JPH02118084A - Hot-dip galvanized alloyed steel sheet excellent in workability and coating property and production thereof - Google Patents

Abstract

PURPOSE:To easily produce the title steel sheet excellent in corrosion resistance, powdering resistance and cratering resistance by hot-dip galvanizing the steel sheet and smoothening the surface and thereafter performing Fe-Mn alloy plating thereon and then heat-treating this plated steel sheet. CONSTITUTION:A steel sheet on which ordinary pretreatment has been performed is immersed in a hot-dip galvanizing bath contg. 0.05-0.3wt.% Al and <=0.2wt.% Pb to perform galvanizing of 30-90g/m<2>. Spangle is made fine by mist spray while the galvanized film is kept in a molten state. After the film is solidified, the surface of the film is smoothened by skin pass treatment. Fe-Mn alloy plating of 0.5-10g/m<2> is performed on the single face or double faces of this steel sheet by an electroplating method. Then the above-mentioned galvanized steel sheet is produced by heating this steel sheet at the temp. not lower than 320 deg.C and not higher than m.p. of Zn for 10min-50 hours in an open-coil state in a batch type annealing furnace of the nonoxidative or reductive atmosphere. The internal layer is constituted of delta1 phase and zeta phase excepting the boundary layer for the basis material having 0.5mum thickness and furthermore Fe and Mn are uniformly distributed in the facial direction and therefore this galvanized steel sheet is made excellent in workability and coating properties.

Description

[Detailed description of the invention] "Industrial application field"
This relates to an eZn alloy plated steel sheet.

[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, taking into consideration workability and paintability in addition to good resistance. It is used. Generally speaking, there are two methods for mass production of 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. However, plating (
= f In order to increase the amount of deposit, 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 FeZn 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 toughened steel plate that has good processability and phase control, ensures high corrosion resistance, and has both workability and paintability.

The most important issue in processability is powdering resistance.
The problem with paintability is cratering resistance, which increases by 2δ.

Powdering is a phenomenon in which a plating film becomes ladle-shaped and falls off during press molding, and cratering is a phenomenon in which the plating film becomes visually visible in the electrocoating process that is performed after chemical conversion treatment. This is a phenomenon in which a crater (crater) is generated.

The former has a high iron content phase (Fe3Z) in the plating film.
n1O+ Fe (20 to 28 wt%) is generated, and this occurs because the plated film is hard and brittle, and the latter occurs due to non-uniformity of the plated film surface (surface shape, oxide film, plated 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 it reaches around 0 wt.%, and Fe is diffused to the plating surface to improve corrosion resistance, especially post-painting corrosion resistance.

That is, after pre-treating (including heat treatment) a steel plate continuously to adjust the material, it is immersed in a zinc-molten plating bath for plating, and then this plated steel plate is heated for 50 min in an alloying furnace.
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 increases, 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 Alloy 1 treatment is divided into two steps: primary and secondary. For example, Tokuko Sho 59-145
In No. 41, in the secondary heating, rapid temperature rise and high temperature heating is performed to remelt the Zn plating film in order to obtain smoothness of the plating film. In this heating, the iron content is 2.2 to 5.5w.
Since the iron content remains in a low range of t%, depending on the result of this secondary heating, secondary heating is performed over a period of time at a low temperature below the melting point of zinc to keep the iron content within the range of 6 to 13 wt%. be. 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 the m-cratering property 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 tamped steel plate with a corrosion-resistant metal layer as an inner layer and a Fe--Zn alloy coating layer with a high iron content on top of the inner layer to improve cationic electrodeposition coating properties. This proposal discloses an alloyed hot-dip galvanized layer which is made into an Fe--Zn alloy by heat treatment after hot-dip plating as the inner corrosion-resistant metal layer.

[Problem to be solved by the invention] However, in the above-mentioned Japanese Patent Publication No. 59-14541,
The iron content on the surface is insufficient for cratering resistance, and for powdering resistance, alloying by rapid temperature rise and high temperature heating after hot-dip galvanizing Because of the processing, it is unavoidable that the alloying reaction progresses unevenly, resulting in the growth of a layer with poor workability, and 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 was dramatically increased, improving the cratering resistance, but since alloying was 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, and at the interface with the steel base where the iron concentration becomes high, "phases grow". In addition, 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 galvanizing that satisfies both properties. No steel plates were obtained.

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

[Means and effects for solving the problem] A means for achieving this object is to heat-treat and form a first layer of hot-dip galvanizing and a second layer of Fe+Mn plating on at least one side of a steel plate. The plating film has a surface layer of the second layer of Fe+Mn plating, and an inner layer of δ1 phase and ζ phase except for the boundary layer with the steel base having a thickness of 0.5 μm. This is an alloyed hot-dip galvanized steel sheet with excellent workability and paintability, characterized by having iron and manganese contents uniformly distributed in the surface direction.

The following methods are available for producing the above-mentioned alloyed hot-dip galvanized steel sheet.

One method is as follows.

(b) Al1.05wt% steel plate subjected to normal pretreatment
A step of applying plating of 30 g/m" or more and 90 r7 m" or less 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) A process in which the spangles are refined while the plating film is still in a molten state.

(c) After the plating film has solidified, perform a skin bath treatment,
A process to smooth the surface of hot-dip galvanized film.

(d) A step of applying Fe-Mn plating of 0.5 g/ri+" to 10 g/ui" on one or both sides of the steel plate.

(e) The steel plate plated in the above step is heated in an open coil state in a batch type annealing furnace maintained in a non-oxidizing or reducing cloud atmosphere at a temperature of 320°C or higher and lower than the melting point of zinc for 10 minutes to 50 minutes. The process of heating for hours.

Another method is as follows.

After the hot-dip galvanizing step (a) above, F e M is applied to one or both sides of the steel sheet while the plating film is in a molten state.
Spray n powder to 0.5g/m” or more 10g/
It includes a step of applying an upper layer plating of less than m2, and then the above (
This is a method for manufacturing an alloyed hot-dip galvanized steel sheet including the steps c), (d), and (e).

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

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

When the surface layer of the plating film is made of FeMn alloy, the generation of craters during electrodeposition coating is prevented. In other words, 2-alloy hot-dip galvanized steel sheets are subjected to cationic electrodeposition coating after phosphate treatment on the plated surface, but phosphophyllite [Zn2Fe] containing Fe is added to the phosphate crystals produced by this chemical conversion treatment. Granular and dense crystals called (PO4)2 4H20] and Fe-free hovite [Zn3 (
There are coarse needle-like crystals called PO4)2 4H20]. If Mn is present in the surface layer during the formation of these scalate crystals, part of the Zn in hopite is replaced with Mn, and the crystals become dense. The presence of Fe also facilitates the formation of phosphophyllite. One of the causes of crater generation is thought to be localized current concentration in the defective areas of the chemical conversion coating, but dense crystalline coatings have fewer defects. Therefore, the surface layer is Fe-
If it is a Mn alloy, craters are less likely to occur.

In the case of alloyed hot-dip galvanized steel sheets, most of the corrosion resistance is determined by the amount of coating and the content of iron and manganese 3 in the coating. By forming the Zn plating film into an FeMn alloy, both bare corrosion resistance and post-painting corrosion resistance are significantly improved.

In the present invention, it is important that the boundary layer between the surface layer and the inner layer has an integral structure formed by mutual thermal diffusion. Due to the thermally diffused integral structure, the concentration of alloy components in the surface layer and inner layer changes continuously to form a FeM n alloyed Zn plating layer, and the mechanical properties and electrochemical properties of the plating film change at the outer edge in adjacent areas. There is no difference between the two types, and the processability and corrosion resistance are excellent.

The inner layer, which makes up the majority of the plating film, is hard and brittle, except for the 05 μm thick boundary layer with the base material.
In addition, if the distribution of iron and mankan content is uniform in the surface direction, powdering during processing can be prevented.

The F phase is likely to form at the boundary between the inner layer and the steel base, but this F phase
A plating film in which no phase is detected has good powdering resistance. It is difficult to detect the r-phase unless it has grown to a thickness of 0.5 μm or more.

Depending on the use, the alloyed hot-dip galvanized steel sheet of the present invention may have no plating film on the other side or may have another plating film formed thereon.

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

A! is usually used in hot-dip galvanizing baths to suppress Fe-Zn alloy reactions and smooth the plated surface. 2 was 0.2% before? k
t3 is added, and Pb is included because of the spangled gB.9 Of these, AI has the effect of suppressing alloy 1, so
Addition of 0.05 wt.% or more prevents Fe-Zn alloy from forming partially and non-uniformly after being immersed in a hot-dip galvanizing bath. In the second step, F e −Z rr @
It is important not to generate gold;
Once this happens, it cannot be corrected in subsequent processes. If the amount of Aρ added is too large, exceeding 0.3 wt%, the effect of suppressing alloying will be excessive, and the subsequent alloying process will take too much time, making it industrially inappropriate. Although PB 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.

The inner layer has a coating weight of 90g/m2 compared to 30g/m2, making it highly resistant to f.
In this case, a high iron content as in the surface layer is not necessary, and a range of 5 wt % to 20 wt % is preferable. If it exceeds 90 g/m2, not only will it result in excessive quality, but it will also take a long time in the subsequent alloying process at low temperatures, reducing productivity. In the case of alloyed hot-dip galvanized steel sheets, powdering is more likely to occur.

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 improved. 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%, there is a risk that the workability of steel sheets for shooting thin plates will be affected.

Fe-Mn alloy plating not only ensures cratering resistance but also diffuses Fe and Mn into the previously applied hot-dip galvanized layer from the side opposite to the steel base during the heat treatment of this rib, resulting in a plating film. This will keep the iron concentration gradient in the inner layer small. The processing method for the alloy plating may be any method such as electroplating, vapor deposition plating, thermal spraying, etc., as long as it is not processed at a temperature higher than the melting point of zinc.
When this alloy plating treatment is performed by alloy powder spraying, 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 surface layer needs to have a coating weight of 0.5 g/m" to 10 g/m'. If it is less than 0.5 g/m", Fe cannot be sufficiently supplied over the entire plated surface. Furthermore, if the amount exceeds 10 g/m'', the effect is saturated and not only is it disadvantageous in terms of cost, but also red rust is likely to occur in terms of corrosion resistance after painting.

The pre-glazed steel sheet that has gone through the above two plating processes is heat treated, but doing so in a non-oxidizing or reducing atmosphere prevents surface oxidation, which may result in non-uniform chemical conversion coating crystals during the chemical conversion treatment before painting. This is to avoid this, and the reason is that the process is carried out in a batch type annealing furnace at a low temperature and over a long period of time. The reason for heating in an open coil state is to uniformly heat the alloy to prevent unevenness from occurring in the alloy, and at the same time to prevent the plating surfaces from coming into contact with 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 plate, making it difficult to obtain a high quality product. Heating is performed at a low temperature, but a temperature of 320° C. or higher is required. If it is lower than 320°C, it takes too much time to obtain the alloy f degree sufficient to ensure corrosion resistance after painting. The temperature is set to the melting point of zinc (419,5℃
), there will be places 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 plates of the open coil may leave marks on the plating surface. FIG. 1 shows the conditions under which both powdering and cratering do not occur within the above temperature range, where the horizontal axis represents the heating time and the vertical axis represents the heating temperature. In the figure, points a and b.

The range surrounded by the line connecting c and d is the preferred condition range 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. Less than 50 hours. When heat treatment is performed under the above heating conditions, Fe diffuses from the steel cable base side and the surface plating side, and Mn diffuses from the surface layer, so proper alloying is achieved without creating a large Fe concentration gradient on the steel cable base side. be done. For this reason, a plating film with good workability is obtained with virtually no phase formation.And since rapid high-temperature heating is avoided, this plating film is uniform even along the surface.Also, The iron content also falls within the range of 5wL% to 20wt%.However, considering the variations in conditions that tend to occur during actual operation, it is particularly preferable to set the heating temperature to 320℃ to 380℃ and the heating time from 30 to 10℃. In this case, the iron content of the plating film ranges from 5 wt% to 1
It falls within the range of 4wt%. Furthermore, this heat treatment
The surface layer and the inner layer become integrally elliptical due to thermal diffusion of F e M n.

[Example] Regarding alloyed hot-dip galvanized steel sheets of 17 examples (Examples) in which two types of steel sheets were used and the hot-dip galvanizing conditions, upper layer plating conditions, and alloying treatment conditions were changed, iron in the plating film was The content was investigated and evaluated by performing a powdering test and a cratering test. For comparison, six examples (comparative examples) treated under conditions outside the scope of the present invention and three examples (conventional examples) according to the prior art were also examined in the same manner.

Details of the conditions are as follows.

The steel plates used were cold-rolled steel plates with a thickness of 0.8 mm, including low-carbon A1 quilt (material A) for thin plates, which is commonly used, and ultra-low carbon titanium, which is said to be prone to powdering due to high processing. jfl (material B).

Table 1 shows each component.

Table 1 (% by weight) Hot-dip galvanizing is carried out in a continuous plating facility equipped with a non-oxidizing furnace and a reduction heating furnace. The spangles were made fine by spraying, and after the plating layer was cooled, a skin pass was performed at an elongation rate of 1.5% to smooth the surface.

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

(1) Electroplating MnSO4・H2O: 20-200g/4Fe
SO4・7H20: 20g/1(NH4)2SO4
: 200g/ρ Na2SO4: 30g/1pH: 3.2
．． Bath temperature: 15°C Cathode current density: 3 A/d m” (2) Plasma spraying Plasma gas: Ar Spraying input pj!: 20KW Spraying path M: 1.0.0 Positive average powder particle size: Approx. 5μm Powder supply Speed: 5 g/nin-dm” (3) Powder spray average powder particle size: Approximately 5 μm Powder feeding rate: 3 g/mm-dm2 The iron and manganese contents in the surface layer and inner layer of the plating film were determined by Auger electron spectrometry and Grimm, respectively. It was investigated by glow discharge emission spectroscopy.

Powdering resistance is determined by bending the product 90 degrees with two radii of curvature, pasting adhesive tape on the inside of the bend, rolling it 411 times, visually observing whether powder has adhered to the adhesive tape, and giving a score. It was evaluated.

The rating criteria is on a five-point scale: 1: no adhesion at all, 2: very little adhesion, 3: slightly adhesion, 4: a little adhesion, and 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. A commercially available cationic electrodeposition paint was also used for electrodeposition, and after preparation, it was stirred for a week and electrodeposition was carried out by instantaneously applying an electrodeposition voltage of 300 V with a distance between electrodes of 4 cm.

The processing conditions and investigation results for each of these examples are shown in Tables 2 to 4.
Shown in the table.

In the Examples, there was no inferiority in powdering resistance even with Material B, and very slight powdering was observed in Example NIL6, which had the limit adhesion amount, and Example N117, which was close to the limit heating time. There is no problem above. In terms of cratering resistance, one or two small craters were found in Example No. 13, where the amount of plating on the surface layer was at the lower limit, 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. Further, the iron content of the inner layer is within the range of 6.0w4% 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, Comparative Example No. 1 with no A(in the bath), Comparative Example No. 2 with excessive heating time, Comparative Example N with a large amount of PB in the bath (L3, Comparative example N with too much adhesion [L4, comparative example Na without upper layer]
5. Comparative Example N [L 6 etc. where heating temperature is too high. There is a problem in either powdering resistance or cratering resistance.

Regarding conventional examples, Conventional Example 1 was alloyed only by rapid heating and rapid heating, and had problems with both properties, while Conventional Example 2 was alloyed at a low temperature after rapid heating and heating, and had poor cratering resistance. Conventional Example 3 is alloyed by rapid heating and a plating layer with a high iron content is applied thereon, resulting in poor powdering resistance. In this way, both characteristics are not satisfied at the same time.

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

Here, an example) J [L 12 alloyed hot-dip galvanized coil (width 1800 mm > width direction, 20
The content of iron and manganese in the plating inner layer was examined at intervals of 0. This result is shown in Figure 2, where the horizontal axis is the distance from the left end of the coil, the vertical axis is the content of iron and manganese, and the ○ marks are plots of the iron content of Examples Nu and 12. , Δ marks are plots of manganese content. In addition, the mark ・ is a plot of the iron content of conventional example Na 2.0 As is clear from the figure, the iron content of Example NILL 2 is 8.0 wt% on average, and all measurement points are 7.8 wt%. It was distributed between 8.2 wt% and 8.2 wt%. Moreover, the manganese content is 0.5 wt% on average, and all measurement points are distributed between 0.4 wt% and 0.6%. In addition, the average iron content of conventional example Nα2 is 8.3.
wt%, and all measurement points range from 8.0 wt% to 9.0
It varies in wt%. Therefore, in the present invention, there is significantly less variation in iron and manganese in the surface direction. Furthermore, to determine whether or not the F phase exists at the bottom of the plating film, approximately two-thirds of the upper layer of the plating film was removed from the samples subjected to the alloying hot-dip galvanizing treatment from Examples Na 1 to N117, and X-ray As a result of diffraction, no phase was detected for any of the samples.

[Effects of the Invention] The plated steel sheet of the present invention has virtually no phase in the plating film, the surface layer and inner layer with a high iron content have an integral structure, and the iron content distribution is has a uniform film in the surface direction, so it has not only sufficient corrosion resistance but also excellent powdering resistance and cratering resistance. This invention has great industrial effects because it can be easily manufactured through a process.

[Brief explanation of drawings]

Fig. 1171 is a diagram showing the relationship between thermal burial conditions and suitability of characteristics for detailed explanation of the present invention, and Fig. 2 is a diagram showing the distribution of iron and manganese content in one embodiment of the present invention.

Claims (3)

[Claims] (1) At least one side of the steel sheet has a plating film formed by heat-treating a first layer of hot-dip galvanizing and a second layer of Fe+Mn plating thereon, and the plating film is
The surface layer is the Fe+Mn plating of the second layer, and the inner layer is composed of δ_1 phase and ζ phase except for the boundary layer with the steel substrate with a thickness of 0.5 μm, and the iron and manganese content is uniform in the surface direction. Alloyed hot-dip galvanized steel sheet with excellent workability and paintability. (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 plate 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 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, (d) A step of applying Fe-Mn plating of 0.5 g/m^2 to 10 g/m^2 on one or both sides of the steel sheet, (
e) The steel plate plated in the above step is heated in an open coil state in a batch type annealing furnace maintained in a non-oxidizing or reducing atmosphere at a temperature between 320°C and above and below the melting point of zinc for 10 minutes to 50 hours. The process of doing. (3) A method for producing an alloyed hot-dip galvanized steel sheet with excellent workability and paintability, which includes the following steps. (b) Steel plate 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 melts. Spray Fe-Mn powder on one or both sides of the steel plate while it is still in the condition to give 0.5g/m
A step of applying an upper layer plating of ^2 or more and 10 g/m^2 or less, (c) A step of smoothing the surface of the hot-dip galvanized film by performing a skin bath treatment after the plating film has solidified, (d) Smoothing in the above step The steel plate was heated in an open coil state in a batch type annealing furnace maintained in a non-oxidizing or reducing atmosphere at a temperature of 320°C or higher and lower than the melting point of zinc.
A process of heating from 0 minutes to 50 hours.