JPS5945757B2 - Manufacturing method for single-sided coated galvanized steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a single-sided coated galvanized steel sheet in which a zinc coating and a steel strip are alloyed.

In recent years, in order to improve the durability of automobiles, galvanized steel sheets or specially coated steel sheets have come to be used for outer panels and undercarriage members.

In the case of steel plates used for undercarriage parts, coatings such as plating can be applied on both sides, but in the case of steel plates used for automobile exterior panels, due to the problem of gloss after painting, there is a demand for single-sided coated steel plates with the surface to be painted as the base steel plate. In addition, weldability and
From the viewpoints of chipping resistance, ED paintability, etc., there is a strong demand for single-sided zinc-coated plated steel sheets that are alloyed with zinc coating. In order to meet these demands, the following (1) to (
111) method was proposed. (1) A method of rapid heating and rapid cooling in a continuous heating furnace at 350 to 800°C for 1 to 30 seconds after electrogalvanizing (Japanese Patent Application Laid-Open No. 47-42
No. 343) (li) 40 by batch annealing after hot-dip plating
Method of heat treatment at 0 to 480°C
No. 0) (111) 450~ by continuous heating furnace after electrogalvanizing
A method of heating at 600 DEG C. and a rate of 2 to 60 DEG C./sec and rapidly cooling (JP-A-50-154126) However, these conventional methods have the following drawbacks.

That is, in both methods (i) and (lli), since the heat treatment is carried out continuously, air tends to get mixed into the heating atmosphere, resulting in the formation of an oxide film on the surface of the product, which deteriorates bonding properties. There is. This is because it is difficult to completely seal the continuous heat treatment equipment (and the equipment is large-scale and expensive). Therefore, the method of item 1) (111) Generally, heat treatment is performed in the air without atmosphere adjustment, and a large amount of oxide film formed on the non-plated surface is removed by a separate pickling process. However, it is difficult to completely remove the oxide film by pickling, and there is a drawback that the bonding process as a pretreatment for subsequent painting is poor. Furthermore, providing a separate pickling process increases equipment costs and processing costs. Next, since the methods described in previous statements 51) and (111) both involve high-temperature heating and rapid cooling, the solid solution carbon in the base steel sheet increases, reducing ductility, making the material brittle, and reducing press workability. There are drawbacks. Also, in terms of the quality of the plating film, there were drawbacks in that the alloy plating layer became rough due to high-temperature and rapid heating, and cracks were likely to occur in the alloy plating layer due to thermal shock during subsequent rapid cooling.
The reason why the alloy plating layer becomes rough is because electrogalvanized steel sheets are used, and unlike hot-dip galvanized steel sheets, the plating film contains A1, which suppresses iron diffusion and forms a uniform alloy plating layer. That's because there isn't. In addition, Maeki 1), (1
Method i) also had operational problems such as the rolls being unable to come into contact with the strip while the zinc was melting.

Furthermore, in method (4), since the material is a hot-dip galvanized steel plate, it is difficult to achieve uniform thinning of less than 30 y/m'' per side, and it is extremely difficult to achieve single-sided plating using hot-dip galvanizing.

Furthermore, heating at 420°C or higher causes problems such as adhesion of the steel plate in the case of tight annealing, and many problems must be overcome before it can be used in actual production.

The present invention was devised in order to solve the above-mentioned drawbacks of the conventional method.
Electrogalvanized at 40V/M2, then heated to 250-350℃ in a non-oxidizing to slightly reducing atmosphere in a batch annealing furnace, and heated to 250-350℃ at that temperature.
The gist of the present invention is a method for producing a single-sided galvanized steel sheet, in which the zinc coating and the steel strip are alloyed by holding for ~15 hours, and then slowly cooled.

Next, the method of the present invention is performed by using a cold-rolled steel strip as a raw material, applying electrolytic zinc mesosa on one side, and converting the plated steel sheet into an open coil, which is then heated for alloying in a batch annealing furnace. and the step of temper rolling the alloyed steel strip.

First, in the single-side electrogalvanizing process, a steel strip that has been previously cold rolled, batch annealed and temper rolled, or continuously annealed is used as a base, and then a normal continuous electrogalvanized method is applied to the steel strip to give a 10-407 /M2 single-sided galvanizing.

