[0001]The present invention relates to a steel sheet and a manufacturing method thereof.
Background technique
[0002]Automotive parts, tool, other machine parts, punching, bending, are manufactured through a machining process such as press working. In that process step, product quality and or stabilizing, in the reduced manufacturing cost, it is necessary to improve the processability of the carbon steel which is a material. In particular, when processing the driving system parts, carbon steel sheet is deformed by high speed rotation, etc., and because it may be broken by ductility insufficient, it is necessary ductility after the heat treatment.
[0003]
　Generally, the carbon steel plate, cold rolling and spheroidizing annealing is applied, as a good workability soft material consisting of ferrite and spheroidized carbides are used carbon steel. Then, heretofore, a technique for improving the processability of the carbon steel sheet have been proposed.
[0004]
　For example, Patent Document 1, C: 0.15 ~ 0.90 wt%, Si: 0.40 wt% or less, Mn: 0.3 ~ 1.0 mass%, P: 0.03 wt% or less, All Al: 0.10 wt% or less, Ti: 0.01 ~ 0.05 wt%, B: 0.0005 ~ 0.0050 wt%, N: 0.01 wt% or less, Cr: 1.2 wt% include the following, has an average carbide grain diameter of 0.4 to spheroidization ratio of 80% or more carbides 1.0μm are dispersed in a ferrite matrix structure, a high carbon steel sheet for fine blanking of the notch tensile elongation 20% or more its preparation is disclosed.
[0005]
　Patent Document 2, C: 0.3 ~ 1.3 mass%, Si: 1.0 wt% or less, Mn: 0.2 ~ 1.5 mass%, P: 0.02 wt% or less, S: containing 0.02 wt% or less, carbides C on the ferrite grain boundary GB carbides number C in the ferrite grain IG C between GB / C IG dispersed carbides such relationship ≦ 0.8 is satisfied has allowed organized, high carbon steel sheet and a manufacturing method, in excellent workability, characterized discloses that cross hardness is less than 160HV.
[0006]
　Patent Document 3, C: 0.30 ~ 1.00 wt%, Si: 1.0 wt% or less, Mn: 0.2 ~ 1.5 mass%, P: 0.02 wt% or less, S: includes 0.02 wt% or less, carbides C on the ferrite grain boundary GB carbides number C in the ferrite grain IG during, C GB / C IG with relation holds a ≦ 0.8, all carbides of 90% or more, there is disclosed a high carbon steel sheet, in which excellent workability characterized by having a structure in which carbides occupied by major axis / minor axis of 2 or less spheroidized carbides are dispersed in ferrite .
[0007]
　In these prior art, it is assumed to be a good enough processability often the proportion of carbides in the ferrite grains.
[0008]
　Patent Document 4, C: 0.1 ~ 0.5 mass%, Si: 0.5 wt% or less, Mn: 0.2 ~ 1.5 mass%, P: 0.03 wt% or less, S: a composition consisting of 0.02 wt% or less, has a structure mainly of ferrite and carbides, S gb = {S on / (S on + S in at)} × 100 (wherein, S on : per unit area of the carbides present, the total area occupied by the carbides present on the grain boundary, S in : of the carbides present per unit area, the ferrite grain boundary carbides, which is defined by the total occupied area) of carbide present in the grain the amount S gb FB workability is characterized in that 40% or more, mold life, and are disclosed steel sheet excellent in formability after FB working.
[0009]
　The technique described in Patent Document 5, by applying a proper hot rolled sheet annealing to the hot rolled sheet having a substantially 100% pearlite structure, thereby promoting spheroidization of carbides, and suppress grain growth of the ferrite, many carbides are characterized by the presence on the ferrite grain boundary.
[0010]
　The technique described in Patent Document 6, ferrite mainly composed of the second phase, suppressing the martensite fraction, as organizational structure consisting mainly of a carbide of cementite or the like, a solid solution of ferrite by actively utilizing the Si ensuring the strength by reinforcement, it is characterized by securing the ductility due to work hardening properties improve ferrite itself.
[0011]
　Patent Document 7, by controlling the ferrite grain size above 10 [mu] m, discloses a technique for producing an excellent soft in the carbon steel plate induction hardening properties. Manufacturing method disclosed in Patent Document 7, 600 ° C. ~ by coarsening the ferrite grains of the steel sheet by box annealing heating to 750 ° C., it is characterized in that achieve softening of the steel sheet.
[0012]
　Disclosed steel sheet in Patent Document 8, 10-50% of the C content is graphitized, the steel structure of the cross section, C wt% of graphite particles of size 3 [mu] m × 10 2 cells / mm 2 or more C wt% 10 × 3 cells / mm 2 containing less, is characterized by a dispersed ferrite phase spheroidized cementite. Manufacturing method disclosed in Patent Document 8, from the viewpoint of graphitization of the steel sheet, a hot-rolled sheet is characterized in that annealing in the range of 600 ℃ ~ 720 ℃.
[0013]
　Steel sheet disclosed in Patent Document 9 comprises 90% or more of bainite phase at an area ratio, of the total Fe-based carbides are precipitated in the bainite phase, it is precipitated in bainitic ferrite grains in Fe system the number of carbides ratio of 30% or more, average particle diameter of the Fe-based carbides are precipitated in the bainitic ferrite grains are characterized by having a tissue is 150nm or less.
[0014]
　Disclosed steel in Patent Document 10, in the region of a steel plate surface to the thickness direction 200 [mu] m, the integration degree of crystal orientations that will fit in parallelism within ± 5 ° with respect to (110) plane is the surface of the steel sheet 2.5 it is characterized in that at least.
CITATION
Patent Literature
[0015]
Patent Document 1: Patent No. 4465057 Patent Publication
Patent Document 2: Japanese Patent No. 4974285
Patent Document 3: Japanese Patent No. 5197076 discloses
Patent Document 4: Japanese Patent No. 5194454 discloses
Patent Document 5: JP 2007-270330 Publication
Patent Document 6 : JP 2012-36497 JP
Patent Document 7: JP 2012-62496 JP
Patent Document 8: JP-A 8-120405 JP
Patent Document 9: JP 2015-160986 JP
Patent Document 10: JP 2015-117406 No.
Summary of the Invention
Problems that the Invention is to Solve
[0016]
　In the technique described in Patent Document 1, aimed coarsening of ferrite grain size and carbide, A for softening C1 has performed an annealing at a temperature of above points, A C1 was annealed at points or more temperatures If, during the annealing, rod-plate-shaped carbides are precipitated. This carbide is because it is said that to lower the workability, even if it is possible to lower the hardness, adversely affect the processability.
[0017]
　Patent Documents 2 and 3, both, it is described that it spheroidization ratio of carbide precipitated at grain boundaries (. Referred to as "grain boundary carbides") is low is the cause to deteriorate the workability. However, the technology described in Patent Documents 2 and 3, neither the object of improving the workability by improving spheroidization ratio of grain boundary carbides. In the technique described in Patent Document 4, only tissue factor is defined, relation between processability and mechanical properties has not been studied.
[0018]
　Patent Document 5-9, the conditions of the annealing step from the viewpoint of promoting deposition of carbides into the ferrite grain boundary is not specified. Further, since the cooling conditions after the annealing process in the Patent Document 5-9 are not identified, in the manufacturing method disclosed in Patent Document 5 to 9, the hardness of the steel sheet austenite formed after annealing is transformed into pearlite is is increased, cold formability may deteriorate.
