Manufacturing process for producing high strength steel product with improved formability
View Patent ↗A flat rolled, high strength, formable steel product has a yield strength of at least about 100 ksi. The product has sufficient formability such that it can withstand a longitudinal or transverse 180° bend of less than 1.0 times its thickness and is preferably comprised of a high strength, low alloy steel composition containing a vanadium-nitride microalloy. The steel product is preferably produced by cold rolling a first steel product having a yield strength of at least about 70 ksi and a n-value from about 0.1 to about 0.16. Cold rolling of the first steel product reduces its thickness and increases its yield strength to at least 100 ksi.
1. A process for producing a steel product comprised of high strength, low alloy steel containing a hardness-promoting vanadium nitride microalloy and having a yield strength of at least about 100 ksi, the process comprising:
(a) casting molten steel to form a solid, as-cast product having a thickness, the as-cast product comprising austenite;
(b) transferring the as-cast product to a first rolling apparatus, wherein a temperature of the as-cast product as it enters the first rolling apparatus is greater than a recrystallization stop temperature of the austenite and above about 1020° C.;
(c) conducting a first reduction step in the first rolling apparatus to reduce the thickness of the as-cast product by a first amount, thereby producing a first thickness-reduced product, wherein a temperature of the as-cast product entering the first rolling apparatus and a temperature of the first thickness-reduced product exiting the first rolling apparatus are above the recrystallization stop temperature and above about 1020° C.;
(d) heating the first thickness-reduced product with a heating apparatus so as to maintain the first thickness-reduced product at a temperature above the recrystallization stop temperature and above about 1020° C., and holding the first thickness-reduced product at said temperature, while heating it with said heating apparatus,
for a time sufficient to permit substantially complete recrystallization of the austenite and thereby reduce a grain size of the austenite;
(e) transferring the first thickness-reduced product to a second rolling apparatus comprised of a plurality of rolling stands;
(f) conducting a second reduction step in the second rolling apparatus to reduce the thickness of the first thickness-reduced product by a second amount, thereby producing a second thickness-reduced product, wherein a temperature of the first thickness-reduced product entering the second rolling apparatus is above the recrystallization stop temperature and above about 1020° C., and a temperature of the second thickness-reduced product exiting the second rolling apparatus is above a phase transformation temperature at which austenite is transformed to ferrite;
(g) cooling the second thickness-reduced product to below the phase transformation temperature, thereby producing a cooled product; and
(h) conducting a third reduction step in a third rolling apparatus to reduce the thickness of the cooled product by a third amount, thereby producing the steel product having a yield strength of at least about 100 ksi.
2. The process of claim 1 wherein the as-cast product produced by casting said molten steel is hot charged into a furnace without first cooling it to ambient temperature, such that the temperature of the as-cast product is maintained above the recrystallization stop temperature and above about 1020° C. between steps (a) and (b) and throughout steps (a) and (b).
3. The process of claim 2 , wherein the temperature of the as-cast product is maintained in the range of above about 1020° C. to about 1200° C. throughout steps (a) and (b) and between steps (a) and (b).
4. The process of claim 1 , wherein the thickness of the as-cast product is from about 30 mm to about 200 mm.
5. The process of claim 1 , wherein the thickness of the as-cast product is from about 50 mm to about 80 mm.
6. The process of claim 1 , wherein the first amount of thickness reduction produced in the first rolling apparatus is from about 40 percent to about 60 percent of the thickness of the as-cast product.
7. The process of claim 1 , wherein the second amount of thickness reduction is greater than the first amount of thickness reduction, wherein the second amount of thickness reduction is measured as a fraction of the thickness of the first thickness-reduced product and the first thickness reduction is measured as a fraction of the thickness of the as-cast product.
8. The process of claim 1 , wherein the temperature of the as-cast product as it enters the first rolling apparatus and the temperature of the first thickness-reduced product exiting the first rolling apparatus is in the range of above about 1020° C. to about 1200° C.
9. The process of claim 1 , wherein the second amount of thickness reduction produced in the second rolling apparatus is from about 80 to about 98 percent of the thickness of the first thickness-reduced product.
10. The process of claim 1 , wherein the thickness of the second thickness-reduced product is from about 1 mm to about 6 mm.
11. The process of claim 1 , wherein the thickness of the second thickness-reduced product is from about 1 mm to about 2 mm.
12. The process of claim 1 , wherein the temperature at which the first thickness-reduced product enters the second rolling apparatus is in the range of above about 1020 C. to about 1200° C.
13. The process of claim 1 , wherein the second thickness-reduced product exits the second rolling apparatus at a temperature In the range from about 820 to about 950° C.
14. The process of claim 1 , wherein the second thickness-reduced product is cooled to a temperature in the range from about 550 to about 700° C. to produce the cooled product.
15. The process of claim 1 , wherein the third amount of thickness reduction is less than the second amount of thickness reduction.
16. The process of claim 1 , wherein the third amount of thickness reduction is from about 2 to about 20 percent of the thickness of the second thickness-reduced product.
17. The process of claim 1 , wherein the cooled product is at ambient temperature when it enters the third rolling apparatus.
18. The process of claim 1 , wherein the cooled product has a yield strength of at least about 70 ksi.
19. The process of claim 1 , wherein the cooled product has a yield strength of at least about 80 ksi (550 MPa).
20. The process of claim 1 , wherein the cooled product has a formability, as measured by n-value, within the range from about 0.1 to about 0.16.
21. The process of claim 1 , wherein the steel product has a yield strength of at least about 100 ksi and a formability such that it can withstand a longitudinal or transverse 180° bend of less than about 0.5 times its thickness without longitudinal or transverse cracking.
22. The process of claim 1 , wherein the steel product has a yield strength of at least about 110 ksi and a formability such that it can withstand a longitudinal or transverse 180° bend of less than about 1 times its thickness without longitudinal or transverse cracking.
23. The process of claim 1 , wherein the first rolling apparatus comprises a rougher.
24. The process of claim 1 , wherein the second rolling apparatus comprises a hot rolling strip mill comprising from four to six of said rolling stands.
25. The process of claim 1 , wherein said heating apparatus of step (d) comprises a heated run-out table or a second furnace and is located between the first rolling apparatus and the second rolling apparatus.
26. The process of claim 1 , wherein the temperature at which the first thickness-reduced product is held in step (d) is from above about 1020° C. to about 1150° C.
27. The process of claim 1 , wherein the thickness of the steel product is from about 1.0 mm to about 4 mm.
28. The process of claim 1 further comprising pickling the cooled product to remove oxides prior to the third reduction step.
29. The process of claim 1 , wherein the steel product contains nitrogen in a sub-stoichiometric ratio relative to vanadium.
30. The process of claim 29 , wherein the steel product contains nitrogen in an amount of about 0.020 wt % and vanadium in an amount from about 0.10 to about 0.12 wt %.
31. The process of claim 29 , wherein the product contains one or more elements selected from the group consisting of carbon, manganese, silicon, molybdenum, niobium and aluminum.
32. The process of claim 1 , wherein said time sufficient to permit substantially complete recrystallization of the austenite is from about 10 to about 30 seconds.
33. The process of claim 32 , wherein said time sufficient to permit substantially complete recrystallization of the austenite is from about 15 to about 25 seconds.
34. The process of claim 1 , wherein said substantially complete recrystallization of the austenite is such that at least about 90 percent of the austenite grains are reduced to a size from about 100 to about 400 microns by said recrystallization.