JP2004099984A - Hot rolled steel strip manufacturing method and manufacturing equipment - Google Patents

Abstract

A method of manufacturing a hot-rolled steel strip and a hot-rolled steel capable of stably manufacturing a hot-rolled steel strip having an ultrafine ferrite structure over the entire length of a coil without deteriorating a sheet thickness profile and a sheet shape. Providing belt manufacturing equipment.
Kind Code: A1 A finish rolling is performed so that the temperature of a steel strip immediately after finish rolling is substantially constant at an Ar ₃ transformation point or higher over the entire length of the steel strip, and immediately thereafter, the steel strip is brought to a temperature not higher than the Ar ₁ transformation point. It is cooled and then heated to a temperature above the Ac ₃ transformation point to induce reverse transformation to austenite, and immediately cooled to a temperature below the Ar ₁ transformation point. It is preferable that the cooling of the steel strip immediately after the finish rolling and immediately after the heating is performed at a cooling rate of 50 ° C./sec or more, and that the rough bar temperature before the finish rolling is substantially constant over the entire length, and the finish rolling is performed. Preferably, the speed is substantially constant over the entire length.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and a facility for manufacturing a hot-rolled steel strip having an ultrafine ferrite structure.
[0002]
[Prior art]
2. Description of the Related Art In recent years, steel materials have been required to have higher strength in response to needs such as reduction in the weight of automobiles and heightening of buildings. In general, increasing the strength of steel reduces toughness, but in the case of strengthening by grain refinement, it is possible to improve strength without decreasing toughness, and various grain refinement techniques have been proposed. ing.
[0003]
As a conventional method for producing a fine-grained structure steel material having a fine-grained ferrite crystal structure with a grain size of 3 to 4 μm as hot rolled as it is, a method of producing a hot-rolled fine-grained steel material in a process of cooling from a temperature range equal to or higher than the Ac ₃ transformation point is performed. In the final stage, a total area reduction rate of one or more times in a temperature range of (Ar ₁ + 50 ° C.) to (Ar ₃ + 100 ° C.) within one second is 50% or more and 95% or more. %, And after completion of the hot working, there is a method of cooling to a temperature range of 600 ° C. or less at a cooling rate of 20 ° C./sec or more and 2000 ° C./sec or less (for example, Patent Document 1 or Patent Document 1). Reference 2).
[0004]
In addition, when the temperature of the material to be rolled passes through one of the rolling stands of the finishing mill train, the hot finish rolling reversely transforms due to heat generated by the rolling process, and the finish rolling temperature becomes Ar ₃ -50 ° C. or higher. In some cases, the winding is finished at a temperature of 550 to 750 ° C. (for example, see Patent Document 3).
[0005]
[Patent Document 1]
Japanese Patent Publication No. 62-7247 (page 2)
[0006]
[Patent Document 2]
JP-B-62-39228 (page 2)
[0007]
[Patent Document 3]
JP-A-10-8142 (page 3)
[0008]
[Problems to be solved by the invention]
The methods of Patent Document 1 and Patent Document 2 perform one-pass large pressure reduction in any stand in the finish rolling mill. However, since the final thickness of the hot-rolled steel strip is about several millimeters, when one-pass large rolling is applied at any of the rolling stands in the finishing mill, a large bending deflection is generated on the rolling roll due to a large rolling load. appear. As a result, the thickness profile of the rolled material is thicker at the center in the width direction, and the cross section of the convex shape, in which the thickness decreases toward the width end, the so-called crown becomes very large, and the ear wave or middle elongation Plate shape defects such as the like easily occur. In addition, in order to perform such a large rolling reduction, a powerful rolling mill that can withstand a large rolling load and a large torque, including a driving system, is necessary.Moreover, in order to secure a finishing temperature and maintain productivity. Requires high-speed rolling by a large-capacity motor, and it is very difficult to realize it with rolling equipment of general specifications.
[0009]
Further, in the method of Patent Document 3, in order to cause reverse transformation utilizing the heat generated in processing in the finishing mill, large machining work in the rolling mill stand, that is, conditions of large pressure and large machining speed are essential. As in the case of the methods of Patent Documents 1 and 2, the deterioration of the sheet thickness profile and the sheet shape as well as the performance of the rolling mill become a serious problem.
