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, 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.
[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]
　In order to enhance the impact energy absorbing capability when the motor vehicle collides, it is effective to increase the yield stress of the steel. Because it is a small amount of deformation, since it is possible to efficiently absorb energy.
[0006]
　Further, on the other hand, even if high-strength steel sheet, the fatigue properties are greatly deteriorated, can not be used as automotive steel sheets.
[0007]
　Further, etc. steel used for underbody member, such as easily exposed to rain, if the thin, since the reduction by corrosion thickness is a major issue, the corrosion resistance is also obtained.
[0008]
　Against challenge good stretch flangeability above, for example, Patent Document 1, by limiting the size of TiC, ductility, stretch flangeability, it is disclosed that can provide a steel sheet excellent in material homogeneity . Further, Patent Document 2, kinds of oxides, by defining the size and number density, it is disclosed that can provide a steel sheet excellent in fatigue properties and stretch-flange formability. In Patent Document 3, by defining the difference in hardness between the area ratio of the ferrite phase and a second phase, the variation in intensity is small and it is disclosed that can provide a steel sheet excellent in the ductility and hole expansion there.
[0009]
　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. However, 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.
[0010]
　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. Conventionally, as a test method for evaluating 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.
[0011]
　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.
[0012]
　As a method of increasing the yield stress, for example, (1) or to work hardening, (2) or a microstructure consisting mainly high low-temperature transformation phase (bainite, martensite) dislocation density, (3) a solid solution strengthening element or added, there is a method or a reinforced (4) precipitated. The method of (1) and (2), because the dislocation density increases, workability is deteriorated significantly. (3) In the method of performing the solid solution strengthening of, there is a limit to the absolute value of the enrichments, it is difficult to increase the yield stress enough to say enough. Therefore, high while achieving processability, to increase the efficiency yield stress, adding Nb, Ti, Mo, an element of V, etc., by performing the precipitation strengthening of these alloys carbonitrides, high yield stress it is desirable to achieve.
[0013]
　From the above point of view, the high strength steel sheet utilizing precipitation strengthening of microalloying elements is being put to practical use, in high-strength steel sheet using this precipitation strengthening, it is necessary to solve the fatigue properties and rust.
[0014]
　For the fatigue properties, the high-strength steel sheet utilizing precipitation strengthening, there is phenomenon of fatigue strength is poor due to softening of the steel sheet surface layer. In the rolling roll and direct contact with the surface of the steel sheet during hot rolling, the heat removal effect of the roll in contact with the steel plate, temperature decreases only the surface of the steel sheet. The outermost layer of the steel sheet Ar 3 below the point, occur coarsening of the microstructure and precipitate, steel outermost layer is softened. This is the main cause of the deterioration of the fatigue strength. Fatigue strength of general steel, as steel sheets outermost layer is cured, improved. Therefore, the high-tensile steel plate using a precipitation strengthening, at present, difficult to obtain a high fatigue strength. To begin with, the purpose of strengthening the steel sheet are the weight reduction of the vehicle body weight, despite increases the strength of the steel sheet, if the fatigue strength is lowered, it is impossible to reduce the thickness. From this viewpoint, it is desirable fatigue strength ratio is 0.45 or more, even in a high-strength hot-rolled steel sheet, it is desirable to keep the tensile strength and fatigue strength well balanced, the high value. Here, the fatigue strength ratio is a value obtained by dividing the tensile strength and fatigue strength of the steel sheet. In general, with the increase of tensile strength, tend to fatigue strength is increased, the higher strength material, the fatigue strength ratio is lowered. Therefore, even with a high tensile strength steel sheet, there is a case where the fatigue strength is not increased, can not be lighter in vehicle weight is an object of high strength.
CITATION
Patent Document
[0015]
Patent Document 1: WO 2013/161090
Patent Document 2: JP 2005-256115 Patent Publication
Patent Document 3: JP 2011-140671 JP
Summary of the Invention
Problems that the Invention is to Solve
[0016]
　The present invention, while a high strength, and to provide a steel sheet excellent and plated steel sheets severe stretch flange formability and fatigue properties and elongation.
Means for Solving the Problems
[0017]
　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.
[0018]
　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.
[0019]
　Further, the present inventors have found that a circle equivalent diameter below 10nm Ti (C, N) Total precipitate density and Nb (C, N) is 10 10 pieces / mm 3 or more, at a depth 20μm from the surface hardness (Hvs), if the ratio of the thickness center of the hardness (Hvc) (Hvs / Hvc) is 0.85 or more, found that excellent fatigue characteristics can be obtained.
