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Aim and problems of paper.
Comparison of medium-values of dimensions for characteristic of initial
Volumetric part of phases in structure of researched samples
Budagyants N.A., Zhizhkina N.A., Gutko Y.I.
THE RESEARCH OF CENTRIFUGAL FORCES’ EFFECT ON PROCESS OF STRUCTURAL FORMATION OF MASSIVE GOODS
This paper is dedicated to working rolls for hot rolling of metals. The operation conditions of the rolling tools determine their durability and reliability. Application of high-effective materials and development of centrifugal casting enable to get a shaping tool which is resistant to wear and breakage as well. As a result of carried out researches rolls with more even level of properties in length and cross-section have been produced. 7 literary sources have been used.
Keywords: centrifugal casting, cast iron, roll, structure, working layer.
The way a problem is put. Rolls are principal working tool in rolling, paper-making, rubber-and-technical productions and food industry. They determine quality and preset sort of output, exit of suitable products and consumption of materials. That is why the rolls must have high operating properties. But each production has its own technological features and conditions of rolls' exploitation.
At the same time in rolling mills rolls are subjected to wear, sign-variable thermal, impact loadings. In the process of exploitation there occurred bending and torsional moments. The results of our explorations of rolling tool's operation [3, 7] have shown that exploitation conditions depend upon the mill's design, specific pressures, one operating time for installation, intensity of cooling and location of rolls. There are roughing and finishing stands in hot sheet rolling mills. Roughing stands are characterized by significant loadings at reducing and by high temperature. Their actions cause crumbling and formation of grid of heat on the rolls' barrel surface. In first finishing stands a grid of cracks and following crumbling of particles form in surface layer of rolls as a result of cooling medium's action, high temperature and an intensive wear. In rolling process separate cracks of fatigue origin gradually increase and may cause breakage of the roll. The cooled sheet comes into the last stands of finishing group. Therefore here working rolls are subjected mainly to mechanical loadings.
Thus the exploitation conditions' analysis of rolls in hot sheet rolling mills has shown that the product at the same time must possess high hardness, strength and plasticity. In addition the level of properties depends on function of the roll. For example, roughing stands need more strong and less hard rolls than finishing stands do. Change of properties in a depth of a rolling toll in of great importance for operating time.
^ Application of high-effective materials and development of new technologies for making rolls ensure rising of exploitation characteristics of the rolling tool. Production of sheet rolling shafts by means of double-layer method pouring into stationary moulds ensure simultaneously getting of high hardness and strength range. Steel and cast-iron of different composition are applied for shaping of working layer and core of rolls. Their choice depends on function and conditions of rolls' exploitation. Alloy cast steel and low-alloyed cast-iron rolls with hardness 64-70 HSD have resulted a good rolling surface with the volume 500000 tons for a roll in roughing stands. High-alloyed cast-iron rolls with higher hardness 74-80 HSD are applied for finishing stands' groups .
Development of centrifugal casting technology enabled to apply new high-hard and wear-resistant materials for the working layer and to ensure the needed strength of rolls' core. Centrifugal casting has widened possibilities in control over the roll's quality by means of various technological parameters . Such method of rolls’ production contributes reduction of quantity of undesirable tinges in working layer’s metal on all its depth .
^ The aim of this paper is research of influence centrifugal casting method on structure of rolls’ working layer. To reach this aim the following problems should be solved:
Materials and results of researches. According to  structure of working layers metal was estimated by method of metallographic analyze with magnification 100x. Specially prepared samples of centrifugal and stationary casting rolls of the same size were researched. High alloyed cast iron for them was prepared in one melting unit.
The analyze of centrifugal forces’ effects on initial crystallizations process based on peculiarity of solidification of rolls material. Size of dendrites and their maximum parts (length of trunks axis (h) and distance between branches’ brim of second order (l)) were estimated. According to  calculation of medium-sizes and variance was executed (tab. 1).
structure of working layer of researched rolls
Results of researches showed that sample of centrifugal casting roll has more compact (dendrites size twice less) and even (small variance of indications) initial structure in comparison with working layer of stationary casting good. It has been established that the centrifugal method exerts a restraining effect on crystallization of longitudinal dendrites.
The linear method of research was used for determination percentage of phases in structure analyzed samples . Results of measuring have been presented in tab. 2.
The table 2 has showed that volumetric part of hard phases in structure of centrifugal casting roll (cementite and martensite) exceeds their quantity in stationary casting product and form 45 %. Among analyzed samples centrifugal casting intermediate product contents of minimum quantity of graphite (1,85%). Pearlite is lacking and small quantity of bainite (17,23 %) is observed in structure it metallic matrix. It is caused by the combination of acting centrifugal force with higher speed of crystallization of the casting.
Conclusions. 1. Resistance and reliability of rolls depend on conditions of their exploitation in a rolling mill. The analysis of the conditions enables accordingly to apply a type of rolls and to raise their resistance.
2. A shaping tool which is resistant to wear and at the same time to breakage can be produced by means of applying of high-effective materials and developing of centrifugal casting. The roll-makers operate the quality of rolls according to demands of customers by means of changing of chemical composition and regulating of technological casting's parameters.
3. Metallographic researches of the working layer for stationary-casted and centrifugal-casted rolls have shown that application of the new technology raises its quality. Centrifugal-casted rolls have more dense and even structure of the working layer. It has been established that the centrifugal method exerts a restraining effect on crystallization of longitudinal dendrites.
4. The structure of centrifugal casting rolls sample is characterized by maximum quantity of more hard parts (cementite, martensite), small quantity of graphite and lack malleable phase of metallic matrix (pearlite). It is caused by the combination of acting centrifugal force with higher speed of crystallization of the casting. As a result of carried out researches rolls with more even level of properties in length and cross-section have been produced. It promotes more effective using of the working layer and significant rise of the roll's resource.
Besides, the increased volumes consumption of metallic goods in the world market and rising of demands to its quality require an improvement of exploitation properties of such rolls. That is why scientific-research work in this plane is going on.
Стаття присвячена робочим валкам для гарячої прокатки металу. Умови експлуатації прокатного інструменту визначають його стійкість та надійність. Застосування високоефективних матеріалів та створення відцентрового лиття дозволяють отримати формуючий інструмент, що одночасно стійкій до зносу та поломок. Результатом досліджень, що проведено, є більш ефективне застосування робочого шару й значне збільшення ресурсу валка. Використано 7 літературних джерел.
Ключові слова: валок, відцентрове лиття, робочий шар, структура, чавун.
Статья посвящена рабочим валкам для горячей прокатки металла. Условия эксплуатации прокатного инструмента определяют его стойкость и надежность. Применение высокоэффективных материалов и создание центробежного литья позволяют получить формующий стойкий одновременно к износу и поломкам инструмент. Результатом проведенных исследований является более эффективное использование рабочего слоя и значительное увеличение ресурса валка. Использовано 7 литературных источников.
Ключевые слова: валок, рабочий слой, структура, чугун, центробежное литье.
Reviewer: Doctor of Engineering, Professor V. I. Sokolov
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