JPH04297563A - Galvannealed steel sheet excellent in workability and its production - Google Patents

Abstract

PURPOSE:To manufacture a galvannealed steel sheet having excellent workability in powdering resistance, flaking resistance and drawability. CONSTITUTION:This is a galvannealed steel sheet in which a galvannealed layer provided at least on one side has a compsn. contg., by weight, 8 to 13% Fe, <0.5% Al, 0.02 to 2.0% Mn and the balance Zn with inevitable impurities, the thickness of a GAMMA phase on the boundary of ferrite is regulated to <=0.5mum, etaand xsi phases are not present on the surface of the coated layer and its coating weight is regulated to 45 to 90g/m<2>. For manufacturing the above galvannealed steel sheet, galvanizing in which the coating weight at least on one side is regulated to the above one is applied by using a galvanizing bath contg. <0.2% Al, 0.01 to 1.0% Mn and the balance Zn with inevitable impurities, and after that, heating is executed at 450 to 500 deg.C for 2 to 40sec and alloying treatment is executed.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet with excellent powdering resistance, flaking resistance and press formability, and excellent workability, and a method for producing the same.

0002

[Prior Art] Conventionally, in order to impart paintability, coating adhesion, weldability, and workability to hot-dip galvanized steel sheets in addition to their corrosion resistance, heat treatment has been applied after hot-dip galvanizing to steel sheets. Alloyed hot-dip galvanized steel sheets in which the plating layer is an iron-zinc alloy are manufactured and used in various fields such as automobiles, home appliances, and architecture.

[0003] When a steel sheet is heat-treated after being hot-dip galvanized in this way, as alloying progresses, mutual diffusion of iron and zinc causes the formation of ζ phase (FeZn13), δ1 phase (FeZn7), Γ phase ( Fe5Zn21) is sequentially generated.

As a result of conventional research on the workability and alloying layer structure of such alloyed hot-dip galvanized steel sheets, the following has been found. In other words, if the η phase or ζ phase exists on the surface of the alloyed plating layer, these phases are relatively soft, so the sliding resistance with the die during press forming becomes large, preventing the steel plate from sliding into the die, and causing the steel plate to deteriorate. This may lead to cutting or burning of the plating layer to the mold. On the other hand, when the Γ phase grows thickly, the Γ phase is hard and brittle, which causes the so-called powdering phenomenon in which the plating layer peels off during press forming.If this powdering phenomenon is significant, the corrosion resistance of the alloyed plating layer deteriorates. Not only will this decrease, but it will also have an adverse effect on press workability. Furthermore, even when the Γ phase is relatively thin, ζ is present on the surface of the plating layer.
If such a phase is present, the soft ζ phase will cause galling during press working, and this shear stress will cause the hard and brittle Γ phase to peel off into flakes, resulting in a so-called flaking phenomenon. Ideally, therefore, it is desirable to have a uniform δ1 phase from the steel plate interface to the surface of the alloyed plating layer, but this is virtually impossible as long as alloying is performed by thermal diffusion treatment.

[0005]

[Problem to be solved by the invention] Conventionally, such δ1
As a technique for obtaining an alloyed plating layer that is close to a single phase, there is a method disclosed in, for example, Japanese Patent Application Laid-Open No. 64-68456, etc., but with this method, alloying is performed so that the η phase and ζ phase do not remain. In order to do this, the effective amount of Al in the plating bath should be 0.1
0% or less, and the steel plate must be immersed in the plating bath at a very high speed of 3 seconds or less, preferably 2 seconds or less, and according to research by the present inventors A heating temperature higher than that is required. However, at this temperature, the growth of the Γ phase is rapid and the Γ phase cannot be suppressed sufficiently;
For example, when the coating amount is 0 g/m2 or more, the Γ phase is 0.5 to 3.
When the particle diameter becomes 0 μm, the powdering resistance becomes insufficient.

On the other hand, in order to suppress the Γ phase, it is desirable to perform alloying at a lower temperature. For example, when alloying at a temperature below 500°C, the growth of the Γ phase is sufficiently suppressed, but the ζ phase tends to remain and it is difficult to press. Moldability and flaking resistance decrease. In this way, the temperature ranges in which the ζ phase and Γ phase can be suppressed are contradictory, so only one of them can be suppressed, and in order to produce an alloyed hot-dip galvanized steel sheet that suppresses both, it is necessary to measure the degree of alloying. However, it needs to be strictly managed. but,
In the current methods, there is no means for preventing the presence of the ζ phase on the surface of the alloyed plating layer and suppressing the Γ phase to a sufficient level for the powling resistance.

