KR101359107B1 - Galvanized steel sheet having excellent coatibility and coating adhesion and method for manufacturing the same - Google Patents

Abstract

One side of the present invention is a hot-dip galvanized steel sheet includes a holding plate, a plurality of uneven parts formed on the surface of the holding plate and a plated layer formed on the holding plate formed with the uneven parts, the total area of the concave portion of the uneven part is the holding steel sheet area Compared to 50 to 95%, the convex portion of the uneven portion may include a metal having a Gibbs free energy change amount per mol of oxygen during the oxidation reaction (delta G) or more than Gibbs free energy change amount per mole of oxygen during the oxidation reaction of Mn. In addition, another method of manufacturing a hot-dip galvanized steel sheet of the present invention comprises the steps of preparing a steel sheet, coating a metal oxide on the prepared steel sheet; Pressing the metal oxide-coated base steel plate with a penetrator to form an uneven portion, annealing the steel plate on which the uneven portion is formed, cooling the annealed steel sheet, and hot-dip galvanizing the cooled steel sheet. It includes a step, the Gibbs free energy change amount (delta G) per one mole of oxygen during the metal oxidation of the metal oxide, may be more than the Gibbs free energy change per mole of oxygen during the oxidation reaction of Mn.

Description

Hot-dip galvanized steel sheet with excellent plating property and plating adhesion and its manufacturing method {GALVANIZED STEEL SHEET HAVING EXCELLENT COATIBILITY AND COATING ADHESION AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a hot-dip galvanized steel sheet and a method for manufacturing the same widely used in automobiles, building structures and home appliances.

Recently, as the safety regulations of automobiles are strengthened and as part of environmentally-friendly efforts to reduce the emission of greenhouse gases, demand for high strength and light weight of automobile steel sheets is increasing. To this end, research is being actively conducted on DP (Dual Phase) steels and TRIP (Transformation Induced Plasticity) steels containing a large amount of non-plating elements such as Si, Mn or Al.

The hot-dip galvanized steel sheet is a plated layer formed by immersing a steel plate in an Al-containing galvanizing bath, thereby forming an alloying inhibitor layer called Fe ₂ Al _{5 at} the interface between the base steel plate and the galvanized layer, and suppressing the alloying. The layer serves to increase the adhesion between the steel sheet and the galvanized layer. However, in the case of a high-strength steel containing a large amount of Si, Mn or Al, Si, Mn or Al diffuse to the surface of the steel sheet to form an oxide in a portion where the alloying suppression layer is not formed, , Thereby causing peeling of the plating layer.

In order to solve this problem, Patent Literature 1 has been proposed. However, as the annealing process is controlled to 750 ° C. or more, the coated Ni forms a Ni-Fe alloy by interdiffusion with the base steel sheet, so that the Ni coating layer is almost present. If not, or the thickness of the coating layer is thin, there is a limit to suppress the surface thickening of Si, Mn or Al. Therefore, there is an urgent need for a technology capable of ensuring excellent plating properties and plating adhesion of high strength hot-dip galvanized steel sheets.

Japanese Patent Publication No. 2008-195987

With respect to hot-dip galvanized steel sheet, there is a need for a method capable of ensuring excellent plating properties and plating adhesion by suppressing the surface concentration and oxidation of Si, Mn or Al, which are non-plating elements contained in steel, during annealing.

One side of the present invention is a hot-dip galvanized steel sheet includes a holding plate, a plurality of uneven parts formed on the surface of the holding plate and a plated layer formed on the holding plate formed with the uneven parts, the total area of the concave portion of the uneven part is the holding steel sheet area Compared to 50 to 95%, the convex portion of the uneven portion may include a metal having a Gibbs free energy change amount per mol of oxygen during the oxidation reaction (delta G) or more than Gibbs free energy change amount per mole of oxygen during the oxidation reaction of Mn.

In addition, another method of manufacturing a hot-dip galvanized steel sheet of the present invention comprises the steps of preparing a steel sheet, coating a metal oxide on the prepared steel sheet; Pressing the metal oxide-coated base steel plate with a penetrator to form an uneven portion, annealing the steel plate on which the uneven portion is formed, cooling the annealed steel sheet, and hot-dip galvanizing the cooled steel sheet. It includes a step, the Gibbs free energy change amount (delta G) per one mole of oxygen during the metal oxidation of the metal oxide, may be more than the Gibbs free energy change per mole of oxygen during the oxidation reaction of Mn.

