US3802866A - Process for producing steels according to the oxygen top-blowing method - Google Patents

Abstract

The invention relates to a process for producing steels, in particular soft, rimming, chromium containing deep drawing steels from chromium containing pig iron according to the oxygen topblowing method. Before or at the onset of blowing 5 to 50 kg of solid chromium-oxide free slag with an analysis of Fe? 10-20%? SiO28-15% Mn4-10% CaO45-55% MgO1- 5% Al2O31- 5% P2O51- 3% and a grain size of 3 to 20 mm, are added to the pig iron per metric ton of metallic input, whereupon within the period of 5 to 20 percent of the total blowing period an amount of CaO is added into the converter as is necessary for reducing to slag any accompanying elements of the pig iron and for adjusting a desired degree of slag basicity. With this process the formation of liquid reactive slag is achieved more rapidly and the time for servicing the blowing lance is reduced. Yield losses are prevented and the life of the refractory lining of the converter is prolonged.

Description

United States Patent [1 1 I,

Rinesch et a].

[ 51 Apr. 9, 1974 PROCESS FOR PRODUCING STEELS ACCORDING TO THE OXYGEN TOP-BLOWING METHOD [73] Assignee: Vereinigte Osterreichische Eisenund Stahlwerke Aktiengesellschaft, Linz, Austria [22] Filed: Feb. 17, 1972 [21] Appl. No.: 227,304

[30] Foreign Application Priority Data Mar. 1, 1971 Austria 1713/71 [52] U.S. Cl 75/52, 75/55, 75/60 [51] Int. Cl C2lc 5/32, C2lc 5/36 [58] Field of Search 75/52-58, 59, 75/60 [56] References Cited UNITED STATES PATENTS 2,440,564 4/1948 Allard 75/52 3,376,130 4/1968 Kootz 75/52 Primary Eraminer-Charles N. Lovell Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or Firm-Brumbaugh, Graves, Donohue & Raymond 57 7 ABSTRACT The invention relates to a process for producing steels, in particular soft, rimming. chromium containing deep drawing steels from chromium containing pig iron according to the oxygen top-blowing method. Before or at the onset of blowing 5 to 50 kg of solid chromiumsidei es eey t a analysis of and a grain size of 3 to 20 mm, are added to the pig iron per metric ton of metallic input, whereupon within the period of 5 to 20 percent of the total blowing period an amount of CaO is added into the converter as is necessary for reducing to slag any accompanying elements of the pig iron and for adjusting a desired degree of slag basicity. With this process the formation of liquid reactive slag is achieved more rapidly and the time for servicing the blowing lance is reduced. Yield losses are prevented and the life of the refractory lining of the converter is prolonged.

4 Claims, No Drawings PROCESS FOR PRODUCING STEELS ACCORDING TO THE OXYGEN TOP-BLOWING METHOD The invention relates to a process for producing steels according to the oxygen top-blowing method, in particular for producing soft, rimming, chromium containing deep drawing steels from chromium containing pig iron, wherein basic slag formers and, if desired, fluxes are added for the formation of a fluid, reactive slag.

In the production of steel according to the oxygen top-blowing method difiiculties arise when a pig iron is refined that contains accompanying elements which are slagged to oxides that melt only with difficulty, whereby the formation of a fluid, reactive slag is delayed. Such a pig iron is for example a chromium containing steel pig iron containing about 4 percent C, 0.5 to 0.8 percent Si, 1.4 percent Mn, up to 0.2 percent P, up to 0.04 percent S and 0.9 to 1.2 percent Cr. It is blown with 0.2 to 0.4 percent chromium for the production of soft, rimming deep drawing steels that are resistant to ageing; see Rinesch R.: Die Verhuttung chromhaltiger Eisenerze bis zur Herstellung kaltgewalzter unberuhigter, alterungsbestandiger Tiefziehbleche mit 0.2 0.4 percent Chrom, Bergund Huttenmannische Monatshefte l 12 (1967), pages 402/412. ln blowing chromium containing pig iron so far a reduced life of the converter lining has been taken for granted for it is necessary to add fluxes to the slag when the process starts; the slag becomes viscuous and crumbly by the forming of Cr O These fluxes then lead at a higher temperature to the formation of too fluid a slag, which is highly aggressive and strongly attacks the refractory lining. A further problem which also is connected with the retarded formation of a liquid slag is the fact that a skull is formed at the blowing lance. So long as the slag is still dry," i.e., crumbly, the converter sprays, whereby iron droplets are flung upwardly and out of the converter. Thereby at the lower part of the blowing lance deposits of metal and slag are formed which have to be removed after each melt, a task that is highly difficult and necessitates a production interruption. Furthermore these skull formations may lead to a damage of the blowing lance. Similar difficulties occur when a pig iron poor in silicium and/or manganese, e.g., a pig iron comprising about 4 percent C, less than 0.3 percent Si, less than 0.6 percent Mn, up to 0.2 percent P and up to 0.04 percent S, is blown. In both pig iron sorts, a liquid slag that ends spraying is formed as a rule only 8 minutes after onset of blowing, i.e., after about 60 percent of the total blowing time, which amounts to 14 minutes.

