Steel part excellent in temper softening resistance

Abstract

The present invention relates to a steel part securing temper softening resistance at 300° C. characterized by (a) the content of Si in the steel being made 0.7% or more, (b) the content of C in the carburized layer being made 0.9 to 1.1% for a hyper eutectic content of C by carburization, (c) the carbides with an outside diameter of 0.1 μm or more (proeutectoid cementite or spherical carbides) not being allowed to be present after carburization, and (d) the amount of residual austenite after carburization, quenching, and tempering being made less than 30%. The gist lies in a steel part excellent in temper softening resistance which is comprised of steel containing predetermined ingredients, has a total content of Mn and Cr of 2.5% or less, and is carburized, quenched, and tempered on its surface, which has a content of C at a part at a 50 μm depth from the surface of 0.9 to 1.1%, does not contain proeutectoid cementite or spherical carbides of an average diameter of 0.1 μm or more or even if containing it, has an area ratio of it of less than 1%, and, further, does not containing residual austenite or even if containing it, has an area ratio of it of less than 30%.

1 . A steel part excellent in temper softening resistance which is comprised of steel containing as chemical ingredients, by mass %, of
 C: 0.1 to 0.3%,   Si: 0.7 to 1.0%,   Mn: 0.2 to 1.5%,   Cr: 1.0 to 1.5%,   S: 0.001 to 0.15%,   Al: 0.001 to 1%, and   N: 0.001 to 0.03%,   satisfying a total content of Mn and Cr of 2.5% or less, and   having a balance of iron and unavoidable impurities, and   which is carburized, quenched, and tempered on its surface,   said steel part characterized by having a content of C at a part at a 50 lam depth from the surface of 0.9 to 1.1%, not containing proeutectoid cementite or spherical carbides of an average diameter of 0.1 μm or more or even if containing it, having an area ratio of it of less than 1%, and, further, not containing residual austenite or even if containing it, having an area ratio of it of less than 30%.   
     
     
         2 . A steel part excellent in temper softening resistance as set forth in  claim 1 , characterized by the steel further containing one or more chemical ingredients, by mass %, of
 Mo: 0.02 to 0.6%,   Ni: 0.1 to 1.0%, and   B: 0.0002 to 0.005%, and   having a total content of Mn, Cr, Mo, and Ni satisfying 2.5% or less.   
     
     
         3 . A steel part excellent in temper softening resistance as set forth in  claim 1  or  2 , characterized by the steel further containing one or more chemical ingredients, by mass %, of
 Ti: 0.01 to 0.05%, 
 Nb: 0.001 to 0.04%, and 
 V: 0.02 to 0.1%. 
 
     
     
         4 . A steel part excellent in temper softening resistance as set forth in  claim 1  or  2  characterized by the steel further containing one or more chemical ingredients, by mass %, of
 Ca: 0.0002 to 0.005%, 
 Zr: 0.0003 to 0.005%, 
 Mg: 0.0003 to 0.005%, and 
 REM (rare earth metals): 0.0001 to 0.005%. 
 
     
     
         5 . A steel part excellent in temper softening resistance as set forth in  claim 3  characterized by the steel further containing one or more chemical ingredients, by mass %, of
 Ca: 0.0002 to 0.005%, 
 Zr: 0.0003 to 0.005%, 
 Mg: 0.0003 to 0.005%, and 
 REM (rare earth metals): 0.0001 to 0.005%.