0001]The present disclosure is excellent related to a steel sheet and a manufacturing method thereof formability, specifically related to the high steel sheet and a manufacturing method thereof of containing Mn concentration and a uniform elongation characteristics and high strength excellent in.
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, the elongation is lowered and the moldability 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 elongation, so far, using the transformation induced plasticity of retained austenite (residual gamma), the so-called TRIP steel has been proposed (e.g., Patent Document 1).
[0004]
　Retained austenite, by concentrating the C 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. 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 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 improve both the elongation and strength of the steel sheet.
[0005]
　Further, the residual austenite amount is large than the TRIP steel, as steel sheet ductility than the TRIP steel, the steel was added 4.0% of Mn has been proposed (for example, Non-Patent Document 1). Since the steel contains a large amount of Mn, weight reduction for the use member also is remarkable. However, the steel has a requirement for a long time heating process, such as box annealing. Therefore, material design in the continuous annealing of such short heating process suitable for the production of high-strength steel sheets subjected to a member for an automobile has not been studied sufficiently, requirements to improve the elongation properties of the case was not clear.
[0006]
　Patent Document 2, a steel prepared by adding 4.0% of Mn cold rolling, subjected to short-time heating of 300 seconds to 1200 seconds, in area%, controlling the ferrite to 30% to 80% by, it discloses steel sheet elongation is significantly improved. However, the steel sheet described in Patent Document 2 has a high content Mn concentration, not work-hardened so rich in non-recrystallized ferrite, as shown in the prior art in FIG. 1, uniform elongation characteristics are inferior. That is, a high steel sheet containing Mn concentration is not capable of having both the tensile strength and formability required for an automobile steel sheet having a structure comprising such a ferrite.
CITATION
Patent Document
[0007]
Patent Document 1: JP-A-5-59429 JP
Patent Document 2: JP 2012-237054 JP
Non-patent literature
[0008]
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
[0009]
　Therefore, it is desired a high content Mn concentration having excellent uniform elongation characteristics and high strength steel sheet.
Means for Solving the Problems
[0010]
　In a high content Mn concentration steel sheet, in order to ensure a good uniform elongation characteristics and high strength, the present inventors have found that in the steel sheet, in area%, 90% 25% or more of martensite tempering or less, ferrite 3% or less, bainite 5% or less, and the residual austenite be included 75% less than 10% was finding valid.
[0011]
　Steel sheet of the present disclosure has been made based on the above findings, its gist the following.
[0012]
　(1) In the steel sheet of the present disclosure in one embodiment,
　by
　mass%, C: 0.10% greater than
　0.55%, Si: less than 0.001%
　3.50%, Mn: 4.00% greater less than 9.00%, and
　sol. Al: less than 0.001% 3.00%,
　containing,
　P: 0.100% or
　less, S: 0.010% or
　less, N: less than 0.050%, and
　O: less than 0.020% or,
　limited to, the balance being iron and unavoidable impurities,
　the metal structure at the 1/4 position of the thickness from the surface in the L cross-section, in area%, martensite tempering of less than 90% more than 25%, less 3% ferrite, containing 10% or more than 75% of residual austenite, and 5% or less of bainite
　steel sheet, characterized in that.
　(2) metal structure at 1/4 position of the thickness from the surface in the L cross-section, in area%, containing 1.0% or less of the cementite, the steel sheet according to (1).
　(3) metal structure at 1/4 position of the thickness from the surface in the L cross-section, comprises a mixed structure consisting of the residual austenite and fresh martensite,
　the mixed structure, the area ratio to the whole of the metallic structure in accounting for 10% more than 75% or less,
　The residual austenite accounts for 10% to 50% in area ratio to the whole of the metallic structure,
　the angle of the aspect ratio in the mixed structure is not less than 1.5 and a major axis makes with the rolling direction There the tissue is 30 degrees to 60 degrees occupies 10% or more in area ratio to the whole of the mixed structure,
　the steel sheet according to (1) or (2).
　(4) in
　mass%, Cr: less than%
　2.00, Mo: 2.00% or
　less, W: 2.00% or
　less, Cu: 2.00% or
　less, Ni: 2.00% or
　less, Ti: 0 .300% or less,
　Nb: 0.300% or
　less, V: 0.300% or
　less, B: 0.010% or
　less, Ca: 0.010% or
　less, Mg: 0.010% or
　less, Zr: 0.010 % or
　less, REM: 0.010% or
　less, Sb: 0.050% or
　less, Sn: 0.050% or less, and
　Bi: 0.050% or less
　of one or more members selected from the group consisting of further steel sheet according to any one of containing, the (1) to (3).
　(5) in
　mass%, Cr: less than 0.01% or more
　2.00%, Mo: 0.01% to 2.00% or
　less, W: 0.01% to 2.00% or
　less, Cu: 0. 2.00% 0.1% or more or less, and
　Ni: 0.01% to 2.00% or less
　further containing one or more members selected from the group consisting of steel sheet according to (4).
　(6)
　mass%, Ti: 0.005% or more 0.300% or
　less, Nb: 0.005% or more 0.300% or less, and
　V: 0.005% or more 0.300% or less
　from the group consisting of further comprising one or two or more selected, steel sheet according to (4) or (5).
　(7) by
　mass%, B: 0.0001% or more than
　0.010%, Ca: 0.0001% or more than
　0.010%, Mg: 0.0001% or more than
　0.010%, Zr: 0. 0.010% 0001% or more or less, and
　REM: 0.0001% to 0.010% or less
　further containing one or more members selected from the group consisting of, either the above (4) to (6) steel plate of crab described.
　(8) in mass%,
　Sb: 0.0005% or more 0.050% or
　less, Sn: 0.0005% or more 0.050% or less, and
　Bi: 0.0005% or more 0.050% or less
　one or selected from the group consisting of further containing more species, the steel sheet according to any one of (4) to (7).
　(9) has a hot-dip galvanizing layer on the surface of the steel sheet, steel sheet according to any one of (1) to (8).
　(10) on the surface of the steel sheet having a galvannealed layer, steel sheet according to any one of (1) to (8).
　(11) In the manufacturing method in one embodiment of the steel sheet of the present disclosure, the (1) and (4) a steel having the chemical composition according to any one of the - (8), hot-rolled steel sheet subjected to hot rolling to be a
　be a cold-rolled steel sheet is subjected to pickling and the cold rolling to the hot-rolled steel sheet,
　the cold-rolled steel sheet, was heated to 650 ° C. at an average heating rate of 5 ~ 30 ° C. / sec 740 ° C. to hold at least 10 seconds or more temperature range,
　the cooling temperature range from a temperature which was maintained at a temperature range of not lower than the 740 ° C. to 500 ° C. or less, an average cooling rate of 2 ° C. / sec or higher 2000 ° C. / sec or less carried out that, in
　the cooling to room temperature after cooling, and
　after cooling to the room temperature, 600 ° C. or higher Ac 3 , be held in a temperature range of less than point 5 seconds or more
　manufacturing method of steel plate characterized by.
　(12) The 600 ° C. or higher Ac 3 in a temperature range of less than points that hold more than 5 seconds, involves raising the temperature range from 500 ° C. to 600 ° C. at an average 2 ~ 10 ° C. / sec, the production of steel sheet according to (11) Method.
　(13) the average cooling rate is 200 ° C. / sec or higher 2000 ° C. / sec or less, the production method of the steel sheet according to (11) or (12).
　In (14) the average cooling rate from the temperature was maintained at a temperature range of not lower than the 740 ° C., to cool the temperature range up to 100 ° C. or less, the (11) to the steel sheet according to any one of (13) Production method.
