JP2020105554A - Alloyed hot-dip galvanized film - Google Patents

Abstract

To provide an alloyed hot-dip galvanized film capable of suppressing formation of Γ1 phase which is a cause of powdering or the like, and improving peeling resistance.SOLUTION: An alloyed hot-dip galvanized film is mainly composed of a Fe-Zn alloy. In the alloyed hot-dip galvanized film, the plating film contains Ag, and the ratio of Zn:Ag in a plating layer is in the range of 10:1 to 30:1 in terms of mass% ratio. Thus, it is found that both of intensity heightening of the plating film itself and improvement of processability are compatible with each other.SELECTED DRAWING: None

Description

The present invention relates to an alloyed hot-dip galvanized coating mainly composed of Fe-Zn alloy.

In recent years, in the automobile industry, there is an increasing demand for higher strength of steel materials used for the purpose of weight reduction of vehicle bodies and improvement of collision safety. Further, since a steel plate used as a member such as an outer plate (body sheet) of an automobile is used in an outdoor environment, it is generally required to have excellent corrosion resistance. On the other hand, increasing the strength of the steel sheet increases the stress that acts on the plating layer during press working, which causes problems such as seizure. Higher strength of the anticorrosion plating layer is also strongly desired.

From such a background, galvanization for sacrificial corrosion used in steel sheets for automobiles is widely used as a galvanized coating made of an alloy of iron (Fe) and zinc (Zn). (For example, Patent Document 1). The steel sheet that has undergone galvannealing has superior surface appearance and corrosion resistance as compared to the galvanized steel sheet that has not undergone alloying treatment. In general, an alloyed hot-dip galvanized steel sheet is obtained by plating hot-dip zinc on the surface of a steel sheet and then immediately heating it to a temperature equal to or higher than the melting point of zinc and diffusing Fe into the Fe-Zn alloy. However, since the alloying rate greatly differs depending on the composition and structure of the steel sheet, its control requires a fairly high degree of technology.

The peeling performance of such galvannealing is greatly affected by the presence or absence of the Γ1 phase, which is one form of the Fe—Zn alloy in the plating layer, and thus the conventional method causes powdering or the like. The plating layer is formed under processing conditions such as heat treatment time and heat treatment temperature that suppress the formation of the Γ1 phase. However, there has been a problem that the conventional control only by the heat treatment condition can form only a phase having insufficient strength, and cannot cope with the high strength of the steel sheet itself.

JP, 2009-68061, A

Therefore, an object of the present invention is to provide an alloyed hot-dip galvanized coating which can suppress the formation of the Γ1 phase that causes powdering and the like by a simple method and has improved peeling resistance. is there.

As a result of intensive studies to solve the above problems, the present inventors have added a trace amount of a metal element other than Zn to the zinc bath, and by controlling the concentration of the alloy element, heat treatment conditions equivalent to those in the conventional case. It has been found that even in the case of using, it is possible to control the formation of the Γ1 phase, which causes the peeling resistance of the galvannealed coating film to be significantly impaired. Furthermore, it has been found that this makes it possible to increase the strength of the plating film itself and improve workability. The present invention has been completed based on these findings.

That is, the present invention, in one aspect, relates to an alloyed galvanized coating,
<1> An alloyed hot-dip galvanized coating mainly composed of a Fe-Zn alloy, wherein the plated coating contains Ag, and the Zn:Ag ratio in the plated layer is 10:1 to 30 by mass. A galvannealed coating characterized by a range of 1:1;
<2> The alloyed hot-dip galvanized coating according to <1> above, wherein the thickness of the Γ1 phase in the plated coating is 1.0 μm or less;
<3> The galvannealed coating film according to <2>, wherein the Γ phase in the plating film has a thickness of 1 to 10 μm;
<4>% by mass, 0.1 to 15.0% of Fe, 80.0 to 95.0% of Zn, and 4.0 to 8.0% of Ag are contained, and the above <1> to <3>. An alloyed hot-dip galvanized coating according to any one of items 1 to 5;
<5>% by mass, further containing 0.05 to 0.5% of Al, the alloyed hot-dip galvanized coating according to any one of the above <1> to <4>; and <6> Γ in the plated coating. An alloyed hot dip galvanized coating according to any one of the above items <1> to <5>, wherein the fracture toughness value of the phase is 0.1 to 0.4 Mpa·m ^1/2 .

