Steel For Machine Structural Use With Excellent Strength, Ductility, And Toughness And Method For Producing The Same

Abstract

A steel for machine structural use with a better strength-ductility-toughness balance than maraging steel and applications thereof are provided. The steel for machine structural use with excellent strength, ductility, and toughness contains, in percent by mass, more than 0.30% to 0.5% of carbon, 1.0% or less of silicon, 1.5% or less of manganese, 0.025% or less of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of boron, and the balance is iron and incidental impurities. The steel has a structure including at least 90% by volume of martensitic structure. The martensitic structure includes blocks having a size of 1.5 μm or less. Dissolved boron is contained in an amount of at least 0.0005% and is present at boundaries of prior austenite grains in a concentration at least 1.5 times that in the prior austenite grains.

1 . A steel for machine structural use with excellent strength, ductility, and toughness, the steel comprising, in percent by mass, more than 0.30% to 0.5% of carbon, 1.0% or less of silicon, 1.5% or less of manganese, 0.025% or less of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of boron, the balance being iron and incidental impurities, the steel having a tensile strength of 2,000 MPa or more and a total elongation of 10% or more.  
   
   
       2 . The steel for machine structural use with excellent strength, ductility, and toughness according to  claim 1 , the steel further comprising, in percent by mass, at least one of 2.5% or less of chromium, 1.0% or less of copper, 2.0% or less of nickel, and 0.5% or less of vanadium.  
   
   
       3 . The steel for machine structural use with excellent strength, ductility, and toughness according to  claim 1 , the steel further comprising, in percent by mass, at least one of 0.1% or less of titanium and 0.1% or less of niobium.  
   
   
       4 . A steel for machine structural use with excellent strength, ductility, and toughness, the steel comprising, in percent by mass, more than 0.30% to 0.5% of carbon, 1.0% or less of silicon, 1.5% or less of manganese, 0.025% or less of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of boron, the balance being iron and incidental impurities, the steel having a structure comprising at least 90% by volume of martensitic structure, the martensitic structure comprising blocks having a size of 1.5 μm or less, wherein dissolved boron is contained in an amount of at least 0.0005% and is present at boundaries of prior austenite grains in a concentration at least 1.5 times that in the prior austenite grains.  
   
   
       5 . The steel for machine structural use with excellent strength, ductility, and toughness according to  claim 4 , the steel further comprising, in percent by mass, at least one of 2.5% or less of chromium, 1.0% or less of copper, 2.0% or less of nickel, and 0.5% or less of vanadium.  
   
   
       6 . The steel for machine structural use with excellent strength, ductility, and toughness according to  claim 4 , the steel further comprising, in percent by mass, at least one of 0.1% or less of titanium and 0.1% or less of niobium.  
   
   
       7 . A steel sheet for machine structural use with excellent strength, ductility, and toughness, the steel sheet comprising the steel for machine structural use according to  claim 1  and having a thickness of 0.5 mm or less.  
   
   
       8 . A metal belt comprising the steel sheet according to  claim 7 , the metal belt having an annular shape.  
   
   
       9 . A method for producing a steel for machine structural use with excellent strength, ductility, and toughness, the method comprising quenching a steel material by heating at a rate of temperature rise of 100° C./s or more and tempering the steel material at 100° C. to 400° C., the steel material comprising, in percent by mass, more than 0.30% to 0.5% of carbon, 1.0% or less of silicon, 1.5% or less of manganese, 0.025% or less of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of boron, the balance being iron and incidental impurities.  
   
   
       10 . The method for producing a steel for machine structural use with excellent strength, ductility, and toughness according to  claim 9 , wherein the steel material further comprises, in percent by mass, at least one of 2.5% or less of chromium, 1.0% or less of copper, 2.0% or less of nickel, and 0.5% or less of vanadium.  
   
   
       11 . The method for producing a steel for machine structural use with excellent strength, ductility, and toughness according to  claim 9 , wherein the steel material further comprises, in percent by mass, at least one of 0.1% or less of titanium and 0.1% or less of niobium.  
   
   
       12 . A method for producing a steel sheet for machine structural use with excellent strength, ductility, and toughness, the method comprising quenching a steel sheet by heating at a rate of temperature rise of 100° C./s or more and tempering the steel sheet at 100° C. to 400° C., the steel sheet comprising, in percent by mass, more than 0.30% to 0.5% of carbon, 1.0% or less of silicon, 1.5% or less of manganese, 0.025% or less of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of boron, the balance being iron and incidental impurities, the steel sheet having a thickness of 0.5 mm or less.  
   
   
       13 . The method for producing a steel sheet for machine structural use with excellent strength, ductility, and toughness according to  claim 12 , wherein the steel sheet further comprises, in percent by mass, at least one of 2.5% or less of chromium, 1.0% or less of copper, 2.0% or less of nickel, and 0.5% or less of vanadium.  
   
   
       14 . The method for producing a steel sheet for machine structural use with excellent strength, ductility, and toughness according to  claim 12 , wherein the steel sheet further comprises, in percent by mass, at least one of 0.1% or less of titanium and 0.1% or less of niobium.  
   
   
       15 . A method for producing a metal belt, the method comprising quenching a metal belt by heating at a rate of temperature rise of 100° C./s or more and tempering the metal belt at 100° C. to 400° C., the metal belt comprising, in percent by mass, more than 0.30% to 0.5% of carbon, 1.0% or less of silicon, 1.5% or less of manganese, 0.025% or less of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of boron, the balance being iron and incidental impurities, the metal belt having a thickness of 0.5 mm or less and having an annular shape.  
   
   
       16 . The method for producing a metal belt according to  claim 15 , wherein the metal belt further comprises, in percent by mass, at least one of 2.5% or less of chromium, 1.0% or less of copper, 2.0% or less of nickel, and 0.5% or less of vanadium.  
   
   
       17 . The method for producing a metal belt according to  claim 15 , wherein the metal belt further comprises, in percent by mass, at least one of 0.1% or less of titanium and 0.1% or less of niobium.  
   
   
       18 . The steel for machine structural use with excellent strength, ductility, and toughness according to  claim 2 , the steel further comprising, in percent by mass, at least one of 0.1% or less of titanium and 0.1% or less of niobium.  
   
   
       19 . The steel for machine structural use with excellent strength, ductility, and toughness according to  claim 5 , the steel further comprising, in percent by mass, at least one of 0.1% or less of titanium and 0.1% or less of niobium.  
   
   
       20 . A steel sheet for machine structural use with excellent strength, ductility, and toughness, the steel sheet comprising the steel for machine structural use according to  claim 4  and having a thickness of 0.5 mm or less.  
   
   
       21 . A metal belt comprising the steel sheet according to  claim 20 , the metal belt having an annular shape.  
   
   
       22 . The method for producing a steel for machine structural use with excellent strength, ductility, and toughness according to  claim 10 , wherein the steel material further comprises, in percent by mass, at least one of 0.1% or less of titanium and 0.1% or less of niobium.  
   
   
       23 . The method for producing a steel sheet for machine structural use with excellent strength, ductility, and toughness according to  claim 13 , wherein the steel sheet further comprises, in percent by mass, at least one of 0.1% or less of titanium and 0.1% or less of niobium.  
   
   
       24 . The method for producing a metal belt according to  claim 16 , wherein the metal belt further comprises, in percent by mass, at least one of 0.1% or less of titanium and 0.1% or less of niobium.