　(15) wherein after cooling at an average cooling rate, holding 100 ° C. or higher 500 ° C. 1000 seconds or less 10 seconds or more or less temperature range, the (11) to the production method of the steel sheet according to any one of (14).
　(16) The 600 ° C. or higher Ac 3 was maintained at a temperature range below point 5 seconds or more, the steel sheet is cooled, subjected to a galvanizing treatment, the steel sheet according to any one of (11) - (15) the method of production.
　(17) after performing the galvanizing treatment, subjected to alloying treatment of the hot-dip galvanizing in a temperature range of 450 ° C. or higher 620 ° C. or less, the production method of the steel sheet according to (16).
Effect of the invention
[0013]
　According to the present disclosure, it is possible to provide a highly containing Mn concentration having excellent uniform elongation characteristics and high strength steel sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014]
[1] Figure 1 is a stress of the steel sheet - a strain curves.
FIG. 2 is a scanning electron micrograph of the metallic structure of the steel sheet obtained in Example.
FIG. 3 is an example of the austenitic zones of the band structure.
DESCRIPTION OF THE INVENTION
[0015]
　Hereinafter, an example embodiment of the steel sheet of the present disclosure.
[0016]
　1. Chemical composition
　The chemical composition of the steel sheet of the present disclosure will be described the reason for defining as described above. In the following description, "%" representing the content of each element means in particular a mass percent unless otherwise specified.
[0017]
　(C: 0.10% greater than 0.55%)
　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 0.10 percent. On the other hand, since impairs the weldability of the steel sheet is excessively containing C, the upper limit of C content is set to less than 0.55%.
[0018]
　The lower limit of C content is preferably 0.15% or more, more preferably 0.20% or more. And the lower limit of C content 0.15% or more, further, by controlling the following 70% 35% area ratio of martensite tempering later, pulling without sacrificing uniform elongation properties strength (TS ) is possible to obtain a steel sheet of a high strength of more than 1180 MPa. Upper limit of the C content is preferably 0.40% or less, and more preferably not more than 0.35%, by the upper limit of the C content in the above range, it is possible to further improve the toughness of the steel sheet .
[0019]
　(Si: 0.001% or more and less than
　3.50%) Si is to strengthen martensite tempering, homogenized tissue is an effective element for improving the processability. Further, Si suppresses the precipitation of cementite, also it has effect of promoting residual austenite. To obtain the above effect, it is necessary to Si content of 0.001% or more. On the other hand, since impairs contained excessively to the plating property and chemical conversion treatability of the steel sheet and Si, and the upper limit of the Si content is set to less than 3.50%.
[0020]
　The lower limit of Si content is preferably 0.01% or more, more preferably 0.30% or more, more preferably 0.50% or more. The lower limit of the Si content by the above-mentioned range, it is possible to further improve the uniform elongation characteristics of the steel sheet. Upper limit of the Si content is preferably 3.00% or less, and more preferably not more than 2.50%.
[0021]
　(Mn: 4.00% greater than
　9.00%) Mn is austenite is stabilized, is an element which enhances the hardenability. In the steel sheet of the present disclosure, to distribute the Mn in austenite, to further stabilize the austenite. In order to stabilize the austenite at room temperature, it is necessary 4.00% of Mn. On the other hand, since the steel sheet impair the ductility is excessively containing Mn, the upper limit of the Mn content is set to less than 9.00%.
[0022]
　The lower limit of Mn content is preferably 4.30% or more, more preferably 4.80% or more. Upper limit of the Mn content is preferably 8.00% or less, and more preferably not more than 7.50%. The lower limit and the upper limit of the Mn content by the above-mentioned range, it is possible to further stabilize the austenite.
[0023]
　(Sol.Al:0.001Pasento more than
　3.00%) Al is a deoxidizing agent, it is necessary to contain 0.001% or more. Further, Al is to extend the two-phase temperature region during annealing, also has the effect of increasing the material stability. Al but increases the effect the greater the content of, the excessive content of Al, surface properties, paintability, and so leads to deterioration such as weldability, sol. The upper limit of Al was less than 3.00%.
[0024]
　sol. The lower limit of Al content is preferably 0.005% or more, more preferably 0.01% or more, more preferably 0.02% or more. sol. Upper limit of the Al content is preferably 2.50% or less, and more preferably not more than 1.80%. sol. The lower limit and the upper limit of the Al content by the above-mentioned range, the deoxidizing effect and material stability improving effect, surface properties, the balance between coating properties and weldability becomes better.
[0025]
　(P: 0.100% or less)
　P is an impurity, steel impairs toughness and weldability if excessively containing P. Thus, the 0.100% or less the upper limit of the P content. Upper limit of the P content is preferably 0.050% or less, more preferably 0.030% or less, more preferably 0.020% or less. Since the steel sheet according to the present embodiment does not require the P, the lower limit of the P content is 0.000%. The lower limit of the P content may be 0.000 percent, or less than 0.001% but, the P content is preferably as small.
[0026]
　(S: 0.010% or less)
　S is an impurity, the steel sheet is excessively containing S, MnS which is stretched by hot rolling to produce, leading to formability deterioration such as bendability and hole expansibility. Therefore, the upper limit of the S content to 0.010% or less. The upper limit of the S content is preferably 0.007% or less, more preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require the S, the lower limit of the S content is 0.000%. The lower limit of the S content may be 0.000% or more than 0.001% or more but, the S content is preferably as small.
[0027]
　(N: less than 0.050%)
　N is an impurity, the steel sheet leads to toughness deteriorate containing 0.050% or more of N. Therefore, the upper limit of the N content is less than 0.050%. The upper limit of the N content is preferably 0.010% or less, more preferably 0.006% or less. Since the steel sheet according to the present embodiment does not require N, the lower limit of the N content is 0.000%. The lower limit of the N content may be 0.000% or more than 0.003% or more but, the N content is preferably as small.
[0028]
　(O: less than 0.020% or)
　O is an impurity, causing the ductility of deterioration when the steel sheet contains O of 0.020% or more. Therefore, the upper limit of the O content to less than 0.020% or. O content of the upper limit is preferably 0.010% or less, more preferably 0.005% or less, more preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require the O, the lower limit of the O content is 0.000%. The lower limit of O content may be used as 0.000% or more than 0.001% or more but, the O content is preferably as small.
[0029]
　Steel sheet of the present embodiment further, Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, 1 kind or is selected from the group consisting of Sn and Bi it may contain two or more. However, the steel sheet according to the present embodiment is Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, does not require Sn and Bi, Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, may not contain Sn and Bi, i.e. the lower limit of the content may be 0%.
[0030]
　(Cr: less than%
　2.00) (Mo: 2.00% or
　less) (W: 2.00% or
　less) (Cu: 2.00% or
　less) (Ni: 2.00% or
　less) Cr, Mo, W , Cu, and Ni, respectively, not an essential element steel sheet according to the present embodiment. However, Cr, Mo, W, Cu, and Ni, because it is an element of improving the strength of the steel sheet, may be contained. To obtain the advantage of improved strength of the steel sheet, steel sheet, Cr, Mo, W, Cu, and selected from the group consisting of Ni 1 or two or more elements each may contain more than 0.01% . However, when the steel sheet is excessively containing these elements, surface flaws of the hot rolling it is easily generated, and further, too high strength of the hot rolled steel sheet, cold-rolling property may deteriorate. Therefore, Cr, Mo, W, Cu, and of one or more elements each content selected from the group consisting of Ni, the upper limit of the Cr content less than 2.00%, Mo , W, Cu, and the upper limit of the content of each of Ni and 2.00% or less.
