JP7748014B2 - steel plate - Google Patents

Description

The present invention relates to a steel plate.

In recent years, the automotive industry has been seeking to reduce the weight of vehicle bodies in order to improve fuel efficiency. Increasing the strength of the steel plates used is one effective way to achieve both a lighter vehicle body and crashworthiness, and this is why the development of high-strength steel plates is progressing. However, as strength increases, the workability of steel plates generally decreases. For this reason, when developing high-strength steel plates, it is important to increase strength while maintaining a certain level of workability.

In this regard, for example, Patent Document 1 describes a steel sheet having a predetermined chemical composition, in which, in a cross section parallel to the rolling direction and thickness direction of the steel sheet, the metal structure at a depth of 1/4 of the sheet thickness from the surface is, in area %, 10% or more retained austenite, 60 to 80% tempered martensite, and less than 20% martensite, and in which the tempered martensite and the martensite have a low-angle grain boundary density of 0.20 to 1.0 μm ⁻¹ where the crystal orientation misorientation is 2 degrees or more but less than 20 degrees, and a high-angle grain boundary density of 0.30 to 0.60 μm ⁻¹ where the crystal orientation misorientation is 20 to 50 degrees, and the ratio A _L /A _{N of the particle density A L in} the rolling direction of the retained austenite to the particle density A _N in the sheet thickness direction is 0.80 to 1.0. Patent Document 1 also teaches that by setting the low-angle grain boundary density to 0.20 to 1.0 μm ⁻¹ , transformation-induced plasticity of austenite occurs at high stress, resulting in an excellent balance of strength and ductility, while by setting the high-angle grain boundary density to 0.30 to 0.60 μm ^{−1 ,} the progression of ductile fracture is suppressed, resulting in excellent impact properties, and further by setting the particle density ratio A _L /A _N to 0.80 to 1.0, retained austenite becomes isotropically dispersed, resulting in excellent bendability and impact properties.

Patent Document 2 describes a high-strength steel plate having a specified chemical composition, in which the steel structure is, in area percentage, 35% to 80% polygonal ferrite and 5% to 25% martensite, and in volume percentage, 8% or more retained austenite, the polygonal ferrite has an average grain size of 6 μm or less, the martensite has an average grain size of 3 μm or less, the retained austenite has an average grain size of 3 μm or less, the polygonal ferrite, the martensite, and the retained austenite each have an average aspect ratio of 2.0 or less, and the amount of Mn (mass %) in the retained austenite divided by the amount of Mn (mass %) in the polygonal ferrite is 2.0 or more. Furthermore, Patent Document 2 teaches that, in order to ensure good ductility, it is necessary to increase the amount of stable retained austenite in which Mn is concentrated, and therefore it is extremely important that the value obtained by dividing the Mn amount (mass%) in the retained austenite by the Mn amount (mass%) in polygonal ferrite be 2.0 or more.

International Publication No. 2021/200169 International Publication No. 2016/067625

As mentioned above, it is known that increasing the strength of steel sheet reduces its workability, specifically its properties such as elongation. In this regard, Patent Document 2 teaches that when the carbon content in the retained austenite, in relation to the manganese content in the retained austenite, satisfies the following relationship: 0.09 x [Mn content] - 0.130 - 0.140 ≤ [C content] ≤ 0.09 x [Mn content] - 0.130 + 0.140 (where [C content]: carbon content in the retained austenite, [Mn content]: manganese content in the retained austenite), a large amount of stable retained austenite is obtained, which allows the deformation-induced transformation (TRIP) phenomenon, a key factor in improving ductility, to occur intermittently until the end of the steel sheet processing, thereby achieving both high strength and better elongation. The invention described in Patent Document 2 is primarily directed to steel sheet with a tensile strength of 590 MPa or more. On the other hand, in the automotive industry and the like, further weight reduction of steel sheets is required, and in order to achieve such weight reduction, it becomes necessary to make the strength of steel sheets higher than ever before. Therefore, there remains a high demand for steel sheets that can improve elongation even when strength is increased to the same level as or higher than conventional steel sheets.

Therefore, the present invention aims to provide a steel sheet that can achieve high strength and improved elongation through a novel structure.

In order to achieve the above-mentioned objectives, the inventors conducted research, focusing particularly on the microstructure of steel sheets. As a result, the inventors discovered that by configuring the microstructure of a steel sheet having a predetermined chemical composition primarily from tempered martensite, ferrite, and retained austenite, high strength can be achieved while improving elongation; furthermore, by limiting the Mn concentration in the ferrite within a predetermined range, solid solution strengthening of the ferrite can be suppressed, and by concentrating Mn in the retained austenite to stabilize the retained austenite, the elongation of the steel sheet can be significantly improved, leading to the completion of the present invention.

The present invention, which has achieved the above object, is as follows.
(1) in mass %,
C: 0.200-0.350%,
Si: 0.01-2.00%,
Mn: 1.40-4.00%,
P: 0.1000% or less,
S: 0.0200% or less,
Al: 2.00% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Cr: 0-2.000%,
Mo: 0-1.000%,
Ti: 0 to 0.500%,
Nb: 0 to 0.500%,
B: 0 to 0.0100%,
Cu: 0 to 1.000%,
Ni: 0-1.000%,
W: 0-0.100%,
V: 0-1.000%,
Ta: 0-0.100%,
Co: 0-3.000%,
Sn: 0-1.000%,
Sb: 0 to 0.500%,
As: 0 to 0.050%,
Mg: 0 to 0.050%,
Zr: 0 to 0.050%,
Ca: 0-0.0500%,
Y: 0 to 0.0500%,
La: 0 to 0.0500%,
Ce: 0 to 0.0500%,
Bi: 0 to 0.0500%, and the balance: Fe and impurities;
The formula satisfies 1.00≦[Si]+[Al]≦2.20, wherein [Si] and [Al] are the contents (% by mass) of each element, and the chemical composition is
In terms of area ratio,
Ferrite: 15 to 40%,
Tempered martensite: 40-65%,
Retained austenite: 10 to 20%
Bainite: 10-30%,
Pearlite: 0 to 10% and As-quenched martensite: 0 to 10%
a value obtained by dividing the average Mn concentration in the ferrite by the Mn concentration in the base material is 0.980 or less;
A steel sheet characterized by having a microstructure in which the value obtained by dividing the average Mn concentration in the retained austenite by the Mn concentration in the base material is 1.150 to 2.000.
(2) The chemical composition is, in mass %,
Cr: 0.001-2.000%,
Mo: 0.001 to 1.000%,
Ti: 0.001 to 0.500%,
Nb: 0.001-0.500%,
B: 0.0001 to 0.0100%,
Cu: 0.001 to 1.000%,
Ni: 0.001 to 1.000%,
W: 0.001-0.100%,
V: 0.001-1.000%,
Ta: 0.001 to 0.100%,
Co: 0.001 to 3.000%,
Sn: 0.001 to 1.000%,
Sb: 0.001 to 0.500%,
As: 0.001 to 0.050%,
Mg: 0.0001-0.050%,
Zr: 0.0001 to 0.050%,
Ca: 0.0001-0.0500%,
Y: 0.0001-0.0500%,
La: 0.0001 to 0.0500%,
Ce: 0.0001 to 0.0500%, and Bi: 0.0001 to 0.0500%
The steel sheet according to (1) above, characterized in that it contains at least one of the following:
(3) The steel sheet according to (1) or (2) above, characterized in that the average C concentration in the retained austenite is 0.80 mass% or more.
(4) The steel sheet according to any one of (1) to (3) above, characterized in that it includes a plate thickness center portion and a surface-softened portion arranged on one or both sides of the plate thickness center portion, and the surface-softened portion has an average thickness of 10 μm or more and an average Vickers hardness of 0.90 times or less the average Vickers hardness at a position halfway through the plate thickness.
(5) The steel sheet according to any one of (1) to (4) above, characterized in that the tensile strength is 1180 MPa or more.
(6) A part comprising the steel sheet according to any one of (1) to (5) above.

The present invention provides steel sheets that can achieve high strength and improved elongation.

<Steel plate>
The steel plate according to the embodiment of the present invention has, in mass%,
C: 0.20-0.35%,
Si: 0.01-2.00%,
Mn: 1.40-4.00%,
P: 0.1000% or less,
S: 0.0200% or less,
Al: 2.00% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Cr: 0-2.000%,
Mo: 0-1.000%,
Ti: 0 to 0.500%,
Nb: 0 to 0.500%,
B: 0 to 0.0100%,
Cu: 0 to 1.000%,
Ni: 0-1.000%,
W: 0-0.100%,
V: 0-1.000%,
Ta: 0-0.100%,
Co: 0-3.000%,
Sn: 0-1.000%,
Sb: 0 to 0.500%,
As: 0 to 0.050%,
Mg: 0 to 0.050%,
Zr: 0 to 0.050%,
Ca: 0-0.0500%,
Y: 0 to 0.0500%,
La: 0 to 0.0500%,
Ce: 0 to 0.0500%,
Bi: 0 to 0.0500%, and the balance: Fe and impurities;
The formula satisfies 1.00≦[Si]+[Al]≦2.20, wherein [Si] and [Al] are the contents (% by mass) of each element, and the chemical composition is
In terms of area ratio,
Ferrite: 15 to 40%,
Tempered martensite: 40-65%,
Retained austenite: 10 to 20%
Bainite: 10-30%,
Pearlite: 0 to 10% and As-quenched martensite: 0 to 10%
a value obtained by dividing the average Mn concentration in the ferrite by the Mn concentration in the base material is 0.980 or less;
The steel is characterized by having a microstructure in which the value obtained by dividing the average Mn concentration in the retained austenite by the Mn concentration in the base material is 1.150 to 2.000.

