TW202037736A - Zinc-coated steel sheet and method of forming the same - Google Patents

Abstract

The present invention relates to a zinc-coated steel sheet and method of forming the same. In the method, a steel sheet is dipped in the zinc bath containing specific amounts of aluminum and magnesium during a hot dip coating step to obtain a zinc-coated steel sheet. The zinc coating of the zinc-coated steel sheet with specific crystalline phases and specification has high hardness and high corrosion resistance ability. Therefore, the zinc-coated steel sheet can be applied in household electric appliances, building materials and cars.

Description

Galvanized steel sheet and manufacturing method thereof

The invention relates to a galvanized steel sheet and a manufacturing method thereof, in particular to a galvanized steel sheet with a coating with high hardness and high corrosion resistance and a manufacturing method thereof.

In order to improve the corrosion resistance of the steel sheet, a layer of zinc metal coating is currently applied to the surface of the steel sheet by hot-dip plating. According to the different hot-dip plating process, the formed coating can be divided into a hot-dip galvanizing (GI) coating and a hot-dip galvanizing (GA) coating.

The smooth surface of GI coating makes it difficult to paint the steel sheet after hot-dip coating. In addition, due to the too soft hardness of the GI coating, during the stamping process, the coating is easily separated from the steel sheet substrate in a large area to form dezincification. Dezincification defects will increase the cleaning frequency of stamping molds, reduce production efficiency, and damage the molds and cause scratches on the surface of the stamping workpiece. In addition, a too soft coating will cause the galvanized steel sheet to generate higher friction with the stamping die during the stamping process, making the surface of the galvanized steel sheet prone to napping defects.

In order to improve the shortcomings of the GI coating that is not easy to paint, generally after the hot-dip plating treatment, the steel sheet is subjected to alloying annealing treatment to obtain the GA coating. Although the surface of the GA coating is rougher and the paintability is improved, however, the GA coating has too hard hardness and therefore does not have good formability, resulting in the coating being easily crushed and peeled locally and evenly during the stamping process, and dezincification will also occur.

In view of this, it is urgent to provide a galvanized steel sheet and a manufacturing method thereof to solve the above-mentioned problems.

Therefore, one aspect of the present invention is to provide a method for manufacturing a galvanized steel sheet by placing the steel sheet in a zinc bath for a hot-dip plating step, and the zinc bath contains a specific content of aluminum and magnesium to obtain Galvanized steel sheet with high hardness and high corrosion resistance.

Another aspect of the present invention is to provide a galvanized steel sheet, which is prepared by the above-mentioned method. The coating of the obtained galvanized steel sheet contains a specific crystal phase and specific specifications, and has high hardness and high corrosion resistance. It can be applied to home appliances, building materials and automobiles.

According to the above aspect of the present invention, a method for manufacturing a galvanized steel sheet is provided. First, a steel sheet is provided, wherein the steel sheet contains: 0.05 wt% (wt%) to 0.5 wt% carbon, 0.1 wt% to 3.0 wt% manganese, 0.01 wt% to 1.2 wt% chromium, 0.1 wt% to 1.2 wt% silicon, 0.0005wt% to 0.01wt% boron, and the balance is iron and unavoidable impurities. Next, an annealing step is performed on the steel sheet to obtain an annealed steel sheet. Then, the annealed steel sheet is immersed in a zinc bath for a hot-dip plating step to obtain a hot-dip galvanized steel sheet, where the zinc bath contains 1.0wt% to 4.0wt% aluminum, 0.5wt% to 4.0wt% magnesium and the rest The amount is zinc and unavoidable impurities. Next, A cooling step is performed on the hot-dip galvanized steel sheet to obtain a galvanized steel sheet, wherein the galvanized steel sheet has a zinc-aluminum-magnesium coating.

According to an embodiment of the present invention, the aforementioned steel sheet is a hot-rolled steel sheet or a cold-rolled steel sheet.

According to an embodiment of the present invention, the aforementioned steel sheet further contains less than 0.22wt% titanium, less than 0.12wt% aluminum, less than 0.12wt% phosphorus, and less than 0.06wt% sulfur.

According to an embodiment of the present invention, the above-mentioned annealing step is performed at an annealing temperature of 500° C. to 900° C. for 20 seconds to 300 seconds.

