Metallurgical consideration and elementary performance tests have been conducted of low yield ratio HT80 steels made by quenching process from α+γ phases termed Q'. Nine kinds of steels subjected to various heat treatments and their welds are used for the present experiments such as tensile tests, Charpy test and bending COD test.
By using TMCP process followed by Q'-quenching and tempering, about 85% of yield ratio of HT80 steel can be realized even if the chemical composition is same as one of conventional HT80 steel. It is made clear from metallurgical observations that the reason why that kind of steel becomes low yield ratio is due to the existence of pure ferrite in the matrix of tempered martensite. The CR-Q'-T steel proposed in the present paper has higher deformability and fracture toughness than those of conventional HT80 steel.
