LOW-CARBON FREE CUTTING STEEL CAPABLE OF OBTAINING SUPERIOR MACHINABILITY BY LIMITING RANGE OF ALLOY CONSTITUTIONS AND ADJUSTING SHAPE OF INCLUSIONS AND IMPROVING CARBURIZING CHARACTERISTICS BY ADJUSTING CONTENT OF Si OR Cr

摘要

PURPOSE: To provide a low-carbon free cutting steel which is excellent in machinability, hot workability and surface characteristics of the tools after cutting and is manufactured inexpensively particularly in case of machining using cemented carbide tools, and a low-carbon free cutting steel with superior carburizing characteristics in addition to the above characteristics. CONSTITUTION: A low-carbon free cutting steel contains 0.05 mass% to less than 0.20 mass% of C, 0.4 to 2.0 mass% of Mn, 0.21 to 1.0 mass% of S, 0.002 to 0.10 mass% of Ti, 0.001 to 0.30 mass% of P, 0.2 mass% or less of Al, 0.001 to 0.03 mass% of O(oxygen), 0.0005 to 0.02 mass% of N and the balance being Fe and impurities, wherein impurities contained steel satisfy the following expressions (i) and (ii): £Expression (i)| (A+B)/C>=0.8 £Expression (ii)| NA>=5 wherein meanings of A, B, C and NA are as follows: A: the total area occupied by substantial MnS inclusions containing Ti carbide and/or Ti carbonitride among inclusions having 1μm or more of circle equivalent diameter per 1 mm¬2 of a cross section parallel to a rolling direction, B: the total area occupied by substantial MnS inclusions containing Ti carbide and Ti carbonitride among inclusions having 1μm or more of circle equivalent diameter per 1 mm¬2 of a cross section parallel to a rolling direction, C: the total area occupied by all inclusions having 1μm or more of circle equivalent diameter per 1 mm¬2 of a cross section parallel to a rolling direction, and NA: the number of substantial MnS inclusions containing Ti carbide and/or Ti carbonitride among inclusions having 1μm or more of circle equivalent diameter per 1 mm¬2 of a cross section parallel to a rolling direction.