EP0870069B1 - Method for galvanising a steel sheet containing oxidizable alloying elements - Google Patents

Abstract

A method wherein a steel sheet is annealed in a non-oxidising annealing atmosphere containing 0.005-1 % of ammonia, and the sheet is immersed in a galvanising bath. The method is particularly suitable for steels containing more than 0.2 % of silicon as the oxidisable alloying element. The wettability of the steel in the galvanising bath is increased while maintaining the original mechanical and/or electrical properties of the steel.

Description

The invention relates to a method for galvanizing steel sheets. containing oxidizable addition elements, in particular silicon.

For certain uses of steel sheets, in particular in the field electrical or applications requiring high strength steels mechanical, steel grades containing addition elements are used oxidizable, in particular silicon.

For these uses, resistance is sometimes also sought high corrosion.

However, it has so far been difficult to obtain corrosion resistance satisfactory by using galvanization.

This difficulty results from poor wettability of these steel sheets in galvanizing baths, linked to the oxidizable addition elements that they contain, in particular in the case of silicon; this poor wettability affects the quality of the coating, including its consistency.

As an indication, in the case of silicon steels, the problems of wettability arises as soon as the silicon content reaches approximately 0.2%, and sometimes even below this content.

Steels with weaker oxidizable additives contents can therefore be galvanized by conventional means: we proceed in a manner known in itself by preparing the steel sheet to be coated by a cleaning treatment, then an annealing treatment under a non-atmospheric oxidizing, even reducing, before dipping the sheet in the bath galvanizing for the coating itself.

The problem of galvanizing therefore arises as soon as one seeks to use steels with a higher content of additives, in order to to obtain a galvanized sheet having both a high resistance to corrosion and high mechanical strength or electrical properties particular.

However, for these steels having oxidizable addition elements at high contents, for example silicon between 0.3% and 3%, we know a galvanizing process in which an additional treatment is carried out sheet before dipping it in the galvanizing bath; the JP document 51 40711 describes in particular additional processing of the type electrochemical which consists in dipping the sheet in a treatment solution to coat it with a very thin composite layer of iron nitride (FeN).

This composite layer with a thickness corresponding to more than 1 g / m2 is applicable on steel sheets with up to 3% silicon and appreciably improves the wettability of the sheet in the galvanizing bath.

But the solution presented by this document requires a step additional in the galvanized sheet manufacturing process, which is penalizing from an economic point of view.

Furthermore, excessive nitriding of the steel at the sheet-layer interface of galvanization and / or an excessively thick layer of iron nitride corresponding to more than 1 g / m2 interface, may cause deterioration of properties mechanical or electrical steel.

However, as already indicated, steels containing elements of oxidizable addition, in particular silicon, are precisely used for their specific mechanical and / or electrical properties.

The object of the invention is to provide a process for galvanizing steel sheets with high content of oxidizable addition elements, in particular of silicon, economical, effective and not detrimental to the mechanical and / or electrical properties of said steel.

The subject of the invention is a process for galvanizing sheet steel. containing oxidizable addition elements, in particular silicon, in which is annealed the sheet under an annealing atmosphere which is non-oxidizing then the sheet is dipped in a galvanizing bath, characterized in that said annealing atmosphere contains between 0.005% and 1% of ammonia.

The invention applies including to steel sheets containing elements addition oxidizable in proportions such that they prevent wettability of the sheet in the galvanizing bath to ensure a homogeneous galvanizing coating.

The invention is particularly applicable to steel sheets with more than 0.2% of silicon.

• ladite atmosphère de recuit contient entre 0,02% et 0,5% d'ammoniac.
• le temps de séjour de ladite tôle dans ladite atmosphère est inférieur ou égal à 3 minutes.

The invention may also have one or more of the following characteristics:

• said annealing atmosphere contains between 0.02% and 0.5% of ammonia.
• the residence time of said sheet in said atmosphere is less than or equal to 3 minutes.

