US1939795A - Method of making chromium steel - Google Patents

Description

Patented Dec. 19, 1933 PATENT OFFICE METHOD OF MAKING CHROMIUM STEEL Kenneth M. Simpson, New York, N. Y., assignor to International Chromium Process Corporation, New York, N. Y., a corporation of Delaware No Drawing. Application January 11, 1932 Serial No. 586,086

10 Claims.

My invention relates to the manufacture of chromium steels, and more particularly has reference to a process which is more economical and practical than those heretofore employed in the production of such alloys.

According to a known method of making lowcarbon, high-chromium steel, steel scrap is melted in an electric furnace and low-carbon, ferrochromium added thereto. After the charge has been heated suificlently for the ferro-chromium to be thoroughly melted and disseminated through the mass of steel, the heat is poured in the usual way. The ferro-chromium employed in this process is of low-carbon content, and generally contains .10% carbon, or less, the purpose being to produce steel containing .16% carbon or less. In chromium steel made by this process, there is always a considerable addition of carbon from the electrodes of the electric furnace in which the process is carried out.

According to another known method of making chromium steel, chrome ore andferro-silicon, instead of ferro-chromium, are added to the melted steel in an electric furnace. Chromium is produced from the ore by the action of the ferrosilicon and works its way down through the bath of steel until the desired percentage of chromium has been attained.

Aside from being an electric furnace operation, the first-mentioned method has the objection that the ferro-chromium employed therein is very expensive. The second known method referred to is objectionable in that the amount of material added to the melted steel is so large and the slags so bulky and troublesome, that the time required for the operation is very long and costly, especially as the furnace must be maintained at high temperature for prolonged periods through the use of electric heat. This second method is difficult to control, and has not been generally adopted.

It is accordingly an object of my invention to provide a process for the manufacture of chromium-steel alloys which is free from the foregoing objections and difficulties of the prior art methods.

My invention has for a further object, the provision of a process of the character set forth, which does not require the use of electric fur- 50 naces, but which can be carried out in the customary fuel-fired furnaces capable of operation at high temperature.

Another object of the invention resides in the provision of a process for making chromium steel which is easily controlled, and wherein no tedious nor expensive methods need be resorted to for regulating the operation.

A still further object of the invention resides in the production of low-carbon chromium steel in the minimum of time and from the cheapest grade of materials, the process being characterized by a high degree of economy in heating and handling charges.

Other objects and advantages of my invention will appear from the more detailed description of the process.

Although the process may be carried out in an electric furnace, I preferably employ a top-fired open-hearth, or any fuel-fired furnace capable of operating at the requisite temperature, this type of furnace being used in preference to an electric furnace for purposes of economy and to avoid increase in carbon content of the product as above pointed out.

The furnace having been brought to high temperature, is charged, according to my invention, with chromium-ore and suitable exothermic reducing agent. The exothermic reducing agent may be ferro-silicon, aluminum, or the like, but I'preferably employ ferro-silicon because of its cheapness. For best operation, the chromiumore and reducing agent should be finely ground and thoroughly mixed before charging into the furnace. The proportions of ore and reducing agent will, of course, depend upon the alloy to be produced and the particular composition of the materials charging to the furnace. When employing ferro-silicon as the reducing agent, no special grade need be selected, and that of cheapest quality, if at all suitable for the purpose, may be employed. The use of low-grade ferro-silicon is possible in my process by reason of the fact that the ferro-chromium, produced by reduction of the ore, goes directly into the steel, and hence the presence of extra iron is of no importance and is not harmful.

The furnace having been charged, as above described, the materials therein are subjected to sufiiciently high temperatures to reduce the chromium-ore by the action of the ferro-silicon. 0 Upon subjection of the materials to the high furnace temperatures for a suflicient time period, for example, between two and three hours, ferrochromium is formed by reduction from the ore and exists in the form of viscous, lumpy masses,

than that of the metal.

At this stage of the process, that is, when the lower viscous layer of metal and the upper layer of fluid slag have formed, the slag, which is always of substantial volume, is withdrawn from the furnace, leaving the ferro-chromium in the furnace. The slag -is run into slag-pots, andif any metal escapes therewith, through the taphole of the furnace, it sinks to the bottom of the slag-pots and may be recovered when the slag is discharged from the latter. The withdrawal of slag at this stage in the operation is of prime importance because it. permits the-application of heat directly to the reduced metal without the necessity of penetrating a thick layer of chromium bearingslagwhich slag is a poor heat conductor. Also this step results in a much smaller slag volume during the last stages of the heat after the steel has been added and makes it possible to produce a much cleaner and more uniform product than is possible when working with too bulky a slag volume. Steels madeby this process are remarkably free from oxides and contained inclusions, and are superior in corrosion resistance and other properties to steels of the same analysis made by other methods.-

The slag, having been thus withdrawn, andthe ferro-chromium retained in the furnace, there is now introduced to the furnace, the requisite amount of clean steel scrap depending upon the composition of the alloy to be produced. Upon continuous heating, the steel scrap is melted down, becomes mixed with the ferro-chromiumand causes the latter to melt and disseminate itself throughout the steel mass; The ferro thereto, as the specific example is given for illustrative purposes only.

