Title of invention: Steel plate
Technical field
[0001]
　The present invention relates to steel sheets suitable for automobile parts.
Background technology
[0002]
　In order to reduce the amount of carbon dioxide emitted from automobiles, the weight of automobile bodies is being reduced by using high-strength steel plates. For example, in order to ensure the safety of passengers, high-strength steel plates are often used for skeletal parts of vehicle bodies. Mechanical properties that have a large effect on collision safety include tensile strength, ductility, ductility-brittleness transition temperature, and 0.2% proof stress. For example, a steel plate used for a front side member is required to have excellent ductility.
[0003]
　On the other hand, the shape of the skeletal component is complicated, and the high-strength steel plate for the skeletal component is required to have excellent hole expansion property and bendability. For example, a steel plate used for a side sill is required to have excellent hole expandability.
[0004]
　However, it is difficult to improve both collision safety and moldability at the same time. Conventionally, techniques for improving collision safety or moldability have been proposed (Patent Documents 1 and 2), but it is difficult to achieve both improvement in collision safety and improvement in moldability.
Prior art literature
Patent documents
[0005]
Patent Document 1:
Japanese Patent Application Laid-Open No. 5589893 Patent Document 2: Japanese Patent Application Laid-Open No. 2013-185196
Patent Document 3: Japanese Patent Application Laid-Open No. 2005-171319
Patent Document 4: International Publication No. 2012/133563
Outline of the invention
Problems to be solved by the invention
[0006]
　An object of the present invention is to provide a steel sheet capable of obtaining excellent collision safety and moldability.
Means to solve problems
[0007]
　The present inventors have made diligent studies to solve the above problems. As a result, it was clarified that in a steel sheet having a tensile strength of 980 MPa or more, excellent elongation is exhibited by setting the surface integrals and forms of retained austenite and bainitic ferrite to predetermined values. Furthermore, when the surface integral of the polygonal ferrite is low, the difference in hardness within the steel sheet is small, and not only excellent elongation but also excellent hole expansion and bendability can be obtained, and embrittlement resistance at a sufficiently low temperature can be obtained. It was revealed that the characteristics and 0.2% proof stress were also obtained.
[0008]
　As a result of further diligent studies based on such findings, the inventor of the present application has come up with various aspects of the invention shown below.
[0009]
　(1) By
　mass%,
　C: 0.1% to 0.5%,
　Si: 0.5% to 4.0%,
　Mn: 1.0% to 4.0%,
　P: 0.015% or less ,
　S: 0.050% or less,
　N: 0.01% or less,
　Al: 2.0% or less,
　Si and Al: 0.5% to 6.0% in total,
　Ti: 0.00% to 0. 20%,
　Nb: 0.00% to 0.20%,
　B: 0.0000% to 0.0030%,
　Mo: 0.00% to 0.50%,
　Cr: 0.0% to 2.0% ,
　V: 0.00% to 0.50%,
　Mg: 0.000% to 0.040%,
　REM: 0.000% to 0.040%,
　Ca: 0.000% to 0.040%, and
　Remaining: It
has a chemical composition represented by Fe and impurities, and in terms of
　area fraction,
　polygonal ferrite: 40% or less,
　martensite: 20% or less,
　It has a metallographic structure represented by bainitic ferrite: 50% to 95% and
　retained austenite: 5% to 50%,
and
　80% or more of the bainitic ferrite has an aspect ratio in terms of area fraction. Consists of bainitic ferrite grains with a ratio of 0.1 to 1.0 and a dislocation density of 8 × 10 2 (cm / cm 3 ) or less in a region surrounded by grain boundaries with an orientation difference angle of 15 ° or more. In
　terms of area fraction, 80% or more of the retained austenite has an aspect ratio of 0.1 to 1.0, a major axis length of 1.0 μm to 28.0 μm, and a minor axis length. Is composed of retained austenite grains of 0.1 μm to 2.8 μm.
[0010]
　(2) The
　metallographic structure has a
　polygonal ferrite: 5% to 20%,
　martensite: 20% or less,
　bainitic ferrite: 75% to 90%, and
　retained austenite: 5% to 20% in terms of surface integral.
The steel plate according to (1) , which is represented by.
[0011]
　(3) The
　metallographic structure has a surface
　integral of polygonal ferrite: more than 20% and 40% or less,
　martensite: 20% or less,
　bainitic ferrite: 50% to 75%, and
　retained austenite: 5% to 30.
The steel sheet according to (1) , which is represented by % .
[0012]
　(4) In the
　chemical composition, in terms of mass%,
　Ti: 0.01% to 0.20%,
　Nb: 0.005% to 0.20%,
　B: 0.0001% to 0.0030%,
　Mo: 0.01% to 0.50%,
　Cr: 0.01% to 2.0%,
　V: 0.01% to 0.50%,
　Mg: 0.0005% to 0.040%,
　REM: 0. 　The steel plate according to any one of (1) to (3), wherein 0005% to 0.040%,
　Ca: 0.0005% to 0.040%,
or any combination thereof holds.
[0013]
　(5)
　The steel sheet according to any one of (1) to (4), which has a plating layer formed on the surface.
Effect of the invention
[0014]
　According to the present invention, since the surface integrals and forms of retained austenite and bainitic ferrite are appropriate, excellent collision safety and moldability can be obtained.
A brief description of the drawing
[0015]
FIG. 1 is a diagram showing an example of an equivalent ellipse of retained austenite grains.
Mode for carrying out the invention
[0016]
　Hereinafter, embodiments of the present invention will be described.
[0017]
　First, the metal structure of the steel sheet according to the embodiment of the present invention will be described. The steel sheet according to the present embodiment has a surface integral of polygonal ferrite: 40% or less, martensite: 20% or less, bainitic ferrite: 50% to 95%, and retained austenite: 5% to 50%. It has a represented metallographic structure. In terms of area fraction, 80% or more of bainitic ferrite has a dislocation density of 8 in the region surrounded by grain boundaries with an aspect ratio of 0.1 to 1.0 and an azimuth difference angle of 15 ° or more. It is composed of bainitic ferrite grains of × 10 2 (cm / cm 3 ) or less. In terms of area fraction, 80% or more of the retained austenite has an aspect ratio of 0.1 to 1.0, a major axis length of 1.0 μm to 28.0 μm, and a minor axis length of 0. It is composed of 1 μm to 2.8 μm residual austenite grains.
[0018]
　(Surface integral of polygonal ferrite: 40% or less)
　Polygonal ferrite has a soft structure. Therefore, the difference in hardness between the polygonal ferrite and the hard structure martensite is large, and cracks are likely to occur at the interface between them during molding. Rhagades may extend along this interface. When the surface integral of the polygonal ferrite is more than 40%, such cracks and elongations are likely to occur, and it is difficult to obtain sufficient hole expansion property, bendability, embrittlement resistance at low temperature and 0.2% proof stress. .. Therefore, the surface integral of the polygonal ferrite is set to 40% or less.
[0019]
　The lower the surface integral of the polygonal ferrite, the less likely it is that C will be concentrated in the retained austenite, improving the hole expandability while reducing the ductility. Therefore, when the hole expandability is more important than the ductility, the surface integral of the polygonal ferrite is preferably 20% or less, and when the ductility is more important than the ductility, the surface integral of the polygonal ferrite is. It is preferably more than 20% and 40% or less. Even when the hole expandability is more important than the ductility, the surface integral of the polygonal ferrite is preferably 5% or more in order to ensure the ductility.
[0020]
　(Surface integral of bainitic ferrite: 50% to 95%)
