CN1276990C - Hot-dip galvanized steel sheet excellent in gloss retention and production method thereof - Google Patents

Abstract

Contact temperature between a plating layer and water flow is controlled through a watercooling step carried out after the plating layer is solidified. Thus, the temperature of band materials during a contact period with the water flow is properly controlled. A small amount of proper easy oxidation elements are further added to plating liquid to stabilize the oxidation state of Al and Mg in a surface layer of the plating layer. Thus, the descending of the surface gloss of a thin steel plate which is hot dipped and plated with Zn-Al-Mg is suppressed.

Description

Hot-dip galvanized steel sheet excellent in gloss retention and production method thereof

technical field

The present invention relates to a technique for preventing deterioration of surface gloss which sometimes occurs during the production of hot-dip-plated Zn-Al-Mg thin steel sheets using a continuous hot-dip plating line.

technical background

JP.Hei-10-226865A owned by the applicant of the present application, JP.Hei-10-306357A, USP6,235,410 (US6,235,410B1) and USP6,379,820 (US6,379,820B1) point out: by adopting a kind of Zn- Al-Mg system hot-dip coating solution and give the coating below the metal structure to prepare Zn-based hot-dip coated steel sheet, can obtain a coated steel sheet with corrosion resistance and surface appearance that fully meet the requirements of industrial products, the Zn- The hot-dipping bath of Al-Mg system contains Al: 4.0-10%, Mg: 1.0-4.0%, Ti: 0.002-0.1%, B: 0.001-0.045%, and the rest is Zn and unavoidable impurities. The structure of the coating contains [primary Al crystal phase] and [Zn single phase] in the matrix of [Al/Zn/Zn ₂ Mg ternary eutectic structure]. The production conditions for obtaining said metal structures are given in the published specification.

During subsequent preparation, during the processing of this hot-dipped Zn-based steel sheet with high Al and Mg contents, the present inventors found that the surface gloss of the coating deteriorated within 2-3 days depending on the preparation conditions. This gloss deterioration of the coating makes the aesthetically pleasing plated surface just after plating faded to a slightly tinge over time (as fast as 2-3 days, sometimes 4-7 days). There are black variegations. The degree of discoloration (a type of blackening) of the outermost surface can be quantitatively determined by measuring surface brightness. For example, a plated surface having a surface brightness (L value) of about L=82 immediately after manufacture decreased to about L=72 within 7 days. Although this decrease in the L value does not degrade the corrosion resistance of the product nor impair the physical and chemical quality of the plated steel sheet, such a decrease is undesirable from the standpoint of surface appearance. This loss of surface gloss does not occur uniformly over the entire surface of the coating, but takes place in the form of blotchy discoloration. This fact especially impairs the aesthetic appearance.

It is considered that this decrease in surface gloss occurs particularly in plated steel sheets with high Al and Mg contents, such as the aforementioned hot-dip Zn-Al-Mg-plated steel sheets. The oxidation degree of Mg enriched on the surface of the outermost layer of the coating and the oxidation state of Al in the surface layer are likely to be the combined reasons for this decline. Hitherto, there has not been published any report on the mechanism of the decrease in surface gloss that occurs in such hot-dipped Zn-Al-Mg thin steel sheets or on the method of suppressing this decrease.

Purpose of the invention

It is therefore an object of the present invention to provide means for suppressing the reduction in surface gloss that occurs particularly in hot-dipped Zn-Al-Mg thin steel sheets.

invention disclosure

In order to overcome the above-mentioned problems, the present inventors conducted extensive experiments and research work. As a result, we determined that the reason for the decrease in surface gloss of hot-dip coated steel sheets is the reaction characteristics of the coating surface with water during water cooling in the cooling step after coating, and that one way to prevent the decrease in surface gloss is to perform after the coating has solidified In the water cooling step of "controlling the contact temperature between the coating and the water flow," in a broad sense, it is very effective to properly control the "strip temperature during contact with the water flow", and, by adding a small amount of appropriate "oxidizable elements" to the plating solution "It can also effectively stabilize the oxidation state of Al and Mg in the surface layer of the coating. The "strip temperature during contact with the water flow" refers to the strip temperature when the coating is solidified and cooled in the cooling step while a water film is formed on the coating surface. Specifically, it is the coating temperature at which water flows over the surface of a completely solidified coating and the coating is cooled by the water film formed on its surface.

In other words, according to the findings of the present inventors, when the steel strip is continuously dipped and taken out in the aforementioned Zn-Al-Mg system hot-dipping bath, and the steel strip is then continuously passed through a water quenching zone for cooling the steel strip At the same time, the completely solidified coating surface is in contact with the water flow (at the same time, a certain amount of water that can form a layer of water film on the coating surface is provided on the coating surface at the same time), by controlling the temperature of the steel strip at the entrance side of the water quenching zone to be lower than 105 °C, the aforementioned deterioration of the gloss of the coating surface can be suppressed.

Although the temperature of the steel strip at the entrance side of the water quenching zone depends on the equipment conditions, it is closely related to the thickness of the sheet. When the thickness of the steel strip is large, it may not be easy to control the temperature of the steel strip on the inlet side of the water quenching zone below 105°C. It has been found that, in this case, by adding a small amount of an element which has a strong chemical affinity to oxygen (i.e. an easily oxidizable element) and which has a stabilizing effect on the oxides of Al to the plating bath, it is not necessary to make the steel strip in water quenching. A similar effect of suppressing deterioration of surface gloss can also be obtained when the temperature on the entrance side of the zone is lower than 105°C (temperature equal to 105°C or higher). Such elements include, for example, rare earth elements, Y, Zr and Si. In practice, it is sufficient to add about 0.002-0.05% by mass of easily oxidizable elements.

