Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
  • C21C5/005—Manufacture of stainless steel

Definitions

• This invention is concerned with a further development of this process, and aims at improving the economy of the process by the use of cheap charge materialfor example, of the steelworks own return scrap. Further aims of the invention are to improve the reliability of the analytical composition of steels produced and to produce high alloy steels, that is steels having a content of more than 5% of alloy elements, for example stainless steels, more economically than hitherto.
• stainless steel is made from liquid crude iron and solid alloy carriers and the liquid part of the charge which forms between 20% and 75% by weight of the total charge is first charged into the blowing crucible and is blown in a first blowing phase beneath a highly basic slag, which is rich in ferric oxide to yield an intermediate product having a phosphorus content of less than 0.025%.
• the slag is then removed and a main part of the solid alloy carriers, which are relatively non-oxidising (e.g.
• nickel or molybdenum alloys when compared to the alloy of another part of the carriers are added to the crucible together with chemical heat carriers such as ferro-silicon if required, after which the blowing is continued in the second phase until the solids are liquified and preferably until the required carbon content is reached and then the other part of the solid alloy carriers which are relatively oxidising (e.g. chromium alloys, preferably ferro-chromium) and contain silicon and carbon, are added and blowing is continued in a third blowing phase beneath a new basic slag having a CaO/Si0 ratio of at least 2.5 or a (CaO-i-MgO)/Si0 ratio of at least 1.5,.
• chemical heat carriers such as ferro-silicon if required
• the slag is treated with reducing agents to reduce the slagged alloy constituents and finally, after removal of the reduced slag, the remainder of the alloy carriers of high quality are added to the crucible.
• blowing is performed to start with at a normal blowing energy corresponding to a lance interval of approximately 90 to 140' cm. and a blast pressure of 6 to atm. g.
• the blowing is subsequently continued during the second half of this phase with application of low blowing energy corresponding to a lance interval from 200 to 350 cm. and a blast pressure from 3 to 8 atm.
• the basicity of the slag in the first phase (CaO/SiO should be more than 3.
• the relatively non-oxidising alloys such as nickel and molybdenum, preferably in the form of high alloy scrap, are added to the melt up to 30% by weight of the total quantity of alloy carriers.
• the liquid charge pig-iron or hot air cupola furnace iron
• chemical heat carriers may also be added to the melt. The latter are however necessary only in exceptional cases.
• the manganese content of the melt is also preferably increased before the start of the second blowing phase by the introduction of relatively non-oxidising manganese carriers.
• fresh slag formers may be added to the charge before starting the second blowing phase.
• the metallurgical functions of the second slag are slight, so that the quantity of slag may be kept small.
• the quantity of these slag formers should be approximately 1 to 4%, and the basicity of the slag should not fall below 2.
• the second blowing phase is performed with a normal lance interval and oxygen pressurefor example, with a lance interval of to cm. and a pressure of 6 to 10 atm. g. It continues at least until the metallic charge is thoroughly liquefied, and preferably until the carbon content desired in the finished steel is attained.
• the readily oxidising alloy carriers are added to the melt.
• any type of chromium alloy may be used.
• Ferrochromium alloys containing at least 5% silicon and 5% carbon are preferred.
• the chromium contained in such a material is far cheaper than in normal ferro-chrornium, and the necessary heat carriers in the form of silicon and carbon are also inserted into the melt at no cost. The latter elements are effective during the entire third blowing phase which follows.
• the chromium content of the melt at the end of the second blowing phase is adjusted to be in accordance with the chromium and carbon contents desired in the finished steel and so that the chromium content is higher by 1 to 3% (absolute) than the chromium content in the finished steel. It is advantageous simultaneously to adjust the manganese content of the charge to the upper limit of the finished steel.
