EP1358360B1 - Steel galvanizing method - Google Patents

Abstract

Hot coating of a steel sheet (1) is carried out by dipping the sheet in a bath of liquid metal (5) utilizing an inlet tube (5) at least partly under a controlled oxidizing atmosphere. A method for the hot coating of a steel sheet (1) by dipping in a bath of liquid metal (5) utilizing an inlet tube (5) at least partly under a controlled oxidizing atmosphere consists of: (a) passing the sheet through a first zone (9) in which the atmosphere of hydrogen and water vapor is non-oxidizing with respect to iron and such that P(H2)/P(H2O) is greater than 70 and P(H2)) is less than 264 ppm; (b) and passing the sheet through a second zone (10) in which the atmosphere of hydrogen and water vapor is oxidizing with respect to the liquid metal and some alloying elements of the steel and such that the H2 content is less than 1% by volume. An Independent claim is also included for the inlet tube under a controlled atmosphere for putting this method into service and incorporating a means for separating the two zones.

Description

Object of the invention

The present invention relates to a novel method of hot-coating a steel sheet by dipping in a bath of liquid metal, in particular containing molten zinc.

More particularly, the invention relates to the control of the oxidation-reduction of the atmosphere of the input horn in the liquid zinc bath, with respect to iron and liquid zinc, more specifically in the vicinity of the wetting line.

State of the art

It is well known to those skilled in the hot-dip galvanizing art that the zinc vapors escaping from the molten zinc bath produce numerous surface defects, by depositing oxides or dusts on the sheet before or during its immersion, which render the product unsuitable for appearance applications.

Thus, it has been proposed to regulate the oxidation state of the atmosphere gas in the tube so that it is not oxidizing for iron but oxidizing only for liquid zinc on the surface of the bath and in the form of steam. This can be done by introducing into the trunk a controlled oxidizing gas, such as a mixture of nitrogen and wet hydrogen for example, optionally in the presence of carbon dioxide or oxygen. This results in the formation of a very thin film of oxide on the liquid metal, which prevents the evaporation of zinc, similar to a film of oil on the surface of the water, which prevents the evaporation of the latter.

Thus, US Pat. No. 4,557,953 (corresponding to EP-A-0 172 681) proposes a method in which an atmosphere of inert gases containing 1-8% hydrogen by volume in combination with 300 to 4500 ppm of water vapor. The ratio H ₂ / H ₂ O must be at least 4.

A similar way of proceeding, by way of example among many others, is described in JP-A-063 30 271.

If it is accepted that these inventions provide a solution to the problem in question, in turn problems are induced by the oxidation state generated by this very fact, in particular in some places of the liquid seal.

In the document JP-A-071 80 014, it is alternatively proposed to suppress the zinc vapors by cooling the surface of the bath to a temperature of about 400 ° C., by blowing nitrogen having a temperature which can reach up to at 200 ° C. This process is however very sharp and difficult to produce industrially, while requiring complex complementary facilities.

With regard to the oxidizing atmospheres in particular, the following two problems are observed. On the one hand, the oxidation-reduction potential recommended in the state of the art for oxidizing the surface of the liquid zinc does not prevent the oxidation of the alloying elements contained in the steel such as the manganese, chromium, silicon, etc. These are commonly used in the production of so-called hard steels high yield strength (HLE). On the other hand, the recommended oxidation-reduction potential also causes excessive oxidation of the surface at the level of the wetting line inside the horn. This results in the formation of large clusters of zinc oxide (ZnO) which always end up detaching, usually unpredictably. This occurs when the viscous driving forces of the surface liquid zinc become greater than those of adhesion and surface tension with respect to the horn.

This problem of ZnO cluster is explained by the fact that in the part of the liquid extremely close to the metal part of the horn, called the shoe, the speed of displacement of the zinc is almost zero at the wetting line. As a result, the formed oxides grow over time until they reach a critical size which then causes them to move towards the band. Finally, these clusters are driven by the web and generate unacceptable defects for the appearance applications of galvanized sheets. It is essentially the face in contact with the bottom roller, called back face, which is affected by these defects, according to a mechanism not fully elucidated to date.

Goals of the invention

The present invention aims to provide a solution to the problems posed in the state of the art and in particular related to the zinc oxidation process which is recommended.

The present invention has the particular object of reducing to a minimum the zone in which the oxidation of liquid zinc and alloying elements of the sheet, with the exception of iron, is achieved, therefore the time corresponding oxidation, before the entry of the strip into the zinc bath.

