JP2687817B2 - Manufacturing method of ERW steel pipe for high strength mechanical structure - Google Patents

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 an electric resistance welded steel pipe for a high strength mechanical structure used for a cylinder, a propeller shaft of an automobile, and the like.

[0002]

2. Description of the Related Art With the recent tendency toward weight reduction of automobiles, introduction of high-strength, thin-walled electric resistance welded steel pipes into propeller shafts and the like has been promoted. The electric resistance welded steel pipe for automotive use, the tensile strength at 490 N / mm ² or more, especially in the propeller shaft is required 690n / mm ² or more strength is further also a high degree of toughness and workability requirements.

Such a high-strength electric resistance welded steel pipe is manufactured, for example, by cold rolling a hot-rolled steel strip to increase its strength and then performing electric resistance welding. However, strengthening by cold rolling is
Often accompanied by reduced toughness and workability. Furthermore,
There are problems that the strength is lowered by the post heat treatment, or that the HAZ portion of the manufactured product steel pipe is softened when arc welding is performed. Therefore, a method for increasing the strength of an electric resistance welded steel pipe without relying on cold working is disclosed in JP-A-2-19752.
It is proposed by Japanese Patent No. 5 publication.

[0004]

However, although cold rolling of a hot rolled steel sheet is accompanied by softening of the HAZ portion and the like, it brings great benefits to electric resistance welding. That is,
The hot-rolled steel strip has a lower plate thickness accuracy in the width direction than the cold-rolled steel strip. When such a hot-rolled steel strip is divided into a plurality of strips to form an electric resistance welded steel pipe, the thickness of both edge portions is different, so that welding defects occur due to non-uniform welding heat input. Further, when the electric resistance welded steel pipe is used as a rotary shaft of a propeller shaft or the like, vibration occurs due to variation in wall thickness in the circumferential direction, which causes vibration and noise. Therefore, the method for producing a high-strength electric resistance welded steel pipe that does not rely on cold working is effective for softening the HAZ portion, but it cannot be said to be a preferable method when viewed comprehensively.

The object of the present invention is to suppress the softening of the HAZ portion and the like, despite adopting the strengthening by cold rolling,
In addition, it is to provide a method for producing an electric resistance welded steel pipe for a high-strength mechanical structure that ensures excellent toughness and workability.

[0006]

[Means for Solving the Problems] In the production of high-strength electric resistance welded steel pipe, cold rolling of hot-rolled steel sheet has many favorable effects on product quality, so it is considered that advanced strengthening by cold rolling is essential. . The present inventors, on the premise of increasing the strength by cold rolling, have conducted research on means for suppressing the adverse effect. As a result, for HAZ softening, cold rolling
Restriction of rolling reduction and composition of hot-rolled steel strip, especially Mo effectively functions, and toughness and workability, heat treatment after electric resistance welding works effectively, and moreover, within the effective temperature range. It has been found that there is a region where softening is hardly a problem.

The present invention has been made based on the above findings, and C: 0.15 to 0.25% by weight and Si: 0.3 by weight.
0% or less, Mn: 1.00 to 1.50%, P: 0.02
5% or less, S: 0.005% or less, Nb: 0.020 to
0.050%, Mo: more than 0.20% and 0.35% or less , S
ol-Al: 0.005 to 0.050%, and if necessary, Ca: 0.0040% or less, Ti: 0.030%
The following and V: 0.10% or less of one or two or more, and a slab of balance Fe and unavoidable impurities was hot-rolled, and the hot-rolled steel sheet was cold-rolled at a rolling reduction of 10 to 35%. After that, ERW welding is performed to form an ERW steel pipe.
The gist is a method for producing an electric resistance welded steel pipe for a high-strength mechanical structure, which is characterized by heat treatment at a temperature of 650 ° C.

[0008]

The reasons for limiting the conditions in the present invention will be described below in the order of slab composition, cold rolling and heat treatment.

(A) Slab composition C: The cheapest element for obtaining strength, but 0.15
If it is less than%, the predetermined strength cannot be obtained, and if it exceeds 0.25%, toughness is deteriorated and arc weldability and workability are deteriorated. Therefore, C is set to 0.15 to 0.25%.

