JP3106634B2 - Method for producing galvannealed steel sheet with excellent press formability and spot 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 an alloyed hot-dip galvanized steel sheet having excellent press formability and spot weldability.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets are widely used as various rust-preventive steel sheets because they have many excellent properties. When this alloyed hot-dip galvanized steel sheet is used as a rust-preventive steel sheet for automobiles, it has excellent corrosion resistance and paint compatibility, as well as excellent press formability and spot weldability required in the vehicle body manufacturing process. It is necessary.

[0003] However, the conventional alloyed hot-dip galvanized steel sheet has the following problems with respect to press formability and spot weldability as compared with a base material steel sheet (for example, a cold-rolled steel sheet).

[0004] When press-forming an alloyed hot-dip galvanized steel sheet using a cold-rolled steel sheet as a base material, a slide formed between a zinc-iron alloy plating layer formed on the surface of the steel sheet and a press die. The sliding resistance force applied to the moving surface is higher than that between the base steel plate and the press die. Therefore, in the alloyed hot-dip galvanized steel sheet, the steel sheet is less likely to flow in a portion where sliding is severe such as a sliding surface with a bead,
The steel sheet is easily broken. As described above, the alloyed hot-dip galvanized steel sheet is more likely to break than the cold rolled steel sheet of the base metal alone. The reasons why the alloyed hot-dip galvanized steel sheet is inferior in press formability are as described above.

[0005] As a method of improving the press formability of an alloyed hot-dip galvanized steel sheet, conventionally, a high-viscosity lubricating oil has been applied to the surface of an alloyed hot-dip galvanized steel sheet, and sliding between the press die has been performed. A method of reducing a sliding resistance force applied to a surface is widely and generally used. However, this method requires an operation of applying lubricating oil before press molding and an operation of degreasing lubricating oil after press molding. Therefore, there is a problem that the cost is high and the environment of the press work place is deteriorated.

On the other hand, in spot welding of an alloyed hot-dip galvanized steel sheet, two superposed alloyed hot-dip galvanized steel sheets are sandwiched between a pair of copper-made electrode tips, and then alloyed with a pair of electrode tips. While the galvannealed steel sheets are pressed against each other, an electric current is applied to the electrode tip, and the two alloyed galvanized steel sheets are mutually subjected to electric resistance welding.

[0007] In spot welding two alloyed hot-dip galvanized steel sheets, the following problems occur. That is,
The welding heat generated at the time of spot welding melts the zinc-iron alloy plating layer of the welded portion of the two galvannealed steel sheets. The galvannealed steel sheet thus molten of the zinc-iron alloy plating layer reacts with the copper contained in the pair of electrode tips, and is hard and brittle on each of the pair of electrode tips. A zinc-copper alloy layer forms. As a result, the electrode tips are severely worn. Therefore, compared with the case where only the cold rolled steel sheet of the base material is spot-welded, the life of the electrode tip is shortened, the number of continuous hit points is reduced, and the spot weldability is reduced.

Conventionally, the following plated steel sheets have been proposed as plated steel sheets having excellent press formability or spot weldability.

(1) JP-A-55-110783 discloses a surface-treated steel sheet having excellent spot weldability, comprising: Aluminum oxide (Al ₂ O ₃ ), silicon dioxide (SiO ₂ ) on the zinc plating layer or zinc alloy plating layer
₂ ) and a surface-treated steel sheet having an oxide film composed of at least one of titanium dioxide (TiO ₂ )
"Prior art 1").

According to the prior art 1 described above, a plated steel sheet having excellent spot weldability can be obtained. An oxide film made of at least one of aluminum oxide (Al ₂ O ₃ ), silicon dioxide (SiO ₂ ) and titanium dioxide (TiO ₂ ) has a high electric resistance and a high melting point, The spot weldability of the galvanized steel sheet can be improved to the same degree as the untreated steel sheet.

(2) JP-A-53-60,332 discloses an alloyed galvanized sheet excellent in spot weldability, comprising: The oxidation rate of the surface layer of the plating layer of the alloyed zinc iron plate is 70% or less (IZnO / IZn + IZnO, where IZnO: photoelectron intensity of the oxide alloy layer by X-ray photoelectron spectroscopy analysis, IZn: unoxidized alloy by X-ray electron spectroscopy analysis) Alloyed galvanized sheet (hereinafter referred to as “prior art 2”) having a photoelectron intensity of the layer.

In the above prior art 2, an oxide film mainly composed of zinc oxide (ZnO) is formed on the surface of a plating layer of an alloyed galvanized steel sheet. A plated steel sheet excellent in weldability can be obtained.

