2015 Volume 55 Issue 9 Pages 1816-1821
Prompted by the oil crisis and global warming, global steelmakers have begun to focus attention on the huge potential of hot charging and direct rolling processes for saving energy and reducing greenhouse gas emissions. New equipment and technologies have been developed in this field. In details, Japan has upgraded its reheating furnaces, applied induction heating and laser welding to endless hot rolling, developed the pair-cross mill for finish rolling and adjusted process parameters to eliminate surface defects of Nb-containing steel. Korea has also achieved endless rolling via super deformation shear joining and developed its own set of systems for the shape control of hot rolled steel products. Europe and America have designed flameless burners and reversing four-high mill stands together with their flying gage change technology and hydraulics systems for reheating and rolling. China has applied chamfered mould technology to eliminate edge and corner defects of microalloyed steel and the semi-endless rolling process to complete its intergrated production. This paper reviews development status of hot rolling and compares technologies and their effects on energy consumption, product quality, productivity and greenhouse gas emissions at global steel works.
Due to the limited energy-saving feature of continuous casting (CC) technology developed in the 1950s,1) various efforts have been made to develop new processes for energy saving in steel industries since the oil crisis in 1973.2) Japanese steelmakers first realized the significance of developing efficient and cost-saving hot rolling processes, and thus proposed and implemented new technologies such as continuous casting-hot charging rolling (CC-HCR), continuous casting-direct hot charge rolling (CC-DHCR), and continuous casting-hot direct rolling (CC-HDR) in the late 1970s.3) By the early 1980s, it was commonly believed by the US iron and steel community that the iron and steel industry would rely on the new hot rolling production processes to improve productivity, reduce energy consumption, and maintain competitiveness in the market in the 1980s. New equipment and techniques for hot charging and direct rolling such as reversing four-high mill stands and endless hot rolling were developed with the hot charge ratio significantly improving at global leading steel works since the late 1980s. In particular, Nippon Steel Corporation (NSC)’s Oita Works recorded an average hot charge ratio of 84.7%, the industry’s top level, in the second half of 1996.4) In the last decade, with global warming becoming increasingly serious,5) global steel industry, as the largest industrial emitter of CO2, has attached greater importance to hot charging and direct rolling due to their emission reduction potential of greenhouse gases.6)
Compared with the traditional continuous casting-cold charging rolling (CC-CCR), charging slabs at an elevated temperature into the reheating furnace of the hot rolling mill is commonly considered to save energy, reduce CO2 emission, improve material quality, reduce material losses, enhance productivity (by up to 6%), and reduce slab stocking.7) Figure 1 shows the flow of equipment for hot charging and direct rolling. Table 1 shows a comparison of CC-CCR, CC-HCR, CC-DHCR, and CC-HDR processes.8) The emission reduction potential of hot charging is estimated to be 30.2 kg CO2/t-rolled steel and energy savings are estimated to be 0.06 GJ/t-rolled steel.9) A total of 2.16 Gt CO2 was directly emitted in steel industry in 2006.10) The energy needs for typical hot rolling are 2–2.4 GJ/t-rolled steel globally.11)
Flow of hot charging and direct rolling equipment.
Tang et al.,8) Park et al.12) and Knoop et al.13) summarized planning and scheduling systems for the modern integrated steel process of steelmaking CC and hot rolling, and discussed some key issues of hot rolling. Alakawa3) and Sato14) compared technologies for steelmaking processes at Asian steel works, including hot charging and direct rolling processes. Ogawa15) reviewed the progress and prospect of rolling technology in the last three decades in Japan. Katoh16) summarized the progress of rolling technology and recommended appropriate solutions for the development of hot rolling. Kneppe17) reviewed the progress and proposed tasks for the new century in hot rolling technology. Joseph18) compared the process and equipment options for hot mill upgrades.
This paper reviews and compares contemporary equipment and technologies for hot charging and direct rolling, and their effects on energy consumption, product quality, productivity and greenhouse gas emissions in the global steel industry.
Despite the economic upheaval following the first oil crisis in 1973, the yeild of hot strip milling increased at an average rate of 0.1% for decades in Japan.19) Owing to the development of high-temperature defect-free slab manufacturing technology,20) CC-HDR was implemented by NSC at its Sakai Works in 1981, and by Nippon Kokan Corporation (NKK) at its Fukuyama Works in 1984.21) With the promotion of hot charging and direct rolling, defects were prone to occur on the slabs of some types of Nb-alloyed steels during the direct rolling processs in the late 1980s.22) Hence, high-temperature mechanical performance of Nb-containing steels was studied and an expansion of applicable steel grades for HCR and HDR was generally achieved in the early 1990s. In 1996, endless hot rolling technology was first implemented at Kawasaki Steel Corporation’s Chiba Works.23) Since the signing of the Kyoto Protocol in 1997, Japanese steel works have been designing and upgrading their equipment and developing new techniques to make full use of the underutilized heat of billets.24)
The running performance of the reheating furnace is critical in determining end-product quality and harvesting energy saving in the steel industry.7) Nakayama Steel Products Corporation is a typical example. Before the renewal of its equipment, the tempetature of billets at its works would fall from 1000°C to about 800°C during the delivery from the casting line to the heating furnace and all the billets had to be reheated to 1130°C before rolling because of the low power capacity of the rolling mill. In 2008, the power capacity of the drive motor was improved by 45%, which makes it possible to roll the billets at a relatively low rolling temperature of 900°C. The delivery time was reduced by 50% and gas consumption was significantly reduced because only the billets at a temperature of below 900°C had to be reheated to 900°C for rolling. Nakayama Steel Products Corporation also replaced 15 traditional burners with 8 recuperative burners, improving the efficiency of the reheating furnace, and shortening the effective furnace length from 22.5 to 7.75 m.25) In general, Nakayama Steel Products Corporation boosted its average direct rolling ratio from 71% to 96% in two months and was awarded by Osaka Prefecture for its achievement of reducing CO2 emission by 16.5% compared with that for 2007.26) NSC’s Oita Works increased rolling speed by about 26% and reduced the cycle time per slab by 0.5 min via an increase in the heating capacity of the reheating furnace and a renewal of the drive motor for the horizontal rolls.27) Compared to furnaces without heat recovery, recuperative burners can reduce fuel consumption by 10–20%, reduce CO2 emission by 35–40%, and make it possible to greatly reduce NOx consumption.28,29)
Endless hot rolling, which was first employed in commercial production in 1996 at Chiba No. 3 continuous hot strip mill, is a production process for joining intermittently supplied rough bars and carrying out finish rolling in an endless manner. Endless rolling can prevent bar bending in the width direction at the head end and pinching at the tail end, which have been operational problems in batch rolling. As a consequence, it permits stable continuous rolling, which results in uniform quality over the entire coil length, leads to increased productivity for its eliminating mill idle time and reduces energy by more than 40% compared to traditional hot rolling. As shown in Fig. 2, bar joining is a key technology for endless rolling.23)
Comparison of (a) traditional, (b) semi-endless, and (c) endless hot rolling processes. (Online version in color.)
The sheet bar joining process via induction heating is illustrated in Fig. 3.30) JFE Steel Corporation has adjusted this method to make it possible to join bars in about 5 s, achieving a 20% increase of the productivity, a 90% decrease of time for making unexpected roll changes due to pinchers, and a drastic reduction of surface defects due to pincher marks.31)
Outline of sheet bar joining (cross section). (Online version in color.)
Using high-power lasers developed by Mitsubishi Heavy Industries, NSC has applied and expanded laser welding to the hot rolling of billets, slabs and plates of low-, medium- and high-carbon steels at its steel works.32) CO2 lasers emitted from two oscillators are reflected and focused on the plasma ejected from the plasma-using nozzle towards the gap between the preceding bar and the abutted following bar. The seam is welded by the heat of the laser beam and the laser-induced plasma is used as a second heat source. The new laser welding method has been proven to improve the bead depth by about 30% compared to that for the conventional welding method and ensure stable welding quality.33)
Primetals Technologies Japan, Ltd. was the first in the world to develop a pair-cross mill (PC mill) with high strip shape control capability in 1991. In a pair cross mill, the top and bottom work roll (WR) and backup roll (BUR) axes are positioned obliquely to each other to change the rolling load distribution between the top and bottom WRs in the width direction and to control their equivalent crown.34) Wide shape and crown control capability can be obtained with a cross angle of as small as 1.5° as shown in Fig. 4. In particular, the PC mill can reduce the required number of plate rolling passes and sharply improve the rooling efficiency. The PC mill is commonly used in hot rolling mills for plates and strips.35,36)
Schematic diagramof PC mill. (Online version in color.)
