JPH04147953A - Production of galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To obtain the steel sheet excellent in rust resistance in an uncoated worked part or in a coated scratched part by exerting Ni precoating and then performing the production of the steel sheet under specific heating conditions. CONSTITUTION:After the surface of a steel sheet is plated with an Ni precoating layer by 0.2-2g/m<2>, rapid heating is done in a nonoxidizing or reducing atmosphere up to 430-500 deg.C sheet temp. at >=30 deg.C/sec temp.-rise rate. This steel sheet is subjected to hot dipping in a Zn plating bath containing 0.05-0.25% Al and then to alloying heating treatment at 470-550 deg.C for 10-40sec at the position right above wiping. By lowering the heating temp. after Ni precoating and raising the temp.-rise rate as mentioned above, the diffusion of the Ni precoated layer into ferrite can be inhibited, and hot dipping and alloying treatment can be performed in the state where the degree of activation and barrier effect of Ni are increased. As a result, Ni distribution in the plated layer can be uniformized and GAMMA-phase in the ferrite interface can be reduced, and the purpose can be accomplished.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing an alloyed hot-dip galvanized steel sheet using the pre-Ni method.

[Prior Art] In the past, alloyed hot-dip Zn-plated steel sheets have already been known as steel sheets for automobiles or constructions that have excellent paint film adhesion, corrosion resistance after painting, and the like. In addition, recently, the special public
As seen in No. 9386, a method for manufacturing an alloyed hot-dip Zn-plated steel sheet using a pre-Ni method is proposed, and specifically, it is disclosed that the method is carried out in a non-oxidation furnace type hot-dip plating line.

[Problems to be Solved by the Invention] Generally, when surface-treated steel sheets for automobiles or construction are used after being painted, phosphate treatment is applied as a pretreatment.
Since Fe exists in the plating layer of alloyed hot-dip Zn-plated steel sheets, the phosphate crystals in the chemical conversion treatment change and become denser, which improves the adhesion of the paint film and prevents scratches after painting. It has advantages such as excellent blister resistance.

However, in the case of a Zn-Fe plating layer, the anti-corrosion effect against corrosion products is small, so for example, the red rust resistance of bare parts that are subjected to severe processing such as the hem of an automobile door and that does not go around the paint, or the painted material. However, the method of manufacturing alloyed hot-dip Zn coated steel sheets using the Ni pre-plating method has the disadvantage of poor red rust resistance in uneven areas, such as when scratches are caused by flying stones. Because a uniform, stable, clean and active surface is obtained, it is possible to obtain a paddy layer with good adhesion to high-strength steel plates containing many alloying elements, and the corrosion resistance is improved by the Ni bure plating method. However, there is still room for improvement in terms of the red rust resistance of parts that have undergone particularly severe processing and the red rust resistance of uneven painted areas, and improvements in these areas have been desired. . Therefore, the present inventors have investigated various ways to improve the red rust resistance of the severely processed parts and painted uneven parts of the plated steel sheet obtained by the manufacturing method of alloyed hot-dip Zn-plated steel sheet using the pre-Ni plating method. However, we have found that the red rust resistance of unpainted parts or uneven painted parts is significantly improved when manufactured under specific heating conditions after pre-N1 plating.

As described above, the present invention provides a method for manufacturing an alloyed hot-dip Zn-plated steel sheet that has excellent red rust resistance in processed parts.

[Means for Solving the Problems] The present inventors first conducted a process for producing an alloyed hot-dip plated steel sheet using the Ni blemish plating method described in Japanese Patent Publication No. 61-9386, after pre-plating. heating to 780℃
An alloyed hot-dip galvanized steel plate was prepared by applying the same process for 18 seconds, and its performance and structure were investigated. Although it showed excellent performance in terms of paint film adhesion, etc., The effect of improving red rust resistance was small. Regarding the structure of the plating layer, it was observed that Ni was not uniformly distributed to the top of the plating layer, and that the r phase (FeaZnzz) at the interface between the steel base was likely to develop. The cause of this is that the heating temperature after pre-plating is too high and the heating rate is slow, which causes the pre-plating layer to diffuse into the steel base during the pre-treatment heating stage, and as a result, during plating and alloying treatment. Because the pre-plating layer reacts with Zn, it is difficult for Ni to diffuse uniformly into the Zn-Fe alloy layer, and if pre-Ni is diffused into the base steel sheet, the barrier effect at the base metal interface is small. This is thought to be due to the fact that the alloy phase grows easily.

