JPH04301033A - Production of small-diameter resistance welded tube having high strength and excellent in toughness and ductility - Google Patents

Abstract

PURPOSE:To produce a small-diameter resistance welded tube of round or special shape having high strength and excellent in toughness and ductility. CONSTITUTION:This method is characterized by subjecting a resistance welded tube, produced by exerting online hardening at hot rolling, using the resulting steel plate with martensitic structure as a stock, and performing roll forming and high frequency welding, to heating up to 500-700 deg.C and to warm drawing into a round or special shape of small diameter. According to the method, tube making can be done without any problem by using an inexpensive high strength coil because the resistance welded tube is a round tube of relatively low t/D, working is facilitated by means of warm working, and a tamper effect is produced because the stock has tartensitic structure. Accordingly, the excellent small-diameter resistance welded tube of round or square shape minimal in the increase or decrease of strength owing to the precipitating effect by Mo, etc., and free from deterioration in ductility and toughness because the influence of working strain is practically prevented can be obtained.

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 small-diameter electric resistance welded steel pipes having high strength and excellent ductility and toughness.

0002

2. Description of the Related Art A conventional method for manufacturing high-strength, small-diameter electric resistance welded steel pipes is shown in FIG. In other words, when manufacturing high-strength steel pipes by directly forming high-strength steel pipes by ERW-welding, as in A, there is a method described in, for example, Japanese Patent Application Laid-open No. 52-114519, and a relatively strong method as in B. In some cases, ERW pipes are made using a low-strength steel plate and then quenched, or when quenching and tempering are performed. In other cases, cold drawing is performed after ERW pipes are made using relatively low-strength steel plates like C. This is a method of obtaining high-strength steel pipes using work hardening.

0003

Table 1 shows a comparison of the ease of manufacturing and the quality of pipe products in the conventional methods described above. Methods A and B, which do not involve cold drawing, are inefficient because the unit weight is small at the time of pipe production. Furthermore, when comparing the workability in pipe making, method A is difficult to form due to the high strength steel plate. In method B, stable quenching is difficult because it is difficult to uniformly heat a square tube when using high-frequency heating. In addition, in the case of heating in a heating furnace, the heating time becomes longer and the austenite grains become larger, which deteriorates the toughness. Regarding the quality of pipe products, method A has poor elongation and toughness because high-strength material is cold-formed, method B has poor toughness when using a heating furnace, and unstable strength when using induction heating. In Method C, the material elongates due to work hardening and the toughness deteriorates, and the elongation and toughness are poorer than those of the same strength that are not processed. The present invention solves the manufacturing problems and quality problems of pipe products in the conventional methods, and provides a method for manufacturing small-diameter or irregular-shaped electric resistance welded steel pipes with high strength, excellent ductility, and toughness.

0004

[Table 1]

[0005]

[Means for Solving the Problems] The gist of the present invention is to provide an electric resistance welded steel pipe manufactured by roller forming and high frequency welding using a steel plate that has been hot-rolled and hardened on-line to have a martensitic structure as a material, and is manufactured by roller forming and high-frequency welding. It is characterized in that it is heated to ~700°C and subjected to warm drawing to obtain a small diameter or irregularly shaped electric resistance welded pipe with high strength, excellent ductility and toughness.

[0006]

