JPS61219488A - Manufacture for electric welded tube having tough welded part - Google Patents

Abstract

PURPOSE:To improve the toughness of a welded part by interposing a metallic thin piece, containing much more alloy elements, between the edge parts of a steel strip, etc. to adhere the piece tightly to the edge part, and butt-welding the opposite edge parts to each other. CONSTITUTION:After treating a steel strip in an entry stage, the strip is continuously formed into a cylindrical open tube P2 by a forming roll. A metallic thin piece M containing C, Mn, Si much more than those of a base metal, is tightly adhered to either of the opposite edge parts (a), (b) of tube P2. Next, the edge parts (a), (b) are butt-welded by a high frequency welder 2 while applying a side pressure to them by squeezing rolls 3 after heating the side edges (a), (b) by the welder 2. At that time, the prescribed alloy elements are replenished from the piece M to the welding part PW, to restrain the grains from growing to coarse ones. Accordingly, the structure of welding part PW is made finer, and the toughness of PW part is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method of manufacturing an electric resistance welded steel pipe, and particularly to a method of manufacturing an electric resistance welded steel pipe having a welded portion with high toughness.
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(Prior technology) A steel pipe is manufactured by continuously forming a steel plate or steel strip into a cylindrical shape using a forming machine, heating the opposing edges of the steel plate or steel strip, and then butt welding them by applying side pressure. The way to do it is
This is generally known as the electric resistance welding steel pipe manufacturing method.

(Problems to be Solved by the Invention) However, in the above method, the opposing edges are locally heated and welded by pressure contact, so the welded part undergoes rapid heating and cooling, resulting in a hardened structure. Become. In addition, since the base metal melts at the center of the weld and is pushed up and down, C, Mn,
Alloying elements such as St are also discharged to the outside, producing a portion called a ferrite band. After welding, the welded portion is heat treated using a seam annealer to soften the hardened structure, and the ferrite band portion is recrystallized. However, since the alloying elements are deviated from and decreased in this part, the crystals become coarse grained and there is a problem that a welded part with low toughness is formed.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for manufacturing an electric resistance welded steel pipe having a welded portion with high toughness.

(Another Means to Solve the Problem) The present invention involves sandwiching a thin metal flake containing one alloying element in a larger amount than the raw material steel between opposing edge portions of a steel plate or steel strip bent into a tubular shape during electric resistance welding. This prevents dilution of alloying elements in the vicinity of the fusion zone and suppresses coarsening of crystal grains, thereby improving the toughness of the weld zone.

That is, in the present invention, a steel plate or steel strip is bent into a cylindrical shape, and an alloying element is added to either the bent steel plate or the opposed edge portion of the steel strip in a larger amount than that of the steel plate or steel strip. The above-mentioned opposing edge portions are butt-welded to each other while closely interposing the metal flakes containing the metal flakes.

An outline of the method for manufacturing an electric resistance welded steel pipe according to the present invention will be explained with reference to FIGS. 1 and 2.

The coiled steel strip P1 is unwound, subjected to side trimming, edge scarfing, etc. in the entry process, and then supplied to the forming process, where it is continuously formed into a cylindrical open band P by the forming roll 1.
It is formed into 2. The open guide P2 is fed to a welding process and joined by resistance or induction.

At this time, according to the present invention, prior to welding, a metal flake M containing a larger amount of alloying elements than the material steel is interposed between the opposing edges a and b (FIG. 2) of the open gib P2. In this state, edge portions a and b are heated, for example, by a high-frequency welder 2, and side pressure is applied by a squeeze roll 3 to butt weld.

The thin metal flake M used in the present invention basically covers the discharged component elements in the welded part (C).

It contains more alloying elements such as Mn and Si than the base material, and it also contains elements effective in suppressing grain coarsening at high temperatures, such as Nb and Ti, and elements effective in improving low-temperature toughness, such as Mn and Ni. In this case, the toughness of the weld can be further improved.

When interposing the thin metal piece M between the opposing edges a and b of the open guide P2, as shown in FIG.
At least between heating coil 2' and welding point Wp,
Desirably, it is in close contact with one of the edge portions (b in the illustrated example). This is to prevent a short circuit from occurring between the edge portion and the thin metal piece M during welding, resulting in poor welding.

