JP2792343B2 - Manufacturing method of galvannealed steel sheet with excellent weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a galvannealed steel sheet having excellent weldability.

[0002]

2. Description of the Related Art Zn is formed on an alloyed hot-dip galvanized steel sheet.
It is known that the presence of an oxide film containing O as a main component improves the weldability of a plated steel sheet.
The optimum value of O is said to be 30 to 3000 mg / m ² (per side). Conventionally, alloying treatment in the manufacturing process of alloyed hot-dip galvanized steel sheet is generally performed by a gas heating furnace method, but in this gas heating furnace method, the contact of the burner flame tends to be uneven in the width direction of the steel sheet, In addition, since the sheet temperature tends to fluctuate in the width direction and the longitudinal direction of the steel sheet, the oxide film generated at the time of alloying shows a non-uniform distribution and tends to be porous, which is not suitable for improving the weldability. .

Conventionally, the following method has been employed to form an oxide film (mainly ZnO as a main component) on the upper layer of a galvannealed steel sheet. A method of immersing in a processing solution containing HNO ₃ or HCl as a main component or spraying the same. In air-water cooling after alloying, ozone is added to the cooling solution to promote oxidation. how to

[0004]

However, these methods are disadvantageous in that it is necessary to secure a space for the treatment, and the equipment cost for the treatment tank, the circulation tank, and the like is high. In addition, there is a problem that surface immersion occurs due to immersion and spray treatment with an acidic solution, and the powdering resistance of the plated steel sheet is deteriorated. In addition, in the method (1), air-water cooling after the alloying treatment is liable to cause a problem such as water leakage, and there is a problem in terms of maintenance. There is also the problem of becoming.

SUMMARY OF THE INVENTION The present invention has been made in view of such conventional problems, and a method for producing an alloyed hot-dip galvanized steel sheet having a dense and uniform oxide film on the upper layer without causing the above-mentioned problems. It is intended to provide.

[0006]

Means for Solving the Problems In the conventional alloying treatment by the gas heating method, heat is applied from the outside of the plating film, so that the heating tends to be non-uniform. It is difficult to form an oxide film uniformly on the upper layer. On the other hand, in the alloying process using the high-frequency induction heating type alloying furnace, the steel sheet itself is directly heated by the high-frequency induction heating, so that the plating film can be uniformly heated from the inside.
Also, the high-frequency induction heating method is different from the gas heating method in that the atmosphere gas can be selected and controlled relatively freely, and the alloying furnace can be kept in an oxidizing atmosphere having a predetermined oxygen concentration or higher.

The present inventors have found that a uniform oxide film can be formed on the plating film in the alloying step by utilizing such characteristics of the high-frequency induction heating method, and the present invention has been completed. It is a thing. That is, the present invention provides an alloyed hot-dip galvanized steel excellent in weldability, characterized in that after hot-dip galvanizing a steel sheet, an alloying treatment is performed in a high-frequency induction heating furnace maintained in an oxidizing atmosphere having an oxygen concentration of 25% or more. This is a method for producing a plated steel sheet.

[0008]

[Function] A steel sheet subjected to a hot-dip galvanizing treatment is treated with an oxygen concentration of 2
By introducing into a high-frequency induction heating furnace kept in an oxidizing atmosphere of 5% or more and performing an alloying treatment, a dense and uniform oxide film (ZnO) is formed on the plating layer. If the oxygen concentration of the atmosphere in the high-frequency induction heating furnace is less than 25%,
The amount of the oxide film deposited is not sufficient, and the formation of the oxide film on each part of the steel sheet tends to be uneven.

FIG. 1 schematically shows an example of a manufacturing process of a galvannealed steel sheet according to the present invention.
1 is a plating bath, 2 is a sink roll, 3 is a wiping nozzle, 4 is an alloying furnace of a high frequency induction heating system, 5 is a preservation zone connected to the alloying furnace, and 6 is a cooling zone.

