Direct steel-making process

Abstract: Steel sheet is formed directly from iron ore concentrate by reducing a bed of iron ore concentrate, particularly high purity magnetized magnetic gamma hematite, to a low density hot cake of carburized metallic iron and then hot rolling the hot cake to a metallic sheet of full density (meeting market specification).

Description: The present invention relates to a direct steel-making process. Steel sheet is conventionally formed by initially casting molten steel into large ingots or slabs, which then are hot and cold rolled to the desired thickness. In US-A-2287663 and US-A-2296498, steel is made by reducing iron ore to form a free-flowing stream of hot iron particles. These are collected and compressed into the desired product. US-A-2 287 663 discloses a method of producing a steel sheet comprising the steps of reducing an iron ore powder and compressing said powder when still hot. One embodiment (Fig. 4) comprises the steps of reduction of the iron oxide powder, forming a hot cake of the thus formed metallic powder and rolling said cake to a steel strip. In accordance with the present invention, there is provided a method of making steel, comprising: reducing a bed of particulate iron ore concentrate to particles of metallic iron; and compacting the metallic iron to a metal sheet, characterised by forming the particles of metallic iron into a porous hot cake in which the particles are sintered together, and compacting the hot cake to the metal sheet. Preferred embodiments of the invention are defined in claims 2-14. Thin steel sheet meeting specifications for most present day industrial uses can be made directly from high purity iron ore concentrates, without melting, using the method described herein. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: Figure 1 illustrates the production of thin steel sheet from a low density metal cake produced by reduction of high purity iron ore, by the use of a rolling mill, in accordance with one embodiment of the invention; Figure 2(a) is a schematic representation of one embodiment of a steel-making apparatus for effecting the process of the invention; Figure 2(b) is a close-up view of the discharge end of the apparatus of Figure 2(a); Figure 2(c) is a sectional view of the pallet section of the apparatus of Figure 2(a); Figure 3 is sectional view of another embodiment of a steel-making apparatus for effecting the process of the invention; Figure 4 depicts graphically the relationships determining ore bed permeability for economic operation; and Figure 5 depicts graphically the relationship of gas pressure to gas volume. Referring to Figures 1 and 2(a) to 2(c), an apparatus 10 for effecting the process of the invention comprises a horizontal travelling grate furnace 12, enclosed within a housing 14 to permit a reducing gas atmosphere to be provided. Other furnaces may be employed such as batch furnaces (see Figure 3) or shaft furnaces. A preheat kiln 16 is provided at the upstream end of the grate to permit particles of iron ore concentrate feed to be preheated to the reducing temperature. The iron ore concentrate may be any desired composition, but preferably comprises magnetic gamma hematite of high purity, preferably +99.5% pure. The magnetic gamma hematite is preferably magnetised. The particle size of the iron ore concentrate determines the hot rolled grain size in the steel sheet produced from the concentrate. In general, the concentrate is preferably sized all minus 10 mesh with less than 15% minus 200 mesh, and more preferably from minus 50 mesh to plus 200 mesh. The preheated particulated iron ore concentrate is deposited as a bed 18 onto articulated grates 20 to the desired depth and bulk density. The grates 20 have a plurality of openings 22 formed therethrough to permit the reducing gas to pass through the bed 18. The bed of iron ore concentrate generally has a bulk density of about 2400 to about 4000 kg/m 3 (about 150 to about 250 pounds per cubic foot), preferably about 2800 to about 3200 kg/m 3 (about 175 to about 200 pounds per cubic foot). The bed depth generally is at least about 6 times the spacing between the gas flow openings 22, so as to develop sufficient back pressure on the reducing gas passing through the openings 22 to distribute the gas between the openings and provide a uniform gas flow and distribution in the bed. For example, there may be provided 1.6 mm (one sixteenth inch) holes through 2.54 cm (one inch) thick grates, with the hole spacing less than one-tenth bed thickness. The iron ore concentrate bed depth preferably is a maximum 2.54 cm (one inch) per 30.5 cm (foot) of bed width. The ore concentrate bed 18 may have any desired transverse and longitudinal dimensions, depending on the size of steel sheet desired to be produced. A reducing gas stream having a temperature