JP2017008390A - Plated steel of high corrosion resistance, and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide plated steel of high corrosion resistance having an increased thickness of a plating layer, and a manufacturing method thereof among two-step hot dip plating methods.SOLUTION: A manufacturing method of plated steel of high corrosion resistance includes a first hot dip plating step for dipping steel in a first molten Zn plating bath containing Al of 1-12 mass% followed by drawing-up, a flux treatment step for applying a flux to the steel treated in the first hot dip plating step, and a second hot dip plating step for dipping the steel treated in the flux treatment step in a second molten Zn plating bath containing Al of 4-6 mass% and being set at a bath temperature of 420°C or lower followed by drawing-up.SELECTED DRAWING: None

Description

  The present invention relates to a highly corrosion-resistant plated steel material and a method for producing the same.

  As the hot dip plating method in the steel field, there are a continuous hot dip plating method in which a steel strip is continuously passed through a hot dip plating bath, and an immersion hot dip plating method in which a steel material processed into a predetermined shape is immersed in the hot dip plating bath and then pulled up. The immersion hot-dipping method is further classified into a one-step plating method in which a steel material is immersed in one type of hot-dipping bath and a two-step plating method in which steel materials are successively immersed in two types of hot-dipping baths. Whether the single-step plating method or the two-step plating method is selected is selected depending on the plating composition and the target thickness.

  As a method for producing a plated steel material by the one-step plating method, galvanization with a high Al concentration such as a Zn-55 mass% Al bath has been put into practical use. This is because it is difficult to control the FeAl alloying reaction.

  On the other hand, it is possible to manufacture Zn-Al plating with a low Al concentration by a one-bath plating method. However, when Si is added to the plating bath to suppress the formation of the FeAl alloy, the thickness of the plating layer is determined by the amount of molten metal remaining on the steel material surface when the material to be plated is pulled up from the plating bath. Further, the amount of molten metal remaining on the steel material surface is determined only by the viscosity of the plating bath. For this reason, when performing low Al concentration Zn-Al plating, a thick plating layer cannot be obtained. When the plating composition is the same, the corrosion resistance of the Zn-based plating is substantially proportional to the amount of plating. Therefore, in the one-bath plating method, even if zinc and Al are alloyed to improve the corrosion resistance of the plating layer, a thick plating layer is hardly obtained, and as a result, high corrosion resistance cannot be obtained.

  Moreover, even if it is a one-step plating method, if it is a plating bath to which Si is not added, a steel material is immersed in the plating bath for a long time, thereby generating a FeAl alloy that is in a solid state at the plating bath temperature on the surface of the steel material. Can do. Since the generation amount of the FeAl alloy increases as the immersion time is increased, thick plating is possible. This is the same reason that a plating adhesion amount is ensured by generating an FeZn alloy by general pure Zn plating. However, it is difficult to properly control the FeAl reaction because the alloying reaction rate varies depending on the steel type and the behavior of the alloy to be produced varies extremely. For this reason, the FeAl alloy is deposited on the plating surface, and it is difficult to obtain a smooth appearance with a metallic luster, which tends to cause a problem that the quality of the plating appearance is deteriorated, which hinders practical use.

  Patent Document 1 discloses that various problems of ZnAl alloy immersion plating can be solved by setting the preheating temperature of the steel material before plating immersion as high as 300 to 700 ° C. However, in immersion plating, steel structures having various shapes may be targeted as objects to be plated. For example, when the preheating temperature of a long material to be plated is increased, it is difficult to uniformly preheat the entire material to be plated, there is a problem that the heat efficiency is low, and further, preheating is required for a long time. Both of these problems can lead to significant cost increases.

  In addition, when heated at an extremely high temperature for a long time, oxides may be generated on the surface of the steel material, which may affect the plating. In order to solve this problem, it is conceivable to heat the steel material in a non-oxidizing atmosphere in advance, but a facility for heating in a non-oxidizing atmosphere is required, which also leads to high costs.

  From the above, the production of ZnAl alloy-plated steel material by the one-step plating method is not carried out because it is not established commercially and industrially at a low Al concentration.

  In addition, high Al concentration Zn-Al alloy plating is carried out by the one bath plating method by adding a small amount of Si to the plating bath to suppress the formation of FeAl alloy and increase the Al concentration. This is because an alloy layer having a thickness on the order of μm can be grown by this, thereby increasing the absolute amount of plating. However, even with this Zn-high Al alloy plating, the absolute amount of plating is not large enough to be satisfactorily satisfied, so that the corrosion resistance is not satisfactory in applications requiring high corrosion resistance. Moreover, since the Al concentration is high and the Zn ratio is low, there is a problem that the sacrificial anticorrosive ability of the plating layer is low and red rust is easily generated from the steel material. For this reason, the present condition is that the high Al concentration Zn-Al alloy plating steel materials manufactured by a one-step plating method are used only for the use in a special environment.

