0001]The present invention has excellent containing Mn denser high-strength steel sheet and a manufacturing method thereof of the bending relates.
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]
　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.
[0005]
　However, the steel to contain a large amount of Mn, Mn segregation during solidification becomes apparent. The Mn segregation is noticeable tissue, hard tissue Mn concentrated area to form a band-like tissue Mashimashi ed.
[0006]
　When the band-like tissue formation in a molding with local variations such as bending, easily occur localization deformation, to become a starting point of deformation concentrated portion is cracked, it is known that the moldability is remarkably deteriorated ing.
[0007]
　Therefore, in order to realize a steel containing a large amount of Mn that bending workability is excellent, it is important to reduce the Mn segregation.
[0008]
　For example, Patent Document 2, as shown in the examples, using a steel plate martensite fraction contains 20% or more by heating, cold rolled, to a temperature range steel sheet once or more 750 ° C. After pickling, the band-like tissue Mn being concentrated dispersed in, reducing the thickness of the band-like tissue, by finely dispersed, steel sheet excellent in formability is disclosed.
[0009]
　Incidentally, in the bending of steel plate, the bending takes large tensile stress in the circumferential direction of the outer peripheral surface portion, on the other hand, since the circumferential surface portion bent such a large compressive stress, bending of the ultra-high strength cold rolled steel sheet to, the state of the surface layer portion has a great influence.
[0010]
　Therefore, to improve the surface of the Mn segregation, tensile stress generated on the surface of the steel sheet during the bending, compressive stress and relaxation, we hypothesized to be able to improve the bending property.
[0011]
　Here, as a means of modifying the surface layer is considered the use of clad plate.
[0012]
　Patent Document 3, while reducing the warp, and the base metal steel, stainless steel, a manufacturing method of clad plate consisting of one kind of Ni and Ni alloy is disclosed.
[0013]
　Further, as a means of modifying the surface layer, it has been proposed the use of cold spray method. Patent Document 4, a substrate portion including Mn, cold-rolled steel sheet is disclosed which has a Mn concentration is low deposition layer formed by cold spray on at least one side of the substrate portion (surface layer).
[0014]
　We, based on the above hypothesis, to produce a clad plate having a reduced than the average Mn concentration in the central layer an average Mn concentration in the surface layer, and the clad plate to investigate cold, the bending of the annealed steel sheet.
[0015]
　Further, as described in some embodiments of the patent document 4, the deposited layer of the substrate portion is higher average Mn concentration before forming the (surface layer) hot rolled, cold to hot rolled substrate portion on by spraying to form the average Mn concentration is low deposition layer (surface layer) was investigated cold rolled, the bending of the annealed steel sheet. Further, as described in other examples of Patent Document 4, the average Mn concentration with high cold-rolled sheet as the substrate portion, the average Mn concentration is low deposition layer by cold spray method on cold-rolled sheet (surface layer) to form, it was also examined bending of annealed steel sheet.
[0016]
　However, despite the relaxed surface of the Mn segregation, bendability was revealed that not improved.
CITATION
Patent Document
[0017]
Patent Document 1: JP-A-5-59429 JP
Patent Document 2: JP 2002-88447 JP
Patent Document 3: JP-A 1-192404 Patent Publication
Patent Document 4: JP 2015-193892 JP
Non-patent literature
[0018]
Non-Patent Document 1: Takashi Furukawa, Author Makoto Matsumura, "formation and mechanical properties of the retained austenite in low-carbon steel which has been subjected to a simple heat treatment" ( "heat treatment", Japan heat treatment Association, 1997, No. 37 winding, No. 4), p. 204
Summary of the Invention
Problems that the Invention is to Solve
[0019]
　The present invention advantageously solves the problem of the prior art suffer described above, suitable as a material for automobile parts, and an object thereof is to provide a steel sheet and a manufacturing method thereof comprising a large amount of Mn having bendability.
Means for Solving the Problems
[0020]
　The present inventors have found that in order to solve the problems associated with bending of the ultra-high strength steel plate, of intensive investigation. First, the present inventors despite using clad steel sheet is reduced from the average Mn concentration in the central layer an average Mn concentration in the surface layer was investigated the factors that bendability does not improve.
[0021]
　As a result, in order to increase the ductility of the central layer containing a large amount of Mn, it is necessary to the annealing temperature after cold rolling a low temperature, on the one hand, the annealing at a low temperature, the surface layer recrystallization is sufficiently done without, hard non-recrystallized structure that is the starting point of cracking was found to be a factor in the bendability deteriorate.
[0022]
　The inventors have also despite using the average Mn concentration cold-rolled steel sheet with reduced than the average Mn concentration of the substrate portion of the formed deposition layer (surface layer) by the cold spray method, factors which bending resistance is not improved for were investigated also.
[0023]
　As a result, form a deposited layer of the substrate portion is higher average Mn concentration before forming the (surface layer) hot-rolling, the average Mn concentration is low deposition layer by cold spray method to hot rolled substrate portion on (a surface layer) and, cold rolling, when obtaining the annealed steel sheets, bendability for recrystallization ratio is poor deposition layer (surface layer) was found not to increase.
[0024]
　The average Mn concentration with high cold-rolled sheet as the substrate portion, if the average Mn concentration is low deposition layer by cold spray method on cold-rolled sheet to form a (surface layer) to obtain annealed steel sheets, deposition layer ( to pores occurs during the surface), the bending resistance was found not to increase. Furthermore, since the crystal grain size of the deposited layer (surface layer) is coarse, sufficient bendability revealed also not be secured.
[0025]
　Therefore, the present inventors have carried out a more detailed investigation. As a result, welding the steel plates with a feature on both surfaces of the base material, hot rolling and cold rolling the cold-rolled sheet under specific conditions, by annealing under certain conditions, to retain the ductility of the central layer while, was able to be improved the most bendability.
[0026]
　The mechanism of this effect, together with reduced surface layer of Mn concentration in the multilayered steel plate Mn segregation is suppressed, and by the fully recrystallized, with localized variations of bending surface is suppressed, the surface layer presumably because cracks is suppressed by the improvement in ductility. Furthermore, since the surface of the recrystallized grain size (grain size of ferrite) becomes finer, it is also conceivable that the bending resistance was further improved.
[0027]
　Gist of the present invention obtained in this way is as follows.
(1) and the plate thickness center,
　a high-strength steel sheet including the surface layer softened portion formed on one side or both sides of the plate thickness center,
　the average Mn concentration in the thickness center portion is 4.0 wt% less than ultra-10.0 wt%,
　the surface layer softened portion has a thickness of 30% of 0.1% of the plate thickness,
　or less average Mn concentration of 2.5% by weight of the surface layer softened portion,
　wherein is the surface layer of the recrystallization of the surface layer softened portion is 90% or more,
　the average crystal grain size of the recrystallized structure of the surface layer softened portion is 0.1μm or more 40μm or less
　and wherein the,
　high-strength steel sheet.
