JPH05148604A - Manufacture of galvanized steel sheet - Google Patents

Abstract

PURPOSE:To economically manufacture a galvanized steel sheet of quality at high productivity by applying an Fe-C plating having a specified compsn. to the surface of a steel at a specified coating weight, thereafter executing galvanizing and, according to necessary, executing alloying treatment. CONSTITUTION:An Fe-C plating contg. 0.001 to 10wt.% C is applied to the surface of a steel sheet at 0.1 to 10g/m<2> coating weight. This plating can be executed by electroplating, e. g. using a sulfate bath in which a water soluble primary iron compound and a carbon compound such as organic acid are dissolved. After that, the steel sheet is applied with galvanizing and is successively subjected to alloying treatment according to necessary. In this way, the objective galvanized steel sheet in which nonplated spots and the delay of the alloying rate do not occur even in a silicon-contg. steel sheet and a phosphorus-contg. steel sheet and excellent in low temp. impact resistance and durability even in thick plating can be obtd.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an improved hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet (in the present specification, both are collectively referred to as hot-dip galvanized steel sheet), for example, automobiles, The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet suitable for applications such as steel instruments, building materials and home appliances. In particular, the present invention relates to a hot-dip galvanized steel sheet suitable for automobiles having a base material of an alloy steel such as a silicon-added steel or a phosphorus-added steel, and further, a hot-dip zinc-based steel sheet suitable for automobiles and building materials capable of thick plating. Manufacturing method.

[0002]

2. Description of the Related Art In recent years, in the industrial fields of automobiles, home appliances, and building materials, hot-dip galvanized steel sheets have been used in large quantities as rust-preventing steel sheets having longer-term rust-preventing ability as the products become more sophisticated. It started to come. In particular, alloyed hot-dip galvanized steel sheets are widely used in terms of economy, their rust preventive function, and performance after coating.

A hot-dip galvanized steel sheet is usually subjected to an appropriate degreasing and washing step, if necessary, preheated in a weakly oxidizing atmosphere or a reducing atmosphere, and then annealed in a hydrogen + nitrogen reducing atmosphere. After that, the steel sheet is cooled to a temperature close to the plating temperature and immersed in a molten zinc bath to manufacture. In the preheating process, it is preferable that an oxide film with a thickness of about 80 nm is formed on the surface of the steel sheet in terms of wettability with molten zinc, but if the thickness of the oxide film is greater than that, dross generation and hot dip coating occur. It is considered that there is an adverse effect in terms of adhesion. In the molten zinc bath, about 0.1 to 0.2% by weight of Al is added in order to suppress the formation of a brittle Fe-Zn alloy at the plating / steel interface and maintain the workability of the plating film. Since Al tends to be enriched in the plating layer during plating, the plating film contains about 0.2 to 0.3% by weight of Al.

The alloyed hot-dip galvanized steel sheet is usually a hot-dip galvanized steel sheet in a heat treatment furnace at 500 to 600 ° C.
The Fe-Zn alloy plating layer is formed by heating the material temperature to 3 to 30 seconds. Plating film is Fe-Zn
The average Fe concentration is 8 to
12% by weight. The plating film usually contains about 0.12 to 0.2% by weight of Al. This is the case when Al is inevitably mixed in during the production of a normal hot-dip galvanized steel sheet that is produced by the same equipment as the alloyed hot-dip galvanized steel sheet, but is alloyed. However, in order to ensure the powdering resistance of the plating film, a small amount of
This is because it is rather appropriate to mix Al, and about 0.08 to 0.11 wt% of Al is present in the plating bath.

In both cases of hot dip galvanizing and alloying hot dip galvanizing, the adhesion amount of the plating layer is
It was about 20 to 70 g / m ² . It is difficult to make the adhesion amount less than 20 g / m ² in a normal hot dip galvanizing operation. Further, if the weight is too thick, the durability against low temperature impact is deteriorated.

