JPH06172916A - Manufacturing of stainless steel - Google Patents

Abstract

PURPOSE: To economically produce a stainless steel with high efficiency by finely pulverizing the oxide of an alloy metal and a reducing agent, intimately mixing them, thereafter forming this mixture into an agglomerated form and adding it to a molten iron bath.

CONSTITUTION: The oxide of an alloy metal in a steel such as Cr, Mn, V, Ni, Co, Mo or the like is finely pulverized or gravitated and is intimtely mixed with a finely pulverized or granulated reducing agent, and this mixture is thereafter formed into an agglomerated form. As for the finely pulverizing or granulating, the maximum grain diameter is regulated to 1 mm, preferably to 75 μm. The oxide can be used in the form of an ore such as a chromite ore or the like. The quantity of the reducing agent is regulated, e.g. to at least about 0.4 times, preferably about 1.3 times the stoichiometric amt. by which the total oxides of Cr and Fe in the oxides are reduced. As the reducing agent, ferrosilicon, ferroalumina-silicon or the like are given. The agglomerated form is briquettes, pellets or the like and is treated at a relatively low temp. according to necessary to promote the solid phase reduction thereof, thereafter, the agglomerated body is added to a molten iron bath or is heated to melt together with solid iron so as to be alloyed.

COPYRIGHT: (C)1994,JPO

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to the production of stainless steels and, more particularly but not exclusively, prior to addition to iron, conventional chromium such as high carbon ferrochrome is produced. Contrary to known methods involving pretreatment of ores, it relates to the production of stainless steel in which a particular ore form, chromium oxide, is added substantially directly to iron.

[0002]

BACKGROUND OF THE INVENTION In this specification, reference is frequently made to the production of stainless steel. It is not intended to limit the scope of the invention in any way. Stainless steels most commonly produce high carbon alloys by alloying iron and high carbon ferrochrome, then subjected to argon-oxygen decarburization to reduce carbon concentrations to acceptable levels. .

It is also possible to avoid the argon-oxygen decarburization step by using low carbon ferrochrome as the chromium-containing alloy component for producing chrome steel.
It is implemented on a relatively small scale.

There are various methods for producing stainless steel in which chrome is added directly in the form of chromite.
In particular, the so-called Hamilton-Evans method involves the melting of iron and the generation of large amounts of slag known as reception slag (a mixture of limestone, fluorspar, millscase (oxide film) and chromite) on molten steel. When the reception slag is melted and heated sufficiently, a chromium ore intimately mixed with high quality ferro-silicon is provided on top of the slag. The use of receptor slag is necessary to supply the heat and chemical buffer to the reduction reaction that takes place and to allow the alloy droplets to reach the melt through the slag. Receptor slag also prevents sequestration of reactants at the surface of the melt, which in turn results in separation of chromite and dissolution of the reducing agent in the melt;
The high basicity of the receptor slag helps prevent silica enrichment of oxide residues and protects the furnace lining from excessive wear.

The disadvantage of this method is the low efficiency with respect to silicon utilization and chromium recovery; the tap-to-tap time is long (5 hours range) due to the bulky mass of molten slag; A considerable amount of Cr ₂ O ₃ is contained in the slag; and excessive wear of the furnace backing results from these drawbacks.

The so-called Wild method is similar to the above except that the silicon content can be kept below 0.5% by the appropriate addition of suitable chromite and iron-silicon.
The slag obtained in the reduction step is sometimes removed, and the time from tapping to tapping is believed to be too long, about 8 hours.

Once it is possible to produce low-carbon ferrochrome, the aforementioned steps are gradually abandoned, firstly by alloying with low-carbon ferrochrome, and then by the above-mentioned current method, namely It appears to be replaced by alloying with high carbon ferrochrome followed by argon-oxygen decarburization.

The required alloying metal can be incorporated substantially directly into the iron substrate in the form of an oxide or ore of such a metal,
It is an object of the present invention to provide an efficient method of manufacturing steel.

[0009]

DETAILED DESCRIPTION OF THE INVENTION Chromium, manganese,
A finely divided oxide obtained by intimately mixing at least one kind of alloying metal for steel selected from vanadium, nickel, cobalt and molybdenum with a finely divided reducing agent, wherein the mixture of the oxide and the reducing agent forms a lump. Including adding in,
A method of making steel is provided.

