| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a non-heat-treated, low-yield ratio, high-tensile thick steel plate having a tensile strength of ≥550 MPa. <P>SOLUTION: The steel plate has a composition comprising, by mass%, 0.05-0.16% C, 1.2-1.8% Mn, 0.005-0.025% Nb, 0.005-0.020% Ti and ≥2.5 Ti/N, has a microstructure comprising ferrite and a hard phase at least on a surface, has an average particle size of ferrite at the surface of 4.0-18.0 μm, has average hardness at the surface of ≤225 HV and has a hardness distribution in the plate thickness direction where the difference of hardness between the surface and a central part of the plate thickness is ≤60 HV. The non-heat-treated, low-yield ratio, high-tensile thick steel plate is excellent in surface ductility and toughness after cold machining. <P>COPYRIGHT: (C)2012,JPO&INPIT |
