Technical field
[0001]
　The present invention relates to a steel sheet and plated steel.
BACKGROUND
[0002]
　Recently, application of the fuel efficiency of an automobile relative demand for weight reduction of various members for the purpose, and thinning with high strength of a steel sheet of an iron alloy or the like used for members, the various members of light metal such as Al alloy It has been promoted. However, when compared to heavy metals such as steel, although a light metal such as Al alloy is advantageous in that the specific strength high, there is a disadvantage that it is significantly more expensive. 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]
　If high strength steel sheet, material properties such as generally moldability (processability) is deteriorated. Therefore, in the development of high-strength steel sheet is an important challenge is to achieve high strength without deteriorating the material properties. Steel, depending on the application, ductility, stretch flange formability, burring formability, ductility, fatigue resistance, impact resistance and corrosion resistance is required, is possible to achieve both of these material properties and strength are important.
[0004]
　For example, after the opening blanking and holes were made by shearing or punching, press forming is performed mainly composed of stretch flanging or burring, good stretch flangeability is required.
[0005]
　Against challenge good stretch flangeability above, for example, Patent Document 1, by limiting the size of TiC, ductility, stretch flangeability, is disclosed that can provide a hot-rolled steel sheet excellent in material homogeneity ing. Further, Patent Document 2, kinds of oxides, by defining the size and number density, it is disclosed that can provide hot-rolled steel sheet excellent in fatigue properties and stretch-flange formability. Further, Patent Document 3, the area ratio of the ferrite phase, and by defining the hardness difference between the ferrite phase and the second phase, small variations in strength, and the hot-rolled steel sheet excellent in the ductility and hole expansion it is disclosed that can provide.
[0006]
　However, in the above Patent Document 1 a technique disclosed, it is necessary to secure a ferrite phase over 95% in the tissue of the steel sheet. Therefore, in order to ensure sufficient strength, even if the 480 MPa class (TS more than 480 MPa), it is necessary to include Ti 0.08% or more. On the other hand, in the steel having a ferrite phase soft 95% or more, to secure the strength of more than 480MPa by precipitation hardening of TiC, decrease in ductility is a problem. Further, in the technique disclosed in Patent Document 2, the addition of rare metals such as La and Ce are essential. Thus, the technique disclosed in Patent Document 2 both have the problem of limitations of the alloying elements.
[0007]
　As described above, in recent years, the automobile member, there is an increasing demand for the application of high-strength steel sheet. When forming a high-strength steel plate and pressed with cold, cracks from the edge portion serving as flanging stretch during molding tends to occur. This is believed to be due to causes proceed hardening only the edge portion by distortion introduced into the end face punching during blanking. The test method for evaluating the conventional stretch flangeability, hole expanding test is used. However, the hole expansion test is to fracture with little distribution strain in the circumferential direction, in the processing of actual part, for the strain distribution is present, the influence of the fracture limit with a gradient of strain and stress around breaks there exist. Therefore, in the case of high strength steel sheet, even it showed sufficient stretch flangeability in hole expanding test, when performing cold press may crack by strain distribution is generated.
[0008]
　Patent Documents 1 and 2, by defining only the tissue to be observed with an optical microscope, it is disclosed that improve the hole expansion. However, whether a sufficient stretch flangeability even when considering strain distribution can be secured is not known. Further, the steel sheet used for such members, flaws or microcracks on the end face formed by shearing or punching processing occurs, crack than those generated flaws or microcracks developed, fatigue fracture it is a concern throughout. Therefore, in the end surface of the steel sheet, there is a need to not cause flaws or microcracks in order to improve the fatigue resistance. As flaws and microcracks that occur these end faces, parallel cracks generated in the thickness direction of the end face. This cracking is referred to as "peeling". This "peeling", especially in the 540MPa class steel sheet, about 80%, produced almost 100% at 780MPa grade steel. Further, this "peeling" occurs without correlation with the hole expanding ratio. For example, the hole expanding ratio is even 50%, even 100%, peeling occurs.
[0009]
　Thus the high strength, in particular for both and various material properties such as formability, for example, Patent Document 4, a steel structure, ferrite is 90% or more, by the balance bainite the method of the steel sheet having both high strength and ductility, and hole expansion is disclosed. However, the present inventors have made additional test, the steel composition disclosed in Patent Document 4, "peeling" occurs after punching.
[0010]
　Further, for example, Patent Documents 2 and 3, by refining the precipitates by adding Mo, yet high strength, the technique of high-tensile hot-rolled steel sheet to achieve superior stretch flangeability disclosed ing. However, the steel sheet according to the technology disclosed in Patent Documents 2 and 3 described above also, the present inventors have made additional test, the steel composition disclosed in Patent Document 5 or 6, "peeling after stamping "There has occurred. Therefore, it can be said that the technique disclosed in Patent Documents 2 and 3, there is no disclosure about a technique for suppressing flaws or microcracks on the end faces which are formed by shearing or punching processing.
[0011]
　Further, while, as described above, to achieve a weight reduction by thinning due to corrosion, there is a tendency that the service life is shortened automobile. Furthermore, in order to improve the rust of the steel sheet, which is stronger demand for coated steel sheet.
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-A 6-293910 Patent Publication
Patent Document 5: JP 2002-322540 JP
Patent Document 6: JP 2002-322541 Patent Publication
Summary of the Invention
Problems that the Invention is to Solve
[0013]
　The present invention is a high strength, has excellent stretch flangeability, and an object thereof is to provide a small steel and plated steel generation of peeling.
Means for Solving the Problems
[0014]
　According to conventional wisdom, the improvement of stretch flangeability (hole expansion), as shown in Patent Documents 1-3, inclusions control, tissue homogenization, between single organization and / or tissue It has been made, such as by reducing the hardness difference. In other words, conventionally, by controlling the tissue being observed by an optical microscope, improvements such as stretch flange formability are achieved.
[0015]
　However, the present inventors have also control only tissue that is observed with an optical microscope, in view of the inability to improve the stretch flangeability when the strain distribution is present, the misorientation in the grains of the grain paying attention to, advanced intensive studies. As a result, the percentage of the total grain misorientation in the crystal grains is 5 ~ 14 ° crystal grain by controlling within a certain range, it found that it is possible to greatly improve the stretch flangeability.
[0016]
　Further, the present inventors have found that the grain boundary density of solid solution C, or the grain boundary density of the total of the solid solution C and solid solution B is one / nm 2 and not more than 4.5 pieces / nm 2 be less , when the average particle size of cementite is precipitated at grain boundaries in the steel sheet is 2μm or less, peeling can be suppressed, it is possible to crack also suppressed from the end face, it found that it is possible to improve further stretch flangeability It was.
[0017]
　The gist of the present invention is as follows.
[0018]
　(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: 0-30%, and 　bainite 70 to 100%, 　have in tissue represented, 　misorientation is surrounded by 15 ° or more grain boundaries And if the circle equivalent diameter of the region is 0.3μm or more is defined as grains, accounting for crystal grains of the whole grain intragranular orientation difference is 5 ~ 14 ° is 20 to 100% area ratio There, 　the grain boundary density of solid solution C, or the grain boundary density of the total of the solid solution C and solid solution B is one / nm 2 and not more than 4.5 pieces / nm 2 or less, 　to precipitate at grain boundaries steel having an average particle diameter of which cementite and wherein the at 2μm or less.

[0019]
　(2)
　tensile strength not less than 480 MPa,
　the tensile strength and steel sheet according to the product of the limit forming height of saddle stretch flange test is equal to or is 19500mm · MPa or more (1).
[0020]
　(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.
[0021]
　(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).
[0022]
　(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).
[0023]
　(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.
[0024]
　(7)
　The plated layer is plated steel sheet according to, characterized in that a galvanized layer (6).
