A METHOD FOR PRODUCING PRECIPITATION STRENGTHENED STEEL STRIP AND STEEL STRIP PRODUCED THEREBY

[0001] The present invention relates to a method for producing precipitation strengthened steel strip and steel strip produced thereby.

[0002] Steels with a carbon content of 0.035% and higher are readily produced by the BOS-process. For producing steels with a carbon content of lower than 0.035% vacuum degassing the steel melt in the BOS-plant is required. Vacuum degassing is a commonly used steelmaking process, used for removing dissolved gases (e.g. hydrogen) from the molten steel. In "Recent Advances in Ultralow-Carbon Steel refining Technology by Vacuum Degassing Processes" from Nippon Steel Technical Report No.61, April 1994 this process is described in more detail. In this process, the steel is exposed to a vacuum which promotes transfer of dissolved gases from the liquid steel to the gas phase. Exposure of steel to vacuum also promotes reactions between oxygen and carbon dissolved in the steel to produce carbon monoxide, by shifting the equilibrium conditions so that the carbon content of the steel is reduced. Extra low carbon steels, having a carbon content of between 0.01 and 0.025% (in weight, or between 100 and 250 ppm C) and ultra low carbon steels, having a carbon content of below 0.01% (below 100 ppm), can thus be produced on an industrial scale. Ultra low carbon steels having carbon contents of below 10 ppm C are attainable with this process.

[0003] The disadvantage of this process is that it requires dedicated installations, such as RH- or DH-degassers, which are capital intensive, and need large amounts of molten steel to function. Additionally, when the carbon has been removed from the steel, it is no longer available in the steel for later use.

[0004] It is an object of the invention to provide a method for producing steels strip but having strengths comparable or greater than extra low or ultra low carbon steels with or ultra low carbon content without vacuum degassing.

[0005] It is also an object of the invention to provide steels ultra low carbon content having an elevated strength in comparison to IF steels but having strengths comparable or greater than extra low or ultra low carbon steels with an ultra low carbon content produced by using the vacuum degassing technique.

[0006] The invention also aims at providing a low-cost method for producing steel strip but having strengths comparable or greater than extra low or ultra low carbon steels with an ultra low carbon content for packaging purposes

[0007] The invention also aims at providing a low-cost method for producing IF steel strip but having strengths comparable or greater than extra low or ultra low carbon steels with an ultra low carbon content for automotive purposes. [0008] According to a first aspect of the present invention a method for producing precipitation strengthened steel strip with an ultra low carbon content without a vacuum- degassing step during steelmaking, comprising the subsequent steps of:

• providing a cold rolled steel strip strengthened with precipitates having a composition consisting of

o carbon 0.12% or less;

o manganese 0.05 to 1.7%;

o silicon 0.6% or less;

o phosphorus 0.1% or less;

o sulphur 0.025% or less;

o aluminium 0.1% or less;

o copper, nickel, chromium each 0.080% or less

o nitrogen 0.015% or less;

o at least one of

- niobium 0.001% to 0.1%;

- titanium 0.001% to 0.15%;

- vanadium 0.001% to 0.2%;

- zirconium 0.001% to 0.1%;

- borium 0.0005 to 0.005%;

o the remainder being iron and unavoidable impurities,

wherein the strip comprises carbon in solute solution and wherein the precipitates consist of one or more of the group of carbides, carbo-nitrides and/or nitrides of niobium, titanium, vanadium, borium, and/or aluminium;

• reducing the solute carbon content of the cold rolled steel strip in a decarburisation process by subjecting the strip to a continuous annealing or a batch annealing process in a gaseous atmosphere comprising nitrogen, hydrogen and water vapour, to remove the carbon in the steel by reacting with the water vapour to produce carbon monoxide, followed by cooling to ambient temperatures;

• and wherein the precipitates comprised in the cold-rolled steel strip are retained in the decarburised steel strip, and wherein the carbon in solid solution is removed from the steel matrix.

