JPH04235266A - Manufacture of alloying galvannealed steel sheet excellent in workability and corrosion resistance - Google Patents

Abstract

PURPOSE:To offer a method for manufacturing a alloying galvannealed steel sheet excellent in workability and corrosion resistance. CONSTITUTION:In a Sendzimir-type hot-dip galvanizing line, the steel sheet to be plated is plated by using a hot-dip galvanizing bath constituted of, by weight, 0.001 to 0.05% Si, 0.05 to 0.20% Al and the balance Zn with inevitable impurities, and after that, the above plated steel sheet is heated and alloyed. Furthermore, at least one face of the steel sheet is coated with an organic film by 0.1 to 4.0g/m<2>, by which a galvannealed steel sheet excellent in workability and corrosion resistance can be manufactured.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a method for manufacturing alloyed hot-dip galvanized steel sheets, which are mainly used for automobiles, in which the plated layer is formed into a Fe-Zn alloy layer by hot-dip galvanizing followed by heat diffusion treatment. .

0002

[Prior Art] So-called alloyed hot-dip galvanized steel sheets, which are hot-dip galvanized and then heat-alloyed, are widely used in automobiles, home appliances, and other applications due to their excellent paintability and weldability. Its production volume is on the rise. Further, properties required for alloyed hot-dip galvanized steel sheets include corrosion resistance, workability, weldability, and paintability. Among these, workability is one of the particularly important characteristics. If the plating layer peels off (flaking, powdering) during processing, processability will decrease. In making this improvement,
Currently, research and development is actively being conducted to optimize the steel type, plating pretreatment, hot-dip plating conditions, alloying heating conditions, etc. It has been confirmed that the workability of alloyed hot-dip galvanized steel sheets deteriorates due to the formation of a thick hard and brittle Γ layer in the alloy layer, and various techniques have been proposed to counter this problem. Currently, no technology has yet been found that embodies the idea of achieving high workability by modifying the form of the alloy layer itself.

[0003] For example, JP-A-56-1, which is galvanized with a very small amount of Al in a hot-dip galvanizing bath and then alloyed.
No. 13470, JP-A-58, in which a steel sheet before galvanizing is pre-plated with Fe, Ni, etc., galvanized, and alloyed.
-104163, JP-A No. 60-110859, etc., however, the alloyed hot-dip galvanized steel sheets obtained from these techniques have a powder-like or It has the disadvantage that it easily peels off in the form of flaking. The cause of this is the alloy layer structure of the plating layer as described above, and the Fe-Zn
The phase structure of the alloy layer caused by interdiffusion is Γ, δ1, and ζ from the base metal side with a high Fe diffusivity toward the plating surface, and these are formed in an orderly layered manner parallel to the base surface. Therefore, it is thought that when a certain processing stress is applied during processing, stress concentration occurs in the hard and brittle Γ phase with the highest Fe content, which causes the plating layer to peel off. Furthermore, regarding corrosion resistance improvement,
Improvements can be made by increasing the amount of plating deposited, but in this case, the Γ layer also becomes thicker, resulting in a problem of reduced workability, making it difficult to achieve both workability and corrosion resistance.

0004

[Problems to be Solved by the Invention] In the conventional techniques described above, the formed alloy layer has a hierarchical structure that is relatively parallel to the base steel plate. For this reason, when processed, stress is concentrated in the most brittle part of the alloy layer, resulting in cracks. As a result, the plating layer peels off from the iron base in a powder form, causing a practical problem. In the present invention, it has been found that the problem of the workability of the alloy layer, which the prior art has, can be solved by applying the following method. In other words, in order to improve the workability of alloyed hot-dip galvanized steel sheets, we believe that it is necessary to change the formation form of the alloy layer to mainly a discontinuous δ1 phase mixed with a ζ phase that can disperse stress. As a result of various studies, we found that a specific amount of Si was added to the hot-dip galvanizing bath.
After plating in a bath to which Al is added, heat alloying treatment is performed to form an alloy phase in which the above-mentioned δ1 phase and ζ phase are mixed, thereby increasing the toughness of the alloy layer. The present inventors have discovered that by applying an organic film to a steel plate, it is possible to significantly improve workability and corrosion resistance by imparting lubricity to the steel plate surface, and have proposed the present invention.

