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]
　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.
[0006]
　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.
[0007]
　Patent Documents 4 to 7, Ti, in steel with the addition of carbide forming elements such as Nb and V, technology for improving the crack and fatigue characteristics of the punching units has been proposed. Patent Document 8 ~ 10, Ti, in steel with the addition of carbide forming elements such as Nb and V, by utilizing B, technology for improving the crack and fatigue characteristics of the punching units has been proposed. Patent Document 11, a ferrite and bainite as the main structure, particle size and fraction of precipitates in the ferrite, and to control the form of bainite, elongation properties, stretch flangeability, a high-strength hot-rolled with excellent fatigue properties steel plate have been described. Patent Document 12, Ti, Nb, in steel with the addition of carbide forming elements V, etc., techniques for improving the surface defects and productivity in the continuous casting process have been proposed.
[0008]
　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.
[0009]
　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.
[0010]
　Patent Documents 1 to 3, by defining a tissue, a technique for improving material properties are disclosed. However, the steel sheet described in Patent Documents 1 to 3, whether a sufficient stretch flangeability even when considering strain distribution can be secured is not known. Further, conventional high strength steel sheet, which has excellent stretch flangeability, fatigue properties of the base material and the punching unit is not satisfactory.
CITATION
Patent Document
[0011]
Patent Document 1: WO 2013/161090 Patent
Patent Document 2: JP 2005-256115 Patent Publication
Patent Document 3: JP 2011-140671 Patent Publication
Patent Document 4: JP 2002-161340 Patent Publication
Patent Document 5: JP 2002-317246 JP
Patent Document 6: JP 2003-342684 Patent Publication
Patent Document 7: JP 2004-250749 JP
Patent Document 8: JP 2004-315857 JP
Patent Document 9: JP 2005-298924 JP
Patent Document 10: JP 2008-266726 JP
Patent Document 11: JP 2007-9322 JP
Patent Document 12: JP 2007-138238 JP
Summary of the Invention
Problems that the Invention is to Solve
[0012]
　The present invention is a high strength, has excellent stretch-flange formability, fatigue properties of the base material and the punching unit and to provide a good steel sheet and plated steel sheet.
Means for Solving the Problems
[0013]
　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.
[0014]
　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.
[0015]
　Further, the present inventors have found that the crystal grain average aspect ratio of the density of the total particle size of the Ti-based carbide and Nb carbide above 20nm in the ferrite grain boundaries on, by a specific range, the mother good fatigue characteristics can be obtained in the timber and punching unit, it found that can prevent injury with irregularities in the punched end face.
[0016]
　The present invention includes a new knowledge about the percentage of total grain misorientation in the crystal grains described above is 5 ~ 14 ° crystal grain, the grain size in average aspect ratio of crystal grains and the grain boundary of ferrite on the 20nm based on the new knowledge on density of the total or more Ti-based carbide and Nb carbide, extensive studies the present inventors have intensively, and have reached the completion.
[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 30 to 95%, and 　bainite: 5% to 70% 　in a tissue represented, 　misorientation is surrounded by 15 ° or more grain boundaries, One when 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 average aspect ratio of equivalent ellipse of the crystal grains is 5 or less, 　the average distribution density of the total particle size of the Ti-based carbide and Nb carbide above 20nm in the ferrite grain boundaries on is 10 pieces / [mu] m or less steel sheet, characterized in that.