sufficient to reduce the metal oxide to metallic form but insufficient to form molten metal from the reduced metal oxide, is passed through the openings 22 in the grate into and through the ore concentrate bed 18. In general, the temperature of the gas stream ranges from about 1150° to about 1300°C, preferably approximately 1200°C. The reducing gas stream preferably is provided by a mixture of carbon monoxide and hydrogen, preferably in a ratio of CO:H 2 of from about 2:1 to about 3:1. The reducing gas mixture preferably is supplied at a minimum rate of about 2500 cubic meters per metric ton of iron produced (80,000 cu ft/2000 lbs of iron). Employing a reducing gas temperature of about 1200°C, reduction of high purity magnetic gamma hematite to hot metallic iron is effected in less than one hour. Following reduction of the metal oxide to metal, the metal cake so formed is maintained at the reduction temperature, i.e. preferably approximately 1200°C, and exposed to a carbon gas atmosphere, such as the equilibrium mixture of carbon monoxide and carbon dioxide, for a sufficient time to provide a desired level of carburizing to control the carbon content of the steel and to sinter the metal particles together to form a hot, light-weight, porous steel cake for discharge from the grate furnace housing 14. The porous steel cake 24 generally is formed with a thickness 6.66 times the desired thickness of hot-rolled sheet to be produced therefrom by rolling the cake to 15% of cake thickness. The cake is passed through the nip 26 between a first pair of counter-rotating rolls 28, which cause compaction of the metal particles in the sheet to the full density (i.e. 7.8 g/cc) of a hot-rolled sheet 30 and at the same time causing elongation of the sheet. The cake 24 is fed through the nip 26 at the same rate as the rate of rotation of the rolls 28 and the desired rate of feed typically is achieved by dropping the cake vertically into the nip 26 and acquiring the required speed by the effect of gravity. A hot 2.54 cm (one-inch) thick steel cake at 1200°C may be reduced 85% in thickness by rolling in the first pass to full density at 3.8 mm (0.15 inches) thickness. The hot steel cake is of low density and high porosity such as to permit all gases contained in the porous structure to be squeezed out of the cake as it is rolled, as seen schematically in Figure 1. However, the hot steel cake should have sufficient strength to permit it to be rolled to a sheet in the first hot pass. The use of a closed die to press the cake to the same thickness would entrap enclosed gases, so that the resulting steel sheet would have less desirable properties than those developed by the direct rolling method described above. In addition to forming individual steel sheets from individual hot, low density steel cakes, a long continuous strip of hot, low density steel cake, for example, 91 cm wide by 2.54 cm thick (three feet wide by one inch thick), may be formed and processed to provide a continuous hot-rolled steel strip, by feeding the continuous cake strip through the rolls 28 at the rate of rotation of the rolls 28. However, when, for example, the product to be made is twenty gauge, 91 cm by 244 cm (three foot by eight foot) corrugated siding, there is no need for a long continuous cake or to provide equipment to make, and store large coils of steel sheet, but rather individual hot cakes may be formed to provide the end product at design dimensions. Pilot plant tests have shown that long furnaces (say 76.2 m (250 feet) long), using a travelling grate system can produce the individual cakes 24 at about 3 m (10 feet) per minute and these individual cakes then can be fed to a standard rolling mill 28 at about 3 m (10 feet) per second. Rolling of steel elongates the sheet and hence the steel sheet must move relative to the roll face as it is compressed and elongated. When rolling low density steel cakes, the relative motion is not as great, and involves a much smaller area of roll face. Edge rolling may be used to limit spread to 10% of cake width. An increase of more than 50% in length is the required level to indicate that the sheet has been sufficiently worked after reaching full density (namely 7.8g/cc). The following Table I provides typical results for a single pass hot rolling of low density porous cake to full density: As mentioned above, it is conventional practice to make steel sheets from large ingots or slabs cast from molten metal. While solidifying and cooling, large crystals or grains grow in the metal. Hot and cold rolling of the steel sheet to the final desired size reduces the grain size to much finer sizes to provide increased strength and toughness. The steel sheet produced by the method of the present invention has a fine grain size in the hot rolled state, mainly due to the fine particle size