Technical field
[0001]
　The present invention relates to a steel sheet and plated steel.
BACKGROUND
[0002]
　Recently, weight reduction of various members for the purpose of improving fuel efficiency of automobiles has been required. Response to the request, and thinner due to the high strength of a steel sheet used for the various members, are applied to the various members of light metal such as Al alloy has been promoted. Light metal such as Al alloy, as compared to the heavy metals such as steel, specific strength is high. However, light metal is significantly expensive compared with heavy metals. Therefore, the application of light metals such as Al alloys are limited to special applications. Therefore, in order to apply the weight of the various components in more inexpensive wide range, thinning by high strength of a steel sheet is required.
[0003]
　In the steel plate used for the various members of the motor vehicle, depending on the application of the member, the strength as well as ductility, stretch flange formability, burring formability, fatigue endurance, material properties such as impact resistance and corrosion resistance is required. However, when increasing the strength of the steel sheet, material properties such as generally moldability (processability) is deteriorated. Therefore, in the development of high-strength steel sheet, it is important to achieve both of these material properties and strength.
[0004]
　Specifically, when manufacturing complex shaped parts with steel, for example, for machining shown below. Subjected to shearing or punching the steel sheet, after the opening blanking and hole, performing press molding and consisting mainly of stretch flanging or burring, the stretch forming. The steel sheet subjected to such processing, good stretch flangeability and ductility is required.
[0005]
　Further, in order to prevent deformation during the automobile parts collision, it is necessary to use a steel sheet having a high yield stress as a material for components. However, the higher the yield stress steel plate, there is a tendency that the ductility is poor. Therefore, the steel sheet used for various members of the motor vehicle, is also required both yield stress and ductility.
[0006]
　Patent Document 1, the steel structure has 95% or more of the ferrite phase at an area ratio, ductility average particle size of Ti carbides precipitated in the steel is 10nm or less, the stretch flange formability, high excellent material homogeneity strength hot rolled steel sheet is described. However, in the steel sheet disclosed in Patent Document 1 has a ferrite phase soft 95% or more, when securing a strength of at least 480 MPa, no sufficient ductility can not be obtained.
[0007]
　Patent Document 2, Ce oxides, La oxides, Ti oxides, Al 2 O 3 high-strength hot-rolled steel sheet excellent in stretch flange formability and fatigue characteristics comprising the inclusions is disclosed. Further, Patent Document 2, high-strength hot-rolled steel sheet area ratio of the bainitic ferrite phase in the steel sheet is 80 to 100% are described. Patent Document 3, the total area ratio of the ferrite phase and the bainite phase, defining the absolute value of the Vickers hardness difference between the ferrite phase and the second phase, the variation in intensity is small and high excellent and ductility and hole expansion strength hot rolled steel sheet is disclosed.
[0008]
　Further, conventionally, there is a steel sheet with a composite structure that combines a hard phase and a soft phase, such as excellent ferrite ductility such as bainite or martensite. Such steel plates are referred to as 2-phase structure (Dual Phase) steel sheet. Dual phase structure steel sheet, the uniform elongation to the intensity is good and is excellent in terms of strength ductility balance. For example, Patent Document 4, was polygonal ferrite + upper bainite structure, it is described a high-strength hot-rolled steel sheet having excellent stretch flangeability and impact properties. Further, Patent Document 5, the tissue is polygonal ferrite, bainite, the strength of low yield ratio of three-phase martensite - high-strength steel sheet having excellent elongation balance and stretch flangeability is described.
[0009]
　Conventional high strength steel sheet, the cold-press forming, it may crack from the edge portion serving as a flanging stretch during molding may occur. This is the time of blanking by distortion introduced into punched end face is believed to be due to work hardening only the edge portion will proceed.
[0010]
　The stretch flangeability test method for evaluating the steel sheet, the hole expansion test is used. However, the hole expansion test, the test piece in a state in which the circumferential direction of the strain distribution is hardly exists to fracture. In contrast, when processing actually steel sheet part shape, strain distribution is present. Strain distribution affects the fracture limit of the component. Thus, even in high-strength steel sheet showing a sufficient stretch flangeability in hole expanding test, by performing cold press, it is estimated that there are cases where cracks occur.
[0011]
　Patent Documents 1-5, by defining a tissue, a technique for improving material properties are disclosed. However, the steel sheet described in Patent Documents 1-5, whether a sufficient stretch flangeability even when considering strain distribution can be secured is not known.
CITATION
Patent Document
[0012]
Patent Document 1: WO 2013/161090
Patent Document 2: JP 2005-256115 Patent Publication
Patent Document 3: JP 2011-140671 Patent Publication
Patent Document 4: JP 58-42726 JP
Patent Document 5: JP HirakiAkira 57-70257 JP
Summary of the Invention
Problems that the Invention is to Solve
[0013]
　The present invention is a high strength, ductile and stretch flangeability good, and to provide a steel sheet and plated steel sheet having a high yield stress.
Means for Solving the Problems
[0014]
　According to conventional wisdom, the improvement of stretch flangeability of the high strength steel plate (hole expansion), as shown in Patent Documents 1-3, inclusions control, tissue homogenization, single organization and / or tissue It has been made, such as by reducing the difference in hardness between. In other words, conventionally, by controlling the tissue being observed by an optical microscope, the improvement of stretch flangeability is improved.
[0015]
　However, even by controlling only the tissue that is observed with an optical microscope, it is difficult to improve the stretch flangeability when the strain distribution is present. Accordingly, the present inventors focused on the orientation difference in the grains of the crystal grains, advanced intensive studies. As a result, by controlling the percentage of the total grain misorientation in the crystal grains is 5 ~ 14 ° crystal grain 20 to 100%, found that it is possible to greatly improve the stretch flangeability.
[0016]
　Further, the present inventors, the structure of the steel sheet, by precipitation state of precipitates in the crystal grains (number density and size) is intended to include two crystal grains different, excellent balance between strength and ductility steel sheet has been found that can be realized. This effect, the tissue of the steel sheet, by configuring to include a large crystal grain of relatively hardness smaller grains and hardness, even without the martensite is present, substantially two-phase structure (Dual functions such as Phase) is estimated to be due to obtain.
[0017]
　The present invention includes a new knowledge about the percentage of total grain misorientation in the crystal grains described above is 5 ~ 14 ° grain, the tissue of the steel sheet, number density and size of the precipitates in the crystal grains it is is based on the new finding by the intended comprising two crystal grains different, repeated studies present inventors have intensively, and have reached the completion.
[0018]
　The gist of the present invention is as follows.
[0019]
　(1)
　in
　mass%,
　C:
　0.008 ~ 0.150%, Si: 0.01 ~
　1.70%, Mn: 0.60 ~ 2.50%, Al: 0.010 ~ 0.60% , 　Ti:
　0 ~ 0.200%, Nb: 0 ~ 　0.200%, Ti + Nb: 0.015 ~ 0.200%, 　Cr: 0 ~ 1.0%, B: 0 ~ 　0.10%, Mo: 　~ 　1.0% 0, 　0 ~ 2.0%, Mg: 0 ~ 0.05%, REM: 　0 ~ 0.05%, Ca: 0 ~ 0.05 　%, Zr: 0 ~ 　0.05%, P: 0.05% or 　less, S: 0.0200% or 　less, N: 0.0060% or less, and 　the balance: Fe and impurities, 　organic in the chemical composition represented and, 　an area ratio, 　ferrite: 5% to 95%, and 　bainite: 5% to 95%, 　in a tissue represented, 　misorientation is surrounded by 15 ° or more grain boundaries, or When the circle equivalent diameter of the region is 0.3μm or more was defined as the crystal grain, a 20-100% percentage of the crystal grains of the whole grain intragranular orientation difference is 5 ~ 14 ° is an area ratio , 　maximum diameter less precipitate 8nm or clusters in said grains, 1 × 10 16 ~ 1 × 10 19 atoms / cm 3 maximum diameter in the hard crystal grains a dispersed by the number density of the crystal grains There is less of a precipitate or cluster 8 nm, 1 × 10 15 atoms / cm 3 include the following a soft crystal grains B dispersed in number density, the volume of the hard grain a% / (vol% of the hard crystal grains a + steel volume% of the soft grain B) is characterized in that from 0.1 to 0.9.

[0020]
　(2)
　tensile strength not less than 480 MPa,
　the tensile strength and the product of the limit forming height of saddle stretch flange test is at 19500mm · MPa or higher,
　that the product of the yield stress and ductility is 10000 MPa ·% or more steel sheet according to the constitution (1).
