Steelemart   Home  l  About Us  l  Advertise  l  FAQs  l  News  l  Contact Us  l  Sitemap  
carbon steelmaking via the minimill route
bulletHow Steel Is Made
bulletPortal Guide
bulletTrading Floor
Buy From Stocks
Reverse Auction
bulletCustomer Services
Trading Prices
Registration Process
Auction Schedule
SMS Update
How Steel Is Made : Carbon Steelmaking via the Minimill Route

Semifinished Steel

An ingot is a block of steel the size of a large refrigerator—or even the size of a car—-formed when the liquid steel is allowed to cool inside a mold. To process, a cast ingot is slowly reheated for 8-10 hours so that the temperature is equal throughout the steel, which is then rolled on a universal, or breakdown, mill into the various semifinished shapes of slab, billet, or bloom.The development and widespread use of continuous casting in more recent years has replaced the multiple steps of ingot casting, cooling, reheating, and rolling to a semifinished form. In a continuous casting operation, the molten steel is poured into a bottomless mold.  As the steel is withdrawn from the bottom of the mold, it forms a continuous strand of steel in various cross-sectional shapes depending on the shape of the mold.Going directly from hot metal to a semifinished form saves both time and energy. Also because of the waste inherent in each step, this process greatly increases the yield, or the percentage of the raw steel that can ultimately be sold or used. A traditional slab has a rectangular cross section typically 4-12 inches thick and 3-5 feet across, though some reach widths of 10½ feet. A slab looks like a very long mattress. Some producers cast “thin-slabs,” which are only two inches thick.When rolled, slabs are usually precursors to the so-called flat products, plate and sheet. The thinner the slab when cast, the less rolling (and expense) required to reduce the steel to the desired thinness. Capital costs for the thinner-slab equipment are also lower. Several producers are working on variations of “thin-strip” casting to go from molten metal in a caster to as near a final shape as possible,perhaps one that is only a fraction of an inch thick.A billet has a square cross section of typically 2-6 inches on a side. It is used to cast longer strands of steel, which are then formed into “long products,” such as bar, rod, wire, rails, structural beams, and seamless pipe. A bloom is essentially an oversized billet with a cross-sectional area greater than 36 square inches and is a more appropriate semifinished material for larger long products.Most domestic companies have more rolling capacity than hot metal or semifinished capacity.

When the market is strong, more finished product can be made if the producer buys semifinished material from various sources. In this way, semifinished slab and billet shapes have a market between producers rather than between a producer and end user. The rolling of steel aligns the grain structure within it, imparting strength and in-ternal consistency to the metal. For this reason, steel directly from a caster needs to be rolled to some degree for almost all applications. While continuously cast slab is the most common flat rolled production route today, ingot casting practice is still the only route for some thicker applications or applications that require several reduction steps to produce the targeted internal properties. The desire to cast ever thinner slabs or nearer to final net shape may actually not produce a broadly usable steel because the thin-slab, or thin-strip, cannot be rolled enough to impart the necessary grain structure within the steel. The trade-off, is cost, as the nearer-to-final-shape castings are cheaper to process—the rolling equipment is less costly and needs less power, and the material requires fewer rolling passes (cycle time).


Carbon Steelmaking via the Minimill Route

Minimills make steel from scrap steel or scrap substitutes in an electric arc fur-nace (EAF). Electric furnace steel, like integrated steel, is then cast into the semi-finished forms of slab, bloom, or billet. Because an integrated process takes more steps (i.e., coke batteries, blast furnaces), is more capital intensive, and traditionally requires more man-hours per ton, the minimills have used their cost advantage to take share away from the integrateds. However, as the price of the raw materials that go into an electric furnace, i.e. scrap, has increased, the cost ad-vantage of the minimills has diminished. The integrateds, too, have brought their costs down at the same time. Generally, if scrap is above $160-170 per ton, most producers would agree the minimill cost advantage disappears. An electric arc furnace is usually filled or charged at the top with scrap steel. A typical 200,000-300,000 ton per year electric furnace would be 25-30 feet in diameter.A large electric current is sent to the furnace via carbon graphite elec-trodes.Between the electrodes an electric arc forms, creating enough heat (3,500 degrees Fahrenheit) to melt the scrap. The electrodes, which look like thick, short telephone poles, are pure carbon. As the electric arc melts the scrap, the steel’s chemistry is periodically tested, and with the addition of iron or alloys into the mix, new steel is made with the desired specifications. As in the integrated process, a ladle met station might be employed after the EAF process for final chemistry trimming before the steel heads to a caster.


