Hot rolled steel sheet and a method of manufacturing thereof
A hot rolled steel sheet having a composition including the elements, expressed in percentage by weight 0.11%≤Carbon≤0.16%, 1%≤Manganese≤2%, 0.1%≤Silicon≤0.7%, 0 02%≤Aluminum≤0.1%, 0.15%≤Molybdenum≤0.4%, 0.15%≤Vanadium≤0.4%, 0.002%≤Phosphorus≤0.02%, 0%≤Sulfur≤0.005%, 0%≤Nitrogen≤0.01%, and can contain one or more of the following optional elements 0%≤Chromium≤0.5%, 0%≤Niobium≤0.05%, 0.0001%≤Calcium≤0.005%, 0%≤Boron≤0.001%, 0%≤Magnesium≤0.0010%, 0%≤Titanium≤0.01%, with 0.3%≤Mo+V+Nb≤0.6%, the remainder composition being composed of iron and unavoidable impurities, the microstructure of steel sheet including in area fraction, 70% to 90% Bainite, 10% to 25% Ferrite wherein the cumulated amounts of Bainite and Ferrite is at least 90% and a cumulated amount of Residual Austenite and Martensite is between 0% and 10%.
1. A hot rolled steel sheet having a composition consisting of the following elements, expressed in percentage by weight:
0.11%≤Carbon≤0.16%,
1%≤Manganese≤2%,
0.1%≤Silicon≤0.7%,
0 02%≤Aluminum≤0.1%,
0.15%≤Molybdenum≤0.4%,
0.15%≤Vanadium≤0.4%,
0.002%≤Phosphorus≤0.02%,
0%≤Sulfur≤0.005%,
0%≤Nitrogen≤0.01%,
0%≤Chromium≤0.5%,
0%≤Niobium≤0.05%,
0.0001%≤Calcium≤0.005%,
0%≤Boron≤0.001%,
0%≤Magnesium≤0.0010%,
with 0.3%≤Mo+V+Nb≤0.6%,
a remainder of the composition being composed of iron and unavoidable impurities,
a microstructure of the hot rolled steel sheet comprising in area fraction, 70% to 90% Bainite, 10% to 25% Ferrite, wherein a cumulated amount of Bainite and Ferrite is at least 90% and a cumulated amount of Residual Austenite and Martensite is between 0% and 10%.
2. The hot rolled steel sheet as recited in claim 1 wherein the composition has 0.2% to 0.6% of Silicon, in percentage by weight.
3. The hot rolled steel sheet as recited in claim 1 wherein the composition has 0.11% to 0.15% of Carbon, in percentage by weight.
4. The hot rolled steel sheet as recited in claim 3 wherein the composition has 0.15% to 0.3% of Vanadium, in percentage by weight.
5. The hot rolled steel sheet as recited in claim 1 wherein the composition has 1.3% to 1.8% of Manganese, in percentage by weight.
6. The hot rolled steel sheet as recited in claim 1 wherein the composition has 0.15% to 0.3% of Molybdenum, in percentage by weight.
7. The hot rolled steel sheet as recited in claim 1 wherein the composition has 0.02% to 0.06% of Aluminum, in percentage by weight.
8. The hot rolled steel sheet as recited in claim 1 wherein the cumulated amount of Residual Austenite and Martensite is between 2% and 10% in area fraction.
9. The hot rolled steel sheet as recited in claim 1 wherein said steel sheet has a tensile strength of 950 MPa or more, and a hole expansion ratio of 40% or more.
10. The hot rolled steel sheet as recited in claim 9 wherein said steel sheet has a tensile strength of 960 MPa or more and a total elongation of 8% or more.
11. A structural or safety part of a vehicle comprising the hot rolled steel sheet as recited in claim 1 .
12. A vehicle comprising the structural or safety part as recited in claim 11 .
13. A method of production of the hot rolled heat treated steel sheet as recited in claim 1 comprising the following successive steps:
providing a semi-finished product having a steel composition consisting of the following elements, expressed in percentage by weight:
0.11%≤Carbon≤0.16%,
1%≤Manganese≤2%,
0.1%≤Silicon≤0.7%,
0.02%≤Aluminum≤0.1%,
0.15%≤Molybdenum≤0.4%,
0.15%≤Vanadium≤0.4%,
0.002%≤Phosphorus≤0.02, %
0%≤Sulfur≤0.005%,
0%≤Nitrogen≤0.01%,
0%≤ Chromium≤0.5%,
0%≤Niobium≤0.05%,
0.0001%≤Calcium≤0.005%,
0%≤Boron≤0.001%,
0%≤Magnesium≤0.0010%,
with 0.3%≤Mo+V+Nb≤0.6%,
a remainder of the composition being composed of iron and unavoidable impurities;
reheating the semi-finished product to a reheating temperature between 1200° C. and 1300° C.;
rolling the semi-finished product in an austenitic range wherein a hot rolling finishing temperature is between 850° C. and 975° C. to obtain a hot rolled steel strip; then cooling the hot rolled steel strip in three step cooling wherein:
in step one of the cooling, cooling starts at a temperature in a temperature range between 850° C. and 975° C. and ends at a temperature in a temperature range between 650° C. and 725° C., with a cooling rate between 40° C./s and 150° C./s;
in step two of the cooling, cooling starts at a temperature in a temperature range between 650°° C. and 725°° C. and ends at a temperature in a temperature range between 620° C. and 690° C., said step two having a duration of 1 s to 10 s and being an air cooling; and
in step three of the cooling, cooling starts at a temperature in a temperature range between 620° C. and 690°° C. and
ends at a temperature in a temperature range between 450°° C. and 550° C. with a cooling rate greater than 20° C./s; thereafter
coiling the hot rolled steel strip at a coiling temperature in a range between 450° C. and 550°° C. to obtain a coiled hot rolled steel strip;
cooling the coiled hot rolled steel strip to room temperature to obtain the hot rolled heat treated steel sheet as recited in claim 1 .
14. The method as recited in claim 13 wherein the reheating temperature for semi-finished product is between 1200° C. and 1275° C.
15. The method as recited in claim 13 wherein the hot rolling finishing temperature is between 880°° C. and 930° C.
16. The method as recited in claim 13 wherein the coiling temperature range is between 470° C. and 530° C.
17. The method as recited in claim 13 wherein the cooling rate for the step one of the cooling is between 40° C./s and 120° C./s.
18. The method as recited in claim 13 wherein the cooling rate for the step three of the cooling is greater than equal to 25° C./s.
19. The method as recited in claim 13 wherein the duration for the step two of the cooling is between 2 seconds and 9 seconds.