The inner and outer beads Pb generated during welding are cut and removed by a bead cutter 4, and an electric resistance welded steel pipe P3 having a straight joint is manufactured. The welded portion Pw of this electric resistance welded steel pipe P3 is heat treated by the seam annealer 5, and the performance of the hardened welded portion is improved.

Thereafter, the electric resistance welded steel pipe P3 is supplied to a sizing process, and after being finished to a specified outer diameter by a sizer roll 6, it is cut into a predetermined length by a cutoff 7.

(Operation) Figure 3 shows the characteristics of the operation of the method of the present invention described in section 2 in relation to the conventional method.
Whereas the opposing edges a and b of P2 are directly joined during welding, according to the method of the present invention, a metal thin piece M containing more alloying elements than the raw steel is interposed between the opposing edges a and b, In this state, butt welding is performed.

As a result, in the conventional method, the base material at the center of the weld zone Pw is melted and extruded upward and downward, and alloying elements such as C, Mn, and Si are simultaneously discharged to form a ferrite band. Therefore, coarse grain crystals are formed in the welded part Pw after heating by the seam annealer, resulting in low toughness. On the contrary,
In the method of the present invention, a predetermined alloying element is replenished into the welding part Pw from the metal flake M interposed between the edge parts a and b, or a useful life element is added to suppress crystal grain coarsening or to improve low-temperature toughness. Since effective elements are added, ferrite bands are hardly formed, and even after heat treatment with a seam annealer, a fine structure is obtained, resulting in a highly tough welded joint. Figure 4 shows a photograph of the structure of the electric resistance weld.

(Example) A stain-sewn steel pipe was manufactured by the method of the present invention using a base material and metal flakes having the components shown in Table 1. For comparison, a conventional example is shown in which a steel pipe was manufactured under the same conditions except that no metal flakes were used. The size of the manufactured steel pipe was 406.4wn in outer diameter and 9.4mm in wall thickness. The size of the metal flakes was 20 mm wide and 04 mm thick.

Table 1 The mechanical properties of the steel pipe obtained by local heat treatment using a seam annealer after welding are shown in Table 2, and the results of the Charpy test at -20°C are shown in Figure 5.

t 7 ) (l) As is clear from Table 2 and FIG. 5, according to the present invention, the strength at the weld and especially the Charpy impact value at the weld are significantly improved compared to the conventional method. I can do it.

(Effects of the Invention) As described above, according to the present invention, it is possible to avoid the formation of ferrite bands in the welded portion, and coarsening of crystal grains in this portion is prevented, resulting in a highly tough welded portion.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an overview of the present invention, FIG. 2 is a diagram showing a metal thin piece interposition method according to the present invention, FIG. 2 is an explanatory diagram showing the effect of the present invention in comparison with a conventional method, and FIG. The figure is a photograph of the structure of a welded part, and FIG. 5 is a diagram showing the Charpy impact values of electric resistance welded steel pipes obtained by the method of the present invention and the conventional method. P 凰...Strip steel, P2...Open knot, P3.
...Erw welded pipe, M...Metal thin piece, Pb...Bead, Pw...Welded part, 1...Forming roll, 2
...High frequency welding machine, 3...Squeeze roll, 4...
・Bead cutter. S...Seam annealer%6...Sizing roll,
7...Cutoff, W...Welding point procedure amendment C method) 1. Display of the case 1985 Patent Application No. 61523 2 Name of the invention Method for manufacturing ERW steel pipes having high toughness welded parts 3 Person making the amendment Relationship to the case Patent applicant 4, Agent 101 6, Target of correction 4, Brief explanation of drawings 7, Contents of Tadashi Chichisode (1) Specification, page 9, line 12: "Figure 2" is corrected to "Figure 3" . (2) Same, page 9, line 14: The phrase ``Figure 4 is a photograph of the structure of the weld'' is corrected to ``Figure 4 is a photograph of the metal structure of the weld.''

Claims (1)

[Claims] (1) A steel plate or steel strip is bent into a cylindrical shape, and a thin metal flake containing an alloying element in a larger amount than that of the steel plate or steel strip is attached to one of the opposing edges of the bent steel plate or steel strip. A method for manufacturing an electric resistance welded steel pipe having a high-toughness welded portion, the method comprising butt welding the opposing edge portions with each other interposed therebetween.