Technical field
[0001]
　The present invention relates to a steel sheet suitable excellent collision characteristics can be obtained in the members of the motor vehicle.
BACKGROUND
[0002]
　When manufacturing the body of an automobile by using a steel sheet, in general, the molding of the steel sheet, welding and painting baking is performed. Therefore, the steel sheets for automobiles are required to have excellent formability and high strength. Conventionally, as a steel sheet used for automobiles, dual phase (DP) steel sheet having a dual phase structure of ferrite and martensite, and transformation induced plasticity (transformation induced plasticity: TRIP) steel sheet and the like. The steel sheet for automobiles, excellent crashworthiness in order to improve the safety of automobiles are also required. In other words, by increasing plastic deformation in the event of an impact from the outside, it is also required to absorb the collision energy.
[0003]
　However, the DP steel and TRIP steel, is a collision characteristic stamping is performed there is a problem that may be reduced. That is, the end face produced by punching (hereinafter sometimes referred to as "punched end face") becomes rough, cracked from punched end face at the time of a collision (hereinafter, sometimes referred to as "end surface cracking") is likely to occur, sufficient it may not be possible to obtain the energy absorption and reaction force characteristic. Sometimes the end face cracks reduce the fatigue properties.
[0004]
　DP steel and TRIP steel sheet is provided with the property of yield strength is increased by paint baking, improvement in yield strength does not become sufficient, may not sufficient reaction force characteristics are obtained.
CITATION
Patent Document
[0005]
Patent Document 1: JP 2009-185355 Patent Publication
Patent Document 2: JP 2011-111672 Patent Publication
Patent Document 3: JP 2012-251239 Publication
Patent Document 4: JP-A 11-080878 JP
Patent Document 5: JP 11-080879 JP
Patent Document 6: JP 2011-132602 Patent Publication
Patent Document 7: JP 2009-127089 JP
Patent Document 8: JP-a 11-343535 JP
Patent Document 9: WO 2010/114083
Summary of the Invention
Problems that the Invention is to Solve
[0006]
　The present invention can suppress the end surface cracks, and to provide a steel sheet can achieve excellent yield strength after baking.
Means for Solving the Problems
[0007]
　The present inventors have conducted extensive studies to solve the above problems. As a result, the following items were found.
　Since the solid solution C contained in (a) steel sheet to enhance segregated in the grain boundary grain boundary, solute as C is large, collision characteristic roughness of the punched end face is better suppressed is obtained, excellent reaction force characteristics after baking is obtained.
　As (b) a specific crystal grain in the total area fraction having a crystal orientation is small, the roughness of the punched end face is suppressed, resulting excellent crashworthiness. The crystal grains having a specific crystal orientation, the normal direction of the plate surface of the steel sheet: crystal orientation parallel to the (normal direction ND) is <111> deviation from direction is the crystal orientation of 10 ° or less crystal grains (hereinafter, "ND // <111> oriented grains" is that), and the crystal orientation parallel to a normal direction of the plate surface of the steel sheet, the crystal orientation deviation of less than 10 ° from the <100> direction certain crystal grains (hereinafter sometimes referred to as "ND // <100> oriented grains") corresponds.
　Because (c) residual austenite causes embrittlement of the punched end face, the more residual austenite is small, the roughness of the punched end face is suppressed, resulting excellent crashworthiness.
[0008]
　The present inventors, as a result of further intensive studies based on these findings, and conceived to aspects of the invention described below.
[0009]
　(1)
　in
　mass%,
　C: 0.05% ~
　0.40%, Si: 0.05% ~ 3.0%, Mn: 1.5%
　~ 3.5%, Al: 1.5% or less ,
　N: 0.010% or
　less, P: 0.10% or
　less, S: 0.005% or
　less, Nb: 0.00% ~ 0.04% or less,
　Ti: 0.00% ~ 0.08% or less ,
　V and Ta:
　0.0% ~ 0.3% in total, Cr, Cu, Ni, Sn and Mo:
　0.0% ~ 1.0% in total, B: 0.000% ~ 0.005% ,
　Ca:
　0.000% ~ 0.005%,
　Ce: 0.000% ~ 0.005%, La: 0.000% ~ 0.005%, and
　balance: Fe and impurities,
in represented by the chemical composition the a,
　in area%,
　number two or more first martensite iron carbide circle equivalent diameter of 2 nm ~ 500 nm in Las: 20% to 95%,
　ferrite: 5%,
　residual austenite 15% or less, and
　Remainder: equivalent circle diameter of bainite or Las number of iron carbides 2 nm ~ 500 nm is the second martensitic or both of less than two
have a steel structure represented
　by, ND // <111> orientation and the grain and ND // <100> oriented grains of the total area fraction of 40% or less,
　the amount of solid solution C is not less than 0.44Ppm,
　the ND // <111> oriented grains is legal plate surface crystal orientation parallel to the line direction is the crystal grains is a crystal orientation deviation of less than 10 ° from the <111> direction,
　the ND // <100> oriented grains is parallel to a normal direction of the plate surface steel crystal orientation, which is a crystal grain is a crystal orientation deviation of less than 10 ° from the <100> direction.
[0010]
　(2)
　In the above chemical composition
　V and Ta: 0.01% ~ 0.3% in total,
　the steel sheet having the constitution that holds (1).
[0011]
　(3)
　In the above chemical
　composition, Cr, Cu, Ni, Sn and Mo: 0.1% ~ 1.0% in total,
　the steel sheet having the constitution that holds (1) or (2).
[0012]
　(4)
　In the above chemical
　composition, B: 0.0003% - 0.005%,
　the steel sheet according to any one of to, characterized in that the holds (1) to (3).
[0013]
　(5)
　In the above chemical
　composition,
　Ca: 0.001% ~ 0.005%,
　Ce: 0.001% ~ 0.005%, La: 0.001% ~ 0.005%, or
　, or any of these steel sheet according to any one of the combinations, characterized in that the holds (1) to (5).
