Steel sheet coated with aluminum-magnesium

Abstract

The present invention relates to an aluminum-magnesium coated steel plate using vacuum coating, wherein an aluminum-magnesium coating layer is constituted by 1 to 45 wt % of magnesium, a balance of aluminum, and other inevitable impurities, and an Al.sub.3Mg.sub.2 alloy phase is formed in the aluminum-magnesium coating layer by performing heat treatment of the steel plate.

Claims

1. A method of forming an aluminum-magnesium alloy layer on a steel plate, comprising: preparing the steel plate; vacuum depositing an aluminum coating layer on an upper portion of the steel plate; vacuum depositing a magnesium coating layer on an upper portion of the aluminum coating layer at least one time or more; and vacuum depositing a secondary aluminum coating layer on an upper portion of the magnesium coating layer at least one time or more; wherein the steel plate where the aluminum-magnesium coating layer is formed is subjected to heat treatment in a heat treatment furnace to perform phase transformation of the aluminum-magnesium coating layer into the aluminum-magnesium alloy layer, wherein the aluminum-magnesium alloy layer includes 12.2 to 27.7 wt % of magnesium based on a total weight of the aluminum-magnesium alloy layer, and wherein the heat treatment is performed under a condition of an inert atmosphere, a temperature in a range of 350 to 450 C., and a heat treatment time of 2 to 10 minutes.

2. The method of claim 1, further comprising vacuum depositing a secondary magnesium layer on an upper portion of the secondary aluminum layer at least one time or more.

3. The method of claim 1, wherein the magnesium coating layer that is vacuum deposited on the steel plate is reacted with iron on the steel plate by diffusion of aluminum that is vacuum deposited on the upper portion of magnesium to be vacuum deposited in a thickness in which an iron-aluminum alloy layer is formed on the coating layer.

4. The method of claim 1, wherein a thickness of an aluminum-magnesium coating layer formed of the aluminum coating layer and the magnesium coating layer is 0.5 to 30 m.

5. The method of claim 1, wherein the aluminum-magnesium coating layer is vacuum deposited by magnetron sputtering.

6. The method of claim 5, wherein the aluminum-magnesium coating layer is vacuum deposited by repeatedly reciprocating the steel plate disposed on upper portions of an aluminum source and a magnesium source.

7. The method of claim 6, wherein a composition of the aluminum-magnesium coating layer is changed by changing a current or a voltage applied to the aluminum source and the magnesium source.

8. The method of claim 4, wherein one or more of the iron-aluminum alloy layer or the aluminum-magnesium alloy layer is formed from the coating layer by the heat treatment.

9. The method of claim 8, wherein an iron component of the steel plate is diffused into the coating layer to form an Al.sub.xFe.sub.y layer, and the Al.sub.xFe.sub.y layer satisfies the following conditions: in the Al.sub.xFe.sub.y layer, x is 1 to 3 and y is 0.5 to 1.5.

10. The method of claim 9, wherein a thickness of the aluminum-iron alloy layer is 0.2 to 1 m.

11. The method of claim 8, wherein in the aluminum-magnesium alloy layer, one or more of an a phase or a phase (Al.sub.3Mg.sub.2) is formed by the heat treatment.

12. The method of claim 9, wherein a thickness of the Al.sub.xFe.sub.y layer is 1 to 50% of a thickness of the aluminum-magnesium coating layer.

13. The method of claim 11, wherein a ratio of the and phases is an XRD intensity ratio of 1(880)/1(111), which is 0.01 to 1.5.