[0020]
　The present invention is based on the new knowledge about the percentage of total grain misorientation in the crystal grains described above is 5 ~ 14 ° crystal grain, and the new findings on the ratio of the hardness, the present inventors diligent investigation, which has led to completion.
[0021]
　The gist of the present invention is as follows.
[0022]
　(1)
　in
　mass%,
　C:
　0.008 ~ 0.150%, Si: 0.01 ~
　1.70%, Mn: 0.60 ~ 2.50%, Al: 0.010 ~ 0.60% , 　Ti:
　0 ~ 0.200%, Nb: 0 ~ 　0.200%, Ti + Nb: 0.015 ~ 0.200%, 　Cr: 0 ~ 1.0%, B: 0 ~ 　0.10%, Mo: 　~ 　1.0% 0, 　0 ~ 2.0%, Mg: 0 ~ 0.05%, REM: 　0 ~ 0.05%, Ca: 0 ~ 0.05 　%, Zr: 0 ~ 　0.05%, P: 0.05% or 　less, S: 0.0200% or 　less, N: 0.0060% or less, and 　the balance: Fe and impurities, 　organic in the chemical composition represented and, 　an area ratio, 　ferrite: 5 to 60% and 　bainite 40 to 95%, 　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 circle equivalent diameter 10nm or less of Ti (C, N) and Nb (C, N) precipitate density is 10 10 pieces / mm 3 or more, 　the hardness (Hvs) in depth 20μm from the surface, the plate the ratio of the thickness center of the hardness (Hvc) (Hvs / Hvc) is steel sheet, characterized in that at least 0.85.

[0023]
　(2)
　the average dislocation density of 1 × 10 14 m -2 steel sheet according to (1) is not more than.
[0024]
　(3)
　tensile strength of not less than 480 MPa,
　the ratio between the tensile strength and yield strength is not less than 0.80,
　in the tensile strength and the product of the limit forming height of saddle stretch flange tested 19500mm · MPa or higher There,
　the steel sheet according to the fatigue strength ratio is equal to or is 0.45 or more (1) or (2).
[0025]
　(4)
　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 any one of to (1) to (3).
[0026]
　(5)
　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 (4).
[0027]
　(6)
　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 (5).
[0028]
　(7)
　(1) plated steel sheet on the surface of the steel sheet according to any one of the - (6), characterized in that the plating layer is formed.
[0029]
　(8)
　The plating layer is plated steel sheet according to, characterized in that a galvanized layer (7).
[0030]
　(9)
　the plating layer is plated steel sheet according to characterized in that it is a galvannealed layer (7).
Effect of the invention
[0031]
　According to the present invention, yet high strength, can be applied to severe ductility and members stretch flangeability is required, and can provide a steel sheet excellent and plated steel fatigue properties. This realizes a steel sheet excellent in collision characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032]
[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
[0033]
　Hereinafter, embodiments of the present invention will be described.
[0034]
"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.
[0035]
"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%.
[0036]
"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%.
[0037]
"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.
[0038]
"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.
[0039]
"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 carbides to fix the C by forming, inhibiting the production of harmful cementite for stretch flangeability. That, Ti and Nb are important in order to TiC and precipitation strengthening during annealing. Although details will be described later, a method utilizing Ti and Nb in the present embodiment, described here. In the manufacturing process, the hot rolling step (step from hot rolling until coiling), partly because there needs to be a solid solution state of Ti and Nb, the coiling temperature at hot rolling, Ti precipitates and Nb precipitates is less difficult to 620 ℃ generation. Then, it is important to introduce a dislocation by applying skin pass rolling before annealing. Next, in the annealing step, the introduced on transposition, Ti (C, N) and Nb (C, N) is precipitated finely. Particularly in the vicinity becomes higher steel sheet surface layer of dislocation density, the effect (Ti (C, N) and Nb (C, N) of the fine precipitation) becomes remarkable. This effect makes it possible to Hvs / Hvc ≧ 0.85, high fatigue properties can be achieved. Further, the precipitation strengthening of Ti and Nb, the ratio between the tensile strength yield strength (yield ratio) can be set to 0.80 or more. The total content of Ti and Nb is less than 0.015%, it is impossible to obtain these effects sufficiently. Therefore, the total content of Ti and Nb is 0.015% or more. The total content of Ti and Nb is preferably 0.020% or more. The total content of Ti and Nb is less than 0.015%, the workability is deteriorated, the frequency of cracks during rolling becomes high. Further, Ti content is preferably 0.025% or more, more preferably 0.035% or more, more preferably 0.025% or more. Further, Nb content is preferably 0.025% or more, more preferably 0.035% or more. On the other hand, the total content of Ti and Nb is more than 0.200% is insufficient proportion of crystal grain misorientation 5 ~ 14 ° in the grains, stretch flangeability is degraded significantly. 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%.
