JPH0711409A - Production of galvanized steel sheet - Google Patents

Abstract

PURPOSE:To produce a galvanized steel sheet effectively provided with corrosion resistance at low costs and also excellent in workability of plating layer by taking the composition, coating weight, and form of precoating, necessary to improve corrosion resistance, into consideration. CONSTITUTION:An Ni-P alloy plating layer, containing 8-15wt.% P, or an Ni-P alloy plating layer, further containing <=15wt.% of one or >=2 elements among W, Mo, Cr, and Cu, is formed on the surface of a steel sheet. The alloy components in these Ni-P alloy plating layers are heat-treated in a nonoxidizing atmosphere, at 500-800 deg.C maximum ultimate temp. for 1-30sec, by which a diffused alloy region is formed. Then, hot-dip galvanizing is applied to form a hot-dip galvanizing layer. By this method, the galvanized steel sheet, excellent in corrosion resistance and also in workability of plating, can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a galvanized steel sheet suitable for automobiles, which has excellent corrosion resistance and plating layer processability.

[0002]

2. Description of the Related Art Rust-preventing steel sheets are mainly used for automobile bodies in terms of reliability for long-term rust prevention. In general, galvanized steel sheets or galvannealed steel sheets are most often used among rust-preventive steel sheets, but there are also corrosion-resistant steel sheets that improve the corrosion resistance of the steel sheets themselves by the components of the steel sheets (for example, Japanese Patent Laid-Open No. 54-75421, JP-A-5
6-9356). In recent years, the required level of corrosion resistance has been increasing more and more, and a method has been proposed in which the surface of a corrosion-resistant steel sheet is plated with zinc to enhance its corrosion resistance (for example, Japanese Patent Laid-Open No. 59-170288).

[0003]

Corrosion resistance of a steel sheet is maintained by a stable rust layer formed on the surface of the steel sheet during the corrosion process. Various additive components must be added to form this stable rust layer, but a considerable amount must be added to obtain a sufficient effect. However, what is utilized for forming a stable rust layer is limited to those added to the surface layer of the additive components which are added at the time of steel making and which are averagely present in the total volume of the steel sheet. That is, in terms of corrosion resistance, it can be said that the components existing in other interpolated portions are wasted. On the other hand, the additive component also has a role of controlling the mechanical properties of the steel sheet. Therefore, the added amount of the corrosion resistance improving component may be limited due to the restriction of the mechanical properties, and the actual condition is that the sufficient corrosion resistance effect is not obtained.

As a method of compensating for the insufficient corrosion resistance of the corrosion-resistant steel sheet, there is a method of forming a zinc-based plated steel sheet on the surface thereof as in JP-A-59-170288. However, the zinc-based plating proposed here is alloy electroplating, and has the drawback that the manufacturing cost thereof is higher than that of hot dipping. On the other hand, hot dip galvanizing has the great advantage that cost reduction can be expected compared to electroplating, but on the other hand, good adhesion and workability of the plating layer cannot be secured depending on the steel plate components and the amount added. There is also a drawback. The reason for this is that the adhesiveness and workability of hot dip galvanization are largely owed to the alloy layer formed by the reaction between the hot dip galvanized steel and the steel sheet, but some of the added components hinder the reaction and impede the formation of the alloy layer. This is for homogenization.

JP-A-3-24255 and JP-A-62-1
In order to improve the plating adhesion of such hot-dip galvanized steel sheets, No. 39860 and the like perform "pre-plating" on the surface of the steel sheet in advance by an electroplating method or the like to improve the adhesion, and then hot-dip galvanizing. A method has been proposed. However, this method is mainly intended to have an adhesion improving effect by "pre-plating", and an effective corrosion resistance imparting effect can hardly be expected. That is, in this method,
No consideration is given to the composition, amount, and morphology of the "pre-plating" required to improve the corrosion resistance.

The present invention has been made in view of the above circumstances, and its purpose is to be effective at low cost by considering the composition, amount, and form of "pre-plating" required for improving corrosion resistance. It is an object of the present invention to provide a method for producing a galvanized steel sheet which produces corrosion resistance and is excellent in the workability of a plated layer.

