0001]The present invention is suitable structural parts and skeleton of the car, as a material of the track frame, excellent strength and toughness, hot-rolled steel sheet excellent in isotropy of toughness, and a manufacturing method thereof.
　The present application, on August 9, 2017, claiming priority based on Japanese Patent Application No. 2017-154294, filed in Japan, the contents of which are incorporated here.
BACKGROUND
[0002]
　From the viewpoint of global environmental protection, automobile exhaust emission regulations and is enhanced, improved fuel efficiency is an issue. Under such circumstances, it has been required a high strength and thinning of automotive steel sheets, as a material for automotive parts, in particular high-strength hot-rolled steel sheet is applied positively. In particular, high-strength hot-rolled steel sheet having the above tensile strength 1180MPa is attracting attention as a material that can greatly increase the fuel efficiency of automobiles.
[0003]
　However, in general, with an increase in the strength of a steel sheet, toughness decreases. Therefore, to impart toughness required as automobile parts, various studies have been made.
[0004]
　For example, Patent Document 1, by mass%, C: 0.05 ~ 0.20%, Si: 0.60% or less, Mn: 0.10 ~ 2.50%, sol. Al: 0.004 ~ 0.10%, Ti: 0.04 ~ 0.30%, B: containing from 0.0005 to 0.0015%, a slab containing the balance of iron and inevitable impurities, at least 1100 ° C. the temperature range of up to solution temperature above 1400 ° C. or less of the heating temperature of the TiC was heated at 0.99 ° C. / h or more heating rate from the the dwelling time of the at heating temperature was less than 30 minutes 5 minutes, between subsequent heat rolling method for producing a high-strength hot-rolled steel sheet have been proposed. In Patent Document 1, as a precipitation strengthening element a small amount of Ti, and the ferritic structure is refined by utilizing solid solution B traces as a stabilizing element of the austenite to lower the transformation temperature in cooling, tensile strength high strength of about 1020 mPa, high-strength hot-rolled steel sheet having a high tenacity of about fracture appearance transition temperature -70 ° C. can be obtained is disclosed.
[0005]
　Patent Document 2, in mass%, C: 0.05 ~ 0.18%, Si: 1.0% or less, Mn: 1.0 ~ 3.5%, P: 0.04% or less, S: 0.006% or less, Al: 0.10% or less, N: 0.008% or less, Ti: 0.05 ~ 0.20%, V: 0.1 super to 0.3% containing the balance the slab consisting of iron and unavoidable impurities was heated to 1200 ° C. or higher, and rough rolling, the cumulative reduction rate at 1000 ° C. or less and 50% or more, the finish rolling end temperature 820 ° C. or higher, finishing and 930 ° C. or less It was subjected to hot rolling comprising rolling, to start cooling within 4.0 seconds, and cooled at an average cooling rate of 20 ° C. / s or higher, 300 ° C. or higher, by winding at 450 ° C. or less, mainly bainite and then, the average lath spacing of bainite laths 400nm or less, and an average major axis length of the lath or less 5.0μm With certain metal structure, the method of producing a high strength hot-rolled steel sheet has been proposed which is excellent in toughness.
[0006]
　Patent Document 3, by mass%, C: 0.08 ~ 0.25%, Si: 0.01 ~ 1.0%, Mn: 0.8 ~ 1.5%, P: 0.025% or less , S: 0.005% or less, Al: 0.005 ~ 0.10%, Nb: 0.001 ~ 0.05%, Ti: 0.001 ~ 0.05%, Mo: 0.1 ~ 1. 0%, Cr: 0.1 ~ 1.0%, B: containing from 0.0005 to 0.0050%, a slab containing the balance of iron and inevitable impurities, were heated to 1100 ~ 1250 ° C., finish rolling after the entry side temperature in the range of 900 ~ 1100 ° C., a finish rolling delivery temperature in the range of 800 ~ 900 ° C., subjected to cumulative rolling reduction of finish and 60-90% rolling in the recrystallization austenite region, immediately start cooling, in martensite critical cooling rate or cooling rate, within 30 seconds from the start of cooling (Ms + 50 ° C.) then cooled to below the cooling stop temperature, and then (after keeping 10 to 60 seconds at a temperature range of the cooling stop temperature ± 100 ° C.), by winding, martensite or tempered martensite, and a main phase, the aspect ratio of prior austenite grains in the rolling direction cross-section has a metal structure is 3 to 18, the method of producing a high strength hot-rolled steel sheet excellent in low temperature toughness has been proposed.
CITATION
Patent Document
[0007]
Patent Document 1: Japanese Patent Laid-Open 5-345917 discloses
Patent Document 2: Japanese Patent 2014-205889 JP
Patent Document 3: Japanese Patent 2011-52321 JP
Summary of the Invention
Problems that the Invention is to Solve
[0008]
　However, in the technology described in Patent Documents 1 and 2, a metal structure ferrite or bainite entity, it may be difficult to manufacture a hot rolled steel sheet having both high strength and high toughness.
　In the technique described in Patent Document 3, Nb, Ti, an addition of Mo and Cr is mandatory, not preferable from the viewpoint of economy, also parallel to the direction (L direction) to the rolling direction of the steel sheet and plate not excellent toughness in both the width direction parallel to the direction (C direction), there is a case anisotropy in toughness is large.
　The present invention shows the above-mentioned solves the problem of the prior art, excellent in strength and toughness, and to provide a relatively low-alloy hot-rolled steel sheet and a manufacturing method thereof excellent in isotropic toughness.
Means for Solving the Problems
[0009]
　The present inventors have found that in order to solve the above problems, the various factors on the toughness of high strength hot rolled steel sheet, and intensive studies. As a result, the present inventors found that the particle size measurement by conventional image analysis as described in Patent Document 3, when the metal structure is a complex tissue including martensite, and the anisotropy of the aspect ratio and toughness of the particle size was knowledge that you can not take the correlation of. Accordingly, the present inventors have focused on sectioning of measuring grain length of one-dimensional for the sample cross-section, the average length crystal grains in the direction parallel to the rolling direction (L direction), parallel to the plate width direction a direction to calculate the average length grain of (C direction), were investigated and their ratio and toughness of anisotropy was found that show their strong correlation. Specifically, the present inventors have defined misorientation of crystal grains adjacent to more than 5 ° 1 single crystal grain, when calculated in section method, the direction parallel (L direction to the rolling direction ) and the average length of the crystal grains is 0.2μm or more 5.0μm less, the crystal grain average length of a direction parallel to the sheet width direction (C direction) is at 0.1μm or 5.0μm or less, and, the ratio of the average length of the L direction of the grain (L direction grain length) and C direction of the crystal grains having an average length (C direction particle length), 0.2 ≦ C direction particle length / L direction grain a length ≦ 5.0, by having a metal structure of martensite main phase, the tensile strength 1180MPa have more strength, and that excellent hot-rolled steel sheet to an isotropic toughness and toughness is obtained and knowledge.