The alloying furnace 4 includes a high-frequency induction heating device 7 and a closed cover 8 covering the high-frequency induction heating device 7. An oxidizing gas is introduced into the closed cover 8 through a gas inlet 9.
The inside of the furnace is set to an oxidizing atmosphere having a predetermined oxygen concentration. The preservation zone 5 is a heat preservation system using a heat transfer ribbon heater.
Sealing devices 10a and 10b are provided at the entrance of the alloying furnace 4 and the outlet of the preservation zone 5, respectively, so that the alloying furnace 4 and the preservation zone 5 communicating therewith are sealed.

In such a production line, the steel sheet S plated in the plating bath 1 is guided to the outside of the bath via the sink roll 2, and the basis weight of the steel sheet S is adjusted by the wiping nozzle 3. And alloyed. An oxidizing gas containing O ₂ , H ₂ O (steam), NOx or SOx and having an oxygen concentration of 25% or more is introduced into the alloying furnace 4 (sealed cover 8) through a gas introduction unit 9, and the furnace pressure is set to a positive pressure. It is kept in. Also, the inside of the tropical zone 5 communicating with the alloying furnace 4 is kept in the same atmosphere. The plated steel sheet S introduced into the alloying furnace 3 is uniformly heated by the high-frequency induction heating device 7, while passing through the alloying furnace 4 and the preservation zone 5, and is densely formed on the plating film by an oxidizing atmosphere in the furnace. In addition, a uniform oxide film is formed. In order to promote oxidation in the alloying furnace 4 and the preservation zone 5, the plate temperature of the steel sheet is preferably maintained at 450 to 600 ° C in the alloying furnace and 350 to 550 ° C in the preservation zone. The plated steel sheet S that has left the preservation zone 5 is introduced into the cooling zone 6 and cooled.

Next, in order to uniformly form an oxide film on the plating film, it is preferable that the plating film itself is smooth. Then, from the viewpoint of the smoothness of such a plating film, it is preferable that the alloyed hot-dip galvanized layer is formed by forming a smooth bulk crystal mainly composed of δ ₁ phase uniformly on the surface of the steel sheet. Further, such an alloyed hot-dip galvanized layer not only enhances the uniformity of the oxide film but also imparts excellent powdering resistance and press formability to the steel sheet. Hereinafter, preferable conditions for obtaining such an alloyed plating film will be described.

The present inventors have formed an alloyed hot-dip galvanized layer in which smooth bulk crystals mainly composed of δ ₁ phase are uniformly formed on the surface of a steel sheet, so that the oxide film has more excellent uniformity and superiority. A method for stably producing an alloyed hot-dip galvanized steel sheet having powdering resistance and press formability was studied, and the following findings were obtained.

[0014] By suppressing the alloying reaction (generation of the ζ phase) in the plating bath and performing the subsequent alloying treatment by using a high-frequency induction heating type heating furnace, the alloying mainly composed of the δ ₁ phase is achieved. A film in which the phase is uniformly formed in the width direction and the longitudinal direction of the strip is obtained.In addition, the alloyed plating film obtained in this way has not only macro uniformity as described above, but also microscopic Since the alloying reaction occurs uniformly, excellent powdering resistance and press formability can be obtained from this aspect as well.

Strict control of the coating is possible by specifying the bath conditions and the sheet temperature conditions on the exit side of the heating furnace of the high-frequency induction heating method. Specifically, the relationship between the low Al bath and the amount of Al in the bath By plating at a low intrusion plate temperature specified in the above, or by plating in a high Al bath and at a high intrusion plate temperature specified in relation to the amount of Al in the bath, and By thickly forming Fe ₂ Al ₅ , which is an anti-oxidation phase, it is possible to appropriately suppress the alloying reaction (generation of ζ phase) in the bath, and further, to perform high-frequency induction heating on such a plated steel sheet. By performing the alloying treatment using a heating furnace of the system by controlling the sheet temperature at the exit side of the heating furnace to be more than 495 ° C. to 520 ° C., the film as described above is obtained,

As a result of an examination based on such knowledge, as a result of performing a hot-dip galvanizing treatment and an alloying treatment under the following conditions (1) or (2), a smooth and uniform alloying as described above is obtained. It was found that a hot-dip galvanized layer was obtained.

