Steel ingots and their Casting during Steelmaking
Steel ingots and their Casting during Steelmaking
Ingot casting is a conventional casting process for liquid steel. Production of crude steel through the ingot casting route constitutes a very small percentage of global crude steel production. However, the method of casting of the liquid steel in ingot moulds is still fundamental for specific low-alloy steel grades and for special forging applications, where products of large dimension, high quality or small lot size are needed. Typical application for conventional ingot casting includes the power engineering industry (e.g. shafts for power generation plants, turbine blades), the oil and gas industry (conveying equipment, seamless tubes), the aerospace industry (shafts, turbines, engine parts), ship building (shafts for engines and drives), tool making and mechanical engineering (heavy forgings, cold, hot and high-speed steels, bearing, drive gears) as well as automotive engineering (shafts, axes).
As the demand of heavy ingot increases nowadays, especially from the power engineering industry and ship industry, there is a tendency of producing extreme large ingots over 600 t and continuous cast strands with thickness over 450 mm and rounds with diameter up to 800 mm, which are mainly applied for pressure retaining components such as reaction vessels for nuclear power plant and rotating components like drive shafts of gas turbines and generator rotors.
The moulds used for casting of ingots are made of cast iron. Cast iron is used for the production of the mould since the thermal coefficient of cast iron is lower than that of steel. Because of this property of cast iron, liquid steel on solidification contracts more than cast iron which makes detachment of ingot easier from the mould. Inner walls of the mould are coated by either tar or fine carbon. The coated material decomposes during solidification and this prevents sticking of solidified ingots with the inner walls of the mould.
Material used for the production of cast iron moulds is generally grey cast iron with lamellar graphite. The typical composition of grey cast iron is C – 3.3 % to 4.0 %, Mn – 0.4 % to 0.9 %, Si – 1.2 % to 2.2 %, P – 0.2 % maximum, and S – 0.05 % maximum. Ductile cast iron or treated pig iron can also be used in the production of moulds.
The moulds employed for the casting of steel ingots have a square, rectangular, round, or polygonal cross-section, in which liquid steel solidifies into a desired shape to be processed by rolling or forging. Ingots with square cross section are used for rolling into billets, rails and other structural sections, whereas, ingots with rectangular cross section, are used for rolling into flat products. Round ingots are used for tube making. Polygonal ingots are used to produce tyres, wheels, etc.
Low capacity steel melting shops with induction furnaces uses very small cross section ingots moulds for casting of liquid steels. Steel ingots produced in these ingot moulds are known as pencil ingots. Typically steel ingot used for the production of rolled products has weight in the range of 5 tons to 35 tons. Pencil ingots are used for the rolling of merchant long products and reinforcement bars and has a weight typically in the range of 100 kg to 200 kg. Steel ingots used for the production by forging of the heavy equipment/components can be extremely large ingots weighing even 600 tons and more.
Moulds used for the casting of steel ingots are basically of two types. They are (i) wide end up or narrow end down moulds, and (ii) narrow end up or big end down moulds. Wide end up moulds are used to produce forging ingots of killed plain carbon (C) or alloy steels. Wide end up moulds may have a solid bottom. Narrow end up moulds are commonly used to produce rimming and semi-killed steel ingots. Narrow end up moulds facilitate easy escape of rimming reaction product which is the carbon mono oxide (CO) gas.
Casting of the liquid steel in cast iron moulds is carried out in the cast house of a steel melting shop. Cast iron moulds (ingot moulds or simply moulds) are placed either on mobile carriages or on a casting field.
Steel ingots are produced either by top pouring or bottom pouring of the liquid steel in the moulds. The increasing demand for quality steels led to the evolution of bottom pouring technology for casting of steel ingots. This process constitutes of a set up involving pouring sprue and runner system to deliver liquid steel into the bottom of one or more cast iron moulds.
Steel is poured into the moulds either directly from steel teeming ladles or via a tundish which is equipped with slide gates in case of top pouring method. In the case of the bottom pouring method, steel is not cast into moulds directly, but via the sprue and runners system, and then it rises evenly in all ingot moulds simultaneously. For this casting method, moulds can also be placed on casting bogies or in casting pits.
Bottom plate is placed under the mould. In case of casting of smaller weight ingots, generally a number of moulds are placed on one plate. Bottom plate is an integral part of the mould assembly and hence its material is to be the same as the material of the mould. Bottom plates are exposed to intense stresses, especially during the initial period of the casting of the liquid steel in the mould. Sometimes the bottom plates are shaped to avoid splatter of the liquid steel.
Fully deoxidized or killed steel used for high quality forgings shrink on solidification and may lead to formation of pipe. Moulds are generally provided with hot top which acts as reservoir to feed the metal and to avoid formation of pipe. Hot tops are used for casting the killed steels and are designed to concentrate the shrink in the head portion of the ingot. The refractory portion of the shell is mostly made of cast steel and lined with (compacted) refractory material with low conductivity which helps to maintain steel in a liquid form as long as possible. Insulating and exothermic materials can also be used for ensuring availability of hot metal towards the end of solidification.
Mould filling of heavy ingot can be performed in two ways namely (i) top pouring, and (ii) bottom pouring. In the top pouring, the liquid steel stream is more exposed to air, suffering from reoxidation problems. As the pouring stream impinges on the melt surface inside the mould, it carries reoxidation products and mould powder, floating on it, back into the bulk, resulting in macro-inclusions. Also, during filling, metal splash adheres to the mould walls and produces surface defects on the ingot skin, which subsequently needs surface conditioning. This makes the top pouring method not suitable for high quality steels, and hence bottom pouring is preferred. This is because, in bottom pouring, the liquid steel flows from the ladle down to the trumpet, passing through the horizontal refractory runner, it enters the nozzle or in-gate upwards, reducing the exposure to air, the entrapment of mould powder and the occurrence of splashing. The bottom pouring needs a controlled velocity during filling in order to avoid turbulences and, consequently, powder entrapment or reoxidation defects
The application of this bottom pouring method for the production of quality ingots has been mainly due to the reduced turbulence of steel in the mould caused by controlled flow of liquid steel in the mould to result in quiet meniscus leading to superior as cast ingot surface, minimal splashing of liquid steel droplets from ladle stream providing freedom from scab type defects, application of steel meniscus during teeming for completely covering the slower teeming rates that reduce turbulence, longitudinal cracks meniscus and maintaining a powder layer throughout the casting formation and minimize laps and ripple marks and finally, process that helped in drastically reducing or even eliminating improved mould life.
The advantages of the top pouring method compared with the bottom pouring method include lower requirement of labour and lower consumption of refractory materials for the preparation of the moulds for the casting, lower loss of steel (the loss of steel results from the solidification of steel in the gating systems which is often called ‘bones’), better location of the heat centre of the solidifying ingot in its upper part, lower potential for the additional contamination due to a contact with the casting ceramic materials, and lower of the temperature of the liquid steel between the ladle and the mould, etc.
Drawbacks of the top pouring method compared with the bottom pouring method include higher potential occurrence of some defects, such as scales, longer time interval for casting the ladle, higher number of ladle closures and therefore increased wear of the ceramic closing mechanism, poorer monitoring and control of the casting speed, and higher wear of moulds, etc.
The mould shape (round, square or multi-fluted cross section) also contributes to the casting quality of the steel ingots. It is chosen according to the expected quality grade and, above all, to the product shape to be forged. Hence, in order to obtain sound ingots, mould shape, runners’ cross-section and length, as well as nozzle diameter and height have to be properly designed. Typically, the design is the result of the factory know-how but, in last decades, numerical simulation has been progressively applied as a useful tool for the optimization of mould shape and process parameters, to further improve the ingot quality.
Mechanism of solidification of liquid steel in ingot mould
The mechanism of the solidification of killed liquid steel in the ingot mould is described below.
- Liquid steel near the mould walls and bottom is chilled by the cold surfaces and a thin shell or skin is formed on the ingot surface. This surface has a fine equiaxed grains and the skin. The formation of skin results in decrease in rate of solidification.
- Due to expansion of mould through the heat transferred from the solidifying steel and contraction of solidified skin an air gap forms between the mould and the skin. This results in decrease in the heat transfer rate, because of the air gap which has a high thermal resistance to heat flow.
- The solidification front perpendicular to the mould faces moves inwards and towards the centre as a result columnar grains form next to the chill surface. The columnar crystals rarely extend to the centre of the mould.
- The central portion of the ingot solidifies as equi-axed grains of bigger size since there is slow rate of solidification in this portion.
The above zones of solidification depend on the evolution of CO gas due to the reaction of C and oxygen (O2) present in the liquid steel. In the case of the semi killed steels, not all the O2 is removed from the liquid steel. However, the O2 content of liquid steel is very low. The necessary super saturation level of the C and O2 reaches towards the end of solidification. As a result the central zone of the equi- axed crystal is disturbed by way of formation of blow holes in the top middle potion of the steel ingot.
In case of solidification of rimming steels, the solidification process is controlled by evolution of CO gas during the solidification process. Since rimming steels are not killed, there is availability of O2 in the liquid steel. The CO gas evolved due to the reaction of C and O2 is at the solid/liquid interface and this stirs the liquid steel during the solidification process. Stirring circulates the liquid steel which brings hot liquid steel to the surface and solidification of steel at top is delayed. Columnar grain formation is prevented due to a more uniform temperature at interior of an ingot. This gives rise to rimming ingots in which gas is entrapped mechanically as blow holes.
Micro-segregation and macro-segregation in steel ingots
The general macrostructure that is often seen in the steel ingot can be divided into three distinct zones namely (i) the outer chill zone with small crystals of approximately equal size, (ii) the intermediate columnar zone with elongated columnar dendrites, and (iii) the central equiaxed zone with relatively large equiaxed grains (Fig 1).
Fig 1 Three distinct macro-structure zones in steel ingots
Besides the referred three zones, a region where the outer columnar dendritic structure transfers to the inner equiaxed grain structure is commonly observed. This region is named as the ‘columnar to equiaxed transition’ (CET) zone. Numerous mechanisms for CET have been proposed, and can be mainly divided into two types namely (i) mechanical blocking (hard blocking), and (ii) solutal blocking (soft blocking). Mechanical blocking mechanism considers the blocking effect of the equiaxed grains on columnar grains that if the equiaxed grains are large enough or their volume fraction is large enough, CET will occur, whereas solutal blocking mechanism takes the solutal interaction into account that if the solute rejected from the equiaxed grains is sufficient to dissipate the undercooling at the columnar front, CET can take place.
Steel ingots have internal discontinuities. They contain non-metallic particles of different chemical composition and size, as well as sites with different chemical composition of the steel. The process of ingot solidification and the internal discontinuities are affected by a number of factors. These factors are (i) shape of the ingot or the mould, (ii) cooling rate or the rate of solidification, (iii) size of the ingot, (iv) temperature of liquid steel and the speed of casting, and (v) chemical composition of the steel.
On the solidification of alloys (liquid steel), solute is partitioned between the solid and liquid to either enrich or deplete the inter-dendritic regions. This naturally leads to variations in the composition on the scale of micro-metres (micro-segregation). Macro-segregation, however, refers to chemical variations over length scales approaching the dimensions of the casting, which for large ingots may be of the order of centimeters or metres. Micro-segregation can be removed by homogenization heat treatments, but it is practically impossible to remove the macro-segregation because of the distances over which species are required to move. Almost all macro-segregation is undesirable since the chemical variations can lead to changeable microstructural and mechanical properties.
It is true that the huge majority of the world’s steel is now continuously cast, but ingot casting is still required for the production of heavy industrial components which comprise large high-cost single-piece sections, such as the pressure vessels required for nuclear power generation. The effects of macro-segregation are critically important in such applications and the ability to predict segregation severity and location is highly sought after.
The first examinations of macro-segregation phenomena in steel ingots were carried out many decades ago and since then the understanding of the processes leading to segregation has improved considerably. However, the macro-segregation can still be observed in ingots made today. The macro-segregation can be in the form of A-segregation, V-segregation and negative base segregation as shown in Fig 2.
Up to the mid-1960s, solute buildup at the tips of advancing solid interface has been believed to be the predominant underlying cause of macro-segregation phenomena in ingots. However, this reason has since been demonstrated to be erroneous by numerous theoretical and experimental investigations. It is now well recognised that the majority of solute is rejected sideways from a growing dendrite, enriching the mushy zone, and that build up in front of dendrite tips is negligible in this regard.
Fig 2 Macro-segregation phenomenon during solidification in ingot mould
All types of macro-segregation are derived from the same basic mechanism which is the mass transfer during solidification. The movement of enriched liquid and depleted solid can occur through a number of processes as shown in the Fig 2 and described below.
- Convective flows due to density gradients caused by temperature and composition variations in the liquid. The thermal and solutal buoyancy contributions can either aid or oppose each other depending on whether local temperature and concentration fields cause liquid density to increase or decrease. The convection due to the coupled action of temperature and solute is known as ‘thermo-solutal’ convection.
- Movement of equiaxed grains or solid fragments which have either nucleated heterogeneously in the liquid steel, become detached from dendrites due to remelting/stress, or have separated from the mould wall after pouring. Equiaxed grains in steels are denser than the surrounding liquid and hence they tend to sink. This mechanism, along with convective fluid flow, is a dominant macro-segregation process in the steel ingots.
- Flow to account for solidification shrinkage and thermal contraction of the liquid and solid on cooling.
