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A0005 Steel Making
by Terry Smith
In this article I will be discussing primary steel making. Primary steel making is done using molten iron from a blast furnace as the main ingredient and a lesser amount of scrap steel and is made in a Basic Oxygen Furnace.
Secondary steel making is generally referred to the process of making steel by melting scrap steel in an electric arc furnace and is the method used by most Mini-mills as the smaller steel plants are referred too.
In primary steel making there is a great deal of control over the finished product of the steel and they can produce almost any type of alloy that the customer may require. The secondary steel making facilities use primarily scrap steel and do not generally produce a large range of alloy steel with the exception of specialized steel making operations. A large amount of the secondary steel production is slated to everyday use products such as re-bar and general purpose carbon steel products. Their carbon content is not as strictly controlled and they may contain traces of other alloys that were present in the scrap used to produce the steel.
Primary steel making, like iron making, has evolved over a number of years from many different methods. It has evolved from Wootz (300 BC), Blister (16th Century), Crucible (refined Blister steel around 1740), Bessemer and open hearth(1858) to Basic oxygen (1950’s) which is the most used process today in world wide primary steel making. The time taken to produce a batch of steel has gone from several days (Wootz) to a mere 40 minutes (Basic Oxygen).
A Basic Oxygen Furnace (BOF) is a large pear shaped furnace that is operated in the upright position with the smaller top portion up. The furnace can be rotated or tipped in either direction to be charged with metal and iron or to pour the finished steel into a ladle. They are called Basic due to the PH of the refractory brick used to line the furnace walls. This refractory is made with calcium oxide and magnesium oxide in order to withstand the high temperatures of the molten metal (up to 5,000 deg F). Furnace capacities can range up to 350 tons of steel per heat. When the furnace is ready to be charged it is tipped towards the floor where the melters control room and workers are and a large overhead crane delivers a charge of scrap in a special bucket to be dumped into the furnace. About 1/5 of the capacity of the furnace (by weight) is charged as scrap. After the scrap has been charged the crane picks up a large ladle of molten iron (which has been filled from the torpedo car from the blast furnace) and pours that into the furnace on top of the scrap. It is vital that the amount of scrap added to the furnace in comparison to the molten iron does not reduce the temp of the molten iron too much or the steel cannot be processed, and is therefore is added in a very controlled process. After the furnace has been charged a large water cooled lance with a thick copper nozzle with three openings in it is lowered down to a level just above the molten iron. Pure oxygen (99%) is injected through the copper nozzle at a speed near MACH 1!
Note: All primary steel making facilities have a plant on-site that produces the pure oxygen required in the steel making process. It is produced twenty four hours a day seven days a week and the oxygen is stored in liquid form in large capacity spheres in order to meet the demands of the steel plant. They also supply argon, and nitrogen for other processes in the plant as well.
When the oxygen is introduced into the furnace it reacts with the carbon in the iron and produces carbon monoxide reducing the carbon content in the iron significantly. Burnt lime (calcium oxide) is also added to the furnace which reacts with the sulfur and other impurities in the iron, such as alumina and silicon to form a slag. A typical charge of iron used in the furnace has a chemistry of 4% carbon ©,0.2 – 0.8% silicon (Si),0.55 -0.75 manganese (Mn), 0.03 -0.09% Phosphorous (P) and 0.025 – 0.05% sulfur (S). There are usually traces of other elements but they are of a low enough level to not affect the steel. After about twenty minutes of blowing the furnace, the lance is withdrawn and the furnace is tilted towards the floor. The temperature of the steel is measured and a sample of steel is drawn and sent to lab for analyses. It only takes the lab about 5 to 6 min. to analyze the sample for chemistry and the results are sent to the melter. If the carbon content is still too high or the other impurities are still too high the furnace is raised and the lance is lowered to continue to blow in oxygen and more lime may be added. The lab will direct the melter as to how much more time the heat needs to be blown or lime is to be added. Again the lance is withdrawn and the furnace tipped for a sample. The reactions of the pure oxygen and carbon also raise the temp. of the steel considerably. It is necessary to have the steel at a high enough temp to properly melt the alloys that are added to the ladle when the steel is poured as some of these alloys melt at a temperature higher than the melting point of steel. When the lab gives its approval for the heat of steel the furnace is tipped in the opposite direction and the steel is poured into a ladle. As the slag that was formed during the steel making process is lighter it floats on top of the steel. The tapping side of the furnace has a type of spout on the edge and the steel flows out through here into the ladle, while the lighter slag floats farther towards the back of the furnace, which is higher than the top end. After the ladle is filled the furnace is rotated back in the opposite direction until the slag runs out into a slag pot to be taken away for disposal. A typical heat of steel, before anything is added, has a chemistry of; 0.3 – 0.6% C, 0.05 – 0.1% Mn, 0.01 – 0.03% Si, 0.01 – 0.03% S and P.
As the steel is poured into the ladle, carbon and other alloys may be added to make the proper grade of steel to meet customer requirements. All the alloys are added by weight in relation to the weight of the particular heat of steel. The carbon is usually in paper bags of pre-determined weight and all the other alloys such as chrome, nickel, tungsten, vanadium and such are all in refined form and stored in hoppers above the steel making floor. At Algoma they were dropped in precisely measured amounts down into a special type of wheelbarrow through chutes. The helpers would then wheel them over to an opening in the floor and dump them down another chute that led into the ladle. The carbon and other alloys where dumped in just after the steel was starting to pour into the ladle so that they would melt and be mixed as the steel was continued to be poured. The whole steel making process took only about forty to sixty minutes to produce a ladle full of steel ready for casting. Although plain carbon steels where often taken to the castor and cast as is, the alloy steels are usually sent to a secondary metallurgy station for more refinement and careful adjustment of the proper alloy before casting. There is considerable oxygen and hydrogen and other minor impurities still dissolved in the steel and these are removed in the secondary process.
In the secondary processing the ladles of steel are covered and a vacumn is created to help draw hydrogen from the steel. Small amounts of aluminum and silicon are added to take up the oxygen in the steel and to help remove some of the other impurities in the steel. These form another slag coating on top of the steel which is then removed prior to going to the castor. The chemistry is checked and small additions of some alloys may be added to bring them up to the required levels in the steel. During these processes, a small lance is inserted in the steel and Argon is pumped in to stir the steel and help mix all the alloys evenly throughout the steel and help remove the hydrogen and oxygen.
Next article in this series will be on continuous casting. Modern steel making uses continuous casting methods for producing the slabs, billets and rounds of steel for their rolling mills. At the time I worked at Algoma (now Essar) they still poured steel into ingot molds. They then re-heated the ingots in soaking pits and rolled them into billets or slabs for the other finishing mills. These two steps have been virtually eliminated with the newer continuous casting process resulting in considerable cost savings in the production of the finished product. They did however have a continuous caster that produced rounds and billets of steel as well, and I am familiar with the process involved.