RU2225885C2 - Method of production of graphitized conversion foundry pig iron - Google Patents

Abstract

FIELD: metallurgy; modification of conversion pig iron intended for re- melting in foundry in production of castings for metallurgy and mechanical engineering. SUBSTANCE: proposed method includes introduction of ferro-silicon in form of powder wire into molten iron before and in the course of pouring to depth no less than 1.5 m at temperature of 1330-1400 C; its specific consumption is set depending on content of carbon and silicon in cast iron. Proposed method makes it possible to update technology of production of graphitized conversion pig iron containing lesser than 0.5% of manganese due to correction of beyond-furnace treatment in pig casting for obtaining graphitized structure in pigs and retaining this effect at repeated re-melting. EFFECT: avoidance of chilling of pig iron; improved physico- mechanical properties; possibility of making effect on positive inheritance of pig iron. 3 tbl

Description

The invention relates to the field of metallurgy and can be used for the modification (graphitization) of pig iron, intended for smelting in iron foundries in the production of metallurgical and engineering castings.

A known method for the production of modified pig iron by introducing crushed ferrosilicon into molten cast iron on the trough when discharged from a blast furnace (see Mishin PP, Kosheleva SA, Petrov GB and others. Production of high-quality cast iron for mechanical engineering .- M.: Metallurgy, 1983, 40 p.). However, this method is inefficient, because in this case, the effect of graphitization of cast iron is not preserved either in pig iron or during subsequent re-melting. In this case, during repeated remelting, cast iron behaves similarly to a conventional blast furnace or cast iron, retaining a number of negative hereditary properties, in particular, a tendency to bleach, which makes it in some cases unsuitable for use in the production of castings, especially thin-walled and chill molds. For this reason, this method is used in practice only for alloying cast iron with silicon, i.e. for synthetic cast iron.

Closest to the proposed one, which we adopted as a prototype, is a method for the production of graphitized foundry grade cast iron, which includes introducing ferrosilicon into liquid cast iron before and during casting process (Russian patent No. 2093586, class C 21 C 1/10, publ. 20.10 .1997).

According to this method, the flow rate of ferrosilicon introduced into molten iron for its graphitization is established depending only on the content of silicon and manganese in the original cast iron.

However, this method is ineffective in graphitizing high-carbon low-manganese pig iron containing less than 0.5% manganese, because in this case, carbon has a more significant effect on the graphitization of cast iron, along with silicon, and the optimal consumption of the graphitizer is determined by the carbon equivalent.

For this reason, the processing according to the known method of the above pig iron does not provide stable values of the mechanical properties and microstructure of cast iron (in particular, the shape and nature of the distribution of graphite) after graphitizing treatment. Cementite appears in the structure of pig iron pigs, which is the reason for the tendency to bleach cast iron during re-melting. This is a significant disadvantage of the known method.

The basis of the present invention is the technical task to improve the method of production of graphitized pig iron of the foundry class by adjusting the technology of out-of-furnace treatment of cast iron by graphitizing additives during casting in pigs in order to obtain a graphitized structure in pig iron and to preserve this effect in cast iron when it is re-melted. This will make it possible to completely eliminate the bleaching of cast iron and increase its physical and mechanical properties, i.e. affect the positive heredity of cast iron.

The problem is solved in that by a known method for the production of graphitized foundry grade cast iron, which includes introducing ferrosilicon into liquid cast iron before and during casting into pigs, ferrosilicon is introduced under the metal level at a temperature of cast iron of 1330 ... 1400 ° C, and its flow rate is set depending on the content of carbon and silicon in cast iron from the ratio

where 0.125 is a constant empirical coefficient, kg / t;

K is the consumption of ferrosilicon, kg / t of pig iron;

m is the silicon content in ferrosilicon,%;

[s], [Si] - respectively, the carbon and silicon content in cast iron,%.

A causal relationship between the totality of all the essential features of the invention and the achieved technical result is that only the use of all the claimed features of the technological process and the indicated graphitizer flow ratios will allow to obtain a graphitized structure in the pig and to preserve this effect in cast iron when it is re-melted, which is confirmed the research below.

Providing a graphitized structure in pig iron of low-manganese pig iron and maintaining this positive effect in pig iron during its re-melting is achieved by introducing a graphitizer (ferrosilicon) under the metal level at a given depth, differentiated specific consumption of graphitizer for processing depending on the carbon and silicon content in cast iron and optimum temperature of cast iron before processing.