[0025]
　(8)
　The plating layer is plated steel sheet according to characterized in that it is a galvannealed layer (6).
Effect of the invention
[0026]
　According to the present invention, a high strength, has excellent stretch flangeability, it can provide the generation of peeling less steel and plated steel. According to the present invention, severe stretch flangeability while a high strength and, in particular, excellent resistance to cracking (peeling) in the member end face which is formed by shearing or punching, 540 MPa grade or higher, further 780MPa grade or higher It can provide surface properties and excellent steel and plated steel sheet burring properties is of steel grade. Steel and plated steel sheet of the present invention can be applied to members that require strict ductility and stretch flangeability yet high strength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027]
[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
[0028]
　Hereinafter, embodiments of the present invention will be described.
[0029]
"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.
[0030]
"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%.
[0031]
"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%.
[0032]
"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.
[0033]
"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.
[0034]
"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. In less than 0.015% total content of Ti and Nb, processability is deteriorated, the frequency is increased to break during rolling. Therefore, the total content of Ti and Nb is set to 0.015% or more, 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, insufficient ratio of grain misorientation is 5 ~ 14 ° in the grains, stretch flangeability is degraded. Therefore, the total content of Ti and Nb is not more than 0.200%, 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%.
[0035]
"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.
[0036]
"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.
[0037]
"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.
[0038]
　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.
[0039]
"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.
[0040]
"B: 0 ~ 0.10%"
　B segregates at the grain boundaries, when present together with the solid solution C, increasing the grain boundary strength. To obtain this effect sufficiently, B content is preferably 0.0002% or more. Also, B improves the hardenability and facilitates the formation of a preferred microstructure continuously cooled transformed structure to burring properties. Therefore, B content is more preferably 0.0005% or more, more preferably 0.001% or more. However, only the solid solution B is present in the grain boundary, if the solid solution C is not present in the grain boundaries, because there is no grain boundary strengthening effect of the more solid solution C, prone to "peeling". Moreover, if they do not contain B, until coiling temperature 650 ° C. or less, some of the B is a grain boundary segregation elements is substituted with solid solution C contributes to the improvement of the strength of the grain boundary, winding the temperature of 650 ° C. greater than the grain boundary density of the total of the solid solution C and solid solution B is one / nm 2 for less than, is estimated to fracture surface cracking occurs. 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%. Also, B content is 0.002 percent, may cause slab cracking. Therefore, B content is preferably not more than 0.002%.
[0041]
"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.
[0042]
"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.
[0043]
"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.
[0044]
"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.
[0045]
"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: 0-30%, and bainite: having 70 to 100%, in tissue represented.
[0046]
: "Ferrite 0-30%"
　not more than 30% ferrite area ratio of, without degrading significantly the burring properties, can be enhanced ductility. Also, ferrite, for transformation with C accumulates in grains tend to solute C is reduced in the grain boundary. On the other hand, when the area ratio of ferrite is more than 30%, the grain boundary density of solid solution C 1 piece / nm 2 and not more than 4.5 pieces / nm 2 becomes difficult to control to the following range. Therefore, the area ratio of ferrite is set to 0-30%.
[0047]
: "Bainite 70-100%"
　by the bainite as a main phase, it can be enhanced stretch flanging, the burring workability. To obtain this effect sufficiently, the area ratio of bainite is 70 to 100%.
[0048]
　The steel sheet of tissue, may include pearlite or martensite, or both. Perlite, like bainite, has good fatigue properties and stretch-flange formability. Comparing the pearlite and bainite, it is good fatigue characteristics of the processed portion towards bainite punching. Area ratio of pearlite is preferably 0 to 15%. When the area ratio of pearlite is within this range, the fatigue characteristics of the punching unit is better steel sheet is obtained. Martensite, since it adversely affects the stretch flangeability, the area ratio of martensite is preferably 10% or less. Ferrite, bainite, the area of ​​the tissue other than the pearlite and martensite is preferably 10% or less, more preferably 5% or less, more preferably 3% or less.
