Gas atomization method of molten steel

Abstract

The present invention relates to a process for producing steel powder, comprising the steps of providing molten iron from a blast furnace, smelting the molten iron in a converter to form molten steel, refining the molten steel in a vacuum arc degasser to obtain refined molten steel containing less than 20-600 ppm C, less than 15-120 ppm S, up to 125 ppm P, up to 80 ppm N and up to 30 ppm O, pouring the refined molten steel into a number of induction furnaces, adding at least one ferroalloy, pouring the molten steel from each induction furnace into a dedicated reservoir connected to at least one gas atomizer, feeding the molten steel under pressure from each reservoir to at least one gas atomizer in each reservoir and gas atomizing the molten steel to form steel powder with a desired composition.

Description

Technical Field

[0001] The present invention relates to the production of steel powder, and more particularly to the production of steel powder by gas atomization for additive manufacturing. The present invention also relates to equipment for producing such steel powder.

Background Art

[0002] The demand for steel powder for additive manufacturing is increasing, and as a result, it is necessary to adapt the manufacturing process.

[0003] In particular, it is known to melt a metal material in an electric furnace or a vacuum melting furnace, refine the composition, and pour the molten steel into a tundish connected to an atomizer. Such a batch process is not suitable for the need to produce large amounts of steel powder, preferably in continuous mode.

Summary of the Invention

Problems to be Solved by the Invention

[0004] Therefore, an object of the present invention is to improve the drawbacks of the prior art equipment and processes by providing a general-purpose process for producing steel powder. In particular, the object is to provide a process that can use different raw materials and produce powders with different steel compositions according to requirements, while operating in continuous mode in some cases.

Means for Solving the Problems

[0005] For this purpose, the first main subject of the present invention consists of a process for producing steel powder comprising the following steps. - Providing molten iron from a blast furnace; - Refining the molten iron in a converter to form molten steel containing up to 600 ppm of C, up to 120 ppm of S, up to 125 ppm of P, up to 50 ppm of N and up to 1200 ppm of O; - A step of refining molten steel in a vacuum arc degassing apparatus to obtain refined molten steel containing less than 20-600 ppm of C, less than 15-120 ppm of S, up to 125 ppm of P, up to 80 ppm of N, and up to 30 ppm of O. - A step of pouring refined molten steel into multiple induction furnaces, - Adding at least one type of ferroalloy to each of the plurality of induction furnaces to adjust the steel composition to the desired steel composition of the steel powder. - A step of pouring molten steel of the desired composition from each induction furnace into a dedicated reservoir connected to at least one gas atomizer, - A step of supplying molten steel from each reservoir under pressure to at least one gas atomizer in each reservoir, and gas atomizing the molten steel to form steel powder of a desired composition.

[0006] The processes according to the present invention may also have the following optional features, which may be considered individually or in combination: - This process is continuous. - Molten iron from the blast furnace is desulfurized to contain less than 50 ppm of sulfur by weight. - Molten steel contains C at 250 ppm or less and / or P at 90 ppm or less and / or N at 25 ppm or less. - During refining in a vacuum arc degassing apparatus, molten steel is decarburized by using the dissolved oxygen in the steel. - During refining in a vacuum arc degassing apparatus, molten steel is deoxygenated until its dissolved oxygen content is 4 ppm or less. - During refining in a vacuum arc degassing apparatus, molten steel is desulfurized by stirring slag against the steel. - The temperature of the refined molten steel at the end of the refining process in a vacuum arc degassing apparatus is between 1580 and 1680°C. - The refined molten steel is poured directly into multiple induction furnaces from a vacuum arc degassing unit. - The refined molten steel is first poured into a tundish, and then poured from the tundish into several induction furnaces. - The tundish allows for the simultaneous pouring of refined molten steel into all induction furnaces. - The temperature inside the tundish is maintained between 1520 and 1620°C. - Purge the tundish with argon to control the oxygen content in the tundish. - The temperature inside multiple induction reactors is maintained between 1500 and 1700°C. - The temperature in at least one of the multiple induction reactors is maintained between 1620 and 1650°C. - The iron alloy added to the induction furnace is not pre-melted. - Scrap or directly reduced iron or silicide alloy or nitride alloy or pure element or mixture thereof is added to at least one of a plurality of induction furnaces. - Induction reactors do not have an atmosphere control. - At least one of the multiple induction reactors is a vacuum induction reactor. - The atmosphere of each dedicated reservoir is argon, nitrogen, or a mixture thereof. - The temperature in each dedicated reservoir is maintained at 1300-1750°C. - The temperature in each dedicated reservoir is at least 150°C above the liquidus temperature of the molten steel.

