Technical field
[0001]
The present invention has excellent formability, i.e. excellent and the uniform elongation was a good weldability, involved in steel sheet having a high strength.
BACKGROUND
[0002]
Of bodies and parts of automobiles, in order to achieve both the weight reduction and safety, high strength of a steel sheet is these materials are underway. In general, when high strength steel sheet, reduced and uniform elongation and hole expansion formability of the steel sheet is impaired. Therefore, in order to use high strength steel plate as a member for an automobile, it is necessary to increase both the strength and formability are conflicting properties.
[0003]
　To improve the uniform elongation, so far, using the transformation induced plasticity of retained austenite (residual gamma), the so-called TRIP steel sheet has been proposed (e.g., see Patent Documents 1 and 2).
[0004]
　Retained austenite, by concentrating the C and Mn, etc. in the austenite, the austenite is obtained by preventing transformed into other tissues even at room temperature. As a technique for stabilizing the austenite, contain a carbide precipitation suppression elements such as Si and Al in the steel, it is proposed to enrich C in austenite during bainite transformation caused steel at the manufacturing stage of the steel sheet there.
[0005]
　In this technique, the more the C content to be contained in the steel sheet austenite further stabilized, it is possible to increase the amount of retained austenite, as a result, it is possible to produce steel sheet excellent both strength and uniform elongation. However, if the steel sheet is used for the structural member, it is often welded to the steel plate is performed, since the weldability is large C content in the steel sheet is deteriorated, such a limitation to the use as a structural member. Therefore, a smaller C content, it is desirable to increase both the uniform elongation and strength.
[0006]
　In order to enhance the hole expansion of the TRIP steel sheet, the steel of bainite as the main structure has been proposed (e.g., Patent Document 3). However, when the low ductility of bainite as the main structure, it ductility of the steel sheet is low, it is difficult fabrication of automotive parts of complex shapes.
[0007]
　On the other hand, the residual austenite amount is large than the TRIP steel, as steel sheet ductility than the TRIP steel, the steel with the addition of 3.0% of Mn has been proposed (for example, Non-Patent Document 1). However, in this type of steel, less example tried to apply to the member for a motor vehicle has not been sufficiently studied the improvement of hole expansion. Including 3.0% of Mn, and the steel sheet having improved hole expansion, etc. it has not yet been developed.
[0008]
　Patent Document 4, a high strength steel sheet mechanical properties were improved by annealing before the cold rolling is performed is disclosed. However, the steel sheet described in Patent Document 4 is excellent DP steel r value, not a steel sheet excellent in strength and ductility. In particular, Patent Document 4, the winding process when the steel sheet ferrite after hot rolling - the pearlite and ferrite to precipitate cementite, a technique to coarsening and spheroidization of cementite is disclosed before cold rolling there. Such steel sheet having a tissue is not capable of having both tensile strength required for the steel sheets for automobiles, ductility, and hole expansion.
CITATION
Patent Document
[0009]
Patent Document 1: Japanese Sho 61-217529 Patent Publication
Patent Document 2: Japanese Patent Laid-Open 5-59429 discloses
Patent Document 3: Japanese Patent 2005-330584 JP
Patent Document 4: Japanese Patent No. 5082451
Non-patent literature
[0010]
Non-Patent Document 1: Takashi Furukawa, Makoto Matsumura, heat treatment, Japan, Japan heat treatment Association, 1997, No. 37 winding, No. 4, p. 204
Summary of the Invention
Problems that the Invention is to Solve
[0011]
　The present invention, excellent uniform elongation and hole expansion, high strength, and an object of the present invention to provide a steel sheet having a good weldability.
Means for Solving the Problems
[0012]
　In order to improve the weldability, it is necessary to lower the C content. Excellent uniform elongation and hole expansion, and to ensure high strength with a low C content, the present inventors have residual austenite of 7 or more area% in the steel sheet is dispersed, further control the form of residual austenite it was knowledge to be effective for. Summary of the such the present invention is as follows.
[0013]
(1) steel sheet according to one embodiment of the present invention is a unit mass%, C: 0.03 ~ 0.40% , Si: 0.01 ~ 5.00%, Mn: 0.50 ~ 12.00% , Al: 0.001 ~ 5.000%, P: 0.150% or less, S: 0.0300% or less, N: 0.0100% or less, O: 0.0100% or less Cr: 0 ~ 5.00 %, Mo: 0 ~ 5.00% , Ni: 0 ~ 5.00%, Cu: 0 ~ 5.00%, Nb: 0 ~ 0.500%, Ti: 0 ~ 0.500%, V: 0 ~ 0.500%, W: 0 ~ 0.500%, B: 0 ~ 0.0030%, Ca: 0 ~ 0.0500%, Mg: 0 ~ 0.0500%, Zr: 0 ~ 0.0500% , REM: 0 ~ 0.0500%, Sb: 0 ~ 0.0500%, Sn: 0 ~ 0.0500%, As: 0 ~ 0.0500%, and Te: 0 ~ 0.05 Containing 0%, the balance being iron and impurities, 1 / 4t of the metal structure comprises retained austenite 4-70 vol%, in the 1 / 4t part, at unit mass% of the retained austenite mean Mn concentration [Mn] gamma average Mn concentration [Mn] in the unit mass% of the total 1 / 4t part ave satisfies expression 1 for, in the 1 / 4t part, an aspect ratio of 2.0 or less volume fraction f of the residual austenite γs volume fraction f of all of the residual austenite γ satisfies the Togashiki 2, C content of at unit mass% [C] and the Mn content [Mn] satisfy the equation 3.
[Mn] gamma / [Mn] ave > 1.5 · · · (Equation
1) f gamma] s / f gamma ≦ 0.30 · · · (Equation
2) [C] × [Mn] ≧ 0.15 · · · (equation
3) steel sheet according to (2) above (1) is a unit mass%, Mn: 3.50 may contain ~ 12.00%.
(3) a steel sheet according to (1) or (2) is a unit mass%, Cr: 0.01% ~ 5.00 %, Mo: 0.01% ~ 5.00%, Ni: 0. 01% ~ 5.00%, Cu: 0.01% ~ 5.00%, Nb: 0.005% ~ 0.500%, Ti: 0.005% ~ 0.500%, V: 0.005% ~ 0.500%, W: 0.005 ~ 0.500%, B: 0.0001% ~ 0.0030%, Ca: 0.0001% ~ 0.0500%, Mg: 0.0001% ~ 0. 0500%, Zr: 0.0005% ~ 0.0500%, REM: 0.0005% ~ 0.0500%, Sb: 0.0050% ~ 0.0500%, Sn: 0.0050% ~ 0.0500% , As: 0.0050% - 0.0500%, and Te: selected from the group consisting of 0.0050% - 0.0500% May contain one or more kinds are.
(4) a steel sheet according to any one of the above (1) to (3) may have a galvanized layer on the surface of the steel sheet.
(5) steel sheet according to any one of the above (1) to (3) may have a galvannealed layer on a surface of the steel sheet.
Effect of the invention
[0014]
　Steel sheet according to the above aspect of the invention, since the C content is 0.40 mass% or less, has excellent weldability. Further, the steel sheet according to the present invention, the tensile product "TS × UEL" with strength and uniform elongation is high. This means that both the tensile strength and the uniform elongation are conflicting properties are excellent. Further, the steel sheet according to the present invention, the product "TS × lambda" between tensile strength and hole expansion is high. This means that both the tensile strength and the hole expansion is contradictory properties is excellent.