[0019]
　Patent Document 10, after winding the steel sheet after finish rolling at a coiling temperature of less than 650 ° C. 400 ° C. or higher, 680 first and annealing of 720 ° C. inclusive ° C., the second time at 790 ° C. or less 730 ° C. or higher perform annealing, after the second annealing, from the viewpoint of spheroidization ratio of the cementite, it discloses the annealed steel sheet at a cooling rate of 20 ° C. / hr. However, in the manufacturing method of Patent Document 10, the finish rolling a 600 ° C. or higher, since terminate below Ae3-20 ° C., so that rolling the steel plate in two-phase region of ferrite and austenite. Therefore, the ferrite phase and the pearlite phase are produced after rolling, the state of dispersion of carbides in the steel sheet after rolling becomes non-uniform, there is a possibility that the hardness is increased.
[0020]
　The present invention, light of the prior art, in the steel sheet, and an object thereof is to improve the cold formability and heat treatment after ductility, and to provide a steel sheet and a manufacturing method thereof to solve the problem.
[0021]
　Here, cold formability of the can, a steel plate, upon which is plastically deformed into a desired shape by cold working and cold forging, the deformability of the steel sheet which can be easily plastically deformed to the required shape without defects means. Further, the heat treatment after the ductility is the ductility of the steel sheet after heat treatment.
Means for Solving the Problems
[0022]
　Wherein the foregoing problems, in order to obtain a steel sheet suitable for materials such as the drive system components, in a steel sheet containing the necessary C to increase the hardenability, increase the grain size of the ferrite, carbide (mainly cementite ) as a suitable particle size, it can be understood that there may be fewer pearlite structure. This is due to the following reasons.
[0023]
　Ferrite phase has a low hardness, high ductility. Accordingly, ferrite mainly as tissue was, by increasing the grain size, it is possible to improve the material formability.
[0024]
　Carbides, by properly dispersed in the metallic structure, while maintaining the material formability, since it is possible to impart excellent wear resistance and rolling fatigue characteristics, must be in the drive system components tissue it is. Also, the carbide in the steel sheet is a strong particles that prevent slipping, the presence of the carbides in the ferrite grain boundaries, to prevent the propagation of slip exceeding a crystal grain boundary, to suppress the formation of shear bands can, to improve the cold forgeability, at the same time, the moldability of the steel sheet improved.
[0025]
　However, cementite is hard and brittle structure, when present in pearlite state a layered structure of ferrite, steel hard, so becomes brittle, should be present in a spherical shape. And cold forgeability, considering the occurrence of cracks during forging, the particle size should be appropriate range.
[0026]
　However, the manufacturing method for realizing the tissue has not been previously disclosed. Accordingly, the present inventors have conducted extensive studies on manufacturing method for realizing the above tissues.
[0027]
　As a result, since the cementite metal structure of the steel sheet after coiling after hot rolling in small fine pearlite or fine ferrite having lamella spacing is the dispersed bainite, a relatively low temperature (400 ℃ ~ 550 ℃) in the take-take. By winding at a relatively low temperature, cementite dispersed in ferrite also easily spheroidized. Subsequently, partially spheroidized cementite with annealing at temperatures just below Ac1 point as annealing in the first stage. Then, in annealing at a temperature between Ac1 point and Ac3 points as annealing in the second stage (two-phase region of the so-called ferrite and austenite), while leaving a portion of the ferrite grains, thereby austenite transformation part. While growing the subsequent slow cooling to leave ferrite grains by ferritic transformation to austenite and there to the nucleus, grain boundary precipitation of cementite while obtaining a large ferrite phase, was found to be able to realize the tissue.
[0028]
　That is, the manufacturing method of the steel sheet to satisfy the hardenability formability at the same time, such as hot rolling conditions and annealing conditions are also difficult to implement to devise in a single, in the so-called integrated process, such as hot rolling, annealing step It was found that can be realized by achieving an optimization.
[0029]
　Thus, the present inventors have optimized in conjunction with the manufacturing conditions in the integrated process leading to annealing the dispersed state and hot-rolled carbide in cold working before the steel sheet structure of optimized steel plate component compositions by controlling the steel sheet structure by an appropriate particle size of the carbide it was found that Dekiru precipitated in ferrite grain boundaries.
[0030]
　Further, the present inventors, the ferrite grain size not less than 5 [mu] m, if the Vickers hardness 170 or less, in the steel sheet was found to be secured to a heat treatment after ductility and excellent cold formability.
[0031]
　The present invention has been made based on the above findings and has as its gist is as follows.
[0032]
　(1) component composition, by
mass%, C:
0.10
~ 0.40%, Si: 0.30 ~ 1.00%, Mn: 0.30
~ 1.00%, Al: 0.001 ~
0.10%, P: 0.02% or
less, S: 0.01% or less
contain, in steel sheet balance being Fe and impurities,
　the ferrite grain boundaries to the number of carbide in ferrite grains (a) carbide beyond the ratio (B / a) is the number of ones (B),
　the ferrite grain size is not less 5μm or 50μm or less,
　an average particle diameter of the carbide is at 0.4μm or 2.0μm or less,
　pearlite area ratio is 6 % or less, and the
　steel sheet Vickers hardness is equal to or less than 170HV than 120 HV.
[0033]
　(2) the steel sheet further contains, by
mass%, N: 0.01% or
less, O: 0.02% or less
according to (1), characterized in that it contains one or more of steel plate.
[0034]
　(3) the steel sheet further contains, by
mass%, Ti: 0.10% or
less, Cr: 0.50% or
less, Mo: 0.50% or
less, B: 0.01% or
less, Nb: 0.10 % or
less, V: 0.10% or
less, Cu: 0.10% or
less, W: 0.10% or
less, Ta: 0.10% or less,
Ni: 0.10% or
less, Sn: 0.05% or less ,
Sb: 0.05% or
less, As: 0.05% or
less, Mg: 0.05% or
less, Ca: 0.05% or
less, Y: 0.05% or
less, Zr: 0.05% or
less, La 0.05% or
less, Ce: 0.05% or less
steel sheet according to (1) or (2), characterized in that it contains one or more.
[0035]
　(4) In the method for manufacturing the steel sheet according to any one of (1) to (3),
the billet component composition according to any one of (i) the (1) to (3) directly or, once heated after cooling were subjected to hot rolling, it takes plated rolled steel sheet completing the finish rolling at 800 ° C. or higher 900 ° C. below the temperature range at 400 ° C. or higher 550 ° C. or less,
(ii) winding dispensing a hot-rolled steel sheet taken, it was subjected to pickling, annealed in the first stage for holding in a temperature range of 650 ° C. or higher 720 ° C. or less for 3 hours or more 60 hours or less, further, 790 ° C. below 725 ° C. or higher subjected to annealing in the second stage for holding in a temperature range 3 hours to 50 hours or less, annealed for 2-step type,
(iii) a hot-rolled steel sheet after the annealing, 30 ° C. / time or less 1 ° C. / time or more cooled to 650 ° C. at a cooling rate controlled to, then cooled to room temperature
it Method for manufacturing a steel sheet characterized.
[0036]
　(5) The method of producing the steel sheet according to (4), wherein the temperature of the steel strip to be subjected to the hot rolling is 1000 ~ 1250 ° C..
