JPH06212384A - Hot dip galvanizing method for silicon-containing steel sheet - Google Patents

Abstract

PURPOSE:To produce a hot dip galvanized or galvannealed steel sheet free from the generation of nonplating by using a steel sheet contg. a specified amt. of Si as a base metal. CONSTITUTION:(1) This is a method for producing a hot dip galvanized steel sheet by using a steel sheet contg. >=0.2wt.% Si as a base metal, and in which the steel sheet after being subjected to hot rolling is pickled, and after that, its surface is ground away by 0.4 to 30g/m<2>. Next, it is subjected to cold rolling and is thereafter subjected to heating treatment to form an iron oxide layer of 0.5 to 5.0g/m<2> expressed in terms of Fe, and it is reduced at 600 to 950 deg.C and is subsequently subjected to hot dip galvanizing. (2) After the application of the hot dip galvanizing by the stage of (1), alloying heat treatment is furthermore executed into a galvannealed steel sheet.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention manufactures a hot-dip galvanized steel sheet or a hot-dip galvanized steel sheet having a base material of a high-strength steel sheet containing silicon (Si), which is particularly suitable as a steel sheet for automobiles. On how to do.

[0002]

2. Description of the Related Art In recent years, a large amount of hot-dip galvanized steel sheets have been used in the industrial fields of home appliances, building materials, automobiles, etc., but they are alloyed for their economic efficiency, rust-preventive function and good performance after painting. Hot-dip galvanized steel sheets are widely used.

Hot-dip galvanized steel sheets are usually preheated in a weakly oxidizing atmosphere or a reducing atmosphere after an appropriate degreasing and washing step or without degreasing and washing, and then protected with hydrogen and nitrogen. Alternatively, it is manufactured by annealing in a reducing atmosphere, cooling to near the plating temperature, and then immersing in a molten zinc bath.

At the time of preheating in the above process, it is preferable that an oxide film having a thickness of about 80 nm is formed on the surface of the steel sheet in terms of wettability with molten zinc, but an oxide film having a thickness larger than that is preferable. Is considered to have an adverse effect of increasing the generation of dross and impairing the adhesion of hot dip coating. 0.08 to 0.18% (hereinafter, unless otherwise specified, "%" means "% by weight") of Al (aluminum) in the hot-dip galvanizing bath, including the manufacturing range of alloyed hot-dip galvanized steel sheet described below.
Is included.

Alloyed hot-dip galvanized steel sheets are usually prepared by continuously hot-dip galvanizing steel sheets 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. The plating layer is made of an Fe-Zn intermetallic compound and generally has an average Fe concentration of 8 to 12%.
The amount of plating applied is usually 25 to 70 g / m ^{2 on} one side, and those below this range are difficult to produce by ordinary means, and those above this range reduce the powdering resistance of the plating layer. It is generally not supplied because it is difficult to secure.

[0006] Al alloying hot-dip galvanized steel sheets often contain about 0.12 to 0.2% Al.
One of the causes is to suppress the formation of an alloy layer at the interface between the plating film and the base steel plate in the production of a normal hot-dip galvanized steel plate, and to maintain the film workability, Al is added in the bath.
Therefore, when an alloyed hot-dip galvanized steel sheet is manufactured using the same bath, Al is unavoidably mixed in the plating film. Another cause is that it is rather desirable to contain Al in the plating bath in order to secure the powdering resistance of the coating film of the galvannealed steel sheet and suppress the generation of dross during the production. Is
This is because a molten zinc bath containing Al of about 0.08 to 0.11% is usually used. Since Al tends to be enriched in the plating layer during plating, plating in the above bath will reduce the Al concentration in the film.
The range is 0.12 to 0.2%.

As the base material of the plated steel sheet as described above,
Conventionally, low carbon Al killed steel sheets and ultra low carbon Ti-added steel sheets have been mainly used. However, in recent years, for automotive materials, for example, as one measure for weight reduction, high strength steel sheets have been demanded, and silicon-containing steel sheets containing 0.2% or more of Si are about to be used. Since Si is an element that improves the strength while ensuring the ductility of steel, it can be said that silicon-added steel is a desirable steel material that meets the above requirements.

However, the silicon-containing steel plate has a major drawback as a base material for hot dip galvanizing. When a silicon-containing steel plate is treated according to the above-mentioned ordinary process, a very small amount of water in the atmosphere reacts with Si in the steel plate in the annealing process, and the Si-Oxide impairs the wettability with molten zinc on the steel plate surface.
Is generated. Therefore, non-plating frequently occurs as the Si concentration in steel increases. It is known that wettability is improved by forming Fe oxide on the surface of a silicon-containing steel sheet by heating it in an oxidizing atmosphere in advance. However, if the Si content exceeds 0.2%, the oxidizing atmosphere in the conventional process (for example, the atmosphere where the air-fuel ratio of the non-oxidizing furnace is set to 1-1.35)
It is difficult to recover the wettability because the Si in the steel has the effect of suppressing the oxidation of the steel, only by preheating with.

Further, in the case of producing a galvannealed steel sheet using a silicon-containing steel sheet as a base material, there is a problem that the alloying treatment speed is remarkably slowed and the production efficiency is lowered.

In order to improve the formability of steel
When Si-added steel, which is based on additive ultra-low carbon steel, is used as the base material, the annealing temperature for recrystallization is 800 ° C or higher, so precipitation of Si-Oxide on the steel sheet surface becomes more pronounced and wets. It becomes more difficult to secure the sex.

As a method for solving the above-mentioned problems, it is conventionally known to apply an undercoat of Ni, Fe or the like prior to hot dipping, but an extra plating step is added, so that the number of steps is increased and the manufacturing is performed. Despite the cost increase,
Steel having a high Si content has a problem that a sufficient effect for improving wettability with molten zinc cannot be obtained, and a uniform alloying treatment is extremely difficult.

As described above, even in the case of silicon-containing steel, which is attractive in terms of material, it is the current situation that there is no practical method for applying suitable hot-dip galvanizing or alloying hot-dip galvanizing.

[0013]

DISCLOSURE OF THE INVENTION According to the present invention, it is impossible to form a satisfactory hot-dip galvanized coating in the conventional hot-dip galvanizing process, and the productivity is very low when alloying treatment is applied. The task was to develop a practical method for producing a plated steel sheet using the contained steel as a base material.

The object of the present invention is to economically perform hot dip galvanization by using a steel plate containing 0.2% or more of Si as a base material for plating without generation of non-plating points, and sufficiently increasing the alloying treatment rate. It is to provide a method for producing a steel sheet and a galvannealed steel sheet.

[0015]

[Means for Solving the Problems] The present inventors
%, Various studies were conducted to improve the wettability between steel and hot-dip galvanizing a hot-dip galvanized steel sheet and to accelerate the processing speed of alloying treatment. As a result, the base steel sheet was hot-rolled, pickled, then its surface was ground at 0.4 g / m ² or more, then cold-rolled, and heat-treated in the hot dip coating line (CGL).
By forming iron oxide of 0.5 g / m ² or more, and then performing ordinary reduction annealing and hot dip coating (and further alloying treatment if necessary), the wettability during hot dip coating is improved and alloying is performed. It has been found that the alloying rate increases when the treatment is applied.

As described above, the wettability at the time of hot dip coating can be improved by introducing the step of heating the base material surface and then performing heat treatment in an oxidizing atmosphere.
By removing the scale remaining on the steel sheet surface by grinding, a work-affected layer is formed at the same time, and during heat treatment in an oxidizing atmosphere, a larger amount of uniform and dense oxide is obtained than the oxide obtained by normal oxidation. A uniform and dense reduced iron layer is also formed during the subsequent reduction annealing that is subsequently performed, and this reduced iron layer acts as a barrier for Si diffusion to the steel sheet surface during annealing and reduces the wettability of Si-Oxide. It is presumed that the generation is suppressed. The work-affected layer is a portion (fine-grained layer) in which crystal grains are changed into fine grains close to amorphous by grinding, and remains as a transformed layer even after cold rolling.

The present invention has been made on the basis of the above findings, and has as its gist the following hot dip galvanizing method or method for producing an alloyed hot dip galvanized steel sheet.

