Gas Removals Obtained by D-H Vacuum Degassing

Transcription

1 Gas Removals Obtained by D-H Vacuum Degassing TIIE APPLICA~I'IUN of vacuun~ degassing in the treatment of liquid steel has gained wide recognition because of the improvements in quality that have been obtained by lowering the oxygen and hydrogen contents. The Dortmund-Horder process installed at,atlas is vacuum treating 10-ton, 30-ton, and 50-ton heats. To date, approximately 1250 heats have been vacuum degassed. These heats have not only been teemed into various ingot sizes from 7 in. to 52 in., but also a large number have been continuously cast..i wide variety of grades have been treated successfully as follo~vs: carbon steels from 0.03 to 1.00 pct carbon; SAE alloy st,eels; alloy tool steels; high-speed tool steels; valve steels; stainless 300, 400, and. 500 series; perma~~oys... Operation The D-H process consists of treating metal in a ladle by exposing it to an evacuated refractory-lined vessel. The heat is tapped from an electric-arc furnace into a conventional ladle and placed in a transfer car. The ladle is positioned below the vessel and 'an..... * Superirisor i\.ielting, Rletallul.gy. and Inspection, Atlas Steels Company, Welland, Ont..' Cai!,nda...., I I I

2 112 Proceedings of '~lectric Furnace Conference, 1963 immersion thern~ocoul~le is used to determine the steel temperature. The nozzle, covered with a slag breaker, is inserted into the liquid steel by raising the ladle hydraulically (Fig 1). The vessel is evacuated by means of a four-stage steam ejector system and the pressure difference between the metal at the surface in the ladle and in the vessel produces a colrrmn of steel 57 in. high, forming a pool covering the bottom of the vessel. The exposure of the metal to the vacuum in the initial stages produces a violent boiling action in the vessel. This reaction is controlled by using only one stage of the steam ejector system initially and then turning on subsequent stages as the boil subsides. Steel is recirculateti by lowering the ladle, holding for a fixed period of time and then LOWER POSITION UPPER POSITION Fig 1-The Dortmund-Horder Process. continuing to cycle the ladle until the pressure in the vessel is below 1 mm. Alloy additions of carbon, manganese, silicon, chromium, alu~ninum, titanium, and boron are made to the heat by means of four addition hoppers attached to the vessel. A liuinber of [nixing cycles are used to ensure that the composit,ion is unifor~n. The treatment is terminated by flooding the vessel with nitrogen returning the steel to the ladle. The flexibility of the D-H process permits heats to be treated that are fully killed, semikilled, or open. The majority of heats are semikilled to opell, and made with a two-slag practice. At slagoff, the heat is blocked with only enough silicon to prevent the heat from opening up before tap. A refining slag of lime and bauxite is used to lower the sulphur content. Bauxite is used in preference to fluorspar as a flux because it has proved to be less erosive on ladle and stopper-rod sleeve refractories. When the clesirecl tapping tenq,erature has been reached ant1 the alloy additions calcul:tted, the heat is tapped. All of the oxidizable elements are added in the vessel to prevent the formation

3 Developments in Electric Furnace Steelmaking 113 of nonmetallic inclusions. Additions of 1111 to 3 pct of the heat weight have been made without an)- difficulty. The temperature losses during the treatment are largely dependent on the size of heat being degassed and the amount of icrroalloys added in the vessel; 10-ton heats in general lose twice as many degrees per cycle as a 50-ton heat. Hc:~ts are tapped from 60" to 150 F, hotter than normal to compensate for temperature losses during degassing and the addition of ferroalloys. Excessive temperature losses are minimized by the use of a resistance heating rod to maintain the vessel at 2800 F. Oxygen The deoxidation of steel by vacuum can be described by the reaction of carbon and oxygen dissolved in the metal to form carbon monoxide gas by the reaction As PC, is reduced, the concentration of osygen is lowered correspo~idingly. Hence oxygen contents lower than the equilibrium values at one atmosphere are obtained by exposing the metal to the reduced pressure. The residual oxygen content of the steel in the ladle mill be determined by the effective carbon monoxide pressure in the vessel. Experiments on open heats by Jackson and Hyamsl have indicated that this pressure is in the order of 100 mm and results at Atlas confirm this value. Vacuum deoxidation is preferred to that obtained by conventional deoxidizers because the oxygen is removed from the system as carbon monoxide gas. Thus, by lowering the oxygen level before additions of deoxidizers are made, such as silicon and aluminum. the frequency and severity of the inclusions formed by thesc elements are minimized. Improvements in cleanliness have been notcd,2 many grades reaching oxygen levels lower than those obtainable with conventional electric furnace ~nclting practices. Vacuum deoxidation, by providing a consistent low oxygen level has reduced the spread of the frequency and severity of indications when measured by the magnaflux test to specifications such as AMS The production of consistently clean heats of SAE for ball-bearing applications has been a continual problem to the electric furnace steelmaker. The detrimental effects of these nonmetallic inclusions on the fatigue life of bearings under very high stress conditions has been described by Hubbell and Pear~on.~ The J-K inclusion limits required for ball-bearing steel quality are shown in Table 1. The improvements in cleanliness obtained by the vacuum deoxidation of SAE heats for bearing applications are illustrated in Fig 2. Table I-J-K Limits far Bearing Quality SAE Inclusion Type Sulfide Aluminate Silicate I Oxide Code... A D Width microns Maxlengt... References are on page 117.

