JP2000160314A - Hot dip galvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control the orientation of a ζ phase in the steel sheet, to improve its adhesion for plating and slidability and to improve its press workability as well by specifying the X-ray diffraction intensity ratio at the specified interplanar spacing of a galvanizing layer contg. a specified amt. of Fe obtd. by plating a steel sheet with hot dip galvanizing and executing alloying. SOLUTION: A steel sheet in which the content of O2 is controlled to about <=40 ppm and which is heated to a temp. higher than that of a plating bath by about 25 to 75 deg.C is applied with hot dip galvanizing to a degree of 20 to 60 g/m2 coating weight and is alloyed to form a galvanizing layer contg., by weight, 8 to 13% Fe and, desirably, about 0.15 to 0.30% Al. In this hot dip galvannealed steel sheet, the X-ray diffraction intensity ratio of the plating layer is allowed to satisfy the inequality of A/B>=0.5 [A denotes the X-ray diffraction intensity in the case the interplanar spacing (d) is 0.154 nm, and B denotes the X-ray diffraction intensity in the case the interplanar spacing (d) is 0.125 nm]. In this way, ζphase crystals 5 are formed on the face of ferrite 2 at the bottom part of recessed parts 4 of plating layer 1 and are not allowed to expose to the surfaces of projecting parts 3.

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, and more particularly to an alloyed hot-dip galvanized steel sheet which is suitable for use as a steel sheet for automobile bodies or parts and has excellent press workability. .

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets (hereinafter sometimes simply referred to as coated steel sheets) are inexpensive and have excellent corrosion resistance and are therefore mainly used as steel sheets for automobile bodies. In addition to the "corrosion resistance", another characteristic required as a steel sheet for an automobile body is that it has excellent "press workability". The “press workability” is a characteristic evaluated based on whether or not a problem occurs during press working. One of the problems is a phenomenon called “powdering” caused by poor “plating adhesion”. This "powdering"
"Phenomenon" is a phenomenon in which a plating layer peels off in a powdery or massive form. When this phenomenon occurs, there is a problem that the corrosion resistance of the peeled portion is deteriorated, and the steel plate itself is flawed by the peeled plated pieces. It is said that “powdering” occurs because a hard and brittle Г phase is formed at the interface between the plating layer and the ground iron.

[0003] On the other hand, another major problem during press working of an alloyed hot-dip galvanized steel sheet is the occurrence of cracks in the steel sheet. The cause of the cracks is that the “slidability” represented by the coefficient of friction is poor (the coefficient of friction is large).
Furthermore, if it is tight, a soft alloy phase, ζ phase, is formed on the surface of the plating layer, and this 引 き 起 こ す phase causes cracking during press working of the plated steel sheet. It is also known that when the amount of the Δ phase is large, foil-like peeling called flaking occurs.

[0004] For example, Japanese Patent Publication No. 3-55544 proposes an alloyed hot-dip galvanized steel sheet in which the Г phase is reduced as much as possible and the ζ phase is not contained. surely,
If the alloyed hot-dip galvanized steel sheet that does not contain the phase, which reduces the “plating adhesion”, reduces the phase as much as possible and deteriorates the “slidability”, can be manufactured in a stable process, the steel sheet for automobile body It is very favorable for the supplier. However, the alloyed hot-dip galvanized steel sheet is manufactured by performing a heat diffusion treatment after hot-dip galvanizing to cause interdiffusion of Fe between the surface plating layer and the ground iron, that is, alloying. It is inevitable that the alloy phase appearing in the Zn binary phase diagram appears in the plating layer. Then, in order to suppress the ζ phase having a low Fe content that appears on the plating surface layer, the Ｆｅ phase having a high Fe content grows thickly at the interface between the plating layer and the ground iron, and on the other hand, the generation of the Г phase is suppressed. In this case, the relation that the ζ phase is generated thickly cannot be avoided.

[0005] Further, Japanese Patent Publication No. 3-55543 discloses that
We have proposed an alloyed hot-dip galvanized steel sheet mainly composed of phase crystals. However, the plated steel sheet has a “sliding property” because the crystal of the above-mentioned Δ phase becomes too large.
And inferior in "flaking resistance".

[0006]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides an alloyed hot-dip galvanized steel sheet having excellent "press workability", that is, both "plating adhesion" and "slidability". It is an object.

[0007]

Means for Solving the Problems In order to achieve the above-mentioned object, the inventor has repeatedly studied to break away from the reciprocal relation regarding the appearance of the Г phase and the ζ phase in the conventional plated steel sheet. As a result, they have found that the orientation of the ζ phase crystal appearing in the plating layer affects the “slidability” of the plated steel sheet. Focusing on this orientation, the present inventors have made intensive efforts to find a means capable of improving the "slidability" even when the ζ phase is present, and have completed the present invention which has escaped the reciprocal relation.

That is, the present invention provides an alloyed hot-dip galvanized steel sheet having a galvanized layer containing 8 to 13% by weight of Fe, wherein the X-ray diffraction intensity of the galvanized layer satisfies the following formula (1). It is an alloyed hot-dip galvanized steel sheet. A / B ≧ 0.5 (1) where: A: X-ray diffraction intensity at a lattice spacing d of 0.154 (nm) B: X at a lattice spacing d of 0.126 (nm) Line Diffraction Strength In the present invention, the plating layer that affects the “press workability” of the alloyed hot-dip galvanized steel sheet is configured as described above, so that the “press workability” is improved. .

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below, taking into account the circumstances leading to the invention. Normally, the coating layer of an alloyed hot-dip galvanized steel sheet is roughly viewed from its surface to the surface of the steel, in the order of ζ phase, δ ₁ phase, and Г phase.
Type alloy phase is present (.GAMMA phase, to be precise, but consists a .GAMMA phase and .GAMMA ₁ phase, since hardly clearly distinguished in the plating layer, for convenience, the .GAMMA phase collectively Do). Conventionally, the identification of these phases, especially their quantification, is not a simple matter, and in tissue observation and the like using an electron microscope,
Insufficient information was obtained. Therefore, the inventor
With the idea of using the X-ray diffraction method together, we established a technique for identifying each of the phases.

First, as a result of examining a method of performing X-ray diffraction and measurement conditions, a normal method of irradiating the surface of the plating layer with X-rays may be used. However, the following method can perform measurement with higher accuracy. Turned out to be more preferable. This highly accurate method is that the surface of the plating layer is attached to the dummy plate with an adhesive (select an adhesive whose adhesion to the plating layer is stronger than the adhesion of the plating layer to the steel substrate), and the dummy is tested with a tensile tester. This is a method in which the plating layer is peeled from the ground iron by pulling the plate, and the plating layer is irradiated with X-rays from the peeled surface side. This is because such a measurement method can measure the X-ray diffraction intensity with higher accuracy. The reason why the accuracy is high is not clear, but the normal alloyed hot-dip galvanized layer has significant irregularities on the surface, so it was measured from the peeled side of the plated layer with less irregularities, and the plated layer was removed from the peeled surface. It is considered that the measurement can be performed based on the positional relationship of each phase crystal in which the influence of the crystal orientation easily appears. In addition, as a tube used for generating X-rays, Co with a small intensity is obtained.
A Cu tube is more suitable than a tube.

The inventor analyzed the plating layer by X-ray diffraction by the above method, and obtained a relationship between the lattice spacing d (unit, nm) and the diffraction intensity which can be used for identifying each phase. . That is, [delta] _1-phase, d = 0.1280,0.2
136, d = 0.1222, 0.1917,
0.2120, 0.2597, d = 0.126 for ζ phase
It has been found that diffraction corresponding to 0, 0.1536, 0.1900 can be used.

Next, the inventor manufactured various kinds of alloyed hot-dip galvanized steel sheets, collected a large number of plated layers, observed them with an electron microscope, and analyzed them by X-ray diffraction. As a result, it was found that the X-ray diffraction intensities at the lattice spacing d of the Г phase forming the plating layer are in a certain proportional relationship with each other. This means that the crystal axes of the Г phase are oriented in various directions in the plating layer, that is, there is almost no crystal orientation, and the “plating adhesion”, that is, [the powdering resistance] is poor. It suggests that it is uniquely determined only by the amount of the phase. On the other hand, it was also found that the X-ray diffraction intensity at each lattice spacing corresponding to the ζ phase can be determined. That is, the inventor can know the orientation of the ζ phase crystal.

Therefore, the inventor carried out electron microscopic observation of various galvannealed steel sheets. The observation result of the conventional galvannealed steel sheet is schematically shown in FIG. 1A and FIG. 2A of an electron micrograph.
From these figures, it was confirmed that the surface of the plating layer had irregularities with a depth of about several tens of μm, and the ζ-phase crystals 5 were oriented in the concave portions 4 with the long axis inclined in the horizontal direction. The ζ-phase crystal 5 is monoclinic and has an elongated hexagonal column shape. In FIG. 2A, a white elongated portion corresponds thereto. The portion of the black is a convex portion 3 of the surface, consisting predominantly [delta] ₁ phase. Further, the lattice spacing d = 0.1
The X-ray diffraction intensity of 536 is (−230), (−82)
The crystal planes of 2) and (-423), where d = 0.260, correspond to the crystal planes of (-604), (820) and (-152).

The observation results of the shape of the coating layer of the conventional galvannealed steel sheet and the measurement results of the X-ray diffraction intensity are as follows:
Considered in comparison with various properties of the plated steel sheet, the inventor:
The following conclusions were made. (A) If the ζ phase crystal 5 is exposed on the surface of the plating layer 1, it is not preferable for “slidability” and “flaking resistance”, that is, “press workability”. (B) Therefore, in order to improve the above characteristics, the ζ-phase crystal 5 is grown substantially perpendicularly from the surface of the ground iron 2 so as not to reach the surface of the plating layer 1, particularly to the convex portion 3. (C) In the plated layer 1 in which the ζ-phase crystals 5 are oriented as in (b), the amount of Γ-phase appears at the interface with the ground iron 2. (D) In the plating layer as in (c), the X-ray diffraction intensity satisfies the following expression (1). The inventor has earnestly studied a plating technique that embodies this idea, and has succeeded in forming the plating layer 1 described in (d) by subjecting the steel sheet to hot-dip galvanizing by the method described below. FIG. 1B shows a schematic surface state of the obtained plating layer 1 and FIG. 2B shows an observation result by an electron microscope. By adjusting the orientation of the ζ phase, it is apparent that the ζ phase crystal has not reached the tip of the convex portion 3.

Here, the expression (1) has a meaning as a means aiming at achieving the above (c). The reason for setting A / B to 0.5 or more is that if A / B is less than 0.5,
Is not achievable by electron microscopic observation and X-ray diffraction results. A / B ≧ 0.5 (1) where: A: X-ray diffraction intensity at a lattice spacing d of 0.154 (nm) B: X at a lattice spacing d of 0.126 (nm) Line diffraction intensity

Subsequently, the inventor manufactured a large number of alloyed hot-dip galvanized steel sheets having a plating layer 1 in which the orientation of the Δ-phase crystal 5 exhibits various states, and examined the relationship between the “slidability” and the orientation. We also investigated "flaking resistance". FIG. 3 shows an example of the result (related to slidability).
In this case, the plating adhesion amount is 30 to 60 g /
m ² , the Fe content of the plating layer is 8 to 13% by weight.
The circles in the figure correspond to the plated steel sheets according to the present invention, and the coefficient of friction representing “slidability” is 0.1%.
It was less than 25. On the other hand, the cross mark indicates that FIG.
(A) and a conventional plated steel sheet shown in FIG.
Their coefficient of friction was greater than 0.125.

