As known to date, cast iron has been in popular use as the basic material for industry. This is because the cast iron has such advantages as excelling in castability, allowing formation of multiple kinds of complicatedly shaped articles, readily yielding to cutting and similar machining works, incurring rather low expenses in procurement of raw materials and execution of melting work, and enjoying ease of manufacture even at a factory of a small scale.
Recently, the electronic industry and the optical industry have advanced to a point where the machine tool, measuring devices, molding dies, and other manufacturing machines which are associated with these industries demand materials of increasingly high accuracy and function. For the purpose of answering this demand, the necessity for materials which are capable of lowering thermal expansion coefficient and repressing thermal deformation to the fullest possible extent besides keeping the characteristic properties of the conventional materials intact is growing all the more in profundity. As metallic materials of low thermal expansion coefficients, an about 36%Ni--Fe alloy (Invar alloy) and an about 30%Ni--5%Co--Fe alloy (Super Invar alloy) which are shown in Table 1-1 and Table 1-2 are known. They have not yet been fully tamed for the utmost use. This is because they are unfortunately deficient in cutting workability, and castability. In recent years, the materials which are obtained by treating the Invar and the Super Invar alloy so as to impart the quality of cast iron thereto and vest them with improved cutting workability and enhanced castability and which, therefore, are relieved of the drawback mentioned above have been attracting growing attention. Table 1-1 and Table 1-2 also show the low-expansion cast iron which has been known as Niresist D5 for a long time, Nobinite cast iron as one example of the low-expansion cast irons developed in the last several years, and the cast iron disclosed in JP-A-62-268,249.
The materials shown in Table 1-1 and Table 1-2, however, are either alloys which have not induced separation of graphite by crystallization or nodular graphite cast irons and mainly have an austenitic structure as a base matrix and, therefore, have tensile strength in the range of from 40 to 55 kgf/mm.sup.2 and Brinell hardness in the neighborhood of HB 120. Where the graphitic structure is formed of graphite flakes or pseudonodular graphite particles, the tensile strength is still lower in the approximate range of from 25 to 35 kgf/mm.sup.2 and the Brinell hardness in the neighborhood of HB 100. When they are applied to such parts as are required to have high accuracy, therefore, the produced parts often pose problems of deformations of various sorts due to insufficient strength. Owing to the softness, they find only limited applications to such sliding parts as are in need of resistance to abrasion.
Besides the materials cited above, JP-A-61-177,356 discloses a low thermal expansion high-nickel content austenite graphite cast iron of the shape of vermicular, JP-A-02-298,236 an alloy having low thermal expansion at a relatively high temperature, JP-A-64-55,364 a low thermal expansion cast iron endowed with improved strength by a heat treatment, JP-B-01-36,548 a low thermal expansion alloy incorporating Ni, Co, V, and Nb therein, JP-A-02-70,040 a low thermal expansion alloy endowed with improved strength by a solid solution treatment, and JP-A-63-433 a graphite cast iron of the shape of vermicular. None of them satisfies both high strength and low expansion; some of them are deficient in strength and others in low expansion.
It is further known that low expansion cast irons having a graphite structure generally incur conspicuous Ni segregation and, because of the liability to have a low Ni concentration in the gap of the Dendrite phase, produce a part deviating from the Invar composition and suffer from deficiency in low expansion as compared with the Invar alloy and the Super Invar alloy which form no graphite. Generally, this problem of Ni segregation is solved by the method of subjecting the low expansion cast iron to a solid solution treatment at a temperature in the range of from 750.degree. C. to 950.degree. C. and then to rapid cooling. This method, however, entails the problem of causing the heat-treated cast iron to deform. Particularly in the case of a low thermal expansion cast iron, since it is an alloy of high Ni content, it has low thermal conductivity as compared with ordinary cast iron and, when hardened in water or oil, shows a large difference in cooling speed between the surface layer and the inner part of a shaped part of the low expansion cast iron and consequently gives rise to a large stress of heat treatment. Thus, the shaped part is destined to retain residual stress if not suffered to induce growth of deformation. Further, since this residual stress is liberated during the course of mechanical fabrication or with the elapse of time, the shaped part of the low expansion cast iron brings about degradation of shape or dimensional accuracy. As a result, it has been necessary for the heat-treated cast iron to undergo a protracted heat treatment which is adapted for the relief of strain.
In association with the recent trend of the products of cast iron toward growth in size and complication in shape, the present inventors have taken notice of the fact that the heat treatment which is given after the step of casting inevitably impairs the reliability of the products. It has been ascertained to them, for example, that the heat treatment which has brought about a favorable effect on the conventional surface plate having 55 cm in diameter and 40 mm in thickness brings about an unfavorable effect of impairing the flatness of surface of a surface plate having 1 m in diameter and 40 mm in thickness.
In the case of such products as are complicated in shape, since they have been fabricated heretofore by machining, the strain which is generated by stress within these products in the process of fabrication has likewise posed a problem. To be relieved of this strain, these products must undergo a time-consuming strain-relieving heat treatment. By reason of the complicatedness of this heat treatment, the desirability of cast products manufacturable without requiring the heat treatment has been finding popular approval. The improvement which is attained in the low expansion property by the heat treatment (particularly for rapid cooling) possibly exerts an adverse effect on the improvement of the dimensional accuracy which constitutes the primary object of the heat treatment. Thus, in the case of the products of complicated shapes which have been heretofore manufactured by machining because the strain-relieving heat treatment applicable thereto is unduly intricate, the desirability of obtaining these products by casting without entailing development of strain due to stress has been finding approval. The cast products, therefore, are desired to retain their inherently low expansibility as cast as much as possible.