[0007]

According to the present invention, at least one surface of a steel sheet is Ni-containing 8 to 15% by weight of P.
A P alloy plating layer or a step of forming a Ni-P alloy plating layer containing 8 to 15% by weight of P and containing 15% by weight or less of one or more of W, Mo, Cr and Cu; A step of thermally diffusing an alloy component of P alloy plating or Ni-P alloy plating to form a diffusion alloy region;
This is a method for producing a galvanized steel sheet having excellent corrosion resistance and processability of the plated layer, which comprises a step of performing hot dip galvanizing to form a hot-dip galvanized layer, and thermal diffusion is the highest in a non-oxidizing atmosphere. Achieved temperature is over 500 ℃ 80
It is performed at 0 ° C. or lower for 1 to 30 seconds.

[0008]

The steel sheet applied to the present invention is a hot rolled steel sheet,
Usually its thickness is between 1.6 and 4.5. The steel sheet of the present invention includes a steel strip. Prior to carrying out the process according to the invention, the steel sheet has been pickled and descaled. Then, a Ni-P based alloy plating layer is formed on one side or both sides of this steel sheet. The Ni-P alloy plating layer has a Ni-P alloy composition containing 8 to 15% by weight of P. The reason for limiting the P content to the above range is as follows. That is, the Ni-P alloy plating of this composition has a structure close to an amorphous structure. When a steel sheet having such a plating layer is heat-treated, a uniform diffusion alloy region is formed in a short time as compared with the case of a general crystalline plating film. When P is less than 8% by weight, the Ni-P alloy plating film is crystalline, and the distribution of P is not uniform, so that the composition of the diffusion alloy region formed when subjected to heat treatment is not uniform, and is initially formed. Stable corrosion resistance cannot be obtained due to insufficient rust homogeneity and density. On the other hand, if P exceeds 15% by weight, the Ni-P alloy plating becomes brittle and its adhesion deteriorates. As a result, plating peeling is likely to occur before forming hot-dip galvanizing. From this, the content rate of P was set to the above range. A preferred P content range is 10 to 13% by weight.

It is also possible to form a Ni-P alloy plating layer, which is a composite of Ni-P and one or more of W, Mo, Cr and Cu in an amount of 15% by weight or less, on the surface of the steel sheet. The roles of Ni and P are as described above, but W, Mo, Cr, and Cu have an inhibitory role on the corrosion of steel, and at the same time, due to the synergistic effect with Ni and P, the denseness of the initially formed rust. It has the effect of further improving the stability and stability. The total content of W, Mo, Cr and Cu was set to 15% by weight or less. Corrosion resistance improves as the content of W, Mo, Cr, Cu increases, but if the total content exceeds 15% by weight, its adhesiveness decreases, so that plating peeling occurs before forming hot-dip galvanizing. Cheap. For these reasons, the W, Mo, Cr, and Cu content rates are in the above range in total. These W, M
A suitable lower limit of the amount of addition for exhibiting the effects of o, Cr and Cu is 0.5% by weight.

The plating amount of this Ni-P alloy plating layer is preferably in the range of 0.5 to 8 g / m ² . 0.
If it is less than 5 g / m ² , the effect of improving the corrosion resistance is not always sufficient, and if it exceeds 8 g / m ² , the cost increase is large with respect to the performance improvement and it is not practically efficient.

Although various methods of forming the Ni-P alloy plating layer are conceivable, electroplating or electroless plating (chemical plating) is preferable in terms of simplicity and uniformity of the obtained film quality. In the case of wet plating, it is possible to carry out after pickling on the outlet side of the pickling line, and by doing so, it is possible to carry out cleaning without additional cleaning on the surface of the hot-rolled steel sheet that has been pickled and activated. A pre-plated film with good adhesion can be effectively formed.

The Fe-Ni-P type diffusion alloy region is formed by heating the steel sheet on which the Ni-P type alloy plating layer (pre-plating layer) is formed. Hot dip galvanizing equipment can be used for this thermal diffusion. For example, a Zenzimer type continuous hot-dip galvanizing facility is provided with a heat treatment furnace in a non-oxidizing atmosphere for the purpose of annealing and surface activation of steel sheet. In this heat treatment furnace, the Ni-P alloy plating layer is appropriately formed. Reacts with the steel base material under various heat treatments, Fe, Ni, P
Form mutual diffusion alloy regions. The maximum plate temperature reached by this heat treatment is preferably 500 ° C. or higher and 800 ° C. or lower. If the temperature is lower than 500 ° C., the diffusion alloy region between the Ni—P alloy plating layer and the surface of the steel sheet is not sufficiently formed, and thus stable initial rust cannot be formed in the corrosion process, so that the corrosion resistance improving effect is small. On the other hand, if the temperature exceeds 800 ° C., the plated metal is likely to be picked up by the roll in the heat treatment furnace, and as a result, surface scratches or the like are likely to occur.

