Patent ID: 9657868
Date: 2017-05-23
CPC Classifications: B23K,C21D,C22C,F16L

Claim:
1. A high strength steel plate having a low yield ratio, the steel plate having a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a metallographic structure including a three-phase structure of mainly a polygonal ferrite phase and a bainite phase in which island martensite is dispersed in surface portions within 5 mm from the upper and lower surfaces, and a duplex-phase structure of mainly a bainite phase in which island martensite is dispersed in a central portion which is other than the surface portions, wherein an area fraction of island martensite is 3% or more and 15% or less, wherein an equivalent circle diameter of the island martensite is 3.0 μm or less, wherein the area fraction of the polygonal ferrite phase in the surface portions is 10% or more and less than 80%, and wherein the remainder of the metallographic structure consists of a bainite phase, a hardness variation in a thickness direction of ΔHV30 or less in terms of Vickers hardness, a hardness variation in a width direction of ΔHV30 or less in terms of Vickers hardness, a maximum hardness in surface portions of the steel plate of HV230 or less in terms of Vickers hardness and a yield ratio of 85% or less and an elongation of 22% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards.