AN IMPROVED PROCESS FOR PRODUCTION OF CHROMIUM ALLOYED SPRING
STEEL BILLETS USING OPEN STREAM BILLET CASTER
FIELD OF INVENTION
The present invention relates to an improved process for the production of Cr-alloyed
spring steel billets through BOF-LF- LTS-CC route using open stream billet caster. The
present disclosure elaborates stringent quality requirements of spring steels, which are
major deterrents against production of spring grades billets through open stream billet
casters. The disclosure also provides the details of processing window used for making
chromium alloyed springs steel cast billets which satisfies all the metallurgical
requirements of final product i.e. leaf spring.
BACKGROUND ART
Spring plates are component meant for the load bearing for an automobile. The springs
in the chassis serve the elastic connection between vehicle bodywork and wheel
suspension. Also they have to absorb roadway impacts . The springs must, therefore,
take up all bending loads, caused by braking, acceleration and turning without
undergoing plastic deformation which implies springs must possess high load bearing
capability, signified by its high hardness. Furthermore, in view of chance of fatigue
failures under the cyclic loadings conditions, inclusion rating of spring steel must be
controlled below critical stringent limits of 2.0 ABCD for both thick and thin series as per
IS 4163 : 1982 specification
Spring steels are difficult to cast using open stream caster owing to oxidation through
atmospheric exposure of metal stream flowing from tundish to mould. Identification of
suitable operating window of alloy design and process control is key for successful
commercial production of spring steel billets free from surface defects and internal
defects like, subcutaneous pin / blow holes, off corner cracks and central piping or
looseness.
Spring steels being used in the automotives 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) while ensuring
absence of any surface / sub-surface cracks or matrix inclusion interface larger than a
critical size where fatigue failure may 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.
Chromium alloying, makes the steel prone to cracking during continuous casting process
due to loss of hot ductility. Open stream billet caster, in the absence of submerged entry
nozzle (SEN), does not check oxidation of metal stream which raises apprehension of
yield loss through (a) choking of metering nozzle if oxygen potential of bath is below
critical level, and (b) formation of scum in the mould which causes not only slag
entrapment on billet surface but also the break-outs under severe conditions. Further,
oxidation of stream affects the cleanliness of steel and the inclusion rating adversely.
Due to use of submerged entry nozzle in bloom casting, cleanliness level for rolled billets
from blooms had been preferred over open stream cast billets. Because of problem of
yield on one hand and chances of non-compliance to inclusion rating, hardness, and
other requirements relating fatigue and impact on the other hand, open stream billet
caster has not been finding favour for making spring steels billets. To meet the stringent
metallurgical requirements of leaf springs, either bottom poured pencil ingots or billets
rolled from ingot or bloom has been in vogue for making of leaf springs.
OBJECTS OF INNOVATION
The primary object of the present invention is to provide a suitable operating window for
processing parameters for open stream caster to cast spring steel billets, which meets
all the metallurgical requirements needed for a leaf spring of Cr-alloyed steel.
The object of the invention provides a process of steel production which meets the
aimed chemical analysis of steel to ensure attainment of the high surface hardness,
requisite hardness throughout the thickness observed through step hardness test, and
stringent inclusion rating.

The objects of the present invention also aims to provide range of parameters such as
chemistry, speed of casting, super heat and other casting parameters that ensures
acceptable quality of cast structure of billet conforming to ASTM 381 C2R2S2
specification or better.
Therefore such as herein described, there is provided the process for production of Cr
alloyed spring steel billets using open stream billet caster comprising the steps of:
preparing an alloy spring steel composition consisting essentially of by weight 0.56-
0.60% carbon 0.85-0.95% manganese, less than 0.03% sulphur and phosphorus, 0.25-
0.35% silicon, less than 0.007% aluminium, 0.85-0.95% chromium, less than 0.3%
copper and the remainder being iron; controlling of carryover slag; maintaining dissolved
oxygen at 20 - 35 ppm at LF dispatch stage; casting speed is controlled at 2.0 - 2.2
m/min; controlling the super heat at less than 50° C; providing electro-mechanical stirring
(EMS) with 275 A current at 8 Hz; and performing the cooling of the mould with water
flow rate of ~ 1400 Ipm.
Yet another object of the present invention is to provide a suitable chemistry of steel
within specified close range so that austenitising - oil quenching -tempering treatment
results in development of suitable microstructure in the final structure having the
requisite hardness.
Further object of the present invention is to provide a macrostructure of the transverse
section of the cast billet which is defect free with respect to central looseness, pin holes
or blow-holes and off comer cracks, so that the final product does not fail prematurely
due to fatigue induced failure.
Another object of the present invention is to provide close control over the operating
practices of continuous casting and electromagnetic stirring (EMS) even with open
stream billet caster.
Another object of the present invention is to provide close control over surface defects,
internal defects of macrostructure like pin / blow holes, off comer cracks, centre
looseness, smooth casting without breakout risks associated with scum generations etc.