Main characteristic elements of the invention

• la tôle passe dans la trompe au niveau d'une première zone dont l'atmosphère, comprenant au moins de l'hydrogène et de la vapeur d'eau, est franchement non oxydante vis-à-vis du fer, la première zone présentant une pression partielle de vapeur d'eau P(H₂O) < 264 ppm et un rapport de pressions partielles P(H₂)/P(H₂O) > 70; cette atmosphère peut éventuellement être faiblement oxydante vis-à-vis du métal liquide et des éléments d'alliage de l'acier constituant la tôle ;
• la tôle passe dans la trompe au niveau d'une deuxième zone dont l'atmosphère, comprenant au moins de l'hydrogène et de la vapeur d'eau, et éventuellement CO₂ et CO, est beaucoup plus oxydante que l'atmosphère de la première zone, vis-à-vis du métal liquide et des éléments d'alliage de l'acier constituant ladite tôle, tout en ne l'étant pas vis-à-vis du fer, la deuxième zone présentant une teneur en H₂ inférieure à 1 % en volume, le pouvoir oxydant du gaz présent dans la deuxième zone étant équivalent à la pression partielle d'oxygène qui correspond à 1 < P (H2) /P (H₂O) ou P(H₂+CO)/P(H₂O+CO₂) <70.

The present invention relates to a novel process for hot-coating steel sheet by dipping in a bath of liquid metal, preferably containing more than 10% zinc, using an inlet horn at least in part in a controlled oxidizing atmosphere, characterized in that it comprises the following successive stages:

• the sheet passes into the trunk at a first zone whose atmosphere, comprising at least hydrogen and water vapor, is frankly non-oxidizing with respect to iron, the first zone having a partial pressure of water vapor P (H ₂ O) <264 ppm and a ratio of partial pressures P (H ₂ ) / P (H ₂ O)>70; this atmosphere may possibly be weakly oxidizing vis-à-vis the liquid metal and alloying elements of the steel constituting the sheet;
• the sheet passes into the trunk at a second zone whose atmosphere, including at least hydrogen and water vapor, and optionally CO ₂ and CO, is much more oxidizing than the atmosphere of the first zone, vis-à-vis the liquid metal and alloying elements of the steel constituting said sheet, while not being vis-à-vis the iron, the second zone having a lower H ₂ content at 1% by volume, the oxidizing power of the gas present in the second zone being equivalent to the partial pressure of oxygen corresponding to 1 <P (H 2) / P (H ₂ O) or P (H ₂ + CO) / P (H ₂ O + CO ₂ ) <70.

According to one characteristic of the invention, an injection of oxidizing gas for the liquid and non-oxidizing metal for iron is carried out in said second zone of said horn. This gas contains for example a mixture of H ₂ O and H ₂ , and / or of H ₂ O and N ₂ , and / or of CO ₂ , CO, H ₂ O, H ₂ and / or O ₂ .

In a particularly advantageous manner, the difference in oxidizing power, expressed in equivalent P (H ₂ ) / P (H ₂ O), between said first zone, non-oxidizing, and said second zone, oxidizing, is at least a factor 8 , that is to say

[P (H ₂ ) / P (H ₂ O)] _oxidant x 8 <[P (H ₂ ) / P (H ₂ O)] _{non-oxidizing} .

Equally advantageously, there is the following relationship between the partial pressures of said first zone, non-oxidizing, and said second oxidizing zone:

[P (H ₂ + CO) / P (H ₂ O + CO ₂ )] _oxidant x 8 <[P (H ₂ + CO) / P (H ₂ O + CO ₂ )] _{non-oxidizing}

Preferably, said first zone is reducing vis-à-vis the iron.

Brief description of the figures

Figure 1 shows schematically a sectional view of an installation type trompe galvanizing steel, according to the state of the art.

Figure 2 schematically shows a sectional view of an installation type steel tubing horn, according to the present invention.

Description of a preferred embodiment of the invention

A very schematic description of a conventional steel galvanizing tube is shown in Figure 1.

The steel strip 1 enters the molten zinc bath 5 at a horn 2 and changes direction, in order to exit the bath again, at the level The atmosphere 7 in the horn 2 above the molten zinc bath 5 is oxidizing at least with respect to zinc, but not iron. Clusters of zinc oxide 4 are formed at the level of the wetting line 6 on the inner surface of the horn 2.

