PURPOSE: To produce a high tensile strength heat treated steel plate excellent in welding performance characteristic and discrimination characteristic by subjecting a slab of a low carbon steel with a specific composition to hot rolling, to cooling under specified temp. condition, or further to heat treatment under specified conditions after reheating.

CONSTITUTION: A slab of a low carbon steel having a composition which contains, by weight, 0.03-0.12% C, <0.60% Si, <2.0% Mn, 0.001-0.1% Al, <0.006% N, and further one or ≥2 kinds among <1.0% Cr, <1.0% Ni, <1.0% Mo, <0.1% V, <0.1% Ti, <0.1% Nb, <0.005% B, <1.0% Cu, and <0.1% rare earth metal element and in which PCM(wt.%) represented by an equation I is regulated to ≤0.22% is hot-rolled and, after hot rolling is finished at (Ar3 point + 100°C), the resulting hot rolled plate is successively cooled from a temp. not lower than the Ar3 point at a cooling velocity not lower than air cooling velocity, or further, this hot rolled plate is reheated to a temp. not lower than the Ac3 point, cooled at a cooling velocity not lower than air cooling velocity, heated and held to and at temps. in the two phase region between the Ac3 point and the Ac1 point, cooled at a cooling velocity not lower than air cooling velocity, and successively subjected to tempering treatment at a temp. in the region not higher than the Ac1 point.