[0031]
　(Ti: 0.300% or
　less) (Nb: 0.300% or
　less) (V: 0.300% or
　less) Ti, Nb, and V is not an essential element in the steel sheet according to the present embodiment. However, Ti, Nb, and V, fine carbide, since an element forming nitrides or carbonitrides, is effective in improving the strength of a steel sheet. Therefore, the steel sheet, Ti, may contain one or more elements selected from the group consisting of Nb, and V. In order to obtain a strength improving effect of the steel sheet, Ti, Nb, and one selected from the group consisting of V or more elements the lower limit of each content to be 0.005% or more preferably . On the other hand, if excessively contained these elements, too strength hot rolled steel sheet rises, cold rolling property may deteriorate. Therefore, Ti, Nb, and one selected from the group consisting of V or more the upper limit value of the elements each content to 0.300% or less.
[0032]
　(B: 0.010% or
　less) (Ca: 0.010% or
　less) (Mg: 0.010% or
　less) (Zr: 0.010% or
　less) (REM: 0.010% or
　less) B, Ca, Mg , Zr, and REM is not an essential element in the steel sheet of the present disclosure. However, B, Ca, Mg, Zr, and REM improves the local ductility and hole expansion of the steel sheet. To obtain this effect, B, Ca, Mg, Zr , and one or more elements of each of the lower limit preferably 0.0001% or more is selected from the group consisting of REM, and more preferably 0 and .001% or more. However, these elements of excess, so degrades the workability of the steel sheet, the upper limit of the content of each of these elements is 0.010% or less, selected B, Ca, Mg, Zr, and from the group consisting of REM it is preferable to one or more of the total content of elements 0.030% or less that.
[0033]
　(Sb: 0.050% or
　less) (Sn: 0.050% or
　less) (Bi: 0.050% or
　less) Sb, Sn, and Bi is not an essential element in the steel sheet of the present disclosure. However, Sb, Sn, and Bi is, Mn in the steel sheet, Si, and / or oxidizable elements such as Al is diffused on the surface of the steel sheet suppresses the formation of a oxide, a surface texture or plating of the steel sheet increase. To obtain this effect, Sb, Sn, and one or the lower limit value of 2 or more elements each content preferably 0.0005% or more is selected from the group consisting of Bi, more preferably 0.001 % or more to be. On the other hand, when the content of each of these elements exceeds 0.050%, the effect thereof is saturated, the upper limit value of the content of each of these elements is 0.050% or less.
[0034]
　2. Metal structure
　will now be described steel sheet metal structure according to the present embodiment.
[0035]
　Metal structure at 1/4 position of the thickness from the surface of the steel sheet according to the present embodiment (also referred to as 1 / 4t part), the area%, 90% 25% or more of the following tempered martensite, 3% of ferrite, 10% or more than 75% of residual austenite, and 5% or less of bainite.
[0036]
　Preferably, the metal structure in the 1 / 4t of the steel sheet, in area%, including 1.0% of cementite.
[0037]
　Preferably, the metal structure in the 1 / 4t of the steel sheet, a tempered martensite as a parent phase, comprises a mixed structure composed of residual austenite and fresh martensite mixed structure is an area ratio relative to the overall metal structure accounting for 10% or more than 75%, residual austenite accounts for 10% to 50% relative to the total metallic structure. That is, the metal structure can comprise 25 to 90% of tempered martensite, 10-50% of residual austenite, and 0 to 65% of fresh martensite.
[0038]
　The fraction of each tissue will vary with the conditions of annealing, gives strength, uniform elongation characteristics, the effect on the material, such as hole expansion. Material is required, for example for changing the parts for automobiles, and select the annealing conditions as needed, it may be controlled tissue fraction within the above range.
[0039]
　The L section of the steel sheet after mirror polishing, 3% nital - corroded (3% nitric acid solution in ethanol), a scanning electron microscope, by observing the microstructure of 1/4 position of the thickness from the surface of the steel sheet, baked martensite, ferrite, cementite, can be measured residual austenite, bainite, and area% of each organization of fresh martensite. For the residual austenite and fresh martensite, firstly, a scanning electron microscope, residual austenite and measuring the area percent of the total fresh martensite, further area% residual austenite by X-ray diffraction method at the thickness 1/4 position to measure. Furthermore, by subtracting the area% residual austenite area% of the total residual austenite and fresh martensite, it calculates the area% of the fresh martensite. The area ratio of residual austenite and fresh martensite include the area ratio of the mixed structure. L section refers to a cut face so as to pass through the central axis parallel to the steel sheet in the thickness direction to the rolling direction.
[0040]
　(Area% of tempered martensite metal structure of 1 / 4t part of the steel sheet: 25-90 area%)
　tempered martensite increases the strength of the steel sheet, a tissue to improve the ductility. Within the scope of the intensity level of interest, in order to maintain preferably both the strength and ductility, and 25-90 area% of the area ratio of martensite tempering. Tempering the lower limit of the content of the martensite preferably 35 area%, more preferably 50% by area. The upper limit of the amount of the martensite tempering is preferably 70 area%. As described above, by the C content to 0.15% or more, further, as described later, by controlling the content of martensite in 35 area% or less than 70 area% tempered, uniform elongation characteristics without compromising the tensile strength (TS) is possible to obtain a steel sheet of a high strength of more than 1180 MPa.
[0041]
　: (Ferrite area ratio of the metal structure of 1 / 4t part of the steel plate 3% or less)
　in the steel sheet according to the present embodiment, it is important that a small amount of ferrite metal structure. So the more ferrite content of metal structure, because the uniform elongation characteristics is remarkably reduced. In order not to decrease significantly the uniform elongation characteristics, the area ratio of ferrite metal structure is 3% or less, more preferably 1% or less, more preferably 0%.
[0042]
　(Area ratio of cementite metal structure of 1 / 4t part of the steel plate 1.0%)
　in the steel sheet according to the present embodiment, preferably, the small amount of cementite metal structure. When reducing the cementite content of metal structure, it improved uniform elongation characteristics, because the still more preferred range is that 15,000 MPa ·% or more tensile strength and the product of the uniform elongation "TS × UEL" is obtained. To improve the uniform elongation characteristics, the area ratio of cementite metal structure preferably 1.0% or less, more preferably 0%.
[0043]
　(Area% of residual austenite of 1 / 4t of the metal structure of the steel sheet: 10% or more than 75%)
　in the steel sheet according to the present embodiment, important that the amount of residual austenite in the metallic structure in is in a predetermined range it is. Residual austenite, the ductility and formability of the steel sheet by transformation-induced plasticity, in particular tissue to enhance the uniform elongation characteristics and hole expansion of the steel sheet. Residual austenite, overhanging working with tensile deformation, drawing, because it can be transformed into martensite by stretch flanging or bending, which contributes to improving the strength of the steel sheet. In order to obtain these effects, the steel sheet according to the present embodiment, in the metal structure, it is necessary to contain the retained austenite 10% or more in area ratio.
[0044]
　The lower limit of the area ratio of residual austenite is preferably 15%, more preferably 18%, more preferably 20%. When the area ratio of residual austenite is 15% or more, the hole expanding property can be further improved. When the area ratio of residual austenite is 18% or more, tensile strength and uniform elongation and the product "TS × UEL" is preferably 13500MPa ·% or more, more preferably 14000MPa ·% or more, more preferably it is 15,000 MPa ·% or more, uniform elongation characteristics so is also maintained at a higher strength.
