Work hardenable yield ratio-controlled steel and method of manufacturing the same

1. A method of manufacturing a yield ratio-controlled steel, comprising the steps of :

subjecting an alloy of steel, comprising 0.10 to 0.40 wt % carbon (C), 0.90 to 1.50 wt % manganese (Mn), 0.50 to 2.50 wt % silicon (Si), and 0.060 wt % or less but exceeding 0 wt % aluminum (Al), with a remainder of iron (Fe) and unavoidable impurities, to rolling or wire drawing, thus obtaining a material;

performing a first heat treatment on the material, in which the material is heated to and maintained for a first predetermined period of time at a first temperature ranging from A c1 to A c3 , which are A c1 and A c3 transformation temperatures; and

immediately after the end of the first predetermine period of time, cooling the material to a second temperature ranging from (25° C+Ms)/2 to Ms, where Ms is a martensite start temperature;

performing a second heat treatment immediately after the material is cooled to the second temperature, in which the material is maintained at the second temperature for a second predetermined period of time; and

generating in the material bainite or martensite based on ferrite.

2. The method of claim 1 , wherein the cooling step is performed by air cooling or water quenching.

3. The method of claim 1 , wherein the yield ratio-controlled steel is a Non-Heat-Treated steel for cold forging with a tensile strength of at least 60 to 140 kgf/mm 2 .

4. The method of claim 1 , wherein the yield ratio-controlled steel is a Non-Heat-Treated steel for cold forging with a tensile strength of 80 kgf/mm 2 , and comprises 0.15 wt % C, 1.5 wt % Mn, 1.5 wt % Si, and 0.050 wt % Al, with the remainder of Fe and unavoidable impurities.

5. The method of claim 1 , wherein the yield ratio-controlled steel is a Non-Heat-Treated steel for cold forging with a tensile strength of 100 kgf/mm 2 , and comprises 0.22 wt % C, 1.5 wt % Mn, 1.5 wt % Si, and 0.050 wt % Al, with the remainder of Fe and unavoidable impurities.

6. The method of claim 1 , wherein the yield ratio-controlled steel is a Non-Heat-Treated steel for cold forging with a tensile strength of 110 kgf/mm 2 , and comprises 0.30 wt % C, 1.5 wt % Mn, 1.5 wt % Si, and 0.050 wt % Al, with the remainder of Fe and unavoidable impurities.

7. A yield ratio-controlled steel, manufactured by the method of claim 1 .

8. The yield ratio-controlled steel of claim 7 , which has a ferrite-based network structure.

9. A method of manufacturing a cold forged part, comprising the steps of:

subjecting an alloy of steel, comprising 0.10 to 0.40 wt % carbon (C), 0.90 to 1.50 wt % manganese (Mn), 0.50 to 2.50 wt % silicon (Si), and 0.060 wt % or less but exceeding 0 wt % aluminum (Al), with a remainder of iron (Fe) and unavoidable impurities, to rolling or wire drawing, thus obtaining a material;

performing first a heat treatment on the material, in which the material is heated to and maintained, for a first predetermined period of time, at a first temperature ranging from A c1 to A c3 , which are A c1 and A c3 transformation temperatures;

immediately after the end of the first predetermine period of time, cooling the material to a second temperature ranging from (25° C+Ms)/2 to Ms, wherein Ms is a martensite start temperature;

performing a second heat treatment immediately after the material is cooled to the second temperature, in which the material is maintained at the second temperature for a second predetermined period of time;

generating in the material bainite or martensite based on ferrite and

subjecting the material to cold forging.

10. A cold forged part, manufactured by the method of claim 9 .

11. The cold forged part of claim 10 , wherein the cold forged part is a bolt, a shaft, a bar, a rod, or a stud.