Effect of the invention
[0037]
　According to the present invention, it is possible to provide a method of manufacturing a steel sheet excellent in ductility after the heat treatment and cold formability. The present invention steel has a high ductility after the heat treatment, is excellent in plate formability before heat treatment, fatigue parts repeated stress is applied, can be suitably used in, for example, an automobile underbody structure parts.
DESCRIPTION OF THE INVENTION
[0038]
　First, a description will be given reasons for limiting the chemical composition of the steel sheet of the present invention. Hereinafter,% means mass%.
[0039]
[C: 0.10 ~ 0.40%]
　C is a carbide is formed and is an element effective for refining the strengthening and ferrite grains of the steel. During cold forming, to prevent the satin occurs on the surface of the steel sheet, in order to ensure the surface appearance of the cold molded article, it is necessary to suppress the coarsening of the ferrite grains. If it is less than 0.10%, insufficient volume fraction of carbides during annealing, can not suppress the coarsening of the ferrite grains, C is 0.10% or more. Preferably at 0.14% or more. On the other hand, if C exceeds 0.40%, the increase in the volume percentage of carbide, the cold formability and heat treatment after ductility is lowered, C is set to 0.40% or less. Preferably not more than 0.38%.
[0040]
[Si: 0.30 ~
　1.00%] Si affects the form of carbides, an element which contributes to the improvement of ductility after the heat treatment. Reducing the number of carbide in ferrite grains, in order to increase the number of the carbide of the ferrite grain boundaries, the two-step type annealing (hereinafter sometimes referred to as "two-stage annealing."), Austenite during annealing to produce a phase, once, after dissolving the carbides, slowly cooled, it is necessary to promote the precipitation of carbides in the ferrite grain boundaries.
[0041]
　When Si is less than 0.30%, the the effect is not sufficiently obtained by the addition, Si is 0.30% or more. Preferably 0.35% or more. On the other hand, when it exceeds 1.00%, the increase in hardness by solid solution strengthening of the ferrite reduces the cold formability, except that cracks are likely to occur, A 3 -point rises and the quenching temperature since it is necessary to increase, Si is not more than 1.00%. Preferably not more than 0.90%.
[0042]
[Mn: 0.30 ~
　1.00%] Mn, in the two-stage annealing, is an element for controlling the form of carbides. If it is less than 0.30%, the 2-stage slow cooling after annealing, the ferrite grain boundaries, since it is difficult to produce a carbide, Mn is set to 0.30% or more. Preferably 0.33% or more. On the other hand, when it exceeds 1.00%, the hardness of the ferrite is increased, since the cold formability is lowered, Mn is not more than 1.00%. Preferably not more than 0.96%.
[0043]
[Al: 0.001 ~
　0.10%] Al is adapted to act as a deoxidizing agent, is an element for stabilizing ferrite. If it is less than 0.001%, wherein the effect is not sufficiently obtained by the addition, Al is 0.001% or more. Preferably 0.004% or more. On the other hand, when it exceeds 0.10%, the number of carbides of the ferrite grain boundaries is reduced, since the cold formability is lowered, Al is not more than 0.10%. Preferably not more than 0.09%.
[0044]
[P: 0.02% or less]
　P segregates in ferrite grain boundaries is an element which forms an action to suppress the formation of carbides in the ferrite grain boundaries. Therefore, the P content is preferably as small as possible, and it may be 0% when reduced to less than 0.0001%, the refining costs increase significantly, it may be 0.0001% or more. The content of P may be not less than 0.0013%. On the other hand, if P exceeds 0.02%, generation of carbides at the ferrite grain boundaries is suppressed, the number of carbides is reduced, since the cold formability is lowered, P is 0.02% or less. Preferably not more than 0.01%.
[0045]
[S: 0.01% or less]
　S is an element which forms a non-metallic inclusion such as MnS. Non-metallic inclusions, since the starting point of cracks at the time of cold forming, S is preferably as small, but may be 0% when reduced to less than 0.0001%, the refining costs increase significantly, 0. it may be 0001% or more. The content of S may be more than 0.0012%. On the other hand, when it exceeds 0.01%, nonmetallic inclusions are generated, since cold formability is lowered, S is set to 0.01% or less. Preferably not more than 0.009%.
[0046]
　The present invention steel sheet, in addition to the above elements, may contain the following elements.
[0047]
[N: 0.01% or less]
　N, when large quantities are present, is an element which embrittle the ferrite. Therefore, N is preferably as small as possible, and the content of N may be 0, but when reduced to less than 0.0001%, the refining costs increase significantly, may be 0.0001% or more. The content of N may be more than 0.0006%. On the other hand, when it exceeds 0.01%, ferrite is brittle, since the cold formability is lowered, N is the set to 0.01% or less. Preferably 0.007% or less.
[0048]
[O: 0.02% or less]
　O, when large quantities are present, is an element which forms a coarse oxide. Therefore, O is preferably as small as possible, and it may be 0% when reduced to less than 0.0001%, the refining costs increase significantly, it may be 0.0001% or more. The content of O can be set to 0.0011% or more. On the other hand, when it exceeds 0.02%, coarse oxides in the steel is produced, since the starting point of cracks at the time of cold forming, O is 0.02% or less. Preferably not more than 0.01%.
[0049]
　In the steel plate of the present invention, in addition to the above elements, further, the following elements may also contain one or more. Note that the following elements, since it is not essential for obtaining the effects of the present invention, the content may be 0%.
[0050]
[Ti: 0.10% or
　less] Ti forms a nitride, an element which contributes to grain refinement. If it is less than 0.001%, the effect is not sufficiently obtained by the addition, Ti is preferably 0.001% or more. More preferably 0.005% or more. On the other hand, when it exceeds 0.10%, to produce coarse Ti nitrides, the cold formability is lowered, Ti is not more than 0.10%. Preferably not more than 0.07%.
[0051]
[Cr: 0.50% or less]
　Cr, while contributing to the improvement of the hardenability, and concentrated in the carbide stabilizing carbides, is an element which forms a stable carbide in the austenite phase. If it is less than 0.001%, because not be obtained hardenability improving effect, Cr is preferably 0.001% or more. More preferably 0.007% or more. On the other hand, when it exceeds 0.50%, to produce a stable carbides in an austenite phase, dissolution of carbides is delayed during quenching, because the required temper strength can not be obtained, Cr is 0.50% or less. Preferably not more than 0.48%.
[0052]
[Mo: 0.50% or less]
　Mo, like Mn, is an element effective form control of carbides, also tissue is miniaturized element which contributes to the improvement of ductility. If it is less than 0.001%, the effect of the addition is not obtained, Mo is preferably 0.001% or more. More preferably not less than 0.017%. On the other hand, when it exceeds 0.50%, reduces the plane anisotropy of r value, the cold formability is lowered, Mo is 0.50% or less. Preferably not more than 0.45%.
[0053]
[B: 0.01% or less]
　B is an element which contributes to the improvement of hardenability. If less than 0.0004%, no effect is obtained by the addition, B is preferably set to 0.0004% or more. More preferably 0.0010% or more. On the other hand, when it exceeds 0.01%, coarse B product is produced, since cold formability is lowered, B is set to 0.01% or less. Preferably 0.008% or less.