(1) A method of manufacturing a hot-dip galvanized steel sheet using a steel sheet containing 0.2% or more of Si as a base material, wherein the hot-rolled steel sheet is pickled and then its surface is 0.4 to 30 g / m 2. ⁽²⁾ Grinding and removing, then cold rolling, heat treatment in an oxidizing atmosphere, and converting it to Fe on the steel plate surface 0.5 to 5.0
A hot dip galvanizing method for a silicon-containing steel sheet, which comprises forming an iron oxide layer of g / m ² and then reducing it at a temperature of 600 to 950 ° C. and then performing hot dip galvanizing.

(2) A method for producing an alloyed hot-dip galvanized steel sheet in which hot dip galvanizing is performed in the step described in (1) above, and then alloying heat treatment is further performed.

[0020]

The function and effect of the conditions defined in the present invention and the reason for limiting the conditions will be described below.

The steel plate targeted by the present invention is Si as described above.
Steel plate with a content of 0.2% or more.

When the Si content is less than 0.2%, it is possible to deal with it by devising conventional techniques, and it is not necessary to dare to apply the method of the present invention.

The scale of the hot rolled steel sheet is removed by pickling, and then the surface thereof is ground. Grinding immediately after hot rolling has an effect, but immediately after hot rolling, not only the surface of the steel sheet is rough, but also the scale is thickly adhered, and grinding exceeding 10 g / m ² Is necessary, which is disadvantageous in terms of cost.

There is no restriction on the grinding method, but a normal brush roll for grinding is relatively suitable. Wet grinding is advantageous in terms of brush life. Grinding amount is limited to 0.4g / m ² to 30g / m ² . If it is less than 0.4 g / m ² , the removal of the Si-based oxide layer that is generated during hot rolling and remains even after pickling is insufficient, and the work-affected layer introduced to the surface of the steel sheet by grinding. The amount of (fine grain layer) is insufficient, and the oxidation rate of Fe during the heat treatment of the next step is small, resulting in insufficient oxidation. If the grinding amount exceeds 30 g / m ² , the effect of removing the Si-based oxide and improving the wettability by forming a fine-grained layer on the surface is saturated, resulting in a large decrease in product yield.

The ground steel sheet is then cold rolled.
This may be the cold rolling that is usually performed when manufacturing cold-rolled steel sheets, and the sheet thickness is rolled from 2.3 to 4.0 mm to 0.4 to 1.6 mm.

Next, if necessary, an alkaline degreasing treatment with a 2 to 3% aqueous solution of NaOH or the like is carried out, and then it is oxidized by heating in an oxidizing atmosphere, and its surface is converted into Fe.
Iron oxide is formed corresponding to an amount of 0.5-5.0 g / m ² . As the atmosphere, an atmosphere containing an oxidizing gas such as CO ₂ , O ₂ and H ₂ O is used. If the amount of oxidation in terms of Fe is less than 0.5 g / m ² , non-plating is likely to occur, and if it exceeds 5.0 g / m ² , reduction of iron oxide in the next step is likely to be insufficient and non-plating is likely to occur. .

The temperature of the oxidation treatment is not particularly limited, but it is suitable that the material temperature is about 500 to 700 ° C. from the viewpoint that the reaction rate is relatively fast and the reaction amount does not become excessive. As for the processing time, if the residence time in this temperature range after the material temperature reaches 500-700 ℃ is about 0-30 seconds, it is possible to obtain a predetermined oxidation amount without changing the line speed. Is.

The steel sheet having the iron oxide layer formed on the surface is subjected to reduction treatment at a temperature of 600 to 950 ° C. in the next step. It should be noted that this reduction treatment is performed also as the annealing treatment after the cold rolling previously performed. For this purpose, a dew point of -5% or more of hydrogen (hereinafter, "%" for gas means "volume%") and a balance of non-oxidizing gas (usually nitrogen) is -15.
It is advisable to use an atmosphere of ℃ or less. If the treatment temperature is lower than 600 ° C, the cold-worked steel sheet may not be sufficiently annealed, and the reduction of oxides may be insufficient, while if it exceeds 950 ° C, the steel sheet softens. It becomes difficult to maintain flatness during operation. The upper limit of the reduction treatment temperature is
The temperature is preferably 820 ° C. The processing time may be 15 to 250 seconds.

