WO2001086006A2

WO2001086006A2 - Improved process for the production of stainless steels and high chromium steels and stainless steelproduced thereby

Info

Publication number: WO2001086006A2
Application number: PCT/IN2001/000095
Authority: WO
Inventors: Ranjan Sen
Original assignee: Ranjan Sen
Priority date: 2000-05-10
Filing date: 2001-05-03
Publication date: 2001-11-15
Also published as: AU5506001A

Abstract

An improved economic process for producing stainless steel which comprises: (a) producing hot metal by reducing a mixture of iron ore and chromite ore by carbon reduction process at a high temperature in the range of 1250 to 1800 degree Centigrade, said hot metal having a chromium content in the range of 5-25 %; (b) decarburising of the product of step (a) in converter; (c) further decarburising the product of step (b) and making the steel which is ready for casting in continuous casting machine or in ingot moulds. The carbon reduction process is carried out in a submerged arc furnace. The feed charge in the submerged arc furnace is is the form of a mixture of iron ore, chromite ore, reductants selected from one or more of coke, coal of various types, electrode paste and graphite electrodes. Iron ore and chromite ore can be used in the form of lump and/or agglomerates. The final decarburisation and degassing of the stainless steel is carried out in VOD unit operating under vacuum.

Description

IMPROVED PROCESS FOR THE PRODUCTION OF STAINLESS STEELS AND HIGH CHROMIUM STEELS AND STAINLESS STEELPRODUCED THEREBY.

FIELD OF INVENTION

The invention relates to an improved process for production of stainless steels and high chromium steels. More particularly it relates to an improved process for production of stainless steel directly from iron ore and chromite ore by carbon reduction process which is cost efficient. The invention also relates to stainless steel produced by the improved process of the present invention.

BACKGROUND OF INVENTION About 16- 18% chromium should be present in steel to render its stainless property. The conventional process-route for making stainless steels involves addition of bulk quantity of ferro chromium, or ferrochrome, the major chromium bearing alloying additive, to the liquid steel with very little or no chromium content. The ferro chromium is made separately in an electric smelting furnace specially designed for making ferro alloys, known as Submerged Arc Furnace (SAF). Chromite ore, which is a mixed oxide of chromium and iron, is reduced in SAF to metallic iron and chromium which is known as ferrochrome and the process demands consumption of a substantial amount of electrical energy which is very expensive in almost all parts of the world including India, resulting in high production cost of ferrochrome. Ferrochrome being one of the major inputs for manufacturing stainless steels, the production cost of stainless steel is also high. With an intention to reduce the cost of production of stainless steels, lot of research and development activities have been carried out mostly in South Africa, Sweden, Japan and Germany to explore alternative routes to reduce chromite ore to ferrochrome by partial or full substitution of electrical energy by cheaper chemical energy. However, commercial success is yet to be achieved in these routes. Nobody is known to have tried to prepare stainless steel directly from chromite ore.

The ultimate necessity is to evolve a process-route which would involve least number of process-steps ensuring least capital investment and at the same time will consume cheap and abundant fuel sources, partially or fully substituting the electrical energy. To cover such a necessity the present work was taken up and the provisional data generated gives the direction that it is possible to produce stainless steels at the least cost not achieved so far.

PRIOR ART

Stainless steel was earlier produced in Electric Arc Furnace (EAF) by melting a mixture of mild steel scrap and stainless steel scrap followed by complete refining in the EAF as disclosed in Technology of Alloy Steel by E.C.Bain & R Paxton.

In the refining step ferro alloys like ferro chrome and nickel or ferro nickel were added in the steel bath, and decarburisation, dephosphorisation and desulphurisation were carried out to finish the chemistry of the bath in line with the final chemical specification of the stainless steel grades.

Ferro chrome used to achieve the specified chromium should have very low carbon. The reason is high carbon ferro chrome which is another variety of ferro chrome would increase the decarburisation load in EAF. The problems to work with high carbon ferro chrome are as follows :

• Requirement of more oxygen to decarburise.