  Next, the two-stage plating method is a method in which a steel material is immersed in a pure Zn plating bath to perform plating, and the steel material is immersed in a ZnAl alloy bath after Zn plating cooling or immediately after Zn plating. The method using the first stage pure Zn plating bath is an industrially established plating technique, and the alloying reaction of Fe and Zn can be controlled almost without any problem. The FeZn alloy produced by the first stage plating is modified to an FeZnAl alloy by the second stage plating. Thereby, the corrosion resistance of the alloy layer is improved. In addition, the pure Zn plating layer present on the FeAl alloy in the first plating is replaced with a ZnAl alloy that is superior in corrosion resistance by the second plating. Thus, through the two-step plating process, the plating layer formed first is converted into an alloy plating layer containing Al. When immersed in the second-stage plating bath, the surface of the plating layer formed by the first-stage plating dissolves in the second-stage plating bath, so the plating layer is slightly thinner, but it is replaced with an alloy with excellent corrosion resistance. Therefore, the corrosion resistance of the plated steel material itself is improved.

  In order to increase the thickness of the plating layer by the two-step plating method, it is necessary to increase the thickness of the plating layer during the plating formation using the first pure Zn plating bath. However, although depending on the material and thickness of the steel material to be plated, the largest plating thickness in immersion Zn plating is about 80 μm of JIS standard HDZ55. It becomes difficult.

  As a specific problem, in order to increase the thickness of the plating layer, when the steel layer is immersed in the plating bath for a long time in the first stage plating to increase the thickness of the plating layer, while the FeZn alloy grows, the growing FeZn alloy Becomes dross from the plating surface, elutes into the plating bath, precipitates at the bottom of the plating bath, and degrades the plating bath.

  Further, in the second stage plating, since the FeZn alloy layer formed in the first stage reacts with Al as the second stage plating component, a part of the first stage plating layer is eluted and the plating layer becomes thin. Become. If the alloy layer formed in the first stage is thick, the bath temperature of the plating bath is increased or the immersion time is increased in order to sufficiently advance the Al substitution reaction in the second stage plating process. Although necessary, both of these promote the elution of the FeZn alloy. Also, in the second-stage plating bath, a part of the FeZn alloy in the first-stage plating layer and the entire ηZn layer on the surface are dissolved, so various bath components of the second-stage plating bath are changed. There are operational and quality issues. Needless to say, the two-step plating method is a high-cost plating method with low equipment efficiency.

JP 2012-241277 A

  The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a highly corrosion-resistant plated steel material capable of increasing the thickness of a plating layer and a method for producing the same in a two-stage plating method of immersion hot dipping. And

  When the present inventors diligently studied to solve the above-mentioned problems, the first-stage plating is not a pure Zn plating bath, but a Zn-Al alloy plating, thereby forming the first-stage plating thicker, By setting the second-stage plating to a composition in the vicinity of the eutectic point of Zn—Al, the bath temperature to 420 ° C. or lower, which is the melting point of pure Zn, and suppressing the thinning of the plating as much as possible by dissolving the first-stage plating, We succeeded in obtaining a thicker plating layer by forming the second plating layer on top of the first plating layer. Thereby, it became possible to obtain plating having high corrosion resistance.

The present invention is as follows.
(1) The steel material surface is provided with a plating layer including a FeAl alloy layer and a Zn alloy layer,
The FeAl alloy layer is formed on the surface of the steel material, includes an FeAl alloy phase and a Zn phase, has an average composition of Fe: 30 to 60% by mass, Al: 15 to 30% by mass, the balance Zn and impurities, Is 100 μm or more,
The Zn alloy layer is a highly corrosion-resistant plated steel material formed on the FeAl alloy layer, made of Al, the balance Zn and impurities, and having a thickness of 5 μm or more.
(2) The high corrosion-resistant plated steel material according to (1), wherein the Zn alloy layer has an average composition of Al: 4 to 6% by mass, the balance Zn and impurities.
(3) The Zn alloy layer
An intermediate layer formed on the FeAl alloy layer and having an average composition of Al: 12% by mass or less, the balance Zn and impurities;
The high corrosion-resistant plated steel material according to (1), which is formed on the intermediate layer and includes an outermost layer having an average composition of Al: 4 to 6% by mass, the balance Zn and impurities.
(4) The high corrosion resistance plated steel material according to (2), wherein the Zn alloy layer further contains 0.1 to 3% by mass of Mg.
(5) The highly corrosion-resistant plated steel material according to (3), wherein the outermost layer further contains 0.1 to 5% by mass of Mg.
(6) a first hot dipping step of forming a layer containing an FeAl alloy layer of 100 μm or more by immersing the steel material in a first hot Zn plating bath containing 1 to 12% by mass of Al,
A flux treatment step of applying a flux to the steel material after the first hot dipping step;
A second hot dipping process in which the steel material after the flux treatment process is dipped in a second hot dipped Zn plating bath containing 4 to 6% by mass of Al and having a bath temperature of 420 ° C. or lower;
A method for producing a highly corrosion-resistant plated steel material comprising:
(7) The method for producing a highly corrosion-resistant plated steel material according to (6), wherein the immersion time of the steel material in the second hot-dip Zn plating bath is in the range of 10 seconds to 120 seconds in the second hot dip plating step.
(8) The method for producing a highly corrosion-resistant plated steel material according to (6) or (7), wherein the second molten Zn plating bath further contains 0.1 to 5% by mass of Mg.