[0009] All compositional percentages are given in wt.% unless otherwise indicated. After decarburisation a distinction is made between the carbon which is bound in the form of carbides or carbo-nitrides, and the carbon content of the matrix in which the precipitates are embedded. The phrase 'removed' in this context means that the method according to the invention endeavours to remove all carbon in solid solution from the matrix (but not from the precipitates). It will be clear that it is physically impractical to remove literally every carbon atom from the matrix, and a negligible amount of carbon may still be present in solid solution in the matrix without affecting the principle of the method which is to remove the carbon from the surrounding matrix as much as possible and retain the strengthening precipitates at the same time. The inventors found that the amount of carbon in the matrix after decarburisation is at the level of ultra low carbon steels , and that the total amount of carbon measured in the steel as a whole corresponds to the amount of carbon that is bound to the precipitates. Internal Friction measurements confirmed that the carbon in solid solution after decarburisation was between 0 and about 1 ppm, in other words, that all carbon still present in the steel was present as a precipitate. So the matrix of the steel strip according to the invention has a carbon content consistent with an ultra low carbon content, but precipitates consistent with a steel having a significantly higher carbon content. It therefore combines the best of two worlds: the high ductility of an IF steel and the high strength of a micro-alloyed low carbon steel.

[OOIO] Decarburisation is a process in which carbon in solid solution is removed from the steel matrix as a result of a reaction with an oxidant at the surface of the steel matrix. In the process according to the invention, the ratio of the hydrogen concentration to the water concentration is such that the iron of the steel matrix is not oxidized, whereas the carbon forms CO (g) and is removed. In itself this decarburisation process is a well-known process, and is (e.g.) used in the production of electrical steels. The texture and desirable magnetic properties of grain-oriented and non-oriented electrical steels are strongly dependent on carbon content as well, and therefore a decarburization stage for these steels is one of the most important phases in the production.

[0011] However, the process according to the invention does not relate to grain- oriented and non-oriented electrical steels, nor does it aim to influence the magnetic properties of the steel. The aim of the process according to the invention is to produce a steel matrix with a formability associated with a very low carbon content, and combine it with the strength of a precipitation or solid solution strengthened steel matrix. It is to be stressed that the decarburisation of the steel matrix according to the invention is a through-thickness decarburisation. A surface decarburisation only will not result in the formability associated with a very low carbon content.

[0012] The process according to the invention requires the production of a cold rolled steel strip having a composition in accordance with the main claim. The production of this cold rolled strip can be performed with any known method of slab or strip casting, followed by hot rolling the slab or strip into a hot rolled coil with the required properties, followed by cold rolling to the final thickness. Pickling and or cleaning steps may be involved in removing hot-rolled scale and/or cleaning the surface of the steel prior to and/or after cold rolling. [0013] It is important to note that the steel up to and including the cold-rolling step has a carbon content up to 0.12%. The minimum carbon content depends on the specific capabilities of the steelmaker, but is in any case at the minimum level consistent with a steelmaking process in which no vacuum degassing step is used. For practical purposes, the minimum carbon content of a steel that is practically and economically achievable without using a vacuum degassing step in the steelmaking process is 0.030%.

[0014] The invention is also embodied in a method for producing precipitation strengthened steel strip with a matrix having an ultra low carbon content without a vacuum-degassing step during steelmaking, comprising the subsequent steps of:

• providing a cold rolled steel strip strengthened with precipitates having a composition consisting of

o carbon 0.12% or less;

o manganese 0.05 to 1.7%;

o silicon 0.6% or less;

o phosphorus 0.1% or less;

o sulphur 0.025% or less;

o aluminium 0.1% or less;

o copper, nickel, chromium each 0.080% or less

o nitrogen 0.015% or less;

o at least one of

- niobium 0.001% to 0.1%;

- titanium 0.001% to 0.15%;

- vanadium 0.001% to 0.2%;

- borium 0.0005 to 0.005%;

o the remainder being iron and unavoidable impurities,

wherein the strip comprises carbon in solute solution and wherein the precipitates consist of one or more of the group of carbides, carbo-nitrides and/or nitrides of niobium, titanium, vanadium and/or borium;

• reducing the solute carbon content of the cold rolled steel strip in a decarburisation process by subjecting the strip to a continuous annealing or a batch annealing process in a gaseous atmosphere comprising nitrogen, hydrogen and water vapour, to remove the carbon in the steel by reacting with the water vapour to produce carbon monoxide, followed by cooling to ambient temperatures;

• and wherein the precipitates that were present in the cold-rolled steel strip prior to decarburisation are retained in the steel strip after decarburisation, and wherein the carbon in solid solution present in the steel matrix prior to decarburisation has been removed after decarburisation, resulting in a microstructure consisting of a precipitation strengthened decarburised steel matrix.

