TWI402360B - And a method for producing the molten Al-plated steel sheet with excellent heat resistance and blackening property - Google Patents

Description

Fused Al-plated steel sheet with excellent resistance to heat and blackening and manufacturing method thereof Technical field

The present invention relates to a molten Al-plated steel sheet and a method for producing the same, which, even if maintained at a high temperature of about 550 ° C, does not blacken the surface of the plating layer, and has excellent heat-resistant blackening property for maintaining high heat reflectivity, and is excellent in workability. .

Background technique

A molten Al-plated steel sheet coated with Al-Si alloy on a steel plate has Si added to the plating layer, so that it can maintain silver white color even at high temperatures and has excellent heat reflection characteristics. Therefore, various types of heat-resistant applications such as silencers for automobiles have hitherto been used. However, once the molten Al-plated steel sheet is exposed to a high temperature of 450 ° C or higher, interdiffusion of Al and Fe occurs, and the Al-Si plating layer becomes an Al-Fe-Si-based intermetallic compound layer and becomes black ( In the following, if there is no special notice, the phenomenon is called "alloying" or "blackening". Moreover, the easiness of blackening is called "blackening".), and the gloss is lost, and the heat reflection is seriously impaired. Sex is well known.

The alloying of the Al plating is closely related to the amount of solid solution nitrogen in the steel sheet, for example, as described in Iron and Steel 70 (1984) S475, etc., in the presence of solid nitrogen. In a steel sheet having a certain amount or more, an AlN layer is formed at the interface between the alloy layer and the steel sheet to suppress the alloying reaction. Further, it is also known that the AlN layer is promoted by plating and annealing the steel sheet containing solid-melting nitrogen, and the blackening temperature is further increased.

Based on this insight, various techniques have been reviewed so far for the technique of inhibiting blackening by alloying. For example, the applicant and the like disclose in Patent Document 1 an Al-plated steel sheet by molten Al plating by a steel having a limit on the amounts of C, Si, N, Al, O, Ti, Nb, V, and B. After the plating of the steel sheet, annealing is performed at 300 to 500 ° C for 2 to 20 hours, and the blackening resistance is maintained.

A corresponding strategy is proposed in Patent Document 2, which is a blackening temperature of 520 ° C with respect to rimmed steel, and a low temperature of 320 ° C from a fully-hardened steel. Solid-melted nitrogen (N) in steel. That is, it is ensured that the solid nitrogen is limited to form Al and Ti which form a stable nitride. Therefore, a method for producing a cast piece for a molten Al-plated steel sheet is disclosed, in which deoxidation conditions and the like are affected, and C, Si, Mn, sol-Al, N, and O are specified to be appropriate. In the scope.

Patent Document 3 proposes a proposal to prevent blackening, which is performed by annealing aluminum at 300 to 500 ° C for 2 to 20 hours, thereby making the Fe-Al-Si-Mg alloy monoclinic. The crystal is formed between the steel sheet and the plating layer, and further forms AlN between the intermetallic compound and the steel material to prevent blackening due to interdiffusion of the elements; the aluminum plating is a component which makes the sol-N remain stable. The steel contains a certain amount of Mg.

Patent Document 4 discloses that Mn and Cr are compositely added to an aluminum plating layer, and after annealing at 300 to 500 ° C for 0.5 hours or more, it is found that these elements are significantly concentrated at the interface between the alloy layer and the plating layer. And this layer will exert an alloying inhibition effect. Therefore, a proposal has been made to achieve a gloss-up effect.

Conventional technical literature Patent literature

Patent Document 1 JP-A-9-195021

Patent Document 2, JP-A-63-109110

Patent Document 3, JP-A-2000-290764

Patent Document 4, JP-A-8-311629

Non-patent literature

Non-Patent Document 1 Iron and Steel, vol. 70 (1984) S475

Even if it is described in the above-mentioned Patent Document 2 that the composition is limited by the total static steel, if the Al plating is still performed, the blackening temperature is about 520 ° C as in the case of the static steel. Therefore, it is impossible to suppress the alloying reaction of Fe-Al at a high temperature of 550 ° C or higher to prevent blackening. The techniques in Patent Documents 1, 3, and 4 are performed by annealing at 300 to 500 ° C for 2 to 20 hours after the Al plating treatment (Post Annealing, also referred to as "additional annealing after plating"). In addition, it is possible to maintain an Al or Al-Si film having excellent light reflectivity, and suppress the cause of blackening--the formation of a Fe-Al intermetallic film having inferior light reflectivity.

It can be seen that this is because the nitrogen (N) in the base material is reacted with Al in the plating layer by additional annealing after plating, and an AlN layer is formed at the plating interface, and the layer becomes an isolation layer. It can suppress the mutual diffusion of elements in the steel and in the plating layer.

However, Post Annealing after plating not only greatly deteriorates the productivity of the steel sheet, but also greatly increases the manufacturing cost, and is a problematic manufacturing method from the viewpoint of energy saving or CO ₂ emission reduction. .

Moreover, once additional annealing is performed after plating, a monoclinic Al-Fe-Si layer is formed at the interface between the base steel sheet and the aluminum plating layer depending on heating conditions. Since the monoclinic Al-Fe-Si layer is harder than the plating layer, there is a disadvantage that cracks are likely to occur during processing.

