DECARBURIZATION OF FERROCHROME AND HIGH ALLOY STEELS WITH OPTIMIZED GAS AND SLAG PHASES TOWARDS IMPROVED Cr RETENTION

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1 J. Min. Metall. Sect. B-Metall. 49 (2) B (213) Jounal of Mining Metallugy Section B: Metallugy DEARBURIZATION OF FERROHROME AND HIGH ALLOY STEELS WITH OPTIMIZED GAS AND SLAG PHASES TOWARDS IMPROVED RETENTION H. Wang a* M.M. Nzotta b L. Teng c S. Seethaaman c a Depatment of Feous Metallugy Univesity of Science Technology Beijing Beijing hina b Reseach & Development (R&D) Pocess Development (Metallugy) Uddeholms AB Hagfos Sweden c Division of Mateials Pocess Science Royal Institute of Technology Stockholm Sweden Abstact (Received 13 August 212; accepted 5 Mach 213) homium is a high value metal the etention of the same duing the efining of high cabon feochome as well as high alloy steel has significant economic envionmental impacts. The loss of chomium duing the decabuization is geneally minimized using agon-oxygen mixtues theeby educing the oxygen patial pessue (P O2 ) of the oxidant gas. In the cuent study expeiments wee caied out in an induction funace O 2 was intoduced with the view to patly educe P O2 patly as an oxidize. Duing these expeiments the decabuization of molten -alloy was conducted using pue O 2 pue O 2 o O 2 +O 2 mixtues. The esults demonstated that the loss can be minimized unde O 2 intoduction. The kinetic analysis showed that the mass tansfe is effective due to the poduction of 2O gas molecules fom one O 2 molecule duing the eaction which will impove the stiing of the bath. Besides O 2 eacts with cabon in melt is an endothemic eaction intoduction of O 2 could be a coole duing the efining pocess hence the tempeatue could be contolled by contolling the diluting gas amount in this case the ove heat of bath efactoy could be pevented the lifetime of efactoy could be extended. Key wods: O 2 ; etention; Patial pessue; Decabuization; Tempeatue contol. 1. Intoduction Taditionally O 2 is selected as an oxidize fo decabuization in feochome efining pocess also Electic Ac Funace (EAF) pocess. Howeve as epoted by ealie eseaches it is ealizable to decabuize with O 2 as well. The pinciple of decabuization of Fe- melts using O 2 has been expounded by Mannion Fuehan [1]. Thei esults showed that nealy one-fifth of O 2 supplied was consumed duing the decabuization eaction. Zughbi [2] epoted highe decabuization ate moe vigoous eaction when O 2 was used as the oxidizing gas instead of O 2. Sain Belton [3] have studied the decabuization of liquid ion by O 2 at geat length. O 2 was also intoduced into the agonoxygen efining pocess by Heise et al.[4]. It is epoted that the cabon emoval efficiency in agonoxygen decabuization pocess (AOD) was impoved by using O 2 agon-oxygen mixtues. Some authos [5-8] also caied out the study on the suface decabuization of Fe-- melts. Besides this thee ae some eseaches who investigated as to how to educe the loss duing feochome efining * oesponding autho: wanghaijuan@ustb.edu.cn DOI:1.2298/JMMB128131W also the high alloy steel making pocess. AOD as a taditional effective way is being used widely in the wold. Steam (H 2 O) is anothe pofitable altenative fo etention duing AOD pocess accoding to UHT s (Uvån Hagfos Teknologi AB Sweden) efeences expeience though its LU technology [9 1]. To the knowledge of the pesent authos no systematic investigation has been caied out with espect to the simultaneous decabuization -loss in Fe-- melts with intoduction of O 2. In the pesent study the etention duing M- LFe making pocess high alloy steel making pocess is investigated with intoducing O 2 in the gas system. 2. Theoetical analysis The ode of oxidation ability fo O 2 O 2 O in Fe- melts is shown in Table 1. The oveall chemical eaction of decabuization with O 2 can be descibed by equation (1). In the pesent calculations ω[i] = 1% (Henian) was chosen as the stad state of O. MOLTEN12 - onfeence Special Issue Ninth Intenational onfeence on Molten Slags Fluxes Salts