Therefore, the present inventors suppressed the diffusion of the pre-Ni layer into the steel base as much as possible by lowering the heating temperature after pre-plating and increasing the temperature increase rate, thereby improving the activity and barrier effect of Ni. We believed that performing hot-dip plating and alloying treatment while maintaining the temperature was the key to manufacturing, and we conducted repeated experiments to find out. As a result, the heating temperature after Ni blur plating was 430~
Rapid heating was performed to 500℃ at a temperature increase rate of 30℃/sec or more, hot-dip plating was performed in a Zn plating bath containing 0.05 to 0.25% A1, and alloying treatment was performed immediately above the wiping.
Only when the process was carried out at 70 to 550°C for 10 to 40 seconds, the Ni distribution in the plating layer became uniform and the r-phase at the substrate interface was suppressed to a thin layer, and compared to the conventional method, the processed parts and uneven painted parts were suppressed. The inventors have discovered that the red rust resistance of the material is significantly improved, and have completed the present invention as described below.

That is, 0.2 to 2 g/Ni blur plating layer is applied to the surface of the steel plate.
After plating, the plate temperature is 430℃ in a non-oxidizing or reducing atmosphere.
After rapid heating to ~500°C at a temperature increase rate of 30°C/sec or more, Zn containing AlO, 05% to 0.25%
Hot-dip plating in a paddy bath, 470 ~ just above the wiping
This is a method for manufacturing an alloyed hot-dip Zn-plated steel sheet, characterized in that alloying heat treatment is performed at 550° C. for 1θ to 40 seconds.

Hereinafter, the present invention will be explained in detail using the drawings.

FIGS. 1(a), 1(b), and 1(c) are diagrams showing the relationship between pretreatment heating plate temperature and red rust resistance of processed parts and uneven painted parts.

Pre-N on hot rolled^2 killed steel plate (1.6mm 5PHC)
After plating the i-plated layer at 0.5g/m2, 0260ppm
, After heating at a heating rate of 70°C/sec to a plate temperature of 400 to 700°C in an 823% N2 atmosphere, ^Q
Hot-dip plating for 3 seconds in a Zn plating bath at 450°C containing 15% O, followed by heat treatment of gold alloy at 520°C for 15 seconds directly above the wiping to give a Fe content of 11% and a coating weight of 60g.
/m2 of alloyed fused Zn-finished steel plate was prepared. The red rust resistance of the machined part was examined by a salt spray test (SST) after deep drawing a test piece into a 25 mm cylindrical overhang, in order to simulate the actual severe machined part. The evaluation was based on the spraying time until the occurrence of %. Also,
In the case of unevenly painted areas, make cross cuts in the electrodeposition coating (film thickness 20 μm) material with a cutter knife in advance, and
Evaluation was made based on the incidence of red rust at the cross-cut portion in 1 week of ST. Each evaluation was made on a five-point scale. The evaluation criteria are as follows.

Evaluation Red rust occurrence time ()Ir) Red rust occurrence time)
5...120~200 Less than 104...90~Less than '120 1O~30
3...70-less than 90 30-502
... 50-less than 70 50-901 ...
...Less than 50 90-100 (*Evaluation of 3 or higher is acceptable) From this figure, it can be seen that the red rust resistance of processed parts and painted uneven parts is good when the heating plate temperature before melt-wiping is in the range of 430-500°C, which is the range of the present invention. It is clear that an alloyed hot-dip Zn polished steel sheet having excellent properties (hereinafter both are collectively referred to as red rust resistance) can be obtained. When the temperature exceeds 500°C, the red rust resistance of the processed portion deteriorates, and good red rust resistance cannot be obtained under the pretreatment heating conditions of the prior art near 780°C. Furthermore, if the temperature is lower than 430°C, non-plating tends to occur during hot-dip plating.

Furthermore, FIG. 2 shows the relationship between heating rate and red rust resistance.

Pre-N on hot rolled A2 killed steel plate (1.6mm 5PHC)
0.5g/m” after plating the 1st layer, 0260ppm
After heating the plate to a temperature of 450°C in an 823% N2 atmosphere, hot-dip plating for 3 seconds in a 450t Zn plating bath containing 15% AnO, and heating the gold alloy for 15 seconds at 520°C directly above the wiping. Through the treatment, an alloyed hot-dip Zn-plated steel sheet with an Fe content of 11% and a coating amount of 60 g/m2 was produced.