According to an embodiment of the present invention, the temperature of the aforementioned zinc bath is 400°C to 550°C.

According to an embodiment of the present invention, the cooling rate of the aforementioned cooling step is 2° C./s to 50° C./s.

According to another aspect of the present invention, a galvanized steel sheet is provided, which is produced by the above-mentioned manufacturing method of galvanized steel sheet, wherein the surface of the zinc-aluminum-magnesium coating contains Zn-Al- with an area ratio greater than 60% MgZn ₂ layered eutectic structure phase, aluminum-rich phase with an area ratio greater than 0.2%, and the remainder are primary crystal zinc phases.

According to an embodiment of the present invention, the interlayer spacing of the Zn-Al-MgZn ₂ layered eutectic structure phase is less than 1000 nm.

According to an embodiment of the present invention, the grain size of the above-mentioned primary zinc phase is less than 3500 nm.

According to an embodiment of the present invention, the above-mentioned zinc-aluminum-magnesium coating contains 1.0 wt% to 4.5 wt% aluminum and 0.5 wt% to 4.5 wt% magnesium.

Applying the galvanized steel sheet of the present invention, the steel sheet is placed in a zinc bath for hot-dip plating, and the zinc bath contains a specific content of aluminum and magnesium to obtain a galvanized steel sheet, and the coating contains a specific crystal Compared with its specific specifications, it has high hardness and high corrosion resistance, and can be used in home appliances, building materials and automobiles.

100‧‧‧Method

110‧‧‧Provide steel sheet

120‧‧‧The steel sheet is annealed to obtain annealed steel sheet

130‧‧‧Hot-dip the annealed steel sheet to obtain the hot-dipped steel sheet

140‧‧‧Cooling step for hot-dipped steel sheet

150‧‧‧Get galvanized steel sheet

In order to make the above and other objects, features, advantages and embodiments of the present invention more comprehensible, the detailed description of the accompanying drawings is as follows: [FIG. 1] is a drawing of a galvanized steel sheet according to an embodiment of the present invention Flow chart of manufacturing method.

Based on the above, the present invention provides a galvanized steel sheet and a manufacturing method thereof. It is to place the steel sheet in a zinc bath for hot-dip plating, and the zinc bath contains a specific proportion of aluminum and magnesium to obtain a galvanized steel sheet. The coating contains a specific crystal phase and a specific specification. Hardness and high corrosion resistance, can be used in home appliances, building materials and automobiles.

Please refer to FIG. 1, which shows a flowchart of a method 100 for manufacturing a galvanized steel sheet according to an embodiment of the present invention. First, as shown in step 110 of the method 100, a steel sheet is provided. The steel sheet here can be selected as hot-rolled steel sheet or cold-rolled steel sheet. The steel sheet contains 0.05 wt% (wt%) to 0.5wt% carbon, 0.1wt% to 3.0wt% manganese, 0.01wt% to 1.2wt% chromium, 0.1wt% to 1.2wt% silicon, 0.0005wt % To 0.01wt% boron, and the remainder is iron and unavoidable impurities.

In the foregoing embodiment, the carbon content of the steel sheet is 0.05 wt% to 0.5 wt%. Carbon can adjust the strength, hardness, wear resistance, toughness and ductility of the steel sheet. When the carbon content is greater than 0.5wt%, the toughness and ductility of the steel sheet will be reduced, making the steel sheet difficult to process. When the carbon content is less than 0.05wt%, the hardness and wear resistance of the steel sheet are too low, which affects the strength of the steel sheet.

In the above embodiment, the manganese content of the steel sheet is preferably 0.1 wt% to 3.0 wt%. Generally speaking, manganese can increase the hardening energy of steel sheets and reduce the quenching temperature. And can slow down the metamorphosis speed of the steel sheet. However, if the content of manganese in the steel sheet is greater than 3.0wt%, more oxide layers will be formed, which will affect the adhesion of the coating during the subsequent hot-dip plating step. Conversely, if the manganese content in the steel sheet is less than 0.1wt%, impurities such as sulfur oxide cannot be effectively removed, and steel sheets with high strength and high elongation cannot be obtained.

