Title of Invention	A STEEL COMPOSITION FOR PRODUCING SUPERIOR QUALITY CONCAST BILLETS FAVOURING REDUCTION OF OFF-CORNER INTERNAL CRACKING AND A METHOD THEREOF
Abstract	A steel composition for producing superior quality concast billets at high speed comprising, Carbon: 0.20-0.25%; Sulphur:0.03%(max); Silicon :0.20 to 0.30 %; Manganese :at least 0.75, selected such that the Mn/Si ratio is maintained at least 2.9 and Mn/S ratio at least 25.0 and or alternatively Carbon: 0.4-0.6%; Sulphur:0.03%(max); Silicon :0.20 to 0.30 %;Manganese :at least 0.80, selected such that the Mn/Si ratio is maintained at least 3.3 and Mn/S ratio at least 26.7 and adapted for reducing the formation of Off-corner crack, blow holes/pin holes and breakout and a method casting seamlessly production of superior quality concast billets at high speed with increased productivity. Said method involving velocity of water cooling in mould is increased to about >11.5 m/sec, providing mould foot roll and modified below mould spray jacket and providing of high upper taper of 2.4%, in multi tapered mould

Title of Invention

A STEEL COMPOSITION FOR PRODUCING SUPERIOR QUALITY CONCAST BILLETS FAVOURING REDUCTION OF OFF-CORNER INTERNAL CRACKING AND A METHOD THEREOF

Abstract

A steel composition for producing superior quality concast billets at high speed comprising, Carbon: 0.20-0.25%; Sulphur:0.03%(max); Silicon :0.20 to 0.30 %; Manganese :at least 0.75, selected such that the Mn/Si ratio is maintained at least 2.9 and Mn/S ratio at least 25.0 and or alternatively Carbon: 0.4-0.6%; Sulphur:0.03%(max); Silicon :0.20 to 0.30 %;Manganese :at least 0.80, selected such that the Mn/Si ratio is maintained at least 3.3 and Mn/S ratio at least 26.7 and adapted for reducing the formation of Off-corner crack, blow holes/pin holes and breakout and a method casting seamlessly production of superior quality concast billets at high speed with increased productivity. Said method involving velocity of water cooling in mould is increased to about >11.5 m/sec, providing mould foot roll and modified below mould spray jacket and providing of high upper taper of 2.4%, in multi tapered mould

