Hot-rolled steel sheet, steel material, and method for producing hot-rolled steel sheet
The present invention provides a hot-rolled steel sheet capable of preventing softening of the strength of a sheet-thickness central portion of the steel sheet in thermal treatment, even in the case where an amount of working performed on the steel sheet is small and a work hardening rate is low. A hot-rolled steel sheet of the present invention consists of chemical components of, in mass %, C: 0.040 to 0.150%, Si: 0 to 0.500%, Mn: 0.10 to 1.50%, P: 0 to 0.050%, S: 0 to 0.020%, Al: 0.010 to 0.050%, N: 0.0010 to 0.0060%, Nb: 0.008 to 0.035%, Cu: 0 to 0.10%, Ni: 0 to 0.10%, Cr: 0 to 0.02%, Mo: 0 to 0.020%, V: 0 to 0.020%, Ca: 0 to 0.0100%, B: 0 to 0.0050%, and the balance: Fe and impurities. The hot-rolled steel sheet contains 0.005 to 0.030% dissolved Nb. An area fraction of ferrite structure is 85% or more, the balance is cementite and/or pearlite structure, and an average crystal grain size of ferrite is equal to or more than 5 μm and equal to or less than 20 μm.
1. A hot-rolled steel sheet consisting of chemical components of, in mass %,
C: 0.040 to 0.150%,
Si: 0 to 0.08%,
Mn: 0.10 to 1.50%,
P: 0 to 0.050%,
S: 0 to 0.020%,
Al: 0.010 to 0.050%,
N: 0.0010 to 0.0060%,
Nb: 0.008 to 0.035%,
Cu: 0 to 0.10%,
Ni: 0 to 0.10%,
Cr: 0 to 0.02%,
Mo: 0 to 0.020%,
V: 0 to 0.020%,
Ca: 0 to 0.0100%,
B: 0 to 0.0050%, and
the balance: Fe and impurities, wherein
the hot-rolled steel sheet contains 0.005 to 0.030% dissolved Nb, and
an area fraction of ferrite structure in a metal structure is 85% or more, the balance of the metal structure is cementite and/or pearlite structure, and an average crystal grain size of ferrite is equal to or more than 5 μm and equal to or less than 20 μm.
2. The hot-rolled steel sheet according to claim 1 ,
wherein Vickers hardness of a sheet-thickness central portion when cold working and thermal treatment of heating at 560 to 620° C. for 120 minutes are performed sequentially on the hot-rolled steel sheet exhibits resistance to softening of 80% or more with respect to Vickers hardness of the sheet-thickness central portion after the cold working.
3. The hot-rolled steel sheet according to claim 1 ,
wherein Vickers hardness of a sheet-thickness central portion when cold working that makes a work hardening rate of Vickers hardness less than 30% and thermal treatment of heating at 560 to 620° C. for 120 minutes are performed sequentially on the hot-rolled steel sheet exhibits resistance to softening of 80% or more with respect to Vickers hardness of the sheet-thickness central portion after the cold working.
4. A steel material comprising the hot-rolled steel sheet according to claim 1 ,
wherein Vickers hardness of a sheet-thickness central portion when cold working and thermal treatment of heating at 560 to 620° C. for 120 minutes are performed sequentially on the hot-rolled steel sheet is 80% or more with respect to Vickers hardness of the sheet-thickness central portion after the cold working.
5. The steel material according to claim 4 ,
wherein Vickers hardness of a sheet-thickness central portion when cold working that makes a work hardening rate of Vickers hardness less than 30% and thermal treatment of heating at 560 to 620° C. for 120 minutes are performed sequentially on the hot-rolled steel sheet is 80% or more with respect to Vickers hardness of the sheet-thickness central portion after the cold working.
6. A method for producing a hot-rolled steel sheet according to claim 1 , the method comprising:
heating a slab to 1200° C. or more;
performing final rolling of finish rolling at a finish rolling temperature of equal to or more than 860° C. and equal to or less than 950° C.;
performing cooling at an average cooling rate of equal to or more than 30° C./sec and equal to or less than 100° C./sec from the finish rolling temperature to 800° C.;
performing cooling at an average cooling rate of equal to or more than 5° C./sec and equal to or less than 100° C./sec from 800° C. to a coiling temperature; and
performing coiling at a coiling temperature of equal to or more than 300° C. and equal to or less than 600° C.,
wherein the slab consists of chemical components of, in mass %,
C: 0.040 to 0.150%,
Si: 0 to 0.08%,
Mn: 0.10 to 1.50%,
P: 0 to 0.050%,
S: 0 to 0.020%,
Al: 0.010 to 0.050%,
N: 0.0010 to 0.0060%,
Nb: 0.008 to 0.035%,
Cu: 0 to 0.10%,
Ni: 0 to 0.10%,
Cr: 0 to 0.02%,
Mo: 0 to 0.020%,
V: 0 to 0.020%,
Ca: 0 to 0.0100%,
B: 0 to 0.0050%, and
the balance: Fe and impurities.
7. The hot-rolled steel sheet according to claim 1 ,
wherein the average crystal grain size of ferrite is equal to or more than 8 μm and equal to or less than 20 μm.