Field of the invention

The present invention relates to a hot rolled steel sheet. In a further aspect the present invention relates to a method of manufacturing a hot rolled steel sheet.

Background of the invention

The property of a steel sheet that needs to be there in places in the bodywork with and without deformation, can be expressed by a waviness parameter. Steel sheets or strips that are highly formable and that show low waviness after forming into an end shape product are required in the automobile industry. Typically they are suitable for the manufacture of automotive body components such as automotive outer panels. Owing to their high formability, interstitial free steel grades can be the steel grades used for such automotive outer panels. Moreover, improvements in paint baking cycles of vehicles have become important in the automobile industry. This enables realising sublime paint appearance, a global improvement of the environment and a reduction of process cost for the paint baking cycle. With respect to realising sublime paint appearance, one of the issues that plays a role is the desired reduction of the thickness of the paint layers whilst keeping or improving the good paint appearance. This can be achieved if the surface of the steel sheet underneath the paint is of excellent surface quality. Thus, there is a need in the automobile industry to develop steel sheets that not only have good surface properties such as low waviness after forming but also have excellent formability.

It is remarked that the terms sheet and strip for a large part can be used interchangeably; in this specification sheet is defined as a part or former part of a strip.

Objectives of the invention

It is an object of the present invention to provide steel sheets that are having a good formability.

It is also an object of the invention to provide steel sheets having good surface properties such as a low waviness after forming.

It is another object of the invention to provide a potentially metal coated, e.g. galvanised, steel sheet which enables realising full finish quality steel products having a low waviness for good appearance.

It is another object of the invention to enable the manufacture of automotive outer panels that show little variance in the waviness due to different grades of deformation at different locations in the panels.

Description of the invention

The state of the art focuses on improving the metal coating process, such as the hot dip coating process, and on skin pass rolling. It is the aim of the present invention to improve the hot rolled steel sheet such that after further processing, such as cold rolling, annealing and galvanizing, the outer surface of a resulting sheet, blank or panel has an optimal surface.

The present invention seeks to provide a reliable solution to improve the formability and to have good surface properties such as a low waviness after forming and low variance of the waviness due to forming of a final product such as an automotive body panel. In this specification if millipercent or ppm is used, these mean millipercent by weight and ppm by weight respectively.

In a first aspect of the present invention, the steel sheet is in hot rolled condition and the steel has a composition in millipercent or where so indicated in ppm:

C: 1-50 ppm;

Mn: max. 200;

Si: max. 100;

Al: 10-200;

Ti: 46-100;

Nb: max. 100;

V: max. 100;

P: max. 20;

S: max. 20;

N: max. 100 ppm; wherein å(Ti+Nb+V) together max. 200; and optionally:

Cr: max. 100;

Ni: max. 100;

B: max. 5 ppm;

Ca: max. 10;

Cu: max. 100;

Mo: max. 100;

Sn: max. 50; the remainder being Fe and unavoidable impurities; characterised in that the composition and the hot rolling conditions are fine-tuned in such a way that:

HRT = 0.752 - 0.351 P + 0.004 G + 0.026 T < 0.10 (Equation A) wherein HRT is a parameter related to the microstructure, texture and final thickness of the steel in its hot rolled condition; wherein P is the total volume fraction of the texture components (112)<110> and (554)<225>, determined using Electron Back-Scatter Diffraction, EBSD, on a sample representing a cross section from the sheet parallel to the rolling direction with a length of at least 1.5 mm through thickness, whereupon the texture coefficients determined by EBSD are used to calculate the said volume fraction; and wherein G is the average grain size of the steel in hot rolled condition in pm based on the scanned area in EBSD; and wherein T is the final thickness of the steel sheet in hot rolled condition in mm, in order to enable attaining a SEP 1941 waviness value Wsa in the final formed product made from the cold rolled, annealed and optionally metal coated hot rolled steel sheet, of 0.32 pm or less, preferably 0.29 pm or less, more preferably of 0.28 pm to enable attaining a delta waviness of 0.1 pm or less, preferably 0.08 pm or less, more preferably 0.06 pm or less, wherein delta waviness is defined as the Wsa cup minus the Wsa flat wherein Wsa cup is the waviness value of a cup made from the cold rolled, annealed and optionally metal coated hot rolled steel sheet by equi-biaxial deformation of 4.5% with a Marciniaktool and Wsa flat is the waviness value of the cold rolled, annealed and optionally metal coated hot rolled steel sheet, wherein the finishing temperature is higher than Ar3 transformation temperature, wherein (112) and (554) are related to the normal direction, and wherein <110> and <225> are related to the rolling direction of the hot rolled steel sheet.

