Steel wire rod for high strength and high toughness spring having excellent cold workability, method for producing the same and method for producing spring by using the same

1. A steel wire rod for a high strength and high toughness spring having excellent cold workability, the steel wire rod having a composition comprising: in weight %, C: 0.4 to 0.7%, Si: 1.5 to 3.5%, Mn: 0.3 to 1.0%, Cr: 0.01 to 1.5%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, B: 0.008 to 0.02%, Al: 0.02 to 0.1%, O: 0.0020% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, remainder Fe, and other unavoidable impurities, having an internal structure formed of ferrite and pearlite, the internal structure in which prior austenite grain size is 8 μm or less, wherein the steel wire rod has a tensile strength less than 1000 MPa after hot rolling and peeling and shaving the hot rolled steel wire rod without annealing thereof, and wherein the unannealed steel wire rod has an unsoftened structure.

2. The steel wire rod of claim 1 , wherein a sum of area fractions of bainite and martensite structures among the internal structure of the steel wire rod is less than 1%.

3. The steel wire rod of claim 1 , wherein the composition of the steel wire rod further comprises: in weight %, V: 0.5% or less and Ti: 0.5% or less.

4. A method of manufacturing a spring by using a high strength and high toughness hot rolled steel wire rod having excellent cold workability, the steel wire rod having a composition comprising: in weight %, C: 0.4 to 0.7%, Si: 1.5 to 3.5%, Mn: 0.3 to 1.0%, Cr: 0.01 to 1.5%, N: 0.01 to 1.0%, Cu: 0.01 to 1.0%, B: 0.008 to 0.02%, Al: 0.02 to 0.1%, O: 0.0020% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, remainder Fe, and other unavoidable impurities, having an internal structure formed of ferrite and pearlite, the internal structure in which prior austenite grain size is 8 μm or less, the method comprising:

peeling and shaving the hot rolled steel wire rod without annealing, wherein the hot rolled steel wire rod has a tensile strength of less than 1000 MPa after peeling and shaving the hot rolled steel wire rod without annealing thereof;

austeniting the peeled and shaved steel wire rod;

oil-cooling the austenited steel wire rod;

tempering the oil-cooled steel wire rod; and

cold working the tempered steel wire rod in a spring shape.

5. A method of manufacturing a spring by using a high strength and high toughness hot rolled steel wire rod having excellent cold workability, the steel wire rod having a composition comprising: in weight %, C: 0.4 to 0.7%, Si: 1.5 to 3.5%, Mn: 0.3 to 1.0%, Cr: 0.01 to 1.5%, N: 0.01 to 1.0%, Cu: 0.01 to 1.0%, B: 0.008 to 0.02%, Al: 0.02 to 0.1%, O: 0.0020% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, remainder Fe, and other unavoidable impurities, having an internal structure formed of ferrite and pearlite, the internal structure in which prior austenite grain size is 8 μm or less, the method comprising:

peeling and shaving the hot rolled steel wire rod without annealing, wherein the hot rolled steel wire rod has a tensile strength of less than 1000 MPa after peeling and shaving the hot rolled steel wire rod without annealing thereof;

hot working the peeled and shaved steel wire rod in a spring shape;

austeniting the hot worked spring;

oil-cooling the austenited spring; and

tempering the oil-cooled spring.

6. The method of claim 4 , wherein an austeniting temperature is 900 to 1000° C.

7. The method of claim 4 , wherein a tempering temperature is 350 to 450° C.

8. The steel wire rod of claim 2 , wherein the composition of the steel wire rod further comprises: in weight %, V: 0.5% or less and Ti: 0.5% or less.

9. The method of claim 5 , wherein an austeniting temperature is 900 to 1000° C.

10. The method of claim 5 , wherein a tempering temperature is 350 to 450° C.