Technical field
[0001]
　The present invention, hot-rolled steel sheet, a method of manufacturing a steel and hot-rolled steel sheet.
Background technique
[0002]
　In order to improve the wear resistance improved and the fatigue strength of the steel material components, hardening treatment on the surface of the steel sheet is subjected. As such curing, for example, carburizing, etc. nitriding or nitrocarburizing treatment, a heat treatment with controlled atmosphere are known.
[0003]
　When subjected to a curing process to the steel sheet surface, while the steel sheet is cured, cured by heating at the time of coarse crystal grains grown in the thickness center portion of the steel sheet, the thickness center portion hardness (strength ) is occurs phenomenon of softening.
[0004]
　As means for suppressing the grain growth of the thickness center portion, it is known to add a small amount of Nb. The addition of Nb in steel, NbC (niobium carbide · Nb and precipitates carbon bonded) is deposited, the NbC is to act as a suppressing pinning grain growth, the plate thickness center at the time of heat treatment it is believed to be able to prevent the growth of crystal grains (for example, see Patent Document 1).
CITATION
Patent Literature
[0005]
Patent Document 1: Laid-Open Patent Publication No. 11-236646
Summary of the Invention
Problems that the Invention is to Solve
[0006]
　Further, when the plastic deformation of the steel sheet in cold, it is possible to increase the strength of the steel sheet by work hardening. Therefore, a steel sheet obtained by adding Nb to increase the strength of the steel sheet undergoes work hardening by plastic deformation in cold, furthermore, when the hardening process on steel surfaces, softening of work hardening of the center of plate thickness while suppressing, it is possible to cure the surface layer.
[0007]
　When the inventors have investigated, if the machining amount before and after the plastic deformation is increased significantly work hardening rate, by heat treating the steel sheet Nb addition, it was possible to suppress the softening of the center of plate thickness. On the other hand, when a low amount of processing is small work hardening rate, even if heat-treated steel sheet of Nb addition, it was found that it is impossible to suppress the softening of the center of plate thickness.
[0008]
　For example, when manufacturing auto parts, after having been subjected to cold working by press molding or the like to the steel sheet, there is a case that soft nitriding the surface. Since automotive parts are a variety of shapes, upon pressing a steel plate, in one part, the processing amount is relatively large portion and the processing amount is relatively small sites and is generated. Here, when a steel sheet containing Nb is by heat treatment during nitrocarburizing, the strength of the plate thickness center in the processing amount is relatively small portion is softened, part strength may be insufficient.
[0009]
　The present invention has been made in view of the above circumstances, an object of the present invention, even when a low amount of processing is small work hardening rate for the steel sheet, the strength of the center of plate thickness of the steel sheet during the heat treatment softening capable of preventing the hot-rolled steel sheet is to provide a method for producing steel and hot-rolled steel sheet.
Means for Solving the Problems
[0010]
(1) in mass% as chemical
components, C:
0.040
~ 0.150%, Si: 0 ~ 0.500%, Mn: 0.10
~ 1.50%, P: 0 ~ 0.050%,
S:
0
~ 0.020%, Al: 0.010
~ 0.050%, N: 0.0010 ~ 0.0060%, Nb: 0.008
~ 0.035%, Cu: 0 ~ 0.10% , Ni:
0 ~ 0.10%, Cr: 0 ~ 0.02%, Mo: 0 ~ 0.020%, V: 0 ~ 0.020%, Ca: 0 ~ 0.0100%, and B: 0 to 0.0050% wherein the solid solution Nb: 0.005 to 0.030% 　the balance being iron and impurities, 　85% or more in the tissue area fraction of ferrite in the metal structure, the remainder of the metal structure is cementite and / or pearlite, the average binding of ferrite Particle size is 5μm or more 20μm or less, the hot-rolled steel sheet. (2) Vickers hardness of the center of plate thickness when relative to the hot-rolled steel sheet was successively subjected to a heat treatment of heating for 120 minutes at cold working and 560 ~ 620 ° C. is 　thickness center after the cold working hot-rolled steel sheet according to (1) to indicate a 80% or more softening resistance against Vickers parts. (3) with respect to the hot-rolled steel sheet, thickness center portion when the cold work hardening rate is less than 30% of the Vickers hardness was successively subjected to a heat treatment of heating for 120 minutes at 560 ~ 620 ° C. Vickers hardness, 　hot rolled steel sheet according to (1) to indicate a 80% or more softening resistance against Vickers hardness of the plate thickness center portion after the cold working. (4) (1) is a steel consisting of hot-rolled steel sheet according to any one of - (3), 　and a heat treatment of heating for 120 minutes at cold working and 560 ~ 620 ° C. relative to the hot-rolled steel sheet Vickers hardness of the center of plate thickness when sequentially done by, 　steel is 80% or more with respect to the Vickers hardness of the center of plate thickness after processing between the cold. (5) (1) to (3) is any steel consisting of hot-rolled steel sheet according to one of, 　cold work-hardening rate of Vickers hardness with respect to the hot-rolled steel sheet is less than 30% machining Vickers hardness of the center of plate thickness of 560 to 620 when sequentially performed and heat treatment of heating for 120 minutes at ℃ is, 　is 80% or more of the Vickers hardness of the center of plate thickness after said cold working steel. (6) mass% as chemical components, C: 0.040 ~ 0.150%, Si: 0 ~ 0.500%, Mn: 0.10 ~ 1.50%, P: 0 ~ 0.050%, S: 0 ~ 0.020%, Al: 0.010 ~ 0.050%, N: 0.0010 ~ 0.0060%, Nb: 0.008 ~ 0.035%, Cu: 0 ~ 0.10% , Ni: 0 ~ 0.10%, Cr: 0 ~ 0.02%, Mo: 0 ~ 0.020%, V: 0 ~ 0.020%, Ca: 0 ~ 0.0100%, and B: 0 to 0.0050% comprises, heating the steel slab and the balance being iron and impurities 1200 ° C. or 　higher, subjected to final rolling finish rolling at a finishing rolling temperature of 860 ° C. or higher 950 ° C. or less, 　from the finish rolling temperature between 800 ° C. and cooled at an average cooling rate of 30 ° C. / sec or higher 100 ° C. / sec or 　less, or 800 ° C. Was cooled until coiling temperature at 5 ° C. / sec or higher 100 ° C. / sec of average cooling speed, 　wound at 300 ° C. or higher 600 ° C. or less of the take-up temperature, method for producing a hot-rolled steel sheet.