(3) JP-A-2-190,483 discloses a galvanized steel sheet excellent in press formability and a method for producing the same, comprising: A galvanized steel sheet with an oxide film formed on the surface of the galvanized steel sheet. A galvanized steel sheet in which an oxide film mainly composed of zinc oxide (ZnO) is formed on the surface of the galvanized steel sheet at a zinc oxide (ZnO) amount of 30 to 3000 mg / m ² (per one side). A method for producing a galvanized steel sheet in which an oxide mainly composed of zinc or iron is formed by electrolytic oxidation, immersion oxidation, or coating oxidation treatment on the surface of a galvanized steel sheet. A method for producing a galvanized steel sheet in which the surface of a plating layer is in a solid phase or the surface of the plating layer is alloyed and roughened, and then heated to form an oxide film mainly composed of zinc oxide (ZnO). (Hereinafter, referred to as “prior art 3”).

In the above-mentioned prior art 3, an oxide film mainly composed of zinc oxide (ZnO) is formed on the surface of a plating layer of a galvanized steel sheet by electrolytic treatment, immersion treatment, coating oxidation treatment or heat treatment. According to Prior Art 3, a galvanized steel sheet having excellent press formability can be obtained.

[0015]

However, the above-mentioned prior art 1 has the following problems. That is,
As described above, the oxide film made of aluminum oxide (Al ₂ O ₃ ), silicon dioxide (SiO ₂ ), titanium dioxide (TiO ₂ ), etc., is coated on the surface of the galvanized steel sheet by aluminum oxide (Al ₂ O _3). ), A solution containing oxides such as silicon dioxide (SiO ₂ ) and titanium dioxide (TiO ₂ ), and then drying the steel sheet. However, prior art 1 has poor adhesion to the paint after painting because the adhesion between the oxide coating formed by such a method and the zinc plating layer or the zinc alloy plating layer is weak.

Prior arts 2 and 3 have the following problems. That is, as described above, the oxide film mainly composed of zinc oxide (ZnO) formed on the surface of the alloyed galvanized layer, upon press molding, the surface between the steel sheet and the press die The effect of reducing the sliding resistance applied to the sliding surface is small, and the press formability is poor.

Accordingly, an object of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet having excellent press formability and spot weldability.

[0018]

SUMMARY OF THE INVENTION From the above-mentioned viewpoint, we have a good paint adhesion and a sliding resistance applied to a sliding surface between a steel sheet surface and a press die during press forming. Power,
That is, the sliding resistance applied to the sliding surface between the zinc-iron alloy plating layer formed on the surface of the steel sheet and the press die is small. The present inventors have intensively studied to develop a method for producing an alloyed hot-dip galvanized steel sheet having excellent press formability and spot weldability without forming a copper alloy layer.
As a result, we obtained the following findings.

(1) Nickel (Ni), manganese, which is harder and has a higher melting point than the zinc-iron alloy plating layer, on the surface of the zinc-iron alloy plating layer of the alloyed hot-dip galvanized steel sheet.
(Mn), molybdenum (Mo), vanadium (V), and copper (Cu) by forming at least one oxide film thereon, the sliding surface applied to the sliding surface between the steel sheet surface and the press die. The dynamic resistance can be reduced to improve press formability, and by preventing direct contact between the copper electrode tip and the zinc-iron alloy plating layer, the top of each of the pair of electrode tips can be improved. In addition, formation of a zinc-copper alloy layer can be prevented, and spot weldability can be improved. (2) By forming the oxide film in the heat treatment step of the alloyed hot-dip galvanized steel sheet, the adhesion between the zinc-iron alloy plating layer and the oxide film is improved, and good paint adhesion is obtained. can get.

The present invention has been made based on the above findings. The method of the present invention comprises passing a steel sheet through a hot-dip galvanizing bath, forming a galvanized layer on at least one surface of the steel sheet, and then forming nickel (Ni) on the surface of the steel sheet on which the galvanized layer is formed. ), Manganese (Mn), molybdenum (Mo), vanadium (V) and copper (Cu) spraying an aqueous solution containing at least one metal ion,
Next, the steel sheet is heated to alloy the galvanized layer and the steel sheet to form a zinc-iron alloy plated layer, and on the surface of the zinc-iron alloy plated layer, on one side of the steel sheet, 10 to 1500 mg / m ^{2 in} terms of the amount of metal ions
Is characterized in that an oxide film of the metal ion is formed in an amount in the range of The aqueous solution is characterized in that it is a nitrate solution, a chloride salt solution or a hydroxide solution. The iron content in the zinc-iron alloy plating layer is in the range of 7 to 15 wt%. The amount of the zinc-iron alloy plating layer is in a range of 20 to 150 g / m ² per one side of the steel sheet.

Hereinafter, the method of the present invention will be described. The steel sheet is passed through a hot-dip galvanizing bath to form a galvanized layer on at least one surface of the steel sheet, and then nickel (Ni), manganese,
(Mn), molybdenum (Mo), vanadium (V) and copper (Cu) are sprayed with an aqueous solution containing at least one metal ion, and then the steel sheet is heated to alloy the galvanized layer and the steel sheet. to form the iron-alloy plated layer, said zinc - - allowed to zinc is on the surface of the iron alloy plated layer, per one surface of steel sheet, in an amount ranging from 10 at the metal ion amount in terms of 1500 mg / m ^2, nickel ( Ni), manganese (Mn), molybdenum (M
o), at least one of vanadium (V) and copper (Cu)
An oxide film of two metal ions is formed.

Nickel (Ni), manganese (Mn) and molybdenum (Mn) are formed on the surface of an alloyed hot-dip galvanized steel sheet.
o), at least one of vanadium (V) and copper (Cu)
The oxide film of one metal ion is, for example, NiO, MnO ₂ ,
It is a coating made of oxides such as MoO ₂ , V ₂ O ₃ , V ₄ O ₇ , and Cu ₂ O, and has excellent adhesion to the surface of the zinc-iron alloy plating layer. Since the oxide film is harder than the zinc-iron alloy plating layer, the sliding resistance applied between the surface of the steel sheet and the press die during press forming is reduced, and the steel sheet is pressed. It easily flows into the mold, and press formability is improved.

Further, since this oxide film has a higher melting point than the zinc-iron alloy plating layer,
This prevents direct contact between the copper electrode tip and the zinc-iron alloy plating layer, thereby suppressing alloying of copper and zinc. Therefore, a zinc-copper alloy layer is not formed on each of the pair of electrode tips, the electrode is less worn, and the spot weldability (continuous spotting property) is improved.

Nickel (Ni), manganese (Mn), molybdenum
An aqueous solution containing at least one metal ion of (Mo), vanadium (V) and copper (Cu) is sprayed onto the surface of the steel sheet before introducing the steel sheet into a heating furnace. The adhesion amount of the oxide film can be adjusted by the concentration of the aqueous solution to be sprayed (hereinafter, referred to as “spray solution”) and the spray amount. The amount of oxide film attached is converted into the amount of metal ions,
It should be in the range of 10 to 1500 mg / m ² per side of the steel sheet. The amount of oxide film attached is 10
If the amount is less than mg / m ² , desired effects cannot be obtained in the above-mentioned action of improving press formability and spot weldability. on the other hand,
The oxide film adhesion amount is 1500 mg / m ² per one side of the steel sheet.
When the value exceeds, the electric resistance at the time of spot welding becomes too large, and the electrode itself generates heat and is easily worn.

The amount of the zinc-iron alloy plating layer of the galvannealed steel sheet of the present invention is preferably 20 g / m ² or more per one side of the steel sheet in order to satisfy corrosion resistance. On the other hand, when it exceeds 150 g / m ² , spot weldability deteriorates.

The iron content of the galvannealed steel sheet of the present invention in the zinc-iron alloy plating layer is preferably in the range of 7 to 15 wt%. If the iron content is less than 7% by weight, the corrosion resistance after painting deteriorates. On the other hand, if the iron content is 15
If it exceeds wt%, powdery peeling of the plating layer during press molding, that is, powdering, occurs.

The coating state of the oxide coating on the surface of the zinc-iron alloy plating layer of the alloyed hot-dip galvanized steel sheet of the present invention is not necessarily required to be uniform. , Has no effect on the above operation. The surface of the zinc-iron alloy plating layer is not smooth, and therefore, if there is an oxide coating on the protruding portion, direct contact between the zinc-iron alloy plating layer and the mold, and contact between the electrode and zinc This is because it is possible to prevent direct contact of the user.

The galvannealed steel sheet of the present invention includes both a single-sided galvanized steel sheet and a double-sided galvanized steel sheet.

As the spraying solution, a nitrate solution, a chloride salt solution, a hydroxide solution and the like are used. The reason is that heat at the time of the alloying treatment causes the water to evaporate and, at the same time, it is necessary to decompose the anion which is a salt. Chloride salt solutions and hydroxide solutions are used.

Nickel (Ni), manganese (Mn), molybdenum
By adding an oxidizing agent to an aqueous solution containing at least one metal ion of (Mo), vanadium (V) and copper (Cu), a more stable oxide film can be formed. Stabilization of the coating means that the oxide in the zinc-iron alloy plating layer is oxidized by the action of the oxidizing agent, so that the oxide coating becomes denser and stronger. As described above, the heat during the alloying treatment evaporates the water, and at the same time, it is necessary to decompose the anions as salts.
Perchlorates and hydrogen peroxide are used as oxidizing agents.

[0031]

Next, the present invention will be described in more detail with reference to examples and comparative examples. In the continuous hot-dip galvanizing line for the alloyed hot-dip galvanized steel sheet, a simple-type spray facility was provided immediately before the heating furnace provided on the plating bath outlet side. Then, an alloyed hot-dip galvanized steel sheet was prepared under the following conditions. That is, a cold-rolled steel sheet having a thickness of 0.8 mm was passed through a hot-dip galvanizing bath to form a galvanized layer on both sides or one side of the steel sheet. Then, the above spray equipment was used to coat nickel on both surfaces of the steel sheet as shown in Table 1.
(Ni), manganese (Mn), molybdenum (Mo), vanadium (V)
And an aqueous solution containing at least one metal ion of copper (Cu) at a temperature of 60 ° C. Next, the steel sheet is heated in a heating furnace to alloy the galvanized layer and the steel sheet to form a zinc-iron alloy plated layer and oxidize the metal ions on the surface of the zinc-iron alloy plated layer. Specimens Nos. 1 to 7 of alloyed hot-dip galvanized steel sheets (hereinafter, referred to as “specimens of the present invention”) within the scope of the present invention were prepared. Plating bath chemical composition: Al; 0.12 wt%, Zn; remaining Plating bath temperature: 460 ° C Plating bath immersion steel plate temperature: 470 ° C Alloying temperature: 510 ° C Alloying time: Predetermined iron content (7 to 15 wt%)
Within the range).

For comparison, the alloyed hot-dip galvanized steel outside the scope of the present invention was prepared under the same conditions as those of the test sample of the present invention except that no spraying was performed on the surface of the steel sheet on which the hot-dip galvanized layer was formed. Specimen of plated steel sheet (hereinafter referred to as “comparative specimen”
No. 1) was prepared.

For the purpose of comparison, the present invention was carried out under the same conditions as the specimen of the present invention except that water was sprayed instead of the aqueous solution of the present invention on the surface of the steel sheet on which the hot-dip galvanized layer was formed. No. 2 (hereinafter referred to as “comparative sample”) of a galvannealed steel sheet was prepared.

For comparison, an aqueous solution containing manganese (Mn) ions was applied on the surface of the steel sheet on which the hot-dip galvanized layer was formed, and the amount of the manganese ion oxide film deposited exceeded the range of the present invention. A specimen (hereinafter, referred to as a “comparative specimen”) No. 3 of an alloyed hot-dip galvanized steel sheet out of the scope of the present invention was prepared under the same conditions as those of the specimen of the present invention except for spraying as described above.

Table 1 shows the adhesion amounts of the zinc-iron alloy plating layer and the oxide film of the prepared inventive specimens Nos. 1 to 7 and comparative specimens Nos. 1 to 3.

[0036]

[Table 1]

The adhesion amount of the oxide film was measured by the following method. That is, the test sample was treated with the reagent grade concentrated hydrochloric acid (12
(Mol / liter) in hydrochloric acid diluted with five times the amount of distilled water (hereinafter referred to as "hydrochloric acid (1 + 5)") to dissolve the oxide film together with the zinc-iron alloy plating layer, and
Quantitative analysis was performed by CP (Inductively Coupled Plasma) emission spectroscopy to determine the amount of metal ions attached. Further, the measurement of the amount of the oxide film formed of the acid-insoluble oxide was carried out by measuring the specimen with hydrochloric acid (1+
5) to dissolve the oxide coating together with the zinc-iron alloy plating layer, then filter the residue in the dissolved solution, and then filter the residue to borax (Na ₂ B ₄ O ₇ );
Sodium carbonate (Na ₂ CO ₃ ) and potassium carbonate (K
₂ CO ₃ ), melted with a mixed flux (borax: sodium carbonate: potassium carbonate = 4: 3: 3), then dissolved with hydrochloric acid (1 + 5) and quantitatively analyzed by ICP emission spectroscopy. The amount of metal ions attached was determined.

Next, the spot weldability and press formability of the thus prepared specimens Nos. 1 to 7 of the present invention and comparative specimens Nos. 1 to 3 were determined by the following method. Examined. Table 1 shows the results.

