JPH07316762A - Method for galvannealing plated base material difficult to be alloyed - Google Patents

Abstract

PURPOSE:To provide a method for galvannealing an Si-contg. steel in which alloying treatment after plating is difficult as a base material by continuous galvanizing equipment. CONSTITUTION:An Si-contg. steel sheet (the content of Si is regulated to 0.1 to 2.0wt.%) is used as a base material, and heating in the same oxidizing furnace is executed in an atmosphere of <=30 deg.C dew point in such a manner that the air-fuel ratio is regulated to 0.8 to 1.25 and the maximum arrival steel sheet temp. in the oxidizing furnace satisfies the following inequality: 700+100XIn [Si] <=T<=830+50XIn [Si]; the oxidizing furnace and [Si] denotes the Si content (wt.%) in the base material steel sheet. By the continuous galvanizing equipment, the galvannealed steel sheet can be obtd. without reducing its productivity. Since this steel sheet has high strength and high workability, when being used as the stock for an automobile body or the like, its thinning and lightening are made possible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for carrying out galvannealing using a steel sheet containing silicon as a plating base material in a continuous hot dip galvanizing facility.

[0002]

2. Description of the Related Art Since galvannealed steel sheets have excellent corrosion resistance, in recent years, there has been an increasing demand as a material for home electric appliances, building materials, and also for automobile bodies. is there. Since the weight of the vehicle body has been reduced from the viewpoints of saving fuel consumption and reducing the total amount of NOx and carbon dioxide (CO ₂ ) emissions by reducing fuel consumption (protecting the environment), it is used for automobile bodies. As a steel plate to be used, a high-strength steel plate capable of reducing the plate thickness while ensuring a predetermined strength is desired. A silicon (Si) -containing steel plate is a steel plate that can meet such demands, has high strength and is excellent in other mechanical properties, and its application range is expected to expand.

However, in general, in a Si-containing steel sheet, when performing galvanizing in a continuous hot dip galvanizing facility, Si in the steel is concentrated on the surface of the steel sheet during the annealing process.
There is a problem that an oxide is generated to deteriorate the wettability of plating.

As a method for solving this problem, for example,
In JP-A-55-122865, JP-A-4-202630 and JP-A-4-202633, a thick oxide film of iron (Fe) is formed on the surface of a Si-containing steel sheet in a non-oxidizing furnace, and this oxidation is performed. A method of applying plating after reducing the coating in a reducing atmosphere containing hydrogen is disclosed. Further, JP-A-5-271891 discloses controlling the atmosphere of a non-oxidizing furnace and a reducing furnace provided in the subsequent stage to control the thickness of an oxide film formed on the surface of a steel sheet to reduce the wettability of plating. How to prevent is shown.

On the other hand, in a high strength steel sheet containing Si, in addition to the above-mentioned deterioration of the wettability of the plating due to the Si oxide concentrated on the surface during annealing and the resulting decrease in the plating adhesion, There is a problem that alloying is significantly delayed when the alloying treatment is performed by heating immediately after plating. If the line speed is reduced to secure the time required for alloying, as described above, Si oxide is generated on the surface of the steel sheet during annealing, which deteriorates the wettability of the plating, which is not preferable. For example, in a steel sheet with a Si content of 0.6% by weight, even if the annealing conditions are the same, it is said that non-plating will occur if the in-furnace time from the annealing furnace in the reducing atmosphere to the immersion in the plating bath is 10 minutes or more. Therefore, when Si-containing steel is used as the plating base material, the in-furnace time should be suppressed to less than 10 minutes, and it is preferable that it be as short as possible.

Further, enlarging the alloying treatment equipment in order to secure a sufficient alloying treatment time requires enormous equipment investment and is substantially impossible.

After hot-dip galvanizing, it may be possible to carry out alloying treatment by batch annealing or a similar method, but this is not preferable because the process becomes complicated and the manufacturing cost increases.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of such a situation, and uses a conventional continuous hot-dip zinc as a base material of Si-containing steel which is difficult to hot-dip galvanize and alloy it. It is an object of the present invention to provide a method for performing galvannealing treatment using a plating facility while changing the operating conditions as little as possible.

[0009]

Means for Solving the Problems The present inventor has carried out alloying hot dip galvanizing with continuous hot dip galvanizing equipment using several types of high strength steel sheets having different Si contents as base materials, and as a result of repeated studies, The following findings were obtained. Incidentally, the continuous hot dip galvanizing equipment, in the preceding stage of the plating equipment, a preheating furnace for preheating the steel sheet in advance, an oxidation-free furnace for further heating the steel sheet to volatilize and disperse the oil and fat on the surface of the steel sheet, and further for the steel sheet surface. It has a reduction furnace that reduces iron oxide to activate the surface and also performs annealing and annealing at the same time, and a continuous annealing facility equipped with a cooling zone, and an alloying treatment facility after the plating facility.

