Steel member, hot-rolled steel sheet for steel member, and production method therefor

Abstract

A steel member, a hot-rolled steel sheet to be used as a material thereof, and production methods therefor are provided. A steel member contains 0.031% to 0.200% Ti, in which 0.005% or more of Ti is precipitated as a precipitate having a particle size of 20 nm or less in the microstructure. A hot-rolled steel sheet contains 0.031% to 0.200% Ti, in which 0.005% or more of Ti is present as dissolved Ti in the microstructure. A method for producing the steel member includes subjecting a hot-rolled steel sheet to forming processing and then performing heat treatment including heating to a temperature of higher than 550° C. and 1,050° C. or lower and then cooling at an average cooling rate of 10° C./s or more in the temperature range of 550° C. to 400° C.

1 . A steel member, comprising, on a percent by mass basis, 0.031% to 0.200% Ti, wherein 0.005% or more of Ti is precipitated as a precipitate having a particle size of 20 nm or less in a microstructure. 
     
     
         2 . The steel member according to  claim 1 , wherein the steel member has a composition containing, on a percent by mass basis:
 C: 0.19% to 0.50%;   Si: 0.002 to 1.5%;   Mn: 0.4% to 2.5%;   Al: 0.01% to 0.19%;   Cr: 0.001% to 0.90%;   B: 0.0001% to 0.0050%;   Ti: 0.031% to 0.200%;   P: 0.019% or less (including 0%);   S: 0.015% or less (including 0%);   N: 0.008% or less (including 0%);   O: 0.003% or less (including 0%); and   Sn: 0.10% or less (including 0%),   and optionally one or two or more selected from:   Nb: 0.001% to 0.15%;   V: 0.001% to 0.15%;   W: 0.001% to 0.15%;   Mo: 0.001% to 0.45%;   Cu: 0.001% to 0.45%;   Ni: 0.001% to 0.45%;   Ca: 0.0001% to 0.005%; and   Sb: 0.0001% to 0.10%,   the balance being Fe and incidental impurities.   
     
     
         3 . (canceled) 
     
     
         4 . The steel member according to  claim 1 , wherein the steel member is a welded steel pipe. 
     
     
         5 . A hot-rolled steel sheet for the steel member according to  claim 1 , comprising, on a percent by mass basis:
 0.031% to 0.200% Ti, wherein 0.005% or more of Ti is present as dissolved Ti in a microstructure.   
     
     
         6 . The hot-rolled steel sheet for the steel member according to  claim 5 , wherein a leading end portion and a trailing end portion are both end portions in a longitudinal direction, and each of the leading end portion and the trailing end portion has a thickness 5% to 50% larger than a central portion in the longitudinal direction. 
     
     
         7 . A method for producing the steel member according to  claim 1 , comprising:
 subjecting a hot-rolled steel sheet to forming processing, the hot-rolled steel sheet containing, on a percent by mass basis, 0.031% to 0.200% Ti, 0.005% or more of Ti being present as dissolved Ti in a microstructure, and then performing heat treatment including heating to a temperature of higher than 550° C. and 1,050° C. or lower and then cooling at an average cooling rate of 10° C./s or more in a temperature range of 550° C. to 400° C.   
     
     
         8 . The method for producing the steel member according to  claim 7 , wherein the hot-rolled steel sheet is produced by, after performing slab extraction of a steel slab containing, on a percent by mass basis, 0.031% to 0.200% Ti at a temperature higher than an equilibrium dissolution temperature T Ti  calculated from formula (1) below, completing finish rolling at a temperature equal to or higher than T Ti —400° C., performing cooling at an average cooling rate of 10° C./s or more in a temperature range of T Ti —400° C. to T Ti —500° C., and performing coiling at a temperature equal to or lower than T Ti —500° C.,
   log([Ti—N×48/14][C])=−7,000/(T Ti (° C.)+273)+2.75  (1)
 
 where Ti, N, and C in formula (1) indicate contents (% by mass) of respective elements in the steel slab. 
 
