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鍛造製品の特性高炭素クロム軸受鋼（ＳＵＪ２）の冷間鍛造

Cold Forging of High-carbon Chromium Bearing Steel (SUJ 2)
難加工材とされるＳＵＪ２を複雑形状、高精度に冷間鍛造で量産を可能にした技術を生産現場の経験から紹介する。
We succeeded in mass-producing the cold forgings of high-carbon chromium
bearing steel (SUJ 2), known as a hardly machinable material, by realizing
complex shape machining with high accuracy. The background is mainly
explained based on field experience.

株式会社タイショーテクノ
代表取締役社長
岡室 養子
Yoko Okamuro, TAISHO TECHNO INC.
大阪府和泉市テクノステージ１丁目５番２号1-5-2 Techno Stage,Izumi City,Osaka,594-1144
TEL : 0725-53-5580
FAX : 0725-53-5570
E-mail:info@taishotechno.co.jp
http://www.taishotechno.co.jp
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屋

全景工 場 内 設 備

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1.2

What is high-carbon chromium bearing steel?

JIS G 4805 prescribes a fully hardened steel with a chemical composition of about 1-percent carbon
and 1.5-percent chromium as high-carbon chromium bearing steel (SUJ).

SUJ stands for (S) steel, (U) use, and (J) journal.
This steel is a proven grade of steel since it has been used for more than half a century withlittle change in its basic composition.

For this special type of steel, JIS specifies the use of an electric furnace for manufacturingmolten steel, meaning that killed steel is subjected to electric vacuum degassing treatment.
This is done to uniformly distribute fine spherical chromium carbide in the structure.

SUJ 1 and 2 are used for small sizes of balls, rollers and races, and account for the most common
usage of SUJ. SUJ 3 is used for medium sizes. For large sizes, SUJ 4 and 5 are used. Molybdenum
is added to SUJ 4 and 5 for good hardenability.
ＰThis ensures appropriate use.2)3)4)

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2.

Process of cold forging of SUJ 2 materials from trial fabrication to automated production

2.1

First stage (1984 to 1986, start-up period)

In 1981, our company was using the SAE 5120 material as the blank to fabricate bearing innerraces as a pipe-shaped product through the following processes:

Bar material → Saw cutting → HAS (heat annealing spheroidizing) → B (lubricating surface
treatment: metal stearate soap coating over zinc phosphate conversion coating, commonly calledbonderite bondalube) → CF (cold forging) → P (piercing)

In 1984, bearing races were made using a SUJ 2 material as the blank. The cutting blank wasgenerally made from a bar material by hot forging with a large draft on the inner bore.
The cutting process proceeded from rough finishing → semi-finishing → finishing, but the yieldwas poor.

Ｐ ５／３４

In 1984, customers requested an improved yield by CF, a substantial reduction in the subsequentcutting process, shortened lead time, and improved productivity.

We searched related literature, surveyed the trade, and attended academic meetings regarding
cold forging of an inner race with a bottom using SUJ 2. However, we were unable to find any
clues to the mass production of SUJ 2. Thus, we had no choice but to try CF by putting a SUJ
2 material in the die, which had been used to form the SAE 5120 material. After several cycles,
the result was buckling of the counter punch, breakage of the upper punch at the base, large
variance in bottom thickness, and poor roundness of the product, thus revealing the poor
workability of SUJ 2 and making an ineffectual start.

Up until 1986, trial and error was repeated concerning die material (heat treatment,hardness,shape, dimensions) and blank shape (weight control). As a result, the die life was
graduallyprolonged and it became possible to forecast production (Table 1 and Figure 1).

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1) Product 1 (Photograph 1)

Product 1

Product 2
OD φ56.38 ID φ39.72H6.1

Photograph 1. Introduction of bearing inner race
(1984 to 2006)
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Table 1. Improvement of upper punch material and hardness
Before improvement
MaterialSKH-9Hardness60∼61 HRC

After improvement
SKH-5564∼65 HRC

Before improvement

After improvement

Figure 1. Improvement of upper punch land shapeＰ

８／３４

2.2 Second stage (1986 to 1996, production increase and improvement period)1) Manufacturing process of Product 3 (Photograph 2)

Top faceOD φ40.3 ID φ27

Rear face
H32.1 P:OD φ5 T9

Photograph 2. Product 3 (1987 to 2008)

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(1)

Selection of manufacturing process

Material

Lubricating
surfacetreatment

Barreltreatment

Heat
treatment

In giving priority to yield, eccentricity and die life, we chose manufacturing process 1(Figure 2). (In manufacturing process 2 shown in Figure 3, the swaging process may be omitted
if the yield effect is not expected or accuracy of the swaged outer diameter is not required.)Cutting

Material

Lubricating
surfacetreatment

Heat
treatment

Barreltreatment

Figure 2. Manufacturing process 1

Cutting

Figure 3. Manufacturing process 2

Ｐ１０／３４

(2) Improved accuracy of CF products
Flattening the bottom angle
Reducing the inner bottom corner radius
Stabilizing the entire length and shape of CF products To push the top face of the product,
the upper punch tip was made straight (Figures 4 and 5).

Before improvement

After improvement

Figure 4. Improvement of upper punch shape

Ｐ１１／３４Reducing eccentricity(0.2 mm → 0.03 mm)(A)

Blank weight control(B)

Outer diameter and thickness accuracy control of CF(C)

Preparation of the die and press so that the punch falls perpendicular to the dieswaging

Improvement of secondary fracture surface of P
(A)

Changing the punch tip shape(B)

Reviewing the drawing clearance(C)

Adding a drawing back pressure

Improved
location

Before improvement

Improved
location

After improvement

Figure 5. Improvement of working method

Ｐ１２／３４(3)Change in die life by heat treatment of die

Because the punch is exposed to high pressure, its life as a tool is shortened by breaking,buckling, cracking, chipping, scuffing and abrasion.
After trial and error was done to achieve such conflicting requirements as high rigidity,
high toughness and high abrasion resistance (Table 2), a compromise was reached. Figure 6
shows the upper punch life upon said compromise. 5）

Table 2. Quench hardness by materials
TypeStandard working Standard working Standard working
hardness by JIS hardness at Hitachi hardness at our(HRC)(HRC)
company (HRC)

JISDaidoHitachi

Koshuha

SKH55

MH55

YXM5HM3564 or more

62∼66

64∼65

SKH57

MH57

XVC5MV1066 or more

63∼67

65∼67

Matrix high-speed steel

⎯

MH85

YXR3

KDMV

⎯

58∼61

61∼62

Powder high-speed steel* Tempered three times

⎯

DEX40

HAP40

⎯

⎯

64∼67

64∼66

Molybdenum-based, high-speed steel

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Number of articles

Quenching conditions
Material

Quench hardness (HRC)

Figure 6. Upper punch life
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Improving the upper punch abrasion resistance
The upper punch was subjected to various kinds of surface treatment. As a result, TiC wasdeemed most suitable for SUJ 2, that is, no scuffing occurred and the abrasion limit wasextended (Figure 7). (The CF product inner dimensions were controlled at ±0.02 mm.)
Improving the die abrasion resistance

Number of articles

The die material was changed to cemented carbide (G3). As a result, the die life was extended
from 0.1 million to 1.3 million cycles. (The CF product outer dimensional accuracy wascontrolled to within ±0.02.)

TiC treatment

TiC treatment
1 9 9 2

1 9 9 3

1 9 9 4

1 9 9 5Figure 7. Change in upper punch life
Ｐ１５／３４

1 9 9 6

1 9 9 7

(4) Shortening the die change time and improving CF productivity
The die assembly was changed from the upper die adjusting type to the lower die adjusting
type (Figure 8). As a result, the die setting time was shortened and productivity wasimproved.
Slide surface

Slide surface
Slidepressure

Bolsterpressure

Bolster surface

Bolster surface

Before improvement
After improvement(upper die adjusting type)(lower die adjusting type)Figure 8. Die assembly drawing
Ｐ１６／３４(5)Stabilizing quality and shortening the measuring time

By ameliorating and improving the inner/outer run-out measuring jigs and dimension measuring
jig for inspection use, the incidence of nonconforming articles due to inspection mistakes
was reduced. The quality was stabilized and inspection time shortened.
(6)

Rationalization by improving HAS of blank SUJ 2

The blank heat treatment process was improved. The HAS hardness, which was around 93 HRB
before improvement, was stably reduced to 84 to 86 HRB after improvement. The carbides were
finely spheroidized and distributed uniformly (Figure 9). (The JIS provision for HAS hardnessof SUJ 2 is 94 HRB or less.)

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Number of articles

Hardness (HRB)

JIS standard
94 or less

1998

Figure 9. Change in HAS hardness

1999

2000

2001

2002

2003

2004

Figure 10. Upper punch life

Ｐ１７／３４2005

(7)Automation

Number of articles

These ameliorations and improvements made field manufacturing automation possible (Photograph3) and improved productivity (Figure 11).