[0015]
　According to the present invention, the C content less than the prior art, than prior art tensile strength, elongation uniform, and it is possible to provide a high strength steel sheet excellent in hole expansibility. With this steel sheet, in particular, increasing both the weight of the car and safety, and, since it becomes possible to be secured ease of welding, the contribution of the industry is extremely remarkable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
[Figure 1] f gamma] s / f gamma is a diagram showing the effect of average heating rate during the first annealing on.
[Figure 2] f gamma] s / f gamma is a diagram showing the effect of maximum heating temperature of the first during annealing on.
[Figure 3] [Mn] gamma / [Mn] ave is a diagram illustrating the effect of maximum heating temperature of the first during annealing on.
[Figure 4] f gamma] s / f gamma is a diagram showing the effect of average heating rate during the second annealing on.
DESCRIPTION OF THE INVENTION
[0017]
　The present inventors have results of extensive studies, the 1 / 4t part of the steel sheet, the average Mn concentration [Mn] in the unit mass% in residual austenite γ average Mn in the unit% by weight of the total of 1 / 4t part concentration [Mn] ave satisfies the following equation (1) with respect to, and an aspect ratio of 2.0 or less residual volume fraction f of the austenite γs and volume fraction f of the total residual austenite γ and satisfies the following formulas (formula 2 ) if it meets the at low C content, found that an excellent high strength steel sheet uniform elongation and hole expansion can be produced. Note that the 1 / 4t part of the steel plate, an area of about 1/4 of the depth of the thickness t of the steel sheet from the rolled surface of the steel sheet. The 1 / 4t part of the steel plate, may be defined as the area between the depth of the surface of the depth of the surface and the 3 / 8t of 1 / 8t from the rolled surface of the steel sheet.
　[Mn] gamma / [Mn] ave > 1.5 · · · (Equation
　1) f gamma] s / f gamma ≦ 0.30 · · · (Equation 2)
[0018]
　Hereinafter, "mean in units mass% Mn concentration" simply referred to as "average Mn concentration", [Mn] gamma / [Mn] ave is sometimes referred to as "Mn thickening degree" a. The feature of the metallographic structure of the steel sheet according to the present embodiment, all, 1 / 4t part of the steel sheet (i.e., a region of 1/4 of the depth of the thickness t of the steel sheet from the rolled surface of the steel sheet are described below which is characteristic of the metal structure in). The rolling surface of the steel plate, 1 / 4t part of the steel sheet present between the central plane in the thickness direction of the steel sheet is believed to have an average metal structure. Thus, in the technical field of the steel plate, the 1 / 4t part of the steel plate, it is usual to be controlled metal structure. If the metal structure of 1 / 4t of the steel sheet according to the present embodiment is controlled as described below, the metal structure of the region other than 1 / 4t of the steel sheet is also preferably controlled, this embodiment tensile strength of the steel sheet according to the uniform elongation, and hole expansion exceeds the target value. Therefore, the structure of 1 / 4t part other than the area of the metal structure of the steel sheet, as long as structure of the metal structure of 1 / 4t part of the steel sheet is within a predetermined range described in the present embodiment is not particularly limited. In the present embodiment, unless otherwise specified, the description regarding the configuration of steel sheet metal structure, to a metal structure of 1 / 4t part of the steel plate.
[0019]
　Hereinafter, the aspect ratio of Mn enriched degree and residual austenite explains why defined as above.
[0020]
([Mn] gamma / [Mn] ave >
　1.5) in the TRIP steel, in the process of annealing, by the ferrite transformation and / or bainite transformation increases the C concentration in the austenite stable at room temperature austenite so that it exists manner. As a simple method for obtaining a steel sheet having higher strength and high uniform elongation, it means for increasing the amount of retained austenite by increasing the C content is known. However, increasing the C content, the deterioration of the weldability of the steel sheet, the steel sheet used for welded structural members and the like required, that approach can not be used.
[0021]
　The present inventors have examined the method for producing a steel sheet capable of increasing the uniform elongation without increasing the C content. As a result, the hot rolling conditions, after hot rolling, by suitably controlling the conditions of the heat treatment prior to cold rolling, increases the Mn concentration in the retained austenite, and to be satisfied (Equation 1) If, while maintaining the strength and C content, it was found to be to improve the uniform elongation.
[0022]
　Although the detailed reason for the above phenomenon occurs is not clear, the present inventors have by thickening the Mn is an austenite stabilizing element in the austenite, it is possible to secure a larger amount of residual austenite, uniform it is believed that growth has risen.
[0023]
　If Mn thickening degree is 1.5 or less, uniform elongation is deteriorated, not obtained mechanical properties required. Here, Mn thickening of the resulting steel sheets under normal production conditions is estimated that on the order of 1.2-1.4. Preferred lower limit of the Mn enrichment degree is 1.52,1.54 or 1.56.
[0024]
　1 / The ratio of Mn concentration in the 4t part, by polishing the cross section perpendicular to the rolling direction and rolling surfaces of the steel sheet, [Mn] in 1 / 4t part of the cross-sectional γ and [Mn] ave topical use such as, FE-SEM to analysis determined by measurement using a device capable.
[0025]
(F gamma] s / f gamma ≦ 0.30)
　Further, the present inventors have found that an aspect ratio of 2.0 or less residual volume fraction f of austenite gamma] s and volume fraction f of the total residual austenite gamma and the above-mentioned (Formula 2 ) When so doing meets the hole expanding ratio was found to increase. With equivalent strength, when compared with the steel sheets not satisfying the steel sheet satisfies the formula 2, towards the steel sheet satisfies Equation 2 was high hole expanding ratio.
[0026]
　Aspect ratio volume fraction f of 2.0 or less of residual austenite γs lowering the reason for hole expansion property is improved is not clear. The present inventors have found that an aspect ratio of 2.0 or less of residual austenite is present in the boundary or the like of the prior austenite boundary and packet, and since the majority having a particle size greater than 1.0 .mu.m, hole expansion processing aspect ratio of 2.0 or less easily stress concentration residual austenite, to assume that because easily broken upon. Therefore, f gamma] s / f gamma a is required to be 0.30 or less. f gamma] s / f gamma is more preferably 0.25 or less, further preferably 0.20 or less.
[0027]
　F in 1 / 4t part gamma] s / f gamma calculation method of the are as follows. First, the aspect ratio of 1 / 4t part of 2.0 or less of residual austenite fraction f gamma] s is determined by EBSP analysis of 1 / 4t part. In this EBSP analysis, and separating the fcc phase and bcc phases regards part fcc phase has become massive one of the residual austenite grains, aspect ratio, the longest width and shortest width of the particle the ratio. Total amount of retained austenite f in 1 / 4t part γ is determined by measurement using the X-ray diffraction at 1 / 4t part.
[0028]
　Next, a description will be given component of the steel sheet according to the present embodiment. Incidentally,% content in the following description means mass%.
[0029]
(C: 0.03 ~ 0.40%)
　C increases the strength of the steel, in order to ensure the retained austenite, is an extremely important element. In order to obtain a sufficient amount of residual austenite, it is necessary C content of 0.03% or more. On the other hand, since it impairs the weldability of the steel sheet is excessively containing C content, the upper limit of the C content is 0.40% or less. Preferred lower limit of the C content is 0.04%, 0.09%, or 0.14%. Preferred upper limit of the C content is 0.36%, 0.32%, or 0.25%.
[0030]
(Mn: 0.50 ~
　12.00%) Mn is austenite is stabilized, is an element which enhances the hardenability. In the steel sheet according to the present embodiment, to distribute the Mn in austenite, to further stabilize the austenite. In order to stabilize the austenite at room temperature, it is necessary 0.50% or more Mn. On the other hand, when the excessive content of Mn so impairs ductility, and 12.00% of the upper limit of the Mn content. Preferred lower limit of the Mn content is 1.50%, 2.30%, 3.00%, or 3.50%. Preferred upper limit of the Mn content is 10.00%, 8.00%, or 6.00%.