  ADVANTAGE OF THE INVENTION According to this invention, in the two-step plating method of immersion hot dipping, the high corrosion resistance plated steel material which made the thickness of the plating layer larger and its manufacturing method can be provided.

No. 15 is a cross-sectional optical micrograph of a plated layer after the first plating of 15 high corrosion resistance plated steel materials. No. The cross-sectional optical microscope photograph of the plating layer after the 2nd plating of 15 high corrosion-resistant plating steel materials.

  Hereinafter, the highly corrosion-resistant plated steel material and its manufacturing method which are embodiments of the present invention will be described.

  The highly corrosion-resistant plated steel material of the present embodiment includes a steel material and a plating layer formed on the steel material surface. The plating layer formed on the steel material surface includes an FeAl alloy layer having a thickness of 100 μm or more and a Zn alloy layer having a thickness of 5 μm or more. The upper limit of the thickness of the entire plating layer is, for example, 305 μm or less. Since the thickness of the plating layer depends on the plating conditions, the lower limit of the thickness is not particularly limited. For example, the thickness is preferably 105 μm or more which is the sum of the lower limit values of the FeAl alloy layer and the Zn alloy layer.

  The FeAl alloy layer having a thickness of 100 μm or more is formed on the surface of the steel material, and includes an FeAl alloy phase and a Zn phase as a structure. The average composition is Fe: 30 to 60% by mass, Al: 15 to 30% by mass, and the balance Zn and impurities.

  Further, the Zn alloy layer having a thickness of 5 μm or more is formed on the FeAl alloy layer, and is composed of Al, the remaining Zn and impurities. The average composition of the Zn alloy layer is Al: 4 to 6% by mass, the balance Zn and impurities.

  Further, the Zn alloy layer of the high corrosion resistance plated steel material of the present embodiment may include an intermediate layer formed on the FeAl alloy layer and an outermost layer formed on the intermediate layer. The intermediate layer is a layer composed of Al: 12% by mass or less with the balance Zn and impurities, and the outermost layer is a layer composed of Al: 4 to 6% by mass with the balance Zn and impurities.

  That is, the plating layer of the highly corrosion-resistant plated steel material of the present embodiment may have a two-layer configuration including an FeAl alloy layer and a Zn alloy layer, or a three-layer configuration including an FeAl alloy layer, an intermediate layer, and an outermost layer. Good. Hereinafter, the steel material and the plating layer will be sequentially described.

  In addition, the high corrosion-resistant plated steel material of the present embodiment includes a first hot-dip plating step in which the steel material is dipped in a first hot-dip Zn plating bath, and a flux treatment step. And a second hot dipping process in which it is dipped in a bath and pulled up. That is, it is manufactured by a so-called two-step plating method.

  There is no particular limitation on the material of the steel material used as the base of the plating layer. Although details will be described later, if general steel or Ni pre-plated steel is used, there is no particular limitation. Al killed steel and some high alloy steels can also be applied, and the shape is not limited.

  The FeAl alloy layer that constitutes the plating layer has an average composition of Fe: 30 to 60% by mass, Al: 15 to 30% by mass, the balance Zn and impurities. The FeAl alloy plating layer is mainly composed of an amorphous FeAl alloy phase, and has a structure in which a Zn phase is dispersed and precipitated in the FeAl alloy phase.

  This FeAl alloy layer is formed by immersing a steel material in a first hot dipping bath having a composition comprising Al: 1 to 12% by mass, the balance Zn and impurities. By immersing the steel material in the first hot dipping bath, Al in the plating bath reacts with Fe of the steel material to form a FeAl alloy.

  The FeAl alloy in the FeAl alloy layer has no sacrificial anticorrosion effect and exhibits exclusively barrier properties. However, the FeAl alloy layer of this embodiment contains Zn as the remainder, and the remainder Zn is dispersed as a Zn phase in the plating structure. For this reason, the FeAl alloy layer according to the present embodiment also exhibits the sacrificial anticorrosive effect of Zn. Even though the FeAl alloy layer contains Fe, even if it corrodes, the corrosion product is stable and hardly generates red rust. Thus, the FeAl alloy layer can protect steel not only by sacrificial corrosion protection but also by the barrier effect.

