JPH11169907A - Continuous hot rolling method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the timing of rolling and joining coincide in the width direction and to uniformize joining strength in the width direction by rolling a sheet with a 1st stand so that the sheet crown ratio in the vicinity of the joining part on the inlet side of the 1st stand of a finishing mill is larger than the sheet crown ratio on the outlet side. SOLUTION: When the sheet crown ratio in the vicinity of the joining part on the inlet side of the 1st stand of the finishing mill is expressed by C0 and that on the outlet side of the 1st stand by C1 , rolling is executed with the 1st stand so as to be C1 <C0 . In the case a convex crown is imparted to a 1st work roll of the finishing mill, it is preferable to impart in the amount of roll crown of >=20 μm/radius per the barrel length and by a curve which is higher than the 1st order and not higher than 2nd order. In the case a concave crown is imparted to the work roll of a roughing mill, it is preferable to be imparted in the amount of crown of >=200 μm/radius and by the curve which is higher than the 1st order and not higher than the 2nd order. Because the initial joining rate is reduced, joining time is shortened.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolling method in which a sheet bar after rough rolling is partially joined in a thickness direction to a preceding sheet bar during finish rolling to continuously roll.

[0002]

2. Description of the Related Art A hot-rolled steel sheet is manufactured by subjecting a sheet bar rolled to a predetermined thickness by a group of rough rolling mills to finish rolling by a group of finishing rolling mills, and winding the sheet bar by a coiler. .

[0003] In recent years, for the purpose of improving productivity and quality, a preceding sheet bar (hereinafter referred to as a preceding material) between a rough rolling mill group and a finishing rolling mill group and a succeeding sheet bar (hereinafter referred to as a subsequent material bar) have been developed. Continuous hot rolling is performed in which finish rolling is continuously performed by joining the sheets. In such continuous hot rolling, several methods have been proposed for a sheet bar joining method.

For example, Japanese Patent Laid-Open Publication No. Sho 60-244401 discloses a method in which sheet bars heated rapidly by a solenoid type coil are pressed together to join them. In this method, when the sheet bar is stopped and heated, the productivity is reduced, and when the sheet bar is heated while traveling, a facility for traveling a joining device including a large heating device is required. There is a problem that the cost increases.

Japanese Patent Application Laid-Open No. 7-256304 discloses a method of joining sheet bars using a laser beam. According to this method, the laser oscillator can be installed off-line, so that the bonding apparatus can be made compact. However, since the sheet bar is melt-bonded from the upper side by the laser beam,
Due to the problem of burn-through, the entire thickness of the sheet bar cannot be welded, and there is an unjoined area that is not joined in the sheet thickness direction. In addition, from the viewpoint of improving productivity, it is desirable to complete the joining in a short time even with a wide material. However, if the joining time is shortened, the welding depth decreases, and the unjoined area in the thickness direction further increases. In this case, there is a problem that the joint portion is broken by the tension between stands during the subsequent finish rolling.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to reduce the joining time in a method of partially joining a preceding sheet bar and a succeeding sheet bar in the thickness direction and continuously performing finish rolling. Therefore, even if the initial joining ratio in the thickness direction is reduced, a method of finish rolling without breaking the joining portion of the sheet bar is provided.

[0007]

According to the present invention, there is provided a method of continuously joining a rear end of a sheet bar after rough rolling to a leading end of a sheet bar after rough rolling in a sheet thickness direction. In the hot rolling method, the crown ratio near the joint at the entrance side of the first stand of the finishing mill is C ₀ , and that at the exit side of the first stand is C ₁ , C ₁ < A continuous hot rolling method characterized in that rolling is performed at a first stand of a finishing mill so as to obtain C ₀ .

That is, as shown in FIG. 1 (a), the work roll crown of the rolling mill is rolled with a convex, and as shown in FIG. 1 (b), the concave roll is formed on the work roll of the rough rolling mill. And the rolling of the first stand of the finishing mill at the entrance material crown of the first mill is performed, or as shown in FIG. Rolling is performed such that the entrance material crown of the first stand is convex and the work roll of the first stand of the finishing mill is a convex crown.

