CLIAMS:We claim:

1. Si-Mn alloyed rolled spring steel billets suitable for manufacture of elastic rail clip (ERC) application comprising
C : 0.47 – 0.63% by wt.;
Mn : 0.76 – 1.04% by wt.;
S:< 0.045%by wt.;
P:<0.045% by wt.;
Si : 1.45 – 1.95% by wt.; and
balance is iron enabling elastic rail clip (ERC) application hardness requirements after austenising–oil quenching-tempering treatment and desired hardenability of steel.
2. Si-Mn alloyed rolled spring steel billets as claimed in claim 1 comprising microstructure of tempered martensite combined with high hardness (> 360 BHN), decarburisation depth of < 0.15 mm and high cleanliness level for stringent inclusion ratings of 3.0 max.
3. Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 1 or 2, having microstructure conforming to ASTM 381 C2R2S2 specification.
4. A process for producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 1 to 3 produced through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route comprising involving said alloy chemistry of
C : 0.47 – 0.63% by wt.;
Mn : 0.76 – 1.04% by wt.;
S:< 0.045%by wt.;
P:<0.045% by wt.;
Si : 1.45 – 1.95% by wt.; and
balance is iron for making of Si-Mn alloyed grade spring steel through THF , teeming into top-poured ingots, and after stripping of the ingots, rolling the ingots into billets using blooming and billet mill.
5. A process for producing Si-Mn alloyed rolled spring steel billets as claimed in claim 4 comprising
(i) Providing steel having composition comprising
C : 0.47 – 0.63% by wt.;
Mn : 0.76 – 1.04% by wt.;
S:< 0.045% by wt.;
P:<0.045% by wt.;
Si : 1.45 – 1.95% by wt.; and
balance is iron.
(ii) Controlling basicity in the range of 2.5 – 3.0;
(iii) Maintain tapping Temperature : ~ 1680o C
(iv) Restricting specific Oxygen consumption to < 60 Nm3/t in THF
(v) Carrying out addition of coke, Si-Mn in the ladle bottom before tapping;
(vi) Carrying out major addition of Fe-Si and Fe-Mn during tapping;
(vii) Gas purging with N2 / Ar for duration > 3 minute;
(viii) maintain Teeming Temperature : ~ 1565 C
(ix) providing 1.75 - 2.0 kg/t of Anti Piping Compound (APC) in top pouring for controlling piping in TP ingots;
6. A process for producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 4 or 5, wherein addition of ferro alloys of about 6t is done by dividing the ferro alloy addition into two parts - ladle bottom as well as direct addition to the bath during tapping to obtain desired recoveries of ferro alloys to achieve the said high levels of alloy content in the resulting spring steel.
7. A process for producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 4 to 6, wherein addition of ferro alloys in spring steel during heat making in absence of any secondary refining facility requires maintaining a very high desired tapping temperature in single attempt involved which is obtained as:
Estimated tap temperature = Liquidus Temp (~1463°C) + Temperature drop due to Fe-alloy addition (~72°C) + Ladle Soaking (~70°C) + Superheat (~60°C) = ~1665°C.
8. A process for producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 4 to 7, involving post ingot teeming steps comprising:
(a) maintaining ingot stripping temperature : > 600 C
(b) keeping track time as low as possible to achieve high ingot-temperature at the time of hot charging;
(c) maintaining the teeming to charging time < 4 hours ;
(d) maintaining initial soaking pit temperature ~ 300 C above the ingot temperature at the time of charging to avoid thermal shock and facilitate slow heating;
(e) spring steel being rich in having Si ~1.7% by wt., heating / cooling is done slowly because of its lower thermal conductivity thus maintaining a minimum time period of 5 hours of Heating + 2 hours of Soaking time for ensuring desired temperature-homogenization;
(f) raising the Soaking pit temperature slowly at the rate of 100 C / hr max.) and maintaining soaking temperature at ~ 1270 C ;
(g) maintaining entry temperature to blooming mill > 1250 C whereas exit-temperature of 225 x 225 mm bloom > 1100 C;
(h) maintaining entry temperature at 1V stand of billet-mill > 1050 C and finish rolling temperature of > 900 C maintained for 96 x 96 mm (6M length) billets of spring steel obtained.

