RU2727391C1 - Method of producing corrosion-resistant painted rolled steel with zinc-aluminum-magnesium coating - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates to metallurgy, namely, to continuous method of steel strip production. Method involves preparation of steel strip before coating, application of Zn-Al-Mg coating by hot immersion, degreasing and conversion of surface of Zn-Al-Mg coating, application of soil on one or two sides of steel strip, application of finishing enamel on one or two sides of steel strip. Concentrations of chemical elements in hot coating bath are, wt%: Al 1.0–1.4, Mg 1.0–1.4, Zn is the rest. Water solution of alkaline salt with pH=10–12 is used for degreasing and removal of oxide film from surface of Zn-Al-Mg coating. For application of conversion layer chromium-free compositions based on dihydrogen hexafluorotitanate or combination of hexafluorotitanate dihydrogen with hexafluorozirconium acid are used. Quality of conversion layer is evaluated by amount of precipitated titanium, which should be 3–12 mg/m. Temperature difference between curing of soil and finishing enamel is 0–5 °C, and the time of hardening of soil and finishing enamel is 0–10 s.EFFECT: invention can be used at metallurgical enterprises when producing a steel strip with a coating having high corrosion resistance.5 cl, 2 dwg, 1 tbl, 1 ex

Description

The invention relates to the metallurgical industry, in particular to a continuous method for the production of steel strip, and can be used at metallurgical enterprises in the production of coated steel strip with increased corrosion resistance, by combining the methods of hot immersion in a bath with a melt from a metal composition and subsequent application of a polymer roller coating.

Steel rolled products with anti-corrosion coating are used in many fields, such as construction, automotive, equipment cases, building exterior panels, etc. Metal zinc coatings or multilayer coatings, in which zinc, primer and enamel are successively applied to the steel base, are most widely used to protect steel from corrosion. The main reason for using zinc as a base for a protective metal coating is its high manufacturability in combination with a low electrode potential, which provides anti-corrosion protection of iron even in uncovered areas due to the consumption of zinc in the course of corrosion processes.

There are also coatings where zinc is combined with other metals to increase the corrosion resistance of the coating itself, for example, with magnesium, aluminum, nickel, etc. A decrease in the proportion of zinc in the coating can negatively affect the cathodic protection of steel in uncoated areas (edges, welded joints). On the other hand, the addition of elements such as magnesium and aluminum improves the corrosion resistance of the metal coating itself by forming an oxide layer involving the added elements. This lowers the electrical conductivity of the surface and, as a consequence, improves the corrosion resistance of the steel after the coating has been applied to the strip surface.

When forming these coatings on the surface of the strip, they strive to obtain not only high corrosion resistance when exposed to various media (moisture condensation, salt fog, atmospheric exposure) (as indicated in EN 13523-7, EN 13523-8 and 13523-26), but also acceptable the level of physical and mechanical properties. The main physical and mechanical properties, indicated, for example, in the standards GOST R 52246-2016, EN 10169, and NFP34301, are the retention of adhesion (no delamination of the coating) when bent to a certain angle, measured as the radius of curvature before delamination occurs relative to the thickness of the strip.

There is a known method for the production of rolled steel with a corrosion-resistant coating, described in patent WO 2013160567 A1 (dated April 25, 2012, Arcelormittal Investigation Y Desarrollo, SL), where an alloy of zinc with aluminum and magnesium is used as a metal coating in the ratio: 0.1-20% aluminum, 0.1-10% magnesium, the rest is zinc. The method involves the following technological operations:

1.preparation of the steel surface before applying coatings using pickling solutions;

2. hot dipping metal plating;

3. cooling of the steel strip;

4. Removal of layers of magnesium oxide or hydroxide formed on the surfaces of the metal coating by sequential treatment in alkaline solutions and subsequent treatment with conversion acid solutions with a pH from 1 to 4;

5. Application of melamine crosslinked polyester paints, isocyanate crosslinked polyesters, polyurethanes and halogenated vinyl polymer derivatives to the surface of the metal coating.

The authors position this method as universal, which allows the production of steel with metal and polymer coatings, where metal coatings can be used in a wide range of ratios of zinc, aluminum and magnesium, and polymer coatings can be of any kind.

However, this method is not devoid of disadvantages, the main one of which is the lack of control over the thickness of the conversion layer on the surface of the steel strip. The authors do not take into account that with too small a thickness of the conversion layer, the anticorrosive properties of subsequent layers of coatings will be significantly reduced, and with too much thickness, the physical and mechanical properties of subsequent layers of coatings will decrease.

Another disadvantage of the disclosed method is the assumption of the use of non-environmentally friendly conversion solutions.

Also, the disadvantages of the method include a wide range of concentrations of coating metals in the melt bath, which significantly complicates the uniformity of properties and phase composition of the zinc-aluminum-magnesium coating along the length of the strip.

There is a known method for the production of rolled steel with a corrosion-resistant coating, described in patent WO 2000071773 A1 (dated May 24, 1999, Nippon Steel Corporation), where an alloy of zinc with aluminum and magnesium is used as a metal coating in the ratio: 2-19% aluminum, 1-10 % magnesium, 0.01-2% silicon, the rest is zinc. The method involves the application of a metal coating by hot dipping a steel strip into a bath with a melt temperature of 450-650 ° C and subsequent cooling at a rate of 0.5 ° C / s. The method allows to obtain the following phase compositions of the coating:

1. a ternary eutectic structure in the form of an Al / Zn / MgZn ₂ matrix with Mg ₂ Si, MgZn ₂ and Zn phases distributed in it;

2. a ternary eutectic structure in the form of an Al / Zn / MgZn ₂ matrix with Mg ₂ Si, MgZn ₂ and Al phases distributed in it;

3. ternary eutectic structure in the form of a matrix Al / Zn / MgZn ₂ with phases Mg ₂ Si, MgZn ₂ , Al and Zn distributed in it;

4. ternary eutectic structure in the form of an Al / Zn / MgZn ₂ matrix with Mg ₂ Si, Al and Zn phases distributed in it.

Before applying paints and varnishes, the surface is treated with conversion chromium-containing compounds based on Cr ³⁺ , Cr ³⁺ + Cr ⁶⁺ , as well as with a solution of CrO ₃ in H ₃ PO ₄ . The amount of applied conversion composition varies from 10 to 300 ml / m ² .

The disadvantage of this method is the use of carcinogenic compounds based on chromium (Cr ⁶⁺ ) to form a conversion layer.

Other disadvantages of this method are similar to those disclosed in WO 2013160567 A1.

In patents KR 101758529 B1 (dated December 24, 2014, Posco Co Ltd), CN 103361588 B (dated March 30, 2012, Angang Steel Company), WO 2009049836 A1 (dated October 10, 2007, Voestalpine Stahl GmbH), WO 2007132007 A1 (dated 15 May 2006, Thyssenkrupp Steel Ag), WO 2015055285 A1 (dated October 15, 2013, Tata Steel Ijmuiden BV), JP 2003138359 A (dated October 29, 2001, Sumitomo Metal Ind Ltd) disclosed methods for the production of zinc-based metal coated steel strip with the addition of aluminum and magnesium. A common disadvantage of the disclosed technical solutions is that the applied materials are anodic in relation to iron and provide anti-corrosion properties only due to the consumption of metals in the coating, the barrier protection against corrosion of such coatings is low, since corrosion products of the coating act as a barrier between the aggressive medium and the coating.

The task for the implementation of which the claimed technical solution is directed is the development of a method for the production of a steel strip with increased anticorrosive characteristics. It is proposed to achieve an improved anti-corrosion effect by combining corrosion protection methods on the surface of the steel strip.

The claimed method for the solution of the problem posed for the production of painted coils from structural steel with a corrosion-resistant zinc-aluminum-magnesium coating is carried out in four stages.

At the first stage, a zinc-aluminum-magnesium coating is applied to a steel strip that is degreased and free from oxide films. Coating is carried out in a molten bath Zn (97.2-98.0%) + Al (1-1.4%) + Mg (1-1.4%) at a temperature of 420-460 ° C and a strip speed of 40-165 m / min. The concentration of Zn, Al and Mg in the molten bath is monitored every 1.0-3 hours by atomic adsorption spectroscopy or inductively coupled plasma mass spectrometry and is maintained in a stable state by adding zinc-based ingots with a Mg content of 1.2-3 , 0% and Al 1.2-2.0%. After passing through the bath, excess liquid metal is removed from the strip surface with gas knives (air or nitrogen), which makes it possible to form a metal coating with a thickness of 4-15 microns. The thickness of the coating is varied by the speed of movement through the bath and the intensity of the gas blowing after passing through the bath. The coated steel strip is cooled at a rate of 1-20 ° C / sec, which forms the final coating structure.

The formed coating is a matrix Al / Zn / MgZn ₂ (no more than 10%) with a Zn phase distributed in it (more than 90%). The high content of zinc in the coating provides cathodic protection of the steel strip even in uncoated areas and in case of damage to the coating during operation, and the corrosion products Al and Mg form double layered hydroxides, which are an additional barrier protection between the corrosive medium and the coating. The corrosion resistance of a steel strip with a coating of this type tested according to GOST 30630.2.5-2013 (ISO 9227: 2012) in a salt spray chamber is 3-10 times higher than that of a steel strip with a standard zinc coating of the same thickness.

In the second stage, the surface of the coated steel is cleaned from organic contaminants and the oxidized surface layer is removed. For cleaning, a composition of an aqueous suspension of alkaline salts at a concentration of 10-28 g / l and an anionic surfactant at a concentration of 0.0-4.0 g / l (processing temperature 50-70 ° C) is used. Such a composition has an alkaline pH of 10-12, which, unlike acid solutions, allows you to effectively clean the surface without dissolving the zinc-aluminum-magnesium coating.

At the third stage, in order to improve anti-corrosion properties, steel with zinc-aluminum-magnesium coating is treated with conversion compounds based on dihydrogen hexafluorotitanate or a combination of dihydrogen hexafluorotitanate with hexafluorozirconic acid. In the process of conversion, insoluble titanium and zirconium compounds are deposited on the surface of the zinc-aluminum-magnesium coating, which form a metal-oxide hydrated film containing TiO ₂ * 2H ₂ O or TiO ₂ * 2H ₂ O + ZrO ₂ * 2H ₂ O. This film provides additional barrier protection against corrosion and increases the wettability of the surface, as a result of which, the adhesion of the organic coating increases. The concentration of active substances and the processing time of the steel strip in the conversion solution are chosen so that the amount of distributed titanium in the conversion layer is 3-12 mg / m ² . When the titanium content is less than 3 mg / m ² , deterioration of the corrosion properties of the coating is observed, and when the titanium content is more than 12 mg / m ² , the deterioration of mechanical properties, such as flexibility.

At the fourth stage, the steel strip is coated with an organic coating (or layer-by-layer organic coatings) using a roller method: first with a polymer primer (or without it), then with a finishing enamel or a single-layer coating. Depending on the assortment produced, the steel strip can be coated both on one side and on both sides with a polymer primer and finishing enamel or a single-layer coating.

When choosing a polymer primer and finishing enamel, it is necessary to take into account the proximity of their temperatures and curing times: the difference in curing temperatures should be 0-5 ° C, and the difference in curing time should be 0-10 s. The choice of these parameters is due to the fact that with the sequential application of polymer coatings, the base of which has different thermophysical properties, the probability of the formation of defects in the process of applying organic layers such as bubbles and craters, which have a negative effect on the corrosion resistance and mechanical properties of the resulting rolled steel with a coating ...

As a primer and finishing enamel, paints and varnishes based on polyesters, polyesters cross-linked with melamine, polyurethanes and hot-curing epoxy resins with a curing temperature of 320-385 ° C, a peak temperature of a metal strip of 200-250 ° C and a curing time in an oven of a polymer coating unit 21 -34 s. In addition, particles of fillers and pigments based on minerals such as TiO ₂ , BaSO ₄ , SiO ₂ , CaCO ₃ , talc, kaolinite, volastonite, iron-manganese spinel and others can be included in the composition of the primer and finishing enamel.

An example of the structure of a fully formed coating is shown in the figure (Fig. 1).

The method disclosed in the claimed technical solution has several advantages:

- the method provides increased corrosion resistance due to the combination of several types of steel protection against corrosion: cathodic, barrier and inhibitor;

- stability of physical and mechanical properties and chemical composition of the coating along the entire length of the strip due to constant monitoring and maintenance of metal concentrations in the melt bath;

- surface cleaning before applying the conversion layer is carried out with an aqueous suspension of alkaline salts (pH = 10-12), which, in contrast to acid solutions (pH <7), avoids damage to the zinc-aluminum-magnesium coating and a decrease in its thickness;

- the method involves the application of a conversion layer of a certain mass per unit area, which provides an optimal combination of corrosion resistance and mechanical properties, after applying an organic coating;

- in the production of coated rolled products, carcinogenic chromium (Cr ⁶⁺ ) containing components prohibited on the territory of the European Union and the United States is not used;

- the use of paints and varnishes with similar thermophysical properties reduces the number of possible rejects, reduces the likelihood of defects in the operation of products from rolled metal products with a coating obtained by the disclosed method, and also increases the productivity of the painting line.

Comparative analysis of scientific, technical and patent literature shows the lack of coincidence of the distinctive features of the proposed method with the features of known technical solutions. On the basis of this, a conclusion is made about the compliance of the proposed technical solution with the criterion of "inventive step".

The claimed technical solution meets the criteria of the invention "Novelty".

The invention is illustrated by the following example:

Example

At the production site of NLMK Group, a pilot batch of painted steel products with zinc-aluminum-magnesium coating was produced on continuous hot-dip galvanizing (ANGTs) and polymer coating (APC) units.

Application of zinc-aluminum-magnesium coating.

For the production of steel strip with zinc-aluminum-magnesium coating in the bath ANGTs was prepared melt containing 97.6 ± 0.4 wt. % zinc, 1.2 ± 0.2 wt. % aluminum and 1.2 ± 0.2 wt. % magnesium. The concentration of aluminum and magnesium was monitored every hour by atomic absorption spectroscopy and was maintained in the range of 1.1-1.3 wt. % aluminum and 1.1-1.3 wt. % magnesium by adding zinc-based ingots with a Mg content of 1.2-3.0 wt. % and Al 1.2-2.0 wt. % as the bath metal is consumed. The melt temperature was maintained at 430 ± 5 ° C.

Degreased and cleaned from oxide films, a strip of 08ps steel, annealed in a preheated continuous furnace to 435 ° C, was immersed at a speed of 165 m / min into the ANGTs melt bath, where a zinc-aluminum-magnesium coating was deposited on its surface. The thickness of the coating after passing through the bath was maintained in the range of 8-10 microns by cutting off excess melt using gas knives with air supply. Next, the coated steel strip was cooled at a rate of 15 ± 2 ° C / s.

Preparation of the strip surface before applying paint and varnish materials (LKM).

The surface preparation of the coated steel strip prior to paint application was as follows:

- Pressure treatment in a temper mill to give a roughness Ra from 0.5 to 1.5 microns.

- Degreasing the surface and removing surface oxides. It was carried out using an alkaline-based composition (pH = 11), consisting of an aqueous suspension of alkaline salts (D1) at a concentration of 25 g / L and any anionic surfactant (S1) at a concentration of 3 g / L.

- Application of a conversion layer. To apply the conversion layer, a composition based on a combination of dihydrogen hexafluorotitanate with hexafluorozirconic acid (under the trade mark Bonderite 1455) was used. The amount of distributed titanium in the surface layer was 10 mg / m ² .

Application of paintwork materials.

The applied coatings were polyester primer according to GOST R 52146-2003 or EN 10169-2010 (R1) and finishing enamel (trade mark Beckrypol 3000) on a polyester base with a relative viscosity according to GOST 8420-74 - 50 s, mass fraction of non-volatile substances 55% and a peak strip temperature of 240 ° C.

The primer and finishing enamel were applied in the APP in succession using a roller method. On the front side - applying a primer, curing the primer in a continuous oven at 340 ° C for 25 s, cooling, applying a finishing enamel, curing the enamel in a continuous oven at 340 ° C for 25 s, cooling. On the reverse side - application of a single coat, curing of a single coat in a continuous oven at 340 ° C for 25 s, cooling.

The painted steel strip obtained by this method was coiled and sent to the warehouse.

The properties of the resulting steel strip were evaluated in accordance with the standards EN 10169, EN 13523-7, EN 13523-8 and 13523-26. The test results are presented in table 1.

The results of corrosion tests are shown in the figure (Fig. 2).

Claims (5)

1. Method for the production of corrosion-resistant painted steel rolled products with zinc-aluminum-magnesium coating, including the preparation of the steel strip before applying the Zn-Al-Mg coating, applying the Zn-Al-Mg coating by hot dipping, degreasing and surface conversion of the Zn-Al-Mg coating , the application of primer on one or two sides of the steel strip, the application of finishing enamel on one or two sides of the steel strip, characterized in that the concentration of chemical elements in the bath of hot coating are, wt. %: Al 1.0-1.4, Mg 1.0-1.4, Zn the rest, for degreasing and removing the oxide film from the surface of the Zn-Al-Mg coating, an aqueous suspension of alkaline salt with pH = 10-12 is used, at application of the conversion layer, a chromate-free composition based on dihydrogen hexafluorotitanate or a combination of dihydrogen hexafluorotitanate with hexafluorozirconic acid is used and the quality of the conversion layer is assessed by the amount of deposited titanium, while the temperature difference between the curing of the soil and the finishing enamel is 0-5 ° C, and the difference in curing time enamel is 0-10 sec. 2. A method according to claim 1, characterized in that a steel strip of cold-rolled annealed, cold-rolled non-annealed or hot-rolled structural steel is used as the substrate for the coating. 3. The method according to claim 1, characterized in that the required concentration of Zn, Al and Mg in the molten bath is maintained by adding zinc-based ingots with a Mg concentration of 1.2-3.0 wt. % and Al 1.2-2.0 wt. % as the bath metal is consumed. 4. The method according to claim 1, characterized in that the alkaline composition is prepared on the basis of an aqueous suspension of alkaline salts at a concentration of 10-28 g / l and an anionic surfactant at a concentration of 0.0-4.0 g / l. 5. The method according to claim 1, characterized in that the amount of titanium in the conversion layer is 3-12 mg / m ² .

Images