Effect of the invention
[0014]
　According to the present invention, the chemical composition, the steel structure, since the area fraction or the like of the specific crystal grain is appropriate, it is possible to suppress the end surface cracks, it is possible to obtain an excellent yield strength after baking.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015]
[1] Figure 1 is a diagram illustrating a hat part.
FIG. 2 is a diagram showing a lid.
FIG. 3 is a diagram showing a specimen.
[4] FIG. 4 is a diagram showing a method of evaluating crack ease of sample.
DESCRIPTION OF THE INVENTION
[0016]
　Hereinafter, embodiments of the present invention will be described.
[0017]
　First, a description will be given chemical composition of the steel used for the steel and the manufacturing according to an embodiment of the present invention. Details will be described later, the steel sheet according to the embodiment of the present invention, hot rolling of steel, cold rolled, annealed, is manufactured through the reheating and temper rolling and the like. Therefore, the chemical composition of the steel sheet and, not only characteristics of the steel sheet, is taken into consideration these processes. 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, by mass%, C: 0.05% ~ 0.40%, Si: 0.05% ~ 3.0%, Mn: 1.5% ~ 3.5%, Al: 1.5% or less, N: 0.010% or less, P: 0.10% or less, S: 0.005% or less, Nb: 0.00% ~ 0.04% or less, Ti: 0.00% ~ 0.08% or less, V and Ta: 0.0% ~ 0.3% in total, Cr, Cu, Ni, Sn and Mo: 0.0% ~ 1.0% in total, B: 0.000% ~ 0.005%, Ca: 0.000% ~ 0.005%, Ce: 0.000% ~ 0.005%, La: 0.000% ~ 0.005%, and balance: tables of Fe and impurities It has a chemical composition that is. As the impurity, those included in raw materials such as ores and scrap, intended to be included in the manufacturing process, is exemplified.
[0018]
　(C: 0.05% ~ 0.40%)
　C contributes to the improvement of the tensile strength, the solid solution C is to strengthen the grain boundary segregated at the grain boundaries. Enhanced grain boundaries, superior crashworthiness roughness of punched end face is suppressed is obtained. The C content is less than 0.05%, sufficient tensile strength, for example, may not tensile strength is obtained than 980 MPa, solid solution C is insufficient or. Therefore, C content is 0.05% or more. In order to obtain more excellent tensile strength and crashworthiness, C content is preferably 0.08% or more. On the other hand, the C content of 0.40 percent, by excessive precipitation increased and iron carbide of residual austenite, the end surface cracking at the time of collision is likely to occur. Therefore, C content is 0.40% or less. In order to obtain more excellent crashworthiness, C content is preferably 0.30% or less.
[0019]
　As described above, the solid solution C contained in the steel sheet to enhance grain boundary segregated at the grain boundaries. Therefore, the more solute C, resulting collision characteristics roughness of punched end face is better suppressed, resulting excellent reaction force properties after baking. Is less than the amount of solute C is 0.44ppm contained in the steel sheet, punched end face becomes rough or not sufficient collision characteristics can be obtained, the reaction force characteristics after sufficient paint baking or not obtained. Reaction force characteristic aging index after baking: Based on (aging index AI) can be evaluated, the amount of dissolved C contained in the steel sheet is less than 0.44Ppm, the desired aging index, e.g. 5MPa or more can not be obtained aging index. Therefore, the amount of solid solution C is not less than 0.44Ppm. For more information about aging index will be described later.
[0020]
　(Si: 0.05% ~ 3.0%) Si
　is or to stabilize the austenite during annealing by suppressing the formation of carbides, the suppression of formation of carbides on securing and grain boundaries of the solid solution C or contribute to. The Si content is less than 0.05%, or not sufficient tensile strength is obtained, the increase in yield ratio due to aging caused by baking solid solution C is insufficient is insufficient sufficient yield ratio, for example, 0. 8 or more of the yield ratio or not obtained. Therefore, Si content is 0.05% or more. In order to obtain more excellent tensile strength and crashworthiness, Si content is preferably 0.10% or more. On the other hand, the Si content of 3.0 percent, ferrite or becomes excessive, the residual austenite may become excessive. Therefore, Si content is at most 3.0%. In view of the slab of season cracking suppression and end crack suppression during hot rolling, Si content is preferably 2.5% or less, more preferably 2.0% or less.
[0021]
　(Mn: 1.5% ~
　3.5%) Mn suppresses the formation of ferrite. The Mn content is less than 1.5%, ferrite is generated excessively, the end surface cracking at the time of collision is likely to occur. Therefore, Mn content is 1.5% or more. In order to obtain more excellent crashworthiness, Mn content is preferably 2.0% or more. On the other hand, the Mn content of 3.5 percent, ND // <111> oriented grains and ND // <100> oriented grains of the total area fraction becomes excessive, the end surface cracking at the time of collision is likely to occur. Therefore, Mn content is 3.5% or less. From the standpoint of weldability, Mn content is preferably 3.0% or less.
[0022]
　(Al: 1.5% or less)
　Al is not an essential element, for example used for deoxidation to reduce the inclusions can remain in the steel. The Al content of 1.5 percent, ferrite is generated excessively, the end surface cracking at the time of collision is likely to occur. Therefore, Al content is 1.5% or less. Costly in reducing the Al content, an attempt to reduce to less than 0.002%, the cost increases considerably. Therefore, Al content may be 0.002% or more. If you make a sufficient deoxidation, it may be Al of 0.01% or more remains.
[0023]
　(N: 0.010% or less)
　N is not an essential element, is contained as an impurity, for example, in the steel. The N content is 0.010% greater, sufficient toughness can be obtained, or become the end surface cracking easily occurs at the time of collision, the yield point elongation may become excessive. Therefore, N content is 0.010% or less. From the viewpoint of moldability, N content is preferably 0.005% or less. Costly in reducing the N content, an attempt to reduce to less than 0.001%, the cost increases considerably. Therefore, N content may be 0.001% or more.