9. A process for manufacture of elastic rail clips or leaf spring comprising:
(i) producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 4 to 8; and
(ii) obtaining the elastic rail clips or leaf spring from said Si-Mn alloyed rolled spring steel billets.

Dated this the 5th day of March, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
,TagSPECI:FIELD OF THE INVENTION

The Present invention relates to Si-Mn alloyed rolled spring steel billets and a process for its production. More particularly, the present invention is directed to providing Si-Mn alloyed rolled spring steel billets produced through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet mill route, suitable for making elastic rail clip (ERC). Advantageously, the present invention provides a selective alloy design and process control for successful commercial production of rolled spring steel billets free from surface defects and internal defects like, subcutaneous pin/blow holes, off corner cracks and central piping or looseness to make it suitable for making elastic rail clip (ERC) and leaf springs favouring wide industrial application.

BACKGROUND OF THE INVENTION

Si-Mn alloyed spring steels suitable for elastic rail clip (ERC) need to have high fatigue resistance in addition to high modulus of elasticity and resilience defined as energy absorbed during elastic deformation depicted by area under stress – strain plot upto elastic limit. In view of the properties required in spring steel, it is necessary to engineer a tempered-martensitic microstructure typified by high hardness (> 360 BHN). Further, it is critical to ensure absence of any surface / sub-surface cracks or matrix inclusion interface larger than a critical size where fatigue failure originate. In view of above, stringent requirement for inclusion rating in the final product assumes significance in addition to defect free macrostructure of cast billets. Usually top poured (TP) ingots has limitation in restricting the defect free surface in form of rolled scabs which is unacceptable for ERC.

Stringent quality requirements are major deterrents against production of ERC grades billets through TP ingot route. In view of problem of yield on one hand and chances of non-compliance to inclusion rating, hardness, and other requirements relating fatigue and impact associated with surface defect and surface decarburisation on the other hand, usually twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet mill route billets have not been considered ideally suited for applications like elastic rail clip or leaf spring. Absence of secondary refinement makes it all the more difficult in meeting the critical inclusion ratings. To meet the stringent metallurgical requirements of ERC, either bottom poured ingots or continuously cast steels have conventionally been in vogue.

Si-Mn grade spring steel is high alloy steel aiming at eutectoid composition through high amount of carbon (C in the range of 0.47 – 0.63%), manganese (Mn in the range of 0.76 – 1.04%) and silicon (Si in the range of 1.45 – 1.95%). As there is no secondary refining facilities at one of the steel plant of the applicants, it was a difficult task to ensure carbon and silicon pick up to a requisite levels. Overall there has been no facility of correcting the chemistry once tapped from THF.

There has been thus a need in the art to developing Si-Mn alloyed spring steel through top poured ingot route overcoming the conventional limitation in respect of surface and sub surface defects as well as compliance to the desired stringent inclusions rating suitable for ERC or leaf spring application by careful selection and trial of the alloy design and control on key process parameters. The above said technological difficulties were overcome by way of the present invention exploiting metallurgical concepts and developing a process technology for the first time for producing ERC grade billets through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route in absence of secondary refining facilities at ISP, Burnpur.

OBJECTS OF THE INVENTION

The basic objective of the present invention is directed to provide Si-Mn alloyed rolled spring steel billets through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route suitable for making elastic rail clip (ERC).

A further object of the present invention is directed to provide Si-Mn alloyed rolled spring steel billets through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route suitable for making elastic rail clip (ERC) through selective alloy design and control of inclusion ratings in final product, meeting all other metallurgical requirements like surface hardness, step hardness, grain size in rolled flats, controlled decarburised depths etc. especially in absence of secondary refining facility.

A still further object of the present invention is directed to provide Si-Mn alloyed rolled spring steel billets through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route suitable for making elastic rail clip (ERC) in which macrostructure of the transverse section of the cast billet must be defect free with respect to central looseness, pin holes or blow-holes and off corner cracks, so that the final product does not fail prematurely due to fatigue induced failure.

