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 technique
[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, excellent formability, it is required to have high strength and excellent crashworthiness after baking. 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.
[0003]
　However, the DP steel and TRIP steel sheet, mechanical properties after baking is disadvantageously may vary within member. That is, in the molding of the steel sheet, because the strain is added in accordance with the shape of the member to be obtained, includes moieties which moieties and distortion distortion is added strongly to the steel sheet after molding is hardly added. Then, as part the added distortion is large, the amount of strain age hardening due to paint baking is increased, hardness is increased. As a result, between the parts most strain and strain is added portion by molding is not added, it may be a large difference in yield strength after baking. In this case, part distortion are hardly added is soft, to or occurred folded in this part, no sufficient reaction force characteristic and the collision characteristics.
CITATION
Patent Literature
[0004]
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: WO 2013/047821
Patent Document 7: JP 2008-144233 JP
Patent Document 8: WO 2012/070271
Summary of the Invention
Problems that the Invention is to Solve
[0005]
　The present invention, while obtaining good moldability, and to provide a steel sheet which can obtain a stable yield strength after baking.
Means for Solving the Problems
[0006]
　The present inventors have conducted extensive studies to solve the above problems. As a result, when the dislocation density in the dislocation density and bainite in the ferrite is high, even in the portion where strain in molding is hardly added, it has been found to increase the yield strength by aging due to baking. When the average particle size of the ferrite and bainite is small, the yield strength was also found that further improved by aging.
[0007]
　The present inventors, as a result of further intensive studies based on these findings, and conceived to aspects of the invention described below.
[0008]
　(1)
　in
　mass%,
　C: 0.05% ~
　0.40%, Si: 0.05% ~ 3.0%, Mn: 1.5%
　~ 4.0%, Al: 1.5% or less ,
　N: 0.02% or
　less, P: 0.2% or
　less, S: 0.01% or
　less, Nb and Ti: 0.005% ~ 0.2% in total,
　V and Ta: a total 0.0
　~ 0.3%%, Cr, Mo, Ni, Cu and Sn:
　0.0% ~ 1.0% in total, B:
　0.00% ~ 0.01%, Ca: 0.000% ~ 0.
　% 005,
　Ce: 0.000% ~ 0.005%, La: 0.000% ~ 0.005%, and
　balance: Fe and impurities,
have in a chemical composition represented,
　the ferrite and bainite in total has a steel structure that contains the area fraction of more than 2%,
　× 3 any average average dislocation density of the dislocation density and the bainite in the ferrite 10 12 m / m 3 1 × 10 ~ 14 m / m 3 a,
　steel sheet, wherein the average particle size of the ferrite and bainite is 5μm or less.
[0009]
　(2)
　the steel structure is an area fraction, the ferrite and bainite: 2% to 60% in total, and martensite: contains 10% to 90%
　area fraction of retained austenite in the steel structure is 15% hereinafter, and the
　steel sheet according to (1), wherein a ratio of the area fraction of ferrite to the area fraction of martensite is 0.03 to 1.00.
[0010]
　(3)
　In the above chemical composition,
　V and Ta: 0.01% ~ 0.3% in total,
　the steel sheet having the constitution that holds (1) or (2).
[0011]
　(4)
　In the above chemical
　composition, Cr, Mo, Ni, Cu and Sn: 0.1% ~ 1.0% in total,
　according to any of characterized in that the holds (1) - (3) steel plate.
[0012]
　(5)
　In the above chemical
　composition, B: 0.0003% - 0.01%,
　the steel sheet according to any one of wherein the holds (1) to (4).
[0013]
　(6)
　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, since the average dislocation density, etc. of the average dislocation density and the bainite in ferrite it is appropriate, it is possible to obtain a stable yield strength even after baking.
DESCRIPTION OF THE INVENTION
[0015]
　Hereinafter, embodiments of the present invention will be described.
[0016]
　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, is produced through the annealing 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% ~ 4.0%, Al: 1.5% or less, N: 0.02% or less, P: 0.2% or less, S: 0.01% or less, Nb and Ti: 0.005% ~ 0.2% in total, V and Ta: 0.0% to 0.3% in total, Cr, Mo, Ni, Cu and Sn: 0.0% ~ 1.0% in total, B: 0.00% ~ 0.01%, Ca: 0. 000% ~ 0.005%, Ce: 0.000% ~ 0.005%, La: 0.000% ~ 0.005%, and the balance has a chemical composition represented by Fe and impurities. As the impurity, those included in raw materials such as ores and scrap, intended to be included in the manufacturing process, is exemplified.