• Decarburisation is accompanied also by loss of costly chromium into the oxidising slag through oxidation of chromium.

• Additional time is required to reduce the chromium oxide and recover chromium from the slag before it is removed to pass on to the next desulphurisation stage, all within the same EAF.

• Increase in the total cycle time of a heat which leads to lower shop productivity. To obviate the above mentioned drawbacks, low carbon ferro chrome was used as the source of chromium as disclosed in page 342, Making, Shaping and Treating of Steel - published by United States Steel. Edited by W. T. Lankford, Jr., N. L. Samways, R. F. Craven & H. E. McGannon. But low carbon ferro chrome is much more expensive compared to high carbon ferro chrome. Therefore, a compromise had always to be made between the use of low carbon ferrochrome and productivity to achieve an optimum cost of production. Such drawbacks were overcome with the advent of Secondary Refining technology and its adoption by a large number of steel makers. This technological modification involves splitting of the entire steelmaking process in two different manufacturing units, as under

Step - 1 (Primary melting) i) Melting down the solid charge in Electric Arc Furnace ( EAF ) ii) Dephosphorisation of the melt in the EAF. iii) Addition of ferro alloys like nickel / ferro nickel and major part of ferrochrome. iv) Partial decarburisation of the melt. v) Tapping of the melt into a ladle without furnace slag.

Step - 2 ( Secondary refining ) i) Addition of basic reducing slag on top of the melt in the ladle, ii) Treatment in a ladle furnace or a converter for desulphurisation, deoxidation, rough alloying including addition of balance ferrochrome and to increase the melt temperature to cover all temperature losses during subsequent treatment steps, iii) Vacuum treatment for further refining, alloy trimming and degassing.

The two-step process has become till now the most economic commercial stainless steel making process due to the following advantages over the earlier process : I. Lower loss of chromium,

II. Ability to use cheaper high carbon ferro chrome instead of costly low carbon ferro chrome,

III. Higher shop productivity, IV. Lower specific cost of production, V. Production of cleaner steels.

Different types of furnaces / converters are in use for secondary refining of stainless steel - they are Vacuum Oxygen Decarburisation Unit (VOD), Argon Oxygen Decarburising Unit (AOD), Metal Refining Process (MRP) converter, Creusot Loire - Udeholm (CLU) converter and others. Depending on the limitations and advantages of the respective units, the selection of the actual secondary refining unit is made. The output of the secondary refining unit is refined liquid stainless steels ready to be shaped either into ingots or continuously cast billets, blooms and slabs.

It is worthwhile to note that the conventional route discussed here applies also for high chromium steels used for various engineering applications. The entire manufacturing process is illustrated in Figure 1 of the accompanying drawings.

Recent Innovative Stainless Steelmaking Processes

With an intention of reduction in the production cost of stainless steelmaking, a number of alternative technologies / process-routes have been further evolved by different steel manufacturers and process equipment suppliers of international repute. In most of such cases the process is based on the use of oxygen blown converters to reduce the bath carbon of the premelt, followed by final treatment in a secondary refining vacuum furnace, mostly Vacuum Oxygen Decarburising (VOD) Unit as disclosed in Outline and Application of VAI Stainless Steelmaking

Technology by J. Steins, E. Fήtz and L. Gould, Iron 85 Steel Review, ISR Publications Ltd., Calcutta, May '97.

Review Of The Processes Adopted Columbus Stainless, South Africa CLU process uses steam which is injected from the bottom of the converter. In no other converter process (like AOD) or ladle process (like VOD) of secondary refining, steam is used as the injecting gas. In AOD and VOD, oxygen is used mainly for decarburisation, and argon, an inert gas is purged, which has two purposes - (i) to promote the process of decarburisation and (ii) to act as the coolant to reduce the temperature of the bath, which otherwise becomes very high consequent to the oxidation of the bath carbon. CLU process was developed by M/s Creusot Loire of France and M/s Uddeholm of Sweden. It is claimed that the steam used in the process acts as a more effective coolant than argon, since steam cracks to hydrogen and oxygen by reacting with bath carbon and cools down the bath as the reaction of cracking of steam is of endothermic nature as disclosed in Vacuum steel degassing processes and secondary metallurgical treatment in ladles and converters by Dr. Wϊlhelm Burgmann, Otfried Wiessner, Jochen Schumann and Rainer Schumann - Metallurgical Plant and Technology - Issue 4/ 1 84.

