Hot rolled steel sheet for ground reinforcement and steel pipe for ground reinforcement, and manufacturing methods thereof

Abstract

The present invention relates to: a hot rolled steel sheet for ground reinforcement and a steel pipe for ground reinforcement, which have excellent strength and formability; and manufacturing methods thereof. An aspect of the present disclosure is to provide a hot-rolled steel sheet for ground reinforcement and a steel pipe for ground reinforcement, having excellent strength and formability, and manufacturing methods thereof.

1 . A hot-rolled steel sheet for ground reinforcement, comprising by weight:
 0.05 to 0.1% of C, 0.1% or less (excluding 0%) of Si, 1.5 to 1.9% of Mn, 0.05 to 0.15% of Ti, 0.03 to 0.1% of Nb, 0.03 to 0.1% of Mo, 0.02% or less (excluding 0%) of P, 0.02% or less (excluding 0%) of S, 0.01% or less (excluding 0%) of N, with a balance of Fe and inevitable impurities, wherein the following Relational expressions 1 and 2 are satisfied,   wherein the hot-rolled steel sheet for ground reinforcement has a microstructure including by area, 90% or more of ferrite,   wherein a grain of the ferrite has an average size of 15 μm or less,   wherein the hot-rolled steel sheet includes by weight, 0.05% or more of a carbide containing Ti, Nb, and Mo alone or in combination thereof,   wherein the carbide has an average size of 20 nm or less,
   0.002≤(Ti/48+Mo/96+Nb/93)≤0.004  [Relational expression 1]
 
   0.002≤(C/12)−(Ti/48+Mo/96+Nb/93)≤0.006  [Relational expression 2]
 
   in the above Relational expressions 1 and 2, where a content of each alloy element refers to weight %.   
     
     
         2 . The hot-rolled steel sheet for ground reinforcement of  claim 1 , wherein the microstructure comprises one or more of pearlite, retained austenite, bainite, and martensite, as a remaining structure thereof. 
     
     
         3 . The hot-rolled steel sheet for ground reinforcement of  claim 1 , wherein the hot-rolled steel sheet has a yield strength (YS) of 700 MPa or more, a tensile strength (TS) of 750 MPa or more, and an elongation (EL) of 15% or more. 
     
     
         4 . A steel pipe for ground reinforcement manufactured using the hot-rolled steel sheet described in  claim 1 . 
     
     
         5 . The steel pipe for ground reinforcement of  claim 4 , the steel pipe has a yield strength (YS) of 800 MPa or more, a tensile strength (TS) of 860 MPa or more, and an elongation (EL) of 10% or more. 
     
     
         6 . The steel pipe for ground reinforcement of  claim 4 , the steel pipe has a deviation in hardness of 15% or less,
 where the deviation in hardness is defined as [(maximum hardness value-minimum hardness value)/maximum hardness value×100] from each hardness value measured at points of 0.5 mm, t/4, and t/2, where t is a thickness of the steel pipe, in a thickness direction from a surface of the steel pipe.   
     
     
         7 . A method for manufacturing a hot-rolled steel sheet for ground reinforcement, comprising:
 reheating a steel slab including, by weight:   0.05 to 0.1% of C, 0.1% or less (excluding 0%) of Si, 1.5 to 1.9% of Mn, 0.05 to 0.15% of Ti, 0.03 to 0.1% of Nb, 0.03 to 0.1% of Mo, 0.02% or less (excluding 0%) of P, 0.02% or less (excluding 0%) of S, 0.01% or less (excluding 0%) of N, with a balance of Fe and inevitable impurities, wherein the following Relational expressions 1 and 2 are satisfied, at a temperature within a range of 1150° C. to 1300° C.;   finish hot rolling the reheated steel slab at a temperature within a range of 800 to 950° C. to obtain a hot-rolled steel sheet; and   coiling the hot-rolled steel sheet at a temperature within a range of 550° C. to 700° C.,
   0.002≤(Ti/48+Mo/96+Nb/93)≤0.004  [Relational expression 1]
 
   0.002≤(C/12)−(Ti/48+Mo/96+Nb/93)≤0.006  [Relational expression 2]
 
   in the above Relational expressions 1 and 2, where a content of each alloy element refers to weight %.   
     
     
         8 - 9 . (canceled)