industrial chemistry for teachers
HAROLD E. McGANNON
Basic Oxygen Steelmaking
Unlted Stater Steel Corporation
P#ttrburgh,Pennrylvan8o
15230
T h e use of the basic oxygen process for m:rliing steel has shown a spectacular growth throughout the world for t,he past decade. At the begirming of INS, it was reported' that the capacit,y for producing stccl by this method in the United States was about 4%; m i l l i o ~tolls, ~ nnd it is estimated the c:rp:rcity will increusc to over S5 milliou toris by t,he start of 1972. Tot:d world capacity for producing steel by the basic oxygen process (including the United St,ates) is expected to grow from about 175 million tons a t the start of 19(X to over 330 million toris by the beginning of 1972. Total world production of raw steel by all steelmaliing processes in l9G7 was nenrly 5.26 million tons. United States production of raw stcel was somewhat over 1" million tons. The furnaccs uscd i n h:rsic oxygen steelmaking have a midc mnge ill cnpncity ; some produce "hcat,s" (hatches) of only :I fclr t o w a t :i time and others produce about 300 tolls. Thc trend is toward usc of thc Iargcr furnaces. The oxygen fnruacc is ahlc to produce n heat of stccl in Icss t,lian a:> hour, comprred with 7 10 hours in openhearth fur~~accs. Thc proccss wtrsists of b l o ~ ~ i nL: g high-velocit,y jet Of 11~:U'ly I)tLT(! (!)!).;)%) g:LSCOW OXygCII through a VCTtic:d, ~wtcr-coolctlI:UICC onto t i cIi:rrgc of molten pig im: :md solid ir011 :~nd steel scr:rp, co~itained in :I rcfractory-lit~rdb:rrrcl-shapcd vcssel that is o p o ~ a t t,hc top. I3urnt lime is :rddcd :rs :i shg-mnking material. Thc frvn:lcc c11:rrgc contains cxccss amounts of carbon, m:~ng:rnesc, silicon, sulfur, and phosphorus that, must bc rcmovcd to the extcnt uccessnry tto convert the ch:rrgc to stcel hnvit~gthe desired composition. These un~v:rtrtede l c m c ~ ~nrc t s removed by oxidation (except sulfur). Carbon in the metal is oxidized to carbon monoxide and lenvcs the system as a gas. The compounds folmed hy ositlatiol~of manganese, silico~~, :md phospliorns ill the nict;rl are taliell up by the slag. Some i n ~ :dso t ~ is oxidized, and the oxidc (considered to he FcOj enters thr slag. At high tcmpcratures nud with suficic.nt rvccss lime (CnO) i n thcsl:rg, sulfur in thc mctd rencts xith some of thc lime to form C:rS that, :dso hrcnmes p:rrt of thc slag. The hasic oxygen process may be considered to be a I
Kniw
l'~.,rew Newrl~~tlel~, Kniser Engitloor.; 1)ivisiotl uf Coq)., I!lM (NO. 4r)).
I ~ d ~ > s l t ~ i ~ sOc:d1er2l,
three-phase system cor~sist,ingof gaseous oxygen, slag, and metal. I n their simplest form, t,he most important chemical react,ions in thc proccss and their equilibrium constants can bc written as
+0
=
FeO (in slag)
We01 KO = -
+0
=
CO (gas)
KO = -. PC" ICI [Ol
Si + 2 0
=
Si02 (in slag)
(SiOl) Ksi = --
Fe C
101
[Si] [012
I t may be noted in col~nectio~i with these formulas t,hat enclosing the symbol for an element or compound ill square bracliet,~indicates that it is present in the metal bath, while enclosure in parentheses signifies t,hat it is present in the slag. The chemical react.ions given above occur in all basic steelmaking processes, including the basic open-hearth and basic electric-furnace processes, and the equilibria relationships indicated above are equally valid. However, the liinetics are considerably different. Gaseous oxygeli has heen used for some time to speed up reactions in basic opcn-hearth arid elect,ric furnaces by injecting it into the molten metal through a pipe extending through the slag and int,o the metal bath. Gaseous oxygcn is also uscd in basic open-hearth furnaces by injecting it a t high velocity onto the bath t.hrough water-cooled lances that extend vertically through the roof of t,he furnace to a short distance above the bath level. All of these uses of gaseous oxygcn speed up the steelmaking processes by hastening the elimination of carbon, agitating tbc bat,h, and promoting attainment of high temperatures. When gaseous oxygen is not used in st,eelmaliing processes, the molten baths are relatively quiescent, and oxidation takes place by relatively slow transfer of oxygen (from added iron oxidcs and the furnace atmosphere) from the slag to the metal. However, in the basic oxygen process, the gaseous oxygen is injected Volume
46, Number 5, May 1969 / 293
directly into the metal and causes the reactants to be brought together rapidly and intimately and extreme turbulence of the slag and metal results. The direct iujection of oxygen aud the turbulent mixing of slag aud metal in the baqic oxygen process greatly increases the vate of the reactions, but this increase in reaction rates can in no way influence the equilibrium values for the reactions. Thus, a knowledge of equilibria makes it possible to predict how changes iu couceutratiou of reactauts cau affect the process, and provides a basis for its understanding and control. As the bath is purified by oxidation of unwanted elements, its melting point becomes higher. More than enough heat is supplied by the oxidation processes to increae the temperature of the bath and keep it molten. A large part of the additional heat is used to heat and melt the solid scrap that was charged into the furnace with the molten pig iron. After blowing is completed, thc steel must be hot enough to melt any solid addition agents (deoxidizers and alloying elements) and also must uot be cooled too much to permit a satisfactory pouring into ingot molds or use in a continuous casting machine. Control of carbon content of the fiuished steel is the
294
/
Journal of Chemical Education
primary purpose in steelmaking, since many of the useful properties of a given steel are related to how much carbon i t contains. Due to the rapidity of the process, sampling of the steel and measurement of its temperature during the operation are difficult. Special temperature-measuring devices are employed, along with rapid methods for determining carbon content and the over-all chemical composition a t the end of the process before the steel is poured out of the furnace into a ladle. Spectrographic analysis is the preferred method for analyzing the finished steel. Again due to the speed of the process, a computer is used to calculate in advance the amounts of molten pig iron, scrap, lime, and other raw materials that are to be charged into the furnace for each "heat" so as to arrive a t the intended composition and temperature of the steel. Based on the results of the chemical analysis of the steel in the furnace after blowing, the computer also determines the quantities of deoxidizers and alloying elements required to beadded to the steel as it is poured into the ladle to give it the exact chemical composition required for the grade of steel being made.
