WO2008025438A1

WO2008025438A1 - Silicon-bearing zinc alloy for zinc-quench galvanisation

Info

Publication number: WO2008025438A1
Application number: PCT/EP2007/006961
Authority: WO
Inventors: Bruno Gay; Eric Robert; Tjakko Zijlema
Original assignee: Umicore
Priority date: 2006-09-01
Filing date: 2007-08-07
Publication date: 2008-03-06

Abstract

The present invention relates to the production of galvanised steel by hot-dip galvanisation, in particular using the so-called zinc-quench technology, which is a variation on the well-known GI process. When the entry temperature of the steel is the bath is particularly high, as is the case in the zinc-quench technology, there is a risk for the formation of undesirable FexZny intermetallics. This problem was solved by the addition of Si to the galvanisation bath. A process for the manufacture of Si-bearing zinc-based alloys that dissolve well in zinc is also proposed. This process is based on the reaction of SiCl4 with molten zinc.

Description

SILICON-BEARING ZINC ALLOY FOR ZIN-QUENCH GALVANISING OF STEEL

The present invention relates to the production of galvanised steel by hot-dip galvanisation, in particular using the so-called zinc-quench technology, which is a variation on the well-known GI process.

In a typical GI process, the steel strip, which is annealed at a temperature between 720 ⁰C and 900 ⁰C, is first cooled between 430 ⁰C to 500 °C and then dipped into a zinc bath, which is at a temperature of 430 ⁰C to 490 ⁰C. An alloyed zinc bath with an Al concentration between 0.14 wt . % and 0.5 wt.% is normally used.

In zinc-quench galvanization, the steel entry temperature is however substantially higher than typical.

Quenching can be applied to increase the productivity of a galvanisation line: the annealed steel strip is hereby only partially cooled, to a moderate temperature of up to 640 ⁰C, before entering the galvanization bath. The shortened cooling cycle allows for a higher line throughput.

Quenching is also applied to enhance the mechanical properties of the steel. The cooling step after annealing is then further shortened or even omitted, resulting in a high steel temperature of more than 640 ⁰C upon dipping. Here, the goal is to impose a thermal shock to the steel, to transform its structure. This technique is particularly interesting for dual-phase steels.

In order to achieve a good formability of the zinc coating, the intermetallic layer between the steel and the coating should be as thin as possible. The high dipping temperature in zinc-quench tends to produce thick intermetallic layers. To counter this, forced convection in the zinc bath has been applied. As illustrated in US 4,752,508, a flow of molten zinc is cooled below the operating temperature of the zinc bath, and directed towards the steel strip. Industrial practice has shown that the entry temperature of the steel should nevertheless be kept below approximately 660 ⁰C.

The present invention prevents the above-mentioned problem by proposing the addition of Si to the zinc bath. It has indeed been found that the addition of Si allows for either a high steel entry temperature if forced convection is applied, or relieves from applying forced convection if a moderate steel entry temperature is selected.

It should be mentioned that the addition of Si to a zinc bath is known from EP-0664838. This document teaches that Si helps in avoiding the formation of floating dross when a hot dip line is switched from the galvannealing (GA) to the galvanization (GI) mode of operation. A recurrent problem in applying this teaching has been that the incorporation of a sufficient quantity of Si in the zinc bath proved to be difficult or even impossible.

Maintaining the proper alloying levels in a zinc bath is not an obvious task, as the consumption of these elements, and particularly that of Al and Si, is proportionally much higher that that of zinc. Consequently, feed ingots with a Si content substantially higher than the Si content of the bath have to be used. The activity of monitoring and correcting the alloying level of the bath using specially prepared feed ingots and pure zinc ingots, is referred to as bath management.

Feed ingots are normally prepared by diluting master alloys, which are preferably zinc-based and highly alloyed, with pure zinc. Adding master alloy ingots directly to the galvanisation bath is not recommended as this may generate excessive concentration inhomogeneities . Providing Si-rich zinc-based alloys has however been problematic. The rate of dissolution of pure Si in liquid zinc is indeed too low to be practicable below 500 ⁰C. Even at higher temperature, and even up to the boiling point of zinc, the dissolution kinetics of Si remain prohibitively slow.

