Technical field
[0001]The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof. More particularly, the present invention is, for the automotive, household appliances, machinery structure, suitable as a material for use in applications such as architectural, about good hot-rolled steel sheet and a manufacturing method thereof processability.
　The present application, on July 7, 2017, claiming priority based on Japanese Patent Application No. 2017-133889, filed in Japan, the contents of which are incorporated here.
BACKGROUND
[0002]The steel sheet which is subjected as a material, such as structural members of transport equipment and various industrial machines including automobiles, strength, workability such as elongation and stretch flange formability, low-temperature toughness and uniformity of their characteristics, such as various characteristics are required such.
[0003]In particular, the inner plate member of an automobile, structural members, steel plates used for the parts such as underbody members, depending on the application, stretch-flangeability, burring workability, ductility, fatigue resistance, impact resistance and corrosion resistance and the like Desired. And these material properties and high strength can be balanced well exhibited at a high level, required for steel sheets used for the members described above. Further, the steel sheet used for the members described above is molded, after which is mounted in an automobile as a part of the member is required to have a disrupted hardly characteristics even when subjected to impact due to collision or the like. In particular, the use temperature is low cold region, since the member is liable to embrittlement, for impact resistance ensured, there is a need to improve also the low temperature toughness of the steel sheet. The low-temperature toughness is a property defined by vTrs (Charpy fracture appearance transition temperature) and the like. The steel sheet used in the parts of the member, as well as excellent processability, low-temperature toughness is obtained as a very important property.
[0004]
　As steel sheets obtained excellent ductility, Dual Phase steel sheet composed of a composite structure of a soft ferrite phase and a hard martensite phase (hereinafter DP steel) it is known. DP steel while excellent ductility, because significantly cracked voids from the interface occurs between the different ferrite phase and martensite phase hardness occurs, there may be poor in hole expansion.
[0005]
　In Patent Document 1, an area ratio, bainitic ferrite 90% or more, less than 5% of martensite, bainite and by 5% or less, and improved the elongation and hole expansion (stretch flangeability), or tensile strength 980MPa high-strength hot-rolled steel sheet have been proposed. However, in the invention described in Patent Document 1, sufficient elongation can not be obtained because it is composed mainly of bainitic ferrite.
[0006]
　In Patent Document 2, after the bainite at an area ratio of 90% or more, martensite remainder austenite, of one or more kinds selected from ferrite and matrix structure, and the cementite dispersed in the tissue It improved the content and widen the hole by controlling the average particle size of (stretch-flangeability), or a tensile strength 980MPa hot-rolled steel sheet have been proposed. However, in the invention described in Patent Document 2, because a winding at 330 ~ 470 ° C. is a transition boiling region, there is a case that a characteristic variation due to temperature variation within the plate surface occurs.
[0007]
　In Patent Document 3, 50 to 95% ferrite fraction, a fraction of the hard second phase is 5-50% consisting of martensite and residual austenite, and interrelationship of the content of carbide forming elements, the relationship between the carbide-forming elements and the C content in terms of the within a predetermined range, the average particle size of the precipitates, defining the fraction of the precipitates, excellent hot-rolled steel sheet fatigue properties has been proposed. However, in the invention described in Patent Document 3, mainly of soft ferrite, since ensuring the strength by precipitation strengthening of fine carbides, there is a case that sufficient low temperature toughness is obtained.
[0008]
　Patent Document 4, a martensite 20-60%, ferrite 40% or more, the and the martensite and the total area ratio of the ferrite is 90% or more, average particle diameter and the martensite, the martensite the ratio between the hardness and the hardness of the ferrite site was within a predetermined range, the high strength hot rolled steel sheet tensile strength is at least 980MPa is proposed. However, in the invention described in Patent Document 4, since the short time required at the end of the finish rolling, there is a case where texture is developed, sufficient elongation and stretch flangeability can not be obtained.
CITATION
Patent Document
[0009]
Patent Document 1: Japanese Patent 2008-255484 JP
Patent Document 2: Japanese Patent 2014-205890 JP
Patent Document 3: Japanese Patent 2009-84648 JP
Patent Document 4: WO 2017/085841
Summary of the Invention
Problems that the Invention is to Solve
[0010]
　The present invention has been made in consideration of the above problems, a high strength and elongation, a manufacturing method capable of stably producing a hot-rolled steel sheets and hot-rolled steel sheet excellent in stretch flange formability and low-temperature toughness an object of the present invention is to provide.
Means for Solving the Problems
[0011]
　The present inventors have found that by optimizing the chemical composition and manufacturing conditions of the hot-rolled steel sheet, by controlling the texture and microstructure of hot-rolled steel sheet, a high strength, and elongation, stretch flange formability and low-temperature toughness It was found that could produce superior hot rolled steel sheet.
[0012]
　The gist of the present invention is as follows.
[0013]
[1] hot rolled steel sheet according to one embodiment of the present invention,
　in
　mass%,
　C: 0.02 ~ 0.20%,
　Si: 0.005 ~ 2.00%, Mn: 1.30 ~ 2.40
　Pasento, P: 0.100 Pasento
　less, S: 0.0100 Pasento
　below, Sol.
　Al:
　0.001 ~ 1.00%, Ti: 0.030 ~ 0.200%,
　N: 0.0010 ~ 0.0100%, Nb: 0
　~ 0.100%, V: 0 ~ 0.50% , 　Mo:
　0 ~ 　0.50%, Cu: 0 ~ 1.00%, Ni: 0 ~ 　1.00%, Cr: 0 ~ 2.00%, B: 0 ~ 　0.0100%, Ca: 0 ~ 　% 0.0100, Mg: 0 ~ 0.0100% and 　REM: 0 ~ 0.0100% 　contained, has a chemical composition the balance being Fe and impurities,

　In 1/4 depth position of the sheet thickness from the surface, the area ratio of ferrite is 10 to 55% 45 to 90% the total area ratio of bainite and martensite, the ferrite, the bainite and the martensite and a total area ratio of 90% or more, the average crystal grain size is less 12.0 .mu.m,
　in texture measured at the center of plate thickness, {100} <011>, {211} <011>, { 311} <011>, {110} <011> and {332} <113> maximum pole density of the orientation group 8.0 or less, and {211} <011> and {332} pole density of <113> total is 10.0 or less,
　hot-rolled steel sheet tensile strength, characterized in that at least 950 MPa.
[0014]
[2] hot-rolled steel sheet according to [1],
　wherein the chemical composition, by
　mass%,
　Nb: 0.001 ~ 0.100%, V: 0.005 ~
　0.50%, Mo: 0.
　~
　0.50% 001, Cu: 0.02 ~ 1.00%,
　Ni: 0.02 ~ 1.00%, Cr: 0.02 ~ 2.00%, and
　B: 0.0001 ~ 0.0100%
　it may contain one or more members selected from the group consisting of.
[0015]
[3] hot-rolled steel sheet according to [1] or [2], the chemical composition, by
　mass%,
　Ca: 0.0002 ~ 0.0100%, Mg: 0.0002 ~ 0.0100%, and
　REM: 0.0002 ~ 0.0100%
　may contain one or more members selected from the group consisting of.
[0016]
[4] hot-rolled steel sheet according to any one of [1] to [3],
　the absolute value of the in-plane anisotropy of r value | [Delta] r | may be 0.35 or less.
　However, [Delta] r = (r 0 + r 90 -2 × r 45 ) / 2
is, r 0 : the rolling direction of the r value, r 90 : the rolling direction perpendicular to the r value, r 45 : 45 ° direction with respect to rolling direction which is the r value. In addition, r value means the Lankford (Lankford) value.