[0007] A second subject of the present invention is an apparatus for producing steel powder, comprising the following: - blast furnace, - A converter capable of refining molten iron to form molten steel containing up to 600 ppm of C, up to 120 ppm of S, up to 125 ppm of P, up to 50 ppm of N, and up to 1200 ppm of O. - A vacuum arc degassing apparatus that can refine molten steel to obtain refined molten steel containing less than 20-600 ppm of C, less than 15-120 ppm of S, up to 125 ppm of P, up to 80 ppm of N, and up to 30 ppm of O. - Multiple induction reactors, - A ferroalloy supply unit capable of supplying at least one type of ferroalloy to the plurality of induction furnaces, - A dedicated reservoir for each induction reactor, each dedicated reservoir being connected to at least one gas atomizer and pressurizable.

[0008] The facility according to the present invention can also have the following optional features, considered individually or in combination. - The facility further comprises a tundish capable of simultaneously pouring molten steel into all induction furnaces. - The tundish is arranged above a plurality of induction furnaces.

Mode for Carrying Out the Invention

[0009] Other features and advantages of the present invention will be described in more detail in the following description.

[0010] This invention is provided for purely illustrative purposes and is not intended to be limiting in any way. It will be better understood by reading the following description.

[0011] In the first step of the process, molten iron (or pig iron) is provided from a blast furnace.

[0012] Conventionally, the blast furnace is supplied with solid materials filled in its upper part, mainly sinter, pellets, iron ore, and carbonaceous materials, generally coke, called the throat of the blast furnace. The iron-containing charged raw materials (sinter, pellets, and iron ore) are conventionally converted to pig iron by reducing iron oxide with a reducing gas (especially containing CO, H2, and N2), and this reducing gas is formed by the combustion of carbonaceous materials in the tuyere located at the lower part of the blast furnace, where air is preheated to a temperature between 1000 and 1300 °C, called hot air, and injected.

[0013] Pig iron and slag are discharged from the molten pool at the bottom of the blast furnace. The pig iron is poured into a ladle, which is then conventionally poured into a converter (or BOF, meaning basic oxygen steelmaking furnace) pre-filled with scrap.

[0014] Pig iron can be transported directly to the converter or can first be pretreated before being poured into the converter. According to one variant of the invention, the pig iron from the blast furnace is sent to a hot metal desulphurization station before being poured into the converter. In that case, the pig iron is preferably desulphurized to contain less than 50 ppm S by weight. This desulphurization step facilitates the refining of the downstream molten steel and thus the desired steel composition is obtained.

[0015] In the second step of the process, the molten iron is refined in the converter to form molten steel containing up to 600 ppm C, up to 120 ppm S, up to 125 ppm P, up to 50 ppm N and up to 1200 ppm O by weight.

[0016] The process of refining iron to steel includes a step of blowing oxygen to decarburize the iron and a post-blowing step of blowing a neutral gas such as argon. Lime and / or dolomite are added to the converter to remove impurities such as silicon, phosphorus, and manganese and to reach the levels of impurities required for the desired steel composition. These additives, together with the impurities extracted from the pig iron, form the converter slag.

[0017] Since the decarburization reaction releases energy, scrap is usually added to control the temperature of the liquid steel being produced. Mineral additives such as lime, dolomite, limestone can be further added to control the chemical composition and temperature of the liquid steel being produced. These mineral additives can also be used to monitor the chemical composition of the slag since the slag composition affects the equilibrium between the liquid steel and the slag and thus the promotion of the reactions occurring in the liquid steel.

[0018] In the present invention, in order to provide a general composition compatible with all possible powder compositions being produced, the composition contains up to 600 ppm C, up to 120 ppm S, up to 125 ppm P, up to 5 ppm N and up to 1200 ppm O by weight at the end of the refining step in the converter, with the balance being iron and inevitable impurities resulting from the process.

[0019] In certain cases where it is necessary to manufacture a powder composition with stringent requirements, the composition may be further limited to C up to 250 ppm and / or P up to 90 ppm and / or N up to 25 ppm.

[0020] The molten steel from the converter is then tapped into a regenerated ladle. Preferably, to minimize slag carryover from the converter, only the first heat of the sequence is tapped into a regenerated ladle to be transported to the next step in the process according to the present invention. The remaining steel and slag are later tapped into a standard steel ladle during the tapping process and transferred to another part of the plant for another process. By minimizing slag carryover, additional deoxidation is prevented and the level of impurities in the molten steel is reduced.

[0021] The molten steel is transferred from the converter to a vacuum arc degassing unit (VAD). In particular, the regenerated ladle is poured into the preheated ladle of the VAD. This transfer is preferably carried out without controlling the atmosphere.

[0022] In the third step of the process, the molten steel is refined in a vacuum arc degassing unit.

[0023] The main purpose of this step is to decarburize, deoxygenate, and desulfurize the molten steel. Optionally, this step may include primary alloying of the molten steel.

[0024] A vacuum arc degassing apparatus is a vacuum chamber or pit in which a preheatable ladle is placed. The chamber or pit completely encloses the ladle. Addition of the chamber is done through a vacuum lock located on the lid of the chamber and directly positioned above the ladle. The lid is also fabricated so that the graphite electrode can move up and down without breaking the vacuum seal, thus allowing arc heating under vacuum. Argon stirring and subsequent purging are usually started as soon as the steel reaches the ladle and continue throughout the entire VAD process. Once the ladle is filled, the lid is fitted from above and sealed. Vacuum is generated by a vacuum pump device through side wall ports.

[0025] In the first substep of the process, the molten steel is reheated to a temperature preferably between 1580 and 1650°C. This is done by electric arc heating by lowering an electrode into the molten steel.

[0026] In the second substep of the process, it is preferable to add flux to cover the molten steel and prevent it from foaming during subsequent substeps.

[0027] In the third substep of the process, a vacuum is applied to reduce the pressure inside the chamber, preferably to less than 1 Torr, to perform initial degassing and deoxygenation.

[0028] In the fourth substep of the process, the steel is preferably decarburized. This can be done by using the dissolved oxygen in the steel, or optionally by using a consumable oxygen lance. Decarburization can be carried out until the carbon content is as low as 20 ppm.

[0029] After decarburization, the vacuum arc degassing apparatus preferably continues to operate under vacuum to clean it.

[0030] In the fourth substep of the process, the steel is preferably deoxygenated. This can be done by adding a desulfurization material to the ladle so that the steel is first deoxygenated and then desulfurized. Examples of such materials are lime, Ca-aluminate flux, and fluorite (fluorospar). The slag is deoxygenated at the same time. Deoxygenation can be carried out until the dissolved oxygen content is 4 ppm or less, or until the total oxygen content is less than 30 ppm.

[0031] In the fifth substep of the process, the steel is preferably desulfurized. This can be done by stirring the slag with respect to the metal (under or without vacuum), and stirring promotes the desulfurization reaction. In this case, it is more preferable to use reducing slag containing a mixture of burnt particulate lime and fluorite (typically 1.5-2.0% of the packing weight).

[0032] In the sixth substep of the process, primary alloying of the molten steel can be optionally carried out by adding an iron alloy, silicide alloy, nitride alloy, pure metal, or a mixture thereof. This primary alloying is particularly interesting when all the different steel powders to be produced in multiple gas atomizers have a given alloying element in common. This can be done either as a single addition or in stages.

