JP3275686B2 - Galvannealed steel sheet with excellent press formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet which exhibits excellent sliding properties between a mold and a plating layer during press forming and exhibits good formability.

[0002]

2. Description of the Related Art In recent years, the use of surface-treated steel sheets in which the surface of a steel sheet has been plated has been increasing. Among these, alloyed hot-dip galvanized steel sheets have been used for their weldability, paintability, and corrosion resistance after painting. Because of their superiority, they are frequently used in automobiles and home appliances. These products are manufactured through steps such as press forming, assembling, and painting. However, during press forming, cracks may occur in the alloyed hot-dip galvanized steel sheet, which is a material, and the product may not be a product.

[0003] These problems are caused not only by the insufficient mechanical properties of the material, but also by the poor sliding condition between the plating layer surface and the press die, and the material not flowing smoothly into the die. No. In order to cope with such a situation, the surface of a mold is conventionally subjected to chrome plating or the like. On the other hand, on the material side, improvements such as improvement of lubricating oil and uniform application have been made, and the phase structure of the plating layer has been improved.

The alloyed hot-dip galvanized layer has a Γ phase (Fe
₃ Zn ₁₀ ) or Γ ₁ phase (hereinafter collectively referred to as “Γ phase”), δ ₁ phase (FeZn ₇ ), and ζ phase (FeZn ₁₃ ). Of these alloy phases, the Γ phase has the highest iron content and is hard,
Then a [delta] _1-phase, zeta-phase is the smallest and most soft iron content. These alloy phases basically have a three-phase structure of Γ phase, δ ₁ phase and ζ phase from
When the degree of alloying is further advanced, the ζ phase disappears and the Γ phase and δ ₁
The phase changes to a two-phase structure.

If the softest ζ phase is formed on the surface of the plating layer, adhesion occurs due to sliding with the mold, frictional resistance increases, and material inflow into the mold is hindered. It is said that the material cracks easily. For these reasons, the purpose of changing the ζ phase to [delta] ₁ phase, as further advance the degree of alloying, and increasing the set temperature of the alloying furnace, the or to heat treatment conditions reduce the line speed Control has been implemented.

[0006]

However, even in the case of an alloyed hot-dip galvanized steel sheet manufactured by the above-mentioned method so as to increase the degree of alloying, the material has sufficient mechanical properties. Nevertheless, at present, the occurrence of material cracks has not been effectively suppressed. Under these circumstances, there is a strong demand for an alloyed hot-dip galvanized steel sheet that can effectively prevent the occurrence of material cracks. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an alloyed hot-dip galvanized steel sheet having a good plating layer with stable sliding characteristics with a mold during press forming. It is in.

[0007]

The alloyed hot-dip galvanized steel sheet according to the present invention, which has solved the above-mentioned problems, has an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer formed on the surface of a base steel sheet. In the above, the alloyed hot-dip galvanized layer has a gist in that 70 or less concave portions having a plated layer thickness of 2 μm or less per 3 mm in a horizontal length of the surface of the plated layer are provided.

[0008] In the galvannealed steel sheet of the present invention, the base steel sheet preferably contains 0.005 to 0.2% of P. Further, the plating layer has
l: It is preferable to contain 0.15 to 0.7%.

The object of the present invention can be attained by adopting the above constitution. If necessary, the surface of the galvannealed layer is coated with a silicic acid or silicate-containing film, Or the silicate is dried S
It is also effective to adopt a configuration that is 1 to 200 mg / m ² in terms of i weight, thereby improving the lubricity of the plating layer and further improving the sliding characteristics of the galvannealed steel sheet. Improvement can be achieved.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In order to obtain a plating layer having stable and good sliding characteristics with a mold, first, as described above, an alloy is manufactured with a relatively high degree of alloying. The cause of material cracking during press forming was investigated for hot-dip galvanized steel sheets. At this time, the mechanical properties of the materials were adjusted to be substantially the same. First, the cross-sectional structure of the plating layer that governs the sliding characteristics was examined in detail.

As a result, the uniformity of the thickness of the plating layer differs for each plated steel sheet. For example, as shown in FIG. 1, the plating layer has a substantially uniform plating thickness (however, the surface is alloyed hot-dip galvanized (A fine unevenness peculiar to the layer is present) [FIG. 1 (a)], and plating in which the thickness of the plating layer is extremely thin locally and the plating thickness is not uniform A plated steel sheet having a layer [FIG. 1 (b)] was observed, and some of the materials had a concave portion at an intermediate level between them.