In some countries, cost of argon is prohibitively high due to restricted availability of the gas. In such countries CLU process can work as a practical economic substitute of other conventional secondary refining processes requiring argon.

The process-route employed is different from conventional process only in respect of adopting CLU converter and ladle treatment practice for secondary refining in place of any conventional units like VOD or AOD.

Advantages I. CLU process has enabled to use Steam instead of Argon gas used in the VOD or AOD Units. At Middelburg in South Africa, where Argon availability is restricted, CLU process has been able to keep the production cost under control.

Limitations

I. Since scrap and ferro-chrome/ charge-chrome are used as the only basic raw materials, any fluctuation in the price of scrap and its availability have a significant impact on the cost of production. II. Use of ferro chrome/ charge-chrome instead of chromite ore is associated with loss of a considerable amount of energy which increases the production cost.

Posco SS Plant No. 2, Pohang, Korea The process is similar to the conventional process with the only variation in the adoption of K-OBM-S converter, mainly to carry out decarburisation, followed by an optional treatment in VOD as disclosed in Outline and Application of VAI Stainless Steelmaking Technology by J. Steins, E. Fritz and L. Gould, Iron 85 Steel Review, ISR Publications Ltd, Calcutta, May '97.

Advantages

I. The process cycle time is likely to be lower than the conventional route due to split up of the tasks of VOD unit between K-OBM-S and VOD unit. This can increase the productivity.

Limitations

Same as mentioned for Columbus Stainless, South Africa.

Microsteel, South Africa In this plant, Induction furnaces have been used as the primary melting unit. Such a plant is rare example of small capacity stainless long product manufacturing plant as disclosed in Outline and Application of VAI Stainless Steelmaking Technology by J. Steins, E. Fritz and L. Gould, Iron & Steel Review, ISR Publications Ltd., Calcutta, May '97.

Advantages

Same as mentioned for POSCO SS Plant No. 2, Pohang, Korea.

Limitations

Same as mentioned for Columbus Stainless, South Africa.

Iscor Pretoria Works, South Africa

The process adopted is one step forward to cost-effective stainless steelmaking since a combination of hot metal and scrap is used as the basic charge-mix. Scrap is premelted in EAF and the crude steel from EAF along with the hot metal (liquid pig iron) is refined in K-OBM-S followed by final treatment in VOD as disclosed in Outline and Application of VAI Stainless Steelmaking Technology by J. Steins, E. Fritz and L. Gould, Iron & Steel Review, ISR Publications Ltd., Calcutta, May '97.

Advantages

I. Use of hot metal helps to increase the productivity and reduce the electric energy and electrode consumption.

II. Some chromite ore is directly used and processed in the converter taking the advantage of the high bath carbon, which also helps to reduce the overall process energy consumption and hence the cost of production.

Limitation

I. Chromite ore charge in the converter has to be necessarily in the form of fines to enable effective reduction of chromite ore by the bath carbon at the steelmaking temperature prevailing in the converter. Acciaierie di Bolzano, Italy

The process is almost similar to that adopted in POSCO, Pohang, Korea, with the same advantages and limitations as disclosed in Outline and Application of VAI Stainless Steelmaking Technology by J. Steins, E. Fritz and L. Gould, Iron & Steel Review, ISR Publications Ltd., Calcutta, May '97.