[0040]
"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.
[0041]
"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.
[0042]
"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.
[0043]
　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.
[0044]
"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.
[0045]
"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%.
[0046]
"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.
[0047]
"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.
[0048]
"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.
[0049]
"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.
[0050]
"Metal structure"
　will be described organization of the steel sheet according to an embodiment of the present invention (metal structure). In the following description, "%" is a unit of the percentage of each tissue (area ratio) is particularly means "area%" unless otherwise specified. Steel sheet according to the present embodiment, ferrite: 5 to 60% and bainite: having 40 to 95%, in tissue represented.
[0051]
: "Ferrite 5-60%"
　When the ferrite area ratio of less than 5%, ductility deteriorates the steel sheet, generally ensuring properties sought in automotive parts and the like becomes difficult. Therefore, the area ratio of ferrite is 5% or more. On the other hand, the area ratio of ferrite is greater than 60%, or becomes difficult stretch flangeability deteriorates, obtain sufficient strength. Therefore, the area ratio of ferrite is 60% or less. Area ratio of ferrite is preferably set to less than 50%, more preferably less than 40%, more preferably less than 30%.
[0052]
"Bainite: 40-95%"
　if the area ratio of bainite is greater than or equal to 40%, can be expected to increase in strength due to precipitation strengthening. That is, as described below, in the manufacturing method of the steel sheet according to the present embodiment, the winding temperature of the hot-rolled steel sheet and 630 ° C. or less, but to ensure a solid solution Ti and solid solution Nb in the steel sheet, the temperature bainite It is in close proximity to the transformation temperature. Therefore, contains many bainite microstructure of a steel sheet, transformation dislocations transformation to be introduced simultaneously because increasing the nucleation sites of TiC and NbC at the time of annealing, higher precipitation strengthening can be achieved. Cooling history during hot rolling, but the area ratio changes greatly, depending on the material properties required, the area ratio of bainite is adjusted. Area ratio of bainite is preferably well over 50% city, strength increase by the further precipitation strengthening by which is increased, reducing the coarse cementite press formability is inferior, press formability is also favorably maintained. Area ratio of bainite is more preferably 60% super cities, more preferably 70 percent. Area ratio of bainite, and 95% or less, preferably 80% or less.
[0053]
　Organization of the steel sheet according to the present embodiment, the balance of the tissue, may comprise a metal structure other than ferrite and bainite. The metal structure other than ferrite and bainite, for example, martensite residual austenite and pearlite and the like. However, the fraction of the remainder of the tissue (area ratio) is large, stretch flangeability degradation is concerned. Therefore, the remainder of the tissue is preferably a total of 10% or less in area ratio. In other words, the sum of ferrite and bainite in the tissue, it is preferred that an area ratio of 90% or more. More preferably, the total of the ferrite and bainite, is 100% in area ratio.
[0054]
　In the method of manufacturing the steel sheet according to the present embodiment, a part of Ti and Nb in the steel sheet in the hot rolling step (step from hot rolling until coiling) leave a solid solution state, the surface layer by skin pass rolling after hot rolling to introduce a strain. Then, in the annealing step, as introduced strain nucleation sites, the surface layer to the Ti (C, N) and Nb (C, N) to precipitate. It is doing to improve the fatigue properties of the above. Therefore, it is important to complete the hot rolled at less Ti and proceeds hardly 630 ° C. precipitation of Nb. That is, it is important to wind the Netsunobezai at 630 ° C. or lower. In tissues of the steel sheet obtained by winding the Netsunobezai (hot rolled stages of tissue), the fraction of bainite is within the above range, may optionally. In particular, the product (high-strength steel sheet, hot dip plated steel sheet, alloyed hot dip plated steel sheet) when desired to increase the growth, it is effective to keep a high fraction of ferrite in the hot rolling.