Therefore, the present invention provides a method for preparing a hot-dip Zn-coated thin steel sheet excellent in gloss retention, which comprises: a steel strip containing Al: 4.0-15% (mass) and Mg: 1.0-4.0% (mass) The steps of continuous immersion and extraction in a Zn-based hot-dip bath, thereby forming a coating on the steel strip, and, thereafter, passing the steel strip continuously through a water quenching zone which cools the steel strip while allowing complete solidification The surface of the coating is in contact with the water flow, at this time, the step of suppressing the deterioration of the gloss of the coating surface by controlling the temperature of the steel strip at the entrance side of the water quenching zone to be lower than 105°C. The Zn-based hot dipping bath preferably comprises, in % (mass): Al: 4.0-15%, Mg: 1.0-4.0%, Ti: 0.001-0.1%, B: 0.001-0.045%, and the rest is Zn and unavoidable impurities.

The present invention also provides a method for preparing a hot-dip Zn-coated thin steel sheet with excellent gloss retention performance, which comprises: a steel strip containing Al: 4.0-15% (mass) and Mg: 1.0-4.0% (mass), and Adding 0.002-0.05% (mass) of at least one selected from the Zn-based hot dipping bath of rare earth elements, Y, Zr and Si easily oxidizable elements for continuous dipping and taking out, thus forming a coating on the steel strip step, and, after that, the steel strip is continuously passed through a water quenching zone, which cools the steel strip while contacting the fully solidified coating surface with water flow, at this time by controlling the temperature of the steel strip on the inlet side of the water quenching zone It is not lower than 105°C but not higher than 300°C, the step of suppressing the deterioration of the surface gloss of the coating. The Zn-based hot dipping bath preferably comprises, in % (mass): Al: 4.0-15%, Mg: 1.0-4.0%, Ti: 0.001-0.1%, B: 0.001-0.045%, at least one of Easily oxidizable elements from rare earth elements, Y, Zr and Si: 0.002-0.05%, the rest is Zn and unavoidable impurities.

As a hot-dip Zn-coated steel sheet obtained by the aforementioned method, the present invention also provides a hot-dip Zn-coated thin steel sheet obtained by hot-dip coating a steel strip with a hot-dip bath and having excellent gloss retention performance. Steel plate, described plating solution comprises, in % (mass):

Al: 4.0-15%, and

Mg: 1.0-4.0%,

optionally,

Ti: 0.001-0.1% and

B: 0.001-0.045%,

Preferably,

At least one easily oxidizable element selected from rare earth elements, Y, Zr and Si: 0.002-0.05%, and

The remainder is Zn and unavoidable impurities, thereby forming a plated layer on the thin steel sheet. The coating has a [primary Al crystal phase] contained in a matrix of [Al/Zn/Zn ₂ Mg ternary eutectic structure], or contained in a matrix of [Al/Zn/Zn ₂ Mg ternary eutectic structure] The metallic structure of [primary Al crystal phase], [Zn phase] and/or [Zn ₂ Mg phase] in .

Brief description of the drawings

Fig. 1 is a schematic diagram illustrating an apparatus for producing a Mg-containing hot-dip Zn-coated steel sheet according to the present invention.

Preferred Embodiments of the Invention

In general, in this specification, the term "steel strip" is used when describing a hot-dip galvanizing line, and the term "sheet steel" is used when describing a product. Steel strip and thin steel plate have the same characteristics.

Figure 1 is a schematic illustration of the device used to prepare Zn-based hot-dip-coated steel sheets. The steel strip 2 passing through the heating furnace 1 maintained at a predetermined temperature is continuously fed into the plating solution 3 . When coming out of the bath 3, the strip passes through a wiping nozzle 4 which adjusts its coating weight, and then through an air jet cooling device 5. The coating solidifies completely during passage through the air jet cooling device 5 . Next, the steel strip passes through air-water cooling zones 6 a , 6 b and an air cooling zone 14 , which can work individually or together, or not, and then passes through a water quenching zone 7 . Afterwards, the steel strip is tempered and cold-rolled on a temper mill 8 , then passed through a tension temper mill 9 , undergoes a testing step, and is crimped on a tension coiler 10 . When carrying out chromate treatment or other such chemical treatments, the steel strip leaving the tension leveler 9 is processed on the roller coater 11, then passes through a drying zone 12 and an air cooling zone 13, and is coiled under tension coiling. On machine 10.

As mentioned above, when preparing a hot-dip Zn-Al-Mg thin steel sheet with a relatively high content of Al and Mg (hereinafter referred to as "Mg-containing hot-dip Zn-coated thin steel sheet"), it is appropriate to consider the surface properties It is essential to control the cooling rate before the coating is fully solidified and where the coating is fully solidified. For this reason, the feeding speed of the steel strip must be strictly controlled, and the temperature of the strip when passing through the air jet cooling device 5 needs to be precisely controlled according to the thickness of the sheet. Next, in order to be able to perform proper temper rolling on the temper mill 8, it is necessary to implement cooling in the air-water cooling zone 6 (and the air cooling zone 14) and/or the water quenching zone 7. The strip temperature is adjusted to a preset temperature level (for example, not higher than 70°C). The cooling intensity at the cooling zone varies according to the speed of travel of the strip and the thickness of the sheet. Generally, water or aqueous solution is sprayed in the air-water cooling zone, air is sprayed in the air spray cooling zone, and in the water quenching zone, a water flow sufficient to instantly form a water film on the surface of the coating is provided. Compared with the first-mentioned cooling step, the latter-mentioned water quenching zone can achieve a faster cooling rate due to the contact of the water flow with the coating surface. Therefore, an effective cooling operation can be performed in the water quenching zone regardless of the change in cooling intensity. Although water or an aqueous solution is sprayed together with the air flow in the air-water cooling zone 6, sufficient water cannot be supplied to form a water film on the surface of the coating. Therefore, this cooling is mainly a result of the release of latent heat by evaporation. This is a different form of heat dissipation from contact with water flow in the water quenching zone, so the cooling rate is different.

