JP2754596B2 - Alloyed hot-dip galvanized steel sheet excellent in workability, paintability, and corrosion resistance, and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a Fe—Zn alloy plated steel sheet used for automobiles, home electric appliances, building materials, and the like.

[Prior art] Galvanized steel sheets are widely used as materials that are inexpensive and have excellent corrosion resistance and strength. Among them, the inner and outer plates of automobiles are made of a large amount of materials that take into account not only corrosion resistance but also workability and paintability. Have been done. In general, there are electroplating and hot-dip galvanizing methods for mass production of galvanized steel sheets.Electroplating processes at low temperatures, so there is no phase change due to heat effects and component control is easy. To increase the volume, the processing time must be increased. On the contrary,
In the hot-dip plating method, the amount of coating can be easily increased without increasing the processing time, but it requires some measures to control the composition of the plating film and the phase formed. In recent years, steel sheets for automobiles have been required to have a higher level of corrosion resistance due to measures against salt damage, etc., and in response to this, the coating composition can be easily secured and the economical hot-dip galvanizing is mainly used. There is a demand for a plated steel sheet that performs well and phase control and ensures high corrosion resistance while also having good workability and paintability.

The most problematic in workability is powdering resistance, and the problem in paintability is cratering resistance. Powdering is a phenomenon in which the plating film becomes powdery and falls off during press molding, and cratering is the unevenness that can be visually observed on the coating film in the electrodeposition coating process after the chemical conversion treatment is performed on the plating film. (Craters). In the former, a Г phase (Fe ₃ Zn ₁₀ , Fe ₂₀ to 28 wt%) with a high iron content is generated in the plating film, which is caused by the hardness and brittleness, and the latter is caused by unevenness of the plating film surface (surface shape, oxidation Film, plating film phase structure, etc.).

Conventionally, alloyed hot-dip galvanized steel sheets used for automobiles have an average iron content of all coating films after hot-dip coating.
Alloying treatment is performed until it reaches about 10 wt%, and Fe is diffused to the plating surface to improve corrosion resistance, especially after coating. That is, after the steel strip is continuously subjected to pretreatment (including heat treatment) to adjust the material, the steel strip is immersed in a plating bath in which zinc is melted, plated, and subsequently, the plated steel strip is placed in an alloying furnace. Raise the temperature rapidly from 500 ° C to 700 ° C and hold for a short time (10 to 30 seconds) to reduce the iron content of the plating film to 10%.
% Alloyed. However, the alloyed hot-dip galvanized steel sheet produced in this way is heated to a high temperature by rapid temperature rise, so that the iron content in the plating film tends to vary depending on the location, and the surface direction and depth direction of the plating film In both cases, the alloying becomes non-uniform,
Problems such as an increase in the iron content at the interface with the steel base to secure the iron content in the surface layer, the inevitable formation of a phase, and the occurrence of thermal stress in the plating film due to high-temperature treatment and rapid cooling. I have

On the other hand, a method has been proposed in which the alloying treatment is divided into two steps, a primary step and a secondary step. For example, Japanese Patent Publication No. 59-14541
In (1), in the temporary heating, rapid heating and high-temperature heating for remelting the Zn plating film is performed to obtain the smoothness of the plating film. In this heating, the iron content stays in the low range of 2.2 to 5.5 wt%. Therefore, according to the result of the temporary heating, the secondary heating is performed at a low temperature below the melting point of zinc, and the iron content is reduced to 6%. It should be within the range of ~ 13wt%.

On the other hand, there has also been proposed one in which the iron content of only the surface layer of the plating film is increased to improve the cratering resistance. For example, Japanese Patent Publication No. Sho 58-15554 proposes a plated steel sheet in which a corrosion-resistant metal layer is provided as an inner layer and a Fe-Zn alloy coating layer having a high iron content is provided thereon to improve the cationic electrodeposition coating property. In this proposal, an alloyed hot-dip galvanized layer in which an Fe—Zn alloy is formed by heat treatment after hot-dip galvanizing is disclosed as the inner corrosion-resistant metal layer.

