The present invention relates to toughness and hole expandability of the above excellent tensile strength 980MPa hot-rolled steel sheet in the balance and a manufacturing method thereof.
BACKGROUND
[0002]
　Recently, in order to improve fuel efficiency and collision safety of automobiles, the vehicle body weight have been worked on intensively by application of high-strength steel sheet. Upon application of high-strength steel sheets is important to ensure the press-formability. Composite structure (Dual Phase) steel sheet (hereinafter DP steel sheet) is constituted by a composite structure of soft ferrite phase and a hard martensite phase, it is generally known to have good press formability. However, DP steel sheet, because it may result in significantly crack voids from the interface of the two different phases hardness occurs, there is a problem of poor hole expandability, high hole expandability of such undercarriage components is required It was not suitable for that purpose.
[0003]
　In Patent Document 1, a hot rolled steel sheet may include a martensite or bainite or the like in addition to ferrite and then, improved hot-rolled steel sheet stretch flange formability is evaluated has been proposed by limiting hole expansion ratio. In Patent Document 2, in order to achieve both elongation and hole expandability, high strength hot rolled steel sheet by controlling the aspect ratio and average particle size of the martensite grain coverage and ferrite grains by ferritic grains has been proposed .
CITATION
Patent Document
[0004]
Patent Document 1: Patent No. 3945367 Patent Publication
Patent Document 2: JP 2015-86415 JP
Summary of the Invention
Problems that the Invention is to Solve
[0005]
　Recently, further weight reduction oriented automobiles, high-strength hot-rolled steel sheet having a higher hole expandability and toughness against a background of complication of parts is required.
[0006]
　In Patent Document 1, Ar 3 point - "Ar 3 performs finish rolling at a temperature of a temperature range of point + 100 ° C." to start cooling within 0.5 seconds after finishing the finish rolling, the finishing temperature " Ar 3 up to point -100 ° C. "be cooled at an average cooling rate of more than 400 ° C. / sec is described. In Patent Document 1, after the completion of such a finish rolling, by performing strong cooling without giving little air time, with ferrite grains are very fine grained, the desired texture is formed, hot-rolled steel sheet plane anisotropy and excellent small workability has been described that the obtained. However, in Patent Document 1, the improvement of toughness, not particularly necessarily sufficient studies have been made from the viewpoint of toughness and stretch flangeability improve, in the hot-rolled steel sheet according to hence the Patent Document 1, the material still there is room for improvement with respect to properties.
[0007]
　In Patent Document 2, it is recrystallized austenite structure in the rolling stand of the previous one of the final stages in the finishing rolling, such as by subsequently introducing strain traces by soft reduction at the grain boundaries of austenite, covering the martensite grains It has been described to control the average particle size and aspect ratio of ferrite grains, finally high-strength hot-rolled steel sheet excellent in balance between elongation and hole expandability are described to be obtained. However, in Patent Document 2, the improvement of toughness, not particularly necessarily sufficient studies have been made from the viewpoint of toughness and stretch flangeability improve, in a high-strength hot-rolled steel sheet according to hence the Patent Document 2, still there is room for improvement in terms of its material properties.
[0008]
　The present invention, while securing the indispensable toughness high strength steel for the above requirements, excellent tensile strength 980MPa or more hot-rolled steel sheet hole expandability capable of satisfying the processability and the manufacturing method thereof an object of the present invention is to provide.