[0054]
[Nb: 0.10% or less]
　Nb is an effective element to form the control of carbides, also tissue is miniaturized element which contributes to the improvement of ductility. If it is less than 0.001%, the effect of the addition is not obtained, Nb is preferably 0.001% or more. More preferably 0.002% or more. On the other hand, when it exceeds 0.10%, to produce a large number of fine Nb carbide, the strength becomes too high, the number of carbides of the ferrite grain boundaries is decreased, since cold formability is lowered, Nb is 0 and .10% or less. Preferably not more than 0.09%.
[0055]
[V: 0.10% or less]
　V, similarly to the Nb, an element effective to form the control of carbides, also tissue is miniaturized element which contributes to the improvement of ductility. If it is less than 0.001%, the effect of the addition can not be obtained, V is preferably 0.001% or more. More preferably 0.004% or more. On the other hand, when it exceeds 0.10%, to produce a large number of fine V carbide, the strength becomes too high, the number of carbides of the ferrite grain boundaries is decreased, since cold formability is lowered, V 0 and .10% or less. Preferably not more than 0.09%.
[0056]
[Cu: 0.10% or less]
　Cu is an element which segregates in the ferrite grain boundary, also an element which contributes to improvement in strength by forming fine precipitates. If it is less than 0.001%, because not be obtained the effect of improving the strength, Cu is preferably 0.001% or more. More preferably 0.004% or more. On the other hand, when it exceeds 0.10%, segregation of the ferrite grain boundaries lead to hot shortness, the productivity of the hot rolling is lowered, and 0.10% or less. Preferably not more than 0.09%.
[0057]
[W: 0.10% or less]
　W also, Nb, similarly to V, is an element effective to form the control of the carbides. If it is less than 0.001%, the effect of the addition can not be obtained, W is preferably 0.001% or more. More preferably 0.003% or more. On the other hand, when it exceeds 0.10%, the fine W carbides many generating and strength too high, the number of carbides of the ferrite grain boundaries is reduced, since the cold formability is lowered, W is 0. to 10% or less. Preferably not more than 0.08%.
[0058]
[Ta: 0.10% or
　less] Ta also, Nb, V, similarly to the W, is an element effective to form the control of the carbides. If it is less than 0.001%, the effect of the addition is not obtained, Ta is preferably set to 0.001% or more. More preferably 0.007% or more. On the other hand, when it exceeds 0.10%, to produce a large number of fine Ta carbides, the strength becomes too high, the number of carbides of the ferrite grain boundaries is reduced, since the cold formability is lowered, Ta is 0. to 10% or less. Preferably, at most 0.09%.
[0059]
[Ni: 0.10% or
　less] Ni is an element effective in improving the ductility. If it is less than 0.001%, the effect of the addition can not be obtained, Ni is preferably 0.001% or more. More preferably 0.002% or more. On the other hand, when it exceeds 0.10%, the number of carbides of the ferrite grain boundaries is reduced, since the cold formability is lowered, Ni is not more than 0.10%. Preferably not more than 0.09%.
[0060]
[Sn: 0.05% or
　less] Sn is an element inevitably mixed from the steel material. Therefore, Sn, since the less preferred, may be 0%, but when reduced to less than 0.001%, the refining costs increase significantly, Sn may be more than 0.001%. The content of Sn may be 0.002% or more. On the other hand, when it exceeds 0.05%, ferrite is brittle, since the cold formability is decreased, Sn is 0.05% or less. Preferably, 0.04% or less.
[0061]
[Sb: 0.05% or less]
　Sb, like Sn, and inevitably mixed from the steel material, it segregates in the ferrite grain boundary, is an element to reduce the number of carbides of the ferrite grain boundaries. Therefore, Sb, since the less preferred, may be 0%. However, when reduced to less than 0.001%, the refining costs increase significantly, Sb may be more than 0.001%. The content of Sb may be 0.002% or more. On the other hand, when it exceeds 0.05%, Sb is segregated on the grain boundary of ferrite, the number of carbides of the ferrite grain boundaries is reduced, since the cold formability is lowered, Sb is 0.05% or less. Preferably not more than 0.04%.
[0062]
[As: 0.05% or
　less] As is, Sn, As with Sb, unavoidably mixed from the steel material, is an element which segregates in the ferrite grain boundaries. Therefore, As, since the less preferred, may be 0%. However, when reduced to less than 0.001%, the refining costs increase significantly, As may be more than 0.001%. Preferably it may be 0.002% or more. On the other hand, when it exceeds 0.05%, As is segregated in ferrite grain boundaries, the number of carbides of the ferrite grain boundaries is reduced, since the cold formability is lowered, As is 0.05% or less. Preferably not more than 0.04%.
[0063]
[Mg: 0.05% or
　less] Mg is an element capable of controlling the form of sulfide by the addition of small amount. If less than 0.0001%, the effect of the addition is not obtained, Mg is preferably made 0.0001% or more. More preferably not less than 0.0008%. On the other hand, when it exceeds 0.05%, ferrite is brittle, since the cold formability is lowered, Mg is set to 0.05% or less. Preferably not more than 0.04%.
[0064]
[Ca: 0.05% or
　less] Ca, like Mg, an element capable of controlling the form of sulfide by the addition of small amount. If it is less than 0.001%, the effect of the addition is not obtained, Ca is preferably 0.001% or more. More preferably 0.003% or more. On the other hand, when it exceeds 0.05%, coarse Ca oxides are produced, since the starting point of cracks at the time of cold forming, Ca is set to 0.05% or less. Preferably not more than 0.04%.
[0065]
[Y: 0.05% or less]
　Y is, Mg, as with Ca, an element that can control the form of sulfide by the addition of small amount. If it is less than 0.001%, the effect of the addition is not obtained, Y is preferably 0.001% or more. More preferably 0.003% or more. On the other hand, when it exceeds 0.05%, coarse Y oxide is generated, since the starting point of cracks at the time of cold forming, Y is not more than 0.05%. Preferably not more than 0.03%.
[0066]
[Zr: 0.05% or
　less] Zr is, Mg, Ca, similar to the Y, is an element capable of controlling the form of sulfide by the addition of small amount. If it is less than 0.001%, the effect of the addition is not obtained, Zr is preferably set to 0.001% or more. More preferably 0.004% or more. On the other hand, when it exceeds 0.05%, coarse Zr oxide is generated, since the starting point of cracks at the time of cold forming, Zr is 0.05% or less. Preferably not more than 0.04%.
[0067]
[La: 0.05% or less]
　La is an element capable of controlling the form of sulfide by the addition of trace amounts, segregated in ferrite grain boundaries, is also an element to reduce the number of carbides of the ferrite grain boundaries. If it is less than 0.001%, since not be obtained form control effect of sulfide, La is preferably set to 0.001% or more. More preferably 0.003% or more. On the other hand, when it exceeds 0.05%, segregation in the grain boundary of ferrite, the number of carbides of the ferrite grain boundaries is reduced, since the cold formability is lowered, La is not more than 0.05%. Preferably not more than 0.04%.
[0068]
[Ce: 0.05% or less]
　Ce, like La, is an element capable of controlling the form of sulfide by the addition of trace amounts, it segregated in ferrite grain boundaries to reduce the number of the carbide of the ferrite grain boundaries there is also an element. If it is less than 0.001%, since not be obtained form control effect of sulfide, Ce is preferably 0.001% or more. More preferably 0.003% or more. On the other hand, when it exceeds 0.05%, segregation in the grain boundary of ferrite, the number of carbides of the ferrite grain boundaries is reduced, since the cold formability is lowered, Ce is not more than 0.05%. Preferably not more than 0.04%.