[0021]
　(3)
　the chemical composition, in
　mass%, Cr: 0.05 ~ 1.0%, and
　B: 0.0005 ~ 0.10%,
characterized in that it comprises one or more selected from the group consisting of steel sheet according to (1) or (2) and.
[0022]
　(4)
　the chemical composition, in
　mass%, Mo:
　0.01 ~ 1.0%, Cu: 0.01 ~ 2.0%, and
　Ni: 0.01% ~ 2.0%,
the group consisting of steel sheet according to any one of comprising at least one selected from (1) to (3).
[0023]
　(5)
　the chemical composition, by
　mass%,
　Ca: 0.0001 ~
　0.05%, Mg: 0.0001 ~ 0.05%, Zr: 0.0001 ~ 0.05%, and
　REM: 0. 0,001 to 0.05%,
the steel sheet according to any one of characterized in that it comprises one or more selected from the group consisting of (1) to (4).
[0024]
　(6)
　(1) plated steel sheet on the surface of the steel sheet according to any one of the - (5), characterized in that the plating layer is formed.
[0025]
　(7)
　The plated layer is plated steel sheet according to, characterized in that a galvanized layer (6).
[0026]
　(8)
　The plating layer is plated steel sheet according to characterized in that it is a galvannealed layer (6).
Effect of the invention
[0027]
　According to the present invention, a high strength, ductility and stretch flangeability is good, can provide a steel sheet having a high yield stress. Steel sheet of the present invention can be applied to a member severe ductility and stretch flangeability, yet high strength is required.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028]
[Figure 1A] Figure 1A is a perspective view showing a saddle molded product used in the saddle-type stretch flange test method.
FIG 1B] Figure 1B is a plan view showing a saddle molded product used in the saddle-type stretch flange test method.
DESCRIPTION OF THE INVENTION
[0029]
　Hereinafter, embodiments of the present invention will be described.
[0030]
"Chemical composition"
　will be described first chemical composition of the steel sheet according to an embodiment of the present invention. In the following description, a unit of content of each element contained in the steel sheet "%" is especially meant to "mass%" unless otherwise specified. Steel sheet according to the present embodiment, C: 0.008 ~ 0.150%, Si: 0.01 ~ 1.70%, Mn: 0.60 ~ 2.50%, Al: 0.010 ~ 0.60 %, Ti: 0 ~ 0.200% , Nb: 0 ~ 0.200%, Ti + Nb: 0.015 ~ 0.200%, Cr: 0 ~ 1.0%, B: 0 ~ 0.10%, Mo : 0 ~ 1.0%, Cu: 0 ~ 2.0%, Ni: 0 ~ 2.0%, Mg: 0 ~ 0.05%, rare earth metal (rare earth metal: REM): 0 ~ 0.05 %, Ca: 0 ~ 0.05% , Zr: 0 ~ 0.05%, P: 0.05% or less, S: 0.0200% or less, N: 0.0060% or less, and the balance: Fe and impurities , in having a chemical composition represented. As the impurity, those included in raw materials such as ores and scrap, intended to be included in the manufacturing process, is exemplified.
[0031]
"C: 0.008 ~ 0.150%"
　C is, Nb, combines with Ti or the like to form precipitates in the steel sheet, contributes to an increase in the strength of the steel by precipitation strengthening. The C content is less than 0.008% or not sufficiently obtained this effect. Therefore, C content is 0.008% or more. C content is preferably 0.010% or more, more preferably 0.018% or more. On the other hand, C content is 0.150% greater than the orientation dispersion tends to increase in bainite, misorientation in the grains is insufficient proportion of crystal grains of 5 ~ 14 °. Also, C content is 0.150% greater, increases harmful cementite for stretch flange formability, stretch flangeability is degraded. Therefore, C content is at most 0.150%. C content is preferably set to 0.100% or less, more preferably at most 0.090%.
[0032]
"Si: 0.01 ~ 1.70%"
　Si functions as a deoxidizer for molten steel. The Si content is less than 0.01%, not sufficiently obtained this effect. Therefore, Si content is less than 0.01%. Si content is preferably 0.02% or more, more preferably 0.03% or more. On the other hand, Si content is 1.70 percent, or degraded elongation flange formability, surface flaws or generated. Further, the Si content 1.70%, the too high is transformation point, need to occur higher rolling temperature. In this case, recrystallization during hot rolling is promoted considerably, misorientation in the grains is insufficient proportion of crystal grains of 5 ~ 14 °. Further, the Si content 1.70%, the surface defects are likely to occur when you are plated layer is formed on the surface of the steel sheet. Therefore, Si content is at most 1.70%. Si content is preferably not more than 1.60%, and more and preferably less 1.50%, more preferably at most 1.40%.
[0033]
"Mn: 0.60 ~ 2.50%"
　Mn is a solid solution strengthening, or by improving the hardenability of steel and contributes to improving the strength of the steel. The Mn content is less than 0.60%, not sufficiently obtained this effect. Therefore, Mn content is 0.60% or more. Mn content is preferably 0.70% or more, more preferably 0.80% or more. On the other hand, the Mn content is 2.50% greater, hardenability becomes excessive, the degree of orientation deviation in bainite increases. As a result, the azimuth difference in the grains is insufficient grain proportion of 5 ~ 14 °, stretch flangeability is degraded. Therefore, Mn content is at most 2.50%. Mn content is preferably not more than 2.30%, more preferably 2.10% or less.
[0034]
"Al: 0.010 ~ 0.60%"
　Al is effective as a deoxidizer for molten steel. The Al content is less than 0.010%, not sufficiently obtained this effect. Therefore, Al content is 0.010% or more. Al content is preferably 0.020% or more, more preferably 0.030% or more. On the other hand, Al content is 0.60 percent, such as weldability and toughness are deteriorated. Therefore, Al content is made 0.60%. Al content is preferably 0.50% or less, more preferably 0.40% or less.
[0035]
"Ti: 0 ~ 0.200%, Nb : 0 ~ 0.200%, Ti + Nb: 0.015 ~ 0.200% "
　Ti and Nb carbides (TiC, NbC) finely precipitated in the steel as, deposition enhancing the strength of steel by strengthening. Moreover, Ti and Nb, the C is fixed by forming a carbide, suppressing the generation of harmful cementite for stretch flangeability. Moreover, Ti and Nb, significantly improves the percentage of grain misorientation is 5 ~ 14 ° in the grains, while improving the strength of the steel, it is possible to improve the stretch flangeability. The total content of Ti and Nb is less than 0.015%, insufficient ratio of grain misorientation is 5 ~ 14 ° in the grains, stretch flangeability is degraded. Therefore, the total content of Ti and Nb is 0.015% or more. The total content of Ti and Nb is preferably 0.018% or more. Further, Ti content is preferably 0.015% or more, more preferably 0.020% or more, more preferably 0.025% or more. Further, Nb content is preferably 0.015% or more, more preferably 0.020% or more, more preferably 0.025% or more. On the other hand, the total content of Ti and Nb is in 0.200 percent, the ductility and formability is deteriorated, the frequency of cracking during rolling becomes high. Therefore, the total content of Ti and Nb is not more than 0.200%. The total content of Ti and Nb is preferably not more than 0.150%. Further, Ti content is 0.200 percent, the ductility is deteriorated. Therefore, Ti content is at most 0.200%. Ti content is preferably not more than 0.180%, more preferably at most 0.160%. Further, Nb content is 0.200%, the ductility is deteriorated. Therefore, Nb content is at most 0.200%. Nb content is preferably not more than 0.180%, more preferably at most 0.160%.
[0036]
"P: 0.05% or less"
　P is an impurity. P is the toughness, ductility, so deteriorating the welding property, P content is preferably as low. When the P content is 0.05 percent, significant stretch flangeability deterioration. Therefore, P content is made 0.05% or less. P content is preferably 0.03% or less, more preferably 0.02% or less. The lower limit of the P content is not particularly defined, but excessive reduction is undesirable from the viewpoint of production cost. Therefore, P content may be 0.005% or more.
[0037]
"S: 0.0200% or less"
　S is an impurity. S not only causes cracking during hot rolling, to form the A type inclusions that degrade the stretch flangeability. Thus, S content is preferably as low. When S content is 0.0200% greater, significant stretch flangeability deterioration. Therefore, S content is at most 0.0200%. S content is preferably not more than 0.0150%, and more preferably 0.0060% or less. The lower limit of the S content is not particularly defined, but excessive reduction is undesirable from the viewpoint of production cost. Therefore, S content may be 0.0010% or more.
