GB2043110A - Deoxidising steel - Google Patents

Abstract

The present invention provides an alloy for the deoxidation of steel, comprising 1 to 10% by weight of calcium and 3 to 20% by weight of silicon, the remainder being aluminium and the usual impurities. The present invention also provides a method of deoxidising steel, by the use of the above alloy, or by the use of an equivalent composition consisting of a mixture of aluminium powder and particulate calcium- silicon.

Description

SPECIFICATION An alloy for the aftertreatment of steel The present invention is concerned with an alloy for the after-treatment of steel.

In addition to silicon and manganese, aluminium is today the most important deoxidation agent used in the production of steel. The present invention is concerned with an improvement of deoxidation aluminium by an alloying addition.

The oxygen contained in steel resulting from the production thereof is effectively bound with aluminium and the steel is, in this manner, killed, i.e. gassing out and liquating phenomena during solidification are avoided.

Furthermore, aluminium binds dissolved nitrogen. These properties of the aluminium are utilised especially in the production of skilled and age-resistant steels, as well as for the production of certain controlled precipitates in the structure, an increased hardness being brought about by these precipitates. Furthermore, they exhibit the grain growth at high temperatures and thus serve for the production of fine-grained steels with increased notch impact strength and improved low temperature behaviour.

However, deoxidation of steel with aluminium suffers from certain disadvantages. The product of the reaction of the aluminium with oxygen, i.e. aluminium oxide, does not melt even at the highest temperatures which occur in the production of steel and frequently a striated arrangement results. In the case of working the steel, for example, by rolling, the oxide accummulations are spread out and impair the mechanical properties of the steel, especially transversely to the direction of working. A further disadvantage of the oxide inclusions produced by normal aluminium deoxidation is their great hardness. Consequently, when working up steels which have been deoxidised with aluminium by cutting processes, these oxide inclusions result in an undesirable wear of the work tools employed.

Processes are admittedly known for overcoming the above-described disadvantages in a subsequent treatment. In addition to other processes, for example, a scavenging gas treatment with or without the use of a vacuum, especially a treatment with alkaline earth metal-containing compounds has a modifying effect on the aluminium oxide inclusions. The aluminium oxide is, by a chemical reaction with the alkaline earth metal oxides formed, in which other oxides can also participate, for example silicon oxide, converted into a substantially harmless form. The compounds formed are liquid at the temperature of steel melt. Consequently, they combine to form quite large droplets and float on the surface of the melt.Any oxides possibly remaining in the liquid steel have, in the cold state, a round shape which, in the case of subsequent working, do not extend longitudinally, a directional dependence of the mechanical properties in the end product thus being avoided. These inclusion-modifying properties, especially of calcium silicon as a source of alkaline earth metal, are known. They are the main reason for the use of calcium silicon in lump or powder form in the production of steel.

Since, in the case of these methods, the calcium silicon is introduced into the melt at a different place and time from the aluminium, it is difficult to ensure that the calcium acts uniformly in all parts of the melt.

Therefore, there is a need to provide an alloy with which the advantages of the main components, aluminium and calcium, can best be utilised.

For the deoxidation treatment of steel, use is made of calcium silicon and of multicomponent alloys with aluminium contents of up to 60% by weight. Commercially available materials contain 7 to 30% by weight of calcium, 24 to 58% by weight of silicon and 10 to 56% by weight of aluminium, which represent comparatively large amounts of calcium and silicon. In the case of these multicomponent alloys, the aluminium and calcium both act simultaneously at the same places of the melt. However, they suffer from the disadvantage of high calcium and silicon contents and of a laborious and expensive production by melting together the components.

Therefore, it is an object of the present invention to overcome the disadvantages of the alloys previously used for the treatment of steel by providing an alloy with a novel composition. A further object of the present invention is to use alloys with a novel composition for the deoxidation of steel.

Thus, according to the present invention, there is provided an alloy for the deoxidation of steel, comprising aluminium and calcium silicon in an amount such that the content of calcium is from 1 to 10% by weight and the content of silicon is from 3 to 20% by weight.

The present invention also provides process for the deoxidation of steel, wherein there is used an alloy containing 1 to 10% by weight of calcium and 3 to 20% by weight of silicon, the remainder being aluminium and the usual impurities which are due to the production process used.