4 114 Proceedings of Electric Furnace Conference, 1963 THIN HEAVY Fig 2-Alumina inclusions in SAE DEGASSED NON DEGASSED CLEANLINESS INDEX CLEANLINESS INDEX Fig 3-Comparison of the cleanliness of degassed and non-degassed Standard 301 Strip.

5 Developments in Electric Furnace Steelmaking 115 It is evident that D-H vacuum degassing increases the frequency of acceptability compared with the results obtained with the conventional electric furnace air melting practice. This is especially true of B-type or alumina inclusions. The use of vacuum deoxidation in the production of SAE has increased the yield from an ingot, because of the reduction in the amount of top and bottom discards required, as compared with that necessary with nondegassed heats to meet the J-K limits. A system was devised to evaluate the cleanliness of stainless strip by rating the frequency and severity of the inclusion lines to prepared standard samples. Each quarter of a coil was rated and the average cleanliness rating was determined for a coil and a heat. This rating system would provide a heat cleanliness index between 0 and 8; the higher the index, the poorer the cleanliness. A series of standard 301 heats, degassed and nondegassed, mere rated to the prepared standard. A large number of heats were rated in each category to prevent the influence of variations in furnace practice. The distribution of the cleanliness index for both categories, degassed and nondegassed, is shown in Fig 3, degassed heats indicating an improvement in cleanliness. Microexamination has revealed that the incidence of heavy inclusions is much more infrequent in degassed heats. Ovygen contents on the average havc heen lo~ver in the degassed stainless heats. Hydrogen The presencc of an evcessive amount of hydrogen in steel is detrimental for the follo~ - ing reasons: (I) the formation of flakes after hot working; (2) the presence of subsurface pinholes in continuously cast billets or slabs. The control of flaking has been possible by vacuum-treating hcltts that are sensitive to the formation of this thermal defect. Not only has flaking not occurred but also the long flake-prevention thermal treatments have been unnecessary. The disposition of hot-worked steel that has I~een vacuum degassed is essentially determined by the minimum cooling rate required to prevent strain cracking. Hydrogen in continuous cast steels has been shown to produce the following ~~roblerns~: 1. Surface and subsurface pinholes. 2. Center ~~orosity. 3. Internal rupturing. 4. Breakouts. The rapid initial solidification in the mold during continuous casting does not permit hydrogen to diffuse into the liquid, producing subsurface pinholes. These defccts, because they are subsurface, are difficult to locate and expensive conditioning operations are necessary to remove them. The utilization of D-H vacuum dcgassing has prevented inho holing from continuous cast slabs. Fig 4 indicates the lowering of the hydrogen level of degassed stainless heats as compared with nontreated heats. The hydrogen samples were taken from the mold during continuous casting. Hydrogen levels are higher on continuous cast heats than on ingot heats, because of the pickup that occurs by the exposure of the steel to the atmosphere when teeming through a tundish. The problem of hydrogen pinholing with continuously cast heats or wild ingot heats is mainly confined to periods of high humidity. Hydrogen control has been established by determining the hydrogen content of the bath approximately one hour before tap. If this result is higher than desired, the tap temperature is adjusted and the heat is degassed.

