JP6831617B2 - Hot-dip galvanized steel sheets with excellent corrosion resistance and alloyed hot-dip galvanized steel sheets and their manufacturing methods - Google Patents

Description

The method of the present invention relates to a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet, which are excellent in corrosion resistance, particularly corrosion resistance of a joint portion of an inner plate member of an automobile.

Hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets have excellent corrosion resistance, coating adhesion, etc., and are widely used as steel sheets for building materials, home appliances, and automobiles.
In recent years, the frequency of use of hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets for the purpose of rust prevention in automobile materials has become extremely high, and demand for them has increased sharply. In addition, from the viewpoint of overseas use and manufacturing, not only domestic rust preventive specifications but also corrosion resistance under severe corrosion environment worldwide has been strongly required.
In particular, the corrosion of materials is severe in areas where snowmelt salt is sprayed, such as in Scandinavia and North America, and salt adheres to suspension members, arms and other undercarriage members, rockers, side sills, floor pans and other vehicle body lower members. As a result, corrosion progresses more significantly than in the domestic environment.
In response to these, automobile manufacturers have been trying to improve corrosion resistance, especially for outer panels, by taking measures such as increasing the amount of zinc plating attached and increasing the coating film thickness.

On the other hand, both the increase in the amount of galvanized adhesion and the increase in the coating film thickness not only bring about an increase in cost, but also increase the coating film thickness because the time required for the painting process of the automobile is extended. There is a problem of deteriorating productivity.
In particular, the effect of increasing the coating film thickness cannot be expected at the undercarriage members such as electrodeposition coating and spray coating, which are difficult to get around, and the joint parts such as welding between steel plates under the vehicle body, and the amount of plating adhered increases. However, it is difficult to take measures because the weldability of the mating part deteriorates.

In order to solve the problem of insufficient corrosion resistance of the laminated portion, a method of improving the corrosion resistance of the plating itself and a steel sheet that has been subjected to an organic coating treatment in advance have been studied as a means for improving the corrosion resistance without relying on the amount of plating adhesion or the increase in coating film thickness. ..

For example, in Patent Document 1, 0.1 to 1.2% by weight of Mg or Sn is added to the galvanized layer to further provide a chromate film layer, and in Patent Documents 2 to 4, various rust preventive pigments are contained. Corrosion resistance is improved by giving an organic film.

However, the invention described in Patent Document 1 is plating in which Mg or Sn is added in a maximum of 1.2% by mass, and has the composition of a conventional alloyed hot-dip galvanized steel sheet (Fe8 to 15% by mass-remaining Zn). Not only is there a considerable deviation, but it is also environmentally unfavorable because it exerts its effect by imparting environmentally unfavorable chromate 10 to 200 mg / m ² to its surface. Furthermore, at this concentration, blackening due to spangles and oxides occurs, which is not suitable for automobile use in terms of appearance quality.
In the inventions described in Patent Documents 2 to 4, the corrosion resistance is improved by applying an organic film to various metal base materials, but since the post-treatment process is obliged to be incorporated after the plating process, the manufacturing cost is increased. Challenges remain.
In addition, when these products are used by users, not only can some of them be used for materials that want to improve the corrosion resistance of the mating parts such as undercarriage members and lower parts of the car body, but also some of the samples can be used for the inner and outer panel panels. Considering that there is a property, it is excessive quality for general use for inner and outer panel panels, and post-processing process cost is incurred there, so there is a problem in economic efficiency on the user side. Remains.
In the invention described in Patent Document 5, one or more of nickel, cobalt, and iron are pre-plated on the surface of the steel sheet before annealing, and nickel, cobalt, copper, and tin are used for the purpose of ensuring the wettability of the plating after annealing the steel sheet. A manufacturing method is known in which a second pre-plating containing the above-mentioned material is performed, and then hot-dip zinc plating is performed to form a heat alloy. The publication states that the maximum second pre-plating amount is 1 g / m ² . For example, assuming a general zinc plating amount of 50 g / m ² as in the examples, the maximum concentration in the plating layer is 2% by mass, which is a noble element than zinc, such as nickel, cobalt, copper and tin. By including an amount on the order of several percent, in an undercarriage environment where it is difficult for the paint to stick around, a local battery is formed and the corrosion resistance is rather deteriorated. Therefore, for the purpose of improving wettability, the invention was made with the corrosion resistance in the part to be painted firmly such as the outer panel in mind, and the undercarriage members that are difficult to get around such as painting and the welding of steel plates under the car body, etc. Corrosion resistance cannot be expected at the mating part.

Japanese Unexamined Patent Publication No. 4-198492 Japanese Unexamined Patent Publication No. 10-128906 Japanese Unexamined Patent Publication No. 11-5269 Japanese Unexamined Patent Publication No. 2004-42222 Japanese Unexamined Patent Publication No. 2012-172216

In view of the above problems, the present inventors have made it possible to use them as rust-preventive steel sheets, which are generally used for automobiles such as inner and outer panel panels, while aiming at improving the corrosion resistance of the mating portions. It provides materials with low cost and low manufacturing cost.

In order to improve the corrosion resistance of the laminated portion, the present inventors have selected a means for improving the corrosion resistance of the plating layer itself without relying on post-treatment, in order not to increase the manufacturing cost. Furthermore, corrosion resistance without significantly deviating from the components of the base galvanized steel sheet so as not to create a new load in the automobile production process of hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that are already used in large quantities. I decided to consider the means to increase. Here, as a range that does not deviate significantly, in order to control alloying and prevent zinc oxidation, aluminum added in the zinc plating bath is unavoidably contained in the plating layer by being brought into the plating layer. The content was set in a range not exceeding the maximum value of 1.0% by mass.

The corrosion resistance of the mated portion means, for example, the corrosion resistance in a slight gap formed by overlapping the flanges of the welded portion, and since phosphate chemical conversion treatment and electrodeposition coating are difficult to adhere to, alloying hot dip galvanizing is used. It is the ability to be exposed without chemical conversion treatment or painting treatment and to be corroded by the penetration of water or the like.

As described above, once water enters the mating portion, drying itself is unlikely to occur due to the narrow gap, air flow is suppressed, and gap corrosion occurs mainly in the vicinity of the spot welded portion at the flange portion. Therefore, as a means for improving the corrosion resistance at the mating portion, it is necessary to prevent the formation of an oxygen concentration cell, which is a direct cause of crevice corrosion, or to perform plating that suppresses the anode reaction at the oxygen-deficient portion.

However, the former measure to suppress oxygen shading is not a realistic measure because it results in not creating a gap in the first place. Therefore, the present inventors decided to investigate the latter means of suppressing the anodic reaction.

The anodic reaction of zinc plating is a reaction in which zinc described by the following formula (1) is eluted to form zinc ions. Zinc ions continue to hydrolyze in the presence of water and change into corrosion products such as hydroxides (Equation (2) below). Furthermore, the presence of halogen ions such as chloride ions stabilizes zinc ions, and it is predicted that the anodic reaction (1) in which zinc elutes will be accelerated.
Anode ^{reaction: Zn → Zn 2+ + 2e -} ····· (1)
Hydrolysis reaction: Zn ²⁺ + 2H ₂ O → Zn (OH) ₂ + 2H ⁺ ... (2)
Cathode _{reaction: (1/2) O 2 + 2H} 2 O + 2e - → 2OH -
(1) and (2) are basically continuous reactions, and increasing the corrosion resistance to the plow means suppressing either or both of the reactions represented by the formulas (1) and (2). Just do it.

The present inventors paid particular attention to the formula (2) among the above two reaction formulas. In equation (2), H ⁺ is released at the tip where corrosion occurs, and acidification occurs at the tip of corrosion. Therefore, it was decided to study a method of suppressing the dissolution of zinc by the corrosive substances generated there by utilizing the acidification by H ⁺ generated.

The simplest means of suppressing zinc corrosion due to acidification is to add a noble alloying element to zinc to bring the plating layer to a noble potential, thereby suppressing the elution and hydrolysis of zinc by H ^+. is there. However, in general, a change in potential due to an alloying element requires the addition of several% or more in mass%. Not only is this likely to deviate significantly from the composition of alloyed hot-dip galvanized steel sheets that have been used in large quantities, but also precious metals (platinum, gold, etc.) that are generally expensive for potentially noble elements. (Silver, copper, etc.) are abundant, and cost difficulties are unavoidable. Therefore, even if measures are taken by adding precious metals to the plating layer, no merit for the automobile industry can be expected so much.