As described above, in the prior art, the additional annealing is performed after the plating to form the separator to suppress the formation of the Fe-Al intermetallic compound, but the workability is poor, and the productivity is deteriorated due to the high temperature and long-time heating, and the production is deteriorated. The cost is high, not only from the processing and economic aspects, but also from the environmental side.

Accordingly, in order to solve the above problems, the present patent application can produce a high temperature of 550 ° C or higher without additional annealing after Al plating, or an additional annealing resistance heating after plating of at least a conventional Al-plated steel sheet. A steel sheet which is blackened and has excellent workability is a problem.

Moreover, in the case where fully killed steels are currently in the mainstream, since it is lower than the solid-melted nitrogen of the unsinkable steel, it is necessary to additionally anneal after plating in order to improve heat resistance. Figure 1 shows the relationship between solid nitrogen and heat resistant temperature. When the solid nitrogen is 46 ppm, it is not static steel. It is well known that when the solid-melting nitrogen is 46 ppm or less, heat resistance can be improved by additional annealing.

On the other hand, a steel containing 46 ppm or more of solid-melting nitrogen is inferior in workability, and the frequency of occurrence of cracks in the case of extrusion processing is high, and thus it is not suitable for processing of complicated shapes.

In the present invention, a low-solid-melting nitrogen steel of 46 ppm or less is proposed to prevent blackening and to improve the workability.

In order to solve the above problems, the inventors of the present invention have found that the interface between the steel material and the Al plating layer (hereinafter referred to as "plating interface") promotes the formation of the AlN layer together with the concentration of nitrogen (N). Carbon (C) will also be concentrated. The existence of C in the austenite former is believed to have some function and contributes to the concentration of N. Therefore, the effect of the N-concentration-promoting effect was investigated by adding a Worthite iron formation factor other than C, and it was found that these Worstian iron-forming factor elements contained an N-concentration-promoting effect. Further, it has been found that the steel sheet of the present invention can also satisfy the processability, and the present invention has been completed. The gist of the following is as follows:

(1) A molten Al-plated steel sheet having excellent heat and blackening resistance, characterized in that the surface of the steel sheet having the following composition has an Al plating layer, and the composition contains, by mass%: C: 0.0005 to 0.01% , Si: 0.0001~0.05%, P: 0.002~0.1%, S: 0.002~0.1%, Al: 0.001~0.01%, N: 0.0015~0.0040%, and O: 0.02~0.08%, and further contains Ni: 0.01~ 0.1% and Cu: 0.01 or 0.1% of one or two, 10×C+Ni+Cu>0.03, the residual part is Fe and unavoidable impurities; further, the composition of the Al plating layer is expressed by mass% Si: 4 to 11%, the residual portion is composed of Al and unavoidable impurities, and the molten Al-plated steel sheet has an Al-Fe-Si alloy layer having a thickness of 5 μm or less at the interface between the Al plating layer and the steel sheet.

(2) The molten Al-plated steel sheet having excellent heat-resistant blackening property as described in (1), wherein an AlN (aluminum nitride) is present at an interface between the steel sheet and the Al-Fe-Si alloy layer, and the aforementioned Al- The Fe-Si alloy layer is a hexagonal Al-Fe-Si alloy layer, and the hexagonal Al-Fe-Si alloy layer has a thickness of 5 μm or less.

(3) The molten Al-plated steel sheet having excellent heat and blackening resistance as described in (1) or (2), wherein the Al plating layer has a Knoop hardness of 90 to 110.

(4) A method for producing a molten Al-plated steel sheet having excellent heat and blackening resistance, characterized in that a steel sheet having a steel composition according to any one of (1) to (3) is used as a plated original plate for Al In the plating, the amount of Si in the Al plating bath is 4 to 11%, the bath temperature is 610 to 650 ° C, and the plating treatment is not performed after the processing.

According to the present invention, it is not necessary to perform post-annealing after plating, and a molten Al-plated steel sheet having excellent heat-resistant blackening property and workability can be obtained at a high temperature of 550 ° C or higher. Therefore, compared with the conventional one, it is possible to achieve extremely excellent productivity and to lower the manufacturing cost, and to achieve an effect of maintaining high gloss with high gloss and good blackening resistance. Moreover, since the heat treatment step is greatly reduced, energy consumption is suppressed, and CO ₂ emission is controlled, the effect of remarkably reducing the environmental load can be obtained.

Simple description of the drawing

Fig. 1 shows the relationship between the amount of nitrogen (N) in steel and the heat resistance of steel.

Figure 2 is a conceptual diagram showing the blackening mechanism of the surface of capped steel and aluminum unsinkable steel. The upper section shows the hooded steel and the lower section shows the aluminum without static steel.

Fig. 3 is a view showing an example of high-frequency wave GDS analysis results of the surface of an Al-plated steel sheet. Fig. 3(a) mainly shows the distribution of aluminum and iron, and Fig. 3(b) shows mainly the distribution of carbon (C) and nitrogen (N).

Fig. 4 is a conceptual diagram showing the relationship between the peak concentration of AlN (the cumulative intensity of N in GDS) and the blackening temperature.

Fig. 5 is a view showing the blackening state of the Al-plated steel sheet of the Al plating bath temperature of the example and the Si concentration in the Al plating bath.