According to one aspect of the present invention, it is possible to suppress the surface concentration and oxidation of Si, Mn or Al to improve the plating property, plating adhesion of the hot-dip galvanized steel sheet. In addition, when Fe oxide (Fe ₂ O ₃ ), a by-product of the pickling process, is used as one of the metal oxides, manufacturing costs can be minimized, which is advantageous in terms of economy.

1 is a schematic diagram of a manufacturing method of a conventional hot-dip galvanized steel sheet.
2 is a schematic diagram of a method of manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present invention.

The present inventors studied a method for providing a hot-dip galvanized steel sheet having excellent plating property and plating adhesion. As a result, the steel sheet was coated in the form of a layer of metal oxide, and the coated layer was formed using an indenter. By forming a concave-convex portion by indentation, it is possible to prevent the formation of concentrates, such as Si, Mn, Al on the surface of the steel sheet during annealing, to improve plating properties, and to prevent the plating layer from falling off during the forming process. Recognizing that it is possible to improve the present invention has been achieved.

As shown in FIG. 1, in the related art, when the base steel sheet 1 is annealed, Si, Mn, Al contained in the base steel sheet 1 is concentrated on the surface, or Si, Mn, Al oxides 2 are formed. When the plating layer 3 was not formed or the plating layer 3 was formed, peeling could easily occur. According to one aspect of the present invention, in order to solve this problem, after forming an uneven portion made of a metal oxide of the appropriate size on the surface of the base steel sheet, and annealing in a reducing atmosphere, the upper portion of the metal oxide It is reduced to metal and the lower part is replaced by Si, Mn and Al oxides by substitution reaction. Therefore, even if the plating is performed, the plating layer is not in direct contact with the non-plating Si, Mn and Al oxide to improve the plating properties, it is possible to minimize the plating peeling. 2 illustrates an embodiment of the present invention, after forming the metal oxide coating layer 5 (including the metal oxide 4) on the steel sheet 1, press-fitted using an indenter (concave portion and Convex portion) and annealing in a reducing atmosphere, the upper portion of the metal oxide is reduced to a metal (for example, reduced Fe (6)), and the lower portion is replaced by Si, Mn and Substituted with Al oxide (7), the plating layer (3) can be easily formed, the plating peelability can also be reduced.

Hereinafter, the hot-dip galvanized steel sheet which is one side of the present invention will be described in detail. The hot-dip galvanized steel sheet may include a plated steel plate, an uneven portion formed on the surface of the plated steel sheet, and a plated layer formed on the plated steel sheet on which the uneven portion is formed. In addition, the plating layer is preferably a galvanized layer, the hot-dip galvanized steel sheet may include an alloyed hot-dip galvanized steel sheet including an alloy heat treatment.

The hot-dip galvanized steel sheet includes a base steel sheet. The steel sheet may include at least 0.5% by weight of one or two or more selected from the group consisting of Si, Mn and Al. However, the present invention can be applied even when the content is lower than the above, and steel grades containing a large amount of the elements are only limited to these lower limits because unplating or plating peeling may be a problem. In addition, the upper limit is not particularly limited, and due to the Si, Mn, Al, and the like, all kinds of steels that are easily caused by unplating or plating peeling may be included.

In addition, it includes a plurality of irregularities formed on the surface of the base steel sheet. The irregularities are preferably controlled to be discontinuously distributed, and in order to effectively improve the wettability and plating adhesion of the zinc layer after the zinc plating process, the interval between the convex portions of the irregularities is preferably set to 5 μm or less. The shape is not specifically limited other than the area mentioned later in this invention.

Then, it is preferable that the area fraction of the concave portion of the unevenness is 50 to 95% of the area of the steel sheet. If the area fraction of the concave-convex portion is less than 50%, the metal oxide is present in the form of a layer. Subsequently, during the reduction annealing process, the metal oxide is reduced to the metal, and the Si, Mn, or Al oxides contained in the steel are plated and plated. It is formed in the form of a layer (layer) at the interface, which causes the plating layer peeling off the plating layer. In addition, when the area fraction of the concave and convex portions of the concave and convex portions exceeds 95%, the amount of metal oxides corresponding to the convex portions of the concave and convex portions distributed on the surface of the steel sheet is small, effectively suppressing the concentration of Si, Mn, or Al contained in the steel to the surface. Since the plating is inferior and the plating layer is peeled off after galvanizing.

