High-strength bolt superior in delayed fracture and resistance and relaxation resistance

Abstract

Disclosed is a high-strength bolt having a tensile strength of 1,200 N/mm 2 or more and superior in delayed fracture resistance and relaxation resistance, prepared by wire-drawing a bolt steel containing the following elements: C: 0.5 to 1.0% (mass %, the same shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2 to 2%, P: 0.03% or less (but not 0%), S: 0.03% or less (but not 0%), and Al: 0.3% or less (but not 0%), and containing proeutectoid ferrite, proeutectoid cementite, bainite and martensite at a total areal rate of less than 20% and pearlite in balance; cold-heading the wire into a bolt shape; and then bluing the bolt in a temperature range of 100 to 500° C.

1 . A high-strength bolt having a tensile strength of 1,200 N/mm 2  or more that is superior in delayed fracture resistance and relaxation resistance, wherein the bolt is prepared by: wire-drawing a bolt steel containing the following elements: C: 0.5 to 1.0% (mass %, the same shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2 to 2%, P: 0.03% or less (but not 0%), S: 0.03% or less (but not 0%), and Al: 0.3% or less (but not 0%), and containing proeutectoid ferrite, proeutectoid cementite, bainite and martensite at a total areal rate of less than 20% and pearlite in balance; cold-heading the wire into a bolt shape; and then bluing the bolt in a temperature range of 100 to 500° C.  
   
   
       2 . The high-strength bolt according to  claim 1 , wherein the bolt steel further comprises at least one of the following elements: Cr: 2.5% or less (but not 0%) and Co: 0.5% or less (but not 0%).  
   
   
       3 . The high-strength bolt according to  claim 1 , wherein the bolt steel further comprises Ni at 1.0% or less (but not 0%).  
   
   
       4 . The high-strength bolt according to  claim 1 , wherein the bolt steel further comprises Cu at 1.0% or less (but not 0%).  
   
   
       5 . The high-strength bolt according to  claim 1 , wherein the bolt steel further comprises at least one element selected from Mo, V, Nb, Ti, and W in a total amount of 0.5% or less (but not 0%).  
   
   
       6 . The high-strength bolt according to  claim 1 , wherein the bolt steel further comprises B at 0.003% or less (but not 0%).  
   
   
       7 . The high-strength bolt according to  claim 2 , wherein the bolt steel further comprises Ni at 1.0% or less (but not 0%).  
   
   
       8 . The high-strength bolt according to  claim 2 , wherein the bolt steel further comprises at least one element selected from Mo, V, Nb, Ti, and W in a total amount of 0.5% or less (but not 0%).  
   
   
       9 . The high-strength bolt according to  claim 2 , wherein the bolt steel further comprises B at 0.003% or less (but not 0%).  
   
   
       10 . The high-strength bolt according to  claim 7 , wherein the bolt steel further comprises at least one element selected film Mo, V, Nb, Ti, and W in a total amount of 0.5% or less (but not 0%).  
   
   
       11 . The high-strength bolt according to  claim 1 , wherein the elements in balance are Fe and inevitable impurities.