In the case where the work roll of the first stand of the finish rolling mill is subjected to finish rolling by giving a convex crown, the roll crown amount is 200 μm / radius or more per cylinder length, and the curve is higher than the primary and less than the secondary curve. Is given. The work roll of the rough rolling mill is provided with a concave crown, that is, the finishing rolling mill 1
When a convex crown is given to the material on the entry side of the stand, the concave crown of the work roll of the rough rolling mill should be 200 μm per body length.
It is preferable that the curve be given by a curve that is higher than m / radius and higher than primary and lower than secondary.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION As shown in FIG. 2, the present inventors joined the sheet bar partially from the upper surface side in the thickness direction, that is, after joining the sheet bar to a certain thickness from the upper surface side. Finish rolling was performed in a state where there was a part that was not joined in the direction (hereinafter, referred to as an unjoined part). Thereafter, the joint after finish rolling was cut out at each width position, and the joint strength was examined. As a result, it was found that the joint strength at the center of the width was lower than that at the plate edge and 1/4 width. Further, in the wide material, the strength at the center of the width was significantly reduced, and a through hole was also observed at the center of the width. Also, by observation of the joint after rolling, the thickness after rolling of the portion joined before rolling by welding (hereinafter, referred to as the initial joint) was constant over almost the entire width except for the plate edge, It has been found that the difference in the joining strength between the central portion and the 1/4 width is due to the difference in the widthwise direction in the strength improvement due to the rolling joining of the unjoined portion.

FIG. 3 shows an initial plate thickness of 40 mm by FEM analysis.
The joining portion of the sheet bar being rolled when the finishing material was subjected to one pass of finish rolling at a rolling reduction of 0.5 with a rolling width of 1300 mm and 1600 mm and a bonding rate of 40% in the thickness direction was viewed from the back side, that is, Although the planar shape of the joined portion is shown, the unjoined portion closes the earliest at a position of 200 to 300 mm from the end of the plate without depending on the plate width, whereas the closing at the central portion of the width is the most delayed. Therefore, the unjoined portion at a position of 200 to 300 mm from the end of the plate is rolled and joined most quickly, and the joining strength after rolling is naturally improved. On the other hand, at the center of the width where the unjoined portion closes slowly, first, 200 to 300 mm from the edge of the plate is closed, and compressive stress acts on this closed portion. Stress acts. Due to this tensile stress, the compressive stress acting on the unjoined portion in the center of the width during rolling decreases, and although the unjoined portion in the center of the width closes by the end of rolling, only weak rolling joining is obtained, and the strength is improved. The margin is smaller than other width positions.

[0012] This phenomenon is caused by a mismatch in the planar shape of the unjoined portion between the trailing end of the preceding material and the leading end of the succeeding material. It was found that the lower the width, the more difficult it was to roll-join at the center of the width as compared to the end of the sheet, and the lower the bonding strength at the center of the width became remarkable.

[0013] In view of the above, the present inventors have studied rolling conditions in which the timings of the roll joining are matched in the width direction, that is, the plane shapes of the trailing end of the preceding material and the leading end of the following material match. As a result, as shown in FIG. 4 (a), by giving a convex crown to the work roll of the first stand of the finishing mill (FIG. 1 (a)), the planar shape of the rear end of the preceding material is changed from the fish tail shape. It was found that the shape could be changed to a tongue shape and could be made to match the plane shape of the leading end of the following material. Since this effect is obtained by changing the width direction distribution of the tension in the rolling direction by giving the rolling reduction distribution in the width direction, when a convex crown is given to the entry side material of the finishing mill ( FIG.
4B, the same effect can be obtained as shown in FIG. Naturally, the same effect can be obtained even when a convex crown is applied to both the work roll and the input side material of the finishing mill.

The present invention focuses on this phenomenon, and imparts a convex crown to the work roll of the first stand of the finishing mill and / or the input material of the finishing mill to change the rolling reduction in the width direction. By doing so, the roll joining timing can be matched in the width direction, and the joining strength can be made uniform in the width direction.

The crown amount given to the work roll of the first stand of the finish rolling mill is such that the larger the crown amount, the easier it is to change the planar shape of the trailing end of the preceding material from the fishtail shape to the tongue shape, even in a wide material, and the larger the crown amount, the more desirable. However, a sufficient effect can be obtained even at 200 μm / radius per body length as shown in FIG.

The roll curve changes the width direction distribution of the tension in the rolling direction even in the case of a wide material, so that it is desirable that the reduction at the center of the width becomes sharply larger than at other width positions, that is, the shape is closer to a straight line. As shown in FIG.
If it is less than mm, a quadratic curve is sufficient.

