JP2776150B2 - Method for producing two-layer alloyed hot-dip galvanized steel sheet with excellent ED resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a two-layer alloyed hot-dip galvanized steel sheet used for an automobile body, underbody parts and the like.
In particular, the present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet having excellent ED coating compatibility and ED spot resistance.

[0002]

2. Description of the Related Art A two-layer alloyed galvanized steel sheet having an Fe-based coating layer on an alloyed hot-dip galvanized layer has excellent corrosion resistance after painting and coating compatibility. Has recently increased, and in recent years, in particular, the thickness of the galvanized film tends to be increased in order to ensure corrosion resistance.

Conventionally, a gas furnace has been widely used for alloying treatment after hot-dip galvanizing. However, a plating film alloyed in such a gas furnace has an upper Fe-based alloy for the following reasons. There is a problem that the plating coverage is poor. On the surface of the plating film alloyed using a gas furnace,
A stable and inactive oxide film layer is generated, and when Fe-rich Fe-Zn alloy electroplating is performed on such an inactive plating film, there are few active points to be a starting point of electrodeposition, It is difficult to obtain a uniform upper plating film. Since gas heating tends to cause uneven heating of each part of the steel sheet, the smoothness and uniformity of the plating film itself are poor, and the coverage of the upper plating is poor.

[0004]

Therefore, when an Fe-based plating is applied to the upper layer of such an alloyed hot-dip galvanized film to ensure good ED coating compatibility, the coverage of the upper plating is ensured. Therefore, the plating adhesion amount must be increased. However, when the deposition amount of the upper plating increases, the hydrogen in the upper plating film causes a defect called so-called ED spot, which opens a mouth at the time of electrodeposition coating by a user (an automobile manufacturer or the like), and therefore, the deposition amount is unnecessarily increased. There is a problem that it cannot be increased.

[0005] As described above, in order to produce a two-layer alloyed hot-dip galvanized steel sheet having excellent ED cratering resistance and also advantageous in generating ED bumps, a small amount of the Fe-based plating in the upper layer is required. In addition, it is necessary to form a uniform coated state, but it is difficult to obtain such a plated steel sheet due to the above-mentioned problems.

[0006]

According to the present invention, there is provided such a system.
It is a result of repeated examination in view of the problem of the coming,Less than
It was made based on the findings and facts as described above.  The alloying process after hot-dip galvanizing is combined with induction heating.
By using a metallurgical furnace, a uniform alloy phase can be obtained.(gas
In the alloying process by heating, heat is applied from outside the plating film.
Heating tends to be non-uniform,
High frequency induction heating while easy to form a uniform alloy phase
In the alloying process by heating, the base steel sheet itself is directly heated
This enables uniform heating of the plating film,
A uniform alloy phase is obtained)This increases the coverage of the upper plating.
ImprovementLet it.  Invitation to freely control the atmosphere inside the furnace
Utilizing the characteristics of the induction heating alloying furnace, a non-oxidizing atmosphere
Alloying in an induction heating type alloying furnace kept at a constant temperature
Surface acid that impairs coatability of upper plating
Prevent formation of oxide filmit can. That is, the present invention is aimed at
High coverage by the upper plating
Stable and inert as produced by thermal alloying
In addition to the oxide film layer, alloying treatment by high-frequency induction heating
Surface acid generated to the extent that processing takes place in air
Can be significantly harmed by the oxide film, but keep it in a non-oxidizing atmosphere.
Alloying treatment in a dipped induction heating alloying furnace
Properly prevents the formation of such surface oxide film on the plating
Thus, a high degree of coverage of the upper plating can be secured.  After alloying, perform skin pass under specified conditions
This improves the smoothness of the galvannealed film.
To improve the coverage of the upper plating.ThanCan enhanceso
Wear.

That is, according to the present invention, after a steel sheet is galvanized in a hot-dip galvanizing bath, it is alloyed in a high-frequency induction heating furnace kept in a non-oxidizing atmosphere.
A method for producing a two-layer alloyed hot-dip galvanized steel sheet having excellent resistance to ED spots, wherein a skin pass is applied at on / mm or more, and then an Fe-based plating is applied to the upper layer of the galvannealed steel sheet. .

[0008]

The details of the present invention and the reasons for limiting the same will be described below. FIG. 1 shows an example of the state of implementation of the plating and alloying treatment in the present invention, which will be described below. In the figure, 1 is a plating bath, 2 is a wiping nozzle,
Reference numeral 3 denotes a high-frequency induction heating type alloying furnace, 4 denotes a tropical zone (for example, a tropical style with a heat transfer ribbon heater), and 5 denotes a cooling zone. The steel sheet S immersed in the plating bath 1 and plated is guided to the outside of the bath via the sink roll 6, the basis weight is adjusted by the wiping nozzle 2, and then introduced into the alloying furnace 3 to be alloyed. In the alloying furnace 3, a reducing gas or an inert gas or a mixed gas thereof (H ₂ , CO, CH ₄ , N ₂ , Al, He, N
e, Kr or the like or a mixed gas thereof) is introduced, and the inside of the alloying furnace 3 is maintained in a positive pressure non-oxidizing atmosphere. The steel sheet S alloyed in the alloying furnace 3 passes through the preservation zone 4 and is introduced into the cooling zone 5 where it is cooled. In this cooling zone 5, usually,
N ₂ is used as a cooling gas, and cooling is performed to a temperature (about 300 ° C. or less) at which the plating film does not oxidize even when exposed to outside air.

The steel sheet temperature in the alloying furnace 3 is 450 to 6
The temperature of the steel plate at about 00 ° C and the temperature of Hot Spring 4 is 350-5
The temperature is set to about 50 ° C. to prevent surface oxidation during the alloying reaction. Sealing devices 8a and 8b for preventing outside air from entering are provided on the inlet side of the alloying furnace 3 and the outlet side of the cooling zone 5, and alloying is provided between the preservation zone 4 and the cooling zone 5. Sealing devices 8c are provided for preventing the atmosphere gases of the furnace / preserving zone and the cooling zone from mixing.

[0010] The plated steel sheet alloyed as described above is subjected to a skin pass. This skin pass is
The rolling force is 0.2 Ton / mm or more. 0.2Ton
If the rolling force is less than / mm, the surface of the plating film cannot be sufficiently smoothed.

After the above skin pass, an Fe-based upper plating is applied in order to improve the coating compatibility. The alloyed hot-dip galvanized steel sheet is liable to cause a defect called cratering during electrodeposition coating, which affects the appearance after final coating. The upper plating prevents the occurrence of such coating defects and enhances the coating compatibility of the coated steel sheet. In order to improve coating compatibility, it is preferable that the upper layer plating be an α single phase.
Since it is a single phase, the Fe content of the upper plating is 50%
It is preferable to make the above.

As described above, the steel sheet is placed in a hot-dip galvanizing bath.
High frequency induction heating maintained in a non-oxidizing atmosphere
Alloying treatment in a furnace, and then a rolling force of 0.2 Ton / m
m and pass skin pass, then alloyed molten zinc
By applying Fe-based plating to the upper layer of the plating layer,
A high degree of coverage of the upper plating is ensured by the action of
It is. In the present invention, since the coverage of the upper plating is improved,
Uniform plating is possible even with an upper layer plating amount of 3 g / m ² or less. However, if the coating amount is less than 1 g / m ² , the coating of the upper plating may not be sufficient even by the present invention, and therefore the lower limit of the coating amount is preferably 1 g / m ² .

Next, from the viewpoint of the smoothness of the plating film, it is preferable that the alloyed hot-dip galvanized layer has a smooth bulk crystal mainly composed of δ ₁ phase uniformly formed on the surface of the steel sheet. That is, such an alloyed galvanized layer is uniformly formed on the steel sheet surface without generating a surface oxide film, and is subjected to skin pass rolling under a predetermined condition, so that the alloyed galvanized layer is extremely uniform. The coating can be made smooth and the coverage of the Fe-based upper plating can be remarkably improved with a small amount of adhesion. Further, such an alloyed hot-dip galvanized layer not only enhances the coverage of the upper plating, but also imparts excellent powdering resistance and press formability. Hereinafter, preferable conditions for obtaining such an alloyed plating film will be described.

The present inventors formed a galvannealed layer in which smooth bulk crystals mainly composed of δ ₁ phase were uniformly formed on the surface of a steel sheet, so that good coverage and excellent coverage of the Fe-based upper layer plating were obtained. A method for stably obtaining the powdering resistance and the press formability was also studied, and the following findings were obtained. Suppress alloying reaction (generation of ζ phase) in plating bath,
Moreover by carrying out the subsequent alloying treatment using a heating furnace of high frequency induction heating method, the width direction of the alloying phase consisting mainly of [delta] ₁ phase strip, is uniformly formed film in the longitudinal direction is obtained In addition, the alloyed plating film thus obtained not only has the macroscopic uniformity as described above, but also has an excellent alloying reaction even microscopically. And press formability

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: 470 ° 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 forming such an alloyed hot-dip galvanized film, an alloying treatment is performed in an alloying furnace in a non-oxidizing atmosphere according to the present invention to prevent the formation of an oxide film and to prevent skin pass under a predetermined condition. By applying, even with a small amount of adhesion, it is possible to form an upper layer plating having remarkably excellent coverage.

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 the bath temperature is set to 470 ° 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, when the temperature of the intruding plate is increased, the reaction temperature immediately after entering the steel plate 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 is heat-treated in a high-frequency induction heating furnace for alloying. Conventionally, generally, gas heating cannot provide 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. Also,
Since the growth rate of the alloy phase is maximized 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 plating structure in which a δ ₁ phase, which is a uniform and smooth massive crystal, is present in the surface layer and an extremely thin Γ phase is present in the lower layer.