In the above embodiment, the chromium content of the steel sheet is 0.01 wt% to 1.2 wt%. Chromium can adjust the acid resistance, heat resistance, hardness and wear resistance of the steel sheet, and can improve the hardenability of the steel sheet, so that the steel sheet can have better mechanical properties in the subsequent quenching process and tempering process. If the chromium content of the steel sheet is greater than 1.2wt%, the steel sheet has greater hardening capacity, weaker toughness, and hardening occurs after heat treatment. If the chromium content of the steel sheet is less than 0.01 wt%, the ductility of the steel sheet is poor.

In the above embodiment, the silicon content of the steel sheet is preferably 0.1 wt% to 1.2 wt%. Silicon is beneficial to increase the elastic limit of the steel sheet, and silicon has the function of deacidification and deoxidation, preventing the generation of pores and increasing the strength of the steel sheet. Only when the silicon content in the steel sheet is greater than 1.2wt%, it will increase the temper brittleness and sensitivity of the steel sheet Sex, and reduce plasticity. In addition, when the silicon content in the steel sheet is less than 0.1wt%, the elasticity and magnetic permeability of the steel sheet are insufficient.

In the above embodiment, the boron content of the steel sheet is 0.0005 wt% to 0.01 wt%. Adding boron to the steel sheet can improve the hardenability of the steel sheet, thereby improving the comprehensive mechanical properties of the steel sheet. When the boron content is greater than 0.01 wt%, the cost of boron is high, resulting in excessively high manufacturing costs. When the boron content is less than 0.0005wt%, the strength of the steel sheet cannot be effectively improved.

In an embodiment, the steel sheet may optionally contain less than 0.22wt% titanium. Titanium has the function of deacidification and deoxidation, which can increase the strength of steel sheet. In one embodiment, the steel sheet may optionally contain less than 0.12wt% aluminum. Aluminum is a powerful deoxidizer, a small amount of aluminum can promote the graphitization of the steel sheet and increase the strength of the steel sheet. In an embodiment, the steel sheet may optionally contain less than 0.12wt% phosphorus. Phosphorus can improve the machinability of steel, and a small amount of phosphorus increases the corrosion resistance of steel. In an embodiment, the steel sheet may optionally contain less than 0.06 wt% sulfur. Sulfur can improve the machinability of steel, and a small amount of sulfur can increase the lubrication effect during cutting.

Next, as shown in step 120 of the method 100, an annealing step is performed on the steel sheet to obtain an annealed steel sheet. The annealing step can improve the workability of the steel sheet. In one embodiment, the annealing step is to heat the steel sheet to an annealing temperature of 500° C. to 900° C. for 20 seconds to 300 seconds.

Then, as shown in step 130 of the method 100, the annealed steel sheet is hot-dipped to obtain a hot-dip coated steel sheet. In the hot-dip coating step, the annealed steel sheet is placed in a zinc bath containing molten zinc, thereby coating the surface of the annealed steel sheet with a coating. Among them, the composition of the zinc bath affects the coating properties of the subsequently formed coating (such as the hardness and corrosion resistance of the coating).

In this embodiment, the zinc bath contains 1.0wt% to 4.0wt% of aluminum, 0.5wt% to 4.0wt% of magnesium, and the remaining amount of zinc and unavoidable impurities, thereby adjusting the specifications of the subsequently formed plating layer to The hardness and corrosion resistance of the coating are optimized. The aforementioned coating structure can be, for example, a Zn-Al-MgZn ₂ layered eutectic structure phase, an aluminum-rich phase, and a primary zinc phase. When the concentration of aluminum or magnesium in the zinc bath is higher than the foregoing range, in addition to the poor properties of the foregoing coating, the temperature of the molten zinc solution is also increased, which is not conducive to the operation of the manufacturing site. When the aluminum concentration or magnesium concentration in the zinc bath is lower than the aforementioned range, the Zn-Al-MgZn ₂ layered eutectic structure phase, aluminum-rich phase, and primary zinc phase structure of specific specifications cannot be formed. In one embodiment, the temperature of the zinc bath is 400°C to 550°C.

Thereafter, as shown in step 140 of the method 100, a cooling step is performed on the hot-dipped steel sheet. Generally speaking, the cooling rate of the cooling step affects the hardness of the coating. The faster the cooling rate, the finer the crystal phase of the coating structure formed, and the higher the hardness of the coating. In one embodiment, the cooling rate of the cooling step is 2° C./s to 50° C./s.