The steel plate treated in the above steps is 380 to 550.
Al when cooled to around ℃ and not alloyed after plating
In a molten zinc bath containing 0.03 to 3% of Al, when performing alloying treatment, it is immersed and plated in a molten zinc bath containing Al of 0.03 to 0.12%, and the amount of deposition is determined by gas wiping within a predetermined range. Is adjusted to become a hot-dip galvanized steel sheet product.

In the case of producing an alloyed hot-dip galvanized steel sheet, the steel sheet plated in the above step is passed through an alloying treatment furnace and heated at 450 to 600 ° C. for 1 to 120 seconds to alloy the plating film.

As described above, by applying the method of the present invention, a sufficient iron oxide layer is formed on the surface of a silicon-containing steel sheet having a small oxidation rate and hard to form iron oxide by utilizing the grinding effect. It is possible to produce a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet which has a function of reducing the diffusion of Si to the surface of the steel sheet and has excellent surface characteristics without unplating.

According to the method of the present invention, after the steel sheet after hot rolling is pickled, the surface of the steel sheet is ground before cold rolling and the subsequent reduction treatment is performed also as annealing after cold rolling. It has a feature that it is possible to reduce the concentration of Si on the surface of the steel sheet at the same reduction (annealing) temperature as compared with the normal method in which grinding is not performed.

[0033]

EXAMPLE A high Si steel having the chemical composition shown in Table 1 was hot-rolled,
Then, after pickling, a whetstone (PVA whetstone manufactured by NIPPON SPECIAL LABOR Co., Ltd .; C1
207M, C320S, etc.) was used to grind in a range of 25 g / m ² or less per side to a plate thickness of 3.6 mm. Then, cold rolling is performed by a pass schedule of 3.6 mm → 2.4 mm → 1.6 mm → 1.2 mm → 1.0 mm → 0.8 mm.
mm, unannealed material), and cut into 250 mm x 100 mm as test materials. This steel plate was solvent degreased with thinner,
After alkaline degreasing in a% NaOH aqueous solution, hot dip plating was performed using a vertical hot dip galvanizing apparatus capable of heat treatment in a predetermined atmosphere and direct hot dipping from a reducing atmosphere.
That is, if necessary, the steel sheet is preheated and oxidized in this apparatus under various atmospheres and heating conditions of a to e shown in Table 2 and then subjected to reduction in an atmosphere of N ₂ + 25% H _{2 to} obtain a steel sheet temperature of 460 ° C. After cooling to 0.degree. C., hot dip plating was performed in a hot dip zinc bath having an Al concentration of 0.03 to 3.0%. Table 3 shows the reducing conditions. The plating time was set to 1 second, and the amount of zinc deposited was adjusted to about 60 g / m ² on one side with a gas wiper.

After plating, the occurrence of non-plating was investigated.
Furthermore, for those plated in a molten zinc bath with an Al concentration of 0.03 to 0.12%, the alloying treatment is performed in a salt bath at 500 ° C, and the time when the Fe concentration in the plating film reaches 10% is set. The time required for alloying was measured. Considering the alloying in the actual plating line, it is judged that rational and productive alloy plating is possible if alloying can be done within 20 seconds. Table 3 shows the time required for alloying for those subjected to plating evaluation and alloying treatment.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3 (1)]

[0038]

[Table 3 (2)]

As can be seen from the results shown in Table 3, all of the examples of the present invention have no non-plating, and all the galvannealed steel sheets can be alloyed in a relatively short time of 20 seconds or less.

[0040]

According to the method of the present invention, a hot-dip galvanized steel sheet containing a silicon-containing steel as a base material or an alloyed hot-dip galvanized steel sheet can be produced without occurrence of non-plating.

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Claims (2)

[Claims] 1. A method for producing a hot-dip galvanized steel sheet using a steel sheet containing Si in an amount of 0.2 wt% or more as a base material, wherein the hot-rolled steel sheet is pickled and then its surface is 0.4 to 30 g / m 2. ⁽²⁾ Grinding and removing, then cold rolling, heat treatment in an oxidizing atmosphere, and converting it to Fe on the steel plate surface 0.5 to 5.0
A hot dip galvanizing method for a silicon-containing steel sheet, which comprises forming an iron oxide layer of g / m ² and then reducing it at a temperature of 600 to 950 ° C. and then performing hot dip galvanizing. 2. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises hot-dip galvanizing in the step of claim 1 and then further subjected to alloying heat treatment.