JPH09122741A - Manufacture of butt-welded steel tube having high-strength and high-quality butt-welded part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a butt-welded steel tube having the butt-welded part without causing such defects as deterioration in strength of the butt-welded part caused by biting scale in the butt-welding surface because of oxygen blow in the same direction as the advancing direction of a skelp just before butt- welding and the generation of an internal bead and external line in the butt- welded part when the butt-welding surfaces having a skelp edge shape at the time of shearing a hot rolled coil with slitters are butted. SOLUTION: After a steel strip 1 heated in a heating furnace 2 is hot-formed into a tubular shape, this steel tube is butt-welded by blowing oxygen against the edge parts of the tubular skelp just before butt-welding in the same direction as the advancing direction of the tubular skelp from the nozzle 7 of a welding hone 5. This method is the manufacturing method of the butt-welded steel tube having the high-strength and high-quality butt-welded part by, in such a case, providing a laser cutting machine 8 between a forming roll 3 and a butt-welding roll 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 producing a forged steel pipe having a high strength and high quality forged portion.

[0002]

2. Description of the Related Art A method for manufacturing a forged steel pipe is shown in FIG. The forged steel pipe is produced by continuously heating a steel strip 1 as a material in a heating furnace 2 for 1200 minutes.
C. to 1300.degree. C., heated by the molding rolls 3 and 4, oxygen is blown from the nozzle 7 of the welding horn 5 just before forging to the edge portion for forge welding. After that, it is successively drawn by a drawing roll and rolled to continuously produce a forged steel pipe having a predetermined outer shape and thickness.

At this time, the oxygen blowing direction is the same as the advancing direction of the tubular skelp, and the position where oxygen is blown from the oxygen nozzle to the edge portion is just before forging. Therefore, the scale of the forged surface not removed and The scale blown off and penetrated into the forged surface is bitten into the forged surface and the strength of the forged part is reduced.

Further, as shown in FIG. 2, there are two types of skelp edge shapes when a hot rolled coil is cut into a predetermined width of steel strip by a slitter, as shown in FIG. . When the forged surfaces having such a shape are butted against each other, an inner surface bead and an outer surface line are generated in the forged portion, and the strength and quality of the forged portion are deteriorated.

In order to prevent the outer surface stripes of the forged portion of the forged steel pipe during the production of the forged welded pipe, the outer surface of the forged welded portion is cut and removed between the forged welded roll and the drawing roll as in JP-A-3-248715. In order to prevent external streaks and to manufacture a forged steel pipe with high strength at the forged part, both edge surfaces of the skelp are preliminarily smoothed to a smooth surface with a roughness of 20 μm or less as in JP-A-55-54286. There is a manufacturing method in which the material is cut, polished, and finished, and then forged.

[0006]

As shown in FIG. 1, in the conventional manufacturing process, the position where oxygen is blown is from the nozzle 7 of the welding horn 5 immediately before the forging portion, and the oxygen blowing direction is a tubular skelp. Is the same as the traveling direction of. For this reason, the scale of the forged surface that has not been removed and the scale that has been blown off and that has entered the forged surface are caught in the forged surface and the strength of the forged portion is reduced.

As shown in FIG. 2, there are two types of skelp edge shapes when a hot rolled coil is cut into a predetermined width of steel strip by a slitter, as shown in FIG. When the forged surfaces having such a shape are butted against each other, an inner surface bead and an outer surface line are generated in the forged portion, and the strength and quality of the forged portion are deteriorated.

By applying the method of the present invention, the scales adhering to the forging surfaces of the edges of the tubular skelp are completely eliminated and the forging surfaces of both edges of the tubular skelp are finished to be smooth surfaces. It is possible to manufacture a forged steel pipe having a high-strength and high-quality forged portion by completely eliminating the scale biting of the forged portion, streak on the outer surface, and beads on the inner surface.

[0009]

According to the present invention, a steel strip 1 which is a material is heated in a heating furnace 2 to hot-form a heated steel strip into a tubular shape, and then an edge portion 11 of a tubular skelp is welded immediately before forging. In the production of a forged steel pipe in which oxygen is blown from the nozzle 7 of the ding horn 5 in the same direction as the traveling direction of the tubular skelp for forging, the scale attached to the forged surface of the edge part 11 of the tubular skelp is completely eliminated. Then, the shape of the forged surface at both edges of the tubular skelp is finished to a smooth surface,
A laser is provided in the middle of the forming rolls 3 and 4 and the forging roll 6 in order to completely eliminate the bite, the outer surface lines, and the inner surface beads.
A method for manufacturing a forged steel pipe having a high-strength and high-quality forged portion characterized by installing a cutting machine 8.

The present invention will be described in detail with reference to FIGS. 3 and 4. In the conventional manufacturing process, the oxygen blowing position is just before forging welding, and since it is in the same direction as the traveling direction of the tubular skelp, the scale of the forging surface is not completely removed, and the blown-out scale becomes the forging surface. There is a problem that it penetrates and bites into the forged part and becomes foreign matter, which deteriorates the strength and quality of the forged part.

The shape of the skelp edge when the hot rolled coil is sheared by a slitter to a predetermined width of the steel strip is shown in FIG.
There are two types, bar up (a) and bar down (b), as shown in Fig. 2, when the edges of such a shape are butted, an inner surface bead and an outer surface line are generated in the forged portion, the strength of the forged portion, There is a problem that the quality deteriorates.