Further, in another aspect, the present invention also relates to a plated steel sheet having an alloyed hot-dip galvanized coating, specifically,
<7> A plated steel sheet having the galvannealed coating film according to any one of the above <1> to <6> on one surface or both surfaces of the steel sheet;
<8> The plated steel sheet according to <7>, wherein the amount of plating adhered on one surface of the steel sheet is 10 to 200 g/m ² .
<9> The plated steel sheet according to <7> or <8>, wherein the steel sheet is a steel sheet containing 90 to 99% by mass of Fe.

According to the present invention, by using an alloyed hot-dip galvanized coating containing a trace amount of Ag in a Fe—Zn alloy, powdering or the like is caused while maintaining the formation of the Γ phase, which is a high-strength phase. It is possible to suppress the formation of the Γ1 phase, which is a factor that significantly impairs the property. As a result, it is possible to provide an alloyed hot-dip galvanized coating that achieves both high strength and improved workability of the plated coating itself. Further, the plating film of the present invention can be formed by a simple method by simply adding a trace amount of Ag element having a specific concentration to the zinc bath without changing the heat treatment conditions and the like unlike the conventional method. It also has the advantage that

FIG. 1 is an SEM image of the element ratio and the surface of a plating film formed by a zinc bath containing no Ag in an Ar—H ₂ gas atmosphere (comparative example). FIG. 2 is an SEM image of the element ratio and the surface of the plating film formed by a zinc bath containing no Ag in an Ar gas atmosphere (comparative example). FIG. 3 is an SEM image of the element ratio and the surface of the plating film formed by the zinc bath with Al added in an Ar gas atmosphere (comparative example). FIG. 4 is an SEM image of the element ratio and the surface of the plating film formed by a zinc bath containing Ag in an Ar gas atmosphere (Example of the present invention). FIG. 5 is a graph showing the formation and growth behaviors of the Γ phase and the Γ1 phase in the cases of Ag addition and no addition. FIG. 6 is a phase diagram of a Fe-Zn-Ag ternary system.

Hereinafter, preferred embodiments of the present invention will be described. The scope of the present invention is not limited to these descriptions, and other than the following examples, the scope of the present invention can be appropriately modified and implemented within a range not impairing the gist of the present invention.

1. Alloyed hot-dip galvanized coating The plated coating of the present invention is a so-called alloyed hot-dip galvanized coating that is mainly composed of Fe-Zn alloy, contains Ag in the plated coating, and Zn:Ag in the plated layer. Is in the range of 10:1 to 30:1 in terms of mass% ratio. That is, as described above, the inclusion of such a small amount of Ag in the plating layer causes powdering or the like while maintaining the formation of the Γ phase, which is a high-strength phase, and is a factor that significantly impairs peeling resistance. This is based on the new finding that the formation of the Γ1 phase can be suppressed.

In the present specification, the “alloying hot-dip galvanized coating” is a plating layer formed on the surface of a steel plate or the like as a base material, and Fe in the steel can diffuse during Zn plating due to an alloying reaction. It means a plating layer mainly composed of Fe-Zn alloy. It has been known so far that alloy layers called ζ phase, δ ₁ phase, Γ ₁ phase, and Γ phase are formed in the alloyed hot-dip galvanized coating film due to the difference in Fe content. The phase structure of the galvannealed coating generally depends on the alloying conditions and the like, but in general, from the steel plate side as the base metal to be plated, the Γ phase, Γ ₁ phase, δ ₁ phase, and ζ phase in this order -Zn intermetallic compounds are present. The δ ₁ phase may be further _classified into two phases, δ _1p phase and δ _1k phase, depending on the difference in lattice constant.