[0031]
(Si:
0.01 ~ 5.00%) (Al: 0.001 ~
　5.000%) Si is a deoxidizing agent, it is necessary to contain 0.01% or more. Al is also a deoxidizing agent, it is necessary to contain 0.001% or more. Also, Si and Al are elements to stabilize ferrite during annealing of a steel sheet, and to increase the C concentration of the austenite by suppressing the cementite precipitation during bainitic transformation, an element which contributes to the securing of residual austenite. Also, Si and Al enhances the hole expansion of the steel sheet. The reason is not clear, and by tempered martensite or ferrite matrix becomes hard and the hardness difference between the martensite and tempered martensite or ferrite transformed from austenite during processing is reduced Presumed.
　Its effect increases the content of Si and Al is high increases, but when excessive content of Si and Al, surface properties, paintability, and so leads to deterioration such as weldability, the upper limit of Si 5.00% follows and, the upper limit of Al is less 5.000%. Further, if the inclusion of Al in an amount exceeding 5.000% delta ferrite remains even at room temperature. The ferrite becomes ferrite stretched during hot rolling, the steel sheet tends to break because tensile stress concentrated at the ferrite in a test or press molding. Preferred lower limit of the Si content is 0.40% 0.90%, or 1.20%. Preferred upper limit of the Si content is 4.00%, 3.50%, or 3.00%.
[0032]
(P: 0.150% or less)
　P is an impurity, excessively when containing impair the ductility and weldability. Therefore, the upper limit of the P content to 0.150% or less. Preferred upper limit of the P content is 0.060% 0.030% or 0.025%. Since the steel sheet according to the present embodiment does not require the P, the lower limit of the P content is 0%. The lower limit of the P content may be 0 percent, or 0.001 percent, P content is preferably as much as possible reduced.
[0033]
(S: 0.0300% or less)
　S is an impurity, if excessively contained, MnS was stretched by hot rolling to produce, leading to formability deterioration such as ductility and hole expansion. Therefore, the upper limit of the S content to less 0.0300%. Preferred upper limit of the S content is 0.0100%, 0.0070%, or 0.0040%. Since the steel sheet according to the present embodiment does not require the S, the lower limit of the S content is 0%. The lower limit of the S content to 0%, or greater than may be 0.0001% but, S content is preferably as much as possible reduced.
[0034]
(N: 0.0100% or less)
　N is an impurity, leading to deterioration of the local ductility exceeds 0.0100%. Therefore, the upper limit of the N content to 0.0100% or less. Preferred upper limit of the N content is 0.0080%, 0.0060%, or 0.0050%. Since the steel sheet according to the present embodiment does not require N, the lower limit of the N content is 0%. The lower limit of the N content greater than 0%, or even may but as 0.0003%, N content is preferably as much as possible reduced.
[0035]
(O: 0.0100% or less)
　O is an impurity, leading to ductility degradation exceeds 0.0100%. Therefore, the upper limit of the O content is set to 0.0100% or less. Preferred upper limit of the O content is 0.0060%, 0.0040%, or 0.0020%. Since the steel sheet according to the present embodiment does not require the O, the lower limit of the O content is 0%. O content of the lower limit greater than 0%, or may be 0.0001% but, O content is preferably as much as possible reduced.
[0036]
　The above is a basic element constituting the steel sheet according to the present embodiment, the steel sheet according to the present embodiment is selected further, Cr, Mo, Ni, Cu, Nb, Ti, V, W, and from the group consisting of B it may contain one or more that. However, the steel sheet according to the present embodiment is Cr, Mo, Ni, Cu, Nb, Ti, V, W, and does not require a B, Cr, Mo, Ni, Cu, Nb, Ti, V, W, and the lower limit of the content of B is 0%.
[0037]
(Cr:

0 ~ 5.00%) (Mo: 0 ~
5.00%) (Ni: 0 ~ 5.00%) (Cu: 0
　~ 5.00%) Cr, Mo, Ni, and Cu, not an essential element in the steel sheet according to an embodiment of the present invention. However, Cr, Mo, Ni, and Cu, since an element of improving the strength of the steel sheet, may be contained in the steel sheet according to the present embodiment. To obtain this effect, the steel sheet is Cr, Mo, Ni, and selected from the group consisting of Cu 1 or two or more elements each may contain 0.01% or more. However, if excessively contained these elements, the strength of the steel sheet becomes too high, which may impair the ductility of the steel sheet. Therefore, Cr, Mo, Ni, and one selected from the group consisting of Cu or more elements of each of the upper limit is 5.00%.
[0038]
(Nb:

0 ~ 0.500%) (Ti: 0 ~
0.500%) (V: 0 ~ 0.500%) (W: 0 ~
　0.500%) Nb, Ti, V, and W, not an essential element in the steel sheet according to an embodiment of the present invention. However, Nb, Ti, since V, and W is an element for generating fine carbides, nitrides or carbonitrides, it is effective in ensuring strength of the steel sheet. Therefore, the steel sheet is Nb, Ti, may contain one or more elements selected from the group consisting of V, and W. To obtain this effect, Nb, Ti, V, and one selected from the group consisting of W or more elements of each of the lower limit be 0.005% preferred. On the other hand, if excessively contained these elements, too increase the strength of the steel sheet, there is a case where the ductility of the steel sheet is lowered. Therefore, Nb, Ti, V, and one or more elements of each of the upper limit 0.500% by selected from the group consisting of W to.
[0039]
(B: 0 ~ 0.0030%) B
　is delay the start of ferrite transformation and bainite transformation, it is possible to enhance the strength of steel, it may be contained in the steel sheet according to the present embodiment as needed . To obtain this effect, it is preferable to set the lower limit of the B content is 0.0001%. On the other hand, excessive amount of B is too enhance hardenability of the steel sheet, since excessive delay the start of ferrite transformation and bainite transformation, which may interfere with the concentration of C into the retained austenite phase. Therefore, the upper limit of B content is 0.0030%.
[0040]
　Steel sheet according to the present embodiment further, Ca, Mg, Zr, and REM may also contain one or more elements selected from the group consisting of (rare earth element). However, the steel sheet according to the present embodiment is Ca, Mg, Zr, and does not require REM, Ca, Mg, the lower limit of the content of Zr, and REM is 0%.
[0041]
(Ca:

0 ~ 0.0500%) (Mg: 0 ~
0.0500%) (Zr: 0 ~ 0.0500%) (REM: 0 ~
　0.0500%) Ca, Mg, Zr, and REM are sulfides and by controlling the shape of oxide, to improve the local ductility and hole expansion of the steel sheet. To obtain this effect, 0.0001% or more Ca, 0.0001% or more of Mg, 1 kind selected from the group consisting of 0.0005% or more Zr, and 0.0005% or more REM or 2 more may be contained in the steel sheet species. However, these elements of excess, so degrades the workability of the steel sheet, the upper limit of each of these elements and 0.0500%. Incidentally, Ca, Mg, Zr, it is preferable that the total content of one or more elements selected from the group consisting of REM or less 0.0500%.
[0042]
　Steel further, Sb, Sn, As, and may contain one or more members selected from the group consisting of Te. However, the steel sheet according to the present embodiment Sb, Sn, As, and does not require a Te, Sb, Sn, the lower limit of the content of As, and Te is 0%.