[0017]
[5] In producing the hot-rolled steel sheet according to any one of [1] to [4], the slab having the chemical composition according to any one of [1] to [3] or producing hot-rolled steel sheet is subjected to multi-pass hot rolling steel strip, a separate method for manufacturing a hot rolled steel sheet according to the embodiment of the present invention,
　the multi-pass the heating temperature in hot rolling 1150 ~ 1350 ° C. and then,
　when expressed as FT finishing temperature in the unit ° C., the FT + 50 ° C. ultra-the FT + 0.99 a total reduction ratio between ° C. 50% or higher, the total reduction ratio between the FT-the FT + 50 ℃ 40 ~ 80% , the the time required for rolling between FT-the FT + 50 ° C. and 0.5 to 10.0 s,
　performs rolling over two passes in each temperature range of the FT + 50 ° C. ultra-the FT + 0.99 ° C. and the FT-the FT + 50 ° C. ,
　the FT, Equation (1) sought Ar 3 or more, and the formula (2) sought TR above, and after completing the finish rolling as 1100 ° C. or less, the water cooling started within 3.0 seconds, the FT ~ 750 ° C. average cooling rate
　was 20 ° C. / sec or more, after stay 5-20 seconds at a temperature range of 750 ~ 600 °
　C., to a cooling stop temperature of less than determined Ms of the formula (3) from 600 ° C., an average cooling rate method for manufacturing a hot-rolled steel sheet, characterized by cooling the 20 ° C. / sec or more.
　Ar 3 (° C.) = 901-325 × [C] + 33 × [Si] -92 × [Mn] + 287 × [P] + 40 × [Al] (1)
　TR (° C.) = 800 + 700 × [Ti] + 1000 × [Nb]
　(2) Ms (° C.) = 561-474 × [C] -33 × [Mn] -17 × [Ni] -21 × [Mo] (3 )
　However, the chemical symbol of the formula (1) to (3) in indicates the content by mass percent of the respective elements.
[0018]
In the method for producing hot-rolled steel sheet according to [6] above [5], the average cooling rate from the Ms to the cooling stop temperature of less than the Ms may be 80 ° C. / s or higher.
[0019]
In [7] the production method of the hot-rolled steel sheet according to the above [5] or [6], after the finish rolling completion, water cooling was started within 0.3 seconds, the average cooling to the FT ~ the FT-40 ° C. speed may perform cooling at 100 ° C. / s or higher.
[0020]
In [8] the production method of the hot-rolled steel sheet according to [7], the step of performing an average cooling rate is 100 ° C. / s or higher cooling to the FT ~ the FT-40 ° C., carried out at rolling stand it may be.
The invention's effect
[0021]
　According to the present invention, a high strength and elongation, the hot-rolled steel sheets and hot-rolled steel sheet excellent in stretch flange formability and low-temperature toughness can provide a manufacturing method capable of stably produced. Inner plate members hot rolled steel sheet according to the present invention of a motor vehicle, the structural member, when used as part of a material such as underbody members, it is easy to process the component shape, withstand use in extreme cold regions it is possible, the contribution of the industry is extremely remarkable.
DESCRIPTION OF THE INVENTION
[0022]
　Hot rolled steel sheet according to the present embodiment (hereinafter, sometimes simply referred to as steel plate), and described in detail below its manufacturing method. In the following description, any% on the chemical composition of the steel is mass%.
[0023]
　 (C: 0.02 ~ 0.20%)
　C is, by generating a hard phase and Ti carbides, such as martensite and bainite, has an effect of improving the strength of steel. The C content is less than 0.02%, it is difficult to sufficiently exhibit the above effect. Therefore, C content is 0.02% or more, preferably 0.04% or more. On the other hand, C content is 0.20 percent, the stretch flangeability and low temperature toughness of the steel sheet is remarkably deteriorated. Also, ferrite transformation after hot rolling is significantly delayed, ferrite desired amount is difficult to obtain. Moreover, weldability occurs becomes remarkable. Therefore, C content is 0.20% or less. C content is preferably 0.15% or less, more preferably 0.12% or less, even more preferably 0.10% or less.
[0024]
　(Si: 0.005 ~
　2.00%) Si has an action to suppress the cementite precipitation with promoting ferrite transformation. Further, Si also has the action of improving the strength of steel by solid solution strengthening. Si content is less than 0.005%, it becomes difficult to exhibit the above effect. Therefore, Si content is 0.005% or more. Si content is preferably 0.40% or more, more preferably 0.80% or more. On the other hand, Si content is 2.00 percent, by surface oxidation in the hot rolling process, the surface properties of the steel sheet is remarkably deteriorated. Therefore, Si content is at most 2.00%. Si content is preferably 1.50% or less, and more preferably not more than 1.30%.
[0025]
　(Mn: 1.30 ~
　2.40%) Mn has an effect of improving the strength of the steel by increasing the solid solution strengthening and hardenability. The Mn content is less than 1.30%, the strength of the above steel sheet 950MPa is difficult to obtain. Therefore, Mn content is 1.30% or more. Mn content is preferably 1.50% or more. On the other hand, the Mn content 2.40 percent, by ferrite transformation in the cooling process after hot rolling is delayed excessively, ferrite desired amount is difficult to obtain. Further, the hardening of martensite and bainite, cracks in the vicinity of the boundary between martensite and bainite and soft ferrite that readily occurs, stretch flangeability and toughness of the steel sheet is lowered.
[0026]
　The present inventors have found that the inclusion in a large amount of Mn, with the increase of in-plane anisotropy of r value of the steel sheet, stretch flangeability was found that in some cases decreases. The reason is not clear, by incorporating Mn in a large amount, the MnS is large amount of precipitation, and local recrystallization and hot during rolling due to Mn segregation, the ferrite transformation after the finish rolling that such variations may occur is presumed to be the cause. From the above, while obtaining a desired amount of ferrite, in order to produce a superior hot-rolled steel sheet stretch flangeability stably, Mn content is at most 2.40%. Mn content is preferably 2.10% or less, more preferably 2.00% or less, even more preferably at most 1.90%.
[0027]
　(P: 0.100% or less)
　P is an element contained in the steel in as an impurity, has the effect of lowering the stretch flangeability and low-temperature toughness of the hot-rolled steel sheet. Therefore, P content is made 0.100% or less. P content is preferably 0.060% or less, more preferably 0.040% or less, even more preferably 0.020% or less. Although P is as impurities from raw materials, it is not necessary to particularly limit the lower limit, in order to ensure the stretch flangeability and low-temperature toughness, the content of P is more preferable lower. However, excessive reduction of P content, manufacturing cost increases. From the viewpoint of production cost, the lower limit of the P content is preferably 0.001%, more preferably 0.005%.
[0028]
　(S: 0.0100% or less)
　S is an element contained as an impurity, has the effect of lowering the workability of hot-rolled steel sheet. Therefore, S content is made 0.0100% or less. S content is preferably 0.0080% or less, more preferably 0.0060% or less, even more preferably 0.0030% or less. Although S is mixed as impurities from raw materials, it is not necessary to particularly limit the lower limit, the S content in order to ensure the workability better less is preferable. However, when excessively reduce the S content, the manufacturing cost is increased. From the viewpoint of production cost, the lower limit of the S content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%.　
[0029]
　(Sol.Al:0.001～1.00Pasento)
　Al is a deoxidizing steelmaking stage cleaned steel, and has an effect of promoting the ferrite transformation. sol. Al content is less than 0.001%, it becomes difficult to exhibit the above effect. Therefore, sol. Al content is 0.001% or more. sol. Al content is preferably 0.01% or more, more preferably 0.02% or more. On the other hand, sol. Also the Al content 1.00 percent, with the effect of the action is saturated, leading to cost increase. Therefore, sol. Al content is at most 1.00%. sol. Al content is preferably 0.80% or less, more preferably 0.60%. In addition, sol. Al refers to acid-soluble Al.