　Bainitic ferrite has a higher density than polygonal ferrite and contains dislocations, which contributes to the improvement of tensile strength. Since the hardness of bainitic ferrite is higher than that of polygonal ferrite and lower than that of martensite, the hardness difference between bainitic ferrite and martensite is the hardness between polygonal ferrite and martensite. Less than the difference. Therefore, bainitic ferrite also contributes to the improvement of hole expandability and bendability. If the surface integral of bainitic ferrite is less than 50%, sufficient tensile strength cannot be obtained. Therefore, the surface integral of bainitic ferrite is set to 50% or more. When the hole expandability is more important than the ductility, the surface integral of bainitic ferrite is preferably 75% or more. On the other hand, when the surface integral of bainitic ferrite exceeds 95%, retained austenite is insufficient and sufficient moldability cannot be obtained. Therefore, the surface integral of bainitic ferrite is set to 95% or less.
[0021]
　(Surface integral of martensite: 20% or less)
　Martensite includes fresh martensite (untempered martensite) and tempered martensite. As described above, the difference in hardness between ferriteal ferrite and martensite is large, and cracks are likely to occur at the interface between them during molding. Rhagades may extend along this interface. When the surface integral of martensite is more than 20%, such cracks and elongations are likely to occur, and it is difficult to obtain sufficient hole expansion property, bendability, embrittlement resistance at low temperature and 0.2% proof stress. Therefore, the surface integral ratio of martensite is 20% or less.
[0022]
　(Surface integral of retained austenite: 5% to 50%)
　Retained austenite contributes to the improvement of moldability. If the surface integral of retained austenite is less than 5%, sufficient moldability cannot be obtained. On the other hand, if the surface integral of retained austenite exceeds 50%, bainitic ferrite is insufficient and sufficient tensile strength cannot be obtained. Therefore, the surface integral of retained austenite is set to 50% or less.
[0023]
　Identification of polygonal ferrite, vanitic ferrite, retained austenite and martensite and identification of area fractions are performed, for example, by scanning electron microscope (SEM) observation or transmission electron microscope (TEM). It can be done by observation. When SEM or TEM is used, for example, a Nital solution and a Repeller solution are used to corrode the sample to obtain a cross section parallel to the rolling direction and thickness direction (cross section perpendicular to the width direction) and / or a cross section perpendicular to the rolling direction. Observe at a magnification of 1000 to 100,000 times.
[0024]
　Electron back scattering diffraction (EBSD) of polygonal ferrite, bainitic ferrite, retained austenite and martensite attached to a field emission scanning electron microscope (FE-SEM). It can also be discriminated by analyzing the crystal orientation by crystal orientation diffraction (FE-SEM-EBSD) using the function or measuring the hardness of a minute region such as Micro Vickers hardness measurement.
[0025]
　For example, in specifying the area fraction of polygonal ferrite and bainitic ferrite, a cross section parallel to the rolling direction and thickness direction of the steel sheet (cross section perpendicular to the width direction) is polished and etched with a nital solution. Next, the area fraction is measured by observing a region where the depth from the surface of the steel sheet is 1/8 to 3/8 of the thickness of the steel sheet with an FE-SEM. Such an observation is carried out for 10 fields of view at a magnification of 5000 times, and the surface integrals of each area of ​​polygonal ferrite and bainitic ferrite can be obtained from the average value of 10 fields of view.
[0026]
　The surface integral of retained austenite can be specified, for example, by X-ray measurement. In this method, for example, a portion from the surface of the steel sheet to 1/4 of the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, and MoKα rays are used as characteristic X-rays. Then, from the integrated intensity ratios of the diffraction peaks of the body-centered cubic lattice (bcc) phases (200) and (211) and the face-centered cubic lattice (fcc) phases (200), (220) and (311), the following The surface integral of retained austenite is calculated using the formula of. Such an observation is performed for 10 fields of view, and the surface integral of retained austenite can be obtained from the average value of the 10 fields of view.
　Sγ = (I 200f + I 220f + I 311f ) / (I 200b + I 211b ) × 100
(Sγ is the surface integral of retained austenite, I 200f , I 220f , and I 311f are the fcc phases (200) and (220), respectively. , (311), I 200b , and I 211b indicate the intensity of the diffraction peaks of (200) and (211) in the bcc phase, respectively.)
[0027]
　The area fraction of martensite can be specified, for example, by field emission-scanning electron microscope (FE-SEM) observation and X-ray measurement. In this method, for example, a region where the depth from the surface of the steel sheet is 1/8 to 3/8 of the thickness of the steel sheet is observed, and a repera liquid is used for corrosion. Since the tissues not corroded by the repera solution are martensite and retained austenite, martensite is subtracted from the area fraction of the region not corroded by the repera solution by the area fraction Sγ of the retained austenite identified by X-ray measurement. The area fraction of can be specified. The surface integral of martensite can also be specified using, for example, an electronic channeling contrast image obtained by SEM observation. In the electron channeling contrast image, martensite is a region having a high dislocation density and substructures such as blocks and packets in the grain. Such an observation is performed for 10 fields of view, and the surface integral of martensite can be obtained from the average value of the 10 fields of view.
[0028]
　(Area fraction of bainitic ferrite grains of a predetermined form: 80% or more of the whole bainitic ferrite)
　Dislocations because bainitic ferrite grains with high dislocation density do not contribute to the improvement of elongation as much as polygonal ferrite The higher the area fraction of the bainitic ferrite grains with higher density, the easier it is for the elongation to decrease. A bainitic ferrite having an aspect ratio of 0.1 to 1.0 and a dislocation density of 8 × 10 2 (cm / cm 3 ) or less in a region surrounded by grain boundaries having an orientation difference angle of 15 ° or more. If the area fraction of the grains is less than 80%, it is difficult to obtain sufficient elongation. Therefore, the surface integral of the bainitic ferrite grains in such a form is 80% or more, preferably 85% or more with respect to the entire bainitic ferrite.
[0029]
　The dislocation density of bainitic ferrite can be identified by microstructure observation using a transmission electron microscope (TEM). For example, the dislocation density of bainitic ferrite can be specified by dividing the number of dislocation lines existing in the crystal grains surrounded by grain boundaries having an orientation difference angle of 15 ° by the area of ​​the crystal grains. ..
[0030]
　(Surface integral of retained austenite grains of a predetermined form: 80% or more of the total
　retained austenite ) Retained austenite is transformed into martensite by process-induced transformation during molding. When retained austenite is transformed into martensite, if this martensite is adjacent to polygonal ferrite or untransformed retained austenite, a large hardness difference will occur between them. A large difference in hardness leads to the occurrence of cracks as described above. Such cracks are particularly likely to occur where stress is concentrated, and stress is likely to be concentrated in the vicinity of martensite transformed from retained austenite with an aspect ratio of less than 0.1. The area fraction of the residual austenite grains having an aspect ratio of 0.1 to 1.0, a major axis length of 1.0 μm to 28.0 μm, and a minor axis length of 0.1 μm to 2.8 μm. If it is less than 80%, cracks are likely to occur due to stress concentration, and it is difficult to obtain sufficient elongation. Therefore, the surface integral of the retained austenite grains in such a form is 80% or more, preferably 85% or more, based on the total retained austenite. Here, the aspect ratio of the retained austenite grains is a value obtained by dividing the length of the minor axis of the equivalent ellipse of the retained austenite grains by the length of the major axis. FIG. 1 shows an example of an equivalent ellipse. Even if the retained austenite grain 1 has a complicated shape, the aspect ratio (L2 / L1) of the retained austenite grain can be obtained from the major axis length L1 and the minor axis length L2 of the equivalent ellipse 2.
