JP2004058149A - Hot rolled steel strip manufacturing method - Google Patents

Abstract

A method of manufacturing a hot-rolled steel strip capable of stably manufacturing a hot-rolled steel strip having an ultrafine ferrite structure having a final ferrite grain size of 3 μm or less without deteriorating a sheet thickness profile and a sheet shape. I will provide a.
A hot slab is subjected to one or more passes of reduction in the thickness direction at a temperature equal to or higher than the Ar3 transformation point, and a reduction rate of at least the last pass of each pass is set to 50% or more. (A), a step of immediately cooling the coarse bar to a temperature lower than the Ar1 transformation point, and a step (B) of heating the cooled coarse bar to a temperature higher than the Ac3 transformation point to reverse to austenite. A step (C) of inducing transformation, a step (D) of immediately applying a reduction in a thickness direction of 50% or more to the heated rough bar in a sheet thickness direction to form a steel strip, and immediately thereafter at 50 ° C./second or more (E) cooling the steel strip at a cooling rate of:
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a hot-rolled steel strip having an ultrafine 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. It is known that the crystal grains are refined by performing a large rolling process. For example, in order to obtain an ultrafine grain structure having an average grain size of 3 to 4 μm or less, a reduction of 50% or more in one pass is required. It is said that it is necessary.
[0003]
For example, a method is disclosed in which an anvil compression process of 50% or more is applied at a temperature equal to or higher than the Ar3 transformation point, and then cooled to produce an ultrafine structure steel having a ferrite having an average particle size of 3 μm or less as a matrix. (For example, see Patent Document 1).
[0004]
Further, as a method for producing a fine-grained structure steel material having excellent ductility having a fine-grained ferrite crystal structure having a grain size of 3 to 4 μm as it is hot-rolled, hot working is performed in a process of cooling from a temperature range of the Ac3 transformation point or higher. In addition, in the final stage, the hot reduction in which the total area reduction rate at least once or twice within the temperature range of (Ar1 + 50 ° C.) to (Ar3 + 100 ° C.) is substantially within 1 second is 50% or more and 95% or less. There is also a method in which a working is performed and after the completion of the hot working, cooling is performed at a cooling rate of 20 ° C./sec or more and 2000 ° C./sec or less to a temperature range of 600 ° C. or less (for example, Patent Document 2 or Patent Reference 3).
[0005]
Further, there is a method in which rolling is performed at a total reduction of 80% or more in the vicinity of the Ar3 transformation point to produce a fine-grain high-strength hot-rolled steel strip (for example, see Patent Document 4).
[0006]
[Patent Document 1]
Japanese Patent Application Laid-Open No. Hei 11-92961
[Patent Document 2]
Japanese Patent Publication No. 62-7247
[Patent Document 3]
Japanese Patent Publication No. 62-39228
[Patent Document 4]
JP-A-58-123823
[Problems to be solved by the invention]
Normally, in the process of manufacturing a hot-rolled steel strip, a high-temperature slab having three or more Ar points reheated in a heating furnace or directly conveyed from a continuous casting line is composed of one or more rolling stands. In a rough rolling process consisting of multi-pass rolling in a rough rolling mill, the thickness is reduced to a coarse bar having a thickness of about 30 to 50 mm. And then wound up in a coiler after a cooling step.
[0011]
When considering the production of a hot-rolled steel strip having an ultrafine grain structure in this normal hot-rolled steel strip production line, for example, as in the above-mentioned Patent Document 1, in the finish rolling process in the prior art, a specific rolling process is performed. In most cases, large pressure is applied in the processing temperature range. However, since the final thickness of the hot-rolled steel strip is about several millimeters, when one-pass large reduction is applied at any of the rolling stands during the finish rolling process, a large bending deflection is caused by the large rolling load. Occurs. For this reason, 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, that is, 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 rolling mill including a driving system and capable of withstanding a large rolling load and a large torque is required, and in order to secure a necessary rolling finishing temperature, the productivity is further increased. In order to prevent the drop, the high-speed rolling by a large-capacity motor is required, and it is very difficult to perform the rolling by a rolling facility having general specifications.