  When the Fe concentration and the Al concentration in the FeAl alloy layer are less than the lower limit of the above range, the amount of FeAl alloy in the FeAl alloy layer decreases, and in particular, the barrier property after the Zn in the FeAl alloy layer is consumed cannot be secured. On the other hand, when the Fe concentration and the Al concentration exceed the upper limits of the above ranges, the Zn content is relatively reduced, and the sacrificial corrosion protection effect is lowered. The Fe concentration in the FeAl alloy layer is more preferably in the range of 40 to 55% by mass. The Al concentration is more preferably in the range of 20 to 27%.

  The thickness of the FeAl alloy layer is at least 100 μm or more. Although an upper limit is not specifically limited, 300 micrometers or less are preferable. When the thickness of the FeAl alloy layer is less than 100 μm, the barrier effect cannot be sufficiently exhibited. In order to make the thickness more than 300 μm, it is necessary to immerse the steel material in the first hot dipping bath for a long time, and the productivity is lowered. Therefore, the upper limit is 300 μm.

  Next, the Zn alloy layer is laminated on the FeAl alloy layer. The Zn alloy layer may have a single-phase structure or a two-layer structure including an intermediate layer and an outermost layer. This difference depends on the surface properties of the plating layer formed in the first hot dipping process and the plating conditions in the second hot dipping process. Although details will be described in the description of the manufacturing method, the Zn alloy layer in the case of the single layer structure is formed by the second hot dipping process. On the other hand, the intermediate layer in the case of the two-layer structure is formed together with the FeAl alloy layer in the first hot dipping process, and the outermost layer is formed in the second hot dipping process.

  When the Zn alloy layer is a single layer, the average composition is Al: 4 to 6% by mass, the balance Zn and impurities. In this case, 0.1 to 3% by mass of Mg may be included.

  Further, when the Zn alloy layer has a two-layer structure of an intermediate layer and an outermost layer, the average composition of the intermediate layer is Al: 12% by mass or less, the remaining Zn and impurities. The average composition of the outermost layer is Al: 4 to 6% by mass, the balance Zn and impurities. In this case, 0.1 to 5% by mass of Mg may be included in the outermost layer.

  The Al concentration of the single-layer structure Zn alloy layer and the Al concentration of the outermost layer in the case of the two-layer structure are in the range of 4 to 6%, respectively. This Al concentration is in the range of ± 1% centering on 5% Al concentration which is the eutectic point of Zn and Al. If the Al content is out of this range, the corrosion resistance is lowered, which is not preferable.

  In the case of the two-layer structure, the Al concentration of the intermediate layer is set to 12% by mass or less. As described above, the intermediate layer is formed on the FeAl alloy layer in the first hot dipping process. Since Al in the intermediate layer is used for forming the FeAl alloy layer before the solidification of the plating, the Al concentration in the intermediate layer is lower than the Al concentration in the FeAl alloy layer. The Al concentration of the intermediate layer is preferably less than 12% by mass, more preferably 11% by mass or less, further preferably 10% by mass or less, and further preferably 8% by mass or less. The lower limit of the Al concentration of the intermediate layer is not particularly limited, but may be 0.1% by mass or more, and may be 0.5% by mass or more. If the Al concentration of the intermediate layer exceeds the upper limit, the corrosion resistance of the intermediate layer is lowered, which is not preferable.

  Further, the corrosion resistance is further improved by adding a small amount of Mg to the Zn alloy layer in the case of a single layer and the outermost layer in the case of a two-layer structure. When Mg exceeds the above upper limit value, the plating layer has a non-uniform thickness, which may reduce the corrosion resistance.

  The thickness of the Zn alloy layer is preferably 5 μm or more for both the one-layer structure and the two-layer structure. The thicker the Zn alloy layer, the better the corrosion resistance. The upper limit of the thickness depends on the viscosity of the plating bath used in the immersion hot dipping method and the structure of the object to be plated, but is about 40 μm at the maximum. If the thickness of the Zn alloy layer is less than 5 μm, sufficient corrosion resistance cannot be obtained.

  Further, the presence of the intermediate layer can be confirmed if the thickness is about 1 μm or more. The outermost layer may be 5 μm or more and 40 μm or less. The corrosion resistance is further improved as the thickness of the intermediate layer and the outermost layer is increased.

  In order to confirm the FeAl alloy layer and Zn alloy layer formed in the plating layer, the cross section is observed with an optical electron microscope, and the structure of the FeAl alloy is observed. Can be distinguished. Further, whether the Zn alloy layer has one structure or two layers can be distinguished by analyzing the Al distribution by surface analysis or line analysis in the cross section of the plating layer. The alloy composition of each layer can be obtained, for example, by analyzing by glow discharge spectroscopy and taking a value in a region where the composition is stable. The thickness of each layer can be determined by microscopic observation, surface analysis or line analysis of constituent elements, glow discharge analysis, or the like. The thickness of the entire plating layer can be measured with an electromagnetic film thickness meter. In addition, the presence of the intermediate layer may be confirmed by observing the cross section of the plating layer with an SEM (scanning electron microscope) or an optical microscope, or by elemental analysis of the cross section of the plating layer and confirming from the distribution state of Al concentration or Zn concentration.

Next, the manufacturing method of the high corrosion resistance plated steel material of this embodiment is demonstrated.
As described above, the manufacturing method of the present embodiment includes a first hot dipping step of forming a layer including a FeAl alloy layer of 100 μm or more by first immersing the steel material in the first hot Zn plating bath and then pulling it up. It comprises a flux treatment step of applying flux to the steel material after the hot dipping step, and a second hot dipping step of lifting the steel material after dipping in the second hot dipping zinc plating bath.

The first molten Zn plating bath used in the first molten plating step is a Zn plating bath containing 1 to 12% by mass of Al. The second hot dipping bath used in the second hot dipping step is a Zn plating bath containing 4 to 6% by mass of Al and having a bath temperature of 420 ° C. or lower.
Hereinafter, each process will be described sequentially.

  In the present embodiment, in the first hot dipping process, the plating layer is formed by using a Zn—Al alloy plating bath instead of the pure Zn plating bath of the prior art.

  When the Al concentration of the first hot dipping bath is large, the corrosion resistance of the resulting plating layer itself is improved, but the melting point is increased and the bath temperature is increased, which makes the Fe-Al alloying reaction intense and difficult to control. Become. In addition, the Zn concentration contained in the obtained plating layer is reduced, and the sacrificial anticorrosive effect is reduced. Moreover, when the Al concentration in the first hot dipping bath is small, the alloying reaction takes time and the productivity is lowered. However, since the Zn concentration increases, the effect of sacrificial corrosion protection is improved. For this reason, Al concentration of a 1st hot dipping bath shall be 1-12 mass%.

  The balance of the first hot dipping bath is Zn and impurities. In addition, it is desirable that the first hot dipping bath does not contain Si. When Si exceeds 100 mass ppm, the alloying reaction of Fe and Al is suppressed, and a plating layer having a thickness of 100 μm or more cannot be formed.

  As described above, the alloy layer structure of the plating obtained in the first hot dipping bath having this composition consists of Fe: 30 to 60% by mass, Al: 15 to 30% by mass, the balance Zn and impurities. An AlZn alloy layer having a regular FeAl alloy phase and a Zn phase is included.

  Since the FeAl alloy obtained in the first hot dipping process is more stable than the FeZn alloy, it is easy to obtain a thick plating of 100 μm or more, which is difficult with pure Zn immersion plating. Depending on the conditions, it may be 200 μm or more.

  Moreover, a Zn plating structure containing Al having a slightly lower concentration than the plating bath due to the progress of the alloying reaction in the cooling process is obtained on the upper layer of the FeAl alloy layer. When this upper plating layer remains in the next second hot dipping process, it becomes an intermediate layer constituting the Zn alloy plating layer. On the other hand, when the Al concentration of the upper plating layer is about the same as the concentration of the plating bath used in the second hot dipping process, that is, when the Al concentration becomes 4 to 6% by mass, the remelting is performed during the second hot dipping process. Will do. In this case, the plated layer of the plated steel material as the final product has a two-layer structure of an FeAl alloy layer and a Zn alloy layer.

  Further, in the first hot dipping bath, Fe is eluted from the steel material and dross is generated. This dross floats on the surface of the plating bath and is easy to remove. There is an advantage over the two-stage plating method of plating.

In addition, it is preferable to implement a flux treatment on the steel material surface before the first hot dipping process. Flux used is not particularly limited as long as it is a flux mainly composed of ZnCl _2. NH ₄ Cl, which is often used other than the flux mainly composed of ZnCl ₂ , is preferably not used because it is incompatible with the ZnAl alloy bath.

  The plating bath temperature and immersion time in the first hot dipping process can be set flexibly in accordance with the heat capacity of the steel material to be plated and the required plating thickness in consideration of the bath temperature drop accompanying immersion. However, from the viewpoint of corrosion resistance, the FeAl alloy layer has a larger barrier effect than sacrificial corrosion protection, and thus it is necessary to generate a thick alloy layer of 100 μm or more by long-time immersion.