(2) the thickness center portion, in
　　mass%, C: 0.05% greater than
　　0.80%, Si: less than 0.001%
　　3.50%, Mn: 4.0% greater than 10.0 less
　　than%, P: 0.10% or
　　less, S: 0.010% or
　　less, sol. Al: less than 0.001% 3.00%, and
　　N: containing less than 0.050%,
　the balance being composed of iron and inevitable impurities, the high strength steel sheet according to (1).
(3) the thickness center portion, further, by
　　mass%, Cr: 0.01% to 2.00% or
　　less, Mo: 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
　, characterized in that it contains at least one selected from the group consisting of high-strength steel sheet according to (2).
(4) the thickness center portion, further, by
　　mass%, Ti: 0.005% or more than
　　0.30%, Nb: 0.005% or more than
　　0.30%, V: 0.005% or more 0 .30% or less, and
　　W: 0.005% or more than 0.30%
　, characterized in that it contains at least one selected from the group consisting of high-strength steel sheet according to (2) or (3) .
(5) the thickness center portion, further, by
　　mass%, B: 0.0001% or more than
　　0.010%, Ca: 0.0001% or more than
　　0.010%, Mg: 0.0001% or more 0 .010% or
　　less, Zr: 0.0001% or more than 0.010%, and
　　REM: 0.0001% or more than 0.010%
　Characterized in that it contains at least one selected from the group consisting of high-strength steel sheet according to any one of the above (2) to (4).
(6) the thickness center portion, further, by
　　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
　, characterized in that it contains at least one selected from the group consisting of high-strength steel sheet according to any one of the above (2) to (5).
(7), wherein the C content of the surface layer softened portion is less than 0.9 times the C content of the plate thickness center, the (2) high according to any one of - (6) strength steel sheet.
Characterized in that (8) the sum of the Cr amount and the Mo content of the surface layer softened portion is less than 0.9 times the total amount of Cr and Mo of the plate thickness center, the (3) - (7 high strength steel sheet according to any one of).
(9) the sum of the surface layer softened portion of the Cu content and the Ni content is equal to or less than 0.9 times the sum of the Cu content and Ni content of the plate thickness center, the (3) to (8 high strength steel sheet according to any one of).
Characterized in that (10) the sum of the Ti content and the Nb content of the surface layer softened portion is less than 0.9 times the sum of the Ti content and the Nb content of the plate thickness center, (4) to (9 high strength steel sheet according to any one of).
(11), wherein the sum of the amount of V and W of the surface layer softened portion is less than 0.9 times the sum of the amount of V and W of the plate thickness center, (4) to (10 high strength steel sheet according to any one of).
(12), wherein the B content of the surface layer softened portion is less than 0.9 times the amount of B the thickness center, the (5) High according to any one of - (11) strength steel sheet.
(13) on the surface of the surface layer softened portion, galvanized layer, characterized in that it comprises galvannealed layer, or an electro-galvanized layer further any one of the above (1) to (12) high-strength steel sheet according to.
(14) In (1) - (13) A method of manufacturing a high strength steel sheet according to any one of
　the surface layer softened portion on one or both surfaces of the base material steel plate constituting the plate thickness center the surface layer softened portion for steel plates formed by laminating to form the multilayered steel,
　heating the multilayered steel sheet heating temperature 1080 ° C. or higher 1300 ° C. or less, and the finish rolling start temperature 800 ° C. 1000 ° C. of less than to hot rolling under conditions,
　a rolled multilayered steel plates between the heat, cooling to 700 ° C. or less 500 ° C. or higher within terminated after 2 seconds of the finish rolling,
　to a temperature of the 500 ° C. or higher 700 ° C. or less after cooling the multilayered steel sheet, to hold more than 3 seconds,
　the pickled and 500 ° C. or higher 700 ° C. or less of the multilayer steel sheet holding temperature at least 3 seconds, then at a reduction ratio of 70% or more and 20% or less the cold rolling,
　between the cold The multilayered steel plates cast, and held 600 ° C. or higher 750 ° C. or less at a temperature of 5 seconds or more, then it, cooling
　, characterized in that it comprises a method for producing a high-strength steel sheet.
(15) wherein characterized in that the winding 500 ° C. or higher 700 ° C. The multilayer steel sheet and held for 3 seconds or more at a temperature lower than at coiling temperature of 600 ° C. or less, the production of high-strength steel sheet according to (14) Method.
(16) the take-up the multi-layer steel sheet, before rolling the cold, and carrying out tempering and held at a temperature of 300 ° C. or higher 550 ° C. or less, high strength according to (15) method of manufacturing a steel plate.
The invention's effect
[0028]
　According to the present invention, excellent and having a strength-ductility balance, it is possible to provide a high strength steel sheet containing Mn concentration having excellent bending properties.
DESCRIPTION OF THE INVENTION
[0029]
　Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.
[0030]
　1. Structure of the surface layer softened portion
　of the structure of the steel sheet of the present invention 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.
[0031]
　Steel sheet of the present invention, the average Mn concentration in the surface layer softened portion having 0.1% of the thickness of 30% thickness is not more than 2.5 mass%, and required recrystallization ratio is 90% or more is there.
[0032]
　(30% of 0.1% or more the thickness of the surface layer softened portion)
　a thickness of the surface layer softened portion can not be obtained a sufficient improvement in bending property is less than 0.1% of the sheet thickness, and tensile greater than 30% deterioration of strength is remarkable. The thickness of the surface layer softened portion is more preferably 20% thickness or less, more preferably 10% or less.
[0033]
　"Superficial softening unit" is determined as follows. First, the current out by nital corrosion the cross-sectional structure of the steel sheet, based on histology obtained from optical microscopy or scanning electron microscopy, and calculates the total thickness of the steel sheet. In the thickness direction of the center of the steel plate, the Vickers hardness in indentation load 100g Weight was measured for five points in the embossing intervals not interfere indentations each other in a direction perpendicular to the thickness direction, the thickness direction and the average value the average Vickers hardness at the center position of the. Then, toward the surface from a thickness direction of the center, the embossing gap at a constant interval of the total thickness 5% of the steel sheet, performing Vickers hardness test of the same five points in each of the thickness direction position. The average Vickers hardness of a certain thickness direction position, when it becomes less than 0.6 times the average Vickers hardness at the center position in the thickness direction is defined as a surface layer softened portion of the surface side from that position. When the 0.6 times the value of the average Vickers hardness at embossing 5% intervals could not define a surface layer softened portion not obtained, between two strokes embossing point surface layer were embossed in the beginning by embossing at any predetermined interval shorter than the interval, to define a surface layer softened portion.
[0034]
　(Average Mn concentration in the surface layer softened portion of 2.5 or less wt%)
　Mn is an element that promotes the formation of the band-like structure. If the average Mn concentration in the surface layer softened portion is at 2.5 wt%, the band-like organization will be formed in the surface layer softened portion, it is impossible to obtain good bending properties. Therefore, the average Mn concentration in the surface layer softened portion is 2.5 wt% or less, preferably 2.0 wt% or less, more preferably 1.5 mass% or less. Since the amount of Mn to small as band-like structure is difficult to form, the lower limit of the average Mn concentration is not particularly defined. Incidentally, the "average Mn concentration in the surface layer softened portion" in the thickness direction center of the tissue cross-section, along the line of the thickness direction perpendicular way, the Mn concentration of the 20 points at 50μm intervals EPMA measurement, the average value obtained from.