[Embodiments and Effects] The present invention facilitates processing and prevents deterioration of ductility and toughness due to processing by performing drawing at a temperature higher than the recrystallization temperature and lower than the austenitizing temperature Ac1. It is characterized by The process of the present invention is shown in FIG. A high-strength coil can be obtained by quenching at a cooling rate of about 15° C./second or more from an Ar point of 3 or higher after finish rolling in hot rolling, and winding the coil at a temperature below the Mf point. The structure of this coil is martensite, which suppresses the precipitation of Mo and the like. Since high strength can be easily obtained by using martensite, alloying elements can be saved and material costs can be reduced. This coil is used to form an electric resistance welded pipe. Since ERW pipes are raw pipes that have been warm drawn,
Since the ratio t/D of the tube wall thickness t to the outer diameter D is small and the tube shape is round, there are no problems in manufacturing. After forming the ERW pipe, warm drawing shown in FIG. 2 is performed. The steel pipe 1 is heated to a predetermined temperature by a heating device 2, and immediately thereafter reduced in diameter or made into a square tube by a drawing device 3. Since the heating temperature of the heating device 2 is in a warm range, the growth of crystal grains is not so much of a problem, and a heating furnace or an induction heating device may be used. Further, the drawing device 3 may be a generally used roller processing device or a die drawing device. Since warm working is easy to process without the negative effects of processing distortion, it is suitable for drawing high-strength pipes that are impossible with cold working, or for rectangular pipes with a large ratio t/R of wall thickness t and bending radius R. Irregularly shaped pipes can also be easily processed. In addition, strength can be ensured by the precipitation effect of Mo and the like through warm working. Examples are shown below.

[0007]

[Table 2] Material properties DI value 9.0 Ac 1 point 720℃ Mf point 390℃

[
<Hot rolling conditions> Heating temperature 1200°C
Finishing temperature 880℃
Winding temperature 100℃
Finished wall thickness 2.5mm (ERW pipe making conditions) Dimensions Outer diameter 50.8mm
Wall thickness 2.5m
m Pipe making speed 60m/min (warm processing) Heating 400-800℃ in heating furnace Drawing processing
Roller drawing speed 60m/min
Pipe products Round pipe Finished outer diameter 42.7mm
(Outer diameter reduction rate 16%)
Finished outer diameter 38.1mm (outer diameter reduction rate 25%)
Square tube finish 31.8 x 31.8 (outer diameter reduction rate 2
7%)
Corner R 5mm Table 3 shows the finished outer diameter of the round tube φ3
The results of changing the heating temperature in the case of 8.1 mm are shown. If it is below 400℃, cracks will occur due to processing, and if it is above 800℃, the strength will decrease, so the heating temperature should be 500 to 700℃.
is desirable. 500°C is above the recrystallization temperature, and 700°C
C is the temperature range in which precipitation strengthening of Mo, Nb, and Ti persists. In this temperature range of 500 to 700°C, the square tube described above could be processed without any problem, and the strength, ductility, and toughness comparable to that of a round tube were obtained.

[0009]

[Table 3]

0010

[Effects of the Invention] According to the present invention as described above, since the ERW pipe is a round pipe with a relatively small t/D using an inexpensive high-strength coil, the pipe can be made without any problems, and by warm processing. It is easier to process, and the material has a martensitic structure, which has a tempering effect. Due to the precipitation effect of Mo, etc., there is little increase or decrease in strength, and there is almost no effect of processing strain, resulting in improved ductility and toughness. Excellent small-diameter, round or square-shaped electric resistance welded steel pipes with no reduction in quality can be obtained.

[Brief explanation of the drawing]

FIG. 1: Process diagram according to the present invention,

FIG. 2 is a diagram showing the warm processing method of the present invention,

FIG. 3 shows a process diagram according to a conventional method.

[Explanation of symbols]

1 Steel pipe 2 Heating device 3 Drawing processing equipment

Claims (2)

[Claims] [Claim 1] Perform online quenching by hot rolling,
A small-diameter electric wire with high strength, excellent ductility, and toughness, which is produced by drawing an ERW steel pipe manufactured by roller forming and high-frequency welding at a temperature of 500 to 700°C using a steel plate with a martensitic structure as a material. Manufacturing method of sewn steel pipe. [Claim 2] Perform online quenching by hot rolling,
This product is characterized by high strength, excellent ductility and toughness, and is produced by drawing an ERW steel pipe produced by roller forming and high frequency welding into a deformed pipe at a temperature of 500 to 700°C, using a steel plate with a martensitic structure as the material. A method for manufacturing an ERW steel pipe to obtain a deformed ERW pipe.