[0007] The present invention solves the problems of the prior art, and eliminates not only the η phase but also the ζ phase on the surface of the alloyed plating layer, and suppresses the Γ phase to a sufficient level of powdering resistance. An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet with excellent workability and a method for manufacturing the same.

[0008]

[Means for Solving the Problems] As a result of intensive research to solve the above problems, the present inventors have found that by adding Mn to the plating bath, the ζ phase can be achieved even at a temperature of 450 to 500°C, which can sufficiently suppress the Γ phase. It was found that the thickness of the Γ phase at the base metal interface is 0.5 μm or less, and when the surface of the alloyed plating layer is measured by X-ray diffraction, there are no η and ζ phases, and They succeeded in developing an alloyed hot-dip galvanized steel sheet with excellent workability, which has an alloyed plating layer with a coating weight of 45 to 90 g/m2 on at least one side, and a method for manufacturing the same.

That is, the alloyed hot-dip galvanized steel sheet with excellent workability according to the present invention contains Fe: 8 to 13% by weight, Al:
less than 0.5% by weight, Mn: 0.02 to 2.0% by weight, with the remainder consisting of Zn and unavoidable impurities, and the thickness of the Γ phase at the interface of the base metal is 0.5 μm or less. There is no η or ζ phase on the surface of the plating layer, and the amount of adhesion is 45 to 90.
It is characterized by having an alloyed plating layer of g/m2 on at least one side. In addition, the method for producing an alloyed hot-dip galvanized steel sheet with excellent workability according to the present invention contains Al: less than 0.2% by weight, Mn: 0.01 to 1.0% by weight, and the remainder is Zn and unavoidable The amount of coating on at least one side is 45 to 90 g/m2 using a hot dip galvanizing bath consisting of impurities.
After hot-dip galvanizing, heat at 450-500℃ for 2-40 seconds to reduce the thickness of the Γ phase at the interface of the base metal to 0.5μ.
m or less and alloying treatment is performed so that η and ζ phases are not present on the surface of the plating layer.

0010

[Function] The reasons for limiting the composition of the hot-dip galvanizing bath used in the present invention, the alloying treatment temperature, and the composition of the plating layer will be explained below. Al: Al is Fe- at the steel plate-plating layer interface.
Zn-F in the plating layer is formed by forming a layer of Al intermetallic compound.
e Effectively suppresses mutual diffusion and lowers the viscosity of the hot-dip galvanizing bath. However, when the Al concentration in the hot-dip galvanizing bath becomes 0.2% by weight or more, Zn
Since the -Fe interdiffusion is extremely suppressed, the alloying reaction is significantly retarded, making alloying in an in-line alloying furnace virtually impossible at low alloying temperatures as in the method of the invention. Therefore, in the method of the present invention, the amount of Al added to the hot-dip galvanizing bath is less than 0.2% by weight.

Next, the composition range of Al in the alloyed plating layer will be described. Generally, when a steel sheet is hot-dip galvanized using a hot-dip galvanizing bath containing Al, a layer of Fe-Al intermetallic compound preferentially precipitates at the steel sheet-plating layer interface, compared to the Al concentration of the hot-dip galvanizing bath. It is known that the Al concentration in the plating layer tends to increase. Exactly the same tendency is observed in the hot-dip galvanizing treatment in the present invention. When hot-dip galvanizing is performed using a hot-dip galvanizing bath having the above-mentioned Al concentration, the Al concentration of the plating layer produced is less than 0.5% by weight. Therefore, the Al concentration in the alloyed plating layer in the alloyed hot-dip galvanized steel sheet according to the present invention was set to be less than 0.5% by weight.

Mn: Mn suppresses the formation of ζ phase and
Added to perform alloying treatment at a low temperature of 0°C or lower. If the amount of Mn added to the hot-dip galvanizing bath is less than 0.01% by weight, the effect of suppressing the formation of the ζ phase will not be sufficient;
When alloyed at temperatures below ℃, the ζ phase tends to remain, and 1.
If more than 0% by weight is added, the plating bath temperature must be raised, making it virtually impossible to suppress the Γ phase to 0.5 μm or less. Therefore, in the method of the present invention, the amount of Mn added to the hot dip galvanizing bath is 0.01
It was limited to ~1.0% by weight.

