JPH0744864A - Production of titanium substrate for magnetic disk - Google Patents

Abstract

PURPOSE:To produce a Ti substrate excellent in flatness and used as a magnetic disk substrate. CONSTITUTION:A Ti substrate subjected to primary rough polishing is prepd. and heated for a heating time (t) (hr) represented by an inequality t>=-0.016T+1.06 [where T is temp. after the heating time (t) and is 350-880 deg.C]. The heated Ti substrate is subjected to secondary rough polishing so that the difference in residual stress between both surface layers of the substrate is regulated to #50MPa.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a titanium substrate used as a magnetic disk substrate and having excellent flatness.

[0002]

2. Description of the Related Art As a substrate material for a magnetic disk, a nonmagnetic, lightweight and high strength material is suitable, and the most commonly used material at present is an aluminum-magnesium alloy. However, there is a demand for a magnetic disk that is smaller and has a large storage capacity, and thus a new material is required for the substrate. At present, various materials are listed as candidates, and one of them is a titanium substrate.

In the manufacturing process of a titanium substrate for a magnetic disk, a plate-shaped titanium is prepared, a doughnut-shaped disk is punched from this, and after strain relief hot correction, precision cutting, rough polishing, plating treatment, and final treatment. Polishing is performed as processing. Here, the hot straightening method after punching is described in, for example, JP-A-2-205204 and JP-A-3-2053.
It is known from Japanese Patent Laid-Open No. 018425 and Japanese Patent Laid-Open No. 03-230814.

However, even if the flatness of the disk is ensured by the hot correction, the flatness may be deteriorated due to the residual stress of the surface layer of the disk due to the precision cutting process by the lathe and the rough polishing process. In particular, in the case of titanium, which is a difficult-to-machine material, the processing load in the rough polishing process must be made considerably higher than that of conventional aluminum alloys, and the residual stress of the disk surface layer after the rough polishing process becomes large. Deterioration of the flatness of the disk due to is a problem.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of manufacturing a titanium substrate having excellent flatness as a magnetic disk substrate. Here, the titanium substrate includes substrates made of pure titanium and various titanium alloys.

With the recent decrease in the diameter of hard disks, the demand for flatness has become strict, and the linear length of 100
A very high degree of flatness is demanded, with a maximum displacement amount per mm (eg, warpage) of 10 μm or less. The reason is that it is necessary to keep the distance between the titanium substrate and the magnetic head constant.

Therefore, it is necessary to maintain the flatness secured by the hot straightening method after punching (for example, Japanese Patent Laid-Open No. 3-230814, etc.) until after the final finishing process. The residual stress on the surface layer of the disk may cause deterioration of the flatness of the disk, which is a serious problem. However, at present, no effective measures have been developed to prevent the deterioration of the flatness of the disk during processing.

Regarding the aluminum alloy magnetic disk substrate, a manufacturing method including a step of applying heat treatment during intermediate surface processing such as rough polishing is proposed in Japanese Patent Application Laid-Open No. 2-121118. The described method is to prevent the swelling of the magnetic film formed on the surface of the aluminum alloy substrate after the final mirror finish. It cannot be applied as a method for restoring the flatness of. The present invention has been made in view of such circumstances.

[0009]

DISCLOSURE OF THE INVENTION The present inventors first sought to find out the cause of the phenomenon that the deterioration of the flatness during the processing of the disk substrate, and as a result of various studies, as a result of the rough polishing, both surfaces of the surface of the disk substrate were It has been found that when the difference in residual stress generated exceeds a specific value, the flatness of the disk substrate secured by hot straightening deteriorates. There are primary rough polishing and secondary rough polishing in the rough polishing process, as a method for removing this residual stress, in the middle of the primary rough polishing and the secondary rough polishing, at an appropriate temperature, and at that temperature On the other hand, it was found that by performing a heat treatment for an appropriate time, the residual stress in the surface layer of the substrate can be released and the flatness obtained by hot correction can be reproduced. Furthermore, when continuing the secondary rough polishing of the substrate after this heat treatment, it was found that by limiting the processing load during polishing, it is possible to process without deteriorating the flatness recovered by the above heat treatment. It was