EAF and Integrated Differences

To some degree, the trend has been for EAF minimills and integrateds to become more alike. In raw materials, for example, traditionally the minimill uses 100% scrap as input into the furnace; integrateds can use 10-25% scrap in their proc-ess.As the price of scrap has risen with the growth of minimill furnaces, more scrap “substitutes” made from virgin iron ore have been developed and used by minimills. Examples of scrap substitutes are direct reduced iron (DRI), hot briquetted iron (HBI), and Iron Dynamics (IDI).In addition to increasingly similar sources for metallics, we are seeing more companies focus on the pairing of a particular level of quality with the lowest cost possible. Not all steel is alike, and not all costs are alike. Typically, better grades of steel products come from virgin iron ore and are rolled a great deal to fully develop internal quality and grain structure. Some steel products require a certain percent of reduction in thickness in order to gain the proper internal characteristics.In the market for midquality steels, the integrateds can offer perhaps more than enough quality but often at too high a cost; the minimills, in contrast, may have the right cost structure but not necessarily the right quality. By altering their equipment configurations, the companies can achieve a better pairing of cost and quality. More integrateds are considering thinner slab casting, and some minimills are adding thicker slab casting, using iron-ore-based substitutes or additional rolling stands in order to develop these middle-market products. These middle-market products satisfy certain product requirements that a traditional minimill (scrap based, thin-slab) may not be able to produce but at a lower cost than an integrated producer (iron ore based, thicker-slab).Considering that the integrateds control the top end of the quality spectrum and minimills have demonstrated they can easily and more cost effectively produce at the low end, the battleground of the future will be in these middle grades. The minimills will continue to attack the integrateds in the middle of the quality spec-trum with purer, more consistent metallic inputs from scrap substitutes. Scrap substitutes, therefore, are growing in popularity not only because of a potential increase in scrap prices, but also because they give the minimills the capability to produce steels of higher quality at potentially lower costs. Partly in response more integrateds will probably add electric arc furnace capability in the future. Additional EAF capacity will give the flexibility of blending different liquid steel chemistries and also set the integrateds up for any coke-oven or blast-furnace expirations in the future.Another benefit of scrap substitutes is assuring input cost. Rather than the cyclic-ity of scrap prices, a minimill producer can have a stable, high-quality input cost through a cycle. The consistency of an input cost allows the minimill to approach the longer-term contract market with consistent selling prices. Enabling contract market participation should also help the minimills in the battle for the middle market against integrateds, which already offer contract options to these customers.