As a base material for such hot-dip galvanized steel sheets, especially alloyed hot-dip galvanized steel sheets, low carbon Al is conventionally used.
Killed steel and ultra-low carbon Ti-added steel were the mainstream. However, with the recent demand for higher strength of automobile materials, 0.2
Attempts have been made to use silicon-containing steel plates containing more than% Si.
Si has the advantage that strength can be improved while maintaining the ductility of steel, and in that sense, silicon-added steel is promising as a steel material.

However, hot dip galvanizing using a silicon-containing steel plate as a base material has the following problems. That is, the silicon-containing steel sheet, according to the normal hot dip galvanizing process, reacts with a very small amount of water in the atmosphere during the annealing process and forms silicon oxide on the steel sheet surface, which increases the Si concentration in the steel. Along with this, the wettability with molten zinc is rapidly lowered, and as a result, non-plating frequently occurs. Further, even if hot-dip plating can be performed, since Si is present in the plating film, alloying is delayed, or uneven alloying treatment occurs. It has been reported that the former problem of decrease in wettability due to silicon oxide is that iron oxide is formed on the surface by preheating in an oxidizing atmosphere to improve wettability and prevent non-plating. But Si
If the content exceeds 0.2% by weight, it is difficult to secure wettability by preheating in an oxidizing atmosphere in the conventional hot dip galvanizing process (for example, setting the air-fuel ratio of the non-oxidizing furnace to 1-1.35). In addition, there is no particular countermeasure for the latter problem of alloying delay, and the delay of alloying processing speed greatly hinders the production efficiency.

In addition, in designing the material, addition of P together with Si can improve bending or drawability.
P may be added to steel because it is cheaper than Si. P does not deteriorate the wettability unlike Si, but the addition of P delays alloying, and the problems of alloying delay and process unevenness are unavoidable like Si.

By the way, the most general method for increasing the rust preventive ability of a hot dip galvanized steel sheet is to increase the amount of coating to make it thicker. However, thick plating generally lowers the low temperature impact resistance against impact load under low temperature (-50 ° C to 0 ° C) environment. Therefore, there is a demand for a hot-dip galvanized steel sheet which has a good low-temperature impact resistance and a high corrosion resistance.

Generally, the cause of the low temperature impact resistance of the hot dip galvanized steel sheet is said to be the hard and brittle Fe-Zn alloy phase generated at the Fe / Zn interface. Therefore, in general, by increasing the amount of Al added in the molten zinc bath, the amount of Fe-Zn-Al ternary alloy layer formed at the Fe / Zn interface is increased to suppress excessive Fe-Zn reaction. Has been taken. However, when the amount of Al in the bath is increased, there is a drawback that the deterioration of the adhesiveness with time due to the aging phenomenon of the plating film (called peeling with time) is likely to occur.

With regard to the galvannealed steel sheet, the brittle Γ phase produced by the excessive Fe-Zn reaction is said to be the cause of the low-temperature impact resistance deterioration of the coating film. Therefore, efforts have conventionally been made to suppress the generation of the Γ phase, and the following method has been adopted. One is to suppress the excessive Fe-Zn reaction in the alloying process by limiting the chemical composition of the base steel sheet. For example, P in steel has the effect of suppressing the Fe-Zn reaction and preventing the formation of brittle Γ phase. However, the addition of P has the problems of delaying alloying and occurrence of process unevenness as described above. On the other hand, since Ti and Mn accelerate the Fe-Zn reaction, it is possible to withhold the use of these elements or to reduce the addition amount, but this limits the chemical composition of the base steel sheet. This naturally limits the mechanical properties of steel.

A method of controlling the alloying process is also effective for improving the durability. For example, a method of generating a plating film composed of a δ ₁ phase single phase rich in ductility can be considered by suppressing the alloying temperature lower than usual. Although this method is effective, it is difficult to achieve the desired plating film structure, that is, the δ ₁ single phase, unless the control conditions are devised depending on the type of base steel sheet, which leads to the result of slowing down the production line speed. Not the right way.

