Technical field
[0001]The present invention, hot rolled steel plate (hereinafter, also referred to as hot-rolled steel sheet) and to a manufacturing method thereof, particularly to toughness excellent tensile strength 1180MPa or more hot-rolled steel sheet anisotropy and a manufacturing method thereof.
BACKGROUND
[0002]Recently, in order to improve and enhance the collision safety of automobile fuel efficiency, efforts to body weight reduction by application of high-strength steel sheet has been actively conducted. However, when increasing the strength of the steel sheet, commonly toughness is deteriorated. Therefore, in the development of high-strength steel sheet is an important challenge is to achieve high strength without deteriorating toughness. In particular, in high-strength steel sheet which is applied to the mobile member, it is important to ensure crashworthiness. Here, in order to improve the toughness and rolling at a low temperature, to improve the toughness by applying the cumulative strain high in non-recrystallized austenite is generally known.
[0003]In contrast, in Patent Document 1, austenite by the proper range reduction ratio between the average strain rate and the at 860 ~ 960 ° C. as the pre-recrystallization region, the volume of the tissue to be transformed from un-recrystallized austenite increased, cold-rolled steel sheet with improved toughness of cold-rolled steel sheet has been proposed by fine tissue yelling made by hot rolling. However, non-rolling reduction in the recrystallization austenite to increase the higher the aspect ratio of prior austenite grains, there is a problem toughness anisotropy becomes strong.
[0004]Hot rolling Patent Document 2, the finishing temperature is high temperature, 1000 ° C. to promote recrystallization austenite by increasing the following reduction ratio was reduced anisotropy by shortening the time until cooling after rolling steel plate has been proposed. However, increasing the reduction rate of 1000 ° C. or less, although to promote recrystallization, in order to perform the finish rolling at a high temperature, recrystallization between stands is promoted, maintaining a high strain at the final stand can not. Therefore, recrystallized austenite grains is not only formed as coarse, there is a problem that toughness becomes inferior.
[0005]In Patent Document 3, the cumulative rolling reduction of 840 ° C. greater with respect to the set to 30% or more, 840 ° C. or less of the reduction ratio of 30% or more, by 75% or less to suppress the aspect ratio of prior austenite grains, crystal grain size of 10μm or more, has been proposed hot rolled steel sheet to 60μm or less. However, when you are rolling at 840 ° C. or less recrystallization does not occur, since occurs grain growth by strain introduced, the crystal grains has a problem that coarse.
CITATION
Patent Document
[0006]Patent Document 1: Japanese Patent No. 3858146
Patent Document 2: Patent No. 5068688 Patent Publication
Patent Document 3: Japanese Patent No. 5556948
Summary of the Invention
Problems that the Invention is to Solve
[0007]
　Recently, there is an increasing demand for further weight reduction of automobiles, high energy absorption during high-speed deformation, automobile parts collision characteristics are good as a high strength steel sheet excellent in toughness anisotropy is required.
[0008]
　The present invention has been made in view of the above problems, the present invention aims to provide a high strength steel sheet excellent in these properties.
Means for Solving the Problems
[0009]
　Conventionally, towards improving toughness of the steel, increasing the cumulative rolling reduction in the pre-recrystallization austenite, various efforts to refine the structure have been made. The present inventors have found that, when increasing the reduction rate of the pre-recrystallization austenite strongly anisotropic tissue, toughness when parallel to the crack and the rolling direction is propagated is noticed that a subordinate, and extensive studies. Such a result, thereby recrystallization after addition of high strain, focusing again recrystallization phenomenon has been avoided conventionally, by using this, in the hot-rolled steel sheet, it is possible to improve the toughness by improving anisotropy It was heading. Specifically, in four or more of the plurality of hot rolling stand for continuous, optimizing the reduction ratio in the last four stands of the plurality of stands, the strain recrystallization temperature capable in the final stand of four stands by controlling the speed, austenite finely recrystallized, it was confirmed that the anisotropy of the tissue is eliminated.
[0010]
　The present invention has been made based on the above findings, it is an aspect of the present invention is as follows.
　(1) in
　mass%, C: 0.10% or more, 0.50% or
　less, Si: 0.10% or more, 3.00% or
　less, Mn: 0.5% or more, 3.0% or
　less, P : 0.100% or
　less, S: 0.010% or
　less, Al: 1.00% or less, and
　N: 0.010% or less
　contained, and the balance Fe and impurities,
　of thickness from the surface in the L section metal structure at 1/4 position, the average value of the aspect ratio of 2.0 or less, an average particle diameter of 0.1μm or more, 3.0 [mu] m or less, and variation coefficient of standard deviation / an average particle diameter of the particle size distribution is 0 prior austenite grains is .40 or higher, and include {001} <110> texture X-ray diffraction intensity ratio of the orientation is 2.0 or more for a random
　sample, more tensile strength 1180MPa
　and having a hot-rolled steel sheet.
