JP2002180223A - Galvanized steel and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a galvanized steel in which adhesion between a plating film and a base material is improved, and which has excellent adhesion so as to withstand severe forming, and a production method therefor. SOLUTION: In the steel, the surface layer part of a base material is provided with an orderly structural layer based on body-centered cubic lattices, and consisting of Fe, Al or the like, and a galvanizing film containing 0.05 to <10% Al is provided thereon. The area % of the ordered structural layer is desirably controlled to >=10%. The steel is produced preferably by using a base material containing a ferritic phase on the surface, and dipping the base material into an Al-containing hot dip plating bath heated at a temp. of 420 to 500 deg.C for 0.1 to 5 sec. It is also possible that the base material is heated at a temperature of 460 to 550 deg.C, and is vapor-deposited and plated with Al.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel material which is suitable as a material for automobiles, home appliances, building materials and the like and has excellent adhesion of a plated film after processing, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, surface-treated steel materials have been used in large quantities in the industrial fields of home appliances, building materials, and automobiles. Particularly, hot-dip galvanizing, which is excellent in economy, rust prevention function, performance after painting, and hot-dip galvanized alloy plating containing less than 10% of Al (hereinafter simply referred to as "hot-dip galvanized plating") Is widely used, but recently there has been an increasing demand for improvement of its rust prevention ability.

For example, in the case of a surface-treated steel sheet for automobiles, it is desired to improve the corrosion resistance (corrosion resistance after painting) of a portion where the coating film after coating is damaged or an end of the coated steel sheet. Further, it is necessary that the plating film does not detach when processing the steel sheet, and that the plating film has good workability. The adhesion of the plating film to the steel sheet is greatly involved in the post-painting corrosion resistance and the workability of the plating film, and the above performance can be improved by improving the adhesion of the plating film.

[0004] A hot-dip galvanized steel sheet is usually subjected to an appropriate degreasing and washing step or without performing degreasing and washing.
After preheating in a non-reducing atmosphere or a reducing atmosphere, the steel sheet is annealed in a reducing atmosphere of hydrogen and nitrogen, and thereafter, the steel sheet is cooled to around a plating temperature and immersed in molten zinc.

A method for improving the adhesion between a plating film and a steel sheet has been proposed. The first is a method of adjusting the components of a plating bath when performing hot-dip plating. Specifically, as proposed in a method of increasing the Al concentration in a hot-dip galvanizing bath, or as proposed in Japanese Patent Application Laid-Open No. 5-31373, the percentage by mass (hereinafter, the% expression representing the chemical composition means mass%). ) Al: 0.05-0.3%, Mg: 0.02-0.3%.
There is a method using a hot-dip galvanizing bath containing 3% and the balance consisting of Zn and unavoidable impurities.

Second, the chemical composition of the steel sheet is adjusted to control the degree of alloying at the interface between the plating film and the base material. For example, there is a method in which the alloying reaction rate is reduced by increasing the P content in steel, or the content of Ti, Mn, or the like, which tends to accelerate the alloying reaction, is reduced.

A third method is to coat a thin metal or an alloy in advance on the surface of a steel sheet with a pure metal or an alloy which is considered to improve plating adhesion. For example, Japanese Patent Application Laid-Open No. 62-139860 discloses a method in which P is coated on the surface of a steel sheet and then hot-dip galvanizing is applied.
A method has been proposed in which hot dip galvanizing is performed after coating with i or Mn.

[0008]

However, according to the first method, for example, when the Al concentration in the bath is increased, fine intermetallic compound particles called dross are formed in the bath, which causes surface defects of the plating film. This is not preferable because the yield of the product is impaired. The second method has a problem that sufficient improvement cannot be achieved because the consistency with a chemical composition for obtaining performance (for example, strength and formability) required for a steel sheet may not be good. Therefore, it is necessary to find a method of improving the plating adhesion without depending on the chemical composition of the steel sheet, without adversely affecting the properties of the steel sheet itself, and without impairing the properties of the plating film.

In the third method, although there is a tendency that the plating adhesion is improved as compared with the conventional method, there is no improvement in the adhesion depending on the coating conditions of the pure metal or alloy, However, this method is not always sufficient because of problems such as a significant delay in the conversion.

The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the adhesion of a plating film to a base material and to provide a molten zinc having excellent adhesion that can withstand severe molding. An object of the present invention is to provide a system-plated steel sheet and a method for producing the same.

