Análisis del Código Laboral y dimensiones de la subjetividad de lo/as

6.1 VACUUM DEGASSED LOW ALLOY STEEL

P rio r to tapping the open-hearth furnace, the steel was relatively free from oxide inclusions.

During tapping, the addition of ferro-m anganese, fe rro -silico n , fe rro - chrom ium and alum inium to the ladje resulted in the form ation of liquid m anganese alum ino-silicate deoxidation products containing sm all amounts of C ^ O g .

These liquid manganese alum ino-silicates w ere also p re sen t in the steel during the early stages of vacuum degassing. With increasing degassing tim e, how ever, th eir num ber gradually decreased as a re su lt of coalescence and flotation until only occasional inclusions of this type w ere observed.

During the la te r stages of degassing, ladle erosion products w ere p resen t in the liquid m etal. In addition, C rgO g-rich and M gO-rich erosion products which had originated from the m agnesite-chrom ium re fra c to rie s of the degassing vessel w ere also p resen t. A lum ino-silicate inclusions containing varying amounts of AI2O3

w ere occasionally observed as w ell. The m ost likely source of these was the high alumina cem ent used on the degassing vessel re fra c to rie s. During vacuum degassing, the total oxygen content of the steel decreased initially and then began to increase again as the ra te of refracto ry erosion exceeded the rate of inclusion flotation. This increase occurred approxim ately 18 min after the com m encem ent of degassing, although the rapid ra te of inclusion rem oval finished after only 14 min of the degassing cy cle.

Complex manganese chrom ium silicate erosion products containing cristo balite and in some cases chrom ium galaxite w ere p resen t in the ladle stream sa m p le s.

They originated from the siliceous m aterial used to patch the ladle and had picked up chrom ium from the liquid steel.

duplex cristobalite/m anganese silicate erosion products. Iu these c a se s, they were holloware erosion products originating from the siliceous cem ent used to joint the runner bricks together.

Despite the generally high level of inclusions in the liquid m etal during tapping, degassing and teem ing, the b illet sam ples from the p resen t tria l cast w ere relatively free from non-m etallic inclusions, only occasional, highly aluminous silicates being observed. It has been suggested that they resulted from erosion of the high alumina cem ent of the degassing v essel, although this could not be confirm ed d irectly . The indications are th erefo re, that in this p articu lar c a st, the m ajority of inclusions p resen t in the molten m etal escaped and w ere not trapped in the ingot during solidi­ fication. N evertheless, the occurrence of certain of the inclusion types, p articu larly the erosion products, could be m inim ised, thus resulting in a cleaner steel p rio r to solidification.

One suggestion for im proving the cleanness of the steel in the ladle would be to add m ore aluminium during tapping, preferably in the form of ferro-alum inium . This should re su lt in the form ation of m ore highly aluminous inclusions and therefore an increase in their ra te of escape.

A fu rther possible im provem ent in the ra te of inclusion rem oval and therefore in steel cleanness in the ladle m ight be achieved by adding the alum inium before the ferro -m an g an ese, ferro -silico n and rem aining ferro-chrom ium providing the m ixing conditions for the la tte r three additions could still be m et. This suggestion is con­ tra ry to some of the ideas discussed in Section 2 .3 .2 w here it was proposed that additions of a relatively weak deoxidiser such as m anganese will d ecrease the degree of super saturation and encourage the form ation of la rg e r inclusions when a strong deoxidiser is subsequently added. However, by adding the alum inium before the fe rro -allo y s, it m ight be possible to form alumina inclusions which should escape very rapidly, particu larly as there is a g reat deal of turbulence during tapping. There would then be less oxygen available for reaction with the ferro -allo y s and,

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th erefo re, fewer liquid m anganese silicate deoxidation products which escape less rapidly than alumina would be form ed. The im plication of this proposal is that inclu­ sion composition is m ore im portant than inclusion size for the rem oval of the deoxi­ dation products and therefore fo r the success of the overall deoxidation p ro c e ss. The only lim itation on this practice is that a certain time interval should occur between the aluminium and ferro -allo y additions to allow the alumina inclusions to e sc a p e . This may not be possible in actual practice because of the relatively sh o rt time available to make the additions.

A further suggestion for im proving steel cleanness is that careful considera­ tion should be given to the condition of the ladle and degassing vessel re fra c to rie s. Erosion of the siliceous m aterial used to re p a ir the bottom of the ladle was a problem in this cast and was aggravated by the locally high concentration of ferro-chrom ium during tapping. Erosion of the m agnesite-chrom ium re fra c to rie s and high alum ina cem ent of the degassing vessel was also a problem . A reduction in the num ber of inclusions arisin g from this source m ight be obtained by low ering the turbulence of the steel flowing through the v essel. As this steel was degassed for cleanness and analytical purposes ra th e r than for hydrogen rem oval, this could possibly be achieved by not using the final steam ejector of the RH unit, i.e . by operating the unit at a slightly higher p re ssu re .