A still further object of the present invention is directed to provide Si-Mn alloyed rolled spring steel billets through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route suitable for making elastic rail clip (ERC ) wherein high levels of alloy content is attained by dividing the ferro alloy addition into two parts; ladle bottom as well as direct addition to the bath during tapping in order to obtain desired recoveries of ferro alloys.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is thus directed to Si-Mn alloyed rolled spring steel billets suitable for manufacture of elastic rail clip (ERC) application comprising
C : 0.47 – 0.63% by wt.;
Mn : 0.76 – 1.04% by wt.;
S:< 0.045%by wt.;
P:<0.045% by wt.;
Si : 1.45 – 1.95% by wt.; and
balance is iron enabling elastic rail clip (ERC) application hardness requirements after austenising–oil quenching-tempering treatment and desired hardenability of steel.

A further aspect of the present invention is directed to said Si-Mn alloyed rolled spring steel billets comprising microstructure of tempered martensite combined with high hardness (> 360 BHN), decarburisation depth of < 0.15 mm and high cleanliness level for stringent inclusion ratings of 3.0 max.

A still further aspect of the present invention is directed to said Si-Mn alloyed rolled spring steel billets having microstructure conforming to ASTM 381 C2R2S2 specification.

A still further aspect of the present invention is directed to a process for producing Si-Mn alloyed rolled spring steel billets produced through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route comprising involving said alloy chemistry of
C : 0.47 – 0.63% by wt.;
Mn : 0.76 – 1.04% by wt.;
S:< 0.045%by wt.;
P:<0.045% by wt.;
Si : 1.45 – 1.95% by wt.; and
balance is iron for making of Si-Mn alloyed grade spring steel through THF , teeming into top-poured ingots, and after stripping of the ingots, rolling the ingots into billets using blooming and billet mill.
Yet another aspect of the present invention is directed to a process for producing Si-Mn alloyed rolled spring steel billets as described above comprising
(i) Providing steel having composition comprising
C : 0.47 – 0.63% by wt.;
Mn : 0.76 – 1.04% by wt.;
S:< 0.045%by wt.;
P:<0.045% by wt.;
Si : 1.45 – 1.95% by wt.; and
balance is iron.
(ii) Controlling basicity in the range of 2.5 – 3.0;
(iii) Maintain tapping Temperature : ~ 1680o C
(iv) Restricting specific Oxygen consumption to < 60 Nm3/t in THF
(v) Carrying out addition of coke, Si-Mn in the ladle bottom before tapping;
(vi) Carrying out major addition of Fe-Si and Fe-Mn during tapping;
(vii) Gas purging with N2 / Ar for duration > 3 minute;
(viii) maintain Teeming Temperature : ~ 1565o C
(ix) providing 1.75 - 2.0 kg/t of Anti Piping Compound (APC) in top pouring for controlling piping in TP ingots;

A still further aspect of the present invention is directed to said process for producing Si-Mn alloyed rolled spring steel billets wherein addition of ferro alloys of about 6t is done by dividing the ferro alloy addition into two parts - ladle bottom as well as direct addition to the bath during tapping to obtain desired recoveries of ferro alloys to achieve the said high levels of alloy content in the resulting spring steel.

A still further aspect of the present invention is directed to said process for producing Si-Mn alloyed rolled spring steel billets wherein addition of ferro alloys in spring steel during heat making in absence of any secondary refining facility requires maintaining a very high tapping temperature is obtained as:
Estimated tap temperature = Liquidus Temp (~1463°C) + Temperature drop due to Fe-alloy addition (~72°C) + Ladle Soaking (~70°C) + Superheat (~60°C) = ~1665°C.

A still further aspect of the present invention is directed to said process for producing Si-Mn alloyed rolled spring steel billets involving post ingot teeming steps comprising:
(a) maintaining ingot stripping temperature : > 600o C
(b) keeping track time as low as possible to achieve high ingot-temperature at the time of hot charging;
(c) maintaining the teeming to charging time < 4 hours ;
(d) maintaining initial soaking pit temperature ~ 300o C above the ingot temperature at the time of charging to avoid thermal shock and facilitate slow heating;
(e) spring steel being rich in having Si ~1.7% by wt., heating / cooling is done slowly because of its lower thermal conductivity thus maintaining a minimum time period of 5 hours of Heating + 2 hours of Soaking time for ensuring desired temperature-homogenization;
(f) raising the Soaking pit temperature slowly at the rate of 100o C / hr max.) and maintaining soaking temperature at ~ 1270o C ;
(g) maintaining entry temperature to blooming mill > 1250o C whereas exit-temperature of 225 x 225 mm bloom > 1100o C;
(h) maintaining entry temperature at 1V stand of billet-mill > 1050o C and finish rolling temperature of > 900o C maintained for 96 x 96 mm (6M length) billets of spring steel obtained.