Means for Solving the Problems
[0009]
　Previously also various efforts to suppress the generation of voids at the interface of the martensite and ferrite toward the material improvement of DP steel sheet have been made. Although increasing the crack propagation path with finer particle size in order to improve the toughness is generally known, each tissue effects and martensite and the ferrite grain size in the composite structure, such as DP steel effect has not been in the clear on. The present inventors focused on the nucleation site or grain growth behavior of the ferrite produced during after hot finish rolling cooling, a result of intensive studies, the average particle size of the ferrite grains covering the martensite grains improve material It was found to be particularly important to improve the toughness and hole expandability of both characteristics. Further, as an effect for each martensite and ferrite, improving the hole expandability by coating the martensite grain, crack necessary for further improvement of toughness by fine average particle size of the ferrite grains the coating It was found to achieve suppression of the propagation. However, the method as described in Patent Document 2, i.e. recrystallized austenite structure, in subsequent method of introducing strain traces by soft reduction at the grain boundaries of austenite, even to control the shape and coverage of the ferrite since austenite grains coarse, tend to ferrite grains even coarse, it is possible to reduce the average particle size of the ferrite grains to fine levels as a result there is often difficult. Accordingly, the present inventors have further investigated, to express the dynamic recrystallization austenite in hot rolling, and found that the crystal grains of the austenite can be introduced with high dislocation density only One austenite grain boundaries fine. Specifically, it is necessary to apply a large strain to express the dynamic recrystallization austenite. In order to reliably express the dynamic recrystallization of austenite in the rolling by rolling stand during finish rolling, rolling load of each rolling stand of the previous one than the rolling load of the rolling stand of the final plurality of consecutive it is important to retain 80% of more. The crystal grains of austenite to a fine in doing so And it is possible to introduce a high dislocation density in the austenite grain boundaries, it is possible to increase the production of fine ferrite grains to improve the frequency of generation ferrite nucleation from austenite grain boundaries during subsequent cooling, whereas in, it is possible to martensite grains miniaturization was transformed from austenite grains during the cooling. Moreover, such for fine martensite grains will be covered by a number of fine ferrite grains of the resulting above upon cooling as well, it also increases significantly the coverage of martensite grains of a ferrite grain it is possible. Thus, not only it is possible to reliably prevent the necessarily sufficient consideration is which do not toughness deterioration in Patent Documents 1 and 2, it is possible to achieve both toughness and hole expandability at a high level.
[0010]
　The present invention has been made based on the above findings, and its gist is as follows.
　(1) in
　mass%, C: 0.02% or more, 0.50% or
　less, Si: 2.0% or
　less, Mn: 0.5% or more, 3.0% or
　less, P: 0.1% or less ,
　S: 0.01% or
　less, Al: 0.01% or more, 1.0% or less, and
　N: 0.01% or less
contain, comprises the balance consisting of Fe and impurities,
　the area fraction in tissue fraction of 10% or more of martensite phase, 40% or less, includes a structure fraction of 60% or more two-phase structure of ferrite phase,
　the mean particle size of the ferrite grains is equal to or less than 5.0 .mu.m,
　by ferrite grains wherein the martensite grain of coverage is 60%, the hot-rolled steel sheet.
　Here, as the martensite grain of coverage by the ferrite grains, which when the whole martensite grain boundary length is 100, and displays the length ratio of the martensite grain boundary occupied by ferrite grains in percentage it is.
　(2) In addition, by
　mass%, Nb: 0.001% or more, 0.10% or
　less, Ti: 0.01% or more, 0.20% or
　less, Ca: 0.0005% or more, 0.0030% or less ,
　Mo: 0.02% or more, 0.5% or less, and
　Cr: 0.02% or more, 1.0% or less
, characterized by containing one or more of the heat according to the above (1) rolled steel sheet.
　(3), wherein an average particle diameter of the ferrite grains is 4.5μm or less, hot-rolled steel sheet according to (1) or (2).
　(4), wherein the coverage is 65% or more, the (1) hot-rolled steel sheet according to any one of - (3).
　(5) the structural fraction of the martensite phase of 10% or more, and less than 20%, hot-rolled steel sheet according to any one of the above (1) to (4).
　(6) above (1) a step of casting a slab having a composition according to any one of - (5),
　the cast slab comprising the steps of hot rolling, at least four consecutive said slab the method comprising finish rolling using a rolling mill having a roll stand, wherein is a respective rolling load of the last three roll stands in the finish rolling is more than 80% of the rolling load of the rolling stand of the previous one, and the average value of the finish rolling temperature at the three rolling stands of the final is 800 ° C. or higher, the process is 950 ° C. or less, and
　finished steel sheet was forcibly cooled rolling, then a winding process, the forced cooling, the finishing is started rolling after completion within 1.5 seconds, the steel sheet to 30 ° C. / sec or more average cooling rate at 600 ° C. or higher, primary cooling to cool to 750 ° C. or less, the primary cooling Process including the steel sheet for more than 3 seconds, intermediate cooling naturally cooled below 10 seconds, and the cooling to a secondary cooling the steel sheet after the intermediate air-cooled to 200 ° C. or less at an average cooling rate of more than 30 ° C. / sec
Characterized in that it comprises a method for manufacturing a hot-rolled steel sheet.