Before the base steel sheet enters the reduction furnace, a preheating furnace,
By promoting the oxidation of the steel sheet surface in a non-oxidizing furnace or a pre-oxidizing furnace such as a direct heating furnace, alloying after plating is promoted.

At this time, Fe in the oxide scale produced by the oxidation in the pre-oxidizing furnace reacts with Zn in the plating film relatively quickly in the initial process of alloying. When the base steel sheet is a Si-containing steel, the diffusion of Fe from the base material into the plating film is very slow, and its rate becomes smaller as the Si content increases.

The formation of scale on the surface of the steel sheet during preoxidation largely depends on the Si content of the base steel sheet and the ambient temperature. Therefore, it is necessary to control the temperature during pre-oxidation according to the Si content of the base steel sheet in order to perform the alloying treatment after plating to obtain an appropriate degree of alloying. In addition, the proper alloying degree is that the coating weight is 30 to 90 g /
It is said that m ² is 6 to 14%, and particularly, 7 to 11% is preferable in the case where the weight is 60 g / m ² or more.

The present invention has been made on the basis of the above findings, and the gist thereof is the following alloying hot dip galvanizing method.

Si content in continuous hot dip galvanizing equipment
When performing galvannealing using a steel sheet of 0.1 to 2.0 wt% as the base material, heating in a pre-oxidizing furnace is performed in an atmosphere with a dew point of 30 ° C or less to an air-fuel ratio of 0.8 to 1.25 and pre-oxidizing. A method of hot dip galvannealing of a hard-to-alloy galvanizing base material, which is carried out so that the maximum reached steel plate temperature in the furnace satisfies the following formula (1).

700 + 100 × ln [Si] ≦ T ≦ 830 + 50 × ln [Si] (1) However, T: maximum attainable steel plate temperature (° C.) in the pre-oxidation furnace [Si]: Si of the base steel plate Content (wt%), ln [Si]
Means the natural logarithm of the Si content of the base steel sheet.

[0016]

The various conditions defined in the present invention will be described below. In addition, "%" means "wt%" unless otherwise specified.

The plating base material targeted by the method of the present invention is Si
It is a steel sheet with a content of 0.1 to 2.0%. Si content 0.1%
If less than, not only the steel sheet excellent in mechanical properties such as strength can not be obtained, in the atmosphere of the pre-oxidation furnace specified by the method of the present invention, a very thick oxide scale is formed on the steel sheet surface, the oxide scale Since the reduced iron layer produced by reduction in the reducing furnace significantly accelerates the alloying reaction, it exceeds the range of the appropriate alloying degree and causes the peeling of the plating film such as powdering during processing, which is not preferable. Further, in the reduction process after the pre-oxidation process, scale may adhere to the hearth roll (conveying roll), which may cause a flaw or the like on the surface of the steel sheet.

On the other hand, if the Si content is more than 2.0%, the effect of promoting alloying after plating cannot be expected even if the pre-oxidation treatment defined by the method of the present invention is performed, and the temperature is raised to form an oxide layer. Since the temperature must be too high during pre-oxidation, and the steel sheet is softened by exceeding the recrystallization temperature.
Even if added, the improvement of mechanical properties cannot be expected.

Regarding carbon (C) contained in the base steel sheet, if its content is less than 0.02%, the wettability of the plating deteriorates and the alloying rate decreases, and if it exceeds 0.15%, the strength increases too much. Therefore, it is preferably 0.02 to 0.15%. Also, phosphorus (P) content is 0.04%
If it exceeds, alloying may be significantly delayed, so 0.
It is preferable to keep it below 04%.

The air-fuel ratio when the above Si-containing steel sheet is treated as a base material in a pre-oxidation furnace is 0.8 to 1.25, preferably 0.9 to 1.2 as described in Examples below. Air-fuel ratio
When it is less than 0.8, the oxidation potential is small, the oxide scale necessary for proper alloying does not occur even if the steel plate temperature (which means the maximum reached steel plate temperature. The same applies below) is generated, and the calorific value is small. Therefore, it becomes difficult to sufficiently raise the steel plate temperature. On the other hand, when the air-fuel ratio exceeds 1.25, the combustion gas is not stabilized, soot deposits on the steel sheet and plating defects occur, or the temperature distribution in the pre-oxidation furnace becomes uneven and the steel sheet surface becomes uneven. If a uniform oxide scale is not generated and alloying treatment is performed, it may cause partial alloying unevenness.

The higher the dew point of the atmospheric gas in the pre-oxidation furnace, the more advantageous it is to produce oxide scale. However, when the dew point is higher than 30 ° C., the effect of promoting the production of oxide scale becomes small and the oxide scale becomes difficult to reduce, which is not preferable. Therefore, the dew point of the atmospheric gas in the pre-oxidation furnace is 30 ° C. or lower, preferably −40 ° C. to 0 ° C.

As a heating method at the time of pre-oxidation, a combustion frame of a burner is discharged from the side surface with a steel plate sandwiched between them, and a non-oxidizing furnace method of rapidly heating the inside of the furnace by the radiant heat, or a combustion frame of the burner is directly applied to the steel plate. Although either a direct fire burner system may be used, a non-oxidizing furnace system using radiant heat is preferable in order to maintain the stability of the atmosphere in the furnace.

The steel plate temperature in the pre-oxidizing furnace is defined as described above because the amount of scale produced during pre-oxidation and the diffusion rate of Fe from the base metal into the plating layer differ depending on the Si content of the steel plate. Because. Steel plate temperature is 700 + 100 × ln [Si]
If it is lower than (℃), it will be difficult to obtain the scale amount necessary for obtaining an appropriate degree of alloying without decreasing the line speed during preoxidation, while the steel plate temperature is 830 + 50.
If it is higher than × ln [Si] (° C.), the amount of oxide scale is too large, the alloying degree exceeds 14%, and plating peeling during processing such as powdering increases, which is not preferable. Further, when the amount of pre-oxidized scale increases, the scale adheres to the hearth roll, which may cause the occurrence of push defects.

According to the above-mentioned method of the present invention, the Si-containing steel, which is difficult to be alloyed after plating, is operated by using the continuous hot dip galvanizing equipment which has been conventionally used, and the operating conditions thereof are largely changed. The hot dip galvanizing and the subsequent alloying treatment can be carried out without any treatment.

[0025]

[Examples] Steel sheets A to G having the chemical composition shown in Table 1 (all have a thickness of 1.6 mm) were used as plating base materials, and hot-dip galvanizing was performed in a continuous hot dip galvanizing equipment by changing the steel sheet temperature during preoxidation. Plating was performed, and then alloying treatment was performed to produce a galvannealed steel sheet, and the degree of alloying was investigated.

A non-oxidizing furnace is used for pre-oxidation, and the air-fuel ratio is set to 0.
95, the dew point was 0 ° C. The processing conditions in the reducing furnace are as follows.

Atmosphere: N ₂ 75% by volume + H ₂ 25% by volume (dew point = −40 ° C.) Steel plate temperature: 790 to 900 ° C., processing time: 120 seconds The plating bath had the following bath temperature and bath composition, and after plating, The basis weight was adjusted to 40 to 55 g / m ² per side. The temperature of the material penetrating into the plating bath was 470 ° C.

Bath temperature: 460 ° C. Bath composition: Al = 0.11 to 0.14% (the balance is zinc) In the alloying treatment after plating, the steel plate temperature is changed in the range of 500 to 630 ° C. in order to adjust the alloying degree. It was

The degree of alloying was evaluated by visual observation and a powdering test. That is, in the visual observation, it is determined whether or not the alloying is visually performed to the surface by visually observing the appearance of the test piece after alloying, and in the powdering test, the following test is performed. The amount of powdering was obtained, and it was determined whether or not the level was practically acceptable.

In the powdering test, a test piece (disc) cut out from a steel sheet after being alloyed is subjected to a cylindrical drawing under the following conditions, and a tape is attached to the outer wall of the processed test piece. After that, the plating film peeled by the processing by peeling is completely removed from the surface of the test piece, and the peeled amount (this is called the amount of powdering is referred to as the amount of powdering) from the difference between the weight of the test piece before processing and the weight of the test piece after tape peeling. That is, test piece 1
It is the amount of powdering per piece). When the powdering amount was 20 mg / piece or less, there was no problem in practical use and the powdering property was considered to be good.

[Cylindrical drawing conditions] Blank diameter: 90 mmφ Punch diameter: 50 mmφ (shoulder R = 5 mm) Die diameter: 54 mmφ (shoulder R = 5 mm) Wrinkle suppressing thickness: 1.0 ton Lubrication: Use rust preventive oil 1 and FIG. FIG. 1 shows the results of an examination of the degree of alloying by visual observation. In the figure, the mark ◯ indicates that the alloy is evenly alloyed to the surface, and the mark * indicates that the alloying is insufficient. Fig. 2 shows the results of evaluating the degree of alloying by the powdering test. The circles in the figure indicate that the amount of powdering is 20.
In the case of less than mg / piece, * indicates that the amount of powdering exceeds 20 mg / piece.