[0012]
In addition, the intermittent large rolling method by anvil compression shown in Patent Document 1 is usually compared with a manufacturing process of a hot-rolled steel strip which is finish-rolled at a speed of several hundred meters to several hundred meters per minute, Since the productivity is very low and it is difficult to obtain a uniform thickness in the longitudinal direction, it is unsuitable as a finish rolling equipment for a hot-rolled steel strip finishing to a final thickness of several mm.
[0013]
Further, in these conventional techniques, even if a large reduction is actively performed, the final ferrite grain size that can be manufactured in the actual hot rolled steel strip manufacturing process is limited to about 3 μm.
[0014]
Furthermore, in recent years, in a method of manufacturing a hot-rolled steel strip, in order to finally manufacture a hot-rolled steel strip having an ultrafine ferrite structure, the austenite crystal grains at the side of the finish rolling step, that is, at the coarse bar stage, are formed. It has been reported that it is important to be as fine as possible.
[0015]
However, in the production process of a general ordinary hot-rolled steel strip, the slab is reduced to about 30 to 50 mm in a rough rolling process of performing multi-pass rolling using a rough rolling mill including one or a plurality of rolling stands. In reducing the thickness to a coarse bar, the rolling reduction in one pass is at most 30%, and the interval between each pass must be very long, from several seconds to several tens of seconds. That is, since the rolling reduction in one pass is small, it is difficult to obtain a fine grain structure by dynamic or static recrystallization induced by rolling. The grain growth rate between passes is also very fast. Usually, between the rough rolling step and the finish rolling step, the coarse bar is kept at a high temperature of about 950 to 1100 ° C. for about one minute, and the austenite grain size of the coarse bar immediately before the finish rolling is 50%. About 100 μm. When the finish rolling is started from the initial austenite grains having such a size, the austenite grain size obtained immediately after the final finish rolling pass is reduced to only about 20 μm, and the transformed ferrite grain size caused by subsequent cooling is 3 μm. About 10 μm.
[0016]
The present invention solves the above-mentioned problems of the prior art and stabilizes a hot-rolled steel strip having an ultrafine ferrite structure having a final ferrite grain size of 3 μm or less without deteriorating the thickness profile and the plate shape. It is an object of the present invention to provide a method for manufacturing a hot-rolled steel strip that can be manufactured by using the method described above.
[0017]
[Means for Solving the Problems]
The present inventors have paid attention to the reduction of the austenite grain size before finish rolling in order to produce a hot-rolled steel strip having an ultrafine grain structure. Then, fine recrystallized austenite grains are precipitated by one-pass large pressure reduction in the rough rolling step, and immediately after that, rapid cooling is performed, so that the fine grain structure precipitated by the large reduction processing can be frozen. Inspired. That is, according to this method, in addition to the effect of grain refinement by one-pass large pressure reduction, the effect of suppressing grain growth from the rough rolling step to the finish rolling step can be obtained.
[0018]
In a rough rolling process in a normal hot-rolled steel strip production line, that is, in a stage where the thickness of the material to be rolled is large, achieving a rolling reduction of 50% or more in one pass requires the friction between the rolling roll and the material. However, it is difficult to reduce the thickness of the roll by a normal rolling method using a roll. In order to realize this, a rolling mill having a very large roll diameter is required, which is not practical. On the other hand, if a reduction is performed by a forging die thickness reduction device (hereinafter, referred to as a thickness reduction press device), a reduction of 50% or more can be realized. Is not necessary, so that it is very preferable for one pass large pressure reduction in the rough rolling step.
[0019]
On the other hand, it is known that fine austenite grains can be obtained by inducing a reverse transformation from ferrite to austenite by processing heat generated spontaneously by external heating or processing.