[0024]
　(P: 0.10% or less)
　P is not an essential element, is contained as an impurity, for example, in the steel. The P content of 0.10 percent, the roughness of the punched end face becomes prominent, the end surface cracking at the time of collision is likely to occur. Accordingly, P content is 0.10% or less. From the standpoint of weldability, P content is preferably 0.05% or less. Costly in reducing the P content, an attempt to reduce to less than 0.001%, the cost increases considerably. Therefore, P content may be 0.001% or more.
[0025]
　(S: 0.005% or less)
　S is not an essential element, is contained as an impurity, for example, in the steel. The S content of 0.005 percent, the roughness of the punched end face becomes prominent, the end surface cracking at the time of collision is likely to occur. Thus, S content is 0.005% or less. In order to suppress the cracking of the welded portion during a collision, S content is preferably 0.003% or less. Costly in reducing S content, an attempt to reduce to less than 0.0002%, cost increases considerably. Therefore, S content may be 0.0002% or more.
[0026]
　Nb, Ti, V, Ta, Cr, Cu, Ni, Sn, Mo, B, Ca, Ce and La are not essential elements, the steel sheet and may optionally elements be appropriately contained in the limits of the predetermined amount to the steel it is.
[0027]
　(Nb: 0.00% ~ 0.04%, Ti: 0.00% ~
　0.08%) Nb and Ti contribute to improvement of the yield strength through grain refining of securing and grain of solid solution C , it is effective in improving the crashworthiness. Therefore, Nb or Ti or both may be contained. However, the Nb content of 0.04 percent, ND // <111> oriented grains and ND // <100> or becomes excessive oriented grains of the total area fraction, excess Nb carbonitride in the grain boundaries and or deposited on, the end surface cracking at the time of collision is likely to occur. Therefore, Nb content is 0.04% or less. The Ti content of 0.08 percent, ND // <111> oriented grains and ND // <100> total area fraction of oriented grains or become excessive, the grain boundary of Ti carbonitride excessively deposited and or, the end surface cracking at the time of collision is likely to occur. Therefore, Ti content is 0.08% or less. In order to ensure the effect of the action, the content of Nb and Ti is preferably 0.01% or above in total. Incidentally, costly in reducing the Nb content, when you try to reduce to less than 0.0002%, cost increases considerably. Therefore, Nb content may be 0.0002% or more. Costly in reducing Ti content, an attempt to reduce to less than 0.0002%, cost increases considerably. Therefore, Ti content may be 0.0002% or more.
[0028]
　(V and Ta: 0.0% ~ 0.3% in total)
　V and Ta, carbides, the formation and comminution of nitride or carbonitride contributes to the improvement of strength. Therefore, V or Ta, or both of these may be contained. However, 0.3 percent content of V and Ta in total, a large amount of carbide or carbonitride is precipitated in grain boundaries, the roughness of the punched end face becomes prominent, the end surface cracking occurred at the time of collision It becomes easier. Therefore, the content of V and Ta is less than 0.3% in total. In view of the slab of season cracking suppression and end crack suppression during hot rolling, the content of V and Ta are preferably 0.1% or less in total. In order to ensure the effect of the action, the content of V and Ta is preferably 0.01% or above in total.
[0029]
　(Cr, Cu, Ni, Sn and Mo:
　total% ~ 1.0 0.0%) Therefore, Cr, Cu, Ni, Sn or Mo, or any combination thereof may be contained. However, Cr, Cu, Ni, In 1.0 percent content of Sn and Mo is in total, processability is remarkably deteriorated, the end surface cracking is likely to occur. Accordingly, the content of Cr, Cu, Ni, Sn and Mo is 1.0% or less in total. From the viewpoint of more reliable suppression of the end surface cracking, the content of Cr, Cu, Ni, Sn and Mo is preferably 0.5% or less in total. In order to ensure the effect of the action, the content of Cr, Cu, Ni, Sn and Mo is preferably 0.1% or more.
[0030]
　(B: 0.000% ~ 0.005%)
　B enhances hardenability of the steel sheet, suppresses the formation of ferrite, prompting the formation of martensite. Thus, B may be contained. However, 0.005 percent content of B in total, there are the end face cracks during collision occurs. Therefore, B content is 0.005% or less. In order to obtain more excellent crashworthiness, B content is preferably 0.003% or less in total. In order to ensure the effect of the action, B content is preferably 0.0003% or more.
[0031]
　(Ca: 0.000% ~ 0.005%, Ce: 0.000% ~ 0.005%, La: 0.000% ~
　0.005%) Ca, Ce and La are oxides in the steel sheet and or finely sulfide, and or alter the properties of the oxides and sulfides, and less likely to occur end surface cracking. Therefore, Ca, Ce or La, or any combination thereof may be contained. However, Ca content, Ce content in any of the La content of 0.005 percent, with unnecessarily cost effective due to the effect is saturated is higher, the moldability decreases. Therefore, Ca content, Ce content, La content are both 0.005% or less. To further suppress a decrease in moldability, Ca content, Ce content, La content are both preferably not more than 0.003%. In order to ensure the effect of the action, Ca content, Ce content, La content are both preferably 0.001% or more. In other words, "Ca: 0.001% ~ 0.005%", "Ce: 0.001% ~ 0.005%" or "La: 0.001% ~ 0.005%" or any combination thereof it is preferably satisfied.
[0032]
　It will now be described steel sheet steel structure according to the embodiment of the present invention. In the following description, the unit of the proportion of phases or tissues constituting the steel structure "%" is especially meant to "area%" of the area fraction unless otherwise noted. Steel sheet according to the embodiment of the present invention, the first martensite number of two or more circle-equivalent diameter of 2 nm ~ 500 nm of iron carbides in Las: 20% to 95%, ferrite 15% or less, residual austenite : 15% or less, and the balance: equivalent circle diameter of bainite or Las has a steel structure represented by both number second of less than two martensitic or of these iron carbide of 2 nm ~ 500 nm .