[0010]
The present invention solves the above-mentioned problems of the prior art, and stably manufactures a hot-rolled steel strip having an ultrafine ferrite structure over the entire length of a coil without deteriorating a sheet thickness profile and a sheet shape. It is an object of the present invention to provide a hot-rolled steel strip manufacturing method and a hot-rolled steel strip manufacturing facility that can perform the method.
[0011]
[Means for Solving the Problems]
The present inventors have found that a hot-rolled steel strip capable of stably producing a hot-rolled steel strip having an ultrafine ferrite structure without deteriorating the sheet thickness profile and the sheet shape in rolling equipment of general specifications. The manufacturing method of was investigated.
[0012]
Generally, steel undergoes a phase transformation from austenite to ferrite or from ferrite to austenite by being heated and cooled. It is known that by appropriately selecting the heating and cooling conditions, the crystal grains can be refined during this phase transformation. The present inventors have conceived of utilizing this characteristic in a method for producing a high-strength, high-toughness hot-rolled steel strip having an ultrafine grain structure.
[0013]
When rapid cooling is performed on the steel sheet immediately after the end of the hot working, a fine structure can be obtained by the effect of increasing the rate of formation of transformation nuclei into ferrite by the rapid cooling, and reducing the rate of grain growth. The present inventors have conducted intensive studies on the effect of the cooling rate. As a result, first, after the final rolling pass of finish rolling, the average grain size is reduced by cooling to a temperature not higher than the Ar ₁ transformation point at a cooling rate of 50 ° C./sec or more. It has been found that a fine ferrite structure of 8 μm or less is obtained. Further, the relative hot rolled strip having a fine ferrite structure, by rapidly heating above Ac ₃ transformation point at continued 100 ° C. / sec or more heating speeds, then induce reverse transformation from ferrite to austenite, fine Austenite can be formed. Immediately after the rapid heating, the hot-rolled steel strip having a fine austenite structure is cooled to an Ar ₁ transformation point or lower at a cooling rate of 50 ° C./sec or higher and wound up, so that the hot-rolled steel strip has an average It has been found that a hot-rolled steel having an ultrafine ferrite structure having a grain size of 3 μm or less can be produced.
[0014]
The present invention has been made based on such findings, and the features thereof are as follows.
[0015]
(1) In the method for producing a hot-rolled steel strip, finish rolling is performed so that the steel strip temperature immediately after the finish rolling is substantially constant at the Ar ₃ transformation point or higher over the entire length of the steel strip. hot rolling to cooling to Ar ₁ transformation point temperature, subsequently heated to Ac ₃ transformation point or above the temperature was induce reverse transformation to austenite, characterized in that immediately cooled to a temperature below Ar ₁ transformation point Steel strip manufacturing method.
[0016]
(2) The hot rolling as described in (1) above, wherein the cooling of the steel strip is performed at a cooling rate of 50 ° C./sec or more immediately after the finish rolling and immediately after heating to a temperature not lower than the Ac ₃ transformation point. Steel strip manufacturing method.
[0017]
(3) The hot-rolled steel strip as described in (1) or (2) above, wherein the rough bar temperature before finish rolling is made substantially constant over the entire length, and the finish rolling speed is made substantially constant over the entire length. Manufacturing method.
[0018]
(4) A hot-rolled steel strip manufacturing facility, wherein a first rapid cooling device for cooling a steel strip after finish rolling is provided on a transfer table between a finishing mill and a coiler, and a rapidly cooled steel strip. And a second rapid cooling device for re-cooling the rapidly heated steel strip.
[0019]
(5) The production equipment for hot-rolled steel strip according to (4), wherein the heating method of the rapid heating device is an induction heating method.
[0020]
(6) The hot rolling as described in (4) or (5) above, further comprising a heating device and / or a heat retaining device for keeping the temperature of the rough bar substantially constant over the entire length on the entrance side of the finishing mill. Steel strip manufacturing equipment.
[0021]
The present application also includes the following inventions.
[0022]
(7) The above (1) to (3), wherein the heating of the steel strip from the temperature below the Ar ₁ transformation point to the temperature above the Ac ₃ transformation point is performed at a heating rate of 100 ° C./sec or more. The method for producing a hot-rolled steel strip according to any one of the above.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
[0024]
(1st Embodiment)
FIG. 2 is a configuration diagram illustrating an example of a hot-rolled steel strip manufacturing facility according to the first embodiment of the present invention.