[0055]
　Organization of the steel sheet hot-rolled stage, because it contains bainite or martensite, having high dislocation density. However, since the tempered bainite or martensite in the annealing, the dislocation density decreases. When the annealing time is insufficient, it will remain the dislocation density is high, the low elongation. Therefore, the average dislocation density of the steel sheet after annealing is 1 × 10 14 m -2 is preferably less. Below Formula (4), in the case of performing annealing under the condition satisfying (5), Ti (C, N) with or Nb (C, N) precipitates, reduction of dislocation density progresses. That is, in the fully Ti (C, N) and Nb (C, N) an advanced state precipitation of the average dislocation density of the steel sheet is reduced. Usually, the decrease in dislocation density leads to a decrease in yield stress of the steel. However, in this embodiment, it decreases with Ti (C, N) of the dislocation density and Nb (C, N) because the precipitated, high yield stress are obtained. In the present embodiment, the measuring method of the dislocation density, CAMP-ISIJ Vol. It performed 17 (2004) P396 in accordance with the "method of evaluating the dislocation density using an X-ray diffraction" described in (110), (211), an average dislocation density from the half-width of (220).
[0056]
　Microstructure, by having the above mentioned features, the steel sheet was subjected to precipitation strengthening by the prior art can achieve high could not be achieved yield ratio and a high fatigue strength ratio. That is, the microstructure in the vicinity of the steel sheet surface layer, different from the microstructure of the center of plate thickness, even if exhibited is and coarse tissue ferrite mainly the hardness of the vicinity of the steel sheet surface layer, in the annealing Ti (C, N) by precipitation of and Nb (C, N), reaches the steel plate center and not inferior hardness. As a result, the generation of fatigue cracks is suppressed, fatigue strength ratio is increased.
[0057]
　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.
[0058]
"Precipitate density"
　in order to obtain a good yield ratio (the ratio between the yield strength and tensile strength), than the transformation strengthening by a hard phase such as martensite, Ya Ti (C, N) precipitated by tempering of bainite nb (C, N) precipitation strengthening due very important. In the present embodiment, the effective circle equivalent diameter to precipitation strengthening is 10nm or less of Ti (C, N) Total precipitate density and Nb (C, N) is 10 10 pieces / mm 3 and more. This realizes a 0.80 or more yield ratio. Here, the circle equivalent diameter 10nm greater precipitate obtained as the square root of (longer diameter × shorter diameter) does not influence on the properties obtained in the present invention. However, as the precipitate size becomes finer, effectively Ti (C, N) and Nb (C, N) precipitation strengthening is obtained due to, thereby, there is a possibility of reducing the amount of alloy elements contained. Thus, the circle equivalent diameter defines the total precipitate density of 10nm or less of Ti (C, N) and Nb (C, N). Observation of precipitates is carried out by observing the replica sample produced according to the method described in JP 2004-317203 with a transmission electron microscope. Field is set at 5000 times to 100,000 times magnification, of three or more field, counting the number of 10nm or less of Ti (C, N) and Nb (C, N). Then, a electrolyte weight from weight change before and after electrolysis, the specific gravity 7.8Ton / m 3 is converted into a volume weight from. Then, by dividing the counted number by the volume to calculate the total precipitate density.
[0059]
"Hardness distribution"
　present inventors have found that in order to improve the fatigue properties and the elongation and collision properties, in high-strength steel sheet that utilize precipitation strengthening by microalloying elements, the hardness of the hardness and the steel plate center in the steel sheet surface layer by 0.85 or the ratio it was found that fatigue properties are improved. Here, the steel sheet surface layer hardness in the steel sheet cross-section, refers to the hardness at the position of depth 20μm from the surface to the inside, which is shown as Hvs. Further, the hardness of the steel sheet center refers to the hardness of the steel sheet surface in steel sheet cross-section a quarter inner position of the plate thickness, which is shown with HVC. It is less than the ratios Hvs / HVC is 0.85, the fatigue property is deteriorated, while in Hvs / HVC 0.85 or more, the present inventors that the fatigue properties are improved have found. Therefore, the the Hvs / HVC 0.85 or more.
[0060]
　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.
[0061]
　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%.
[0062]
　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.
[0063]
　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)".
[0064]
　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".
[0065]
　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.
[0066]
　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.
[0067]
　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.
[0068]
　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.
[0069]
　According to the steel sheet according to the present exemplary embodiment, the yield strength of at least 380MPa can be obtained. That is, excellent yield strength is obtained. The upper limit of the yield strength is not particularly limited. However, in the composition range of this embodiment, the upper limit of the substantial yield strength is about 900 MPa. Yield 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, it can be measured.
[0070]
　According to the steel sheet according to the present embodiment, 0.80 or more yield ratio (the ratio between the tensile strength yield strength) can be obtained. That is, excellent yield ratio. The upper limit of the yield ratio is not particularly limited. However, in the composition range of this embodiment, the upper limit of the substantial yield ratio is about 0.96.