[0043]
(Sb:

0 ~ 0.0500%) (Sn: 0 ~
0.0500%) (As: 0 ~ 0.0500%) (Te: 0 ~
　0.0500%) Sb, Sn, As, and Te, It suppressed the Mn in the steel sheet, Si, and / or oxidizable elements such as Al is diffused on the surface of the steel sheet to form an oxide, enhanced surface properties and plating of the steel sheet. To obtain this effect, Sb, Sn, As, and one member selected from the group consisting of Te or more elements of each of the lower limit may be 0.0050%. On the other hand, the content of each of these elements is more than 0.0500%, its effect is saturated, and the upper limit of each of these elements and 0.0500%.
[0044]
([C] × [Mn] ≧ 0.15)
　chemical composition of the steel sheet according to the present embodiment, C content of at unit mass% [C] and the Mn content [Mn] is necessary to satisfy the following expression 3 there is.
[C] × [Mn] ≧ 0.15 ··· ( Equation 3)
　none of C and Mn, during heat treatment included in the method of producing the steel sheet, the austenite contained in the steel sheet stabilizes, finally obtained It has the effect of increasing the amount of retained austenite steel sheet to be. If the product of the C content and the Mn content is 0.15 or more, the amount of residual austenite was 4% or more, and it is possible to satisfy equation 1 by enrich Mn in residual austenite. Preferred lower limit of the [C] × [Mn] is 0.30 or 0.50. It is not particularly necessary to define the upper limit of the [C] × [Mn], may be 4.80, which is calculated from the upper limit value of the C content and Mn content described above.
[0045]
　Next, a description will be given steel sheet metal structure according to the present embodiment.
[0046]
　Metal structure of the steel sheet according to the present embodiment includes a residual austenite, other organizations ferrite, residual austenite, fresh martensite, bainite, composed of one or more such tempered martensite.
　The fraction of each tissue will vary with the conditions of annealing, the strength, impact the material in such ductility and hole expansion. The material is, for example, since the changes by the component for an automobile, to select the annealing conditions as needed, may be controlled tissue fraction.
[0047]
(The amount of residual austenite in the metallic structure in 1 / 4t part of the steel sheet: 4 to 70 vol%)
　in the steel sheet according to the present embodiment, it is important to control the amount of residual austenite. Residual austenite, the ductility of the steel sheet by transformation-induced plasticity, a tissue particularly enhance the uniform elongation of the steel sheet. Further, residual austenite, since transformed into martensite by the processing, which contributes to improving the strength of the steel sheet. In order to obtain these effects, 1 / 4t of the steel sheet according to the present embodiment, it should contain at least 4% of retained austenite in volume fraction. Preferably, the lower limit of the volume fraction of residual austenite is 5%, 7%, 9%, or 12%. Incidentally, as described above, the configuration of the metal structure of the region other than 1 / 4t of the steel sheet, as long as structure of the metal structure of 1 / 4t part of the steel sheet is within a predetermined range described in the present embodiment, in particular but it is not limited. The volume fraction of retained austenite in the region other than the 1 / 4t part of the steel sheet is not limited. In the present embodiment, unless otherwise specified, the description of the content of such residual austenite shows the content of residual austenite, etc. metal structure of 1 / 4t part of the steel plate.
[0048]
　The volume fraction of retained austenite is preferably as high as possible. However, the steel sheet having the above chemical components, it is difficult to contain residual austenite 70% by volume. The retained austenite to 70 volume percent, it is necessary to include C 0.40 percent, in this case, the weldability of the steel sheet is impaired. Therefore, the upper limit of the volume fraction of residual austenite is 70% or less.
[0049]
　Unless the amount of residual austenite above is within the prescribed range, the balance of the metal structure of the steel sheet according to the present embodiment is not particularly limited, may be suitably selected depending on the properties required. Tissue that may be included in the remainder of the metal structure, ferrite, fresh martensite, bainite, and the like tempered martensite may include tissue and inclusions than these. Ferrite, fresh martensite, bainite, and upper and lower limits of the tempered martensite is not particularly limited, describing these preferred upper and lower limits as follows.
[0050]
　Ferrite is an organization to improve the ductility of the steel plate. The ferrite content may be 0% by area, but within the range of intensity levels of interest, in order to maintain preferably both the strength and ductility, the area ratio of ferrite is preferably 10 to 75%. A further preferred lower limit of the content of ferrite is 15 area%, 20 area%, or 25% by area. A further preferred upper limit of the content of ferrite is 50 area%, 65 area%, or 70% by area.
[0051]
　Metal structure of the steel sheet according to this embodiment, fresh martensite (i.e., martensite is not tempered) may contain. Fresh martensite is a hard organization, is effective in ensuring the strength of the steel sheet. However, when the content of the fresh martensite is small, ductility of the steel sheet is increased. Therefore, the content of fresh martensite may be 0% by area. The steel sheet according to the present embodiment, in order to secure the ductility may be 25% area ratio of the upper limit of the amount of the fresh martensite. Preferred lower limit of the content of the fresh martensite 0.5 area%, 1 area%, or 2% by area. Preferred upper limit of the content of fresh martensite 20 area%, 15 area%, or 12% by area.
[0052]
　Metal structure of the steel sheet according to the present embodiment may include a tempered martensite obtained by tempering the fresh martensite formed after final annealing. The content of tempered martensite may be 0 area%, tempered martensite is a structure to improve both the strength of the steel sheet and hole expansion. The content of tempered martensite, can be appropriately selected depending on the intensity level of interest, when the upper limit of the content of tempered martensite was 25%, it can be preferably ensured ductility of the steel sheet. A further preferred lower limit of the content of the tempered martensite is 3 area%, 5 area%, or 7 is area%. A further preferred upper limit of the content of the tempered martensite is 22 area%, 20 area%, or 18 is area%.
[0053]
　Metal structure of the steel sheet according to the present embodiment may include bainite. During bainite transformation, C is concentrated in the austenite, the austenite is stabilized. That is, by performing the heat treatment to include the bainite steel sheet finally obtained, the residual austenite is stabilized. Further, bainite is also a tissue can be improved both strength and hole expansion of the steel sheet. Content of bainite may be 0 area%, but in order to obtain these effects, it is preferable to contain 5% or more by area of ​​bainite. The steel sheet according to the present embodiment, because it contains at least 0.5% of Mn, it is difficult to bainite to 50 area%. Therefore, the upper limit of the content of bainite may be 50% by area. A further preferred lower limit of the content of the bainite 7 area%, 8 area%, or 10% by area. A further preferred upper limit of the content of the bainite 45 area%, 42 area%, or 40% by area.
[0054]
　Metal structure of the steel sheet according to the present embodiment may include perlite. Pearlite during cooling during annealing, and sometimes transforms from austenite alloying moderate. The upper limit of the amount of the pearlite is preferably 10% by area. If the upper limit of the pearlite content is 10 area%, prevents residual austenite amount is 4 area% or less, it is possible to secure the strength and ductility. A further preferred upper limit of the content of perlite 3 area%, 2 area%, or 1% by area. Further, since the content of perlite lower is better, the lower limit of the content of the perlite is 0 area%.
[0055]
　Incidentally, if either or both of ferrite and tempered martensite are included in the steel sheet, it is preferable that the average crystal grain size of the ferrite and tempered martensite 10μm or less. In this case, the steel sheet because tissue becomes finer to high strength. Furthermore, since the tissue is uniform, work strain becomes to be uniformly introduced into the steel sheet, thereby improving the uniform elongation of the steel sheet. The average grain size of ferrite and tempered martensite is more preferably 7μm or less, still more preferably 5μm or less.
[0056]
　The following shows the method of identifying the tissue. Incidentally, the identification method described below is performed at 1 / 4t of all steel.
　Ferrite observations, using the cut surface of the polishing and Nital corroded samples, carried out by light microscopy. In a typical optical microscope image, since ferrite appear white, and the fraction of ferrite by measuring the area ratio of the white portion. Measurement of area ratio, performs structure photographs with image analysis.
