Hot-rolled steel sheet with excellent press formability and production method thereof
View Patent ↗The issue of the present invention is to provide a hot-rolled steel sheet with excellent press formability and method for producing the steel sheet, wherein the steel sheet has not only hole expandability but also stretch flanging workability by not assessing hole expandability for stretch flanging as conventional but an actual phenomena of side-bend elongation. To solve the issue, it is confirmed that the steel sheet are excellent in hole expandability and stretch flanging workability, wherein the steel sheet with a certain content of C, Si and Mn is characterized in that, in a metallic structure of said steel sheet, the area fraction of ferrite is 70% or more, the area fraction of bainite is 30% or less, the area fraction of either one or both of martensite and retained austenite is 2% or less, and with regard to respective average intervals (L θ , L i and L MA ), average diameters (D θ , D i and D MA ) and number densities of a cementite, an inclusion and either one or both of martensite and retained austenite (n θ , n i and n MA ), a void formation/connection index L defined by formula 1 is 11.5 or more: L = n θ L θ / D θ 2 + 2.1 n i L i / D i 2 + n MA L MA / D MA 2 n θ + n i + n MA ( formula 1 )
1. A hot-rolled steel sheet with excellent press formability, consisting of, in mass %,
C: 0.03 to 0.10%,
Si: 0.6 to 1.5%,
Mn: 0.5 to 2.0%,
Nb: 0-0.08%,
Ti: 0-0.2%,
W: 0-0.5%,
Mo: 0-0.4%,
Cu: 0-1.2%,
Ni: 0-0.6%,
Cr: 0-1.0%,
B: 0-0.005%,
Ca: 0-0.01%, and
REM: 0-0.01%, and
having a balance of Fe and unavoidable impurities,
as impurities,
P: limited to 0.05% or less,
S: limited to 0.01% or less,
Al: limited to 0.30% or less,
N: limited to 0.01% or less,
wherein the hot-rolled steel sheet has a tensile strength of at least 540 MPa, and wherein in the steel sheet, the X-ray random intensity ratios of {211} plane parallel to a surface of the steel sheet at the ½ thickness position, the ¼ thickness position, and the ⅛ thickness position in the thickness direction from the surface are 1.5 or less, 1.3 or less, and 1.1 or less, respectively, and
wherein in the metallic structure of said steel sheet, the area fraction of ferrite is 70% or more, the area fraction of bainite is 30% or less, the area fraction of either one or both of martensite having an area of 0.1 μm 2 or more and retained austenite having an area of 0.1 μm 2 or more is 2% or less, and
with regard to respective average intervals, average diameters and number densities of a cementite having an area of 0.1 μm 2 or more, an inclusion having an area of 0.05 μm 2 or more and either one or both of the martensite and the retained austenite, a void formation/connection index L defined by formula 1 is 11.5 or more:
L
=
n
θ
L
θ
/
D
θ
2
+
2.1
n
i
L
i
/
D
i
2
+
n
MA
L
MA
/
D
MA
2
n
θ
+
n
i
+
n
MA
(
formula
1
)
n θ , n i and n MA : number densities of the cementite, the inclusion and either one or both of the martensite and the retained austenite, respectively, and the unit is pieces/μm 2 ;
D θ , D i and D MA : average diameters of the cementite, the inclusion and either one or both of the martensite and the retained austenite, respectively, and the unit is μm; and
L θ , L i and L MA : average intervals of the cementite, the inclusion and either one or both of the martensite and the retained austenite, respectively, and the unit is μm.
