High-resistance steel strip comprising a temperature homogenisation chamber

Abstract

Method for thermally treating a scrolling steel strip (5), said method comprising the following steps: heating the strip (5) in a zone for heating with a direct flame (10); temperature homogenisation of the strip (5) in a homogenisation chamber (20) comprising at least one radiant heating tube (25), so as to homogenise the strip (5) in temperature after the passing thereof into the zone for heating with a direct flame (10) of the preceding step; oxidation of the strip (5) in an oxidation chamber (30) with an oxidising atmosphere having an oxygen volume concentration greater than 1%; reduction of the strip (5) in a reduction zone (40).

Claims

1. A method for thermally treating a scrolling high-resistance steel strip, said method comprising the following steps: a) heating the strip in a zone for heating with a direct flame, the heating step being carried out with an atmosphere having an oxygen volume concentration less than 0.01% by volume; b) temperature homogenisation of the strip in a homogenisation chamber comprising at least one radiant heating tube, so as to homogenise the strip in temperature after the passing through the zone for heating with a direct flame, the temperature homogenisation step being carried out with an atmosphere having an oxygen volume concentration less than 0.01% by volume; c) after the temperature homogenization of the strip, oxidation of the strip in an oxidation chamber with an oxidizing atmosphere to form an oxide layer on the strip, wherein the temperature homogenisation results in growth of a more homogenous oxide layer thickness over the surface of the strip as compared to a thickness of an oxide layer without the temperature homogenisation; d) after the oxidation of the strip, a further thermal treatment step which increases a temperature of the strip and comprising reduction of the strip in a reduction zone to remove the oxide layer.

2. The method according to claim 1, wherein the reduction zone has a reducing atmosphere having a hydrogen volume concentration greater than 3%.

3. The method according to claim 1, wherein the oxidation step is carried out at a strip temperature of between 650 C. and 750 C.

4. The method according to claim 1, wherein said oxidizing atmosphere has an oxygen volume concentration between 1.5% and 5%.

5. The method according to claim 1, wherein the temperature homogenisation step is carried out at a strip temperature of between 650 C. and 750 C.

6. The method according to claim 1, wherein the heating step a) is carried out so as to obtain a strip temperature of between 650 C. and 750 C.

7. The method according to claim 1, wherein the temperature homogenisation step is carried out with an atmosphere without oxygen.

8. The method according to claim 1, wherein the heating step a) is carried out with an atmosphere without oxygen.

9. The method according to claim 1, wherein the temperature homogenisation step is carried out by the scrolling of the strip in the proximity of said at least one radiant heating tube.

10. The method according to claim 1, wherein said homogenisation section comprises two radiant heating tubes and in that the strip scrolls between said two radiant heating tubes.

11. The method according to claim 1, wherein said oxidizing atmosphere has an oxygen volume concentration between 2% and 5%.

Description

DESCRIPTION OF THE DRAWINGS

(1) These aspects of the invention, as well as others will be clarified in the detailed description of particular embodiments of the invention, reference being made to the drawings of the figures, in which:

(2) FIG. 1 shows an embodiment according to the invention;

(3) FIG. 2 shows an embodiment according to the invention;

(4) FIG. 3 shows a schematic view of the supply of a strip to a temperature homogenisation chamber, then to an oxidation chamber and the progression of the strip to a reduction zone.

(5) The drawings of the figures are not to scale and are not limiting. Generally, similar elements are referenced by similar references in the figures. The presence of reference numbers in the drawings cannot be considered as limiting, including when these numbers are indicated in the claims.

DETAILED DESCRIPTION

(6) FIG. 1 shows a schematic illustration of the furnace 1 according to the second aspect of the invention making it possible to implement the method according to the first aspect of the invention. The furnace 1 comprises, in the scrolling direction of the strip 5, a zone for heating with a direct flame 10, a temperature homogenisation chamber 20, an oxidation chamber 30 and a reduction zone 40 for reducing oxide and the thermal treatment of the strip. The furnace 1 comprises a direct heating furnace section 2 comprising the zone for heating with a direct flame 10 and a radiant heating furnace section 3 comprising the temperature homogenisation chamber 20, the oxidation chamber 30 and the reduction zone 40.

