Steelmaking is the process for producing steel from iron and ferrous scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, and excess carbon are removed from the raw iron, and alloying elements such as manganese, nickel, chromium and vanadium are added to produce different grades of steel. Limiting dissolved gases such as nitrogen and oxygen, and impurities (termed “inclusions”) in the steel is also important to ensure the quality of the products cast from the liquid steel. There are two major processes for making steel, namely basic oxygen steelmaking which has liquid pig-iron from the blast furnace and scrap steel as the main feed materials, and electric arc furnace (EAF) steelmaking which uses scrap steel or direct reduced iron (DRI) as the main feed materials. Oxygen steelmaking is fuelled predominantly by the exothermic nature of the reactions inside the vessel where as in EAF steelmaking, electrical energy is used to melt the solid scrap and/or DRI materials. In recent times, EAF steelmaking technology has evolved closer to oxygen steelmaking as more chemical energy is introduced into the process.
Blast Furnace / Basic Oxygen Furnace
Coke, limestone flux, and iron ore (iron oxide) are charged into the top of the blast furnace, allowing the carbon to precisely bond with the iron to create high-carbon, liquid pig-iron. The largest blast furnaces have a volume around 5,580 m3 (190,000 cu. ft.) and can produce around 80,000 MT of steel per week
The basic oxygen furnace (BOP or BOF) is the process in which a mixture of scrap (25-30%) and hot pig-iron (70-75%) from the blast furnace is exposed to oxygen blown at a high velocity. Such furnaces may be top-blown, bottom-blown, or a combination of the two. The basic principle is the same in all cases; the oxygen combines with unwanted elements to form oxides that either leave the bath as gases or a slag layer on top of the bath. This is a very high productivity process; a typical 220-ton heat can be produced in 45 minutes or less.
Another steelmaking process widely used today is the electric furnace. The furnace charge is generally 100% scrap, and an electric arc is used as the heat source for the refining process. The electric furnace is generally not used for deep drawing steel or very light gauge steel because of its inability to reduce the residual element levels typically found in ferrous scrap to the low levels required. However, where carefully selected, high-quality scrap is available, steel produced in the electric furnace can be quite satisfactory for many applications.
Casting, Hot Rolling & Pickling
Once the heat of steel meets desired chemical and temperature requirements, the steelmaking furnace is tapped, and the molten steel is poured into a ladle. The steel may be given additional treatments in a separate vessel, such as a degasser, to fine tune its properties, following which it is cast.
The continuous casting process transforms the molten steel in the ladle into a slab in one continuous operation. Molten steel is poured from the ladle into a tundish, or a holding vessel, at the top of the continuous caster. The steel is metered from the tundish into a water-cooled open-ended mold for initial solidification, from which it is withdrawn, supported by a massive set of rolls and guiding equipment. As the steel slowly moves through the continuous caster, it is transformed from a molten state into solid slab form. The transformation from molten steel into slab is measured in minutes, not days. This process makes a slab with excellent quality attributes and is highly energy and cost efficient.
Hot Strip Mill
The slabs, while still hot from the caster or the slabbing mill or after reheating, are reduced in thickness and greatly elongated by rolling through the series of roughing and finishing stands comprising the hot strip mill. The strip passes through a series of stands, each consisting of four or six rolls in a vertical stack. The two rolls touching the strip are called work rolls, and the larger rolls applying uniform pressure to the work rolls are called back-up rolls. The stands are synchronized to compensate for ever-increasing speeds as the material is elongated and reduced in gauge. In the manufacture of tinmill products or other light gage products, a slab enters the hot strip mill 6 to 9 inches thick and is reduced to a hot rolled band about 0.07 to 0.10 inch thick, now in coil form. Careful control of the finishing temperature at the last rolling stand and the coiling temperature is necessary to achieve optimum properties in the final product.
Pickling & Oiling
Next, the hot rolled band must be cleaned of oxides formed during the hot rolling process. This process, referred to as a pickling, passes the hot rolled band through a series of tanks filled with diluted hydrochloric or sulfuric acid. After the oxides are removed, the pickling unit then rinses the strip, dries the strip, oils the strip for both rust protection and to add lubricants for further processing, and recoils the strip, all in a continuous operation.