[0045]
　Area ratio of residual austenite is preferably as high as possible. However, the steel sheet having the chemical components described above, 75% by area ratio is the upper limit of the amount of residual austenite. Be contained in 9.0% of Mn, but the residual austenite can be 75 area percent, in this case, it is impaired ductility and castability of the steel sheet. From the standpoint of hole expansion improved, the area ratio of residual austenite is preferably 50% or less.
[0046]
　(Bainite area ratio of the metal structure of 1 / 4t part of the steel sheet: 5%)
　in the steel sheet according to the present embodiment, the bainite is present in the metal structure, MA constituent is hard tissue during bainite site is inherent. Uniform elongation characteristics are deteriorated when the island martensite is inherent in the bainite. In order not to lower the uniform elongation characteristics, the area ratio of bainite is 5% by area or less, preferably 0% by area.
[0047]
　(1 / 4t part mixed structure area ratio of residual austenite and fresh martensite metal structure of the steel sheet: 10% or more than 75%)
　in the steel sheet according to the present embodiment, preferably, the residual of the metal structure mixed structure of austenite and fresh martensite account for 75% or less than 10% in area ratio to the overall metal microstructure. Mixed structure of residual austenite and fresh martensite by strain induced transformation of retained austenite, becomes substantially one hard fresh martensite. Further, the residual austenite alone or by strain induced transformation, a substantially one hard fresh martensite. That is, the mixed structure, the area ratio of the fresh martensite also means that the residual austenite single structure in the case of 0%. Thus, by controlling the amount and orientation of the tissue in the amount of mixed structure or residual austenite single residual austenite and fresh martensite, hole expansion is improved. Therefore, in terms of hole expandability improvement, not only the amount of residual austenite alone, the amount of mixed structure behaves as substantially one fresh martensite structure after processing-induced transformation is also important.
[0048]
　By the area ratio of the mixed structure of fresh martensite and residual austenite is 10% or more, the area ratio of retained austenite is increased hole expanding property to become 10% or more. The area ratio of the mixed structure by 75% or less, at the time of processing-induced transformation can be suppressed that the voids at the interface of the residual austenite and fresh martensite is generated, it is possible to maintain excellent hole expansion . Therefore, preferably, the area ratio of the mixed structure of retained austenite and fresh martensite and 75% or less than 10%. That is, as long as in the range area ratio of residual austenite of 10-50% and the mixed structure area ratio of in the range of 10 to 75% is possible area ratio of fresh martensite is 0 to 65% it can. Area ratio of mixed structure is preferably 70% to 15% or less, more preferably 65% ​​or less than 20%.
[0049]
　The fresh martensite, a martensite that has not been tempered. Fresh martensite is a hard organization, is effective in ensuring the strength of the steel sheet. However, the smaller the amount of fresh martensite, bending of the steel sheet is increased. On the other hand, often reduce the flow stress during molding in order to reduce the spring back during bending deformation, it is preferable to reduce the yield ratio, better area ratio of fresh martensite is large for that. Therefore, while maintaining the bendability of the steel sheet, in terms of reducing the yield ratio, the metal structure of the steel sheet, an area ratio, preferably from 0 percent, more preferably 1% or more, more preferably 2% or more, further more preferably fresh martensite more than 3%. The upper limit of the amount of the fresh martensite, in order to ensure the bending resistance, preferably 55% by area ratio, more preferably 45%, still more preferably 20%.
[0050]
　Tempered martensite, ferrite, cementite, residual austenite, bainite, and as the remaining structure other than fresh martensite is preferably a bainite tempering. Measurement of the area ratio of bainite tempering can be carried out above the tempered martensite, ferrite, cementite, the residual austenite, bainite, and measurement of the area ratio of the fresh martensite as well as scanning electron microscopy.
[0051]
　In the steel sheet according to the present embodiment, preferably, mixed structure of retained austenite and fresh martensite comprises a tissue having a 1.5 or more aspect ratios. Mixing the measurement of the aspect ratio of the tissue can be performed based on the observation of the microstructure by the scanning electron microscope. Mixed with the aspect ratio of the tissue, it refers to the aspect ratio of the region (tissue) the contrast looks uniform in scanning electron microscopy.
[0052]
　Orientation of the mixed structure having a 1.5 or more aspect ratio greatly affects the hole expansion. If the major axis of the mixed structure has an angle of less than 30 degrees to the rolling direction, since the mixed structure exhibits an oriented structure in the rolling direction, impairing the homogeneity. If the major axis of the mixed structure has a 60 degree angle than to the rolling direction, mixed structure impairs the homogeneity for exhibiting oriented structure in the thickness direction. Therefore, the long axis of the mixing tissue when having an angle of 30 degrees to 60 degrees to the rolling direction, good hole expansion is obtained. Measurements of angles with respect to the rolling direction of the long axis of the mixed structure may be based on observing the microstructure by the scanning electron microscope.
[0053]
　Mixed with the long axis of the tissue, the same as the longitudinal length in the measurement of the aspect ratio of the mixed structure.
[0054]
　Further, if the value aspect ratio is the major axis of 1.5 or more and the area of ​​the mixed structure is 30 degrees to 60 degrees to the rolling direction divided by the area of ​​the total mixed structure is 10% or more, mixed can long axis of the tissue can be inhibited to become oriented structure in the rolling direction, hole expansion can be improved, can be obtained SF value of more than 10% in stretch flange test. Tissue angle aspect ratio makes with it and the major axis is the rolling direction is 1.5 or more is less than 60 degrees 30 degrees, the ratio of the mixed structure is an area ratio of 10% or more, preferably 20% or more it is. Tissue angle and is an aspect ratio of 1.5 or more major axis makes with the rolling direction is 30 degrees to 60 degrees is, measurement of the percentage of the mixed structure is the microstructure by the scanning electron microscopy it can be performed based on the observation.
[0055]
　From microstructural image by scanning electron microscopy, to derive the direction of the aspect ratio and major axis of each mixed structure, specifically, for example, it may be performed as follows. First, to calculate the second moment of area of ​​the mixed structure of interest. Then, to calculate the principal axis of inertia and the main inertia moments of the second moment. The square root of the direction of the principal moments of inertia (inertia main axis of the larger principal moments of inertia) first principal axis of inertia, the aspect ratio for the square root of the principal moments of inertia of the second principal axis of inertia (inertia main axis towards the main moments of inertia is small) Direction the ratio. The first principal axis direction is the long axis direction.
[0056]
　In the steel sheet according to the present embodiment (also referred to as a distance) pitch austenitic zone of the band structure is preferably 12μm or less. By spacing austenite zone is within the range, the steel sheet has a better hole expansion. 3 shows an example of austenitic zone of the band structure. 3, portions indicated by the arrows are austenite zone. Figure 3 is a distribution image of austenite and ferrite in the range of 40μm to 80μm and a sample thickness direction to a sample rolling direction measured by EBSD (Electron Back Scatter Diffraction Patterns). Spacing of austenite band band tissue is the pitch between the arrow and the arrow shown in FIG. In this application, the spacing of the austenite zone band tissue, 80 [mu] m and a distribution image of the range of 40μm measured sample thickness direction, the specimen thickness direction of the sample thickness direction of length 40μm to sample the rolling direction by using the EBSD it can be calculated by dividing the number of peaks in the austenite volume fraction profiles along.
[0057]
　Next, a description will be given mechanical properties of the steel sheet according to the present embodiment.
