Secondary refining (ladle metallurgy)
Secondary refining technology is applied to steel melted and refined in a converter or electric arc furnace to adjust the steel’s chemical composition and temperature.
Secondary refining is necessary to maximize the capacity of the primary refining furnace and produce high-purity and high-cleanliness steel. Secondary refining requires the following capabilities:
(1) Remove hydrogen, carbon, and nitrogen under vacuum (reduced pressure).
(2) Stir and deoxidize the molten steel to uniformize the temperature of the molten steel.
(3) Use flux and slag for refining, desulfurization, deoxidation, and inclusion control (chemical and foaming slag).
(4) Raise the temperature of the molten steel using electric arc, aluminum oxide, or gas burners.
(5) Add alloys to adjust the chemical composition.
From the late 1950s to 1960, many types of vacuum refining equipment were developed. The Bochumer process (droplet degassing method) was the first commercial degassing equipment developed by the Bochumer Alliance in 1952 to eliminate hydrogen defects in large cast and forged ingots.
The DH method was developed by Dortmund Horder Huttenunion in 1956, and the RH method was developed by Ruhrstahl and Heraeus in 1958 as suction-type vacuum degassing equipment. Vacuum arc degassing (VAD) and vacuum oxygen decarburization (VOD) were developed in 1968, while argon oxygen decarburization (AOD) and ladle furnace degassing (LF) were developed in 1967 and 1971, respectively.
Vacuum Metallurgy
Vacuum refining is broadly categorized into two types: RH and VD. RH, VD, and DH [80,81] are widely used, as shown in Figure 11. The DH method, installed in 1961 at Yawata Steel (now Nippon Steel), was Japan’s first vacuum refining facility. Although the DH method has been widely used in Japan, it has some disadvantages compared to the RH method, such as the use of a lifting vacuum container, complex equipment, and low degassing capacity. In 1998, Nippon Steel developed the REDA method to address the shortcomings of the DH method. The REDA method is used in the production of ultra-low carbon and stainless steel, and is more efficient than VOD refining in producing high-purity stainless steel.
RH process
The first RH system was installed in 1963 at Fuji Steel’s Hirohata Works (now Nippon Steel Hirohata) in Japan, becoming the world’s second RH facility. From the late 1970s to the late 1980s, Japan’s steel production process rapidly changed, with continuous casting replacing the solidification process in mold casting. Under these circumstances, the RH process also saw rapid progress and development, including the following points.
(1) Add ultrasonic testing of aluminum killed steel degassing to produce aluminum killed steel wide and thick plates.
(2) General steel grades use RH, taking advantage of its efficient vacuum decarburization and stirring capabilities to reduce the converter load.
(3) Applied to ultra-low carbon steel to increase productivity and improve product quality.
In the RH process, the circulation of molten steel is caused by argon gas supplied from the immersion tube (vent pipe), utilizing the gas lift effect. The actual plant’s circulation rate was measured using the gold isotope Au198 as a tracer and copper additions. The technical team analyzed the changes in the composition of the continuous alloy additions and compared the results with reported data from other companies. They proposed the equation (4) for the circulation rate, Q. Figure 12 shows the relationship between the calculated results and the actual measurements of the RH equipment.
Vacuum refining decarburization is mainly used for the following two purposes:
(1) Decarburization within the low carbon range while suppressing [Cr] oxidation in stainless steel production.
(2) Mass production of ultra-low carbon steel for automotive panels.
VOD is the primary vacuum refining process used in stainless steel production. Much research into ultra-low carbon steel began in the 1980s; in particular, the RH process’s ability to decarburize within the ultra-low carbon range, and its use in converters for the mass production of ultra-low carbon and stainless steel, has made significant contributions to technological advancement.
The technical team established a decarburization model assuming continuous equilibrium, which approximates perfect mixing between the ladle and vacuum container (2 containers) under RH environment. Using this model, the impact of operating conditions on decarburization was analyzed, as shown in Figure 13.
The analysis results are as follows:
(1) 90% of the decarburization is carried out in the vacuum chamber and the remaining 10% is carried out in the upper ventilation pipe.
(2) In the low carbon region, i.e., [C] ≤ 40 ppm, the decarburization rate stagnates, ak ≤ 10 m3/min, and K ≤ 0.15 min−1. The rate-controlling step of the decarburization in the vacuum chamber is a combination of the mass transfer of the decarburization reaction and the circulation rate.
Recent RH equipment has higher circulation rates and shorter mixing times. Studies have shown that there are no dead zones with uneven concentration distribution within the ladle. When [C] > 40 ppm, the rate-determining steps are (i) the rate of bubble nucleation on the steel surface in the vacuum chamber and (ii) the rate of bubble growth and flotation separation (aggregation and unification). In the low-carbon region ([C] ≤ 40 ppm), (ii) is the rate-determining step. Therefore, to accelerate the decarburization of ultra-low carbon steel in an RH environment, the following points are important:
(1) Rapidly increase the vacuum degree in the early stage of decarburization by increasing the RH exhaust volume.
(2) Increase the circulation rate of the molten steel.
(3) Strongly stir the surface of the molten steel in the later stage of decarburization.
RH-OB (O2 injection) was developed at Nippon Steel Muroran Steel Works to facilitate the decarburization of stainless steel. A dual-layered tuyeres are located at the bottom of the vacuum chamber, with oxygen injection supplied by the inner tube of the oxygen lance and oil mist supplied by the outer tube.
Controlling the RH treatment time after decarburization, heating, and oxygen injection is crucial for the optimal flotation and removal of inclusions generated by the oxygen injection process.
The RH-KTB (RH-Kawatetsu) oxygen top-blowing method was developed at Kawasaki Steel’s Chiba Works. The distance between the top lance and the molten steel surface is 1.6m and 4.5m. The oxygen lance height is controlled based on the carbon content target, and oxygen is blown to the steel surface for decarburization. The RH-KTB’s functions are as follows:
(1) The use of carbon post-combustion reduces the temperature drop and reduces the workload of edge processing.
(2) The decarbonization rate increases (the capacity coefficient increases).
(3) Due to the use of RH-KTB, the carbon content is increased as much as possible at the end of the converter blowing, the steel tapping temperature and the total iron content (%T.Fe) are reduced, and the unit value is improved.
The RH-MFB (RH Multi-Function Burner) was developed at Nippon Steel’s Hirohata Works. It supplies oxygen and liquefied natural gas from a top lance, heating the refractory and molten steel during processing and waiting times.
The RH-PTB (Powder Charge Top Injection) is a blast furnace technology developed at Sumitomo Metal (now Nippon Steel)’s Wakayama Works for the production of ultra-low sulfur and low-carbon steel. Its characteristic is the injection of fine iron ore within a [C] range of ≤40 ppm for decarburization.
Research suggests that entraining fine iron ore in the molten steel increases the reactive interface area, acting as an oxygen source and providing nucleation conditions for CO bubbles.
At Kawasaki Steel in Chiba (now JFE Steel East (Chiba)), a mixture of hydrogen and argon was injected through a circulating gas nozzle to examine the decarburization behavior of ultra-low carbon steel. The results revealed that hydrogen dissolved into the molten steel, decarburizing the steel in the low-carbon region, where hydrogen provided nucleation for degassing.
The effects of hydrogen injection were investigated, revealing that the decarburization rate increased in the ultra-low carbon region (C) ≤ 15 ppm, reaching the ultimate low carbon level.
The technical team conducted cold model experiments to study the oxygen injection behavior of the Laval nozzle under reduced pressure and compared the measured jet dynamic behavior with the numerical analysis results.
The denitrification reaction is influenced by chemical composition, particularly by surface-active elements. It is known that the apparent denitrification rate constant, represented by the [N] secondary reaction, decreases with increasing [O] and [S], with [O] being more effective than [S]. The controlling factors in the denitrification rate are the removal rate of nitrogen adsorbed on the steel surface and the desorption of N2 (g).
In the RH process, nitrogen absorption and removal occur simultaneously because air leaks out from cracks in the vent pipe. Therefore, nitrogen absorption caused by air leakage should be minimized. The following technologies have been reported to reduce nitrogen absorption:
(1) Steel is tapped without deoxidation in the converter.
(2) Increase the circulating (Ar) gas flow rate.
(3) Improve the argon sealing performance of the pipeline and flange.
(4) Use of flangeless pipeline
Other examples have been reported. Research has been conducted on mixing CO with circulating gas (Ar) to promote nitrogen removal. Non-deoxidation steelmaking, suppression of gas leakage from flanges, and the use of sealed tundishes have been reported as countermeasures to prevent nitrogen absorption.
As a non-vacuum denitrification method, the denitrification effect of slag was investigated.
CaO-Al2O3 slag was used to control its chemical composition, thereby increasing the slag’s nitrogen capacity and, consequently, the nitrogen removal rate. Nitrogen removal from the slag was conducted by allowing nitrogen to enter the gas phase. Furthermore, fundamental research was conducted on the incorporation of nitrogen from molten steel into the slag.
The PERM (Pressure Elevation Method) was developed to facilitate inclusion removal by flotation. This technology utilizes the difference in nitrogen solubility between atmospheric and reduced pressure to generate nitrogen bubbles.
This technology was applied to a 50-ton VOD production plant for carbon steel and stainless steel, and a 250-ton RH production plant for bearing steel. The oxygen removal rate was more than doubled, and both the total oxygen content and average inclusion size in the slab were reduced.
VOD (Vacuum Oxygen Decarburization)
The VOD process was developed by Witten in Germany in 1967, and the first equipment in Japan was installed at Nisshin Steel in 1968 for stainless steel refining. Examples of developments include the production of ultra-low carbon and ultra-low nitrogen stainless steels using the VOD process. The use of single-hole nozzles, replacing porous gas bricks, became the mainstream in ladle refining at the time. Furthermore, research was conducted on the effect of oxidant powder injection on denitrification.
VCR (Vacuum Conversion Refining: Vacuum AOD)
In stainless steel refining, AOD can be used to decarburize high-carbon molten steel. However, AOD’s decarburization rate is low in the low-carbon range, so the final carbon content does not meet ultra-low carbon requirements.
Although the introduction of large amounts of Ar gas can reduce the CO partial pressure, this increases costs. Daido Special Steel developed VCR, a modified AOD process with vacuum refining capabilities. The company began stainless steel production in 1991, using a 70-ton AOD-VCR process.
The VCR process utilizes dissolved oxygen and oxides in the slag to decarburize the steel under vacuum conditions without oxygen blowing. Nippon Steel introduced the V-AOD stainless steel refining process in 1996. The V-AOD process uses oxygen blowing (without dilution) under reduced pressure (C≦0.6%) to improve decarburization conditions, shorten refining time, and reduce the consumption of silicon alloy required to reduce chromium in the slag.
Secondary refining without vacuum treatment (ladle metallurgy)
Secondary refining without vacuum treatment is initiated by injecting argon through special lances or bottom permeable bricks for various purposes, including adjusting chemical composition, deoxidation, removing inclusions, desulfurization and homogenizing the molten steel temperature.
Methods for creating bubbles in molten steel, such as the scanning gun method, the TN (Thyssen Nieder Rhein) method, and the CaSi wire feeding method, began to be commercialized in the 1980s. ASEA-SKF was the first method to use an electromagnetic induction coil located outside the ladle for stirring.
Nippon Steel developed CAS-OB (Sealed Argon Bubble Oxygen Blowing), a simplified secondary refining process that uses argon bubbles for deoxidation and chemical composition adjustment. Nippon Steel also developed the KIP (Kimitsu Injection Process) for flux spray desulfurization. These technologies were later modified to include vacuum capabilities, creating the reduced-pressure CAS-OB and V-KIP.
The LF (Ladle Furnace) process was developed in the 1970s. In this process, molten steel is deoxidized and desulfurized while being stirred in an argon atmosphere. The addition of alloys lowers the temperature of the molten steel, allowing for arc heating using graphite electrodes. In the 1980s, the NKK-AP (NKK Arc Refining Process) was commercialized for the production of ultra-low sulfur and ultra-low oxygen steels using a flux gun and graphite electrode heating.
Inclusion Control by LF Method
The composition of inclusions with a size of 1 μm or larger was investigated using a scanning electron microscopy technique called ASPEX Explorer in a case hardened steel (SCM) (C/0.22, Si/0.26, Mn/0.68, Ni/1.62, Cr/0.56 (mass %)) produced by the EAF (150 t)-LF-RH-CC process.
At Sanyo Special Steel, by controlling the addition of CaO and CaF2, the composition of oxide inclusions in RH is controlled to the MgO-Al2O3 system, which helps reduce the total oxygen content through flotation (Figure 15). By fully controlling the EAF-LF-RH-CC process, a high-yield process technology for ultra-clean bearing steel has been developed.
Inclusion Control
Numerous studies have been conducted on alumina inclusions. Fractal theory has been used to analyze the agglomeration and flotation behavior of alumina clusters. The effects of argon bubbles on inclusion removal in RH processes have been studied. Furthermore, various fundamental studies have reported on the formation and removal of alumina.
A model for inclusion formation based on a coupled reaction model is proposed. Oxide metallurgy is a technique that uses inclusions and precipitates to control the microstructure of steel.
This paper reports on the use of fine secondary oxide inclusions generated during solidification to achieve a finer microstructure. This secondary oxide inclusion can be exploited in thin slab and strip casting, particularly at higher solidification rates. Future research is needed to further investigate the role of inclusions and precipitates in controlling microstructure.
Problems and Prospects of Ladle Metallurgy
To meet the demand for high-quality steel products, new technologies for producing high-purity, high-cleanliness steel must be further developed.
While decarburization in the low-carbon range is already achieved through vacuum refining, precise reaction modeling, data science techniques, and rapid heating technologies will be required to further control the carbon, oxygen, and total oxygen content (C), as well as the temperature.
For denitrification, simplified electric arc furnace processes, slag utilization, and slag denitrification technologies are needed. Regarding desulfurization, ultra-low sulfur steel can already be produced, but high-speed desulfurization and heating technologies are needed to meet the growing demand. Research on inclusions and precipitates in ultra-clean steel is needed to enhance and improve the microstructure of steel products.
#Refractory brick for steel making