[0049]
　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.
[0050]
　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.
[0051]
　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%.
[0052]
　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.
[0053]
　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)".
[0054]
　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".
[0055]
　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.
[0056]
　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.
[0057]
　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.
[0058]
　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.
[0059]
　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.
[0060]
　The steel sheet according to the present embodiment, the grain boundary density of solid solution C, or the grain boundary density of the total of the solid solution C and solid solution B is one / nm 2 and not more than 4.5 pieces / nm 2 or less . The grain boundary density of solid solution C, or grain boundary density of the total of the solid solution C and solid solution B 1 piece / nm 2 and not more than 4.5 pieces / nm 2 by less, generating a "peeling" not let, it is possible to improve the stretch flangeability. This is believed to solute C and solute B is to strengthen grain boundaries. Therefore, in order to obtain this effect sufficiently, the grain boundary density of solid solution C, or grain boundary density of the total of the solid solution C and solid solution B 1 piece / nm 2 and more. On the other hand, the grain boundary density of solid solution C, or the solute C solid solution and grain boundary density of the total B is 4.5 atoms / nm 2 exceeds, stretch flangeability is degraded. This is too much solute C and solute B at the grain boundaries, it is presumed to be because the grain boundary becomes brittle. Therefore, the grain boundary density of solid solution C, or the grain boundary density of the total of the solid solution C and solid solution B is 4.5 atoms / nm 2 or less.
[0061]
　The steel sheet according to the present embodiment, the average particle size of the cementite is precipitated in the grain boundaries is 2μm or less. The average particle size of cementite is precipitated at the grain boundaries by a 2μm or less, it is possible to improve the stretch flangeability. The stretch flangeability, during molding, voids are generated, by connecting, cracks. Thus, the presence of coarse cementite at the grain boundaries, cementite cracking during molding, voids are likely to occur. Even cementite, which forms a lamellar pearlite is no problem even if present. This or be those not break easily the shape of the cementite, because cementite is sandwiched α phase, believed because those less likely to void. The average particle size of cementite, since preferably smaller because, preferably a 1.5μm or less, more preferably 1.0μm or less.
[0062]
　The average particle size of cementite is precipitated in the grain boundaries, a transmission electron microscope sample was taken from the place of 1/4 thickness of the specimen cut out from 1 / 4W or 3 / 4W position of the steel sheet width of test steels , field emission electron gun acceleration voltage of 200kV (field emission gun: FEG) observed by equipped with a transmission electron microscope. Precipitates observed at the grain boundaries, it is cementite by analyzing the Diffraction pattern can be confirmed. The average particle diameter of cementite in this embodiment, by measuring the particle size of the total cementite was observed in one field of view is defined as an average value calculated from the measured value.
[0063]
　To measure the solid solution C and solid solution B that are present in the grain boundaries and within the grains, using a three dimensional atom probe method. The three-dimensional atom probe method, position sensitive atom probe (Position Sensitive Atom Probe, PoSAP) is used. Position sensitive atom probe, A. Oxford University in 1988 It is a device developed by Cerezo et al. This device can measure the position sensitive detector as a detector of the atom probe includes a (position sensitive detector), the flight time and the position of the reached atom detector without using an aperture during simultaneous analysis it is a device.