[0038]
"N: 0.0060% or less"
　N is an impurity. N is preferentially than C, to form a with Ti and Nb precipitate, decreasing the effective Ti and Nb for fixing the C. Therefore, N content is preferably as low as. If N content is 0.0060% greater, significant stretch flangeability deterioration. Therefore, N content is made 0.0060% or less. N content is preferably 0.0050% or less. The lower limit of the N content is not particularly defined, but excessive reduction is undesirable from the viewpoint of production cost. Therefore, N content may be 0.0010% or more.
[0039]
　Cr, B, Mo, Cu, Ni, Mg, REM, Ca and Zr is not an essential element, a good optional elements be contained appropriately limit the predetermined amount on the steel sheet.
[0040]
"Cr: 0 ~ 1.0%"
　Cr contributes to the improvement of the strength of the steel. Although Cr may lack a desired purpose is achieved, in order to obtain this effect sufficiently, Cr content is preferably 0.05% or more. On the other hand, Cr content is 1.0 percent, economics and the effect is saturated is reduced. Therefore, Cr content is 1.0% or less.
[0041]
"B: 0 ~ 0.10%"
　B increases the hardenability and increases the structural fraction of the low-temperature transformation product phase is a hard phase. Although, even though it does not contain B intended purpose is achieved, in order to obtain this effect sufficiently, B content is preferably 0.0005% or more. On the other hand, B content is 0.10 percent, economics and the effect is saturated is reduced. Therefore, B content is at most 0.10%.
[0042]
"Mo: 0 ~ 1.0%"
　Mo has the effect of increasing the strength by forming a carbide improves the hardenability. Although, even though it does not contain Mo intended purpose is achieved, in order to obtain this effect sufficiently, Mo content is preferably 0.01% or more. On the other hand, Mo content is 1.0 percent, it may ductility and weldability is decreased. Therefore, Mo content is 1.0% or less.
[0043]
"Cu: 0 ~ 2.0%"
　Cu, as well as increase the strength of the steel sheet to improve the corrosion resistance and scale peelability. Although, even though it does not contain Cu intended purpose is achieved, in order to obtain this effect sufficiently, Cu content is preferably 0.01% or more, more preferably 0.04% or more . On the other hand, Cu content is 2.0 percent, sometimes surface defects may occur. Therefore, Cu content is 2.0% or less, preferably 1.0% or less.
[0044]
"Ni: 0 ~ 2.0%"
　Ni, as well as increase the strength of the steel sheet to improve the toughness. Although, even though it does not contain Ni intended purpose is achieved, in order to obtain this effect sufficiently, Ni content is preferably 0.01% or more. On the other hand, Ni content is 2.0 percent, the ductility is reduced. Therefore, Ni content is 2.0% or less.
[0045]
"Mg: 0 ~ 0.05%, REM : 0 ~ 0.05%, Ca: 0 ~ 0.05%, Zr: 0 ~ 0.05% "
　Ca, Mg, Zr and REM are both sulfide controls and the shape of the oxide to improve the toughness. Ca, Mg, but even if they do not contain Zr and REM intended purpose is achieved, in order to obtain this effect sufficiently, Ca, Mg, 1 or more selected from the group consisting of Zr and REM the content of preferably 0.0001% or more, more preferably 0.0005% or more. On the other hand, Ca, Mg, the content of either Zr or REM 0.05 percent, stretch flangeability is degraded. Therefore, the content of Ca, Mg, Zr and REM are both 0.05% or less.
[0046]
"Metal structure"
　will be described organization of the steel sheet according to an embodiment of the present invention (metal structure). In the following description, "%" is a unit of the percentage of each tissue (area ratio) is particularly means "area%" unless otherwise specified. Steel sheet according to the present embodiment, ferrite: 5% to 95%, and bainite: with 5 to 95%, in tissue represented.
[0047]
"Ferrite: 5% to 95%"
　ferrite area ratio is less than 5%, ductility is deteriorated, securing properties required in automotive parts and the like becomes difficult generally. Therefore, the area ratio of ferrite is 5% or more. On the other hand, the area ratio of ferrite is 95%, or becomes difficult stretch flangeability deteriorates, obtain sufficient strength. Therefore, the area ratio of ferrite is 95% or less.
[0048]
"Bainite: 5% to 95%"
　bainite area ratio of less than 5% stretch flangeability is degraded. Therefore, the area ratio of bainite is 5% or more. On the other hand, the area ratio of bainite in the 95%, ductility is deteriorated. Therefore, the area ratio of bainite is 95% or less.
[0049]
　The steel sheet of tissue, for example, martensite residual austenite may be included perlite and the like. If it exceeds 10% tissue area ratio of other than ferrite and bainite in total, stretch flangeability degradation is concerned. Therefore, the area of ​​the tissue other than ferrite and bainite is preferably 10% or less in total. In other words, the area ratio of ferrite and bainite is preferably a total of 90% or more, more preferably 100%.
[0050]
　The proportion of each tissue (area ratio) is determined by the following methods. First, etching a sample taken from the steel plate at nital. To structure photograph obtained by viewing the 300 [mu] m × 300 [mu] m in 1/4 depth position of the plate thickness using an optical microscope after etching, performing image analysis. The image analysis area ratio of the ferrite area ratio of pearlite, as well as the total area fraction of bainite and martensite is obtained. Then, using the Repera corroded samples, with respect to structure photograph obtained by viewing the 300 [mu] m × 300 [mu] m at the position of 1/4 depth of thickness with an optical microscope, and performs image analysis. This image analysis, the total area fraction of retained austenite and martensite is obtained. Further, using the cutting plane from the rolled surface direction normal to 1/4 depth of thickness sample by X-ray diffraction measurement determining the volume fraction of retained austenite. The volume fraction of retained austenite are the equivalent to the area ratio, which is the area ratio of residual austenite. Then, the area ratio of the martensite was obtained by subtracting the area ratio of residual austenite from the total area fraction of retained austenite and martensite, bainite by reducing the area ratio of martensite from the total area fraction of bainite and martensite area ratio is obtained. In this way, it is possible to obtain ferrite, bainite, martensite, the respective area ratios of residual austenite and pearlite.
[0051]
　The steel sheet according to the present embodiment, when the azimuth difference is surrounded by 15 ° or more grain boundaries, and the circle equivalent diameter of the region is 0.3μm or more is defined as crystal grains, intragranular orientation differences 5-14 percentage of the total grains ° a crystal grain is 20 to 100% area ratio. Misorientation in the grains the crystal orientation analysis in many electron beam backscatter diffraction pattern analysis used (electron back scattering diffraction: EBSD) technique is determined using. Misorientation in the grains, in tissue, the boundary misorientation is 15 ° or more and the grain boundary is a value when defining the region surrounded by the grain boundaries and crystal grains.
[0052]
　Grain misorientation in the grains is 5 ~ 14 ° is effective for obtaining a steel sheet having excellent balance between strength and workability. By increasing the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains, while maintaining the desired strength of the steel sheet, it is possible to improve the stretch flangeability. When percentage of the crystal grains of the whole grain intragranular orientation difference is 5 ~ 14 ° is in the area of ​​20% or more, the desired steel sheet strength and stretch flangeability is obtained. The proportion of crystal grain misorientation is 5 ~ 14 ° in the grains, since the higher may be, the upper limit is 100%.
[0053]
　As described later, by controlling the strain accumulation of the subsequent three stages of the finishing rolling, the crystal orientation difference occurs in grains of ferrite and bainite. Given this cause as follows. By controlling the accumulation distortion, dislocation in austenite increases, high density can dislocation walls within the austenite grains, several cell blocks are formed. These cell blocks have different crystal orientations. By transformation Thus a high dislocation density, and austenite containing the cell blocks of different crystal orientations, ferrite and bainite also be in the same particle, there is a crystal orientation difference, and also high dislocation density it is considered that. Thus, the crystal orientation difference in the grains are considered to be correlated with the dislocation density contained in the crystal grains. Generally, an increase in the dislocation density in grain, while providing improved strength, reducing the processability. However, the crystal grains misorientation in the grains is controlled to be 5 ~ 14 °, it is possible to improve the strength without reducing the workability. Therefore, the steel sheet according to the present embodiment, the heading difference in grain to grain ratio of 5 ~ 14 ° to 20% or more. Misorientation in the grains is less than 5 ° crystal grain is excellent in workability is difficult to strengthening. Misorientation in the grains is 14 ° more than the crystal grains, since deformability in the crystal grains are different, it does not contribute to the improvement of stretch flangeability.