[0010]
　Further, the present inventors have found that in order to produce a hot rolled steel sheet having a metallic structure described above is, C, Si, Mn, P , S, Al, N, and Ti in terms of adjusted with proper range, respectively, Not cumulative rolling reduction in the recrystallization γ region is 70% or more and interpass time is 10.0 seconds 0.2 seconds, in each pass, a value 0.05 ≦ represented by the following formula (1) after having been subjected to finish rolling satisfies a ≦ 23.0, immediately start cooling, coiling temperature 300 ° C. important that wound below in martensite critical cooling rate V (℃ / s) or more cooling rate It was found that is.
[Equation 1]

　where n in the above formula (1) is a roll rotation speed (rpm), r is the reduction ratio (%), H is the rolling entry side thickness (mm).
[0011]
　The present invention is based on the above findings, it has been completed by repeatedly further study. That is, the gist of the present invention is as follows.
[1] hot rolled steel sheet according to one embodiment of the present invention, chemical components, by
mass%, C: 0.06% to 0.20% or
less, Si: 1.0% or
less, Mn: 1.5% ultra 3.5% or
less, P: 0.040% or
less, S: 0.004% or
less, Al: 0.10% or
less, N: 0.004% or
less, Ti: 0.04% or more 0.20% hereinafter,
Nb: 0% or more than
0.04%, Mo: 0% to 1.0% or
less, Cu: 0% to 0.5% or
less, Ni: 0% to 0.5% or less,
containing, the balance being Fe and impurities,
　Metal structure in the position and sheet width center of the sheet thickness 1/4 depth, and 90% by volume or more of martensite, consists of a 0 vol% to 10 vol% of the remaining structure, the remaining structure is bainite or ferrite and one or both, the average length of the L direction of the crystal grains is a direction parallel to the rolling direction is at 0.2μm or 5.0μm or less, is a direction parallel to the plate width direction C of the and the direction of 0.1μm or more 5.0μm or less the average length of the crystal grains, and, in the L direction of the crystal grains having an average length of an average L direction particle length is the length of the crystal grains and the C direction there ratio of the C direction particle length is 0.2 ≦ C direction particle length / L direction particle length ≦ 5.0,
　the tensile strength is not less than 1180 MPa.
[0012]
[2] hot-rolled steel sheet according to [1], in the metal structure at the position and sheet width center of the sheet thickness 1/4 depth, the average length of the prior austenite grains of the L direction L direction old γ and grain length, wherein, when the average length of C direction of the former austenite grains and the C direction old γ grain length, the L direction old γ grain length and the C direction old γ ratio between the particle length is 0.03 ≦ C direction former γ may be a grain length / L direction old γ grain length ≦ 0.40.
[0013]
[3] hot-rolled steel sheet according to [1] or [2], the chemical composition, by mass%, Nb: 0.01% or more than 0.04%, Mo: 0.01% or more 1. 0% or less, Cu: 0.01% to 0.5% or less, Ni: 0.01% to 0.5% or less, may contain one or more selected from among.
[0014]
[4] The method for producing a hot rolled steel sheet according to another aspect of the present invention has the following of (a) ~ (d) Step:
a steel material consisting of chemical components described in (a) above [1] heating step is heated to 1200 ° C. or higher 1350 ° C. or less;
and (b) a said steel material after heating continuously to a plurality of rolling stands passed through the finish is rolling by the rolling process, the finish rolling start temperature 800 ° C. and above, the a value defined by the following formula (1) in the rolling stand of the is rolled so as to satisfy the 0.05 ≦ a ≦ 23.0, the interpass time between rolling stands 0.2 seconds and 10.0 seconds or more, further, the final rolling stand outlet side temperature 800 ° C. or higher 950 ° C. or less, and finishing the cumulative reduction rate at 800 ° C. or higher 950 ° C. or less and 70% or more rolling
step; (c) the cooling within 10.0 seconds after the finish rolling Started, and the following formula (2) and the following formula (3) martensite critical cooling rate V specified by the cooling process for cooling at (° C. / s) or more average cooling rate; and
(d) after cooling, the coiling temperature taking up winding process at 300 ° C. or less,
[Expression

　2] V = 10 2.94-0.75 　(beta-1) ... (2)
　beta = 2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + Mo ... ( 3)
　However, the n in the formula (1) is a roll rotation speed (rpm), r is the reduction ratio (%), H is the rolling entry side thickness (mm), C in the formula (3) , Si, Mn, Ni, Mo is the mass% of each element, Ni, if not containing Mo substitutes 0 wt% Ni, the section Mo.
The invention's effect
[0015]
　According to this aspect of the present invention is excellent in strength and toughness can be obtained hot rolled steel sheet of comparatively low alloy excellent in isotropic toughness. According to this aspect of the present invention, for example, ductile - brittle transition temperature has a high toughness below both -60 ° C. in a direction parallel (L direction) and the sheet width direction parallel (C direction) in the direction to the rolling direction hot-rolled steel sheet can be obtained. Thus, by applying the hot-rolled steel sheet according to the embodiment of the present invention structural components of an automobile, skeleton or to a frame or the like of the track, can reduce vehicle weight while ensuring the safety of motor vehicles, reducing the environmental impact it is possible.
　Further, according to the another aspect of the present invention, tensile has a high strength of at least strength 1180 MPa, and excellent hot-rolled steel sheet to an isotropic toughness and tenacity can stably produce, industrial remarkable it is possible to achieve the effect.
DESCRIPTION OF THE INVENTION
[0016]
　Hereinafter, the hot-rolled steel sheet and a manufacturing method thereof according to the present embodiment will be specifically described.
　First, the hot rolled steel sheet according to the present embodiment (hereinafter, sometimes simply referred to as steel sheet) will be described the reasons for limitation of the chemical components of. Incidentally,% represents the following chemical components, it means all mass%.
[0017]
C: 0.06% to 0.20% or less
　C improves the hardenability of steel, is an element necessary to obtain the strength of the hot-rolled steel sheet to produce a martensite is low temperature transformation phase. To obtain the desired strength is required C content of more than 0.06%. On the other hand, when the C content exceeds 0.20%, it degrades the workability and weldability of the steel sheet. Therefore, C content is 0.20% or less than 0.06%. Preferably, C content is 0.08% or more and is not more than 0.18%.
[0018]
Si: 1.00% or less
　Si suppresses the formation of coarse oxides and cementite degrading the toughness of the steel sheet, although an element which contributes to solid solution strengthening, Si content exceeds 1.00% and surface properties of the steel sheet is remarkably deteriorated, chemical conversion treatability and corrosion resistance decreases. Therefore, Si content is at most 1.00%. From the viewpoint of contributing to strengthening suppressing the formation of coarse oxides and cementite solid solution, Si content is preferably 0.01% or more, more preferably 0.40% or more. Further, Si content is preferably 0.80% or less.