[Problems to be Solved by the Invention] However, Japanese Patent Publication No. 59-14541 does not satisfy the cratering resistance. Regarding the cratering resistance, the iron content of the surface is insufficient, and the powdering resistance is also non-uniform since the alloying treatment is performed by rapid temperature rise and high temperature heating after hot dip galvanizing. It is inevitable to proceed, and as a result, a layer with poor workability grows. Further, in some cases, a portion that is not alloyed and a portion where alloying has progressed are mixed, and a phenomenon of so-called uneven burning is exhibited. As described above, since the primary heating is likely to be non-uniform, the secondary heating conditions based on the result of the primary heating become extremely complicated, and there are great difficulties in carrying out the actual operation.

In Japanese Patent Publication No. 58-15554, cratering resistance is improved because the iron concentration on the plating surface is dramatically increased, but alloying is completed by heat treatment after hot-dip galvanizing. Similar to -14541, there is a problem of non-uniform alloying. In addition, the iron concentration gradient in the plating film becomes large, and the Г phase grows at the interface with the steel base where the iron concentration becomes high. Further, thermal stress due to rapid thermal quenching is not preferable for powdering resistance.

As described above, efforts have been made to satisfy powdering resistance and durable cratering properties, but a hot-dip galvanized steel sheet satisfying both properties has not yet been obtained.

In order to solve this problem, the present invention has been made, and an object of the present invention is to provide a plated steel sheet which satisfies not only corrosion resistance but also powdering resistance and cratering resistance, and a method for producing the same.

[Means and Actions for Solving the Problems] Means for achieving this object are as follows: at least one surface of the steel sheet has an adhesion amount of 0.5 g / m ² or more and 10 g / m ² or less and an Mn content of 3
The adhesion amount and the outer layer of 0 wt% or more of Mn-Zn alloy 30 g / m ² to 9
Except for the boundary between the base steel having a thickness of 0.5μm at 0 g / m ² or less made of an inner layer portion of the Mn-Fe alloy Zn containing no Г phase, both layers portions thereof are mutually thermally diffused at the boundary An alloyed hot-dip galvanized steel sheet having excellent workability and paintability, characterized in that it has a plated film that forms an integrated structure and that has a uniform distribution of the alloy component content in the plane direction. The following is a method for producing the alloyed hot-dip galvanized steel sheet.

One of them is a method for producing an alloyed hot-dip galvanized steel sheet having excellent workability and coatability, which includes the following steps.

(A) A steel strip that has been subjected to a normal pretreatment is Al 0.05 wt% or more and 0.3 w
t% or less and Pb0.2Wt% step was immersed in a molten zinc plating bath subjected to 30 g / m ² or more 90 g / m ² or less of galvanized containing the following.

(B) A step of performing spangle refining while the plating film is in a molten state.

(C) After the plating film is solidified, a skin pass treatment is performed.
A step of smoothing the surface of the hot-dip galvanized film.

(D) Adhesion amount on one or both sides of this galvanized steel strip
A step of applying a Mn-Zn alloy plating of not less than 0.5 g / m ^{2 and not} more than 10 g / m ^{2 and not} less than 40 wt% of Mn.

(E) Heating the steel strip plated in the above process in a batch type annealing furnace maintained in a non-oxidizing or reducing atmosphere in an open coil state at a temperature in the range of 320 ° C or more and the melting point of zinc for 10 minutes to 50 hours. Process.

The other is a method for producing an alloyed hot-dip galvanized steel sheet having excellent workability and coatability, which includes the following steps.

(A) A steel strip that has been subjected to a normal pretreatment is Al 0.05 wt% or more and 0.3 w
t% or less and Pb0.2Wt% or less by immersion in molten zinc plating bath containing 30 g / m ² or more ~90g / m ² step of applying the following plating.

(B) plating film step of performing one or both surfaces Mn40wt% or more Mn-Zn alloy by spraying powder 0.5 g / m ² or more 10 g / m ² or less of the upper layer plating of the steel strip while it is molten.

(C) A step of smoothing the surface of the hot-dip galvanized film by performing a skin pass treatment after the plating layer is solidified.

(D) In a batch type annealing furnace in which the steel strip having the film smoothed in the above step is maintained in a non-oxidizing or reducing atmosphere, in an open coil state at a temperature within the range of 320 ° C or more and the melting point of zinc or less. Heating for minutes to 50 hours.

 The above means will be described in detail, including the operation thereof.