[0033]
　(First martensite number of iron carbide circle equivalent diameter of 2 nm ~ 500 nm in Las is 2 or more: 20% to 95%)
　circle equivalent diameter in the lath the number of iron carbides 2 nm ~ 500 nm 2 more than five first martensite, the tensile strength improvement of and contribute to securing the solid solution C, by securing solid solution C, the yield ratio is improved by aging due to baking, the end surface cracking during collision suppressing It is. Iron carbide on the lath boundaries do not correspond to the iron carbides in the class. Iron carbide, not only those composed of Fe and C, also applicable but also the other elements. Other elements, Mn, Cr and Mo are exemplified.
[0034]
　Martensite circle equivalent diameter in Las there is no more iron carbides 2nm, and martensite even number that is less than two and a circle equivalent diameter in the lath not exist or iron carbide 2nm has a tensile strength It can not contribute sufficiently to improving and securing the solid solution C. Be equivalent circular diameter in the lath exist 2nm or more iron carbides least two, martensite those following 500nm Among them is less than two is due to the influence of coarse iron carbide, excessive yield point elongation or cause, or to inhibit the improvement of tensile strength.
[0035]
　Then, the area fraction of the first martensite is less than 20%, not enough to improve the the yield ratio by aging due to baking. Therefore, the area fraction of the first martensite is 20% or more. To obtain a higher yield ratio, area fraction of the first martensite is preferably 30% or more. On the other hand, the area fraction of the first martensite is 95%, the ductility is insufficient, with or without a punched end face, large cracks from sites prone deformed during a collision. Therefore, the area fraction of the first martensite is 95% or less. To obtain a better ductility, the area fraction of the first martensite is preferably 90% or less.
[0036]
　(Ferrite: 15% or less)
　ferrite, although improving the formability of the steel sheet, or prone to end surface cracks at the time of collision, or to inhibit the increase of the yield ratio by baking, or reduce the reaction force characteristic . Then, when the area fraction of ferrite is 15%, the occurrence of the end face cracks, a decrease in inhibition and reaction force characteristic of improving the yield ratio is remarkable. Therefore, the area fraction of ferrite is 15% or less. In order to obtain more excellent crashworthiness, area fraction of ferrite is preferably 10% or less, more preferably 6% or less.
[0037]
　(Residual austenite 15% or less)
　residual austenite, but contributes to the absorption of improving the moldability and impact energy, the punched end face by embrittlement easily occurs end face cracks at the time of collision. The area fraction of retained austenite in the case of more than 15%, generation of the end face cracks is remarkable. Therefore, the area fraction of retained austenite is 15% or less. In order to obtain more excellent crashworthiness, area fraction of retained austenite is preferably not more than 12%. The area fraction of retained austenite is less than 3%, the cracking of the flange portion extending in a collision occurs. Therefore, the area fraction of retained austenite is preferably 3% or more.
[0038]
　(Balance: second martensitic or both of these equivalent circle diameter of bainite or Las is the number of iron carbides 2 nm ~ 500 nm less than 2)
　first martensite, other than ferrite and residual austenite balance, bainite or the second martensitic or both. If it contains bainite, enrichment of C is promoted, the area fraction easily obtained residual austenite of 3% to 15%.
[0039]
　In the present application, the ferrite, polygonal ferrite (.alpha.p), pseudo-polygonal ferrite (.alpha.q) and particulate bainitic ferrite (alpha B) is included, the bainite, lower bainite, upper bainite and bainitic ferrite (alpha ° B) are included. Granular bainitic ferrite has recovered dislocation substructures without lath bainitic ferrite is a structure in which no precipitation of carbides lath becomes bundle, old γ grain boundaries remain intact (ref : "steel of bainite Photos -1" Japan Iron and steel Institute (1992) p.4, reference). In this reference, there is a description that; "conserving the prior austenite grain boundary sheaf-like with laths but no carbide", "Granular bainitic ferrite structure dislocated substructure but fairly recovered like lath-less" described that and.
[0040]
　The second martensite, martensite circle equivalent diameter in Las there is no more iron carbide 2nm, be equivalent circle diameter in Las is present more than iron carbide 2nm in number than two circle equivalent diameter martensite, and during lath is be present 2nm or more iron carbides two or more, those following 500nm among the martensite of less than two is applicable. The area fraction of 3% of the second martensite may not sufficiently yield ratio obtained after baking. Therefore, the area fraction of the second martensite is preferably 3% or less.
[0041]
　Ferrite, bainite, the area ratio of martensite and pearlite, for example, an optical microscope or scanning electron microscope (scanning electron microscopy: SEM) using a photograph of the steel structure taken by, can be measured by a point count method or image analysis. Determination of granular bainitic ferrite (alpha B) and bainitic ferrite (α ° B) performs the tissue observation by SEM and transmission electron microscope (TEM), can be performed on the basis of the description in the above references. Circle equivalent diameter of iron carbides in martensite lath can be measured by performing a tissue observation by SEM and TEM. The amount of solute C may be measured, for example, by internal friction method. The contents of the internal friction method, for example, "The Japan Institute of Metals Journal (1962), vol. 26, (1), 47" are described in.
[0042]
　Area fraction of retained austenite, for example, electron backscatter diffraction can be measured by (electron backscatter diffraction EBSD) method or X-ray diffraction method. When measured by X-ray diffraction method, using a Mo-K [alpha ray diffraction intensity of the (111) plane of the ferrite (alpha (111)), the diffraction intensity of the (200) plane of the residual austenite (gamma (200)) , the diffraction intensity of (211) plane of the ferrite (alpha (211)), and the diffraction intensity of the (311) plane of the residual austenite (gamma (311)) were measured, the area fraction of retained austenite from the following equation (f a can be calculated).
f A = (2/3) {100 / (0.7 × alpha (111) / gamma
　　　(200) +1)} + (1/3) {100 / (0.78 × alpha (211) / gamma (311 ) +1)}
[0043]
　Next, a description will be given ND // <111> oriented grains and ND // <100> total area fraction of oriented grains in the steel sheet according to the embodiment of the present invention. The present inventors, ND // <111> oriented grains and ND // <100> oriented grains of the total area fraction was found to greatly affect the end surface cracks at the time of collision. That is, the total area fraction of 40-percent, the end surface cracking during collision is found to be likely to occur. Therefore, the total area fraction of 40% or less. Crystal orientation can be identified by EBSD method. Incidentally, ND // <111> oriented grains and ND // <100> total area fraction of oriented grains are relative to the total crystal grains in the observation plane, it is distinguished from the area fraction of the steel structure. In other words, in between which is the denominator is different, not necessary to these sum to 100%.