[0031]
　Next, the chemical composition of the steel sheet according to the embodiment of the present invention and the slab used for manufacturing the steel sheet will be described. As described above, the steel sheet according to the embodiment of the present invention is manufactured through hot rolling, pickling, cold rolling, first annealing, second annealing and the like. Therefore, the chemical composition of the steel sheet and the slab considers not only the characteristics of the steel sheet but also these treatments. In the following description, "%", which is a unit of the content of each element contained in the steel sheet and the slab, means "mass%" unless otherwise specified. The steel plate according to the present embodiment and the slab used for manufacturing the same are C: 0.1% to 0.5%, Si: 0.5% to 4.0%, Mn: 1.0% to 4 in mass%. 0.0%, P: 0.015% or less, S: 0.050% or less, N: 0.01% or less, Al: 2.0% or less, Si and Al: 0.5% to 6.0 in total %, Ti: 0.00% to 0.20%, Nb: 0.00% to 0.20%, B: 0.0000% to 0.0030%, Mo: 0.00% to 0.50%, Cr: 0.0% to 2.0%, V: 0.00% to 0.50%, Mg: 0.000% to 0.040%, REM (rare earth metal): 0.000% It has a chemical composition represented by ~ 0.040%, Ca: 0.000% to 0.040%, and the balance: Fe and impurities.
[0032]
　(C: 0.10% to 0.5%)
　Carbon (C) contributes to the improvement of the strength of the steel sheet and the improvement of the elongation through the improvement of the stability of the retained austenite. If the C content is less than 0.10%, it may be difficult to obtain sufficient strength, for example, tensile strength of 980 MPa or more, or the stability of retained austenite may be insufficient to obtain sufficient elongation. .. Therefore, the C content is 0.10% or more, preferably 0.15% or more. On the other hand, when the C content exceeds 0.5%, the transformation from austenite to bainitic ferrite is delayed, so that the bainitic ferrite grains of a predetermined form are insufficient and sufficient elongation cannot be obtained. Therefore, the C content is 0.5% or less, preferably 0.25% or less.
[0033]
　(Si: 0.5% to 4.0%)
　Silicon (Si) contributes to the improvement of the strength of steel and the improvement of elongation through the improvement of the stability of retained austenite. If the Si content is less than 0.5%, these effects cannot be sufficiently obtained. Therefore, the Si content is 0.5% or more, preferably 1.0% or more. On the other hand, when the Si content exceeds 4.0%, the strength of the steel becomes too high and the elongation decreases. Therefore, the Si content is set to 4.0% or less, preferably 2.0% or less.
[0034]
　(Mn: 1.0% to 4.0%)
　Manganese (Mn) contributes to the improvement of steel strength and suppresses the polygonal ferrite transformation that occurs during the cooling of the first annealing or the second annealing. Or When the hot-dip galvanizing treatment is performed, the polygonal ferrite transformation that occurs during the cooling of the treatment is also suppressed. If the Mn content is less than 1.0%, these effects may not be sufficiently obtained, or polygonal ferrite may be excessively generated to deteriorate the hole expandability. Therefore, the Mn content is 1.0% or more, preferably 2.0% or more. On the other hand, if the Mn content exceeds 4.0%, the strength of the slab and the hot-rolled steel sheet becomes too high. Therefore, it is set to 4.0% or less, preferably 3.0% or less.
[0035]
　(P: 0.015% or less)
　Phosphorus (P) is not an essential element and is contained as an impurity in steel, for example. P segregates in the central portion of the steel sheet in the thickness direction to reduce the toughness or embrittle the welded portion. Therefore, the lower the P content, the better. In particular, when the P content exceeds 0.015%, the toughness is significantly reduced and the weldability is embrittled. Therefore, the P content is 0.015% or less, preferably 0.010% or less. Reducing the P content is costly, and attempts to reduce it to less than 0.0001% significantly increase the cost. Therefore, the P content may be 0.0001% or more.
[0036]
　(S: 0.050% or less)
　Sulfur (S) is not an essential element and is contained as an impurity in steel, for example. S lowers the manufacturability of casting and hot rolling, or forms coarse MnS to lower the hole expandability. Therefore, the lower the S content, the better. In particular, when the S content exceeds 0.050%, the decrease in weldability, the decrease in manufacturability, and the decrease in hole expandability are remarkable. Therefore, the S content is set to 0.050% or less, preferably 0.0050% or less. Reducing the S content is costly, and attempts to reduce it to less than 0.0001% significantly increase the cost. Therefore, the S content may be 0.0001% or more.
[0037]
　(N: 0.01% or less)
　Nitrogen (N) is not an essential element and is contained as an impurity in steel, for example. N forms a coarse nitride and deteriorates bendability and hole expansion property, and causes blow holes during welding. Therefore, the lower the N content, the better. In particular, when the N content exceeds 0.01%, the bendability and hole expansion property are significantly reduced, and the occurrence of blow holes is remarkable. Therefore, the N content is 0.01% or less. Reducing the N content is costly, and attempts to reduce it to less than 0.0005% significantly increase the cost. Therefore, the N content may be 0.0005% or more.
[0038]
　(Al: 2.0% or less)
　Aluminum (Al) functions as a deoxidizing material and suppresses the precipitation of iron-based carbides in austenite, but is not an essential element. When the Al content exceeds 2.0%, the transformation from austenite to polygonal ferrite is promoted, and polygonal ferrite is excessively generated, resulting in deterioration of hole expandability. Therefore, the Al content is 2.0% or less, preferably 1.0% or less. Reducing the Al content is costly, and attempts to reduce it to less than 0.001% significantly increase the cost. Therefore, the Al content may be 0.001% or more.
[0039]
　(Si and Al: 0.5% to 6.0% in total)
　Both Si and Al contribute to the improvement of elongation through the improvement of the stability of retained austenite. If the total content of Si and Al is less than 0.5%, this effect cannot be sufficiently obtained. Therefore, the total content of Si and Al is 0.5% or more, preferably 1.2% or more. Only either Si or Al may be contained, and both Si and Al may be contained.
[0040]
　Ti, Nb, B, Mo, Cr, V, Mg, REM and Ca are not essential elements but optional elements that may be appropriately contained in the steel plate and the slab up to a predetermined amount.
[0041]
　(Ti: 0.00% to 0.20%)
　Titanium (Ti) contributes to the improvement of steel strength through dislocation strengthening caused by precipitation strengthening and fine grain strengthening. Therefore, Ti may be contained. In order to sufficiently obtain this effect, the Ti content is preferably 0.01% or more, more preferably 0.025% or more. On the other hand, when the Ti content exceeds 0.20%, the carbonitride of Ti is excessively precipitated and the moldability of the steel sheet is lowered. Therefore, the Ti content is 0.20% or less, preferably 0.08% or less.
[0042]
　(Nb: 0.00% to 0.20%)
　Niobium (Nb) contributes to the improvement of steel strength through dislocation strengthening caused by precipitation strengthening and fine grain strengthening. Therefore, Nb may be contained. In order to sufficiently obtain this effect, the Nb content is preferably 0.005% or more, more preferably 0.010% or more. On the other hand, when the Nb content exceeds 0.20%, the carbonitride of Nb is excessively precipitated and the moldability of the steel sheet is lowered. Therefore, the Nb content is 0.20% or less, preferably 0.08% or less.
