US20090246065A1

US20090246065A1 - Alloy, shaft made therefrom, and motor with shaft

Info

Publication number: US20090246065A1
Application number: US12/255,890
Authority: US
Inventors: Hiroshi Yamada
Original assignee: Minebea Co Ltd
Current assignee: Minebea Co Ltd
Priority date: 2008-03-26
Filing date: 2008-10-22
Publication date: 2009-10-01
Also published as: JP2009256765A

Abstract

A nonmagnetic austenitic stainless steel consists of, all by weight %, 1.5 to 3.5% of Cu, 8.5 to 9.5% of Mn, 0.18 to 0.22% of C, 0.5 to 1.0% of Si, 16.5 to 17.5% of Cr, 0.15 to 0.2% of N, 0.13 to 0.3% of S, and the balance of Fe and inevitable impurities.

Description

This application claims priority under 35 U.S.C. §119 to Japanese Patent Application Nos. JP2008-079670 filed on Mar. 26, 2008, and JP2008-173649 filed on Jul. 2, 2008, the entire contents of which are hereby incorporated by references.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an alloy that may be used as a material for the shafts of motors, for example. Specifically, the present invention relates to a nonmagnetic austenitic stainless steel having a superior machinability. The nonmagnetic austenitic stainless steel may be suitably used as a shaft material for spindle motors of hard disk drives of personal computers, fan motors, motors incorporated in various home electric appliances and precision apparatuses, and the like. Moreover, the present invention relates to a shaft made of the austenitic stainless steel and relates to a motor using the shaft, such as stepping motors.
2. Description of the Related Art
A nonmagnetic stainless steel having an austenitic structure is conventionally used as a shaft material for motors so that a shaft is not magnetized by a magnet of a motor and thereby maintaining the rotation performance. SUS303Cu is a typical example of the nonmagnetic stainless steel and is superior in corrosion resistance and machinability, thereby being suitably used as a shaft material. However, SUS303Cu includes Ni in which the price has been rising recently, whereby use of SUS303Cu increases the production cost. An austenitic stainless steel disclosed in Japanese Examined Patent Publication No. 54-20444 may be known as an alternative to SUS303Cu. A high-manganese steel disclosed in Japanese Examined Patent Publication No. 56-8096 and Japanese Unexamined Patent Application Laid-open No. 7-126809 may be mentioned as a steel material having a nonmagnetic characteristic other than austenitic stainless steels. This material has high work hardening and is inferior in machinability.
For ordinary steel materials, Pb and S are publicly known as components for improving machinability, but Pb is not preferable from an environmental point of view. In adding S to a high-manganese steel, MnS is formed, and the machinability is thereby improved. A nonmagnetic austenitic stainless steel, which may be suitably used for motor shafts and needs not have expensive Ni added thereto, has not yet been sufficiently investigated. Accordingly, it was necessary to investigate an austenitic stainless steel having a nonmagnetic characteristic among steel materials as an alternative to SUS303Cu.

SUMMARY OF THE INVENTION

In view of the above circumstances, it is an object of the present invention to provide a nonmagnetic austenitic stainless steel that may be suitably used as an alloy for motor shafts. The nonmagnetic austenitic stainless steel is formed so as to have a nonmagnetic characteristic, a superior machinability, and an appropriate hardness, without using an expensive Ni and without using Pb that is effective for improving the machinability. Accordingly, the nonmagnetic austenitic stainless steel has high durability, and an increase in production cost can be prevented. Moreover, it is another object of the present invention to provide a motor shaft made of the alloy and to provide a motor using the shaft, such as stepping motors.
The inventors have conducted intensive research on development of an alloy so as to achieve the above objects, and the inventors have discovered the following. In producing a nonmagnetic austenitic stainless steel, by adding appropriate amounts of Mn and S to a certain amount of Cu and also adding appropriate amounts of C, Si, Cr, and N to Cu, the stainless steel has a nonmagnetic characteristic and machinability that is improved. The present invention provides a nonmagnetic austenitic stainless steel based on this, and the nonmagnetic austenitic stainless steel consists of, all by weight %, 1.5 to 3.5% of Cu, 8.5 to 9.5% of Mn, 0.18 to 0.22% of C, 0.5 to 1.0% of Si, 16.5 to 17.5% of Cr, 0.15 to 0.2% of N, 0.13 to 0.3% of S, and the balance of Fe and inevitable impurities.
Hereinafter, bases of contents of the chemical components (elements) of the present invention will be described. It should be noted that the following symbol “%” indicates a weight %. In the present invention, the balance other than the following components consists of Fe and inevitable impurities.

C: 0.18 to 0.22%

C is a useful austenite-forming element and is effective for improving the strength of a steel, but C may cause a decrease in the corrosion resistance when the content of C is large. In addition, the content of C must be selected in view of workability and machinability that are required of a stainless steel. According to these points of view, the lower limit of the content of C is set so as to reliably improve the strength, and the upper limit of the content of C is set so as to secure the corrosion resistance and obtain the workability and the machinability that are required.

Mn: 8.5 to 9.5%

Mn is used as an alternative to Ni and is an element that produces a nonmagnetic austenitic structure, and Mn is the most important element for maintaining a low magnetic permeability. In order to obtain a stable austenitic structure, the content of Mn is set to be relatively large in the present invention. According to Table 1 and FIG. 1, the content of Mn must be not less than 8.5% so that an alloy stably exhibits a nonmagnetic characteristic when the magnetic permeability is not more than 1.01. As shown in Table 1, 3, and 4, when the content of Mn is more than 9.5%, spot rusting occurs, the corrosion resistance is decreased, and the amount of work hardening is greater than those in stainless steels including Mn at 9.5% or less. In addition, although Mn is less expensive than Ni, the content of Mn is desirably not more than 9.5% so as to reduce the production cost.

Cu: 1.5 to 3.5%

Addition of Cu is effective for decreasing the magnetic permeability, and the lowest content for efficiently obtaining the effect of Cu is 1.5%. Since further effects of Cu are not obtained even when the content of Cu is 3.5% or more, the upper limit of the content of Cu is set to be 3.5% so that cracks do not occur during hot rolling.

Si: 0.5 to 1.0%

Si is an element necessary as a deoxidizer agent, specifically, as a deoxidizer agent used in refining, but causes decreases in the corrosion resistance and cold workability as the content of Si increases. Therefore, the lower limit of the content of Si is set to be 0.5% in which the effect of Si as a deoxidizer agent is obtained, and the upper limit of the content of Si is set to be 1.0% so as to ensure the corrosion resistance and the cold workability.

Cr: 16.5 to 17.5%

Cr is an element necessary for forming a passivation film so as to improve the corrosion resistance, and Cr is a useful ferrite-forming element. When the content of Cr is less than 16.5%, a stainless steel tends to have insufficient corrosion resistance. On the other hand, when the content of Cr is more than 17.5%, a stainless steel may have an unstable austenitic phase. Accordingly, the content of Cr is set to be 16.5 to 17.5%.

N: 0.15 to 0.2%

N is a useful austenite-forming element and increases the hardness of a stainless steel. In order to add N to an alloy, the alloy is melted in a melting furnace under a nitrogen atmosphere, and N can be included in the alloy at approximately 0.15 to 0.20% in the melting furnace under an ordinary atmospheric pressure. When the content of N is less than 0.15%, an austenite is not sufficiently stabilized, and when the content of N is more than 0.2%, blowholes may occur. In order to add N to an alloy at more than 0.2% under an ordinary atmosphere, a special apparatus is required, and the production cost is thereby increased. Therefore, the content of N is set to be 0.15 to 0.2% in the present invention.