[0030]
　(Ti: 0.030 ~
　0.200%) Ti has an effect of refining the structure by forming a Ti nitride. Further, Ti is precipitated carbides, it has the effect of strengthening the steel. Ti content above effect becomes hard to be exerted is less than 0.030% or. Therefore, Ti content is 0.030% or more. Ti content is preferably 0.040% or more, more preferably 0.060% or more. On the other hand, if excessively contained a Ti, by coarse nitrides and carbides are produced, stretch flangeability and toughness of the steel sheet is lowered. Furthermore, since Ti has also has the effect of increasing recrystallization temperature of austenite, the excessive content of Ti, the recrystallization temperature becomes excessively high, since the anisotropy of the r value is increased, the stretch flangeability of the steel sheet There is reduced. Therefore, Ti content is at most 0.200%. Ti content is preferably 0.160% or less, and more preferably not more than 0.140%.
[0031]
　(N: 0.0010 ~ 0.0100%)
　N is to suppress the coarsening of the austenite to form a Ti nitride slab reheating or during hot rolling, has an effect of refining the microstructure . N content is difficult to exhibit the effect is less than 0.0010%. Therefore, N content is 0.0010% or more. N content is preferably 0.0015% or more, more preferably 0.0020% or more. On the other hand, N content is 0.0100% greater, to form a coarse Ti nitrides and deteriorates stretch flangeability of the steel sheet. Therefore, N content is 0.0100% or less. N content is preferably 0.0060% or less.
[0032]
　(Nb: 0 ～ 0.100
　Pasento) Nb is an optional element. Nb serves to suppress the coarsening of the crystal grain size of the hot-rolled steel sheet, a ferrite grain size finer, has the effect of increasing the strength of the hot rolled steel sheet by precipitation strengthening of NbC. To obtain these effects, the Nb content is preferably made 0.001% or more. Nb content is more preferably 0.005% or more. On the other hand, when the Nb content exceeds 0.100%, the effect described above is saturated, which may cause an increase in rolling load of the hot finish rolling. Therefore, Nb content is preferably 0.100% or less. Nb content is preferably 0.060% or less, more preferably 0.030% or less.
[0033]
　(V: 0 ～ 0.50 Pasento)
　V is an optional element. V has with a solid solution in the steel increase the strength of the hot rolled steel sheet, carbides and nitrides, the effect of precipitates in the steel as carbonitrides, etc., to precipitation strengthening. To obtain these effects, the V content is preferably made 0.005% or more. V content is more preferably 0.01% or more. On the other hand, may cause a reduction in the steel sheet toughness V content exceeds 0.50%. Therefore, V content is preferably set to 0.50% or less. V content is more preferably 0.30% or less.
[0034]
　(Mo: 0 ～ 0.50
　Pasento) Mo is an optional element. Mo is to increase the hardenability of steel, has an effect of increasing the strength of the hot rolled steel sheet to form a carbide or carbonitride. To obtain these effects, the Mo content is preferably made 0.001% or more. Mo content is more preferably 0.005% or more. On the other hand, when the Mo content exceeds 0.50%, the susceptibility to cracking of the slab increases. Therefore, the content of Mo is preferably set to 0.50% or less. Mo content is more preferably not more than 0.30%.
[0035]
　(Cu: 0 ～ 1.00
　Pasento) Cu is an optional element. Cu has an effect of enhancing the effectiveness and intensity of improving the toughness of the steel. To obtain these effects, the Cu content is preferably made 0.02% or more. Cu content is more preferably 0.08% or more. On the other hand, there is a case where an excessive Cu is contained weldability of the steel sheet is lowered. Therefore, Cu content is preferably set to 1.00% or less. Cu content is more preferably 0.50% or less, even more preferably 0.30% or less.
[0036]
　(Ni: 0 ～ 1.00
　Pasento) Ni is an optional element. Ni has an effect of enhancing the effectiveness and intensity of improving the toughness of the steel. To obtain these effects, the Ni content is preferably made 0.02% or more. Ni content is more preferably 0.10% or more. On the other hand, if excessively contained the Ni Kasami alloy cost, also, there are cases where toughness of the heat affected zone of the steel sheet is degraded. Therefore, Ni content is preferably not more than 1.00%. Ni content is more preferably 0.50% or less, even more preferably 0.30% or less.
[0037]
　(Cr: 0 ～ 2.00
　Pasento) Cr is an optional element. Cr has the effect of promoting the formation of such martensite by increasing the hardenability of the steel. To obtain this effect, the Cr content is preferably made 0.02% or more. Cr content is more preferably 0.05% or more. On the other hand, if excessively contained the Cr, ferrite transformation in the cooling process after hot rolling ends up excessively delayed, there is a case where ferrite desired amount is difficult to obtain. Therefore, Cr content is preferably not more than 2.00%. Cr content is more preferably 1.50% or less, more further preferably 1.00% or less, particularly preferably not more than 0.50%.
[0038]
　(B: 0 ～ 0.0100 Pasento)
　B is an optional element. B, as well as increase the grain boundary strength, it has the effect of improving the toughness of the steel. Also, B has the effect of precipitation strengthening steel by nitride. To obtain these effects, B content that is preferably set to 0.0001% or more. B content is more preferably 0.0003% or more. On the other hand, the above effect is saturated even contain a B exceeding 0.0100% alloy cost increases. Therefore, B content is preferably set to 0.0100% or less. B content is more preferably 0.0050% or less, even more preferably 0.0030% or less, particularly preferably 0.0010% or less.
[0039]
　(Ca: 0 ～ 0.0100
　Pasento) Ca is an arbitrary element. Ca causes the number dispersing fine oxides in the molten steel has an effect of refining the metal structure of the steel sheet. Further, Ca is to fix the S in the molten steel as CaS spherical, by inhibiting the production of stretched inclusions such as MnS, has the effect of improving stretch flangeability of the hot-rolled steel sheet. To obtain these effects, the Ca content is preferably 0.0002% or more. Ca content is more preferably 0.0005% or more. On the other hand, when the Ca content exceeds 0.0100%, the CaO in the steel increases, adversely affecting the toughness of the steel sheet. Therefore, Ca content is preferably set to 0.0100% or less. Ca content is more preferably 0.0050% or less, even more preferably is 0.0030% or less.
[0040]
　(Mg: 0 ～ 0.0100
　Pasento) Mg is an optional element. Mg has the effect of forming an oxide or sulfide in the molten steel as with Ca, suppresses the formation of coarse MnS, it is dispersed a large number of fine oxides, refining the structure of the steel sheet. To obtain these effects, the Mg content is preferably 0.0002% or more. Mg content is more preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0100% increases the oxide in the steel, adversely affects the toughness of the steel sheet. Therefore, Mg content is preferably set to 0.0100% or less. Mg content is more preferably 0.0050% or less, even more preferably is 0.0030% or less.
[0041]
　(REM: 0 ～ 0.0100
　Pasento) REM is an optional element. REM likewise as Ca, to form an oxide or sulfide in the molten steel, to suppress the formation of coarse MnS, it is dispersed a large number of fine oxides, has the effect of refining the structure of the steel sheet. To obtain these effects, the REM content is preferably 0.0002% or more. REM content is more preferably 0.0005% or more. On the other hand, oxides in the steel increases the REM content exceeds 0.0100%, the adverse effect on the toughness of the steel sheet. Therefore, REM content is preferably set to 0.0100% or less. REM content is more preferably 0.0050% or less, even more preferably is 0.0030% or less.
　Here, the REM (rare earth), Sc, refers to a total of 17 elements consisting of Y and lanthanoid. In the present embodiment, the content of REM refers to the total content of these elements.
[0042]
　The chemical composition of the hot rolled steel sheet according to the present embodiment, other than the elements, Fe and impurities. An impurity in the present embodiment is a component to be mixed in the course of the components, or manufacturing contained in the raw material, means intentionally components not intended to be contained in the steel.
