Title of Invention

AN IMPROVED PROCESS FOR DENITROGENATION OF STEEL

Abstract An improved process for denitrogenation of steel and in particular to a process for denitrogenation of low carbon steel by gas generation in-situ. The improved process comprising subjecting the liquid steel to treatment with carburized iron and thereby generate carbon monoxide gas in situ such as to provide large interfacial area and requisite low partial pressure of nitrogen at the reaction site and effectuate removal of nitrogen. In the above process gas is generated in situ inside the liquid steel bath by reaction of oxygen present in the steel with carbon which is present in carburized iron added to the bath. Oxygen in steel reacts with carbon and carbon monoxide gas is generated in situ in the form of large number of tiny bubbles which provide large interfacial area and requisite low partial pressure of nitrogen at the reaction site. The process thus provides for an alternative approach to reduce nitrogen content of steel under atmospheric conditions and without having to use expensive fluxes. The above process of denitrogenation of steel further assist in deoxidation of the steel tapped from BOF. The process is simple and can be attended without the need for expensive equipment and/or expensive special fluxes.
Full Text The present invention relates to an improved process for denitrogenation of steel and in particular to a process for denitrogenation of low carbon steel by gas generation in-situ.
There has been an increasing demand of steel containing low levels of impurities. By reducing carbon and nitrogen contents to low levels, vis., carbon and nitrogen together not exceeding 150 parts per million (ppm) improves toughness and resistance to pitting corrosion of steel. Reducing carbon and nitrogen to very low levels improve deep drawability of thin sheets for continuous annealing process. Low nitrogen ( The processes used at present for removing nitrogen from steel are (i) use of vacuum with flushing gas and/or (it) use of special fluxes of high nitride capacity
In the first approach reported in an article entitled "Removal of Impurities in Molten Steel by the powder top blowing method under reduced pressure" by Kaoru Shinme et al. published in The Sumitomo Search No. 46 April 1991, there is disclosed a new refining method in SMI for accelerating the reactions of the decarburization, the nitrogen removal, the hydrogen removal and the desylphurization using a 2.5t vacuum induction furnace and the 50t EF-VOD process. In this method, named the VOD-PB (Powder Top Blowing) method, the powdered particles are blown and penetrated into the molten steel through the nozzle of a top lance with argon gas under reduced pressure. As a result of applying the method to the industrial EF-VOD process, the following steels were produced : (1) Ultra low carbon ( The above known process required reducing the pressure inside the vessel containing steel to create an environment in the bath such that equilibrium partial pressure of nitrogen in steel is greater than the partial pressure of nitrogen

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present in the environment. So nitrogen transfer takes place from steel to the gas phase.
In another article entitled "Removal of Nitrogen from steel using novel fluxes" by K. Nomura et al. published in Metallurgical Transaction B Volume 22 B December 1991-783, the feasibility of denitrogenation of steel using lime-based slags, including additives to increase the basicity (BaO) or which have a strong affinity for nitrogen (TiO2) is reported. In particular, the nitride capacity of CaO - AI2O3 -TiO2 slags and CaO - BaO - AI2O3 - TiO2 slags in such process of removal of nitrogen from steel is reported.
in the above approach for denitrogenation of steel, fluxes used do form nitride. The equilibrium pressure of nitrogen in nitride so formed is lower than the equilibrium partial pressure of nitrogen in steel. Therefore, nitrogen takes place from steel to the flux.
It would be evident from the above known methods of denitrogenation of steel that the vacuum process requires expensive vacuum equipment and injection facility while the special flux practice requires expensive fluxes. Therefore, the presently available processes are difficult to carry out and are also cost-extensive.
It is thus the basic objective of this invention to find an alternative approach to reduce nitrogen content of steel under atmospheric conditions and without having to use expensive fluxes.
Another objective is to provide for denitrogenation of steel without the need for expensive equipment and/or expensive special fluxes.
Yet another objective is to provide for a process of denitrogenation of steel which would also assist in deoxidation of the steel tapped from BOF.
Thus according to the present invention there is provided an improved process for denitrogenation of steel comprising :

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subjecting the liquid steel to treatment with carburiied iron and thereby generate carbon monoxide gas in situ such as to provide large interfacial area and requisite tow partial pressure of nitrogen at the reaction site and effectuate removal of nitrogen.
In the above process gas is generated in situ inside the liquid steel bath by reaction of oxygen present in the steel with carbon which is present in carburized iron added to the bath. Oxygen in steel reacts with carbon and carbon monoxide gas is generated in situ in the form of large number of tiny bubbles which provide large interfacial area and requisite low partial pressure of nitrogen at the reaction site.
In accordance with one aspect the process of the invention comprising :
melting mild steel scrap in furnace maintaining the temperature in the range of 1580-1650°C;
introducing carburized iron into the melt from the top to facilitate in-situ generation of carbon monoxide gas and favour the driving off of the nitrogen.
In accordance win another aspect the process of the invention comprises:
i) providing requisite amount of carburized iron in the bottom of the ladle ;
ii) pouring liquid steel from the primary furnace in said ladle to thereby effectuate a reaction between the dissolved oxygen in steel and the carbon to generate in-situ carbon monoxide gas bubbles to facilitate removal of nitrogen from steel.
Importantly in the above processes the rate of generation of gas in-situ should be high in order to create an environment of low partial pressure of nitrogen in the
bath and the size of gas bubbles which provides large interfacial area to facilitate the kinetics of denitrogenation process. The rate of gas generation usually depend on the high oxygen content of the bath and low carbon content of steel.

