ROLLS Production of HSS Rolls for use in Narrow Hot Strip Mills and Rod Mills. British Rollmakers (China) Ltd

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1 Production of HSS Rolls for use in Narrow Hot Strip Mills and Rod Mills presented by British Rollmakers (China) Ltd ROLLS Advances in Mill Roll Technology April 1999 International Convention Centre Birmingham, UK 1

2 The Centrifugal Casting of HSS Rolls for Narrow Strip Mills and Rod Mills Zhang Xin * Michael C. Perks ** ABSTRACT This paper records the development and production of the latest HSS quality, horizontally centrifugally cast, with the core material in spheroidal graphite iron. The composition and subsequent heat treatment ensures the hardness of the working layer of the HSS material will reach 80/85 shore, with virtually no fall off in hardness throughout the working layer, while the structure of complex carbides in a martensitic matrix ensures uniform wear as well as high wear resistance. Heat treatment as well as producing a satisfactory wear resistant shell also allows the core material to maintain its high strength properties. By using the CC duplex HSS roll in the finishing stands of narrow strip mills, or in the pre-finishing stands of high speed wire rod mills, gives significant productivity gains both in roll life, and roll shop/mill operations. INTRODUCTION The introduction of high speed continuous mills together with increased customer demands in surface finish and shape, the rollmakers are faced with the dual challenge of producing roll qualities for increased productivity as well as end product quality. In recent years new roll qualities have been developed, as well as different manufacturing processes needed to successfully produce these rolls. The conventional AIC roll in some stands has been replaced by High Chrome alloys and more recently by HSS qualities. Quantum increases in performance have been recorded and yet still further development is continuing. The foundry is especially designed for the production of small to medium sized centrifugally cast rolls. There are in line 8 horizontal centrifugal casting machines, each one capable of casting 3 rolls per hour of varying sizes. To feed these casting machines there are 5 in-line induction furnaces, medium frequency of varying capacity from 1 tonne to 5 tonnes. Liquid metal is weighed into the moulds by electronic crane mounted weighers, and metal composition is controlled by a. Quantavac. Metal temperatures are measured by standard thermocouples, while metal solidification is measured by black body radiation thermocouples. A heat treatment shop is computer controlled to very close temperature ranges ±5 C. Current annual production for small rolls and rings amounts to approximately 8,000 t.p.a. The quality control system is independent from production, and produces certificates of assurance for dimensions, hardness, structure, and ultrasonics as well as monitoring the production systems. THE EFFECT OF ALLOYING ELEMENT ON THE STRUCTURE OF THE WORKING LAYER Carbon Carbon, Iron and Chromium when in specific ratios, combine to form complex carbides M7C3. At the same time carbon will also combine with Mo, V, Nb, Cr, and W, to form various high hardness carbides such as MC, M6C, and M2C. To obtain the most suitable wear resistant and castable structure, the carbon level should be between 1.6%-2.0%. Increasing the carbon above this level promotes the formation of more and continuous carbide, thus reducing the ductility, heat and crack resistance. * Ms. Zhang Xin, Deputy Gen. Manager, BRC-Lian Qiang Foundry, Tangshan, China ** Mr. Michael C. Perks, Technical Director, British Rollmakers (China) Ltd., Hong Kong, China 2

3 Silicon Silicon together with carbon helps with the fluidity during casting, maintains ductility and strength, and should be at a level of % Manganese Manganese helps to keep the oxygen level of liquid HSS composition to a minimum, while at the same time combining with the residual Sulphur to form MnS, preventing grain boundary embrittlement. Manganese level should be % Chromium Cr, FE, V, Nb, Mo, W and Carbon combine together to form complex carbides during solidification and subsequent heat treatment. Some of the Chromium is dissolved in the matrix, and increases the wear resistance and hardenability. Corrosion resistance is also increased. To give the best wear resistance while at the same time maintaining the ductility, the Chromium level should be between % Tungsten Fe and C combine together with W, Cr, Mo, Nb and V to form duplex carbides which enhance the high temperature and tempering characteristics and subsequent hardness of these HSS qualities. W is usually present in the Fe4W2C form. Eutectic M6C carbides have a fish bone type shape and due to the low quenching temperature cannot be easily be broken down into the non continuous form. W2C type carbides have a theoretically high density of 17.2 which under centrifugal force can easily be changed.w level is set at % to get the best wear resistance, ductility, heat and crack resistance. It also helps to maintain the hot hardness during the rolling operation, and increases the corrosion resistance at elevated temperature. Molybdenum Molybdenum has a similar function when combined with Fe and C, as W. Eutectic Mo2C shape carbides form in the shape of rods/sticks or features, easily dispersed. The theoretical density of Mo2C is 9, 18 and can partially be substituted for W when the total amount of carbides and variance analysis is considered. It is not advisable to substitute all the W by Mo as some of the oxidation and corrosion properties will be diminished. The Mo level is ideally set at % Vanadium and Niobium Both V and Nb combine with Fe, Cr, Mo to form MC carbides, which have very high hardness levels and will precipitate as fine carbides, thus playing a major role in the rolling process, increasing the wear and crack resistance, while at the same time strengthening the ductility. To be effective the level of both alloys should be % but above this level segregation will occur during solidification using the centrifugal process. Nickel While Nickel is used extensively in steels to increase the hardenability, it is not used in HSS qualities because of its tendency to increase the residual austenite levels after transformation treatments have been carried out. Therefore to maintain the desired mechanical property levels, we suggest that the level of Nickel be % 3

