JP2803566B2 - Alloyed galvanized steel sheet with excellent film destruction resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an alloyed galvanized steel sheet which can be formed into various shapes.
The present invention also relates to a galvanized steel sheet having excellent low-temperature chipping resistance after painting.

[0002]

2. Description of the Related Art A so-called alloyed galvanized steel sheet (hereinafter referred to as GA steel sheet) in which a hot-dip galvanized steel sheet is heat-treated and the entire coating film is made of an Fe-Zn alloy has excellent corrosion resistance, paintability and weldability. Therefore, it is widely used in the fields of automobiles, household electric appliances, building materials and the like.

[0003] GA used as a material for automobile bodies
The steel sheet is subjected to press working to form into various shapes such as bending / unbending deformation, sliding deformation, and shrinking deformation.

[0004] The plating film obtained by performing the alloying treatment is composed of an intermetallic compound of Fe and Zn, and has a problem that it has a low deformability for processing and is inferior in workability and formability. Furthermore, considering that the GA steel sheet is used in a severely corrosive environment, it is necessary to increase the coating weight. However, the increase in the coating weight is likely to impair the workability and formability of the GA steel sheet. Work on.

[0005] On the other hand, there is a phenomenon in which a plating film is peeled off due to rock jumps during traveling of an automobile, which is observed in cold regions. When the temperature of the coating film applied on the plating film becomes low, the deformability becomes small at a low temperature, and the force for shrinking due to the increase in elastic force increases, so that a large compressive stress acts on the plating film. Therefore, when impacts such as stone bombs are applied to the surface of the coated plated steel sheet, cracks are formed from the coating film to the plating film, and further to the interface between the plating film and the steel plate, and the shear stress that peels off the plating film acts to form the coating. Peeling (low temperature chipping phenomenon). In addition, when the impact causes deformation to the steel plate as the plating substrate, the plating film having low deformability is restrained by the coating film having low deformability and cannot follow the deformation of the steel plate, and the film peels. In any case, since the peeling of the plating film leads to a decrease in corrosion resistance, the occurrence of the low-temperature chipping phenomenon is a great obstacle to the use of the GA steel sheet in a cold region.

[0006] workability, there are many proposals in the past with respect to excellent GA steel and a manufacturing method thereof formability, for example, in JP-A 1-68456 and JP-of base steel interface gamma (Fe ₃
There has been proposed an alloyed hot-dip galvanized steel sheet having a plating layer comprising a Zn ₁₀ ) phase having a thickness of 0.1 μm or less and a balance of a δ ₁ phase and having excellent powdering resistance and flaking resistance. However, although the workability of this steel sheet is good, it cannot be said that it is still sufficient.In addition, in order to manufacture a steel sheet having a plating layer having such a film configuration with current alloying treatment equipment, low temperature is required. Heating for a long time is necessary, and there is a disadvantage that productivity is poor.

Regarding the adhesion of the plating film to the shearing stress, the present applicant has found that an alloyed galvanized steel sheet having good adhesion can be obtained by setting the thickness of the Δ phase to 0.2 μm or more. (JP-A-58-130265)
No.). Improving the adhesion of the plating film is considered to be an even more important issue in view of the trend that adhesives are used for joining automobile body panels and the like.

In general, the Δ phase tends to promote the powdering of the plating film of the GA steel sheet and reduce the workability.
This is because the Γ (Fe ₃ Zn ₁₀ ) phase grows and the Γ ₁ (Fe ₅ Zn ₂₁ ) phase also thickens, deteriorating the powdering resistance. In the chemical treatment, attention has been focused on suppressing the growth of the Γ phase. However, as described above (Japanese Patent Application Laid-Open No. 58-130265), in order to enhance the adhesion of the plating film to shear stress, it is necessary to grow the Γ phase thickly, so that the powdering resistance and the shear stress are reduced. It was not possible to obtain a GA steel sheet having excellent adhesion.

[0009] Forming into various shapes such as bending / unbending deformation, sliding deformation, and shrinking deformation, and various impacts in a use environment after being incorporated into a product such as an automobile body (for example, an automobile in a cold region) There is a demand for the development of a GA steel sheet having a plating film that can withstand various types of processing and impact, such as rock jumping or shearing during running.