[0058]
　TS of steel sheet according to the present embodiment is preferably 780MPa or more, more preferably 1000MPa or more, more preferably 1180 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, it is preferable that uniform elongation (UEL) is excellent. In this case, TS × UEL is preferably 12000 MPa ·% or more, more preferably 13500MPa ·% or more, more preferably 14000MPa ·% or more, even more preferably 15,000 MPa ·% or more. Further, in order to provide a steel sheet according to the present embodiment for press molding, it is desirable also excellent hole expansion. Hole expansion can be assessed in SF value, SF value is preferably 10% or more, more preferably 12% or more, more preferably 15% or more.
[0059]
　Steel sheet of the present disclosure as described above, has a high strength, a more uniform elongation characteristics good, is also preferably hole expansion satisfactory, since the excellent moldability, automobiles such as pillars and cross members which is ideal for structural component applications. Moreover, the steel sheet of the present disclosure because of the high content Mn concentration, so also contribute to weight reduction of the automobile, the contribution of the industry is extremely remarkable.
[0060]
　3. Manufacturing method
　Next, a method for manufacturing a steel sheet according to the present embodiment.
[0061]
　Steel sheet according to the present embodiment, a steel having a chemical composition described above was melted in a conventional manner, the casting to produce a slab or steel ingot, by heating the hot-rolled, obtained hot rolled steel sheet after pickling, it produced annealed.
[0062]
　Hot rolling may be performed in a conventional continuous hot rolling line. In the production method of the steel sheet according to the present embodiment, the annealing can be performed in a continuous annealing line, it is excellent in productivity. Satisfies the below conditions, it may be conducted in either annealing furnace and a continuous annealing line. Further, the steel sheet after cold rolling rolling may be performed skin pass rolling.
[0063]
　To obtain a metallic structure of the steel sheet of the present disclosure, heat treatment conditions, the particular annealing conditions, carried out within the range shown below.
[0064]
　As long as the steel sheet according to the present embodiment has a chemical composition described above, the molten steel may be those which are melted in conventional blast furnace process, as steel created in the electric furnace method, a large amount of scrap raw materials it may be intended to include in. 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. Temperature of the steel material to be subjected to hot rolling is preferably set to 1100 ° C. or higher 1300 ° C. or less. By the temperature of the steel material to be subjected to hot rolling to above 1100 ° C., it is possible to further reduce the deformation resistance during hot rolling. On the other hand, by setting the temperature of the steel material to be subjected to hot rolling to 1300 ° C. or less, it is possible to suppress a decrease in yield due to scale loss increases.
[0066]
　Time for maintaining a temperature range of 1100 ° C. or higher 1300 ° C. or less before the hot rolling is not particularly limited, in order to improve the bending resistance is preferably set to 30 minutes or more, further to be more than 1 hour preferable. Further, preferably to less than 10 hours in order to suppress excessive scale loss, further preferably not more than 5 hours. Incidentally, in the case where a hot direct rolling or direct rolling, without performing heat treatment may be supplied directly to hot rolling.
[0067]
　Finish rolling start temperature is preferably 700 ° C. or higher 1000 ° C. or less. Finish rolling start temperature is more preferably 850 ° C. greater, still more preferably 900 ° C. or higher. The finish rolling start temperature by a 700 ° C. or higher, it is possible to reduce the deformation resistance during rolling. The finish rolling start temperature, more preferably 850 ° C. greater, by more preferably above 900 ° C., generation embrittlement organization of ferrite preferentially by generating and hot-rolled steel sheet to high-angle grain boundaries of the martensitic structure since also suppressed, and narrowing the spacing austenitic zone of the band structure, it is possible to improve the hole expanding property. On the other hand, by the finish rolling start temperature to 1000 ° C. or less, it is possible to suppress the deterioration of the surface properties of the steel sheet due to the grain boundary oxidation. The residual austenite has the same effect as the embrittlement tissue is synonymous to the mixed structure and brittle structure of the residual austenite and fresh martensite.
[0068]
　The hot-rolled steel sheet obtained by performing finish rolling is cooled, the winding can be in the coil. The coiling temperature after cooling is preferably set to 700 ° C. or less. By the coiling temperature to 700 ° C. or less, internal oxidation is suppressed, the subsequent pickling becomes easy. Coiling temperature is more preferably 650 ° C. or less, more preferably 600 ° C. or less. In order to suppress the breakage of the cold rolling, after cooling to room temperature, it may be tempered hot rolled sheet below 600 ° C. 300 ° C. or higher before cold rolling.
[0069]
　Hot-rolled steel sheet, after being subjected to pickling in a conventional manner, cold rolling is performed, it is a cold-rolled steel sheet.
[0070]
　When modifying the shape by performing a mild rolling about 0% ultra -5% before or after a by pickling before cold rolling, since it is advantageous in terms of flatness securing preferred. Also, pickling property is improved than performing the mild rolling before pickling, removing surface concentrating element is promoted, the effect of improving the chemical conversion treatability and plating resistance.
[0071]
　Tissues were finer steel sheet after annealing, from the viewpoint of narrowing the distance between austenite band of the band structure, the reduction ratio of cold rolling is preferably 20% or more. From the viewpoint of suppressing breakage during cold rolling, the reduction ratio of cold rolling is preferably 70% or less. Interval of the austenite zone is preferably 12μm or less. By spacing austenite zone to 12μm or less, it is possible to further improve the hole expansion.
[0072]
　Heating the cold-rolled steel sheet obtained through the above-described hot rolling step and cold rolling step, 5 ~ 30 ℃ / sec average heating rate in the temperature was raised to 650 ° C. for 10 seconds at a temperature range of not lower than 740 ° C. held above, then, the temperature range of the temperature was maintained at a temperature range of not lower than 740 ° C. to 500 ° C. or less and cooled at an average cooling rate of 2 ° C. / sec or higher 2000 ° C. / sec or less, after cooling to room temperature, again heated to, 600 ° C. or higher Ac 3 held in a temperature range below point 5 seconds or more. Heat treatment of the cold rolled steel sheet is preferably carried out in a reducing atmosphere, a reducing atmosphere and more preferably containing nitrogen and hydrogen, such as nitrogen 98% and hydrogen 2% reducing atmosphere. By heat treatment in a reducing atmosphere, it is possible to prevent the scale on the surface of the steel sheet is adhered, can be sent directly to the plating process without requiring acid cleaning. Holding 100 ° C. or higher 500 ° C. 1000 seconds or less 10 seconds or more or less temperature range, then cooled to room temperature, then heated again, 600 ° C. or higher Ac 3 is preferably maintained at a temperature range below point 5 seconds or more .
[0073]
　(Annealing conditions after cold rolling 5 ~ 30 ° C. / sec average heating rate at 650 ° C. until a temperature range of more than to 740 ° C. Atsushi Nobori 10 seconds or longer)
　after cold rolling, 5 ~ 30 ℃ / sec It maintains an average heating rate at 650 ° C. until a temperature range of more than to 740 ° C. Atsushi Nobori 10 seconds performing first annealing. By above 740 ° C. The annealing temperature after cold rolling, it is possible to reduce the ferrite content in and the steel sheet can be a distribution of ferrite in the steel sheet after annealing more uniform, uniform elongation characteristics , it is possible to improve the hole expandability, and strength. At that time, by raising the temperature to 650 ° C. at an average heating rate of 5 ~ 30 ° C. / sec, it is possible to further reduce ferrite content of metal structure, 3% or less and the area ratio of ferrite in the metal structure , more preferably 1% or less, it is possible to more preferably to 0%.