14. A method of forming an aluminum-magnesium alloy layer on a steel plate, comprising: preparing the steel plate; vacuum depositing an aluminum coating layer on an upper portion of the steel plate; vacuum depositing a magnesium coating layer on an upper portion of the aluminum coating layer at least one time or more; and vacuum depositing a secondary aluminum coating layer on an upper portion of the magnesium coating layer at least one time or more; wherein the steel plate where the aluminum-magnesium coating layer is formed is subjected to heat treatment in a heat treatment furnace to perform phase transformation of the aluminum-magnesium coating layer into the aluminum-magnesium alloy layer, wherein the aluminum-magnesium alloy layer includes 12.2 to 27.7 wt % of magnesium based on a total weight of the aluminum-magnesium alloy layer, and wherein the heat treatment is performed under a condition of an inert atmosphere, a temperature in a range of 350 to 450 C., and a heat treatment time of 2 to 10 minutes wherein the magnesium coating layer that is vacuum deposited on the steel plate is reacted with iron on the steel plate by diffusion of aluminum that is vacuum deposited on the upper portion of magnesium to be vacuum deposited in a thickness in which an iron-aluminum alloy layer is formed on the coating layer.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates vacuum coating equipment used in an exemplary embodiment according to the present invention.

(2) FIG. 2 is a graph illustrating a change of a coating layer according to a heat treatment temperature and a heat treatment time.

(3) FIG. 3 illustrates an XRD (X-ray diffraction analysis) test result of a specimen used in FIG. 2.

(4) FIG. 4 is a scanning electron microscope (SEM) photograph illustrating a structure change of the specimen used in FIG. 2 according to heat treatment.

(5) FIG. 5 illustrates a GDLS (glow discharge light spectroscopy) analysis result of the specimen used in FIG. 2 according to heat treatment.

(6) FIG. 6 is a graph illustrating a result of a neutral salt spray test performed in order to evaluate a corrosion characteristic of a steel plate where an aluminum-magnesium coating layer is formed with respect to the specimen partially taken from the specimen used in FIG. 2.

(7) FIG. 7 is a graph illustrating an evaluation result of adhesion force of a steel plate of an aluminum-magnesium alloy layer.

(8) FIG. 8A is a transmission electron microscope (TEM) photograph illustrating a structure of the aluminum-magnesium alloy layer formed on the steel plate.

(9) FIG. 8B is a partially enlarged transmission electron microscope (TEM) photograph of FIG. 8A.

MODE FOR INVENTION

(10) Advantages and features of the present invention and methods to achieve them will be elucidated from exemplary embodiments described below in detail with reference to the accompanying drawings.

(11) However, as those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention, and on the contrary, exemplary embodiments introduced herein are provided to make disclosed contents thorough and complete and sufficiently transfer the spirit of the present invention to those skilled in the art. Therefore, the present invention will be defined only by the scope of the appended claims.

(12) Hereinafter, the present invention will be described in more detail with reference to the drawings.

(13) FIG. 1 illustrates a schematic diagram of vacuum coating equipment used to apply aluminum-magnesium on a steel plate.

(14) According to an exemplary embodiment of the present invention, in order to apply aluminum-magnesium on the steel plate, for example, a vacuum coating method may be used. In the vacuum coating method, as compared to an existing plating method, a process cost is high but a coating layer having a small thickness may be rapidly manufactured, and thus the vacuum coating method may have competitiveness in terms of productivity.

(15) In FIG. 1, magnetron sputtering is used to form an aluminum-magnesium coating layer. Aluminum and magnesium sources are simultaneously operated, and a reciprocating motion or a rotation motion of a substrate is performed over a sputtering source to form the coating layer.

(16) According to the exemplary embodiment of the present invention, as an aluminum-magnesium coating substrate, for example, a cold-rolled steel plate 200 may be used. Herein, the cold-rolled steel plate is preferably low carbon steel having a carbon content of 0.3 wt % or less, and is preferably used as a steel plate for vehicles, a steel plate for home appliances, or a steel plate for building materials.

(17) Since the cold-rolled steel plate 200is coated with a rust preventive oil, a degreasing process for removing rust preventive oil is required.

(18) The degreasing process of the steel plate 200 may be performed by using, for example, a surfactant. After degreasing of the steel plate 200, ultrasonic wave washing is performed with, for example, alcohol and acetone, and the steel plate is then equipped in vacuum coating equipment.

