Cold-rolled steel sheet and process for manufacturing same
A high-strength cold-rolled steel sheet having excellent ductility and stretch flangeability includes: a chemical composition consisting, in mass %, C: 0.06 to 0.3, Si: 0.6 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, total of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to 1%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003% and the remainder of Fe and impurities; a microstructure having a main phase including at least 40 area % in total of martensite and/or bainite; and a texture in which proportion of an average X-ray intensity in an {100}<011> to {211}<011> orientations relative to an average X-ray intensity of a random structure not having a texture is less than 6.
1. A cold-rolled steel sheet characterized by having:
a chemical composition comprising, in mass % of C: 0.06 to 0.3, Si: 0.6 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, total of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to 1%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003% and the remainder of Fe and impurities;
a microstructure having a main phase of either or both of martensite and bainite which comprises at least 40 area % in total; and
a texture in which proportion of an average X-ray intensity for {100}<011> to {211}<011> orientations relative to the average X-ray intensity of a random structure which does not have a texture at a depth of ½of a sheet thickness is less than 6,
wherein the microstructure has secondary phases including ferrite and retained austenite, the ferrite comprises 3 to 19 area % and satisfies Equation (1) and the retained austenite comprises at least 3 area % and satisfies Equations (2) and (3):
d F ≤4.0 (1);
d As ≤1.5 (2); and
r As ≥50 (3),
where d F is an average grain diameter in μm of ferrite defined by high angle grain boundaries having a tilt angle of at least 15°, d As is an average grain diameter in μm of retained austenite having an aspect ratio of less than 5 and r As is an area fraction in % of the retained austenite having an aspect ratio of less than 5 relative to all retained austenite.
2. The cold-rolled steel sheet as forth in claim 1 , wherein the main phase comprises 71 area % or more.
3. The cold-rolled steel sheet as set forth in claim 1 , wherein the chemical composition comprises one or two elements selected from, in mass % Ti: 0.005 to 0.08% and Nb: 0.003 to 0.04%.
4. The cold-rolled steel sheet as forth in claim 3 , wherein the main phase comprises 71 area % or more.
5. The cold-rolled steel sheet as set forth in claim 1 , wherein the chemical composition comprises, in mass %, sol.Al: 0.1 to 2.0%.
6. The cold-rolled steel sheet as forth in claim 5 , wherein the main phase comprises 71 area % or more.
7. The cold-rolled steel sheet as set forth in claim 1 , wherein the chemical composition comprises one or more elements selected from, in mass % Cr: 0.03 to 1%, Mo: 0.01 to 0.3% and V: 0.01 to 0.3%.
8. The cold-rolled steel sheet as forth in claim 7 , wherein the main phase comprises 71 area % or more.
9. The cold-rolled steel sheet as set forth in claim 1 , wherein the chemical composition comprises, in mass %, B: 0.0003 to 0.005%.
10. The cold-rolled steel sheet as forth in claim 9 , wherein the main phase comprises 71 area % or more.
11. The cold-rolled steel sheet as set forth in claim 1 , wherein the chemical composition comprises one or two elements selected from, in mass %, Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.003%.
12. The cold-rolled steel sheet as forth in claim 11 , wherein the main phase comprises 71 area % or more.
13. The cold-rolled steel sheet as set forth in claim 1 , comprising a plating layer on a surface of the cold-rolled steel sheet.
14. The cold-rolled steel sheet as forth in claim 13 , wherein the main phase comprises 71 area % or more.
15. Process for manufacturing a cold-rolled steel sheet according to claim 1 , the process comprising the following steps (A) and (B):
(A) a cold rolling step in which a hot-rolled steel sheet having a chemical composition comprising, in mass % of C: 0.06 to 0.3, Si: 0.6 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, total of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to 1%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003% and the remainder of Fe and impurities is cold rolled to obtain a cold-rolled steel sheet; and
(B) an annealing step in which the cold-rolled steel sheet obtained in the step (A) is heated under conditions that the cold-rolled steel sheet is heated at an average heating rate condition of at least 15° C./sec so that an proportion of an unrecrystallization of a region not transformed to austenite at a time of reaching (Ac 1 point +10° C.) becomes at least 30 area %, and is then held in a temperature range of at least (0.3×Ac 1 point+0.7×Ac 3 point) and at most (Ac 3 point+100° C.) for at least 30 seconds, and the steel sheet is then cooled at an average cooling rate of at least 10° C./sec for a temperature range of at most 650° C. and at least 500° C.
16. The process for manufacturing a cold-rolled steel sheet as set forth in claim 15 , wherein after completion of hot rolling, the hot-rolled steel sheet is coiled at a temperature of at most 300° C. and then subjected to heat treatment in a temperature range of 500° C. to 700° C.
17. The process for manufacturing a cold-rolled steel sheet production as set forth in claim 15 , further comprising the step of plating the cold-rolled steel sheet after the step (B).
18. The process for manufacturing a cold-rolled steel sheet as set forth in claim 15 , wherein the hot-rolled steel sheet is a steel sheet in which average grain diameter of a BCC phase defined by high angle grain boundaries having a tilt angle of at least 15° is at most 6 μm, the steel sheet being obtained by a hot rolling step of cooling at a cooling rate (Crate) satisfying following Equation (4) for a temperature range from a temperature at the completion of rolling to (temperature at the completion of rolling −100° C.) after completion of hot rolling in which hot rolling is completed at at least Ar 3 point:
IC
(
T
)
=
0.1
-
3
×
10
-
3
·
T
+
4
×
10
-
5
·
T
2
-
5
×
10
-
7
·
T
3
+
5
×
10
-
9
·
T
4
-
7
×
10
-
11
·
T
5
∫
0
-
100
-
dT
Crate
(
T
)
·
IC
(
T
)
<
4
(
4
)
where Crate (T) is a cooling rate (° C./s) (positive value),
T is a relative temperature (° C., negative value) with the temperature at the completion of rolling as zero, and
if there is a temperature at which Crate is zero, a value obtained by dividing a holding time (Δt) at the temperature by IC (T) is added as an integral for the section.
19. The process for manufacturing a cold-rolled steel sheet as set forth in claim 18 , wherein the cooling in the temperature range includes starting cooling at a cooling rate of at least 400° C./sec and cooling at the cooling rate in a temperature range of at least 30° C.
20. The process for manufacturing a cold-rolled steel sheet production as set forth in claim 18 , wherein the cooling in the temperature range includes starting water cooling at a cooling rate of at least 400° C./sec and cooling at the cooling rate for a temperature section of at least 30° C. and at most 80° C., and then stopping a water cooling stop time of 0.2 to 1.5 seconds to measure a shape of the sheet during the time, and subsequently cooling at a rate of at least 50° C./sec.