[0019]
Mn: 1.5% greater than 3.5%
　Mn, together with a solid solution in the steel contributes to improving the strength of the steel, is an element to enhance hardenability. The In order to obtain such effects, it is necessary to 1.5 percent of Mn content. On the other hand, when the Mn content exceeds 3.5%, not only the effect is saturated, workability and delayed fracture resistance of the steel sheet to form a band-like structure by solidification segregation is reduced. Therefore, the Mn content is 1.5 percent 3.5% or less. Mn content is 1.8% or more, preferably at least 2.0%, also preferably not more than 3.0%.
[0020]
P: 0.040% or less
　P is an element which contributes to solid solution in the steel strength improvement of steel, grain boundaries, particularly segregates at prior austenite grain boundaries, lowering of low-temperature toughness and workability of the steel sheet there is also an element cause. Therefore, P content is preferably reduced as much as possible, it is preferable to 0%, P content of up to 0.040% is acceptable. Accordingly, P content is 0.040% or less. However, since the effect corresponding to the increase of even refining costs excessively reduced P content is not obtained, the P content is 0.003% or more, it is preferable that 0.005% or more. Also, P content is 0.030% or less, it is preferable that 0.020% or less.
[0021]
S: 0.004% or less
　S combines with Ti and Mn in the steel forms coarse sulfide is an element that reduces the workability of hot-rolled steel sheet. Therefore, S content is preferably reduced as much as possible, it is preferable to 0%, S content of up to 0.004% or is acceptable. Thus, S content is 0.004% or less. However, since the excessively reduced S content commensurate with the increase in the refining costs effect can not be obtained, the S content is 0.0003% or more, 0.0005% or more, it is 0.001% or more It is preferred. Further, S content is 0.003% or less, it is preferable that 0.002% or less.
[0022]
Al: 0.10% or less
　Al acts as a deoxidizer in steelmaking stage, is an element effective for improving the cleanliness of the steel. However, it causes an increase in oxide inclusions and is contained excessively Al, along with lowering the toughness of the hot-rolled steel sheet, causing flaws occurred. Therefore, Al content is 0.10% or less. Al content is 0.005% or more, preferably at least 0.01%, also preferably 0.08% or less.
[0023]
N: 0.004% or less
　N is precipitated in steel as nitride by binding to nitride formation elements, since an element which contributes to grain refinement, N content is 0.0005% or more it is preferable that the. However, N is the liable precipitates as coarse nitrides combined with Ti at elevated temperatures, coarse nitrides lowers the toughness of the hot-rolled steel sheet. Therefore, the N content is 0.004% or less. N content is more preferably 0.001% or more, and preferably 0.003% or less.
[0024]
Ti: 0.04% to 0.20% or less
　Ti, by refining the grain by forming a fine carbonitride in the steel, improve the strength and toughness of hot-rolled steel sheet. To express such effect, it is necessary that the Ti content is 0.04% or more. On the other hand, if the Ti content exceeds 0.20%, after which the effect is saturated, coarse precipitates by many precipitates in the steel, the toughness of the hot-rolled steel sheet is reduced. Therefore, Ti content is 0.20% or less than 0.04%. Ti content is 0.05% or more, preferably from 0.05 percent, also preferably from 0.10% or less.
[0025]
　The above is the basic component of the hot rolled steel sheet according to the present embodiment, the hot rolled steel sheet according to the present embodiment, for the purpose of further improving the toughness and high strength for example, Nb optionally, Mo, Cu and It may contain one or more selected from among and Ni. If that does not contain these elements, the lower limit of these elements is 0%.
[0026]
Nb: 0% or more than 0.04%
　Nb is an element that improves the strength of steel by forming a carbonitride. Such effects, in order to express the a Nb content is preferably set to 0.01% or more. On the other hand, when the Nb content exceeds 0.04%, the deformation resistance is increased, rolling load of hot rolling is increased during manufacture, rolling operation itself is difficult too large burden on the rolling mill there is a case to be. Further, when the Nb content exceeds 0.04%, the form of coarse precipitates in steel toughness of hot-rolled steel sheet is reduced. Therefore, Nb content is preferably 0.04% or less than 0.01%. Nb content is more preferably 0.02% or more and 0.03% or less.
[0027]
Mo: 0% to 1.0% or less
　Mo increases the hardenability of steel, an element which contributes to the strength of a steel sheet. The In order to obtain such effects, the Mo content is preferably set to 0.01% or more. However, Mo has a high alloy cost, the inclusion much Mo causes an increase in cost and weldability of the steel sheet when the Mo content exceeds 1.0% may decrease. Therefore, Mo content is preferably not more than 1.0% 0.01%. Mo content is more preferably 0.02% or more and 0.4% or less.
[0028]
Cu: 0% to 0.5% or less
　Cu is an element which contributes to solid solution in the steel for improving the strength of the steel. Further, Cu is also an element improving the hardenability. To obtain these effects, the Cu content is preferably set to 0.01% or more. However, there are cases where Cu content decreases the surface quality of the hot-rolled steel sheet exceeds 0.5%, chemical conversion treatability and corrosion resistance decreases. Therefore, Cu content is preferably 0.5% or less than 0.01%. Cu content is more preferably 0.05% or more and 0.3% or less.
[0029]
Ni: 0% to 0.5% or less
　Ni is dissolved in the steel contributes to the strength increase of the steel, also improving the hardenability. To obtain these effects, the Ni content is preferably set to 0.01% or more. However, Ni is higher alloy cost, the inclusion number of Ni causes an increase in cost and Ni content may decrease the weldability of the steel sheet exceeds 0.5%. Therefore, Ni content is preferably 0.5% or less than 0.01%. Ni content is more preferably 0.02% or more and 0.3% or less.
[0030]
　The elements other than the elements mentioned above, may be included in the steel sheet within a range which does not impair the effects of the present invention. In other words, the balance may be substantially iron. The steel sheet according to the present embodiment, for example, in order to improve the delayed fracture resistance, Ca, and REM, etc. may be contained 0.005% or less, respectively. Further, the steel sheet according to the present embodiment can also contain trace elements improve the hot workability and the like.
[0031]
　Next, a description will be given reasons for limiting the metallographic structure of the hot rolled steel sheet according to the present embodiment.
　Metal structure of the hot rolled steel sheet according to the present embodiment is one that includes martensite as a main phase, and more preferably consists of a single phase of martensite. If the orientation difference between adjacent grains is defined more than a 5 ° with one crystal grain, the average length of the crystal grains 0.2μm in the direction parallel to the rolling direction calculated by the intercept method (L direction) above 5.0μm or less, and the crystal grain average length of a direction parallel to the sheet width direction (C direction) is 0.1μm or more 5.0μm or less, the average length of the L direction of the grain (L direction the ratio of the grain length) and C direction of the crystal grains having an average length (C direction grain length) has a metal structure which is 0.2 ≦ C direction particle length / L direction particle length ≦ 5.0.