'Inthis operation there was made a l-ton heat of chromium steel containingfrom 16% to 18% chromiumand the operation was as follows:

The furnace was first brought to a temperature of approximately 3100 F., and then charged with Y a mixture of finely ground chrome ore and finely silicon. The mixture was charged into the fur-' nace as rapidly as possible, and the latter, a silica brick structure, was maintained at a maximum temperature of about 3100 F. The reaction between C1'203 and the silicon takes place rapidly with the evolution of considerable heat. The treatment was completed in about two hours, the term-chromium then lying on the hearth in irregular, viscous masses covered by a relatively deep bath of fluid slag. The latter was then withdrawn from the furnace through the tap-hole and the ferro-chromium retained on the hearth. The tap-hole was then stopped and the furnace immediately charged with 2,000 lbs. of low-carbon steel scrap. The furnace was now run at its maximum temperature until the steel and ferrochromium were melted and thoroughly mixed and distributed. The steel was then tapped from the furnace into a ladle in the usual way and analyzed .10% carbon and 16% to 18% chromium.

From the foregoing, it will be seen that I have devised a, practical process for manufacturing chromium steel in fuel-fired furnaces, and in which the time required to reduce the ferrochromium from the ore is very brief. The process is further characterized by the fact that no tedious nor expensive manipulations need be practiced, as is the case when ore and ferro-silicon are added to a steel bath, and further by the fact that the cheapest grade of ore and ferro-silicon can be used with the production of extremely low-carbon steels. Since the slag is removed from the furnace at an intermediate point in the operation, it will be seen that the amount of material to be heated during the final stages of the process is considerablyreduced, and, the ferro-chromium being retained in the furnace, there are no heat losses and handling charges as would be incurred if it were withdrawn, cooled and then put back into the furnace to be reheated.

Once the expected yield for any given ore has been established, the matter of calculating the charges for the furnace is simple. For example, in order to produce a certain number of tons of steel containing a certain percentage or chromium, it is only necessary to figure the amount of ore necessary to charge with the mixed ferrosilicon to yield the quantity of chromium neces- ,sary to be melted with the steel in order to give the desired percentage of chromium in the final steel. By operating in the manner herein described, I have been able to produce steels with a carbon-content as low as .03%, and I have further been able to make steels conforming to standard specifications quite readily. The steels which I have made by the process of the present invention have contained between 12% and 20% chromium.

Having thus described the invention, I claim: 1. A process for manufacturing chromium steel which comprises charging a furnace with chromium ore and an exothermic reducing agent, heating the charge by fuel firing to reduce the ore thereby forming slag and metal, removing the slag from the furnac while retaining the reduced metal therein, adding steel to the reduced metal in the furnace and disseminating the reduced metal therethrough.

2. A process for manufacturing chromium steel which comprises charging a furnace with chrome ore and a reducing agent, reducing the ore by heating the charge, and removing resultant slag from the furnace while retaining the reduced metal in the furnace, then adding scrap steel to the metal in the furnace and thoroughly mixing the steel and said metal by continued heating.

3. A process for manufacturing chromium steel which comprises charging a furnace with chromium ore and an exothermic reducing agent, heating the charge by fuel firing sufilciently to reduce the' ore and to effect a separation of reduced chromium from the slag, removing the slag from the furnace while retaining the chromium therein, then adding steel to the chromium in the furnace and alloying the chromium with the steel by continued heating.

4. A process for manufacturing chromium steel which comprises charging a furnace with chromium ore and an exothermic reducing agent, heating the charge. until the ore is reduced and the charge separated into a lower layer of reduced chromium and an-upper layer of slag, removing the slag layer from the furnace while retaining the reduced chromium layer therein, then adding scrap steel to the retained layer in the furnace, melting same, and alloying the chromium with the steel by continued heating.

5. The process as defined in claim 2 further characterized in that the charge is heated by fuel firing.

6. The method of making chromium steel which comprises charging chrome ore and a reducing agent to a furnace, heating the charge by fuel flring in a manner to form a viscous substantially unfused mass of metal and a fluid slag, withdrawing the slag from the furnace while retaining the mass of metal therein, subsequently introducing steel to the furnace and continuing the heating to distribute said metal therethrough.

7. The method of making chromium steel which comprises charging chrome ore and ferrosilicon to a furnace, heating the charge sufllcient- 1y to form a viscous mass of ferro-chromium and a fluid slag, withdrawing the slag from the fur nace while retaining the ferro-chromium therein, then introducing scrap steel to the furnace and continuing the heating to distribute the ferro-chromium therethrough.

8. A process for the production of chrome steel which comprises introducing to a furnace a charge of chrome ore and farm-silicon, heating the charge by fuel-firing until it separata into a lower layer of ferro-chromium and an upper layer of fluid slag, withdrawing the slag layer from the furnace while retaining the ferro-chromium layer therein, then charging the furnace with scrap steel and continuing the heating until the steel is melted and the chromium thoroughly distributed therein.

9. In the art of producing chromium steel wherein chrome ore, a reducing agent and steel are heated in a furnace, the improvement which comprises heating the ore and reducing agent by fuel firing to reduction temperature in the furnace in the absence of the steel and removing resultant slag from the furnace, and then charging the steel into the furnace.

10. In the art of producing chromium steel wherein chromium ore, ferro-silicon and steel are heated in a furnace, the improvement which comprises converting the chromium in the ore to ferro-chromium by the action ,of the ferro-silicon and removing resultant slag from the furnace prior to the charging of the steel into the furnace, subsequently charging scrap steel to the furnace and melting the same therein, and distributing the farm-chromium through the molten steel.

KENNETH M. SIMPSON.