Manufacturing Process of Vermicular Graphic Cast-Irons Through Double Modification

This invention relates to a process for manufacturing vermicular graphite cast-iron by a double modification. Processes for manufacturing vermicular graphite cast-irons are already known. One process modifies the liquid cast-iron in a single phase with modifying alloys which contain both compactizing (Mg, Ce) and anticompactizing (Ti, Al) elements for the graphite. A second known process modifies the iron in a single step with rare-earth elements. A third process is to treat liquid cast iron with metallic zirconium or with an alloy containing zirconium and magnesium.

These processes have the disadvantage that the alloy or modifying element ratio must be adjusted depending on the initial sulphur content of the liquid cast-iron, the modifying temperature and the holding time in the liquid state after modifying, and that inconsistent vermicular graphite ratios in the cast-iron structure result and thus the mechanical characteristics of the poured cast iron are not constant.

The process for manufacturing vermicular graphite cast-irons through double modification, according to this invention, uses in the first phase of modification an alloy of the Fe Si Mg Ti Al Ce type, which is introduced into the bottom of a pocket ladle where castiron having a temperature of about 1500.degree. C. and a sulphur percentage ranging between 0.01-0.04% is poured, removes the disadvantages mentioned above, that is, in order to obtain some cast-irons having in their structure a minimum of 85% of vermicular graphite and high mechanical characteristics. The first modification of the liquid cast-iron is done by adding 1.1% modifying alloy, followed by a differentiated thermal analysis in order to determine the graphite shape obtained, after which the second modification is done with an addition of 0.25% anticompactizing modifier if the graphite's preponderant shape after the first modification is nodular; usual neutral modifier if the respective graphite shape is vermicular or compactizing modifier if the graphite shape is flaked, after which the obtained cast-iron is poured into parts.

Here is an example of carrying out the invention: A certain quantity of cast-iron was melted in an induction furnace up to a temperature of 1500.degree. C. The liquid cast-iron thus prepared was poured into a pocket ladle of the type of those used in modifying cast-irons by the Sandwich process. Beforehand, the following were put into the pocket: 1.1% modifying alloy with the following chemical composition: 51.8% Si; 2.69% Al; 2.05% Ca; 4.64% Mg; 4.97% Ti; 0.3% Mischmetall, the rest Fe. After completion of the modifying reaction, a sample was taken for the differentiated thermal analysis. The form of the differentiated cooling curve showed that the amount of the nodular graphite in the structure is of 42% and that of the vermicular graphite is of 58% So, the cast-iron had a vermicular graphite content that is too small which should be of a minimum of 85% and a nodular graphite content that was too high. As a consequence, we conducted an anticompactizing post-modification, that is 0.25% of modifying alloy with a chemical composition of 48.4% Si; 0.8% Ca; 8% Ti was introduced after which the cast-iron was poured into parts. The tests made, showed that a vermicular graphite cast-iron with 92% of vermicular graphite in the structure, the rest being nodular graphite having the following mechanical characteristics: tensile strength of 392 N/mm.sup.2 ; elongation 6.4%; hardness 149 HB. If a compactizing post-modification is required instead, a modifying alloy having: 0.5-2% Mg; 0.5-2% Ce; 40-90% Si, and the rest Fe is used in the same conditions. A particularly preferred modifying alloy for anticompacting modification has 1 to 15% Ti; 40 to 90% Si and the rest Fe.

The present invention has the following advantages:

liquid cast-irons can be used, having a sulphur content ranging between 0.01 and 0.04% at a temperature ranging between 1350.degree. and 1520.degree. C.;

vermicular graphite cast-irons are obtained with a minimum of 85% of vermicular graphite in the structure, with high mechanical characteristics, constant and reproducible;

the possible time of waiting after the first modification can be increased before pouring the cast-iron into parts, using a compactizing modifier, added before pouring; and

the invention allows correction of the graphite shape before pouring the cast-iron into parts, thus reducing considerably the waste ratio, a fact which leads to significant metal savings.