Also, "press workability" other than "slidability"
Also satisfied the values required for steel sheets for automobile bodies. The Fe content in the plating layer is set to 8% by weight to 13% by weight. If the amount is less than 8% by weight, the appearance amount of the ζ phase becomes too large and the “slidability” deteriorates. If the amount exceeds 13% by weight, the amount of the Г phase increases and the “powdering resistance” deteriorates. I do. If the content of Al in the plating layer is too low, the formula (1) tends to be unable to be satisfied.
0.15 to 0.30% by weight is preferable because alloying cannot be performed.

The amount of plating is 20 to 60 g /
The range of m ² is appropriate from the viewpoints of corrosion resistance, weldability, and the like, and when the amount of adhesion is large, the 傾向 phase and the ζ phase tend to increase easily. Components other than Fe and Al, for example, Pb, Cd, Sn, Mg, Mn, Cr, Ni, REM,
Even if Bi, Sb, B, S, O, etc. were inevitably or added in small amounts, there was essentially no effect on the production of the hot-dip galvanized steel sheet according to the present invention.

Next, a plating method for producing the galvannealed steel sheet according to the present invention will be described. First, the inventor considered that the surface condition of a steel strip (including a steel sheet) as a material to be plated was important after the rolling and recrystallization annealing was completed and immediately before plating.
The reason is that the ζ phase crystal 5 grows in a direction substantially perpendicular to the surface of the ground iron 2 and does not reach the tip of the convex portion 3.
This is because it is necessary to cause an alloying reaction in a direction substantially perpendicular to the surface of the ground iron 2. The surface of the steel strip after annealing is often oxidized, but if the surface of the base iron 2 is oxidized even slightly before plating, alloying at that part becomes faster after the plating adheres, and Crystal growth occurs. Therefore, the plated steel strip according to the present invention keeps the oxygen concentration in the atmosphere at a low level of 40 ppm or less, preferably 10 ppm or less in a temperature range from 500 ° C. to 600 ° C. so that the steel strip surface does not oxidize before plating. It is preferable to do so. Further, in order to prevent slight oxidation, the hydrogen concentration in this atmosphere is preferably increased to 4% by volume, preferably 7% by volume or more.

In plating, the temperature of the plating bath was usually about 460 ° C., and the temperature of the steel strip (plated material) immersed therein was almost the same. However, in the present invention, the steel strip temperature to be immersed is 25 to 75 times lower than the temperature of the bath with respect to the plating bath temperature.
° C. That is, as the steel strip temperature at the time of infiltration into the plating bath, for example, the temperature of the plating bath is 46
In the case of 0 degreeC, 485-535 degreeC is preferable. Although such an effect of the temperature of the steel strip entering the plating bath was completely unexpected, the cause is presumed as follows.

