JPH04341550A - Production of galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To improve the finishing appearance in the coating of a steel sheet by heating a plated steel sheet to a specified temp., thereafter heating it to a specified temp. as unalloyed and next executing rapid cooling at a specified cooling rate. CONSTITUTION:A steel sheet is plated by a galvanizing bath constituted of, by weight, 0.04 to O.12% Al and the balance Zn with inevitable impurities, and after the regulation of the coating weight, it is subjected to allaying treatment under heating by an allaying furnace. At this time, after the regulation of the coating weight, allaying under heating in a primary stage is executed by an allaying furnace. In this stage, it is heated at the sheet temp. of 500 to 600 deg.C. After that, by alloying under heating in a secondary stage as unalloyed, this steel sheet is heated in the sheet temp. range of 510 to 470 deg.C to complete the allaying. Next, the steel sheet is rapidly cooled at >=30 deg.C/S cooling rate. The coating weight is regulated to 45 to 90g/m<2>, the thickness of a xsi phase on the surface of the plated layer to <=0.5mum and the thickness of a gamma phase on the boundary of the iron base to <=0.7mum. In this way, the product of quality can stably be manufactured at low cost in high efficiency.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an alloyed hot-dip galvanized steel sheet. Specifically, the present invention is a method for producing an alloyed hot-dip galvanized steel sheet that has excellent peeling resistance of the plating layer during molding and excellent electrodeposition coating finish after molding.

0002

BACKGROUND OF THE INVENTION Alloyed hot-dip galvanized steel sheets are produced by heating after hot-dip plating to diffuse iron in the base steel sheet into the plating layer, thereby forming an iron-zinc alloy layer. Compared to general galvanized steel sheets, it has superior coating corrosion resistance and weldability.
Widely used in automobiles and home appliances. Recently, there has been an increasing demand for paint corrosion resistance, and the use of alloyed hot-dip galvanized steel sheets is rapidly increasing. Moreover, as the number of areas in which they are used expands, they are being applied to areas that undergo more severe processing, and there is a strong desire for improved workability such as pressability. In addition, electrodeposition coating and powder coating have recently become common practice, but since the finish quality of these coatings is poor, there is a strong desire for improvement.

[0003] In order to improve the peeling resistance during molding and the coating finish after molding, it is necessary to improve the moldability of the plating layer. In other words, since alloyed hot-dip galvanized steel sheets are made of hard and brittle intermetallic compounds, the plating layer may lift or peel off during forming, and if it peels off, it transfers or deposits on the die or steel sheet during forming. , resulting in quality defects such as press scratches and mold scratches. In addition, if the plating layer lifts up, the reaction products generated in this area during the degreasing and chemical conversion treatment processes will generate moisture due to dehydration reaction during the curing process during baking of electrodeposition coating and powder coating, so "bumps" will occur.
This causes uneven paint defects called . There is a strong desire to solve the above-mentioned problems associated with forming, and there is a strong desire to develop an alloyed galvanized steel sheet with good pressability and paint finish.

0004

[Problems to be Solved by the Invention] Since alloyed hot-dip galvanized steel sheets are manufactured by thermal diffusion treatment, there is a hard and brittle Γ phase (Γ phase or Γ1 phase) at the interface with the iron substrate.
(hereinafter collectively referred to as the Γ phase) is generated. Since the peeling resistance deteriorates as the Γ phase thickness increases, there is a problem that the plating peels off during compression processing such as bending back processing and shrinkage flange processing. On the other hand, in areas where sliding deformation occurs due to high surface pressure, such as drawing beads, the plating layer tends to peel off due to frictional shearing force, but in this case, the hardness of the plating surface layer is important, and the soft ζ phase and As more phases remain, frictional resistance increases and plating peeling increases. If the plating peels off during molding, the molding quality and molding workability will be significantly impaired.
It is important to improve the peeling resistance of the plating layer by suppressing the phase and ζ phase and creating a plating structure mainly composed of the δ1 phase.