[0074]
　Annealing temperature after cold rolling is 740 ° C. or higher and Ac 3 is preferably at least points. The annealing temperature after cold rolling 740 ° C. or higher and Ac 3 by the above points, it is possible to promote the recrystallization significantly, the ferrite content in the steel sheet can be reduced to 0%. Here, results of investigation at a heating rate 0.5 ~ 50 ° C. / sec, Ac 3 the following formula as a
　point: Ac 3 = 910-200√C + 44Si-25Mn + 44Al
　is obtained, Ac using this equation 3 calculates the points can do.
[0075]
　On the other hand, the upper limit of the annealing temperature after cold rolling is preferably 950 ° C. or less. The annealing temperature by a 950 ° C. or less, and suppress damage to the annealing furnace, thereby improving the productivity. Annealing temperature after cold rolling is preferably 800 ° C. or less. By the annealing temperature after cold rolling to 800 ° C. or less, it is possible to further reduce the content of ferrite and cementite metal structure.
[0076]
　The non-recrystallized complete removal, in order to ensure a stable and good toughness, the annealing time of 10 seconds or more, preferably 40 seconds or more. From the viewpoint of productivity, it is preferable to set the annealing time is within 300 seconds.
[0077]
　(Cooling conditions after annealing: Average temperature range 740 ° C. the temperature was maintained at a temperature range of not lower than up to 500 ° C. or less cooling rate 2 ° C. / sec or higher 2000 ° C. / cooling sec or less)
　in the cooling after annealing, 740 ° C. the temperature range up to 500 ° C. or less from the temperature was maintained at above temperature range, cooled at an average cooling rate of 2 ° C. / sec or higher 2000 ° C. / sec or less. The average cooling rate in the temperature range from the temperature held at a temperature range of 740 ° C. or more after annealing up to 500 ° C. or less by a (hereinafter, the average referred to also as the cooling rate after annealing) 2 ° C. / sec or more, the grain boundary it is possible to improve the suppressing bendability segregation, also, it is possible to suppress the formation of ferrite in the cold rolled steel sheet, and narrowing the spacing austenitic zone of the band structure, improve the hole expansion be able to.
[0078]
　Average cooling rate after annealing is preferably 20 ° C. / sec or more, more preferably 50 ° C. / sec or more, more preferably 200 ° C. / sec or more, even more preferably 250 ° C. / sec or more. By the average cooling rate after annealing and 200 ° C. / sec or more, it is cooled in the critical cooling rate or higher, since the entire steel material after the cooling can be tissue martensite mainly control the microstructure after the final heat treatment easily can increase material stability, tensile intensity variations can be reduced, also, the fragile tissue can be improved more even hole expansion so oriented in the austenite grain boundaries.
[0079]
　Average cooling rate after annealing can be used water quenching cooling method or a mist jet cooling method, since it is difficult to be controlled to more than 2000 ° C. / sec, substantial upper limit of the average cooling rate after annealing is 2000 ° C. / made in seconds.
[0080]
　In cooling after the annealing, the cooling stop temperature of the average cooling rate within the above range, preferably 100 ° C. or less, more preferably 80 ° C. or less, more preferably below 50 ° C.. Cooling at an average cooling rate in the above range, by the cooling stop temperature in the above temperature range, because embrittlement tissue preferentially generated high angle grain boundaries of the martensitic structure, it is possible to improve the hole expansion it can.
[0081]
　(After cooling holding conditions below hold 10 seconds 1000 seconds 100 ° C. or higher 500 ° C. or less of the temperature range)
　is preferably, after the cooling after the annealing, 100 ° C. or higher 500 ° C. in 10 seconds or more or less the temperature range 1000 seconds to hold below. With the holding time of 10 seconds or more in the above temperature range, C distribution to austenite progresses sufficiently, the austenite to a final heat treatment before the tissue more can be produced. As a result, that the austenite massive in structure after final heat treatment to produce controlled more effectively, it is possible to further suppress the fluctuation of the strength properties. On the other hand, even in the holding time of 1000 seconds, greater than the effect of the action saturated and productivity is lowered. Holding time at the temperature range is more preferably not less than 30 seconds. From the viewpoint of productivity, the holding time in the temperature range, more preferably not more than 300 seconds.
[0082]
　Holding temperature preferably 100 ° C. or higher in the above temperature range, more preferably by more than 200 ° C., it is possible to improve the efficiency of the continuous annealing line. Meanwhile, preferably the holding temperature by a 500 ° C. or less, it is possible to suppress the grain boundary segregation, thereby improving bendability.
[0083]
　After cooling after the annealing, after preferably held at a temperature range of 100 ° C. or higher 500 ° C. or less, cooling the steel sheet to room temperature.
[0084]
　(Annealing conditions after cooling 600 ° C. or higher Ac 3 in a temperature range of less than point 5 seconds or longer)
　after the annealing cooling, preferably after cooling to room temperature, more preferably at a temperature range 100 ° C. or higher 500 ° C. after holding in a temperature range of 100 ° C. or higher 500 ° C. or less was cooled to or room temperature after cooling to room temperature and held and heated again, 600 ° C. or higher Ac 3 in a temperature range of less than point 5 seconds or longer to. The annealing temperature Ac 600 ° C. or higher 3 by less than points, it is possible to improve the uniform elongation characteristics and hole expansion. Cementite is more reliably dissolved, in order to ensure stable good toughness, an annealing time of 5 seconds or more, preferably 30 seconds or more, more preferably 60 seconds or more. Further, from the viewpoint of productivity, it is preferable to set the annealing time is within 300 seconds. Preferably, 600 ° C. or higher Ac 3 when heated to a temperature range below point, the temperature is raised to a temperature range from 500 ° C. to 600 ° C. at an average heating rate of 2 ~ 10 ° C. / sec. The temperature range of 500 ° C. ~ 600 ° C. By heating at an average heating rate of 2 ~ 10 ° C. / sec, it is possible to reduce the cementite content of metal structure. By annealing the second, the area ratio of cementite metal structure than 1.0%, more preferably, to 0%.
[0085]
　Cooling after the annealing, when 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.
[0086]
　When producing a hot-dip galvanized steel sheet galvanized on the surface of the steel sheet, stops the cooling after the annealing in the temperature range of 430 ~ 500 ° C., then immersed cold-rolled steel sheet in a plating bath of molten zinc performing molten zinc plating treatment and. Conditions of the plating bath may be within the normal range. After the plating treatment may be cooled to room temperature.
[0087]
　In the production of galvannealed steel sheet is subjected to galvannealing on the surface of the steel sheet, after galvanized steel sheet, prior to cooling the steel sheet to room temperature, the 450 ~ 620 ° C. performing alloying treatment of hot-dip galvanizing temperature. Alloying treatment conditions may be within the normal range.
[0088]
　By producing steel sheet as described above, the tensile strength (TS) is preferably 780MPa or more, and more preferably it is possible to obtain a steel sheet of a high strength of at least 1180 MPa. Thus, when using steel sheets as automobile materials, reducing the thickness by a high strength, which can contribute to weight reduction. Furthermore, it is possible to improve the uniform elongation (UEL), the TS × UEL, preferably it is possible to obtain a steel sheet excellent in high strength and uniform elongation properties of above ·% 12000 MPa.
[0089]
　Steel sheet produced by the production method of the present disclosure as described above, has a high strength, more uniform elongation properties are also good, so is excellent in moldability, suitably structural component applications of automobile pillars, etc. it can be used. Further, after the annealing of the cold rolled steel sheet by a cooling stop temperature and 100 ° C. or less, it is possible to obtain a high strength and excellent in addition to the uniform elongation characteristics excellent in hole expansion and the steel sheet, the cross member it can be suitably used for structural parts automotive applications which require stretch flanging the like.
