In March 2012, Danieli successfully started the melt shop

Size: px
Start display at page:

Download "In March 2012, Danieli successfully started the melt shop"

Transcription

1 A new benchmark in special steel casting: new twin-strand vertical caster at POSCO Specialty Steel, Korea The new Danieli 2-strand Vertical Caster (VCCM) producing 420 x 530mm blooms at Posco Specialty Steel, Korea, produces a very wide range of carbon, stainless, and tool steel grades. As these are traditionally extremely difficult to cast, special design and operating solutions have been used. Excellent bloom quality has been achieved, both in terms of surface with no need for scarfing and internally, in terms of equiaxed zone, centre porosity and segregation indicees. Authors: Antonio Sgro, Marco Rinaldi and Giovanni Accardo Danieli Plant item Value Capacity 227,300t/yr No. of strands 2 Dummy bar type Rigid top feeding Casting section 420 x 530mm Cutting lengths 4-6m (main cutting system) 2-3m (secondary cutting system) Mould Plate type, 700mm long Oscillator type Hydraulic (INMO- type) Cutting system Oxy-cutting with powder Caster productivity 81.6t/hr max. Metallurgical length ~25.5m Casting platform height 19m from zero level Bottom of caster depth -24m from zero level Electromagnetic stirrer M-EMS and F-EMS (movable type) Secondary cooling system Zone 1: water Zone 2 and 3: air mist 2 loop regulation (inner/ outer radius lateral sides) for each zone Withdrawal unit 8 modules per strand, two rolls each Soft reduction units 8 modules per strands (+bloom thickness measuring stand) r Table 1 Main characteristics of the VCCM In March 2012, Danieli successfully started the melt shop at POSCO Specialty Steel, Changwon Works, Korea, and in April 2012, delivered the first bloom from the new caster. The melt shop is designed to produce 450,000t/yr of liquid steel to feed either the VCCM or the ingot casting facilities. The 70t EAF is a full-platform EBT FastArc-2 and is equipped with a Static Var Compensator (SVC) and a 72-MVA transformer, sidewall Danarc Module system for oxygen and carbon injection, Robox door-mounted multiple lance manipulator for oxygen, carbon and FeSi powder injection and roof-mounted lime injectors. Secondary metallurgy is provided by a single-position ladle furnace (15MVA) and a twin-tank single-cover VD/ VOD station. The ladle furnace is equipped with a watercooled inert type roof and a full package of auxiliaries and two ladle cars. The VD/VOD station is equipped with a steam-ejector pump with suction capacity of 400kg/hr. CASTER DESIGN HIGHLIGHTS The main design aspects of the caster and product range are shown in Table 1. The target production is 227,300t/yr of high quality 420 x 530mm rectangular blooms. The product mix includes plain carbon, carbon alloyed, stainless, tool and other special-purpose steel grades, austenitic stainless steels and carbon alloyed steel representing the majority of grades produced (see Table 2). The wide range of special steel grades and the high quality requirements meant POSCO Specialty Steels developed a vertical caster solution, despite the huge investment, mainly in civil works, instead of a classic curved caster. A 3D-computer schematic is shown in Figure 1. Ladle-Tundish The tundish has been designed using CFD analysis (see Figure 2) in order to establish the most suitable 82

2 STEELMAKING AND CASTING r Fig 1 3D overview of the VCCM r Fig 2 Tundish CFD simulation: velocity magnitude on refractory lining [m/s] Group Steel grade Plain carbon and , alloy steel 1530, , Stainless steel 4301, 4306, 4436, 4435, 4405, , 4571, 4587, 4550, 4462, , 4029, 4021, 4028, 4104, 4016 SUH3, -11, -35, -660(JIS) 4922, SCA10, -12, etc (Heat resistant) 310 (Full austenite) 630 (precipitation hardening) Tool Steel D2, D11, 2080, 2379, 2344, 2343A2, 23F85, O1, 2362, 2365, Carbon tool steel 2311, 2312, 2738 Special purpose steel INVAR (36%, 42% -Ni), COVAR Incoloy, Inconel, SUPER % C-5% Cr-0.6% Mo Nimonic70A, H13, 18CrNiMo6 r Table 2 Steel grades produced set of performance parameters. It has a capacity of 25t, a working level of 900mm and optimum dam location and impact pad to provide fluid dynamics able to control the thermal and chemical homogenisation of liquid steel and to remove inclusions by the slag layer. To ensure product quality, it is equipped with an argon sealing system and tundish-to-mould SENs. Stable casting conditions are guaranteed by automatic control of liquid steel: from ladle to tundish by ladle slide gate control and tundish weighing system detection, and from tundish to mould by an electro-mechanic stopper rod control system (see Figure 3) and eddy current liquid steel level detector. Moreover, an automatic powder feeding system keeps the proper powder thickness as a function of the cast steel grade and a continuous tundish temperature measuring device feeds the process control system. Mould-Oscillator The mould is a CuCrZr copper plate design with embedded thermocouples to provide the additional functions of mould temperature monitoring as well as an anti-breakout system (see Figure 4). It is moved vertically by a Q-INMO+ hydraulic oscillator, a patented solution with a unique rolling element technology, which means that the system motion is almost frictionless (see Figure 5). This ensures the system always moves accurately, with no deviation from pass line, even at high oscillation frequencies up to 490 cycles per minute. The oscillator allows machine operation using the optimum oscillation stroke, frequency and wave form, ensuring an excellent bloom surface quality, as well as fast alignment. Danieli Rotelec supplied the stirring equipment, which consisted of a Mould Stirrer (M-EMS) and Final Stirrer (F-EMS). The F-EMS is installed on a positioning system to allow for different F-EMS positions, according to casting a 83

