Steel for pressure vessel having excellent surface quality and impact toughness, and method for manufacturing same

1. A steel material for a pressure vessel, the steel material comprising:

by weight percentage, wt %, 0.1 to 0.15% of carbon (C), 0.15 to 0.5% of silicon (Si), 1.2 to 1.8% of manganese (Mn), 0.01% or less, excluding 0%, of phosphorus (P), 0.01% or less, excluding 0%, of sulfur (S), 0.01 to 0.05% of aluminum (Al), 0.01 to 0.05% of niobium (Nb), 0.01 to 0.25% of nickel (Ni), 0.1% or less, excluding 0%, of copper (Cu), 0.01 to 0.1% of molybdenum (Mo), 0.01 to 0.05% of vanadium (V), 0.003% or less, excluding 0%, of titanium (Ti), 5 ppm or less, excluding 0 ppm, of boron (B), 20 to 100 ppm of nitrogen (N), a balance of iron (Fe) and inevitable impurities,

a composite structure of ferrite and bainitic ferrite as a microstructure, and

the steel material satisfying Relational Expression 1 below,

0.5<[(Ti+Nb+B)/3.5N+(Cu/Ni)]<2.5  [Relational Expression 1]

where each element refers to a content by wt %.

2. The steel material of claim 1 , wherein when a thickness of the steel material is greater than 35 mm, the steel material comprises ferrite having an area fraction of 40 to 80% and retained bainitic ferrite as the microstructure, and

wherein when the thickness of the steel material is 35 mm or less, the steel material comprises ferrite having an area fraction of 20 to 60%, tempered martensite having an area fraction of 20% or less, including 0%, and retained bainitic ferrite as the microstructure.

3. The steel material of claim 1 , wherein the steel material comprises an M(C,N) carbonitride having an average grain diameter of 100 nm or less in a microstructure, the M is niobium (Nb), titanium (Ti), or vanadium (V), and a ratio of the Nb is 20% or more.

4. The steel material of claim 1 , wherein a number of star cracks in a surface of the steel material is less than or equal to 1 per 50 m 2 unit area.

5. The steel material of claim 1 , wherein in a ¼t point, a tensile strength is 480 MPa or more, and Charpy impact absorption energy at a temperature of −50° C. is 150J or more, where t refers to a thickness, in mm, of the steel material.

6. The steel material of claim 1 , wherein in a ½t point, tensile strength is 480 MPa or more, and Charpy impact absorption energy at a temperature of −50° C. is 100J or more, where t refers to a thickness, in mm, of the steel material.

7. The steel material of claim 1 , wherein the steel material is subjected to a post weld heat treatment (PWHT).

8. The steel material of claim 1 , wherein the steel material has a thickness of 10 to 150 mm.

9. A method of manufacturing the steel material for the pressure vessel of claim 1 , the method comprising:

reheating a steel slab to a temperature of 1050 to 1200° C., the steel slab including, by weight percentage, wt %, 0.1 to 0.15% of carbon (C), 0.15 to 0.5% of silicon (Si), 1.2 to 1.8% of manganese (Mn), 0.01% or less, excluding 0%, of phosphorus (P), 0.01% or less, excluding 0%, of sulfur (S), 0.01 to 0.05% of aluminum (Al), 0.01 to 0.05% of niobium (Nb), 0.01 to 0.25% of nickel (Ni), 0.1% or less, excluding 0%, of copper (Cu), 0.01 to 0.1% of molybdenum (Mo), 0.01 to 0.05% of vanadium (V), 0.003% or less, excluding 0%, of titanium (Ti), 5 ppm or less, excluding 0 ppm, of boron (B), 20 to 100 ppm of nitrogen (N), a balance of iron (Fe) and inevitable impurities, and satisfying Relational Expression 1 below;

rough rolling the reheated steel slab within a temperature range of Ar3+100° C. to 1200° C.;

finish hot rolling the rough-rolled steel slab at a temperature of Ar3+30° C. or higher to manufacture a hot-rolled steel sheet;

air cooling the hot-rolled steel sheet to room temperature;

reheating the air-cooled hot-rolled steel sheet to a temperature of Ac3 or higher, and then maintaining the reheated hot-rolled steel sheet for (1.3t+20) minutes or more, where t refers to a thickness, in mm, of the steel material;

cooling the hot-rolled steel sheet to room temperature at a cooling rate of 2° C./sec or higher after the maintaining;

performing a tempering process in which the cooled hot-rolled steel sheet is maintained within a temperature range of 600 to 700° C. for ( 1 . 9 t+ 30) minutes or more; and

air cooling the hot-rolled steel sheet to room temperature after performing the tempering process,

0.5<[(Ti+Nb+B)/3.5N+(Cu/Ni)]<2.5  [Relational Expression 1]

where each element refers to a content by wt %.

10. The method of claim 9 , wherein the steel slab is a continuous casting steel slab obtained through continuous casting at a temperature of Ar3 or higher, or a forging steel slab obtained by forging the continuous casting steel slab to a thickness decreased by 10 to 60% of an initial thickness.

11. The method of claim 9 , further comprising:

performing a post weld heat treatment (PWHT) on the hot-rolled steel sheet, air-cooled to room temperature after performing the tempering process, within a temperature range of 595 to 635° C. for 120 minutes or more.