The invention's effect
[0011]
　According to the present invention, it is possible to provide an excellent hot-rolled steel sheet in the balance between toughness and hole expandability, it is possible to provide a hot rolled steel sheet suitable for press parts requiring high processing. Further, hot-rolled steel sheet of the present invention has a tensile strength of at least 980 MPa, because the balance of toughness and hole expandability can is excellent at a high level, weight reduction of the vehicle body due to thinning of the body material, such as an automobile, integrally molded of parts, shortening of processing steps are possible, improvement in fuel consumption, it is possible to reduce the manufacturing cost, it has high industrial value.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012]
FIG. 1 is a conceptual diagram illustrating the coverage of martensite grains of a ferrite grain.
DESCRIPTION OF THE INVENTION
[0013]

　The present invention focuses on the nucleation site or grain growth behavior of the ferrite produced during cooling after hot finish rolling, the ratio of the ferrite grains to cover the average particle size and martensite grains of ferrite grains controlling the is to provide toughness and hole expandability of the hot-rolled steel sheet excellent high strength balanced by. Hot-rolled steel sheet of the present invention has a predetermined composition, in terms of area fraction, tissue fraction of 10% or more of martensite phase, 40% or less, includes a structure fraction of 60% or more two-phase structure of ferrite phase , an average particle size of the ferrite grains is equal to or less than 5.0 .mu.m, martensite grains coverage by the ferrite grains are characterized by a greater than 60%.
[0014]
　Will be described in detail for each constituent element of the present invention are described below. First, we describe reasons for limiting the components of the present invention (composition). % Of component content means mass%.
[0015]
[C: 0.02% or more, 0.50% or less]
　C is an important element that determines the strength of the steel sheet. In order to obtain a desired intensity must be contained 0.02% or more. Preferably 0.03% or more, more preferably 0.04% or more. However, in order to degrade the toughness 0.50% are ultra-containing, the upper limit is 0.50%. C content may be 0.45% or less, or 0.40% or less.
[0016]
: [Si 2.0% or
　less] Although Si is effective for raising the strength as a solid solution strengthening element, to cause the toughness deterioration, and 2.0% or less. Preferably 1.5% or less, more preferably less than 1.2% or 1.0%. Si may not contain, i.e. the Si content may be 0%. For example, Si content is 0.05% or more, may be 0.10% or more, or 0.20% or more.
[0017]
[Mn: 0.5% or more, 3.0% or
　less] Mn is effective for increasing strength as hardenability and solid solution strengthening element. To obtain the desired intensity is required than 0.5%. Preferably 0.6% or more. Excessive addition to produce harmful MnS in hole expandability, so the upper limit to 3.0% or less. Mn content may be less or 2.0% or less 2.5%.
[0018]
: [P 0.1% or less]
　desirable than low P, with an adverse effect on the workability and weldability 0.1 percent super-containing, so fatigue properties also decrease, to 0.1% or less. Preferably 0.05% or less, more preferably 0.03% or less. P content may be 0%, but excessive reduction since the cost is increased, preferably 0.0001% or more.
[0019]
[S: 0.01% or less]
　Since S is to generate low as desired, too large, the toughness isotropic harmful inclusions, such as MnS in resistance, the need to be 0.01% or less. Severe when the low temperature toughness is required, preferably set to 0.006% or less. S content may be 0%, but excessive reduction since the cost is increased, preferably 0.0001% or more.
[0020]
[Al: 0.01% or more,
　1.0%] Al is an element necessary for deoxidation is added 0.01% or more. For example, Al content may be 0.02% or more, or 0.03% or more. However, excessive addition generates alumina precipitated on a cluster-like, in order to deteriorate the toughness, the upper limit is made 1.0%. For example, Al content may be less or 0.6% or less 0.8%.
[0021]
[N: 0.01% or less]
　N forms coarse Ti nitrides at high temperatures, deteriorating the toughness. Therefore, to 0.01% or less. For example, N content may be less or 0.005% 0.008%. N content may be 0%, but excessive reduction since the cost is increased, preferably 0.0001% or more.
[0022]
　Although not required to satisfy the required properties, or to reduce the manufacturing variation, in order to further improve the strength, one or more of the following elements in order to further improve the toughness and / or hole expandability it may be added.
[0023]
[Nb: 0.001% or more, 0.10% or
　less] Nb is a possible to reduce the crystal grain size of the hot-rolled steel sheet, it is possible to increase the strength by NbC. Nb content is obtained, the effect at 0.001% or more. For example, Nb content may be 0.01% or more, or 0.02% or more. On the other hand, 0.10 percent the effect is to saturate, so the upper limit is 0.10%. For example, Nb content may be less or 0.06% or less 0.08%.
