| 摘要 |
PURPOSE:To produce the extra low yield point high tensile strength steel of low yield point ahd superior ductility by cooling the sheet of the specific composition at the critical cooling rate controlled under fixed conditions. CONSTITUTION:The steel containing C; 0.005-0.2%, Mn; 0.3-2.5%, S<0.02%, Al<0.1% as basic components is contained, if necessary, independently or in combination with three kinds of additives, one or two more kinds of Si<2%, Cr<1%, Mo<0.5%, Ni<0.7%, Cu<0.7%, B<0.008%, P<0.2%, one or two or more kinds of rare earth metals, Ca and Zr at 0.1% or less, one or two or more kinds of Nb and TiV at 0.1%. The process of heating the thin film of the steel of the abovementioned composition to the temperature between Ac1-Ac3 transformation point then cooling the same at above the critical cooling rate CR determined by formulas (1)(2) is repeated more than twice. This yields the extra low yield point high tensile strength steel plate having a yield stress of 20kg/mm<2> or more and a yield ratio of 50% or less. |