The fact that this steel was not degassed for the purpose of hydrogen rem oval but for cleanness and analysis reasons ra ise s the question as to w hether equivalent resu lts might not be obtained by sim ple in ert gas purging through a porous plug in the bottom of the ladle. This would allow the degassing unit to concentrate m ore on those qualities for which hydrogen rem oval was essen tial. The only problem is that the ladle slag is an oxidising one and argon bubbling could prom ote phosphorus rev ersio n as a resu lt of slag-m etal m ixing. This is not too much of a problem during R .H . degassing because the slag tends to rem ain relatively quiescent.

It is also clear from the p resen t trial cast that approxim ately 15 min after the s ta rt of degassing, the total oxygen content reached a minimum and then began to increase again as a re su lt of refractory erosion. T herefore, some consideration should be given to reducing the degassing tim e.

A final suggestion is that careful attention should be paid to the condition of the holloware re fra c to rie s, particu larly the siliceous m aterial used to joint the trum pet and runner b ric k s, as a m ajor source of inclusions in the p resen t tria l cast was holloware erosion during uphill-teem ing.

6 .2 LOW SILICON LEADED FREE-CUTTING STEEL

On the basis of the silicon contents of the sam ples taken before and afte r tapping, neither the silicon p resen t in the ladle additions nor silicon p re sen t in the form of siliceous slag inclusions mixed into the m etal during tapping appeared to contribute significantly to the overall silicon content of the steel.

Pick-up of silicon in the form of silicate inclusions occurred in the ladle after tapping. The m ajority of these silicates originated from erosion of the ladle re fra c to rie s. They p ersisted throughout the teem ing period and w ere p resen t in the ladle stre am , ingot and b illet sam ples. There was no d irect evidence that slag entrainm ent during tapping was a problem .

The bottom billet sam ples w ere extrem ely d irty , containing long alum ino- silicate strin g ers w hereas by com parison, the middle and top b illet sam ples from these ingots w ere relatively clean. This difference in inclusion content between the bottom 30% of the ingot and the rem ainder was a re su lt of the bottom cone segregation of alum ino-silicate inclusions.

There was no significant difference in cleanness between the uphill-teem ed ingots and the direct-teem ed o nes. Manganese alum ino-silicates resu ltin g from

refracto ry erosion w ere p resen t in all of them . This indicates therefore that hollow are erosion during uphill-teem ing was not a problem . R ather the pick-up silicon a fte r tapping and the presence of alum ino-silicates in the billets w ere a re su lt of ladle

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erosion. This probably included erosion of the stopper and nozzle as well as the actual ladle lining. Thus, because of the very high Mn:Si ra tio , this steel is very susceptible to attack by manganese on alum ino-silicate ladle re fra c to rie s. The con­ dition of the ladle lining had a m arked effect on the degree of ladle erosion and th e re ­ fore on the extent of the pick-up of silicon. Ladles which had been used about four tim es previously and which had not been repaired or patched w ere found to give the b e st re su lts.

The m ain recom m endation th erefo re, for im proving the cleanness of the steel is that careful attention should be paid to the condition of the ladle re fra c to rie s , as the m ajor source of inclusions was ladle erosion. In this re sp ec t, it m ight prove beneficial from the steel cleanness point of view to reinforce the re fra c to rie s in the region of the ladle bottom , e .g . the use of m ore highly aluminous re fra c to rie s for the bottom p a rt of the ladle and stopper rod, and a m agnesite nozzle or in sert together with a m agnesite or high alumina ram m ing compound.

6 .3 CARBON-MANGANESE-NIOBIUM STEEL

F or the two developm ent c a s ts , the m odifications in steelm aking p ractice w ere very successful in improving the through-thickness ductility of the p la te s.

Although it was not possible to quantify the differences in inclusion content at corresponding stages during the three tria l c a s ts , optical m etallographic exam ination indicated that, in general, there w ere few er inclusions p resen t in the liquid m etal for the two development casts than for the original standard p ractice c a st.

The use of a full aluminium deoxidation practice instead of one involving a ferro -silico n block, resulted in a m arked reduction in the num ber of deoxidation products p resen t during refining.

