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Not all RHFs are created equal Rotary Hearth Furnace Primer

Jim McClelland

The author describes a method for forming a beam blank from a rectangular section, continuously cast slab using a bilaterally acting forging press with four tools to form the flanges and web. The resultant beam blank can then be rolled to the finished beam in a conventional beam mill. This process has been found to be technically and economically feasible and produces beams of equivalent quality to beams produced from conventional ingots and specially shaped blanks produced in continuous casting machines. The method is considered to have good potential in converter-continuous caster complexes where heavy sections and plates could be produced alongside each other from continuously cast slabs. The layout of the equipment of a rolling complex and design the press, realizing offered technology are submitted. At present, heavy I-beams are rolled either from ingots or from specially shaped continuously cast blanks. The first method tends to give low yields and the second is not widely used owing to problems in casting technology and the complexity of the plant. The idea has been put forward to using flat slabs, which can be most efficiently produced by continuous casting, as feedstock for rolling I-beams. In Japan, for example, a technique has been developed for rolling beams from I-blanks formed from flat slabs by rolling on an edge with numerous small reductions. In the present author' view, however, the metal discard losses are high owing to uneven deformation over the slab cross-section in this method and there is a distinct probability of surface defects forming owing to the unfavourable stress-strain distribution. At the Uralmash Industrial Association, a new method has been devised for producing I-beam blanks from continuously cast slabs by upsetting the slabs in two mutually perpendicular directions simultaneously over the full slab length. Instead of producing beams from ingots preworked in the blooming mill and roughing stand of the beam mill, the method consists in forming the I-beam blank in a press. The blank can then be subsequently rolled directly in universal stands without intermediate reheating. This beam blank is formed by reduction of the narrow and wide faces of the slab simultaneously along the entire slab length in plane strain (planar deformation) conditions. This practice completely eliminates discard at the blank forming stage. To check the basic feasibility of this method of forming I-beam blanks, a number of different production trials were carried out to determine the technical and economic / parameters of the process. The initial stock consisted of 150 x 1000 mm section, continuously cast slabs of steel produced at the Donetsk Iron - and Steelworks. The experimental investigations were carried out in a special device, which prevented elongation and created conditions for planer deformation of the slab. The slabs were upset on a press with a force of 29.4 MN. The shape of the upset I-beam blanks fitted the pass grooves in the 900 stand of the rail and section mill at the Nizhnii Tagil' Combine (NTMK) for rolling no. 60 beams. Results of measurements of deformation parameters showed that blanks of high dimensional accuracy could be produced equal to those rolled in a blooming mill, It should be noted that uniform metal flow was obtained across the flange width and that the process was stable as regards changes in friction conditions in the zones of contact of the stock and the press tools. No tearing or other defects hindering subsequent rolling were observed on the flange surfaces in zones of free spread of the metal or in the end faces of the blanks. The trial I-beam blanks were rolled down in the Nizhnii Tagil' Combine rail and section mill. Before rolling, the blanks were heated in accordance with works specification and the heating temperature was checked by optical pyrometer. The absence of edge fins and the symmetrical shape of the trial blanks ensured efficient biting conditions in the first rolling pass. Rolling proceeded smoothly without slip. No defects were observed on the trial beams. The surface condition met the specifications of GOST 535-58 and the beam section and geometrical dimensions were in compliance with COST 8239-72. To assess the quality of the trial beam steel, metallographic studies were carried out on transverse slices cut from the I-beam blanks and the beams themselves. The microstructure was investigated after cold etching and etching with Oberhoffer's reagent; sulphur prints, microstructure, and grain size distribution over the section were also studied. The I-beam blanks were found to have a finer grain size than the starting slabs. No columnar dendrite zones were observed. The pattern of distribution of central segregation in the zone subjected to minimum deformation was identical to that in the starting slab. A segregation zone was observed at the point of formation of the flange elements, but this zone was largely elongated in the direction of metal flow. The I-beam blanks had low contents of sulphide inclusions ( like the starting slabs). No metallurgical type defects were observed. The microstructure in the blanks differed considerably from that in the starting slabs (the grain was more homogeneous and equiaxed). Depending on the position of the microsection, the grain size ranged from 3 to 4. The quality of the I-beam blanks was generally good and equal to that of beam blanks produced by current methods. The macrostructure of the trial beams was dense and fine grained. Metallurgical defects and discontinuities