Steel Making Methods | Advantages| Disadvantages| Basic Oxygen Furnaces| * Very high production rates and low residual element * Does not burn fuel| * Good efficiency requires large amount of pig iron to continue production. * Requires costly filtering process due to high levels of pollutants produced. * High refurbishing costs. * High dependence on blast furnace/coking. | Electric Arc Furnaces| * Minimal emissions/pollution. * Filtering of scrap not necessary. * Easy temperature control. * Precise alloying. * Economical to use scrap metal. * Contamination free. * Simultaneous deep deoxidising and desulfurization actions. * Excessive electricity required. * Requires a steady supply of scrap metal * High transportation cost * Enclosures to reduce high sound levels * Dust collector for furnace off-gas * Slag production * Cooling water demand * Heavy truck traffic for scrap, materials handling, and product| The first step in the process, is to make the steel itself. The most common method of steel making, constituting for over 60% of worldwide production uses a Basic Oxygen Furnace (BOF). This process includes taking over 75% pig iron and reducing it to a low-carbon steel in an abundance of oxygen.
The second type utilises Electric Arc Furnaces (EAFs). This involves melting up to 100% recycled scrap and reforming it using the heat produced from electrical arcs between highly charged electrodes. Figure 2 (right & below): EAF Process Figure 2 (right & below): EAF Process Figure 1 (left): BOF Process Figure 1 (left): BOF Process From the table above it is clear to see that without an established, effective transport system that allows for large amounts of scrap metal to be processed, the Basic Oxygen Method has fewer disadvantages.
However, as TATA Steel already has an efficient system in place, the most feasible method would be using Electric Arc Furnaces. Despite initial costs, using EAFs save on energy and raw materials, making it more environmental and cost friendly in the long run. ‘Whilst a typical integrated (ie. BOF-route) steel mill today costs about $1100 per tonne of installed capacity, a medium-size EAF-route mini-mill today costs under $300 per tonne in terms of the initial capital outlay. ‘1 Casting Methods The next stage in the process is to shape the steel and this is done by casting.
Casting involves allowing molten metal to be poured into a mould to it can cool and solidify into a desired shape. The two most common methods of casting are Ingot Casting and Continuous Casting. The first of which is a traditional method that has largely been discontinued in mass production since the 1950s. It involves moulding the steel into bars (or ingots) before being reshaped and treated. Continuous casting however misses out the ingot stage and skips straight to having the metal in the form of slabs, billets or blooms for subsequent rolling in the finishing mills.
Figure 4 (left): Continuous Casting Process Figure 4 (left): Continuous Casting Process Figure 3 (left): Ingot Casting Process Figure 3 (left): Ingot Casting Process Because Continuous Casting is basically an “evolved” version of Ingot Casting, there are now little or no advantages of Ingot Casting. Continuous Casting is more advantageous because: * Reduced overall costs * Improved quality of steel due to less variability in chemical composition both along the thickness and along the length and surface has fewer defects. * Increased yield, since it is not necessary to crop the ends of continuously cast slabs. Reduced energy costs because the slabs are sent directly to hot rolling and do not require pits for reheating. Also, the thicknesses of continuously cast slabs are half the thickness of ingot castings and thus require lower energy for hot rolling. * Less pollution/emissions. * More amenability over the dimensions. Because of all this, the clearly logical method to use for mass production is Continuous Casting because for something as mass produced as Automotive Gears, the initial investment spent on start up costs would be quickly made up.
Case Hardening Methods Case hardening crucial for steel components that are subjected to severe or continuous impacts, high temperatures and high pressures. It is a heat treatment process that produces the required attributes of a hard, wear and fatigue resistant surface layer whilst maintaining a tough, durable core that allows for high stress situations. These properties are achieved by altering the chemical, metallurgical and physical properties of the components exterior without affecting its more ductile interior.
For gears, case hardening is required to prevent pitting and deformation of the gears teeth under cyclic stresses. This method is preferred to through hardening, which is the uniform hardening of the entire component, as hardened metal is relatively less ductile and although strong, would not offer the same degree of toughness desired at its core. There are several different case hardening techniques used in the manufacturing industry. The different methods determine which physical properties, (such as surface hardness, strength, ductility, case depth and wear resistance) the component gets.
This can be done by altering temperatures, heat source, time period, and quench media. Carburising This is a diffusion-based process used on low-carbon or mild steels where a component is subjected to thermochemical phases. The component is packed in a carbon-rich environment at high temperatures, commonly between 870oC and 1010oC, for over a period of time until the carbon composition of the surface layer has chemically increased. At this stage the iron phase changes from ferrite to austenite, a state that is able to dissolve more carbon.
The component is then quenched in water or a oil based solution, which is a rapid cooling process that produces a hard surface layer, where volume expansion on the surface is greater than the core thereby compressing the surface, locking the carbon atoms, transforming the iron phase to a martensitic state which ultimately improves its overall tensile and yield strength. This method requires the entire component to be heated and quenched, therefore protecting the component with a protective layer to case harden specific sections is necessary.
