LOW ALLOY STEEL FOR STEEL FORGING AND CRANK SHAFT

PROBLEM TO BE SOLVED: To provide a low alloy steel for steel forging, having yield strength and toughness required for large steel forging and excellent in machinability.SOLUTION: The low alloy steel for steel forging of the present invention has a composition containing as a basic component, C:0.30 mass% to 0.40 mass%, Si:over 0 mass% to 0.45%, Mn:0.50 mass% to 1.90 mass%, Ni:0.70 mass% to 2.00 mass%, Cr:1.00 mass% to 2.30 mass%, Mo:over 0 mass to 0.35 mass%, V:over 0 mass% to 0.20 mass% and the balance Fe with inevitable impurities, 17% or more of a bainite structure, a martensite structure or a combination structure thereof and the balance a pearlite structure, a ferrite structure or a combination structure thereof and satisfies following formulation. (1.5×Mn+2×Ni)/Cr