Some types of steel, especially Nb-microalloyed steel, are highly crack sensitive when charged at a relatively high temperature or rolled directly. Thus, Japanese academics and steel industries began to study the mechanical properties of Nb-containing steels and the influence of charging temperature, aim temperature and drop-out temperature on the precipitation of Nb(C,N), and then carried out a series of industrial experiments.37,38,39,40) Take Nb-containing low-carbon steel as an example. It was concluded that when the hot rolling starting temperature was at 1130°C and the finishing temperature was lower than 850°C, the tensile strength and toughness of the hot band produced by direct rolling processes were almost the same as those produced by reheating process, and precipitates of Nb(C,N) effectively formed and were useful for grain refining.37) Via adjustment of process parameters, the region of embrittlement was avoided and the high-ductility, high-temperature region was ultilized. As a result, surface defects of Nb-containing steels were eliminated at Japanese steel works in the early 1990s.
Korea started to develop its steel industry in the 1960s and founded the Pohang Iron and Steel Company (POSCO) in 1968. Through more than 20 years of continuous efforts to increase facility efficiency and productivity, POSCO became the world’s leading steel company based on crude steel production in the 1990s.41) POSCO had successfully established an integrated system of CC, hot rolling and cold rolling processes in three years since the layout design was first launched in 1986, achieving a yield of 250000 tons of hot-rolled coils per year in 1989.14) Teamed with a Japanese supplier, POSCO started to develop an integrated process control system in 199642) and succeeded in testing the world`s first endless hot rolling process using two overlapping bars in 2006.43) Since then, POSCO has built its own system of shape control and demonstrated its superior quality of hot-rolled steel products.44)
A joining machine based on super-deformation joining technology, a solid-state joining process accomplished by overlapping two rough bars and then bringing together a pair of knives to shear the joint position, was incorporated into the No. 2 hot rolling mill at the Pohang Works on August 7, 2006. Figure 5 schematically illustrates the joining process from start to finish.45,46) This process reduces the time and offers firmer and thinner steel plates economically compared to that of Japan. The shearing process on the surface of the small cross-tower district reduces the area of the oxide scale and reduces the connection area ratio from 100% to 10%, thereby forming a solid connection in the entire height.47) POSCO now produces 100000 tons of coils per month by endless rolling, and has set a record for continuously carrying out endless rolling for one hour, producing 45 bars (more than 1000 tons).45)
Schematic illustration of the joining process from start to finish, showing changes in state. (Online version in color.)
POSCO is leading in the field of looper and tension control, which affects both the dimensional quality and the mass flow of a strip. Important factors of the quality for the product in a hot strip mill include thickness, width, strip shape.48) Width shrinkage has a negative effect on productivity. The strip width of the hot strip mill can be controlled in a roughing mill and finishing mill process. Automatic width control (AWC) in the roughing mill adjusts the gap of the vertical mill to diminish the width difference from a target width. A speed disturbance of the actuator, such as by mass-flow unbalance, can be corrected by operator intervention, automatic gauge control (AGC) or automatic shape control (ASC).49) The use of AWC, AGC or ASC in PC mills produces a tradeoff between dimensional quality and mass flow.44)
Hot charging and direct rolling processes were successively applied at the Great Lakes Works of United State Steel Corporation, German Bremen Steel Works, Austrian Linz Steel Works, and other European and American steel works in the 1980s after they were implemented in Japan. Reversing four-high mill stands equiped with hydraulics systems were designed and soon widely used in the 1990s. Following the Kyoto Protocol, Europe and America developed flameless oxidation burners for reheating to reduce NOx emission and reduce energy consumption.