There are two types of carburizing methods used in the manufacturing industries, namely atmosphere carburizing and vacuum carburising, the former being the more commonly used as it has the ability to produce high volume output and has lower capital equipment costs, while vacuum carburising offers a more uniform case depth which in turn reduces distortion as well as the ability to reach higher temperatures therefore reducing processing times. Induction Heating This is a process of passing an alternating current through a coil around the component to generate a magnetic field, where eddy currents are induced.
This along with the resistivity of steel components generates heat, austenitising the surface of the component. The depth to which case hardening occurs is determined by the frequency of the current, such that lower frequencies creates a deeper hardened material. This method allows for localised case hardening of the gear tooth with its core material still unaltered. The gear surface is then similarly quenched in water or an oil based solution, transforming it to a martensite.
Single-shot hardening is where the entire component is heated in one procedure whilst progressive hardening involves the heating and quenching processes progressively. Induction heating is a relatively fast process that offers accurate heating at precise sections, minimising distortion as well as causes minimal changes to the geometry of the gear, as well as faster cooling rates that creates harder surface layers. Figure 5 (left): Carburising Process Figure 5 (left): Carburising Process Figure 6 (right): Induction Heating Process Figure 6 (right): Induction Heating Process The Strength of Automotive Gears
The simplest method of calculating the strength necessary of any gear is to consider the maximum load on the tip of a single gear tooth. The Lewis Equation can be used to calculate a relatively accurate minimum UTS needed from the steel tooth with non-complex dimensions. In the automotive industry, varieties of different steels are made specifically for different components in different vehicles. The steel grades used on conventional cars can generally withstand a minimum of 750MPa whereas motorsport and military vehicles are made with much more superior grades, some able to withstand up to 2050MPa.
Hardenability Results Using SEP1664 The SEP1664 model can be used to find the hardness at a series of depths, following case hardening, for 11 different steel types. The Rockwell hardness (HRC) at a given depth is found using the following equation: HRC=a0+a1mC+a2mSi+a3mMn+a4mP+a5mS+a6mCr+a7mMo+a8mNi+a9mAl+a10mCu+a11mN+a12mB+a13mTi+a14mV a0-14 are coefficients available in the SEP1664 tables and mX is the mass proportion of additive X. A spreadsheet was created which used a VBA macro to find steel compositions that met the hardenability criteria by trial and error.
Several percentage masses for each additive within the range specified by the SEP1664 tables were tested. Solutions were sorted in order of increasing raw material cost. This macro was run for each of the 11 steel types. Three steel types were found to be too hard. Of the remaining eight, there was sufficient data to evaluate hardness at all the required depths for four. For the other four it was possible to infer from the hardness trend that steel could be produced which was suitable for all depth levels.
The cheapest result was for the steel type specified as being approximately 1% chromium by mass (tables 1a and 1b from the SEP1664 model) at $161. 67 per tonne. For this alloy the hardness at 11mm depth was borderline acceptable. The additives making the greatness contribution to hardness were determined so they could be varied to give a greater margin for error. The importance of each additive at each depth could be found from the equation by multiplying the coefficient by the additive amount (i. e. evaluating the relevant anmX term) and calculating its percentage contribution to the total hardness.
The three most important additives for each depth level and the relative importance of the different additives at 11mm depth are shown in Table 1 and Table 2 respectively. Table 2: The Three Most Important Additives In Terms Of Contribution To HRC For Each Depth Importance| 1. 5| 3| 5| 7| 9| 11| 13| 15| 20| 25| 30| #1| C| C| C| Cr| Cr| Cr| Cr| Cr| Cr| Cr| Cr| #2| Mn| Cr| Cr| C| C| C| C| C| C| C| C| #3| N| Mn| N| Mn| Mn| Mn| Mn| Mn| Mn| Mn| Mn| Table 3: Relative Additive Contributions To HRC At 11mm Depth C| Si| Mn| P| S| Cr| Mo| Ni| Al| Cu| N| 40. 1%| 0. 7%| 13. 9%| 0. 0%| 0. %| 40. 6%| 0. 0%| 0. 1%| 4. 3%| 0. 2%| 0. 0%| Adjusting the appropriate additives gave a greater margin for error for a small increase in cost ($2. 32 per tonne, total cost $163. 99 per tonne). The composition of this steel is shown in Table 3. A plot of HRC against depth is shown in Figure 1 along with the hardenability criteria, it can be seen that the hardness, which tends to decrease with depth following case hardening, is very unlikely to exceed the stipulated maximum hardness at the depths for which data is unavailable, no minimum hardness is stipulated for these depths.
A Jominy test is recommended on a sample of this steel, once it has been manufactured, to ensure that the hardenability criteria are met. The amount of carbon, chromium and manganese must be controlled to within 3% of the given values, tight control is not necessary for other additives. Table 4: Chosen Steel Composition Additive| C| Si| Mn| P| S| Cr| Mo| Ni| Al| Cu| N| Amount| 0. 248%| 0. 02%| 0. 63%| 0. 004%| 0. 038%| 0. 947%| 0. 005%| 0. 010%| 0. 051%| 0. 017%| 0. 0148%| Figure 7: Hardenability Curve For Chosen Steel (Blue) And Cheapest Steel (Red), The Criteria Are Shown In Black