JP2000219948A - Galvannealed steel sheet excellent in workability and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a galvannealed steel sheet applicable to the steel sheet for motor car body facing, in which stable press formability is obtd. even in the case of using no lubricator, increase of the surface roughness can be prevented and further, degradation of the appearance at the temper rolling time is not observed, and this producing method. SOLUTION: This hot-dip metal coating and alloying treatment is executed under condition of 0.08-0.13 wt.% [Al] as the effective Al concn. in the galvanizing bath, 450-500 deg.C steel sheet temp. during dipping, 510-560 deg.C max. reaching temp. [T1] and 415-480 deg.C holding temp. for alloying [T2], and the condition: 840+1000×[Al]<=[T1]+[T2]<=890+1000×[Al]. In this way, the galvannealed steel sheet regulated to in the ranges of 8-13 wt.% Fe content, 0.5-1.4 μm [Ra] surface roughness at the center line after coating (cutoff value = 0.8 mm), <=10 g/m2 ζ (Zeta) phase on the surface layer and [Ra] (μm)×[ζ value] (g/m2)<=5.0, is obtd.

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 having excellent workability, which is mainly used for an automobile body or the like which requires severe powdering resistance and slidability during press forming. Regarding the production method, it is possible to secure stable press formability without performing special post-treatment, after once cleaning the steel sheet with a cleaning oil or the like for the purpose of removing dirt on the surface of the steel sheet. The present invention relates to an alloyed hot-dip galvanized steel sheet for automotive body exteriors having excellent workability and a method for producing the same.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets have been widely used as corrosion-resistant steel sheets for automobile bodies because of their excellent corrosion resistance after painting. However, it has been difficult to apply the alloyed hot-dip galvanized steel sheet to a steel sheet for automobile body exteriors. Compared to electro-galvanized steel sheet, electro-zinc alloy-plated steel sheet, etc., the basis weight of plating is large, so the powdering resistance is inferior, the amount of peeling of plating increases during press working, and the peeled plating powder is pushed in during pressing This is because there is a problem that the appearance is impaired.

[0003] An alloyed hot-dip galvanized steel sheet is heated immediately after hot-dip galvanizing the steel sheet to promote the diffusion of iron in the base material, thereby forming a zinc-iron alloy layer on the plating film. Therefore, there is an iron concentration gradient in the depth direction of the plating film, and from the base metal to the plating film surface direction,
Iron-rich 鉄 phase, Γ ₁ phase, δ ₁ phase, ζ phase (or
When untreated, a pure zinc η phase) is formed. However, the alloying treatment by heating after galvanizing film formation (hereinafter GA treatment) is due to lack of stability, the alloy phase formed on the outermost surface changes greatly in plating film, zeta Airyo, gamma _1-phase However, there is a problem that the balance of the Γ phase greatly fluctuates.

[0004] Since the ζ phase is a soft phase, it tends to seize with a mold during press molding, increases the sliding resistance (friction coefficient) at the time of press molding, and is liable to generate press cracks. There is a problem that it is inferior. Meanwhile, Γ ₁ phase, Γ
Since the phase is a hard phase, the plating film peels off from the 剥離 phase and Γ ₁ phase as a starting point due to the unfolding deformation during press molding, and the plating powder is likely to be generated, resulting in poor powdering resistance. is there.

[0005] Usually, alloyed hot-dip galvanized steel sheets for automobile bodies are required to ensure not only rust prevention but also slidability.
Rust-preventive oil, which also has a lubricating function, is applied to compensate for the decrease in slidability during press molding. However, mainly in the case of steel sheets for automobile body exterior use, cleaning oil with poor lubricating function is used to wash off foreign substances on the steel sheet surface. Excellent slidability is required for galvanized steel sheets themselves.

[0006] As described above, from the viewpoint of workability, particularly, it is necessary to ensure both powdering resistance and stability of sliding properties. It was difficult to use the steel sheet as it was.

Conventionally, when an alloyed hot-dip galvanized steel sheet is used as a steel sheet for the exterior of an automobile body, in order to ensure slidability, for example, a method disclosed in Japanese Patent Application Laid-Open No. Hei 3-191045 is used. Two-layer plating in which an iron-based flash plating is applied on an alloyed hot-dip galvanized steel sheet has been used. By applying such a hard iron-based plating, even if there are many ζ phases in the surface layer of the alloyed plating film, good slidability can be secured.
A method has been adopted in which only the formation of the Γ phase is suppressed.

[0008] However, since the two-layer plating has a problem of production cost, it is strongly demanded that the alloyed hot-dip galvanized steel sheet itself ensure powdering resistance and slidability without post-treatment. ing.

[0009] For example, in Japanese Patent Application Laid-Open No. Hei 3-191045, even if there is no upper plating, it is possible to optimize the amount of phase and to improve the surface roughness (R
max), powdering resistance and flaking resistance
(Sliding property) can be ensured, but the amount of haze is relatively large, and if the amount of hue is extremely reduced, it becomes difficult to ensure powdering resistance, and powdering resistance
It can be seen that it is difficult to secure an appropriate range of alloying degree for ensuring slidability (reducing ζ phase). That is, it is difficult to reduce the ζ phase as much as possible while ensuring the powdering resistance by using a GA-processed heat pattern that is simply heated and held as conventionally studied.

To improve the slidability, it is effective to reduce the soft 鉄 phase having a low iron content or the η phase of pure zinc, as described above. In order to suppress this, it is effective to reduce the hard Γ phase having a high iron content formed at the plating / base metal interface.
It is effective to form a GA film close to _one- phase single-phase. See JP-A-64-68456, JP-B-3-55544 and JP-A-2709173.