[0033] Iron alloys refer to various alloys of iron with one or more other elements in high proportions, such as silicon, niobium, boron, chromium, aluminum, manganese, and molybdenum. The main alloys are FeAl (usually containing 40-60 wt% Al), FeB (usually containing 17.5-20 wt% B), FeCa, FeCr (usually containing 50-70 wt% Cr), FeMg, FeMn, FeMo (usually containing 60-75 wt% Mo), FeNb (usually containing 60-70 wt% Nb), FeNi, FeP, FeS, FeSi (usually containing 15-90 wt% Si), FeSiMg, FeTi (usually containing 45-75 wt% Ti), FeV (usually containing 35-85 wt% V), and FeW (usually containing 70-80 wt% Mo).

[0034] Silicide alloys can be MnSi, CrSi, or CaSi in particular. Nitride alloys can be MnN.

[0035] Pure metals can include iron, copper, nickel, cobalt, chromium, calcium, and rare earth metals.

[0036] After primary alloying of the molten steel, it is preferable to perform a final rinse to improve the cleanliness of the steel. The rinse is a bubbling of argon at a lower flow rate to suspend larger inclusions (typically larger than 100 μm). This final rinse can be continued for up to 10 minutes.

[0037] At the end of the refining process in the VAD, the molten steel is preferably reheated by electric arc heating, which involves lowering an electrode into the molten steel. More preferably, the molten steel is reheated to a temperature between 1580 and 1680°C.

[0038] Refining in the VAD further adjusts the composition of the molten steel to provide a versatile composition that is compatible with all possible powder compositions to be produced. At the end of the refining step in the VAD, the steel composition contains, by weight, 20 to less than 600 ppm of C, 15 to less than 120 ppm of S, up to 125 ppm of P, up to 80 ppm of N, and up to 30 ppm of O.

[0039] In the fourth step of the process, the molten steel from the VAD is poured into multiple induction furnaces.

[0040] An induction furnace is an electric furnace in which heat is applied by induction heating of metal. An induction furnace consists of a non-conductive crucible that holds a filling of molten metal, surrounded by a coil of copper wire. A strong alternating current flows through the wire. The coil generates a rapidly reversing magnetic field that penetrates the metal.

[0041] Thanks to multiple induction furnaces, the steel powder manufacturing process can be easily carried out continuously.

[0042] Each induction reactor can be operated independently of the others. It can be shut down specifically for maintenance or repair while other induction reactors are still in operation. In addition, the induction lines can be supplied with ferroalloys, scrap, directly reduced iron (DRI), silicide alloys, nitride alloys, or pure elements in different quantities for each induction reactor.

[0043] The number of induction furnaces is adjusted to match the desired flow of molten steel coming from the vacuum arc degassing unit and / or the steel powder at the bottom of the atomizer.

[0044] According to one modification of the present invention, molten steel from a vacuum arc degassing apparatus is poured directly into multiple induction furnaces. "Directly" in this case includes the use of ladles for transferring the molten steel to the multiple induction furnaces.

[0045] According to another modification of the present invention, molten steel from a vacuum arc degassing unit is first poured into a tundish, and then poured from the tundish into a plurality of induction furnaces. This configuration allows for easy distribution of molten steel to the induction furnaces as needed. The tundish is primarily used as a storage container. It can be batch-fed by the vacuum arc degassing unit and supplied independently to each induction furnace. In particular, molten steel can be poured into all induction furnaces simultaneously. One way to achieve this capability is to provide the tundish with as many injection means as there are induction furnaces. The injection means may be injection holes and corresponding stopper rods.

[0046] The temperature inside the tundish is preferably maintained between 1520 and 1620°C.

[0047] The tundish is preferably purged with argon to control the oxygen content in the tundish.

[0048] In the fifth step of the process, at least one type of ferroalloy is added to each of the multiple induction furnaces to adjust the steel composition to the desired steel powder composition.