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Carbon and oxygen of bath are related and their ratio is dictated by thermodynamic equilibrium of carbon-oxygen reactions.
Preferably an oxidised steel bath with low carbon is essential for the process to work.
While the above processes can be carried out in air, since atmospheric air contains about 80% nitrogen, nitrogen pick up from atmosphere for low nitrogen steel is possible and for process efficiency preferably the process should be carried out in a nitrogen free environment.
The details of the invention, its objectives and advantages are explained hereunder in greater detail in relation to non-limiting exemplary embodiments under the following examples.
EXAMPLE 1
i) Raw materials, equipment required to be used in the processes are as follows :
a) liquid steel to be treated having the approximate composition,
C - 0.04%
O - 800 ppm or higher N - 50 ppm or higher Temperature : 1580-1650°C Pressure: Atmospheric
b) Carbon containing Reagent: Iron bearing 4-6% carbon
c) Equipment: Typical equipment may consist of a steel ladle with some
free board to accommodate gas generated. Reagent is kept at the
bottom of the ladle. Liquid steel is poured from the top to the ladle.
c)
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ii) Procedure to be followed :
a. Requisite amount of carburized iron 2-4% of steel with carbon content
4.4 to 4.9% was placed in the bottom of the ladle.
b. Liquid steel 1580-1650°C with dissolved oxygen of 800-1200 ppm
carbon 0.025-0.04% and nitrogen of 50-125 ppm from the primary
furnace e.g., BOF which contains dissolved oxygen of about 800 ppm
is poured in ladle.
Reaction between dissolved oxygen in steel and carbon dissolving from the reagent producing carbon monoxide gas bubbles facilitating removal of nitrogen from steel.
EXAMPLE 2
Under this Example the following process steps were followed :
a. 50 kg of mild steel scrap was melted in a 100 kg air induction furnace.
b. Temperature of the bath was adjusted between 1580-1650°C. Oxygen
gets picked up from air. Oxygen was greater than 800 ppm.
c. Carburized cast iron containing 4.4-4.9% C was introduced into the
melt from the top.
d. Bath surface was covered with lime powder to reduce nitrogen pick up
from air.
As cast iron dissolved, carbon reacted with oxygen in steel producing a large volume of carbon monoxide gas in the form of tiny bubbles providing conditions for denitrogenation of steel.
Typical results of denitrogenation achieved following the above process under Example 2 by gas generation in-situ are detailed hereunder in Table 1

6 TABLE 1

Expt.No.
Bath Temp °C
Amount of carburized iron, kg
Initial oxygen ppm
Initial nitrogen ppm
Initial %C
Final %C
Equilibrium partial pressure of initial N atmosphere
Final N ppm
1
1583
1.70
809
76
0.026
0.050
29x10-2
38
2
1597
1.96
1112
94
0.034
0.056
4.42x10-2
57
3
1583
1.50
825
79
0.023
0.052
3.12.x 10-2
58
4
1609
1.20
992
97
0,048
0.036
4.70x10-2
71
5
1581
1.00
1251
53
0.028
0.016
1.4.x 10-2
48
In the above process, of the invention a large volume of gas is produced in situ inside the bath with very high interfacial surface area. The equilibrium partial pressure of nitrogen in steel exceeds the partial pressure of nitrogen in the gas generated at the reaction site; although, the partial pressure of nitrogen in the atmospheric environment is of the order of 0.8 atmosphere. Since denitrogenation process is an interfacial process, large interfacial area of bubbles accelerates the kinetics of the process.
It would thus be evident from the above that the process of the invention provides for simple and cost effective denitrogenation of liquid steel. Also, simultaneous deoxidation and denitrogenation of an oxidised bath of low carbon steel is possible by generating flushing gas with large interfacial area inside the bath itself by chemical reactions. Some deoxidiser can be saved. Importantly, denitrogenation of low carbon steel is feasible under atmospheric condition. No vacuum equipment is required and also no special flux is required for removal of nitrogen from steel following the above cost effective process of the invention.