[0043]
　
　hot rolled steel sheet according to the present embodiment, at a depth position of 1/4 of the sheet thickness from the surface of the steel sheet, ferrite area ratio of the total area ratio of "bainite and martensite" defining "ferrite, bainite and martensite" total area ratio, the range of the average grain size. Here, ferrite, Area Ratio and crystal grain size of bainite and martensite, to define the steel structure of a depth position of 1/4 of the sheet thickness from the surface of the steel sheet, the depth position, the surface of the steel sheet in the an intermediate point between the thickness center position, except for the texture, the steel structure in the position, the representative of the steel structure of the hot-rolled steel sheet (showing the average steel structure of the entire hot-rolled steel sheet) is there.
[0044]
　In the present embodiment, the ferrite and, in addition to the polygonal ferrite comprises acicular ferrite and quasi-polygonal ferrite, bainitic ferrite constituting the ferrite or bainite structures constituting the pearlite structure does not include. Bainitic ferrite in the present embodiment, treated as bainite.
　Hereinafter will be described the reason for defining the respective range.
[0045]
　(Ferrite area ratio of 10 to 55%)
　soft ferrite phase is tissue required to obtain a good ductility of the steel sheet. The area ratio of ferrite is less than 10%, elongation of the steel sheet is lowered. Therefore, the area ratio of ferrite is 10% or more. Area ratio of ferrite is preferably 15% or more. On the other hand, if ferrite is excessively deposited, the chemical composition of the steel sheet according to the present embodiment, it is difficult to obtain a tensile strength of at least 950 MPa. Therefore, the area ratio of ferrite is not more than 55%. Area ratio of ferrite is preferably less than 40%, more preferably 38% or less, particularly preferably 36% or less.
[0046]
　(Total area ratio of bainite and martensite: 45 ~ 90%)
　hard bainite or martensite is a tissue required to obtain a high strength. The total area ratio is less than 45% of bainite and martensite, it is difficult to obtain a tensile strength of at least 950MPa with the chemical composition of the steel sheet according to the present embodiment. Therefore, the total area fraction of bainite and martensite is 45% or more. The total area fraction of bainite and martensite is preferably 60 percent, more preferably 62% or more, further preferably 64% or more. In the present embodiment, the martensite, auto-tempered been tempered martensite is also included, the bainite includes bainitic ferrite. On the other hand, if the total area fraction of bainite and martensite is more than 90%, insufficient ferrite area ratio of elongation can not be obtained formability of the steel sheet decreases. Therefore, the total area fraction of bainite and martensite is 90% or less. The total area fraction of bainite and martensite is preferably 85% or less.
[0047]
　(Ferrite, the total area fraction of bainite and martensite: 90% or more (other tissues: 10% or less))
　ferrite, other than the bainite and martensite, as other tissue is any tissue, residual austenite, pearlite and grain boundary there is such as cementite. When the area ratio of the other tissue exceeds 10%, the these tissues can裂起point, stretch flangeability and low temperature toughness of the steel sheet is lowered. Therefore, the area ratio of the other tissues is 10% or less. Area ratio of other organizations, preferably 8% or less, more preferably 5% or less. The area ratio of other organizations may be 0%. In other words, ferrite, and martensite and the total area ratio of bainite is 90% or more, preferably 92% or more, more preferably 95% or more, may be 100%.
　Residual in other tissues austenite, it significantly degrades the stretch flangeability of the steel sheet in the transformation very hard martensite by pre processing such as punching. Therefore, among other tissues, it is preferable that the area ratio of residual austenite of 3% or less. Area ratio of residual austenite, 2% and more preferably less, and even more preferably 1% or less, particularly preferably 0%.
[0048]
　: (Average crystal grain size 12.0μm or less)
　the average crystal grain size of the block size of the ferrite particle diameter and martensite and bainite is coarse, fracture unit at break is increased, the low temperature toughness of the steel sheet is reduced . Therefore, the average crystal grain size is not more than 12.0 .mu.m. The average grain size is preferably not more than 10.0 [mu] m, more preferably not more than 7.0 .mu.m. It lowers the average crystal grain size is preferably as small is not particularly limited. However, in a normal hot rolling average crystal grain size is comminuted as below 1.0μm is because it is technically difficult, in general is 1.0μm or more.
　Incidentally, the average crystal grain size in the present embodiment, one crystal structure of bcc, namely ferrite, bainite, martensite and 0.3μm or more regions in the crystal orientation difference 15 ° or more and a circle equivalent diameter of the pearlite grains means the average grain size was defined as the residual austenite, the grain size of grain boundary cementite is not included in the average grain size.
[0049]
　In the present embodiment, the average crystal grain size and area ratio of each organization, parallel steel cross-section in the rolling direction and the thickness direction, the tissue at 1/4 depth position of the sheet thickness from the surface of the steel sheet, a thermal field emission using EBSD analysis device constituted by a type scanning electron microscope and EBSD detector, scanning electron microscope (SEM) observation and EBSD (electron Back scattering diffraction: electron backscatter diffraction method) obtained by the analysis.
[0050]
　In SEM observation, the residual austenite is present between bainite and martensite lath or block and the packet, because it is difficult to distinguish residual austenite and bainite and martensite were measured included the bainite and martensite, ferrite, "bainite, martensite (and residual austenite)", and measuring the area ratio of the remaining structure (pearlite and grain boundary cementite).
[0051]
　The EBSD analysis, 200 [mu] m, distinguishes between fcc and bcc at 0.2μm intervals region of 100μm in the thickness direction was measured crystal orientation information to the rolling direction around the 1/4 depth position of the sheet thickness, EBSD using included software (AMETEK Inc. "OIM analysis (TM)") of the analysis device, a 0.3μm or more regions is defined as a crystal grain in the crystal orientation difference 15 ° or more and a circle equivalent diameter, fcc (residual determine the area ratio of austenite), also determine the average crystal grain size of the bcc by a method using the following Equation 1].
[0052]
　It was determined by SEM observation from the area ratio of "bainite, martensite (and residual austenite)", by subtracting the area ratio of the fcc (residual austenite) obtained by EBSD analysis to obtain the area ratio of bainite and martensite.
[0053]
　The average crystal grain size of bcc, determine the value calculated by the equation shown below [Equation 1]. Wherein, D is the average grain size, N is the number of crystal grains contained in the evaluation area of ​​the average crystal grain size, Ai is i-th (i = 1,2, ··, N) grain area, di indicates a circle equivalent diameter of i-th grain.
[0054]
[Number 1]

[0055]
　Boundaries having a crystal orientation difference of 15 ° or more is mainly ferrite grain boundary, a block boundary of martensite and bainite. JIS G 0552: In the measurement method of the ferrite grain size in compliance with 2013, there are cases where the crystal orientation difference from being calculated particle size also ferrite grains of less than 15 °, furthermore, a block of martensite and bainite is not calculated . Therefore, the average crystal grain diameter in the present embodiment, a value obtained by EBSD analysis.
[0056]
　
　hot rolled steel sheet according to the present embodiment, in the center of plate thickness of the steel sheet, {100} <011>, {211} <011>, {311} <011>, {110} <011> and {332} <113> maximum pole density of the orientation group, and {211} defining the total pole density of <011> and {332} <113>. The thickness center portion in the present embodiment, the range (from the surface of the steel sheet 1/2 depth position of the sheet thickness) thickness center position from about 1/10 of the respective plate thickness in Tables direction and back direction of the steel sheet means. For example, the thickness of the steel sheet if 2 mm, and the thickness center portion, means ranges of about 100μm bordering the thickness center position in front direction and the back direction.
　The reason for defining the texture in the sheet thickness center is because where the texture and mechanical properties of the center of plate thickness correlate well. The reason is not clear, but the inventors speculate as follows. Hot-rolled steel sheet, the friction between the roll and the steel sheet during rolling, reverse shear deformation occurs in the front and back of the steel sheet, the plane strain deformation occurs at the center of plate thickness. Texture of the hot-rolled steel sheet, plate change in the thickness direction in accordance with this variant, because the direction of shear deformation at the front and back of the steel sheet is reversed, the direction of the symmetrical front and back even texture develops. Therefore, the results cancel the influence of texture on the mechanical properties on the front and back, and the texture and mechanical properties of the center of plate thickness corresponding well.
