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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys

Definitions

• Chromium alloyed steel is at present normally manufactured by melting steel scrap circulating in the factory together with alloyed or unalloyed merchant scrap and ferro alloys in an arc furnace, after which the melt is metallurgically processed to obtain the desired type of steel.
• the price of ferro alloys increases as the percentage of impurities decreases.
• Gaseous oxygen is used for this purpose in order to oxidize not only the carbon but also the silicon content of the charge.
• This high-quality reducing agent is thus only used to generate heat.
• the silica formed is absorbed in a calcium silicate slag which, even if it is tapped at the start of the blowing, causes considerable metal losses since with conventional oxygen refining iron and chromium are oxidized to a considerable extent at the same time as the silicon.
• the silica formed in the initial stage causes a considerable increase in the quantity of slag, particularly if a high CaOzSiO ratio is desired in the final stage.
• the large quantity of slag causes several other disadvantages; for instance it takes up great quantities of chromium oxidized during the carbon refining which must then be reduced to chromium again after completed refining. For this reason ferro chromium alloys poor in silicon are preferred as the chromium raw material, but these are more expensive to manufacture than those rich in silicon.
• the silicon in the ferro chromium is used, possibly in combination with silicon in the melt or added to the melt, or with other substances having greater afiinity to oxygen than chromium, in order to re- 3,711,278 Patented Jan. 16, 1973 "ice cute chromium from chromium ore and thus increase the chromium content of the melt.
• this process can be carried out so that the slag formed has low percentages of chromium and iron and can thus be separated with low metal losses. Because the silica formed is removed before the carbon refining, a considerably smaller quantity of slag is formed during the refining than with other methods.
• the invention thus makes it possible, without any drawbacks, to use cheap metallic chromium raw materials such as ferro chromium alloys having high percentages of silicon and carbon which are cheaper to manufacture, and also makes it possible to utilize the silicon content of these products to reduce chromium from chromium ore, thus further decreasing the costs of the raw materials for thesteel.
• a carbonaceous iron metal is used as starting material which contains alloying substances having a greater afiinity to oxy gen than chromium.
• this melt is brought by intensive stirring into intimate contact with a material containing chromium oxide which is added in such quantities that substantially the entire mass of said alloying substances is oxidized and turned to slag with added slag former, while substantially all the chromium oxide added is reduced to metal and is consequently dissolved in the iron melt.
• the melt is deslagged and in a second stage it is refined with oxygen or oxygen-enriched air, the carbon content of the metal at the start of the refining being at least 2% and the temperature of the melt during the refining being permitted to increase as the carbon content drops so that the temperature is at least 1700 when the carbon content has fallen to 0.20%.
• the refining process in the second stage can be carried out under the conditions stipulated with considerably less slag formation thus decreasing the chromium oxidation, and thus also the necessity for re-reduction of oxidized chromium after completed refining.
• the original iron melt should contain alloying substances having a greater aflinity to oxygen than chromium, and one such suitable substance is silicon.
• the starting melt may suitably consist of a siliceous pig iron.
• a favourable method of producing siliceous pig iron is to melt scrap in a cupola furnace with acid lining. This pig iron may possibly be desulphurized. Alloying substances having greater alfinity to oxygen than chromium may also be added, suitably partly in the form of iron alloys with high percentages of carbon and silicon, such as charge chrome.
• the material containing chromium oxide which is added in the first stage according to the invention, suitably consists of chromium ore. According to the invention it is advisable to adjust the quantity of alloying substances having greater affinity to oxygen than chromium so that they are substantially consumed for the reduction of chromium oxide and other similar or more easily reduced oxides in the material containing chromium oxide and so that in this way a chromiferrous melt is obtained after the first stage which has such a chromium content that, possibly after further additions of chromium during the second stage, the desired final percentage of chromium is obtained in the material. It is then unnecessary to make any great adjustment of the chromium content in the melt after the second stage.
• the easily reduced oxides which may exist in the material containing chromium oxide may be mentioned particularly iron and nickel oxides.
• the temperature during the first stage should suitably be above 1450 C. and may possibly be adjusted by the combustion of hydrocarbon, preferably with oxygen, above the surface of the melt. By preheating and possibly melting the additives it is possible to further increase the quantity of chromium which can be reduced from the ore.
• basic slag formers are suitably added, preferably containing CaO and MgO,to produce a basic, easily flowing slag which is removed after the first stage.
• coolant is suitably added, preferably material containing chromium or chromium oxide, thus further increasing the chromium content in the melt.
• the ratio between the temperature of the melt and its carbon content which should be aimed at in order to give the least possible chromium oxidation can be obtained from the theoretically determined conditions of equilibrium in such melts.
• the temperature in the Charge should not increase more rapidly of higher than necessary as this causes increased wear on the lining.
• slag former is added so that a slag with a basic value greater than 3.5 is obtained.
• basic value is meant in this specification the ratio (Ca+MgO) :SiO these substances being indicated in percent by weight. Since the alloying substances having a greater affinity to oxygen than chromium are removed in the first stage and,
• the econd step can be carried out with very little slag, thus facilitating the carbon refining.
• the second stage is carried out entirely or partly during strong mechanical stirring.
• Chromium which may have been oxidized is suitably recovered during the second stage by reducing the slag by the addition of reducing agent having a greater aflinity to oxygen than chromium, for example ferro-silicon or some other silicon alloy having low carbon content.
• the reduction after the second stage suitably takes placeduring strong mechanical stirring.
• the intensive mechanical stirring which should be used in the first stage and partly or entirely during the second stage and also during the reduction after the second stage can be elfected by using a rotary furnace whose axis of rotation slopes at an angle of, at the most 45, to the horizontal plane, and preferably rotating the furnace rapidly, that is at more than rpm.
• the speed may, of course, not be so high that the stirring effect is lost due to the influence of centrifugal forces on the melt.
• part of the refining process takes place when the furnace is stationary and standing in vertical position and oxygen gas is blown from above towards the surface, thus making it possible to effect a so-called burning spot where the carbon isrefined at extremely high temperatures.
• the intensive mechanical stirring of the metal can also be effected by producing circular movements in the melt in a substantially cylindrical container having a substantially vertical axis.
• a substantially cylindrical container having a substantially vertical axis One example of such a means is the so-called shaking ladle.
• part of the refining process may be carried out While-the container is stationary.
• the temperature of the melt was approximately 1550 C.
• the final temperature can be kept lower if desired by additional cooling with NiO Within the limits for the stipulated final Ni content of the steel or by means of ordinary steel scrap, preferably of known composition.
• the melt pre-treated in this way was then used to manufacture a chromium alloyed steel according to the invention.
• the furnace was rotated rapidly while 325 kg. charge chrome (6% C, 8% Si, 54% Cr, 32% Fe),
• the melt was cooled to a temperature of around 1600- C. by the addition of approximately 300 kg. stainless scrap having a similar composition to the finished steel,
• EXAMPLE 2 A crude iron having the following composition was manufacturfed in a blast-furnace from chromium ore, iron ore and raw material, for instance nickel oxide, con- The crude iron was charged at a temperature of about 1500 C. in liquid form into an oxygen gas converter of LD/KALDO type in which the oxygen gas refining can take place with the converter either in stationary, vertical position or rotating with a sloping axis.
• the melt of liquid crude iron was processed according to stage 1 of the invention by allowing the furnace to rotate and successively adding 180 kg. chromium ore and 70 kg. quicklime. The chromium was thus reduced from the ore by the Si in the crude iron. After minutes an iron melt with the following composition had been obtained:
• the slag had a basic value of 1.24 and contained 3.7% Cr O
• the converter was placed in vertical position and oxygen gas blowing was initiated while about 200 kg. stainless scrap of 18/ 8 type and 30 kg. quick-lime were successively added per ton of charged crude iron. Because of the low silicon content in the charge very little slag was formed and the re fining took place extremely rapidly, forming an extremely hot burning spot, thus reducing the oxidation of chromium.
• oxygen gas per ton crude iron had been blown onto the melt, it had approximately the following composition:
• a suitable melting furnace for example a cupola furnace with acid lining or an arc furnace 750 kg
• hematite pig iron containing 3.5% C and 1.5% Si was melted together with 230 kg. unrefined chromium containing 6% C, 8% Si and 54% Cr.
• the pig iron may be replaced by a corresponding quantity of scrap and carburizer, for example coke.
• the melt was tapped and, possibly after desulphurizing, charged into a shaking ladle at a temperature of 1450 C.
• chromium ore (44% Cr O 24.5% FeO) was added.
• the ladle was made to shake so that the reaction between lime and ore on the one hand and the silicon in the melt on the other hand was completed and a calcium silicate slag poor in chromium and iron was formed.
• the metal melt contained 3.9% C, 0.2% Si, 16.8% Cr, and a slag could be tapped off containing 3.2% Cr -O and 2% fe and having a basic value of 1.35.
• a method for manufacturing chromium alloyed steel comprising: admixing intimately, during a first chromium oxide reducing stage, while intensively stirring a carbonaceous iron melt with silicon as an alloying substance having a greater affinity to oxygen than chromium said silicon being furnished to the melt from compositions selected from the group consisting of silicon, ferrosilicon, and charge chrome and also admixing said melt with a charge comprising chromium oxide in a quantity whereby substantially the entire silicon in said alloying substance is oxidized and converted, by an added slag former of CaO or MgO, into slag, and whereby substantially all the chromium oxide in the material containing chromium oxide is reduced to metal; adjusting the quantity of silicon in the iron melt whereby the chromium reduced by silicon, together with any chromium already in the iron melt and chromium added during the continued treatment of the melt produce adesired final percentage of chromium in the material; maintaining during said first oxidizing stage a temperature of

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Mechanical Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Treatment Of Steel In Its Molten State (AREA)

Applications Claiming Priority (1)

Publications (1)

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Country Status (5)

Cited By (6)

• 1969
  • 1969-04-23 SE SE05803/69A patent/SE339700B/xx unknown
• 1970
  • 1970-02-17 US US00012143A patent/US3711278A/en not_active Expired - Lifetime
  • 1970-03-02 CA CA076190A patent/CA934967A/en not_active Expired
  • 1970-04-10 DE DE19702017210 patent/DE2017210A1/de active Pending
  • 1970-04-14 AT AT341070A patent/AT320704B/de not_active IP Right Cessation

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