Therefore, we have made extensive studies on methods for suppressing the dissolution of zinc with the produced zinc corrosion products. In particular, the addition amount to the galvanized layer is limited to a maximum of 1.0% by mass so that the addition of elements to the plating does not significantly deviate from the zinc plating composition. A method that can improve the corrosion resistance of the mated parts, maintain various performances such as normal post-painting corrosion resistance, plating adhesion, and weldability required for inner and outer panel applications, and provide a method that can be provided without a new treatment process such as post-treatment. investigated.

At the tip of corrosion, the dissolution of zinc proceeds as the anode reaction (1), but as a means to avoid this, it is necessary to add an element lower than zinc. Specifically, aluminum, magnesium, etc. can be considered. Although it is an element that is not necessarily disadvantageous in terms of cost, the former is still added to the galvanized bath in a certain amount for bath management and alloying control, but it becomes an oxide of aluminum. As a result, it becomes a noble potential and the effect is not exhibited. Magnesium can be expected to be effective, but since it is an element that is easily oxidized, there are major operational problems. In any case, after all of these elements are anodic-dissolved, the anodic dissolution reaction of zinc proceeds as a result.

Next, a means for suppressing the hydrolysis reaction (2) was considered. Since it is acidified at the tip of corrosion, we investigated a method to improve corrosion resistance by dissolving some element under acidic conditions to form a corrosion product and coexisting with the corrosion product of zinc. However, an element having a lower potential than zinc is expected to elute unilaterally and lose its effect like magnesium described above, so it is a noble element than zinc and is under acidic conditions. However, we diligently examined what could be eluted with zinc. As a result, it was found that among the amphoteric metals, tin and lead are particularly effective when added in a small amount.

The present invention has been made based on the above findings, and the gist of the present invention is
(1) In mass%, tin or both tin and lead are contained in the steel in a total amount of more than 0.1% and 0.625% or less, and 8 to 15% of iron and both tin or tin and lead are contained. An alloyed hot-dip galvanized steel sheet having excellent corrosion resistance, which comprises 0.001 to 0.05% in total in the plating layer.
(2) A steel sheet containing tin or both tin and lead in a total amount of more than 0.1% and 0.625% or less in mass% is passed through a hot-dip zinc bath containing 0.10 to 0.50% of aluminum. After squeezing the sheet and then wiping for adjusting the texture, the steel sheet is heated using a heating device to diffuse iron and zinc, thereby adding 8 to 15% of iron and tin or both tin and lead to a total of 0. A method for producing an alloyed hot-dip galvanized steel sheet having excellent corrosion resistance, which comprises 001 to 0.05% contained in the plating layer.
Is.

As described above, the present invention has made it possible to provide a highly corrosion-resistant steel sheet that does not depend on the amount of plating adhered or the coating film thickness to solve the problem of insufficient corrosion resistance of the joint portion of the undercarriage member and the lower member of the vehicle body of the automobile. Yes, especially the contribution to the automobile industry is extremely large.

A sample example for the corrosion resistance test of the mating part is shown.

The hot-dip galvanized steel sheet having excellent corrosion resistance in the present invention needs to contain tin or tin or tin and lead in total of 0.001% by mass or more and 0.05% by mass or less in the plating layer. If it is less than 0.001% by mass, the corrosion resistance to crevice corrosion due to oxygen shading at the mating portion is insufficient. Further, if it is added in excess of 0.05% by mass, the plating adhesion deteriorates, the sample may turn black and the appearance quality may deteriorate, and the upper limit is 0.05% by mass.

The iron composition in the alloyed hot-dip galvanized layer in the present invention is preferably 8 to 15% by mass. If it is less than 8% by mass, the continuous dot weldability, which is one of the characteristics of alloyed hot-dip galvanizing, is inferior. Further, if it exceeds 15% by mass, the plating adhesion deteriorates.

In addition to tin, lead, and iron contained in the plating layer in the present invention, the method for analyzing aluminum added as a component of a galvanizing bath is to dissolve the plating layer in hydrochloric acid, and perform ICP emission spectroscopic analysis or ICP-MS. It can be measured by means such as emission spectroscopic mass spectrometry. Further, in order to suppress the dissolution of the base material at the time of dissolution, hydrochloric acid containing 5% or less of an inhibitor can be used.

