JPS6367551B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a hot-dip aluminum plated steel plate, and more specifically, a plated steel plate immediately after hot-dip galvanization is subjected to a two-step rapid cooling treatment, and then annealed.
A skin pass is then performed to eliminate spangles in the plated metal and to improve mechanical properties after annealing. In molten aluminum plating, a steel plate is passed through an aluminum (Al) plating bath and raised above the bath, then slowly cooled by air or natural cooling, and then rolled up to improve its mechanical properties. An over-aging annealing treatment is performed, and a skin pass mill is also performed for the purpose of eliminating spangles in the dazzling metal and correcting the shape of the steel plate. However, molten aluminum plated steel sheets obtained through conventional conventional processes have relatively coarse spangles in the plated metal, so in order to eliminate the spangles, the rolling force of the skin pass mill must be increased, or
Or you have to increase the number of passes. Therefore, the skin pass elongation rate increases, and the mechanical properties (especially elongation properties) of the galvanized steel sheet deteriorate significantly. In addition, cooling immediately after plating is done by blower cooling or natural cooling, and the cooling rate is 20℃/20℃.
It is about seconds. For this reason, precipitation of solid solution C in the base steel sheet progresses, and the amount of supersaturated solid solution C decreases, requiring a long time for subsequent overaging treatment to improve mechanical properties. The present invention has been made to solve the above problems. The method for producing molten aluminum-plated steel sheets of the present invention involves rapidly solidifying the plated metal to a temperature around the freezing point (approximately 580°C) immediately after molten aluminum plating, thereby suppressing the growth of plated metal crystals and creating fine spangles. Allow to form, then heat to a temperature below 450℃ for 30 minutes.
By cooling at a cooling rate of ~100℃/s,
Precipitation of solid solution C in the base steel sheet during cooling is suppressed, and then over-aging annealing is performed at a temperature range of 350 to 450°C to improve mechanical properties, followed by a skin pass mill to completely eliminate spangles. It is characterized by the fact that it is made as follows. In the present invention, cooling of the plated steel plate immediately after molten aluminum plating may be carried out using water as a coolant, but the plated metal (approximately
(600 to 680°C) is in a molten state, so if a large amount of water is sprayed at once, water drop marks may remain on the plated surface, damaging the surface quality. In such cases, first spray a relatively small amount of water to cool the plated metal to a temperature around the freezing point, then spray a large amount of water from that temperature to achieve a cooling rate of 30 to 100°C/sec. It is best to cool it down. In addition to water, for example, an aqueous ammonium hydrogen phosphate solution is preferably used as the refrigerant. The temperature reached by the above rapid cooling is 450
℃ or less, but in actual operation, a temperature in the range of 350 to 450°C is appropriate, taking into account the temperature of the overaging annealing treatment that is performed subsequent to quenching. If the rapid cooling is performed in two stages in this way, a beautiful surface is obtained without water drop marks, minimized spangles are formed, and the rapid cooling sufficiently prevents the precipitation of supersaturated carbon in the matrix of the base steel sheet. is suppressed and a large number of fine precipitates are formed. The obtained aluminum plated steel plate is heated to 350 to 450℃.
When annealing is performed at a temperature of 2 to 3 minutes,
As a result of the progress of precipitation of supersaturated C in the matrix with a large number of fine precipitates as nuclei, a remarkable improvement in mechanical properties, especially elongation, is observed. Furthermore, since the spangles of the plated metal are very fine, the required surface adjustment can be achieved with a skin pass at a low elongation rate. In the present invention, the reason why the cooling rate from around the solidification temperature of aluminum is limited to 30 to 100°C/sec is that if the cooling rate is less than 30°C/sec, the amount of supersaturated solid solute C before overaging will decrease. This is because the annealing temperature must be increased or the overaging annealing time must be increased, which is not practical. On the other hand, if the temperature exceeds 100°C/sec, a kind of hardening effect occurs and the mechanical properties do not recover even if over-aging treatment is performed. The reason for limiting the overaging treatment temperature range to 350 to 450℃ is that at temperatures below 350℃, overaging treatment takes a long time (usually 5 minutes or more), making it unsuitable for short-time annealing. Annealing must be used. In addition, after over-aging treatment, it is necessary to prevent hardening.
Since it is necessary to slowly cool the product to about 150°C, if the overaging temperature is high, the cooling equipment will be long to slow down to about 150°C, which is impractical. Furthermore, if the overaging temperature becomes high, alloying between the plating layer and the steel sheet progresses, which is undesirable. For these reasons, the appropriate temperature range for the overaging treatment is 350 to 450°C. In the present invention, other plating conditions are the same as those of normal ones, except that the plated steel plate is rapidly cooled immediately after hot-dip galvanizing, and if necessary, the amount of metal deposited on the surface of the steel plate is adjusted on the plating bath. Control or surface chemical conversion treatment may be carried out in the same manner as usual. To explain an example of the present invention, in a continuous molten aluminum plating line, a plate thickness of 0.8
mm / plate width 914mm low carbon rimmed steel cold rolled steel strip (C:
0.045%, Mn: 0.27%, P: 0.018%, S: 0.019
%, Si:tr. Aluminum deposition amount: 80g/ ^m2 )
is passed through a reduction furnace for surface purification while the bath temperature is 670°C.
°C molten aluminum plating bath (Al-10%Si)
The plate was pulled up onto the bath while being continuously passed through the bath, gas wiped to control the amount of metal deposited, and water cooling was performed using a cooling device placed directly above the bath. Water cooling is divided into two stages: first, the temperature is cooled down to 580°C by spraying at a cooling rate of 30°C/sec, then the temperature is cooled down to 450°C by water spraying at a cooling rate of 30 to 100°C/sec, and then the temperature is evaporated at 350 to 450°C. Aging treatment (processing time 2
~3 minutes), and then the surface was adjusted using a skin pass. Table 1 shows the spangle size and mechanical properties of the obtained damped steel plate. Note that the surface of the plated steel plate was beautiful with no water drop marks. As a comparative example, similar measurements were carried out on a laminated steel sheet manufactured according to the conventional method under the same conditions as above, except that cooling immediately after plating was performed by allowing it to cool. In the table, the symbol "Surface skin rank" means the following evaluation. A: Skin with no visible spangles, B: Skin with some spangles remaining, C: Skin with spangles remaining. As shown in the table, the plated steel sheets of the examples of the present invention have excellent elongation and strength, and have minimized spangles. In addition, in the conventional example, since the spangles were coarse, 2 to 3 passes were required for surface preparation using a skin pass, but in the example of the present invention, 1 pass was required.
We were able to achieve surface conditioning in just one pass. Note that although the original plate of the above-mentioned sample material is a low carbon rimmed steel, it goes without saying that a similar improvement in mechanical properties can be seen in the case of a low carbon killed steel.

[Table] As described above, the aluminized steel sheet obtained by the present invention is given excellent mechanical properties by annealing. Furthermore, since the spangles are fine, subsequent surface adjustment work is greatly reduced.

Claims (1)

[Claims] 1. A plated steel sheet immediately after being lifted from a molten aluminum plating bath is rapidly cooled to the freezing point temperature of aluminum to form fine spangles, and then heated at a cooling rate of 30 to 100 °C/sec to 450 °C. Rapid cooling to a temperature below ℃, then 350 ~
A method for producing molten aluminum plated steel sheets, which is characterized by performing over-aging annealing treatment in a temperature range of 450°C, and then using a skin pass mill to eliminate fine spangles on the plated surface.