2. A method for producing a hot-rolled steel sheet with excellent press formability, comprising:
a step of reheating a slab to a temperature of 1,150° C. or more and holding the slab for 120 minutes or more and 180 minutes or less, thereafter performing rough rolling the slab,
a step of performing finish rolling such that the end temperature becomes between Ae 3-30 ° C. and Ae 3 +30° C., wherein a total length of time between passes in a final 4 passes in said finish rolling is 3 seconds or less,
a step for performing primary cooling to a temperature between 510 and 650° C. at a cooling rate of 50° C./s or more, the primary cooling being started between 0.9 seconds and 2.0 seconds after the completion of the finish rolling,
a step of performing air cooling for 2 to 5 seconds,
a step of performing secondary cooling at a cooling rate of 30° C./s or more,
a step of performing coiling at a temperature of 500 to 600° C., and
a step of performing cooling to 200° C. or less at an average cooling rate of 30° C./h or more to obtain a steel sheet, wherein:
Ae 3 =937−477C+56Si−20Mn−16Cu−15Ni−5Cr+38Mo+136Ti−19Nb+198A1+3315B (formula 2)
wherein C, Si, Mn, Cu, Ni, Cr, Mo, Ti, Nb, Al and B represent the contents of respective elements, and the unit is mass %:
wherein the slab is made of a steel consisting of, in mass %,
C: 0.03 to 0.10%,
Si: 0.6 to 1.5%,
Mn: 0.5 to 2.0%,
Nb: 0-0.08%,
Ti: 0-0.2%,
W: 0-0.5%,
Mo: 0-0.4%,
Cu: 0-1.2%,
Ni: 0-0.6%,
Cr: 0-1.0%,
B: 0-0.005%,
Ca: 0-0.01%, and
REM: 0-0.01%, and
having a balance of Fe and unavoidable impurities,
as impurities,
P: limited to 0.05% or less,
S: limited to 0.01% or less,
Al: limited to 0.30% or less,
N: limited to 0.01% or less, wherein the hot-rolled steel sheet has a tensile strength of at least 540 M Pa, and wherein in the steel sheet, the X-ray random intensity ratio of {211} plane parallel to a surface of the steel sheet at the ½ thickness position, the ¼ thickness position, and the ⅛ thickness position in the thickness direction from the surface are 1.5 or less, 1.3 or less, and 1.1 or less, respectively.
3. The method for producing a hot-rolled steel sheet with excellent press formability according to claim 2 , wherein with regard to respective average intervals, average diameters and number densities of a cementite having an area of 0.1 μm 2 or more, an inclusion having an area of 0.05 μm 2 or more and either one or both of the martensite and the retained austenite in the metallic structure of said steel sheet, the void formation/connection index L defined by formula 1 is 11.5 or more:
L
=
n
θ
L
θ
/
D
θ
2
+
2.1
n
i
L
i
/
D
i
2
+
n
MA
L
MA
/
D
MA
2
n
θ
+
n
i
+
n
MA
(
formula
1
)
n θ , n i and n MA : number densities of the cementite, the inclusion and either one or both of the martensite and the retained austenite, respectively, and the unit is pieces/μm 2 ;
D θ , D i and D MA : average diameters of the cementite, the inclusion and either one or both of the martensite and the retained austenite, respectively, and the unit is μm; and
L θ , L i and L MA : average intervals of the cementite, the inclusion and either one or both of the martensite and the retained austenite, respectively, and the unit is μm.