[0043]
　(B: 0.0000% to 0.0030%)
　Boron (B) strengthens grain boundaries and suppresses the polygonal ferrite transformation that occurs during the cooling of the first annealing or the second annealing. When the hot-dip galvanizing treatment is performed, the polygonal ferrite transformation that occurs during the cooling of the treatment is also suppressed. Therefore, B may be contained. In order to sufficiently obtain this effect, the B content is preferably 0.0001% or more, and more preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0030%, the effect of the addition is saturated and the manufacturability of hot rolling is lowered. Therefore, the B content is 0.0030% or less, preferably 0.0025% or less.
[0044]
　(Mo: 0.00% to 0.50%)
　Molybdenum (Mo) contributes to the strengthening of steel and suppresses the polygonal ferrite transformation that occurs during the cooling of the first annealing or the second annealing. .. When the hot-dip galvanizing treatment is performed, the polygonal ferrite transformation that occurs during the cooling of the treatment is also suppressed. Therefore, Mo may be contained. In order to sufficiently obtain this effect, the Mo content is preferably 0.01% or more, more preferably 0.02% or more. On the other hand, if the Mo content exceeds 0.50%, the manufacturability of hot rolling is lowered. Therefore, the Mo content is 0.50% or less, preferably 0.20% or less.
[0045]
　(Cr: 0.0% to 2.0%)
　Chromium (Cr) contributes to the strengthening of steel and suppresses the polygonal ferrite transformation that occurs during the cooling of the first annealing or the second annealing. .. When the hot-dip galvanizing treatment is performed, the polygonal ferrite transformation that occurs during the cooling of the treatment is also suppressed. Therefore, Cr may be contained. In order to sufficiently obtain this effect, the Cr content is preferably 0.01% or more, more preferably 0.02% or more. On the other hand, if the Cr content exceeds 2.0%, the manufacturability of hot rolling is lowered. Therefore, the Cr content is set to 2.0% or less, preferably 0.10% or less.
[0046]
　(V: 0.00% to 0.50%)
　Vanadium (V) contributes to the improvement of steel strength through dislocation strengthening caused by precipitation strengthening and fine grain strengthening. Therefore, V may be contained. In order to sufficiently obtain this effect, the V content is preferably 0.01% or more, more preferably 0.02% or more. On the other hand, when the V content exceeds 0.50%, the carbonitride of V is excessively precipitated and the moldability of the steel sheet is lowered. Therefore, the Nb content is 0.50% or less, preferably 0.10% or less.
[0047]
　(Mg: 0.000% to 0.040%, REM: 0.000% to 0.040%, Ca: 0.000% to 0.040%)
　Magnesium (Mg), rare earth metal (REM) and calcium (Mg) Ca) is present in steel as an oxide or sulfide and contributes to the improvement of hole expandability. Therefore, Mg, REM or Ca or any combination thereof may be contained. In order to sufficiently obtain this effect, the Mg content, REM content and Ca content are all preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, when the Mg content, the REM content or the Ca content exceeds 0.040%, a coarse oxide is formed and the hole expandability is lowered. Therefore, the Mg content, the REM content, and the Ca content are all 0.040% or less, preferably 0.010% or less.
[0048]
　REM (rare earth metal) refers to a total of 17 elements of Sc, Y and lanthanoids, and "REM content" means the total content of these 17 elements. REM is added, for example, with mischmetal, which may contain lanthanoids in addition to La and Ce. A simple substance of a metal such as metal La or metal Ce may be used for adding REM.
[0049]
　Examples of impurities include those contained in raw materials such as ore and scrap, and those contained in the manufacturing process. Specifically, P, S, O, Sb, Sn, W, Co, As, Pb, Bi and H are exemplified as impurities. The O content is preferably 0.010% or less, the Sb content, Sn content, W content, Co content and As content are preferably 0.1% or less, and the Pb content and Bi content are The H content is preferably 0.005% or less, and the H content is preferably 0.0005% or less.
[0050]
　According to this embodiment, excellent collision safety and moldability can be obtained. For example, the hole expandability is 30% or more, the ratio (R / t) of the minimum bending radius (R (mm)) to the plate thickness (t (mm)) is 0.5 or less, the total elongation is 21% or more, and 0. It provides mechanical properties with a 2.2% proof stress of 680 MPa or more, a tensile strength of 980 MPa or more, and a ductile-brittle transition temperature of -60 ° C or less. In particular, when the surface integral of the polygonal ferrite is 5% to 20% and the surface integral of the bainitic ferrite is 75% or more, a hole expandability of 50% or more can be obtained, and the area fraction of the polygonal ferrite can be obtained. When the rate is more than 20% and 40% or less, a total growth of 26% or more can be obtained.
[0051]
　Next, a method for manufacturing a steel sheet according to the embodiment of the present invention will be described. In the method for producing a steel sheet according to the embodiment of the present invention, hot rolling, pickling, cold rolling, first annealing and second annealing of a slab having the above chemical composition are performed in this order.
[0052]
　(Hot rolling) In
　hot rolling, rough rolling, finish rolling and winding of slabs are performed. As the slab, for example, a slab obtained by continuous casting or a slab made of thin slab casters can be used. The slab may be subjected to a hot rolling facility while being held at a temperature of 1000 ° C. or higher after casting, or may be cooled to a temperature of less than 1000 ° C. and then heated and used in a hot rolling facility.
[0053]
　The rolling temperature of the final pass of rough rolling is 1000 ° C to 1150 ° C, and the rolling reduction of the final pass is 40% or more. If the rolling temperature of the final pass is less than 1000 ° C., the austenite particle size after finish rolling becomes excessively small. In this case, the transformation from austenite to polygonal ferrite is excessively promoted, the uniformity of the metal structure is lowered, and sufficient moldability cannot be obtained. Therefore, the rolling temperature of the final pass is set to 1000 ° C. or higher. On the other hand, when the rolling temperature of the final pass exceeds 1150 ° C., the austenite particle size after finish rolling becomes excessively large. Also in this case, the uniformity of the metal structure is lowered, and sufficient moldability cannot be obtained. Therefore, the rolling temperature of the final pass is set to 1150 ° C. or lower. If the rolling reduction of the final pass is less than 40%, the austenite particle size after finish rolling becomes excessively large, the uniformity of the metal structure is lowered, and sufficient formability cannot be obtained. Therefore, the reduction rate of the final pass is set to 40% or more.
[0054]
　The rolling temperature of finish rolling shall be Ar 3 points or more. If the rolling temperature is less than 3 Ar points, the metal structure of the hot-rolled steel sheet contains austenite and ferrite, and the mechanical properties differ between austenite and ferrite, so that sufficient formability cannot be obtained. Therefore, this rolling temperature is set to Ar 3 points or more. When this rolling temperature is set to Ar 3 points or more, the rolling load during finish rolling can be relatively reduced. In the finish rolling, a plurality of rough-rolled plates obtained by rough rolling may be joined and continuously rolled. After the rough-rolled plate is once wound, finish rolling may be performed while rewinding.
[0055]