[0032]
　In the hot rolled steel sheet according to the present embodiment, when containing martensite as a main phase in the metal structure further includes a remaining structure. Further, if the metal structure is a single phase of martensite is not included in the remaining structure.
　Here, the "90% by volume or more of martensite" may contain only martensite than 90 vol%, both martensite and tempered martensite may be included in a total 90% or more by volume . Even aspects of both in the present embodiment, it is possible to secure the isotropy of excellent strength and toughness, there is no need to distinguish between martensite and tempered martensite.
　Incidentally, tempered martensite, there is the martensite is tempered, the dislocation density than martensite is lower martensite. Manufacturing method according to this embodiment described below does not include a heating step for the purpose of tempering after quenching, there is a case where tempered martensite is produced by recuperation after quenching and winding.
[0033]
　The term "main phase" in the present embodiment refers to a case where the phase is 90% or more by volume fraction. By martensite main phase, it is possible to obtain a desired high strength. Note that the remaining structure other than the main phase, comprising bainite and / or ferrite. When the volume fraction of the remaining structure is increased, the strength of the steel sheet is reduced, making it impossible to obtain the desired strength. Thus, the remaining structure is 10% or less in volume fraction. The remaining structure is preferably 5% or less, more preferably 1% or less.
[0034]
　The "single phase" in the present embodiment, merely forms the volume fraction of the phases of the "main phase" means that 100%. The volume fraction of the remaining structure when the metal structure is a single phase of martensite is 0%.
[0035]
　Measurement of the metal structure, first, the position and sheet width center of the plate thickness 1/4 depth of the hot-rolled steel sheet, as a section parallel is observed surface in the rolling direction and the sheet width direction, a scanning electron microscope a test piece for observation is taken. In this embodiment, the position of the sheet thickness 1/4 depth, a position advanced by the length of 1/4 of the plate thickness toward the thickness direction from the surface of the steel sheet. The observation surface after mirror polishing, and corroded in a 3% nital solution, using a scanning electron microscope, photographing a 3 field in 2000-fold magnification. Each measurement field of view and 500μm × 500μm. Thereafter, image processing to measure the area fraction of the type and metal structure of the metal structure. Since the area fraction and the volume fraction are substantially the same, the area fraction of each metal structure obtained, the volume fraction of each metal structure.
[0036]
　The hot rolled steel sheet according to the present embodiment, the metal structure of the position and sheet width center of the sheet thickness 1/4 depth, the average length of the crystal grains 0.2μm in a direction parallel to the rolling direction (L direction) above 5.0μm or less, is not less 0.1μm or 5.0μm or less grain average length of a direction parallel to the sheet width direction (C direction), and, L direction of the crystal grains having an average length ( L direction particle length) and C direction of the crystal grains having an average length (the ratio of the C direction grain length) is 0.2 ≦ C direction particle length / L direction particle length ≦ 5.0. L direction and / or the average length of the C direction of the crystal grains in the 5.0μm greater than the toughness of the L-direction and / or the C direction deteriorated. The average length of L direction of the crystal grains is less than 0.2 [mu] m, or when the average length of the C direction of the crystal grains is less than 0.1 [mu] m, the effect of toughness improvement by fine crystal grains is saturated. On the other hand, when the ratio of the L direction particle length and C direction particle length (C direction particle length / L direction grain length) is less than or 0.2 more than 5.0, the anisotropy of toughness increases, L direction and excellent toughness can not be obtained in both the C direction. Therefore, L direction particle length (average length) 0.2 [mu] m or more 5.0μm or less, C direction particle length (average length) of 0.1μm or more 5.0μm or less, and 0.2 ≦ C direction particle length / a L direction particle length ≦ 5.0.
[0037]
　The average length of the crystal grains by intercept method, for example, a photograph of the sample cross-section, pulling the 100-150 line segments of each full-length L in the L direction and the C direction, the crystal grains of the line segment crosses calculated by determining the number n, and calculates the L / n of each of the line segment drawn on the photograph, and the average value as the average length of the L direction and C direction respectively of the crystal grains.
　In the present embodiment, the position and sheet width center of the plate thickness 1/4 depth of the hot-rolled steel sheet, as a section parallel is observed surface in the rolling direction and the plate width direction, electron backscatter diffraction (EBSP) the use specimens were taken, after polishing the observation surface, to appear tissue with electrolytic polishing, using a backscattered electron diffraction apparatus (EBSP apparatus), three field taken at 8000 × magnification. Each measurement field of view and 500μm × 500μm. Then, using the EBSP measurement data analysis software, orientation difference between adjacent grains is defined as one crystal grain of not less than 5 °. Then, 100-150 This pulling in a direction parallel to the respective L direction and C direction of a line segment of the full-length 100μm on the image, obtains the L / n from the number of grains across each linear, L direction and the average value and the average length of C directions of the crystal grains.
[0038]
　In the present embodiment, "parallel to the rolling direction" is intended to include a range of ± 5 ° to the rolling direction. Similarly, "parallel to the sheet width direction" is intended to include a range of ± 5 ° with respect to a direction parallel to the sheet width direction.
[0039]
　In the hot rolled steel sheet according to the present embodiment, although factors is not clear which is fine grain in L direction and C direction respectively, it is inferred as follows. By performing the finish rolling very large cumulative rolling reduction, prior austenite grain is elongated in the L direction (rolling direction), but the dislocation density is increased to be introduced into the prior austenite grains, when the martensitic transformation occurs increases the tendency of block size generated by the different Las directions jumbled in parallel aligned lath population is miniaturized. As a result, not only the C direction of the block size, block size L direction which is extended by rolling are also intended to be miniaturized. Therefore, as an index indicating that the dislocation is sufficiently introduced into the prior austenite grains, the L direction old γ grain length is the average length of the prior austenite grains of an aspect ratio (L direction of the former austenite grains, the C direction the ratio between the C direction old γ grain length is the average length of the prior austenite grains), with the aspect ratio of the 0.03 ≦ C direction old γ grain length / L direction old γ grain length ≦ 0.40 it is preferable to satisfy. If L direction old γ grain length and the ratio between the C direction old γ grain length (C direction old γ grain length / L direction old γ grain length) is 0.40 greater, insufficient accumulation of strain at the time of manufacture There, it may not be possible to obtain a desired tissue in hot-rolled steel sheet after manufacturing. If L direction old γ grain length and the ratio between the C direction old γ grain length (C direction old γ grain length / L direction old γ grain length) is less than 0.03, the restraint of the old austenite grains greatly extended in the direction L martensite generation site transformation becomes difficult and it is difficult to fine martensite lath is generated in the L direction so as to divide the L direction within the austenite grains produced by. Further, since the shape of the prior austenite grain boundary is complicated, it is sometimes impossible to obtain a desired tissue in hot-rolled steel sheet after manufacturing.