The specific means for releasing the above residual stress and restoring the flatness will be described below. (1) The invention of claim 1 is a method of manufacturing a titanium substrate for a magnetic disk, which comprises the following steps. (A) a step of preparing a titanium substrate that has been subjected to primary rough polishing; (b) a step of heat-treating the titanium substrate for a time defined by the following formula (however, the minimum time is 5 minutes); and t ≧ −0. 016T + 10.6 However, t: heating time (hour), T: heating temperature (° C) 3
It is 50-880 degreeC. (C) Secondary rough polishing of the heat-treated titanium substrate.

(2) The invention according to claim 2 is such that, in the secondary rough polishing step, the polishing load is controlled to perform polishing, and the residual stress difference between the surface layers of the titanium-made substrate is 50 MPa or less. The method for producing a titanium substrate for a magnetic disk as described above.

[0012]

[Function] The titanium substrate used as the material is a substrate that has been subjected to primary rough polishing. This substrate is a substrate that is roughly rough-polished by punching a doughnut-shaped disc from plate-shaped titanium, hot-correcting the strain and then performing precision cutting. The primary rough polishing step here is a grindstone with a small abrasive grain count (large abrasive grain size) for reducing the thickness of the disk substrate, or polishing by lapping with relatively high surface pressure using the above abrasive grains. It is a process.

The secondary rough polishing refers to a grinding stone lapping and lapping polishing process using a grain having a small grain size prior to the plating process. In addition, the finish polishing means polishing after plating the titanium substrate.

In the lapping polishing in the primary rough polishing, for example, a hard cast iron surface plate is subjected to a certain relatively high surface pressure, for example, 200 g / cm ² , and polishing is performed by using abrasive grains. Therefore, residual stress is generated on the substrate surface, and the flatness of the substrate is deteriorated by this residual stress. Therefore, in order to release the above-mentioned residual stress, heat treatment as described below is performed. If the temperature of this heat treatment is less than 350 ° C., a sufficient flatness recovery effect cannot be obtained.

On the other hand, the phase transformation (α → β) temperature of titanium is 8
Since the temperature is 82 ° C., when the heating temperature exceeds 880 ° C., the flatness is deteriorated accompanying the phase transformation, and the above object cannot be achieved. Therefore, the heat treatment temperature range is 350 to 8
The temperature is 80 ° C.

Next, the heat treatment time is within the range represented by the following formula from the viewpoint of sufficiently obtaining the effect of flatness recovery, and this range is shown in FIG. t ≧ −0.016T + 10.6 However, t: heating time (hour), T: heating temperature (° C.) 3
It is 50-880 degreeC. However, regarding the lower limit of the processing time, a minimum soaking time is required to be 5 minutes in order to make the temperature of the entire disk uniform. The upper limit is not particularly limited, but it is desirable to be within 20 hours in consideration of the effect of flatness recovery and cost.

Regarding the condition of the secondary rough polishing after the heat treatment, the difference in residual stress remaining on both surfaces of the disk after processing is 50 MPa so that the flatness recovered by the heat treatment is not deteriorated again. It is necessary to set the processing load as follows. As described above, this secondary polishing refers to a grindstone lapping and lapping polishing process using a small abrasive grain size prior to the plating treatment, but the processing load is such that the difference in residual stress is 50 MPa or less. The processing load for lapping is 100 g /
It is desirable to be less than or equal to cm ² .

Since the yield stress of pure titanium is 274 MPa, it is considered in principle that the disk is not deformed if the residual stress is less than this value, but it is actually less than the yield stress. However, if it shows a flatness value that does not satisfy the specifications of the disk substrate, it cannot be used as the disk substrate. Therefore, as a result of various studies on the residual stress and the flatness, if the difference in the residual stress on both surfaces of the disk after processing is 50 MPa or less regardless of the processing method, the above-mentioned flatness, that is, 10 μm or less per 100 mm can be achieved. .