Cold Rolling & Annealing
The hot rolled pickled and oiled band must now be further reduced into lighter gauges. This is performed on a tandem cold reduction mill. These mills consist of a series of 5 or 6 stands with rolls 4- or 6-high set in tandem. As the strip passes through the mill, it is continuously reduced. The hot rolled band charged into the cold mill can be reduced to nominal gauges as light as 0.006 inches, for an approximate 80% reduction. The product exiting the cold mill is properly termed “full hard.” Material in this state is too hard or stiff for forming and must be annealed (that is, softened) to obtain the desired strength and formability.
Box & Continuous Annealing
The full hard coil must be cleaned of all rolling and coolant solutions used on the cold reduction mill before it is annealed or softened. This cleaning is performed electrolytically as the strip is passed through an alkaline bath. Cleaning is performed either on a separate unit for the product to be box annealed or as part of a continuous annealing line.
Box annealing (or batch annealing), for softer tempers, is accomplished by placing cleaned coils in a box anneal furnace. The coils are stacked on bases, covered by the furnace, sealed, then exposed to a carefully controlled temperature cycle and atmospheric environment. The coils are slowly heated to a specific temperature, sustained at that temperature for a predetermined time, then cooled. Both the temperature and duration of the cycle are critical determinants of the final properties of the coil.
In a continuous annealing line, a full hard coil is unwound and fed continuously through a unit that cleans the strip, heats to a designated temperature, maintains that temperature for a specified period of time, then cools the strip and recoils. This cycle is measured in minutes, versus days for the batch anneal process. Because of the short heating cycle, the process is best suited for stiffer, higher strength specifications. Continuously annealed steel exhibits very uniform properties throughout the coil.
Single reduced product is processed on a 2-stand, 4-high temper mill that improves flatness and shape, imparts the final desired stiffness (or temper), and imparts the desired surface texture or finish to the material.
More specifically, in the temper mill the final gage is slightly reduced no more than 1.5% of its original thickness. This minimal reduction ensures that shape and mechanical properties, namely hardness, are within specification, and it eliminates discontinuous yielding, or fluting, during subsequent forming. Additionally, the finish or surface texture of the steel can be varied from very smooth to very rough by using rolls of varying roughness on the temper mill. The finish of the material is specified by the customer and will vary by the end use of the product and the manufacturing processes of the end user.
Double Cold Reduction Mill
For tinmill gauges usually below 0.0077 inches, a three-step production procedure is utilized, which involves cold reduction, annealing, and a second cold reduction to final gauge. Such products could not be made on a conventional 5- or 6-stand tandem mill because of the excessively high reductions that would be required from conventional hot-band gauges. Double-reduction mills are typically 2- or 3-stand, 4-high mills, with the majority of the reduction taken in the first stand. Either box annealing or continuous annealing may be used, with the latter giving slightly higher strength to the finished product. The amount of the second reduction will vary from about 15 to 35 percent of the original thickness, depending on the final properties desired. These steels exhibit high strengths, but have adequate formability for canmaking.
The rolls used in hot- and cold-rolling mills have to withstand the high loads, temperatures, and deformations involved in rolling, while imparting a carefully controlled shape and surface finish to the strip. After conventional machining to near-final dimensions, rolls (which may be cast iron, cast steel, or forged steel depending on the application) are finished by precision grinding in a computer- controlled roll grinder. The grinding operation provides the proper shape to the roll barrel and the desired texture (roughness) to its surface. For rolls with rougher surfaces than can be obtained by grinding, shot blasting with cast iron or steel shot is employed as a final finishing operation.
Electrolytic Tin Plating
Black plate (basically a clean, light gauge cold rolled steel) destined for coating goes directly from either the temper mill or double cold reduction mill to the coating line. The sequence of operations that occurs in a tinplating line, at speeds up to 2000 feet per minute, are the following:
- Charging the Coil: The black plate coil is placed on an uncoiling mandrel and fed into the unit. The charged coil is then welded to the preceding coil in order to run the facility continuously. Looping towers accumulate a length of material that is varied to enable the welding of two coils while not stopping the operation.
