JP2727595B2 - Alloyed hot-dip galvanized steel sheet excellent in workability and paintability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is used for automobiles, home electric appliances, building materials and the like.
The present invention relates to an Fe—Zn alloy plated steel sheet.

[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 thermal effects and component control of the plating film is easy. In order to increase the amount of plating, the processing time must be increased. On the other hand, in the hot-dip plating method, the amount of adhesion can be easily increased without increasing the processing time, and a Fe—Zn alloy can be easily produced by performing a heat treatment after plating. However, some contrivance is required to control the composition of the plating film and the phase generated. 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 a plating film becomes powdery and falls off during press molding, and cratering is a phenomenon that can be visually observed on a coating film in an electrodeposition coating process performed after a chemical conversion treatment is performed on the plating film. (Craters).

The former is a phase with high iron content (Fe ₃ Zn ₁₀ ,
Fe20-28 wt%) is generated because it is hard and brittle, and the latter is caused by unevenness of the plating film surface (surface shape, oxide 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, the steel sheet is continuously subjected to pretreatment (including heat treatment) to adjust the material, and then immersed in a plating bath in which zinc is melted to perform plating. Subsequently, the plated steel sheet is heated to 500 ° C. in an alloying furnace. 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, and in addition, the iron concentration gradient in the plating film increases, and the iron content at the interface with the steel substrate increases to secure the iron content in the surface layer.
There is a problem that generation of a phase is unavoidable, and that thermal stress is generated in the plating film by high-temperature treatment and rapid cooling.

On the other hand, a method has been proposed in which the alloying treatment is divided into two steps, primary and secondary. For example, in Japanese Patent Publication No. 59-14541, in the primary heating, rapid heating and high temperature heating for remelting a Zn plating film is performed in order to obtain smoothness of the plating film. In this heating, the iron content is kept in a low range of 2.2 to 5.5 wt%. Therefore, according to the result of the primary heating, the secondary heating is performed at a low temperature equal to or lower than the melting point of zinc, and the iron content is reduced to 6%. It should be within the range of ~ 13wt%. It discloses that this method provides an alloyed hot-dip galvanized film having a smooth surface, excellent appearance, and no peeling or powdering during processing.

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. 58-15554 proposes a plated steel sheet in which a corrosion-resistant metal layer is used as an inner layer, and a Fe-Zn alloy coating layer having a high iron content is applied 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.

[Problem to be Solved by the Invention] However, Japanese Patent Publication No. 59-14541 does not satisfy the cratering resistance. Regarding cratering resistance, the iron content of the surface is insufficient,
Also, regarding the powdering resistance, the alloying treatment is performed by rapid temperature rise and high temperature heating after hot-dip galvanizing, so that the alloying reaction inevitably proceeds in a non-uniform manner, resulting in poor workability. Resulting in. 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, the iron concentration on the plating surface is dramatically increased, so that the cratering resistance is improved.
Since the alloying is completed by heat treatment after hot-dip galvanizing, there is a problem of non-uniformity of alloying as in Japanese Patent Publication No. 59-14541, and in addition, the iron concentration gradient in the plating film increases, The Γ phase grows at the interface with the steel substrate where the iron concentration is high. Further, thermal stress due to rapid thermal quenching is not preferable for powdering resistance.

As described above, efforts have been made to satisfy the powdering resistance and the cratering resistance, 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 method for producing a plated steel sheet that satisfies both powdering resistance and cratering resistance in addition to corrosion resistance.

[Means and Actions for Solving the Problems] A means for achieving this object is to provide at least one surface of a steel sheet with a first layer formed by hot-dip galvanizing and a Fe + Mn layer thereon.
A plating layer formed by heat-treating the second layer by plating, the plating layer having a surface layer of Fe + Mn plating of the second layer and an inner layer forming a boundary layer with a 0.5 μm-thick steel substrate. It is an alloyed hot-dip galvanized steel sheet having excellent workability and paintability, characterized by comprising a δ ₁ phase and a ζ phase except that the iron and manganese contents are uniformly distributed in the plane direction.

The following is a method for producing the galvannealed steel sheet.

 One method is as follows.

(B) A steel sheet that has been subjected to normal pretreatment is treated with Al
a step of dipping in a hot-dip galvanizing bath containing t% or less and Pb of 0.2 wt% or less to apply a 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) A step of subjecting one or both sides of the steel sheet to Fe-Mn plating of 0.5 g / m ² or more and 10 g / m ² or less.

(E) In a batch annealing furnace in which the plated steel sheet is kept 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.
A step of heating from 0 minutes to 50 hours.

 Another method is as follows.

