Patent Application
20230323492
Typically, vacuum degassers utilized in the steel industry are either a Ruhrstale and Heraeus (RH) degasser or a Vacuum Tank Degasser (VTD). The RH degasser may be used for carbon removal and the tank degasser may be used for nitrogen and sulfur removal. The liquid steel tapped out of an Electric Arc Furnace (EAF) may contain higher contents of nitrogen and sulfur compared to that of a Basic Oxygen Furnace (BOF). As such, plants utilizing an EAF may choose to install a tank degasser, and BOF plants may choose to install an RH degasser.
The vacuum processes conducted outside a primary steelmaking furnace may be utilized to further remove carbon and decrease the solubility of hydrogen and nitrogen by lowering the partial pressures of CO, H2, and N2. Because of its high carbon removal efficiency, an RH degasser may be installed in shops intended for mass production of ultra-low carbon steel products. However, conventional RH degasser process has essentially no capability to remove sulfur and has very limited capability to remove nitrogen. Even if auxiliary equipment such as those to enable powder injection is installed at the RH degasser, the sulfur removal capability is very limited.
The intensifying global effort to reduce carbon dioxide emission has tremendous impact on the electrical steel market and manufacturing. As a result, the demand for electrical steel, especially electrical steel grades with low core loss and high permeability, is increasing at a very rapid pace. Electrical steel includes Grain-oriented (GO) and Non-grain-oriented (NGO) or Non-oriented (NO) steels. Grain-oriented steel is normally used in static equipment such as transformers. Non-grain-oriented steel is normally used in rotating equipment such as motors and generators. The electrification of the transportation worldwide, for example, means that internal combustion engines are steadily being replaced by electrical motors. High grade NGO are used in those motors to increase the energy efficiency.
The steel industry is also working to reduce carbon dioxide emission. Transitioning steelmaking from the traditional blast furnace+basic oxygen furnace (BF+BOF) to electric arc furnace (EAF) can reduce the CO2 emmison by 60% to 90%, depending on the feed materials of the EAF and the source of the electricity, and many steel companies around the world are doing just that. However, the production of electrical steel is dominated by the BF+BOF process route plants due to the stringent requirement on control of residual elements in electrical steel. The present disclosure provides a method to produce electrical steel with ultra-low levels of residual elements such as carbon, nitrogen, sulfur, and oxygen required in high-grade NGO steel in an EAF steelmaking shop.
A method of making NGO steel A method of making non-grain oriented (NGO) steel, comprising the steps of; tapping the liquid steel out of an EAF furnace; deoxidizing the liquid steel before or after transferring the liquid steel to a ladle metallurgy furnace; removing sulfur at the ladle metallurgy furnace; skimming off ladle slag and/or adding fluxes to the ladle slag to prevent sulfur reversion; transferring the desulfurized liquid steel from the ladle metallurgy furnace to a RH degasser; removing nitrogen from the desulfurized and deoxidized liquid steel at the RH degasser with fast vacuum pump down; adding fluxes into the vacuum vessel to fortify the bottom layer of the ladle slag against sulfur reversion; removing carbon by oxygen blowing and prevents carbon pick up during the step of adding fluxes and arcing for sulfur removal if sulfur removal is carried out at the LMF after carbon removal at the RH degasser in the conventional process; adding alloys to achieve chemistry specifications of the NGO steel; and recirculating the steel at the RH to remove inclusions.
A NGO steel sheet product comprising as low as less than 0.003 wt. % weight percent carbon, 0.003 weight percent nitrogen and 0.003 weight percent sulfur, wherein the steel product has been subjected to deoxidation and sulfur removal at a ladle metallurgy furnace prior to decarbonization at an RH degasser thereby preventing carbon pickup during sulfur removal.