Kawasaki Chiba Works No. 4, Japan

Out of all the processes discussed herein, the most recent process for the stainless steelmaking is practised by Kawasaki Steel Corporation (KSC), Japan at their Chiba works no. 4 as disclosed in Outline and Application of VAI Stainless Steelmaking Technology by J. Steins, E. Fritz and L. Gould, Iron 8& Steel Review, ISR Publications Ltd., Calcutta, May '97. Three significant features of this process route, which makes it different from all other conventional as well as innovative stainless steel making routes, are given below :

1) Chromite ore is used directly to substitute a substantial part of purchased FeCr ;

2) Normal hot metal from blast furnace is used, which is, in the next step, converted to chromium rich hot metal by smelting of chromium ore fines followed by refining ;

3) Due to adoption of this process route, a cost saving of US$ 50 per tonne of steel has been reported.

For the above three reasons it is considered that any comparison of the new route proposed herein, if at all is to be made, should be done with Kawasaki process since so far it is reported to be the most cost effective route for stainless steel making. Accordingly, the process route of KSC is described below. The flow sheet for the KSC process is presented in Figure 2 of the accompanying drawings. Details of the process adopted by Kawasaki Steel Corporation

In the first step normal pig iron in the form of hot metal is tapped from the blast furnace which is taken to a combined blowing converter known as KMS-S as disclosed in New technologies for the production of stainless steel by E. Fritz and J. Steins, Iron 8s Steel Review, ISR Publications Ltd., Calcutta, ANNUAL '97 Issue. In KMS-S chromium ore fines and coke is charged and oxygen is blown at a high rate. The carbon present in the hot metal as well as the carbon charged into the KMS-S converter reduces the chromite ore fines to metallic chromium. This is continued up to a stage when the chromium content of the bath is in the range of 9 to 13%. At this stage the metal is tapped. This can be termed as the premelt for stainless steel making, which in the next stage is charged into another combined blowing converter named as K-OBM-S. In this converter oxygen is blown and the operating conditions are controlled in a manner to allow decarburisation and desulphurisation. Carbon is blown from a starting level of average 5.5% to about 0.12% to limit chromium oxidation. Final decarburisation is carried out under vacuum conditions in the VOD unit. In this way it has been possible to produce all important grades of stainless steel including AISI 304. It is reported that this process-route enables to achieve a reduction in the cost of production to the tune of US$ 50 per tonne of liquid steel, depending on the site conditions.

Advantages I. Cost of charge materials is less

II. No Scrap is required, thus the route is independent of the price fluctuations of scrap

III. Substantial amount of chromite ore is used, so effect of increasing price of electrical energy is less dominant. Limitations

I. Chromite ore is charged into the converter in the form of fines. No report is available about use of lump ore. Lump ore is likely to affect the productivity as well as the consumption of coke and oxygen adversely. II. Too many process plants involving higher investment cost and more maintenance.

But none of the processes described in the before mentioned pages, was able to use the chromite ore and iron ore directly for making the premelt in a single step, suitable for refining to produce stainless steel. Furthermore the conventional route involves wastage in energy. This refers to the fact that the ferrochrome which comes out of the SAF is in liquid form. This is cooled to solidify to facilitate transportation of ferrochrome to stainless steel making plants located mostly at areas different from the ferrochrome plants. The heat content of the liquid ferrochrome is completely wasted at this stage. In the steelmaking plant solid ferrochrome is used as the chromium source, where it needs to be melted and heated up again to the steelmaking temperature (about 1700° C) This demands expenditure of extra energy. Thus the conventional route has become energy intensive through loss of energy and consumption of energy which cannot be avoided in the conventional route for stainless steel making.

It is possible to substantially economise the manufacturing of stainless steel by producing hot metal with chromium content close to the specification requirement of stainless steel directly in one step through the submerged arc furnace or any other similar reduction / smelting - reduction furnace, using a mixture of iron ore and chromite ore, particularly lumpy and/or agglomerated chromite ore. There is a tremendous potential of generation of electrical power from the submerged arc furnace off-gas and power credit will also be accordingly available. This would further bring down the production cost of stainless steel. OBJECT OF INVENTION

Thus the principal object of the invention is to provide a process for the direct conversion of iron ore and chromite ore into stainless steel.