[0044]
　Next, a description will be given mechanical properties of the steel sheet according to an embodiment of the present invention.
[0045]
　Steel sheet according to the present embodiment preferably has a tensile strength of at least 980 MPa. It is less than the tensile strength 980 MPa, because it is difficult to obtain the benefits of weight reduction by strengthening the member.
[0046]
　Steel sheet according to the present embodiment, preferably 5MPa or more, more preferably 10MPa or more aging index: having (aging index AI). The aging index is less than 5 MPa, the yield ratio after baking is low, because it is difficult to obtain a good reaction force characteristic. Aging index referred to here is added 10% tensile prestrain is the difference between the yield strength and aging prior to the yield strength after the aging 60 minutes at 100 ° C. is performed, the amount of increase in yield strength due to aging It corresponds to. Aging index is affected by the amount of solute C in the steel sheet.
[0047]
　Steel sheet according to the present embodiment is preferably 3% or less, more preferably has a yield point elongation of 1% or less. The yield point elongation of 3 percent, because the easily broken with the local strain concentration at the time of molding and during the collision.
[0048]
　Steel sheet according to the present embodiment is preferably 0.80 or more, more preferably has a yield ratio after aging due to baking of 0.88 or more. The yield ratio is less than 0.80 after aging, not sufficient collision characteristics can be obtained because it is difficult to obtain the benefits of weight reduction of the member. Yield ratio after aging referred to herein is measured as follows. First, by adding a 5% tensile prestrain the steel sheet, performing an aging treatment for 20 minutes at 170 ° C. corresponding to baking finish. Thereafter, to get the tensile strength and yield strength by a tensile test, to calculate the yield ratio from the tensile strength and yield strength. Are you tensile prestrain size and 5%, in the manufacture of components for the skeleton of an automobile, it is taken into consideration that generally the bent portion and forming distortion of 5% or more to the drawn portion is introduced .
[0049]
　Next, a method for manufacturing a steel sheet according to the embodiment of the present invention. In this production method, hot rolling of steel having the above chemical composition, cold rolling, annealing, the reheating and temper rolling are performed.
[0050]
　First, to produce a slab having the above chemical composition, performing hot rolling. Slab subjected to hot rolling, for example, it can be produced by continuous casting, blooming method or thin slab caster or the like. Continuous casting is carried out immediately hot rolled after casting - may be employed processes such as direct rolling.
[0051]
　In hot rolling, performing rough rolling and finish rolling. Finish rolling begins at (960+ (80 × [% Nb] + 40 × [% Ti])) ℃ or higher. [% Nb] is Ni content, [% Ti] is the Ti content. Temperature starting finish rolling (finish rolling start temperature: HST) in the (960+ (80 × [% Nb] + 40 × [% Ti])) than ℃, ND // <100> oriented grains and ND // <111 > total area fraction of oriented grains becomes excessive, the roughness of the punched end face becomes prominent, the end surface cracking is likely to occur at the time of collision. Finish rolling is terminated at (880+ (80 × [% Nb] + 40 × [% Ti])) ℃ or higher. Temperature to terminate the finish rolling (finish rolling end temperature: HFT) In the (880+ (80 × [% Nb] + 40 × [% Ti])) than ℃, ND // <100> oriented grains and ND // <111 > total area fraction of oriented grains becomes excessive, the roughness of the punched end face becomes prominent, the end surface cracking is likely to occur at the time of collision. Finish rolling is preferably completed at (890+ (80 × [% Nb] + 40 × [% Ti])) ℃ or higher.
[0052]
　After the end of the final rolling, to cool the steel plate. In this cooling, the finish rolling end temperature (HFT) a first average cooling rate (CR1) between up (HFT-20 ℃) not more than 10 ° C. / s, Ar 3 second between point to 700 ° C. the average cooling rate of the (CR2) and 30 ° C. / s or higher. In the first average cooling rate of 10 ° C. / s greater, ND // <100> oriented grains and ND // <111> oriented grains of the total area fraction becomes excessive, the roughness of the punched end face becomes prominent, end surface cracking is likely to occur at the time of collision. First average cooling rate is preferably not more than 8 ° C. / s. In the second average cooling rate is less than 30 ° C. / s, and can not be obtained enough solid solution C after annealing, may not be sufficiently improved yield ratio by baking, the roughness of the punched end face significant or it becomes.
[0053]
　The take-up after the finish rolling is carried out at 670 ℃ or less. The coiling temperature (CT) is 670 ° C. greater, it is impossible to obtain a sufficient solid solution C after annealing, may not yield ratio is sufficiently improved even by baking, or become roughness of the punched end face significant to. Coiling temperature, preferably 620 ° C. or less.
[0054]
　After the winding, performs pickling and cold rolling. Conducted by cold rolling 75% or less of reduction ratio. The reduction ratio of cold rolling is greater than 75%, the roughness of the punched end face becomes prominent, the end surface cracking is likely to occur at the time of collision.
