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

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.

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.

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.

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.

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.

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.