FIELD OF THE INVENTION

The present invention relates to an improved process technology of deoxidation and refining of steels having end application in making non-rimming variety of welding electrodes i.e. Electrode Welding Non-rimming (EWNR). The proposed process has resulted in producing defect free billets of EWNR as well as elimination of nozzle clogging problem in these type of grades.
BACKGROUND OF INVENTION:
Electrode quality (EQ) steels consist of both rimming and non-rimming (killed) quality steels which are used as the core wire of covered electrodes. Rimming quality steel is preferred material for covered welding electrodes used in manual metal arc welding (MMAW). The starting material for rimming steel core wire is ingots whereas for non-rimming quality, it is either bloom or billet.

EWNR (Electrode Welding Non Rimming) is a non rimming continuously cast substitute of conventional rimming steel produced through ingot casting route. Due to compositional requirement of grade (Table-1), the amounts of major deoxidants are restricted, which in turns makes it very difficult to control dissolved oxygen in liquid steel. The main challenge of manufacturing this grade is to meet the chemistry as specified by the customers, maintain low dissolved oxygen level during steelmaking operation, trouble free casting and billets free from blow holes and pin holes.

Table 1-Specification of EWNR Grade Steel
C Si Mn P S Al
0.1 max 0.03 max 0.38-0.62 0.03 max 0.025 max 0.012 max

Sticking of alumina as well as spinel inclusions inside the sub entry nozzles (SEN) and in tundish nozzle is perennial problem during continuous casting of aluminum killed steel through billet casters. This results in restriction or completely blockage of flow of liquid steel through the nozzles eventually leading to abortion of sequence in billet casters and stopping of continuous casting machine. Nozzle clogging not only restricts the productivity by restraining the casting sequence, intermittent extrication of clogged alumina particles or dislodged refractory materials are a significant source of non-metallic macro-inclusions in the cast sections of billet casters. If these inclusions are not removed completely during secondary refining of steel they causes excessive clogging mainly in low carbon Al killed steels. In other grades of Al killed steel cogging is also very prominent if the deoxidation and secondary refining is not carried out properly.
OBJECTIVE OF INVENTION:
For low carbon Al-killed (LCAK) steel grades nozzle clogging is a big problem. Continuous casting of low carbon aluminum killed grades is more susceptible for nozzle clogging. Normally in the various process technologies adopted for heat making of EWNR grade the billets made are rejected due to severe blow holes and pin holes (because of excess dissolved oxygen content). Cobbling and cracking of billets during wire rolling was also occurred due to inferior internal soundness of billets. Also, problems of nozzle clogging in LCAK grades especially in EWNR (electrode welding non rimming quality) were very severe. In many cases the casting was aborted in 2nd heat only due to heavy clogging in sub entry nozzle (SEN).
The present invention is related to an improved process technology of deoxidation and refining of steels having end application in making non rimming variety of welding electrodes (EWNR). The proposed improved method has resulted producing defect free billets of EWNR as well as elimination of nozzle clogging problem in these type of grades.
Therefore such as herein described there is provided an improved method of de-oxidation and refining of steels comprising the steps of controlling the dissolved oxygen in steel ~ 30 ppm by killing steel during tapping only and avoiding any aluminium wire addition during secondary refining, adding aluminium cube/block in a single stage during initial stage of de-oxidation, floating out the alumina inclusions formed near the slag metal interface and preventing chances of Calcium Sulphide (CaS) formation by , making Hot Metal Desulphurisation Station (HMDS) mandatory for all heats.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 illustrates the Sub Entry Nozzle Clogging during casting of EQ grade as a prior art.
DETAILED DESCRIPTION OF THE INVENTION
Non metallic inclusions in steel which are solid at continuous casting temperatures are the prime cause for nozzle clogging of SEN. Upon analysis we can see that various types of different solid inclusions are found in SEN clogged material. Major inclusions types are alumina (Al2O3), spinel types (MgO-Al2O3), different forms of calcium aluminates like CaO-Al2O3 (solid or semi-solid), CaS, CaO-Al2O3-CaS, CaO-Al2O3-MgO etc. Reoxidation of alloys like Al also plays a very critical role in nozzle clogging. Sometimes undue heat loss of steel in tundish during continuous casting can also stimulate nozzle clogging due to chilling or freezing of metal in SEN. Many process technologies are adopted at different steel plants for solving the problem nozzle clogging. Some of the established technological practices to prevent nozzle clogging of SEN are mentioned below:
Ca-Treatment Practice- Proper treatment of steel with Ca results in conversion of solid oxide inclusions like alumina into liquid inclusions of calcium aluminates, thus as a result nozzle clogging is minimized.
Careful selection of deoxidizer and deoxidation method – Deoxidation practice adopted plays a major role in controlling the genesis of solid non metallic inclusions. Not only this nature and morphology of inclusions generated also results from the selection of deoxidizers, deoxidation practice like sequence of ferro alloys additions. Selection of suitable combination of deoxidation practice like pet coke deoxidation, complex deoxidation& use of low Al for deoxidation helps in reduction of nozzle clogging;
Protection from Reoxidation and Argon injection in the SEN- Reoxidised alumina inclusions are very small in sizes and have got a tendency of forming chains of alumina. Argon injection in the SEN through mono block stopper prevent molten steel from reoxidation and also helps to push any clog material buildup into mould, so nozzle clogging is reduced.

