US2937084A - Process for production of high-grade cast-iron - Google Patents

Description

United States Patent PROCESS FORPRODUCTION OF HIGH-GRADE CAST-IRON Fritz Klepp and Richard Werner, Liezen, Austria, assignors to Vereinigte Osterreichische Eisenuntl Stahlwerke Aktiengesellschaft, Linz, Austria, a company of Austria .No Drawing. Application November 18, 1957 Serial No. 696,922

2 Claims. (Cl. 75-130) be present and the silicon content of the starting material should be less than 1%. If the silicon content is higher, a subsequent addition of substances which cause the formation of .thespherical graphite structure and are usually employed in form of alloys with silicon may increase the silicon content to such a degree as to cause losses in quality, 7

In order to fulfill the requirements set forth, high-grade cast-iron hasubeen produced .by remelting soft-iron or steel scrapin electric are or high-frequency furnaces, followed by carburizing. If, however, an acid cupola furnace should be used for the production of a high-grade I cast-iron, such as spheroidal graphite cast-iron, a selected special charge free of disturbing elements and having a low silicon contentis absolutely necessary. This requirement is always emphasized by the licensors of processes for the production of spheroidal or nodular graphite castiron.

It is an object of the present invention to improve the economic efiiciency of the production of high-grade castiron. More particularly, it is an object of the invention to enable the production of high-grade cast-iron from a normal charge in the acid cupola furnace, without need for a special selection of the charged materials as regards their silicon content and any content of disturbing elements.

According to the invention this problem is solved in Whereas it is generally impossible to remove phosphorus from normal cast-iron, a preferred embodiment of the invention enables also a dephosphorizing of the charge. The dephosphorization proceeds according to the which are inequilibrium with each other. The equilibrium constant is known to be proportional to the;FeO and CaO contents of the slag, and has a negative temperature coefiicient so that the dephosphorization will be the-better the higher are the FeO and C210 content or the basicity of the slag and the lower is the reaction temperature,

provided that the temperature is sufficient for producing a reactive slag. The dephosphorization of molten cupola iron is normally disturbed, however, .byv the silicon and carbon contents. The slagged silicon, which is available in the form of SiO;;, may cause a rephosphorization by,

a decomposition of phosphates which have been formed. This is highly undesirable. On the other hand, it is known from surface blowing steel-making processes thata dephosphorization will not occur until the carbon content is reduced below 2% because the slag will be sufliciently reactive only at this time. It is obvious that this carbon content is too low for cast-iron because'it would require, a subsequent carburization. For this reason grey. cast-y iron has not been dephosphorized in most cases.

According to theinvention an effective dephosphorization and at the same time a reduction of the-sulphur content, is effected by adding aniron oxide carrier together with a fine grained material containing Ca0 and Na CO during the blowing of oxygen to the ,bath. The

ironoxide carrier may preferably consistof'the dust obtained in knownsurface blowing processes 'used for making steelfrom crude iron. Becauseit is important that theadditions should be as finely divided as possible, in

order to achieve a quick reaction, it is preferable to introduce the iron oxide carrier as well as the basic additions into the bath with the aid of the blowing gas itself. Part of the sulphur may be'rendered harmless by a pretreatment of the spout iron with sodium carbonate, magnesium or calcium carbide before oxygen is blown thereon.

As an example for the practice of the process according'to the invention a cupola iron charge was composed of the following starting materials:

5 40% scrap of spheroidal graphite caSt-iron containing that iron which has been made by melting a normal charge in the acid cupola furnace is treated by blowing it with oxygen or oxygen-enriched gas, preferably in the presence of a basic slag, for reducing the silicon content, by which treatment the temperature of the bath is increased and the disturbing elements which are detrimental for a subsequent processing to spheroidal graphite cast-iron are also removed. g v w The oxidizing treatment of the charge of cupola iron is desirably effected by blowing technically pure oxygen onto the surface of the bath of molten iron under such blowing conditions that the silicon content is reduced to less than 1%, preferably to about 0.5% and the simultaneous oxidation of carbon does not exceed 0.20% For instance, an oxygen blowing tube having a diameter of about 10 mm. may be used and approximately 10 cubic metres of oxygen per ton of the bath may be supplied to the bath under a pressure of 2-10 kg./sq. cm. above atmospheric pressure, depending on the amount of the charge. In that case the temperature on the surface of the bath will amount to about 1500 deg. C. and the removal of silicon and of the disturbing elements will be effected within a few minutes, the temperature of the bath increasing by about 150 deg. C.

15% iron mould breakage containing 0.12% P and A hot blast was employed and the resulting spout iron contained 3.60% C, 0.07% P and 1.65% Si.