were absent. The flange metal showed virtually no segregation-induced macro heterogeneity apart from a few diffuse areas in the joint between the flange and web. The central segregation zone of the starting slab was almost entirely concentrated in a narrow central zone from top to bottom of the web. The microstructure was studied in micro sections cut from the edge of the flange at about one-third of the distance from the web surface and from the point at which the web joined the flange. The microstructure showed a much finer grain than that in the beam blank. The grain size at the flange edge at about one-third of the distance from the web was 8-9 and in the flange-web junction zone it was 7. The formation of a coarser grain in the flange-web junction zone was attributed to the higher temperature and smaller degree of deformation in this zone as compared with the other parts of the section. All the mechanical properties met the specifications of GOST 380-71, regardless of sampling position in the trial beam section. The results of statistical analysis confirmed the normal distribution of all mechanical properties of the beams with low asymmetry and excess, and insignificant scatter in the mechanical property values (the coefficient of variation ranged from 0.511% to 14.89%). The dependence of mechanical properties on sampling position evidently depends on the degree of deformation and finish-rolling temperature of each component of the beam section. The properties of the trial beams obtained from continuously cast slabs were compared with those of conventionally produced beams from NTMK. Samples were taken at one-sixth of the distance along the flange width from its edge, in accordance with GOST 7564-73. Comparative analysis of the average data showed that the tensile properties in the trial beams were no lower and the ductile properties were 15 per cent higher than those of beams rolled at NTMK. The steel quality and mechanical properties of beams produced from continuously cast slabs, therefore, met the requirements of GOST 380-71. The above investigations showed the technical and practical feasibility of producing good quality I-beams from flat-faced, continuously cast slabs. The proposed production method allows the simplest design of continuous casters for rectangular slabs to be used. The efficiency of the process is also improved considerably owing to complete elimination of discard in the blank forming operation and a marked reduction in discard during rolling of the beam. The quality of the beams is also improved owing to creation of favourable stress-strain conditions in the deformation zone, while the energy requirement of the process is reduced by 4-6kWh/t due to elimination of the need for intermediate heating of the I-beam blanks, partial utilization of the heat in the continuously cast slabs, and improvement of the degree of utilization of the heating furnaces. The proposed method for producing I-beams from continuously cast slabs can be recommended for use in plants producing beams and other flanged sections of larger size with wide flanges which, if produced by conventional rolling, would require heavy ingots or specially profiled continuously cast blanks. Owing to the flexibility of this process and the possibility of combining it with plate production, it will be most effective in minicomplexes using basic oxygen converters and continuous casting machines producing semi products. On the basis of the newly developed practice, a technical proposal has been drafted for the design of a minicomplex (the machines of continuously cast - press - universal beam mill) with an output of 800 kt beams/year with a suitable plant kit and section layout, assessment of amounts of engineering equipment and overall drive capacities, establishment of basic process parameters, and provision of several design variants for bilaterally acting hydraulic presses. The technical project of press provides mechanization of change blocks heads and giving of slabs in working space, devices for removal of dross, system of cooling of the technological tool and automatics. Press is intended for manufacturer from continuously cast slabs shaped beam blanks in length up 10m. The maximal effort develops in a vertical direction - 196MN in horizontal is 147MN. Depending on number of beam blank and it geometrical sizes the cycle of formation on press varies from 7 (before 100s. Hence, press car provide productivity mill up to a million tonne beams in a year. In our opinion, intermediate, less capital-intensive variant of application press before break-down stand of the universal beam mill for formation of ender zones beam blanks is perspective also with the purpose of sharp (minimum in 3-4 times) reductions crop ends. The introduction of this new process technology would reduce capita costs by 22 per cent and production costs by 5.5 per cent. Annual operating costs would fall by six per cent. The technology of pressing is most economic for manufacture of large-sized building (for example, columned, etc.) profiles, with height of section 1000mm and more. Conclusion Theoretical and experimental studies, design and development work, costing calculations, and comparative analysis have all confirmed the high efficiency and viability of the proposed method of creating plant complexes to produce heavy flanged sections and beams from continuously cast slabs of rectangular section. The offered new technology allows to create highly effective and slightly wastes mini complexes and is universal beam mills for manufacture from continuous-cast slabs of the wide range of sizes I-beam (including, building, columned) sections.