On the basis of previous experience, it was not to have been expected that, by means of an alloy of the kind according to the present invention, the aluminium oxide inclusions contained in steel could be effectively modified. It was, therefore, surprising that even such small additions as 1 to 10% by weight calcium and 2 to 20% by weight silicon to deoxidation aluminium suffice in order to convert the aluminium oxide inclusions into a form which is harmless for the steel. Thus, on the basis of known melt diagrams, in the case of the given amount ratio of calcium oxide, aluminium oxide and/or silicon oxide, no liquid compounds should result.However, experiments have shown that the oxide content of a steel which has been treated with a deoxidation aluminium to which, according to the present invention, there has been alloyed only about 2% by weight of calcium and about 6% by weight of silicon, is already decisively improved. A preferred composition of the alloy according to the present invention for the treatment of steel contains 2 to 6% by weight of calcium and 4 to 15% by weight of silicon, the remainder being aluminium.

Furthermore, the alloy according to the present invention has the advantage that it is simple to produce. Thus, for example, calcium silicon can be alloyed directly with aluminium present in a molten state, an additional energy-consuming working step for the melting of these two components thereby being unnecessary. The alloy according to the present invention containing 1 to 10% by weight of calcium and 3 to 20% by weight silicon, the remainder being aluminium, can be cast in the usual manner. Thus, the casting of pigs and of spheroidal or drop-shaped pieces, such as are usual for deoxidation aluminium, can readily be carried out. It merely transpired that a somewhat elevated casting temperature impaired the casting. The handling of the calcium silicon used also raises no special requirements for care on the part of the melter.A particular advantage of the new alloy according to the present invention is that the calcium introduced only burns away very slowly from the alloy and its calcium content is only slightly reduced, even after several hours.

Apart from the mentioned pigs and spheroidal and drop-shaped particles, the alloy according to the present invention can also be used to produce granules, wires, rods or powder for the treatment of steel. In the case of the production of powder for injection or blowing in processes, the composition according to the present invention can be produced either by pulverising the alloy according to the present invention or by mixing aluminium powder with ground calcium silicon.

The following Examples are given for the purpose of illustrating the present invention: Example 1 A series of 1 8 melts of a heat-treatable steel, 41 Cr4, such as is used, for example, for drop-cast parts for motor vehicles, was treated in each case in 70 tonne capacity pans with 0.7 kg./tonne of an alloy containing 1.8% calcium and 6% silicon, the remainder being aluminium, instead of with the conventionally used deoxidation aluminium. A substantially improved degree of oxide purification was achieved in comparison with the values usually obtained with aluminium alone. The K4 value determined according to the Stahl-Eisen Prüfblatt VDEh 1570/71 was, in the case of 93% of the test samples, below 10.

Example 2 6 Melts of a work steel, 55NiCrMoV6, such as is used, for example, for the construction of pressure moulds and which contained about 0.02 to 0.03% by weight of sulphur to facilitate workability, was subjected to a blowin treatment in a 80 to 100 tonne capacity pan. In this case, 0.6 kg./tonne of a mixture of 85% by weight aluminium and 15% by weight finely ground calcium silicon was blown into the steel melt with the use of argon. After the treatment, the sulphur content had only decreased by 0.003% by weight. However, an ususually low content of oxide impurities was ascertained, the K4 values of all samples, according to Stahl-Eisen Prüfblatt VDEk 1570/71 being below 10.

Treatment with pure calcium silicon powder also gave a high degree of oxide purity but the calcium silicon also removed the desired sulphur content of the steel. Deoxidation with aluminium alone, however, resulted in substantially higher contents of oxide impurities than in the case of the use of the alloy according to the present invention.

Claims (10)

1. An alloy for the deoxidation of steel, comprising 1 to 10% by weight of calcium and 3 to 20% by weight of silicon, the remainder being aluminium and the usual impurities.

2. An alloy according to claim 1, comprising 2 to 6% by weight of calcium and 4 to 15% by weight of silicon, the remainder being aluminium and the usual impurities.

3. An alloy according to claim 2, comprising about 2% by weight of calcium and about 6% by weight of silicon, the remainder being aluminium and the usual impurities.

4. An alloy according to claim 1 for the deoxidation of steel, substantially as hereinbefore described and exemplified.

5. A method of deoxidising steel, wherein an alloy according to any of claims 1 to 4 is introduced into a steel melt.

6. A method according to claim 5, wherein the alloy is introduced into a steel melt in the form of pigs, spheroidal or dropshaped particles, granules, wires, rods or powder.

7. A modification of the method according to claim 5, wherein a mixture of aluminium powder and of ground calcium silicon is used instead of the alloy.

8. A method of deoxidising steel according to any of claims 5 to 7, substantially as hereinbefore described and exemplified.

9. Steel, whenever deoxidised with an alloy according to any of claims 1 to 4.

10. Steel, whenever deoxidised by the method according to any of claims 5 to 8.