Process for production of 780mpa-grade high-tensile-strength steel plates excellent in low-temperature toughness

Abstract

A process for the production of 780PMa-grade high-tensile -strength steel plates excellent in low-temperature toughness, which comprises heating a steel bloom which contains by mass C: 0.06 to 0.15%,Si: 0.05 to 0.35%, Mn: 0.60 to 2.00%, P: 0.015% or less, S: 0.015% or less, Cu: 0.1 to 0.5%, Ni: 0.1 to 1.5%, Cr: 0.05 to 0.8%, Mo: 0.05 to 0.6%, Nb: less than 0.005%, V: 0.005 to 0.060%, Ti: less than 0.003%, Al: 0.02 to 0.10%, B: 0.0005 to 0.003%, and N: 0.002 to 0.006% to a temperature of 1050 to 1200 DEG C; completing the hot rolling of the bloom at 870 DEG C or above; cooling, after a lapse of 10 to 90 seconds, the obtained plate at a cooling rate of 5 DEG C / s or above from a temperature of 840 DEG C or above to a temperature of 200 DEG C or below; and then tempering the resulting plate at a temperature of 450 to 650 DEG C for 20 to 60 minutes.

Description

technical field [0001] The present invention relates to a method for manufacturing a 780 MPa grade high-tensile steel plate for steel for marine structures and pressure water pipes, which is excellent in low-temperature toughness. Background technique [0002] It is considered to be an effective method to refine the quenched structure (lower bainite or martensite) for producing a steel material having a tensile strength of 780 MPa class and excellent low-temperature toughness. In order to refine the quenched structure, it is necessary to refine the austenite grain size before cooling the steel material before becoming the quenched structure. [0003] Especially in the case of direct quenching (DQ) manufacturing, the austenite grain size can be controlled by controlled rolling, and the austenite grain size before the quenching structure can be made by rolling in the austenite recrystallization zone. diameter miniaturization. [0004] However, it is difficult to grasp the au...

AI Technical Summary

Problems solved by technology

However, as described in Japanese Patent Application Laid-Open No. 2000-8135, if the steel contains 0.2% or more of C for the purpose of ensuring strength, TiC, which is a very hard particle, will be formed in the base material and welded part, which may degrade the toughness. The problem

[0010] As mentioned above, the reality is that no method for manufacturing 780MPa-class high-tensile steel sheet that is Nb-free and Ti-free and has both high strength and excellent low-temperature toughness has not been proposed so far.

Images