[0064]
　With this device, not only it is possible to display all configuration elements in the alloy present on the sample surface as a two-dimensional map with a spatial resolution of an atomic level, a monoatomic layer of the sample surface using field evaporation phenomenon by evaporation by going to extend the two-dimensional map in the depth direction can be displayed and analyzed as a three-dimensional map. The grain boundary observed, to the needle shape using FIB (focused ion beam) apparatus (manufactured by Hitachi, Ltd. FB2000A), the cut sample by electrolytic polishing in order to produce a needle-like sample for AP containing grain boundaries made to be needle tip grain boundaries in an arbitrary shape the scanning beam. The sample, taking advantage of the contrast occurs in different crystal grains having orientation in channeling phenomenon of SIM (scanning ion microscope), to identify the grain boundaries while observing, cut with the ion beam. Position sensitive atom probe is a CAMECA Co. OTAP. Measurement conditions were a sample position temperature of about 70K, a probe total voltage is 10 ~ 15kV, the pulse ratio is 25%. The grain boundary of each sample were measured respectively three times the intragranular, as a representative value and the average value. Value obtained by removing the measured value from the background noise and the like is defined as the atomic density per unit grain boundary area, which grain boundary number density (grain boundary segregation density) (number / nm 2 and). Therefore, the solid solution C that exists at the grain boundaries, just refers to a C atoms present in the grain boundary. Moreover, the solute B existing at the grain boundaries, just refers to the B atoms present in the grain boundary.
[0065]
　The grain boundary density of solid solution C in the present embodiment is defined as the number per grain boundary unit area of ​​solid solution C that exists at the grain boundary (density). The grain boundary density of solid solution B in the present embodiment, defined as the number per grain boundary unit area of ​​solid solute B existing in the grain boundary (density). According to the three-dimensional atom probe method, since the distribution of three-dimensionally atoms in atomic map is found, it can be confirmed often the number of C atoms and B atoms in the grain boundary position. Incidentally, if precipitates and can be specified in atomic number, positional relationship between the other atoms (such as Ti).
[0066]
　Next, a method for manufacturing a steel sheet according to the embodiment of the present invention. In this way, performing hot rolling, air cooling, the first cooling and the second cooling in this order.
[0067]
"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.
[0068]
　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.
[0069]
　After the slab heating, it performs rough rolling against the slab extracted from the heating furnace without waiting particular, the rough bar is obtained. When the end temperature of the rough rolling is below 1000 ° C., and increases the hot deformation resistance in the rough rolling, the operation of the rough rolling may lead to failure. Therefore, the end temperature of the rough rolling is to 1000 ° C. or higher. On the other hand, if the end temperature of the rough rolling exceeds 1150 ° C., the grain boundary density of solid solution C in grain boundaries 1 / nm 2 may become less. This, Ti and Nb in austenite, precipitates as coarse TiC and NbC, solid solution C is presumed to be due to decrease. Further, when the end temperature of the rough rolling exceeds 1150 ° C., hot rolled sheet strength may be deteriorated. This is because the TiC and NbC is coarse precipitates.
[0070]
　If the time from the end of rough rolling to the start of finish rolling exceeds 150 seconds, the solid solution C amount grain boundary density in the grain boundary 1 / nm 2 may become less. This, Ti and Nb in austenite, precipitates as coarse TiC and NbC, solid solution C is presumed to be due to decrease. Moreover, the hot rolled sheet strength is also lowered. This is because the TiC and NbC is coarse precipitates. On the other hand, when the time from the end of rough rolling to the start of the finish rolling is less than 30 seconds, scales between the surface scales of the steel sheet locations iron between before the beginning of finish rolling and path, blister serving as a starting point for spindle scale defects order but to occur, that these scale defects are easily generated.
[0071]
　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.
[0072]
　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.
[0073]
　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.
[0074]
　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.
[0075]
　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.
[0076]
　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. The maximum temperature of the steel sheet during finish rolling is Ar 3 exceeds + 0.99 ° C., and γ grains are coarsened grown until the start of cooling after the finish rolling end, the grain boundary density of solid solution B and solid solution C in grain boundaries There is increased.
[0077]
　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.
[0078]
　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%.
