JP2013007071A - Al-Zn ALLOY-PLATED STEEL PLATE, AND METHOD AND DEVICE FOR MANUFACTURING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an Al-Zn alloy-plated steel plate excelling in workability and corrosion resistance without causing drastic reduction of facility life, degradation of productivity and significant cost increase.SOLUTION: A plated layer is formed by rapidly coagulating molten plating by performing rapid cooling treatment by a cooling rate ≥300°C/sec to the molten plating adhering to a surface of a steel plate pulled up from a plating pot. Thereby, a crack generated due to Si particles can be made minute by refining Si particles in a plated layer. In addition, a sacrifice anticorrosive effect of Zn can be exerted around the crack over a long term by increasing the number of layers of a Zn-rich phase by making the metal structure finer.

Description

  The present invention relates to an Al—Zn alloy-plated steel sheet excellent in workability and corrosion resistance, a manufacturing method thereof, and a manufacturing apparatus.

  Al-Zn alloy-plated steel sheet has higher corrosion resistance than Zn-plated steel sheet and the like. Among them, 55% Al-Zn alloy-plated steel sheet containing 55% by mass of Al in the plating layer is suitable for exteriors such as wall materials and roofing materials. Widely used as a material. The reason why the corrosion resistance of the Al—Zn alloy-plated steel sheet is high is that Al exhibits a coating anticorrosion effect and Zn exhibits a sacrificial anticorrosion effect.

  The plated layer of the Al—Zn alloy-plated steel sheet is brittle unless specially devised, and cracks are generated by bending or the like. Moreover, when the coating film formed on the surface of the plating layer is torn by the crack, the base steel plate is exposed and the appearance is deteriorated, and the anticorrosion effect by Al cannot be obtained. While Zn exists around the crack, corrosion of the exposed base steel sheet can be suppressed to some extent by the sacrificial anticorrosive action of Zn, but red rust occurs when Zn wears down over time.

  The brittleness of the Al—Zn alloy-plated steel sheet is due to its alloy composition and metal structure. When the hot-dip plating is attached to the base steel plate and cooled at a normal cooling rate (100 ° C./sec or lower), the hot-dip plating is supercooled to the melting point or lower while maintaining the molten state, and the Al primary crystal is dendritic (dendritic) ). Al dendrite grows by an Al—Si eutectic reaction as the first stage of solidification and subsequently by an Al—Zn eutectic reaction as the second stage. Dendritic growth proceeds by extending 6 or 8 primary arms from the solidification nucleus and extending secondary and tertiary arms branched from the primary arm. The distance between the secondary and tertiary arms depends on the cooling rate and the alloy composition of the plating. In addition, Al dendrite can be confirmed with the naked eye and exhibits a pattern like a snow crystal. This is generally called a spangle pattern and gives the plated steel sheet a unique appearance. Depending on the application, this appearance is said to have a design, and a well-organized spangle pattern may be required.

  Next to primary Al, Si particles crystallize as eutectic. At this time, since Si is discharged from the primary crystal Al toward the unsolidified liquid phase, most of the Si particles are crystallized between the Al dendrite arms. In the final stage of solidification, the Al-Zn eutectic reaction separates an Al-rich phase with a high Al concentration and a Zn-rich phase with a high Zn concentration. As described above, the plating layer of the Al—Zn alloy-plated steel sheet is composed of three phases of an Al-rich phase, a Zn-rich phase, and Si particles.

  The precipitated Si particles are plate-like at a cooling rate of 100 ° C./sec or less and are distributed in the thickness direction of the plating layer, but these Si particles are known to be brittle. On the other hand, since many fine Zn particles are precipitated in the Al-rich phase, the Al-rich phase is tougher than the Si particles. Therefore, when a tensile load is applied to the surface of the plated steel sheet, the Al-rich phase hardly deforms, the Si particles are brittlely broken, and the Si particles act as crack starting points and propagation paths that divide the plated layer. By such a mechanism, many cracks are generated in the plating layer in the process of processing the Al—Zn alloy plated steel sheet.

  As described above, Si particles serve as the starting point and propagation path of cracks, but it is essential to add Si in order to improve the workability of the plated steel sheet. That is, when the base steel sheet is immersed in hot dip plating with a relatively high Al concentration, an Fe—Al compound is formed as an alloy layer on the surface of the steel sheet. This alloy layer grows quickly, and even if the steel plate is immersed in the plating bath for several seconds, a thick alloy layer is formed, and the workability of the plated steel plate is significantly reduced. In order to prevent this, Si having an effect of suppressing the growth of the Fe—Al compound is added.

