Steel and part

Abstract

Steel improved in all of the hardenability, toughness, surface-originated micropitting life, and bending fatigue strength and a part manufactured using such steel are provided having predetermined constituents having an Fn1 defined by the following formula (1) of 0.20 to 0.65 and having an Fn2 defined by the following formula (2) of 0.50 to 1.00: Fn1=4.2×[Cr]/(7.0×[Si]+16.0×[Mn])  (1) [Element]: mass % of element Fn2=A1/A2  (2) A1: total area (μm 2 ) of sulfide-based inclusions containing 1.0 mol % or more of Ca with respect to the total number of moles in the sulfides and having a circle equivalent diameter of 1.0 μm or more in observation regions of a total area of 4.0 mm 2 A2: total area (μm 2 ) of sulfide-based inclusions having a circle equivalent diameter of 1.0 μm or more in observation regions of a total area of 4.0 mm 2

1 . A steel having a chemical composition comprising, by mass %,
 C: 0.10 to 0.30%,   Si: 0.01 to 0.25%,   Mn: 0.20 to 1.50%,   P: 0.001 to 0.015%,   S: 0.001 to 0.010%,   Cr: 0.50 to 2.00%,   Mo: 0.10 to 0.50%,   Al: 0.005 to 0.100%,   Ca: 0.0002 to 0.0010%,   N: 0.005 to 0.025%,   O: 0.0015% or less,   Cu: 0 to 0.20%,   Ni: 0 to 0.20%,   B: 0 to 0.005%,   Nb: 0 to 0.05%,   Ti: 0 to 0.10%, and   a balance of Fe and impurities, wherein   Fn1 defined by the following formula (1) is 0.20 to 0.65, and   Fn2 defined by the following formula (2) is 0.50 to 1.00:
   Fn1=4.2×[Cr]/(7.0×[Si]+16.0×[Mn])  (1)
 
   [Element]: mass % of element
   Fn2=A1/A2  (2)
 
   A1: total area (μm 2 ) of sulfide-based inclusions containing 1.0 mol % or more of Ca with respect to the total number of moles in the sulfides and having a circle equivalent diameter of 1.0 μm or more in observation regions of a total area of 4.0 mm 2      A2: total area (μm 2 ) of sulfide-based inclusions having a circle equivalent diameter of 1.0 μm or more in observation regions of a total area of 4.0 mm 2 .   
     
     
         2 . The steel according to  claim 1  wherein the chemical composition includes, by mass %, at least one of Cu: 0.20% or less, Ni: 0.20% or less, and B: 0.005% or less. 
     
     
         3 . The steel according to  claim 1  wherein the chemical composition includes, by mass %, at least one of Nb: 0.05% or less and Ti: 0.10% or less. 
     
     
         4 . The steel according to  claim 1 , wherein the steel is a steel rod. 
     
     
         5 . A part excellent in surface-originated micropitting life and bending fatigue strength, a chemical composition at a region of a depth of 500 μm or more from the surface of the part comprising, by mass %,
 C: 0.10 to 0.30%, 
 Si: 0.01 to 0.25%, 
 Mn: 0.20 to 1.50%, 
 P: 0.001 to 0.015%, 
 S: 0.001 to 0.010%, 
 Cr: 0.50 to 2.00%, 
 Mo: 0.10 to 0.50%, 
 Al: 0.005 to 0.100%, 
 Ca: 0.0002 to 0.0010%, 
 N: 0.005 to 0.025%, 
 O: 0.0015%, 
 Cu: 0 to 0.20%, 
 Ni: 0 to 0.20%, 
 B: 0 to 0.005%, 
 Nb: 0 to 0.05%, 
 Ti: 0 to 0.10%, and 
 a balance of Fe and impurities, wherein 
 Fn1 defined by the following formula (1) is 0.20 to 0.65, and 
 Fn2 defined by the following formula (2) is 0.50 to 1.00,
   Fn1=4.2×[Cr]/(7.0×[Si]+16.0×[Mn])  (1)
 
 
 [Element]: mass % of element
   Fn2=A1/A2  (2)
 
 
 A1: total area (μm 2 ) of sulfide-based inclusions containing 1.0 mol % or more of Ca with respect to the total number of moles in the sulfides and having a circle equivalent diameter of 1.0 μm or more in observation regions of a total area of 4.0 mm 2    
 A2: total area (μm 2 ) of sulfide-based inclusions having a circle equivalent diameter of 1.0 μm or more in observation regions of a total area of 4.0 mm 2 . 
 
     
     
         6 . The part according to  claim 5 , wherein the chemical composition includes at least one of, by mass %, Cu: 0.20% or less, Ni: 0.20% or less, and B: 0.005% or less. 
     
     
         7 . The part according to  claim 5 , wherein the chemical composition includes at least one of, by mass %, Nb: 0.05% or less and Ti: 0.10% or less. 
     
     
         8 . The part according to  claim 5  wherein at a center part, an absorption energy vE20 is 43 J/cm 2  or more. 
     
     
         9 . The steel according to  claim 2  wherein the chemical composition includes, by mass %, at least one of Nb: 0.05% or less and Ti: 0.10% or less. 
     
     
         10 . The steel according to  claim 2 , wherein the steel is a steel rod. 
     
     
         11 . The steel according to  claim 3 , wherein the steel is a steel rod. 
     
     
         12 . The steel according to  claim 9 , wherein the steel is a steel rod. 
     
     
         13 . The part according to  claim 6 , wherein the chemical composition includes at least one of, by mass %, Nb: 0.05% or less and Ti: 0.10% or less. 
     
     
         14 . The part according to  claim 6  wherein at a center part, an absorption energy vE20 is 43 J/cm 2  or more. 
     
     
         15 . The part according to  claim 7  wherein at a center part, an absorption energy vE20 is 43 J/cm 2  or more. 
     
     
         16 . The part according to  claim 13  wherein at a center part, an absorption energy vE20 is 43 J/cm 2  or more.