| Customization: | Available |
|---|---|
| Type: | Ordinary Sand Casting |
| Application: | Steel Ingot Making |
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| Graphite Type | Mainly A type graphite, most of the graphite is flake, a small amount of E type | ||
| Spheroidization Grade | / | Rate of spherodization | / |
| Graphite length | <25 | Grade | 4 |
| Pearlite content | ≥90% | Grade | 3 |
| Cementite content | <2% | Grade | Cement 1 |
| Tensile strength(MPa) | 100-300 | Specified plasticity extension strength | / |
| Percentage of elongation after fracture(%) | / | ||
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 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.
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.
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.
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.
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.
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.
We are a reliable supplier that can make forging parts and casting parts. Here are some details of the products for your reference.
1. Main products: Custom Open Die Forging, steel ingot mould ,
2. Materials: Alloy steel,pig iron
3.Supplying range: Shafts, Sleeves, Rings, Cylinders, Blocks, Modules etc
4. Process: Forging/Casting - Normalizing & Tempering - Proof Machining- Quenching & Tempering - Finish Machining
We can offer you in various process conditions.
5. QA DOC.: Chemical Composition Report, Mechanical Properties Report, UT Report, Heat Treatment Report, Dimensions Check Report
The data on chemical composition report and mechanical properties report are approved by third party, Luoyang Ship Material Research Institute, CSIC.
UT test: 100% ultrasonic test according to EN10228-3, SA388, Sep 1921 C/c etc.
Heat Treatment Report: provide original copy of heat treatment curve/time table.
Steel Ingot: EAF-LF-VD/ESR. Material Certificate according to En10204-3.1 is requested from ingot supplier.
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