[0024]
: [Ti 0.01% or more, 0.20% or less]
　with the Ti to precipitation strengthening ferrite, the rate of transformation is delayed, since the control is enhanced, is an effective element for obtaining a ferrite fraction of aim . To obtain a balance of good toughness and hole expandability is necessary to add 0.01% or more. However, the 0.20% super addition to the TiN inclusions are produced which was caused, since the hole expandability is deteriorated, the content of Ti is 0.01% or more and 0.20% or less. For example, Ti content should be 0.02% or more, or 0.03% or more, may be 0.15% or less, or 0.10%.
[0025]
[Ca: 0.0005% or more, or less
　0.0030%] Ca Many dispersing fine oxides in the deoxidation of molten steel, the tissue with which is a suitable element for refining, steel in the desulfurization of the molten steel the S in the fixed as CaS spherical, is an element that improves the hole expandability by suppressing the formation of stretched inclusions such as MnS. These effects are the amount added is obtained from 0.0005%, to saturate at 0.0030% or the content of Ca is 0.0005% or more and 0.0030% or less. For example, Ca content may also be 0.0010% or more, or 0.0015% or more, may be 0.0025% or less.
[0026]
[Mo: 0.02% or more, 0.5% or
　less] Mo is an element effective as a precipitation strengthening of ferrite. In order to obtain this effect is desirably added in 0.02% or more. For example, Mo content may be 0.05% or more, or 0.10% or more. However, addition of a large amount for the handling of the slab increases the susceptibility to cracking of the slab becomes difficult, so the upper limit is 0.5%. For example, Mo content may be less or 0.3% or less 0.4%.
[0027]
[Cr: 0.02% or more, 1.0% or
　less] Cr is an element effective for improving the strength of the steel sheet. In order to obtain this effect it is necessary to add 0.02% or more. For example, Cr content may be 0.05% or more, or 0.10% or more. However, addition of a large amount to 1.0% of upper to lower the ductility. For example, Cr content may be less or 0.5% or less 0.8%.
[0028]
　In the hot rolled steel sheet of the present invention, the balance other than the above components, Fe and impurities. Here, the impurities, in producing the hot rolled steel sheet industrially, starting raw materials such as ores and scrap, a component mixed by various factors of the manufacturing process, hot rolling of the present invention it is intended to include those that are not intentionally added components against steel. Further, the impurity, be an element other than components described above, even an element to which the element-specific operations and effects are included in the hot-rolled steel sheet at a level that does not affect the characteristics of the hot rolled steel sheet according to the present invention it is intended to embrace.
[0029]
　Next, a description will be given crystal structure of the hot rolled steel sheet of the present invention.
[0030]
[Martensitic phase structure fraction of 10% or more, 40% or less, of the two-phase structure at least 60% tissue fraction of the ferrite phase]
　hot rolled steel sheet of the present invention, comprises a two-phase structure of martensite phase and ferrite phase . Here, "two-phase structure" in the present invention, is intended to refer to tissue total martensite phase and ferrite phase is an area ratio of 90% or more. For the remainder, it may contain perlite and bainite.
[0031]
　The steel sheet containing the above two-phase structure, the soft are hard martensite is dispersed in the high ferrite elongation in, thereby achieving a high elongation despite high strength. However, in such a steel sheet, high strain near the hard tissue is concentrated, there is a disadvantage that resistance hole expansion for crack propagation speed is increased is lowered. Therefore, although consideration is often on the size phase fraction and martensite grains of ferrite and martensite, ferrite grains size and martensite grains actively controlled to steel an array of ferrite grains covering the material examples of examining the possibility of improvement is little. The present invention, by appropriately controlling the sequence of ferrite grains to cover the average particle size and martensite grains of ferrite grains in a two-phase structure consisting of martensite phase and ferrite phase, excellent balance between toughness and hole expandability and there is provided a hot rolled steel sheet of high strength. According to the present invention, the hot-rolled steel sheet, martensite of 10% or more in area fraction of the steel sheet structure, containing 40% or less, it is necessary to contain the ferrite phase of 60% or more. For example, the martensite phase may also be in an area fraction of 12% or more or 14% or more, it may be 35% or less, or 30% or less. Further, the ferrite phase may be 70% or 80% in area fraction, the upper limit is 90% or less, or may be 85% or less. In particular fraction of martensite phase toughness and hole expandability balance is excellent 10% or more and less than 20% or 18% or less. When the fraction of martensite phase is less than 10%, an average particle size of the ferrite grains is inevitably increased, the toughness is lowered. Martensite phase fraction is greater than 40% when it comes to the ductility of the poor martensite phase is lowered hole expandability for the principal. The fraction of the ferrite phase is less than 60% not sufficient strain relief of a ferrite grain, and because the workability can not be secured, it is impossible to achieve both the toughness and hole expandability at a high level.