Furthermore, the impact resistance at low temperature does not depend only on the bonding force between the Fe substrate and the plating film,
Microcracks generated in the plating film due to collision propagate and change direction at the Fe / Zn interface, whether or not the cracks concentrate and run along the interface, and further plating of the extremely thin part near this interface The fracture resistance of the layer is considered to be the decisive factor, and its theoretical elucidation is still in the dark stage. In addition, the blistering resistance of the impact flaws and the red rust proofing ability, which are shown when the deformed and destroyed plating film is subjected to repeated dry and wet corrosion in a salt water environment, are important factors for overall practicality. is doing.

In order to deal with such a problem, conventionally, a pure metal such as Fe or Ni is preliminarily plated on the steel sheet (pre-plating) prior to the hot dip galvanizing, so that the Fe substrate / plating layer interface is It is known to improve the adhesiveness of A technique of pre-plating a steel plate with pure Fe is disclosed in, for example, JP-A-57-79
No. 160, No. 57-70268, and No. 58-120772.

The hot-dip galvanized steel sheet on which ultra-thin undercoating is applied before hot dip coating shows some improvement in the adhesion of the plating film at room temperature as compared with the case where it is not applied, but at low temperature and high speed, pebbles, etc. The durability against the low-temperature impact is extremely insufficient. Moreover, sufficient plating wettability cannot be obtained with steel having a high Si content, and Si and P
It has not been possible to solve the problems such as the delay of alloying when the content is high and the difficulty of uniform alloying.

[0016]

Therefore, alloy steels which are attractive in terms of material, especially Si-containing steels, are difficult to be coated with molten zinc in a practical sense in the conventional molten zinc process. When the alloying treatment is performed using the steel base material containing P or P, there is a problem that the alloying is delayed or becomes non-uniform. Furthermore, the conventional hot-dip galvanizing technology has no means for preventing the deterioration of low-temperature durability against impact, especially in the case of thick plating.

It is an object of the invention to alloy steels, especially Si> 0.2%.
Of steel is galvanized without forming unplated spots, and
Not only for the additive steel but also for the P additive steel, the alloying treatment rate is sufficiently improved, and such a steel base material is used to produce economically high-quality hot-dip galvanized steel sheet with high productivity. It is to provide a method.

Another object of the present invention is to solve the problem of deterioration of low temperature durability against impact, which is observed in hot-dip galvanized steel sheet especially for thickening the rust preventive ability. It is an object of the present invention to provide a method capable of producing a hot-dip galvanized steel sheet excellent in low temperature impact resistance.

[0019]

Means for Solving the Problems The present inventors have examined the wettability between steel and molten zinc in hot dip galvanization of Si-added steel which is particularly prone to non-plating. as a result,
Oxidizing the steel sheet before reduction annealing, which has been performed as a conventionally known plating pretreatment, and means for forming a surface layer of iron oxide in advance are effective for improving the wettability between the steel sheet and the zinc after reduction annealing. I reconfirmed that. However, only with this oxidizing means, when the Si concentration increases, the formation of iron oxide does not proceed sufficiently, and the desired amount of iron oxide in the steel plate preheating step (Fe.
It was found that the amount of iron oxide was not more than half that of 0.05 to 0.25 g / m ^{2 with} respect to 5 g / m ² ), which was insufficient.

On the other hand, pre-plating before reduction annealing was also examined. However, with the conventional pure Fe pre-plating, there is some improvement even when combined with the oxidation treatment before reduction annealing, but there is a limit to the improvement, and as the Si content increases, sufficient iron oxide content increases. Proved to be difficult to obtain.

