JP2001247949A - Hot dip galvannealed steel sheet excellent in press formability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip galvannealed steel sheet capable of stably obtaining excellent press formability because of its small sliding resistance and excellent powdering resistance in press forming. SOLUTION: In this hot dip galvannealed steel sheet, the surface of plating is provided with a flat part, and grooves with a depth of 0.1 to 5.0 μm are present in 0.1 to 30.0% by area ratio. Moreover, the area ratio of the flat part in the surface of the plating is 10 to 90%. In the case the flat part on the surface of the steel sheet is formed, a die is directly contacted with the flat part at the time of press forming. When the above grooves are formed on the flat part, lubricating oil infiltrates into the grooves and is retained, so that its slidability improves.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvannealed steel sheet having excellent press formability.

[0002]

BACKGROUND OF THE INVENTION Alloyed hot-dip galvanized steel sheets have better paintability and weldability than galvanized steel sheets.
Widely used for automobiles and home appliances.

[0003] Such an alloyed hot-dip galvanized steel sheet is
It is subjected to press molding and used for the intended use. But,
The alloyed hot-dip galvanized steel sheet has a drawback that press formability is inferior to cold-rolled steel sheet. This is because the sliding resistance between the alloyed hot-dip galvanized steel sheet and the press die is greater than that of the cold-rolled steel sheet. That is, in a portion where the sliding resistance between the bead and the galvanized steel sheet is extremely large, the galvannealed steel sheet hardly flows into the press die, and the steel sheet is easily broken.

[0004] As a method for improving the press formability of a galvanized steel sheet, a method of applying a high-viscosity lubricating oil is generally widely used. However, in this method, there are problems such as the occurrence of coating defects due to poor degreasing in the coating process due to the high viscosity of the lubricating oil, and the unstable press performance due to running out of oil during pressing. In order to solve the above problem, it is necessary to minimize the amount of lubricating oil applied, and for that purpose, it is necessary to improve the press formability of the galvannealed steel sheet.

As a method for solving the above-mentioned problem, Japanese Patent Laid-Open Publication No. Sho 53-60332 and Japanese Patent Laid-Open Publication No. 2-19048
No. 3 discloses that an oxide film mainly composed of ZnO is formed by subjecting a surface of a galvanized steel sheet to electrolytic treatment, immersion treatment, coating oxidation treatment, or heat treatment to improve weldability or workability. Disclosure technology.

JP-A-4-88196 discloses that a galvanized steel sheet is immersed in an aqueous solution containing 5 to 60 g / l of sodium phosphate and having a pH of 2 to 6 on the surface of a galvanized steel sheet.
There is disclosed a technique of forming an oxide film mainly composed of P oxide by spraying the aqueous solution and electrolytic treatment to improve press formability and chemical conversion treatment.

[0007] Japanese Patent Application Laid-Open No. Hei 3-191093 discloses a press-formability and a chemical conversion by forming a Ni oxide on a surface of a galvanized steel sheet by electrolytic treatment, dipping treatment, coating treatment, coating oxidation treatment, or heating treatment. A technique for improving processability is disclosed.

Japanese Patent Application Laid-Open No. 7-18402 discloses a technique for forming a concave portion on the surface of an alloyed hot-dip galvanized steel sheet so that lubricating oil can be easily held on the steel sheet surface and press formability is improved. I have.

[0009]

However, when the above-described prior art of providing a surface oxide film is applied to an alloyed hot-dip galvanized steel sheet, the effect of improving press formability cannot be obtained stably. The inventors have studied in detail the cause. As a result, it has been found that the alloyed hot-dip coated steel sheet is caused by non-uniform surface reactivity due to non-uniform Al oxide, and large surface irregularities. That is, when the above-described prior art is applied to an alloyed hot-dip coated steel sheet, the reactivity of the surface is non-uniform. It is difficult to form the oxide layer uniformly, and the thickness of the oxide layer becomes thin in a portion having low reactivity (a portion having a large amount of Al oxide). In addition, since the surface irregularities are large, the direct contact with the press mold at the time of press molding is the convex part of the surface, but in the convex part, the contact part between the thin part of the oxide layer and the mold. , The sliding resistance is increased, and the effect of improving press formability cannot be sufficiently obtained.

[0010] Further, regarding the technique for forming the concave portion,
It turned out that sufficient press moldability cannot be obtained by this alone. This is considered to be because lubricating oil easily accumulates in the concave portion, but conversely, lubricating oil hardly accumulates in the convex portion, which has a large effect on slidability.

The present invention has been made to solve the above problems, and has as its object to provide an alloyed hot-dip coated steel sheet having excellent press formability.

