US2137945A - Process for producing titanium steel - Google Patents

Description

Patented Nov. 22, 1938 UNITED STATES PATENT OFFICE PROCESS FOR PRODUCING TITANIUM STEEL Walther Mathesius, Berli'n-Nikolassec, Germany No Drawing. Original application March 29,

1935, Serial No. 13,757. Divided and this application January 22, 1937, Serial No. 121,907. In Germany April 7, 1934 4 Claims.

has now been found by extensive experiment that a foundation material of the best steel properties is formed if silicon is practically excluded as a steel producer and the carbon present is united to titanium in the form of carbide (TiC). such a foundation steel, both by suitable choice of the carbon content as Well as by alloying of known steel-improving metals or non-metals, highly valuable steels can be obtained, all derived from a foundation steel which is practically free from silicon and contains carbon only in the form of titanium carbide.

No such foundation material has yet been produced by the numerous known proposals for adding titanium to iron as a steel former.

A process is already known for making a carbon-free titanium steel which consists in fining or decarburizing carbon-containing iron under a slag layer or cover of lime and titanium ironstone and adding to the bath after the completion of the fining as much aluminium as appears necessary for the reduction of the quantity of titanium from the slag cover required in the final product. what high expenditure in aluminium since the reducing action of the same is, besides the in tended reduction of titanic acid, not limited in addition to the iron and manganese oxides present iri the slag as well but extends also to the silicic acid, so that a silicon-free foundation'material in the sense mentioned above could not be obtained.

Experiments have now shown that a slag which is .poor in silicic acid, consisting of about '70 per cent T102 and about 30 per cent C210 is sufiiciently thinlyliquid at a temperature of 1450- 1500 C. in order withits. aid to be able to carry out a steel fusion process in the Siemens-Martin furnace, and that at this high titanic acid con: centration the quantities of titanium necessary for the formation of titanium steel enter into the steel bath even without the use of aluminium.

A slag of aproximately this composition is obtained when an ordinary steel bath is decarburized in the Siemens-Martin furnace or electric furnace with a suitable, best briquetted, mixture From This known process requires a some-' of titanium ironstone and lime as soon as a state of equilibrium is set up between the steel bath and the slag.

The fact that in the course of such a metallurgical process certain amounts of FeO and S102 are unavoidably incorporated in the slag as well raises the thin liquidity of the latter and consequently in general its utility.

It has further been observed that in a steel bath the affinity of titanium for nitrogen, sulphur, and oxygen is greaterv than for carbon and iron and any other alloy constituents the con sequence of which is that when alloying titanium in a steel bath first of all the titanium compounds of the three said non-metals are always formed (they pass into the slag) before titanium can appear in the steel as iron titanide or as free alloy constituent.

However, it also follows from these affinity relationships that after the saturation of nitrogen, sulphur and oxygen by titanium only titanium carbides can in addition be present in the steel if the ratio of titanium to carbon corresponds at least to that of the formula TiC. If the titanium content is smaller there remain in the steel, in addition to titanium carbides, also iron carbides, by the simultaneous presence of which the properties of the steel are then influenced.

The afiinity of silicon and phosphorus for titanium in an iron melt is, according to the results of researches carried out hitherto, so small that the existence of compounds between these bodies has not yet been proved.

On the other hand practical observation has taught that the solubility of titaniumcarbides silicon and still more in the presence of aluminium in the melt.

. From these experimental results arises the possibility of making a series of hitherto unknown steels in practical steel works operation,

present then the iron carbide still present in addition to titanium carbide will unfavorably infiuence the steel bath, chiefly in the way of influence on texture.

The properties of carbon steels and of titanium v in steel is strongly diminished in the presence of y steels are decisively influenced by the difl'erent behaviour of iron carbides and of titanium carbides in iron alloys. Both carbides are soluble in iron to a certain extent, the degree of their solubility depending on the other alloy constituents. a

The most outstanding difierence in their behaviour consists in the known pearlite formation in the case of carbon steels, which does not appear in the case of titanium steel. Every slowly cooled carbon steel, therefore, consists at least of two constituents diifering considerably from one another both chemically and physically,

ferrite and cementite, whereas titanium steel is in this sense a unitary single substance consisting merely of iron containing certain quantities of titanium carbides in solution, corresponding to the solution of iron carbide in iron in the case of hardened carbon steel.