It has been found that the gloss retention of the coating surface varies with the temperature of the strip on the entry side of the water quenching zone. When the temperature is 105° C. or higher, surface gloss deterioration tends to occur. Although the cause thereof has not been determined, the inventors have concluded that the cause involves factors such as: when the strip temperature is 100° C. or higher when entering the water quenching zone, a boiling phenomenon occurs to increase the reactivity of the coating surface, and , at atmospheric pressure, in the presence of water, the properties of Al are different at temperatures above and below about 110°C, that is, above this temperature, Al ₂ O ₃ ·H ₂ O (or AlOOH) compounds are stable, while at low temperatures At this temperature, the Al ₂ O ₃ ·3H ₂ O (or Al(OH) ₃ ) compound is stable, and as a result, the Al compound formed on the coating surface in the water quenching zone varies depending on the inlet side strip temperature.

The metal structure of the _{aforementioned} Mg-containing hot-dip Zn-coated thin steel sheet includes [primary Al crystal phase], [Zn phase] and/or [Zn ₂ Mg phase], through the above-mentioned chemical reaction, the outermost surface of the ternary eutectic crystal structure becomes an easily oxidizable form, and, it is considered, this is why the 2 or The cause of surface gloss deterioration that produces brightness changes occurs within 3 days.

In conclusion, the present inventors have found that hot-dip Zn-coated thin steel sheets containing Mg can be significantly prevented by adjusting the temperature of the strip entering the water quenching zone below 105°C, as demonstrated by the examples given later. Deterioration of surface gloss occurs. In this case, when performing the cooling operation before entering the water quenching zone, do not supply enough water to the fully solidified coating surface to form a quenching water film as in the water quenching zone, but perform a slow cooling operation , for example with air-water cooling (spray) or air cooling (eg spray air cooling). This is because it is meaningless to adjust the temperature of the steel strip entering the water quenching zone to a lower temperature of 105°C if the quenching operation is carried out to form a water film as in the water quenching zone before the steel strip enters the water quenching zone. Depending on sheet thickness and strip travel speed, it is even possible to adjust the strip temperature below 105°C on the inlet side of the water quenching zone by air-water cooling.

However, when the thickness of the sheet is thick, sometimes the temperature of the strip on the inlet side of the water quenching zone cannot be adjusted below 105°C. Although sufficient cooling can be achieved in the air-water cooling zone by reducing the strip travel speed even at larger sheet thicknesses, this is undesirable as it creates freezing point control problems and reduces productivity. However, as demonstrated by the examples given hereinafter, by the presence of an appropriate amount of an easily oxidizable element which has a strong chemical affinity for oxygen and has a stabilizing effect on Al oxides, e.g. Rare earth elements, Y, Zr or Si, are effective in suppressing deterioration of surface gloss even when the strip temperature on the inlet side of the water quenching zone is 105°C or higher. In practice, it is sufficient to add about 0.002-0.05% by mass of said readily oxidizable elements. Although there may be other elements having similar effects to rare earth elements, Y, Zr or Si, none has been confirmed.

These readily oxidizable elements have the property of being enriched in the outermost surface portion of the coating during or even after solidification of the coating on the Mg-containing hot-dip Zn-plated steel sheet. Therefore, even when only a small amount is added to the plating solution, these elements can reach a relatively high concentration at the outermost surface. It is believed that this enables these elements to suppress surface reactions in the water quenching zone that are detrimental to surface gloss retention. However, when the addition amount of these easily oxidizable elements in the plating solution is less than 0.002% by mass, the suppressing effect cannot be exhibited. Therefore, the addition amount of these easily oxidizable elements in the plating solution must not be less than 0.002% (mass). On the other hand, when the addition amount is too much, the solubility in the plating solution is reduced, and even if it is completely dissolved, its suppression effect on the reduction of surface gloss is saturated, because the excess part of the element is only precipitated in the surface part of the coating. Near the grain boundary. Therefore, these elements are added in an amount not higher than 0.10% by mass, in some cases not higher than 0.08% by mass, preferably 0.05% by mass, more preferably 0.03% by mass.

As mentioned earlier, when the strip temperature on the inlet side of the water quenching zone is 105°C or higher, the effect of inhibiting the deterioration of surface gloss by adding one or more easily oxidizable elements is most effective. However, even when the temperature of the strip on the inlet side of the water quenching zone is below 105°C, one or more easily oxidizable elements can be safely added to the bath.

When the temperature of the strip on the inlet side of the water quenching zone is 105°C or higher, it should preferably be controlled not higher than 300°C, because when it is higher than 300°C, the cooling load in the water quenching zone is too large to make the flat cooling The temperature on the rolling entry side is sufficiently lowered.

In an ordinary production line, if the thickness of the strip is less than 1.6 mm, it is relatively easy to keep the temperature of the strip at the entrance side of the water quenching zone lower than 105°C, thereby ensuring good gloss retention. When the thickness of the strip is 1.6mm or thicker, in terms of practical operation, it is better to add easily oxidizable elements than to make the temperature on the inlet side of the water quenching zone lower than 105°C by performing a forced cooling zone on the steel strip. to ensure gloss retention performance.