Based on the above results, as a result of various studies on the pre-plating of the Fe-based alloy, the Fe-C pre-plating is effective in improving the wettability, accelerating the alloying rate, and further improving the low temperature impact resistance. I found that. That is, if Fe-C pre-plating is applied as a thin film before hot dip galvanizing, the wettability of Si-added steel with hot-dip zinc is improved and non-plating is eliminated, and alloys such as Si-added steel and P-added steel are used. It is possible to prevent the delay of alloying and the occurrence of process unevenness, which are problems in the case of using a steel type whose alloying is delayed as a base material. Further, regardless of the steel type, it is possible to prevent the deterioration of low temperature resistance to impact, which has been recognized especially in thick plating.

In the present invention, the C content on the steel sheet is 0.0
Fe-C plating of 01 to 10 wt% is applied at an adhesion amount of 0.1 to 10 g / m ² , followed by hot dip galvanizing, or hot dip galvanizing followed by alloying treatment. The gist is a method of manufacturing a plated steel sheet.

[0023]

The operation of the present invention will be described below together with its operation. In the following description,% means% by weight unless otherwise specified.

The main feature of the present invention is that, before hot dip galvanizing, preferably before annealing of the base steel sheet, which is performed as a pretreatment, pre-plating with an extremely thin amount of 0.1 to 10 g / m ² is applied. , And this pre-plated film together with Fe element is 0.001
˜10 wt% of C element is included. There are no particular restrictions on other conditions, such as the steel type of the base steel sheet, the pretreatment conditions for plating, the composition of hot dip galvanizing, the amount of deposition, and the alloying treatment conditions, and existing techniques may be applied. However, as can be seen from the background of the completion of the present invention described above, when the present invention is applied to a specific base material steel type or a specific hot-dip galvanized amount, the effect becomes more remarkable than in other cases. is there.

The base steel sheet used in the method of the present invention is usually a cold-rolled steel sheet, but the present invention is not limited to this, and the steel type of the base steel sheet is not particularly limited, either carbon steel or ultra low carbon steel. It may be appropriately selected from arbitrary steel types including low alloy steel and high alloy steel according to the product application. Base material steel sheet is C, Si, Mn, Ti,
It may contain one or more alloying elements usually added to steels such as Nb, S, P, Al, Ni, Cr, Mo, V and B. In one aspect, the present invention is applied to the production of a hot-dip galvanized steel sheet using a steel sheet containing Si: 0.2% or more as a base material. In this case, non-plating due to a decrease in wettability, which is a problem in the conventional method, can be prevented.

In another aspect, the invention provides a Si: 0.2
% Or more, P: 0.03% or more, which is applied to the production of alloyed hot-dip galvanized steel sheet containing a steel sheet containing one or both of them as a base material. To be prevented.

If necessary, the base steel sheet is degreased and / or pickled by a conventional method to clean the surface. For surface cleaning, for example, organic solvents (trichlene, thinner, etc.)
After degreasing, the electrolytic degreasing treatment is carried out in a 2 to 10% sodium orthosilicate aqueous solution, and further pickling is carried out in a 5 to 20% hydrochloric acid aqueous solution.

Then, Fe-C plating is applied as pre-plating before hot dip galvanizing the base steel sheet, preferably before preheating or reduction annealing performed in pre-treatment of plating. This Fe-C
The plating can be performed by electroplating, vacuum deposition, thermal spraying or the like. The case of using typical electroplating will be described below, but the electro-Fe-C plating can be carried out in accordance with normal electro-plating. For example, by using a sulfate bath, a chloride bath, a borofluoride bath, or the like, by dissolving an appropriate carbon compound (eg, organic acid) in addition to the water-soluble ferrous iron compound in the bath, the electric Fe -C plating can be formed. An example of a suitable electrolytic bath is Fe ²⁺
Contains 10 to 240 g / l of ions, and one or more organic acids (eg, oxalic acid, formic acid, glycine, malonic acid, glycolic acid, lactic acid, malic acid, acetic acid, maleic acid, citric acid) , Ascorbic acid, etc.) are added. Instead of the organic acid or in combination with the organic acid, another organic compound such as a carbonyl compound, polyethylene glycol, or thiourea can be used as the C supply source.