　The volume fraction of retained austenite is determined by X-ray diffraction method. The volume ratio obtained by this means can be considered to area ratio to be approximately the same.
　Observation of fresh martensite, with a cutting surface of the polishing and Repera corroded samples, carried out by light microscopy.
　Perlite, abrasive and nital and observed the cross section of the corroded samples SEM, and area ratio of pearlite by measuring the area ratio of the region composed of lamellar structure.
[0057]
　Bainite, and tempered martensite, with SEM, the cut surface of the suitably prepared sample can be determined by observing at magnification of 5000 times. Ferrite, martensite, residual austenite, a tissue other than pearlite regards tissue cementite therein are precipitated in one direction and bainite, tissue cementite are precipitated along two or more directions it can be considered the the tempered martensite.
　On the other hand, possible in order to omit the determination operations described above, according to the method for producing a tempered martensite, bainite, and also appropriately selecting the method for identifying a tempered martensite.
　An example of a technique to produce tempered martensite steel sheet according to the present embodiment, and that the temperature of the steel sheet before austempering process described later to a temperature of Ms ~ Mf point. Another example of a method to produce steel tempered martensite in the present embodiment, it is exemplified to perform tempering on the steel sheet after annealing completion. When not performing such a tempering process is absent tempered martensite.
[0058]
　If the tempered martensite is not present, ferrite, martensite, residual austenite, the area ratio of the tissues other than the pearlite, can be regarded as bainite area ratio.
　If to produce a tempered martensite performing tempering after annealing completed, measuring the amount of martensite tempering pretreatment of steel sheet, the amount of martensite, the amount of tempered martensite obtained after tempering it can be considered.
　When producing tempered martensite temperature of the steel sheet before austempering treatment to a temperature of Ms ~ Mf point may estimate the amount of tempered martensite based on a change amount of the volume expansion of the steel sheet. When austenite is transformed into martensite or bainite, because the volume is increased. The increase in volume of the temperature Ms ~ steel within 0.1 seconds of the temperature of the Mf of the steel sheet is regarded as the amount of tempered martensite, the amount of increase in the subsequent volume can be regarded as the amount of bainite .
[0059]
　Next, a description will be given mechanical properties of the steel sheet according to the present embodiment.
　Tensile strength of the steel sheet according to the present embodiment is preferably at least 440 MPa. This when using steel as an automotive material, reducing the thickness by a high strength, in order to contribute to weight reduction. Further, in order to provide a steel sheet according to the present embodiment for press molding, the uniform elongation (UEL) and hole expansion (lambda) and it is desired that excellent. Product "TS × UEL" between tensile strength and uniform elongation is at 20000 MPa ·% or more, the product "TS × lambda" between tensile strength and hole expansion it is desirable that the 20000 MPa ·% or more. By the structure of the metal structure of the component and 1 / 4t part in the range described above, such mechanical properties.
[0060]
　Incidentally, the steel sheet according to the present embodiment may be a galvanized steel sheet having a galvanized layer provided on the surface thereof, galvannealed with galvannealed layer provided on the surface thereof it may be a plated steel sheet. Galvanized layer or a galvannealed layer has a dramatically enhancing effect and corrosion resistance and appearance of the steel sheet according to the present embodiment. On the other hand, galvanized layer or galvannealed layer does not impair the mechanical properties of the steel sheet according to the present embodiment.
[0061]
　Next, a method for manufacturing a steel sheet according to the present embodiment.
　Steel sheet according to the present embodiment, melting the ingredients of the steel, such as in a conventional manner, the casting to create a slab or steel ingot, by heating the hot-rolled, obtained hot rolled steel sheet after the pickling, after first annealing, producing subjected to further second annealing. Between the first annealing and the second annealing may be performed cold rolling as needed.
[0062]
　Hot rolling may be performed in a conventional continuous hot rolling line. It said first annealing and the second annealing, satisfies the below conditions, may be performed in either annealing furnace and a continuous annealing line. Further, the steel sheet after cold rolling rolling may be performed skin pass rolling.
[0063]
　Cooling conditions after hot rolling, winding condition, annealing conditions of the first annealing, by limiting the cold rolling reduction ratio, and a second annealing the annealing conditions within the range shown below, respectively, formulas 1 and it is possible to obtain a metal structure that satisfies the provisions.
[0064]
　Unless the chemical component is in the range of steel sheet according to the present embodiment described above, the molten steel may be those which are melted in conventional blast furnace process, as steel created in the electric furnace method, raw materials There may be those containing scrap in large quantities. Slab, it may be one produced by conventional continuous casting process, or may be those produced by thin slab casting.
[0065]
　Heating the slab or ingot of the above, performing the hot rolling. The heating temperature and hot rolling temperature at that time not specified.
[0066]
　The hot-rolled steel sheet obtained by performing finish rolling is cooled, winding, and coil. Suppression of pearlite transformation, and for refinement of the crystal grain size, it cooling rate after hot rolling to coiling start is large are preferred. Therefore, after hot rolling, and the average cooling rate of the temperature range from 800 ° C. to a coiling temperature 10 ° C. / s or higher. Furthermore, in order to refine the grain size after hot rolling, is preferably 30 ° C. / s or higher average cooling rate of temperature ranging from 800 ° C. to coiling temperature. After hot rolling, the upper limit of the average cooling rate temperature range from 800 ° C. to coiling temperature is preferably 100 ° C. / s or less in order to accurately control the coiling temperature. Incidentally, the "average cooling rate of the temperature range from 800 ° C. to a coiling temperature", 800 ° C. and the difference between the coiling temperature, elapsed time steel took to be cooled from 800 ° C. to a coiling temperature is a value obtained by dividing the. The average cooling rate in the temperature range when finish rolling of hot rolling is completed at below 800 ° C., the average cooling rate of the temperature range from finish rolling immediately after the temperature to the coiling temperature, from 800 ° C. to a coiling temperature instead, it may be controlled.
[0067]
　The coiling temperature after cooling to 600 ° C. or less. When the coiling temperature exceeds 600 ° C., hot rolled tissue ferrite - becomes uneven tissue by coarsening of pearlite, coarse cementite occurs on subsequent first annealing. This coarse cementite, or become retained austenite after the second annealing coarse, remains cementite after the second annealing, there is a case where the amount of retained austenite or reduced. In either case, the mechanical properties of the steel sheet is degraded. Coiling temperature, more preferably 550 ° C. or less, more preferably 500 ° C. or less.
[0068]
　Hot-rolled steel sheets obtained in this manner is annealed after being cooled to room temperature.
　The hot annealing after rolling (first annealing) is an important process in the manufacturing method of the steel sheet according to the present embodiment. In the final structure, said to satisfy (Equation 1) uses the annealing, it is necessary to enrich Mn in residual austenite. Annealing performed after cold rolling (second annealing) only it is impossible to satisfy (Equation 1).
[0069]
　In the first annealing, heating the hot-rolled steel sheet, held for 30 minutes or more to the temperature range of 550 ~ 750 ° C., and cooled to below the Ms point. By performing the annealing steel sheet that satisfies both of (Equation 1) and (2) is obtained.
　If the first annealing temperature is a temperature range below 550 ° C., the residual austenite in the structure of the finally obtained steel sheet does not satisfy the equation (2). The cause is unknown, if the first annealing temperature is less than 550 ° C., coarse cementite during the first annealing occurs prior austenite grain boundaries and packet boundaries, the coarse cementite second aspect ratio when the annealing is estimated to be because changes in 2.0 following coarse austenite.