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WE CLAIM :
1. An improvedprocess^ for denitrogeifiation of steel comprising :
subjecting the liquid steel to treatment with carburized iron and thereby generate carbon monoxide gas in situ such as to provide large interfacial area and requisite low partial pressure of nitrogen at the reaction site and effectuate removal of nitrogen.
2. An improved process as claimed in claim 1 wherein the carburized iron used
is iron bearing 4-6% carbon.
3. An improved process as claimed in anyone of claims 1 to 2 comprising :
melting mild steel scrap in furnace maintaining the temperature in the range of
1580-1650°C;
introducing carburized iron into the melt from the top to facilitate in situ generation of carbon monoxide gas and favour the driving off of the nitrogen.
4. An improved process as claimed in anyone of claims 1 to 3 wherein after the
step of addition of the carburized iron, the bath surface is covered with lime
powder to reduce nitrogen pick-up from air.
5. An improved process as claimed in anyone of claims 1 or 2 comprising :
t) providing requisite amount of carburized iron in the bottom of the ladle ;
ii) pouring liquid steel from the primary furnace in said ladle to thereby effectuate a reaction between the dissolved oxygen in steel and the carbon to generate in situ carbon monoxide gas bubbles to facilitate removal of nitrogen from steel.

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6. An improved process as claimed in anyone of claims 1 to 5 wherein the
molten steel has a dissolved oxygen content of greater than 800 ppm
7. An improved process as claimed in anyone of claims 5 or 6 wherein the
temperature of steel is maintained in the range of 1580-1650°C.
8. An improved process as claimed in anyone of claims 1 to 7 wherein the
molten steel has a dissolved oxygen content of 800-1200 ppm,
9. An improved process as claimed in anyone of claims 1 to 8 wherein the
carbon content of steel is in the range of 0,025 - 0.04% and initial nitrogen
content of 50-125 ppm.
10.An improved process as claimed in anyone of claims 1 to 9 wherein the amount of carburized iron used is 2-4% of steel
11. An. improved process as claimed in anyone of claims 1 to 10 wherein the rate
of generation of gas in situ is higher compared to the rate at which gas can be
injected from outside as flushing agent in order to create an environment of
low partial pressure of nitrogen in bath and the small size of gas bubbles
which provide large interfacial area to facilitate the kinetics of denitrogenation
process.
12. An improved process as claimed in anyone of claims 1 to 11 wherein the rate
of gas generation depends upon the high oxygen content of the bath and low
carbon content of steel to be treated.
13. An improved process as claimed in anyone of claims 1 to 12 wherein the ratio
of carbon and oxygen maintained in the bath after reaction is close to
thermodynamic equilibrium of carbon oxygen reaction in steel.
14. An improved process as claimed in anyone of claims 1 to 13 wherein an
oxidised steel bath under low carbon is used.

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15.An improved process as claimed in anyone of claims 1 to 14 wherein the same is carried out in nitrogen free environment.
16. An improved process for denitrogenation of low carbon steel substantially as hereindescribed and illustrated with reference to the accompanying examples.

ANJAN SEN
Of S. MAJUMDAR&CO.
Applicants' Agent
Dated this 10th day of January 2001

An improved process for denitrogenation of steel and in particular to a process for denitrogenation of low carbon steel by gas generation in-situ. The improved process comprising subjecting the liquid steel to treatment with carburized iron and thereby generate carbon monoxide gas in situ such as to provide large interfacial area and requisite low partial pressure of nitrogen at the reaction site and effectuate removal of nitrogen. In the above process gas is generated in situ inside the liquid steel bath by reaction of oxygen present in the steel with carbon which is present in carburized iron added to the bath. Oxygen in steel reacts with carbon and carbon monoxide gas is generated in situ in the form of large number of tiny bubbles which provide large interfacial area and requisite low partial pressure of nitrogen at the reaction site. The process thus provides for an alternative approach to reduce nitrogen content of steel under atmospheric conditions and without having to use expensive fluxes. The above process of denitrogenation of steel further assist in deoxidation of the steel tapped from BOF. The process is simple and can be attended without the need for expensive equipment and/or expensive special fluxes.

Documents:

00014-cal-2001-abstract.pdf

00014-cal-2001-claims.pdf

00014-cal-2001-correspondence.pdf

00014-cal-2001-descritpion(complete).pdf

00014-cal-2001-form-1.pdf

00014-cal-2001-form-18.pdf

00014-cal-2001-form-2.pdf

00014-cal-2001-form-3.pdf

00014-cal-2001-p.a.pdf

14-cal-2001-granted-abstract.pdf

14-cal-2001-granted-claims.pdf

14-cal-2001-granted-correspondence.pdf

14-cal-2001-granted-description (complete).pdf

14-cal-2001-granted-examination report.pdf

14-cal-2001-granted-form 1.pdf

14-cal-2001-granted-form 18.pdf

14-cal-2001-granted-form 2.pdf

14-cal-2001-granted-form 3.pdf

14-cal-2001-granted-letter patent.pdf

14-cal-2001-granted-pa.pdf

14-cal-2001-granted-reply to examination report.pdf

14-cal-2001-granted-specification.pdf


Patent Number 194165
Indian Patent Application Number 14/CAL/2001
PG Journal Number 30/2009
Publication Date 24-Jul-2009
Grant Date 29-Jul-2005
Date of Filing 10-Jan-2001
Name of Patentee STEEL AUTHORITY OF INDIA LIMITED
Applicant Address RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI
Inventors:
# Inventor's Name Inventor's Address
1 RADHESHYAM SAU RESEARCH & DEVELOPMENT CENTRE FOR IRON AND STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834002
PCT International Classification Number C12C 5/38
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA