JPS591635A - Production of electricaly welded steel pipe - Google Patents

Abstract

PURPOSE:To prevent the hardening in the weld zone of a steel pipe without using seam annealing in the stage of welding both edge parts of a tubular- formed body of a belt steel contg. C at a specific rate, by heating only the weld zone right after the welding then cooling the same at a specific rate. CONSTITUTION:A belt steel 11A contg. >=0.30% carbon or >=0.50% carbon equiv. is formed to a circular shape; thereafter, both edge parts thereof are welded by a high frequency to form a blank pipe 11B. After the outside beads of the pipe 11B are chipped away, only the weld zone 11C is reheated with a reheater 14. The reheater 14 is constituted of gas burners 16 which are connected with a gas supply pipe 15 and can heat the zone 11C and a heat insulation pipe 17 which encloses the pipe 11B, and can prevent the cooling by heat radiation of the pipe 11B with a heat insulating material 17A. The cooling rate of the zone 11C is thus controlled in such a way that no martensite is precipitated in the zone 11C and that bainite is precipitated as a primary crystal.

Description

[Detailed description of the invention] The present invention relates to a method of manufacturing an electric resistance welded steel pipe.

In general, electric resistance welded steel pipes utilize the proximity effect and skin effect of high-frequency welding, that is, the effect of effectively heating only the part of the welded part that requires heating, to further heat both edges of a steel strip formed into a tubular shape. The wood pipe is formed by frequency welding. However, since the proximity effect and skin effect in high-frequency welding cause local heating of the welded part, the welded part inevitably undergoes rapid heating and cooling.
Austenite in the weld zone is likely to transform into martensite, resulting in a hardened structure in the weld zone, and particularly in high carbon steel pipes, high manganese steel pipes, etc., the hardening of the weld zone tends to be further accelerated.

- If significant hardening occurs in the welded area as in F.
In the steel pipe cutting process after welding, the life of the knife may deteriorate,
There is a risk that cracks may occur in the hardened weld during the roll holding work in the diameter setting process and bend straightening process after welding.

Therefore, conventionally, welds that harden as described above are reheated by seam annealing of the weld t.
The structure is improved by converting martensite in the weld into austenite, and then converting the austenite into ferrite, pearlite, or bainite.

However, when applying the above seam annealing to a relatively small diameter raw pipe, since the diameter of the raw pipe is small,
By reheating by seam annealing, not only the welded part where the structure needs to be improved, but also the entire raw pipe is heated, and the roll lubricant and impeder cooling water inside the raw pipe are boiled and A so-called steam-back phenomenon occurs. This steam-back phenomenon has the problem of lowering the temperature of the raw pipe at the liquid level 14 and causing a so-called cold weld (cold weld) in which an unwelded part remains in the welded part.

The present invention has been made in view of the above-mentioned conventional problems, and is a method for manufacturing an ERW steel pipe that can prevent hardening of welded parts in high carbon steel pipes or high carbon equivalent steel pipes without using seam annealing. The purpose is to provide

In order to achieve the above object, the present invention provides 0.30%
In a method for making an electric resistance welded steel pipe in which a steel band containing the above carbon content or a carbon equivalent of 0.50% or more is formed into a tubular shape, and both edges of the steel band are high-frequency welded to form a base pipe,
Immediately after welding the raw pipe, only the I@liquid contact area is reheated, and the cooling rate of the welded area is adjusted so that bainite is precipitated as a pro-eutectoid without precipitating Y rutinsite in the welded area. It is designed to adjust.

The present invention will be explained in detail below.

First, the present invention is applied to a raw pipe obtained by forming a steel strip with a carbon content of 0.30% or more and then high-frequency welding both edge portions of the steel strip. The present invention provides, in the CCT diagram specified by the steel material, in the air cooling zone of the raw pipe made of the steel material,
In order to ensure that the temperature and cooling rate in the wall coating zone are appropriate for precipitating bainite from austenite as a pro-eutectoid without the austenite transforming into martensite, only the welded area should be heated using gas, electricity, laser, etc. immediately after welding. The heating means and heat retention means are used for reheating. That is, the above-mentioned reheating needs to be performed before martensitic transformation occurs, that is, before the temperature of the weld zone reaches the M5 point, and the reheating temperature also needs to be set at sub1ζ higher than the MS point. Also, after reheating,
In order to sustain the bainite transformation, it is necessary to keep the weld zone warm and maintain the temperature of the weld zone higher than the M3 point.