(19) Then, exhaustion is performed until a pressure in a vacuum vessel 100 is approximately 10.sup.5 torr or less. After this vacuum exhaustion, argon gas may be injected into the vacuum vessel 100 to apply a DC voltage of about 800 V to the steel plate 200, when the degree of vacuum approaches 1*10.sup.2 torr, and thus glow discharging is caused, thereby purifying a surface of a specimen.

(20) When purification of the specimen is finished, the vacuum equipment is subjected to exhaustion until the pressure becomes approximately 10.sup.5 torr that is a basic pressure, and aluminum-magnesium are then applied on the steel plate by using magnetron sputtering sources 300 and 400.

(21) When aluminum and magnesium are applied on the steel plate by sputtering, aluminum and magnesium are sequentially applied. In this case, it is preferable that aluminum be first applied on the steel plate. However, magnesium may be first applied on the steel plate and aluminum may then be applied, but as long as a thickness of a magnesium coating layer may be controlled to be small by subsequent heat treatment or the like to react aluminum deposited on an upper portion of magnesium with iron on the steel plate by diffusion and thus form an iron-aluminum alloy layer on the steel plate, magnesium may be first applied on the steel plate and an aluminum layer may then be applied.

(22) Further, when aluminum and magnesium are applied on the steel plate by sputtering, in the case where aluminum and magnesium are sequentially applied, a mode where after a magnesium layer is first applied on an upper portion of the aluminum layer, the magnesium layer is further applied, the aluminum layer is applied thereon, and the aluminum layer is further applied may be performed, that is, two magnesium layers and two aluminum layers may be sequentially applied on the upper portion of the aluminum layer.

(23) As described above, in the case where the same material layers are repeatedly and sequentially applied on the same material layer (for example, application is performed in the order of AlMgMgAlAlMg), it is preferable that the substrate, that is, the steel plate 200, disposed on upper portions of the sputtering sources 300 and 400 of two materials, be repeatedly reciprocated.

(24) In addition, it is preferable that a thickness of an aluminum-magnesium protective film applied on the steel plate 200 be 0.5 to 30 m. Further, by changing a current applied to the aluminum and magnesium sputtering sources 300 and 400, evaporation ratios of aluminum and magnesium may be different from each other to change an aluminum-magnesium composition.

(25) The aluminum-magnesium coating layer is formed on the steel plate 200 by the aforementioned sputtering method, and the aluminum-magnesium coating layer formed on the steel plate forms a multi-layered structure.

(26) In this case, a content of magnesium in the aluminum-magnesium coating layer is preferably 1 to 45 wt %, and more preferably 9 to 40 wt %.

(27) As described above, it is preferable that the steel plate where the aluminum-magnesium coating layer is formed be subjected to heat treatment in a vacuum heat treatment furnace.

(28) As the vacuum heat treatment furnace, a heat treatment furnace formed by continuously connecting a preheating furnace, a heat treatment furnace, and a soaking pit may be used. In this case, it is preferable that in the preheating furnace, the heat treatment furnace, and the soaking pit, a blocking film blocking spaces of the furnaces at each connection portion and a door for moving the steel plate in the blocking film be formed.

(29) After the heat treatment furnace is subjected to exhaustion into a vacuum state, an inert gas, for example, nitrogen gas, may be provided as an atmospheric gas.

(30) In heat treatment of the steel plate where the aluminum-magnesium coating layer is formed, heat treatment is performed by first charging the steel plate into the preheating furnace, and then moving the steel plate to the heat treatment furnace in a state where a temperature is stabilized by heating the steel plate to a heat treatment temperature.

(31) It is preferable that heat treatment of the steel plate where the coating layer is formed be performed at 350 to 450 C. for 2 to 10 minutes. If heat treatment is performed at 350 C. or less within 2 minutes, the aluminum-magnesium layer does not form the aluminum-magnesium alloy, and if heat treatment is performed at 450 C. or more for more than 10 minutes, an iron component of the steel plate is diffused into the coating layer or magnesium is diffused into the surface of the coating layer, and thus these are not preferable.