Effect of the invention
[0011]
　According to the present invention, even when a low amount of processing is small work hardening rate for the steel sheet, which can prevent the softening of the strength of the center of plate thickness of the steel sheet during the heat treatment, hot-rolled steel sheet, steel and hot rolled steel sheet it is possible to provide a method of manufacturing.
DESCRIPTION OF THE INVENTION
[0012]
　Prior to detailed description of the preferred embodiments of the present invention, the following, the principles of the present invention will be described while performing guess.
[0013]
　Binding of the steel sheet are present NbC in the steel structure during the time of cold working, when the cold working under conditions work hardening rate increases, NbC existing in the steel and Nb and C in accordance with the plastic deformation It is solved, finely dispersed in the steel sheet divided into solid solution Nb and C. Furthermore, the heat treatment of the steel sheet after cold working, are formed NbC and solute Nb and C is attached again, the pinning effect of the newly formed NbC, grain growth of the center of plate thickness There is prevented, softened during heat treatment is suppressed.
[0014]
　On the other hand, when the cold working under conditions work hardening rate is reduced, since the deformation experienced by the NbC existing in the steel is small, NbC which the bond between the Nb and C is solved it becomes extremely small, due to subsequent heat treatment solute Nb to produce a fine NbC becomes less state. Therefore, the effect of delaying the movement of dislocations due to the pinning action of NbC is not increased so much, without being prevented growth of crystal grains, the effect of suppressing the recrystallization becomes as small.
[0015]
　Thus, formed when the conventional NbC-rich steel sheet work hardening rate cold working under conditions such that small, since there is less NbC binding between Nb and C is released, the particles of NbC is in the hot-rolled large remains relatively that is becomes mainly. Thereafter, when the heat treatment, since the solid solution Nb is small, the number of particles of NbC precipitated by heat treatment is reduced, decreasing the effect of pinning action of the newly formed NbC, center of plate thickness at the time of heat treatment can not prevent the grain growth, it is presumed that it can not suppress the heat softening of the center of plate thickness during the heat treatment.
[0016]
　Based on the above consideration, the present inventor has by including solute Nb in many previously in the steel, without depending on work-hardening rate at the time of plastic working by cold steel plate was subjected to heat treatment after plastic working even if found to be able to prevent the softening of the center of plate thickness.
[0017]
　Solute Nb contained in advance in the steel, so there is no biased in the steel sheet, when the solid solution Nb during heat treatment NbC is produced by combining with C, NbC is dispersed finely in the steel plate since so present, the pinning action of NbC, it is presumed that it is possible to prevent the grain growth of the center of plate thickness during the heat treatment.
[0018]
　Solute Nb is particularly because of the property of generating a large amount of NbC in the vicinity of dislocations generated in the steel by plastic working cold, if it is steel sheet cold working has been performed, a plate of the steel sheet during the heat treatment it is advantageous in that it prevents the softening of the strength of the thick center portion. That is, a steel sheet is present solute Nb in steel, when the heat treatment After cold working, when heated for example to a soft nitriding treatment temperature 500 ~ 600 ° C., the solute Nb and C There NbC is produced by compounds. However, the steel sheet NbC is present in the absence of a solute Nb in steel, skip cold working, if the heat treatment the steel sheet remains was cast heat, a new fine NbC is too since the heating of the generated state without only pinned using only coarse and fewer NbC generated when producing a hot-rolled steel sheet will not act. Therefore, the effect of delaying the movement of dislocations in the crystal grains of the re-crystallization of more than 550 ° C., which is the starting temperature is small, can not prevent the growth of crystal grains thickness center portion in the heat treatment, the thickness center at the time of heat treatment it is presumed that it can not suppress the heat softened parts.
[0019]
　To facilitate the generation of NbC to prevent grain growth of the thickness center portion of the heat treatment process, first, it is effective to allowed to remain solid solution Nb in steel. As described above, by the NbC in steel solid solution Nb by a high cold working, rather than suppressing the thermal softening of the center of plate thickness during the heat treatment, in the present invention, to produce a hot rolled steel sheet by leaving the solute Nb in steel, we have found a method of suppressing the thermal softening of the plate thickness center at the time of heat treatment upon. Then, introduced to force the dislocations steel remaining solid solution Nb, that a large amount to produce a NbC from solute Nb in the vicinity of dislocations during the heat treatment, softening of the intensity of the center of plate thickness of the steel sheet during the heat treatment inventors have found that it is effective in preventing.
[0020]
　The amount of force the dislocation which has been introduced in order to promote the formation of NbC can be expressed by the curing of Vickers hardness by cold working. In the present invention, it is preferable to cure at least 10% with respect to the Vickers hardness of the material before the material cold working.
[0021]
　As described above, the hot rolled steel sheet of the present invention can be particularly preferably used in the case of performing heat treatment such as surface hardening, such as nitrocarburizing treatment after cold working.
[0022]
　Hereinafter, the hot-rolled steel sheet of the present embodiment, a method for manufacturing the resulting steel and hot rolled steel sheet hot-rolled steel sheet was cold-working and heat treatment will be described.
[0023]
　First described chemical components of the hot rolled steel sheet of the present embodiment. The content of each component is mass%. Moreover, the scope herein includes upper and lower limit values ​​unless otherwise mentioned.
[0024]
(C: 0.040 ~ 0.150%)
　C is an element effective to maintain the strength. During the heat treatment of the cold processed hot-rolled steel sheet (e.g., soft nitriding treatment) in order to prevent the reduction in strength of the center of plate thickness by adequately generate NbC is required C amount is more than 0.040% is there. On the other hand, the press formability of the hot-rolled steel sheet when the C content exceeds 0.150% of the upper limit of 0.150% to decrease. C content is preferably 0.040 to 0.10%, more preferably from 0.040 to 0.090%.
[0025]
: (Si 0 ~
　0.500%) is Si is an element to enhance the deoxidation and strength of the steel, in the present embodiment is added as a strength adjustment. When the Si content is high, defects are easily generated surface oxides on the surface of the steel sheet during hot rolling is generated. Further, also decreases press formability. Therefore, Si content is set to less 0.500%.
Si content is preferably 0.10% or less, more preferably 0.08% or less. On the other hand, Si is because they are contained in the iron ore is usually unavoidably component present. Therefore, the lower limit of Si amount can be 0.001%. In order to enhance the deoxidation and strength of the steel, the amount of Si for example 0.090% or more, preferably it is 0.200% or more.