　(2) in
　mass%, Ti: 0.02% or more, 0.20% or
　less, Nb: 0.00% or more, 0.10% or
　less, Ca: 0.0000% or more, 0.0060% or
　less, Mo : 0.00% or more, 0.50% or less, and
　Cr: 0.0% or more, 1.0% or less
　, further characterized by containing one or two or more selected from the group consisting of hot rolled steel sheet according to (1).
　(3) above (1) or (2) A method of producing hot-rolled steel sheet according to method for producing a hot rolled steel sheet, which comprises a step (a) ~ (e) below :
(a) above (1) or (2) in a slab having the component composition 1100 ° C. or more, wherein the heating step is heated to less than 1350 °
C.; a (b) the slab after said heating, four or more of the plurality of a rolling step of rolling using a rolling mill having a stand, a total length of the last four stands of the plurality of stands is not more than 18m, reduced plate thickness before and after the last four stands There step satisfies the following formula 1
　　1.2 ≦ ln (t 0 /T)≦3.0 (equation 1)
　where, t 0 is the thickness immediately before entering the last four stands, t is the thickness of the immediately after exiting from the last four stands;
(c) the last And the rolling temperature in the final stand and strain rate at the final stand of four stands, satisfying formula 2 and formula 3 below step
　　11.0 ≦ log (v × exp ( 33000 / (273 + T)) ≦ 15.0 (equation 2)
　　T ≧ Ar 3 point (equation 3)
　Here, v is the strain rate (/ s) in the final stand, T is a rolling delivery temperature (℃) in the final stand;
start cooling (d) within the rolling after the end of 1.0 seconds to the cooling step to cool the temperature range from finish rolling temperature to 750 ° C. at 100 ° C. / s or more average cooling rate; and
(e) the cooled process, the winding step for winding.
Effect of the invention
[0011]
　According to this aspect of the present invention, it is possible to absorb energy during high-speed deformation is high, auto parts collision characteristics are good as, provides a hot-rolled steel sheet of high strength with excellent toughness anisotropy. According to this hot-rolled steel sheet, reducing vehicle weight, such as an automobile, integral molding of components, and it is possible to shorten the processing step, it is possible to improve and reduce the manufacturing cost of the fuel consumption, the present invention is a high industrial value.
DESCRIPTION OF THE INVENTION
[0012]
　For hot rolled steel sheet according to an embodiment of the present invention will be described. Hot rolled steel sheet according to the present embodiment controls the recrystallization and grain growth behavior during hot finish rolling. Adjust the amount of distortion in the successive stand, by reaching the strain critical need for recrystallization at the final stand to form a fine recrystallized grains, no finely and anisotropic shape of the crystal grains are polygonal of it is possible to build in the organization. Even after recrystallization, by as short as possible the time until cooling start time, suppressing the grain growth of recrystallized grains. By fabricate a polygonal of austenite grains fine at hot rolling step, it is possible to obtain an excellent hot-rolled steel sheet toughness. Moreover, further processed cold rolled steel sheet or heat treatment for steel plates, the steel sheet excellent in toughness hot rolled steel sheet. Specifically, the hot rolled steel sheet according to the present embodiment has a tensile strength of at least a predetermined chemical composition and 1180 MPa, the metal structure has an average value of the aspect ratio of 2.0 or less, an average particle diameter of 0. 1μm or more, 3.0 [mu] m or less, and a particle size distribution with a standard deviation / mean a particle diameter variation coefficient is 0.40 or more prior austenite grains, and {001} <110> orientation X-ray diffraction intensity ratio of to the random sample but including in a texture 2.0 or higher.
[0013]
　Hereinafter, each constituent element of the present invention will be described in detail. First, we describe reasons for limiting the chemical composition of the hot rolled steel sheet according to the present embodiment (chemical composition). % Of component content means mass%.
[0014]
　
　C is an important element for improving the strength of the steel sheet. To obtain the desired intensity, it is necessary that the C content is 0.10% or more. C content is preferably 0.25% or more. However, C content deteriorates toughness of the steel sheet to be 0.60 percent. Therefore, the C content is 0.60% or less. C content is preferably 0.50% or less.
[0015]
　 Si is an element having an effect of improving the strength of the steel sheet. To obtain this effect, the Si content is set to 0.10% or more. Si content is preferably 0.50% or more. On the other hand, when the Si content is 3.00 percent, the toughness of the steel sheet is degraded. Therefore, the Si content is set to less 3.00%. Si content is preferably not more than 2.50%.
[0016]
　 Mn is by improving and solid solution strengthening of the hardenability, is an element effective for improving the strength of the steel sheet. To obtain this effect, the Mn content is 0.5% or more. Mn content is preferably 1.0% or more. On the other hand, Mn content is harmful MnS is generated isotropic comes to toughness of 3.0 percent. Therefore, the Mn content is 3.0% or less. Mn content is preferably 2.0% or less.
[0017]
　
　P is an impurity, P content is as low as desired. That is, on P content decrease in workability and weldability becomes 0.100 percent is significantly, fatigue properties also decreases. The reason P content is limited to 0.100% or less. P content is preferably 0.050% or less.
