JP5815947B2 - How to produce a coating on a steel strip - Google Patents

Description

  The present invention relates to a strip, generally a steel strip having a corrosion resistant metal alloy coating.

  In particular, the present invention relates to a corrosion-resistant metal alloy coating containing aluminum-zinc-silicon-manganese as a main element in the alloy, and hereinafter referred to as an “Al—Zn—Si—Mg alloy”. This metal coating may contain other elements present as intentional alloying additions or as inevitable impurities. Thus, the term “Al—Zn—Si—Mg alloy” is understood to cover alloys containing such other elements as intentional alloying additions or as inevitable impurities. The metallized strip can be sold as a final product, or it can be sold as a painted final product with a paint coating applied on one or both sides.

  The present invention is particularly, but not limited to, a method for increasing the ductility of an Al-Zn-Si-Mg coating on a steel strip.

  The present invention is not limited to this, but in particular it may be coated with the above Al-Zn-Si-Mg alloy, optionally coated with paint, and then end-use products such as building materials (e.g. relates to steel strips that are cold-formed (eg by roll forming) into profiled walls and roofing sheets. The ductility of the coating is an important issue for such end-use products (painted and unpainted), especially in areas that are directly cold formed (eg tensile bending).

Typically, the Al—Zn—Si—Mg alloy of the present invention comprises aluminum elements, zinc elements, silicon elements and magnesium elements in the following weight percent ranges:
Aluminum: 40-60%
Zinc: 40-60%
Silicon: 0.3-3%
Magnesium: 0.3-10%
Containing.

  Typically, the corrosion resistant metal alloy coating of the present invention is produced on a steel strip by a hot dipping process.

  In conventional molten metal plating methods, the steel strip generally passes through one or more heat treatment furnaces and then enters a bath of molten metal alloy that is held in a coating pot. A heat treatment furnace adjacent to the coating pot has an outlet snout that extends downward toward a position proximate to the top surface of the vessel.

  This metal alloy is usually maintained in a molten state in the coating pot by using a heating inductor. The strip typically exits the heat treatment furnace through an exit end section in the form of an elongate furnace exit chute or snout immersed in a bath. Within the bath, the strip passes around one or more sink rolls, is taken up from the bath, and is coated with a metal alloy as it passes through the bath.

  After leaving the hot dip bath, the metal alloy coated strip is passed through a coating thickness control station, such as a gas knife or gas wiping station, where the coated surface is exposed to a jet of wiping gas to coat the coating thickness. To control.

  The metal alloy coated strip then passes through the cooling station and undergoes forced cooling.

  Then, if necessary, the cooled metal alloy coated strip is passed through the skin strip rolling section (also known as the temper rolling section) and the tension leveling section, if necessary. Condition may be adjusted. The conditioned strip is coiled at a coiling station.

  Depending on the end use application, the metallized strip can be coated on one or both sides of the strip, for example using polymer paint.

  A corrosion-resistant metal coating composition that is widely used in building materials in Australia and abroad, particularly for deformed walls and roofing sheets, is a 55% Al-Zn coating composition containing Si. Profiled sheets are usually manufactured by cold forming painted metal alloy coated strips. Typically, profiled sheets are manufactured by rolling a painted strip.

  The addition of Mg to this known 55% Al—Zn—Si coating composition has been proposed in the patent literature for many years (see, eg, US Pat. No. 6,635,359 in the name of Nippon Steel Corporation). However, Al-Zn-Si-Mg coatings on steel strips are not commercially available in Australia.

  When Mg is included in a 55% Al-Zn coating composition, Mg has been shown to provide beneficial effects on product performance, such as improved cut edge protection.

  The above discussion is not regarded as an accepted matter known in Australia and abroad.

FIG. 1 shows a sample with a 55% Al—Zn—1.5% Si-2% Mg coating that was held at various temperatures for 30 minutes and then cooled to 80 ° C. at a rate of 0.5 ° C./min. The critical bending strain of is shown. FIG. 2 shows the CSR of a sample with a heat treated 55% Al—Zn—1.5% Si-2% Mg coating as a function of hold temperature. FIG. 3 shows the aging behavior of a 150 g / m ² 55% Al—Zn—1.5% Si-2% Mg coating after batch annealing, paint baking cycle, and a combination of both heat treatments.