[0019]
　(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,
　it brittle fracture rate of the punching fracture surface is less than 20% steel sheet according to the constitution (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, it has excellent stretch-flange formability, fatigue properties of the base material and the punching unit can provide a good steel sheet. Steel sheet of the present invention, a strict stretch flange formability yet high strength, can be applied to members that require the fatigue properties of the base metal and punching unit, clearance strict, severe working conditions using worn Shah and punch in even when subjected to punching it can be prevented from being damaged with the irregularities in the punched end face.
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.
FIG. 2 is a diagram showing a method of calculating the average aspect ratio of crystal grains.
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. 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%.
[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 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%.
[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: with 5 to 70%, in tissue represented 30 to 95%, and bainite.
[0046]
: "Ferrite 30-95%"
　When the ferrite area ratio of less than 30%, sufficient fatigue property can not be obtained. Therefore, the area ratio of ferrite is 30% or more, preferably 40% or more, more preferably 50% or more, more preferably 60% 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.
[0047]
"Bainite: 5 to 70%"
　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 70%, ductility is deteriorated. Therefore, the area ratio of bainite is 70% or less, preferably 60% or less, more preferably 50% or less, more preferably 40% or less.
[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 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.
[0056]
　The average aspect ratio of crystal grains of the corresponding ellipse in tissue is related to the generation behavior of cracks, bumps and dips of the punching edge. If the average aspect ratio of equivalent ellipse of the crystal grains is more than 5, cracks becomes significant, fatigue cracking is likely to occur that starting from the punched portion. Accordingly, the average aspect ratio of equivalent ellipse of the crystal grains is 5 or less. Its average aspect ratio is preferably 3.5 or less. Thus, it is possible to prevent the occurrence of cracks even in a more severe punching. The lower limit of the average aspect ratio of crystal grains of the corresponding ellipse is not particularly limited, but 1 to a circle equivalent are substantially the lower.
[0057]
　The average aspect ratio, tissue was observed L section (parallel to the rolling direction cross-section) was measured for 50 or more crystal grains (ellipse major axis) / (the ellipse minor axis length), is an average value. Here, the grain refers to a grain surrounded by the grain boundary inclination angle 10 ° or more high angle grain boundary.
[0058]
　There are fine Ti-containing carbide or Nb system carbides in the ferrite grain boundaries on the tissue, and the crystal grains are flattened, brittle fracture rate of the punching fracture surface increases, the fatigue characteristics deteriorate. According to our observations, Ti-containing carbide and Nb carbide above particle size 20nm on ferrite grain boundaries, they tend to induce voiding when tectonic considered to be responsible for intergranular fracture. Ferrite grain boundaries on the 20nm or more Ti carbides and Nb system carbides, when present at greater than 10 per grain boundary length 1μm in average distribution density of the total, brittle fracture rate increases, the fatigue properties of the member It leads to a decrease. Therefore, the average distribution density of the total particle size of the Ti-based carbide and Nb carbide above 20nm in the ferrite grain boundaries on was 10 pieces / [mu] m or less, preferably not more than 6 / [mu] m. The average distribution density of the total particle size of the Ti-based carbide and Nb carbide above 20nm in the ferrite grain boundaries on the, the lower from the viewpoint of brittle fracture surface suppression preferred. If the average distribution density of the total particle size of the Ti-based carbide and Nb carbide above 20nm in the ferrite grain boundaries on is less than 0.1 or / [mu] m, brittle fracture will not substantially occur. The average distribution density of the total Ti-containing carbide and Nb carbide on the grain boundary of ferrite as a result of the cutting sample of L cross-section (parallel to the rolling direction cross-section) was observed with a scanning electron microscope (SEM) calculated using the.
[0059]
　Fracture form of punched fracture surface correlates with the occurrence behavior of irregularities and microcracks punched fracture surface, it affects the fatigue properties of the member having the punched portions. If brittle fracture rate in the fracture surface is 20% or more, unevenness of fracture is increased, minute cracks due to prone, the occurrence of fatigue cracks of the punching portion is promoted. According to this embodiment, the brittle fracture rate of less than 20% is obtained, sometimes less than 10% of the brittle fracture surface ratio. Brittle fracture rate in the fracture plane, punched specimen steel plate shear or punch at a clearance conditions thickness of 10-15% is a value measured by observing the formed fracture surfaces.
[0060]
　Texture of the steel sheet, through the influence of the cracking and residual stress distribution of the punching fracture surface, affects the fatigue properties of the punching unit. When {112} <110> orientation and {332} <113> orientation X-ray random intensity ratio of the plate surface at the center of plate thickness exceeds 5 respectively, there are cases where cracking of the fracture surface of the punching portion occurs . Thus, X-rays random intensity ratio of the orientation are preferably set to 5 or less, more preferably 4 or less. If X-ray random intensity ratio of the orientation is 4 or less, cracks even punched with worn punch used in mass production is unlikely to occur. X-ray random intensity ratio of the orientation is completely random 1 is substantially the lower.
[0061]
　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.
[0062]
　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.
[0063]
　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.
[0064]
　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.
[0065]
　According to the steel sheet according to the present embodiment, the brittle fracture rate of less than 20% and 0.4 or more fatigue limit ratio. That is, it is possible to obtain the fatigue properties in excellent preform and punching unit.
[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]