The holding time at the maximum plate temperature depends on the temperature, but is preferably 1 to 30 seconds. If it is less than 1 second, it is difficult to form a sufficient diffusion alloy region, and the effect of improving corrosion resistance is not sufficiently exhibited. On the other hand, if it exceeds 30 seconds, the interface layer becomes brittle due to excessive diffusion alloying, and the adhesion and processing of the plating layer May deteriorate.

The diffusion alloy region preferably has a depth of 0.1 to 20 μm in order to effectively exert the effect of adhesion and workability of the plating layer. When the Ni-P alloy plating layer is heat-treated, a part of the Ni-P alloy plating layer forms a diffusion alloy region, and a steel plate / diffusion alloy region / Ni-P alloy plating layer is formed. Forming a region,
In some cases, the present invention includes both the steel plate and the diffusion alloy region.

After forming the diffusion alloy region in this way, the steel sheet is introduced into a molten zinc bath. It is desirable that the amount of hot-dip galvanized be ²⁰ to 120 g / m ² . If the adhesion amount is too small, good corrosion resistance cannot be obtained, and if it is too large, the workability of the plating layer is deteriorated. As the hot-dip galvanizing bath, one containing a trace amount of Al, which has been widely used in the past, can be used. For applications that require even better coating properties, the galvanized layer can be heat alloyed after forming a predetermined amount of hot-dip galvanizing. At this time, the alloying ratio (the ratio of Fe and a part of the components of the Ni—P alloy layer in the hot-dip galvanized layer: wt%) is preferably 7 to 15 wt%. If it is less than 7% by weight, it is difficult to obtain the effect of improving the coating property, and if it exceeds 15% by weight, the workability of the plating layer tends to be deteriorated.

The galvanized steel sheet according to the present invention thus obtained has better corrosion resistance than the galvanized steel sheet obtained by the conventional method. The reason for this is considered to be that in the present invention, a diffusion alloy region containing Fe-Ni-P as a main component is formed at the interface between the steel sheet substrate and the plating layer, and this intermediate layer contributes to the improvement of corrosion resistance. Although the mechanism for improving corrosion resistance has not been clarified yet, the inventor presumes that the intermediate diffusion alloy region suppresses the cathode reaction due to the formation of homogeneous and dense Fe-based rust in the initial stage of corrosion. There is.

[0017]

EXAMPLES Examples of the present invention will be described below. Ni-P using three types of (A, B, C) steel plates (plating base plates)
System alloy plating was performed, and then the Ni-P system alloy plating layer was thermally diffused and then hot-dip plated. Table 1 shows steel plates (A,
The main components of B, C) are shown in Table 2 in the pre-plating composition (1 to 1).
5) and Table 3 and Table 4 show the thermal diffusion and hot dip plating conditions and the corrosion resistance evaluation results of the obtained plated steel sheet. All of these examples were carried out on a small test piece using a plating simulator in a laboratory. Samples 13 to 15, 19,
Nos. 20, 40 to 46 show comparative examples when they are out of the range of the present invention, and Samples 13 and 45 are samples 14, 15 and 14 when Ni-P alloy plating (pre-plating) is not performed.
Nos. 40 and 41 are samples whose pre-plating amount is outside the range of the present invention, Samples 19, 20, 43 and 44 are samples whose Zn content is outside the range of the present invention, and Sample 46 is Zn-Ni.
The case of alloy plating is shown respectively.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

[0021]

[Table 4]

(Hot dipping conditions) (1) Heat treatment atmosphere: 25% H ₂ -75% N ₂ (2) Plating bath component: 0.13 ± 0.01 wt% Al (3) Plating bath temperature: 460 ° C. ± 10 ° C (4) Plating alloying conditions: 500 to 550 ° C (plate temperature) x
Controlled by 3 to 10 seconds (holding time) (evaluation method / standard) (1) Corrosion resistance: Corrosion depth of uncoated steel sheet after 60 days was measured in a corrosive environment where dry water and moisture were combined with salt spray, and It was evaluated according to the standard.