In order to secure the alloy phase of such an alloyed plating film, a production method in which the immersion time in a hot-dip galvanizing bath is shortened and the GA treatment is rapidly performed is described in, for example,
-81662.

Japanese Patent No. 2770825 and the like specify the amount of Al in a hot-dip galvanizing bath and the temperature at which a steel sheet penetrates into a galvanizing bath to reduce the base material crystal grain boundaries in the plating film. A method for suppressing the outburst reaction, which is local abnormal alloy phase growth, and then rapidly alloying by high-frequency induction heating and maintaining the state, thereby suppressing the growth of the ζ phase. ing.

In order to improve the slidability, it is considered that the surface roughness of the galvanized steel sheet itself is also important. Even in the case of an alloyed hot-dip galvanized steel sheet, the surface roughness of the steel sheet is Ra = 0.5 to
JP-A-6-153302 describes that it is effective to reduce the surface roughness of a work roll during temper rolling so that the thickness can be maintained at 1.0 μm.

Further, from the viewpoint of powdering resistance, for example, JP-A-61-223174 discloses that after hot-dip galvanized film is formed, it is heated rapidly and at a high temperature, and then heated to a predetermined temperature range. A GA treatment heat pattern for cooling and holding is also recommended.

[0015]

[Problems that the Invention is to Solve also alloy phase of a conventional alloying plating film with only by controlling the form close to the [delta] ₁ single phase,
While good press-formability can be ensured when rust-preventive oil is applied, it is difficult to ensure stable press-formability when oil lubricity cannot be expected. In particular, it has been found that there is a problem that stable slidability cannot be ensured if the alloyed hot-dip galvanized film remains in a state where a cleaning oil generally used as a steel sheet for automobile body exteriors is applied. did.

Further, regarding the surface roughness, as described later, in the case of the alloyed hot-dip galvanized steel sheet, the surface roughness increases due to non-uniform iron diffusion during the GA treatment, so that the temper rolling is performed. It is difficult to ensure stable steel plate surface roughness only by adjusting the surface roughness of the work roll at the time, and it is necessary to suppress non-uniformity of iron diffusion during GA processing. found.

Further, if a work roll having a low roughness is used at the time of temper rolling, the peeled plating powder and the like adhere to the roll, and it is easy to cause an indentation flaw on a steel sheet, so that automobiles requiring beautiful appearance are required. There is a problem in using it as a vehicle exterior steel sheet.

Thus, an object of the present invention is to ensure stable press formability even in a state in which lubricating effects due to lubricating oil cannot be expected, and to reduce surface roughness due to non-uniform iron diffusion during GA processing. Can be prevented from increasing, and
An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet which can be applied as a steel sheet for automobile body exteriors and which prevents deterioration in appearance due to indentation flaws experienced during temper rolling, and a method for producing the same.

[0019]

Means for Solving the Problems The present inventors have conducted various investigations on the state of an alloyed hot-dip galvanized film that is optimal for securing stable sliding properties, and as a result of intensive study, have found that With regard to the sliding property of the plated steel sheet, it was found that the amount of phase in the surface layer of the plating film and the shape of the outermost surface of the plating (surface roughness) had a great influence.

That is, only the individual control of the Δ phase and the surface roughness, which have been conventionally proposed, can be applied to an alloyed hot-dip galvanized steel sheet for automobile body exteriors in a state where a cleaning oil having poor lubricity is applied. It is difficult to ensure stable slidability during press molding, and it is necessary to control the amount of ζ phase and surface roughness under a certain correlation.

Furthermore, such an alloy phase structure (ζphase amount)
As a result of studying the factors attributable to the variation in surface roughness, during the plating immersion, first, the Al richness formed by the Fe-Al alloying reaction between Fe on the steel sheet surface and Al added to the hot-dip galvanizing bath. It was also found that the amount of ζ phase and the surface roughness greatly fluctuated on the surface by the subsequent alloying process.

That is, according to the present invention, in forming an alloyed hot-dip galvanized film, the Al concentration when a metal material to be treated such as a steel sheet is immersed in a hot-dip galvanizing bath is controlled, whereby the plated film is formed. Controlling the amount of Al in the plating film, and controlling the GA heat treatment pattern at the level of the amount of Al in the plating film in consideration of the alloying reaction behavior that fluctuates depending on the level of Al in the plating film when performing the subsequent alloying treatment By doing so, the GA alloy phase formed thereafter and the surface shape after the GA conversion process are optimized, and as a result, processing that improves the slidability and powdering properties of the alloyed coating itself Thus, an alloyed hot-dip galvanized steel sheet having excellent properties can be obtained.

In the prior art, since heating is performed from a low-temperature region to a high-temperature region in the process of GA formation, a reduction in the 期待 phase can be expected by rapid heating, but some ζ phase growth is inevitable. In order to suppress the ζ phase, it is effective to heat the ζ phase to around 520 ° C., which is the eutectic temperature. Many proposals have been made to form a GA below ℃. The temperature at which the 温度 phase disappears is expected to be insufficient.

Further, in the alloyed hot-dip galvanized steel sheet, during the growth process of the alloyed layer, the iron diffusion rate varies depending on the ferrite crystal orientation of the base material, and irregularities occur in the alloyed plated film in units of base material crystal grains. A micro-depression phenomenon that occurs. Due to the existence of the micro depressions, there is a problem that it is difficult to secure such a surface roughness having the expected effect even when the work roll of the temper rolling is specified.