Next, as shown in step 150 of the method 100, a galvanized steel sheet is obtained. The galvanized steel sheet obtained by the above-mentioned manufacturing method 100 has a zinc-aluminum-magnesium coating.

In one embodiment, the surface of the galvanized steel sheet obtained by the foregoing manufacturing method includes a specific crystal phase and a specific specification. The specific crystal phase referred to herein can be, for example, a Zn-Al-MgZn ₂ layered eutectic structure phase, an aluminum-rich phase, a primary zinc phase, or any combination of the above. The specific specifications referred to herein can be, for example, specific components corresponding to the area ratio, interlayer spacing, and crystal grain size of the coating surface. In this embodiment, the zinc-aluminum-magnesium coating of the galvanized steel sheet obtained by the foregoing manufacturing method contains a Zn-Al-MgZn ₂ layered eutectic phase with an area ratio greater than 60% and an area ratio greater than 0.2%. The aluminum-rich phase and the remainder are the primary zinc phase. The Zn-Al-MgZn ₂ layered eutectic structure phase and aluminum-rich phase can improve the hardness and corrosion resistance of the coating. In one embodiment, the interlayer spacing of the Zn-Al-MgZn ₂ layered eutectic structure phase is less than 1000 nm, preferably 350 nm to 1000 nm. In one embodiment, the grain size of the primary zinc phase is less than 3500 nm.

In the above embodiments, such as specific crystal phases (Zn-Al-MgZn ₂ layered eutectic structure phase, aluminum-rich phase and primary zinc phase) and specific specifications (area ratio of the coating surface, interlayer spacing and crystal If the grain size falls outside the above range, the hardness of the coating will be too hard or too soft, which will cause dezincification defects in the subsequent stamping process and the corrosion resistance of the coating will be insufficient.

In one embodiment, the zinc-aluminum-magnesium coating of the galvanized steel sheet includes 1.0 wt% to 4.5 wt% aluminum and 0.5 wt% to 4.5 wt% magnesium.

In one embodiment, the coating of the galvanized steel sheet obtained above has high hardness and high corrosion resistance, and can be applied to household appliances, building materials and automobiles.

Several embodiments are used below to illustrate the application of the present invention, but they are not used to limit the present invention. Those with ordinary knowledge in the technical field of the present invention can make various changes and modifications without departing from the spirit and scope of the present invention. Retouch.

Examples 1 to 6

First, a cold-rolled steel sheet is provided, which contains 0.05wt% to 0.5wt% carbon, 0.1wt% to 3wt% manganese, 0.01wt% to 1.2wt% chromium, and 0.1wt% to 1.2wt% silicon , 0.0005wt% To 0.01wt% of boron, less than 0.22wt% of titanium, less than 0.12wt% of aluminum, less than 0.12wt% of phosphorus, less than 0.06wt% of sulfur, and the remainder is iron and unavoidable impurities. Then, the cold-rolled steel sheet is subjected to a continuous annealing step for 20 seconds to 300 seconds at an average annealing temperature of 500° C. to 900° C. to obtain an annealed steel sheet. Then, the annealed steel sheet is subjected to a hot dip coating step in a zinc bath at 400° C. to 550° C. to obtain a hot dip coated steel sheet. As shown in Table 1, this zinc bath contains 1.0wt% to 4.0wt% aluminum, 0.5wt% to 4.0wt% magnesium, and the remainder is zinc and unavoidable impurities. Afterwards, the hot-dip galvanized steel sheet is subjected to a cooling step at a cooling rate of 2° C./s to 50° C./s to form a hot-dip galvanized (GI) coating to obtain a galvanized steel sheet with a zinc-aluminum-magnesium coating.

The thickness of the coating layer of the above-mentioned galvanized steel sheet is 7.8 microns (μm) to 13.2 μm.

Comparative examples 1 to 2

Comparative Examples 1 to 2 are prepared by the same manufacturing method as Examples 1 to 6, except that the zinc bath in the hot-dip coating step contains aluminum, zinc and inevitable impurities, but does not contain magnesium (such as Table 1). The coating thickness of the galvanized steel sheets of Comparative Examples 1 to 2 was 7.8 μm to 12.6 μm.

Comparative example 3

Comparative Example 3 is the same manufacturing method as Comparative Examples 1 to 2 for preparing galvanized steel sheets, except that after the hot-dip coating step and before the cooling step, an alloying annealing treatment step is performed to form a hot-dip zinc alloy化 (GA) plating. Through the alloying annealing process, the iron atoms of the hot-dipped steel sheet can be diffused into the coating to form a zinc-iron phase. The coating thickness of the galvanized steel sheet of Comparative Example 3 is the same as that of Comparative Examples 1 to 2.

Crystal phase and parameters of the coating

Table 1 shows that the plating layers of Examples 1 to 6 contain 1.7wt% to 3.4wt% aluminum and 0.7wt% to 3.1wt% magnesium, while the plating layers of Comparative Examples 1 to 3 contain 0.12wt% to 0.19wt% Of aluminum, and none of them contain magnesium.

In addition, the microstructure of the zinc-aluminum-magnesium coating of the galvanized steel sheets of Examples 1 to 6 contains 68.7% to 82.7% of the Zn-Al-MgZn ₂ layered eutectic phase and 0.2% to 0.6% of the aluminum-rich phase The balance is the primary zinc phase. And the interlayer spacing of the Zn-Al-MgZn ₂ layered eutectic phase is 580nm to 700nm, and the crystal grain size of the primary zinc phase is 1680nm to 2200nm. The coating microstructures of the galvanized steel sheets of Comparative Examples 1 to 3 do not contain the Zn-Al-MgZn ₂ layered eutectic structure phase and the aluminum-rich phase structure.

Hardness measurement

Utilize the Vicker hardness in the American Society for Testing and Materials (ASTM) standard number E384 "Standard Test Method for Microindentation Hardness of Materials" (Standard Test Method for Microindentation Hardness of Materials) The micro-hardness test method is to measure the surface hardness of galvanized steel sheets. It is to press an indenter with a square pyramid with a diagonal of 136° into the surface of the hot-dip galvanized steel sheet. After maintaining the load for a certain period of time, remove the load and measure the diagonal length of the square indentation on the surface of the material. In this embodiment, the measured load is 15 grams to 25 grams, the indentation time is 10 seconds, and the average of the measured values of 5 random points is used to convert the Vickers Hardness of the hot-dipped steel sheet coating. Number, HV).

The results showed that Example 1 was HV 95; Example 2 was HV 104; Example 3 was HV 117; Example 4 was HV 122; Example 5 was HV 135; Example 6 was HV 138; Comparative Example 1 was HV 58; Comparative Example 2 is HV 55; Comparative Example 3 is HV 242.

From the above results, it can be seen that the coating hardness of the galvanized steel sheets of Examples 1 to 6 is moderate, which can avoid dezincification defects and picking problems during the stamping process. The coating hardness of the galvanized steel sheets of Comparative Examples 1 to 2 was too soft, resulting in the formation of dezincification defects and napping problems during the stamping process. However, the galvanized steel sheet of Comparative Example 3 had too hard coating hardness, which would also cause dezincification defects during the stamping process.

Corrosion resistance test

Utilize the standard number B117 "Standard Practice for Operating Salt Spray (Fog) Apparatus" of the American Society for Testing and Materials and the general standard of the Chinese National Standards (CNS) of the Bureau of Standards, Inspection and Quarantine of the Ministry of Economic Affairs The neutral salt spray test method in section 6.2.1 of CN S8886 "Salt spray test method" is to test the corrosion resistance of galvanized steel sheets. It is a 5% saline solution with a pH of 6.5 to 7.2 and is continuously sprayed on the galvanized steel sheet in a fine mist to record the time when white rust and red rust start to form on the surface of the coating. In addition, cut the edges of the galvanized steel sheet to expose the steel sheet substrate, and record the time when the red rust starts to form. The corrosion resistance of the galvanized steel sheet is judged by the rusting time of the galvanized steel sheet. The longer the rusting time of the galvanized steel sheet, the better the corrosion resistance. In this embodiment, based on the common range specified by the aforementioned two standards for the amount of fog, the amount of fog is set to be 1.0 mL/80 cm ² /hr to 1.5 mL/80 cm ² /hr.

Please refer to Table 1. The white rust generation time of Examples 1 to 6 is 24.5 to 36.7 times longer than that of Comparative Examples 1 to 3, and the red rust generation time of Examples 1 to 6 is longer than that of Comparative Examples 1 to 3. From 13.3 times to 11.3 times, the cutting edge red rust generation time of Examples 1 to 6 is 6.4 to 6.8 times longer than that of Comparative Examples 1 to 3.

Compared with Comparative Examples 1 to 3, the galvanized steel sheets with zinc-aluminum-magnesium coatings of Examples 1 to 6 have significantly improved corrosion resistance.

It can be seen from the above-mentioned embodiments that the galvanized steel sheet of the present invention and its manufacturing method, the steel sheet is hot-dipped in a zinc bath containing a specific content of aluminum and magnesium, and the coating of the obtained galvanized steel sheet contains specific crystals. Compared with its specific specifications, it has high hardness and high corrosion resistance, and can be used in home appliances, building materials and automobiles.

Although the present invention has been disclosed in several embodiments as above, it is not intended to limit the present invention. Anyone with ordinary knowledge in the technical field to which the present invention pertains can make various modifications without departing from the spirit and scope of the present invention. Modifications and modifications, therefore, the scope of protection of the present invention shall be subject to the scope of the attached patent application.

100‧‧‧Method

110‧‧‧Provide steel sheet

120‧‧‧The steel sheet is annealed to obtain annealed steel sheet

130‧‧‧Hot-dip the annealed steel sheet to obtain the hot-dipped steel sheet

140‧‧‧Cooling step for hot-dipped steel sheet

150‧‧‧Get galvanized steel sheet

Claims (10)

A method for manufacturing a galvanized steel sheet includes: providing a steel sheet, wherein the steel sheet contains: 0.05 wt% (wt%) to 0.5 wt% carbon; 0.1 wt% to 3.0 wt% manganese; 0.01 wt% to 1.2wt% chromium; 0.1wt% to 1.2wt% silicon; 0.0005wt% to 0.01wt% boron; and the balance is iron and inevitable impurities; an annealing step is performed on the steel sheet to obtain an annealing Steel sheet; the annealed steel sheet is immersed in a zinc bath for a hot-dip galvanizing step to obtain a hot-dip galvanized steel sheet, wherein the zinc bath contains 1.0wt% to 4.0wt% aluminum, 0.5wt% to 4.0wt % Of magnesium and the remainder are zinc and unavoidable impurities; and performing a cooling step on the hot-dip galvanized steel sheet to obtain the galvanized steel sheet, wherein the galvanized steel sheet has a zinc-aluminum-magnesium coating. The method for manufacturing a galvanized steel sheet as described in item 1 of the scope of patent application, wherein the steel sheet is a hot-rolled steel sheet or a cold-rolled steel sheet. The method for manufacturing a galvanized steel sheet as described in item 1 of the scope of the patent application, wherein the steel sheet further contains less than 0.22wt% titanium, less than 0.12wt% aluminum, less than 0.12wt% phosphorus and less than 0.06wt% sulfur. According to the method for manufacturing a galvanized steel sheet as described in item 1 of the patent application, the annealing step is performed at an annealing temperature of 500° C. to 900° C. for 20 seconds to 300 seconds. The method for manufacturing a galvanized steel sheet as described in item 1 of the scope of patent application, wherein a temperature of the zinc bath is 400°C to 550°C. The method for manufacturing a galvanized steel sheet as described in item 1 of the scope of patent application, wherein one of the cooling steps has a cooling rate of 2°C/s to 50°C/s. A galvanized steel sheet, which is produced by the method for manufacturing a galvanized steel sheet according to any one of items 1 to 6 in the scope of the patent application, wherein a surface area ratio of the zinc-aluminum-magnesium coating is greater than 60 % Of the Zn-Al-MgZn ₂ layered eutectic structure phase, the aluminum-rich phase with an area ratio greater than 0.2%, and the remainder is the primary zinc phase. The galvanized steel sheet described in item 7 of the scope of patent application, wherein the interlayer spacing of the Zn-Al-MgZn ₂ layered eutectic structure phase is less than 1000 nm. The galvanized steel sheet as described in item 7 of the scope of patent application, wherein a grain size of the primary zinc phase is less than 3500 nm. The galvanized steel sheet described in item 7 of the scope of the patent application, wherein the zinc-aluminum-magnesium coating contains 1.0wt% to 4.5wt% aluminum and 0.5wt% to 4.5wt% magnesium.

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