[0054]
　The proportion of crystal grain misorientation is 5 ~ 14 ° in the grains can be measured by the following method. First, the rolling direction vertical section of the 1/4 depth position of the sheet thickness t from the surface of the steel sheet (1 / 4t part), 200 [mu] m in the rolling direction, the area of ​​100μm in the rolling surface normal direction in measured intervals 0.2μm EBSD obtain crystal orientation information by analyzing. Here EBSD analysis, using a thermal field emission scanning electron microscope (JEOL Ltd. JSM-7001F) and EBSD detector (TSL manufactured HIKARI detector) device constituted by, carried in the analysis speed of 200 to 300 points / sec to. Next, the crystal orientation information obtained by the 0.3μm or more regions misorientation 15 ° or more and a circle equivalent diameter is defined as the grain, it calculates the mean misorientation in grain grain, determining the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains. Average misorientation of crystal grains and the grains defined above can be calculated using the software provided with EBSD analyzer "OIM Analysis (registered trademark)".
[0055]
　The definitive embodiment "intragranular orientation difference" represents the azimuthal distribution of the crystal grains "Grain Orientation Spread (GOS)". The value of grain misorientation "Analysis of misorientation in plastic deformation of the stainless steel by EBSD method and X-ray diffraction method", Hidehiko Kimura other, of the Japan Society of Mechanical Engineers (A ed), Vol. 71, 712 No. 2005 , p. 1722-1728, as described in, is determined as an average value of misorientation between the crystal orientation and all the measurement points as a reference in the same crystal grains. In this embodiment, crystal orientation as a criterion is orientation obtained by averaging all the measured points in the same grain. GOS of value can be calculated by using the software that came with the EBSD analyzer "OIM Analysis (registered trademark) Version 7.0.1".
[0056]
　In the steel sheet according to the present embodiment, the area ratio of the tissue to be observed with an optical microscope organizations such as ferrite or bainite, the grain ratio of the misorientation is 5 ~ 14 ° in the grains, those directly related is not. In other words, for example, even if a steel sheet having an area ratio and the area ratio of bainite of the same ferrite, not necessarily the ratio of crystal grain misorientation is 5 ~ 14 ° in the grains it is the same. Accordingly, only by controlling the area ratio and the area ratio of the bainite ferrite can not be obtained a characteristic corresponding to the steel sheet according to the present embodiment.
[0057]
　The steel sheet according to the present embodiment, the maximum diameter in the crystal grains is less precipitates or clusters 8 nm, 1 × 10 16 ~ 1 × 10 19 atoms / cm 3 and a hard grain A dispersed by the number density of the crystal maximum diameter less precipitate 8nm or clusters in the grain, 1 × 10 15 atoms / cm 3 include the following a soft crystal grains B dispersed in number density, the volume of the hard grain a% / (hard grain a volume percent of + vol% soft grains B) is 0.1 to 0.9. The volume% of the hard grains A and soft vol% of crystal grains B are preferably a total of 70% or more, more preferably 80% or more. In other words, 1 × 10 15 atoms / cm 3 Ultra 1 × 10 16 atoms / cm 3 in volume percent of dispersed grains less than the number density of 30 percent, to obtain a characteristic corresponding to a steel sheet according to the embodiment hard to be there is. Thus, 1 × 10 15 atoms / cm 3 Ultra 1 × 10 16 atoms / cm 3 less than the grain volume% dispersed by the number density of, preferably 30% or less, more preferably 20% or less.
[0058]
　The size of the "precipitates or clusters" in the hard grains A and soft grain B, for a plurality of precipitates by a measuring method described later, respectively to measure the maximum diameter, the value obtained by calculating the average value is there. Maximum diameter of precipitates and the precipitates or if the cluster is of spherical in diameter, in the case of plate is defined as the diagonal length.
[0059]
　Precipitates or clusters in the crystal grains contributes to strengthening improvement of the steel sheet. However, the maximum diameter of the precipitates exceeds 8 nm, the distortion is concentrated on the precipitate ferrite tissue during processing of the steel sheet, for a possibility that the ductility deteriorates become a source of voids is increased, undesirably. The lower limit of the maximum diameter of precipitates is not particularly necessary to limit, in order to sufficiently exhibit the effect of improving the strength of the steel sheet due to the pinning force of dislocations in the crystal grains stably shall be 0.2nm or more it is preferable.
[0060]
　Precipitates or clusters in the present embodiment, Ti, Nb, carbides of one or more precipitate forming elements selected from the group consisting of Mo and V, to be formed of a nitride or carbonitride preferably . Here, the carbonitride, means a carbide nitrogen is mixed into a carbide, the composite precipitates carbides. Further, in the present embodiment, it allowed to contain in a range that does not inhibit the characteristic corresponding to a steel sheet carbides of the precipitate-forming elements, nitrides, or other precipitates than carbonitrides, according to this embodiment It is.
[0061]
　The steel sheet according to the present embodiment, based on the order to increase both tensile strength and ductility of the steel sheet for the purpose, the number density of precipitates or clusters of grains in the hard grains A and soft grain B below mechanisms to limit Te.
[0062]
　Any of the hard grains A and soft grain B, as the number density of precipitates in the crystal grains becomes high, it is considered that the hardness of the crystal grains is increased. Conversely, none of the hard grains A and soft grain B, as the number density of the carbide precipitates in the crystal grains is lowered, it is considered that the hardness of the crystal grains is reduced. In this case, elongation of the grains (total elongation, uniform elongation) is increased, the contribution to the strength is small.
[0063]
　When the number density of crystal grains of the precipitates of the hard crystal grains A and the soft grain B is approximately the same, the elongation for the tensile strength is reduced, sufficient strength ductility balance (YP × El) is obtained. On the other hand, if the number density difference of the hard crystal grains A and the soft grain precipitates in the crystal grains in the B large elongation for the tensile strength is increased, good strength ductility balance can be obtained. Hard grains A is responsible for the effect of increasing primarily strength. Soft crystal grains B is responsible for acting primarily enhance ductility. The present inventors have found that the strength ductility balance (YP × El) to obtain a good steel sheet, the number density of precipitates of hard crystal grains A 1 × 10 16 ~ 1 × 10 19 atoms / cm 3 and the number density of precipitates of soft grain of B 1 × 10 15 atoms / cm 3 was experimentally found that there needs to be less.
[0064]
　The number density of the precipitates of the hard crystal grains A is × 10 1 16 atoms / cm 3 is less than the strength of the steel sheet becomes insufficient and strength ductility balance can not be sufficiently obtained. The number density of precipitates of hard grains A is 1 × 10 19 atoms / cm 3 by weight, the effect of improving the strength of the steel sheet by the hard grains A is saturated and the cost increases due to the addition amount of precipitate forming elements cause or become, flange stretch toughness of ferrite and bainite is deteriorated or degraded.
[0065]
　The number density of precipitates soft crystal grains B is × 10 1 15 atoms / cm 3 by weight, the ductility of the steel sheet becomes insufficient and strength ductility balance can not be sufficiently obtained. For these reasons, in the present embodiment, the number density of precipitates of hard grain A 1 × 10 16 ~ 1 × 10 19 atoms / cm 3 and, 1 × 10 the number density of precipitates of soft grain B 15 pieces / Cm 3 or less.
[0066]
　Tissue in the present embodiment, the volume of the volume percent ratio {hard grain A hard grain A in the total volume of the steel sheet microstructure% / (vol% of the hard crystal grains A + soft vol% of crystal grains B) " } is in the range of 0.1 to 0.9. The volume% of the hard crystal grains A to the total volume of the steel sheet structure is from 0.1 to 0.9 strength ductility balance of the steel sheet as a target can be stably obtained. When the volume% ratio of hard grain A in the total volume of the steel sheet structure is less than 0.1, decreases the strength of the steel sheet, it is difficult to ensure a higher tensile strength 480MPa strength. If the volume percent of the ratio of the hard crystal grains A is more than 0.9, ductility of the steel sheet is insufficient.
[0067]
　Note that in the steel sheet according to the present embodiment, and it tissue is hard crystal grains A or soft grains B, and it is bainite or ferrite, not correspond. For example, if the steel sheet according to the present embodiment is a hot-rolled steel sheet, a rigid crystal grains A is mainly bainite, liable to be soft, crystal grains B is predominantly ferrite. However, it may be included much ferrite hard grains A hot-rolled steel sheet, it may be included in many bainite soft grain B. The area ratio of bainite or ferrite in the tissue, and the proportion of the hard crystal grains A and the soft grain B can be adjusted by such annealing.
[0068]
　The number density of precipitates or maximum diameter of the clusters, and the maximum diameter is less precipitate 8nm or cluster in the crystal grains in the structure of the steel sheet according to the present embodiment can be measured using the following method.
