Steel sheet

Abstract

Provided is a steel sheet having a predetermined chemical composition and a microstructure comprising, by area ratio, ferrite: 15 to 40%, tempered martensite: 40 to 65%, retained austenite: 10 to 20%, bainite: 10 to 30%, pearlite: 0 to 10%, and as-quenched martensite: 0 to 10%, wherein a value of an average value of an Mn concentration in the ferrite divided by an Mn concentration of a base material is 0.980 or less, and a value of an average value of an Mn concentration in the retained austenite divided by the Mn concentration of the base material is 1.150 to 2.000.

Description

FIELD

[0001] The present invention relates to a steel sheet.BACKGROUND

[0002] In recent years, in the automobile industry, lighter weight of car bodies has been sought from the viewpoint of improvement of fuel economy. To achieve both lighter weight of car bodies and collision safety, increasing the strength of the steel sheet used would be one effective method. Due to such a background, development of a higher strength steel sheet has been promoted. On the other hand, along with higher strength, the formability of a steel sheet generally falls. For this reason, in the development of a high strength steel sheet, securing a certain level of formability or more while increasing the strength is important.

[0003] In relation to this, for example, PTL 1 describes a steel sheet having a predetermined chemical composition and having a microstructure at a sheet thickness ¼ position from the surface in a cross-section parallel to the rolling direction of the steel sheet and sheet thickness direction containing, by area %, retained austenite: 10% or more, tempered martensite: 60 to 80%, and martensite: less than 20%, in the tempered martensite and martensite, a low angle grain boundary density with a crystal misorientation of 2 degrees or more and less than 20 degrees being 0.20 to 1.0 μm−1, a high angle grain boundary density with a crystal misorientation of 20 to 50 degrees being 0.30 to 0.60 μm−1, and a ratio AL / AN of a particle density AL of retained austenite in the rolling direction and a particle density AN of retained austenite in the sheet thickness direction being 0.80 to 1.0. Further, PTL 1 teaches by making the above low angle grain boundary density 0.20 to 1.0 μm−1, transformation induced plasticity of austenite occurs due to the high stress and an excellent strength-ductility balance is obtained, on the other hand, by making the above high angle grain boundary density 0.30 to 0.60 μm−1, progression of ductile fracture is suppressed and excellent crash properties are obtained, and, furthermore, by making the above particle density ratio AL / AN 0.80 to 1.0, the retained austenite is dispersed isotropically and excellent bendability and crash properties are obtained.

[0004] PTL 2 describes a high strength steel sheet having a predetermined chemical composition and having a steel structure comprising, by area ratio, polygonal ferrite: 35% or more and 80% or less and martensite: 5% or more and 25% or less and, by volume fraction, retained austenite: 8% or more, further, the average crystal particle size of the polygonal ferrite being 6 μm or less, the average crystal particle size of the martensite being 3 μm or less, the average crystal particle size of the retained austenite being 3 μm or less, the average aspect ratios of the crystal grains of the polygonal ferrite, martensite, and retained austenite being respectively 2.0 or less, and, furthermore, the value of the amount of Mn in the retained austenite (mass %) divided by the amount of Mn in the polygonal ferrite (mass %) being 2.0 or more. Further, PTL 2 teaches that to secure good ductility, it is necessary to increase the amount of stable retained austenite with concentrated Mn, therefore it is extremely important that the value of the amount of Mn in the retained austenite (mass %) divided by the amount of Mn in the polygonal ferrite (mass %) be 2.0 or more.CITATION LISTPatent Literature

[0005] [PTL 1] WO2021 / 200169

[0006] [PTL 2] WO2016 / 067625SUMMARYTechnical Problem

[0007] As explained above, it is known that along with higher strength, the formability of a steel sheet falls and that, specifically, the elongation and other properties fall. In relation to this, PTL 2 teaches that if the amount of C in the retained austenite satisfies 0.09×[amount of Mn]−0.130−0.140≤[amount of C]≤0.09× [amount of Mn]−0.130+0.140 (where [amount of C]: amount of C in the retained austenite and [amount of Mn]: amount of Mn in the retained austenite) in relation to the [amount of Mn] in the retained austenite, a large amount of so-called stable retained austenite, which intermittently causes the phenomenon of strain induced transformation (TRIP), the main factor in improvement of ductility, up to the end of working of the steel sheet, is obtained and that thereby a high strength and a much better elongation can be achieved. Here, the invention described in PTL 2 mainly covers a steel sheet with a tensile strength of 590 MPa or more. On the other hand, in the automobile industry, etc., further lightening of weight of a steel sheet is being sought. To achieve such lightening of weight, a steel sheet must be made higher in strength than even before. Therefore, there is still a high need for a steel sheet enabling improvement of the elongation when increasing the strength to a level equal to the past or higher than the same.

[0008] Therefore, the present invention has as its object to provide a steel sheet which is high strength and can achieve improved elongation by a novel constitution.Solution to Problem

[0009] The inventors engaged in studies so as to achieve the above object focusing on in particular the microstructure of a steel sheet. As a result, the inventors discovered that by making the microstructure of the steel sheet having a predetermined chemical composition one mainly comprised tempered martensite, ferrite, and retained austenite, it is possible to achieve high strength while improving elongation and, furthermore, by limiting the Mn concentration in the ferrite to within a predetermined range, it is possible to suppress the solution strengthening of the ferrite and make Mn concentrate in the retained austenite to stabilize the retained austenite and remarkably improve the elongation of the steel sheet and thereby completed the present invention.

[0010] The present invention able to achieve the above object is as follows:

[0011] (1) A steel sheet having a chemical composition comprising, by mass %,

[0012] C: 0.200 to 0.350%,

[0013] Si: 0.01 to 2.00%,

[0014] Mn: 1.40 to 4.00%,

[0015] P: 0.1000% or less,

[0016] S: 0.0200% or less,

[0017] Al: 2.00% or less,

[0018] N: 0.0200% or less,

[0019] O: 0.0200% or less,

[0020] Cr: 0 to 2.000%,

[0021] Mo: 0 to 1.000%,

[0022] Ti: 0 to 0.500%,

[0023] Nb: 0 to 0.500%,

[0024] B: 0 to 0.0100%,

[0025] Cu: 0 to 1.000%,

[0026] Ni: 0 to 1.000%,

[0027] W: 0 to 0.100%,

[0028] V: 0 to 1.000%,

[0029] Ta: 0 to 0.100%,

[0030] Co: 0 to 3.000%,

[0031] Sn: 0 to 1.000%,

[0032] Sb: 0 to 0.500%,

[0033] As: 0 to 0.050%,

[0034] Mg: 0 to 0.050%,

[0035] Zr: 0 to 0.050%,

[0036] Ca: 0 to 0.0500%,

[0037] Y: 0 to 0.0500%,

[0038] La: 0 to 0.0500%,

[0039] Ce: 0 to 0.0500%,

[0040] Bi: 0 to 0.0500%, and

[0041] balance: Fe and impurities, and

[0042] satisfying 1.00≤[Si]+[Al]≤2.20, where [Si] and [Al] are the contents of the elements (mass %), and

[0043] a microstructure comprising, by area ratio,

[0044] ferrite: 15 to 40%,

[0045] tempered martensite: 40 to 65%,

[0046] retained austenite: 10 to 20%,

[0047] bainite: 10 to 30%,

[0048] pearlite: 0 to 10%, and

[0049] as-quenched martensite: 0 to 10%, wherein

[0050] a value of an average value of an Mn concentration in the ferrite divided by an Mn concentration of a base material is 0.980 or less, and

[0051] a value of an average value of an Mn concentration in the retained austenite divided by the Mn concentration of the base material is 1.150 to 2.000.

[0052] (2) The steel sheet according to (1), wherein the chemical composition includes, by mass %, at least one of

[0053] Cr: 0.001 to 2.000%,

[0054] Mo: 0.001 to 1.000%,

[0055] Ti: 0.001 to 0.500%,

[0056] Nb: 0.001 to 0.500%,

[0057] B: 0.0001 to 0.0100%,

[0058] Cu: 0.001 to 1.000%,

[0059] Ni: 0.001 to 1.000%,

[0060] W: 0.001 to 0.100%,

[0061] V: 0.001 to 1.000%,

[0062] Ta: 0.001 to 0.100%,

[0063] Co: 0.001 to 3.000%,

[0064] Sn: 0.001 to 1.000%,

[0065] Sb: 0.001 to 0.500%,

[0066] As: 0.001 to 0.050%,

[0067] Mg: 0.0001 to 0.050%,

[0068] Zr: 0.0001 to 0.050%,

[0069] Ca: 0.0001 to 0.0500%,

[0070] Y: 0.0001 to 0.0500%,

[0071] La: 0.0001 to 0.0500%,

[0072] Ce: 0.0001 to 0.0500%, and

[0073] Bi: 0.0001 to 0.0500%.

[0074] (3) The steel sheet according to the above (1) or (2), wherein an average value of a C concentration in the retained austenite is 0.80 mass % or more.

[0075] (4) The steel sheet according to any one of the above (1) to (3), wherein the steel sheet includes a middle part in sheet thickness and a soft surface layer arranged at one side or both sides of the middle part in sheet thickness, the soft surface layer has an average thickness of 10 μm or more and an average Vickers hardness of 0.90 time or less of an average Vickers hardness of a sheet thickness ½ position.

[0076] (5) The steel sheet according to any one of the above (1) to (4), wherein a tensile strength is 1180 MPa or more.

[0077] (6) A part containing the steel sheet according to any one of the above (1) to (5).Advantageous Effects of Invention

[0078] According to the present invention, it is possible to provide a steel sheet which is high strength and can achieve improved elongation.DESCRIPTION OF EMBODIMENTS<Steel Sheet>

[0079] The steel sheet according to an embodiment of the present invention has a chemical composition comprising, by mass %,

[0080] C: 0.20 to 0.350%,

[0081] Si: 0.01 to 2.00%,

[0082] Mn: 1.40 to 4.00%,

[0083] P: 0.1000% or less,

[0084] S: 0.0200% or less,

[0085] Al: 2.00% or less,

[0086] N: 0.0200% or less,

[0087] O: 0.0200% or less,

[0088] Cr: 0 to 2.000%,

[0089] Mo: 0 to 1.000%,

[0090] Ti: 0 to 0.500%,

[0091] Nb: 0 to 0.500%,

[0092] B: 0 to 0.0100%,

[0093] Cu: 0 to 1.000%,

[0094] Ni: 0 to 1.000%,

[0095] W: 0 to 0.100%,

[0096] V: 0 to 1.000%,

[0097] Ta: 0 to 0.100%,

[0098] Co: 0 to 3.000%,

[0099] Sn: 0 to 1.000%,

[0100] Sb: 0 to 0.500%,

[0101] As: 0 to 0.050%,

[0102] Mg: 0 to 0.050%,

[0103] Zr: 0 to 0.050%,

[0104] Ca: 0 to 0.0500%,

[0105] Y: 0 to 0.0500%,

[0106] La: 0 to 0.0500%,

[0107] Ce: 0 to 0.0500%,

[0108] Bi: 0 to 0.0500%, and

[0109] balance: Fe and impurities, and

[0110] satisfying 1.00≤[Si]+[Al]<2.20, where [Si] and [Al] are the contents of the elements (mass %) and

[0111] a microstructure comprising, by area ratio,

[0112] ferrite: 15 to 40%,

[0113] tempered martensite: 40 to 65%,

[0114] retained austenite: 10 to 20%,

[0115] bainite: 10 to 30%,

[0116] pearlite: 0 to 10%, and

[0117] as-quenched martensite: 0 to 10%, wherein

[0118] a value of an average value of an Mn concentration in the ferrite divided by an Mn concentration of a base material is 0.980 or less, and

[0119] a value of an average value of an Mn concentration in the retained austenite divided by the Mn concentration of the base material is 1.150 to 2.000.