As mentioned above, it is known that the workability of steel sheets decreases as their strength increases, specifically with a decrease in properties such as elongation. In relation to this, TRIP (Transformation Induced Plasticity) steel sheets, which utilize the transformation-induced plasticity of retained austenite, are known as a means of achieving both high strength and improved elongation. When retained austenite is included in the microstructure, the elongation of the steel sheet is generally improved due to the TRIP effect, in which the steel sheet transforms to martensite through strain-induced transformation during deformation. Therefore, the inventors conducted research, focusing particularly on microstructures containing retained austenite, in order to achieve both high strength and improved elongation in steel sheets.

First, the inventors discovered that optimizing the chemical composition of a steel sheet and configuring the steel sheet's microstructure to primarily contain tempered martensite, a hard and tough structure, ferrite, a soft structure, and retained austenite exhibiting the TRIP effect—more specifically, configuring the steel sheet to contain, by area ratio, 40-65% tempered martensite, 15-40% ferrite, and 10-20% retained austenite—can improve elongation while maintaining sufficient strength. In particular, with regard to optimizing the chemical composition of the steel sheet, the inventors discovered that, in addition to including predetermined proportions of C and Mn, which are effective in increasing the strength of the steel sheet, controlling the total amount of Si and Al to within a range of 1.00-2.20% by mass is effective in both increasing strength and improving elongation. It is believed that controlling the total amount of Si and Al within this range allows C to be concentrated in the retained austenite. More specifically, the microstructure of the steel sheet according to the embodiment of the present invention includes tempered martensite, ferrite, and retained austenite, as well as 10 to 30% bainite in terms of area ratio. Here, since bainite cannot fully dissolve the C contained in the steel when it transforms from austenite, C is released from the bainite into the surrounding microstructure as the bainite transformation progresses. Specifically, the C released from the bainite either concentrates in the untransformed austenite or forms carbides. However, the presence of a certain amount of Si and Al in the steel makes it difficult to form carbides, which is thought to result in C being concentrated in the untransformed austenite. In this way, C is concentrated in the retained austenite in the final microstructure. The concentrated C not only stabilizes the retained austenite and contributes to improving the elongation of the steel sheet, but also increases the hardness when the retained austenite transforms to martensite through stress-induced transformation, thereby contributing to the high strength of the steel sheet.

Next, in order to further improve the elongation of steel sheets, the inventors conducted research focusing on the specific forms of ferrite, a soft microstructure that can particularly contribute to improving elongation, and retained austenite, which exhibits the TRIP effect. As a result, the inventors discovered that by limiting the Mn concentration in ferrite within a predetermined range, more specifically by controlling the average Mn concentration in ferrite divided by the Mn concentration in the base material (i.e., the Mn content of the steel sheet) to 0.980 or less, and thereby making the Mn concentration in ferrite lower than the average Mn concentration in the entire steel sheet, it is possible to significantly suppress solid solution strengthening of ferrite by Mn, thereby improving the elongation of the steel sheet. Additionally, the present inventors have found that, in relation to reducing the Mn concentration in ferrite, it is possible to stabilize the retained austenite by concentrating Mn in the retained austenite, more specifically by controlling the value obtained by dividing the average value of the Mn concentration in the retained austenite by the Mn concentration in the base material (i.e., the Mn content of the steel sheet) to 1.150 to 2.000, and that, in combination with the effect of reducing the Mn concentration in ferrite, it is possible to significantly improve the elongation of the steel sheet.

While not intending to be bound by any particular theory, it is believed that stabilizing retained austenite through such Mn enrichment delays stress-induced transformation toward higher strains during deformation of the steel sheet, thereby reducing the likelihood of necking. As a result, it is believed that the TRIP effect can be properly exerted up to the higher strains during deformation, and the timing of necking and stress-induced transformation is optimized, significantly improving the elongation of the steel sheet. More specifically, for example, if the level of Mn enrichment is too high and the retained austenite is overly stabilized, even if necking occurs, stress-induced transformation from retained austenite to martensite may not properly occur, and the retained austenite may remain until the steel sheet fractures. Similarly, for example, if the stability of retained austenite is low due to insufficient Mn enrichment, stress-induced transformation may occur before necking occurs during deformation. In such cases, it is natural that the elongation of the steel sheet cannot be improved. Therefore, in the steel sheet according to the embodiment of the present invention, it is extremely important to moderately concentrate Mn in the retained austenite, i.e., to control the value obtained by dividing the average Mn concentration in the retained austenite by the Mn concentration in the base material to 1.150 to 2.000. Furthermore, by reducing the Mn concentration in ferrite in addition to this feature, i.e., by controlling the value obtained by dividing the average Mn concentration in ferrite by the Mn concentration in the base material to 0.980 or less, it is believed that the elongation of the steel sheet can be significantly improved due to the specific combination of moderate stabilization of the retained austenite and suppression of solid solution strengthening of ferrite. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent elongation, and is therefore particularly useful in the automotive field, where both of these properties are required.

The steel sheet according to an embodiment of the present invention will be described in more detail below. In the following description, the unit of content of each element, "%," means "mass %" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values before and after it are included as the lower and upper limits, unless otherwise specified.

[C:0.200-0.350%]
C is an element that ensures a predetermined amount of martensite and improves the strength of the steel sheet. To fully obtain this effect, the C content is set to 0.200% or more. The C content may be 0.210% or more, 0.220% or more, 0.240% or more, or 0.260% or more. On the other hand, excessive C content may result in excessively high strength and reduced elongation. Alternatively, the rolling load during production may become excessive, increasing the load on equipment such as a rolling mill and reducing manufacturability. For this reason, the C content is set to 0.350% or less. The C content may be 0.320% or less, 0.300% or less, or 0.280% or less.

[Si: 0.01 to 2.00%]
Si is an element that improves the strength of steel sheet through solid solution strengthening. To fully obtain this effect, the Si content is set to 0.01% or more. The Si content may be 0.10% or more, 0.30% or more, 0.50% or more, or 0.80% or more. On the other hand, excessive Si content may make it difficult to remove scale formed during hot rolling, which may lead to deterioration of appearance. Therefore, the Si content is set to 2.00% or less. The Si content may be 1.80% or less, 1.50% or less, 1.20% or less, less than 1.20%, 1.10% or less, or 1.00% or less.

[Mn: 1.40-4.00%]
Mn is an element that improves hardenability and contributes to improving steel sheet strength. Furthermore, Mn is an element that concentrates in retained austenite to stabilize the retained austenite, thereby improving elongation. To fully achieve these effects, the Mn content is set to 1.40% or more. The Mn content may be 1.60% or more, 1.80% or more, 2.00% or more, 2.20% or more, 2.50% or more, or 2.80% or more. On the other hand, excessive Mn content may excessively stabilize the retained austenite, thereby reducing elongation and/or promoting solid solution strengthening of ferrite, which may similarly reduce elongation or increase the load on rolling mills and other equipment during production, thereby reducing manufacturability. Therefore, the Mn content is set to 4.00% or less. The Mn content may be 3.80% or less, 3.50% or less, 3.20% or less, or 3.00% or less.

[P: 0.1000% or less]
P is an impurity element that embrittles welds and deteriorates galvanizability. Therefore, the P content is set to 0.1000% or less. The P content may be 0.0600% or less, 0.0200% or less, 0.0150% or less, or 0.0100% or less. The lower the P content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, reducing the P content to less than 0.0001% in practical steel sheets significantly increases production costs, which is economically disadvantageous. Therefore, the P content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[S: 0.0200% or less]
S is an impurity element that impairs weldability and also impairs manufacturability during casting and hot rolling. Therefore, the S content is set to 0.0200% or less. The S content may be 0.0150% or less, 0.0120% or less, 0.0100% or less, or 0.0080% or less. The lower the S content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, if the S content of a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which will be economically disadvantageous. Therefore, the S content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[Al: 2.00% or less]
Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel. The Al content may be 0%, but to fully obtain these effects, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.01% or more, 0.10% or more, more than 0.20%, 0.25% or more, or 0.30% or more. On the other hand, excessive Al content may form coarse oxides, which may reduce toughness. Therefore, the Al content is set to 2.00% or less. The Al content may be 1.80% or less, 1.50% or less, 1.30% or less, or 1.00% or less.

[N: 0.0200% or less]
N is an element that causes blowholes during welding. Therefore, the N content is set to 0.0200% or less. The N content may be 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. The lower the N content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, if the N content of a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which is economically disadvantageous. Therefore, the N content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[O: 0.0200% or less]
O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0200% or less. The O content may be 0.0180% or less, 0.0150% or less, 0.0100% or less, or 0.0080% or less. The lower the O content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, if the O content of a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which is economically disadvantageous. Therefore, the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

The basic chemical composition of the steel sheet according to an embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as needed, for the purpose of improving properties. For example, the steel sheet may contain Cr: 0-2.000%, Mo: 0-1.000%, Ti: 0-0.500%, Nb: 0-0.500%, B: 0-0.0100%, Cu: 0-1.000%, Ni: 0-1.000%, W: 0-0.100%, V: 0-1.000%, Ta: 0-0.100%, Co: 0-3.000%, Sn At least one of the following may be included: 0 to 1.000%, Sb: 0 to 0.500%, As: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.0500%, Y: 0 to 0.0500%, La: 0 to 0.0500%, Ce: 0 to 0.0500%, and Bi: 0 to 0.0500%. These optional elements will be described in detail below.