Rimming quality steel is preferred material for covered welding electrodes used in manual metal arc welding (MMAW). The main challenge of manufacturing EWNR grade is to meet the chemistry as specified by the customers, maintain low dissolved oxygen level during steelmaking operation, and trouble free casting and billets free from blow holes and pin holes. Also for low carbon Al-killed (LCAK) steel grades nozzle clogging is a big problem. Continuous casting of low carbon aluminum killed grades is more susceptible for nozzle clogging. Normally in the various process technologies adopted for heat making of EWNR grade the billets made were rejected due to severe blow holes and pin holes (because of excess dissolved oxygen content). Cobbling and cracking of billets during wire rolling was also occurring due to inferior internal soundness of billets. Problems of nozzle clogging in LCAK grades especially in EWNR (electrode welding non rimming quality) were very severe. In many cases the casting was aborted in 2nd heat only due to heavy clogging in sub entry nozzle (SEN). The present work is related to an improved process technology of deoxidation and refining for steels having end application in making non rimming variety of welding electrodes (EWNR). The proposed improved method has resulted producing defect free billets of EWNR grade as well as elimination of nozzle clogging problem in these types of grades.
Conventional heat making practice of EWNR grade
EWNR grade of steel are also known as electrode quality (EQ-non rimming) steels used for making welding electrodes are having low percentage of carbon, silicon and aluminum (Table-1). Further sulphur and phosphorus control is very important in EQ steels as manganese level is also on lower side. It is also required to keep sulfur and phosphorous content as low as possible as they can cause undesirable reduction in mechanical properties like brittleness in the weld metal.
Table 1-Specification of EWNR Grade Steel
C Si Mn P S Al
0.1 max 0.03 max 0.38-0.62 0.03 max 0.025 max 0.012 max
In conventional practice of heat making of EWNR grade the nozzle clogging was very serious (Figure 1), there were only one or two heats in one casting sequence. Problems of SEN nozzle clogging in EWNR grade was a bottleneck in shop’s productivity due to lower casting speed, break-out due to biased flow and loss in time due to replacement of nozzle as shown in Fig 1. The cost of production was also increased due to nozzle change, tundish change etc. Also, the quality of cast product was not up to the mark due to entrapment of dislodged non-metallic and undesirable flow patterns leading to powder entrapment & shell thinning.
Conventional practice involved addition of ferro-manganese and few quantity of Al bar (~100 kg) during tapping at BOF. Large quantity of Al wire (~150 kg) was added during LF refining and around 0.4 kg/tcs of CaSiwas injected in LF. In the final chemistry the Al was kept ~0.005% and Ca was ~25 ppm.
Work Carried Out
Modifications in flux and deoxidation and steel refining practice
In the conventional deoxidation practice for EQ grade at ISP addition of only 300-400 kg of HCFeMn and 100-150 kgs of Al bar (in ladle bottom) was added during tapping. The fluxing practice was addition of lime around 500 kgs, 200 calcined dolo addition and 100 kg calcined bauxite addition. In secondary refining i.e in ladle furnaces around 150 kg (400-500 mtrs) of Al wire was added to fully kill the steel.
Higher amount of Al wire addition in LF leads to higher clogging due to formation of fine alumina inclusions inside the melt which is generally difficult to float out and insufficient flotation time.
The deoxidation practice was modified to control the dissolved oxygen in steel ~ 30 ppm by killing steel during tapping only, thus avoiding/minimizing wire addition during secondary refining. Addition of Al cube/block addition during initial stage of deoxidation has beneficial effect of clogging. When aluminium is added at the initial stages of tapping it also helps in reduction of the oxide components like FeO & MnO (slag deoxidation) in the slag. Since the alumina inclusions are formed near the slag metal interface it is easy to float them out. Late addition of Al during secondary refining and additions of Al in separate different batches is not advisable because they increase the clogging tendency of the melt. Therefore it was recommended to add Al in single batch and as early as possible to give inclusions time to coagulate and float out. It has been reported that if Al is added late it results in formation of angular, faceted type of inclusion morphology having clustering and chain forming tendency and they have very weak tendency to float out. At IISCO Steel Plant it was also observed that last stage Al addition in the form of Al wire at the ladle furnace processing was leading to severe clogging.

In order to make the slag fluid and also inclusion friendly the flux practice was modified to get a soft, not corrosive ladle top slag by introduction of calcined dolomite & calcined bauxite. In conventional practice only lime addition of ~600 kg was practiced, which was modified (stage-wise) to 800 kg lime addition during tapping + 200 kg calcined dolomite and 250 kg calcined bauxite addition. The modified practice helped in reduction in slag melting point and fluidity of ladle top slag and making it suitable for refining of steel.
Optimization of Calcium Treatment for EQ grades
Modification of calcium treatment practice was carried out based on the grade chemistry and total oxygen level in steel the liquid window was indentified using FACTS age software. To prevent any chances of CaS formation, HMDS was made mandatory for all heats.
It was decided to keep Al in the range of 0.006-0.01% so as to keep dissolved oxygen level ~30 ppm. For this Al and oxygen range, the identified Ca in steel was ~30-40 ppm. Accordingly CaSi injection was adjusted as ~0.6 kg/tcs, targeting final Ca in tundish not < 25 ppm. For removal of the deoxidation product and transformed calcium aluminates after calcium treatment extended soft purging for 7-8 minutes was carried out after secondary refining.
Usefulness of the Invention
The work is based on the actual work carried out in industrial scale to eliminate the difficulties faced in production of non rimming variety of electrode quality wire rods. Blowhole defects in billets due higher oxygen potential of melt leading to rejections at billet stage or cobbling of wires during rolling is a perennial problem for steel making shops. The present work describes methodologies adopted to improve the processing parameters to produce defect free billets. The work described in the specification is pertinent to all steel industries and will also be helpful in solving their problem of nozzle clogging.
Industrial Applicability
The modified practice for steel deoxidation and refining can be utilized in any steel melting shop with modifications of flux and de-oxidation practice and selection of suitable Ca wire injection range. The modified practice was used for production of EWNR grade steel at IISCO Steels Plant, Burnpur. SEN Nozzle clogging during of casting of low carbon aluminium killed steel was a bottleneck for ISP. Technical interventions helped in almost eliminating nozzle clogging tendency in EWNR grade and also helped in producing defect free billets with sound macro properties. With these interventions it is, now it is possible to produce steel billets for non rimming variety of electrode quality wire rods grade of steel.
Improved method of deoxidation and fluxing practice for EWNR Grade
Continuous casting of low carbon aluminum killed grades is more susceptible to nozzle clogging. The most important requirement of low carbon aluminium killed steel is to minimise generation of alumina and removal of de-oxidation product mainly alumina from liquid steel. As we can see from (table-1) the chemistry of EWNR grade there is a restriction of Si (0.03%max) and in Al (0.012% max). With this restriction it is very difficult to control the level of deoxidation of melt. The under deoxidized melt will have a tendency of boiling of metal in mould during casting as well as the cast product will have inferior macro soundness (high blow holes and pin holes). The fully killed condition of melt also poses difficulties during continuous casting, inferior cleanliness level and chances of grade deviation due to higher Al in final chemistry.
In the developed method, the sequence and quantity of ferro alloy/deoxidizer addition was optimized. Ferro alloy addition sequence, made during tapping of steel in ladle was addition of strongest deoxidizer as first in the sequence (i.e Al bars to be added first) and to be followed by appropriate amount of addition of FeMn. This practice lead to higher Al2O3 in the slag (proper killing of steel and a raised alumina content ensured fluid ladle top slag) and less content of Al in liquid steel. As the bath got killed sufficiently there was very less or no requirement of Al wire addition in secondary refining.
The developed flux and deoxidation practice has been designed to suit the grade requirement of EWNR steel with sound macro as well as higher casting sequence length due to no clogging of SEN.
Development of standard practice for steel refining & inclusion flotation treatment
Low Carbon Aluminium Killed (LCAK) steels are prone to clogging. Earlier attempts of making EWNR grade with conventional method viz. steel de-oxidation and aluminium buildup in LF lead to ladle nozzle and SEN clogging during casting. Developed standard practices for steel de-oxidation and its refining in LF helped to produce clean steel free from alumina as well as with sound macro. The calcium treatment practice has been optimized considering the residual oxygen level in steel to transform the alumina into liquid calcium aluminates. The liquid window for addition of Ca has been worked out for EQ grade under ISP conditions. Special extended soft purging of bath with a suitably calculated purging regime was been used to maximize the floatation of transformed alumina.