[0035]
　(Surface layer more than 90% softened portion of the recrystallization ratio)
　at the surface layer of the recrystallization is unsatisfactory, the non-recrystallized structure is present, non-recrystallized structure is a starting point of cracking during bending deformation for poor ductility. When the recrystallization of the surface layer softened portion is less than 90%, it is impossible to obtain good bending properties, the recrystallization of the surface layer softened portion is 90% or more. Preferably, 95% or more.
[0036]
　(Average crystal grain size of the surface layer softened portion is 0.1μm or 40μm or less)
　the average crystal grain size of the surface layer of the recrystallized structure (ferrite) is coarse, in order to promote uneven deformation during bending deformation, It becomes a factor of bending degradation. Therefore, the average crystal grain size of the surface layer softened portion, and 40μm or less. The average crystal grain size of the surface layer softened portion is more preferably, 30 [mu] m or less, more preferably, 25μm or less. On the other hand, when the average crystal grain size is 0.1μm or less, the ductility of the surface layer softened portion is significantly degraded, the average crystal grain size of the surface layer softened portion, a 0.1μm or more. The average crystal grain size of the surface layer softened portion is more preferably at 0.5μm or more, more preferably 1μm or more.
[0037]
　"Re-crystallization rate" is determined in the following manner. In the recrystallization rate measurement test by SEM / EBSD method, the surface layer softened portion which is defined by the method described above, as the center position in the thickness direction becomes the measurement surface, subjected to mirror polishing and colloidal polishing the surface of the steel sheet, field emission using a scanning electron microscope (FE-SEM) and OIM crystal orientation analysis apparatus to acquire the crystal orientation data group at 0.2μm intervals for measurement plane 100μm square area. The resulting crystal orientation data groups were analyzed by analysis software (TSL OIM Analysis), Kernel Average Misorientation between the first proximity measurement point (KAM value) is defined as the recrystallization texture of the region of 1.0 ° or less, the calculating the area ratio to the total area of ​​the region.
[0038]
　The average crystal grain size of the surface layer softened portion is determined as follows. In the mean crystal grain size measurement test by SEM / EBSD method, the crystal grain boundary by analyzing the crystal orientation data group obtained by the above method with analysis software (TSL OIM Analysis), with a misorientation or misorientation 15 ° the particle size when defined as one crystal grain of a region surrounded by calculated in area Fraction method to calculate the mean particle size of the entire observation area.
[0039]
　2. The chemical composition of the center of plate thickness
　Following describes the chemical composition of the desired thickness center part in order to obtain the effect of the present invention (central layer). Incidentally, "%" for the content of element, in particular means "% by mass" unless otherwise specified.
[0040]
　(C: 0.05% greater than 0.80%)
　C increases the strength of the steel, in order to ensure the retained austenite, is an extremely important element. While retaining sufficient strength, in order to obtain the amount of retained austenite, it is necessary C content of at least 0.05 percent. On the other hand, since impairs the weldability of the steel sheet is excessively containing C, the upper limit of the C content is less than 0.80%. The content of C is preferably in the range of 0.60%, and more preferably in the range of 0.50% or less.
[0041]
　(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 low-temperature toughness of the steel sheet is excessively containing Si, the upper limit of the Si content is less than 3.50%. The lower limit of Si is preferably 0.01%, more preferably 0.30%, more preferably from 0.50%. 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%, more preferably 2.50%.
[0042]
　(Mn: 4.00% greater than
　10.0%) Mn is austenite is stabilized, is an element which enhances the hardenability. In the steel sheet of the present invention, 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 impairs low-temperature toughness steel sheet excessively containing Mn, the upper limit of the Mn content is less than 10.0%. The lower limit of Mn content is preferably 4.30%, more preferably 4.80%. Upper limit of the Mn content is preferably 8.00%, more preferably 7.50%. By the lower limit and the upper limit of the Mn content is in the above range can be obtained more stable ductility. Incidentally, the "average Mn concentration in the thickness center portion" in the thickness direction center of the plate thickness center of the tissue cross-section, along the line of the thickness direction perpendicular way, the Mn concentration of the 20 points at 50μm intervals EPMA measured, calculated from the average value.
[0043]
　(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 stability of the material. 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%. sol. The lower limit of Al content is preferably 0.005%, more preferably 0.01%, more preferably 0.02%. sol. Upper limit of the Al content is preferably 2.50%, more preferably 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, paintability, and balance between the weldability becomes better.
[0044]
　(P: 0.10% or less)
　P is an impurity, steel impairs toughness and weldability if excessively containing P. Therefore, the upper limit of the P content to 0.10% or less. Upper limit of the P content is preferably 0.050%, more preferably 0.030% or, more preferably 0.020% or. 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 P content may be 0.000% or more than 0.001% but, the P content is preferably as small.
[0045]
　(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 Anahiroge properties. 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, more preferably 0.003%. 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 0.0001% but, the S content is preferably as small.
[0046]
　(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%, more preferably 0.006% or. 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 0.0003% but, the N content is preferably as small.
[0047]
　(Cr: 0.01% to 2.00% or less, Mo: 0.01% to 2.00% or less, Cu: 0.01% to 2.00% or less, and Ni: 0.01% or more 2. 100% or
　less) Cr, Mo, Cu, and Ni, respectively, not an essential element steel sheet according to the present embodiment. However, Cr, Mo, 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, Cu, and selected from the group consisting of Ni 1 or two or more elements each may contain 0.01% or more. 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, Cu, and selected from the group consisting of Ni 1 or two or more elements of the upper limit of the content of each of the 2.00% to.
[0048]
　(Ti: 0.005% or more than 0.30%, Nb: 0.005% or more than 0.30%, V: 0.005% or more than 0.30%, and W: 0.005% or more 0. 30% or
　less) Ti, Nb, V, and W is not an essential element in the steel sheet according to the present embodiment. However, Ti, Nb, V and W, fine carbide, since an element forming nitrides or carbonitrides, is effective in improving the strength of a steel sheet. Therefore, the steel sheet, Ti, Nb, may contain one or more elements selected from the group consisting of V, and W. In order to obtain a strength improving effect of the steel sheet, Ti, Nb, the lower limit of one or more elements each content is selected from the group consisting of V and W be a 0.005% preferred . On the other hand, if excessively contained these elements, too strength hot rolled steel sheet rises, cold rolling property may deteriorate. Therefore, Ti, Nb, 1 kind selected from the group consisting of V and W or more the upper limit value of the elements each content to 0.30%.
[0049]
　(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%) B, Ca, Mg, Zr, and REM is not an essential element. 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 preferably 0.0001% of one or more elements each lower limit is selected from the group consisting of REM, and more preferably 0. to 001%. 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% chosen B, Ca, Mg, Zr, and from the group consisting of REM it is preferable to one or more elements the total content of 0.030% or less.