That is, it is difficult to secure a stable surface roughness even if the surface roughness of the temper rolling roll is reduced unless the micro-depression is reduced. However, it is difficult to always sufficiently reduce the amount by the manufacturing method described above.

The micro-depression largely depends on the amount of Al-enriched formed during plating immersion and the temperature in the subsequent GA treatment process, and the reduction of the amount of Al-enriched in the plating film, The reduction of the effective Al content in the hot-dip galvanizing bath (= the total Al content in the bath minus the dissolved Fe content in the bath) and a decrease in the temperature of the intruding material reduce the micro-depression. On the other hand, GA formation temperature is also effective in reducing micro depressions,
When GA was applied, it was found that heating reduced the iron diffusion rate difference between the base material grains and reduced micro-depression.

According to the effective Al amount level in the plating bath,
By changing the GA formation temperature, it is possible to simultaneously reduce the ζ phase and the surface roughness, and it is possible to secure stable slidability.

Further, the secure powdering resistance, but reduces the gamma phase and gamma ₁ Airyo is important, this depends substantially alloyed. To reduce phase in conventional technology
At the GA formation temperature, an appropriate degree of alloying is easily ensured, but as described above, at this temperature, the effect of reducing the ζ phase and the effect of reducing the surface roughness are insufficient. (4) Heating at a higher temperature is effective for reducing the amount of phase and surface roughness. However, if the temperature is maintained as it is, it is difficult to secure an appropriate degree of alloying, which is not preferable. It is considered that the growth of the alloyed phase from the molten zinc state occurs in a relatively short time, and once the temperature has been raised to the GA formation temperature, it is immediately cooled and held, so that an appropriate alloyed phase can be secured. In addition, it is possible to prevent a reduction in powdering resistance. However, even after increasing the GA formation temperature, it is expected that molten zinc will remain slightly, and if the cooling temperature is too low, the molten zinc solidifies,
It is necessary to maintain the temperature above 415 ° C. where the remaining zinc can exist in a molten state.

The concept of rapidly heating to a high GA temperature and then maintaining the temperature at a low temperature is described in JP-A-61-223174. In this invention, however, the GA temperature and the holding temperature are high, and However, it is insufficient from the viewpoint of ensuring excellent powdering resistance. The present inventors have conducted intensive studies and found that GA
When the crystallization temperature exceeds 560 ° C, it is harder than the
There is a problem that _one phase begins to grow and the powdering property is remarkably reduced, and it has been found that it is not preferable to extremely increase the GA temperature from the viewpoint of powdering resistance.

Thus, according to the present invention, the lubricating effect can not be expected, the alloyed hot-dip galvanized
In order to ensure good and stable press formability, the amount of Al phase in the bath is controlled to control the amount of phase and surface roughness, and to control the GA rate and the amount of enriched Al that affects the surface roughness. After adjusting the infiltration material temperature, the high-temperature GA depends on the Al level in the bath.
Then, by adopting a GA heat pattern that immediately cools down, a proper alloying degree is ensured, and at the same time, the reduction and stabilization of the 、 phase and the surface roughness are also ensured.

Here, the gist of the present invention is as follows. (1) Fe content: 8 to 13% by weight, surface roughness after plating is center line roughness (cut-off value = 0.8 mm): [Ra], 0.5 μm or more, 1.4 μm or less, Δ phase amount: Processing characterized by having an alloyed hot-dip galvanized film having [ζ] of 10 g / m ² or less and [Ra] (μm) × [ζ value] (g / m ² ) ≦ 5.0. Alloyed hot-dip galvanized steel sheet with excellent resistance.

(2) Effective Al concentration in plating bath: [Al] is 0.08
In hot-dip galvanizing bath of ~ 0.13wt%, plate temperature 450 ~
The steel sheet is immersed at 500 ° C., and immediately after the immersion plating, the alloy is heated to the maximum temperature of 510 to 560 ° C. at the plate temperature: [T ₁ ], and then immediately heated to 415 ° C.
Alloying holding temperature of 480480 ° C .: cooling to [T ₂ ], holding at this temperature range, and cooling again,
A method for producing a galvannealed steel sheet having excellent workability, wherein [T ₁ ], [T ₂ ] and [Al] are set so as to satisfy the following formula. 840 + 1000 × [Al] ≦ [T ₁ ] + [T ₂ ] ≦ 890 + 1000 × [Al]

[0033]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. Fe content: Fe content in alloyed plating film (degree of alloying)
Is important for ensuring powdering resistance and slidability. If it is less than 8%, it is impossible to completely eliminate the ζ phase. If it exceeds 13%, the Γ phase will increase too much,
Powdering resistance deteriorates. Therefore, the content of Fe is preferably 8 to 13%, and more preferably 9 to 11%.

(3) Amount of phase: The amount of the phase present in the plating film is important for ensuring the slidability. If it exceeds 10 g / m ² , the slidability is ensured even if the surface roughness is reduced. Because it will be difficult
10 g / m ² or less. Preferably it is 6.0 g / m ² or less. In addition, if the appropriate degree of alloying and the amount of the Γ phase can be ensured, there is no lower limit value for the ζ phase, but if the amount of the ζ phase is reduced, the degree of alloying and the Γ phase increase, so that the ζ phase is substantially 0.5 g / m ² or more exists. Therefore, the preferred amount of the solid phase is 0.5 to 6.0 g / m ² .

Here, the ζ phase is present in the surface layer of the plating film, and this “surface layer” is a region where the ζ phase exists in the alloyed plating film, and specifically, ζ The region up to the depth where the phase disappears, usually 0.1 to 1.
This is a region having a depth of 0 μm.