Flameless air-fuel combustion uses air as the oxidizer, and flameless oxy-fuel uses commercial oxygen as an oxidant. This technology carries out combustion under diluted oxygen conditions using internal flue gas recirculation, with flame becoming invisible. Flameless oxy-fuel gives high thermal efficiency, higher levels of heat flux, and reduced fuel consumption compared to those of conventional oxy-fuel. These benefits are combined with low NOx emission and better thermal uniformity. Since 2003, more than 30 furnaces within the U.S. steel industry have been equipped with flameless oxidation burners. In addition to decreasing the total heating time by 15%, the flameless oxy-fuel gives more uniform heating, an additional fuel reduction of 17%, and 5–20% less scaling compared to conventional oxyfuel in reheating operation.50,51,52)
In heavy plate rolling, heated plates are plastically deformed by rolling mills. For this task, reversing four-high mill stands consisting of two WRs and two BURls were developed in the U.S. and EU. The deformation of the plate takes place between the two WRs. The high rolling forces during the deformation cause the rolls to bend which in turn yields a nonuniform thickness profile of the final product. The two BURs reduce the bending deflection of the WRs. Additionally, the mills are equipped with devices for bending compensation like hydraulic bending systems and/or specifically shaped rolls. These measures ensure a plain roll gap and thus a uniform lateral thickness profile of the final product. In particular, Nucor’s reversing four-high mill stands could work as the rougher and the finishing mill simultaneously in its castrip process.53,54,55)
The first implementation of flying gage change (FGC) was by TMEIC GE corporation at a plant in Holland. FGC is an advanced technology used in these mills to change the strip thickness on the fly. As shown in Fig. 6, FGC produces a tapered section with a transition length, which is determined by the rolling speed, the system response time, and equipment capabilities. The mill cannot be slowed down to make changes, so FGC is a highly coordinated control procedure involving the actuator regulators. A mill with an FGC system works with very long slabs and produces many coils from one slab, changing the gage at the beginning and the end, sometimes requiring to change gage part way through. It reduces the production cost by avoiding large tension and thickness variations.56,57)
FGC system. (Online version in color.)
High-performace hydraulic screwdowns can easily be installed on most reversing four-high mill stands with little or no modification. The fast response derived from the hydraulic power matches ideally with the modeling of mill stretch in electronic controls, obtaining thickness deviation of less than 1.5% at the exit of each stand. In particular, ThyssenKrupp Steel installed new hydraulic side guides upstream and downstream of the rougher in combination with hydraulic screwdowns in the rougher, resulting in accurate shape control without the assist of sabers and considerable facility cost and space saving.58)
The steel industry in China started in the late 1970s. Most leading Chinese steel works first applied HCR to production in the middle 1980s and then made efforts to improve the charging temperature and the hot charge ratio, achieving a high hot charge ratio of over 90% and a low average charging temperature of about 500°C in the late 1990s. Semi-endless rolling technology was widely applied for integrated hot rolling over the last decade. However, the formation of billet cracks, especially corner cracks, occurred frequently during CC-HCR. Thus, chamfered mould technology was developed and applied at several steel works to solve this issue since 2010, which has attracted the attention of many other global steel companies.59)
The mould with chamfered corners, as shown in Fig. 7. was proposed by Chinese Iron and Steel Research Institute and first successfully applied at ShouGang Corporation’s Jingtang Steel Works after many industrial trials.60) The adoption of the chamfered mould slows down the heat transfer of the corner, producing a uniform temperature distribution on the surface, which relieves the stress on the corner of the billets during hot rolling, makes corner cutting unnecessary, and effectively reduces the risk of edge and corner defects of microalloyed steel by more than 85%.61) The technology is estimated to reduce material loss by about 2.0% and reduce energy needs by 0.42 GJ/t-rolled steel, reducing CO2 emission by 38.31 kg/t-rolled steel for Chinese plants.62)
Schematic diagram of chamfered mould. (Online version in color.)
Lianyang Steel Works successfully applied the semi-endless rolling process to realize continuous hot rolling, the first in China, in 2004. Compared to traditional hot rolling, the process adopts high-speed shear technology to shear the slabs into subsections before coiling, uses FGC technology to control the strip thickness for stable rolling, and optimizes the speed of delivery and the structure of the reheating furnace for temperature uniformity of ultra-long slabs, as shown in Fig. 2. Semi-endless rolling is widely used and has been proven to increase strip yield by 0.5%–1.0%, increase productivity by 10%–15% and dramaticlly improve product quality at many Chinese steel works.63,64,65)
In order to meet with the ever-increasing demand of hot-rolled steel products and enhance market competitiveness, the global steel industry has to establish a low-cost and high-productivity system for hot charging and direct rolling processes. A direct rolling ratio of 100% and a yield of 100% are the future trends with the development of precision control in the hot rolling process, which means no delay between casting and rolling and defect-free products. As for global warming, developed countries have already realized the significance of CO2 emission reduction and have made efforts to optimize the energy structure and improve efficiency, but with many potential reductions not fully exploited. Different departments of the steel industry should be managed better and work together with energy, environmental protection, and other industries. However, for most developing countries, how to elevate the hot charge temperature and increase the hot charge ratio without degrading product quality is still an unsloved problem and thus more studies on steel properties and temperature control need to be done.
The authors would like to thank the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (2012), the Key Projects in the National Science & Technology Pillar Program (2013BAE07B00) and the State Natural Science Fund Projects of China (51474142) for supporting this work.