  Although the plating layer obtained in the first hot dipping process depends on the plating conditions, the surface is low in metallic luster because the FeAl alloy is easily exposed on the plating surface. In particular, when a thick layer is formed by growing a FeAl alloy layer, the exposed area of the FeAl alloy is increased, the appearance is grayed out, the unevenness is increased, and the surface smoothness may be lowered. Such a dip plating in which a thick FeAl alloy layer containing no Si is produced is not commercially produced. In the present embodiment, the above-described problem of the FeAl alloy layer is solved by forming another plating layer on the FeAl alloy plating layer by the subsequent second hot dipping process.

  The second-stage plating in the conventional two-stage plating method is performed at a temperature higher than 420 ° C., which is the Zn melting point, in order to react the ZnFe alloy layer with Al. However, in this embodiment, in order to prevent dissolution of the plating layer obtained in the first hot dipping process, the second hot dipping bath is made as low as possible. The plating composition of the second hot dipping bath is preferably a Zn-5% Al bath, which is the eutectic point of the ZnAl alloy. In an actual operation, it is difficult to maintain the Al concentration in the bath strictly at the eutectic point, so it is desirable to manage it in the range of about 4 to 6% by mass. Since the melting point of the Zn-5% Al bath is about 380 ° C., it is lower by 30 ° C. than the melting point of pure Zn. Therefore, by setting the bath temperature to 420 ° C. or lower, preferably 400 ° C. or lower, of the melting point of pure Zn, remelting of the plating layer formed in the first hot dipping process is minimized, and Zn is further formed thereon. Plating is performed so as to place a -4 to 6% Al plating layer.

  That is, in the prior art, the plating is thinned by the two-stage plating as described above, but in the present embodiment, the plating layer is thickened by the second-stage plating. In this way, the first plating layer is thicker than in the case of the conventional two-step plating, and the plating layer is further thicker in the second step. The above thick plating becomes possible.

  Since the bath temperature is limited to a temperature that is slightly higher than the melting point, the plating bath may solidify when the bath temperature is lowered. For this reason, in actual operation, the bath temperature is based on 400 ° C. or less, but it is important to manage the heat capacity of the steel material, the heat capacity of the plating bath, and the preheating temperature before plating of the steel material.

Prior to the second hot dipping process, the steel after the first hot dipping process is subjected to flux treatment. After the steel material after the first hot dipping process is cooled, a flux is applied, and after drying and preheating, a second hot dipping process is performed. In this case, it is desirable not to use NH ₄ Cl, which is often used for flux, because it is incompatible with the ZnAl alloy bath. For this reason, the flux used for the flux treatment is preferably a flux containing ZnCl ₂ as a main component. The flux concentration may be made thinner than the flux treatment before the first hot dipping process.

  The immersion time in the second hot dipping process is 10 seconds or more and less than 120 seconds. The immersion time should be as short as possible, but according to experiments, a minimum of 5 to 10 seconds is required for the flux reaction, and more time may be required depending on the heat capacity of the steel material. Depending on the shape of the steel material, it may be difficult to control the immersion time in seconds, but the plating is performed in a short time as much as possible by adjusting the preheating temperature and bath temperature of the steel material. Since the plating layer containing Al of the surface layer obtained in the first hot dipping process may partially dissolve depending on the plating conditions of the second hot dipping process as described above, the immersion time ( The reaction time is as short as possible.

  The Zn alloy layer formed by the second hot dipping process is formed by the thickness of the surface non-alloy layer (ηZn layer) obtained by normal immersion galvanizing or the second plating in the conventional two-step plating method. Like the surface plating layer, the thickness cannot be controlled by the dipping conditions, and is determined by the viscosity of the molten metal in the second hot dipping bath. However, in this embodiment, since the surface of the plating layer after the first hot dipping process is not smooth, the molten metal in the second hot dipping bath is easily held on the plating surface, so the Zn alloy layer of this embodiment is smooth. The plating thickness is larger than when plating on the surface.

  Mg, which is effective for improving the corrosion resistance of zinc-based plating, can be added to the second hot dipping bath. The plating formation reaction in the second hot dipping process is a reaction between a plating layer containing zinc as a base and a second hot dipping bath that is a ZnAl bath, so that plating can be performed in a bath containing up to 5% by mass of Mg. It is. This Mg addition can further improve the corrosion resistance.

  In addition, when a plating bath containing Mg is used in a flux-type immersion plating method in which the base is a steel material, the plating property is very poor, and thus plating defects such as non-plating are likely to occur. For this reason, it is not preferable to add Mg to the first hot dipping bath.

  After the second hot dipping process, it is desirable to carry out a general chemical conversion treatment used in Zn-based plating in order to prevent the occurrence of white rust in the obtained plating layer at an early stage.