Photograph 3. Automation

1983

1986

1990

1996

1998

(Single)(Single)(Single)(Single)(Automation)

Figure 11. Change in productivity (per hour)
Ｐ１８／３４

3.1 Thin-walled product
When a thin-walled product is quenched after machining, there is a high incidence of quench
bending, resulting in a high percentage of defective products. By making the residual stress
distribution as uniform as possible during the CF working, quench bending can be converselyreduced.
In terms of shape, the fine details of the die can be transferred to the product. By using thisadvantage, subsequent working can be substantially reduced. One problem with CF working wasthat lubricating surface treatment with conventional B resulted in poor slippage. Therefore,
the material fluidity is poor and the dimensional target can hardly be attained. The transferperformance consequently becomes poor. As a countermeasure, B was changed to BM (coating ofa mixture of graphite and molybdenum disulfide (MoS2)i) over zinc phosphate conversion coating).
(The popular name BMS refers to Bonderite MoS2.) Care must be taken because BM is more sensitive
to moisture than B and the duration of effect is short compared to B．
One advantage of the chip-less working of CF is that the CF finished skin can be used as is.
For this purpose, the surface finish conditions of the die and the lubricating surface treatment
conditions of the blank are important factors.
Ｐ１９／３４

Examples of thin-walled products of SUJ 2 material (Photographs 4 and 5).

Blank 3.3-mm thick → CF1 process → Finished productPhotograph 4. Product 4 (1996 to 2008)

Blank 3.3-mm thick → CF1 process
Photograph 5. Product 5 (1998 to 1999)
Ｐ２０／３４

1)Manufacturing process of Product 5(Figures 12 and 13)

Figure 12. Drawing of CF of Product 5

Figure 13. Product 5 in finished stateＰ２１／３４

(1)Change of manufacturing process

With manufacturing process table(cutting only) shown in Figure 14, the quench bending waslarge and the percentage of defectives abnormally high.
With manufacturing process table(CF + Cutting of end face only), there was little quench
bending and the desirable results were obtained.
As a cause for such little quench bending, the residual stress might be made uniform by CF
working.

Pipe

Cut-off

Cutting

PlateFigure 14.
Manufacturing process table

Lubricating
surfacetreatment

Heat
treatment

Regarding the accuracy per ball, that is, both roundness (Figure 16) and surface roughness(Figure 17), the desirable results were obtained and the limit of thin wall working of SUJ
material was extended.

Blank drawing

Figure 15. Manufacturing process table(cutting only)
Ｐ２２／３４(CF + Cutting of end face only)After CF
LS roundness results
Feature name
Measurement number

ScaleZ-height
Datum:Spindle
Filter type

After quenching
FilterShape

LS roundness results
Feature name

Measurement mode

Measurement number

Date ofmeasurement

Scale
Z-height
Datum:Spindle

FilterShape
Measurement mode

Filter type

Date ofmeasurement

Figure 16. Roundness data

Horizontal scale: 500.000 per one division
Horizontal magnification:
× 20.0–
Vertical scale: 500.000 per one division
Vertical magnification:
× 20.0–Shape start point:0.256 mm
Shape end point:3.833 mm
Note: All shape data are not printed out.

Horizontal scale: 500.000 per one division
Horizontal magnification:
× 20.0–
Vertical scale: 500.000 per one division
Vertical magnification:
× 20.0–Shape start point:
0.482 mm
Shape end point:4.084 mm
Note: All shape data are not printed out.

Figure 17. Surface roughness data

Ｐ２３／３４3.2

Products of deformed or complex shape

The surface finish of the CF die must be fine in order to use a product having an oval or irregular
inner shape as a commercial product without further processing. Not only that, the die assembly
should become complex for easy control and maintenance of the corner radius of the upper/lower
dies and punches.
It is desirable to improve the accuracy of the fracture surface by the last process of T (trimming)
or P. In T, the drawing burr allowance must be as thin as possible or continuously reduced.
In P, the CF worked surface must remain as much as possible. To avoid primary and secondary
sheared surfaces, and make a beautiful skin, contrivances are required of the corner radius andclearance of the drawing tool, holding down of the blank，

Ｐ２４／３４

Examples of deformed or complex shape products of
SUJ 2 in Products 6 to 13 (Photograph 6 to 11)

Top face
Cut surface
Top face
Cut surface
Product 6 OD φ29H30
Product 7 OD φ24.3 H24
Photograph 6. (1999 to 2008, SUJ 2)

