Challenges in Special Steel Making

Special bar quality [SBQ] is a long steel product where an assured quality is delivered by the steel mill to its customer. The bars have enhanced tolerance to higher stress application and it is demanded for specialised component making. The SBQ bars are sought for component making processing units such as closed die hot forging, hot extrusion, cold forging, machining, heat treatment, welding operations. The final component quality of the secondary processing units depends on the quality maintained at the steel maker end along with quality maintained at the fabricator end. Thus, quality control is ensured at every unit process stages. The various market segments catered to by SBQ steel segment is ever growing and is reviewed. Steel mills need adequate infrastructure and technological capability to make these higher quality steels. Some of the critical stages of processing SBQ and the critical quality maintenance parameters at the steel mill in the manufacture has been brought out.


INTRODUCTION:
Steel long product produced, may be divided into Merchant Bar Quality [MBQ] and Special Bar Quality [SBQ]. MBQ steels, is manufactured to specified sizes with appropriate chemical limits to meet a set of properties, where the end use is non-critical. MBQ bars, is made from unconditioned billets. The bars may have liberal tolerances controls. The surface and core defects are wide and not well quantified. The bar manufacture may involve mild bending, hot forming, punching and welding. The quality norms in terms of internal porosity, surface seams are also liberal.
Special Bar Quality is a term used in long product industry, where an assured stress level in the application can be met by the steel [1,2]. They are long steel products manufactured to meet tough applications. Stress tolerance of the steel is enhanced which provide higher level of consistency and integrity. To achieve the desired level of performance chemical composition and cleanliness of the steel are critical to achieve the desired mechanical properties. While the chemistry achieved is as per the alloy design principles, the achievement of cleanliness levels is critical to achieve the special bar quality and they have a direct impact on the dynamic properties such as fatigue life of a component. The SBQ steel bars, are produced using processes that enhance quality parameters required for end use and customer specifications. The quality in the bars produced may be required to meet with the lowest levels of chemical segregation, inclusion rating, internal defects and surface defects.
Heterogeneity in a bulk steel is inherent in the manufacturing processes of the steel and SBQ has minimal heterogeneity acceptable to a customer. The heterogeneities in a steel include non-metallic inclusions [3,4], macro-segregation [5], micro segregation [6], micro-void or discontinuities that was generated in the casting [7] and that did not heal during subsequent hot deformation processes [8,9]. There is a quantified and assured acceptance level of the heterogeneity in the steel in terms of the quality of the steel bars. In terms of application, the SBQ steel performs in a highly loaded condition due to lower level of defect initiation sites. Hence, several safety critical high warranty prone components, where strength, fatigue life and durability requirements are there, fall in this category. The components are subjected to rotation, twisting and bending type of load application calls for SBQ steels.   Island and Turbine  Island Aerospace Landing gears, Missile parts, forgings are widely used in commercial jets, helicopters, pistonengine planes, military aircraft and spacecraft, bulkheads, wing roots and spars, hinges, engine mounts, brackets, beams, shafts, landing gear cylinders and struts, wheels, brake carriers and discs and arresting hooks, shafts, couplings, manifolds, wheel lever forging, fan forging, landing gear main leg forging Industrial Hook, Chains, Mooring chains for offshore platform, Input/output shafts, Pump Bodies, Wheel hubs and spindles, Ring gears and pinions, Valve bodies, Balls, Tees, Seats, Hammer union nuts, Check valves, crane wheels, and machine parts. , pins, studs, bolts, gears, shafts, axles, bolts, studs, crane wheels, and machine parts.
Tool & Die steel Hand tools and hardware: Pliers, hammers, sledges, wrenches and garden tools, as well as wirerope clips, sockets, hooks, turnbuckles and eye bolts, Dies for plastic moulds, die casting, forging tools and dies, hammers, screwdrivers and wrenches SBQ quality steel bars can be produced using rimmed, capped, semi-killed, or fully killed deoxidation practice. The appropriate type is dependent on chemical composition, quality, and customer specifications. In addition, specification may demand quantified austenitic grain size, microstructure and mechanical properties and dimension. The tensile strength ranges in SBQ is similar to those applicable for the MBQ steels, while there could be enhanced fracture resistance properties such as impact or fracture toughness. There no specified size ranges established for SBQ steels. The bars can be squares, rounds, ovals, half-ovals, half-rounds, octagons, and other special special shapes such as beam blanks. A customer needed to give specific quality parameters for his application in unambiguous numerical value of acceptance depending on his end use application. Some of the typical parameters sought for SBQ steel quality is, given in Table 2. The Table brings out the quality parameters, typical equipment needed, critical parameters that need to be controlled and the requirements to qualify the steel under SBQ category. The SBQ steels have to pass through narrow chemistry bands and higher levels of customer specification. More than cost performance is the basis a customer evaluates a SBQ producer. Grade wise even simple carbon steels are enabled to perform at higher performance levels [life or load enhancement], when it is subjected to SBQ processing.