  In order to reduce the occurrence of cracks due to Si particles, several methods have been conventionally proposed. One is a method of adding an element that makes Si particles spherical, for example, Sr as described in Patent Document 1. When Sr is added and the Si particles crystallized in the solidification process are spheroidized, the Si particles become difficult to act as a crack starting point / propagation path, so that the generation of cracks can be reduced.

  Another method is a post-annealing method. For example, as described in Patent Document 2, a plated steel sheet wound in a coil shape is inserted into a batch-type heating furnace and heated and held at a temperature of 100 to 300 ° C. for several hours. This treatment promotes Ostwald growth of fine Zn particles contained in the Al-rich phase, and softens the Al-rich phase. As a result, stress concentration on the Si particles is alleviated, so that the generation of cracks due to the Si particles can be reduced. Similar contents are also described in Patent Document 3, in which the plated steel sheet is heated and held at a temperature of 80 to 235 ° C. for a predetermined time or more.

  Further, in connection with the present invention, Patent Document 4 describes that mist is sprayed on the hot-dip plating on the surface of the steel sheet and cooled to 400 ° C. or lower. Specifically, the cooling rate between 600 ° C. and 500 ° C. is set to 100 ° C./second or more, and the cooling rate between 500 ° C. to 400 ° C. is set to 50 ° C./second or more. However, Patent Document 4 is intended to refine the spangle pattern, and is essentially different from the present invention aimed at improving workability and corrosion resistance by reducing the occurrence of cracks.

JP 2007-284718 (paragraph number 0023) JP 2002-194519 A (paragraph numbers 0018 to 0019) JP 2003-213397 A (paragraph number 0037) JP 2003-013193 A (paragraph numbers 0036 to 0037)

  The method of adding Sr as in Patent Document 1 has a problem that a large amount of bottom dross is generated although there is no significant cost increase because the addition amount is only a few (several tens of ppm) relative to Si. Sr reacts with Si in the plating bath to form an intermetallic compound, which is deposited in large quantities at the bottom of the plating pot. In the production of Al-Zn alloy-plated steel sheet, a grooved induction furnace is often used for heating the plating bath. However, if bottom dross accumulates at the bottom of this grooved induction furnace, the flow path is blocked, greatly reducing the life. It becomes a factor.

  In order to perform post-annealing as in Patent Documents 2 and 3, a batch-type heating furnace is required in addition to the plating pot, and the cost increases accordingly. In addition, the amount that can be processed at once in the heating furnace is about 100 t in many cases, and since it takes a long time for heating and holding and heating and cooling before and after that, productivity is greatly reduced. Furthermore, the cost increase due to the use of city gas for heating is inevitable.

  An object of the present invention is to obtain an Al—Zn alloy-plated steel sheet excellent in workability and corrosion resistance without causing a significant reduction in equipment life, a reduction in productivity, and a significant increase in cost.

  In the plated steel sheet according to the present invention, the plating layer on the steel sheet surface includes Al: 20 to 95% by mass, Si: 0.6 to 3.0% by mass, and the balance is made of an alloy composed of Zn. It relates to an alloy-plated steel sheet. The plating layer is characterized in that it is formed by subjecting the hot dipped plating pulled up from the plating pot to a rapid cooling treatment at a cooling rate of 300 ° C./sec or more and rapidly solidifying it.

  Moreover, the manufacturing method of the plated steel plate which concerns on this invention is from the alloy by which the plating layer of the steel plate surface contains Al: 20-95 mass%, Si: 0.6-3.0 mass%, and the remainder consists of Zn. It is related with the manufacturing method of the Al-Zn alloy plating steel plate which becomes. It is characterized in that a plating layer is formed by rapidly solidifying the hot-dip plating by applying a rapid cooling treatment at a cooling rate of 300 ° C./sec or more to the hot-dip plating adhering to the surface of the steel plate pulled up from the plating pot. And

  In the manufacturing method of said plated steel plate, it is preferable to complete | finish a rapid cooling process, after rapidly_cool | quenching to below the temperature which the Al-Si eutectic reaction in hot dipping completes.

