JP2004130375A - Method and equipment for manufacturing hot-rolled steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and equipment for manufacturing a hot-rolled steel strip which enable a hot-rolled steel strip having superfine ferritic structure to be manufactured. <P>SOLUTION: By the method for manufacturing the hot-rolled steel strip, rolling is performed in the hot finish rolling of a steel strip using a finishing mill composed of a plurality of rolling stands by setting each draft at one rolling stand or two or more adjacent rolling stands including the final rolling stand of the finishing mill to be ≥ 40% and also shape straightening is applied to the hot-rolled steel strip with levelers arranged on the outlet side of each rolling stand which performs rolling with a draft of 40% or more. <P>COPYRIGHT: (C)2004,JPO

Description

【００３５】
図１は、第５圧延スタンド〜第７圧延スタンドでの総圧下率（％）に対する熱延鋼帯のフェライト粒径（μｍ）の変化を示したものである。第５圧延スタンド〜第７圧延スタンドでの総圧下率にほぼ比例してフェライト粒径が微細化していることが判った。
【００３６】
表２は、表１の試験の１ケースとして、第５圧延スタンド〜第７圧延スタンドにて各々圧下率５０％の圧下を行っているＮｏ．１０について、パススケジュ−ルと各圧延スタンドでの圧延荷重、圧延動力の実績値を示した表である。また、図３はその時の板クラウン比率変化と急峻度を示したものである。
【００３７】
板クラウン比率変化は各圧延スタンド間プロフィルメ−タで測定したデ−タ、急峻度は板クラウン比率変化から計算した値であるが、レベラーを使用したケ−スでは目視による板形状評価を行い、板形状不良が認められなかったことから急峻度を０として評価している。また、通常の仕上圧延機ではモ−タ−定格出力の１７５％程度までの負荷がかかる圧延は可能であるが、表２より、特に第７圧延スタンドでの圧延負荷がほぼ定格出力の１７５％となっており、これ以上の大圧下や速度増が困難であることが判る。この点で、第５圧延スタンド〜第７圧延スタンドでの総圧下率を上げ、歪の累積効果を利用することが有効である。
【００３８】
【表２】

【００３９】
また、図３より、本圧延条件では第５〜第７圧延スタンドにて板クラウン比率変化が形状不感帯外となるため耳波形状となる。レベラーを使用しない場合、第５〜第７圧延スタンドにて急峻度がそれぞれ２〜３％となり、大きな耳波形状となったことを確認した。これに対し、第５圧延スタンドと第６圧延スタンド間、第６圧延スタンドと第７圧延スタンド間、第７圧延スタンド後に設置された各レベラーにて形状矯正を施した場合、目視にて大きな板形状不良が発生していないことを確認した。
【００４０】
【発明の効果】
以上説明したように、本発明によれば、超微細なフェライト組織を有し、強度・靭性に優れた平坦な板形状を有する熱延鋼帯の製造が可能となる。
【図面の簡単な説明】
【図１】本発明による熱延鋼帯製造方法によるフェライト粒径微細化効果を示す図
【図２】本発明の熱延鋼帯の製造設備列の一実施形態を示す説明図
【図３】本発明による熱延鋼帯製造方法での圧延実績例を示す図
【符号の説明】
１　　加熱炉
２　　スラブ
３　　粗圧延機
４　　コイルボックス
５　　仕上圧延機
６ａ　レベラー
６ｂ　レベラー
６ｃ　レベラー
７　　冷却装置
８　　巻取り機[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to a method and a facility for manufacturing a hot-rolled steel strip having a flat plate shape having a fine ferrite structure and excellent strength and toughness.
[0002]
[Prior art]
As a method for producing a fine-grained steel material having a fine-grained ferrite crystal structure having a grain size of 3 to 4 μm as hot-rolled and having excellent ductility, hot working is performed in a process of cooling from a temperature range of the Ac ₃ transformation point or higher. In the final stage, the total area reduction rate of once or twice or more in a temperature range of (Ar ₁ + 50 ° C.) to (Ar ₃ + 100 ° C.) substantially within 1 second is 50% or more and 95% or less. A method is known in which a certain hot working is performed, and after the hot working is completed, the 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, see Patent Documents 1 and 2.)
[0003]
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 is Ar ₃ -50 ° C. or higher. After finishing at 550 to 750 ° C., a method for producing a steel sheet for cans with good workability and without roughening by performing scale removal, cold rolling, continuous annealing, and temper rolling is provided. Are known. (For example, see Patent Document 3).
