JP3120709B2 - Manufacturing method of welded steel pipe - 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 a welded steel pipe, and more particularly to a method for preventing hot cracking of a weld metal and liquefaction cracking of a weld heat affected zone during seam welding of a steel pipe welded and manufactured after cold forming. The present invention relates to a method for producing a welded steel pipe by submerged welding.

[0002]

2. Description of the Related Art Conventionally, a steel pipe is formed by forming a steel sheet into a tube by a UOE process, a roll bend, a press bend, or the like, and welding an end face butt portion from an inner surface to an outer surface.

[0003] When a steel pipe is manufactured by such a method, depending on the type of steel (especially when steel having a high C content is used) and the welding method, hot cracking of the weld metal and liquefaction cracking of the weld heat affected zone may occur. May occur. The hot crack is a crack generated along the dendrite of the weld metal, and the liquefaction crack is a crack caused by melting of the crystal grain boundary of the weld heat affected zone by welding heat and heat shrinkage during cooling.

[0004] As a method of preventing hot cracking in welding, for example, JP-A-58-53377, JP-A-60-18280,
There are a method for controlling the chemical composition and structure of the weld metal, and a method for controlling the welding method and welding conditions as disclosed in JP-A-61-9979. Further, as disclosed in Japanese Patent Application Laid-Open No. 2-1269, there has been proposed a method of mechanically relaxing solidification shrinkage causing hot cracking to prevent cracking.

[0005]

However, all of these techniques take into account only hot cracking of the weld metal, and a method for simultaneously preventing both hot cracking of the weld metal and liquefaction cracking of the weld heat affected zone. Is not shown.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and in a steel pipe containing carbon as high as 0.25 to 0.55% by weight, hot cracking and welding of a weld metal on the premise of submerged welding. An object of the present invention is to provide a method for manufacturing a welded steel pipe capable of simultaneously preventing both liquefaction cracks in a heat-affected zone.

[0007]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a method of manufacturing a steel pipe using steel containing 0.25 to 0.55% by weight of C. C
The difference in the amount, C (base metal) -C (weld metal) is 0.25% or less, and the oxygen amount in the weld metal is 350 ppm or more and 700 pp
m, welding heat input (HI, kJ / cm) and welding speed (W
S, cm / min) satisfy the following equation, and the welding current is 1
Provided is a method for manufacturing a welded steel pipe, characterized by performing submerged welding under a condition of 000 A or less.

The present inventors performed submerged arc welding under various conditions using a steel having a carbon content of 0.25 to 0.55% to obtain high-temperature cracking of a weld metal portion and the effect of welding heat. The portion was examined for liquefaction cracks. Oxygen content in the weld metal is 350ppm or more and 700p
FIG. 1 shows the effect of welding heat input and welding speed on the occurrence of hot cracking in a weld metal of less than pm. In the figure, a black circle indicates that hot cracking of the weld metal has occurred, and a white circle indicates that no hot cracking of the weld metal has occurred. As shown in this figure, to prevent hot cracking of the weld,
It is necessary to control the welding heat input and the welding speed, and as the welding heat input increases, the welding speed, which is the limit of the occurrence of hot cracking, tends to decrease, and the limit can be expressed by the following equation (1). I found it.

WS ≦ −1.5 HI + 95 (1) In addition, the occurrence of liquefaction cracks in the heat-affected zone of the weld is determined by the C of the weld metal.
It has been found that the difference between the content and the C content of the base metal has an effect, and that controlling this difference can prevent such a difference. The present invention has been completed based on such findings of the present inventors.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. In the present invention, it is assumed that a steel pipe is manufactured using steel containing 0.25 to 0.55% by weight of C, and the difference between the amounts of C in the base metal and the weld metal, C (base metal) -C (weld Metal) is less than 0.25% and the oxygen content in the weld metal is 350 ppm
More than 700 ppm, welding heat input (HI, kJ / cm)
And the welding speed (WS, cm / min) satisfy the following expression (1), and perform the submerged welding under the condition that the welding current is 1000 A or less.