[0069]
　In the present invention steel, the remainder of the chemical composition is Fe and inevitable impurities.
[0070]
　In the steel plate of the present invention, in addition to the above chemical composition, it exceeds 1 (a) the ratio of the number of ferrite grain boundary carbide to the number of carbide in ferrite grains (A) (B) (B / A), (b ) ferrite grain size is at 5μm or 50μm or less, (average particle diameter of c) carbides is at 0.4μm or 2.0μm or less, (d) and the pearlite area ratio is less 6%, (e) Vickers hardness characterized requirement to or less than Saga 120HV than 170 Hv.
[0071]
　The present invention steel sheet, in addition to the above chemical composition, by comprising the features above requirement (a) to (e), it is possible to have a post-heat treatment and excellent cold formability ductility. This is the present inventors is a novel finding found. It will be described below.
[0072]
[Features Requirement (a)]
　of the present invention steel sheet structure is substantially, tissue composed of ferrite and carbides. Then, the ratio of the number of ferrite grain boundary carbide to the number of carbide in ferrite grains (A) (B) (B / A) is in excess of 1 tissue.
[0073]
　Incidentally, carbides, cementite (Fe is a compound of iron and carbon 3 in addition to C), the Fe atoms in cementite, Mn, compounds or substituted with alloy elements such as Cr, alloy carbides (M 23 C 6 , M 6 C, MC, etc. [M: Fe, and other added metal element as an alloy is a).
[0074]
　When forming the steel sheet into a predetermined shape, shear zones are formed in the macrostructure of the steel sheet, in the vicinity of the shear zone, caused by slip deformation is concentrated. Slip deformation is accompanied by proliferation of dislocation, in the vicinity of the shear zone, the region of high dislocation density is formed. With the increase of the strain amount applied to the steel sheet, slip deformation is promoted, the dislocation density increases. In order to improve the cold formability, it is effective to suppress the formation of shear bands.
[0075]
　In terms of microstructure, formation shear zone, slip occurs in one certain crystal grains, overcame grain boundaries, is understood as a phenomenon that continuously propagated to neighboring grains. Therefore, in order to suppress the formation of shear bands, it is necessary to prevent the propagation of slip exceeding a grain boundary. Carbides in the steel sheet is a strong particles that prevent slipping, the presence of the carbides in the ferrite grain boundaries, can be prevented the propagation of slip exceeding a grain boundary, to suppress the formation of shear zones, it is possible to improve the cold formability.
[0076]
　Based on theory and principles, cold formability is believed to strongly affected by the coverage of the ferrite grain boundary carbide, the highly accurate measurement is required. However, the measurement of the coverage of the ferrite grain boundary carbide in three-dimensional space, repeatedly performs observation and sample cutting with FIB with a scanning electron microscope in serial sectioning SEM observation or mandatory becomes 3D EBSP observation , together requires extensive measuring times, the accumulation of technical know-how is indispensable.
[0077]
　The present inventors have not adopted as not a general analysis method the observation technique, it was sought more convenient and higher metric precision. As a result, if the ratio of the number of ferrite grain boundary carbide to the number of carbide in ferrite grains (A) (B) and (B / A) as an index, quantitative evaluation can be cold formability, and, When the ratio (B / a) is more than 1, it was found that the cold formability is remarkably improved.
[0078]
　Occurs during the cold forming of steel plate, buckling, folding, none of the convolution, since those caused by localized distortion due to the formation of the shear zones, by the presence of carbides in the ferrite grain boundaries, shear zone the formation and localization of strain relaxation, buckling, folding, generation of convolution is suppressed.
[0079]
[Features Requirement (b)]
　in the steel sheet structure after annealing, by making the ferrite grain size and higher 5 [mu] m, it is possible to improve the cold formability. When the ferrite grain diameter is less than 5 [mu] m, to increase the hardness, a crack or cracks are likely to occur during cold forming, ferrite grain size shall not be less than 5 [mu] m. Preferably it is 7μm or more. On the other hand, when the ferrite grain diameter exceeds 50 [mu] m, and the number of carbide suppresses grain boundary propagation of slip decreases, since cold formability is lowered, the ferrite grain diameter is set to 50 [mu] m or less. Preferably is less than or equal to 38μm.
[0080]
[Features Requirement (c)]
　When the average particle diameter of the carbide contained in the present invention steel sheet structure is less than 0.4 .mu.m, the hardness increased remarkably in steel sheet, so cold formability is lowered, the carbide the average particle diameter be at least 0.4 .mu.m. Preferably is 0.6μm or more. On the other hand, the average particle diameter of the carbide contained in the tissue of the present invention the steel sheet is more than 2.0 .mu.m, since carbides during cold forming becomes a starting point of cracking, the average particle size of the carbide and 2.0 .mu.m or less . Preferably is less than or equal to 1.95μm.
[0081]
[Features Requirement (d)]
　the pearlite area ratio is more than 6%, the hardness is significantly increased in the steel, since cold formability is lowered, pearlite area ratio is 6% or less. Preferably 5% or less.
[0082]
[Features Requirement (e)]
　The Vickers hardness of the steel sheet by a 120HV or more 170HV or less, it is possible to improve the cold formability. When Vickers hardness is less than 120 HV, since buckling is likely to occur during cold forming, the Vickers hardness is not less than 120 HV. Preferably is equal to or greater than 130HV. On the other hand, the Vickers hardness exceeds 170 Hv, ductility is lowered and the internal cracking is likely to occur at the time of cold forming, the Vickers hardness is not more than 170 Hv. Preferably it is less than 160HV.
[0083]
　The following describes the observation and measurement method of the tissue.
[0084]
　Observation of carbide is carried out with a scanning electron microscope. Prior to observation, the sample for microstructure observation was polished by diamond abrasive grains having an average particle size of wet grinding and 1μm by emery paper, after finishing the observation plane mirror 3% nitric acid - the tissue at alcohol solution etching. Observation magnification, in 3000 times, selects a magnification that can determine the ferrite and carbides of tissue. The selected magnification, capturing a plurality of visual field of 30 [mu] m × 40 [mu] m in the sheet thickness 1/4 layer randomly. For example, to shoot eight or more areas that do not overlap with each other.
[0085]
　The resulting tissue image, measuring the area of the carbide. Equivalent circle diameter from the area of the carbide (= 2 × √ (area /3.14)) sought, and the average value and the carbide particle size. The measurement of the area of the carbide, image analysis software (e.g., Win ROOF manufactured by Mitani Corporation) was used, may be measured in detail the area of the carbide contained in the analysis region. Incidentally, in order to suppress the expansion of measurement error due to noise, the area is 0.01 [mu] m 2 is less carbide excluded from the evaluation.
[0086]
　Using the aforementioned tissue image by counting the number of cementite present in the ferrite grain boundary, from all the carbides number, subtracts the number of ferrite grain boundary carbide, calculates the number of carbide in ferrite grains. Based on the counting and calculated the number of carbides, it calculates the ratio of the number of ferrite grain boundary carbide to the number of carbide in ferrite grains (A) (B) and (B / A). The area is 0.01 [mu] m 2 or less of carbides, not count.
