JP3366816B2 - Manufacturing method of hot rolled steel sheet - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for hot rolling of steel materials, and in particular, in hot rolling, a plurality of steel materials after rough rolling (hereinafter referred to as rough bars) are joined together,
The present invention relates to a method for producing a hot-rolled steel sheet by continuously performing finish rolling without breaking.

[0002]

2. Description of the Related Art In a conventional hot rolling line, slabs (slabs) heated in a heating furnace are rough-rolled and finished-rolled one by one. Since it is difficult to control the temperature and cooling during hot rolling, the material variation is likely to occur.Furthermore, this part moves in the cooling zone until winding after finishing rolling without tension, causing unstable flapping. Defects are more likely to occur, leading to a decrease in yield. Therefore, in recent years, in order to solve this problem, a rough bar is joined before finishing continuous rolling,
A method has been adopted in which finish rolling is performed on a plurality of rolls. A joining method for making hot-rolled steel continuous is proposed in Japanese Patent Laid-Open No. 61-144201. This joining method discloses a method of abutting the rear end portion of the preceding rolled material and the leading end portion of the trailing rolled material to press and join both ends, or a method of joining by welding from the upper surface.

However, in the case of both ends joining as in Japanese Patent Laid-Open No. 61-144201, breakage occurs frequently because the central portion is not crimped, and as a countermeasure against this, in order to prevent scale generation in the crimped portion which causes breakage. Has the problem of requiring expensive equipment. In addition, in the method of welding from the upper surface of the butt portion, high-speed welding is required to prevent the temperature drop of the steel material, and it is necessary to adjust the welding depth so that the molten portion does not pierce. Unable to join thickness. For this reason, a non-joint portion is formed at the back surface position of the joined portion, and as with the case of both-end joining, unless measures for preventing scale generation are taken, crimping by rolling becomes difficult, and fracture of the joined portion occurs during rolling. There is.

[0004]

SUMMARY OF THE INVENTION Therefore, the present invention solves the above-mentioned problems, and does not require a large amount of equipment such as scale preventive measures, and does not cause breakage during rolling, so that the rear end and the end of the rough bar are not broken. By joining the hot-rolled steel sheets, it is possible to manufacture hot-rolled steel sheets by a hot-rolling continuous process with high productivity.

[0005]

Means for Solving the Problems The gist of the present invention for solving the above-mentioned problems is that when the pre-joining (welding) position of a rough bar is the upper surface, when the upper surface is welded and the first rolling of the joining portion is performed, The upper surface of the trailing material comes into contact with the rolling roll earlier than the lower surface, so that the non-bonding surface of the lower surface is subjected to pressure bonding stress.

Alternatively, when the pre-joining (welding) position of the rough bar is the lower surface, the lower surface of the following material comes into contact with the rolling roll earlier than the upper surface at the time of the first rolling of the joined portion after welding the lower surface. The control is performed so that the pressure bonding stress acts on the non-bonding surface of the upper surface.

The specific means of solving the present invention is as follows.

(1) A hot rolling method in which the trailing end portion of the preceding material of the rough bar and the leading end portion of the following material are cut, and then both ends are butted and welded from the upper surface to the lower surface of the steel sheet to be joined and then rolled. In the steel sheet manufacturing method, the joint part is
The height of the tip of the trailing material on the welding side of the
In addition to making it higher than the height of the edge, during the first rolling after welding,
Bonding is performed so that the upper surface of the following material comes into contact with the rolling roll earlier than the lower surface, and the crimp stress due to bending stress is applied to the non-bonded surface of the lower surface.
A method for manufacturing a hot-rolled steel sheet , comprising rolling so as to act .

(2) A hot rolling process in which the trailing end portion of the preceding material of the rough bar and the leading end portion of the following material are cut, and then both ends are butted and welded from the lower surface to the upper surface of the steel sheet to join and then rolled. In the steel sheet manufacturing method, the joint part is
The height of the tip of the trailing material on the welding side of the
It is lower than the height of the edge, and during the first rolling after welding,
Bond the lower surface of the following material so that it comes into contact with the rolling roll earlier than the upper surface, and the crimp stress due to bending stress is applied to the non-bonded surface of the upper surface.
A method for manufacturing a hot-rolled steel sheet , comprising rolling so as to act .