2 176 H. Wang et al. / JMM 49 (2) B (213) Table 1. The oxidation ability fo O 2 O 2 O. Reactions No. G H T S ( G J/mol (1873K) G + 1 O2 ( g) O(g) (1) G T Weak O exothemic G +O 2 (g) 2 O(g) (2) G T Weak endothemic G G T +O O(g) (3) Weak exothemic G O ( g) O ( s) (4) G= T Stong exothemic 3 ) 2 3O ( g) O ( s) 3O( g) (5) G T Weak exothemic (. It is seen in Table 1 that O 2 is a weake oxidizing Hee / p agent compaed to O 2. Fom a themodynamic point log 1 fc e [ ] % e [ ] % (9) of view it is possible to use O 2 as an oxidize fo decabuization ealizing that will also be oxidized log 1 f e [ ] % e [ ] % (1) by O 2 duing this pocess fom eaction (5). One point to be noticed is that eactions (1) (3) (4) (5) The inteaction paamete values of e e e e ae exothemic while eaction (2) is an endothemic can be obtained fom liteatues [11-13]. e eaction. With mixing O 2 in the gas system the The Gvalue of the Gibbs. enegy change fo eaction tempeatue of the bath could be deceased also (6) thus is given by: G contolled in some extent T T Except fo the possibility to use O 2 as a 66. [ ] % 714. [ ] % 3lg decabuize the etention duing the [ ] % 2lg [ ] % 3lg( p / p O ) decabuization pocess needs to be consideed as (11) well. It is also well known that in ode to pevent The citical tempeatue Tc was calculated at in the molten metal phase fom oxidation at the same diffeent situations the esult is pesented in Table time decabuization eaction (6) given below should 2. can be kept in the metal bath only if the be pevented. This can be illustated by intoducing tempeatue of the melt exceeds Tc. This indicates that the paamete efeed to in the pesent wok as the a lowe value of Tc is bette fo the etention of if citical tempeatue Tc fo. itical the bath tempeatue is constant. tempeatue is the tempeatue at which G= 6. 3 Table 2. The citical tempeatue T fo diffeent [] []. 3O(g)+2=2(s)+3 (6) The stad states of omposition of G6( J / mol) wee chosen as ω [i] = 1% (Henian). In the case of 2 the slag pue No. steel P O / Pa G Tc /K T substance stad state was taken. The stad state ω []% ω []% ( J / mol) of 1 atm was fo O. G of equation (6) is given T 1538 by equation (7) T 168 G T( J / mol) (7) T 171 Assuming that the activity of ( 2 ) is T 1871 G l (pecipitated as pue 2 without consideing the T 1561 dissolution 6 in the slag phase) 3 a a T 1514 (2O 3) G6 G6 RT ln 2 3 a ( p / p ) T f [ ] % G6 RTln (8) f [ ] % ( po / p ) whee l ca efes to themodynamic activities f efes to Henian activity coefficients; p O is the pessue of O gas p θ efes to total pessue. O Figue 1 illustates the vaiation of Tc with espect to contents. Fom this figue it can be seen clealy that Tc inceases with the inceasing of content when content is a constant. Futhe Tc deceases with the inceasing of content when

3 H. Wang et al. / JMM 49 (2) B (213) content is stable. Thus if the tempeatue of the melt is kept constant -loss will incease with the pogess of decabuization. O in case the tempeatue is kept constant if the patial pessue of O is deceased the will be kept in the melt the will be emoved. To ealize decease P O dilute gas such as A could be used (the pinciple to AOD pocess) fom the themodynamics point of view. Besides themodynamic aspects kinetic aspects should also need to be consideed. A intoduction could incease the stiing of the bath hence incease the contact between cabon 2 that eventually is oxidized theeby estoing the back to the melt. In the pesent study O 2 was used as the stiing gas. It should be noted that the activity of 2 was assumed to be unity in the above calculations. If the activity of chomium oxide is deceased due to the dissolution in the slag chomium oxidation will be favoed. compaed to O 2. It may be also noted that fo evey O 2 bubble injection two O bubbles ae fomed; the numbe of O bubbles will be twice that coesponding to O 2 injection. This can facilitate good mixing between metal slag phases leading subsequently towads a nea-equilibium situation coesponding to equations (12) (13). In the above consideations the tempeatue of the metal bath was assumed to be constant. In eality decabuization with O 2 injection (eaction (1)) would be exothemic while eaction (2) will be endothemic. Thus decabuization with O 2 would equie additional enegy input in ode to keep the tempeatue of the metal bath constant. Fo efining of feochome o high alloy steel the tempeatue must be contolled duing decabuization peiod to ensue that it is neithe too low to lead oxidized stongly no too high to lead funace lining damaged. The eal poduction expeience showed that the tempeatue aound 17 o is optimum accoding to Rick [14] this tempeatue is nomally used in high alloy steel efining as well as Medium Low cabon Fe efining. 