Full Text	FIELD OF THE INVENTION: The present invention is directed to a process of faster production of CONCAST billets to meet the quality and quantity requirements of the end users. More particularly, the invention is directed to superior quality billets for purposes like forging, spring making and wire rods at higher casting speeds and improved productivity, eliminating detrimental strand breakout events in continuous casting and the causes responsible for such phenomenon, leading to production loss, machine damage and serious safety hazards to operating personnel. The invention is also directed to a method and operational practices to restrict strand rupture/breakout occurrences in continuous casting eliminating the factors responsible for such event such as shell sticking, slag/scum entrapment, off-corner shell thinning and formation of severe longitudinal off-corner cracks and the like. The method of arresting the strand breakout of billets produced at faster casting speed involving operating parameters like-increased water velocity in mould to reduce mould distortion at higher speed, increased upper mould taper to avoid off-corner crack formation, providing below mould foot roll for extra support, improved below mould spray jacket, modified cooling water distribution and extension of secondary cooling zone length to eliminate halfway/midway/reheating cracks, modified oscillation stroke, developing nomogram for optimization of steel chemistry, such that the acceptance level of special quality steel billets has gone up from existing 50% to nearly 100% and the strand breakout frequency is reduced substantially from 1.8% per strand to less than 1.0 % per strand, improving productivity in CONCAST plant. BACKGROUND ART: It is well known in the art of steel products produced in steel plants, there is a growing demand of CONCAST billets. The quality of such products shall also meet stringent internal quality requirements for processing of steels of special applications at the consumers end such as forging, spring manufacturing, wire rods of varied sections. To meet the growing demand for good quality steel, higher casting speeds are required on one hand to achieve higher productivity and to identify and eliminate the casting defects on the other hand, is also extremely important for maintaining the quality/acceptance standard of cast billets produced at faster speed. The major defects occurring during casting of billets are longitudinal off-corner cracks, midway or reheating cracks, centerline cracks, pinholes and 2 blow holes etc. Longitudinal off-corner cracks are the defects usually occurring at or near the corner of the billets and initiate in or just below the mould. Local thinning of the solid shell near the corner region, higher corner temperature, in-mould bulging or mould distortion and steel composition are the main reasons responsible for the formation of longitudinal off-corner cracks. The halfway or the midway cracks are formed primarily between growing columnar dendrites and are aligned perpendicular to strand surface. Such cracks are formed during solidification of molten metal and are usually filled in by the residual elements like Sulphur and Phosphorus having lower solidification temperature, which reduces intrinsic toughness properties. Low alloy steel grades such as the forging quality and high carbon chrome bearing steel for automobile application and high carbon steel for agricultural appliances, when produced as billets in continuous casting plant, frequently suffer from strand breakouts which is as high as 1.6 to 1.8 % even at low and moderate casting speeds in the range of 2.0-2.2m/min, for reasons like severe longitudinal off-corner cracks and occurrence of slag breakouts caused due to unfavorable Mn/Si ratio in steel. The flow velocity of cooling water through the mould wall play an important role in mould heat extraction and thereby controlling the hot as well as cold face temperature of operating mould tube which in turn influence the rate of cooling and solidification of the cast billets. Water flow velocity in mould water channel for casting of 125mm square billets at the existing value of Electromagnetic stirring (EMS), leading to defects like the mould distortion due to high hot face temperature, and nucleation boiling of water in the mould water channel leading to poor heat transfer inducing formation of crack due to higher strand temperature. Further, the existing secondary zone comprising MS zone-0.3 m long with cooling water distribution rate of 165 Ipm, zone -I -1.5m long having water distribution of 180 Ipm and the zone-IIA -1.9m long having the water distribution of 80 Ipm; thus the total length of secondary cooling spreading over 3.7 m, was found to be insufficient as compared to the computed Crater length of 8-9m, up to end of solidification, while casting was carried out at a speed of 2.2-2.6m/min; further the secondary cooling water distribution/flow rate was independent of casting speed resulting in strand reheating to an extent of 120°C and leading to formation of halfway and centerline cracks. This arrangement needed a relook at determining the proper zone length and distribution of cooling water to avoid the defects as 3 mentioned. In addition to these factors, surface reheating due to improper secondary cooling tends to increase in tensile stresses at solid/liquid interfaces, giving rise to crack formation tendency. The centerline cracks on the other hand, originate during end of solidification due to uncontrolled excess of shrinkage of bulk casting body. Other defects such as the pinholes and blow holes occur due to mainly improper chemistry, impurities and excessive use of lubricating oil in mould. It is also a common experience in continuous casting of billets, the occurrences of breakout of CONCAST billets as a deleterious event. Breakout results in loss of production, yield loss, machine damage and in addition serious safety hazards to the operating personnel. Thus to identify and rectify/improve operational practices responsible for occurrence of breakout is of extreme importance in deciding indices for setting criteria in continuous casting. Reasons like the shell sticking, slag/scum entrapment, off-corner shell thinning and formation of longitudinal off-corner cracks and the like, are attributable to the event of breakout in continuous casting. Loss of steel contact with the mould wall either in the of-corner region or along the narrow face results in localized shell thinning which contributes to the formation of off-corner cracks and in extreme situation causes shell rupturing and breakout. In most of the integrated steel plants, the billets of section such as 125 mm sq., is produced for conversion to forging quality and spring steel products, through BOF-LTS/LF-CC route. The stringent quality requirements for these product grades lead to lower macro acceptability and consequently, higher rate of diversion to lower grades. A typical macro structure of billet samples of 125mm sq. section shows presence of defects like off-corner, midway/reheating and centerline cracks and also longitudinal off-corner related breakout is common during continuous casting of such billets. There has been therefore, a continuous need for developing an improved method for production of CONCAST billets at higher speed with necessary technical measures including modification/incorporation of process steps/parameters directed to identify the reasons and eliminate/reduce the occurrences of various types of cracks as observed in casting of billets and also to avoid breakout due to reasons like the shell sticking, slag/scum entrapment, off- corner shell thinning and formation of longitudinal off-corner cracks causing shell rupturing and breakout in extreme situation, and thereby avoid stopping production and improve productivity and yield, with increased acceptability of product quality. 4 OBJECT OF THE INVENTION: It is thus the basic object of the present invention to provide a method of producing superior quality Concast billets at high speed aiming to avoid/reduce shell rupturing and breakout caused due to shell sticking, slag/scum entrapment, off-corner shell thinning or formation of longitudinal off-corner cracks and the like, such that the acceptance level of quality for special grade billets is increased with improved productivity. A further object of the present invention is directed to a method of producing superior quality Concast billets wherein a Nomogram is developed for optimum steel chemistry for the concast billets with regard to Mn/Si and Mn/S ratios to reduce formation of mould scum and to enable solidified shell strength for reduction of off-corner crack, blow-holes or pin- holes and thus avoiding frequent breakout. A further object of the present invention is directed to developing a method of producing superior quality Concast billets at high speed wherein appropriate water velocity is maintained/established in the water channel of mould to reduce mould distortion at high speed, leading to reduction of mould distortion and enhancement of solidified shell strength in the mould. A still further object of the present invention is directed to developing a method of producing superior quality Concast billets at high speed wherein the upper mould taper is suitably increased to reduce the occurrence of negative taper with a view to reduce off- corner crack and strand breakouts at high speed. A still further object of the present invention is directed to developing a method of producing superior quality Concast billets at high speed wherein foot rolls are installed beneath the mould for reducing billet vibrations inside the mould and to provide extra support below the mould to avoid/reduce chances of formation of off-corner crack and strand breakouts at high speed casting of billets. A still further object of the present invention is directed to developing a method of producing superior quality Concast billets at high speed wherein suitably altered mould oscillation stroke to accommodate suitable speed range and to improve surface quality is incorporated. 5 A still further object of the present invention is directed to developing a method of producing superior quality Concast billets at high speed wherein secondary cooling zone length is extended and the secondary cooling flow rate is controlled/optimized in order to eliminate halfway/midway/reheating cracks. A still further object of the present invention is directed to developing a method of producing superior quality Concast billets at high speed wherein the continuous casting parameters, such as the composition, speed, below mould cooling, oscillation stroke etc are controlled in a manner that the concast breakout is restricted to a minimum of 1.0% per strand and thus improving quality of concast billets to a acceptance level nearly 100% and moreover, providing means to cast carck prone steel grades. SUMMARY OF THE INVENTION Thus according to the basic aspect of the present invention there is provided a steel composition for producing superior quality concast billets at high speed comprising: Carbon: 0.20-0.25% Sulphur:0.03%(max); Silicon :0.20 to 0.30 %; Manganese :at least 0.75 % , selected such that the Mn/Si ratio is maintained above 2.9 and Mn/S ratio above between 25.0 and adapted for reducing the formation of Off-corner crack, blow holes/pin holes and breakout. A still further aspect of the present invention directed to a steel composition for producing superior quality concast billets at high speed comprising: Carbon: 0.4-0.6%; 6 Sulphur: 0.03%(max); Silicon :0.20 to 0.30 %; Manganese :at least 0.80 % , selected such that the Mn/Si ratio is maintained at least 3.3 and Mn/S ratio at least 26.7 and adapted for reducing the formation of Off-corner crack, blow holes/pin holes and breakout. According to a further aspect of the present invention directed to a method of casting directed to ensuring superior quality concast billets at high speed comprising: providing optimized steel composition with selective Mn/Si and Mn/S ratios to reduce formation of mould scum and enable solidified shell strength for reduction of off-corner crack, blow hole/pin hole and break out; increasing the water velocity in mould to reduce mould distortion at higher speed; increasing upper mould taper to reduce strand breakout and off-corner crack formation; optimizing the oscillation parameters for billet caster; and incorporation of extended and modified secondary cooling length. A still futher aspect of the present invention directed to a method of casting directed to ensuring superior quality concast billets at high speed comprising: providing mould foot roll and modified below mould spray jacket and providing of high upper taper in multi tapered mould. Another aspect of the present invention involving said method of casting directed to ensuring superior quality concast billets at high speed ,wherein the velocity of water cooling in mould is increased to > 11.5 m/sec. 7 According to a further aspect of the present invention directed to a method of casting directed to ensuring superior quality concast billets at high speed , wherein (a) the oscillation parameters for billet caster comprised: negative strip time : 0.12-0.15s; negative strip ratio : 60-70%; mould lead : 3-5 mm and (b) said multi taper mould configuration comprise modified triple tapered mould comprised: upper taper : about 2.4 %; middle taper: about 0.9%; and lower taper: about 0.5%. A still further aspect of the present invention directed to a method of casting directed to ensuring superior quality concast billets at high speed , wherein the modified secondary cooling zone length was extended by about 3 m by introducing two more zones such that the water distribution provided for restricting the strand reheating to The present invention and its objects and advantages are described in greater details with reference to the accompanying non limiting figures and illustrations. BRIEF DESCRIPTION OF FIGURES Figure l:is the illustration of the section of conventional concast billet showing presence of the off-corner crack, centerline crack and midway/halfway crack at specific locations. Figure 2: is the illustration of the phenomenon of strand breakout in concast billet due to severe off-corner crack propagation or shell thinning and similar other reasons. 8 Figure 3: is the illustration of the foot roll assembly for the continuous casting mould wherein foot rolls are installed beneath the mould for reducing billet vibrations inside the mould and providing extra support below the mould to avoid strand breakouts due to off- corner cracking etc., according to the present invention. Figure 4: is the illustration of the modified below mould spray jacket according to the present invention favoring desired cooling and arresting longitudinal off-corner shell cracking, causing rupture and strand breakout and improve acceptance level of Concast billets. Figure 5: is the illustration of the typical micro-graph of special quality billet sample showing non-occurrence of internal cracks or blow holes in concast billets by implementing the method according to the invention. Figure 6: is the illustration of the improvements in macro acceptability of special quality cast billets upon implementation of the method of the present invention. Figure 7: is the comparative illustration of the lowering in the occurrences of lower strand breakout frequency on implementing the method of the present invention as compared to prior practice. DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING FIGURES The present invention relates to a method of casting superior quality concast billets at high casting speed, avoiding the occurrence of the event of shell rupturing and strand breakouts for the continuous cast billets, caused due to a number of reasons comprising shell sticking, slag/scum entrapment, off-corner shell thinning and formation of longitudinal off-corner cracks. Such events of rupture and breakout of billets are highly detrimental, expensive and leading to frequent interruption of production, resulting in poor productivity and yield. Conventionally, the forging and spring steel quality billets are produced in 125mm sq. section following the BOF-LTS/LF-CC route. The stringent quality requirements for these 9 grades tend to lower macro acceptability and consequently, higher rate of diversion to lower grades. Typical macro-structure of billet samples of 125mm square section shows occurrences of various cracks such as off-corner, mid-way/reheating and centerline cracks. In order to eliminate occurrences of these cracks in continuous casting of superior quality billets at high speed and improve the percent acceptance level of CC billets, the improved method of the present invention has been developed, reducing/avoiding the strand breakout phenomenon and the resultant loss of production and yield. The present invention is directed to a method of continuous casting of superior quality concast billets at fast speed avoiding rupture and strand breakouts comprising the following technical measures: a. Increasing water velocity in mould to reduce mould distortion at higher speed; b. Increase of upper mould taper to reduce strand breakout and off-corner crack formation; c. Provisioning of below mould foot rool assembly for extra support; d. Provisioning of modified below mould spray jacket for efficient cooling of the strand controlling cooling rate to avoid cracks; e. Modifying secondary cooling water distribution and extension of secondary cooling zone length to reduce halfway/midway/reheating cracks. f. Modification of mould oscillation stroke to accommodate suitable speed range and to improve surface quality. g. Developing nomogram for optimization of steel chemistry. The method of the present invention ascertains the range of process parameters and related hardware means for implementation of the above said improvements in technological steps and procedure such that the cast billets quality acceptance goes up to nearly 100% from the existing 50% and the rate of occurrences of shell rupture or strand breakouts reduced from existing 1.8% to less than 1.0%. Reference is first invited to the accompanying Figure 1 that illustrates the occurrences of the various types of cracks in a typical macrosection of 125mm sq. standard billet produced by following conventional route of CONCAST billets wherein the off-corner cracks, centerline cracks and midway/reheating cracks have been shown. The accompanying Figure 2 illustrates the phenomenon of longitudinal off-corner cracks propagated to develop strand breakout for a continuous cast 125mm sq. billet. The method of the present invention is directed to eliminating the event of crack occurrences and the consequent shell rupture and breakouts by adopting the measures details of which are as follows: 10 Nomogram for achieving desired steel composition: The present invention is directed to developing Nomogram between Manganese(Mn), Silicon(Si), Sulphur(S) and Carbon(C) in steel composition, among the commercially available steel grades produced by the steel industries comprising superior quality forging and spring steel grades which are subjected to frequent breakouts in continuous casting strands. Low alloy steel grades such as the forging quality and high carbon chrome bearing steel for automobile application and high carbon steel for agricultural appliances, when produced as billets in continuous casting plant, frequently suffer from strand breakouts which is as high as 1.6 to 1.8 % even at low and moderate casting speeds in the range of 2.0-2.2m/min, for reasons like severe longitudinal off-corner cracks and occurrence of slag breakouts caused due to unfavorable Mn/Si ratio in steel. To avoid midway/reheat cracks, blow holes/pin holes and severe off corner cracks leading to strand breakouts at high speed casting of billets, a nomogram for adjustment of steel composition was prepared according to the present invention, based on the thermodynamic calculations. Mn-Si deoxidation practices are followed for reducing billets usually through open pour casting by metering nozzles in billet casters. To maintain desired flowability of liquid metal in mould, Mn/Si ratio is generally maintained at least 3.0. A low ratio of solid inclusions rich in silica are formed which in turn leads to formation of high melting point scum, tending to get entrapped between the mould walls and the solidifying shell of the steel. This viscous slag penetrates the mould-strand gap leading to poor lubrication and low mould heat transfers just beneath the mould meniscus. This leads to lowering of shell thickness along the regions where mould scum is entrapped. Due to such localized thinning of shells at the exit of mould fail to sustain ferrostatic pressure and thereby causing shell rupture and breakouts. Although the higher Mn/Si ratio(>3.0) is maintained, as a thumb rule approach to deal with the problem and do not provide a comprehensive guide line for maintaining a chemistry in terms of C, Mn, Si in steel to form liquid inclusions. Thus for formation of liquid Mn-Si oxide inclusions, it is required to have different Mn/Si ratios compatible with carbon content in steel. As already discussed, the presence of off-corner cracks of varied intensity leads to internal quality problems causing higher rejection of the superior quality billets such as the forging 11 quality and spring steel quality billets and the associated costs incurred. Presence of one or more severe longitudinal off-corner cracks causes shell puncturing/rupturing or breakout at mould exit. Solidification of the cast billets free of any entrapment or inclusions or blow holes/pin holes and cracks provide desired shell rigidity and strength, avoiding frequent occurrences of breakouts. Increasing the Mn/S ratio is observed to have an important role for increasing shell strength and thereby reduces susceptibility to crack formation and propagation, responsible for strand rupture/breakouts. A nomogram among Silicon, manganese and Sulphur for different range of Carbon content in special grade steels, is prepared based on thermodynamic calculations, to ensure a chemistry providing desired Mn/Si corresponding to the chemistry such as to restrict occurrences of breakouts in continuous casting of billets at high speed. The nomogram is presented in following Table 1 And Table 2. Table 1: Nomogram between Mn, Si and S (Carbon:0.20-0.25 wt%, Sulphur-0.03%) Si, wt % Mn, wt %(min) Mn/Si S, wt % Mn/S 0.20 0.75 3.7 0.03 25.0 0.21 0.75 3.6 0.03 25.0 0.22 0.76 3.5 0.03 25.3 0.23 0.77 3.3 0.03 25.7 0.24 0.79 3.3 0.03 26.3 0.25 0.80 3.2 0.03 26.7 0.26 0.81 3.1 0.03 27.0 0.27 0.83 3.1 0.03 27.7 0.28 0.84 3.0 0.03 28.0 0.29 0.85 2.9 0.03 28.3 0.30 0.87 2.9 0.03 29.0 12 Table 2: Nomogram between Mn, Si and S (Carbon:0.40-0.60 wt%, Sulphur-0.03%) Si, wt % Mn, wt %(min) Mn/Si S, wt % Mn/S 0.20 0.80 4.0 0.03 26.7 0.21 0.81 3.9 0.03 27.0 0.22 0.82 3.7 0.03 27.3 0.23 0.84 3.7 0.03 28.0 0.24 0.85 3.5 0.03 28.3 0.25 0.86 3.4 0.03 28.7 0.26 0.87 3.4 0.03 29.0 0.27 0.90 3.3 0.03 30.0 0.28 0.92 3.3 0.03 30.7 0.29 0.95 3.3 0.03 31.7 0.30 0.98 3.3 0.03 32.7 The method of the present invention is thus directed to maintain steel chemistry of liquid steel for continuous casting of billets at high speed, by selectively maintaining Mn/Si >3.0 and Mn/S ranging 25 to 32 depending on carbon content of low alloy and special alloy steel for forging or spring steel applications, following the above nomogram developed in order to obtain rigidity and strength of appropriate order for desired control/stop occurrences of off- corner cracks in cast billets leading to strand rupture/breakout. Mould water flow velocity: The flow velocity of cooling water through the mould wall play an important role in mould heat extraction and thereby controlling the hot as well as cold face temperature of operating mould tube which in turn influence the rate of cooling and solidification of the cast billets. It has been observed that a high hot face temperature causes mould distortion near meniscus region whereas the high cold face temperature results in nucleation boiling of mould cooling water. The nucleation boiling of cooling water is a phenomenon leading to poor and non- uniform heat transfer causing non-uniform shell thickness and higher strand temperature inside mould. The situation gets further aggravated at high casting speeds and due to use of mould electromagnetic stirring (EMS). Water flow velocity in mould water channel for casting of 125mm square billets has been increased from existing value of 13 to be inadequate for high speed casting using EMS, to a velocity of >11.5 m/sec, based on heat transfer calculations. Provisioning of mould Foot rolls and modified below-mould Spray jacket: While casting billets of superior quality at high speed in continuous caster moulds, there is a general tendency for the billets cast at high speeds subjected to lower shell thickness and high shell temperature of the solidifying strand. This in turn, results in lower shell strength. Moreover the shell thickness is found to be lowest at off-corner location due to in mould bulging. These are the reasons for which off-corner cracking occur in high-speed continuous casting of billets. The present invention is directed to arrest occurring of such longitudinal off-corner cracks leading to strand breakouts, by providing extra support to the thin and hot shell just below the mould, in the form of foot rolls for casting of 125mm sq. billets. Reference is now invited to the accompanying Figure 3, wherein the position and mounting arrangement of the Foot rolls for supporting the solidified strand is illustrated, that shows that roll support is provided from all four sides of the