[Cr: 0-2.000%]
Like Mn, Cr is an element that improves hardenability and contributes to improving the strength of the steel sheet. The Cr content may be 0%, but to obtain the above-mentioned effects, the Cr content is preferably 0.001% or more. The Cr content may be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, if Cr is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the Cr content is preferably 2.000% or less, and may be 1.500% or less, 1.000% or less, or 0.500% or less.

[Mo: 0-1.000%]
Like Cr, Mo is an element that contributes to increasing the strength of steel sheets. This effect can be achieved even with a small amount. The Mo content may be 0%, but to achieve the above effect, the Mo content is preferably 0.001% or more. The Mo content may be 0.010% or more, 0.020% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Mo content may deteriorate hot workability and reduce productivity. Therefore, the Mo content is preferably 1.000% or less. The Mo content may be 0.800% or less, 0.400% or less, or 0.200% or less.

[Ti: 0 to 0.500%]
Ti is an element effective in controlling the morphology of carbides. Ti can promote an increase in the strength of ferrite. The Ti content may be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may be 0.002% or more, 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if Ti is contained excessively, the effect saturates and there is a risk of an increase in manufacturing costs. Therefore, the Ti content is preferably 0.500% or less, and may be 0.400% or less, 0.200% or less, or 0.100% or less.

[Nb: 0 to 0.500%]
Like Ti, Nb is an element effective in controlling the morphology of carbides and is also effective in refining the structure to improve the toughness of steel sheets. These effects can be obtained even with trace amounts. The Nb content may be 0%, but to obtain the above effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.005% or more or 0.010% or more. On the other hand, excessive Nb content may generate coarse carbides in the steel, reducing the toughness of the steel sheet. Therefore, the Nb content is preferably 0.500% or less. The Nb content may be 0.200% or less, 0.100% or less, or 0.060% or less.

[B: 0 to 0.0100%]
B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of martensite. Furthermore, B is a beneficial element for increasing the strength of steel. These effects can be achieved even with trace amounts. The B content may be 0%, but to achieve the above effects, the B content is preferably 0.0001% or more. The B content may be 0.0005% or more or 0.0010% or more. On the other hand, excessive B content may result in a decrease in toughness and/or weldability. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[Cu: 0-1.000%]
Cu is an element that contributes to improving the strength of steel sheets. This effect can be achieved even with trace amounts. The Cu content may be 0%, but to achieve the above effect, the Cu content is preferably 0.001% or more. The Cu content may be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, excessive Cu content may cause red shortness and reduce productivity in hot rolling. Therefore, the Cu content is preferably 1.000% or less. The Cu content may be 0.800% or less, 0.600% or less, 0.300% or less, or 0.100% or less.

[Ni: 0-1.000%]
Ni is an element effective in improving the strength of steel sheet. The Ni content may be 0%, but to obtain the above effect, the Ni content is preferably 0.001% or more. The Ni content may be 0.005% or more or 0.010% or more. On the other hand, excessive Ni content may reduce the weldability of the steel sheet. Therefore, the Ni content is preferably 1.000% or less. The Ni content may be 0.800% or less, 0.400% or less, or 0.200% or less.

[W: 0-0.100%]
W is an element effective in controlling the morphology of carbides and improving the strength of steel sheets. The W content may be 0%, but to obtain these effects, the W content is preferably 0.001% or more. The W content may be 0.005% or more or 0.010% or more. On the other hand, excessive W content may cause a decrease in weldability. For this reason, the W content is preferably 0.100% or less. The W content may be 0.080% or less, 0.040% or less, or 0.020% or less.

[V: 0-1.000%]
Like Ti and Nb, V is an element effective in controlling the morphology of carbides and is also effective in refining the structure to improve the toughness of the steel sheet. The V content may be 0%, but to obtain the above effect, the V content is preferably 0.001% or more. The V content may be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, excessive V content may cause the formation of a large amount of precipitates, which may reduce toughness. Therefore, the V content is preferably 1.000% or less. The V content may be 0.400% or less, 0.200% or less, or 0.100% or less.

[Ta: 0-0.100%]
Ta, like W, is an element effective in controlling the morphology of carbides and improving the strength of steel sheet. The Ta content may be 0%, but to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.005% or more or 0.010% or more. On the other hand, even if Ta is contained in an excessive amount, the effect saturates, and adding more than necessary to the steel sheet increases the manufacturing cost. For this reason, the Ta content is preferably 0.100% or less. The Ta content may be 0.080% or less, 0.040% or less, or 0.020% or less.

[Co: 0-3.000%]
Co, like Ni, is an element effective in improving the strength of steel sheet. The Co content may be 0%, but to obtain the above effect, the Co content is preferably 0.001% or more. The Co content may be 0.005% or more, 0.010% or more, or 0.100% or more. On the other hand, excessive Co content may deteriorate hot workability and increase raw material costs. Therefore, the Co content is preferably 3.000% or less. The Co content may be 2.000% or less, 1.000% or less, 0.500% or less, or 0.200% or less.

[Sn: 0 to 1.000%]
Sn is an element that can be contained in steel sheets when scrap is used as a raw material for the steel sheets. Furthermore, Sn may cause ferrite embrittlement. Therefore, the lower the Sn content, the better, and it is preferably 1.000% or less. The Sn content may be 0.100% or less, 0.040% or less, or 0.020% or less. The Sn content may be 0%, but reducing the Sn content to less than 0.001% results in an excessive increase in refining costs. Therefore, the Sn content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[Sb: 0 to 0.500%]
Like Sn, Sb is an element that can be contained in steel sheet when scrap is used as a raw material for the steel sheet. Furthermore, Sb may strongly segregate at grain boundaries, potentially causing embrittlement of the grain boundaries. Therefore, the lower the Sb content, the better, and it is preferably 0.500% or less. The Sb content may be 0.100% or less, 0.040% or less, or 0.020% or less. The Sb content may be 0%, but reducing the Sb content to less than 0.001% would result in an excessive increase in refining costs. Therefore, the Sb content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[As: 0 to 0.050%]
Like Sn and Sb, As is an element that can be contained in a steel sheet when scrap is used as a raw material for the steel sheet. Furthermore, As is an element that strongly segregates at grain boundaries, and a lower As content is preferable. The As content is preferably 0.050% or less, and may be 0.040% or less, or 0.020% or less. The As content may be 0%, but reducing the As content to less than 0.001% results in an excessive increase in refining costs. Therefore, the As content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[Mg: 0 to 0.050%]
Mg controls the morphology of sulfides and oxides and contributes to improving the bending formability of steel sheets. This effect can be achieved even with a small amount. The Mg content may be 0%, but to achieve the above effect, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0005% or more, 0.001% or more, or 0.005%. On the other hand, even if an excessive amount of Mg is contained, the effect saturates, and adding more Mg than necessary to the steel sheet increases the manufacturing cost. Therefore, the Mg content is preferably 0.050% or less. The Mg content may be 0.040% or less, 0.020% or less, or 0.010% or less.

[Zr: 0 to 0.050%]
Zr is an element that can control the morphology of sulfides with a small amount. The Zr content may be 0%, but to obtain the above effect, the Zr content is preferably 0.0001% or more. The Zr content may be 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, even if Zr is contained in an excessive amount, the effect saturates, and containing more Zr than necessary in the steel sheet increases the manufacturing cost. Therefore, the Zr content is preferably 0.050% or less. The Zr content may be 0.040% or less, 0.020% or less, or 0.010% or less.

[Ca: 0-0.0500%]
[Y: 0 to 0.0500%]
[La: 0 to 0.0500%]
[Ce: 0 to 0.0500%]
Ca, Y, La, and Ce are elements that can control the morphology of sulfides in trace amounts. The Ca, Y, La, and Ce contents may be 0%, but to obtain the above effects, the Ca, Y, La, and Ce contents are preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, 0.0020% or more, or 0.0030% or more. On the other hand, even if these elements are contained in excess, the effect saturates, and containing more than necessary in the steel sheet increases the manufacturing cost. Therefore, the Ca, Y, La, and Ce contents are preferably 0.0500% or less, and may be 0.0200% or less, 0.0100% or less, or 0.0060% or less.

[Bi: 0 to 0.0500%]
Bi is an element that has the effect of improving formability by refining the solidification structure. The Bi content may be 0%, but to obtain this effect, the Bi content is preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, even if Bi is contained in an excessive amount, the effect saturates, and adding more Bi than necessary to the steel sheet increases the manufacturing cost. Therefore, the Bi content is preferably 0.0500% or less, and may be 0.0400% or less, 0.0200% or less, or 0.0100% or less.

In the steel sheet according to the embodiment of the present invention, the balance excluding the above elements consists of Fe and impurities. Impurities are elements that are mixed in from the steel raw materials and/or during the steelmaking process, and whose presence is permitted to the extent that they do not impair the properties of the steel sheet according to the embodiment of the present invention.

[1.00≦[Si]+[Al]≦2.20]
The chemical composition of the steel sheet according to the embodiment of the present invention must satisfy the following formula.
1.00≦[Si]+[Al]≦2.20
In the formula, [Si] and [Al] are the contents (mass%) of each element. In addition to the effects described for each element, Si and Al are effective elements for concentrating C in retained austenite, as mentioned above. When bainite transforms from austenite, C is released from the bainite into the surrounding microstructure. The presence of Si and Al in the steel makes it difficult for the C released from bainite to form carbides, which is thought to lead to C being concentrated in the untransformed austenite. Like Mn, the concentrated C not only stabilizes the retained austenite and contributes to improving the elongation of the steel sheet, but also increases the hardness when the retained austenite transforms to martensite through stress-induced transformation, thereby contributing to the strength of the steel sheet. To fully achieve these effects, the chemical composition of the steel sheet according to the embodiment of the present invention is controlled so that the total content of Si and Al is 1.00% or more, i.e., [Si] + [Al] ≧ 1.00. From the viewpoint of further enhancing these effects, the total content of Si and Al is preferably 1.20% or more, and may be 1.40% or more or 1.60% or more. On the other hand, if the total content of Si and Al is too high, excessive strength may be obtained and/or excessive stabilization of retained austenite may occur, resulting in reduced elongation and/or an excessive rolling load during cold rolling, etc., which may cause cracks in the steel sheet. Therefore, the total content of Si and Al is set to 2.20% or less, i.e., [Si] + [Al] ≦ 2.20. The total content of Si and Al may be 2.10% or less, 2.20% or less, 1.90% or less, 1.80% or less, or 1.70% or less.