[0087]
　Ferrite grain size is in the above procedure, after polishing the observation surface of the sample to a mirror, 3% nitric acid - etched in alcoholic solution, the etching tissues were observed under an optical microscope or a scanning electron microscope, photographed image it can be measured by applying a line segment method.
[0088]
　Next, the present invention will be described manufacturing method.
[0089]
　The present invention production process, the conditions of the hot rolling step, the conditions of the process conditions and two-stage annealing process consistently cooperation to manage up firewood, and performing tissue control of the steel sheet.
[0090]
　The required billet of molten steel chemical composition was continuously cast, directly or once heated after cooling were subjected to hot rolling, at a temperature range of 800 ° C. or higher 900 ° C. or less, completing finish rolling of the hot rolled to. Such hot rolling by applying to the steel strip, it is possible to obtain a steel sheet structure composed of fine pearlite and bainite.
[0091]
　The hot-rolled steel sheet completing finish rolling take seeded in a temperature range of 400 ° C. or higher 550 ° C. or less. Dispensing a hot-rolled steel sheet taken Maki, was subjected to pickling, subjected to two-stage annealing, after annealing, was cooled to 650 ° C. at 1 ° C. / time or 30 ° C. / time cooling rate was controlled below, then, at room temperature until cool.
[0092]
　The two-stage annealing process, the hot rolled steel sheet, in the first stage of the annealing step, and held in a temperature range of 650 ° C. or higher 720 ° C. or less for 3 hours or more 60 hours or less, in the second stage of the annealing step, 725 ° C. or higher 790 ° C., i.e., an annealing step of holding 50 hours or less for more than 3 hours at a temperature range.
[0093]
　Hereinafter, the hot rolling step (in particular, the finishing rolling step) will be described in detail and Maki-up process.
[0094]
[Hot Rolling Step]
　If heated after once cooling the slab subjected to hot rolling, the heating temperature is preferably 1000 ° C. or higher 1250 ° C. or less, the heating time is preferably 3 hours or less than 0.5 hours. The steel slab, directly when subjected to hot rolling, the steel strip temperature is preferably 1000 ° C. or higher 1250 ° C. or less.
[0095]
　Billet temperature or slab heating temperature exceeds 1250 ° C., or, when the steel slab heating time exceeds 3 hours, significantly decarburization from the surface layer billet, when before quenching heating, the steel sheet surface layer austenite grains abnormal growing cold formability is lowered. Therefore, the steel strip temperature or billet heating temperature is preferably 1250 ° C. or less, the steel slab heating time is preferably 3 hours or less. More preferably 1200 ° C. or less, or less 2.5 hours.
[0096]
　Billet temperature or slab heating temperature is less than 1000 ° C., or, when the steel slab heating time is less than 0.5 hours, not eliminated micro-segregation and macro segregation generated in casting, inside the slab, alloying elements such as Si and Mn is locally thickened region is remaining cold-forming property is lowered. Therefore, the steel strip temperature or billet heating temperature is preferably 1000 ° C., the steel slab heating time is preferably 0.5 hour or more. More preferably 1050 ° C. or higher, at least 1 hour.
[0097]
[Finish rolling step in the hot rolling]
　finish rolling of hot rolling is completed at a temperature range of 800 ° C. or higher 900 ° C. or less. When finishing temperature is lower than 800 ° C., been an increase in the deformation resistance of the steel sheet, rolling load considerably increases, also the roll wear amount is increased, productivity is lowered. Therefore, the finishing temperature in the present invention is to 800 ° C. or higher. Preferably at 830 ℃ or more.
[0098]
　When finishing temperature exceeds 900 ° C., a thick scale is formed during passage through the Run Out Table (ROT), due to the scale, scratches occur on the surface of the steel sheet, during cold forming, cracks flaws starting There occur. Therefore, the finishing temperature is set to 900 ° C. or less. Preferably at 870 ℃ or less.
[0099]
[After finish rolling, the temperature conditions of up to Maki-up process of hot-rolled steel sheet]
　When cooling the hot-rolled steel sheet after finish rolling in the ROT, the cooling rate is preferably 10 ° C. / sec or higher 100 ° C. / sec or less. When the cooling rate is less than 10 ° C. / sec, thick scale is generated during cooling, can not suppress the occurrence of caused by flaws in it, the cooling rate is preferably at least 10 ° C. / sec. More preferably 15 ° C. / sec or more.
[0100]
　Over the inside from the surface layer of the steel sheet, and cooled at a cooling rate exceeding 100 ° C. / sec, the outermost layer is excessively cooled, low temperature transformation structure such as bainite or martensite occurs. After removal Maki, when paying out a hot-rolled steel coil cooled to 100 ° C. ~ room temperature, fine cracks occur in the low-temperature transformation structure. The micro cracks, it is difficult to remove by pickling. At the time of cold forming, cracks are generated microcracks starting from. To prevent the low-temperature transformation structure such as bainite or martensite in the outermost surface layer portion occurs, the cooling rate is preferably 100 ° C. / sec or less. More preferably at most 90 ° C. / sec.
[0101]
　Incidentally, the cooling rate after passing through the hot-rolled steel sheet without irrigation interval after the finish rolling, from the time of receiving water cooled in the water injection section, at the time it is cooled on ROT to the target temperature of the winding-up, each water injection section points to a cooling capacity to receive from the cooling equipment does not indicate average cooling rate until the temperature is coiling the coiling machine from injection start point.
[0102]
[Maki-up Step]
　coiling temperature is set to 400 ° C. or higher 550 ° C. or less. When coiling temperature is less than 400 ° C., transformed into untransformed a an austenitic stiff martensite before winding-up, during payout of hot-rolled steel coil, cracks occur in the surface layer of the hot-rolled steel sheet, cold moldability is lowered. To suppress the transformation, coiling temperature is set to 400 ° C. or higher. Preferably at 430 ℃ or more.
[0103]
　When coiling temperature exceeds 550 ° C., a large pearlite generates a lamellar spacing, high thermal stability, thick acicular carbides are generated. The needle-shaped carbides are also remains after two-stage annealing. During cold forming of steel plate, a crack is generated as a starting point the needle-shaped carbides, coiling temperature is set to 550 ° C. or less. Preferably at 520 ℃ or less.
[0104]
　Hereinafter, more detailed description of two-stage annealing process of the present invention production process.
[0105]
　Dispensing a hot-rolled steel coils, after being subjected to pickling, the 2-step type to keep the two temperature ranges annealing (2-step annealing) performed. By applying a two-stage annealing hot-rolled steel sheet, to control the stability of the carbides, as well as promoting the formation of carbides in the ferrite grain boundary, it is possible to increase the spheroidization ratio of the carbide of the ferrite grain boundaries. Incidentally, after dispensing the hot-rolled steel coil, until a two-stage annealing process and two-step annealing process after the cooling process is completed, not the hot-rolled steel sheet and cold rolling. By cold rolling, is fine of ferrite grains, the steel sheet becomes difficult to soften it may Vickers hardness of the steel sheet does not become less 170HV than 120 HV.
[0106]
[1-stage annealing process]
　annealing the first-stage, A C1 conducted in a temperature range of below points. This annealing causes a coarsening of the carbides By concentrating alloying elements enhance the thermal stability of the carbide. Then, A C1 points or more A 3 raised to a temperature range below point to produce an austenitic in structure. Then, gradually cooled, austenite was transformed to ferrite, increasing the concentration of carbon in austenite.