P is the total volume fraction of the texture components (112)<110> and (554)<225> . Here <110> and <225> are the directions that are related to the rolling direction. The (112) and (554) are related to the normal direction. The volume fraction of P is the sum of volume fraction of two orientations; (112)<110> and (554)<225>. The volume fraction of each component is expressed dimensionless. In the calculation of volume fraction, an angular spread of 11 degrees is used. Due to the thickness gradient of the microstructure and the texture, the volume fraction measured at different thickness positions may be different. All the measurements that are carried out here are over the whole thickness. The texture is determined using Electron Back-Scatter Diffraction, EBSD, on a sample representing a cross section from the sheet parallel to the rolling direction with a length of at least 1.5 mm through thickness, where upon the texture coefficients determined by EBSD are used to calculate the said volume fraction; and the average grain size of the steel in hot rolled condition in pm based on the scanned area in EBSD. An intermediate stage after hot rolling is cold rolling and skin passing, where the surface properties after final cold rolling and after skin passing are important to achieve the desired waviness. During the cold rolling part the steel is cold rolled wherein the roughness Ra as measured with a cut-off threshold of 2.5 mm, herein abbreviated to Ra2.5, of the work roll of the last stand of the cold rolling mill is lower than 4.5 pm but higher than 0.6 pm. The skin-pass operation may be carried out using an electric discharge textured work roll (EDT) for which the work surfaces have a roughness Ra2.5 comprised between 2.0 pm and 3.5 pm, preferably between 1.8 pm to 3.5 pm, more preferably between 1.8 pm to 2.5 pm. The elongation of the metal sheet during the skin-pass operation is comprised between 0.5% and 2%.

In the present invention, one of the objectives is to enable production of a strip, sheet or blank that in the cold rolled and annealed condition or in the cold rolled, annealed and metal coated condition has a low waviness value expressed as Wsa (1-5) according to standard SEP 1941 : 2012, ‘Measurement of the waviness characteristic value Wsa (1-5) on cold rolled metallic flat products’, designated hereinafter by the abbreviation Wsa. A low waviness Wsa of such a strip, sheet or blank allows reduction of the thickness of the paint layer used for attaining a given quality of paint appearance or, for a certain thickness of the paint layer, an improvement in the quality of the paint appearance. It is remarked that in SEP 1941 it is stated that the description is valid for non-deformed sheet metal in connection with changes in the waviness parameters due to forming operations. In an aspect of the present invention it is this change in the waviness that is defined herein as delta waviness that is sought to be minimised. It was found that if the features of the invention as claimed above are observed, an optimal steel sheet in hot rolled condition is realised, that can be advantageously used to manufacture cold rolled, optionally metal coated, steel sheet e.g. for the manufacture of automotive body parts. Wsa in the final formed product is then 0.30 pm or less, preferably 0.29 pm or less, more preferably of 0.28 pm or less and/or delta waviness is then 0.1 pm or less, preferably 0.08 pm or less, more preferably 0.06 pm or less. For a given thickness T, a suitable P or G has to be chosen to comply with Equation A (herein also referred to as Eqn (A)).

In a second embodiment of the invention, in the steel sheet at least one of the following elements in the steel is present in the mentioned ranges given in millipercent or where so indicated in ppm:

C: 1-30 ppm;

Mn: 10-200, preferably 40-180;

Si: 1-50, preferably 2-15;

Al: 10-100;

Ti: 46-95, preferably 46-90;

Nb: max. 90, more preferably max. 10;

V: max. 90, preferably max. 50, more preferably max. 10;

P: max. 15;

S: max. 15;

N: max. 80, preferably max. 60 ppm; wherein å(Ti+Nb+V) together max. 100, preferably max. 90; and optionally:

Cr: max. 60;

Ni: max. 60;

B: max. 4 ppm;

Ca: max. 5;

Cu: max. 60;

Mo: max. 60;

Sn: max. 30.