Surface roughness: Surface roughness is also important for ensuring slidability. If the surface roughness is large, the resistance (coefficient of friction) during sliding increases, and the sliding property decreases. In addition, if the surface roughness is small, it is difficult to secure an appropriate oil film between the press die and the surface of the plated steel sheet (holding the oil film), causing metal contact with the press die and causing a seizure phenomenon. . Further, when the surface roughness is reduced, the plating powder peeled off by the powdering phenomenon at the time of the press working becomes easy to be pushed in and causes poor appearance. In the present invention, Ra (cut-off value = 0.8 mm) is adopted as an index of the surface roughness, and Ra is optimally 0.5 to 1.4 μm. Preferably 0.6
1.01.0 μm.

Correlation between surface roughness and ζ phase amount: Even if the ζ phase amount is small, if the surface roughness is large, an improvement effect on slidability cannot be expected. In addition, if the surface roughness is at least large, the effect of improving slidability cannot be expected. In other words, when press forming is performed when there are irregularities on the surface of the steel sheet, there is a locally high surface pressure part (projection of surface roughness), and the substantial surface pressure is extremely high in that part. Get higher. For parts with high surface pressure
It is considered that the presence of a soft phase significantly impairs slidability when a soft phase is present.

Therefore, when the surface roughness (Ra) is small,
Even if the ζ phase is present, the contact area with the press die becomes large, so that the actual surface pressure decreases and good press formability can be secured. On the other hand, even if the amount of ζ phase is small (even if it does not substantially exist), when the surface roughness (Ra) is large, the contact area with the press die becomes small, and the actual surface pressure increases,
It is considered that press formability deteriorates.

Further, even when the amount of the phase and the surface roughness are extremely small, the workability tends to be reduced. Although the reason for this is not clear, an extreme decrease in the ζ phase leads to an increase in the degree of alloying and, consequently, to a reduction in the powdering resistance, which tends to easily cause plating exfoliation powder during processing.
At this time, if the surface roughness is extremely small, it is considered that plating peeling powder tends to accumulate during processing, the plating powder builds up (seizure) in the mold, and the mold comes off. If the surface roughness is large, the plating powder generated during processing is easy to peel off, making it difficult to build up in the mold.
It is presumed that it is not necessarily effective in improving workability.

From the above viewpoints, in order to ensure good slidability, the relationship between the surface roughness and the amount of ζ phase is expressed as [Ra] (μm) ×
If the [ζphase amount] (g / m ² ) is not less than 5.0, stable and good slidability cannot be secured. Preferably it is 1-3 or less.

In the present invention, if the degree of alloying, the amount of ζ phase, and the surface roughness can be ensured, particularly with respect to the basis weight,
Although it does not matter, usually, from the viewpoint of rust prevention, 25 to 70
g / m ² is sufficient. Next, the appropriate ζ
Manufacturing conditions that can ensure the phase amount and surface roughness will be described.

Effective Al concentration: The amount of Al in the plating bath is important in the present invention. In hot-dip galvanizing, aluminum is usually added to a plating bath in order to ensure adhesion of a plating film. This is because, after annealing, an Fe-Al alloy phase is formed on the surface of the steel sheet immersed in the plating bath, the abnormal growth of the Zn-Fe alloy phase is suppressed, and the adhesion of the plating film is ensured.

Therefore, when the amount of Al in the bath is small, there is a problem that the growth of the alloy phase increases during the immersion of the plating bath, and the plating adhesion decreases. On the other hand, if the amount of Al in the bath is large, the Fe-Al alloy phase formed on the steel sheet surface during plating immersion is strongly formed, and in the subsequent alloying reaction,
There is a problem that diffusion is difficult to occur and proper alloying is difficult. Further, if the plating film contains a large amount of Al, the alloying becomes non-uniform, so that there is a problem that the surface roughness increases.

Therefore, the effective amount of Al in the plating bath is
The concentration (= total Al content−eluted Fe content) is preferably 0.08 to 0.13 wt%. If it is less than 0.08%, the adhesion of the plating film is reduced, and the powdering resistance is reduced. On the other hand, if the content is less than 0.08%, the iron diffusion rate at the time of alloying becomes high.
When alloying at high temperature is performed to suppress phase formation,
It becomes difficult to secure the phase amount, and the powdering resistance deteriorates.

On the other hand, when the effective Al concentration exceeds 0.13%,
The diffusion speed is slow, and it becomes difficult to suppress the ζ phase, and the slidability decreases. If the alloying temperature is increased to suppress the ζ phase, the alloying temperature exceeds the appropriate alloying temperature, and there is a problem that the powdering resistance is deteriorated, which is not preferable. Further, since the Fe diffusion reaction becomes non-uniform, the surface roughness increases, and there is also a problem that it is difficult to secure an appropriate surface roughness. Therefore, the effective Al concentration in the bath is 0.08 to 0.13 wt
%, Preferably 0.10-0.11 wt%.

Intruded material plate temperature: The plate temperature of the intruded material (also called the intruded material temperature or the intruded material temperature) is determined by enrichment during plating bath immersion.
Affects Al content. Even in the same bath Al amount, when the intruding material temperature rises, the enriched Al amount increases. Therefore, the penetration plate temperature
If the temperature exceeds 500 ° C, as in the case where the bath Al content is high, the Fe diffusion rate is delayed, making it difficult to achieve GA even at high temperatures, increasing the amount of phase, and further increasing the slidability due to the increase in surface roughness. Degradation is a problem. If the temperature is lower than 450 ° C., the diffusion rate of Fe is promoted as in the case of a low Al content in the bath. Powdering property deteriorates.