To work out the optimal parameters of the method, special experiments were carried out in laboratory and industrial conditions.

The low-manganese blast-furnace pig iron with the carbon content of 4.0 ... 4.8%, silicon 0.50 ... 1.0% and manganese 0.25 ... 0.45% was subjected to graphitizing treatment. Testing was carried out in ladles, the mass of pig iron in the ladle was 1.2 ... 1.5 tons and industrial 65 ... 75 tons during laboratory experiments. To maintain the effect of graphitization, pig iron was processed immediately before and during casting. FSi 75 grade ferrosilicon was used as a graphitizing modifier. It was introduced into cast iron in the form of flux-cored wire by means of a tribing apparatus or in crushed form from a feed hopper. The flux-cored wire was introduced under the metal level to the maximum possible depth. Crushed ferrosilicon was fed to the surface of cast iron in a ladle or to a stream of metal during casting into ingots.

Under laboratory conditions, only the method of introducing a graphitizer into molten iron was worked out. The results are shown in table. 1.

As follows from the data table. 1. The maximum positive effect of graphitization of cast iron is achieved only when ferrosilicon is introduced below the metal level. When a graphitizer is fed to the surface of cast iron in a ladle or to a stream of metal, the degree of absorption of silicon is unstable and low, and the effect of graphitization of cast iron is practically absent. This is evidenced by the high level of technological samples with whitening (92%).

Later, when working out the optimal parameters of the technological process in industrial conditions, a graphitizer (ferrosilicon) was introduced into cast iron in the form of cored wire under the metal level in the ladle to a depth of at least 1.5 m.

The specific consumption of ferrosilicon for processing was varied within 1.7 ... 2.4 kg per ton of cast iron. Cast iron was processed at its temperature in the range of 1330 ... 1420 ° C.

During casting of pig iron and after re-melting of graphitized cast iron, standard wedge samples were taken for bleaching and samples for mechanical tests. Re-melting of graphitized cast iron was carried out in an induction electric furnace. The obtained experimental data are given in table. 2.

As follows from the data given in table. 2, the greatest suspicious effect (minimal bleaching, higher mechanical properties of cast iron, lower specific consumption of graphitizer) is achieved only when implementing the method within the parameters of the claimed invention (experiments 1 ... 6).

When the temperature of the treated cast iron is above the upper limit (above 1400 ° C), the tendency of cast iron to bleach increases, and the mechanical properties deteriorate (experiments 7 ... 9).

Cast iron with a temperature below 1330 ° C was not processed, because this worsens the conditions of the technological process of casting iron into ingots, the surface quality of ingots is removed.

With a specific consumption of graphitizer (ferrosilicon) for processing less than the calculated value in accordance with the claims, the effect of graphitization of cast iron is not achieved (experiments 10 ... 15). With a greater consumption of ferrosilicon for processing, the positive effect of graphitization of cast iron does not increase, i.e. positive hereditary properties of cast iron during repeated remelting do not improve. However, in this case, the specific consumption of ferrosilicon increases and the economic indicators of the process deteriorate (experiments 16 ... 21).

According to the proposed method in an industrial environment produced more than 1,500 tons of pigmented low-manganese graphitized cast iron. In the table. 3 shows the main indicators of production of an industrial batch of cast iron according to the proposed and known methods.

Given in the table. 3 data show that the implementation of the proposed method for the production of graphitized pig iron of the foundry class, ceteris paribus, almost completely eliminates its tendency to bleach. provides stable and higher mechanical properties at a lower specific consumption of the modifier, which improves the economic performance of the technology. The use of graphitized cast iron produced by the proposed method provides high quality castings, including thin-walled.

Claims (1)

A method for the production of graphitized pig iron of a foundry grade containing less than 0.5% manganese, comprising introducing ferrosilicon in the form of cored wire into molten iron before and during casting to the metal level in the ladle at a given temperature, characterized in that the ferrosilicon is introduced to a depth of not less than 1.5 m at a temperature of cast iron (1330 ÷ 1400) ° C and set its specific consumption depending on the carbon and silicon content in cast iron from the ratio

where K is the consumption of ferrosilicon, kg / t of pig iron; m is the silicon content in ferrosilicon,%; [C], [Si] - respectively, the carbon and silicon content in cast iron,%; 0.125 - constant empirical coefficient, kg / t.