(1) Al content in bath: 0.05% or more;
Less than 13%, bath temperature: 460 ° C or less, and Al in the bath
437.5 × [Al%] + 428> T ≧ 437.5 × [Al%] + 408 where [Al%]: Al amount in bath (%) T: Hot-dip galvanizing under conditions satisfying the penetration plate temperature (° C.) suppresses the Fe—Zn alloying reaction in the bath, and after plating, the plate temperature on the exit side of the heating furnace in the high-frequency induction heating furnace is reduced. More than 495 ° C to 520 ° C
Heat so that

(2) Al content in the bath: 0.13% or more, bath temperature: 470 ° C. or less, and the Al content in the bath and the temperature of the steel sheet entering the plating bath are 571 × [Al% + 416 ≧ T ≧ 571 × [Al%] + 396, where [Al%]: Al amount in bath (%) T: Alloying in bath by plating under conditions that satisfy: The inhibitory phase, Fe ₂ Al _5, is actively formed to form Fe-
After the Zn alloying reaction is suppressed, and after plating, the sheet is heated in a high-frequency induction heating furnace so that the sheet temperature on the exit side of the heating furnace is more than 495 ° C to 520 ° C.

That is, the alloying reaction in the bath is suppressed as much as possible and the alloying treatment by high-frequency induction heating is performed under specific conditions on the plating film in which the alloying is suppressed as described above. phase is small, and smooth alloying phase consisting mainly of [delta] ₁ phase in the steel sheet each section is uniformly formed, yet has excellent powdering resistance as a whole by the microscopic uniformity of the film structure, further pressing A plated steel sheet excellent in formability can be obtained. Therefore, when such an alloyed hot-dip galvanized film is formed, an extremely uniform oxide film is formed on the upper surface of the plated film by alloying in an alloying furnace having a predetermined oxidizing atmosphere according to the present invention. Can be.

It is presumed that, under such plating-alloying conditions, the plating film having the excellent characteristics as described above is obtained for the following reasons. First, by using the high-frequency induction heating method in the alloying treatment, the steel sheet itself can be directly heated, and the interface in contact with the plating film is heated most. The Fe-Zn reaction occurs uniformly in a short time and independently of the position on the strip,
Therefore, partial no residual peracetic alloy or ζ phase on the steel sheet, homogeneous [delta] ₁ phase is formed, thereby smoothness and uniformity and uniform powdering resistance and press formability of the coating It is presumed to be obtained.

Second, since the high-frequency induction heating is heating from the steel plate side as described above, it is presumed that a uniform alloying reaction occurs microscopically. That is, in the conventional alloying treatment by gas heating,
Since heat is applied from the outside of the film, the heating is likely to be non-uniform, and the alloying reaction is likely to be microscopically non-uniform. In particular, since the crystal grain boundaries are highly reactive, the so-called outover
A strike reaction is likely to occur, and when an outburst structure is generated in this way, a Γ phase starts to grow from this portion, and the formation of the Γ phase deteriorates the powdering resistance. On the other hand, since high-frequency induction heating is heating from the steel plate side, there is little local variation in the alloying as described above,
Oxide on steel plate surface and alloying inhibitor (Fe ₂
Since Al ₅ ) is also easily diffused, it is considered that a uniform alloyed film can be obtained even from a microscopic viewpoint.

Third, the high frequency induction heating can shorten the growth time of the Γ phase because the plating can be alloyed in a short time. In the present invention, since the generation of the Γ phase in the bath can be suppressed, the final formation amount of the 少 な く phase is small, which is also considered to contribute greatly to the improvement of the powdering resistance.