Numerous attempts and proposals have been made to accelerate the formation of a fluid slag in the oxygen top-blowing method. Thus, it is possible e.g., to increase the distance between the bath surface and the blowing lance and to blow soften however, then the CO formed during refining is partially burnt in the con verter so that the refractory lining in the upper part of the converter is worn to a greater degree. According to another proposal fine granular fluxes, such as soft burnt lime, are continuously blown onto the bath together with the blowing stream as this is described e.g., in the British patent specification No. 883,958 and US. Pat. No. 3,004,847. However, for supplying the solid materials an expensive apparatus is necessary which is also liable to disturbances. Furthermore such solid supply plants necessitate partly an extremely fine granular material the preparation of which is expensive. The supply of dust-like lime for a rapid slag formation is therefore only justifiable when pig iron sorts containing a high amount of phosphorus are refined, because this solves above all metallurgical difficulties relative to dephosphorization and there is no problem of skull formation at the blowing lance. From the British patent specification No. 883,958 and US. Pat. No. 3,004,847 it is furthermore known that in refining Thomas pig iron, which takes place in two phases, the slag containing a high amount of P 0 being drawn off in between, the final slag is retained in the converter and used for the next charge. This liquid end slag as a rule contains a high amount of CaO so that it is used for the next charge also in order to save lime. Working with retaining the final slag or part thereof leads to difficulties when the liquid pig iron of the following charge is filled into the converter because strong, even explosion-like reactions may occur between the carbon of the pig iron and the iron oxide of the slag. A further disadvantage lies in that the weight and the composition of the final slag does not remain constant so that disturbances may arise in the course of the process of the following melt and faulty analyses may be obtained. Also the temperature of the end slag and its viscosity may be very different at the beginning of blowing of the subsequent m'elt so that great delays may occur with regard to the period when a reactive slag starts to be formed. Apart from these operational difficulties, this process, in which the final slag is used again for the following melt, cannot be used in refining pig iron sorts containing about 1 percent chromium, because they are refined in a one-step process so that the end slag contains the overall amount of the chromium reduced to slag as Cr O and thus cannot be used again. For overcoming the difficulties and for accelerating the slag formation in oxygen top blowing converters the German provisional publication No. 1,608,310 suggests the use of synthetic slag formers instead of powderized lime or pieces of lime or limestone; these slag formers are to be produced from a mixture of a calcium oxide with powderized, partly or completely sintered iron ore. The production of synthetic slags necessitates certain raw materials and is expensive so that its use has not been justified on a large industrial scale. According to a further proposal synthetic slag produced from red mud obtained from aluminum industry, from dolomite, lime and bauxite, is used in an oxygen top blowing converter in order to increase the life of the refractory lining by providing for a rapid formation of a fluid slag.

Finally, in this connection a proposal contained in the German printed application No. 1,783,013 should be mentioned, according to which, in the production of steel with carbon contents of 0.3 to 1.0 percent as catch charges according to the oxygen top-blowing method, for obtaining low P-contents part of the lime demand is added in the form of a lime-iron oxide mixture, which is also to be produced synthetically, within a period starting before onset of blowing and lasting until half of the blowing time; pellets of lime and red mud or of lime and blowing dust mud are to be produced and used; this, however, is likewise expensive.

The invention is aimed at creating an inexpensive process that may easily be carried out, with which the formation of skulls at the blowing lance and losses of yields by spraying of the metal at the onset of blowing are prevented and with which also the life of the refractory lining of the converter is to be improved.

Thus, in a process of the kind defined in the introduction, the invention resides in that to the pig iron before or at the onset of blowing per metric ton of metallic input 5 to 50 kg, preferably 20 to 30 kg, of solid chromium oxide free slag having an analysis of Fe -20% si02 8-! 5% Mn 4- i 0% CaO 45-5 5% MgO 1-5 7. ALO, l-5% 150 14% and a grain size of 3 to mm, preferably 3 to 8 mm, are added, whereupon within a period of 5 to 20 percent of the total blowing period, calculated from the onset of blowing, an amount of CaO in the form of burnt lime is added into the converter as is necessary for reducing to slag any accompanying elements of the pigiron and for adjusting a desired degree of slag basicity.

