JPH0273953A - Alloying hot dip galvanized steel sheet excellent in workability and painting suitability and its production - Google Patents

Abstract

PURPOSE:To provide a steel sheet excellent in corrosion resistance, powdering resistance, and cratering resistance by providing an outer layer part composed of an Fe-Zn alloy with a specific component in a specific coating weight and an inner layer part composed of alloyed zinc in a specific coating weight to a steel sheet, applying thermal diffusion to the boundary between the above layer parts to form an integral structure, and forming a plating film on the above. CONSTITUTION:At least one side of a steel sheet is provided with an outer layer composed of an Fe-Zn alloy of >=40wt.% iron content in which coating weight is regulated to 0.5-10g/m 2 and an inner layer composed of alloyed zinc of 0.5mum thickness which consists of delta1-phase and zeta-phase except in the boundary layer between a steel as base metal and this layer and in which coating weight is regulated to 30-90g/m<2>. The boundary between both layers mentioned above is subjected to mutual thermal diffusion, by which the above layers are formed into an integral structure. Further, the distribution of iron content is uniformized in a facial direction to form a plating film. By this method, the steel sheet excellent in corrosion resistance, powdering resistance, and cratering resistance can be provided.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is directed to the production of fibers used in automobiles, home appliances, building materials, etc.
This relates to e-Zn alloy plated steel sheets.

[Prior art] Galvanized steel sheets are widely used as materials that are inexpensive and have excellent corrosion resistance and strength.In particular, galvanized steel sheets are used in large quantities for the interior and exterior panels of automobiles, considering their workability and paintability in addition to their corrosion resistance. It is being said. Generally speaking, there are two methods for mass producing galvanized steel sheets: electroplating and hot-dip plating.Since electroplating is processed at low temperatures, there is no phase change due to heat effects, and it is easy to control the composition of the plating film. In order to increase the amount of plating deposited, the processing time must be increased. On the other hand, with the hot-dip plating method, the amount of adhesion can be easily increased without increasing the processing time, and an Fe-Zn alloy can be easily produced by performing heat treatment after plating. However, it requires some effort to control the composition of the plating film and the phases produced. In recent years, steel sheets for automobiles have been required to have a higher degree of corrosion resistance in order to deal with salt damage, etc. In response to this, we have developed plating compositions, mainly hot-dip galvanizing, which can easily secure a coating amount and is economical. There is a need for a plated steel sheet that has excellent corrosion resistance, good phase control, and good workability and paintability while also ensuring high corrosion resistance.

The most important problem in processability is powdering resistance.
An issue with 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 in which the plating film becomes powdery and falls off. Cratering is a phenomenon in which the plating film becomes visually visible in the electrocoating process that is performed after chemical conversion treatment. (crater) is a phenomenon that occurs. The former has a high iron content phase (Fe3) in the plating film.
Zn1o, Fe2O ~ 28 wt%) is generated, which occurs because it is hard and brittle, and the latter occurs due to the uneven surface shape of the plating film surface, oxide film, plating film phase structure, etc.).

Conventionally, alloyed hot-dip galvanized steel sheets used for automobiles have an average iron content of 1 after hot-dipping.
Alloying treatment is performed until the Fe content reaches around 0 wt%, and Fe is diffused to the plating surface to improve corrosion resistance, especially post-painting corrosion resistance.

That is, after the steel strip is subjected to continuous pretreatment (including heat treatment) to prepare the material, it is plated by immersing it in a molten zinc bath, and then the plated steel strip is placed in an alloying furnace. and 50
Rapidly raise the temperature from 0℃ to 700℃ for a short period of time (10
~30 seconds) to alloy the plating film with an iron content of around 10%. However, since the alloyed hot-dip galvanized steel sheets produced in this way are heated to high temperatures due to rapid temperature rise, the iron content in the plating film tends to vary depending on the location, and the iron content in the plating film tends to vary in the surface direction and depth direction. In addition to this, the iron concentration gradient within the plating film becomes large, and in order to secure the iron content in the surface layer, the iron content at the interface with the steel base increases, resulting in the formation of "phases". Furthermore, high temperature treatment and rapid cooling generate thermal stress in the plating film.

