Claims:We Claim:

1. A cored wire for clean steel production comprising:
CaFeAl cored wire having composition of 5 to 15 % Al + 30 to 50 % Ca and the rest % Fe.

2. A cored wire as claimed in claim 1 comprising Ca-40%, Al 10% and Rest Fe.

3. A cored wire as claimed in anyone of claims 1 to 2 having linear density in the range of 0.225 to 0.235 kg/mtr.

4. A process for clean steel production comprising of step of secondary steel making comprising
carrying out said secondary steel making of Ca treatment at high temperature in the range of 1580 to 15900C involving CaFeAl cored wire having composition of 5 to 15 % Al + 30 to 50 % Ca and the rest % Fe.

5. A process as claimed in claim 4 comprising:

injecting said CaFeAl cored wire at said temperature range of 1580 to 15900C after desulphurization in the bath avoiding formation of CaS inclusions Ca losses at early stage and ensuring soft purging rate (< 10 Nm3/hr) for inclusion floatation.

6. A process as claimed in anyone of claims 4 or 5 comprising maintaining optimum amount of Al in the bath in the range of 0.025 to 0.035 % prior to Ca treatment.

7. A process as claimed in anyone of claims 4 to 6 comprising:

carrying out said Ca treatment after desulphurization to the required rate in the bath of Sulphur drop by < 0.01% along with an Al of 0.035 % in the bath ;
said cored wire injected after time gap of 6 to 10 minutes preferably minimum 7 minutes after Aluminium wire addition to avoid the formation of CaS inclusions Ca losses at early stage; and
after said Ca treatment maintaining said soft purring rate (< 10 Nm3/hr) for inclusion floatation.

8. A process as claimed in anyone of claims 4 to 7 comprising:

providing low carbon steel processed in ladle having a composition of 0.05% max of C, 0.03% max of Si, 0.1-0.15 % of Mn, 0.025-0.045% of Al, 0.007% max of S and the remainder of Fe with a shielding of slag;
carrying out said Ca treatment involving said cored wire by adding to the molten steel at high temperature in the range of 1580 to 15900C and achieving desired Al content drop and the sulfur content drop and maintaining residual amount of calcium of 0.0018-0.0025% in the bath.

9. A process as claimed in anyone of claims 4 to 8 comprising involving reduced average length of CaFeAl wire injected upto 325 mtrs. only and enabling casting of thus Ca treated metal free of any stopper rise.

10. A process as claimed in anyone of claims 4 to 9 comprising said cored wire Ca treatment enabling Ca recovery of 16 to 19 %.

Dated this the 7th day of May, 2016 Anjan Sen

Of Anjan Sen & Associates
Applicants Agent

, Description:FIELD OF THE INVENTION

The present invention relates to cored wire for clean steel production and a process for clean steel production. More particularly, the present invention is directed to advancements in CaFeAl composite cored wire for benefit utilization in injection process in secondary steelmaking such as to achieve desired Ca recovery in steel without significant drop in Al and avoiding generation of Al2O3 inclusions and related problems of nozzle clogging and/or stopper rise in casting stage. The advancement is targeted at simple and cost-effective manner of achieving secondary steel making more efficiently and with better control over the refinement process.

BACK GROUND OF THE INVENTION

Calcium is added to aluminum killed steels during secondary steel making for the modification of solid alumina inclusions into liquid calcium aluminates which also have high dissolvability for sulphide inclusions. This is beneficial for the smooth continuous casting process by avoiding nozzle clogging problems but also contribute for the clean steel by removing sulphide type inclusions.

At Secondary steel making Ca treatment is carried out for:
(i) To improve the castability of Aluminum killed steels. Alumina modification by Calcium into calcium Aluminates reducing the risk of clogging by producing liquid inclusions at casting temperature and by reducing the oxygen content.
(ii) Inclusion modification and reduce the inclusion number in the bath.
(iii)Desulphurization and modification of iron and manganese sulphide inclusions into globular inclusions.

