High-tension steel plate excellent in base metal toughness and HAZ toughness

摘要

The steel plate of the invention is a high-tension steel plate that has a tensile strength of 1100 MPa or more and is excellent in base metal toughness and HAZ toughness and preferably in abrasion resistance. The steel plate satisfies a predetermined requirement of components in the steel. The Ceq (IIW) represented by the following equation ranges from 0.40 to 0.45 both inclusive:
Ceq(IIW)=[C]+{⅙×[Mn]}+{⅕×([Cr]+[Mo]+[V])}+{ 1/15+([Cu]+[Ni])}
in which each parenthesis-symbol [ ] means the content by percentage of an element in the parentheses. Oxide grains having a maximum diameter of 2 μm or less are present in a number density of 200/mm2 or more in the steel. The steel is composed of 29% or more by volume of martensite microstructure, and bainite microstructure as the balance.

主权项

1. A steel plate, comprising the following as components in the steel:
C: 0.10 to 0.16% provided that the symbol “%” means “% by mass” and hereinafter the same matter is applied to any symbol “%” described in connection with each of the components, Si: 0.2 to 0.5%, Mn: 1 to 1.4%, P: 0.03% or less, S: 0.01% or less, Al: 0.010 to 0.08%, Cr: 0.03 to 0.25%, Mo: 0.25 to 0.4%, Nb: 0.01 to 0.03%, B: 0.0003 to 0.002%, N: 0.006% or less, REMs: 0.0005 to 0.0030%, Zr: 0.0003 to 0.0020%, Fe and one or more inevitable impurities as the component-balance of the steel, wherein the Ceq (IIW) represented by the following equation ranges from 0.40 to 0.45 both inclusive:
Ceq(IIW)=[C]+{⅙×[Mn]}+{⅕×([Cr]+[Mo]+[V])}+{ 1/15+([Cu]+[Ni])}in which each parenthesis-symbol [ ] means the content by percentage of one of the elements in the parentheses, and
grains of one or more oxides that each have a maximum diameter of 2 μm or less are present in a number density of 200/mm2 or more in the steel, martensite microstructure is contained in a proportion of 29% or more by volume, and the microstructure-balance of the steel is bainite microstructure, the steel plate having a plate thickness of 6 mm or more and a tensile strength of 1100 MPa or more.