[0011]

Means for Solving the Problems The present inventors have investigated the adhesion of a plating film of a hot-dip Zn-coated steel sheet manufactured by applying a hot-dip zinc-based plating to a base material and the crystallographic morphology near the base material surface. Was studied in detail using a high-resolution transmission electron microscope. As a result, a body-centered cubic (b
cc) A layer based on a lattice and exhibiting an ordered structure mainly composed of Fe and Al atoms (substitution type ordered solid solution.
It has been found that the presence of the "ordered structure layer") provides excellent adhesion to the plating film thereon.

This ordered structure often forms a so-called B2 ordered lattice in which Al atoms are substituted at the body center of the body-centered cubic lattice, which is a crystal structure of αFe, to form a solid solution.
Such a regular structure is determined by analyzing regular lattice reflection in an X-ray or electron diffraction pattern. Also,
It can also be confirmed by directly observing the atomic arrangement with a high-resolution electron microscope.

The regularly arranged atoms of the ordered solid solution come to be randomly arranged as the temperature increases, and
It is said to cause irregular metamorphosis. The intermetallic compound is
It maintains a regular arrangement until it melts, and is distinguished from an ordered solid solution in this respect. The existence of the above-mentioned ordered structure at the plating interface of the base material in the hot-dip coated steel material has not been recognized so far.

The reason why the adhesion between the plating films is improved when the ordered structure layer is present on the surface of the steel sheet is not yet clear, but the Fe-A generated when the plating film is formed is not clear.
l It is presumed that this is because the consistency and affinity between the intermetallic compound, the Fe-Zn intermetallic compound, or the plating itself and the steel sheet are improved.

In order to ensure the adhesion of the plating film, a regular structure layer is generated at the time of immersion in a plating bath, and the regular structure layer does not cause random arrangement (irregularization) of the regular structure in a subsequent step. You need to do that.

It is considered that the ordered structure layer on the surface of the base material is generated by diffusion of elements such as Al and Zn contained in the plating bath into the base material and replacing Fe atoms constituting the base material. . Therefore, in order to generate an ordered structure layer during hot-dip plating, it is necessary to properly control the Al concentration in the hot-dip plating bath, the plating bath temperature, and the like.

Al sources for generating the ordered structure layer include:
In addition to the method using Al contained in the plating bath, for example, a chemical coating method such as alloy plating or molten salt plating, or a physical coating method such as a vapor deposition method, an ion plating method, or an ion implantation method. For example, Al is coated on the surface of a base material provided with a body-centered cubic lattice, and Al atoms are diffused into the base material by heating, or Al contained in the base material is diffused to the surface and segregated. It does not matter how.

The disordering of the ordered structure layer proceeds at a high temperature.
Therefore, in order to maintain the ordered structure layer obtained at the time of plating in the plated product, the plating bath temperature, the immersion time in the plating bath, the cooling rate after plating, etc. are properly controlled so that the disordering does not proceed. It is important.

The present invention has been completed on the basis of these findings, and the gist of the present invention is to provide a galvanized steel material described in the following (1) and (2) and a manufacturing method thereof described in (3) and (4). It is in.

(1) Al is added to the surface of the base material by 0.0% by mass.
A hot-dip galvanized steel material having a hot-dip coating film containing 5% or more and less than 10%, with the balance being substantially composed of Zn, wherein a body-centered cubic lattice is formed in the interface region between the plating film and the base material. A hot-dip galvanized steel material comprising a layer having a regular structure mainly composed of Fe atoms and Al atoms.

(2) The existence ratio of the layer having a regular structure is
The hot-dip galvanized steel material according to the above (1), wherein the area ratio is 10% or more of the surface of the base material. (3) a base material having at least a ferrite phase on its surface,
0.15 seconds or more and 5 seconds or less immersed in a hot-dip plating bath containing Al in an amount of 0.05% or more and less than 10% by mass% and substantially consisting of Zn and having a temperature of 420 ° C or more and 500 ° C or less. The method for producing a hot-dip galvanized steel material according to the above (1) or (2), wherein the hot-dip galvanized steel material is cooled after adjusting the amount of adhesion and cooling.

(4) Al is coated on at least the surface to be plated of a base material having a ferrite phase on its surface and heated, and then contains Al in an amount of 0.05% or more and less than 10% by mass%, and the balance substantially consists of (1) The above (1), wherein the coating is immersed in a hot-dip plating bath made of Zn at a temperature of 420 ° C. or more and 500 ° C. or less for 0.1 second or more and 5 seconds or less, plated, adjusted in adhesion amount, and cooled. Or the method for producing a hot-dip galvanized steel material according to (2).