Fourth, as an advantage of high-frequency induction heating, uniform heating can be performed in the width and length directions of the steel sheet, so that strict control of the sheet temperature on the exit side of the heating furnace is possible, and gas Unlike an atmosphere heating method such as a furnace, there is no rise in the heated atmosphere gas (draft effect), so it is considered that overalloy hardly occurs even without special cooling.

In the method (2), the Fe—Zn reaction is suppressed by forming Fe ₂ Al ₅ as an alloying suppressing phase in a bath, and the δ ₁ phase is formed in the subsequent heat treatment. Since the high-frequency induction heating is heating from the steel sheet side, Fe ₂ Al ₅ is easily diffused during alloying and δ
Form _one phase. In other words, even if Fe ₂ Al ₅ is formed thick to appropriately suppress the Fe—Zn reaction, it can be diffused reliably and uniformly during alloying. As a result, the alloying becomes microscopically uniform, and the formation of a thick Fe ₂ Al ₅ suppresses the generation of a Γ phase in the bath, which results in a uniform alloy phase and excellent powdering resistance. It is considered that the property can be obtained.

In the above methods (1) and (2), in order to minimize the alloying reaction in the plating bath, the amount of Al in the plating bath, the sheet temperature of the steel sheet when entering the plating bath, and the bath temperature are reduced. Stipulated. That is, in order to suppress the alloying reaction in the plating bath, the method (1) uses a low Al bath and
and the lower invading plate temperature defined in relation to the
In the method (2), the temperature of the infiltration plate is set to be a high Al bath and a higher penetration plate temperature defined in relation to the amount of Al in the bath.

The reasons for limiting the respective plating conditions will be described below. Under the condition (1), plating is performed in a low Al bath at a low penetration plate temperature in order to suppress the alloying reaction (generation of the ζ phase) in the plating bath.
If the amount is less than 0.05%, since there is no effect of suppressing alloying by Fe ₂ Al ₅ , an outburst reaction occurs in the bath, and the powdering resistance deteriorates. Therefore, the Al content in the bath is set to 0.05% or more. On the other hand, if the Al content is 0.13% or more, the Fe-Zn alloying reaction is excessively suppressed in the bath, and therefore it is necessary to cause a rapid alloying reaction in the subsequent alloying treatment. The rapid alloy reaction deteriorates the smoothness and uniformity of the plating film and the powdering resistance. Therefore, the Al content in the bath is set to less than 0.13%.

It is necessary that the penetration plate temperature satisfies the condition of the following relational expression in relation to the amount of Al in the bath. 437.5 × [Al%] + 428> T ≧ 437.5 × [Al%] + 408 where [Al%]: Al amount in bath (%) T: Temperature of penetrating plate (° C.) Temperature of penetrating plate is Al in bath If the above upper limit is exceeded in relation to the amount,
Ζ phase occurs alloying reaction in the bath is formed, not eventually alloyed phase mainly composed of [delta] ₁ phase of interest is obtained.
On the other hand, if the penetration plate temperature is lower than the above lower limit, Fe ₂ Al ₅ is generated non-uniformly, and a local alloying reaction occurs, so that the smoothness and uniformity of the plating film and the powdering resistance deteriorate. Resulting in.

If the plating bath temperature is high, the alloying reaction in the bath is accelerated, so that the bath temperature is set to 460 ° C. or lower.
On the other hand, if the bath temperature is too high, structures immersed in the bath are eroded, causing problems such as the generation of dross.

Next, under the above condition (2), Al in the plating bath forms Fe ₂ Al ₅ on the steel sheet surface immediately after entering the bath,
Suppress the generation of Fe-Zn alloy. Al content is 0.13%
Small Such inhibitory effect is less than, is formed ζ phase in the bath, not eventually alloyed phase mainly composed of [delta] ₁ phase of interest is obtained. Therefore, the amount of Al in the bath is set to 0.13% or more.