[0020]
Then, the present inventors performed a rapid cooling immediately after the rough rolling step (step (A)) (step (B)) to obtain a coarse bar having a fine-grained ferrite structure in a finish rolling step (step (D)). ) It was conceived that rapid transformation on the entry side (step (C)) induces a reverse transformation from ferrite to austenite, and the initial austenite structure in finish rolling can be refined.
[0021]
The present inventors obtain a coarse bar having initial austenite grains having a grain size of 20 μm or less on the entry side of finish rolling by combining the large reduction in the rough rolling step and the thermomechanical treatment utilizing this reverse transformation. The rough bar having such an initial austenite grain size can be finished and rolled immediately at a predetermined rolling reduction or more (step (D)), and immediately thereafter, rapid cooling is performed (step (E)). As a result, it has been found that a hot-rolled steel strip having an ultrafine ferrite structure of 3 μm or less, which was the conventional limit, can be manufactured.
[0022]
The present invention has been made based on these findings, and has the following features.
[0023]
(1) A method for producing a hot-rolled steel strip, comprising: a step (A) of applying a reduction in a thickness direction to a hot slab to form a coarse bar; and a step (B) of cooling the coarse bar. In the step (A), the hot slab is subjected to one or more passes of reduction in the thickness direction at a temperature equal to or higher than the Ar3 transformation point, and a reduction rate per time of at least the last pass of the hot slab is reduced to 50%. %, And the step (B) is a step of cooling the coarse bar to a temperature not higher than the Ar1 transformation point immediately after the step (A) at a cooling rate exceeding 15 ° C./sec. A method for producing a hot-rolled steel strip.
[0024]
(2) A step (A) of applying a reduction in the thickness direction to the hot slab to form a coarse bar, a step (B) of cooling the coarse bar, and a step (C) of heating the cooled coarse bar. Wherein the step (A) comprises applying one or more passes of reduction in the thickness direction to the hot slab at a temperature not lower than the Ar3 transformation point; The step (B) is performed at a cooling rate exceeding 15 ° C./sec. Immediately after the step (A), in which at least the rolling reduction per pass of the last pass is 50% or more. A step of cooling the coarse bar to a temperature not higher than the Ar1 transformation point, wherein the step (C) comprises, after the step (B), heating the cooled coarse bar to a temperature higher than the Ac3 transformation point to induce reverse transformation to austenite. A method for producing a hot-rolled steel strip.
[0025]
(3) a step (A) of applying a reduction in the thickness direction to the hot slab to form a coarse bar, a step (B) of cooling the coarse bar, and a step (C) of heating the cooled coarse bar. A method for producing a hot-rolled steel strip, comprising: a step (D) of applying a reduction in the thickness direction to the heated rough bar to form a steel strip; and a step (E) of cooling the steel strip. In the step (A), the hot slab is subjected to one or more passes of reduction in the thickness direction at a temperature equal to or higher than the Ar3 transformation point, and a reduction rate per time of at least the last pass of the hot slab is reduced to 50%. %, And the step (B) is a step of cooling the coarse bar immediately after the step (A) to a temperature equal to or lower than the Ar1 transformation point, and the step (C) is a step (C). After B), the cooled coarse bar is heated above the Ac3 transformation point to induce reverse transformation to austenite. The step (D) comprises, after the step (C), immediately applying a reduction in a thickness direction of 50% or more in total thickness to the heated coarse bar to form a steel strip; E) is a step of cooling the steel strip at a cooling rate of 50 ° C./second or more immediately after the step (D).
[0026]
(4) In the step (A), as a means for applying a reduction in the sheet thickness direction in which a reduction rate per operation is 50% or more, a forging die reduction device for reducing a hot slab with upper and lower dies is used. The method for producing a hot-rolled steel strip according to any one of the above (1) to (3).
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
Drawing 3 is an explanatory view showing one embodiment of a manufacturing equipment row of a hot-rolled steel strip used for carrying out the present invention, and is equipment which manufactures a hot-rolled steel strip from a slab cast by continuous casting equipment. .
