Low-thermal expansion cast steel with excellent machinability

Details Low-thermal expansion cast steel with excellent machinability Low-thermal expansion cast steel with excellent machinability

2000-07-07

2002-02-05

Ip, Sikyin (Department: 1742)

Metal treatment

Stock

Ferrous

C148S442000, C148S505000, C420S094000, C420S095000

Reexamination Certificate

active

06344095

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a low-thermal expansion cast steel having a high Ni content, more particularly to a low-thermal expansion cast steel having excellent machinability.
DESCRIPTION OF PRIOR ART
Recent development of industries in electronics and optics requires low-thermal expansion materials suffering from only small size variation due to thermal expansion and shrinkage by temperature changes near room temperature, for members constituting high-precision machine tools, high-precision measurement apparatuses, etc. To meet this demand, there have been an invar alloy of Fe—Ni
36
(by mass) having a linear thermal expansion coefficient of about 1.0×10
−6
/° C. near room temperature, and a super invar alloy of Fe—Ni
32
—Co
5
(by mass) having a linear thermal expansion coefficient of about 0.5×10
−6
/° C. near room temperature.
However, the above Fe—Ni alloys and Fe—Ni—Co alloys are relatively soft, poor in machinability. Accordingly, in conventional cast products, graphite is crystallized or precipitated mainly in an austenitic matrix structure, such that graphite exhibits lubrication effects between cutting tools and the work made of low-thermal expansion materials, thereby achieving good machinability. Typical examples are cast steel ASTM A-436, TYPE 5 and A-439, TYPE D-5 containing carbon at a cast iron level (2% by mass or more) for the crystallization of graphite, and cast steel described in Japanese Patent Laid-Open No. 63-162841, in which the carbon content is increased to 0.8 weight % to precipitate graphite.
Among the above conventional low-thermal expansion materials, cast steel of ASTM A-439, TYPE D-5 has drastically improved machinability because of the precipitation or crystallization of a large amount of graphite as a machinability-improving component. However, it has as large an average linear thermal expansion coefficient as 4.0×10
−6
/° C. or more in a range of 30-100° C. This is due to the fact that the cast steel has an increased level of micro-segregation of Ni serving to increase a thermal expansion coefficient because of the inclusion of about 2% by mass of C, and that the cast steel contains about 2% by mass of Si serving to increase a linear thermal expansion coefficient by 1.0×10
−6
/° C. per 1% by mass. In members constituting apparatuses required to have higher precision such as semiconductor production apparatuses and semiconductor test apparatuses, an average linear thermal expansion coefficient of less than 4.0×10
−6
/° C. is needed in a range of 30-100° C., making such low-thermal expansion cast iron as ASTM A-439, TYPE D-5 unsuitable therefor.
With respect to the cast steel disclosed by Japanese Patent Laid-Open No. 63-162841, it is suitable for members requiring high precision, because its average linear thermal expansion coefficient is 2.5×10
−6
/° C. or less. However, it is much poorer in machinability than the cast iron of ASTM A-439, TYPE D-5, because the amount of graphite as a machinability-improving inclusion is only about ⅓ as compared with the cast iron of ASTM A-439, TYPE D-5.
OBJECT OF THE INVENTION
Accordingly, an object of the present invention is to provide a low-thermal expansion cast steel having a low thermal expansion coefficient and excellent machinability.
SUMMARY OF THE INVENTION
To have an average linear thermal expansion coefficient of less than 4.0×10
−6
/° C. in a range of 30-100° C. and machinability not lower than that of the cast iron of ASTM A-439, TYPE D-5, the amounts of C and Si should be controlled to minimize increase in a thermal expansion coefficient, thereby increasing the amounts of machinability-improving inclusions. Here, there may be two or more types of machinability-improving inclusions. The machinability-improving inclusions may be MnS, MnSe, Pb, etc. in addition to the above graphite, though Se and Pb should be avoided because of strong toxicity, causing environmental contamination.
In view of this, the inventors have found that a low-thermal expansion cast steel having excellent machinability and a suppressed linear thermal expansion coefficient in a range of room temperature to 100° C. can be obtained by having an austenitic matrix structure in which both graphite and MnS having different functions to improve machinability are precipitated, and by minimizing the amount of elements dissolving in the matrix, which serve to increase a thermal expansion coefficient, thereby suppressing the micro-segregation of Ni. The present invention has been completed based on this finding.
Thus, the first low-thermal expansion cast steel with excellent machinability according to the present invention contains 0.3-0.9% by mass of C and 25-40% by mass of Ni, and having 0.5-3%, as an area ratio, of graphite and 0.02-0.3%, as an area ratio, of granular MnS in an austenitic matrix structure, whereby said cast steel has an average linear thermal expansion coefficient of less than 4.0×10
−6
/° C. in a range of room temperature to 100° C.
The second low-thermal expansion cast steel with excellent machinability according to the present invention contains 0.3-0.9% by mass of C and 25-40% by mass of Ni, and having 0.5-3%, as an area ratio, of graphite, 0.02-0.3%, as an area ratio, of granular MnS, and 10-700, per 1 mm
2
, of plate-like MnS having a length of 8 &mgr;m or more in an austenitic matrix structure, whereby said cast steel has an average linear thermal expansion coefficient of less than 4.0×10
−6
/° C. in a range of room temperature to 100°.
In a preferred embodiment of the present invention, the low-thermal expansion cast steel with excellent machinability has a chemical composition (by mass) comprising 0.3-0.9% of C, 1.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 25-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying S≦(1/4) Mn.
In another preferred embodiment of the present invention, the low-thermal expansion cast steel with excellent machinability has a chemical composition (by mass) comprising 0.3-0.9% of C, 1.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 25-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying (1/4) Mn<S≦(1/4) Mn+0.05.
In a further preferred embodiment of the present invention, the low-thermal expansion cast steel with excellent machinability has a chemical composition (by mass) comprising 0.4-0.8% of C, 0.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 30-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying S≦(1/4) Mn.
In a still further preferred embodiment of the present invention, the low-thermal expansion cast steel with excellent machinability has a chemical composition (by mass) comprising 0.4-0.8% of C, 0.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 30-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying (1/4) Mn<S≦(1/4) Mn+0.05.
The low-thermal expansion cast steel preferably contains 12% by mass or less, more preferably less than 4% by mass, of Co. It also preferably contains 4% by mass or less of Cr.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[1] Composition of Low-thermal Expansion Cast Steel
The low-thermal expansion cast steel of the present invention contains at least 0.3-0.9% by mass of C and 25-40% by mass of Ni.
In a preferred embodiment, the chemical composition (by mass) of the low-thermal expansion cast steel is 0.3-0.9% of C, 1.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 25-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying S≦(1/4) Mn.
In another preferred embodiment, the chemical composition (by mass) of the low-thermal expansion cast steel is 0.

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