The present invention pertains to composite cored wire having a reactive metal as one of the constituent of its core material and beneficial effect of core material compositional change on the steel cleanliness and cost of operation.

The beneficial effect of adding calcium to steel in clean steel technology is to convert solid alumina inclusions into liquid calcium aluminates. Various methods are available for adding calcium to liquid steel like pure calcium wire, CaSi wire, CaFe wire and CaFeAl wire. The method of adding these wires in liquid steel bath was discussed in detail in US Patent No:4481032. Existing CaFeAl cored wire contains Al in the level of 25% by weight.

It is also well known fact that when pure calcium metal cored wire is added to steel bath, it creates more agitation and splashing of the bath due to its high vapor pressure and low boiling temperature. This high metal splashing creates operational and safety related problems during the calcium treatment. It also results in low calcium retention time giving low recovery of calcium. It was found that the reactivity of calcium can be significantly reduced if calcium is combined with aluminium as aluminum lowers the vapour pressure of calcium and increases its retention time in the bath.

US Patent No: 2085802 discuss the steel purification and inclusion removal by the insertion of calcium core inserted in a sheath of steel tube.

In US patent No: 4097268, it has been claimed that composite calcium clad wire consisting of steel or aluminum sheath with a mixture of calcium, aluminum and rare-earth elements as cored materials improves deoxidation, desulphurization and steel cleanliness.

US Patent No. 2,819,956 discloses an addition agent for the treatment of steel, in which a core of a mixture of calcium and aluminum is encapsulated by iron to reduce non-metallic inclusions.

US patent No: 6346135 discuss the treatment of molten metal bath by fabricating a cored wire with an outer steel jacket having a higher melting point than an inner core material and the inner core material is a composite material of calcium and aluminum. The composite core material, having lower melting temperature melts and forms an alloy prior to the melting of outer jacket.

The above mentioned patents pertain to either composite cored wire consisting of aluminum and calcium wire or a mixture of aluminum and calcium powders at various proportions. However, it has been found by way of the present advancement that the higher percentage of aluminum in cored wire used in the above said patents creates certain disadvantages during steel processing as detailed below. Nozzle clogging or stopper rise in thin slab casters resulted from the treating of steel with cored wire with high aluminum content, reduces productivity and increases surface and subsurface defects in hot rolled coils. Therefore, the present invention is aimed to reduce the creation of additional inclusions during secondary steel making by calcium treatment.
Some disadvantages of CaFeAl wire with higher Al percentage include:
• Increase in the percentage of aluminum in the cored wire results in the formation of more alumina inclusions either in ladle furnace or continuous caster leading to SEN clogging by inclusion deposition.
• Processing time at ladle furnace is increased due to bath soft purging requirement in case of abnormal rise in bath aluminum beyond the grade specification limits.
• Inconsistent calcium recovery and more CaFeAl cored wire consumption.
• Poor steel cleanliness.
• Higher procurement cost with 25% Al in the CaFeAl wire.

The present advancement thus attempts to provide a clean steel production process involving cored wire with selective amounts reduced of Al along with Ca and balance Fe so that Al and Ca are maintained in desired level in the treated steel while also eliminating problems of casting due to inclusions with the advantage of reduced cored wire consumption and resultant reduction in cost of finished steel.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide a cored wire for clean steel production enabling maintaining correct aluminum content as per the grade specification without the requirement of compositional adjustment after calcium treatment and a process for clean steel production comprising of step of secondary steel making.

A further object of the present invention is directed to advancement in constitution of CaFeAl cored wire of selective composition in secondary refining of steel for better castability, cleaner steel and less surface defect along with cost saving.

A still further object of the present invention is directed to advancements in constitution of CaFeAl cored wire of selective composition in secondary refining of steel for improved cleanliness of Steel by reducing Al2O3 inclusion, reduced % FeO in slag and increased %S in slag indicating higher desulphurization efficiency.