Fig. 6 is a view showing the occurrence of blackening of the Al-plated steel sheet in the Al plating bath temperature of the example and the Si concentration in the Al plating bath.

Figure 7 is a conceptual diagram of a draw bead test.

Form for implementing the invention

Hereinafter, a proper embodiment of the present invention will be described in detail.

First, according to the conventional technique, it is considered based on the reason that the blackening property is improved by additional annealing after Al plating (heating and blackening becomes difficult). Figure 2 contains a simplified illustration of its mechanism.

The upper part of Fig. 2 shows a capped steel containing a relatively high concentration of solid-melted nitrogen (N), and the lower part shows an example of an Al-unsinkable steel containing a low concentration of solid-melted nitrogen.

The capped steel containing a relatively high concentration of solid molten nitrogen (N) has improved blackening by the following mechanism.

x) First, if Al plating is applied to the cap metal 10 that becomes a bare metal, an AlN separator 11 and a hexagonal Al-Fe-Si alloy may be formed between the Al plating layer 13 and the bare metal 10 after plating. Layer 12.

y) In the subsequent heating at 550 ° C, the hexagonal Al-Fe-Si alloy layer 12 is changed to the monoclinic Al-Fe-Si alloy layer 12'.

In the present invention, the hexagonal Al-Fe-Si alloy layer 12 is also referred to as (Al-Fe-Si)H, and the monoclinic Al-Fe-Si alloy layer 12' is also referred to as (Al-Fe-Si). )M. Any of these is an intermetallic compound formed by an Al-Fe-Si ternary system, each of which has a crystal structure of Hexagonal and Monoclinic. Although the correct chemical formula is still discussed, it is said that the hexagonal Al-Fe-Si alloy layer is Al ₈ Fe ₂ Si, and the monoclinic Al-Fe-Si alloy layer is Al ₅ FeSi.

Moreover, at this time, the AlN layer 15 is formed at the plating interface (the interface between the base material and the plating layer), and this layer becomes a separator, and the mutual diffusion of the elements of the steel material and the plating layer is suppressed. Therefore, the plating layer does not change to an Al-Fe alloy (intermetallic compound), and a surface having good light reflectivity can be obtained (second drawing in the second drawing).

On the other hand, when Al is not static steel having a low solid-melting nitrogen concentration (second stage in Fig. 2), if Al plating is applied to the unstained steel 10' which becomes a bare metal, since the solid-melted nitrogen is small, it is isolated in AlN. The layers (i.e., the separators other than the foregoing), the elements between the steel sheet and the plating layer may mutually diffuse. As a result, it is considered that the hexagonal Al-Fe-Si alloy layer 12 becomes a monoclinic Al-Fe-Si alloy layer, and further, since it is diffused into the Al plating layer 13, it changes to a θ phase or an η phase 14, thereby The Fe in the plating becomes high and blackening occurs (the lower part of Fig. 2).

As a result, the inventors of the present invention paid attention to the plating interface and tried to observe and explain the phenomenon occurring at the plating interface. As shown in Fig. 3, when the composition of the plating interface is analyzed, the nitrogen (N) in which AlN is formed at the interface is concentrated, and the concentration of carbon (C) is determined. In Fig. 3, after the Al plating, only the Al plating layer was electrolytically peeled off to expose the alloy layer, and the surface was analyzed by high-frequency wave GDS. The high-frequency wave GDS is an analysis device for measuring the element distribution in the depth direction by using an Ar gas sputtering surface, wherein the horizontal axis represents the sputtering time, and the vertical axis represents the signal intensity proportional to the concentration.

At the plating interface (correctly the interface between the plating layer and the alloy (intermetallic compound) layer), the C forming element of the Worth is already concentrated. The solid melting of N is much larger in the Worthfield iron than in the Ferrite. In other words, it is possible to form an element by Worthite iron, and by adding an element which is easily concentrated on the surface, so that only a slight thickness of the outermost surface is fermented by Vostian, so the N concentration here rises (N concentration) ). The element having such a property may, for example, be Cu or Ni. It can be seen that these elements also have the same effect, so the effects of these elements are reviewed.

As a result, it was confirmed that when Cu or Ni was added, an AlN layer and a hexagonal Al-Fe-Si alloy layer of about 3 μm were formed at the plating interface.

Figure 4 shows the peak concentration of AlN (the cumulative intensity of N with DGS) versus the blackening temperature. As can be seen from Fig. 4, the higher the peak concentration of AlN becomes, the higher the blackening temperature becomes. In other words, once a strong AlN barrier layer is formed, the interdiffusion of elements between the steel sheet and the Al plating layer can be suppressed without generating an Fe-Al intermetallic compound.

In other words, it is known that even if the solid nitrogen is about 20 ppm and a low steel grade, a high concentration of AlN and a hexagonal Al-Fe-Si alloy layer can be formed as in the conventional unsinkable steel. Thus, even without post-plating post-annealing, it is possible to produce an Al-plated steel sheet which is not blackened.

In other words, if Cr is added to the steel sheet, Cr is concentrated on the surface of the steel sheet. Since Cr is a ferrite-grain-forming element, once Cr is concentrated, it will hinder the concentration of C, N, Cu, and Ni in the formation of iron in the Worthfield, resulting in a decrease in the peak concentration of AlN. Therefore, it is better to add Cr as much as possible, and try not to add it. Similarly, it is preferable to add other ferrite-forming elements such as Mo.