[0015] In an embodiment of the invention the cooling to ambient temperatures is performed in a non-oxidising atmosphere. By means of this process the surface of the strip after decarburisation is free, or substantially free, from oxides. This clean surface may serve as a substrate for coating with an inorganic coating, such as a zinc or zinc alloy based coating, or an organic coating, without the need for pickling or cleaning, or at least for a milder pickling or cleaning. The cooling process can also be performed in an oxidising atmosphere in which case a more severe pickling or cleaning may be required, unless the steel can be used as produced.

[0016] In an embodiment the process according to the invention provides a cold rolled steel strip having a carbon content of at least 0.030% and preferably of at least 0.035%.

[0017] Manganese, silicon, phosphorus, sulphur and aluminium are all selected to be consistent with an aluminium-killed, silicon-killed or aluminium-silicon killed Nb, V, Ti or Zr-containing HSLA steel (or combinations of Nb, V, Ti or Zr), or a boron containing steel or a boron containing HSLA steel, or a carbon steel strengthened with nitrogen. Nitrogen is present in all steels as an inevitable impurity, or as an alloying addition to form nitrides or carbonitrides or to act as a solid solution strengthening addition. These additions ensure that the steel after decarburisation still retains the strength associated with the aforementioned high strength steels.

[0018] Prior to the decarburisation, the carbon in the steel is typically either in solid solution in the steel matrix, or bound as carbides or carbonitrides in small precipitates, such as NbC. These carbides or carbonitrides can be formed at various stages such as during the hot rolling (deformation induced precipitation), during cooling after hot rolling on the run-out table of the hot strip mill (transformation induced precipitation or just as a result of the increased supersaturation upon cooling), or during cooling on the hot-rolled coil. These precipitates deliver an increase of strength compared to the steel matrix without these precipitates.

[0019] In the process according to the invention the decarburisation process after cold rolling ensures that the carbon in solid solution is removed from the whole steel matrix, so that the matrix becomes as ductile as the matrix of an ULC steel, but wherein the precipitates are still present in the matrix, thus ensuring that this ductile matrix has a strength which is considerably higher than the strength of an ULC steel. In the steels according to the state of the art, such a soft matrix is only attainable on an industrial scale by means of the vacuum degassing step in the steelmaking process, which is an expensive step, with significant associated capital investments and potential process and product control issues. Moreover, this vacuum degassing also removes the carbon before it has had a chance to precipitate as a strength enhancing carbide. [0020] Consequently, the decarburisation process is tailored to remove the carbon in solid solution and to retain the carbon in the precipitates. In an embodiment of the invention the carbon in solid solution, i.e. the carbon in the matrix, is at most 5 ppm.

[0021] However, not only must the strip be decarburised, the cold rolled strip must also be recrystallised after cold-rolling. Depending on annealing process conditions, the recrystallisation can occur before, during or after the decarburisation. If the recrystallisation happens before decarburisation, a comparison between an annealed structure and an annealed and decarburised structured show no relevant difference in texture. Otherwise, when there is an overlap between the recrystallisation and the decarburisation, or if the recrystallisation starts after the decarburisation, a change in texture impairing the formation of the a-fibre is observed..

[0022] The process according to the invention can be used in a batch annealing (BA) mode and in a continuous annealing (CA) mode. Although the timescales for a BA are much longer (hours instead of seconds), it is possible to arrive at a decarburised matrix with retained precipitates after BA. However, due to the long timescales, the decrease in strength and increase in ductility is more pronounced in the BA process, partly also due to the larger grains in a BA-microstructure.

[0023] Measurements of precipitate sizes before and after decarburisation revealed that the size distribution of the precipitates does not change. This is evidence of the persistence of the precipitates during the decarburisation and evidence of the decarburisation taking place by removing the carbon in solid solution only.