The deposition amount of this electric Fe-C plating film can be controlled by the current density and the energization time, and the C content can be controlled by the kind and the addition amount of the carbon compound to be added. In addition, the C content is 0.001 ~
The composition and conditions of the electroplating bath are controlled so that the lower layer Fe—C plating film having a deposition amount of 0.1 to 10 g / m ² is formed at 10%. The current density in this electric Fe-C plating is generally 0.1
Within the range of up to 100 A / dm ² .

The amount of the Fe-C plating film as the lower layer deposited is 0.1 to
Within the range of 10 g / m ² . If the adhesion amount is less than 0.1 g / m ² ,
The lower layer plating is not uniformly coated on the steel sheet, the structure of the steel sheet itself is exposed, and due to the diffusion of the additive elements in the steel sheet to the surface in the heat treatment in the reducing atmosphere performed as a pretreatment for hot dip galvanization, the purpose of the present invention And the effect cannot be obtained. If the deposition amount of the lower layer Fe-C plating exceeds 10 g / m ² , the effect is saturated and large equipment is required, which is not advantageous in terms of manufacturing cost.

The C content in the Fe-C plating film is 0.001 to
10% When the C content is less than 0.001%, the obtained lower layer plating exhibits only the same performance as the pure Fe plating film,
The effect of improving the wettability of molten steel with Si-added steel and Si /
The effect of preventing the alloying delay in the P-added steel becomes insufficient, and the object of the present invention is not achieved. Moreover, the effect of improving the low temperature impact resistance of the hot dip galvanized coating is small. On the other hand, if the C content exceeds 10%, the Fe—C coating itself becomes brittle and the workability of the plating coating deteriorates. Therefore, it is not a good idea to increase the C content unnecessarily. The preferred C content is 0.
It is from 01 to 1.0%.

The Fe-C plating film contains Al, S
i, Nb, Mn, Mg, Mo, Ta, Cu, Ni, Zn, Co, Sn, Sb, T
One or two or more trace alloy components selected from i, Cr, Cd, Pb, Tl, In, V, W, P, S, B, N, etc. may be contained, and such trace alloys are included. Fe-containing components
Forming a C plating film is also included in the "Fe-C plating" of the present invention. The total amount of these trace components in the plating film is usually 0.01% or less, especially 0.005%.
The following is preferable. These trace components are chlorides, nitrates, sulfates, acetates, carbonates, molybdates, pyrophosphates, hypophosphites, organometallic salts, or solutions in which a metal is dissolved in an acid. Can be added to.

After the Fe-C plating is pre-plated on the steel sheet in this way, it is usually washed with water and dried, and then a pretreatment which is usually performed for activating the surface of the steel sheet is performed by hot dip galvanizing. This pretreatment can be carried out by preheating the steel sheet in a weak oxidizing atmosphere or a reducing atmosphere, if necessary, and then annealing it in a nitrogen + hydrogen reducing atmosphere. Specifically, for example, after preheating at 500 to 700 ° C. for 60 seconds or less in an N ₂ + O ₂ atmosphere having an oxygen concentration of 0.001 ppm or more and 20 vol% or less, a hydrogen concentration of 10 to 30 vol%, Dew point is-
Reduction annealing is performed for 30 to 90 seconds in an N ₂ + H ₂ atmosphere at 30 ° C. or less.

Then, hot dip galvanizing is performed, and if necessary, further alloying treatment is performed to obtain the hot dip galvanized steel sheet and the hot galvannealed steel sheet of the present invention. These can be performed according to a conventional method. For example, the hot-dip galvanizing method can be any method including a continuous hot-dip galvanizing line of Zenzimer type. Hot-dip galvanizing can be performed at a bath temperature of about 460 ° C. The amount of hot-dip galvanized coating is not particularly limited. Conventional 20-70
Although high-quality hot-dip galvanizing can be performed even with an adhesion amount within the range of g / m ² , according to the present invention, low temperature impact resistance can be ensured even with thick plating. Is 15
One of the features is that it can be expanded to 0 g / m ² and high-quality hot-dip galvanizing is performed even with thick plating.