[0017]
　(C: 0.05% ~ 0.40%)
　C contributes to the improvement of the tensile strength. The C content is less than 0.05%, no sufficient tensile strength, for example, 980MPa or more tensile strength. Therefore, C content is 0.05% or more. In order to obtain higher tensile strength, C content is preferably 0.08% or more. On the other hand, the C content of 0.40 percent, can not be obtained rearrangement of sufficient density in the ferrite, also difficult to obtain a preferable steel structure. Therefore, C content is 0.40% or less. From the standpoint of weldability, C content is preferably 0.35% or less.
[0018]
　(Si: 0.05% ~
　3.0%) Si affects the formation of iron carbide and age hardening associated therewith. The Si content is less than 0.05%, not provide a sufficient amount of solute C, no yield strength sufficiently increased even by aging due to baking. Therefore, Si content is 0.05% or more. To increase the yield strength further, Si content is preferably 0.10% or more. On the other hand, the Si content of 3.0 percent, can not be obtained rearrangement of sufficient density in the ferrite, also difficult to obtain a preferable steel structure. 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.
[0019]
　(Mn: 1.5 Pasento ～ 4.0
　Pasento) Mn is, to suppress the transformation to ferrite from austenite, contributes to the improvement of the tensile strength. The Mn content is less than 1.5%, no sufficient tensile strength, for example, 980MPa or more tensile strength. Therefore, Mn content is 1.5% or more. In order to obtain higher tensile strength, Mn content is preferably 2.0% or more. On the other hand, Mn content is 4.0 percent, sufficient formability can not be obtained. Therefore, Mn content is 4.0% or less. In order to obtain more excellent formability, Mn content is preferably not more than 3.5%.
[0020]
　(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 is not obtained ferrite or bainite having an average dislocation density in the range described below. 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.
[0021]
　(N: 0.02% or less)
　N is not an essential element, is contained as an impurity, for example, in the steel. The N content of 0.02 percent is not obtained sufficient moldability nitride large amount deposited. Therefore, N content is 0.02% 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.
[0022]
　(P: 0.2% or less)
　P is not an essential element, is contained as an impurity, for example, in the steel. The P content of 0.2 percent, not obtained sufficient formability P compound a large amount of precipitation. Accordingly, P content is 0.2% or less. From the standpoint of weldability, P content is preferably 0.07% 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.
[0023]
　(S: 0.01% or less)
　S is not an essential element, is contained as an impurity, for example, in the steel. The S content of 0.01 percent, sufficient formability can be obtained by a large amount of precipitated sulfides. Thus, S content is 0.01% or less. To further suppress a decrease in moldability, 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.
[0024]
　(Nb and Ti: 0.005% ~ 0.2% in
　total) Nb and Ti contributes to refinement and precipitation strengthening of the grain of the ferrite or bainite. Nb and Ti (Ti, Nb) to form a carbonitride, depending on the content of Nb and Ti, solid solution C amount and the solute N amount after annealing is changed. Is less than 0.005% content of Nb and Ti in total, not obtained ferrite or bainite having an average particle size in the range described below, no yield strength sufficiently increased even by aging due to baking. Therefore, the content of Nb and Ti is 0.005% or more in total. In order to increase sufficiently the yield strength by age, the content of Nb and Ti is preferably 0.010% or more in total. On the other hand, 0.2 percent content of Nb and Ti in total, (Ti, Nb) is not obtained sufficient formability carbonitride and a large amount of precipitation. Therefore, the content of Nb and Ti is 0.2% or less in total. The content of Nb and Ti is preferably 0.1% or less in total.
[0025]
　V, Ta, Cr, Mo, Ni, Cu, Sn, B, Ca, Ce and La is not an essential element, a steel plate and may optionally elements be appropriately contained in the limits of the predetermined amount to the steel.
[0026]
　(V and Ta: 0.0% ~ 0.3% in total)
　V and Ta, carbides, the grain refinement of formation and ferrite and bainite nitride or carbonitride contributes to the improvement of strength. Therefore, V or Ta, or both of these may be contained. However, the content of V and Ta in 0.3 percent in total, precipitated a large amount of carbonitride, ductility is lowered. 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.
[0027]
　(Cr, Mo, Ni, Cu and Sn:
　total% ~ 1.0 0.0%) Therefore, Cr, Mo, Ni, Cu or Sn or any combination thereof may be contained. However, Cr, Mo, Ni, 1.0 percent at a content of Cu and Sn is total, processability is remarkably deteriorated, elongation is reduced. Accordingly, the content of Cr, Mo, Ni, Cu and Sn is not more than 1.0% in total. From the standpoint of manufacturability, the content of Cr, Mo, Ni, Cu and Sn are preferably less than 0.5% in total. In order to ensure the effect of the action, the content of Cr, Mo, Ni, Cu and Sn is preferably 0.1% or more.