Processing of SBQ
In the manufacture of SBQ, one has to assess the type of infrastructure a plant has towards manufacture of the same. It is necessary to have infrastructure that has quality control at every stage of its operation. The infrastructure should address to some of the enhanced quality requirement demanded by the steel. JSW Salem works is a leading SBQ steel producer in India. The steel plant should have a means to produce steels with lowest levels of impurities within a narrow chemistry range. The infrastructure the plant has towards making SBQ has to be, examined by a customer. The typical processing route of JSW Salem Works is, given in Fig.1. The steel in the plant is made through, the blast furnace route with controlled input charge material, with quality control at every unit process with adequate equipment. The steel in JSW is made from the blast furnace -EOF route with least addition of external scrap, which is a source of certain defect causing tramp impurities such as Cu, Sn, As. Excess residual Cu can lead to surface defects that defeats the purpose of SBQ [12]. The steel made from blast furnace route is devoid of harmful residual elements unlike a recycled EAF steel.

Steel Making
The blast furnace hot metal pig iron is, extracted with the desired level of Si content [0.5 to 1.2%] and carbon content [~ 4.3%]. Lower levels of P, S and Ti, is desired for SBQ steels. The exothermic oxidation of the carbon and Si with oxygen in an EOF produces heat sufficient for steel making. The oxidation of all solute elements in hot metal, is carried out in the EOF. The elements with higher oxide stability [lower standard free energy of oxide] preferentially get oxidised first. While all the alloying elements are oxidized, the iron purity increases to >99.5% pure iron with dissolved oxygen. It is, desired to ensure that the tap carbon level is controlled, to achieve the lowest possible equilibrium oxygen in the bath. It is desirable to maintain EOF melt oxygen<600 ppm for SBQ type clean steel. The low phosphorous levels required by the SBQ steels is, generated by promoting P mass transfer from molten steel to slag at this stage. Some of the stringent mechanical property requirement in power plant, aerospace and defence sectors, demand extremely low P level as low P content enhances the fracture toughness and DBTT of the of the heat treated alloy steel [13]. The levels may be as low as 0.007% in power plant rotor steel forgings. The slag basicity and FeO content is controlled, to ensure dephosphorisation at the primary melting stages such as EOF furnace [14].The FeO content in the slag has to be maintained so that the equilibrium oxygen in the pure iron tapped does not increase. If the FeO content in slag is large then, the equilibrium oxygen in steel bath is higher during tap. This promotes more number of inclusions in the steel at the secondary steel making stage. Low inclusion levels are desired by SBQ steels in ladle furnace process ensure management of easily reducible oxides (FeO+MnO) towards lowest levels.
The steel from EOF is, transferred to a ladle with deoxidisers to bring down the initial dissolved oxygen in steel. Here, there are several strategies to control the inclusions. Addition of carbon along with the tapping stream, prevent ingress of air to molten steel bath due to carbon oxidation. In some of the experimental heats made by the author, observed, a high Al recovery along with high carbon pick up. This implied that the carbon is able to go into solution in molten steel and contributes to the lowering of oxide inclusions in the bath. The residual oxygen in the tapped steel is killed with a combined Si-Mn deoxidation or with large amounts of Al additions in the ladle. This initial deoxidation during tapping is called as blocking the steel heat, which brings down the oxygen level in molten steel, low enough, so that alloying elements added subsequently are fully recovered without oxidation. The initial Al killing gives lowest oxygen levels during blocking [15]. Alloying using quality controlled ferroalloys is carried out in ladle furnace, depending on the steel grade. In some practices, initial slag skimming followed by synthetic slag addition is carried out [16,17]. The slag in LF has to have adequate lime content >50% to ensure a white slag practice, the reducing condition along with good basicity for refining reaction. Vacuum Degassing with a pressure <1mbar is carried out, where excellent degassing is promoted by intense gas stirring under reduced pressure. Nitrogen levels fall down along with hydrogen to low values and in SBQ steel [18]. The FeO content of the slag falls to very low levels <0.5% [reducing condition], which along with high slag basicity, promote excellent desulphurisation. Some of the steels, where transverse toughness properties are critical sulphide inclusions have to be reduced and hence very low levels of S <0.005% is targeted [19]. Desulphurisation takes place intensely at vacuum degassing stage where again the FeO content is monitored to the lowest level possible to achieve low dissolve oxygen content in the bath [20]. In the case of resulphurised steel, sulphur wire is subsequently added to meet the required sulphur levels [21].Usually, deoxidant addition, especially Al is carried out within vacuum as this is the element most reactive and may fluctuate in Ladle furnace. After Vacuum degassing treatment, a period of soft rinsing, at low Ar flow rates has to be carried out. Here, the flow rate of Ar has to be at low rates, which ensure formation of large plume of fine bubbles. Each of this bubbles as they ascend through the melt provide heterogeneous interfaces for inclusions to interact with each other and float to melt surface. Often, stirring efficiency is a parameter optimised during alloying and soft rinsing [22]. It must be ensured, that the duration of soft rinsing does not result in temperature fall that warrants re-arcing. Appropriate initial ladle temperature should be maintained to, account for this loss in temperature.
If there is a requirement of Ca treatment, the LF processing window in adjusted with suitable slag tailoring and Ca addition maintaining a Ca/Al ratio suitable enough to promote CaO.Al2O3inclusion which is liquid at the temperature of steel making [23]. If the required liquid phase is not achieved high temperature calcium aluminate inclusions populate the steel bath as inclusions. Usually, for high quality steel, after VD arcing is not permitted as, it exposes the melt to atmospheric oxygen during turbulent arcing. After VD the steel is very clean with least amount of gaseous elements. Sometimes, deoxidant addition such as Al content is specified and this has to be made up preferably in the VD stage only. The residual Al is important depending on the grade as this combines with N to form fine AlN inclusions that ensures fine grain sizes desired in an SBQ [24] . Fine grain sizes enhance the strength and toughness and optimum level is required.

Casting of Steel
SBQ involves both ingot and continuous casting processes and the later process predominates due to higher yield, productivity and quality in certain areas. Ingot casting is preferred where the product requires highly recrystallized grain size that warrants high reduction ratio.
Once the liquid steel is made, it is important to maintain its quality during subsequent processing. The tundish has to have a good refractory that is stable and least reactive to the melt chemical and physical interaction and should have flow behaviour where macro inclusions float out [25]. The refractory with high MgO content is preferred. It is subjected to pre-determined prolonged heating to ensures that the lining is free of moisture. The molten bath, when transferred to tundish may react with moisture to introduce hydrogen and oxygen [26]. Hydrogen measurement at this stage is a critical parameter. Appropriate post processing treatments such as stack cooling or anti flaking treatments depend on the hydrogen levels at this stage [27]. Also the nitrogen pick-up at this stage is monitored as there is a correlation between N pick up and oxygen pick up from atmosphere [28]. Casting is carried out with proper tundish flux through an SEN. Free opening of the ladle is desired and the SEN is properly shrouded to prevent ingress of air to oxidise the bath. The SEN with upward angle nozzle is preferred as the flow is smoothened and the flow pattern enables flotation of inclusion towards the top mould surface layers, where the mould powders have the ability to absorb inclusion [29]. The mould powder choice is critical to the grade. For example, the peritectic grade demand a different condition of solidification compared to a high carbon steel grade [30,31]. The wrong choice of mould powder can lead to sub-surface seams that may escape the notice during surface grinding yet show up during hot rolling. The super heat needs to be optimised [32]. High super heat favours inclusion flotation during casting, but macro segregation and centre line porosity tends to enhance. Too low a super heat tends to clog the nozzle and prevent inclusion from floating. Appropriate parameters of casting speed, primary and secondary cooling are to be ensured [33,34,35]. The casting mould is subjected to proper oscillation frequency to ensure smooth stripping of the billet and improper choice of condition may lead to severe ripples in the steels in surface, especially at the corners. It may also lead to transverse crack and hooke defect [36]. Another important phenomenon that will ensure proper solidification with freedom from surface and subsurface inclusions is, obtained by the electromagnetic stirring in the mould or strand. The stirring ensures better heat transfer that gives better solidification macro structure and the lighter inclusions are, pushed to the core due to the centripetal force acting on the particles [37,38]. Different grades have different characteristics in secondary cooling. In-appropriate choice of this parameter again results in seam cracks. The temperature during unbending in certain grades leads to embrittlement associated with AlN precipitation [39]. Automatic mould level controller is another control parameter that ensure proper turbulence free meniscus solidification [40] that results in superior surface quality with mould powder smoothly filled between the solidifying steel shell and water cooled mould.
The bloom or billet solidification structure is usually heterogeneous with a chill layer, columnar zone and core equiaxed zone [41] . The columnar zone is minimised and efforts to maximize equiaxed grains. The deformation of columnar zone product results in banded microstructure with anisotropic mechanical properties especially transverse toughness and ductility. Efforts to maximize the equiaxed zone is maximised by proper choice of superheat, casting speed, primary and secondary cooling in a continuous casting process. The structural banding is an important aspect of alloy steel and it is seldom possible to completely eliminate the banded microstructure caused by the interdendritic region rich in solute and dendrite core devoid of alloying elements [42,43]. The acceptable level of banding is specified to be about 5 BHN difference in

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International Conference on Advances in Metallurgy, Materials and Manufacturing IOP Publishing IOP Conf. Series: Materials Science and Engineering 314 (2018) 012016 doi:10.1088/1757-899X/314/1/012016 some of the standards. Depending on the grade, one may encounter core central porosity or macro segregation. Efforts to minimize the same by optimising super heat of the grade being cast, casting speed, and EMS current.
Secondary cooling characteristics govern the initiation of surface cracks and one needs to optimise the parameters of the same for a given grade. There could be formation of embrittling phases during unbending in continuous casting that can initiate surface flaws [44]. Although much lower in volume, special steels produced by Electro-slag remelting, vacuum arc remelting etc. Are, classified under SBQ type steels. These steels may be demanded for use in high end applications in nuclear, defence and aerospace.

Hot Rolling of Steel
The cast blooms or billets made are hot scarfed, in some plant, while they are subject to cold grinding before being hot rolled [45]. In some plants hot charging is carried out which, enables conservation of energy apart from preventing phase transformation during complete cooling that involves volume changes and associated stresses in the bloom or billet [46,47]. The blooms have to be heated appropriately before hot deformation in reheating furnace as a temperature gradient will result in non-uniform flow strength and inhomogeneous deformation. The thermal cycling is such that the core achieves the required temperature [48] . During the bloom re-heating, apart from choice of heating parameters, oxidation of the bloom and the surface decarburisation are important aspects that are crucial to SBQ product [49]. Scales generated in the reheating operation reduces the steel yield, which needs to be curtailed, and in some grades can be potential sites for crack initiation. The choice of the burning fuel and fuel:air ratio are factor optimised in a furnace. The soaked bloom is, subjected to initial deformation in a blooming mill where the cast structure is heavily broken down. The deformation at this stage is large enough to convert the cast dendritic microstructure to recrystallized grain structure. Phenomenon such as static, dynamic and metadynamic recrystallization refines the grain structure [50]. The reduction ratio is a monitored measure of the extent of deformation that has enabled the cast product to become a wrought product. The fine grain structure obtained has to be retained especially after the finish rolling. The deformation strain, pass schedule and rpm are critical parameters apart from the surface and core quality of the input bloom. Post rolling, the blooms are cooled in cooling table or slow cooling boxes or pits or vermiculite, to ensure achievement of suitable hardness and also diffusing out hydrogen in steel in some grades. The final hardness achievement in the bars, suitable for machining, may be specified by some of the customer, where slow cooling needs to get a softer product. Certain high alloy steel prone for hydrogen flaking needs to be given a proper anti-flaking heat treatment below A1 temperature [51]. Hydrogen level in excess of 1.2 ppm in high hardenability steel such as C-Mn, Cr-Mo and Ni-Cr-Mo steels are prone for hydrogen flaking problem.