[0023]

Embodiments of the present invention will be described below in detail. Base material: In order to interpose the ordered structure layer at the interface between the base material and the plating film, the ordered structure layer is provided in the boundary region, that is, in the surface layer portion of the base material below the plating film. For that purpose, the base material only needs to be a steel having a ferrite phase on at least its surface,
For example, mild steel for drawing such as extremely low C steel and low C steel, or high tensile steel containing various alloy elements can be used. Retained austenitic steel with ferrite phase and austenite phase, ferrite phase and bainite phase,
Alternatively, a composite structure steel or the like comprising a ferrite phase and a martensite phase may be used. It is desirable that the ratio of the ferrite phase is 10% or more in area%. The form of the base material is
In addition to thin steel sheets such as cold rolled steel sheets and hot rolled steel sheets, steel pipes,
Arbitrary materials such as a shaped steel, a bar, a wire, a thick steel plate and the like can be used.

Regular structure layer of the base material: Fe is mainly formed on the surface layer of the base material under the plating film of the plated steel material, that is, in the boundary region.
It is assumed that a regular structure layer composed of atoms and Al atoms exists. Mainly, the composition of the ordered structure layer is mainly composed of Fe atoms and Al atoms, but Zn is used instead of Al.
, Si, P and the like may be contained in a small amount as an impurity or an additive element.

The ordered structure layer of the present invention can be confirmed by observing the plated steel material in a sectional direction using a transmission electron microscope and analyzing an electron diffraction pattern obtained from a surface layer portion of the base material.
For example, when a layer having the ordered structure of the present invention is present on a steel surface where the base material shows a body-centered cubic structure, electron diffraction patterns from the <110> direction of the base material include [110], [200], [ Diffraction spots from crystal planes such as [112] can be seen, but diffraction spots from the [100] plane can be confirmed in addition to these in the electron diffraction pattern near the base metal surface. Can be confirmed.

The presence of the ordered structure layer improves the adhesion of the plating film, but in order to obtain better adhesion, the coverage of the ordered structure layer on the surface of the base material is determined by reducing the surface of the base material provided with the plating film. (Hereinafter,% representing the ordered structure layer)
(The indication means the above-mentioned area ratio). More preferably, it is 50% or more, further preferably 80% or more.

The above-mentioned coverage was determined by conducting a mapping analysis (area analysis) using Al and Fe elements on a steel material in which the existence of the ordered alloy layer was confirmed by an Auger electron spectrometer, and determining the area of the region confirmed to be the ordered alloy layer. It can be determined from the area ratio.

Chemical composition of plating film; Al: Al concentration is 0.05% or more during hot-dip plating;
Use a plating bath that is less than 10%. If the Al concentration of the plating bath is less than 0.05%, the formation of the ordered structure layer on the surface of the base material when immersed in the plating bath becomes insufficient, and disappears during the later alloying treatment, and the product base material is lost. The ordered structure layer cannot be formed on the surface. Preferably 0.08%
As described above, in order to further suppress the formation of the Fe-Zn intermetallic compound, the content is desirably 0.1% or more.

If the Al concentration in the plating bath exceeds 10%, the workability of the plating film obtained is impaired. For this reason, the Al concentration in the plating bath is set to less than 10%. It is preferably at most 6%. However, as described later, in the case where the base material surface is coated with Al prior to plating and then heated to introduce a regular structure, the plating bath may not contain Al.

The balance is substantially Zn. Substantially means, besides inevitable impurities, Ti and Mg for promoting alloying of 0.5% or less, Si for controlling alloy phase of 2% or less, and Ni of 0.5% or less. , Mn is 0.2% or less,
This means that 0.2% or less of Pb or Sb may be contained for controlling the surface appearance.

The amount of plating applied is not particularly limited, but is preferably 20 g / m ² or more per plated surface in order to obtain desired corrosion resistance. The product surface after plating may be subjected to a known chromate treatment, chemical conversion treatment, or the like.

Manufacturing method: A preferable manufacturing method of the plated steel material of the present invention will be described below by taking a case where a thin steel sheet is used as a base material as an example.

Pretreatment: The base material is first subjected to a pretreatment, but there is no problem if it is within the range of the prior art. Usually, alkali degreasing is performed. Thereafter, if the base material requires annealing, it is annealed in a reducing atmosphere by a conventional method,
Then, it is cooled to the vicinity of the plating bath temperature, and A
Immersion in a hot-dip plating bath containing l. If annealing is not required, heat to around the plating bath temperature by the usual method,
It is immersed in a hot-dip plating bath to apply hot-dip plating.

