JP2001113301A - Rolling method for preventing rolling crack on continuously cast billet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rough rolling method of a bar steel whose rolling crack at the inner and the outer parts is hardly generated by using flat rolls, even when a continuously cast billet is directly fed into a rolling line without cooling and continuously rolling after welding the cast billets. SOLUTION: In the continuous rolling of the bar steel continuously rolled after welding the cast billets, when the bar steel is produced with the rolling by directly feeding the continuously cast billet, this rolling method is executed by using the continuously cast billet having the round cross section, and using caliber rolls at the first pass for rough rolled forming pass and the flat rolls on and after the second pass for rougher roll. The rolling method has the peculiarity, in which the light rolling reduction is executed to the side surfaces of the continuously cast billet in the perpendicular direction to the rolling direction of the first pass at the inlet side of the first pass rolls for rough rolled forming pass to prevent the rolling crack on the continuously cast billet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for rolling a steel bar such as a bar and a wire rod, and more particularly to a method for continuously rolling a steel bar by welding a billet slab manufactured by a continuous casting method.
The present invention relates to a rolling method capable of preventing a billet from rolling on the inner and outer surfaces and producing a high-quality strip.

[0002]

2. Description of the Related Art Generally, billet slabs are manufactured by reducing the cross-sectional size by continuous casting. Although the slab shape after billet continuous casting is generally a square cross section, there was a problem of internal cracking due to unevenness in the solidification process. In Japanese Patent Application Laid-Open No. 5-42304, uniformity of solidification in a mold is maintained by using a slab having a round cross-sectional shape obtained by continuous casting as a strip steel rolling material, and the solidified structure is caused by non-uniformity. There has been proposed a method for preventing cracking during solidification.

[0003] As a method for rolling a strip or wire rod using billet slabs, a rolling method using a hole-shaped roll is generally used.
For the purpose of improving the operation rate, a method called caliber-less rolling, in which rolling is performed in a plurality of passes using a flat roll in the rough rolling and intermediate rolling stages and a hole-shaped roll is used in the finishing pass, for example, is disclosed in Japanese Patent Publication No. -37582, JP-A-58-58
No. 23502, JP-A-58-68402 and the like. This rolling method has a low area reduction rate,
There is no need to change the roll depending on the product size,
Excellent economy. However, on the other hand, it is necessary to strongly reduce the pressure in the first pass of the rough rolling in order to increase the area reduction efficiency, and the flat roll is used, so that there is a problem that rolling cracks are easily generated on the billet side surface.

A technique of continuously welding and cutting billet slabs manufactured and cut by continuous casting in recent years using flash butt welding or the like is disclosed in Japanese Patent Laid-Open No. 9-66.
No. 301, JP-A-9-66302, JP-A-9-302
-85301.

[0005]

In rolling bar steel, billet slabs having a rectangular cross section manufactured by continuous casting are generally used. There is a problem that internal cracks are easily generated during rolling. However, even when this problem is avoided by using a billet slab having a round cross section, when the caliber-less rolling method using the flat roll is applied thereto,
In the rough rolling by the flat roll, the round section has a problem that the tensile stress in the rolling direction generated on the side face of the billet is larger than that of the square section, and the round section tends to cause rolling cracks.

When the caliber-less rolling method is applied at a high temperature without cooling the continuous cast billet irrespective of the cross-sectional shape, the billet slab is not transformed and has large crystal grains. There was a problem that the susceptibility to rolling crack was high.

[0007] Therefore, when rolling by a flat roll without cooling a continuous cast billet having a round cross section without cooling, there is a problem that rolling cracks are very likely to occur. When continuous rolling is applied here, in addition to the above-mentioned problems, the welded portion obtained by welding the billet slab has a high temperature, so that there is a problem that the material strength of the welded portion is reduced and further rolling cracks are likely to occur. .

[0008] The present invention solves the above-mentioned problems, and sends a continuously cast billet slab directly to a rolling line without cooling.
It is an object of the present invention to provide a method for rough rolling of a steel bar using flat rolls, in which even when a billet slab is welded to be continuous and rolled, internal and external rolling cracks are less likely to occur.

[0009]

The above object is achieved by the following invention. The first invention of the present invention is a method for producing a steel bar by direct-feed rolling from a continuous cast billet slab. In the continuous rolling of a steel bar in which the billet slab is welded and continuously rolled, the continuous cast billet slab having a round cross section is used. Using a hole-type roll in the first pass of the rough rolling forming pass,
The rough rolling after the second pass is a rolling method using a flat roll, and the billet slab is rolled in a direction perpendicular to the rolling direction of the first pass on the entry side of the first pass of the rough rolling forming pass. This is a rolling method for preventing rolling cracks in a continuously cast billet slab, characterized by performing light reduction on a side surface.