Another aspect of the invention is that the finely divided oxide has a maximum particle size of 1 mm, preferably less than 75 microns,
In this case, the maximum particle size of the reducing agent is about 1 millimeter, preferably less than about 75 microns; the reducing agent is about 0 of the theoretical amount required to reduce all alloying metal and iron oxidants in the oxide. .4 times and preferably about 1.3 times the stoichiometric amount present; and for oxides it is provided as an ore.

The bulk mixture of chromium oxide and reducing agent can be in the form of briquettes, pellets or other agglomerate units and can be added to the bath of molten iron present.
Alternatively, it can be heated with the solidifying iron to be alloyed with the alloying metal. Any conventional or suitable binder can be used for the aggregates.

In each case, experiments have shown that melting occurs with increasing temperature of the agglomerating units after solid state reduction of the alloying metal oxide.

The test shows that solid state reduction starts at about 1200 ° C. and progresses rapidly toward completion at 1300 ° C. and 1400 ° C., while melting of the material occurs at 1500 ° C. and 1550 ° C.
It happens at ℃. Solid state reduction occurs very rapidly and batches of stainless steel can be processed using, for example, the method of the present invention.
It can be manufactured in a short time.

If the alloying metal is chromium, it may be added in the form of chromite and the reducing agent may be ferrosilicon or ferro-aluminum-silicon. In such cases, the reduction can be reduced to 14% even in oxidizing environments such as air.
Substantial completion is reached at 00 ° C.

The production of steel is carried out by any suitable furnace, including electric arc furnaces, plasma arc furnaces, induction furnaces, etc., in order to supply the minimum energy needed to make up the reduction process and heat losses. Can be done at. In order that the present invention may be more fully understood, various tests conducted to date are set forth.

Detailed description of the tests carried out Three tests were carried out, the first one being a small scale crucible test and the other one being a further scale-up 2
Tests on a 00 kVA DC transfer plasma arc furnace and a 300 kVA three-phase AC electric arc furnace. The results of these tests are described below.

Crucible Batch Test Composite pellets were prepared using fine particles of chromite and ferrosilicone, often using bentonite as a binder, and various tests were conducted on it.
It will be appreciated that the composition of the pellets is as shown in Table 2 under "Charge (g)" and that the different ingredients were used in different proportions. The raw material analysis is shown in Table 1.

[0018]

[Table 1]

In each case, the moist pellets 120
The oven dried at 24 ° C. for 24 hours and the weighed composite pellets and iron flakes were heated to 1550 ° C. for 1 hour in an alumina crucible in a small vertical tube furnace in an air atmosphere. The compositions of the produced metal and slag were analyzed, and the results are shown in Table 2.

[0020]

[Table 2]

[Table 3]

DC Plasma Arc Test This test was carried out in a 200 kVA furnace manufactured by the applicant. The furnace was a known DC plasma arc furnace using a single graphite electrode centered on the furnace bath.
Supplying DC power to form a molten bath and make it an anode,
The graphite electrode was the cathode. Outer diameter 830mm,
A furnace with a refractory coating thickness of 140 mm was coated with a refractory material with an Al ₂ O ₃ content of about 90%. Hearth made of similar material 300 mm
Thickly coated, a number of mild steel rods were used to create a direct current (anode) current path from the molten bath through the hearth to the anode cable.

The charge material consisted of steel scrap, nickel and chromite granules and composite pellets made from ferrosilicon using bentonite as a binder. The composition of the pellets is shown in Table 4, and the raw material analysis is shown in Table 3.
Prior to testing, the pellets are air dried for 24 hours and further heated to 800 ° C for about 4 hours to strengthen the pellets.

Each heating or test consists of two stages. First
The stage is to melt steel scrap and nickel for about 30 minutes, and the second stage is to feed and melt chromite and ferrossili composite pellets for an additional 1.5 hours. In both stages, the furnace power and energy are adjusted accordingly to maintain the melt temperature between 1550 ° C and 1600 ° C. All feed materials were hand-fed to the furnace through a charging port on the roof of the furnace. The compositions of the produced metal and slag were analyzed, and the results are shown in Table 4.

[0024]

[Table 4]

[Table 5]

[Table 6]

Three-Phase AC Open Arc Furnace Test This test was conducted using a 300 kVA three-phase AC open arc furnace manufactured by Applicant. The furnace was of the known type using a 3 graphite electrode with a 500 mm PCD centered on the bath. Three-phase AC power supply was provided to the molten bath by electrodes.
The furnace has an outer diameter of 965 mm and has an outer lining of magnesia and an inner lining of alumina refractory brick (Al ₂ O ₃ , 90%). The hearth is lined with cast alumina, which has holes with plugs to minimize the amount of material retained in the furnace during testing and to eliminate dilution effects.