4. A hot-rolled steel sheet with excellent press formability, comprising, in mass %,
C: 0.03 to 0.10%,
Si: 0.6 to 1.5%,
Mn: 0.5 to 2.0%,
Nb: 0-0.08%,
Ti: 0-0.2%,
W: 0-0.5%,
Mo: 0-0.4%,
Cu: 0-1.2%,
Ni: 0-0.6%,
Cr: 0-1.0%,
B: 0-0.005%,
Ca: 0-0.01%, and
REM: 0-0.01%, and
having a balance of Fe and unavoidable impurities,
as impurities,
P: limited to 0.05% or less,
S: limited to 0.01% or less,
Al: limited to 0.30% or less,
N: limited to 0.01% or less,
wherein the hot-rolled steel sheet has a tensile strength of at least 540 MPa, and wherein in the steel sheet, the X-ray random intensity ratio of {211} plane parallel to a surface of the steel sheet at the ½ thickness position, the ¼ thickness position, and the ⅛ thickness position in the thickness direction from the surface are 1.5 or less, 1.3 or less, and 1.1 or less, respectively, and with the proviso that said steel sheet contains no vanadium;
wherein the hot-rolled steel sheet has a tensile strength of at least 540 MPa, and wherein in the steel sheet, the X-ray random intensity ratio of {211} plane parallel to a surface of the steel sheet at the ½ thickness position, the ¼ thickness position, and the ⅛ thickness position in the thickness direction from the surface are 1.5 or less, 1.3 or less, and 1.1 or less, respectively, and wherein in the metallic structure of said steel sheet, the area fraction of ferrite is 70% or more, the area fraction of bainite is 30% or less, the area fraction of either one or both of martensite having an area of 0.1 μm 2 or more and retained austenite having an area of 0.1 μm 2 or more is 2% or less, and
with regard to respective average intervals, average diameters and number densities of a cementite having an area of 0.1 μm 2 or more, an inclusion having an area of 0.05 μm 2 or more and either one or both of the martensite and the retained austenite, a void formation/connection index L defined by formula 1 is 11.5 or more:
L
=
n
θ
L
θ
/
D
θ
2
+
2.1
n
i
L
i
/
D
i
2
+
n
MA
L
MA
/
D
MA
2
n
θ
+
n
i
+
n
MA
(
formula
1
)
n θ , n i and n MA : number densities of the cementite, the inclusion and either one or both of the martensite and the retained austenite, respectively, and the unit is pieces/μm 2 ;
D θ , D i and D MA : average diameters of the cementite, the inclusion and either one or both of the martensite and the retained austenite, respectively, and the unit is μm; and
L θ , L i and L MA : average intervals of the cementite, the inclusion and either one or both of the martensite and the retained austenite, respectively, and the unit is μm.
5. A method for producing a hot-rolled steel sheet with excellent press formability, comprising:
a step of reheating a slab to a temperature of 1,150° C. or more and holding the slab for 120 minutes or more and 180 minutes or less, thereafter performing rough rolling the slab,
a step of performing finish rolling such that the end temperature becomes between Ae3-30° C. and Ae3+30° C.,
a step for performing primary cooling to a temperature between 510 and 650° C. at a cooling rate of 50° C./s or more, the primary cooling being started between 0.9 seconds and 2.0 seconds after the completion of the finish rolling,
a step of performing air cooling for 2 to 5 seconds,
a step of performing secondary cooling at a cooling rate of 30° C./s or more,
a step of performing coiling at a temperature of 500 to 600° C., and
a step of performing cooling to 200° C. or less at an average cooling rate of 30° C./h or more to obtain a steel sheet, wherein:
Ae 3 =937−477C+56Si−20Mn−16Cu−15Ni−5Cr+38Mo+136Ti−19Nb+198A1+3315B (formula 2)
wherein C, Si, Mn, Cu, Ni, Cr, Mo, Ti, Nb, Al and B represent the contents of respective elements, and the unit is mass %, and
wherein the slab is made of a steel comprising, in mass %,
C: 0.03 to 0.10%,
Si: 0.6 to 1.5%,
Mn: 0.5 to 2.0%,
Nb: 0-0.08%,
Ti: 0-0.2%,
W: 0-0.5%,
Mo: 0-0.4%,
Cu: 0-1.2%,
Ni: 0-0.6%,
Cr: 0-1.0%,
B: 0-0.005%,
Ca: 0-0.01%, and
REM: 0-0.01%, and
having a balance of Fe and unavoidable impurities,
as impurities,
P: limited to 0.05% or less,
S: limited to 0.01% or less,
Al: limited to 0.30% or less,
N: limited to 0.01% or less,
with the proviso that said steel sheet contains no vanadium, wherein the hot-rolled steel sheet has a tensile strength of at least 540 MPa, and wherein in the steel sheet, the X-ray random intensity ratio of {211} plane parallel to a surface of the steel sheet at the ½ thickness position, the ¼ thickness position, and the ⅛ thickness position in the thickness direction from the surface are 1.5 or less, 1.3 or less, and 1.1 or less, respectively.