[0032]
　In the present invention, structural fraction of the ferrite phase and martensite phase is determined as follows. First, a sample is taken as an observation plane parallel plate thickness cross section in the rolling direction of the hot-rolled steel sheet, after the corrosion with a reagent such as nital and Repera polished the observation surface, field emission scanning electron microscope (FE-SEM ) image analysis using an optical microscope or the like, more specifically tissue 1/4 position of the sheet thickness was observed by an optical microscope at 1000 times magnification, image analysis it in the field of view of 100 × 100 [mu] m . The average of these measurements at 10 or more visual fields is determined as the structural fraction of the ferrite phase and martensite phase, respectively.
[0033]
[Martensite grain coverage by the ferrite grains is greater than 60%]
　In the present invention, one of the most important features is an array of ferrite grains. Ferrite grains in the present invention is arranged in a form surrounding the martensite grains. Figure 1 is a conceptual diagram illustrating the coverage of martensite grains of a ferrite grain. As shown in FIG. 1, of the martensite grain boundary, it defines the ratio of the total martensite grain boundary length of the portion occupied by the ferrite grains and coverage. In the present invention, the length of the portion occupied by all martensite grain boundary length and the ferrite grains is determined using an optical microscope, for example, electron back scattering pattern analysis (Electro BackScattering Diffraction: EBSD) Quantitative using it can be obtained in. In the present invention, the coverage of martensite grains by ferritic grains, and organizational 1/4 position of the sheet thickness select the field of view of random 100 × 100 [mu] m, EBSD like the 500 or more martensite grains definitive than 10 field All martensite grain boundary length using an optical microscope (martensite unoccupied "sum of the ferrite grains of the outer peripheral length corresponding to the martensite grain boundary occupied by ferrite grain" and the "ferrite grains total) and the length of the portion occupied by the ferrite grains of the length of the site grain boundary "(" sum of the ferrite grains of the outer peripheral length corresponding to the martensite grain boundary occupied by ferrite grains " ) is calculated by determining the. When martensite grains coverage by the ferrite grains exceeds 60% increases the connectivity of the ferrite, it is possible to suppress the generation of voids generated during processing, thereby improving the toughness and hole expandability. If coverage is low, coupling of the ferrite is lowered, i.e. the more the gap between the ferrite grains covering the martensite grains, since the stress in such a gap during processing may occur cracks concentrated , it is preferable that the coverage is a higher value, for example 65% or more, 68% or more, or may be 70% or more. 70% or more in the molding undergoing more severe working It is desirable to. Moreover, the coverage may be 100%, for example, may be not more than 98% or less, or 95%.
[0034]
Average particle size of the ferrite grains is less 5.0 .mu.m]
　While, in increasing the fraction of ferrite phase in order to increase the coverage, toughness becomes inferior when the average particle size of the ferrite grains increases. Therefore, the average particle size of the ferrite grains is required to be less 5.0 .mu.m. For example, the average particle size of the ferrite grains, may also be 0.5μm or more or 1.0μm or more, and / or 4.5μm or less, 4.0 .mu.m or less, there is 3.5μm or less or 3.0μm or less at best, preferably, 0.5 [mu] m or more and 3.0μm or less. Therefore, the ferrite grain refinement by increasing the nucleation sites of ferrite transformation becomes important. In the present invention, the average particle size of the ferrite grains is measured as follows using the EBSD. The EBSD, for example, using an apparatus constituted by FE-SEM and EBSD detector, a tissue 1/4 position of the plate thickness was observed at 1000-fold magnification, image analysis in the field of view of it 100 × 100 [mu] m . Then, a boundary angle difference of the crystal grain boundary is 5 ° or more and the grain boundary, a region surrounded by the grain boundaries of the grain size of ferrite grains measured by equivalent circular diameter as grain, these in more than 10 visual field of the average of the measured values and the average particle size of the ferrite grains.
[0035]
　In hot-rolled steel sheet of the present invention, as described above, it is possible to martensite grains miniaturization as well ferrite grains. The average particle size of martensite grains is not particularly limited, for example, 1.0 .mu.m or more, it may also be 3.0μm or more or 6.0μm or more, and / or 20.0μm or less, 18.0 or less, 15 .0μm may be less than or equal to less than or 10.0μm. In Figure 1, the martensitic grains are illustrated for greater aspect than ferrite grains, hot rolled steel sheet of the present invention is not limited to such embodiments, the ferrite grain average particle size of martensite grains it is intended to encompass larger than the average grain size.
