Hot press forming parts having excellent bending properties and method for manufacturing the same
Provided is a method for manufacturing an HPF part having excellent bending properties including: manufacturing a hot rolled steel sheet; coiling the hot rolled steel sheet within a range of 450° C. to 750° C. for a time satisfying Expression 1; cold rolling, annealing and then hot dip aluminum plating on the annealed steel sheet; heating the hot dip aluminum plated steel sheet to a temperature of 850° C. to 1000° C. and maintaining the hot dip aluminum plated steel sheet for a certain period of time; and hot forming the heated steel and cooling at a temperature within a range of 200° C. or lower at a cooling speed of 20° C./s to 1000° C./s at the same time to manufacture an HPF part, [Expression 1] 190,000≤[coiling temperature (CT)×Time (min)]/2≤350,000, in the Relational Expression 1, Time refers to a time taken to reach 200° C. from coiling temperature.
1. A method for manufacturing an HPF part, the method comprising:
manufacturing a hot rolled steel sheet comprising C: 0.18% to 0.25%, Si: 0.1% to 0.5%, Mn: 0.9% to 1.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.01% to 0.05%, Cr: 0.05% to 0.5%, Ti: 0.01% to 0.05%, B: 0.001% to 0.005%, N: 0.009% or less, a balance of Fe and other impurities by wt %;
coiling the hot rolled steel sheet at a temperature within a range of 450° C. to 750° C. for a time satisfying Relational Expression 1 to form a coiled steel sheet;
cold rolling and annealing the coiled steel sheet to form an annealed steel sheet, and then conducting hot dip aluminum plating on the annealed steel sheet to form an aluminum plated steel sheet;
heating the aluminum plated steel sheet to a temperature of 850° C. to 1000° C. and then maintaining the aluminum plated steel sheet at the temperature of 850° C. to 1000° C. for a certain period of time to form a heated aluminum plated steel sheet; and
hot forming the heated aluminum plated steel sheet and, simultaneously with the hot forming, cooling the heated aluminum plated steel sheet to 200° C. or lower at a cooling speed of 20° C./s to 1000° C./s to manufacture an HPF part,
190,000≤[coiling temperature(CT)×Time(min)]/2≤350,000 [Relational Expression 1]
in the Relational Expression 1, Time refers to a time taken to reach 200° C. from coiling temperature.
2. The method of claim 1 , wherein the certain period of time is 1 second to 1000 seconds.
3. The method of claim 1 , wherein the annealing is performed at a temperature within a range of 700° C. to 900° C.
4. The method of claim 1 , wherein a cold reduction ratio during the cold rolling is in a range of 30% to 80%.
5. The method of claim 1 , wherein the HPF part comprises a base steel sheet and a hot dip aluminum layer formed on a surface of the base steel sheet, a ferrite phase is continuously or discontinuously formed on a surface layer of the base steel sheet to a thickness of 50 μm or thinner, a percentage of the ferrite phase in the surface layer is 5% or less; and carbide particles having a size of 1 μm or smaller are dispersively distributed in the surface layer of the base steel sheet so as to occupy 90% or greater in a whole carbide particle distribution.
6. The method of claim 5 , wherein an amount of the carbide particles in a size range of 1 μm to 10 μm is 5 or less per 10 mm 2 in the surface layer.
7. The method of claim 1 , wherein the hot rolled steel sheet further comprises Mo+W: 0.001% to 0.5%.
8. The method of claim 1 , wherein the hot rolled steel sheet further comprises at least one of Nb, Zr and V: within a range of 0.001% to 0.4%.
9. The method of claim 1 , wherein the hot rolled steel sheet further comprises Cu+Ni: within a range of 0.005% to 2.0%.
10. The method of claim 1 , wherein the hot rolled steel sheet further comprises at least one of Sb, Sn and Bi: 0.03% or less.