[0026]
(Mn: 0.10 ~
　1.50%) Mn is an element improving the strength to increase the hardenability of steel, in the present embodiment is added as a strength adjustment. Mn content embrittlement due S in steel is likely to occur in less than 0.10%. Moreover, the press formability is degraded when Mn content exceeds 1.50%. Mn content is preferably 0.1 to 1.3%, more preferably 0.1 to 1.10%.
[0027]
(P: 0 ~ 0.050%)
　P tends cause embrittlement, it is lower in order to ensure the press workability. Therefore P content is set to the upper limit 0.050%. P content is preferably 0.03% or less, more preferably 0.02% or less. Meanwhile, P is, because it is contained in the iron ore is usually unavoidably component present. Therefore, 0.001% the lower limit of the P content, and more specifically may be a 0.002%.
[0028]
(S: 0 ~ 0.020%) S
　tends cause embrittlement as well as P, it is lower in order to ensure the press workability. Thus S content is set to the upper limit 0.020%. S content is preferably 0.015% or less, more preferably 0.010% or less. On the other hand, S is, because it is contained in the iron ore is usually unavoidably component present. Therefore, the lower limit of the S amount may be 0.001%.
[0029]
(Al: 0.010 ~
　0.050%) Al has an effect of increasing the surface hardness and generates nitrides AlN becomes the steel sheet surface in the nitrocarburizing treatment. Therefore, Al amount is excessive 0.010%. On the other hand, in order to maintain a high press formability and the upper limit 0.050%. Al content is preferably 0.010 to 0.040%, more preferably from 0.015 to 0.030 percent.
[0030]
(N: 0.0010 ~ 0.0060%)
　N, like Al, an element necessary for generating the Al nitride surface of the steel sheet in the nitrocarburizing treatment, it is preferable to be contained 0.0010% or more. On the other hand, when N is a large amount present in the steel sheet prior to pressing reduction of ductility is increased, the workability of the steel sheet is lowered. Therefore the N content is preferably lesser, the upper limit 0.0060%. N amount is preferably from 0.0010 to 0.0040 percent, more preferably 0.0010 to 0.0030%.
[0031]
(Nb: 0.008 ~ 0.035%)
(solid solution Nb: 0.005 ~ 0.030%)
　hot-rolled steel sheet of the present embodiment, by having a solid solution Nb, nitrocarburizing after cold working saved when temperature is raised in the process, starting from the dislocations introduced by the cold working, by changing the deposition composed NbC solid solution Nb, the movement of dislocations is delayed, the work hardening caused by cold working can do. In order to achieve this, it is necessary first 0.005% or more of dissolved Nb. To the solid solution Nb to 0.005% or more is, Nb amount is excessive 0.008% or. Since the effect of solute Nb is saturated at 0.030% or, the upper limit of the solid solution Nb 0.030%. On the other hand, Nb in the steel press formability is degraded by increasing. Therefore, the upper limit of the Nb content is set to 0.035%. Nb content is preferably 0.010 to 0.030 percent, more preferably from 0.010 to 0.025 percent. Solute Nb content is preferably 0.005 to 0.030 percent, more preferably from 0.008 to 0.030 percent.
[0032]
　Nb amount of solid solution in the steel sheet can be calculated from the residue obtained by electrolytic extraction. For example, at the position of the plate width 1/4 or 3/4 of the cooled steel plate to room temperature after winding, 30 mm square (30 × 30 mm = 900 mm 2 to) the size of the test piece was taken as the electrolytic solution 10 % acetylacetone-1% tetramethylammonium chloride - methanol solution was used to cause a constant current electrolysis in an electrolytic solution. The residue after constant current electrolysis remained in the electrolytic solution was collected and filtered through a filter of 0.2 [mu] m, as well as measuring the mass of harvested residue after residue acid decomposition treatment, ICP emission spectroscopy (Inductively Coupled Plasma Atomic Emission Spectroscopy: by ICP-AES), measures the mass of Nb in the residue. Then, Nb of the residue is, assuming that was present as precipitates carbides or nitrides of Nb, determining the amount obtained by subtracting the amount of Nb in the residue from the total Nb content of the steel sheet as solute Nb content.
[0033]
(Cu: 0 ~
　0.10%) Cu is optionally added for strength adjustment. In order not to lower the workability to a maximum of 0.10%. Cu amount, in order to increase the strength without reducing the workability, preferably 0.01 to 0.08%, more preferably 0.02 to 0.05%.
[0034]
(Ni: 0 ~
　0.10%) Ni is, in manufacturing the steel Cu is contained, it added to prevent embrittlement cracking during hot rolling. The addition amount of Ni is preferably degree than half the Cu content. Since the Ni content is reduced workability of the steel sheet exceeds 0.10%, the upper limit is 0.10%. Ni amount, to prevent embrittlement cracking without lowering workability, preferably 0.01 to 0.08%, more preferably 0.02 to 0.05%.
[0035]
(Cr: 0 ~
　0.02%) Cr is added as required for Likewise intensity adjustment and Cu. In order not to lower the workability to a maximum of 0.02%. Cr content in order to increase the strength without reducing the workability, preferably from 0.005 to 0.020 percent, more preferably from 0.010 to 0.015%.
[0036]
(Mo:
0 ~ 0.020%) (V: 0 ~
　0.020%) Mo, V is optionally added to the well as intensity adjustment and Cu. In order not to lower the workability and the respective upper limit 0.020%. Mo amount in order to increase the strength without reducing the workability, preferably from 0.005 to 0.020 percent, more preferably from 0.010 to 0.018%.
[0037]
(Ca: 0 ~
　0.0100%) Ca is possible to prevent the embrittlement due to S, is added as needed to prevent local ductility decreases due to coarsening of MnS. Ca is to saturate the effect 0.0100%, which is the upper limit. Ca amount, in order to prevent embrittlement without reducing the workability, preferably from 0.002 to 0.010 percent, more preferably from 0.002 to 0.008%.
[0038]
(B: 0 ~ 0.0050%)
　B is added as needed to to prevent aging by N to prevent a decrease in ductility. Order to saturate the effect 0.0050%, C is by binding is B, the amount of NbC is lowered, to lower the softening resistance during heat treatment, which is the upper limit. B quantity, in order to prevent aging by N without lowering the softening resistance is preferably 0.0003 to 0.0030%, more preferably from 0.0004 to 0.0020 percent.
