Process for producing high strength shaft

Details Process for producing high strength shaft Process for producing high strength shaft

1999-10-15

2001-08-07

Ip, Sikyin (Department: 1742)

Metal treatment

Process of modifying or maintaining internal physical...

Heating or cooling of solid metal

C148S590000, C148S593000, C148S328000

Reexamination Certificate

active

06270596

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to a process for producing a boron-containing shaft having improved machinability, cold workability, inducing hardenability, and torsional strength, and more particularly to a process for producing a shaft, such as a drive shaft of automobiles.
BACKGROUND OF THE INVENTION
In order to realize a reduction in production cost and an improvement in fuel consumption, there is a demand for a reduction in weight of components through an improvement in steels corresponding to S40C (JIS (Japanese Industrial Standards)) which have hitherto been used as steels for shafts of automobiles. To this end, a further increase in strength is required of S40C.
For example, Japanese Patent Publication No. 32946/1996 discloses a boron steel having improved machinability and cold workability, wherein, as compared with S40C, the content of silicon and the content of manganese have been reduced and the hardenability has been compensated for by boron. This steel has a carbon content of not more than 0.45% by weight which is on the same level of the content of carbon in S40C. Therefore, this steel has not been intentionally attempted to increase the strength of S40C.
In order to increase the strength, it is necessary to further increase the carbon content. Studies conducted by the present inventors, however, have revealed that a carbon content of not less than 0.47% by weight causes transition of fracture mode from ductile fracture to brittle (intergranular) fracture in a torsional strength test, inhibiting the improvement of the torsional strength.
Thus, in order to increase the torsional strength, it is necessary to increase the carbon content. Increasing the carbon content, however, brings about transition of fracture mode from ductile fracture to brittle fracture, inhibiting the improvement of the torsional strength. Therefore, the brittle fracture should be inhibited in order to improve the torsional strength.
DISCLOSURE OF THE INVENTION
In this description, JIS grain size classifications are defined according to Table A below.
TABLE A
JIS Grain Size Classification
Grain
Number of
Average
Size
Crystal Grains
Sectional Area
Number
in Area of
of Crystal
(N)
1 mm
2
(n)
Grain (mm
2
)
−3  
1
1
−2  
2
0.5
−1  
4
0.25
0
8
0.125
1
16
0.0625
2
32
0.0312
3
64
0.0156
4
128
0.00781
5
256
0.00390
6
512
0.00195
7
1024
0.00098
8
2048
0.00049
9
4096
0.000244
10 
8192
0.000122
The number (n) of austenite crystal grains in a unit sectional area of 1 mm
2
is represented by n=2
N+3
, wherein N denotes grain size number. (Japanese Industrial Standard, Category No. G, 0551-1997.)
The present inventors have found additive elements, which are important for inhibiting the brittle fracture, particularly molybdenum, and the amount thereof added, while taking into consideration the machinability and cold workability.
Accordingly, it is an object of the present invention to provide a process for producing a shaft having improved strength without sacrificing the machinability and cold workability of S40C.
Thus, according to one aspect of the present invention, there is provided a process for producing a high strength shaft, comprising the steps of:
rolling or forging an alloy as a starting material at a heating temperature of AC
3
to 1050° C. with a reduction in area of not less than 30%, said alloy comprising by weight carbon: 0.47 to 0.55%, silicon: 0.03 to 0.15%, manganese: 0.20 to 0.50%, molybdenum: 0.08 to 0.30%, sulfur: 0.005 to 0.035%, boron: 0.0005 to 0.005%, titanium: 0.05 to 0.20%, nitrogen: not more than 0.01%, aluminum: 0.005 to 0.05%, and manganese+molybdenum: 0.45 to 0.70% with the balance consisting of iron and unavoidable impurities, thereby producing a steel product having a hardness after rolling or forging of 85 to 97 HRB; and
induction hardening the steel product to obtain a shaft having a hardening depth ratio (distance from the surface to a position of 500 HV/radius of component) of not less than 0.25 and an austenitic grain size number as specified in JIS G 0551 of not less than 7.
In order to inhibit the brittle fracture, 0.0005 to 0.005% of boron and 0.08 to 0.30% of molybdenum were added to improve the intergranular strength.
Further, 0.05 to 0.20% of titanium was added to finely precipitate titanium carbides or titanium carbonitrides in the steel. This reduced the ferrite grain size after rolling and inhibited the growth of austenitic grains in the course of quenching and tempering, such as induction quenching and tempering, thereby bringing the austenitic grain size number as specified in JIS to not less than 7. This contributed to the refinement of austenitic grains and improved intergranular strength. The addition of these additive elements in combination could inhibit the brittle fracture in a carbon content of 0.47 to 0.55%. Furthermore, in order to improve the machinability and cold workability, the contents of silicon and manganese, ferrite strengthening elements, were lowered. Further, the heating temperature in the rolling or forging was specified to finely precipitate titanium carbides or titanium carbonitrides, resulting in the refinement of austenitic grains at the time of rolling or forging to increase the percentage area of ferrite. These effects in combination brought the hardness after rolling or forging to 85 to 97 HRB and improved the machinability and cold workability.


REFERENCES:
patent: 5803993 (1998-09-01), Yoshida et al.
patent: 62-112727 (1987-05-01), None
patent: 10195589 (1998-07-01), None

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