By increasing the temperature of the infiltrated steel strip, an alloying reaction is intentionally caused at the interface between the ground iron and the plating layer. In particular, an alloying called an outburst is formed on the grain boundaries of iron crystal grains. A fast-acting convex portion is formed. At the same time, Al may be higher on the surface in the ground iron crystal grains, and an Al-rich layer for suppressing alloying is uniformly generated. Therefore, on the inner surface of the grain, the ζ phase is perpendicular to the ground iron surface, but it is considered that it does not reach the surface. For example, when the temperature of the plating bath is 460 ° C., if the temperature of the infiltrated steel strip is lower than 485 ° C., it is difficult to satisfy the above formula (1).
If the temperature exceeds 5 ° C., the formula (1) is satisfied, but the temperature of the plating bath is too high, which is unsuitable for plating. Furthermore, the temperature for alloying the plating layer applied after plating, that is, the heating temperature of the steel strip is preferably in the range of 460 to 500 ° C, more preferably 470 to 490 ° C. If the temperature is lower than 460 ° C., a large amount of the ζ phase tends to appear, and if the temperature exceeds 500 ° C., the amount of the Г phase increases even if the ζ phase decreases.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Continuously cast ultra-low carbon Ti-Nb added steel (U
The steel strip obtained by rolling LC-IF) is used as a material to be plated.
An alloyed hot-dip galvanized steel sheet according to the present invention was manufactured. The equipment used is a continuous galvanizing line (CGL). Therefore, the steel strip is moved at a running speed of 80 to 150 m / m.
After performing recrystallization annealing at 800 to 850 ° C. while moving in, hot dip galvanizing on both sides, and immediately after pulling, heating at 480 to 490 ° C. for 10 to 20 seconds, alloying treatment of the plating layer Was done. Al in plating layer
For the concentration, the Al concentration in the plating bath was adjusted, and for the Fe content in the plating layer, the heating conditions of the alloying furnace were appropriately selected and adjusted to the above ranges. The steel sheet after the alloying treatment was examined for “plating adhesion” and “slidability” in the following tests, and evaluated as a steel sheet for an automobile body.

Evaluation test of "powdering resistance as plating adhesion" Evaluation test: 90 degree bending and returning of an alloyed hot-dip galvanized steel sheet, pressure bonding and peeling of a tape to a plating layer, and peeling amount of zinc Was measured by X-ray fluorescence. The evaluation was performed according to the following ranking based on the count number of the zinc. The count number (cps) by fluorescent X-rays is 0 to 500 ... Rank 1 (good) 500 to 1000 ... Rank 2 (somewhat good) 1000 to 2000 ... Rank 3 (normal) 2000 to 3000 ... Rank 4 (somewhat poor) 3000 or more ... Rank 5 (poor).

Evaluation Test of "Slidability" In a sliding test using a known flat plate, the same surface was repeatedly slid 10 times with a holding pressure of 200 kgf, and the friction coefficient was determined from the result obtained at the 10th time. Further, regarding the orientation state of the coarse ζ phase crystal in the plating, the Cu tube, the tube voltage 50
At kV and a current of 250 mV, the plating layer obtained by the above-mentioned peeling method was subjected to X-ray diffraction to determine the A / B intensity ratio. The evaluation results are shown in Table 1 collectively. Table 1 shows that the alloyed hot-dip galvanized steel sheet according to the present invention has better results in both "powdering resistance" and "slidability" than the comparative example.

[0025]

[Table 1]

[0026]

As described above, according to the present invention,
It has become possible to supply a high-quality galvannealed steel sheet having excellent "press workability", that is, both "plating adhesion" and "slidability".

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating a surface state of an alloyed hot-dip galvanized steel sheet by observing a plating layer with an electron microscope;
(A) corresponds to a conventional steel sheet, and (b) corresponds to a steel sheet according to the present invention.

FIGS. 2A and 2B are photographs showing a state in which a plating layer of an alloyed hot-dip galvanized steel sheet is observed with an electron microscope. FIG. 2A corresponds to a conventional steel sheet, and FIG. 2B corresponds to a steel sheet according to the present invention.

FIG. 3 is a diagram showing the results of an investigation on the relationship between the X-ray diffraction intensity of a plating layer and the “slidability” of an alloyed hot-dip galvanized steel sheet having such a plating layer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Plating layer 2 Ground iron (surface) 3 Protrusion of plating layer 4 Depression of plating layer 5 ζ phase crystal

Claims (1)

[Claims] 1. A galvannealed steel sheet having a galvanized layer containing 8 to 13% by weight of Fe, wherein the X-ray diffraction intensity of the galvanized layer satisfies the following expression (1). To galvannealed steel sheet. A / B ≧ 0.5 (1) where: A: X-ray diffraction intensity at a lattice spacing d of 0.154 (nm) B: X at a lattice spacing d of 0.126 (nm) Line diffraction intensity