6 11 6 Proceedings of Electric Furnace Conference, 1963 The level of hydrogen achieved by D-H vacuum degassing is determined by the following factors: 1. The initial oxygen content. 2. The initial hydrogen content. 3. Thc number of cycles or recirculations. On heats having the same carbon content, lorrer hydrogen values have been obtained with a higher starting oxygen level; that is, lower hydrogen results in open heats as compared with those that are silicon killed. A survey of ct plain-carbon steel heats indicates that the average hydrogen in the hot top of D-H-vacuum-degassed open heats is 1.7 ppm, whereas treated killed heats have an average hydrogen content of 2.2 ppm. HYDROGEN IN THE CONTINTUOUS CASTING MOULO (PPM) Fig 4-Hydrogen in Stainless 301, 302, and 304. In both, approximately the same number of cycles was used. The lower hydrogen level achieved on open heats is undoubtedly due to the lower initial hydrogen level and thc flushing action of the carbon boil provided by the higher oxygen content. The number of cycles or recirculations is a factor when hydrogen or oxygen removal is in qnestion. The gas removal, however, is higher during the initial recirculations and levels off toward the end of the treatment. Hence excessively long treatments are not necessltry to reach a hydrogen level below 2 pprn on flake-sensitive grades. Nitrogen The residual nitrogen level has not been altered by D-H vacuum degassing. This level is essentially determined by the melting practice and the composition of the steel. In cases where nitrogen has been added in excess of the normal residual level, approximately 30 pct has been removed during a treatment.

7 Developments in Electric Furnace Steelmaking 117 Conclusions 1. The D-H vacuum-degassing unit has improved the quality of various grades of steel by removing harmful amounts of hydrogen and oxygen. 2. Vacuum deosidation has provided a,higher level of cleanliness than that obt:~ine:l I)y using conventional deosidizers in the electric furnace. 3. Vacuum degassing has been successful in removing excess hydroge:~ and there!,^ preventing flaking in forgings and pinholing in continuously cast sections. References 1. Jackson, P. L., and W. M. Hyams: D-H Carbon-Oxygen Reaction. AISI Pittsburgli ltegional Technical hieeting. October Kollmann. W. C., and C. D. Preusch: The D-H Process. Elec. Fur. Steel Proc., AIME (1961) 19, Hubbell. H., and P. I<. Pearson: Nonmetallic Inclusions and Fatigue under Very High Stress Conditions. Qu:~lity Requirements of Superduty Steels, AIME, Nemethy, L.. E. Stock, and W. B. F. RlacKay: Progress in Continuous Casting ol Stainless Steel Slabs at Atlas Steels Limited. AIhlE-ASM Fall Meeting, Detroit, October Discussion C. D. PREU~C~I (Crucible Steel Comlx~ny of America, Pittsburgh, 1'ennsylvaiiia)- Many of the results outlined Ily Mr. Bury have also been encountered by us. One highlighted difference that I might point out is with respect to the amount of superheat required. This is very much a function of heat size, and for our situation, for example, superheat above normal tapping temperatures is only in the order of about 10' to 30' for our larger heats of 90 to 180 tons. I would like to ask whether they have found the same degree of improvement on steels cast into conventional ingot molds, as well as on steels that are continuous cast. F. G. BURY-All of our stainless 300 series, processecl at Atlas, are processed by continuous casting. We do process some wide strip that is converted elsewliere. We have not attempted to evaluate this strip produced from ingots. LOUIS BERTRAND (E. 1. dul'ont de Nemours & Co., Wilmington, Delaware)-I am curious as to the process itself. How many times does one recycle from the ladle up into the vacuum ch:lmber, and what is the vacuum chamber capacity as a per cent of that in the ladle? F. G. BuRY-T~~ number of cycles used is essentially determined by the size of the heat. We use many more cycles on a 50-ton heat than we would on an 81 or 10-ton heat. Normally, for a 10-ton heat, we use, on the average, around ten: on the 50-ton heat, it would be in the order of 30 to 35 cycles. Normally we draw into the vessels on a 50-ton heat, anywhere from 5 to 8 tons. C. W. SUNDBERG (Minneapolis Electric Steel Castings Company, Minneapolis, Minnesota)-What improvement in the impact values mould you expect from this method of handling steel? If you do have data, do you have them for high strength steels; for example, type 4340 with strength levels up around 240,000 psi?

8 1 18 Proceedings of Electric Furnace Conference, 1963 F. G. BURY-I do not have these figures with me, but they were outlined in a paper given by Coleman and Preusch in the 1961 Transactions, and the results at Atlas are fairly close to these values. J. R..ITK~NSON (Dofasco, Hamilton, Ontario, Canada)-When you talk about loss in temperature, would your loss be greater with open steel than killed steel for the same sized heat? F. G. BURY-No, Ive have not noticed this difference.