The invention also relates to a sheet galvanizing device for implementing the method according to the invention of the type comprising means sheet annealing under controlled atmosphere, which lead downstream to means for quenching the annealed sheet in a galvanizing bath, and means for controlling said atmosphere, characterized in that the means for atmosphere control are adapted to introduce and maintain in said ammonia atmosphere at a content between 0.005 and 1% and in this that said ammonia is introduced into said atmosphere near the entrance of sheet metal in the annealing means.

The invention also relates to a steel sheet containing oxidizable additives, in particular silicon, galvanized by method according to the invention of the type having an interface layer, between said steel and galvanizing layer, containing nitrogen in solid solution characterized in that the thickness of said interface layer corresponds to less than 1 gram per square meter of interface surface.

The invention may also have as a characteristic at least 0.2% silicon as an additive in said steel.

The invention will be better understood on reading the description which will follow, given by way of nonlimiting example.

The galvanizing installation that is used includes means for annealing of sheet metal strip under controlled atmosphere, which lead downstream to means for dipping the sheet in a galvanizing bath, and means to control said atmosphere.

The annealing means can be divided into several zones corresponding to different temperatures, in particular an area of cooling before entering the galvanizing bath.

The atmosphere of this cooling zone can communicate with the atmosphere above the surface of the galvanizing bath.

In each of these zones, the atmosphere is controlled.

The annealing means and the sheet dipping means as well as the atmosphere control means are known in themselves.

The atmosphere control means are adapted in a way known per se for introducing ammonia into the annealing zone and to control the ammonia content of the annealing atmosphere between 0.005 and 1%.

According to a preferred variant of the invention, ammonia is introduced in the annealing atmosphere near the entry of the sheet in the means annealing.

This particular provision concerning the galvanizing device goes to contrary to conventional designs of gas supply to atmospheres annealing, according to which, in particular for supplying, for example, nitrogen and in hydrogen, the gas is introduced essentially at the outlet of the sheet metal annealing means, so that the gas flows against the current of sheet metal in the annealing means.

We will now describe the galvanizing process according to the invention.

The method according to the invention is applicable to steel sheets containing oxidizable addition elements.

The method according to the invention is particularly applicable to steel sheets silicon, especially more than 0.2% silicon.

To galvanize a silicon steel sheet, proceed as follows in the installation described above.

We clean the silicon sheet to be galvanized in a manner known per se, for example by passing the bare sheet in a flame to degrease the surface to be coated.

The sheet is then annealed in the annealing means of the installation, in conditions of temperature, duration and composition of the atmosphere of annealing adapted in a manner known per se to obtain in particular the desired recrystallization of steel and, as necessary, reduction of iron oxides possibly present on the surface of the sheet after cleaning.

According to the invention, under the aforementioned conditions concerning the composition of the annealing atmosphere, a condition relating to the content of ammonia from said atmosphere which must be between 0.005% and 1%, which can be reached by means of atmosphere control previously defined.

Preferably, said ammonia content is maintained between 0.02% and 0.5%.

The annealing time, i.e. the time it takes for the sheet to pass through the annealing means is preferably less than 3 minutes.

The annealing temperature is generally between 500 ° C and 900 ° C.

Said annealing conditions also include, as a need, a cooling step in an area of the annealing means provided for this purpose, before soaking the sheet in the galvanizing bath.

After annealing under this atmosphere comprising ammonia, said annealing therefore comprising any cooling of the sheet, we proceed to the galvanizing the annealed sheet itself in a manner known in herself.

So, for example, we dip the sheet in a galvanizing bath, we extracts the sheet from the bath, we wring the bath layer entrained on the sheet to obtain the required thickness of galvanizing coating, then cool the sheet metal.

Thus, according to the invention, even though the ammonia content is very low in the annealing atmosphere and remains below 1%, the effect is obtained expected improvement of the wettability of the sheet in the bath galvanizing for steels containing oxidizable additives, especially silicon at more than 0.2%.

This effect is therefore obtained without any additional step compared to a conventional manufacturing process of galvanized sheet, and without prejudice to mechanical and / or electrical properties of the steel used.

Surprisingly, this effect is obtained with very low contents in ammonia even if the duration of annealing, and therefore the duration of exposure of the sheet with ammonia, is less than or equal to 3 minutes.