A further object of the present invention is directed to advancements in process for clean steel production involving CaFeAl cored wire even with reduced Al content so that any additional treatment required to adjust aluminum after calcium treatment that adds cost and deteriorates quality of steel is reliably avoided.

A still further object of the present invention is directed to provide a process for clean steel production involving CaFeAl cored wire with reduced Al content to improve castability by reducing additional generation of Al2O3 inclusions during/after calcium treatment and improvement in the surface quality of final product.

Another object of the present invention is directed to provide a process for clean steel production involving CaFeAl cored wire with reduced Al content to limit the soft rinsing duration as per SOP and delivery the heat with right chemistry & temperature.

Yet another object of the present invention is directed to provide a process for clean steel production involving CaFeAl cored wire with reduced Al content to improve the cleanliness of steel with reduced procurement cost of CaFeAl wire by incorporating selective chemical composition.

A still further object of the present invention is directed to provide a process for clean steel production involving CaFeAl cored wire with reduced Al content to enable delayed release of Ca at higher depth of molten metal bath, thus reducing wire consumption while ensuring desired Ca recovery.

SUMMARY OF THE INVENTION

Thus according to the basic aspect of the present invention the same is directed to a cored wire for clean steel production comprising:
CaFeAl cored wire having composition of 5 to 15 % Al , 30 to 50 % Ca and the rest % Fe.

A further preferred aspect of the present invention is directed to said cored wire comprising Ca-40%, Al 10% and Rest Fe.

A still further aspect of the present invention is directed to said cored wire having linear density in the range of 0.225 to 0.235 kg/mtr.

Another aspect of the present invention is directed to a process for clean steel production comprising of step of secondary steel making comprising
carrying out said secondary steel making of Ca treatment at high temperature in the range of 1580 to 15900C involving CaFeAl cored wire having composition of 5 to 15 % Al + 30 to 50 % Ca and the rest % Fe.

A further aspect of the present invention is directed to said process comprising:

injecting said CaFeAl cored wire at said temperature range of 1580 to 15900C after desulphurization in the bath avoiding formation of CaS inclusions Ca losses at early stage and ensuring soft purging rate (< 10 Nm3/hr) for inclusion floatation.

A still further aspect of the present invention is directed to said process comprising maintaining optimum amount of Al in the bath in the range of 0.025 to 0.035 % prior to Ca treatment.

Yet another aspect of the present invention is directed to said process comprising:

carrying out said Ca treatment after desulphurization to the required rate in the bath of Sulphur drop by < 0.01% along with an Al of 0.035 % in the bath ;
said cored wire injected after time gap of 6 to 10 minutes preferably minimum 7 minutes after Aluminium wire addition to avoid the formation of CaS inclusions Ca losses at early stage; and
after said Ca treatment maintaining said soft purring rate (< 10 Nm3/hr) for inclusion floatation.

A further aspect of the present invention is directed to said process comprising:

providing low carbon steel processed in ladle having a composition of 0.05% max of C, 0.03% max of Si, 0.1-0.15 % of Mn, 0.025-0.045% of Al, 0.007% max of S and the remainder of Fe with a shielding of slag;
carrying out said Ca treatment involving said cored wire by adding to the molten steel at high temperature in the range of 1580 to 15900C and achieving desired Al content drop and the sulfur content drop and maintaining residual amount of calcium of 0.0018-0.0025% in the bath.

A still further aspect of the present invention is directed to said process comprising involving reduced average length of CaFeAl wire injected upto 325 mtrs. only and enabling casting of thus Ca treated metal free of any stopper rise.

A still further aspect of the present invention is directed to process comprising said cored wire Ca treatment enabling Ca recovery of 16 to 19 %.