[0057]
　({100} <011> at the center of plate thickness, {211} <011>, {311} <011>, the maximum pole of {110} <011> and {332} <113> orientation component group (main orientation component group) density: 8.0)
　{100} <011> as a main orientation to develop in the texture of the center of plate thickness of the hot rolled steel sheet, {211} <011>, {311} <011>, {110} <011 > and {332} is <113>. Even only one of these orientations group develops, tensile strength, yield strength of the hot rolled steel sheet, stretch-plane anisotropy of the various mechanical properties such as r value is increased, over the entire circumferential direction stretch flangeability be deformed are particularly significantly reduced. Thus, in this embodiment, to suppress the development of all these orientation component group, it is important that the texture becomes more random. In this embodiment, {100} <011> at the center of plate thickness, {211} <011>, {311} <011>, {110} <011> and {332} <113> each pole orientation group by calculating the density, obtaining the maximum value. Maximum pole density that is low, it means that the ratio of the tissue of the random orientation is high, {100} <011>, {211} <011>, {311} <011>, {110} <011> and {332} <113> orientation component group of texture means that not developed. Therefore, to 8.0 or less the maximum pole density of the orientation component group. Maximum pole density of the orientation component group is preferably 7.0 or less, more preferably 6.0 or less. The maximum electrode density of the orientation component group, because if you do not have a texture is 1.0, and more preferably close to 1.0.
　Pole density can be obtained by crystal orientation information by EBSD analysis, but synonymous with X-ray random intensity ratio.
[0058]
　(Total pole density of {211} <011> and {332} <113> at the center of plate thickness: 10.0)
　of the orientation component group mentioned above, especially {211} <011> and {332} <113 with the development of>, stretch flangeability of the steel sheet is remarkably lowered. Therefore, to 10.0 or less the sum of the pole density of {211} <011> and {332} <113>. {211} total pole density of <011> and {332} <113> is more preferably 8.0 or less. The pole total density is preferably as small as possible, because if you do not have the texture are each pole density of 1.0, and more preferably a value close to 2.0.
[0059]
　Incidentally, {hkl} crystal plane parallel to the rolling plane, representing the crystal direction parallel to the rolling direction. That is, the crystals are oriented is to the plate surface normal direction {hkl}, in the rolling direction and the {hkl} .
　Further, in this embodiment, the pole densities of the crystal orientations in the sheet thickness center, using scanning electron microscopy and EBSD analysis combining the device apparatus and AMETEK Co. OIM Analysis (TM), by EBSD analysis , the plate thickness center at (thickness center position from the (1/2 depth position from the surface of the plate thickness of the steel sheet), tables direction and each thickness about 1/10 the range back direction of the steel sheet), fcc and bcc to distinguish the door to measure the crystal grain orientation information of more than 1000 bcc, determined by ODF analysis using power series expansion method (harmonic series expansion).
[0060]
　
　(Tensile Strength: 950 MPa or more)
　hot rolled steel sheet according to the present embodiment has the control of steel structure and texture, high strength, and excellent low-temperature toughness, the elongation and stretch flangeability. However, the tensile strength of the hot rolled steel sheet is small, a small effects such as body weight and improved rigidity. Therefore, the tensile strength of the hot rolled steel sheet according to the present embodiment (TS) is not less than 950 MPa. The tensile strength is preferably 980MPa or more.
[0061]
　Elongation of hot-rolled steel sheet, JIS Z 2241: evaluated by a defined breaking total elongation to 2011 (El), it is preferable that TS × El as an index of the balance between the strength and elongation of 1400 MPa ·% or more, more preferably 15,000 MPa ·% or more.
　Stretch flangeability of the hot-rolled steel sheet will be described later | [Delta] r | in addition, JIS Z 2256: evaluated by a defined hole expansion rate 2010 (λ), TS × indicative of the balance between strength and stretch flangeability preferably λ is 50000 mPa ·% or more, more preferably 55000MPa ·% or more.
　Low temperature toughness of hot-rolled steel sheet, JIS Z 2242: fracture appearance transition temperature in a Charpy impact test specified in 2005 (vTrs) is preferably not -40 ℃ less.
[0062]
　The hot rolled steel sheet according to the present embodiment, from the viewpoint of obtaining sufficient stretch flangeability, which is an index of in-plane anisotropy of r value, | [Delta] r | (absolute value of [Delta] r) the smaller is preferred. plane anisotropy of r value | [Delta] r | is preferably 0.40 or less, and more preferably not more than 0.35, more preferably 0.30 or less, particularly preferably 0.25 or less . more plane anisotropy of r value small as possible preferably, 0 is most preferable.
　Δr is, (r 0 + r 90 -2 × r 45 represented by) / 2, r 0 : the rolling direction of the r value, r 90 : the rolling direction perpendicular to the r value, r 45 : 45 ° direction with respect to rolling direction which is the r value. In addition, r value means the Lankford (Lankford) value.
[0063]
　
　Next, explaining the reasons for limiting manufacturing conditions of hot-rolled steel sheet according to the present embodiment.
　The present inventors have found that hot-rolled steel sheet according to the present embodiment, it was confirmed that obtained by the production method comprising hot rolling and cooling as follows.
[0064]
　First, to produce a hot-rolled steel sheet is subjected to inter-multi-path heat rolling a slab having the chemical composition described above. Slab subjected to hot rolling, but may be those obtained by continuous casting and casting-blooming rolling, or may be they were added to hot working or cold working. Multi-pass hot rolling can be carried out using the Rebasumiru or tandem mill, from the viewpoint of industrial productivity, at least the last few stages of it is preferable to use a tandem mill.
[0065]
　(Heating temperature in hot rolling: 1150 ~ 1350 ℃)
　The temperature of the slab or billet to be subjected to hot rolling is less than 1150 ° C., becomes insufficient solution of Ti carbide, strength and workability of the steel sheet is lowered. On the other hand, when the temperature is 1350 ° C. than the slab or billet to be subjected to hot rolling, or cause a decrease in yield to produce thick scale, when heating the slab or billet in a heating furnace, significant in a heating furnace damage there is a case for or given. Accordingly, the temperature of the slab or billet to be subjected to hot rolling to 1150 ~ 1350 ° C..
　The temperature of the slab or billet to be subjected to hot rolling, sufficient if the above temperature range, was heated to the temperature range was charged into the heating furnace steel ingot or steel slab was less than 1150 ° C. in addition to the cases subjected to hot rolling, it may be subjected to hot rolling without performing a heat treatment while maintaining the high temperature state above 1150 ° C. the slab or slabbing the steel slab obtained obtained by continuous casting.
[0066]
　: (FT + 50 ° C. The total reduction ratio between super ~ FT + 150 ℃ 50% or higher)
　in the present embodiment, as the FT a finishing temperature in the unit ° C., by increasing the total reduction ratio in hot rolling between FT + 50 ° C. ultra ~ FT + 0.99 ° C. , it can be refined recrystallized austenite grains in the steel sheet. To refine the recrystallized austenite grains in the steel sheet, the total reduction ratio between FT + 50 ° C. ultra ~ FT + 0.99 ° C. is 50% or more. The total rolling reduction is less than 50% in the temperature range, since the austenite is not sufficiently fine, together with the tissue becomes coarse after transformation, the recrystallization rolling pass during rolling between subsequent FT ~ FT + 50 ℃ by delay, texture after transformation will be developed. Although total reduction rate is preferably as high among FT + 50 ° C. ultra ~ FT + 0.99 ° C., since the industrially limited to about 90%, may be less than 90%.
