JPH11156433A - High carbon electric resistance welded steel tube of high workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the high carbon electric resistance welded steel tube of high workability capable of suppressing a high temp. crack in resistance welding and supressing lowering of a product yield in an ultrasonic flaw inspection due to a hook crack. SOLUTION: A high carbon steel slab, in which a P concentration of a center segregation part continuously cast of a high carbon steel containing, by weight, 0.3-0.6% C, 0.15-0.35% Si, 0.3-1.5% Mn, <=0.035% S, <=0.035% Al, <=0.012% P and the balance Fe with inevitable impurities satisfies P <=-0.32×C% +0.244, is hot rolled to a high carbon steel rolled coil, which is used for a stock of resistance welding.

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 a high-workability, high-carbon steel ERW pipe that suppresses high-temperature cracking during ERW and has a reduced yield.

[0002]

2. Description of the Related Art The strength of carbon steel for mechanical structures specified in JIS G4051 increases as the amount of carbon increases, but elongation decreases and P segregation tends to increase. The segregated portion of P causes a reduction in hot workability.
For this reason, when subjected to severe processing such as bulge forming, in which a compressive force is applied in the pipe axis direction while applying hydraulic pressure to the inner surface of the pipe material to expand and deform along the outer mold, the P segregation portion becomes brittle. It is also conceivable to cause cracks.

[0003] In the production of ERW steel pipe of high carbon steel, if there is segregation in the welded portion, shearing occurs in the vertical flow of the metal flow at the welding point, causing hot cracking in the segregated portion having a low melting point, and May cause cracks. As shown in FIG. 2, the high-temperature cracking near the welding point during ERW welding is caused by the fact that the solidus temperature of the P segregation part is lower than that of the normal part, and the edge melting occurs when the molding stress is applied. This is because the segregation part 2 is melted together with the part 1 and a shearing force also acts there. The hook crack causes a defect in an ultrasonic flaw detection test to deteriorate the product yield, and significantly impairs workability in a subsequent process. For this reason, the conventional high-carbon steel hot-rolled coil is difficult to perform the electric resistance welding, and even if it is possible, only a thin material of 2 to 3 mm or less can be formed.

As a high carbon steel electric resistance welded steel pipe excellent in workability, C: 0.4 to 0.8%, Si: 0.15 to 0.35
%, Mn: 0.3 to 2.0%, P: 0.030% or less,
S: 0.035% or less, Al: 0.035% or less, the balance is reduced to segregation by adding 0.05 to 0.15% of Mo in a high carbon steel containing Fe and inevitable impurities. High carbon steel electric resistance welded steel pipe made of a material which facilitates hot workability (Japanese Patent Application Laid-Open No. Hei 4-263039)
Has been proposed.

[0005]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. 4-263 is disclosed.
The Mo-added high carbon steel electric resistance welded steel pipe disclosed in Japanese Patent No. 039-39, even when an expensive Mo alloy is used, does not eliminate the center segregation of the nonmetallic inclusions by only Mo addition. There is a problem that the product yield in the ultrasonic test is low.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art, suppress high-temperature cracking during electric resistance welding, and reduce the reduction in product yield in ultrasonic flaw detection tests. An object of the present invention is to provide a method for manufacturing a steel pipe.

[0007]

According to the first aspect of the present invention, there is provided a method for producing a high-workability high-carbon steel ERW steel pipe, wherein C: 0.3-0.
6%, Si: 0.15 to 0.35%, Mn: 0.3 to
1.5%, S: 0.035% or less, Al: 0.035%
Hereinafter, a high carbon steel containing P: 0.012% or less, the balance being Fe and unavoidable impurities, and the P concentration [P] of the continuously segregated central segregation portion satisfies the following formula (1). A high-carbon steel hot-rolled coil obtained by hot rolling a steel slab is used as a material. [P] ≦ {−0.32 × [C (%)] + 0.244} (1)

As described above, a high-carbon steel hot-rolled coil obtained by hot rolling a high-carbon steel slab in which the P concentration [P] of the center segregation portion of the high-carbon steel satisfies the above formula (1) is used as a raw material. By using, the segregation of P is reduced, the hot workability of the segregated part is improved, the high temperature cracking at the time of electric resistance welding is suppressed, and the reduction of the product yield in the ultrasonic inspection test due to the hook crack is reduced. In addition to being able to suppress, it is possible to improve the workability in a subsequent step.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the chemical components of high carbon steel for machine structures are limited for the following reasons. C
Is an element necessary to increase the mechanical strength of steel, but if it is less than 0.3%, the mechanical strength of a high carbon steel ERW pipe for machine structure cannot be secured, and 0.6% If it exceeds, the elongation is reduced, the workability is deteriorated, and the formability and cutability of the ERW steel pipe are deteriorated.

[0010] Si is an element effective as a deoxidizing element in steel, but if its content is less than 0.15%, its effect is not sufficient.
On the other hand, if the content exceeds 0.35%, oxygen in the atmosphere is taken in at the welding surface during the electric resistance welding to become SiO ₂ , causing a penetrator and exceeding the JIS standard of carbon steel for machine structural use. , 0.15 to 0.35%.

Mn is an indispensable element for securing the strength and toughness of steel, but if it is less than 0.3%, hot brittleness due to S cannot be avoided, and 1.5% If it exceeds, workability deteriorates and penetrator defects due to the MnO component are liable to occur at the time of electric resistance welding.
It was set to 3 to 1.5%.

The content of S is preferably as small as possible because it deteriorates the hot workability of steel. However, the content of S is set to 0.035% or less in accordance with the JIS standard of carbon steel for machine structural use.

[0013] Al is an element necessary for deoxidation, but if it is increased, the workability and weldability are deteriorated due to the formation of alumina clusters.

When P segregates at the grain boundary, the bonding force at the grain boundary is weakened, and the grain boundary may become brittle when bulging is performed. Therefore, the smaller the content of P, the more preferable. P segregation of high carbon steel slabs in order to reduce the segregation of P in steel to 0.012% or less and to prevent high-temperature cracking during ERW, and to reduce the reduction in yield in ultrasonic testing. Part of P concentration is [P] ≦ ０．３−0.32 × [C
(%)] + 0.244 °.

In the present invention, the P concentration of the segregated portion of the high carbon steel slab is [P] ≦ ｛−0.32 × [C (%)] + 0.2
The angle of 44 ° can be satisfied by performing electromagnetic stirring during continuous casting or lowering the degree of superheat of molten steel.

According to the method for producing a high carbon steel ERW steel pipe of the present invention, the P concentration in the high carbon steel is 0.012% or less, and the P concentration in the segregated portion of the high carbon steel slab is [P] ≦ ｛− 0.32x
By using a high-carbon steel hot-rolled coil of [C (%)] + 0.244 ° as a material, the solidus temperature difference between the segregated portion and the normal portion is reduced, and hot cracking during electric resistance welding is suppressed. In addition, the yield of ultrasonic flaw detection can be significantly improved.

In the present invention, the P concentration of the segregated portion of the high carbon steel slab is [P] ≦ ｛−0.32 × [C (%)] + 0.2
The reason why the angle is set to 44 ° is as follows. That is, C:
0.20 to 0.95%, P: 0.01 to 0.20%, S
i: 0.15 to 0.35%, Mn: 0.3 to 1.5%,
S: 0.035% or less, Al: 0.035% or less, a remainder is cut out from a segregated portion of a high carbon steel slab composed of Fe and unavoidable impurities with a diameter of 8 mm and a length of 140 mm, and specified in JIS Z2241. A tensile test at a high temperature of 1300 ° C. was carried out in accordance with the tensile test method for metallic materials described above, and the drawing (RA) was determined by the following equation (2). RA = (A ₀ −A) / (A ₀ ) × 100 (2) where RA: squeezing (%), A: minimum cross-sectional area measured by carefully matching the fractured surfaces of the test pieces ( mm ² ), A ₀ :
Original cross-sectional area (mm ² )

As shown in FIG. 1, C (%) in steel and P
The relationship between (%), reduction (%), and the occurrence of grain boundary embrittlement is determined, and a boundary line of the presence or absence of the occurrence of grain boundary embrittlement is drawn at a reduction (RA) of 80% or more indicating high workability. The above equation showing the relationship between the P concentration and C (%) in the segregated portion where no generation occurred was determined.

[0019]

EXAMPLE Steel No. 1 shown in Table 1 was used. After smelting a high-carbon steel of 1 to 16 chemical components, the P concentration of the central segregation part was measured for each slab in the case of continuous casting with electromagnetic stirring and the case of continuous casting without electromagnetic stirring. The [P] value was calculated by the equation (1). In addition, each slab is subjected to normal hot rolling to form a hot-rolled coil having a thickness of 5.0 mm, using both the hot-rolled coils as a material, and forming both edges of an open pipe continuously formed into a cylindrical shape by a forming roll. ERW welding was performed using a high-frequency current of 280 KHz to produce an ERW steel pipe having an outer diameter of 38.1 mm and a wall thickness of 5.0 mm. Each of the obtained electric resistance welded steel pipes was subjected to normalizing heat treatment and bending correction processing, and then subjected to an ultrasonic flaw detection test to determine an ultrasonic flaw detection yield. Table 2 shows the results.

[0020]

[Table 1]

[0021]

[Table 2]

As shown in Table 2, the steel of the present invention was subjected to continuous casting with electromagnetic stirring. 1 to 7, the P concentration of the segregated portion was [P] ≦ 部 −0.32 × [C (%)] + 0.24
4%, and the ultrasonic inspection yield was 9
8.0% or more. On the other hand, steel no. 4 Test N of the steel of the present invention and comparative steel
o. 9 to 16, the P concentration of the segregation part is [P] ≦
{−0.32 × [C (%)] + 0.244} was not satisfied, and the ultrasonic flaw detection yield was greatly reduced to 94.1% or less in all cases.

[0023]

The method for producing a high-workability, high-carbon steel ERW pipe according to the present invention is characterized in that a high-carbon steel slab in which central segregation is suppressed by suppressing the amount of P to a low level in accordance with the amount of carbon is hot-rolled. To manufacture high carbon steel ERW pipes by using high-carbon steel hot-rolled coils as a material and performing electric resistance welding to suppress the decrease in yield due to rejection of ultrasonic inspection test due to hook cracks Can be. Further, since the P concentration in the segregated portion is small, even when severe processing such as bulge forming is performed, the possibility of the occurrence of embrittlement cracking in the segregated portion is reduced.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship among C (%) and P (%), reduction RA (%), and the presence or absence of grain boundary embrittlement in steel.

FIG. 2 is an explanatory view of a cause of occurrence of a hot crack in a high carbon steel ERW steel pipe.

[Explanation of symbols]

 1 Edge melting part 2 P segregation part

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[Procedure amendment]

[Submission date] January 6, 1999

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Name of invention

[Correction method] Change

[Correction contents]

[Title of Invention] high formability high carbon steel electric sewing steel tube

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0001

[Correction method] Change

[Correction contents]

[0001]

The present invention relates to relates to a high formability high carbon steel ERW <br/> steel pipe less yield decreased to suppress hot cracking during electric resistance welding.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0006

[Correction method] Change

[Correction contents]

[0006] An object of the present invention is to solve the above-mentioned disadvantages of the prior art, suppress high-temperature cracking during electric resistance welding, and reduce the reduction in product yield in ultrasonic inspection tests. and to provide a steel pipe.

[Procedure amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0007

[Correction method] Change

[Correction contents]

[0007]

High formability high carbon steel electric sewing steel tube according to claim 1 of the present invention In order to achieve the above object, according, C: 0.3~0.6%, Si: 0.15~0.35
%, Mn: 0.3 to 1.5%, S: 0.035% or less, Al: 0.035% or less, P: 0.
Hot-rolled high carbon steel slab containing 012% or less, the balance being Fe and unavoidable impurities, and the P concentration [P] of the continuously-segregated central segregation part satisfies the following formula (1) Ru Tei and high carbon steel hot rolled coils formed by a material. [P] ≦ {−0.32 × [C (%)] + 0.244} ……… (1)

[Procedure amendment 6]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction contents]

[0008] As described above, the high-carbon steel hot-rolled coil obtained by hot rolling a high-carbon steel slab satisfying the above-mentioned formula (1) with the P concentration [P] of the central segregation portion of the high-carbon steel is used as a material. by the lower child, segregation of P is reduced, hot cracks during electric resistance welding hot workability of the segregation area is improved is suppressed, a decrease in product yield in the ultrasonic testing due to hook cracks In addition to being able to suppress, it is possible to improve the workability in a subsequent step.

[Procedure amendment 7]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction contents]

The high carbon steel electric sewing steel pipe of the present invention, P of high carbon steels
The P concentration in the segregated part of the high carbon steel slab
By having a high-carbon steel hot rolled coils with [P] ≦ {-0.32 × [ C (%)] +0.244} and the material, the solidus temperature difference of segregation and the normal portion becomes small, electric resistance welding Hot cracking at the time is suppressed, and the yield of ultrasonic flaw detection can be remarkably improved.

[Procedure amendment 8]

[Document name to be amended] Statement

[Correction target item name] 0022

[Correction method] Change

[Correction contents]

As shown in Table 2, in Test Nos. 1 to 7 in which the steel of the present invention was continuously cast with electromagnetic stirring, the P concentration of the segregated portion was [P] ≦
{−0.32 × [C (%)] + 0.244} was satisfied, and the ultrasonic inspection yield was 98.0% or more in each case. In contrast, test No. 8 of steel No. 4 continuously cast without electromagnetic stirring and steel No. 4
In Test Nos. 9 to 16 of Nos. 9 to 16, the P concentration in the segregation
[P] ≦ {−0.32 × [C (%)] + 0.244} was not satisfied, and the ultrasonic flaw detection yield was greatly reduced to 94.1% or less in all cases.

[Procedure amendment 9]

[Document name to be amended] Statement

[Correction target item name] 0023

[Correction method] Change

[Correction contents]

[0023]

Effects of the Invention high formability high carbon steel electric sewing steel pipe of the present invention,
By the amount of P high carbon steel slabs with suppressed center segregation is suppressed to a low level as a material a high carbon steel hot rolled coils formed by hot rolling, performing electric resistance welding in accordance with the amount of C, the hook the yield reduction due to failure of the ultrasonic flaw detection test by the crack can suppress Wins Rukoto. In addition, since there is little P enrichment in the segregated part, even when subjected to severe processing such as bulging,
The likelihood of embrittlement cracking at the segregated portion is reduced.

Claims (1)

[Claims] 1. C: 0.3-0.6%, Si: 0.15
0.35%, Mn: 0.3-1.5%, S: 0.03
5% or less, Al: 0.035% or less, P: 0.012%
The P concentration of the central segregation part continuously cast in a high carbon steel containing the following, with the balance being Fe and inevitable impurities, [P]
Using a hot-rolled high-carbon steel coil obtained by hot rolling a high-carbon steel slab satisfying the following formula (1) as a raw material. [P] ≦ {−0.32 × [C (%)] + 0.244} (1)