[0004]
[Patent Document 1]
Japanese Patent Publication No. 62-7247 [0005]
[Patent Document 2]
JP-B-62-39228 [0006]
[Patent Document 3]
JP-A-10-8142
[Problems to be solved by the invention]
In the methods of Patent Document 1 and Patent Document 2, one-pass large rolling is performed at any of the rolling stands in the finishing mill, but the final thickness of the hot-rolled steel strip is about several mm. When one pass large reduction is applied at one of the rolling stands in the finishing rolling line, bending of the rolling roll occurs due to a large rolling load, and the thickness profile of the rolled material becomes thicker at the center in the width direction. In addition, the convex cross-sectional shape in which the plate thickness decreases toward the plate width end, that is, the so-called plate crown, becomes extremely large, and a plate shape defect such as an ear wave or middle elongation tends to occur.
[0008]
In addition, in the method of Patent Document 3, in order to cause reverse transformation using the heat generated by processing in a finish rolling mill, conditions of large processing work in the rolling stand, that is, large pressure reduction and a large processing speed are essential. As in the case of Patent Literature 1 and Patent Literature 2, deterioration of the thickness profile and the shape of the plate has been a serious problem. When the shape of the plate is deteriorated due to a large reduction at a position other than the final rolling stand of the finish rolling mill, the steel plate is squeezed or perforated. In the worst case, the plate may be broken in the finish rolling mill. In addition, the one-pass large reduction in the latter stage of the hot finish rolling is problematic in that the rolling load, particularly the rolling power, becomes excessively large. It is limited by the rolling reduction.
[0009]
The present invention solves the above-mentioned problems of the prior art, and provides a method and apparatus for manufacturing a hot-rolled steel strip capable of stably manufacturing a hot-rolled steel strip having an ultrafine ferrite structure. With the goal.
[0010]
[Means for Solving the Problems]
The present inventors have studied a hot-rolled steel strip manufacturing method and a manufacturing facility capable of stably manufacturing a hot-rolled steel strip having an ultrafine ferrite structure having a good plate shape.
[0011]
Usually, in the finish rolling of a hot-rolled steel strip, if the finish rolling is completed in a two-phase region of austenite and ferrite below the Ar ₃ transformation point, a mixed grain structure is not obtained and desired mechanical properties cannot be obtained. In order to avoid this, the steel strip temperature at the exit side of the final finish rolling is set to the Ar ₃ transformation point or higher. It is also known that in hot working of steel materials, crystal grains can be refined by applying a large strain in one pass, and this refining effect is particularly effective at a temperature near the Ar ₃ transformation point. Large reductions are known to be effective. In other words, in order to obtain an ultra-fine ferrite structure by finish rolling for producing a hot-rolled steel strip, one-pass or multiple-pass large-pressure reduction is performed in a subsequent rolling stand in which the temperature of the rolled material approaches the Ar ₃ transformation point. It is very effective to add. However, as described above, it is inevitable that the shape of the plate is greatly disturbed under a one-pass large pressure reduction in a state where the plate thickness is small.
[0012]
Usually, a finish rolling mill for producing a hot-rolled steel strip is provided with various actuators for controlling a plate shape, such as a roll bender, a roll shift, and a roll cloth. In actual operation, products of various dimensions are manufactured, and there are various fluctuation factors such as thermal expansion of the roll and abrasion of the roll surface according to the rolling schedule. Therefore, setting of the shape control actuator by setup calculation In addition, the operator visually judges the plate shape between the rolling stands during the passing of the sheet, and changes the setting of these actuators online, or the actuator before the next coil rolling. This is reflected in the operation volume setting. However, when a large rolling reduction of 50% or more is applied at the final rolling stand to obtain a finished plate thickness of, for example, about 2 mm or less, the plate shape is kept flat only by control by these shape control actuators. It is very difficult.
[0013]
Therefore, the present inventors installed a leveler used for plate shape correction processing in-line of the finish rolling line, and performed rolling while correcting the plate shape defect generated under one-pass large pressure in-line. Inspired. Large grain rolling at the final rolling stand that performs rolling near the Ar ₃ transformation point is most effective for grain refinement, but rolling reduction that can be achieved in one pass due to restrictions on rolling load, rolling power, and the like. The amount is limited. In order to produce a hot-rolled steel strip having finer crystal grains, not only the final rolling stand, but also a downstream side of a finishing mill such as a rolling stand on one upstream side of the final rolling stand or another upstream side thereof. By applying a large reduction continuously at the plurality of rolling stands, a large refining effect can be obtained by utilizing the effect of accumulating strain. At this time, the plate shape defect generated by the large rolling reduction is manifested in a region where the plate thickness is thin, so that a leveler is installed after the rolling stand performing the large rolling reduction to correct the deterioration of the plate shape as needed, Stable threading is possible.