Alloying heating temperature: The alloying temperature is 510 to 56.
0 ° C. is preferred. As described above, if the temperature exceeds 560 ° C., a harder phase ₁ is formed than the phase ₁ ; therefore, the powdering resistance is deteriorated, which is not preferable. And there is a problem in slidability because the ζ phase remains. If the temperature is lower than 510 ° C., the effect of suppressing micro-depression of the alloyed film cannot be exhibited as described above, and there is a problem from the viewpoint of surface roughness. In order to more effectively obtain the target surface condition of the present invention, the alloying temperature is 520 to 54.
0 ° C. is preferred.

Alloying holding temperature: The reason for holding immediately after cooling after alloying is that the degree of alloying increases with alloying at a high temperature and the powdering resistance deteriorates as described above. This is to prevent it. The holding temperature at that time is 48
The reason why the temperature is specified below 0 ° C is to prevent the powdering property from deteriorating due to overtreatment, and the reason why the temperature is specified above 415 ° C is that generation of the η phase and generation of the ζ phase This is for suppressing and securing slidability.

Alloying Heat Pattern: However, GA
In the heat treatment pattern, it is not sufficient to simply specify the maximum attained temperature [T ₁ ] and the holding temperature [T ₂ ] independently in order to secure an appropriate degree of alloying. That is, [T ₁ ]
If [T ₂ ] is high, it is difficult to obtain the expected degree of alloying, and there is a problem that the degree of alloying is too high and the powdering resistance decreases, while [T ₁ ] and [ T ₂ ] is low,
There is a problem that the degree of alloying is low, the ζ phase tends to remain, and the slidability is reduced.

That is, in order to secure an appropriate degree of alloying, it is necessary to control the total heat input until the completion of GA formation, and the maximum GA formation temperature [T ₁ ] and the holding temperature [T ₂ ] It is important to be within the proper temperature range.

Further, regarding the temperature during the alloying treatment,
It is also greatly affected by the amount of Al enrichment in the plating film. Al in bath
If the temperature and the steel sheet penetration temperature are high, a strong Fe-Al phase is formed in the initial stage of plating immersion, and the amount of enriched Al increases, so that alloying is delayed. Therefore, it is necessary to further raise the alloying temperature in order to secure the temperature. Further, if the amount of Al in the bath and the temperature of the steel sheet intruding material are low, the Fe-Al layer in the initial stage of plating immersion is thin, and the amount of Al-enriched material is reduced, so that alloying is promoted. In order to secure the phase amount and the surface roughness, it is necessary to lower the alloying temperature.

From the above viewpoints, the heat pattern during the alloying treatment can be changed in accordance with the change in the amount of Al in the bath in the relationship between the maximum attained plate temperature [T ₁ ] and the holding temperature [T ₂ ]. It is important to obtain the appropriate degree of alloying, the amount of phase, and the surface roughness specified in the present invention.

In the production method according to the present invention, in the bath
As a suitable heat pattern when changing the GA heat pattern in accordance with the Al amount, the following relationship is important.

If it is less than the lower limit of 840 + 1000 × [Al] ≦ [T ₁ ] + [T ₂ ] ≦ 890 + 1000 × [Al] [T ₁ ] + [T ₂ ], the amount of Δ phase is reduced and the surface roughness is reduced. The effect is insufficient and the slidability deteriorates, which is not preferable. On the other hand, if the value exceeds the upper limit, it is difficult to secure an appropriate degree of alloying, and the powdering resistance is lowered. In the relationship of [T ₁ ] + [T ₂ ], it is preferable to satisfy the following expression.

860 + 1000 × [Al] ≦ [T ₁ ] + [T ₂ ] ≦ 870 + 1000 × [Al] In the present invention, the heating rate for securing such a GA heat pattern, or cooling once immediately after alloying treatment The cooling rate is not particularly limited as long as the temperature can be ensured, but it is considered that it is preferable to increase the cooling rate as much as possible in order to more effectively exert the effects of the present invention. It is preferable that the cooling rate is at least 10 ° C / sec.

The method of heating the alloying furnace for obtaining such a heat pattern is not particularly limited either, and as long as the temperature of the present invention can be ensured, it does not matter whether it is radiant heating using a normal gas furnace or high-frequency induction heating. In view of the fact that rapid heating is possible, the high-frequency induction heating method is more preferable. Also, it does not matter how to cool immediately from the highest reached plate temperature after the alloying heating. If the heat is shut off by air seal etc. after alloying, it is enough to leave it open, and it cools more quickly. There is no problem with gas cooling.

[0057]

Next, the function and effect of the present invention will be described more specifically with reference to examples and comparative examples.

(Example 1) For the purpose of clarifying the influence of workability and surface condition, various kinds of materials manufactured by a continuous hot-dip galvanizing line using ultra-low carbon IF steel having a thickness of 0.65 to 0.85 mm as a material were used. Alloyed hot-dip galvanized steel sheet material (Amount = 38-62g /
For m ² ), powdering resistance and sliding properties were investigated.

At this time, the surface roughness (Ra) and the amount of ζ phase were selected according to the present invention as factors affecting processability, and the relationship between the balance and processability was examined. The surface roughness was measured using a surface roughness meter according to a center line roughness [Ra] (cutoff value = 0.8 mm) which is a surface roughness parameter.