[0050]
　(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) Sb, Sn, and Bi is essential not the elements. However, Sb, Sn, and Bi is, Mn in the steel sheet, Si, and / or easily oxidizable elements such as Al, are diffused on the surface of the steel sheet, suppresses the formation of an oxide, the surface properties of the steel sheet Ya improve the plating properties. To obtain this effect, Sb, Sn, and one or more lower limit elements each content preferably 0.0005% is selected from the group consisting of Bi, more preferably 0.001% to. 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%.
[0051]
　3. The chemical composition of the surface layer softened portion
　steel sheet in the present invention, for the elements other than Mn, sometimes the surface layer softened portion and the center of plate thickness chemical composition is different. In such cases, the preferred chemical composition of the surface layer softening unit is as follows.
[0052]
　(C: 0.9 times or less of the C content of the plate thickness center)
　C is intended to increase the strength of the steel sheet, are added to improve the strength of high-strength steel sheet. C content of the surface layer softened portion is preferably not more than 0.9 times the C content of the thickness center portion. This is to the hardness of the surface layer softened portion lower than the hardness of the center of plate thickness. Larger than 0.9 times, there may not be obtained excellent bendability. More preferably, the C content of the surface layer softened portion than 0.7 times the C content of the thickness center portion, even more preferably 0.5 times or less, and most preferably not more than 0.3 times. For preferred content of C in the center of plate thickness is less than 0.80%, the content of the preferred C of the surface layer softened portion is less than 0.72%. Preferably less than 0.5%, more preferably less than 0.3%, most preferably less than 0.1%. The lower limit of the C content is not particularly defined. When using industrial ultra-low C steel, but the order of 0.001% is a substantial lower limit, from the viewpoint of solid solution C amount was completely eliminated solute C by using a Ti or Nb, it may be used Interstitial Free steel.
[0053]
　(Si: 0.001% or more and less than
　3.50%) Si is a ferrite stabilizing element, since increasing the Ac3 transformation point, it is possible to form a large amount of ferrite at a wide annealing temperature, It is added from the viewpoint of structure control improvement. To achieve such advantages, it is necessary to set the Si content to 0.001% or more. However, the addition of 3.50 percent in order to degrade the surface quality of the steel sheet is less than 3.50%.
[0054]
　(P: 0.10% or less)
　P causes embrittlement of the welded portion. Since exceeds 0.10% becomes remarkable embrittlement of the weld zone has limited its proper range below 0.10%. The lower limit of the content of P is not specified, but be less than 0.001%, it is economically disadvantageous.
[0055]
　(S: 0.010% or less)
　S is weldability and during casting and adversely affect the productivity of the hot rolling. Therefore, to the upper limit 0.010% or less. The lower limit of the content of S is not specified, but be less than 0.0001%, it is economically disadvantageous.
[0056]
　(Sol.Al:0.001Pasento more than
　3.00%) Al acts as a deoxidizer, it is preferably added in deoxidation process. In order to obtain such an effect, sol. The Al content must be 0.001% or more. On the other hand, sol. When Al content is 3.00% or more, the risk of slab cracking during continuous casting is increased, and less than 3.00%.
[0057]
　(N: 0.050% or less)
　N forms coarse nitrides, from degrading the bendability, it is necessary to suppress the addition amount. This is because when N exceeds 0.050% Since this tendency becomes remarkable, and the range of the N content is 0.050% or less. In addition, N is the, better less since it causes blowhole occurrence during welding. The lower limit of the N content, the effect of the present invention is exhibited without being specifically defined by the content of N to less than 0.0005% leads to a significant increase in manufacturing cost.
[0058]
　Surface softened portion is preferably, by
　　mass%, C: less than
　　0.72%, Si: 0.001% or more and less than
　　3.50%, Mn: 2.5% or less
　　P: 0.10% or
　　less, S: 0 .010% or
　　less, Sol. Al: less than 0.001% 3.00%, and
　　N: containing less than 0.050%,
　with the balance consisting of iron and unavoidable impurities. Surface softening unit may further comprise the following ingredients.
[0059]
　(Cr: 0.01% to 2.00% or less, Mo: 0.01% to 2.00% or less, Cu: 0.01% to 2.00% or less, and Ni: 0.01% or more 2. 100% or
　less) Cr, Mo, Cu, and Ni, because it is an element of improving the strength of the steel sheet, may be contained. Steel sheet, Cr, Mo, Cu, and selected from the group consisting of Ni 1 or two or more elements each may contain 0.01% or more. However, when the steel sheet is excessively containing these elements, too high strength of the steel sheet, there are cases where surface defects during rolling is easily generated. Therefore, Cr, Mo, Cu, and selected from the group consisting of Ni 1 or two or more elements of the upper limit of the content of each of the 2.00% to.
[0060]
　Further, it is preferable that the total amount of Cr and Mo of the surface layer softened portion is less 0.9 times the total amount of Cr and Mo of thickness center portion. Total amount of Cr and Mo amounts to stabilize the carbides, the thickness center portion of the Cr amount and amount of Mo large, coarse carbides is likely to remain than 0.9 times, causing deterioration of the surface properties. More preferably, 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times.
[0061]
　A sum of Cu content and Ni content of the surface layer softened portion is preferably not more than 0.9 times the sum of the Cu content and Ni content of the thickness center portion. Cu amount to improve the hardenability, and the Ni content of the sum is greater than 0.9 times the Cu content and Ni content of the thickness center portion, the low-temperature transformation structure is likely to occur, causing bendability deteriorate. More preferably 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times.
[0062]
　(Ti: 0.005% or more than 0.30%, Nb: 0.005% or more than 0.30%, V: 0.005% or more than 0.30%, and W: 0.005% or more 0. 30% or less)
　Ti, Nb, V, and W, fine carbide, since an element forming nitrides or carbonitrides, is effective in improving the strength of a steel sheet. Therefore, the steel sheet, Ti, Nb, may contain one or more elements selected from the group consisting of V, and W. In order to obtain a strength improving effect of the steel sheet, Ti, Nb, V, and one selected from the group consisting of W or more elements the lower limit of each content be 0.005% preferable. On the other hand, if excessively contained these elements, too strength hot rolled steel sheet increases, there is a possibility that cracking during cold rolling. Therefore, Ti, Nb, V, and one or more elements of the upper limit of the content of each of the 0.30% is selected from the group consisting of W to.
[0063]
　A sum of Ti content and Nb content of the surface layer softened portion is preferably not more than 0.9 times the sum of the Ti content and the Nb content of the thickness center portion. Total amount of Ti and Nb amount is larger than 0.9 times the Ti amount and Nb amount of the center of plate thickness surface layer tends to harden, causing bendability deteriorate. More preferably, 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times.
[0064]
　A sum of W amount and V amount of surface softening portion is preferably not more than 0.9 times the sum of the W amount and V amount of the center of plate thickness. Since W and V are formed easily elemental carbides sum of W amount and V amount is, the larger than 0.9 times the W amount and V amount of thickness center portion, coarse carbides are formed in the surface layer becomes a factor of bendability deteriorate. More preferably, 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times.
[0065]
　(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%) 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 preferably 0.0001% of one or more elements each lower limit is selected from the group consisting of REM, and more preferably 0. to 001%. On the other hand, B, Ca, Mg, Zr, and REM amount is 0.9 times larger than the surface layer of the thickness center portion is excessively hardened, which may deteriorate the surface properties. Accordingly, B, Ca, Mg, Zr, and one or the upper limit of 2 or more elements 0.009% is selected from the group consisting of REM, preferably 0.006% or less.