When a convex crown is given to the input material,
What is necessary is just to use what provided the concave crown to the work roll of the rough rolling mill. At this time, the amount of the concave crown applied to the work roll of the rough rolling mill is the same as that of the case where the convex crown is applied to the work roll of the first stand of the finishing mill (200 μm / radius), and the change in the crown ratio is the same. As a result of a search in consideration of the elastic deformation of the roll, it was found that the radius was 200 μm / radius per body length.

[0018]

[Example] The entire width of the preceding and succeeding materials having a width of 1300 mm is
A sheet bar (thickness 40 mm, length 9000 mm) laser-bonded 30% from the upper surface in the thickness direction is heated to 1200 ° C., and a convex crown is formed under the conditions shown in Table 1 on a work roll of the first stand of the finishing mill. (Diameter: 850 mm (width center)) or a 1.2-degree curve was given to each of the entry-side materials, and finish rolling was performed to a plate thickness of 3 mm by a finishing mill group. Table 2 shows the state of the joint after rolling in each example. As is evident from Table 2, according to the present invention, there is no break in the joint after finish rolling and no decrease in strength at the center of the width.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

According to the present invention, in continuous hot rolling in which sheet bars are partially joined in the sheet thickness direction and finish rolling is performed, finish rolling is performed so that the sheet crown ratio is reduced at the first stand of the finishing mill. By performing the rolling, the strength of the joined portion can be made uniform in the width direction by matching the rolling joining timing of the unjoined portion. As a result, even when the initial joining ratio in the sheet thickness direction is low, during the finish rolling, Can be prevented from breaking at the joint. Further, since the initial joining ratio can be reduced, the joining time can be shortened.

[Brief description of the drawings]

FIG. 1 shows a schematic diagram of the method of the invention. (A) is a case where the work roll of the first stand of the finishing mill is rolled with a convex crown, (b) is a case where the entrance side material is rolled with a convex crown, and (c) is a case where the work roll is mixed with the work roll. This is the case where a convex crown is applied to any of the side materials and rolling is performed.

FIG. 2 shows a cross section in the rolling direction in which a leading material and a succeeding material are butted and partially joined in a sheet thickness direction.

FIG. 3 is a plan view of an unjoined portion during one-pass rolling.
(A) is a plate width of 1300 mm, (b) is a plate width of 1600
mm.

FIG. 4 (a) shows a plan shape of an unbonded portion during rolling when a convex crown is given to a work roll of a first stand of a finishing mill, and FIG. 4 shows a planar shape of an unbonded portion during rolling when a convex crown having the same crown ratio change as the work roll crown of the first stand of the finishing mill (FIG. 4A) is applied to the incoming material.

5A and 5B show a difference in a planar shape of an unjoined portion due to a difference in a convex crown amount of a first stand of a finishing mill; FIG.
μm / radius, (b) is a case where a convex crown of 400 μm / radius is given by a quadratic curve.

6A and 6B show a difference in a planar shape of an unjoined portion due to a difference in a roll curve, wherein FIG. 6A shows a case of a 1.2 degree curve and FIG. 6B shows a case of a quadratic curve.

Claims (4)

[Claims] 1. A hot rolling method in which a front end of a subsequent rough-rolled sheet bar is partially joined in a thickness direction to a rear end of a preceding rough-rolled sheet bar to continuously roll. The plate crown ratio in the vicinity of the joint at the entrance of the first stand of the finishing mill is C ₀ , and that at the exit of the first stand is C ₁
A continuous hot rolling method characterized by rolling at a first stand of a finishing mill so that C ₁ <C ₀ . 2. A work roll provided with a convex crown having a diameter of 200 μm / radius or more per barrel length in a curve higher than the first order and lower than the second order as the first stand of the finishing mill. Item 4. The continuous hot rolling method according to Item 1. 3. The final pass of the rough rolling includes 200
2. The continuous hot rolling method according to claim 1, wherein a work roll provided with a concave crown having a diameter of not less than μm / radius higher than the first order and not more than the second order is used. 4. A work roll provided with a concave crown having a curve higher than the first order and a curve not higher than the second order in the final pass of the rough rolling, and a higher degree than the first order in the first stand of the finishing mill. 2. The continuous hot rolling method according to claim 1, wherein a work roll provided with a convex crown having a quadratic or lower curve is used.