[0039]
　The rest of the hot-rolled steel sheet is iron and impurities. The hot-rolled steel sheet, iron, for example, 97.40 to 99.84 percent, preferably contained 98.10 to 99.83%.
[0040]
　It will now be described hot-rolled steel sheet metal structure.
　Metal structure of the hot rolled steel sheet of the present embodiment is a ferrite organization 85% or more in area fraction, the balance is cementite and / or pearlite. Further, the average crystal grain size of the ferrite is 20μm or less in the range of 5 [mu] m.
[0041]
　Once the tissue area fraction of ferrite is less than 85%, since the formability of the steel sheet is undesirably reduced. Area fraction of ferrite is more preferably 90% or more, further preferably 92% or more. Also, the remaining structure is either one or both of cementite or pearlite. Bainite is preferably not included in the organization. Area fraction of ferrite, obtains the area fraction of white appears portion is observed corrode steel sheet surface with nital. The area ratio of the remaining structure determines the area fraction of black appears portion is observed corrode steel sheet surface with nital.
[0042]
　The average grain size of the ferrite is good 5μm or 20μm or less. Is less than the average crystal grain size of 5 [mu] m, the strength of the steel sheet becomes excessively high, the elongation EL (%) is reduced, the workability is deteriorated. If the average crystal grain size exceeds 20 [mu] m, surface texture of the steel sheet after pressing becomes orange peel (peel) shape, the surface roughness is increased. The average grain size of the ferrite is preferably 6μm least 15μm or less, more preferably 8μm or more 15μm or less.
[0043]
　The thickness of the hot rolled steel sheet of the present embodiment is not particularly limited, but is preferably 2.0mm or more 9.0mm or less. Steel having a thickness of less than 2.0mm, there is a possibility that the cured layer is formed to a thickness of the center portion of the steel sheet in the nitrocarburizing treatment, if the effect of the present invention that softening resistance improvement of the heat treatment is not required is there. Further, the application of hot-rolled steel sheet of the present embodiment, the use of steel sheet thickness exceeds 9.0mm, since not assumed, can be the upper limit of the thickness of 9.0mm.
[0044]
　The tensile strength TS of the hot rolled steel sheet of the present embodiment is more 400 MPa 640 MPa or less. In addition, the elongation EL (%) is 25.0% or more. Tensile strength TS (MPa), elongation EL (%), according to JIS Z 2241 (2011) Method of tensile test for metallic materials.
[0045]
　As for the anisotropy during processing of the steel sheet, as the height of the ear when the steel sheet has a cylindrical deep drawing a is 2mm or less. Ear height, diameter 200 mm, the steel sheet was cut into a circle having a thickness of 4.5 mm, punch inner diameter 100 mm in diameter, punch shoulder R3mm, and deep drawing of a cylinder clearance with conditions that 1.4 times the thickness of the steel sheet Occasionally, the difference between the highest height and the lowest height of the cylindrical portion after deep drawing the height of the ear. To the ear height 2mm or less, it is desirable that the finish rolling temperature in the range of 900 ~ 950 ° C..
[0046]
　Next, a method for manufacturing a hot-rolled steel sheet of the present embodiment.
　Hot-rolled steel sheet of the present embodiment, a slab having a chemical composition described above (the steel slab) is heated to 1200 ° C. or higher, subjected to final rolling finish rolling at a finishing rolling temperature of less than 950 ° C. 860 ° C. or higher, finishing between rolling temperature of 800 ° C. and cooled at an average cooling rate of 30 ° C. / sec or higher 100 ° C. / sec or less, the average cooling rate of 5 ° C. / sec or higher 100 ° C. / sec or less between from 800 ° C. to a coiling temperature in cooling, it is prepared by winding at 300 ° C. or higher 600 ° C. or less of the winding temperature.
[0047]
　The heating temperature of the slab may be at 1200 ° C. or higher, but is preferably more preferably less 1300 ° C. 1200 ° C. or higher 1280 ° C. 1220 ° C. or more or less. The heating temperature here is the temperature of the slab thickness center section. The slab in after casting, Nb is due to the presence in a compound such as NbC, Nb is heated at the slab center 1200 ° C. or higher in order to form a solid solution in the steel. On the other hand, if the heating temperature is too high, scale the slab surface is excessively generated during heating, flaws on the surface of the steel sheet after hot rolling may occur. There is also a possibility that the yield decreases. Thus the upper limit of the heating temperature is set to 1300 ° C..
[0048]
　Rolling temperature finish in the final rolling finish rolling is set to 950 ° C. or less 860 ° C. or higher. Finish rolling temperature is measured temperature of the steel sheet surface. The Nb which is dissolved by heating in order not to precipitate as a carbide, it is necessary that the finish rolling temperature to 860 ° C. or higher. To express the isotropic when pressing the hot-rolled steel sheet, it is desirable that the finish rolling temperature above 900 ° C..
　On the other hand, finishing the rolling temperature is too high, will crystal grains is too grown, the hot-rolled steel sheet from the anisotropy becomes remarkable when the pressing, it is necessary to set the upper limit to 950 ° C. or less. Rolling temperature finish in the final rolling finish rolling, but may be within the range described above, preferably 900 ° C. or higher 940 ° C. or less, more preferably 900 ° C. or higher 930 ° C. or less.
[0049]
　Average cooling rate between finishing rolling temperature of up to 800 ° C. is a 30 ° C. / sec or higher 100 ° C. / sec or less. The average cooling rate is an average cooling rate at the center of plate thickness of the steel sheet. Temperature range from finish rolling temperature to 800 ° C., since the solid solution Nb is the temperature range that likely precipitated NbC, in order to this temperature range to pass as soon as possible, the average cooling rate of between finish rolling temperature to 800 ° C. regulatory. When the average cooling rate of this temperature range is 30 ° C. / sec or more, Nb is reduced by the solid solution Nb increases relatively precipitated. On the other hand, if the average cooling rate is too high, if the average crystal grain size of the ferrite structure is too small, the area fraction of ferrite is decreased, to a maximum of 100 ° C. / sec. Average cooling rate between finishing rolling temperature of up to 800 ° C., although may be within the range described above, preferably 40 ° C. / sec or higher 100 ° C. / sec or less, more preferably 50 ° C. / sec or higher 100 ° C. / sec less.