[0055]
　Performing the annealing after the cold rolling. Maximum temperature (ST) is (Ac this annealing 3 in -60) below ° C., or ND // <100> oriented grains and ND // <111> oriented grains of the total area fraction is 40 percent, area fraction of the ferrite may become 15 percent. As a result, roughness of the punched end face becomes prominent, the end surface cracking is likely to occur at the time of collision. In time is less than 3 seconds annealing, for the same reason, the roughness of the punched end face becomes prominent, the end surface cracking is likely to occur at the time of collision. Accordingly, the maximum temperature is (Ac 3 and -60) In order to obtain more excellent crashworthiness, the maximum temperature is preferably (Ac 3 and -40) On the other hand, the highest temperature is (Ac 3 in the +70) Accordingly, the maximum temperature is preferably (Ac 3 and +70) ° C.. The annealing, for example, using a continuous annealing facility having a continuous annealing equipment or plating equipment.
[0056]
　Transformation temperature Ac 3 the value of (℃) [% C] is the C content is [% Si] of Si content, a [% Mn] is Mn content is [% Cu] is the Cu content, [% Ni] is Ni a content is [% Cr] is the Cr content is [% Mo] is Mo content, [% Ti] is the Ti content is [% Nb] is Nb content [ % V] is V content, [% Al] is the Al content.
　Ac 3 (° C.) = 937.2-436.5 [% C] +56 [% Si] -19.7 [% Mn] -16.3 [% Cu] -26.6 [% Ni] -4.9 [% Cr] +38.1 [% Mo ] +136.3 [% Ti] -19.1 [% Nb] +124.8 [% V] +198.4 [% Al]
[0057]
　The cooling after the annealing, the fourth average cooling rate between the first three average cooling rate of between 700 ° C. to 500 ° C. The (CR3) and 10 ° C. / s or higher, 0.99 ° C. from 300 ° C. (CR4 ) be the 10 ℃ / s or more. In a third average cooling rate is less than 10 ° C. / s of, or the area fraction and 15% of ferrite, and may not be able to obtain sufficient solid-solution C, sufficient yield ratio by baking It does not improve in. Third average cooling rate of, preferably 20 ° C. / s or higher. In the fourth average cooling rate is less than 10 ° C. / s, and can not be obtained a sufficient solid solution C, no yield ratio is sufficiently improved even by baking.
[0058]
　Thereafter, the re-heating of more than 10 seconds at 300 ° C. or higher 530 ° C. or less of the temperature range. Iron carbide is grown martensite within lath during the reheating. In the holding temperature (Tr) is less than 300 ° C., can not be obtained sufficient iron carbides, not sufficiently improved yield ratio by baking, the end surface cracking easily occurs at the time of collision, the energy absorption is low, sufficient reaction force characteristics can not be obtained. Held at the time is less than 10 seconds can not be obtained excellent crashworthiness the same reason. The holding temperature is 530 ° C. greater than iron carbides are coarsened, the yield point elongation or becomes excessive, the tensile strength or insufficient.
[0059]
　Plated on the steel sheet may be carried out during the reheating. Plating treatment, for example, may be carried out provided in continuous annealing equipment plating equipment may be performed in a different plating dedicated equipment and continuous annealing equipment. The composition of the plating is not particularly limited. The plating can be performed, for example, a hot dipping process, an alloying hot dipping treatment or electroplating treatment.
[0060]
　After reheating, it carried out temper rolling (skin pass rolling) in elongation 0.2% or more. The elongation rate is less than 0.2%, it is impossible to yield point elongation obtain 3% ultra next, sufficient reaction force characteristics. On the other hand, the growth rate of 2.0 percent, it may moldability decreases. Therefore, the growth rate preferably 2.0% or less.
[0061]
　In this way, it is possible to produce a steel sheet according to the embodiment of the present invention.
[0062]
　According to this embodiment, the chemical composition, the steel structure, since the area fraction or the like of the specific crystal grain is appropriate, it is possible to suppress the end surface cracks, it is possible to obtain an excellent yield strength after baking.
[0063]
　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
[0064]
　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.
[0065]
　In this test, Steels having chemical compositions shown in Table 1 to produce a steel slab, heating the slab to 1200 ° C. ~ 1250 ° C., was subjected to hot rolling. In hot rolling the billet was subjected to rough rolling and finish rolling. Blank in Table 1 indicates that the content of the element is less than the detection limit, the balance being Fe and impurities. Underlined in Table 1 indicates that the value is out of range of the present invention.
[0066]
[Table 1]

[0067]
　Using seven stands in the finish rolling, the most upstream side of the first stand of the ingress temperature, i.e. the temperature immediately before rolling and the most downstream side of the seventh stand delivery temperature, i.e. to measure the temperature immediately after rolling. Entry side temperature of the first stand corresponds to the finish rolling start temperature (HST), delivery temperature of the seventh stand corresponds to the finish rolling end temperature (HFT). These are shown in Table 2.
[0068]
　The hot-rolled steel sheet is cooled after finish rolling, it was wound up. In this cooling and winding, the first average cooling rate (CR1), Ar between the finish rolling end temperature (HFT) to (HFT-20 ° C.) 3 second average cooling rate between points to 700 ° C. the (CR2) and coiling temperature (CT) shown in Table 2.
[0069]
　After winding, to remove the scale by performing the pickling of hot-rolled steel sheet. Thereafter, by performing cold rolling at a reduction ratio of 45% to 70%, thickness was obtained cold-rolled steel sheet of 1.2 mm. This was followed by the annealing of cold-rolled steel sheet using a continuous annealing equipment. In the annealing, the fourth table the average cooling rate (CR4) of between the third average cooling rate (CR3) and 300 ° C. to 0.99 ° C. between the highest temperature (ST), from 700 ° C. to 500 ° C. 2 shows.
[0070]
　Was then reheated cooled steel sheet to a temperature of 0.99 ° C. or less. In this reheating, holding temperature (Tr) and retention time (tr) shown in Table 2. Then, refining was carried out rolling (skin pass rolling). Elongation rate (SP) shown in Table 2 in the temper rolling.