The chemical composition of the steel sheet according to the embodiment of the present invention may be measured using a common analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.

[Microstructure]
Next, the microstructure of the steel plate according to the embodiment of the present invention will be described. Hereinafter, the structure fraction will be expressed as an area fraction, and the unit "%" of the structure fraction means area %. As will be described later, the microstructure is controlled at 1/4 of the plate thickness of the steel plate. The 1/4 of the plate thickness of the steel plate means the region between the surface at 1/8 depth and the surface at 3/8 depth of the plate thickness from the rolled surface of the steel plate. Hereinafter, unless otherwise specified, all of the structure fractions refer to values at 1/4 of the plate thickness.

[Ferrite: 15 to 40%]
Ferrite is a soft structure and is easily deformed, contributing to improved elongation. To fully obtain this effect, the area fraction of ferrite is set to 15% or more. From the viewpoint of improving elongation, a higher area fraction of ferrite is preferable, and may be, for example, 20% or more, 25% or more, or 30% or more. On the other hand, if ferrite is contained in an excessive amount, the steel sheet may not achieve the desired strength. Therefore, the area fraction of ferrite is set to 40% or less. The area fraction of ferrite may be 38% or less, 36% or less, or 34% or less.

[Tempered martensite: 40 to 65%]
Tempered martensite is a hard structure that contributes to improving strength. Furthermore, tempered martensite is a tougher structure than as-quenched martensite, which is hard but relatively brittle, and therefore also contributes to improving elongation. To fully obtain these effects, the area fraction of tempered martensite is set to 40% or more. From the viewpoint of improving strength, the higher the area fraction of tempered martensite, the more preferable it is, and it may be, for example, 42% or more, 45% or more, or 48% or more. On the other hand, if tempered martensite is contained in an excessive amount, the strength may become too high and the elongation may decrease. Therefore, the area fraction of tempered martensite is set to 65% or less. From the viewpoint of improving elongation, the lower the area fraction of tempered martensite, the more preferable it is, and it may be, for example, 60% or less, 55% or less, or 50% or less.

[Residual austenite: 10 to 20%]
Retained austenite is a structure that improves the strength and elongation of a steel sheet due to the TRIP effect, in which the retained austenite transforms to martensite through stress-induced transformation during deformation of the steel sheet. To fully obtain this effect, the area fraction of the retained austenite content is set to 10% or more. From the viewpoint of improving elongation, a higher area fraction of the retained austenite is preferable, and may be, for example, 12% or more, 14% or more, or 16% or more. On the other hand, if retained austenite is contained excessively, the proportion of other structures such as ferrite and tempered martensite decreases, and as a result, the desired strength and/or elongation may not be achieved. Therefore, the area fraction of the retained austenite is set to 20% or less. The area fraction of the retained austenite may be 19% or less or 18% or less.

[Bainite: 10 to 30%]
The microstructure of the steel sheet according to the embodiment of the present invention is primarily composed of the above-described ferrite, tempered martensite, and retained austenite. These structures primarily contribute to improved strength and elongation. However, in addition to these structures, bainite is also included as an essential structure. If the area fraction of bainite is too low, the amount of carbon released into austenite during bainite transformation may be insufficient, resulting in insufficient carbon enrichment in the retained austenite, which may adversely affect the improvement of strength and/or elongation. Therefore, the area fraction of bainite is set to 10% or more. The area fraction of bainite may be 12% or more, 15% or more, 18% or more, or 20% or more. On the other hand, since bainite is a relatively hard structure, it may contribute to improving strength. However, if it is included in excess, the proportions of other structures, such as ferrite, tempered martensite, and retained austenite, may decrease, resulting in failure to achieve the desired strength and/or elongation. Therefore, the area fraction of bainite is set to 30% or less. The area fraction of bainite may be 28% or less, 25% or less, or 22% or less.

[Perlite: 0 to 10%]
[As-quenched martensite: 0 to 10%]
The area fraction of the remaining structure other than ferrite, tempered martensite, retained austenite, and bainite may be 0%, but if a remaining structure exists, the remaining structure is pearlite and as-quenched martensite. From the viewpoint of ensuring the above-described effects based on ferrite, tempered martensite, retained austenite, and bainite, the area fractions of the remaining structure, i.e., pearlite and as-quenched martensite, may each be 10% or less, for example, 8% or less, 6% or less, 4% or less, or 2% or less. On the other hand, achieving 0% of the area fraction of these structures requires advanced control in the steel plate manufacturing process, which may result in a decrease in yield. Therefore, the area fractions of pearlite and as-quenched martensite may each be 0.5% or more or 1% or more.

[Identification of Microstructure and Calculation of Area Ratio]
Identification of each metal structure and calculation of the area ratio are carried out by EBSD (Electron Backscattering Diffraction), X-ray measurement, corrosion using Nital reagent or Lepera solution, and by observing a 100 μm × 100 μm region of the steel sheet cross section perpendicular to the sheet surface at a magnification of 1,000 to 50,000 times using a scanning electron microscope. Note that when measuring the area ratio of each structure, three measurement locations are used and the average value is calculated.

[Calculation of ferrite area ratio]
The ferrite area fraction is measured by the following method. That is, using an EBSD attached to a scanning electron microscope, measurements are made at intervals (pitch) of 0.2 μm over a range of 1/8 to 3/8 thickness from the surface of the steel sheet, centered at a position of 1/4 of the sheet thickness. The value of the local misorientation average (Grain Average Misorientation: GAM) is calculated from the measurement data. Then, regions where the local misorientation average value is less than 0.5° are considered to be ferrite, and their area and area fraction are measured. Here, the local misorientation average is a value obtained by calculating the misorientation between adjacent measurement points in a region surrounded by grain boundaries with a crystal misorientation of 5° or more, and averaging the calculated values for all measurement points within the crystal grain.

[Calculation of bainite area ratio]
The area ratio of bainite is determined by taking a sample from a cross section of the steel plate perpendicular to the plate surface, polishing the observation surface, etching it with nital solution, and observing the area ranging from 1/8 to 3/8 of the plate thickness, centered at 1/4, with a field emission scanning electron microscope (FE-SEM). The area ratio is calculated using known image analysis software. For example, the "Analyze" function of "ImageJ" can be used as the image analysis software to calculate the area ratio. "ImageJ" is open-source, public domain image processing software that is widely used by those skilled in the art. In FE-SEM observation, for example, the structure of an observation surface, which is a square with sides of 30 μm, is classified as follows: Bainite is a collection of lath-shaped crystal grains that either does not contain iron-based carbides with a major axis of 20 nm or more, or contains iron-based carbides with a major axis of 20 nm or more, where the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction. Here, a group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by 5° or less. Bainite surrounded by grain boundaries with a misorientation of 15° or more is counted as one bainite grain.

[Calculation of area ratio of tempered martensite]
The area fraction of tempered martensite is calculated using the same observation surface and measurement method as used to calculate the area fraction of bainite. In tempered martensite, cementite exists within the martensite laths. However, since there are two or more types of crystal orientations of martensite laths and cementite, and cementite has multiple variants, tempered martensite can be identified. The area fraction of tempered martensite identified in this way is calculated using the point counting method.

[Calculation of area ratio of as-quenched martensite]
To determine the area fraction of as-quenched martensite, first, an observation surface similar to the observation surface used to identify the ferrite is etched with a repeller solution, and the same region as that used to identify the ferrite is designated as the observation region. Corrosion with the repeller solution does not corrode the as-quenched martensite or retained austenite. Therefore, the observation region corroded by the repeller solution is observed with an FE-SEM, and the uncorroded region is designated as the as-quenched martensite and retained austenite. The total area fraction of the as-quenched martensite and retained austenite identified in this manner is then calculated using the point-counting method. Next, the volume fraction of the retained austenite calculated as follows is considered to be the area fraction of the retained austenite, and this area fraction is subtracted from the total area fraction to calculate the area fraction of the as-quenched martensite.

[Calculation of area ratio of retained austenite]
The area fraction of retained austenite is calculated by measuring the diffraction intensity using X-rays on a sample obtained by removing a 100 μm region from the surface in the sheet thickness direction by electrolytic polishing or chemical polishing. Specifically, measurement is performed using MoKα rays as characteristic X-rays, and the volume fraction of retained austenite is calculated from the integrated intensity ratio of the obtained diffraction peaks of (200) and (211) of the bcc phase and (200), (220), and (311) of the fcc phase.

[Calculation of pearlite area ratio]
The area ratio of pearlite can be obtained by taking a sample from a cross section of the steel plate perpendicular to the plate surface, polishing the observation surface, corroding it with Nital reagent, and observing and photographing a secondary electron image of a range from 1/8 to 3/8 of the plate thickness, centered at a position 1/4 of the way down from the surface of the steel plate, using a scanning electron microscope. In the secondary electron image, carbides are observed with a relatively brighter contrast than other steel structures. In the photographed image, the region where plate-like carbides are arranged in rows at intervals of 0.5 μm or less is defined as pearlite, and the area ratio of pearlite is calculated using the "Analyze" function of the image analysis software "ImageJ" mentioned above.