[0012]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in order to achieve the above-mentioned object, and as a result, have formed a flat portion on the galvannealed steel sheet and further formed a flat portion on the flat surface. It has been found that by forming the groove, excellent press formability can be obtained stably.

The flat portion on the surface of the galvannealed steel sheet exists as a convex portion as compared with the surroundings.
An electron micrograph of the plating surface is shown in FIG. In FIG. 1, the portion seen with black contrast is the flat portion. The actual contact with the press mold during press molding is mainly due to the flat part that can be seen with this black contrast, so if the sliding resistance in this flat part is reduced, the press formability can be stably improved. it can.

In order to reduce the sliding resistance in the flat portion, it is effective to prevent adhesion between the plating layer and the mold.
For that purpose, it is effective to improve the slipperiness of the plating layer surface. As a result of study based on this, it was found that it is effective to form a groove in a flat portion.

The present invention has been made based on the above findings. The first invention has a flat portion on a plating surface, and the flat portion has a depth of 0.1 μm or more and 5.0 μm or less. The present invention provides an alloyed hot-dip galvanized steel sheet characterized in that the grooves are present in an area ratio of 0.1% or more and 30.0% or less.

According to a second aspect of the present invention, there is provided the galvannealed steel sheet according to the first aspect, wherein an area ratio of the flat portion on the plating surface is 10% or more and 90% or less.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION An alloyed hot-dip galvanized steel sheet has irregularities on the plating surface due to the difference in reactivity between the steel sheet and the plating interface during the alloying treatment. In the present invention, temper rolling is performed on the galvannealed steel sheet. By performing the temper rolling, the unevenness of the plating surface is alleviated and the surface is smoothed, and at the same time, the projections on the plating surface are flattened. The flat part of the surface of the alloyed hot-dip galvanized steel sheet formed as described above is a part where the mold is in direct contact at the time of press forming, so it is important to improve the slipperiness of this part in improving the sliding property. is there.

As a result of examination from this viewpoint, it was found that forming a groove in the flat portion was effective for improving the slidability. This is considered to be because lubricating oil permeates and is retained in the groove. FIG. 2 is an example of an electron micrograph of a flat portion of a galvannealed steel sheet according to an embodiment of the present invention. In FIG. 2, a portion that is seen in black contrast (the end of the arrow in the figure) is a groove existing in the flat portion.

The above-mentioned groove has a diameter of 0.1 μm or more.
A groove having a depth of 0 μm or less is defined as an area ratio of 0.
When the content is 1% or more and 30.0% or less, an alloyed hot-dip galvanized steel sheet exhibiting good sliding properties can be obtained. The reason why the depth of the groove is limited to 0.1 μm or more is that if the depth is less than 0.1 μm, the effect of improving the slidability is small and sufficient press moldability cannot be obtained. The reason for limiting the area ratio to 0.1% or more is the same. The reason for limiting the depth of the groove to 5.0 μm or less and the area ratio to 30.0% or less is that if it exceeds this, the film becomes brittle and the powdering resistance deteriorates.

In the present invention, the groove in the flat portion is
The shape as viewed from the surface refers not only to an elongated so-called groove-shaped depression, but also to a depression existing in a flat portion, such as a polygonal depression (see FIG. 2), as viewed from the surface. , Refers to the average depth of the continuous depression.

The depth and area ratio of the groove can be determined by using an atomic force microscope or 3
It can be measured by a scanning electron microscope for dimensional observation.

The area ratio of the flat portion on the plating surface is desirably 10% or more and 90% or less. 1
If it is less than 0%, of the area that actually contacts the mold, the contact area between the part (recess) excluding the flat part and the mold becomes large, and the flat part and the mold that can exhibit the slidability improvement effect by the groove. Therefore, the effect of improving press formability is reduced. Also, the portion excluding the flat portion has a role of retaining lubricating oil during press molding. The area ratio of the portion excluding the flat portion is less than 10% (the area ratio of the flat portion is 90%
), The oil tends to run out during press molding, and the effect of improving press formability is reduced. Among these, the area ratio of the flat portion is preferably in the range of 20% to 80%, more preferably in the range of 30% to 70%.

The flat portion of the plating surface can be easily identified by observing the surface with an optical microscope or a scanning electron microscope. The area ratio of the flat portion on the plating surface can be determined by image analysis of the micrograph.

In the present invention, although the Fe concentration and the Al concentration of the plating film of the galvannealed steel sheet are not particularly specified, the structure of the plating layer mainly composed of δ phase and further containing ζ phase is ideal. It is a target. If the ζ phase or η phase with a low Fe concentration is the main component, the plating film becomes soft with a low melting point, so-called flaking during press molding, the plating film is deposited on the mold, and the product yield decreases due to press scratches. In addition, the production efficiency may decrease due to the increase in the frequency of mold and mold care. Further, when the Γ phase and Γ ₁ phase having a high Fe concentration are mainly used, powdering in which the plating film peels off during press working is apt to occur, and pressing scratches due to flaking are liable to occur. Therefore,
The δ phase is mainly used as the plating film.