[0024] As the decarburisation is a diffusion driven process, the carbon starts to decrease at the surface. The speed of the decarburisation is determined by a combination of the dew point of the atmosphere in which the decarburisation takes place, the temperature at which decarburisation takes place, and the extent of the decarburisation after choosing these parameters is time dependent.

[0025] As a definition of the extent of decarburisation the following is taken : the carbon concentration at the start of the decarburisation is the sum of the solute carbon (C _s (t=0)), and the carbon bound in precipitates (C _p (t=0)). The premise is that the concentration of carbon bound in precipitates does not change during the decarburisation process (C _p (t=0) = C _p (t=∞)), wherein

C _p = Minimum [X, Y] + Maximum [Z, 0] + 12/93*Nb + 12/91*Zr + 12/51*V (1) wherein

X = 2*12/(2*32)*S;

Y = 2*12/(4*48)*(Ti-48/14*N)

Z = 12/48*(Ti -48/14*N + 4*48/(2*32)*S); and wherein Minimum[X, Y] = lower value of X and Y

Minimum[X, Y] = zero if Y is negative;

Maximum[Z, 0] = higher value of 0 and Z; and C _s = C '|max (2)

So a measure of the extent of the decarburisation is the decrease of Cs. Ideally, after complete decarburisation the solute carbon Cs(t=∞) is 0. So a measure of the decarburisation is:

C _D eca= C _p + Cs (t)/Cs(t=0) (3)

At the start (t=0) this value is C _max , at the end, the value is C _p . In the intermediate stages of decarburisation, a carbon gradient exists over the thickness of the strip, the Cs being practically zero at the surface, and being between the start value and zero in the centre. The carbon values in eq. (3) are average carbon contents over the thickness of the strip.

[0026] As a result of the decarburisation annealing of the cold rolled strip a degree of recovery of the cold-rolled full-hard microstructure is almost inevitable. In an embodiment of the invention a method is provided wherein the continuous decarburisation annealing or a batch decarburisation annealing process also causes the cold rolled steel strip to recrystallise.

[0027] In an embodiment of the invention a method is provided for producing steel strip with an ultra low carbon content wherein the average solute carbon content of the steel strip after the decarburisation process is at most 5 ppm, preferably at most 2 ppm, and more preferably at most 1 ppm and even more preferably 0 ppm. The lower the average solute carbon content, the more the matrix resembles an ultra low carbon matrix with the associated high ductility, but with the retained precipitates to ensure strength.

[0028] In an embodiment the inert or reducing gaseous atmosphere consists of:

(i) . HNX with a hydrogen content of between 1 to 12 vol.%, preferably at most 5.5 vol.%, preferably at least 1.5 vol.%, or

(ii) . Hydrogen only, or

(iii) . Hydrogen diluted with up to 20 vol.% nitrogen.

The hydrogen and optionally nitrogen are the main constituents of the inert or reducing gaseous atmosphere, although it is practically unavoidable that trace amounts of other gases are present as well. However, as long as these trace amounts do not affect the properties of the inert or reducing gaseous atmosphere these trace amounts may be allowable. The presence of water vapour affects the dewpoint of the gaseous atmosphere, and at the high temperature of the annealing process the water oxidises the carbon from the steel and drives it off as CO. The higher the dewpoint, the 'wetter' the gas, and the more rapid the decarburisation occurs. In a preferred embodiment the dew point during decarburisation is between -30 and + 40°C, preferably between -30 and + 15°C. Preferably, in case of using HNX, the gaseous atmosphere is HNX with a hydrogen content of between 1 to 12 vol.%, preferably of at most 5.5 vol.% and/or preferably of at least 1.5 vol.%.

[0029] In a preferred embodiment the decarburisation annealing is performed in a continuous annealing line, wherein the annealing is performed by subjecting the cold- rolled steel strip to the gaseous atmosphere at a temperature of between 700 and 850 °C, preferably between 700 and 800 and more preferably between 700 and 760 °C, for a period of between 100 and 480 seconds at a dew point between -15 and +40°C.

