JP3223839B2 - Galvannealed steel sheet - Google Patents

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 used in automobiles, home appliances, building materials and the like, particularly after press forming and roll forming, and more particularly to an alloyed hot-dip galvanized steel sheet having improved powdering resistance during forming. It relates to a galvanized film.

[0002]

2. Description of the Related Art Conventionally, several techniques described below have been disclosed for the coating structure of an alloyed hot-dip galvanized steel sheet, but few of them are actually applied due to their industrial difficulty. .

[0003] Japanese Patent Application Laid-Open No. 1-68456 (hereinafter referred to as "prior art 1") discloses a technique relating to an alloyed hot-dip galvanized steel sheet having excellent powdering resistance and flaking resistance. There is disclosed an alloyed hot-dip galvanized film in which the amount and the Fe and Al concentrations are specified, the Γ phase at the base iron interface is 1.0 μm or less, and the η phase and ζ phase do not exist on the surface of the plating layer.

Japanese Patent Application Laid-Open No. 3-249162 (hereinafter referred to as Prior Art 2) discloses a technique relating to an alloyed hot-dip galvanized steel sheet having both sliding properties, surface treatment properties, and powdering resistance. , Consisting of the δ1 phase alone or the δ1 phase and the Γ1 phase having a thickness of 1 μm or less on the surface of the cold-rolled steel sheet,
Further, an alloyed hot-dip galvanized film having a bulk crystal Fe—Zn alloy film having an average aspect ratio of the surface layer crystal of 3 or less is disclosed.

Japanese Patent Laid-Open No. 4-83859 (hereinafter referred to as Prior Art 3) discloses a technique relating to an alloyed hot-dip galvanized steel sheet having excellent low-temperature impact adhesion after coating. It has a three-layer structure of ζ phase, δ1 phase, and ζ phase, the alloy layer amount CW, Γ phase thickness tg (μm) in the alloy layer.
Is set in a range represented by CW> 46 · tg + 12, which discloses an alloyed hot-dip galvanized film.

Japanese Patent Application Laid-Open No. Hei 6-17221 (hereinafter referred to as Prior Art 4) discloses a technique relating to an alloyed hot-dip galvanized steel sheet having excellent adhesive bonding properties. And the ratio of the total thickness of the {1} and {2} phases to the total thickness of the plating layer is 0.10 or more, and the thickness of the {1} phase relative to the {1} phase An alloyed hot-dip galvanized coating having a ratio of 0.05 to 0.50 is disclosed.

[0007] Japanese Patent Application Laid-Open No. 7-34213 (hereinafter referred to as Prior Art 5) discloses that the steel sheet surface has an Fe concentration of 7-1.
5% is an alloyed hot-dip galvanized film in which Fe is enriched at the interface. A convex portion having a maximum diameter of 10 μm or less and a height of 0.1 to 2 μm is formed on the surface of the steel sheet. 5
alloying melt having excellent interfacial adhesion, characterized by the presence of a Γ phase of 0.5 μm or less, a Γ1 phase of 0.5 to 3.5 μm, and a coating of δ1 phase or δ1 + ζ phase thereon. A galvanized steel sheet is disclosed.

[0008]

In the manufacturing process of an alloyed hot-dip galvanized steel sheet, an annealed and surface-conditioned (generally hydrogen-reduced) steel sheet is plated by immersing it in a zinc bath containing a small amount of Al. After the adjustment of the adhesion amount, an alloying reaction occurs between Zn and Fe in the alloying furnace, and the film is formed by Zn-F
An e-type intermetallic compound. The Zn—Fe-based intermetallic compound has four types of phases called a Γ phase, a 相 1 phase, a δ1 phase, and a ζ phase, and the state of presence of these phases greatly affects quality. In particular, the thicker the Γ phase and the Γ1 phase, the more easily the film is peeled off. Therefore, a film structure in which the thickness is reduced has been proposed as described above.