If the total area ratio of each tissue obtained using the above identification and calculation methods is different from 100%, the area ratio of each tissue shall be the value obtained by multiplying the area ratio of each tissue by 100/(total area ratio of each tissue).

[Value obtained by dividing the average Mn concentration in ferrite by the Mn concentration in the base material (Mn _F /Mn _A ): 0.980 or less]
In an embodiment of the present invention, the value obtained by dividing the average Mn concentration in ferrite by the Mn concentration in the base material (i.e., the Mn content of the steel sheet) is controlled to 0.980 or less. By sufficiently reducing the Mn concentration in ferrite to 0.980 times or less the average Mn concentration in the entire steel sheet, solid solution strengthening of ferrite by Mn can be significantly suppressed, thereby improving the elongation of the steel sheet. From the viewpoint of improving elongation, the lower this value is, the more preferable it is, and it may be, for example, 0.970 or less, 0.960 or less, or 0.950 or less. Although the lower limit is not particularly limited, since it is difficult to achieve a Mn concentration of 0% in ferrite, the value obtained by dividing the average Mn concentration in ferrite by the Mn concentration in the base material may be, for example, 0.800 or more, 0.830 or more, 0.850 or more, 0.870 or more, 0.890 or more, 0.900 or more, or 0.910 or more.

The average Mn concentration in ferrite is determined using an electron probe microanalyzer (EPMA) as follows. Specifically, a sample is first taken from a steel plate at a cross-section perpendicular to the plate surface. A 100 μm x 100 μm region is observed within the 1/8 to 3/8 plate thickness range, centered at 1/4 plate thickness, in an electron channeling contrast image taken with a field emission scanning electron microscope (FE-SEM). The location of ferrite within this observation region is then determined. Next, the observation region is wet polished with emery paper and polished with diamond abrasive grains with an average particle size of 1 μm, followed by chemical polishing. To determine the measurement region, a Vickers hardness test is performed to create an indentation in a 100 x 100 μm square in the observation region, which serves as a marker. The Mn concentration in ferrite is then measured using the EPMA. The measurement was performed using a JXA-8500F manufactured by JEOL. Crystal orientation information was obtained at an acceleration voltage of 7 kV and a measurement point interval of 80 nm, and the portion of the observation area determined to be BCC was measured. From the data obtained in this manner, the Mn concentration in the ferrite was determined using a calibration curve method, and the obtained value was determined as the average Mn concentration in the ferrite.

[Average Mn concentration in retained austenite divided by Mn concentration in base metal (Mn _γ /Mn _A ): 1.150 to 2.000]
In an embodiment of the present invention, the value obtained by dividing the average Mn concentration in the retained austenite by the Mn concentration in the base material (i.e., the Mn content of the steel sheet) is controlled to 1.150 to 2.000. By increasing the Mn concentration in the retained austenite to 1.150 to 2.000 times the average Mn concentration in the entire steel sheet, the retained austenite can be appropriately stabilized. As a result, the TRIP effect can be appropriately exerted up to the high strain side during deformation of the steel sheet, and the timing of the occurrence of necking and deformation-induced transformation is optimized. This, in combination with the suppression of solid solution strengthening of ferrite, enables a significant improvement in the elongation of the steel sheet. From the viewpoint of improving elongation, the higher this value, the better, and it may be, for example, 1.200 or more, 1.250 or more, or 1.300 or more. On the other hand, if the degree of Mn enrichment is too large and the retained austenite is excessively stabilized, even if necking occurs, the strain-induced transformation from the retained austenite to martensite may not occur properly, and the retained austenite may remain until the steel sheet breaks. Therefore, the value obtained by dividing the average Mn concentration in the retained austenite by the Mn concentration in the base material is set to 2.000 or less, and may be, for example, 1.800 or less or 1.600 or less.

The average Mn concentration in the retained austenite is also determined using EPMA. Specifically, the FCC phase is separated using crystal orientation information obtained in the same manner as described above to determine the average Mn concentration in ferrite. The separated FCC phase is the retained austenite. Next, the Mn concentration in the retained austenite is measured using EPMA. The measurement is performed using a JXA-8500F manufactured by JEOL. Crystal orientation information is obtained at an acceleration voltage of 7 kV and a measurement point spacing of 80 nm, and the portion of the region determined to be FCC is measured. From the data obtained in this measurement, the Mn concentration in the retained austenite is determined using the calibration curve method, and this value is determined to be the average Mn concentration in the retained austenite.

[Average C concentration in retained austenite: 0.80 mass% or more]
According to a preferred embodiment of the present invention, the average C concentration in the retained austenite is controlled to 0.80 mass% or more. By controlling the average C concentration in the retained austenite to 0.80 mass% or more, the effects of the C enrichment in the retained austenite described above, i.e., the improvement in elongation due to the stabilization of the retained austenite and the increase in strength due to the increase in hardness of the deformation-induced martensite, can be particularly pronounced. From the viewpoint of improving elongation and increasing strength, a higher average C concentration in the retained austenite is preferable, and may be, for example, 0.85 mass% or more or 0.90 mass% or more. The upper limit is not particularly limited, and the average C concentration in the retained austenite may be, for example, 1.00 mass% or less, 0.98 mass% or less, 0.96 mass% or less, or 0.95 mass% or less.

The average C concentration in the retained austenite is also determined using EPMA, as before. Specifically, in the same manner as described above to determine the average Mn concentration in the retained austenite, crystal orientation information is obtained and the parts of the region determined to be FCC are measured. From the data obtained in this measurement, the C concentration in the retained austenite is determined using the calibration curve method, and this value is determined to be the average C concentration in the retained austenite.

[Surface softening part]
According to another preferred embodiment of the present invention, the steel sheet includes a thickness center portion and a surface-softened portion located on one or both sides of the thickness center portion, the surface-softened portion having an average thickness of 10 μm or more and an average Vickers hardness of 0.90 times or less the average Vickers hardness at the half-thickness position. Providing such a softened portion on the surface layer of the steel sheet can improve the bendability of the steel sheet. Therefore, by combining this improved bendability with the improved elongation described above, a steel sheet can be obtained that not only improves workability during component manufacturing but also provides excellent crash resistance for the manufactured components. In this embodiment, an average thickness of 10 μm or more can fully demonstrate the effects of providing a surface-softened portion on one or both sides of the steel sheet. The average thickness of the surface-softened portion may be any value of 10 μm or more, such as 15 μm or more, 30 μm or more, 40 μm or more, 50 μm or more, 70 μm or more, or 100 μm or more. Although there is no particular upper limit, the average thickness of the surface-softened portion is generally 30% or less of the plate thickness. For example, the average thickness of the surface-softened portion may be 25% or less, 20% or less, 15% or less, or 10% or less of the plate thickness, and more specifically, may be 450 μm or less, 400 μm or less, 300 μm or less, 250 μm or less, 200 μm or less, or 150 μm or less.

The average Vickers hardness of the surface-softened portion may be any average Vickers hardness not greater than 0.90 times the average Vickers hardness at the half-thickness position. Controlling the average Vickers hardness of the surface-softened portion to not greater than 0.90 times the average Vickers hardness at the half-thickness position, i.e., lowering the hardness of the surface portion relative to the center of the plate, can reliably improve the bendability of the steel plate. To further enhance this effect of improving bendability, for example, the average Vickers hardness of the surface-softened portion may be not greater than 0.85 times, not greater than 0.80 times, not greater than 0.70 times, or not greater than 0.60 times the average Vickers hardness at the half-thickness position. While there is no particular lower limit, the average Vickers hardness of the surface-softened portion is generally not less than 0.10 times the average Vickers hardness at the half-thickness position, and may be, for example, not less than 0.15 times or not less than 0.20 times.

In this invention, the "average thickness of the surface-softened portion" and "average Vickers hardness of the surface-softened portion" are determined as follows. First, the Vickers hardness is measured at regular intervals (e.g., every 5% of the thickness, or, as needed, every 3%, 2.5%, 1%, or 0.5% of the thickness) in the thickness direction from the half-thickness position toward the surface of the surface-softened portion using an indentation load of 100 g. Next, the Vickers hardness is measured at three or more points (e.g., five or ten points) along a line perpendicular to the thickness direction from the half-thickness position, using an indentation load of 100 g. The average of these measurements is taken as the average Vickers hardness at that thickness direction. Preferably, the distance between each measurement point in the thickness direction and perpendicular to it should be at least four times the distance of the indentation, if possible. "A distance of at least four times the distance of the indentation" means a distance of at least four times the diagonal length of the rectangular opening of the indentation made by the diamond indenter during Vickers hardness measurement. If it is difficult to stamp linearly from the surface in the thickness direction while maintaining a distance of at least four times the indentation between each measurement point, stamping may be performed in a zigzag pattern from the surface in the thickness direction while maintaining a distance of at least four times the indentation between each measurement point. When the average Vickers hardness at a certain thickness direction position is 0.90 times or less the average Vickers hardness at a similarly measured half-thickness position, the surface side of that position is defined as a surface-softened zone, and the average thickness (μm) and its proportion (%) of the thickness are determined. The Vickers hardness of 10 randomly selected points within the surface-softened zone thus determined is measured with an indentation load of 100 g, and the average value is calculated to determine the average Vickers hardness of the surface-softened zone. When surface-softened zones are located on both sides of the center of the thickness, the average thickness and average Vickers hardness of the surface-softened zone on the other side are determined by measurements similar to those described above.

[Thickness]
The steel plate according to the embodiment of the present invention has a thickness of, for example, 0.6 to 6.0 mm, but is not particularly limited thereto. The thickness may be, but is not particularly limited to, 0.8 mm or more, 1.0 mm or more, or 1.2 mm or more. Similarly, the thickness may be 4.0 mm or less, 3.0 mm or less, 2.5 mm or less, or 2.0 mm or less. The thickness of the steel plate is measured using a micrometer.

[Plating]
The steel sheet according to the embodiment of the present invention may further have a plating layer on its surface for the purpose of improving corrosion resistance, etc. The plating layer may be any appropriate plating layer, for example, a hot-dip plating layer or an electroplated layer. The hot-dip plating layer may be, for example, a hot-dip galvanized layer, a hot-dip zinc alloy plating layer (a hot-dip plating layer composed of an alloy of zinc and additional elements such as Si and Al), or a galvannealed layer (alloyed plating layer) obtained by alloying these platings. The hot-dip galvanized layer and hot-dip zinc alloy plating layer preferably contain less than 7% by mass of Fe, and the alloyed plating layer preferably contains 7% by mass or more and 15% by mass or less of Fe. In the hot-dip galvanized layer, hot-dip zinc alloy plating layer, and alloyed plating layer, the components other than zinc and Fe are not particularly limited, and various compositions within the usual range can be adopted. The plating layer may also be, for example, an aluminum plating layer. The coating weight of the plating layer is not particularly limited and may be a common coating weight.
The aforementioned steel sheet characteristics, such as chemical composition, microstructure fraction, and softened surface areas, do not apply to the surface plating. In other words, evaluation of the chemical composition, microstructure fraction, and softened surface areas of plated steel sheets is performed by immersing the steel sheets in a strong alkaline solution and removing the surface plating layer.

[Mechanical properties]
Steel sheets according to embodiments of the present invention can achieve high tensile strength, for example, tensile strength of 980 MPa or more. The tensile strength is preferably 1080 MPa or more or 1180 MPa or more, more preferably 1250 MPa or more or 1350 MPa or more. The upper limit is not particularly limited, but the tensile strength may be, for example, 1780 MPa or less, 1700 MPa or less, 1600 MPa or less, or 1500 MPa or less. In addition, despite having such extremely high tensile strength, steel sheets according to embodiments of the present invention can reliably and sufficiently improve elongation compared to steel sheets that do not include such a specific combination due to the specific combination of chemical composition and microstructure described above. For example, steel sheets according to embodiments of the present invention can achieve a total elongation of 14% or more, preferably 15% or more, and more preferably 16% or more. The upper limit is not particularly limited, but the total elongation may be, for example, 30% or less or 25% or less. The tensile strength and total elongation are measured by conducting a tensile test in accordance with JIS Z 2241:2011 using a JIS No. 5 test piece taken in a direction such that the longitudinal direction of the test piece is parallel to the direction perpendicular to the rolling direction of the steel plate.

The steel sheet according to the present invention may be, in particular, a cold-rolled steel sheet. As described above, the steel sheet according to the present invention can reliably achieve both the contradictory properties of high strength and excellent elongation. In addition, certain preferred embodiments of the present invention can significantly improve bendability. Therefore, the steel sheet according to the present invention is useful for use in parts in technical fields that require both high strength and workability, and is particularly useful for use in parts in the automotive field. In a preferred embodiment, an automobile part is provided that includes the steel sheet according to the present invention. Examples of automobile parts include frame parts, bumpers, and other structural and reinforcing parts that require strength. At least a portion of these parts may include the steel sheet according to the present invention, and therefore at least a portion of these parts will satisfy the chemical composition and metallographic characteristics described above. In areas of the steel sheet that are not in direct contact with the mold during press forming or other forming and that are subjected to a relatively small degree of processing, the metallographic characteristics do not change significantly before and after forming.

<Steel sheet manufacturing method>
Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, but is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

A method for manufacturing a steel sheet according to an embodiment of the present invention includes:
a hot rolling step comprising: heating a slab having the chemical composition described above in relation to the steel sheet to a temperature of 1200 to 1400°C, finish rolling the slab, and then coiling the slab at a temperature of 500 to 700°C, and dwelling the coiled coil in a temperature range of 600 to 750°C for 1.0 to 5.0 hours, wherein the end temperature of the finish rolling is 900°C or higher;
a pickling step of pickling the obtained hot-rolled steel sheet;
The method is characterized by including a cold rolling step of cold-rolling a pickled hot-rolled steel sheet at a reduction ratio of 20 to 90%, and a step of annealing the resulting cold-rolled steel sheet, wherein the annealing comprises heating the cold-rolled steel sheet and holding it at a maximum heating temperature of 780 to 900°C for 30 to 500 seconds, followed by cooling and soaking, and the cooling comprises primary cooling from the maximum heating temperature to a primary cooling stop temperature of 650°C or higher at an average cooling rate of 1.0 to 20.0°C/second, and then secondary cooling to a secondary cooling stop temperature of 100 to less than 300°C at an average cooling rate of 20°C/second or higher, and the soaking comprises heating the secondarily cooled cold-rolled steel sheet and soaking it in a temperature range of 300 to 450°C for 100 seconds or longer. Each step will be described in detail below.

[Slab]
The slab to be subjected to hot rolling may be any cast slab, and is not limited to a specific cast slab. For example, it may be a continuously cast slab or a slab produced by a thin slab caster.

[Slab heating temperature: 1200 to 1400°C]
When a slab that has been cooled after casting is heated and then subjected to hot rolling, the slab is heated to 1200 to 1400°C. The slab used in manufacturing a steel plate according to an embodiment of the present invention contains a relatively large amount of alloying elements. Therefore, before the slab is subjected to hot rolling, it is necessary to heat the slab to dissolve the alloying elements in the slab. If the heating temperature of the slab is less than 1200°C, the alloying elements will not be sufficiently dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. Therefore, the heating temperature of the slab is set to 1200°C or higher. The upper limit of the heating temperature of the slab is not particularly limited, but is set to 1400°C or lower in consideration of the heating capacity and productivity of the heating equipment.

[Rough rolling]
In this method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[Finishing rolling temperature: 900°C or higher]
The heated slab, or the slab that has been subjected to rough rolling as needed, is then subjected to finish rolling. As described above, the slab used to manufacture the steel sheet according to the embodiment of the present invention contains a relatively large amount of alloying elements, and therefore, a large rolling load is required during hot rolling. For this reason, hot rolling is preferably performed at a high temperature. In particular, the finish rolling temperature is important in terms of controlling the microstructure of the steel sheet. If the finish rolling temperature is in the two-phase temperature range (austenite + ferrite), the microstructure may become more non-uniform, and formability after heat treatment may be reduced. For this reason, the finish rolling temperature is set to 900°C or higher. Although there is no particular upper limit, the finish rolling temperature is preferably set to, for example, 1100°C or lower in order to suppress coarsening of austenite.

[Winding temperature: 500-700℃]
Next, the finish-rolled hot-rolled steel sheet is coiled at a temperature of 500 to 700°C. If the coiling temperature of the hot-rolled steel sheet exceeds 700°C, the non-uniformity of the microstructure increases, and formability after heat treatment is likely to deteriorate. For this reason, the coiling temperature is set to 700°C or less. If the coiling temperature is less than 500°C, the strength of the hot-rolled sheet becomes excessively high, impairing cold rollability, so the lower limit of the coiling temperature is set to 500°C.

[Coil retention temperature after winding: 600 to 750°C]
Controlling the coil retention after coiling is very important for controlling the value obtained by dividing the average Mn concentration in ferrite by the Mn concentration in the base material (Mn _F /Mn _A ) and the value obtained by dividing the average Mn concentration in retained austenite by the Mn concentration in the base material (Mn _γ /Mn _A ) within desired ranges in the microstructure of the steel sheet obtained at the end. More specifically, ferrite, pearlite, and bainite are obtained by first coiling a finish-rolled hot-rolled steel sheet at a temperature of 500 to 700°C. Next, by retaining the coil having such a microstructure at 600 to 750°C, Mn can be concentrated from bainitic ferrite in the ferrite or bainite into the carbides contained in the pearlite or bainite, thereby generating a distribution of Mn concentration. The Mn-enriched carbides are likely to become retained austenite after subsequent annealing, and Mn increases chemical stability and contributes to improved elongation. If the retention temperature is less than 600°C, Mn cannot be sufficiently diffused, making it impossible to promote the enrichment of Mn in carbides contained in pearlite and bainite. Therefore, in the microstructure of the finally obtained steel sheet, the average Mn concentration in the retained austenite cannot be sufficiently increased, and relatedly, the average Mn concentration in ferrite cannot be sufficiently reduced. As a result, the desired Mn _F /Mn _A value and/or Mn _γ /Mn _A value cannot be obtained in the microstructure of the finally obtained steel sheet. Therefore, the lower limit of the retention temperature is set to 600°C. On the other hand, if the retention temperature exceeds 750°C, Mn is excessively enriched, making it difficult to exhibit the TRIP effect, so the upper limit of the retention temperature is set to 750°C. The method of retaining the coil after winding at 600 to 750°C is not limited to a specific method, and may involve reheating the coil or covering the coil in a highly insulating box.