[0069]
　Maximum diameter less precipitates 8nm in the crystal grains, but also on the defect density in the tissue, it is difficult to generally its quantified by observation with a transmission electron microscope (TEM). Therefore, the maximum diameter is preferable to measure the maximum diameter and the number density of precipitates in the crystal grains with the three-dimensional atom probe (3D-AP) method is suitable for observing the following deposit 8nm . Furthermore, among the precipitates in order to accurately measure the maximum diameter and the number density of the cluster more size is small observation method according to 3D-AP is preferred.
[0070]
　Maximum diameter and the number density of precipitates or clusters in the crystal grains, using the observation method by 3D-AP, for example, can be measured as follows. First, cut a rod samples of 0.3 mm × 0.3 mm × 10 mm steel plate to be measured, and needle-like processed by electrolytic polishing method, a sample. Using this sample, it was measured over 500,000 atoms by 3D-AP in an arbitrary direction in the crystal grains, and quantitative analysis was visualized by three-dimensional map. The measurement of such optional direction, different crystal grains is performed for 10 or more, precipitation per volume number density (observation region of the maximum diameter and the maximum diameter is less precipitate 8nm precipitates contained in each grain obtaining object number) and the average value. The maximum diameter of precipitates in the crystal grains, the shape is obvious precipitates, shaped like a rod the length of the rod, a plate-like ones are diagonal length, those of spherical and diameter. Of precipitates, particularly small clusters of size, since in many cases the shape is not clear, the precise size measurement method utilizing the electrolytic evaporation of field ion microscope (FIM), the maximum diameter of precipitates and clusters it is preferable to determine.
[0071]
　More optional grain from any direction of the measuring results, precipitation state can know of precipitates in each crystal grain, different from the grains of distinguishing the precipitation state of precipitates, is knowing these volume ratio it can.
[0072]
　In addition to the above measurement method, it is possible to wider field of view, it is also possible to use a combination of field ion microscope (FIM) techniques. FIM, by a high voltage is applied to the sample in the needle, introducing an inert gas, a method of imaging an electric field distribution of the surface two-dimensionally. In general, it precipitates in the steel material, giving a bright or dark contrast than ferrite matrix. Perform field evaporation of a specific atomic planes one atomic plane, by observing the occurrence disappearance of contrast precipitates, depth size of the precipitates can be the estimated accurately.
[0073]
　In the present embodiment, stretch flangeability was used saddle shaped article is evaluated by the saddle-type stretch flange test method. 1A and 1B are views showing a saddle molded product used in the saddle-type stretch flange test method in the present embodiment, FIG. 1A is a perspective view, FIG. 1B is a plan view. In the saddle-type stretch flange test method, specifically, a saddle-shaped molded article 1 of the stretch flange shape imitating comprising a straight portion and an arc portion as shown in FIGS. 1A and 1B by pressing, the limit of the time evaluating the stretch flangeability using a forming height. In the saddle-type stretch flange test method in this embodiment, the curvature radius R of the corner portion 2 with a 50 ~ 60 mm, saddle molded article 1 was the opening angle θ of the corner portions 2 120 °, punching the corner portion 2 measuring the critical forming height H (mm) when the clearance at the time 11%. Here, the clearance between represents the ratio between the thickness of the gap between the test piece of the punching die and a punch. Clearance, since actually determined by a combination of punching tool and the plate thickness, and 11%, which means that within the ranges of 10.5 to 11.5%. Determination limit forming height H observes the presence or absence of cracks with more than one-third of the length of the plate thickness by visual observation after molding, the forming height limit cracks is not present.
[0074]
　Conventionally, hole expanding test which is used as a test method corresponding to stretch flange formability, the circumferential strain is to fracture with little distribution. Therefore, the time of actual stretch flanging strain or stress gradient in the peripheral breaks are different. Further, the hole expansion test, such as an evaluation at the time of breakage of the through thickness occurs, not in evaluation reflecting the original stretch flangeability. On the other hand, in the saddle-type stretch flange tests used in this embodiment, since it is possible to evaluate the stretch flangeability considering strain distribution, it can be evaluated that reflect the original stretch flangeability.
[0075]
　According to the steel sheet according to the present embodiment, tensile strength of at least 480MPa can be obtained. In other words, excellent tensile strength can be obtained. The upper limit of the tensile strength is not particularly limited. However, in the composition range of this embodiment, the upper limit of the substantial tensile strength is about 1180 MPa. Tensile strength, to produce a No. 5 test piece described in JIS-Z2201, by performing the tensile test according to the test method described in JIS-Z2241, can be measured.
[0076]
　According to the steel sheet according to the present embodiment, the product of the marginal forming height in tensile strength and saddle stretch flange test above 19500mm · MPa is obtained. In other words, excellent stretch flangeability is obtained. The upper limit of the product is not particularly limited. However, in the composition range of this embodiment, the upper limit of the substantial this product is about 25000 mm · MPa.
[0077]
　According to the steel sheet according to the present embodiment, the product of the above 10000 MPa ·% of yield stress and ductility is obtained. That is, it is possible to obtain excellent strength ductility balance.
[0078]
　Next, a method for manufacturing a steel sheet according to the embodiment of the present invention. In this way, performing hot rolling, the first cooling and the second cooling in this order.
[0079]
"Hot rolling"
　hot rolling includes a rough rolling and finish rolling. In hot rolling, heating the slab (slab) having the above chemical composition, performing rough rolling. Slab heating temperature is, and SRTmin ° C. or higher 1260 ° C. or less represented by the following formula (1).
SRTmin = [7000 / {2.75- log ([Ti] × [C])} - 273) + 10000 / {4.29-log ([Nb] × [C])} - 273)] / 2 ·· - (1)
　where, [Ti] in the formula (1), [Nb], [C] represents Ti in mass%, Nb, the content of C.
[0080]
　When the slab heating temperature is less than SRTmin ° C., Ti and / or Nb is not sufficiently solution. When Ti and / or Nb at a slab heating is not solution, carbides of Ti and / or Nb (TiC, NbC) as by fine precipitation, it is difficult to improve the strength of steel by precipitation strengthening. Further, when the slab heating temperature is less than SRTmin ° C., carbides (TiC, NbC) to secure the C by the formation of, it becomes difficult to suppress the formation of harmful cementite to burring properties. Further, when the slab heating temperature is less than SRTmin ° C., the crystal orientation difference in the grains is likely insufficient proportion of crystal grains of 5 ~ 14 °. Therefore, the slab heating temperature is set to more than SRTmin ° C.. On the other hand, when the slab heating temperature is 1260 ° C. greater than the yield by the scale off is reduced. Therefore, the slab heating temperature is set to 1260 ° C. or less.
[0081]
　Crude bars obtained by the rough rolling. Thereafter, the hot-rolled steel sheet obtained by finish rolling. The proportion of crystal grain misorientation is 5 ~ 14 ° in the grains to 20% or more, was 0.5-0.6 cumulative strain of later three stages (final three passes) at the finish rolling above, for cooling to be described later. This is due to the following reasons. Misorientation in the grains is 5 ~ 14 ° a crystal grain is produced by the transformation at para equilibrium at a relatively low temperature. Therefore, the limitation to a certain range the dislocation density of austenite before transformation in hot rolling, by limiting the range of the subsequent cooling rate, the crystal grain misorientation of the grains is 5 ~ 14 ° You can control the generation.
[0082]
　That is, by controlling the cumulative strain and subsequent cooling in the subsequent three stages of the finishing rolling can be controlled nucleation frequency of the crystal grains and the subsequent growth rate misorientation is 5 ~ 14 ° in the grains. As a result, it is possible to control the crystal grain area ratio of misorientation is 5 ~ 14 ° in the grains in the steel sheet obtained after cooling. More specifically, the dislocation density of austenite introduced by the finish rolling is involved primarily in the nucleation frequency, the cooling rate after rolling is mainly involved in the growth rate.
[0083]
　The cumulative strain of the subsequent three stages of the finishing rolling is less than 0.5, the dislocation density of austenite to be introduced is not sufficient and the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains is less than 20% . Therefore, subsequent three-stage cumulative strain of 0.5 or more. On the other hand, when the accumulation of subsequent three stages of the finishing rolling strain exceeds 0.6, recrystallization austenite during hot rolling occurs, the accumulation dislocation density during the transformation is reduced. As a result, the proportion of the crystal grains is less than 20% orientation difference in the grains is 5 ~ 14 °. Therefore, subsequent three-stage cumulative strain of 0.6 or less.
[0084]
　The subsequent three stages of the finishing rolling cumulative strain (Ipushironeff.) Is obtained by the following equation (2).