The sheet obtained by the process according to the invention, present on the surface of the steel sheet after annealing, or at the steel-galvanizing layer interface, a interface layer composed of steel containing nitrogen in solid solution and / or composed of iron nitride.

Preferably, this interface layer has a thickness corresponding to less than 1 g / m2.

This low thickness of the interface layer contributes to maintaining the mechanical and electrical properties of steel.

Thanks to the invention, it therefore becomes possible to improve the resistance to corrosion and surface appearance of galvanized silicon steel sheets while maintaining the starting mechanical and / or electrical properties.

When using a galvanizing device in which, according to a preferred variant of the invention described above, ammonia is introduced in the annealing atmosphere near the entrance of the sheet, it improves still the wettability of steel.

The invention also advantageously applies to steels having other oxidizable addition elements than silicon, in particular titanium, manganese, chromium and aluminum.

The following examples illustrate the invention.

Comparative example 1 :

The purpose of this example is to illustrate the problem that the invention seeks to solve.

The sample to be galvanized is a steel sheet containing 0.3% silicon.

For galvanizing, the sample is annealed under a reducing atmosphere containing 15% hydrogen, the rest being nitrogen and unavoidable impurities.

The other conditions for galvanizing are established in itself known manner to obtain a zinc coating of the order of 140 g / m2.

The result of this comparative example is that we observe problems wettability of the sample in the galvanizing bath, which is detrimental to the adhesion and the homogeneity of the coating, therefore to the resistance to corrosion that it brings.

Example 1 :

The purpose of this example is to illustrate the galvanizing process according to the invention.

On a sample of the same nature as in Comparative Example 1, we proceed as in Comparative Example 1 with the difference that the annealing atmosphere contains, according to the invention, 0.03% of ammonia.

No problem of wettability of the sample is observed in the galvanizing bath, which allows the zinc coating to protect with effectiveness of the sample against corrosion.

Thus, a very low concentration of ammonia in the atmosphere of annealing solves the wettability problem encountered in comparative example 1 and there is no need to add a step additional in the manufacturing process of galvanized sheet.

Example 2:

The purpose of this test is to illustrate that, according to the invention, the presence low concentration ammonia in the front annealing atmosphere galvanizing does not significantly modify the mechanical properties of the sheets silicon steel, often called "HR" steel, ie "High Strength".

We measure the mechanical properties of steel sheet samples which are galvanized after annealing in atmospheres differing only in the ammonia content.

Apart from this ammonia content ("NH3 content"), the conditions for galvanizing are identical to those of Example 1.

The steel grade used corresponds to the steel called "SOLDUR 490" from SOLLAC, which contains between 0.2 and 0.3% silicon.

As in Example 1, no wettability problem is observed (at least in cases where the annealing atmosphere contains ammonia, even at very low contents).

To evaluate the mechanical properties, the limiting stress of elastic behavior - called R _p-0.2 - and the breaking stress - called R _m are measured in a conventional manner; the unit of measurement is the MegaPascal (MPa).

The results obtained are shown in the table below: NH3 content: Mechanical measurement 0% 0.5% 1% 2% 5% 10% R _p-0.2 (MPa) 498 511 510 542 535 578 R _m (MPa) 564 576 579 632 603 630

Provided therefore that, according to the invention, the ammonia content in the annealing atmosphere remains well below or equal to 1%, so we do not see no significant change in mechanical properties.

As soon as the ammonia content in the annealing atmosphere exceeds 1%, that is to say as soon as we place ourselves in any conditions already described in the prior art, there is a significant degradation of mechanical properties.

Comparative example 2:

The purpose of this example is to illustrate the drawbacks associated with the application to a steel sheet of a galvanizing process in which the steel is annealed under an atmosphere containing ammonia in a higher proportion than in the invention, as in GB 1 396 419.

This example also aims to illustrate the drawbacks related to application, to steels other than those containing elements addition in high proportions, of a galvanizing process in which is annealed the steel under an atmosphere containing weak ammonia proportion as in the present invention.