The invention is a breakthrough in preventing the surface luster reduction of Mg-containing hot-dip Zn-coated steel sheets, and therefore, can be used for Mg-containing hot-dip Zn-coated steel sheets with surface gloss reduction. This Mg-containing hot-dip Zn-coated steel sheet is typically a hot-dip Zn-Al-Mg-coated steel sheet excellent in corrosion resistance and surface appearance obtained by using a Zn-based hot-dip bath, the Zn The basic hot-dipping bath contains basic components Al: 4.0-10% and Mg: 1.0-4.0%, plus Ti: 0.001-0.1% and B: 0.001-0.045%, and the rest is Zn and unavoidable impurities, so The metal structure of the obtained coating consists of [primary Al crystal phase] in the matrix of [Al/Zn/Zn ₂ Mg ternary eutectic structure] or in the matrix of [Al/Zn/Zn ₂ Mg ternary eutectic structure] The metallic structure of [primary Al crystal phase], [Zn phase] and/or [Zn ₂ Mg phase] in . The metal structure containing [primary Al crystal phase] in the matrix of [Al/Zn/Zn ₂ Mg ternary eutectic structure] is preferably [Al/Zn/Zn ₂ Mg ternary eutectic structure] + [primary Al crystal phase] The total amount is not less than 80% (volume) of the metal structure, more preferably not less than 95% (volume) of the metal structure. The remainder may consist of [Zn phase] and [Zn ₂ Mg phase], plus, in some cases, small amounts of [Zn/Zn ₂ Mg binary eutectic] and/or [Al//Zn ₂ Mg binary eutectic]. When Si is added, a small amount of [Si phase], [Mg ₂ Si phase], [Al/Mg ₂ Si binary eutectic], etc. may also exist.

Therefore, the present invention provides a kind of hot-dip Zn thin steel sheet that obtains excellent luster retention by hot-dipping a steel strip, and the hot-dip coating solution used when hot-dipping a steel strip contains, in % (mass):

Al: 4.0-15%,

Mg: 1.0-4.0%,

Ti: 0.001-0.1%,

B: 0.001-0.045%,

At least one easily oxidizable element selected from rare earth elements, Y, Zr and Si: 0.002-0.05%, and

The remainder is Zn and unavoidable impurities, thereby forming a plated layer on the thin steel sheet. The coating has a [primary Al crystal phase] contained in a matrix of [Al/Zn/Zn ₂ Mg ternary eutectic structure], or contained in a matrix of [Al/Zn/Zn ₂ Mg ternary eutectic structure] The metallic structure of [primary Al crystal phase], [Zn phase] and/or [Zn ₂ Mg phase] in . Its structural feature is that one or more easily oxidizable elements are enriched in the outermost surface layer of the coating.

According to the present invention, the addition amounts, functions and influences of Al, Mg, Ti, B, etc. in the plating solution remain unchanged in the plating solution in which at least one easily oxidizable element is added. Adding Ti and B _{is beneficial, because these elements play a role in inhibiting the generation and growth of Zn 11 Mg 2 phase .} Corrosivity has adverse effects. In order to obtain the above effect by adding Ti, B or Ti-B alloy or compound to the plating solution, it is sufficient to make the plating solution contain Ti: 0.001-0.1% and B: 0.001-0.045%. When the content is higher than the above range, the precipitated phase grows in the plating layer, forming "bumps" (called butsu in Japanese) in the plating layer that impair the surface appearance.

The presence of Al in the plating layer improves the corrosion resistance of the plated steel sheet and can suppress the generation of scale during the preparation of the plated steel sheet. When the Al content is less than 1.0% by mass, the effect of improving corrosion resistance is insufficient, and furthermore, the effect of suppressing the generation of Mg oxide-based scale is also low. It is preferable to make the Al content not less than 4.0% by mass. On the other hand, when the Al content exceeds 15% by mass, a Fe-Al alloy layer is remarkably formed between the coating and the base steel sheet, reducing the adhesion of the coating. The Al content is preferably 4.5-13.0% by mass, more preferably 5.0-10.0% by mass, most preferably 5.0-7.0% by mass.

The presence of Mg in the plating layer can generate uniform corrosion products on the surface of the plating layer, and, thus, it has the effect of remarkably improving the corrosion resistance of the plated steel sheet. When the Mg content is less than 1.0%, the effect of uniformly generating such corrosion products is insufficient. On the other hand, when the Mg content exceeds 4.0%, the effect of improving the corrosion resistance by Mg is saturated, and undesired Mg oxide-based scales are more likely to occur. Therefore, the Mg content is limited to 1.0-4.0% by mass. Preferably the Mg content is 1.5-4.0% by mass, more preferably 2.0-3.5% by mass, most preferably 2.5-3.5% by mass.

It has been found that the aforementioned problem of reduced surface gloss also occurs when chemical treatment is performed at the final stage of the production line for hot-dip Zn-coated steel sheets containing Mg, for example by using a roll coater 11 as shown at the bottom of FIG. It does not matter whether a chromate coating, a chromate-free conversion coating, a chromic acid-containing organic resin film, or a chromic acid-containing silicate film, or other such films are formed. Just as a decrease in brightness and a decrease in surface gloss may (or may not) occur without chemical treatment, depending on production conditions, a plated product with a film produced by chemical treatment may (or may not) appear after a few days A phenomenon in which brightness decreases and surface gloss decreases. It has been found that this problem can likewise be overcome according to the invention by controlling the temperature of the strip on the inlet side of the water quenching zone and by adding at least one easily oxidizable element to the bath.

More specifically, also in the case where the steel strip passes through the water quenching zone and then passes through the chemical treatment zone, as in the aforementioned method of producing Mg-containing hot-dip Zn-coated steel sheet, by controlling the strip on the inlet side of the water quenching zone If the material temperature is lower than 105°C or 0.002-0.05% (mass) easily oxidizable elements are added to the plating solution, the surface gloss of the chemically treated Mg-containing hot-dip Zn-coated steel sheet can be suppressed from decreasing.