[0018]
　
　S is an impurity, S content is as low as desired. That, S content is generated becomes significant harmful inclusions, such as MnS isotropic toughness exceeds 0.010%. Therefore, the S content is limited to 0.010% or less. When the particularly severe low temperature toughness is required, it is preferable that the S content is 0.006% or less.
[0019]
　 Al is an element necessary for deoxidation at steelmaking process. However, if the Al content exceeds 1.00%, alumina deposited on the cluster shape is produced, the toughness is degraded. Therefore, the Al content to 1.00% or less. Preferably not more than 0.50%.
[0020]
　
　N is an impurity. When the N content is 0.010%, the coarse Ti nitrides are formed at high temperatures, toughness of the steel sheet is degraded. Therefore, the N content is 0.010% or less. N content is preferably 0.006% or less.
[0021]
　Hot rolled steel sheet according to the present embodiment contains the above chemical components, and the balance basically in that it consists of Fe and impurities. Although not an essential element to satisfy the required properties, or to reduce the manufacturing variation, in order to improve the strength, Ti, Nb, Ca, Mo, and one or more members selected from the group consisting of Cr the may further contain in the range below. However, Nb, Ca, Mo, and since Cr is not essential to meet any demand characteristics, the lower limit of the content thereof is 0%. Here, the impurities, in producing the steel industrially, refers to the components to be mixed ore raw material scraps and other factors. Nb, Ca, Mo, and the content of Cr, is less than the lower limit of the content shown below, it can be regarded as an impurity, substantial effect on the effect of the hot rolled steel sheet according to the embodiment there is no.
[0022]
　 Ti is an effective element for suppressing the recrystallization and grain growth of austenite between the stand (between passes). By suppressing the recrystallization of austenite between the stand, it is possible to further accumulate strain. Ti it is possible to obtain the effect of suppressing the recrystallization and grain growth of austenite by adding 0.02% or more. Ti content is preferably 0.08% or more. On the other hand, if the Ti content is 0.20 percent, inclusions originating from TiN is formed and the toughness of the steel sheet is degraded. Therefore, the content of Ti to 0.20% or less. Ti content is preferably less 0.16%.
[0023]
　 Nb is an effective element for suppressing the recrystallization and grain growth of austenite between stands. By suppressing the recrystallization of austenite between the stand, it is possible to further accumulate strain. To obtain the effect of suppressing the recrystallization and grain growth of austenite between stands substantially has the Nb content is preferably set to 0.01% or more. On the other hand, Nb content is 0.10 percent, the effect is saturated. Therefore, even if the inclusion of Nb, and 0.10% the upper limit of Nb content. More preferable upper limit of the Nb content is 0.06% or less.
[0024]
　 Ca disperses a large number of fine oxides at the time of deoxidation of molten steel, which is an element having an effect of refining the structure of the steel sheet. Also, Ca has the S in the steel is fixed as CaS spherical, is an element improving the anisotropy of toughness by suppressing the formation of stretched inclusions such as MnS. In order to obtain substantially the these effects, it is preferable that the Ca content is 0.0005% or more. On the other hand, the effect Ca content be greater than 0.0060% is saturated. Therefore, even if the inclusion of Ca, the upper limit of the content of Ca and 0.0060%. More preferable upper limit of the Ca content is 0.0040%.
[0025]
　 Mo is an effective element for precipitation strengthening of ferrite. In order to obtain this effect substantially, it is preferable that the Mo content is 0.02% or more. Mo content is more preferably 0.10% or more. On the other hand, when the Mo content is excessive slab cracking susceptibility handling of the slab it becomes difficult to grow. Therefore, even if the inclusion of Mo, and 0.50% the upper limit of the Mo content. More preferable upper limit of the Mo content is 0.30%.
[0026]
　 Cr is an element effective for improving the strength of the steel sheet. In order to obtain this effect substantially, it is preferable that the Cr content is 0.02% or more. Cr content is more preferably 0.1% or more. On the other hand, the ductility is deteriorated when the Cr content is excessive. Therefore, even if to be contained, and 1.0% the upper limit of the Cr content. More preferable upper limit of the Cr content is 0.8%.
[0027]
　Next, a description tissue for the hot rolled steel sheet according to the present embodiment.
[0028]
　Hot rolled steel sheet according to the present embodiment has a former austenite grains are finer recrystallized structure. A tensile strength of 1180MPa or higher class, the average particle size of prior austenite grains from relying heavily on toughness, transformed tissue, i.e. does not matter for the steel sheet structure. Single phase is preferable in order to reduce the absolute value and the anisotropy of the toughness, the martensite single phase is often used in high strength steel.