[0079]
　If rolling reduction in the final pass in the finish rolling is less than 3%, passing plate shape deteriorates, the winding shape of the coil during the hot coiling may adversely affect concern product thickness accuracy. On the other hand, reduction ratio of the final pass in the finish rolling exceeds 20%, the dislocation density of the steel sheet inside the introduction of excessive strain increases more than necessary. After the end of the final rolling, areas of high dislocation density, because the strain energy is high, tends to transform into ferrite structure. Such transformation ferrite formed by, in order to deposit without solid solution much carbon, easily concentrated on the interface between the carbon austenite and ferrite was contained in the mother layer, dissolution of the grain boundary C in addition to the grain boundary density of increases, carbide coarse Nb and Ti is likely to precipitate at the interface. Solute N in this way the finish rolling, if Ti is reduced, for the reasons described above, can not be expected that improving the strength of the steel sheet, "peeling" is likely to occur. Therefore, the reduction ratio of the final pass in the finish rolling is controlled to be in the range of 20% less than 3%.
[0080]
　When the rolling speed of the final pass in the finish rolling is less than 400Mpm, gamma grains grow coarse, the grain boundary density of solid solution C in grain boundaries is increased. Therefore, the rolling speed of the final pass in the finish rolling is a more 400Mpm. On the other hand, although the effect of the present invention without particular limitation on the upper limit of the rolling speed, equipment constraints on 1800mpm less is practical. Therefore, the rolling speed of the final pass in the finish rolling is less 1800Mpm.
[0081]
"Air-cooled"
　In this manufacturing method, perform the air cooling of hot-rolled steel plate only 2 seconds or less of the time from the end of the finish rolling. The cooling time is in 2 seconds, greater than the grain boundary density of solid solution B and solid solution C in grain boundaries is increased. Therefore, the air cooling time is less than 2 seconds.
[0082]
"First cooling, the second cooling"
　2 seconds after the following air-cooling, 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 400 ~ 600 ° C. at a cooling rate higher than 30 ° C. / s. Between the first cooling and the second cooling, the hot rolled steel sheet to hold 0-10 seconds in the first temperature range. It is preferable to cool the hot-rolled steel sheet after the second cooling.
[0083]
　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 70% bainite area ratio, the crystal orientation difference in the grains is 5 ~ 14 ° grains the percentage is insufficient. Further, when the holding time at 600 ~ 750 ° C. greater than 10 seconds, it harmful cementite is likely to generate the burring properties, In addition, the average particle size of cementite is precipitated in the grain boundaries is often greater than 2μm . 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 70% of bainite in an area ratio of 5 ~ 14 ° crystals the proportion of grain is insufficient.
[0084]
　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 400 ° C. cooling, the or a 600 ° C. greater than the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains is insufficient.
[0085]
　If the coiling temperature exceeds 600 ° C., the grain boundary density of solid solution C is one / nm 2 becomes less than, fracture surface cracking occurs. In addition, the area ratio of the ferrite also increases. Therefore, the coiling temperature is set to 600 ° C. or less, preferably 550 ° C. or less. On the other hand, when the coiling temperature is less than 400 ° C., an average particle size of cementite is precipitated at grain boundaries for more than 2 [mu] m, the hole expansion value deteriorates. Therefore, the coiling temperature is set to 400 ° C. or higher, preferably 450 ° C. or higher.
[0086]
　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.
[0087]
　It can be obtained steel sheet according to this way this embodiment.
[0088]
　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.
[0089]
　In order to take the scale of the surface, it may also be pickled. If as conditions of the hot rolling and cooling, subsequently, cold rolling, heat treatment (annealing), even if the plating and the like can achieve the same effect.
[0090]
　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. 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 strength and tensile strength. Accordingly, the cold rolling can be used to adjust the intensity. Cold-rolled steel sheet obtained by cold rolling.
[0091]
　If the temperature exceeds 840 ° C. heat treatment (annealing), tissue yelling made by hot rolling by austenitizing, thus being canceled. Also, in general, after annealing, in order to cool down to room temperature in a shorter time than in the hot rolling, the number martensite tends to stretch flangeability is degraded significantly. Therefore, the annealing temperature is preferably between 840 ° 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.
[0092]
　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.
[0093]
　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.
[0094]
　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.
[0095]
　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.
[0096]
　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
[0097]
　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.