     
     
         9 . A method for producing the hot-rolled steel sheet according to  claim 5 , comprising:
 after performing slab extraction of a steel slab containing, on a percent by mass basis, 0.031% to 0.200% Ti at a temperature higher than an equilibrium dissolution temperature T Ti  calculated from formula (1) below, completing finish rolling at a temperature equal to or higher than T Ti —400° C., performing cooling at an average cooling rate of 10° C./s or more in a temperature range of T Ti —400° C. to T Ti —500° C., and performing coiling at a temperature equal to or lower than T Ti —500° C.,
   log([Ti—N×48/14][C])=−7,000/(T Ti (° C.)+273)+2.75  (1)
 
   where Ti, N, and C in formula (1) indicate contents (% by mass) of respective elements in the steel slab.   
     
     
         10 . The steel member according to  claim 2 , wherein the steel member is a welded steel pipe. 
     
     
         11 . A hot-rolled steel sheet for the steel member according to  claim 2 , comprising, on a percent by mass basis:
 0.031% to 0.200% Ti, wherein 0.005% or more of Ti is present as dissolved Ti in a microstructure.   
     
     
         12 . The hot-rolled steel sheet for the steel member according to  claim 11 , wherein a leading end portion and a trailing end portion are both end portions in a longitudinal direction, and each of the leading end portion and the trailing end portion has a thickness 5% to 50% larger than a central portion in the longitudinal direction. 
     
     
         13 . A method for producing the steel member according to  claim 2 , comprising:
 subjecting a hot-rolled steel sheet to forming processing, the hot-rolled steel sheet containing, on a percent by mass basis, 0.031% to 0.200% Ti, 0.005% or more of Ti being present as dissolved Ti in a microstructure, and then performing heat treatment including heating to a temperature of higher than 550° C. and 1,050° C. or lower and then cooling at an average cooling rate of 10° C./s or more in a temperature range of 550° C. to 400° C.   
     
     
         14 . A method for producing the steel member according to  claim 4 , comprising:
 subjecting a hot-rolled steel sheet to forming processing, the hot-rolled steel sheet containing, on a percent by mass basis, 0.031% to 0.200% Ti, 0.005% or more of Ti being present as dissolved Ti in a microstructure, and then performing heat treatment including heating to a temperature of higher than 550° C. and 1,050° C. or lower and then cooling at an average cooling rate of 10° C./s or more in a temperature range of 550° C. to 400° C.   
     
     
         15 . A method for producing the steel member according to  claim 10 , comprising:
 subjecting a hot-rolled steel sheet to forming processing, the hot-rolled steel sheet containing, on a percent by mass basis, 0.031% to 0.200% Ti, 0.005% or more of Ti being present as dissolved Ti in a microstructure, and then performing heat treatment including heating to a temperature of higher than 550° C. and 1,050° C. or lower and then cooling at an average cooling rate of 10° C./s or more in a temperature range of 550° C. to 400° C.   
     
     
         16 . The method for producing the steel member according to  claim 13 , wherein the hot-rolled steel sheet is produced by, after performing slab extraction of a steel slab containing, on a percent by mass basis, 0.031% to 0.200% Ti at a temperature higher than an equilibrium dissolution temperature T Ti  calculated from formula (1) below, completing finish rolling at a temperature equal to or higher than T Ti —400° C., performing cooling at an average cooling rate of 10° C./s or more in a temperature range of T Ti —400° C. to T Ti —500° C., and performing coiling at a temperature equal to or lower than T Ti —500° C.,
   log([Ti—N×48/14][C])=−7,000/(T Ti (° C.)+273)+2.75  (1)
 
 where Ti, N, and C in formula (1) indicate contents (% by mass) of respective elements in the steel slab. 
 