[0025]
The hot-rolled steel strip manufacturing equipment shown in FIG. 2 includes a rough rolling mill 3 for reducing a slab 2 reheated in a heating furnace 1 to a coarse bar having a predetermined thickness, and a coarse bar having a predetermined thickness. And a finishing mill 5 for reducing the thickness of the hot-rolled steel strip. Further, on the transport table on the downstream side, a first rapid cooling device 6 for rapidly cooling the hot-rolled steel strip, which is located immediately adjacent to the exit side of the finishing mill 5, and cooling water on the surface of the steel strip is removed. Water removing device 7, a rapid heating device 8 for rapidly heating a hot-rolled steel strip after rapid cooling, and a second rapid cooling device 9 for rapidly cooling a rapidly heated hot-rolled steel strip. And a winder 10 for winding the hot-rolled steel strip.
[0026]
The first rapid cooling device 6 performs rapid cooling of the steel strip immediately after the end of hot working to reduce the grain growth rate and the transformation nucleation rate to ferrite to obtain a finer structure. Cooling device. Therefore, in order to enable rapid cooling immediately after finish rolling, the first rapid cooling device 6 is desirably disposed as close as possible to the exit side of the finish rolling mill 5, and the steel strip after finish rolling is Ar _1. It is preferable to have the ability to cool to a temperature below the transformation point at a cooling rate of 50 ° C / sec or more.
[0027]
The cooling water removing device 7 is for removing cooling water remaining on the surface of the steel strip that has passed through the first rapid cooling device 6, and for maintaining the subsequent rapid heating device 8, the rapid heating device is used. It is preferable to provide it on the entry side of 8. In particular, when the rapid heating device 8 is an induction heating device, such a device is necessary because the cooling water on the steel strip is easily scattered in the induction heating device and electric leakage is likely to occur. In the present embodiment, the air purge device is provided to face both the upper and lower surfaces of the steel strip, but the present invention is not limited to this, and may be, for example, a drain roll.
[0028]
The rapid heating device 8 is a device for rapidly heating a rapidly cooled steel strip having a temperature of not more than the Ar ₁ transformation point to a temperature of not less than the Ac ₃ transformation point to induce reverse transformation to obtain a fine austenite structure. It is. If the heating rate at that time is low, the length of the heating device for giving the required heat-up amount becomes long, and a fine structure is hardly obtained due to the growth of grains during heating. It is preferable to use an induction heating device having high heating efficiency and good temperature control. If the induction heating device is used, the steel strip can be rapidly heated at a heating rate of 100 ° C./second or more. Therefore, the rapid heating device 8 can be made compact, and the steel strip can be heated without grain growth. Can and is desirable.
[0029]
The second rapid cooling device 9 performs rapid cooling on the steel strip immediately after the reverse transformation to austenite occurs, thereby slowing down the grain growth rate and increasing the transformation nucleus generation rate to ferrite, and A cooling device for obtaining tissue. Therefore, in order to enable rapid cooling immediately after the occurrence of the reverse transformation, it is desirable that the second rapid cooling device 9 be disposed as close as possible to the outlet side of the rapid heating device 8. It is preferable to have the ability to cool a steel strip having a temperature of ₃ transformation points or more to a temperature of ₁ Ar transformation point or less at a cooling rate of 50 ° C./sec or more.
[0030]
Next, a method of manufacturing a hot-rolled steel strip using the hot-rolled steel strip manufacturing equipment according to the present embodiment configured as described above will be described with reference to FIG.
[0031]
The slab 2 usually has a thickness of about 200 to 300 mm. In the present embodiment shown in FIG. 2, the slab 2 is directly sent from the continuous casting apparatus or reheated in the heating furnace 1, and has a temperature equal to or higher than the Ac ₃ transformation point. A slab (usually 1100 to 1250 ° C) is used. Then, the slab 2 is reduced to a predetermined thickness (usually about 30 to 50 mm) by the rough rolling mill 3 at a temperature equal to or higher than the Ar ₃ transformation point, and is processed into a rough bar. The number of passes in the rough rolling and the number of rough rolling mills 3 to be installed are appropriately determined so as to obtain a predetermined coarse bar thickness.