[0071]
　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.
[0072]
　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.
[0073]
　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. 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.
[0074]
　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.
[0075]
　Next, a method for manufacturing a steel sheet according to the embodiment of the present invention. In this way, the hot rolling, the first cooling, the second cooling, the first skin pass rolling, performing annealing and a second skin pass rolling in this order.
[0076]
"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.
[0077]
　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.
[0078]
　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.
[0079]
　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.
[0080]
　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.
[0081]
　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.
[0082]
　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.
[0083]
　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.
[0084]
　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.
[0085]
　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%.
[0086]
"First cooling, the second cooling"
　In this manufacturing method, carried out after completion of finish rolling, the 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 ~ 630 ° 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 seconds than 10 seconds or less in the first temperature range.
[0087]
　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 an area ratio obtaining 40% or more of bainite, misorientation of the grains is 5 ~ 14 ° grains the percentage is insufficient. From the viewpoint of obtaining a high fraction of bainite, the cooling stop temperature in the first cooling, and 750 ° C. or less, preferably a 740 ° C. or less, and more preferably set to 730 ° C. or less, more preferably 720 ° C. or less.
[0088]
　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 in the area ratio obtaining 40% of bainite is 5 ~ 14 ° crystals the proportion of grain is insufficient. From the viewpoint of obtaining a high fraction of bainite, retention time was less than 10.0 seconds, preferably not more than 9.5 seconds, more preferably not more than 9.0 seconds, more preferably less 8.5 seconds . 600 the holding time at ~ 750 ° C. is 0 seconds, it becomes difficult to obtain a ferrite area ratio of 5% or more, the crystal orientation difference in the grains is insufficient proportion of crystal grains of 5 ~ 14 ° .
[0089]
　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 obtain more than 5% ferrite area ratio, the crystal grain ratio of the misorientation is 5 ~ 14 ° in the grains Run short. On the other hand, if the cooling stop temperature in the second cooling is at 630 ° C. greater than or insufficient proportion of grain misorientation is 5 ~ 14 ° in the grains, the area ratio to obtain more than 40% bainite in many cases it may become difficult. From the viewpoint of obtaining a high fraction of bainite, the cooling stop temperature in the second cooling, and 630 ° C. or less, preferably a 610 ° C. or less, and more preferably set to 590 ° C. or less, more preferably 570 ° C. or less.
[0090]
　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.
[0091]
　Winding the hot-rolled steel sheet after the second cooling. By the coiling temperature and 630 ° C. or less to suppress the precipitation of alloy carbonitrides at the stage of the steel sheet (steps from hot rolling until coiling).
[0092]
　As described above, the heating of the hot rolling, and cooling history by more highly controlling the coiling temperature, can achieve the desired heat Nobuhara plate.
[0093]
　The heat Nobuhara plate, an area ratio, have a tissue containing from 5 to 60% ferrite and 40 to 95% bainite, misorientation is surrounded by 15 ° or more grain boundaries, and the circle equivalent diameter of 0. If the region is 3μm or more was defined as the crystal grain, a percentage of the crystal grains of the whole grain intragranular orientation difference is 5 ~ 14 ° is 20 to 100% area ratio.
[0094]
　In this 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, that is, even when controlling the condition or cooling condition of hot rolling alone, it can not obtain the desired heat-rolled plate, important to appropriately control both conditions of hot rolling and cooling it is. 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.
[0095]
"First skin pass rolling"
　In the first skin pass rolling, the hot-rolled steel sheet was pickled, subjected to skin pass rolling at an elongation rate of from 0.1 to 5.0% of the steel sheet after pickling. By applying skin pass rolling the steel sheet, it is possible to impart strain on the surface of the steel sheet. During the annealing in the subsequent step, the alloy carbonitrides onto translocation through the strain is liable to nucleation, surface layer is cured. If the growth rate of the skin pass rolling is less than 0.1% can not provide sufficient distortion, surface hardness Hvs does not rise. On the other hand, if the elongation rate of the skin pass rolling exceeds 5.0%, even strain is imparted by the steel plate center part not surface only, poor workability of the steel sheet. If ordinary steel sheet, ferrite Subsequent annealing recrystallized to improve the elongation and hole expansion. However, having a chemical composition in the present embodiment, and during hot rolled steel sheet winding is performed at 630 ° C. or less, Ti, Nb, Mo, V has a solid solution, re-ferrite these by annealing crystals significantly delay, does not improve elongation and hole expansion after annealing. Therefore, the growth rate of skin pass rolling to 5.0% or less. The skin pass distortion in accordance with the elongation rolling is applied, from the viewpoint of improvement in fatigue properties, precipitation strengthening in the vicinity steel surface layer in annealing in accordance with the distortion amount of the steel sheet surface proceeds. Therefore, the elongation rate is preferably 0.4% or more. Further, from the viewpoint of workability of the steel sheet, in order to prevent the processability of the degradation due to distortion of the application to the steel inside the elongation it is preferably set to 2.0% or less. If the growth rate of skin pass rolling of 0.1 to 5.0% and improved Hvs / HVC, it can be seen that 0.85 or more. Further, if the case of not performing skin pass rolling (elongation 0% skin pass rolling) or to skin pass rolling elongation exceeds 5.0 percent, it can be seen that the Hvs / Hvc <0.85.