[0010] The second invention of the present invention is characterized in that light reduction is performed also in the same direction as the rolling direction of the first pass of the rough rolling forming pass, and the continuous cast billet slab according to the first invention of the present invention is characterized in that This is a rolling method for preventing rolling cracks.

According to a third aspect of the present invention, there is provided the first and second aspects of the present invention, characterized in that the welding portion of the billet slab is clamped synchronously with the billet slab to perform a light reduction. This is a rolling method for preventing rolling cracks in a continuously cast billet slab.

In the present invention, the direct rolling refers to rolling a cast billet at a high temperature without cooling it to a temperature below the A3 transformation point, and includes a case where a small amount of heating is performed for reheating before rolling. It is. In the present invention, since a round cross-sectional shape is used as a billet slab, internal cracks are less likely to occur during the solidification process and rough rolling than when a square cross-section is used. Furthermore, by using a grooved roll in the first pass of rough rolling, the billet side surface is compressed in the width direction by the grooved slope,
The tensile stress in the rolling direction generated when the side surface of the billet is stretched in the rolling direction is smaller than that in flat roll rolling, so that even when a round cross-section billet is used, rolling cracks on the side surface of the billet are less likely to occur. In addition, due to the restraint of the grooved roll, a surface reduction efficiency equivalent to that of the flat roll can be obtained with a small rolling reduction, and the tensile stress in the rolling direction generated on the side surface of the billet becomes smaller than that of the flat roll rolling. Rolling cracks on the side surface are less likely to occur. The temperature of the side of the welded portion of the billet slab where the material strength is reduced due to the high temperature is reduced by heat removal by the hole-shaped slope of the hole-shaped roll, so that the material strength is increased and rolling cracks occur. It becomes difficult.

At this time, the billet slab is further subjected to light reduction using a non-driven guide roll or the like on the entry side of the first pass of the rough rolling forming pass. It is effective to slightly reduce the side surface of the billet slab in a direction perpendicular to the rolling direction of the first pass of the rough rolling forming pass. The lightly reduced portion of the billet slab is refined by recrystallization to improve the breaking strength and reduce the rolling crack susceptibility. Further, the temperature decreases due to heat removal in contact with the roll, and the material strength of the welded portion of the billet slab also increases. The amount of reduction under light pressure is 5 times the diameter of the round billet.
% Or less.

If this light reduction is performed not only in the direction perpendicular to the rolling direction of the first pass of the rough rolling forming pass but also in the rolling direction at the same time, the grain refinement by recrystallization proceeds further,
The temperature is also reduced, and rolling cracks are less likely to occur.

Further, the light reduction can be performed by clamping a welded portion of a billet slab. The welding part is clamped while moving in synchronization with the moving speed of the billet slab and lightly reduced, and then rough rolling is performed. Also in this case, the welded portion may be clamped in only one direction perpendicular to the rolling direction of the first pass of the rough rolling forming pass, but at the same time, in a direction parallel to the rolling direction of the first pass of the rough rolling forming pass. It is more effective if sandwiched between the two directions. Since the welded part of the billet slab is particularly hot and the material strength is low, this welded part is clamped and heat is removed from the clamp part,
Since the material strength is increased, recrystallization occurs under light pressure and the grains are refined, the breaking strength is improved, and the rolling crack susceptibility is reduced, so that rolling cracks in the welded portion are less likely to occur. Also,
The light pressure reduction also has an effect of shaping the welded portion, so that rolling cracks due to poor welding of the billet slab welded portion are less likely to occur.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION A billet having a round cross section used in the present invention is manufactured by a continuous casting method. A round billet cast by a continuous casting machine and cut to a predetermined length is directly fed at a high temperature without cooling, joined by flash butt welding, and then rolled. The welding for the continuity may be high-frequency heating welding, a laser welding method, or the like. After welding, the joint is deburred and continuously rolled by a rolling mill train to produce a bar product. If the temperature of the billet decreases during the transportation, heating may be performed before rolling. FIG. 1 shows an embodiment of the present invention. Fig. 1 shows the change in the shape of the billet in the cross-sectional direction in each rolling step (pass) of rough rolling. In the first rolling, light rolling in the lateral direction (or upper and lower surfaces in the longitudinal direction) is performed, and rolling is performed in the longitudinal direction (or the transverse direction) by the hole type roll 2 in the first rolling (rough one pass) of the rough rolling. Then, it is rolled in the transverse direction (or longitudinal direction) by the flat roll 3 of the second rolling (rough 2 pass) of the rough rolling, and then the flat roll is moved directly to the previous pass in the subsequent rough rolling process. And rough rolling is performed. For the subsequent finish rolling, bar and wire products will be manufactured using the commonly used hole rolls. In the intermediate rolling, a flat roll may be used, or a grooved roll may be used. In the subsequent finish rolling, a flat roll may be used halfway and a grooved roll may be used for the minimum number of passes, or a bar / wire product may be manufactured using a holed roll for all passes. . If a roll of an oval caliber or a box caliber is used as the roll of the hole type, there is an effect of preventing cracking.