[0032]
The rough bar is subsequently reduced by the finishing mill 5 to a predetermined hot-rolled steel strip thickness at a temperature equal to or higher than the Ar ₃ transformation point. Here, in the present embodiment, the amount of temperature decrease between the time when the leading end portion of the coarse bar is passed through the finishing mill 5 and the time when the tail end portion is passed is compensated, and the entire length of the hot-rolled steel strip is compensated for. Accelerated rolling is performed to keep the temperature immediately after finish rolling substantially constant. In other words, the setting calculation including the temperature calculation of the rolled material is performed so that the temperature immediately after the finish rolling is substantially constant in advance over the entire length of the steel strip in advance, and the rolling conditions such as the threading speed of the rough bar tip, the acceleration rate, and the like are determined. And finish rolling. Note that the temperature immediately after finish rolling is “substantially constant” means a range of about ± 10 ° C. with respect to the target finish temperature, but the temperature near the edge of the sheet width is lower than that near the center of the sheet width. Because of the large size, it is preferable to provide an edge heater (not shown) on the upstream side of the finishing mill 5 so as to compensate for the temperature near the end of the sheet width.
[0033]
The finish-rolled hot-rolled steel strip is then cooled by the first rapid cooling device 6 at a cooling rate of 50 ° C./sec or more immediately after the final rolling pass of the finish rolling from a temperature above the Ar ₃ transformation point to the Ar ₁ transformation. Rapidly cooled to a temperature below the point. The cooling water removal device 7 removes the remaining cooling water on the steel strip surface injected by the first rapid cooling device 6, and the rapid heating device 8 at a heating rate of 100 ° C./second or more at the Ac ₃ transformation point or higher. , Is rapidly cooled by the second rapid cooling device 9 at a cooling rate of 50 ° C./sec or more to a temperature below the Ar ₁ transformation point and wound up by the winder 10.
[0034]
Further, from the viewpoint of material adjustment, the temperature at which the film is wound by the winder 10 is also important. In the present embodiment, the winding temperature is set to about 600 ° C.
[0035]
In the present embodiment, the passing speed of the hot-rolled steel strip on the runout changes from the leading end to the trailing end of the coil. Therefore, a plurality of thermometers are installed on the inlet side and the outlet side of each of the first rapid cooling device 6, the rapid heating device 8, and the second rapid cooling device 9, and the temperature measurement results and the final finish rolling mill are used. The information of the rolling speed controls the amount of cooling water, heating output, and the like to realize the temperature pattern shown in FIG. 1 over the entire length of the hot-rolled steel strip.
[0036]
As described above, according to the present embodiment, the hot-rolled steel strip immediately after finish rolling is rapidly cooled to the Ar ₁ transformation point or less to form a fine ferrite structure, and further heated to the Ac ₃ transformation point or more to induce reverse transformation. After that, the hot-rolled steel strip having an ultrafine ferrite structure can be obtained by rapid cooling to the Ar ₁ transformation point or less again. Furthermore, since the temperature immediately after the finish rolling is made substantially constant over the entire length of the steel strip by performing the accelerated rolling in the finish rolling, an ultrafine ferrite structure that is substantially uniform over the entire length can be obtained.
[0037]
(Second embodiment)
Drawing 3 is a lineblock diagram showing an example of the manufacturing equipment of the hot-rolled steel strip concerning a 2nd embodiment of the present invention. The same parts as those in FIG. 2 are denoted by the same reference numerals and description thereof will be omitted.
[0038]
The facility for manufacturing a hot-rolled steel strip shown in FIG. 3 is configured in the same manner as the first embodiment, except that a coil box 4 for winding a coarse bar and keeping heat is provided.
[0039]
The coil box 4 is provided in order to keep the temperature of the coarse bar on the entry side of the finishing mill 5 substantially constant over the entire length by winding and keeping the temperature of the coarse bar.
[0040]
In the accelerated rolling described in the first embodiment, the temperature of the steel strip immediately after finish rolling can be made substantially constant over the entire length, but it is inevitable that the heat histories including the time element will differ. However, for uniformity of the material, it is desirable that the heat history is constant over the entire length of the steel strip.
[0041]
Therefore, in this embodiment, from the viewpoint of uniformity of the material and controllability of rapid cooling and rapid heating on the runout, the temperature drop at the tail end of the rough bar is compensated, and the rough bar temperature at the entrance of the finishing mill 6 is compensated. The coil box 4 is provided in order to make the length substantially constant over the entire length. Then, by performing the finish rolling at a constant speed, the temperature of the hot-rolled steel strip at the exit side of the finish rolling mill 6 is made substantially constant over the entire length. The means for compensating for this temperature drop is not limited to the coil box 4, but may be any heating device or heat retention device that makes the rough bar temperature substantially constant over the entire length. For example, a tunnel furnace, an induction heating device, or the like may be provided.