[0010]

DISCLOSURE OF THE INVENTION The present invention has been made in view of such circumstances, and has as its object to have good workability (powdering resistance) and to be applied to a base material for a coated steel sheet. An object of the present invention is to provide a GA steel sheet having a plating film which is excellent in shearing workability in the case where it is used, and film adhesion to an impact such as a stone jump in a cold region when used as an automobile body panel.

[0011]

The inventors of the present invention have conducted various studies to solve the above-mentioned problems, and as a result, for example, have found that powder deformation can be caused by bending / unbending deformation, sliding deformation, shrinking deformation, etc. by pressing. If the ring occurs, crack was confirmed that occurring in the vicinity of the interface within or gamma _1-phase and gamma-phase of gamma ₁ phase. This may be because the stress concentration occurs in dislocations and twin defects present in gamma ₁ Aiuchi.

[0012] On the other hand, adhesive shear fracture caused when an adhesive is used to join GA steel sheets, and low-temperature chipping generated due to impact on a coated plated steel sheet (as described above, the impact of the plating film on In the shearing failure (the peeling including the plating film = enamel hair) that occurs during the shearing of a coated steel sheet with a hard coating applied to a GA steel sheet, the shear stress that separates works). Destruction occurs. This is for shear stress
This indicates that the interface between the Γ phase and the base steel sheet is the weakest. It has been found that by growing the Γ phase, the unevenness of the interface between the Γ phase and the base steel sheet increases, and the shear adhesive strength is improved.In general, if the Fe content of the plating film is increased,密 着 Adhesion at the interface between the phase and the base steel sheet can be improved. However, the gamma phase is grown thick, even gamma _1-phase becomes thicker accordingly, powdering resistance is lowered.

Therefore, the bending stress, shearing stress, etc. generated in the plating film in response to the force applied from the outside to the GA steel sheet.
In order to obtain a film that can withstand any stress, the thickness of the Γ phase and Γ ₁ phase must be specified, and a plated film that has a good balance between workability (powdering resistance) and film adhesion must be used. It is necessary to form it on the surface of the steel sheet by the chemical treatment.

The present invention has been made as a result of repeated studies based on such a policy, and the gist lies in the following GA steel sheet (alloyed galvanized steel sheet).

[0015] At least one side of the steel sheet has a coating weight
20~90g / m ^2, the average content of Al in the plating film 0.1 to 0.8
In weight%, and the average thickness ratio of the gamma phase to the entire plating film thickness satisfies the following formula (1), the average thickness ratio of the gamma _1-phase with respect to the average total thickness of gamma phase and gamma ₁ phase following ( 2) An alloyed galvanized steel sheet with excellent film destruction resistance characterized by satisfying the formula.

[0016] 0.01 ≦ gamma phase thickness / total film thickness (per one side) ≦ 0.4 ··· (1) 0.001 ≦ Γ 1 phase thickness / (gamma ₁ phase thickness + gamma phase thickness) ≦ 0.5 ··· (2), where The term “film-breaking resistance” refers to, for example, film-breaking (powdering), adhesion-shear failure, and low-temperature caused by applying bending, unbending deformation, sliding deformation, shrinkage deformation, etc. to a plated steel sheet by pressing. It means resistance to various forms of film destruction such as chipping and shearing destruction (enamel hair).

[0017]

The constituent features of the present invention and the functions and effects thereof will be described below.

The basic idea of the present invention is to provide improved shear adhesive strength to form irregularities as much as possible to grow a gamma phase at the interface of the steel sheet and gamma phase, the growth of cause and prone gamma ₁ phase powdering such It is to control.

[0019] to a coating weight per one surface 20~90g / m ^2, in less than 20 g / m ² poor corrosion performance which is the original purpose of the GA steel sheet, on the other hand, when it exceeds 90 g / m ^2, the plating This is because even if the structure of the film satisfies the conditions defined in the present invention, sufficient resistance to film destruction is not ensured. If the corrosiveness of the usage environment of the GA steel sheet is not so strong, the coating weight
There is no problem even if it is less than g / m ² . Further, from the viewpoint of film destruction resistance, it is desirable to set the upper limit of the coating weight to 70 g / m ² .

Regarding the average Al content in the plating film,
The higher the value, the more desirable the film composition is. However, if the content exceeds 0.8% by weight, the effect is saturated, and the alloying time during the production of GA steel sheet increases, the line speed decreases, and the production cost increases. Problems such as adhesion of dross to the car will occur. On the other hand, if the Al content is less than 0.1% by weight, a desired coating composition cannot be obtained. Therefore, the average Al content in the plating film is 0.1 to 0.8% by weight, preferably 0.15 to
0.5% by weight.

FIG. 1 shows that C: 0.003%, Si: 0.019%, Mn: 0.
32%, P: 0.04%, Ti: 0.01%, S: 0.004%, N: 0.002
%, Sol.Al: 0.04% (“%” means “% by weight”), cold rolled steel sheet consisting of the balance Fe (sheet thickness 0.8 mm, sheet width)
120mm) as a base metal, hot-dip galvanizing and alloying treatments under various conditions to obtain a coating weight of 50-60g /
FIG. 4 is a diagram illustrating the results obtained by examining the powdering property, the shearing property, and the low-temperature chipping property of a GA steel sheet having an m of ² by the method used in Examples described later. The circles, triangles, and crosses in the figure indicate the following states, respectively.

Regarding the powdering property, 印: the peeling amount per unit area of the processed portion / total adhesion amount is 0.2 or less △ mark: 超 え more than 0.2 0.5 or less × mark: 超 え more than 0.5 For the shearing property, 印 mark: cut end face Peeling length of 0.2mm or less △ mark: 超 え Over 0.2mm 0.5mm or less × mark: 超 え Over 0.5mm For low temperature chipping properties ○ mark: Average plating peel diameter 2mm or less △ mark: 〃 Over 2mm 4mm or less × mark : 超 え Over 4 mm As shown in Fig. 1, Γ phase thickness / total film thickness (per side) is 0.01 to 0.4, 〜 ₁ phase thickness / (Γ ₁ phase thickness + 相 phase thickness)
When it is within the range of 0.001 to 0.5 (the range surrounded by the solid line in the figure), all of the powdering property, the shearing property and the low-temperature chipping property are good, and a plating film having excellent film breaking resistance can be obtained.

If the thickness of the phase is less than 1% of the total thickness (per side) of the plating film, no irregularities are formed at the interface between the phase and the base steel sheet, and the shear adhesive strength is low. As shown, the shearing property and the low-temperature chipping property are poor.上限 The upper limit of the phase thickness is not limited by the deterioration of the shear bond strength, but Γwith the increase of the phase thicknessΓIt is limited by the increase of the ₁ phase thickness, and the Γphase thickness exceeds 40% of the total plating film thickness If it does not become a desirable Γ ₁ phase thickness.
However, a certain thickness or more in gamma phase grows, if the ratio of gamma Aiatsu to the total plating film thickness becomes 0.5 or more, relatively gamma ₁ Aiatsu gamma is to the total plated film thickness smaller ₁ Since the ratio of phase thickness is 0.5 or less, Γ ₁ phase thickness / (Γ
₍₁ phase thickness + Γ phase thickness) ≤ 0.5, resulting in a good plating film for any processing or impact. However, the Fe concentration in the film increases, which causes a problem in the corrosion resistance, which is the original purpose of the GA steel sheet. Therefore, the ratio of the Δ phase thickness to the total plating film thickness is in the range defined by the above equation (1), that is, 0.01 to 0.4.

When the steel sheet is used in a corrosive environment where corrosion resistance is not so required, a GA steel sheet having a plating film having a phase thickness / total film thickness (per side) of 0.7 or more can be applied.

Γ ₁ phase thickness / (Γ ₁ phase thickness + Γ phase thickness) is 0.5
If it exceeds the value, the powdering resistance will be degraded. On the other hand, if it is less than 0.001, sufficient growth of the green phase cannot be expected with the current equipment, and the shear adhesive strength will decrease.

For the GA steel sheet of the present invention, for example, as a heating condition at the time of alloying after subjecting the base steel sheet to hot-dip galvanizing, the maximum attained material temperature is set to 550 ° C. or more, and the average rise until reaching the temperature is reached. It can be manufactured by setting the temperature rate to 25 ° C./sec or more. The cooling rate after heating is desirably 15 ° C./sec or more. More preferably, the temperature is set to 20 ° C./sec or more. Temperature range which is the gamma ₁ phase grows (≦ 550 ° C.)
This is to avoid as much as possible.

