Crankshaft and method for manufacturing same

Abstract

A surface of a steel, as a material for a crankshaft, is nitrocarburized. The steel contains, as alloy elements C having a content 0.10 mass % or more 0.30 mass % or less, Si having a content 0.5 mass % or more and 0.3 mass % or less, Mn having a content 0.3 mass % or more and 1.5 mass % or less, Mo having a content 0.8 mass % or more and 2.0 mass % or less, Cr having a content 0.1 mass % or more and 1.0 mass % or less, and V having a content 0.1 mass % or more and 0.5 mass % or less, with a remainder consisting of Fe and inevitable impurities. The contents of the alloy elements fall within ranges: 2.0 mass %≦Mn+Cr+Mo≦3.0 mass %, 2.3 mass %≦C+Mo+5V ≦3.7 mass %, and 2.7 mass %≦2.16 Cr+Mo+2.54V≦4.0 mass %. If a steel sample extracted from a central portion of the nitrocarburized steel free from an influence of the nitrocarburizing treatment is austenitized at 1200° C. for one hour, and cooled to a room temperature so that a cooling rate at which the steel sample passes through a temperature range between 900° C. and 300° C. is 0.5° C./second, then an area percentage of a bainite structure in steel structures is 80% or more and a Vickers hardness measured at a cross section is 260 Hv or more and 330 Hv or less. A surface hardness of a nitrocarburized layer is 650 Hv or more, a formation depth of the nitrocarburized layer is 0.3 mm or more, and a hardness of the central portion is 340 Hv or more. Thereby a crankshaft which is excellent both in the machinability and in fatigue strength, even after nitrocarburizing treatment on the surface, is provided.

1 . A crankshaft consisting of a steel having a surface subjected to a nitrocarburizing treatment, comprising: 
 a pin; and    a journal, wherein    the steel contains, as alloy elements:    C having a content equal to or more than 0.10 mass % and equal to or less than 0.30 mass %,    Si having a content equal to or more than 0.05 mass % and equal to or less than 0.3 mass %,    Mn having a content equal to or more than 0.3 mass % and equal to or less than 1.5 mass %,    Mo having a content equal to or more than 0.8 mass % and equal to or less than 2.0 mass %,    Cr having a content equal to or more than 0.1 mass % and equal to or less than 1.0 mass %, and    V having a content equal to or more than 0.1 mass % and equal to or less than 0.5 mass %, with a remainder consisting of Fe and inevitable impurities;    the contents of the alloy elements fall within the following ranges:    2.3 mass %≦C+Mo+5V≦3.7 mass %,    2.0 mass %≦Mn+Cr+Mo≦3.0 mass %, and    2.7 mass %≦2.16 Cr+Mo+2.54V≦4.0 mass %;    if a steel sample extracted from a central portion of the nitrocarburized steel free from an influence of the nitrocarburizing treatment is austenitized at 1200 degrees Centigrade for one hour, and cooled to a room temperature so that a cooling rate at which the steel sample passes through a temperature range between 900 and 300 degrees Centigrade is 0.5° C./second, then an area percentage of a bainite structure in steel structures is equal to or higher than 80 percent and a Vickers hardness measured at a cross section is equal to or higher than 260 Hv and equal to or lower than 330 Hv, and    the pin and the journal have a surface hardness of a nitrocarburized layer is equal to or higher than 650 HV, a formation depth of the nitrocarburized layer is equal to or larger than 0.3 millimeters, and a hardness of the central portion is equal to or higher than 340 Hv.    
   
   
       2 . The crankshaft according to  claim 1 , wherein 
 a content of Pb is equal to or less than 0.03 mass %.    
   
   
       3 . The crankshaft according to  claim 1 , wherein 
 the steel contains one or more of:    Nb having a content equal to or more than 0.02 mass % and equal to or less than 0.2 mass %,    Ti having a content equal to or more than 0.005 mass % and equal to or less than 0.2 mass %, and    Al having a content equal to or more than 0.003 mass % and equal to or less than 0.1 mass %.    
   
   
       4 . The crankshaft according to  claim 1 , wherein 
 the steel contains one of or both of:    S having a content equal to or more than 0.01 mass % and equal to or less than 0.1 mass %, and    Ca having a content equal to or more than 0.0010 mass % and equal to or less than 0.010 mass %.    
   
   
       5 . A method for manufacturing a crankshaft according to  claim 1 , wherein 
 after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and    thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.    
   
   
       6 . The crankshaft according to  claim 2 , wherein 
 the steel contains one or more of:    Nb having a content equal to or more than 0.02 mass % and equal to or less than 0.2 mass %,    Ti having a content equal to or more than 0.005 mass % and equal to or less than 0.2 mass %, and    Al having a content equal to or more than 0.003 mass % and equal to or less than 0.1 mass %.    
   
   
       7 . The crankshaft according to  claim 2 , wherein 
 the steel contains one of or both of:    S having a content equal to or more than 0.01 mass % and equal to or less than 0.1 mass %, and    Ca having a content equal to or more than 0.0010 mass % and equal to or less than 0.010 mass %.    
   
   
       8 . The crankshaft according to  claim 3 , wherein 
 the steel contains one of or both of:    S having a content equal to or more than 0.01 mass % and equal to or less than 0.1 mass %, and    Ca having a content equal to or more than 0.0010 mass % and equal to or less than 0.010 mass %.    
   
   
       9 . The crankshaft according to  claim 6 , wherein 
 the steel contains one of or both of:    S having a content equal to or more than 0.01 mass % and equal to or less than 0.1 mass %, and    Ca having a content equal to or more than 0.0010 mass % and equal to or less than 0.010 mass %.    
   
   
       10 . A method for manufacturing a crankshaft according to  claim 2 , wherein 
 after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and    thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.    
   
   
       11 . A method for manufacturing a crankshaft according to  claim 3 , wherein 
 after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and    thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.    
   
   
       12 . A method for manufacturing a crankshaft according to  claim 6 , wherein 
 after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and    thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.    
   
   
       13 . A method for manufacturing a crankshaft according to  claim 4 , wherein 
 after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and    thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.    
   
   
       14 . A method for manufacturing a crankshaft according to  claim 7 , wherein 
 after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and    thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.    
   
   
       15 . A method for manufacturing a crankshaft according to  claim 8 , wherein 
 after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and    thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.    
   
   
       16 . A method for manufacturing a crankshaft according to  claim 9 , wherein 
 after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and    thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.