CASTING STEEL STRIP

• US 20070114002A1
• Filed: 10/11/2006
• Published: 05/24/2007
• Est. Priority Date: 10/10/2003
• Status: Active Grant

• Current Assignee: Nucor Corporation
• Original Assignee: Nucor Corporation
• Inventors: BLEJDE, Walter, SCHLICHTING, Mark, MAHAPATRA, Rama, SOSINSKY, David
• Granted Patent: US 7,484,551 B2
• Time in Patent Office: Days
• US Class Current: 164/480
• CPC Class Codes: B22D 11/001 of specific alloys
• CPC Class Codes: B22D 11/0622 formed by two casting wheels

Abstract

A method of controlling heat flux between the casting pool and the surfaces of the casting roll in a strip steel casting process includes controlling the concentration of nitrogen in the steel melt. The concentration of nitrogen may be controlled to achieve an operational heat transfer to the casting roll. The beat flux to the casting rolls may be controlled by maintaining the sum of partial pressures calculated from the nitrogen and hydrogen concentrations in the casting pool below 1.15 atmospheres. Decreasing the concentration of nitrogen can increase heat flux at the casting rolls.

Claims

1. A method of casting thin steel strip comprising the steps of:

determining a desired heat flux set point in casting thin cast strip from molten metal from a casting pool between casting rolls in a twin roll caster;

calculating the heat flux in the casting thin cast strip from molten metal from the casting pool in the twin roll caster; and

changing the nitrogen concentration in the casting pool to adjust the heat flux to the desired heat flux set point.

2. The method of casting thin steel strip as claimed in claim 1 further comprising:

controlling the nitrogen concentration in the casting pool during the casting campaign to adjust for changes roughness of the casting rolls and other operating conditions to maintain the calculated heat flux to the desired heat flux set point or some desired adjustment therefrom.

3. The method of casting thin steel strip as claimed in claim 2 where the controlling step is done automatically by a proportional controller.

4. A method of casting steel strip comprising:

introducing molten plain carbon steel on casting surfaces of at least one casting roll with the molten steel having a nitrogen content below about 120 ppm and a hydrogen content below about 6.9 ppm and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;

forming a casting pool of molten metal adjacent the casting surfaces of the casting roll;

causing the nitrogen level in the molten metal in the casting pool to vary depending on casting speed and strip thickness to control heat flux between the casting pool and the surfaces of the casting roll to a desired value; and

solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to form thin steel strip.

5. The method of claim 4 where the sum of partial pressure of nitrogen and partial pressure of hydrogen calculated from the nitrogen and hydrogen concentration in the introduced molten metal is no more than 1.0 atmosphere.

6. The method of casting steel strip of claim 5 where the sum of the partial pressures of hydrogen and nitrogen is between 0.2 and 0.8 atmospheres.

7. The method of casting steel strip of claim 4 comprises the steps of correlating heat flux with the sum of the partial pressures of hydrogen and nitrogen in the casting pool, measuring the hydrogen and nitrogen concentrates in the molten metal going into the casting pool, calculating the sum of the partial pressures from the measured hydrogen and nitrogen concentration, and changing the concentration of nitrogen in the casting pool to provide the sum of the partial pressure of the hydrogen and nitrogen to provide the desired correlated heat flux.

8. The method of casting steel strip of claim 4 where the hydrogen content is between 1.0 and 6.5 ppm.

9. A method of casting steel strip comprising:

assembling a pair of cooled casting rolls having a nip between them and confining end closures adjacent to ends of the casting rolls;

introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on the casting rolls with the end closures confining the pool, with the molten steel having a nitrogen content below about 120 ppm and a hydrogen content below about 6.9 ppm and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;

causing the nitrogen level in the molten metal in the casting pool to vary depending on casting speed and strip thickness to control heat flux between the casting pool and the surfaces of the casting roll to a desired value;

counter-rotating the casting rolls and solidifying the molten steel to form metal shells on casting surfaces of the casting rolls having nitrogen and hydrogen levels reflected by the content of the molten steel to provide for the formation of thin steel strip; and

forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip.