• new basic slag formers are added to the charge thus prepared. Their quantity depends upon the SiO produced during the third blowing phase. The basicity (CaO/SiO of the slag produced should not fall below the value 2.5. For steels which must be blown to extremely low carbon contents, it is appropriate, in preparing the crucible to replace a portion of the lime by dolomite; in this case, the slag ratio (CaO+MgO)/Si0 may be lowered to the value 1.5.
• the lance interval which is below normal or else to use a lance which generates a more compact oxygen jet than is otherwise customary.
• the choice of the quantity of oxygen to be blown per unit of time is dependent upon the lump size and composition of the ferro-chromium added. The latter dissolves continuously. By an appropriate control of the oxygen supply, it should be ensured that the active oxygen is only equal in quantity to that required for the combustion of the carbon and silicon liberated.
• the blowing is performed to a carbon content which is below the carbon content prescribed for the finished steel.
• the crucible After completing the blowing, the crucible is tilted. In this position, reducing agents in the form of for example Al, Si or CaSi, are applied to the slag present on the charge in order to reduce oxidised chromium and manganese.
• the proportion of reducing agents depends upon the level of the preceding addition of chromium, and upon the carbon content of the bath. It may be determined empirically for each individual case.
• the addition of the reducing agent may also be performed through a chute or the like with the vessel upright or obliquely positioned.
• oxygen is preferably blown onto the bath for approximately 1 minute with a reduced lance interval and a reduced oxygen supply, in order to accelerate the progress of the slag reduction by mechanical turbulence.
• the reduction process is usually complete after 5 to minutes.
• the crucible is tilted back into the horizontal position, a sample is taken, and a portion of the slag is removed by vigorous letting off. Not later than 5 minutes after the sampling, the crucible is positioned obliquely and 10 to 40% according to the temperature of return scrap, of the same or of a similar quality to the steel to be manufactured is added. Owing to the poor thermal conductivity of the high alloy steels, the heat losses in the stationary bath are low, as the thermal content of the melt is fully sufficient to dissolve the added scrap. Neither the iron content nor the alloy content of the scrap sufiers losses during its dissolution.
• the crucible After adding the scrap, the crucible is restored into the horizontal position and the residue of the slag is removed.
• the further treatment is governed by the quality of steel desired.
• slag formers e.g., in the form of lime and ingot iron, which can be heated with gas burners or oil burners.
• a preferred example of the invention consists in the treatment of the fining slag with reducing agents.
• a white disintegrating slag is produced which causes a difiusion deoxidation and a vigorous disulphurisation.
• the steel may also be deoxidised with aluminum or silicon in a conventional manner.
• the sulphur content of the steel is also reduced to approximately 0.004%.
• a process for the production of stainless steel from crude iron and solid alloy carrier consisting at least in part of chromium, nickel and molybdenum by oxygen top blowing said process including the steps of charging liquid crude iron, which forms between 20% and 75% by weight of the total charge, into a blowing crucible, said crude iron being selected from the group consisting of pig iron and hot air cupola furnace iron, blowing said liquid crude iron in a first blowing phase beneath a highly basic slag, which is rich in ferric oxide, to yield an intermediate product having a phosphorus content of less than 0.025%, removing said slag, adding a main portion of solid alloy carrier bearing nickel and molybdenum together with a heat carrier to said crucible, continuing blowing said charge in a second phase until said main portion of solid alloy carriers is liquefied, and a predetermined carbon content is reached adding another portion of alloy carrier bearing chromium, silicon and carbon, continuing blowing in a third blowing phase beneath a new basic slag having a CaO

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Citations (8)

• 1962
  • 1962-09-18 AT AT742062A patent/AT237000B/de active
  • 1962-11-16 DE DE19621433535 patent/DE1433535A1/de active Pending
• 1963
  • 1963-07-17 LU LU44082D patent/LU44082A1/xx unknown
  • 1963-08-14 GB GB32154/63A patent/GB1025230A/en not_active Expired
  • 1963-08-21 SE SE9141/63A patent/SE305661B/xx unknown
  • 1963-08-26 US US304683A patent/US3282679A/en not_active Expired - Lifetime

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