(32) This heat treatment is preferably performed at 350 C. for 10 minutes or at 400 C. for 4 minutes.

(33) If the steel plate where the coating layer is formed is subjected to heat treatment, the iron component of the steel plate is diffused into the coating layer at an interface between the steel plate and the coating layer to form an Al.sub.xFe.sub.y layer, and a phase change of the aluminum-magnesium coating layer into the aluminum-magnesium alloy layer is performed.

(34) Herein, in the Al.sub.xFe.sub.y layer, it is preferable for x to be 1 to 3 and y to be 0.5 to 1.5, and it is preferable for a thickness of the Al.sub.xFe.sub.y layer to be 0.2 to 1 m.

(35) In the Al.sub.xFe.sub.y layer, since x and y values affect brittleness in an AlFe alloy phase by diffusion, the AlFe phase (e.g.: Fe.sub.3Al, FeAl, and the like) improves adhesion force between the steel plate and the aluminum-magnesium alloy layer in a range where an alloy phase having poor mechanical properties (e.g.; FeAl.sub.2, Fe.sub.2Al.sub.5, FeAl.sub.3, and the like) is not generated, x is 1 to 3, and y is 0.5 to 1.5, and thus the x and y values are limited to the aforementioned range.

(36) Further, the reason why a layer thickness of the AlFe alloy phase is limited to 0.2 to 1 m is because if the thickness of the AlFe layer is increased, since a content of Al is relatively limited but a content of Fe is increased, the AlFe alloy phase having brittleness is generated, and thus mechanical properties of the coating layer may be reduced.

(37) In this case, the Al.sub.xFe.sub.y layer formed at an interface between the steel plate and the coating layer is an aluminum-iron alloy layer including magnesium in a small amount, and it is preferable for the Al.sub.xFe.sub.y layer to be formed in a thickness that is 1 to 50% of a thickness of the aluminum-magnesium coating layer in a coating layer direction in the steel plate.

(38) Herein, the reason why the thickness of the Al.sub.xFe.sub.y layer is limited to 1 to 50% of the thickness of the coating layer is because if the Al.sub.xFe.sub.y layer is formed in a thickness that is larger than 50% of the thickness of the coating layer, since the content of Fe is increased, the alloy phase having poor mechanical properties may be generated.

(39) In addition, the aluminum-magnesium alloy layer subjected to the phase change by heat treatment is in a state where and phases are mixed. Herein, the phase means an aluminum phase of a face-centered cubic lattice (FCC), and the phase means Al.sub.3Mg.sub.2 of the face-centered cubic lattice. As described above, in the formed aluminum-magnesium alloy layer, a ratio of the and phases is an XRD intensity ratio, that is, I (880)/I (111), and is preferably 0.01 to 1.5.

(40) As described above, in the aluminum-magnesium alloy layer, the ratio of the and phases (I/I) is set to 0.01 to 1.5 in order to limit the content of Mg at the phase that is generated, because in the case where the AlMg coating layer is subjected to heat treatment, the XRD peak intensity of the generated AlMg alloy phase ( phase) is differently exhibited according to the content of Mg.

(41) Further, in the aluminum-magnesium alloy layer subjected to the phase change by heat treatment, crystal grains are formed, and it is preferable that a size of the crystal grains be 0.2 to 1 m.

(42) Herein, the reason why the size of the crystal grains is limited to 0.2 to 1 m is because in the case where the size of the crystal grains of the AlMg alloy layer is 0.2 m or less, the size is not easy to obtain by controlling a heat treatment condition, and if the size is first increased to 1 m or more, division into the AlFe layer and the Mg layer occurs, and the division is not preferable.

(43) In addition, it is preferable for the area ratio of the phase/ phase of the crystal grains of the aluminum-magnesium alloy layer formed as described above to be 10 to 70%.

(44) Herein, the reason why the area ratio of the phase/ phase in the crystal grain of the aluminum-magnesium alloy is limited to 10 to 70% is because, in the case where the area ratio deviates from the aforementioned range, the AlMg alloy phase ( phase) is not formed, which is not preferable.

