JPH08290277A - Method for joining hot rolled billet - Google Patents

Abstract

PURPOSE: To provide such a method for joining a hot rolled billet capable of speeding up tack welding in laser welding or reducing the laser output. CONSTITUTION: In a method in which a billet is joined by abutting the rear end of a preceding billet 1 on the tip end of a succeeding billet 3 and tack- welding 7 the abutting part 5 by laser welding and then joining the part by pressure welding, the optical axis 21 of the laser beam is inclined backward against the welding direction, approximately in the plane 18 including the abutting face of the billets 1, 3, so that the laser beam scans along the abutting line 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for joining hot rolled steel pieces such as a sheet bar and a slab.

[0002]

2. Description of the Related Art In the hot rolling of steel slabs such as sheet bars and slabs, the rear end of the preceding steel slab and the front end of the following steel slab are joined together. These steel pieces have a thickness of 20 to
The width is about 50 mm and the width is about 600 to 2000 mm. The temperature of the billet is around 1000 ° C. As one of the joining methods of this steel slab, the rear end face of the preceding steel slab and the front end face of the trailing steel slab are abutted, and temporary welding is performed along the abutting portion, and then the temporarily welded steel slab is pressure welded. A method of joining both steel pieces is well known.

It is also known that laser welding is used for the tack welding. For example, WO. Japanese Laid-Open Patent Publication No. 94/6838 discloses a method in which a trailing end portion of a preceding rolled material and a leading end of a trailing rolled material being rolled are abutted with each other, and the abutted portion is welded with a laser beam to perform continuous rolling. . Laser welding can increase the energy density by focusing a laser beam in a circular shape with a lens or a concave mirror. For this reason, the steel in the focused irradiation portion is instantly evaporated, and a keyhole is generated just below the surface of the billet. In laser welding, this keyhole is moved along the welding line to weld the steel pieces. The steel around the keyhole melts to form a molten pool, and the movement of the keyhole causes the molten steel in the molten pool to solidify behind and the welding is completed. The front end and the rear end of the steel slab are cut with a running shear or the like to form a joint surface.

The joint portion of the billet must have sufficient joint strength so as not to break in the subsequent rolling process. Therefore, the penetration depth of the tack welding at the butt portion must be a sufficient depth. If the laser power is constant,
It is known that the penetration depth is proportional to the -0.5th power of the welding speed, as shown in FIG. Of course, if the welding speed is constant, the penetration depth increases as the laser output is increased. However, if the welding speed is lowered to obtain the required penetration depth, the welding time becomes longer and the cost of the product increases. Further, if a high-power laser oscillator is used to increase the welding speed, joining equipment becomes expensive and power consumption also increases.

[0005]

SUMMARY OF THE INVENTION The present invention is intended to provide a method for joining hot-rolled steel slabs capable of accelerating tack welding by laser welding or reducing laser output.

[0006]

According to the method for joining hot-rolled steel pieces of the present invention, the trailing end portion of the preceding steel piece and the leading end portion of the following steel piece are butted, and the butted portion is temporarily welded by laser welding. Then, in the method of joining the steel pieces by pressing the butt portion together,
The optical axis of the laser beam is tilted rearward with respect to the welding direction and the laser beam is scanned along the abutting line in a plane substantially including the abutting surface of the steel slab. At this time, as described later, the angle α formed by the optical axis 21 of the laser beam and the normal line 23 to the surface of the billet.
It is more effective to irradiate the laser beam so that (see FIG. 1) is 5 to 20 degrees.

The steel piece to be joined in the present invention has a thickness of 2
It is a sheet bar or slab of 0 to 50 mm, and the temperature is 900 to 1200 ° C. The rear end portion of the preceding steel piece and the front end portion of the following steel piece may be shaped into a shape by a running shear or a laser cutting device, and then abutted to perform laser welding. Laser welding is, for example, output 30 to 4
A 5 kW continuous wave CO ₂ laser oscillator may be used, and the irradiation position of the laser beam may be scanned along the abutting portion of the steel slab running on the welding line. The welding speed may be 5 to 20 m / s. The laser beam is preferably focused and irradiated so that the beam spot diameter on the surface of the billet is 0.5 to 2.0 mm.