[0030] In an embodiment the strip is provided in the form of a coil and the decarburisation annealing is performed in a batch annealing furnace, wherein the annealing is performed by subjecting the cold-rolled steel coil to an annealing process until the cold spot of the coil has reached a temperature of between 500 and 700°C, and wherein the dew point of the gaseous atmosphere in the batch annealing furnace is between -30 and + 40°C, preferably between -30 and + 15°C, preferably wherein the coil of cold-rolled steel strip is provided in the form of an Open coil' in order to facilitate the decarburisation. However, even a tightly coiled strip can be successfully decarburised in a BA process, but the decarburisation is and faster if the access of the gaseous atmosphere is better, which is the case for an open coil, which consists of a coiled strip with spacers in between subsequent wraps of the coil.

[0031] In a preferred embodiment the cold rolled steel strip prior to the decarburisation process has a carbon content of at most 0.09 %, preferably at most 0.08%, or at most 0.07 or at most 0.06 or even at most 0.05 %. The lower the carbon content to start with, the shorter the decarburisation time, or the more complete the decarburisation in a given time. However, to retain sufficient strength the carbon content prior to the decarburisation process must be sufficient to obtain the desired strength level by creating enough and effective precipitates.

[0032] According to a second aspect of the invention a steel strip according to claim 9 is provided and its preferable embodiments according to the dependent claims 10 to 13.

[0033] In an embodiment the decarburised steel strip is cooled after the decarburisation annealing, and the cooling is preferably performed with dry HNX or hydrogen (the hydrogen optionally being diluted with up to 50 vol.% nitrogen). A dry gas can be characterised by having a dew point of between -40 and -80 °C. This cooling step will prevent oxidation of the strip.

[0034] In an embodiment the decarburised steel strip is further provided with one or more metallic and/or organic coatings after the decarburisation annealing. The metallic coating may be a known coating e.g. for use in automotive or building applications, such as a zinc-based or zinc-alloy-based hot dip galvanising step, or an electroplating step. Also, e.g. for packaging applications, the metallic coating may be useful, such as a tin layer, a nickel layer, a chromium layer, a chromium/chromium oxide layer, or an FeSn 1 : 1 layer. This metallic coating, which may consist of one or more different layers, is optionally combined with an organic coating. By means of a non-limiting example a decarburised steel strip according to the invention may be provided with a chromium/chromium oxide layer and subsequently with a polyester layer for canmaking purposes.

[0035] In an embodiment the organic coating on the decarburised steel strip consists of either a thermoset organic coating, or a thermoplastic single layer coating, or a thermoplastic multi-layer polymer coating.

[0036] In a preferred embodiment the thermoplastic polymer coating is a polymer coating system comprising one or more layers comprising the use of thermoplastic resins such as polyesters or polyolefins, but can also include acrylic resins, polyamides, polyvinyl chloride, fluorocarbon resins, polycarbonates, styrene type resins, ABS resins, chlorinated polyethers, ionomers, urethane resins and functionalised polymers. For clarification :

Polyester is a polymer composed of dicarboxylic acid and glycol. Examples of suitable dicarboxylic acids include therephthalic acid, isophthalic acid, naphthalene

dicarboxylic acid and cyclohexane dicarboxylic acid. Examples of suitable glycols include ethylene glycol, propane diol, butane diol, hexane diol, cyclohexane diol, cyclohexane dimethanol, neopentyl glycol etc. More than two kinds of dicarboxylic acid or glycol may be used together.

Polyolefins include for example polymers or copolymers of ethylene, propylene, 1- butene, 1-pentene, 1-hexene or 1-octene.

Acrylic resins include for example polymers or copolymers of acrylic acid, methacrylic acid, acrylic acid ester, methacrylic acid ester or acrylamide.

Polyamide resins include for example so-called Nylon 6, Nylon 66, Nylon 46, Nylon 610 and Nylon 11.

Polyvinyl chloride includes homopolymers and copolymers, for example with ethylene or vinyl acetate.

Fluorocarbon resins include for example tetrafluorinated polyethylene, trifluorinated monochlorinated polyethylene, hexafluorinated ethylene-propylene resin, polyvinyl fluoride and polyvinylidene fluoride.

Functionalised polymers for instance by maleic anhydride grafting, include for example modified polyethylenes, modified polypropylenes, modified ethylene acrylate copolymers and modified ethylene vinyl acetates.