[0049] Iron alloys refer to various alloys of iron with one or more other elements in high proportions, such as silicon, niobium, boron, chromium, aluminum, manganese, and molybdenum. The main alloys are FeAl (usually containing 40-60 wt% Al), FeB (usually containing 17.5-20 wt% B), FeCa, FeCr (usually containing 50-70 wt% Cr), FeMg, FeMn, FeMo (usually containing 60-75 wt% Mo), FeNb (usually containing 60-70 wt% Nb), FeNi, FeP, FeS, FeSi (usually containing 15-90 wt% Si), FeSiMg, FeTi (usually containing 45-75 wt% Ti), FeV (usually containing 35-85 wt% V), and FeW (usually containing 70-80 wt% Mo).

[0050] The relative amounts of the ferroalloy mixture and each of the ferroalloys are, if applicable, adjusted to achieve the desired steel powder composition. The ferroalloys added to the induction furnace are preferably not pre-melted.

[0051] Optionally, scrap, directly reduced iron, silicide alloys, nitride alloys, pure elements, or mixtures thereof may be added to facilitate composition adjustment.

[0052] Directly reduced iron is produced by the direct reduction of iron ore (in the form of lumps, pellets, or powder) to iron using reducing gas or elemental carbon produced from natural gas or coal.

[0053] Silicide alloys can be MnSi, CrSi, or CaSi in particular. Nitride alloys can be MnN.

[0054] Pure elements can be, in particular, carbon and pure metals such as iron, copper, nickel, cobalt, chromium, calcium, and rare earth metals.

[0055] This step can be carried out independently and asynchronously in each induction furnace. As described above, different steel compositions can be prepared in different induction furnaces to obtain different steel powders.

[0056] To ensure proper melting of the ferroalloy and homogenization of the composition, the temperatures in the multiple induction furnaces are preferably maintained between 1500 and 1700°C, more preferably between 1620 and 1700°C. The temperature in at least one of the multiple induction furnaces is more preferably maintained between 1580 and 1650°C to extend the crucible and refractory life of the induction furnace.

[0057] It is preferable not to control the atmosphere of each induction furnace. However, in one modified version of the present invention, the atmosphere of at least one of the induction furnaces can be controlled. In particular, this is a vacuum induction furnace.

[0058] The minimum duration in each induction furnace is controlled by the atomization rate and the rate at which the liquid steel can be discharged from the reservoir.

[0059] In the sixth step of the process, for each induction furnace, molten steel of the desired composition is poured into a dedicated reservoir connected to at least one gas atomizer. “Dedicated” means that the reservoir is paired with a given induction furnace. However, multiple reservoirs can be dedicated to a single given induction furnace. For clarity, each induction furnace has its own production flow, along with at least one reservoir connected to at least one gas atomizer. Using such parallel and independent production flows, the process for producing steel powder is versatile and can be easily made continuous.

[0060] Injection can be carried out directly from the induction reactor to the reservoir, or by using a transfer ladle.

[0061] A reservoir is primarily a storage tank that allows for atmosphere control, heating of molten steel, and pressurization.

[0062] The atmosphere of each dedicated reservoir is preferably argon, nitrogen, or a mixture thereof to avoid oxidation of the molten steel.

[0063] The steel composition poured into each reservoir is heated above its liquidus temperature and maintained at this temperature. This overheating prevents clogging of the atomizer nozzles. Furthermore, the reduction in the viscosity of the molten composition helps to obtain a powder with high sphericity and no satellites, and with an appropriate particle size distribution.

[0064] The composition is preferably heated to a temperature at least 150°C above its liquidus temperature so that its viscosity is sufficiently reduced. However, since surface tension increases with temperature, it is preferable not to heat the composition to a temperature more than 450°C above its liquidus temperature.

[0065] Preferably, the composition is heated to a temperature 200 to 300°C higher than its liquidus temperature.

[0066] In one modification of the present invention, the composition is heated between 1300 and 1750°C, preferably between 1550 and 1750°C, which represents a good compromise between viscosity reduction and surface tension increase.