The plating bath may be either an alkaline bath or an acid bath. The reason why plating is limited to one side is because, as mentioned above, when used on the exterior of an automobile, it is preferable to use a steel plate on the outside because a glossy coating can be applied. However, there is no technical relationship between the plating being limited to one side and the subsequent alloying treatment conditions. The reason why the plating amount is set to 10 to 40 y/m is that if it is less than 107/m, sufficient corrosion resistance cannot be obtained, and if it exceeds 407/m, it is not preferable in terms of cost to produce by electrogalvanizing. In the next heating process for alloying, the steel strip galvanized on one side as described above is wound into an open coil, and then the plating layer is alloyed in a batch annealing furnace in a non-oxidizing or weakly reducing atmosphere. Perform heat treatment for

The reason for using an open coil is to uniformly heat the steel strips, prevent uneven alloying, and prevent the steel strips from adhering to each other.

In other words, when a tight steel strip is heated in a batch-type furnace,
Temperature distribution becomes partially non-uniform, iron atoms diffuse partially differently, and the iron content in the plating coating tends to become uneven locally, especially in the longitudinal direction, making the plating coating with a uniform iron content It is not possible to obtain a product with On the other hand, when similar heating is performed using an open coil, the heat is easily transmitted to the inside of the coil, making it possible to obtain a product having a plating coating with a uniform iron content. The relationship between the heating temperature and holding time in the annealing furnace is determined by the plate thickness and the amount of plating.

Plate thickness 0.7~1.2mm 1 plating amount 10~407/m"
For example, take the case of an automobile exterior panel that is galvanized on only one side.
As shown in Figure 1, at a heating temperature of 250°C, the holding time is 2~
It takes 15 hours, and at 300℃ it takes 1 to 10 hours, 35 hours.
It can be seen that 1 to 2 hours are required at 0°C. In short, a heating temperature of 250 to 350°C requires a holding time of 1 to 15 hours. Here, set the heating temperature to 250
The reason why the temperature was set at ~350°C is that below 250°C, the diffusion rate of atoms is slow, so the holding time for alloying must be considerably long, resulting in a lack of productivity. This is because if the temperature exceeds 350°C, the diffusion rate of atoms is too fast, making it difficult to control the holding time for proper alloying. In addition, the reason why the holding time was set to 1 to 15 hours is that under the above heating conditions, if the holding time is shorter than 1 hour, alloying will not be completed, and even if alloying is completed, the holding time will be short, so it will take longer to raise the temperature. This is because it becomes difficult to manage the holding time and it is difficult to operate while keeping the heat treatment conditions constant. On the other hand, if the time is longer than 15 hours, alloying will progress too much and cracks or powdering will occur in the plating layer during processing, which is not preferable. Automotive outer panels are usually blanked, pressed, phosphated, and ED.
Since it is attached to the vehicle body through a process such as painting (electrodeposition), paint adhesion and corrosion resistance after ED painting are important issues.

In other words, if the base steel sheet and the plated surface are oxidized during the alloying process, the phosphate treatment properties will deteriorate, and the performance after the subsequent ED painting will deteriorate, so it is important to adjust the atmosphere during the alloying process. be. Therefore, in the alloying process, it is necessary to make the atmosphere in the furnace non-oxidizing or weakly reducing in order to prevent oxidation of the steel sheet base and the plated surface. The open coil that has undergone the prescribed heat treatment as described above is cooled in the furnace to approximately 150°C, then taken out of the furnace and cooled to near room temperature.
Rewind into a tight coil.

The reason why the present invention performs heat treatment at a relatively low temperature and then slowly cools is to reduce the amount of solid solute carbon in the manufactured steel sheet, improve its ductility, and improve its press workability. In the next step of temper rolling, the tightly wound coil as described above is subjected to temper rolling of about 1% in order to correct the shape and prevent the occurrence of stretcher strain.

A single-sided coated galvanized steel sheet manufactured through such a process of single-sided electrogalvanizing, alloying treatment, and temper rolling has an iron content in the alloy layer of 107/M2 for a steel sheet with a plating amount of 6.
．． 5-25%, plating amount 407/6.5 for TI steel plate
~18%, excellent workability, no uneven baking,
Formation of a uniform alloy layer is observed over the entire length and width of the coil.

Since the atmosphere has been adjusted, the oxidation of the plated surface and the steel sheet substrate has progressed, so it showed the same phosphate treatment properties as ordinary cold-rolled steel sheets. Next, examples of the manufacturing method of the present invention will be shown.

A steel strip with a thickness of 0.7 mm was electrogalvanized on one side with Zn4O7/M2, and after being wound into an open coil, it was placed in a batch annealing furnace (however, the atmosphere in the furnace was 7% H2, 93% N2, The temperature was raised to 300℃ over 3 hours at a dew point of -15℃, and the alloying treatment was performed at various times between 0 and 25 to 10 hours at this temperature, followed by furnace cooling to 150℃ and extraction. It was cooled to room temperature outside the furnace and subjected to 1% temper rolling to produce an alloyed single-sided galvanized steel sheet.