In one embodiment of the invention, the value of å(Ti+Nb+V) together is max. 100 or preferably max. 90. The advantage of having such a value is to avoid any clogging during continuous casting.

In an alternative embodiment of the invention, in the steel sheet at least one of the elements in the steel is present in the mentioned ranges given in millipercent or where so indicated in ppm:

C: 1-22 ppm;

Mn: 10-150;

Si: 1-13;

Al: 20-80;

Ti: 46-70;

P: 1-13;

S: 1-13;

N: 10-60 ppm; and optionally:

Nb: max. 3;

V: max. 5;

Cr: max. 50;

Ni: max. 50; B: max. 3 ppm;

Ca: max. 2;

Cu: max. 50;

Mo: max. 40;

Sn: max. 20.

In another embodiment of the invention, in the steel sheet at least one of the following elements in the steel is present in the mentioned ranges given in millipercent or where so indicated in ppm:

C: 1-21 ppm;

Mn: 40-130;

Si: 2-13;

Al: 30-70;

Ti: 50-70;

P: 2-13, preferably 1-5;

S: 3-13;

N: 10-40 ppm; and optionally:

Nb: max. 2;

V: max. 4;

Cr: max. 40;

Ni: max. 40;

B: max. 2 ppm;

Ca: max. 1 ;

Cu: max. 40;

Mo: max. 20;

Sn: max. 10.

The present invention in a further aspect relates to a steel sheet, which is characterised in that

HRT = 0.752 - 0.351 P + 0.004 G + 0.026 T < 0.09.

If the HRT value is lower, the waviness value and/or the delta waviness value of the resulting product is also lower. A further embodiment of the present invention relates to a steel sheet where the G is 22 pm or smaller, preferably 20 pm or smaller. If the value of G is lower, then the value of HRT is lower. It allows to achieve a fine graine size, which is favourable for the final product.

A further aspect of the present invention relates to a method for manufacturing a steel sheet as described above for each of the compositions described. The manufacturing method to produce a product from the hot rolled steel sheet according to the present invention embodiments may after the hot rolling step comprise steps such as a pickling step, a cold rolling step, an annealing step, a galvanising step, and a temper rolling or skin pass step. The steel with the specified composition according to the embodiments of the present invention is smelted, for example, in a converter and formed into a slab by a continuous casting process or the like.

The slab to be used is preferably produced by a continuous casting process to prevent macrosegregation of the components. The slab to be used may be produced by an ingot-making method or a thin slab casting process. Alternatively, after a slab is produced, in addition to a conventional method in which a slab is first cooled to a room temperature and then heated again, an energy saving process, such as hot direct rolling or direct rolling, may be applied. The energy saving process may comprise placing a slab into a heating furnace whilst keeping the slab temperature without cooling to a room temperature, or performing rolling immediately after keeping the temperature for a short time.

The slab used in the hot rolling step may be heated. In heating, the slab heating temperature is preferably as low as possible for the aim of energy saving. If however the heating temperature falls below 1150°C, the carbide is not sufficiently dissolved. In terms of an increase in the scale loss with increasing oxidation weight gains, the slab heating temperature is desirably 1250°C or lower.

In hot rolling, the slab is rolled at a hot rolling finishing temperature that is at or higher than the Ar3 transformation temperature, then cooled at an average cooling rate of 30°C/s or higher, and then coiled. Here, the Ar3 temperature is the temperature at which ferrite transformation starts in the cooling. If the hot rolling finishing temperature falls below the Ar3 temperature, both a and g phase are generated in the rolling, and as a result, the grain size of ferrite becomes large and the grains will be less uniform through the thickness. Therefore, the hot rolling finishing temperature is the Ar3 temperature or higher.

As a maximum hot rolling finishing temperature 970°C is preferred. In one embodiment of the invention, the method comprises a step of hot rolling the steel sheet; wherein the hot rolling finishing temperature is lower than 960°C. Thus, the method comprises a step of hot rolling the steel sheet; wherein the hot rolling finishing temperature is lower than 960°C, wherein the finishing temperature is higher than Ar3 transformation temperature. The maximum value of hot rolling finishing temperature is preferably Ar3+70°C . A further embodiment of the present invention relates to the method for manufacturing a steel sheet as described above, wherein the hot rolling finishing temperature is lower than 945°C.