The amount of phase was determined by a galvanostatic electrolysis method in which a galvannealed steel sheet was immersed in an electrolytic solution under the conditions described in Table 1 and a potential was measured when a predetermined current value was passed. Measured. FIG. 1 shows the results qualitatively. As shown in FIG. 1, the amount of ζ phase was determined from the melting time at the melting potential of each alloy layer at that time.

[0061]

[Table 1]

As a method of evaluating workability, powdering resistance and slidability were examined. As an evaluation method of the powdering resistance in this example, a cylindrical drawing test was performed under the conditions and specifications described in FIG. 2, the side surface after processing was peeled off with a tape, and the processing was performed based on the weight difference before and after processing. The amount of peeling of the plating powder was measured. As a determination method at that time, in view of the powdering resistance performance of the steel sheet for vehicle body exterior, the amount of peeling was ranked according to the level described in Table 2, and a circle or higher was determined to achieve the target level.

[0063]

[Table 2]

The slidability was determined by measuring the coefficient of friction by a flat plate pull-out test performed as shown in FIG.
A comparative study of slidability was conducted. As a determination method at that time, in consideration of the actual results of the press cracking of the steel sheet for vehicle body exterior and the friction coefficient at that time, the levels were classified as shown in Table 3, and it was determined that the target level was achieved when ○ or more.

[0065]

[Table 3]

FIG. 4 is a graph summarizing the results of the evaluation of the workability by the above-mentioned powdering resistance test and slidability test, and shows the Δ phase amount and center line roughness of various galvannealed steel sheets. 6 illustrates the evaluation results of workability when the value [Ra] fluctuates.

As shown in Table 4, the workability comprehensive evaluation criterion in this case is as follows.
The level that satisfies the target value is defined as △,
Both of the slidability are in the best range ◎, the slidability is the best, the powdering resistance has reached the target level ○, the powdering resistance is the best, the slidability has reached the target level Is indicated by ●, and any one of powdering resistance and slidability which does not reach the target level is indicated by ×.

[0068]

[Table 4]

From FIG. 4, it can be seen that in order to achieve both good powdering resistance and slidability in a state where the cleaning oil is applied, only the Δ phase amount and the surface roughness, which have been conventionally studied, are reduced. Is not sufficient, the amount of ζ phase is 10 g / m ² or less, and Ra is 0.5
It can be seen that the region is in the range of ~ 1.4 µm and the product of the amount of ζ phase and Ra is 5.0 or less. Especially, the amount of ζ phase is 0.5-0.6g / m
^2, Ra in is 0.6 to 1.0 [mu] m region, and a region of the product is from 1.0 to 3.0 of ζ Airyo and Ra is, powdering resistance, has excellent sliding properties both turned out to be the best suitable amount range did.

An example of a manufacturing method for obtaining such a surface property capable of having such excellent workability will be described based on the results of a survey on a galvannealed steel sheet actually used.

(Example 2) A material prepared by applying galvanizing with a basis weight of 45 g / m ² in a continuous galvanizing line using a very low carbon IF steel having a thickness of 0.80 mm was prepared. (Al concentration = 0.10%, penetration temperature = 480 ° C)
After alloying, dull roll (surface roughness: Ra = 1.
(7 μm), after 0.8% temper rolling, changes in powdering resistance, slidability, and surface roughness were examined.

[0072] At this time, in order to clarify the influence of the Fe content (alloy degree) at a heating rate = 20 ° C. / sec, to change the T ₁ (alloying highest temperature), T ₁ temperature Immediately after reaching,
The cooling was performed at a cooling rate of 10 ° C./sec, and the temperature was lowered to a holding temperature of 450 ° C., and the degree of alloying was changed so that a series of alloying treatment times was completed in 20 seconds.

Measurement of phase amount (FIG. 1) and surface roughness, and powdering resistance (FIG. 2) and slidability (FIG. 3)
The investigation was performed in the same manner as in Example 1. The evaluation and judgment of the powdering resistance and the sliding property were performed according to the criteria described in Tables 2 and 3. Table 5 shows the results of these surveys.

No. 15 and No. 16 show that the upper limit of the degree of alloying is 13% or less from the result of the investigation on powdering resistance. The optimum value is 11% or less from No.12 and No.11. No. 4 and No. 5 show that the lower limit of the degree of alloying is 8% or more from the results of slidability, and that No. 6 and No. 7 preferably 9% or more.

From No. 15 and No. 16, when the maximum alloying ultimate temperature T ₂ exceeds 560 ° C. and [T ₁ ] + [T ₂ ] exceeds 990 ° C.,
T ₁ is 560, since it cannot be suppressed to 13% or less, which is an appropriate degree of alloying for powdering resistance.
No. 11 and No. 12 show that the temperature is preferably 540 ° C. or less.

Looking further at the relationship between the maximum temperature T _{1 for} alloying and the surface roughness, T ₁ is 510 ° C. or higher, preferably 520 ° C.
If it is above, the surface roughness is greatly reduced. Also,
The amount of ζ phase also sharply drops from 510 ° C. or higher, indicating that the T ₁ temperature is preferably 510 ° C. or higher, preferably 520 ° C. or higher.

[0077]

[Table 5]

(Embodiment 3) Next, an extremely low carbon sheet having a thickness of 0.70 mm
Prepare a material with a galvanized weight of 45 g / m ² in a continuous hot-dip galvanizing line using IF steel (effective Al concentration in the plating bath = 0.10%, intruding material temperature = 470 ° C), and use the test equipment. ,
After alloying, dull roll (surface roughness: Ra = 1.7 μ
m), after passing 0.8% temper rolling, powdering resistance,
The slidability and changes in surface roughness were investigated.