[0107]
　By slow cooling, carbon atoms adsorbed on carbide remaining in the austenite, carbides and austenite covers the grain boundary of ferrite, finally, the steel sheet microstructure, spheroidized carbides in many existing tissue in grain boundaries of ferrite be able to.
[0108]
　A C1 point or more A 3 when held at points below the temperature range, the residual carbides is small, during cooling, pearlite, and, rod carbide, the plate-like carbides produced. Perlite, and rod-like carbides, the plate-like carbides are generated, cold forming of the steel sheet is remarkably lowered. Therefore, A C1 points or more A 3 by holding in the temperature range below point, it is important in improving the cold formability of increasing the number of residual carbides.
[0109]
　In the steel sheet structure to form in the above first stage of the annealing step, A C1 temperature range below point, the thermal stabilization of carbides is promoted, A described above C1 points or more A 3 temperature range below point in retention in, it is possible to increase the number of residual carbides.
[0110]
　1-stage annealing at annealing temperature (first stage annealing temperature) is set to 650 ° C. or higher 720 ° C. or less. When the first stage annealing temperature is lower than 650 ° C., but the stabilization of carbides is sufficiently, when the second-stage annealing, it is difficult to leave carbides in the austenite. Therefore, the first stage annealing temperature is set to 650 ° C. or higher. Preferably at 670 ℃ or more. On the other hand, when the first stage annealing temperature exceeds 720 ° C., austenite is produced before the stability of the carbides increases, since the control of the aforementioned tissue changes difficult, the first stage annealing temperature is set to 720 ° C. or less . Preferably 700 ° C. or less.
[0111]
　1-stage annealing at annealing time (first stage annealing time) is not more than 60 hours or more 3 hours. When the first stage annealing time is less than 3 hours, rather than stabilization of carbides is sufficiently, at the time of the second-stage annealing, it is difficult to leave carbides in the austenite. For this reason, the first stage annealing time shall not be less than 3 hours. Preferably it is greater than or equal to 5 hours. On the other hand, when the first stage annealing time exceeds 60 hours, not be expected more stabilization of carbides, further, the productivity is lowered, the first stage annealing time is less 60 hours. Preferably not more than 55 hours.
[0112]
[2-stage annealing process]
　second stage annealing temperature (second stage annealing temperature) in the annealing is set to 790 ° C. or less 725 ° C. or higher. When the second stage annealing temperature is lower than 725 ° C., the amount of austenite is small, the number of carbides in the ferrite grain boundary (B) is lowered. Therefore, the second stage annealing temperature is set to 725 ° C. or higher. Preferably less than or equal to 715 ℃. On the other hand, when the second stage annealing temperature exceeds 790 ° C., it becomes difficult to leave the carbide austenite, since the control of the aforementioned tissue changes difficult, second stage annealing temperature is set to 790 ° C. or less. Preferably at 770 ℃ or less.
[0113]
　2-stage annealing at annealing time (second stage annealing time) is not more than 50 hours or more 3 hours. Is less than the second stage annealing time is 3 hours, the amount of austenite is small, and is not sufficient dissolution of carbides in the ferrite grains, it is difficult to increase the number of carbides of the ferrite grain boundaries. For this reason, the second stage annealing time shall not be less than 3 hours. Preferably it is greater than or equal to 6 hours. On the other hand, the second stage annealing time is more than 50 hours, so it is difficult to leave the carbide austenite, the second stage annealing time to 50 hours or less. Preferably not more than 45 hours.
[0114]
　After the two-stage annealing, steel sheet is cooled to 650 ° C. at 1 ° C. / time or 30 ° C. / time cooling rate was controlled to below. The resulting austenite at annealing in the second stage was gradually cooled, with to transform to ferrite, to adsorb the carbon to carbide remaining in the austenite. Although the cooling rate is slower preferably, it is less than 1 ° C. / time, increases the time required for cooling, since productivity is lowered, the cooling rate is set to 1 ° C. / time or more. Preferably at 5 ° C. / time or more.
[0115]
　On the other hand, if the cooling rate exceeds 30 ° C. / time, austenite transforms to pearlite, increases the hardness of the steel sheet, since cold formability is lowered, the cooling rate is less 30 ° C. / hour. It is preferably at most 26 ° C. / hour.
[0116]
　The steel sheet after annealing and cooled to 650 ° C. at the cooling rate cools to room temperature. In cooling to room temperature, the cooling rate is not particularly limited.
[0117]
　Incidentally, annealing and annealing in the second stage of the first stage may be either box annealing or continuous annealing. Box annealing may be performed by using a box-type annealing furnace. The atmosphere in the two-stage annealing, particularly, but not limited to a specific atmosphere. For example, an atmosphere of 95% nitrogen, an atmosphere of hydrogen over 95%, may be any atmosphere of air atmosphere.
[0118]
　As described above, according to the present invention production process, substantially has the structure of the following ferrite and spheroidized carbides grain size 5μm or 50 [mu] m, the ferrite grain boundaries to the number of carbide in ferrite grains (A) of exceeding the ratio (B / a) is the number of ones carbide (B), furthermore, it can be Vickers hardness of less 170HV than 120 HV, obtain steel sheet having excellent cold formability and heat treatment after ductility.
Example
[0119]
　Next is a description of examples of embodiments, conditions in the examples is an example of the conditions employed for confirming the workability and effects of the present invention, the present invention is, in this single condition example the present invention is not limited. The present invention does not depart from the gist of the present invention, as long as to reach the present invention purposes, it is capable of adopting various conditions.
[0120]
　(Example 1)
　To examine the influence of the chemical composition, Table 1-1, Table 1-2 (component composition of the steel sheet of the present invention) and Table 2-1, components shown in Table 2-2 (composition of Comparative steel) the continuous casting composition slab (slab), to implement the steps from hot rolling step to the two-stage annealing process under the following conditions, a sample for property evaluation indicated in Table 3 (invention steels A- the 1 ~ Z-1 and the comparative steels AA-1 ~ AZ-1) was prepared. Incidentally, Table 1-1, billet No. A ~ Z of Table 1-2, all have a chemical composition of the steel sheet of the present invention. On the other hand, Table 2-1, the component composition of the steel pieces No.AA ~ AZ in Table 2-2, all of which are outside the scope of the chemical composition of the steel sheet of the present invention.
[0121]
[Table 1-1]

[0122]
[Table 1-2]

[0123]
[table 2-1]

[0124]
[Table 2-2]

[0125]
　That was heated for 1.8 hours at each slab component composition shown in Table 1 and Table 2 1240 ° C., subjected to hot rolling to complete the finish rolling at a finishing temperature 820 ° C.. Then, subsequently, it cooled at a cooling rate of 45 ° C. / sec on ROT, to produce a hot-rolled steel sheet coil performs up Maki at coiling temperature 510 ° C.. Next, dispensing the hot-rolled steel sheet coil, after pickling, the hot-rolled steel sheet coil after pickling in order to perform the annealing in the first stage and placed in a box type annealing furnace, a 95% hydrogen and 5% nitrogen by controlling the annealing atmosphere so as to include, and held heated for 36 h to 705 ° C. from room temperature to equalize the temperature distribution in the hot-rolled steel coil. Then, the mixture was heated to 760 ° C. in order to perform the annealing in the second stage and held for 10 hours, then to 650 ° C., then cooled at a cooling rate of 10 ° C. / time, then furnace cooled to room temperature, for property evaluation the samples were prepared.