Preferably a slag is used whose pyrometric cone fall point (Seger) lies between l,300 and l,370C and whose flow point lies below l,380C.

Individual analyses of slags used in operational charges are listed below; their pyrometric cone fall point (Seger) lies between l,340 and l,350C and their flow point lies at 1,360C.

Such slags are obtained when chromium free steel pig iron sorts of customary composition are refined; since these slags are inexpensive, only the costs for their pulverisation have to be calculated for their use according to the invention. Thus a mill product may be used that is already present and need not be produced synthetically; in the scope of the process according to the invention it is its main task to form a slag protective layer preventing spraying at the onset of blowing so that the blowing lance is protected. Since in such steel work slags the CaO is for its greatest part bound to the remaining oxides, and the free lime portion participating in the metallurgical reactions is negligible, such a slag functions essentially only as a protective layer against the spraying of the converter before the metallurgical slag proper is formed. The solid chromium oxide free steel work slag may also be added already into the pig iron charge ladle so that it is preheated at the beginning of blowing and is liquefied yet more quickly. Only after the onset of blowing, namely within the period of 5 to 20 percent of the total blowing time, calculated from the onset of blowing, the lime necessary for the metallurgical reaction (dephosphorising, desulphurisation), is added, if desired with the addition of fluxes. When calculating the lime amount, obviously the Ca0- amount contained in the added solid slag is considered.

In blowing chromium containing pig iron sorts it has been possible by this method to reduce the time for the formation of a liquid reactive slag by at least 2 minutes; the slag formation thus may be ended within a period of about 40 percent of the overall blowing time. While customarily ore is added at the beginning of blowing to provide for a more rapid liquefaction of the slag this is no longer necessary and results in an increase of the scrap amount and thus in a reduction of the input costs. Also the addition of fluxes, which are always unfavourable for the refractory lining, may be reduced when a solid slag is used. Finally the reduction of the time necessary for servicing, i.e., for removing skull formations at the lance, is expressed in a considerable increase in efficiency. Also the addition of ferro-manganese into the casting ladle in the production of rimming deep drawing steels with 0.2 to 0.4 percent chromium may be lowered, because, when solid slag is used for the formation of a protective layer for the pig iron bath, the

outthrow of metal and slag droplets is prevented to a large degree, which droplets at the beginning of the process have a high manganese content; the manganese content of the steel is therefore somewhat higher prior to tapping than with steels which are produced in customary manner. In the following table operational results are listed which illustrate the advantages of the process according to the invention.

without the with the addition of addition of solid slag solid slag number of mclts (30 mt tapping 243 267 weight) total servicing time owing to skull 455 min 60 min formation at the blowing lance number of melts/day 42.8 48.8 yield pig iron good blocks 83.34 88.69 input per mt block steel fluid pig iron 944.37 kg 892.02 kg scrap 206.47 kg 260.37 kg Fe from ore 13.83 kg 0 kg lime 50.60 kg 43.37 kg fluorspar 4.48 kg 2.70 kg solid slag (3-8 mm) 0 kg 28.36 kg ladle input per rnt block steel Ferro-manganese 1.82 kg What we claim is:

l. A method for producing steel from pig-iron containing chromium by a single continuous oxygen topblowing process carried out in a converter, comprising the steps of:

adding to said pig-iron, prior to blowing, 5 to 50 Kg of solid chromium-oxide free slag per metric ton of metallic input, said slag being selected from basic slag formers and fluxes and having an analysis of Fe 10-20% Ste, 8-] 5% Mn 4-1o% CaO 45-55% MgO l-5% Ago, 1-s% P 0, 14%

and a grain size of 3 to 20 mm,

blowing oxygen into the converter, and

simultaneously therewith adding within a period of 5 to percent of the total blowing time, calculated from the onset of blowing, an amount of CaO in the form of burnt lime as is necessary for reducing to slag any accompanying elements of the pig-iron and for adjusting a desired degree of slag basicity.

2. The process set forth in claim 1, wherein said slag is added in an amount of 20 to kg per metric ton of

Claims (3)

1. 2. The process set forth in claim 1, wherein said slag is added in an amount of 20 to 30 kg per metric ton of metallic input.
2. 3. The process set forth in claim 1, wherein the grain size of said slag amounts to 3 to 8 mm.
3. 4. The process set forth in claim 1, wherein a slag is used whose pyrometric cone fall point lies between 1,300* and 1,370*C and whose flow point lies below 1,380*C.