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Casting of large diameter round blooms at Jangsu HuaigangA 1Mt/yr large diameter rounds (380-500mm) bloom caster was successfully commissioned in early 2007. Key design features are mould taper and strand cooling to avoid thermal distortion, and progressive unbending and withdrawal pressure to assist in minimising central segregation and cracking. The caster is linked to a new special bar quality bar mill.

Huaigang Company dates back to 1970, when Qingjiang I & S Works was founded. After several

property and name changes, in December 2006, its name became Jiangsu Shagang Group Huaigang Special Steel Co Ltd. In 2006, the outputs of steel products and pig iron were 1.6 and 1.2Mt/yr. Plans are in progress to reach 3Mt/yr output by 2010.

On 31 December 2006, a new 1Mt/yr, 14m radius, 6-strand Danieli conticaster for jumbo-size round blooms was successfully started. The main steel grades to be produced are bearing, spring, alloy and quality carbon structural and tubular, steel for fasteners, HSLA, corrosion resistant, cold heading, and surface hardening. Detailed specifications are shown in Table 1.

MACHINE DESCRIPTIONKey plant characteristics are shown in Table 2. Some key design points are expanded below:

Casting speed/sizes The conticaster has been designed to cast at speeds of 0.48-0.7m/min for 380mm diameter rounds, 0.3-0.55m/min for 450mm diameter rounds and 0.27-0.45mpm for 500mm diameter rounds, according to steel grade and tundish superheat. This equates to 186-241t/hr.

TuNDISH The advanced tundish design allows smooth operation according to the specific steelmaking process needs, ensuring that no defects are introduced during casting and enhancing the tendency to improve overall steel cleanliness (see Figure 1). For these reasons the tundish design takes into account the thermo-physical requirements of the continuous caster.

These can be summarised as follows:

` Thermal stability The liquid steel has to be maintained at constant temperature during the casting process, and uniform in all the tundish zones

Author: Carlo TravagliniDanieli Centro Met

r Fig 1 Liftable ladle turret and liftable tundish car

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r Fig 2 Cooling chamber (6-strand casting, round 380mm)

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` Physical conditions The tundish is a buffer between one heat and the next as well as a vessel to remove inclusions. For this reason the residence time has to be more than 10 minutes. Working steel level is 800mm.

Mould cooling and taper The heart of the continuous casting process is the mould. The aim of mould cooling is to keep the copper tube temperature low in order to avoid thermal distortion and to prevent water boiling (which reduces heat extraction).

Mould cooling has been improved by increasing water velocity, resulting in increased mould heat flux. To ensure a regular thermal flow between mould and solidifying shell, it is important to have a uniform contact between the copper tube and the shell. For these reasons the mould taper has to accompany the solidified shell along the whole 780mm mould length.

SECONDARy COOLINgLiquid steel solidification when the bar leaves the mould is as important as the initial steel skin formation within the mould. Therefore, it is necessary that the spray cooling system is correctly designed and controlled to avoid excessive thermal shocks that may result in cracks and depressions. The secondary cooling pattern is also fundamental to producing good internal quality. In this connection, there are three main items to consider when designing a secondary cooling system:

` Number and length of the spray zones Reheating

grade C Si Mn P S Ni Cr Mo Cu Ti B20 0.17 0.17 0.35 ≤0.035 ≤0.035 ≤0.25 ≤0.25 0.23 0.37 0.65 45 0.42 0.17 0.50 ≤0.035 ≤0.035 ≤0.25 ≤0.10 ≤0.25 0.45 0.35 0.80 20CrMo 0.17 0.17 0.40 ≤0.035 ≤0.035 ≤0.30 0.80 0.15 0.24 0.37 0.70 1.10 0.25 20CrMnTi 0.17 0.17 0.80 ≤0.035 ≤0.035 ≤0.30 1.00 0.04 0.23 0.37 1.10 1.30 0.10 40Cr 0.37 0.17 0.50 ≤0.035 ≤0.035 ≤0.30 0.80 0.44 0.37 0.80 1.10 40MnB 0.37 0.17 1.10 ≤0.035 ≤0.035 ≤0.30 ≤0.08 0.0005 0.44 0.37 1.40 0.003560Si2MnA 0.56 1.50 0.60 ≤0.030 ≤0.030 ≤0.35 ≤0.35 ≤0.06 ≤0.25 0.64 2.00 0.90 Y12 0.08 0.15 0.70 0.08 0.10 0.10 0.35 1.00 0.15 0.20 GCr15 0.95 0.15 0.25 ≤0.025 ≤0.020 ≤0.30 1.45 ≤0.08 ≤0.25 1.05 0.35 0.45 1.65 28MnCr5 0.25 ≤0.12 0.60 ≤0.035 0.020 ≤0.15 0.80 ≤0.10 ≤0.25 0.35 0.80 0.035 1.00

r Table 1 Chemical composition of steel grades to be cast

r Fig 3 CCM during casting operation (6-strands, 500-mm round bloom)

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of the bloom surface has to be avoided to minimise the risk of internal cracks

` Spray nozzle arrangement Specially developed in order to assure uniform cooling of blooms

` Water distribution and control strategy Secondary cooling is dynamically controlled by the supervisor computer according to casting speed.

