Ultra-high strength weathering steel for hot-stamping applications

Patent metadata

Field	Value
Publication number	US-11773465-B2
Application number	US-202016944377-A
Country	US
Kind code	B2
Filing date	Jul 31, 2020
Priority date	Sep 19, 2019
Publication date	Oct 3, 2023
Grant date	Oct 3, 2023

Abstract

Disclosed herein is a light-gauge, ultra-high strength weathering steel sheet with a composition, material properties, and surface characteristics that make it suitable for hot-stamping applications and making hot-stamped products. Also disclosed herein is a high friction rolled carbon alloy steel strip free of prior austenite grain boundary depressions and having a smear pattern. Still further disclosed herein is a high friction rolled carbon alloy steel strip that has been surface homogenized to provide a thin cast steel strip free of a smear pattern.

First claim

What is claimed is:

1. A continuously cast ultra-high strength steel sheet with a corrosion resistance of a weathering steel for use in hot-stamping applications comprising:
a carbon alloy thin cast steel strip cast at a cast thickness less than or equal to 2.5 mm having a composition comprising:
(i) by weight, between 0.20% and 0.40% carbon, between 0.1% and 3.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.1% and 3.0% nickel, and silicon killed containing less than 0.01% aluminum, and
(ii) the remainder iron and impurities resulting from melting;
wherein bainite is formed from prior austenite within the thin cast steel strip by cooling the thin cast steel strip at less than 100° C./s to produce a microstructure of primarily bainite, a yield strength of between 620 and 800 MPa, a tensile strength of between 650 and 900 MPa, an elongation of between 3% and 10%, and having a corrosion index of 6.0 or greater independent of an additional coating.

2. The steel sheet of claim 1 wherein bainite is formed from the prior austenite within the thin cast steel strip by cooling the thin cast steel strip at less than 100° C./s to produce a microstructure of substantially bainite.

3. The steel sheet of claim 1 wherein the carbon alloy thin cast steel strip comprises, by weight, between 0.2% and 0.39% copper.

4. The steel sheet of claim 1 wherein the carbon alloy thin cast steel strip comprises, by weight, between 1.0% and 3.0% nickel.

5. The steel sheet of claim 1 wherein the carbon alloy thin cast steel strip comprises, by weight, between 0.2% and 0.39% copper and between 1.0% and 3.0% nickel.

6. The steel sheet of claim 1 wherein the thin cast steel strip is capable of undergoing an austenitizing condition at between 780° C. and 950° C. to austenitize the thin cast steel strip.

7. The steel sheet of claim 6 wherein the austenitizing condition is for a period of between 1 minute and 30 minutes.

8. The steel sheet of claim 6 wherein the austenitizing condition is for a period of between 6 minutes and 10 minutes.

9. The steel sheet of claim 1 wherein the thin cast steel strip is capable of undergoing an austenitizing condition at between 900° C. and 930° C. to austenitize the thin cast steel strip.

10. The steel sheet of claim 9 wherein the austenitizing condition is for a period of between 1 minute and 30 minutes.

11. The steel sheet of claim 9 wherein the austenitizing condition is for a period of between 6 minutes and 10 minutes.

12. The steel sheet of claim 1 wherein the cast thickness is solidified at a heat flux greater than 10.0 MW/m 2 and cooled in a non-oxidizing atmosphere to below 1100° C. and above the Ar3 temperature at a cooling rate greater than 15° C./s before the bainite is formed from prior austenite.

13. The steel sheet of claim 1 having a reduced thickness of between 15% and 50% reduction by hot rolling the as-cast thickness before forming the bainite.

14. The steel sheet of claim 1 having a reduced thickness of between 15% and 50% reduction and having a pair of opposing exterior side surfaces primarily free of prior austenite grain boundary depressions by high friction hot rolling the opposing exterior side surfaces before forming the bainite.

15. The steel sheet of claim 14 wherein the pair of opposing exterior side surfaces are substantially free of prior austenite grain boundary depressions by high friction hot rolling the opposing exterior side surfaces before forming bainite.

16. The steel sheet of claim 14 wherein the pair of opposing exterior side surfaces further comprise a smear pattern of prior austenite grain boundaries, the smear pattern formed under shear through plastic deformation from high friction hot rolled prior austenite grain boundaries of the pair of opposing exterior side surfaces, the smear pattern extending in a direction of the high friction hot rolling.

17. The steel sheet of claim 16 wherein the pair of opposing exterior side surfaces are surface homogenized to eliminate the smear pattern.

18. The steel sheet of claim 1 wherein the composition has no purposeful addition of boron.

19. The steel sheet of claim 1 wherein the thin cast steel strip is formed with less than 5 ppm boron.

20. The steel sheet of claim 1 that is uncoated by an additional coating.

21. The steel sheet of claim 1 further comprising an additional coating.

22. The steel sheet of claim 1 comprising, by weight, between 0.1% and 1.0% chromium.

23. The steel sheet of claim 1 that is substantially free of scale when reheated to above an austenitizing temperature.

24. A method for making a hot-stamped product from a continuously cast ultra-high strength steel sheet with a corrosion resistance of a weathering steel comprising the steps of:
(a) preparing a molten steel melt comprising:
(i) by weight, between 0.20% and 0.35% carbon, between 0.1% and 3.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.1% and 3.0% nickel, silicon killed with less than 0.01% aluminum, and
(ii) the remainder being iron and impurities resulting from melting;

(b) forming the melt into a casting pool supported on casting surfaces of a pair of cooled casting rolls having a nip there between;
(c) counter rotating the casting rolls and solidifying at a heat flux greater than 10.0 MW/m 2 into a thin cast steel sheet to less than 2.5 mm in thickness delivered downwardly from the nip and cooling the sheet in a non-oxidizing temperature to below 1100° C. and above the Ar3 temperature at a cooling rate greater than 15° C./s;
(d) slowly cooling the thin cast steel strip at less than 100° C./s to produce a microstructure of primarily bainite from prior austenite within the thin cast steel strip, a yield strength of between 620 and 800 MPa, a tensile strength of between 650 and 900 MPa, an elongation of between 3% and 10%, and having a corrosion index of 6.0 or greater independent of an additional coating; and
(e) hot-stamping the thin cast steel strip to austenitize the thin cast steel strip to form a product with a yield strength and a tensile strength in excess of the yield strength and the tensile strength of the thin cast steel strip.

25. The method of claim 24 where the step of cooling the thin cast steel strip at less than 100° C./s forms a product comprising a microstructure of substantially bainite.

26. The method of claim 24 further comprising the step of:
austenitizing the thin cast steel strip at between 780° C. and 950° C.

27. The method of claim 26 wherein the step of austenitizing is for a period of between 1 minute and 30 minutes.

28. The method of claim 26 where the step of austenitizing is for a period of between 6 minutes and 10 minutes.

29. The method of claim 24 where the thin cast steel strip is substantially free of scale after the step of austenitizing.

30. The method of claim 24 further comprising the step of:
austenitizing the thin cast steel strip at between 900° C. and 930° C.

31. The method of claim 30 wherein the step of austenitizing is for a period of between 1 minute and 30 minutes.

Assignees

• Nucor Corp

Inventors

• Mishra Kishlay
• Kelly Paul
• Wang Tao

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C21D9/52 Primary

  for wires; for strips {; for rods of unlimited length} · CPC title

• B21D22/022

  by heating the blank or stamping associated with heat treatment (C21D takes precedence) · CPC title

• C21D6/004

  containing Cr and Ni · CPC title

• C21D6/005

  containing Mn · CPC title