Crankshaft

Abstract

A crankshaft is made of steel wherein straightening can be carried out with ease even though normalizing after hot forging is eliminated, and wherein cutting work can be carried out with ease even though lead (Pb) content of the steel is low. The steel is to be treated with ferritic nitrocarburizing after being machined to a crankshaft, and contains following chemical compositions: C: not less than 0.35 mass % (mass percentage) and not more than 0.45 mass %, Si: not less than 0.1 mass % and not more than 0.4 mass %, Mn: not less than 0.4 mass % and not more than 0.7 mass %, S: not less than 0.04 mass % and not more than 0.07 mass %, Ca: not less than 0.0005 mass % and not more than 0.0050 mass %, Ti: not less than 0.0050 mass % and not more than 0.0120 mass %, N: not less than 0.0042 mass % and not more than 0.0480 mass %, and balanced Fe and inevitable impurities, wherein Ti/N (the ratio of mass percentages of Ti and N) is controlled in the range of not less than 0.25 to not more than 1.2, and Pb is regulated to not more than 0.03 mass %. In addition, the cross-sectional microstructure except the ferritic nitrocarburized layer consists of ferrite+pearlite. In this ferrite+pearlite microstructure, the total number of sulfides, the size of which exceed 1 μm (micro-meters) observed per 160 mm 2 of the viewing field, is counted to be not less than 12,000 and not more than 31,000, and the averaged grain size of pearlite lies in the range of not less than 14 μm to not more than 20 μm.

1 . A crankshaft made of steel and treated with ferritic nitrocarburizing, the steel containing: 
 C: not less than 0.38 mass % (mass percentage) and not more than 0.42 mass %;    Si: not less than 0.15 mass % and not more than 0.35 mass %,    Mn: not less than 0.45 mass % and not more than 0.6 mass %,    S: not less than 0.04 mass % and not more than 0.06 mass %,    Ca: not less than 0.0010 mass % and not more than 0.0050 mass %,    Ti: not less than 0.0050 mass % and not more than 0.0120 mass %,    N: not less than 0.0042 mass % and not more than 0.0480 mass %, and    balanced Fe and inevitable impurities, wherein Ti/N (the ratio of mass percentages of Ti and N) exists in the range of not less than 0.25 but not more than 1.2, wherein microstructure of a cross-section except the ferritic nitrided layer consists of ferrite+pearlite, and the total number of sulfides, the size of which is not less than 1 μm (micro-meters), observed in said ferrite+pearlite microstructure is in the range of not less than 12,000 to not more than 31,000 per 160 mm 2  of viewing fields,    and wherein the averaged grain size of pearlite consisting said ferrite+pearlite is in the range of not less than 14 μm to not more than 20 μm.    
     
     
         2 . The crankshaft according to  claim 1 , wherein said steel contains Pb (lead), the content of which is not more than 0.03 mass %.  
     
     
         3 . The crankshaft according to  claim 1 , wherein said steel contains O (oxygen), the content of which is not less than 0.0005 mass % and not more than 0.01 mass %.  
     
     
         4 . The crankshaft according to  claim 2 , wherein said steel contains O (oxygen), the content of which is not less than 0.0005 mass % and not more than 0.01 mass %.  
     
     
         5 . The crankshaft according to  claim 1 , wherein WS/WO of said steel is controlled to be not less than 8 and not more than 50, where WS represents the sulfur content in mass percentage and WO represents the oxygen content in mass percentage.  
     
     
         6 . The crankshaft according to  claim 2 , wherein WS/WO of said steel is controlled to be not less than 8 and not more than 50, where WS represents the sulfur content in mass percentage and WO represents the oxygen content in mass percentage.  
     
     
         7 . The crankshaft according to  claim 3 , wherein WS/WO of said steel is controlled to be not less than 8 and not more than 50, where WS represents the sulfur content in mass percentage and WO represents the oxygen content in mass percentage.  
     
     
         8 . The crankshaft according to  claim 4 , wherein WS/WO of said steel is controlled to be not less than 8 and not more than 50, where WS represents the sulfur content in mass percentage and WO represents the oxygen content in mass percentage.