Si: Mn-Si, which is added as a deoxidizer at the steelmaking stage, is called a penetrator during electric resistance welding.
Since it causes the formation of non-metallic inclusions in the system, 0.3
0% or less.

Mn: An element that obtains strength relatively inexpensively, but if it is less than 1.0%, the prescribed strength cannot be obtained. Also,
If it exceeds 1.50%, the strength becomes too high and the segregation in the steel becomes large, so that the workability and the like deteriorate. Therefore,
Mn was set to 1.0 to 1.50%.

P: 0.025% or less because it is an element existing in steel as an impurity and easily segregated.

Like S: P, it exists as an impurity and particularly deteriorates the workability and toughness, so the content was made 0.005% or less.

Nb: An element that improves toughness by making the particles finer while ensuring strength. If it is less than 0.020%, the effect of addition cannot be obtained, and if it exceeds 0.050%, the strength increases,
It has no effect on grain refinement and causes cost increase. Therefore, Nb is set to 0.020 to 0.050%.

Mo: A main element in the present invention. Although this element contributes to strength, it is considered to be an unfavorable element from the viewpoint of toughness. However, it is effective as a softening resistance element in the arc welded HAZ part of the manufactured electric resistance welded steel pipe.
FIG. 1 is a diagram showing the effect of Mo on the hardness of the HAZ portion and the base material portion when arc welding is performed on the manufactured electric resistance welded steel pipe. The hardness of the base metal part increases as the amount of Mo increases, but the hardness of the HAZ part increases more than that, and Mo becomes H
It can be seen that it effectively functions to prevent softening of the AZ portion.
When the amount of Mo is 0.20% or less , this effect is not sufficiently obtained, and arc welding after pipe making causes remarkable HAZ softening. On the other hand, if it exceeds 0.35%, deterioration of toughness becomes a problem. Therefore, Mo is included in a relatively large amount of more than 0.20% and 0.35% or less, and the toughness reduction due to this is that Nb is less than 0.
This is supplemented by atomizing by adding in the range of 020 to 0.050% and heat treatment at 400 to 650 ° C. after pipe forming.

Sol-Al: It is added as a deoxidizing agent at the stage of pipe making, but if the addition amount is too small, deoxidation deficiency occurs, and conversely, if it is too large, alumina inclusions increase, and in all cases steel cleaning Is impaired, the content is set to 0.005 to 0.050%.

Ca: Improves the cleanability and the workability by reducing the sulfide inclusions in the steel and making them spherical. However, if it is too large, it becomes a cause of impairing the cleanability as non-metallic inclusions. Therefore, the upper limit of addition is 0.00
It was set to 40%.

Like Ti: Nb, it increases strength and promotes grain refinement. However, if it is too large, the toughness may be deteriorated, and the upper limit in the case of adding is set to 0.030% .

V: Has the same effect as Nb and Ti,
Since the effect does not increase even if it exceeds 0.10%, the upper limit for addition is set to 0.10%.

(B) Cold Rolling The hot rolled steel strip is cold rolled to increase its strength. Further, by improving the wall thickness accuracy in the plate width direction, the wall thickness of both edge portions during electric resistance welding is made uniform, and the welding quality is improved. Further, the wall thickness accuracy of the manufactured electric resistance welded steel pipe in the circumferential direction is improved, and the vibration during rotation is suppressed. If the reduction ratio in cold rolling is less than 10%, the effect of increasing the strength is small, and if it exceeds 35%, the HAZ part becomes significantly softened and the toughness and workability are extremely deteriorated. Even heat treatment cannot improve these performances to a satisfactory level (see Table 2).
See) .

(C) Heat Treatment In the electric resistance welded steel pipe, the electric distortion welded portion is hardened by the processing strain due to forming and the rapid cooling due to the electric resistance welding, and a large hardness difference is generated between the electric resistance welded portion and the base material portion. Therefore, the toughness and workability are essentially not good, and in the present invention, the toughness and the workability are deteriorated by cold rolling. Therefore, heat treatment is applied to the electric resistance welded steel pipe, but if the temperature is less than 400 ° C ,
In addition to marked softening, the effect of improving toughness and workability is not sufficient, and if it exceeds 650 ° C, HAZ softening and
And decrease in toughness are suppressed, but significant decrease in strength is caused (Table
2) . That is, if the temperature is 650 ° C or lower, the HAZ softening is suppressed without impairing the effect of strengthening by cold rolling.
Control is possible, it is not possible to improve the toughness and workability.