A further object of the invention is to provide a process in which reduction of overall process electrical energy, and thus producing the steel at a lower cost is achieved.

A further object of the invention is to provide a process in which reduction in wastage of expensive energy which is associated with the conventional route is achieved.

A still further object of the invention is to evolve a technology which permits use of iron ore instead of steel scrap as iron ore of good quality is available in abundance in India.

Yet another object of the invention is to provide a process which eliminates or reduces the use of steel scrap which has its usual erratic supply situation and fluctuating price.

SUMMARY OF THE INVENTION

It has been particularly found that increased energy efficiency may be achieved by switching over to carbon reduction of a mixture of suitable proportion of iron ore, chromite ore and fluxing materials in a single reduction furnace like submerged arc furnace at a higher temperature to produce crude liquid steel with chromium content very close to the final chemistry of stainless steel, followed by refining.

Thus the present invention relates to an improved process for producing stainless steel and high chromium steels at a lower cost which comprises: a) producing hot metal by reducing a mixture of iron ore 85 chromite ore, both of any form, i.e., lumpy and/or agglomerated fines, by carbon reduction process at a high temperature in the range of 1250 to 1800°C, said hot metal having a chromium content in the range of 5-25%; b) decarburising of the product of step (a) in converter;

further decarburising the product of step (b) and making the steel which is ready for casting in continuous casting machine or in ingot moulds.

The product of step (a) is subjected to dephosphorisation, if necessary.

In this method the following steps have been avoided :

Preparing ferrochrome or chargechrome for its subsequent addition to liquid steel to produce stainless steel

Addition of chromite ore and its smelting in a bath of normal liquid pig iron to produce the premelt having chromium content close to that of stainless steel, before its final refining

Definition of Agglomerates

The agglomerates as proposed for use are defined as Briquettes, Pellets (sintered or otherwise strengthened) and Sinters made out of chromite ore fines, friable lumpy chrome ore, chrome ore concentrate, iron ore fines, Blue Dust and other forms of fines of chrome ore and iron ore. A suitable binder is used to impart the necessary strength in cold condition for handling and also in hot condition for avoiding disintegration inside the furnace.

Refining

The sequence of the refining operations needs to be carefully selected because of the higher carbon content of the bath compared to the crude stainless steel conventionally obtained from the primary electric arc furnaces. The refining operations which will be suitable for treatment of hot metal of above mentioned analysis, can be subdivided into the following three stages which will be carried out in three different stations.

Stage I - Dephosphorisation of hot metal (if required) Stage II - Decarburisation in a suitably designed oxygen blown converter Stage III - Final decarburisation in Vacuum Oxygen Decarburisation (VOD) converter.

Dephosphorisation of hot metal ( Stage I )

In the event of phosphorous content of hot metal being of the order of 0.03% as indicated hereinafter under the heading 'Analysis of hot metal', it is observed that no separate step for dephosphorisation would be required, since the specified levels of phosphorus content of almost all grades of stainless steels are 0.045% max.

However, in case the P content of the hot metal is found to be around 0.1%, it is advisable to treat the hot metal in a dephosphorisation unit where removal of phosphorous is done by injecting calcium carbide into the hot metal. Complete or intense dephosphorisation is not recommended as the efficiency of dephosphorisation becomes progressively lower with the progress of dephosphorisation. Treatment upto about 0.05% P is adequate. The refining operation proposed to be done in the next stage (Stage II) can take care of removal of the balance phosphorous quite effectively. However, in case the phosphorous content of hot metal is 0.06% or below, it is recommended to skip Stage I by starting the refining operation from Stage II.

Decarburisation in converter ( Stage II )

The hot metal after treatment in Stage I or from the submerged arc furnace ( depending on the phosphorous content of hot metal ) will be charged into a specially designed converter in which Oxygen is blown into the bath through bottom tuyeres and preferably also through top lance. The converter can handle the following metallurgical processes :

• Decarburisation

• Dephosphorisation

• Desulphurisation • Deoxidation

It needs to be mentioned here that the design of the converter is of great importance because the converter is required to carry out the decarburisation of the hot metal having carbon as high as 5 - 6%.