Therefore, the present invention provides a kind of hot-dip Zn thin steel sheet that obtains excellent luster retention by hot-dipping a steel strip, and the hot-dip coating solution used when hot-dipping a steel strip contains, in % (mass):

Al: 4.0-15%,

Mg: 1.0-4.0%,

Ti: 0.001-0.1%,

B: 0.001-0.045%,

At least one easily oxidizable element selected from rare earth elements, Y, Zr and Si: 0.002-0.05%, and

The remainder is Zn and unavoidable impurities, whereby a plated layer is formed on the thin steel sheet, and the plated layer is chemically treated.

Example 1

Contain about 6% (mass) Al, about 3% (mass) Mg, about 0.05% (mass) Ti and about 0.01% (mass) B Zn-based hot dip coating solution, and prepare a Mg-containing hot-dip Zn-plated steel sheet (thickness: 0.8-1.0 mm) with a coating whose metal structure is contained in a matrix of [Al/Zn/Zn ₂ Mg ternary eutectic structure] [Primary Al crystal phase], [Zn phase] and [Zn ₂ Mg phase] in the. At this time, the temperature of the strip at the outlet side of the jet air cooler 5 is 335° C. or lower so that the coating is completely solidified in the cooler. In zone 7, the cooling conditions and strip travel speed remained essentially unchanged. The strip temperature on the inlet side of the water quenching zone 7 is 100°C or lower, and the steel strip passes through the temper rolling mill at about 70°C or lower. The obtained Mg-containing hot-dip Zn-plated steel sheet was excellent in surface gloss, and a phenomenon in which the gloss was lowered was not observed.

A steel strip of 1.6 mm or thicker is passed under substantially the same conditions as previously described. Although the temperature of the strip on the inlet side of the water quenching zone 7 rose to about 120-150°C, there was no problem in the operation itself, and a Mg-containing Zn-coated steel sheet was produced with the same excellent surface gloss. However, after 2 or 3 days, the surface gloss of the steel sheet decreased slightly, and some lightness L values were observed to drop from 82 just after the end of manufacture to about 75 after two days. Afterwards, the cooling capacity of the air-water cooling zones 6a, 6b is increased in order to reduce the strip temperature on the inlet side of the water quenching zone 7 to below 105°C. As a result, the decrease in surface gloss was no longer observed.

In addition, when the chromate treatment is performed, whether or not the decrease in surface gloss occurs again depends on the temperature of the strip on the inlet side of the water quenching zone 7 . From this, it can be seen that when the chromate treatment is performed in the same manner as when the chromate treatment is not performed, surface deterioration also occurs.

Example 2

Based on the experience obtained in the foregoing examples, the degree of gloss reduction of the coating surface was examined by conducting tests under various conditions. The laboratory method (Lab mehtod) L value measured by the spectrophotometer is used as the brightness (L), and the brightness value is used to evaluate the surface gloss.

First, during the preparation of hot-dip Zn-Al-Mg thin steel sheets under the following "coating conditions", air-water cooling (spraying) and water quenching (water jetting) were performed under the following "cooling conditions" to make the strip The temperature from when the coating is fully solidified drops to about 30°C. By measuring the L value immediately after plating and after the constant temperature and humidity test, the degree of surface gloss degradation of the sample taken from the obtained plated steel sheet was evaluated, wherein, in the constant temperature and constant humidity test, the just-plated The subsequent samples were kept in a thermohygrostat at a temperature of 60° C. and a relative humidity of 90% for 20 hours.

Plating conditions

Steel strip processed: 2.0mm thick hot-rolled steel strip

Plating bath composition: Al=9.0% (mass), Mg=2.3% (mass), balance=Zn

Bath temperature: 430°C

Coating weight: 90g/m ²

Strip travel speed: 80m/min

cooling condition

(1) Air-water cooling

Nozzle: Dual Fluid Nozzle

Fluid: water (pressure = 12.5kPa) + air (pressure = 4.0kPa)

Spray water flow: as shown in Table 1

Spray air flow: as shown in Table 1

Average radius of fog particles: about 50μm

Strip temperature at the start of spraying: as shown in Table 1

Strip temperature at the end of spraying: as shown in Table 1

(2) Water quenching

Water jet: 7 rows of nozzles, each row includes 10 flat nozzles, and the interval between nozzles in the strip width direction is 150mm

Fluid: water (pressure = 2.5kgf/cm ² )

Water flow: as shown in Table 1

Strip temperature at the start of water jetting: as shown in Table 1

Strip temperature at the end of water jetting: as shown in Table 1

Table 1 shows the plated steel sheets obtained under the aforementioned conditions, different strip temperatures at the beginning of water spraying (strip temperature on the side of the water quenching inlet zone), immediately after coating and after 20 hours of constant temperature and constant humidity L value measurement results after the test. The metallic structure of the coating was examined by microscopic observation of a cross-section of the coating on each plated steel sheet. The metal structure of each thin steel plate coating contains [primary Al crystal phase] in the matrix of [Al/Zn/Zn ₂ Mg ternary eutectic structure], and sometimes [Zn phase] and [Zn ₂ Mg phase ] (hereinafter referred to as "Zn ₂ Mg-based" metal structure).

Table 1

Plating bath composition: Al=9.0% (mass), Mg=2.3% (mass), balance=Zn serial number cooling condition surface gloss Example type air-water cooling water quenching L value immediately after plating L value after 20 hours constant temperature and constant temperature test Spray water flow m ³ /h Spray air flow m ³ /min Strip temperature at the start of spraying °C Strip temperature when spraying is stopped °C Water flow m ³ /h Strip temperature when the water jet starts to spray °C Strip temperature °C when water jetting stops A-1 10 1000 198 131 150 92 30 82 82 this invention A-2 10 1000 280 207 150 101 30 82 81 this invention A-3 8 600 280 223 150 112 30 82 74 Comparative example A-4 4 600 280 245 150 132 31 81 70 Comparative example

It can be seen from the results in Table 1 that when the temperature of the strip on the side of the water quenching inlet is higher than 105°C, as the temperature increases, the decrease trend of the brightness L value is stronger. In contrast, when the temperature was lower than 105° C., the brightness immediately after plating remained unchanged with time, confirming that a Mg-containing hot-dip Zn-plated steel sheet excellent in gloss retention was obtained.

