High strength hot-rolled steel sheet having excellent workability, and method for manufacturing the same
Provided is a steel material that may be used for arms, frames, beams, brackets, reinforcing materials, etc. of chassis parts of a vehicle and, more specifically, to a high strength hot-rolled steel sheet having excellent workability and a method for manufacturing same. The steel sheet includes: by weight %, 0.1-0.15% of C, 2.0-3.0% of Si, 0.8-1.5% of Mn, 0.001-0.05% of P, 0.001-0.01% of S, 0.01-0.1% of Al, 0.7-1.7% of Cr, 0.0001-0.2% of Mo, 0.02-0.1% of Ti, 0.01-0.03% of Nb, 0.001-0.005% of B, 0.1-0.3% of V, 0.001-0.01% of N, and a balance of Fe and inevitable impurities, wherein tensile strength (TS) is 1180 MPa or more, a product (TS×El) of tensile strength and elongation is 20,000 MPa % or more, and a product (TS×HER) of tensile strength and hole expandability is 30,000 MPa % or more.
1 . A high strength hot-rolled steel sheet having excellent formability, comprising:
by weight %, 0.1-0.15% of C, 2.0-3.0% of Si, 0.8-1.5% of Mn, 0.001-0.05% of P, 0.001-0.01% of S, 0.01-0.1% of Al, 0.7-1.7% of Cr, 0.0001-0.2% of Mo, 0.02-0.1% of Ti, 0.01-0.03% of Nb, 0.001-0.005% of B, 0.1-0.3% of V, 0.001-0.01% of N, and a balance of Fe and inevitable impurities,
wherein [relational expression 1] and [relational expression 2] are satisfied,
wherein a microstructure of the hot-rolled steel sheet includes, by an area fraction, 5-15% of ferrite, 5-20% of retained austenite, and 10% or less of inevitable structure, in addition to a bainite matrix structure, and
wherein tensile strength (TS) is 1180 MPa or more, a product (TS×El) of tensile strength and elongation is 20,000 MPa % or more, and a product (TS×HER) of tensile strength and hole expandability is 30,000 MPa % or more,
20≤Hγ≤50
Hγ=194.5−(428[C]+11[Si]+45[Mn]+35[Cr]−10[Mo]−107[Ti]−56[Nb]−70[V]) [Relational expression 1]
where [elemental symbol] indicates a content (weight %) of each element
0.7 ≤a p ≤3.5
a p =([Mo]+[Ti]+[Nb]+[V])×[C] −1 [Relational expression 2]
where [elemental symbol] indicates a content (weight %) of each element.
2 . The high strength hot-rolled steel sheet of claim 1 , wherein the ferrite has an average hardness value of 200 Hv or more.
3 . The high strength hot-rolled steel sheet of claim 1 , wherein the inevitable structure is one or more of martensite, martensite austenite constituent (MA), and austenite.
4 . The high strength hot-rolled steel sheet of claim 1 , wherein, in a microstructure of the hot-rolled steel sheet, the number of precipitates having a diameter of 5 nm or more in ferrite present within 100 μm from a retained austenite grain boundary is 5×10 n /mm 2 (1≤n≤3).
5 . The high strength hot-rolled steel sheet of claim 4 , wherein the precipitates are carbide including one or more of Mo, Ti, Nb and V, or nitride including one or more of Mo, Ti, Nb and V.
6 . A high strength hot-rolled steel sheet having excellent formability, comprising:
by weight %, 0.1-0.15% of C, 2.0-3.0% of Si, 0.8-1.5% of Mn, 0.001-0.05% of P, 0.001-0.01% of S, 0.01-0.1% of Al, 0.7-1.7% of Cr, 0.0001-0.2% of Mo, 0.02-0.1% of Ti, 0.01-0.03% of Nb, 0.001-0.005% of B, 0.1-0.3% of V, 0.001-0.01% of N, and a balance of Fe and inevitable impurities,
wherein [relational expression 1] and [relational expression 2] are satisfied,
wherein, in a microstructure of the hot-rolled steel sheet, the number of precipitates having a diameter of 5 nm or more in ferrite present within 100 μm from a retained austenite grain boundary is 5×10 n /mm 2 (1≤n≤3), and
wherein tensile strength (TS) is 1180 MPa or more, a product (TS×El) of tensile strength and elongation is 20,000 MPa % or more, and a product (TS×HER) of tensile strength and hole expandability is 30,000 MPa % or more,
20≤Hγ≤50
Hγ=194.5−(428[C]+11[Si]+45[Mn]+35[Cr]−10[Mo]−107[Ti]−56[Nb]−70[V]) [Relational expression 1]
where [elemental symbol] indicates a content (weight %) of each element
0.7 ≤a p ≤3.5
a p =([Mo]+[Ti]+[Nb]+[V])×[C] −1 [Relational expression 2]
where [elemental symbol] indicates a content (weight %) of each element.
7 . The high strength hot-rolled steel sheet of claim 6 , wherein the precipitates are carbide including one or more of Mo, Ti, Nb and V, or nitride including one or more of Mo, Ti, Nb and V.