It may be observed here that some amount of chromite ore fines may be charged into the converter at this stage. The carbon content of the bath can be made to reduce the chromite ore to metallic chromium to increase the chromium level of the bath. Chromite fines would also act as the coolant to reduce the temperature of the bath which would go up consequent to oxygen blowing. The plant return stainless steel scrap can also be added at this stage. This would also act as coolant. A proper material balance exercise taking into account the chromium input from chromite ore fines and plant return scrap, will determine the actual composition of the charge- mix into the SAF.

Referring back to page 9 of this patent specification, it can be observed that a K-OBM-S converter is being used successfully at the Chiba Works of Kawasaki Steel Corporation, Japan for decarburising the hot metal with average 5.5% C to produce stainless steel. The high carbon hot metal input to the K-OBM-S converter in Kawasaki Works is, however obtained by a Duplex route which is different from the single step submerged arc furnace process being proposed in this patent specification.

Decarburisation in VOD Unit (Stage III)

It is advisable to go for partial refining upto a bath carbon level of about 0.10% - 0.15% in the converter mentioned in Stage II above, and then transfer the metal to the final ladle. This would cut down the cycle time and also improve the recovery of chromium. The ladle containing the semifinished output from the converter is to be treated in a VOD unit which will perform the following tasks :

• remove further carbon by blowing oxygen under vacuum, thus ensuring nominal loss of chromium

• Produce Extra Low Carbon (ELC) stainless steel grades by "deep decarburisation" .

• render the steel free of dissolved gases to enhance the metallurgical quality of the product

After the treatment is over in the VOD unit, the steel will be ready for casting to produce billets, blooms or slabs through continuous casting machine, or to produce ingots through casting into ingot moulds.

DETAILED DESCRIPTION OF THE INVENTION

The process flow sheet for the new technology is presented in Figure 3 of the accompanying drawings.

Raw Materials The basic ingredients of stainless steels, viz., iron and chromium is made available from the primary natural sources such as iron ore and chromite ore. In the invented process, iron ore and chromite ore are mixed in the ratio of about 1.0-2.5 : 0.5-1.5 and preferably 1.5 : 1.0. However, the exact ratio will depend on the following factors:

i) the actual chemical analysis of iron ore and chromium ore to be used ii) the percentage of chromium desired in the crude stainless steels or high chromium steels iii) the amount of plant return stainless steel scrap available for remelting iv) the amount of chromite ore fines, if used, and its chemical analysis. The iron and chromite ores are in the form of lumps and/or agglomerates.

The typical analysis of iron ore and chromite ore under Indian conditions, are given in Table - 1 and Table - 2.

The invention will now be described in greater details for a better understanding of the invention and such description is non-limiting.

DETAILS OF EXPERIMENTS CARRIED OUT

The chromite ore and iron ore used in the experiment, were carried out and the results obtained are given below.

Chromite ore Iron ore

Cr₂0₃ 44.99% Fe₂0₃ 92.00%

Si0₂ 6.78% Si0₂ 1.75%

AlaOa 6.00 % AI2O3 2.50%

CaO 0.07% P 0.048%

MgO 13.4%

Na₂0 0.30%

K₂0 0.20% s Trace

P Trace

Total Fe 13.22% ^c;r/pe ratio 2.33

Atmosphere Maintained in Experiments.

In the case of ironmaking in blast furnace, high volume of pressurised hot air blast is sent into the blast furnace mainly to burn the coke to carbon monoxide gas which take part in the indirect reduction of lower oxide of iron to metallic iron. Before the formation of lower oxide of iron (FeO), higher oxides (Fe₂θ3 and Fe3θ4) are reduced to lower oxides by the solid carbon present in the coke. The lower oxide of iron, i.e., FeO is then mainly reduced by gaseous carbon monoxide, which is called Indirect Reduction.