In the present invention, the ζ phase is a metal having a composition of FeZn ₁₃ and having a monoclinic crystal structure with lattice constants of a=13.4Å, b=7.6Å, c=5.06Å, β=127.3. It is the phase of the intermetallic compound. The δ ₁ phase is a phase of an intermetallic compound having a composition of FeZn ₇ , and in the δ _1p phase, the hexagonal crystal has lattice constants a=12.8Å and c=57.4Å, and the δ _1k phase is , Δ _1p phase is said to have a lattice constant of 3 times the period.

Further, in the present invention, the Γ1 phase is a phase of an intermetallic compound having a composition of Fe ₅ Zn ₂₁ or FeZn ₄ , a face-centered cubic crystal, and a lattice constant of a=17.96Å. The Γ phase is a phase of an intermetallic compound having a composition of Fe ₃ Zn ₁₀ , a body-centered cubic crystal, and a lattice constant of a=8.97Å.

Of these, the Γ1 phase is known to cause powdering and the like, which causes the peeling resistance of the plating layer to be significantly impaired. In the alloyed hot-dip galvanized coating of the present invention, the formation of the Γ1 phase can be suppressed by adding a small amount of Ag, and as a result, the thickness of the Γ1 phase in the plated coating is 1.0 μm or less. it can. Preferably, the thickness of the Γ1 phase can be 0.5 μm or less, more preferably 0 to 0.1 μm or less.

On the other hand, the Γ phase has high strength, and it is considered that the existence of the Γ phase can impart strength to the plating layer. According to the present invention, peeling resistance and the like can be improved without impairing the strength of the plating layer by suppressing the formation of an unfavorable Γ1 phase while maintaining the formation of the Γ phase. In the galvannealed coating of the present invention, the thickness of the Γ phase in the plated coating can be preferably 1 to 10 μm.

From the viewpoint of suppressing the formation of the Γ1 phase and maintaining the formation of the Γ phase, in the alloyed hot-dip galvanized coating of the present invention, the Zn:Ag ratio in the plated layer is 10:1 to 30:% by mass. Ag is present in the range of 1. Preferably, the Zn:Ag ratio is in the range 10:1 to 25:1, more preferably 10:1 to 20:1. The Zn:Ag ratio can be adjusted by the amount of Ag added to the zinc bath in the plating forming step.

As described above, the alloyed hot-dip galvanized coating of the present invention has an Ag content such that the Zn:Ag ratio in the plated layer falls within a specific range, while suppressing the formation of the Γ1 phase, Although the effect of maintaining the formation of the Γ phase can be provided, the total metal content in the plating layer is typically 0.1 to 15.0 mass% for Fe and 80.0 for Zn. ˜95.0% by mass and Ag in an amount of 4.0 to 8.0% by mass can be contained. However, it is not limited to this range.

Regarding the Fe composition in the plating film, if the Fe content is too low, a soft Zn-Fe alloy may be formed on the plating surface, deteriorating press formability, while if the Fe content is too high, , A brittle alloy layer develops too much at the plating/steel sheet interface, and the plating adhesion may deteriorate. Therefore, the Fe content is preferably in the range of 5 to 12 mass %. However, the Fe content can be appropriately adjusted according to the application of the plated steel sheet and the like.

In a preferred embodiment of the present invention, the plating layer may further contain Al for the purpose of adjusting the Fe content. By adding the Al together with Ag to the zinc bath, the melting of Fe from the base steel sheet can be controlled. If the amount of Al added is too small, Zn—Fe alloying may proceed excessively during alloying treatment, and a brittle alloy layer may develop too much at the plating/steel plate interface, resulting in poor plating adhesion. If the added amount is too large, the Fe—Al—Zn-based barrier layer is formed too thick and alloying does not proceed during the alloying treatment, so that the desired iron content may not be obtained. Therefore, the Al content is preferably in the range of 0.05 to 0.5 mass%, more preferably in the range of 0.06 to 0.45 mass%.