In addition, the convex portion is preferably made of a metal oxide, the metal element of the metal oxide is preferably the Gibbs free energy change (delta G) per mol of oxygen during the oxidation reaction is more than the Gibbs free energy change per mol of oxygen during the oxidation reaction of Mn. Do. Specifically, it is more preferable that it is one kind or two or more kinds of Ni, Fe, Co, Cu, Sn, and Sb. This is because the metal has a larger value than the Gibbs free energy change per mole of oxygen during the oxidation reaction of Si, Mn or Al, so it is easy to cause a substitution reaction with Si, Mn or Al. In addition, Fe ₂ O ₃ which is a by-product of the pickling process may be used as one of the metal oxides in the present invention, and thus manufacturing cost may be reduced.

And it is preferable to control the height of the said convex part to 0.1-1.0 micrometer. When the thickness is less than 0.1 μm, the amount of metal oxide coated on the surface of the base steel sheet is so small that all of it is reduced to Fe in the early stage of annealing in a reducing atmosphere, so that Si, Mn or Al contained in a high strength steel is concentrated to the surface. As a result, the plating property is lowered by not suppressing the formation of the oxide. On the other hand, when the height of the convex portion is increased, the upper limit is not only economical but also because the height limit of the indenter does not discontinuously short-circuit the metal oxide coating layer, resulting in peeling of the metal oxide layer in the form of a layer. It is preferable to set it as 1.0 micrometer.

Hereinafter, the manufacturing method of the hot-dip galvanized steel sheet which is another aspect of this invention is demonstrated in detail. The manufacturing method comprises the steps of preparing a steel plate, coating a metal oxide on the prepared steel plate, the metal oxide-coated steel plate to form a concave-convex portion by indentation (Penetrator), steel plate formed with the concave-convex portion Annealing, cooling the annealed steel sheet and hot-dip galvanizing the cooled steel sheet may include.

First, prepare the steel sheet. As described above, the base steel sheet is not particularly limited, but a steel grade containing a large amount of Si, Mn or Al may be applied.

Metal oxide is coated on the prepared steel sheet. The metal oxide may be coated on the surface of the steel sheet by immersing the steel sheet in a solution containing the metal oxide. The solution containing the metal oxide preferably includes 1) metal oxide and water or 2) metal oxide and organic solvent. In addition, the metal element of the metal oxide is preferably a metal in which the Gibbs free energy change amount (delta G) per one mole of oxygen during the oxidation reaction is more than the Gibbs free energy change amount per one mole of oxygen during the oxidation reaction of Mn. In addition, the metal is more preferably one or two or more of Ni, Fe, Co, Cu, Sn and Sb. The content of the metal oxide in the solution containing the metal oxide is preferably limited to 10 to 30% by weight. If the metal oxide is less than 10% by weight in the solution, the amount of the metal oxide contained in the coating layer is not sufficient. It is difficult to expect the effect of the present invention due to the small amount of discontinuous metal oxide after indentation with the indenter. If the metal oxide exceeds 30% by weight in the solution, the possibility of exceeding the upper limit of the thickness of the coating layer to be controlled increases. In addition, due to the difference in specific gravity, a considerable amount of the metal oxide in the solution sinks, leading to the disadvantage that solution management is required rather than increasing the effect of the present invention.

In addition, the thickness of the metal oxide coating layer is preferably controlled to 0.1 ~ 1.0㎛. When the thickness of the metal oxide coating layer is less than 0.1 μm, the amount of metal oxide coated on the surface of the steel sheet is so small that all of it is reduced to Fe in the early stage of annealing in the reducing atmosphere. The plating property is lowered by failing to suppress the concentration on the surface and forming the oxide. On the other hand, when the metal oxide coating layer is thick, the upper limit is not only economical but also because the height limit of the indenter does not discontinuously short-circuit the metal oxide coating layer, resulting in peeling of the metal oxide layer in the form of a layer. It is preferable to control to 1.0 micrometer. At this time, if the height of the concave-convex portion (convex portion) to be described later and the thickness of the coating layer are the same, since the coating layer is hardly completely discontinuously, the height of the concave-convex portion should be about 6/5 of the thickness of the coating layer. Painting is effectively done.