[0028]
　(B: 0.00% ~ 0.01%)
　B enhances hardenability of the steel sheet, suppresses the formation of ferrite, prompting the formation of martensite. Thus, B may be contained. However, in 0.01 percent content of B in total is not obtained sufficient formability boride is a large amount of precipitation. Therefore, B content is 0.01% or less. To further suppress a decrease in ductility, 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.
[0029]
　(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, processability, particularly inhibit a decrease in elongation. 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.
[0030]
　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. The steel structure of the steel sheet according to an embodiment of the present invention ferrite and bainite is contained in more than 2% of the area fraction in total. The average dislocation density of the average dislocation density and the bainite in ferrite Both × 10 3 12 m / m 3 ~ 1 × 10 14 m / m 3 is, the average particle size of the ferrite and bainite is 5μm or less.
[0031]
　As described above, the present inventors, when the dislocation density in the dislocation density and the bainite in the ferrite is high, even in places where strain at the time of molding is hardly added, the yield strength is improved by aging due to baking it has become clear that the. The average dislocation density or both of these average dislocation density or the bainite in ferrite 3 × 10 12 m / m 3 is less than the yield strength of the part distortion during molding is hardly added is not sufficiently improved by prescription , not enough collision characteristics. Therefore, the average dislocation density of the average dislocation density and the bainite in ferrite 3 × 10 none 12 m / m 3 at least. In order to obtain more excellent crashworthiness, average dislocation density of the average dislocation density and the bainite in ferrite are both preferably × 10 6 12 m / m 3 at least. The average dislocation density or both of these average dislocation density or the bainite in the ferrite 1 × 10 14 m / m 3 in greater than lowered moldability, yield strength of the part distortion during molding is hardly added not sufficiently improved by prescription, sufficient collisions characteristics may not be obtained. Therefore, the average dislocation density of the average dislocation density and the bainite in the ferrite 1 × 10 none 14 m / m 3 or less. In order to obtain more excellent formability and crashworthiness, the average dislocation density of the average dislocation density and the bainite in ferrite are both preferably × 10 8 13 m / m 3 or less.
[0032]
　The average dislocation density of the average dislocation density and the bainite in the ferrite, for example, transmission electron microscopy (transmission electron microscopy: TEM) can be obtained using photographs. That is, to prepare a TEM photograph of the thin film sample, optionally draw a line on the TEM photograph, in order to obtain an average dislocation density in the ferrite counts the occurrences of this line intersects the dislocation lines in the ferrite . Then, the length of the lines in the ferrite L, and the number of locations where the line in the ferrite and the dislocation line intersects N, when the thickness of the sample and t, the dislocation density in the ferrite in the thin film sample "2N / represented by (Lt) ". Using a TEM photograph taken at a plurality of positions of the thin film sample, the mean value of the dislocation density obtained from the plurality of TEM photographs are obtained as an average dislocation density in the ferrite. The thickness t of the sample, may be used the measured value, may be used simply to 0.1 [mu] m. The average dislocation density in bainite, the portion that intersects counted in bainite, by using the length of the line in the bainite can be obtained in a similar manner to obtain the average dislocation density in the ferrite.
[0033]
　As described above, the present inventors, when the particle size of the ferrite and bainite is small, the yield strength was found to be further improved by aging. In 5μm exceeds an average particle size of the ferrite and bainite, the yield strength of the part distortion during molding is hardly added is not sufficiently improved by aging, insufficient collision characteristics. Therefore, the average grain size of ferrite and bainite is 5μm or more. In order to obtain more excellent crashworthiness, the average particle size of the ferrite and bainite is preferably 3μm or less.
[0034]
　Both the average dislocation density of the average dislocation density and the bainite in ferrite 3 × 10 12 m / m 3 ~ 1 × 10 14 m / m 3 is, and the average particle size of the ferrite and bainite is not more 5μm or less also, it is less than 2% area fraction of ferrite and bainite is total, or not sufficient formability can not be obtained, sufficient collisions performance may not be obtained. Therefore, the area fraction of ferrite and bainite is not less than 2% in total. To obtain better moldability and crashworthiness, area fraction of ferrite and bainite is preferably 5% or more in total.
[0035]
　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.
[0036]
　Ferrite and bainite also contributes to the improvement of the formability of the steel sheet. However, in more than 60% area fraction of ferrite and bainite in total, may be sufficient collision characteristics can not be obtained. Therefore, the area fraction of ferrite and bainite is preferably 60% or less in total. In order to obtain more excellent crashworthiness, area fraction of ferrite and bainite is more preferably 40% or less in total.