[0071]
　For some of the steel sheet, after continuous annealing, during or continuous annealing, performed galvanizing treatment or galvannealing treatment, the steel sheet of some other, was galvanized process after continuous annealing. It shows a steel species corresponding to the plating process shown in Table 2. Table "GI" in 2 represents a galvanized steel sheet galvanizing treatment is performed, "GA" represents an alloyed hot-dip galvanized steel sheet alloying galvanizing treatment is performed, "EG" refers to shows the electro-galvanized steel sheet galvanized processing is performed, "CR" represents a cold-rolled steel sheet plating is not performed. For example, sample No. 30, sample No. In 31, CR3 cooling of 30 ° C. / s, galvanizing treatment (GI) or galvannealed process (GA), CR4 has performed the cooling and reheating of 15 ° C. / s in this order.
[0072]
[Table 2]

[0073]
　Samples were produced of steel plate in this way. Underlined in Table 2 indicates that the numerical value is outside the appropriate range of production conditions. Then, it was observed steel microstructure of the sample. In the steel structure observation, the area fraction of ferrite (f F ), the area fraction of the first martensite (f MP ), area fraction of retained austenite (f A measured), the type of these other tissues It was specific. This observation, portions of the 1/4 thickness of the steel sheet was analyzed by the analysis or X-ray diffraction method according to point counting method or image analysis using an optical microscope photograph or a SEM photograph. Discrimination from optical micrographs and SEM photographs are difficult to tissue performs a specific crystal orientation by TEM observation and EBSD method was determined based on the description of the reference. Circle equivalent diameter of iron carbides was measured by SEM observation, the circle equivalent diameter of hard small iron carbides determine the SEM observation was measured by TEM observation.
[0074]
　Measurement of ND // <100> oriented grains and ND // <111> oriented grains total area fraction were also performed. In this measurement, in the section including the normal direction of the rolling direction (RD) and the plate plane (ND), the area between 1/2 position from the 1/4 position of the plate thickness 5000 .mu.m 2 EBSD method for more areas the analysis by went. Furthermore, the amount of solute C was measured by the internal friction method.
[0075]
　The results are shown in Table 3. Underlined in Table 3 indicates that the numerical value is outside the scope of the present invention. Table "B" in the column of "other tissue" 3 indicates bainite, "P" indicates pearlite, "M" denotes the second martensite.
[0076]
[table 3]

[0077]
　This was followed by a tensile test according to JIS Z 2241 for each sample. In this tensile test, using a tensile test specimen conforming to JIS Z 2201 to the plate width direction (direction perpendicular to the rolling direction) and longitudinal direction. Then, each sample was measured yield strength YS, tensile strength TS, yield point elongation YPE and uniform elongation UEL. In this tensile test, for each sample, tensile tensile specimen subjected to aging treatment predistortion at 170 ° C. After adding 5% for 20 minutes also prepared, the yield strength YS and tensile strength TS after aging after aging measured and calculated yield ratio YR after aging.
[0078]
　For each sample was measured aging index AI. In the measurement of aging index AI, by adding 10% tensile prestrain performs aging for 60 minutes at 100 ° C., it was measured yield strength Subsequent to tensile test. Leave measured yield strength by tensile testing prior to the aging was calculated increase from the yield strength before aging yield strength after aging.
[0079]
　The evaluation of cracking ease of each sample were carried out. 1 to 4 are views showing a method of evaluating crack ease. In this evaluation, it was first prepared lid 21 shown in hat part 11 and 2 shown in FIG. Longitudinal length of the hat part 11 and the lid 21 was 900 mm. Length in the width direction of the lid 21 was 100 mm. The height of the top of the hat part 11 is set to 50mm, it is a 50mm length in the width direction, and both the width direction of the lengths of the two flange portions 25 mm, the curvature radius of the curved portion was 5 mm. Hat type center diameter of the part 11 is formed 10mm hole 12, centered on the diameter of the cover 21 is formed a hole 22 for 10mm. Hole 12 and hole 22 is formed with 15% clearance in the punch punching. Hole 12 is formed before forming of the hat-shaped element 11. Then, as shown in FIG. 3, superimposed a flange portion and the lid 21 of the hat part 11, it was joined by spot welding to obtain a specimen 31. Thereafter, as shown in FIG. 4, the hole 12 is the upper surface, as hole 22 is positioned on the lower surface, placing the specimen 31 on the pedestal 41 free gap. Longitudinally of the gap size of the specimen 31 is 700 mm. Then, it dropped from a height of 3m the weight 42 of the cylindrical weight 500kg in central specimen 31, to confirm the presence or absence of cracks from cracking and hole 22 from the hole 12.
[0080]
　The results are shown in Table 4. Underlined in Table 4 shows that out of range of that number to the target.
[0081]
[Table 4]

[0082]
　As shown in Table 4, an invention example Sample No. 1, No. 3, No. 12, No. 14, No. 16, No. 17, No. 28 ~ No. 34 because it has provided the requirements of the present invention exhibited excellent properties.
[0083]
　Sample No. 2, No. 4, No. 5, No. 18 ~ No. In 20, ND // <111> for oriented grains and ND // <100> oriented grains of the total area fraction was over, the end surface cracking occurs due to the influence of impact. Sample No. 6, No. 7, No. 10, No. 21, No. 22, No. In 25, since solid solution C was too small, does not so much increase the yield strength by prescription, sufficient yield ratio can not be obtained after aging. Sample No. 8, an excessive ferrite area fraction of, ND // <111> for oriented grains and ND // <100> oriented grains of the total area fraction was excessive, sufficient yield ratio after aging is obtained , the end surface cracking occurs due to the influence of impact. Sample No. 9, No. 24, since the area fraction of ferrite was excessive, sufficient yield ratio obtained after aging, the end surface cracking occurs due to the influence of impact. Moreover, since solid solution C was too small, does not so much increase the yield strength by prescription, sufficient yield ratio can not be obtained after aging. Sample No. 11, No. 13, No. 26, No. 27, since the area fraction of the first martensite was too small, no sufficient yield ratio obtained after aging, the end surface cracking occurs due to the influence of impact. Sample No. 15, since the area fraction of the first martensite was excessively, sufficient yield ratio obtained after aging, the end surface cracking occurs due to the influence of impact.