For this reason, one has resorted to commercially available Si- rich Al-based alloys, typically containing about 13 wt.% Si, which is an eutectic composition. These alloys, which have a Si to Al ratio of only 0.15 to 1, do dissolve in a zinc bath, but automatically carry high amounts of Al, leading to an excessive Al concentration in the galvanisation bath. Other Al-based alloys with a higher Si content are available, but they have proven very difficult to dissolve in zinc, and cannot be used in practice.

It has now been found that suitable Si-rich zinc-based alloys can efficiently be prepared by relying on the reduction of SiCl₄ by zinc itself or by any other element, such as Al, prone to reduce SiCl₄ and to generate chlorides.

Hence, a first object of the invention concerns a process for the manufacture of a Si-bearing zinc-based alloy, comprising the steps of:

- selecting a starting metal comprising at least 80 wt.% of zinc; - heating the starting metal to a temperature at or above its melting point;

- injecting SiCl₄ into the melt so as to obtain an alloy with a Si concentration between 0.5 and 3 wt.%, and metal chlorides;

- separating the metal chlorides; - cooling the alloy until its solidification, which is typically complete .

If an Al and Si bearing zinc-based alloy is desired, then an excess of Al has to be present in the starting metal, as the SiCl₄ will preferentially react with Al to form the volatile AlCl₃. In this case, a starting alloy comprising up to 20 wt.% Al, with rest zinc and unavoidable impurities, would be appropriate. If no Al is desired in the alloy, then either a stoichiometric or substoichiometric (vs. SiCl₄) amount of Al should be selected. Obviously, one could then also opt for pure zinc, such as SHG

(Super High Grade) zinc as a starting metal. The SiCl₄ will then react with zinc and form ZnCl₂.

It is advisable to maximise the Si concentration in the zinc-based alloy, especially if it going to be used as a master alloy for the preparation of feed ingots. Less than 0.5 wt.% is therefore not recommended. However, a concentration of more than 3 wt.%, that is much in excess of the 2.5 wt.% solubility limit in molten zinc, is not recommended because excessively large Si crystals may develop in the melt.

The injection and separation steps can be performed simultaneously, by selecting a melt temperature above the boiling point of the formed chlorides. If the starting material is essentially pure zinc, then ZnCl₂ is formed, a species that evaporates at 732 ⁰C. If the starting material contains significant amounts of Al, then the very volatile AlCl₃ is formed.

The preferred melt temperature is between 750 and 850 ^CC. Under 750 ⁰C the solubility of Si decreases markedly, while above 850 ⁰C the zinc losses by evaporation may become prohibitive. A temperature lower than 732 °C can nevertheless be used, but any ZnCl₂ will then have to be separated by skimming.

The injection of SiCl₄ into a molten zinc melt is further described in application PCT/EP2006/002937, which 's content is incorporated here by reference in its entirety.

The cooling of the Si-bearing alloy is preferably performed rapidly, at more than 10 °C/min. Granulating the alloy in water is even more preferred. Quenching of the alloy indeed ensures that small-sized Si crystals, with a d50 (mean particle diameter) of 10 um or less, are formed upon solidification: such finely divided crystals are advantageous, as they dissolve rapidly when added to molten zinc.

Another object of the invention therefore concerns a zinc-based alloy containing between 0.5 and 3 wt.% of Si, the Si being present as small crystals, preferably with a d50 of 10 um or less. It should be mentioned that EP253776 discloses a Zn- 55% Al - 1.6 % Si alloy for galvanisation, but for the preparation of this alloy it is said that "known" alloying methods were used, and hence from the above it is evident that no small crystals with a d50 of 10 um or less were disclosed.

A further object is a zinc-based alloy consisting of zinc, up to 20 wt.% of Al, and unavoidable impurities, with a Si to Al weight ratio between 0.15 and 5. These ratios indeed allow for the direct preparation of feed ingots adapted to the relative consumption of Si and Al during galvanisation. Again, a major part of the Si is preferably present as small crystals with a d50 of 10 um or less.

By "unavoidable impurities", it is indicated that sufficiently pure zinc should be used, such as SGH (Super High Grade) zinc. The alloying elements Si and Al should also be pure, e.g. with a purity of 99 wt.% or better.