Yet another aspect of the present invention is directed to a process for manufacture of elastic rail clips or leaf spring comprising:
(i) Process technology for producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 4 to 8; and
(ii) obtaining the elastic rail clips or leaf spring from said Si-Mn alloyed rolled spring steel billets as claimed in claim 9.
The objects and advantages of the present invention are described hereunder in greater details with reference to following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: shows the unsatisfactory results of macro of intermediate stage rolled bloom of
Si-Mn grade spring steels (before innovation).
Figure 2: shows photographs of samples after upsetting tests depicting evidence of adequate hot workability of the steel.
Figure 3: shows a typical Macro of rolled billet made with suitable operating parameters produced according to the present invention.
Figure 4: depicts microstructures at different stages of making of ERC, wherein inclusion rating and final surface hardness of ERC have been shown.
Figure 5: shows the step hardness measurements/test results of the Si-Mn steel flats.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide Si-Mn alloyed rolled spring steel billets through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route suitable for making elastic rail clip (ERC) through selective alloy design and precise control of process parameters for control of inclusion ratings in final product, meeting all other metallurgical requirements like surface hardness, step hardness, grain size in rolled flats, controlled decarburised depths etc., especially in absence of secondary refining facility.

High hardness, high elastic limit, fatigue resistance, impact resistance and resilience are characteristics needed for a good spring material. Microstructure of tempered martensite combined with high hardness (> 360 BHN), decarburisation depth (< 0.15 mm) and high cleanliness level for stringent inclusion ratings of 3.0 max. A B C D for both thin and thick series ensures resistance to small deformation as well as the desired mechanical properties. Surface / internal defect, inclusion level and depth of decarburisation is critical for fatigue resistance. To meet the stringent requirements, it is necessary to ensure chemistry of steel within specified close range so that austenitising – oil quenching –tempering treatment results in development of suitable microstructure in the final ERC reflected by specified hardness. Further, the macro of billets conform to ASTM 381 C2R2S2 specification, i.e. macrostructure of the transverse section of the cast billet must be defect free with respect to central looseness, pin holes or blow-holes and off corner cracks, so that the final product does not fail prematurely due to fatigue induced failure.

Inherent hardenability of designed alloy chemistry of Si-Mn alloyed ERC grade steel (C, Mn, Si) helps in achieving the tempered martensitic microstructure during austenitising-oil quenching-tempering treatment of ERC process. Under appropriate window of operating practices of Si-Mn alloyed rolled spring steel billets suitable for making elastic rail clip (ERC) through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route, problem of surface defects, internal defects of macrostructure like pin / blow holes, off corner cracks, centre looseness, is eliminated in the billet on one hand, while on the other, scum generations and piping apprehended in TP ingot route spring steels is controlled. Further, for meeting the stringent inclusion rating, steel making and teeming practice needs close control.

Process technology developed by the applicants ensures all the quality requirements of ERC grade spring steels not only at billet stage but also at the finished product stage. The optimized conditions of process parameters have been arrived after number of trials and validated through processing of these billets into ERC at the customers-end.

With the help of developed process technology, the applicants’ manufacturing facility is in a position to commercially supply ERC grade spring steel billets free from blow holes, off corner cracks, piping, central looseness. Macro of the billets conform to the ASTM 381 C2R2S2 specification. Microstructure, inclusion rating, depth of decarburisation and hardness of hot rolled round-rods conformed to the stipulated norm of ERC. ERC made from these rolled round-rods showed remarkable consistency with respect to surface hardness at final stage. Customer feedback on performance trial was good. Product is accepted in the market.