　On the other hand, when the first annealing temperature exceeds 750 ° C., the martensite is increased at the end of the first annealing, is not satisfied (Equation 1). The cause is unknown, the not satisfy the equation (1), the structure of the steel sheet during the first annealing becomes substantially the austenite single phase, it is estimated that segregation of Mn is because not occur. Therefore, the upper limit of the first annealing temperature of 750 ° C..
[0070]
　Also, when heating the steel sheet to a first annealing temperature, it is necessary to make the average heating rate in the temperature range of 300 ~ 550 ° C. and 1 ° C. / s or higher. If not satisfied this condition, the metal structure of the steel sheet can not satisfy the equation (2), hole expansion property deteriorates. The reason is not clear, in the case of less than 1 ° C. / s, during heating, cementite is precipitated in the austenite grain boundaries and packet boundaries, the cementite, the aspect ratio of 2.0 or less austenite it is presumed to be because made. This limited the average heating rate is not only the first annealing, it is necessary to satisfy at the second annealing. The reason is the same as in the first annealing. Note that "average heating rate in the temperature range of 300 ~ 550 ° C." is the difference between the upper and lower limits of the temperature range (250 ° C.), the steel sheet temperature divided by the time required to pass through the temperature range is the value.
[0071]
　During annealing, there is no need to strictly isothermal hold temperature of the steel sheet. During the annealing, the temperature of the steel sheet for 30 minutes or more, sufficient if the temperature range of 550 ~ 750 ° C., the temperature of the steel sheet within this range may vary. When the time for holding the steel sheet temperature in the range of 550 ~ 750 ° C. is less than 30 minutes can not metal structure of the steel sheet satisfy the equation (1). This diffusion length of Mn by the lack of retention time is insufficient, sufficient Mn in austenite is estimated to be can not be concentrated.
[0072]
　Cold rolling is not essential for the production of steel sheet according to the present embodiment. However, thickness adjustment, may be subjected to cold-rolled steel sheet to adjust the shape. Cold rolling, the metal structure of the steel sheet after annealing finer and thereby an effect of improving the mechanical properties. However, if the reduction ratio of cold rolling is too large, the local ductility decreases.
[0073]
　After the first anneal, or any after cold rolling, the second annealing the steel sheet is subjected. In the method of manufacturing the steel sheet according to the present embodiment, the annealing temperature in the second annealing, a temperature at which coexist ferrite and austenite.
　When the annealing temperature of the second annealing is lower than 550 ° C. is austenite amount obtained in the second annealing is reduced, it is impossible to leave sufficient residual austenite in the finally obtained steel sheet. When the second annealing temperature exceeds 800 ° C., since Mn which was enriched in the austenite in the first annealing is diffused again, finally obtained steel sheet does not satisfy the equation (1). Further, with respect to steel 800 ° C. is an austenite single-phase region, when subjected to a second annealing at annealing temperatures above 800 ° C., it is impossible to obtain steel sheet of tissue according to the present embodiment.
[0074]
　Further, (holding time at 550 ℃ ~ 800 ℃) temperature holding time in the second annealing is 5 seconds or more. If the temperature holding time of the second annealing is less than 5 seconds, carbides are not dissolved completely, the amount of the finally obtained residual austenite is reduced, also, the case of performing the cold rolling also proceed recrystallization since no ductility of the finally obtained steel sheet deteriorates significantly. It is not necessary to define the upper limit of the temperature holding time of the second annealing, in consideration of productivity, it is preferable to set the upper and 1000 seconds.
[0075]
　Second cooling after annealing, encourage transformation from austenite phase to ferrite phase, it is important in order to stabilize the austenite by concentrating the C in the austenite phase of the untransformed. When the average cooling rate in the cooling after the second annealing below 1 ° C. / s, pearlite is generated, the amount of the finally obtained residual austenite is reduced. The steel sheet according to the present embodiment, since by concentrating the Mn in residual austenite is allowed to slow the average cooling rate in the cooling after the second annealing up to 1 ° C. / s. On the other hand, the average when the cooling rate is 200 ° C. / s greater is the cooling after the second annealing, because it is impossible to sufficiently proceed ferrite transformation, the upper limit of the average cooling rate in cooling after the second annealing it is preferable that the 200 ° C. / s. In the case of plated steel sheet, the average cooling rate is in the second cooling after annealing, the average cooling rate of the temperature range up to the cooling stop temperature to be described later from the second annealing temperature (i.e., a second annealing temperature and the difference between the cooling stop temperature is a second value divided by the time required to cool the steel sheet from the annealing temperature to the cooling stop temperature of). Otherwise plated steel sheet, the average cooling rate is in the second cooling after annealing, the average cooling rate of the temperature range from the second annealing temperature to 430 ° C. (i.e., the difference between the second annealing temperature and room temperature and a second value divided by the time taken to cool the steel sheet from the annealing temperature to room temperature).
[0076]
　Cooling after the second annealing, if not plated steel sheet, it is sufficient as it is carried out to room temperature. Also, when the plated steel sheet is manufactured as follows.
[0077]
　When producing a hot-dip galvanized steel sheet, the cooling after the second annealing stops in the temperature range of 430 ~ 500 ° C., immersed in a molten zinc plated cold rolled steel sheet in a plating bath of molten zinc subsequent do. Conditions of the plating bath may be within the normal range. After the plating treatment may be cooled to room temperature.
[0078]
　In the production of galvannealed steel sheet, after galvanized steel sheet, before cooling the steel sheet to room temperature, performing an alloying treatment at a temperature of 450 ~ 600 ° C. in a steel sheet. Alloying treatment conditions may be within the normal range.
[0079]
　By the manufacturing method described above, in the steel sheet according to the present embodiment, Mn is because it is concentrated in the austenite, only cooled to the second annealing after it room temperature, it is possible to obtain a residual austenite that is stable at room temperature . Further, as the conventional TRIP steel listed in Patent Document 1 stops the cooling after the second annealing at room temperature or higher, no problem even when held at that temperature range.
[0080]
　By producing steel sheet as described above, it can be obtained steel sheet according to the present embodiment, galvanized steel sheet, and galvannealed steel sheet.
Example
[0081]
　Hereinafter, further examples illustrate the invention.
Example 1
[0082]
　First, to investigate the influence of heating rate in the first annealing, an experiment was conducted as described below.
[0083]
　In unit mass%, C: 0.10%, Si : 1.0%, Mn: 4.3%, P: 0.010%, S: 0.0020%, Al: 0.03%, N: 0 .0020%, and O: containing 0.0012%, the balance being a slab having a chemical composition is iron and impurities, were prepared using a vacuum melting furnace. The resulting slab was heated to 1200 ° C., after hot rolling at a finishing temperature 921 ° C., cooling the average cooling rate in the temperature range from 800 ° C. to coiling starting temperature of 50 ℃ / s, 450 ℃ in the wound. The wound hot rolled steel sheet, after cooling to room temperature, was first annealing. In heating before the first annealing, an average heating rate in the temperature range of 300 ~ 550 ° C. and with various values in the range of 0.01 ~ 30 ℃ / s. First annealing at the annealing temperature (maximum heating temperature) was 570 ° C.. Temperature holding time at the annealing temperature was 7200 seconds (120 minutes) in all the hot-rolled steel sheet. After the temperature holding completion, it was cooled hot-rolled steel sheet to room temperature. Thereafter, rolling reduction 52% cold rolling the hot-rolled steel sheet was subjected to second annealing cold-rolled steel sheet thus obtained. In heating before the second annealing, the average heating rate in the temperature range of 300 ~ 550 ° C. and 3 ° C. / s, as 620 ° C. The annealing temperature in the second annealing, the retention time was 600s. Then, the second cold-rolled steel sheet after annealing, and the average cooling rate of the temperature range from the second annealing temperature to 430 ° C. and cooled as 10 ° C. / s. Various steel sheets obtained, [Mn] gamma / [Mn] ave and f gamma] s / f gamma was determined by the following means.