Next, it is discussed why the hexagonal Al-Fe-Si alloy layer has an effect on blackening.

In the case where no annealing is performed after plating, it is considered that AlN is formed during the cooling process after Al plating. At this time, the alloy layer is formed, so the solid nitrogen in the steel reacts with the Al of the alloy layer to form AlN. However, it can be seen that the hexagonal Al-Fe-Si alloy layer reacts more easily with the solid-melted nitrogen in the steel than the monoclinic Al-Fe-Si alloy layer, and as a result, AlN is formed.

In other words, since the interface between the AlN and the Al plating layer has a hexagonal Al-Fe-Si alloy layer (instead of the monoclinic Al-Fe-Si alloy layer), AlN is easily formed, and mutual diffusion inhibition of Fe-Al can be expected. The multiplication effect of the isolation effect. That is, the hexagonal Al-Fe-Si alloy layer is effective for the formation of AlN.

However, since the hexagonal Al-Fe-Si alloy layer has high hardness, if the layer is thick, the ductility of the steel sheet itself is hindered, and cracks are likely to occur when the plated steel sheet is formed. Therefore, it is preferable that the thickness of the hexagonal Al-Fe-Si alloy layer is controlled to 5 μm or less.

The control of the thickness of the alloy layer roughly determines the amount of Si in the bath and the bath temperature. If the bath temperature is too high, the alloy layer will grow. Therefore, it is also known that the Si concentration in the plating bath is 4 to 11%, and the plating bath temperature is kept at a lower temperature of 610 to 650 ° C to form a stable formation of the AlN and the hexagonal Al-Fe-Si alloy layer. It is effective.

If it is discussed from the viewpoint of the Si temperature in the bath, as previously shown, the chemical formula can be presumed, and if the hexagonal Al-Fe-Si alloy layer is compared with the monoclinic Al-Fe-Si alloy layer, the Si content is different. It contains about 10% Si relative to the former, and the latter contains about 15% Si. Therefore, when the amount of Si in the bath exceeds 11%, a monoclinic Al-Fe-Si alloy layer is mainly formed, and when the amount of Si in the bath is 4 to 11%, a hexagonal Al-Fe-Si alloy layer is easily formed. When the amount of Si in the bath is less than 4%, it is easy to form an Al-Fe compound containing no Si.

Fig. 5 shows the blackening state (photograph) according to the bath temperature of the Al plating bath and the Si concentration in the Al plating bath, and Fig. 6 shows the Si content in the bath and the blackening state depending on the bath temperature. The line in the figure indicates that the Si content is 4 to 11% and the bath temperature is 610 to 650 °C. The components which become the base steel material at this time are shown in Table 1.

Still, the following numbers in Fig. 5 indicate the Si concentration and bath temperature in each bath.

Further, the evaluation of the blackening of Fig. 6 is ○: no blackening, Δ: partial blackening, and ×: total blackening. Even if it is a rating of △, it is still not practical for use because it is partially blackened.

Next, the reason for limiting the components of the present invention will be described.

First, describe the composition of the steel. In fact, the units of the steel components are all in mass%.

C: If the concentration of the solid-melting nitrogen is the same, the less the C content, the more the workability of the steel sheet is improved. On the other hand, since the components of the present invention necessarily contain solid-melting nitrogen, the workability is slightly inferior. Therefore, it is preferred to have a low C in terms of processability. In the present invention, it is limited to 0.01% or less. However, the above-mentioned subject matter is preferably 0.005% or less, more preferably 0.004% or less, more preferably 0025% or less, still more preferably 0.001% or less. Further, in order to secure the strength of the steel material, the lower limit is preferably 0.0005%.

Si:Si reacts with oxygen in the steelmaking stage to remove oxygen from the molten steel. Also, in the steel strip manufacturing step, it is also possible to react with the solid molten oxygen (O) in the steel. Further, Si reacts with N in steel to form Si ₃ N ₄ , SiN or the like to reduce solid nitrogen. Further, once the amount of Si is increased, it is concentrated as an oxide on the surface during heating in the hot-dip plating step, thereby causing an unplating phenomenon. In short, the desired element is preferably as low as 0.05% or less, preferably 0.041% or less, preferably 0.021% or less, more preferably 0.01% or less, and even more preferably 0.004% or less. The lower limit is preferably about 0.001%.

N: In order to prevent the blackening after Al plating to maintain gloss, it is necessary to leave the steel sheet as solid-melting nitrogen. More than 0.0015% of N is required for this purpose. It is preferably 0.0019% or more, more preferably 0.024% or more, and more preferably 0.0031% or more. On the other hand, as the solid-melting nitrogen increases, the steel sheet hardens, and the endurance and tensile strength are greatly increased, and the stretching is lowered. Moreover, the press formability also deteriorates. Therefore, the upper limit of the amount of N is 0.0040%. In the present invention, such as the following, since the Al concentration in the steel material is low, the surface which contacts the Al plating layer is excluded, and AlN is not formed. Therefore, the amount of N is approximately equal to the solid nitrogen.