3. Expeiment A seies of expeiments wee caied out in an induction funace with samples masses of 1kg metal with a slag phase above. The schematic diagam of expeimental set-up is illustated in Figue 2. Figue 1. The effect of contents on Tc Anothe aspect to be consideed with espect to the eduction of 2 fomed by the cabon dissolved in the steel is the fomation of 3 molecules of O gas accoding to eaction (12): (12) 2(s)+3=3O(g)+2 (FeO) (l) + = Fe + O(g) (13) The fomation of 3 molecules of O by the eduction of 2 the melt is simila to the oxidation of in the melt as epesented by eaction (5). The O gas bubbles thus fomed eithe by eaction (5) o by eaction (12) will not futhe eact with the components in the metal bath. One of the significant diffeences between O 2 O 2 injections is that fo evey molecule of O 2 injected 2 moles of fom the melt can be emoved; howeve fo evey mole O 2 only one cabon atom in the melt will be emoved. Accodingly in ode to achieve the same metal composition at the end of the blowing one needs to blow twice the amount of O 2 Figue 2. A schematic dawing of the induction funace An Al 2 cucible of 45mm I.D. 15mm in height was adopted in the expeiment to cay metal slag samples it was positioned inside an oute gaphite cucible heated in an induction funace. The decabuizing gas was blown into the liquid melt though an alumina lance fom the melt top lance was immeged in the melt duing expeiment. Synthetic slag with the composition: ao 4 mass% MgO 1 mass% SiO 2 4 mass% Al 2 1 mass% was used to cove the metal. The slag composition coesponds to Al 2 satuation so that

4 178 H. Wang et al. / JMM 49 (2) B (213) composition changes due to cucible dissolving in the slag could be avoided. The cucible was specially designed poduced in the pesent laboatoy. A Pt.3% Rh Pt. 6%Rh themocouple was place inside the wall of the cucible with the aim to monito the tempeatue of the melt accuately. The cucibles wee peheated in a funace at 573 K ovenight in ode to emove adsobed moistue then peheated to 1273 K in a muffle funace at a heating ate of 3 K/min. The slag sample was melted at 1773 K in an induction funace. The amount of slag added was appoximately 5% of the metal mass. The mothe alloy was poduced in significant amounts by pemelting alloy mixtue of tageted composition in an induction funace in ode to get homogenous alloy. It was then cut into even pieces of was used in each expeiment. Duing the decabuization unning A atmosphee was intoduced duing the melting of the steel. Afte totally melted oxidant gas was blown into the liquid metal at 1873K. Samples of the steel melt wee taken at appopiate time intevals using quatz sample tubes of 6mm I.D. quenched in cold wate. The samples wee analyzed to obtain cabon chomium compositions. 4. Results Discussions Table 3 pesents a selected pat of the esults fom the significant numbe of expeiments caied out in the pesent wok. The initial cabon contents of Fe alloy ae selected as.3mass% 3. mass% the esults obtained ae pesented in this table. Only pue O 2 pue O 2 wee tested in cuent expeiments. The expeiments wee caied out in two diffeent scales one with 5g mateials the othe with1g mateials both at 1873K. 4.1 The elationship between content duing the efining pocess It is illustated clealy in Figue 3 that the ate fo loss with decabuization pocess while blowing pue O 2 is slightly lage than that blowing pue O 2 when the initial cabon content in the melt was.3wt%. This indicates that O 2 may have supeioity Figue 3. The elationship between duing efining pocess with the initial cabon content at.3 mass% in etention. Howeve compaing the slopes of those above cuves stating fom cabon.3wt% to.15wt% they ae vey simila. Hence the O 2 intoduction seems have no impacts on saving duing low cabon level. This needs futue investigation. Howeve if one compaes the effect of O 2 O 2 to etention at highe initial cabon content as shown in Figue 4 it is vey diffeent fom that with low cabon content. It is seen in Figue 4 that when blowing pue O 2 the content almost kept constant duing decabuzaiton while with oxygen blowing the loss is maximum 2% with high initial cabon content (appoximately 3. mass%). That indicates the advantage to use O 2 fo etention especially with high initial cabon content melt. That indicates O 2 may have big advantages fo decabuization of high Table 3. The change duing efining pocess at 1873K 1%O 2 1%O 2 1%O 2 1%O 2 Time 5g steel 5g steel 1kg steel 1g steel Time 9ml/min gas 9ml/min gas 136ml/min gas 136ml/min gas (mass%) (mass%) (mass%) (mass%) (mass%) (mass%) (mass%) (mass%) min min min min min min min min min min min min min min

5 H. Wang et al. / JMM 49 (2) B (213) Figue 4. The elationship between duing efining pocess with the initial cabon content at 3. mass% Figue 5. The -loss duing efining when blowing pue O 2 pue O 2 cabon feochome (HFe) to make medium cabon feochome (MFe) o low cabon feochome (LFe) in feoalloy industy. That is becasese nomally HFe contains 7-8 mass% cabon 5 mass% of the aim cabon fo M-LFe is anging fom 4 mass% cabon to.5 mass% cabon o even lowe. That cabon egion is good fo use O 2 as the decabuize based on the pesent esult. Howeve fo this melt content is much highe than that in the high alloy steel melt which may enhance the loss to the slag. The pesent autho will keep investigate this in the futue. 4.2 The effect of O - etention at low initial cabon content The same initial content was taken to investigate the loss in pue O 2 O 2 at low cabon contant (aound.25%). Fo 1%O 2 the initial steel composition was :.257wt%11.1wt%; fo 1%O 2 the initial composition of steel was :.253wt%8.5wt%.The -loss in these two situations is shown in Figue 5. It is obvious that the -loss ate in 1%O 2 1%O 2 is almost the same at the beginning 7 mins. But with the pogess of efining -loss ate with blowing 1% O 2 is less than that blowing 1%O 2. The maximum -loss with blowing 1%O 2 is about 3% at 17.5min at the same time blowing 1%O 2 the -loss even each 75% which is moe than twice that of blowing 1%O 2. This indicates that O 2 oxidation ability is stonge than O 2. And if one consides both decabuization -oxidation Figue 6 may illustate clealy. It can be found the decabuization ate by blowing O 2 is about twice of that blowing pue O 2 duing 3.5min to 14min. while the ates in both cases ae faily constant afte about 3.5 min. With espect to Figue 6. The decabuization ate -loss ate when blowing pue O 2 pue O 2 -loss ate the ate with blowing pue O 2 is about 1.6 times that of blowing pue O 2 when the gas was injected into the bath fo 3.5min.With the pogess ate of blowing pue O 2 is seen to incease shaply even ove /min but the -loss ate is almost constant at about /min. This would imply that when blowing O 2 the -loss is aound 1/3 of the value coesponding to O 2 blowing. Howeve the decabuization ate with O 2 is just aound 2 times of that with O 2. That indicates O 2 is good fo keeping inside the steel fo the same decabuization level. 4.3 The utilization atio of O 2 O 2 in decabuization pocess Fo blowing O 2 the effective pat which is used in decabuization oxidation is O as shown in equation (14). Hence fo blowing pue O 2 the effective pat of O 2 (O 2 can be calculated as ½ O 2 ) was taken to plot. The amount of effective pat of gas (O 2 ) eact with metal was calculated plotted in

6 18 H. Wang et al. / JMM 49 (2) B (213) Figue 7 Figue 8 fo two diffeent initial cabon content melts. O2( ads ) O( ads ) O( ads )...(14) Figue 7. (a) The O2 amount eacting with alloy when blowing 9ml/min O min (Initial.26% 11.1%; Final.3% 3.3%) (b) The O2 amount eacting with alloy when blowing 9ml/min O2 21min (Initial.29% 1.3%; Final.145% 16.8%) It is seen that at low initial cabon contents most of the O2 O2 ae used fo oxidizing just a mino pat of the injected gas is used fo oxidizing. This indicates that fo low cabon melt loss is stong. Howeve if conside the oveall gas utilization (effective pats used fo eaction) the utilization atio of O2 is moe than O2 especially in the fist 7.min almost all the effective pat of O2 is used fo oxidizing but just half of O2 is used fo oxidation the est may have been used fo oxidizing Fe o escaped un-eacted. Fo high initial cabon content the utilization of gas is also investigated illustated in the following figues. The oveall utilization of the gases at high initial cabon content is shown in Figue 8(a) Figue 8(b) is simila to that decabuization of low initial Figue 8. (a) The O2 amount eacting with alloy when blowing 136ml/min O2 36 min (Initial 3.2% 14.6%; Final 1.31% 