  Moreover, the apparatus for producing a plated steel sheet according to the present invention includes an alloy in which the plating layer on the steel sheet surface includes Al: 20 to 95% by mass, Si: 0.6 to 3.0% by mass, and the balance is made of Zn. It is related with the manufacturing apparatus of the Al-Zn alloy plating steel plate which becomes. Including a plating pot for receiving a hot-dip plating in which the alloy is melted, and a rapid cooling facility for performing a rapid cooling process at a cooling rate of 300 ° C./sec or higher on the hot-plating adhered to the surface of the steel sheet. Thus, a plating layer is formed.

  In the apparatus for producing a plated steel sheet, the quenching equipment can be sprayed with water mist.

  In the present invention, a plating layer is formed by rapidly solidifying the molten plating by subjecting the molten plating attached to the surface of the steel plate pulled up from the plating pot to a rapid cooling treatment at a cooling rate of 300 ° C./sec or more. To do. According to this, the Si particle in a plating layer can be refined | miniaturized and the crack which originates in Si particle | grains can be refined | miniaturized. Therefore, when the plated steel sheet is bent, the coating film formed on the surface of the plated layer is reduced from being torn by cracks, and the appearance of the bent portion can be kept good. Further, by making the cracks fine, the exposed area of the base steel sheet can be narrowed and the amount of red rust generated can be suppressed.

  As described above, the plating layer is composed of the three phases of the Al-rich phase, the Zn-rich phase, and the Si particles, and the Zn-rich phase is formed so as to fill between the dendrite arms of the Al-rich phase. is there. When the hot dipping is rapidly solidified, the metal structure is refined, the arm interval of the Al dendrite is narrowed, and the frequency with which the Al-rich phase is observed in the thickness direction of the plating layer increases. The Zn-rich phase formed so as to fill the space between the Al-rich phases is also observed more frequently. In a plated steel sheet produced by a normal method not subjected to quenching treatment, the Zn-rich phase is about 1 to 2 layers, but in a plated steel sheet subjected to quenching treatment, 2 to 4 layers of Zn-rich phase are formed. When the number of Zn-rich phases increases, the probability that a Zn-rich phase exists around the cracks increases, and the sacrificial anticorrosive effect of Zn is exerted around the cracks over a long period of time. Therefore, generation | occurrence | production of red rust in the exposed part of the base steel plate by a crack can be suppressed over a long period of time.

  As described above, in the present invention, improvement in workability of the plated steel sheet by refining cracks and improvement in corrosion resistance in the exposed portion of the base steel sheet due to cracks are realized by performing a rapid cooling treatment. According to this, unlike the method of adding Sr in Patent Document 1, the life of the grooved induction furnace is not shortened due to the generation of bottom dross. In addition, since the rapid cooling process can be performed at low cost and in a short time by spraying mist or the like, there is no need to provide a batch-type heating furnace as in the post-annealing process of Patent Documents 2 and 3, and productivity is reduced. There is no significant cost increase. According to the present invention, an Al—Zn alloy-plated steel sheet excellent in workability and corrosion resistance can be obtained without significantly shortening the equipment life, reducing the productivity, and significantly increasing the cost.

  The reason why the cooling rate is set to 300 ° C./sec or more is that when the cooling rate is lower than this, coarse Si particles are crystallized, and the effect of the present invention cannot be obtained. The cooling rate is more preferably 360 ° C./sec or more.

  When the rapid cooling treatment is performed until the temperature reaches the temperature at which the Al—Si eutectic reaction is completed or less, the hot dipping can be solidified at once, and the metal structure of the plating layer can be refined more reliably. Therefore, an Al—Zn alloy plated steel sheet excellent in workability and corrosion resistance can be obtained more reliably. Moreover, if the rapid cooling process is terminated after the rapid cooling to a temperature at which the Al—Si eutectic reaction is completed or less, the cost required for the rapid cooling process can be reduced. Even if the rapid cooling treatment is performed until the temperature is significantly lower than the temperature at which the Al—Si eutectic reaction is completed, the effect of refinement of the metal structure is saturated, but the cost required for the rapid cooling treatment is increased, which is not preferable.

  If the quenching equipment is to spray water with a mist, the quenching equipment can be simplified and the cost can be reduced, and the high cooling heat of water can be used to easily achieve the required cooling rate. Obtainable.