The guide roll arranged on the entry side of the first pass of the rough rolling forming pass is kept open until the tip of the billet at the start of the continuous rolling bites into the rolling roll of the first pass, and the tip of the billet is kept in the open state. After passing through the rolling roll in the first pass, the gap between the pair of guide rolls is gradually narrowed, and a predetermined light pressure is applied to the side surface of the billet (contact of the rolling roll perpendicular to the rough rolling surface in the first pass). Not free side). The light reduction amount may be 5% or less of the diameter of the round billet, and approximately 1 to 2% is sufficiently effective. Further, it is desirable that the contact surface of the guide roll with the free side surface of the billet conforms to the shape of the billet. The effect is great if it is formed in a round or oval shape having a radius of curvature approximately 1.5 times the billet radius. Since the billet slab is welded and continuous, the billet slab can be lightly reduced without driving by the pulling force of the rolling mill.

If the light reduction is performed not only in the direction perpendicular to the rolling direction of the first pass of the rough rolling forming pass but also in the rolling direction at the same time, the refining by recrystallization proceeds more,
The temperature is also reduced, and rolling cracks are less likely to occur.

The light reduction can be performed by clamping the welded portion of the billet slab while synchronizing with the movement of the welded portion. In this case, a clamp in only one direction may be used, but it is more effective if the clamp is sandwiched from two directions. After the billet slab is welded to be continuous, the welded portion is clamped by a hydraulic drive or the like before deburring of the joined portion and before entering the rolling mill row, and a slight reduction is applied. It is desirable that the shape of the clamp surface conforms to the shape of the billet.

[0020]

[Example 1] [Example 1] A billet with a round section produced by continuous casting was directly fed, and each billet was sequentially joined and integrated by flash butt welding, and after removing burrs at a welded portion, the billet was conveyed to a row of rolling mills. Then, continuous rolling was performed. A non-driven guide roll is disposed on the entry side of the first pass of the rough rolling forming pass, and a rough roll is formed by using a hole roll for the first rolling (rough one pass) and a flat roll for the following rolling. When the production method of the present invention is used (example of the present invention), a non-driven guide roll is not used, and rough rolling is performed using a flat roll for both the first rolling (rough one pass) and the following rolling. The comparison was made in the case of using the conventional technique for performing the above (Comparative Example). The billet temperature before passing through the guide roll immediately before rolling was 1020 ° C, and the temperature of the welded portion was 1145 ° C. In the non-driven guide roll, the diameter of the flange portion is 150 mm, and the radius of curvature of the contact surface with the free side surface of the billet is 150 mm. FIG. 2 shows a shape 8 after rolling when a continuous cast round section billet 7 is longitudinally rolled by a hole roll 6 in one coarse pass in the example of the present invention. The billet slab 7 has a round cross section having an initial diameter D of 200 mm, and the hole-shaped roll 6 having a coarse pass has an oval caliber (radius of curvature R = 0.8D). The total reduction by the guide roll was set to 3 mm, and the roll gap of one coarse pass was changed, and the presence or absence of cracks after one coarse pass was observed. Table 1 shows the results. Thereafter, rolling was performed with a flat roll up to three passes of finish rolling, and after the slitting, round bars having two kinds of diameters were manufactured using a grooved roll. No cracks occurred in any of the manufacturing processes.

[0021]

[Table 1]

FIG. 3 shows a comparative example, in which a rough pass and
7 shows a post-rolled shape 11 when the continuous cast round section billet 10 is rolled in the longitudinal direction by a screw 9. The round billet slab has a round cross section with a diameter of 200 mm and a flat roll diameter of 80 mm.
0 mm. Table 2 shows the results of rolling tests performed under various roll gap reduction conditions. As can be seen from Table 2,
When rolling down the round billet using a flat roll, rolling cracks occur frequently. Compared to FIG. 2, it can be seen that in FIG. 3, the deformation of the side surface of the billet after rolling is large, which causes rolling cracks on the side surface.