[0005] Poor paint finish due to particles, which is a problem in electrodeposition coating and powder coating, occurs when cracks occur at the interface between the plating layer and the iron base during molding, and the plating layer lifts up without peeling off. The reaction product generated in this area during the pre-painting treatment process generates moisture through a dehydration reaction during the baking and curing process of the paint film, creating water vapor gas bubbles within the paint film, resulting in a "bumpy" appearance. It is estimated that this will cause paint film defects. Therefore, in this case as well, it is important to improve peeling resistance and crack propagation resistance during molding. As mentioned above, in order to have excellent moldability and improve the paint finish after molding, the Γ phase thickness is 0.7 m or less, the ζ
It is necessary that the phase thickness is 0.5 μm or less. In order to improve the peeling resistance and crack propagation resistance of the coating layer, the present invention provides a method for manufacturing an alloyed hot-dip galvanized steel sheet in which the amount of Γ phase and ζ phase produced is suppressed as much as possible, and the coating structure is mainly composed of δ1 phase. This is what we provide.

[0006]

[Means for solving the problems] The present invention provides Al0.04~
After plating with a galvanizing bath consisting of 0.12 wt%, residual zinc and unavoidable impurities, and then controlling the basis weight, heating in an alloying treatment furnace is performed.As the first step, 500% The plate is heated to a temperature of 600°C or higher between 510°C and 600°C, and in the unalloyed state immediately before alloying is completed, it is cooled to a temperature of 470°C or higher at a temperature of 510°C or lower for alloying. Complete it and then 30
This is a method for producing an alloyed hot-dip galvanized steel sheet, which is characterized by rapidly cooling to a temperature range of 400°C or less at a cooling rate of 0°C/s or more. In addition, during the temperature raising process of the alloying furnace, heat treatment must be carried out to the maximum temperature mentioned above within 7 seconds after immersion in the bath, and during the alloying process of the alloying furnace, the heating time must be 510°C or lower and 470°C or higher for 3 times. Alloying should be achieved within seconds, the basis weight should be 45 to 90 g/m2, the Γ phase thickness should be 0.7 μm or less, and the ζ phase thickness should be 0.5 μm or less, and iron should be added to the upper layer of the alloyed galvanized steel sheet. It is preferable to perform electroplating consisting mainly of zinc and the remainder zinc.

[0007]

[Action] 0.04 to 0.12w of Al in the galvanizing bath
The reason for using t% (expressed here as the effective Al concentration in the bath, the same applies hereinafter) is that in this composition range, an Al~Fe~Zn alloy layer is locally formed on the steel sheet surface and is short-lived. but,
It has the effect of suppressing the reaction between the iron base and zinc. This suppressing effect has the effect of suppressing the formation of the ζ phase that is likely to be formed in alloying reactions in the low temperature range. That is, when the Al concentration is lower than 0.04 wt%, the amount of alloy layer mainly composed of ζ phase increases in the plating bath, and even if the present invention is applied, the ζ phase remains thick on the plating surface layer, or Raising the temperature or lengthening the heating time until the phase disappears is ineffective because the Γ phase grows thicker. Further, since a large amount of Fe elutes into the bath, dross generation increases, which impairs plating quality, which is not preferable.

On the other hand, when Al exceeds 0.12 wt%,
Since a ternary alloy layer with a high Al concentration is generated over the entire surface of the steel sheet in the plating bath, the alloying reaction is significantly suppressed. For this reason, the alloying heat treatment time increases significantly, requiring an extremely long alloying furnace, resulting in excessive equipment costs, and in a normal alloying furnace, the threading speed is reduced and the heating time is increased. This is not practical as it would significantly reduce productivity. In addition, if the alloying speed is increased by increasing the temperature, it will be necessary to heat the alloy to a temperature higher than 600°C, so there is a problem in that excessive heating capacity is required, and the surface layer of the plating may burn due to the high temperature. ,
There is a problem that the plating layer partially drips and the plating thickness becomes non-uniform, and the appearance of the plating is extremely deteriorated, resulting in a decrease in commercial value. Moreover, the thickness of the Γ phase becomes excessive in the subsequent cooling process, so there is a problem that the application effect of the present invention is lost.