[0067] The reservoir is either continuously pressurized or can be pressurized when molten steel is supplied. The means for pressurizing the reservoir are designed accordingly. Continuous pressurization of each reservoir is advantageous for having a continuous flow from the reservoir to at least one atomizer connected to the reservoir. The pressure of each dedicated reservoir is adjusted to keep the metal flow constant. The pressure setting depends on several parameters, which can be adjusted as needed by those skilled in the art.

[0068] The reservoir can comprise a single chamber or multiple chambers that can be pressurized independently of each other. Multiple chambers allow the steel powder manufacturing process to be more easily continuous.

[0069] In the seventh step of the process, once the dedicated reservoir is pressurized, the molten steel can flow from the reservoir to at least one of the gas atomizers connected to the reservoir.

[0070] The molten composition is atomized into fine metal droplets by forcing a flow of molten metal through an orifice (nozzle) at the bottom of a reservoir under moderate pressure and impacting it with a jet of gas. The gas is introduced into the metal flow as it exits the nozzle, and the accompanying gas expands (due to heating), creating turbulence as it exits into the atomization tower, which has a large collection volume. The latter is filled with an inert gas to prevent the powder from oxidizing. The metal droplets are cooled as they fall within the atomization tower. Gas atomization is preferred because it is advantageous for producing powder particles with a high degree of roundness and a small amount of satellite. These particles are also less oxidized than those produced by water atomization.

[0071] The atomizing gas is preferably argon or nitrogen. Both increase the melt viscosity more slowly than other gases, such as helium, which promotes the formation of smaller particle sizes. They also play a role in controlling the purity of the chemical, avoiding undesirable impurities, and ensuring a good form of powder. Since the molar weight of nitrogen is 14.01 g / mol compared to 39.95 g / mol of argon, finer particles can be obtained by using argon rather than nitrogen. On the other hand, the specific heat capacity of nitrogen is 1.04 J / (gK) compared to 0.52 of argon. Therefore, nitrogen increases the cooling rate of the particles. In some cases, argon may be preferred over nitrogen to avoid contamination of the composition by nitrogen.

[0072] The gas flow affects the particle size distribution and microstructure of the metal powder. In particular, the cooling rate increases with higher flow rates. Therefore, the gas flow rate (m 3 The gas-to-metal ratio, defined as the ratio between the gas flow rate (kg / hour) and the metal flow rate (kg / hour), is preferably maintained between 1 and 5, more preferably between 1.5 and 3.

[0073] The nozzle diameter affects the flow rate of the molten metal, and therefore the particle size distribution and cooling rate. The maximum nozzle diameter is preferably limited to 6 mm to limit the increase in average particle size and the decrease in cooling rate. The nozzle diameter is more preferably between 2 and 3 mm to more precisely control the particle size distribution and to work favorably for the formation of the desired microstructure.

[0074] The metal powder obtained by atomization can be sieved to retain particles whose size is better suited to the subsequent technology to be used, particularly additive manufacturing. For example, in the case of additive manufacturing by powder bed fusion, a range of 15 to 50 μm is preferred. In the case of additive manufacturing by laser metal deposition or direct metal deposition, a range of 45 to 150 μm is preferred.

[0075] Parts manufactured from metal powder produced by this process can be obtained by additive manufacturing techniques such as powder bed fusion (LPBF), direct metal laser sintering (DMLS), electron beam melting (EBM), selective heating sintering (SHS), selective laser sintering (SLS), laser metal deposition (LMD), direct metal deposition (DMD), direct metal laser melting (DMLM), direct metal printing (DMP), laser cladding (LC), and binder jetting (BJ). Coatings made from metal powder according to the present invention can also be obtained by manufacturing techniques such as cold spraying, thermal spraying, and high-speed oxygen fuel spraying. They can also be obtained by conventional powder metallurgy such as press forming and sintering.