A further objective of invention was to search window of suitable operating parameters
and develop suitable process technology for production of Cr alloyed spring steel billets
using open stream billet caster.
The attainment of the foregoing and related objects, advantages and features of the
invention should be more readily apparent to those skilled in the art, after review of the
following more detailed description of the invention, taken together with the drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The accompanying drawings, which are incorporated in and form a part of this
specificaticn, illustrate embodiments of the invention and, together with the description,
serve to explain the invention.
Fig. 1 (a) and (b) illustrates the macro of concast billets of Cr-alloyed spring steel
pertaining to unsuitable operating parameters used for open stream billet caster.
Fig.2 illustrates the typical macro of concast billet of Cr-alloyed spring steel made in an
open stream billet caster with suitable operating parameters.
DETAILED DESCRIPTION
High hardness, high elastic limit, fatigue resistance, impact resistance and resilience are
characteristics needed for a good spring material. In addition high alternating flexure
strengths, storable energies and reciprocal damping are required in application of good
spring material. Inherent hardenability of designed alloy chemistry of Cr - alloyed spring
steel (C, Mn, Si, Cr, B) helps in engineering the tempered martensitic microstructure
during auslenitising-oil quenching-tempering treatment of leaf spring making process.
The proces s developed for DSP ensures all the quality requirements of spring steels not
only at cast billet stage but also at the finished leaf spring stage. Cr-alloyed spring steel
is made through BOF-LF-LTS-CC route and cast into 125 X 125 mm billets using the
EMS facility under optimized conditions of process parameters, which also include close
control of chemistry. The optimized conditions of process parameters have been arrived

after number of trials and validated through processing of these billets into leaf springs at
the customers-end.
With the help of developed process technology, DSP is in a position to commercially
supply Cr-alloyed spring grade billets free from blow holes, off comer cracks, piping,
central looseness. The produced macro's of the cast billets using the disclosed process
conform to the ASTM 381 C2R2S2 specification. The microstructure, inclusion rating,
depth of decarliurisation and hardness conformed to the stipulated norms. Leaf springs
made from the se rolled flats showed remarkable consistency with respect to surface
hardness at final stage. Results of load rate test and endurance test were within norms.
Customer feedback on performance trials was good.
To meet the stringent requirements of inclusion rating and other metallurgical attributes
of Cr-alloyed spring grade steel billets produced commercially through BOF-LF-LTS-CC
route at DSP had been challenging task in view of the open stream billet caster and the
limitations of lowering S and P at DSP. Salient features of key control parameters used
in the developed process technology are ■.
• Aim Chemistry (in percentage)

• Control of carryover slag through use of dart
• Dissolved Oxygen 15-35 ppm at LF dispatch stage
• Mn / S ratio > 25
• Mn / Si ratio > 2.6
• Casting Speed : 2.0-2.2 M/min.
• Super Heal:: < 50°C
• EMS Current: 275 - 300 A at 8 Hz
• Mould Cocling : ~1400 Ipm
Composition of developed Cr-alloyed grade spring steel aimed at eutectoid composition
(C in the range of 0.55 - 0.60%), because presence of pro-eutectoid femite would be

detrimental to spring steel requirements. In view of hardness requirements after
austenitising-oil quenching-tempering treatment, for getting adequate hardenability of
steel, both Cr and Mn content were kept in the range of 0.8 - 0.95. Mn/S and Mn/Si ratio
was adjuster to control cracking tendency and castability of steel. To ensure
uninterrupted casting while using metering nozzle of open stream caster, oxygen in the
bath was kept between 15-35 ppm. Application of EMS aimed at eliminating internal
defects in the cast billet.
Under proper window of operating practices of continuous casting and electromagnetic
stirring (EMS) even with open stream billet caster, problem of surface defects, internal
defects of macrostructure like pin / blow holes, off corner cracks, centre looseness, is
eliminated in the cast billet on one hand, while on the other, smooth casting is ensured
without breakout risks often associated with scum generations apprehended during
casting of spring steels through open stream billet casters. Further, for meeting the
stringent inclusion rating, steel making and casting practice needs close control.
Microstructure of tempered martensite combined with high hardness (> 360 BHN),
decarburisation depth (< 0.15 mm) and high cleanliness level for stringent inclusion
ratings of 2.0 max. A B C D for both thin and thick series ensure (a) resistance to small
deformation which implies a stable microstructure, and (b) the desired mechanical
properties in terms of hardness, resilience and fatigue resistance. 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 leaf spring reflected by specified
hardness. Further, the macro of cast billet must 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 comer
cracks, so that the final product does not fail prematurely due to fatigue induced failure.
Cr-alloyed spring grade steel were made and cast into 125 x 125 mm billets using BOF-
LF-CC route. These billets were inspected for macro and surface defects. Fig. 1 shows
the unsatisfactory results of macro of billets cast under normal conditions. Process of
leaf spring manufacture involves cutting of billets into smaller length (30") pieces and hot