Next, the composition range of Mn in the alloyed plating layer will be described. Mn tends to precipitate preferentially in the plating layer and become higher in concentration than in the plating bath. When hot-dip galvanizing is performed using a hot-dip galvanizing bath with the above-mentioned amount of Mn added, the Mn concentration of the plating layer produced is 0.
02 to 2.0% by weight. Therefore, the Mn concentration in the alloyed plating layer in the alloyed hot-dip galvanized steel sheet of the present invention is 0.02 to 2.0% by weight.

Fe: In the coating layer structure of the alloyed hot-dip galvanized steel sheet with excellent workability according to the present invention, there are no η and ζ layers on the surface of the coating layer, and the thickness of the Γ layer at the base metal interface is 0. ．．
5 μm or less, and the Fe concentration in the alloyed plating layer with such a configuration is 8 to 13% by weight, so it is 8 to 13% by weight.
And so.

[0015] Alloying treatment temperature and heating time: When the plating layer of a hot-dip galvanized steel sheet is alloyed by heating above 500°C, a Γ layer is likely to be formed, and the Γ layer at the interface of the base metal is 0.5 μm thick.
It is not preferable because it exceeds. On the other hand, when alloyed at a temperature below 450°C, the effect of suppressing the formation of the ζ layer becomes weaker at the Mn concentration of the above composition, and the ζ layer tends to remain on the surface of the plating layer.
The plating layer structure of the alloyed hot-dip galvanized steel sheet according to the present invention is characterized by the formation of an alloy layer in which η phase and ζ phase do not exist on the surface of the plating layer and the Γ phase at the base metal interface is 0.5 μm or less. I can't. Therefore, the alloying temperature in the method of the present invention was set at 450 to 500°C. However, this 45
Even at an alloying temperature of 0 to 500°C, if the heating time is short, the η phase and ζ phase will remain on the surface of the plating layer, and if the heating time is too long, the Γ phase at the base metal interface will be 0.5 μm or more thick. As a result of various studies, the inventors found that 2 to 40
It is necessary to heat the substrate in such a way that the thickness of the Γ phase at the substrate interface is 0.5 μm or less and the η and ζ phases are not present on the surface of the plating layer.

[0016] Thickness of alloyed hot-dip galvanized layer: In the alloyed hot-dip galvanized steel sheet according to the present invention, the applicable coating weight is 45 to 90 g/m2. 45g
/m2, it is possible to produce an alloyed hot-dip galvanized steel sheet that satisfies powdering resistance, flaking resistance, and press formability using conventional techniques, and the steel sheet of the present invention is not particularly advantageous. Moreover, if it exceeds 90 g/m2, the flaking resistance and press formability will be satisfactory, but the powdering resistance will decrease, so the applicable coating weight of the alloyed hot-dip galvanized steel sheet according to the present invention is set at 45 to 90 g/m2.
And so. However, if flaking resistance and press formability are satisfied, it can be applied up to 45 to 150 g/m2. When the coating weight exceeds 150 g/m2, it becomes practically impossible to produce a coating layer in which the Γ phase at the substrate interface is 0.5 μm or less and the η and ζ phases are not present on the plated surface. Other compositions of the alloyed plating layer: Although only Fe, Al, and Mn are specified as the composition of the alloyed plating layer, the effects of the present invention will not change even if small amounts of other components such as Pb and Sb are added. be.

[0017]

EXAMPLES Next, examples of alloyed hot-dip galvanized steel sheets with excellent workability according to the present invention will be described in more detail together with comparative examples. By varying the amounts of Al and Mn introduced into the plating bath of a continuous hot-dip galvanizing line using a Sendzimer-type non-oxidizing furnace, low-carbon cold-rolled steel sheets with a thickness of 0.7 mm and a width of 1,000 mm were used as base plates for plating. , various hot-dip galvanized steel sheets having a coating weight within the range of 45 to 90 g/m2, which is the range of the coating weight of the alloyed hot-dip galvanized layer in the present invention, were then alloyed. Various alloyed hot-dip galvanized steel sheets were heated and alloyed for various times in a processing furnace to have different Fe concentrations in the alloy layers, and were tested using the method for testing the workability of the plating layer described below. The results are shown in Tables 1 and 2.