[0096]
　When first skin pass rolling elongation of 0.1 to 5.0% excellent elongation can be obtained. Also, when the first skin pass rolling elongation ratio exceeds 5.0%, poor elongation, poor press formability. If the first skin pass rolling elongation ratio exceeds 0% or 5%, the fatigue strength ratio is poor.
[0097]
　When first skin pass rolling elongation of 0.1 to 5.0% if the tensile strength is almost the same, it can be seen that substantially the same elongation and fatigue strength ratio. If the first skin pass rolling elongation exceeding 5% (high skin pass region), tensile strength is not less than 490 MPa, elongation is low, it can be seen even lower fatigue strength ratio.
[0098]
"Annealing"
　after performing the first skin pass rolling, annealing the steel sheet. In addition, it is also possible to use a leveler, such as the shape correction to the purpose. Purpose of the annealing is not to perform tempering of the hard phase, Ti was dissolved in the steel sheet, Nb, Mo, and V is to be deposited as an alloy carbonitrides. Therefore, the control of maximum heating temperature (Tmax) and the holding time at the annealing step is important. By controlling the maximum heating temperature and the holding time within a predetermined range, not only increases the tensile strength and yield stress, improve surface hardness, performed to improve the fatigue properties and the collision characteristics. If the temperature and holding time during annealing is unsuitable, either because carbonitrides does not precipitate, or coarsening of precipitates carbonitride takes place, to limit as follows maximum heating temperature and holding time.
[0099]
　Maximum heating temperature during annealing is set within a range of 600 ~ 750 ° C.. The maximum heating temperature is less than 600 ° C., the time required for precipitation of the alloy carbonitrides becomes very long, it is difficult to produce in a continuous annealing equipment. Thus, the maximum heating temperature is set to 600 ° C. or higher. Further, the maximum heating temperature of 750 ° C. greater than occur coarsening of alloy carbonitrides, can not be sufficiently obtained intensity increase due to precipitation strengthening. Further, when the maximum heating temperature is above Ac1 point, becomes two-phase region of ferrite and austenite, the strength increase due to precipitation strengthening can not be sufficiently obtained. Thus, the maximum heating temperature is set to 750 ° C. or less. As described above, the main purpose of this annealing is not to perform tempering of the hard phase is to deposit a Ti or Nb, which had been dissolved in the steel sheet. At this time, final strength, is determined by the fraction of each phase in the microstructure of the alloy components and steel of the steel material, the improvement of improvement yield ratio of fatigue characteristics due to surface curing, Ya alloy components of the steel material in no way be affected to each phase of the fraction in the microstructure of the steel sheet.
[0100]
　The present inventors have, as a result of extensive experiments, 600 ° C. or more retention time in the annealing (t) is the following formula with respect to maximum heating temperature during annealing (Tmax) (4), ( 5) by satisfying the relationship has been found that satisfactory high yield stress and 0.85 or more Hvs / HVC.
　× Tmax ≦ 530-0.7 T ≦ 3600-3.9 × Tmax · · ·
　(4) T> 0 · · · (5)
[0101]
　If the maximum heating temperature is in the range of 600 ~ 750 ℃, Hvs / Hvc is 0.85 or more. Steel sheet according to the present embodiment, both the retention time at 600 ° C. or higher (t) is Equation (4), which is manufactured under the condition satisfying the range of (5). Steel sheet according to the present embodiment, the retention time (t) is Equation (4), a case, Hvs / HVC 0.85 or more to satisfy the range of (5). Steel sheet according to the present embodiment, when Hvs / HVC is 0.85 or more, fatigue strength ratio is 0.45 or more. If the maximum heating temperature is in the range of 600 ~ 750 ° C., the surface layer is hardened by precipitation hardening, Hvs / HVC is 0.85 or more. By setting the retention time at the maximum heating temperature and 600 ° C. or higher within the range shown above, as compared with the hardness of the steel sheet center, the surface layer is sufficiently cured. Accordingly, the steel sheet according to the present embodiment ratios fatigue strength is 0.45 or more. This is because the curing of the surface layer, is because it is possible to delay the occurrence of fatigue cracks, the higher the surface hardness, the effect is large.