When a plating bath containing no Al is used, or when it is desired to enhance the adhesion, it is preferable to provide an Al coating layer on the base material prior to plating. In this case, the Al coating method is optional. For example, a known chemical coating method such as alloy plating or molten salt plating, or a known physical coating method such as a vapor deposition method, an ion plating method, or an ion implantation method. A preferred method is to coat the base material surface with Al by a method or the like and then to heat the base material to diffuse Al atoms into the base material.

In this case, the Al coating layer has a thickness of 0.1 μm.
A degree is enough. The method of diffusing Al atoms into the base material is, for example, in a temperature range of 400 to 500 ° C., 1 to 6
A method such as heating in the range of 0 seconds is suitable.

The hot-dip plating is preferably performed by using a known continuous hot-dip plating equipment, but other conventional methods such as a batch processing method may be used. The Al content of the plating bath may be the one containing a desired amount of Al in the plating film.

The plating bath temperature is set to 420 ° C. or more and 500 ° C. or less. When the temperature is lower than 420 ° C., the hot-dip plating becomes difficult.
Is insufficiently diffused into the base material, and the ordered structure is not sufficiently formed. If the temperature exceeds 500 ° C, Zn vapor increases, contaminates the steel sheet before plating and causes defects, and dross increases in the plating bath due to an increase in the amount of Fe eluted into the bath, and adheres to the plating film. Operational problems such as surface defects.

The immersion time in the plating bath is 0.1 second or more,
Seconds or less. If the immersion time is less than 0.1 second, the formation of the ordered structure is not sufficient, and if it exceeds 5 seconds, the production efficiency decreases, which is not good.

After being immersed in the plating bath, it is pulled up, the amount of adhesion is adjusted by a known method such as gas wiping, and then cooled by a conventional method. The cooling rate is not particularly limited. After the cooling, post-treatments such as temper rolling, chromate treatment, and chemical conversion treatment may be performed by a conventional method.

The method for producing a plated steel material of the present invention described above does not require any special equipment and is a method for controlling the production conditions within an appropriate range, so that it can be carried out efficiently and at low production cost. .

[0041]

EXAMPLE A cold-rolled steel strip having a thickness of 0.8 mm and a width of 200 mm as defined in JIS-G3141 was used as a base material, which was degreased in a 10% aqueous sodium hydroxide solution. Using a hot-dip plating simulator, 10
% By volume, the balance consisting of nitrogen gas, heated to 760 to 800 ° C. in an atmosphere having a dew point of −30 ° C. or less, subjected to recrystallization annealing for 60 seconds, cooled to near the plating bath temperature, and Immersed in hot-dip bath at various temperatures for various times,
Pull up and use gas wiping method to apply 60
It was adjusted to g / m ² and cooled to room temperature. The Al concentration in the plating bath was varied.

Some base materials were degreased with trichloroethane at 70 ° C., and then heated in a weakly oxidizing atmosphere at 500 ° C. for 30 seconds, and then heated in a mixed gas atmosphere of nitrogen and hydrogen at 760 to 800 ° C. for 60 seconds. did. After that, the steel plate is 460-55
After cooling to 0 ° C., Al was deposited on the surface of the base material by a sputter deposition method using Al having a purity of 99.9% as a target.
In the vicinity of the surface, based on body-centered cubic (bcc) lattice, mainly Fe
And a regular structure layer composed of Al atoms. A shielding plate having holes in various patterns was inserted between the base material and the Al evaporation source to change the area coverage of the ordered structure layer.

A test steel sheet subjected to these treatments was subjected to a predetermined A
It was immersed in a hot-dip galvanizing bath at a concentration of 460 ° C. for 3 seconds, and hot-dip galvanized so that the amount of coating per one side was 60 g / m ² .

The performance of the obtained steel sheet was evaluated by the following method. Confirmation of ordered structure: For a cross-section sample of a plated steel sheet for observation with an electron microscope, a focused ion beam processing device was used to observe a region of about 15 μm × 15 μm including the interface between the plating film and the steel sheet at about 100 nm (nanometer) ). Using a transmission electron microscope, electron diffraction patterns are collected from selected regions on the interface of the base metal side, and regular lattice diffraction spots of the body-centered cubic lattice ([100],
[111], [210], etc.), the presence or absence of a rule structure layer was determined. Observations were made for 10 or more visual fields per sample.

With respect to the steel sheet whose ordered structure was confirmed by the above observation, mapping analysis (area analysis) using Al and Fe elements was performed using an Auger electron spectrometer, and the area ratio of the region confirmed to be ordered was determined. investigated.

Edge corrosion resistance after coating: using a shearing device in which the clearance of the cutting blade is adjusted so that the burrs of the cut end surface become 10% of the plate thickness, width: 70 mm, length: 150 mm
Of test pieces were collected. After degreasing, surface conditioning and chemical conversion treatment, these were subjected to cationic electrodeposition coating with a thickness of 20 ± 1 μm, and baked at 175 ° C. for 25 minutes. After that, intermediate coating (40 μm) of alkyd paint for automobiles, baking,
Top coat of melamine polyester paint (40μm),
Three coats of baking and three bake coatings were applied.
Spray the test piece with salt water (5% saline at 35 ° C, 7 hours)
→ Dry (50 ° C, 20 minutes) → Wet (RH 85%, 50
(C, 15 hours) as one cycle, and after performing 60 cycles of the corrosion cycle test, the red rust occurrence area ratio of the end face was evaluated in the following stages. No occurrence of red rust: 、, 5% or less: ○, more than 5%, 10% or less: Δ, more than 10%, 30% or less: ×, more than 30%: XX.

Corrosion resistance of the part having coating flaws: A test piece coated in the same manner as in the method for evaluating end face corrosion resistance after coating was provided with cut flaws reaching the base material base with a cutter knife, and the same corrosion cycle test as described above was conducted. After the cycle, the red rust generation area ratio of the flaw was evaluated based on the same criteria as the end face corrosion resistance after coating.

Adhesion of plating film: A circular blank having a diameter of 90 mm was sampled from a plated steel sheet, deep drawn into a cylindrical cup having a diameter of 50 mm and a depth of 28 mm, and an adhesive tape was applied to the plating film on the side wall surface. The area ratio of the peeled pieces adhered to the pressure-sensitive adhesive tape was visually judged and evaluated according to the following criteria. No peeling: 、, less than 10%: 、, more than 10%, less than 30%: Δ, 30% or more: x.

Table 1 shows the plating conditions, the Al content of the plating film, the coverage of the ordered structure layer, and the results of the adhesion test.

[0050]

[Table 1]

As shown in Table 1, when the ordered structure layer was provided, the corrosion resistance at the end face, the corrosion resistance at the flaws, and the adhesion were good. In particular, the coverage of the ordered structure layer is 20%
Those having good adhesiveness had good adhesion, and those having 70% or more had particularly excellent end face corrosion resistance and flaw portion corrosion resistance. On the other hand, when the ordered structure layer was not formed as in Test No. 19 or 21, or in Test No. 20 in which the Al content of the plating film was too high, these performances were not good.

[0052]

The hot-dip galvanized steel material of the present invention is excellent in the adhesion of the plating film at the time of processing, and is also excellent in the corrosion resistance of the cut end face and the flawed portion. Further, the steel material of the present invention can be efficiently and inexpensively manufactured by optimizing the manufacturing conditions by using known equipment.
Therefore, it is extremely useful as a material for automobiles, home appliances, building materials, and the like.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Masahiko Hori 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. F-term (reference) 4K027 AA02 AA22 AB02 AB07 AB26 AB32 AB42 AC12 AC15 AC18 AE02 AE03 AE11 AE18

Claims (4)

[Claims] (1) 0.05% by mass of Al on the surface of a base material
A hot-dip galvanized steel material having a hot-dip galvanized film containing less than 10% and a balance of substantially Zn, based on a body-centered cubic lattice in an interface region between the plated film and the base material, A hot-dip galvanized steel material comprising a layer having a regular structure mainly composed of Fe atoms and Al atoms. 2. The hot-dip galvanized steel material according to claim 1, wherein the abundance ratio of the layer having the ordered structure is 10% or more of the area of the base material in terms of area ratio. 3. A base material having at least a ferrite phase on its surface, containing Al in an amount of 0.05% or more and less than 10% by mass%, and the balance substantially consisting of Zn.
The hot-dip galvanized plating according to claim 1 or 2, wherein the coating is immersed in a hot-dip plating bath at a temperature of 00 ° C or lower for 0.10 seconds or more and 5 seconds or less, plated, adjusted in adhesion amount, and then cooled. Method of manufacturing steel. 4. Al is coated on at least the surface to be plated of a base material having a ferrite phase on the surface and heated, and thereafter, Al is contained in an amount of 0.05% or more and less than 10% by mass%, and the balance is substantially Zn. The plating is performed by dipping in a hot-dip plating bath of 420 ° C. or more and 500 ° C. or less for 0.1 second or more and 5 seconds or less, adjusting the amount of adhesion, and then cooling. 3. The method for producing a hot-dip galvanized steel material according to item 1.