JPS6048570B2 - Continuous over-aging treatment method for continuous molten aluminized steel sheets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous over-aging treatment method for hot-dip aluminum plated steel sheets that have undergone continuous annealing.

A hot-dip aluminized steel plate made from a continuously annealed material is inevitably harder than one made from a batch-annealed material, regardless of whether it is in-line annealed or outline annealed. This is a continuous annealing process that involves heating, recrystallization annealing,
This is due to the fact that it is subjected to a short thermal cycle of cooling and has a particularly fast cooling rate, so it contains more supersaturated solid solution carbon and nitrogen than batch annealed materials (slow cooling materials). However, the manufacturing method of aluminum-plated steel sheets (Sendzimir method), in which continuously annealed material is hot-dip plated, has many advantages in terms of mass productivity and quality control, so it is suitable for applications where hardness is not a major problem. It has made great progress as a material. However, in recent years, it has become desirable to take advantage of the excellent properties of aluminum-plated steel sheets for applications that require complex or advanced processing. There has been a strong demand for the development of technology to provide this. In order to soften the aluminum-plated steel sheet (hereinafter referred to simply as continuous hot-dip aluminum-plated steel sheet) and improve press formability, the above-mentioned supersaturation process is basically required. The purpose is to precipitate solid solution carbon as a stable carbide and eliminate solid solution hardening.

As a method for softening this continuous hot-dip aluminum-plated steel sheet, for example, as described in Japanese Patent Publication No. 12968/1982, a predetermined temperature (204 to 45
A method is known in which low-temperature box annealing is performed at a temperature of 4° C. for a long time (at least about 4 hours). However, if the over-aging J-effect treatment is performed in such a batch type annealing furnace, the advantage of continuous annealing and short-time annealing is halved, and the advantages of a continuous line are lost.
Furthermore, as a method for softening a continuously annealed material by continuously overaging it, for example, the method described in Japanese Patent Publication No. 334/1984 is known. In this method, after recrystallization annealing,
The material is rapidly cooled from that temperature to 500 DEG C. or lower at a cooling rate of 50 DEG C./Sec or more, and then held at 300 DEG to 500 DEG C. for 1 stroke or more, thereby carrying out a soaking and overaging treatment. However, by applying this method to the Sendzimier type plating line,
By directly connecting this over-aging furnace to the continuous annealing furnace and adopting a line that performs immersion plating after the over-aging treatment, the over-aging furnace can be installed even for steel types that do not require over-aging treatment. As a result, as a line that can handle a wide range of steel types, the line becomes over-equipped overall and is not necessarily beneficial for improving operability and productivity. The present invention provides a method for continuously overaging a continuous molten aluminum plated steel sheet that has been immersion plated through continuous annealing, and the method is to provide a method for continuously overaging a continuous molten aluminum plated steel sheet that has been subjected to continuous annealing and immersion coating. In this heat treatment furnace, the plated steel plate is rapidly heated to 500 to 580°C, maintained within this temperature range for less than 2 seconds, and then slowly cooled. Features. Hereinafter, the method of the present invention will be specifically explained with reference to the drawings. FIG. 1 shows the overaging treatment conditions for a continuous hot-dip aluminum plated steel sheet, and is intended to explain the range of conditions employed in the present invention.

Figure 1. In the figure, the curve AA' indicates the boundary line of the alloying start region of the aluminized layer. This curve A-A
In order to obtain A1 containing a small amount of Sl (approximately 9%) after in-line annealing using the normal Sendzimir method,
Heat-resistant plated by immersion in a plating bath; aluminized steel plate (
In the case of a coating weight of 40 g/d (both sides), approximately 10% (wet analysis) of Fe is usually detected in the plating layer, but the boundary is set when this Fe% exceeds 15%. This is because if the Fe% in the plating layer exceeds 15%, alloying will progress 3 and press workability will be greatly impaired. On the other hand, the curve B-B in FIG.
' indicates the lower limit necessary to almost achieve the over-aging treatment of the steel sheet, and was determined by examining the mechanical properties of the steel sheet after the over-aging treatment. Therefore, this curve A-
This is the area between A' and curve B-B'. 7. If you do not expect the effect of complete overage, curve B
- It may be slightly lower than B'. From the drying speed of the continuous overaging heat treatment furnace used in the method of the present invention and the setting conditions of each zone, the conditions that are most stable in terms of quality and have good operability are, as will be described later, a temperature of 500 to 580°C and a holding period of About 10 to 2 is good. FIG. 2 is a layout diagram of the overaging furnace used in the present invention, with the upper stage showing a horizontal furnace and the lower stage showing a vertical furnace.

At the stage of investigating the effects of the present invention, a reduced model horizontal furnace was used, but if thermal efficiency is to be given priority, it is advantageous to employ a vertical furnace like the one shown in the lower stage. The main components of the heat treatment furnace are induction heating zone 1 in the order of flow of aluminized steel sheets.
, a cooling zone (in some cases, a soaking zone) 2, and a cooling zone 3. One feature of the invention is that the induction heating zone i
The objective is to rapidly heat the steel strip to 500-580°C in zone 1, and in the external heating method normally used for heating steel strip,
Since the aluminized steel plate has low radiation activity, the heat cycle of the present invention cannot be suitably carried out. In other words, by internal heating type induction heating, aluminum plated steel plate is heated to 500 to 58
An important requirement of the present invention is to raise the temperature to 0° C. in a short time. Since the cooling zones 2 and 3 are for thermal management of gradual cooling, any heating method may be used. The reason why the cooling adjustment zone is divided into two is to provide flexibility in the range of application of this heat treatment furnace. This is because it can be used appropriately for hot-dip galvanized copper plates and the like. In FIG. 2, numeral 4 indicates an aluminized copper plate, 5 indicates an auxiliary device before the heat treatment furnace, and 6 indicates an auxiliary device after the heat treatment furnace. Figure 3 shows a typical heat cycle for overaging treatment according to the present invention, and the numbers in the figure correspond to the heating and cooling in each zone 1, 2, and 3 in the heat treatment furnace in Figure 2. .

As is clear from FIG. 3, the heat cycle according to the method of the present invention is completed in an extremely short time. 500~580°C
The time to maintain this temperature after rapid heating to 370°C is preferably within 2°C, and after that, in order to prevent alloying, the cooling process is preferably started at a cooling rate of about 5°C/Sec or less to about 370°C. , the entire cycle takes less than 1 minute (in some cases less than 5 minutes).

Therefore, it is extremely advantageous because the line speed can be increased and the overaging treatment line can be operated with a short length. Representative examples will be described below.

Smelted in a 90 ton converter, C: 0.06%, Mn: 0.
28%, Si; Tr) P; 0.016%, S; 0.01
Rimmed steel of 6% is cold rolled to 0.8W using the normal method.
After finishing it to f1 thickness, it was placed in a Sendzimir type continuous hot-dip aluminum plating machine (the plating bath was 90% N and 9% Si) with a plating weight of 40 g/go (both sides).
So, I passed the board.

This continuous hot-dip aluminized steel sheet was passed through a horizontal furnace of the Oshio model shown in the upper row of FIG. 2, and the heat cycle shown in FIG. 3 was applied. After that, a 1.2% skin bath was applied.
Table 1 shows various test values for the copper plate subjected to this over-aging treatment and the steel plate before the over-aging treatment. As is clear from the results in Table 1, continuous aging treatment by the method of the present invention
The descending point (Y.S) and tensile strength (T.S) decreased, the total elongation (E1) increased, and it became significantly softer, as evidenced by the conical cup test value (CCV). It has good properties.

[Brief explanation of drawings]

Figure 1 is a time-temperature relationship diagram showing the overaging treatment conditions for aluminized steel sheets, and Figure 2 is according to the present invention. FIG. 3 is a heat cycle diagram showing an example of the heat cycle of the method of the present invention. 1...Induction heating zone, 2...Cooling control zone, 3...Cooling control zone.

Claims (1)

[Claims] 1. Continuously annealed and dip-plated continuous hot-dip aluminum-plated steel sheets are passed continuously in a belt shape through a heat treatment furnace including an induction heating zone and a cooling zone, and in this heat treatment furnace, the plated steel sheets are A continuous over-aging treatment method for a continuous hot-dip aluminum plated steel sheet, which is characterized by applying a heat cycle of rapidly heating to 580° C., holding it in this temperature range for within 20 seconds, and then slowly cooling it.