　εeff. = Σεi (t, T) ··· (2)
　where,
　.epsilon.i (t, T) = Ipushiron'ai0 / exp {(t / .tau.R) 2/3 },
　.tau.R = .tau.0 · exp (Q / RT),
　.tau.0 = × 10 8.46 -9
　,
　Q = 183200J, a = 8.314 J R / K · mol,,
　Ipushiron'ai0 represents the strain logarithm of time pressure, t denotes the accumulated time immediately before cooling in the path, T indicates the rolling temperature in the path.
[0085]
　The rolling end temperature Ar 3 When less than ° C., rising to excessive dislocation density of austenite before transformation, misorientation in the grains is grain is 5 ~ 14 ° and it is difficult to 20% or more. Therefore, the end temperature of finish rolling is Ar 3 and ℃ or higher.
[0086]
　Finish rolling, a plurality of rolling mill was linearly arranged, it is preferably carried out using a continuous rolling in one direction tandem mill to obtain a predetermined thickness. Also, when performing finish rolling by using a tandem rolling mill, by performing cooling between the rolling mill and the rolling mill (between stand cooling), the steel sheet temperature during the finish rolling is Ar 3 ° C. or higher ~ Ar 3 + 0.99 ° C. or less controlled to be in the range. The maximum temperature of the steel sheet during finish rolling is Ar 3 exceeds + 0.99 ° C., toughness is feared that degradation to the particle size is too large.
[0087]
　By performing the hot rolling conditions as above, to limit the dislocation density range before austenite transformation can misorientation in the grains to obtain a crystal grain which is 5 ~ 14 ° in the desired proportions.
[0088]
　Ar 3 is based on the chemical components of the steel sheet is calculated by the following equation in consideration of the influence of the transformation point by pressure (3).
　Ar 3 = 970-325 × [C] + 33 × [Si] + 287 × [P] + 40 × [Al] -92 × ([Mn] + [Mo] + [Cu]) - 46 × ([Cr] + [ Ni]) ··· (3)
　where, [C], [Si] , [P], [Al], [Mn], [Mo], [Cu], [Cr], [Ni] , respectively indicates C, Si, P, Al, Mn, Mo, Cu, Cr, the content by mass percent and Ni. For elements that are not contained, calculated as 0%.
[0089]
　"First cooling, the second cooling"
　after hot rolling, performing a first cooling and the second cooling hot-rolled steel sheet in this order. In the first cooling, cooling the hot-rolled steel sheet to a first temperature range of 600 ~ 750 ° C. at 10 ° C. / s or more cooling rate. In a second cooling, to cool the hot-rolled steel sheet to a second temperature range of 450 ~ 650 ° C. at a cooling rate higher than 30 ° C. / s. Between the first cooling and the second cooling, the hot rolled steel sheet for 1 to 10 seconds in the first temperature range. It is preferable to cool the hot-rolled steel sheet after the second cooling.
[0090]
　The cooling rate of the first cooling is less than 10 ° C. / s, the crystal orientation difference in the grains is insufficient proportion of crystal grains of 5 ~ 14 °. Further, when the cooling stop temperature of the first cooling is lower than 600 ° C., it becomes difficult to obtain more than 5% ferrite area ratio, the crystal orientation difference in the grains is 5 ~ 14 ° grains the percentage is insufficient. Further, when the cooling stop temperature of the first cooling is at 750 ° C. greater, it becomes difficult to obtain more than 5% of bainite area ratio, the crystal orientation difference in the grains is 5 ~ 14 ° grains the percentage is insufficient.
[0091]
　If the holding time at 600 ~ 750 ° C. is more than 10 seconds, harmful cementite is likely to generate the burring properties. Further, when the holding time at 600 ~ 750 ° C. greater than 10 seconds, the crystal orientation difference when many, the more particle to be difficult to obtain more than 5% of bainite in an area ratio of 5 ~ 14 ° crystals the proportion of grain is insufficient. 600 When the holding time at ~ 750 ° C. is less than 1 second, it becomes difficult to obtain a ferrite area ratio of 5% or more, insufficient ratio of crystal grains the crystal orientation differences 5 ~ 14 ° in the grains to.
[0092]
　The cooling rate of the second cooling is less than 30 ° C. / s, with harmful cementite is likely to generate the burring properties, crystal orientation difference in the grains is insufficient proportion of crystal grains of 5 ~ 14 °. Or a second cooled below stop temperature is 450 ° C. cooling, the or a 650 ° C. greater than the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains is insufficient.
[0093]
　The upper limit of the cooling rate in the first cooling and the second cooling is not particularly limited, it may be a plant capacity of the cooling equipment as follows 200 ° C. / s in consideration.
[0094]
　The temperature difference between the first cooling stop temperature and the cooling stop temperature in the second cooling of the cooling, it is effective to a 30 ~ 250 ° C.. When the temperature difference between the first cooling stop temperature and the cooling stop temperature in the second cooling of cooling is less than 30 ° C., the volume of the volume% {hard grain A hard grain A in the total volume of the steel sheet microstructure % / (vol% of the hard crystal grains a +% by volume of the soft grain B)} is less than 0.1. Therefore, the cooling stop temperature of the first cooling and the temperature difference between the cooling stop temperature in the second cooling and 30 ° C. or higher, preferably a 40 ° C. or more, more preferably 50 ° C. or higher. When the temperature difference between the first cooling stop temperature and the cooling stop temperature in the second cooling of cooling exceeds 250 ° C., vol% of the hard crystal grains A to the total volume of the steel sheet structure is greater than 0.9. Therefore, the cooling stop temperature of the first cooling and the temperature difference between the cooling stop temperature in the second cooling and 250 ° C. or less, preferably a 230 ° C. or less, more preferably 220 ° C. or less.
[0095]
　Further, the temperature difference between the first cooling stop temperature and the cooling stop temperature in the second cooling of the cooling, by a 30 ~ 250 ° C., tissue, maximum diameter in the crystal grains is less precipitate 8nm or cluster 1 × 10 16 ~ 1 × 10 19 atoms / cm 3 and a hard grain a dispersed by the number density of maximum diameter in the crystal grains is less precipitate 8nm or cluster 1 × 10 15 atoms / cm 3 made to include a soft crystal grains B dispersed in the following number density.
[0096]
　It can be obtained steel sheet according to this way this embodiment.
[0097]
　In the above-described manufacturing method, by controlling the conditions of hot rolling, introducing the machining dislocation austenite. On these, by controlling the cooling conditions, it is important to leave moderately the introduced processed dislocation. That is, even when controlling the condition or cooling condition of hot rolling alone, can not be obtained steel sheet according to the present embodiment, it is important to appropriately control both the conditions of the hot rolling and cooling is there. The conditions other than the above, for example, wound in a known manner after the second cooling, may be a known method, not particularly limited. Further, by dividing the temperature region to be deposited, it is possible to disperse the above-mentioned hard crystal grains A and the soft grain B.
[0098]
　In order to take the scale of the surface, it may also be pickled. If as hot rolling and cooling conditions are described above, followed by cold rolling, heat treatment (annealing), even if the plating and the like can achieve the same effect.
[0099]
　In cold rolling, the reduction rate is preferably set to 90% or less. If rolling reduction in the cold rolling exceeds 90%, the ductility is reduced. Portion which is collapse large hard grains A and soft grain B by cold rolling, recrystallized grains during annealing after cold rolling, was hard grains A and soft grain B after hot rolling both were encroachment of presumably because disappears crystal grains having two types of hardness. May not be performed cold rolling, the lower limit of the rolling reduction in cold rolling is 0%. As described above, while the heat-rolled plate, has excellent moldability. On the other hand, on dislocation introduced by cold rolling, a collection Ti remains solid solution, Nb, Mo and the like, by precipitation, it is possible to improve the yield point (YP) and tensile strength (TS). Accordingly, the cold rolling can be used to adjust the intensity. Cold-rolled steel sheet obtained by cold rolling.
[0100]
　The temperature of the heat treatment (annealing) after the cold rolling is preferably 840 ° C. or less. During annealing, enhanced by that could not be precipitated at the stage of hot rolling Ti and Nb are precipitated, complex phenomena such as softening by coarsening of dislocation recovery, precipitation occurs. When the annealing temperature exceeds 840 ° C., a large effect of coarsening of precipitates, shortage with a maximum diameter of less following deposit 8 nm, the crystal orientation difference in the grains is the proportion of crystal grains of 5 ~ 14 ° to. Annealing temperature is more preferably set to 820 ° C. or less, further preferably 800 ° C. or less. The lower limit of the annealing temperature is not particularly provided. As described above, while the heat-rolled plate is not carried out annealing, in order to have excellent formability.