[0028]
The hot-rolled steel strip manufacturing equipment shown in FIG. 3 includes a rough rolling mill 4 that rolls a slab 3 that has been cast by a continuous casting apparatus 1 or that has been reheated in a heating furnace 2 after casting to a predetermined thickness. Subsequently, the thickness reduction press device 5 capable of continuously reducing the slab 3 in the thickness direction with a reduction rate of 50% or more per time, and the rapid cooling of the rough bar immediately after the thickness reduction press are performed. A rapid cooling device 6a, a rapid heating device 7 for rapidly heating a rough bar to a predetermined temperature, a finishing mill 8 for reducing the thickness of the rough bar to a predetermined thickness, and a hot rolling steel strip immediately after finish rolling. A rapid cooling device 6b for rapidly cooling to a predetermined temperature, a cooling device 9 for adjusting a winding temperature of the hot-rolled steel strip after the rapid cooling, and a coiler 10 for winding the hot-rolled steel strip are provided. ing.
[0029]
The plate thickness reduction press device 5 is a device for repeatedly performing a press operation for opening and closing a mold provided above and below the slab 3 while sandwiching the slab 3 while sequentially sending out the slab, and processing the entire length of the slab to a desired thickness. . According to such a plate thickness reduction press apparatus, unlike the conventional rough rolling using a rolling roll, a large reduction with a reduction rate of about 70% per operation is possible. In the present invention, one or a plurality of thickness reduction presses capable of performing a large reduction of 50% or more per reduction are provided.
[0030]
The rough rolling mill 4 is for rolling a slab to a predetermined thickness by one or a plurality of rolling mills, and is provided on the upstream side of the plate thickness reduction press device 5. However, when only the plate thickness reduction press device 5 can be used to reduce the slab 3 to a coarse bar having a predetermined thickness, it is not necessary to particularly install the device. In addition, it is desirable to install a heat retaining cover or the like between the rough rolling mill 4 and the plate thickness reduction press device 5 to prevent a temperature drop at the tail end of the material to be rolled.
[0031]
The rapid cooling device 6a is a device that performs rapid cooling immediately after a large reduction in the plate thickness reduction press device 5. The rapid cooling device 6a is desirably disposed as close as possible to the exit side of the plate thickness reduction press device 5.
[0032]
As the rapid heating device 7, an induction heating device having good temperature control is usually used so that the coarse bar can be rapidly heated in a short time. Further, an edge heater may be provided adjacent to the rapid heating device 7 to heat the edge of the coarse bar. It is desirable to install a heat retention cover or a coil box between the coarse bar rapid cooling device 6a and the rapid heating device 7, particularly for preventing a temperature drop at the tail end of the coarse bar. Further, the control of the heating output by the rapid heating device 7 calculates the cross-sectional average temperature from the surface temperature of the coarse bar measured by the thermometers 11a and 11b, and becomes equal to or higher than the Ac3 transformation point over the entire length and the entire thickness of the coarse bar. As such, and considering the pass schedule and the rolling speed in the finish rolling, the heating output may be adjusted from the leading end to the tail end of the coarse bar so that a predetermined finishing temperature can be secured at the finishing mill outlet. .
[0033]
The cooling device 6b is a device for performing rapid cooling immediately after the finishing mill, and is desirably arranged as close as possible to the exit side of the finishing mill 8.
[0034]
Further, from the viewpoint of material adjustment, the temperature at the time of winding on the coiler 10 is also important. In the embodiment of FIG. 3, a cooling device 9 for winding temperature adjustment is arranged immediately before the coiler 10.
[0035]
Hereinafter, an embodiment of the method of the present invention using the above-described apparatus configuration will be described with reference to a rolled material temperature transition diagram of FIG.
[0036]
The slab 3 after continuous casting is usually about 200 to 300 mm in thickness. In the embodiment of FIG. 3, the slab 3 is directly sent from the continuous casting apparatus 1 or at a temperature not lower than the Ac3 transformation point in the heating furnace 2 (normally). Uses a slab reheated to 1100 to 1250 ° C).