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 order to obtain smoothness of the plating film, rapid heating and high temperature heating is performed to remelt the Zn plating film in the second heating. In this heating, the iron content was reduced to 2.
Since the iron content remains in a low range of 2 to 5.5 wt%, depending on the result of this secondary heating, secondary heating is performed at a low temperature below the melting point of zinc over time to bring the iron content to a range of 6 to 13 wt%. It can be paid to. It is also disclosed that by this method, it is possible to obtain an alloyed hot-dip galvanized film with a smooth surface, excellent appearance, and no peeling or powdering during processing.

On the other hand, it has also been proposed to improve cratering resistance by increasing the iron content only in the surface layer of the plating film. for example,
The proposal of Japanese Patent Publication No. 58-15554 is a plated steel sheet with a corrosion-resistant metal layer as an inner layer and a Fe--Zn alloy coating layer with a high iron content attached thereon to improve cationic electrodeposition coating properties. This proposal discloses an alloyed hot-dip galvanized layer that is formed into an Fe-Zn alloy by heat treatment after hot-dip galvanizing as the corrosion-resistant metal layer that is the inner layer.

[Problems to be Solved by the Invention] However, the above-mentioned Japanese Patent Publication No. 59-14541 does not satisfy the cratering resistance. Regarding cratering resistance, the iron content on the surface is insufficient, and regarding powdering resistance, the alloying reaction is uneven because the alloying treatment is performed by rapid heating at high temperature after hot-dip galvanizing. As a result, an r layer with poor workability grows. Further, in some cases, unalloyed portions and highly alloyed portions coexist, resulting in a so-called uneven burning phenomenon. As described above, secondary heating tends to be non-uniform, so secondary heating conditions based on the results of secondary heating become extremely complicated, and implementation thereof is very difficult in actual operation.

In Japanese Patent Publication No. 58-15554, the iron concentration on the plating surface is dramatically increased, which improves the cratering resistance, but since alloying is completed by heat treatment after hot-dip galvanizing, Similar to No. 14541, there is the problem of non-uniform alloying, and in addition, the iron concentration gradient within the plating film becomes large, resulting in phase growth at the interface with the steel base where the iron concentration is high. Thermal stress caused by rapid heating and cooling is also unfavorable for powdering resistance.Thus, efforts have been made to satisfy powdering resistance and cratering resistance, but there is still no hot-dip galvanized steel sheet that satisfies both properties. has not been obtained.

In order to solve this problem, this invention was made, and the object is to provide a plated steel sheet that satisfies both powdering resistance and cratering resistance in addition to corrosion resistance, and a method for manufacturing the same. .

[Means and effects for solving the problem] The means for achieving this purpose is to coat at least one side of the steel plate with an adhesion amount of 0.5 g/♂ or more and 10 g/m” or less, and an iron content of 40 wt% or more. Alloying consisting of δ, phase and ζ phase, except for the boundary layer between the outer layer of the FeZn alloy layer and the steel substrate with a coating weight of 30 g/m" or more and 90 g/m2 or less and a thickness of 0.5 μm. An alloyed hot-dip galvanized steel sheet that has an inner layer of zinc and a plating film in which both layers are thermally diffused to each other at the boundary to form an integral structure, and the distribution of iron content is uniform in the surface direction. be.

The method for manufacturing the above-mentioned alloyed hot-dip galvanized steel sheet includes the following.

One method is as follows.

(b) Steel strip subjected to normal pretreatment with Ag3.05wt%
A step of applying plating of 30 g/m'' to 90 g/m'' by immersing it in a hot-dip galvanizing bath containing Pb of 0.3 wt% or less and Pb of 0.2 wt% or less, (b) The plating film is in a molten state. (c) Performing a skin pass treatment after the plating film has solidified,
A step of smoothing the surface of the hot-dip galvanized film;
g/m" or more and 10 g/rn" or less Fe30wt
% or more of Fe-Zn alloy plating, (e) the steel strip plated in the above step is heated at 320°C or higher in an oven coil state in a batch annealing furnace maintained in a non-oxidizing or reducing atmosphere. This is a method for producing an alloyed hot-dip galvanized steel sheet, which includes a step of heating at a temperature within the range of melting point or lower for 10 minutes to 50 hours.