[0014]
The present invention has been made based on such findings, and the features thereof are as follows.
[0015]
(1) In hot finish rolling of a steel strip using a finish rolling mill composed of a plurality of rolling stands, each rolling reduction in one or two or more adjacent rolling stands of a finishing rolling mill including a final rolling stand is 40%. A method for producing a hot-rolled steel strip, wherein the hot-rolled steel strip is subjected to shape correction by a leveler disposed on an outlet side of each rolling stand for rolling at a rolling reduction of 40% or more while performing the rolling as described above.
[0016]
(2) In a finishing rolling mill including a plurality of rolling stands, a leveler for performing shape correction processing of a hot-rolled steel strip is arranged on two or more rolling stand exit sides adjacent to a finishing rolling mill including a final rolling stand. Characteristic hot rolled steel strip manufacturing equipment.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 2 is an explanatory view showing one embodiment of a hot-rolled steel strip manufacturing equipment row of the present invention.
[0018]
The hot-rolled steel strip production line shown in FIG. 2 winds the rough bar and a rough rolling mill 3 for reducing the slab 2 reheated in the heating furnace 1 to a rough bar having a predetermined thickness. A coil box 4 for keeping heat, a finishing mill 5 for reducing a coarse bar into a hot-rolled steel strip having a predetermined thickness, and a position between a rolling stand at a latter stage of three adjacent finishing mills including a final rolling stand. An upstream leveler 6a and a downstream leveler 6b for correcting the shape of the hot-rolled steel strip, a leveler 6c positioned immediately after the final rolling stand for correcting the shape of the hot-rolled steel strip, and a hot-rolled steel. A cooling device 7 for rapidly cooling the strip and adjusting the winding temperature, and a winding machine 8 for winding the hot-rolled steel strip are provided.
[0019]
The coil box 4 is used for winding a rough bar and keeping the temperature constant at the finish rolling entry side of the rough bar. In the finish rolling according to the present invention, since a large reduction is applied at the latter stage of the finish rolling where the rolling speed is necessarily increased, the rolling power at the later stage of the finish rolling machine becomes excessive. For this reason, it is preferable to make the motor of the rolling mill downstream of the finishing mill as large as possible, but it is possible to reduce the temperature drop at the tail end of the coarse bar by winding the coarse bar into a coil box. It is not necessary to perform high-speed rolling at the tail end by accelerated rolling that is usually performed, and by performing constant-speed rolling from the leading end to the trailing end of the rough bar, it is possible to prevent the rolling power from becoming excessive. As means for compensating for the temperature drop at the tail end of the coarse bar, a tunnel furnace, an induction heating device, or the like may be arranged instead of the coil box.
[0020]
Further, as another embodiment, a form of continuous hot rolling between the coil box 4 and the finishing mill 5 by welding or pressing the tail end of the front coarse bar and the front end of the next coarse bar. The present invention is applicable also to the above.
[0021]
The levelers 6a, 6b, 6c are arranged to correct a plate shape defect generated by large rolling reduction by bending back. Each of the levelers 6a, 6b, and 6c in the embodiment shown in FIG. 2 is composed of five rolls of three upper rolls and two lower rolls, and the elongation of about 0.3% is obtained by adjusting the inter-mesh. It is possible to give a rate. Further, a roll water cooling mechanism for reducing the thermal expansion of the leveler roll, a strong cooling mechanism for the leveler roll bearing, and the like are provided. Further, in order to reduce the abrasion of the surface of the leveler roll, it is desirable to use a material having high wear resistance, such as high-speed steel or super steel, as the leveler roll material.
[0022]
As described above, in order to make the temperature of the hot-rolled steel strip immediately above the Ar ₃ transformation point immediately after the final finish rolling from the viewpoint of material uniformity, for example, by adjusting the slab heating temperature in a heating furnace or the like at the finish rolling entry side. In order to adjust the temperature of the coarse bar and utilize the effect of accumulating strain in the subsequent rolling stand, it is preferable to use positive temperature control in the finishing mill by means of spraying between stands or the like. In other words, in order to utilize the effect of accumulating strain, it is preferable to perform continuous reduction directly above the transformation point of Ar _{3 as} much as possible. By performing rolling while cooling at, continuous large pressure reduction at the stage after finishing has a great effect on refining crystal grains.