     
     
         17 . The method for producing the steel member according to  claim 14 , wherein the hot-rolled steel sheet is produced by, after performing slab extraction of a steel slab containing, on a percent by mass basis, 0.031% to 0.200% Ti at a temperature higher than an equilibrium dissolution temperature T Ti  calculated from formula (1) below, completing finish rolling at a temperature equal to or higher than T Ti —400° C., performing cooling at an average cooling rate of 10° C./s or more in a temperature range of T Ti —400° C. to T Ti —500° C., and performing coiling at a temperature equal to or lower than T Ti —500° C.,
   log([Ti—N×48/14][C])=−7,000/(T Ti (° C.)+273)+2.75  (1)
 
 where Ti, N, and C in formula (1) indicate contents (% by mass) of respective elements in the steel slab. 
 
     
     
         18 . The method for producing the steel member according to  claim 15 , wherein the hot-rolled steel sheet is produced by, after performing slab extraction of a steel slab containing, on a percent by mass basis, 0.031% to 0.200% Ti at a temperature higher than an equilibrium dissolution temperature T Ti  calculated from formula (1) below, completing finish rolling at a temperature equal to or higher than T Ti —400° C., performing cooling at an average cooling rate of 10° C./s or more in a temperature range of T Ti —400° C. to T Ti —500° C., and performing coiling at a temperature equal to or lower than T Ti —500° C.,
   log([Ti—N×48/14][C])=−7,000/(T Ti (° C.)+273)+2.75  (1)
 
 where Ti, N, and C in formula (1) indicate contents (% by mass) of respective elements in the steel slab. 
 
     
     
         19 . A method for producing the hot-rolled steel sheet according to  claim 6 , comprising:
 after performing slab extraction of a steel slab containing, on a percent by mass basis, 0.031% to 0.200% Ti at a temperature higher than an equilibrium dissolution temperature T Ti  calculated from formula (1) below, completing finish rolling at a temperature equal to or higher than T Ti —400° C., performing cooling at an average cooling rate of 10° C./s or more in a temperature range of T Ti —400° C. to T Ti —500° C., and performing coiling at a temperature equal to or lower than T Ti —500° C.,
   log([Ti—N×48/14][C])=−7,000/(T Ti (° C.)+273)+2.75  (1)
 
   where Ti, N, and C in formula (1) indicate contents (% by mass) of respective elements in the steel slab.   
     
     
         20 . A method for producing the hot-rolled steel sheet according to  claim 11 , comprising:
 after performing slab extraction of a steel slab containing, on a percent by mass basis, 0.031% to 0.200% Ti at a temperature higher than an equilibrium dissolution temperature T Ti  calculated from formula (1) below, completing finish rolling at a temperature equal to or higher than T Ti —400° C., performing cooling at an average cooling rate of 10° C./s or more in a temperature range of T Ti —400° C. to T Ti —500° C., and performing coiling at a temperature equal to or lower than T Ti —500° C.,
   log([Ti—N×48/14][C])=−7,000/(T Ti (° C.)+273)+2.75  (1)
 
   where Ti, N, and C in formula (1) indicate contents (% by mass) of respective elements in the steel slab.   
     
     
         21 . A method for producing the hot-rolled steel sheet according to  claim 12 , comprising:
 after performing slab extraction of a steel slab containing, on a percent by mass basis, 0.031% to 0.200% Ti at a temperature higher than an equilibrium dissolution temperature T Ti  calculated from formula (1) below, completing finish rolling at a temperature equal to or higher than T Ti —400° C., performing cooling at an average cooling rate of 10° C./s or more in a temperature range of T Ti —400° C. to T Ti —500° C., and performing coiling at a temperature equal to or lower than T Ti —500° C.,
   log([Ti—N×48/14][C])=−7,000/(T Ti (° C.)+273)+2.75  (1)
 
   where Ti, N, and C in formula (1) indicate contents (% by mass) of respective elements in the steel slab.