[0084]
　1 / The ratio of Mn concentration in the 4t part, by polishing the cross section perpendicular to the rolling direction and rolling surfaces of the steel sheet, [Mn] In this section γ and [Mn] ave calculated by the measured using FE-SEM obtained.
[0085]
　F in 1 / 4t part gamma] s / f gamma were as follows. First, the aspect ratio of 1 / 4t part of 2.0 or less of residual austenite fraction f gamma] s were asked by EBSP analysis of 1 / 4t part. In this EBSP analysis, and separating the fcc phase and bcc phases regards part fcc phase has become massive one of the residual austenite grains, aspect ratio, the longest width and shortest width of the particle It was the ratio. Total amount of retained austenite f in 1 / 4t part γ was determined by measurement using the X-ray diffraction at 1 / 4t part.
[0086]
　The following describes the test results. As shown in FIG. 1, the average heating rate during the first annealing is obtained by the manufacturing method is lower than 1 ° C. / s sample did not satisfy Expression (2). TS × lambda of samples does not satisfy the formula (2) was significantly reduced compared with other samples.
Example 2
[0087]
　Next, to investigate the effect of annealing temperature in the first annealing, an experiment was conducted as described below.
[0088]
　Heating the slab used in Example 1 in 1200 ° C., finish after hot rolling at a temperature 921 ° C., an average cooling rate of the temperature range up to coiling starting temperature of 800 ° C. and cooled as 50 ° C. / s, the take-up treatment was carried out at 450 ℃. After cooling the coiled hot rolled steel sheet to room temperature, it was first annealing. Before the first annealing, (if the annealing temperature is below 550 ° C., 300 ° C. ~ within the range of the annealing temperature) range of 300 ~ 550 ° C. The average heating rate of at was 3.5 ° C. / s. Then, the heating was terminated for each sample at 450 ~ 850 ° C., then, each sample was 7200s held at each annealing temperature, and then air-cooled the sample to room temperature. Thereafter, cold rolling cold rolling ratio 52% in each sample was subjected to a second anneal to each sample. The average heating rate in the range of 300 ~ 550 ° C. in the second annealing and 3.5 ° C. / s, the annealing temperature was 620 ° C., annealing time was 600s. After the end of the second annealing, each sample, and the average cooling rate of the temperature range from the second annealing temperature to 430 ° C. and cooled as 10 ° C. / s.
[0089]
　As shown in FIG. 2, the sample heating temperature during the first annealing is obtained by the manufacturing method is lower than 550 ° C. did not satisfy Expression (2). As a result, the sample heating temperature during the first annealing is obtained by the manufacturing method is lower than 550 ° C. is, TS × lambda is compromised. On the other hand, as shown in FIG. 3, the samples obtained by the production process the heating temperature during the first annealing exceeds 750 ° C., did not satisfy Expression (1). As a result, the sample heating temperature during the first annealing is obtained by the production method of greater than 750 ° C. is, TS × UEL is compromised.
Example 3
[0090]
　Next, to investigate the influence of heating rate in the second annealing, an experiment was conducted as described below.
　Implementing the slab used in Example 1 was heated to 1200 ° C., after hot rolling at a finishing temperature 921 ° C., cooling the average cooling rate in the section from 800 ° C. to coiling starting temperature of 50 ℃ / s, 450 the take-up treatment was carried out at ℃. Thereafter, at a first annealing by heating each sample an average heating rate of 300 ~ 550 ° C. as 2.5 ° C. / s, each sample at 590 ° C. After 7200s held, air cooled each sample to room temperature. Then, after performing cold rolling to each sample in cold reduction ratio of 52%, it was heated each sample at an average heating rate 0.01 ~ 30 ℃ / sec in the range of 300 ~ 550 ° C.. Further in the second annealing temperature 620 ° C., after which each sample was 600s retained, and the average cooling rate of the temperature range from the second annealing temperature to 430 ° C. to cool the sample as a 10 ° C. / s.
　As shown in FIG. 4, when the average heating rate during the second annealing is lower than 1 ° C. / s, not satisfied equation (2). As a result, TS × lambda is compromised.
Example 4
[0091]
　Steel A ~ P a melting and casting to create a slab, these slabs was hot-rolled to create a hot-rolled steel sheet, the take-up of these hot-rolled steel sheet, pickling, a first annealing, cold rolling , and subjected to a second annealing, and subjected to arbitrarily plating and alloying treatment, which gave a steel sheet 1-25. Chemical composition of steel A ~ P are as shown in Table 1-1 and Table 1-2, the production conditions of the steel plate 1 to 25 are as shown in Table 2, the structure of the state of the steel sheet 1-25 is as shown in Table 3, the mechanical properties of the steel sheet 1 to 25 were as shown in Table 4. Table 1-1 and unit numbers of chemical components of the steel A ~ P shown in Table 1-2 is the mass%, the remainder of the chemical components was iron and impurities. If the plating process is performed, and the cooling stop temperature before immersion in the plating bath and 460 ° C.. Further, if the alloying treatment is performed, alloying temperature was 520 ° C..
　Incidentally, listed in Table 2 as "first annealing time" test materials No. 4 and the test material No. 7 except the time at which the temperature of the test material has been in the range of 550 ~ 750 ° C.. Test material No. 4 and the test material No. 7 of the "first annealing time" is the time at which the temperature has been the "first annealing temperature" test material. Listed in Table 2 as "second annealing time" test materials No. 8 and test materials No. 21 except a time when the temperature of the test material has been in the range of 550 ~ 800 ° C.. Test material No. 8 and test materials No. 21 of the "second annealing time" is the temperature of the test materials is "second annealing temperature" and once was time. The "second average cooling rate after annealing", as plated test piece had an average to a second 460 ° C. from the annealing temperature (i.e. cooling stop temperature before immersion in above-described plating bath) a cooling rate, when not plated sample material is the average cooling rate from the second annealing temperature to 430 ° C..
[0092]
　Ferrite volume ratio of 1 / 4t part, the structural photograph of 1 / 4t part of the cross section of the polishing and Nital corroded samples taken with an optical microscope and determined by the structure photograph image analysis. The value obtained by this technique is the area ratio of ferrite, is considered to be substantially the same value as the area ratio and volume fraction.
　Pearlite volume ratio of 1 / 4t part, the structural photograph of 1 / 4t part of the cross section of the polishing and Nital corroded samples taken with SEM, it determined by the structure photograph image analysis. In the image analysis, it was considered a region having a lamellar structure and pearlite.
　1 / 4t part volume fraction of tempered martensite, when to produce a tempered martensite performing tempering after annealing finished, to determine the amount of martensite in 1 / 4t of the tempering pretreatment of the steel sheet, the the amount of martensite was obtained by regarding the amount of tempered martensite 1 / 4t part obtained after tempering. If the temperature of the steel sheet before austempering caused the tempered martensite in the temperature of Ms ~ Mf point, 1 / 4t part volume fraction of tempered martensite is the temperature of the steel sheet to a temperature of Ms ~ Mf from measuring the increase in volume of the steel sheet within 0.1 seconds, the increase amount was obtained by regarding the amount of tempered martensite 1 / 4t part obtained after tempering.