The procedure is as in Example 2, but on steel samples called "IF Ti", that is to say "Intersticial Free" (that is to say without elements interstitial), containing less than 250 ppm of silicon; this "IF Ti" steel is obtained in a manner known per se by the introduction of titanium to the production of liquid steel, titanium being intended to precipitate carbon content and prevent it from getting into the interstices.

The mechanical results obtained are shown in the table below. NH3 content: Mechanical measurement 0% 0.5% 1% 2% 5% 10% R _p-0.2 (MPa) 110 252 192 301 399 409 R _m (MPa) 298 401 426 426 453 507

It can therefore be seen that annealing under ammonia very significantly modifies the mechanical properties of sheet steel.

This result is likely to dissuade the skilled person from the galvanizing, annealing under ammonia, in particular with a view to galvanizing, steels which we want to control (and keep at initial) mechanical properties.

GB 1,396,419, mentioned above, describes a method of nitriding of steel wire, which includes an annealing treatment under a reducing atmosphere containing high proportions of ammonia (> 15%), this which has the effect of nitriding the surface to be coated.

This particular method of galvanizing applies here to wires (and not to sheets), to so-called effervescent steels; effervescent steels are non "deoxidized" steels; therefore they do not contain addition elements oxidizable and therefore do not pose problems of wettability in a bath galvanizing.

In this document, the effect sought by nitriding the surface of the steel before galvanizing is to improve the adhesion of the coating of galvanizing, which is a particularly critical problem on steel wires.

Thus, the means described in this document, namely the introduction ammonia in high proportion in the annealing atmosphere, applies to other steels than those of the field of the present invention and does not provide no particular wettability result, since moreover this Wettability is no problem for this type of steel.

What is more, the application of the means described in this document to steels not containing oxidizable additives, such as IFS steels (IF = "Insterticial Free Steel", or steel without middleware), has the disadvantage of degrade the mechanical properties, as shown by the results of the table above (NH3 content: 2, 5 and 10%).

This is why a person skilled in the art of galvanizing would be dissuaded to apply this nitriding means to solve the wettability problem of steels with high mechanical resistance, because it would fear to degrade the mechanical properties.

As shown in particular by the results obtained with low ammonia contents in accordance with the invention (NH3 content: 0.5 and 1%), the teaching of Comparative Example 2 therefore goes against that of Example 2 according to the invention, in terms of the change in properties mechanical, well preserved in the case of Example 2 for the weak ammonia contents but significantly deteriorated in the case of the example Comparative 2, including for low ammonia contents.

Claims (6)

1. Process for galvanizing steel sheet containing oxidizable alloying elements, particularly silicon, in which the sheet is annealed in a non-oxidizing annealing atmosphere and then the sheet is hot-dipped in a galvanizing bath, characterized in that the said annealing atmosphere contains between 0.005% and 1% ammonia.
2. Process according to Claim 1, characterized in that the said annealing atmosphere contains between 0.02% and 0.5% ammonia.
3. Process according to Claim 1 or 2, characterized in that the residence time of the said sheet in the said atmosphere is less than or equal to 3 minutes.
4. Sheet-galvanizing apparatus for implementing the process according to any one of the preceding claims, of the type comprising means for annealing the sheet in a controlled atmosphere which feed downstream into means for hot-dipping the annealed sheet in a galvanizing bath, and means for controlling the said atmosphere, characterized in that the atmosphere-control means are designed to introduce into the said atmosphere, and to maintain therein, ammonia at a concentration of between 0.005 and 1% and in that the said ammonia is introduced into the said atmosphere near the point where the sheet enters the annealing means.
5. Steel sheet containing oxidizable alloying elements, particularly silicon, which is galvanized by the process according to any one of Claims 1 to 3, of the type having, between the said steel and the galvanizing layer, an interlayer containing nitrogen in solid solution, characterized in that the thickness of the said interlayer corresponds to less than 1 gram per square metre of interface surface.
6. Steel sheet according to Claim 5, characterized in that the said oxidizable alloying elements comprise silicon with a concentration of greater than or equal to 0.2% in the said steel.