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US 6,638,371 B1Granted
Cold-rolled steel sheet having ultrafine grain structure and method for manufacturing the same
AssigneeKawasaki Steel Corporation
InventorsTetsuo Mochida, Kazuhiro Seto, Kei Sakata, Tomohisa Oonishi
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Abstract
A cold-rolled steel sheet having an ultrafine grain structure including a ferrite phase provided. The cold-rolled steel sheet contains C, Si, Mn, Ni, Ti, Nb, Al, P, S, N and Fe and incidental impurities. The ferrite phase has a content of 65 percent by volume or more and an average grain size of 3.5 μm or less.
Claims14
A cold-rolled steel sheet having an ultrafine grain structure including a ferrite phase, the cold-rolled steel sheet comprising:
- 0.03 to 0.16 mass percent of C;
- 2.0 mass percent or less of Si;
- at least one of 3.0 mass percent or less of Mn and 3.0 mass percent or less of Ni;
- at least one of 0.2 mass percent or less of Ti and 0.2 mass percent or less of Nb;
- 0.01 to 0.1 mass percent of Al;
- 0.1 mass percent or less of P;
- 0.02 mass percent or less of S;
- 0.005 mass percent or less of N;
- and
- Fe and incidental impurities,
- wherein the ferrite phase has a content of 65 percent by volume or more and an average grain size of 3.5 μm or less, and the C, Si, Mn, Ni, Ti, and Nb satisfy expressions (1), (2), and (3):
- 637.5+4930(Ti*+(48/93)·[%Nb])>A1 (1)
- A3<860 (2)
- [%Mn]+[%Ni]>1.3 (3)
- where
- Ti*=[%Ti]−(48/32)·[%S]−(48/14)·[%N];
- A1=727+14[%Si]−28.4[%Mn]−21.6[%Ni];
- A3=920+612.8[%C]2−507.7[%C]+9.8[%Si]3−9.5[%Si]2+68.5[%Si]+2[%Mn]2−38[%Mn]+2.8[%Ni]2−38.6[%Ni]+102[%Ti]+51.7[%Nb];
- and
- [%M] represents element M content (mass %).
17 more elements
A method for manufacturing a cold-rolled steel sheet having an ultrafine grain structure, the method comprising:
- heating a starting steel material to a temperature of 1200° C. or more;
- hot-rolling the starting steel material;
- cold-rolling the hot-rolled material;
- performing recrystallization annealing at a temperature in the range of A3° C. to (A3+30)° C.;
- and
- cooling the annealed material to 600° C. or less at a rate of 5° C./s or more,
- wherein the starting steel material comprises:
- 0.03 to 0.16 mass percent of C;
- 2.0 mass percent or less of Si;
- at least one of 3.0 mass percent or less of Mn and 3.0 mass percent or less of Ni;
- at least one of 0.2 mass percent or less of Ti and 0.2 mass percent or less of Nb;
- 0.01 to 0.1 mass percent of Al;
- 0.1 mass percent or less of P;
- 0.02 mass percent or less of S;
- 0.005 mass percent or less of N;
- and
- Fe and incidental impurities, and
- wherein the C, Si, Mn, Ni, Ti, and Nb satisfy expressions (1), (2), and (3):
- 637.5+4930(Ti*+(48/93)·[%Nb])>A1 (1)
- A3<860 (2)
- [%Mn]+[%Ni]>1.3 (3)
- where
- Ti*=[%Ti]−(48/32)·[%S]−(48/14)·[%N];
- A1=727+14[%Si]−28.4[%Mn]−21.6[%Ni];
- A3=920+612.8[%C]2−507.7[%C]+9.8[%Si]3−9.5[%Si]2+68.5[%Si]+2[%Mn]2−38[%Mn]+2.8[%Ni]2−38.6[%Ni]+102[%Ti]+51.7[%Nb];
- and
- [%M] represents element M content (mass %).
24 more elements
Dependencies are inferred from the claim text.