The composition of the hot dip galvanizing bath, for example,
There is also no particular limitation on the Al concentration. The method of the present invention uses Al in the bath.
The effect can be exhibited regardless of the concentration. Therefore, the "hot dip galvanizing" of the present invention can be applied to the production of zinc alloy plated steel sheets such as a galvalume steel sheet using a Zn-55wt% Al bath and a galfan steel sheet using a Zn-5wt% Al bath. Even in that case, the effect of the present invention is exhibited. However, in the present invention, since the reaction between Zn and the steel sheet is enhanced by providing the lower Fe-C pre-plating layer, if the amount of Al is too small, this reaction can be controlled during plating. In some cases, it may be difficult to control the adhesion amount. In that sense, the amount of Al in the bath is 0.03%
The above is preferable. Further, as is well known to those skilled in the art, in the production of galvannealed steel sheet, if the Al concentration in the plating bath becomes excessive, it is generated during plating.
The Fe-Al alloy layer becomes too thick, which hinders alloying.
Therefore, when alloying, the Al concentration in the bath is preferably 3% or less.

The alloying treatment also conforms to the prior art and does not particularly define the atmosphere. 420 to 600 ℃
It can be carried out by heating for 10 to 30 seconds.

According to the present invention, since the wettability by hot-dip zinc is poor, the steel sheet of Si-added steel containing 0.2% or more of Si, which is a steel type that cannot be plated by the normal hot-dip galvanizing method, is not plated. Hot dip galvanizing can be performed uniformly without causing this. When a hot dip galvanized steel sheet is manufactured by the method of the present invention using a Si-added steel sheet as a base material, Al
Non-plated products called GI, which are products with a concentration of 0.12% or more as they are, and various galvanized products containing the above-mentioned galfan and galvalume and having an Al concentration in the bath of about 0.03 to 55%. Hot dip galvanizing can be performed without any trouble.

The present invention is also advantageous for steel containing 0.2% or more of Si and / or 0.03% or more of P (referred to as Si / P-added steel), which is likely to cause delay of alloying and uneven treatment. Can be applied to. That is, when an alloyed hot-dip galvanized steel sheet is manufactured by using such a steel sheet of Si / P-added steel as a base material, the alloying treatment can be performed quickly and uniformly, and the production efficiency is significantly improved. In this case, Al in the bath
The concentration is preferably 0.03% or more and 3.0% or less.
As described above, if the Al concentration exceeds 3.0%, alloying is hindered, and if it is less than 0.03%, it becomes difficult to control plating.

In the present invention, the effect described above (prevention of non-plating,
The details of the reason why the promotion of alloying and the improvement of low temperature impact resistance by improving adhesion are achieved are not clear, but the effects of crystal orientation / grain size near the interface, crystal toughness, and micro irregularity shape of the interface, etc. Or, it is caused by the chemical properties of C itself (for example, it reacts with elements in the steel and changes the mechanism that adversely affects each), and the reactivity between Zn and the steel sheet due to the Fe-C pre-plated layer. Is considered to be involved.

[0040]

Example 1 Various ultra-low carbon steel cold-rolled steel sheet unannealed materials (sheet thickness 0.8 mm) having the compositions shown in Table 1 were cut into 250 × 100 mm as test materials. This steel sheet was previously degreased with trichlene, electrolytically degreased in a 2 to 10% sodium orthosilicate aqueous solution at 60 ° C., pickled in 10% hydrochloric acid, and then plated for 0.1 to 30 seconds under the plating conditions shown in Table 2. Fe-C electroplating or electric Fe plating for comparison was applied.

[0041]

[Table 1]

[0042]

[Table 2]

In FC2-4, after preparing the bath, 90 ° C
Heat for 10-20 minutes at room temperature, add sulfuric acid after cooling, and p
H was adjusted to 2. The plating temperature was kept constant at 40 ° C. The amount of Fe-C and Fe electroplating deposited was varied by changing the energization time and the current density.

The following Table 3 shows the results of examining the adhesion amount and C content (weight% in the film and C adhesion amount) of the above-mentioned pre-plated film.
Shown in.

[0045]

[Table 3]

Using a vertical hot dip galvanizing apparatus (manufactured by Lesca) capable of heat treatment in a predetermined atmosphere and direct hot dipping from a reducing atmosphere, the pre-plated steel sheet obtained above and a pre-painted sheet for comparison were used. Hot-dip galvanizing was performed on a steel sheet without plating. Each test steel sheet was preheated and annealed in this apparatus in advance, and then hot-dipped. Preheating is 5 to 15% oxygen for Fe-C pre-plated steel sheet and non-pre-plated steel sheet.
In the nitrogen atmosphere containing vol%, and in the case of the Fe pre-plated steel sheet for comparison, the nitrogen atmosphere containing 8000 ppm to 15 vol% oxygen is performed at 500 ° C for 0 to 60 seconds, and the annealing is performed at a dew point of -30. ℃, oxygen concentration 2ppm, N ₂ + 40% H ₂
It was carried out at 850 ° C for 60 seconds in the atmosphere. After annealing, after cooling to a steel plate temperature of 520 ° C., hot dip galvanizing was performed using molten zinc having various Al concentrations. The plating time was 1 second, and the Zn adhesion amount was adjusted to about 60 g / m ² (per surface) with a gas wiper. After plating, the Si-added steel was visually inspected for the occurrence of non-plating, and the wettability of the steel sheet was evaluated. Further, each of the obtained hot-dip galvanized steel sheets was alloyed in a salt bath at 500 ° C., and the time until alloying was completed (alloying time) was measured. That is, the time required for the Fe concentration in the hot-dip galvanized film to reach 9 to 12% was measured and used as the alloying time. The results are shown in Table 4 below.

[0047]

[Table 4]

As can be seen from the results shown in Table 4, by preliminarily applying the ultra-thin Fe-C pre-plating according to the present invention, the wettability of the Si-added steel is improved and the non-plating is prevented. , Also effective in improving the alloying delay in Si / P-added steel. On the other hand, in the comparative example subjected to Fe pre-plating, the alloying time is completely the same as that in the comparative example without pre-plating, and the effect of shortening the alloying time is completely in the Fe pre-plating despite the presence of the pre-plating layer. Not obtained. From this, the unexpected effect of Fe-C pre-plating is clear.

Example 2 Using various cold-rolled steel sheets (sheet thickness 0.8 mm) having the components shown in Table 5, degreasing and washing were carried out in the same manner as in Example 1, and then electroplating with an electrolytic solution having the composition shown in Table 6 A bath was used and electrolytic Fe-C plating was performed under the same electrolytic conditions as shown in Table 6. Also, for the purpose of comparison, Table 6 also shows the electrolysis conditions when electroplating Fe.

[0050]

[Table 5]

[0051]

[Table 6]

The C content in the electric Fe-C plating film was changed by controlling the concentration of formic acid, and the plating film thickness was changed by adjusting the amount of electricity supplied. In addition, a trace amount of alloying component was contained in a part of the Fe-C plating film. Microalloy components are chlorides, nitrates, sulfates, acetates, carbonates, molybdates, pyrophosphates of each metal in the electrolytic solution.
The hypophosphite, the organometallic salt, or the metal itself was dissolved in an acid in advance and added in an amount such that a plating film having a target composition could be obtained.

