Hot-rolled steel sheet having excellent cold formability and hardenability and method for manufacturing the same
Hot-rolled steel has a chemical composition containing, by mass %, C: 0.18% or more and 0.29% or less, N: 0.0050% or less, Ti: 0.002% or more and 0.05% or less, B: 0.0005% or more and 0.0050% or less, and appropriately controlled amounts of Si, Mn, P, S, Al, and a tensile strength of 500 MPa or less with a variation in tensile strength of 60 MPa or less throughout a region including the edges in the width direction of the steel sheet, and having excellent cold formability and hardenability.
1. A hot-rolled steel sheet having a tensile strength of 500 MPa or less and, the steel sheet having a chemical composition comprising, by mass %,
C: 0.18% or more and 0.29% or less,
Mn: 0.88% or less,
S: 0.03% or less,
N: 0.0050% or less,
Ti: 0.002% or more and 0.05% or less,
B: 0.0005% or more and 0.0050% or less, and
Si: 0.4% or less,
P: 0.1% or less,
sol.Al: 0.1% or less,
the balance being Fe and inevitable impurities and a microstructure wherein a fraction of ferrite and pearlite with respect to the whole microstructure is 95% or more in terms of a sum of volume fractions of both ferrite and pearlite, wherein mean grain diameter of the ferrite is 7.0 μm or more and 15.0 μm or less, and wherein a volume fraction of the ferrite with respect to the whole microstructure is 50% or more.
2. The hot-rolled steel sheet according to claim 1 , wherein the chemical composition further comprises, by mass %, one or both of Nb and V: 0.1% or less in total.
3. The hot-rolled steel sheet according to claim 1 , wherein the chemical composition further comprises, by mass %, one or more of Ni, Cr and Mo: 1.5% or less in total.
4. The hot-rolled steel sheet according to claim 1 , wherein the chemical composition further comprises, by mass %, one or both of Sb and Sn: 0.1% or less in total.
5. The hot-rolled steel sheet according to claim 1 , wherein the steel sheet has a variation in tensile strength ATS of 60 MPa or less throughout a region that is on an inner side 5 mm from edges in a width direction of the steel sheet.
6. A method for manufacturing a hot-rolled steel sheet having a microstructure wherein a fraction of ferrite and pearlite with respect to the whole microstructure is 95% or more in terms of a sum of volume fractions of both ferrite and pearlite, wherein mean grain diameter of the ferrite is 7.0 μm or more and 15.0 μm or less, and wherein a volume fraction of the ferrite with respect to the whole microstructure is 50% or more, comprising:
hot-rolling a steel having a chemical composition comprising, by mass %, C: 0.18% or more and 0.29% or less, Si: 0.4% or less, Mn: 0.88% or less, P: 0.1% or less, S: 0.03% or less, sol.Al: 0.1% or less, N: 0.0050% or less, Ti: 0.002% or more and 0.05% or less, B: 0.0005% or more and 0.0050% or less and the balance being Fe and inevitable impurities with a finishing temperature of 800° C. or higher and 900° C. or lower and thereafter,
cooling the resulting hot-rolled steel sheet at a mean cooling rate of 20° C./s or less, and
coiling the resulting hot-rolled steel sheet at a coiling temperature CT of 500° C. or higher.
7. The method according to claim 6 , wherein the chemical composition further comprises, by mass %, one or both of Nb and V: 0.1% or less in total.
8. The method according to claim 6 , wherein the chemical composition further comprises, by mass %, one or more of Ni, Cr and Mo: 1.5% or less in total.
9. The method according to claim 6 , wherein the chemical composition further comprises, by mass %, one or both of Sb and Sn: 0.1% or less in total.
10. The hot-rolled steel sheet according to claim 2 , wherein the chemical composition further comprises, by mass %, one or more of Ni, Cr and Mo: 1.5% or less in total.
11. The hot-rolled steel sheet according to claim 2 , wherein the chemical composition further comprises, by mass %, one or both of Sb and Sn: 0.1% or less in total.
12. The hot-rolled steel sheet according to claim 3 , wherein the chemical composition further comprises, by mass %, one or both of Sb and Sn: 0.1% or less in total.
13. The hot-rolled steel sheet according to claim 2 , wherein the steel sheet has a variation in tensile strength ΔTS of 60 MPa or less throughout a region that is on an inner side 5 mm from edges in a width direction of the steel sheet.
14. The hot-rolled steel sheet according to claim 3 , wherein the steel sheet has a variation in tensile strength ΔTS of 60 MPa or less throughout a region that is on an inner side 5 mm from edges in a width direction of the steel sheet.
15. The hot-rolled steel sheet according to claim 4 , wherein the steel sheet has a variation in tensile strength ΔTS of 60 MPa or less throughout a region that is on an inner side 5 mm from edges in a width direction of the steel sheet.
16. The method according to claim 7 , wherein the chemical composition further comprises, by mass %, one or more of Ni, Cr and Mo: 1.5% or less in total.
17. The method according to claim 7 , wherein the chemical composition further comprises, by mass %, one or both of Sb and Sn: 0.1% or less in total.
18. The method according to claim 8 , wherein the chemical composition further comprises, by mass %, one or both of Sb and Sn: 0.1% or less in total.