[0019]

EXAMPLES Examples of the present invention will be shown below. Example 1 A disc was punched from a cold rolled plate of CP2 type pure titanium having a thickness of 0.51 mm, and a disc having a diameter of 1.89 inches and a final finished thickness of 0.381 mm was subjected to hot working at 600 ° C. for 5 hours. A flat plate having a flatness of 5 μm or less was used as a processing original plate. The original plate was reduced to 60 μm in thickness by lapping of primary rough polishing, and further polished to 40 μm by grinding with a grindstone. At this stage, the flatness is measured once, the discs are heat treated in an electric resistance heating furnace in the air, and after the heat treatment, polishing is performed to the extent that oxide scale on the disc surface layer is removed. The measurement was performed to examine the change in the flatness of the disk before and after the heat treatment.

Table 1 shows the results. In Table 1, x: when the flatness exceeds 5 μm, ◯: when the flatness is 5 μm or less. The flatness required for a 1.89-inch disk is usually 5 μm or less. Here, the flatness of the disk substrate after the primary rough polishing was almost 10 μm or more. From the results of Table 1, the flatness of the disk substrate is better as the temperature of the heat treatment is higher and the treatment time is longer. From this result, the range appropriate for the relationship between the temperature and time of the heat treatment is shown in FIG.
The range indicated by 1 is the range represented by the formula shown in claim 1.

[0021]

[Table 1]

Example 2 A disc was punched from a cold-rolled sheet of CP2 type pure titanium having a thickness of 0.51 mm to obtain a disc having a diameter of 1.89 inches and a final finished thickness of 0.381 mm, at 600 ° C. for 5 hours. Was subjected to hot straightening and the flatness of which was 5 μm or less was used as a processing original plate. A heat treatment was performed at 500 ° C. for 3 hours using the original plate having a thickness reduced by 60 μm by lapping of primary rough polishing, and then 10 μm was polished by lapping and grindstone polishing for secondary rough polishing. With respect to these discs, the processing strain remaining on the disc substrate was measured by the X-ray diffraction method (sin ² ψ-2θ diagram by the tilt method).

Specifically, in order to remove a portion of the outermost layer that interferes with the stress measurement, the sample is etched by about 5 μm with an appropriate etching solution, and then X
The residual stress on both sides of the disk was measured by the line diffraction method. The results are shown in Table 2. From Table 2, when the processing load during lapping exceeds 100 g / cm ² , the flatness of the disk after processing deteriorates.

On the other hand, in the case where the same steps up to lapping were carried out by grindstone polishing (Samples 8 to 12), the residual stress difference value and the flatness of the disk were almost the same as the values after lapping under the same conditions. It is the same, and it can be seen that a large residual stress difference that would deform the titanium substrate does not occur depending on the grinding stone polishing. This is because in the grindstone polishing, the abrasive grains are dispersed in a sponge and the polishing is performed with this sponge, so that the load applied to the titanium substrate is small. From Table 2, when the processing load in lapping was 100 g / cm ² or less, the flatness was 5 μm or less, which was good.

[0025]

[Table 2]

[0026]

EFFECTS OF THE INVENTION By using the method of the present invention, it is possible to eliminate the deformation of the disk generated during the primary rough polishing process, and obtain a titanium substrate for a hard disk having excellent flatness.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between temperature and time of heat treatment of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuichi Hirata 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Inventor Masaki Omura 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Date Main Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. A method of manufacturing a titanium substrate for a magnetic disk, which comprises the following steps. (A) a step of preparing a titanium substrate that has been subjected to primary rough polishing; (b) a step of heat-treating the titanium substrate for a time defined by the following formula (however, the minimum time is 5 minutes); and t ≧ −0. 016T + 10.6 However, t: heating time (hour), T: heating temperature (° C) 3
It is 50-880 degreeC. (C) Secondary rough polishing of the heat-treated titanium substrate. 2. The titanium substrate for a magnetic disk according to claim 1, wherein in the secondary rough polishing step, the polishing processing load is controlled to perform polishing so that the difference in surface residual stress between both surfaces of the titanium substrate is 50 MPa or less. Production method.