High strength steel plate and method for production thereof

Abstract

The high strength steel plate according to the present invention contains 0.02 to 0.08% C, by mass, and has substantially a two phase microstructure of ferrite and bainite. The ferrite contains precipitates having particle size of 30 nm or smaller grain size. The steel plate has yield strength of 448 MPa or higher. The method for manufacturing the high strength steel plate comprises the steps of hot rolling, accelerated cooling, and reheating. The accelerated cooling is conducted down to the temperature of 300 to 600° C. at a cooling rate of 5° C. / s or higher. The reheating is conducted up to temperature of 550 to 700° C. at a heating rate of 0.5° C. / s or higher.

Description

TECHNICAL FIELD [0001] The present invention relates to a steel plate having excellent resistance to hydrogen-induced cracking (HIC resistance), used for manufacturing a steel pipe or the like, and to a method for manufacturing the same. BACKGROUND ART [0002] Line pipes used for transporting crude oil or natural gas, containing hydrogen sulfide, are required to have what is called “sour-resistance” including HIC resistance and resistance to stress corrosion crack (SCC resistance), adding to strength, toughness, and weldability. The phenomenon of hydrogen-induced cracking (HIC) of steel is based on a process in which hydrogen ions generated by corrosion reaction are adsorbed on the surface of steel, penetrate into steel as atomic hydrogen, and diffuse and accumulate around non-metallic inclusions such as MnS and hard second phase of steel, thus triggering crack initiation by an increase in internal pressure. [0003] To prevent that type of HIC, JP-A-54-110119, (the term “JP-A” referre...

AI Technical Summary

Benefits of technology

The present invention provides a high strength steel plate with excellent resistance to HIC (hydrogen-induced cracking) at low cost without adding large amounts of alloying elements. The steel plate has a yield strength of 448 MPa or higher and consists of a two phase microstructure of ferrite and bainite. The hardness difference between ferrite and bainite is reduced to a specific level or below, which improves HIC resistance. The steel plate contains fine precipitates with particle size of -30 nm or smaller, which enhance toughness and HIC resistance. The hardness difference between bainite and ferrite is preferably -70 or smaller in Vickers scale (HV) to maintain excellent HIC resistance. The bainite has a hardness of HV 320 or smaller to avoid formation of MA (martensite-austenite constituent) that promotes cracking. The area percentage of bainite is preferably 10 to 80% to ensure sufficient HIC resistance. The ratio of C content to the sum of Mo and Ti contents by atom percentage is important to achieve fine precipitates with particle size of -10 nm or smaller, which strengthen the steel plate.

Problems solved by technology

If, however, C content is less than 0.02%, sufficient strength cannot be attained, and if C content exceeds 0.08%, toughness and HIC resistance degrade.

Images