4 Table I: The chemical compositions in the working layer of HSS rolls (%) C Si Mn W Cr Mo V Nb Ni 1.6/ / / / / / / / /1.5 HSS/Shell MICROSTRUCTURE The high speed steel shell is closest to a D2 type analysis, the microstructure consisting of primary and eutectic carbides in a martensitic matrix. Strong carbide stabilising elements, such as Vanadium, Tungsten, Molybdenum and Chromium are added to improve the hardness and resistance to wear, oxidation and roughness. The type, morphology and volume fraction (see table II) of carbides as well as the characteristics of the matrix all effect the properties. Hardness is affected by the intercellular carbides and their distribution. Wear and surface quality are directly influenced by carbide morphology. High amounts of Vanadium give the best wear properties but compromise the surface roughness characteristic of the roll. The carbide stabilising elements are balanced to ensure the correct morphology and volume fraction whilst retaining adequate solid solution strengthening. The high spinning speeds necessary to produce small diameter rolls limits the vanadium content, due to potential problems of segregation. The transition or white iron zone is made up of complex Chromium and Vanadium carbides. The temperature profile across the interface has to be carefully controlled to avoid the formation of intercellular micro-porosity. The core is an SG cast iron, well spheroidised with a high nodule count. The excellent feeding characteristics of the SG ensures centreline soundness even in the thinnest sections. Figures I and II Show the typical shell microstructure at x 500 and x 200. Figure I Figure II Shell x 500 Shell x 200 4

5 Table II: The carbide composition & mechanical properties of several quality of duplex rolls Roll Carbides Tensile Carbide Matrix Wear Mod of Type Form Area Size Strength AV HV HV Resis Elasticity (%) (µm) (Mpa) (HSC) /x103 Mpa HSS VC C MC 7-15 < M2C 2000 M6C 1800 HiCr M7C C 1600 HiNiCr M3C C 1100 Table III: The chemical composition of the core materials of CC duplex HSS rolls Quality C Si Mn Ni Cr Mo Mg SGP Cast Iron 3.2/ / / /1.0 Š / CORE COMPOSITION & PROPERTIES Figure III Core x 100 5

6 The tensile strength of the roll neck has to be high enough to withstand the rolling loads as well as the heat treatment. Chemical compositions are chosen very carefully according to the specific working conditions of the rolls and the strength required for the core and the necks, to withstand melting, casting conditions and heat treatment effect. The chemical compositions of the cast iron pearlitic core and CC duplex HSS roll core are listed in Table III while the mechanical properties are listed in Table IV. The microstructure is shown in Figure III. Table IV: The mechanical properties of the core materials of CC duplex HSS rolls Quality Tensile Strength Elongation Bending Strength Hardness (Mpa) (%) (Mpa) (HSC) SGP Cast Iron SHELL CORE INTERFACE To ensure successful roll life and avoid mechanical failure, a complete fusion between the shell and the core is vital. In manufacturing HSS rolls by horizontal CC duplex casting method, the working layer will be removed from the casting machine after solidification. The position of the working layer will change from horizontal to vertical. The positions of the chill and the upper and lower necks are matched, and casting of the core material can then proceed. To obtain an optimum working layer, various factors need to be considered. (1) The temperature of the inner surface of the shell (2) The protection from oxidation of the inner surface (3) The casting temperature of the shell and core iron (4) Time interval between casting shell & core (5) Dressing thickness & Cooling rate. It is essential to get a good fusion between the inner core and outer shell but avoiding excessive mixing the working layer and the core. The temperature of the internal layer should be between 1,000-1,100 C measured by radiation thermocouple, while the core iron to be within 1,350-1,450 C dependent upon liquidus in order to obtain the required optimum conditions for fusion of the shell and core, but not to mix the working layer & core. In practice, when the inner surface of the shell is oxidized, fusion of the shell/core requires a higher temperature of core iron but this is not recommended. This oxidation will increase the difficulty of the fusion between the core and the inner surface of the working layer. Thus, in our CC casting technology, we are using a special kind of material to act as a flux protection layer under the high temperature of the inner surface of the shell. This protection layer carries the function of anti-oxidation of the inner surface of the shell and also preserving uniform temperature. This will enables the shell metal to have a unidirectional solidification from the surface to the inner layer. After aligning all the chills (barrel & necks) in position, core metal will be top poured or bottom poured into the mould. Due to the wash action of the high temperature core metal and the buoyant force, the flux protection layer will separate from the solidified working layer and float to the upper neck chills. A 3-10mm thick solidified working layer will be remelted and fused with the molten iron of the core. The final sodification of the shell and the core is a metallurgical bond. One of the characteristics of CC duplex HSS roll is that the interface between the shell and the core is very distinct. The quality of the interface and the depth of the working layer is checked by ultrasonics. Indepth investigation was carried out to determine the integrity of the shell/core bond. We considered it necessary for this interface to be narrow, free from porosity, and of suitable structure to withstand the rolling loads. Electron probe micro analysis together with a quantitative line scan was carried out by Swinden Technology Centre, Corus, across the interface and into the shell and core at either side. 6