[0090]
　Moreover, the steel sheet of the present disclosure because of the high content Mn concentration, so also contribute to weight reduction of the automobile, the contribution of the industry is extremely remarkable.
Example
[0091]
　The steel sheet of the present disclosure will be described more specifically with reference to examples. However, the following examples are examples of the steel sheet and a manufacturing method thereof of the present disclosure, the steel sheet and a manufacturing method thereof of the present disclosure is not intended to be limited to the aspects of the following examples.
[0092]
　1. Production of evaluation steel sheet
　steel having the chemical components shown in Table 1 were melted in a converter furnace, to obtain a 245mm thick slab by continuous casting.
[0093]
[Table 1]

[0094]
　The resulting slab was hot rolled under the conditions shown in Table 2, were manufactured plate hot-rolled steel sheet of 2.6mm thickness, then pickling the resulting hot-rolled steel sheet, then cold rolling, 1 It was manufactured plate of cold-rolled steel sheet of .2mm thickness.
[0095]
[Table 2-1]

[Table 2-2]

[0096]
　The obtained cold-rolled steel sheet, to prepare annealed cold-rolled steel sheet is subjected to heat treatment conditions shown in Table 3. Heating the cold-rolled steel sheet, and held at 740 ° C. or higher temperature range 10 seconds or more, then, 740 ° C. or higher temperature range of temperature to 500 ° C. or less was maintained in a temperature range of the average cooling rate 2 ° C. / sec It was cooled in not more than 2000 ℃ / sec. Then, in some instances, it was held 100 ° C. or higher 500 ° C. or less at a temperature range 10 seconds to 1000 seconds or less. After cooling to room temperature, heated again, 600 ° C. or higher Ac 3 was maintained at a temperature range below point 5 seconds or more. Heat treatment of the cold rolled steel sheet was performed in a reducing atmosphere of 98% nitrogen and 2% hydrogen.
[0097]
[Table 3-1]

[Table 3-2]

[0098]
　For some annealed cold-rolled steel sheet example, after the final annealing, the cooling after annealing is stopped at 460 ° C., was immersed for 2 seconds in a plating bath of molten zinc of the cold-rolled steel sheet 460 ° C., molten the zinc plating process was carried out. Conditions of the plating bath is the same as that of the conventional. If not subjected to alloying processing described later, after the holding of 460 ° C., and cooled to room temperature at an average cooling rate 10 ° C. / sec.
[0099]
　For some annealed cold-rolled steel sheet example, after the galvanizing treatment was performed without cooling to room temperature, followed by alloying treatment. Was heated to 520 ° C., carried out and held for 5 seconds alloying at 520 ° C., then cooled to room temperature at an average cooling rate 10 ° C. / sec.
[0100]
　Thus obtained annealed cold-rolled steel sheet to temper rolling at a 0.1% rate of elongation and was prepared various evaluations steel plate.
[0101]
　2. Evaluation Method
　for annealing cold-rolled steel sheet obtained in each example, the microstructure observation, a tensile test, uniform elongation test, conducted stretch flange tests, tempered martensite, ferrite, cementite, residual austenite, bainite, tempered bainite , and the area ratio of the fresh martensite, the total area ratio of the residual austenite and fresh martensite, angle aspect ratio and major axis makes with the rolling direction of the mixed structure, tensile strength (TS), uniform elongation characteristics, stretch flangeability (hole expansion), and were evaluated interval of austenitic zone. Method of each evaluation are as follows.
[0102]
　Tempered martensite, ferrite, cementite, residual austenite, bainite, tempered bainite, and the area ratio of the fresh martensite was calculated from a tissue observation and X-ray diffraction measurement using a scanning electron microscope. For L cross section taken parallel to the steel sheet in the thickness direction and the rolling direction, subjected to mirror polishing and then to appear a microstructure with 3% nital, at 5000 magnification of the scanning electron microscope, the surface 1/4 the microstructure was observed at the position, the area of ​​0.1mm by × 0.3 mm range image analysis for the (Photoshop (registered trademark)), tempered martensite, ferrite, cementite, bainite, tempered bainite, and fresh martensite rate, as well as to calculate the total area ratio of the residual austenite and fresh martensite. Moreover, the resulting steel sheet width from 25 mm, was cut out length 25 mm, Atsushi reduced the thickness ¼ subjected to chemical polishing in the specimen, to the surface of the chemical polishing after the specimen , it was performed three times a X-ray diffraction analysis using a Co tube, analyzing the obtained profile, on average, respectively to calculate the area ratio of residual austenite, the total area of ​​the retained austenite and fresh martensite by subtracting the area ratio of residual austenite ratio was calculated area ratio of fresh martensite.
[0103]
　Angle aspect ratio and the long axis of the mixed structure is formed with the rolling direction, as well as the overall mixed structure of the tissue and is an aspect ratio of 1.5 or more long axis is less than 60 degrees angle of more than 30 degrees formed between the rolling direction area ratio was measured using image analysis software ImageJ. First, to obtain a SEM image (24μm × 18μm) was observed at a magnification of 5000 times the microstructure at 1/4 position from the surface using SEM, an SEM image is rolling direction was arranged so that the horizontal. Then, using the ImageJ, to form a 1280 × 960 pieces of divided regions in the SEM image. For each divided area, a mixed structure black, the other regions subjected to the binarization processing such that the white. Binarization threshold, "Glasbey, CA (1993)," An analysis of histogram-based thresholding algorithms ", CVGIP: Graphical Models and Image Processing 55: 532-537," an average value of the luminance value described in It was determined using the method employed as the threshold value. This algorithm is implemented in ImageJ, and automatically binarized by using the Auto threshold The function and determination method of Method = Mean threshold. That is, binarization threshold, Method = Mean in ImageJ, as radius = 15, each pixel value is replaced with the average pixel value within a radius of 15 pixels around the attention to pixels, histogram after smoothing It was automatically determined from. Using the image after binarization processing for each mixed structure which is labeled, the number of pixels (area), by outputting the long axis direction, and the aspect ratio is the aspect ratio is 1.5 or more, major axis was calculated the ratio of the mixed structure angle is 60 ° C. or less than 30 degrees formed between the rolling direction. Note that the area ratio of residual austenite and fresh martensite include the area ratio of the mixed structure.
[0104]
　Spacing of austenite zone, EBSD measuring the distribution image of the range of 40μm to 80μm and a sample thickness direction to the sample the rolling direction with the austenite volume fraction of the sample thickness direction of length 40μm along the sample thickness direction by dividing the number of peaks in the profile, it was calculated.
[0105]
　(Mechanical properties of the test methods)
　to JIS5 No. tensile specimen from a direction perpendicular to the rolling direction of the steel sheet was taken to measure the tensile strength (TS) and the uniform elongation (UEL). Tensile test was conducted by the method specified in JIS-Z2201 using a JIS5 No. tensile specimens. Uniform elongation test was conducted by the method specified in JIS-Z2201 using a JIS5 No. 5 test piece parallel portion length 50 mm.
