High carbon steel sheet superior in tensile strength and elongation and method for manufacturing the same

1. A high carbon steel sheet comprises 0.2 to 1.0 wt % carbon (C), greater than about 1 wt % and less than or equal to 3.0 wt % silicon (Si), greater than 0 wt % and less than or equal to 3.0 wt % manganese (Mn), greater than 0 wt % and less than or equal to 3.0 wt % chromium (Cr), greater than 0 wt % and less than or equal to 3.0 wt % nickel (Ni), greater than 0 wt % and less than or equal to 0.5 wt % molybdenum (Mo), greater than about 1 wt % and less than or equal to 3.0 wt % aluminum (Al), greater than 0 wt % and less than or equal to 0.01 wt % boron (B), greater than 0 wt % and less than or equal to 0.5 wt % titanium (Ti), and the remainder substantially being iron (Fe) and inevitable impurities,

the contents of carbon, manganese, chromium, and nickel satisfy the following Equation 1, and the contents of silicon and aluminum satisfy the following Equation 2, and the carbon (C), manganese (Mn), chromium (Cr), nickel (Ni), and aluminum (Al) satisfy the following equation 3:

(3.0−2.5×C)wt %≦(Mn+Cr+Ni/2)8.5 wt %  (Equation 1)

Si+Al> about 2.0 wt %  (Equation 2),

Log 10 [50% transformation time (sec)]=−2.742+3.561×C+0.820×Mn+0.416×Cr+0.402×Ni−0.332×Al+1330/(T+273)≦Log 10 [3×3600]  (Equation 3),

wherein T is a temperature in degrees Celsius and represents a transformation temperature, and 50% transformation time is a minimum time required for 50% transformation into bainite.

2. The high carbon steel sheet of claim 1 , wherein

the high carbon steel sheet comprises a fine microstructure, the fine microstructure comprises residual austenite, and the volume percentage of the residual austenite in the fine microstructure ranges from 15 vol % to 50 vol %.

3. The high carbon steel sheet of claim 2 , wherein

the fine microstructure further comprises bainite, and the bainite ranges from 50 vol % to 85 vol %.

4. The high carbon steel sheet of claim 3 , wherein

the tensile strength of the high carbon steel sheet is greater than 1000 MPa, and the elongation thereof is greater than 10%.

5. The high carbon steel sheet of claim 4 , wherein

the titanium (Ti) and nitrogen (N) satisfy the following Equation 6:

Ti(wt %)>N(wt %)×3.42  (Equation 6).

6. The high carbon steel sheet of claim 4 , wherein

the carbon (C) in the composition of the high carbon steel sheet ranges from 0.4 wt % to 1.0 wt %, and

the contents of the manganese (Mn), chromium (Cr), and nickel (Ni), satisfy the following equation:

1.5 wt %≦(Mn+Cr+Ni/2)≦8.5 wt %.

7. The high carbon steel sheet of claim 4 , wherein

the carbon (C) in the composition of the high carbon steel sheet ranges from 0.2 wt % to 0.7 wt %, and

the contents of the manganese (Mn), chromium (Cr), and nickel (Ni) satisfy the following equation:

3.0 wt %≦(Mn+Cr+Ni/2)≦8.5 wt %.

8. A method for manufacturing a high carbon steel sheet, comprising:

i) preparing a high carbon steel sheet comprising 0.2 to 1.0 wt % carbon (C), greater than about 1 wt % and less than or equal to 3.0 wt % silicon (Si), greater than 0 wt % and less than or equal to 3.0 wt % manganese (Mn), greater than 0 wt % and less than or equal to 3.0 wt % chromium (Cr), greater than 0 wt % and less than or equal to 3.0 wt % nickel (Ni), greater than 0 wt % and less than or equal to 0.5 wt % molybdenum (Mo), greater than about 1 wt % and less than or equal to 3.0 wt % aluminum (Al), greater than 0 wt % and less than or equal to 0.01 wt % boron (B), greater than 0 wt % and less than or equal to 0.5 wt % titanium (Ti), and the remainder substantially being iron (Fe) and inevitable impurities;

ii) austenitizing the high carbon steel sheet;

iii) cooling the high carbon steel sheet while maintaining the austenite structure; and

iv) isothermally transforming the austenitized high carbon steel sheet in a temperature range from 150° C. below the bainite transformation starting temperature to the bainite transformation starting temperature,

wherein the contents of carbon, manganese, chromium, and nickel satisfy the following Equation 1, and the silicon and the aluminum satisfy the following Equation 2, and the carbon (C), manganese (Mn), chromium (Cr), nickel (Ni), and aluminum (Al) satisfy the following Equation 3:

(3.0−2.5×C)wt %≦(Mn+Cr+Ni/2)≦8.5 wt %  (Equation 1)

Si+Al> about 2.0 wt %  (Equation 2),

Log 10 [50% transformation time (sec)]=−2.742+3.561×C+0.820×Mn+0.416×Cr+0.402×Ni−0.332×Al+1330/(T+273)≦Log 10 [3×3600]  (Equation 3),

wherein T is a temperature in degrees Celsius and represents a transformation temperature, and 50% transformation time is a minimum time required for 50% transformation into bainite.

9. The method of claim 8 , wherein, in the isothermal transformation, the isothermal transformation is carried out for one minute to 48 hours.

10. The method of claim 9 , wherein, during the isothermal transformation, the bainite transformation of the high carbon steel sheet is finished at greater than 50 vol % and less than 100 vol %.

11. The method of claim 10 , wherein the time taken to complete 50 vol % bainite transformation of the high carbon steel sheet is more than one minute and less than three hours.

12. The method of claim 9 , wherein the bainite transformation starting temperature satisfies the following equation:

bainite transformation starting temperature (Bs) (° C.)=830−270×C(wt %) 90×Mn(wt %)−37×Ni(wt %)−70×Cr(wt %)−83×Mo(wt %).

13. The method of claim 8 , wherein the cooling is performed on a run-out table at a cooling speed of 10-50°/sec.

14. The method of claim 13 , wherein the isothermal transformation is performed by coiling the high carbon steel sheet.

15. The method of claim 14 , wherein

the bainite transformation starting temperature satisfies the following equation:

bainite transformation starting temperature (Bs) (° C.)=830−270×C(wt %)−90×Mn(wt %)−37×Ni(wt %)−70×Cr(wt %)−83×Mo(wt %).

16. The high carbon steel sheet of claim 1 , wherein the content of at least one of silicon and aluminum is greater than 1.0 wt % and less than or equal to 3.0 wt %.

17. The method of claim 8 , wherein the content of at least one of silicon and aluminum is greater than 1.0 wt % and less than or equal to 3.0 wt %.