3 r Fig 3 Stopper rod mechanism r Fig 4 Mould thermal map speeds, and to minimise axial carbides and segregation for different steel grades. The positioning system (see Figure 6) consists of a frame moved by hydraulic cylinder and a position control loop: the system can be automatically or manually moved to any position in a range of 1,500mm following the setup given by the liquid pool control (LPC) system (see later). A dedicated set of refractory telescopic panels, moving up and down along the frame stroke, protects the guides from scale and water. r Fig 5 Q-INMO+ hydraulic oscillator r Fig 6 F-EMS positioning system 84 Porosity and segregation The main aspects that affect the internal quality of a bloom are internal porosity and segregation, and they are strictly related to the solidification of the melt. There are two main types of porosity: gas or micro porosity, and shrinkage or macro porosity, such as millimeter-sized centreline holes. Gas porosity is a result of the condensation of dissolved gases in the melt on freezing, caused by the difference in gas solubility in the liquid and solid phases. It can be eliminated by an appropriate melt treatment, such as degassing. Shrinkage porosity occurs because the phase transformation induces volume differences inside the material and the mass flow is unable to fill this difference. It is quite difficult to avoid: one way is to compress the mushy zone during freezing. In areas of a casting under compression, the deformation of the solid can counterbalance the increase in density, thus avoiding porosity, even if there is no liquid feeding. During solidification, some locations are subject to tension rather than compression. In this case, the deformation of the solid skeleton adds to the solidification shrinkage, thereby inducing even more liquid suction in the mushy zone. A lack of feeding of mushy regions under tension

4 STEELMAKING AND CASTING will result in hot cracks, a defect similar to the shrinkage porosity, but that also requires tensile stresses. Segregation is defined as non-uniform distribution of chemical composition from the solidification process and is classified into two categories: micro- and macrosegregation. Micro-segregation initiates from the different solubility of chemical composition in the solid and liquid phase and appears on the length scale of the dendrite arm spacing. Macro-segregation is closely related to the macroscopic transportation of the chemical composition and is on a scale of millimeters. Whereas micro-segregation can be eliminated by homogenisation treatment, macrosegregation cannot be eliminated due to the large diffusion length required. Of the various sources of macro-segregation, four mechanisms are distinguished: solidification shrinkage, natural or forced convection, grain sedimentation and deformation of the solid. As investigated for a number of casters [1] (see Figure 7), secondary dendrite arms spacing increases with the increasing distance from the chill surface, corresponding to decreasing cooling rates. The larger the section size the coarser the dendrite spacing and associated porosity and micro-segregation. The correlation between distribution of microscopic shrink holes and secondary dendrite arm spacing shows that there is an optimum secondary dendrite arm spacing corresponding to an optimum solidification time which causes minimal micro-segregation as result of two opposite effects: larger volume, and wider channels between single dendrite arms [2]. In the case of large blooms, micro-segregation should be evaluated by segregation model (several are available from the literature) [3] and the solid liquid interface calculated in combination with thermo-mechanical models [4]. r Fig 7 Secondary dendrite arm spacing as a function of distance from chill surface of steel for various low carbon and stainless steels Mechanical soft reduction According to the above, porosity can be avoided by adding extra compressive stress to the solid. Forces are applied at certain locations according to defined times and roll tapering. The reduced roll gap squeezes the solid shell toward the centre and so the solid phase feeds and reduces the porosity level. Soft reduction performs the mechanical control of the section size in order to compensate the solidification shrinkage and, consequently, the control of the liquid steel migration inside the mushy zone, which causes the centre segregation in high carbon and high alloy grades. In theory, soft reduction can eliminate segregation completely because it cancels any movement inside the mushy zone. However, practically, the setting of the reduction zone, amount of reduction and reduction rate under practical casting conditions, is very a r Fig 8a Spray cooling effect 85