After plating in the above-mentioned zinc plating bath, the area weight is controlled by a generally known method such as a gas wiping method, and a heat alloying treatment is performed. In this case, the reason why the alloying temperature in the heating process is set to 500℃ or more and 600℃ or less is that 500℃
At lower temperatures, the ζ phase tends to form, and the plating structure consisting mainly of the δ1 phase, which is the object of the present invention, cannot be ensured under any alloying conditions. Furthermore, if the temperature is higher than 600°C, there is a problem that the zinc vapor on the surface of the plating layer will burn and the appearance of the plating will deteriorate, and the fluidity of zinc will increase, causing the unalloyed plating layer to flow and sag. However, there are problems in that the plating thickness becomes non-uniform and the appearance of the plating is impaired due to the appearance of dripping patterns. Additionally, if the alloying process is completed at a temperature higher than 600°C, in the subsequent cooling process,
No matter what heating conditions or industrially possible cooling rates are applied, alloying cannot be achieved under conditions where the Γ phase thickness is maintained at 0.7 μm or less, which is unsuitable.

By the way, it is not desirable to achieve complete alloying in the alloying treatment carried out in the above temperature range. In other words, when alloying heat treatment is performed in this high temperature range, cooling is performed successively with the η phase remaining on the plating surface.
It is important to achieve alloying in a relatively low temperature range of below 10°C and above 470°C. In this case, it is generally preferable that the η phase remaining on the plating surface layer be 5 μm or less. If it is more than 5 μm, it is disadvantageous because the ζ phase tends to grow during the subsequent alloying heat treatment in a low temperature range.

[0011] If the η phase is less than 5 μm, the first stage 5
In heat alloying treatment in a relatively high temperature range of 00 to 600°C, the Fe concentration in the η phase becomes sufficiently high, making it difficult to generate ζ phase in subsequent heat alloying treatment in a low temperature range. There is. Furthermore, when the η phase is present in the plating surface layer, the Γ phase is difficult to grow even in such a relatively high temperature range. This is because zinc reacts with Fe at the interface of the iron substrate through grain boundary diffusion of the δ1 phase, which grows primarily in this temperature range, making it difficult to form a Г phase with a high Fe concentration, and This is mainly due to the fact that the phase is easy to grow.

By the way, the reason why there is an optimal temperature range in alloying treatment at low temperatures is that, although alloying at a temperature higher than 510° C. can suppress the formation of ζ phase, in the cooling process after alloying is completed, for example, Even when industrially used rapid cooling methods such as air-water cooling are applied, the growth of the Γ phase cannot be suppressed and a Γ phase thickness of 0.7 μm or less cannot be ensured, which is not preferable.

Furthermore, a temperature lower than 470° C. is not preferable because the η phase remaining in the plating surface layer forms a ζ phase during the alloying process. If the alloying heat treatment is performed until the ζ phase once generated disappears, the Г phase will grow at a fairly rapid rate, making it impossible to secure the plating structure of the present invention. Furthermore, the lower the temperature, the longer the heating time required to complete alloying, resulting in a reduction in productivity.

Incidentally, the alloying time required to alloy the η phase of 5 μm or less varies depending on the alloying temperature, but is generally within 3 seconds. In other words, in the second stage of heating, the heating time in the high temperature range of 500 to 600 °C in the previous stage must be ensured so that heating in the intermediate temperature range of 470 to 510 °C is completed within 3 seconds. , it is possible to perform an alloying reaction that suppresses the ζ and Г phases. In this case, the higher the heating temperature in the high temperature range, the more advantageous the growth of the δ1 phase will be promoted. In any case, the η phase becomes 5μ due to heating in the high temperature range.
Heating to below m is essential for the subsequent heating in the intermediate temperature range in the second stage.

By the way, after controlling the basis weight, the time required to heat the steel sheet to a temperature range of 600°C from 500°C to 600°C during the heating alloying treatment is 7 days after the steel plate is immersed in the plating bath.
Preferably within seconds. When a steel plate is immersed in a plating bath, it preferentially reacts with a small amount of Al added to the bath to form an Al-Fe-Zn alloy layer with a high Al concentration (herein referred to as an Al barrier layer). This has the effect of suppressing the reaction between the base iron and Zn. This suppression time is extremely short, within 7 seconds after immersion, so within this time 500
It is desirable to heat to a temperature range of 600°C or higher. If it is longer than 7 seconds, the reaction suppression time will pass and the alloying reaction will start in a low temperature range of 500° C. or lower. If the temperature is below 500°C, ζ phase is likely to be generated, and the effects of the present invention cannot be obtained. It is sufficient to heat to 500° C. or higher within 7 seconds, which is the alloying reaction suppression time by the Al barrier layer, but the shorter the heating temperature rise time, the more pronounced the effect of suppressing the ζ phase.