[0050]
　The average cooling rate of between 800 ° C. to coiling temperature is set to 5 ° C. / sec or higher 100 ° C. / sec or less. The average cooling rate is an average cooling rate at the center of plate thickness of the steel sheet. Temperature range from 800 ° C. to coiling temperature, because the solute Nb is the temperature range to stably exist, in this temperature range may be alleviated cooling rate than the temperature range up to 800 ° C.. Therefore, the average cooling rate in this temperature range is in the above range. When the average cooling rate is 5 ° C. / sec or more, it is possible to lower the temperature of the steel strip before the winding of the steel sheet to the upper limit of the coiling temperature. On the other hand, if the average cooling rate is too high, since a ferrite area fraction is reduced the ductility decreases, to a maximum of 100 ° C. / sec. The average cooling rate of between 800 ° C. to coiling temperature, but may be within the range described above, preferably 15 ° C. / sec or more 100 ° C. / sec or less, more preferably 15 ° C. / sec or more 60 ° C. / sec less.
[0051]
　Winding temperature of the steel sheet after cooling to 300 ° C. or higher 600 ° C. or less. The coiling temperature is a surface temperature of the steel plate. Hot-rolled steel sheet of the present embodiment, remains wound at a low temperature when NbC precipitation is suppressed Nb is solid-solved, although the workability decreases, improves softening resistance during the heat treatment. On the other hand, when the wound at a high temperature, but the workability was improved elongation of hot-rolled steel sheet is improved, the upper limit for the remaining less of solute Nb is 600 ° C.. Therefore, in the present embodiment restricts the coiling temperature in the above range. Winding temperature of steel sheet, it may be within the range described above, and preferably 400 ° C. or higher 600 ° C. or less, 580 ° C. or less and more preferably 450 ° C. or higher.
　As described above, it is possible to manufacture a hot-rolled steel sheet of the present embodiment.
[0052]
　Hot-rolled steel sheet of the present embodiment is molded into a predetermined product shape by cold working such as press forming, then carburizing treatment, nitriding treatment, carbonitride treatment, surface hardening treatment such as nitrocarburizing process is performed by, it is steel constituting the automobile parts and the like. Surface hardening process is a process of heat treating the hot-rolled steel sheet after cold working in a predetermined atmosphere. Hot-rolled steel sheet of the present embodiment, even when the heat treatment after cold working is applied, small amount of decrease in Vickers hardness of the plate thickness center before and after the heat treatment, has a softening hard characteristic.
[0053]
　Cold working, press working, hole expansion processing, may be a plastic working of cold bending, or the like. Also, when representing the degree of work amount during cold working at the work hardening rate [Delta] R (%), in the present embodiment, the cold working may be applied in any work-hardening rate [Delta] R (%) is, [Delta] R if (%) is 10% or more, NbC is dislocation is sufficiently introduced to precipitation, the effect of softening resistance tends to be exhibited. In the present embodiment, and has a large work-hardening rate, [Delta] R (%) refers to the case of 30% or more. Moreover, the work hardening rate is low, [Delta] R (%) refers to the case of less than 30%. Hot-rolled steel sheet of the present embodiment, even if the [Delta] R (%) of less than 10-30%, showing a softening hard characteristic before and after the heat treatment.
[0054]
　Atmosphere in the surface hardening treatment is not particularly limited. As an example, NH 3 concentration 35%, CO 2 concentration 5%, N 2 atmosphere at a concentration of 60% can be exemplified. Hot-rolled steel sheet of the present embodiment, the heat treatment temperature in the range of 560 ~ 620 ° C., even exhibit sufficient softening resistance as heat-treated at a heat treatment time of 120 minutes. The temperature range to be applied in actual surface hardening treatment in the range of 500 ~ 600 ° C., the heat treatment time is about 60 to 180 minutes. In such conditions, hot-rolled steel sheet of the present embodiment exhibits sufficient softening resistance.
[0055]
　Hot-rolled steel sheet of the present embodiment, Vickers hardness of the center of plate thickness of the case of sequentially performing a heat treatment of heating for 120 minutes at cold working and 560 ~ 620 ° C. is the center of plate thickness after cold working It is as shown 80% or more softening resistance against Vickers hardness. In particular, even when the cold work-hardening rate of the Vickers hardness is less than 30% the processing has been performed, the Vickers hardness of the center of plate thickness after heat treatment, the center of plate thickness after cold working It shows 80% or more softening resistance against the Vickers hardness.
[0056]
　Incidentally, the work hardening rate in the present embodiment is as follows.
　The Vickers hardness of the center of plate thickness of the front cold working the hot-rolled steel sheet and Hv (before cold working), the Vickers hardness of the center of plate thickness after cold working was Hv (after cold working) when, work hardening amount ΔWHv is represented by the following (alpha) expression, work hardening rate [Delta] R (%) is expressed by the following (beta) expression.
[0057]
ΔWHv = Hv (cold room after processing) -HV (cold room before working) ... ([alpha])
[Delta] R (%) = ΔWHv / Hv (cold room before processing) × 100 ... (β)
[0058]
　Further, the hardness variation after heat treatment are as follows. Heat treatment is a case of performing heating for 120 minutes at the heat treatment temperature. Hot-rolled steel sheet of the present embodiment, ΔHv (%) is as shown 80% or more.
[0059]
　When the Vickers hardness of the center of plate thickness before the heat treatment of the cold processed hot-rolled steel sheet was Hv (after heat treatment), curing amount ΔTHv after heat treatment is represented by the following (gamma) type, hardness after heat treatment change rate Delta] HV (%) is expressed by the following ([delta]) expression.
[0060]
ΔTHv = Hv (after heat treatment) -HV (cold room before working) ... (gamma])
Hv (%) = ΔTHv / ΔWHv × 100 ... ([delta])
[0061]
　Delta] HV (%) of the upper limit is not 100%, including the case where the steel sheet by heat treatment becomes harder. For example, solute C in the steel to form NbC by heat treatment, thereby, in some cases the strength is increased.
[0062]
　Incidentally, the Vickers hardness of the plate thickness center of the hot-rolled steel sheet, the Vickers hardness test method defined in JIS Z 2244 (2009), is a hardness measured by weight of 100g (0.9807N) in micro-Vickers meter . The measurement in the region of the range of the thickness center in the thickness direction ± 100 [mu] m of the hot-rolled steel sheet is performed more than three times the hardness test, an average value.
[0063]
　Steel cold working and surface hardening is manufactured is subjected to a hot-rolled steel sheet, hardness change ratio ΔHv after heat treatment (%) may be indicators of more than 80%.