[0036]

　Next, a method for manufacturing the hot-rolled steel sheet of the present invention.
[0037]
　Hot-rolled steel sheet of the present invention comprises the steps of casting a slab having the same composition as the hot-rolled steel sheet, the cast slab comprising the steps of hot rolling, comprising at least four consecutive rolling stands the slab the method comprising finish rolling using a rolling mill, said is a respective rolling load of the last three roll stands in the finish rolling is more than 80% of the rolling load of the rolling stand of the previous one, and the last three rolling average of the finish rolling temperature in the stand 800 ° C. or higher, the process is 950 ° C. or less, and then finish rolled forced steel plate cooling, then a winding process, the forced cooling, the finish rolling after the completion is initiated within 1.5 seconds, the steel sheet to 30 ° C. / sec or more average cooling rate at 600 ° C. or higher, the primary cooling to 750 ° C. or less cooling, the steel sheet after the primary cooling 3 seconds or more, produced by a method comprising the step of including a secondary cooling for cooling the intermediate cooling naturally cooled below 10 seconds, and at an average cooling rate steel plate of more than 30 ° C. / sec after the intermediate air-cooled to 200 ° C. or less be able to.
[0038]
　Such manufacturing methods, those skilled in the art can be implemented using a variety of rolling technology is well known, is not particularly limited, for example, it is carried out by an endless rolling or the like for connecting up to rolling from casting preferable. It is possible to rolling described high load below in finish rolling by performing an endless rolling.
[0039]
[Casting slab]
　cast slab is not limited to a specific method. As the slab having the same composition as described above for the hot rolled steel sheet of the present invention is obtained, followed melting by blast furnace and electric furnace or the like, performs various secondary refining to adjust the chemical composition, followed by normal it may be cast by continuous casting or ingot method. In addition, it may be cast by a method such as thin slab casting. Although may be used scrap as a raw material for cast slab, it is necessary to adjust the chemical composition.
[0040]
[Hot rolling]
　According to the present invention, the cast slab is then subjected to hot rolling, the hot rolling tandem mill or the like to cast slabs with at least four successive roll stands using a rolling mill, each of the rolling load of the last three roll stands comprises such that 80% or more finish rolling of rolling load of the rolling stand of the previous one. Slab hand, by applying a continuously high load in the final three rolling stands in the finish rolling, it is possible to express the dynamic recrystallization austenite in the steel sheet. By expressing dynamic recrystallization austenite can be introduced a high dislocation density and finer crystal grains of the austenite and the austenite grain boundaries. As a result, it is possible to increase the production of fine ferrite grains to improve the frequency of generation ferrite nucleation from austenite grain boundaries during the subsequent forced cooling, while the austenite grains during the forced cooling transformation to martensite grains also can be miniaturized. Moreover, since such martensite grains are coated with a number of fine ferrite grains of the produced during Similarly forced cooling, it is possible to increase significantly even coverage of martensite grains of a ferrite grain .
[0041]
　When each of the rolling load of the last three roll stands is less than 80% with respect to the rolling load of the rolling stand of the previous one, static recrystallization or recovery is accelerated in rolling pass rolling stand, the dynamic strain unable to accumulate necessary recrystallization. In more detail, for example, even when subjected to hot rolling at a higher reduction rate in each rolling stand, the time between each rolling pass is increased, while the strain introduced in each rolling pass to the next rolling pass resulting in recovery. As a result, it becomes impossible to accumulate strain required for dynamic recrystallization. Therefore, when controlling the hot rolling at a reduction ratio, it is necessary to control strictly the interpass time specific short time. Moreover, even tightly controlled tentatively paths between time specific short time, when the final three of any one of rolling reduction of rolling stands is low, to satisfy the course 80% of the rolling load can not, it is impossible to accumulate the required strain in the same manner as dynamic recrystallization. In contrast, in the manufacturing method of the hot-rolled steel sheet of the present invention, by controlling the rolling load rather than a reduction ratio of hot rolling, it is possible to reliably accumulate strain. More specifically, with the accumulation of strain, load required rolling is high. Thus, by controlling the hot rolling within a certain rolling load, strain required for dynamic recrystallization is surely accumulated, and it is possible to control the accumulation amount. It does not specify the upper limit of the rolling load in particular, that of building the plate-shaped exceeding 120% for one rolling load before rolling stands becomes difficult, it is strip breakage between rolling passes increases like , made many challenges in manufacturing. Therefore, rolling load is 80% or more, preferably 85% or more, and / or 120% or less, preferably 100% or less. In general, the more and more subsequent rolling stands, a large effect on the accumulation of strain. Therefore, if it can not achieve more than 80% of the rolling load at subsequent rolling stands from among the last three roll stands, an average particle diameter of ferrite grains becomes larger, those Tend to martensite grains coverage by the ferrite grains become smaller. Furthermore, speaking in terms of rolling reduction is not particularly limited, hot rolling according to the method of the present invention, the reduction ratio by the final rolling stand is generally 25% or more, the range preferably of 25 to 40% It is performed such that the inner.