To meet the stringent requirements of inclusion rating and other metallurgical attributes of ERC grade Si-Mn alloyed spring steel billets produced commercially through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route particularly in absence of secondary refining facilities at the applicants’ manufacturing facility, salient features of key control parameters used in the developed process technology are as follows:

(i) Composition of the steel achieved is as follows:
C Mn S & P Si
0.47-0.63 0.76-1.04 <0.045 1.45-1.95
having Liquidus : ~ 1472o C
(ii) Control of basicity in the range of 2.5 – 3.0;
(iii) Tapping Temperature : ~ 1680o C;
(iv) Specific Oxygen consumption restricted to < 60 Nm3/t in THF;
(v) Addition of coke, Si-Mn in the ladle bottom before tapping;
(vi) Major addition of Fe-Si and Fe-Mn during tapping;
(vii) Gas purging with N2/Ar for duration > 3 minute
(viii) Teeming Temperature : ~ 1565o C
(ix) Use 1.75 - 2.0 kg/t Anti Piping Compound (APC) in TP for controlling piping in TP ingots;

The normal operational practice in THF includes fettling/repair of hearth, charging of fluxes scrap and iron ore, heating of solid charge, hot metal pouring, flushing of initial siliceous slag, melting of scrap, refining and then tapping. The normal charge consists of 85% hot metal and 15% scrap. The design duration of the whole cycle is 5 hours, with tapping at every 150 minutes. Impurities like phosphorus and sulphur etc. are removed by entrapping them in a high basicity slag. The slag basicity is aimed at around 2.0. After fettling, sandwich type charging sequence is adopted. In normal conditions six boxes (~12 ton) of scrap is charged followed by limestone and iron ore. Then remaining amount of scrap is charged. In normal course, six boxes of limestone (~10.8 ton) and 1 box of iron ore (~ 2 ton) is charged in furnace. In case of higher opening phosphorous content in bath, feed of limestone is given to take care of phosphorous in the furnace. Normally 110t of hot metal and 12 boxes of scrap are charged in the furnace. After heating for 1 hour, hot metal is taken and poured into the furnace. Oxygen lancing started after hot metal pouring. Refining time varies between 2.5 to 3.0 hours depending on the refining load. The heat is tapped in the ladle after attaining desired levels of carbon, phosphorous and temperature.

Si-Mn grade spring steel is high alloy steel aiming at eutectoid composition through high amount of carbon (C in the range of 0.47 – 0.63%), manganese (Mn in the range of 0.76 – 1.04%) and silicon (Si in the range of 1.45 – 1.95%). As there is no secondary refining facilities at the applicants manufacturing facility, it was a difficult task to ensure carbon and silicon pick up to a requisite levels. Overall there has been no facility of correcting the chemistry once tapped from THF.

Since composition of spring steel grade 55Si7 has high levels of alloy content as is mentioned above, huge amount of (~6 t) ferro-alloys needs to be added. In normal cases the addition of ferro alloys is done in the ladle bottom, but for making spring steel it was felt necessary to divide the ferro alloy addition into two parts; ladle bottom as well as direct addition to the bath during tapping. This was done mainly to obtain desired recoveries of ferro alloys.

Due to ~6 t addition of ferro alloys in spring steel during heat making on one hand and non-availability of any secondary refining facility on the other, it has been very critical to achieve desired tapping temperature in single attempt. The desired tap temperature was estimated on the following basis:

Estimated tap temperature = Liquidus Temp (~1463°C) + Temperature drop due to Fe-alloy addition (~72°C) + Ladle Soaking (~70°C) + Superheat (~60°C) = ~1665°C

Making spring steel billets through THF - TP Ingot – Blooming & Billet Mill route, with consistent metallurgical attributes like acceptable macro, has been the challenging tasks. Control of inclusion ratings in final product, meeting all other metallurgical requirements like surface hardness, step hardness, grain size in rolled flats, controlled decarbed depths etc. especially in absence of secondary refining facility at applicants’ production facility have been the critical issues.

Composition of Si-Mn grade spring steel aims at eutectoid composition because presence of pro-eutectoid ferrite is detrimental to requirements of ERC grade spring steel. In view of hardness requirements after austenitising-oil quenching-tempering treatment, for getting adequate hardenability of steel, C, Mn and Si were kept in the range of 0.47 – 0.63, 0.76 – 1.04 and 1.45 – 1.95 respectively.

Si-Mn alloyed grade spring steel were made through THF and teemed into top-poured ingots. After stripping, these ingots were rolled into billets of 96 x 96 mm using blooming and billet mill. During initial trials, surface cracks were observed during bloom stage. Accompanying Figure 1 shows the unsatisfactory results of macro of intermediate stage rolled bloom. Metallurgical investigation of cross section of the crack established the genesis of these defects to be linked with surface cracks originating from ingot teeming and stripping practice. Accompanying Figure 2 shows evidence of adequate hot workability of the steel. Corrective measures were identified and subsequent trial were conducted with modified practice. Post ingot teeming, following salient parameters were adhered to :
1. Aim Ingot stripping temperature : > 600o C
2. Attempt keeping track time as low as possible to achieve high ingot-temperature at the time of hot charging
3. The teeming to charging time < 4 hours
4. Initial soaking pit temperature : ~ 300o C above the ingot temperature at the time of charging to avoid thermal shock and facilitate slow heating. (e.g. for ingot temperature of 500o C, initial soaking pit temperature to be 800o C)
5. Spring steel being rich in Si (~1.7%) needs to be heated / cooled slowly because of its lower thermal conductivity. A minimum time period of 5 + 2 hours of Heating + Soaking time ensured temperature-homogenization
6. Soaking pit temperature raised slowly at the rate of 100o C / hr max.) and soaking temperature was maintained at ~ 1270o C
7. Entry temperature to blooming mill > 1250o C whereas exit-temperature of 225 x 225 mm bloom > 1100o C
8. Entry temperature at 1V stand of billet-mill > 1050o C and finish rolling temperature of > 900 C aimed for 96 x 96 mm (6M length) billets of spring steel

By adopting the above controlled process parameters led to billets with sound macro which is shown in the accompanying Figure 3. Performance trial was conducted with the billets with desired macrostructure. No edge crack was observed on the rolled rounds obtained of these billets. Microstructure, inclusion rating, depth of decarburisation and hardness conformed to the stipulated norm. Accompanying Figure 4 depicts microstructures at different stages of making of ERC, inclusion rating and final surface hardness of ERC. Flats were experimentally rolled from the 96 x 96 mm billet rolled billets and after heat treatments simulating austenitising, oil-quenching and tempering. Step hardness measurements of the flats as shown in accompanying Figure 5 met all the requirements stipulated for leaf spring also. Si-Mn alloyed rolled spring steel billets made through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route are suitable for both ERC and leaf spring applications.

It is thus possible by way of the present invention to provide Si-Mn alloyed rolled spring steel billets through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route suitable for making elastic rail clip (ERC) and leaf spring applications through selective alloy design and precise control of process parameters for control of inclusion ratings in final product, meeting all other metallurgical requirements like surface hardness, step hardness, grain size in rolled flats, controlled decarburised depths etc., especially in absence of secondary refining facility. Advantageously, the macrostructure of billets produced according to the present invention conform to ASTM 381 C2R2S2 specification, i.e. macrostructure of the transverse section of the cast billet must be defect free with respect to central looseness, pin holes or blow-holes and off corner cracks, so that the final product does not fail prematurely due to fatigue induced failure. The billets so produced being free from surface defects and internal defects like, subcutaneous pin/blow holes, off corner cracks and central piping or looseness to make it suitable for making elastic rail clip (ERC).

We claim:

1. Si-Mn alloyed rolled spring steel billets suitable for manufacture of elastic rail clip (ERC) application comprising
C : 0.47 – 0.63% by wt.;
Mn : 0.76 – 1.04% by wt.;
S:< 0.045%by wt.;
P:<0.045% by wt.;
Si : 1.45 – 1.95% by wt.; and
balance is iron enabling elastic rail clip (ERC) application hardness requirements after austenising–oil quenching-tempering treatment and desired hardenability of steel.
2. Si-Mn alloyed rolled spring steel billets as claimed in claim 1 comprising microstructure of tempered martensite combined with high hardness (> 360 BHN), decarburisation depth of < 0.15 mm and high cleanliness level for stringent inclusion ratings of 3.0 max.
3. Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 1 or 2, having microstructure conforming to ASTM 381 C2R2S2 specification.
4. A process for producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 1 to 3 produced through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet Mill route comprising involving said alloy chemistry of
C : 0.47 – 0.63% by wt.;
Mn : 0.76 – 1.04% by wt.;
S:< 0.045%by wt.;
P:<0.045% by wt.;
Si : 1.45 – 1.95% by wt.; and
balance is iron for making of Si-Mn alloyed grade spring steel through THF , teeming into top-poured ingots, and after stripping of the ingots, rolling the ingots into billets using blooming and billet mill.
5. A process for producing Si-Mn alloyed rolled spring steel billets as claimed in claim 4 comprising
(i) Providing steel having composition comprising
C : 0.47 – 0.63% by wt.;
Mn : 0.76 – 1.04% by wt.;
S:< 0.045% by wt.;
P:<0.045% by wt.;
Si : 1.45 – 1.95% by wt.; and
balance is iron.
(ii) Controlling basicity in the range of 2.5 – 3.0;
(iii) Maintain tapping Temperature : ~ 1680o C
(iv) Restricting specific Oxygen consumption to < 60 Nm3/t in THF
(v) Carrying out addition of coke, Si-Mn in the ladle bottom before tapping;
(vi) Carrying out major addition of Fe-Si and Fe-Mn during tapping;
(vii) Gas purging with N2 / Ar for duration > 3 minute;
(viii) maintain Teeming Temperature : ~ 1565 C
(ix) providing 1.75 - 2.0 kg/t of Anti Piping Compound (APC) in top pouring for controlling piping in TP ingots;
6. A process for producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 4 or 5, wherein addition of ferro alloys of about 6t is done by dividing the ferro alloy addition into two parts - ladle bottom as well as direct addition to the bath during tapping to obtain desired recoveries of ferro alloys to achieve the said high levels of alloy content in the resulting spring steel.
7. A process for producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 4 to 6, wherein addition of ferro alloys in spring steel during heat making in absence of any secondary refining facility requires maintaining a very high desired tapping temperature in single attempt involved which is obtained as:
Estimated tap temperature = Liquidus Temp (~1463°C) + Temperature drop due to Fe-alloy addition (~72°C) + Ladle Soaking (~70°C) + Superheat (~60°C) = ~1665°C.
8. A process for producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 4 to 7, involving post ingot teeming steps comprising:
(a) maintaining ingot stripping temperature : > 600 C
(b) keeping track time as low as possible to achieve high ingot-temperature at the time of hot charging;
(c) maintaining the teeming to charging time < 4 hours ;
(d) maintaining initial soaking pit temperature ~ 300 C above the ingot temperature at the time of charging to avoid thermal shock and facilitate slow heating;
(e) spring steel being rich in having Si ~1.7% by wt., heating / cooling is done slowly because of its lower thermal conductivity thus maintaining a minimum time period of 5 hours of Heating + 2 hours of Soaking time for ensuring desired temperature-homogenization;
(f) raising the Soaking pit temperature slowly at the rate of 100 C / hr max.) and maintaining soaking temperature at ~ 1270 C ;
(g) maintaining entry temperature to blooming mill > 1250 C whereas exit-temperature of 225 x 225 mm bloom > 1100 C;
(h) maintaining entry temperature at 1V stand of billet-mill > 1050 C and finish rolling temperature of > 900 C maintained for 96 x 96 mm (6M length) billets of spring steel obtained.

9. A process for manufacture of elastic rail clips or leaf spring comprising:
(i) producing Si-Mn alloyed rolled spring steel billets as claimed in anyone of claims 4 to 8; and
(ii) obtaining the elastic rail clips or leaf spring from said Si-Mn alloyed rolled spring steel billets.

Dated this the 5th day of March, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
ABSTRACT

TITLE: Si-Mn ALLOYED ROLLED SPRING STEEL BILLETS SUITABLE FOR MANUFACTURE OF ELASTIC RAIL CLIP(ERC) AND LEAF SPRING APPLICATIONS AND A PROCESS FOR ITS PRODUCTION.

The Present invention is directed to providing Si-Mn alloyed rolled spring steel billets and a process for its production through twin hearth furnace (THF) – top poured (TP) ingot – blooming and billet mill route, suitable for making elastic rail clip (ERC) comprising alloy steel with C : 0.47 – 0.63% by wt.; Mn : 0.76 – 1.04% by wt.; S:< 0.045%by wt.; P:<0.045% by wt.; Si : 1.45 – 1.95% by wt.; and balance is iron enabling elastic rail clip (ERC) in meeting hardness requirements and desired hardenability in alloy steel after austenitising–oil quenching-tempering treatment. Advantageously, the present invention provides a selective alloy design and process control for successful commercial production of rolled spring steel billets free from surface defects and internal defects like, subcutaneous pin / blow holes, off corner cracks and central piping or looseness to make it suitable for making elastic rail clip (ERC) and the leaf springs favouring wide industrial application.