Al: Al is usually used as an oxygen scavenger for the molten steel in the steel making step. However, the Al remaining here will react with the solid nitrogen to become AlN in the steel strip manufacturing step. The AlN is dispersed in the steel sheet and is different from AlN present at the interface between the steel sheet and the plating. As a result, since the amount of solid-melting nitrogen is small, the concentration of AlN formed at the interface is small, and the blackening prevention property after Al plating is deteriorated. Therefore, the amount of Al is preferably low. Therefore, the upper limit is limited to 0.01%. It is preferably 0.005% or less, more preferably 0.003% or less, and is more preferably 0.002% or less. Let the lower limit be 0.001%.

O: Since there is an inclusion in the steel if there is oxygen, it is generally deoxidized by Al, Si, etc. in the steel making stage. In the present invention, the steel contains preferably 0.03% or more of oxygen, more preferably 0.042% or more, more preferably 0.050% or more. This reason is as described above because if O is sufficient in the steel, it has a stability effect of resistance to heat and blackening.

This is the effect of oxygen at 0.03%. However, when the oxygen content is still increased, the workability is deteriorated by the inclusions, so the upper limit of ○ is 0.08%, and preferably 0.065%.

Ti, B: These elements form a compound with N. Therefore, it is better to ensure that the molten nitrogen is less.

P, S: These are known as debris that is easy to surface deflection. For the refining of the economic side, the lower limit of P and S is 0.002%.

On the other hand, P is an element which causes ductility and brittleness of the steel sheet, and S is an obstacle to the ductility of the steel sheet. Therefore, the respective upper limit is 0.1%. Further, the preferred upper limit of P is 0.066%, and the upper limit of S is preferably 0.081%.

Ni, Cu: These elements are Worstian iron forming elements which are easy to be surface-concentrated, and are important elements for causing an upward effect of resistance to heat and blackening as described above.

That is, it is known that at the interface between the steel sheet and the aluminum plating, the C of the Worstian iron forming element is concentrated at the interface, which may contribute to the concentration of N.

Then, the present inventors further added Cu or Ni which forms the element of the Worthite iron, and investigated the effect. As a result, it was confirmed that the AlN layer was easily formed by adding Cu or Ni. On the other hand, in the case where Cr which is one of the ferrite-forming elements is not present, the aforementioned effects occur even if the amount is small, but the above effect is lost once the presence of Cr, and therefore it is not preferable to use Cr in combination. Therefore, let Cr be 0.02% or less, which is an unavoidable debris.

The lower limit of Ni is 0.01%, preferably 0.018%, and more preferably 0.029%. Further, the lower limit of Cu is 0.01%, preferably 0.022%, more preferably 0.041%. Since the excessive addition of Ni and Cu causes generation after hot rolling, the upper limit is made 0.1%. It is possible to confirm the formation of AlN by satisfying the lower limit, and it is possible to blacken the suppression.

Furthermore, let 10 × C + Ni + Cu > 0.03. This is to specify the three elements of the aforementioned Worstian iron stabilization element and surface-concentration elements. Although Mn is also a stabilizer for the Worthite iron, it is excluded because the concentration on the surface is not large. These elements are added to form AlN at the alloy layer-steel plate interface, and it is possible to suppress blackening up to 550 ° C without additional annealing after plating.

The element other than the above elements is not particularly limited, but Mn may generally contain about 0.2 to 0.8%.

(About Al plating)

Next, the reason why Si in the Al plating layer and the molten Al plating bath is limited will be described. In still, the unit is % by mass (marked by % in the following description). In Al plating which does not contain Si, an Al-Fe intermetallic compound layer (generally referred to as an alloy layer: FeAl ₃ or Fe ₂ Al ₅ ) tends to grow very thick, and a grown alloy layer causes plating during processing. Peeling. Generally, Si is added to suppress the growth of the alloy layer. In order to reduce the alloy layer, the amount of Si must be 4% or more. On the other hand, the effect is saturated at about 11%, and the addition of the above ratio causes a decrease in corrosion resistance and workability. Therefore, the upper limit of the amount of Si in the plating bath is 11%, and the lower limit is 4%. Further, inevitable elements other than Al and Si in the bath generally contain about 2% of Fe (melted from a plated steel sheet or a plating machine). There is no particular limitation on this.

In the present invention, it has been found that the amount of Si in the Al plating bath is 4 to 11%, and the bath temperature is particularly preferably 610 to 650 °C. Under the conditions of aluminum plating, it is possible to exhibit heat-resistant blackening resistance at 550 ° C without applying post-plating and post-annealing. And in the case of aluminizing, the molten nitrogen in the steel reacts with the plating component to form AlN in the Al interface between the steel sheet and the plating bath, and under this condition, the alloy layer becomes a hexagonal Al-Fe-Si alloy layer, which is easier. Generate AlN. Further, since the bath temperature is too low, the bath viscosity is high, and the control of the amount of adhesion becomes difficult. Therefore, it is difficult to operate at a temperature lower than 610 °C.

The addition elements of the Al plating layer and the plating bath other than this may be Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, Bi, a bismuth alloy, etc., as long as the plating layer is Al is the main body and can be applied. The addition of Zn and Mg is effective in the sense that it is difficult to produce red rust. However, excessive addition of these elements with high vapor pressure causes Zn and Mg smoke generation and powdery substances on the surface (cause). In the case of the formation of Zn or Mg), it is not preferable to add Zn: 30% by mass or more and Mg: 5% by mass or more.