The physical properties of slowly cooled titanium steel, therefore, correspond extensively to those of hardened carbon steel, e. g. the position of the elasticity limit at 80 to 90 per cent of the strength. Slowly cooled titanium steel cannot be made soft by a heating or annealing operation, but it can be hardened if its content of titanium carbide is greater than corresponds to the natural solubility of titanium carbide in iron. Its hardening temperature lies at about 1000 C., and it is noteworthy that such a titanium steel abruptly chilled in water at this temperature shows in breaking tests, in addition to a considerable increase in strength, also an extension of about 10 per cent.

The special steels obtained from this foundation steel by alloying known improving metals have, in addition to their specific properties, the

outstanding fundamental properties of titanium carbide steel, such, as for example, little tendency to form pipes, freedom from segregation, resistance to wear and others, which are attributable to the very fine and homogeneous texture of the titanium carbide steel. The following are examples of steels according to this invention tanium carbides in making the granule finer exceeds the contrary action of the iron phosphides.

Steels for metal sheets for deep drawing can be made particularly soft by addition of titanium if by a measured content of silicon and aluminium the solvent capability of the iron for titanium carbides is suitably diminished.

This action is made useful to a still greater degree for the purpose of extensively diminishing the hysteresis in the case of transformer stampings. It scarcely requires emphasis that for all these reasons the titanium steel is pro-eminently suited for the'production of large forged pieces and of high value steel castings.

The introduction of titanium into iron alloys has been attempted for many years. The results, however, have always been unsatisfactory since when covering iron baths with oxidic slags, or slags which are rich in silicic acid, a perfectly irregular titanium loss unavoidably occurred which appeared within the limits of 50 to 100' per cent and apart from the question of economy made impossible the production of steels with an accurately determinable titanium content.

The limitation of the loss to a small amount and the consequent regulated and economical production of titanium steel is possible however if, as above described, the iron baths are covered with a slag which is poor in silicic acid and which consists essentially of titanic acid and lime. There 'are various ways available in practical steel works operation of attaining this aim.

The lining operation can be carried out in a Martin furnace or an electric furnace merely with the use of lime and titanium ironstone which is poor in silicic acid. Then, particularly, if the charge possessed a higher manganese content and the quantity of ore with respect to the bath has been so regulated that with the production of the desired carbon content the iron oxides of the slag were consumed as well, a reduction of the titanic acid from the slag will already set in as a result of the high temperature prevailing in the furnace. This action can be suitable for particular purposes or having parraised by the addition of petrol coke, oil coke, or ticular properties: the like to the slag, or carbon can be added at the gggg Ti Special constituents Hardenable steels 0.3 and above. 1.5 and above. }All steel-improylng Structural steels 0.l0.2 metals. Antlcorrosion steels Below 0.l About 0.5"... 0.51% P- Stcels for metal sheets for Below 0.l About 0.5... About 0.3% Si and deep drawing. about 0.1% It]. Steels for transformer Below 0.l About 0.5"... About 4% S1 and stampings. about 0.1-0.5% Al.

In the case of all these steels the union of nitrogen, sulphur and carbon with titanium produces a great increase in the quality.

In the case of hardenable steels the increase in the wear strength and a temperature resistiviity far exceeding that of carbon steels before any considerable letting down of the hardness takes place is worthy of note. By addition of titanium the strength of the structural steels may be increased up to about 90 kg. (with about 80 kg. elasticity limit). Further strengthincreases may be attained by means of known alloy metals without the formation of hard separate carbides being able to occur, since the carbon is as good as completely bound to titanium.

The anticorrosion steels may be given a content of 0.5-1 per cent of phosphorus without cold brittleness occurring, since the action of the tistart to the slag-forming mixture of titanium ironstoneand lime, or briquettes may be formed of these three constituents.

However, the fining process can also be car'- ried out wholly or partially in a Martin furnace or a converter in the usual manner without effecting deoxidation, then separating the bath from the oxidic slag by tapping off or emptying the converter, and subjecting it to an after treatment in an electric furnace in which a slag of titanium ironstone and li'me; from which the iron oxides have been removed by reduction, has been made liquid.