  The inventor has also demonstrated that the addition of Mg to a 55% Al—Zn coating composition has a significant adverse effect on the ductility of the coating. This is caused by the formation of a rough intermetallic phase in the coating microstructure and the hardening effect of Mg on the Al-rich dendrite and Zn-rich interdendritic regions in the coating microstructure.

  In particular, in the context of the hardening effect, Applicants have observed that an age hardening reaction occurred following solidification of the 55% Al-Zn-1.5% Si metal coating and was dissolved in the Al-rich phase in the coating. It was noticed that excess Zn precipitates as a metastable phase. This results in an increase in strength of the Al-rich phase and consequently increases the effectiveness of potential crack initiation sites. This age hardening reaction results in a significant increase in coating hardness within 2-4 weeks of solidification of the coating, and cold forming (eg rolls) of this metal alloy coated steel (including painted metal coated steel). If molding is not performed immediately after the coating has solidified, an increase in bend cracking can occur. In some cases, this can be a big problem.

  Applicants have discovered that this age hardening also occurs in Al-Zn-Si coatings containing Mg.

  The present invention is a coating of an Al-Zn-Si-Mg alloy on a steel strip that is applied by hot dip plating and then heat treated to improve the ductility of the coating.

  Applicants have discovered that the resulting coating can be cold formed with a low cracking level in tensile bending compared to a coating that is not heat treated. Applicants have also discovered that the benefits obtained during heat treatment are long lasting. In particular, improved ductility can be maintained for over 12 months.

  Accordingly, the present invention provides an Al—Zn—Si—Mg alloy coated steel strip produced by hot dipping a steel strip with an alloy and then heat treating the coated strip.

According to the present invention,
(A) passing a steel strip through a hot dipping bath containing Al, Zn, Si, and Mg and optionally another element to produce an alloy coating on the strip; and (b) heat treating the coated strip. There is also provided a method for producing a coating of corrosion resistant Al—Zn—Si—Mg alloy on a steel strip comprising the step of improving the ductility of the coating.

  Preferably, the method includes the step of heat treating the coated strip at a hold temperature of at least 150 ° C.

  The term “hold temperature” is understood herein to mean the maximum temperature at which the coated strip is heated and held during the course of the heat treatment cycle.

  More preferably, the method includes the step of heat treating the coated strip at a hold temperature of at least 200 ° C.

  Typically, the method includes heat treating the coated strip at a hold temperature of at least 225 ° C.

  Preferably, the method includes heat treating the coated strip at a hold temperature of less than 300 ° C.

  More preferably, the method includes heat treating the coated strip at a hold temperature of less than 275 ° C.

  Preferably, the method includes the step of holding the coated strip for 45 minutes or less at the hold temperature.

  More preferably, the method includes the step of holding the coated strip at hold temperature for 30 minutes or less.

  Preferably, the method includes slowly cooling the heat treated coated strip from a hold temperature to a temperature of 100 ° C. or lower.

  Applicants desire that the cooling rate of the heat-treated coated strip affects the duration of the softening effect obtained by heat treatment, i.e. improved ductility, and that the cooling rate is a "slow" cooling rate. I discovered that.

  More preferably, the method includes slowly cooling the heat treated coated strip from a hold temperature to a temperature of 80 ° C. or lower.

  Preferably, the cooling rate is 40 ° C./hour or less.

  More preferably, the cooling rate is 30 ° C./hour or less.

  The heat treatment step of this method may be performed on a batch basis or on a continuous basis.

Typically, the Al—Zn—Si—Mg alloy of the present invention comprises aluminum elements, zinc elements, silicon elements, and magnesium elements in the following weight percent ranges:
Aluminum: 40-60%
Zinc: 40-60%
Silicon: 0.3-3%
Magnesium: 0.3-10%
Containing.

  Preferably, the magnesium concentration is 8 wt. %.

  Preferably, the magnesium concentration is 3 wt. %.

  Preferably, the magnesium concentration is at least 0.5 wt. %.

  Preferably, the magnesium concentration is 1 wt. % To 3 wt. %.

  More preferably, the magnesium concentration is 1.5 wt. % To 2.5 wt. %.

  Preferably, the silicon concentration is 3.0 wt. %.

  Preferably, the silicon concentration is 1.6 wt. %.

  Preferably, the silicon concentration is 1.2 wt. %.

  Preferably, the silicon concentration is 0.6 wt. %.

  Preferably, the aluminum concentration is at least 45 wt. %.

  Typically, the aluminum concentration is at least 50 wt. %.

  Al-Zn-Si-Mg alloys do not contain intentional additions, i.e., additions above the concentration level considered to be chromium and / or manganese impurity levels.

  The Al—Zn—Si—Mg alloy may contain other elements as impurities or may be intentionally added.

  Preferably, the coating on the strip is 30 microns or less.

  According to the present invention, a metal-coated steel strip produced by the above method is also provided.

  Preferably, the metal coated steel strip is cold formed into end use products such as building materials (eg, profiled walls and roofing sheets).

According to the present invention,
(A) passing the steel strip through a hot dipping bath containing Al, Zn, Si, and Mg and optionally other elements and producing an alloy coating on the strip;
(B) heat treating the coated strip to improve the ductility of the coating;
(C) slowly cooling the heat-treated coated strip from a hold temperature to a temperature of 100 ° C. or lower; and (d) producing a paint coating on the cooled heat-treated coated strip. A method of forming a steel strip is also provided.

  Preferably, the Al—Zn—Si—Mg alloy and the heat treatment step are as defined above.

  According to the present invention, there is also provided a painted metallized steel strip produced by the above method.

  Preferably, the metal coated steel strip is cold formed into end use products such as building materials (eg, profiled walls and roofing sheets).

  The present invention is an example base made by the applicant.

In particular, the examples
(A) whether the improvement in ductility of 55% Al-Zn-1.5% Si-2% Mg coating is achieved by annealing heat treatment,
(B) Optimal holding temperature, and (c) to determine the aging behavior of the heat treatment coating (including the heat treatment coating that was subjected to the next paint baking cycle (PBC) heat treatment simulation).

Examples for steel strip samples coated with 55% Al-Zn-1.5% Si-2% Mg alloy with a coating density of 150 g / m ² (ie 75 g / m ^{2 on} each side of the strip sample) The sample was then heated to various hold temperature ranges, the sample was held at that temperature for a predetermined time of 30 minutes, and then heat treated by cooling the heat treated sample to ambient temperature.

  The example also included a paint baking cycle (PBC) heat treatment simulation for several samples. The PBC treatment involved heating the sample to a peak metal temperature of 230 ° C. at about 7 ° C./second, followed by a water quenching step.

FIG. 1 shows 55% Al—Zn—1.5% Si held at various temperatures for the predetermined time (30 minutes) and then cooled to 80 ° C. at a rate of 0.5 ° C./min. 2 shows the critical bend strain (CBS) of the sample with a −2% Mg (coating density 150 g / m ² ) coating, ie, the strain of the coating required to initiate cracking.

  FIG. 1 shows a coated sample in which CBS was heat treated at a hold temperature ranging from 5.3% of the as-received coated sample (ie, sample point at ambient temperature) to 225-250 ° C. It shows that it increased to a maximum of 8.3%. This corresponds to a 56% increase (significant improvement) in the ductility of the coating. This figure also shows that the CBS begins to increase at a hold temperature of 150 ° C.

  A semi-quantitative measurement of cracking severity was also used to evaluate the coating ductility of the sample.

  Crack Degree Evaluation (CSR) is a dimensionless tensile bend crack degree system commonly used as a measure of coating ductility in a 55% Al-Zn coating field. A 2T bend is produced and viewed under a stereo microscope at a magnification of 15 times. As such, bend cracking is compared to a standard set and assigned a number between 0 and 10 (0 indicates no cracking is visible and 10 indicates severe cracking). Therefore, a low CSR rating is preferred over a high rating.

FIG. 2 shows the CSR of a sample with a heat treated 55% Al—Zn—1.5% Si-2% Mg (150 g / m ² ) coating as a function of hold temperature. From the figure, it is clear that 225 ° C. is the optimum hold temperature in this test. It is also clear from the figure that the CSR began to improve at a hold temperature of 150 ° C.

  FIG. 3 shows (a) 55% Al-Zn-1.5% Si-2 that has been aged for 3 months or less and heat-treated at the above-determined optimum hold temperature of 225 [deg.] C. for the predetermined time of 30 minutes. A sample with a coating of% Mg alloy, (b) a sample according to item (a) subjected to a paint baking cycle, (c) as obtained 55% Al-Zn-1.5% Si-2% Mg alloy And (d) shows the aging behavior of a sample with a coating of 55% Al-Zn-1.5% Si-2% Mg alloy that has undergone only a paint baking cycle treatment.

  For the heat treated coating, no significant reversion to as-obtained ductility was observed in 3 months when the annealed coating was subjected to the next paint baking cycle heat treatment. Extrapolation of these results leads to the conclusion that a heat treatment over a predetermined time of 30 minutes at a hold temperature of 225 ° C. is effective for a period longer than 12 months.

  The above examples show that the heat treatment of the 55% Al-Zn-1.5% Si-2% Mg alloy coating on the strip improved the ductility of the coating.

  Many modifications may be made to the invention without departing from the spirit and scope of the invention.

  As an example, the example was performed on a 55% Al-Zn-1.5% Si-2% Mg coating, but the present invention is also generally applicable to Al-Zn-Si-Mg coatings. is there.

Claims (11)

(A) passing the steel strip through a hot dipping bath containing Al, Zn, Si, and Mg and optionally other elements to produce an Al-Zn-Si-Mg alloy coating on the strip;
(B) cooling the coated strip; and
(C) heat treating the coated strip to improve the ductility of the coating;
Including
The Al—Zn—Si—Mg alloy has the following weight% range of aluminum element, zinc element, silicon element, and magnesium element:
Aluminum: 40-60%
Zinc: 40-60%
Silicon: 0.3-3%
Magnesium: 1 to 2.5%
Contain,
The heat treatment step (c) heats the coated strip from a temperature below hold temperature to a hold temperature of at least 200 ° C .; holds the coated strip at the hold temperature; and removes the coated strip from the hold temperature to 100 ° C. A method of producing a coating of corrosion resistant Al-Zn-Si-Mg alloy on a steel strip , comprising slow cooling at a cooling rate of 40 ° C / hour or less to the following temperature .   The method of claim 1, comprising heat treating the coated strip at a hold temperature of less than 300 degrees Celsius.   The method of claim 1 comprising heat treating the coated strip at a hold temperature of less than 275 ° C. Comprising the step of holding 30 minutes or less the coated strip at-hold temperature, method of claim 1.   The method according to claim 1, comprising gradually cooling the heat-treated coated strip from a hold temperature to a temperature of 100 ° C. or less at a cooling rate of 30 ° C./hour or less.   The magnesium concentration is 1.5 wt. % To 2.5 wt. The method of claim 1, wherein the method is%.   The silicon concentration is 3.0 wt. The method of claim 1, wherein the method is less than%.   The aluminum concentration is at least 45 wt. The method of claim 1, wherein the method is%.   The method of claim 1, wherein the Al—Zn—Si—Mg alloy does not contain intentional addition of chromium and / or manganese.   The method of claim 1, wherein the Al—Zn—Si—Mg alloy comprises another element as an impurity or as an intentional addition. (A) passing the steel strip through a hot dipping bath containing Al, Zn, Si, and Mg to produce an Al-Zn-Si-Mg alloy coating on the strip;
(B) by heat-treating the coated strip process for improving the ductility of the coating, in parallel Beauty
(C) generating a coating of paint on the cooled heat treated coated strip;
The Al—Zn—Si—Mg alloy has the following weight% range of aluminum element, zinc element, silicon element, and magnesium element:
Aluminum: 40-60%
Zinc: 40-60%
Silicon: 0.3-3%
Magnesium: 1 to 2.5%
Contain,
The heat treatment step (b) heats the coated strip from a temperature below the hold temperature to a hold temperature of at least 200 ° C .; holds the coated strip at the hold temperature; and removes the coated strip from the hold temperature to 100 ° C. A method of producing a painted metal-coated steel strip comprising slow cooling to a temperature of:

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