S: 0.13% to 0.3%

S forms sulfides with elements such as Mn, and cutting resistance is decreased by the sulfides that are disposed in a steel by dispersing. When the content of S is more than 0.3%, mechanical strength and hot workability of a stainless steel are decreased. When the content of S is less than 0.13%, the cutting resistance is not effectively decreased. Therefore, the content of S is set to be 0.13 to 0.3%.
The above elements are essential components of the alloy of the present invention, but the following component besides the above elements may be included in the alloy of the present invention.

P: Not More Than 0.045%

When the content of P is high, intergranular segregation may occur, whereby the corrosion resistance, the workability, and the toughness are decreased. In this case, when the content of P is more than 0.045%, the corrosion resistance, the workability, and the toughness are extremely decreased. Therefore, the content of P is set to be not more than 0.045%.
The above elements are components related to a nonmagnetic austenitic stainless steel for motors of the present invention, and this alloy can be obtained by mixing the components and melting them, for example. When a wire rod material or a bar material is commercially produced as a raw material for shafts, a steel material melted in a melting furnace is passed through several steps such as slabbing, hot-rolling, cold-rolling, annealing, and pickling, whereby the wire rod material or the bar material may be shipped at a reduction of approximately 0 to 20%. Therefore, it is required that the magnetic permeability μ be not more than 1.01 even when the reduction is changed. The present invention provides a motor shaft produced from the alloy, and a motor using the shaft, such as stepping motors.
According to the present invention, by adding appropriate amounts of Mn and S to a certain amount of Cu and also adding appropriate amounts of C, Si, Cr, and N thereto, an austenitic structure having a nonmagnetic characteristic can be reliably obtained without adding Ni. Moreover, by adding an appropriate amount of S to Cu, the machinability can be improved. Furthermore, by using Mn, which is lower in cost, instead of using Ni, the production cost of the nonmagnetic austenitic stainless steel can be decreased.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a Schaeffler diagram.

EXAMPLES

The present invention will hereinafter be illustrated by way of the preferred embodiments.
According to the Schaeffler diagram shown in FIG. 1, addition amount of each alloy component such as C, Cr, and Mn was investigated so that an alloy is formed with an austenitic single phase. Each sample, having chemical components (by weight %) shown in Table 1, was melted in a vacuum melting furnace, and samples Nos. 1 to 5, in which each of the samples weighed 20 kg, were obtained. Each sample Nos. 1 to 5 included Fe and inevitable impurities as balance besides the chemical components shown in Table 1.

TABLE 1

Chemical components of samples (by weight %)

	C	Si	Mn	P	S	Cr	Cu	N

No. 1	0.21	0.61	8.53	0.001	0.124	16.78	0.95	0.165
No. 2	0.21	0.61	8.8	0.002	0.138	16.91	1.93	0.172
No. 3	0.21	0.61	8.89	0.004	0.174	16.93	2.97	0.173
No. 4	0.21	0.63	8.08	0.002	0.174	17.13	1.93	0.166
No. 5	0.21	0.63	10.11	0.002	0.181	16.93	1.95	0.161
No. 6	0.22	0.63	11.14	0.002	0.172	16.85	1.95	0.165