[0029]
　In order to improve the toughness, it can be finer austenite structure conventionally known to be effective. As a means to increase the cumulative rolling reduction of unrecrystallized austenite, it is common to a flat tissue. However, in the case with complex modified as collision characteristics of automobile steel sheet, the only high unidirectional toughness can not be obtained good characteristics, it is necessary to improve the anisotropy for the rolling direction. The present inventors have conducted intensive studies, toughness of crack propagation characteristics found to depend largely on the shape of the old austenite structure in order to reduce the anisotropy recrystallized austenite, by polygonal of It has also been found that it is effective. Furthermore, when a high temperature the hot rolling temperature in order to promote recrystallization, the crystal grains become coarse, in order to suppress this, the strain rate at the final stand of the hot rolling and the rolling temperature proper I found a method in the range. This method to express recrystallized only in the final stand, it is possible to obtain a fine austenitic recrystallized structure, it is possible to obtain a steel sheet having a and excellent toughness have more tensile strength 1180MPa Become.
[0030]
　 X-ray diffraction intensity ratio comprises 2.0 or more in which the texture of {001} <110> to the random sample
　from the surface in the L cross-section of the steel sheet of the present embodiment having a thickness of 1 / metal structure in 4 position, the average value of the aspect ratio of 2.0 or less, an average particle diameter of 0.1μm or more, 3.0 [mu] m or less, and variation coefficient of standard deviation / an average particle diameter of the particle size distribution is 0.40 prior austenite grains is not less than, as well as texture X-ray diffraction intensity ratio of the {001} <110> to the random sample is 2.0 or more.
[0031]
　The aspect ratio of prior austenite grains is divided by the ratio of the average crystal grain size in the rolling direction with an average grain size in the thickness direction. L section refers to a cut face so as to pass through the central axis parallel to the steel sheet in the thickness direction to the rolling direction.
[0032]
　The average value of 2.0 than the prior austenite grain aspect ratio, resulting anisotropy of toughness, parallel crack propagation properties in the rolling direction is inferior. The aspect ratio of prior austenite grains, it accumulates strain is insufficient, it rolling temperature is low, or both, there is a high tendency when recrystallization ratio of the austenite is not sufficiently obtained. To eliminate anisotropy smaller or completely, the aspect ratio of prior austenite grains is preferably 1.7 or less, more preferably 1.5 or less, more preferably 1.3 or less, even more preferably 1. 1 or less, even more preferably 1.0.
[0033]
　The average particle size of prior austenite grains is an average value of the circle equivalent diameter.
[0034]
　The average particle size is less than 0.1μm of prior austenite grains, because the work hardening characteristics of a steel sheet is lost, and when the coil after hot rolling, cracks when unwinding the coil is likely to occur in the next step. On the other hand, when it exceeds 3.0 [mu] m, low temperature toughness is inferior in high strength steel sheet. The average particle size of prior austenite grains is preferably 0.5μm or more, 2.5 [mu] m or less, more preferably 0.7μm or more, 2.4 [mu] m or less, more preferably 1.0μm or more and less 2.3 .mu.m.
[0035]
　Coefficient of variation is calculated by "standard deviation" / "average particle size" of the particle size of prior austenite grains. When recrystallization high strain is applied during hot rolling is expressed grew after immediately recrystallized grains and recrystallized and grain are mixed. Therefore, the standard deviation of the particle size of prior austenite grains is increased, the coefficient of variation is large. Since the crack propagation is suppressed by fine regions, toughness of the steel sheet higher fines and the coefficient of variation is improved. Toughness variation coefficient is excellent 0.40 or more is obtained. Coefficient of variation is preferably 0.45 or more, more preferably 0.50 or more, more preferably 0.55 or more. The upper limit of the variation coefficient is not particularly limited, for example may be 0.80.
[0036]
　1/4 position of the thickness of a L section of the steel sheet from the surface of the steel sheet after the mirror polishing, 3% nital - corroded (3% nitric acid in ethanol), observing the microstructure with a scanning electron microscope (SEM) to the aspect ratio of prior austenite grains can be measured average particle size, and the standard deviation of the particle size distribution. Specifically, the range of crystal grains in one field of view is about 10,000 observed by imaging by SEM observation, and image analyzed using image analysis software (WinROOF), the average grain size of prior austenite grains, aspect ratio can be calculated average value, and standard deviation of the particle size distribution.
[0037]
　Metal structure at 1/4 position of the thickness from the surface in the L cross-section of the steel sheet of the present embodiment also, the {001} <110> orientation X-ray diffraction intensity ratio of the relative random sample (hereinafter, also referred to as X-ray random intensity ratio) but including in a texture 2.0 or higher.
[0038]
　More X-ray random intensity ratio in the rolling direction and parallel to {001} <110> orientation perpendicular to the rolling surface is large, the influence of the crystal orientation to be given to the toughness in the rolling direction and the vertical direction is reduced, L direction and C direction of anisotropy is reduced. {001} <110> orientation X-ray random intensity ratio of for the random sample is preferably 3.0 or more, more preferably 4.0 or more.