cast and solidified billet strand just below the mould and thus preventing in mould bulging and off-corner longitudinal cracking to avoid strand breakouts. The accompanying Figure 4, illustrates the modification effected in the below mould spray jacket to accommodate the Foot roll assemblies. Modification of Mould oscillation parameter: The method of casting superior quality concast billets at high speed according to the present invention, further takes into account the impact of mould oscillation parameter on the caster performance and product quality. It has been observed in the art of continuous casting, that the oscillation parameters have significant influence on caster performance and also on surface quality of solidifying strand in terms of laps/bleeds, when cast at high speed. Hot face temperature of mould tube depends on oscillation parameters. High mould oscillation stroke length increases shell deformation below the meniscus leading to sticker formation due to increased mould strand interaction. Moreover, high stroke length increases negative strip time causing deterioration of surface quality. Optimum oscillation parameters for billet caster for superior castings at high speed are experimentally standardized as follows: 14 Negative strip time :0.12-0.15 sec Negative strip ratio :60-70% Mould lead :3-5 mm In the conventional caster operation, a mould oscillation parameter value having stroke 12mm at a frequency of 150 cycles per minute, found to be not suitable for the speed range of 2.3-2.8 m/min. On contrary, the oscillation stroke of 12mm is suitable for a very high casting speed of >2.5 m/min as per the conventional practice, the corresponding figures shown in bold figures in the following Table 3. However, the improvement in terms of superior billet quality and low breakout occurrences is achieved by adjusting/modifying the mould oscillation stroke to 11 mm. The modified oscillation parameters with modified stroke are given in the following Table 4. It is apparent from the Table 4, that the oscillation parameters are within the desired acceptable range of values in the casting speed range of 2.3-2.8m/min. Table 3: Conventional Oscillation parameters Speed m/min Tn NSR Mould lead 2.0 0.15 76.97 6.09 2.1 0.15 75.77 5.83 2.2 0.15 74.55 5.83 2.3 0.15 73.32 5.34 2.4 0.14 72.08 5.10 2.5 0.14 70.83 4.86 2.6 0.14 69.57 4.62 2.7 0.14 68.29 4.39 2.8 0.13 67.00 4.17 15 Table 4: Modified Oscillation parameters relating to the invention. Speed m/min Tn NSR Mould lead 2.0 0.15 74.77 5.16 2.1 0.15 73.43 4.91 2.2 0.14 72.08 4.67 2.3 0.14 70.72 4.43 2.4 0.14 69.34 4.20 2.5 0.14 67.94 3.97 2.6 0.13 66.53 3.75 2.7 0.13 65.10 3.53 2.8 0.13 63.65 3.31 Provisioning of higher upper taper in multi tapered mould: Conventionally, mould taper is provided to compensate the shrinkage of the solidifying strand in the mould. In addition, taper also compensates the mould distortion that takes place in the upper region of the mould due to high mould hot face temperature. The high hot face temperature is mainly due to high heat extraction in the upper part of the mould leading to the formation of the negative taper. The problem gets further complex at higher casting speeds. The provision of negative taper in the upper portion of the mould causes in mould bulging and excessive gap formation resulting in poor strand-mould interaction. This phenomenon has an adverse effect leading to the formation of deeper oscillation marks on the strand surface, off-squareness of billet and formation of off-corner cracks. As an viable better alternative, Triple tapered mould is used in existing method for casting of 125mm sq. section billet having the configuration as follows: Upper Taper: 2.1%; Middle Taper :0.9%; Lower taper :0.5%; Taper configuration was modified to increase upper taper for compensating the high mould distortion at selective high casting speed exceeding 2.5m/min. Thus the present invention is directed to apply triple tapered mould with preferred higher upper taper of 2.4% as compared to existing 2.1% in the mould, to overcome the problems of crack or poor surface of strand, while middle and lower taper are maintained at the conventional values. 16 Extension and modification of secondary cooling zone length: The length of secondary cooling zones and the water spray/distribution pattern has the importance in respect of formation of halfway/midway or reheating cracks, the nature, origin and location of such cracks tend to use these names synonymously, and also the centerline cracks in continuous casting of superior quality billets. Improper secondary zone length selection and inadequate cooling water distribution therein, tends to strand reheating resulting in the high thermal stresses and strains at the solidifying front. When the thermal strains developed due to reheating exceeds a critical value, cracks may initiate perpendicular to the billet surface. The present invention involve the secondary cooling zone modification due to the following reasons: The existing secondary zone comprising MS zone-0.3 m long with cooling water distribution rate of 165 Ipm, zone -I -1.5m long having water distribution of 180 Ipm and the zone-IIA - 1.9m long having the water distribution of 80 Ipm; thus the total length of secondary cooling spreading over 3.7 m, was found to be insufficient as compared to the computed Crater length of 8-9m, up to end of solidification, while casting was carried out at a speed of 2.2-2.6m/min; further the cooling water distribution/flow rate was independent of casting speed resulting in strand reheating to an extent of 120°C and leading to formation of halfway and centerline cracks. The present invention thus removed the deficiency of the secondary cooling method by extending the length of the secondary cooling zone by 3.1m by introducing two additional cooling zones IIB and zone III. Moreover, the water distribution corresponding to different casting speeds were moderated at different zones of secondary cooling such that the strand reheating is avoided and the strand reheating is restricted to water distribution in different zones of the modified secondary cooling zone, directed to eliminating formation of halfway/reheat and centerline cracks in concast billets are presented in the following Table 5. 17 Table 5: Modified Secondary Cooling Distribution: Speed (m/min) MS Zone (Ipm) Zone-I (Ipm) Zone-IIA (Ipm) Zone-IIB (Ipm) Zone-Ill (Ipm) 1.5 165 170 70 65 28 1.7 165 183 78 65 28 1.9 165 195 85 65 28 2.1 165 208 93 70 31 2.3 165 220 100 77 34 2.5 165 233 107 84 37 2.7 165 247 113 91 40 2.9 165 260 120 99 43 3.1 165 273 127 106 46 3.3 165 287 133 113 49 3.5 165 300 140 120 52 The present invention is thus directed to a method for high speed casting of superior quality concast billets led to a significant reduction of internal defects of cast billets in the form of longitudinal off-corner cracks, halfway/midway/reheat cracks and centerline cracks and blow holes or pin holes. Thus the resulting cast billets was having higher rigidity and strength such as to provide a macro section free of appearance of any crack or negligible cracks, as illustrated in the accompanying Figure 5. Macro acceptance of special quality billets improved from 1.8% per strand to Figure 7, respectively. It is thus possible by way of the present invention directed to a method of casting of superior concast billets at high speed, avoiding occurrences of strand rupture/breakout is capable to ensure higher acceptance standard of the superior quality concast billets, more particularly, the forging and spring steel quality cast steel billets by imposing control on selective chemistry and process parameters, such that the shell thinning, longitudinal off- corner or centerline crack and the like, responsible for the strand frequent breakouts can be eliminated and a superior quality billets of higher rigidity and strength is obtained, ensuring higher plant availability, operational safety and improved productivity at lower costs due to lesser rejection and as such providing prospects for wide industrial applications. 18 WE CLAIM: 1. A steel composition for producing superior quality concast billets at high speed comprising: Carbon: 0.20-0.25% Sulphur:0.03% max; Silicon :0.20 to 0.30 %; Manganese :at least 0.75 % , selected such that the Mn/Si ratio is maintained at least 2.9 and Mn/S ratio at least 25.0 and adapted for reducing the formation of Off-corner crack, blow holes/pin holes and breakout. 2. A steel composition for producing superior quality concast billets at high speed as claimed in claim 1 comprising : Nomogram between Mn, Si and S (Carbon:0.20-0.25 wt%, Sulphur-0.03%) Si, wt % Mn, wt %(min) Mn/Si S, wt % Mn/S 0.20 0.75 3.7 0.03 25.0 0.21 0.75 3.6 0.03 25.0 0.22 0.76 3.5 0.03 25.3 0.23 0.77 3.3 0.03 25.7 0.24 0.79 3.3 0.03 26.3 0.25 0.80 3.2 0.03 26.7 0.26 0.81 3.1 0.03 27.0 0.27 0.83 3.1 0.03 27.7 0.28 0.84 3.0 0.03 28.0 0.29 0.85 2.9 0.03 28.3 0.30 0.87 2.9 0.03 29.0 19 3. A steel composition for producing superior quality concast billets at high speed comprising: Carbon: 0.4-0.6%; Sulphur:0.03% (max); Silicon :0.20 to 0.30 %; Manganese :at least 0.80; selected such that the Mn/Si ratio selectively varies between 3.3 to 4.0 and Mn/S ratio selectively varies between 26.7 to 32.7 and adapted for reducing anyone or more of the formation of Off-corner crack, blow holes/pin holes and breakout. 4. A steel composition for producing superior quality concast billets at high speed as claimed in claim 3 comprising : Nomogram between Mn, Si and S (Carbon:0.40-0.50 wt%, Sulphur-0.03%) Si, wt % Mn, wt %(min) Mn/Si S, wt % Mn/S 0.20 0.80 4.0 0.03 26.7 0.21 0.81 3.9 0.03 27.0 0.22 0.82 3.7 0.03 27.3 0.23 0.84 3.7 0.03 28.0 0.24 0.85 3.5 0.03 28.3 0.25 0.86 3.4 0.03 28.7 0.26 0.87 3.4 0.03 29.0 0.27 0.90 3.3 0.03 30.0 0.28 0.92 3.3 0.03 30.7 0.29 0.95 3.3 0.03 31.7 0.30 0.98 3.3 0.03 32.7 20 5. A method of casting directed to ensuring superior quality concast billets at high speed comprising: providing optimized steel composition with selective Mn/Si and Mn/S ratios to reduce formation of mould scum and enable solidified shell strength for reduction of off-corner crack, blow holes/pin holes and break outs; increasing the water velocity in mould to reduce mould distortion at higher speed; increasing upper mould taper to reduce strand breakout and off-corner crack formation; optimizing the oscillation parameters for billet caster; and incorporation of extended and modified secondary cooling length. 6. A method of casting directed to ensuring superior quality concast billets at high speed as claimed in claim 5 comprising: providing mould foot roll and modified below mould spray jacket and providing of high upper taper in multi tapered mould. 7. A method of casting directed to ensuring superior quality concast billets at high speed as claimed in anyone of claims 5 or 6 wherein the velocity of water cooling in mould is increased to about >11.5 m/sec. 8. A method of casting directed to ensuring superior quality concast billets at high speed as claimed in anyone of claims 5 to 7 wherein (a) the oscillation parameters for billet caster comprised: negative strip time : 0.12-0.15s; negative strip ratio : 60-70%; mould lead : 3-5 mm, and (b) said multi taper mould configuration comprise modified triple tapered mould comprised: upper taper : about 2.4 %; middle taper: about 0.9%; and 21 lower taper: about 0.5%. 22 9. A method of casting directed to ensuring superior quality concast billets at high speed as claimed in anyone of claims 5 to 8 wherein the modified secondary cooling zone length was extended by about 3 m by introducing two more zones such that the water distribution provided for restricting the strand reheating to 10. A steel composition for producing superior quality concast billets at high speed and a method of casting directed to ensuring superior quality concast billets at high speed substantially as hereindescribed and illustrated with reference to the accompanying examples and figures. A steel composition for producing superior quality concast billets at high speed comprising, Carbon: 0.20-0.25%; Sulphur:0.03%(max); Silicon :0.20 to 0.30 %; Manganese :at least 0.75, selected such that the Mn/Si ratio is maintained at least 2.9 and Mn/S ratio at least 25.0 and or alternatively Carbon: 0.4-0.6%; Sulphur:0.03%(max); Silicon :0.20 to 0.30 %;Manganese :at least 0.80, selected such that the Mn/Si ratio is maintained at least 3.3 and Mn/S ratio at least 26.7 and adapted for reducing the formation of Off-corner crack, blow holes/pin holes and breakout and a method casting seamlessly production of superior quality concast billets at high speed with increased productivity. Said method involving velocity of water cooling in mould is increased to about >11.5 m/sec, providing mould foot roll and modified below mould spray jacket and providing of high upper taper of 2.4%, in multi tapered mould