Although the foregoing description of the present invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of illustration by way of examples and description and is not intended to be exhaustive or to limit the invention to the particular embodiments and applications disclosed. The particular embodiments and applications were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such changes, modifications, variations, and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled..

WE CLAIM:

1. An improved method of de-oxidation and refining of steels comprising the steps of :
controlling the dissolved oxygen in steel ~ 30 ppm by killing steel during tapping only and avoiding any aluminium wire addition during secondary refining;
adding aluminium cube/block in a single stage during initial stage of de-oxidation;
floating out the alumina inclusions formed near the slag metal interface; and
preventing chances of Calcium Sulphide (CaS) formation by , making Hot Metal Desulphurisation Station (HMDS) mandatory for all heats.

2. The method of de-oxidation and refining of steels as claimed in claim 1, wherein the proportion of ingredients include
C Si Mn P S Al
0.1 max 0.03 max 0.38-0.62 0.03 max 0.025 max 0.012 max
and balance iron.

3. The method of de-oxidation and refining of steels as claimed in claim 1, wherein the aluminium was kept in the range of 0.006-0.01% so as to keep dissolved oxygen level ~30 ppm.

4. The method of de-oxidation and refining of steels as claimed in claim 1, wherein the identified Ca in steel is ~30-40 ppm.

5. The method of de-oxidation and refining of steels as claimed in claim 1, wherein the CaSi injection is adjusted as ~0.6 kg/tcs, targeting final Ca in tundish not < 25 ppm.

6. The method of de-oxidation and refining of steels as claimed in claim 1, wherein order to remove the deoxidation product and transformed calcium aluminates after calcium treatment extended soft purging for 7-8 minutes is carried out after secondary refining.

7. The method of de-oxidation and refining of steels as claimed in claim 1, wherein the said method is configured for production of non rimming variety of electrode quality wire rods (EWNR).

8. The method of de-oxidation and refining of steels as claimed in claim 1, wherein the said method is configured to produce billets free from Blowhole defects.

9. The method of de-oxidation and refining of steels as claimed in claim 1, wherein the said steel melt is free from nozzle clogging defects.