Steel sheet for submerged arc welding

Abstract

A steel sheet for submerged arc welding, comprising, by mass, 0.03 to 0.09% C, 1.5 to 2.5% Mn, 0.005 to 0.025% Nb, 0.005 to 0.02% Ti, 0.01 to 0.06% Al, 0.0005 or less B, 0.001 to 0.008% N, 0.015% or less P, 0.015% or less S, 0.006% or less O and the balance Fe and impurities. This steel sheet for submerged arc welding exhibits excellent low-temperature toughness in the area bordering the base material and the area close to molten line of HAZ, and also exhibits excellent low-temperature toughness in the base material and welded metal.

Description

technical field [0001] The present invention relates to a steel plate for submerged arc welding. More specifically, the present invention relates to a steel plate for submerged arc welding suitable for a steel pipe base material of a welded steel pipe used in pipelines for transporting oil and natural gas. Background technique [0002] In pipelines for transporting oil and natural gas, there is a constant need to improve transport efficiency. The transportation efficiency can be improved by increasing the operating pressure, but in order to increase the operating pressure, it is necessary to increase the strength of the welded steel pipes constituting the pipeline. Among the methods for increasing the strength of the welded steel pipe, there are methods of improving the strength of the steel pipe material itself and methods of increasing the thickness of the welded steel pipe. [0003] When the strength is increased by thickening the welded steel pipe, the following two pro...

AI Technical Summary

Problems solved by technology

First, the increase in wall thickness of welded steel pipes leads to a large amount of heat input during pipe welding, thereby reducing the toughness of the HAZ due to the coarsening of the structure of the welded heat affected zone (Heat Affected Zone: hereinafter referred to as HAZ)

Second, the toughness of the HAZ and the base material decreases due to the increased plastic restraint of the thick wall

[0008] However, in the steels disclosed in these documents, the low-temperature toughness of the weld metal in the case of submerged arc welding is not measured.

Therefore, even if the low temperature toughness of HAZ is good, when welding by submerged arc welding, the low temperature toughness of the weld metal may decrease

In addition, the HAZ needs to have excellent low-temperature toughness not only in the vicinity of the welding line, but also in the interface with the base metal (hereinafter referred to as the outer layer of the HAZ). Evaluate the low temperature toughness of both sides of the HAZ outer layer

Therefore, even if the portion near the melting line has good low-temperature toughness, the low-temperature toughness may decrease in the outer layer of the HAZ

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