The prior art 1 relates to a δ1 + Γ structure, and has a feature in a film structure in that the η phase and the ζ phase do not exist and the Γ phase is limited to 1.0 μm or less. However, as described in the text of the specification,
In order to manufacture the above film, the immersion time in the plating bath is 3
It is necessary to reduce the effective Al content in the bath to 0.1% or less in seconds, preferably 2 seconds or less, and it is difficult to produce industrially unless a new process is used.

The prior art 2 relates to a coating structure composed of the δ1 phase alone or the δ1 phase and the Γ1 phase having a thickness of 1 μm or less. If the penetration temperature of the steel sheet into the bath and the alloying conditions are properly selected, the conventional technique 2 can be used. It can be manufactured by a manufacturing process. However, a Γ phase may be generated depending on the type of steel or the surface condition of the steel sheet, and strict control of production conditions is required to obtain the above-mentioned film structure in an industrially stable manner.

The prior art 3 is intended to improve low-temperature impact adhesion after coating, and in a three-layer structure of Γ phase, δ1 phase, and ζ phase, the thickness of the Γ phase is determined by the total alloy layer thickness. The feature is that the value is set to be less than the value. That is, it discloses that the thickness of the plating layer is increased in order to absorb the impact energy by the film, and the thickness of the Γ phase, which is a starting point of a crack under the impact, is reduced. However, increasing the thickness of the plating layer leads to an increase in the amount of peeling when forming a steel sheet, and is not preferable from the viewpoint of powdering resistance (SAE Technical Paper Series 890349 (1989) [The Pe
eling-Off Resistance of Zinc and Iron-Zinc Coated
Steel Sheets in Stamping]).

The prior art 4 is proposed for improving the adhesive bonding property, in which a Γ phase exists at the steel sheet interface and a Γ1 phase exists on the surface, and the total thickness of the Γ1 phase and the Γ phase is more than a predetermined value. Further, an alloyed hot-dip galvanized film in which the thickness of the (1) phase is 2 to 20 times the thickness of the (1) phase is shown. That is,
The inventors found that when the Γ1 and Γ phases coexist,
It was found that the shear tensile strength required for adhesive bonding was improved to act as a buffer phase, and the above-mentioned technology was completed based on the finding.
(2) Both phases must be equal to or more than a predetermined amount, which is not preferable for powdering resistance as described above. Furthermore, in order to achieve this film structure, it is necessary to perform pretreatment such as Fe-C electroplating or carburizing on the steel sheet in advance, and there is a problem that the manufacturing cost increases.

Prior art 5 is proposed to improve interfacial adhesion. For this purpose, a specific convex portion is formed on the surface of a steel sheet, and a Γ phase of 1.5 μm or less on the surface, It forms a film on which a 3.5 μm Γ1, δ1 or δ1 + ζ phase is formed, and this does not improve the powdering resistance. It is described therein that when the growth of the Γ phase is suppressed and the Γ1 phase thicker than the Γ phase is formed thereon, the powdering resistance is improved.

The prior art is summarized as follows. (1) For powdering resistance, a δ1 single phase structure is preferable, but when a Γ phase or a Γ1 phase is present, it is better that they are thin. (2) In order to improve low-temperature impact adhesion, it is necessary to reduce the thickness of the Δ phase and increase the thickness of the plating film. However, the thickening of the coating deteriorates the powdering resistance. (3) To improve adhesive bonding and interfacial adhesion, use # 1
And Γ must coexist positively, and Γ1 must be thicker than Γ.

As described above, the methods of the prior arts 1 and 2 exist to improve the powdering resistance. However, these methods are difficult to manufacture by the conventional process, or the optimum manufacturing conditions vary depending on the type of steel or the surface condition of the steel sheet. Therefore, it is difficult to stably produce it industrially.
The film structure of No. 5 is not necessarily optimized for powdering resistance, and thus does not always satisfy the demands of consumers.

The present invention has been made in view of the above circumstances, and has as its object to provide an alloyed hot-dip galvanized steel sheet having a plating film having good powdering resistance during molding.