5 r Fig 8b Solidified thickness r Fig 8c Solidification cone r Fig 9 LPC system interface delicate, so soft reduction can usually only reduce the maximum and average value of porosity and centreline segregation so that the overall carbon concentration is more homogeneous. If wrongly operated, however, soft reduction may worsen the internal quality. Several simulations have been carried out on most of the contractual steel grades through an offline program using a high-resolution mesh (6.5 million volumes on a 30m machine). The program has addressed the development of the machine design according to the technological requirements in terms of stresses, deformation analysis, sizing of the mechanical soft reduction devices, secondary cooling, temperature contour, solidification along the strand and roll forces to be applied (see Figure 8). As a result of these extensive design activities, the VCCM has been provided with eight soft reduction modules covering a reduction zone of 7.7m as well as mould and movable final EMS and an online solidification model called Liquid Pool Control (LPC). This online process program calculates the solidification along the strand according to a given mesh, as well as the nodal temperatures. Figure 9 gives a typical screen view, showing the temperature pattern from the surface to the inner core as well as the solid, mushy and liquid phases along the strand. The LPC system is able to drive the tapering of roll gaps to squeeze the bloom according to the location of the end of the solidification cone and to the reduction strategy defined. To find the optimum reduction for each steel grade it has been necessary to make extensive analysis of the macro-etched samples as well as to review some parameters and setup patterns. Figure 10 illustrates typical internal quality achieved for alloy, ferritic, austenitic, and tool steels. 86

6 STEELMAKING AND CASTING CONCLUSIONS The new Danieli 2-strand vertical bloom caster at Posco Specialty Steel is casting a very wide range of carbon, stainless and tool steel grades. These are traditionally extremely difficult to cast, so special design and operational solutions have been used, including vertical casting, INMO oscillation and optimised tundish, mould and cooling designs. Mechanical soft reduction and movable final stirrers are optimised through a dynamic set-up based on a real-time solidification model. The targets defined in the contract were quite challenging for some of the steel grades and for the levels of internal quality, carbon segregation and central porosity required. As a consequence of the above, good results are being achieved for surface quality with no need for scarfing, and internal quality with excellent equiaxial zone, centre porosity and segregation indices is produced in all casting conditions. On carbon steels, carbon alloyed steels and tools steels, 90% of contractual values have been achieved using only one test. The combination of M-EMS, movable F-EMS and mechanical soft reduction, has been shown to be a powerful tool to achieve required performances. The optimisation of equipment parameters has meant reconsideration of some limits in the force application. Consequently, for some steel grades, the reduction applied has been larger than initially calculated, showing the benefits in terms of segregation and porosity, but without the impact of cracks. During the first year of operation, the VCCM has cast more than 900 heats. MS r Fig 10a Ferritic stainless steel r Fig 10b Austenitic stainless steel S316 r Fig 10c Alloy steel AISI 4140 Antonio Sgro and Marco Rinaldi are with Danieli Centro Met, Buttrio, Italy. Giovanni Accardo is with Danieli Service, Rayong, Thailand CONTACT: info@danieli.com [1] A W Cramb, Casting of Near Net Shape Products, pp , TMS, 1988 [2] K Harste, B Weisgerger, K Tacke, J Gnauk, M Bobadilla, G Lovato, M Crocenzo and T Hätönen, Improvement of internal quality by controlling the microstructure of microalloyed cast steel, EU Science Research Development, pp1-100, 2001 [3] T Matsumiya, H Kajioka, S Mizogushi, Y Ueshima and H Esaka, Mathematical analysis of segregations in continuously-cast slabs, in Transaction ISIJ, 24, pp , 1984 [4] K S Oh, J. D Lee, C H Moon and J Choi, The Effect of Soft Reduction Conditions on Internal Quality of a Large High Carbon Steel CC Bloom, Metec InsteelCon 2011, CC technology session 5, pp1-6 r Fig 10d Austenitic stainless steel S304 r Fig 10e Tool steel TC3 r Fig 10 Examples of cast bloom quality 87