FIG. 2 is a table showing the relationship between the heat treatment temperature and the tensile strength of the electric resistance welded steel pipe, using the cold reduction rate as a parameter. Although the tensile strength increases as the rolling reduction in cold rolling increases, the effect of increasing the strength by cold rolling is not impaired at any rolling reduction up to 650 ° C. FIG. 3 is a table showing the effects of cold reduction and heat treatment in the flatness test and push-expansion test shown in FIGS. 4 (A) and 4 (B). Cold rolling exacerbates these test data, but heat treatment recovers to a problem-free level.

[0023]

Example A slab having the composition shown in Table 1 was hot-rolled, descaled, and cold-rolled. Further, the steel strip was divided into multiple strips to form an electric resistance welded steel pipe having an outer diameter of 60.5 mm and a wall thickness of 1.6 mm, which was heat treated. Table 2 shows the results of an examination of the tensile properties and toughness of the electric resistance welded steel pipe after the heat treatment and the hardness reduction amount of the arc welded HAZ part. The electric resistance welded steel pipe manufactured under the conditions of the present invention has a tensile strength of 800 N / mm ² or more, good toughness and workability, and little softening of the HAZ part. In addition,
All the hot rolling was performed at a heating temperature of 1230 ° C.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

As is clear from the above description, in the method for manufacturing an electric resistance welded steel pipe for a high strength mechanical structure of the present invention, since the strength is increased by cold working, the high strength can be easily obtained. . Further, the cold working improves the plate thickness accuracy, improves the quality of the electric resistance welded portion, and improves the dimensional accuracy of the product steel pipe in the circumferential direction. And, despite adopting high strength by cold working, excellent toughness and workability are secured, and HAZ when arc welding is performed on a product steel pipe.
Suppress softening of parts.

[Brief description of the drawings]

FIG. 1 is a table showing the effect of Mo on the hardness of a HAZ part and a base metal part when arc welding is performed on a manufactured electric resistance welded steel pipe.

FIG. 2 shows the relationship between the heat treatment temperature and the tensile strength of ERW steel pipe,
It is a chart which shows a cold rolling reduction as a parameter.

FIG. 3 is a table showing the effects of cold reduction and heat treatment in flattening tests and push-expanding tests.

FIG. 4 is a schematic diagram for explaining a flatness test and a push-expansion test.

Claims (2)

(57) [Claims] 1. C: 0.15 to 0.25% by weight, S
i: 0.30% or less, Mn: 1.00 to 1.50%,
P: 0.025% or less, S: 0.005% or less, Nb:
0.020-0.050%, Mo: over 0.20% 0.3
A slab containing 5% or less of Sol-Al: 0.005 to 0.050% and the balance of Fe and unavoidable impurities was hot-rolled, and the hot-rolled steel sheet was cold-rolled at a rolling reduction of 10 to 35%. After that, ERW welding is performed to form an ERW steel pipe.
A method of manufacturing an electric resistance welded steel pipe for high-strength mechanical structures, characterized by performing heat treatment at a temperature of 650 ° C. 2. C: 0.15 to 0.25% by weight and S
i: 0.30% or less, Mn: 1.00 to 1.50%,
P: 0.025% or less, S: 0.005% or less, Nb:
0.020-0.050%, Mo: over 0.20% 0.3
5% or less , Sol-Al: 0.005 to 0.050%
And further Ca: 0.0040% or less, Ti: 0.030
% Or less and V: 0.10% or less of one or two or more, and a slab composed of the balance Fe and unavoidable impurities is hot-rolled, and the hot-rolled steel sheet is cold-rolled at a rolling reduction of 10 to 35%. After that, it is welded by electric resistance to form an electric resistance welded steel pipe.
A method for producing an electric resistance welded steel pipe for high-strength mechanical structures, characterized by performing heat treatment at a temperature of ˜650 ° C.