An online clean technology database

Direct casting for iron and steel sector

The iron and steel sector is the second-largest industrial user of energy, consuming 616 Mtoe in 2007 and is also the largest industrial source of CO2 emissions. The five most important producers – China, Japan, the United States, the European Union and Russia – account for over 70% of total world steel production. A main technology in the iron and steel industry is the direct casting, which is the process of creating finished iron and steel products using moulds to shape the molten metal. 

Introduction top

Steel making is a complicated processes involving many stages and yielding thousands of by-products. Steel can be produced either from steel scrap or iron. The iron and steel sector is the second-largest industrial user of energy, consuming 616 Mtoe in 2007 and is also the largest industrial source of CO2 emissions. The five most important producers – China, Japan, the United States, the European Union and Russia – account for over 70% of total world steel production. Steel is produced through several processes, which depend on various factors of production, such as available raw materials, energy supply and investment capital. There are three principal modern processing routes (IEA, 2008):

- the blast furnace (BF) and basic oxygen furnace (BOF) method, based on 70% to 100% iron ore, with the remainder scrap, for the iron input;
- the direct reduced iron (DRI)/EAF method, based on iron ore and often scrap for the iron input;
- the scrap/electric arc furnace (EAF) method, based on scrap for the iron input.

Iron and Steel Casting is the process of creating finished iron and steel products using moulds to shape the molten metal. Pig Iron, the product of smelting iron ore in a blast furnace, has too many impurities to be forged. These impurities make the metal too brittle. However, the low melting point of pig iron means that it can be cast, which is why it sometimes referred to as cast iron even before it has been moulded. Steel, on the other hand, is purer than pig iron and so can be both cast and forged.

Currently, most steel is continuously cast into slabs, billets or blooms, which have to be reheated when they are later rolled into final shape. Direct casting (i.e. nearnet- shape casting and thin-strip casting) integrates the casting and hot-rolling of steel into one step, thereby reducing the need to reheat the steel before rolling it (IEA, 2008). In the direct continuous steel casting, liquid steel is directly cast into semi-finished products, which eliminates the need for primary rolling of ingots. The crude steel or liquid steel is poured into a reciprocating refractory-lined receptacle, called a Tundish. Below the Tundish are water-cooled copper moulds of desired size. The steel solidified in the moulds is slowly pulled out to produce an "endless" strand, which is gas-cut to desired lengths. This steel is called semi-finished steel. The semi-finished steel is fed in to re-rolling mills to get finished steel products.

illustration © climatetechwiki.org

Figure 1: Casting flow diagram (Source: US DOE, 2001)

Feasibility of technology and operational necessities top

The main processes in the steel direct casting are:

  • Cast pipe fitting manufacturing
  • Cast steel chain manufacturing
  • Direct casting manufacturing
  • Iron casting manufacturing
  • Moulded cast iron pipe or tube manufacturing
  • Stainless steel cast, seamless pipe or tube manufacturing
  • Steel casting manufacturing
  • Steel die-casting manufacturing
  • Valve or valve parts, steam, gas or water manufacturing (ferrous metal)

Depending on the product end-use, various shapes are cast. In recent years, the melting/casting/rolling processes have been linked while casting a shape that substantially conforms to the finished product. The Near-Net-Shape cast section has most commonly been applied to Beams and Flat Rolled products, and results in a highly efficient operation. The complete process chain from liquid metal to finished rolling can be achieved within two hours (see http://www.steel.org/AM/Template.cfm?TEMPLATE=/CM/ContentDisplay.cfm&CONTENTID=22641).

Current technologies include ingot, thick slab, thin slab, billet and bloom casting. Future developments will lead to ultra-thick slab casting for thick plates, direct strip casting for sheets 0.03 to 0.15 inches thick, continuous casting products with fewer inclusions, rod casting, rapid prototyping of complex geometries via droplet consolidation or laser/wire technologies, rheocasting, and direct part fabricating via computer-controlled casting/milling machines (US DOE, 2001).