WS ≦ −1.5 HI + 95 (1) Here, the reason why the steel pipe containing 0.25 to 0.55% of C is used to manufacture a steel pipe is limited to such a high carbon content.
This is because, particularly in the content, high-temperature cracking of the weld metal portion and liquefaction cracking of the weld heat-affected zone pose a problem.

Difference in C content between base metal and weld metal, C (base metal) −
The reason why C (weld metal) is set to 0.25% or less is that if the C content exceeds this, liquefaction cracking of the heat affected zone occurs. The lower limit does not need to be specified.

The condition (1) that the condition is satisfied is as follows.
This is because if the welding speed WS exceeds -1.5 HI + 95, hot cracking of the weld metal occurs. The lower limit of the welding speed does not need to be particularly limited. The welding heat input does not need to be particularly limited as long as the above formula (1) is satisfied, but is 5 to 40 kJ / cm.
The degree is a preferred range.

The amount of oxygen in the weld metal is 350 ppm or more.
The reason why the oxygen content in the weld metal is less than 35 ppm is less than 00 ppm.
If it is less than 0 ppm, the tendency of hot cracking is increased, and it becomes difficult to prevent hot cracking even if welding is performed under welding conditions satisfying the formula (1). On the other hand, if it is more than 700 ppm, the toughness of the weld metal deteriorates. is there.

The reason why the welding current is set to 1000 A or less is that if the welding current exceeds this value, it becomes difficult to prevent welding hot cracking. From the viewpoint of preventing welding hot cracking, it is not necessary to specify the lower limit of the welding current.

According to the present invention, 0.25-0.
Even if a steel containing a high C such as 55% is used, seam welding of a steel pipe can be performed without causing high-temperature cracking of the weld metal and liquefaction cracking of the weld heat-affected zone.

[0017]

EXAMPLE A welded steel pipe was manufactured by submerged arc welding using a base metal and a weld metal having the carbon contents shown in Table 1, and the welded portions were examined. Table 1 shows the difference between the C content of the base metal and the weld metal, the oxygen content, the welding current, the welding heat input, the welding speed, the welding speed at which no hot cracking occurs,
The results of the evaluation of the weld are also shown.

[0018]

[Table 1]

The difference in the amount of carbon between the weld metal and the base metal, the amount of oxygen in the weld metal, and the welding conditions satisfy the conditions of the present invention.
In 1,4,5,8,9,10, no high-temperature cracking of the weld metal or liquefaction cracking of the weld heat affected zone was observed, and a sound steel pipe was obtained.

On the other hand, the difference in C content is 0.25%
No. exceeding In No. 2, liquefaction cracking of the heat affected zone was observed. In addition, No. 3 having an oxygen content of less than 350 ppm. No. 3, welding current exceeding 1000A. No. 6, the welding speed exceeds the limit welding speed. In No. 7, hot cracking of the weld metal occurred. Further, No. 2 having a large amount of oxygen in the weld metal. No. 11 shows that the weld metal has a low toughness value.

[0021]

As described above, according to the present invention,
Using steel having a carbon content of 0.25 to 0.55% by weight, a welded steel pipe can be manufactured without causing high-temperature cracking of the weld metal or liquefaction cracking of the weld heat-affected zone.

[Brief description of the drawings]

FIG. 1 is a diagram showing the influence of welding heat input and welding speed on hot cracking of weld metal.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Houji Mifune 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-58-77778 (JP, A) JP-A-2 JP-A-99279 (JP, A) JP-A-54-107450 (JP, A) JP-A-63-43773 (JP, A) JP-A-5-337648 (JP, A) (58) Fields investigated (Int. . ^7, DB name) B23K 9/18 B21C 37/08 B23K 9/095 B23K 35/30

Claims (1)

(57) [Claims] When producing a steel pipe using steel containing 0.25 to 0.55% by weight of C, C of a base metal and a weld metal is used.
The difference in the amount, C (base metal) -C (weld metal) is 0.25% or less, and the oxygen amount in the weld metal is 350 ppm or more and 700 pp
m, welding heat input (HI, kJ / cm) and welding speed (W
S, cm / min) satisfy the following equation, and the welding current is 1
A method for manufacturing a welded steel pipe, comprising performing submerged welding under conditions of 000 A or less. WS ≦ −1.5HI + 95