Steel material for seismic damper having superior impact toughness, and manufacturing method for same

Abstract

The present invention provides a steel material for a seismic damper, and a manufacturing method for same. The steel material comprises, in wt %, at most 0.006% of C, at most 0.05% of Si, at most 0.3% of Mn, at most 0.02% of P, at most 0.01% of S, 0.005 to 0.05% of Al, at most 0.005% of N, 48/14 ×[N] to 0.05% of Ti (here, [N] is the nitrogen content in wt %), 0.04 to 0.15% of Nb, and the remainder in Fe and other unavoidable impurities, and has a ferrite single structure, and has a ferrite grain average particle size of 150 to 500 μm in a region of a surface layer part, corresponding to 30% of the total thickness from the surface thereof.

1 . A steel material for a seismic damper, comprising by weight:
 0.006% or less of C, 0.05% or less of Si, 0.3% or less of Mn, 0.02% or less of P, 0.01% or less of S, 0.005 to 0.05% of Al, 0.005% or less of N, 48/14×[N] to 0.05% of Ti, where, [N] refers to a weight of nitrogen (weight %), and 0.04 to 0.15% of Nb, with a balance of Fe and other unavoidable impurities, having a ferrite single structure,   wherein an average ferrite grain size in a surface layer portion from a surface thereof to a region corresponding to 30% of the total thickness is 150 to 500 μm.   
     
     
         2 . The steel material for a seismic damper of  claim 1 , wherein an average ferrite grain size in an inner region other than the surface layer portion is 10 to 60 μm. 
     
     
         3 . The steel material for a seismic damper of  claim 1 , wherein an R1 value defined by the following Relational Expression 1 is within a range of 0.8 to 150,
   R1=[Nb]/[Si]  [Relational Expression 1]
   In Relational Expression 1, [Nb] represents a content of Nb (weight %) and [Si] represents a content of Si (weight %).   
     
     
         4 . The steel material for a seismic damper of  claim 1 , wherein an R2 value defined by the following Relational Expression 2 is within a range of 4 to 200,
   R2=([Ti]+[Nb])/[Si]  [Relational Expression 2]
   In the Relational Expression 2, [Ti] represents a content of Ti (weight %) and [Si] represents a content of Si (weight %).   
     
     
         5 . The steel material for a seismic damper of  claim 1 , wherein a ratio (Ds/Dt) of a thickness Ds of the surface layer portion of a total thickness Dt of the steel material is within a range of 0.1 to 0.3. 
     
     
         6 . The steel material for a seismic damper of  claim 1 , wherein the steel material has a yield strength of 120 MPa or less. 
     
     
         7 . A manufacturing method for a steel material for a seismic damper, comprising:
 heating a steel slab including by weight: 0.006% or less of C, 0.05% or less of Si, 0.3% or less of Mn, 0.02% or less of P, 0.01% or less of S, 0.005 to 0.05% of Al, 0.005% or less of N, 48/14×[N] to 0.05% of Ti, where, [N] refers to a content of nitrogen (weight %), and 0.04 to 0.15% of Nb, with a balance of Fe and other unavoidable impurities, to a temperature within a range of 1050 to 1250° C.;   subjecting the heated steel slab to finish rolling in a temperature range of Ar3-80° C. or higher and Ar3 or lower; and   performing a shot blasting treatment operation on a surface of the finish-rolled steel material,   wherein the shot blasting treatment operation is performed so that a metallic ball or a non-metallic ball is rotated at a rate of 1500 to 2500 rpm and sprayed on a surface of the plate material at a rate of 60 to 100 m/s.   
     
     
         8 . The manufacturing method for a steel material for a seismic damper of  claim 7 , wherein the shot blasting treatment operation is performed for 10 to 30 minutes. 
     
     
         9 . The manufacturing method for a steel material for a seismic damper of  claim 7 , wherein a diameter of the metallic ball or non-metallic ball is 0.8 to 1.2 mm. 
     
     
         10 . The manufacturing method for a steel material for a seismic damper of  claim 7 , further comprising, after the shot blasting treatment operation:
 performing a heat treatment so that an LMP value defined by the following Relational Expression 3 satisfies a range of 23.5 to 24.5,
     LMP=T ×[log( t )+20]/1000  [Relational Expression 3]
 
   in Relational Expression 3, T represents a heat-treatment temperature, wherein a unit thereof is ° C. In addition, t represents a heat treatment time, wherein a unit thereof is minutes.   
     
     
         11 . The manufacturing method for a steel material for a seismic damper of  claim 10 , wherein the heat treatment operation is performed in a range of 850 to 900° C.