[0042]
　In addition, it is important in the process of the finish rolling at a temperature (finish rolling temperature) also present invention, as specifically low average value of the finishing rolling temperature in the three rolling stands of the final, during the forced cooling it can be introduced with high dislocation density by finer vital grain boundary martensite grain size. However, when the average value of these finish rolling temperature is too low, ferrite transformation proceeds rapidly, it can not be secured structure fraction of 10% or more of martensite phase. On the other hand, if this average value is high, the dislocation density of austenite grain boundaries is reduced, the coverage is reduced. From the above, the average value of the finish rolling temperature at the three rolling stands final 800 ° C. or higher, and 950 ° C. or less. The hot rolling with three rolling stands of the final in the present invention, there is the temperature rises by the processing heat generation due to the high rolling loads, such high temperatures are advantageous for expression of the dynamic recrystallization . On the other hand, since it is disadvantageous to the cumulative distortion and high temperatures at a later stage, the final temperature after rolling by the rolling stand (finish rolling end temperature) is not particularly limited, it is preferably, for example, 850 ° C. or higher. Further, the finish rolling end temperature may be for example 1000 ° C. or less.
[0043]
(Rough rolling)
　in the method of the present invention, for example, for the thickness adjustment, with respect to the cast slab may be subjected to rough rolling before the finish rolling. Such rough rolling is not particularly limited, for example, after the cast slab directly or once cooled, be carried out reheating for dissolution such as homogenization or Ti carbonitride optionally can. When performing reheating, the temperature of homogenization at less than 1200 ° C., dissolving both insufficient, which may cause a reduction in degradation and processing of the intensity. On the other hand, if the temperature of the reheating is higher than 1350 ° C., manufacturing cost, productivity is lowered, also tends eventually to mixed grain by initial austenite grain diameter increases. Therefore, re temperature of the heating is preferably set to 1200 ° C. or higher for the dissolution of such homogenization and / or Ti carbonitride, preferably less than 1350 ° C..
[0044]
Force cooling and take-up]
　After the end of the final rolling is better that was quickly forced cooling. Between the end of the final rolling up the forced cooling start strain is recovered, ferrite grains produced by transformation upon subsequent forced cooling by grain growth occurs, tends to coarse also austenite grains. Further, since the dislocation density of austenite grain boundaries introduced by dynamic recrystallization during finish rolling is decreased, the subsequent forced martensite grains coverage by the ferrite grains upon cooling may deteriorate. Although recovery amount of strain until the forced cooling starts may vary depending rolling temperature and the rolling rate, the time to forced cooling starts from the end of the final rolling it can be prevented from being completely recovered if it is within 1.5 seconds . It is preferred to utilize the distortion due to rolling efficiently is within 1 second. After the end of the final rolling, primary cooling as the average cooling rate 30 ° C. / sec or more at 600 ° C. or higher, and cooled to 750 ° C. or less, more than 3 seconds, 10 seconds or less natural cooling (hereinafter referred to as an "intermediate cooling") do. During this period takes place ferrite formation, by the diffusion of C, C enrichment of the austenite occurs. The upper ductility by formation of ferrite is increased, C was concentrated to austenite is important to contribute to the strength of the martensite by the subsequent forced cooling. Is less than the average cooling rate is 30 ° C. / sec, caused a coarsening of the austenite grains is delayed ferrite transformation during the intermediate cooling is not structural fraction of the ferrite phase of the object can be obtained. The intermediate air cooling start temperature exceeds 750 ° C., on structural fraction of the ferrite phase can not be sufficiently achieved, the particle becomes too large, the final martensite grains also tends to increase. Is less than the intermediate air cooling start temperature of 600 ° C. or intermediate air cooling time is less than 3 seconds, the fractions of the structures given ferrite phase can not be obtained, the higher the structural fraction of the martensite phase. On the other hand the intermediate air cooling time exceeds 10 seconds tissue fraction of martensite phase is lowered in. It is preferably less than or equal to 8 seconds in view of ensuring the structural fraction of the martensite phase.