[Coil residence time after winding: 1.0 to 5.0 hours]
As described above, by retaining the wound coil at 600 to 750°C, Mn can be concentrated from the bainitic ferrite in ferrite or bainite into the carbides contained in pearlite or bainite, thereby creating a distribution of Mn concentrations. However, if the retention time is less than 1.0 hour, Mn cannot be sufficiently diffused, making it impossible to promote Mn concentration in the carbides contained in pearlite or bainite. Therefore, in the microstructure of the steel sheet obtained at the end, the average Mn concentration in the retained austenite cannot be sufficiently increased, and relatedly, the average Mn concentration in the ferrite cannot be sufficiently reduced. As a result, the desired Mn _F /Mn _A value and/or Mn _γ /Mn _A value cannot be obtained in the microstructure of the steel sheet obtained at the end. Therefore, the lower limit of the retention time is set to 1.0 hour. On the other hand, if the retention time exceeds 5.0 hours, Mn is excessively concentrated, making it difficult to exhibit the TRIP effect, so the upper limit of the retention time is set to 5.0 hours.

[Acid washing process]
In the pickling process, oxides on the surface of the hot-rolled steel sheet are removed to improve the chemical conversion treatability and plating property of the cold-rolled steel sheet. The solution used for pickling may be any solution used in ordinary pickling, such as a solution containing 5 vol. % or more of hydrochloric acid or sulfuric acid. Pickling may be performed once, or multiple times as necessary.

[Cold rolling process]
The pickled hot-rolled steel sheet is subjected to cold rolling at a reduction ratio of 20 to 90% to obtain a cold-rolled steel sheet. By setting the cold-rolling reduction ratio to 20% or more, the shape of the cold-rolled steel sheet can be kept flat, thereby suppressing a decrease in ductility of the final product. The cold-rolling reduction ratio is preferably 30% or more. On the other hand, by setting the cold-rolling reduction ratio to 90% or less, it is possible to suppress the rolling load from becoming excessively large, which makes rolling difficult. The cold-rolling reduction ratio is preferably 80% or less. The number of rolling passes and the reduction ratio per pass are not particularly limited, and may be set appropriately so as to be within the above range.

[Annealing process]
[Maximum heating temperature: 780-900℃]
By setting the maximum heating temperature to 780 to 900°C, austenite is generated during annealing, making it easier to obtain a predetermined amount of tempered martensite as the final structure. This makes it easier for the steel sheet to meet the desired tensile strength. If the maximum heating temperature is less than 780°C, sufficient austenite is not generated, and the predetermined amount of tempered martensite cannot be obtained after cooling, so the lower limit of the maximum heating temperature is set to 780°C. On the other hand, if the maximum heating temperature exceeds 900°C, there is no problem with the properties of the steel sheet, but productivity decreases. For this reason, the maximum heating temperature is set to 900°C or less, and preferably 850°C or less.

[Holding time: 30-500 seconds]
As described above, austenite is generated at the maximum heating temperature in the annealing process, making it easier to obtain a predetermined amount of tempered martensite as the final structure. If the holding time is less than 30 seconds, sufficient austenite is not generated, and the predetermined amount of tempered martensite cannot be obtained after cooling, so the lower limit of the holding time is set to 30 seconds. The upper limit of the holding time does not affect the material properties, but if it is too long, productivity will decrease, so the upper limit is set to 500 seconds.

[Dew point]
In the annealing process, the dew point of the furnace atmosphere during holding at the maximum heating temperature may be increased to -30°C or higher. By performing the annealing process in such an atmosphere, it is possible to promote the decarburization reaction from the steel sheet surface. As a result, it is possible to form a surface softened portion having an average thickness of 10 μm or more and an average Vickers hardness of 0.90 times or less the average Vickers hardness at the half-thickness position of the sheet. By providing such a softened portion in the surface layer of the steel sheet, it is possible to improve the bendability of the steel sheet. By adding this improvement in bendability to the improvement in elongation described above, it is possible to obtain a steel sheet that not only improves the workability during part manufacturing but also has excellent crash resistance in the manufactured parts.

[Primary cooling]
[Average cooling rate from maximum heating temperature to primary cooling stop temperature of 650°C or higher: 1.0 to 20.0°C/sec]
In order to obtain the desired ferrite area ratio, it is necessary to appropriately perform primary cooling from the maximum heating temperature. More specifically, the primary cooling is performed at an average cooling rate of 1.0 to 20.0°C/s from the maximum heating temperature to a primary cooling stop temperature of 650°C or higher. If the primary cooling stop temperature is less than 650°C, excessive ferrite is generated, the area ratio of tempered martensite cannot be obtained sufficiently, and strength is reduced. For this reason, the lower limit of the cooling stop temperature is set to 650°C. Even if the average cooling rate from the maximum heating temperature to the primary cooling stop temperature is less than 1.0°C/s, excessive ferrite is generated, the area ratio of tempered martensite cannot be obtained sufficiently, and strength is reduced. For this reason, the lower limit of the average cooling rate is set to 1.0°C/s. If the average cooling rate exceeds 20.0°C/s, ferrite cannot be generated, and the effect of primary cooling cannot be exerted.

[Secondary cooling]
[Secondary cooling stop temperature: 100 to less than 300°C]
Next, the cold-rolled steel sheet is cooled from the primary cooling stop temperature to 100 to less than 300°C in secondary cooling. In secondary cooling, martensite can be obtained by quenching. The martensite produced in secondary cooling is tempered in the dwell operation described below to become tempered martensite. Furthermore, in order to promote bainite transformation and obtain retained austenite in the dwell operation, a certain amount of untransformed austenite must be left in secondary cooling. The lower the secondary cooling stop temperature, the more martensite increases and the amount of untransformed austenite decreases, making it impossible to obtain the desired amount of retained austenite. Therefore, the lower limit of the secondary cooling stop temperature is set to 100°C. On the other hand, if the secondary cooling stop temperature is 300°C or higher, sufficient martensite cannot be obtained in the secondary cooling stage. Therefore, the desired amount of tempered martensite cannot be obtained even with the subsequent dwell operation. In this case, a relatively large amount of untransformed austenite may remain after the quenching operation, and this untransformed austenite may subsequently be cooled and ultimately produce a large amount of as-quenched martensite. Therefore, the upper limit of the secondary cooling stop temperature is set to less than 300°C.

[Average cooling rate: 20°C/sec or more]
As described above, martensite is obtained in the secondary cooling from the primary cooling stop temperature. If the average cooling rate during this period is less than 20°C/sec, quenching is difficult, a large amount of bainite is formed, and the desired amount of tempered martensite cannot be obtained in the final microstructure. Therefore, the average cooling rate during the secondary cooling is set to 20°C/sec.

[Staying in the temperature range of 300 to 450°C for 100 seconds or more]
After secondary cooling, the cold-rolled steel sheet is allowed to dwell in the temperature range of 300 to 450°C for 100 seconds or more, thereby tempering the martensite obtained by secondary cooling and obtaining tempered martensite. Furthermore, by transforming untransformed austenite that has not been transformed during secondary cooling into bainite and promoting C enrichment in the austenite, a sufficient amount of retained austenite can be obtained and the C concentration in the retained austenite can also be increased. If the dwell temperature is less than 300°C, tempering is insufficient, and as-quenched martensite tends to remain, resulting in poor elongation. Therefore, the lower limit of the dwell temperature is set to 300°C. On the other hand, if the dwell temperature exceeds 450°C, excessive tempering occurs, and sufficient strength cannot be obtained. Therefore, the upper limit of the dwell temperature is set to 450°C. The dwell time is necessary to promote bainite transformation. If the dwell time is less than 100 seconds, the bainite transformation is not completed, and sufficient retained austenite cannot be obtained, and/or the desired C concentration in the retained austenite cannot be obtained.

[Plating and surface treatment]
The steel sheet may be subjected to a plating treatment such as electroplating or vapor deposition plating, and further, an alloying treatment may be performed after the plating treatment. The steel sheet may also be subjected to a surface treatment such as formation of an organic coating, film lamination, organic salt or inorganic salt treatment, or non-chromium treatment.

When hot-dip galvanizing is performed on a steel sheet as a plating process, the steel sheet is heated or cooled to a temperature at least 40°C lower than the temperature of the galvanizing bath and not higher than 50°C, and then passed through the galvanizing bath. This hot-dip galvanizing process results in a steel sheet with a hot-dip galvanized layer on its surface, i.e., a hot-dip galvanized steel sheet. The hot-dip galvanized layer has a chemical composition, for example, of 7% to 15% by mass of Fe, with the balance being Zn, Al, and impurities. The hot-dip galvanized layer may also be a zinc alloy.

When alloying treatment is performed after hot-dip galvanizing treatment, for example, the hot-dip galvanized steel sheet is heated to a temperature of 460°C or higher and 600°C or lower. If this temperature is lower than 460°C, alloying may be insufficient. On the other hand, if this temperature is higher than 600°C, alloying may be excessive, resulting in deterioration of corrosion resistance. This alloying treatment results in a steel sheet with an alloyed hot-dip galvanized layer on the surface, i.e., an alloyed hot-dip galvanized steel sheet.

The above-described exemplary methods can be used to manufacture steel sheets according to embodiments of the present invention. Note that the above-described embodiments merely illustrate specific examples of how the present invention can be implemented, and the technical scope of the present invention should not be interpreted as being limited by these examples. In other words, the present invention can be implemented in various forms without departing from its technical concept or main features.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples in any way.

In the following examples, steel plates according to embodiments of the present invention were manufactured under various conditions, and the tensile strength and elongation properties of the resulting steel plates were investigated.

