JPH05263193A - High strength electrically welded steel tube for boiler excellent in softening resistance at the time of stress relieving annealing - Google Patents

Abstract

PURPOSE:To provide a high strength electrically welded steel tube for a boiler excellent in softening resistance at the time of stress relieving annealing. CONSTITUTION:In an electrically welded steel tube having a componental compsn. constituted of, by weight, 0.10 to 0.30% C, 0.05 to 0.50% Si, 0.25 to 2.0% Mn, 0.01 to 0.25% Mo and 0.01 to 0.1% V as fundamental components, furthermore added with 0.02 to 0.1% Cu and 0.001 to 0.004% Ca, and the balance Fe with inevitable impurities, normalizing is executed after tube making into a high strength electrically welded steel tube for a boiler excellent in softening resistance at the time of stress relieving annealing. In this way, the objective electrically welded steel tube free from the deterioration in strength even if stress relieving annealing is executed for a long time and excellent in workability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength electric resistance welded steel pipe for a boiler.

[0002]

2. Description of the Related Art Steel pipes for boilers used as evaporator pipes and superheater pipes in boiler furnaces are under increasing pressure to improve efficiency, and one of the measures is to increase the strength of these pipes. Is being done. Conventionally, the manufacturing method of high strength ERW steel pipe is STB in JIS G 3461.
35, 42, 52 in the manner described in C,
This is a method of manufacturing a so-called carbon steel containing Si, Mn, P and S as basic components by annealing after pipe forming. Further, as a case where the alloying element is added, there is a method described in JIS G 3462.

[0003]

A boiler tube manufactured as a tube is bent, expanded, reduced in diameter and then welded when assembled as a boiler. During this welding, the welded portion is hardened and residual strain occurs. For this reason, the strain relief annealing is carried out after welding, but since it is heated for a long time, generally not only the welded portion but also other portions have lower strength. FIG. 1 shows the relationship between the time for strain relief annealing and the tensile strength, and the strength decreases with time.
Further, FIG. 2 shows the relationship between the temper parameter and the strength, and the temper parameter becomes large,
That is, as the temperature increases and the time increases, the strength decreases. As described above, the method described in the related art has two problems.
Although there is a type, in the method of JIS G 3461, since so-called five elements C, Si, Mn, P and S are used as basic components, strength reduction during strain relief annealing cannot be avoided. Therefore, it is necessary to increase the components of the base strength in consideration of the decrease in strength, which causes a drawback that workability and weldability are significantly reduced. Also, JISG
In the method of 3462, the strength reduction during strain relief annealing is relatively small, but the cost is high and the weldability is poor because a large amount of alloying elements are added.

The present invention solves the above problems in the present situation, and an object thereof is to provide a high strength electric resistance welded steel pipe for a boiler by adjusting alloy components.

[0005]

In order to achieve the above object, the present invention provides (1) component composition by weight of C: 0.10 to 0.10.
0.30%, Si: 0.05 to 0.50%, Mn: 0.
25-2.0%, Mo: 0.01-0.25%, V:
An electric resistance welded steel pipe having 0.01 to 0.1% as a basic component and the balance Fe and unavoidable elements, and the electric resistance welded part and the base metal part have a uniform structure by normalizing after pipe forming. ERW steel pipe for high-strength boiler having excellent softening resistance during strain relief annealing, characterized by (2) component composition by weight C: 0.10-0.10
0.30%, Si: 0.05 to 0.50%, Mn: 0.
25-2.0%, Mo: 0.01-0.25%, V:
An electric resistance welded steel pipe containing 0.01 to 0.1% and Cu: 0.02 to 0.1% as a basic component, and the balance Fe and unavoidable elements. An electric resistance welded steel pipe for a high-strength boiler having excellent softening resistance during strain relief annealing, characterized in that the base metal part has a uniform structure, and (3) component composition by weight C: 0.10 to 0 .30%, Si: 0.
05-0.50%, Mn: 0.25-2.0%, Mo:
0.01-0.25%, V: 0.01-0.1%, C
u: 0.02-0.1%, Ca: 0.001-0.00
It is an electric resistance welded steel pipe having 4% as a basic component and the balance Fe and unavoidable elements, and the strain relief is characterized in that the electric resistance welded part and the base metal part have a uniform structure by normalizing after the pipe making. ERW steel pipe for high-strength boiler with excellent softening resistance during annealing.

The present invention will be described in detail below. The boiler tube manufactured as a tube as described above is welded after being subjected to bending, expanding, reducing the diameter and the like when assembled as a boiler. During this welding, the welded portion is hardened and residual strain occurs. For this reason, the strain relief annealing is carried out after welding, but since it is heated for a long time, generally not only the welded portion but also other portions have lower strength. Therefore, the present invention defines an electric resistance welded steel pipe for a boiler that does not cause strength reduction even if such strain relief annealing is performed for a long time.