In addition, as a post-plating treatment, a chemical conversion coating, a resin coating, or the like may be applied for the purpose of primary rust prevention and lubricity. As for the chromate treatment, if a hexavalent chromium limit is considered in recent years, it is preferable to use a trivalent treatment film. Further, it is also possible to apply a subsequent treatment other than chromate in an inorganic system. To impart lubricity, wax, alumina, ceria, molybdenum disulfide (MoS ₂ ), or the like may be used in advance for surface treatment.

The amount of adhesion to the Al plating layer is not particularly limited, and is generally from 80 to 120 g/m ² on both sides, and there is no particular problem in the adhesion amount.

A well-preserved Al-plated steel sheet having excellent heat and blackening resistance has been subjected to additional annealing treatment after plating. According to the additional annealing, the hardness of the Al plating layer is reduced. This corresponds to the fine precipitation of Fe which is solid-melted in Al, and the hardness of the Al plating layer before the additional annealing is 90-110 in the Rope hardness, and is reduced to 50-80 after the additional annealing. Here, the Rope hardness means that the shape of the indenter of the Vickers hardness is different, and the test method is specified in JIS (Japanese Industrial Standard) Z2251 (2009). When the hardness of the plated section of 10-30 μm is measured, since it is difficult to measure the Vickers indenter, it is defined by the Rope hardness. It is considered that the Al-plated steel sheet is liable to cause scratches during press forming, and it is more likely to cause such erosion when it is subjected to annealing treatment, and thus has been said to be a problem. In the present invention, since the heat-resistant blackening property is improved without applying an annealing treatment, the pressurization molding can be expected to be improved.

(Evaluation of resistance to blackening)

The evaluation of blackening was performed at 520 ° C to 580 ° C for 20 hours at 10 ° C for each hour, and the blackening of the surface was visually observed. Further, it is known that in the heating temperature at the time of the evaluation, AlN is not generated and only blackening is performed. Table 2 shows the results of evaluation of the resistance to heat blackening and the evaluation of the workability of the examples of the present invention. As is clear from the results of Table 2, the present inventors have confirmed that blackening does not occur until 550 ° C without annealing. Compared with the conventional product (refer to the patent literature), the blackening temperature of the annealed material after plating is 520 ° C and 530 ° C, which indicates that heat resistance (ie, heat resistance to blackening) has been improved from the viewpoint of blackening. . Moreover, the effect of the present invention was verified by the fact that the blackening temperature after annealing was substantially the same as that of the conventional product (see Patent Document).

It is not preferable to subject the steel sheet of the present application to annealing (Post Annealing, also referred to as additional annealing) by box annealing or the like after plating. As described above, the hardness of the Al plating from the annealing treatment is lowered, and the corrosion is likely to occur at the time of press molding. Further, since the shape of the steel sheet is torn apart due to warpage or the like when the box annealing treatment is applied, there is a need for a skin pass and a finishing line through the sheet, and it becomes necessary to increase the thickness of the steel sheet. 3 steps. This is not appropriate from the viewpoint of productivity and manufacturing cost.

[Examples]

Hereinafter, the present invention will be described in more detail by way of examples.

(Example 1)

The material was a cold-rolled steel sheet having a steel composition as shown in Table 2 (having a general hot rolling step and a cold rolling step) (plate thickness: 0.8 mm), and molten Al plating was performed. For the molten Al plating, a line of a non-oxidizing furnace-reduction furnace type was used, and after plating, the amount of plating adhesion was adjusted to about 80 g/m ^{2 on} both sides by a gas wiping method, followed by cooling. The annealing temperature at this time was about 800 ° C, and the plating bath was composed of Al-9% Si-2% Fe. The Fe in the bath is supplied from a plating machine or a strip in a bath, which is unavoidable. Also, the bath temperature has been set to 645 °C. The plating has a good appearance and cannot be plated. One part of the prepared sample was further subjected to a post-plating annealing treatment in a box annealing furnace at 380 ° C for 10 hours, and thereafter a 1% temper rolling was performed. A barrel roll was used for the rolling at the time of temper rolling.

Thus, the sample characteristics that have been prepared are evaluated.

(1) Heat and blackening resistance

In a box annealing furnace, samples (50 mm × 100 mm) were annealed at respective temperatures of 520 to 580 ° C for 200 hours. The presence or absence of blackening was determined by visual observation after annealing and observation of cross-sectional structure.

The evaluation of the resistance to heat blackening has been ○: no blackening, △: partial blackening, ×: total blackening. Even if it is a rating of △, since the part is blackened, it cannot be used for practical use.

The temperature conditions required for the resistance to heat and blackening differ depending on the exposed environment of the member to be used. The temperature required for home appliances such as toasters and grills is usually lower at 500 °C, heaters and kerosene stoves are around 550 °C, and silencers used in automobiles or locomotives require blackening temperatures exceeding 550 °C. . In this application, temperatures in excess of 600 ° C should be required, but the design considerations can reduce the required temperature of the material. For example, an insulating material can be added to make the material temperature 550 ° C. Conversely, by increasing the blackening temperature of the material, the design relaxation space can be increased, and the reduction of the heat insulating material can also reduce the cost.

(2) Original plate processing

After applying the pressurized oil, the blank diameter: 100 mm, the punch diameter: 50 mm (drawing ratio is 2.0), and the extrusion process is performed to determine whether it is squeezable. .

The original plate processing property rating is ○: no abnormality, ×: cracking occurs.

(3) Recognition method of AlN and hexagonal Al-Fe-Si

Let the presence or absence of AlN be based on the use of GDS to detect the N-peak of the alloy layer-steel interface. Moreover, the GDS system was measured by plating Al by electroplating and removing it. On the other hand, the Al-Fe-Si alloy layer is added to the same material as that described for (Al-Fe-Si)H, and this may be recognized by X-ray diffraction from the surface after Al plating is electrolytically stripped and removed. .

(4) Plating processability

The test piece was tested on a test piece having a thickness of 0.8 mm and a size of 30 × 200 mm. The shape of the mold at this time is shown in Fig. 6. The surface roughness of the mold was expressed by Ra to be about 1.2 μm. After applying the pressurized oil, 10 consecutive strands of bead were formed, and the occurrence of scratches in the 10th sample was visually determined. The pressurization weight at this time was 500 kfg, and the plate thickness reduction rate was about 12%. However, the test was not carried out under the condition that the original sheet processing property was evaluated.

Judging criteria: ○: no scratches, △: a part of the sample was scratched, and ×: the sample was completely scratched.

(5) Alloy layer type, coating hardness

In order to identify the type of the alloy layer, the composition of the alloy layer was measured from the cross section. The portion corresponding to the alloy layer of the sample after the cross-section polishing was measured at an arbitrary 7 points by an electron probe microanalyzer (EPMA), and the value of Si/(Al + Fe + Si) was calculated. At this time, it is calculated in mass%. When the value is 8-11%, it is a monoclinic Al-Fe-Si alloy layer, and at 12-16%, it is a hexagonal Al-Fe-Si alloy layer. When it is not returned to any of them, when 7 When five or more points of the point measurement were hexagonal Al-Fe-Si alloy layers, it was judged that the alloy layer was a hexagonal Al-Fe-Si alloy layer. On the contrary, when 5 or more points in the 7-point measurement were monoclinic Al-Fe-Si alloy layers, it was judged that the alloy layer was a monoclinic Al-Fe-Si alloy layer. When both the hexagonal Al-Fe-Si alloy layer and the monoclinic Al-Fe-Si alloy layer are 4 points or less, both are generated. In the display of Tables 3 and 4, only a single one of the foregoing is only represented by H or M, and both the hexagonal Al-Fe-Si alloy layer and the monoclinic Al-Fe-Si alloy layer are formed. The time is expressed as H+M.

The hardness of the Al plating layer was also measured using a cross-section sample, and the Rope hardness was measured for the Al portion of the Al plating layer. Five points were measured and the average value was calculated. Let the weight at this time be 3gf. The Rope hardness was measured using a micro hardness meter MVK-G3 manufactured by Akashi Seisakusho Co., Ltd.

Table 2 summarizes the specifications and evaluation results of the samples.

In Table 2, the portion of the component value is circled to indicate that the component of the present application is exceeded.

As shown in Table 2, if the amount of C, Si, P, S, O, and N is too large, the processing property of the original plate will be hindered (numbers 1 to 4, 8, and 9). As for the heat-resistant blackening property, the steel components (Nos. 11 to 17 in Table 3) of the invention examples can prevent the blackening of the alloying at 540 ° C without annealing, and add Ni or Cu by a predetermined amount or more. It can prevent blackening up to 550 °C. Obviously, in Comparative Nos. 11 to 13, it was confirmed that the addition of Ni or Cu to the steel increased the heat resistance and blackening property.

The action of Ni and Cu is presumed to be multiplied by C to make AlN easy to form. In No. 10, blackening was prevented at 530 ° C, and addition of Ni and Cu confirmed the effect of increasing the blackening temperature at 20 ° C. 550 ° C is a blackening temperature that cannot be achieved without the annealing step. On the other hand, numbers 18 to 24 show an evaluation result, and the characteristics when additional annealing is applied after plating are evaluated. The blackening temperature was further increased by 20 ° C after the application of annealing.

However, at this time, the hardness of the Al plating is lowered, and a pressure scratch is generated. The reason is considered to be that the hardness of the Al plating layer is lowered. However, by applying annealing, it was detected that the alloy layers were all monoclinic Al-Fe-Si alloy layers. As described in detail, the monoclinic Al-Fe-Si alloy layer is judged to be a lower temperature and a stable phase than the hexagonal Al-Fe-Si alloy layer, and is formed by deterioration in the annealing step.

(Example 2)

Using the steel L of Table 1 (corresponding to the composition of the present invention), the amount of Si in the Al plating bath was changed to the bath temperature, and plating was performed. The adhesion amount was 80 g/m ² as in Example 1. Thus, the samples produced were evaluated. The evaluation conditions and evaluation criteria were the same as in the first embodiment. However, before the Al processing, the annealing treatment after the plating treatment was not performed, and the original plating was evaluated. Table 4 summarizes the relationship between plating conditions (Si amount in the bath, bath temperature), heat resistance to blackening, and workability. At this time, the thickness of the alloy layer was measured from the section lens barrel and shown in Table 4.