EXAMPLE

(45) First, as the specimen used in the experiment, a steel plate including 0.12 wt % or less of C (but 0% was excluded), 0.50 wt % or less of Mn (but 0% was excluded), 0.04 wt % or less of P (but 0% was excluded), 0.040 wt % or less of S (but 0% was excluded), the balance of Fe, and other inevitable impurities, and rolled to a thickness of 0.8 mm through hot rolling and cold rolling, was prepared.

(46) In order to remove the rust preventive oil from the steel plate prepared as described above, degreasing was performed by using an olefin-based surfactant.

(47) The steel plate subjected to degreasing was subjected to ultrasonic wave treatment with alcohol and then ultrasonic wave treatment with acetone to perform washing by ultrasonic waves, and then equipped in the vacuum coating equipment.

(48) Next, after exhaustion was performed until the pressure in the vacuum vessel became 10.sup.5 torr or less, the argon gas was injected into the vacuum vessel and the DC voltage of 800 V was applied to the steel plate when the degree of vacuum approached 1*10.sup.2 torr, and thus glow discharging was caused, thereby purifying the surface of the specimen.

(49) In addition, the steel plate where purification was finished was charged in the magnetron sputtering device where the aluminum source 300 and the magnesium source 400 were equipped, and exhaustion was performed until the pressure became 10.sup.5 torr that was the basic pressure of the sputtering device.

(50) Next, the magnetron sputtering device was operated to sequentially deposit aluminum and magnesium on the upper portion of the steel plate. In this case, the deposition amount was controlled by adjusting the voltage in the state where the aluminum source and the magnesium source were fixed, and the steel plate was repeatedly horizontally reciprocated to sequentially deposit aluminum-magnesium-magnesium-aluminum-aluminum-magnesium on the upper portion of the steel plate.

(51) The deposition conditions of aluminum and magnesium deposited on the steel plate were the same as those of the following Table 1.

(52) TABLE-US-00001 TABLE 1 Deposition source Al Mg Intensity of power supply 0.6-8 Kw 0.2-2.5 Kw Process pressure 10 mTorr (Ar 80 SCCM) Distance between steel plate and source 70 mm

(53) In the composition of the aluminum-magnesium coating layer deposited on the upper portion of the steel plate, the deposition ratio of the aluminum source (target) 500 and the magnesium source (target) 600 was controlled by controlling the intensity (kW) of the input power supply, and the composition of the aluminum-magnesium coating layer was controlled to 3.74 wt %, 5.69 wt %, 7.65 wt %, 12.25 wt %, 16.71 wt %, 20.97 wt %, 21.20 wt %, 27.72 wt %, and 31.50 wt % based on the content of magnesium in the entire coating layer. Further, the entire coating layer was deposited at a thickness of 5 m.

(54) As described above, after the aluminum-magnesium coating layer was deposited on the upper portion of the steel plate, each specimen was charged into the heat treatment furnace to perform heat treatment.

(55) As the heat treatment furnace used for heat treatment of the coating layer, a vacuum heat treatment furnace where a preheating room and a heat treatment room were connected was used.

(56) Heat treatment was performed by first charging the steel plate where the coating layer was formed into the preheating furnace, and then moving the steel plate to the heat treatment furnace in a state where the temperature was stabilized by heating the steel plate to the heat treatment temperature. In both the preheating furnace and the heat treatment furnace, the inert atmosphere was formed by the nitrogen gas, and the steel plate where the coating layer was formed was sufficiently preheated to the heat treatment temperature in the preheating furnace, and then moved to the heat treatment furnace.

(57) In the heat treatment furnace, heat treatment of the steel plate where the coating layer was formed was performed at 350 to 450 C. for 2 to 10 minutes according to the composition of the coating layer while the heat treatment condition was changed.