[0101]
　On the surface of the steel sheet of the present embodiment, the plating layer may be formed. That is, the plated steel sheet can be mentioned as another embodiment of the present invention. Plating layer, for example, electroplated layer, a hot-dip plated layer or an alloyed hot-dip plating layer. The melt plating layer and alloyed hot-dip plated layer, for example, a layer consisting of at least one of zinc and aluminum. Specifically, galvanized layer, galvannealed layer, molten aluminum plating layer, alloyed molten aluminum plating layer, melt Zn-Al plating layer, and the like alloyed molten Zn-Al plating layer. In particular, from the viewpoint of the plating easiness and corrosion resistance, hot-dip galvanized layer and the galvannealed layer.
[0102]
　Dip plated steel sheets and alloyed hot dip plated steel sheet is produced by subjecting a molten plating or alloyed hot dip plated against steel sheet according to the present embodiment described above. Here, the alloying hot dip plating, to form a molten plating layer on the surface is subjected to hot-dip plating, and then refers to the molten plating layer alloyed hot dip plating layer is subjected to alloying treatment. Steel sheet plating may be a hot-rolled steel sheet, it may be a steel sheet subjected to the annealing and cold rolling the hot-rolled steel sheet. Dip plated steel sheets and alloyed hot dip plated steel sheet has a steel sheet according to the present embodiment, and the molten plating layer and alloyed hot-dip plating layer is formed on the surface, along with effects of the steel sheet according to the embodiment , excellent corrosion resistance can be achieved. Before plating, as a pre-plating may be with a Ni or the like on the surface.
[0103]
　When performing heat treatment steel sheet (annealing), after heat treatment, it is immersed in the galvanizing bath, it may form a galvanized layer on the surface of the steel sheet. In this case, the original plate of the heat treatment may be a hot-rolled steel sheet may be cold-rolled steel sheet. After forming the hot-dip galvanizing layer, reheated, and a plating layer and the base steel by performing an alloying treatment for alloying, it may form a galvannealed layer.
[0104]
　Plated steel sheet according to the embodiment of the present invention, since the plating layer is formed on the surface of the steel sheet has excellent corrosion resistance. Thus, for example, by using the plated steel sheet of the present embodiment, the member of the automobile when thin, that the service life of the automobile is shortened by corrosion of the members it can be prevented.
[0105]
　The above embodiments are all merely illustrate concrete examples of implementing the present invention, in which technical scope of the present invention should not be limitedly interpreted. That is, the present invention is its technical idea or without departing from the essential characteristics thereof, can be implemented in various forms.
Example
[0106]
　Next, a description will be given of an embodiment of the present invention. Conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to this single example of conditions. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0107]
　Table 1 and smelted steel having the chemical composition shown in Table 2 to produce a steel slab, the resulting steel pieces were heated to a heating temperature shown in Table 3 and Table 4, it performs rough rolling in hot It was subsequently subjected to finish rolling under the conditions shown in Table 3 and Table 4. Thickness of hot-rolled steel sheet after finish rolling was 2.2 ~ 3.4 mm. Blank in Table 1 and Table 2 means that analytical values ​​were below the detection limit. Table 1 and underlined in Table 2 indicates that the value is out of range of the present invention, underlined in Table 4 show that is out of the range suitable for the manufacture of steel sheet of the present invention.
[0108]
[Table 1]

[0109]
[Table 2]

[0110]
[table 3]

[0111]
[Table 4]

[0112]
　Ar 3 (° C.) was determined using Equation (3) from components shown in Table 1 and Table 2.
　Ar 3 = 970-325 × [C] + 33 × [Si] + 287 × [P] + 40 × [Al] -92 × ([Mn] + [Mo] + [Cu]) - 46 × ([Cr] + [ Ni]) ··· (3)
[0113]
　Finishing three stages cumulative strain was determined from the equation (2).
　εeff. = Σεi (t, T) ··· (2)
　where,
　.epsilon.i (t, T) = Ipushiron'ai0 / exp {(t / .tau.R) 2/3 },
　.tau.R = .tau.0 · exp (Q / RT),
　.tau.0 = × 10 8.46 -9
　,
　Q = 183200J, a = 8.314 J R / K · mol,,
　Ipushiron'ai0 represents the strain logarithm of time pressure, t denotes the accumulated time immediately before cooling in the path, T indicates the rolling temperature in the path.
[0114]
　Then, Table 5 and the first cooling under the conditions shown in Table 6, held at the first temperature range, performing a second cooling, Test No. To obtain a hot-rolled steel sheet of 1-44.
[0115]
　Test No. The hot-rolled steel sheet 21 is subjected to cold rolling at a reduction ratio shown in Table 5, it was subjected to a heat treatment at a heat treatment temperature shown in Table 5, to form a galvanized layer, further subjected to alloying treatment, the surface It was formed galvannealed layer (GA) to. Test No. The hot-rolled steel sheet of 18 to 20, 44, was subjected to heat treatment in the heat treatment temperature shown in Table 5 and Table 6. Test No. Hot-rolled steel sheets 18-20 are subjected to a heat treatment to form a galvanized layer (GI) to the surface. It underlined in Table 6 show that is out of the range suitable for the manufacture of steel sheet of the present invention.
[0116]
[table 5]

[0117]
[Table 6]

[0118]
　Each steel sheet (hot-rolled steel test No.1 ~ 17 and 22 ~ 43, the hot-rolled steel sheet of the test No.18 ~ 20, 44 that has been subjected to the heat treatment, cold rolled steel sheets of test No.21 subjected to heat treatment) for, by the following method, it was determined ferrite, bainite, martensite, structural fraction of pearlite (area ratio), and the proportion of crystal grain misorientation is 5 ~ 14 ° of intragranular. The results are shown in Table 7 and Table 8. If it contains martensite and / or pearlite, as described in parentheses in the column of "bainite area ratio" in the table. Underlined in Table 8 indicates that the value is out of range of the present invention.
[0119]
"Ferrite, bainite, martensite, structural fraction of pearlite (area ratio)"
　First, the samples taken from the steel sheet was etched with nital. To structure photograph obtained by viewing the 300 [mu] m × 300 [mu] m in 1/4 depth position of the plate thickness using an optical microscope after etching, and then image analysis was carried out. The image analysis area ratio of the ferrite, pearlite area ratio, as well as to give a total area ratio of bainite and martensite. Then, using the Repera corroded samples, with respect to structure photograph obtained by viewing the 300 [mu] m × 300 [mu] m at the position of 1/4 depth of thickness with an optical microscope, and then image analysis was carried out. This image analysis, to give a total area fraction of retained austenite and martensite. Further, using the cutting plane from the rolled surface direction normal to 1/4 depth of thickness sample, it was determined volume fraction of residual austenite by X-ray diffraction measurement. The volume fraction of retained austenite are the equivalent to the area ratio, which was used as a area ratio of residual austenite. Then, the total of area ratios obtained area ratio of martensite by reducing the area ratio of residual austenite, the area of the bainite by reducing the area ratio of martensite from the total area fraction of bainite and martensite of retained austenite and martensite to obtain the rate. There was thus obtained ferrite, bainite, martensite, the respective area ratios of residual austenite and pearlite.
[0120]
"Misorientation proportion of crystal grains is 5 ~ 14 ° in the grains,"
　the rolling direction vertical section of the 1/4 depth position of the sheet thickness t from the surface of the steel sheet (1 / 4t part), 200 [mu] m in the rolling direction, rolling the 100μm region of the surface normal direction and EBSD analysis measurement interval 0.2μm was obtained crystal orientation information. Here, EBSD analysis, thermal field emission scanning electron microscope (JEOL Ltd. JSM-7001F) and EBSD detector using an apparatus comprised of (TSL manufactured HIKARI detector), the analysis speed of 200 to 300 points / sec Carried out. Next, the crystal orientation information obtained by defining a 0.3μm or more regions misorientation 15 ° or more and a circle equivalent diameter and grain, calculates the mean misorientation in grain grain, grain misorientation inner was determined the ratio of 5 ~ 14 ° a crystal grain. Average misorientation of crystal grains and the grains defined above was calculated using the software provided with EBSD analyzer "OIM Analysis (registered trademark)".
[0121]
　Each steel plate on (hot-rolled steel test No.1 ~ 17 and 22 ~ 43, the hot-rolled steel sheet of the test No.18 ~ 20, 44 that has been subjected to the heat treatment, cold rolled steel sheets of test No.21 subjected to heat treatment), by the following method,
and the maximum diameter of precipitates or clusters in the crystal grain, the maximum diameter was measured and the number density of the following precipitates or clusters 8 nm. Further, using the obtained measurement values, calculates the volume percent of the volume percent and the soft grain B of the hard crystal grains A, the volume of the hard grain A% / (volume of the hard grain A% + soft crystals was determined particle volume percent of B) {volume ratio a / (a + B)} . The results are shown in Table 7 and Table 8.