[0017]

As described above, an alloyed hot-dip galvanized film is formed by an alloying reaction between hot-dip zinc and a steel sheet. Generally, the film structure formed by these alloying reactions is formed. Has a film structure in which Γ phase, Γ1 phase, δ1 phase and ζ phase are mixed. Therefore, the present inventors, Γ phase,
The relationship between the film structure and the powdering resistance was investigated in detail in order to control the film structure composed of the Γ1, δ1 and ζ phases to obtain the most suitable film structure for the powdering resistance. As a result, the following conclusions were obtained.

(1) A processing mode in which the sliding of the coating surface and the deformation of the steel plate itself occur simultaneously as in press forming or roll forming, that is, a processing mode in which shear strain and tensile / compression strain are simultaneously applied to the coating. Then, as shown in FIG. 1 (a), cracks perpendicular to the steel plate / coating interface (hereinafter referred to as vertical cracks) occur at predetermined intervals mainly in the δ1 phase due to tensile stress in the early stage of processing, at a predetermined interval determined by the hardness of the coating. I do.
Due to the occurrence of vertical cracks, the strain stored in the film is
Once released, when processing proceeds further and the film cannot follow the deformation of the steel sheet, as shown in FIG. 1 (b), cracks parallel to the steel sheet / film interface (hereinafter, horizontal) Cracks) occur. When the horizontal crack reaches the adjacent vertical crack, peeling of the film occurs.

(2) The state of such cracks is greatly affected by the film structure. (3) When a soft ζ phase or η phase exists in the surface layer of the film, the vertical cracks generated in the δ1 phase do not penetrate to the surface of the film and stop in a region where the ζ phase exists. (FIG. 2 (a)).

(4) On the other hand, the occurrence and propagation of horizontal cracks are affected by the phase structure near the steel sheet / coating interface. That is, when only the Γ phase exists at the steel plate / film interface, the propagation path of the horizontal crack is the steel plate / Γ phase interface (FIG. 2).
(B)) When the amount of strain is the same, (1) the propagation distance increases as the phase thickness increases. On the other hand, when the Γ1 phase exists on the Γ phase, the propagation path of the parallel crack changes depending on the existence state of the Γ1 phase. That is, the Γ1 phase grows in the δ1 phase in a needle-like form in the early stage of generation, and changes into a wedge-shaped or even a block-shaped as the alloying proceeds. That is, the interface with the Γ phase increases. The powdering resistance is best when it is needle-shaped, and deteriorates when it becomes wedge-shaped or block-shaped.

This is because, at the needle-like stage, the propagation path of the parallel crack has a complicated appearance as shown in FIG. 2C, but when it becomes wedge-shaped, as shown in FIG. This is because the cracks propagate along the Γ phase / Γ1 phase interface, and film peeling, that is, powdering easily occurs.

(5) The change in the powdering resistance due to the change in the horizontal crack propagation path corresponds well to the shape of the # 1 crystal, that is, the average aspect ratio (the ratio between the long side and the short side).

The present invention has been completed on the basis of the above-mentioned findings, and has a Γ phase, a Γ1 phase and a δ1 phase on the steel sheet surface from the steel sheet side.
An alloyed hot-dip galvanized steel sheet in which a δ1 phase or a δ1 phase and a ζ phase are sequentially formed, wherein the average thickness of the Γ phase is 1.5 μm or less, and the average of the mixed layer of the Γ1 phase and the δ1 phase. Provided is an alloyed hot-dip galvanized steel sheet characterized in that the thickness is less than twice the average thickness of the Γ phase and the average aspect ratio of the Γ1 crystal (the ratio of the long side to the short side by cross-sectional observation) is 2 or more. Is what you do.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. Unlike simple “bending,” “tensile,” and “compression,” in press forming and roll forming, shear stress and tensile and / or compressive stress simultaneously act in the film. In such a case, the influence of the film structure on the powdering resistance also differs.

In order to improve the powdering resistance,
A method of generating a phase to stop vertical cracks in the film is conceivable.However, since the ζ phase has a low melting point and is abundant on the film surface, in some cases, it reduces slidability and reduces press formability. May deteriorate.

