METHOD OF PRODUCING METAL COATED STEEL STRIP

Abstract

A method of forming a coating of a metal alloy on a steel strip to form a metal alloy coated steel strip is disclosed. The method includes a hot dip coating step of dipping steel strip into a bath of molten metal alloy and forming a metal alloy coating on exposed surfaces of the steel strip. A native oxide layer as defined herein forming on the metal alloy coating of the metal alloy coated strip emerging from the metal coating bath. The method includes controlling the method downstream of the hot dip coating step and/or selecting the metal coating composition to maintain the native oxide layer at least substantially intact on the metal alloy coating during the downstream steps.

Claims

1. A method of forming a coating of a metal alloy on a steel strip to form a metal alloy coated steel strip, the method including a hot dip coating step of dipping steel strip into a bath of molten AlZnSiMg alloy and forming a metal alloy coating of the AlZnSiMg alloy on exposed surfaces of the steel strip, with exposed surfaces of the metal alloy coating oxidising and forming a native oxide layer forming on the metal alloy coating of the metal alloy coated strip after the metal alloy coated strip emerges from the metal coating bath, and a step of cooling the metal alloy coated strip with the native oxide layer with cooling water, and the method including controlling one or more than one of pH of cooling water to be in a range of pH 5-9, temperature of cooling water to be in a range of 25-80 C., and chemical composition of cooling water in the strip cooling step to maintain the native oxide layer intact on the metal alloy coating during the downstream steps.

2. The method defined in claim 1 including a step of treating the metal alloy coated strip with a passivation solution, and the method including controlling the method between the hot dip coating step and the passivation step to maintain the native oxide layer at least substantially intact on the metal alloy coating.

3. The method defined in claim 1 wherein the strip cooling step includes controlling the pH of cooling water to be less than 8.

4. The method defined in claim 1 wherein the strip cooling step includes controlling the pH of cooling water to be less than 7.

5. The method defined in claim 1 wherein the strip cooling step includes controlling the pH of cooling water to be greater than 6.

6. The method defined in claim 1 wherein the strip cooling step includes controlling the temperature of cooling water to be less than 70 C.

7. The method defined in claim 1 wherein the strip cooling step includes controlling cooling water temperature to be less than 60 C.

8. The method defined in claim 1 wherein the strip cooling step includes controlling cooling water temperature to be less than 50 C.

9. The method defined in claim 1 wherein the strip cooling step includes controlling cooling water temperature to be greater than 40 C.

10. The method defined in claim 1 wherein the strip cooling step includes controlling the pH by adding acid to the cooling water.

11. The method defined in claim 1 wherein the strip cooling step includes cooling the coated strip with cooling water to a temperature range of 30-50 C.

12. The method defined in claim 1 wherein the AlZnSiMg alloy includes the following ranges in % by weight: Al: 2 to 19% Si: 0.01 to 2% Mg: 1 to 10% Balance Zn and unavoidable impurities

13. A metal alloy coated steel strip produced by the method defined in claim 1.

14. The method defined claim 1 wherein the native oxide layer includes Mg oxide, Al oxide, and a small amount of oxides of other elements of the AlZnSiMg alloy coating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0081] The present invention is described further by way of example with reference to the accompanying drawings of which:

[0082] FIG. 1 is a schematic drawing of a continuous metal coating line for forming an AlZnSiMg alloy coating on steel strip in accordance with the method of the present invention; and

[0083] FIGS. 2(a) to 2(d) are graphs illustrating the results of x-ray photoelectron spectroscopy analysis of the metal alloy coating surfaces of metal alloy coated steel strip samples.

DESCRIPTION OF EMBODIMENTS

[0084] With reference to FIG. 1, in use, coils of cold rolled low carbon steel strip are uncoiled at an uncoiling station 1 and successive uncoiled lengths of strip are welded end to end by a welder 2 and form a continuous length of strip.

[0085] The strip is then passed successively through an accumulator 3, a strip cleaning section 4 and a furnace assembly 5. The furnace assembly 5 includes a preheater, a preheat reducing furnace, and a reducing furnace.

[0086] The strip is heat treated in the furnace assembly 5 by careful control of process variables including: (i) the temperature profile in the furnaces, (ii) the reducing gas concentration in the furnaces, (iii) the gas flow rate through the furnaces, and (iv) strip residence time in the furnaces (i.e. line speed).

[0087] The process variables in the furnace assembly 5 are controlled so that there is removal of iron oxide residues from the surface of the strip and removal of residual oils and iron fines from the surface of the strip.