In this case also, titanium can be transferred into the bath either by reduction of the titanic acid from the slag or the desired titanium content can be produced by alloying ferrotitanium.

Of particular importance is the introduction taining raw materials. As is well known, in

Bessemer converters, in the acid Siemens-Martin furnace or in electric furnaces lined with acidic V blocks only iron with a minimum phosphorus content can be worked up since the simultaneous presence of phosphorus and carbon in the iron produces a coarse granule and such material possesses the property of cold brittleness.

It has now been found that this drawback dis-'- appears if a titanium carbide content is given to the phosphorus-containing iron baths, e. g.- according to the processes as described above. It has been found that a. sufficient content of titanium or titanium carbide checks the unfavorable action of the phosphorus and permits steels to be made containing carbon,-titanium and phopshorus which, in spite of the phosphorus content, are highly valuable and have very specific properties.

A scientific explanation for these facts can apparently be found in the following:

As is well known, titanium possesses a very high aflinity for combining with carbon. It is probable, therefore, that 'with i a sufiicient titanium content as compared with carbon and also sulphur and nitrogen, all the carbon present in the iron is combined as titanium carbide of the formula TiC.

Now, titanium carbides possess only a moderate solubility in iron. The titanium carbides dissolved or suspended in the fused state, because of the high temperature beyond the prevailing solubility limits must be separated out relatively early, therefore in consequence of their high fusion temperture. It is'to be assumed that these fine crystals which appear in masses at definite temperatures form crystallization nuclei uniformly distributed in the setting steel and provide the cause of the formation of a fine granule. The iron phosphides setting only at a considerably lower temperature are. now no longer able to set in cohering larger crystals and thus to bring about the formation of a coarse granule.

' The union of the carbon with titanium also raises the solubility of the iron phosphides in.

ferrite to a considerable extent. A uniformity or homogeneity of the alloying of a phosphoruscontaining steel is thus obtained which is not attainable in any other way.

It is probable that this is why such steels have a very fine texture in spite of their phosphorus content.

Such steels with, forexample, 0.5-1 per centphosphorus are very resistant to corrosion. On

account of their high phosphorus contenttheir sion steels with 0.1-0.2 per cent 0, 0.5-1 per cent Ti, 0.5-1 percent P. In both varieties of steel the elasticity limits lie at about 75 per cent of the strength which is proof of the fact that in them the ferrite has been hardened to a 'con-' siderable extent by alloy constituents present in solution with maintenance of an extension of' 25-30 per cent. A particular advantage of the invention lies in this that the process, as it has been described above, enables highly valuable varieties. of steel to be made from phosphoruscontainingraw material in'a furnace with acidic slag supply and acidic stone material.

The known steel-improving additions may be made to both varieties of steel, for example, in order to produce a further raising in strength or other special properties.

What I claim is:--

1. A method of producing titanium steels consisting in covering an iron bath with a slag which is as free as possible from silicic acid and contains titanic acid and lime in the proportion of :30, and out of this, by reduction with carbon, introducing titanium into the iron bath in a quantity which is at least sufficient to remove the nitrogen, sulphur and oxygen content in the bath into the slag in the form of titanium compounds and then to convert all the carbon of the iron phorus-containing iron bath with a layer of slag.

which is as free as possible of silicic acid and contains titanic acidand lime in the proportion of 70:30, and out of this, by reduction with carbon,

introducing titanium into the iron bath in a quantity which is at least sufficient to remove the nitrogen, sulphur and oxygen content in the bath into the slag in the form of titanium compounds and then to convert all the carbon of the iron bath into titanium carbide corresponding to the formula TiC, the titanium content or the resulting steel bath being 0.5 to 1%. n

4. A method of producing titanium steels for deep drawing sheet and transformer stampings consisting in covering an iron bath with a slag which is as free as possible from silicic acid and contains titanic acid and lime in the proportion of 70:30, and out of this by reduction with carbon, introducing titanium into the iron bath in a quantity which is at least sufficient to remove the nitrogen, sulphur and oxygen content in the bath into the slag in the form of titanium compounds and then to convert all the carbon of the iron bath into titanium carbide corresponding to the formula TiC, and alloying the resulting steel, containing not more than 0.5% titanium, with 0.3 to 4% of silicon and 0.1 to 0.5% of aluminium.

WALTHER MA'I'HESIUS.