Bend pipe and process for producing the same

Abstract

A high-strength hot bend pipe which has a balance between an excellent strength of at least X70 grade and toughness and which has excellent tensile properties and a weld metal with excellent low temperature toughness is manufactured. A UOE steel pipe having a base metal with a composition of C: 0.03-0.12%, Si: 0.05-0.50%, Mn: 1.4-2.2%, S: at most 0.01 %, Al: at most 0.06%, N: at most 0.008%, and a remainder of Fe and impurities, with the carbon equivalent (Ceq) being at most 0.36% and the weld cracking parameter (Pcm) being at most 0.22%, and having a weld metal with a weld cracking parameter(Pcm) of at most 0.28%, a B content of at most 5 ppm, and an O content of at most 280 ppm is heated to a temperature range of 900-1100 DEG C and subjected to bending, and then is immediately cooled to a temperature range of 300 DEG C or lower at a cooling rate of at least 3 DEG C / sec, and then is tempered in a temperature range of 300-500 DEG C.

Description

technical field [0001] The present invention relates to bent pipes and methods for their manufacture. More specifically, the present invention relates to a bent pipe that uses a welded steel pipe having a welded portion facing the axial direction as a raw material, has strength of API standard X70 or higher, and has excellent low-temperature toughness of the weld metal, and a method for manufacturing the same. Background technique [0002] In recent years, there has been an increasing demand for reducing pipeline costs. In addition, due to advances in manufacturing technology, high-strength steel pipes are used for pipelines. Most of the high-strength steel pipes (straight pipes) used for pipelines meet API specification X70 grade. There are only a few cases of high-strength steel pipes with API specification X80 in the world. In particular, bent pipes (curved pipes) produced by bending welded steel pipes having welded portions facing the axial direction are indispensable...

AI Technical Summary

Problems solved by technology

[0007] However, in the inventions disclosed in Patent Documents 1 and 2, it is very difficult to ensure the yield strength on the compression deformation side (referred to as "abdominal side" in this specification) that is the inner side of the bent portion.

To be precise, in order to ensure the low-temperature toughness of the weld metal, it is necessary to reduce the Pcm and optimize the composition, but if tempering is not performed after bending, it is difficult to ensure the yield strength on the belly side of the bend

[0008] In addition, if tempering is performed at a high temperature of 500°C or higher, the tensile strength will be low

In order to compensate for the decrease in tensile strength, it is necessary to increase the content of alloy elements in the base metal, which increases the cost and makes it difficult to use for pipelines

[0009] In addition, with the increase in strength, it is difficult to ensure the low-temperature toughness of the weld metal in the bent portion

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