If the concentration of aluminum added in the bath is less than 0.10% by mass, the base metal and zinc are likely to be diffusion-alloyed, resulting in deterioration of plating adhesion in hot-dip galvanized steel sheets and heating alloys in alloyed hot-dip galvanized steel sheets. There is a concern that operational variations will occur at the time of conversion. The upper limit needs to be 0.50% by mass or less. Above this, the surface appearance with exposed zinc grain boundaries called spangles is likely to appear, and the hot-dip galvanized steel sheet impairs the dignity for automobile applications.

As a method of containing a total of 0.001% by mass or more and 0.05% by mass or less of both tin or tin and lead in the plating layer, both tin or tin and lead having the same concentration are contained in the hot-dip galvanizing bath. Can be produced by adding.

In addition, tin or both tin and lead are added to the steel sheet, and after hot-dip galvanizing, it is heated and diffused, and iron diffuses into zinc to add up both tin or tin and lead in the plating layer. It is possible to contain 0.001% by mass or more and 0.05% by mass or less.

When tin or both tin and lead are contained, it is necessary that the total content is 0.05 to 0.625% by mass . In the process of heating after hot-dip galvanizing and diffusing iron into zinc, tin or both tin and lead are simultaneously contained in the alloyed hot-dip galvanizing layer in the relationship represented by the following formula (1). Can be done. The same effect can be obtained with tin or both tin and lead diffused from the steel sheet.
Concentration of tin or both tin and lead in the plating layer = Iron concentration in alloyed hot-dip galvanizing layer ×
Tin or both tin and lead ... (1)

Since the iron composition of the alloyed hot-dip galvanized steel sheet is preferably 8 to 15% by mass, tin or both tin and lead are diffused into the plated layer as a ratio in the steel sheet to this concentration. Therefore, when both tin or tin and lead in the steel sheet are less than 0.007% by mass, the tin or tin in the plating layer at the time when the iron composition of the alloyed hot-dip galvanized layer is 8 to 15% by mass The concentration of both leads is insufficient, and the combined corrosion resistance effect is not exhibited. In consideration of this, the concentration of tin or both tin and lead in the steel sheet is set to more than 0.1% by mass. If it exceeds 0.625 mass%, the amount of tin and lead diffusing into the plating layer increases, you deterioration coating adhesion and appearance quality.
The iron concentration of a high-quality alloyed hot-dip galvanized steel sheet having a good overall balance of plating adhesion, corrosion resistance after painting, weldability, etc. is more preferably 10 to 12% by mass, and is therefore contained in the steel sheet. More preferred concentrations of tin or both tin and lead are greater than 0.1 to 0.42% by weight.

It is not clear why tin or both tin and lead, which are noble elements compared to zinc, are particularly effective in corrosion resistance in the environment where the mating part is, but the mating part is in an environment where oxygen concentration corrosion occurs. It is known that the most advanced part where the corrosion progresses is in an acidic environment, and it is presumed that tin or both tin and lead exert some corrosion suppressing effect in the acidic environment.

Carbon, silicon, manganese, phosphorus, sulfur, aluminum, titanium, niobium, boron, nitrogen, in addition to one or both of the above tin or lead, for the purpose of improving strength, elongation, hole expandability, etc. as a base material component. There is no problem in the expression of the present invention even if chromium or the like is added. Further, in the annealing step of the steel sheet, there is no problem in the expression of the present invention even if various hydrogen concentration, oxygen concentration and dew point are adopted for the purpose of improving the wettability of hot dip galvanized steel sheet.

For the method of continuously heating the steel sheet after hot-dip galvanizing to obtain an alloyed hot-dip galvanized steel sheet, the currently used heating method can be used, and induction heating, energization heating, gas heating, etc. may be used alone or in combination. Absent.

The temperature of the hot-dip galvanizing bath may be the conditions conventionally applied, and for example, conditions such as 440 ° C to 480 ° C can be applied. By applying hot-dip galvanizing in this way at 30 to 200 g / m ² , it can be applied to various uses.
Further, as elements contained in the hot-dip galvanizing bath, tin or both tin and lead in the present invention, aluminum added for preventing oxidation of the zinc plating bath and adjusting the alloying reaction, and other elements are dissolved from the base metal. It shall inevitably contain iron and other components.

Various electroplating, chromate treatment, and lubrication are required on the surfaces of the hot-dip zinc-plated steel sheet and the alloyed hot-dip zinc-plated steel sheet thus obtained for the purpose of improving coatability, weldability, lubricity, corrosion resistance, and the like. It is possible to perform a property improving treatment, a phosphate treatment, a resin coating treatment, a weldability improving treatment and the like.
Next, an example of the present invention will be described.

