JPH05263192A - High strength electrically welded steel tube for boiler excellent in high temperature property - Google Patents

Abstract

PURPOSE:To provide a high strength resistance welded steel tube for a boiler excellent in high temp. properties. CONSTITUTION:This is a high strength resistance welded steel tube for a boiler excellent in high temp. properties characterized by executing normalizing after tube making in a resistance welded steel tube having a componnetal compsn. of, by weight, 0.15 to 0.30% C, 0.05 to 0.50% Si, 0.25 to 1.5% Mn, 0.005 to 0.010% N, 0.02 to 0.10% Cu and 0.001 to 0.004% Ca or furthermore added with 0.01 to 0.10% Mo, and the balance Fe with inevitable elements. In this way, the objective resistance welded steel tube for a boiler high in creep strength as well as excellent in workability and weldability and moreover inexpensive in cost 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 In steel pipes for boilers used as evaporator pipes and superheater pipes in boiler furnaces, the pressure is increasing in order 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]

When assembled as a boiler, the boiler tube manufactured as a tube is bent, expanded, reduced in diameter, etc. and then welded and assembled. On the other hand, the boiler needs creep strength because it is used at high temperature for a long time. There are two types of steel pipes described in the prior art as described above, but JIS G
In the 3461 steel pipe, so-called five elements C, Si, M
Since n, P, and S are the basic components, it is necessary to increase the amount of the components and increase the base strength in order to secure the creep strength. However, increasing the base strength adversely affects weldability and workability. Also, JIS
In the method of G 3462, since a large amount of alloying elements are added, the creep strength is secured, but for this reason, there are disadvantages that the cost is high and welding is difficult.

The present invention solves the above problems at present, and provides an electric resistance welded steel pipe for a high-strength boiler excellent in high temperature characteristics such as creep strength and weldability by adjusting the steel composition. That is the purpose.

[0005]

In order to achieve the above-mentioned object, the present invention provides (1) component composition by weight of C: 0.15 to 0.15.
0.30%, Si: 0.05 to 0.50%, Mn: 0.
25-1.5%, N: 0.005-0.010%, C
u: 0.02-0.10%, Ca: 0.001-0.0
An electric resistance welded steel pipe containing 04% as a basic component and the balance Fe and unavoidable elements has excellent high-temperature properties, characterized by the fact that the electric resistance welded part and the base metal part have a uniform structure due to normalization after pipe making. ERW steel pipe for high-strength boiler, and (2) component composition by weight C: 0.15 to 0.30%, Si: 0.0
5 to 0.50%, Mn: 0.25 to 1.5%, N: 0.
005-0.010%, Cu: 0.02-0.10%,
Ca: 0.001 to 0.004%, Mo: 0.01 to
High temperature characteristics of the electric resistance welded steel pipe having 0.10% as a basic component and the balance Fe and unavoidable elements, in which the electric resistance welded part and the base metal part have a uniform structure by normalizing after the pipe making. This is an electric resistance welded steel pipe for high strength boilers.

The present invention will be described in detail below. As described above, the boiler tube manufactured as a tube is bent, expanded, reduced in diameter, etc. and then welded and assembled when assembled as a boiler. On the other hand, the boiler needs creep strength because it is used at high temperature for a long time. However, with conventional steel pipes, the base strength is increased,
Since many alloy elements are added, workability and weldability deteriorate. Therefore, the present invention defines an electric resistance welded steel pipe for a boiler, which has high creep strength and excellent workability and weldability.

First, the reasons for limiting 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 required strength cannot be obtained, so the lower limit was made 0.15%. or,
If it exceeds 0.30%, cold workability such as formability at the time of pipe making, toughness, and weldability tend to deteriorate, and the upper limit is 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, 0.50% was made the upper limit.

When Mn is less than 0.25% in terms of strength, strength becomes insufficient, and when it exceeds 1.5%, cracks may occur due to insufficient ductility during processing such as forming during pipe forming. Therefore, the lower limit is 0.25% and the upper limit is 1.5%.

N is an effective element for ensuring the creep strength. Figure 1 shows the relationship between the N content and the estimated creep strength at 100,000 hours. To obtain the effect, the lower limit is 0.005.
%. Further, if it exceeds 0.010%, blue heat brittleness (that is, strength increase at about 200 to 300 ° C.) becomes remarkable,
Since the hot workability deteriorates, 0.010% was made the upper limit.

Ca is an important element for controlling the morphology of inclusions, which is important for workability, but if less than 0.001%, it is not effective, and it is not necessary to add more than 0.004%. Therefore, the lower limit is 0.001% and the upper limit is 0.004%
And

Cu is an important element for corrosion resistance, especially for preventing pitting corrosion, and if it exceeds 0.10%, the effect is the same or less, so the upper limit was made 0.10%. Also, 0.02%
If it is less than 0.1%, there is no effect, so the lower limit was made 0.02%.

[0012] Next, claim 2 is a method in which Mo is added to claim 1, but Mo is an important element for preventing a decrease in strength during strain relief annealing, and when welding a joint between pipes. However, if it is less than 0.01%, it has no effect on the strength reduction resistance and softening resistance, and there is no need to add it in excess of 0.10%. Therefore, the lower limit is 0.01% and the upper limit is 0.1%. It was set to 10%.

Next, the electric resistance welded steel pipe of the present invention is produced by normalizing the steel having the above-mentioned components after pipe forming. By normalizing, the hardened part of the welded part during electric resistance welding is homogenized with the base metal part other than the welded part, and the structure is stabilized, preventing the strength from decreasing during long-term use at high temperature. , To secure the creep strength. 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]

EXAMPLE A diameter of 1 according to the present invention and the conventional composition steel shown in Table 1
An electric resistance welded steel pipe having a diameter of 01.6φ and a thickness of 4.4 t was manufactured. The outline of the production method is shown in FIG. The characteristics of the electric resistance welded steel pipe subjected to the normalizing treatment are also shown in Table 1. It can be seen that 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 for a boiler having high creep strength, excellent workability and weldability, and low cost.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the amount of N and the estimated creep strength at 100,000 hours.

FIG. 2 is a diagram showing a basic manufacturing process of the steel pipe of the present invention.

Claims (2)

[Claims] 1. The weight ratio of the components is C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.25 to 1.5%, N: 0.005 to 0. 0.010%, Cu: 0.02 to 0.10%, Ca: 0.001 to 0.004% as a basic component, and the balance is Fe and an unavoidable element, which is an electric resistance welded steel pipe. ERW steel pipe for high-strength boilers with excellent high-temperature characteristics, characterized in that the ERW welded part and the base metal part have a uniform structure. 2. The component composition by weight is C: 0.15 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.25 to 1.5%, N: 0.005 to 0. 0.010%, Cu: 0.02 to 0.10%, Ca: 0.001 to 0.004%, Mo: 0.01 to 0.10% as a basic component, and the balance Fe and an unavoidable element. An electric resistance welded steel pipe for a high-strength boiler having excellent high-temperature characteristics, characterized in that the electric resistance welded part and the base metal part have a uniform structure by normalizing after pipe making.