Method for the production of very high strength martensitic steel and sheet or part thus obtained
The present invention provides a method for the fabrication of a steel sheet with a completely martensitic structure which has an average lath size of less than 1 micrometer and an average elongation factor of the laths is between 2 and 5. The elongation factor of a lath is defined as a maximum dimension 1 max divided by and a minimum dimension 1 max . The steel sheet has a yield stress greater than 1300 MPa and a mechanical strength greater than (3220(C)+958) megapascals. A composition of a semi-finished steel product includes, expressed in percent by weight, is, 0.15%≤C≤0.40%, 1.5%≤Mn≤3%, 0.005%≤Si≤2%, 0.005%≤Al≤0.1%, 1.8%≤Cr≤4%, 0%≤Mo≤2%, whereby: 2.7%≤0.5 (Mn)+(Cr)+3(Mo)≤5.7%, S≤0.05%, P≤0.1%, optionally: 0%≤Nb≤0.050%, 0.01%≤Ti≤0.1%, 0.0005%≤B≤0.005%, 0.0005%≤Ca≤0.005%. The semi-finished product is reheated to a temperature T 1 in the range between 1050° C. and 1250° C., then subjected to a roughing rolling at a temperature T 2 in the range between 1000 and 880° C., with a cumulative rate of reduction ε a greater than 30%, to obtain a sheet with a completely recrystallized austenitic structure with an average grain size less than 40 micrometers and preferably less than 5 micrometers. The sheet is then partially cooled to prevent a transformation of the austenite at a rate V R1 greater than 2° C./s to a temperature T 3 between 600° C. and 400° C. in the metastable austenitic range, and subjected to a finishing hot rolling at the temperature T 3 of the partially cooled sheet, with a cumulative rate of reduction ε b greater than 30% to obtain a sheet that is then cooled at a rate V R2 which is greater than the critical martensitic quenching rate.
1. A method for fabrication of a steel part with a completely martensitic structure having an average lath size of less than 1 micrometer, an average elongation factor of the laths being between 2 and 5, the elongation factor of a lath with a maximum dimension 1 max and minimum dimension 1 mm being defined by
l
max
l
min
,
the method comprising the following:
obtaining a steel blank, a composition of the steel blank including, whereby the contents are expressed by weight,
0.15%≤C≤0.40%,
1.5%≤Mn≤3%,
0.005%≤Si≤2%,
0.005%≤Al≤0.1%,
1.8%≤Cr≤4%,
0%≤Mo≤2%,
whereby
2.7%≤0.5 (Mn)+(Cr)+3(Mo)≤5.7%,
S≤0.05%,
P≤0.1%,
a remainder of the composition including iron and inevitable impurities resulting from processing,
heating the blank to a temperature T 1 in a range between A C3 and A C3 +250° C. so that an average austenitic grain size is less than 40 micrometers;
transferring the heated blank to a hot stamping press or a hot forming device;
cooling the blank to a temperature T 3 in a range between 600° C. and 400° C. at a rate V R1 which is greater than 2° C./s to prevent a transformation of austenite;
hot stamping or hot forming the cooled blank at the temperature T 3 by a quantity ε c greater than 30% in at least one zone, to obtain a part, ε c being defined by
ɛ
c
_
=
2
3
(
ɛ
1
2
+
ɛ
1
ɛ
2
+
ɛ
2
2
)
,
where ε 1 and ε 2 are principal deformations accumulated over all of the deformation steps at the temperature T 3 ; and
cooling the part at a rate V R2 which is greater than a critical martensitic quenching rate.
2. The method for the fabrication of a part as recited in claim 1 , wherein the blank is hot-stamped to obtain a part, the part is held in a stamping tool to cool the part at a rate V R2 which is greater than a critical martensitic quenching rate.
3. The method for the fabrication of a steel part as recited in claim 1 , wherein the blank is pre-coated with aluminum or an aluminum-based alloy.
4. The method for the fabrication of a steel part as recited in claim 1 , wherein the blank is pre-coated with zinc or a zinc-based alloy.
5. The method for the fabrication of steel part as recited in claim 1 , further comprising the step of subjecting the part to a tempering heat treatment at a temperature T 4 which is between 150 and 600° C. for a period of time between 5 and 30 minutes.
6. The method for the fabrication of a steel part as recited in claim 1 , wherein the average grain size less is less than 5 micrometers.
7. The method for the fabrication of a steel part as recited in claim 1 , wherein the transferring step may occur before or after the step of cooling the blank to a temperature T 3 .
8. The method for the fabrication of a steel part as recited in claim 1 , wherein the composition of the steel blank includes 0%≤Nb≤0.050%.
9. The method for the fabrication of a steel part as recited in claim 1 , wherein the composition of the steel blank includes 0.01%≤Ti≤0.1%.
10. The method for the fabrication of a steel part as recited in claim 1 , wherein the composition of the steel blank includes 0.0005%≤B≤0.005%.
11. The method for the fabrication of a steel part as recited in claim 1 , wherein the composition of the steel blank includes 0.0005%≤Ca≤0.005%.