After the hot dip galvanizing step of (a), while the plating film is in a molten state, Fe-Mn powder is sprayed on one or both surfaces of the steel sheet to perform upper plating of 0.5 g / m ² or more and 10 g / m ² or less. Process, and then (c), (d), (e)
This is a method for producing an alloyed hot-dip galvanized steel sheet including the step of:

The above means, including its operation, will be described in detail below.

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

When the surface layer of the plating film is made of an Fe-Mn alloy, crater generation during electrodeposition coating is prevented. That is, the alloyed hot-dip galvanized steel sheet is subjected to cationic electrodeposition coating after subjecting the plated surface to phosphate treatment. Phosphitelite [Zn] containing Fe is added to phosphate crystals generated by this chemical conversion treatment. ₂ Fe (P
_{O 4) 2 · 4H 2 O} ] and referred hopeite free of dense crystals and Fe in the granular [Zn ₃ (PO ₄₎ is the _{2 ·} 4H 2 O] and referred coarse needles. If Mn is present in the surface layer during the formation of these phosphate crystals, part of Zn in the whipite is replaced by Mn, and the crystal becomes denser. The presence of Fe also facilitates the formation of phosphophyllite. One of the causes of crater generation is considered to be local current concentration on the chemical conversion coating defect, but a dense crystal coating has few defects. Therefore, if the surface layer is an Fe-Mn alloy, craters are less likely to occur.

In the case of an alloyed hot-dip galvanized steel sheet, most of the corrosion resistance is determined by the coating weight and the iron and manganese contents in the film. By forming the Zn plating film into an Fe-Mn alloy, both the bare corrosion resistance and the corrosion resistance after painting are significantly improved.

In the present invention, it is important that the boundary layer between the surface layer and the inner layer has an integrated structure formed by thermal diffusion to each other.
The alloy component concentration of the surface layer and the inner layer is continuously changed by the heat-diffused integrated structure to form a Fe-Mn alloyed Zn plating layer,
The plating film does not have extremely different mechanical properties and electrochemical properties at adjacent portions, and is excellent in workability and corrosion resistance.

The inner layer that occupies the majority of the plating film does not contain a hard and brittle Γ phase, except for the boundary layer with the 0.5 μm thick steel substrate, and the distribution of the iron and manganese content is uniform in the plane direction. And powdering during processing can be prevented. The Γ phase is easily formed at the boundary between the inner layer and the steel base, but the plating film in which this Γ phase is not detected has good powdering resistance. It is difficult to detect that the Γ phase has not grown to a thickness of 0.5 μm or more.

The alloyed hot-dip galvanized steel sheet of the present invention may have no plating film on the other surface or may have another plating film depending on the application.

Hereinafter, the production method of the present invention will be described. 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 wt%, the effect of suppressing alloying becomes excessive,
The subsequent alloying process takes too much time and is industrially unsuitable. Pb does not directly participate in the alloying reaction, but a large amount of Pb
Should be limited to 0.2% by weight or less, since it reduces powdering resistance.

The inner layer has a coating weight of 30 g / m ² to 90 g / m ² which is suitable for high corrosion resistance. In this case, a high iron content is not required as in the surface layer, and the range of 5 wt% to 20 wt% is preferable. If the amount exceeds 90 g / m ² , not only the quality becomes excessive, but also the alloying treatment performed at a low temperature in a later step requires a long time, and lowers productivity. In general, when the plating film is thick, the film may be broken or peeled off during processing, and powdering is likely to occur in the case of an alloyed hot-dip galvanized steel sheet.

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 cratering resistance can be efficiently improved, and the sharpness after painting can also be improved. When the skin pass is performed at an elongation of 0.3% or more, the plated surface becomes smooth. However, when the elongation is too large and exceeds 5%, the workability of a general steel sheet for thin sheets may be affected.

The Fe-Mn alloy plating ensures cratering resistance, and in the subsequent heat treatment, diffuses Fe and Mn from the surface opposite to the steel base into the previously applied hot-dip galvanized layer, resulting in the inner layer of the plating film. The iron concentration gradient is kept small. The above-described alloy plating treatment method may be any method such as electroplating, vapor deposition plating, or thermal spraying, as long as the treatment is not performed at a temperature higher than the melting point of zinc. When performing this alloy plating process by spraying alloy powder,
If the above-mentioned hot-dip galvanized layer is performed in a molten state, the spangles are miniaturized at the same time, and one step can be omitted.

The surface layer needs to have a coating weight of 0.5 g / m ² to 10 g / m ² . If it is less than 0.5 g / m ² , it is not possible to sufficiently supply Fe over the entire plating surface. In addition, when the amount exceeds 10 g / m ² , the effect is saturated, and not only is the cost disadvantageous, but also red rust is easily generated in the corrosion resistance after coating.

The coated steel sheet that has undergone the two plating steps described above is subjected to a heat treatment. Performing the treatment in a non-oxidizing or reducing atmosphere prevents the surface from being oxidized and makes the conversion film crystals non-uniform during the chemical conversion treatment before painting. This is done in a batch-type annealing furnace, because it takes a long time at a low temperature.
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 sheet, and it is difficult to obtain a high quality product. Heating is performed at a low temperature, but requires a temperature of 320 ° C. or higher. If the temperature is lower than 320 ° C., it takes too much time to obtain a degree of alloying sufficient to secure corrosion resistance after painting. 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 cannot be ignored. Furthermore, the spacer inserted between the steel plates 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. Up to the time coordinate, ie
It will be 10 minutes or more and 50 hours or less. When heat treatment is performed under the above heating conditions, Fe diffuses from the steel substrate side and the surface plating side, and Mn
Is diffused from the surface layer, so that an appropriate alloying can be achieved without a large Fe concentration gradient on the steel substrate side. Therefore, the Γ phase is not substantially generated, and a plating film having good workability can be obtained. And since this plating film avoids rapid high-temperature heating, it becomes uniform even along the surface. Also, iron content
It is in the range of 5 wt% to 20 wt%. However, in consideration of the variation in conditions that are likely to occur during actual operation, it is particularly preferable that the heating temperature be from 320 ° C. to 380 ° C. and the heating time be from 30 minutes to 10 hours. In this case, the iron content of the plating film falls within the range of 5 wt% to 14 wt%. Further, by this heat treatment, the surface layer and the inner layer have an integrated structure due to the thermal diffusion of Fe-Mn.

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

The steel plate used is a cold-rolled steel plate with a thickness of 0.8 mm, which is commonly used for low-carbon Al-killed steel (material A) for thin plates and 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 Fe-Mn alloy plating, and each was processed under the following conditions.

(1) Electroplating _{MnSO 4 · H 2 O: 20~200g} / FeSO 4 · 7H 2 O: 20g / (NH 4) 2 SO 4: 200g / Na 2 SO 4: 30g / pH: 3.2, bath temperature: 15 ℃ Cathode current density: 3A / dm ² (2) Plasma spray Plasma gas: Ar Thermal spray input: 20KW Spray distance: 100mm Average powder particle size: about 5μm Powder supply speed: 5g / min ・ dm ² (3) Powder spray average Powder particle size: about 5 μm Powder supply rate: 3 g / min · dm ² The iron and manganese contents in the surface layer and the inner layer of the plating film were examined by Auger electron spectrometry and Grim-glow discharge emission spectroscopy, respectively.

Powdering resistance is evaluated after bending to 90 degrees with a radius of curvature of 2 mm, then sticking an adhesive tape inside the bend, peeling it off, visually observing the situation where powder adhered to this adhesive tape, scoring and evaluating did.

The evaluation criteria are as follows: 1; no adhesion, 2; extremely slight adhesion, 3; slight adhesion, 4; slight adhesion, 5; considerable adhesion.

Cratering 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 used, but after mixing, stirring was performed for one week.
Electrodeposition was performed by instantaneously applying an electrodeposition voltage of 300 V at a distance between the electrodes of 4 cm.

Tables 2 to 4 show the processing conditions and investigation results of each of these examples.

In the examples, even in the case of the material B, the powdering resistance was not inferior, and in the example No. 6 having the limit adhesion amount and the example No. 17 close to the limit heating time, extremely slight powdering was observed. However, there is no problem in practical use. In the cratering resistance, the coating amount of the surface layer is the lower limit in Example N.
One or two small craters were found in o.13,
This is 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. Further, the iron content of the inner layer is in the range of 6.0 wt% to 13 wt%, which ensures sufficient corrosion resistance after painting.

On the other hand, in Comparative Examples treated under conditions outside the scope of the invention, Comparative Example No. 1 without Al in the bath, Comparative Example with excessive heating time
No.2, Comparative example No.3 with much Pb in the bath, Comparative example No.4 with too much adhesion, Comparative example No.5 with no upper layer, Comparative example No.6 with too high heating temperature Powdering resistance etc. There is a problem in either the resistance or cratering resistance.

In the conventional example, the conventional example 1 was alloyed only by rapid heating and high-speed heating, and there was a problem in both characteristics. The conventional example 2 was alloyed and adjusted at low temperature after rapid heating and high-speed heating, and the cratering resistance was low. Conventional Example 3 is alloyed by rapid heating and high-speed heating and is provided with a plating layer having a high iron content thereon, and is inferior in powdering resistance. Thus, both characteristics are not satisfied at the same time.

Next, the distribution of iron and manganese contents in the inner layer of the plating film according to the present invention was examined.

Here, in the width direction of the alloyed hot-dip galvanized coil (width 1800 mm) of Example No. 12, the iron and manganese content of the plating inner layer was examined at intervals of 200 mm. The result is shown in FIG. In the figure, the horizontal axis is the distance from the left end of the coil,
The vertical axis indicates the content of iron and manganese, and the circles indicate examples.
It is a plot of the iron content of No. 12, and the symbol △ plots the manganese content. In addition, the mark ● plots the iron content of Conventional Example No. 2. As is clear from the figure, the iron content of Example No. 12 was 8.0 wt% on average.
And all the measurement points were distributed between 7.8 wt% and 8.2 wt%. The average manganese content is 0.5wt%,
All measurement points are distributed between 0.4 wt% and 0.6%. The iron content of Conventional Example No. 2 is 8.3 wt% on average,
All measurement points vary from 8.0 wt% to 9.0 wt%. Therefore, in the present invention, variation of iron and manganese in the plane direction is remarkably small. Further, as to whether or not a Γ phase is present at the bottom of the plating film, about two-thirds of the upper layer of the plating film was subjected to the alloyed hot-dip galvanizing treatment of Examples No. 1 to No. 17. As a result of X-ray diffraction after removal, no Γ phase was detected in any of the samples.

[Effect of the Invention] The plated steel sheet of the present invention has substantially no Γ phase in the plating film, and the surface layer having a high iron content and the inner layer have an integral structure, and the distribution of the iron content is low. Since it has a uniform film in the plane direction, it has both excellent corrosion resistance and excellent powdering resistance and cratering resistance in addition to sufficient corrosion resistance. It is an invention which has a great industrial effect because it can be easily manufactured.

[Brief description of the drawings]

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

Continuation of the front page (56) References JP-A-2-88752 (JP, A) JP-A-2-73953 (JP, A) JP-A-61-253397 (JP, A) JP-A-60-67690 (JP) JP-A-57-79160 (JP, A) JP-A-57-114692 (JP, A) JP-A-58-39792 (JP, A) JP-A-61-119663 (JP, A) 58-34169 (JP, A) JP-A-56-158864 (JP, A) JP-B-58-15554 (JP, B2) JP-B-59-14541 (JP, B2) Iron and steel, 72 [13] ( 1986), (Showa 61-9-9) p. S1331

Claims (3)

(57) [Claims] A plating film formed by heat-treating a first layer formed by hot-dip galvanizing and a second layer formed thereon by Fe + Mn plating on at least one side of a steel sheet, wherein the plating layer has a surface layer of the first layer. It is a two-layer Fe + Mn plating in which the inner layer is composed of δ ₁ phase and 境界 phase except for the boundary layer with a 0.5 μm thick steel substrate, and the iron and manganese content is uniformly distributed in the plane direction. A galvannealed steel sheet with excellent workability and paintability. 2. A method for producing an alloyed hot-dip galvanized steel sheet having excellent workability and paintability, comprising the following steps. (B) A steel sheet that has been subjected to normal pretreatment is treated with Al
a process of immersing in a hot-dip galvanizing bath containing t% or less and Pb 0.2 wt% or less to apply a plating of 30 g / m ² or more and 90 g / m ² or less. (b) Spangle while the plating film is in a molten state (C) performing a skin pass treatment after the plating film is solidified,
(D) a step of applying 0.5 g / m ² or more to 10 g / m ² or less of Fe-Mn plating on one or both surfaces of the steel sheet; In a batch type annealing furnace where the applied steel sheet is maintained in a non-oxidizing or reducing atmosphere, in an open coil state at a temperature within a range of 320 ° C or higher and a melting point of zinc or lower.
A step of heating from 0 minutes to 50 hours. 3. Workability characterized by including the following steps:
Manufacturing method of galvannealed steel sheet with excellent paintability. (B) A steel sheet that has been subjected to normal pretreatment is treated with Al
a step of immersing in a hot-dip galvanizing bath containing t% or less and Pb 0.2 wt% or less to apply a plating of 30 g / m ² or more and 90 g / m ² or less, (b) steel sheet while the plating film is in a molten state Spray Fe-Mn powder on one or both sides of 0.5g / m ² or more 10g / m
Step of applying an upper layer plating of ² or less, (c) a step of smoothing the surface of the galvanized coating subjected to skin pass process after the plating film has solidified, (d) a non-oxidizing or reducing the smoothed steel sheet in the step Heating in an open coil state at a temperature within the range of 320 ° C. or more and the melting point of zinc in a batch type annealing furnace maintained in a neutral atmosphere for 10 minutes to 50 hours.