[0005]

[Means for Solving the Problems] The gist of the present invention is to use a Sendzimer type hot-dip galvanizing line to coat a steel plate with Si: 0.001 to 0.2% by weight and Al: 0.05 to 0% by weight.
．． After plating using a hot-dip galvanizing bath consisting of 20% by weight, the balance being Zn and unavoidable impurities, the plated steel sheet is heated and alloyed, and at least one side of the steel sheet is further coated with an organic film of 0.1 to 4.0 g/m2. This is a method for producing an alloyed hot-dip galvanized steel sheet with excellent workability and corrosion resistance.

[0006]

[Function] In the present invention, Si and Al in the plating bath are Fe-
During the generation or growth process of the Zn alloy layer, Fe becomes concentrated and segregated at least in the δ1 phase at the base metal interface and its surface, and this Si and Al segregation zone acts as a barrier to suppress excessive diffusion of Fe into the ζ phase in the upper layer of δ1. This is to suppress excessive diffusion of Fe into the ζ phase, where processing stress tends to be dispersed, and at the same time to promote the mixing phenomenon of the ζ phase and δ1 phase, where processing stress is easy to disperse. This is to prevent the formation of Fe over-diffusion alloy layers such as Γ1 phase and Γ phase, and at the same time to suppress the layered growth thereof. If the concentration of Si in the plating bath is less than 0.001% by weight, this effect will not be sufficiently exhibited. If it exceeds 0.2% by weight, Si in the plating bath is taken into dross and disappears, resulting in a decrease in Si addition yield and is not economical. The Al
If the concentration of Al in the plating bath is less than 0.05% by weight, this effect will not be fully exhibited, and if it exceeds 0.20% by weight, the effect of suppressing Fe diffusion by Al will be strong, and it will take a long time to complete the alloying process of the plating layer. It takes too much time and is not practical. Furthermore, even if Pb, Cd, and Sb, which are often used as spangle adjusting elements, are added to the plating bath in a total amount of less than 0.30% by weight, the effects of the present invention will not be affected in any way. isn't it. The plating amount is 2
A range of 0 to 100 g/m2 is preferred. If it is less than 20 g/m2, corrosion resistance will be insufficient, and if it exceeds 100 g/m2, the film will be too thick, requiring time for alloying treatment, and productivity will be impaired, which is not very preferable.

Furthermore, an organic coating is applied to at least one side of the steel plate. The organic film herein refers to a layer formed of an organic compound such as epoxy resin, polyacrylic resin, polyurethane resin, polyethylene, polystyrene, polyacetylene, polyester, nylon, or polyimide. The presence of the organic film can suppress adhesion of the plated metal to the mold, resulting in improved press workability and improved corrosion resistance. The amount of organic film deposited is preferably 0.1 to 4.0 g/m2. 0.1
If it is less than g/m2, it is difficult to completely cover the lower plating layer, and adhesion of the exposed portion of the lower layer to the mold may occur, which is not preferable. If it exceeds 4.0 g/m2, it is not preferable because it tends to deteriorate the electrodeposition coating property, which is an important characteristic required for anticorrosion steel sheets for car bodies, which is the main application of alloyed galvanized steel sheets. As the resin for the organic coating, an epoxy resin is preferable because it has excellent adhesion to the steel sheet, but since the surface condition of the alloyed hot-dip galvanized steel sheet is rich in irregularities, there is no problem in adhesion with any resin. Also, it does not particularly matter whether it is solvent-based or water-based. The presence of pigment additives such as silica in the organic film is effective as long as the lubricity is not impaired. Further, addition of wax is preferable because it improves lubricity. Additionally, chemical conversion treatments such as phosphate treatment and chromate treatment (electrolytic chromate, reactive chromate, and coating chromate) are effective as pre-painting treatments to improve the adhesion of organic films, and these are also within the scope of the method of the present invention. Enter.