As the test material, a cold-rolled steel sheet having a thickness of 0.8 mm whose components are shown in Table 1 was used. Using the composition of the hot-dip galvanized bath shown in Table 2, hot-dip galvanized (GI) or alloyed hot-dip galvanized steel sheet (GA) containing various iron concentrations was prepared.

The bath temperature was 450 ° C. For hot-dip galvanizing, the plate passing time in the bath was set to 3 seconds in both Examples and Comparative Examples, and the amount of zinc adhered was adjusted to 45 g / m ² with an N ₂ gas wiper. An induction heating type heating facility was used to prepare the alloyed hot dip galvanizing. After hot-dip galvanizing or alloying heat treatment, it was continuously cooled by a mist cooling device.
For the evaluation, the appearance, corrosion resistance of the mating part, corrosion resistance after painting, and plating adhesion were examined.
The appearance of the evaluation was visually confirmed, and those without non-plating or patterns, which were suitable for automobile use, were marked with ◯, and those with non-plating, blackening, spangle patterns, etc., which were not suitable for automobile use, were marked with x.
As for the corrosion resistance of the mating part, as shown in FIG. 1, the plate laminating material is spot-welded to prepare a sample in which there is a gap in the mating part, and a phosphate chemical treatment (manufactured by Nihon Parkerizing Co., Ltd., PB-SX35) and electrodeposition are performed. The coating was 20 μm (manufactured by Nippon Paint Co., Ltd., PN1010), and the erosion depth of the mating surfaces after 120 cycles of the combined cycle test (JASO609-91) was evaluated. The evaluation was as follows: ⊚ for erosion depth of 0.1 mm or less, ◯ for 0.3 mm or less, and x for more than 0.3 mm.
Corrosion resistance after coating is obtained by subjecting unprocessed plated steel sheet to phosphate chemical treatment (Nihon Parkerizing Co., Ltd., PB-SX35), electrodeposition coating 20 μm (Nippon Paint Co., Ltd., PN1010), cross-cutting, and composite cycle test (composite cycle test). JASO609-91) The bulge width of the cut portion after 120 cycles was evaluated. The evaluation was ⊚ for 3 mm or less, ◯ for 5 mm or less, and x for more than 5 mm.
Regarding the plating adhesion, for the hot-dip galvanized steel sheet, after 2T ball impact, the plating was peeled off with an adhesive tape, and the one without peeling was marked with ◯, and the one with peeling was marked with x.
The alloyed hot-dip galvanized steel sheet was subjected to V-bending and unbending at 60 ° and then plated and peeled with an adhesive tape, and the peeling width was measured. The evaluation was ⊚ when the peel width was 3 mm or less, ◯ when the peel width was 5 mm or less, and x when the peel width was more than 5 mm.
The results are shown in Table 3.

All of the examples of the present invention in Table 3 were excellent in corrosion resistance at the mating portion, corrosion resistance after painting, and plating adhesion.
On the other hand, in Comparative Examples 16, 21, and 24 in which the abundance of tin and lead in the plating layer was insufficient, no improvement was observed in the corrosion resistance of the combined portion, while in Comparative Examples 17, 18, 19, 20, and 23, which were too large. In No. 25, the appearance, plating adhesion, and corrosion resistance after painting were inferior. In Comparative Example 22, the Al concentration in the bath composition was too high, and the appearance and corrosion resistance after painting were inferior.

Claims (2)

In mass%, both tin or tin and lead are contained in the steel in a total amount of more than 0.1% and 0.625% or less, and 8 to 15% of iron and both tin or tin and lead are contained in a total of 0. An alloyed hot-dip galvanized steel sheet having excellent corrosion resistance, which comprises 001 to 0.05% in the plating layer. A steel sheet containing tin or both tin and lead in a total amount of more than 0.1% and 0.625% or less in% by mass was passed through a hot-dip galvanized bath containing 0.10 to 0.50% of aluminum. Then, after wiping for adjusting the texture, the steel sheet is heated using a heating device to diffuse iron and zinc, so that 8 to 15% of iron and tin or both tin and lead are 0.001 to 0 in total. A method for producing an alloyed hot-dip galvanized steel sheet having excellent corrosion resistance, characterized in that 0.05% is contained in the plating layer.

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