[0033]

EXAMPLES Examples of the present invention are shown in Tables 1 to 6. In this example, a cold-rolled steel sheet manufactured from the following steel types, ie, Al-killed steel and IF steel, was used as a material and subjected to hot-dip galvanizing, alloying heat treatment, skin pass, and upper layer plating under the conditions shown in Tables 1 to 4. Was. 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 (T
i, Nb-added IF steel) Tables 5 and 6 show the properties of the obtained galvannealed steel sheet.

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

The Fe% in the film 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 air volume of the cooling blower and the amount of mist are adjusted,
Fe% in the film was controlled. Tests and evaluation methods for each characteristic are as follows.

量 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 in 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.

ＥＤ Resistance to ED spots After a phosphate film was formed on each surface of the material of the present invention and the comparative material by immersion treatment, a cationic type electrodeposition coating was applied under the following conditions. Voltage: 300 V Bath temperature: 26.5 ° C. Specimen area / anode area: 1/1 Coating thickness: 20 μm Baking temperature: 170 ° C. Baking time: 20 minutes Specimen subjected to electrodeposition coating as described above The spot-like defects generated in the coating film were visually inspected, and the following evaluation was performed based on the number of defects generated in 48 mmφ. ：: bump-like defect 0 △ to Δ: bump-like defect less than 2 △: bump-like defect 2 or more and less than 5 ×: bump-like defect 5 or more

Cratering Resistance After a phosphate film was formed on each surface of the material of the present invention and the comparative material by immersion treatment, a cationic type electrodeposition coating was applied under the following conditions. Voltage: 300 V Bath temperature: 26.5 ° C. Specimen area / anode area: 1/1 Coating thickness: 20 μm Baking temperature: 170 ° C. Baking time: 20 minutes Specimen subjected to electrodeposition coating as described above The crater-like defects generated in the coating film were visually inspected, and the following evaluation was performed based on the number of crater-like defects generated in 48 mmφ. ○: 5 or less crater-like defects △: more than 5 crater-like defects to 20 or less ×: more than 20 crater-like defects

○ P ratio This is an evaluation of the suitability of the outer surface of the plated steel sheet (mainly, the coatability of the upper layer of the Fe system). A phosphate film is formed by immersion treatment, and the film is analyzed by X-ray diffraction to determine the phosphophyllite. The peak values of the crystal and the whipite crystal were measured, and the P ratio was defined by the following equation. P ratio = (Phosphophyllite crystal diffraction peak height) /
[(Phosphophyllite crystal diffraction peak height) + (Hopite crystal diffraction peak height)] The P ratio was evaluated as follows. : P ratio of 0.8 or more △ to Ｐ: P ratio of 0.7 to less than 0.8 △: P ratio of 0.5 to less than 0.7 ×: P ratio of less than 0.5

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

[Table 5]

Remarks * 1 ED cratering resistance and P ratio are poor due to poor upper layer plating coverage. * 2 Suitable range for conventional technology (upper plating 3-4 g / m
² ) * 3 Appropriate range in conventional technology (upper plating 3-4 g / m
² ) * 4 Poor ED resistance due to large amount of upper layer plating. * 5 Poor powdering resistance due to partial thickening of the Γ phase, and poor coatability of the upper plating due to difficulty in adjusting the atmosphere temperature in the furnace. * 6 Due to the partial thickening of the Γ phase, the powdering resistance is poor, and it is difficult to adjust the atmosphere temperature in the furnace.
Also poor in coatability of upper layer plating.

[0050]

[Table 6]

Remarks * 7 ED cratering resistance and P ratio are poor due to poor upper layer plating coverage. * 8 Appropriate range for conventional technology (upper plating 3-4 g / m
² ) * 9 Appropriate range for conventional technology (upper plating 3-4 g / m
² ) * 10 Poor ED spot resistance due to large amount of upper plating. * 11 Due to insufficient skin pass rolling force, the surface is rough and the upper layer coverage is poor. * 12 Uneven due to uneven grilling

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an example of an implementation state of a plating-alloying process in the present invention.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiro Iwabuchi 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-60-224791 (JP, A) JP-A-61 JP-A-207561 (JP, A) JP-A-1-177353 (JP, A) JP-A-2-225652 (JP, A) JP-A-61-253397 (JP, A) JP-A-1-177352 (JP, A) (JP) 52-48342 (58) Fields investigated (Int.Cl. ⁶ , DB name) C23C 28/00-30/00 C23C 2/00-2/26 C21D 1/42

Claims (1)

(57) [Claims] 1. A steel sheet is plated in a hot-dip galvanizing bath, alloyed in a high-frequency induction heating furnace maintained in a non-oxidizing atmosphere, and then subjected to a skin pass with a rolling force of 0.2 Ton / mm or more. Thereafter, a method for producing a two-layer alloyed hot-dip galvanized steel sheet having excellent ED spot resistance, wherein Fe-based plating is applied to an upper layer of the alloyed hot-dip galvanized layer.