(3) The method for producing a hot-rolled steel sheet according to the above (1), wherein when the upper surface of the succeeding material comes into contact with the rolling roll, the lower surface does not come into contact with the rolling roll.

(4) The method for producing a hot-rolled steel sheet according to the above (2), wherein the upper surface of the succeeding material is not in contact with the rolling roll when the upper surface is in contact with the rolling roll.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors preliminarily bond the trailing end of the preceding material and the leading end of the following material by laser welding or arc welding at the joint portion of the rough bar, and then perform rolling, and the rolling condition. While observing the above, a tensile test was performed on the joint after rolling, and the joining state was investigated.

At this time, the shape of the joint end face was intentionally changed, and welding-rolling was repeated with various combinations.
As a result, it was found that the joining condition after rolling could be extremely good or bad. The arrangement of the end faces is summarized as follows.

1) When the height of the tip of the trailing material on the welding side of the joint is higher than the height of the trailing edge of the preceding material on the non-welding side with respect to the rolling direction, the joining condition tends to be good. There is.

2) Conversely, when the height of the preceding material on the welding side of the joint is higher than the height of the rear end of the trailing material on the welding side, the joining condition tends to deteriorate.

Therefore, further observation was carried out using a laboratory rolling mill to examine how the above-mentioned differences in conditions 1) and 2) appear during rolling. FIG. 1 is a diagram schematically showing a state of a rough bar joint portion during rolling by a laboratory rolling mill. As a result, in the case of 1) described above, as shown in FIG. 1A, the upper surface of the trailing material 2 is more than the tip portion 3 of the lower surface of the trailing material 2 when viewed from the welding side at the time of biting in rolling. The high tip 4 comes into contact with the rolling roll 5 to hold down the non-welded portion and is indicated by the arrow .
Is crimped stress non-hermetic surface by bending stress so as to effect rolling, in the case of 2) Conversely, the non-welded end of the following material 2 as shown in FIG. 1 (b), i.e., the lower surface of the distal portion 4 comes into contact with the rolling roll 6 first, so that the bending stress indicated by the arrow
It was clarified that rolling is performed with the non-bonded part open.

As described above, the difference in the abutting state of the joined portion determines whether the non-joined portion is in the open state or the pressed state during the first rolling after joining. FIG. 2 is a photomicrograph of a cross section in the longitudinal direction of the joint portion of the rough bar rolled in the state shown in FIGS. The material rolled at a bonding rate of 40% in the state shown in (b) of FIG. 1 is in a state where the non-bonded portion 9 is open during the first pass as shown in (b) of FIG. 2 and is flat after the second pass. Therefore, the material in which the non-bonded portion 9 is opened is not pressure-bonded, and as the rolling progresses, it may be broken due to the tension between the stands. However,
The material rolled in the state shown in (a) at a bonding rate of 40% is rolled by pressing the non-bonding portion 8, and the non-bonding portion 8 is closed in one pass as shown in (a) of FIG. In this state, as the rolling progresses, the crimping progresses while crushing and stretching the scale, so there is no breakage.

As described above, in order to create a state in which the leading end of the trailing material on the welding side comes into contact with the rolling roll earlier than the leading end on the non-welding side, shearing cuts are used to make a difference in height between the end face butting positions. It is possible to adopt a method or a method of making a height difference by changing the height position at the time of preliminary joining.

FIG. 4 (a) is a perspective view of the joint portion of the rough bar, and FIG. 4 (b) is a sectional view schematically showing the longitudinal section of the joint portion of the rough bar.

That is, as shown in FIG. 4, the cutting portion is deformed by shear cutting, and the front end of the preceding material 1 has a lower upper surface and a lower lower surface as viewed from the center line. Subsequent material 2
Is higher on the upper surface side of the tip portion and lower on the lower surface side as viewed from the center line, contrary to the preceding material 1. In this way, when the upper surface side of the tip end portion of the trailing material 2 is made higher than the center line at the abutting position (h1> h2) and joined by welding from the upper surface and rolled, the trailing material 2 and the rolling roll are rolled at the time of rolling. The position where and come into contact first comes into contact with the high portion of the upper surface of the following material.