Example 3

Under the following conditions, using the same cooling conditions after solidification of the coating as in Example 2, a Mg-containing hot-dip Zn-coated steel sheet was prepared, and the decline in surface gloss was detected. The results are shown in Table 2.

Plating conditions

Steel strip processed: 3.2mm thick hot-rolled steel strip

Bath composition: Al=6.3% (mass), Mg=3.2% (mass), Ti=0.008% (mass), B=0.002% (mass), balance=Zn

Bath temperature: 390°C

Coating weight: 120g/m ²

Strip travel speed: its changes are shown in Table 2

cooling condition

(1) Gas cooling (cooling with air jet cooler only)

Nozzle: Plate nozzle with a gap width of 5mm

Gas: Air (pressure=4kPa)

Air flow: as shown in Table 2

Strip temperature at the start of air cooling: as shown in Table 2

Strip temperature at the end of air cooling: as shown in Table 2

(2) Water quenching

Water jet: 7 rows of nozzles, each row includes 10 flat nozzles, and the interval between nozzles in the strip width direction is 150mm

Fluid: water (pressure = 2.2kgf/cm ² )

Water flow: as shown in Table 2

Strip temperature at the start of water jetting: as shown in Table 2

Strip temperature at the end of water jetting: as shown in Table 2

Table 2

Bath composition: Al=6.3% (mass), Mg=3.2% (mass), Ti=0.008% (mass), B=0.002% (mass), balance=Zn serial number Strip traveling speed m/min cooling condition surface gloss Example type air cooling water quenching L value immediately after plating L value after 20 hours constant temperature and humidity test Air flow m ³ /min Strip temperature at the beginning of cooling °C Strip temperature at the end of cooling °C Water flow m ³ /h Strip temperature when the water jet starts to spray °C Strip temperature °C when water jetting stops B-1 twenty three 0 - - 120 82 30 81 81 this invention B-2 31 2500 167 126 120 82 30 81 81 this invention B-3 40 2500 185 140 120 102 30 81 80 this invention B-4 40 1200 185 158 120 118 30 81 73 Comparative example B-5 56 2500 212 164 120 131 31 81 71 Comparative example

It can be seen from the results in Table 2 that, as in Example 2, in this example, when the temperature of the strip on the side of the water quenching inlet is higher than 105°C, as the temperature rises, the decreasing trend of the brightness L value is more intense . However, when the temperature was lower than 105° C., the luminance immediately after plating remained unchanged even after a 20-hour constant temperature and humidity test. The metallic structure of the coating was examined by microscopic observation of the coating's cross-section for each plated steel sheet. A "Zn ₂ Mg system" metallic structure was obtained in each thin steel sheet without exception.

Example 4

The water-quenched plated surfaces of samples numbered B-2 and B-5 in Example 3 were chemically treated under the following conditions. Using the method in Example 2, the brightness of the sample was measured immediately after the chemical treatment and after a 20-hour constant temperature and humidity test. The results are shown in Table 3.

Chemical treatment condition A

Coating method: spraying and squeeze roller method

Treatment solution: Zinchrome 3387N, produced by Nihon Parkerizing Co., Ltd. (total concentration of chromium in the solution: 10g/L)

Chromium coating weight: 10mg/m ²

Chemical treatment condition B

Coating method: roller coating method

Treatment solution: Zinchrome 3387N with 1g/L zirconium fluoride added (total concentration of chromium in the solution: 20g/L)

Chrome coating weight: 40mg/m ²

Chemical treatment condition C

Coating method: roller coating method

Treatment solution: an aqueous solution mainly containing 50g/L magnesium phosphate, 10g/L potassium fluotitanate and 3g/L organic acid

Coating weight of metal parts: 50mg/m ²

Chemical treatment conditions D

The coating is formed by the following two-step method

bottom layer

Coating method: spray and squeeze roller method

Treatment solution: Zinchrome 3387N (the total concentration of chromium in the solution: 10g/L)

Chromium coating weight: 10mg/m ²

top floor

Coating method: roller coating method

Organic coating: polyurethane resin (thickness: 1.5μm)

table 3 Plating after chemical treatment Chemical treatment conditions surface gloss Example type L value immediately after chemical treatment L value after 20 hours constant temperature and humidity test No.B-2 A 81 81 this invention No.B-4 A 81 72 Comparative example No.B-3 B 81 80 this invention No.B-5 B 81 70 Comparative example No.B-3 C 81 81 this invention No.B-4 C 81 73 Comparative example No.B-2 D. 81 81 this invention No.B-5 D. 81 72 Comparative example

It can be seen from the results in Table 3 that when the temperature of the strip on the side of the water quenching inlet is 105°C or higher, regardless of whether chemical treatment is performed or not, the brightness L decreases with time. In contrast, when the temperature was lower than 105° C., the brightness immediately after plating remained unchanged.

Example 5

Plating is performed under the following plating conditions and cooling conditions, in which one or more easily oxidizable elements are added to the plating solution. The degradation of the surface gloss of each coating is detected. The results are shown in Table 4.