From FIG. 1, the steel sheet temperature (T) at the time of preoxidation required for alloying up to the surface is such that the Si content is 0.1 to 2.0%.
It is understood that T ≧ 700 + 100 × ln [Si] (° C.) for the range of. Further, as shown in FIG. 2, the steel sheet temperature during pre-oxidation with good powdering property was T ≦ 830 + 50 × l for the same range of Si content of 0.1 to 2.0%.
n [Si] (° C.).

That is, the steel sheet temperature at the time of pretreatment for attaining an appropriate degree of alloying satisfies the cases shown in FIGS. 1 and 2, and the lower limit of the steel sheet temperature is that the alloying is uniformly performed up to the surface by visual observation. It is the lowest steel plate temperature judged, and the upper limit is the highest steel plate temperature evaluated to show good powdering properties. The range defined by the lower limit and the upper limit is the steel plate temperature represented by the equation (1).

Next, using the steel sheet of C shown in Table 1 as a plating base material, hot dip galvanizing was performed in a continuous hot dip galvanizing facility, and an alloying treatment was performed to obtain a galvannealed steel sheet. The effect of the air-fuel ratio and the dew point of the atmospheric gas on the pre-oxidation was investigated.

The steel sheet temperature in the non-oxidizing furnace was fixed at 750 ° C. The processing conditions and plating conditions in the reducing furnace are the same as those in the above test, and in the alloying process, the steel plate temperature is 560 ° C.
Fixed to.

In the investigation of the alloying degree, the alloying state was examined by visual observation as in the case of the above-mentioned test, and the alloying degree of the plating film was obtained. The degree of alloying (content of Fe in the alloy layer) was determined by dissolving the plating film and measuring the concentrations of Fe, Zn and Al by the atomic absorption method. Further, the presence or absence of abnormality such as the occurrence of defects in the plating film was also visually observed.

The survey results are shown in Table 2. In the column of evaluation in the table, a circle indicates an alloying degree of 7 to 11% (as described above, a range that is particularly preferable for thick basis weight), and a triangle indicates an alloying degree of 6% or more and less than 7% or 11 % To 14%, x indicates alloying degree 6
% Or more than 14%, the degree of alloying is 6
Approximately 14% was defined as an appropriate degree of alloying.

As shown in Table 2, the air-fuel ratio is less than 0.8 (N
In o.1), alloying treatment is insufficient and the air-fuel ratio is 1.25.
If it is larger than that (No. 11), plating defects due to soot adhesion due to incomplete combustion occur, and the degree of alloying exceeds the proper range, resulting in a defect. No.2 and N
In o.10, when the air-fuel ratio is the lower limit and the upper limit, respectively,
The appearance of the plating film was good, but the degree of alloying is marked with a triangle, and the air-fuel ratio for marking a degree of alloying with a circle is
It can be seen that 0.9 to 1.2 is preferable.

When the dew point exceeds 30 ° C. (No. 7), the alloying degree exceeds the appropriate range because the alloying progresses too much. When the dew point is No. 6 (dew point is 30 ° C.), the alloying degree is evaluated. Was a triangle. In order to give an evaluation of ○, it is desirable to keep the dew point at 0 ° C or lower.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

According to the method of the present invention, even if Si-containing steel, which is difficult to alloy after plating, is used as the plating base material,
The continuous hot dip galvanizing equipment makes it possible to obtain an alloyed hot dip galvanized steel sheet without lowering productivity.

This steel sheet is a request for the development and practical application of an alloyed hot-dip galvanized steel sheet of high-strength steel which has high strength, high workability, and is required to be rust-proof, mainly in automobile manufacturers. In addition, it is possible to reduce the thickness and weight of automobile bodies and the like, and has great industrial and economic effects such as improved fuel efficiency and resource saving.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of the Si content of the base steel sheet and the steel sheet temperature during pre-oxidation on the proper alloying degree, as a result of the examination of the alloying degree by visual observation.

FIG. 2 is a diagram showing an evaluation result of an alloying degree by a powdering test, which shows an influence of a Si content of a base steel sheet and a steel sheet temperature during pre-oxidation on an appropriate alloying degree.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C22C 38/02

Claims (1)

[Claims] 1. A continuous type hot dip galvanizing facility has a Si content of
When performing galvannealing using a steel sheet of 0.1 to 2.0 wt% as the base material, heating in a pre-oxidizing furnace is performed in an atmosphere with a dew point of 30 ° C or less to an air-fuel ratio of 0.8 to 1.25 and pre-oxidizing. A method for hot dip galvannealing of a hard-to-alloy galvannealed base material, which is performed so that the maximum steel plate temperature reached in the furnace satisfies the following formula (1). 700 + 100 × ln [Si] ≦ T ≦ 830 + 50 × ln [Si] ・ ・ ・ (1) However, T: maximum attainable steel plate temperature in the pre-oxidation furnace (℃) [Si]: Si content of the base steel plate ( weight%)