Method and furnace installation for heat treating metal strip

Abstract

A method for heat-treating a metal strip, where the metal strip is pre-heated continuously in a pre-heating zone with the aid of hot gas and subsequently undergoes further heat treatment in a directly fired furnace in a reducing and/or oxidizing atmosphere. The metal strip is pre-heated in the pre-heating zone with hot inert gas and further heated with an electric heating system before entering the directly fired furnace. A furnace plant for implementing the process and a related heat recovery system are also disclosed.

Claims

1. A method for treating a metal strip (1), comprising: (a) pre-heating the metal strip (1) with a hot inert gas (6) in a pre-heating zone (3) to reach a pre-heated metal strip; (b) further heating the pre-heated metal strip after step (a) in a directly fired furnace (8) in one or more of a reducing atmosphere and an oxidizing atmosphere, the directly fired furnace (8) generating exhaust gases (11), wherein the metal strip (1) is further heated by an electric heating system (5) after step (a) of pre-heating with inert gas (6) and before entering the directly heated furnace (8) in step (b), and heat in the exhaust gases (11) from the directly fired furnace (8) is used to pre-heat the inert gas (6) for use within the pre-heating zone (3).

2. The method of claim 1, wherein the electric heating system is an induction heating system.

3. The method of claim 1, wherein the metal strip (1) is heated to more than 200° C. in the inert gas atmosphere in the pre-heating zone (3).

4. The method of claim 3, wherein the metal strip (1) is heated to up to approximately 300° C. in the inert gas atmosphere in the pre-heating zone (3).

5. The method of claim 3, wherein the metal strip (1) is heated to more than approximately 350° C. by the electric heating system.

6. The method of claim 5, wherein the metal strip (1) is heated to more than approximately 500° C. by the electric heating system.

7. The method of claim 1, wherein the electric heating system (5) heats the metal strip (1) in an inert gas atmosphere.

8. The method of claim 7, wherein the inert gas atmosphere is a nitrogen atmosphere.

9. The method of claim 7, wherein heat in the exhaust gases (11) from the directly fired furnace (8) is used to pre-heat the combustion air (13) for the burners in the directly fired furnace (8).

10. The method of claim 1, wherein the electric heating system (5) heats the metal strip (1) in a reducing atmosphere.

11. The method of claim 10, wherein the reducing atmosphere is a nitrogen atmosphere containing 2-3% hydrogen.

12. The method of claim 10, wherein heat in the exhaust gases (11) from the directly fired furnace (8) is used to pre-heat the combustion air (13) for the burners in the directly fired furnace (8).

13. The method of claim 1, wherein heat in the exhaust gases (11) from the directly fired furnace (8) is used to pre-heat the combustion air (13) for the burners in the directly fired furnace (8).

14. A method of treating a metal strip (1), comprising: (i) conveying the metal strip (1) to a pre-heating zone (3) within which the metal strip (1) is pre-heated to above 200° C. with a hot inert gas (6), thereby forming a pre-heated metal strip; (ii) conveying the pre-heated metal strip through an electric heating system (5) positioned downstream of pre-heating zone (3) for further heat treatment in either an inert gas atmosphere or a reducing atmosphere of an inert gas with hydrogen to more than approximately 350° C., thereby forming an electrically heated metal strip; and (iii) feeding the electrically heated metal strip through a directly fired furnace (8) positioned downstream of the electric heating system (5) for treatment in one or more of a reducing atmosphere and an oxidizing atmosphere, the directly fired furnace (8) generating exhaust gases (11), wherein heat in the exhaust gases (11) from the directly fired furnace (8) is used to pre-heat the inert gas (6) for use in the pre-heating zone (3).

15. The method of claim 14, wherein the electric heating system heats the pre-heated metal strip in a reducing atmosphere of nitrogen with approximately 2-3% hydrogen.

16. The method of claim 14, wherein the electric heating system heats the pre-heated metal strip via induction to more than 500° C.

17. The method of claim 14, wherein heat in the exhaust gases (11) from the directly fired furnace (8) is used to pre-heat the combustion air (13) for the burners in the directly fired furnace (8).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, an embodiment of the invention is described on the basis of two drawings. In these drawings:

(2) FIG. 1 shows a schematic arrangement of the furnace plant;

(3) FIG. 2 shows a schematic arrangement of the heat recovery system of the furnace plant.

DETAILED DESCRIPTION

(4) The same reference numerals in the two figures refer to the same components or material flows in each case.

(5) In FIG. 1, the metal strip 1 runs continuously at an inlet temperature of approximately 50° C. over a pair of sealing rolls 2 into the pre-heating zone 3 of the furnace plant and is heated there to approximately 200° C. with the aid of hot nitrogen 6. In the pre-heating zone 3, hot nitrogen 6 is blown through nozzles directly onto the metal strip 1. Deflector rolls 4 guide the metal strip 1 through the furnace plant.

(6) Following the pre-heating zone 3, the metal strip 1 is heated further to approximately 500° C. in an inert gas atmosphere with the aid of an electric induction heating system 5. Nitrogen is also fed into this area through the pipe 7. Then the metal strip enters the directly fired furnace 8 and is further heated there in the initial area 8a before being freed of the oxide layer in the reducing zone 8b at approximately 720° C. Immediately after this, the bright metal strip 1 is exposed to an oxidizing atmosphere 8c at approximately 760° C. in which internal oxidation processes take place preferably in the basic material, where silicon oxide is formed. After this, the metal strip leaves the furnace plant via the sealing roll pair 2 and is then fed to a galvanizing plant, for example after further heat treatment and cooling.

(7) FIG. 2 shows a schematic of a heat recovery system in the furnace plant in FIG. 1. The hot, waste gases 11 from the directly fired furnace 8 are enriched here with combustion air 13 and fed to a post-combustion plant 9. In the two heat exchangers 10a and 10b and in the boiler 12, heat is extracted from the hot, exhaust gases 11 before they are discharged through the chimney 14. Of course, a suitable exhaust gas cleaning process can be implemented beforehand.

(8) In the first heat exchanger 10a, the combustion air 13 for the furnace 8 burners is heated to approximately 560° C. by the hot exhaust gas, which has a temperature of approximately 950° C. In the second heat exchanger 10b, the nitrogen from the pre-heating zone is heated again from approximately 350° C. to approximately 450° C. and then returned to the pre-heating zone in order to heat the metal strip 1.

(9) As the surface of the metal strip 1 is wet in places from water or hydrocarbons when it enters the pre-heating zone 3, water vapour and hydrocarbons would gather in the pre-heating zone 3 if the nitrogen loop were closed. In order to avoid this, some of the nitrogen is removed from the pre-heating zone 3 and replaced by fresh nitrogen, as illustrated by the two arrows in FIG. 2.