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...

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Benefits of technology

[0007] An object of the present invention is to provide a high strength steel plate for line pipe having excellent HIC resistance not only against HIC due to center segregation but also against HIC generated at near surface or around inclusions, at low cost without adding large amount of alloying elements.

[0010] The above-described ferrite contains fine precipitates having particle size of 30 nm or smaller. Ferrite has excellent toughness and HIC resistance, while, normally low strength to give low hardness. Accordingly, when the steel consists of ferrite and bainite, the large difference in hardness between ferrite and bainite makes the interface therebetween act as an origin of crack and as a route of crack propagation, thus HIC resistance becomes poor. For the above-described high strength steel plate, HIC resistance is improved by reducing the hardness difference between ferrite and bainite to a specific level or below. The hardness difference can be reduced by increasing hardness of ferrite. That is, the hardness difference between ferrite and bainite can be decreased by strengthening the ferrite by finely dispersing precipitates. If, however, particle size of precipitates exceeds 30 nm, the strengthening of ferrite by dispersing precipitates becomes insufficient, failing in reducing the hardness difference between ferrite and bainite, thus the particle size of precipitates is specified to 30 nm or smaller. In addition, to strengthen the ferrite more effectively by adding smaller amount of alloying elements and also to establish excellent HIC resistance, the particle size of precipitates is preferably 10 nm or smaller, and more preferably 5 nm or smaller.

[0015] In the above-described steel plate, further high strength increase effect is attained by co-addition of Mo and Ti to finely precipitate complex carbides containing basically Mo and Ti in steel compared with the case of MoC and / or TiC precipitation strengthening. The strong effect for strength increase is attributed to fine precipitates having particle size of 10 nm or smaller.

[0027] The present invention also provides a method for manufacturing a high strength steel plate having yield strength of 448 MPa or higher comprising the steps of: hot rolling; accelerated cooling; and reheating.

[0032] Furthermore, the present invention provides a method for manufacturing a high strength steel plate having yield strength of 448 MPa or higher comprising the steps of: hot rolling; accelerated cooling; and reheating.

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.

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