　The winding temperature is 750 ° C or lower. When the winding temperature exceeds 750 ° C., coarse ferrite or pearlite is formed in the structure of the hot-rolled steel sheet, the uniformity of the metal structure is lowered, and sufficient formability cannot be obtained. Oxides may be thickly formed on the surface to reduce pickling properties. Therefore, the winding temperature is set to 750 ° C. or lower. The lower limit of the winding temperature is not particularly limited, but it is difficult to wind at a temperature lower than room temperature. A coil of hot-rolled steel sheet is obtained by hot rolling of a slab.
[0056]
　(Pickling) After
　hot rolling, pickling is performed while rewinding the coil of the hot-rolled steel sheet. Pickling should be done once or more than once. Pickling removes oxides on the surface of the hot-rolled steel sheet, improving chemical conversion treatment and plating properties.
[0057]
　(Cold rolling) Cold
　rolling is performed after pickling. The rolling reduction ratio for cold rolling is 40% to 80%. If the reduction rate is less than 40%, it may be difficult to keep the shape of the cold-rolled steel sheet flat, or sufficient ductility may not be obtained. Therefore, this reduction rate is set to 40% or more, preferably 50% or more. On the other hand, when this reduction ratio exceeds 80%, the rolling load becomes excessive, the recrystallization of ferrite is excessively promoted, coarse polygonal ferrite is formed, and the surface integral of the polygonal ferrite exceeds 40%. Or Therefore, this reduction rate is set to 80% or less, preferably 70% or less. The number of rolling passes and the rolling reduction rate for each pass are not particularly limited. A cold-rolled steel sheet is obtained by cold-rolling the hot-rolled steel sheet.
[0058]
　(First annealing)
　The first annealing is performed after cold rolling. In the first annealing, the cold-rolled steel sheet is first heated, first cooled, second cooled, and first held. The first annealing can be performed, for example, on a continuous annealing line.
[0059]
　The annealing temperature of the first annealing is 750 ° C. to 900 ° C. If the annealing temperature is less than 750 ° C., the surface integral of the polygonal ferrite becomes excessive or the surface integral of the bainitic ferrite becomes too small. Therefore, the annealing temperature is 750 ° C. or higher, preferably 780 ° C. or higher. On the other hand, when the annealing temperature exceeds 900 ° C., the austenite grains become coarse and the transformation from austenite to bainitic ferrite or tempered martensite is delayed. Then, due to the delay of this transformation, the surface integral of bainitic ferrite becomes too small. Therefore, the annealing temperature is 900 ° C. or lower, preferably 870 ° C. or lower. The annealing time is not particularly limited, and is, for example, 1 second or more and 1000 seconds or less.
[0060]
　The cooling stop temperature of the first cooling is 600 ° C. to 720 ° C., and the cooling rate up to this cooling stop temperature is 1 ° C./sec or more and less than 10 ° C./sec. If the cooling stop temperature of the first cooling is less than 600 ° C., the surface integral of the polygonal ferrite becomes excessive. Therefore, the cooling stop temperature is set to 600 ° C. or higher, preferably 620 ° C. or higher. On the other hand, when the cooling stop temperature exceeds 720 ° C., the surface integral of retained austenite is insufficient. Therefore, the cooling stop temperature is set to 720 ° C. or lower, preferably 700 ° C. or lower. If the cooling rate of the first cooling is less than 1.0 ° C./sec, the surface integral of the polygonal ferrite becomes excessive. Therefore, this cooling rate is 1.0 ° C./sec or higher, preferably 3 ° C./sec or higher. On the other hand, when this cooling rate is 10 ° C./sec or more, the surface integral of retained austenite is insufficient. Therefore, this cooling rate is set to less than 10 ° C / sec, preferably 8 ° C / sec or less.
[0061]
　The cooling stop temperature of the second cooling is 150 ° C. to 500 ° C., and the cooling rate up to this cooling stop temperature is 10 ° C./sec to 60 ° C./sec. When the cooling stop temperature of the second cooling is less than 150 ° C., the lath width of bainitic ferrite or tempered martensite becomes fine, and the residual austenite remaining between the laths becomes a fine film. As a result, the surface integral of the retained austenite grains of the predetermined form becomes too small. Therefore, the cooling stop temperature is set to 150 ° C. or higher, preferably 200 ° C. or higher. On the other hand, when the cooling stop temperature exceeds 500 ° C., the formation of polygonal ferrite is promoted and the surface integral of the polygonal ferrite becomes excessive. Therefore, the cooling stop temperature is set to 500 ° C. or lower, preferably 450 ° C. or lower, and more preferably about room temperature. Further, the cooling stop temperature is preferably Ms point or less depending on the composition. If the cooling rate of the second cooling is less than 10 ° C./sec, the formation of polygonal ferrite is promoted and the surface integral of the polygonal ferrite becomes excessive. Therefore, this cooling rate is set to 10 ° C./sec or higher, preferably 20 ° C./sec or higher. On the other hand, when the cooling rate exceeds 60 ° C./sec, the surface integral of retained austenite becomes less than the lower limit. Therefore, this cooling rate is set to 60 ° C./sec or less, preferably 50 ° C./sec or less.
[0062]
　The method of the first cooling and the second cooling is not limited, and for example, roll cooling, air cooling, water cooling, or any combination thereof can be performed.
[0063]
　After the second cooling, the cold-rolled steel sheet is held at a temperature of 150 ° C. to 500 ° C. for a time of t1 second to 1000 seconds defined by the following formula (1). This holding (first holding) is performed as it is, for example, after the second cooling without lowering the temperature to less than 150 ° C. In the formula (1), T0 is the holding temperature (° C.), and T1 is the cooling stop temperature (° C.) of the second cooling.
　t1 = 20 × [C] + 40 × [Mn] -0.1 × T0 + T1-0.1 (1)
[0064]
　During the first retention, the diffusion of C into the retained austenite is promoted. As a result, the stability of retained austenite is improved, and it becomes possible to secure retained austenite in a surface integral of 5% or more. If the retention time is less than t1 second, C will not be sufficiently concentrated in the retained austenite, and the retained austenite will be transformed into martensite during the subsequent temperature decrease, resulting in an excessive surface integral of the retained austenite. Therefore, the holding time is t1 second or more. When the holding time exceeds 1000 seconds, the decomposition of retained austenite is promoted, and the surface integral of retained austenite becomes too small. Therefore, the holding time is set to 1000 seconds or less. An intermediate steel sheet is obtained by the first annealing of the cold-rolled steel sheet.
[0065]
　The first holding may be performed, for example, by lowering the temperature to a temperature of less than 150 ° C. and then reheating to a temperature of 150 ° C. to 500 ° C. When the reheating temperature is less than 150 ° C., the lath width of bainitic ferrite or tempered martensite becomes fine, and the residual austenite remaining between the laths becomes a fine film. As a result, the surface integral of the retained austenite grains of the predetermined form becomes too small. Therefore, this reheating temperature is set to 150 ° C. or higher, preferably 200 ° C. or higher. On the other hand, when the reheating temperature exceeds 500 ° C., the formation of polygonal ferrite is promoted and the surface integral of the polygonal ferrite becomes excessive. Therefore, this reheating temperature is set to 500 ° C. or lower, preferably 450 ° C. or lower.
[0066]
　The intermediate steel sheet has, for example, a polygonal ferrite: 40% or less, a bainitic ferrite and / or tempered martensite: 40% to 95% in total, and a retained austenite: 5% to 60% in terms of surface integral. It has a metallographic structure represented by. Further, for example, in terms of surface integral, 80% or more of the retained austenite is composed of retained austenite grains having an aspect ratio of 0.03 to 1.00.
[0067]
　(Second annealing)