[0067]
　(FT ~ FT + 50 total reduction ratio between ° C.:
　　40 ~ 80%) (FT ~ FT + 50 ° C. between the time required for rolling: 0.5 to 10 seconds)
　in the present embodiment, the total reduction ratio and between FT ~ FT + 50 ℃ by properly controlling the time required for rolling can I cooling conditions coupled with after hot rolling to be described later, to obtain a good hot-rolled steel sheet and workability and toughness.
　The total reduction rate is less than 40% between FT ~ FT + 50 ℃, structure becomes coarse after transformation, recrystallization with delays after rolling pass and between the rolling finishing, deformation of the inner steel plate becomes uneven, transformation specific orientation that would develop, stretch flangeability of the steel sheet is lowered later. Therefore, the total reduction ratio between FT ~ FT + 50 ℃ is 40% or more. On the other hand, the total reduction ratio in the temperature range exceeds 80%, the recrystallized texture even if significantly developed, stretch flangeability of the steel sheet is lowered. Therefore, the total reduction ratio between FT ~ FT + 50 ℃ is 80% or less.
　In the present embodiment, furthermore, it is important to properly control the time required for the rolling of said temperature range. If the time required for the rolling of the temperature range is too short will be rolling strain is accumulated excessively without proceeding recrystallization between passes, the desired texture by specific orientation develops becomes difficult to obtain. Therefore, the time required for the rolling of the temperature range is set to more than 0.5 seconds. Preferably at least 1.0 seconds, more preferably not less than 2.0 seconds. On the other hand, if the time required for the rolling of the temperature range is too long, the recrystallized grains in rolling paths will be grain growth structure after transformation becomes coarse. Therefore, the time required for the rolling of the temperature range is not more than 10.0 seconds. Preferably 8.0 seconds or less, and more preferably not more than 6.0 seconds.
[0068]
　FT + 50 ° C. rolling between super ~ FT + 0.99 ° C., in any of the rolling of the rolling between FT ~ FT + 50 ℃, since is possible to repeat the processing and recrystallization is critical, 2-pass or 3 passes or more in each of the temperature range subjected to rolling. Largest in terms of development inhibiting texture of the steel sheet, rolling reduction per pass between FT + 50 ° C. ultra ~ FT + 0.99 ° C. is preferably 60% or less, more preferably 55% or less. Maximum per pass reduction rate between FT ~ FT + 50 ℃ is preferably 50% or less, 45% and more preferably less, more preferably 40% or less, and most preferably 35% or less.
　Note that the total reduction ratio, relative to the inlet plate thickness before the first pass at a given temperature range, the total rolling reduction in a temperature range (the temperature range first pass before the inlet plate thickness Metropolitan in rolling the percentage of the difference between the exit thickness Metropolitan after the final pass in the rolling temperature range).
[0069]
　(Finishing temperature FT: Equation (1) sought Ar 3 or more, and the formula (2) obtained TR or more and 1100 ° C. or
　less) Ar 3 (° C.) = 901-325 × [C] + 33 × [Si] -92 × [Mn] + 287 × [P] + 40 × [Al]
　(1) TR (° C.) = 800 + 700 × [Ti] + 1000 × [Nb] (2)
　However, the above formula (1) and (2) in the atomic symbol represents the content by mass percent of the respective elements.
　In the present embodiment, by repeating the processing and recrystallization of the austenite phase in the finish rolling, achieving suppression of development of the texture as well as structure finer. Therefore, the finishing temperature FT is, Ar obtained by Equation (1) 3 or more, and a more TR obtained by equation (2). Here, the finishing temperature FT, refers to a surface temperature of the steel sheet after final rolling.
[0070]
　FT is Ar 3 is less than, ferrite transformation proceeds during finish rolling, deformed ferrite that is generated by elongation and stretch flange formability of the steel sheet is lowered. Also, the FT is less than TR, the austenite becomes remarkably flat before after hot rolling cooling, the hot rolled steel sheet of the final product, becomes elongated in the rolling direction tissue, by plastic anisotropy increases, elongation and stretch flangeability is degraded. By the FT or more TR, moderately encourage recrystallization of worked austenite in the rolling path, can be finer recrystallized austenite grains achieved, after the hot rolling, after hot rolling to be described later of I cooling conditions coupled with, can be obtained is hot-rolled steel sheet having a suitable steel structure and texture in the low-temperature toughness and stretch flangeability. FT is preferably TR + 20 ° C. or higher, more preferably TR + 40 ° C. or higher.
　On the other hand, when FT exceeds 1100 ° C., the tissue ends up coarsening, low temperature toughness of the steel sheet is lowered. Therefore, FT is set to 1100 ° C. or less. Preferably 1080 ° C. or less, more preferably 1060 ° C. or less. The temperature during the finish rolling refers to the surface temperature of the steel material, it can be measured by a radiation thermometer or the like.
[0071]
　In this embodiment, the FT within a predetermined range, and the time required for rolling between the above-described FT ~ FT + 50 ° C. By within a predetermined range, rolling path in a predetermined temperature range and after the finish rolling recrystallization austenite is promoted, weakened ferrite transformation promoting effect by processing austenite, the area ratio of ferrite can be controlled to 55% or less. Furthermore, it is possible to refine the austenite grain size, with fine grain size is obtained, by recrystallization of austenite progresses, it can facilitate a reduction in electrode density.
[0072]
　(After completing the finish rolling, the time until the start of water cooling: 3.0 seconds or less)
　after the finish rolling is completed, since by utilizing strain accumulated by rolling miniaturization of tissue within 3.0 seconds water-cooled to a start to. The water cooling may be performed in a plurality of stages. After the finish rolling completion, the time until the water-cooling start is 3.0 seconds more than will to recover the strain in austenite, it becomes difficult to obtain the desired tissue. After finish rolling completion, the time until the start of water cooling, preferably within 2.0 seconds, more preferably within 1.0 seconds, more preferably within 0.5 seconds. After finish rolling completion, the time until the start of water cooling, preferably not less than 0.05 seconds to recrystallize the austenite after finish rolling completion.
[0073]
　: (FT ~ average cooling rate of 750 ° C. 20 ° C. / sec or more)
　the hot-rolled steel sheet after finish rolling completion temperature completing the finish rolling: Mean when cooled from (finishing temperature FT (° C.)) to 750 ° C. the cooling rate is an important process conditions in order to obtain the desired tissue. Incidentally, the average when the calculation of the cooling rate, as the time after the completion of finish rolling, includes the time to water-cooling start. If the average cooling rate in the temperature range is lower than 20 ° C. / sec, a fine tissue formation is difficult, and ferrite or pearlite during cooling precipitates, stretch flangeability and low temperature toughness of the steel sheet is lowered. Therefore, the average cooling rate in the temperature range to 20 ° C. / sec or more. Preferably 30 ° C. / sec or more, more preferably 40 ° C. / sec or more. The upper limit is not particularly necessary to limit, it is preferable from the viewpoint of thermal strain by the plate warp suppressing at 300 ° C. / sec or less.
[0074]
　Further, in a temperature range of FT ~ 750 ° C., by rapid cooling of the hot zone after the finish rolling end, it is possible to structure finer, low temperature toughness of the steel sheet is further improved. For this purpose, after completing the finish rolling, to start water cooling within 3.0 seconds, an average cooling rate of FT ~ 750 ° C. In addition to the 20 ° C. / sec or more, up to FT ~ FT-40 ℃ preferably to an average cooling rate 100 ° C. / sec or more. In this case, the water cooling is not preclude the performed in a plurality of cooling steps of performing steps and subsequent cooling for the purpose of quenching temperature range of FT ~ FT-40 ℃. The average cooling rate of FT ~ FT-40 ℃ the above effect becomes difficult to obtain less than 100 ° C. / sec. The average cooling rate of FT ~ FT-40 ℃ is preferably 120 ° C. / sec or more, more preferably 0.99 ° C. / sec or more. The upper limit is not critical and it is preferred from the viewpoint of the temperature variation suppression in the steel sheet is 1000 ° C. / sec or less.