As described above, the uneven portion is formed on the steel sheet coated with the metal oxide. By using the indenter (Penetrator), by pressing the steel sheet coated with the metal oxide, one or a plurality of irregularities are formed. In the present invention, the shape of the indenter is not particularly limited except for the height thereof. It is preferable to control the height of the indenter to 1.0 to 2.0 µm. If the height of the indenter is less than 1.0 μm, the metal oxide coating layer cannot be penetrated and the grooves are not grooved into the base metal, resulting in discontinuous distribution of the metal oxide. Accordingly, the Si, Mn, Al contained in the steel is concentrated on the surface during the reduction annealing process to form a layer (Si), Mn, Al oxide in the layer (layer) to cause galvanization, plating peeling. In addition, when the height of the indenter exceeds 2.0㎛ the metal oxide coating layer can be sufficiently discontinuously short-circuited, but the grooves are deeply dug deep into the base steel sheet, the plating surface is inferior after galvanization, the plating quality is poor. Therefore, the height of the indenter for discontinuously controlling the metal oxide coating layer coated on the surface of the steel sheet to control the discontinuous distribution of the metal oxide coating layer and to minimize the surface irregularities of the steel sheet to prevent the heat quality of the plating quality is 1.0 It is preferable to control to -2.0 micrometers.

As described above, the steel sheet on which the irregularities are formed can be annealed in a reducing atmosphere. In the annealing process, the upper portion of the metal oxide is reduced to the metal due to the reducing atmosphere. In the lower part of the metal oxide, Si, Mn or Al causes a substitution reaction with the metal oxide in the process of diffusing a large amount of Si, Mn or Al contained in the steel sheet surface. Through this, unplating can be prevented by preventing the oxide of Si, Mn or Al from being concentrated to the surface. The annealing in the reducing atmosphere is preferably performed at 750 to 850 ° C. If the heating temperature in the annealing process is lower than 750 ° C., it becomes difficult to secure material properties such as tensile strength or elongation of the steel as well as metal. The reduction rate of the oxide can cause the wettability of zinc and dropping of the plating layer due to the remaining metal oxides. If it exceeds 850 ° C, the reduction rate of the metal oxide is faster than that of the non-plating element, and the diffusion rate of the non-plating element is increased. Increasingly, it is not effective in suppressing the surface thickening and oxidation of the non-plating element by Fe metal oxide. Therefore, it is preferable to control the heating temperature in the annealing process to 750 ~ 850 ℃.

Through such a process, it is preferable that the total area fraction of the concave portion of the uneven portion is 50% or more and 95% or less than that of the steel sheet. As described above, through this, it is possible to manufacture a high strength hot dip galvanized steel sheet excellent in plating properties and plating adhesion.

The steel sheet may be cooled after the annealing step. The method of cooling here is not specifically limited, Any method may be used.

The cooled steel plate may be immersed in a plating bath to form a plating layer. The temperature of the plating bath is preferably controlled to 440 ~ 460 ℃, if the temperature of the plating bath is less than 440 ℃ the viscosity of the plating bath is increased to reduce the mobility of the roll (roll) winding the steel sheet and It causes slippage between the rolls, which causes defects in the steel sheet. In addition, when the temperature of the plating bath exceeds 460 ℃ to promote the dissolution of the steel sheet to accelerate the generation of dross in the form of Fe-Zn compound to generate an unplated. Therefore, in order to minimize the occurrence of defects of the steel sheet, it is preferable to control the temperature of the plating bath to 440 ~ 460 ℃.