[0042]
Next, a method of manufacturing a hot-rolled steel strip using the hot-rolled steel strip manufacturing equipment according to the present embodiment configured as described above will be described with reference to FIG. The detailed description of the same parts as those in the first embodiment will be omitted.
[0043]
As in the first embodiment, the slab 2 is reduced in thickness to a predetermined thickness by the rough rolling mill 3 and processed into a rough bar at a temperature equal to or higher than the Ar ₃ transformation point. Next, the coarse bar is once wound up by the coil box 4 and kept at a substantially constant temperature over the entire length.
[0044]
The coarse bar wound up in the coil box 4 is then discharged from the coil box 4 and subsequently reduced in thickness by the finishing mill 5 to a predetermined hot-rolled steel strip thickness at a temperature not lower than the Ar ₃ transformation point. Here, in this embodiment, finish rolling is performed at a constant rolling speed. That is, by performing finish rolling at a constant rolling speed on a rough bar having a substantially constant temperature over the entire length by being kept in the coil box 4, the hot-rolled steel strip temperature immediately after the finish rolling is made substantially constant over the entire length. can do. The term “substantially constant” over the entire length of the coarse bar means a range of the target coarse bar temperature of about ± 10 ° C., but the innermost circumference and the outermost circumference of the coarse bar wound around the coil box are referred to. Even if the portion is wound into a coil, a slight temperature decrease is inevitable due to being exposed to the atmosphere. Also, as in the first embodiment, a slight temperature drop is inevitable near the edge of the plate width. Therefore, it is preferable to install a rough bar heating device and an edge heater (both not shown) between the coil box 4 and the finishing mill 5 so as to compensate the temperature of these unsteady portions.
[0045]
The finish-rolled hot-rolled steel strip is then subjected to Ar ₃ cooling at a cooling rate of 50 ° C./sec or more immediately after the final rolling pass of finish rolling by the first rapid cooling device 6, similarly to the first embodiment. It is rapidly cooled from a temperature above the transformation point to a temperature below the Ar ₁ transformation point. The cooling water removal device 7 removes the remaining cooling water on the steel strip surface injected by the first rapid cooling device 6, and the rapid heating device 8 at a heating rate of 100 ° C./second or more at the Ac ₃ transformation point or higher. Is rapidly cooled to a temperature below the Ar ₁ transformation point by the second rapid cooling device 9 at a cooling rate of 50 ° C./sec or more, and is immediately taken up by the winder 10.
[0046]
As another embodiment, between the coil box 4 and the finishing mill 6, a continuous hot working is performed by welding or pressing the tail end of the preceding coarse bar and the leading end of the subsequent coarse bar. The present invention is applicable to the form of rolling. In particular, when the finished plate thickness is, for example, 2.0 mm or less, high-speed constant speed rolling by continuous hot rolling is effective.
[0047]
As described above, according to the present embodiment, the hot-rolled steel strip immediately after finish rolling is rapidly cooled to the Ar ₁ transformation point or less to form a fine ferrite structure, and further heated to the Ac ₃ transformation point or more to induce reverse transformation. After that, the hot rolled steel strip having an ultrafine ferrite structure can be obtained by rapid cooling again to the Ar ₁ transformation point or lower. Furthermore, in the finish rolling, the temperature at the entrance of the finishing mill is made substantially constant, and furthermore, constant speed rolling is carried out. Steel strip can be obtained.
[0048]
In the two embodiments described above, an example in which the rough rolling step is performed by a general method has been described. However, in order to obtain a hot-rolled steel strip having an ultrafine ferrite structure, the smaller the initial austenite grain size on the entry side of the finish rolling, the better, and the effect of implementing the present invention will be greater. Therefore, when carrying out the present invention, it is desirable to minimize the austenite grain size on the entry side of finish rolling.
[0049]
【Example】
A hot-rolled steel strip was manufactured according to the above-described second embodiment of the present invention using the hot-rolled steel strip manufacturing equipment row shown in FIG. 3. The transformation points of Ar ₃ , Ac ₃ , and Ar ₁ of the steel used were 850 ° C., 860 ° C., and 720 ° C., respectively.