Next, the present inventors have examined the relationship between the concave portion and the material crack. At this time, in order to quantify the generation amount of the concave portion, the number of the concave portion where the plating film thickness was 2 μm or less was observed per 3 mm in the horizontal length of the plating layer cross section, and the relationship with the material crack was investigated. At this time, the reason why the concave portion was a portion having a plating film thickness of 2 μm or less was 2 μm.
This is because the portion of m or less can be clearly distinguished as an extremely thin portion with respect to the film thickness of a normal alloyed hot-dip galvanized steel sheet of about 4 to 8 μm.

FIG. 2 shows the results. As is clear from FIG. 2, material cracking increases as the number of concave portions increases. When the number is 70 or less, no material cracking is observed, but when the number exceeds 70, material cracking starts to occur. . Therefore, it is understood that the number of the concave portions needs to be 70 or less in order to effectively prevent the material crack.

The inventors of the present invention have conducted intensive studies on the mechanism of generation of the concave portions and specific means for preventing the generation of the concave portions, and have found the following. That is, the concave portion is an outburst reaction (Zn) occurring on the grain boundary of the base iron (the grain boundary of the base steel sheet) in the initial stage of plating or alloying.
-Zn-Fe causing local abnormal growth of Fe alloy layer
The above-mentioned concave portion is formed because zinc existing on the ferrous crystal grains having a low alloying reaction rate is diffused and moved to the Zn—Fe alloy layer grown by the alloying reaction) by the action of alloying heating. There was found. That is, the plating layer has a convex shape on the ground crystal grain boundary having a high alloying reaction rate, and has a concave portion in the ground crystal grain having a low alloying reaction rate.

As described above, since the surrounding zinc diffuses and moves to the convex plating layer, the amount of zinc in this portion increases, and a large amount of ζ phase having a low iron concentration is generated on the surface of the partial plating layer. It is considered that the surface of the plating layer is liable to adhere to the mold by sliding with the mold, the frictional resistance is increased and the inflow of the material into the mold is hindered, and the material is easily cracked. In addition, the reason why the alloying plating rate on the base iron grain boundary is increased is that the amount of impurities is smaller than that in the base iron crystal grain boundary, the state becomes extremely clean, the reactivity between Zn and Fe becomes high, It is considered that a Zn—Fe alloy layer is formed and grows. Therefore, in order to suppress the formation of the concave portions, it is considered necessary to reduce the difference in the alloying reaction rate between the base iron crystal grain boundary and the base iron crystal grain.

The present inventors have also studied specific means based on these findings. First, in order to lower the purity of the base metal grain boundaries in order to reduce the alloying speed at the base metal grain boundaries, the addition of various elements that are easily segregated at the grain boundaries to the base steel plate was examined. As a result, as shown in FIG. 3, a clear correlation was observed between the P content in the base steel sheet and the number of formed recesses, and it was clarified that the number of formed recesses was significantly reduced by the addition of P. .

As is apparent from FIG. 3, when the P content in the base steel sheet is less than 0.005%, the number of recesses generated exceeds 70 which causes material cracking during press forming.
When the content is 0.005% or more, the number is 70 or less, and it can be seen that the number of the concave portions decreases as the P content increases.

On the other hand, the formation of the concave portion requires the Al in the plating layer.
The content was found to be relevant. FIG. 4 is a graph showing the relationship between the Al content in the plating layer and the number of recesses formed. As is apparent from this figure, the number of concave portions generated greatly increased as the Al content increased, and the Al content was 0.7%.
%, The number of recesses exceeds 70, which causes material cracking at the time of pressing. However, at 0.7% or less, the number of recesses becomes 70 or less, and the number of recesses decreases as the Al content decreases. ing.

The cause of the change in the number of concave portions generated depending on the Al content in the plating layer could be considered as follows. Al in the plating layer is Al added to the plating bath.
The higher the Al concentration in the plating bath, the higher the Al content in the plating layer. Since the Al added to the plating bath forms an extremely thin Fe-Al alloy layer by the reaction of the iron and steel alloy at the plating stage, the alloying process causes a diffusion reaction of Fe and Zn across this layer. . Here, the concentration of Al in the plating bath, in other words, if the Al content in the plating layer is low, the amount of this generation decreases, and a large amount of Zn in the plating layer passes through this layer and the grain boundaries and crystal grains of the base iron. Regardless of the inside, the alloying reaction occurs violently, so that there is no difference in the alloying reaction between the crystal grain boundaries and the inside of the crystal grains, the outburst reaction is suppressed, and the number of recesses in the plating layer cross section is reduced. Conceivable. On the other hand, when the Al concentration in the plating bath, that is, the Al content in the plating layer is increased, F
The generation amount of the e-Al alloy layer increases, Zn in the plating layer reaching the ground iron surface is greatly restricted, and a small amount of Zn causes an alloying reaction in the crystal grain boundaries of the ground iron and in the crystal grains. It is thought that the difference between the alloying reaction rate in the crystal grain boundary and the inside of the crystal grain becomes apparent, the outburst reaction at the crystal grain boundary is promoted, and the number of recesses formed in the plating layer increases. .

The greatest feature of the present invention is that the number of recesses formed on the surface of the plating layer is adjusted to 70 or less, whereby excellent sliding between the mold and the plating layer during press molding is achieved. In order to prevent the occurrence of material cracks by exhibiting good forming workability by exhibiting characteristics, it is preferable to set the P content in the base steel sheet to 0.005% or more as described above. However, when the P content is excessive, P may be greatly concentrated at the grain boundaries of the base iron, and the toughness of the material may be impaired. From such a viewpoint, the P content is 0.2%
It is preferable to set the following. A more preferable range than the P content is about 0.01 to 0.15%.

On the other hand, as described above, it is preferable that the Al content in the plating layer be 0.7% or less in order to reduce the number of concave portions formed on the plating surface to 70 or less. When the Al content becomes very small,
If it is less than 5%, the reaction rate of the alloying becomes extremely fast, and it becomes impossible to suppress the growth of the Γ phase which adversely affects the plating peeling resistance. From such a viewpoint, the Al content in the plating layer is preferably 0.15 to 0.7%, and more preferably about 0.2 to 0.6%.

In the plating layer, Fe is inevitably contained at the time of alloying, and the content of Fe is 8%.
It is preferable that the content be less than 8%. If the content is less than 8%, a large amount of ζ phase is present in the plating layer, which causes material cracking. In addition, the content of Fe becomes excessive and 17%
If it exceeds 300, it becomes impossible to suppress the growth of a phase which adversely affects the plating resistance. From such a viewpoint, the Fe content in the plating layer is preferably set to 8 to 17%.
More preferably, it is good to be about 9 to 16%.

As described above, the surface of the plated layer of the galvannealed steel sheet of the present invention may be coated with a silicic acid or silicate-containing film, if necessary, whereby the effect of the plated layer described above is obtained. Thus, the sliding characteristics can be further improved. In other words, this film is a strong hard film composed of fine silica particles. By forming such a film, the lubricity of the surface of the plating layer is improved, and the sliding applied to the surface layer of the plating during press molding. The deformation resistance can be reduced. Further, by forming this film, the chemical conversion treatment property is also improved.

In order to exert the effect of the above-mentioned film, as described above, the silicic acid or silicate in this film is
1 to 200 mg / m ² in terms of dry SiO ₂ weight
It is necessary to be. The reason will be described in detail with reference to the drawings.

FIG. 5 shows SiO ₂ when a coating containing silicic acid or silicate is applied to a galvannealed steel sheet.
1 is a graph showing the relationship between the content of, and lubricity and chemical conversion treatment. At this time, the coefficient of friction was used as an index of lubricity, and the chemical conversion treatment was represented by phosphoric acid treatment. The alloyed hot-dip galvanized steel sheet has a P content of 0.06% in the base steel and an Al content of 0.4 in the plating layer.
0%, the content of Fe in the plating layer is 11.0%, and the number of concave portions on the plating surface is 35 (3 m in the horizontal direction of the plating layer surface).
m).

Among them, the friction coefficient was measured by the following plane sliding test. [Sample size]: 40 × 300 mm [Tool]: Flat tool (18 × 20 mm) [Pressing force]: 5 kg / mm ² [Sliding speed]: 300 mm / min [Sliding length]: 150 mm [Coating oil]: Knox Last 550 (manufactured by Parker Kosan),
2 g / m ² Specifically, the pull-out load was measured, and the friction coefficient was calculated from the surface pressure and the pull-out load.

The phosphate treatment was evaluated by the following method. [Phosphate treatment solution]: SD5000 (manufactured by Nippon Paint Co., Ltd.) [Step]: Degreasing → washing → surface conditioning → phosphate treatment [Determination of phosphate film]: Observation of film by SEM,
Classification was performed according to the following evaluation criteria. ：: Uniform film formation △: Partial film formation ×: No film formation

As is apparent from FIG. 5, when the SiO ₂ content is 1 mg / m ² or more, the friction coefficient is significantly reduced. However, when the SiO ₂ content is 2
When it exceeds 00 mg / m ² , the lubricating property is good, but the phosphating property is remarkably deteriorated. As is clear from the results, when a silicic acid or silicate-containing film is coated, the silicic acid or silicate in the film is
1 to 200 mg / m ² in terms of dry SiO ₂ weight
It is necessary to be. The preferred lower limit of the SiO ₂ content is 20 mg / m ² , more preferably 40 mg / m ² . The preferable upper limit of the content of SiO ₂ is 100 mg / m ² , and more preferably 80 mg / m ² .