　The second annealing is performed after the first annealing. In the second annealing, a second heating, a third cooling and a second holding of the intermediate steel sheet are performed. The second annealing can be performed, for example, on a continuous annealing line. By performing the second annealing under the following conditions, the dislocation density of bainitic ferrite is reduced, and the area fraction of the bainitic ferrite grains of a predetermined form having a dislocation density of 8 × 10 2 (cm / cm 3 ) or less is achieved. Can be enhanced.
[0068]
　The annealing temperature of the second annealing is 760 ° C to 800 ° C. If the annealing temperature is less than 760 ° C., the surface integral of the polygonal ferrite becomes excessive, the surface integral of the bainitic ferrite grains, the surface integral of the retained austenite, or both of them become too small. Therefore, the annealing temperature is 760 ° C. or higher, preferably 770 ° C. or higher. On the other hand, when the annealing temperature exceeds 800 ° C., the surface integral of austenite increases with the austenite transformation, and the surface integral of bainitic ferrite becomes too small. Therefore, the annealing temperature is set to 800 ° C. or lower, preferably 790 ° C. or lower.
[0069]
　The cooling stop temperature of the third cooling is 600 ° C. to 750 ° C., and the cooling rate up to this cooling stop temperature is 1 ° C./sec to 10 ° C./sec. If the cooling stop temperature is less than 600 ° C., the surface integral of the polygonal ferrite becomes excessive. Therefore, the cooling stop temperature is set to 600 ° C. or higher, preferably 630 ° C. or higher. On the other hand, when the cooling stop temperature exceeds 750 ° C., the surface integral of martensite becomes excessive. Therefore, the cooling stop temperature is set to 750 ° C. or lower, preferably 730 ° C. or lower. If the cooling rate of the third cooling is less than 1.0 ° C./sec, the surface integral of the polygonal ferrite becomes excessive. Therefore, this cooling rate is 1.0 ° C./sec or higher, preferably 3 ° C./sec or higher. On the other hand, when this cooling rate exceeds 10 ° C./sec, the surface integral of bainitic ferrite becomes too small. Therefore, this cooling rate is set to 10 ° C./sec or less, preferably 8 ° C./sec or less.
[0070]
　When the hole expandability is more important than the ductility, the cooling stop temperature is preferably 710 ° C. or higher, more preferably 720 ° C. or higher. This is because the surface integral of the polygonal ferrite is likely to be 20% or less. When ductility is more important than hole expandability, the cooling stop temperature is preferably less than 710 ° C, more preferably 690 ° C or lower. This is because the surface integral of the polygonal ferrite is likely to be more than 20% and 40% or less.
[0071]
　After the third cooling, the steel sheet is cooled to a temperature of 150 ° C. to 550 ° C. and held at that temperature for 1 second or longer. During this retention (second retention), the diffusion of C into the retained austenite is promoted. If the retention time is less than 1 second, C will not be sufficiently concentrated in retained austenite, the stability of retained austenite will decrease, and the surface integral of retained austenite will be too small. Therefore, the holding time is set to 1 second or longer, preferably 2 seconds or longer. If the holding temperature is less than 150 ° C., C is not sufficiently concentrated in the retained austenite, the stability of the retained austenite is lowered, and the surface integral of the retained austenite is too small. Therefore, the holding temperature is 150 ° C. or higher, preferably 200 ° C. or higher. On the other hand, when the holding temperature exceeds 550 ° C., the transformation from austenite to bainitic ferrite is delayed, so that the diffusion of C into the retained austenite does not proceed, the stability of the retained austenite decreases, and the surface integral of the retained austenite. The rate is too low. Therefore, the holding temperature is set to 550 ° C or lower, preferably 500 ° C or lower.
[0072]
　In this way, the steel sheet according to the embodiment of the present invention can be manufactured.
[0073]
　In the embodiment of the present invention described so far, a part of austenite is transformed into ferrite by controlling the primary cooling rate of the first annealing to 1 ° C./s or more and less than 10 ° C./s. With the formation of ferrite, Mn diffuses and concentrates in untransformed austenite. Due to the concentration of Mn in austenite, the yield stress of austenite increases in the second retention during the second annealing, and the transformation stress generated by the transformation to bainitic ferrite is relaxed. Favorable crystal orientation is preferentially generated. Therefore, the strain introduced into the bainitic ferrite is reduced, and the dislocation density can be controlled to 8 × 10 2 (cm / cm 3 ) or less. By controlling the dislocation density of vanitic ferrite to 8 × 10 2 (cm / cm 3 ) or less, the work effectiveness at the time of plastic deformation can be enhanced, so that excellent ductility can be obtained. The mechanism by which ductility is improved by reducing the dislocation density of bainitic ferrite is as follows. In TRIP steel, when martensite is formed from retained austenite by work-induced transformation, dislocations are introduced into adjacent bainitic ferrite and work hardening is performed. If the dislocation density of bainitic ferrite is low, the work hardening rate can be maintained high even in a region where the strain is large, so that uniform elongation is improved.
[0074]
　The steel sheet may be subjected to a plating treatment such as an electroplating treatment or a vapor deposition plating treatment, and further, an alloying treatment may be performed after the plating treatment. The steel sheet may be subjected to surface treatment such as formation of an organic film, film lamination, treatment with organic salts / inorganic salts, and non-chromium treatment.
[0075]
　When hot-dip galvanizing a steel sheet is performed as a plating process, for example, the temperature of the steel sheet is heated to a temperature 40 ° C. lower than the galvanizing bath temperature and 50 ° C. higher than the galvanizing bath temperature. Allow to cool and pass through a galvanized bath. By the hot-dip galvanizing treatment, a steel sheet having a hot-dip galvanized layer on the surface, that is, a hot-dip galvanized steel sheet can be obtained. The hot-dip galvanized layer has, for example, a chemical composition represented by Fe: 7% by mass or more and 15% by mass or less, and the balance: Zn, Al and impurities.
[0076]
　When the alloying treatment is performed after the hot-dip galvanizing treatment, for example, the hot-dip galvanized steel sheet is heated to a temperature of 460 ° C. or higher and 600 ° C. or lower. If this temperature is less than 460 ° C., alloying may be insufficient. If this temperature exceeds 600 ° C., alloying may be excessive and corrosion resistance may deteriorate. By the alloying treatment, a steel sheet having an alloyed hot-dip galvanized layer on the surface, that is, an alloyed hot-dip galvanized steel sheet can be obtained.
[0077]
　It should be noted that all of the above-described embodiments are merely examples of implementation of the present invention, and the technical scope of the present invention should not be construed in a limited manner by these. That is, the present invention can be implemented in various forms without departing from the technical idea or its main features.
Example
[0078]
　Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to this one condition example. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0079]
　(First Test) In
　the first test, slabs having the chemical compositions shown in Tables 1 to 3 were produced. The blanks in Tables 1 to 3 indicate that the content of the element was below the detection limit, and the balance was Fe and impurities. Underlines in Tables 1 to 3 indicate that the values ​​are outside the scope of the present invention.
[0080]
[table 1]