[0037]
First, in the step (A), at a temperature equal to or higher than the Ar3 transformation point, the slab is reduced by a rough rolling mill 4 in one or more passes to reduce the thickness to, for example, about 100 mm. Further, the sheet thickness reduction press device 5 further applies a reduction in the sheet thickness direction with a reduction rate of 50% or more per press (press working), thereby forming a coarse bar having a sheet thickness of about 30 to 50 mm. In the step (A), the number of rolling passes by the rough rolling mill 4 and the thickness reduction pressing by the thickness reduction press device 5 are performed in consideration of the reduction amount and the uniformity of the strain distribution given by the reduction. The number of passes is determined. Further, the number of these passes can be adjusted by the number of installed rough rolling mills 4 and the thickness reduction press devices 5 and the number of times of reverse.
[0038]
However, in the present embodiment, at least in the last pass in the step (A), the sheet thickness reduction press device 5 applies a reduction of 50% or more to the slab at one time at a temperature equal to or higher than the Ar3 transformation point. The grain refinement mechanism of the present invention needs to be at or above the Ar3 transformation point in order to utilize the austenite recrystallization phenomenon, and it is effective to apply large strain to grain refinement, This is because under a pressure of less than 50%, the effect of grain refinement is small. Further, such reduction is performed at least in the last pass of the step (A) because a reduction in the thickness direction of 50% or more is applied to refine the crystal grains, and then the rapid cooling in the step (B) is immediately performed. It is for doing.
[0039]
Next, in the step (B), the rough bar is rapidly cooled to a temperature equal to or lower than the Ar1 transformation point of the material to be rolled by using the rapid cooling device 6a provided immediately near the exit side of the plate thickness reduction press device 5. Here, the material to be rolled is rapidly cooled to a temperature lower than the Ar1 transformation point because the structure after press working is austenite. This is because it is necessary to rapidly cool to the temperature and transform it into ferrite. As the cooling rate increases, the number of transformation nuclei of ferrite precipitates increases, and the ferrite grain size also decreases. Further, since the higher the cooling rate, the more the grain growth is hindered, it is preferable to set the cooling rate to more than 15 ° C./sec.
[0040]
Through the steps (A) and (B) as described above, a coarse bar having a fine-grained ferrite structure can be obtained.
[0041]
Further, in the step (C), the rough bar conveyed to the entry side of the finishing mill 8 at a temperature below the Ar1 transformation point is rapidly heated by the rapid heating device 7 to a temperature above the Ac3 transformation point of the material to be rolled. Do. Thereby, reverse transformation from ferrite to austenite is induced, and a fine-grained austenite structure can be obtained. The step (C) is preferably performed immediately before the finish rolling step (step D). This is because if the time until the finish rolling step becomes longer, the grain size of the fine-grained austenite structure generated by the reverse transformation increases due to grain growth.
[0042]
Through the steps (A) to (C) described above, the initial austenite structure before finish rolling can be refined.
[0043]
Subsequently, in the step (D), the finishing mill 8 reduces the thickness to a predetermined finished plate thickness. In the step (D), depending on the amount of reduction to the finished plate thickness or the like, the finish rolling may be performed by a finishing rolling mill including one rolling stand, or may be performed by a finishing rolling mill including a plurality of rolling stands. In some cases, finish rolling is performed, but in either case, a reduction in the thickness direction of 50% or more in total reduction is applied.
[0044]
Then, in the step (E), rapid cooling is performed at a cooling rate of 50 ° C./second or more by the rapid cooling device 6 b located immediately near the exit side of the finishing mill 8. It is adjusted and cooled so that it is wound up by the coiler 10. At this time, the cooling rate by the rapid cooling device 6b is set to 50 ° C./sec or more to transform and precipitate ferrite having a small grain size by rapid cooling. In addition, the regulated cooling by the cooling device 9 is not always necessary. If the rapid cooling device 6b can cool the material to a predetermined temperature required for building the material, it may be directly wound after the rapid cooling. Good.