[0023]

[Table 2]

As described above, in the comparative example in which flat rolls are all used for the rough rolling, which is the prior art, cracks occurred at the stage of one rough pass. However, when the method of the present invention was used, cracks occurred. Could be prevented.

[Example 2] A round-section billet produced by continuous casting was directly fed, and each billet was sequentially joined and integrated by flash butt welding. After removing burrs at a welded portion, the billet was transferred to a rolling mill row. To perform continuous rolling. After removing the burr of the welded part, before the rough rolling forming pass, using a clamp device that moves in synchronization with the moving speed of the billet slab, the welded part of the billet slab was clamped and a slight reduction was applied. . The clamping device was hydraulically driven, the radius of curvature of the clamping surface was 150 mm, and the total reduction (the amount of decrease in diameter) on the side surface of the round billet by the clamping device was 4.0 mm. Billet temperature before clamping before rolling was 1020
° C and the temperature of the weld was 1145 ° C. Thereafter, rolling was performed using the manufacturing method of the present invention, in which the first rolling (rough one pass) was performed using a hole-shaped roll, and the following rolling was performed using a rough roll using a flat roll. FIG. 4 shows a state where the welded portion of the billet slab is clamped, which is an example of the present invention. The continuous casting round section billet 12 is clamped to the clamping surface 13 of the clamping device.
This is the case where both the side surface and the upper and lower surfaces are clamped. The billet slab has a round cross section with a diameter of 200 mm, and the clamping surface has a curvature to increase the contact area with the billet slab. The rough one-pass hole type roll was the same as in Example 1, and was made oval caliber (radius of curvature R = 0.8D). The roll gap was changed and the presence or absence of cracks after one rough pass was observed. Table 3 shows the results. Thereafter, rolling was performed with a flat roll up to three passes of finish rolling, and after the slitting, round bars having two kinds of diameters were manufactured using a grooved roll.
No cracks occurred in any of the manufacturing processes.

[0026]

[Table 3]

The use of the method of the present invention in which a slight reduction was applied using a clamp device also prevented the occurrence of cracks.

[0028]

According to the production method of the present invention, internal cracks are less likely to occur during the solidification process of the billet slab and during rough rolling, and cracks on the inner and outer surfaces during rough rolling can be prevented. In particular, rolling cracks on the side surfaces of the billet hardly occur. Therefore, rolling using flat rolls can be applied to continuous rolling of the bar steel, so that the product yield can be improved and the production efficiency can be increased.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing one embodiment of the present invention.

FIG. 2 is a diagram showing a rolled shape of a rough one-pass round cross-section billet of the present invention.

FIG. 3 is a diagram showing a rolled shape of a conventional rough one-pass round cross-section billet.

FIG. 4 is a view showing a state where a welded portion of a billet slab of the present invention is clamped.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 No-drive guide roll 2 Coarse 1 pass hole roll 3 Coarse 2 pass flat roll 4 Coarse 3 pass flat roll 5 Coarse 4 pass flat roll 6 Coarse 1 pass hole roll 7 Continuous cast round section billet shape before coarse 1 pass rolling 8 Continuous cast round section billet shape after coarse 1 pass rolling 9 Flat roll of coarse 1 pass 10 Continuous cast round sectional billet shape before coarse 1 pass rolling 11 Coarse 1 Continuously cast round section billet shape after pass rolling 12 Continuously cast round section billet 13 Clamp surface of clamp device

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Takashi Ariizumi, 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd. (72) Susumu Okawa 1-2-1, 1-2, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Hiroshi Wakasa 1-2-1 Marunouchi, Chiyoda-ku, Tokyo F-term inside Nippon Kokan Co., Ltd. 4E002 AA04 AC12 AC14 BA04 BB01 BB06 BC01 BC05 BC10 BD02 BD05 CB01 CB08

Claims (3)

[Claims] 1. A method of manufacturing a steel bar by direct-feed rolling from a continuous cast billet slab, wherein a continuous cast billet slab having a round cross section is used in continuous rolling of a steel bar in which the billet slab is welded and continuously rolled. A rolling method in which a hole roll is used in the first pass of the rough rolling forming pass, and a flat roll is used in the rough rolling in the second and subsequent passes. A rolling method for preventing rolling cracks in a continuously cast billet slab, wherein light rolling is performed on a side surface of the billet slab in a direction perpendicular to a rolling direction in a first pass. 2. The rolling crack of the continuous cast billet slab according to claim 1, wherein the light rolling is performed in the same direction as the rolling direction of the first pass of the rough rolling forming pass. Rolling method. 3. The continuous cast billet slab according to claim 1, wherein a light reduction is performed by clamping a welded portion of the billet slab in synchronization with the billet slab. A rolling method that prevents rolling cracks.