Full Text

FIELD OF THE INVENTION:
The present invention is directed to a process of faster production of CONCAST billets to
meet the quality and quantity requirements of the end users. More particularly, the
invention is directed to superior quality billets for purposes like forging, spring making and
wire rods at higher casting speeds and improved productivity, eliminating detrimental
strand breakout events in continuous casting and the causes responsible for such
phenomenon, leading to production loss, machine damage and serious safety hazards to
operating personnel. The invention is also directed to a method and operational practices to
restrict strand rupture/breakout occurrences in continuous casting eliminating the factors
responsible for such event such as shell sticking, slag/scum entrapment, off-corner shell
thinning and formation of severe longitudinal off-corner cracks and the like. The method of
arresting the strand breakout of billets produced at faster casting speed involving operating
parameters like-increased water velocity in mould to reduce mould distortion at higher
speed, increased upper mould taper to avoid off-corner crack formation, providing below
mould foot roll for extra support, improved below mould spray jacket, modified cooling
water distribution and extension of secondary cooling zone length to eliminate
halfway/midway/reheating cracks, modified oscillation stroke, developing nomogram for
optimization of steel chemistry, such that the acceptance level of special quality steel billets
has gone up from existing 50% to nearly 100% and the strand breakout frequency is
reduced substantially from 1.8% per strand to less than 1.0 % per strand, improving
productivity in CONCAST plant.
BACKGROUND ART:
It is well known in the art of steel products produced in steel plants, there is a growing
demand of CONCAST billets. The quality of such products shall also meet stringent internal
quality requirements for processing of steels of special applications at the consumers end
such as forging, spring manufacturing, wire rods of varied sections. To meet the growing
demand for good quality steel, higher casting speeds are required on one hand to achieve
higher productivity and to identify and eliminate the casting defects on the other hand, is
also extremely important for maintaining the quality/acceptance standard of cast billets
produced at faster speed. The major defects occurring during casting of billets are
longitudinal off-corner cracks, midway or reheating cracks, centerline cracks, pinholes and
2

blow holes etc. Longitudinal off-corner cracks are the defects usually occurring at or near
the corner of the billets and initiate in or just below the mould. Local thinning of the solid
shell near the corner region, higher corner temperature, in-mould bulging or mould
distortion and steel composition are the main reasons responsible for the formation of
longitudinal off-corner cracks. The halfway or the midway cracks are formed primarily
between growing columnar dendrites and are aligned perpendicular to strand surface. Such
cracks are formed during solidification of molten metal and are usually filled in by the
residual elements like Sulphur and Phosphorus having lower solidification temperature,
which reduces intrinsic toughness properties.
Low alloy steel grades such as the forging quality and high carbon chrome bearing steel for
automobile application and high carbon steel for agricultural appliances, when produced as
billets in continuous casting plant, frequently suffer from strand breakouts which is as high
as 1.6 to 1.8 % even at low and moderate casting speeds in the range of 2.0-2.2m/min, for
reasons like severe longitudinal off-corner cracks and occurrence of slag breakouts caused
due to unfavorable Mn/Si ratio in steel.
The flow velocity of cooling water through the mould wall play an important role in mould
heat extraction and thereby controlling the hot as well as cold face temperature of operating
mould tube which in turn influence the rate of cooling and solidification of the cast billets.
Water flow velocity in mould water channel for casting of 125mm square billets at the
existing value of Electromagnetic stirring (EMS), leading to defects like the mould distortion due to high hot
face temperature, and nucleation boiling of water in the mould water channel leading to
poor heat transfer inducing formation of crack due to higher strand temperature.
Further, the existing secondary zone comprising MS zone-0.3 m long with cooling water
distribution rate of 165 Ipm, zone -I -1.5m long having water distribution of 180 Ipm and
the zone-IIA -1.9m long having the water distribution of 80 Ipm; thus the total length of
secondary cooling spreading over 3.7 m, was found to be insufficient as compared to the
computed Crater length of 8-9m, up to end of solidification, while casting was carried out at
a speed of 2.2-2.6m/min; further the secondary cooling water distribution/flow rate was
independent of casting speed resulting in strand reheating to an extent of 120°C and leading
to formation of halfway and centerline cracks. This arrangement needed a relook at
determining the proper zone length and distribution of cooling water to avoid the defects as
3

mentioned. In addition to these factors, surface reheating due to improper secondary
cooling tends to increase in tensile stresses at solid/liquid interfaces, giving rise to crack
formation tendency. The centerline cracks on the other hand, originate during end of
solidification due to uncontrolled excess of shrinkage of bulk casting body. Other defects
such as the pinholes and blow holes occur due to mainly improper chemistry, impurities and
excessive use of lubricating oil in mould.
It is also a common experience in continuous casting of billets, the occurrences of breakout
of CONCAST billets as a deleterious event. Breakout results in loss of production, yield loss,
machine damage and in addition serious safety hazards to the operating personnel. Thus to
identify and rectify/improve operational practices responsible for occurrence of breakout is
of extreme importance in deciding indices for setting criteria in continuous casting. Reasons
like the shell sticking, slag/scum entrapment, off-corner shell thinning and formation of
longitudinal off-corner cracks and the like, are attributable to the event of breakout in
continuous casting. Loss of steel contact with the mould wall either in the of-corner region
or along the narrow face results in localized shell thinning which contributes to the
formation of off-corner cracks and in extreme situation causes shell rupturing and breakout.
In most of the integrated steel plants, the billets of section such as 125 mm sq., is produced
for conversion to forging quality and spring steel products, through BOF-LTS/LF-CC route.
The stringent quality requirements for these product grades lead to lower macro
acceptability and consequently, higher rate of diversion to lower grades. A typical macro
structure of billet samples of 125mm sq. section shows presence of defects like off-corner,
midway/reheating and centerline cracks and also longitudinal off-corner related breakout is
common during continuous casting of such billets.
There has been therefore, a continuous need for developing an improved method for
production of CONCAST billets at higher speed with necessary technical measures including
modification/incorporation of process steps/parameters directed to identify the reasons and
eliminate/reduce the occurrences of various types of cracks as observed in casting of billets
and also to avoid breakout due to reasons like the shell sticking, slag/scum entrapment, off-
corner shell thinning and formation of longitudinal off-corner cracks causing shell rupturing
and breakout in extreme situation, and thereby avoid stopping production and improve
productivity and yield, with increased acceptability of product quality.
4

OBJECT OF THE INVENTION:
It is thus the basic object of the present invention to provide a method of producing
superior quality Concast billets at high speed aiming to avoid/reduce shell rupturing and
breakout caused due to shell sticking, slag/scum entrapment, off-corner shell thinning or
formation of longitudinal off-corner cracks and the like, such that the acceptance level of
quality for special grade billets is increased with improved productivity.
A further object of the present invention is directed to a method of producing superior
quality Concast billets wherein a Nomogram is developed for optimum steel chemistry for
the concast billets with regard to Mn/Si and Mn/S ratios to reduce formation of mould scum
and to enable solidified shell strength for reduction of off-corner crack, blow-holes or pin-
holes and thus avoiding frequent breakout.
A further object of the present invention is directed to developing a method of producing
superior quality Concast billets at high speed wherein appropriate water velocity is
maintained/established in the water channel of mould to reduce mould distortion at high
speed, leading to reduction of mould distortion and enhancement of solidified shell strength
in the mould.
A still further object of the present invention is directed to developing a method of
producing superior quality Concast billets at high speed wherein the upper mould taper is
suitably increased to reduce the occurrence of negative taper with a view to reduce off-
corner crack and strand breakouts at high speed.
A still further object of the present invention is directed to developing a method of
producing superior quality Concast billets at high speed wherein foot rolls are installed
beneath the mould for reducing billet vibrations inside the mould and to provide extra
support below the mould to avoid/reduce chances of formation of off-corner crack and
strand breakouts at high speed casting of billets.
A still further object of the present invention is directed to developing a method of
producing superior quality Concast billets at high speed wherein suitably altered mould
oscillation stroke to accommodate suitable speed range and to improve surface quality is
incorporated.
5

A still further object of the present invention is directed to developing a method of
producing superior quality Concast billets at high speed wherein secondary cooling zone
length is extended and the secondary cooling flow rate is controlled/optimized in order to
eliminate halfway/midway/reheating cracks.
A still further object of the present invention is directed to developing a method of
producing superior quality Concast billets at high speed wherein the continuous casting
parameters, such as the composition, speed, below mould cooling, oscillation stroke etc are
controlled in a manner that the concast breakout is restricted to a minimum of 1.0% per
strand and thus improving quality of concast billets to a acceptance level nearly 100% and
moreover, providing means to cast carck prone steel grades.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a steel
composition for producing superior quality concast billets at high speed comprising:
Carbon: 0.20-0.25%
Sulphur:0.03%(max);
Silicon :0.20 to 0.30 %;
Manganese :at least 0.75 % ,
selected such that the Mn/Si ratio is maintained above 2.9 and Mn/S ratio above between
25.0 and adapted for reducing the formation of Off-corner crack, blow holes/pin holes and
breakout.
A still further aspect of the present invention directed to a steel composition for producing
superior quality concast billets at high speed comprising:
Carbon: 0.4-0.6%;
6

Sulphur: 0.03%(max);
Silicon :0.20 to 0.30 %;
Manganese :at least 0.80 % ,
selected such that the Mn/Si ratio is maintained at least 3.3 and Mn/S ratio at least 26.7
and adapted for reducing the formation of Off-corner crack, blow holes/pin holes and
breakout.
According to a further aspect of the present invention directed to a method of casting
directed to ensuring superior quality concast billets at high speed comprising:
providing optimized steel composition with selective Mn/Si and Mn/S ratios to reduce
formation of mould scum and enable solidified shell strength for reduction of off-corner
crack, blow hole/pin hole and break out;
increasing the water velocity in mould to reduce mould distortion at higher speed;
increasing upper mould taper to reduce strand breakout and off-corner crack formation;
optimizing the oscillation parameters for billet caster; and
incorporation of extended and modified secondary cooling length.
A still futher aspect of the present invention directed to a method of casting directed to
ensuring superior quality concast billets at high speed comprising:
providing mould foot roll and modified below mould spray jacket and
providing of high upper taper in multi tapered mould.
Another aspect of the present invention involving said method of casting directed to
ensuring superior quality concast billets at high speed ,wherein the velocity of water cooling
in mould is increased to > 11.5 m/sec.
7

According to a further aspect of the present invention directed to a method of casting
directed to ensuring superior quality concast billets at high speed , wherein
(a) the oscillation parameters for billet caster comprised:
negative strip time : 0.12-0.15s;
negative strip ratio : 60-70%;
mould lead : 3-5 mm and
(b) said multi taper mould configuration comprise modified triple tapered mould comprised:
upper taper : about 2.4 %;
middle taper: about 0.9%; and
lower taper: about 0.5%.
A still further aspect of the present invention directed to a method of casting directed to
ensuring superior quality concast billets at high speed , wherein the modified secondary
cooling zone length was extended by about 3 m by introducing two more zones such that
the water distribution provided for restricting the strand reheating to The present invention and its objects and advantages are described in greater details with
reference to the accompanying non limiting figures and illustrations.
BRIEF DESCRIPTION OF FIGURES
Figure l:is the illustration of the section of conventional concast billet showing presence of
the off-corner crack, centerline crack and midway/halfway crack at specific locations.
Figure 2: is the illustration of the phenomenon of strand breakout in concast billet due to
severe off-corner crack propagation or shell thinning and similar other reasons.
8

Figure 3: is the illustration of the foot roll assembly for the continuous casting mould
wherein foot rolls are installed beneath the mould for reducing billet vibrations inside the
mould and providing extra support below the mould to avoid strand breakouts due to off-
corner cracking etc., according to the present invention.
Figure 4: is the illustration of the modified below mould spray jacket according to the
present invention favoring desired cooling and arresting longitudinal off-corner shell
cracking, causing rupture and strand breakout and improve acceptance level of Concast
billets.
Figure 5: is the illustration of the typical micro-graph of special quality billet sample showing
non-occurrence of internal cracks or blow holes in concast billets by implementing the
method according to the invention.
Figure 6: is the illustration of the improvements in macro acceptability of special quality cast
billets upon implementation of the method of the present invention.
Figure 7: is the comparative illustration of the lowering in the occurrences of lower strand
breakout frequency on implementing the method of the present invention as compared to
prior practice.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
The present invention relates to a method of casting superior quality concast billets at high
casting speed, avoiding the occurrence of the event of shell rupturing and strand breakouts
for the continuous cast billets, caused due to a number of reasons comprising shell sticking,
slag/scum entrapment, off-corner shell thinning and formation of longitudinal off-corner
cracks. Such events of rupture and breakout of billets are highly detrimental, expensive and
leading to frequent interruption of production, resulting in poor productivity and yield.
Conventionally, the forging and spring steel quality billets are produced in 125mm sq.
section following the BOF-LTS/LF-CC route. The stringent quality requirements for these
9

grades tend to lower macro acceptability and consequently, higher rate of diversion to lower
grades. Typical macro-structure of billet samples of 125mm square section shows
occurrences of various cracks such as off-corner, mid-way/reheating and centerline cracks.
In order to eliminate occurrences of these cracks in continuous casting of superior quality
billets at high speed and improve the percent acceptance level of CC billets, the improved
method of the present invention has been developed, reducing/avoiding the strand breakout
phenomenon and the resultant loss of production and yield. The present invention is
directed to a method of continuous casting of superior quality concast billets at fast speed
avoiding rupture and strand breakouts comprising the following technical measures:
a. Increasing water velocity in mould to reduce mould distortion at higher speed;
b. Increase of upper mould taper to reduce strand breakout and off-corner crack
formation;
c. Provisioning of below mould foot rool assembly for extra support;
d. Provisioning of modified below mould spray jacket for efficient cooling of the strand
controlling cooling rate to avoid cracks;
e. Modifying secondary cooling water distribution and extension of secondary cooling
zone length to reduce halfway/midway/reheating cracks.
f. Modification of mould oscillation stroke to accommodate suitable speed range and to
improve surface quality.
g. Developing nomogram for optimization of steel chemistry.
The method of the present invention ascertains the range of process parameters and related
hardware means for implementation of the above said improvements in technological steps
and procedure such that the cast billets quality acceptance goes up to nearly 100% from
the existing 50% and the rate of occurrences of shell rupture or strand breakouts reduced
from existing 1.8% to less than 1.0%.
Reference is first invited to the accompanying Figure 1 that illustrates the occurrences of
the various types of cracks in a typical macrosection of 125mm sq. standard billet produced
by following conventional route of CONCAST billets wherein the off-corner cracks, centerline
cracks and midway/reheating cracks have been shown. The accompanying Figure 2
illustrates the phenomenon of longitudinal off-corner cracks propagated to develop strand
breakout for a continuous cast 125mm sq. billet. The method of the present invention is
directed to eliminating the event of crack occurrences and the consequent shell rupture and
breakouts by adopting the measures details of which are as follows:
10

Nomogram for achieving desired steel composition:
The present invention is directed to developing Nomogram between Manganese(Mn),
Silicon(Si), Sulphur(S) and Carbon(C) in steel composition, among the commercially
available steel grades produced by the steel industries comprising superior quality forging
and spring steel grades which are subjected to frequent breakouts in continuous casting
strands. Low alloy steel grades such as the forging quality and high carbon chrome bearing
steel for automobile application and high carbon steel for agricultural appliances, when
produced as billets in continuous casting plant, frequently suffer from strand breakouts
which is as high as 1.6 to 1.8 % even at low and moderate casting speeds in the range of
2.0-2.2m/min, for reasons like severe longitudinal off-corner cracks and occurrence of slag
breakouts caused due to unfavorable Mn/Si ratio in steel.
To avoid midway/reheat cracks, blow holes/pin holes and severe off corner cracks leading to
strand breakouts at high speed casting of billets, a nomogram for adjustment of steel
composition was prepared according to the present invention, based on the thermodynamic
calculations.
Mn-Si deoxidation practices are followed for reducing billets usually through open pour
casting by metering nozzles in billet casters. To maintain desired flowability of liquid metal
in mould, Mn/Si ratio is generally maintained at least 3.0. A low ratio of solid inclusions rich in silica are formed which in turn leads to formation of high melting
point scum, tending to get entrapped between the mould walls and the solidifying shell of
the steel. This viscous slag penetrates the mould-strand gap leading to poor lubrication and
low mould heat transfers just beneath the mould meniscus. This leads to lowering of shell
thickness along the regions where mould scum is entrapped. Due to such localized thinning
of shells at the exit of mould fail to sustain ferrostatic pressure and thereby causing shell
rupture and breakouts. Although the higher Mn/Si ratio(>3.0) is maintained, as a thumb
rule approach to deal with the problem and do not provide a comprehensive guide line for
maintaining a chemistry in terms of C, Mn, Si in steel to form liquid inclusions. Thus for
formation of liquid Mn-Si oxide inclusions, it is required to have different Mn/Si ratios
compatible with carbon content in steel.
As already discussed, the presence of off-corner cracks of varied intensity leads to internal
quality problems causing higher rejection of the superior quality billets such as the forging
11

quality and spring steel quality billets and the associated costs incurred. Presence of one or
more severe longitudinal off-corner cracks causes shell puncturing/rupturing or breakout at
mould exit. Solidification of the cast billets free of any entrapment or inclusions or blow
holes/pin holes and cracks provide desired shell rigidity and strength, avoiding frequent
occurrences of breakouts. Increasing the Mn/S ratio is observed to have an important role
for increasing shell strength and thereby reduces susceptibility to crack formation and
propagation, responsible for strand rupture/breakouts.
A nomogram among Silicon, manganese and Sulphur for different range of Carbon content
in special grade steels, is prepared based on thermodynamic calculations, to ensure a
chemistry providing desired Mn/Si corresponding to the chemistry such as to restrict
occurrences of breakouts in continuous casting of billets at high speed. The nomogram is
presented in following Table 1 And Table 2.
Table 1: Nomogram between Mn, Si and S
(Carbon:0.20-0.25 wt%, Sulphur-0.03%)

Si, wt % Mn, wt %(min) Mn/Si S, wt % Mn/S
0.20 0.75 3.7 0.03 25.0
0.21 0.75 3.6 0.03 25.0
0.22 0.76 3.5 0.03 25.3
0.23 0.77 3.3 0.03 25.7
0.24 0.79 3.3 0.03 26.3
0.25 0.80 3.2 0.03 26.7
0.26 0.81 3.1 0.03 27.0
0.27 0.83 3.1 0.03 27.7
0.28 0.84 3.0 0.03 28.0
0.29 0.85 2.9 0.03 28.3
0.30 0.87 2.9 0.03 29.0
12

Table 2: Nomogram between Mn, Si and S
(Carbon:0.40-0.60 wt%, Sulphur-0.03%)

Si, wt % Mn, wt %(min) Mn/Si S, wt % Mn/S
0.20 0.80 4.0 0.03 26.7
0.21 0.81 3.9 0.03 27.0
0.22 0.82 3.7 0.03 27.3
0.23 0.84 3.7 0.03 28.0
0.24 0.85 3.5 0.03 28.3
0.25 0.86 3.4 0.03 28.7
0.26 0.87 3.4 0.03 29.0
0.27 0.90 3.3 0.03 30.0
0.28 0.92 3.3 0.03 30.7
0.29 0.95 3.3 0.03 31.7
0.30 0.98 3.3 0.03 32.7
The method of the present invention is thus directed to maintain steel chemistry of liquid
steel for continuous casting of billets at high speed, by selectively maintaining Mn/Si >3.0
and Mn/S ranging 25 to 32 depending on carbon content of low alloy and special alloy steel
for forging or spring steel applications, following the above nomogram developed in order to
obtain rigidity and strength of appropriate order for desired control/stop occurrences of off-
corner cracks in cast billets leading to strand rupture/breakout.
Mould water flow velocity:
The flow velocity of cooling water through the mould wall play an important role in mould
heat extraction and thereby controlling the hot as well as cold face temperature of operating
mould tube which in turn influence the rate of cooling and solidification of the cast billets. It
has been observed that a high hot face temperature causes mould distortion near meniscus
region whereas the high cold face temperature results in nucleation boiling of mould cooling
water. The nucleation boiling of cooling water is a phenomenon leading to poor and non-
uniform heat transfer causing non-uniform shell thickness and higher strand temperature
inside mould. The situation gets further aggravated at high casting speeds and due to use of
mould electromagnetic stirring (EMS). Water flow velocity in mould water channel for
casting of 125mm square billets has been increased from existing value of 13

to be inadequate for high speed casting using EMS, to a velocity of >11.5 m/sec, based on
heat transfer calculations.
Provisioning of mould Foot rolls and modified below-mould Spray jacket:
While casting billets of superior quality at high speed in continuous caster moulds, there is a
general tendency for the billets cast at high speeds subjected to lower shell thickness and
high shell temperature of the solidifying strand. This in turn, results in lower shell strength.
Moreover the shell thickness is found to be lowest at off-corner location due to in mould
bulging. These are the reasons for which off-corner cracking occur in high-speed continuous
casting of billets. The present invention is directed to arrest occurring of such longitudinal
off-corner cracks leading to strand breakouts, by providing extra support to the thin and hot
shell just below the mould, in the form of foot rolls for casting of 125mm sq. billets.
Reference is now invited to the accompanying Figure 3, wherein the position and mounting
arrangement of the Foot rolls for supporting the solidified strand is illustrated, that shows
that roll support is provided from all four sides of the cast and solidified billet strand just
below the mould and thus preventing in mould bulging and off-corner longitudinal cracking
to avoid strand breakouts. The accompanying Figure 4, illustrates the modification effected
in the below mould spray jacket to accommodate the Foot roll assemblies.
Modification of Mould oscillation parameter:
The method of casting superior quality concast billets at high speed according to the present
invention, further takes into account the impact of mould oscillation parameter on the caster
performance and product quality. It has been observed in the art of continuous casting, that
the oscillation parameters have significant influence on caster performance and also on
surface quality of solidifying strand in terms of laps/bleeds, when cast at high speed. Hot
face temperature of mould tube depends on oscillation parameters. High mould oscillation
stroke length increases shell deformation below the meniscus leading to sticker formation
due to increased mould strand interaction. Moreover, high stroke length increases negative
strip time causing deterioration of surface quality. Optimum oscillation parameters for billet
caster for superior castings at high speed are experimentally standardized as follows:
14

Negative strip time :0.12-0.15 sec
Negative strip ratio :60-70%
Mould lead :3-5 mm
In the conventional caster operation, a mould oscillation parameter value having stroke
12mm at a frequency of 150 cycles per minute, found to be not suitable for the speed range
of 2.3-2.8 m/min. On contrary, the oscillation stroke of 12mm is suitable for a very high
casting speed of >2.5 m/min as per the conventional practice, the corresponding figures
shown in bold figures in the following Table 3. However, the improvement in terms of
superior billet quality and low breakout occurrences is achieved by adjusting/modifying the
mould oscillation stroke to 11 mm. The modified oscillation parameters with modified
stroke are given in the following Table 4. It is apparent from the Table 4, that the
oscillation parameters are within the desired acceptable range of values in the casting speed
range of 2.3-2.8m/min.
Table 3: Conventional Oscillation parameters

Speed
m/min Tn NSR Mould lead
2.0 0.15 76.97 6.09
2.1 0.15 75.77 5.83
2.2 0.15 74.55 5.83
2.3 0.15 73.32 5.34
2.4 0.14 72.08 5.10
2.5 0.14 70.83 4.86
2.6 0.14 69.57 4.62
2.7 0.14 68.29 4.39
2.8 0.13 67.00 4.17
15

Table 4: Modified Oscillation parameters relating to the invention.

Speed
m/min Tn NSR Mould lead
2.0 0.15 74.77 5.16
2.1 0.15 73.43 4.91
2.2 0.14 72.08 4.67
2.3 0.14 70.72 4.43
2.4 0.14 69.34 4.20
2.5 0.14 67.94 3.97
2.6 0.13 66.53 3.75
2.7 0.13 65.10 3.53
2.8 0.13 63.65 3.31
Provisioning of higher upper taper in multi tapered mould:
Conventionally, mould taper is provided to compensate the shrinkage of the solidifying
strand in the mould. In addition, taper also compensates the mould distortion that takes
place in the upper region of the mould due to high mould hot face temperature. The high
hot face temperature is mainly due to high heat extraction in the upper part of the mould
leading to the formation of the negative taper. The problem gets further complex at higher
casting speeds. The provision of negative taper in the upper portion of the mould causes in
mould bulging and excessive gap formation resulting in poor strand-mould interaction. This
phenomenon has an adverse effect leading to the formation of deeper oscillation marks on
the strand surface, off-squareness of billet and formation of off-corner cracks. As an viable
better alternative, Triple tapered mould is used in existing method for casting of 125mm sq.
section billet having the configuration as follows:
Upper Taper: 2.1%; Middle Taper :0.9%; Lower taper :0.5%;
Taper configuration was modified to increase upper taper for compensating the high mould
distortion at selective high casting speed exceeding 2.5m/min. Thus the present invention is
directed to apply triple tapered mould with preferred higher upper taper of 2.4% as
compared to existing 2.1% in the mould, to overcome the problems of crack or poor surface
of strand, while middle and lower taper are maintained at the conventional values.
16

Extension and modification of secondary cooling zone length:
The length of secondary cooling zones and the water spray/distribution pattern has the
importance in respect of formation of halfway/midway or reheating cracks, the nature,
origin and location of such cracks tend to use these names synonymously, and also the
centerline cracks in continuous casting of superior quality billets. Improper secondary zone
length selection and inadequate cooling water distribution therein, tends to strand reheating
resulting in the high thermal stresses and strains at the solidifying front. When the thermal
strains developed due to reheating exceeds a critical value, cracks may initiate
perpendicular to the billet surface. The present invention involve the secondary cooling zone
modification due to the following reasons:
The existing secondary zone comprising MS zone-0.3 m long with cooling water distribution
rate of 165 Ipm, zone -I -1.5m long having water distribution of 180 Ipm and the zone-IIA -
1.9m long having the water distribution of 80 Ipm; thus the total length of secondary
cooling spreading over 3.7 m, was found to be insufficient as compared to the computed
Crater length of 8-9m, up to end of solidification, while casting was carried out at a speed of
2.2-2.6m/min; further the cooling water distribution/flow rate was independent of casting
speed resulting in strand reheating to an extent of 120°C and leading to formation of
halfway and centerline cracks.
The present invention thus removed the deficiency of the secondary cooling method by
extending the length of the secondary cooling zone by 3.1m by introducing two additional
cooling zones IIB and zone III. Moreover, the water distribution corresponding to different
casting speeds were moderated at different zones of secondary cooling such that the strand
reheating is avoided and the strand reheating is restricted to water distribution in different zones of the modified secondary cooling zone, directed to
eliminating formation of halfway/reheat and centerline cracks in concast billets are
presented in the following Table 5.
17