The above and other objects and advantages are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings and example.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig 1: showing graphically the Stopper position of heat processed with 10% Al based CaFeAL cored wire according to the present invention;

Fig 2: showing graphically the Stopper position of heat processed with conventional 25% Al based CaFeAL wire;

Figure 3: showing graphically (a) reduction in casting problems on account of stopper rise, nozzle clogging etc was observed with the use of new CaFeAl wire with reduced Al as compared to earlier used wire with 25% AL; and (b) improvement observed in HR coil surface quality such as defects related to Al streaks, mould slag/ powder entrapment in case of new CaFeAl wire.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS

The present invention is directed to provide a cored wire for clean steel production in order to maintain correct aluminum content as per the grade specification without the requirement of compositional adjustment after calcium treatment and a process for clean steel production comprising of step of secondary steel making.

Secondary steel making conventionally requires Al wire addition (99.9% Al) for de-oxidation and de-sulphurisation. At secondary steel making after checking the initial bath chemistry, lime and Al wire is introduced into the bath until the sulphur drops below 0.007%. So after sulphur drop, Ca treatment is carried out maintaining a gap of at least about 7 minutes.

The usage of CaFeAl wire is mostly being opted for Calcium treatment of aluminum killed steels. However, the usage of CaFeAl has also got some deficiencies as mentioned hereinbefore.

It has been observed that when a conventional CaFeAl wire with 25% aluminum, around 16 kgs (0.08 kgs/t) of Al when injected into steel, it reacts with the scarce O2 in the bath or the O2 entrapped during wire feeding and generates around 30 kgs of Al203. The more Al addition into the bath the more Al2O3 inclusions are generated in steel.

The challenging task is to develop a material for cored wire that serves the purpose of inclusion modification as well as to reduce the generation of inclusions. Usage of pure calcium could serve the purpose of inclusion morphology change with no extra generation of inclusions in steel, but, its high procurement cost, its vigorous reaction and splashing when added to steel and associated aluminum drop by 0.012% limits its usage in the steel industry. This abnormal aluminum drop indicates the reaction of soluble aluminum with oxygen to form around 45 kgs of Al2O3 inclusions in a heat size of 200 tons.

The proposed CaFeAl wire with 5-15% aluminum is found to be selectively beneficial for heat processing with lesser inclusion generation in steel.

Some of the critical aspects governing secondary steel refining process stages for Ca treatment involving the selective CaFeAl wire with 5-15% aluminum according to the present invention comprise the following:

1.0.025 to 0.035 % Al is optimum amount of Al required in the bath prior to Ca treatment.
2. to carry out Ca treatment at high temperature (1580-1590 °C) to avoid temperature drop after Ca treatment.
3. Ca treatment is carried out after desulphurization to the required rate in the bath;
4. avoiding formation of CaS inclusions, Ca losses at early stage;
5. After Ca treatment, ensure soft purging rate (< 10 Nm3/hr) for inclusion floatation.

The nature of the invention, its objects and advantages are explained hereunder in greater detail in relation to the following non-limiting exemplary illustrations traversed hereunder:

EXAMPLE

Trials were conducted with various Al% compositions (10,12,15,18,25% Al) in Cored wire in Low carbon steel grades –Cold rolled & Cold galvanized with Al 0.025-0.045 % and restriction of Si.

Low carbon steel processed in ladle (Magnesia lining ladle) having a composition of 0.05% max of C, 0.03% max of Si, 0.1-0.15 % of Mn, 0.025-0.045% of Al, 0.007% max of S and the remainder of Fe with a shielding of slag , and then cored wire (modified) composed of a sheath of a steel tube having an outer diameter of 13 mm, composed of mixture having 40% Ca, 10% Al and 50% Fe which had been prepared by cold rolling was added to the molten steel at 1,585° C. As the result of this treatment, 5 minute after the addition of the wire, the Al content drop by 0.002% and the sulfur content drop by 0.001%.

The residual amount of calcium is 0.0018-0.0025% in the bath, which is persistent even in case of 25 % Al cored wire indicating that lower % Al in wire still enabled to reduce the local vapour pressure of Ca effectively maintaining the same Ca PPM in the bath.