[0066]
　Further, the B content of the surface layer softened portion is preferably not more than 0.9 times the amount of B the thickness center. Since B is formed easily element the low temperature transformation phase becomes large and the surface layer is harder than 0.9 times the amount of B thickness center portion, causing not obtained excellent bending properties. More preferably 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times. Each lower limit is not specified.
[0067]
　4. Organization of the steel sheet
　will be described steel sheet of tissue according to the present embodiment.
[0068]
　Thickness center portion of the tissue, preferably, ferrites, have a structure consisting of martensite or bainite, and residual austenite, and more preferably, ferrite, martensite, or bainite tempering, quenching martensite, and residual austenite and it has a structure consisting.
[0069]
　Surface softening of the tissue preferably has the tissue consists essentially of ferrite.
[0070]
　Surface softening unit have less porosity than 1% preferably at an area ratio, and more preferably substantially 0%.
[0071]
　5. Mechanical properties of the steel sheet
　will be described mechanical properties of the steel sheet according to the present embodiment.
[0072]
　Tensile strength of the steel sheet according to the present embodiment is preferably 780MPa 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. TS × UEL is preferably 12000 MPa ·% or more, more preferably 14000MPa ·% or more.
[0073]
　With respect to the bending resistance, the strength with steels of less than 780 MPa 1180 MPa,
a direction perpendicular to the rolling direction, the V bending test according to JIS Z2248, as (bending ridgeline coincides with the rolling direction) longitudinal becomes when performing, it is preferable to limit the bending radius R is 1.0mm or less, and more preferably a 0.8mm or less. The strength of 1180MPa or more types of steel, the V bending limit testing bending radius, preferably made of a 2.0mm or less, and more preferably a 1.5mm or less.
[0074]
　6. Manufacturing method
　Next, a method for manufacturing a steel sheet according to the present embodiment. The following description is for modifications thereof are merely illustrative of the process for obtaining a high-strength steel sheet of the present invention, multi-layer steel sheet with high strength steel plate, were laminated two steel plates as described below of the present invention it is not intended to be limited to. For example, in place of the lamination process described below, it may be formed a surface layer softened portion by the cold spray method onto the base steel sheet.
[0075]
　Steel sheet according to the present embodiment, on one side or both sides of the base material steel plate which constitutes the center of plate thickness having the above chemical composition, the average Mn concentration of 2.5 wt% or less, and a steel sheet for the outer layer softened portion laminated forming a multilayered steel Te, a multilayer steel sheet with a welded around, then hot rolled, after rolling, after the post-holding immediately cooled at a high temperature, the cooled hot rolled steel sheet was pickled, cold rolled and be prepared annealed.
[0076]
　(Formation of multilayered steel:. On one side or both sides of the base material steel plate was degreased surface constituting the thickness center portion, a steel sheet satisfying the chemical composition of the surface layer softened portion are laminated to weld around)
　above in the base steel sheet satisfying the chemical composition of the center of plate thickness, by laminating steel sheets satisfying the chemical composition of the surface layer softened portion to the surface to form a multilayered steel by welding the periphery. These steel sheets, if they meet the above chemical composition, may be one produced by any production method.
[0077]
　(Heating temperature of the multilayered steel: 1080 ° C. or higher 1300 ° C. or less)
　at the heating temperature before hot rolling is 1080 below ° C., the hot deformation resistance during working becomes high, operation becomes difficult. On the other hand, if the heating temperature is 1300 ° C. greater than the scale loss, the yield is lowered. Therefore, the heating temperature is set to 1300 ° C. or less 1080 ° C. or higher. Time for maintaining a temperature range of 1080 ° C. or higher 1300 ° C. or less prior hot rolling is not particularly limited, in order to improve the hole expansion is preferably 30 minutes or more, be one hour or more A further preferred. In order to suppress the excessive scale loss, preferably to less than 10 hours, more preferably not more than 5 hours. Incidentally, in the case of performing the hot direct rolling or direct rolling may be subjected to hot rolling while maintaining the above temperature range. In this specification, the temperature is the temperature measured at the central position of the steel sheet surface.
[0078]
　(Finish rolling start temperature: 800 ° C. or higher 1000 ° C. or less)
　finish rolling start temperature is preferably 800 ° C. or higher 1000 ° C. or less. The finish rolling start temperature by a 800 ° C. or higher, it is possible to reduce the deformation resistance during rolling. 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.
[0079]
　(After rolling cooling cooled to 500 ° C. or higher 700 ° C. or less within 2 seconds)
　within 2 seconds after completion of finish rolling, cooled to 500 ° C. or higher 700 ° C. or less. This is an important condition in the present invention, the fine old γ grains of the surface layer softened portion, and generating uniformly fine ferrite grains produced during cooling, in a subsequent annealing step, thoroughly surface softened portion it is possible to re-crystallization.
[0080]
　After the completion of the finish rolling, the time until cooling to 500 ° C. or higher 700 ° C. or less than 2 seconds, prior austenite grain size becomes coarse, the annealing process after no surface layer softened portion is sufficiently recrystallized . Therefore, after the finish rolling, the time until cooling to 500 ° C. or higher 700 ° C. or less and within 2 seconds. Preferably within 1.8 seconds, more preferably within 1.5 seconds. The shorter the time until cooling is short, the old γ grain size is fine reduction, it becomes easy to recrystallization, but the lower limit is provided, 0.1 seconds constraint of the production process becomes substantially the lower.
[0081]
　If the cooling rate is satisfied the above conditions, it may be any speed, but as the cooling rate is fast, easy fine effect of the old γ grain size is obtained. Therefore, it is preferable that the cooling rate is 20 ° C. / s or higher, further preferably 50 ° C. / s or higher.
[0082]
　If the cooling stop temperature after rolling is less than 500 ° C., a part of the surface layer softened portion becomes the low temperature transformation structure. When having a plurality of structure of ferrite and low temperature transformation structure, since the uneven deformation during cold rolling is introduced, does not occur uniformly recrystallization, non-recrystallized structure is likely to remain. The cooling stop temperature is delayed ferrite transformation in the surface layer softened portion at 700 ° C. or higher, it is impossible to accumulate sufficient strain in the surface layer softened portion by cold rolling step after. Thus, the cooling stop temperature is set to 500 ° C. or higher 700 ° C. or less.
[0083]
　(500 ° C. or higher 700 ° C. or less of the holding time after cooling to temperature of more than 3 seconds)
　when 500 retention time at ° C. or higher 700 ° C. or less of the temperature is less than 3 seconds, no ferrite surface layer softened portion is sufficiently generated . Preferably, the retention time is at least 5 seconds, more preferably not less than 10 seconds.