The feature of the present invention is to utilize the Al barrier layer formed on the surface of the steel plate by a trace amount of Al added to the bath.
It substantially suppresses the alloying reaction in the low temperature range below 500°C where the ζ layer is likely to be formed, and is rapidly heated to 500°C or above within the alloying suppression time of the Al barrier layer, where the δ1 phase is likely to precipitate and form. The purpose is to carry out a heating diffusion reaction at temperatures above ℃.

In applying the present invention, external heating methods such as gas burners and electric heaters, and internal heating methods such as induction heating and resistance heating can be used as heating methods. It is desirable to have a method that allows for easy rapid heating and for easily controlling heat treatment by following changes in the heat capacity rate such as changes in the size and speed of the steel plate. That is, when comprehensively evaluating heating controllability, responsiveness, temperature uniformity, rapid heating ability, etc., an induction heating method is more advantageously applicable than, for example, a gas heating method.

By the way, when applying the present invention, there is no particular restriction on the basis weight range, but 45 g/m2 or more and 9
It is more advantageous to apply it below 0 g/m2. In other words, if the basis weight is less than 45g/m2,
Even without applying the present invention, plated steel sheets with good quality can be manufactured. Moreover, even if the present invention is applied to a plated steel sheet thicker than 90 g/m2, the ζ phase thickness is 0.5 μm or less and Γ1
It is difficult to ensure a phase thickness of 0.7 μm or less, which is not effective.

By the way, better quality can be ensured by applying an electroplating layer consisting mainly of iron and the remainder zinc on the upper layer of the alloyed hot-dip galvanized steel sheet produced by the above method. In other words, zinc-based alloy plating mainly composed of iron is
Due to its high hardness, it is easy to ensure lubricity during molding. For this reason, for parts that require severe molding, it is common to apply such a hard upper layer plating. For this reason. It is desired that the plating layer of the alloyed hot-dip galvanized steel sheet serving as the lower layer has extremely good peeling resistance. especially,
Since the lubricity is improved by the upper layer plating, it becomes easier to shrink and deform, making it easier for cracks to occur in the plating layer.
This tends to cause a problem in which the finish quality of the coating deteriorates due to a decrease in peeling resistance and the resulting lifting of the plating layer.

[0020] By the method described above, the ζ phase thickness is 0.5 μm or less,
The peeling resistance of the plating layer is significantly improved by applying a zinc-based alloy plating mainly made of iron by electroplating to the surface of the plating layer of an alloyed hot-dip galvanized steel sheet with a phase thickness of 0.7 μm or less. However, the peeling of the plating during molding and the appearance of the painted finish are extremely good. Note that the upper layer plating may be of any type for improving lubricity, and there is no need to particularly limit the type of the plating layer. For example, in addition to iron, Ni, Co,
It is also effective when applying zinc-based alloy plating or zinc-based composite plating containing P, B, etc. alone or in combination of two or more. In addition, the basis weight of the upper layer plating is generally 1.
It is effective to carry out the treatment within the range of ~10 g/m2. 1
If it is less than 10 g/m2, the lubricating effect cannot be obtained, and if it is more than 10 g/m2, the lubricating effect is saturated, which is not preferable because it is economically disadvantageous.

[0021]

[Examples] Examples of the present invention are summarized in Table 1 together with comparative examples. As shown by the results, it is clear that by applying the present invention, the ζ phase and the Г phase can be suppressed, and an alloyed hot-dip galvanized steel sheet with good peeling resistance and painted appearance can be produced.