13.5%) (b) The O2 amount eacting with alloy when blowing 136ml/min O2 36 min (Initial 2.99% 15.1%; Final.91% 15.1%) cabon content. The effective pat of pue O2 used fo eacting with is just half the gas the est is oxidizing Fe o just flows away howeve fo pue O2 almost 1% of the effective gas is used fo eaction in this case aound 1% O2 is used fo decabuization since is almost not oxidized. It is also seen in above figues that at high initial cabon content most of the gas ae used fo oxidizing only a mino pat is used fo oxidizing fo both two gas cases. Howeve fo blowing pue O2 at the beginning 24mins the gas amount to eact with ae vey simila but fo blowing pue O2 the gas amount oxidizing is much lage than that eacts with which indicates that O2 is much moe effective in decabuization at high initial cabon content also efficient fo etention. 5. onclusions 1) The expeimental esult shows that O2 is effective fo etention especially fo high initial

7 H. Wang et al. / JMM 49 (2) B (213) cabon content situation. O 2 can be as diluting gas to educe patial pessue of oxygen. The endothemicity of the Boudouad eaction can be advantageously used to contol the bath contol tempeatue which would extend the efactoy life. 2) Fo low initial cabon content the oveall utilization atio of O 2 is highe than O 2 especially at the initial stage of decabuization almost all the effective pat of O 2 is used fo oxidizing on the othe h fo O 2 as decabuize appoximate half of O 2 is used fo oxidation. 3) Fo high initial cabon content O 2 could be an effective diluting gas fo decabuization since in this situation O 2 is vey effective fo decabuization. At lowe initial cabon content the supeioity of O 2 fo decabuization etention is not obvious. 6. Futue wok 1) The slag now used is synthesized in ode to potect the Al 2 cucibles howeve in the eal poduction MgO o MgO- 2 lining ae nomally used in Fe efining pocess high alloy steel efining pocess. Hence the slag composition is diffeent needs to be optimized so that the activity of the oxidation poduct viz. 2 would be high in the slag phase. In this case loss could be educed. 2) Based on positive esults obtained now pilot plant tials ae cuently being planned at Uddeholms AB Hagfos Sweden. In ode to maximize decabuization a compute pogam based on a kinetic model was developed as pat of this wok. This is being adopted to contol the gas composition flow ate. The pogam will also take into account the exta enegy need in the pocess in view of the endothemicity of the above eaction by contolling the atio of O 2 +O 2. This also would enable the pevention of oveheating would have a positive impact on the efactoy life. These tials ae expected to be followed by plant tials at Uddeholms AB whee the pocess is expected to be implemented. 3) Fo high cabon feochome efining the use of O 2 as a decabuize needs to be futueinvestigated. The cuent esults show that O 2 is moe efficient at highe cabon content. Fo high cabon feochome the cabon content is aound 7wt% hence the decabuization with O 2 appeas to be pomising thei big suppot fo unning pilot expeiments in the plant. Refeences [1] F. J. Mannion R. J. Fuehan. Metallugical Tansactions B (Pocess Metallugy) 2(6) (1989) [2] H. D. Zughbi. Scinavian Jounal of Metallugy 32(4) ( 23) [3] D. R. Sain G. R. Belton. Metallugical Tansactions B (Pocess Metallugy)7B(2) (1976) [4] B. H. Heise R. N. Dokken.United States Patent No [5] [Y. Nii K. Ito K. Sano. Testu-to-Hagane 55(6) ( 1969) 437. [6] J.. Fulton S. Ramachan. Poceedings of the Electic Steelmaking onfeence Univesity Pess [7] T. Kuwano S. Mauhashi Y. Aoyama. Testu-to- Hagane 59(7) ( 1973) 863. [8] L. F. Banhadt. Ph.D Thesis 1965 MIT ambidge. [9] -J Rick. Nodic steel mining eview 3(29) [1] [11] G. K. Sigwoth J. F. Elliott. Metal Science 8 (1974) [12] K. Nakamua T. Ohno K. Segawa. Poceedings of Intenational onfeence on the Science Technology of Ion Steel 1(197) 456. [13] J. hipman. J. Ion Steel Inst. 18(1955) 97. [14] -J Rick. The twelfth Intenational Feoalloys ongess June Helsinki Finl pp Acknowledgement Swedish Foundation fo Stategic Envionmental Reseach (MISTRA) though Jenkontoet Sweden is sinceely acknowledged fo patial financial suppot of this wok. The authos wish also to thank Uddeholms AB Hagfos Sweden fo help with chemical analysis of samples useful discussions