[0081]
[Table 2]

[0082]
[Table 3]

[0083]
　Then, after cooling once or without cooling, the slab was directly heated to 1100 ° C. to 1300 ° C. and hot-rolled under the conditions shown in Tables 4 to 7 to obtain a hot-rolled steel sheet. Then, it was pickled and cold-rolled under the conditions shown in Tables 4 to 7 to obtain a cold-rolled steel sheet. The underline in Tables 4 to 7 indicates that the numerical value is out of the range suitable for manufacturing the steel sheet according to the present invention.
[0084]
[Table 4]

[0085]
[Table 5]

[0086]
[Table 6]

[0087]
[Table 7]

[0088]
　Subsequently, the cold-rolled steel sheet was first annealed under the conditions shown in Tables 8 to 11 to obtain an intermediate steel sheet. The underline in Tables 8 to 11 indicates that the numerical value is out of the range suitable for manufacturing the steel sheet according to the present invention.
[0089]
[Table 8]

[0090]
[Table 9]

[0091]
[Table 10]

[0092]
[Table 11]

[0093]
　Next, the metallographic structure of the intermediate steel sheet was observed. In this observation, the area fraction of polygonal ferrite (PF), the area fraction of bainitic ferrite or tempered martensite (BF-tM), and the area fraction of retained austenite (residual γ) were measured, and further. The area fraction of the retained austenite grains of a predetermined form was calculated from the shape of the retained austenite. These results are shown in Tables 12 to 15. The underline in Tables 12 to 15 indicates that the numerical value is out of the range suitable for manufacturing the steel sheet according to the present invention.
[0094]
[Table 12]

[0095]
[Table 13]

[0096]
[Table 14]

[0097]
[Table 15]

[0098]
　Then, the intermediate steel sheet was second-annealed under the conditions shown in Tables 16 to 19 to obtain a steel sheet sample. Manufacturing No. 150 and No. In 151, a plating treatment was performed after the second annealing, and the production No. 151 was obtained. In 151, an alloying treatment was performed after the plating treatment. As the plating treatment, hot-dip galvanizing treatment was performed, and the temperature of the alloying treatment was set to 500 ° C. The underlines in Tables 16 to 19 indicate that the numerical values ​​are out of the range suitable for manufacturing the steel sheet according to the present invention.
[0099]
[Table 16]