[0064]
　As described above, according to the hot-rolled steel sheet of the present embodiment, even when a low amount of processing is small work hardening rate for the steel sheet, can be prevented softening of the strength of the center of plate thickness of the steel sheet during the heat treatment .
　Further, according to the manufacturing method of the hot-rolled steel sheet of the present embodiment can be produced an excellent hot-rolled steel sheet softening resistance during the heat treatment.
Example
[0065]
　It will now be described in more detail by way of examples the invention. Note that the embodiments described below is a mere example of the present invention, the present invention is not intended to be limited to the following examples.
　Smelted steel by converter, to produce a slab by continuous casting. Table 1A and Table 1B shows the components 1-44 as chemical components of the slab.
[0066]
　The resulting slab was heated to a predetermined heating temperature, subjected to final rolling finish rolling at a predetermined finish rolling temperature, the average cooling rate and 800 ° C. between the finish rolling temperature of 800 ° C. during up to coiling temperature It cooled by variously changing the average cooling rate, by winding at a predetermined coiling temperature, to produce a hot rolled steel sheet S01 ~ S84. Table 2A ~ Table 2C, showing a heating temperature in the production of hot-rolled steel sheet, the finish rolling temperature, the average cooling rate and coiling temperature. Also shows the thickness of the hot rolled steel sheet obtained in accordance with the Table 2A ~ Table 2C. Incidentally, in Tables 2A ~ Table 2C, an average cooling rate between finishing rolling temperature of 800 ° C., and wherein the average cooling rate I, the average cooling rate of between 800 ° C. to a coiling temperature, average cooling rate It was described as II.
[0067]
　Then, by performing press working with respect to hot-rolled steel sheet thus obtained, to produce a press-molded product. Press working, diameter 200 mm, hot-rolled steel sheet was cut into a circle having a thickness of 4.5 mm, and a condition for punch inner diameter 100 mm in diameter, punch shoulder R3mm, and 1.4 times the thickness of the clearance. By deep drawing of a cylinder in this condition, to produce a cup-shaped press-formed product height 52 mm. Further, in order to examine the influence of the plate thickness is changed, for the hot-rolled steel sheet of 9.0mm from a thickness 2.0 mm, was subjected to the same press working.
[0068]
　Next, the press molded article was subjected to soft nitriding. Atmosphere nitrocarburizing process, NH 3 concentration 35%, CO 2 concentration 5%, N 2 and an atmosphere of 60% concentration. Heating rate to 0.7 ° C. / minute, heat treatment temperature was 570 ~ 625 ° C., the heat treatment time was and 120, after heating air cooled. The heat treatment temperature of soft-nitriding are shown in Tables 3A ~ Table 3C.
[0069]
(Microstructure of the hot-rolled steel sheet)
　The obtained hot rolled steel sheets, by microscopic observation by cross nital etching treatment, tissue morphology, to obtain an average crystal grain size of the area fraction and ferrite ferrite structure. The results are shown in Tables 2A ~ Table 2C.
[0070]
(Hot rolled solute Nb content of the steel plate)
　was also measured by the method described below a solute Nb content in the hot-rolled steel sheet. First, at the position of the plate width 1/4 of hot-rolled steel sheet is cooled to room temperature after winding, 30 mm square (30 × 30 mm = 900 mm 2 were taken) the size of the test piece. Then, 10% acetylacetone-1% tetramethylammonium chloride as an electrolyte - to prepare a methanol solution, and the test specimen in an electrolyte solution by constant current electrolysis. The residue after constant current electrolysis remained in the electrolytic solution was collected and filtered through a filter of 0.2 [mu] m, as well as measuring the mass of harvested residue after residue acid decomposition treatment, ICP emission spectrometry
(Inductively Coupled Plasma Atomic Emission Spectroscopy: by ICP-AES), was measured mass of Nb in the residue. Nb in the residue assumes that was present as a precipitate of carbides or nitrides of Nb, the amount obtained by subtracting the amount of Nb in the residue from the total Nb content of the steel sheet was solute Nb amount. The results are shown in Tables 2A ~ Table 2C.
[0071]
(Tensile strength and elongation)
　were also determined for tensile strength TS and elongation EL of the hot-rolled steel sheet obtained (%). Tensile strength TS (MPa), elongation EL (%) was measured by JIS Z 2241 (2011) Method of tensile test for metallic materials. The results are shown in Tables 2A ~ Table 2C. TS is a good 400 ~ 640MPa, EL was good for more than 25.0%.
[0072]
(Press Press Cracking presence of mold) of
　the nitrocarburizing treatment before the press-molded article was evaluated the presence or absence of the occurrence of cracks as a press cracking evaluation. The evaluation results are shown in the "E", "S", "E, S", "N". The contents of the "E" - "N" is as follows. The results are shown in Tables 3A ~ Table 3C.
[0073]
E: There are cracks in the ends of the molded article.
S: there is a crack in the shoulder R portion.
E, S: the end of the molded article is broken, there is a crack in the shoulder R portion.
N: No cracking.
[0074]
(Press ear occurrence or non-occurrence)
　for soft-nitriding treatment before the press-molded article, it was to evaluate the presence or absence of the occurrence of the ear. The difference between the highest of the press-molded product height and the lowest height and ear height. Results of evaluation are shown in "A", "B", "C", "D". The contents of "A" to "D" is as follows. B and A was determined as good. It should be noted that, as the press cracking has occurred, was not carried out the measurement of the press ear. The results are shown in Tables 3A ~ Table 3C.
[0075]
A: ear height is more than 0mm 1mm or less.
B: ear height is 1mm ultra 2mm below.
C: ear height of 2mm ultra-3mm or less.
D: ear height is 3mm greater.
[0076]
(Roughening occurrence or non-occurrence)
　for soft-nitriding after the press-rub the sides of the molded article in the circumferential direction in the grindstone of # 400, gave a streak-like defects. At that time, it is determined streaky scratches good if entered in a straight line, and the rough skin occurs (orange peel occurs) is not (A). Meanwhile, shading occurs or streaky scratches, when it is separated were to have rough skin occurs (occurrence of orange peel) (B). The results are shown in Tables 3A ~ Table 3C.
