JPH0355544B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a thick alloyed hot-dip galvanized steel sheet with excellent powdering resistance and flaking resistance. (Prior art) Alloyed hot-dip galvanized steel sheets are produced by heating a hot-dip galvanized steel sheet after plating to diffuse iron in the base steel sheet into the plating layer, forming an iron-zinc alloy. Because it has superior paint corrosion resistance compared to galvanized steel sheets, it is widely used as a material for automobiles, building materials, home appliances, etc. (Problems to be Solved by the Invention) In recent years, demands for corrosion resistance have become stronger and stronger, and thick alloyed hot-dip galvanized steel sheets have been desired. However, since alloyed hot-dip galvanized steel sheets are manufactured by thermal diffusion treatment as mentioned above, as the basis weight increases, the iron concentration gradient in the galvanized layer increases, and the Fe concentration at the interface with the base steel increases. Highly concentrated and brittle Γ
phase is formed and the layer becomes thicker, while the Fe concentration near the surface of the plating layer decreases, leaving the η phase (pure Zn phase) with incomplete alloying, or an alloy phase with a low Fe concentration, i.e. The ζ phase tends to be thick. When the Γ phase becomes thicker, the plating layer tends to peel off during press processing, causing so-called powdering, which causes eaves to appear in the product, resulting in lower yields and lower efficiency due to increased frequency of mold cleaning. Get out. On the other hand, if the η phase or ζ phase remains on the surface of the plating layer, these phases are relatively soft and are likely to cause mold galling during press working, becoming so-called flaking that accumulates on the mold bead, etc. This causes a decrease in yield and efficiency in the pressing process. Therefore, ideally from the substrate interface to the surface of the plating layer.
It is desirable to have a uniform δ ₁ phase with no Fe concentration gradient, but this is virtually impossible as long as alloying is performed by thermal diffusion treatment. At low area weights, alloyed hot-dip galvanized steel sheets mainly consisting of the δ ₁ phase, which is close to the ideal type, have been manufactured and put into practical use, but at thicker area weights of 45 g/m2 or ^more , powder resistance is required in the pressing process. There is no existing alloyed hot-dip galvanized steel sheet that has satisfactory ring properties and flaking resistance, and there is a strong need for its development. The present invention provides an alloyed hot-dip galvanized steel sheet that advantageously solves the above-mentioned problems. (Means for solving the problems) The first invention for solving the above problems is as follows:
The composition is Fe: 8-12%, Al: 0.05-0.25%, and the balance is Zn, and the Γ phase at the base metal interface is 1.0 μm.
The following is the basis weight where ηζ phase does not exist on the surface of the plating layer.
This is an alloyed hot-dip galvanized steel sheet with excellent powdering resistance and flaking resistance, having a plating layer of 45 to 90 g/m ² on at least one side. Further, the second invention has Fe: 8 to 12%, Al:
Alloyed hot-dip galvanized metal having a composition of 0.05 to 0.25% Zn with the balance being Zn, a Γ phase at the base metal interface of 1.0 μm or less, and an area weight of 45 to 90 g/m ² with no η phase present on the surface of the plating layer. On top of the layer, Fe: 60% or more, balance Zn
This is an alloyed hot-dip galvanized steel sheet with excellent powdering resistance and flaking resistance, which has a two-layer plating layer on at least one side, which is an alloy plating layer formed from the following. (Function) The features of the present invention will be described by comparing it with conventional techniques. Conventionally, the effective amount of Al in the molten zinc bath (Al% - Fe%)
For example, the steel strip is plated by passing it through a bath containing 0.09 to 0.15%, and after adjusting the basis weight by gas wiping etc., the plate is passed through an alloying furnace until the metallic luster on the plated surface disappears, that is, the surface In manufacturing, the alloy is heat-treated until alloying is completed, and then immediately cooled to control the degree of alloying. (Unexamined Japanese Patent Publication No. 61-223174) The composition of this plating layer is Fe: 8 to 13%,
Al: 0.25-0.35%, balance Zn. However, when hot-dip galvanized steel sheets with a basis weight of 45 g/m2 or ^more are alloyed in this process, the thickness of the Γ phase formed at the interface between the base metals is, for example, about 1 to 37 μm, and the powdering resistance is insufficient. . Therefore, the effective amount of Al in the bath was reduced to about 0.10% or less, and the Fe-Al alloy layer formed in the bath was made thinner.