FIG. 2 shows the experimental results of investigating the variation of the penetration depth when the angle at which the laser beam is applied to the steel slab is changed within the abutting plane while the laser beam is scanned at a constant speed. . Experimental conditions are as follows: Steel billet type: SS41, Steel billet temperature: 1000 ° C., Laser oscillator: Continuous wave CO ₂ laser oscillator with output of 45 kW, Laser beam scanning speed: 10 m /
min, beam spot diameter on the surface of the billet is about 0.7 mm
Is. The laser beam irradiation angle α in this experiment refers to the angle formed by the optical axis of the laser beam and the normal to the surface of the billet, as shown in FIGS. 3 (a) and 3 (b). When the optical axis of the laser beam is tilted forward with respect to the welding direction as shown in FIG. 3 (a), the irradiation angle α> 0, and when tilted backward as shown in FIG. 3 (b), the irradiation angle α is set. <0.

As shown in FIG. 2, the laser beam irradiation angle α
The penetration depth changes when is changed. In particular, -2
In the region of 6.5 <α <0, the penetration depth becomes larger than that in the case where the laser beam is vertically irradiated (α = 0). Therefore, the angle between the optical axis of the laser beam and the normal to the surface of the billet is 5 to 20 ° (the laser beam irradiation angle α in the experiment).
Then, the penetration depth can be made deeper by irradiating the laser beam so that the penetration depth is −20 to −5 °). Further, when the same laser beam focusing optical system is used, since the penetration depth and the bead width have a positive correlation, the present invention makes it possible to increase the bead width without lowering the welding speed. Therefore, it is possible to prevent the laser beam from coming off the butting portion.

FIG. 5 shows the relationship between the welding rate and the fracture of the joint. In FIG. 5, when the welding rate is 30% or more, the joint is not broken. Therefore, from the viewpoint of bonding strength, it is desirable that the welding rate is about 30% or more of the thickness of the steel billet. The optical axis of the laser beam may be tilted about ± 5 ° with respect to the surface including the abutting surface of the steel piece.

[0011]

FIG. 4 (a) is a longitudinal sectional view of a melting region when a laser beam irradiation position is scanned while a laser beam is vertically irradiated on the surface of a steel slab as in the prior art, and FIG. 4 (b) shows the present invention. The vertical cross-sectional views of the fusion zone by the method of FIG.
A keyhole, which is an evaporation hole, is formed at the laser beam irradiation position, and the keyhole moves in synchronization with the scanning of the laser beam irradiation position. In the method of the present invention, since the laser beam is irradiated with the optical axis of the laser beam tilted backward with respect to the welding direction, more energy is given to the front side in the moving direction of the keyhole. The front of the keyhole in the moving direction is the keyhole generation surface, and by giving more energy to it, the depth of the keyhole increases,
Along with that, the penetration depth also increases. For example, when the laser beam irradiation angle α is set to −10 ° in the above experiment, the penetration depth is increased by about 7% as compared with the case where the laser beam is vertically irradiated, and the wider bead width is It is possible to prevent the laser beam from coming off the butt line.

It is known that the penetration depth is proportional to the -0.5th power of the velocity as described above when the laser power is fixed. For example, if the angle between the optical axis of the laser beam and the normal to the surface of the billet is 10 ° in the method of the present invention, the same penetration depth can be obtained even if the welding speed is increased by about 15% compared to the conventional speed. As a result, the speed of laser tack welding can be increased.

[0013]

EXAMPLES Examples of the present invention will be described by taking the joining of hot rolled sheet bars as an example. FIG. 6 is a side view schematically showing the sheet bar joining equipment provided in the continuous hot rolling equipment, and FIG. 7 is a plan view of the equipment. Note that FIG. 1 is a perspective view for explaining laser tack welding in the same equipment. Figure 6
In the seat bar joining equipment shown in FIG. 7, the front and rear ends of the seat bars 1 and 3 are cut off along the bar width direction by the running shear 11 to form a straight joining surface. Then, the rear end surface of the preceding sheet bar 1 and the front end surface of the following sheet bar 3 are butted, and the linear butted portion 5 is temporarily welded 7 by laser welding. Following the tack welding 7, the rolling mill train 16 continuously rolls both the sheet bars 1 and 3 to complete the joining of the sheet bars 1 and 3. Seat bars 1 and 3 have a length of 20 m and a width of 11
The thickness is 00 mm and the thickness is 42 mm. The temperature of the sheet bars 1 and 3 at the time of forming the joint surface is 1000 ° C. The feed speed of the sheet bars 1 and 3 is 80 m / min.

