Hot-rolled steel sheet having excellent impact resistance, steel pipe, member, and manufacturing methods therefor

Abstract

A preferable aspect of the present invention provides: a hot-rolled steel sheet containing, by weight, 0.35-0.55% of C, 0.7-1.5% of Mn, 0.3% or less of Si, 0.03% or less of P, 0.004% or less of S, 0.04% or less of Al, 0.3% or less (excluding 0%) of Cr, 0.3% or less of Mo, one or two of 0.1-1.0% of Ni and 0.1-1.0% of Cu, 0.4% or more of Cu+Ni, 0.006% or less of N, and the balance Fe and other impurities, the alloy elements satisfying relational formulas 1 to 3 below, wherein a microstructure of the hot-rolled steel sheet comprises, by volume, 10% or more of ferrite and 90% or less of pearlite; a steel pipe and a member each using the same; and manufacturing methods therefore. [Relational formula 1] (Mn/Si)≥3 (weight ratio) [Relational formula 2] (Ni+Cu)/(C+Mn)≥0.2 (weight ratio) [Relational formula 3] (Ni/Si)≥(weight ratio).

1 . A hot-rolled steel sheet having excellent impact resistance, comprising:
 by weight %, 0.35-0.55% of C, 0.7-1.5% of Mn, 0.3% or less (excluding 0%) of Si, 0.03% or less (including 0%) of P, 0.004% or less (including 0%) of S, 0.04% or less (excluding 0%) of Al, 0.3% or less (excluding 0%) of Cr, 0.3% or less (excluding 0%) of Mo, one or two of 0.1-1.0% of Ni and 0.1-1.0% of Cu, 0.4% or more of Cu+Ni, 0.006% or less (excluding 0%) of N, and a balance of Fe and other impurities, the alloy elements satisfying relational formulae 1-3 as below, where a microstructure includes, by volume %, 10-30% of ferrite and 70-90% of pearlite.
   (Mn/Si)≥3 (a weight ratio)  [Relational Formula 1]
 
   (Ni+Cu)/(C+Mn)≥0.2 (a weight ratio)  [Relational Formula 2]
 
   (Ni/Si)≥1 (a weight ratio)  [Relational Formula 3]
 
   
     
     
         2 . The hot-rolled steel sheet of  claim 1 , further comprising:
 one or two or more selected from a group consisting of 0.04% or less (excluding 0%) of Ti, 0.005% or less (excluding 0%) of B and 0.03% or less (excluding 0%) of Sb.   
     
     
         3 . The hot-rolled steel sheet of  claim 1 , wherein the hot-rolled steel sheet has tensile strength of 600-1000 MPa. 
     
     
         4 - 6 . (canceled) 
     
     
         7 . A steel pipe, comprising:
 by weight %, 0.35-0.55% of C, 0.7-1.5% of Mn, 0.3% or less (excluding 0%) of Si, 0.03% or less (including 0%) of P, 0.004% or less (including 0%) of S, 0.04% or less (excluding 0%) of Al, 0.3% or less (excluding 0%) of Cr, 0.3% or less (excluding 0%) of Mo, one or two of 0.1-1.0% of Ni and 0.1-1.0% of Cu, 0.4% or more of Cu+Ni, 0.006% or less (excluding 0%) of N, and a balance of Fe and other impurities, the alloy elements satisfying relational formulae 1-3 as below, where a microstructure includes, by volume %, 10-60% of ferrite and 40-90% of pearlite.
   (Mn/Si)≥3 (a weight ratio)  [Relational Formula 1]
 
   (Ni+Cu)/(C+Mn)≥0.2 (a weight ratio)  [Relational Formula 2]
 
   (Ni/Si)≥1 (a weight ratio)  [Relational Formula 3]
 
   
     
     
         8 . The steel pipe of  claim 7 , wherein the steel pipe further includes one or two or more selected from a group consisting of 0.04% or less (excluding 0%) of Ti, 0.005% or less (excluding 0%) of B, and 0.03% or less (excluding 0%) of Sb. 
     
     
         9 - 12 . (canceled) 
     
     
         13 . A member, comprising:
 by weight %, 0.35-0.55% of C, 0.7-1.5% of Mn, 0.3% or less (excluding 0%) of Si, 0.03% or less (including 0%) of P, 0.004% or less (including 0%) of S, 0.04% or less (excluding 0%) of Al, 0.3% or less (excluding 0%) of Cr, 0.3% or less (excluding 0%) of Mo, one or two of 0.1-1.0% of Ni and 0.1-1.0% of Cu, 0.4% or more of Cu+Ni, 0.006% or less (excluding 0%) of N, and a balance of Fe and other impurities, the alloy elements satisfying relational formulae 1-3 as below, where a microstructure includes one or two of 90% or more of martensite and tempered martensite, and 10% or less of retained austenite.
   (Mn/Si)≥3 (a weight ratio)  [Relational Formula 1]
 
   (Ni+Cu)/(C+Mn)≥0.2 (a weight ratio)  [Relational Formula 2]
 
   (Ni/Si)≥1 (a weight ratio)  [Relational Formula 3]
 
   
     
     
         14 . The member of  claim 13 , wherein the member further includes one or two or more selected from a group consisting of 0.04% or less (excluding 0%) of Ti, 0.005% or less (excluding 0%) of B, and 0.03% or less (excluding 0%) of Sb. 
     
     
         15 . The member of  claim 13 , wherein the member has yield strength of 1400 MPa or higher and tensile strength of 1800 MPa or higher. 
     
     
         16 - 20 . (canceled)