Another method is as follows.

After the hot-dip galvanizing process in (a) above, 30wt of Fe is applied to one or both sides of the steel strip while the plating film is in a molten state.
Q, 5g by spraying Fe-Zn alloy powder of % or more
This is a method for manufacturing an alloyed hot-dip galvanized steel sheet, which includes a step of applying an upper layer plating of at least 10 g/m'' and then steps (c), (d), and (e).

The above means will be described in detail below, including their effects.

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

When the iron content of the outer layer of the plating film is 50 vt% or more, the generation of craters during electrodeposition coating is prevented. In other words, alloyed hot-dip galvanized steel sheets are subjected to cationic electrodeposition coating after phosphate treatment on the plated surface, but phosphophyllite [Zn2] containing Fe is added to the phosphate crystals produced by this chemical conversion treatment. F e (PO4)2 ・4 H20
] and coarse acicular crystals called hopite [Zn5(POa)z 4H20] which does not contain Fe. One of the causes of cratering is thought to be local concentration of current at defective areas in the chemical conversion coating, but the coating formed with phosphophyllite is denser than that of hopite and has fewer defects. Therefore, if enough Fe is supplied on the plating surface to facilitate the formation of phosphophyllite, craters will be less likely to occur. When the iron content on the plating surface becomes high and approaches 40 wt%, the occurrence of craters rapidly decreases.

The outer layer needs to have a coating weight of 0.5 g/yn" to 10 g/7 m". If it is less than 0.5 g/m2, it will not be possible to sufficiently supply Fe to the entire plating surface, and if it is deposited in excess of 10 g/m2, the effect will be saturated and it will only become disadvantageous in terms of cost. Moreover, red rust is more likely to occur in corrosion resistance after painting.

In the case of alloyed hot-dip galvanized steel sheets, the corrosion resistance is mostly determined by the coating weight and the iron content in the coating.

Inner layer from 30 g/m2 to 90 g/rnz
The amount of iron deposited is appropriate for high corrosion resistance.
A range of t% is preferred. If it exceeds 90 g/m", not only will it result in excessive quality, but it will also take a long time in the subsequent alloying process at a low temperature, reducing productivity. In addition, in general, the thicker the plating film, the harder it is to process. Occasionally, the coating may break or peel, and in the case of alloyed hot-dip galvanized steel sheets, powdering is likely to occur.

In the present invention, it is important that the boundary layer between the outer layer and the inner layer has an integral structure formed by mutual thermal diffusion. The thermally diffused integrated structure creates a state in which the iron concentration of the outer layer and inner layer changes continuously, and the material of the inner layer, which makes up the majority of the plating film, is separated from the 0.5 μm thick steel base. The material of the inner layer is made to consist of hard and brittle "δ", "ζ" and "ζ" phases, and the distribution of iron content is uniform in the surface direction. Powdering is prevented by the material of the inner layer. The "phase" is generated in the boundary layer between the inner layer and the steel base, but the plating film in which this "phase is not detected has good powdering resistance.
It is difficult to detect unless it has grown to a thickness of μm or more. The iron content in the plating film has an important meaning in terms of processability and post-painting corrosion resistance, and these properties can only be fully demonstrated if this content is uniformly controlled over the entire plated surface. Due to this structure, the mechanical properties and electrochemical properties of the plated film do not differ significantly between adjacent parts, and the plated film has excellent workability and corrosion resistance.

The alloyed hot-dip galvanized steel sheet of the present invention may or may not have a plating film on the other side depending on the use.

The manufacturing method of the present invention will be described below.

A hot-dip galvanizing bath usually contains around 0.2% Aρ for suppressing Fe-Zn alloy reactions and smoothing the plated surface, and also contains PB for spangle adjustment.

Among these, Aρ has the effect of suppressing alloying, so 0.05w
Fe-Zn after addition of t% or more and immersed in hot-dip galvanizing bath
Prevents partial and non-uniform formation of alloy. It is important that the Fe--Zn alloy is not formed non-uniformly in this process, and once it becomes non-uniform, it cannot be corrected in subsequent steps. The amount of A and R added is too large, 0.3wt.
If it exceeds %, the effect of suppressing alloying becomes excessive, and the subsequent alloying treatment takes too much time, making it unsuitable for industrial use. p
Although b does not directly participate in the alloying reaction, a large amount of pb reduces powdering resistance, so it must be limited to 0.2 wt% or less.