It is clear that the red rust resistance is good when rapid heating is performed at a temperature increase rate of 30 t/sec or more according to the present invention. If the heating rate is 30 t/sec without grooves, the red rust resistance will deteriorate. The deterioration in red rust resistance at a temperature increase rate corresponding to the conventional technology range is considered to be due to the inability to obtain good plating adhesion.

Based on these results, in the present invention, a low heating temperature after blur Ni plating and a high temperature increase rate are important for producing alloyed hot-dip Zn-plated steel sheets with excellent red rust resistance. This is a major point. The method of rapid heating is not particularly limited, but various methods can be applied, such as a method of directly heating the steel plate with electricity, and an induction heating method.

[Function] The results of analyzing the structure of the plating layer obtained by the manufacturing method of the present invention and the results of the prior art are schematically shown in FIG. In the case of the pre-treatment heating plate temperature and temperature increase rate after Ni blur plating in the range of the present invention, almost no diffusion of the pre-Ni layer into the base steel is observed during heating, whereas in the conventional technology range In the case of heating temperature and heating rate of , most of Ni diffuses into the steel base during heating and changes to a solid solution layer of Fe-Ni. It has been found that this difference in the state of Ni during heating seems to cause a difference in the plating layer structure during the subsequent hot-dip plating and alloying treatments.

That is, Zn, Fe, Nl, and A1 are relatively uniformly distributed in the plating layer of the alloyed hot-dip Zn-plated steel sheet manufactured by the method of the present invention, and the Zn-N1-An-Fe quaternary alloy layer It had a more structured structure. In addition, the r phase at the substrate interface is also 0
．． The thickness was suppressed to within 5 μm. Although the details are not yet clear, the reason why the growth of the r-phase was suppressed in the case of the present invention is that the pre-Ni layer that remains as it is during heating forms a N1-AIL-Zn-based barrier layer during hot-dip plating. Since it was observed that it was formed, it is thought that it acts as a barrier to r-phase growth at the stage of alloying treatment. The improvement in red rust resistance is due to improved plating adhesion due to the suppression of the r-phase, and the uniform inclusion of Ni and Al1 in the plating layer to the surface layer, which reduces corrosion products during the salt spray test. In other words, the stabilizing effect of non-conductive basic zinc chloride, which makes it difficult for the oxygen reduction reaction which is the cathodic reaction of the corrosion reaction, and the effect of suppressing the oxygen diffusion reaction (barrier effect) work synergistically. It seems to be.

The reason why the deposition amount of the blur N1 plating was set to 0.2 g/ya' or more is because the effect of improving the corrosion resistance of the processed area due to Ni was observed when the amount was more than 0.2 g/ya'. In addition, if it is less than 0.2 g/m", unplated parts are likely to occur when fitting. The upper limit is 2 g/m".
The reason for choosing m' is that if this value is exceeded, the Ni content in the plating layer becomes too high, and during the corrosion process, Ni, which is electrochemically responsible, forms a local battery, which actually deteriorates the corrosion resistance. . In addition, the lower limit of A standing in the bath was set at 0.05% because if it is less than this, alloying will proceed too much during alloying treatment, too much R phase will be formed at the interface of the base metal, and the plating of the alloy layer will occur. This is because the adhesion and red rust resistance of the processed part do not improve.

Also, the upper limit of A sentence in the bath was set to 0°25% because AN was 0.
．． If it exceeds 25%, Nj-AU-Z during plating
This is because a Fe-An-Zn-based barrier layer is likely to be formed in addition to n, and alloying does not proceed during alloying treatment, making it impossible to obtain good coating film adhesion.

The optimum temperature for alloying treatment is 470 to 550°C. 47
If it is below 0℃, alloying will be difficult to proceed and good coating adhesion will not be obtained.If it exceeds 550℃, alloying will proceed too much, R phase will develop easily at the base metal interface, and plating adhesion will deteriorate. The alloying time is determined by the balance with the alloying temperature, but a range of 10 to 40 seconds is appropriate. If it takes less than 10 seconds, alloying will be difficult to proceed and good coating film adhesion will not be obtained. Moreover, if it exceeds 40 seconds, alloying progresses too much, the r phase tends to develop, and plating adhesion and red rust resistance deteriorate.

There are no particular restrictions on the amount of plating deposited, but from the viewpoint of corrosion resistance it is preferably 10 g/m2 or more, and from the viewpoint of workability it is preferably 150 g/+n2 or less.