In order to coat the alloyed hot-dip galvanized steel sheet with a silicic acid or silicate-containing film, specifically, a silicate colloid solution or a silicate aqueous solution may be applied to the steel sheet and dried. As the silicate colloid solution used at this time, commercially available colloidal silica (for example, Nissan Chemical's Snowtex series) may be used. Examples of the aqueous silicate solution include sodium silicate, potassium silicate, and lithium silicate.

The method of applying the above-mentioned silicate colloid solution or silicate aqueous solution to the surface of the galvannealed steel sheet is not particularly limited, but may be, for example, a method of dipping in the above-mentioned aqueous solution or a roll coater. In addition to a coating method and a spray coating method, various commonly known general methods can be appropriately selected.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following Examples do not limit the present invention.
Modifications and alterations that do not depart from the spirit described below are all included in the technical scope of the present invention.

[0032]

【Example】

Example 1 Using a very low carbon steel containing 0.04% of Ti, a hot-dip galvanizing line and a galvanizing treatment were performed in a hot-dip galvanizing line, and as shown in Table 1 below, the P content in the base steel sheet, Various alloyed hot-dip galvanized steel sheets in which the Al content and the Fe content in the plating layer were changed were manufactured. With respect to the obtained galvannealed steel sheet, the number of recesses formed on the surface of the plating layer (the number per 3 mm of the horizontal length of the surface of the plating layer) was measured, and the steel sheet was subjected to an actual automobile press test, and material cracking occurred. The situation was investigated.

The results are also shown in Table 1. Examples of the examples satisfying all the requirements specified in the present invention show excellent press formability, and the occurrence of material cracks is effectively prevented. You can see that it is.

[0034]

[Table 1]

Example 2 A very low carbon steel containing 0.04% of Ti was subjected to a hot dip galvanizing process and an alloying process in a hot dip galvanizing line, and as shown in Table 2 below, P Various alloyed hot-dip galvanized steel sheets having different contents, Al contents and Fe contents in the plating layers were manufactured.

With respect to this galvannealed steel sheet,
After measuring the number of recesses formed on the surface of the plating layer (the number per 3 mm of the horizontal length of the surface of the plating layer), silicic acid (SiO ₂ colloid solution) or silicate was added to the surface of the galvannealed steel sheet. And using a squeezing roll.
After the application, the coating was dried at 80 ° C. to form a hard SiO ₂ film.

The obtained material was subjected to an actual automobile press test to investigate the occurrence of material cracks, and the friction coefficient and phosphatability were evaluated by the above-described methods. The results are also shown in Table 2, and all of the examples satisfying all the requirements specified in the present invention show excellent press formability and that the occurrence of material cracks is effectively prevented. I understand. In addition, it can be seen that the coating with the silicic acid or silicate-containing film provides excellent lubricity and excellent phosphatability.

[0038]

[Table 2]

[0039]

According to the present invention, there is provided an alloy capable of improving the sliding characteristics of a plating layer with a mold to improve press formability and preventing the occurrence of material cracks during press forming. A hot-dip galvanized steel sheet can be realized, and this steel sheet is particularly useful for automobiles.

[Brief description of the drawings]

FIG. 1 is a view schematically showing the surface properties of an alloyed hot-dip galvanized steel sheet.

FIG. 2 is a graph showing the relationship between the number of concave portions present in a plating layer and material cracking.

FIG. 3 is a graph showing the relationship between the P content in steel and the number of recesses formed.

FIG. 4 is a graph showing the relationship between the Al content in a plating layer and the number of recesses formed.

FIG. 5 is a graph showing the relationship between the content of SiO _{2 in} a coating film and lubricity and chemical conversion treatment properties.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-253849 (JP, A) JP-A-7-18403 (JP, A) JP-A 8-27556 (JP, A) (58) Field (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (4)

(57) [Claims] 1. An alloyed hot-dip galvanized steel sheet in which an alloyed hot-dip galvanized layer is formed on a surface of a base steel sheet, the alloyed hot-dip galvanized layer has a recess having a plating layer thickness of 2 μm or less on the surface of the plating layer. 7 per 3 mm in horizontal length
An alloyed hot-dip galvanized steel sheet having excellent press formability, wherein the number is zero or less. 2. The base steel sheet according to claim 1, wherein P: 0.005 to 0.5.
The galvannealed steel sheet according to claim 1, which contains 2% (mean% by mass, the same applies hereinafter). 3. The plating layer according to claim 1, wherein Al: 0.15 to 0.1.
The galvannealed steel sheet according to claim 1 or 2, which contains 7%. 4. The surface of the alloyed hot-dip galvanized layer is coated with a silicic acid or silicate-containing film, and the silicic acid or silicate in the film is 1% in terms of SiO ₂ weight after drying.
Galvannealed steel sheet according to claim 1 is to 200 mg / m ^2.