7 Figure IV show the alloy distribution and concentration in the area of scan. Electron scan microscopy quantitatively confirmed these conclusions that the interface was approximately 3mm wide and there is some minor chrome carbide concentration within this area, but it is not continuous. See Table VI. From these results we can draw a number of conclusions : - The shell/core interface is narrow The interface does not contain any porosity and is sound There is little or no alloy migration into the core material & little shell solution into the core The core material remains in this case normal pearlitic nodular graphite. Tensile tests across this bond show levels of 600 MPa. Table VI: % Carbide levels (Electron Scan) Distance from Interface VK CRK NiK MoK WK mm SHELL CORE HEAT TREATMENT HSS rolls have high hardness, good wear resistance at high temperatures due to quenching, tempering and heat treatment. The quenching temperature for CC duplex HSS rolls is governed by the alloy composition. We are required to maintain a good working depth of the rolls as well as to maintain the quality of the core materials, especially when using alloy 7

8 spheroidal nodular graphite. If the quenching temperature is too high, the core will be distorted or even melted. Thus, the foundry controls the alloy content and subsequent quenching temperature up to the point that the alloy carbides just transform to become austenite. Adopting forced air cooling, the material will be transformed into bainitic and martensitic form, which is then tempered. The Mo, W, V and Nb dissolved in the austenitic matrix then become stable component of carbides. This enacts the release of micro carbides and leads to the hardening effect. The outer shell is then formed with high hardness and good wear resistance at high temperatures. This will not affect the ductility of the inner layer while the tensile strength of the neck is greater than 550 Mpa. A standard production HSS roll #A was used as a sample for heat treatment testing. Test pieces were cut from the barrel in the dimensions of 50mm x 30 mm x 10 mm. There are 10 testing pieces, the 10th one is as cast. The first to ninth testing pieces are separated into 3 groups and undergo quenching in 3 different temperature : 950 C, 1,050 C and 1,150 C. After quenching, tempering is also done in 3 different temperature : 500 C, 550 C and 600 C. The hardness after quenching and tempering are listed in Table VII. From Table VII we can see that at quenching temperature 1,150 C and tempering temperature 550 C, the HSS roll has hardening effect. The hardness after tempering is 3 HSC higher than the hardness after quenching. For other temperature for quenching and tempering, the hardness drops, especially when tempering is done at 600 C, the hardness drop is the most radical. Though at 950 C quenching C tempering there is also hardness drop, the final hardness can still attain 83 HSC. According to the findings from the above testing, there are two suitable handling for heat treatment : (1) 1,150 C quenching C tempering (2) 950 C quenching C tempering but (2) is recommended as being more practical. Subsequent tempering can be carried out to give correct structure. After heat treatment of the HSS rolls in accordance with the above testing parameters, the hardness profile of the roll is illustrated in Table VIII below. Table VII: The hardness change under different temperature for quenching and tempering Sample Quenching Hardness after Quench Tempering Hardness after Temper Temp ÞC HRC HSC Temp ÞC HRC HSC casting state Table VIII: Roll hardness testing results Roll# Barrel Hardness in Casting State Barrel Hardness after Heat Treatment A75 A B C Ave A B C Ave

9 Table IX: Performance Data AIC/SG HSS Area Size Roll Type Strands Tonnes Dressing Tonnes Dressing Improvement 312 x 500 Strip mm x 10 China 310 x 150 Ring mm mm x x 600 Strip x x 50 Ring Carbide Replaceme Ex-China 280 x 400 Rod x x 710 Rod Multi mm x 5.5 PRECAUTIONS IN USE 1. Good water supply both in volume and pressure, properly directed. 2. Good shape from previous stands. Directional control of guides critical. 3. Crack removal after redressing. 4. Regular Ultrasonic testing of interface. 5. Correct tension and drafting between stands. BENEFITS WHEN USING HSS ROLLS 1. Longer rolling campaigns. 2. Less dressing per campaign. 3. Improved mill and roll shop productivity. (Less mill down time) 4. Better shape and size to the sizing mill. 5. Reduced roll cost/tonne rolled. 6. Lower roll inventory. CONCLUSION The use of HSS rolls can show quautum increases in benefits to both the mill and roll shops in productivity and quality. To make full use of these benefits, some changes in practices will be necessary particularly in inspection, records and rescheduling. Roll qualities in earlier stands will have to be reviewed to minimise mill downtime and even maintainance schedules may change. ACKNOWLEDGEMENT The authors wish to thank their colleagues at British Rollmakers China and Lian Qiang Foundry for their assistance towards this paper. John Mees, Sheffield Forgemasters Rolls Technical Department 9