Therefore, as a result of performing a rolling experiment in the state where these height differences are provided and observing, it is confirmed that in any case, the non-bonded portion is pressed and rolled to increase the strength of the bonded portion. It was

Further, the relationship between the joining ratio of both end surfaces of the preceding material and the following material and the strength ratio of the base material (tensile strength of the joint / tensile strength of the base material) was obtained by an experiment. The result is shown in FIG.

As shown in FIG. 5, in the range of the joining rate of 40 to 55%, the base metal strength ratio is obtained by rolling the tip of the trailing material on the welding side in contact with the rolling roll first. 85
% Or more (marked by ● in the figure), but the strength ratio of the base metal is low when rolling is performed without the tip of the trailing material on the welding side coming into contact with the rolling roll first (marked by ○ in the figure).

When the joining rate exceeds 65%, the influence of the rolling method on the strength ratio of the base metal is small because there are few non-joint parts. However, in order to perform welding exceeding 65%, high speed welding is required. It is not practical because it is necessary to adjust the welding depth so that the welded and melted parts do not penetrate.

Therefore, in the present invention, the bonding rate is 30.
% Or more, especially 40% or more is desirable, but rolling is possible even if it is less than 40%.

An outline of implementing the present invention will be described with reference to FIG.

FIG. 6 is a diagram showing an outline of a process of joining a leading material and a trailing material of a rough bar and continuously rolling them to manufacture a hot rolled steel sheet.

As shown in FIG. 6, a steel slab (a slab) obtained by continuous casting is heated to a hot rolling temperature in a heating furnace 10 and hot rolled in a rough rolling mill 11. The rough rolled rough bar is wound into a rough rolled coil 12, and the tip end portion of the unwound rough bar is cut by a cutting machine 13 such as a shear cutting machine to form a tip groove suitable for welding. And similarly, the cutting machine 1
The rear end portion is abutted with the rear end portion of the preceding rough bar that has been cut at 3, and is joined from the upper surface to the lower surface of the steel sheet by a welding device 14 such as laser or arc welding.

At the time of this joining, as described with reference to FIG. 1 (a), at the time of the first finish rolling, the upper surface of the succeeding material comes into contact with the upper finishing rolling roll earlier than the lower surface, and at the time of the contact, the lower surface of the succeeding material is lower. Bond it so that it does not come into contact with the finishing rolling roll.

The joined rough bar is finish-rolled by a finish rolling mill 15 and a cooling device 16 provided on a run-out table.
After being cooled to a coiling temperature by, the coiler is wound by a coiler 18.

When the steel sheet is wound into a coil having a predetermined size, it is cut by a cutting machine 17. As for the cutting position, it is desirable to cut at the joining position.

[0032]

EXAMPLES Examples of the present invention will be described in comparison with comparative examples.

A rough bar joining experiment was conducted using a 7-stand tandem rolling mill. The rough bar thickness is 20 to 50 mm, and the shear is used to cut the trailing edge of the preceding material and the leading edge of the trailing material. The bonding experiment was performed. The rough bar was made of steel having a thickness of 20 to 50 mm and pre-bonded by welding (laser-arc welding) from above the shear and the rough bar. Rolling at the leading end of the trailing material was performed separately in three ways: top, bottom, and top and bottom, and the state of breakage during passage was observed. Then, the material passed through was subjected to a tensile test after being made into a cold piece, and compared with the strength of the base material to determine the strength ratio of the base material. The pre-bonding rate was 40% in all cases. The results are shown in Table 1 and FIG.

FIG. 3 is a diagram schematically showing the butt cross-sectional shape of the joint portion. Experimental materials 1, 2, and 3 are A in FIG.
This is an embodiment of the present invention in which the upper portion of the following material comes into contact with the rolling roll before the lower portion at the time of initial rolling as shown in the butt cross-sectional shape of the joints B and C. In the examples of the present invention, the steel sheets were passed without breakage, and the base material strength ratio was as high as 80% or more. Experimental materials 6 and 7 in which the upper and lower parts simultaneously contacted the rolling rolls (Fig. 3
Although the F and G cross-sections of (1) and (2) were passed through without breaking, it was not observed that the non-bonded portion of the rolled material was pressure bonded, and the base metal strength ratio was about 50%. Further, the experimental material 4 (the cross-sectional shape of D in FIG. 3) in which the lower part of the trailing material comes into contact with the rolling roll earlier than the upper part is broken during rolling, and the experimental material 5 (the cross-sectional shape of E in FIG. 3) is Although the steel sheet was passed through, it was not observed that the non-bonded portion of the rolled material was pressure bonded, and the base material strength was 40%.