[0045]
FIG. 2 is a diagram showing a change in crystal grain size in the process of manufacturing a hot-rolled steel strip. This figure is based on calculation results obtained by a simulation program that describes changes in the microstructure of a material as a function of changes in temperature and strain due to processing and time.
[0046]
The calculation conditions as an example of the production of the hot-rolled steel strip according to the method of the present invention described above are as follows. That is, the low-carbon steel slab 3 having a thickness of 250 mm is heated to 1100 ° C. in the heating furnace 2 using the row of hot-rolled steel strip manufacturing equipment shown in FIG. Then, the slab 3 is reduced to 100 mm by applying a three-pass reduction by the rough rolling mill 4, and further reduced to 30 mm by applying a reduction of 70% at a time by the thickness reduction press device 5. Thick and coarse bars (step (A)). Next, immediately after the reduction by the sheet thickness reduction press device 5, the material is cooled by the rapid cooling device 6a at a cooling rate of 30 ° C./sec to 700 ° C. which is below the Ar1 transformation point of the material (step (B)). The coarse bar is rapidly heated by the rapid heating device 7 at a heating rate of 70 ° C./sec to 900 ° C. which is equal to or higher than the Ac3 transformation point of the material to cause reverse transformation (step (C)). Then, in the finishing mill 8, the thickness is reduced to 2 mm (total rolling reduction of 93%) (step (D)). Immediately after finishing rolling, the cooling rate is reduced to 200 ° C./sec by the rapid cooling device 6b. And rapidly cooled to 600 ° C. (step (E)).
[0047]
On the other hand, calculation conditions as a comparative example are as follows. That is, the low-carbon steel slab 3 having a thickness of 250 mm is heated to 1100 ° C. in the heating furnace 2 similarly to the present invention. Then, instead of the step (A) of the example of the present invention, the slab 3 is reduced by 7 passes by the rough rolling mill 4 to reduce the thickness to 30 mm to obtain a coarse bar. The rapid cooling (step (B) of the present invention) and the rapid heating (step (C) of the present invention) are not performed on the coarse bar. Then, similarly to the present invention, the thickness is reduced to 2 mm (total rolling reduction of finish rolling 93%) in the finishing mill 8 and immediately after finishing rolling is completed, to 600 ° C at a cooling rate of 200 ° C / sec. After rapid cooling, it is wound up by the coiler 10.
[0048]
As a result of the calculation under the above conditions, as shown in FIG. 2, the austenite average particle size of the slab which is about 210 μm on the rough rolling mill entry side is finished by passing through the steps (A) to (C) of the present invention. At the entry side of the rolling mill, it is refined to an average particle size of about 10 μm. Further, it can be seen that a hot-rolled steel strip having an ultrafine ferrite crystal structure of about 1 to 2 μm can be obtained by further performing the steps (D) to (E). On the other hand, in the comparative example which does not go through the steps (A) to (C) of the present invention, the average particle size on the entrance side of the finishing mill is about 80 μm, and then the steps (D) to (E) of the present invention It can be seen that even under the same conditions, only a ferrite structure having an average particle size of about 8 μm can be obtained.
[0049]
As described above, according to the production method of the present invention, compared with the conventional production method, the grain size of the coarse bar on the entrance side of the finish rolling mill becomes finer, and further, the hot-rolled steel It turns out that a belt is obtained.
[0050]
【Example】
A hot-rolled steel strip was manufactured by performing the process used for the above-described simulation calculation. Similarly, for the comparative example, the process used for the above-described simulation calculation was performed to manufacture a hot-rolled steel strip.
[0051]
When the structure of the hot-rolled steel strip after cooling to room temperature was examined, the hot-rolled steel strip of the comparative example obtained by the conventional rough rolling method had a ferrite structure with an average grain size of about 8 μm, whereas the method of the present invention It was found that the hot-rolled steel strip produced by the above method had an ultrafine ferrite crystal structure of about 1 to 2 μm, and the effect of the method of the present invention was confirmed.
[0052]
Further, the production of the hot-rolled steel strip could be performed stably without deteriorating the sheet profile and the sheet shape.