Therefore, as another method, a method of giving resistance to the propagation of a horizontal crack becomes important. Regarding the propagation behavior of horizontal cracks, the followings were used:
If the crack propagates linearly at one interface, the crack propagates relatively easily. However, when the average aspect ratio (the ratio between the long side and the short side) of the Γ1 crystal is 2 or more, ( As shown in c), the crack propagation path is bent and, at the same time, the resistance to crack propagation increases, so that the powdering resistance is also improved.

The cause is considered as follows. $ 1
The phase nucleates at the interface between the Γ phase and the δ1 phase and grows in the δ1 phase. With the growth of the # 1 phase, the shape changes to a needle-wedge-block shape, and the aspect ratio of the # 1 crystal decreases. However, since the crystal is grown in the solid phase, as shown in FIG. Internal stress builds up in the phase and the δ1 phase. When this internal stress increases, Γ / Γ1 and δ1 /
(4) The shear strength at the interface becomes relatively weak, and horizontal cracks easily propagate.

FIG. 4 is an electron micrograph showing a peeled state when the wedge-shaped or block-shaped {1} phase is present. It can be seen that the peeling occurs at the Γ / Γ1 and δ1 / Γ interfaces.

On the other hand, the propagation path is Γ / Γ from the steel plate interface.
Considering the transition stage when changing to one interface and the δ1 / Γ interface, horizontal cracks are not linear but sew along weak interfaces because there is a part with weak bonding force at each interface. become. For this reason, it is considered that a large amount of energy is required for peeling the film, and the film is hardly peeled.

As described above, the thinner the Δ phase is, the better for the powdering resistance. In the present invention, the thickness of the Δ phase is specified to be 1.5 μm or less as an average thickness. On the other hand, for the # 1 phase, the lower limit is determined from the viewpoint that the propagation path of the horizontal crack changes, and the upper limit is determined from the viewpoint of reduction in shear strength.

As described above, the internal stress generated in the Γ1 phase and the δ1 phase depends on the shape or aspect ratio of the Γ1 crystal. If the aspect ratio of the Γ1 crystal is 2 or more, the internal stress becomes small. The resistance to crack propagation increases, and the powdering resistance is improved.
The aspect ratio of one crystal is defined as 2 or more.

Further, since the Γ / Γ1 interface area increases with the growth of the Γ1 phase and the interface strength decreases, the thickness of the Γ1 phase (Γ1) is considered from the viewpoint of maintaining the interface strength at a sufficient value.
Phase average crystal height) is defined as less than twice the average thickness of the Δ phase.

The average thickness of the (1) -phase and (1) -phase can be measured by corroding the cross section of the coating with nital or the like, and then observing it with a scanning electron microscope (SEM), preferably at a magnification of 5000 or more. The average aspect ratio of the # 1 crystal is quantified as the average value of the ratio dl / dc between the long side dl and the short side dc. FIG. 5 shows the definition of the film structure at that time.

In the present invention, the method for producing the target alloyed hot-dip galvanized steel sheet is not particularly limited, but it can be suitably produced using the following method.

First, plating is performed by immersing a steel sheet cooled to 470 ° C., preferably 450 ° C. or lower, in a zinc plating bath containing 0.05 to 0.3% of Al. At this time, the Al concentration and the penetration plate temperature are set to the above values so as not to cause an Outburst reaction in the bath. This is O
This is because, when the utburst reaction occurs, the Γ phase nucleates immediately after that, and as a result, the Γ phase thickness increases and the 増 大 1 phase grows easily.

It is important to delay the start of the Outburst reaction in order to obtain the film structure of the present invention. Therefore, when using IF steel in which the Outburst reaction is likely to occur as a plating base plate, the Al content in the bath should be 0.1%.
It is preferable to increase it to 3% or more. Also, Outbur
In order to delay the start of the st reaction, the bath temperature may be set to 450 ° C. or lower, or a chemical solution containing C may be applied to the surface of the plated steel sheet, followed by annealing and plating.