Plating conditions

Steel strip processed: 1.6mm thick hot-rolled steel strip

Bath composition: as shown in Table 4

Bath temperature: 450°C

Coating weight: 190g/m ²

Strip travel speed: 60-120m/min

cooling condition

(1) Air-water cooling

Nozzle: Dual Fluid Nozzle

Fluid: water (pressure = 1.0-3.5kgf/cm ² ) + air (pressure = 2.5-5.0kgf/cm ² )

Spray water flow: 0-8m ³ /h

Spray air flow: 0-600m ³ /h

Average radius of fog particles: 10-30μm

(2) Gas cooling

Nozzle: Plate nozzle with a gap width of 5mm

Gas: Air (pressure=4kPa)

Air flow: 0-3500m ³ /min

(3) Water quenching

Water jet: 7 rows of nozzles, each row includes 10 flat nozzles, and the interval between nozzles in the strip width direction is 150mm

Fluid: water (pressure = 3.0kgf/cm ² )

Water flow: 180m ³ /h

Strip temperature at the start of water jetting: as shown in Table 4

Table 4 serial number Plating solution composition (% (mass), balance Zn) Strip temperature at the inlet side of the water quenching zone °C chemical treatment surface gloss Example type al Mg Ti B easily oxidized elements L value just after plating or chemical treatment L value after 20 hours constant temperature and humidity test type content C-1 13.8 2.2 0.046 0.020 Y 0.030 198 none 81 80 this invention C-2 8.2 2.9 0.012 0.005 Y 0.009 96 none 82 82 this invention C-3 9.4 3.3 0.009 0.010 LaCe 0.0070.010 155 none 82 81 this invention C-4 4.5 1.3 0.037 0.038 Zr 0.028 237 none 81 80 this invention C-5 6.1 3.3 0.024 0.009 Zr 0.003 131 none 82 81 this invention C-6 10.3 3.7 0.005 0.006 Zr 0.029 103 none 82 82 this invention C-7 7.8 3.2 0.011 0.002 Si 0.041 138 none 82 82 this invention C-8 6.3 3.0 0.050 0.003 Si 0.023 100 none 82 82 this invention C-9 5.2 2.6 0.008 0.004 Si 0.004 83 none 82 82 this invention C-10 5.6 2.5 0.006 0.001 Zr 0.001 146 none 81 72 Comparative example C-11 11.5 2.0 0.073 0.015 La 0.001 174 none 81 66 Comparative example C-12 6.1 3.3 0.024 0.009 Zr 0.003 131 Yes (Type B) 82 81 this invention C-13 13.8 2.2 0.046 0.020 Y 0.030 198 Yes (Type C) 81 80 this invention C-14 6.3 3.0 0.050 0.003 Si 0.023 100 Yes (Type C) 82 82 this invention C-15 5.6 2.5 0.006 0.001 Zr 0.001 146 Yes (Type B) 81 73 Comparative example C-16 4.4 1.8 - - Zr 0.021 109 none 82 82 this invention C-17 6.3 2.9 - - YSi 0.0050.012 145 none 82 82 this invention C-18 5.7 3.2 - - Si 0.028 167 Yes (Type B) 81 81 this invention

As shown in Table 4, in addition to adding at least one easily oxidizable element in the plating solution, under the same conditions under other conditions, the brightness after the 20-hour constant temperature and humidity test in Examples 2 and 3 was prepared. The plated steel sheet in which the value L decreased to about 71 was similar to the plated steel sheet, but at this time, the decrease in the lightness L value was suppressed by adding an easily oxidizable element. When using ESCA to analyze the elemental composition of the coated surface layer of the Mg-containing hot-dip Zn-coated thin steel sheet numbered C-7 with added Si, it was found that almost all of the Si was concentrated in the outermost surface part of the coating , and almost no Si exists inside the coating. Comparison of the ESCA analysis results of the coated steel sheet (Code C-10) with a decrease in the brightness L value in the 20-hour constant temperature and humidity test with the results of the coated steel sheet without a decrease in the brightness L value (Code C-5) , found that: Mg tends to exist in a large amount in the outermost surface layer portion of the previous plated steel sheet coating.

Table 4 includes examples of chemical treatments for water-quenched coatings (numbers C-12 through C-15). The degree of decrease in the luminance L value differs depending on whether an easily oxidizable element is added, and it can be seen that the decrease in the luminance L value is suppressed when an easily oxidizable element is added.

As mentioned above, the present invention can effectively prevent the decrease of surface gloss unique to the Mg-containing hot-dip Zn-plated thin steel sheet. Therefore, it is possible to provide a Mg-containing hot-dip Zn-plated steel sheet that is excellent in corrosion resistance and also excellent in gloss retention performance.

Claims (9)

1. one kind prepares the hot dip process Zn steel-sheet method that gloss keeps excellent performance, and it comprises:

Steel band carries out continuous impregnating and taking-up in containing the Zn base hot dipping plating bath of Al:4.0-15 quality % and Mg:1.0-4.0 quality %, form the step of coating thus on steel band, and,

Afterwards, make steel band continuously by a shrend district, cool off steel band the coating surface that solidifies is fully contacted with current in this shrend district, wherein is lower than 105 ℃ by the control steel band in the temperature of shrend district inlet side this moment, suppresses the step of coating surface glossy deterioration.

2. one kind prepares the hot dip process Zn steel-sheet method that gloss keeps excellent performance, and it comprises:

Steel band carries out continuous impregnating and taking-up in Zn base hot dipping plating bath, on steel band, form the step of coating thus, described Zn base hot dipping plating bath contains, in quality %: Al:4.0-15%, Mg:1.0-4.0%, Ti:0.001-0.1%, B:0.001-0.045% the rest is Zn and unavoidable impurities; And,

Afterwards, make steel band continuously by a shrend district, cool off steel band the coating surface that solidifies is fully contacted with current in this shrend district, wherein is lower than 105 ℃ by the control steel band in the temperature of shrend district inlet side this moment, suppresses the step of coating surface glossy deterioration.

3. one kind prepares the hot dip process Zn steel-sheet method that gloss keeps excellent performance, and it comprises:

Steel band is containing Al:4.0-15 quality % and Mg:1.0-4.0 quality %, and carry out continuous impregnating and taking-up in the Zn base hot dipping plating bath of at least a easy oxidation element that is selected from rare earth element, Y, Zr and Si that adds 0.002-0.05 quality %, on steel band, form the step of coating thus, and

Afterwards, make steel band continuously by a shrend district, cool off steel band the coating surface that solidifies is fully contacted with current in this shrend district, wherein is not less than 105 ℃ by the control steel band in the temperature of shrend district inlet side this moment, suppresses the step of coating surface glossy deterioration.

4. one kind prepares the hot dip process Zn steel-sheet method that gloss keeps excellent performance, and it comprises:

Steel band carries out continuous impregnating and taking-up in Zn base hot dipping plating bath, on steel band, form the step of coating thus, described Zn base hot dipping plating bath comprises, in quality %: Al:4.0-15%, Mg:1.0-4.0%, Ti:0.001-0.1%, B:0.001-0.045%, at least a easy oxidation element that is selected from rare earth element, Y, Zr and Si: 0.002-0.05% the rest is Zn and unavoidable impurities; And,

Afterwards, make steel band continuously by a shrend district, cool off steel band the coating surface that solidifies is fully contacted with current in this shrend district, wherein is not less than 105 ℃ but be not higher than 300 ℃ by the control steel band in the temperature of shrend district inlet side this moment, suppresses the step of coating gloss deterioration.

5. the hot dip process Zn steel-sheet method that keeps excellent performance according to any one the preparation gloss among the claim 1-4, it further comprises a step that makes the steel band that passes through the shrend district by the chemical treatment district.

6. one kind forms the hot dip process Zn steel sheet that gloss that coating obtains keeps excellent performance thereon thus by adopting a kind of hot dipping plating bath that steel band is carried out the hot dipping plating, and described plating bath comprises, in quality %:

Al：4.0-15％，

Mg：1.0-4.0％，

At least a easy oxidation element that is selected from rare earth element, Y, Zr and Si: 0.002-0.05%, and

The rest is Zn and unavoidable impurities,

Described steel sheet obtains by the method that may further comprise the steps:

Steel band carries out continuous impregnating and taking-up in described hot dipping plating bath, form the step of coating thus on steel band; And,

Afterwards, make steel band continuously by a shrend district, cool off steel band the coating surface that solidifies is fully contacted with current in this shrend district, wherein is not less than 105 ℃ by the control steel band in the temperature of shrend district inlet side this moment, suppresses the step of coating surface glossy deterioration.

7. one kind forms the hot dip process Zn steel sheet that gloss that coating obtains keeps excellent performance thereon thus by adopting a kind of hot dipping plating bath that steel band is carried out the hot dipping plating, and described plating bath comprises, in quality %:

Al：4.0-15％，

Mg：1.0-4.0％，

Ti：0.001-0.1％，

B：0.001-0.045％，

At least a easy oxidation element that is selected from rare earth element, Y, Zr and Si: 0.002-0.05%, and

The rest is Zn and unavoidable impurities,

Described steel sheet obtains by the method that may further comprise the steps:

Steel band carries out continuous impregnating and taking-up in described hot dipping plating bath, form the step of coating thus on steel band; And,

Afterwards, make steel band continuously by a shrend district, cool off steel band the coating surface that solidifies is fully contacted with current in this shrend district, wherein be not less than 105 ℃ but be not higher than 300 ℃ by the control steel band in the temperature of shrend district inlet side this moment, suppress the step of coating surface gloss deterioration.

8. one kind by adopting a kind of hot dipping plating bath that steel band is carried out the hot dipping plating, form coating thus thereon and coating is carried out chemical treatment, and the gloss that obtains keeps the hot dip process Zn steel sheet of excellent performance, and described plating bath comprises, in quality %:

Al：4.0-15％，

Mg：1.0-4.0％，

At least a easy oxidation element that is selected from rare earth element, Y, Zr and Si: 0.002-0.05%, and

The rest is Zn and unavoidable impurities,

Described steel sheet obtains by the method that may further comprise the steps:

Steel band carries out continuous impregnating and taking-up in described hot dipping plating bath, form the step of coating thus on steel band,

Afterwards, make steel band continuously by a shrend district, cool off steel band the coating surface that solidifies is fully contacted with current in this shrend district, wherein is not less than 105 ℃ by the control steel band in the temperature of shrend district inlet side this moment, suppress coating surface glossy deterioration step and

Make the step of steel band by the chemical treatment district by the shrend district.

9. one kind by adopting a kind of hot dipping plating bath that steel band is carried out the hot dipping plating, form coating thus thereon and coating is carried out chemical treatment, and the gloss that obtains keeps the hot dip process Zn steel sheet of excellent performance, and described plating bath comprises, in quality %:

Al：4.0-15％，

Mg：1.0-4.0％，

Ti：0.001-0.1％，

B：0.001-0.045％，

At least a easy oxidation element that is selected from rare earth element, Y, Zr and Si: 0.002-0.05%, and

The rest is Zn and unavoidable impurities,

Described steel sheet obtains by the method that may further comprise the steps:

Steel band carries out continuous impregnating and taking-up in described hot dipping plating bath, form the step of coating thus on steel band,

Afterwards, make steel band continuously by a shrend district, cool off steel band the coating surface that solidifies is fully contacted with current in this shrend district, wherein be not less than 105 ℃ but be not higher than 300 ℃ by the control steel band in the temperature of shrend district inlet side this moment, suppress coating surface gloss deterioration step and

Make the step of steel band by the chemical treatment district by the shrend district.

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