Due to temperature gradient through thickness of hot rolled sheet in the final hot rolling step, it is recommended that if the gauge thickness of hot rolled sheet is thicker, the preferred hot rolling finishing temperature is lower. For example, fora thickness gauge of 3.5 mm, a finishing temperature of Ar3+50°C is chosen, while for thickness gauge of 4.7 mm, a finishing temperature of Ar3+30°C is preferred.

A further embodiment of the present invention relates to a method for manufacturing a steel sheet as described above, wherein reduction in the last stand of the hot rolling finishing mill is more than 15%, preferably more than 20%. By an increased reduction, the grain size G is finer and the texture component P is increased.

In a further embodiment, the present invention relates to the method for manufacturing a steel sheet as described above, wherein the steel sheet is cold rolled and wherein the roughness Ra measured with a cut-off threshold at 2.5 mm, herein abbreviated to Ra2.5, of the work roll of the last stand of the cold rolling mill is lower than 4.5 pm but higher than 0.6 pm.

During cold rolling, a total cold rolling reduction is generally comprised between 50% and 85%, so as to obtain a substrate with a thickness which is for example comprised between 0.2 and 2 mm. In some embodiments, an annealing step may be performed. The cold rolled substrate is then subjected to annealing conducted in a conventional way in an annealing furnace under a reducing atmosphere, with a view to recrystallization after the work hardening which it has undergone during the cold rolling operations. The annealing step is a step of heating the cold rolled steel sheet to a temperature of 650°C to 900°C. The annealing step may be performed preferably in a range of 780°C - 820°C. After recrystallization annealing, the steel sheet is cooled down to a temperature close to the bath temperature. After entry in the bath, the steel sheet or the substrate is metal coated on its two sides with a Zn based coating. The coating weight per surface can be 35 to 45 g/m ² . Then the metal sheet is subjected to wiping by means of nozzles, which project a wiping gas, placed on either side of the metal sheet. The wiping gas is ejected from each nozzle along a direction horizontal and orthogonal to the metal sheet. The running speed of the substrate in the production line and therefore in front of the nozzle is between 80 m/min and 160 m/min. This can be preferably greater than 100 m/min, or even 120 m/min.

The outlet of the nozzle is generally positioned at a distance 6 mm to 12 mm from the metal sheet along the main ejection direction. The outlet generally appears as a rectangular slot which extends, perpendicularly to the running direction of steel sheet substrate and over a width at least equal to the width of the metal sheet. The distance of the nozzle outlet to the metal sheet is preferably not greater than 10 mm and more preferably below 8 mm.

When the metal coated steel sheet is completely cooled, the metal sheet may undergo a temper rolling or tension levelling operation for giving texture to the outer surfaces of the metal coating. The transferred surface texture to the outer surfaces of the metal coating with sufficient roughness enables the metal sheet to retain sufficient amount of oil applied on the metal sheet, in order for its forming process to be properly carried out. The other purpose ofthis transferred surface texture is that it provides the metal sheet with desired low waviness properties.

The skin pass operation may be carried out using an electric discharge textured work roll (EDT) for which the work surfaces have a roughness Ra2.5 comprised between 2.0 pm and 3.5 pm, preferably between 1.8 pm to 3.5 pm; more preferably between 1.8 pm to 2.5 pm. The elongation of the metal sheet during the skin pass operation is comprised between 0.5% and 2%. All steel sheets are given a waviness Wsa of less than 0.35 pm and preferably less than 0.30 pm. The steel sheet optionally having been temper rolled or skin passed may then be cut out and undergoes a forming process, for example by stretching, drawing or bending, in order to form a part which may then be painted on each coating. After the deformation process, the outer surfaces of the metal coating of the part have a waviness Wsa value of less than or equal to 0.32 pm, or even less than or equal to 0.30 pm, or even less than or equal to 0.28 pm. This waviness may for example be measured after equi-biaxial deformation of 4.5% in the rolling direction and the transverse direction on the sheet plane, or approximately 9% through thickness direction.