[0100]
[Table 17]

[0101]
[Table 18]

[0102]
[Table 19]

[0103]
　Subsequently, the metallographic structure of the steel sheet sample was observed. In this observation, the area fraction of polygonal ferrite (PF), the area fraction of bainitic ferrite (BF), the area fraction of retained austenite (residual γ), and the area fraction of martensite (M) were measured. Further, the area fraction of the retained austenite grains of the predetermined form and the area fraction of the bainitic ferrite grains of the predetermined form were calculated from the shapes of the retained austenite and the bainitic ferrite. These results are shown in Tables 20 to 23. Underlines in Tables 20 to 23 indicate that the values ​​are outside the scope of the present invention.
[0104]
[Table 20]

[0105]
[Table 21]

[0106]
[Table 22]

[0107]
[Table 23]

[0108]
　Next, the mechanical properties of the steel plate sample (total elongation, 0.2% proof stress, tensile strength (maximum tensile strength), hole expansion value, ratio R / t of bending radius to plate thickness, and ductility-brittle transition temperature) are examined. It was measured. In the measurement of total elongation, 0.2% strength and tensile strength, JIS No. 5 test pieces with the direction perpendicular to the rolling direction (plate width direction) as the longitudinal direction are collected from the steel sheet sample, and a tensile test conforming to JIS Z 2242 is performed. Was done. In the measurement of the hole expansion value, a hole expansion test of JIS Z 2256 was performed. For the measurement of the ratio R / t, the test of JIS Z 2248 was performed. For the measurement of ductility-brittle transition temperature, JIS Z 2242 was tested. These results are shown in Tables 24 to 27. The underline in Tables 24 to 27 indicates that the value is out of the desired range.
[0109]
[Table 24]

[0110]
[Table 25]

[0111]
[Table 26]

[0112]
[Table 27]

[0113]
　As shown in Tables 24 to 27, Test Nos. Within the scope of the present invention. 1 and No. In the fourth invention example, excellent elongation, 0.2% proof stress, tensile strength, hole expansion value, ratio R / t and ductility-brittle transition temperature were obtained.
[0114]
　On the other hand, manufacturing No. 2 and No. The surface integral of polygonal ferrite such as 3 etc. is excessive, the surface integral of bainitic ferrite is insufficient, the surface integral of retained austenite is insufficient, and the proportion of retained austenite grains of a predetermined form is insufficient. In the comparative example in which the proportion of the bainitic ferrite grains of the predetermined form was insufficient, the elongation, the hole expansion value, and the ratio R / t were low. Manufacturing No. 5 and No. The surface integral of bainitic ferrite such as 6 is insufficient, the surface integral of martensite is excessive, the proportion of retained austenite grains of a predetermined form is insufficient, and the proportion of bainitic ferrite grains of a predetermined form is insufficient. In the insufficient comparative example, the elongation, the hole expansion value and the ratio R / t were low. Manufacturing No. 30 and No. In a comparative example in which the proportion of retained austenite grains of a predetermined form was insufficient, such as 37, the elongation was low. Manufacturing No. 70 and No. The surface integral of bainitic ferrite such as 85 is insufficient, the surface integral of martensite is excessive, the proportion of retained austenite grains of a predetermined form is insufficient, and the proportion of bainitic ferrite grains of a predetermined form is insufficient. In the insufficient comparative example, the elongation, the hole expansion value, and the ratio R / t were low.
Industrial applicability
[0115]
　The present invention can be used, for example, in industries related to steel sheets suitable for automobile parts.
The scope of the claims
[Claim 1]
　By mass%,
　C: 0.10% to 0.5%,
　Si: 0.5% to 4.0%,
　Mn: 1.0% to 4.0%,
　P: 0.015% or less,
　S: 0.050% or less,
　N: 0.01% or less,
　Al: 2.0% or less,
　Si and Al: 0.5% to 6.0% in total,
　Ti: 0.00% to 0.20%,
　Nb: 0.00% to 0.20%,
　B: 0.0000% to 0.0030%,
　Mo: 0.00% to 0.50%,
　Cr: 0.0% to 2.0%,
　V: 0.00% to 0.50%,
　Mg: 0.000% to 0.040%,
　REM: 0.000% to 0.040%,
　Ca: 0.000% to 0.040%, and the
　balance: Fe It
has a chemical composition represented by and impurities, and in terms of
　area fraction,
　polygonal ferrite: 40% or less,
　martensite: 20% or less,
　vanitic ferrite: 50% to 95%, and
　Residual austenite: It
has a metal structure represented by 5% to 50%, and
　80% or more of the bainitic ferrites have an aspect ratio of 0.1 to 1.0 and an aspect ratio of 0.1 to 1.0 in terms of area fraction. It is composed of bainitic ferrite grains with a dislocation density of 8 × 10 2 (cm / cm 3 ) or less in the region surrounded by grain boundaries with an orientation difference angle of 15 ° or more, and is composed of bainitic ferrite grains with an
　area fraction of the retained austenite. 80% or more of the retained austenite grains have an aspect ratio of 0.1 to 1.0, a major axis length of 1.0 μm to 28.0 μm, and a minor axis length of 0.1 μm to 2.8 μm. A steel plate characterized by being composed of.
[Claim 2]
　The metal structure, the area fraction,
　polygonal ferrite: 5% to 20%,
　martensite: 20% or less,
　bainitic ferrite: 75% to 90%, and
　residual austenite: 5% to 20%,
in Table The steel plate according to claim 1, wherein the steel plate is made.
[Claim 3]
　The metal structure, the area fraction,
　polygonal ferrite: 20 percent less than 40%,
　martensite: 20% or less,
　bainitic ferrite: 50% to 75%, and
　residual austenite: 5% to 30%,
in The steel plate according to claim 1, wherein the steel plate is represented.
[Claim 4]
　In the chemical composition, in terms of mass%,
　Ti: 0.01% to 0.20%,
　Nb: 0.005% to 0.20%,
　B: 0.0001% to 0.0030%,
　Mo: 0.01. % To 0.50%,
　Cr: 0.01% to 2.0%,
　V: 0.01% to 0.50%,
　Mg: 0.0005% to 0.040%,
　REM: 0.0005% to 　The steel plate according to any one of claims 1 to 3, wherein 0.040%,
　Ca: 0.0005% to 0.040%,
or any combination thereof holds.
[Claim 5]
　The steel sheet according to any one of claims 1 to 4, wherein the steel sheet has a plating layer formed on the surface thereof.