Steel sheet for hot press forming having low-temperature heat treatment property, method of manufacturing the same, method of manufacturing parts using the same, and parts manufactured by the same

1. A steel sheet for hot press forming, comprising: by weight,

carbon (C): 0.15 to 0.35%;

silicon (Si): 0.5% or less;

manganese (Mn): 1.5 to 2.2%;

phosphorus (P): 0.025% or less;

sulfur (S): 0.01% or less;

aluminum (Al): 0.01 to 0.05%;

nitrogen (N): 100 to 200 ppm;

titanium (Ti): 0.005 to 0.05%;

tungsten (W): 0.005 to 0.1%; and

boron (B): 1 to 50 ppm,

wherein Ti/N: less than 1.1, where Ti/N is the atomic ratio of the corresponding elements, Ceq expressed by the following formula ranges from 0.48 to 0.58, and temperature Ar3 ranges from 670° C. to 725° C., wherein the steel sheet has a microstructure consisting of ferrite and pearlite, and

wherein the steel sheet exhibits a tensile strength of 750 MPa or less before hot press forming or before cold forming and post heat treatment after cold forming and a tensile strength of 1470 MPa or more after hot press forming or after post heat treatment after cold forming,

Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+V/14  [Formula]

where C, Si, Mn, Ni, Cr and V indicate the contents (wt %) of the respective elements.

2. The steel sheet of claim 1 , further comprising at least one selected from the group consisting of by weight: niobium (Nb): 0.005 to 0.1%; vanadium (V): 0.005 to 0.1%; copper (Cu): 0.1 to 1.0%; and nickel (Ni): 0.05 to 0.5%.

3. The steel sheet for hot press forming of claim 1 , wherein a content of manganese (Mn) is 1.55 to 2.2%.

4. The steel sheet of claim 1 , wherein the steel sheet is coated with zinc.

5. The steel sheet of claim 1 , wherein the steel sheet is coated with aluminum.

6. The steel sheet of claim 1 , wherein the atomic ratio of Ti/N is 1.085 or less.

7. The steel sheet of claim 1 , wherein Nitrogen (N) is 130 to 200 ppm.

8. A structural part for a motor vehicle, which is manufactured from a steel sheet by hot press forming, or post-heat treatment after cold forming, in which:

the steel sheet has a composition of, by weight, carbon (C): 0.15 to 0.35%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 2.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.05%, nitrogen (N): 100 to 200 ppm, titanium (Ti): 0.005 to 0.05%, tungsten (W): 0.005 to 0.1%, and boron (B): 1 to 50 ppm, wherein Ti/N: less than 1.1, where Ti/N is the atomic ratio of the corresponding elements, Ceq expressed by the following formula ranges from 0.48 to 0.58, and temperature Ar3 ranges from 670° C. to 725° C.;

wherein a final microstructure of the structural part includes, by area fraction, martensite of 90% or more, and the balance of at least one selected from bainite and ferrite, and

wherein the structural part has a bake hardenability of 100 MPa or more and a tensile strength of 1470 MPa or more

Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+V/14  [Formula]

where C, Si, Mn, Ni, Cr and V indicate the contents (wt %) of the respective elements.

9. The structural part of claim 8 , wherein the steel sheet structural part is coated with zinc.

10. The structural part of claim 8 , wherein the steel sheet structural part is coated with aluminum.

11. The structural part of claim 8 , wherein the atomic ratio of Ti/N is 1.085 or less.

12. The structural part of claim 8 , wherein Nitrogen (N) is 130 to 200 ppm.

13. A method of manufacturing a hot rolled steel sheet for hot press forming, comprising:

heating a steel slab to a temperature from 1150° C. to 1250° C., the steel slab having a composition of, by weight, carbon (C): 0.15 to 0.35%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 2.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.05%, nitrogen (N): 100 to 200 ppm, titanium (Ti): 0.005 to 0.05%, tungsten (W): 0.005 to 0.1%, and boron (B): 1 to 50 ppm, wherein Ti/N: less than 1.1, where Ti/N is the atomic ratio of the corresponding elements, Ceq expressed by the following formula ranges from 0.48 to 0.58, and temperature Ar3 ranges from 670° C. to 725° C.; and

rolling the heated steel slab via a roughing mill process and a finishing mill process to form the steel sheet,

wherein the finishing mill process includes:

rolling the steel sheet above Ar3 temperature; and

cooling and coiling the steel sheet at a temperature from 600° C. to 700° C., and

wherein the steel sheet has a microstructure consisting of ferrite and pearlite, and

wherein the steel sheet exhibits a tensile strength of 750 MPa or less before hot press forming or before cold forming and post heat treatment after cold forming and a tensile strength of 1470 MPa or more after hot press forming or after post heat treatment after cold forming,

Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+V/14  [Formula]

where C, Si, Mn, Ni, Cr and V indicate the contents (wt %) of the respective elements.

14. The method of claim 13 , wherein the steel slab further comprises at least one selected from the group consisting of by weight: niobium (Nb): 0.005 to 0.1%; vanadium (V): 0.005 to 0.1%; copper (Cu): 0.1 to 1.0%; and nickel (Ni): 0.05 to 0.5%.

15. The method of claim 13 , wherein a content of manganese (Mn) is 1.55 to 2.2%.

16. A method of manufacturing a cold rolled steel sheet for hot press forming,

comprising:

pickling a hot rolled steel sheet, the hot rolled steel sheet having a composition of, by weight, carbon (C): 0.15 to 0.35%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 2.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.05%, nitrogen (N): 100 to 200 ppm, titanium (Ti): 0.005 to 0.05%, tungsten (W): 0.005 to 0.1%, and boron (B): 1 to 50 ppm, wherein Ti/N: less than 1.1, where Ti/N is the atomic ratio of the corresponding elements, Ceq expressed by the following formula ranges from 0.48 to 0.58, and temperature Ar3 ranges from 670° C. to 725° C.;

cold-rolling the pickled steel sheet to manufacture full hard steel sheet; and

continuously annealing the full hard steel sheet,

wherein, the temperature of the continuous-annealing is controlled to be within a range of 750° C. to 850° C., and temperature of a following over aging section is controlled to be within a range of 450° C. to 600° C., and

wherein the steel sheet has a microstructure consisting of ferrite and pearlite, and

wherein the steel sheet exhibits a tensile strength of 750 MPa or less before hot press forming or before cold forming and post heat treatment after cold forming and a tensile strength of 1470 MPa or more after hot press forming or after post heat treatment after cold forming,

Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+V/14  [Formula]

where C, Si, Mn, Ni, Cr and V indicate the contents (wt %) of the respective elements.

17. The method of claim 16 , wherein the hot rolled steel sheet further comprises at least one selected from the group consisting of by weight: niobium (Nb): 0.005 to 0.1%; vanadium (V): 0.005 to 0.1%; copper (Cu): 0.1 to 1.0%; and nickel (Ni): 0.05 to 0.5%.

18. The method of claim 17 , further comprising coating the steel sheet with zinc.

19. The method of claim 18 , wherein the coating of the steel sheet with zinc includes one selected from hot-dip galvanizing, galvannealing, zinc or zinc-iron electroplating.

20. The method of claim 16 , further comprising coating the steel sheet with zinc.

21. The method of claim 20 , wherein the coating of the steel sheet with zinc includes one selected from hot-dip galvanizing, galvannealing, zinc or zinc-iron electroplating.

22. The method of claim 16 , wherein a content of manganese (Mn) is 1.55 to 2.2%.

23. A method of manufacturing an aluminum coated steel sheet for hot press forming, comprising:

pickling a hot rolled steel sheet, the hot rolled steel sheet having a composition of, by weight, carbon (C): 0.15 to 0.35%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 2.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.05%, nitrogen (N): 100 to 200 ppm, titanium (Ti): 0.005 to 0.05%, tungsten (W): 0.005 to 0.1%, and boron (B): 1 to 50 ppm, wherein Ti/N: less than 1.1, where Ti/N is the atomic ratio of the corresponding elements, Ceq expressed by the following formula ranges from 0.48 to 0.58, and temperature Ar3 ranges from 670° C. to 725° C.;

cold-rolling the pickled steel sheet to manufacture full hard steel sheet;

annealing the full hard steel sheet at a temperature from 750° C. to 850° C.; and

dipping the annealed steel sheet in a hot aluminum or aluminum alloy bath so as to cool the coated steel sheet to room temperature at a cooling rate from 5° C./sec to 15° C./sec,

wherein the steel sheet has a microstructure consisting of ferrite and pearlite, and

wherein the steel sheet exhibits a tensile strength of 750 MPa or less before hot press forming or before cold forming and post heat treatment after cold forming and a tensile strength of 1470 MPa or more after hot press forming or after post heat treatment after cold forming,

Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+V/14  [Formula]

where C, Si, Mn, Ni, Cr and V indicate the contents (wt %) of the respective elements.

24. The method of claim 23 , wherein the aluminum coated steel sheet further comprises at least one selected from the group consisting of by weight: niobium (Nb): 0.005 to 0.1%; vanadium (V): 0.005 to 0.1%; copper (Cu): 0.1 to 1.0%; and nickel (Ni): 0.05 to 0.5%.

25. A method of manufacturing structural parts for a motor vehicle, comprising:

preparing a blank made of a steel sheet for hot press forming, the steel sheet having a composition of, by weight, carbon (C): 0.15 to 0.35%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 2.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.05%, nitrogen (N): 100 to 200 ppm, titanium (Ti): 0.005 to 0.05%, tungsten (W): 0.005 to 0.1%, and boron (B): 1 to 50 ppm, wherein Ti/N: less than 1.1, where Ti/N is the atomic ratio of the corresponding elements, Ceq expressed by the following formula ranges from 0.48 to 0.58, and temperature Ar3 ranges from 670° C. to 725° C.;

heating the blank at a temperature of 820° C. to 950° C.;

maintaining the heated blank for 60 seconds or more, and extracting the maintained blank;

transferring the extracted blank into a prepared die, and performing the hot press forming; and

cooling hot press formed part to a temperature of 200° C. or less at a cooling rate of 20° C./sec or more in the die,

wherein a final microstructure of the structural part includes, by area fraction, martensite of 90% or more, and the balance of at least one selected from bainite and ferrite, and

wherein the structural part has a bake hardenability of 100 MPa or more and a tensile strength of 1470 MPa or more,

Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+V/14  [Formula]

where C, Si, Mn, Ni, Cr and V indicate the contents (wt %) of the respective elements.

26. The method of claim 25 , wherein the steel sheet for forming further comprises at least one selected from the group consisting of by weight: niobium (Nb): 0.005 to 0.1%; vanadium (V): 0.005 to 0.1%; copper (Cu): 0.1 to 1.0%; and nickel (Ni): 0.05 to 0.5%.

27. A method of manufacturing parts, comprising:

preparing a blank or a tube made of a steel sheet for post-heat treatment, the steel sheet having a composition of, by weight, carbon (C): 0.15 to 0.35%, silicon (Si): 0.5% or less, manganese (Mn): 1.5 to 2.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.01% or less, aluminum (Al): 0.01 to 0.05%, nitrogen (N): 100 to 200 ppm, titanium (Ti): 0.005 to 0.05%, tungsten (W): 0.005 to 0.1%, and boron (B): 1 to 50 ppm, wherein Ti/N: less than 1.1, where Ti/N is the atomic ratio of the corresponding elements, Ceq expressed by the following formula ranges from 0.48 to 0.58, and temperature Ar3 ranges from 670° C. to 725° C.;

cold-forming the prepared blank or tube into a shape of the part;

heating the manufactured part at a temperature of 820° C. to 950° C.;

maintaining the heated part for 60 seconds or more, and extracting the maintained part; and

cooling the extracted part to a temperature of 200° C. or less at a cooling rate of 20° C./sec or more,

wherein a final microstructure of the structural part includes, by area fraction, martensite of 90% or more, and the balance of at least one selected from bainite and ferrite, and

wherein the structural part has a bake hardenability of 100 MPa or more and a tensile strength of 1470 MPa or more,

Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+V/14  [Formula]

where C, Si, Mn, Ni, Cr and V indicate the contents (wt %) of the respective elements.

28. The method of claim 27 , wherein the steel sheet for forming further comprises at least one selected from the group consisting of by weight: niobium (Nb): 0.005 to 0.1%; vanadium (V): 0.005 to 0.1%; copper (Cu): 0.1 to 1.0%; and nickel (Ni): 0.05 to 0.5%.