As described above, after the hot-dip plating is performed while suppressing the Outburst reaction, the adhesion amount control and the alloying treatment are performed. At this time, the maximum reached sheet temperature of the steel sheet was 450
C. to 550 ° C. or lower. Since the Γ1 phase exists stably in a temperature range of 550 ° C. or less, when alloying and heating for a predetermined time so that the Fe content in the plating film becomes 8% or more and 14% or less, the interface between the Γ phase and the δ1 phase is formed. Γ1 phase nucleates. Therefore, it is important not to grow the needle-like Γ1 phase immediately after the generation of the nuclei in a wedge shape or a block shape. For this purpose, the temperature must be 25 ° C up to a temperature of 350 ° C or less and 250 ° C or more.
Rapid cooling may be performed at a cooling rate of at least / s.

If the maximum temperature of the steel sheet is 550 ° C. or higher, the Γ phase undesirably grows excessively during the alloying heating. After quenching to a temperature of 350 ° C. or less and 250 ° C. or more, slow cooling or soaking heat treatment may be performed within the above temperature range in order to alleviate distortion in the film.

By manufacturing under such conditions, the film structure near the interface between the film and the steel sheet has an average Δ phase thickness of 1.5.
μm or less, the average thickness of the mixed layer of the Γ1 phase and the δ1 phase is less than twice the average thickness of the Γ phase, and the average aspect ratio of the Γ1 crystal is 2
The above conditions of the present invention are satisfied.

The surface structure of the film is δ1 phase,
A structure in which phases are mixed may be used. The former structure is preferable when the film requires slidability, and the latter structure is preferable when the film does not require much slidability.制 御 In order to control the presence or absence of a phase, Japanese Patent Application Laid-Open No. Hei 4-23526 has already proposed the present inventors.
5 and JP-A-4-232240 may be used. That is, by specifying bath conditions such as the amount of Al in the bath, the intrusion plate temperature, and the bath temperature according to the type of the base steel, the ζ phase formation reaction in the bath is controlled, and the processing temperature of the alloying heating furnace is reduced. The final coating structure is controlled by the specification.

In producing the alloyed hot-dip galvanized steel sheet according to the present invention, it is necessary that Al is added to the bath, but the components added to the bath other than Al are not particularly limited. That is, in addition to Al, Fe, Pb, Sb, S
Even if i, Sn, Mg, Mn, Ni, Ti, Li, Cu, etc. are contained or added, the effects of the present invention are not impaired. Furthermore, in the present invention, the components of the original plating plate are not limited at all. Further, an upper layer film such as a Ni-Fe-O-based or Fe-Zn-based film may be provided on the film surface.

[0042]

EXAMPLES Cold rolled steel sheets having the components shown in Table 1 (sheet thickness: 0.8 m)
m) was passed through CGL to produce galvannealed steel sheets having various coating structures.

[0043]

[Table 1]

The production conditions at this time are as follows: the annealing temperature is 850 ° C., the temperature of the plate entering the bath, the Al concentration in the bath and the bath temperature are 440 ° C. to 465 ° C. and 0.120 to 0.145 wt.
% And 450-470 ° C. Subsequently, the amount of plating was controlled to 45 to 90 g / m ² per side, and 460
After performing alloying heating so that the maximum attained sheet temperature of ~ 540 ° C is reached, the average cooling rate from the heating temperature to 300 ° C is 25.
Rapid cooling was performed at a rate of ３５35 ° C./s. Then, 0.
Investigation rolling of 5% was performed and sampling for film inspection was performed.

The examination of the film structure was carried out as follows.
First, the cross section of the steel plate was polished and corroded with 5% nitric alcohol (nital). Next, photographs of three visual fields were taken from an arbitrary position using a scanning electron microscope, and two photographs were taken for each visual field.
The thickness of the Δ phase was measured at 10 points at intervals of μm, and the average value of all the measured values was defined as the average thickness of the Δ phase.