The Huaigang secondary cooling configuration has four cooling zones for a total water cooling length of 6.46m (see Figure 2).

MACHINE RADIuS AND WITHDRAWAL/STRAIgHTENINg uNITProgressive unbending of the bloom, as well as an accurate control of the pressure at the withdrawal/unbending unit, are provided to achieve high internal quality and accurate size tolerance of the cast round bloom. The Huaigang bloom caster has a five module-type withdrawal and straightening unit to extract the bloom and straighten it from nominal radius (14m), through unbending at 19m and 34m radii (see Figures 3, 4 & 5).

Automation system The new conticaster is equipped with Level 1 automation based on three Siemens S7 PLCs performing all sequences. Level 2 is structured in a three-layer architecture performing the production management function and is equipped with mathematical models for casting sequences, cutting optimisation and quality control system.

The new architecture structured in the equipment control layer, operator interface layer and database and process control layer, is able to work at any level even without the presence of the upper level (which might not be working for a number of reasons), thus guaranteeing the best performance and high reliability for the production target. The software for both Level 1 and Level 2 was developed

r Fig 4 Continuous casting machine: front view

r Fig 5 Discharge area: pawls cooling bed

at Danieli Automation and is based on Danieli Centro Met knowledge of casting technology.

QuALITy RESuLTSThe machine has already cast at the maximum casting speeds without problems or defects in the final product, performing at productivity levels of 217t/hr of cold steel.

Even though the commissioning period has not yet been concluded, we can summarise some quality details on the 380mm product:Product size tolerance Data from 16 heats of 380mm generated a diameter tolerance ((D-380)/380) x 100 based on 4 measurements around the diameter of 0.86% and an ovality ((Dmax-Dmin)/Dmin) x 100 of 0.99%.

r Fig 6 Macro-etch of cross section round 380mm

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Casting

These are well within the contractual tolerances of 1.25% on diameter, and 1.5% ovality, with respect to nominal.

Internal quality 100% of samples seen (380mm) are free of cracks (internal, half-way, subsuperficial; see Figure 6).

Surface quality Oscillation marks are regular, no surface defects appear during normal condition casting (see Figure 7).

DEVELOPMENTSA significant further plant expansion was the start-up in late 2007 of the new 800,000t/yr Danieli special bar quality (SBQ) rolling mill, directly connected with the bloom caster, for hot charging of the just-cast blooms. Quenching box equipment will be put into full operation at the caster delivery side (at present, only tested), thus enabling online bloom quenching after straightening. This will further increase and enhance the quality of the final rolled product (which means reducing the non-conformity after rolling) on low carbon steels.

Linked operation of the bloom caster and new rolling mill will create the second Direct Casting/Rolling plant at Jiangsu Huaigang, following the Danieli Black-Box mini-mill, which has been operating successfully there since the late 1990s.

CONCLuSIONSThe machine was designed with the aim of producing high- quality steel at low cost. This target has been obtained. The caster has performed well from the very beginning,

Machine type 2BLC1406Number of strands 6Machine radius (nominal) 14mUnbending radius 19m, 34mStrand distance 1,700mmMachine productivity 186-241t/hr with 6 strandsMetallurgical length 25m (max)Casting sections 380mm, 450mm, 500mm diameterLadle capacity 90t avg, 100t nominalLadle support Ladle turret with ladle lifting and weighing systemTundish nominal capacity 35tTundish steel level 800mm Tundish level control Load cells on tundish carTundish support Tundish car with lifting systemTundish stream control Electromechanical stopper rodSteel stream protection Submerged shroudMould tube Curved, 780mm longMould lubrication PowderMould level control Radioactive (Cs 137)Electromagnetic stirrer External mould EMS and final EMSOscillating unit HydraulicSecondary cooling system Water on foot rollers, air mist on strandStraightening Five modules per strandWithdrawal speed range 0.20-3.2m/minQuenching box Before cutting device, approx. 1m longBloom cutting Oxy-cutting torchCutting length 6mDummy bar type Flexible

r Table 2 Main caster details

both in terms of productivity and quality. Production and quality indices are close to unit values, meaning that the applied Danieli technology guarantees simple machine management that allows automatic control of the conticaser in every phase of the production process. MS

Carlo Travaglini is Director of Engineering at Danieli Centro Met, Buttrio, Italy.

CONTACT: a.fragiacomo@danieli.it

r Fig 7 500mm bloom round, 16Mn