Slag-m etal mixing during tapping resulted in the entrainm ent of calcium alum inates in the m etal. These inclusions contained considerable am ounts of MgO

as a re su lt of contamination by m agnesite refracto ry from the furnace, taphole or launder. These M gO-rich inclusions w ere also p resen t in the ingots during teem ing.

Final sulphur contents w ere very sim ilar in all three c a s ts . The indications a re therefore that the im provem ent in through-thickness ductility of the two develop­ m ent casts was a re su lt of a lower oxide inclusion content ra th e r than a reduction in total sulphur content. In p a rtic u la r, the incidence of gross alum inate clu sters at the bottom of the ingot and A - segregates of manganese sulphide at the top was consi­ derably reduced. The composition and morphology of the alum inate p a rtic le s in the clu sters indicated that they w ere not p rim ary aluminium deoxidation products but secondary deoxidation/reoxidation inclusions together with alum inium /alum ino-silicate reaction p ro d u cts. Segregation and agglom eration of these inclusions in the bottom of the ingot occurred during cooling and solidification. However, the sev erity of this segregation was le ss , and th erefo re, the through-thickness ductility of the plate g re a te r, for the two developm ent casts than fo r the original standard c a st. As the teem ing conditions, in p articu lar the tem p eratu re, w ere approxim ately the sam e in each case, this difference in quality was presum ably a re su lt of an im provem ent in steel cleanness, i.e . a low er volume fraction of inclusions in the liquid m etal during teem ing. The fact that, for the sam e m etal sulphur content, there was also a c o rre s ­ ponding im provem ent in ductility at the top of the ingot as a re su lt of w eaker A - segregate form ation, im plies that the ingot solidification pattern was influenced by the oxide inclusions in the liquid m etal. Oxide p articles possibly acted as nuclei for the precipitation of iron d en d rites, thus prom oting a m ore equiaxed ingot stru c tu re and th erefo re, according to McDonald and H unt^^^), m ore favourable conditions, i.e . increased solid/liquid reg io n s, for the form ation of A - seg reg ates.

Although there w ere no obvious indications that argon shrouding of the teem ing stream was beneficial from the point of view of steel cleanness, the p ractice should be continued until m ore detailed resu lts are available.

resulted in considerable entrainm ent of the additive in the liquid m etal. This was not the case in the two development casts w here the mould additive was suspended about 30 cm from the bottom of the mould so that it was added after the s ta rt of teem ing when there was a pool of m etal in the mould.

M agnesite refracto ry erosion was evident in all three c a sts, but p articu larly in cast C .2 . It was not possible to determ ine the individual contribution from the m agnesite lining, taphole or launder, but the cast to cast variation corresponded with the variation in furnace lining life .

Although the higher AI2O3 ladle bricks still gave rise to some alum ino-

silicate erosion p ro d u cts, the indications w ere that this was less than with the norm al fireb rick ladle.

Although a practice for the production of C-Mn-Nb steel with through-thickness p rop erties n ecessary to m eet the required specifications has been developed, it is considered that fu rth er im provem ents in both through-thickness ductility and ingot to plate yield could be achieved by additional after-furnace treatm ent of the m olten stee l. Accepting that slag-m etal mixing during tapping is unavoidable in term s of

obtaining a sulphur content of about 0.005%, then argon purging should help to accelerate the elim ination of the entrained slag p a rtic le s . In addition, effective treatm en t of the steel with a calcium -containing deoxidant should further inhibit the form ation of alum ina agglom erates as well as detrim ental type II manganese sulphide inclusions. The latter can still cause problem s, even with a total sulphur content of only 0.005%, because of the segregation ch aracteristics of the steel solidifying in the 2 0 t slab

m oulds. T rials are to be carried out to investigate the effects of these fu rth er modifications on steel quality.

6 .4 GENERAL CONCLUSIONS AND RECOMMENDATIONS FOR

FURTHER WORK

From the resu lts of the investigations reported in this th e s is , it is apparent that the entrainm ent of exogenous inclusions, p articu larly refracto ry erosion p ro d u c ts,

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in the liquid steel during tapping and teem ing is a m ajor source of oxide inclusions and can lead to quality problem s (rejections and/or reduced yield). These problem s a re likely to become w orse in the future as specifications and qualities become m ore exacting. Consequently, there is a need for fu rth er work to be c arried out on the form ation of inclusions as a re su lt of:-

(i) R efractory E rosion - with higher m anganese s te e ls , attack by the m anganese

on fireclay re fra c to rie s is a serious problem . The indications are that this problem can be alleviated to some extent by the use of alum ino-silicate re fra c to rie s having higher AI2O3 contents and this requ ires further investigation, both on a laboratory

scale and in production h e a ts .