In addition to Fe, Zn, Ag, and Al, the alloyed hot-dip galvanized coating of the present invention may contain a small amount of Pb, Sb, Si, Sn, Mg, Mn, from the viewpoint of further improving corrosion resistance and workability. One or more selected from the group consisting of Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, I, and C may be contained.

The alloyed hot-dip galvanized coating of the present invention preferably has a Γ phase fracture toughness value of the plated coating of 0.1 to 0.4 Mpa·m ^1/2 , more preferably 0.15 to 0.3 Mpa. -M ^1/2 . This defines the excellent property that the alloyed hot-dip galvanized coating of the present invention can maintain the grain boundary strength of the high strength Γ phase while suppressing the formation of the Γ1 phase.

2. Process for Forming Alloyed Hot Dip Galvanized Coating and Method for Manufacturing Plated Steel Sheet Next, the process for forming a galvannealed galvanized coating of the present invention will be described. This step also corresponds to the method for producing a plated steel sheet having the plated coating of the present invention on its surface. However, the steps described below are merely typical examples, and other methods can be used as long as they can form the alloyed hot-dip galvanized coating of the present invention.

As the steel sheet which is the base material (base), both hot-rolled steel sheet and cold-rolled steel sheet can be used, and particularly when used as an automobile outer panel, it is desirable to use an ultra-low carbon steel sheet having good press formability. In the present invention, the steps up to obtaining the base steel sheet are not particularly limited.

In the formation of the alloyed hot-dip galvanized coating of the present invention, the process of forming the alloyed hot-dip galvanized layer on the surface of the base steel sheet, the known non-oxidizing furnace method or all radiant hot-dip method such as can be applied, Typically, a method including a hot dip galvanizing step and an alloying treatment step can be used. The process conditions will be described below.

a) Hot-dip galvanizing step Hot-dip galvanizing is performed according to a method known in the art by immersing the base steel sheet in a hot-dip galvanizing bath continuously or discontinuously. In this case, the temperature of the hot-dip galvanizing bath is usually the melting point of Zn (about 420° C.) or higher. However, when the temperature is close to the melting point, Zn may be locally solidified due to fluctuations in the bath temperature, and the operation may become unstable. Therefore, it is generally preferable to set the temperature to 440° C. or higher. On the other hand, if the temperature of the molten metal becomes too high, a Fe—Al—Zn phase that hinders alloying may be generated. Therefore, the temperature is preferably 500° C. or less, for example.

By adding Ag to the zinc plating bath in the plating step, the alloyed hot-dip galvanized coating of the present invention containing Ag in the plating layer can be obtained. For example, the Ag content in the zinc bath is set such that the Zn:Ag ratio is in the range of 10:1 to 30:1 in terms of mass% ratio. Preferably, the Ag content in the zinc bath is set such that the Zn:Ag ratio is in the range of 10:1 to 25:1, more preferably 10:1 to 20:1. As described above, the inclusion of such a small amount of Ag causes powdering and the like in the plating film obtained after the alloying treatment described later while maintaining the formation of the high-strength phase Γ phase Formation of the Γ1 phase, which is a factor that significantly impairs the

In addition to Zn and Ag, a small amount of Al, Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi may be added to the zinc plating bath. , Sr, I, Cs, and REM can be contained alone or in combination. Depending on the amounts of these other components, it is preferable in that the corrosion resistance and workability are improved. In addition, from the viewpoint of corrosion resistance and cost, the amount of the alloyed hot-dip galvanized film deposited is preferably 10 to ²⁰⁰ g/m ² in terms of the amount of coating deposited on one surface of the steel sheet. Such an adhesion amount can be achieved by appropriately adjusting the immersion time (passing speed), bath temperature, and the like.

