JPH05140711A - Manufacture of galvanized steel sheet - Google Patents

Abstract

PURPOSE:To manufacture a uniformly plated steel sheet by previously subjecting the surface of a steel sheet to Fe-N composite coating treatment and thereafter applying galvanizing thereto. CONSTITUTION:The surface of a steel sheet whose kind is a one particularly low in plating wettability such as Si-added steel and contg. 0.03 to 3% SiO2 is subjected to Fe-N composite coating treatment and is then galvannealed. In this way, since it is excellent in coating corrosion resistance, press formability and weldability, the surface plating of a galvannealed steel sheet as the material used in a wide industrial field such as automobiles, house appliances and building materials can uniformly be applied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing a hot dip galvanized steel sheet, which improves the coating wettability of steels such as Si-added steels, especially steels having low coating wettability, and industrially produces them. It aims to be easy.

[0002]

2. Description of the Related Art Hot-dip galvanized steel sheets are materials used in a wide range of industrial fields such as automobiles, home appliances, and building materials because of their excellent coating corrosion resistance, press formability, and weldability. The manufacturing method is usually to heat the cold-rolled steel sheet or hot-rolled steel sheet in an oxidizing or non-oxidizing atmosphere to burn off dirt such as oil adhering to the surface, and then clean the surface in a reducing atmosphere. After that, it is carried out in the step of applying hot dip galvanizing.

[0003]

In the Si-added steel used as a high-strength steel sheet, if the Si content in the steel is high,
Si oxide is generated on the surface of the steel sheet, which impedes wettability with molten zinc. As a solution to this problem, it is disclosed that the steel plate surface is plated with Fe prior to plating (Japanese Patent Laid-Open No. 59-2.
No. 3858), but the effect is found to be insufficient, and the present invention proposes a method for improving the effect.

[0004]

The present invention has been made in order to advantageously solve the above-mentioned problems, and is characterized by a novel plating method for further enhancing the effect of pre-plating before hot dipping. The present invention is a method for producing a hot dip galvanized steel sheet, which comprises subjecting a steel sheet surface to an Fe-N composite coating treatment in advance, and then performing hot dip galvanizing plating. The steel sheet surface containing Si 0.03 to 3% is Fe- Method for producing hot-dip galvanized steel sheet, characterized by performing N composite coating treatment, and then hot-dip galvanizing plating, using an acidic aqueous solution bath containing one or more Fe ions and one or more of the following N components Fe- according to claim 1 or 2, wherein the steel plate is electrolyzed at the cathode.
Manufacturing method of N composite coating treatment. The N content is 1 to 200 g / l of nitrate ions and / or 0.01 g / l to 20 g / l of an organic compound containing N such as a polyamine derivative.

[0005]

The Fe-N composite coating layer refers to a coating layer having a structure in which N-based material is alloyed, mixed, or occluded in Fe. An example of the cathodic electrolysis method is shown below as a method of forming the Fe-N composite coating. If necessary, degreasing the steel plate by the usual method,
After the pickling treatment, the solution was introduced into an aqueous solution of ferrous sulfate or ferrous chloride containing iron nitrate and / or nitrate ions (nitrate ion concentration of 1 g / l to 20 g / l), and a current density of 30 to 300 A / using the steel sheet as the cathode By performing an electrolytic treatment of dm ² for 0.1 to 30 seconds, a nitric acid decomposed product which is an N substance is co-deposited as N in the Fe coating layer by 0.01 to 1%. In addition, F
Although the form of the e coating layer varies depending on the electrolysis conditions, it is made of iron oxide, iron hydroxide, or metallic iron.

An example of a plating method for obtaining the Fe-N composite layer is shown below. If necessary, the steel sheet is degreased and pickled by a usual method, and then introduced into a bath containing 0.01 to 10 g / l of polyamine in an aqueous solution of ferrous sulfate or ferrous chloride, and the steel sheet is used as a cathode for electric current. Density 30-300A /
By performing electrolytic treatment of dm ² for 0.1 to 30 seconds, F
In the plating layer, polyamine, which is an N substance, has a N content of 0.
01-0.5% eutectoid. Next, when the above-prepared one is heated to 500 to 700 ° C. in a non-oxidizing furnace and held at 500 to 800 ° C. in a reducing furnace for heat treatment, iron oxide is reduced to iron, and an Fe plating layer is formed. Along with recrystallization, nitric acid decomposition products and polyamines are decomposed to form a Fe-N composite plating layer.

When no nitrogen component is present, the Fe plating layer recrystallizes to form crystal grains similar to those of the base steel sheet, but Si of the base steel sheet reaches the surface of the Fe plating layer through the grain boundaries. Spreads easily. Si forms an oxide on the surface of the steel sheet during the heat treatment process in an oxidizing or non-oxidizing atmosphere and a reducing atmosphere, and has a significant effect on the subsequent wettability with molten zinc. Since the amount of the material varies depending on the steel type, the wettability with molten zinc is different. For this reason, the effect of reducing the amount of Si oxide was insufficient in Fe pre-plating.