[0088] The heat treated strip is then passed via an outlet snout downwardly into and through a molten bath containing an AlZnSiMg alloy held in a coating pot 6 and is coated with the AlZnSiMg alloy. Typically, the AlZnSiMg alloy in the coating pot 6 comprises in % by weight: Zn: 30 to 60%, Si: 0.3 to 3%, Mg: 0.3 to 10%, and balance Al and unavoidable impurities. It is noted that the AlZnSiMg alloy may contain other ranges of these elements. It is also noted that the AlZnSiMg alloy may contain other elements as deliberate additions or as impurities. For example, the coating pot 6 may also contain Ca for dross control in the molten bath. The

[0089] AlZnSiMg alloy is maintained molten in the coating pot at a selected temperature by use of heating inductors (not shown). Within the bath the strip passes around a sink roll and is taken upwardly out of the bath. The line speed is selected to provide a selected immersion time of strip in the coating bath. Both surfaces of the strip are coated with the AlZnSiMg alloy as it passes through the bath.

[0090] After leaving the coating bath 6 the strip passes vertically through a gas wiping station (not shown) at which its coated surfaces are subjected to jets of wiping gas to control the thickness of the coating.

[0091] The exposed surfaces of the AlZnSiMg alloy coating oxidise as the coated strip moves through the gas wiping station and a native oxide layer forms on the exposed surfaces of the coating. As indicated above, the native oxide is the first oxide to form on the surface of the metal alloy coating, with its chemical make-up being intrinsically dependent on the composition of the metal alloy coating, including Mg oxide, Al oxide, and a small amount of oxides of other elements of the AlZnSiMg alloy coating.

[0092] The coated strip is then passed through a cooling section 7 and is subjected to forced cooling by means of a water quench step. The forced cooling may include a forced air cooling step (not shown) before the water quench step. The water quench step is, by way of example, a closed loop in which water sprayed onto coated strip is collected and then cooled for re-use to cool coated strip. The cooling section 7 includes a coated strip cooling chamber 7a, a spray system 7b that sprays water onto the surface of the coated strip as it moves through the cooling chamber 7a, a water quench tank 7c for storing water that is collected from the cooling chamber 7b, and a heat exchanger 7d for cooling water from the water quench tank 7c before transferring the water to the spray system 7b. In accordance with one embodiment of the present invention (a) the pH of the cooling water supplied to the spray system 7b is controlled to be in a range of pH 5-9, typically in a range of 6-8, and (b) the temperature of the cooling water supplied to the spray system is controlled to be in range of 30-50 C. The applicant has found that both control steps (a) and (b) minimise removal of the native oxide layer on the AlZnSiMg alloy coating on the coated strip.

[0093] The pH and temperature control may be achieved, by way of example, by using a pH probe and a temperature sensor in an overflow tank of the water quench tank 7c and supplying data from the probe/sensor to a PLC and calculating required acid additions to maintain the pH at predetermined set points for pH and the water temperature, with any acid additions and temperature adjustments being made so that the water in the water quench tank 7c is controlled to the set points for pH and temperature. This is not the only possible option for achieving pH and temperature control.

[0094] The pH, temperature, and chemical control may also be achieved, by way of example, by using a once through water cooling system where the quench water is not recirculated and the input water has pH and temperature properties as described above.

[0095] The cooled, coated strip is then passed through a rolling section 8 that conditions the surface of the coated strip. This section may include one or more of skin pass and tension leveling operations.

[0096] The conditioned strip is then passed through a passivation section 10 and coated with a passivation solution to provide the strip with a degree of resistance to wet storage and early dulling.

[0097] The coated strip is thereafter coiled at a coiling station 11.

[0098] As discussed above, the applicant has conducted extensive research and development work in relation to AlZnSiMg alloy coatings on steel strip.

[0099] As discussed above, the applicant has found in the research and development work that the native oxide layer that forms as the metal alloy coated strip moves through the gas wiping station is important in terms of minimising corrosion of the underlying metal alloy coating as the coated strip is processed downstream of the bath.

[0100] In particular, the applicant has found that it is important to maintain the native oxide layer at least substantially intact in order to maintain a metal alloy coating that has a suitable surface quality for passivation with a passivation solution.

[0101] More particularly, the applicant has found that total removal of the native oxide layer can lead to corrosion of the metal alloy coating before a downstream passivation step, with the corrosion including any one of the following surface defects of crevices, pits, black spots, voids, channels, and speckles.

[0102] The research and development work relevant to the native oxide issue included x-ray photoelectron spectroscopy (XPS) depth profiling analysis to assess the conditions of the surfaces of a series of metal alloy coatings.