[0022]

[Table 1] Note 1: Plating bath temperature 460°C (Pb0.08wt%, F
e0.03wt%) Note 2: Steel plate: plate thickness 0.7 mm, plate width 1050 mm Note 3:
Threading speed: 100 mpm Note 4: Induction heating with a frequency of 10 kHz is used for heating. A gas burner heating method was used for heat-retaining alloying. Note 5: Cooling after completion of alloying is done by air-water cooling method to 30 to 8
It was cooled to 380°C at a cooling rate in the range of 0°C/sec, and then cooled to around 60°C by air cooling and water cooling. Note 6: The upper layer was plated by electroplating in the same line after alloying and cooling. The plating composition is an alloy plating (4 g/m2) of 85 wt% Fe and the balance Zn. Note 7: Peeling resistance of the plating layer was evaluated by performing a tape test on the inner bent part of the 1T bending test, and evaluating the peeling width and blackness of the plating layer that was peeled off and attached to the tape on a five-point scale. Peeling resistance rating: (Good) 1-2-3-4-5 (Poor) Note 8:
The painted appearance is achieved by cationic electrodeposition coating after cylindrical drawing.
The uneven state of the surface of the coating film was observed using a stereoscopic microscope with a magnification of 10 times, and evaluated on a five-point scale. Cylindrical drawing conditions: Steel plate size (100 x 70) Punch diameter 50φ Die shoulder radius 50mm Drawing height 40m
Electrodeposition coating: Degreasing treatment commonly performed in the automotive field,
A chemical conversion treatment was performed and cationic electrodeposition coating was performed at 280V for 2 minutes. Note 9: The phase thickness is determined by cross-sectional polishing with an inclination angle of 30 degrees and a magnification of 2.
The measurement was performed by observing with an optical microscope at a magnification of 1,000 times. Note 10: The ζ phase thickness is determined by the commonly used electrolytic stripping method.
The electrical quantity of the ζ-phase potential was converted into thickness. Note 11: The presence or absence of the η phase was measured using a reflectance meter and an X-ray diffraction device installed between the high-temperature heating zone and the low-temperature heating zone in the furnace. That is, when the η phase remains, it has a metallic luster, but when the alloy layer grows to the plating surface, the degree of blackening increases and the reflectance decreases. For reference, X
The determination was made by also measuring the peak intensity of diffracted X-rays corresponding to the η phase using a line diffractometer.

[0023]

[Effects of the Invention] By applying the present invention, the formation of Г phase and ζ phase in alloyed hot-dip galvanized steel sheets is suppressed,
It becomes possible to have a plating layer structure consisting mainly of the δ1 phase,
It is possible to improve the peeling resistance of the plating layer during molding, and to reliably improve the appearance and finish of the coating when performing electrodeposition coating, powder coating, high solid coating, etc. after molding. The present invention eliminates the conventional quality weaknesses of alloyed hot-dip galvanized steel sheets, enables stable production of high-quality products at low cost and with high efficiency, and is extremely useful industrially.

Claims (5)

[Claims] [Claim 1] Plating is performed in a galvanizing bath consisting of Al: 0.04 to 0.12 wt%, residual Zn and unavoidable impurities, and after controlling the basis weight, heating alloying treatment is performed in an alloying furnace. After completing the basis weight control, in the first stage heating alloying process in the alloying furnace,
The plate is heated to a temperature of 600°C or less in the above manner, and then, in the second stage of heat alloying treatment, the alloying is completed by heating the plate in a temperature range of 510°C or lower and 470°C or higher while remaining in an unalloyed state. , followed by rapid cooling at a cooling rate of 30° C./s or more. 2. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 1, wherein the steel sheet is heated to the maximum temperature within 7 seconds after being immersed in the bath during the heating process of the alloying furnace. 3. In the second stage alloying process of the alloying treatment furnace, the heating time is 3 times at 510°C or lower and 470°C or higher.
The method for manufacturing an alloyed hot-dip galvanized steel sheet according to claim 1 or 2, wherein the manufacturing time is within seconds. 4. According to any one of claims 1 to 3, the basis weight is 45 to 90 g/m2, the ζ phase thickness on the surface of the plating layer is 0.5 μm or less, and the Γ phase thickness at the interface with the iron substrate is 0.7 μm or less. A method for producing an alloyed hot-dip galvanized steel sheet as described in . [Claim 5] The upper layer of the alloyed galvanized steel sheet is electroplated mainly with iron and with the balance being zinc.
4. The method for producing an alloyed hot-dip galvanized steel sheet according to any one of 4.