(1) Spot weldability: No. 1 specimen of the present invention
To 7 and Comparative Sample Nos. 1 to 3 were continuously spot-welded with a pair of electrode tips. The evaluation of the spot weldability is based on the pair of electrodes capable of forming an appropriate nugget having a diameter equal to or larger than a predetermined diameter on the joint portion of each of the test pieces of the galvannealed steel sheet under the following test conditions. The number of welding times of the tip (the number of continuous hit points) was examined, and the welding was performed based on the results. The test conditions were as follows. Electrode: Tip diameter 6 mm, dome shape Pressure: 250 Kg Welding time: 12 cycles Welding current: 11.0 kA Welding speed: 1 point / sec Electrode life: The number of welding (the number of continuous spots) when the nugget diameter was less than Equation 1 The life of the electrode. Here, t: plate thickness.

[0040]

(Equation 1)

(2) Press formability: The friction coefficients of the test specimens Nos. 1 to 7 of the present invention and the test specimens Nos. 1 to 3 were measured by the following measuring methods. The press formability of the specimen was evaluated. FIG. 1 is a front view showing a friction coefficient measuring device. The friction coefficient measuring device includes a horizontally movable slide table 2 on which the specimen 1 is placed, a contact 3 for pressing the specimen 1 on the slide table 2 from above, and a lower part of the slide table 2. , A vertically movable slide table support 5 having a roller 4 in contact with the lower surface of the slide table 2, provided at a position symmetrical to the contact 3, and a slide table support 5 attached to the slide table support 5. And a second load cell 7 for measuring a sliding resistance force F of the slide table 2 attached to the end of the slide table 2 in the horizontal movement direction. ing. 8
Is a rail. The drive mechanism (not shown) pushes up the slide table support 5 to bring the specimen 1 placed on the slide table 2 into contact with the contact 3 with a pressing load N as shown by an arrow, and not shown. By another drive mechanism, the slide table 2 is horizontally moved together with the specimen 1 with a sliding resistance force F as shown by an arrow at a drawing speed shown below. From the ratio between the pressing load N and the sliding resistance F during the horizontal movement, the friction coefficient μ was calculated by the following equation. μ = F / N The measurement conditions were as follows. Contact: Diameter 10mmφ Pressing load: 400 Kgf Pulling speed: 100 cm / min.

As is clear from Table 1, the comparative sample N
o.1 was inferior in spot weldability and press formability because the aqueous solution was not sprayed.

The comparative sample No. 2 was inferior in spot weldability and press formability because water was sprayed without spraying the aqueous solution.

In Comparative Sample No. 3, since the amount of oxide film attached was out of the range of the present invention and high, the electrical resistance during spot welding became too large and the spot weldability was extremely poor. . Further, the electrode itself generated heat and sparks, and the electrode was melted.

On the other hand, the test specimens Nos. 1 to 7 of the present invention
Was good in both spot weldability and press formability.

[0046]

As described above, according to the method of the present invention, an alloyed hot-dip galvanized steel sheet having excellent press formability and spot weldability can be obtained, and thus has an industrially useful effect.

[Brief description of the drawings]

FIG. 1 is a front view showing a friction coefficient measuring device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Specimen 2 Slide table 3 Contact 4 Roller 5 Slide table support 6 First load cell 7 Second load cell 8 Rail.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-2458 (JP, A) JP-A-3-287788 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (4)

(57) [Claims] 1. A steel sheet is passed through a hot-dip galvanizing bath to form a galvanized layer on at least one surface of the steel sheet, and then nickel (Ni) is formed on the surface of the steel sheet on which the galvanized layer is formed. An aqueous solution containing at least one metal ion of manganese (Mn), molybdenum (Mo), vanadium (V) and copper (Cu) is sprayed, and then the steel sheet is heated to form the galvanized layer and the Along with forming a zinc-iron alloy plating layer by alloying with a steel sheet, on one surface of the steel sheet, on the surface of the zinc-iron alloy plating layer,
An amount in the range of 10 to 1500 mg / m ^{2 in} terms of the amount of metal ions,
A method for producing an alloyed hot-dip galvanized steel sheet having excellent press formability and spot weldability, comprising forming an oxide film of the metal ion. 2. The method according to claim 1, wherein the aqueous solution is a nitrate solution, a chloride salt solution or a hydroxide solution. 3. The method according to claim 1, wherein the content of iron in the zinc-iron alloy plating layer is in a range of 7 to 15 wt%. 4. The method according to claim 1, wherein the amount of the zinc-iron alloy plating layer is in a range of 20 to 150 g / m ² per one surface of the steel sheet.