This article reviews the development of steelmaking technology over the past 60 years and provides an outlook for future developments. In Japan, hot metal pretreatment began in the 1960s with the goal of reducing refining costs and improving product quality. This focus has now evolved to include reducing processing time, reusing steelmaking slag, and utilizing a variety of iron sources. In converter refinement, in addition to high-speed decarburization, the integration of phenomenon visualization and sensor modeling technologies with data science techniques is becoming increasingly important.
In ladle metallurgy (secondary refining), technologies for achieving high-speed processing and heating of molten steel are key issues. This article also briefly discusses the necessity of process revolution for a sustainable social and environmental development.
In 2014, the Iron and Steel Institute of Japan (ISIJ) published a review entitled “Steelmaking Technology for the Last 100: Toward Highly Efficient Massproduction Systems for High-Quality Steel,” outlining the history of steel production technology. This year, ISIJ International (formerly Transactions of ISIJ) celebrated its 60th anniversary. During this period, the international steel association provided information on the development of Japanese steel production processes to relevant parties in other countries and made relevant technological recommendations. As part of a special issue commemorating its 60th anniversary, ISIJ International published this article reviewing the past 60 years of development in refining processes, particularly in Japan’s long-process steel mills.
Hot metal pretreatment
Development and evolution of hot metal pretreatment
Hot metal desulfurization
Hot metal desulfurization begins with off-furnace refining, where sodium hydroxide or soda is added to the hot metal container. Subsequently, calcium carbide is added from the top, nitrogen is blown from the bottom, and the hot metal ladle is stirred and shaken. However, these technologies have been gradually eliminated due to reaction efficiency deviations, the generation of acetylene gas, and the difficulty of using them on large-tonnage hot metal containers. In the torpedo car injection method, the desulfurizer is dispersed in the hot metal to react, and the reactants float on the hot metal. The desulfurizer is calcium carbide or sodium carbonate powder. Due to the weak stirring ability of this process, the desulfurized slag cannot be reused. Kawasaki Steel (now JFE Steel) developed a calcium oxide powder containing a surfactant as a substitute for calcium carbide. [2] In 1965, Fuji Steel (now Nippon Steel) developed a mechanical stirring method called KR (Kanbara reactor). In the KR process, the desulfurizer enters the hot metal through the vortex generated by the rotating impeller and reacts with the hot metal interface. The KR process has a higher reaction efficiency than the injection method. At 1400°C, the CaO-10% CaF2 KR process can reduce the sulfur content in hot metal to below 5 ppm. The KR process has been a popular standard technology since the 1970s. However, a challenge with the KR process is reducing the amount of desulfurizer required. Desulfurization to below 50 ppm can be achieved using only CaO without CaF2. Newer technologies will be discussed below. Mg-CaO hot metal desulfurization also achieved industrialization during this period.
Hot metal dephosphorization and its differences in functions
The double-slag method is a technology that uses a converter to remove phosphorus from molten iron. However, the high-phosphorus slag produced by desiliconization and dephosphorization requires intermediate slag discharge, which leads to problems such as large slag volume, slag splashing, poor molten iron recovery, and low production efficiency.
In response to the growing demand for low-phosphorus steel, a hot metal dephosphorization process has been developed to reduce the burden of converter dephosphorization. Desiliconization is performed before dephosphorization to reduce the specific consumption of the dephosphorization agent, CaO. In addition to oxygen, which acts as an oxidizer for CaO, mill scale, sintered ore fines, and iron ore fines can also be used as oxygen sources.
Dephosphorization in the torpedo car occurs as a short-term reaction with the slag floating above the molten iron, while the desulfurization reaction occurs between the high-basicity slag and the molten iron, which is a long-term reaction process. [5, 6] This process has been commercialized since the early 1980s. During this process, the dephosphorization slag needs to be discharged, and the dephosphorized molten iron is decarburized in the converter.
Research has also been conducted on dephosphorization using soda ash.[7-9] While soda ash has good dephosphorization effects, it presents challenges such as severe Na2O corrosion on refractory materials, difficulty in handling Na2O slag, evaporation of Na (g), and refining costs. These issues have limited the application of soda ash, while CaO-based fluxes have gained widespread use.