- Side Trimming: The material is side trimmed to the final width. The scrap is automatically baled, removed, and recycled to the steelmaking furnaces.
- Clean & Pickle: The strip is next electrolytically cleaned, pickled, and rinsed by running through a series of tanks to ensure the surface is clean and suitable to accept the electro-deposition of tin.
- Electroplating: The strip passes through tanks containing tin anodes on either side of the strip and tin electrolyte. Tin dissolves from the anode and is transported through the electrolyte to deposit on the strip. The quantity of tin that deposits on the strip is determined by the quantity of electric current flowing between the anode and the strip. To produce differential coatings, different current settings are used for each of the anode banks. The dull as-plated tinplate is sometimes referred to as having a matte finish.
- Melting Tower (Tin Reflow): The electrolytic tinplate next passes through a melting tower. In this section, the temperature of the strip is raised by resistance or induction heating to just above the melting point of tin, then is immediately quenched in cold water. The tin begins to melt and reflows uniformly across the strip. The product now takes on the more typical bright or shiny surface appearance. Should a customer require a matte or unmelted tin finish, the melting tower can be turned off. When differentially coated tinplate is being produced, an identifying mark may be placed on either side just prior to melting.
- Chemical Treatment: A sodium dichromate solution is next applied to the electrolytic tinplate to create a light protective chromium oxide film. This passivation process protects the surface from the formation of tin oxides, which will decrease lacquerability. Two commonly used chemical treatments for electrolytic tinplate that have been adopted by the industry are:
- Sodium Dichromate Dip (SDCD): Has a moderate resistance to tin oxide formation with limited storage stability and is used where a highly passivated tin surface is not required or is detrimental to the end use. Sodium Dichromate Dip has an aim not to exceed 0.15 mg/sq. ft. of chromium on the surface.
- Cathodic Sodium Dichromate (CDC): With the addition of a cathodic electric current, a highly passivated surface against the formation of tin oxide is provided. Cathodic Sodium Dichromate treatment normally has an aim of 0.5 mg/sq. ft. of chromium on the surface.
- Oiling: A very thin film of oil is then electrostatically applied to the finished tinplate. The oil is applied to minimize scratching the tin surface in transit and handling. The primary function of oiling is not prohibiting rust formation as it is with uncoated products. The oil applied to electrolytic tinplate, almost exclusively, is acetyl tributyl citrate or ATBC. The oil is applied uniformly to both sides.
- Recoiling: The finished tinplate product is finally recoiled at the exit end of the electrolytic tin line on a take up reel that most commonly has a 16.5 inch inside diameter (ID).
- Quality Inspection: The entire process is monitored automatically and manually to ensure that the material conforms to specification and meets customer expectations.
Electrolytic Tin Coating
Electrolytic tinplate has a maximum thickness of 135 lb. base weight or 0.0149 inch nominal thickness. Electrolytic tin coating starts at 0.0150 inch nominal thickness and is produced up to 0.0359 inch nominal thickness.
Electrolytic Chromium Coated Steel
Electrolytic chromium coated steel, developed for food packaging in the 1960’s, offer superior lacquer adhesion and good storage properties. Unlike tinplate with its multiplicity of coating weights, only a single standardized chromium metal and chromium oxide coated product is manufactured.
This product follows the same processing sequence as electrolytic tinplate, but during the electrolytic deposition process, chromium metal and chromium oxide are deposited instead of tin. Metallic chromium coating of 5.0 mg/sq. ft. is applied on each surface. The outer oxide film ranges from 0.7 to 2.0 mg/sq. ft., but is generally on the lower side of this range.
Unlike tin, the chromium layers cannot be reflowed, therefore a coating line dedicated to chromium coating will not have melting towers as used on the tin line to reflow the tin into a bright state.
The product is also electrostatically oiled before it exits the coating line. Historically, the industry used butyl stearate oil (BSO), which was developed for its lubricity to prevent scratching. In some instances, it has been determined that acetyl tributyl citrate (ATBC) oil used on electrolytic tinplate is more compatible with some specific lacquering and paint systems. In this case, ATBC has been preferred at the expense of the greater lubricity of the BSO.