By refining the spangles while the hot-dip galvanized film is in a molten state and further performing a skin pass treatment after the galvanized film has solidified, a smooth plated surface is obtained, and the coverage of the subsequent upper layer plating is improved. the result,
Not only can the cratering resistance be efficiently improved, but also the sharpness after painting can be obtained. If the skin pass is performed at an elongation rate of 0.3% or more, the mating surface will be smooth, but if the elongation rate is too large and exceeds 5%, the workability of general steel materials for thin plates may be affected.

Fe-Zn alloy plating with an iron content of 40 wt% or more not only ensures cratering resistance, but also allows Fe to be diffused into the previously applied hot-dip galvanized layer from the opposite side of the steel base during subsequent heat treatment. As a result, the iron concentration gradient in the inner layer of the plating film can be kept small. The processing method for the alloy plating may be any method such as electroplating, vapor deposition plating, thermal spraying, etc., as long as the processing is not performed at a temperature higher than the melting point of zinc. When this alloy plating treatment is performed by spraying alloy powder, if it is performed while the previous hot-dip galvanized layer is in a molten state, the spangles will be made finer at the same time, and one step can be omitted.

The plated steel strip that has gone through the above-mentioned two plating processes is heat treated, but doing so in a non-oxidizing or reducing atmosphere prevents surface oxidation and prevents the chemical conversion coating crystals from forming unevenly during the chemical conversion treatment before painting. This is to avoid such problems, and the reason why the process is carried out in a batch type annealing furnace is because the process is carried out at a low temperature over a long period of time. The purpose of heating with an oven coil is to prevent unevenness in alloying by uniformly heating, and at the same time to prevent the plating surfaces from adhering to each other and causing defects. In a tight coil state, the temperature distribution becomes non-uniform, and there are parts where the alloying rate is high and parts where the alloying rate is low. In particular, this non-uniformity occurs in the longitudinal direction of the steel strip, making it difficult to obtain a high quality product. Heating is done at a low temperature, but
A temperature of 320° C. or higher is necessary. If the temperature is lower than 320°C, it takes too much time to obtain a degree of alloying sufficient to ensure corrosion resistance after painting. When the temperature is made higher than the melting point of zinc (<419.5° C.), there appear locations where alloying progresses rapidly and the formation of F phase cannot be ignored. Furthermore, there is a risk that the spacer inserted between the steel strips of the oven coil may leave marks on the plating surface. Figure 1 shows the conditions under which both powdering and cratering do not occur within the above temperature range.
The horizontal axis represents heating time and the vertical axis represents heating temperature. In the figure, points a and b
,c.

The range surrounded by the line connecting d is the preferred range of conditions for actual operation that does not cause powdering or cratering, and the heating time is from the time coordinate of point A to the time coordinate of point C, that is, 10 minutes or more. It will take less than an hour. When the heat treatment is performed under the above heating conditions, Fe diffuses from the steel base side and the outer layer plating side, so that appropriate alloying is achieved without creating a large Fe concentration gradient on the steel cable base side. As a result, a plating film consisting only of the δ1 phase and ζ phase is obtained, with virtually no phase formation.And since rapid high-temperature heating is avoided, this plating film is uniform even along the surface. In addition, the iron content falls within the range of 5 wt% to 20 wt%.

However, considering the variations in conditions that tend to occur during actual operation, it is particularly preferable that the heating temperature is between 320°C and 38°C.
The heating time is 30 minutes to 10 hours to 0°C.

In this case = 18 The iron content of the composite plating film falls within the range of 5wt% to 14W%. Further, by this heat treatment, the outer layer portion and the inner layer portion become an integral structure due to thermal diffusion of Fe-Zn.