On the other hand, in the Fe-N composite coating treatment, amorphous-like fine crystals are generated in the recrystallization process,
No clear grain boundaries are observed. Further, N diffuses in the grain faster than Si and tends to be concentrated at grain boundaries and surfaces.
Therefore, the diffusion of Si to the surface of the plating layer is significantly suppressed, the formation of Si oxide is suppressed even if the Si content in steel is high, and good plating wettability is secured.

Next, a method of treating the Fe-N composite coating will be described. Although it is a method based on the cathodic electrolysis method, an aqueous solution of ferrous sulfate or ferrous chloride containing iron nitrate and / or nitrate ions (nitrate ion concentration of 1 g / l to 200 g /
Using the acid treatment bath of l), electrolytic treatment is performed using a steel sheet as a cathode. Current density 30-300 A / dm ² , 0.1-3
By performing the electrolytic treatment for 0 second, a nitric acid decomposed product which is an N substance is co-deposited as 0.01 to 1% as N in the Fe coating layer.
The form of the Fe coating layer differs depending on the electrolysis conditions, but is made of iron oxide, iron hydroxide, or metallic iron. Nitrate ions are reduced and decomposed on the surface of the steel sheet by cathodic electrolysis, and are uniformly contained in the Fe coating layer as N-type substances. Although the amount of nitrate ion added is less than 1 g / l, almost no effect is observed. 20
Even if it exceeds g / l, the N content in the Fe coating layer is saturated and the improvement effect is saturated.

Next, the electrolysis conditions will be described. It is preferable to subject the steel sheet to electrolytic treatment at a current density of 30 A / dm ² or more. When it is 30 A / dm ² or less, it takes time to obtain a sufficient Fe—N coating amount for improving the plating wettability, which is not industrial. The flow velocity of the plating solution is 10 to 200 m / min as the relative velocity to the steel strip, and the plating temperature is 40 to 7
0 ° C is suitable.

The coating amount is 0.2 to 10 g / m ² of the coating layer. When the coating amount is less than 0.2 g / m ² ,
The effect is small, and if it exceeds 10 g / m ² , the effect is almost unchanged and it is disadvantageous in terms of cost.
² or less is preferable. The amount of N in the Fe-N coating is preferably 0.01% or more and 1.0% or less. If the amount of N is less than 0.01%, the effect on the alloying reaction is small, and if it exceeds 1.0%, the plating layer becomes brittle and peels off in the furnace, which is not preferable. If necessary, the steel sheet may be degreased and pickled by a conventional method before plating.

Next, as a method by electroplating, an anionic surfactant, a cationic surfactant containing a polyamine derivative or N as an N component in an aqueous solution of ferrous sulfate and / or ferrous chloride, One or more nonionic surfactants and amphoteric surfactants are added in an amount of 0.01 to
Using an acidic plating bath added with 20 g / l, electrolytic treatment is performed using the steel sheet as a cathode.

These N-mass substances are ether groups and amines.
Adsorption to active sites is high due to the action of polar groups such as
Therefore, it is easily and uniformly contained in Fe plating. this
The additives are generally R²-X- (R¹-Y) n-R³  And / or R²-(R¹-X) n-R³  Indicates a compound represented by. Here, X, Y: O (ether group), NH, NR (amine group),
NR₂+ (Ammonium group) R¹: Alkylene group and / or R², R³: H, alkyl group, phenyl, naphthyl, etc.
Aryl group and / or its derivative (carboxylic acid, sulfuric acid,
Functional group such as phosphoric acid, salt, ester) n = 1 to 2000 Even if these compounds are linear compounds such as polyamines
Well, of crown ether and sorbitan monolaurate
Cyclic compounds such as amine derivatives may also be used.
A linear / cyclic mixed compound such as rufon may be used.

[0014] By way of specific example, & primary amine RNH2 R: alkyl secondary amine R ₂ NH R: alkyl tertiary amines RNR'2 R, R ': alkyl

[0015]

[Chemical 1]

Diamine RNH (CH ₂ ) ₃ NH ₂ R: alkyl

[0017]

[Chemical 2]

[0018]

[Chemical 3]

[0019]

[Chemical 4]

Urea and its derivatives CO (NH₂)₂  Etc.

The addition amount of these additives in the plating bath is 0.
01 to 20 g / l is a preferable range. 0.01 g /
If it is less than 1, almost no effect is observed. 20g /
When it exceeds 1, the content of N is saturated and the solubility in the plating bath is restricted. The above additives may be used alone or in combination of two or more. The Fe ² + ion concentration is not particularly limited, but it is usually in the range of 10 to 150 g / l, and optimum conditions can be adopted for the target plating in consideration of current efficiency. In addition, Na +, K +, NH
_The addition of extraneous salts such as ₄ + and Mg ² + ions is effective for increasing the electric conductivity of the bath. Depending on the purpose, Ni, Co, Cr, Mn, Cu, Cd, Sn, P
Even if a small amount of ions such as b are added to cause a small amount of the third component to co-deposit on the plating layer, the effect of the present invention is essentially unchanged.