　Crude bars obtained by the rough rolling. When the end temperature of the rough rolling is below 1000 ° C., there are cases where the crystal grains after the finish hot rolling cracking occurs in the fracture surface of the punched portion is flattened. Therefore, the end temperature of the rough rolling is to 1000 ° C. or higher.
[0070]
　After rough rolling may be subjected to heat treatment until the completion of the finish rolling. By performing the heat treatment, the temperature in the width direction and the longitudinal direction of the rough bar becomes uniform, variation in the material in the product of the coil is reduced. Heating method in the heat treatment is not particularly limited. For example, a furnace heating, induction heating, electrical heating may be performed by a method such as high frequency heating.
[0071]
　After rough rolling, until the completion of the finish rolling, it may be subjected to a de-scaling. The descaling, the surface roughness is reduced, in some cases to improve the fatigue characteristics. Method of de-scaling is not particularly limited. For example, it can be performed by high-pressure water jet.
[0072]
　Time from the end of rough rolling to the start of the finish rolling, through recrystallization behavior of austenite during rolling influences the fracture form of punched fracture surface. If the time from the end of rough rolling to the start of the finish rolling is less than 45 seconds, sometimes brittle fracture rate of the punching edge is large. Therefore, the time from the end of rough rolling to the start of the finish rolling is 45 seconds or more. By this time 45 seconds or more, recrystallization of austenite is further promoted, it can be more spherical crystal grains, fatigue properties of the punching portion becomes better.
[0073]
　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.
[0074]
　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.
[0075]
　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.
[0076]
　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.
[0077]
　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.
[0078]
　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.
[0079]
　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.
[0080]
　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%.
[0081]
"Air cooling"
　In this manufacturing method, performs air cooling of hot-rolled steel sheet by 2 seconds than 5 seconds or less from the end of finish rolling. The cooling time affects the flattening of the crystal grains after transformation in connection with recrystallization austenite. If air cooling time is less than 2 seconds, brittle fracture rate of the punching edge is large. Therefore, the air cooling time is set to 2 seconds, preferably above the 2.5 seconds or more. If air cooling time exceeds 5 seconds, with is difficult securing coarse TiC and / or NbC is precipitated strength properties of the punched end face is deteriorated. For this reason, air cooling time is not more than 5 seconds.
[0082]
"First cooling, the second cooling"
　2 seconds than 5 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 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.
[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 an area ratio obtained more than 30% of ferrite, crystal orientation difference in the grains is 5 ~ 14 ° grains the percentage is insufficient. As the cooling stop temperature in the first cooling is high, it tends to be high ferrite fraction. From the viewpoint of obtaining a high ferrite fraction, cooling stop temperature of the first cooling, and 600 ° C. or higher, preferably between 610 ° C. or higher, more preferably a 620 ° C. or higher, more preferably to 630 ° C. or higher. 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 ratio or insufficient, the average distribution density of the Ti-containing carbide and Nb carbide on the ferrite grain surface may become excessive.
[0084]
　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 If ~ 750 retention time at ℃ is less than one second, it becomes difficult to obtain ferrite with an area ratio of 30% or more, insufficient ratio of crystal grains the crystal orientation differences 5 - 14 ° in the grains to. As the holding time is long, it tends to be a higher fraction of ferrite. From the viewpoint of obtaining a high ferrite fraction, retention time was 1 second or more, preferably not less than 1.5 seconds, more preferably not less than 2 seconds, more preferably 2.5 seconds or more.
[0085]
　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 °. If the cooling stop temperature in the second cooling is lower than 450 ° C., it becomes difficult to an area ratio obtained more than 30% of ferrite, crystal grain ratio of the misorientation is 5 ~ 14 ° in the grains Run short. As the cooling stop temperature in the second cooling is high, it tends to be high ferrite fraction. From the viewpoint of obtaining a high ferrite fraction, cooling stop temperature in the second cooling, and 450 ° C. or higher, more preferably a 510 ° C. or higher, further preferably 550 ° C. or higher. On the other hand, if the cooling stop temperature in the second cooling is at 650 ° C. greater, it becomes difficult to obtain more than 5% of bainite area ratio, orientation difference in grain crystal grains is 5 ~ 14 ° the percentage is insufficient.
[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. The area ratio of ferrite and bainite is first cooled, second cooling and compositely depending on the conditions of holding between them, can not be controlled only by these individual conditions, for example, as follows there is a trend. That is, the first long cooling stop temperature is 610 ° C. or higher and the area ratio of ferrite easily with more than 40% of the cooling, the ferrite area ratio of if 620 ° C. easily with more than 50%, if 630 ° C. ferrite the area ratio of easily with more than 60%.
[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 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.
[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 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.
[0091]
　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., the effect of coarsening of the precipitates is large, the crystal orientation difference in the grains is insufficient proportion of crystal grains of 5 ~ 14 °. 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.
[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 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, shown in Tables 3 and 4 performs rough rolling under conditions, 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 3 and "elapsed time" in Table 4 is the elapsed time until the start of the finish rolling from the end of rough rolling. 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.
[0099]
[Table 1]