Table 1 shows samples Nos. 2 and 3 that are examples of the present invention and shows samples Nos. 1 and 4 to 6 that are comparative examples that are not examples of the present invention. The sample No. 1 was formed by adding 8.53% of Mn to 0.95% of Cu. The sample No. 2 was formed by adding 8.8% of Mn to 1.93% of Cu. The sample No. 3 was formed by adding 8.89% of Mn to 2.97% of Cu, and the addition amount of Cu was increased. The sample No. 4 was formed by adding 8.08% of Mn to 1.93% of Cu. The sample No. 5 was formed by adding 10.11% of Mn to 1.95% of Cu, and the addition amount of Mn was increased. The sample No. 6 was formed by adding 11.14% of Mn to 1.95% of Cu, and the addition amount of Mn was increased.
The material of the present invention was developed based on the following four points.
(a) The magnetic permeability μ being not more than 1.01 even when the material is processed at a reduction of 20%.
(b) Adding N to the material so as to stabilize an austenite.
(c) Adding Cu and Mn to the material so as to avoid an increase in the magnetic permeability.
(d) Adding S to the material so as to improve the machinability.
In the examples Nos. 2 and 3 of the present invention and the comparative examples Nos. 1, 4, 5, and 6, change in the magnetic permeability according to the reduction, and a relationship between the reduction and the hardness, were measured respectively. The results are shown in Tables 2 and 3. In this case, the reduction is a percentage of a value calculated by dividing the difference between a cross section of a raw material before processing and a cross section of the raw material after the processing by the cross section of the raw material before the processing. For example, when a bar material having a cross section of 100 cm²is formed by drawing process, and a bar material having a cross section of 80 cm²is thereby obtained, the reduction is 20%. The raw materials of the samples were formed by drawing process so as to decrease the diameter thereof, and the reduction was varied from 5% to 20% in increments of 5%.

TABLE 2

Relationship between reduction (5%, 10%, 15%,
and 20%) and magnetic permeability

	5%	10%	15%	20%

No. 1	1.03	1.15	1.5	2.5
No. 2	<1.01	<1.01	<1.01	<1.01
No. 3	<1.01	<1.01	<1.01	<1.01
No. 4	<1.01	1.03	1.04	1.1
No. 5	<1.01	<1.01	<1.01	<1.01
No. 6	<1.01	<1.01	<1.01	<1.01

TABLE 3

Relationship between reduction (0%, 5%, 10%,
and 20%) and hardeness (HV)

	0%	5%	10%	15%	20%

No. 1	219	247	270	310	340
No. 2	212	248	276	320	345
No. 3	213	246	272	315	332
No. 4	216	238	258	290	330
No. 5	204	255	288	322	350
No. 6	222	267	315	330	370

A salt spray test prescribed by Japanese Industrial Standard JIS Z2371 was performed on each of the samples Nos. 1 to 6, and the results are shown in Table 4.

TABLE 4

Results of salt spray test

No. 1	No. 2	No. 3	No. 4	No. 5	No. 6

Rust did	Rust did	Rust did	Rust did	Spot	Spot
not occur	not occur	not occur	not occur	rusting	rusting
				occurred	occurred

According to Tables 1 and 2, in the sample No. 1 including Cu at less than 1.5%, the magnetic permeability μ was increased to not less than 1.01 as the reduction was increased. In the sample No. 4 including Cu at more than 1.5% and including Mn at not more than 8.5%, the magnetic permeability μ was increased to not less than 1.01 as the reduction was increased. On the other hand, in the samples Nos. 2, 3, 5, and 6, in which the addition amount of Cu was 1.5 to 3.5% and the addition amount of Mn was more than 8.5%, the magnetic permeability μ was less than 1.01, which is the same as the magnetic permeability of SUS303Cu, even when the reduction was 20%. According to the results of the salt spray tests shown in Table 4, when the addition amount of Mn was 10.11% or more, spot rusting occurred, and the corrosion resistance was deteriorated. According to Table 3, when the addition amount of Mn was 10.11% or more, the amount of the work hardening was extremely large, compared to the samples including Mn at 8.5 to 9.5%.
As described above, when a wire rod material or a bar material is commercially produced as a raw material for shafts, a steel material melted in a melting furnace is passed through several steps such as slabbing, hot-rolling, cold-rolling, annealing, and pickling, whereby the wire rod material or the bar material may be shipped at a reduction of approximately 0 to 20%. Therefore, it is required that the magnetic permeability μ be not more than 1.01 even when the reduction is changed.
Next, in order to investigate the machinability of the examples Nos. 2 and 3 of the present invention and the comparative examples Nos. 1, 4, 5, and 6, cutting resistance of each sample was measured by lathe processing using a superhard coating chip. At that time, the primary component force was measured, and the values are shown in Table 5. In the lathe processing, a tool for a high-manganese steel and a tool for a stainless steel were used. As shown in Table 5, the samples Nos. 5 and 6 exhibited a large primary component force, because the amount of the work hardening was great when the addition amount of Mn was not less than 9.5%. On the other hand, the samples Nos. 1 to 4 exhibited a higher primary component force than that of SUS303Cu, but the degree of the primary component force thereof does not cause serious problems in cutting. If S is added to a steel material, the S forms sulfide (MnS) by reacting with Mn, and the sulfide is dispersed into the steel material, whereby the cutting resistance is deceased, and the machinability is improved. In the samples Nos. 2 and 3, the machinability was good with respect to each chip, whereby a chip can be easily selected for processing.