[0103] The graphs of FIGS. 2(a) to 2(d) are the results of XPS analysis of various materials, representing a set of possible metal coating surface conditions.

[0104] The graph of FIG. 2(a) is an XPS depth profile of the surface of an AlZnSiMg alloy coated steel panel produced on the Hot Dip Process Simulator (HDPS) at the research facilities of the applicant. The HDPS is a state-of-the-art unit purpose-built to the specifications of the applicant by Iwatani International Corp (Europe) GmbH. The HDPS unit comprises a molten metal pot furnace, an infrared heating furnace, gas wiping nozzles, de-drossing mechanisms, gas mixing and dewpoint management functions, and computerized automatic control systems. The HDPS unit is capable of simulating a typical hot dip cycle on a conventional metal coating line. The horizontal axis of FIG. 2(a) marked Etchine time refers to the etching time in the analysis and indicates the depth of the coating from the surface of the coating. Each of the lines in the Figure indicates a different atomic component in the coating. FIG. 2(a) indicates that a thin oxide layer of approximately 9 nm thickness was detected on the AlZnSiMg alloy coated steel panel. The oxide layer consisted primarily of aluminium and magnesium oxides. The HDPS has gas cooling but no water quench, and thus the oxide layer is representative of oxides forming on the surface of the molten coating at elevated temperatures. One characteristic of the oxide layer is the presence of a small portion of calcium oxide (2 at % Ca) resulted from a low level of Ca addition in the molten coating bath for dross control. The oxide is described as a native oxide by the applicant, as it is the first oxide to form on the surface of the metal coating and its chemical make-up is intrinsically dependant on the composition of the metal coating.

[0105] The graph of FIG. 2(b) is an XPS depth profile of the surface of an AlZnSiMg alloy coated steel panel produced on one of the applicant's metal coating lines where there is a water quench step in the production loop and the pH and temperature of the quench water is controlled. The pH was controlled with nitric acid addition to be pH 5-8 and the temperature was controlled to be 35-55 C. FIG. 2(b) shows that a small portion only of the native oxide was removed due to the water quench. However, the presence of Ca indicates that the native oxide was not totally removed. Moreover, there was no corrosion of the underlying AlZnSiMg alloy coating. Significantly, FIG. 2(b) also indicates that under the particular water quench conditions, it was possible to maintain a partial native oxide layer.

[0106] The graph of FIG. 2(c) is an XPS depth profile of the surface of an AlZnSiMg alloy coated steel panel produced on another metal coating line of the applicant, where there is also a water quench step in the production loop. The pH was controlled to be pH 5-8 and the temperature was controlled to be 35-55 C. FIG. 2(c) shows that the conditions of the water quench were such that there was partial removal of the native oxide layer and Ca was detected at levels lower than that in FIG. 2(a) or 2(b). Some new oxide formed on the surface of the AlZnSiMg alloy coating, presumably during or following the quench process. Nevertheless, there was no corrosion attack on the underlying structure of the AlZnSiMg alloy coating.

[0107] The graph of FIG. 2(d) is an XPS depth profile of the surface of an AlZnSiMg alloy coated steel panel produced on yet another metal coating line of the applicant, where there is also a water quench step in the production loop. The pH was controlled to be greater than pH 9 and the temperature was controlled to be greater than 50 C. FIG. 2(d) shows that the water quench conditions resulted in complete removal of the native oxide layer and obvious corrosion attack on the underlying structure of the AlZnSiMg alloy coating. The new oxide layer that formed on the surface of the metal coating was characterized by a substantial presence of zinc oxide (corrosion product) in the layer and a much greater layer thickness. This resulted in an unsatisfactory passivation outcome.

[0108] The research and development work described with reference to FIGS. 2(a) to 2(d) indicates that water quench conditions that maintain the integrity of the underlying structure of a metal alloy coating allow the metal alloy coating to achieve a satisfactory passivation outcome, whereas water quench conditions that cause any corrosion attack to the underlying structure of the metal coating impair the ability of the metal alloy coating to be properly passivated.

[0109] Many modifications may be made to the present invention described above without departing from the spirit and scope of the invention.

[0110] By way of example, whilst the embodiment of the metal coating line shown in FIG. 1 includes a coated strip cooling section 7 that includes water sprays, the present invention is not so limited and extends to any suitable water cooling system, such as dunk tanks.

[0111] By way of further example, whilst the description of the invention in relation to the Figures focuses on control of a water cooling step in a metal coating line, the invention is not so limited and the control may be otherwise achieved and may, for example, include selection of metal alloy coating compositions that form more resistant native oxide layers.