First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions shown in Table 1. These slabs were then heated to the heating temperatures shown in Table 2 and hot-rolled. Hot rolling was performed by rough rolling and finish rolling. More specifically, the rough rolling conditions were the same for all Examples and Comparative Examples, and the finish rolling end temperature and coiling temperature were as shown in Table 2. Next, the coiled coil was held at the maximum temperature shown in Table 2 for the time shown in Table 2. The obtained hot-rolled steel sheets were pickled and then cold-rolled at the reduction shown in Table 2 to obtain cold-rolled steel sheets with a thickness of 1.4 mm. Next, the obtained cold-rolled steel sheets were annealed under the conditions shown in Table 2, consisting of heating, primary cooling, secondary cooling, and holding operations. The dew points in the furnace during holding at the maximum heating temperature were as shown in Table 2. Finally, hot-dip galvanizing was performed as appropriate, and some of the slabs were further subjected to an alloying treatment.

The properties of the obtained steel plates were measured and evaluated using the following methods.

[Tensile strength (TS) and total elongation (El)]
The tensile strength (TS) and total elongation (El) were measured by conducting a tensile test in accordance with JIS Z 2241:2011 using a JIS No. 5 test piece taken from a direction in which the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the steel plate.

[Bendability]
Bendability is evaluated by the ratio R/t of the critical bending radius R to the sheet thickness t. To determine the critical bending radius R, a No. 1 test piece described in JIS Z 2204:1996 was prepared so that the direction perpendicular to the rolling direction was the longitudinal direction (the bending ridge line coincided with the rolling direction), and a V-bending test was performed in accordance with JIS Z 2248:2022. The angle between the die and punch was set to 60°, and the bending test was performed while changing the punch tip radius in 0.5 mm increments. The punch tip radius at which the sheet could be bent without cracking was determined as the critical bending radius R.

[Collision resistance properties]
The crashworthiness was evaluated based on TS, El, and R/t. Specifically, crashworthiness was evaluated as follows: TS of 980 MPa or more, El of 14% or more, and R/t of 1.5 or less were all satisfied, with a double circle; two were satisfied with a circle; and one or less was satisfied with an x.

Steel sheets with a TS of 980 MPa or more and an El of 14% or more were evaluated as being capable of achieving high strength and improved elongation. The results are shown in Table 3.

Referring to Tables 1 to 3, Comparative Examples 22 and 26 had low C and Mn contents, respectively, resulting in low TS. Comparative Examples 23 and 27 had high C and Mn contents, respectively, resulting in excessively high strength of the hot-rolled sheet, making it impossible to properly perform cold rolling. Comparative Example 24 had a low total Si and Al content, which is thought to have resulted in insufficient C enrichment in untransformed austenite during bainite transformation. As a result, the desired amount of retained austenite could not be obtained, and El decreased. Comparative Example 25 had a high total Si and Al content, which resulted in excessive strength and/or excessive stabilization of retained austenite, resulting in excessive rolling load during cold rolling and cracking of the steel sheet. Comparative Examples 28 and 29 had a short coil dwell time after coiling, which is thought to have resulted in insufficient diffusion of Mn and failure to promote enrichment of Mn in carbides contained in pearlite and bainite. As a result, the generation of retained austenite from the carbides was insufficient, and the average Mn concentration in the retained austenite could not be sufficiently increased while the average Mn concentration in the ferrite could not be sufficiently reduced, resulting in a decrease in El. In Comparative Example 30, the maximum temperature (retention temperature) of the coil after coiling was low, which similarly prevented Mn from being sufficiently diffused and prevented the concentration of Mn in the carbides contained in pearlite and bainite from being promoted. As a result, the generation of retained austenite from the carbides was insufficient, and the average Mn concentration in the retained austenite could not be sufficiently increased while the average Mn concentration in the ferrite could not be sufficiently reduced, resulting in a decrease in El.

In Comparative Example 31, the maximum heating temperature in the annealing process was low, resulting in insufficient austenitization and the desired amount of tempered martensite could not be obtained. As a result, TS decreased. In Comparative Example 32, the holding time at the maximum heating temperature in the annealing process was short, resulting in insufficient austenitization and the desired amount of tempered martensite could not be obtained. As a result, TS decreased. In Comparative Example 33, the primary cooling stop temperature in the annealing process was low, resulting in excessive ferrite formation. As a result, the desired amount of tempered martensite was not obtained, and TS decreased. In Comparative Example 34, the average cooling rate in the primary cooling process was slow, resulting in excessive ferrite formation. As a result, the desired amount of tempered martensite was not obtained, and TS decreased. In Comparative Example 35, the secondary cooling stop temperature in the annealing process was low, resulting in excessive tempered martensite formation. As a result, retained austenite was not obtained, and El decreased. In Comparative Example 36, the secondary cooling stop temperature in the annealing process was high, which is thought to have resulted in the formation of a large amount of as-quenched martensite from the untransformed austenite remaining after the dwell operation, resulting in a decrease in El. In Comparative Example 37, the dwell time in the annealing process in the temperature range of 300 to 450°C was short, which is thought to have resulted in insufficient C enrichment in the untransformed austenite during bainite transformation. As a result, the desired amount of retained austenite could not be obtained, and El decreased. In Comparative Example 38, the average cooling rate in the secondary cooling process was slow, which resulted in the formation of a large amount of bainite, making it impossible to obtain the desired amount of tempered martensite. As a result, TS decreased. In Comparative Example 39, the maximum temperature (dwell temperature) of the coil after coiling was low, while the dwell time of the coil after coiling was long, which is thought to have resulted in the inability to achieve appropriate Mn enrichment. As a result, although the average Mn concentration in the retained austenite could be increased, the average Mn concentration in the ferrite could not be sufficiently reduced, resulting in a decrease in El. In Comparative Example 40, the coil was held for a short time after winding, which is thought to be why Mn could not be sufficiently diffused and the concentration of Mn in the carbides contained in pearlite and bainite could not be promoted. As a result, the average Mn concentration in the retained austenite could not be sufficiently increased, and El decreased accordingly.

In contrast, the steel sheets of all the examples had a specified chemical composition, comprising, by area ratio, 40-65% tempered martensite, 15-40% ferrite, and 10-20% retained austenite. Furthermore, the average Mn concentration in ferrite divided by the Mn concentration in the base material was controlled to 0.980 or less, and the average Mn concentration in retained austenite divided by the Mn concentration in the base material was controlled to 1.150-2.000. This significantly improved the elongation of the steel sheets, despite their extremely high tensile strength of 980 MPa or more. In particular, in Examples 1 to 15, 19 to 21, 41, and 42, which had a surface-softened portion having an average thickness of 10 μm or more and an average Vickers hardness of 0.90 times or less the average Vickers hardness at the half-thickness position of the plate, R/t was 1.5 or less, and therefore bendability was high. Furthermore, TS was 980 MPa or more and El was 14% or more, so very high crash resistance characteristics were achieved.

Claims (6)

In mass%,
C: 0.200-0.350%,
Si: 0.01-2.00%,
Mn: 1.40-4.00%,
P: 0.1000% or less,
S: 0.0200% or less,
Al: 2.00% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Cr: 0-2.000%,
Mo: 0-1.000%,
Ti: 0 to 0.500%,
Nb: 0 to 0.500%,
B: 0 to 0.0100%,
Cu: 0 to 1.000%,
Ni: 0-1.000%,
W: 0-0.100%,
V: 0-1.000%,
Ta: 0-0.100%,
Co: 0-3.000%,
Sn: 0-1.000%,
Sb: 0 to 0.500%,
As: 0 to 0.050%,
Mg: 0 to 0.050%,
Zr: 0 to 0.050%,
Ca: 0-0.0500%,
Y: 0 to 0.0500%,
La: 0 to 0.0500%,
Ce: 0 to 0.0500%,
Bi: 0 to 0.0500%, and the balance: Fe and impurities;
The formula satisfies 1.00≦[Si]+[Al]≦2.20, wherein [Si] and [Al] are the contents (% by mass) of each element, and the chemical composition is
In terms of area ratio,
Ferrite: 15 to 40%,
Tempered martensite: 40-65%,
Retained austenite: 10 to 20%
Bainite: 10-30%,
Pearlite: 0 to 10% and As-quenched martensite: 0 to 10%
a value obtained by dividing the average Mn concentration in the ferrite by the Mn concentration in the base material is 0.980 or less;
A steel sheet characterized by having a microstructure in which the value obtained by dividing the average Mn concentration in the retained austenite by the Mn concentration in the base material is 1.150 to 2.000. The chemical composition is, in mass %,
Cr: 0.001-2.000%,
Mo: 0.001 to 1.000%,
Ti: 0.001 to 0.500%,
Nb: 0.001-0.500%,
B: 0.0001 to 0.0100%,
Cu: 0.001 to 1.000%,
Ni: 0.001 to 1.000%,
W: 0.001-0.100%,
V: 0.001-1.000%,
Ta: 0.001 to 0.100%,
Co: 0.001 to 3.000%,
Sn: 0.001 to 1.000%,
Sb: 0.001 to 0.500%,
As: 0.001 to 0.050%,
Mg: 0.0001-0.050%,
Zr: 0.0001 to 0.050%,
Ca: 0.0001-0.0500%,
Y: 0.0001-0.0500%,
La: 0.0001 to 0.0500%,
Ce: 0.0001 to 0.0500%, and Bi: 0.0001 to 0.0500%
The steel sheet according to claim 1, characterized in that it contains at least one of the following: The steel sheet according to claim 1 or 2, characterized in that the average C concentration in the retained austenite is 0.80 mass% or more. 3. The steel sheet according to claim 1, comprising a plate thickness center portion and a surface-softened portion arranged on one or both sides of the plate thickness center portion, wherein the surface-softened portion has an average thickness of 10 μm or more and an average Vickers hardness of 0.90 times or less the average Vickers hardness at a position halfway through the plate thickness. 3. The steel sheet according to claim 1, wherein the tensile strength is 1180 MPa or more. A component, characterized in that it comprises a steel sheet according to claim 1 or 2 .