The final preferred deoxidation products after Ca treatment are C12A7 (12CaO.7Al2O3) and C3A (3CaO.Al2O3) which have a lower melting point of 1455 °C and 1535 °C respectively.

The excess %Al in the wire is of not much use (leads to the formation of inclusions), and also an increase in linear density of wire we were able to get the required Ca PPM at lower consumption rate.

Importantly the trials conducted with different Al% Compositions surprisingly ended up with the same improved results as the % Al in the wire has reduced. The said residual amount in cored wire of the present invention is much beneficial in terms of quality and commercially.

The comparison of conventional 25% Al cored wire with various lesser percentages of aluminum wires is presented in Table 1. It can be observed from the results presented in Table 1 that the lower the aluminum percentage in the cored wire the lesser is the length of cored wire required to raise the calcium to the same level of 25-26 ppm as the overall linear density of wire is increased.

Table 1:
Parameter Units Ca Treatment
Al in CaFeAl wire Wt % Ca-40 Ca-40 Ca-40 Ca-40 Ca-40 Ca-30
Al-10 Al-12 Al-15 Al-18 Al-25 Al-0
Fe-50 Fe-48 Fe-45 Fe-42 Fe-35 Fe-70
Linear Density kg/mtr 0.224 0.22 0.218 0.215 0.206 0.28
Average of Ca PPM 25.14 25.84 24.97 25.65 26.72 25.73
Average length of CaFeAl wire injected mtrs 310 310 325 325 345 536
Average Ca added kgs 27.78 27.28 28.34 27.95 28.43 25
Average heat size tons 190 190 190 190 190 190
Average of Ca recovery % 17.2 18 16.74 17.44 17.86 11

Samples with varying compositions of aluminum were analyzed just before and after calcium treatment for both 25% Al and 10 % Al wires to find out any aluminum and sulphur drops.

Drop in Al during processing is a common phenomenon but Cored wire with Al will compensate the loss of aluminum during processing. Following Table 2 shows the sample properties after Ca treatment.

Table 2:
Samples just before & after Ca treatment
Al in CaFeAl wire %

Ca-40 Ca-40 Ca-30
Al-25 Al-10 Al-0
Fe-35 Fe-50 Fe-70
Linear Density Kg/mtr 0.206 0.224 0.28
Average of S Drop % 0.001 0.001 0.001
Average of Al Drop % 0.002 0.002 0.005

The slag analysis results of trial heats are given in following Table 3. The percentage of FeO in slag is reduced for 10% Al wire than 25% Al wire and the percentage of sulphur in slag increased indicating the better desulphurization with 10% Al cored wire.
Table 3:
Slag Analysis %Al in CaFeAl
10 25
Average of CaO 52.51 52.75
Average of MgO 8.49 8.41
Average of SiO2 2.56 2.28
Average of Al2O3 33.02 33.20
Average of FeO 0.65 0.78
Average of S in Slag 0.77 0.57
Average of Basicity 2.88 2.92
Average of S (LF In) 0.02 0.02
Average of S ( LF Out) 0.005 0.005
Average of % De-S 73.48 69.73

Also surprisingly significant reduction in casting problems on account of stopper rise, nozzle clogging etc was observed with the use of new CaFeAl wire (5-15 % Al content) as compared to earlier used wire (25% Al contents). An improvement observed in HR coil surface quality such as defects related to Al streaks, mould slag/ powder entrapment in case of new CaFeAl wire. From the data presented in following Table 4, it could be inferred that the castability is increased and HR coil defects reduced with 10-15% Al cored wire than 25% cored wire.
Table 4:
Casting Issues %Al in CaFeAl HRC Defects %Al in CaFeAl
25 10—15 25 10--15
Total No of Trial Heats 631 202 Total No of coils in Trial Heats 5911 2038
Total No of Heats having Casting Problems 63 10 No of coils having Al streak defect 59 11
% Heats having casting problems 9.98 4.95 % Heats having defects 1 0.54