As described above, according to the highly corrosion-resistant plated steel material of the present embodiment, a plating layer composed of a FeAl alloy layer having a thickness of 100 μm or more and a Zn alloy layer having a thickness of 5 μm or more is formed on the steel material surface. In addition, since the thickness of the entire plating layer can be increased and the corrosion resistance of the FeAl alloy layer and the Zn alloy layer is excellent, it is possible to realize a plated steel material whose corrosion resistance is significantly improved as compared with the prior art.
Moreover, since the average composition of the Zn alloy layer is Al: 4 to 6% by mass, the remaining Zn and impurities, and has a composition excellent in corrosion resistance, it is possible to realize a plated steel material whose corrosion resistance is significantly improved as compared with the conventional one.
Moreover, it is provided with an intermediate layer made of a Zn alloy having an Al content of 12% by mass or less and an outermost layer made of a Zn alloy having an Al content of 4 to 6% by mass, and both the intermediate layer and the outermost layer have a composition excellent in corrosion resistance. Therefore, it is possible to realize a plated steel material whose corrosion resistance is significantly improved as compared with the conventional case.
Furthermore, since Mg is contained in the outermost layer of the Zn alloy layer or the Zn alloy layer, the corrosion resistance of the outermost layer of the Zn alloy layer or the Zn alloy layer can be further improved.

Moreover, according to the manufacturing method of the highly corrosion-resistant plated steel material of the present embodiment, the hot-dip plated layer containing the FeAl alloy is formed on the steel material surface in the first hot-dip process, and then the bath temperature is 420 ° C. in the second hot-dip process. A steel material is immersed in a Zn plating bath adjusted as follows to form a hot-dip plating layer made of a Zn alloy containing Al. At this time, since the bath temperature is set to 420 ° C. or lower in the second hot dipping process, the hot dipped layer formed in the first hot dipping process may melt the surface layer or may hardly melt. is there. Therefore, without reducing the thickness of the hot dip plating layer formed in the first hot dip plating step, the hot dip plating layer can be further formed in the second hot dip plating step, and a plating layer having a film thickness as a whole can be formed.
The hot dip plating layer containing the FeAl alloy obtained by the above method and the hot dip plating layer made of the Zn alloy containing Al each become a plating layer excellent in corrosion resistance.
Furthermore, since the immersion time of the steel material is set in the range of 10 seconds to 120 seconds in the second hot dipping step, a hot dipped layer made of a Zn alloy can be formed while preventing the hot dipping layer formed first from being dissolved.
Moreover, since Mg is contained in the second hot dipping bath, the corrosion resistance of the hot dipped layer made of a Zn alloy can be further improved.

The contents of the present invention will be described in detail based on examples.
In this example, the material to be plated was a hot-rolled steel sheet with black skin (SS400) of 150 mm × 70 mm × 6.0 mm. The steel sheet was cleaned with a commercially available alkaline degreasing agent, then immersed in a 10% HCl aqueous solution at 40 ° C. for about 5 minutes to remove the scale on the surface, washed with hot water at 60 ° C., and then washed with 60 ° C. flux (ZnCl ₂ / It was immersed in NaCl / SnCl ₂ = 210/25/6 (g / L) for about 1 minute, and heat-dried in an atmosphere at 200 ° C. for 5 minutes in an air atmosphere.

  The steel sheet is immersed in a Zn—Al (—Mg) plating bath (first hot dip galvanizing bath (first stage plating bath)) for a predetermined time, and then pulled up, allowed to cool naturally, and the plating is completely solidified. And then water cooled. The plated steel sheet is immersed in a flux for 10 seconds under the same conditions as the first stage, and is heated and dried in a 200 ° C. heating furnace in an air atmosphere for 5 minutes, and then a Zn—Al (—Mg) plating bath (second molten zinc). After being immersed in a plating bath (second-stage plating bath) for a predetermined time and plated, it was pulled up and cooled.

  The plating thickness was measured with an electromagnetic film thickness meter after the first stage plating, and was measured with an optical microscope having a cross-sectional structure after the completion of the second stage plating. Results were recorded in units of 10 μm. (“Intermediate layer thickness: 0 μm” indicates that the intermediate layer could not be confirmed by microscopic observation.) The composition of the plating layer was analyzed by glow discharge spectroscopy, and the value of the region where the composition was stable was taken. It was.

  The obtained plated steel sheet was evaluated for appearance and corrosion resistance. Appearance (surface properties) was evaluated by visual judgment, and corrosion resistance was evaluated visually by JASO M609-91, which is a cyclic corrosion test. “Spotted red rust” is a red rust having a diameter of 1 to 2 mm or less, and “entire red rust” is a state where 50% or more of the test surface has red rust.

