High strength press-formed member and method for manufacturing the same
A high strength press-formed member includes a steel sheet constituting the member including a composition including by mass %, C: 0.12% to 0.69%, Si: 3.0% or less, Mn: 0.5% to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, N: 0.010% or less, Si+Al: at least 0.7%, and remainder as Fe and incidental impurities, wherein a microstructure of the steel sheet includes martensite, retained martensite, and bainite containing bainitic ferrite, an area ratio of said martensite with respect to the entire microstructure of the steel sheet is 10% to 85%, at least 25% of said martensite is tempered martensite, content of retained austenite is 5% to 40%, area ratio of said bainitic ferrite in said bainite with respect to the entire microstructure of the steel sheet is at least 5%, total of area ratios of said martensite, said retained austenite, and said bainitic ferrite in said bainite with respect to the entire microstructure of the steel sheet is at least 65%, and average carbon concentration in the retained austenite is at least 0.65 mass %.
1. A high strength press-formed member obtainable by hot press-forming comprising a steel sheet constituting the member, the steel sheet comprising a composition including by mass %,
C: 0.281% to 0.69%,
Si: 3.0% or less,
Mn: 0.5% to 3.0%,
P: 0.1% or less,
S: 0.07% or less,
Al: 3.0% or less,
N: 0.010% or less,
Si+Al: at least 0.7%, and
remainder as Fe and incidental impurities,
wherein a microstructure of the steel sheet comprises martensite, retained austenite; bainite containing bainitic ferrite,
an area ratio of said martensite with respect to the entire microstructure of the steel sheet is 10% to 85%,
at least 25% of said martensite is tempered martensite,
content of retained austenite is 6% to 40%,
area ratio of said bainitic ferrite in said bainite with respect to the entire microstructure of the steel sheet is at least 5%,
total of area ratios of said martensite, said retained austenite, and said bainitic ferrite in said bainite with respect to the entire microstructure of the steel sheet is at least 65%, and
average carbon concentration in the retained austenite is at least 0.65 mass %.
2. The high strength press-formed member of claim 1 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Cr: 0.05% to 5.0%;
V: 0.005% to 1.0%; and
Mo: 0.005% to 0.5%.
3. The high strength press-formed member of claim 1 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ti: 0.01% to 0.1%; and
Nb: 0.01% to 0.1%.
4. The high strength press-formed member of claim 1 , wherein the composition further comprises by mass %, B: 0.0003% to 0.0050%.
5. The high strength press-formed member of claim 1 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ni: 0.05% to 2.0%; and
Cu: 0.05% to 2.0%.
6. The high strength press-formed member of claim 1 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ca: 0.001% to 0.005%; and
REM: 0.001% to 0.005%.
7. A method for manufacturing a high strength press-formed member, comprising:
preparing a steel sheet having the composition of claim 1 ;
heating the steel sheet to a temperature of 750° C. to 1000° C. and retaining the steel sheet in that state for 5 seconds to 1000 seconds;
subjecting the steel sheet to hot press-forming at a temperature of 350° C. to 900° C.;
cooling the steel sheet to a temperature of 50° C. to 350° C.;
heating the steel sheet to a temperature in a temperature region of 350° C. to 490° C.; and
retaining the steel sheet at temperature in the temperature region for 5 seconds to 1000 seconds.
8. The high strength press-formed member of claim 2 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ti: 0.01% to 0.1%; and
Nb: 0.01% to 0.1%.
9. The high strength press-formed member of claim 2 , wherein the composition further comprises by mass %, B: 0.0003% to 0.0050%.
10. The high strength press-formed member of claim 3 , wherein the composition further comprises by mass %, B: 0.0003% to 0.0050%.
11. The high strength press-formed member of claim 2 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ni: 0.05% to 2.0%; and
Cu: 0.05% to 2.0%.
12. The high strength press-formed member of claim 3 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ni: 0.05% to 2.0%; and
Cu: 0.05% to 2.0%.
13. The high strength press-formed member of claim 4 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ni: 0.05% to 2.0%; and
Cu: 0.05% to 2.0%.
14. The high strength press-formed member of claim 2 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ca: 0.001% to 0.005%; and
REM: 0.001% to 0.005%.
15. The high strength press-formed member of claim 3 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ca: 0.001% to 0.005%; and
REM: 0.001% to 0.005%.
16. The high strength press-formed member of claim 4 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ca: 0.001% to 0.005%; and
REM: 0.001% to 0.005%.
17. The high strength press-formed member of claim 5 , wherein the composition further comprises by mass % at least one element selected from the group consisting of:
Ca: 0.001% to 0.005%; and
REM: 0.001% to 0.005%.