A comparison of casting parameters for 10% and 25% aluminum cored wire are shown graphically in Figure 1 and Figure 2 respectively. Smooth casting without any stopper rise was observed in heats processed with CaFeAl (10% Al). Whereas high stopper rise was observed in heats processed with CaFeAl (25%Al). These are the system generated data and graphs showing casting performance in terms of stopper rise. Fig 1 shows no stopper rise during casting of heat processed with 10 % Al CaFeAl wire (New advanced wire). This is showing good castability of heat processed with 10 % Al CaFeAl wire. Fig 2 shows significant stopper rise during casting of heat processed with 25% CaFeAl wire.
Accompanying Figure 3 shows the reduction in casting related defects by reduced Al content in cored wire as compared to the conventional 25% Al wire wherein with the help of bar chart (a) shows the reduction in % heats having casting parameter related issues; and (b) shows the percent defects in casting.
Significant reduction in casting problems on account of stopper rise, nozzle clogging etc was observed with the use of new CaFeAl wire as compared to earlier used wire. An improvement observed in HR coil surface quality such as defects related to Al streaks, mould slag/ powder entrapment in case of new CaFeAl wire. From the accompanying Figure 3, it could be inferred that the castability is increased and HR coil defects reduced with 10-15% Al cored wire than 25% cored wire.

From the above trials, it could be concluded that CaFeAl cored wire with composition of 5 to 15 % Al + 30 to 50 % Ca and the rest % Fe, preferably Ca/Al/Fe in 40/10/50 ratio, has significantly special and surprisingly special benefits of secondary steel manufcature. The selective percentage of aluminum is required in CaFeAl wire to dilute the vapor pressure of calcium in liquid steel and improve the consistency in recovery of calcium. Optimum level of aluminum leads to the better recovery of calcium and compensate the loss of aluminum due to argon purging at the time of calcium injection. Moreover, higher level of consistency in calcium recovery with 5-15% aluminum also contributes to close control of calcium in steel and reduction in its consumption. The Ca treatment by way of secondary steel production according to the present invention involving CaFeAl cored wire of selective composition (5 to 15% Al) thus was found to be capable to offer the following benefits:
1. No significant drop in aluminum was observed even with lower Al% in CaFeAl wire. As the optimum amount of Al already present in the bath and the rate of purging that was maintained.
2. Due to low %Al in the cored wire lower will be the generation of Al2O3. Al has affinity for Oxygen so the lower the Al injected in the bath cleaner the steel.
3.Due to increase in linear density of wire we were able to get the required Ca PPM at lower consumption rate why because the Fe in the wire is generally added for increasing the bulk density of wire for deeper penetration, so as the Al content varied the bulk density varied in our trials (Ca % kept constant -40).
4. Reduction in stopper fluctuations/nozzle blockages for smooth casting instances has reduced when compared with CaFeAl-25% wire as the % of Al in the wire gradually reduce the so the chances of inclusion formation at the end of secondary steelmaking has reduced.
5. Improvement in the properties of casting powder. The properties of casting powder is important for smooth casting, excess Al2O3 in the steel affects the viscosity of the casting powder.
6. Lower % Al in wire still enabled to reduce the local vapour pressure of Ca effectively maintaining the same Ca PPM in the bath might be due to the level of % Al maintained in the bath allowing for its retention, the purging rate maintained, the speed of the wire and the depth it has reached inside the ladle.
7. Improvement in the Yield and desulphurization of steel. The yield mainly depends on the input level of Fe, so obviously the yield will improve and the desulphurization rate indicating the lower levels of oxygen attained in the bath during heat processing.
8. Overall procurement/treatment cost will be reduced as the % of Al mixture in the wire has reduced which is adding to the cost.

It is thus possible by way of the present advancement to provide for a simple and cost-effective manner of achieving secondary steel making more efficiently and with better control over the refinement process.