First, the steel strip for plating may be a cold-rolled steel strip or a hot-rolled steel strip, and may be subjected to an annealing treatment together with a surface adjustment as a normal pretreatment.

When the content of Mn in the outer layer portion of the plating film is 30 wt% or more, crater generation during electrodeposition is prevented. That is, the alloyed hot-dip galvanized steel sheet is subjected to a cation electrodeposition after a phosphate treatment is performed on the plated surface. Phosphate crystals are generated by the chemical conversion treatment. At this time, the plating surface
When only Zn is present, although hopeite _{[Zn 3 (PO 4) 2} 4H 2 O] and referred coarse needle-shaped crystals are produced, if Mn is present which is eluted replaces the part of Zn in the crystal the crystal Becomes dense. On the other hand, one of the causes of crater generation is considered to be local current concentration on the chemical conversion film defect, but if the crystal is dense, the number of defect portions is small and craters are unlikely to occur. Mn
This effect becomes remarkable when the Mn concentration becomes 30 wt% or more, and the generation of craters sharply decreases. Regarding the amount of the outer layer portion having a high Mn concentration, an adhesion amount of 0.5 g / m ² is necessary to sufficiently supply Mn over the entire plating surface, but 10 g / m ²
If it exceeds, the effect saturates, and on the contrary, it becomes disadvantageous in terms of cost.

The inner layer of the plating film does not need to have a higher Mn content than the outer layer, but when Mn is alloyed with Zn, the bare corrosion resistance is dramatically improved, and when Mn or Fe is alloyed, the corrosion resistance after painting is improved. Is improved. Therefore, in the case of Mn-Fe alloyed hot-dip galvanizing, corrosion resistance is ensured even if the amount of adhesion is smaller than that of zinc plating, but an amount of adhesion of 30 g / m ² or more is required to cope with high corrosion resistance. On the other hand, if the amount of adhesion is too large, not only the quality becomes excessive, but also a long time is required in the alloying treatment, and the productivity is reduced. Also, in general, when the plating film becomes thicker, the film may be broken or peeled off during processing, and in the case of an alloyed hot-dip galvanized steel sheet, powdering is likely to occur, so it is advisable to adhere over 90 g / m ² is not. The inner layer is in contact with the steel sheet and a Г phase may be formed at the boundary with the steel substrate. However, the plating film in which this Г phase is not detected has good powdering resistance. It is difficult to detect that the Г phase has not grown to a thickness of 0.5 μm or more. Regarding the powdering resistance, the most significant influence is given to the material of the inner layer that occupies most of the plating film, and the hard and brittle Mn—Fe alloyed hot-dip galvanized film in which no Г phase is detected does not cause powdering.

The inner layer and the outer layer described above are not clearly separated from each other, but will be described in detail later. However, the amount of adhesion between the two layers cannot be clearly expressed by numbers, and the amount of adhesion in the two-layer state is defined as an outer layer portion and an inner layer portion. If both layers thermally diffuse with each other and become an integrated structure, the composition is continuous and there is no break,
There is no extreme difference in mechanical properties between adjacent parts of electrochemical properties, and there is no weak point in workability or corrosion resistance.

In the case of a material such as a plated steel sheet, if there is a non-uniform part in the coating, it may be the starting point of corrosion or the starting point of breakage during processing, but if the coating is uniform in the plane direction, These starting points are eliminated, and the quality is extremely stable and the properties are maximized.

Depending on the usage, smoothness of appearance may be required only on one side. In this case, the other surface does not require an outer layer portion having a Mn content of 30% by weight or more, and only the hot-dip galvanized film is sufficient.

The operation of the manufacturing method will be described below. Usually, about 0.2% of Al is added to the hot-dip galvanizing bath for suppressing the reaction of the Fe-Zn alloy and smoothing the plated surface, and contains Pb for adjusting the spangle. Of these, Al has an alloying suppression effect, so that it is added in an amount of 0.05 wt% or more to prevent the Fe-Zn alloy after immersion in the hot-dip galvanizing bath from being partially and non-uniformly formed. It is important not to generate the Fe-Zn alloy non-uniformly in this step, and once it is made non-uniform, it cannot be corrected in a later step. If the addition amount of Al is too large and exceeds 0.3% by weight, the effect of suppressing alloying becomes excessive, and the subsequent alloying process takes too much time, which is industrially unsuitable. Although Pb does not directly participate in the alloying reaction, a large amount of Pb lowers the powdering resistance, so it must be limited to 0.2 wt% or less.