(7) The method according to the invention comprises the implementation of step a) for heating the strip 5 by direct flame in the zone for heating with a direct flame 10. The method then comprises the implementation of step b), i.e. the scrolling of the strip 5 in the proximity of at least one radiant heating tube 25 so as, for example, to leave time for the strip 5 preheated to a target temperature, to be homogenised in temperature, while preserving said target temperature. According to another possible scenario, the strip 5 can be heated in the homogenisation chamber 20 so as to have a (homogenised) outlet temperature greater than the inlet temperature. The method then comprises the implementation of the oxidation step c), i.e. the scrolling of the strip 5 in the oxidation chamber 30 comprising an oxygen volume concentration greater than 1% and preferably between 1.5% and 5%. During step c), an oxide layer is formed on the surface of the strip 5. The oxide formed is mainly iron oxide II, II-III, or III, generally. The method for thermally treating a steel strip 5 comprises, after step c), step d) during which, the steel strip 5 oxidised in step c), undergoes a thermal treatment at a strip temperature up to 800 C. and preferably up to 850 C. During this step d), the strip 5 is subjected to a reducing atmosphere preferably comprising a hydrogen volume concentration greater than 3%, and more preferably, between 3% and 5%. The remaining volume fraction being generally nitrogen. The temperature of the thermal treatment in the reduction zone during step d) can be modified, relatively easily, without steps a), b) and c) being greatly modified.

(8) FIG. 2 shows a view of the whole of a furnace 1 according to the second aspect of the invention, with a schematic representation of the progression of the strip 5 through the zone for heating with a direct flame 10, the homogenisation chamber 20, the oxidation chamber 30 and the reduction zone 40 comprised in the furnace 1. The strip 5 describes a succession of vertical passes during which it scrolls through the direct heating furnace section 2, then the radiant heating furnace section 3. After having scrolled through the zone for heating with a direct flame 10, the strip 5 enters into the radiant heating furnace section 3 through the homogenisation chamber 20. In the non-limiting example shown in FIG. 2, the zone for heating with a direct flame 10 comprises two pass lines. Then, the strip 5 is directed towards the temperature homogenisation chamber 20.

(9) The pass line comprising the temperature homogenisation chamber 20 and the oxidation chamber 30 is situated in the RTF section (radiant heating furnace section) of the furnace 1. Thus, the oxidation chamber 30 is at a similar temperature of the RTF section which surrounds it while being preferably isolated at the level of the oxygen and hydrogen content.

(10) After having left the oxidation chamber 30, the strip 5 enters into the reduction zone 40 for the thermal treatment thereof. The reduction zone 40 comprises a series of vertical passes surrounded by radiant heating tubes 25 making it possible for an adjustment of the temperature of the strip 5 in order to carry out a desired thermal treatment of the high-resistance steel strip 5.

(11) FIG. 3 shows a schematic view of the supply of the strip 5 to the temperature homogenisation chamber 20, then to the oxidation chamber 30 and the progression of the strip 5 to the reduction zone 40. FIG. 3 shows a particular embodiment of the temperature homogenisation chamber 20 which illustrates, by way of example, three radiant heating tubes 25 arranged such that the strip 5 passes, in the proximity during the scrolling thereof, into the temperature homogenisation chamber 20. The temperature homogenisation chamber 20 illustrated makes it possible for good homogenisation of the temperature of the strip 5 at a target temperature, the target temperature being defined according to the composition of the steel. Thus, an oxide thickness specifically defined and homogenous over the whole of the surface of the strip 5 can be obtained.

(12) For example, in operation, a steel strip 5 is supplied in a zone for heating with a direct flame 10 and is heated under reducing conditions, in the presence of carbon monoxide and hydrogen, preferably so as to reach a strip temperature of between 650 C. and 750 C. The steel strip is then brought towards the oxidation chamber 30 which is confined in the section of the radiant heating furnace (RTF), where the oxidation occurs with an oxygen content greater than 1%. This oxidation step makes it possible for the formation on the surface of an iron oxide layer, for example. Then, the oxide layer is removed during the step of thermally treating in a reducing atmosphere, in order to proceed with the galvanisation step according to a method well-known to a person skilled in the art.

(13) The present invention has been described in relation to specific embodiment, which have a purely illustrative value and must not be considered as limiting. Generally, the present invention is not limited to the examples illustrated and/or described above. The use of the verbs comprise, include, involve or any other variant, as well as the conjugations thereof, cannot, in any manner, exclude the presence of elements other than those mentioned. The use of the indefinite article one, a or an, or the definite article the, to introduce an element does not exclude the presence of a plurality of these elements. The reference numbers in the claims do not limit the scope thereof.

(14) In summary, the invention can also be described as follows. Method for thermally treating a scrolling high-resistance steel strip 5 and comprising the following steps: a) heating the strip 5 in a zone for heating with a direct flame 10; b) temperature homogenisation of the strip 5 in a homogenisation chamber 20 comprising at least one radiant heating tube 25, so as to homogenise a temperature of the strip 5 after the passing thereof into the zone for heating with a direct flame 10; c) oxidation of the strip 5 in an oxidation chamber 30 with an oxidising atmosphere having an oxygen volume concentration greater than 1%; d) reduction of the strip 5 in a reduction zone 40 with a reducing atmosphere having a hydrogen volume concentration greater than 3%.