Figure 2 shows the schematic galvanizing installation adapted to the features of the present invention.

The present invention relates to a device for producing a bell or horn 2 in two distinct sections by means of a separation zone 8, the atmospheres 9,10 respectively corresponding to the two sections, said zone 1 and zone 2, being of nature different. Zone 1 corresponds to a non-oxidizing atmosphere 9 and zone 2 corresponds to an oxidizing atmosphere 10 vis-a-vis liquid zinc.

According to a preferred embodiment of the invention, the zone 2 has an injector 12 of oxidizing gas with typically a ratio H ₂ / H ₂ O, or equivalent (H ₂ + CO) / (CO ₂ + H ₂ O), from 1 to 70, with an H ₂ content <1% by volume and a flow rate of between 5 and 50 Nm ³ / h.

Still according to the invention, zone 2 is as short as possible. Zone 2 has a length typically between 10 and 2000 mm and advantageously between 100 and 300 mm, in order to optimize the distribution of oxidizing gas over the width of the zone and to simultaneously minimize the oxidation time of the sheet. When the line rotates at a speed of 1 m / s for example, the oxidation time of the substrate is typically 0.1 to 0.2 seconds.

The present invention thus makes it possible to obtain the oxidation of liquid zinc in zone 2 and at the same time to limit the oxidation time available for the sheet before entering the zinc bath, by the presence of a zone 1 non-oxidizing. In other words, the invention consists in reducing to a minimum the zone in which the intentional oxidation of zinc is carried out in order to reduce the undesired available oxidation time for the alloying elements contained in the sheet metal. steel.

In addition, it will be possible to prevent the formation of oxide clusters at the level of the horn at the wetting line by protecting the inner part of the horn at this point by means of a reducing material, such as carbon for example. Thus, this reducing material makes it possible to limit as much as possible the adhesion of the boundary layer of the liquid metal to the tube, at the level of the wetting zone, by locally preventing the oxidation of zinc therein.

Claims (6)

1. Method for hot-coating a steel sheet (1) by quenching in a bath of liquid metal (5), preferably comprising over 10% zinc, using an intake pump (2) at least partially under controlled oxidising atmosphere, characterised in that it comprises the following successive stages:

   - the sheet (1) passes in the pump (2) at the level of a first zone (9) whose atmosphere, comprising at least hydrogen and water steam, is definitely non-oxidising with regard to iron, the first zone (9) having a partial pressure of water steam P(H₂O) < 264 ppm and a ratio of partial pressures P(H₂)/P(H₂O) > 70;

   - the sheet (1) passes in the pump (2) at the level of a second zone (10) whose atmosphere, comprising at least hydrogen and water steam and possibly CO₂ and CO, is oxidising with regard to the liquid metal and to the alloy elements of the steel making up said sheet, the second zone (10) having an H₂ level lower than 1% by volume, the oxidising power of the gas present in the second zone (10) being equivalent to the partial pressure of oxygen which corresponds to 1 < P(H₂)/P(H₂O) or P(H₂+CO)/P(H₂O+CO₂) < 70.
2. Method according to Claim 1, characterised in that an injection (12) of gas that is oxidising with regard to the liquid metal and non-oxidising with regard to iron occurs in said second zone (10) of said pump (2).
3. Method according to Claim 2, characterised in that said oxidising gas comprises a mixture of H₂O and H₂, and/or H₂O and N₂, and/or CO₂, CO, H₂O, H₂, and/or O₂.
4. Method according to any one of Claims 1 to 3, characterised in that the difference in oxidising power, expressed in equivalent P(H₂)/P(H₂O), between said non-oxidising first zone (9) and said oxidising second zone (10) is at least a factor of 8, i.e.:
   
           [P(H₂)/P(H₂O)] _oxidising x 8 < [P(H₂)/P(H₂O)]_{non-oxidising}.
   
   
5. Method according to any one of Claims 1 to 3, characterised in that the following relationship exists between the partial pressures of said non-oxidising first zone (9) and said oxidising second zone (10):
   
           [P(H₂+CO) / P (H₂O+CO₂)] _oxidising x8< [ P (H₂+CO) /P(H₂O+CO₂)]_{non-oxidising}.
   
   
6. Method according to any one of the preceding claims, characterised in that said first zone (9) is reducing with regard to iron.

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