While this hot-dip galvanized film is in a molten state, the spangles are refined, and a smooth plated surface is obtained by performing skin pass treatment after the plated film is solidified.
The coverage of the upper plating applied thereafter is improved. As a result, the anti-cretering property can be efficiently improved, and the sharpness after painting can be obtained. When the skin pass is performed at an elongation ratio of 0.3% or more, the plated surface becomes smooth. However, when the elongation ratio is too large and exceeds 5%, the workability of a general thin steel sheet material may be affected.

The Mn-Zn alloy plating with a Mn content of 30 wt% or more is performed to ensure the creeping resistance and to diffuse Mn into the hot-dip galvanized layer previously applied in the subsequent heat treatment. That is, since Mn diffuses from the upper layer in the inner layer during heating, the amount of Fe diffusion from the steel substrate can be suppressed, and the opportunity for the generation of the Γ phase at the boundary of the steel substrate is reduced. At the same time, the inner layer is made of an Fe-Mn-Zn alloy, so that the corrosion resistance is greatly improved.

The upper layer alloy plating may be performed by any method such as electroplating, vapor deposition plating, and thermal spraying, as long as the method is not a method of processing at a temperature higher than the melting point of zinc. When this alloy plating process is performed by spraying alloy powder, if the preceding hot-dip galvanized layer is performed while it is in a molten state, the spangles can be miniaturized at the same time, and one step can be omitted.

Heating the coated steel strip after the two plating steps described above is performed in a non-oxidizing or reducing atmosphere to prevent oxidation of the surface and to make the conversion film crystals non-uniform during the chemical conversion treatment before painting. This is because the treatment is performed in a batch-type annealing furnace at a low temperature for a long time.
The reason why the heating is performed in the state of the open coil is to prevent unevenness in the alloying due to the uniform heating and also to prevent the plating surfaces from adhering to each other and generating defects. In the state of the tight coil, the temperature distribution becomes non-uniform, and a part having a high alloying rate and a part having a small alloying rate are partially formed. In particular, this unevenness occurs in the longitudinal direction of the steel strip, and it is difficult to obtain a high quality product. Heating is performed at low temperatures, but temperatures above 320 ° C are required. If the temperature is lower than 320 ° C., it takes too much time to obtain the Mn and Fe contents sufficient to secure the corrosion resistance after coating. When the temperature is higher than the melting point of zinc (419.5 ° C.), a portion where alloying proceeds rapidly appears, and the formation of a Γ phase at the boundary between the inner layer and the steel base cannot be ignored. Furthermore, the spacer inserted between the steel strips of the open coil may leave traces on the plating surface. FIG. 1 shows the conditions under which neither powdering nor craters occur in the above temperature range. The horizontal axis represents the heating time and the vertical axis represents the heating temperature. In the figure, a range surrounded by a line connecting points a, b, c, and d is a preferable condition range for practical operation in which powdering and craters do not occur. The time is up to the time coordinate, that is, 10 minutes or more and 50 hours or less.
When the heat treatment is performed under the above heating conditions, the plating film is not subjected to rapid high-temperature heating, so that the alloy component content becomes uniform even along the surface. Also, the Fe content is 3 wt% in the inner layer.
% To 20% by weight, and Mn in the outer layer portion is 30% or more, so that powdering during press molding does not occur and cratering during painting can be prevented. However, considering the variability of conditions that tend to occur during actual operation, it is particularly preferable that the heating temperature be from 320 ° C to 380 ° C.
The heating time is from 30 minutes to 10 hours. Further, by this heat treatment, the boundary between the upper layer portion and the inner layer portion has a continuous composition due to the diffusion of Mn-Zn to form an integrated structure.

[Example] For 17 alloyed hot-dip galvanized steel sheets using two types of steel sheets and changing the hot-dip galvanizing conditions, upper layer plating conditions, and alloying treatment conditions, the alloy component content in the plating film was determined. It was evaluated by conducting a powdering test and a cratering test. For comparison, six cases (comparative example) and three cases (conventional example) according to the prior art, which were processed under conditions outside the scope of the present invention, were similarly examined. Details of the conditions are as follows.