In the laser tack welding in the sheet bar joining equipment shown in FIG. 6, the laser oscillator 13 is a continuous wave C
It is an O ₂ laser oscillator and has a steady output of 45 kW.
The laser beam emitted from the laser oscillator 13 is transmitted through the laser beam transmission optical system 14 to the laser processing head 1
5 is transmitted. The optical axis 21 of the laser beam irradiated from the processing head 15 to the butt section 5 is on the plane 18 including the butt surface, and forms an irradiation angle α with the normal line 23 of the surface of the sheet bar (see FIG. 1). The irradiation angle α is 10 °. Although the laser oscillator 13 is fixed, the laser processing head 15 linearly scans the abutting portion 5 at 11.5 m / min while moving in synchronization with the sheet bars 1 and 3.

By the laser tack welding performed in this way, 30% or more of the cross-sectional area of the butt portion was welded, and the joint portion was not broken by finish rolling in the subsequent stage. When the laser beam is vertically irradiated under the same conditions as in the conventional case, 30% or more of the butt section cross-sectional area cannot be welded unless the laser beam scanning speed is set to 10 m / min or less, and the rolled plate breaks during finish rolling. There was something I did.

[0016]

According to the present invention, the penetration depth and bead width of laser tack welding can be increased without lowering the welding speed. Therefore, the abutting portion can be welded deeper without impairing the bonding strength, and the laser beam from the abutting portion can be prevented from slipping off. Further, even if the welding speed is increased by about 15% as compared with the conventional one, laser tack welding with the same penetration depth as that of the conventional one can be performed, so that the welding speed can be increased. On the contrary, if the welding speed is the same as the conventional one, the output of the laser oscillator can be reduced.

[Brief description of drawings]

FIG. 1 is a perspective view illustrating laser tack welding.

FIG. 2 is a diagram showing a relationship between a laser beam irradiation angle and a penetration depth.

FIG. 3 is a diagram showing a definition of a sign of a laser beam irradiation angle α.

FIG. 4 (a) is a vertical cross-sectional view of a melting region in the case where a laser beam irradiation position is scanned while a surface of a steel slab is irradiated with a laser beam perpendicularly. (B) It is a longitudinal cross-sectional view of the fusion zone by the method of the present invention.

FIG. 5 is a diagram showing a relationship between a welding rate and a fracture of a joint.

FIG. 6 is a side view schematically showing an example of a steel piece joining facility for carrying out the method of the present invention.

FIG. 7 is a plan view of the equipment shown in FIG.

FIG. 8 is a diagram showing the relationship between welding speed and penetration depth.

[Explanation of symbols]

 1 Leading Seat Bar 3 Trailing Seat Bar 5 Butt Section 6 Butt Line 7 Temporary Welding 11 Running Shear 13 Laser Oscillator 14 Laser Beam Transmission Optical System 15 Laser Processing Head 16 Rolling Machine Row 21 Laser Beam Optical Axis 23 Sheet Bar Surface Normal angle α Irradiation angle

Claims (3)

[Claims] 1. A method of joining a steel slab by butt-joining a trailing end of a preceding steel slab and a leading end of a trailing steel slab, temporarily welding the butt by laser welding, and welding the butt by pressure welding the butt.
A method for joining hot-rolled steel pieces, characterized in that the optical axis of the laser beam is tilted rearward with respect to the welding direction and the laser beam is scanned along the butt line in a plane that substantially includes the butt surface of the steel pieces. . 2. The method for joining steel pieces according to claim 1, wherein an angle formed by an optical axis of the laser beam and a normal line to the surface of the steel pieces is 5 to 20 °. 3. The method for joining steel pieces according to claim 1, wherein about 30% or more of the thickness of the steel pieces is tack-welded.