Reductive dephosphorization using Ca-CaF2 or CaC2 -CaF2 fluxes has also been studied,[10,11] but this process has not yet been commercialized due to the need to stabilize the reduction product Ca3P2 and other issues.
Development of Mechanical Stirring (KR) Method in Hot Metal Desulfurization
The KR mechanical stirring method was commercialized in 1965. Compared to the injection method, the desulfurizer consumption is higher because the flux is dispersed into the molten iron multiple times. Its higher stirring energy allows for high-speed desulfurization, resulting in low sulfur content in the molten iron, meeting the demand for low-sulfur and ultra-low-sulfur steel. Due to this advantage, the injection spray method has been largely replaced by the KR method since the 2000s.
Soda ash and CaO-CaF2 are used as desulfurizers for torpedo car spray desulfurization. However, since Na and F will cause environmental pollution when the desulfurization slag is recycled, their use is prohibited. This is also the main reason for the transition from spraying to mechanical stirring.
Developing high-efficiency hot metal
The mechanical stirring desulfurization (KR) method uses propane gas injection and oxygen top blowing technology to reduce the oxygen potential, and brings the additive into the molten iron at this low oxygen potential location, resulting in improved desulfurization efficiency and reduced desulfurization agent usage.
In the KR process, fine CaO powder less than 1 mm is added from the top. Since calcium oxide is mainly in solid state under the conditions of molten iron desulfurization, the added calcium oxide powder will agglomerate to form larger spherical particles with a particle size ranging from a few millimeters to tens of millimeters. This is a disadvantage because most of the agglomerated CaO desulfurizer will not react with sulfur.
To address this issue, a desulfurizer injection technology was developed. Figure 117 shows the results of this technology (300 tons/furnace, N2 = 6 to 20 Nm3/min, CaO = 100 to 400 kg/min, distance between the spray gun and the molten iron surface from 0.3 to 1.4 m, and molten iron temperature from 1523 to 1663 K). This technology suppressed the agglomeration and growth of CaO powder.
Under the conditions of CaO and molten iron wetting, when the velocity of the CaO powder exceeds the critical entrainment velocity, the powder is entrained into the molten iron in the form of fine particles, as shown in Figure 2. The desulfurization efficiency of calcium oxide increased by 1.3 times, and the amount of desulfurization slag used was reduced by 20%. Nakai’s team quantitatively analyzed the effectiveness of CaO desulfurization by simulating the agglomeration behavior of solid CaO in molten iron. The calcium oxide injection technology has been implemented at all JFE Steel plants. After desulfurization, the slag can be recycled in a hot state, utilizing the unreacted CaO for desulfurization.
Hot metal dephosphorization
Research progress on thermodynamics and kinetics of hot metal dephosphorization
Fluorspar (CaF2) is widely used to improve dephosphorization efficiency in the production of low-phosphorus hot metal, and this has been reported. Although CaF2 promotes CaO melting, increasing oxygen potential and Ca2+ activity, environmental standards for soil dephosphorization have been revised, and since the late 1990s, CaF2-free dephosphorization technologies have become increasingly important. To analyze the reaction rates of hot metal pretreatment, a coupled reaction model was developed to account for the simultaneous multicomponent reactions occurring during hot metal pretreatment. The coupled reaction model was used to analyze the effects of stirring energy, hot metal temperature, and flux composition on hot metal dephosphorization. The coupled reaction model is an important tool for analyzing hot metal dephosphorization rates. The model was extended to account for solid phase crystallization associated with changes in slag composition during hot metal dephosphorization. The extended model also considers mass transfer between the solid slag (2CaO·SiO2), liquid slag, and hot metal phases.
Observations of CaO particles melting into slag revealed that metal droplets in the slag expand due to CO gas, increasing their residence time in the slag (this is known as the “expanding droplet model”). These results suggest that accurately understanding actual phenomena through direct observation and measurement will become increasingly important in the future.
During the dephosphorization process of molten iron, oxidation reactions such as FeO formation and decarburization proceed simultaneously under non-equilibrium conditions. Therefore, this is a very complex but important phenomenon because it affects the melting effect of CaO, stirring of the reaction site and iron metal yield. Reaction sensing technology and further research are needed to understand its mechanism.
Evolution of dephosphorization of hot metal in converters
In 1983, JFE Steel conducted tests on dephosphorization of hot metal in a Q-BOP converter, commercializing the process using the intense stirring capabilities of the Q-BOP and bottom-blown oxygen and CaO powder. The hot metal (1370°C, 4.5wt% C, 0.2wt% Si, 0.40wt% Mn, 0.14wt% P, 0.02wt%) was dephosphorized to 3.7wt% C, trace Si, 0.30wt% Mn, 0.010wt% P, and 0.010wt% S. The treatment time was 3 minutes, using 20kg/t of CaO, 3kg/t of CaF2, and 6 Nm3 of oxygen. The hot metal temperature did not drop. However, without CaF2, it was difficult to maintain P2O5 and FetO.
In 1979, Nippon Steel commercialized the SMP (Minimum Slag Process), which first desiliconizes the molten iron and then dephosphorizes it in the converter, thereby reducing the amount of slag.
Kobe Steel commercialized the “hot metal pretreatment furnace” in 1983, utilizing the LD converter as the designated vessel for dephosphorization and desulfurization. Sumitomo Metal (now Nippon Steel) developed the SRP (Simplified Refining Process with Slag Recovery Process) in 1987, whereby desiliconized and desulfurized hot metal are dephosphorized and decarburized in separate converters, with the decarburized slag recycled to the dephosphorization converter.
In 1989, Nippon Steel Nagoya Works commercialized a process called “LD-ORP” (Optimized Refining Process), which first removes phosphorus from molten iron using bottom-blown limestone powder, and then desulfurizes it using bottom-blown soda ash. In 2000, Nippon Steel developed the Multi-Refining Converter (MURC), which combines desulfurized molten iron and scrap steel in a converter for desiliconization and dephosphorization, followed by deslagging and decarburization, as shown in Figure 3.
JFE Steel has commercialized the Dual Slag Refining Process (DRP), as shown in Figure 4. This process involves feeding molten iron and scrap into a converter for desiliconization. After desiliconization, the SiO2-rich slag is discharged, and the molten iron is dephosphorized. The heat generated by silicon oxidation is used to melt the scrap, and the discharge of the SiO2-rich slag reduces CaO consumption. The DRP process can also be used to produce ultra-low phosphorus steel.
During hot metal dephosphorization, promoting and controlling FetO generation is crucial for facilitating the early melting of CaO, which replaces CaF2. JFE Steel has developed a technology for dynamically controlling FeO generation. FetO generation can be directly detected by measuring the oxygen balance, as shown in Figure 5. A mathematical model for hot metal dephosphorization quantifies the impact of FetO generation, enabling the production of stable, low-phosphorus hot metal, as shown in Figure 6. In 2015, a new converter (330 tons/furnace) was installed at the Third Steelmaking Works of the West Japan Works (Fukuyama) to increase the dephosphorization capacity of the DRP hot metal.
Hot metal jet dephosphorization method (hot metal torpedo car, ladle)
The advantage of flux injection is its ability to promote transient reactions. Pioneering research on transient reactions has been conducted, studying the behavior of particles injected into the molten pool. In torpedo car dephosphorization, the limited free space results in low flux injection velocities, leading to long operation times. Inadequate mixing and stirring conditions prevent sufficient reaction between the top slag and the molten iron, resulting in dead zones. To address these issues, deslagging during injection and the use of dual injection lances have been reported.
The current hot metal dephosphorization process is mainly concentrated on the converter type. The original intention of developing the converter type was to make full use of the converter’s turning capacity. Because the LD converter has a larger free space, the oxygen top blowing lance can be used to the maximum extent. The difference from the previous double slag method is as follows:
(1) Bottom blowing can promote the reaction, make the molten iron temperature uniform, and shorten the processing time.
(2) Used in series with advanced desulfurization processes, it expands the concentration range of [Si] before treatment. By utilizing the oxidation heat of this Si, it is expected to improve the scrap steel melting capacity.
(3) Maintaining and controlling a high FetO content in the slag promotes dephosphorization.
(4) High-temperature decarburization slag can be recycled, which is beneficial to CaO melting, improving iron metal recovery and reducing slag production.
(5) During the dephosphorization oxygen blowing process, unburned converter flue gas can be recycled.
Converter refining process
The principle of the converter steelmaking process is based on the bottom-blown air method invented by H. Bessemer in 1856. In Bessemer’s converter, dephosphorization was difficult due to the use of acidic refractories. The Thomas process enabled the use of alkaline refractories to dephosphorize hot metal produced from high-phosphorus iron ore in Europe.
The LD converter (oxygen top-blown converter) trial was carried out in 1949 on a 2-ton furnace. In November 1952, a 30-ton converter was put into operation. Six months later, a converter of the same size was put into operation at Donawitz OEAMG. In Japan, the first LD converter was put into operation by NKK (now JFE Steel) and Hatta Steel (now Nippon Steel) in 1957. The rapid popularization of LD technology corresponded to the emergence of integrated steel mills in Japan and the increase in crude steel production during the same period. Subsequently, LD converter technology made significant rapid progress, including the development of pure oxygen production technology, flue gas recovery systems and porous top-blowing lances. However, the LD converter was eventually replaced by a top-bottom composite converter at Yawata Steel in 1979, replacing the converter installed in Kawasaki Steel in 1976. The 230-ton Q-BOP[45] converter has excellent metallurgical properties. The top-bottom composite converter is currently the mainstream of converter steelmaking, accounting for about 70% of the world’s crude steel production.
Development of converter refining technology
Stirring and combination blowing converter
A double-layer tuyeres (oxygen and propane) for bottom-blown permeable bricks was developed and commercialized. In the Q-BOP process, CaO powder is transported through the inner tube and oxygen is supplied, while propane is supplied through the outer tube of the double-layer tuyeres. Dissolved oxygen (O) levels in the bottom zone are lower than those in the LD converter. Even in the low [C] zone, dissolved oxygen levels in the Q-BOP are lower than in the LD converter.
At that time, the effects of oxygen bottom blowing on metallurgical properties were discussed, using tracer detection and mixing in reactors such as RH, VOD, and ASEA-SKF. It was pointed out that the complete mixing time τ (s) is a function of the stirring energy density ε (W/t-steel), which is independent of the mixing method.
To evaluate the effects of blowing methods (number and arrangement of tuyeres) and bath depth on stirring energy density, studies were conducted and numerous empirical formulas were developed. Fluid motion and mass transfer were also analyzed. These studies led to the development of top-bottom combined-blowing converters in Japan. Two types of top-bottom combined-blowing converters exist: one with oxygen blowing from both the top and bottom, and the other with top oxygen blowing and bottom blowing limited to inert gas stirring. Steelmakers developed nozzle designs and refractory materials for both methods, and now all Japanese converters, with the exception of Q-BOP converters, are top-bottom combined-blowing converters.
Development and construction of top and bottom blown converter
ISCO (Index of Selective Carbon Oxidation, see Equation (2)) was used as an indicator to explain the reactions in the converter based on the study of the metallurgical properties of 5 t scale and commercial scale converters and the measurement of mixing time using a copper tracer.
Except for the ultra-low carbon region where [C] < 0.02 mass%, the CO partial pressure of the bottom-blowing gas has a relatively small effect on selective decarburization. Extensive research on the top-to-bottom blowing ratio has shown that when the bottom-blowing ratio is greater than 10%, metallurgical properties close to those of Q-BOP can be achieved.
Converter high-speed refining technology
Development of oxygen lance for top-blown converter
During the decarburization process of dephosphorized hot metal in a converter, the amount of slag produced is small, but splashing and dust increase iron loss. Technicians studied the dust generation mechanism and found that iron dust in the early stages of decarburization is primarily caused by bubble collapse, while in the later stages of decarburization, it is primarily caused by smoke (evaporation).
When using multi-hole nozzles, if the gas nozzles overlap too much, splashing increases. A new nozzle arrangement was designed, using nozzles of different diameters and angles side by side to reduce overlap on the molten steel surface, as shown in Figure 8. Sumitomo Metal Wakayama Works (now Nippon Steel) used a furnace with a higher free space and a newly designed lance to achieve high-speed decarburization (9 minutes) at a higher oxygen flow rate (5 Nm3/t/min).
Based on the cold model and 6-ton hot model furnace experiments, the nozzle was designed under over-expansion conditions to achieve soft blowing conditions, and it was shown that the design reduced dust generation.
Converter detection technology
Various methods for detecting slag splashing during the blowing process have been reported, including monitoring with cameras at the furnace mouth, measuring furnace vibrations with accelerometers, and detecting with acoustic signals.
As process modeling using data science is expected to advance in the near future, accurately understanding this phenomenon through the development of new sensors, direct observation, and modeling techniques will become even more important.
Converter method for producing stainless steel
In 1972, Nippon Steel established a process for producing ferritic stainless steel using molten iron produced in a blast furnace, replacing the electric arc furnace (scrap melting) process. Nippon Steel’s process actively used the RH-OB method and molten steel and utilized aluminum oxidation heating technology, which could produce ferritic stainless steel for the long time required.
Kawasaki Steel’s Chiba Works (now JFE Steel East Japan Works (Chiba)) employed a dual refining process using two top-bottom blown converters (K-BOP) and RH-KTB.
The K-BOP process primarily involves smelting and reduction, slag removal, FeCr melting, and decarburization. Subsequently, a new process using chromium ore instead of chromium pellets was developed to produce ferritic stainless steel (see Figure 9). This process utilizes two converters (SRF: smelting reduction furnace and DCF: decarburization furnace), with secondary refining using the VOD process to meet the requirements for ultra-low carbon stainless steel and high quality.
A vertical bending continuous caster and a J-First (JFE Stainless Steel Flexible Repository) holding furnace with heating/melting functions were installed. Carbon serves as a reducing agent during the smelting and reduction of chromium ore. To compensate for heat losses caused by the reduction reaction, heat is supplied by oxidation of the carbon and CO gas. However, when using higher oxygen supply rates to increase productivity, dust generation from carbon post-combustion and refractory erosion are issues.
To address these issues, a special burner using hydrogen-based fuel was developed in 2012. In this technology, chromium ore pellets are heated within the burner flame, achieving high heating efficiency. Figure 10 shows a comparison of the original and developed heat transfer methods. Because the chromium ore pellets are heated within the burner flame, heating efficiency reaches 80%. This burner promotes chromium ore melting, compensating for heat loss during the reduction reaction, and achieving a 20% improvement in energy efficiency compared to comparable heating conditions. In addition to reducing FeCr alloy consumption, the reduced use of carbon as a heat source also reduces CO2 emissions, thereby reducing environmental impact.
Melting scrap steel using a top-blown and bottom-blown converter
Nakayama Steel began using the NSR process (Nakayama scrap melting process) in 2002. This process uses a conventional top-blown converter with top coal loading to melt 100% scrap steel.
The KS converter uses a top- and bottom-blown converter to melt 100% of scrap steel. Nippon Steel’s Hirohata Works installed the KS converter after its blast furnace was decommissioned. Coal is injected through the bottom tuyere, and carbon black from waste tires is recycled as a heat source instead of coal. The rubber and steel cords from the waste tires are also collected as byproduct gas and iron, respectively.
The “National Project on Recycling of Waste and Scrap, Reduction of Dioxin Gas Emissions, Energy Saving and Selective Expansion of Energy” conducted in Japan from 1991 to 2000 showed that when energy recovery is taken into account, primary energy can be reduced by 25% compared to the electric arc furnace process.
In the near future, it will be necessary to develop a new process that can minimize carbon dioxide emissions while maintaining the productivity and quality of steel products. To meet this challenge, scrap steel sorting technology and carbon recovery technology should be developed to expand the use of cold iron resources.