b) Alloying treatment step Next, the alloying treatment step to be performed is a step for diffusing Fe from the base material steel plate into the hot-dip galvanized layer formed on the surface of the base material steel plate in the previous step. It may be heated to a temperature in the range of 650° C. and kept at the temperature in that range, or may be heated to a temperature in the range of 470 to 650° C. and gradually cooled to the solidification temperature of Zn (about 420° C.). .. Here, if the heating temperature for the alloying treatment is less than 470° C., it becomes difficult to sufficiently diffuse Fe in the base material steel sheet into the plating layer, or due to the diffusion of a sufficient amount of Fe. It may take a long time and reduce productivity. On the other hand, if the heating temperature for alloying treatment exceeds 650° C., there arises a problem that coarse iron-based carbide is generated inside the steel sheet. Therefore, the heating temperature for the alloying treatment can be preferably set in the range of 470 to 650°C. When the alloying treatment is carried out by heating and holding at a temperature in the range of 470 to 650°C, the holding time is preferably in the range of 10 to 1600 seconds.

c) Other steps In some cases, after the a) hot dip galvanizing step and the b) alloying step, a diffusion process may be performed to further reduce the concentration gradient of the Fe content in the plated layer. The step of further forming a film on the surface of the plated film from phosphorus oxide or phosphorus-containing composite oxide may be performed.

3. Galvanized Steel Sheet Having Alloyed Hot Dip Galvanized Coating In another aspect, the present invention also relates to a galvanized steel sheet having the aforementioned galvannealed galvanized coating on one side or both sides of the steel sheet.

As described above, the steel sheet that is the base material (base) is not particularly limited, and both hot-rolled steel sheets and cold-rolled steel sheets can be used, and steel sheets having any composition can be used.

For example, the steel sheet which is the base material of the present invention can be an iron and steel sheet containing Fe as the main component, preferably containing 90 to 99% by mass of Fe, and the balance being other elements and unavoidable impurities. .. The base steel sheet contains a small amount of C, Si, Mn, P, S, Cr, Ni, Cu, Nb, V, Mo, W, B, Al, Ti, O, and N from the viewpoint of high strength. It may contain one kind or two or more kinds. Further, from the viewpoint of formability, a small amount of one or more of Ca, Ce, Mg, Zr, and Hf may be included as other elements. The content of these elements other than Fe can be appropriately adjusted according to the intended use, member, and the like. When used as an outer panel for automobiles, it is desirable to use an ultra-low carbon steel sheet having good press formability.

The steel sheet which is the base material of the present invention may be subjected to acid cleaning treatment, cold rolling, annealing treatment or the like depending on the case. Further, the plate thickness and the like of the steel plate serving as the base material of the present invention and the plated steel plate after the plating treatment are not particularly limited, and any size can be used.

From the viewpoint of corrosion resistance and cost, the coating weight of the alloyed hot-dip galvanized coating can be preferably 10 to 200 g/m ² in terms of the coating weight per one side of the steel sheet.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the scope of the present invention is not limited to the following Examples.

1. Formation of Plating Layer by Ag Addition A Fe—Zn alloyed hot dip galvanized coating was formed on the surface of the steel sheet using a zinc bath containing Ag, and the formation state of Γ phase and Γ1 phase was evaluated. As a comparative example, an alloyed hot-dip galvanized coating was similarly formed using a zinc bath containing no Ag and a zinc bath containing Al, and the formation conditions of the Γ phase and the Γ1 phase were evaluated.

The zinc bath used is as follows.
1) Ag-added zinc bath Ag: 6% by mass, Zn: 94% by mass
2) Al-added zinc bath Al: 0.5 mass%, Zn: 99.5 mass%
3) Zinc bath without addition Zn: 100% by mass
The heat treatment time was 2000 s, 5000 s, and 10000 s. Ar gas and Ar—H ₂ gas were used as the gas of the heat treatment atmosphere.