In order to manufacture the galvannealed steel sheet according to the present invention, plating is performed in a galvanizing bath, alloying treatment is performed, and temper rolling is further performed. It is necessary that Al is added to the galvanizing bath, but additional element components other than Al are not particularly limited. That is, in addition to Al, F
e, Pb, Sb, Si, Sn, Mg, Mn, Ni, T
Even if i, Li, Cu, etc. are contained or added, the effects of the present invention are not impaired. In the alloying treatment, the plating film is mainly adjusted to δ by adjusting the alloying treatment conditions.
The structure consists of a phase and further contains a phase. Next, a flat portion is formed on the plating surface by temper rolling. At that time, it is desirable that the rolling conditions be adjusted so that the area ratio of the flat portion is in the range described above.

Next, a groove is formed in the flat portion of the plating surface. The method for forming the grooves is not particularly limited, and methods such as immersion in an acidic solution, electrolytic treatment, and surface grinding can be employed.

[0027]

Next, the present invention will be described in more detail with reference to examples. On a cold-rolled steel sheet having a thickness of 0.8 mm, plating and alloying are performed in a galvanizing bath containing Al by an ordinary galvannealing method, and the coating weight is 60 g / m ² ,
A plating film having a predetermined Fe concentration was formed, and further temper rolling was performed. At this time, the area ratio of the flat portion on the surface was changed by changing the rolling load of the temper rolling. Subsequently, in order to form a groove in the flat part, the alloyed hot-dip coated steel sheet was immersed in a hydrochloric acid solution having a pH of 3.0 and a bath temperature of 50 ° C. Here, the density and the processing time were changed to various predetermined values, and the area ratio of the groove having a depth of 0.1 μm or more and 5.0 μm or less in the flat portion was adjusted.

Next, with respect to the test pieces (test materials Nos. 2 to 11) prepared as described above, measurement of the depth and area ratio of the groove in the flat portion, the measurement of the area ratio of the flat portion, press formability test, and powdering resistance A sex test was performed. The measurement of the depth and area ratio of the groove, the press formability test and the powdering resistance test were performed as follows.

Further, for comparison, an alloyed galvanized steel sheet which was not subjected to any of the temper rolling, the alkali treatment, and the projection forming treatment was prepared (test material No. 1), and the same investigation was conducted.

(1) Measurement of Groove Depth and Area Ratio Atomic Force Microscope (Digital Instrument)
The average depth of the groove and the area ratio of the groove where the average depth of the groove was in the range of 0.1 to 5.0 μm were measured using Nanoscope 2) manufactured by t Company. The depth of the groove was determined by using the upper surface of the flat portion adjacent to the groove as a reference surface, and using this position as a reference, the average depth of the groove (n = 5).

(2) Evaluation test for press formability "Test for measuring friction coefficient" In order to evaluate press formability,
The friction coefficient of each specimen was measured as follows.

FIG. 3 is a schematic front view showing a friction coefficient measuring device. As shown in the figure, a sample 1 for measuring a coefficient of friction collected from a specimen is fixed to a sample table 2, and the sample table 2 is
It is fixed to the upper surface of the horizontally movable slide table 3. On the lower surface of the slide table 3, a vertically movable slide table support 5 having rollers 4 in contact therewith
The first load cell 7 for measuring the pressing load N of the bead 6 against the friction coefficient measurement sample 1 by being pushed up is attached to the slide table support 5. A second load cell 8 for measuring a sliding resistance force F for moving the slide table 3 in the horizontal direction while the pressing force is applied is attached to one end of the slide table 3. As a lubricating oil, Noxlast 55 manufactured by Nippon Parkerizing Co., Ltd. was used.
The test was performed by applying 0HN to the surface of Sample 1.

FIGS. 4 and 5 are schematic perspective views showing the shapes and dimensions of the beads used. The bead 6 slides while being pressed against the surface of the sample 1. The shape of the bead 6 shown in FIG.
The lower part of both ends in the sliding direction is a curved surface with a curvature of 4.5 mmR, and the lower surface of the bead on which the sample is pressed is 10 m in width.
m, a plane having a length of 3 mm in the sliding direction. The shape of the bead 6 shown in FIG. 5 has a width of 10 mm and a length 69 in the sliding direction of the sample.
mm, the lower part at both ends in the sliding direction is a curved surface with a curvature of 4.5 mmR, and the lower surface of the bead on which the sample is pressed is 10 m in width.
m, a plane having a length of 60 mm in the sliding direction.

