METHOD FOR RECRYSTALLISATION ANNEALING OF A NON-GRAIN-ORIENTED ELECTRIC STRIP

Abstract

A method for the recrystallisation annealing of a non-grain-oriented electric strip (2) in a continuous annealing and coating line (1) is presented. Therein, the electric strip (2) is heated in an induction furnace (5) to a temperature of at least 680° C. at a heating rate of at least 80 K/s and then, in an optional second continuous furnace (8), to a temperature of at least 820° C. at a heating rate of at most 20 K/s. The electric strip (2) is initially heated before the induction furnace (5) via a first continuous furnace (3) to a temperature of at least 300° C. at a heating rate of at most 60 K/s.

Claims

1.-10. (canceled)

11. A method for recrystallization annealing of a non-grain-oriented electric strip (2) in a continuous annealing and coating line (1), comprising: initially heating the electric strip (2) by a first continuous furnace (3) to a temperature of at least 300° C. at a heating rate of at most 60 K/s; thereafter heating the electric strip (2) in an induction furnace (5) to a temperature of at least 680° C. at a heating rate of at least 80 K/s; thereafter heating the electric strip (2) in a second continuous furnace (8) to a temperature of at least 820° C. at a heating rate of at most 20 K/s.

12. The method according to claim 11, wherein the electric strip (2) passes through the first continuous furnace (3) with a dwell time of at least 5 s.

13. The method according to claim 11, wherein the electric strip (2) is heated in the first continuous furnace (3) to a temperature of at most 500° C.

14. The method according to claim 11, wherein the electric strip (2) is heated in the first continuous furnace (3) to a temperature of at most 450° C.

15. The method according to claim 11, wherein the heating rate in the first continuous furnace (3) is at most 50 K/s.

16. The method according to claim 11, wherein the heating rate in the first continuous furnace (3) is at most 30 K/s.

17. The method according to claim 11, wherein the first continuous furnace (3) is continuously flushed with a flushing gas.

18. The method according to claim 17, wherein the flushing gas has a hydrogen content of 20 to 50% by volume and flows in a counterflow direction opposite a movement of the electric strip (2).

19. The method according to claim 11, wherein heating the electric strip (2) in an induction furnace (5) is performed in two stages by heating the electric strip (2) initially in a first stage (6) to a temperature in the range of 680-00° C. and then in a second stage (7) to a temperature in the range of 700-950° C.

20. The method according to claim 19, wherein heating the electric strip (2) in the first stage (6) is performed by a longitudinal field inductor and wherein heating the electric strip (2) in the second stage (7) is performed by a transverse field inductor.

21. The method according to claim 11, further comprising annealing the electric strip (2) in the second continuous furnace (8) at a temperature in the range of 820-1050° C. for at least 5 s.

22. The method according to claim 21, wherein the annealing is carried out initially in a first step at a temperature in the range of 820 to 880° C. and then in a second step at a temperature in the range of 900 to 1150° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 shows an embodiment of an annealing and treatment line for carrying out the method for the recrystallization annealing of a non-grain-oriented electric strip, and

[0024] FIG. 2 shows an embodiment of the temperature profile of the method for the recrystallization annealing of a non-grain-oriented electric strip.

DETAILED DESCRIPTION

[0025] FIG. 1 shows an embodiment of an annealing and treatment line 1 for carrying out the method for the recrystallization annealing of a non-grain-oriented electric strip, in which, for example, a non-grain-oriented electric strip 2 with a width of 1200 mm and a strip thickness of 500 μm is subjected to a temperature treatment at a strip speed of 150 m/min, as shown for example in FIG. 2.

[0026] The annealing and treatment line 1 comprises a first continuous furnace 3 with electric heating elements 4, by means of which the electric strip 2 is heated. The first continuous furnace 3 has a length of 30 m. Downstream in the direction of strip travel is an induction furnace 5, which is formed from two separate stages 6, 7 and has a total length of 6 m. A second continuous furnace 8, which is also conventionally heated by heating elements 4, follows to the induction furnace 5. The second continuous furnace 8 has a length of 42 m. A holding zone 9 with a length of 54 m is also provided downstream of the second continuous furnace 8.

[0027] As shown in FIG. 1, the non-grain-oriented electric strip 2 initially enters the first radiation-heated continuous furnace 3 via an entry channel 10. The electric strip 2 is heated to 450° C. within 30 seconds. Up to this temperature, the microstructure recovery is negligible. The low heating rate of 15 K/s is advantageous in order to evaporate the unavoidable residues from the pre-process on the electric strip 2 slowly and without cracking. The hot inner walls of the first continuous furnace 3 guarantee that the residues now in the gas atmosphere do not condense. The evaporation of the residues from the electric strip 2 usually leads to a strong emission of smoke. Therefore, the first continuous furnace 3 is continuously flushed in counterflow with a hydrogen-rich flushing gas, which is supplied to the first continuous furnace 3 through an inlet opening 11a and discharged through an outlet opening 11b.

[0028] Alternatively and/or additionally, the flushing gas can also be supplied to the treatment line 1 further downstream, for example via the second continuous furnace 8 and/or via the holding zone 9, as indicated by the arrows 13.

[0029] The electric strip 2 then passes through induction furnace 5, where it is initially heated up to 700° C. in a longitudinal field inductor 6 and then up to 800° C. in a transverse field inductor 7. Furnace rollers 12 are located between the individual stages 6, 7 in order to keep the strip sag low and to ensure precise guidance of the electric strip 2 through the narrow gaps of the two inductors 6, 7.

[0030] Since the furnace roller 12 is located outside the inductors 6, 7, there is an unavoidable interruption in the heating. However, the resulting average heating rate in the induction furnace 5 of 180 K/s is high enough to reliably suppress microstructure recovery and thus non-uniform grain growth.

[0031] Further heating to 1,000° C. takes place in the radiation-heated second continuous furnace section 8. The heating rate in this continuous furnace 8 is insignificant in terms of quality and thus depends only on its construction length and on ensuring sufficient heating time.

[0032] In the holding zone 9, the desired grain size is set in the microstructure by holding the strip temperature at 1,000° C. with a dwell time of presently 26 s.

[0033] This is followed by a slow cooling system and a rapid cooling system (not shown in FIG. 1), which are designed in a manner familiar to the skilled person.

LIST OF REFERENCE SIGNS

[0034] 1 Annealing and treatment line

[0035] 2 Electric strip

[0036] 3 First continuous furnace

[0037] 4 Electric heating elements.

[0038] 5 Induction furnace

[0039] 6 First stage/longitudinal field inductor

[0040] 7 Second stage/transverse field inductor

[0041] 8 Second continuous furnace

[0042] 9 Holding zone

[0043] 10 Entry channel

[0044] 11a Inlet opening for flushing gas

[0045] 11b Outlet opening for flushing gas

[0046] 12 Furnace rollers

[0047] 13 Arrow