It is a schematic block diagram of the continuous hot dipping equipment which concerns on this invention. It is an enlarged photograph of the section of the plating layer of a plating steel plate concerning a comparative example (left side) and an example (right side). It is a graph which shows the average width of a crack with respect to the bending degree of a plated steel plate which concerns on a comparative example and an Example. It is an enlarged photograph of the surface of the plating layer in the bending process part of a plated steel plate which concerns on a comparative example (left side) and an Example (right side). It is the photograph which image | photographed after the wet test the surface of the bending process part of a plated steel plate which concerns on a comparative example (left side) and an Example (right side).

  Next, a method for producing an Al—Zn alloy plated steel sheet (hereinafter simply referred to as a plated steel sheet) according to the present invention will be specifically described with reference to the drawings. As shown in FIG. 1, the continuous hot dipping apparatus includes a plating pot 1 that holds a plating bath, a metal roll 2 installed in the plating pot 1, and a pair of gas wiping nozzles 3 installed above the plating pot 1. The cooling system 4 is installed above the gas wiping nozzle 3 and the forced air cooling system 5 is installed above the quenching system 4.

  As the steel strip S that passes through the continuous hot dipping equipment, pickled hot rolled steel sheets or cold rolled steel sheets that are cold rolled to a predetermined thickness after pickling are applied. Before being immersed in the plating pot 1, the steel strip S is heated in an annealing furnace in a hydrogen / nitrogen mixed atmosphere and heat-treated so as to become a predetermined material, and the surface oxide is reduced by hydrogen or the like. And dirt are removed. The steel strip S immersed in the plating pot 1 through the annealing furnace is turned upward by the metal roll 2. The gas wiping nozzle 3 blows gas to the steel strip S immediately after rising from the plating bath to adjust the amount of hot-dip plating.

  The steel strip S that has passed through the gas wiping nozzle 3 is subjected to a quenching process in the quenching facility 4. The timing of starting the rapid cooling process depends on the temperature of the plating bath, but needs to be within 2 seconds and at most 5 seconds after the steel strip S passes the gas wiping nozzle 3. If the start is delayed more than this, the temperature of the steel strip S decreases due to the heat release from the steel strip S, the Al-Si eutectic reaction proceeds, and coarse Si particles may crystallize in the plating layer. It is. The distance from the gas wiping nozzle 3 to the rapid cooling equipment 4 is set according to the conveyance speed of the steel strip S so that the rapid cooling process can be started early. For example, when the conveyance speed is 130 m / min, if the distance from the gas wiping nozzle 3 to the quenching equipment 4 is set within about 10 m, the quenching process can be started within 5 seconds after passing through the gas wiping nozzle 3. it can.

  As the quenching equipment 4, a mist spraying device, an air-water spraying device, a metal particle spraying device, or the like can be applied. Hereinafter, a mist spraying device that sprays water by mist will be described as an example. When mist is sprayed on the hot dipped plating adhered to the surface of the steel strip S, the hot dipping and the steel strip S are rapidly cooled by the endothermic action when water is evaporated. That is, the amount of heat removal from the hot dip plating 4 and the steel strip S in the quenching equipment 4 is determined by the amount of latent heat (evaporation heat) of the mist evaporated in the quenching equipment 4. When the dew point temperature in the quenching equipment 4 rises due to the evaporation of water, the evaporation rate of water decreases and the cooling rate of the steel strip S decreases, so it is necessary to exhaust the steam to avoid this. As an exhaust method, there are a method of installing a blower that blows outside air into the quenching facility 4, a method of installing a blower that sucks out the humid air in the quenching facility 4, and these two methods may be used in combination.

  What is sprayed by the quenching equipment 4 is a minute mist having an average particle diameter of 30 μm or less. This is because if the particle diameter of the mist is larger than this, a satin-like surface appearance defect called a water drop mark may occur on the surface of the plating layer. As the nozzle of the mist spraying device, any nozzle can be used as long as it can realize an average particle diameter of 30 μm or less. For example, a commercially available one-fluid nozzle or two-fluid nozzle can be applied.

  The amount of water per unit time sprayed by the quenching equipment 4 is the temperature of the steel strip S when entering the quenching equipment 4, the dimensions (thickness / width) of the steel strip S, the transport speed of the steel strip S, and the required cooling rate. And the temperature after cooling. If the mist is sprayed excessively, mists collide with each other and coarse water particles are likely to be generated, causing the above-mentioned water drop mark to be generated, and causing the water to scatter to the surrounding equipment and causing a failure. It is better to avoid it. The appropriate amount of water varies depending on the structure of the quenching facility 4 and therefore needs to be determined by experiment.