[0084]
　(Coiling temperature: 600 ° C. or less)
　preferably performs winding at coiling temperature of 600 ° C. or less. By performing winding the coiling temperature as 600 ° C. or less, it becomes easy low temperature transformation phase is formed in the thickness center portion, since the strain amount of distribution of the surface layer in the cold rolling step after the coiling is increased, the surface softening part recrystallization easily, and tends to further fine the crystal grain size. Further, by setting the coiling temperature and 600 ° C. or less, in pickling after coiling, it is easier to remove scale. Coiling temperature is more preferably 500 ° C. or less, still more preferably 400 ° C. or less.
[0085]
　In order to suppress the breakage of the cold rolling, after cooling to room temperature, it may be tempered hot-rolled sheet at 300 ° C. or higher 600 ° C. or less.
[0086]
　(Reduction ratio of cold rolling: 20% to 70%)
　hot-rolled steel sheet is subjected to a pickling in a conventional manner, cold rolling is performed. When the reduction ratio of cold rolling is less than 20%, sufficient strain can not be introduced into the surface layer softened portion, in a later annealing step, not the surface layer softened portion is sufficiently recrystallized. On the other hand, when the reduction ratio of cold rolling is 70 percent, since there are cases where the steel sheet is broken during rolling, the reduction ratio of cold rolling is 70% or less than 20%.
[0087]
　(Annealing heat treatment after cold rolling: After holding rolled multilayered steel plates 600 ° C. or higher 750 ° C. 5 seconds or more at a temperature below room temperature until cool)
　a multilayer steel sheet cold rolled, 600 ° C. or higher 750 ° C. carry out the annealing by heating to a temperature of less than or equal to. When the heating retention temperature is lower than 600 ° C., on the surface softened portion is not sufficiently recrystallized cementite thickness center portion is not sufficiently dissolved, not stable residual γ fraction obtained. When it is 750 ° C. greater, it is difficult to produce a ferrite thickness center section. Therefore, heating and holding temperature is set to 600 ° C. or higher 750 ° C. or less.
[0088]
　If the holding time is less than 5 seconds, no non-recrystallized structure of the surface layer softened portion is sufficiently recrystallized. To completely remove the non-recrystallized structure, it is preferable that the annealing time is 10 seconds or more, more preferably 15 seconds or more. From the viewpoint of productivity, the annealing time is preferably set to less 3600 seconds.
[0089]
　Cooling stop temperature after the heating and maintaining, at the center of plate thickness, in order to produce a low temperature transformation structure, preferably at 550 ° C. or less, more preferably 300 ° C. or less, and most preferably at 100 ° C. or less .
[0090]
　After the cooling, to stabilize the softening and residual austenite low-temperature transformation structure may be tempered at a temperature of 300 ° C. or higher 550 ° C. or less.
[0091]
　When producing a hot-dip galvanized steel sheet galvanized on the surface of the steel sheet, and stops in the temperature range of cooling to 430 ~ 500 ° C. after annealing at above 600 ° C. or higher 750 ° C. or less of the temperature, and then cold performing molten zinc plating treatment by immersing a rolled steel sheet in a plating bath of molten zinc. Conditions of the plating bath may be within the normal range. After the plating treatment may be cooled to room temperature.
[0092]
　Subjected to galvannealing the surface of the steel sheet, when manufacturing a galvannealed steel sheet, after galvanized steel sheet, prior to cooling the steel sheet to room temperature, 450 steel - at a temperature of 620 ° C. performing alloying treatment of galvanizing. Alloying treatment conditions may be within the normal range.
[0093]
　By producing steel sheet as described above, it is possible to obtain a steel sheet according to the present embodiment.
Example
[0094]
　The steel sheet of the present invention will be described more specifically with reference to examples. However, the following examples are examples of the steel sheet of the present invention, the steel sheet of the present invention is not intended to be limited to the aspects of the following examples.
[0095]
　1. Preparation of the evaluation steel
　sample preparation method of the multi-layer steel sheet according to Table 2 is a cladding method was prepared according to the following method. For continuously cast slab having a thickness of 20mm with the chemical compositions shown in Table 1 (steel sheet thickness center portion), after removing the surface oxide by grinding the surface on one or both sides, the chemical compositions shown in Table 1 It layered in arc welding the surface layer steel plate (surface layer softened portion) having a. Heating temperature shown in table 2, holding time before hot rolling, finish rolling start temperature, cooling completion time, the cooling stop temperature, holding time after cooling, and at a coiling temperature, heating, after heating and holding, the hot rolling , cooling, cooling after holding, and to obtain a laminated hot-rolled steel sheet by performing the winding in the coil. Thereafter, usual manner pickled, tempering temperature shown in Table 2, cold rolling rate, annealing temperature, and annealing time, perform tempering, cold rolling, and annealing, and cooled to room temperature.
[0096]
　Sample preparation method of the multi-layer steel sheet according to Table 2 is a cold spray was prepared according to the following method.
[0097]
　When the substrate and the hot rolled sheet, the sheet thickness 20mm continuous casting slab having the chemical compositions shown in Table 1 (steel sheet thickness center section), the heating temperature, holding time before hot rolling are shown in Table 2 , finish rolling start temperature, cooling completion time, the cooling stop temperature, holding time after cooling, and at a coiling temperature, heating, after heating and holding, hot rolling, cooling, performed after cooling hold, and winding the coil, to prepare a hot rolled sheet, after removing the surface oxide by grinding the surface to produce a formed steel sheet deposition layer (surface layer) on the surface layer in the cold spray method to one or both sides. Thereafter, tempering temperature shown in Table 2, cold rolling rate, annealing temperature, and annealing time, perform tempering, cold rolling, and annealing, and cooled to room temperature.
[0098]
　On the other hand, when the substrate and cold rolled sheet, the hot-rolled plate produced by the above method, by a conventional method and pickling, performed tempering and cold rolling at a tempering temperature and rolling reduction are shown in Table 2 to prepare a cold-rolled sheet, then at least one surface deposition layer of cold-rolled sheet (surface layer) was formed using a cold spray method, performed annealing at the annealing temperature and annealing time are shown in Table 2, was cooled to room temperature.
[0099]
　Iron-based particles used in the cold spray method used was a component and particle size shown in Table 4. Particles used in the cold spray method, in accordance with the prior art, ground, repeatedly classification with a sieve was used after adjusted so as to have a predetermined particle diameter. Further, nitrogen gas was used as the working gas. The working gas in the heater was heated to 700 ° C., and mixed by supplying the iron-based particles from the particle feeder, a spray nozzle, spraying the substrate, to obtain a multi-layer steel sheet. Note that the working gas pressure was 3MPa constant. The scanning speed of the nozzle was adjusted by the machine control.
[0100]
[Table 1-1]

[Table 1-2]

[0101]
[Table 2-1]

[Table 2-2]

[0102]
　For some annealed cold-rolled steel sheet, after final annealing, the cooling after annealing is stopped at 460 ° C., and immersed for 2 hours in a plating bath of molten zinc of the cold-rolled steel sheet 460 ° C., molten zinc a plating treatment 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.
[0103]
　For some annealed cold-rolled steel sheet, after the galvanizing treatment was performed without cooling to room temperature, followed by alloying treatment. It 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.
[0104]
　Thus obtained annealed cold-rolled steel sheet to temper rolling at a 0.1% rate of elongation and was prepared various evaluations steel plate.