of the starting ore concentrate feed. The direct steel process described herein provides a means of controlling hot rolled grain size in the steel, by controlling the iron ore concentrate particle size. The hot-rolled steel sheet 30 has the same grain structure and ductility as standard hot rolled steel has after the standard steel sheet has been further cold rolled and annealed to refine and improve its grain structure and ductility. The hot-rolled steel sheet 30 may be subjected to further hot or cold rolling and annealing methods to meet particular specifications, as carried out for any other steel of the same analysis. For example, the sheet may be cold-rolled by additional counter-rotating rolls 32 to decrease the thickness from 3.8 mm to 2.5 mm (0.15 inch to 0.1 inch) to form a steel sheet 34. In Figure 3, there is illustrated a batch furnace 50 wherein there is upward gaseous flow through first a coke bed 52 and then through two permeable beds 54, 56 of magnetized maghemite iron ore, to form a hot porous steel cake by reduction. Figure 5 illustrates the required gas pressure for various bed depths to provide a desired gas flow of 0.03 m 3 /kg (0.5 cu ft/lb) of ore for an iron concentrate feed of known bulk density of 1618 kg/m 3 (100 lb/cu ft). Figure 4 illustrates the practical limits of ore concentrate bulk density and bed porosity for operating the system profitably. While the method of the invention mainly is useful in producing a single layer steel sheet, the procedure also may be employed to form a multiple layer structure, by providing a bed 18 of suitable layers of oxides. One novel cladding material which may be made by forming a three layer rust-resistant steel sheet which is formed from a layered oxide mass comprising: First layer - nickel oxide/concentrate mix - 15% Second layer - concentrate - 70% Third layer - nickel oxide/concentrate - 15% Such cladding mixture may contain 4% nickel oxide or 12% nickel oxide.

Claims: A method of making steel, comprising: reducing a bed of particulate iron ore concentrate to particles of metallic iron; and compacting the metallic iron to a metal sheet, characterised by forming the particles of metallic iron into a porous hot cake in which the particles are sintered together, and compacting the hot cake to the metal sheet. A method as claimed in claim 1, wherein the iron ore concentrate is magnetic gamma hematite containing less than 0.5 wt% impurities. A method as claimed in claim 2, wherein the magnetic gamma hematite is magnetised. A method as claimed in any one of claims 1 to 3, wherein the bed of particulate iron ore concentrate has a bulk density in the range of from 2400 to 4000 kg/m 3 (150 to 250 pounds per cubic foot) and is composed of particles of iron ore concentrate sized in the range of from minus 50 mesh to plus 200 mesh. A method as claimed in any one of claims 1 to 4, wherein the bed of particulate iron ore concentrate is reduced by passing therethrough a stream of reducing gas which comprises a mixture of carbon monoxide and hydrogen at a temperature which is at least sufficient to effect the reduction but not sufficient to melt the iron. A method as claimed in claim 5, wherein the reducing gas stream comprises a mixture of carbon monoxide and hydrogen having a ratio of CO:H 2 in the range of from 2:1 to 3:1. A method as claimed in any preceding claim, wherein the reduction is effected at a reduction temperature in the range of from 1150 °C to 1300 °C. A method as claimed in claim 7, wherein the reduction temperature is 1200 °C. A method as claimed in claim 7 or claim 8, wherein the hot cake is formed by maintaining the particles of metallic iron at the reduction temperature to sinter the particles together, and exposing the particles of metallic iron to a carbon gas atmosphere to carburise the metal in the cake. A method as claimed in claim 9, wherein the carbon gas atmosphere is provided by a mixture of carbon monoxide and carbon dioxide. A method as claimed in any preceding claim, wherein compaction of the cake expels gases therefrom and provides a metal sheet having a full density of 7.8 g/cc. A method claimed in any preceding claim, wherein the cake is compacted by feeding the cake through a nip between counter-rotating rollers at a linear speed corresponding to the rotational speed of the rollers. A method as claimed in any preceding claim, wherein the cake is hot rolled to form a sheet having a thickness of no greater than 15% of the thickness of the cake. A method as claimed in any preceding claim, wherein the cake is hot rolled to form a sheet having a length at least 50% greater than the length of the cake.

Inventors: CAVANAGH PATRICK E, CAVANAGH, PATRICK E.
Applicants: VIRGIN METALS CANADA LTD, VIRGIN METALS (CANADA) LIMITED
Application year: 1993
Main classification: C21B13
IPC codes: C21B13, C21B11