- Deformation of the solid network due to thermal stresses, shrinkage stresses and metallo-static head (the pressure provided by the liquid steel above).
- Imposed flows due to pouring, applied magnetic fields, stirring, rotation etc.
The tendency of elements to segregate is expressed by the equilibrium distribution coefficient, K0=Cs/Cl where Cs is the concentration of the element in the solid phase and Cl is the concentration of the element in the liquid phase. This is because of the different solubility of elements in liquid and solid steel.
Defects of killed steel ingots
There are two types of defects namely (i) external defects or surface defects, and (ii) and internal defects. The surface defects are external scales, longitudinal tears, transverse, skewed, or zigzag tears, cracks, cold shuts, superficial voids, and Slag and sand nodes on the surface. Other than segregation, the internal defects are pipe formation, blow holes, flakes, and exogenous and endogenous inclusions. Some of these defects are described below.
Pipe formation – Liquid steel contracts on solidification. The volumetric shrinkage leads to formation of pipe. In killed steels pipe formation occurs toward the end of solidification. Fig 3 shows primary and secondary pipe in narrow end up mould and in wide end up mould while casting killed steel. Only primary pipe can be seen in wide end up mould. Rimming and semi-finished steels show very less tendency for pipe formation. Wide end up moulds show smaller pipe as compared with narrow end up mould. The portion of ingot containing pipe has to be discarded which affects yields. The remedy for the pipe formation is the use of hot top on the mould. The volume of the hot top is 10 % to 15 % higher than ingot volume. Pipe formation is restricted in the hot top which can be discarded. Use of exothermic materials in the hot top keeps the liquid steel hot in the top portion and pipe formation can be avoided. Another method is to pour extra mass of metal.
Fig 3 Pipe formation during solidification of liquid steel in ingot mould
Blow holes – The reason for formation of blow holes in steel ingot is the evolution of gas during solidification of the liquid steel. Entrapment of gas produces blow holes in the steel ingot. Blow holes located inside the ingot can be welded during rolling. Rimming steels show blow holes due to rimming reaction between C and O2. The rimming reaction produces CO, which when is unable to escape during solidification, produces blow holes. Semi-killed steels also show tendency to the formation of blow holes. The remedy for the formation of blow-holes is the control of gas evolution during solidification so that blow-hole forms only within the ingot skin of adequate thickness.
Non-metallic inclusions – Non-metallic inclusions are inorganic oxides, sulphides and nitrides formed by reaction between metal like iron, titanium, zirconium, manganese, silicon, and aluminum with non-metallic elements like O2, nitrogen, sulphur etc. An inclusion is a mismatch with the steel matrix. Fine size inclusions when distributed uniformly are not harmful. Non deformable inclusions such as Al2O3 are undesirable. Inclusion modification is the remedy to alleviate the harmful effect of inclusions on properties of steel.
Ingot cracks – Surface cracks are formed due to friction between mould and ingot surface. The improper design of mould taper and corner radius cause surface cracks. Different types of cracks are (i) transverse cracks, (ii) longitudinal cracks also known as panel cracks, (iii) restriction cracks and (iv) Sub-cutaneous cracks.
Transverse cracks are parallel to the base of ingot and are formed due to longitudinal tension in the ingot skin. As the aspect ratio of the ingot increases, tendency to transverse crack formation increases.
Longitudinal cracks are formed due to lateral tension in the skin. They are parallel to vertical axis of ingot. Alloy steels are more prone to longitudinal cracks than mild steels. Panel crack formation in static-cast steel ingots is a problem that has plagued the steel industry for several decades. Although the defect is intermittent and affects less than 2 % of susceptible steel grades, the problem is persistent and expensive since affected ingots must be scrapped. Panel cracks are due to two distinct types of cracking problems, referred to as ‘mid-face’ and ‘off-corner’ panel cracks, respectively. Mid-face panel cracks are found exclusively in small, medium C, hypo-eutectoid, and pearlitic steel ingots and usually show a single, continuous, longitudinal fracture down the centre of one of the ingot faces. Certain alloy steels are particularly prone to this defect and are affected at slightly lower C contents. Off-corner panel cracks frequently form rough oval, discontinuous, crack patterns on the wide faces of large ingots. They affect only low C steels with high Mn content and are usually first observed when they open up during hot rolling. Both types of defect affect only killed, aluminum-treated steels and appear as deep, inter-granular cracks that follow prior austenite grain boundaries.
Restriction cracks can be near the corner radius of the ingot. Smooth corners of the mould and gradual curvature minimize restriction cracks.
Sub- cutaneous cracks are internal fissures close to the surface. The cracks are formed due to thermal shocks.