[0075]
　Quenching of the high temperature zone after the finish rolling completion of above (cooling FT ~ FT-40 ℃) is not limited to after the final stand of the finish rolling may be performed between the rolling stands. In other words, it is not rolling stand after the rapid cooling, or for the purpose of such straightening and cooling control, the rolling reduction may be added to rolling than 8%. In this case, rolling after quenching is not included in the finish rolling process.
[0076]
　(750 ~ 600 ° C. residence time: 5-20 seconds)
　hot-rolled steel sheet after finish rolling reaches a temperature range of 750 ~ 600 ° C., transformation from austenite to ferrite becomes active. Therefore, the hot-rolled steel sheet by staying over 5 seconds in the above temperature range, to promote transformation to ferrite from austenite to obtain the desired ferrite area ratio. When the residence time in the temperature range is less than 5 seconds, the transformation from austenite to ferrite does not proceed sufficiently, to obtain the desired ferrite area ratio becomes difficult. Therefore, the residence time in the temperature range is 5 seconds or more. Preferably it is greater than or equal to 7 seconds. On the other hand, when the residence time in the temperature range exceeds 20 seconds, ferrite or excessively precipitated, pearlite and cementite or precipitation. Therefore, the residence time in the temperature range is not more than 20 seconds. Preferably 17 seconds or less, and more preferably not more than 14 seconds.
　In the present embodiment, the residence time of 750 ~ 600 ° C., after reaching the temperature of hot-rolled steel sheet after finish rolling within 750 ° C., indicates the time until the temperature reached 600 ° C. decreased, this not necessarily always be cooled steel sheet at the time range.
[0077]
　(Average cooling rate from 600 ° C. to a cooling stop temperature of less than Ms: 20 ° C. / sec or
　more) Ms (° C.) = 561-474 × [C] -33 × [Mn] -17 × [Ni] -21 × [ Mo] (3)
　a 750 to untransformed austenite remaining in the hot-rolled steel sheet after stay 5-20 seconds to a temperature range of 600 ° C., to obtain a tensile strength of at least 950MPa by transformed into martensite and bainite for the, and the average cooling rate from 600 ° C. to the above formula (3) cooling stop temperature below the martensitic transformation start temperature Ms of the formula 20 ° C. / sec or more. The average cooling rate in the temperature range below 20 ° C. / sec, the desired tissue is difficult to obtain by such pearlite is excessively formed during cooling, resulting 950MPa or more tensile strength is difficult to obtain. Average cooling rate in the temperature range is preferably 40 ° C. / sec or more, more preferably 50 ° C. / sec or higher. The upper limit of the average cooling rate is not specifically limited in the above temperature range, from the viewpoint of thermal strain by the plate warp suppressing, preferably 300 ° C. / sec or less.
[0078]
　(Average cooling rate from Ms to the cooling stop temperature of less than Ms: 80 ° C. / s or higher)
　in order to increase the tensile strength of the steel sheet, the average cooling rate from Ms to the cooling stop temperature below Ms 80 ° C. / sec it is preferable that the above. More preferably 100 ° C. / sec or more, even more preferably 120 ° C. / sec or more. The upper limit is not necessary to particularly limited, it is preferable from the viewpoint of uniformity in the thickness direction of the tissue at 500 ° C. / sec or less. More preferably at most 400 ° C. / sec. The cooling stop temperature of less than Ms is preferably Ms-20 ° C. or less, and more preferably not more than Ms-50 ° C..
[0079]
　After cooling to cooling stop temperature below Ms is generally performing winding.
　When manufacturing a hot rolled steel sheet according to the present embodiment, for example, straightening may be applied appropriately known temper rolling for the purpose. Furthermore, the present invention may be plated steel sheet plated. Plating may be either electroplating and hot dip plating is not particularly limited plating species, is generally a zinc-based plating containing zinc plating and zinc alloy plating. Examples of the plated steel sheet, electro-galvanized steel sheets, electrolytic zinc - nickel alloy plated steel sheet, galvanized steel sheet, galvannealed steel sheets, fused zinc - aluminum alloy coated steel sheet is exemplified. Coating weight may be a conventional amounts.
[0080]
　The thickness of the hot rolled steel sheet according to the present embodiment is not particularly limited, if the thickness is too thick, since the tissue to be generated in the steel sheet surface layer and the inner differ significantly, preferably not more than 6.0mm . Meanwhile, since the plate thickness is too thin and difficult passage plate of hot rolling is, in general, preferably at least 1.0 mm. More preferably, 1.2 mm or more, more preferably 1.5mm or more.
Example
[0081]
　After the chemical composition shown in Table 1 Steel having (% by weight) was cast and melted, and a slab of 30mm thickness by hot forging. The resulting heated steel slab at a test compact tandem mill, subjected to multiple rolling also two to four passes in any of rolling rolling and FT ~ FT + 50 ℃ between FT + 50 ° C. ultra ~ FT + 0.99 ° C., subjected to hot rolling under the conditions shown in Table 2-1 and Table 2-2, was finished to a thickness of 2.5 ~ 3.5 mm. Shows the production conditions are shown in Table 2-1 and Table 2-2. Incidentally, it underlined bold indicates outside the scope of the present invention.
[0082]
[Table 1]

[0083]
[table 2-1]

[0084]
[Table 2-2]

[0085]
　The obtained hot rolled steel sheets, by scanning electron microscopy and EBSD analysis, observing the steel sheet cross section parallel to the rolling direction and the thickness direction, the area of the tissue at the 1/4 depth position of the sheet thickness from the steel sheet surface the average crystal grain size and, and to determine the pole density of each crystal orientation of the center of plate thickness.
　The sample used in the EBSD analysis, after mirror polishing, to remove work strain in the surface layer of the observation surface by electrolytic polishing. EBSD analysis, using the EBSD analysis apparatus is composed of a thermal field emission scanning electron microscope and EBSD detector, 200 [mu] m in the rolling direction around the 1/4 depth position of the sheet thickness from the surface of the steel sheet, plate in 100μm region of the thickness direction, distinguishes between fcc and bcc at intervals of 0.2μm to measure crystal orientation information, using the supplied software (AMETEK Inc. "OIM analysis (TM)") of the EBSD analysis device , misorientation 15 ° or more and a 0.3μm or more regions in the circle equivalent diameter is defined as the crystal grain was determined area ratio of the average grain size and fcc of bcc (residual austenite). Note that the average crystal grain size of bcc, was determined value calculated by the formula shown below Equation 1]. Wherein, N is the number crystal grains contained in the evaluation area of the average crystal grain size, Ai is i-th (i = 1,2, ··, N ) grain area, di is the i th grain shows a circle equivalent diameter.
[0086]
[Number 1]

[0087]
　Ferrite, the area ratio of "bainite, martensite (and residual austenite)", and the remaining structure (pearlite and grain boundary cementite) was determined by SEM observation. Here, the residual austenite is present between lath and blocks of bainite and martensite, since it was difficult to distinguish the residual austenite and bainite and martensite, the "bainite, martensite (and residual austenite)" included. It was determined by SEM observation from the area ratio of "bainite, martensite (and residual austenite)", by subtracting the area ratio of the fcc (residual austenite) obtained by EBSD analysis to obtain the area ratio of bainite and martensite.
[0088]
　Using the same apparatus, the thickness center portion, the EBSD analysis, the center of plate thickness from (the thickness center position (1/2 depth position of the sheet thickness from the surface of the steel sheet), front direction and the back direction of the steel sheet in the ranges of thickness about 1/10), distinguishes between fcc and bcc measured grain orientation information 4500 to 5500 amino bcc, pole densities of the crystal orientation by ODF analysis using the series expansion method I was asked.