As in Sample 1, in Table 4, when the amount of Si in the bath is less than 2%, since the melting point of the plating bath becomes high, it must be used as a high bath temperature. Further, when the amount of Si is 2%, alloying of Al and Fe is likely to occur, and the alloy layer grows in the bath. Since the alloy layer is hard, it hinders the ductility of the steel sheet itself. Therefore, the workability of the original plate in the sample 1 was lowered. At this time, the resistance to heat and blackening is also inferior.

Al plating conditions affect the resistance to heat and blackening. In Nos. 2 to 11, the blackening resistance at the time of changing the amount of Si in the plating bath and the bath temperature were evaluated, and when the amount of Si was 15%, the resistance to heat blackening was inferior. The alloy layer at this time becomes a monoclinic Al-Fe-Si alloy layer. When the bath temperature is less than 610 ° C, the bath temperature becomes too high and it is difficult to apply Al plating. However, the alloy layer numbered 1 does not conform to the hexagonal Al-Fe-Si alloy layer or the monoclinic Al-Fe-Si alloy layer. As described, it was judged from the analysis result that it was Fe ₂ Al ₅ . In No. 9, the bath temperature was raised to increase the thickness of the alloy layer. Under these conditions, the alloy layer becomes too thick, which hinders the formability of the steel sheet.

Heretofore, the preferred embodiments of the present invention have been described, but the present invention is not limited to the examples. It is obvious to those skilled in the art that various modifications and alterations are possible within the scope of the patent claims, and it is of course within the technical scope of the present invention.

Industrial utilization possibility

In the case of using a steel material having a high temperature of about 550 ° C, the present invention can be utilized for users who pay special attention to the aesthetic appearance of the product. According to the present invention, it is possible to manufacture a steel material which is used at a high temperature of about 550 ° C and which is aesthetically pleasing, and which can be manufactured at low cost and with good productivity.

10. . . Hooded steel 10

10’. . . Quanjing Steel 10’

11. . . AlN isolation layer 11

12. . . Hexagonal Al-Fe-Si alloy layer 12

12’. . . Monoclinic Al-Fe-Si alloy layer 12'

13. . . Al plating

14. . . θ phase or η phase

15. . . AlN layer

Fig. 1 shows the relationship between the amount of nitrogen (N) in steel and the heat resistance of steel.

Figure 2 is a conceptual diagram showing the blackening mechanism of the surface of capped steel and aluminum unsinkable steel. The upper section shows the hooded steel and the lower section shows the aluminum without static steel.

Fig. 3 is a view showing an example of high-frequency wave GDS analysis results of the surface of an Al-plated steel sheet. Fig. 3(a) mainly shows the distribution of aluminum and iron, and Fig. 3(b) shows mainly the distribution of carbon (C) and nitrogen (N).

Fig. 4 is a conceptual diagram showing the relationship between the peak concentration of AlN (the cumulative intensity of N in GDS) and the blackening temperature.

Fig. 5 is a view showing the blackening state of the Al-plated steel sheet of the Al plating bath temperature of the example and the Si concentration in the Al plating bath.

Fig. 6 is a view showing the occurrence of blackening of the Al-plated steel sheet in the Al plating bath temperature of the example and the Si concentration in the Al plating bath.

Figure 7 is a conceptual diagram of a draw bead test.

Claims (4)

A molten Al-plated steel sheet having excellent heat and blackening resistance, characterized in that the surface of the steel sheet having the following composition has an Al plating layer, and the composition contains, by mass%: C: 0.0005 to 0.01%, Si: 0.0001~0.05%, P: 0.002~0.1%, S: 0.002~0.1%, Al: 0.001~0.01%, N: 0.0015~0.0040% and O: 0.02~0.08%, and more Ni: 0.01~0.1% and Cu: one or two of 0.01 to 0.1%, and satisfy the relationship of 10×C+Ni+Cu>0.03, the residual part is Fe and unavoidable impurities; and the composition of the Al plating layer is Si in mass% : 4 to 11%, the residual portion is composed of Al and unavoidable impurities, and the Al plating has a Knoop hardness of 90 to 110; and the molten Al-plated steel sheet is attached to the Al plating layer. The interface of the steel sheet has an Al-Fe-Si alloy layer having a thickness of 5 μm or less. A molten Al-plated steel sheet having excellent heat-resistant blackening property as in the first aspect of the patent application, wherein an AlN (aluminum nitride) is present at an interface between the steel sheet and the Al-Fe-Si alloy layer, and the aforementioned Al-Fe The -Si alloy layer is a hexagonal Al-Fe-Si alloy layer, and the hexagonal Al-Fe-Si alloy layer has a thickness of 5 μm or less. A molten Al-plated steel sheet having excellent heat and blackening resistance according to the first or second aspect of the patent application, wherein the molten Al-plated steel sheet having excellent heat and blackening resistance is not subjected to annealing treatment after plating. A method for producing a molten Al-plated steel sheet having excellent heat and blackening resistance, characterized in that a steel sheet having a steel component according to any one of claims 1 to 3 is used as a plating original plate for Al plating. In the case where the amount of Si in the Al plating bath is 4 to 11%, the bath temperature is 610 to 650 ° C, and the plating treatment is not applied, the annealing treatment is performed.