[0053]
【The invention's effect】
As described above, according to the present invention, it is possible to manufacture a hot-rolled steel strip having an ultrafine ferrite structure with a grain size of 3 μm or less, which was conventionally difficult to manufacture.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing a transition of a rolled material temperature in an embodiment of a method of manufacturing a hot-rolled steel strip according to the present invention; FIG. 2 is an explanatory view showing a change in crystal grain size in a process of manufacturing a hot-rolled steel strip; 3 is an explanatory view showing one embodiment of a row of equipment for manufacturing a hot-rolled steel strip used in the practice of the present invention.
DESCRIPTION OF SYMBOLS 1 Continuous casting apparatus 2 Heating furnace 3 Slab 4 Rough rolling machine 5 Thickness reduction press apparatus 6a, 6b Rapid cooling apparatus 7 Rapid heating apparatus 8 Finish rolling mill 9 Cooling apparatus 10 Coils 11a, 11b Thermometer

Claims (4)

A method for producing a hot-rolled steel strip, comprising: a step (A) of applying a reduction in a thickness direction to a hot slab to form a coarse bar; and a step (B) of cooling the coarse bar.
In the step (A), a reduction in the thickness direction of one or more passes is applied to the hot slab at a temperature equal to or higher than the Ar3 transformation point, and a reduction rate per time of at least the last pass of the hot slab is determined. A step of forming a coarse bar with 50% or more;
In the step (B), immediately after the step (A), a step of cooling the coarse bar to a temperature equal to or lower than the Ar1 transformation point at a cooling rate exceeding 15 ° C./sec.
A method for producing a hot-rolled steel strip. A method comprising: a step (A) of applying a reduction in the thickness direction to a hot slab to form a coarse bar; a step (B) of cooling the coarse bar; and a step (C) of heating the cooled coarse bar. A method for manufacturing a rolled steel strip, comprising:
In the step (A), a reduction in the thickness direction of one or more passes is applied to the hot slab at a temperature equal to or higher than the Ar3 transformation point, and a reduction rate per time of at least the last pass of the hot slab is determined. A step of forming a coarse bar with 50% or more;
In the step (B), immediately after the step (A), a step of cooling the coarse bar to a temperature equal to or lower than the Ar1 transformation point at a cooling rate exceeding 15 ° C./sec.
In the step (C), after the step (B), a step of heating the cooled coarse bar to an Ac3 transformation point or more to induce a reverse transformation to austenite;
A method for producing a hot-rolled steel strip. A step (A) of applying a reduction in the thickness direction to the hot slab to form a coarse bar, a step (B) of cooling the coarse bar, a step (C) of heating the cooled coarse bar, and the heating A method for producing a hot-rolled steel strip, comprising: a step (D) of applying a reduction in the thickness direction to the roughened bar to form a steel strip; and a step (E) of cooling the steel strip.
In the step (A), a reduction in the thickness direction of one or more passes is applied to the hot slab at a temperature equal to or higher than the Ar3 transformation point, and a reduction rate per time of at least the last pass of the hot slab is determined. A step of forming a coarse bar with 50% or more;
The step (B) is a step of immediately cooling the coarse bar to a temperature equal to or lower than the Ar1 transformation point after the step (A);
In the step (C), after the step (B), a step of heating the cooled coarse bar to an Ac3 transformation point or more to induce a reverse transformation to austenite;
The step (D) is a step of immediately after the step (C), applying a reduction in a thickness direction of 50% or more in a total reduction ratio to the heated coarse bar to form a steel strip;
The step (E) is a step of cooling the steel strip at a cooling rate of 50 ° C./sec or more immediately after the step (D);
A method for producing a hot-rolled steel strip. In the step (A), as a means for applying a reduction in a sheet thickness direction in which a reduction rate per operation is 50% or more, a forging die reduction device for reducing a hot slab with upper and lower dies is used. Item 4. The method for producing a hot-rolled steel strip according to any one of Items 1 to 3.

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