[0050]
After heating a low carbon steel slab having an initial plate thickness of 250 mm to about 1200 ° C. in the heating furnace 1, the slab was reduced to 30 mm by 7-pass rolling in a rough rolling mill 3 to obtain a coarse bar. The rough bar was wound around a coil box 4 and kept at a substantially constant temperature, and then was subjected to seven passes of finish rolling by a finishing mill 5 at a constant rolling speed from the front end to the tail end while sequentially unwinding. The finish plate thickness is 2 mm, the finish rolling speed is 700 mpm, and the target finish temperature is 850 ° C. Subsequently, the steel strip is cooled to about 720 ° C. at a cooling rate of 200 ° C./sec by the first rapid cooling device 6 installed at a position immediately adjacent to the exit side of the final finishing mill, and the steel purge is performed on the steel strip by the air purge device 7. After removing the remaining cooling water, the temperature was raised to about 870 ° C. at a rate of 150 ° C./sec using the induction heating type rapid heating device 8. Immediately thereafter, it was cooled to about 600 ° C. at a cooling rate of 200 ° C./sec using the second rapid cooling device 9, and was taken up by the winder 10.
[0051]
On the other hand, investigation was also made on the crystal grain size in the method of manufacturing a hot-rolled steel strip by the conventional method. That is, a low-carbon steel slab having an initial plate thickness of 250 mm was heated to about 1200 ° C. in a heating furnace, and then this slab was reduced to 30 mm by 7-pass rolling in a rough rolling mill to obtain a coarse bar. The rough bar was subjected to seven-pass accelerated finish rolling in which the rolling speed at the tip end was 700 mpm and the rolling speed at the tail end was 1000 mpm. The finish plate thickness is 2 mm, and the target finish temperature is 850 ° C. Subsequently, it was cooled to about 600 ° C. at a cooling rate of 40 ° C./sec and wound up by a winder.
[0052]
In both the present invention and the conventional method, the structure of the hot-rolled steel strip was examined after winding and cooling to room temperature by air cooling, and the average particle size was about 8 to 12 μm in the conventional method. In contrast to the ferrite crystal structure having a large variation toward the end, the method according to the present invention has a uniform ultrafine ferrite crystal structure with an average particle size of about 2 μm over the entire length of the coil, The method of the present invention was confirmed.
[0053]
【The invention's effect】
As described above, according to the present invention, a hot-rolled steel strip having an ultrafine ferrite structure having a grain size of 3 μm or less can be stably manufactured, and a steel strip having high strength and high toughness can be manufactured. It becomes possible.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a transition of a rolled material temperature in a method of manufacturing a hot-rolled steel strip according to the present invention. FIG. 2 is a configuration diagram illustrating an example of a hot-rolled steel strip manufacturing facility according to the first embodiment of the present invention. FIG. 3 is a configuration diagram showing an example of a hot-rolled steel strip manufacturing facility according to a second embodiment of the present invention.
DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Slab 3 Rough rolling mill 4 Coil box 5 Finish rolling mill 6 First rapid cooling device 7 Moisture removal device (air purge device)
8 Rapid heating device 9 Second rapid cooling device 10 Winding machine

Claims (6)

The method of manufacturing a hot rolled strip, the finishing strip temperature immediately after rolling is performed to finish rolling so as to be substantially constant at Ar ₃ transformation point or more over the entire length of the steel strip, and immediately thereafter the steel strip Ar ₁ transformation cooled to a temperature below the point, subsequently Ac ₃ by heating above the transformation point temperature to induce reverse transformation to austenite, the hot rolled strip immediately characterized by cooling to a temperature below Ar ₁ transformation point Production method. Production of hot rolled strip according to the cooling of the steel strip, to claim 1, characterized in that a cooling rate of more than 50 ° C. / sec to perform immediately after heating immediately after finish rolling and Ac ₃ transformation point or more of the temperature Method. The method for producing a hot-rolled steel strip according to claim 1 or 2, wherein the rough bar temperature before finish rolling is made substantially constant over the entire length, and the finish rolling speed is made substantially constant over the entire length. A hot-rolled steel strip manufacturing facility, wherein a first rapid cooling device for cooling a steel strip after finish rolling is provided on a transfer table between a finish rolling mill and a coiler, and the rapidly cooled steel strip is heated. A hot-rolled steel strip manufacturing facility comprising: a rapid heating device; and a second rapid cooling device for cooling the rapidly heated steel strip again. The hot-rolled steel strip manufacturing equipment according to claim 4, wherein the heating method of the rapid heating device is an induction heating method. The production of the hot-rolled steel strip according to claim 4 or 5, wherein a heating device and / or a heat retaining device for keeping the temperature of the rough bar substantially constant over the entire length is provided on the entrance side of the finishing mill. Facility.

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