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STS bloom caster for special steels Izhstal, Russia, celebrated its 250-year anniversary in 2010 and marked it by the commissioning of an STS-supplied continuous casting machine designed to meet the highest quality bloom, mini-slab and billets for the most demanding of applications. Performance guarantees were particularly rigorous, but all grades were approved.
In 2010, STS supplied and commissioned a continuous casting machine for Izhstal, in Russia, which was tailored
to its particular melting shop layout and designed to produce the highest quality blooms, mini-slabs and billets (see Figure 1). Izhstal was very demanding of its quality targets, aiming to exceed current world standards. STS got the order on a turnkey basis including initial consumables, with performance guarantees, based on strict standards for more than 150 steel grades covering some of the most extreme and wide-ranging industrial applications.
From the fi rst meeting it was clear that it was important to have the best available technologies installed on a casting plant for the largest Russian steel company on its most important and historical site. Thus, the caster has been generously equipped and effi ciently completed for high-quality production, with the most advanced automation and controls for online monitoring and bloom management.
The three-strand caster is served by two 30t EAFs and a 42t LF, and is designed to produce sizes ranging from 100x100mm up to 180x180mm.
The main characteristics of the plant are as follows:
MAIN TECHNICAL FEATURES` Machine radius 7m` Number of strands Three` Steel grades Low, medium and high
carbon steels, stainless, medium and high alloyed
` Ladle capacity Maximum 42t with sliding gate system and porous plugs
` Sequencing facility Ladle car for two ladles with hydraulic lifting system
` Weighing management Tundish and ladle monitoring by load cells
` Tundish 10t capacity operating with protected or open stream, controlled by stopper rods or nozzle changers
Authors: Giorgio Cabai, Fabrizio Cabai and Matteo Bellina STS
r Fig 1 Caster discharge area
` Tundish car Fully cantilevered, equipped with hydraulic lifting system
` Supplied casting sizes 125x125mm and 140x180mm
` Mould type Curved, multi-tapered 1,000mm long with electromagnetic stirrer
` Speed control With minimum radioactive emission and automatic start
` Mould lubrication Either oil proportioning pump with variable fl ow or automatic powder delivery system
` Mould oscillator Hydraulically controlled with adjustable stroke and frequency
` Guiding apron In-line electromagnetic stirrers, pneumatically pressurised guiding rolls with progressive force according to billet size and casting speed. Optical billet temperature monitoring
` Withdrawal straightener Double radius 7/13m, two withdrawal powered rolls, computer pneumatic pressure control during
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monitor the moulds, and in a safe manner. For conventional open stream casting, a variable speed
dosing pump assures the correct oil fl ow in any casting condition in the same way and with the same safety or need of mould continuous human monitoring.
Argon feeding – tundish to mould The system comprises a dedicated argon supply installed on the tundish and equipped with fl ow controls to deliver the right amount of purging gas to the connection between tundish nozzle and submerged shroud.
This enables the guaranteed values over gas pick-up in the fi nal product to be achieved.
Multi-taper copper tubes The copper tube design was critical due to the wide variety of steel grades to be cast and hence the wide range of casting speeds. The best solution was a complex multi-taper design to assure good contact for all the grades to be cast, but with the priority for the most critical ones.
Automatic mould level control with fast start-up The STS-developed control system has been utilised for fast response even with a low power radioactive source, and allows full control of all working parameters for fast set-up and immediate performance just after start-up. A very capable mould level control system producing a stable
casting according to cast size and speed or re-stranding function
` Dummy bar system Solid type with overhead tilting/parking, pneumatically controlled
` Cutting system High speed automatic oxy-gas cutting torches for minimum metal loss and minimum scarf
` Scarf removal Automatic blade-type system` Cutting length 4-12m with very strict
tolerances of ±5mm` Billet identifi cation 12 character impact type
computer controlled marking machine installed on transfer roller table combined with billet weighing
` Billet transfer Single rollers for all strands` Billet discharge Overhead bridge crane
picking up each billet, and transferring it to cooling bed. Same bridge can feed hot billet transfer or short billet sorting system
` Cooling system Hydraulic turnover walking beam cooling bed with grouping of billets for easy removal and storing by overhead crane (provision for future application of billet slow cooling system for special steels treatment)
` Pollution control Fume extraction in the torch area
` Auxiliary equipment Controlled cooling furnaces designed for a wide range of qualities
` Plant supervision HMI level 2
MACHINE DETAILSMould powder addition system This automatic system is independent for each strand and is installed on top of the tundish cars, as shown in Figure 2.
