1. Field of the Invention
The present invention relates to a process of forging a steel, particularly steel articles having a complicated shape such as connecting rods and other load bearing parts used for the foot assembly of automobiles and construction equipment.
2. Description of the Related Art
The conventional processes for producing machine parts by forging steel include hot forging, warm forging, and cold forging. Small articles having a simple shape are produced by cold forging and large articles having a complicated shape are produced by hot forging. Warm forging is partially used for the high precision forming of stainless steel and other materials having a high resistance to deformation.
The recent trend of minimizing the weight of machine parts including those of automobiles necessitates steel materials with greater strength achieved by the addition of alloying elements in steel, resulting in an increased resistance to deformation under which a tool cannot stand. Moreover, a section modulus compensating for a reduction in stiffness due to weight reduction requires a complicated article shape causing a further reduction in the life of the tools used for forming thereof.
To solve this problem, it might be possible to reduce the resistance to deformation by using an elevated forging temperature higher than the conventional temperature of from 1000.degree. to 1250.degree. C., but this is not practically advantageous and is not actually used because the elevated temperature causes an intense oxidation of steel during heating and forging thereof with a resulting degradation in product yield, article precision and surface quality and because the formability of steel is not remarkably improved as expected because of a rapid drop of the material temperature when brought into contact with a forging die.
Such an elevated temperature forging is only reported on page 11 of "SEISAN-KENKYU (Study of Manufacture)", February, 1990, vol. 42, No. 2, published by Institute of Industrial Science, University of Tokyo, in which a cast iron is heated to a half-molten state and forged. The half-molten state enables a material which is otherwise unforgeable to be forged without the occurrence of cracking. A cast iron can be brought into a half-molten state by heating to about 1000.degree. C., which is not higher than normal temperatures used in forging of steels, and no particular measures are taken to control the heating condition and atmosphere for suppressing the oxidation and the working condition for improving the formability.
A steel has a melting point far higher than that of a cast iron and is not forged at a temperature close to the melting point thereof because of the above-mentioned problems.
A cast iron is, of course, not applicable as a material for strength parts or load bearing parts necessary for automobiles, etc.