The present invention relates to copper alloys and molds made of the copper alloys especially for use in continuous casting apparatuses.
Throughtout the specifications and claims, by the term "mold temperature" is meant the temperature at which the mold is used and the percentages used in connection with the alloy composition are all by weight.
Conventionally, deoxidized copper having a high thermal conductively has been widely used for the molds of continuous casting apparatuses. With the use of a large-sized continuous casting apparatus adapted for a high-speed and efficient operation, the mold has become more prone to a trouble such as deformation or wear when employed relatively few times for casting operation. Such deformation or wear of the mold impedes an improvement in the efficiency of the continuous casting apparatus.
In an attempt to overcome the foregoing problem, we have carried out various experiments and researches with the following finding.
The relationship between the solidification constant K (mm.min.sup..sup.-1/2) of steel and the thermal conductivity (Kcal/m.hr. .degree. C) of the mold is expressed by: EQU K = 22.9.lambda. .sup.0.036
The above equation indicates that the thermal conductivity of the mold exerts hardly any influence on the solidification constant of molten steel in the mold. Since the thermal conductivity of pure copper is 290 Kcal/m.hr. .degree. C, the solidification constant of steel within a mold made of pure copper is about 28. If the thermal conductively reduces to one half the above-mentioned value, the solidification constant is still about 27. Whereas it has generally been believed that the mold must be made of a highly heat-conductive material to promote solidification, the equation shows that the thermal conductivity need not be considered so critical.
The deoxidized copper mold conventionally used has a high thermal conductivity and is therefore subject to the trouble described, since deoxidized copper is not fully satisfactory in high-temperature characteristics. Inasmuch as the thermal conductivity does not exert a noticeable influence on the solidification constant, it is desired to provide a mold which is made of a material having a high softening temperature and great strength at high temperatures although the mold may have a lower thermal conductivity than deoxidized copper molds heretofore used extensively.
Our researches have revealed that the occurrence of trouble in the mold relates to the mold temperature as well as to the thermal stress attributable to that temperature. This invention has been accomplished through researches subsequently conducted on the relationship between the softening temperature of material of the mold and mold temperature and on the relationship between the high-temperature strength of the mold material and the internal thermal stress of the mold.