[0076] The process according to the present invention can be carried out using equipment including the following: - blast furnace, - A converter capable of refining molten iron to form molten steel containing up to 600 ppm of C, up to 120 ppm of S, up to 125 ppm of P, up to 50 ppm of N, and up to 1200 ppm of O. - A vacuum arc degassing apparatus that can refine molten steel to obtain refined molten steel containing less than 20-600 ppm of C, less than 15-120 ppm of S, up to 125 ppm of P, up to 80 ppm of N, and up to 30 ppm of O. - Multiple induction reactors, - A ferroalloy supply unit capable of supplying at least one type of ferroalloy to the plurality of induction furnaces, - A dedicated reservoir for each induction reactor, each dedicated reservoir being connected to at least one gas atomizer and pressurizable.

[0077] The equipment may further include a tundish from which molten steel or refined molten steel can be poured simultaneously into all induction furnaces. Such a tundish facilitates the storage of molten steel and the supply of induction furnaces on demand. The tundish is preferably positioned above the multiple induction furnaces to further facilitate supply.

[0078] The induction furnaces are preferably movable in and out of their positions and tiltable to deslag and pour molten steel in the reservoir. They are preferably located on one floor of the steelworks, more preferably on one floor below the tundish. They are preferably located above the corresponding reservoir and atomizer to further facilitate feeding.

[0079] At least one of the multiple induction furnaces can be a vacuum induction furnace to satisfy a particular steel composition of powder.

[0080] The ferroalloy supply unit preferably comprises a storage silo for each containing one type of ferroalloy, and a transport means capable of transporting each ferroalloy to each induction furnace, and optionally to a ladle smelting furnace. The ferroalloy supply unit may also comprise a storage means for silicide alloys and / or nitride alloys and / or pure elements, and a transport means capable of transporting these materials to each induction furnace and optionally to a vacuum arc degassing unit. The transport means may be supply pipes. They may reach each induction furnace directly, or reach a mixing unit where mixtures of ferroalloys, silicide alloys, nitride alloys, and pure elements are prepared before being transported to each induction furnace. The ferroalloy supply unit may also comprise a supply means for scrap and direct reduced iron.

[0081] Each dedicated reservoir is preferably connected to at least two gas atomizers so that one gas atomizer can be stopped, for example, for maintenance or repair, to collect the powder at its bottom, while maintaining continuous production of steel powder.

[0082] Each reservoir is preferably connected to at least one gas atomizer by a supply pipe. More preferably, the supply pipe is heated, for example, by induction heating, to maintain proper overheating of the molten steel and thereby prevent clogging of the atomizer nozzle. The supply pipe can be closed by a closing mechanism such as a stopper rod operated from inside the reservoir or a stopper located inside the supply pipe.

Claims

1. A method for producing steel powder having a desired composition, comprising the following steps: - Step of supplying molten iron from the blast furnace, - A step of refining the molten iron in a converter to form refined molten steel having a composition containing up to 600 ppm of C, up to 120 ppm of S, up to 125 ppm of P, up to 50 ppm of N, and up to 1200 ppm of O. - A step of refining the refined molten steel in a vacuum arc degassing apparatus to obtain further refined molten steel having a composition containing 20 to less than 600 ppm of C, 15 to less than 120 ppm of S, up to 125 ppm of P, up to 80 ppm of N, and up to 30 ppm of O. - A step of pouring the further refined molten steel into multiple induction furnaces, - A step of adding at least one type of ferroalloy to each of the plurality of induction furnaces, and further adjusting the composition of the refined molten steel to a desired composition of steel powder, thereby obtaining a refined molten steel having a desired composition in the plurality of induction furnaces. - A step of pouring the adjusted and refined molten steel from each of the plurality of induction furnaces into each dedicated reservoir of the plurality of reservoirs, wherein each dedicated reservoir of the plurality of reservoirs is connected to each gas atomizer, - A step of supplying molten steel prepared and refined from a corresponding dedicated reservoir under pressure to the gas atomizer of each of the plurality of reservoirs, and using the gas atomizer of each dedicated reservoir to gas atomize the prepared and refined molten steel to obtain steel powder having the desired composition.

2. The method according to claim 1, wherein the composition of the refined molten steel includes up to 250 ppm of C and / or up to 90 ppm of P and / or up to 25 ppm of N.