[0040]
　The ratio of the L direction old γ grain length is the average length of the prior austenite grains in the L direction and C direction old γ grain length is the average length of the C direction of the former austenite grains is measured by the following methods.
　A plate position of 1/4 depth and sheet width center of the thickness of the hot rolled steel sheet, the cross section perpendicular to the sheet width direction (L cross section), and a cross section perpendicular to the rolling direction (C cross section) each is viewing surface as such, two collecting optical microscope specimen. For both samples the sample and C cross-sectional observation for the L cross-section observation, after mirror polishing, the observation surface was corroded with nital solution, 500 [mu] m in the thickness direction using an optical microscope, in the thickness direction and the direction perpendicular to shoot the field of view of the 2000μm. From a sample capturing photos for L cross-section observation, the old austenite grains having an average length of L direction was measured (L direction old γ grain length), from the photographed picture of a sample for the C cross-section observation, C the direction of the former austenite grains the average length of (C direction old γ grain length) is measured. However, it measured at the 100 crystal grains in each captured photograph, by averaging, to measure the L direction old γ grain length and C directions the old γ grain length. It may make it easier to observe the crystal grains, the field of view of 500 [mu] m × 500 [mu] m measurement is adjacent in each cross section 4 perimetry, by connecting them, when observing the field of view of 500 [mu] m × 2000 .mu.m.
[0041]
　Hot rolled steel sheet according to the present embodiment, having the above chemical components and metallographic structure. If the tensile strength is 1180MPa or more, in the case of applying the hot-rolled steel sheet according to the present embodiment structural part of a motor vehicle, skeletal, etc. to the track frame, it is possible to reduce the thickness while securing the desired strength , it is possible to contribute to the improved fuel efficiency of the car.
　Incidentally, the thickness of the hot rolled steel sheet according to the present embodiment is not particularly limited, as structural steel plate of an automobile, may be 1.0mm or 3.6mm or less.
[0042]
　Next, a method for manufacturing the hot rolled steel sheet according to the present embodiment.
　Method for producing a hot rolled steel sheet according to the present embodiment includes a heating step of heating a steel material having the above chemical components (a), rolling to finish rolling process finishing the steel material after heating (b), finish rolling has after the martensite critical cooling rate V (℃ / s) cooling step of cooling the above average cooling rate (c), after cooling, winding process for winding at a coiling temperature 300 ° C. or less and (d) ing. It is also possible to have a rough rolling step between the heating step (a) and the finish rolling step (b). It will be described in detail a method for manufacturing the hot rolled steel sheet according to the present embodiment.
[0043]
(A) heating process
　in the heating step, heating the steel material consisting of chemical ingredients described above in 1350 ° C. or less 1200 ° C. or higher. Method for producing a steel material, not critical and the molten steel having the chemical components described above, was melted in a converter furnace or the like, a steel material such as a slab by casting method such as continuous casting, conventional methods are It can be applied. Incidentally, ingot making - it may be used slabbing method.
[0044]
　In the steel material or the like slab, most carbonitride forming elements such as Ti is in a non-uniform distribution in the steel material is present as coarse carbonitrides. Coarse precipitates present in a non-uniform distribution (carbonitride) are properties of the hot-rolled steel sheet (e.g., tensile strength, toughness, hole expansion, etc. processability) degrade. Therefore, by heating the steel material before the hot rolling, to form a solid solution coarse precipitates. The coarse precipitates to fully dissolved before hot rolling, it is necessary that the heating temperature of the steel material 1200 ° C. or higher. However, if the heating temperature of the steel material is too high, generation of surface flaws, to cause a decrease in yield due to scale off, the heating temperature of the steel material to 1350 ° C. or less.
[0045]
　The steel material is heated to a heating temperature above 1200 ° C. for a predetermined time, the retention time exceeds 4800 seconds, the results of the scale generation amount increases, easily scale biting or the like occurs in the subsequent finish rolling process , there is a case where the surface quality of the hot-rolled steel sheet is degraded. Therefore, the retention time of the steel material in the temperature range of not lower than 1200 ° C., preferably not more than 4800 seconds.
[0046]
　Rough rolling step
　between the heating step and the finish-rolling step may be carried out rough rolling against steel material. Rough rolling, it is only necessary to obtain the desired sheet bar dimension, the conditions are not particularly limited.
[0047]
(B) finish-rolling process
　the steel material after the the steel material or the rough rolling heated in the heating step to, performing finish rolling. In rolling during the rolling stand of the previous finish rolling or finish rolling, it is preferable to perform descaling.
[0048]
　The finish rolling step, the rolling by passing continuously a steel material after heating or after rough rolling to a plurality of rolling stands. Further, in the finish rolling step, in a temperature range of 800 ° C. or higher 950 ° C. or less, is rolling at a cumulative reduction rate of 70% or more. Final rolling stand outlet side temperature is set to 800 ℃ more than 950 ℃ or less. In each rolling stand is rolled to A value satisfies 0.05 ≦ A ≦ 23.0 stipulated by the following formula (1). Furthermore, the path time between rolling stands is not more than 10.0 seconds 0.2 seconds. n in formula (1) is a roll rotation speed (rpm) in each rolling stand, r is rolling reduction (%) in each rolling stand, H is is rolled thickness at entrance side of each rolling stand (mm) . Hereinafter, explaining the reasons for limitation of the finish rolling step.
[0049]
[Number 3]

[0050]
(Finish rolling start temperature 800 ° C. or higher)
　in the finish rolling, but is rolling by passing continuously a steel material after heating to a plurality of rolling stands, the starting temperature of finish rolling is set to 800 ° C. or higher. When the finish rolling start temperature is lower than 800 ° C., rolling in some of the plurality of rolling stands (especially rolling stand the first half) is performed in a two-phase region temperature of ferrite and austenite, and processing tissue after finish rolling residual hot rolling strength and toughness of the steel sheet is lowered. Accordingly, the finish rolling start temperature is set to 800 ° C. or higher. Finish rolling start temperature is the inlet side temperature of the rolling stand which steel is first passed through a and the surface temperature of the steel sheet. Incidentally, with the finish rolling start temperature above 800 ° C., by defining described below as the final rolling stand outlet side temperature 800 ° C. or higher 950 ° C. or less, in a temperature range of not lower than 800 ° C. In all rolling stand so that rolling is performed. The upper limit of the finish rolling start temperature is to suppress the coarsening of austenite may be the 1100 ° C..