  Table 1 shows the detailed plating conditions of the plated steel sheet and the investigation results of the increase / decrease in the thickness of the entire plated layer after the second plating with respect to the plating thickness of the first plating layer. Table 2 shows the results of the investigation of the composition of the plating layer and the appearance and corrosion resistance.

  From Table 1, it can be seen that when the second stage plating condition is 420 ° C. or lower and the immersion time is 120 seconds or shorter, the upper layer plating of the first stage plating is not lost by the second stage plating and the plating can be thickened.

  From Table 2, it can be seen that a plating thickness of 100 μm or more is a condition for obtaining high corrosion resistance. Some of the examples have point-like red rust occurring at an early stage, but when the plating is thick, it is a feature of the plating layer of this embodiment that the red rust spreads very slowly. The corrosion behavior is different from that of the continuously plated high corrosion-resistant plated steel sheet. Moreover, although an Example has what has an intermediate | middle layer (No.1-14, 20-23) and what does not exist (No.15-19), even if it is any case, favorable corrosion resistance is shown. ing. No. In No. 32, since the first plating layer was a pure Zn plating layer, it had sacrificial anticorrosion properties but low barrier properties, and thus red rust was generated on the entire surface. In addition, it was confirmed by microscopic observation of the cross-sectional structure of the corrosion test piece that “dotted red rust” was caused by the alloy layer and that the steel was not corroded, and that “corrosion of the steel was started in“ entire red rust ”. It was done.

1 and FIG. The cross-sectional optical microscope photograph of 15 plated steel materials is shown. FIG. 1 is a cross-sectional photograph of the plated layer after the first-stage plating, and FIG. 2 is a cross-sectional photograph after the second-stage plating. As shown in FIG. 1, in the first stage plating step, a first stage FeAl alloy layer B is formed on the steel material A, and a surface layer plating C having a lower Al concentration than the FeAl alloy layer is formed on the surface layer. I understand that. It can be seen that the FeAl alloy layer B includes a plurality of phases. Further, it can be seen that the surface plating C has a metal structure clearly different from that of the FeAl alloy layer B. In addition, as shown in FIG. 2, it can be seen that the second-stage Zn alloy layer D is formed on the first-stage FeAl alloy layer A by the second-stage plating step. According to FIG. 2, it can be confirmed that the second stage Zn alloy layer D contains a relatively large block phase, and it is determined from the optical micrograph that the structure form is different from that of the FeAl alloy layer A. The Thus, it is shown from the optical micrograph that the Zn alloy layer D and the FeAl alloy layer A can be distinguished.

Claims (8)

The steel material surface is provided with a plating layer including a FeAl alloy layer and a Zn alloy layer,
The FeAl alloy layer is formed on the surface of the steel material, includes an FeAl alloy phase and a Zn phase, has an average composition of Fe: 30 to 60% by mass, Al: 15 to 30% by mass, the balance Zn and impurities, Is 100 μm or more,
The Zn alloy layer is a highly corrosion-resistant plated steel material formed on the FeAl alloy layer, made of Al, the balance Zn and impurities, and having a thickness of 5 μm or more.   The high corrosion-resistant plated steel material according to claim 1, wherein the Zn alloy layer has an average composition of Al: 4 to 6% by mass, the balance Zn and impurities. The Zn alloy layer is
An intermediate layer formed on the FeAl alloy layer and having an average composition of Al: 12% by mass or less, the balance Zn and impurities;
The high corrosion-resistant plated steel material according to claim 1, comprising: an outermost layer formed on the intermediate layer and having an average composition of Al: 4 to 6% by mass, the balance Zn and impurities.   The high corrosion-resistant plated steel material according to claim 2, wherein the Zn alloy layer further contains 0.1 to 3% by mass of Mg.   The high corrosion-resistant plated steel material according to claim 3, wherein the outermost layer further contains 0.1 to 5 mass% of Mg. A first hot dipping step of forming a layer containing an FeAl alloy layer of 100 μm or more by immersing the steel material in a first hot Zn plating bath containing 1 to 12% by mass of Al;
A flux treatment step of applying a flux to the steel material after the first hot dipping step;
A second hot dipping process in which the steel material after the flux treatment process is dipped in a second hot dipped Zn plating bath containing 4 to 6% by mass of Al and having a bath temperature of 420 ° C. or lower;
A method for producing a highly corrosion-resistant plated steel material comprising:   The method for producing a highly corrosion-resistant plated steel material according to claim 6, wherein, in the second hot dipping step, the immersion time of the steel material in the second hot dipped Zn plating bath is in the range of 10 seconds to 120 seconds.   The method for producing a highly corrosion-resistant plated steel material according to claim 6 or 7, wherein 0.1 to 5% by mass of Mg is further contained in the second molten Zn plating bath.