be improved. Furthermore, even when performing a painting application after the metal plating on the steel sheet directly or on the base sheet for use in laminating the steel sheet which has been coated with an organic membrane, as have come into use in recent years, it is possible to increase the adhesiveness of the surface through the effects of the present invention.

The exemplary embodiments of the present invention may be used in all types of containers, whether they are 2-piece cans or 3-piece cans, and it is possible to use the present invention even when the above types of challenges are present in the application.

As an example of one exemplary embodiment of the present invention, a steel sheet coating with Sn, which is one of the most common types of steel sheet for use in containers, may be used, its color tone, surface coating adhesiveness and weldability can be evaluated.

The adhesiveness was measured through performing a T-type separation test on a test piece where a nylon adhesive was used to attach two layers of sheet on which a 25 mg/m2 layer of an epoxy-phenol paint had been applied on both surfaces. The adhesive was heated in application, this test piece was wet with tap water, and the separation strength was measured. Of course, items where the separation strength was high were determined to have good adhesiveness, and were judged to be excellent.

For color tone, 10, um may be painted on a transparent polyester resin which was then dried, and using a spectrophotometric colorimeter, the obtained value of L can be identified. If the value of L is high, this shows excellent color tone characteristics, and based on this value, the sample may be evaluated.

For weldability, a seam weld can be performed as is used in conventional 3-piece cans, changing the weld current, and seeking the weldability current range from the surface damage in the weld location due to the arc current between the surface of the steel sheet and the polar ring during welding as well as the strength of the weld location through splash generation (dust generation) during welding and through the peel test (Hyne test), it is possible to determine the width of the range and the lower limit value.

A broader range results in higher stability in manufacturing, and when the lower limit is low, there is unlikely to be property changes or separation of the plating due to the

increased temperature in the weld location. Using these facts, the sample can be evaluated. The roughness was measured using a laser three-dimensional roughness meter.

The steel sheet after nitriding treatment did have some change in N concentration in the thickness direction, but it is possible to use the average value of thickness in the present invention.

Performing hot-rolling, cold-rolling and recrystallization annealing on the steel of each component shown in Table 1, it is possible to manufacture each type of steel sheet. The amount of N shown in Table 1 is the average amount of N in the thickness prior to nitriding treatment. For one part of the materials, by controlling the temperature and the atmosphere of the nitriding furnace continuing after the high-temperature retention furnace used in the second recrystallization annealing, the sheet according to the conditions shown in Table 1 may be passed, and a nitriding performed. Nitriding-treatment was performed during or after the annealing, and the conditions were such where the recrystallization may be considered to have been complete prior to beginning nitriding.

Furthermore, the steel sheets can be manufactured using temper rolling. The rolling reduction conditions, the final sheet thickness, the analysis results of the amount of nitriding and the results of the evaluation of the characteristics for each steel is shown in Table 2. By controlling the state in the thickness direction within the range prescribed in the present invention by using the manufacturing method of the present invention, it can be confirmed that it is possible to obtain favorable color tone, surface coating adhesiveness and weldability. For one part of the materials where the nitriding was not performed, it may be possible to adjust the surface roughness by specifying the temper rolling conditions, but due to wastage of the roll and to the number of passes,

effective production was inhibited (the column for productivity notes"Poor").