Top face

Rear face

Top face

Cut surface

Photograph 7. Product 8
Photograph 8. Product 9(1999 to 2008, SAE 5120)(2003, Completed trial production of SUJ 2)(Products 8 and 9: OD 27.9 mm, H 30 mm, same size)
Ｐ２５／３４

Blank 4.3-mm thick → Product→Scrap

Photograph 9. Product 10 (2003 to 2008, SCM 415)

Blank 4.5-mm thick → Product → Scrap
Photograph 10. Product 11(2003, Completed trial production of SUJ 2)(Products 11 and 12 are the same size.)
Ｐ２６／３４

Viewed obliquely from above

Cut surface

Product 12

Cut surface
Product 13(2005 to 2008,SAE 5120)(2006, Completed trial production of SUJ 2)

Photograph 11. (Products 12 and 13: OD 38.55 mm, H 37 mm, same size)

Ｐ２７／３４

4.Future tasks for CF
Our company fabricates thin, long, small, deformed and complex shape articles, and gears andsprockets by cold forging on a net shape or chipless basis. The materials include SC material,
alloy steel, aluminum and duralumin. In the bearing industry, cutting and grinding have long
been done on proven manufacturing lines.

Consequently, the competition was so severe that theworkload is measured in units of seconds. Improving yield or reducing the cutting process alone
cannot solve the problem. The advent of breakthrough technology is being awaited, and suchtechnology will be achieved by a joint project that includes the blank, CF, cutting, lubricating
surface treatment and heat treatment.

Ｐ２８／３４Given the globalization of the manufacturing industry, the shift of mass production overseas
is naturally proceeding. To enhance the advantage of domestic production, high-grade products
must be produced with high accuracy and high quality by multi-product, small-lot production.
The necessary technology must be easy to transfer and completed as early as possible. Improving
shop productivity is yet another task.
When considering the distance from the blank to end product, CF working time can be shortenedcompared to cutting, grinding and buffing, depending on how CF is done. With CF, uniform andstable accuracy can be obtained, and surface roughness controlled. Thus, this process can beconsidered a promising working technology having high potential for a breakthrough.

Ｐ２９／３４

Therefore, three-dimensional simulation analysis, tribology (analysis of the mechanism of thecontact part between the die and workpiece) and CF working after heat treatment are importantbut remain lacking. Thus, we should challenge these technologies. Moreover, a technology wehave cultivated for many years is available to control the material flow during working. Theultimate concept of CF is “feel what the pressed material feels.” Taking this as the basic
stance, we should attain high quality, high speed and low cost, which will become increasingly
required from this point on.
In the field of CF manufacturing, staff should be trained in and master higher production
technology. At the same time, it is also important to foster skills, sense and feeling gainedfrom experience, then brush up and hand them over.

Ｐ３０／３４

Finally, Photograph 12 shows some new and old products that we have fabricated.

Photograph 12. Some new and old products

Ｐ３１／３４

5.Conclusion
The author, together with our staff, sincerely thank professor Takashi Ishikawa (of NagoyaUniversity) for giving us this opportunity, Mr. Shuzo Matsumoto (former senior executive director
at Shimano, Inc. and former manager of Shimano Laboratory) for rearing us in a strict manner
regarding forging, Mr. Kazuo Shima (former executive director at former Koyo Seiko and former
board chairman at DAIBEA Co., Ltd.) for giving us the opportunity to encounter SUJ 2, and ourcustomers for always giving us subjects.

Ｐ３２／３４References1)

Hideaki Kudo: 45th Lecture of the Japan Society for Technology of Plasticity “Fundamentals
and practice of forging working” (July 19 to 21, 1989, Osaka)

2)

Seizo Nagasaki et al.: Dictionary of Material Name [Second Edition], AGNE Gijutsu Center(February 10, 2006)

3)

Metal Material Manual, Taiga Shuppan Co., Ltd. (May 15, 2005)

4)

Eiichi Watabayasi and Hisayoshi Tahara: Bearing Story, Japanese Standards Association(November 28, 2003)

5)

The Japan Society for Heat Treatment: Heat Treatment Guidebook, Taiga Shuppan Co., Ltd.
(January 27, 2005)

6)

Toshio Sakurai: Tribology Library 1, Second Edition, “Physical Chemistry of Lubrication,
new edition,” Saiwai Shobo (March 30, 2004)

Ｐ３３／３４今後共、ご指導ご鞭撻の程よろしくお願い申し上げます。
ご清聴ありがとうございました。

２００８年

６月２０日

株式会社タイショーテクノ
代表取締役社長

Ｐ３４／３４岡室養子