[0045]
　C thickening the austenite in order to martensite, after cooling to 200 ° C. or less as a secondary cooling after intermediate cooling, it is important that the wound. The average cooling rate at this time is required to be 30 ° C. / sec or more. When the coiling temperature exceeds 200 ° C., there are cases where bainite phase and / or pearlite phase in winding together with the generated elongation decreases, a two-phase structure of ferrite phase and martensite phase can not be obtained. Average when the cooling rate is less than 30 ° C. / sec bainite phase and / or pearlite phase is formed during cooling, two-phase structure of ferrite phase and martensite phase can not be obtained.
[0046]
　After casting a slab having the same composition as described for hot-rolled steel sheet of the present invention, the rough rolling performed if necessary, and then to finish rolling as described above, the subsequent forced cooling and winding operation carried out doing, the area fraction, tissue fraction of 10% or more of martensite phase, 40% or less, includes a structure fraction of 60% or more two-phase structure of ferrite phase, the average grain diameter of ferrite grains is 5.0μm or less, martensite grains coverage by the ferrite grains can be reliably manufactured hot-rolled steel sheet is 60 percent. Therefore, according to the above-described manufacturing method, it is possible to provide a hot rolled steel sheet excellent tensile strength 980MPa or more high intensity balance of toughness and hole expandability.

WE claims

[Requested item 1]
　By
　mass%, C: 0.02% or more, 0.50% or
　less, Si: 2.0% or
　less, Mn: 0.5% or more, 3.0% or
　less, P: 0.1% or
　less, S: 0.01% or
　less, Al: 0.01% or more, 1.0% or less, and
　N: 0.01% or less
contain, comprises the balance consisting of Fe and impurities,
　in terms of area fraction, Martens site-phase structure fraction of 10% or more, 40% or less, includes a structure fraction of 60% or more two-phase structure of ferrite phase,
　the mean particle size of the ferrite grains is equal to or less than 5.0 .mu.m,
　martensite grains of a ferrite grain wherein the coverage is 60%, the hot-rolled steel sheet.
　Here, as the martensite grain of coverage by the ferrite grains, which when the whole martensite grain boundary length is 100, and displays the length ratio of the martensite grain boundary occupied by ferrite grains in percentage it is.
[Requested item 2]
　Furthermore, by
　mass%, Nb: 0.001% or more, 0.10% or
　less, Ti: 0.01% or more, 0.20% or
　less, Ca: 0.0005% or more, 0.0030% or
　less, Mo: 0.02% or more, 0.5% or less, and
　Cr: 0.02% or more, 1.0% or less
, characterized by containing one or more of, hot-rolled steel sheet according to claim 1.
[Requested item 3]
　Wherein the average particle size of the ferrite grains is 4.5μm or less, hot-rolled steel sheet according to claim 1 or 2.
[Requested item 4]
　Characterized in that the coverage is 65% or more, hot-rolled steel sheet according to any one of claims 1-3.
[Requested item 5]
　The tissue fraction of martensite phase of 10% or more, and less than 20%, hot-rolled steel sheet according to any one of claims 1-4.
[Requested item 6]
　A step of casting slab having a composition according to any one of claims 1 to 5,
　the cast slab comprising the steps of hot rolling, comprising at least four consecutive rolling stands the slab rolling the method comprising finish rolling using a machine, the respective rolling load of the last three roll stands in the finish rolling is not less than 80% of the rolling load of the rolling stand of the previous one, and three rolling the final average of rolling temperature finish in stand 800 ° C. or higher, the process is 950 ° C. or less, and
　then finish rolled steel plate forced cooling and then a winding process, the forced cooling after the finish rolling termination 1 is initiated within .5 seconds, the steel sheet to 30 ° C. / sec or more average cooling rate at 600 ° C. or higher, primary cooling to cool to 750 ° C. or less, the steel sheet after the primary cooling 3 Above, the step of including a secondary cooling for cooling the intermediate cooling naturally cooled below 10 seconds, and at an average cooling rate steel plate of more than 30 ° C. / sec after the intermediate air-cooled to 200 ° C. or less
, characterized in that it comprises a manufacturing method of hot-rolled steel sheet.