In order to solve these problems, as shown in FIGS. 3 and 4, the welding horn 5 immediately before forging is used.
A laser cutting machine 8 is installed in the middle of the forming rolls 3 and 4 and the forging roll 6 in addition to the blowing of oxygen from. The laser cutting machine 8 removes the scale adhering to the forging surface of the edge portion 11 of the tubular skelp during the heating furnace 2 and hot forming and finishes the shape of the forging surface of the edge portion of the tubular skelp into a smooth surface. It is installed for the purpose.

The nozzle 9 of the laser beam can be mounted in double so that both edges 11 of the skelp can be cut at the same time. Laser for cutting all pipe sizes
The nozzle 9 of the cutting machine can be freely moved vertically and horizontally and at an angle θ. The method for cutting the edge portion 11 of the tubular skelp is to guide the laser beam taken out from the laser oscillator in a predetermined direction by using a reflecting mirror and to focus it as a minute spot on the skelp edge portion 11 through a condenser lens. Let it cut.

At this time, at the same time, the assist gas is ejected through the small diameter nozzle 10 mounted coaxially with the laser beam. Oxygen, air, or an inert gas is used as the assist gas. When oxygen is used as the assist gas, the exothermic reaction with oxygen is accompanied and the cutting efficiency is remarkably improved, and the melted material melted by laser irradiation is blown off at the same time, and the cut part is finished to have a smooth surface. When air or an inert gas is used as the assist gas, the melted material melted by laser irradiation is blown off, and the cut portion is finished to have a smooth surface.

Welding hoses just before forging welding in the conventional method
The oxygen blown from the nozzle 5 is the scale attached to the forged surface.
It is necessary to satisfy the two effects at the same time: to blow away the solder and to heat the forged surface instantaneously by the heat of oxidation reaction. By installing the laser cutting machine 8 between the forming roll 3.4 and the forging roll 6, the skelp edge portion 11 is formed.
The forged welded surface was completely smooth and the forged welded surface was smooth.

From this fact, the oxygen blower from the welding horn 5 just before the forging can be used only for the purpose of raising the temperature of the forging surface just before the forging, and as a result, a sufficient forging temperature can be maintained. This makes it possible to set manufacturing conditions easily.

From the above, by applying the present invention, it is possible to manufacture a forged steel pipe having a high-strength and high-quality forged portion by completely eliminating the scale biting of the forged portion, the outer surface line, and the inner bead. Becomes

[0018]

[Examples] Table 1 shows the results of the investigation of the outer surface line length and the occupancy ratio of foreign matter in the forged portion in a practical test by the manufacturing process of the conventional method shown in FIG. Table 2 shows the examination results of the outer surface line length and the forged portion occupancy ratio in the practical test by the manufacturing process of the method of the present invention shown in FIGS. 3 and 4.

[0019]

[Table 1]

[0020]

[Table 2]

The methods of calculating the flatness ratio, the expansion ratio, and the forged portion occupancy ratio shown in Tables 1 and 2 are shown below.

Flatness = (1-H / D) × 100 (%),
D: Outer diameter of raw pipe, H: Flat height, expansion ratio = D '/ D, D: Outer diameter of base pipe, D': Outer diameter of pipe after expansion, Occupied part foreign matter occupancy ratio = L / t x 100 (%), L:
Total foreign matter length at forged part, t: Pipe wall thickness

[0023]

In the manufacturing process of the forged steel pipe according to the conventional method, the position where oxygen is blown is from the nozzle of the welding horn just before forging, and the oxygen blow direction is the same as the skelp advancing direction. For this reason, the scale of the forged surface that has not been removed and the scale that has been blown off and that has entered the forged surface are caught in the forged surface and the strength of the forged portion is reduced. When the skelp-edge forged welded surfaces are abutted when the hot-rolled coil is sheared by a slitter to a predetermined width of the steel strip, an inner surface beat and an outer surface streak are generated in the forged portion, and the strength and quality of the forged portion are deteriorated.

By applying the method of the present invention, it becomes possible to manufacture a forged steel pipe having a high-strength and high-quality forged portion by completely eliminating the scale biting, the outer surface line and the inner bead of the forged portion. It was

[Brief description of the drawings]

FIG. 1 is a plan view showing a manufacturing process of a conventional forged steel pipe.

FIG. 2 is a diagram showing a skelp edge shape when cut by a slitter.

FIG. 3 is a plan view showing a method for manufacturing a forged steel pipe according to the present invention.

FIG. 4 is a cross-sectional view of the vicinity of a laser cutting machine device of the method of the present invention.

[Explanation of symbols]

1: Steel strip 2: Heating furnace 3: Forming roll 4: Forming roll 5: Welding horn 6: Forging roll 7:
Nozzle 8: Laser cutting machine 9: Laser beam nozzle 10: Assist gas nozzle 11: Edge part of tubular skelp

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Daigo Sumimoto 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corporation Kimitsu Steel Co., Ltd.

Claims (1)

[Claims] 1. A steel strip heated in a heating furnace is hot-formed into a tubular shape, and then oxygen is blown to the edge of the tubular skelp from the nozzle of a welding horn just before forging in the same direction as the traveling direction of the tubular skelp. In the production of forged steel pipe for forge welding, a forged steel pipe having a high strength and high quality forged welded portion characterized by installing a laser cutting machine for skelp edge cutting between the forming roll and the forging welded roll. Manufacturing method