On the other hand, since the Γ1 phase appears as an individual crystal, the lengths of the long side and the short side of each crystal were measured at 20 points each, and the average value of the ratio was defined as the average aspect ratio of the Γ1 crystal. .

The presence or absence of the phase was determined by X-ray analysis.
The peak ratio between (−4, 2, 1) and δ1 (2, 4, 9) was measured and judged from the value of I (ζ) / I (δ1). I
When the value of (ζ) / I (δ1) is 0.4 or less, substantially ζ
It can be assumed that no phases are present.

The powdering resistance was evaluated by a bead pull-out test and an actual press test shown in FIG. In bead pulling, the peeling amount also changes depending on the amount of film adhesion, so the bead pulling test measures the amount of plating peeling on the surface that has been wrung with a male bead,
The evaluation was made as a percentage with respect to the coating weight before the test.

In FIG. 6, reference numeral 1 denotes a test piece, 2 denotes a female bead, and 3 denotes a male bead. In an actual test, the test piece 1 is sandwiched between the female bead 2 and the male bead 3, and the test piece is pulled out while being pressed with a constant pressure.
And the amount of peeling was measured from the weight change before and after the test.
At this time, the pressing force is 500 kgf and the female bead shoulder R
Was 1.0 mm, and the tip R of the male bead was 0.5 mm.

Further, in the actual press test, a fender mold of a passenger car was used, and after 150 sheets of continuous press, evaluation was made on the state of occurrence of defects on the surface of the pressed article and the amount of powder adhering to the mold. The evaluation criteria were as follows. ◎: Pressed product: No surface defects occurred Powder amount: Minor △: Pressed product: Pressed scratch due to powdering (10
%) Powder amount: small amount ×: Pressed product: generation of pressing scratch due to powdering (10
% Or more) Powder amount: large amount These results are summarized in Table 2.

[0051]

[Table 2]

As shown in Table 2, all of the examples of the present invention have good powdering resistance, whereas all of the comparative examples outside the scope of the present invention have insufficient powdering resistance. there were. In addition, it was confirmed that sufficient powdering resistance was not obtained for the above-mentioned conventional techniques 1 to 5. From the above results, the effect of the present invention was confirmed.

[0053]

As described above, according to the present invention,
Γ phase, Γ1 phase and δ1 phase mixed layer on steel sheet surface from steel sheet side,
Assuming that the δ1 phase or the film structure in which the δ1 phase and the 順次 phase are sequentially formed, an alloyed hot-dip galvanized steel sheet having a film structure most suitable for powdering resistance can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a powdering phenomenon under shear, tensile, and compressive stress.

FIG. 2 is a schematic view showing a film structure and a powdering phenomenon.

FIG. 3 is a schematic diagram showing growth of a single phase and internal stress of a film.

FIG. 4 is an electron micrograph showing a peeled state of the galvannealed film.

FIG. 5 is a schematic diagram for explaining the definition of a film structure.

FIG. 6 is a sectional view showing a bead pull-out test device.

[Explanation of symbols]

 1 ... Test piece 2 ... Female bead 3 ... Male bead

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Ahmed Tea Alpas 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-7-292452 (JP, A) JP-A Heihei 7-34213 (JP, A) JP-A-6-17221 (JP, A) JP-A-4-301061 (JP, A) JP-A-3-249162 (JP, A) JP-A-2-170959 (JP, A) A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. The steel sheet surface has a Γ phase, a Γ1 phase and a δ phase
A mixed layer of one phase, an alloyed hot-dip galvanized steel sheet in which a δ1 phase or a δ1 phase and a ζ phase are sequentially formed, wherein an average thickness of the Γ phase is 1.5 μm or less, and a mixed layer of Γ1 phase and δ1 phase. An alloyed hot-dip galvanized steel sheet wherein the average thickness is less than twice the average thickness of the Γ phase, and the average aspect ratio of the Γ1 crystal (the ratio of the long side to the short side by cross-sectional observation) is 2 or more.