Next, specific embodiments of the present invention will be described with reference to the drawings. FIGS. 1 and 2 are explanatory diagrams showing an apparatus used for carrying out the present invention. After the steel strip 11A is formed into a circular shape, both edges thereof are high-frequency welded using squeeze rolls 12 to form a blank pipe 11B. Raw tube 11B
After the outer bead is removed by the bead cutting device 13, only the welded portion 11C is reheated by the reheating device 14. The bed heating device @14 is connected to a gas supply pipe 15 and includes a gas burner 16 that can heat the welded part 11C. ! : , surrounding the raw pipe 11B and the heat insulating material 17
A and a heat-retaining tube 17 that can prevent the base tube 11B from cooling due to heat radiation.

That is, the reheating device 14
The cooling rate of the welded part 11C can be adjusted so that the welded part 11C is reheated immediately after welding and bainite is precipitated as a pro-eutectoid without martensite being precipitated in the welded part 11C.

Next, after welding five steel pipes with an outer diameter of 34 m and a wall thickness of 5 points made of the materials shown in Table 1 at a mill speed of 25 m/min, the welded portions were welded using the equipment shown in Figures 1 and 2 above. We will explain the implementation results that made it possible to prevent the hardening of.

Table 1 The CCT diagram of this steel material is as shown in Figure 3.
Curve A in the figure shows the cooling process of the steel pipe manufactured based on the present invention, and curve B shows the cooling process of the steel pipe manufactured based on the conventional method.

That is, the steel pipe according to curve IA is reheated by the reheating device 14 7 seconds after welding, and the subsequent cooling rate is adjusted to 37° C./second or less, without precipitating martensite in the welded part. Bainite is precipitated as a pro-eutectoid. The steel pipe according to curve B is simply air-cooled after welding, and martensite is precipitated in the welded portion. Note that both the steel pipes according to curve A and the steel pipes according to curve B are water-cooled after air cooling, allowing sizing at room temperature. According to the above implementation results, the hardness of the welded part of the steel pipe according to curve B based on the conventional method was 653 (HV), while that of the steel pipe according to curve A based on the method of the present invention. The hardness of the welded part was found to be 347 (HV).

That is, according to the method of manufacturing an electric resistance welded steel pipe of the present invention, even when the steel pipe material is made of a highly hardenable material, it is possible to prevent the hardening of the steel pipe without using seam annealing after welding, and to prevent the hardening of the steel pipe during cutting in the subsequent process. There is no risk of shortening the life of the cutter or causing cracks in the welded part during roll holding during the diameter setting process and bend straightening process.

As described above, the present invention involves forming a steel band containing a carbon content of 0.30% or more or a carbon equivalent of 0.50% or more into a tubular shape, and then high-frequency welding both edges of the steel band. In a method for manufacturing an electric resistance welded steel pipe that forms a pipe, immediately after welding the base pipe, only the welded part is reheated so that bainite is precipitated as proeutectoid crystals without precipitating martensite in the welded part. Since the cooling rate of the welded portion is adjusted, it is possible to prevent the hardening of the welded portion from IF in high carbon steel pipes or high carbon pixel steel pipes without using seam annealing.

[Brief explanation of drawings]

FIG. 1 shows 1! used in carrying out the present invention. ! ! 2 is a sectional view taken along the line n-Il in FIG. 1, and FIG. 3 is a CCT diagram showing a specific embodiment of the present invention. 11A... band hook, 11B... raw pipe, 11C... welded part, 14... reheating device. Agent: Patent Attorney Osamu Shiokawa

Claims (1)

[Claims] (110,3 (ERW steel pipe in which a steel band containing carbon content of 1 or more or carbon equivalent of 0.50% or more is formed into a tubular shape, and both edges of the steel band are high-frequency welded to form a base pipe. In the manufacturing method, immediately after welding the raw pipe, only the welded part is reheated so that bainite is precipitated as proeutectoid crystals without precipitating martensite in the welded part,
A method for manufacturing an electric resistance welded steel pipe, characterized by a shear for adjusting the cooling rate of the welded part.