10. The method of casting steel strip of claim 9 where the nitrogen level in the molten metal in the casting pool is controlled by calculating the sum of the partial pressures of nitrogen and hydrogen from the measured concentration of hydrogen and nitrogen in the casting pool, correlating the sum of the partial pressures of hydrogen and nitrogen with a desired level of heat flux, and adding or reducing nitrogen levels in the achieved desired sum of the partial pressures associated with target heat flux.

11. The method of claim 9 where the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.0 atmosphere.

12. The method of casting steel strip of claim 9 where the sum of the partial pressures of hydrogen and nitrogen is between 0.2 and 0.8 atmospheres.

13. The method of casting steel strip of claim 12 comprises the steps of correlating heat flux with the sum of the partial pressures of hydrogen and nitrogen in the casting pool, measuring the hydrogen and nitrogen concentrates in the molten metal going into the casting pool, calculating the sum of the partial pressures from the measured hydrogen and nitrogen concentration, and changing the concentration of nitrogen in the casting pool to provide the sum of the partial pressure of the hydrogen and nitrogen to provide the desired correlated heat flux.

14. The method of claim 9 where the hydrogen content is between 1.0 and 6.5 ppm.

15. A method of casting steel strip comprising:

introducing molten plain carbon steel on casting surfaces of at least one casting roll with the molten steel having a nitrogen content below about 100 ppm and a hydrogen content below about 6.9 ppm and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;

forming a casting pool of molten metal adjacent the casting surfaces of the casting roll;

causing the nitrogen level in the molten metal in the casting pool to vary depending on casting speed and strip thickness to control heat flux between the casting pool and the surfaces of the casting roll to a desired value; and

solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to form thin steel strip.

16. The method of claim 15 where the sum of partial pressure of nitrogen and partial pressure of hydrogen calculated from the nitrogen and hydrogen concentration in the introduced molten metal is no more than 1.0 atmosphere.

17. The method of casting steel strip of claim 15 where the sum of the partial pressures of hydrogen and nitrogen is between 0.2 and 0.8 atmospheres.

18. The method of casting steel strip of claim 17 comprises the steps of correlating heat flux with the sum of the partial pressures of hydrogen and nitrogen in the casting pool, measuring the hydrogen and nitrogen concentrates in the molten metal going into the casting pool, calculating the sum of the partial pressures from the measured hydrogen and nitrogen concentration, and changing the concentration of nitrogen in the casting pool to provide the sum of the partial pressure of the hydrogen and nitrogen to provide the desired correlated heat flux.

19. The method of casting steel strip of claim 15 where heat flux is correlated with the sum of the partial pressures of hydrogen and nitrogen in the casting pool, the hydrogen and nitrogen concentrates in the molten metal going into the casting pool in measured and the sum of the partial pressures calculated, and the concentration of nitrogen in the casting pool is changed to provide the sum of the partial pressure of the hydrogen and nitrogen to provide the desired correlated heat flux.

20. The method of casting steel strip of claim 15 where the hydrogen content is between 1.0 and 6.5 ppm.

21. A method of casting steel strip comprising:

assembling a pair of cooled casting rolls having a nip between them and confining end closures adjacent to ends of the casting rolls;

introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on the casting rolls with the end closures confining the pool, with the molten steel having a nitrogen content below about 120 ppm and a hydrogen content below about 6.9 ppm and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;

causing the nitrogen level in the molten metal in the casting pool to vary depending on casting speed and strip thickness to control heat flux between the casting pool and the surfaces of the casting roll to a desired value;

counter-rotating the casting rolls and solidifying the molten steel to form metal shells on casting surfaces of the casting rolls having nitrogen and hydrogen levels reflected by the content of the molten steel to provide for the formation of thin steel strip; and

forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip.

22. The method of casting steel strip of claim 21 where the nitrogen level in the molten metal in the casting pool is controlled by calculating the sum of the partial pressures of nitrogen and hydrogen from the measured concentration of hydrogen and nitrogen in the casting pool, correlating the sum of the partial pressures of hydrogen and nitrogen with a desired level of heat flux, and adding or reducing nitrogen levels in the achieved desired sum of the partial pressures associated with target heat flux.

23. The method of claim 21 where the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1 atmosphere.

24. The method of casting steel strip of claim 21 where the sum of the partial pressures of hydrogen and nitrogen is between 0.2 and 0.8 atmospheres.

