Metallu~ist, Vol. 43, Nos. 11-12, 1999

INTEGRATED TECHNOLOGY FOR CASTING-CRYSTALLIZATION HEAT TREATMENT IN THE CONTINUOUS CASTING OF H I G H - S P E E D STEELS

S. L. Paren'kov, R. V. Kakabadze, V. P. Pavlov, A. V. Supov, and N. M. Aleksandrova

The combination of casting-crystallization, deformation, and rate-temperature-dependent processes in a single production cycle to the maximum extent possible reflects the essence of current trends in the technical development of metallurgy [ 1]. This approach promises significant organizational and economic benefits (lower production costs, higher labor productivity, saving of time and material resources). Also, practical experience has shown that combining the above processes creates a fundamentally new method of forming the structure and properties of metal products. The integration of these multifaceted operations offers broad possibilities for improving the quality of metal products and at the same time achieving a high level of consistency in regard to product quality. Work in this direction has been ~ooin,,~on for some time in this country and abroad. For example, different types of thermomechanical treatment (TMT) have been proven effective in practice. These treatments combine deformation processes (rolling) and solid-state phase transformations (heat treatment) in one production cycle. Such widely used metallurgical technologies as controlled rolling and the heat treatment of hot-rolled products made of rail steel have gained acceptance. In connection with the qualitative improvements in existing methods and equipment for continuous casting, rapid strides are now being made in integrated technologies that combine the operations of casting and rolling. Provoking considerable interest among experts are the reports on work being done on units whose operation can be characterized as "casting on a continuous caster rolling into sheet or strip" (so-called CSP-technologies) [2]. Theoretical substantiation of such units and work on their practical realization are also being done in our country [3]. In this article, we present a new variant of an integrated metallurgical treatment that combines the crystallization of a melt - such as during casting on a continuous caster - and phase transformations in the solid state (heat treatment of the crystallizing semifinished product). The essence of such integration lies in the tact that the crystallizing semifinished product is cooled quickly but not monotonically, as is usually done in the manufacture of any metal product. Instead, the cooling of the semifinished product is discontinuous, with abrupt cooling being followed by and alternated with heating. Impulsive cooling is done within the temperature ranges in which primary and secondary recrystallization take place. We have called the new method of obtaining cast metal products the "ICCC method" (impulsive-continuouscrystallization of the casting) and have termed the process of obtaining a continuous-cast semifinished product by the ICCC method as "casting-crystallization heat treatment" (CCHT). Both the ICCC method and the CCHT production process have been licensed in Russia [4]. The CCHT technology, which has been introduced at the Moscow factory Serp i Molot in the casting of steels on a radial continuous caster, has made it possible to solve two main problems: 9 it has sharply reduced the incidence and size of typical casting defects (coarse dendrites, chemical segregation, grain-boundary accumulations of coarse particles of primary phases, etc.), which has significantly improved the process ductility of the cast steel;

Metallurgical Plant Serp i Molot, N. 1~. Bauman Moscow State Technical University, G. V. Kurdyumov Institute of Metal Physics and Functional Materials, and the Central Scientific Research Institute of Ferrous Metallurgy (a State Science Center). Translated from Metallurg, No. 11, pp. 39-41, November, 1999. 0026-0894/99/1112-0485522.00 9

Kluwer Academic/Plenum Publishers

485

Fig. 1. Microstructure of high-speed steel R6M5 cast in an ingot mold (a) and on a continuous caster (b).

9 it has significantly reduced the number of conversions required and appreciably lowered the cost of making the finished rolled product compared to casting in ingot molds. Results such as these have made it possible, for the first time in Russia, to continuously cast trial batches of high-speed steel and then immediately roll the cast steel. The above-mentioned problems were resolved as a result of a qualitative change in the structure of continuous-cast steel R6M5 compared to the structure of the same steel when cast in an ingot mold (Fig. 1). The reduction in the percentage of the eutectic component during CCHT and refinement of the ledeburite inclusions give the continuous-cast steel a substantial reserve of toughness and ductility (Fig. 2). The level of the viscoplastic properties of such steel allows problem-free withdrawal of the semifinished product during casting on the continuous caster and its delbrmation in the subsequent rolling conversions. The production of steel R6M5 by this technology obviates the need for high-waste forging, which is a necessary operation in the standard technology (i.e., when the steel is cast in ingot molds). Other operations that are no longer necessary when continuous casting is used are the production and assembly of ingot molds, assembly and disassembly of gating systems, annealing of the ingots, the use of costly and cumbersome forging equipment, and most intra- and inter-factory transport operations. A detailed analysis of the advantages of the new technology in relation to the standard technology can be made by comparing the two flow charts for the production of high-speed steel R6M5 in Fig. 3. The biggest advantage is that continuous casting reduces the number of conversions by half - from six to three - and reduces the number of operations from 18 to five, i.e., more than threefold. One important feature turns out to be the elimination of the most time-consuming operations, such as the homogenizing annealing of the ingots, the forging conversion, and many of the transport operations. For the conditions at the Serp i Molot 486

Fig. 2. Change ill the percentage of the ductile component (d.c.) in relation to the method of casting: casting in an ingot mold - 10% d.c. in the ingot (a);

casting on a continuous caster - 20% d.c. in the continuous-cast

semifinished product (b).

plant, it takes at [east 82 h to cast one 10-ton heat of high-speed steel in ingot molds. Use of the CCHT technology shortens this process to 10 h. Here, 30 rain is spent on casting and the rest of the time is taken up by the slow cooling of the steel after it leaves the continuous caster. The losses of metal to cropping, oxidation, and conditioning range up to 30% of the weight of the ingot in the conventional technology and amount to less than 18% in the new technology. It should be emphasized that the above-described C C H T technology was realized on a radial continuous caster with a vertical mold. Casters of this type allow vertical exit of the semifinished product from the mold and its subsequent rotation on a horizontal path. Such forced deformation during radial travel of the semifinished product and its movement onto the horizontal section was often the reason for the failure of previous attempts to cast high-speed steel on radial continuous casters. Due to the high degree of brittleness of high-speed steels in the temperature range close to the liquidus, even small deformations inevitably lead to fracture of the semifinished product as it is being withdrawn.

The greater ductility of

high-speed steel which has been subjected to C C H T makes such cracking impossible. Thus, none of the semifinished products obtained in the commercial trials at the Serp i Molot plant fractured on the sections where the cast steel undergoes bending and unbending. After being cast on the continuous caster, semifinished products with a cross section of 160 x 160 or 140 x 140 mm and a length ranging from 1.6 to 3.15 m are sent directly to the rolling mill, without intermediate annealing or conditioning of the sides and with the forging operation being eliminated. 487

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Fig. 3. Flow charts for the metallurgical production of high-speed steels by the existing technology (casting into ingots) (a) and the new technology (continuous casting by the CCHT method) (b): I) refining in all electric-arc furnace; II) preparation of ingot molds; III) casting; IV) processing of ingots after casting; V) forging on a rollforging machine (RFM) with a large reduction; VI) rolling; 1) refining and pouring of the steel from a ladle; 2) manufacture of ingot molds; 3) assembly of ingot molds on a platform for casting ingots (15 ingot molds tor each 10 tons of steel); 4) assembly of the gating systems; 5) transport of the plattorm with prepared ingot molds to the steel-casting shop; 6) casting of the steel; 7) cooling of the ingot molds; 8) disassembly of the ingot molds, extraction of the ingots, and evacuation of the molds in the mold yard; 9) removal of gates; 10) heat treatment of the ingots (homogenizing annealing); 11) assembly and transport of a batch of ingots to the plant pertorming the forging on the RFM; 12) cropping of the top and bottom of the ingots; 13) heating to the forging temperature and deformation on the RFM; 14) annealing of the forged semifinished products; 15) return of the lorged semifinished products to the metallurgical plant; 16) heating for the rolling operation; 17) hot rolling into sections or sheets;

18) annealing.

In producing hot-rolled sheet, sheet bars (25 mm thick) were rolled on a 560 mill. Then finish rolling was done on a 1500 mill to obtain sheet 2 ~ mm thick. As in the case of conventional forged semifinished products, rolling temperature was 1150-1050~ The metal was heated in continuous gas-fired pusher furnaces belore rolling on each mill. The finished sheet bars and sheet were cooled completely after rolling. The final operation was a softening annealing performed by the same technology that the factory uses for sheet obtained from forged semifinished products of steel R6M5 that is cast in ingot molds. The quality of the sheets was thoroughly checked in accordance with the requirements of GOST 19265-73. An evaluation was made of grain size, points tor carbide nonunifonnity, the character of the nonmetallic inclusions, hardness, and impact toughness. All of the test parameters of the R6M5 steel obtained by the new technology were comparable to the same parameters of steel made by conventional technologies (Table 1). 488

TABLE 1. Quality Indices of Hot-Rolled 4-mm-thick Sheet of Steel R6M5 Made by Different Technologies Grain size after quenching t dq, ~tm)

Point rating for carbide nonunilonnity

Hardness in the annealed state

after quenching and triple tempering at 56()~

Conventional technology,with casting into ingots and subsequentforging and rolling <0.6 2-3 230-250 HB 63 HRC <0.3

Continuouscasting with the use of CCHT <2 24(I-260 HB

63 HRC

In addition, none of the specimens taken from the continuous-cast semifinished products contained surface defects or internal defects that could warrant rejection of the steel. No internal discontinuities were found by ultrasonic testing perlbrmed on batches of sheet from different heats. Sheets of steel R6M5 ranging in thickness from 2 to 4.5 mm and made by the new technology have been sent to many machine-buildingplants to make tools and conduct durability tests. In particular, such sheets have been sent to the plants BELFREZ (in Belgorod) and AMO ZIL (Moscow). The results of tests of the tools demonstrate the high quality of the steel obtained by the CCHT technology. The properties of this steel are comparable to or better than those of steel of the same grade produced by existing methods. For exampie, according to data from the tests conducted by AMO ZIL, experimental cutters made of steel produced by the CCHT method had a wear resistance 20-30% greater than that of regular cutters. Similar results on tool life have been obtained at other plants. Conclusions 1. A new method has been developed for obtaining cast metal products. The method is based on combining crystallization processes and processes which occur during heat treatment of the crystallizing metal in a single production cycle (CCHT). 2. For the tirst time in domestic practice, combined cycles of CCHT have been used in the continuous casting of high-speed steel R6M5 on a radial continuous caster. The casting was done at the Serp i Molot plant. 3. Replacing casting in ingot molds by contitmous casting shortens the production cycle lbr high-speed steels by a factor of more than eight and decreases the number of processing operations and conversions threefold. It has also been shown that it is possible to roll high-speed steel cast on a continuous caster by the proposed technology, without the need for tbrging. 4. The quality of finished products of steel R6M5 made by the new technology meets the requirements of GOST 19265-73. Tools made from an experimental batch of the steel lasted 20% longer than tools made from steel produced by the usual technology.

REFERENCES D. V. Henning, E Kyeper, E Pleschiutschnigg, and E. Tomanek, CSP - The Status and Latest Innovation o f the Slab

2,

3. 4.

Casting Technology, 3rd European Conference on Continuous Casting, Madrid, Spain, October 20--23, 1998, pp. 357-375. A. Cristallini, A. Spaccarotella, R. Capotosti, G. Flemming, and J. Sucker, Thin Slab CSP PlvcessJbr Specialty Steels. 3rd European Confi, rence on Continuous Casting, Madrid, Spain, October 20-23, 1998, pp. 357-375. M. R Galkin, G. S. Nikitin, and R. I. Ritman, "Compact casting-rolling units for making sheets of steel and alloys," Metallurg, No. 8, 25-30 (1999). A. V. Supov, N. M. Aleksandrova, S. L. Paren'kov, R. V. Kakabadze, and V. E Pavlov, "Metallographic problems in the production of continuous-cast high-speed steels," Metalloved. Term. Obrab. Met., No. 9, 6-13 (1998).

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