The steel sheet used is a cold-rolled steel sheet with a sheet pressure of 0.8 mm. It is a low-carbon Al killed (material A) for thin sheets that is widely used, and an ultra-low-carbon titanium-containing steel (for high-processing, which is said to be prone to powdering). Material B). Each component is shown in Table 1.

Hot-dip galvanizing is performed in a continuous plating facility equipped with a non-oxidizing furnace and a reduction heating furnace, the amount of coating is adjusted by a gas throttle device provided immediately after the plating bath, and then spangles are refined by mist spraying. After cooling, the plating layer was skin-passed at an elongation of 1.5% to smooth the surface.

Electroplating, plasma spraying, or powder spraying was used for the Mn-Zn alloy plating, and each was processed under the following conditions.

(1) Electroplating _{ZnSO 4 · 7H 2 O: 50~150g} /, MnSO 4 · H 2 O: 50~150g /, Na 3 C 6 H 5 O 7 · 2H 7 O: 180g /, bath temperature: 50 ° C. , Cathode current density: 30-50A / dm ² (2) Plasma spraying Plasma gas: Ar, spraying heat input: 20KW, spraying distance: 100mm, average powder particle size (Mn70%): about 5μ
m, powder supply rate: 5 g / min · dm ² (3) powder spray Average powder particle size (Mn 70%): about 5 μm, powder supply rate:
The content of the alloying component in the 3 g / min · dm ² plating film was determined for the outer layer portion of the plating and the inner layer portion of the plating by Grimm-glow discharge emission spectroscopy.

The powdering resistance was evaluated by bending the sheet 90 degrees at a radius of curvature of 2 mm, applying an adhesive tape to the inside of the bend, peeling the adhesive tape off, visually observing the state in which the powder adhered to the adhesive tape, and giving a score. The evaluation criteria are as follows: 1; no adhesion, 2; extremely slight adhesion, 3; slight adhesion, 4; slight adhesion, 5; considerable adhesion.

The anti-cretering resistance is achieved by subjecting the plated surface to a chemical conversion treatment.
Next, electrodeposition coating was performed, and the number of craters generated at this time was evaluated. For the chemical conversion treatment, a commercially available immersion type phosphate treatment agent was used. For the electrodeposition coating, a commercially available cationic electrodeposition coating was also used, but after mixing, the mixture was stirred for one week.
Electrodeposition was performed by instantaneously applying an electrodeposition voltage of 300 V at a distance between the electrodes of 4 cm.

Table 2 shows the processing conditions and results of each of these examples.

In the examples, there is no inferior powdering resistance even in the material B, and in the case of No. 6 of the critical adhesion amount and Example No. 17 close to the critical heating time, extremely slight powdering was observed. There is no problem in practical use. Regarding the resistance to creeping, no more than two small craters were found in No. 11 in which the Mn content of the outer layer was near the lower limit, but this was not a problem in practical use. Thus, in the examples, all the alloyed hot-dip galvanized steel sheets have both powdering resistance and cratering resistance. In addition, the content of the alloy component in the inner layer is 3wt% to 1wt.
It is within the range of 0 wt%, and sufficiently ensures corrosion resistance after painting.

On the other hand, in Comparative Examples treated under conditions outside the scope of the invention, No. 1 containing no Al in the hot dip galvanizing bath, No. 2 with excessive heating time, No. 3 with a large amount of Pb in the bath, adhesion amount Too many N
o.4, No.5 without outer layer, No.6 with too high heating temperature, etc. There is a problem in either powdering resistance or cratering resistance.

In the conventional example, No. 1 was alloyed only by rapid heating and high temperature heating, and there was a problem in both characteristics.Example No. 2 was alloyed and adjusted at low temperature after rapid heating and high temperature, and was resistant to cratering. No. 3 is alloyed by rapid temperature rise and high temperature heating and is provided with a plating layer having a high iron content thereon, which is inferior in powdering resistance. Thus, both characteristics are not satisfied at the same time.