[0106]
　Stretch flange test, cut a stretch flange test strip of 120 mm × 120 mm, was cut a hole having a diameter of 10mm in the center by machining. Extruding the apertured test piece cylindrical punch, reamed, the test is stopped at the time when the hole edge cracks therein has progressed, and measured the hole diameter d (unit mm). SF (stretch flange test values) are the following
　formula: SF = 100 × (d-10) / 10
　represented by. The mixed structure accounts for 75% or less than 10% in area ratio to the overall metal microstructure, accounting for 50% or less than 10% by area ratio for the entire residual austenite metallographic, in mixed structure in the case where the aspect ratio occupies a major axis and is 1.5 or higher than 10% by area ratio to the whole of the angle formed by the rolling direction is 30 degrees to 60 degrees tissue mixed structure, the band tissue when the pitch of the austenite zone of 12μm greater, 10 can be obtained to 12% of the SF value, when the pitch of the austenite zone is 12μm or less, it is possible to obtain 12% or more of the preferred SF values, the austenite zone can pitch is at below 11 [mu] m, to obtain a more preferable SF value of 15% or more.
[0107]
　3. Evaluation results
　The results of the above evaluation are shown in Table 4. In an embodiment, 12000 MPa ·% or more TS × UEL was obtained. 1, a comparative example of Example No. 3 (prior art), the stress of the steel sheet of the embodiment of Example No. 1 - shows a strain curve. 2, the steel sheet obtained in the Examples Example No. 18 shows a scanning electron micrograph of the metal structure at the 1/4 position of the thickness from the surface in the L section. Portion enclosed by a dotted line is a mixed structure of residual austenite and fresh martensite. The rest of the black part is a martensite tempering. Solid arrow represents the long axis direction of the mixed structure of the rolling direction, and residual austenite and fresh martensite, broken lines, the angle formed by the rolling direction represents a range of 30 degrees to 60 degrees, the solid line double arrow represents the major axis and the length of the minor axis of the mixed structure. The long axis of the mixed structure is represented as an angle formed between the rolling direction theta.
[0108]
[Table 4-1]

WE CLAIM

[Requested item 1]
　By
　mass% C: 0.10% greater than
　0.55% Si: less than 0.001%
　3.50% Mn: 4.00% greater than 9.00%, and
　sol. Al: less than 0.001% 3.00%,
　containing,
　P: 0.100% or
　less, S: 0.010% or
　less, N: less than 0.050%, and
　O: less than 0.020% or,
　limited to, the balance being iron and unavoidable impurities,
　the metal structure at the 1/4 position of the thickness from the surface in the L cross-section, in area%, martensite tempering of less than 90% more than 25%, less 3% ferrite, containing 10% or more than 75% of residual austenite, and 5% or less of bainite
　steel sheet, characterized in that.
[Requested item 2]
　The L metal structure at 1/4 position of the thickness from the surface in cross-section, in area%, containing 1.0% or less of the cementite, the steel sheet according to claim 1.
[Requested item 3]
　Metal structure at 1/4 position of the thickness from the surface in the L cross-section, comprises a mixed structure consisting of the residual austenite and fresh martensite,
　the mixed structure is preferably 10% by area ratio to the whole of the metallic structure or 75 percent or less,
　the residual austenite, the accounts for 50% or less than 10% by area ratio for the entire metal structure,
　the aspect ratio in the mixed structure is 1.5 or more and long axis organization but angle between the rolling direction is 30 degrees to 60 degrees, occupies more than 10% in area ratio to the whole of the mixed structure,
　the steel sheet according to claim 1 or 2.
[Requested item 4]
　By
　mass%, Cr: less than%
　2.00, Mo: 2.00% or
　less, W: 2.00% or
　less, Cu: 2.00% or
　less, Ni: 2.00% or
　less, Ti: 0.300%
　hereinafter, Nb: 0.300% or
　less, V: 0.300% or
　less, B: 0.010% or less,
　Ca: 0.010% or
　less, Mg: 0.010% or
　less, Zr: 0.010% or less,
　REM: 0.010% or
　less, Sb: 0.050% or
　less, Sn: 0.050% or less, and
　Bi: 0.050% or less
　further containing one or more members selected from the group consisting of, steel sheet according to any one of claims 1-3.
[Requested item 5]
　By
　mass%, Cr: less than 0.01% or more
　2.00%, Mo: 0.01% to 2.00% or
　less, W: 0.01% to 2.00% or
　less, Cu: 0.01% or more 2.00% or less, and
　Ni: 0.01% to 2.00% or less
　further containing one or more members selected from the group consisting of steel plate of claim 4.
[Requested item 6]
　By
　mass%, Ti: 0.005% or more 0.300% or
　less, Nb: 0.005% or more 0.300% or less, and
　0.005% or more 0.300% or less: V
　is selected from the group consisting of further comprising one or more kinds, steel sheet according to claim 4 or 5.
[Requested item 7]
　By
　mass%, B: 0.0001% or more than
　0.010%, Ca: 0.0001% or more than
　0.010%, Mg: 0.0001% or more than
　0.010%, Zr: 0.0001% or more 0.010% or less, and
　REM: 0.0001% or more than 0.010%
　, further one or more selected from the group consisting of containing, according to any one of claims 4-6 steel sheet.
[Requested item 8]
　By
　mass%, Sb: 0.0005% or more 0.050% or
　less, Sn: 0.0005% or more 0.050% or less, and
　0.0005% or more 0.050% or less: Bi
　is selected from the group consisting of further comprising one or more, the steel sheet according to any one of claims 4-7.
[Requested item 9]
　Steel sheet according to any one of the on the surface of the steel sheet having a galvanized layer, claims 1-8.
[Requested item 10]
　Steel sheet according to any one of the on the surface of the steel sheet having a galvannealed layer, claims 1-8.
[Requested item 11]
　The steel having the chemical composition according to any one of claims 1 and 4-8, be a hot-rolled steel sheet subjected to hot rolling,
　is subjected to pickling and the cold rolling to the hot-rolled steel sheet be a cold-rolled steel sheet,
　the cold rolled steel sheet, to hold 5 ~ 30 ° C. / sec average heating rate at 650 ° C. until the temperature was raised 740 ° C. or higher at a temperature range 10 seconds or more,
　more than the 740 ° C. the cooling temperature range from a temperature which was maintained at a temperature range up to 500 ° C. or less, be carried out at an average cooling rate 2 ° C. / sec or higher 2000 ° C. / sec or less,
　cooling to room temperature after the cooling, and
　then cooled to the room temperature after, 600 ° C. or higher Ac 3 , be held in a temperature range of less than point 5 seconds or more
　manufacturing method of steel plate characterized by.
[Requested item 12]
　The 600 ° C. or higher Ac 3 be held in a temperature range of less than point 5 seconds or more, comprising heating the temperature range from 500 ° C. to 600 ° C. at an average 2 ~ 10 ° C. / sec, according to claim 11 the method of manufacturing the steel sheet.
[Requested item 13]
　The average cooling rate is 200 ° C. / sec or higher 2000 ° C. / sec or less, the production method of the steel sheet according to claim 11 or 12.
[Requested item 14]
　Wherein the average cooling rate from the temperature was maintained at a temperature range of not lower than the 740 ° C., to cool the temperature range up to 100 ° C. or less, the production method of the steel sheet according to any one of claims 11 to 13.
[Requested item 15]
　After cooling in the average cooling rate, holding 100 ° C. or higher 500 ° C. or less at a temperature range 10 seconds to 1000 seconds or less, the production method of the steel sheet according to any one of claims 11 to 14.
[Requested item 16]
　The 600 ° C. or higher Ac 3 was maintained at a temperature range below point 5 seconds or more, the steel sheet is cooled, subjected to a galvanizing treatment, the production method of the steel sheet according to any one of claims 11-15.
[Requested item 17]
　After having been subjected to the galvanizing treatment, in a temperature range of 450 ° C. or higher 620 ° C. or less subjected to alloying treatment of the galvanized method of steel sheet according to claim 16.