After the plating step, the hot-dip galvanized steel sheet may be additionally alloyed and heat treated. By controlling the alloying heat treatment temperature to 480 ° C. or higher, the Fe content can be sufficiently secured in the galvanized layer, and by controlling the temperature to 600 ° C. or lower, the powdering phenomenon of the plating layer falling off during the processing of excessive Fe content in the plating layer can be prevented. have.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

The steel sheet was cold rolled, and the surface of the steel sheet was cleaned by degreasing and pickling, and then Fe ₂ O ₃ metal oxide was mixed with an organic solvent and applied by a bar coater. At this time, the thickness of the coating layer was controlled as described in Table 1 below. In order for the metal compound to adhere well to the steel sheet, the organic resin was dissolved in a solution and a binder was added. In order to form the uneven portion, the indenter was press-fitted in a direction perpendicular to the steel plate at an appropriate load, and the height of the indenter was controlled as described in Table 1 below. Thereafter, by blowing nitrogen gas containing 5% hydrogen in a reducing atmosphere, the steel sheet subjected to the annealing process for 60 seconds was immersed in a galvanizing bath for 3 seconds according to the temperature shown in Table 1 below. The amount of plating deposited on the surface was maintained at 60 g / m 2 through wipping).

In order to measure the area fraction of the concave and convex portions formed on the steel sheet after the metal oxide coating process, the measurement was performed using a scanning electron microscope (SEM), and the values thereof are shown in Table 1 below. In addition, the thickness of the coating layer and the height of the indenter were measured and shown in Table 1 below. In addition, in order to evaluate the plating property of the steel plate after the plating process, the coating area ratio of the zinc plated layer to the total surface area of the plated was measured, and the values thereof are shown in Table 1 below. In addition, for cross-sectional observation, the specimen was cut to 15 × 15 mm 2, the cross section was polished, and the plated layer was observed by scanning electron microscope. In addition, in order to measure the plating adhesion of the steel sheet, a bending test was performed after bending the specimen having a size of 30 × 80 mm 2 at 180 ° angle. Depending on the material properties of the steel sheet, 0T or 1T bending was performed in a range where the material did not break. When the transparent vinyl tape was attached to the bending part and peeled off, the plating layer appeared as 'peeling', and when the plating layer did not come out at all, it was described in Table 1 as 'non-peeling'.

division Coating layer thickness
(탆) Indenter
Height (㎛) Annealing temperature
(℃) Concave
Area fraction (%) Zinc plated layer
Coverage area ratio (%) Plated
Adhesiveness Inventory 1 0.8 1.0 810 51 98 Non-peel Inventive Example 2 0.75 1.15 835 52 97.5 Non-peel Inventory 3 1.0 1.1 830 54 98.5 Non-peel Honorable 4 0.65 1.9 820 60 99 Non-peel Inventory 6 0.4 1.2 815 53 95 Non-peel Comparative Example 1 2.2 1.1 800 8 83 Exfoliation Comparative Example 2 1.0 1.3 890 52 84 Exfoliation Comparative Example 3 0.8 0.4 825 5 80 Exfoliation Comparative Example 4 0.7 1.2 710 55 82 Exfoliation Comparative Example 5 0.9 2.5 830 58 86 Exfoliation Comparative Example 6 0.08 1.1 813 56 85 Exfoliation Comparative Example 7 0.85 1.3 824 38 87 Exfoliation

As shown in Table 1, Inventive Examples 1 to 6, the thickness of the metal oxide coating layer, the height of the indenter, the heating temperature in the annealing process all satisfy the conditions controlled by the present invention, the metal oxide coated on the surface of the steel sheet The area fraction of the uneven portion controlled discontinuously was more than 50%. As a result, the coating area ratio of the zinc plated layer was all 95% or more, showing excellent plating property, and no peeling part was found.

On the contrary, in Comparative Example 1, the thickness of the coating layer exceeded the range controlled by the present invention, and the metal oxide coating layer was not discontinuously shorted due to the height limit of the indenter as well as the economic aspect. It caused the plating to peel off.

In addition, in Comparative Example 2, the annealing temperature exceeds the range controlled by the present invention, so that the reduction rate of the Fe metal oxide is faster than that of the non-plating element, and the diffusion rate of the non-plating element is increased, thereby increasing the non-plating element by the Fe metal oxide. It was not effective in inhibiting surface thickening and oxidation. As a result, the coating area ratio of the zinc plated layer was low as 84%, causing the plating layer to fall off due to Si, Mn or Al oxide.