By making the formation of the Fe--Zn alloy phase relatively easy, alloyed hot-dip galvanized steel sheets can be produced with lower temperature heat treatment. The composition of the plating layer is Fe: 6 to 11%, Al: 0.05 to 0.25%, and the balance is Zn. However, the basis weight is 45
g/m ² or more, there is a condition that the thickness of the Γ phase is 1 μm or less, but there are also η phases and η phases on the surface of the plating layer.
The ζ phase tends to remain, and flaking resistance is not sufficient. The above two examples show that it is not possible to satisfy both powdering resistance and flaking resistance only by controlling the effective amount of Al in the bath. However, according to studies conducted by the present inventors, not only the thickness but also the quality of the Fe-Al alloy phase is important.
That is, in the latter example, the Fe-Al alloy phase is thin and has insufficient performance as a barrier for Fe diffusion, and the Fe-Al film is already partially destroyed before the alloying heat treatment.
The formation of Fe-Zn alloy metals such as the ζ phase has begun. In the present invention, by setting the steel strip immersion time in the bath to 3 seconds, preferably 2 seconds or less, the Fe-Al alloy phase is guided to the alloying furnace in a healthy state, and the η
phase, ζ phase does not exist, and the Γ layer thickness at the base metal interface is
We have discovered that it is possible to produce an alloyed hot-dip galvanized steel sheet with a small Fe concentration gradient of 1 μm or less, and succeeded in achieving good powdering resistance and flaking resistance. The condition of immersing the steel strip in the bath for 3 seconds, preferably within 2 seconds, is extremely difficult to achieve with the general continuous hot-dip galvanizing equipment currently in operation. In other words, the dipping roll diameter needs to be at least 600 mm to prevent the steel strip from bending, and if the press formability of the steel strip is taken into consideration, it is 800 mm.
It is preferable to design as above. Therefore, the steel strip path length in the bath is 3 m or more, preferably 4 m or more, which cannot be achieved with the steel strip speed of 60 m/min, which is the normal operating condition. It would be better to further increase the steel strip speed, but
It requires a huge investment to extend the vertical alloying furnace,
Alternatively, an immersion time of 3 seconds, preferably 2 seconds or less can be achieved by passing the molten zinc bath through a special short path, but in any case, it is necessary to use a new process that has not existed before. The present invention has an effective Al content in the bath of 0.10% or less and a dipping time of 3.
By setting the heating time to 2 seconds, preferably 2 seconds or less, Fe: 8 to 12% and Al: 0.05 to 0.25% can be achieved in the plating layer. If Fe is less than 8%, the ζ phase tends to remain on the surface of the plating layer, and in extreme cases, the η phase also remains.
If Fe exceeds 12%, the Γ phase tends to exceed 1 μm, which is not preferable. If Al: less than 0.05%, the ζ phase tends to remain on the surface of the plating layer, and on the other hand, if alloying is performed at a high temperature to avoid the ζ phase remaining, the Γ phase on the surface of the base metal will exceed 1 μm, which is not preferable. When Al exceeds 0.25%, the Γ phase becomes 1μ when heat treated until the alloying is completed to the surface.