The feedstock consists of steel scrap, nickel and composite pellets. The composite pellet is formed by using bentonite or sodium silicate as a binder from chromite fine particles and ferrosilicon. The bentonite pellets are air dried for 24 hours and then heated at 800 ° C. for 4 hours to cure. The sodium silicate pellets are used after being air-dried for 12 hours and then oven-dried at 120 ° C. for 12 hours. The composition of chromite, ferrosilicon and scrap is as shown in Table 5,
The pellet composition for each test is shown in Table 6.

[0027]

[Table 7]

As in the case of the 200 kVA DC plasma arc test, each heating or test consists of two stages: melting and melting. During the melting, the steel scrap and nickel were hand-fed for about 30 minutes and then the composite pellets were fed with an automated feeder for about 1 hour. The power supply to the furnace was adjusted to maintain a working temperature of 1550 ° C. The furnace opened after each test to remove most of the material and the solids and analytical results are shown in Table 6.

[0029]

[Table 8]

[Table 9]

[0030]

[Table 10]

[Table 11]

200 kVA to illustrate the method of the invention
Results of tests conducted using a plasma arc furnace and a 300 kVA electric arc furnace show lower chromium recovery in the melt than expected from the initial crucible test. It is considered that the reason for this is that the working conditions are different between the case of the plasma arc furnace or the electric arc furnace and the case of the tubular furnace used in the crucible test. In the case of plasma arc furnaces and electric arc furnaces, the maximum temperature is higher and faster than in the case of tubular furnaces.
The faster the melt forms, the shorter the solid phase reaction time in the pellet and the greater the proportion of chromium entering the slag layer.

For the above reasons, the benefits of the present invention include the use of a two-step process in which the agglomerated material is initially treated at a relatively low temperature to facilitate solid state reduction and thereafter charged, for example, in a high temperature electric arc furnace. It is believed to be maximal by doing. Rotary kilns and rotary floor kilns are suitable. In such a way, the alloying metal will become a higher proportion of the melt. From the results presented above, it will be apparent that the direct use of chromite as a chromium source as in the present invention produces good quality stainless steel.

The scope of the invention is not limited solely to the method described herein, but is agglomerates of chromium oxide as an intermediate and a suitable reducing agent and, if desired, a binder sold to steel producers. Is also intended to be included. Furthermore, the present invention is likewise applicable to the production of other alloy steels containing chromium, manganese, vanadium, nickel, cobalt or molybdenum alone or in combination.

Claims (14)

[Claims] 1. An alloy metal with at least one steel selected from the group consisting of chromium, manganese, vanadium, nickel, cobalt and molybdenum, in the form of an oxide obtained by pulverizing or alloying the alloy metal. And in the form of intimate admixture and agglomeration with a finely divided or finely divided reducing agent. 2. The method according to claim 1, wherein the finely divided or finely divided oxide has a maximum particle size of 1 mm. 3. The method according to claim 1, wherein the finely divided or finely divided reducing agent has a maximum particle size of 1 mm. 4. The method of claim 2 wherein the maximum particle size of the oxide is 75 microns. 5. The method of claim 3 wherein the maximum particle size of the reducing agent is 75 microns. 6. The reducing agent is present in an amount of at least about 0.4 times the stoichiometric amount required to reduce all of the chromium oxides and iron oxides in the oxide. The method described in any one of. 7. The stoichiometric amount of the reducing agent required to reduce all of the chromium oxides and iron oxides in the oxide is about 1.3.
7. The method according to claim 6, which is present in a doubled amount. 8. The oxide according to claim 1, which is in the form of an ore.
7. The method according to any of items 7. 9. The method according to claim 1, wherein the aggregated state of the mixture of oxide and reducing agent is briquette, pellet or other similar shape. 10. The method according to claim 1, wherein an aggregate of oxide and reducing agent is added to the molten iron bath. 11. The oxide and reducing agent agglomerates are heated with the solid iron with which the alloy metal is to be alloyed.
Item 11. The method according to any one of item 10. 12. The solid-state reduction is promoted prior to addition to the molten iron bath by initially treating the aggregate of oxide and reducing agent at a relatively low temperature. the method of. 13. The method according to claim 1, wherein the oxide is chromite ore. 14. The method according to claim 1, wherein the reducing agent is at least one of ferrosilicon and ferroaluminosilicon.