After the completion of the electro-Fe-C plating, the obtained pre-plated steel sheet is dried, and subsequently, using the same apparatus as in Example 1, a weak oxidizing property (N ₂ containing 5 ppm of oxygen) is used. After preheating the steel sheet at 500 ° C for 1 second in the atmosphere, N ₂ + 20%
It was annealed by heating at 850 ° C. for 20 seconds in an atmosphere of H ₂ . Then, dip for 3 seconds in a hot dip galvanizing bath containing a specified amount of Al at 460 ℃, and about 60 g / m ² per side.
Hot-dip galvanized. In addition, some test materials
The hot dip galvanization was followed by an alloying treatment at 500 ° C until the metallic luster of the zinc disappeared.

The hot dip galvanized steel sheet (G
The low-temperature impact resistance of the plating films of the respective test materials of the abbreviated I and the galvannealed steel sheets (abbreviated as GA) was evaluated by the following method.

Test method: Zinc phosphate treatment (using CF168 treatment liquid from Chemfil Co.) on the plating film on one side of the test material
→ Cationic electrodeposition coating (PPG uses Uniprime paint, film thickness
30μm) → intermediate coating (using the company's epoxy ester paint, film thickness 15μm) → topcoat paint (using the company's acrylic / enamel paint, film thickness 45μm). Set this on the test bench and place diamond particles (diameter about 3 mm) at a low temperature of -40 ° C at a speed of 120
After giving an impact of 10 collisions at a speed of km / h, it was subjected to the corrosion resistance exposure test described below. The impacted test material was continuously exposed to the industrial area environment (Amagasaki City, Hyogo Prefecture) while immersing it in a 3% NaCl aqueous solution for 30 minutes once a month, and this exposure test was continued for 5 years. did. For the evaluation of low temperature impact resistance, the maximum diameter of the coating film blister at the collision point is 5 mm or more (×), 3 mm or more and less than 5 mm
(△), 1 mm or more and less than 3 mm (○), less than 1 mm (⊚).

The test results are shown as the type of steel sheet used and the lower layer Fe.
Table 7 to Table 9 together with the C content of the -C electroplating film, the film thickness and the amount of trace elements, and the amount of Al in the hot dip galvanizing bath.

[0057]

[Table 7]

[0058]

[Table 8]

[0059]

[Table 9]

As is clear from the results shown in these tables, in the hot dip galvanized steel sheet and the galvannealed galvannealed steel sheet which have been subjected to electric Fe-C pre-plating, the conventional pre-plating is Fe electroplating. Compared with the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, and the hot-dip galvanized steel sheet which is not pre-plated and the hot-dip galvanized steel sheet, the low temperature impact resistance is extremely good. Further, as can be seen from the test results of the comparative example in which the C content or the film thickness of the lower layer electric Fe-C plating is outside the range specified by the present invention, the film thickness is too thin, or the C amount is too large or too small. If it is too high, sufficient low temperature impact resistance cannot be obtained.

[0061]

INDUSTRIAL APPLICABILITY As described above, the method of the present invention improves the plating wettability of the steel sheet in hot dip galvanizing to prevent non-plating, and accelerates the subsequent alloying treatment to improve the alloying treatment. It is possible to prevent the occurrence of delay and uneven processing.
At the same time, the obtained plating film has excellent film durability against low-temperature impact load even with thick plating. Therefore,
According to the present invention, high-quality hot-dip galvanized steel sheets and hot-dip galvanized steel sheets can be economically produced regardless of the type of base steel sheet. Therefore, the hot-dip galvanized steel sheet produced by the method of the present invention can be remarkably extended in life when it is applied to, for example, vehicle bodies of automobiles and home electric appliances, and is extremely useful industrially.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Tetsuaki Tsuda 4-53-3 Kitahama, Chuo-ku, Osaka Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] 1. Fe with a C content of 0.001 to 10 wt% on a steel plate
A method for producing a hot-dip galvanized steel sheet, which comprises performing hot-dip galvanizing after applying C-plating at an adhesion amount of 0.1 to 10 g / m ² or performing an alloying treatment subsequent to hot-dip galvanizing.