[0039]
　X-ray random intensity ratio in X-ray diffraction measurement, with respect to the X-ray intensity of a powder sample having a random orientation distribution, the intensity ratio of the X-ray intensity of the hot rolled steel sheet sample to be measured, a suitable X-ray tube subjected to X-ray diffraction intensity measurement of alpha {002} surface using a diffractometer method using a ball, it is determined by comparison with the diffraction intensity of the random sample.
[0040]
　If the measurement is difficult due to X-ray diffraction, using EBSD (Electron Back Scattering Diffraction Pattern) method, 1/5 or less of the average particle size measurement interval of pixels in a region where crystal grains can be measured more than 5000 measured may be measured X-ray random intensity ratio from the distribution of the pole figure or ODF (Orientation distribution Function).
[0041]
　
　hot rolled steel sheet according to the present embodiment, assuming application to improve or body weight of the collision safety of automobiles and the like, the tensile strength and more than 1180 MPa. Tensile limit strength is not particularly provided, it is preferably not more than 2000MPa were evaluated in the toughness.
[0042]
　Next, a method for manufacturing the hot rolled steel sheet according to the present embodiment.
[0043]
　Method for producing a hot rolled steel sheet according to the present embodiment includes the steps of following (a) ~ (e):
　(a) the slab having the component composition described above 1100 ° C. or higher, the heating step of heating to below 1350 ° C. ;
　(b) the slab after heating, four or more a rolling step of rolling using a rolling mill having a plurality of stands, the total length of the last four stands of the plurality of stand below 18m There, thickness reduction before and after the last four stand step satisfies the following formula 1
　　1.2 ≦ ln (t 0 /T)≦3.0 (equation 1)
　where, t 0 is the last a plate thickness immediately before the four stands, t is the plate is thick immediately after exiting from the last four stands;
　and rolling temperature in strain rate and final stand in the final stand of four stands (c) of the last but satisfies Equations 2 and 3 below Degree
　　11.0 ≦ log (v × exp ( 33000 / (273 + T)) ≦ 15.0 ( Equation 2)
　　T ≧ Ar 3 point (Equation 3)
　Here, v is an strain rate in the final stand (/ s) , T is a rolling delivery temperature of the final stand (° C.);
　(d) the start of the cooling within 1.0 seconds after the finish rolling, the temperature range from finish rolling temperature to 750 ℃ 100 ℃ / s or higher cooling step to cool at an average cooling rate; and
　(E) after the cooling step, the winding step for winding.
　Hereinafter, the respective steps will be described.
[0044]
　
　before hot rolling (hot rolling), heat against the slab. When heating the slab having the same chemical composition as the hot rolled steel sheet according to the embodiment, obtained by continuous casting or the like, the temperature of the heating is less than 1100 ° C., homogenization of the slab becomes insufficient. In this case, the strength and workability of the resulting steel sheet is lowered. On the other hand, if the heating temperature is above 1350 ° C., the initial austenite grain size is increased, the average particle size of prior austenite grains becomes difficult to fabricate the structure of the steel plate to be less than 3.0 [mu] m. Therefore, the heating temperature, 1100 ° C. or more and less than 1350 ° C..
[0045]
　
　In the rolling step, the continuously tandem rolling for rolling steel plate using a rolling mill having four or more of the plurality of stands, among the plurality of stands, the total distance of the last four stands, cumulative strain rolling in the four stands with (thickness reduction), it is important to control the rolling temperature and strain rate at the final stand. Mill for the tandem rolling, the strain if appropriate strain range of four successive roll stands at the rear end is accumulated. Also, in the final stand, by optimizing the strain rate and rolling temperature can be recrystallized austenite by strain accumulated. Usually, the finishing stands of hot rolling 6 stages or seven stages are mainstream. It is, of course, not limited to this number of stages, in the present invention, among the plurality of stands, to the proper range strain amount and strain rate by controlling the rolling of the last four stages.
[0046]
　Specifically, the four or more of the plurality of stands, the total length of the last four stands are arranged such that below 18m. Because steel is a tandem rolling linked, if it is the optimized strain rate at the final stand of the four or more stands, interpass time between the last four stand (3), strain it can be adjusted to the rolling speed and reduction ratio which can accumulate. That is, the final stand delivery side of the rolling speed and rolling reduction Kimare, rolling speed of the previous stand determines. For example, the final one rolling speed × rolling speed = final stand of the previous stand (reduction ratio of 1 final stand). Further, the path between the time = path distance / final one rolling speed of the previous stand. Therefore, since the strain true accumulating the path distance (down plate thickness), it is possible to obtain the interpass time and strain rate of all of the stand. Since the total length of the last four stand interpass time is long in 18m greater, it is impossible to accumulate the required strain for recrystallization, the aspect ratio of prior austenite grains is increased, X-rays random intensity ratio Chisanaru is. The lower limit of the total length of the last four stands, in view of easily controlling the inter-path is preferably 10m or more.
[0047]
　In the last four stands, to impart strain of formula 1.