[0077]
(Hardness before and after cold working)
　was measured Vickers hardness of the center of plate thickness of the hot rolled steel sheet in press forming before and after. Vickers hardness of the center of plate thickness after pressing was the Vickers hardness of the plate thickness center at the side portion of the cup-shaped press-formed product. Work-hardening rate of the press-molded product varies by measurement positions. To work hardening rate is examined Vickers hardness before and after heat treatment in less than 30%, measured at 3 ~ 7 mm position from the bottom surface of the press-molded product, since the work hardening rate is examined Vickers hardness before and after heat treatment in 30% or more in, it was measured in 25mm and 35mm position from the bottom surface of the press-molded product. Table 3A ~ Table 3C, Vickers hardness Hv (before cold working) of the center of plate thickness of the front and rear cold working, showing the Hv (after cold working). Further, the measurement position of the post-cold working Vickers hardness Hv (after cold working). Further illustrates work hardening rate ΔR (%) of. Work hardening rate [Delta] R (%) was determined by the (alpha) expression and (beta) expression. Incidentally, what the press cracking occurs, hardness measurement was not performed.
[0078]
(Hardness before and after the heat treatment)
　was measured Vickers hardness of the center of plate thickness of the hot rolled steel sheet in the heat treatment before and after work hardening amount ΔTHv before and after the heat treatment, and was determined the hardness change ratio ΔHv before and after the heat treatment. Work hardening amount ΔTHv and heat treatment before and after the hardness change ratio ΔHv was determined by the (gamma) expression and ([delta]) expression.
　Then, Delta] HV is 80% or more is B and A, less than 80%. Incidentally, what the press cracking occurs, hardness measurement was not performed. The results are shown in Tables 3A ~ Table 3C.
[0079]
　It is shown in Tables 2A ~ Table 2C and Table 3A ~ Table 3C the above results.
[0080]
　Steel S01 ~ S42, S70, S72, S73 is a hot rolled steel sheet manufactured by the manufacturing conditions specified slabs having the chemical ingredients of the present invention in the present invention, the hardness variation after heat treatment showed 80% or more and which, seen to be excellent in softening resistance after heat treatment.
[0081]
　S79 and S80 are hot-rolled steel sheet manufactured by the manufacturing conditions specified slabs having the chemical ingredients of the present invention in the present invention. Specifically, the S79 and S03, the same type of steel is an example of hot-rolled under the same conditions, likewise, S80 and S18, is an example of hot rolling the same steel type under the same conditions. S79 and S80, since higher heating temperature during nitrocarburizing against S03 and S18, the hardness variation after heat treatment is less than 80%. However, by setting the heating temperature at the time of soft-nitriding to these steels S79 and S80 to 620 ° C. or less, the hardness variation after heat treatment becomes 80% as shown in S18 and S03.
[0082]
　Steel S43 ~ steel step S54 is an example that deviates from the chemical components of the present invention.
　That is, the steel S43 is less C content, the amount of NbC is reduced during the nitrocarburizing treatment, it could not be secured hardness. In addition, the ferrite grain becomes coarse, rough skin has occurred. Steel S44, since the C content was excessive, press cracking occurred EL is lowered. Steel S45, since the Si content was over, press cracking occurred EL is lowered. Steel S46 is less Mn content, ferrite grains become coarse, rough skin occurred. Steel S47 is, Mn amount is excessive, also, the area fraction of ferrite is generated bainite decreases, press cracking occurred EL is lowered. Steel S48 is excessive P content, also, the area fraction of ferrite is generated bainite decreases, press cracking occurred EL is lowered. Steel S49, since the S content was excessive, press cracking occurred EL is lowered. Steel step S50, less Al content, also ferrite crystal grains become coarse, rough skin occurred. Steel S51, since the Al content was excessive, press cracking occurred EL is lowered. Steel S52, since the N content was excessive, press cracking occurred EL is lowered. Steel and S53, small Nb content, Therefore solute Nb is lowered, we could not secure the hardness after nitrocarburizing. Steel step S54, Nb amount is excessive, also, the area fraction of ferrite is generated bainite decreases, press cracking occurred EL is lowered.
[0083]
　Steel S55 is low heating temperature at the time of hot rolling, solute Nb is lowered, we could not secure the hardness after nitrocarburizing.
[0084]
　Steel S56 has a high cooling rate to 800 ° C., the less ferrite area fraction of press cracking occurred EL is lowered.
[0085]
　Steel S57 is, B is greater than the upper limit, the ear of the press goods has increased. Furthermore, by C binds is B, reduces the yield of NbC, could not ensure the hardness after nitrocarburizing.
[0086]
　Steel S58 is greater cooling rate after finish rolling until coiling is the coiling temperature is also lowered, and decreases ferrite area fraction of bainite is also generated, press cracking occurred becomes EL low.
[0087]
　Steel S59, since the cooling speed is slow, the average crystal grain size of the ferrite is coarse roughening occurs and also could not ensure the hardness after nitrocarburizing becomes solid solution Nb low.
[0088]
　Steel step S60, large cooling rate to 800 ° C., the less ferrite area fraction of press cracking occurred EL is lowered.
[0089]
　Steel S61 is low heating temperature at the time of hot rolling, solute Nb is lowered, we could not secure the hardness after nitrocarburizing.
[0090]
　Steel S62 has a high finish rolling temperature, the solid solution Nb is reduced, could not secure the hardness after nitrocarburizing. On the other hand, the steel S63 is finish rolling temperature is low, middle coarse flat ferrite hot rolled occurs. Therefore anisotropy at the time of press working increases, EL was also reduced.
[0091]
　Steel step S64, large cooling rate to 800 ° C., the area fraction of ferrite is generated bainite decreases, TS becomes high, EL also decreased. On the other hand, the steel S65 has a low cooling rate to 800 ° C., the solid solution Nb is reduced, could not secure the hardness after nitrocarburizing.
[0092]
　Steel S66 is, because of the high cooling rate of up to 800 ℃ ~ coiling temperature, the area ratio of ferrite is low, press cracking has occurred become EL low. On the other hand, the steel S67 has a low cooling rate to 800 ° C. ~ coiling temperature, the solid solution Nb is reduced, could not secure the hardness after nitrocarburizing.
[0093]
　Steel S68 has a high coiling temperature, the solid solution Nb is reduced, could not secure the hardness after nitrocarburizing. On the other hand, the steel S69 is the coiling temperature is low, also bainite generated decreases the ferrite area fraction of press cracking occurred EL is lowered.
[0094]
　Steel S71 is low heating temperature at the time of hot rolling, solute Nb is not sufficiently generated. Since solute Nb is small, even the hardness and the soft nitriding treatment at a high temperature can not be ensured.