[0084]
　Sample No. In 35, since the C content was too small, a sufficient tensile strength was not obtained, et al. Sample No. In 36, since the C content was excessive, excessive area fraction of retained austenite, the end surface cracking occurs due to the influence of impact. Sample No. In 37, since Si content was too small, the tensile strength can not be obtained a minute, also, is not significantly increased yield strength by age, sufficient yield ratio can not be obtained after aging. Sample No. In 38, since Si content was excessive, an excessive area fraction of ferrite and the area fraction of retained austenite sufficient yield ratio can not be obtained after aging. Sample No. In 39, since the Mn content was too small, an excessive ferrite area fraction of, sufficient yield ratio obtained after aging, the end surface cracking occurs due to the influence of impact. Sample No. In 40, since the Mn content was excessive, ND // <111> oriented grains and ND // <100> is excessive oriented grains of the total area fraction of the end face cracks caused by the influence of the impact. Sample No. In 41, since Al content was excessive, an excessive ferrite area fraction of a sufficient yield ratio can not be obtained after aging. Sample No. In 42, since the N content was excessive, the end surface cracking due to the influence of the impact occurs, the yield point elongation becomes excessive. Sample No. In 43, since the P content was excessive, the end surface cracking occurs due to the influence of impact. Sample No. In 44, since S content was excessive, the end surface cracking occurs due to the influence of impact. Sample No. In 45, since Ti content was excessive, the end surface cracking occurs due to the influence of impact. Sample No. In 46, since the Nb content was excessive, the end surface cracking occurs due to the influence of impact.
[0085]
　Focusing on the manufacturing method, the sample No. 2, sample No. In 19, since the start temperature and end temperature of finish rolling is low, ND // <111> oriented grains and ND // <100> oriented grains of the total area fraction becomes excessive. Sample No. 4, No. In 18, for termination temperature of finish rolling is low, ND // <111> oriented grains and ND // <100> oriented grains of the total area fraction becomes excessive. Sample No. 5, No. In 20, since the first average cooling rate was high, ND // <111> oriented grains and ND // <100> oriented grains of the total area fraction becomes excessive. Sample No. 6, No. In 21, since the second mean cooling rate is low, solid solution C becomes too small. Sample No. 7, No. In 22, since the coiling temperature was higher, the solid solution C becomes too small. Sample No. 8, No. In 23, since the peak temperature of annealing is low, the area fraction of ferrite becomes excessive, D // <111> oriented grains and ND // <100> oriented grains of the total area fraction becomes excessive. Sample No. 9, No. In 24, since the third average cooling rate is low, the area fraction of ferrite becomes excessive, solid solution C becomes too small. Sample No. 10, No. In 25, since the average cooling rate of the fourth was low, solid solution C becomes too small. Sample No. 11, No. In 26, for holding the temperature of the reheating it is low, the area fraction of the first martensite becomes too small. Sample No. 14, No. In 27, because the retention time of reheating is shorter, the area fraction of the first martensite becomes too small. Sample No. In 17, because they did not perform reheating, area fraction of the first martensite becomes excessive.
Industrial Applicability
[0086]
　The present invention is, for example, can be used in industry in which a suitable steel to the body of an automobile.

claims
[Requested item 1]
　By
　mass%,
　C: 0.05% ~
　0.40%, Si: 0.05% ~ 3.0%, Mn: 1.5%
　~ 3.5%, Al: 1.5% or
　less, N: 0.010% or
　less, P: 0.10% or
　less, S: 0.005% or
　less, Nb: 0.00% ~ 0.04% or
　less, Ti: 0.00% ~ 0.08% or less,
　V and ta:
　0.0% ~ 0.3% in total, Cr, Cu, Ni, Sn and Mo:
　0.0% ~ 1.0% in total, B: 0.000% ~
　0.005%, Ca:
　% ~ 0.005 0.000%,
　Ce: 0.000% ~ 0.005%, La: 0.000% ~ 0.005%, and
　balance: Fe and impurities,
have in a chemical composition represented ,
　in area%,
　the first martensite number of equivalent circle diameter of 2 nm ~ 500 nm of iron carbides in Las is 2 or more: 20% to 95%,
　ferrite 15% Lower,
　residual austenite 15% or less, and
　Remainder: equivalent circle diameter of bainite or Las number of iron carbides 2 nm ~ 500 nm is the second martensitic or both of less than two
have a steel structure represented
　by, ND // <111> orientation and the grain and ND // <100> oriented grains of the total area fraction of 40% or less,
　the amount of solid solution C is not less than 0.44Ppm,
　the ND // <111> oriented grains is legal plate surface crystal orientation parallel to the line direction is the crystal grains is a crystal orientation deviation of less than 10 ° from the <111> direction,
　the ND // <100> oriented grains is parallel to a normal direction of the plate surface steel crystal orientation, which is a crystal grain is a crystal orientation deviation of less than 10 ° from the <100> direction.
[Requested item 2]
　In the above chemical composition
　V and Ta: 0.01% ~ 0.3% in total,
　the steel sheet according to claim 1, characterized in that holds to.
[Requested item 3]
　In the chemical
　composition, Cr, Cu, Ni, Sn and Mo: 0.1% ~ 1.0% in total,
　steel sheet according to claim 1 or 2, characterized in that holds to.
[Requested item 4]
　In the chemical
　composition, B: 0.0003% ~ 0.005%,
　the steel sheet according to any one of claims 1 to 3, characterized in that hold.
[Requested item 5]
　In the chemical
　composition,
　Ca: 0.001% ~ 0.005%,
　Ce: 0.001% ~ 0.005%, La: 0.001% ~ 0.005%, or
　, or any combination thereof is established steel sheet according to any one of claims 1 to 4, characterized in that.