Cold rolled steel sheet having aging resistance and superior formability, and process for producing the same
A cold rolled steel sheet, and a method of manufacturing the same, designed to have aging resistance and excellent formability suitable for use in automobile bodies, electronic appliances, and the like. The cold rolled steel sheet comprises in weight %: 0.003% or less of C, 0.003˜0.03% of S, 0.01˜0.1% of Al, 0.02% or less of N, 0.2% or less of P, at least one of 0.03˜0.2% of Mn and 0.005˜0.2% of Cu, and a balance of Fe and other unavoidable impurities. When the steel sheet comprises one of Mn and Cu, the composition of Mn, Cu, and S satisfies at least one relationship: 0.58*Mn/S≦10 and 1≦0.5*Cu/S≦10, and when the steel sheet comprises both Mn and Cu, the composition of Mn, Cu, and S satisfies the relationship: Mn+Cu≦0.3 and 2≦0.5*(Mn+Cu)/S≦20. Participates of MnS, CuS, and (Mn, Cu)S in the steel sheet have an average size of 0.2 μm or less. Since carbon content in a solid solution state in a crystal grain is controlled by fine precipitates of MnS, CuS, or (Mn, Cu)S, the steel sheet has enhanced aging resistance and formability, and has excellent yield strength and strength-ductility.
1. A cold rolled steel sheet for an automobile having aging resistance, comprising in weight %: 0.003% or less of C; 0.005˜0.03% of S; 0.01˜0.1% of Al; 0.02% or less of N; 0.2% or less of P; 0.05˜0.2% of Mn; 0.2˜1.2% Cr and the balance of Fe and other unavoidable impurities, wherein a composition of Mn and S satisfies the relationship: 0.58*Mn/S≦10, wherein the steel sheet comprises precipitates of MnS having an average size of 0.2 μm or less, and wherein the steel has a yield strength of 197 MPa or more, an r m -value of 1.47 or more, and an aging index of 27 MPa or less.
2. The steel sheet as set forth in claim 1 , wherein the steel sheet comprises 0.004% or less of N.
3. The steel sheet as set forth in claim 1 , further comprising 0.1˜0.8% of Si.
4. The steel sheet as set forth in claim 3 , further comprising 0.01˜0.2% of Mo.
5. The steel sheet as set forth in claim 4 , further comprising 0.01˜0.2% of V.
6. The steel sheet as set forth claim 3 , further comprising 0.01˜0.2% of V.
7. The steel sheet as set forth in claim 1 , wherein the steel sheet comprises 0.005˜0.02% of N and 0.03˜0.06% of P.
8. The steel sheet as set forth in claim 7 , wherein the composition of Al and N satisfies the relationship: 1≦0.52*Al/N≦5.
9. The steel sheet as set forth in claim 1 , further comprising 0.01˜0.2% of Mo.
10. The steel sheet as set forth in claim 1 , further comprising 0.01˜0.2% of V.
11. The steel sheet as set forth in claim 10 , wherein the composition of V and C satisfies the relationship: 1≦0.25*V/C≦20.
12. The steel sheet as set forth in claim 1 , wherein the steel sheet comprises 0.015% or less of P.
13. The steel sheet as set forth in claim 1 , wherein the steel sheet comprises 0.03˜0.2% of P.
14. A cold rolled steel sheet for an automobile having aging resistance comprising in weight %: 0.0005˜0.003% of C; 0.003˜0.025% of S; 0.01˜0.08% of Al; 0.02% or less of N; 0.2% or less of P; 0.052˜0.2% of Cu; and the balance of Fe and other unavoidable impurities, wherein a composition of Cu and S satisfies the relationship: 1≦0.5*Cu/S≦10, wherein the steel sheet comprises precipitates of CuS having an average size of 0.1 μm or less, and wherein the steel has a yield strength of 166 MPa or more, an r m -value of 1.45 or more, and an aging index of 28 MPa or less.
15. The steel sheet as set forth in claim 14 , wherein the steel sheet comprises 0.015% or less of P.
16. The steel sheet as set forth in claim 14 , wherein the steel sheet comprises 0.004% or less of N.
17. The steel sheet as set forth in claim 14 , wherein the composition of Cu and S satisfies the relationship: 1≦0.5*Cu/S≦3.
18. The steel sheet as set forth in claim 14 , wherein the steel sheet comprises 0.03˜0.2% of P.
19. The steel sheet as set forth in claim 14 , further comprising 0.2˜1.2% of Cr.
20. The steel sheet as set forth in claim 19 , further comprising 0.01˜0.2% of Mo.
21. The steel sheet as set forth in claim 20 , further comprising 0.01˜0.2% of V.
22. The steel sheet as set forth claim 19 , further comprising 0.01˜0.2% of V.
23. The steel sheet as set forth in claim 14 , wherein the steel sheet comprises 0.005˜0.02% of N and 0.03˜0.06% of P.
24. The steel sheet as set forth in claim 23 , wherein the composition of Al and N satisfies the relationship: 10.52*Al/N≦5.
25. The steel sheet as set forth in claim 14 , further comprising 0.01˜0.2% of Mo.
26. The steel sheet as set forth in claim 14 , further comprising 0.01˜0.2% of V.
27. The steel sheet as set forth in claim 26 , wherein the composition of V and C satisfies the relationship: 1≦0.25*V/C≦20.
28. A cold rolled steel sheet for an automobile having aging resistance, comprising in weight %: 0.0005˜0.003% of C; 0.003˜0.025% of S; 0.01˜0.08% of Al; 0.02% or less of N; 0.2% or less of P; 0.03˜0.2% of Mn; 0.005˜0.2% of Cu; and the balance of Fe and other unavoidable impurities, wherein a composition of Mn, Cu, and S satisfies the relationship: Mn+Cu≦0.3 and 2≦0.5*(Mn+Cu)/S≦20, wherein the steel sheet includes precipitates of MnS, CuS, and (Mn, Cu)S having an average size of 0.2 μm or less, wherein the steel sheet has an Aging Index (AI) of 30 MPa or less, and wherein the steel sheet has a yield strength of 162 MPa or more and an r m -value of 1.55 or more.
29. The steel sheet as set forth in claim 28 , wherein the steel sheet comprises 0.015% or less of P.
30. The steel sheet as set forth in claim 28 , wherein the steel sheet comprises 0.004% or less of N.
31. The steel sheet as set forth in claim 28 , wherein the composition of Mn, Cu and S satisfies the relationship: 2≦0.5*(Mn+Cu)/S≦7.
32. The steel sheet as set forth in claim 31 , wherein the steel sheet comprises 0.03˜0.2% of P.
33. The steel sheet as set forth in claim 31 , further comprising at least one of 0.1˜0.8% of Si and 0.2˜1.2% of Cr.
34. The steel sheet as set forth in claim 33 , further comprising 0.01˜0.2% of V.
35. The steel sheet as set forth claim 33 , further comprising 0.01˜0.2% of V.
36. The steel sheet as set forth in claim 31 , wherein the steel sheet comprises 0.005˜0.02% of N and 0.03˜0.06% of P.
37. The steel sheet as set forth in claim 36 , wherein the composition of Al and N satisfies the relationship: 1 0.52*Al/N≦5.
38. The steel sheet as set forth in claim 28 , further comprising 0.01˜0.2% of Mo.
39. The steel sheet as set forth in claim 38 , further comprising 0.01˜0.2% of V.
40. The steel sheet as set forth in claim 28 , further comprising 0.01˜0.2% of V.
41. The steel sheet as set forth in claim 40 , wherein the composition of V and C satisfies the relationship: 1≦0.25*V/C≦20.