WHAT IS CALIMED IS:
1. A method of manufacturing a high-carbon hot-rolled steel sheet,
comprising the steps of:
preparing high-carbon steel materials comprising C: 0.7 to 0.9%, Si: 0.5% 5 or less, Mn: 0.1 to 1.5%, Cr: 0.5% or less, P: 0.05% or less, and S: 0.03% or less in wt%, remaining Fe, and other inevitable impurities;
heating the high-carbon steel materials again and manufacturing a steel
sheet by performing hot rolling in an austenite region in which a finishing
temperature for the hot rolling is an Ar3 transformation temperature or higher;
10 rapidly cooling the steel sheet at 520 to 620°C before phase
transformation is started in a Run-Out Table (ROT);
uniformly maintaining a cooling retention temperature so that the
cooled steel sheet is subject to phase transformation in any one temperature
between 520 to 620°C;
15 winding the steel sheet in the cooling retention temperature.
2. The method of claim 1, wherein in the step of rapidly cooling the steel
sheet, the steel sheet has a phase transformation fraction of 10% or less during
the cooling.
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3. The method of claim 2, wherein in the cooling retention temperature,
the steel sheet remains uniform in a range of ±20°C of the cooling retention
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temperature.
4. The method of claim 2, wherein in the cooling retention temperature,
the steel sheet remains uniform in a range of ±5°C of the cooling retention.
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5. The method of claim 3, wherein in the step of winding the steel sheet,
the steel sheet is wound when a phase transformation fraction is 70% or more.
6. The method of claim 5, wherein in the step of uniformly maintaining a
10 cooling retention temperature, an upper part of the steel sheet passing through
the ROT is cooled by air and a lower part of the steel sheet passing through the ROT is cooled by water.
7. The method of any one of claims 1 to 6, wherein in the step of
15 performing hot rolling, the steel sheet is subject to hot rolling in a thickness of
1.4 mm to 4.0 mm.
8. The method of any one of claims 1 to 6, wherein in the step of rapidly
cooling the steel sheet, cooling speed of the steel sheet is 50 to 300°C/sec.
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9. The method of any one of claims 1 to 6, wherein in the step of uniformly
maintaining a cooling retention temperature, the steel sheet is maintained for 5
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seconds to 60 seconds.
10. The method of any one of claims 1 to 6, wherein one or more processes
selected from a pickling and oiling line process, a spheroidizing annealing
5 process, and a primary cold-rolled process is not performed on the wound steel sheet.
11. The method of any one of claims 1 to 6, further comprising a step of
performing cold rolling on the wound steel sheet at a reduction ratio of 70% or
10 more by omitting a heat treatment process.
12. The method of claim 7, further comprising a step of performing cold
rolling on the wound steel sheet at a reduction ratio of 70% or more by omitting
a heat treatment process.
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13. A high-carbon hot-rolled steel sheet, comprising high-carbon steel
materials comprising C: 0.7 to 0.9%, Si: 0.5% or less, Mn: 0.1 to 1.5%, Cr: 0.5% or
less, P: 0.05% or less, and S: 0.03% or less in wt%, remaining Fe, and other
inevitable impurities, wherein a microstructure of the steel materials comprises
20 a fine pearlite phase having a lamellar structure in which an interlayer interval between stratified carbide layers is 50 to 200 nm.
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14. The high-carbon hot-rolled steel sheet of claim 13, wherein the interlayer interval between the stratified carbide layers of the fine pearlite phase has a uniform size within ±20 nm.
5 15. The high-carbon hot-rolled steel sheet of claim 13, wherein an average colony size or particle size of the fine pearlite phase is 1 to 5µm.
16. The high-carbon hot-rolled steel sheet of any one of claim 14 to claim 15,
wherein the fine pearlite phase has a volumetric fraction of 70% or more.
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17. The high-carbon hot-rolled steel sheet of claim 16, wherein a sum of
volumetric fractions of the fine pearlite phase and a bainite phase is 90% or
more.
15 18. The high-carbon hot-rolled steel sheet of any one of claims 13 to 15 and 17, wherein the hot-rolled steel sheet has Vickers hardness of 300 to 400 HV.
19. A high-carbon cold-rolled steel sheet cold-rolled using a hot-rolled steel
sheet according to any one of claims 13 to 15 and 17.
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20. A high-carbon cold- and hot-rolled steel sheet manufactured using the
method of claim 12.
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