　Bainite volume ratio of 1 / 4t part, when tempered martensite is not present, ferrite, martensite, residual austenite, and considers the tissues other than the pearlite to be bainite, 1 / 4t of ferrite, martensite, residual austenite, and was calculated based on the volume fraction of pearlite. If to produce a tempered martensite performing tempering after annealing completion considers the bainite volume fraction of 1 / 4t part, ferrite, martensite, and residual austenite, pearlite, and tissues other than tempered martensite bainite , 1 / 4t of ferrite, martensite, residual austenite, pearlite, and was calculated based on the volume fraction of tempered martensite. If the temperature of the steel sheet before austempering caused the tempered martensite in the temperature of Ms ~ Mf point, bainite volume ratio of 1 / 4t part, 0 the temperature of the steel sheet in the temperature of Ms ~ Mf. the increase in the volume of the steel sheet after one second was calculated by regarding the bainite volume fraction of 1 / 4t part.
　Residual austenite volume fraction of 1 / 4t part was determined by X-ray diffraction method.
　Fresh martensite volume fraction of 1 / 4t part, a structure photograph of a cross section of polishing and Repera corroded samples taken with an optical microscope and determined by the structure photograph image analysis.
[0093]
　Tensile strength (TS), uniform elongation (u-EL), and ductility (t-EL) was determined by a tensile test of the steel sheet conforming to JIS Z 2241. The hole expansion lambda, using a test piece of 80mm square was measured by hole expansion test according to Japanese Tetsuren standard JFST1001-1996. Steel TS × UEL and TS × lambda is 20000 MPa ·% or more were considered mechanical properties are excellent steel sheet.
[0094]
[Table 1-1]

[0095]
[Table 1-2]

[0096]
[Table 2]

[0097]
[table 3]

[0098]
[Table 4]

[0099]
　Chemical composition and production conditions were appropriate in Example 1,3,5,6,9,11,13,15,17 and 20, the residual austenite volume fraction, [Mn] gamma / [Mn] ave , and f gamma] s / f gamma is properly controlled and excellent mechanical properties.
[0100]
　On the other hand, Comparative Example Comparative Example 2 annealing time in the first annealing is insufficient, and the annealing temperature in the first annealing was excess 4, since Mn is not sufficiently concentrated in residual austenite, TS × uEL is insufficient.
　Comparative Example 7 annealing temperature in the first annealing is insufficient, the aspect ratio of the volume fraction f of 2.0 or less of residual austenite γs so did not decrease sufficiently, TS × lambda is insufficient.
　Comparative Example 8 annealing temperature in the second annealing is insufficient, does not contain residual austenite, TS × UEL is insufficient.
　Comparative Example 10 the average cooling rate after the second annealing is insufficient, since the amount of retained austenite is insufficient, TS × UEL is insufficient.
　Comparative Example 12 annealing time in the second annealing is insufficient, since the amount of retained austenite is insufficient, TS × UEL is insufficient.
　Comparative Example 14 the average cooling rate after the first annealing is insufficient, the aspect ratio of the volume fraction f of 2.0 or less of residual austenite γs so did not decrease sufficiently, TS × lambda is insufficient.
　Comparative Example 16 in which the first average heating rate of the annealing before the is insufficient, the aspect ratio of the volume fraction f of 2.0 or less of residual austenite γs so did not decrease sufficiently, TS × lambda is insufficient.
　Comparative Example 21 annealing temperature in the second annealing was excessive, since Mn which was enriched in the austenite in the first annealing is diffused again, Mn is not sufficiently concentrated in the residual austenite, TS × uEL is insufficient.
[0101]
　Comparative Example 22 C content is insufficient and Mn content Comparative Example 23 was insufficient, since Mn is not sufficiently concentrated in residual austenite, TS × UEL is insufficient.
　Comparative Example 24 C × Mn is insufficient, because Mn was not sufficiently concentrated in residual austenite, TS × UEL is insufficient.
　Comparative Example 25 Mn content was excessive, ductility is deteriorated, TS × UEL and TS × lambda is insufficient.

The scope of the claims
[Requested item 1]
　で. Unit
　mass%,
　C:
　0.03 ~ 0.40%, Si: 0.01 ~
　5.00%, Mn: 0.50 ~ 12.00%, of Al: from 0.001 ~
　5.000%, P : 0.150% or
　less, S: 0.0300% or
　less, N: 0.0100% or
　less, O: 0.0100% or
　less,
　of Cr: 0 ~ 5.00%, of Mo: 0 ~
　5.00%, of Ni :
　0
　~ 5.00%,
　a Cu: 0 ~ 5.00%, of Nb: 0 ~ 0.500%,
　of Ti: 0 ~ 0.500%, V: 0 ~
　0.500%, W is: 0 ~ 0.
　%
　500,
　B: 0.0030 ~
　0%, of Ca: 0.0500 ~ 0%, of Mg: 0 ~
　0.0500%, Zr: 0 ~ 0.0500%, the REM: 0 ~
　0.0500%, Sb:
　~ 0.0500% 0, of
　Sn: 0.0500 ~ 0%, of As: 0 ~ 0.0500%, andび
　Te: 0 ~ 0.0500%
wo shi comprising, Portion ga Tieおyoびimpurities kara na ri,
　1 / 4t partのmetallic structure ga residue Bioーsu Te naイSuites wo 4 to 70% by volume containing Mi,
　before note 1 / 4t part niお~ te, before referred residue Bioーsu Te naイSuites inの. Unit mass%でのaverage Mn concentration [Mn] γ note 1 / 4t part of allの. unit mass%でのaverage Mn concentration before ga [Mn] Ave ni Dui shi te formula 1 wo the Man ta shi,
　before note 1 / 4t part niお~ te, manufactured Spectrapore ku Suites ratio ga 2.0 or lessのbefore referred residue Bioーsu Te naイSuitesのvolume ratio F [gamma] sとbefore full teのreferred residue Bioーsu Te naイSuitesのvolume fraction F gamma]とga formula 2 wo the Man ta shi,
　radiolabeling bit mass%でのC content [C] andびMn content [Mn] ga formula 3 wo the Man ta su
koとwo special Zhiとsuru plate.
　[Mn] gamma] / [Mn] Ave > for 1.5 · · · (Formula.
　1) F [gamma] s / F gamma] ≦ 0.30 · · · (Formula
　2) [C] × [Mn] ≧ 0.15 · · · (Equation 3)
[Requested item 2]
　In unit
　mass%, Mn: 3.50 ~ 12.00%
steel sheet according to claim 1, characterized in that it contains.
[Requested item 3]
　In unit
　mass%,
　Cr:
　0.01% ~ 5.00%, Mo: 0.01% ~ 5.00%, Ni:
　0.01% ~ 5.00%, Cu: 0.01% ~ 5.
　Pasento 00,

　Nb: 0.005 Pasento ～ 0.500 Pasento, Ti: 0.005 Pasento ～ 0.500 Pasento,
　V: 0.005 Pasento ～ 0.500 Pasento, W: 0.005 ～ 0.500 Pasento,
　B:
　0.0001%
　~ 0.0030%, Ca: 0.0001% ~
　0.0500%, Mg: 0.0001% ~ 0.0500%, Zr: 0.0005% ~
　0.0500%, REM:
　%
　~ 0.0500 0.0005%, Sb: 0.0050% ~
　0.0500%, Sn: 0.0050% ~ 0.0500%, As: 0.0050% ~ 0.0500%, and
　Te: 0 .0050% .0500%
one or more members selected from the group consisting of
containing Steel sheet according to claim 1 or 2, characterized in Rukoto.
[Requested item 4]
　Steel sheet according to any one of claims 1 to 3, characterized in that it has a hot-dip galvanizing layer on the surface of the steel sheet.
[Requested item 5]
　Steel sheet according to any one of claims 1 to 3, characterized in that it has a galvannealed layer on a surface of the steel sheet.