Free machining steel for machine structural use having improved chip disposability

Abstract

Disclosed is a free machining steel for machine structural use, which can realize very good chip disposability without incorporation of any harmful material such as lead (Pb). This free machining steel is a hot rolled or hot forged free machining steel having improved chip disposability. The free machining steel comprises, by mass, carbon (C): 0.01 to 0.70%, silicon (Si): 0.05 to 2.00%, manganese (Mn): 0.20 to 3.50%, calcium (Ca): 0.0003 to 0.01%, sulfur (S): 0.020 to 0.300%, aluminum (Al): 0.002 to 0.300%, nitrogen (N): 0.003 to 0.035%, and oxygen (O): 0.0010 to 0.0080% with the balance consisting of iron (Fe) and unavoidable impurities. The steel comprises sulfides having the major axis of from 0.5 μm to 20 μm, exclusive, and comprising MnS as the main component, in a number of not less than 30% of the total number of sulfides. The steel also comprises oxide inclusions having the major axis of from 0.5 μm to 50 μm, exclusive, and being present either together with sulfides or singly, in a number of not less than 10 per mm 2 of the inspection area.

1 . A free machining steel having improved chip disposability, wherein the steel is hot rolled or hot forged and comprises, by mass, 
 carbon (C): 0.01 to 0.70%,    silicon (Si): 0.05 to 2.00%,    manganese (Mn): 0.20 to 3.50%,    calcium (Ca): 0.0003 to 0.01%,    sulfur (S): 0.020 to 0.300%,    aluminum (Al): 0.002 to 0.300%,    nitrogen (N): 0.003 to 0.035%, and    oxygen (O): 0.0010 to 0.0080%,    the balance consisting of iron (Fe) and unavoidable impurities,    wherein the steel comprises sulfides having the major axis of from 0.5 μm to 20 μm, exclusive, and comprising MnS as the main component, in a number of not less than 30% of the total number of sulfides, and    wherein the steel comprises oxide inclusions having the major axis of from 0.5 μm to 50 μm, exclusive, and being present either together with sulfides or singly, in a number of not less than 10 per mm 2  of the inspection area.    
     
     
         2 . The free machining steel according to  claim 1 , which further comprises one or more members selected from, by mass, chromium (Cr): 0.20 to 2.50%, molybdenum (Mo): 0.05 to 1.50%, nickel (Ni): 0.05 to 3.50%, vanadium (V): 0.01 to 0.50%, niobium (Nb): 0.01 to 0.10%, and titanium (Ti): 0.01 to 0.50%.  
     
     
         3 . The free machining steel according to  claim 1  or  2 , which further comprises one or more members selected from, by mass, magnesium (Mg): 0.0003 to 0.01%, zirconium (Zr): 0.0005 to 0.30%, bismuth (Bi): 0.01 to 0.30%, and boron (B): 0.0003 to 0.015%.