Starting a continuous casting machine involves placing a dummy bar (essentially a curved metal beam) up through the spray chamber to close off the base of the mould. Metal is poured into the mould and withdrawn with the dummy bar once it solidifies. It is extremely important that the metal supply afterwards be guaranteed to avoid unnecessary shutdowns and restarts, known as 'turnarounds'. Each time the caster stops and restarts, a new tundish is required, as any uncast metal in the tundish cannot be drained and instead freezes into a 'skull'. Avoiding turnarounds requires the meltshop, including ladle furnaces (if any) to keep tight control on the temperature of the metal, which can vary dramatically with alloying additions, slag cover and deslagging, and the preheating of the ladle before it accepts metal, among other parameters.

Status of the technology and its future market potential top

Continuous casting, introduced in the 1970s and 1980s, saves both energy and material, and now accounts for 88% of global steel production (IISI, 2005). The total potential energy saving in the iron and steel industry is 133 Mtoe, equivalent to 421 Mt CO2 on the basis of current production levels. These potentials are technical and the economic potentials are significantly below these levels as achieving these savings will require re-build or major refurbishments. In some regions with small-scale production and low-quality indigenous coal and iron ore, the reduction potential will be particularly difficult to achieve. China accounts for 55% of the potential energy saving, although a number of other countries have higher potential in terms of energy reductions per unit of steel produced. The average global potential is 4.1 GJ per tonne of crude steel, equivalent to 0.3 tCO2/tonne of steel produced (IEA, 2010).

There are several examples of successful direct casting for iron and steel. For instance, in December 2004, Deacero SA de CV awarded Danieli & C an order for a second melt shop and wire rod mill as a major expansion of its Celaya minimill in Mexico to double its present 1Mt/y capacity (see http://www.allbusiness.com/primary-metal-manufacturing/iron-steel-mills-ferroalloy/859509-1.html). Deacero was the first in the Americas to try Endless Welding Rolling and the billet-welding unit has been in operation at Celaya since mid 2000. This resulted in significant benefits in material yield and productivity (+2% and +4.5% respectively, in straight bars production). An EWR billet welding unit now also operates at the Deacero Saltillo wire rod mill starting since 2005. At present the Danieli EWR scorecard lists a total of 14 units supplied worldwide.

Improvements in materials flow management focus on the increased recovery of steel scrap, the development of new steel types and the design of new steel products. For example, more steel can be recovered from municipal solid waste through mechanical waste separation. For new steel types, significant developments will be needed in the design of alloys and testing procedures.
Crude steel production is estimated to increase from 1 351 Mt in 2007 to 2 408 Mt and 2 857 Mt in 2050 in the low- and high-demand cases respectively. In both cases, China will remain the main crude steel producer, accounting for about 30% of world production. India, other developing Asia, and Africa and Middle East will have the strongest growth rates, with the result that between 32% and 35% of all production in 2050 will be from those countries/regions (IEA, 2010). The main challenges for the further development of direct casting technology relate to the quality of the product and its usability by steel processors and users. Increased reliability, control and the adaptation of the technology to larger-scale production units will benefit its wider application. To date, productivity problems with direct casting at a large steel-maker have eliminated the expected efficiency gains.
Contribution of the technology to economic development (including energy market support) top

Direct casting technology for iron and steel leads to considerable savings of capital and energy. According to the IEA (2008), energy savings may amount to 1 GJ to 3 GJ per tonne of steel. Direct casting may also lead to indirect energy savings because of reduced material yield losses. Compared to a current, state-of-the-art casting and rolling facility, the specific energy savings of direct-cast technologies are estimated at about 90%. Estimates for the possible reduction of capital costs range from 30% to 60%. If the use of direct casting can be expanded, upstream emissions could be reduced by up to 100 Mt per year and costs could be reduced at the same time. Total energy savings will depend on the speed at which strip and near-net-shape casters enter the market.

illustration © climatetechwiki.org

Figure 2: Energy intensity in direct casting process (Source: Natural Resources Canada)

Compared to a current, state-of-the-art casting and rolling facility, the specific energy savings of direct-cast technologies are estimated at about 90%. Estimates for the possible reduction of capital costs range from 30% to 60%. If the use of direct casting can be expanded, upstream emissions could be reduced by up to 100 Mt per year and costs could be reduced at the same time. Total energy savings will depend on the speed at which strip and near-net-shape casters enter the market (IEA, 2010).