(58) That is, in the case where the heat treatment temperature was low, the heat treatment time was controlled to be slightly long, and in the case where the heat treatment temperature was high, the heat treatment time was controlled to be slightly short.

(59) FIG. 2 illustrates a change of the coating layer according to the heat treatment temperature and the heat treatment time. In the specimen used in FIG. 2, the composition of magnesium of the coating layer was 39.0 wt %.

(60) In addition, in each graph of FIG. 2, the red line (central line) of the left upper side represents a concentration change of aluminum, the blue line (leftmost line) of the left lower side represents a concentration change of magnesium, and the right green line represents a concentration change of iron.

(61) As seen from FIG. 2, in the coating layer, the change according to diffusion of each component element exhibits the same effect for a short time as the temperature is increased. For example, it can be seen that in the case where heat treatment of the coating layer is performed at 350 C. for 2 minutes or more and performed at 450 C. for 10 minutes or less, the component elements of the coating layer are diffused into each other to form the aluminum-magnesium alloy layer.

(62) However, it can be seen that in the case where heat treatment of the coating layer is performed at 350 C. for 2 minutes, since diffusion of the component elements is not sufficient, the aluminum-magnesium alloy is not well formed, and in the case where heat treatment is performed at 450 C. for 10 minutes, reverse diffusion of magnesium occurs.

(63) FIG. 3 illustrates an XRD (X-ray diffraction analysis) test result of the specimen used in FIG. 2. In FIG. 3, a horizontal axis of each graph is an angle (2) at which a peak is observed, a vertical axis represents the intensity, and a unit is represented by a normalized arbitrary unit (a.u.).

(64) As seen from FIG. 3, in the case where the coating layer is subjected to heat treatment at 350 C. for 2 minutes or more, the aluminum-magnesium alloy layer (Mg or Al.sub.12Mg.sub.17) is formed, in the case where the coating layer is subjected to heat treatment at 450 C. for 10 minutes, the aluminum-iron alloy is formed at an interface of the steel plate, and under the temperature and the time condition therebetween, the phase (Al) and the phase (Al.sub.3Mg.sub.2) are mixed while the aluminum-magnesium alloy layer is appropriately formed.

(65) As described above, in the formed aluminum-magnesium alloy layer, the ratio of the and phases was an XRD intensity ratio, that is, I (880)/I (111), and was in the range of 0.01 to 1.5.

(66) Next, FIG. 4 illustrates a scanning electron microscope (SEM) observation result of a structural change of the specimen used in FIG. 2 according to heat treatment.

(67) As seen from FIG. 4, it could be observed that before the coating layer was subjected to heat treatment, in the aluminum-magnesium coating layer, the columnar structure was observed under the condition where the content of magnesium was 10 wt % or less and the state where the structure was dense was observed in the state where the content of magnesium was 10 wt % or more, but in the case where the coating layer was subjected to heat treatment, the crystal grains were formed while phase transformation of the coating layer into the aluminum-magnesium alloy layer was performed. In the aluminum-magnesium coating layer, the size of the crystal grains tended to be reduced if the content of magnesium was increased, and the size was in the range of 0.2 to 1 m.

(68) Next, FIG. 5 illustrates a GDLS (glow discharge light spectroscopy) analysis result of the specimen used in FIG. 2 according to heat treatment.

(69) As seen from FIG. 5, in the aluminum-magnesium coating layer, before and after heat treatment, in the steel plate, a composition change of the coating layer is not large in a direction of the coating layer (in each graph of FIG. 5, a direction from the left side to the right side), that is, a depth direction of the coating layer. This means that by heat treatment, a phase change of the coating layer occurs but the composition change is not large. That is, even though the phase change of the coating layer occurs by heat treatment, before and after the phase change occurs, the total amount of Al and Mg existing in the AlMg coating layer is not changed.

(70) In the above, the observation result of the change of the coating layer of the steel plate where the aluminum-magnesium coating layer was formed according to heat treatment was described, and hereinafter, evaluation results of a corrosion experiment of the coating layer subjected to heat treatment and adhesion force of the coating layer will be described.