[0098]
　Table were melted a steel having a chemical composition shown in 1 to manufacture a steel strip, the resulting steel pieces were heated to a heating temperature shown in Table 2 and Table 3, after the rough rolling in hot, and a subsequent finish rolling under the conditions shown in Table 2 and Table 3. Thickness of hot-rolled steel sheet after finish rolling was 2.2 ~ 3.4 mm. Table 2 and "elapsed time" in Table 3 is the elapsed time until the start of the finish rolling from the end of rough rolling. Table 1 blank means that analytical values ​​were below the detection limit. Table 1 in underline indicates that the number is out of range of the present invention, underlined in Table 3 show that is out of the range suitable for the manufacture of steel sheet of the present invention.
[0099]
[Table 1]

[0100]
[Table 2]

[0101]
[table 3]

[0102]
　Ar 3 (° C.) was determined using Equation (3) from components shown in Table 1.
　Ar 3 = 970-325 × [C] + 33 × [Si] + 287 × [P] + 40 × [Al] -92 × ([Mn] + [Mo] + [Cu]) - 46 × ([Cr] + [ Ni]) ··· (3)
[0103]
　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.
[0104]
　For hot-rolled steel sheet obtained by the method shown below, structural fraction of each tissue (area ratio), and misorientation of intragranular it was determined the percentage of the crystal grains is 5 ~ 14 °. The results are shown in Table 4 and Table 5. Underlined in Table 5 indicates that the number is out of range of the present invention.
[0105]
"Structural percentage of each tissue (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.
[0106]
"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)".
[0107]
　Next, the 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. The results are shown in Tables 4 and 5. Underlined in Table 5 indicates that the number is out of range of the present invention.
[0108]
　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.
[0109]
　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.
[0110]
　To investigate the extent of peeling, carried out punching of the steel sheet was observed in its end face. Punching conditions, was carried out in accordance with the hole expanding test (JFS T 1001-1996). Punched steel plate 10 sites, fracture surface cracking of the following two positions is determined that OK, it is determined as NG and more than three places. An average particle size of cementite is precipitated in the grain boundary, the grain boundary density of solid solution C, or the grain boundary density of the total of the solid solution C and solid solution B was observed by the above-mentioned method. The results are shown in Tables 4 and 5. Underlined in Table 5 indicates that the number is out of range of the present invention.
[0111]
[Table 4]

[0112]
[table 5]

[0113]
　Invention sample (Test No.1 ~ 21), more tensile strength 480 MPa, and the product of the limit forming height of 19500mm · MPa or higher tensile strength and saddle stretch flange tested were obtained.
[0114]
　Test No. 22-27, the chemical component is a comparative example outside the scope of the present invention. Test No. 28-47, as a result of outside the range manufacturing conditions is desired, the tissue to be observed with an optical microscope, the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains, the average grain size of the cementite, the solute C grain boundary density, any one or more of the grain boundary density of the total of the solid solution C and solid solution B are comparative examples that did not satisfy the scope of the present invention. In these examples, the index of stretch flangeability or did not satisfy the target value, peeling was or occurred. In addition, tensile strength in some cases was also lower.
Industrial Applicability
[0115]
　According to the present invention, it is possible to provide that can be applied to members that strict stretch flange formability yet high strength is required, a high-strength hot-rolled steel sheet having excellent stretch flangeability. These steel sheets, in order 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: 0-30%, and
　bainite 70 to 100%,
　in a tissue represented,
　misorientation is surrounded by 15 ° or more grain boundaries, and When the equivalent 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 grain boundary density of solid solution C, or the grain boundary density of the total of the solid solution C and solid solution B is one / nm 2 and not more than 4.5 pieces / nm 2 or less,
　is precipitated in the grain boundary steel sheet average particle diameter of cementite and wherein the at 2μm or less.
[Requested item 2]
　Tensile strength not less than 480 MPa,
　the tensile strength and the steel sheet according to claim 1, the product of the marginal forming height of saddle stretch flange test is equal to or is 19500mm · MPa or higher.
[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.