JP2808569B2 - Manufacturing method of galvannealed steel sheet with excellent corrosion resistance after painting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0101]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet having improved corrosion resistance after coating by preventing cracks in a plated layer due to temper rolling.

[0092]

2. Description of the Related Art In recent years, from the viewpoint of improving corrosion resistance, the use of zinc alloy-plated steel sheets instead of conventional cold-rolled steel sheets has been generalized in automobile bodies. The main components of this zinc alloy plated steel sheet are electric Zn-Ni alloy plated steel sheet, electric Zn-F
Although e-alloy plated steel sheets and the like are generally expensive, some have attempted to use inexpensive alloyed hot-dip galvanized steel sheets.

This alloyed hot-dip galvanized steel sheet is a gas reduction type hot-dip galvanizing line using a hydrogen-containing gas for pretreatment such as a Sendzimer type, non-oxidizing furnace type or a seal type, or a radiant tube after electrolytic degreasing. -In the hot-dip galvanizing line of gas reduction using the indirect heating method by
After pre-treating the steel sheet and plating it in a zinc bath, the amount of coating is adjusted and 450-550 ° C before the plating layer solidifies.
And the plating layer is made into a Zn-Fe alloy.

[004] plating layer after alloying is of a [delta] _1-phase resistance Paudarin resistance and frame - since the excellent King resistance, but are alloyed with the goal of this during the alloying treatment, after the alloying It is proceeded alloying addition by steel heat workability inferior Γ phase is generated between the [delta] _1-phase and the steel basis material, comes to grow. Therefore, immediately after the alloying treatment, a mist of water is sprayed together with the compressed air to forcibly cool the mixture to 400 ° C. or less where no Γ phase grows. This forced cooling was usually performed at a cooling rate of about 20 ° C./sec, since the plating layer did not peel off or powdered during the processing even if it was not cooled very quickly.

However, when the alloying treatment is performed, the shape and mechanical properties of the steel sheet are impaired. 30
% Temper rolling is performed. The purpose of using the dull roll in the temper rolling is to prevent the coating film anchoring effect due to the specific surface roughness of the galvannealed steel sheet from disappearing. The Darroll transfer rate refers to the area ratio of the transfer portion measured using an image analyzer after observing the surface of the plating layer at 500 times using a scanning electron microscope at a magnification of 500 times, and calculating the average value as a percentage. It is represented by

[0086]

However, when a galvannealed steel sheet subjected to temper rolling with a dal roll is subjected to electrodeposition coating and subjected to a corrosion resistance test, it is compared with a steel sheet not subjected to temper rolling. There is a problem that coating film blisters occur.

007

Accordingly, an object of the present invention is to provide an alloyed hot-dip galvanized steel sheet which does not cause the blistering of the coating film. That is, according to the present invention, the forced cooling is
An alloyed hot-dip galvanized steel sheet was manufactured by performing the cooling at a cooling rate of 5 to 80 ° C./sec and performing the temper rolling at a transfer rate of dull roll of 40 to 90%.

[0098]

The present inventors have conducted various investigations on the cause of coating blistering caused by temper rolling, and as a result, cracks that reach the steel base during temper rolling are generated in the plating layer. Na ^+, Cl ^-, corrosion promoting substance such as water from entering therein, and found that in order to generate a blistering coating.

That is, during temper rolling, the steel sheet elongates by 0.2 to 2.0% due to the rolling force and tension, but the Zn—Fe alloy plating layer is inferior in plastic deformation ability, so it does not follow the elongation of the steel sheet. It was found that cracks occurred in the direction of the width of the portion where the transfer of the Darroll was not received, as schematically shown in FIG. And it was found that this crack is more likely to occur as the Γ phase grows.

Based on this finding, the present inventors produced samples in which the forced cooling after the alloying treatment was strengthened to suppress the Γ phase growth of the plating layer, and temper rolling was performed at various elongation rates. As a result, the generation of cracks could not be completely prevented. However, instead of the elongation rate, which is a general condition, the temper rolling conditions are focused on the transfer rate of the dal roll, and samples having different growth of the Γ phase are tempered with dal rolls having different surface roughness. As a result of rolling, some of them did not have cracks.

The inventors of the present invention have examined the alloying treatment conditions, forced cooling conditions, and Darroll transfer rate without cracks, and found that the forced cooling conditions and the Darroll transfer rate were controlled within a certain range. For example, they found that cracks could be prevented. As a result of various experiments for both ranges, the forced cooling was enhanced to a cooling rate of 35 to 80 ° C./sec, and the transfer rate of dull roll during temper rolling was increased to 40 to 90%. I found something good.

When the forced cooling conditions and the Darroll transfer rate are set as described above, the growth of the Δ phase having poor workability is suppressed, and the elongation of the plating layer is improved. Further, since the portion where the dull roll has not been transferred is reduced, the stress applied to the plating layer is reduced even with the same elongation. By these synergistic effects, cracks do not occur in the plating layer. The transfer rate of dull roll can be controlled by using a microscopic photograph of the surface of the steel sheet so that the transfer site can be read.