3. The method according to claim 1 or 2, wherein, in the vacuum arc degassing apparatus, the refined molten steel is decarburized during the refining of the refined molten steel by using dissolved oxygen in the steel.

4. The method according to any one of claims 1 to 3, wherein, in the vacuum arc degassing apparatus, the refined molten steel is deoxygenated during the refining of the refined molten steel until the dissolved oxygen content of the further refined molten steel is 4 ppm or less.

5. The method according to any one of claims 1 to 4, wherein, in the vacuum arc degassing apparatus, the refined molten steel is desulfurized during the refining of the refined molten steel by stirring slag against the refined molten steel.

6. The method according to any one of claims 1 to 5, wherein the temperature of the further refined molten steel is between 1580 and 1680°C.

7. The method according to any one of claims 1 to 6, wherein the further refined molten steel is poured directly from the vacuum arc degassing apparatus into the plurality of induction furnaces.

8. The method according to any one of claims 1 to 6, wherein the further refined molten steel is first poured from the vacuum arc degassing apparatus into a tundish, and then poured from the tundish into the plurality of induction furnaces.

9. The method according to claim 8, wherein the tundish is configured to simultaneously pour the further refined molten steel into each of the plurality of induction furnaces.

10. The method according to claim 8 or 9, wherein the tundish is maintained at a temperature between 1520 and 1620°C.

11. The method according to any one of claims 8 to 10, wherein the tundish is purged with argon to control the oxygen content in the tundish.

12. The method according to any one of claims 1 to 11, wherein each of the plurality of induction furnaces is maintained at a temperature between 1500 and 1700°C.

13. The method according to any one of claims 1 to 12, wherein at least one of the plurality of induction furnaces is maintained at a temperature between 1620 and 1650°C.

14. The method according to any one of claims 1 to 13, wherein the at least one ferroalloy added to each of the plurality of induction furnaces is not pre-melted.

15. The method according to any one of claims 1 to 14, wherein scrap or directly reduced iron or silicide alloy or nitride alloy or pure element or mixture thereof is added to at least one of the plurality of induction furnaces.

16. The method according to any one of claims 1 to 15, wherein none of the induction furnaces among the plurality of induction furnaces are atmosphere-controlled.

17. The method according to any one of claims 1 to 16, wherein at least one of the plurality of induction furnaces is a vacuum induction furnace.

18. The method according to any one of claims 1 to 17, wherein each of the plurality of reservoirs has an atmosphere comprising argon, nitrogen, or a mixture thereof.

19. The method according to any one of claims 1 to 18, wherein each of the plurality of reservoirs is maintained at a temperature between 1300 and 1750°C.

20. The method according to any one of claims 1 to 19, wherein each of the plurality of reservoirs is maintained at a temperature at least 150°C above the liquidus temperature of the adjusted and refined molten steel poured into the reservoir.

21. Equipment for producing steel powder having a desired composition, comprising the following: - Blast furnaces that supply molten iron, - A converter configured to refine the molten iron and form refined molten steel having a composition containing up to 600 ppm of C, up to 120 ppm of S, up to 125 ppm of P, up to 50 ppm of N, and up to 1200 ppm of O, - A vacuum arc degassing apparatus configured to further refine the refined molten steel to obtain further refined molten steel containing 20 to less than 600 ppm of C, 15 to less than 120 ppm of S, up to 125 ppm of P, up to 80 ppm of N, and up to 30 ppm of O. - Multiple induction reactors, - A ferroalloy supply unit configured to supply at least one type of ferroalloy to the plurality of induction furnaces, - A plurality of reservoirs, each including a dedicated reservoir for each of the plurality of induction furnaces, wherein each of the plurality of reservoirs is connected to and pressurized by at least one gas atomizer.

22. The apparatus according to claim 21, further comprising a tundish configured to simultaneously pour the further refined molten steel into each of the plurality of induction furnaces.

23. The apparatus according to claim 22, wherein the tundish is positioned above a plurality of induction furnaces.