[0100]
[Table 2]

[0101]
[table 3]

[0102]
[Table 4]

[0103]
　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)
[0104]
　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.
[0105]
　Then, Table 5 and cooling of the hot rolled steel sheet under the conditions shown in Table 6, a first cooling, holding at a first temperature range, performing a second cooling, Test No. To obtain a hot-rolled steel sheet of 1 to 45. Air cooling time corresponds to the time from the end of finish rolling to the start of the first cooling.
[0106]
　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, 45, 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.
[0107]
[table 5]

[0108]
[Table 6]

[0109]
　Each steel sheet (hot-rolled steel test No.1 ~ 17 and 22 ~ 44, the hot-rolled steel sheet of the test No.18 ~ 20, 45 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 they contain martensite and / or pearlite were listed in the "remaining structure" in the table. Underlined in Table 8 indicates that the value is out of range of the present invention.
[0110]
"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.
[0111]
"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)".
[0112]
　Each steel plate on (hot-rolled steel test No.1 ~ 17 and 22 ~ 44, the hot-rolled steel sheet of the test No.18 ~ 20, 45 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 an average aspect ratio of crystal grains of corresponding elliptical particle size in ferrite grain boundaries on it was determined and the average distribution density of the total Ti-containing carbide and Nb carbide of more than 20 nm. The results are shown in Table 7 and Table 8.
[0113]
"Crystal grains having an average aspect ratio of equivalent ellipse"
　the L section (parallel to the rolling direction cross-section), tissue was observed using the above EBSD, respectively, for 50 or more grains (ellipse major axis) / ( calculating an ellipse minor axis length), to obtain an average value of the calculated value. Figure 2 is a diagram showing a method of calculating the average aspect ratio of crystal grains. Crystal grains 2 14 is the particle surrounded by the grain boundary inclination angle 15 ° or more high angle grain boundary. As shown in FIG. 2, the elliptical major axis 12, of the straight line connecting between any two points on the grain boundaries 11 of each crystal grain 14 observed using the above EBSD, it means the longest straight line. The ellipse minor axis 13, of the straight line connecting between any two points on the grain boundaries 11 of each crystal grain 14 observed using the above EBSD, a point bisecting the length of the ellipse major axis 12 as means a straight line perpendicular to the ellipse major axis 12.
[0114]
"Average distribution density of the total particle size of the Ti-based carbide and Nb carbide above 20nm in the ferrite grain boundaries on"
　the L cross section, observed with a SEM, to measure the length of the ferrite grain boundaries, further that ferrite particle size on the grain boundaries was measured the total number of the Ti-containing carbide and Nb carbide of more than 20 nm. With total number of Ti-containing carbide and Nb carbide measured to calculate the average distribution density is the sum number of the Ti-containing carbide and Nb carbide per 1μm length of the ferrite grain boundaries. Note that the particle size of the Ti-containing carbide and Nb carbide refers to a circle equivalent radius of the Ti-based carbide and Nb carbide.
[0115]
[Table 7]