Contribution of the technology to protection of the environment top

The SO2 and VOC emissions are of particular concern in refining and casting operations. According to US DOE (2001), during ingot casting, particulate emissions are generated when molten steel is poured (teemed) into the molds. The major emissions, including iron and other oxides (FeO, Fe2O3, SiO2, CaO, and MgO), are controlled by collection devices. Operational changes in ingot casting, such as bottom pouring instead of top pouring, can reduce emissions. Bottom pouring exposes much less of the molten steel to the atmosphere than top pouring, thereby reducing the formation of particulate matter (Marsosudiro, 1994). Applied water rates for the contact systems are typically about 3,600 gallons/ton of cast product; discharge rates for the better controlled casters are less than 25 gallons/ton (EPA, 2000). The principal pollutants are total suspended solids, oil and grease, and low levels of particulate metals. As with vacuum degassing, chromium, copper, and selenium may be found in continuous casting wastewater. Wastewater treatment includes scale pits for mill scale recovery and oil removal, mixed- or single-media filtration, and high-rate recycle (US EPA, 1995).

Climate top

On a global scale, a full deployment of direct casting in the iron and steel sector can have a significant effect on the reduction of GHG emissions. More in detail, IEA (2008) estimates that the yearly reduction of CO2 (in Gt) can be 0-0.01, 0-0.03 and 0-0.1 for the periods 2008-2015, 2015-2030 and 2030-2050, respectively. 

The total potential energy saving in the iron and steel industry is 133 Mtoe, equivalent to 421 Mt CO2 on the basis of current production levels. These potentials are technical and the economic potentials are significantly below these levels as achieving these savings will require re-build or major refurbishments. In some regions with small-scale production and low-quality indigenous coal and iron ore, the reduction potential will be particularly difficult to achieve. China accounts for 55% of the potential energy saving, although a number of other countries have higher potential in terms of energy reductions per unit of steel produced. The average global potential is 4.1 GJ per tonne of crude steel, equivalent to 0.3 tCO2/ tonne of steel produced (IEA, 2010).

Financial requirements and costs top

According to the IEA (2008), the investment costs required for the direct casting method is estimated to 200 and 150-200 USD/t produced for the periods 2008-2015 and 2015-2050, respectively. Investment costs for a traditional continuous caster and hot-rolling mill are about 70 USD/t higher than for direct casting. In general, the total investment costs for the financing of business as usual solutions amount to between USD 2.0 trillion and USD 2.3 trillion between now and 2050 (IEA, 2010). 

References top

Energetics, 2003. Steel Industry Technology Roadmap - Barriers and pathways for yield improvements. Available at: http://www.energetics.com/resourcecenter/products/roadmaps/samples/Pages/steel-roadmap.aspx

IEA, 2008. Energy Technology Perspectives - Scenarios and Strategies to 2050. International Energy Agency, Paris, France.

IEA, 2010. Energy Technology Perspectives - Scenarios and Strategies to 2050. International Energy Agency, Paris, France.

IISI, 2005: World steel in figures, 2005: International Iron and Steel Institute (IISI), Brussels.

Marsosudiro, P.J., 1994. Pollution Prevention in the Integrated Iron and Steel Industry and its Potential Role in MACT Standards Development. TRC Environmental Corporation. Available at: http://www.p2pays.org/ref/15/14423.pdf

US Department of Energy, 2001. Steel Industry Technology Roadmap. AISI’s Strategic Planning for Research and Development committee. Available at: http://www1.eere.energy.gov/industry/steel/roadmap.html

US EPA, 1995. Preliminary Study of the Iron and Steel Category 40 CFR Part 420 Effluent Limitations Guidelines and Standards. Washington, D.C., USA. Available at: http://www.epa.gov/waterscience/guide/ironsteel/pstudy.html