[0037]
　Martensite contribute to ensuring the tensile strength. The area fraction of martensite is less than 10%, sufficient tensile strength, for example, may not tensile strength is obtained than 980 MPa, the average dislocation density in the ferrite is 3 × 10 12 m / m 3 to or is less than there is. Therefore, the area fraction of martensite is preferably 10% or more. In order to obtain more excellent tensile strength and crashworthiness, area fraction of martensite and more preferably not less than 15%. On the other hand, the area fraction of martensite is 90%, the average dislocation density or both of these average dislocation density or the bainite in the ferrite 1 × 10 14 m / m 3 or a greater, sufficient ductility to obtain you may or may not be. Therefore, the area fraction of martensite is preferably 90% or less. To obtain a better crashworthiness and ductility area fraction of martensite and more preferably not more than 85%. The martensite contains martensite and tempered martensite as quenched, it is preferable 80 or more area% of the total of martensite is tempered martensite.
[0038]
　Area fraction f of martensite M area fraction f of ferritic against F ratio of (f F / f M in) is less than 0.03, the average dislocation density in ferrite × 10 1 14 m / m 3 becomes greater or, it is possible that a sufficient ductility can not be obtained. Therefore, the ratio (f F / f M ) is preferably 0.03 or more. To obtain a better crashworthiness and ductility ratio (f F / f M ) is more preferably 0.05 or more. On the other hand, the ratio (f F / f M In) is 1.00 greater than the average dislocation density in the ferrite × 10 3 12 m / m 3 may be less than. Therefore, the ratio (f F / f M ) is preferably 1.00 or less. In order to obtain more excellent crashworthiness, the ratio (f F / f M ) is more preferably 0.80 or less.
[0039]
　Retained austenite is effective for improving the enhancement and impact energy absorption characteristics of moldability. Residual austenite, contributes to the improvement of the strain aging hardening amount at baking. However, in 15 percent area fraction of retained austenite has an average dislocation density in ferrite × 10 1 14 m / m 3 may be or become greater than steel plate after molding or embrittlement. Therefore, the area fraction of retained austenite is preferably 15% or less. In order to obtain more excellent crashworthiness and toughness, area fraction of retained austenite is more preferably 12% or less. When the area fraction of retained austenite is 2% or more can be expected the effect of improving the strain aging hardening amount.
[0040]
　Ferrite, bainite, pearlite may be mentioned as examples of those contained in the steel structure other than martensite and retained austenite. Area fraction of perlite is preferably 2% 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 of the reference.

claims.1
　By
　mass%,
　C: 0.05% ~
　0.40%, Si: 0.05% ~ 3.0%, Mn: 1.5%
　~ 4.0%, Al: 1.5% or
　less, N: 0.02% or
　less, P: 0.2% or
　less, S: 0.01% or
　less, Nb and Ti: 0.005% ~ 0.2% in total,
　V and Ta: 0.0% in total 0
　% .3, Cr, Mo, Ni, Cu and Sn: 　0.0%
　~ 1.0% in
total, B: 0.00% ~ 0.01%, Ca: 0.000% ~ 0.005%,
　ce:
　0.000% ~ 0.005%, La: 0.000% ~ 0.005%, and
　balance: Fe and impurities,
in having a chemical composition represented,
　more than 2% of ferrite and bainite in total It has a steel structure that contains the area fraction of
　the average dislocation density and the average dislocation density 3 × any 10 in bainite in ferrite 12 m / m 3 ~ 1 × 10 14 m / m 3 a,
　steel sheet, wherein the average particle size of the ferrite and bainite is 5μm or less.
[Claim 2]
　The steel structure is an area fraction, the ferrite and bainite: 2% to 60% in total, and martensite: contains 10% to 90%
　area fraction of retained austenite in the steel structure is located at 15% or less ,
　steel sheet according to claim 1, the ratio of the area fraction of ferrite to the area fraction of martensite is characterized in that it is a 0.03 to 1.00.
[Claim 3]
　In the chemical composition,
　V and Ta: 0.01% ~ 0.3% in total,
　steel sheet according to claim 1 or 2, characterized in that holds to.
[Claim 4]
　In the chemical
　composition, Cr, Mo, Ni, Cu and Sn: 0.1% ~ 1.0% in total,
　the steel sheet according to any one of claims 1 to 3, characterized in that hold.
[Claim 5]
　In the chemical
　composition, B: 0.0003% ~ 0.01%,
　the steel sheet according to any one of claims 1 to 4, characterized in that holds to.
[Claim 6]
　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 5, characterized in that.