JPH1029013A - Production of high strength, high quality butt-welded steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high strength and high quality butt-welded tube by arranging an assistant heating device in the midway between a forming roll and a butt-welding roll, blowing specific flow rate of inert gas after heating the surface layer parts of both edge parts of a tubular skelp and executing the butt-welding. SOLUTION: A steel sheet 1 is heated in a heating furnace 2 and the heated steel sheet is formed into a tubular shape by forming rolls 3, 4 and the surface layer parts of edge surface parts are uniformly heated at >=1380 deg.C by the assistant heating device 8 to fuse surface scales on the surface parts. Then, the inert gas of 0.2-2.5m<3> /min is blown from a nozzle 7 of a welding horn 5 to blow off the fused scale and the butt-welded steel tube is produced by the butt- welding rolls 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 manufacturing a high-strength, high-quality forged steel pipe.

[0002]

2. Description of the Related Art FIG. 2 shows a method of manufacturing a forged steel pipe. 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., forming by the forming rolls 3, 4, oxygen is sprayed to both edges from the nozzle 7 of the welding horn 5 immediately before forging, and forging is performed by the forging roll 6. Then, squeezing sequentially with squeezing rolls, adding rolling to a predetermined outer diameter,
Continuous production of thick wrought steel pipe. However, the scale adhering to both edges is not completely removed only by oxygen spraying immediately before forging, but is directly engaged with the forging and the strength of the forging is significantly reduced. In addition, since oxygen promotes the formation of scale by an oxidative exothermic reaction with base iron, it is important to set appropriate oxygen spraying conditions. In order to improve the quality of a forged portion of a forged steel pipe, Japanese Patent Application Laid-Open No. 55-766 has been proposed.
As in 91, there is a manufacturing method in which both edges of a steel strip are heated to a forging temperature by a plasma arc immediately before forging, and subsequently, both edges are sprayed with oxygen to forge and form a pipe.

[0003]

In the method for producing a forged steel pipe, after a steel strip heated in a heating furnace is hot-formed into a tubular shape, oxygen is blown immediately before forge welding. (2) The scale adhering to the edge is melted and scattered. However, the scale adhered before the oxygen spraying is difficult to remove, which causes a problem that the scale is caught in the forged portion.

[0004] The present invention provides a steel pipe having excellent characteristics of a forged portion by reducing the scale biting into the forged portion.

[0005]

A conventional method for producing a forged steel pipe is as follows. A steel pipe is heated from 1200 ° C. to 1300 ° C. in a heating furnace, then formed into a tubular shape with a forming roll, and oxygen is blown to both edges to form a forged roll. It produces forged steel pipes. However, in the conventional manufacturing method, the scale is blown off by oxygen blowing in order to remove the scale at the edge portion. However, actually, impurities are left in the forged portion. In addition, since the entire steel strip is heated only by the heating furnace, (1) the fuel consumption of the heating furnace is large, and a large cost is required. (2) The amount of generation of scale increases and iron loss increases, and a large number of man-hours are required for processing for scale peeling. (3) Since the temperature in the heating furnace is high, there is a drawback that the brick is extremely damaged and a large amount of equipment maintenance cost is required. In order to avoid such drawbacks, the present invention shares the roles of heating the steel strip and scattering the scale, and allows extraction from the heating furnace at a low temperature.

The specific solution of the present invention is as follows.

[0007] In the production of a forged steel pipe, a steel strip heated to a hot working temperature in a heating furnace is hot-formed into a tube and then forged.
An auxiliary heating device is installed between the forming roll and the forging roll to heat the surface layer at both end portions to 1380 ° C. or more, and then blow an inert gas at a flow rate of 0.2 to 2.5 m ³ / min. This is a method for producing a forged steel pipe.

[0008]

Next, an embodiment of the present invention will be described in detail with reference to FIG.

The steel strip 1 is heated from 700 ° C. to 1300 in the heating furnace 2.
C., and the heated steel strip is formed into a tube by the forming rolls 3 and 4. The auxiliary heating device 8 uniformly heats the surface layer on the end face of the edge to 1380 ° C. or more to melt the surface scale of the edge. After that, an inert gas having a flow rate of 0.2 to 2.5 m ³ / min is blown from the nozzle 7 of the welding horn 5 to blow off the molten scale and forged by the forging roll 6 to form a steel pipe.

The reason why the surface layer on the end face of the edge portion is heated to 1380 ° C. or more is that the melting point of the scale (FeO) attached to the surface is about 1380 ° C. and the scale is melted. The melted scale is easily scattered easily by blowing the inert gas immediately thereafter. But,
When the flow rate is 0.2 m ³ / min or less, the molten scale is not completely removed from the edge portion, and the flow rate is 2.5 m ³ / m
The flow rate of the proper inert gas since the scale and molten at in more remains in the solidified edge portion by the cooling effect by blowing inert gas 0.2~2.5m ³ / m
in.

The above-mentioned auxiliary heating device includes (1) a high-frequency induction heating device, and (2) a plasma arc device.

(1) In the case of the high frequency induction heating device,
The high frequency induction heating coil uniformly heats the surface layer at the edge to the appropriate forging temperature. This is achieved by generating a high frequency current with a high frequency oscillator and flowing a high frequency current through the high frequency induction heating coil to form the edge of the tubular skeleton. Induced current is induced in the part. This induced current forms a loop in the tubular skeleton and heats the tubular skeleton with the Joule heat of this current.However, the current density near the edge is significantly higher than the current density in the other parts, so only the vicinity of the edge is Is rapidly heated. In addition, the magnetic flux density induced by the high-frequency induction heating coil is highest at the edge portion, rapidly decreases toward the center, and the difference in magnetic flux density passing through the center of the thickness of the edge portion is small. Can be uniformly heated.

From the above, the present invention is to provide a steel pipe having excellent forged joint characteristics by reducing the scale biting of the forged joint.

[0014]

EXAMPLE Table 2 shows the results of the practical test and the occupancy ratio of the forged portion in the conventional manufacturing process shown in FIG. Table 2 shows the results of the practical test and the occupancy ratio of the forged portion in the manufacturing process of the method of the present invention shown in FIG. The following formulas show the calculation methods of the flatness ratio, the push-out expansion ratio, and the occupancy ratio of the forged joint foreign matter shown in Tables 1 and 2, respectively.

Flatness = (1−H / D) × 100 (%) D: Outer diameter of base tube H: Flat height Push-out expansion ratio = D ′ / D D: Outer diameter of base tube D ′: After expansion Pipe outer diameter Forged joint foreign matter occupancy = L / t x 100 L: Length of all forged welded foreign matter t: Pipe wall thickness As shown in Table 1, according to the method of the present invention, the foreign matter occupancy is 1 /
It turns out that it has decreased to 3 to 1/4.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

According to the method of manufacturing a forged steel pipe, after hot-forming a steel strip heated in a heating furnace into a tubular shape, oxygen is blown immediately before forge welding. There are two effects of causing an exothermic reaction and (2) melting and scattering scale attached to the edge. However, the scale adhered before the oxygen spraying is difficult to remove, which causes a problem that the scale is caught in the forged portion. In addition, since the entire steel strip is heated only by the heating furnace, (1) the fuel consumption of the heating furnace is large, and a large cost is required. (2) The amount of generated scale increases, iron loss increases, and a large number of man-hours are required for the treatment for scale peeling.
(3) Since the temperature in the heating furnace is high, there is a drawback that the brick is extremely damaged and a large amount of equipment maintenance cost is required.

The present invention has made it possible to provide a steel pipe having excellent characteristics of the forged portion by reducing the scale biting of the forged portion of the forged steel tube.

[Brief description of the drawings]

FIG. 1 is a plan view showing a process for manufacturing a forged steel pipe according to the method of the present invention.

FIG. 2 is a plan view showing a process for manufacturing a forged steel pipe according to a conventional method.

[Explanation of symbols]

 Reference Signs List 1 steel strip 2 heating furnace 3 forming roll 4 forming roll 5 welding hole 6 forging roll 7 nozzle 8 auxiliary heating device

Claims (1)

[Claims] 1. A method for manufacturing a forged welded steel pipe in which a steel strip heated by a heating furnace is hot formed into a tube and then forged by welding, wherein an auxiliary heating device is provided between a forming roll and a forged welding roll, and both edge portions of the tubular skeleton are provided. A method for producing a high-strength and high-quality forged steel pipe, characterized in that the surface layer of (1) is heated to 1380 ° C. or more, and then an inert gas having a flow rate of 0.2 to 2.5 m ³ / min is blown forged.