The alloys according to the invention can be added directly to the galvanizing bath or, preferably, can be used indirectly, as master alloys to prepare feed ingots. In the latter case, the alloys are added to pure zinc or to a zinc-based Al alloy in an alloying furnace. Small-sized Si crystals can again be obtained by cooling the feed ingots at a rate of at least 10 °C/min. Hence, a further object of the invention concerns the direct or indirect use of alloys as defined above to manage a galvanization bath comprising 0.01 to 0.05 wt.% of Si and 0.01 to 0.5 wt.% of Al, both as solutes, i.e. in a dissolved form.

The invention also concerns a process for the continuous galvanization of steel, comprising the step of quenching the steel from a temperature of more than 500 °C, preferably more than 530 ⁰C, by dipping it in a galvanization bath prepared as described above, and heated at a temperature of less than 490 ⁰C.

The invention further concerns the process for the continuous galvanization of steel, comprising the step of quenching the steel from a temperature of more than 500 ⁰C, preferably more than 530 ⁰C, by dipping it in a galvanization bath comprising 0.01 to 0.05 wt.% of Si and 0.01 to 0.5 wt.% of Al, which is heated at a temperature of less than 490 °C.

If the galvanization bath is equipped with forced convection, the entrance temperature of the steel can be increased to at least 580 ⁰C, even to at least 620 ⁰C, or to at least 660 ⁰C.

Preferably, the steel used in the galvanization process is a dual- phase steel.

When a steel sheet is dipped in a galvanisation bath, a reaction layer is formed. If the temperature is appropriate, this layer is very thin and shows good ductility. This layer is called 'inhibition layer', and contains Fe, Al and zinc. If the Al- content in the bath is between 0.14 and 1 wt.%, then the layer is composed of small Fe₂Al₅Zn_x crystals.

However, if the temperature of the steel sheet and/or of the zinc bath is too high, or if the Al-content is lower than 0.14 wt.%, large intermetallic Fe_xZn_y intermetallics appear. These are undesirable because of their very poor ductility; they further impair the visual aspect to the corresponding coating.

There is a real risk for the formation of Fe_xZn_x intermetallics in the following 3 industrial situations.

In a continuous galvanizing line, the steel sheet is annealed at about 800 ⁰C. In order to produce a galvanized steel sheet with controlled properties, the steel sheet is cooled before entering the zinc bath. The temperature of the steel sheet should not exceed the bath temperature, which is about 460 ⁰C, by more than 30 °C. Although the cooling systems adequately lower the temperature of the steel sheet most of the time, temperature spikes may occur intermittently, e.g. during process transitions.

A similar problem may occur when galvanizing thick hot rolled steel sheets. Due to the thermal inertia of thick sheets, the galvanisation layer remains liquid for an appreciable amount of time after exiting the bath, and chemical reactions continue to develop. The Al may hereby get depleted in the boundary layer, possibly dropping to less than 0.14 wt . % .

Finally, Fe_xZn_y may also appear when selecting a high bath entry temperature on purpose, according to the zinc-quench technology mentioned above.

It has been found that the addition of Si to the galvanizing bath blocks the formation of Fe_xZn_x at the interface between the steel substrate and the zinc coating, up to a steel sheet entry temperature of 750 °C. Instead of Fe_xZn_y, an inhibition layer is formed containing about 30 at.% of Si, 20 at . % of Al and 50 at.% Fe.

The invention will now be illustrated in the following examples. Example 1: Preparation of a master alloy

1000 kg zinc is melted in a reactor, and heated to 820 °C. 130 kg of SiCl₄, which is liquid at room temperature, is injected in the zinc through a submerged tube, at a rate of about 50 kg/h. The injection is performed substantially at the bottom of the vessel. The SiCl₄ is heated in the tube by the surrounding molten zinc, and is actually injected as a gas. The end of the injection tube is equipped with a porous plug or fritted glass to ensure an efficient dispersion of the SiCl₄. A conversion yield of 95 to 100% is achieved. During the injection of the SiCl₄, ZnCl₂ is formed and evaporated. A zinc based master alloy containing 2 wt.% of Si is obtained. It is then granulated in water.

