PURPOSE:To produce the high-strength steel plate having a low yield ratio by applying the specific heat treatment of low carbon steel material to make its structure to ferrite and low temperature transformed structure. CONSTITUTION:The steel containing C; 0.01-0.15%, Si<3.0%, Mn; 1.0-4.0%, S<0.015%, sol.Al; 0.001-0.10%, or the steel further containing, if necessary, one or more kinds of Cu<1.0%, Ni<1.0%, Cr<1.0%, Mo<0.4%, Nb<0.10%, V<0.15%, Ti<0.15%, Zr<0.10%, rare earth elements <0.10%, Ca<0.01%. This steel material is heated to the temperature between the AC1 point and AC3 point, after which the co-existence temperature region of ferrite and austenite is slow cooled as far as possible at a cooling rate of 10 deg.C/sec. or less. Next, in order to cause low-temperature transformation of the austenite portion, it is quenched at a cooling rate of 30 deg.C/sec. or more. This yields the steel material for automobiles of tensile strength 50kg/mm<2> or more and a low yield ratio.