TABLE 5

Cutting resistance (primary component force)

	Primary component force
	(N) using a tool for high-	Primary component force (N)
	manganese steel	using a tool for stainless steel

No. 1	108	110
No. 2	110	111
No. 3	105	109
No. 4	99	102
No. 5	126	128
No. 6	138	140
SUS303Cu	84	86

According to Tables 1 and 2, in the nonmagnetic austenitic stainless steel of the present invention, the addition amount of Cu is set to be not less than 1.5% so as to decrease the magnetic permeability and is set to be not more than 3.5% so that cracks do not occur during hot rolling. According to Tables 3 and 5, the amount of Mn is preferably decreased in view of the machinability, but the steel does not have a nonmagnetic characteristic when the amount of Mn is decreased to approximately 7%. In addition, in view of the corrosion resistance shown in Table 4, the amount of Mn is set to be 8.5 to 9.5%. N is an austenite-stabilizing element, and the amount of N is preferably increased, but the amount of N is set to be 0.15 to 0.2% in view of the limit of dissolution from the air. In view of the amount of Mn and N, C is set to be 0.18 to 0.22%, Cr is set to be 16.5 to 17.5%, and Si is set to be 0.5 to 1.0%, so that the amounts thereof are within the austenitic region of the Schaeffler diagram shown in FIG. 1. The addition amount of S is set to be not less than 0.13 % so as to form MnS and is set to be not more than 0.3% so that cracks do not occur during hot rolling. The addition amount of P is set to be not more than 0.045% so that the steel is not embrittled by intergranular segregation of P.
The nonmagnetic austenitic stainless steel of the present invention is preferably used for shafts of ordinary motors. Specifically, a stepping motor in the motors has been reduced in size recently, and an extremely compact stepping motor having a diameter of 3 mm or 6 mm has been proposed, for example. Since this kind of motor has a very small shaft, the workability of the shaft is very important, and good machinability is thereby required. In addition, since the small shaft must have a sufficient strength, the material of the shaft is required to have an appropriate hardness. The nonmagnetic austenitic stainless steel of the present invention has a nonmagnetic characteristic and is superior in machinability and strength, and the nonmagnetic austenitic stainless steel is less expensive than a conventional material. Accordingly, the nonmagnetic austenitic stainless steel is preferably used for shaft of stepping motors that will be commercially produced. The application of the alloy of the present invention is not limited to shafts of stepping motors, and the alloy of the present invention can be used as a nonmagnetic steel for processing rotating shafts of the other various precision electronics.

Claims

1. A nonmagnetic austenitic stainless steel consisting of, all by weight %, 1.5 to 3.5% of Cu, 8.5 to 9.5% of Mn, 0.18 to 0.22% of C, 0.5 to 1.0% of Si, 16.5 to 17.5% of Cr, 0.15 to 0.2% of N, 0.13 to 0.3% of S, and the balance of Fe and inevitable impurities.

2. A shaft for a motor made of said nonmagnetic austenitic stainless steel recited in claim 1.

3. A motor using said shaft recited in claim 2.

4. The motor according to claim 3, wherein said motor is used as a stepping motor.