Mixtures of two or more resins can be used. Further, the resin may be mixed with antioxidant, heat stabiliser, UV absorbent, plasticiser, pigment, nucleating agent, antistatic agent, release agent, anti-blocking agent, etc. The use of such thermoplastic polymer coating systems have shown to provide excellent performance in can-making and use of the can, such as shelf-life.

[0037] In a preferable embodiment the decarburised precipitation strengthened steel strip produced according to the invention has a solute carbon content of the decarburised and recovered and/or recrystallised steel of at most 5 ppm, wherein the decarburised matrix is strengthened by precipitates, which were formed prior to decarburisation, consisting of one or more of the group of carbides, carbo-nitrides and/or nitrides of niobium, titanium, vanadium and/or boron, is provided. The lower the average solute carbon content, the more the matrix resembles an ultra low carbon matrix with the associated high ductility, but with the retained precipitates to ensure strength.

[0038] The amount of carbon containing precipitates of the steel strip according to the invention is consistent with the carbon content of the steel prior to decarburisation, whereas the carbon content of the matrix of the steel strip according to the invention is consistent with the steel after decarburisation. So when the steel strip according to the invention is compared to an ultra low carbon steel with a carbon content in the matrix comparable to that of the steel according to the invention after decarburisation, the amount of precipitates in the ultra low carbon steel is much lower than in the steel according to the invention after decarburisation, because the decarburisation has preserved the precipitates which had already formed in the steel prior to decarburisation, but the decarburisation has removed the carbon in solute solution. So for the same carbon in solute solution, the steel according to the invention has a higher strength, and for the same strength the steel according to the invention has a better uniform elongation. This combination of low carbon in the matrix but with precipitates concentrations consistent with a much higher carbon content in the matrix can only be reached by the method according to the invention.

[0039] The invention is also embodied in a decarburised precipitation strengthened steel strip produced by decarburising a precipitation strengthened cold-rolled strip with a matrix having an ultra low carbon content, the precipitation strengthened cold-rolled strip comprising (in wt.%) :

o carbon 0.12% or less;

o manganese 0.05 to 1.7%;

o silicon 0.6% or less;

o phosphorus 0.1% or less;

o sulphur 0.025% or less;

o aluminium 0.1% or less;

o copper, nickel, chromium each 0.080% or less

o nitrogen 0.015% or less; o at least one of

- niobium 0.001% to 0.1%;

- titanium 0.001% to 0.15%;

- vanadium 0.001% to 0.2%;

- borium 0.0005 to 0.005%;

o the remainder being iron and unavoidable impurities,

wherein the cold-rolled strip before decarburisation comprised carbon in solute solution and wherein the precipitates providing the precipitation strengthening consisted of one or more of the group of carbides, carbo-nitrides and/or nitrides of niobium, titanium, vanadium and/or borium ;

• wherein the decarburised precipitation strengthened steel strip comprises a microstructure consisting of precipitates that were present in the cold-rolled steel strip prior to decarburisation and retained in the steel strip after decarburisation, and a steel matrix from which the carbon in solid solution present in the steel matrix prior to decarburisation has been removed after decarburisation, resulting in a final microstructure consisting of a precipitation strengthened decarburised steel matrix.

[0040] According to a third aspect the invention is also embod ied in the use of the steel strip according to the invention for the production of cans or containers for the packaging of food, beverages or non-food products or batteries or fuel cells or for the production of steel parts and/or for automotive applications. These types of applications often require good formability and a high strength .

[0041 ] The invention will now be further explained by means of the following non- limitative examples.

Table 1 : Grades (in wt% (otherwise in ppm), other elements are at impurity level and balance Fe). Cp and Cs computed from eq . ( 1) and eq . (2).