[0122]
"Precipitates or maximum diameter of the clusters, and the maximum diameter measured in the number density of less precipitates or clusters 8nm in grain"
　maximum diameter and the number density of precipitates or clusters in the crystal grains is by 3D-AP observation method was used to measure in the following manner. Cut rod-like sample of 0.3 mm × 0.3 mm × 10 mm from the measured steel sheet, and a needle-like processed by electrolytic polishing to prepare a sample. Using this sample, it was measured over 500,000 atoms by 3D-AP in an arbitrary direction in the crystal grains, and quantitative analysis was visualized by three-dimensional map. The measurement of such optional direction, different crystal grains is performed for 10 or more, precipitation per volume number density (observation region of the maximum diameter and the maximum diameter is less precipitate 8nm precipitates contained in each grain object number) and was determined as an average value. The maximum diameter of precipitates in the crystal grains, the shape is obvious precipitates, shaped like a rod the length of the rod, a plate-like ones are diagonal length, those of spherical and the diameter. Of precipitates, particularly small clusters of size, since in many cases the shape is not clear, the precise size measurement method utilizing electrolysis evaporation of field ion microscope (FIM), the maximum diameter of precipitates and clusters Were determined.
[0123]
　In addition to the above measurement method, it is possible to wider field of view, using the combination of field ion microscope (FIM) techniques. FIM, by a high voltage is applied to the sample in the needle, introducing an inert gas, a method of imaging an electric field distribution of the surface two-dimensionally. Or brighter than the ferrite matrix, or dark contrast was precipitate. Perform field evaporation of a specific atomic planes one atomic plane, by observing the occurrence disappearance of contrast precipitate was estimated depth direction size of the precipitates.
[0124]
[Table 7]

[0125]
[Table 8]

[0126]
　Test No. Hot-rolled steel sheet of 1 to 17, 22 to 43, the test was subjected to a heat treatment No. Hot-rolled steel sheet of 18 to 20, 44, the test was subjected to a heat treatment No. For cold-rolled steel sheet 21, in a tensile test, determined and a yield strength tensile strength, the saddle stretch flange tested to determine the limits forming height of the flange. Then, the tensile strength (MPa) and the flange of the indicator elongation product of the marginal forming height (mm), if the product is more than 19500mm · MPa, is determined that excellent stretch flangeability. The tensile strength (TS) is the case where more than 480 MPa, was judged to be high strength. Also, the product of the yield stress (YP) and ductility (EL) is the case where 10000 MPa ·% or more, strength ductility balance is determined to be good. The results are shown in Tables 9 and 10. Underlined in Table 10 indicates that the numerical value is outside the desired range.
[0127]
　Tensile tests were taken from the direction perpendicular to the rolling direction JIS5 No. Tensile test pieces, using the test pieces were tested in accordance with JISZ2241.
[0128]
　Saddle stretch flange test uses R60mm curvature radius of the corner, and the opening angle theta 120 ° the saddle-type molded article was performed with 11% clearance when punching the corner portion. Limit forming height is visually after molding, to observe the presence or absence of cracks with more than one-third of the length of the sheet thickness, and the forming height limit cracks is not present.
[0129]
[Table 9]

[0130]
[Table 10]

[0131]
　Invention sample (Test No.1 ~ 21), more tensile strength 480 MPa, the product of the limit forming height in tensile strength and saddle stretch flange test above 19500mm · MPa, and 10000 MPa ·% or more yield stress and ductility the product of the was obtained.
[0132]
　Test No. 22-28, the chemical component is a comparative example outside the scope of the present invention. Test No. 22-24 and test No. 28, an index of stretch flangeability did not satisfy the target value. Test No. 25, since a small total content of Ti and Nb, the product of the stretch flangeability and yield stress and (YP) and ductility (EL) did not satisfy the target value. Test No. 26, since the total content of Ti and Nb is large, workability is deteriorated, cracks occurred during rolling.
[0133]
　Test No. 28-44, as a result of outside the range manufacturing conditions is desired, the number of tissues, the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains, precipitates hard crystal grains A are observed with an optical microscope density, number density of precipitates of soft grain in B, either one or more volume ratio {volume of hard grain a% / (vol% of the hard crystal grains a + soft vol% of crystal grains B)} There is a comparative example that did not satisfy the scope of the present invention. Test No. 29-41 and test No. 44, the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains is less, the product and / or stretch flangeability indication of yield stress and (YP) and ductility (EL) is satisfied the target value There was no. Test No. 42-43 is larger in volume ratio {A / (A + B)}, product and stretch flangeability indication of yield stress and (YP) and ductility (EL) did not satisfy the target value.
Industrial Applicability
[0134]
　According to the present invention, a high strength, ductility and stretch flangeability is good, can provide a steel sheet having a high yield stress. Steel sheet of the present invention can be applied to members that strict stretch flange formability yet high strength is required. Steel sheet of the present invention is a material suitable for lightweight by thinning of automotive parts, to contribute to improved fuel efficiency of an automobile, has high industrial applicability.

The scope of the claims
[Requested item 1]
　By
　mass%,
　C:
　0.008 ~ 0.150%, Si: 0.01 ~
　1.70%, Mn: 0.60 ~ 2.50%, Al: 0.010 ~
　0.60%, Ti:
　~
　0.200% 0, Nb: 0 ~
　0.200%, Ti + Nb: 0.015 ~ 0.200%,
　Cr: 0 ~ 1.0%, B: 0 ~ 0.10%,
　Mo: 0 ~ 1
　%
　.0,
　Cu: 0 ~
　2.0%, Ni: 0 ~ 2.0%, Mg: 0 ~ 0.05%,
　REM: 0 ~ 0.05%, Ca: 0 ~
　0.05%, Zr : 0 ~
　0.05%, P: 0.05% or
　less, S: 0.0200% or
　less, N: 0.0060% or less, and
　the balance: Fe and impurities,
　have in a chemical composition represented,
　the area at the rate,
　ferrite: 5% to 95%, and
　bainite: 5% to 95%,
　in a tissue represented,
　misorientation is surrounded by 15 ° or more grain boundaries, and Ensho If the diameter is the region is 0.3μm or more was defined as the crystal grain, a 20-100% percentage of the crystal grains of the whole grain intragranular orientation difference is 5 ~ 14 ° is an area ratio,
　the maximum diameter in the crystal grains is less precipitates or clusters 8nm, 1 × 10 16 ~ 1 × 10 19 atoms / cm 3 and a hard grain a dispersed by the number density of the maximum diameter in said crystal grains 8nm the following precipitates or clusters, 1 × 10 15 atoms / cm 3 and a following number density soft grain dispersed in B, the volume of the hard grain a% / (volume of the hard grain a% + soft crystals steel volume% particle B) is characterized in that from 0.1 to 0.9.
[Requested item 2]
　Tensile strength not less than 480 MPa,
　the product of the limit forming height in tensile strength and saddle stretch flange test is at 19500mm · MPa or higher,
　and wherein the product of the yield stress and ductility is 10000 MPa ·% or more steel sheet according to claim 1.
[Requested item 3]
　The chemical composition, in
　mass%, Cr: 0.05 ~ 1.0%, and
　B: 0.0005 ~ 0.10%,
characterized in that it comprises at least one selected from the group consisting of wherein steel sheet according to claim 1 or 2.
[Requested item 4]
　The chemical composition, in
　mass%, Mo:
　0.01 ~ 1.0%, Cu: 0.01 ~ 2.0%, and
　Ni: 0.01% ~ 2.0%,
is selected from the group consisting of steel sheet according to any one of claims 1 to 3, characterized in that it comprises one or more that.
[Requested item 5]
　The chemical composition, by
　mass%,
　Ca: 0.0001 ~
　0.05%, Mg: 0.0001 ~ 0.05%, Zr: 0.0001 ~ 0.05%, and
　REM: 0.0001 ~ 0 .05%,
the steel sheet according to any one of claims 1 to 4, characterized in that it comprises one or more selected from the group consisting of to.
[Requested item 6]
　Plated steel sheet on the surface of the steel sheet according to any one of claims 1 to 5, characterized in that the plating layer is formed.
[Requested item 7]
　Plated steel sheet according to claim 6 wherein the plating layer is, which is a galvanized layer.
[Requested item 8]
　Plated steel sheet according to claim 6 wherein the plating layer is, which is a galvannealed layer.