Steel material excellent in resistance of ductile crack initiation from welded heat affected zone and base material and method for manufacturing the same

Abstract

A steel material has a composition of C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.5%, P: 0.05% or lower, S: 0.05% or lower, Al: 0.1% or lower, and N: 0.01% or lower and, as required, one or two or more elements selected from Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower, V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca: 0.01% or lower, and REM: 0.1% or lower in terms of % by mass, and the balance Fe with inevitable impurities, in which the microstructure at the ¼ position of the plate thickness contains ferrite and a hard phase, the area fraction of the hard phase is 50 to 90%, and the average aspect ratio of the ferrite is 1.5 or more.

1 . A steel material excellent in resistance of ductile crack initiation from welded heat affected zone and a base material,
 comprising:   a composition of C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.5%, P: 0.05% or lower, S: 0.05% or lower, Al: 0.1% or lower, and N: 0.01% or lower in terms of % by mass, and the balance Fe with inevitable impurities,   having a microstructure at ¼ position of plate thickness containing ferrite and a hard phase,   an area fraction of the hard phase of 50 to 90%, and   an average aspect ratio of the ferrite of 1.5 or more.   
     
     
         2 . The steel material according to  claim 1 , further comprising, in the chemical composition, one or more elements selected from the group consisting of Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower, V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca: 0.01% or lower, and REM: 0.1% or lower in terms of % by mass. 
     
     
         3 . The steel material according to  claim 1 , wherein the microstructure on a surface of a steel plate contains ferrite and a hard phase, the area fraction of the ferrite exceeds 40%, and the average aspect ratio of the ferrite grain size exceeds 2. 
     
     
         4 . A method for manufacturing a steel material excellent in resistance of ductile crack initiation from welded heat affected zone and a base material comprising:
 reheating a steel base material of  claim 1  to 1000° C. or more,   rolling the same such that a rolling reduction rate in a temperature range of 900° C. or more is 50% or more and a rolling finish temperature is Ar 3  point to Ar 3 -50° C.,   starting water cooling at Ar 3 -10° C. to Ar 3 -70° C., and   terminating the water cooling at 500° C. or lower.   
     
     
         5 . The method according to  claim 4  further comprising, after the water cooling, performing tempering treatment at a temperature lower than the highest heating temperature Ac 1  point. 
     
     
         6 . The steel material according to  claim 2 , wherein the microstructure on a surface of a steel plate contains ferrite and a hard phase, the area fraction of the ferrite exceeds 40%, and the average aspect ratio of the ferrite grain size exceeds 2. 
     
     
         7 . A method for manufacturing a steel material excellent in resistance of ductile crack initiation from welded heat affected zone and a base material comprising:
 reheating a steel base material of  claims 2  to  1000 ° C. or more,   rolling the same such that a rolling reduction rate in a temperature range of 900° C. or more is 50% or more and a rolling finish temperature is Ar 3  point to Ar 3 -50° C.,   starting water cooling at Ar 3 -10° C. to Ar 3 -70° C., and   terminating the water cooling at 500° C. or lower.