At that time, in order to clarify the range of the holding temperature, T ₁ = 510, 515, 520 at a heating rate of 20 ° C./sec.
The temperature was fixed at 525 ° C., immediately cooled at a cooling rate of 10 ° C./sec, and the performance when the holding temperature T ₂ was changed was investigated. As in Example 2, a series of alloying treatment times was completed in 20 seconds to change the degree of alloying.

Measurement of phase amount (FIG. 1) and surface roughness, powdering resistance (FIG. 2) and slidability (FIG. 3)
The investigation was performed in the same manner as in Example 1. The evaluation and judgment of the powdering resistance and the sliding property are the same as those described in Tables 2 and 3. Table 6 shows the results of the survey.

From No. 1 to No. 3, it can be seen that if the holding temperature is lower than 415 ° C., the target performance cannot be maintained because the amount of the Δ phase is large and the slidability is reduced. Also, from No.19 to No.20,
Be lowered to T ₁ temperature, the T ₂ temperature 480 ° C. greater than alloyed rises, powdering resistance is lowered, the target level of performance is found to be not be ensured.

[0082] From these results, according to the present invention, unlike the case of Example 2, also raise the T ₂ temperature, Ra is not significantly reduced, relative decrease in Ra, raising the T ₁ temperature It has been found that it is effective to raise the T ₁ temperature in order to secure a suitable slidability, in order to reduce the necessary haze and decrease the surface roughness, and thereafter, to ensure a suitable powdering resistance is lowered T ₂ temperature, it can be seen that it is necessary to ensure proper alloying degree.

[0083]

[Table 6]

(Example 4) In order to clarify the effect of the intruding material temperature on the effect of Al in the plating bath, the effective Al concentration in the bath and the intrusion were investigated using an ultra-low carbon IF steel having a thickness of 0.70 mm. For powdering resistance and sliding property when the material temperature is changed,
An alloyed hot-dip galvanized steel sheet was prepared and examined in a test plating bath. The basis weight at that time was 50 g / m ^2, and the heat pattern of the alloying treatment was T ₁ temperature = 540 ° C. (heating rate =
40 ° C./sec), T ₂ temperature = 430 ° C. (cooling rate = 15 ° C./sec), and a series of alloying treatment times were fixed at 20 seconds.

After the alloying treatment, the steel sheet was subjected to 0.8% temper rolling with a dull roll (surface roughness: Ra = 0.8 μm), and then subjected to powdering resistance (under the same conditions as in Example 1). (Fig. 2), slidability (Fig. 3), ζ phase amount (Fig. 1), and surface roughness were investigated. The results of the survey are shown in Table 7.

From Nos. 4 to 38, it can be seen that when the effective Al concentration of the bath is 0.08 to 0.13%, good workability in which both powdering resistance and slidability are compatible can be obtained. It is also found that the temperature of the intruding material at that time needs to be 450-500 ° C. In particular, from No. 16-19 and No. 22-25,
It can be seen that when the effective Al concentration is in the range of 0.10 to 0.11%, optimum workability in which both powdering resistance and slidability are suitable is obtained.

From Nos. 1 and 2, when the effective Al concentration is low, the alloying degree cannot be kept in an appropriate range even if the intruding material temperature is increased, and the powdering resistance may be inferior. I understand. From No.39 and No.40, when the effective Al concentration is high, the alloying process is delayed even if the intruding material temperature is lowered, and it becomes difficult to secure an appropriate degree of alloying. However, it can be seen that the surface roughness becomes large and slidability cannot be ensured.

Further, for example, No. 7, No. 13, No. 19
, No. 24, effective A even with almost the same degree of alloying
l As the concentration decreases, the surface roughness decreases,
Further, it can be seen that it is difficult to secure proper slidability because the amount of ζ phase increases and the slidability decreases.
This is because, when immersed in the plating bath, the amount of Al-enriched material that delays alloying is reduced, so that the diffusion of Fe is accelerated. It is presumed that this indicates that the amount of the ζ phase increases until the temperature reaches.

[0089]

[Table 7]

(Example 5) In order to clarify the effects of the Al content in the plating bath and the alloying temperature, an extremely low carbon IF having a thickness of 0.70 mm was used.
Using steel as a raw material, alloyed hot-dip galvanized steel sheets were prepared and examined in a test plating bath for powdering resistance and sliding properties when the effective Al content in the bath and the intruding material temperature were varied.

At that time, the basis weight was 50 g / m ^2, and the temperature of the intruding material was fixed at 480 ° C. GA of heat pattern changes the T ₂ temperature (heating rate = 40 ° C. / sec), T ₂ temperature = 430 ° C.
(Cooling rate = 15 ° C./sec), and a series of alloying treatment times were fixed at 20 seconds.

After the alloying treatment, the alloy was subjected to a 0.8% temper rolling with a dull roll (surface roughness: Ra = 0.8 μm), and then subjected to a workability test. The workability evaluation at that time is shown in Table 4.
In accordance with the overall workability evaluation evaluation. The results are shown in Table 7 and FIG.

According to Table 7, the effective Al concentration was 0.08 to 0.13%, T ₁
It can be seen that in the temperature range of 510 to 560 ° C., the target levels of powdering resistance and slidability can be secured. However, even with the effective Al concentration in that range, 840 + 1000 × [Al] ≤ [T ₁ ] + [T ₂ ]
[T ₁ ] +, which is the material temperature at the time of GA conversion, within the range of ≦ 890 + 1000 × [Al]
It can be seen that powdering resistance and slidability cannot be ensured unless [T ₂ ] is contained.