[0051]
(Final rolling stand outlet side temperature 800 ° C. or higher 950 ° C. or less)
　in the delivery temperature of the final rolling stand is finish rolling temperature is lower than 800 ° C., since the rolling is performed in a two-phase region temperature of ferrite and austenite, the rolling worked structure strength and toughness of hot rolled steel sheets remain is reduced after. On the other hand, in the steel material having a chemical composition according to the present embodiment, the non-recrystallized austenite region is approximately 950 ° C. or less of the temperature range. Therefore, when the delivery temperature of the final rolling stand is higher than 950 ° C., austenite grains grow, grain length of martensite hot-rolled steel sheet obtained after cooling increases. As a result, it is difficult to obtain a desired tissue, the strength and toughness of the hot-rolled steel sheet is reduced. Therefore, final rolling stand outlet side temperature is set to 800 ° C. or higher 950 ° C. or less. Note that the temperature referred to herein, is intended to refer to the surface temperature of the steel sheet.
[0052]
(800 ° C. or higher 950 ° C. cumulative rolling reduction in the following: 70%)
　as described above, in the steel material having a chemical composition according to the present embodiment, the non-recrystallized austenite region is at approximately 950 ° C. below the temperature range Therefore, the delivery temperature of the final rolling stand and 950 ° C. or less. If the temperature range of the finish rolling at (800 ° C. or higher 950 ° C. or less) cumulative reduction rate of delivery temperature of the final rolling stand of the finish rolling start temperature is less than 70%, the dislocation density introduced into the non-recrystallized austenite is small Become. When the dislocation density introduced into the non-recrystallized austenite becomes small, it is difficult to obtain a desired tissue, the strength and toughness of the hot-rolled steel sheet is reduced. Thus, the cumulative reduction rate at 800 ° C. or higher 950 ° C. or less by a plurality of rolling stands in the finish rolling is 70% or more. However, when the cumulative rolling reduction at 800 ° C. or higher 950 ° C. or less exceeds 97%, there are cases where the shape of the steel sheet is degraded, the cumulative rolling reduction in the temperature range is preferably set to 97% or less.
　Note that the cumulative rolling reduction at 800 ° C. or higher 950 ° C. or less in the present embodiment, the final in the total rolling reduction in the temperature range (inlet plate thickness before the first pass in the rolling of the temperature range and the rolling of the temperature range the percentage of the difference between the exit thickness Tokyo) after paths.
[0053]
: (Interpass time between rolling stands 0.2 seconds to 10.0 seconds or less)
　in the finish rolling step, the rolling by passing the steel material after heating continuously the plurality of rolling stands. When interpass time between rolling stands exceeds 10.0 seconds to recover and proceed recrystallization between passes, the cumulative strain becomes difficult, it is impossible to obtain the desired tissue. Time preferably shorter between passes, but the shortening of the path between the time, because there is a restriction in terms of installation space and the rolling speed of the rolling stand, and 0.2 seconds or more.
[0054]
(A value of each rolling stand: 0.05 ≦ A ≦ 23.0)
　A value specified by the above formula (1) is a value calculated on the basis of the rolling conditions, indicating the magnitude of the dislocation density by this be able to. Dislocation density is also increased, which is introduced into the austenite as A value is higher, the A value exceeds 23.0, the temperature of the steel strip is increased by working heat generation amount is significantly path between the rolling stands during time is difficult to accumulation of strain even less than 0.2 seconds 10.0 seconds. On the other hand, when the A value is less than 0.05, even interpass time is 10.0 seconds 0.2 seconds between the rolling stands, the dislocation density is reduced to be introduced into the austenite. As a result, it is difficult to obtain a desired tissue, the strength and toughness of the hot-rolled steel sheet is reduced. Therefore, the path between the time between rolling stands of finish rolling and 0.2 seconds 10.0 seconds or less, and it is desirable to roll such that 0.05 ≦ A ≦ 23.0 at each rolling stand . A more preferred range of A value, 0.20 or more and 20.0 or less. Further, it is more preferable to the A value in the final stand 10.0 or higher.
[0055]
(C) cooling step
　in the cooling step, to start cooling within 10.0 seconds after the finish rolling completion, cooled in martensite critical cooling rate V (℃ / s) or more average cooling rate.
[0056]
　In the present embodiment, it established the cooling equipment downstream of the finishing rolling mill, to provide cooling while passing the steel sheet after finish rolling to this cooling facility. Cooling equipment, martensite critical cooling rate V (℃ / s) capable cooled steel sheet in the above average cooling rate equipment is desirable. For example, as such cooling equipment, it can be exemplified water-cooling equipment using water as the cooling medium.
[0057]
　The average cooling rate in the cooling step, a temperature drop range of the steel sheet from the cooling start to the end of cooling, and divided by the required time from the time of the cooling start to the end of cooling. Cooling at the start and is, and at the time of the introduction of the steel plate to the cooling equipment, cooling and end will be at the time of the derivation of the steel plate from the cooling equipment.
　Further, the cooling facilities, equipment and there is no air interval in the middle, there is a facility having a middle to one or more cooling sections. In the present embodiment, it may be either of the cooling equipment. Even when using a cooling facility having the air cooling section, the average cooling rate from the cooling starting to the end of cooling is sufficient if the martensite critical cooling rate V (℃ / s) or more.
[0058]
　Hereinafter, explaining the reasons for limitation of the cooling conditions. The cooling stop temperature is 300 ° C. or less, this condition will be described in the winding step.
[0059]
(Cooling start time: finish 10.0 seconds or less after rolling)
　finish after rolling, immediately start cooling. More specifically, within after finish rolling 10.0 seconds, more preferably within 5.0 seconds, more preferably cooling is started within 1.0 seconds. When the cooling start time is delayed, recrystallization proceeds, it is performed cooled in a state in which the strain is released, it is impossible to obtain the desired tissue.
[0060]
(Average cooling rate: more martensite critical cooling rate V (℃ / s))
　The average cooling rate, the martensite critical cooling rate V (° C. / s) or more. Doing cooling at an average cooling rate of less than martensite critical cooling rate V (℃ / s), becomes bainite and ferrite is easily formed, the volume fraction of martensite becomes smaller. Martensite critical cooling rate V in the present embodiment (° C. / s) is the minimum cooling rate martensite fraction of the metal structure after cooling is 90% or more. Specifically, martensite critical cooling rate V in the present embodiment (° C. / s) is calculated by the following formula (2) and (3). However, atomic symbols in the following formula (3) is the content of the element (mass%). Incidentally, Ni, if not containing Mo substitutes 0 wt% Ni, the section Mo. Martensite critical cooling rate V (℃ / s) or more cooling is preferably performed to a cooling stop temperature.
[0061]
= 10 　V 2.94-0.75 　(b-1) ... (2)
　b = 2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + Mo ... (3)
[0062]
(D) winding process
　described above in the cooling step is cooled to a cooling stop temperature steel sheet, wound at 300 ° C. or less. Since the winding immediately steel sheet after cooling is performed, the coiling temperature is approximately equal to the cooling stop temperature. When the coiling temperature exceeds 300 ° C., polygonal ferrite or bainite strength is reduced to produce. Therefore, the coiling temperature of the cooling stop temperature is set to 300 ° C. or less.