[0105]
　2. Evaluation method
　The obtained annealed cold-rolled steel sheet, thickness measurement, structure observation, porosity measurements of the surface layer softened portion, Vickers hardness test, the recrystallization rate measurement test by SEM / EBSD method, the average crystal grain size of the surface layer softened portion measuring, tensile test was carried out uniform elongation test, and V bending test.
[0106]
　Observation method of tissue thickness center portion was performed as follows. Subjected to mirror polishing and colloidal polishing the surface of the steel sheet as the center of plate thickness becomes the measurement surface, using a field emission scanning electron microscope (FE-SEM) and OIM crystal orientation analysis device, the measuring surface 100μm square area 0 It was obtained the crystal orientation data groups in .2μm intervals. The resulting crystal orientation data groups were analyzed by analysis software (TSL OIM Analysis), were classified tissue. In Phase-MAP, and determines the discriminated region and austenite phase and residual austenite. In Phase-MAP, martensite tempering non austenite phase, bainite, quenching martensite, and the ferrite phase and the discrimination region of tissue containing the ferrite, based on the secondary electron image observed at 3000-fold magnification It was determined as further described below. In the ferrite phase, among those with lower tissue in grains, tissue including cementite therein is determined and the martensite or bainite tempering. In the ferrite phase, among those with lower tissue in grains, tissue free of cementite therein was determined to martensite while quenching. In the ferrite phase, to a region that does not include the infrastructure in the grains it is determined with ferrite.
[0107]
　Perform structure observation by the above method, the center of plate thickness of the tissue, ferrite, were classified martensite or bainite tempering, quenching martensite, and residual austenite.
[0108]
　The structural observation of the surface layer softened portion, except that the surface layer softened portion is set to be the measurement surface, were performed in the same manner as the tissue observation of the center of plate thickness. Surface softening of the tissue was substantially ferrite.
[0109]
　The porosity of the surface layer softened portion, identified pore portion of the polishing surface, was calculated as an area ratio by image processing. The porosity of the surface layer softened section was observed by 1000-fold magnification scanning electron microscope, by image processing diameter by detecting the above pore 0.01 [mu] m, and calculating the area ratio of the sum.
[0110]
　Vickers hardness test, as described above, performed to define the surface softened portion. First, the current out by nital corrosion the cross-sectional structure of the steel sheet, based on histology obtained from optical microscopy or scanning electron microscopy, and calculates the total thickness of the steel sheet. In the thickness direction of the center of the steel plate, the Vickers hardness in indentation load 100g Weight was measured for five points in the embossing intervals not interfere indentations each other in a direction perpendicular to the thickness direction, the thickness direction and the average value and the average Vickers hardness at the center position of the. Then, toward the surface from a thickness direction of the center, the embossing gap at a constant interval of the total thickness 5% of the steel sheet was subjected to Vickers hardness test of the same five points in each of the thickness direction position. The average Vickers hardness of a certain thickness direction position, when it becomes less than 0.6 times the average Vickers hardness at the center position in the thickness direction was defined as the surface layer softened portion of the surface side from that position. When the 0.6 times the value of the average Vickers hardness at embossing 5% intervals could not define a surface layer softened portion not obtained, between two strokes embossing point surface layer were embossed in the beginning by embossing at any predetermined interval shorter than the interval defined the surface softened portion. The ratio of the thickness of the surface layer softened portion for steel for plate thickness of the center of plate thickness for steel plates are shown in Table 3, "the ratio of the surface layer softened portion (one side) (%)".
[0111]
　SEM / recrystallization rate measurement test by the EBSD method, the center position of the surface layer softened portion which is defined by the method described above, subjected to mirror polishing and colloidal polishing the surface of the steel sheet so that the measurement surface, the field emission scanning electron microscope ( using FE-SEM) and OIM crystal orientation analysis device and the measuring surface 100μm square area to get the crystal orientation data group at 0.2μm intervals. The resulting crystal orientation data groups were analyzed by analysis software (TSL OIM Analysis), Kernel Average Misorientation between the first proximity measurement point (KAM value) is defined as the recrystallization texture of the region of 1.0 ° or less, the and calculating the area ratio to the total area of ​​the region.
[0112]
　The average crystal grain size of the surface layer softened portion was measured as follows. In the mean crystal grain size measurement test by SEM / EBSD method, the crystal grain boundary by analyzing the crystal orientation data group obtained by the above method with analysis software (TSL OIM Analysis), with a misorientation or misorientation 15 ° the particle size when defined as one crystal grain of a region surrounded by calculated in area Fraction method to calculate the average particle size of the entire observation area.
[0113]
　To the rolling direction perpendicular to the direction of the steel plate taking a long axis were taken JIS5 No. No. 5 test pieces were measured tensile strength (TS) and the uniform elongation (UEL). Tensile test was conducted by the method specified in JIS Z 2241 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.
[0114]
　Critical bending radius R creates a No. 1 test piece described in JIS Z2204 as the direction perpendicular the longitudinal direction (bending ridge is coincident with the rolling direction) to the rolling direction, V bending according to JIS Z2248 test was carried out. For samples with surface softening unit only on one side, bent so as to outwardly bend surface having a surface softening unit. The angle of the die and punch and 60 °, perform bending test by changing the tip radius of the punch in 0.1mm increments, cracks were determined as a limit bending radius R of the punch tip radius can be bent without causing. The intensity with grades of less than 780 MPa 1180 MPa, critical bending radius R is 1.0mm greater than the bending resistance not intended (sign ×), following the bending of good ones 1.0mm (sign ○), following 0.8mm things was the bending of Yu (◎). The strength of 1180MPa or more grades, critical bending radius R bendability not intended 2.0mm greater (sign ×), 2.0mm bendability good following ones (code ○), 1.5 mm or less of those It was evaluated as the bending of Yu (◎).
[0115]
　The obtained steel sheet was measured and the chemical composition of the thickness direction of the center position of the surface layer softened portion as defined above, the chemical composition of the central position in the thickness direction, the steel sheet surface layer softened portion shown in Table 1, respectively and it was similar to the chemical composition of the base material steel plate.
[0116]
　The average Mn concentration in the thickness direction center of each of the center of plate thickness and surface softening of the tissue section, along the line of the thickness direction perpendicular way, the Mn concentration of the 20 points at 50μm intervals EPMA measurement, that it was determined from the average value. As a result, the average Mn concentration of the center of plate thickness, and the average Mn concentration in the surface layer softened portion, respectively the base steel sheet shown in Table 1, and was approximately the same as the Mn concentration in the surface layer softened portion for steel plates.
[0117]
　3. Evaluation results
　The results of the evaluation are shown in Table 3.
[0118]
[Table 3-1]

[Table 3-2]

[0119]
[Table 4]

[0120]
　Numerical values ​​underlined in Tables 1 to 3 show that the content indicated by that number, condition or out of range mechanical properties are desired.
[0121]
　Steel sheet examples in Tables 2 and 3, the average Mn concentration in the center of plate thickness is less than 4.0 mass percent and 10.0 mass%, 30% of the thickness from 0.1 percent of the surface layer softened portion is steel having of the average Mn concentration is 2.5% or less, wherein it is the surface softening of the recrystallization ratio is 90%, that has excellent TS × UEL balance, characterized with high content Mn concentration steel while having a has excellent bendability.