[Example] Regarding the alloyed hot-dip galvanized steel sheets of 17 examples (Examples) that were treated using one type of steel and changing the hot-dip galvanizing conditions, upper layer plating conditions, and alloying treatment conditions, the The iron content was investigated and evaluated by performing a powdering test and a cratering test. For comparison, 6 examples (comparative examples) treated under conditions outside the scope of the present invention and 3 examples (conventional examples) according to the prior art will also be examined in the same manner. Details of the conditions are as follows.

The steel used was cold-rolled steel with a plate thickness of 0.8 mm, including low carbon A, which is commonly used for thin plates, &killed (material A), and super, which is said to be prone to powdering due to high processing. It is a low carbon titanium containing steel (Material B).

Table 1 shows each component.

Table 1 (wt%) Hot-dip galvanizing is performed in a continuous plating facility equipped with a non-oxidizing furnace and a reduction heating furnace, and the coating amount is adjusted using a gas throttle device installed after the plating bath. After cooling the plating layer, perform a skin pass at an elongation rate of 1.5% to make the surface smooth.

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

(1) Electroplating Fe 2 SO4 7 H20380g / IZ
n SO4・7 H2015~150 g/ (IC
H3COONa 20g/lNa2S○
4 30 g / 1 bath temperature
50℃ cathode current density
50A/dm2 (2) Plasma spraying Plasma gas Ar spraying heat input 20KW spraying distance
100+*m average powder particle size (Fe80%
) Approximately 5μm powder supply rate, 5g/w
in-dm2 (3) Powder spray Average powder particle size (Fe80%) Approximately 5 μm Powder supply rate 3 g/win・d m” The iron content in the plating film was determined by grim glow discharge emission spectroscopy. The inner layer was investigated.

Powdering resistance is determined after bending 90 degrees with a radius of curvature of 2.
Adhesive tape was pasted on the inside of the bend, peeled off, and visually observed to see if the powder had adhered to the adhesive tape.
Evaluate by giving points. The scoring criteria is on a five-point scale: 1: no adhesion, 2: very little adhesion, 3: slightly adhesion, 4: a little adhesion, 5: considerable adhesion.

Cratering resistance was evaluated by applying a chemical conversion treatment to the plated surface, followed by electrodeposition coating, and evaluating the number of craters generated at this time. A commercially available dipping type phosphate treatment agent was used for the chemical conversion treatment. For electrodeposition coating, a commercially available cationic electrodeposition paint was used, and after preparation, it was stirred for one week and electrodeposition was carried out by instantaneously applying an electrodeposition voltage of 300 V with an electrode spacing of 1114 CI.

The processing conditions and investigation results for each of these examples are shown in Table 2.

In the examples, there was no inferiority in powdering resistance even with material B, and slight powdering was observed in Example Na 6, which was close to the limit adhesion amount, and Example N [Ll7, which was close to the limit heating time. , there is no problem in practical use. In terms of cratering resistance, the iron content in the upper layer is close to the limit Example No.
, 11 , N[L L 2 , N113, one to two small lators were found, but this also poses no problem in practice. In this way, all the alloyed hot-dip galvanized steel sheets in the examples have both powdering resistance and cratering resistance. Also, the iron content of the inner layer is 5.5w.
The content is within the range of t% to 13wt%, which ensures sufficient corrosion resistance after painting.

On the other hand, in the comparative examples treated under conditions outside the scope of the invention, there is Comparative Example Nα1 with no Aβ in the bath, Comparative Example No. 2 with excessive heating time, and Comparative Example N113 with a large amount of Pb in the bath. Comparative example No. with too much adhesion [L4, comparative example No. without upper layer,
5. Comparative Example N where the heating temperature was too high [L6, etc.] There is a problem in either powdering resistance or cratering resistance.

In the conventional example, Conventional Example N
The conventional examples NQ and 3 are alloyed by rapid heating at high temperature and then alloyed at low temperature, and have inferior cratering resistance, while conventional examples NQ and 3 are alloyed by rapid heating at high temperature and then have a plating layer with a high iron content on top. and has poor powdering resistance. In this way, both characteristics are not satisfied at the same time.

Next, the iron content distribution in the inner layer of the plating film according to the present invention was investigated.