○ Maximum corrosion depth is 0.05 mm or less △ Maximum corrosion depth is more than 0.05 mm and 0.2 mm or less × Maximum corrosion depth is more than 0.2 mm (2) Workability: bending by 180 degree bending test The damage condition of the plating film at the tip was observed and evaluated according to the following criteria.

○ Degree of zero damage or fine cracks △ Large cracks or partial peeling of plated pieces × Plating peeling is observed in a wide range (3) Paintability: Phosphate treatment is performed and cations are applied. A steel plate coated with a type of electrodeposition was scratched with a cutter knife to reach the substrate, and exposed to the same corrosive environment as (1) for 100 days. The swelling of the coating from the scratched portion was observed and evaluated according to the following criteria.

"Double circle" The maximum bulge width on one side is 1 mm
Below ○ Maximum bulge width on one side is over 1 mm, 3 mm or less △ Maximum bulge width on one side is over 3 mm From the results of Tables 3 and 4, according to the present invention, a galvanized steel sheet having good corrosion resistance is effectively produced. You can see that you can. According to the method of the present invention, by adding a trace amount of the corrosion resistance enhancing component to the plating / steel plate interface, the corrosion resistance is significantly improved without adding a large amount of such a component to the steel plate. This gives a great advantage to the steel plate manufacturing cost, but not only that, as seen in Samples 45 and 46, its corrosion resistance is the same as that of the conventional corrosion resistant steel plate (high C
It is better than the one in which Zn-based plating is applied to (r, Mo added). It is presumed that this is because the interface diffusion layer formed by “pre-plating” is superior to the conventional method in forming stable and dense initial rust. Also in terms of workability of the plating layer, good results were shown compared to the conventional method. This is because Ni in the method of the present invention promotes a uniform reaction between steel and zinc,
It is presumed that the alloy layer with good adhesion was formed.

In addition, according to the present invention, it is possible to reduce the plating amount of the galvanized or alloyed galvanized layer while maintaining the corrosion resistance, which further improves the workability of the galvanized layer and the weldability (for example, Improvement of blowhole resistance in arc welding) can be expected. From a long-term perspective, it is also effective in terms of recycling by reducing the amount of zinc in scrap.

[0027]

As described above, according to the present invention, without unnecessarily adding a large amount of the corrosion resistance enhancing component to the steel sheet,
It is possible to effectively produce a galvanized steel sheet having good corrosion resistance, which gives a great advantage to the steel sheet production cost. Furthermore, according to the present invention, the amount of galvanized or alloyed galvanized layer is reduced, and the amount of zinc in scrap is reduced. For example, blowhole resistance in arc welding) can be improved.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C25D 5/50 // C23C 18/36 18/50 C25D 3/56 AC (72) Inventor Shiobara Komitsu 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Tube Co., Ltd.

Claims (4)

[Claims] 1. A P plate containing 8 P on at least one surface of a steel plate.
A step of forming a Ni-P alloy plating layer containing 15 to 15% by weight, and a step of thermally diffusing the alloy components of the Ni-P alloy plating layer to form a Fe-Ni-P-based diffusion alloy region on the steel sheet. And a step of forming a hot-dip galvanized layer by performing hot-dip galvanizing after this step, and a method for producing a galvanized steel sheet excellent in corrosion resistance and workability of a plated layer. 2. A P plate on at least one surface of a steel plate.
A step of forming a Ni-P alloy plating layer containing 15% by weight to 15% by weight and 15% by weight or less of one or more of W, Mo, Cr and Cu, and an alloy component of the Ni-P alloy plating layer. Corrosion resistance and plating layer processing including a step of thermally diffusing to form a Fe-Ni-P-based diffusion alloy region on a steel sheet and a step of performing hot dip galvanizing to form a hot dip galvanized layer after this step A method for producing a galvanized hot rolled steel sheet having excellent properties. 3. The method for producing a galvanized steel sheet excellent in corrosion resistance and plating layer workability according to claim 1 or 2, wherein the thermal diffusion is performed in a non-oxidizing atmosphere at a maximum attainable temperature of 500 ° C. or higher and 800 ° C. or lower. 4. The method for producing a galvanized steel sheet excellent in corrosion resistance and plating layer workability according to claim 3, wherein the maximum ultimate temperature is held for 1 second to 30 seconds.