Method of manufacturing hot press formed parts having excellent powdering resistance
Provided is a method of manufacturing an HPF part comprising: preparing a steel sheet comprising C: 0.18% to 0.25%, Si: 0.1% to 1.0%, 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, and a balance of Fe and impurities; heating the steel sheet at 550° C. to 850° C. followed by maintaining at 640° C. to 680° C., and immersing the steel sheet in a hot dip aluminum plating bath, and a balance of Al and impurities; cooling the plated steel sheet at 15° C./s or faster; heating the cooled hot dip aluminum plated steel sheet at 880° C. to 930° C. followed by maintaining for a certain time; and hot forming the alloyed hot dip aluminum plated steel sheet and quenching at a range of 300° C. or lower at the same time.
1. A method of manufacturing an HPF part having an improved powdering resistance during hot press forming, the method comprising:
a process of preparing a steel sheet comprising C: 0.18% to 0.25%, Si: 0.1% to 1.0%, 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, and a balance of Fe and other impurities by wt %;
a process of heating the steel sheet at a temperature of 550° C. to 850° C. followed by being maintained at 640° C. to 680° C. to form a heated steel sheet;
a process of immersing the heated steel sheet in a hot dip aluminum plating bath for hot dip aluminum plating to form a hot dip aluminum-plated steel sheet, the hot dip aluminum plating bath comprising Si: 9% to 11%, Fe: 3% or less, and a balance of Al and other inevitable impurities by wt %;
a process of cooling the hot dip aluminum-plated steel sheet at an average cooling speed of faster than 15° C./s until a plated layer of the hot dip aluminum-plated steel sheet is solidified to form a cooled hot dip aluminum-plated steel sheet;
a process of heating the cooled hot dip aluminum-plated steel sheet at a temperature of 880° C. to 930° C. followed by being maintained for a predetermined period of time to alloy a hot dip aluminum plated layer of the cooled hot dip aluminum-plated steel sheet to form an alloyed hot dip aluminum-plated steel sheet; and
a process of hot forming the alloyed hot dip aluminum-plated steel sheet and being quenched to a temperature range of 300° C. or lower simultaneously with the process of hot forming to manufacture an HPF part,
wherein the alloyed hot dip aluminum-plated layer comprises: a soft diffusion layer and a hard alloyed layer; the alloyed layer comprises: Fe 2 Al 5 matrix phase and a tau phase comprised of Fe—Al based alloy phase; in the alloyed layer, the tau phase exists at area % within a range of 10% to 30%; and the tau phase comprises: Si of 10% or greater and Cr of 0.2% or greater by wt % thereof.
2. The method of claim 1 , wherein the tau phase comprises: Si: 10% to 12%, Mn+Cr: 1.3% to 2.0%, and a balance of Fe and Al by wt % thereof.
3. The method of claim 1 , wherein the alloyed layer and the diffusion layer have a thickness ratio in a range of 1.5 to 3.0.
4. The method of claim 1 , wherein the steel sheet is a cold rolled steel sheet or a hot rolled steel sheet.
5. The method of claim 1 , further comprising: a process of cooling the alloyed hot dip aluminum-plated steel sheet to a temperature range of 700° C. to 780° C. at a cooling speed of 20° C./s to 100° C./s, before the process of hot forming.
6. The method of claim 1 , wherein the steel sheet further comprises: Mo+W: 0.001% to 0.5%.
7. The method of claim 1 , wherein the steel sheet further comprises: at least one of Nb, Zr, and V: within a range of 0.001% to 0.4% (as a sum).
8. The method of claim 1 , wherein the steel sheet further comprises: Cu+Ni: within a range of 0.005% to 2.0%.
9. The method of claim 1 , wherein the steel sheet further comprises: at least one of Sb, Sn, and Bi: 0.03% or less.