According to the above experimental results, in the comparative example belonging to the conventional method, breakage occurred during rolling and the base material strength ratio was low, but in the present invention example, there was no breakage during rolling, and The base metal strength ratio was also high.

[0036]

[Table 1]

[0037]

As described above, according to the method of the present invention, the pressure-bonding can be promoted even at the non-bonded portion by rolling, the fracture of the bonded portion can be prevented, and the hot rolling continuous with high productivity can be achieved. Becomes possible.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a state of a rough bar joint portion during rolling by a laboratory rolling mill.

FIG. 2 is a micrograph of a longitudinal section of a joint portion of a rough bar rolled in the state shown in FIGS. 1 (a) and 1 (b).

FIG. 3 is a diagram schematically showing an abutting cross-sectional shape of a joint portion.

FIG. 4A is a perspective view of a joint portion of a coarse bar,
(B) is a sectional view schematically showing a longitudinal section of the rough bar.

FIG. 5 is a diagram showing a relationship between a joining ratio of both end surfaces of a preceding material and a following material and a base material strength ratio (a tensile strength of a joint portion / a tensile strength of a base material).

[Figure 6] Continuous rolling by joining the leading and trailing materials of the rough bar
It is a figure which shows the outline of the process of manufacturing a hot rolled steel sheet.

[Explanation of symbols]

1 preceding material 2 trailing materials 3 tip 4 Tip (first contact with rolling roll) 5 rolling rolls 6 rolling rolls 7 welds 8 Non-joint part 9 Non-joint 10 heating furnace 11 rough rolling mill 12 Rough rolling coil 13 cutting machine 14 Welding equipment 15 Finishing mill 16 Cooling system 17 cutting machine 18 Winder

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinji Matsuo 1st Nishinosu, Oita-shi, Nippon Steel Co., Ltd. Inside the Oita Works (72) Inventor Seiji Uri, 1st Nishinosu, Oita-shi Nippon Steel Oita Works (56) References JP-A-60-184481 (JP, A) JP-A-7-290108 (JP, A) JP-A-7-284809 (JP, A) JP-A-4-351210 (JP, A) JP-A-4-322805 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B21B 1/00-11/00 B21B 15/00-15/20 B23K 20 / 00-20/26 B23K 15/00-15/10

Claims (4)

(57) [Claims] 1. A hot-rolled steel sheet which is obtained by cutting the trailing end portion of a leading material of a rough bar and the leading end portion of a trailing material, then abutting the both ends to weld and join from the upper surface to the lower surface of the steel sheet, and then rolling. soluble in the manufacturing method, the joint portion to the rolling direction
The height of the tip of the trailing material on the contact side is
The height is higher than the height, and at the first rolling after welding, the upper surface of the trailing material is joined so that it comes into contact with the rolling roll earlier than the lower surface, and the crimp stress due to bending stress acts on the non-bonded surface of the lower surface.
A method for manufacturing a hot rolled steel sheet, comprising: 2. A hot-rolled steel sheet which is obtained by cutting the trailing end portion of the leading material of the rough bar and the leading end portion of the trailing material, then abutting the both ends, welding and joining from the lower surface to the upper surface of the steel sheet, and then rolling. soluble in the manufacturing method, the joint portion to the rolling direction
The height of the tip of the trailing material on the contact side is
In addition to making it lower than the height, at the time of the first rolling after welding, the lower surface of the following material is joined so that it contacts the rolling roll earlier than the upper surface, and the crimp stress due to bending stress acts on the non-bonded surface of the upper surface.
A method for manufacturing a hot rolled steel sheet, comprising: 3. The method for producing a hot-rolled steel sheet according to claim 1, wherein when the upper surface of the following material contacts the rolling roll, the lower surface does not contact the rolling roll. 4. The method for producing a hot-rolled steel sheet according to claim 2, wherein when the upper surface of the following material comes into contact with the rolling roll, the upper surface does not come into contact with the rolling roll.