(1) Presence or absence of η, ζ phase The measurement was made by X-ray diffraction, and judgment was made based on whether or not a peak value indicating the presence of η, ζ phase appeared.

(2) Powdering resistance test: With the test surface facing inside, bend the test piece 180 degrees so that the test surface has a circular arc portion with a diameter six times the thickness t of the test piece, as shown in Figure 1. After bending back, apply cellophane adhesive tape to the test surface, then peel off the cellophane adhesive tape and visually check the amount of powdered plating metal that adheres to the cellophane adhesive tape, judging according to the following criteria. did. 5: No adhering plating metal 4: Small amount of adhering plating metal 3: Medium amount of adhering plating metal 2: Large amount of adhering plating metal 1: A large amount of powdery plated metal peeling off without tape Based on this standard, a rating of 5 to 3 is a practical problem There is no range.

(3) Press formability test: Evaluation was made by the outer diameter ratio in a cup drawing test under the conditions shown in FIG. 2 using the same rust preventive oil. Diameter of disk before test piece drawing (D0): 75mm Thickness of steel plate used for drawing: tmm Diameter of punch used for mold drawing (d): 40mm Punch tip radius used for drawing: 5mm Drawing Die shoulder radius used for forming: 5tmm Wrinkle holding force during drawing:
Condition after 1,000 kgf test Depth of drawing by drawing: 20 mm Diameter of flange after drawing: D1 mm Outer diameter ratio: D1/D0 This outer diameter ratio of 0.734 to 0.743 is at the level of cold rolled steel plate. .

(4) Flaking resistance test: As shown in Fig. 3, a sample S of 300 mm width x 260 mm length was wrinkled using a die 1 having a through hole with an inner diameter of 42 mm and a wrinkle presser 2 with a bead of 3 mm height. The specimen was clamped with a pressing force of 500 kgf and drawn to a molding height of 50 mm using a punch 3 with a diameter of 40 mm, and the peeling state of the plated metal was visually observed and evaluated according to the following criteria. 4: No peeling 3: Small amount of peeling 2: Medium amount of peeling 1: Large amount of peeling Based on this standard, an evaluation of 4 to 3 is within a range that causes no practical problems.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

Effects of the Invention As detailed above, the alloyed hot-dip galvanized steel sheet with excellent workability according to the present invention has a coating weight of 45 to 9.
Since it has excellent workability even when coated as thick as 0 g/m2, the use of alloyed hot-dip galvanized steel sheets can be expected to expand, and the method for producing alloyed hot-dip galvanized steel sheets with excellent workability according to the present invention This is an innovative method that can industrially stably and continuously manufacture alloyed hot-dip galvanized steel sheets with excellent properties without the need for high-speed threading as in the conventional technology described above. It has great industrial value.

[Brief explanation of drawings]

FIG. 1 is a diagram illustrating a powdering resistance test method.

FIG. 2 is a diagram illustrating a press formability test method.

FIG. 3 is a diagram illustrating a test method for flaking resistance.

[Explanation of symbols]

1 Dice 2 Wrinkle presser 3 Punch S sample

Claims (2)

[Claims] [Claim 1] Fe: 8 to 13% by weight, Al: 0.5
less than % by weight, Mn: 0.02 to 2.0% by weight, with the remainder consisting of Zn and unavoidable impurities, and the thickness of the Γ phase at the base metal interface is 0.5 μm or less, and the plating layer No η and ζ phases exist on the surface and the amount of adhesion is 45 to 90 g/m
1. An alloyed hot-dip galvanized steel sheet with excellent workability, characterized in that it has an alloyed plating layer of No. 2 on at least one side. Claim 2: Al: less than 0.2% by weight, Mn: 0.
After performing hot-dip galvanizing with a coating weight of 45-90 g/m2 on at least one side using a hot-dip galvanizing bath containing 01-1.0% by weight and the remainder consisting of Zn and unavoidable impurities, Processing characterized by alloying treatment by heating at 500°C for 2 to 40 seconds so that the thickness of the Γ phase at the substrate interface is 0.5 μm or less and the η and ζ phases are not present on the surface of the plating layer. A method for manufacturing alloyed hot-dip galvanized steel sheets with excellent properties.