High strength thin steel material for API having excellent resistance to deformation and method of manufacturing same
An embodiment of the present invention provides a high strength thin steel material for API having excellent resistance to deformation and a method of manufacturing same, the steel material comprising, in weight %, C: 0.05-0.15%, Si: 0.5% or less (0% excluded), Mn: 0.5-2.5%, Nb: 0.05% or less (0% excluded), V: 0.004% or less (0% excluded), Mo: 0.03-0.2%, Cr: 0.1-0.3%, P: 0.03% or less (0% excluded), S: 0.015% (0% excluded), Al: 0.05% or less (0% excluded), N: 0.01% or less (0% excluded), and the balance being Fe and other inevitable impurities, wherein the microstructure of the steel material comprises, in area %, ferrite: 10~30% and the balance being bainite, the ferrite having an average crystal grain size of 15-30 μm and including 3,000/μm 2 or less V-based precipitates.
1 . A steel material, the steel material comprising:
by weight %, C: 0.05-0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.5-2.5%, Nb: 0.05% or less (excluding 0%), V: 0.004% or less (excluding 0%), Mo: 0.03-0.2%, Cr: 0.1-0.3%, P: 0.03% or less (excluding 0%), S: 0.015% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), and a balance of Fe and other inevitable impurities,
wherein a microstructure of the steel material includes, by area %, ferrite: 10-30% and a balance of bainite,
wherein ferrite has an average grain size of 15-30 μm, and
wherein 3,000/μm 2 or less of V-based precipitates are included.
2 . The steel material of claim 1 , wherein the other inevitable impurities include at least one of Ni and Cu, and a content thereof is suppressed to Ni: 0.05% or less (excluding 0%) and Cu: 0.05% or less (excluding 0%).
3 . The steel material of claim 1 , wherein a carbon equivalent (Ceq) of the steel material, defined by [Equation 1] as below, is 0.4 or less:
Ceq =[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5 (1)
where [C], [Mn], [Cu], [Ni], [Cr], [Mo] and [V] may refer to contents (% by weight) of corresponding elements, respectively.
4 . The steel material of claim 1 , wherein the microstructure further includes 10% or less of pearlite.
5 . The steel material of claim 1 , wherein bainite has an average packet size of 5-20 μm.
6 . The steel material of claim 1 , wherein the V-based precipitate has an average diameter of 5-10 nm.
7 . The steel material of claim 1 , wherein the V-based precipitate has a maximum diameter of 20 nm or less.
8 . The steel material of claim 1 , wherein the steel material has yield strength: 500-700 MPa, tensile strength: 600-800 MPa, yield ratio: 80-85%, elongation: 20-30%, impact toughness at −30° C.: 80 J or more.
9 . A method of manufacturing a steel material, the method comprising:
reheating a slab including, by weight %, C: 0.05-0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.5-2.5%, Nb: 0.05% or less (excluding 0%), V: 0.004% or less (excluding 0%), Mo: 0.03-0.2%, Cr: 0.1-0.3%, P: 0.03% or less (excluding 0%), S: 0.015% (excluding 0%), Al: 0.05% or less (excluding 0%), N: 0.01% or less (excluding 0%), and a balance of Fe and other inevitable impurities at 1200-1400° C.,
obtaining a hot-rolled steel material by rough-rolling the reheated slab and finishing-rolling the slab at an austenite single-phase temperature;
water-cooling the hot-rolled steel material to a temperature of 650-750° C. at a rate of 40-60° C./sec and air-cooling the steel material for 3-7 seconds; and
water-cooling the air-cooled hot-rolled steel material to a temperature of 450-600° C. at a rate of 30-50° C./sec and winding the steel material.
10 . The method of claim 9 , wherein the other inevitable impurities include at least one of Ni and Cu, and a content thereof is suppressed to Ni: 0.05% or less (excluding 0%) and Cu: 0.05% or less (excluding 0%).
11 . The method of claim 9 , wherein the reheating the slab is performed for 100-400 minutes.
12 . The method of claim 9 , wherein a thickness of the rough-rolled slab relative to a thickness of the reheated slab is 10-25%.
13 . The method of claim 9 , wherein a temperature of the finishing-rolling is 800-1000° C.