  In order to uniformly cool the steel strip S in the width direction in the quenching equipment 4, the variation in the spray amount of mist in the width direction is adjusted to within 10%. If there is a larger variation than this, the shape of the steel strip S may collapse due to thermal stress in the steel strip S. In order to sufficiently cool the edge portion of the steel strip S, the mist is sprayed with a width wider than that of the steel strip S.

  The nozzle header 4a of the mist spraying device is arranged so as to sandwich the steel strip S from both sides. At this time, it is not necessary to arrange the nozzle header 4a on one side and the other side of the steel strip S symmetrically with respect to the steel strip S. In particular, at the edge portion of the steel strip S, mists sprayed outward from the edge portion collide with each other to form coarse water particles, and water droplet marks are likely to be generated. In order to avoid this, the nozzle headers 4a may be arranged in a staggered manner with the nozzle headers 4a being vertically displaced.

  The steel strip S subjected to the rapid cooling process in the rapid cooling facility 4 is further cooled in the forced air cooling facility 5 as necessary. The forced air cooling equipment 5 cools the steel strip S by blowing air, and the cooling rate is about 10 to 100 ° C./sec. When sufficient cooling is performed by the rapid cooling facility 4, the forced air cooling facility 5 may be omitted. The steel strip S that has passed through the forced air cooling facility 5 is subjected to various treatments by a plating adhesion meter, a skin pass mill, a tension leveler, a chemical conversion treatment facility, and the like, and is then wound into a coil shape.

<Example 1> Next, Example 1 of this invention is shown. Here, the steel strip S that had been subjected to a predetermined heat treatment in an annealing furnace was immersed in the plating pot 1, and the molten plating was adhered to the surface of the steel strip S. Next, after adjusting the amount of plating applied by the gas wiping nozzle 3, the steel strip S is passed through the quenching equipment 4 and subjected to quenching treatment, and then subjected to the various treatments described above before being wound in a coil shape. I took it. Thereafter, an acrylic paint was applied to the surface of the plating layer to obtain a final product.

  The plating bath of the plating pot 1 contains 55% by mass of Al, 1.6% by mass of Si, and melts an alloy made of Zn with the balance being kept at 600 ° C. The melting point of this alloy is about 580 ° C. The plate thickness of the steel strip S was 0.35 mm, the plate width was 914 mm, and the conveyance speed of the steel strip S was set to 130 m / min.

  The rapid cooling equipment 4 was constituted by a mist spraying device that mists water to spray it. This mist spraying device can spray a maximum of 50 l / min of mist. The nozzle header 4a on one side and the other side of the steel strip S is shifted by 1 m in the conveying direction of the steel strip S in order to avoid collision of mists sprayed outside the edge portion and forming coarse water particles. Are arranged. A slit for taking outside air into the quenching equipment 4 is provided around the nozzle header 4a, and a blower is installed outside the slit.

  The quenching equipment 4 was installed at a position 2 m above the gas wiping nozzle 3. Since the conveying speed of the steel strip S is 130 m / min, the steel strip S reaches the quenching equipment 4 in about 1 second after passing through the gas wiping nozzle 3 and is subjected to quenching treatment. The temperature of the steel strip S immediately before entering the quenching facility 4 was 570 ° C. The cooling rate in the rapid cooling equipment 4 was set to 360 ° C./sec, and the amount of mist sprayed was set accordingly. The steel strip S passes through the quenching equipment 4 in 0.5 seconds. In other words, the steel strip S is rapidly cooled for 0.5 seconds. The temperature of the steel strip S that passed through the quenching equipment 4 was 390 ° C., which was almost equal to 380 ° C., which is the temperature at which the Al—Si eutectic reaction was completed.

Example 2 A plated steel sheet was formed under the same conditions as in Example 1 except that the amount of mist sprayed in the quenching facility 4 was 120% of Example 1. The cooling rate in the quenching equipment 4 was 420 ° C./sec, and the temperature of the steel strip S that passed through the quenching equipment 4 was 360 ° C.

Example 3 A plated steel sheet was formed under the same conditions as in Example 1 except that the amount of mist sprayed in the rapid cooling facility 4 was 80% of Example 1. The cooling rate in the quenching equipment 4 was 300 ° C./sec, and the temperature of the steel strip S that passed through the quenching equipment 4 was 420 ° C.