[0102]
"Second skin pass rolling"
　After annealing, subjected to the second skin pass rolling with respect to the steel plate. Thus, the fatigue properties can be further improved. In the second skin pass rolling, the elongation is 0.2 to 2.0% preferably 0.5 to 1.0%. The elongation is less than 0.2%, the improvement and the surface layer only of work hardening of sufficient surface roughness not obtained, no fatigue characteristics are sufficiently improved. Therefore, elongation of the second skin pass rolling is 0.2% or more. On the other hand, when the elongation ratio exceeds 2.0%, the steel sheet is excessively work hardened, there is a case where the press formability is inferior. Therefore, elongation of the second skin pass rolling is 2.0% or less.
[0103]
　It can be obtained steel sheet according to this way this embodiment. That is, by control over the chemical composition and production conditions including alloying elements, conventionally excellent formability could not be achieved, fatigue properties and having a collision safety, and a tensile strength of more than 480MPa high strength steel sheet It can be produced.
[0104]
　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
[0105]
　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.
[0106]
　Table 1 and smelted steel having the chemical composition shown in Table 2 to produce a steel slab, the resulting steel pieces were thermally to rough rolling to a heating temperature shown in Table 3 and Table 4, and subsequently, It was finished 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 2 means that analytical values ​​were below the detection limit. Table 1 and underlined in Table 2 indicates that the value is out of range of the present invention, underlined in Table 4 show that is out of the range suitable for the manufacture of steel sheet of the present invention.
[0107]
[Table 1]

[0108]
[Table 2]

[0109]
[table 3]

[0110]
[Table 4]

[0111]
　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)
[0112]
　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.
[0113]
　Next, by first cooling the hot-rolled steel sheet under the conditions shown in Table 5 and Table 6, held at the first temperature range, a second cooling, a first skin pass rolling, annealing and a second skin pass rolling, test No. To obtain a hot-rolled steel sheet of 1-46. The heating rate of the annealed with 5 ° C. / s, and the cooling rate from the maximum heating temperature was 5 ° C. / s. Also, for some experimental examples, following the annealing, galvanizing, and performs alloying treatment to produce a galvanized steel sheet (GI and forth) and galvannealed steel sheets (GA and forth). In the case of producing a hot-dip galvanized steel sheet, the second skin pass is carried out after galvanizing, when manufacturing alloy hot-dip galvanized steel sheet, the second skin pass was carried out after the alloying process. It underlined in Table 6 show that is out of the range suitable for the manufacture of steel sheet of the present invention.
[0114]
[table 5]

[0115]
[Table 6]

[0116]
　Then, for each steel plate, by the following method, ferrite, bainite, martensite, structural fraction of pearlite (area ratio), percentage of grain misorientation is 5 ~ 14 ° in the grains, precipitate density and to determine the dislocation density. 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.
[0117]
"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.
[0118]
"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)".
[0119]
"Precipitate density"
　by observing the replica sample produced according to the method described in JP 2004-317203 JP by a transmission electron microscope to observe the precipitate. Field is set at 5000 times to 100,000 times magnification, of three or more fields were counted and the number of 10nm or less of Ti (C, N) and Nb (C, N). Then, a electrolyte weight from weight change before and after electrolysis, the specific gravity 7.8Ton / m 3 in terms of the volume weight from by dividing the volume of the counted number, to calculate the total precipitate density.
[0120]
"Dislocation density"
　CAMP-ISIJ Vol. 17 (2004) to measure the dislocation density in accordance with the "method of evaluating the dislocation density using an X-ray diffraction" according to P396, the average dislocation density from the half-width of (110), (211), (220) calculated.
[0121]
[Table 7]

[0122]
[Table 8]

[0123]
　Next, the tensile test, determined and a yield strength tensile strength, the saddle stretch flange tested to determine the limits forming height. Also, evaluated as an index of stretch flangeability the product of tensile strength and (MPa) limit molding height as (mm), if the product is more than 19500mm · MPa, is determined that excellent stretch flangeability.
[0124]
　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. The adequate range of elongation corresponding to the intensity level of the tensile strength defined by the following equation (6) to evaluate the elongation (EL). Specifically, acceptance ranges of elongation, in consideration of the balance between tensile strength and right sides of the values over the range of the formula (6) below.
　Elongation [%] ≧ 30-0.02 × tensile strength [MPa] ··· (6)
[0125]
　Also, the saddle-type stretch flange test, the curvature radius R of the corner portion 60 mm, the opening angle θ of a corner portion using the saddle type molding with 120 °, was performed with 11% clearance when punching the corner portion . Further, the limit forming height observes the presence or absence of cracks with more than one-third of the length of the plate thickness by visual observation after molding, and the forming height limit cracks is not present.
[0126]
　Relates evaluation of hardness, using Ltd. Akashi Seisakusho MVK-E micro Vickers hardness meter, it was measured cross-sectional hardness of the steel sheet. As the steel sheet surface layer hardness (Hvs), the hardness was measured position of depth 20μm from the surface to the inside. Further, as the hardness of the steel sheet center (HVC), the hardness was measured in 1/4 inner position of the sheet thickness from the steel sheet surface. At each position, three times the hardness measurement, the average value of the measured values ​​hardness (Hvs, HVC) and the (average value of n = 3). It should be noted, it was set load onto 50gf.
[0127]
　Fatigue strength conforms to JIS-Z2275, was measured using a Schenk plane bending fatigue tester. Stress load during measurement was set the speed of the test in both swing will be 30Hz. Further, in accordance with the conditions and Schenk plane bending fatigue tester, to measure the fatigue strength at 107 cycles. The fatigue strength at 107 cycles was calculated fatigue strength ratio was divided by the tensile strength measured by a tensile test described above. Fatigue strength ratio was as acceptable 0.45 or more.
[0128]
　These results are shown in Tables 9 and 10. Underlined in Table 10 indicates that the numerical value is outside the desired range.
[0129]
[Table 9]

[0130]
[Table 10]

[0131]
　Invention sample (Test No.1 ~ 21), more tensile strength 480 MPa, (ratio between the tensile strength yield strength) 0.80 or more yield ratio, tensile or 19500mm · MPa strength and saddle stretch flange Test the product of the limit forming height, and 0.45 or more in fatigue strength ratio was obtained in.
[0132]
　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 as well as the C 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.
[0133]
　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, precipitate density, either the hardness ratio 1 One or more of a comparative example that did not satisfy the scope of the present invention. Test No. 28-40, since the proportion of crystal grain misorientation is 5 ~ 14 ° in the grains is small, indicators and fatigue strength ratio of stretch flangeability did not satisfy the target value. Test No. 41, 43 to 46 or fewer precipitate density, the hardness ratio is low or the fatigue strength ratio did not satisfy the target value.
Industrial Applicability
[0134]
　According to the present invention, it is possible to provide a high strength, yet application to severe members stretch flangeability is required is possible, a steel sheet excellent high strength stretch-flange formability and fatigue properties. 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: 5 to 60% and
　bainite 40 to 95%,
　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,
　circle equivalent diameter 10nm or less of Ti (C, N) and Nb (C, N) precipitate density is 10 10 pieces / mm 3 or more,
　the hardness (Hvs) in depth 20μm from the surface, the thickness center steel ratio of hardness (HVC) of (Hvs / HVC), characterized in that at least 0.85.
[Requested item 2]
　The average dislocation density of 1 × 10 14 m -2 steel sheet according to claim 1, wherein the or less.
[Requested item 3]
　Tensile strength not less than 480 MPa,
　the ratio between the tensile strength and yield strength is not less than 0.80,
　the product of the limit forming height in the tensile strength and saddle stretch flange test is at 19500mm · MPa or higher,
　fatigue steel sheet according to claim 1 or 2 intensity ratio is equal to or is 0.45 or more.
[Requested item 4]
　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 any one of claim 1 to 3.
[Requested item 5]
　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 4, characterized in that it comprises one or more that.
[Requested item 6]
　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 5, characterized in that it comprises one or more selected from the group consisting of to.
[Requested item 7]
　Plated steel sheet on the surface of the steel sheet according to any one of claims 1 to 6, characterized in that the plating layer is formed.
[Requested item 8]
　Plated steel sheet according to claim 7 wherein the plating layer is, which is a galvanized layer.
[Requested item 9]
　Plated steel sheet according to claim 7 wherein the plating layer is, which is a galvannealed layer.