The effective Al content is 0.10 to 0.11% T ₁ temperature = 52
0 to 540 ° C and 850 + 1000 × [Al] ≤ [T ₁ ] + [T ₂ ] ≤ 870
Within the range of + 1000 × [Al], it can be seen that the best optimum regions exist for both the powdering resistance and the slidability.

(Example 6) For the purpose of reconfirming whether the effects of the present invention were actually realized in a continuous annealing line, a very low carbon IF steel having a sheet thickness of 0.65 to 0.85 mm was used. A study was conducted on the degree of alloying, the amount of phase, the surface roughness, and the powdering resistance and slidability when various alloying treatment conditions were changed.

The basis weight of the plating at that time was controlled to 45 to 50 g / m ² , and the effective Al concentration in the plating bath, the intruding material temperature, the alloying temperature
([T ₁ ] and [T ₂ ]) were varied. The temper rolling after the alloying treatment was performed with dull rolls (surface roughness: Ra = 1.2 μm). Table 8 shows the results.

From No. 1, No. 24 and No. 25, even if various alloying treatment conditions are changed, even under severe lubrication conditions, which is the expected effect of the present invention, it is necessary to ensure good workability. Indicates that 0.08 to 0.13 wt% is required as the effective Al concentration in the plating bath. In addition, the intruding material temperature is No. 2,
From No.23, it can be seen that the intruding material temperature also needs to be 450-500 ° C. Moreover, alloying conditions also, No.10, from No.15, [T _1] is 510 to 560 ° C. is, [T _2] is No.4, from No.21,
It turns out that 415-480 ° C is required.

For example, from [No. 6 and No. 20], even if [T ₁ ] and [T ₂ ] are within the proper range, [T ₁ ] + [T ₂ ] is 840
It can also be seen that the expected workability level cannot be ensured unless the range is between + 1000 × [Al] and 890 + 1000 × [Al].

Further, within the operating condition range in which the effects expected by the present invention are expected, ζ phase ([(value]), surface roughness ([Ra]), and [Ra] × It can be seen that [ζvalue] can be secured within the optimal range, and the expected results can be obtained.

[0100]

[Table 8]

[0101]

According to the present invention, when an alloyed hot-dip galvanized steel sheet is applied to a steel sheet for exterior of an automobile body, the steel sheet is cleaned with a generally used cleaning oil having poor lubricity to clean the steel sheet surface. Also in the press forming of the present invention, it is possible to secure good and stable press formability without performing any special post-treatment and as-alloyed hot-dip coated steel sheet. The effect of improving the workability of the alloyed hot-dip galvanized steel sheet manufactured according to the present invention is extremely remarkable, and the industrial value thereof is extremely large in the industry where improvement in productivity and stabilization of product performance are required. is there.

The alloyed hot-dip galvanized steel sheet of the present invention can be used in a conventional state even when lubricating is imparted by lubrication by post-treatment after lubrication, as well as when lubricating rust preventive oil is applied. The alloyed hot-dip galvanized steel sheet has good and stable workability as compared with the case where similar processing is performed.

[Brief description of the drawings]

FIG. 1 is a graph illustrating a method of measuring a ζ phase amount by an electrolytic potential method.

FIG. 2 is an explanatory diagram of cylindrical drawing processing conditions when evaluating powdering resistance by a plating peeling amount by a cylindrical drawing.

FIG. 3 is an explanatory diagram of a flat plate pull-out test condition when slidability is evaluated by a friction coefficient by a flat plate pull-out test.

FIG. 4 is a graph illustrating an optimal region of Δ phase amount and surface roughness: Ra with respect to workability capable of achieving both powdering resistance and slidability.

FIG. 5 is a graph illustrating an optimum area of an effective Al concentration and a maximum attained sheet temperature in a GA heat pattern with respect to workability capable of achieving both powdering resistance and slidability.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Jiro Iwase, 3F, Oaza Hikari, Kashima City, Ibaraki Prefecture Sumitomo Metal Industries, Ltd. Inside Kashima Works, Ltd. (72) Inventor Koichi Nishio 3, 3 Oaza Hikari, Kashima City, Ibaraki Prefecture 4K027 AA02 AA22 AB02 AB28 AB36 AB42 AC73 AE03 AE12 AE18 AE25 AE27

Claims (2)

[Claims] 1. Fe content: 8 to 13% by weight, surface roughness after plating is center line roughness (cutoff value = 0.8 mm): [Ra],
0.5 μm or more, 1.4 μm or less, ζ phase amount: alloying melting in which [ζ] is 10 g / m ² or less and [Ra] (μm) × [ζ value] (g / m ² ) ≤ 5.0 An alloyed hot-dip galvanized steel sheet with excellent workability characterized by having a galvanized film. 2. The effective Al concentration in the plating bath: [Al] is 0.08 or more.
In hot-dip galvanizing bath of 0.13wt%, plate temperature 450 ~ 50
The steel sheet is immersed at 0 ° C, and immediately after the immersion plating, the alloying heating is performed. At this time, the steel sheet is heated once to a maximum temperature of 510 to 560 ° C: [T ₁ ], and then immediately heated to 415 ° C at the sheet temperature.
Alloying holding temperature of 480480 ° C .: cooling to [T ₂ ], holding in this temperature range, and cooling again,
A method for producing a galvannealed steel sheet having excellent workability, wherein [T ₁ ], [T ₂ ] and [Al] are set so as to satisfy the following formula. 840 + 1000 × [Al] ≦ [T ₁ ] + [T ₂ ] ≦ 890 + 1000 × [Al]