US2026071292A1PendingUtilityA1
Hot rolled and heat-treated steel sheet and method of manufacturing the same
Est. expiryDec 19, 2039(~13.4 yrs left)· nominal 20-yr term from priority
C21D 8/02C22C 38/14C22C 38/12C22C 38/06C22C 38/04C22C 38/02C22C 38/002C22C 38/001C21D 2211/008C21D 2211/005C21D 2211/001C21D 8/0263C21D 8/0226C21D 6/008C21D 6/005C21D 1/26B21B 2001/225B21B 1/22C21D 2211/004C21D 2211/002C21D 9/46C21D 1/18C21D 8/0247
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Claims
Abstract
A method for manufacturing a hot rolled and heat-treated steel sheet includes casting to obtain a semi-finished product having a composition comprising, by weight percent: C: 0.12-0.25%, Mn: 3.0-8.0%, Si: 0.7-1.5%, Al: 0.3-1.2%, B: 0.0002-0.004%, S≤0.010%, P≤0.020%, N≤0.008%, and iron; reheating at a Treheat between 1150° C. and 1300° C.; hot rolling the reheated semi-finished product with a finish rolling temperature (Tnr-100° C. to 950° C.), coiling the hot rolled steel sheet at a Tcoil (20° C. and 700° C.) and cooling to obtain a microstructure comprising martensite and bainite, M+B>80%, F<20%, and <20% of the sum of martensite-austenite (M-A) islands and carbides, and having the multiplication of PAGS <1000 μm2 and a pancaking index <5; reheating, quenching, reheating to a partitioning temperature, holding for 1 to 1000 seconds, and cooling.
Claims
exact text as granted — not AI-modifiedWhat is claimed is:
1 . A method for manufacturing a hot rolled and heat-treated steel sheet, comprising the following successive steps:
casting a steel to obtain a semi-finished product, said semi-finished product having a composition comprising, by weight percent:
C: 0.12-0.25%
Mn: 3.0-8.0%
Si: 0.7-1.5%
Al: 0.3-1.2%
B: 0.0002-0.004%
S≤0.010%
P≤0.020%
N≤0.008%
and optionally one or more of the following elements, in weight percentage:
Mo≤0.5%
V≤0.2%
Nb≤0.06%
Ti≤0.05%
a remainder of the composition being iron and unavoidable impurities resulting from processing; reheating the semi-finished product at a temperature T reheat between 1150° C. and 1300° C.; hot rolling the reheated semi-finished product with a finish rolling temperature FRT between Tnr-100° C. and 950° C. to obtain a hot rolled steel sheet, Tnr being the non-recrystallisation temperature defined as
825
+
2300
⋆
%
Nb
+
710
⋆
%
Ti
+
150
⋆
%
Mo
+
120
⋆
%
V
+
8
⋆
%
Mn
;
coiling the hot rolled steel sheet at a coiling temperature T coil comprised between 20° C. and 700° C. and cooling to room temperature so to obtain, a microstructure comprising martensite and bainite, the sum of which being greater than 80%, strictly less than 20% of ferrite, and strictly less than 20% of the sum of martensite-austenite (M-A) islands and carbides, and having the multiplication of PAGS in rolling direction by PAGS in normal direction lower than 1000 μm 2 and a pancaking index lower than 5;
reheating the hot rolled steel sheet to a temperature TA1 strictly lower than Ae3 and higher than (Ae1+Ae3)/2, and maintaining the steel sheet at the annealing temperature TA1 for a holding time tA1 between 3 s and 1000 s, Ae1 and Ae3 temperature being defined as
Ae
1
=
670
+
15
⋆
%
Si
-
13
⋆
%
Mn
+
18
⋆
%
Al
Ae
3
=
890
-
20
⋆
√
%
C
+
20
⋆
%
Si
-
30
⋆
%
Mn
+
130
*
%
Al
quenching the hot-rolled steel sheet to a quenching temperature TQ lower than (Ms-50° C.), to obtain a quenched steel sheet, Ms being defined as
Ms
=
560
-
(
30
*
%
Mn
+
13
⋆
%
Si
-
15
⋆
%
Al
+
12
⋆
%
Mo
)
-
600
⋆
(
1
-
exp
(
-
0
,
96
⋆
%
C
)
)
;
reheating the quenched steel sheet to a partitioning temperature TP comprised between 350° C. and 550° C., and maintaining the quenched steel sheet at the partitioning temperature for a partitioning time between 1 s and 1000 s; and
cooling the steel sheet to the room temperature to obtain a hot rolled and heat-treated steel sheet.
2 . The method as recited in claim 1 wherein the manganese content is comprised between 3.0% and 5.0%, by weight.
3 . The method as recited in claim 1 wherein the silicon content is comprised between 0.8% and 1.3%, by weight.
4 . The method as recited in claim 1 wherein a yield strength of the hot rolled and heat-treated steel sheet is higher than 950 MPa.
5 . The method as recited in claim 1 wherein a tensile strength of the hot rolled and heat-treated steel sheet is higher than 1180 MPa.
6 . The method as recited in claim 1 wherein a uniform elongation of the hot rolled and heat-treated steel sheet is higher than 10%.
7 . The method as recited in claim 1 wherein a hole expansion ratio of the hot rolled and heat-treated steel sheet is higher than 25%.
8 . The method as recited in claim 1 , wherein the hot rolled and heat-treated steel sheet has a microstructure consisting of, in surface fraction: between 5% and 45% of ferrite; between 25% and 85% of partitioned martensite, the partitioned martensite having a carbide density less than 2×10 6 /mm 2 ; between 10% and 30% of retained austenite; less than 8% of fresh martensite, a part of said fresh martensite being combined with retained austenite in the shape of martensite-austenite (M-A) islands in total surface fraction less than 10%; and a pancaking index lower than 5.
9 . The method as recited in claim 8 wherein fresh martensite and martensite-austenite islands size of the hot rolled and heat-treated steel sheet are less than 0.7 μm.
10 . The method as recited in claim 8 wherein the microstructure of the hot rolled and heat-treated steel sheet includes the fresh martensite and the M-A islands.
11 . The method as recited in claim 8 wherein the M-A islands of the hot rolled and heat-treated steel sheet have an aspect ratio lower or equal to 2.
12 . The method as recited in claim 8 wherein the fresh martensite conent of the hot rolled and heat-treated steel sheet is 2% to less than 8%.
13 . The method as recited in claim 8 wherein the microstructure of the hot rolled and heat-treated steel sheet consists of 5% to less than 10% of the M-A islands in total surface fraction.Join the waitlist — get patent alerts
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