[0008]

[Example] After annealing low carbon titanium-added steel that was cold-rolled to a plate thickness of 0.8 mm and a plate width of 914 mm on a Sendzimer hot-dip galvanizing line, it was immersed in a hot-dip galvanizing bath and fixed to the specified size using a gas wiping method. Immediately after plating to a coating amount of Chromate treatment was carried out so that the amount of Cr deposited was 40 to 60 mg/m2, and an organic coating was further applied using a roll coater. Table 1 shows a list of the steel plate manufacturing conditions and evaluation results at that time.

[0009]

[Table 1]

[0010] Example No. of the present invention. 1 to 3 are Fe in the alloy layer
Effect of weight %, no. No. 4 and 5 are influenced by the plating bath Si concentration. No. 6 and 7 are influenced by the plating bath Al concentration. 8,
No. 9 was tested for the influence of the amount of coating, and all had good alloy layer structure, workability, and corrosion resistance. Also No. 10,11
This study investigated the effects when Pb, Cd, and Sb, which are spangle adjusting elements, were added, and it was found that there were no particular problems. No. Nos. 12 and 13 are Nos. 12 and 13 when the organic film thickness is thin. No. 14 indicates a thick case; 15, 16, 17
This study investigated resins other than epoxy, and all of them had good processability and corrosion resistance. On the other hand, comparative example No.
．． 1 is Si, No. 2 has no added Al, so
The alloy layer structure is a hierarchical alloy layer parallel to the base material, resulting in poor workability. Also No. In No. 3, the alloying reaction did not occur because the Al concentration was too high. No. Sample No. 4 was a case in which no organic film was applied, and the flaking resistance and corrosion resistance were poor in workability.

[0011] The cross-section of the alloy layer type plating layer was observed under a microscope, and a score was given based on the following criteria. ○: δ1-based alloy phase in which δ1 phase is discontinuous due to ζ phase △: Γ phase appears and slightly hierarchical alloy layer ×: Γ,
Hierarchical alloy layer in which the δ1 and ζ phases are aligned and generated parallel to the substrate XX: Workability without alloying Powdering resistance 60 degree V bending and peeling tape, and score based on the following criteria Ta. ○: Peeling width on one side is less than 2 mm Δ: Peeling width on one side is 2 mm or more and less than 5 mm ×: Peeling width on one side is 5 mm or more Flaking resistance Evaluated by tension molding with square beads. 2.0 kgf/cm2 between punch and die (plug size 0.7
75 x 280 mm), and then passed through the bead portion while pulling the test piece. 200 sheets were repeatedly molded, and the degree of deposition of the plating layer metal on the steel plate or bead portion was evaluated relative to the other. (Good) ○-△-× (Poor) Corrosion resistance cycle corrosion test (5% salt water immersion for 10 minutes → 5
0℃, RH95% or more humidity 10 minutes → 60℃, RH40
% or less drying for 10 minutes for 1 cycle) After 2000 cycles, the amount of reduction in board thickness was measured and scored based on the following criteria. ○: 0.1mm or less △: More than 0.1mm, 0.2mm or less ×: More than 0.21mm

0012

As explained above, the present invention changes the plating layer structure of an alloyed hot-dip galvanized steel sheet by adding Si and Al to the hot-dip galvanizing bath, and processes it by applying an organic film to the upper layer. This greatly improves the properties and corrosion resistance of galvanized steel sheets, and expands the applications of galvanized steel sheets.
This has great industrial effects.

Claims (1)

[Claims] Claim 1: In a Sendzimer type hot-dip galvanizing line, a steel plate to be plated is heated in a molten state consisting of 0.001 to 0.2% by weight of Si, 0.05 to 0.20% by weight of Al, and the balance being Zn and unavoidable impurities. After plating using a galvanizing bath, the plated steel sheet is heated and alloyed, and at least one side of the steel sheet is further coated with an organic film of 0.1 to 4.0 g/m2, which has excellent workability and corrosion resistance. A method for producing alloyed hot-dip galvanized steel sheets.