　　　≦ ln 1.2 (t 0 /T)≦3.0 (Equation 1)
　where, ln (t 0 / t) represents strain true accumulating decreased plate thickness (the logarithmic strain), t 0 is the last plate thickness of just before entering the four stand, t is the thickness of the immediately after exiting from the last of the four stand. ln (t 0 in value is less than 1.2 / t), the strain necessary for recrystallization at the final stand is not granted, the aspect ratio of prior austenite is increased. ln (t 0 in value than 3.0 in / t), since the inter-path is too large decrease thickness time becomes longer, sufficient strain can not be applied in the final stand, it can be recrystallized no, the aspect ratio of prior austenite increases.
[0048]
　In the final stand of the last four stands, it is rolling at a strain rate and the rolling temperature satisfy the following formula 2 and formula 3.
　　　11.0 ≦ log (v × exp ( 33000 / (273 + T)) ≦ 15.0 ( Equation 2)
　　　T ≧ Ar 3 point (Equation 3)
　Here, v is the strain rate of the final stand (/ s), . T is a rolling delivery temperature of the final stand (℃) equation 2, Zener's-Hollomon factor (Z factor) is a function of strain rate and temperature:

It was derived based on the. The value is less than 11.0 log (v × exp (33000 / (273 + T)), or rolling temperature strain rate slow because of the high or both, an average particle size of prior austenite grains obtained becomes coarse the value 15.0 Ultra .log (v × exp (33000 / (273 + T)), because the strain rate rolling temperature is low or both or fast, the austenite can not be recrystallized, large aspect ratio becomes, X-rays random intensity ratio becomes smaller. Moreover, strain rate also affects the growth time of recrystallized grains of austenite. that is, the standard deviation of the recrystallized grain size slower strain rate increases. on the other hand, can no longer be assured the time required when the strain rate is too fast to recrystallization during hot finish rolling will not cause recrystallization. in addition, the strain rate and the rolling temperature The relationship long as it satisfies the above equation 2, there is no limitation on a respective values. However, in order to put the aspect ratio of prior austenite grain size in a predetermined range, it is necessary to recrystallized austenite single phase. When the ferrite is generated during rolling, recrystallized austenite inhibited by ferrite, the crystal grain size becomes flat, the rolling delivery side is essential be carried out in single-phase austenite. final stand of the last four stands in satisfies the equation 2, and .T need to satisfy equation 3 is rolling delivery temperature of the final stand, T method of manufacturing a hot rolled steel sheet according to the present embodiment Ar 3 by at least points , .Ar can be obtained more tensile strength 1180 MPa 3 points following
　formula: Ar 3 = 901-325 × C + 33 × Si-92 × Mn + 287 × P
　It is calculated by.
[0049]
　
　After the end of rolling, in order to keep the recrystallized austenite structure to elaborate made by rolling fine to start cooling within 1.0 seconds, the finish rolling temperature temperature range of up to 750 ° C. The 100 ° C. / s cooling the above average cooling rate. The cooling start time of 1.0 seconds greater, since the time from the recrystallization was expressed to start cooling is applied, the fine region is absorbed by the coarse grains prior austenite grains is increased by Ostwald growth, small coefficient of variation now, the toughness is reduced. The cooling rate is less than 100 ° C. / s, also occurs grain growth austenite during cooling, the average particle size of prior austenite grains are coarsened, the coefficient of variation is small. Cooling rate of less than 750 ° C., since the influence of the austenite grain size is small, the cooling rate for obtaining a hot rolled tissue of interest can be freely selected.
[0050]
　The upper limit of the cooling rate is not particularly limited, considering the equipment constraints, etc., also, in order to the tissue distribution in the thickness direction more uniform, is preferably at most 600 ° C. / s. Since the cooling stop temperature is maintained stably by fine prior austenite grain size, it is preferable to cool to 550 ° C. or less.
[0051]
　
　is in the transformation from austenite structure yelling made in the cooling step organization there is no restriction. If the hot rolled steel sheet while hot rolled products, in order to more stably secure the tensile strength of at least 1180 MPa, preferably wound below 550 ° C.. When performing cold rolling in the next step, 550 ° C. or more for decreasing the load of cold rolling, coiling at less than 750 ° C., it is preferred to soften.
[0052]
　(Other Steps)
　hot-rolled steel sheet of the present embodiment, pickling, cold rolling, and subsequent processing is not essential, the hot-rolled steel sheet produced, pickling, may be cold-rolled.
[0053]
　For example, in order to remove scale on the surface of the hot-rolled steel sheet, subjected to a pickling process, the plate thickness of the steel sheet may be adjusted by subjecting the cold-rolled process. The conditions of the cold process is not particularly limited, processability, in terms of accuracy of plate thickness, cold rolling rate of 30% or more, preferably 80% or less. Cracks and the steel edge by a cold rolling rate 80% or less, an excessive increase in strength work hardening can with your k suppress.