The total content of elements other than Zn and Al mixed in the plating film due to the hot-dip galvanizing, and elements other than Fe mixed into the coating film from the base steel sheet during the alloying treatment, in the plating film is total. Up to 5% by weight, there is no effect on the properties of the film. In addition, as such an element,
Ni, Co, Sn, Mn, Pb, Sb, Sr, V, Cr, Ti, Si, Li, L
a, Ce, etc. Further, there is no problem even if Fe is contained in the plating bath to perform plating.

[0028]

[Example] C: 0.002%, Si: 0.01%, Mn: 0.18%, P:
0.01%, Ti: 0.04%, S: 0.005%, N: 0.002%, sol.A
l: 0.06% (“%” means “% by weight”)
A cold-rolled steel sheet (full-hard material) with a thickness of 0.8 mm and a width of 120 mm made of Fe with the balance being Fe is used as the base metal steel sheet, subjected to normal pretreatment, hot-dip galvanized, and then alloyed A GA steel sheet was produced by performing a chemical treatment. FIG. 2 shows a heat pattern during the period from the pretreatment to the alloying treatment.

The hot-dip galvanizing was performed by subjecting the base steel sheet to alkaline immersion degreasing, followed by drying and continuous plating without exposing the steel sheet to the outside air. The hot dip galvanizing bath had an Al concentration of 0 to 0.5% by weight, a bath temperature of 460 ° C., and a plating bath immersion time of 1 second.

The additional element other than Al is a single metal or Zn
And Al were added to the bath as appropriate.

The alloying treatment was performed using an infrared furnace, the rate of temperature rise was changed in the range of 10 to 35 ° C./sec, the maximum attained material temperature was changed in the range of 480 to 580 ° C., and the alloying treatment time was also changed appropriately. . In addition,
The GA steel sheet of the present invention was obtained only when the maximum attained material temperature was 550 ° C. or more and the heating rate was 25 ° C./sec or more.

Tables 1 and 2 show the coating weight and the composition of the plating film. The plating adhesion amount indicates the adhesion amount on the surface to be evaluated. The additional elements in the column of the composition of the plating film indicate elements other than Zn, Fe, and Al contained in the plating film, and the values in parentheses indicate weight% in the film. The Γ ratio is the ratio (%) of the average thickness of the Γ phase to the total plating film thickness.
Gamma ₁ ratio, gamma to the average total thickness of gamma phase and gamma _1-phase
It means the ratio (%) of the average thickness of _one phase. In addition, the thickness of these intermetallic compounds was determined in several places where the cross section of the GA steel plate was mirror-polished by oil polishing, etched with nital (0.05%) for 40 seconds, washed with ethanol, identified by SEM observation, and arbitrarily observed. It was determined from the average thickness. A powdering test was performed on the GA steel sheet to evaluate workability.

As shown in FIG. 3, a circular test piece (disc) cut from a GA steel plate was subjected to a cylindrical drawing process so that the evaluation surface became a convex portion of the cup, and the cup shape after the process was used. A cellophane tape is applied to the outer wall of the test piece and then peeled off.The amount of peeling of the plating film is determined from the difference between the weight of the test piece before processing and the weight of the test piece after tape peeling, and the peeling relative to the amount of plating applied is obtained. It was evaluated by the ratio of the amounts. The processing conditions and evaluation criteria are as follows (marked with ◎ and ○).

[Processing conditions] Coating oil: rust-preventive oil Blank diameter: 60 mmφ Punch diameter: 33 mmφ Die diameter: 35 mmφ Die roughness: Rmax = 0.3 μm Pressing pressure: 200 kgf Overhanging height: 20 mm [Evaluation criteria] Peeling amount / total adhesion amount (per unit area of processed part) is 0.1 or less… ◎ Exceeds 0.1 and 0.2 or less… ○ Exceeds 0.2 and 0.5 or less… △ Exceeds 0.5 and 0.7 or less… × 0.7 Exceeding ... XX Moreover, the following coating was applied to the above GA steel sheet to form a coated steel sheet, which was cut by shearing, and the peel length from the cut end face was measured to evaluate the shearing resistance.

The coating is performed only on the plated surface with a bar coater using a chromate (NRC manufactured by Nippon Paint Co., Ltd.).
300: 60 to 80 mg / m ^{2 in} terms of Cr), a primer (FLC-P600 manufactured by Nippon Paint Co., Ltd .: thickness 7 to 8 μm) and a top coat (FLC-100 HQ manufactured by Nippon Paint Co., Ltd.):
(Thickness: 16 to 18 μm) in this order, and baking was performed at 140 ° C., 215 ° C., and 220 ° C. at the highest ultimate plate temperature, respectively.

The evaluation criteria for the resistance to shearing destruction are as follows (marked as good).

[Evaluation Criteria] Peeling length from the cut end surface is 0.2 mm or less .... more than 0.2 mm and 0.5 mm or less .... △ 0.5 mm or more. , A low-temperature chipping test was performed to evaluate low-temperature chipping resistance.

In the coating, the unprocessed flat plate of the GA steel sheet was degreased with a degreasing agent FC4336 (manufactured by Nippon Parkerizing Co., Ltd.).
A chemical conversion treatment was performed using PZT (manufactured by Nippon Parkerizing Co., Ltd.), and then an epoxy-based cationic electrodeposition coating (paint U-80: manufactured by Nippon Parkerizing Co., Ltd.) having a thickness of 20 ± 1 μm was performed. Baking was performed at 25 ° C. for 25 minutes.

Thereafter, a melamine alkyd lugabake middle coat and a top coat (manufactured by Kansai Paint Co., Ltd.) were applied only on the surface by spray coating, and a middle coat and a top coat were applied. The coating thickness is 35 ± 2μ for both middle coat and top coat.
m. In addition, baking after painting, middle coat, top coat,
All were carried out at 140 ° C. for 30 minutes.

In the low-temperature chipping test, the coated surface of the coated plate (coated plate after intermediate coating and top coating) cooled to −20 ° C. has a diameter of 3 mm.
Five limestones of ~ 4 mm were collided with a compressed air of ² kg / cm ² , and the average peeling diameter of the plating film after peeling off the tapered part (measurement was performed 5 times, the average of the maximum peeling diameter in each time) was obtained.
The low-temperature chipping resistance was evaluated based on the following evaluation criteria (marked with ◎).

[Evaluation Criteria] The average peel diameter is 1 mm or less ◎ を More than 1 mm and 2 mm or less ○ More than 2 mm and 4 mm or less △ More than 4 mm × × Test results are shown in Tables 1 and 2. Also shown. As is clear from this result, when at least one of the conditions defined in the present invention is not satisfied, one of powdering resistance, shearing resistance and low-temperature chipping resistance,
Or everything was bad.

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

The GA steel sheet of the present invention has good workability (powdering resistance), and also has excellent shear workability when applied to a base material for coated steel sheets, and excellent adhesion to a film against impact. It is suitable as a material for various kinds of coated steel plates including those for automobile bodies.

[Brief description of the drawings]

Fig. 1 shows the effect of Γ phase thickness / total film thickness (per side) and Γ ₁ phase thickness / (Γ ₁ phase thickness + Γ phase thickness) on film destruction resistance. (B) shows the results of investigation on the shearing property, and (c) shows the results of investigation on the low-temperature chipping property.

FIG. 2 is a diagram showing a heat pattern during a period from annealing of a steel sheet used in an example to alloying treatment.

FIG. 3 is a diagram schematically showing a method for evaluating powdering properties.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-256857 (JP, A) JP-A-6-88192 (JP, A) JP-A-6-88185 (JP, A) JP-A-6-88185 17221 (JP, A) JP-A-4-297563 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] Claims: 1. The coating weight of at least one side of a steel sheet is
20~90g / m ^2, the average content of Al in the plating film 0.1 to 0.8
In weight%, and the average thickness ratio of the gamma phase to the entire plating film thickness satisfies the following formula (1), the average thickness ratio of the gamma _1-phase with respect to the average total thickness of gamma phase and gamma ₁ phase following ( 2) An alloyed galvanized steel sheet with excellent film destruction resistance characterized by satisfying the formula. 0.01 ≦ Γphase thickness / total film thickness (per side) ≦ 0.4 ・ ・ ・ (1) 0.001 ≦ Γ ₁ phase thickness / (Γ ₁ phase thickness + Γphase thickness) ≦ 0.5 ・ ・ ・ (2)