Next, the plating conditions will be described. It is preferable to plate the steel sheet at a current density of 30 A / dm ² or more. When it is 30 A / dm ² or less, it takes a long time to obtain a sufficient Fe—N plating amount for improving alloying, which is not industrial. The flow velocity of the plating solution is 10 to 200 m / min as the relative velocity to the steel strip, and the plating temperature is 40 to 70.
℃ is suitable.

The plating amount is 0.2 to 10 g / m ²
Plating. If the plating amount is less than 0.2 g / m ² , the effect is small, and if it is more than 10 g / m ² , the effect is almost unchanged and the cost is disadvantageous.
/ M ² or less is preferable. The amount of N in the Fe-N plating is preferably 0.01% or more and 1.0% or less. N
If the amount is less than 0.01%, the effect on the alloying reaction is small, and if it exceeds 1.0%, the plating layer becomes brittle and tends to peel off in the furnace, which is not preferable. If necessary, the steel sheet may be degreased and pickled by a conventional method before plating. Further, it is also possible to add the nitride fine particles as they are or together with the above-mentioned surfactant.

Next, examples of modes other than the above will be shown. The electroplating method may be performed immediately before the hot dipping method. After electroplating, if necessary, flux may be applied and heated to 400 to 500 ° C. to lead to a hot dip plating bath, or directly to the hot dip bath at room temperature and heated in the bath for plating. In this case, the N-containing substance of the Fe-N composite plating layer is hot-deposited in an undecomposed or partially-decomposed state, but has a sufficient suppressing effect on Si diffusion in the hot-dip plating bath. F
The e-N composite plating can also be applied by a physical vapor deposition method that targets Fe and N, and a chemical vapor deposition method that decomposes and deposits using a vaporizable Fe salt and an organic vapor. Fe- by these methods
The mechanism of action of the N composite plating is the same as above, and will be obvious.

The thickness of the Fe-N composite plating layer of the present invention is
0.2-10 g / m < ² > is suitable. If it is less than 0.2 g / m ² , the effect is unclear, and if it is more than 10 g / m ² , the effect is almost unchanged and the cost is disadvantageous. In the Si-added steel, a steel sheet having a Si content of 0.2 to 3% has a great effect of improving the wettability with molten zinc. 0.2%
If it is less than the same level as ordinary steel, if it exceeds 3% Si, the effect is insufficient even with the Fe-N composite plating of the present invention.

The hot dip galvanizing bath of hot-dip galvanized steel sheet usually contains Al in an amount of 0 to 0.2%.
b, Cd, Sb, Sn may be present in a small amount, and for the purpose of quality improvement, Ni, Mn, Ti, Zr, M
Although a small amount of g, Ca, Li, lanthanide, etc. may be added, the method of the present invention is essentially applicable to all the steps of alloying. Fe-N
In complex plating, other elements such as Ni, Zn,
Even if a small amount of Mn, Cu, P, B, O, C, N, Cl, H, Na, etc. is mixed, it is essentially the same as the present invention.

[0027]

EXAMPLES Next, examples of the present invention will be given together with comparative examples. Tables 1A and 1B show plating conditions in advance. After the plating, heat treatment is performed at 650 ° C. (plate temperature) on the outlet side of a direct-fired non-oxidizing furnace of a continuous hot-dip galvanizing equipment pretreatment furnace, and at 750 to 800 ° C. for 30 seconds in a reduction heat treatment furnace, and then a zinc-based plating bath. It was introduced to and plated. Table 2 shows the chemical composition of each steel sheet. Table 3A and Table 3B show the plating wettability of the plated steel sheet. X indicates that the plating is not adhered to most of the steel sheet, Δ indicates that the plating is not adhered on part, and ◯ indicates that the plating is adhering well.

[0028]

[Table 1A]

[0029]

[Table 1B]

[0030]

[Table 2]

[0031]

[Table 3A]

[0032]

[Table 3B]

[0033]

As described above, according to the present invention, this can be easily performed even for a steel type such as a high-strength steel sheet having a high Si content, which has low wettability of plating and is difficult to be hot-dipped. By doing so, it is industrially stable even for high-strength steel sheets with a high Si content, which was previously difficult to hot-dip galvanize,
Moreover, hot dip plating can be reliably performed.

Claims (3)

[Claims] 1. A method for producing a hot dip galvanized steel sheet, which comprises subjecting a steel sheet surface to a Fe-N composite coating treatment in advance and then hot dip galvanizing. 2. A method for producing a hot dip galvanized steel sheet, which comprises subjecting a surface of a steel sheet containing 0.03 to 3% of Si to an Fe—N composite coating treatment, and then hot dip galvanizing. 3. The steel sheet is electrolyzed at the cathode using an acidic aqueous solution bath containing one or more of Fe ions and any one of the following N-contents.
The method for producing the Fe-N composite coating treatment according to claim 1. The N component is an organic compound containing nitrate ions of 1 to 200 g / l and / or N, such as a polyamine derivative.