Martensitic steel with delayed fracture resistance and manufacturing method
A cold rolled and annealed martensitic steel sheet is provided. The steel sheet includes by weight percent, 0.30≤C≤0.5%, 0.2≤Mn≤1.5%, 0.5≤Si≤3.0%, 0.02≤Ti≤0.05%, 0.001≤N≤0.008%, 0.0010≤B≤0.0030%, 0.01≤Nb≤0.1%, 0.2≤Cr≤2.0%, P≤0.02%, S≤0.005%, Al≤1%, Mo≤1% and Ni≤0.5%. The remainder of the composition includes iron and unavoidable impurities resulting from melting. The microstructure is 100% martensitic and a prior austenite grain size is lower than 20 μm. The steel sheet has a delayed fracture resistance of at least 24 hours during an acid immersion U-bend test. A method, part, structural member and vehicle are also provided.
1. A martensitic steel sheet, directly obtained after cold rolling, annealing and cooling, comprising, by weight percent:
0.30≤C≤0.5%;
0.2≤Mn≤1.5%;
0.5≤Si≤3.0%;
0.02≤Ti≤0.05%;
0.001≤N≤0.008%;
0.0010≤B≤0.0030%;
0.01≤Nb≤0.1%;
0.2≤Cr≤2.0%;
P≤0.02%;
S≤0.005%;
Al≤1%;
Mo≤1%; and
Ni≤0.5%;
the remainder of the composition being iron and unavoidable impurities resulting from melting;
a microstructure being 100% tempered martensitic with a prior austenite grain size lower than 20 μm;
the steel sheet having a delayed fracture resistance of at least 24 hours during an acid immersion U-bend test.
2. The cold rolled and annealed martensitic steel sheet according to claim 1 wherein 0.01≤Nb≤0.05%.
3. The cold rolled and annealed martensitic steel sheet according to claim 1 wherein 0.2≤Cr≤1.0%.
4. The cold rolled and annealed martensitic steel sheet according to claim 1 wherein Ni≤0.2%.
5. The cold rolled and annealed martensitic steel sheet according to claim 1 wherein Ni≤0.05%.
6. The cold rolled and annealed martensitic steel sheet according to claim 1 wherein Ni≤0.03%.
7. The cold rolled and annealed martensitic steel sheet according to claim 1 wherein 1≤Si≤2%.
8. The cold rolled and annealed martensitic steel sheet according to claim 1 wherein a tensile strength is at least 1700 MPa, a yield strength is at least 1300 MPa and a total elongation is at least 3%.
9. The cold rolled and annealed martensitic steel sheet according to claim 1 wherein the delayed fracture resistance is at least 100 hours during the acid immersion U-bend test.
10. The cold rolled and annealed martensitic steel sheet according to claim 1 wherein the delayed fracture resistance is at least 300 hours during the acid immersion U-bend test.
11. The cold rolled and annealed martensitic steel sheet according to claim 1 wherein the delayed fracture resistance is at least 600 hours during the acid immersion U-bend test.
12. A method for producing a cold rolled and annealed martensitic steel sheet according to claim 1 comprising the steps of:
casting a steel so as to obtain a slab;
reheating the slab at a temperature T reheat above 1150° C.;
hot rolling the reheated slab at a temperature above 850° C. to obtain a hot rolled steel;
cooling the hot rolled steel until a coiling temperature T coiling between 500 and 660° C.;
coiling the hot rolled steel cooled at T coiling ;
de-scaling the hot rolled steel;
cold rolling the steel so as to obtain a cold rolled steel sheet;
heating up to a temperature T anneal between Ac3° C. and 950° C.,
annealing at T anneal for a time between 40 seconds and 600 seconds so as to have a 100% austenitic microstructure with a grain size below 20 μm;
cooling the cold rolled steel to room temperature or tempering temperature at a cooling rate CR quench of at least 100 C/s; and
tempering the cold rolled sheet at a temperature between 180° C. and 300° C. for at least 40 seconds.
13. The method for producing a cold rolled and annealed martensitic steel sheet according to claim 12 wherein the cooling rate CR quench is at least 200° C./s.
14. The method for producing a cold rolled and annealed martensitic steel sheet according to claim 13 wherein the cooling rate CR quench is at least 500° C./s.
15. The method for producing a cold rolled and annealed martensitic steel sheet according to claim 12 wherein the austenitic grain size formed during annealing at T anneal for a time between 40 seconds and 600 seconds is below 15 μm.
16. A part for a vehicle comprising: the cold rolled and annealed martensitic steel according to claim 1 .
17. A structural member comprising:
a cold rolled and annealed martensitic steel according to claim 1 .
18. A vehicle comprising:
a part made of a cold rolled and annealed martensitic steel according to claim 1 .
19. The method for producing a cold rolled and annealed martensitic steel sheet according to claim 12 further comprising the step of applying a cooling step to the cold rolled steel from the annealing temperature down to a temperature T 1 of at least 820° C. at a cooling rate of at least 1° C./s.
20. The method for producing a cold rolled and annealed martensitic steel sheet according to claim 12 further comprising the step of tempering the cold rolled steel at a temperature between 180° C. and 300° C. for at least 40 seconds.
21. The method for producing a cold rolled and annealed martensitic steel sheet according to claim 12 wherein the steps are performed in succession.
22. A part for a vehicle comprising: the cold rolled and annealed martensitic steel produced according to claim 12 .
23. A structural member comprising:
a cold rolled and annealed martensitic steel produced according to claim 12 .
24. A vehicle comprising:
a part made of a cold rolled and annealed martensitic steel produced according to claim 12 .