25. The method of casting steel strip of claim 24 comprises the steps of correlating heat flux with the sum of the partial pressures of hydrogen and nitrogen in the casting pool, measuring the hydrogen and nitrogen concentrates in the molten metal going into the casting pool, calculating the sum of the partial pressures from the measured hydrogen and nitrogen concentration, and changing the concentration of nitrogen in the casting pool to provide the sum of the partial pressure of the hydrogen and nitrogen to provide the desired correlated heat flux.

26. The method of casting steel strip of claim 21 where heat flux is correlated with the sum of the partial pressures of hydrogen and nitrogen in the casting pool, the hydrogen and nitrogen concentrates in the molten metal going into the casting pool in measured and the sum of the partial pressures calculated, and the concentration of nitrogen in the casting pool is changed to provide the sum of the partial pressure of the hydrogen and nitrogen to provide the desired correlated heat flux.

27. The method of claim 21 where the hydrogen content is between 1.0 and 6.5 ppm.

28. A method of casting steel strip comprising:

introducing molten plain carbon steel on casting surfaces of at least one casting roll with the molten steel having a nitrogen content below about 85 ppm and a hydrogen content below about 6.9 ppm and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;

forming a casting pool of molten metal adjacent the casting surfaces of the casting rolls;

causing the nitrogen level in the molten metal in the casting pool to vary depending on casting speed and strip thickness to control heat flux between the casting pool and the surfaces of the casting roll to a desired value; and

solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to form thin steel strip.

29. The method of claim 28 where the sum of partial pressure of nitrogen and partial pressure of hydrogen calculated from the nitrogen and hydrogen concentration in the introduced molten metal is no more than 1.0 atmosphere.

30. The method of casting steel strip of claim 28 where the sum of the partial pressures of hydrogen and nitrogen is between 0.2 and 0.8 atmospheres.

31. The method of casting steel strip of claim 30 comprises the steps of correlating heat flux with the sum of the partial pressures of hydrogen and nitrogen in the casting pool, measuring the hydrogen and nitrogen concentrates in the molten metal going into the casting pool, calculating the sum of the partial pressures from the measured hydrogen and nitrogen concentration, and changing the concentration of nitrogen in the casting pool to provide the sum of the partial pressure of the hydrogen and nitrogen to provide the desired correlated heat flux.

32. The method of casting steel strip of claim 28 where the hydrogen content is between 1.0 and 6.5 ppm.

33. A method of casting steel strip comprising:

assembling a pair of cooled casting rolls having a nip between them and confining end closures adjacent to ends of the casting rolls;

introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on the casting rolls with the end closures confining the pool, with the molten steel having a nitrogen content below about 120 ppm and a hydrogen content below about 6.9 ppm and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;

causing the nitrogen level in the molten metal in the casting pool to vary depending on casting speed and strip thickness to control heat flux between the casting pool and the surfaces of the casting roll to a desired value;

counter-rotating the casting rolls and solidifying the molten steel to form metal shells on casting surfaces of the casting rolls having nitrogen and hydrogen levels reflected by the content of the molten steel to provide for the formation of thin steel strip; and

forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip.

34. The method of casting steel strip of claim 33 where the nitrogen level in the molten metal in the casting pool is controlled by calculating the sum of the partial pressures of nitrogen and hydrogen from the measured concentration of hydrogen and nitrogen in the casting pool, correlating the sum of the partial pressures of hydrogen and nitrogen with a desired level of heat pressures associated with target heat flux.

35. The method of claim 33 where the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.0 atmosphere.

36. The method of casting steel strip of claim 33 where the sum of the partial pressures of hydrogen and nitrogen is between 0.2 and 0.8 atmospheres.

37. The method of casting steel strip of claim 36 comprises the steps of correlating heat flux with the sum of the partial pressures of hydrogen and nitrogen in the casting pool, measuring the hydrogen and nitrogen concentrates in the molten metal going into the casting pool, calculating the sum of the partial pressures from the measured hydrogen and nitrogen concentration, and changing the concentration of nitrogen in the casting pool to provide the sum of the partial pressure of the hydrogen and nitrogen to provide the desired correlated heat flux.

38. The method of claim 33 where the hydrogen content is between 1.0 and 6.5 ppm.