In a further embodiment the present invention relates to a method for manufacturing a steel sheet as described above, wherein the steel is hot dip galvanised and temper rolled and wherein the roughness Ra2.5 of the work roll of the last stand of the temper rolling mill is in a range of 1.0 pm-5.0 pm. In a further aspect, the present invention relates to a steel sheet wherein the hot rolled sheet gauges are of thickness between 3 mm to 5 mm. Examples

The invention will now be further explained by means of the following, non-limiting examples. Steels having a chemical composition shown in Table 1 and the balance being Fe and unavoidable impurities were smelted in a vacuum melting furnace and slab material having a thickness of 225 mm was obtained.

Table 1: steel compositions all in milli percent except C and N in ppm. The I.E. represents invention examples and C.E. represents comparative examples. The underlined numbers are out of the scope of the present invention.

In the examples the slab thickness is approximately 225 mm, the transfer gauge is in the range of 35 mm to 40 mm, and the final gauge thicknesses of the hot rolled strips is in the range of 3 mm to 5 mm. The average target cooling rate after the finish rolling is at least 30°C/s, so that the microstructure of the hot rolled steel sheet becomes a more uniform microstructure. The coiling temperature is 750°C or lower, but higher than 550°C. Thus the microstructure of the hot rolled steel sheet becomes a more uniform microstructure and the scale loss due to oxidation is reduced. In all of the experiments, a pickling step is performed. The pickling step is a step of removing an oxide scale of a surface of a hot rolled steel sheet obtained in the hot rolling step by performing pickling. The cold rolling step is a step of cold rolling a pickled sheet after the pickling step. In some experiments, at least the last cold rolling pass is carried out with relatively smooth work rolls. In such a case, the surfaces of the rectified and non-etched rolls of the rolling mill directly in contact with the above mentioned sheet for ensuring its deformation, have a roughness Ra2.5 less than 0.5 pm. The aim for the waviness Wsa (1-5) of the cold rolled sheet is less than 0.40 pm. In the examples listed below, all steel sheets are cold rolled with such a surface characteristic. In the examples listed below, the roughness Ra2.5 of the work surfaces of the EDT work rolls for temper rolling is comprised between 2.50 pm and 3.0 pm. The elongation during the temper rolling operation is in the range of 1.0%-1.5%.

The hot rolling was performed with the process conditions shown in Table 2. The rest of process; pickling, cold rolling, annealing, galvanising and temper rolling were conducted as described in the previous section. The waviness after temper rolling, and thereafter biaxial stretching using a Marciniak tool at a strain of 4.5% are also shown in Table 2. From example 7 to example 15, Ra2.5, CR (surface roughness of the work roll used for cold rolling CR in the last stand of the cold rolling mill) is 0.5 pm and Ra2.5, TR (surface roughness of the work roll used for temper rolling TR of the temper rolling mill) is 2.8 pm.

Table 2.

Dependent on whether the microstructure and texture satisfy the HRT criterion as defined according to the present invention, the waviness Wsa after biaxial stretching turns out to be lower or higher, which leads to a bad (B), good (G) or excellent (E) surface appearance as can be seen in Table 2. The waviness values Wsa of the steel sheet surfaces are measured after the skin pass operation (flat) and after stretching (cup). The latter is carried out by equi-biaxial deformation of 4.5% with a Marciniak tool. The results of the measurements of Wsa are grouped in table 2. The use of parameter HRT satisfying Equation A gives the possibility of attaining a waviness Wsa after a skin pass and equi- biaxial deformation as described above of smaller than or equal to 0.30 pm. When the hot band microstructure and texture do not satisfy the HRT criterion, the waviness becomes considerably higher, as can be seen in Table 2. For higher values of hot rolled T, a lower value of finishing temperature can be selected. For samples 7,14,15, the hot rolled T has a higher value. In such cases it may be possible to lower the finishing temperatures to a lower value to comply with Eqn (A). Even though for sample 8, the hot rolled T has a higher value (4.69 mm), a lower the finishing temperature such as 921 °C is chosen so that it complies with Eqn (A). The waviness Wsa as mentioned herein is as defined in standard SEP 1941: 2012. The present invention has been described above with reference to several exemplary embodiments. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.