It is not preferable because it exceeds m. The plating composition of the present invention is characterized by a large Fe/Al ratio when compared with the plating composition of conventional alloyed hot-dip galvanized steel sheets produced with an immersion time of 3 seconds or more. It is preferable that the Γ phase has a thickness of 1 μm or less in order to improve powdering resistance. If it exceeds 1 μm and the basis weight is 45 g/m ² or more, the powdering resistance will deteriorate, causing actual damage to the press. Although there are various methods for measuring the Γ phase, the following method was adopted here. That is, the cross section of the sample was analyzed along the direction perpendicular to the steel plate surface using _an Identified. Measurement accuracy can be improved by using the inclined polishing method. The presence or absence of the η phase and ζ phase is most appropriately determined by potential measurement in relation to sensitivity and pressability. The following method was adopted here. That is,
Constant current electrolysis is carried out using the sample as an anode at a current density of 20 mA/cm ² in an electrolytic solution containing 100 g of ZnSO ₄ .7H ₂ O and 200 g of NaCl, and the potential is measured using Ag as a reference electrode. At this time, if the potential of the plating surface layer is less than -750 mV with respect to the Ag electrode, it is determined that the η phase exists, and if it is between -700 mV and -750 mV, it is determined that the ζ phase exists. (The potential of the bare iron is around -270 mV) If it is -670 mV or higher, it is determined that the ζ phase does not substantially exist. The reason for applying current is to remove the influence of oxide films and the like. 2 described later
In the case of layer plating, the potential of the Fe-rich upper layer is indicated immediately after the current is applied, but the potential of the lower layer is immediately indicated, so slight cracks may occur in the upper layer, which is judged based on the potential at this point. For example, even if the upper layer is present, the potential of the lower layer, which is electrically less noble, is measured. Even if it is determined by potential measurement that no ζ phase exists and the surface is δ ₁ phase, a small amount of ζ phase or η phase may be detected by X-ray diffraction.
However, since the flaking resistance is related to the mechanical properties of the surface layer of the plating layer, the macroscopic potential measurement method better matches the actual press. If the ζ phase measured in this way exists, the soft ζ phase will "gall" when sliding on the bead of the mold, for example when molding a car fender.
This is undesirable because it is removed, deposits on the bead, and aggregates, causing eaves. If the surface of the plating layer is substantially in the _δ1 phase, flaking as described above will not occur. Another aspect of the present invention is Fe: 60% or more,
2 with an alloy plating layer made of Zn on the top layer
It is a layer-plated alloyed hot-dip galvanized steel sheet. The upper plating layer is an extremely hard layer with a Vickers hardness of about 400, and its presence can suppress adhesion to the mold and improve flaking resistance. The thickness of the upper plating layer is preferably 1 to 10 g/m ² . 1g/
If it is less than m ² , it is difficult to completely cover the lower plating layer, and adhesion from the exposed portion of the lower layer to the mold may occur, which is not preferable. If it exceeds 10 g/m ² , corrosion resistance tends to deteriorate, which is not preferable. In addition,
If the Fe content of the upper layer is less than 60%, the plating layer becomes brittle, which is not preferable. If it is 60% or more, the film will be hard and have good powdering properties. As an example of upper layer plating, electroplating has good throwing power, so it can be easily coated on an alloyed hot-dip galvanized layer with severe fine irregularities. In addition to Fe and Zn, the upper plating layer contains Ni, Co, Cr, Mn,
Even if a small amount of Al, Si, Zr, Cu, Mo, Ti, P, C, O, S, etc. is contained, the effect on press forming does not substantially change. In the case of two-layer plating, the presence of the ζ phase in the lower layer is substantially permissible. That is, the presence of the upper layer can suppress adhesion to the mold. However, the presence of the η phase is not preferred. If anything,
If an extremely soft η phase (Vitkers hardness of about 30 to 50) exists directly under the upper layer, the η phase will be destroyed when the mold bead slides, and the η phase of the peeled pieces will become a binder and cause secondary agglomeration. It is from. The thickness of the alloyed melt-plated layer of the present invention is within the applicable range of 45 to 90 g/m ² in terms of basis weight. 45
If less than g/m ² , powdering resistance cannot be achieved using conventional technology.
It is possible to produce an alloyed hot-dip galvanized steel sheet that has satisfactory flaking resistance, and the present invention is not particularly advantageous. If it exceeds 90 g/m ² , it is virtually impossible to produce a plated layer in which the Γ phase is 1 μm or less and the η phase or ζ phase is not present in the plating surface layer. Although only Fe and Al are specified as the composition of the alloyed melt-plated layer, other components such as Pb, Cd, Sn,
In, Li, Sb, As, Bi, Mg, La, Ce, Ti, Zr,
Even if small amounts of Ni, Co, Cr, Mn, P, S, O, etc. are added or unavoidably mixed, the effects of the present invention essentially remain unchanged. In addition, the plating layer of the present invention is preferably applied to both sides in the case of a rust-preventing steel plate with a basis weight of 45 to 90 g/m ² on both sides, but it is preferable to apply the plating layer on both sides with a basis weight of 45 to 90 g/m ² on one side and on the other side ^. In the case of a differentially thickened steel plate with a surface of less than 45 g/m ² , it can also be applied only to the thickened surface. In the case of a single-sided plated steel plate, this applies only to the plated side. (Example) Next, examples of the present invention will be described together with comparative examples. CC-AI-K steel (0.7 ^t ×
Immediately after plating in a non-oxidation furnace type continuous hot-dip galvanizing line, the alloy was continuously heated and alloyed using an alloying furnace using a 1200 ^W coil). The Al composition in the plating layer was determined by the Al concentration in the plating bath, and the Fe concentration in the plating layer was determined by appropriately selecting the heating conditions of the alloying furnace. The plate threading speed is 40 to 70 m/min, and the immersion time is 1 to 70 m/min.
Plating was performed for 5 seconds. However, in all Examples, the time is in the range of 1 to 3 seconds. On the other hand, on the rear exit side of the line, upper layer plating was performed using the electroplating method. A normal sulfuric acid bath was used, and the upper layer plating composition and area weight were controlled by the Zn/Fe ion ratio and current density in the plating bath. In addition, when upper layer plating was not applied, the upper layer plating tank was filled with water, passed through the plate, and then dried with hot air. Next, the method for testing the workability of the plated layer will be described. (1) Powdering resistance test Before processing, apply vinyl tape to the bent part.
A bending combination (two-way bending) was performed with the tape surface facing inside, the tape was opened again, and the tape was peeled off.The plating layer was judged based on the degree of blackening caused by adhesion to the tape. (Good) ◎−○−△−× (Poor) (◎, ○ indicates no practical problem) (2) Flaking resistance test Evaluation was performed by tension forming with a square bead as shown in FIG. The weight between punch 1 and die 2 is 2.0Kg.
f/cm ² (plug size 0.7 x 75 x 280 mm) and test piece 3, and then pull test piece 3 to pass through the bead portion. 200 sheets were repeatedly formed, and the degree of deposition of the plating layer metal on the steel plate or bead portion was evaluated relative to each other. (Good) ◎−○−△−× (Poor) (◎, ○ indicates no practical problem) (3) Actual press test A fender part for an ordinary passenger car was molded using an actual press. 300 sheets were repeatedly molded, and the degree of adhesion and accumulation of the plating layer metal on the steel plate or press mold was evaluated relative to the degree of adhesion. Evaluation was made by attaching a tape to each part, peeling it off, and determining the degree of blackening of the metal powder transferred to the tape. (Good) ◎−○−△−× (Poor) (◎, ○ indicates no practical problem)
Show it on the table.

[Table] (Effects of the invention) As explained above, the galvanized steel sheet of the present invention improves both workability and flaking property, expands the uses of hot-dip galvanized steel sheet, and has great industrial effects. .

[Brief explanation of drawings]

FIG. 1 is an explanatory cross-sectional view showing a flaking resistance test. 1...Punch, 2...Dice, 3...Test piece.

Claims (1)

[Claims] 1 Fe: 8-12%, Al: 0.05-0.25%, balance Zn
and the Γ phase at the base metal interface is
Alloyed hot-dip galvanizing with excellent powdering resistance and flaking resistance, having a plating layer on at least one side with a coating weight of 45 to 90 g/ ^m2 with a thickness of 1.0 μm or less and no ηζ phase present on the surface of the plating layer. steel plate. 2 Fe: 8-12%, Al: 0.05-0.25%, balance Zn
and the Γ phase at the base metal interface is
An alloyed galvanized layer consisting of Fe: 60% or more and the balance Zn is created on an alloyed hot-dip galvanized layer with a coating weight of 45 to 90 g/m ² with a thickness of 1.0 μm or less and no η phase on the surface of the plated layer. An alloyed hot-dip galvanized steel sheet having excellent powdering resistance and flaking resistance and having a two-layered galvanized layer on at least one side.