The element ratios and SEM images of the surfaces of the plating films obtained when the heat treatment time is 2000 s are shown in FIGS. In the case where Ag was not added (FIGS. 1 and 2), the formation of the Γ1 phase having a thickness of 2.0 μm or more was observed in each case. Similarly, when only Al was added (FIG. 3 ), the formation of the Γ1 phase having unevenness but the same thickness as when Ag was not added was observed. On the other hand, in the case of the Ag-added zinc bath (FIG. 4), formation of the Γ1 phase was not observed. Since Ag is solid-dissolved in the δ _1k phase, it is suggested that Ag suppresses the formation of Γ1 phase.

Similarly, it was confirmed that even when the heat treatment time was 5000 s and 10000 s, the formation of the Γ1 phase was significantly suppressed when the Ag-added zinc bath was used, as compared with the other cases.

FIG. 5 shows the formation and growth behaviors of the Γ phase and the Γ1 phase obtained from these results when Ag is added or not added. From these results, it was found that by adding Ag to the zinc bath, the formation of the Γ1 phase was delayed without delaying the formation of the Γ1 phase and the formation of the Γ1 phase was suppressed.

Note that, in addition to Ag, Ti, Ni, Mg, and Sn were each added to a zinc bath for plating formation, but in the case of metals other than Ag, the effect of suppressing the formation of the Γ1 phase was not confirmed.

2. Dependence of Ag addition amount 1. Similarly, as a result of confirming the formation of the Γ1 phase by using an Ag addition amount different from the above, it was confirmed that the growth behavior of the Γ1 phase was significantly different depending on the addition amount of Ag (Ag addition amount: 3% by mass).

Further, based on these results, the Fe-Zn-Ag ternary system phase diagram was obtained from the concentration at the heterophasic interface between the Γ1 phase and the polyphase. The obtained state diagram is shown in FIG. As shown in FIG. 6, since the composition width of the Γ1 phase becomes narrower as the Ag of the δ _1k phase becomes thicker, Ag makes the Γ1 phase unstable and/or forms the Γ1 phase. It is thought to suppress growth. The fact that the formation of the Γ1 phase was slightly observed at the heat treatment time of 2000 s when the amount of Ag added was small suggests that the concentration of Ag may not be sufficient for the formation of the Γ1 phase.

3. Examination of grain boundary strength of Γ phase
A strain of 8% or more was applied to a plating layer formed from a zinc bath with or without addition of Ag, and the fracture toughness value of the Γ phase was calculated from the crack spacing. As a result, in each case, the fracture toughness value was a value near 0.20 Mpa·m ^1/2 , and it was found that the grain boundary strength of the Γ phase was maintained even when Ag was added.

Claims (9)

An alloyed hot-dip galvanized coating mainly composed of an Fe-Zn alloy, wherein the plated coating contains Ag, and the Zn:Ag ratio in the plated layer is 10:1 to 30:1 by mass. An alloyed hot-dip galvanized coating characterized by a range. The galvannealed coating according to claim 1, wherein the thickness of the Γ1 phase in the plated coating is 1.0 μm or less. The galvannealed coating according to claim 2, wherein the thickness of the Γ phase in the plated coating is 1 to 10 µm. The mass content of Fe is 0.1 to 15.0%, Zn is 80.0 to 95.0%, and Ag is 4.0 to 8.0%. Alloyed hot-dip galvanized coating. The alloyed hot-dip galvanized coating film according to any one of claims 1 to 4, further comprising 0.05 to 0.5% by mass of Al. The galvannealed coating according to any one of claims 1 to 5, wherein the fracture toughness value of the Γ phase in the plated coating is 0.1 to 0.4 Mpa·m ^1/2 . A plated steel sheet having the galvannealed coating film according to any one of claims 1 to 6 on one side or both sides of the steel sheet. The plated steel sheet according to claim 7, wherein the coating weight per one surface of the steel sheet is 10 to 200 g/m ² . The plated steel sheet according to claim 7 or 8, wherein the steel sheet is a steel sheet containing 90 to 99% by mass of Fe.