[0063]
　Note that after winding, in a conventional manner to hot-rolled steel sheet may be subjected to temper rolling, also, the scale formed on the surface subjected to pickling may be removed. Or further, galvanizing, plating or the like electro-galvanized, may be subjected to chemical conversion treatment.
Example
[0064]
　The molten steel chemical compositions shown in Table 1 were melted in a converter furnace, and a slab (steel material) by a continuous casting method. Incidentally, Table 1, Table 2A, the "critical cooling rate (° C. / s)" in Table 2B is a martensite critical cooling rate V (° C. / s), is calculated by the following formula (2) and (3) that. However, atomic symbols in the following formula (3) is the content of the element (mass%). Ni, when containing no Mo was substituted for 0 wt% Ni, the section Mo.
[0065]
= 10 　V 2.94-0.75 　(b-1) ... (2)
　b = 2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + Mo ... (3)
[0066]
　Then these steel material was heated under the conditions shown in Table 2A and Table 2B, after the rough rolling, a finish rolling under the conditions shown in Table 2A and Table 2B (total seven passes, rolling stands F1 ~ F7) went. Finish rolling start temperature was 800 ° C. or higher in all the steel material. After the completion of the finish rolling, cooled under the conditions shown in Table 2A and Table 2B, the winding is cooled to coiling temperature shown in Table 2A and Table 2B, to obtain a hot rolled steel sheet thickness shown in Table 2A and Table 2B .
[0067]
　The cumulative rolling reduction in Tables 2A and 2B, showing the cumulative rolling reduction at 800 ° C. or higher 950 ° C. or less in the rolling stands F1 ~ F7 of finish rolling. Further, "A" is A value in each path calculated by the above formula (1), "P / s" is the path between the time (in seconds). For example, it was listed in the F1 "P / s" indicates the interpass time between rolling stand F1 and rolling stand F2.
[0068]
　Cooling after finish rolling is to be due to water-cooling was carried out by passing the steel sheet having no water-cooling equipment air cooling section in the middle. Cooling rate in Table 2A and Table 2B are a temperature drop range of the steel sheet from Water-equipment introduced up when water cooling facility derivation, the average cooling rate obtained by dividing the required transit time of a steel sheet for a water-cooled equipment.
[0069]
　The resulting collected specimen from hot-rolled steel sheet, microstructure observation, a tensile test was conducted Charpy impact test. Results of each test were shown in Table 2C and Table 2D. Incidentally, the volume fraction of martensite and M phase of the metal structure column in Table 2C and the table 2D, showing bainite or ferrite, or the volume fraction of both, the remaining structure. Also, tissue observation method and the various test methods were as follows.
[0070]
Structure Observation: volume fraction of the metal structure
　positions the plate thickness 1/4 depth of the hot-rolled steel sheet and the sheet width center, as parallel section is observed surface in the rolling direction and the plate width direction, scanning electron We were taken microscope test piece. The viewing surface was mirror-polished, corroded with 3% nital solution, using a scanning electron microscope, and 3-field taken at 2000 × magnification. The measurement field of view was 500μm × 500μm. Thereafter, image processing, the kind of metal structure, the area fraction of each phase and the metal tissues were measured. The area fraction of each metal structure obtained was a volume fraction of each tissue.
[0071]
Structure Observation: grain average length of (L direction particle length and C directions particle length)
　from the position and sheet width center of the plate thickness 1/4 depth of the hot-rolled steel sheet, parallel to the rolling direction and the plate width direction cross section so it becomes observation surface was collected backscattered electron diffraction (EBSP) for test pieces. After polishing the observation surface, to appear tissue with electrolytic polishing, using a backscattered electron diffraction apparatus (EBSP system), and 3-field taken at 8000 × magnification. The measurement field of view was 500μm × 500μm. Then, using the EBSP measurement data analysis software, orientation difference between adjacent grains define a more than 5 ° with one crystal grain was determined grain length by sectioning.
　In section method, in a direction parallel to the respective L direction and C direction, 133 present pulling of a line segment of the full-length 100μm on the image, obtains the L / n from the number of grains across each linear, and the average value L and the average particle length in a direction and C direction respectively of the crystal grains.
[0072]
　The ratio of the L direction old γ grain length is the average length of the prior austenite grains in the L direction and C direction old γ grain length is the average length of the C direction of the former austenite grains was measured by the following method.
　First, a plate position of 1/4 depth and sheet width center of the thickness of the hot rolled steel sheet, the cross section perpendicular (L cross section) in the sheet width direction, and a cross section perpendicular (C cross section) in the rolling direction, respectively observation plane and so that, with the optical microscope specimen was two sampled. For both samples the sample and C cross-sectional observation for the L cross-section observation, after mirror polishing, the observation surface was corroded with nital solution, 500 [mu] m in the thickness direction using an optical microscope, in the thickness direction and the direction perpendicular taken the field of view of the 2000μm. From a sample capturing photos for L cross-section observation, the old austenite grains having an average length of L direction was measured (L direction old γ grain length), from the photographed picture of a sample for the C cross-section observation, C the direction of the former austenite grains the average length of (C direction old γ grain length) was measured. However, it measured at the 100 crystal grains in each captured photograph, by averaging, to measure the L direction old γ grain length and C directions the old γ grain length. Incidentally, observation photograph in each section, the field of view of the adjacent 500 [mu] m × 500 [mu] m 4 and perimetry, by connecting them to observe the visual field of 500 [mu] m × 2000 .mu.m.
[0073]
Tensile test
　From the hot-rolled steel sheet, the tensile direction is taken JIS5 test piece No. so in a direction parallel to the rolling direction, JIS Z 2241: perform tensile compliance test with the provisions of 2011, tensile strength (TS) I was determined.
　Where tensile strength is more than 1180 MPa, and judged to be acceptable as having the desired strength in the present invention. Where the tensile strength is less than 1180 MPa, it is determined that the failed as not having the desired strength in the present invention.
[0074]
Charpy impact test
　from hot-rolled steel sheet, the longitudinal direction of the rolling direction parallel to the direction of the test piece (L direction), and sheet width such that the direction (C direction) parallel to the direction of the thickness of 2.5mm subsize specimen (V notch) were collected, respectively, JIS Z 2242: in compliance with the provisions of the 2005 performs Charpy impact test at -198 ° C. from room temperature, L-direction and C direction respectively ductile - brittle transition temperature (DBTT: ductile-brittle transition temperature) was determined. Here, the thickness of the specimen, a specimen was produced a sheet thickness as 2.5mm hot-rolled steel sheet at double-side grinding. The Table 2C and in Table 2D, L direction and C direction ductile - shows the brittle transition temperature of each "transition temperature (L)" and "Transition Temperature (C)".
