Ultra-high-strength hot-rolled steel sheet, steel pipe, member, and manufacturing methods therefor
A preferable aspect of the present invention provides: an ultra-high-strength hot-rolled steel sheet containing, by weight, one or two of 0.40-0.60% 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, 0.9-1.5% of Ni, and 0.9-1.5% of Cu, 1.1% or more of Cu+Ni, 0.04% or less (excluding 0%) of Ti, 0.005% or less (excluding 0%) of B, 0.006% or less (excluding 0%) of N, and the balance Fe and other impurities, the alloy elements satisfying relational formulas 1 and 2 below, wherein a microstructure of the hot-rolled steel sheet comprises, by volume, 7% or more of ferrite and 93% or less of perlite; a steel pipe and a member each using the same; and manufacturing methods therefor. [Relational formula 1] (Mn/Si) #3 (weight ratio) [Relational formula 2] (Ni/Si) #1 (weight ratio)
1. A hot-rolled steel sheet for a hot press member comprising, by weight, C: 0.43 to 0.60%, Mn: 0.7 to 1.5%, Si: 0.3% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (excluding 0%), Cr: 0.3% or less (excluding 0%), Mo: 0.3% or less (excluding 0%), Ni: 0.099 to 1.5%, Cu: 0.096 to 1.5%, Cu+Ni: 1.1% or more, Ti: 0.04% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.006% or less (excluding 0%), and a balance of Fe and impurities,
wherein the alloying element satisfies the following relationships 1 and 2,
the hot-rolled steel sheet includes a microstructure comprising, by volume, 7 to 30% of ferrite and 70 to 93% of pearlite, and
the hot-rolled steel sheet has a tensile strength of 710 MPa or more:
(Mn/Si)≥3 (weight ratio) [Relationship 1]
15.8≥(Ni/Si)≥1 (weight ratio) [Relationship 2]
2. The hot-rolled steel sheet for a hot press member according to claim 1 , wherein the hot-rolled steel sheet has a tensile strength of 710 to 1000 MPa.
3. The hot-rolled steel sheet for a hot press member according to claim 1 , comprising, by weight, C: 0.487 to 0.60%.
4. The hot-rolled steel sheet for a hot press member according to claim 1 , comprising, by weight, Mn: 0.7 to 0.912%.
5. The hot-rolled steel sheet for a hot press member according to claim 1 , comprising, by weight, Ni: 1.21 to 1.5%.