[0054]
　The cold-rolled steel sheet may be annealed. Since the austenite grain size yelling made by hot rolling to suppress the coarsening, the maximum temperature of the annealing is preferably at 900 ° C. or less. On the other hand, from the viewpoint of productivity to prevent such a long time of building a rolled structure by recrystallization, preferably at least 500 ° C.. After annealing, the straightening and surface roughness adjustment may be subjected to the temper rolling purposes. The temper rolling, so as not to leave rolling tissue, it is preferable that the reduction ratio of 1.0% or less.
[0055]
　Hot-rolled steel sheet, for surface improving corrosion resistance, electroplating, may be subjected to hot dipping, or alloy hot-dip plating. In the plating process, to give heat to the austenite grain size yelling made by hot rolling process is prevented from being coarsened, is preferably 900 ° C. or less. After plating, straightening or roughness adjustment may be further subjected to temper rolling for the purpose of. The temper rolling process, to avoid leaving rolling tissue, it is preferable that the reduction ratio of 1.0% or less. If the hot-rolled steel sheet cold rolled, cold rolled steel sheet, the above electrical plating may be carried out hot dipping, or alloy hot-dip plating, and temper rolling.
Example
[0056]
　Hereinafter, the hot-rolled steel sheet of the present invention will be specifically described by way of examples. However, conditions in the examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to the following examples. Without departing from the gist of the present invention, as long as they achieve the object of the present invention, it is also possible to put into practice after appropriate modifications or variations within the scope adaptable to the gist. Accordingly, the present invention can employ various conditions so that they are all included in the technical features of the present invention.
[0057]
　Chemical composition and Ar shown in Table 1 3 a steel having a point were melted in a converter furnace, and the slab thickness 230mm at continuous casting. Thereafter, the slab is heated to a temperature of 1200 ° C. ~ 1250 ° C., the billet was subjected to rough rolling, the heating temperature shown in Table 2, finishing temperature, cooling rate, and coiling temperature, heating, finish rolling, cooling, and winding conduct is taken up, to produce a hot-rolled steel sheet.
[0058]
　Also Table 2 shows steel grade components, finish rolling conditions, and the plate thickness of the steel sheet was used. In Table 2, the "strain rate" strain rate of the final stand of the finishing rolling stand for successive "IriAtsu" rolling mill finishing four or more of the plurality of stands are consecutive, immediately prior to entering the last four stands entry side thickness of, "Deatsu" is the delivery side thickness immediately after leaving the last four stands, "stand length", the total length of the last four stands of the plurality of stands, "start time" time from the completion of the finish rolling of the final stand to the start cooling, the average cooling rate from the "cooling rate" is the finish rolling temperature to 750 ° C., and "coiling temperature" is the coiling temperature after the end of cooling.
[0059]
[Table 1]

[0060]
[Table 2]

[0061]
　The steel sheet thus obtained, the position of 1/4 of the sheet thickness from the surface after mirror polishing the L cross section of the steel sheet, 3% nital - corroded (3% nitric acid in ethanol), the crystalline to one field of view by imaging the range SEM observation particle is about 10,000 observation, and image analyzed using image analysis software (WinROOF), the average grain size of prior austenite grains, the standard deviation of the particle size distribution, and average aspect ratio value was calculated. It was calculated variation coefficient by dividing the standard deviation of the particle size distribution by the average grain size.
[0062]
　In the central portion in the 1/4 position of the thickness from the surface in the L cross-section of the steel sheet of the present embodiment, by using the EBSD (Electron Back Scattering Diffraction Pattern) method, 1/5 or less of the measurement interval is the average particle size of the pixels, in the region where crystal grains can be measured more than 5000, the distribution of pole figures or ODF (orientation distribution Function), to measure the {001} <110> orientation X-ray random intensity ratio of.
[0063]
　The tensile test of the steel sheet, a JIS5 test piece No. rolling width direction (C direction) of the steel sheet was taken, according to JISZ2241, the tensile strength was evaluated for TS (MPa).
[0064]
　It was measured ductile brittle transition temperature as toughness evaluation of steel plates. Measurement of ductile brittle transition temperature is V-notch test piece of 2.5mm subsize specified in JISZ2242, perform Charpy impact test C direction notch, the temperature at which the brittle fracture rate becomes 50% and the ductility-brittle transition temperature did. Also, final thickness of the steel sheet was measured in total thickness for that of less than 2.5 mm. Ductile brittle transition temperature is as acceptable as long as -50 ° C. or less. The anisotropy, the absorbed energy of the C-direction notch and L direction notch was measured at -60 ° C., to calculate the ratio (L direction / C direction), 0.6 or more, if 1.0 or less, It was excellent in anisotropy.
[0065]
　Table 2, prior austenite grain size (formerly γ grain size), the coefficient of variation of the prior austenite grains, the aspect ratio of prior austenite grains, {001} <110> orientation X-ray random intensity ratio of tensile strength, ductility-brittle transition temperature , and it shows the measurement results of anisotropy. As shown in Table 2, in the example of the present invention tensile strength of more than 1180 MPa, the transition temperature is at -50 ° C. or less, were excellent in strength and toughness.