[0126]
　Tissue of the sample was observed by the method described above, the ferrite grain size, and were measured the number of carbides. Then, charged with the sample to an atmosphere furnace, at 950 ° C., and 20 minutes retention was conducted Hojogo, a 50 ° C. oil cooling. Then, hardness was tempered to be 400 HV. After heat treatment ductility searches the surface of the sample after annealing sense, to prepare a JIS5 No. 5 test piece thickness 2 mm, it was obtained by performing a tensile test at room temperature. The distance between the target point and 50mm, was subjected to a tensile test at a test speed of 3mm / min. 10% or more was considered good.
[0127]
　Table 3, a ferrite grain size ([mu] m), Vickers hardness (HV), the ratio of the number of ferrite grain boundary carbide to the number of carbide in ferrite grain (grain boundary carbides number / intragranular number carbide), and heat treatment shows the rear ductility (%).
[0128]
[table 3]

[0129]
　As shown in Table 3, in the present invention steel plate (A-1 ~ Z-1), both, and Vickers hardness is 170HV or less, the ferrite grain boundaries to the number of carbide in ferrite grains of carbides number of ratio (grain boundary carbides number / intragranular number carbide) is greater than 1. Since hardness is an indication of cold formability, the steel sheet of the present invention (A-1 ~ Z-1) is found to be excellent in cold formability.
[0130]
　In contrast, many Si amount in the comparative steel sheets AA-1, more amount of C in Comparative steel AB-1, many Mn amount in Comparative steel AD-1, in either a Vickers hardness of 170HV It is beyond the.
[0131]
　Small amount of C in Comparative steel AH-1, A 3 for points higher, quenching is impossible. Small amount of Si in Comparative steel AE-1, Vickers hardness not only was less than 120 HV, the heat treatment after the ductility is lowered. In other comparative steel sheets, component composition, it is beyond the scope of the chemical composition of the steel sheet of the present invention, after heat treatment ductility is reduced.
[0132]
　(Example 2)
　finishing rolling of hot rolling, in order to examine the effect of each condition of the coiling step and two-step annealing process of the steel plate, the test steel plate of No. A-2 ~ Z-2 as follows It was produced. That is, first, the respective billets No. A ~ Z component composition shown in Table 1-1 and Table 1-2, was heated for 1.8 hours at 1240 ° C., subjected to hot rolling are shown in Table 4 the condition to complete the finish rolling of hot rolling, then cooling at a cooling rate of 45 ° C. / sec on ROT, taken plated at coiling temperatures shown in Table 4, the plate thickness 3.0mm hot-rolled steel coil It was prepared.
[0133]
　After pickling the hot-rolled steel sheet coils, in the annealing conditions shown in Table 4, it was subjected to box annealing in the two-stage stepped. From the hot-rolled steel sheet after annealing, and collect data for evaluation characteristics of sheet thickness 3.0 mm, ferrite grain size ([mu] m), Vickers hardness (HV), the ferrite grain boundary carbide to the number of carbide in ferrite grains ratio number of (grain boundary carbides number / intragranular number carbide), and were measured ductility (%) after heat treatment. The results are shown in Table 5.
[0134]
[Table 4]

[0135]
　As shown in Table 5, in the present invention steel sheet, both, Vickers hardness is not more than 170 Hv, the ratio of the carbides number of ferrite grain boundaries for carbides number in ferrite grains is greater than 1. Hardness because it is an indicator of cold formability, any steel sheet of the present invention, it can be seen that excellent cold formability. Further, any steel sheet of the present invention, because it has a 10% or more after heat treatment ductility, it can be seen that a good respect after heat treatment ductility.
[0136]
　In contrast, in the comparative steel sheet manufacturing conditions, since it is outside the scope of manufacturing conditions of the manufacturing method of the present invention, the Vickers hardness is increased. Further, in some comparative steel has fallen grain boundary carbides number / intragranular number carbide.
[0137]
[table 5]

Industrial Applicability
[0138]
　As described above, according to the present invention, it is possible to provide a method of manufacturing a steel sheet excellent in ductility after the heat treatment and cold formability. Accordingly, the present invention has high applicability in the steel sheet production and utilization industry.

claims

[Claim 1]Component composition, by
mass%,
C:
0.10 ~ 0.40%, Si: 0.30 ~ 1.00%,
Mn: 0.30 ~ 1.00%, Al: 0.001 ~ 0.10
%, P: 0.02% or
less, S: 0.01% or less
contain, in steel sheet balance being Fe and impurities,
　the number of carbides of the ferrite grain boundaries to the number of carbide in ferrite grains (a) ( beyond the ratio (B / a) is 1 B),
　the ferrite grain size is not less 5μm or 50μm or less,
　an average particle diameter of the carbide is at 0.4μm or 2.0μm or less,
　pearlite area ratio is 6% or less There,
　the steel sheet Vickers hardness is equal to or less than 170HV than 120 HV.
[Claim 2]
　It said steel sheet further contains, by
mass%, N: 0.01% or
less, O: 0.02% or less
steel sheet according to claim 1, characterized in that it contains one or more.
[Claim 3]
[Correction 25.05.2017 under Rule 91]　
　The steel plate further contains, by
mass%, Ti: 0.10% or
less, Cr: 0.50% or
less, Mo: 0.50% or
less, B: 0.01%
hereinafter, Nb: 0.10% or
less, V: 0.10% or
less, Cu: 0.10% or
less, W: 0.10% or less,
Ta: 0.10% or
less, Ni: 0.10% or less,
sn: 0.05% or
less, Sb: 0.05% or
less, As: 0.05% or
less, Mg: 0.05% or
less, Ca: 0.05% or
less, Y: 0.05% or
less, Zr: 0.05% or
less, La: 0.05% or
less, Ce: 0.05% or less
steel sheet according to claim 1 or 2, characterized in that it contains one or more of.
[Claim 4]
　A method of manufacturing a steel sheet according to any one of claims 1 to 3,
a slab component composition according to any one of (i) claims 1 to 3, directly, or, subjected to hot rolling heat and then cooled, the hot rolled steel sheet completing the finish rolling at a temperature range of 800 ° C. or higher 900 ° C. or less taken plated at 400 ° C. or higher 550 ° C. or less,
hot-rolled steel sheet was wound (ii) the payout, after applying pickling, annealed in the first stage for holding in a temperature range of 650 ° C. or higher 720 ° C. or less for 3 hours or more 60 hours or less, further 3 hours at a temperature range of 790 ° C. or less 725 ° C. or higher subjected to 2-stage annealing holding 50 hours inclusive, annealed for 2-step type,
(iii) the cooling rate of the hot-rolled steel sheet after the annealing was controlled to below 1 ° C. / time or 30 ° C. / time in cooled to 650 ° C., then cooled to room temperature
, characterized in that Method of manufacturing a steel sheet.
[Claim 5]
　Method for producing a steel sheet according to claim 4 in which the temperature of the steel strip subjected to rolling between the heat is characterized in that the 1000 ~ 1250 ° C..