Comparative Example 1 A plated steel sheet was formed under the same conditions as in Example 1 except that the amount of mist sprayed in the quenching facility 4 was 60% of that in Example 1. The cooling rate in the quenching equipment 4 was 200 ° C./sec, and the temperature of the steel strip S that passed through the quenching equipment 4 was 470 ° C.

<Comparative example 2> The plated steel plate was formed on the same conditions as Example 1 except the amount of mist spraying in the rapid cooling equipment 4 having been 20% of Example 1. The cooling rate in the quenching equipment 4 was 40 ° C./sec, and the temperature of the steel strip S that passed through the quenching equipment 4 was 550 ° C.

Comparative Example 3 A plated steel sheet was formed under the same conditions as in Example 1 except that the amount of mist sprayed in the rapid cooling equipment 4 was 5% of Example 1. The cooling rate in the quenching equipment 4 was 20 ° C./sec, and the temperature of the steel strip S that passed through the quenching equipment 4 was 560 ° C.

<Comparative example 4> The plated steel plate was formed on the same conditions as Example 1 except not spraying mist in the rapid cooling equipment 4. FIG.

  Table 1 shows the performance of the plated steel sheets of Examples 1 to 3 and Comparative Examples 1 to 4. Based on the performance of the plated steel sheet of Comparative Example 4, the workability and corrosion resistance in the table were evaluated as Δ if there was no superior difference, ○ if it was excellent, and ◎ if it was more excellent.

<Observation of metal structure>
The plated layer of the plated steel sheet of each Example and Comparative Example was confirmed with a scanning electron microscope (hereinafter referred to as SEM). Specifically, the plated steel sheet was cut and mechanically polished, then precisely polished with an ion beam, and the cross section of the plated layer was observed with an SEM. The SEM photograph of the cross section of the plating layer which concerns on the comparative example 4 and Example 1 is shown in FIG. In the SEM photograph according to Comparative Example 4 on the left side, reference numeral 6 indicates Si particles, reference numeral 7 indicates a Zn-rich phase, and reference numeral 8 indicates an Al-rich phase. As is clear from this, coarse Si particles were crystallized in the plating layer according to Comparative Example 4, and the Zn-rich phases that could be confirmed were 1 to 2 layers in the thickness direction of the plating layer. Comparative Examples 1 to 3 were the same results as Comparative Example 4. On the other hand, coarse S particles were not seen in the SEM photograph according to Example 1 on the right side of FIG. 2, and it was confirmed that the Zn-rich phase had 2 to 4 layers. Similar results were confirmed in Examples 2 and 3.

<Evaluation of workability> The plated steel plate before applying the paint was bent along the reference line, and the workability of the plated steel plate was evaluated based on the size of cracks generated in the plating layer on the surface of the bent portion. Specifically, five test lines having a length of 1 mm perpendicular to the reference line were drawn in a visual field range of 3 mm × 2 mm, and the average width of the cracks was calculated from the number and size of cracks crossing the test line. The SEM was used for the observation of cracks, and this was performed in five fields of view. Moreover, said measurement was performed in the some bending process part from which a bending process degree differs.

  FIG. 3 is a graph summarizing the measurement results for the plated steel sheets of Example 1 and Comparative Example 3. In each bending degree, the plated steel sheet according to Example 1 has a smaller value of the average crack width. ing. In addition, the symbol of 0T-5T which concerns on a bending process degree has shown the magnitude | size of the curvature of a bending process part, and a curvature is so small that a number is large. FIG. 4 is an SEM photograph of the surface of the plating layer in the bent portion, and shows that the crack in Example 1 on the right side was smaller than that in Comparative Example 3 on the left side.

  Regarding the size of the crack generated in the bent portion, in Comparative Examples 1 to 3, no improvement was observed as compared with Comparative Example 4 in which mist was not sprayed. On the other hand, in Examples 1-3, a crack became small compared with each comparative example, and it has confirmed that the plated steel plate concerning Examples 1-3 was excellent in workability. The reason why the cracks are small and excellent in workability is considered to be because brittle and coarse Si particles are not seen in the plating layer (see FIG. 2).