Specialty Steelmaking

Specialty steels are defined by their alloy content, which change the physical qualities of steel. Stainless steel, for example , not only has carbon steel’s qualities of strength, durability, and malleability, but it is also stays corrosion resistant in many harsh environments, maintains its strength at high operating temperatures, and can provide an attractive, easily maintained surface appearance. Stainless steel technically means steel with a chromium content of at least 10%.Chromium makes the steel rust and stain resistant. Adding nickel to chromium stainless makes the steel easier to fabricate. Molybdenum can be added to make the steel harder. Stainless steel is typically produced by melting stainless steel scrap in an electric arc furnace and is therefore minimill based. Because the stainless steel market is smaller and grades are more specialized, steelmaking operations lend them-selves to smaller furnaces on average. In addition, stainless steelmaking is often more batch oriented than continuous compared with carbon steelmaking. Scrap stainless is used as a source for much of the needed alloy ingredients. Stainless steel scrap is relatively undesirable to a carbon steel producer, as it is quite expensive to remove the alloys once they are in the scrap. A specialty steel maker will add virgin alloys to the furnace to get the recipe close to the desired mixture. Molten stainless steels today are refined further after the electric furnace in order to perfect the alloy levels. The refining vessels are referred to as AODs (argon oxygen decarburization) and VODs (vacuum oxygen decarburization). The benefits of the decarburization process are primarily a wider choice of alloys, greater chemical process efficiency, and higher consistency and control. The use of a secondary refining vessel separate from the melting furnace also allows a cleaner, more economic use of the primary melt facility while still achieving the targeted metallurgical chemistry. Producers who make carbon steels and stainless in the same furnace use more alloy additions in the AOD or VOD in order to bring the stainless steel to specification. Producing stainless in this way usually means the producer melts only carbon scrap in the EAF at the end of a stainless production run in order to “rinse” remaining alloys from the furnace. Residual chrome content in carbon steel, for example, would be an undesirable result. Adding virgin alloys into the AOD or VOD refining vessel can be a slightly more expensive route to making stainless,both in terms of the cost of the ingredients and the time necessary for the refiningto occur. The variety of stainless steel grades produced is a result of the different combinations of chromium and other alloys. The ferritic grades of stainless, which account for approximately 32% of U.S. production, have chromium content between 10% and 27% and no nickel. The high chrome content provides maximum oxidation (rust) resistance at high temperatures. Ferritic steels are classified under product I.D. numbers in the 400s. The main ferritic grade is 409 stainless, which accounts for about 65% of U.S. ferritic production and about 21% of the total U.S.stainless steel market. The 409 stainless grade is used for highly polished trim applications and in places where environments and atmospheres are relatively mild. Examples of markets for 409 stainless are food processing equipment, flat-ware,and vehicle trim. The largest 409 market by far, however, is in auto exhaust systems, a market that has been an important growth area. AK Steel is the leading U.S. producer of the “auto chrome” grades with approximately 85% market share. Austenitic stainless, along with its minimum 10% chromium content, is 4-35% nickel. The austenitic grades, often referred to as the 300 series to denote their product I.D. numbers, account for two-thirds of domestic stainless production.They offer a wide variety of physical qualities (strength, ductility, weldability, etc.),which can be maintained during further processing. Because they contain nickel,the austenitic steels are more expensive to make than ferritics, but because of their attractive physical and mechanical properties, the austenitic grades are more widely used. A basic grade in the austenitics is 304 stainless, which has a chrome content of 18-20% and a nickel content of 8.0-10.5%. The 304 stainless grade is sometimes referred to as “18-8” stainless to indicate its chrome and nickel chemistry. Approximately 30% of total U.S. stainless production is of the 304 grades, and in its cold rolled form, 304 stainless is the bellwether stainless grade for investors.


Electrical Steels

Electrical steels have no chrome, and thus,technically speaking are specialty, rather than stainless steels. Grain-oriented electrical steel is treated in such a way as to align the atomic structure or grains in the steel, greatly increasing its ability to conduct electricity with less resistance and heat generation. These products, which sell for around $1,500 per ton, are used in transformers both at the power station (25% of the grain-oriented market) and distribution stages(50% of the market) of the electric utility grid. Demand for grain-oriented steel is therefore dependent on housing starts and utility capital spending. Non-oriented electrical steel is treated differently and is typically found in electric motors and appliances. The non-oriented electrical steels sell for about $1,000 per ton and are driven by general economic activity.


Special Melting and Remelting Processes

To improve the quality and/or purity of a steel, it can be remelted under a con-trolled atmosphere or environment that essentially acts to further remove impurities or imperfection in the steel. Electroslag remelting (ESR), and vacuum arcremelting (VAR) are two types of remelting operations, each of which imparts different characteristics to the remelted steel. Vacuum induction melting (VIM) is a special way to make particularly high-purity steels. Some steels will pass through more than one of these remelting steps; what is lost in time and expense is gained in a product that can perform as the customer desires. In the ESR process, a large, often cylindrical, block or ingot of steel is hung over a bath of slag. An electric arc is struck through the slag, which begins to melt the bottom of the steel ingot column. Since the steel is heavier, it drips through the slag as impurities are drawn into the slag. The slag acts to block air from affecting the refining process as the steel collects below the slag and is allowed to cool and solidify. In a VAR process, the remelting takes place in a vacuum, which keeps even more of the damaging air from the steel. VAR is useful in reducing hydrogen, nitrogen,and oxygen as well as other elements in the steel’s chemistry.

Vacuum induction melting (VIM) involves both steelmaking and casting in an enclosed vacuum tank. This process is relatively time-consuming (6-10 hours perheat) and is typically done in small batches. Because the steel stays under vacuumlonger, steel that has gone through the VIM process has an even lower gasand residuals content than steel processed through VAR.

back Back Next next

  Disclaimer  l  Privacy Policy