Cold rolled steel sheet for flux-cored wire, and manufacturing method therefor
The purpose of one aspect of the present disclosure is to provide: a cold rolled steel sheet for a flux-cored wire, having excellent low temperature toughness, welding workability and processability; and a manufacturing method thereof. One embodiment of the present disclosure provides: a cold rolled steel sheet for a flux-cored wire, comprising, by wt %, 0.005-0.10% of C, 0.05-0.25% of Mn, 0.05% or less of Si (excluding 0%), 0.0005-0.01% of P, 0.008% or less of S (excluding 0%), 0.005-0.06% of Al, 0.0005-0.003% of N, 0.8-1.7% of Ni, 0.1-0.5% of Cr, and a balance of Fe and inevitable impurities, and having 0.10-0.75 of WN defined by Relationship 1 below; and a manufacturing method therefor. WN=(31×C+0.5×Mn+20×Al)×(Ni)×(0.6×Cr). Relationship 1
1. A cold rolled steel sheet for a flux-cored wire, comprising, by wt %:
0.035% to 0.10% of carbon (C), 0.05% to 0.25% of manganese (Mn), more than 0% and 0.05% or less of silicon (Si), 0.0005% to 0.01% of phosphorus (P), more than 0% and 0.008% or less of sulfur (S), 0.005% to 0.06% of aluminum (Al), 0.0005% to 0.003% of nitrogen (N), 0.8% to 1.7% of nickel (Ni), 0.1% to 0.5% of chromium (Cr), and a balance of iron (Fe) and inevitable impurities, and having 0.10 to 0.75 of W N defined by Relationship 1 below,
W N =(31×C+0.5×Mn+20×Al)×(Ni)×(0.6×Cr), Relationship 1
where a unit for a content of each element in Relationship 1 is weight %, and
wherein the cold rolled steel sheet has a segregation index of a welded portion that is less than 0.15%.
2. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet has a microstructure consisting of, by area %, 1% to 6% of cementite and a balance of ferrite.
3. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet has elongation of 40% or above.
4. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet has an impact energy of 50 J or greater at −40° C.
5. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet comprises 0.06 to 0.24% of manganese (Mn).
6. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet comprises more than 0% and 0.04% or less of silicon (Si).
7. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet comprises 0.001% to 0.009% of phosphorus (P).
8. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet comprises more than 0% and 0.0075% or less of sulfur (S).
9. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet comprises 0.007% to 0.05% of aluminum (Al).
10. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet comprises 0.008% to 0.0029% of nitrogen (N).
11. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet comprises 0.8% to 1.65% of nickel (Ni).
12. The cold rolled steel sheet of claim 1 , wherein the cold rolled steel sheet comprises 0.13% to 0.45% of chromium (Cr).
13. A method for manufacturing the cold rolled steel sheet of claim 1 comprising:
heating a slab comprising, by wt %, 0.035% to 0.10% of carbon (C), 0.05% to 0.25% of manganese (Mn), more than 0% and 0.05% or less of silicon (Si), 0.0005% to 0.01% of phosphorus (P), more than 0% and 0.008% or less of sulfur (S), 0.005% to 0.06% of aluminum (Al), 0.0005% to 0.003% of nitrogen (N), 0.8% to 1.7% of nickel (Ni), 0.1% to 0.5% of chromium (Cr), and a balance of iron (Fe) and inevitable impurities, and having 0.10 to 0.75 of W N defined by Relationship 1 below to 1100° C. to 1300° C.;
hot rolling the heated slab such that a finish rolling temperature is 880° C. to 950° C. to obtain a hot rolled steel sheet;
coiling the heated hot rolled steel sheet in a temperature range of 550° C. to 700° C.;
cold rolling the coiled hot rolled steel sheet at a rolling reduction ratio of 50% to 85% to obtain a cold rolled steel sheet; and
continuously annealing the cold rolled steel sheet in a temperature range of 700° C. to 850° C.,
W N =(31×C+0.5×Mn+20×Al)×(Ni)×(0.6×Cr), Relationship 1
where a unit for a content of each element in Relationship 1 is weight %.
14. The manufacturing method of claim 13 , further comprising pickling the coiled hot rolled steel sheet before the cold rolling.
15. The manufacturing method of claim 13 , further comprising skin-pass rolling the continuously annealed cold rolled steel sheet.