[0089]
　In order to evaluate the mechanical properties of hot-rolled steel sheet, the tensile strength TS (MPa), breaking total elongation El (%) is, JIS Z 2241: comply with the 2011, r value, | Δr | as, JIS Z 2254: 2008 conform to, stretch flangeability, JIS Z 2256: was assessed by rate widened bore which is measured according to 2010 λ (%). Low temperature toughness was evaluated by fracture appearance transition temperature vTrs (℃), JIS Z 2242 : conforms to 2005, subjected to Charpy impact test using a V-notch test pieces obtained by processing the steel sheet to 2.5mm subsize specimen It was evaluated Te.
　Table 3-1 and the steel structure in Table 3-2 shows the investigation results of the texture and mechanical properties. Incidentally, {100} <011>, {211} <011>, {311} <011>, the maximum electrode density of {110} <011> and {332} <113> orientation component group is 3-1 and Table Table shows a "maximum pole density in the main orientation component group" in 3-2.
[0090]
　Tensile strength, and as acceptable if more than 950MPa as a high strength, | [Delta] r | is as acceptable as excellent stretch flangeability the case of 0.40 or less, vTrs (° C.), the low temperature -40 ℃ less It was passed as excellent toughness. Workability, break total elongation El, | [Delta] r | other indicators, strength - was also assessed by stretch flangeability balance (TS × λ) - breaking total elongation balance (TS × El) and strength. TS × El (MPa ·%) is evaluated as acceptable as excellent in the case of more than 14000MPa ·% to have elongation high strength, TS × λ (MPa ·%) is located where it is 50000 mPa ·% or more with high strength It was passed as excellent stretch flange formability.
[0091]
[Table 3-1]

[0092]
[Table 3-2]

[0093]
　Table 3-1 and Table 3-2, in the invention examples in accordance with the present invention, tensile strength of at least 950 MPa, 0.40 following | [Delta] r |, to have -40 ℃ following vTrs from the strength, stretch flangeability, it can be seen that excellent low-temperature toughness. Furthermore, 14000MPa ·% or more TS × El, since it has a 50000 mPa ·% or more TS × lambda, it can be seen that combines the high strength and elongation and stretch flangeability. According to the present invention, the strength, elongation, and can be obtained excellent hot-rolled steel sheet stretch flangeability and low temperature toughness.
　In contrast, the chemical composition, the steel structure other than the texture, or Comparative Example texture is outside the scope of the present invention, the strength, elongation is inferior either stretch flange formability and low-temperature toughness.

The scope of the claims

[Requested item 1]By
　mass%,
　C: 0.02
　~ 0.20%, Si: 0.005 ~ 2.00%, Mn: 1.30
　~ 2.40%, P: 0.100% or
　less, S: 0.0100 % or
　less, Sol.
　Al:
　0.001 ~ 1.00%, Ti: 0.030 ~
　0.200%, N: 0.0010 ~ 0.0100%, Nb: 0
　~ 0.100%, V: 0 ~ 0.50% , 　Mo:
　0 ~ 　0.50%, Cu: 0 ~ 1.00%, Ni: 0 ~ 　1.00%, Cr: 0 ~ 2.00%, B: 0 ~ 　0.0100%, Ca: 0 ~ 　% 0.0100, Mg: 0 ~ 0.0100% and 　REM: 0 ~ 0.0100% 　contained, has a chemical composition the balance being Fe and impurities, 　1/4 depth position of the sheet thickness from the surface in, the area ratio of 10 to 55% of ferrite, a total area ratio of 45 to 90% of bainite and martensite, and in the ferrite, the total area fraction of the bainite and the martensite is 90% or more, average crystal grain size is less 12.0 .mu.m,

　Maximum in texture measured at the center of plate thickness, {100} <011>, {211} <011>, {311} <011>, the {110} <011> and {332} <113> orientation component group pole density 8.0 or less, and the sum of the pole density of {211} <011> and {332} <113> is 10.0 or less,
　hot-rolling, wherein the tensile strength is not less than 950MPa steel sheet.
[Requested item 2]
　The chemical composition, by
　mass%,
　Nb: 0.001
　~ 0.100%, V: 0.005 ~ 0.50%, Mo: 0.001
　~ 0.50%, Cu: 0.02 ~ 1.
　% 00,
　Ni: 0.02 ~ 1.00%, Cr: 0.02 ~ 2.00%, and
　B: 0.0001 ~ 0.0100%
　containing one or more compounds selected from the group consisting of hot-rolled steel sheet according to claim 1, characterized in that.
[Requested item 3]
　The chemical composition, by
　mass%,
　Ca: 0.0002 ~ 0.0100%, Mg: 0.0002 ~ 0.0100%, and
　REM: 0.0002 ~ 0.0100%
　1 kind selected from the group consisting of or hot-rolled steel sheet according to claim 1 or claim 2, characterized in that it contains two or more.
[Requested item 4]
　[Delta] r | | is hot rolled steel sheet according to any one of claims 1 to 3, characterized in that 0.35 or less the absolute value of in-plane anisotropy of r value.
　However, [Delta] r = (r 0 + r 90 -2 × r 45 ) / 2
is, r 0 : the rolling direction of the r value, r 90 : the rolling direction perpendicular to the r value, r 45 : 45 ° direction with respect to rolling direction which is the r value.
[Requested item 5]
　In producing the hot-rolled steel sheet according to any one of claims 1 to 4, the multi-pass hot rolling a slab or steel strip having a chemical composition according to any one of claims 1 to 3 subjecting a method for producing a hot-rolled steel sheet to produce a hot-rolled steel sheets,
　the heating temperature in the multi-pass hot rolling and 1150 ~ 1350 ° C.,
　when expressed as FT finishing temperature in the unit ° C., the FT + 50 ° C. than ~ the FT + 0.99 a total reduction ratio between ° C. 50% or higher, the FT-the FT + 50 the total reduction ratio between ° C. 40 ~ 80%, the FT-the FT + 50 ° C. 0.5 - the time required for rolling between 10. and 0 seconds,
　the FT + 50 ° C. in each of the temperature range of ultra-the FT + 0.99 ° C. and the FT-the FT + 50 ° C. performs rolling over two passes,
　the FT, the formula (1) obtained Ar 3 or more, and wherein (2) sought TR above, and after completing the finish rolling as 1100 ° C. or less, the water cooling started within 3.0 seconds, an average cooling rate of the FT ~ 750 ° C.
　more,and 750 after staying 5-20 seconds in a temperature range of ~ 600 ° C.,
　and characterized in that cooling from 600 ° C. to a cooling stop temperature of less than Ms obtained by the equation (3), the average cooling rate of 20 ° C. / sec or higher manufacturing method of hot-rolled steel sheet to be.
　Ar 3 (° C.) = 901-325 × [C] + 33 × [Si] -92 × [Mn] + 287 × [P] + 40 × [Al]
　(1) TR (° C.) = 800 + 700 × [Ti] + 1000 × [ Nb] (2)
　Ms (℃) = 561-474 × [ C] -33 × [Mn] -17 × [Ni] -21 × [Mo] (3)
　where each element symbol in the above formula (1) to (3) in the indicates the content by mass percent of the respective elements.
[Requested item 6]
　Method for manufacturing a hot-rolled steel sheet according to claim 5, characterized in that the average cooling rate of 80 ° C. / s or more until the cooling stop temperature of less than the Ms from the Ms.
[Requested item 7]
　After finish rolling completion, water cooling was started within 0.3 seconds, the FT ~ claim 5 or 6 average cooling rate to FT-40 ° C. which is characterized in that for cooling is 100 ° C. / s or higher method for manufacturing a hot-rolled steel sheet according to.
[Requested item 8]
　Method for manufacturing a hot-rolled steel sheet according to claim 7, characterized in that the step of performing the average cooling cooling rate is 100 ° C. / s or more until the FT ~ the FT-40 ° C., performed between rolling stands.