High-Strength Steel and Metal Bolt Excellent In Character of Delayed Fracture

Abstract

A high-strength steel having excellent character of delayed fracture can be manufactured at a low cost without adding expensive alloy elements such as Ni, Co or V in large quantities. The steel including a steel microstructure in which a ratio of martensite phase is 90% or more and a diameter of prior austenite grains is 10 μm or less, the steel microstructure being made by hardening and then tempering at 100° C. to 400° C., the steel including C: more than 0.30% and not more than 0.50% by mass, Si: not more than 1.0% by mass, Mn: not more than 1.5% by mass, Ti: not more than 0.1% by mass, Mo: not less than 0.3% and not more than 0.50% by mass, B: not less than 0.0005% and not more than 0.01% by mass, and a balance composed of Fe and inevitable impurities.

1 . A high-strength steel being excellent in character of delayed fracture, the high-strength steel comprising a steel microstructure in which a ratio of martensite phase is 90% or more and a diameter of prior austenite grains is 10 μm or less, the steel microstructure being made by hardening and then tempering at 100° C. to 400° C., the steel including C: more than 0.30% and not more than 0.50% by mass, Si: not more than 1.0% by mass, Mn: not more than 1.5% by mass, Ti: not more than 0.1% by mass, Mo: not less than 0.3% and not more than 0.50% by mass, B: not less than 0.0005% and not more than 0.01% by mass, and a balance composed of Fe and inevitable impurities. 
     
     
         2 . The high-strength steel being excellent in character of delayed fracture according to  claim 1 , wherein the steel further includes at least one element selected from the group consisting of Al: not more than 1.0% by mass, Cr: not more than 2.5% by mass, Cu: not more than 1.0% by mass, Ni: not more than 2.0% by mass, and V: not more than 0.5% by mass. 
     
     
         3 . The high-strength steel being excellent in character of delayed fracture according to  claim 1 , wherein the steel further includes at least one element selected from the group consisting of W: not more than 1.0% by mass and Nb: not more than 0.1% by mass. 
     
     
         4 . The high-strength steel being excellent in character of delayed fracture according to  claim 1 , wherein the hardening is performed using induction heating. 
     
     
         5 . A metal bolt being excellent in character of delayed fracture, the metal bolt including a steel microstructure in which the ratio of martensite phase is 90% or more and the diameter of prior austenite grains is 10 μm or less, the steel microstructure being made by preparing a bolt using the steel according to  claim 1 , hardening the bolt, and tempering the bolt at 100° C. to 400° C. 
     
     
         6 . The high-strength steel being excellent in character of delayed fracture according to  claim 2 , wherein the steel further includes at least one element selected from the group consisting of W: not more than 1.0% by mass and Nb: not more than 0.1% by mass. 
     
     
         7 . The high-strength steel being excellent in character of delayed fracture according to  claim 2 , wherein the hardening is performed using induction heating. 
     
     
         8 . The high-strength steel being excellent in character of delayed fracture according to  claim 3 , wherein the hardening is performed using induction heating. 
     
     
         9 . The high-strength steel being excellent in character of delayed fracture according to  claim 6 , wherein the hardening is performed using induction heating. 
     
     
         10 . A metal bolt being excellent in character of delayed fracture, the metal bolt including a steel microstructure in which the ratio of martensite phase is 90% or more and the diameter of prior austenite grains is 10 μm or less, the steel microstructure being made by preparing a bolt using the steel according to  claim 2 , hardening the bolt, and tempering the bolt at 100° C. to 400° C. 
     
     
         11 . A metal bolt being excellent in character of delayed fracture, the metal bolt including a steel microstructure in which the ratio of martensite phase is 90% or more and the diameter of prior austenite grains is 10 μm or less, the steel microstructure being made by preparing a bolt using the steel according to  claim 3 , hardening the bolt, and tempering the bolt at 100° C. to 400° C. 
     
     
         12 . A metal bolt being excellent in character of delayed fracture, the metal bolt including a steel microstructure in which the ratio of martensite phase is 90% or more and the diameter of prior austenite grains is 10 μm or less, the steel microstructure being made by preparing a bolt using the steel according to  claim 4 , hardening the bolt, and tempering the bolt at 100° C. to 400° C. 
     
     
         13 . A metal bolt being excellent in character of delayed fracture, the metal bolt including a steel microstructure in which the ratio of martensite phase is 90% or more and the diameter of prior austenite grains is 10 μm or less, the steel microstructure being made by preparing a bolt using the steel according to  claim 7 , hardening the bolt, and tempering the bolt at 100° C. to 400° C. 
     
     
         14 . A metal bolt being excellent in character of delayed fracture, the metal bolt including a steel microstructure in which the ratio of martensite phase is 90% or more and the diameter of prior austenite grains is 10 μm or less, the steel microstructure being made by preparing a bolt using the steel according to  claim 8 , hardening the bolt, and tempering the bolt at 100° C. to 400° C. 
     
     
         15 . A metal bolt being excellent in character of delayed fracture, the metal bolt including a steel microstructure in which the ratio of martensite phase is 90% or more and the diameter of prior austenite grains is 10 μm or less, the steel microstructure being made by preparing a bolt using the steel according to  claim 9 , hardening the bolt, and tempering the bolt at 100° C. to 400° C.