[0122]
　On the other hand, subjected in Tables 2 and 3 試材 No. 2,3,6,16,18,22,24,46,49,50, and 52, recrystallization of the surface layer softened portion, deviates from the range defined in the present invention, to obtain a superior bendability It is not.
[0123]
　Test material No. 14 has a lower average Mn concentration in the thickness center portion, not obtained excellent TS × UEL balance.
[0124]
　Test material No. 32, the annealing temperature is high, not obtained excellent TS × UEL balance.
[0125]
　Test material No. 33 has a high average Mn concentration in the surface layer softened portion, not obtained excellent bending properties.
[0126]
　Test material No. 38 has a small thickness of the surface layer softened portion, not obtained excellent bending properties.
[0127]
　Test material No. 39 has a large thickness of the surface layer softened portion, the strength is low.
[0128]
　Test material No. 46,49,50, and 52, the hot rolled sheet as a substrate, is a multi-layer steel sheet produced by cold spray deposition layer recrystallization ratio of (surface layer) is low, we have obtained excellent bendability Absent.
[0129]
　Test material No. 47, 48, and 51, the cold-rolled sheet as the substrate, but multiple layer steel sheet produced by cold spray method, pores are generated in the deposited layer (surface layer), not obtained a good bending properties. Further, the average crystal grain size of the recrystallized structure of the surface layer softened portion is large, not obtained excellent bending properties.
Industrial Applicability
[0130]
　According to the present invention, a high strength steel sheet of high Mn content with excellent suitable bending workability as automotive materials, can be obtained with good production efficiency, the advantages of the industry is large.

WE CLAIM

And the plate thickness center,
　a high-strength steel sheet including the surface layer softened portion formed on one side or both sides of the plate thickness center,
　the average Mn concentration in the thickness center portion is 4.0 wt percent 10. 0 less than mass%,
　the surface layer softened portion has a thickness from 0.1% thickness 30%,
　the average Mn concentration in the surface layer softened portion is more than 2.5 mass%,
　the surface layer softened and a recrystallization ratio of the parts is more than 90%,
　an average grain size of the recrystallized structure of the surface layer softened portion is 0.1μm or more 40μm or less
　and wherein the,
　high-strength steel sheet.
[Requested item 2]
　The thickness center portion, in
　　mass% C: 0.05% greater than
　　0.80% Si: 0.001% or more and less than
　　3.50% Mn: 4.0% greater than 10.0%,
　　P: 0.10% or
　　less, S: 0.010% or
　　less, sol. Al: less than 0.001% 3.00%, and
　　N: containing less than 0.050%,
　the balance being composed of iron and inevitable impurities, the high strength steel sheet according to claim 1.
[Requested item 3]
　The thickness center portion, further, by
　　mass%, Cr: 0.01% to 2.00% or
　　less, Mo: 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
　, characterized in that it contains at least one selected from the group consisting of high-strength steel sheet according to claim 2.
[Requested item 4]
　The thickness center portion, further, by
　　mass%, Ti: 0.005% or more than
　　0.30%, Nb: 0.005% or more than
　　0.30%, V: 0.005% or more 0.30% or less and
　　W: 0.005% or more than 0.30%
　, characterized in that it contains at least one selected from the group consisting of high-strength steel sheet according to claim 2 or 3.
[Requested item 5]
　The thickness center portion, further, by
　　mass%, B: 0.0001% or more than
　　0.010%, Ca: 0.0001% or more than
　　0.010%, Mg: 0.0001% or more 0.010%
　　hereinafter, Zr: 0.0001% or more than 0.010%, and
　　REM: 0.0001% or more than 0.010%
　, characterized in that it contains at least one selected from the group consisting of, claims 2 to the steel plate according to any one of 4.
[Requested item 6]
　The thickness center portion, further, by
　　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
　at least, characterized in that it contains a type, high-strength steel sheet according to any one of claims 2-5 selected from the group consisting of.
[Requested item 7]
　Wherein the C content of the surface layer softened portion is less than 0.9 times the C content of the plate thickness center, high-strength steel sheet according to any one of claims 2-6.
[Requested item 8]
　Wherein the total amount of Cr and Mo of the surface layer softened portion is less than 0.9 times the total amount of Cr and Mo of the plate thickness center, any one of claims 3-7 high-strength steel sheet according to.
[Requested item 9]
　Wherein the sum of the Cu content and Ni content of the surface layer softened portion is less than 0.9 times the sum of the Cu content and Ni content of the plate thickness center, any one of claims 3-8 high-strength steel sheet according to.
[Requested item 10]
　Wherein the sum of Ti content and Nb content of the surface layer softened portion is less than 0.9 times the sum of the Ti content and the Nb content of the plate thickness center, any one of claims 4-9 high-strength steel sheet according to.
[Requested item 11]
　Wherein the sum of the amount of V and W of the surface layer softened portion is less than 0.9 times the sum of the amount of V and W of the plate thickness center, any one of claims 4-10 high-strength steel sheet according to.
[Requested item 12]
　Wherein the B content of the surface layer softened portion is less than 0.9 times the amount of B the thickness center, high-strength steel sheet according to any one of claims 5-11.
[Requested item 13]
　On the surface of the surface layer softened portion, galvanized layer, and further comprising galvannealed layer, or an electro-galvanized layer, high-strength steel sheet according to any one of claims 1 to 12 .
[Requested item 14]
　A method of manufacturing a high strength steel sheet according to any one of claims 1 to 13,
　the surface layer softened portion constituting the surface layer softened portion on one or both surfaces of the base material steel plate constituting the plate thickness center forming a multilayered steel of use steel sheets laminated,
　the heating of the multilayered steel to 1080 ° C. or higher 1300 ° C. or less, and finish it to hot rolling under conditions of rolling start temperature 800 ° C. or higher 1000 ° C. or less ,
　the rolled multilayered steel plates between the heat, cooling to 500 ° C. or higher 700 ° C. or lower within terminated after 2 seconds of the finish rolling,
　after cooling the multilayered steel to a temperature of the 500 ° C. or higher 700 ° C. or less , keeping more than 3 seconds,
　the 500 ° C. pickled the held multilayered steel at least 3 seconds at more than 700 ° C. or less of the temperature and then to cold rolling at a reduction rate of 70% more than 20% or less,
　the cold between rolled multilayered steel plates, 00 holds ° C. or higher 750 ° C. temperature below 5 seconds or more, then it, cooling
　, characterized in that it comprises a method for producing a high-strength steel sheet.
[Requested item 15]
　Characterized in that winding the multilayer steel holding the 500 ° C. or higher 700 ° C. at a temperature at least 3 seconds at the coiling temperature of 600 ° C. or less, the method of producing a high strength steel sheet according to claim 14.
[Requested item 16]
　The multilayered steel was the winding, before rolling the cold, and carrying out tempering and held at a temperature of 300 ° C. or higher 550 ° C. or less, the method of producing a high strength steel sheet according to claim 15 .