Grade C Mn Al s Nb V Ti N Cs *

(ppm) (ppm)

A 0.075 0.20 0.040 0.005 -- - -- 0.0033 0 750 R

C 0.045 0.5 0.030 0.006 0.013 - -- 0.0040 16.7 433 I

D 0.045 0.5 0.030 0.006 0.013 - -- 0.0120 16.7 433 I

E 0.070 0.50 0.030 0.004 0.026 - -- 0.0040 33.6 566 I

F 0.045 0.60 0.030 0.005 0.013 0.13 -- 0.0080 322.6 127 I

G 0.002 0.18 0.040 0.005 -- -- -- 0.0017 — 20 R

H 0.005 0.35 0.040 0.004 0.040 -- -- 0.0030 51.6 0 R

I 0.002 0.13 0.030 0.005 -- -- 0.055 0.0030 93 0 R

J 0.033 0.52 0.004 0.0021 0.006 0.009 0.032 0.001 92.5 237 I

K 0.027 0.10 0.057 0.0016 -- 0.005 0.039 0.001 94.7 175 I L 0.058 0.60 0.002 0.0019 -- -- -- 0.010 0 580 R

M 0.063 0.91 0.030 0.005 -- 0.109 0.050 0.0028 306 324 I

I : inventive example; R: reference example

[0042] A steel is a low-carbon steel used for comparison with C steel. Steel C and L are niobium based micro-alloyed steels, and D is a niobium based micro-alloyed steel with extra nitrogen. Steel E is a niobium based micro-alloyed steel with a higher carbon and niobium content compared to steel C. Steel F is a niobium and vanadium based micro- alloyed steel. Steel G, H and I are ultra-low carbon steels, one without microalloying elements, one microalloyed with niobium, and the other microalloyed with titanium. Grades F, G, H and I are comparative samples and are all produced using vacuum degassing in the steelmaking process. Steel J and K are Ti-V based steels, and steel M is a higher carbon and high Ti-V microalloyed steel.

[0043] Samples of these materials were all heat treated in the same way. The only variable in the heat treatment was the dew point. It was set to -60 °C for the samples which had to remain non-decarburised and it was varied between -5 °C to + 10 °C to obtain decarburised steels.

Table 2 : Comparison of mechanical properties of decarburised and non-decarburised grades (CA = continuous annealing at 750°C for 170 s, BA = batch annealing with cold spot at 640°C). The samples were JIS5 format and tested according to according to EN 10002-1/ISO 6892-1.

Grade BA/CA Gauge (mm) YS (MPa) TS (MPa) Ag (%) Decarburised

Al CA 0.25 330 345 26 Yes

A2 CA 0.25 355 390 22 No

CI CA 0.27 255 380 22 Yes

C2 CA 0.27 380 450 15 No

Dl CA 0.99 320 420 19 Yes

D2 CA 0.27 300 405 19.5 Yes

D3 CA 0.99 390 480 15.5 No

D4 CA 0.27 395 490 16 No

D5 CA 0.27 305 410 17.7 No

El CA 0.25 230 365 19 Yes

E2 CA 0.25 390 410 12 No

Fl CA 0.28 280 400 19 Yes

F2 CA 0.28 383 490 14.6 No

Cl l BA 0.27 240 380 22 Yes

C12 BA 0.27 200 410 20.4 No

G CA 0.23 220 352 15.6 No H CA 0.33 195 350 20 No

I CA 0.30 150 328 26 No

With YS= yield stress, TS= tensi e stress and Ag= uniform elongation.

[0044] Note that sample CI (decarburised) and A2 (not decarburised) show a similar tensile strength. This supports the hypothesis that the precipitates in CI are able to compensate for the strength loss of as a result of the decarburisation. The increase in tensile strength of about 40 MPa for CI compared to Al is the result of the presence of precipitates. Figure 2 show that the loss of strength after decarburisation originate essentially from the removal of the carbon in solid solution, because the precipitate distribution before and after decarburisation is largely unaffected by the decarburisation (compare E2 with El) .No obvious loss of precipitate after decarburisation is observed. Moreover, Figure 3 shows that after decarburisation the carbon concentration remaining through the entire sample is equal to the carbon bound to carbides (C _p ). This supports that the loss of strength after decarburisation is mainly due to carbon removal. The comparison between CI and D2 shows that the presence of N not only increases the strength, but also improves the work hardening (YS/TS) of the decarburised grades. Figure 4 shows the GDEOS profile for C and Mn of non decarburised and partially decarburised D samples, The partially decarburised samples has not a flat profile (higher C content in the sample strip thickness centre). The comparison between samples D2 fully decarburised and D5 partially decarburised indicates that an incomplete decarburisation reduces the strength but has a limited effect on the uniform elongation.