<Evaluation of Corrosion Resistance> Here, a wet test was performed on the coated steel sheet that was coated and bent to evaluate the corrosion resistance of the bent portion. Specifically, four bending portions having a bending degree of 0T, 2T, 3T, and 4T (see FIG. 3) were formed on the plated steel sheet. The conditions of the wet test were in accordance with JIS-K2246, and exposed to an atmosphere having a temperature of 50 ° C. and a relative humidity of 95% for 500 hours, and the occurrence of red rust in each bent portion was observed. FIG. 5 is a photograph of the surface of the bent portion of the plated steel sheet after the wet test, and it can be confirmed that the amount of red rust is less in Example 1 on the right side than Comparative Example 4 on the left side.

  Regarding the amount of red rust generated after the wet test, in Comparative Examples 1 to 3, no improvement was observed as compared to Comparative Example 4 in which mist was not sprayed. On the other hand, in Examples 1-3, generation | occurrence | production amount of red rust decreased compared with each comparative example, and it has confirmed that the plated steel plate which concerns on Examples 1-3 was excellent in corrosion resistance. The excellent corrosion resistance is because the cracks are small and the base steel sheet is less exposed (see FIG. 4). Furthermore, as a result of the formation of 2 to 4 layers of Zn-rich phase, the sacrificial anticorrosive effect of Zn around the cracks over a long period of time. This is considered to be demonstrated.

  As shown in Table 1, the plated steel sheets according to Examples 1 and 2 exhibited further workability and corrosion resistance superior to Example 3. The cooling rate in Examples 1 and 2 is larger than that in Example 3, and the temperature of the steel strip S after the rapid cooling treatment in Examples 1 and 2 is the temperature at which the Al—Si eutectic reaction is completed (380 ° C.). The reason may be that it is almost equal to or less than.

  As is clear from the above evaluation, when a large amount of mist is sprayed with the quenching equipment 4 to increase the cooling rate, a plated steel sheet having excellent workability and corrosion resistance can be obtained. Since the rapid cooling equipment 4 is only for mist-forming and spraying water, the cost increase by introducing this is small. The present invention is suitable as a method for improving the workability and corrosion resistance of a plated steel sheet without a large cost increase.

DESCRIPTION OF SYMBOLS 1 Plating pot 2 Metal roll 3 Gas wiping nozzle 4 Rapid cooling equipment 4a Nozzle header 5 Forced air cooling equipment 6 Si particle 7 Zn rich phase 8 Al rich phase S Steel strip

Claims (5)

The plated layer on the surface of the steel sheet is an Al—Zn alloy plated steel sheet made of an alloy containing Al: 20 to 95% by mass, Si: 0.6 to 3.0% by mass, and the balance being composed of Zn,
The Al—Zn alloy plating is characterized in that the plating layer is formed by subjecting the hot-dip plating pulled up from the plating pot to a rapid cooling treatment at a cooling rate of 300 ° C./sec or more and rapidly solidifying it. steel sheet. The method for producing an Al—Zn alloy-plated steel sheet, comprising a plated layer on the surface of the steel sheet, comprising Al: 20 to 95 mass%, Si: 0.6 to 3.0 mass%, and the balance being composed of Zn. And
It is characterized in that a plating layer is formed by rapidly solidifying the hot-dip plating by applying a rapid cooling treatment at a cooling rate of 300 ° C./sec or more to the hot-dip plating adhering to the surface of the steel plate pulled up from the plating pot. A method for producing an Al—Zn alloy-plated steel sheet.   The method for producing an Al-Zn alloy-plated steel sheet according to claim 2, wherein the quenching treatment is terminated after quenching to a temperature below the temperature at which the Al-Si eutectic reaction in hot-dipping is completed. An apparatus for producing an Al—Zn alloy-plated steel sheet, in which the plating layer on the surface of the steel sheet contains Al: 20 to 95 mass%, Si: 0.6 to 3.0 mass%, and the balance is composed of Zn. And
A plating pot for receiving a hot dipped plating in which the alloy is melted, and a rapid cooling facility for performing a rapid cooling process at a cooling rate of 300 ° C./sec or higher for the hot dipping adhered to the steel sheet surface,
An apparatus for producing an Al-Zn alloy-plated steel sheet, wherein a plating layer is formed by rapidly solidifying hot-dip plating.   The apparatus for producing an Al—Zn alloy-plated steel sheet according to claim 4, wherein the rapid cooling equipment mists and sprays water.