However, in case of SAF, no air blast is used and, therefore, the oxides of iron present in the ore will be reduced to metallic iron entirely by carbon, which is known as Direct Reduction. In case of reduction of chromic and chromous oxides, entire reduction is through direct reduction by solid carbon, or in other words, there is no role of carbon monoxide gas in the reduction of the two oxides of chromium as well as the oxides of iron. It is for this reason the ore mixture was intimately mixed with solid coke and the mixture was heated within the furnace to allow only direct reduction of chromic oxide and also the oxides of iron. In the process of this reduction at high temperature carbon monoxide gas would be generated. The furnace was continuously flushed by argon which is an inert gas, with an inlet and an outlet at the bottom and top of the furnace respectively. This has helped to dilute the CO gas and in turn to reduce the partial pressure of CO gas.

Experimental Tests

Altogether three (3) tests were done and the result are shown in Examples 1 to 3. All the raw materials, viz. iron ore, chromite ore, coke and flux, were ground to 125 mesh. size.

EXAMPLE - 1

250 gms. of a mixture of the raw materials was prepared for this test. The constituents and their respective weights are given below.

Iron ore 85.0 gms. Chromite ore 53.0 gms.

Coke 77.0 gms.

Limestone 31.0 gms. Quartzite 1.6 gms. Manganese ore 2.4 gms. TOTAL ^"250.0 gms.

The sample was put into a crucible and placed in an electrically operated furnace. The furnace is equipped with a programming unit to achieve the attempted heating rate. The objective of this test was to record the temperatures at which some of the characteristic changes were likely to take place. The results and observations are as follows :

RESULTS

Softening point ... 1225°C

Fusion point ... 1297°C

Melting point ... 1409°C

OBSERVATIONS

1) The charge was found to be completely molten at 1409°C on holding it at this temperature for 25 minutes. 2) Slag was completely liquid.

3) Metal was completely liquid.

4) Complete separation of metal and slag was observed.

No analysis of metal and slag was, however, attempted in this test.

EXAMPLE - 2

250 gms of the mixture of same composition as used in Example - 1 was heated to 1450°C and the temperature was maintained for 20 minutes. After solidification the slag was removed and the chromium and carbon content of the metal was analysed.

Cr : 3.07%

C : 2.60%

From the above the recovery of chromium is estimated to be about 15%.

EXAMPLE - 3

In Example - 3 the temperature and the holding time was increased to ensure higher recovery of chromium. The temperature was increased to 1600°C and the holding time to 25 minute. The quantity of coke in the charge was also increased by 25% and the composition of the charge in Example - 3 was as follows : Iron ore 79.0 gms. Chromite ore 49.2 gms. Coke 89.4 gms.

Limestone 28.7 gms. Quartzite 1.5 gms. Manganese ore 2.2 gms.

TOTAL

These changes have improved the course of reaction significantly, thereby increasing the recovery of chromium substantially. Analysis of metal and slag are as follows:

Analysis of metal

C 6.3 %

Cr 14.8 %

Si 1.2 %

Analysis of slag

CaO 24.00 % Si0₂ 19.20 % Cr₂0₃ 1.68 % Basicity 1. .25 %

The recovery of chromium as estimated in this case is 78%.

Thus with a slag basicity (CaO/Si0₂) of 1.25, 78% recovery of chromium can be achieved and it is possible to get about 15% chromium in the hot metal, which is already close to the chromium content of stainless steel grades like AISI 201 and 316 having about 17% Cr. Even with 15% Cr. in the hot metal, stainless steel grades like AISI 410 and 420 can be made where very little or no ferro chrome addition may be required.

In view of the results of the above mentioned three (3) tests, it is observed that hot metal with chromium content up to any desired level to suit the chromium content of different grades of stainless steels, can be produced in a submerged arc furnace. Specific Consumption of Raw Materials

The specific consumption of the raw materials per tonne of metal (with 20% Cr ) tapped from submerged arc furnace are estimated to be as follows :

Iron ore (lumpy and agglomerates) 1040 kg /

Chromium ore (lumpy and agglomerates) ... 700 kg / T

Coke / other reductants 450 kg / T

Limestone 390 kg / T

Quartzite 40 kg / T

Power 3100KWh/ T

NOTE : All consumption figures are net on dry basis charged to the submerged arc furnace.