In a bath containing 0.13% or more of Al, the reaction temperature immediately after entering the steel sheet increases when the temperature of the entering sheet increases,
Fe ₂ Al ₅ is formed to be thick. As a result, the reaction of the Fe—Zn alloy in the bath is suppressed. However, the penetration plate temperature needs to satisfy the condition of the following relational expression in relation to the Al content in the bath. 571 × [Al%] + 416 ≧ T ≧ 571 × [Al%] + 396 where [Al%]: Al amount in bath (%) T: Temperature of intruding plate (° C.)

As described above, the method (2) uses a high Al bath,
Although it is based on a high penetration plate temperature, if the penetration plate temperature exceeds the above upper limit in relation to the amount of Al in the bath, the diffusion rate of Fe increases, and the suppression effect by Fe ₂ Al ₅ becomes insufficient. Since the outburst structure is partially generated in the bath, the smoothness and uniformity of the plating film and the powdering resistance are deteriorated. On the other hand, if the penetration plate temperature is below the lower limit, the amount of Fe ₂ Al ₅ formed is not sufficient, and the Fe-
The effect of suppressing the Zn alloy reaction cannot be obtained properly.

When the invading plate temperature exceeds 520 ° C., F
Since e ₂ Al ₅ tends to be locally excessively generated, baking unevenness occurs, and the uniformity and the powdering resistance of the plating film deteriorate. In addition, an increase in the amount of heat input to the pot requires additional equipment such as a bath temperature cooling unit, and further, increases the amount of dross generated in the bath and causes problems such as frequent occurrence of surface defects. For this reason, it is preferable that the penetration plate temperature be 520 ° C. or less regardless of the amount of Al in the bath. Further, similarly to the above (1), the bath temperature is 470 ° C. or less.

The plated steel sheet needs to be subjected to alloying treatment in a high-frequency induction heating furnace, and a gas alloying method generally performed in the related art cannot obtain a target alloyed plating film at all. In this alloying treatment, the sheet is heated so that the sheet temperature on the outlet side of the furnace becomes more than 495 ° C. to 520 ° C., and is cooled after holding for a predetermined time. In order to form the δ ₁ phase, heating at a temperature exceeding 495 ° C. is necessary, and the plating in which the alloying in the bath is suppressed is alloyed here, and the bulk crystal alloy mainly composed of the δ ₁ phase is formed. Allow a phase to form. However, if the heating temperature exceeds 520 ° C., the Γ phase is formed, and the powdering resistance deteriorates. Therefore, the upper limit of the heating temperature is 520 ° C. The reason why the sheet temperature at the exit side of the high-frequency induction heating furnace is controlled is that the temperature becomes the highest sheet temperature in the alloying heat cycle. In addition, since the growth rate of the alloy phase becomes maximum in this vicinity, it is possible to cause an alloying reaction at that temperature by controlling the outlet sheet temperature.

The alloyed plating film thus obtained has a uniform and smooth bulk crystal δ ₁ phase on the surface layer side, an extremely thin Δ phase under the layer, and a uniform layer on the uppermost layer. The plating structure has a fine oxide film (ZnO).

[0035]

【Example】

Example 1 In the line shown in FIG.
mm cold-rolled steel sheet is hot-dip galvanized (coating weight: 60 g / m ² per side), and then alloyed in a high-frequency induction heating type alloying furnace maintained in the furnace atmosphere shown in Table 1.
An alloyed hot-dip galvanized steel sheet in which the Fe% in the alloyed plating layer was 10 ± 0.5 wt% was produced. Table 1 also shows the results of a weldability test based on the oxide film amount of the upper layer (ZnO amount) of the plating film and the number of continuous spot welding points of the obtained steel sheet.

[0036]

[Table 1]

Example 2 Al-killed steel of the following steel type, I
A cold-rolled steel sheet manufactured from F steel was used as a material, and hot-dip galvanizing and alloying heat treatment were performed under the conditions shown in Tables 2 and 3. The alloying heat treatment was performed by a gas heating method or a high-frequency induction heating method. Steel type A: 0.03% C-0.02% Sol. Al (Al
Killed steel) Steel type B: 0.0025% C-0.04% Sol. Al-
0.07% Ti (Ti-added IF steel) Steel type C: 0.0024% C-0.06% Sol. Al
-0.06% Ti-0.007% Nb (Ti, Nb-added IF steel) The properties of the obtained galvannealed steel sheet are shown in Tables 4 and 5.

In the present embodiment, the temperature at which the steel sheet enters the plating bath is the surface temperature of the steel sheet immediately before immersion measured by a radiation thermometer. The sheet temperature on the exit side of the heating furnace is the surface temperature of the steel sheet measured by a radiation thermometer. In addition, A
The 1 amount is the effective Al concentration defined by the following equation. [Effective Al concentration] = [Total Al concentration in bath] − [Iron concentration in bath] +0.03 Fe% in the coating depends on bath conditions, heating conditions and cooling conditions. The cooling conditions hardly affect the macro or micro uniformity of the film structure, which is one of the features of the present invention, but affect the characteristics by changing the degree of alloying (Fe% in the film). Therefore, in this embodiment, the amount of air and the amount of mist of the cooling blower are adjusted, and the amount of Fe in the film is adjusted.
% Was controlled. Tests and evaluation methods for each characteristic are as follows.

○ Amount of phase in product film: X obtained film
And ray diffraction, for ζ phase peak of d = 1.900 - click strength Iζ the _(421), also peak of d = 1.990 for [delta] ₁ Phase - takes click intensity i? ₁ a _(249), respectively, under The amount of ζ phase in the film was represented by the peak strength ratio shown in the equation. In addition,
Ibg is a background, and if Z / D is 20 or less, substantially no ζ phase exists. Z / D = (Iζ ( ₄₂₁ ) −Ibg) / (Iδ ₁ ( ₂₄₉ ) −
Ibg) × 100

○ Powdering resistance: 1 g / rust-proof oil (Knoxlast 530F manufactured by Parker Kosan Co., Ltd.) was added to the test piece.
After applying m ² , a bead pull-out test was performed with a bead radius R: 0.5 mm, a pressing load P: 500 kg, and an indentation depth h: 4 mm. The amount was calculated. The numerical values in the table are average values of a plurality of measured values (5 × 5 = 25).

○ Maximum deviation in powder width direction of powdering resistance:
5 points in coil length direction, 5 points in coil width direction (both edges, 1/4 position and center
Part), the powdering resistance was measured, and the difference between the maximum value and the minimum value was determined.

○ Coefficient of friction: After applying 1 g / m ² of rust-preventive oil (Knoxlast 530F, manufactured by Parker Kosan Co., Ltd.) to the test piece, an indenter made of tool steel SKD11 was pressed with a load of 400 kg to obtain 1 m / m ^2. The drawing was performed at a drawing speed of min, and the ratio between the drawing load and the pressing load was defined as the friction coefficient. The numerical values in the table represent a plurality of measured values (5 × 5 = 25).
).

The maximum deviation of the coefficient of friction in the sheet width direction: The coefficient of friction was measured at the same location as the powdering resistance, and the difference between the maximum value and the minimum value was determined.

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

[Table 4]

[0047]

[Table 5]

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an example of a manufacturing process according to the method of the present invention.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-337059 (JP, A) JP-A-2-263970 (JP, A) JP-A-2-170961 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] An alloyed hot-dip galvanized steel excellent in weldability, characterized in that a steel sheet is hot-dip galvanized and then alloyed in a high-frequency induction heating furnace kept in an oxidizing atmosphere having an oxygen concentration of 25% or more. Manufacturing method of plated steel sheet.