[0023]
In the cooling after hot working, the higher the cooling rate, the faster the rate of transformation nucleation to ferrite, the slower the grain growth rate, and the like, and the cooling device 7 operates from immediately after the leveler 6c to a predetermined winding temperature. It is preferable to have the ability to perform cooling at a cooling rate of 50 ° C./sec or more.
[0024]
In order to enable rapid cooling immediately after the rolling mill, the cooling device 7 is desirably disposed as close to the leveler 6c exit side as possible. In the embodiment of FIG. 2, the cooling device 7 is disposed near the leveler. The leveler 6c may be installed downstream of the cooling device 7 in some cases, or the leveler 6c and the cooling device 7 may be combined into one structure, and the steel strip being cooled may be subjected to leveler processing.
[0025]
Hereinafter, an embodiment of the method of the present invention using the above-described apparatus configuration will be described.
[0026]
In the hot-rolled steel strip production line shown in FIG. 2, a slab 2 having a thickness of usually 200 to 300 mm is reheated to about 1200 ° C. in a heating furnace 1, and has a thickness of 30 to 50 mm in a plurality of passes in a rough rolling mill 3. To a coarse bar. The reduced coarse bar is once wound up in the coil box 4 and finished into a hot-rolled steel strip having a thickness of several mm by 7-pass rolling in a finishing mill 5 composed of 7 stands. In the present embodiment, a leveler 6a positioned between the fifth rolling stand and the sixth rolling stand for correcting the shape of the hot-rolled steel strip, and a leveler positioned between the final rolling stand and the sixth rolling stand one upstream thereof. A leveler 6b for correcting the shape of the hot-rolled steel strip and a leveler 6c for correcting the shape of the hot-rolled steel strip located immediately after the final rolling stand are installed. At 7 rolling stands, a large reduction of 40% or more is applied in each pass, and leveling machines 6a, 6b, and 6c provided immediately after each are performing a straightening process with an elongation rate of about 0.3%. Here, the reason why the elongation rate is set to about 0.3% is based on an experimental result that it is possible to correct most shape defects by giving an elongation rate of about 0.3%. Then, the cooling device 7 immediately cools the hot-rolled steel strip after the leveler immediately, and performs a controlled cooling to a winding temperature of about 600 ° C. I do. The rapid cooling rate is preferably 50 ° C./sec or more.
[0027]
In the above-described embodiment, the fifth rolling stand to the seventh rolling stand apply a large pressure reduction of 40% or more for each pass, and a leveler is installed immediately after each to correct the plate shape. The present invention is not limited to this embodiment. The present invention provides rolling of one or more adjacent rolling stands of a finishing rolling mill including a final rolling stand at a rolling reduction of 40% or more, and output of each rolling stand for rolling at a rolling reduction of 40% or more. This is a method for producing a hot-rolled steel strip in which a hot-rolled steel strip is subjected to shape correction by a leveler arranged on the side.
[0028]
The hot-rolled steel strip thus obtained is excellent in strength and toughness, has a fine-grained ferrite crystal structure with a grain size of 5 μm or less, and has a flat plate shape.
[0029]
【Example】
A hot-rolled steel strip was manufactured by the method of the present invention using the hot-rolled steel strip manufacturing equipment row shown in FIG. That is, a low carbon steel slab having a thickness of 250 mm and a width of 1000 mm is heated to about 1200 ° C. in a heating furnace, and the slab is reduced to 30 mm by a 5-pass rolling using a rough rolling machine to form a coarse bar. . The coarse bar was once wound into the entire length of a coil box, and then subjected to seven passes of finish rolling at a constant finish rolling speed of 700 mpm while sequentially unwinding the coarse bar wound into the coil box. At this time, the withstand load of each rolling stand is 4000 Tonf, and the rated motor output is 10,000 KW.
[0030]
Table 1 shows that the finished plate thickness was 2 mm, and in cases 2 to 10 of the present invention except for case 1, one or more adjacent two or more rolling stands (the seventh rolling stand, Rolling is performed by a pass schedule in which the rolling reductions at the sixth and seventh rolling stands, the fifth to seventh rolling stands) are 40% or more, and the rolling reductions at the other rolling stands are uniform, and the final rolling is performed. This is a result of immediately examining the average ferrite grain size of the hot-rolled steel strip immediately after cooling to about 600 ° C. and further cooling to room temperature. When the reduction from the fifth rolling stand to the seventh rolling stand is reduced by 40% or more, the shape of the hot-rolled steel strip is corrected by a leveler located immediately after the rolling stand.