Some Bauxite may be added, depending on the AI2O3 / MgO ratio of the ores (lump/ agglomerates).

Analysis of hot metal

The analysis of the hot metal output from submerged arc furnace is observed to be approximately as follows : C : 4.8 - 5.2% Cr : 18.0 - 20%, Si : 2.7 - 3.0%

Mn : 0.4 - 0.5% P ~ 0.03%, 0.036% Fe : About 72% TABLE -1 TYPICAL ANALYSIS OF IRON ORE (Under Indian condition)

TABLE - 2 - TYPICAL ANALYSIS OF CHROMITE ORE - LUMPY/FINES/CONCENTRATES (Under Indian condition)

Claims

1. An improved process for producing stainless steel and high chromium steels at a lower cost which comprises:

a) producing hot metal by reducing a mixture of iron ore 8& chromite ore, both of any form, i.e., lumpy and/or agglomerated fines, by carbon reduction process at a high temperature in the range of 1250 to 1800°C, said hot metal having a chromium content in the range of 5-25%;

b) decarburising of the product of step (a) in converter;

c) further decarburising the product of step (b) and making the steel which is ready for casting in continuous casting machine or in ingot moulds.

2. An improved process as claimed in claim 1 in which carbon reduction process is carried out in a submerged arc furnace.

3. An improved process for producing stainless steel as claimed in claim 1 in which feed charge in the submerged arc furnace is in the form of a mixture of iron ore, chromite ore, reductants selected from one or more of coke, coal of various types, wood chips, electrode paste, broken pieces of electrodes, and fluxing materials in the ratio of (18-25) : (10-15) : (6-12) : (5- 12).

4. An improved process for producing stainless steel as claimed in any preceding claims in which fluxing materials are selected from Lime, Limestone, Quartzite, Manganese ore, Bauxite, or mixtures thereof.

5. An improved process for producing stainless steel as claimed in claim 1 in which for producing the hot metal, the range of Cr2θ3 content of the chromite ore for charging in the submerged arc furnace is in the range of 25% to 60%.

6. An improved process for producing stainless steel as claimed in claim 1 in which for producing the hot metal, total iron content of the iron ore for charging into submerged arc furnace is in the range of 50-80%.

7. An improved process for producing stainless steel as claimed in any preceding claims in which the temperature of the submerged arc furnace is maintained in the range of 1250- 1800°C.

8. An improved process for producing stainless steel as claimed in claim 1 in which dephosphorisation of the hot metal is carried out by injecting suitable compounds based on calcium and / or magnesium to the hot metal when the phosphorous content of the hot metal before dephosphorisation is in the range of 0.05% to 0.15%.

9. An improved process for producing stainless steel as claimed in claim 1 in which dephosphorisation by injection of suitable compounds based on calcium and / or magnesium to the hot metal is stopped when the phosphorus content of the hot metal reaches upto 0.05%.

10. An improved process for producing stainless steel as claimed in claim 1 in which dephosphorisation, decarburisation, desulphurisation 85 deoxidation is carried out in a converter to the level of 0.10% to 0.15% carbon level.

11. An improved process for producing stainless steel as claimed in claim 1 in which plant return stainless steel scrap is added at the decarburisation stage into the converter.

12. An improved process for producing stainless steel as claimed in claim 1 in which chromite ore fines are charged at the decarburisation stage into the converter.

13. An improved process for producing stainless steel as claimed in claim 1 in which the final decarburisation and degassing of the stainless steel is carried out in VOD unit operating under vacuum.

14. An improved process for producing stainless steel at a substantially lower cost as herein described.

15. Stainless steel and high chromium steels when produced by the improved process claimed in any of claims 1 - 14.