(71) First, the corrosion experiment of the coating layer subjected to heat treatment will be described.

(72) FIG. 6 illustrates a result of a neutral salt spray test performed in order to evaluate a corrosion characteristic of the steel plate where the aluminum-magnesium coating layer is formed with respect to the specimen partially taken from the specimen used in FIG. 2.

(73) The neutral salt spray test was performed in 5% NaCl at 35 C. according to the ASTM B117 regulation. A horizontal axis of a graph of FIG. 6 represents the content of magnesium (the unit is wt %) of the coating layer, and a vertical axis represents a time (the unit is hours) at which red rust occurs.

(74) In FIG. 6, a bar represented by diagonal lines represents a result of the specimen before heat treatment (non-heat treated), and a bar represented by crossed lines represents a result of the specimen subjected to heat treatment at 400 C. for 10 minutes. In addition, a bar represented by parallel lines represents a result of the neutral salt spray test of an electric zinc-plating steel plate having a thickness of 5.6 m for comparison.

(75) As described in FIG. 6, before heat treatment, in the steel plate where the aluminum-magnesium coating layer is formed, as the content of magnesium is increased, corrosion resistance tends to be slightly improved. However, in the steel plate where the aluminum-magnesium coating layer is formed was subjected to heat treatment at 400 C. for 10 minutes, the result where corrosion resistance was significantly improved in most ranges was exhibited.

(76) As shown in FIG. 6, in the steel plate where the aluminum-magnesium coating layer was formed, corrosion resistance was significantly improved at the content of magnesium of 21.2 wt %, but at the content of 27 wt % or more, corrosion resistance was slightly reduced, but nevertheless, corrosion resistance was improved as compared to that of the specimen before heat treatment.

(77) As described above, in the case where the steel plate where the aluminum-magnesium coating layer was formed was subjected to heat treatment, in the case where the content of magnesium of the coating layer was 7.6 to 31.5 wt %, excellent corrosion resistance was exhibited. This improvement of corrosion resistance is evaluated as a characteristic according to densification of the structure of the coating layer as the content of magnesium is increased and formation of the phase (Al.sub.3Mg.sub.2) in the aluminum-magnesium alloy phase generated due to heat treatment. In this result of the neutral salt spray test, it can be confirmed that, as compared to the electric zinc-plating steel plate, performance is improved by about 10 times or more.

(78) However, if the content of magnesium of the coating layer is increased to 45 wt % or more, the aluminum-magnesium coating layer became unstable, and corrosion resistance was reduced.

(79) FIG. 7 is a graph illustrating an evaluation result of adhesion force of the steel plate of the aluminum-magnesium alloy layer.

(80) This adhesion force test was performed according to the ASTM D522 regulation.

(81) As shown in FIG. 7, it can be seen that in the case where the content of magnesium of the coating layer is low, excellent adhesion force is exhibited, but as the content of magnesium of the coating layer is increased, adhesion force is reduced. As described above, the increase in adhesion force of the steel plate of the coating layer is evaluated to be caused by formation of the aluminum-iron alloy layer on the surface of the steel plate.

(82) As described above, in order to observe the structure of the aluminum-magnesium alloy layer formed on the steel plate, the case where the specimen having the content of magnesium of 20.2 wt % of the coating layer was subjected to heat treatment at 400 C. for 10 minutes was checked by the transmission electron microscope (TEM).

(83) FIGS. 8A and 8B illustrate this TEM result.

(84) As seen from FIG. 8A, as the coating layer, the aluminum-iron alloy layer is formed by heat treatment between the steel plate and the aluminum-magnesium alloy layer. In addition, it can be seen that in the aluminum-magnesium alloy layer, the phase and the phase (Al.sub.3Mg.sub.2) are mixed while the crystal grains are formed. Although the exemplary embodiments of the present invention have been described with reference to the accompanying drawings, it will be apparent to those skilled in the art that various modifications and changes may be made thereto without departing from the technical spirit or essential feature of the invention.