Example 2 : Preparation of feed ingots

Feed ingots with a certain range of Al and Si contents are needed for the proper management of a galvanisation bath. Such ingots are prepared by melting SHG zinc in an alloying furnace, to which Al is added in the form of 99,7% Al platelets, and Si in the form of the granulated master alloy according to Example 1. A typical set of 1000 kg feed ingots is prepared according to Table 2. Ingots with a very low Si content, or even with only Al, are used to correct incidental excursion of the Si content of the galvanisation bath. Conversely, ingots with Si only can be useful.

Table 2 : Feed ingots

Example 3: Zinc-quench galvanization

In this experiment, the effect of the addition of Si to a classical GI bath containing 0.3 wt.% Al on the Fe-Zn reactivity is illustrated for a Si addition of 500 ppm. This level is slightly above the solubility limit of 350 to 400 ppm. The Si to Al weight ratio is 0.166. The bath temperature is varied between 460 and 490 °C, and the steel sheet entry temperature from 500 to 750 °C. The Fe content in the boundary layer of the galvanized steel sheet, which is an indicator for the formation of undesired Fe_xZn_y intermetallics, is measured. The results are given in Table 2.

Table 2: Fe in the boundary layer of galvanized steel sheet

Table 2 clearly shows the effect of the Si addition: even in extreme working conditions, with a steel strip at 750 °C and a bath at 490 ⁰C, Fe in the boundary layer is sufficiently suppressed. The inhibition layer composition has been measured by Auger analysis and the approximate composition is as follow: 30 at.% of Si, 20 at.% of Al, 50 at . % Fe, and less than 2 at . % Zn. The very low zinc content of the inhibition layer corroborates the absence of any significant amount of Fe_xZn_y in the boundary layer.

Claims

1. Process for the manufacture of a Si-bearmg zinc-based alloy, comprising the steps of: - selecting a starting metal comprising at least 80 wt.% of zinc;

- heating the starting metal to a temperature above its melting point;

- injecting SiCl₄ into the melt so as to obtain an alloy with a Si concentration between 0.5 and 3 wt.%, and metal chlorides; - separating the metal chlorides;

- cooling the alloy until solidification.

2. Process according to claim 1, whereby the injection and separation steps are performed simultaneously, by selecting a melt temperature above the boiling point of the metal chlorides, preferably between 750 °C and 850 ⁰C.

3. Process according to claims 1 or 2, whereby the solidification step is performed at a cooling rate of more than 10 °C/mm.

4. Process according to claim 3, whereby the cooling step is performed by granulation of the liquid alloy in water.

5. A Si-bearing zinc-based alloy containing between 0.5 and 3 wt.% of Si, the Si being present as small-sized crystals with a d50 of 10 μm or less.

6. A Si-bearing zinc-based alloy according to claim 5, consisting of zinc, up to 20 wt.% of Al, and unavoidable impurities, with a Si to Al weight ratio between 0.15 and 5.

7. Use of a Si and Al-bearing zinc-based alloy according to any one of claims 5 or 6, to manage a zinc galvanization bath comprising 0.01 to 0.05 wt.% of Si and 0.01 to 0.5 wt.% of Al, both as solutes.

8. Process for the continuous galvanization of steel, comprising the step of quenching the steel from a temperature of more than 500 °C, preferably more than 530 °C, by dipping said steel in the galvanization bath prepared according to claim 7, which is heated at a temperature of less than 490 ⁰C.

9. Process for the continuous galvanization of steel, comprising the step of quenching the steel from a temperature of more than 500 ⁰C, preferably more than 530 °C, by dipping said steel in a galvanization bath comprising 0.01 to 0.05 wt.% of Si and 0.01 to 0.5 wt.% of Al, which is heated at a temperature of less than 490 ⁰C.

10. Process for the continuous galvanization of steel, according to claim 8 or 9, wherein the steel is quenched from a temperature of more than 580 ⁰C, preferably more than 620 ⁰C, and most preferably more than 660 ⁰C.

11. Process according to any one of claims 8 to 10, wherein the steel is a dual-phase steel.