[0095]
　Steel S74, both steels S75 and steel S76 are hot-rolled steel sheet was hot rolled low Nb content slabs under the same conditions. These differences are examples of changing the rate of work hardening by changing the measuring position of Vickers hardness in the press molded articles. In either case, the solid solution Nb is not sufficiently generated. Therefore, the steel S74, as the steel S75, can not be secured hardness after nitrocarburizing at high machining area, and as the steel S76, could not secure the hardness after nitrocarburizing at low machining area .
[0096]
　Steel S77 and steel 78, the solid solution Nb but is highly reduced content of Nb steel, when work hardening rate is large, it is possible to secure the hardness after nitrocarburizing. On the other hand, steel S59, steel S61, steel S62, steel S65, steel S67, steel S68, as the steel S84, when the solid solution Nb is even higher less Nb content steel, work-hardening rate is small, soft it is impossible to ensure the hardness after nitriding.
[0097]
　Steel S81 and S82 are the hot-rolled steel sheet and hot rolled at approximately the same conditions is low Nb content slab, which by pressing, is an example of heat treatment at 620 ° C. greater than the high temperature. The difference of the steel S81 and S82 changes the measurement position of the Vickers hardness in the press molded article is an example of changing the work-hardening rate. Further, the difference between S53, S74 ~ S76 is that the heat treatment at a high temperature of 620 ° C. greater. In either case S81 and S82, the solid solution Nb for Nb content is very small is not sufficiently generated. Therefore, as the steel S81, it can not be secured hardness after nitrocarburizing at high machining area, and as the steel S84, could not secure the hardness after nitrocarburizing at low machining area.
[0098]
　Steel S83, albeit solid solution Nb, those C content is low. Therefore, the amount of NbC when on the heat treatment of the soft-nitriding is less failed to ensure the hardness be heat treated at a high temperature of 620 ° C. greater.
[0099]
[Table. 1A]

[0100]
[TABLE 1B]

[0101]
[Table. 2A]

[0102]
[Table 2B]

[0103]
[Table 2C]

[0104]
[Table. 3A]

[0105]
[Table 3B]

[0106]
[Table 3C]

[0107]
　Having described in detail preferred embodiments of the present invention, the present invention is not limited to such an example. It would be appreciated by those skilled in the relevant field of technology of the present invention, within the scope of the technical idea described in the claims, it is intended to cover various changes and modifications , also such modifications are intended to fall within the technical scope of the present invention.

The scope of the claims
[Claim 1]
　In mass% as chemical
components,
C:
0.040 ~ 0.150%, Si: 0 ~ 0.500%,
Mn: 0.10 ~ 1.50%, P: 0 ~
0.050%, S: 0
0.020%

Al:~, 0.010 ~ 0.050%, N: 0.0010 ~ 0.0060%,
Nb: 0.008 ~ 0.035%, Cu: 0 ~ 0.10%,
Ni:
~
0.10% 0,
Cr: 0 ~ 0.02%, Mo: 0 ~ 0.020%,
V: 0 ~ 0.020%, Ca: 0 ~ 0.0100%, and
B: 0 ~ 0. 0050%,
wherein the
solid solution Nb: 0.005 to 0.030%
　the balance being iron and impurities,
　not less than 85% by tissue area fraction of ferrite in the metallographic structure, the metal structure the remainder is cementite and / or pearlite, the average crystal grain size of the ferrite It is 5μm or more 20μm or less, the hot-rolled steel sheet.
[Claim 2]
　Vickers hardness of the center of plate thickness when subjected to a heat treatment of heating for 120 minutes at cold working and 560 ~ 620 ° C. relative to the hot-rolled steel sheet sequentially is,
　Vickers center of plate thickness after the cold working hot-rolled steel sheet according to claim 1, characterized in that show more than 80% of softening resistance against hardness.
[Claim 3]
　To the hot-rolled steel sheet, the cold work hardening rate of the Vickers hardness is less than 30%, the Vickers hardness of the center of plate thickness when sequentially performed and heat treatment of heating for 120 minutes at 560 ~ 620 ° C. Saga,
　hot rolled steel sheet according to claim 1, characterized in that show more than 80% of softening resistance against Vickers hardness of the plate thickness center portion after the cold working.
[Claim 4]
　A steel consisting of hot-rolled steel sheet according to any one of claims 1 to 3,
　in the case of sequentially performing a heat treatment of heating for 120 minutes at cold working and 560 ~ 620 ° C. relative to the hot-rolled steel sheet Vickers hardness of the center of plate thickness is,
　the steel is 80% or more with respect to the Vickers hardness of the center of plate thickness after processing between the cold.
[Claim 5]
　A steel consisting of hot-rolled steel sheet according to any one of claims 1 to 3,
　the work hardening rate is cold working and 560 ~ 620 ° C. to less than 30% of Vickers hardness with respect to the hot-rolled steel sheet in Vickers hardness of the center of plate thickness when sequentially performed and heat treatment for heating 120 minutes,
　the steel is 80% or more with respect to the Vickers hardness of the center of plate thickness after said cold working.
[Claim 6]
　In mass% as chemical
components,
C:
0.040 ~ 0.150%, Si: 0 ~ 0.500%,
Mn: 0.10 ~ 1.50%, P: 0 ~
0.050%, S: 0
0.020%

Al:~, 0.010 ~ 0.050%, N: 0.0010 ~ 0.0060%,
Nb: 0.008 ~ 0.035%, Cu: 0 ~ 0.10%,
Ni:
~
0.10% 0,
Cr: 0 ~ 0.02%, Mo: 0 ~ 0.020%,
V: 0 ~ 0.020%, Ca: 0 ~ 0.0100%, and
B: 0 ~ 0. 0050%,
wherein the heating the steel slab and the balance being iron and impurities 1200 ° C. or higher,
　subjected to final rolling finish rolling at a finishing rolling temperature of less than 950 ° C. 860 ° C. or higher,
　from the finish rolling temperature of 800 ° C. during cooling at an average cooling rate of 30 ° C. / sec or higher 100 ° C. / sec or less,
　the take-up the temperature from 800 ° C. The until time cooled at 5 ° C. / sec or higher 100 ° C. / sec of average cooling speed,
　wound at 300 ° C. or higher 600 ° C. or less of the take-up temperature, method for producing a hot-rolled steel sheet.