Heat Treatment Testing & Quality Control:
Many of the SBQ steels should show favourable response to heat treatment. They have specified hardenability, fine grains, flawless surfaces etc. This has to be consistently maintained by the steel mill to ensure consistent heat treatment response and mechanical property requirements in production at customer end. Some of the SBQ steels are inclusion engineered, where inclusion modification is carried out using Ca, Te, Pb or re-sulfurised [52 to 54]. They form fine evenly distributed secondary phases, which can enhance machinability. SBQ may also have non-heat treated steel grades such as microalloyed steel [55]. These steels, post forging achieves high strength properties.
In terms of quality control, the steels have to conform to chemistry, inclusion rating, grain size, microstructure, macrostructure, surface flaw by Magnaflux or MPI, core defect by ultra-sonic testing, decarburisation, banding level and mechanical properties at the customer specified conditions. There are other physical attributes such as the dimension, multiple length, straightness, ovality in round product or rhombodity in a square product [56]. Steel mills, which make SBQ has the specialised infrastructure to ensure dimension and defect free surfaces. The machines include straightening machines, surface grinding machines for blooms, annealing, peeling units etc.

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International Conference on Advances in Metallurgy, Materials and Manufacturing IOP Publishing IOP Conf. Series: Materials Science and Engineering 314 (2018) 012016 doi:10.1088/1757-899X/314/1/012016 Often SBQ steels undergo direct machining or further processing such as closed die forging, extrusion operation or cold drawing operation in a spherodize annealed condition [57] or a heat treatment after fabrication or surface hardening heat treatment post fabrication. Quality steel mill understands the requirement of subsequent processing industry and tailors the SBQ steel bar quality to suit the end use applications so that the products can be realized with consistency and least failures.
SBQ steels are, subjected to annealing, normalising, hardening and tempering heat treatments. Minimising defect initiation sites in the steel making operation enable achievement of consistent properties. Properties such as transverse impact or fracture toughness are superior in SBQ steels. The embrittlement generating impurities are least in the steel. There is a good microstructural homogeneity due to control over parameters such as hardenability. In inclusion engineered steels, there is a consistency in terms of machinability. Surface heat treatment response of the SBQ steel bars show consistency in the hardness and depth of layers and grain size in processes such as, carburising, nitriding, induction hardening etc, [58][59]. The surface quality of the bars influence the flaws generated in heat treatment and these steels and that is one reason SBQ is preferred for heat treated products.
In spite of additional costs associated with additional infrastructural necessity in manufacture and testing, productivity losses associated with higher refining times, and yield losses due to higher discards, the customer prefers SBQ to ensure assured performance. The SBQ steel market is growing. Customers generally assesses, the capability of the product by its performance in final applications.

Conclusion:
A review of various processing conditions in a steel mill in the manufacture of SBQ steels has been brought out. Special bar quality requires assured mechanical and quality parameters which needs to be adhered to by the steel mills. Proper infrastructure and processing technological capability and scientific understanding is needed for the manufacture of SBQ. JSW is a leading producer of SBQ with excellent infrastructure for the manufacture and quality assurance of the products.