[0120] As stated earlier, it is known that along with higher strength, the formability of a steel sheet falls, specifically the elongation and other properties fall. In relation to this, as means for achieving both higher strength and improved elongation, a TRIP (transformation induced plasticity) steel sheet utilizing the transformation induced plasticity of retained austenite is known. If retained austenite is included in the microstructure, in general the elongation of a steel sheet is improved due to the TRIP effect of transformation to martensite by the strain induced transformation during deformation of the steel sheet. Therefore, the inventors engaged in studies to realize both higher strength and improved elongation of a steel sheet focusing in particular on a microstructure containing retained austenite.

[0121] First, the inventors discovered by rectifying the chemical composition of a steel sheet and making the microstructure of the steel sheet one including mainly the hard and tough structure tempered martensite, the soft structure ferrite, and retained austenite having a TRIP effect, more specifically making it include, by area ratio, tempered martensite: 40 to 65%, ferrite: 15 to 40%, and retained austenite: 10 to 20%, it is possible to secure sufficient strength while improving the elongation. In particular, in relation to rectification of the chemical composition of the steel sheet, the inventors discovered that in addition to including in predetermined ratios C and Mn so effective for increasing the strength of a steel sheet, controlling the total amount of Si and Al to, by mass %, a range of 1.00 to 2.20% is effective for both increasing the strength and improving the elongation. By controlling the total amount of Si and Al to within such a range, it is believed possible to make the C concentrate in the retained austenite. More specifically, the microstructure of the steel sheet according to an embodiment of the present invention includes, by area ratio, bainite: 10 to 30% in addition to the tempered martensite, ferrite, and retained austenite. Here, when bainite transforms from austenite, since the C contained in the steel is not sufficiently solid-soluble in bainite, C is discharged from the bainite to the surrounding microstructure as the bainite transformation proceeds. Specifically, the C discharged from the bainite either concentrates in the untransformed austenite or forms carbides. However, the presence of predetermined amounts of Si and Al in the steel makes it difficult to form the carbides, and therefore as a result it is believed that C is concentrated in the untransformed austenite. In this way, C concentrates in the retained austenite in the final microstructure. It is believed that the concentrated C not only stabilizes the retained austenite to contribute to improvement of the elongation at the steel sheet, but also contributes to higher strength of the steel sheet since the hardness also increases when the retained austenite is transformed to martensite by strain induced transformation.

[0122] Next, the inventors engaged in studies to further improve the elongation of a steel sheet focusing on the specific forms of the ferrite as a soft structure particularly able to contribute to improvement of elongation in the microstructure and of the retained austenite exhibiting the TRIP effect. As a result, the inventors discovered that by controlling the Mn concentration in the ferrite to within a predetermined range, more specifically by controlling the value of the average value of Mn concentration in the ferrite divided by the Mn concentration of the base material (i.e, the Mn content of the steel sheet) to 0.980 or less to make the Mn concentration in the ferrite smaller than the average Mn concentration of the steel sheet as a whole, it is possible to remarkably suppress the solution strengthening of ferrite by the Mn and thereby possible to improve the elongation of a steel sheet. In addition, the inventors discovered that, in relation to lowering the Mn concentration in the ferrite, by making Mn concentrate in the retained austenite, more specifically controlling the value of the average value of Mn concentration in the retained austenite divided by the Mn concentration of the base material (i.e., the Mn content of the steel sheet) to 1.150 to 2.000, it is possible to stabilize the retained austenite and, by combination with the effect due to the drop in Mn concentration in the ferrite, possible to remarkably improve the elongation of the steel sheet.

[0123] While not intending to be bound by any specific theory, it is believed that by such Mn concentration stabilizing the retained austenite, the strain induced transformation when deforming the steel sheet becomes delayed to the high strain side making necking (constriction) harder to occur. As a result, it is believed that it becomes possible to suitably obtain the TRIP effect until the high strain side at the time of deformation and that occurrence of necking and the timing of strain induced transformation are also made suitable and the elongation of the steel sheet can be remarkably improved. Explained in more detail, for example, if the degree of Mn concentration is too large and the retained austenite is excessively stabilized, even if necking occurs, strain induced transformation from retained austenite to martensite does not suitably occur and sometimes retained austenite remains until fracture of the steel sheet. Similarly, for example, if the Mn is not sufficiently concentrated and therefore the stability of the retained austenite is low, sometimes strain induced transformation occurs before the occurrence of necking. In such a case, only naturally, the elongation of the steel sheet can no longer be improved. Therefore, in the steel sheet according to an embodiment of the present invention, it is extremely important to suitably make the Mn concentrate in the retained austenite, i.e., control the value of the average value of Mn concentration in the retained austenite divided by the Mn concentration of the base material to 1.150 to 2.000. Further, in addition to this feature, by lowering the Mn concentration in the ferrite, i.e., by controlling the value of the average value of Mn concentration in the ferrite divided by the Mn concentration of the base material to 0.980 or less, it is believed possible to remarkably improve the elongation of a steel sheet due to the specific combination of suitable stabilization of the retained austenite and suppression of solution strengthening of ferrite. Therefore, according to the steel sheet according to an embodiment of the present invention, it is possible to reliably realize the contradictory properties of high strength and excellent elongation, therefore the steel sheet according to an embodiment of the present invention is particularly useful in use in the automobile industry where achievement of both of these properties is sought.

[0124] Below, the steel sheet according to an embodiment of the present invention will be explained in more detail. In the following explanation, the “%” of the units of contents of the elements, unless otherwise indicated, means “mass %”. Further, in this Description, the “to” showing a numerical range, unless otherwise indicated, is used in the sense of the numerical values described before and after the same being included as the lower limit value and the upper limit value.[C: 0.200 to 0.350%]

[0125] C is an element securing a predetermined amount of martensite and improving the strength of a steel sheet. To sufficiently obtain such an effect, the C content is 0.200% or more. The C content may also be 0.210% or more, 0.220% or more, 0.240% or more, or 0.260% or more. On the other hand, if excessively including C, sometimes the strength becomes too high and the elongation falls. Alternatively, the rolling load at the time of production becomes excessive and the burden on the rolling mill and other facilities becomes high and sometimes the productivity falls. For this reason, the C content is 0.350% or less. The C content may also be 0.320% or less, 0.300% or less, or 0.280% or less.[Si: 0.01 to 2.00%]

[0126] Si is an element enhancing the strength of a steel sheet by solution strengthening. To sufficiently obtain such an effect, the Si content is 0.01% or more. The Si content may also be 0.10% or more, 0.30% or more, 0.50% or more, or 0.80% or more. On the other hand, if excessively including Si, removal of the scale formed in the hot rolling becomes difficult and sometimes deterioration of the appearance is invited. For this reason, the Si content is 2.00% or less. The Si content may also 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 to 4.00%]

[0127] Mn is an element improving the quenchability and contributing to improvement of the steel sheet strength. Further, Mn is an element which concentrates in the retained austenite to stabilize the retained austenite and thereby improve the elongation. To sufficiently obtain these effects, the Mn content is 1.40% or more. The Mn content may also 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, if excessively including Mn, the retained austenite is excessively stabilized and therefore conversely sometimes a drop in the elongation is invited and / or the ferrite is strengthened by solution strengthening and similarly a drop in the elongation is invited or the load on the rolling mill or other facility at the time of production rises and sometimes the productivity falls. For this reason, the Mn content is 4.00% or less. The Mn content may also be 3.80% or less, 3.50% or less, 3.20% or less, or 3.00% or less.[P: 0.1000% or Less]

[0128] P is an impurity element and an element causing embrittlement of welded parts and deterioration of the plateability. For this reason, the P content is 0.1000% or less. The P content may also be 0.0600% or less, 0.0200% or less, 0.0150% or less, or 0.0100% or less. The P content may also be as small as possible. The lower limit is not particularly prescribed, but may also be 0%. On the other hand, in actual steel sheet, if reducing the P content to less than 0.0001%, the production costs would greatly rise and the result would become disadvantageous economically. For this reason, the P content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.[S: 0.0200% or Less]

[0129] S is an impurity element and an element obstructing weldability and further obstructing productivity at the time of casting and the time of hot rolling. For this reason, the S content is 0.0200% or less. The S content may also be 0.0150% or less, 0.0120% or less, 0.0100% or less, or 0.0080% or less. The S content is preferably as small as possible. The lower limit is not particularly prescribed and may also be 0%. On the other hand, in actual steel sheet, if reducing the S content to less than 0.0001%, the production costs would greatly rise and the result would become disadvantageous economically. For this reason, the S content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.[Al: 2.00% or Less]

[0130] Al is an element functioning as a deoxidizer and an element effective for increasing the strength of steel. The Al content may also be 0%, but to sufficiently obtain these effects, the Al content is preferably 0.001% or more. The Al content may also 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, if excessively including Al, coarse oxides are formed and sometimes the toughness is decreased. Therefore, the Al content is 2.00% or less. The Al content may also be 1.80% or less, 1.50% or less, 1.30% or less, or 1.00% or less.[N: 0.0200% or Less]

[0131] N is an element becoming a cause for formation of blowholes at the time of welding. For this reason, the N content is 0.0200% or less. The N content may also be 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. The N content may also be as small as possible. The lower limit is not particularly prescribed and may also be 0%. On the other hand, in actual steel sheet, if reducing the N content to less than 0.0001%, the production costs would greatly rise and the result would become disadvantageous economically. For this reason, the N content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.[O: 0.0200% or Less]

[0132] O is an element becoming a cause for formation of blowholes at the time of welding. For this reason, the O content is 0.0200% or less. The O content may also be 0.0180% or less, 0.0150% or less, 0.0100% or less, or 0.0080% or less. The O content is preferably as small as possible. The lower limit is not particularly prescribed and may also be 0%. On the other hand, in actual steel sheet, if reducing the O content to less than 0.0001%, the production costs would greatly rise and the result would become disadvantageous economically. For this reason, the O content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0133] The basic chemical composition of the steel sheet according to an embodiment of the present invention is as explained above. Furthermore, the steel sheet may, according to need, further contain at least one of the following elements in place of part of the balance of Fe for the purpose of improving the properties. For example, the steel sheet may contain at least one of Cr: 0 to 2.000%, Mo: 0 to 1.000%, Ti: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0100%, Cu: 0 to 1.000%, Ni: 0 to 1.000%, W: 0 to 0.100%, V: 0 to 1.000%, Ta: 0 to 0.100%, Co: 0 to 3.000%, Sn: 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 explained in detail below.[Cr: 0 to 2.000%]

[0134] Cr, like Mn, is an element improving the quenchability and contributing to improvement of the steel sheet strength. The Cr content may also be 0%, but to obtain the above effect, the Cr content is preferably 0.001% or more. The Cr content may also be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, even if excessively containing Cr, the effect becomes saturated and a rise in the production costs is liable to be invited. Therefore, the Cr content is preferably 2.000% or less and may also be 1.500% or less, 1.000% or less, or 0.500% or less.[Mo: 0 to 1.000%]

[0135] Mo, like Cr, is an element contributing to higher strength of steel sheet. This effect can be obtained even with a trace amount. The Mo content may also be 0%, but to obtain the above effect, the Mo content is preferably 0.001% or more. The Mo content may also be 0.010% or more, 0.020% or more, 0.050% or more, or 0.100% or more. On the other hand, if excessively including Mo, the hot formability will fall and the productivity will sometimes fall. For this reason, the Mo content is preferably 1.000% or less. The Mo content may also be 0.800% or less, 0.400% or less, or 0.200% or less.[Ti: 0 to 0.500%]

[0136] Ti is an element effective for control of the form of carbides. Due to Ti, increased strength of ferrite can be promoted. The Ti content may also be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may also be 0.002% or more, 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, even if excessively containing Ti, the effect becomes saturated and a rise in the production costs is liable to be invited. Therefore, the Ti content is preferably 0.500% or less and may also be 0.400% or less, 0.200% or less, or 0.100% or less.[Nb: 0 to 0.500%]

[0137] Nb, like Ti, is an element effective for control of the form of carbides and an element also effective for refinement of the structure to improve the toughness of the steel sheet. These effects can be obtained even with a trace amount. The Nb content may also be 0%, but to obtain the above effect, the Nb content is preferably 0.001% or more. The Nb content may also be 0.005% or more or 0.010% or more. On the other hand, if excessively including Nb, coarse carbides, etc., are formed in the steel and sometimes the toughness of the steel sheet is lowered. For this reason, the Nb content is preferably 0.500% or less. The Nb content may also be 0.200% or less, 0.100% or less, or 0.060% or less.[B: 0 to 0.0100%]

[0138] B is an element suppressing the formation of ferrite and pearlite from austenite in the process of cooling and promoting the formation of martensite. Further, B is an element advantageous for increasing the strength of steel. These effects can be obtained even with a trace amount. The B content may also be 0%, but to obtain the above effect, the B content is preferably 0.0001% or more. The B content may also be 0.0005% or more, or 0.0010% or more. On the other hand, if excessively including B, sometimes the toughness and / or weldability falls. For this reason, the B content is preferably 0.0100% or less. The B content may also be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.[Cu: 0 to 1.000%]

[0139] Cu is an element contributing to improvement of the strength of steel sheet. This effect can be obtained even with a trace amount. The Cu content may also be 0%, but to obtain the above effect, the Cu content is preferably 0.001% or more. The Cu content may also be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if excessively including Cu, red shortness is invited and the productivity in hot rolling is liable to be reduced. For this reason, the Cu content is preferably 1.000% or less. The Cu content may also be 0.800% or less, 0.600% or less, 0.300% or less, or 0.100% or less.[Ni: 0 to 1.000%]

[0140] Ni is an element effective for improvement of the strength of steel sheet. The Ni content may also be 0%, but to obtain the above effect, the Ni content is preferably 0.001% or more. The Ni content may also be 0.005% or more, or 0.010% or more. On the other hand, if excessively including Ni, sometimes the weldability of the steel sheet falls. For this reason, the Ni content is preferably 1.000% or less. The Ni content may also be 0.800% or less, 0.400% or less, or 0.200% or less.[W: 0 to 0.100%]

[0141] W is an element effective for control of the form of carbides and improvement of the strength of the steel sheet. The W content may also be 0%, but to obtain these effects, the W content is preferably 0.001% or more. The W content may also be 0.005% or more, or 0.010% or more. On the other hand, if excessively including W, sometimes the weldability falls. For this reason, the W content is preferably 0.100% or less. The W content may also be 0.080% or less, 0.040% or less, or 0.020% or less.[V: 0 to 1.000%]

[0142] V, like Ti and Nb, is an element effective for control of the form of carbides and is an element also effective for refinement of the structure and improvement of the toughness of steel sheet. The V content may also be 0%, but to obtain the above effect, the V content is preferably 0.001% or more. The V content may also be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if excessively including V, a large amount of precipitates is formed and sometimes the toughness is made to fall. For this reason, the V content is preferably 1.000% or less. The V content may also be 0.400% or less, 0.200% or less, or 0.100% or less.[Ta: 0 to 0.100%]

[0143] Ta, like W, is an element effective for control of the form of carbides and improvement of the steel sheet strength. The Ta content may also be 0%, but to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may also be 0.005% or more, or 0.010% or more. On the other hand, even if excessively containing Ta, the effect becomes saturated and inclusion in steel sheet more than necessary invites a rise in production costs. For this reason, the Ta content is preferably 0.100% or less. The Ta content may also be 0.080% or less, 0.040% or less, or 0.020% or less.[Co: 0 to 3.000%]

[0144] Co, like Ni, is an element effective for improvement of the strength of steel sheet. The Co content may also be 0%, but to obtain the above effect, the Co content is preferably 0.001% or more. The Co content may also be 0.005% or more, 0.010% or more, or 0.100% or more. On the other hand, if excessively including Co, sometimes the hot formability falls and an increase in material costs is led to. For this reason, the Co content is preferably 3.000% or less. The Co content may also be 2.000% or less, 1.000% or less, 0.500% or less, or 0.200% or less.[Sn: 0 to 1.000%]

[0145] Sn is an element able to be contained in steel sheet when using scrap as the raw material for the steel sheet. Further, Sn is liable to trigger embrittlement of ferrite. For this reason, the Sn content is preferably as small as possible and is preferably 1.000% or less. The Sn content may also be 0.100% or less, 0.040% or less, or 0.020% or less. The Sn content may also be 0%, but reducing the Sn content to less than 0.001% would invite an excessive increase in refining costs. For this reason, the Sn content may also be 0.001% or more, 0.005% or more, or 0.010% or more.[Sb: 0 to 0.500%]

[0146] Sb, like Sn, is an element able to be contained in steel sheet when using scrap as the raw material for the steel sheet. Further, Sb strongly segregates at the grain boundaries and is liable to invite embrittlement of the grain boundaries. For this reason, the Sb content is preferably as small as possible and is preferably 0.500% or less. The Sb content may also be 0.100% or less, 0.040% or less, or 0.020% or less. The Sb content may also be 0%, but reducing the Sb content to less than 0.001% would invite an excessive increase in refining costs. For this reason, the Sb content may also be 0.001% or more, 0.005% or more, or 0.010% or more.[As: 0 to 0.050%]

[0147] As, like Sn and Sb, is an element able to be contained in steel sheet when using scrap as the raw material for the steel sheet. Further, As is an element strongly segregating at the grain boundaries. The As content is preferably as small as possible. The As content is preferably 0.050% or less and may also be 0.040% or less, or 0.020% or less. The As content may also be 0%, but reducing the As content to less than 0.001% would invite an excessive increase in refining costs. For this reason, the As content may also be 0.001% or more, 0.005% or more, or 0.010% or more.[Mg: 0 to 0.050%]

[0148] Mg controls the form of sulfides and oxides and contributes to improving the bendability of steel sheet. This effect can be obtained even with a trace amount. The Mg content may also be 0%, but to obtain the above effect, the Mg content is preferably 0.0001% or more. The Mg content may also be 0.0005% or more, 0.001% or more, or 0.005%. On the other hand, even if excessively containing Mg, the effect becomes saturated and inclusion in steel sheet more than necessary invites a rise in production costs. For this reason, the Mg content is preferably 0.050% or less. The Mg content may also be 0.040% or less, 0.020% or less, or 0.010% or less.[Zr: 0 to 0.050%]

[0149] Zr is an element enabling control of the form of sulfides in a trace amount. The Zr content may also be 0%, but to obtain the above effect, the Zr content is preferably 0.0001% or more. The Zr content may also be 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, even if excessively containing Zr, the effect becomes saturated and inclusion in steel sheet more than necessary invites a rise in production costs. For this reason, the Zr content is preferably 0.050% or less. The Zr content may also be 0.040% or less, 0.020% or less, or 0.010% or less.[Ca: 0 to 0.0500%][Y: 0 to 0.0500%][La: 0 to 0.0500%][Ce: 0 to 0.0500%]

[0150] Ca, Y, La, and Ce are elements enabling control of the form of the sulfides in trace amounts. The Ca, Y, La, and Ce contents may also be 0%, but to obtain the above effect, the Ca, Y, La, and Ce contents are preferably respectively 0.0001% or more and may also be 0.0005% or more, 0.0010% or more, 0.0020% or more, or 0.0030% or more. On the other hand, even if excessively containing these elements, the effect becomes saturated and inclusion in steel sheet more than necessary invites a rise in production costs. Therefore, the Ca, Y, La, and Ce contents are preferably respectively 0.0500% or less and may also be 0.0200% or less, 0.0100% or less, or 0.0060% or less.[Bi: 0 to 0.0500%]

[0151] Bi is an element having the action of improving the formability by refinement of the solidified structure. The Bi content may also be 0%, but to obtain such an effect, the Bi content is preferably 0.0001% or more and may also be 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, even if excessively containing Bi, the effect becomes saturated and inclusion in steel sheet more than necessary invites a rise in production costs. Therefore, the Bi content is preferably 0.0500% or less and may also be 0.0400% or less, 0.0200% or less, or 0.0100% or less.

[0152] In the steel sheet according to an embodiment of the present invention, the balance besides the above elements is comprised of Fe and impurities. The “impurities” are elements entering from the steel raw materials and / or in the steelmaking process and allowed to be included in a range not obstructing the properties of the steel sheet according to an embodiment of the present invention.[1.00≤[Si]+[Al]≤2.20]

[0153] The chemical composition of the steel sheet according to an embodiment of the present invention has to satisfy the following formula:1.00≤[Si]+[Al]≤2.20where [Si] and [Al] are contents of the elements (mass %). Si and Al are elements which, in addition to the effects explained regarding the individual elements, as mentioned earlier, are effective for making C concentrate in the retained austenite. When bainite transforms from austenite, C is discharged from the bainite to the surrounding microstructure. The C discharged from the bainite has difficulty forming carbides due to Si and Al being present in the steel, therefore it is believed C is concentrated in the untransformed austenite. The concentrated C not only, in the same way as the case of Mn, stabilizes the retained austenite to contribute to improvement of the elongation of steel sheet, but also increases the hardness when the retained austenite transforms to martensite by strain induced transformation, and therefore can also contribute to higher strength of the steel sheet. To sufficiently obtain these effects, the chemical composition of the steel sheet according to an 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 is satisfied. From the viewpoint of further increasing these effects, the total content of the Si and Al is preferably 1.20% or more and may also be 1.40% or more, or 1.60% or more. On the other hand, if the total content of the Si and Al is too high, the excessive increase in strength and / or excessive stabilization of the retained austenite sometimes causes the elongation to fall and / or the rolling load at the time of cold rolling, etc., to become excessive and the steel sheet to crack. Therefore, the total content of the Si and Al is 2.20% or less, i.e., [Si]+[Al]≤2.20. The total content of the Si and Al may also be 2.10% or less, 2.20% or less, 1.90% or less, 1.80% or less, or 1.70% or less.

[0155] The chemical composition of the steel sheet according to an embodiment of the present invention may be measured by a general analysis method. For example, the chemical composition of the steel sheet may be measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-nondispersive type infrared absorption method.[Microstructure]

[0156] Next, the microstructure of the steel sheet according to an embodiment of the present invention will be explained. Below, the structural fractions are shown by area ratios, therefore the units “%” of the structural fractions mean area %. Further, as stated later, the microstructure is controlled at the sheet thickness ¼ part of the steel sheet. The “sheet thickness ¼ part of the steel sheet” means the region between the plane at the ⅛ depth of sheet thickness from the rolled surface of the steel sheet and the plane at the ⅜ depth. Below, unless particularly indicated otherwise, a “structural fraction” means a value at the sheet thickness ¼ part in all cases.[Ferrite: 15 to 40%]

[0157] Ferrite is a soft structure, therefore easily deforms and contributes to improving the elongation. To sufficiently obtain such an effect, the area ratio of the ferrite is 15% or more. From the viewpoint of improving the elongation, the higher the area ratio of ferrite, the more preferable. It may also be for example 20% or more, 25% or more, or 30% or more. On the other hand, if excessively containing ferrite, at the steel sheet, sometimes the desired strength cannot be achieved. Therefore, the area ratio of ferrite is 40% or less. The area ratio of ferrite may also be 38% or less, 36% or less, or 34% or less.[Tempered Martensite: 40 to 65%]

[0158] Tempered martensite is a hard structure, therefore is a structure contributing to increasing the strength. Further, tempered martensite is a structure tougher than the hard, but relatively brittle as-quenched martensite, therefore also contributes to improving the elongation. To sufficiently obtain these effects, the area ratio of the tempered martensite is 40% or more. From the viewpoint of increasing the strength, the higher the area ratio of the tempered martensite, the more preferable. It may also be for example 42% or more, 45% or more, or 48% or more. On the other hand, if excessively containing tempered martensite, the strength becomes too high and sometimes the elongation falls. Therefore, the area ratio of tempered martensite is 65% or less. From the viewpoint of improving the elongation, the lower the area ratio of tempered martensite, the more preferable. It may also be for example 60% or less, 55% or less, or 50% or less.[Retained Austenite: 10 to 20%]

[0159] Retained austenite is a structure increasing the strength and elongation of steel sheet by the TRIP effect of transformation to martensite by strain induced transformation during deformation of steel sheet. To sufficiently obtain such an effect, the area ratio of retained austenite content is 10% or more. From the viewpoint of improving the elongation, the higher the area ratio of retained austenite, the more preferable. It may also be for example 12% or more, 14% or more, or 16% or more. On the other hand, if excessively including retained austenite, the ratio of ferrite and tempered martensite or other structures falls and as a result sometimes it is not possible to achieve the desired strength and / or elongation. Therefore, the area ratio of retained austenite is 20% or less. The area ratio of retained austenite may also be 19% or less or 18% or less.[Bainite: 10 to 30%]

[0160] The microstructure of the steel sheet according to an embodiment of the present invention is mainly comprised of the above ferrite, tempered martensite, and retained austenite. Increasing of strength and improvement of elongation are mainly achieved by these structures, but bainite is included as an essential structure in addition to these structures. If the area ratio of bainite is too small, at the time of bainite transformation, the amount of C discharged into the austenite becomes insufficient, the concentration of C in the retained austenite becomes insufficient, and sometimes the increasing of strength and / or improvement of elongation are disadvantageously affected. Therefore, the area ratio of bainite is 10% or more. The area ratio of bainite may also be 12% or more, 15% or more, 18% or more, or 20% or more. On the other hand, bainite is also a relatively hard structure, therefore an contribute to enhancement of strength, but if excessively included, the ratios of other structures of ferrite, tempered martensite, and retained austenite fall and as a result sometimes the desired strength and / or elongation cannot be achieved. Therefore, the area ratio of bainite is 30% or less. The area ratio of bainite may also be 28% or less, 25% or less, or 22% or less.[Pearlite: 0 to 10%][As-Quenched Martensite: 0 to 10%]

[0161] The balance structure besides the ferrite, tempered martensite, retained austenite, and bainite may be, by area ratio, 0%, but if a balance structure is present, the balance structure is pearlite and as-quenched martensite. From the viewpoint of securing the above effect by ferrite, tempered martensite, retained austenite, and bainite, the area ratios of the balance structure, i.e., pearlite and as-quenched martensite, are respectively 10% or less, for example, may be 8% or less, 6% or less, 4% or less, or 2% or less. On the other hand, making the area ratio of these structures 0% requires sophisticated control in the process of production of steel sheet, therefore a drop in yield is sometimes invited. Therefore, the area ratios of pearlite and as-quenched martensite may also respectively be 0.5% or more or 1% or more.[Identification of Microstructures and Calculation of Area Ratios]

[0162] The microstructures are identified and the area ratios are calculated by EBSD (electron backscattered diffraction), X-ray measurement, corrosion using a Nital reagent or LePera solution, and examination by a scan type electron microscope of a 100 μm×100 μm region of a cross-section of the steel sheet vertical to the sheet surface by a power of 1000 to 50000×. Note that, in measurement of the area ratio of any of the structures as well, three locations may be measured and the average value calculated.[Calculation of Area Ratio of Ferrite]

[0163] The area ratio of ferrite is measured by the following method. That is, EBSD attached to the scan type electronic microscope is used to measure a range of ⅛ to ⅜ thickness centered at the position of ¼ of the sheet thickness from the surface of the steel sheet at 0.2 μm intervals (pitch). The value of the grain average misorientation (GAM) is calculated from the measurement data. Further, a region with a value of the grain average misorientation of less than 0.5° is deemed ferrite and the area and area ratio are measured. Here, the “grain average misorientation” is the value obtained by calculating the misorientations between adjoining measurement points in regions surrounded by grain boundaries with a crystal misorientation of 5° or more and obtaining an average for all of the measurement points in the crystal grains.[Calculation of Area Ratio of Bainite]

[0164] For the area ratio of bainite, a sample is taken having a sheet thickness cross-section vertical to the sheet surface of the steel sheet as an examined surface, the examined surface is polished and is etched by a Nital solution, a range of ⅛ to ⅜ thickness centered at ¼ of sheet thickness is examined by a field emission scanning electron microscope (FE-SEM), and known image analysis software is used for calculation. Note that as an image analysis software, for example, it is possible to use an “Analyze” function of “ImageJ” to calculate the area ratio. Here, the “ImageJ” is an open source public domain image processing software which is widely utilized among persons skilled in the art. Note that, in the examination by an FE-SEM, for example, structures at an examined surface of a rectangular shape with sides of 30 μm are differentiated as follows. Bainite is a collection of lath shaped crystal grains not including inside it long axis 20 nm or more iron-based carbides or containing inside it long axis 20 nm or more iron-based carbides but the carbides belonging to a single variant, i.e., a group of iron-based carbides stretching in the same direction. Here, “group of iron-based carbides stretching in the same direction” means a difference in stretching direction of the group of iron-based carbides of within 5°. In bainite, bainite surrounded by grain boundaries with a misorientation of 15° or more is counted as a single bainite grain.[Calculation of Area Ratio of Tempered Martensite]

[0165] The area ratio of the tempered martensite is calculated by the examined surface and measurement method used for calculation of the area ratio of bainite above. With tempered martensite, there is cementite present inside the martensite laths, but there are two or more types of crystal orientation of the martensite laths and cementite and the cementite comes in several variants, therefore it is possible to identify the tempered martensite. The area ratio of the tempered martensite identified in this way is calculated by the point counting method.[Calculation of Area Ratio of As-Quenched Martensite]

[0166] The area ratio of the as-quenched martensite is calculated by first etching by a LePera solution an examined surface similar to the examined surface used for identification of ferrite and using a region similar to identification of ferrite as an examined region. With corrosion by the LePera solution, the as-quenched martensite and retained austenite are not corroded. For this reason, the examined region corroded by the LePera solution is examined by an FE-SEM and the noncorroded regions are deemed as-quenched martensite and retained austenite. Further, the total area ratio of the as-quenched martensite and retained austenite identified in this way is calculated by the point counting method. Next, the volume fraction of the retained austenite calculated in the following way is deemed as the area ratio of the retained austenite and the area ratio is subtracted from the total area ratio to calculate the area ratio of the as-quenched martensite.[Calculation of Area Ratio of Retained Austenite]

[0167] The area ratio of the retained austenite is calculated by measuring the diffraction intensity using X-rays in a sample from which a region of 100 μm has been removed in the sheet thickness direction from the surface layer by electrolytic polishing or chemical polishing. Specifically, this is measured using MoKα rays as the characteristic X-rays. The volume fraction of the retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the obtained bcc phase and (200), (220), and (311) of the fcc phase.[Calculation of Area Ratio of Pearlite]

[0168] For the area ratio of pearlite, a sample is taken having a sheet thickness cross-section vertical to the sheet surface of the steel sheet as an examined surface, the examined surface is polished and is etched by a Nital solution, and a range of ⅛ to ⅜ thickness centered at ¼ of sheet thickness from the surface of the steel sheet is examined and photographed using a secondary electron image by a scanning electron microscope. In the secondary electron image, the carbides are observed by a contrast relatively brighter than other steel structures. In the photographed image, regions with plate-shaped carbides arranged in lines at 0.5 μm or less intervals are defined as pearlite. The “Analyze” function of the above image analysis software “ImageJ” is used to calculate the area ratio of pearlite.

[0169] If the total area ratio of the structures obtained by the above identification and calculation methods differs from 100%, the values obtained by multiplying the area ratios of the structures by 100 / (total area ratio of structures) are made the area ratios of the structures.[Value (MnF / MnA) of Average Value of Mn Concentration in Ferrite Divided by Mn Concentration of Base Material: 0.980 or Less]

[0170] In an embodiment of the present invention, the value of the average value of the Mn concentration in the ferrite divided by the Mn concentration of the base material (i.e., the Mn content of the steel sheet) is controlled to 0.980 or less. By making the average value of the Mn concentration in the ferrite a sufficiently smaller 0.980 time or less of the average Mn concentration of the steel sheet as a whole, it is possible to remarkably suppress the solution strengthening of ferrite by the Mn and thereby possible to improve the elongation of the steel sheet. From the viewpoint of improving the elongation, the lower the value, the more preferable. It may also be for example 0.970 or less, 0.960 or less, or 0.950 or less. The lower limit is not particularly prescribed, but it is difficult to make the Mn concentration in the ferrite 0%, therefore the value of the average value of the Mn concentration in the ferrite divided by the Mn concentration of the base material may, for example, also be 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.

[0171] The average value of Mn concentration in the ferrite is determined in the following way using an electron probe micro analyzer (EPMA). Specifically, first, a sample is taken with the sheet thickness cross-section vertical to the sheet surface of the steel sheet as the examined surface. In an electron channeling contrast image by FE-SEM (field emission type scanning electron microscope), a 100 μm×100 μm region is observed within a sheet thickness ⅛ to ⅜ range centered about sheet thickness ¼ to thereby obtain a grasp of the position of the ferrite in the observed region. Next, the examined surface is wet polished by emery paper and polished by a diamond abrasive having a 1 μm average particle size, then is chemically polished. To determine the measured region, in the Vickers hardness test, indentations are made at a 100×100 μm square in the examined region and those indentations are used as markings. Next, an EPMA is used to measure the Mn concentration in the ferrite. The apparatus used for measurement is the JXA-8500F made by JEOL. Portions judged to be BCC in the above examined region by obtaining crystal orientation information under conditions of an acceleration voltage of 7 kV and measurement point interval of 80 nm are measured. From the data obtained by measurement in this way, the Mn concentration in the ferrite is found using the calibration curve method. The obtained value is determined as the average value of Mn concentration in the ferrite.[Value (MnΓ / MnA) of Average Value of Mn Concentration in Retained Austenite Divided by Mn Concentration of Base Material: 1.150 to 2.000]

[0172] In an embodiment of the present invention, the value of the average value of Mn concentration in the retained austenite divided by the Mn concentration of 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 of the steel sheet as a whole, it is possible to suitably stabilize the retained austenite. As a result, it becomes possible to suitably realize the TRIP effect up to the high strain side at the time of deformation of the steel sheet, the timing of occurrence of necking and timing of strain induced transformation is also rectified, and, due to combination with suppression of solution strengthening of ferrite, elongation of the steel sheet can be remarkably improved. From the viewpoint of improving the elongation, the higher the value, the more preferable. It may also 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 concentration is too large and the retained austenite is excessively stabilized, even if necking occurs, strain induced transformation from retained austenite to martensite will not suitably occur and sometimes retained austenite will remain until fracture of the steel sheet. Therefore, the value of the average value of Mn concentration in the retained austenite divided by the Mn concentration of the base material is 2.000 or less, for example, may be 1.800 or less or 1.600 or less.

[0173] The average value of Mn concentration in the retained austenite is similarly determined using an EPMA. Specifically, crystal orientation information obtained in the same way as explained above for determining the average value of Mn concentration in the ferrite is used to separate the FCC phase. Here, the separated FCC phase is retained austenite. Next, the EPMA is used to measure the Mn concentration in the retained austenite. The apparatus used for measurement is the JXA-8500F made by JEOL. Portions judged to be FCC in the region by obtaining crystal orientation information under conditions of an acceleration voltage of 7 kV and measurement point interval of 80 nm are measured. From the data obtained by measurement in this way, the Mn concentration in the retained austenite is found using the calibration curve method. The obtained value is determined as the average value of Mn concentration in the retained austenite.[Average Value of C Concentration in Retained Austenite: 0.80 Mass % or More]

[0174] According to a preferred embodiment of the present invention, the average value of the C concentration in the retained austenite is controlled to 0.80 mass % or more. By controlling the average value of the C concentration in the retained austenite to 0.80 mass % or more, the effect due to the concentration of C in the retained austenite explained previously, i.e., improving the elongation due to the stabilization of retained austenite and the higher strength due to increasing the hardness of strain induced martensite, can be made particularly remarkable. From the viewpoint of improving elongation and higher strength, the higher the average value of C concentration in the retained austenite, the more preferable. It may also be for example 0.85 mass % or more or 0.90 mass % or more. The upper limit is not particularly prescribed, but the average value of C concentration in the retained austenite may, for example, be 1.00 mass % or less, 0.98 mass % or less, 0.96 mass % or less, or 0.95 mass % or less.

[0175] The average value of the C concentration in the retained austenite is also determined by an EPMA in the same way as before. Specifically, in the same way as explained above for determining the average value of Mn concentration in the retained austenite, crystal orientation information is acquired to measure the portions judged to be FCC in the regions. From the data obtained by measurement in this way, the C concentration in the retained austenite is found using the calibration curve method. The obtained value is determined as the average value of C concentration in the retained austenite.[Soft Surface Layer]

[0176] According to a separate preferred embodiment of the present invention, the steel sheet includes a middle part in sheet thickness and a soft surface layer arranged at one side or both sides of the middle part in sheet thickness. The soft surface layer has an average thickness of 10 μm or more and has an average Vickers hardness of 0.90 time or less of the average Vickers hardness of the sheet thickness ½ position. By providing such a softened part at a surface layer of the steel sheet, it is possible to improve the bendability of the steel sheet. Therefore, by improving the bendability in addition to improving the elongation explained above, it is possible to obtain steel sheet excellent in crashworthiness at the part after production in addition to improving the formability at the time of part production. In the present embodiment, by having the average thickness of 10 μm or more, the effect of providing a soft surface layer at one side or both sides of the steel sheet can be sufficiently realized. The average thickness of the soft surface layer may be any value of 10 μm or more. For example, it may be 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. The upper limit is not particularly prescribed, but in general, the average thickness of the soft surface layer is 30% or less of the sheet thickness. For example, the average thickness of the soft surface layer may be 25% or less, 20% or less, 15% or less, or 10% or less of the sheet thickness, 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.

[0177] The average Vickers hardness of the soft surface layer may be any average Vickers hardness of 0.90 time or less of the average Vickers hardness of the sheet thickness ½ position. By controlling the average Vickers hardness of the soft surface layer to 0.90 time or less of the average Vickers hardness of the sheet thickness ½ position, i.e., by lowering the hardness of the surface layer part relative to the middle part in sheet thickness, it becomes possible to reliably improve the bendability of the steel sheet. To better improve such an effect of improving the bendability, for example, the average Vickers hardness of the soft surface layer may be 0.85 time or less of the average Vickers hardness of the sheet thickness ½ position, 0.80 time or less, 0.70 time or less, or 0.60 time or less. The lower limit is not particularly prescribed, but in general the average Vickers hardness of the soft surface layer is 0.10 time or more of the average Vickers hardness of the sheet thickness ½ position, for example, may be 0.15 time or more or 0.20 time or more.

[0178] In the present invention, the “average thickness of the soft surface layer” and the “average Vickers hardness of the soft surface layer” are determined in the following way. First, at certain interval in the sheet thickness direction from the sheet thickness ½ position of the steel sheet toward the surface at the soft surface layer side (for example, every 5% of sheet thickness, in accordance with need, every 3%, every 2.5%, every 1%, or every 0.5%), the Vickers hardness at that sheet thickness direction position is measured by an indentation load of 100 g. Next, the Vickers hardnesses of a total of three points or more, for example, five points or 10 points, are similarly measured by an indentation load of 100 g on a line from that position in a direction vertical to the sheet thickness. The average value of these is made the average Vickers hardness at that sheet thickness direction position. The interval of the measurement points aligned in the sheet thickness direction and the direction vertical to the same is, when possible, a distance of four times or more of an indent. The “distance of four times or more of an indent” means a distance of four times or more of the length of a diagonal at a rectangular shaped opening of an indent formed by a diamond indenter when measuring the Vickers hardness. If making the interval of the measurement points a distance of four times or more of the indents and making indents linearly in the sheet thickness direction from the surface would be difficult, it is also possible to make the interval of the measurement points a distance of four times or more of the indents and make indents zigzag in the sheet thickness direction from the surface. When the average Vickers hardness at a certain sheet thickness direction becomes 0.90 time or less of the similarly measured average Vickers hardness at the sheet thickness ½ position, a surface side from that position is defined as the soft surface layer and the average thickness of the soft surface layer (μm) and the ratio (%) in the sheet thickness are determined. The Vickers hardnesses at 10 random points in the thus determined soft surface layer are measured by an indent load of 100 g and the average value of these is calculated to determine the average Vickers hardness of the soft surface layer. If soft surface layers are arranged at the two sides of the middle part in sheet thickness, the average thickness and average Vickers hardness of the soft surface layer at the other side are determined by measurement in the same way as explained above.[Sheet Thickness]

[0179] The steel sheet according to an embodiment of the present invention is not particularly limited, but, for example, has a 0.6 to 6.0 mm sheet thickness. While not particularly limited, the sheet thickness may also be 0.8 mm or more, 1.0 mm or more, or 1.2 mm or more. Similarly, the sheet thickness may also be 4.0 mm or less, 3.0 mm or less, 2.5 mm or less, or 2.0 mm or less. The sheet thickness of the steel sheet is measured by a micrometer.[Plating]

[0180] The steel sheet according to an embodiment of the present invention may further have a plating layer on its surface for the purpose of improving the corrosion resistance, etc. The plating layer may be any suitable plating layer. For example, it may be either of a hot dip coated layer and an electroplated layer. The hot dip coated layer, for example, may be a hot dip galvanized layer, hot dip galvannealed layer (hot dip coated layer comprised an alloy of zinc and Si and Al or other additional elements), or alloyed hot dip galvanized layer obtained by alloying these platings (alloyed plating layer). The hot dip galvanized layer and hot dip galvannealed layer are preferably plating layers containing less than 7 mass % of Fe. Further, the alloyed plating layer is preferably a plating layer containing Fe in 7 mass % or more and 15 mass % or less. In the hot dip galvanized layer, hot dip galvannealed layer, and alloyed plating layer, the constituents besides zinc and Fe are not particularly limited. Various constitutions can be employed within the usual scope. Further, the plating layer may also, for example, be an aluminum plating layer, etc. Further, the amount of deposition of the plating layer is not particularly limited and may be a general amount of deposition.

[0181] Note that, the chemical composition, structural fractions, and soft surface layer explained above as features of the steel sheet do not cover the plating on the surface. That is, the chemical composition, structural fractions, and soft surface layer in plated steel sheet are evaluated for steel sheet which has been dipped in a strong alkali aqueous solution and had the plating layer of the surface removed.[Mechanical Properties]

[0182] According to the steel sheet according to an embodiment of the present invention, it is possible to achieve a high tensile strength, for example, a 980 MPa or more tensile strength. 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 prescribed, but for example the tensile strength may also be 1780 MPa or less, 1700 MPa or less, 1600 MPa or less, or 1500 MPa or less. In addition, according to the steel sheet according to an embodiment of the present invention, despite having such an extremely high tensile strength, by the specific combination of the chemical composition and microstructure explained above, it is possible to reliably and sufficiently improve the elongation compared with steel sheet not including such a specific combination. For example, the steel sheet according to an embodiment of the present invention can achieve a 14% or more total elongation and can preferably achieve a 15% or more, more preferably a 16% or more total elongation. The upper limit is not particularly prescribed, but for example the total elongation may be 30% or less or 25% or less. The tensile strength and total elongation are measured by performing a tensile test compliant with JIS Z 2241:2011 based on a JIS No. 5 test piece taken from an orientation where a longitudinal direction of the test piece becomes parallel with the rolling perpendicular direction of the steel sheet.

[0183] The steel sheet according to an embodiment of the present invention in particular may be cold rolled steel sheet. According to the steel sheet according to an embodiment of the present invention, as explained above, it is possible to reliably obtain both the contradictory properties of high strength and excellent elongation. In addition, according to a specific preferred embodiment of the present invention, it is possible to remarkably improve the bendability. For this reason, the steel sheet according to an embodiment of the present invention is useful for use in parts, etc., of technical fields in which both high strength and formability are sought. In particular, it is useful for use in parts in the field of automobiles, etc. In a preferred embodiment, an auto part including the steel sheet according to an embodiment of the present invention is provided. As examples of auto parts, frame parts, bumpers, and other structural parts and reinforcement parts, etc., in which strength is required may be mentioned. These parts need only include the steel sheet according to an embodiment of the present invention at least in part. For this reason, at least portions of these parts will satisfy the features of the chemical composition and microstructure explained above. At portions not directly contacting the dies in press forming and other shaping operations where the degree of work is also relatively low, the features of the microstructure do not particularly change before and after shaping.<Method of Production of Steel Sheet>

[0184] Next, a preferable method of production of the steel sheet according to an embodiment of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for producing the steel sheet according to an embodiment of the present invention and is not intended to limit the steel sheet to one produced by the method of production such as explained below.

[0185] The method of production of the steel sheet according to an embodiment of the present invention comprises

[0186] hot rolling including heating a slab having a chemical composition explained above in relation to the steel sheet to a 1200 to 1400° C. temperature and finish rolling it, then coiling it at a 500 to 700° C. temperature, and making the obtained coil dwell at a temperature region of 600 to 750° C. for 1.0 to 5.0 hours, an end temperature of the finish rolling being 900° C. or more,

[0187] pickling an obtained hot rolled steel sheet,

[0188] cold rolling the pickled hot rolled steel sheet by a 20 to 90% rolling reduction, and

[0189] annealing the obtained cold rolled steel sheet, in which annealing step, the annealing including heating the cold rolled steel sheet and holding it at a 780 to 900° C. highest heating temperature for 30 to 500 seconds, then cooling and making it dwell there, the cooling including primary cooling from the highest heating temperature to a 650° C. or more primary cooling stop temperature by an average cooling speed of 1.0 to 20.0° C. / s, then secondary cooling down to a 100 to less than 300° C. secondary cooling stop temperature by an average cooling speed of 20° C. / s or more, the dwelling including heating the secondary cooled cold rolled steel sheet and making it dwell in a temperature region of 300 to 450° C. for 100 seconds or more. Below, the steps will be explained in detail.[Slab]

[0190] The slab supplied for the hot rolling need only be a cast slab. It is not limited to a specific slab. For example, it may be a continuously cast slab or a slab produced by a thin slab caster.[Heating Temperature of Slab: 1200 to 1400° C.]

[0191] If heating the cast, then once cooled slab, then supplying it for hot rolling, the slab is heated to 1200 to 1400° C. The slab used for production of the steel sheet according to an embodiment of the present invention contains a relatively large amount of alloy elements. For this reason, before hot rolling the slab, it is necessary to heat the slab to make the alloy elements dissolve in the slab. If the heating temperature of the slab is less than 1200° C., the alloy elements will not sufficiently dissolve in the slab thereby causing coarse alloy carbides to remain and sometimes resulting in brittle fracture. For this reason, the heating temperature of the slab is 1200° C. or more. The upper limit of the heating temperature of the slab is not particularly prescribed, but from the viewpoint of the heating capacity of the heating facilities and productivity, it is 1400° C. or less.[Rough Rolling]

[0192] In the present method, for example, the heated slab may be rough rolled before the finish rolling so as to adjust the sheet thickness, etc. The rough rolling need only secure the desired sheet bar dimensions. The conditions are not particularly limited.[Finish Rolling End Temperature: 900° C. or More]

[0193] The heated slab, or the slab additionally rough rolled in accordance with need, is next finish rolled. In this way, the slab used for the production of the steel sheet according to an embodiment of the present invention contains a relatively large amount of alloy elements, therefore it is necessary to increase the rolling load at the time of hot rolling. For this reason, the hot rolling is preferably performed at a high temperature. In particular, the end temperature of the finish rolling is important in terms of control of the microstructure of the steel sheet. If the end temperature of the finish rolling is in the dual phase temperature region of (austenite+ferrite), the unevenness of the microstructure becomes greater and the formability after heat treatment sometimes falls. For this reason, the end temperature of the finish rolling is 900° C. or more. The upper limit is not particularly prescribed, but for suppressing coarsening of the austenite, the end temperature of the finish rolling is preferably, for example, 1100° C. or less.[Coiling Temperature: 500 to 700° C.]

[0194] Next, the finish rolled hot rolled steel sheet is coiled at a 500 to 700° C. temperature. If the coiling temperature of the hot rolled steel sheet is more than 700° C., the microstructure becomes increasingly uneven and the formability after heat treatment easily deteriorates. For this reason, the coiling temperature is 700° C. or less. With a coiling temperature of less than 500° C., the hot rolled sheet strength becomes excessively high and the cold rollability is impaired, so the lower limit of the coiling temperature is 500° C.[Dwell Temperature After Coiling: 600 to 750° C.]

[0195] Dwell control of the coil after obtained is extremely import in control of the value (MnF / MnA) of the average value of the Mn concentration in the ferrite divided by the Mn concentration of the base material and the value (MnΓ / MnA) of the average value of the Mn concentration in the retained austenite divided by the Mn concentration of the base material to within the desired ranges in the microstructure of the finally obtained steel sheet. Explained in more detail, first, the finish rolled hot rolled steel sheet is coiled at 500 to 700° C. temperature, whereby ferrite, pearlite, and bainite are obtained. Next, if the coil having such a microstructure is made to dwell at 600 to 750° C., it is possible to promote the concentration of Mn in the carbides contained in pearlite and bainite from the ferrite and the bainitic ferrite in the bainite and possible to create a spread of Mn concentration. Carbides in which Mn is concentrated easily become retained austenite after the later annealing, rise in chemical stability due to the Mn, and contribute to improvement of the elongation. If the dwell temperature is less than 600° C., Mn cannot be made to sufficiently disperse, therefore it becomes no longer possible to promote the concentration of Mn in the carbides contained in the pearlite and bainite. Therefore, it is not possible to sufficiently increase the average value of the Mn concentration in the retained austenite in the microstructure of the finally obtained steel sheet and in turn no longer possible to sufficiently reduce the average value of the Mn concentration in the ferrite. As a result, it becomes no longer possible to obtain the desired MnF / MnA value and / or Mnγ / MnA value in the microstructure of the finally obtained steel sheet. Therefore, the lower limit of the dwell temperature is 600° C. On the other hand, if the dwell temperature is more than 750° C., Mn excessively concentrates and the TRIP effect becomes harder to realize, and therefore the upper limit of the dwell temperature is 750° C. The method of making the obtained coil dwell at 600 to 750° C. may be reheating the coil or may be covering the coil by a box with a high heat insulating ability. The invention is not limited to any specific method.[Dwell Time of Coil After Coiling: 1.0 to 5.0 Hours]

[0196] As stated above, the obtained coil is made to dwell at 600 to 750° C., whereby it is possible to promote the concentration of Mn from the ferrite or the bainitic ferrite in the bainite into the carbides contained in the pearlite and bainite and possible to form a spread of Mn concentration. However, with a dwell time of less than 1.0 hour, Mn cannot be sufficiently dispersed, therefore it becomes no longer possible to promote the concentration of Mn in the carbides contained in the pearlite and bainite. Therefore, in the microstructure of the finally obtained steel sheet, it is not possible to sufficiently increase the average value of Mn concentration in the retained austenite and in turn it becomes no longer possible to sufficiently reduce the average value of Mn concentration in the ferrite. As a result, in the microstructure of the finally obtained steel sheet, it becomes no longer possible to obtain the desired MnF / MnA value and / or Mnγ / MnA value. Therefore, the lower limit of the dwell time is 1.0 hour. On the other hand, with a dwell time of more than 5.0 hours, the Mn excessively concentrates and the TRIP effect becomes difficult to realize, therefore the upper limit of the dwell time is 5.0 hours.[Pickling Step]

[0197] In the pickling step, the oxides on the surface of the hot rolled steel sheet are removed and the chemical convertability and plateability of the cold rolled steel sheet are improved. The solution used for the pickling need only be a solution used for usual pickling. For example, a 5 vol % or more hydrochloric acid or sulfuric acid may be mentioned. Further, the pickling may be performed at one time or divided into several times in accordance with need.[Cold Rolling Step]

[0198] The pickled hot rolled steel sheet is supplied to the cold rolling for a rolling reduction of 20 to 90% to obtain cold rolled steel sheet. By making the rolling reduction of the cold rolling 20% or more, it is possible to maintain the shape of the cold rolled steel sheet flat and suppress a drop in ductility of the final product. The rolling reduction of the cold rolling is preferably 30% or more. On the other hand, by making the rolling reduction of the cold rolling 90% or less, it is possible to keep the rolling load from becoming excessive and the rolling from becoming difficult. The rolling reduction of the cold rolling is preferably 80% or less. The number of rolling passes and the rolling reductions of each pass are not particularly limited and may be suitably set so that the rolling reduction of the cold rolling becomes the above range.[Annealing Step][Highest Heating Temperature: 780 to 900° C.]

[0199] By the highest heating temperature being 780 to 900° C., austenite is formed during the annealing and a predetermined amount of tempered martensite is easily obtained as a final structure. For this reason, the steel sheet easily satisfies the desired tensile strength. If the highest heating temperature is less than 780° C., sufficient austenite is not formed and a predetermined amount of tempered martensite cannot be obtained after cooling, therefore the lower limit of the highest heating temperature is 780° C. On the other hand, if the highest heating temperature is more than 900° C., while there are no problems in the properties of the steel sheet, the productivity falls. For this reason, the highest heating temperature is 900° C. or less and is preferably 850° C. or less.[Holding Time: 30 to 500 Seconds]

[0200] As stated above, at the highest heating temperature of annealing step, austenite is produced and a predetermined amount of tempered martensite becomes easily obtained as a final structure. If the holding time is less than 30 seconds, sufficient austenite is not formed and after cooling, a predetermined amount of tempered martensite cannot be obtained, therefore the lower limit of the holding time is 30 seconds. The upper limit of the holding time does not have an effect on grade, but if too long, the productivity is lowered, therefore 500 seconds is the upper limit.[Dew Point]

[0201] In the annealing step, the dew point in the furnace atmosphere at the time of holding at the highest heating temperature may be increased to −30° C. or more. By performing the annealing step in such an atmosphere, it becomes possible to promote a decarburization reaction from the steel sheet surface. As a result, it is possible to form a soft surface layer having an average thickness of 10 μm or more and having an average Vickers hardness of 0.90 time or less of the average Vickers hardness at the sheet thickness ½ position. By providing such a softened layer at the surface layer of the steel sheet, it becomes possible to improve the bendability of steel sheet. By adding such improvement of bendability to the improvement of elongation explained above, aside from improvement of the formability at the time of production of a part, it is possible to obtain steel sheet excellent in crashworthiness in the part after production.[Primary Cooling][Average Cooling Speed From Highest Heating Temperature to 650° C. or More Primary Cooling Stop Temperature: 1.0 to 20.0° C. / s]

[0202] To obtain the desired ferrite area ratio, it is necessary to suitably perform primary cooling from the highest heating temperature. More specifically, the primary cooling is performed from the highest heating temperature to the 650° C. or more primary cooling stop temperature by an average cooling speed of 1.0 to 20.0° C. / s. If the primary cooling stop temperature becomes less than 650° C., ferrite excessively forms, the area ratio of tempered martensite can no longer be sufficiently obtained, and the strength falls. For this reason, the lower limit of the cooling stop temperature is 650° C. Even with an average cooling speed from the highest heating temperature to the primary cooling stop temperature of less than 1.0° C. / s, ferrite excessively forms, the area ratio of tempered martensite is no longer sufficiently obtained, and the strength falls. For this reason, the lower limit of the average cooling speed is 1.0° C. / s. With an average cooling speed of more than 20.0° C. / s, ferrite cannot be formed, therefore the effect of primary cooling cannot be realized.[Secondary Cooling][Secondary Cooling Stop Temperature: 100 to Less Than 300° C.]

[0203] Next, the cold rolled steel sheet is cooled from the primary cooling stop temperature to 100 to less than 300° C. as secondary cooling. In the secondary cooling, martensite can be obtained by quenching. The martensite formed by the secondary cooling is tempered in a later explained dwelling operation to thereby become tempered martensite. Further, to proceed with the bainite transformation and obtain retained austenite in the dwelling operation, it is necessary to leave a certain amount of untransformed austenite in the secondary cooling. The lower the secondary cooling stop temperature, the more the martensite increases and the less the amount of untransformed austenite, therefore it becomes no longer possible to obtain the desired amount of retained austenite. Therefore, the lower limit of the secondary cooling stop temperature is 100° C. On the other hand, with a secondary cooling stop temperature of 300° C. or more, at the secondary cooling stage, sufficient martensite cannot be obtained. For this reason, even with a subsequent dwelling operation, the desired amount of tempered martensite cannot be obtained. Further, in this case, after the dwelling operation, untransformed austenite remains in a relatively large amount. Such untransformed austenite sometimes finally forms a large amount of as-quenched martensite after subsequent cooling. Therefore, the upper limit of the secondary cooling stop temperature is less than 300° C.[Average Cooling Speed: 20° C. / s or More]

[0204] As stated above, with the secondary cooling from the primary cooling stop temperature, martensite is obtained. With an average cooling speed in this period of less than 20° C. / s, quenching becomes difficult, bainite is produced in a large amount, and, in the final microstructure, the desired amount of tempered martensite cannot be obtained. Therefore, the average cooling speed of the secondary cooling is 20° C. / s.[Dwelling at Temperature Region of 300 to 450° C. for 100 Seconds or More]

[0205] After the secondary cooling, the cold rolled steel sheet can be made to dwell in the temperature region of 300 to 450° C. for 100 seconds or more to temper the martensite obtained by the secondary cooling and obtain tempered martensite. Furthermore, at the time of secondary cooling, the untransformed austenite can be made to transform to bainite to promote concentration of C in the austenite and thereby obtain retained austenite in a sufficient amount and the C concentration in the retained austenite can also be increased. With a dwell temperature of less than 300° C., tempering becomes insufficient, as-quenched martensite easily remains, and the elongation deteriorates. For this reason, the lower limit of the dwell temperature is 300° C. On the other hand, with more than 450° C., tempering excessively proceeds, and sufficient strength cannot be obtained. For this reason, the upper limit of the dwell temperature is 450° C. The dwell time is necessary in order to make the bainite transformation proceed. With a dwell time of less than 100 seconds, the bainite transformation does not finish, sufficient retained austenite cannot be obtained, and / or the desired C concentration in the retained austenite also cannot be obtained.[Plating Treatment and Surface Treatment]

[0206] Further, electroplating treatment, vapor deposition plating treatment, or other plating treatment may be applied to the steel sheet. Furthermore, after the plating treatment, alloying treatment may be performed. Further, organic coating, film lamination, treatment by an organic salt or inorganic salt, nonchrome treatment, or other surface treatment may also be applied to the steel sheet.

[0207] If performing hot dip galvanization treatment on the steel sheet as plating treatment, for example, the steel sheet is heated or cooled to a temperature 40° C. lower than the temperature of the galvanization bath or more and a temperature 50° C. higher than the temperature of the galvanization bath or less, then the steel sheet is run through the galvanization bath. Due to such a hot dip galvanization treatment, steel sheet provided at its surface with a hot dip galvanized layer, i.e., hot dip galvanized steel sheet, is obtained. The hot dip galvanized layer, for example, has a chemical composition expressed as Fe: 7 mass % or more and 15 mass % or less and balance: Zn, Al, and impurities. Further, the hot dip galvanized layer may also be galvannealed.

[0208] If performing the alloying treatment after hot dip galvanization treatment, for example, the hot dip galvanized steel sheet is heated at a 460° C. or more and 600° C. or less temperature. If this temperature is less than 460° C., the alloying sometimes becomes insufficient. On the other hand, if this temperature is more than 600° C., the alloying becomes excessive and sometimes the corrosion resistance deteriorates. Due to such an alloying treatment, steel sheet provided at its surface with a hot dip galvannealed layer, i.e., hot dip galvannealed steel sheet, is obtained.

[0209] The method illustrated above can be used to produce the steel sheet according to an embodiment of the present invention. Note that the above embodiments only show concrete examples when working the present invention. The technical scope of the present invention may not be interpreted limitatively due to the same. That is, the present invention can be worked in various forms without departing from its technical idea or its main features.

[0210] Below, examples will be used to explain the present invention in more detail, but the present invention is not limited to these examples in any way.EXAMPLES

[0211] In the following examples, steel sheets according to an embodiment of the present invention were produced under various conditions and investigated for the properties of tensile strength and elongation of the obtained steel sheets.

[0212] First, molten steels were cast by the continuous casting method to form slabs having the various chemical compositions shown in Table 1. These slabs were heated to the heating temperatures shown in Table 2 and hot rolled. The hot rolling was performed by rough rolling and finish rolling. More specifically, the rough rolling was performed under the same conditions in all of the examples and comparative examples while the end temperature and coiling temperature of the finish rolling were as shown in Table 2. Next, the obtained coils were allowed to dwell at the maximum temperatures shown in Table 2 over the time periods shown in Table 2. The obtained hot rolled steel sheets were pickled, then were cold rolled by the rolling reductions shown in Table 2 to obtain cold rolled steel sheets having 1.4 mm sheet thicknesses. Next, the obtained cold rolled steel sheets were annealed by a heating, primary cooling, secondary cooling, and dwelling operation under the conditions shown in Table 2. The dew points in the furnace at the time of holding at the highest heating temperatures were as shown in Table 2. Finally, the steel sheets were suitably hot dip galvanized as plating treatment and further several among them were subjected to alloying treatment.TABLE 1-1Chemical composition (mass %), balance: Fe and impuritiesSteel no.CSiMnPSAlNOCrMoTiNbBCuNiA0.2331.052.800.00150.00140.830.00140.0101B0.2410.801.600.00090.00220.300.00100.00160.1200.080C0.3421.943.900.00300.00310.240.01070.00390.0460.0017D0.2100.722.200.00220.00180.530.01680.00610.3200.166E0.2671.751.400.00190.00180.040.00110.00181.5000.1440.0470.093F0.2821.212.600.01020.00100.610.00350.00150.1040.0210.0028G0.2900.652.630.02100.00230.910.00390.00240.0500.0300.0015H0.1500.961.900.00160.01020.160.00250.0163I0.3900.462.320.01470.01500.720.01520.0016J0.2300.302.800.00110.00170.400.00150.0026K0.2301.682.800.00110.00170.570.00150.0026L0.2301.561.200.00110.00170.020.00150.0026M0.2301.565.010.00160.01020.020.00250.0163Bold underlines indicate outside scope of present invention.TABLE 1-2SteelChemical composition (mass %), balance: Fe and impuritiesno.WVTaCoSnSbAsMgZrCaYLaCeBiSi + AlA1.88B0.0050.0090.01501.10C0.0970.0390.00202.18D0.0671.25E0.0040.0371.79F0.0170.0110.00800.00600.00601.82G1.56H1.12I1.18J0.70K2.25L1.58M1.58Bold underlines indicate outside scope of present invention.TABLE 2Hot rolling stepColdAnnealing stepDwellingrollingHeatingSlabFinish600~stepHighestheatingrollingCoilingHighest750° C.RollingheatingHoldingDewProductiontemp.end temp.temp.temp.dwell timereductiontemp.timepointconditions(° C.)(° C.)(° C.)(° C.)(h)(%)(° C.)(s)(° C.)112609305406772.056790 700212409305406683.056790 700312409305406604.056790 700412609205706902.040800100−5512509205706923.040800100−5612509205706834.040800100−5712809406007102.05083015010812709406007053.06083015010912709406007004.060830150101013009105506603.043810 80−301113009105506504.043810 80−301213059105506805.043810 80−301312309505206503.566850120151412409505206404.566850120151512209505206555.066850120151612409306107201.050825210−401712509306107102.050825210−401812609306107003.050825210−401912109205906802.07180030502012209205906753.07180030502112109205906854.07180030502212509206007003.05080020002312509306007053.05080020002412509306007103.05080020002512509306007003.05080020002612509106007053.05080020002712509206007153.05080020002812609305406900.56080010002912609305406950.56080010003012609305405203.06080010003112609305406903.06275010003212609305406903.062800 1503312609305406903.06280010003412609305406903.06280010003512609305406903.06280010003612609305406903.06280010003712609305406903.06280010003812609305406903.06280010003912609305405806.06080010004012609305407300.56080010004112309505206503.566850450154212309505206503.56685012030Annealing stepPrimary coolingSecondary coolingDwellingCoolingAverageCoolingAverage300~stopcoolingstopcooling450° C.Productiontemp.speedtemp.speeddwell timeconditions(° C.)(° C. / s)(° C.)(° C. / s)(h)PlatingAlloying173012.019335320YesYes273012.019335320YesYes373012.019335320YesYes470010.024643250NoNo570010.024643250NoNo670010.024643250NoNo7750 8.014152300YesNo8750 8.014152300YesNo9750 8.014152300YesNo10720 6.026148400NoNo11720 6.026148400NoNo12720 6.026148400NoNo1374013.027050290NoNo1474013.027050290NoNo1574013.027050290NoNo1678015.0197100 550NoNo1778015.0197100 550NoNo1878015.0197100 550NoNo1970020.023740300YesYes2070020.023740300YesYes2170020.023740300YesYes2275013.029555180NoNo2375013.024155180NoNo2475013.027655180NoNo2575013.015555180NoNo2675013.017055180NoNo2775013.014055180NoNo2875015.021050350NoNo2975015.021050350NoNo3075015.021050350NoNo3175015.021050350NoNo3275015.021050350NoNo3362015.021050350NoNo34704 0.521050350NoNo3575015.0 5040250NoNo3675015.034040250NoNo3775015.020040 10NoNo3875015.0200 5250NoNo3975015.021050350NoNo4075015.021050350NoNo4174013.027050290NoNo4274013.027050290NoNoBold underlines indicate outside preferable range.The properties of the obtained steel sheets were measured and evaluated by the following methods:[Tensile Strength (TS) and Total Elongation (El)]The tensile strength (TS) and total elongation (El) were measured by taking a JIS No. 5 test piece from an orientation where a longitudinal direction of the test piece becomes parallel to a rolling perpendicular direction of the steel sheet and performing a tensile test compliant with JIS Z 2241:2011 based on the same.[Bendability]

[0215] The bendability was evaluated by a ratio R / t of a limit bending radius R and a sheet thickness “t”. The limit bending radius R was determined by preparing a No. 1 test piece described in JIS Z 2204:1996 so that the direction vertical to the rolling direction became the longitudinal direction (bending ridgeline matching rolling direction) and performing a V-bending test compliant with JIS Z 2248:2022. The bending test was performed with an angle of the die and punch of 60° and with the radius of the tip of the punch changed in 0.5 mm units. The radius of the tip of the punch where bending was possible without cracks was found as the limit bending radius R.[Crashworthiness]

[0216] The crashworthiness was judged as good or bad by the TS, El, and R / t. That is, cases where the TS being 980 MPa or more, the El being 14% or more, and R / t being 1.5 or less all were satisfied were evaluated as “VG (very good)” in crashworthiness, cases where two were satisfied were evaluated as “G (good)”, and cases where only 1 or less was satisfied were evaluated as “P (poor).

[0217] Cases with a TS of 980 MPa or more and an El of 14% or more were evaluated as steel sheet achieving high strength and improved elongation. The results are shown in Table 3.TABLE 3AverageAverageMn conc.value MnγMicrostructure fractions (area %)value MnFMnA inof MnAs-(mass %) ofbaseconc. inTestProd.TemperedRetainedquenchedMn conc.materialMnF / retained γno.SteelconditionsFerritemartensiteγBainitePearlitemartensitein ferrite(mass %)MnA(mass %)1A139411010002.742.800.9793.342A239411010002.702.800.9643.453A339411010002.602.800.9293.824B428481311001.551.600.9691.855B528481311001.501.600.9381.886B628481311001.461.600.9131.917C724471514003.823.900.9794.498C824471514003.803.900.9744.529C924471514003.703.900.9494.5510D1022571110002.152.200.9772.5411D1122571110002.132.200.9682.5812D1222571110002.112.200.9592.6313E1322501612001.371.400.9791.7314E1422501612001.341.400.9571.9215E1522501612001.311.400.9362.1116F1621551410002.532.600.9733.0217F1720561410002.502.600.9623.1118F1822541410002.462.600.9463.2119G1921411218262.582.630.9763.1320G2021411218262.562.630.9733.1621G2122411217262.552.630.9703.2222H2232431015001.831.900.9632.3423I23Hot rolled sheet strength too high so cold rolling not possible24J243148 120002.742.800.9793.3325K25Cracking occurred at cold rolling making subsequent running impossible26L2655 0 0351001.131.200.942—27M27Hot rolled sheet strength too high so cold rolling not possible28A283840 811032.782.800.9932.8529A293742 810032.752.800.9822.8430A303741 812022.792.800.9962.8131A317020 010002.742.800.979—32A3243321114002.742.800.9793.3433A334828 915002.742.800.9793.3534A3443291112502.742.800.9793.3335A353466 0 0002.742.800.979—36A3634 01528023 2.742.800.9793.3137A373445 3 5013 2.742.800.9793.2938A3834151536002.742.800.9793.3139A3934401211032.802.801.0003.540A4034411111032.682.800.9573.141E1332401612001.371.400.9791.7342E1330421612001.371.650.8301.93AverageAverageAverage Vickersvalue Cγthicknesshardness (Hv)of C(μm)SoftSheetconc. inSoftsurfacethick. ½Bend-TestMnγ / retained γsurfacelayerpositionHvs / TSElabilityCrash-no.MnA(mass %)layer(Hvs)(Hvc)Hvc(MPa)(%)(R / t)worthinessRemarks11.1930.89801284170.311207161.5VGEx.21.2320.91811224190.291212171.5VGEx.31.3640.92781264240.301227171.5VGEx.41.1560.93761354310.311250141.5VGEx.51.1750.94791364310.321250141.5VGEx.61.1940.93801354310.311250151.5VGEx.71.1510.841101255030.251480141.0VGEx.81.1590.851161235030.241480141.0VGEx.91.1670.811181225030.241480141.0VGEx.101.1550.87651274090.311182141.5VGEx.111.1730.86631294090.321182141.0VGEx.121.1950.88681224090.301182141.0VGEx.131.2360.901211154300.271249141.0VGEx.141.3710.911231164300.271249141.0VGEx.151.5070.901251104300.261249141.0VGEx.161.1620.87124754771.001398152.5GEx.171.1960.86104564830.941418152.0GEx.181.2350.8984654710.991379142.0GEx.191.1900.88751364300.321249141.0VGEx.201.2020.90761354300.311249141.0VGEx.211.2240.91741324290.311244141.0VGEx.221.2320.8275693440.20972141.0GComp. ex23Hot rolled sheet strength too high so cold rolling not possibleComp. ex.241.1890.2975893690.24105391.0GComp. ex.25Cracking occurred at cold rolling making subsequent running impossibleComp. ex.26——76782730.29745201.0GComp. ex.27Hot rolled sheet strength too high so cold rolling not possibleComp. ex.281.0180.83761254070.311173131.5GComp. ex.291.0140.84771224180.291210121.5GComp. ex.301.0040.88781274070.311173101.5GComp. ex.31——761322840.46782221.5GComp. ex.321.1930.84804603201.44896182.0PComp. ex.331.1960.85702143450.62976192.0PComp. ex.341.1890.84722343420.69965172.0PComp. ex.35——723245890.55175662.0PComp. ex.361.1820.78751254630.27135261.0GComp. ex.371.1750.68731225670.22168591.0GComp. ex.381.1820.78771223430.36971181.5GComp. ex.391.2500.83761204000.301169131.5GComp. ex.401.1070.83761243980.311160121.5GComp. ex.411.2360.90205954300.221207161.0VGEx.421.1700.911671024300.241225151.0VGEx.Bold underlines indicate outside scope of present invention.

[0218] Referring to Tables 1 to 3, in Comparative Examples 22 and 26, the respective C and Mn contents were low, and therefore in each, the TS fell. In Comparative Examples 23 and 27, the respective C and Mn contents were high, and therefore in each, the strength of the hot rolled sheet became too high and the cold rolling could not be suitably performed. In Comparative Example 24, the total content of Si and Al was low, and therefore it is believed that C was not sufficiently concentrated in the untransformed austenite at the time of bainite transformation. As a result, the desired amount of retained austenite could not be obtained and the El fell. In Comparative Example 25, the total content of Si and Al was high, and therefore the strength was excessively increased and / or the retained austenite was excessively stabilized causing the rolling load to become excessive at the time of cold rolling and and the steel sheet to crack. In each of Comparative Examples 28 and 29, the dwell time of the coil after coiling was short, and therefore it is believed that Mn could not be made to sufficiently disperse and concentration of Mn in the carbides contained in the pearlite and bainite could not be promoted. As a result, insufficient retained austenite was formed from the carbides. Further, the average value of Mn concentration in the retained austenite could not be sufficiently increased and the average value of Mn concentration in the ferrite could not be sufficiently reduced and in turn the El fell. In Comparative Example 30, the highest temperature (dwell temperature) after coiling was low, and therefore similarly Mn could not be made to sufficiently disperse and concentration of Mn in the carbides contained in the pearlite and bainite could not be promoted. As a result, insufficient retained austenite was formed from the carbides. Further, the average value of Mn concentration in the retained austenite could not be sufficiently increased and the average value of Mn concentration in the ferrite could not be sufficiently reduced and in turn the El fell.

[0219] In Comparative Example 31, the highest heating temperature in the annealing step was low, and therefore austenization became insufficient and the desired amount of tempered martensite could not be obtained. As a result, the TS fell. In Comparative Example 32, the holding time at the highest heating temperature in the annealing step was short, and therefore similarly the austenization became insufficient and the desired amount of tempered martensite could not be obtained. As a result, the TS fell. In Comparative Example 33, the primary cooling stop temperature in the annealing step was low, and therefore ferrite was excessively formed and in turn the desired amount of tempered martensite could not be obtained and the TS fell. In Comparative Example 34, the average cooling speed of the primary cooling in the annealing step was slow, and therefore similarly ferrite was excessively formed and in turn the desired amount of tempered martensite could not be obtained and the TS fell. In Comparative Example 35, the secondary cooling stop temperature at the annealing step was low, and therefore tempered martensite was formed in a large amount and in turn retained austenite could not be obtained and the EL fell. In Comparative Example 36, the secondary cooling stop temperature in the annealing step was high, and therefore it is believed that as-quenched martensite was formed in a large amount from the untransformed austenite remaining after the dwell operation. As a result, the El fell. In Comparative Example 37, the dwell time at the temperature region of 300 to 450° C. in the annealing step was short, and therefore it is believed that C was not sufficiently concentrated in the untransformed austenite at the time of bainite transformation. As a result, the desired amount of retained austenite could not be obtained and the EL fell. In Comparative Example 38, the average cooling speed of the secondary cooling in the annealing step was slow, and therefore bainite was formed in a large amount and the desired amount of tempered martensite could not be obtained. As a result, the TS fell. In Comparative Example 39, the highest temperature (dwell temperature) of the coil after coiling was low, but the dwell time of the coil after coiling was long, and therefore it is believed suitable concentration of Mn could not be achieved. As a result, the average value of Mn concentration in the retained austenite could be increased, but average value of Mn concentration in the ferrite could not be sufficiently reduced and in turn the El fell. In Comparative Example 40, the dwell time of the coil after coiling was short, and therefore it is believed Mn could not be made to sufficiently disperse and concentration of Mn in the carbides contained in the pearlite and bainite could not be promoted. As a result, insufficient retained austenite was formed from the carbides. As a result, the average value of Mn concentration in the retained austenite could not be sufficiently increased and in turn the El fell.

[0220] In contrast to this, the steel sheets according to all of the examples were configured having the predetermined chemical compositions, including, by area ratio, tempered martensite: 40 to 65%, ferrite: 15 to 40%, and retained austenite: 10 to 20%, having values of the average value of Mn concentration in the ferrite divided by the Mn concentration of the base material controlled to 0.980 or less and further having values of the average value of Mn concentration in the retained austenite divided by the Mn concentration of the base material controlled to 1.150 to 2.000 and thereby having 980 MPa or more extremely high tensile strengths, yet were able to be remarkably improved in the elongation of the steel sheets. In particular, in each of Examples 1 to 15, 19 to 21, 41, and 42 provided with a soft surface layer having an average thickness of 10 μm or more and having an average Vickers hardness of 0.90 time or less of the average Vickers hardness at the sheet thickness ½ position, the R / t was 1.5 or less and therefore the bendability was high and, furthermore, the TS was 980 MPa or more and the El was 14% or more, and therefore an extremely high crashworthiness could be achieved.

Claims

1. A steel sheet having a chemical composition comprising, by mass %,C: 0.200 to 0.350%,Si: 0.01 to 2.00%,Mn: 1.40 to 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 to 2.000%,Mo: 0 to 1.000%,Ti: 0 to 0.500%,Nb: 0 to 0.500%,B: 0 to 0.0100%,Cu: 0 to 1.000%,Ni: 0 to 1.000%,W: 0 to 0.100%,V: 0 to 1.000%,Ta: 0 to 0.100%,Co: 0 to 3.000%,Sn: 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%,Bi: 0 to 0.0500%, andbalance: Fe and impurities, andsatisfying 1.00≤[Si]+[Al]≤2.20, where [Si] and [Al] are the contents of the elements (mass %), anda microstructure comprising, by area ratio,ferrite: 15 to 40%,tempered martensite: 40 to 65%,retained austenite: 10 to 20%,bainite: 10 to 30%,pearlite: 0 to 10%, andas-quenched martensite: 0 to 10%, whereina value of an average value of an Mn concentration in the ferrite divided by an Mn concentration of a base material is 0.980 or less, anda value of an average value of an Mn concentration in the retained austenite divided by the Mn concentration of the base material is 1.150 to 2.000.

2. The steel sheet according to claim 1, wherein the chemical composition includes, by mass %, at least one ofCr: 0.001 to 2.000%,Mo: 0.001 to 1.000%,Ti: 0.001 to 0.500%,Nb: 0.001 to 0.500%,B: 0.0001 to 0.0100%,Cu: 0.001 to 1.000%,Ni: 0.001 to 1.000%,W: 0.001 to 0.100%,V: 0.001 to 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 to 0.050%,Zr: 0.0001 to 0.050%,Ca: 0.0001 to 0.0500%,Y: 0.0001 to 0.0500%,La: 0.0001 to 0.0500%,Ce: 0.0001 to 0.0500%, andBi: 0.0001 to 0.0500%.

3. The steel sheet according to claim 1, wherein an average value of a C concentration in the retained austenite is 0.80 mass % or more.

4. The steel sheet according to claim 1, wherein the steel sheet includes a middle part in sheet thickness and a soft surface layer arranged at one side or both sides of the middle part in sheet thickness, the soft surface layer has an average thickness of 10 μm or more and an average Vickers hardness of 0.90 time or less of an average Vickers hardness of a sheet thickness ½ position.

5. The steel sheet according to claim 1, wherein a tensile strength is 1180 MPa or more.

6. A part containing the steel sheet according to claim 1.