PURPOSE: To produce efficiently mechanical structural parts having high quality with a simplified process and lessened manpower by heating and hot rolling a steel contg. a specific ratio of C, Si, Mn, Nb, solAl, B and N under specific conditions.

CONSTITUTION: A steel contg., by weight %, 0.04W0.10% C, ≤0.50% Si, 0.80W 2.50% Mn, 0.01W0.10% Nb, 0.01W0.05% solAl, 0.0005W0.0035% B, and ≤0.0045% N and consisting of the balance Fe and unavoidable impurities is heated to 1,000W1,200°C. The steel is then hot rolled at ≥60% cumulative draft at 850W 1,000°C and a finishing temp. of 800W950°C. The steel for machine structural parts having combined high strength and good toughness is thus obtd. without requiring a softening treatment prior to cold working and a hardening treatment in the stage of tempering.