It has dedicated silos with capacity suffi cient for multiple sequences. The silos have level controls and are equipped with a heating system to assure moisture-free powder can be automatically fed into the mould using argon as a carrier gas. A pneumatically actuated dosing system delivers the right amount of powder according to casting requirements, so providing consistency and reliability.
With this system only a single operator is required to
r Fig 2 Automatic mould powder addition system
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and level surface, was of prime importance because of the need for high surface quality of the blooms. The precise and easily adjustable response of the controller also allows use of radioactive sources that with other systems must be changed earlier because of decay.
Electromagnetic stirring Due to the stringent bloom internal quality and segregation requirements, one mould stirrer and one intermediate strand stirrer with external coils for rapid cast size change and caster fl exibility are used. The mould stirrer provides improvement on the surface and primary solidifi cation structure, and the lower one, sited along the curve, contributes to bloom internal quality soundness and minimises segregation. This stirrer is moved according to cast size, casting condition and steel grade to comply with solidifi cation and metallurgical requirements. Figure 3 shows the range of movement along the strand so as to be positioned at correct solidifi cation point in any casting condition and precisely for segregation-critical steels.
Cooling water management This is completely automatic for both primary and secondary cooling, with dedicated nozzles on secondary cooling cages. Water is adjusted automatically via control valves, and suitable sensors following the specifi c casting condition and steel grade according to specifi c recipes inserted in the system. Feedback of bloom temperature measured at the exit of the cooling chamber is provided to the system.
Cutting torches Torches were chosen rather than shears to avoid bloom deformation typically created by shears. For stainless steel and special high alloyed steels, an iron powder addition and fume extraction system is also employed (see Figure 4).
Bloom slow cooling system Given the wide variety of steel grades produced, provision has been made for future installation of a slow cooling system to prevent cracking in hard grades and ensure correct metallurgical structure for further processing. The slow cooling system has been designed with two dedicated insulated and covered beds designed for 10m and 12m bloom lengths.
The bloom handling system has been designed to be suitable for discharging the blooms on dedicated roller tables for immediate transfer to the slow cooling beds.
Bloom weighting system Because of the wide variety of steels produced and the need to optimise bloom length and weight for production control and rolling optimisation, in-line weighting of the blooms was requested. The weighting system is installed on the transfer roller table and is served by a lifter that lifts up the bloom from the roller table and
r Fig 3 Adjustable strand stirrer
r Fig 4 Cutting torches
r Fig 3 Adjustable strand stirrer
r Fig 4 Cutting torches
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sits it on load cells (see Figure 5). The weight is registered in the production tracking system and can be recovered at any time from the historical data base.
Bloom marking system on transfer roller table Following weighing, with the aim of avoiding costly control systems and bloom mixing, the caster is equipped with punch-type marking machines. These are completely automatic and provide a suitable code by the control system as illustrated in Figure 6.
Bloom de-burring Because of the need for very high product quality and to eliminate the need for time-consuming bloom conditioning, STS was asked to install a device to clean off the little burr created on the lower face of the blooms by torch cutting. A simple pneumatically actuated knife device was designed for the bloom bottom surface, cleaning it very effi ciently. An example is shown in Figure 7.
Bloom discharge system The design of the discharge system allows billets and blooms to be discharged to the cooling bed, to a roller table for immediate transportation for hot charging, or to the slow cooling beds. On the opposite side of the cooling bed a suitable frame has been designed to receive short billets or those deemed unsuitable for quality reasons.
Quality control and tracking system The full set of information available by all sensors and controls on the machinery are used to provide online monitoring of the process via software. All production parameters, together with all surveyed parameters, are recorded from the fi eld sensors and collected dynamically for every 10mm slice of bloom/billet to provide an historical record of data suitable for easy classifi cation of all materials produced (casting speed, temperatures, water fl ows, bloom temperature, withdrawal pressures, cutting data, bloom weights, marked codes, etc.). This data is displayable on computer screens in various formats.
MACHINE TEST PHASE AND PERFORMANCEBefore plant start-up full operator training covering the continuous casting theory, the machinery mechanical details and the automation and controls was completed, enabling all operators to operate, manage and maintain the machinery.
Theoretical training was complemented by practical operation on the caster during the cold testing period and during fi rst hot testing. After erection, the machine cold testing procedures were conducted and all parameters verifi ed.
Following cold commissioning, the plant start-up took very little time, and then the performance tests commenced as per the agreed procedure.
r Fig 5 Bloom weighing
r Fig 7 140x180mm bloom after cutting and de-burring
r Fig 6 Typical bloom identifi cation
r Fig 5 Bloom weighing
r Fig 6 Typical bloom identifi cation
r Fig 7 140x180mm bloom after cutting and
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For casting purposes, the steel grades to be produced were divided into groups or families having similar processing behaviour, and ‘recipes’ produced. In this way, the caster set-up is easier and requires fewer changes. The wide range of steels to be produced as shown in Table 1 did complicate this preliminary phase of work, but it was all satisfactorily completed.
For each one of the above groups, specific information has been listed in the Operation & Technological Manual for machine set-up, casting parameters and operations details.
The set of information and process data comprises:` Tundish preparation procedures` Tundish pre-heating cycle and final temperature` Tundish nozzle characteristics` Covering powder type and adding procedure` Mould lubricant (oil or powder) and addition
procedure/quantity` Argon flushing procedure and set data` Mould-level set point and regulation` Primary cooling water characteristics and set points` M-EMS set point data` Oscillation frequency and stroke ` Secondary cooling nozzle set-up` Secondary cooling water characteristics and set points` F-EMS position and set point data` Bloom exit temperature data` Casting speed ` Cutting torch pressure set points` Cutting torches iron powder set points` Bloom cut length` Bloom marking code` Bloom weight
Some of the data are automatically set from the caster control system by selecting the steel grade and cast size on the recipes page on the HMI.
The operators were free to modify-enter-adjust the set point parameters on the STS user-friendly and very intuitive MHI interface. Some adjustments were made easily during
r Table 1 Izhstal steel product mix
Steel group Steel type Annual production, t Annual production, t 125x125mm 140x180mm1 Peritectic 50,000 5,0002 Medium carbon 64,000 18,0003 High carbon 50,000 12,0004 4a: Alloyed with Cr/Cr-Si/Cr-Si-V/Cr-B-Mn 89,500 60,000 4b: Alloyed with Cr+Si+Ni 5 Alloyed with Cr-Mo /Cr-Mo-Ni-V-Ti 24,000 40,0006 Bearings/tools 2,500 9,0007 Spring and structural steel with Cr-Si 8,000 4,0008 Stainless steel and Cr-Ni steel 2,000 12,000
ParameterGeneral parameters Number of heats and sections Section sizes and steel grades Casting speed Casting yield Productivity testsDimensional tolerances Cross section Face bulging / depression Rhomboidity (square sections) Billet length Billet straightness Billet twistSurface quality Macrostructure Axial (centre) porosity Centre carbon segregation Equiaxed zone Light contour (white band) Internal cracks Nitrogen pick upCasting machine Re-stranding time Tundish life and sequence Mould changing time Time for section change
r Table 2 Guarantee parameters during test phase
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fi rst testing, with the aim of optimising the bloom quality through fi ne-tuning set points and parameters. Because the product quality specifi cations were so tight, every aspect of operation and process was analysed, tested and fi nally set for the best result. A short summary of the guaranteed parameters that have been considered during testing phase is shown in Table 2.
All the above parameters had to be tested and verifi ed for both cast sizes and for selected steels in each steel group and covering practically each family of steel in production and also represented by the most critical grades.
Although stainless steel was not initially included, we agreed to include this for completeness and full satisfaction of both parties in machine testing. The testing period lasted some months, during which more than 500 heats were cast. The surface quality of the billets was immediately good and only some small internal cracks on some of the more critical grades required some tuning.
All parameters were met with complete satisfaction.Typical cross sections of some representative sample
blooms are shown in Figure 8.
CONCLUSIONS By investing in one of their most important and historical sites, and aiming for very high targets for both production and quality, the strategic new caster in Izhstal was a challenge for the Mechel group. The wide range of steel grades to be produced and the high quality targets would have been a test for any plant supplier.
The challenge was picked up by STS knowing that its standard, simple, sturdy and maintenance-free continuous casting machine, developed over many years, could meet these very demanding performances, and knowing that the fl exibility of design gives it the possibility to accessorise for the most critical and stringent production and quality requirements.
The completed casting plant, fully accessorised for this specifi c project, was thus installed and successfully commissioned, thanks also to the help of Izhstal personnel. This caster is a practical success and an example in how the long tradition of STS machinery designed to reduce maintenance and increase reliability, can be equipped with the most advanced features to meet the most critical quality requirements.
After long and comprehensive testing on a wide range of products, performance guarantees were met and the acceptance certifi cate was received. MS
Giorgio Cabai is President, Fabrizio Cabai is Managing Director and Matteo Bellina is Technical Manager, all at STS s.r.l, Udine, Italy
CONTACT: [email protected]
r Fig 8a Rectangular Bloom 140x180mm peritectic steel
r Fig 8b Rectangular bloom 140x180mm alloy steel with Cr Ni and Ti r Fig 8b Rectangular bloom 140x180mm alloy steel
r Fig 8a Rectangular Bloom 140x180mm