A method of making NGO steel product comprising as low as less than 0.003 weight percent carbon, 0.003 weight percent nitrogen and 0.003 weight percent sulfur, the method comprising; tapping the liquid steel out of a BOF furnace; transferring the liquid steel to a ladle metallurgy furnace; removing sulfur from the liquid steel in the ladle metallurgy furnace; skimming off ladle slag and/or adding fluxes to the ladle slag to prevent sulfur reversion at the ladle metallurgy furnace; and transferring the liquid steel from the ladle metallurgy furnace to an RH degasser for carbon removal, wherein the removal of oxygen and sulfur prior to transferring the liquid steel to the RH degasser facilitates nitrogen removal and prevents carbon pick up during the step of removing sulfur.
Compared to the BF+BOF route, liquid steel tapping out of an EAF in general contains a higher concentration of nitrogen and sulfur. Depending on the scrap market conditions and availability of various metallic charge materials, it is often advantageous from a cost standpoint for an EAF steel shop to tap out steel with relatively high sulfur and nitrogen. In the BF+BOF route, the combination of deep desulfurization in hot metal pretreatment and scrap selection can lower the sulfur content in the liquid steel tapping out of the BOF around 0.005% or even lower. At the RH degasser, with the powder injection either through the top lance or the tuyeres, or bulk flux addition, it is possible to remove a few tens of ppms of sulfur, to achieve the ultra-low sulfur required in NGO steel grades. However, the sulfur content in liquid steel tapping out of an EAF is generally around 0.025% or even higher. To rely on the sulfur removal at the RH degasser to achieve the ultra-low sulfur levels is very difficult, if not impossible in an EAF shop. For nitrogen, the liquid steel tapping out of the BOF is generally between 20 to 30 ppm, and hence no nitrogen removal is necessary in the downstream secondary steelmaking steps. Nitrogen control in the EAF steelmaking has improved significantly in the past few decades, but even with the best practices the nitrogen level in the liquid steel tapping out of EAF are in the 40 ppm level, and normally expensive scrap mix is needed to achieve such low nitrogen content. Therefore, nitrogen removal is necessary in an EAF shop to produce high grade NGO steels. The present disclosure provides a method to efficiently remove sulfur and nitrogen in an EAF shop to produce high grade NGO steel using the EAFLMF
RH
CC route, as shown in
The present disclosure provides a method to produce NGO steel with ultra-low carbon (as low as less than 30 ppm), ultra-low nitrogen (as low as less than 30 ppm), and ultra-low sulfur (as low as less than 30 ppm), possible through a RH degasser, even from liquid steel tapped out of EAF with high levels of sulfur and nitrogen.
The present disclosure includes a method of using the LMF to remove large amounts of sulfur prior to vacuum decarburization at the RH and utilizes techniques to prevent and/or minimize sulfur reversion from the slag to the steel during RH treatment. Ultra-low sulfur in steel produced using this method ensures an extremely low concentration of sulfide based inclusions in the steel and hence lowers the core loss and improves the permeability of the NGO steel.
The present disclosure utilize the ultra-low sulfur level achieved at the LMF to accelerate nitrogen removal at the degasser. Therefore, significant amount of nitrogen can be removed at the RH degasser and that enables the production of ultra-low nitrogen steel even from the higher nitrogen levels in liquid steel tapping out of an EAF furnace. The ultra-low nitrogen in steel produced using this disclosure ensures low nitride-based inclusion concentration in the steel and hence lowers the core loss and improves the permeability of the NGO steel.
The present disclosure uses oxygen blowing to introduce oxygen into liquid steel that was deoxidized before the RH treatment to achieve carbon removal under vacuum. The ultra-low carbon in steel produced using this disclosure prevents age hardening of the NGO steel as well as core loss increase due to carbide formation.
The present disclosure utilize the RH degasser for inclusion control to produce clean steel with ultra-low total oxygen content to achieve the low core loss and high permeability required in high grade NGO steel.
The ultra-low sulfur in the liquid steel and the low oxygen content of the steel (killed steel) arriving at the RH degasser may enable rapid nitrogen removal at the RH degasser under vacuum. It may also enable fast pump down, e.g., as fast as the vacuum system can do, without steel splashing which often occurs in the RH vessel in the conventional RH treatment with open (unkilled) heat. When carbon removal is desired, oxygen may be blown through the top lance or side tuyeres into the heat. Once the aluminum in the steel is removed by oxygen, decarburization may follow. The oxygen blow/injection may be controlled such that the dissolved oxygen level in the liquid steel is high enough that the decarburization reaction will not be hindered, but low enough to not to promote sulfur reversion from the slag. That level may be around 200 ppm dissolved oxygen in the liquid steel. A model was developed to calculate the volume of oxygen blowing. Inputs of the model include steel mass, carbon, aluminium and silicon contents in the steel, and aim carbon content in the steel. The oxygen blowing may be divided into two or more batches to control the peak dissolved oxygen level in the steel. Following decarburization, the heat may be killed and alloys containing manganese, silicon, aluminum, chromium, etc., may be added into the RH degasser vessel to achieve desired steel grade chemistry specifications. Sufficient recirculation time in the RH before and after the alloy addition may allow inclusion floatation to produce clean steel. Depending on the type of caster, further treatment such as calcium treatment for thin slab caster may be desirable before sending the heat to the caster.
Deoxidizing the liquid steel may be accomplished before or after transferring the steel to a LMF to remove sulfur. In various embodiments of the present disclosure the heat does not have to be oxidized (not killed) for vacuum decarburization: it may be killed (for sulfur removal) and then blown “open” (using, for example, top blown oxygen) for vacuum decarburization. Embodiments of the present disclosure may make it possible to produce steel with ultra-low carbon (as low as less than 30 ppm), ultra-low nitrogen (for example as low as less than 30 ppm), and ultra-low sulfur (for example as low as less than 30 ppm), consistently and rapidly through an RH degasser, even from liquid steel tapped out of the primary steelmaking furnace with high levels of sulfur and nitrogen such as those from an EAF furnace.
In various embodiments of the present disclosure, a process to remove sulfur from steel before carbon removal in the degasser may be used to prevent carbon pickup from flux materials added for sulfur removal and from electrode during arcing in the process that removes sulfur after carbon removal at the RH degasser.
In various embodiments of the present disclosure the LMF may be used to remove sulfur prior to vacuum decarburization. This process may result in: (1) a larger amount of sulfur (e.g., a few hundreds of ppm) may be removed; and (2) distribution of the two functions (sulfur removal and carbon removal) at two facilities that can operate in parallel or concurrently (LMF and RH) and hence increased shop productivity. Embodiments of the present disclosure may allow cheaper EAF scrap mix than that otherwise required to tap out lower sulfur steel.
Embodiments of the present disclosure utilize techniques to prevent/minimize sulfur reversion from the slag to the steel during subsequent decarburization. Embodiments of the present disclosure utilize the ultra-low sulfur level achieved at the LMF to accelerate nitrogen removal at the degasser. Therefore, a significant amount of nitrogen may be removed at the RH degasser to produce ultra-low nitrogen steel even from liquid steel with higher nitrogen levels such as those tapped out of an EAF furnace. Embodiments of the present disclosure may allow cheaper EAF scrap mix than that otherwise required to tap out lower nitrogen steel. Embodiments of the present disclosure may shorten the tap to cast time and hence reduce energy consumption (less electrical arcing at the LMF) and reduce the ladle refractory consumption.
An RH degasser may be installed in a BOF shop in which the liquid steel tapped from the BOF contains a lower sulfur level (e.g., less than 0.01 wt. % and as low as less than 0.003 wt. % comparing to over 0.02 wt. % and as high as over 0.03 wt. % from an EAF), and lower nitrogen level and hence sulfur and nitrogen removal may not be necessary; In prior processes, sulfur removal in RH may be possible by flux addition or injection when ultra-low sulfur is desired. The amount of sulfur removal may be limited and it may be difficult to use a starting sulfur level as high as those tapped from an EAF. The fluxes may contain a high percentage of calcium fluoride (CaF2) which are detrimental to the life of RH degasser refractories: the lower vessel and snorkel may last only for, for example, 2-3 heats.
Oxygen blowing/injection may be utilized with an RH degasser to accelerate decarburization or to increase steel temperature.
Table 1 shows the average and standard deviation of C, N, S and Ti in lower grade electrical steel (Cold Rolled Motor Lamination or CRML steel) heats made using a prior process. The average carbon, nitrogen, and sulfur in the liquid steel as measured from samples taken from the steel ladle after tapping from the EAF are 0.0286%, 0.0042%, and 0.0231%. Higher percentages of pig iron and HBI are used in the EAF to make those grades. As a result, the 0.0042% average nitrogen is lower than that in other steel grades in which nitrogen is not critical. Higher percentages of those “virgin” materials normally means higher cost.
For carbon, as the RH degasser is very efficient for carbon removal, the average carbon content in liquid steel after the RH degasser treatment, as measured in the first sample taken at the LMF after the heat returned to the LMF, is 0.0014% with a standard deviation of 0.0009%. As shown in
For nitrogen, as shown in Table 1 and
For sulfur, there is no removal during the first round of the LMF treatment and the RH treatment as demonstrated in Table 1. When the heat returned to the LMF, flux additions and strong stirring were applied to remove sulfur. There is higher silicon (Si) and aluminum (Al) concentration in CRML grade steel than in most other grades which also promotes the sulfur removal. The sulfur was lowered to 0.0013% on average. The sulfur level remains steady, with a very slight drop, from the LMF to the caster as shown in Table 1. Overall, the sulfur performance is excellent even for high grade NGO steel production.
Titanium (Ti) is another important residual elements to be controlled in electrical steel. Residual titanium in electrical steel forms small precipitates in the final products that are detrimental to the electrical steel properties—namely, increased core loss and decreased magnetic permeability. As shown in Table 1, titanium in the CRML steel made using the prior art increased significantly from the after RH sample to the final LMF sample. As a residual element, titanium may be introduced into the steel with other alloys such as ferrosilicon or it may be reverted to the steel from the slag due to the reduction of titanium oxide in the slag during desulfurization. With the prior art, desulfurization was carried out after the RH treatment, that resulted in significant increase in titanium content in the liquid steel as shown in Table 1. In the embodiments of the present disclosure, oxygen blowing at the RH degasser, which occurs after the desulfurization of the steel at the LMF, oxidizes and hence lowers the content of titanium in the steel from the prior desulfurization step. That is another advantage of the present disclosure.
The following examples are intended to illustrate various aspects of the present disclosure, and are not intended to limit the scope of the disclosure.
The general process after the heat was tapped out of the EAF furnace follows the six steps described above.
In this particular heat, the final EAF celox measurement showed 3099° F. and 624 ppm dissolved oxygen for the liquid steel in the EAF furnace. The 163 short ton heat was tapped out of the EAF, without any additions during tapping, at 10:01 AM on Feburary 10th, 2022. The heat was then sent to the LMF. Argon stirring from both two porous plugs in the steel ladle were initiated when the heat approaches the LMF on the transfer car on the V-track from the EAF to the LMF. Once the heat arrived at the LMF, the measurement at 10:02 AM showed the liquid steel is 3068° F. with 501 ppm dissolved oxygen. The heat was killed using 603 pounds of aluminum. With strong stirring from both argon plugs, batches of fluxes were added to remove sulfur. Fluxes additions include 3240, 503, 172, 37 pounds of lime, calcium aluminate, slag deoxidizer, and calcium carbide, respectively. After the sulfur removal process and the sample showed 0.0019% sulfur, as shown in Table 2, the heat was arced for 10 minutes to bring the temperature to 3030° F. Then the heat was arced for 4 more minutes to compensate the temperature loss from the final flux additions. The final flux additions include 806 pounds of lime and 203 pounds of slag deoxidizer. The lime was added to thicken the slag and the slag deoxidizer was added to be trapped in the slag and react with oxygen diffused into the slag layer later at the RH when oxygen is blow into the heat. Both measures were taken to minimize sulfur reversion during the later treatment at the RH degasser. The LMF treatment finished at 11:05 AM.
The heat was then shipped to the RH degasser via crane. The vacuum treatment started at 11:37:09 AM. At the RH, the vacuum was pumped down as fast as the system is capable of can as there is no danger splashing of metal inside the vessel that caused by the initial violent carbon-oxygen reaction in un-killed heat in previous process. Higher lifting gas flow was also used from the start, rather than ramp up later in previous process. In embodiments of the present disclosure, 10 to 15 minutes of deep vacuum may be used as the first stage of the vacuum treatment to remove nitrogen. In this particular heat, 15 minutes elapsed from start of the vacuum to the start of oxygen blow. Near the end of that period, a steel sample was taken and analyzed. The carbon, aluminum, and silicon contents from the analysis were fed into a model to calculate the oxygen required for decarburization. The goal was set to control the dissolved oxygen content at the end of decarburization cycle to be 100 ppm to 200 ppm. In some embodiments, the oxygen was also blown in one or two or three batches in order to lower the peak oxygen content in the steel to minimize sulfur reversion. In this particular heat, 172 Nm3 of oxygen was blown into the heat via the top lance in three batches. Near the end of decarburization, the measurement showed 2881° F. with 104 ppm oxygen. The heat was killed at 12:22:47 PM. After alloy additions, the vacuum was broken, and the heat was sent out to the trim station at the RH facility for calcium treatment. Calcium treatment may be used for the heat to be castable on a thin slab caster.
The carbon, nitrogen, sulfur and titanium content throughout the process are listed in Table 2. For carbon, this trial heat ended 0.0017% at the caster, significantly lower than the 0.0051% average in the previous process. As shown in Table 1 and
This heat was a NGO grade with high silicon, aluminum levels and moderate manganese level. As the contents of alloy elements such as silicon, alumninum, manganese in the NGO grades are not subject to the current disclosure, they are not listed in this disclosure.
Similar to that in the previous example (heat 2201684), after tapping out from the EAF, the heat was killed at the LMF and desulfurized. The final sample at the LMF showed a sulfur of 0.0019%. Near the end of the treatment at the LMF, 800 pounds of lime and 200 pounds of deoxidizer was added into the ladle slag as part of the “slag conditioning” practices to prevent sulfur reversion during the RH treatment.
At the RH degasser, the heat was first subjected to 15 minutes under deep vacuum for nitrogen removal. During that period, 1200 pounds of lime was added into the RH vessel to further condition the ladle slag, especially the bottom layer of the ladle slag, as depicted in
Following similar procedures as that in heat 2202772, four more NGO heats with high silicon, alumnium levels and moderate level of manganese were produced. The final caster chemistries on carbon, nitrogen, sulfur, and nitrogen are listed in Table 4.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.
Also, it may be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.
In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and/or” unless specifically stated otherwise, even though “and/or” may be explicitly used in certain instances. In this application and the appended claims, the articles “a,” “an,” and “the” include plural referents unless expressly and unequivocally limited to one referent.
As used herein, “including,” “containing” and like terms are understood in the context of this application to be synonymous with “comprising” and are therefore open-ended and do not exclude the presence of additional undescribed or unrecited elements, materials, phases or method steps. As used herein, “consisting of” is understood in the context of this application to exclude the presence of any unspecified element, material, phase or method step. As used herein, “consisting essentially of” is understood in the context of this application to include the specified elements, materials, phases, or method steps, where applicable, and to also include any unspecified elements, materials, phases, or method steps that do not materially affect the basic or novel characteristics of the disclosure.
Whereas particular embodiments of this disclosure have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present disclosure may be made without departing from the disclosure.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 63/237,636 filed Aug. 27, 2021, the benefit of U.S. United Patent application Ser. No. 17/897,824 filed Aug. 29, 2022, which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63237636 | Aug 2021 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 17897824 | Aug 2022 | US |
| Child | 18132521 | US |