In addition, in the width direction of the alloyed hot-dip galvanized coil (width 1800 mm) of Example No. 14, the alloy component content of the inner layer portion of the plating film was examined at intervals of 200 mm. This result is compared with the conventional example
FIG. 2 shows a comparison with No. 2. In FIG. 2, the horizontal axis represents the distance from the left end of the coil, and the vertical axis represents the content of Fe, which is an alloying component. In the example, the average value of the iron content is 8.2 wt%
And all the measurement points were distributed between 8.0 wt% and 8.4 wt%. On the other hand, in the conventional example, the average value was 8.3 wt%, and the measured value was in a range from 7.9 wt% to 8.9 wt%.

Also, as to whether or not there is a Γ phase at the bottom of the plating film, about two-thirds of the upper layer was removed from the samples subjected to the galvannealing treatment of Examples 1 to 17, and X-ray diffraction was performed. As a result, no Δ phase was detected in any of the samples.

[Effects of the Invention] According to the present invention, there is substantially no brittle Γ phase in the plating film, the surface layer has a high Mn content, and the inner layer is Fe-Mn-Zn.
Since it is an alloyed hot-dip galvanized steel sheet having an even coating in the plane direction, it has excellent powdering resistance and cratering resistance in addition to sufficient corrosion resistance. The industrial effect of the present invention in which such a surface-treated steel sheet can be manufactured without going through a complicated process is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between heat treatment conditions and proper characteristics for explaining the main part of the present invention, and FIG. 2 is a diagram showing the distribution of iron content in one embodiment of the present invention.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C23C 2/06 C23C 2/26-2/28 C23C 28/02

Claims (3)

(57) [Claims] Claims: 1. An adhesion amount of 0.5 g / m on at least one surface of a steel sheet.
^{2 to} 10 g / m ² and Mn content 30 wt% or more Mn-Zn alloy outer layer and adhesion amount 30 g / m ^{2 to} 90 g / m ² and thickness 0.5 μm
m, except for the boundary with the steel base of m, the inner layer of Mn-Fe alloyed Zn that does not contain a Г phase, and both layers form an integrated structure thermally diffused mutually at the boundary, and An alloyed hot-dip galvanized steel sheet having excellent workability and coatability, characterized by having a plating film having a uniform component content distribution in the plane direction. 2. A method for producing an alloyed hot-dip galvanized steel sheet having excellent workability and paintability, comprising the following steps. (A) A steel strip that has been subjected to a normal pretreatment is Al 0.05 wt% or more and 0.3 w
a step of immersing in a hot-dip galvanizing bath containing t% or less and Pb 0.2 wt% or less to apply a zinc plating of 30 g / m ² or more and 90 g / m ² or less. (B) A step of performing spangle refining while the plating film is in a molten state. (C) After the plating film is solidified, a skin pass treatment is performed.
A step of smoothing the surface of the hot-dip galvanized film. (D) Adhesion amount on one or both sides of this galvanized steel strip
A step of applying a Mn-Zn alloy plating of not less than 0.5 g / m ^{2 and not} more than 10 g / m ^{2 and not} less than 40 wt% of Mn. (E) Heating the steel strip plated in the above process in a batch type annealing furnace maintained in a non-oxidizing or reducing atmosphere in an open coil state at a temperature in the range of 320 ° C or more and the melting point of zinc for 10 minutes to 50 hours. Process. 3. A method for producing an alloyed hot-dip galvanized steel sheet having excellent workability and coatability, comprising the following steps. (A) A steel strip that has been subjected to a normal pretreatment is Al 0.05 wt% or more and 0.3 w
t% or less and Pb0.2Wt% or less by immersion in molten zinc plating bath containing 30 g / m ² or more ~90g / m ² step of applying the following plating. (B) plating film step of performing one or both surfaces Mn40wt% or more Mn-Zn alloy by spraying powder 0.5 g / m ² or more 10 g / m ² or less of the upper layer plating of the steel strip while it is molten. (C) A step of smoothing the surface of the hot-dip galvanized film by performing a skin pass treatment after the plating layer is solidified. (D) In a batch type annealing furnace in which the steel strip having the film smoothed in the above step is maintained in a non-oxidizing or reducing atmosphere, in an open coil state at a temperature within the range of 320 ° C or more and the melting point of zinc or less. Heating for minutes to 50 hours.