[0116]
[Table 8]

[0117]
　Each steel plate on (hot-rolled steel test No.1 ~ 17 and 22 ~ 44, the hot-rolled steel sheet of the test No.18 ~ 20, 45 that has been subjected to the heat treatment, cold rolled steel sheets of test No.21 subjected to heat treatment), according JIS Z2275, performed plane bending fatigue test under conditions of stress ratio = -1, was evaluated by fatigue limit. Test No. Hot-rolled steel sheet of 1 to 17, 22 to 44, the test was subjected to a heat treatment No. Hot-rolled steel sheet of 18 to 20, 45, 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. Further, brittle fracture rate during punching is less than 20%, and, when the fatigue limit ratio is 0.4 or more, the fatigue characteristics of the base metal and the punching unit 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.
[0118]
　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.
[0119]
　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.
[0120]
　Brittle fracture rate during punching, punched into a circular shape of 20 to 50 samples steel plate shear or punch at a clearance conditions thickness of 10-15%, the formed fracture surface, respectively, using a microscope the observed. Then, a portion of the metallic gloss and brittle fracture surface was measured circumferential length of the brittle fracture surface. Here, the length in the circumferential direction of the brittle fracture surface, refers to the length in the circumferential direction from the end region became brittle fracture surface to the edge. Then, the ratio of the total circumferential length of the brittle fracture surface of all circumferential length observed was brittle fracture rate. For example, when punching the 20 samples steel sheet punch having a diameter of 10 mm, the total circumferential length becomes 20 × 10 × πmm. There are 20 only one brittle fracture of the sample steel sheet, and if the circumferential length of the brittle fracture surface was 1 mm, brittle fracture rate is 1 / (20 × 10 × π ) and a.
[0121]
　Fatigue limit ratio was calculated by dividing the tensile strength values ​​of the fatigue limit of the steel sheet measured by the above method (fatigue limit (MPa) / Tensile strength (MPa)).
[0122]
[Table 9]

[0123]
[Table 10]

[0124]
　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, brittle fracture at less than 20% punching rate, and 0.4 or more fatigue ratio was obtained.
[0125]
　Test No. 22-27, the chemical component is a comparative example outside the scope of the present invention. Test No. 22-24, an index of stretch flangeability did not satisfy the target value. Test No. 25, since a small total content of Ti and Nb, the index and the tensile strength of the stretch flangeability has not satisfied the target value. Test No. 26, since the total content of Ti and Nb is large, workability is deteriorated, cracks occurred during rolling. Test No. 27, because many total content of Ti and Nb, an index of stretch flangeability did not satisfy the target value.
[0126]
　Test No. 28-46, 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 aspect ratio, one of the density of carbides one or more is a comparative example that did not satisfy the scope of the present invention. Test No. 28 to 40, 45, the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains is small, the index of stretch flangeability did not satisfy the target value. Test No. 41-44, since the average aspect ratio of the equivalent ellipse of the crystal grains is large, brittle fracture rate during punching became more than 20%.
Industrial Applicability
[0127]
　According to the present invention, a high strength, it has excellent stretch-flange formability, fatigue properties of the base material and the punching unit can provide a good steel sheet. Steel sheet of the present invention, clearance is severely, even when a punching under severe processing conditions used worn Shah and punch, it is possible to prevent damage associated with irregularities in the punched end face. Steel sheet of the present invention can be applied to high strength and tough stretch flangeability yet, the base material and the punching unit of the fatigue properties and the required member. 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,
　the ferrite 30 to 95%, and
　bainite: 5% to 70%
　in a tissue represented,
　misorientation is surrounded by 15 ° or more grain boundaries, and circle In the event that the 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,
　the average aspect ratio of crystal grains corresponding ellipse is 5 or less,
　the average distribution density of the total particle size of the Ti-based carbide and Nb carbide above 20nm in the ferrite grain boundaries on the 10 or / [mu] m or less steel sheet which is characterized.
[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 brittle fracture rate of the punching fracture surface is less than 20% 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.