The resulting spout iron had a temperature of 1330 deg. C., measured with a pyropto instrument. It was tapped into a ladle and treated with a small amount of sodium-carbonate for preliminary desulphurization. A small amount of Elektron (a known magnesium alloy) may be used instead of the sodium carbonate. Then lime was added in small lumps to the bath and an oxygen jet was blown onto the surface of the bath under a pressure of 5 kg./ sq. cm. above atmospheric pressure from a nozzle having a diameterof 10 mm. After a blowing period of four minutes an injector valve was opened whereby a powder supply container, which contained a mixture of dust from surface blowing steelrnaking processes and dust slag which contained CaO and Na CO was connected to the oxygen supply conduit; the blowing was continued.

Patented May '17, 1960- After an additional blowing period of 3 minutes the blowing gaswas shut ofi and the bath was slagged off. At this time the bath had a carbon content of 3.42%, a phosphorus content of 0.055% and-a silicon content of 0.52%. The temperature measured with the Pyropto was 1450 deg. C." The iron having this composition was suitable low specific gravity and the high vapour pressure of the" magnesium at the temperatures of the molten iron. Various methods, such as the immersion-bell method or the pressure-ladle method, have been proposed for introducing pure magnesium. In view of the difficulty involved in maintaining thetemperature and for safety reasons, however, it is always preferred to introduce magnesium into the bath in the form of silicon-containing master alloyspsuch as SiMgFe, whereby the vapour pressure is reduced. These alloys, however, have the disadvantage of introducing a larger amount of silicon into the bath, which adversely affects the toughness of the cast material. As has already been mentioned, these difficulties are eliminated accordingto the invention by reducing the silicon content of the bath to a value which permits the use of silicon-containing magnesium alloys for producing the spherical graphite structure or the addition of inoculating silicon without disturbing the toughness properties. Surprisingly it has been found that independently of the method employed for the introduction ofmagnesium the magnesiumcontent or output obtained with 'melts which had previously-been treated according to the invention with oxygen was higher than with melts which came froman' electric furnace. 'Whereas the spheroidal graphite cast-iron from the-electric furnace, containing about 0.02% S, has a;final magnesium content of 0.04-0.06 if the usual amount of MgSi alloy has been added, a magnesium content of 0.09% was achieved under the same temperature conditions with an iron which had been blown with oxygen although the melt had an initial sulphur content of 0.04%.

Even'if the cast-iron produced according to the invention has no spheroidal graphite structure it has greatly improved strength properties and a lower mass influence compared to ordinary grey cast-iron or to iron which has been superheated in the electric furnace.

Typical characteristic values obtained With a specimen 30 mm. in diameter and consisting of iron containing 3.3% C, 2.2%.Si and.0.60% Mn are indicated hereinafter. The first column contains the values for ordinary grey cast-iron, the second column contains the values for cast-iron which has been superheated in the electric furnace and the third column indicates the values for oxygentreated cast-iron.

Column Column Column I II III Ultimate tensile stress (kg/sq. mm.) 14-24 22-28 31 Bending strength (kg/sq. mm.) 40-50 4555 Deflection 8-12 12-15 23 blowing said bath with asubstance selected from the group consisting of oxygen and oxygen-enriched gas in the presence of a basic slag to increase the temperature of the bath and to reduce the silicon content to less than 1% reducing the phosphorous content during blowing by introducing into said bath finely divided material containing iron oxide, calcium oxide and sodium carbonate, and adding to said bath silicon-containing materials which cause the formation of a spheroidal graphite structure during solidification of the bath.

2. A process according to claim 1 wherein said finely divided material containing iron oxide, calcium oxide and sodium carbonate is entrained in' said treating gas.

References Cited the file of this patent UNITED STATES PATENTS France Sept. 30, 1953

Claims (1)

1. A PROCESS FOR THE PRODUCTION OF SPHEROIDAL GRAPHITE CAST-IRON COMPRISING PROVIDING A MOLTEN IRON BATH CONTAINING SILICON AND PHOSPHOROUS, SAID BATH BEING PRODUCED BY MELTING A NORMAL CHARGE IN AN ACID CUPOLA FURNACE, BLOWING SAID BATH WITH A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF OXYGEN AND OXYGEN-ENRICHED GAS IN THE PRESENCE OF A BASIC SLAG TO INCREASE THE TEMPERATURE OF THE BATH AND TO REDUCE THE SILICON CONTENT TO LESS THAN 1%, REDUCING THE PHOSPHOROUS CONTENT DURING BLOWING BY INTRODUCING INTO SAID BATH FINELY DIVIDED MATERIAL CONTAINING IRON OXIDE, CALCIUM OXIDE AND SODIUM CARBONATE, AND ADDING TO SAID BATH SILICON-CONTAINING MATERIALS WHICH CAUSE THE FORMATION OF A SPHEROIDAL GRAPHITE STRUCTURE DURING SOLIDIFICATION OF THE BATH.