Table 1 shows the results of examining the surface appearance, workability, bonding processability, and operability of the product after manufacture. In addition, the workability test is 2m
After bending to a radius of 900 m, the occurrence of cracks on the inner side was observed. Bonding properties were determined by bonding the non-plated surface using a commercially available dip-type phosphate treatment solution under normal treatment conditions and observing the crystalline state of the bonding film using the sump method.
Example 1: Plate thickness, plating amount, batch annealing furnace atmosphere, etc.
Under the same conditions as above, the temperature was raised to 350°C over 3 hours,
Alloying treatment was carried out at this temperature for various holding times between 0.1 and 5 hours, cooled in a furnace to 150°C, extracted, cooled to room temperature outside the furnace, and subjected to 1% temper rolling. A galvanized steel sheet coated on one side and subjected to alloying treatment was produced.

After manufacturing, the surface appearance, processability, bonding processability, and operability of the product were investigated, and the results are shown in Table 2. Processability test was also carried out in Example 1.
The same method was used as (the same applies hereinafter).

Example 1: Plate thickness, plating amount, batch annealing furnace atmosphere, etc.
Under the same conditions as above, the temperature was raised to 375°C over 3 hours,
Alloying treatment was carried out at this temperature for various holding times between 0.1 and 1 hour, cooled in a furnace to 150 iso C. After extraction, cooled to room temperature outside the furnace, and 1% temper-rolled. A galvanized steel sheet coated on one side and subjected to alloying treatment was manufactured by performing the following steps.

Table 3 shows the results of examining the surface appearance and workability of the manufactured products. A steel strip with a thickness of 0.7 mm was electrogalvanized on one side with ZnlO7/wl, and after being wound into an open coil, it was heated at 300°C in a batch annealing furnace (the atmosphere in the furnace was the same as in Example 1). The temperature is raised over 3 hours until the temperature reaches 3 hours, and the alloying process is performed at this temperature for various times between 0.25 and 10 hours, cooled in a furnace to 150°C, and after extraction, cooled to room temperature outside the furnace. It was cooled and subjected to 1% temper rolling to produce an alloyed single-sided coated galvanized steel sheet.

Table 4 shows the results of examining the surface appearance and processability of the products after manufacture. Figure 2 diagrammatically shows the alloying treatment cycle (cycle of heating temperature and holding time) of Examples 1 to 4, and the alloying cycle is a triangular heating and cooling cycle as shown in Figure 3. The cycle only needs to be within the temperature and time limits specified by the method of the present invention, and it is also possible to take the modified cycle shown in FIG. 2 in this way.

Next, a comparative example in which the heating atmosphere is different from that of the present invention will be shown.

A steel strip with a thickness of 0.7mT1t was electrogalvanized on one side with Zn4O7/TI, and after being wound into an open coil, it was heated in a batch annealing furnace in the atmosphere for 3 hours to determine the temperature and holding time. Alloying treatment was carried out by changing the temperature at 150°C.
After cooling in a furnace and extracting the material, it was subjected to 1% temper rolling to produce an alloyed single-sided zinc-coated steel sheet. Table 5 shows the results of examining the surface appearance, bonding processability, and workability of the manufactured product. Therefore, it can be seen that the heating atmosphere needs to be a non-oxidizing or weakly reducing atmosphere as in the present invention. Single-sided coated galvanized steel sheets produced by the method of the present invention not only have excellent corrosion resistance and paintability when used as automobile outer panels and undercarriage components, but also have excellent weldability and workability. It shows suitability.

[Brief explanation of drawings]

1 is a diagram showing the relationship between heating temperature and holding time in the method of the present invention, FIG. 2 is a diagram showing the alloying cycle in the method of the present invention, and FIG. 3 is a diagram showing the relationship between heating temperature and holding time in the method of the present invention. FIG. 3 is a diagram showing an example of a modification of the alloying cycle shown in FIG.

Claims (1)

[Claims] 1 One side of a cold-rolled steel strip is electrolytically galvanized with 10 to 40 g/m^2 of Zn, and then heated to 250 to 35% Zn in an open coil state in a non-oxidizing to weakly reducing atmosphere in a batch annealing furnace.
Production of a single-sided coated galvanized steel sheet characterized by heating to 0°C, holding at that temperature for 1 to 15 hours to alloy the zinc coating and the steel strip, then slowly cooling and temper rolling. Law.