[0031]
According to Table 1, each rolling stand that performs rolling at one or more adjacent two or more rolling stands of the finishing mill including the final rolling stand at a rolling reduction of 40% or more, and rolls at the rolling reduction of 40% or more. The hot-rolled steel strip is subjected to shape correction by a leveler disposed on the exit side of the ferrite to have a ferrite crystal structure with a grain size of 1.5 μm (Case 10) to 5 μm (Case 2) and a flat plate shape. It was found that a hot-rolled steel strip having
[0032]
In any case, the finish rolling temperature is within 820 ° C. to ± 10 ° C., which is the Ar ₃ transformation point of the material to be rolled. In the correction by the leveler, the inter-mesh was adjusted so that the elongation rate was set to about 0.2 to 0.3%.
[0033]
Case 1 as a comparative example was a case where the rolling reduction was uniform at 32.1% in all the rolling stands. In this case, a hot-rolled steel strip having a ferrite crystal structure with a particle size of 8.7 μm was obtained.
[0034]
[Table 1]

[0035]
FIG. 1 shows the change in the ferrite grain size (μm) of the hot-rolled steel strip with respect to the total rolling reduction (%) in the fifth to seventh rolling stands. It was found that the ferrite grain size was reduced in proportion to the total rolling reduction in the fifth to seventh rolling stands.
[0036]
Table 2 shows, as one case of the test of Table 1, No. 5 in which the rolling reduction was performed at a rolling reduction of 50% at each of the fifth to seventh rolling stands. 10 is a table showing the pass schedule and the actual values of the rolling load and the rolling power at each rolling stand. FIG. 3 shows the change in the plate crown ratio and the steepness at that time.
[0037]
The change in strip crown ratio is data measured by a profile meter between rolling stands, and the steepness is a value calculated from the change in strip crown ratio. Since no plate shape defect was recognized, the steepness was evaluated as 0. In a normal finishing mill, it is possible to perform rolling with a load up to about 175% of the rated motor output. From Table 2, it can be seen that the rolling load at the seventh rolling stand is almost 175% of the rated output. This indicates that it is difficult to further reduce the pressure and increase the speed. In this regard, it is effective to increase the total draft in the fifth to seventh rolling stands and use the effect of accumulating strain.
[0038]
[Table 2]

[0039]
Further, from FIG. 3, under the present rolling conditions, the plate crown ratio change is outside the shape dead zone at the fifth to seventh rolling stands, so that the shape becomes an ear wave shape. When a leveler was not used, the steepness was 2-3% in each of the fifth to seventh rolling stands, and it was confirmed that a large ear wave shape was obtained. On the other hand, when the shape was corrected by the levelers installed between the fifth rolling stand and the sixth rolling stand, between the sixth rolling stand and the seventh rolling stand, and after the seventh rolling stand, a large plate was visually observed. It was confirmed that no shape defect occurred.
[0040]
【The invention's effect】
As described above, according to the present invention, it is possible to produce a hot-rolled steel strip having a flat plate shape having an ultrafine ferrite structure and excellent strength and toughness.
[Brief description of the drawings]
FIG. 1 is a diagram showing the effect of reducing the ferrite grain size by the hot-rolled steel strip manufacturing method according to the present invention. FIG. 2 is an explanatory view showing one embodiment of a hot-rolled steel strip manufacturing equipment row according to the present invention. Diagram showing an example of actual rolling results in the method of manufacturing a hot-rolled steel strip according to the present invention.
DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Slab 3 Rough rolling mill 4 Coil box 5 Finishing rolling mill 6a Leveler 6b Leveler 6c Leveler 7 Cooling device 8 Winding machine

Claims (2)

In hot finish rolling of a steel strip using a finish rolling mill composed of a plurality of rolling stands, rolling is performed by setting each rolling reduction in one or two or more adjacent rolling stands of a finishing rolling mill including a final rolling stand to 40% or more. A method for producing a hot-rolled steel strip, wherein the hot-rolled steel strip is subjected to shape correction by a leveler disposed on an outlet side of each rolling stand for rolling at a rolling reduction of 40% or more. In a finishing rolling mill comprising a plurality of rolling stands, a leveler for performing shape correction processing of a hot-rolled steel strip is arranged on two or more rolling stand exit sides adjacent to a finishing rolling mill including a final rolling stand. Hot rolled steel strip manufacturing equipment.

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