The above results were explained for the case of only Zn plating bath, but Ni, Sb,
Similar results were obtained for alloyed hot-dip Zn-plated steel sheets containing Pb alone or in combination in a trace amount of 0.2% or less. In addition, regarding the bath temperature, even if it is a Zn bath or a case where Zn contains a trace amount of alloying elements, the normal 43
Conditions of 0 to 500°C can be used.

As the base steel plate, both hot-rolled steel plate and cold-rolled steel plate can be used, including ^2 killed steel plate, Al1-5i killed steel plate, Ti-5
ulc, P-TiSulc low carbon steel plate, high St, M
Various materials such as n-based high-tensile steel plates can be used.

[Examples] Table 1 shows examples of the manufacturing method of the present invention and the steel plates obtained. The * mark is a comparative material made by a manufacturing method other than the present invention. A pickling material for hot rolled steel plate 5PHC (1.6 mm) was used as the base, and Ni plating was performed by electroplating in a sulfuric acid acid bath. After pretreatment heating (0260 ppm, H23%, balance Nz), hot-dip plating was performed at a bath temperature of 450° C. for 3 seconds. After wiping, alloying heat treatment is performed at 450~
Testing was carried out at 550° C. for 5 to 40 seconds to prepare samples with various plating layer compositions. The coating weight was 60g/m''.The red rust resistance was evaluated according to the evaluation criteria described above.
The 50% time of occurrence of red rust on the cylindrical drawn material and the incidence of red rust on the cross-cut portion of the painted material were investigated, and each was evaluated using a 5-point method, and a comprehensive evaluation was made using a 5-point method considering both results. A score of 3 or more was considered a pass. In addition, the blister resistance (paint film adhesion) of the cross-cut portion of the painted material after 4 weeks of SST was also evaluated. The rating in this case is that the blister width is 3
○ for 11II11 or less, △ for 3 mm to 4 mm or less,
A value of more than 4 mm was evaluated as x, and a value of 6 or more was evaluated as a pass.

As shown in No. 1 to 18, Ni blur plating layer 0.2 to
2 g/m2, heating plate temperature 430-500℃, heating rate 3
0'e/see or more, AI in bath 0.05-0.15%
The plated steel sheet obtained under the manufacturing conditions of the present invention, in which the alloying heating conditions are 470 to 550° C. for 10 to 40 seconds, has excellent red rust resistance in processed parts and uneven painted parts, and excellent blister resistance of the coated material. Compared to this, the case without blurring Ni (No.
, 21), when the amount of deposited Ni plating layer, heating plate temperature, temperature increase rate, An in the bath, and alloying treatment conditions deviate from the scope of the present invention (No. 19 to 30), the processed part and The red rust resistance of uneven painted areas is poor.

Furthermore, Nos. 31 to 33 were cases in which a small amount of other alloying elements were contained in the plating bath, and excellent performance was also shown in this case.

[Effect of the invention] As described above, according to the manufacturing method of the present invention, alloyed molten Z
It has extremely great industrial significance because it provides red rust resistance in processed parts and uneven parts of the coating film, which is unprecedented for n-plated steel sheets.

[Brief explanation of drawings]

Figures 1 (a), (b), and (c) are diagrams showing the relationship between pretreatment heating plate temperature and red rust resistance of processed areas and unevenly painted areas.Figure 2 shows the relationship between heating rate, processed areas, and uneven painted areas. FIG. 3 is a diagram schematically showing the structure of a matted layer obtained by the manufacturing method of the present invention. The other 4 people's value is ＼ｼC) ｺｻﾉﾉﾉﾉﾉｲﾆﾆﾆﾆ; Praise W4 = to Obe R to L cloud 5 carp: sword B; herobe t) L back C blue cloud haze map: j” Hft G @l!I familiar 4 poems (this invention) 430-500 ℃ figure (prior art) 780℃

Claims (1)

[Claims] 1 Ni pre-plating layer of 0.2 to 2 g/m on the surface of the steel plate
^2 After plating, the plate temperature is 43℃ in a non-oxidizing or reducing atmosphere.
Zn containing 0.05 to 0.25% Al after rapid heating to 0 to 500 °C at a temperature increase rate of 30 °C/sec or more
Hot-dip plating in a plating bath, 470~ just above the wiping
A method for manufacturing an alloyed hot-dip Zn-plated steel sheet, comprising performing alloying heat treatment at 550°C for 10 to 40 seconds.