Table 5: Modified Secondary Cooling Distribution:

Speed
(m/min) MS Zone
(Ipm) Zone-I
(Ipm) Zone-IIA
(Ipm) Zone-IIB
(Ipm) Zone-Ill
(Ipm)
1.5 165 170 70 65 28
1.7 165 183 78 65 28
1.9 165 195 85 65 28
2.1 165 208 93 70 31
2.3 165 220 100 77 34
2.5 165 233 107 84 37
2.7 165 247 113 91 40
2.9 165 260 120 99 43
3.1 165 273 127 106 46
3.3 165 287 133 113 49
3.5 165 300 140 120 52
The present invention is thus directed to a method for high speed casting of superior quality
concast billets led to a significant reduction of internal defects of cast billets in the form of
longitudinal off-corner cracks, halfway/midway/reheat cracks and centerline cracks and blow
holes or pin holes. Thus the resulting cast billets was having higher rigidity and strength
such as to provide a macro section free of appearance of any crack or negligible cracks, as
illustrated in the accompanying Figure 5. Macro acceptance of special quality billets
improved from 1.8% per strand to Figure 7, respectively.
It is thus possible by way of the present invention directed to a method of casting of
superior concast billets at high speed, avoiding occurrences of strand rupture/breakout is
capable to ensure higher acceptance standard of the superior quality concast billets, more
particularly, the forging and spring steel quality cast steel billets by imposing control on
selective chemistry and process parameters, such that the shell thinning, longitudinal off-
corner or centerline crack and the like, responsible for the strand frequent breakouts can be
eliminated and a superior quality billets of higher rigidity and strength is obtained, ensuring
higher plant availability, operational safety and improved productivity at lower costs due to
lesser rejection and as such providing prospects for wide industrial applications.
18

WE CLAIM:
1. A steel composition for producing superior quality concast billets at high speed
comprising:
Carbon: 0.20-0.25%
Sulphur:0.03% max;
Silicon :0.20 to 0.30 %;
Manganese :at least 0.75 % ,
selected such that the Mn/Si ratio is maintained at least 2.9 and Mn/S ratio at least 25.0
and adapted for reducing the formation of Off-corner crack, blow holes/pin holes and
breakout.
2. A steel composition for producing superior quality concast billets at high speed as
claimed in claim 1 comprising :
Nomogram between Mn, Si and S
(Carbon:0.20-0.25 wt%, Sulphur-0.03%)

Si, wt % Mn, wt %(min) Mn/Si S, wt % Mn/S
0.20 0.75 3.7 0.03 25.0
0.21 0.75 3.6 0.03 25.0
0.22 0.76 3.5 0.03 25.3
0.23 0.77 3.3 0.03 25.7
0.24 0.79 3.3 0.03 26.3
0.25 0.80 3.2 0.03 26.7
0.26 0.81 3.1 0.03 27.0
0.27 0.83 3.1 0.03 27.7
0.28 0.84 3.0 0.03 28.0
0.29 0.85 2.9 0.03 28.3
0.30 0.87 2.9 0.03 29.0
19

3. A steel composition for producing superior quality concast billets at high speed
comprising:
Carbon: 0.4-0.6%;
Sulphur:0.03% (max);
Silicon :0.20 to 0.30 %;
Manganese :at least 0.80;
selected such that the Mn/Si ratio selectively varies between 3.3 to 4.0 and Mn/S ratio
selectively varies between 26.7 to 32.7 and adapted for reducing anyone or more of the
formation of Off-corner crack, blow holes/pin holes and breakout.
4. A steel composition for producing superior quality concast billets at high speed as
claimed in claim 3 comprising :
Nomogram between Mn, Si and S
(Carbon:0.40-0.50 wt%, Sulphur-0.03%)

Si, wt % Mn, wt %(min) Mn/Si S, wt % Mn/S
0.20 0.80 4.0 0.03 26.7
0.21 0.81 3.9 0.03 27.0
0.22 0.82 3.7 0.03 27.3
0.23 0.84 3.7 0.03 28.0
0.24 0.85 3.5 0.03 28.3
0.25 0.86 3.4 0.03 28.7
0.26 0.87 3.4 0.03 29.0
0.27 0.90 3.3 0.03 30.0
0.28 0.92 3.3 0.03 30.7
0.29 0.95 3.3 0.03 31.7
0.30 0.98 3.3 0.03 32.7
20

5. A method of casting directed to ensuring superior quality concast billets at high speed
comprising:
providing optimized steel composition with selective Mn/Si and Mn/S ratios to reduce
formation of mould scum and enable solidified shell strength for reduction of off-corner
crack, blow holes/pin holes and break outs;
increasing the water velocity in mould to reduce mould distortion at higher speed;
increasing upper mould taper to reduce strand breakout and off-corner crack formation;
optimizing the oscillation parameters for billet caster; and
incorporation of extended and modified secondary cooling length.
6. A method of casting directed to ensuring superior quality concast billets at high speed as
claimed in claim 5 comprising:
providing mould foot roll and modified below mould spray jacket and
providing of high upper taper in multi tapered mould.
7. A method of casting directed to ensuring superior quality concast billets at high speed as
claimed in anyone of claims 5 or 6 wherein the velocity of water cooling in mould is
increased to about >11.5 m/sec.
8. A method of casting directed to ensuring superior quality concast billets at high speed as
claimed in anyone of claims 5 to 7 wherein
(a) the oscillation parameters for billet caster comprised:
negative strip time : 0.12-0.15s;
negative strip ratio : 60-70%;
mould lead : 3-5 mm, and
(b) said multi taper mould configuration comprise modified triple tapered mould
comprised:
upper taper : about 2.4 %;
middle taper: about 0.9%; and
21

lower taper: about 0.5%.
22
9. A method of casting directed to ensuring superior quality concast billets at high speed as
claimed in anyone of claims 5 to 8 wherein the modified secondary cooling zone length
was extended by about 3 m by introducing two more zones such that the water
distribution provided for restricting the strand reheating to 10. A steel composition for producing superior quality concast billets at high speed and a
method of casting directed to ensuring superior quality concast billets at high speed
substantially as hereindescribed and illustrated with reference to the accompanying
examples and figures.

A steel composition for producing superior quality concast billets at high speed comprising,
Carbon: 0.20-0.25%; Sulphur:0.03%(max); Silicon :0.20 to 0.30 %; Manganese :at least
0.75, selected such that the Mn/Si ratio is maintained at least 2.9 and Mn/S ratio at least
25.0 and or alternatively Carbon: 0.4-0.6%; Sulphur:0.03%(max); Silicon :0.20 to 0.30
%;Manganese :at least 0.80, selected such that the Mn/Si ratio is maintained at least 3.3
and Mn/S ratio at least 26.7 and adapted for reducing the formation of Off-corner crack,
blow holes/pin holes and breakout and a method casting seamlessly production of superior
quality concast billets at high speed with increased productivity. Said method involving
velocity of water cooling in mould is increased to about >11.5 m/sec, providing mould foot
roll and modified below mould spray jacket and providing of high upper taper of 2.4%, in
multi tapered mould

Documents:

00244-kol-2008-abstract.pdf

00244-kol-2008-claims.pdf

00244-kol-2008-correspondence others.pdf

00244-kol-2008-description complete.pdf

00244-kol-2008-drawings.pdf

00244-kol-2008-form 1.pdf

00244-kol-2008-form 2.pdf

00244-kol-2008-form 3.pdf

244-KOL-2008-(22-10-2014)-ABSTRACT.pdf

244-KOL-2008-(22-10-2014)-CLAIMS.pdf

244-KOL-2008-(22-10-2014)-CORRESPONDENCE.pdf

244-KOL-2008-(22-10-2014)-FORM-2.pdf

244-KOL-2008-(22-10-2014)-OTHERS.pdf

244-KOL-2008-CORRESPONDENCE OTHERS 1.1.pdf

244-KOL-2008-PA.pdf

abstract-00244-kol-2008.jpg

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Patent Number

264517

Indian Patent Application Number

244/KOL/2008

PG Journal Number

01/2015

Publication Date

02-Jan-2015

Grant Date

01-Jan-2015

Date of Filing

12-Feb-2008

Name of Patentee

STEEL AUTHORITY OF INDIA LIMITED

Applicant Address

RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI

Inventors:

#	Inventor's Name	Inventor's Address
1	BIRUDAVOLU BALAKRISHNA REDDY	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI-834002
2	PATWARI KUNTAL	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI-834002
3	PAL BISWARANJAN	DURGAPUR STEEL PLANT, DURGAPUR-713203
4	CHARYULU MUSTIGUDALA PILLA	DURGAPUR STEEL PLANT, DURGAPUR-713203
5	PRADHAN NIRMAL	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI-834002

PCT International Classification Number

C22C

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA