JP2616944B2 - Manufacturing method of hot-dip galvanized steel sheet with differential thickness - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a hot-dip galvannealed steel sheet having excellent properties as a steel sheet for automobiles.

(Prior art) A galvannealed steel sheet is apt to undergo plating peeling called powdering during press working.

For this reason, Japanese Patent Application Laid-Open No. Sho 61-22374 proposes a technique for controlling the alloying heating conditions so that the Fe concentration in the plating layer is made appropriate to improve the powdering resistance.

(Problems to be Solved by the Invention) The alloying reaction is a diffusion reaction of Fe into Zn, and the amount of Fe diffusion must be controlled. Since the heating conditions act equally on the front and back of the steel sheet, the amount of Fe diffusion on the front and back is equal, and it is possible to have the proper Fe concentration on both sides with equal thickness plating, but on either side with differential pressure plating Exceeding the Fe concentration, as a result, the powdering resistance is deteriorated, or the alloying is insufficient, resulting in appearance defects called burn spots. Both sides of the steel sheet of the present invention have an appropriate Fe concentration in the plating layer and have excellent powdering resistance. It is an object of the present invention to provide a method for producing a hot-dip galvanized steel sheet having a different thickness.

(Means for solving the problem) To achieve this purpose, the alloying reaction rate on the thicker side was made larger than that on the thinner side, and the idea of increasing the amount of Fe diffusion was conceived. It has been found that it is effective to spray molten zinc with different Al concentrations on the thin side and the thick side on both sides of the steel sheet immediately after infiltration.

That is, the present invention provides a method of reducing and annealing a steel sheet to remove an oxide film on the surface, immersing the steel sheet in a hot-dip galvanizing bath, performing plating, adjusting the amount of coating so that the amount of plating on the front and back surfaces is different, and subsequently, In the method of producing a hot-dip galvannealed steel sheet in which a heat treatment is performed, immediately after the steel sheet enters the plating bath, hot-dip zinc containing Al is sprayed on both sides of the steel sheet, in which case the thinner side is from the thicker side. Is a method for producing a galvannealed steel sheet with a different thickness, characterized by increasing the Al concentration.

FIG. 1 shows an example of a continuous hot-dip galvanizing line, in which 1 is a pay-off reel, 2 is an entrance looper, 3 is a cleaning, 4 is an annealing furnace, 5 is a plating bath, 6 is a wiping nozzle, and 7 is an alloying furnace. , 8 is a water cooling device, 9 is a skin pass, 10
Is a tension leveler, 11 is a chemical conversion treatment, 12 is an exit looper,
13 is a tension reel.

FIG. 2 shows one embodiment of the plating bath 5 to which the present invention is applied. Zinc is sprayed by the A nozzle 21 and the B nozzle 22 immediately after the strip 14 enters the plating bath 5. The zinc supplied to each nozzle is pumped from the plating bath by a pump 23, and the Al concentration is changed as follows by an Al concentration adjusting device 24 as necessary. For example, when the A nozzle 21 side is thicker, zinc whose Al concentration has been reduced by the Al concentration adjuster 24 is supplied to the A nozzle 21 and the Al concentration adjuster
Zinc whose Al concentration is increased by 24 'is supplied to the B nozzle 22.

In another embodiment shown in FIG. 3, two baths 33 and 34 for adjusting the Al concentration are provided, respectively, and the A and B nozzles 21 and 22 are
To supply zinc.

(Operation) In the galvanizing bath, Zn-Fe
In order to prevent the alloy layer from forming thick, generally 0.10 ~
Contains 0.20% Al. When a steel sheet is immersed in a plating bath, an alloy layer called an Al-enriched layer is formed on the surface of the steel sheet, but the thickness of the Al-enriched layer depends on the Al concentration in the bath, and the thickness increases as the Al concentration increases. .

The alloying reaction rate after plating depends on the thickness of the Al-enriched layer. The thicker the Al-enriched layer, the lower the alloying reaction rate (Fe diffusion rate).

Since the Al-enriched layer is formed immediately after the steel sheet enters the plating bath, the thickness of the Al-enriched layer can be changed by blowing molten zinc containing different concentrations of Al onto the steel sheet at this time. it can.

When producing a hot-dip galvanized steel sheet with a different thickness, the thicker surface, which requires a greater amount of Fe diffusion during alloying, is sprayed with low-Al-concentration molten Zn and the thickness of the Al-enriched layer is increased. Thinner. On the other hand, by spraying molten Zn with a high Al concentration on the thinned surface where Fe diffusion must be suppressed, a thick Al-enriched layer is formed, so that even under the same alloying heating conditions, In both cases, an alloyed hot-dip galvanized steel sheet having an appropriate Fe concentration can be manufactured.

(Example) Hot-dip galvanizing was performed by the equipment shown in FIG. 1 using the plating bath 5 shown in FIG. 2, and subsequently, alloying treatment was performed to produce a hot-dip galvannealed steel sheet with a different thickness. Plating bath Al
The concentration was 0.15%, the bath temperature was 450 ° C, and the temperature of the plate entering the bath was 470 ° C.

The plated steel sheet was subjected to alloying treatment by changing the amount of adhesion on the front and back, and the Fe concentration of the plated layer was investigated and a powdering test was performed. Table 1 shows the results.

In the powdering test, the coated steel sheet was bent by 90 °, and then the tape was attached to the compression side. After being bent back, the tape was peeled off and the powdering resistance was determined based on the amount of Zn attached to the tape. An index of 1 indicates that the powdering resistance is the best, and that as the index increases, the powdering resistance deteriorates.

As is clear from Table 1, by applying the present invention, it is possible to produce a galvannealed steel sheet having a different thickness with good thickening and thinning, exhibiting good powdering resistance, and free from burn spots.

(Effect of the Invention) According to the present invention, it is possible to produce a galvannealed steel sheet having excellent powdering resistance on both the thicker side and the thinner side and suitable as a steel sheet for automobiles.

[Brief description of the drawings]

 FIG. 1 is an explanatory view of a continuous galvanizing line, FIG. 2 is a detailed view of a plating bath, and FIG. 3 is a detailed view of another plating bath. 5 Plating bath, 21 A nozzle 22 B nozzle

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Minoru Sakamoto 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. Chome (without address) Inside Kawasaki Steel Corporation Mizushima Works (56) References JP-A-63-62855 (JP, A) JP-A-62-44563 (JP, A)

Claims (1)

(57) [Claims] 1. After reducing and annealing a steel sheet to remove an oxide film on the surface, the steel sheet is immersed in a hot-dip galvanizing bath to perform plating, and the coating amount is adjusted so that the coating amount on the front and back surfaces is different. In the method for producing a hot-dip galvannealed steel sheet in which a heat treatment is performed, immediately after the steel sheet enters the plating bath, hot-dip zinc containing Al is sprayed on both sides of the steel sheet, with the thinner side being the thicker side. A method for producing a hot-dip galvanized steel sheet having a different thickness, wherein the Al concentration is higher than that of the hot-dip galvanized steel sheet.