In Comparative Example 3, the height of the indenter was less than the range controlled by the present invention, so that the metal oxide coating layer could not be penetrated and the grooves were not penetrated into the base steel sheet. As a result, the metal oxide was not discontinuously distributed. Accordingly, after galvanization, Si, Mn, Al contained in the steel was concentrated on the surface to form an oxide layer, so that the coating area ratio of the galvanized layer was low as 80%, which caused plating peeling.

In addition, in Comparative Example 4, the annealing temperature is less than the range controlled by the present invention, so it is difficult to secure material properties such as tensile strength or elongation of the steel excellently, and the reduction rate of Fe oxide is low so that the wettability of zinc due to the remaining Fe oxide is low. And it can cause the plating layer to fall off and accordingly the area fraction of the zinc plated layer is also low as 82% and eventually the plating peeling occurs, the plating adhesion was not good.

In addition, in Comparative Example 5, although the height of the indenter exceeds the range controlled by the present invention, the metal oxide coating layer may be sufficiently discontinuously shorted, but after the galvanization, the plating surface is heated after the groove is deeply dug into the steel sheet. For this reason, the coating area ratio of the zinc plated layer was low as 86%, resulting in poor plating quality.

In Comparative Example 6, the thickness of the coating layer was so thin that the amount of metal oxide coated on the surface of the steel sheet was so small that all of it was reduced to Fe in the early stage of annealing in the reducing atmosphere. The coating area ratio of the zinc plated layer was 85%, causing the plating peeling phenomenon together with the unplated due to the fact that it was not inhibited from forming on the surface and forming oxide.

In addition, in Comparative Example 7, even if the thickness of the metal oxide coating layer, the height of the indenter, and the annealing temperature meet the range controlled by the present invention, the area fraction of the concave-convex portions of the concave-convex portions controlling the metal oxide coating layer to be discontinuously distributed is less than 50%. Due to this, Si, Mn, and Al oxides, which are concentrated on the surface in the process of reducing annealing by connecting metal oxides with each other, are formed in a layer form, resulting in inferior wettability of zinc, and the coating area ratio of the zinc plating layer is 87%. As it is less than 95%, the plating property is not good, and the plating layer is dropped.

1. steel sheet,
2.Si, Mn, Al oxides,
3. plating layer,
4. metal oxides,
5. metal oxide coating layer,
6. Reduced Fe
7. Si, Mn, Al oxides.

Claims (10)

Base steel sheet; A plurality of irregularities formed on the surface of the steel sheet; And a plating layer formed on the base steel sheet on which the uneven portion is formed.
The total area of the concave portion of the concave-convex portion is 50 to 95% of the area of the holding steel sheet, and the convex portion of the concave-convex portion has a change in Gibbs free energy per mole of oxygen during the oxidation reaction of delta G per oxidized reaction of Mn. Hot-dip galvanized steel sheet comprising a metal that is above.
The hot dip galvanized steel sheet according to claim 1, wherein the convex metal element is at least one of Ni, Fe, Co, Cu, Sn, and Sb.
The hot dip galvanized steel sheet according to claim 1, wherein the height of the convex portion is 0.1 to 1.0 µm.
The hot dip galvanized steel sheet according to claim 1, wherein the convex portion has a spacing of 5 µm or less (excluding 0).
Preparing a base steel sheet; Coating a metal oxide on the prepared steel sheet; Forming a concave-convex portion by press-fitting the base steel plate coated with the metal oxide with an indenter; Annealing the steel sheet on which the irregularities are formed; Cooling the annealed steel sheet; And hot-dip galvanizing the cooled steel sheet,
The Gibbs free energy change amount (delta G) per mol of oxygen during the metal oxidation of the metal oxide is more than the Gibbs free energy change per mol of oxygen during the oxidation reaction of Mn.
The method of claim 5, wherein the metal oxide coating layer has a thickness of 0.1 μm to 1.0 μm.
The method of manufacturing a hot-dip galvanized steel sheet according to claim 5, wherein the indenter has a height of 1.0 to 2.0 µm.
The method of claim 5, wherein the annealing is performed at 750 ° C. to 850 ° C. 7.
The method of claim 5, wherein in the hot dip galvanizing, the temperature of the zinc plating bath is 440 ~ 460 ℃.
The method of claim 5, further comprising, after the hot dip galvanizing, alloying heat treatment at 480 ~ 600 ℃.

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