[0045] The crystallographic texture evolution between the decarburised and non- decarburised grades has been determined using the Orientation Distribution Function (ODF) based on XRD-measurements and is used for a qualitative comparison of the texture before and after decarburisation annealing. If recrystallisation occurs before decarburisation, then the texture will be that of a higher carbon matrix, and be similar to that of the not-decarburised sample and if recrystallisation occurs after decarburisation then the crystallographic texture will have more resemblance to an ultra low carbon steel. The latter results in a decrease of the a-fibre compared to the non-decarburised samples.

[0046] Brief description of the drawings: Figure 1 describes the uniform strain (%) as a function of the tensile strength (MPa) for de-carburised low-carbon and microalloyed material compared to material that was already low in carbon before the annealing. So even if the carbon contents of the standard material and the decarburised material is comparable, the strength of the decarburised material is still higher whilst the strain values remain good. [0047] Figure 2 shows the number of particles (Y-axis) against equivalent particle diameter (in nm, X-axis) as determined by quantitative TEM-analysis, showing that the precipitate distribution is largely unaffected by the decarburisation. The left hand graph is non decarburised (E2), and the right hand graph is decarburised (El).

[0048] Figure 3 and figure 4 shows the results of a Glow Discharge Optical Emission Spectroscopy (GDOES) measurement for carbon and manganese. With this the chemical composition as a function of depth can be determined. An argon plasma above the surface of the sample is ignited and material is removed at a relatively high rate. The removed atoms are excited and therefore emit light in the visible range and this is measured with a variety of spectrometers. The GDOES-equipment used has a detection limit of 0.001wt% for carbon. According to these measurements and disregarding the artificial surface effect, the carbon content after decarburisation is equal to Cp (eq. 1), i.e. the amount of carbon bound to carbides for sample D2 and has a flat profile. This finding was consistent for all decarburised steels. In figure 4 the carbon levels of the undecarburised and partially decarburised steel are shown as well. The carbon level of uncarburised steel reaches the bulk value quite rapidly whereas for the partially decarburised steel the profile is not flat. For the partially decarburised steel the amount of carbon measured by GDOES was 40 ppm, whereas the amount of carbon bound to the Nb was 16.7 ppm. So the remaining carbon in solid solution for the partially decarburised steel is estimated to be C _s = C _me as-C _p = 40 - 16.7 = 23.3 ppm (with total C concentration measured after decarburisation).. It is noted that the The manganese level was identical before and after decarburisation showing that the decarburisation does not affect the manganese level. This finding was consistent for all metallic alloying elements for all decarburised steels.

[0049] Figure 4 shows some further GDOES measurements of steel D and E (the artificial surface effect should be disregarded). Steel E is fully decarburised (there is a flat and horizontal line from the surface to the centre of the strip), meaning that the amount of carbon measured is the amount present as precipitates, whereas steel D shows a lower value of carbon at the surface and an increasing level towards the centre of the strip. It is clear that the decarburisation is not yet complete because of the gradient in carbon content. The reason for steel D being below E at the surface (e.g. at 20 μιτι depth) is that steel E contains twice as much niobium, resulting in more carbon bound to Nb (Cp is higher).

[0050] Figure 5 shows the ODF-texture components of the various fibres in the Phi2=45° section. Figure 6, 7 and 8 show measurements of the intensities in the Phi2=45° section for Cl l and C12, for CI, C2 and for Al, A2 as well as the maximum intensity. There is no significant difference in the texture between CI (BA) and C2 (BA) (figure 6), as well as between Al (CA) and A2 (CA) (figure 8). However, the intensity of the a (=rotated cube) has dropped in C2 (CA) from 23 compared to 16.6 for CI (CA) (fig. 7). This is believed to have been caused by the fact that for sample A2 (CA) and C2 (BA) the recrystallisation is achieved at low temperature (full recrystallisation estimated at 680 °C for A2 (CA) and at 600 °C for C2 (BA) and for sample C2 (CA) the full recrystallisation is estimated at 725 °C, whereas the grade is already much more decarburised at that temperature.