Furthermore, there are instances where the evaluated value of the roughness of the steel sheet made through this special rolling meets the level of the steel in the present invention, but the characteristics of those materials did not reach the level of the optimal material of the steel according to the exemplary embodiment of the present invention. (Table 1) Steei Element (mass %) Nitriding beginning* ! Nitriding beginning *2'°", P)) C Si Mn S Al N Temp Time NH3 Temp Time NH3 NH3 Concentration oc Sec. % Sec. % (, C/% al 0. 001 0. 03 0. 1 0. 004 0. 003 0. 01 0. 0028 a2 0. 001 0. 03 0. 1 0. 004 0. 003 0. 01 0. 0028 750 2 10 550 10 1020 a3 0 001 0. 030. 1 0. 004 0. 003 0. 01 0. 0028 650 2 10 650 10 10 10 a4 0. 001 0. 03 01 0. 004 0. 003 0. 01 0. 0028 550 2 10 750 10 10 0 bl 0. 012 0. 03 0. 3 0. 011 0. 035 0. 06 0. 0011- : b2 0. 012 Q. 03 0. 3 0. 011 0. 035 0. 06 0. 0011 650 1 30 650 1 30 3 b3 0. 012 0. 03 0. 3 0. 011 0. 035 0. 06 0. 0011 650 1 30 650 3 30 3 b4 0. 012 0. 030. 3 0. 011 0. 035 0. 06 0. 0011 650 1 30 650 5 30 3 b5 0. 0 1 2 0. 03 0. 3 0. 0 1 l 0. 035 0. 06 0. 0011 650 1 10 650 5 30 to cl 0. 046 0. 45 0. 5 0. 0Q9 Q. 01 3 0. 15 0. 0043- c2 0. 046 0. 45 0. 5 0. 009 0. 013 0. 15 0. 0043 600 5 40 600 120 23 c3 0. 046 0. 45 0. 5 0. 009 0. 013 0. 15 0. 0043 400 5 60 600 360 20-3 c4 0. 046 0. 45 0. 5 0. 009 0. 013 0. 15 0. 0043 600 5 40 600 600 20 1 dl 0. 025 0. 02 0. 2 0. 033 0. 010 0. 08 0. 0022------ d2 0. 025 0. 02 0. 2 0. 033 0. 010 0. 08 0. 0022 750 2 0. 05 650 30 0. 05 4000 d3 0. 025 0. 020. 2 0. 033 0. 010 0. 08 0. 0022 600 2 0. 2 650 30 0. 2 250 d4 0. 025 0. 02 0. 2 0. 033 0. 010 0. 08 0. 0022 650 2 0. 2 650 30 0. 2 500 el 0. 036 0. 02 1. 5 0. 014 0. 010 0. 04 0. 0029---.-- e2 0. 036 0. 02 1. 5 0. 014 0. 010 0. 04 0. 0029 600 1 2 600 15 2 25 e3 0. 036 0. 02 1. 5 0. 014 0. 010 0. 04 0. 0029 650 1 2 600 ! 5 23 ? fl 0. 034 0. 92 0. 8 0. 022 0. 009 0. 02 0. 0094-- gl 0. 004 0. 120. 3 0. 011 0. 007 0. 001 0. 0134------ hl 0. 068 0. 250. 4 0. 007 0. 021 0. 02 0. 0242 *1, *2: The nitriding furnace is divided into an early stage and a late stage, and the plate temperature and gas concentration for each stage<BR> can be modified independently. (Table 2) t'ost-Temper Product nitriding N added Surface Surface region Region Ealuation Area ralfing plate Steel total N (mass aitrified. density.. _ _ _ _ amount ppm) ratio (n/o3 (unitslEun2) dhesivene Weldabil (mass %). _. mass % ai 0. 00280<0. 5i. Q <0. 0011 0. 15 0. 45 2601 b b c f Comparison a2 0. 0045 17 2 40. Q 0. 01 1 0. 1'0. 41 340 a b b e Invention a3 0. 0451 423 2 (>200 0. 06 0. 15 0. 35 370 a a a e Invention a4 0. 0244 2lfi 30 10. U O. I3 I 0. 15 t1. 45 310 b a a e Invention b] 0. OOn0<0. 51. 0<0. 00 ! 3 O. IS 0. 76 160 d d c e Comparison b2 0. 0031202803 CU5 0. 52, 390 b b a e Invention b3 0. 0167 156 5 30. 0 0. 5 3 0. 15 0. 31 430 a a a e Invention b4 0. 0299 288 10 60. 0 0. 9 3 0. 1S 0. 25 540 a a a e Invention b5 0. 0299 2884 10. 003 0. 15 0. 33 350 a a b e Invention cl 0. 00430 <0. 51. 0<0. 0011 0. 22 0. 32 300 b b b f Comparison c2 0. 0437 394 4t ? 4. 0 1. 5 1 0. 22 0. 16 350 a b a c Invention c3 0. 1175 1 132 8 (2. 0 4. 5 I 0. 22 0. 25 520 a b b e Invention c4 0. 2121 2078 >90 1. 0 2. 5 1 0. 2= 0. 33 310 b b b e Invention dl 0. 0022 _ eO. 5 1. 0 <0. 001 la olo 0. 25 150 d c e Comr) arison d2 0. 0027 5 1 3. 0-0. 01 10 0. 13 0. 25 350 b b a e Invention d3 0. 0046 24 7 13. 0 0. 03 10 0. 131 0. 29 390 a a a e Invention d4 0. 0046243400. 0210 0. 13 0. 20 260 b a b e Invention el 0. 029 0 <0. 5 I. 0 <O. OI 1 0. 35 0. 59 240 b c c e Comparison e2 0. 0032 3 30 140. 0 0. 40 1 0. 35 0. 63 380 a a a e Invention c3 0. 0032 3 IS 20. 00. 20I 0. 35 0. 49 330 a a Invention fl 0. 0094 0 <0. 5 1. 0 <0. OOI 1 0. 22 0. 48 230 c c c e Comparison gl 0. 0154 0 <0. 5 1. 0<0. 00tI 0. 25 0. 24 160 d c d e Comparison hl 0. 0242 00. 5 LOO. OOI 1 0. 20 0. 30 220 c d c e Comparison A: (Nitrified area ratio on the surface) I (nitrified area ratio on the cross-section at a thickness of 1/4 depth)<BR> a: Very good<BR> b ; Good<BR> c : Acceptable conventional level<BR> d: Unacceptable conventional level<BR> e: No problems in terms of productivity<BR> f : May cause inhibition of productivity

Above, the exemplary embodiments and variants of the present invention have been described. However, the present invention is not limited to these exemplary embodiments and variants. It is possible for additions, omissions, replacements and other modifications to be made to the structure without deviating from the purpose of the present invention. In addition, all references, publications and patent applications referenced above are incorporated here by reference in their entireties.

INDUSTRIAL APPLICABILITY According to the steel sheet for use in containers and the manufacturing method therefor in the present invention, it is possible to improve the color tone of the container, the surface coating adhesiveness and the weldability without needing to perform complex treatments after the nitriding treatment which could adversely affect productivity. In this way, it is possible to deterioration of the productivity of the manufacture of steel sheets for ultra-thin containers as well as to extraordinary efficacy in the manufacturing process.