[0066]
　In contrast, the value of formula 1 in Test No. 6 is less than 1.2, the austenite to cumulative strain is insufficient in the last four stands can not be recrystallized, the aspect ratio becomes 2.0 than . Therefore, anisotropy has become less than 0.6.
[0067]
　In Test No. 15, the value is 3.0 ultra next to Formula 1, the plate thickness decreased is too large at the end of four stands, because the path between the time is long, the strain necessary for recrystallization can not be imparted, the aspect ratio of 2.0 than the high, anisotropy has become less than 0.6.
[0068]
　In Test No. 17, the value of the finish rolling temperature be lower expression 2 is above 15.0, for austenite could not be recrystallized, accumulation of high aspect ratio X-ray random intensity ratio is small (texture degree is low), anisotropy has become a less than 0.6.
[0069]
　In Test No. 24, the value of the expression 2 for the slow high strain rate rolling finishing temperature has become less than 11.0, the average particle size of the austenite grains are coarsened, the transition temperature be -50 ° C. greater , a toughness is inferior.
[0070]
　In Test No. 28, the cooling start time 1.0 second than a long, since the time from the recrystallization was expressed to start cooling is standing, with fine region is absorbed by the coarse grains prior austenite grains by Ostwald growth become large, because the coefficient of variation is small, it is toughness inferior.
[0071]
　In Test No. 32, a greater stand length of the last four stand 18m, long interpass time, in order to strain necessary for recrystallization can not be accumulated, a large aspect ratio X-ray random intensity ratio is small ( low degree of integration of the texture), anisotropy has become a less than 0.6.
[0072]
　In Test No. 34, the finish temperature Ar according to Table 1 3 tensile strength because it is below the point it becomes lower. Furthermore, the cumulative strain is small in equation 1 the value of the last four stand is less than 1.2, more exceeds the value of the rolling finishing temperature is low Equation 2 is 15.0, the aspect ratio is large X-ray random intensity ratio is small (low degree of integration of the texture), anisotropy becomes less than 0.6.
[0073]
　In Test No. 35, the cumulative strain is small value of formula 1 the last four stand is less than 1.2, more stand-length of the last four stands have exceeded 18m, high aspect ratio X linear random intensity ratio is decreased (low degree of integration of the texture). Therefore, anisotropy has become less than 0.6.

The scope of the claims
[Requested item 　By 1]
　mass%, C: 0.10% or more, 0.50% or
　less, Si: 0.10% or more, 3.00% or
　less, Mn: 0.5% or more, 3.0% or
　less, P: 0. 100% or
　less, S: 0.010% or
　less, Al: 1.00% or less, and
　N: 0.010% or less
　contained, and the balance Fe and impurities,
　of thickness from the surface in the L section 1/4 metal structure at position, the average value of the aspect ratio of 2.0 or less, an average particle diameter of 0.1μm or more, 3.0 [mu] m or less, and variation coefficient of 0.40 or more is a standard deviation / average particle diameter of the particle size distribution in a prior austenite grain, and includes a {001} <110> orientation texture X-ray diffraction intensity ratio is 2.0 or more for the random sample,
　more tensile strength 1180MPa
　hot rolling and having a steel sheet.
[Requested item 2]
　By
　mass%, Ti: 0.02% or more, 0.20% or
　less, Nb: 0.00% or more, 0.10% or
　less, Ca: 0.0000% or more, 0.0060% or
　less, Mo: 0. 100% or more, 0.50% or less, and
　Cr: 0.0% or more, 1.0% or less
　, further characterized by containing one or two or more selected from the group consisting of, claim 1 hot-rolled steel sheet according to.
[Requested item 3]A method of manufacturing a hot rolled steel sheet according to claim 1 or 2, the manufacturing method of the hot rolled steel sheet comprising the steps (a) ~ (e) below:
(a) claims the slab having a composition as set forth in 1 or 2 1100 ° C. or more, the heating step is heated to less than 1350 ° C.;
a (b) the slab after the heating, by using a rolling mill having four or more multiple stand rolling step a rolling step, the total length of the last four stands of the plurality of stands is not more than 18m, the thickness reduction before and after the last four stand satisfies equation 1 below to
　　≦ ln 1.2 (t 0 /T)≦3.0 (equation 1)
　where, t 0 is the thickness immediately before entering the last four stands, t exits from the last four stands It is the thickness of the immediately after;
(c) the last four stands The rolling temperature in the final stand and strain rate at the final stand, satisfies equation 2 and equation 3 below step
　　11.0 ≦ log (v × exp ( 33000 / (273 + T)) ≦ 15.0 ( Equation 2)
　　T ≧ Ar 3 point (equation 3)
　here, v is the strain rate (/ s) in the final stand, T is a rolling delivery temperature (℃) in the final stand;
And (d) to start cooling on the within the end of rolling after 1.0 seconds, the cooling step the temperature range from finish rolling temperature to 750 ° C. and cooled at an average cooling rate of more than 100 ° C. / s; and
(e) the after the cooling process, the winding step of performing the winding.