rolling into rolled flats of leaf spring which is later subjected to austenitising and oil
quenching followed by tempering treatment. Incidence of blow-holes and off corner
cracks make cast billets unsuitable for spring making process. On rolling of billets
containing these defects, hair-line cracks and short longitudinal cracks near the edge of
the rolled flats were observed. Detailed investigation including macro of cast billets,
microstructure of rolled flat, chemical analysis, EPMA analysis of failed area and
theoretical analysis underlined the need to improve the cast structure. Evidence of
internal oxidation observed near the defects established that the genesis of the defects
lied with pre-existing defects in the form of occasional incidence of blow-holes and off-
corner cracks extending to the surface.
The EMS current and other processing parameters were modified to get sound macro.
Technological intervention led to billets with sound macro (Fig. 2). Performance trial was
conducted. No edge crack was observed on the rolled flats. Microstructure, inclusion
rating, depth of decarburisation and hardness conformed to the stipulated norm. Leaf
springs mads from these rolled flats showed remarkable consistency with respect to
surface hard ness at final stage. Results of load rate test and endurance test were within
norms. Results of performance trial were satisfactory.
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 and description and is not intended to be
exhaustive or to limit the invention to the particular embodiments and applications
disclosed. It will be apparent to those having ordinary skill in the art that a number of
changes, modifications, variations, or alterations to the invention as described herein
may be made, none of which depart from the spirit or scope of the present invention.
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 process for production of Cr alloyed spring steel billets using open
stream billet caster comprising the steps of: preparing an alloy spring steel composition
consisting essentially of by weight 0.56-0.60% carbon 0.85-0.95% manganese, less than
0.03% sulphur and phosphorus, 0.25-0.35% silicon, less than 0.007% aluminium, 0.85-
0.95% chromium, less than 0.3% copper and the remainder being iron; Controlling of
carryover slag; maintaining dissolved Oxygen at 15-35 ppm at LF dispatch stage;
casting speed is controlled at 2.0 - 2.2 m/min; controlling the super heat at less than
50° C; providing electro-mechanical stirring (EMS); and performing the cooling of the
mould.
2. An improved process for production of Cr alloyed spring steel billets as claimed
in claim 1, comprising a further step of maintaining of Mn / S ratio > 25.
3. An improved process for production of Cr alloyed spring steel billets as claimed
in claim 1, comprising a further step of maintaining of Mn / Si ratio > 2.6.
4. An improved process for production of Cr alloyed spring steel billets as claimed
in claim 1, wherein the control of carryover slag is done through use of dart.
5. An inproved process for production of Cr alloyed spring steel billets as claimed
in claim 1, wherein the EMS current is maintained at275-300A at 8 Hz.
6. An improved process for production of Cr alloyed spring steel billets as claimed
in claim 1, wherein the cooling of the mould is carried out at the rate of ~ 1400 Ipm
7. An improved process for production of Cr alloyed spring steel billets as claimed
in claim 1, wherein the microstructure of tempered martensite is combined with high
hardness (> 360 BHN), decarburisation depth (< 0.15 mm) and high cleanliness level for
stringent inclusion ratings of 2.0 maximum, ABCD for both thick and thin series as per
IS 4163 : 1932 specification.

8. An improved process for production of Cr alloyed spring steel billets using open
stream billet caster substantially as herein described with particular reference to
accompanying drawings.

The present invention relates to an improved process for the production of Cr-alloyed
spring steel billets through BOF-LF- LTS-CC route using open stream billet caster. The
present disclosure elaborates stringent quality requirements of spring steels, which are
major deterrents against production of spring grades billets through open stream billet
casters. The disclosure also provides the details of processing window used for making
chromium alloyed springs steel cast billets which satisfies all the metallurgical
requirements of final product i.e. leaf spring.