FIG. 2 shows the thickness of the Γ phase of the plating layer and the state of crack generation by temper rolling according to the cooling rate after the plating layer alloying treatment. The cooling rate was 35 ° C./sec.
By doing so, no crack occurs in the plating layer. The cooling rate is preferably as fast as possible to suppress Γ phase growth,
If the speed is higher than 80 ° C./sec, the shape of the steel sheet will be damaged.

The temper rolling in FIG. 2 was carried out at a transfer rate of dull roll of 50 to 60%. Cracks in the plating layer were observed by observing the plating layer surface with a scanning electron microscope.
The method was performed by measuring the total crack length in a visual field of 500 times.
If the sum is within 100 μm, use the symbol △.
Those having a total sum of 100 μm or more were evaluated with the symbol x.

The alloying treatment method for producing the galvannealed steel sheet includes a method using a heating furnace alone and a method using a heating and holding furnace placed continuously or close to the heating furnace to alloy the entire width direction of the steel sheet. In some cases, the forced cooling in the present invention may be performed at the cooling rate described above.

FIG. 3 shows that the cooling rate after the alloying treatment of the plating layer was constant at 40 ° C./sec, and the elongation rate of the temper rolling was 0.1%.
This figure shows the state of occurrence of cracks in the plating layer due to the transfer rate of the Darroll during temper rolling at a constant of 5%, which was measured in the same manner as in the case of FIG. 2. However, when the transfer rate was increased to 40% or more. In this case, no crack occurs in the plating layer. However, in order to increase the transfer rate of the dal roll, it is necessary to increase the rolling reduction. If the rolling reduction is too high, the elongation rate of the steel sheet increases and the work hardens. For this reason, the transfer rate needs to be 90% or less.

[0173]

[Example] A heating furnace, a heating and holding furnace, a water mist and a forced air blower with compressed air were continuously arranged immediately above a zinc bath in a non-oxidizing furnace type continuous hot-dip galvanizing line. - through plate Shinano in de 110m / min to La
After reducing with an H ₂ -N ₂ mixed gas, the sample was immersed in a zinc bath at a bath temperature of 460 ° C., and the adhesion amount was 57 to 60 g /
m ² (one side). And immediately in the heating furnace 50
Heat to 0 ° C for 6 seconds, hold at that temperature for 16 seconds in a heating and holding furnace, cool immediately after leaving the heating and holding furnace at various cooling rates with a forced cooling device, and then change the transfer rate of Darroll. Temper rolling was performed.

Next, a test piece was sampled from the galvannealed steel sheet manufactured as described above, and the surface of the plating layer was measured for the total crack length in a visual field of 500 times with a scanning electron microscope. Those with no cracks are indicated by the symbol ○, those whose sum is within 100 μm are indicated by the symbol △,
Further, those having a total sum of 100 μm or more were evaluated with the symbol x.

The test piece was SD5 manufactured by Nippon Paint Co., Ltd.
Degreasing at 00, phosphate treatment with SD5000 manufactured by the company, and cationic electrodeposition coating of PTU-80 manufactured by the company
After the application of μm, a cross cut reaching the steel base is put, and a salt spray test (JIS Z 2371) is performed for 480 hours to measure the blister width of the coating film from the cross cut portion. , And those smaller than 2 to 4 mm were evaluated with the symbol △, and those with 4 mm or more were evaluated with the symbol x. Table 1 shows the results.

[0202]

[Table 1]

[0219]

As described above, according to the production method of the present invention, a galvannealed steel sheet having excellent corrosion resistance after painting can be produced.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a state in which cracks occur in a portion where dull roll has not been transferred in temper rolling.

FIG. 2 is a graph showing a Γ phase thickness of a plating layer according to a cooling rate after a plating layer alloying treatment and a state of occurrence of cracks due to temper rolling.

FIG. 3 shows a cooling rate of 40 ° C./sec after a plating layer alloying process.
FIG. 4 is a plot diagram showing a relationship between a transfer rate of dull roll and a state of occurrence of a crack in a plating layer under a condition where the elongation rate of the temper rolling is constant and 0.5% is constant.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroaki Demachi 5th Ishizu Nishimachi, Sakai City, Osaka Nisshin Steel Co., Ltd. Sakai Works (56) References JP-A-4-381 (JP, A) JP-A Heisei 2-274860 (JP, A) JP-A-2-274856 (JP, A) JP-A-2-175004 (JP, A) JP-A-2-170959 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] (1) A hot dip galvanizing method is applied to a steel sheet to adjust the amount of coating, and then an alloying treatment is applied to the coating layer by heating, followed by forced cooling and temper rolling by dal roll. The method of manufacturing a galvanized steel sheet, wherein the forced cooling is performed at a cooling rate of 35 to 80 ° C./sec, and the temper rolling is performed with a transfer rate of dull roll of 40 to 90%. Manufacturing method of galvannealed steel sheet with excellent corrosion resistance.