Here, in the width direction of the alloyed hot-dip galvanized coil (width 800 mm) of Example N [L14], the iron content of the inner layer of the plating was investigated at 200-meter intervals. In this case, conventional example N1
Compared with L2. The results are shown in FIG. In the figure, the horizontal axis is the distance from the left end of the coil, and the vertical axis is the iron content.
The O mark is a plot of Example Na14, and the * mark is a plot of Conventional Example 2. As is clear from the figure, the average iron content of Example No. 14 was 9.7 wt%, and all measurement points were distributed between 9.5 wt% and 9.9 wt%. On the other hand, the average iron content of conventional example Nα2 is 8.3 wt%, and all measurement points are 7.9 wt%.
t% to 9. It was distributed between OwL% and had a large variation.

Furthermore, to determine whether or not a phase exists at the bottom of the plating film, approximately two-thirds of the upper layer was removed from the samples subjected to the alloying hot-dip galvanizing treatment from Example No. 2 to Na 17. As a result of diffraction, no 1° phase was detected in any of the samples.

[Effects of the invention] The plated steel sheet of the present invention has substantially no phase in the plating film, and has an integral structure of an outer layer portion and an inner layer portion with a high iron content. Since the film has a uniform content distribution in the surface direction, it has not only sufficient corrosion resistance but also excellent powdering resistance and crater link resistance. This invention is industrially very effective because the plated steel sheet described above can be easily manufactured through a simple process.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between heat treatment conditions and appropriate characteristics for detailed explanation of the present invention, and FIG. 2 is a diagram showing the distribution of iron content in one embodiment of the present invention.

Claims (3)

[Claims] (1) At least one side of the steel plate has an adhesion amount of 0.5 g/m
The outer layer of the Fe-Zn alloy layer has an iron content of 40 wt% or more with a thickness of ^2 or more and 10 g/m or less, and an adhesion amount of 30 g/m
The inner layer of alloyed zinc consisting of the δ_1 phase and the ζ phase, except for the boundary layer with the steel substrate with a thickness of 0.5 μm and ^2 or more and 90 g/m^2 or less, and those two layers mutually at the boundary. An alloyed hot-dip galvanized steel sheet with excellent workability and paintability, characterized by having a plating film that forms an integral structure through heat diffusion and has a uniform distribution of iron content in the surface direction. (2) A method for producing an alloyed hot-dip galvanized steel sheet with excellent workability and paintability, the method comprising the following steps. (b) Steel strip subjected to normal pretreatment with Al0.05wt%
A step of applying plating of 30 g/m^2 to 90 g/m^2 by immersing it in a hot-dip galvanizing bath containing Pb of 0.3 wt% or less and Pb of 0.2 wt% or less; (b) The plating film is melted. (c) Performing a skin pass treatment after the plating film has solidified,
A step of smoothing the surface of the hot-dip galvanized film;
A step of applying Fe-Zn alloy plating with Fe50wt% or more of g/m^2 or more and 10 g/m^2 or less, (e) A batch in which the steel strip plated in the above step is maintained in a non-oxidizing or reducing atmosphere. A process of heating an open coil in a type annealing furnace at a temperature of 320°C or higher and lower than the melting point of zinc for 10 minutes to 50 hours. (3) A method for producing an alloyed hot-dip galvanized steel sheet with excellent workability and paintability, the method comprising the following steps. (b) Steel strip subjected to normal pretreatment with Al0.05wt%
A step of applying plating of 30 g/m^2 to 90 g/m^2 by immersing it in a hot-dip galvanizing bath containing Pb of 0.3 wt% or less and Pb of 0.2 wt% or less; (b) The plating film is melted. A step of spraying Fe-Zn alloy powder containing 50 wt% or more of Fe on one or both sides of the steel strip while the steel strip is still in the condition to form an upper layer plating of 0.5 g/m^2 or more and 10 g/m^2 or less, (c) Plating film After solidification, a step of smoothing the surface of the hot-dip galvanized film by skin pass treatment; (d) batch annealing in which the steel strip with the galvanized film smoothed in the above step is maintained in a non-oxidizing or reducing atmosphere; A process of heating in an open coil in a furnace at a temperature within the range of 320°C or higher and lower than the melting point of zinc for 10 minutes to 50 hours.