The friction coefficient measurement test was performed under the following two conditions. The friction coefficient μ between the specimen and the bead was calculated by the equation: μ =
It was calculated by F / N.

[Condition 1] Using the bead shown in FIG. 4, the pressing load N was 400 kgf, and the sample withdrawing speed (horizontal moving speed of the slide table 3) was 100 cm / min. At this time, the evaluation was performed based on the coefficient of friction.
Less than 0.135, ○: 0.135 or more, less than 0.15,
Δ: 0.15 or more, less than 0.17, ×: 0.17 or more,
And

[Condition 2] Using the bead shown in FIG. 5, the pressing load N: 400 kgf, the withdrawal speed of the sample: 20 c
m / min. At this time, the evaluation was performed based on the coefficient of friction, ◎: less than 0.18, ：: 0.18 or more, 0.20
Less than, Δ: 0.20 or more, less than 0.23, ×: 0.23
That is all.

(3) Powdering resistance test For the powdering resistance, a draw bead test was performed to measure the amount of peeling of the film per unit area. A film peeling amount of less than 10 g / m ² was regarded as acceptable. Here, the draw bead test is a test method in which a steel plate coated with a lubricating oil is pulled out while being sandwiched between a bead and a die, and then a tape peeling test is performed, and a peeling amount of the plating film is evaluated from a weight difference before and after the test. It is. The bead used was a triangular bead having a tip angle of 90 °, the molding height was 4 mm, and the pressing load between the bead and the die was 500 kgf.

Further, based on the friction coefficient measurement test results obtained above and the powdering resistance test, comprehensive evaluation was made as follows. ×: The rating of the poorer coefficient of friction under the conditions 1 and 2 was ×. Δ: The rating of the poorer coefficient of friction under the conditions 1 and 2 was Δ,
Or, when the peeling amount of the film in the draw bead test is 10 g / m ² or more and the coefficient of friction is inferior, the evaluation is 〜 to ○. ○: The peeling amount of the film in the draw bead test is less than 10 g / m ² ,劣: The peeling amount of the film in the draw bead test was less than 10 g / m ² , and the friction coefficients of the conditions 1 and 2 were all ◎. The test results are shown in Table 1.

[0039]

[Table 1]

From the test results, the following matters are clear. (1) Comparative Example 1 has no flat portion because it has not been subjected to temper rolling, and thus has a high friction coefficient and poor slidability. (2) In Comparative Examples 2 and 3, since the area ratio of the groove is out of the range of the present invention, the amount of film peeling is large and the powdering resistance is poor. (3) In Examples 1 to 8 of the present invention, the area ratio of the groove is within the range of the present invention, the coefficient of friction is reduced, the slidability is improved, and the powdering resistance is excellent. Invention Examples 3 to
In No. 8, since the area ratio of the flat portion is in the range of 10 to 90%, the coefficient of friction is further reduced, and the effect of improving the slidability is more excellent.

[0041]

The alloyed hot-dip galvanized steel sheet of the present invention comprises:
Since the sliding resistance at the time of press molding is small and the powdering resistance is also excellent, stable and excellent press moldability can be obtained.

[Brief description of the drawings]

FIG. 1 is a scanning secondary electron microscope photograph of the galvannealed steel sheet.

FIG. 2 is a scanning secondary electron micrograph showing grooves existing in a flat portion of a galvanized surface of a galvannealed steel sheet according to an embodiment of the present invention.

FIG. 3 is a schematic front view showing a friction coefficient measuring device.

FIG. 4 is a schematic perspective view showing a bead shape and dimensions in FIG. 3;

FIG. 5 is a schematic perspective view showing another bead shape and size in FIG. 3;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sample for coefficient of friction measurement 2 Sample stand 3 Slide table 4 Roller 5 Slide table support 6 Bead 7 1st load cell 8 2nd load cell 9 Rail N Pressing load F Sliding resistance force P Tensile load

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Shoichiro Hira 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Yoshiharu Sugimoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun 4K027 AA02 AA05 AA22 AB02 AB09 AB13 AB28 AB36 AB37 AB44 AC32 AC73 AC82 AC86 AC87 AE25

Claims (2)

[Claims] 1. A flat portion is provided on a plating surface, and a groove having a depth of 0.1 μm or more and 5.0 μm or less exists in the flat portion in an area ratio of 0.1% or more and 30.0% or less. An alloyed hot-dip galvanized steel sheet. 2. An area ratio of a flat portion on a plating surface is 1
2. The method according to claim 1, wherein the amount is not less than 0% and not more than 90%.
2. A galvannealed steel sheet according to claim 1.