　L direction and C direction ductile - where brittle transition temperature is -60 ° C. or less, it is determined that excellent isotropic toughness and tenacity.
[0075]
[Table 1]

[0076]
[Table. 2A]

[0077]
[Table 2B]

[0078]
[Table 2C]

[0079]
[TABLE 2D]

[0080]
　In the embodiment shown in Table 2C and Table 2D, since both cooling the critical cooling rate V (℃ / s) or more average cooling rate includes 90% martensite by volume%. The remainder being one or both of bainite or ferrite. However, No. As will be described later 27, martensite is not sufficiently generated, bainite is generated number.
[0081]
　Further, as shown in Table 2C and Table 2D, hot rolled steel sheets of Example, the desired tensile strength (above 1180 MPa), good toughness (L direction, ductility in the direction C both - brittle transition temperature -60 ° C. or less) It has become a hot-rolled steel sheet having both the door.
[0082]
　On the other hand, hot-rolled steel sheets of the comparative examples outside the scope of the present invention, whether a predetermined tensile strength is not secured, sufficient toughness is not be ensured.
[0083]
　No. 6, since the final rolling stand delivery temperature is in the 980 ° C., without accumulation of distortion occurs because the coarsening of austenite occurs, it is impossible to obtain a sufficiently miniaturized martensite, tensile has become an example of strength and toughness is insufficient.
　No. 13, rolling due to the long path between the time between the stand F1 and rolling stand F2, has become not possible to obtain a sufficiently miniaturized martensite, the tensile strength and toughness is insufficient example.
　No. 16 is that cumulative rolling reduction at 950 ° C. or less is less than 70%, because it can not accumulated sufficient strain can not be obtained sufficiently miniaturized martensite, the tensile strength and toughness It has become a deficient example.
　No. 18 is obtained in the first pass of rolling (F1), since A value less than 0.05, during rolling, reduced dislocation density introduced into the austenite, a sufficiently miniaturized martensite it is not possible, it has become the example toughness is insufficient.
[0084]
　No. 20, after finish rolling, since a long time to start cooling, stress introduced into austenite is released, it is impossible to obtain a sufficiently miniaturized martensitic structure, so the example toughness is insufficient there.
　No. 23, because the path between the time between the rolling stand F1 and rolling stand F2 was longer, stress introduced into austenite is released, it is impossible to obtain a sufficiently miniaturized martensite, lack toughness It has become the example.
　No. 27 has been cooled in the martensitic critical speed V (℃ / s) or higher cooling rate, for the coiling temperature a cooling stop temperature exceeds 300 ° C., the martensite is not sufficiently generated, the tensile strength is insufficient It has become the example.
[0085]
　No. 31, 7 A value in the pass rolling (F7) is above 23.0, large machining fever final rolling stand outlet side temperature becomes high by occurs, some of the strain released to the start cooling because it was, it has become the example tensile strength is insufficient.
　No. 35, since the C content in the steel is lower than the predetermined component range, and is an example in which the tensile strength is insufficient.
　No. 36, Ti content in the steel is higher than the predetermined component range, the precipitate formed such coarse TiC and TiN, which is an example of toughness is insufficient.

WE CLAIM

Chemical composition, in
mass%, C: 0.06% to 0.20% or
less, Si: 1.0% or
less, Mn: 1.5% greater than
3.5%, P: 0.040% or less,
S: 0.004% or
less, Al: 0.10% or
less, N: 0.004% or
less, Ti: 0.04% to 0.20% or
less, Nb: 0% or more than 0.04%,
Mo: 0% to 1.0% or
less, Cu: 0% to 0.5% or
less, Ni: 0% to 0.5% or less,
containing the balance being Fe and impurities,
　the sheet thickness 1/4 depth position and the metal structure in the sheet width center position of the 90% by volume or more of martensite, consists of a 0 vol% to 10 vol% of the remaining structure, the remaining structure is one or both of bainite or ferrite, 0.2μm average length of L direction of the crystal grains is a direction parallel to the rolling direction than And at 5.0μm or less, is at 0.1μm or more 5.0μm or less average length of C direction of the crystal grains is the direction parallel to the sheet width direction, and the average length of the L direction of the crystal grains a a a L direction particle length and the C direction of the ratio ≦ 0.2 ≦ C direction particle length / L direction particle length between C direction particle length is the average length of the crystal grains 5.0 is,
　tensile strength There is greater than or equal to 1180MPa, the hot-rolled steel sheet.
[Requested item 2]
　In the metal structure at the position and sheet width center of the sheet thickness 1/4 depth, the average length of the prior austenite grains of the L direction and L direction old γ grain length, the average length of the prior austenite grains of the C direction when was the C direction old γ grain length of the L direction old γ ratio of the grain length and the C direction old γ grain length is 0.03 ≦ C direction old γ grain length / L direction old γ grain length ≦ 0 it is .40, hot rolled steel sheet according to claim 1.
[Requested item 3]
　The chemical composition, by
mass%, Nb: 0.01% or more than
0.04%, Mo: 0.01% to 1.0% or
less, Cu: 0.01% to 0.5% or
less, Ni: 0.5% 0.01% or less,
containing one or more selected from among, hot-rolled steel sheet according to claim 1 or claim 2.
[Requested item 4]
　A step of following (a) ~ (d), the manufacturing method of the hot-rolled steel sheet according to any one of claims 1 to 3:
(a) a steel material consisting of chemical components according to claim 1 heating step is heated to 1200 ° C. or higher 1350 ° C. or less;
and (b) a said steel material after heating continuously to a plurality of rolling stands passed through the finish is rolling by the rolling process, the finish rolling start temperature 800 ° C. and above, the a value defined by the following formula (1) in the rolling stand of the is rolled so as to satisfy the 0.05 ≦ a ≦ 23.0, the interpass time between rolling stands 0.2 seconds and 10.0 seconds or more, further, the final rolling stand outlet side temperature 800 ° C. or higher 950 ° C. or less, and finishing the cumulative reduction rate at 800 ° C. or higher 950 ° C. or less and 70% or more rolling
step; (c) the cooling within 10.0 seconds after the finish rolling Started to, and the following formula (2) and cooling step for cooling with martensite critical cooling rate V (° C. / s) or more average cooling rate defined by the following formula (3); and
(d) after cooling, the coiling temperature taking up winding process at 300 ° C. or less,
[Expression

　1] V = 10 2.94-0.75 　(beta-1) ... (2)
　beta = 2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + Mo ... ( 3)
　However, the n in the formula (1) is a roll rotation speed in rpm, a reduction rate r is%, H is rolled thickness at entrance side at the mm, C in the formula (3) , Si, Mn, Ni, Mo is the content by mass percent of the respective elements, Ni, if not containing Mo substitutes 0 wt% Ni, the section Mo.