First, the reasons for defining the components of the steel sheet used in the present invention will be explained. In claim 1, if the C content is small, the ductility is good and the workability is excellent, but the lower limit is set to 0.10% because the required strength cannot be obtained. or,
If it exceeds 0.30%, cold workability such as formability at the time of pipe making and toughness tend to be deteriorated, and the heat-affected zone is hardened at the time of pipe making welding of the electric resistance welded steel pipe, and workability is deteriorated. Therefore, the upper limit was set to 0.30%.

In the case of killed steel, it is difficult to control Si to less than 0.05% in steel making technology, and if it exceeds 0.50%, ductility is adversely affected, and scale formation causes deterioration of surface properties. From the point, 0.05% as the lower limit, 0.5
The upper limit was 0%.

With respect to Mn, if the strength is less than 0.25%, the strength becomes insufficient, and if it exceeds 2.0%, cracks may occur due to insufficient ductility during processing such as molding during pipe forming. Therefore, the lower limit is 0.25% and the upper limit is 2.0.
%.

[0010] Mo is an important element for preventing strength reduction during strain relief annealing, and prevents softening during joint welding of pipes, but if it is less than 0.01%, strength reduction resistance / softening resistance. Therefore, the lower limit was set to 0.01% and the upper limit was set to 0.25%.

In order to improve hot workability (particularly bending workability), V precipitates solid solution N, which causes blue embrittlement, as VN. If it is less than 0.01%, there is no effect on strength reduction resistance and softening resistance, and there is no need to add more than 0.10%, so the lower limit was made 0.01% and the upper limit was made 0.10%.

[0012] In claim 2, Cu is added to claim 1, but Cu is an important element for preventing corrosion resistance, particularly pitting corrosion. If it exceeds 0.1%, the effect is less than that. Therefore, the upper limit was set to 0.1%. Further, if less than 0.02%, there is no effect, so the lower limit was made 0.02%.

In claim 3, Ca is added to claim 2, but Ca is an important element for controlling the morphology of inclusions that are important for workability, and if less than 0.001%, it is effective. Moreover, since there is no need to add more than 0.004%, the lower limit was made 0.001% and the upper limit was made 0.004%.

The electric resistance welded steel pipe having the above components is subjected to normalization after pipe forming. This is to normalize the hardened part of the welded part during electric resistance welding with the base metal part other than the welded part by stabilizing the structure and prevent the strength from decreasing during strain relief annealing. Is. The characteristic of the above-mentioned steel pipe is an electric resistance welded steel pipe for a boiler which does not cause strength reduction even after performing strain relief annealing for a long time and has excellent workability.

[0015]

EXAMPLES Table 1 shows the components of the electric resistance welded steel pipes of the conventional example and the present invention example. Table 1 also shows the properties of the steel pipes annealed after pipe forming. As is apparent from the table, the steel of the present invention is superior to the conventional steel in terms of workability, weldability and manufacturing cost.

[0016]

[Table 1]

[0017]

As described above, according to the present invention, it is possible to obtain an electric resistance welded steel pipe which is less likely to have a decrease in strength during strain relief annealing after welding, has excellent workability, and is inexpensive.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between time for strain relief annealing and tensile strength.

FIG. 2 is a diagram showing the relationship between temper parameters and tensile strength.

Claims (3)

[Claims] 1. The weight of the component composition is C: 0.10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.25 to 2.0%, Mo: 0.01 to 0. 0.25%, V: 0.01 to 0.1% as a basic component, and the balance Fe and inevitable elements are the electric resistance welded steel pipes, and the electric resistance welded part and the base metal part are uniform due to normalization after pipe making. ERW steel pipe for high-strength boiler with excellent softening resistance during strain relief annealing characterized by having a structure. 2. The component composition by weight is C: 0.10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.25 to 2.0%, Mo: 0.01 to 0. 25%, V: 0.01 to 0.1%, Cu: 0.02 to 0.1% as a basic component, and the balance Fe and unavoidable elements, which is an electric resistance welded steel pipe, and is fired after pipe forming. ERW steel pipe for high-strength boilers with excellent softening resistance during strain relief annealing, characterized in that the ERW welded part and the base metal part have a uniform structure according to the standard. 3. Component composition by weight: C: 0.10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.25 to 2.0%, Mo: 0.01 to 0. 0.25%, V: 0.01 to 0.1%, Cu: 0.02 to 0.1%, Ca: 0.001 to 0.004% as a basic component, and the balance Fe and an unavoidable element. ERW steel pipe for high-strength boiler, which is a sewn steel pipe and has excellent softening resistance during strain relief annealing, characterized in that the ERW welded part and the base metal part have a uniform structure by normalizing after pipemaking. ..