THE PROCESS OF THE MULTI-PART FORGING OF THE CONTINUOUS CAST SLAB MODELLING

Transcription

1 THE PROCESS OF THE MULTI-PART FORGING OF THE CONTINUOUS CAST SLAB MODELLING Jan Sińczak, Wiesław Madej Faculty of Metallurgy and Materials Science University of Mining and Metallurgy 2-59 Kraków, Al. Mickiewicza 3, Poland 1. INTRODUCTION A very wide range of steel grades is used for die forgings manufacturing. Weight of die forgings ranges from several grams to dozens of kilograms. Occasionally their weight reaches about 1 Mg. Apart from adequate chemical composition forging billet must meet requirements regarding structure, plasticity and the surface quality. In the case of continuously cast slab, crystals distribution in its cross-section should also be taken into consideration. Unfavorable as-cast structure significantly influences forging cross-section structure. Significant inhomogeneity of the primary structure may magnify effect of local strain nonuniformity resulting from a shape of forging. In the result, the product is likely to be defected. The conventional die forging technologies of as-rolled slab resulted in primary inhomogeneity of the structure, which was compensated with large deformation during rolling. In case of continuously cast slab, problem of its quality must be satisfactorily solved at the stage of crystallization and plastic forming process including preheating prior to forging and final heat treatment following forging process. Preparation of forging billet by rolling solves the problems of inhomogeneity and unfavorable quality of cast structure, however the price of the rolled slab is higher. What is more, elimination of the rolling process requires determining of minimum amount of reduction that ensures satisfactory level of forgings quality. Currently, there are no univocal statements regarding the problem. As far as die forging stock is concerned, requirements about grain size and distribution and cementite form in beforehand rolled slab are emphasized. Improvement of the continuous casting process, in ever increasing extent, enables usage of non-worked stock (prior to forging) for die forgings [1]. In general, die forging process brings about large degree of deformation, which assures required level of mechanical and plastic properties of die forgings. A character of structural transformation is the resultant from the primary structure of an ingot, the initial temperature, the time of forging process, number of forging stages, the length of intervals between them and the chemical composition of steel. In association with the degree of deformation and its distribution these factors dictate final operational properties of forged parts. From abovementioned one may conclude that decision of using cheaper stock from continuously cast slab for die forging depends on the shape, the application, the size and the quality of the stock. Therefore, in making proper decision, analysis based on numerical computation in the range of metal flow and stresses and strain value and distribution throughout forging cross section [2,3] may be helpful. This paper presents results of numerical computations for axi-symmetrical forging of a collar and steel cast in a form of ingot and broached/stretch-forged with various degrees of forging [4,5]

2 2. NUMERICAL COMPUTATION Applying of continuously cast slab was investigated in terms of minimal amount of reduction deformed for die forged parts. In case of flat die forging, minimal forging reduction ratio equals 3, whereas for as-rolled slab it should be at least 1,5. Evaluation of forging reduction ratio is carried out baring on the ratio of the stretch-forged stock [4]. If the forging is obtained in upsetting process, which is common for the most of axi-symmetrical forgings, the above mentioned rule is impracticable. In such cases it is useful to estimate the effective strain distributoon in the longitudinal section of the forging. An advantage of this method is possibility of effective strain evaluation for any shape of forging. Simultaneously it should be mentioned that effective strength and amount of deformation aren't synonymous. Amount of deformation which ensures required mechanical properties is determined basing on researches [3]. It is particularly important in case of using the continuously cast slab. Therefore, information acquired from after-mentioned numerical computation may be taken into consideration while establishing distribution of properties of axi-symmetrical forging made of continuously cast slab. Weight of the model forging of the collar is,214 Mg. Including flash, weight of the stock amounts to,223 Mg. To establish effect of amount of deformation on value and distribution of calculated parameters, three shapes of square stock were used (table 1). Stock slenderness for equivalent diameters, calculated to maintain constant volume, corresponding to reduced cross sections of the stock, are 2,7; 1,5;,9 (tab. 1). Numerical computations were performed using program Form2D, based on finite element method assuming the visco-plastic model of deformed material, with following boundary conditions taken to the analysis: steel 2, starting stock temperature 115 C, temperature of tools 25 C, friction coefficient,3. Table 1. Parameters of the stock from continuously cast slab, taken to the numerical computation of the forging process Mark Cross section Diameter D, mm Height H, mm Slenderness H/D a Square ,7 b Square ,5 c Square ,9 Forging of the investigated collar is conducted in two stages. The first stage is upsetting with reduction ensuring filling of die cavities with minimum surplus of metal flowing into the flash. For the examined shape of the model forging numerical computations prove it is more convenient to use a billet of less reduction ratio in the first stage (figure 1). Typical unfilling of the bottom die impression (figure 1a) is the result of undesirable metal flow to the gutter, in the initial stage of forging. The stock with less amount of upset fills up die cavities, however, technological conditions should take into account energy parameters of forging process (fig. 2). With a less amount of upset in the last stage, less forging pressure occurs (figure 2, curve b), which is essential from die impression durability standpoint. Having regarded above-mentioned conditions, numerical calculations of the two stages forging for three stock slendernesses were carried out, with a stock upset to the height of 1 mm prior to each of the forging processes

3 a) b) Fig. 1. Model forging process of a stock upset to the height of: a - 5 mm; b - 1 mm. In the pictures of the forgings, vertical lines of the net shown only Load, MN a b Stroke, % Fig. 2. Forging pressure/load calculated numerically for die forging of a stock upset to the height of: a - 1 mm; b - 5 mm. Values of effective strain observed in cross-sectional areas during upsetting and die forging for examined stock slendernesses are shown on figure 3, areas of constant difference being taken to the analysis. The values of effective strain were divided into eight ranges. As foreseen, in both forging operations effective strain increases with an increase of stock slenderness. In upsetting process more nonuniformity of effective stress, resulting from stock slenderness, was observed (fig.3a). For this reason, forging in impressions of extricate configuration is advantageous, as less nonuniformity of mechanical properties of forgings can be expected, independently of stock slenderness. Simultaneously, complex shape of forgings is propitious to nonuniformity of effective stress (figure 3b). In case of analysed forging, gradient of effective stress is approximately 4, irrespective of the stock slenderness

4 A) Effective strain ei 2 1,5 1,5 a b c Number of field B) Effective strain ei a b c Number of field Fig. 3. Values of effective strain through the cross section of forgings from a stock of slenderness: a - 2.7; b - 1.5; c -.9 (A upsetting, B - die forging) Essential meaning for the forging process has distribution of mean stresses. Positive values of mean stresses (fig. 4) may cause fractures in continuously cast slab. In case of the analysed forging, the most disadvantageous is the last stage of cavity filling process. For each of the examined stock slendernesses, location of positive mean stresses areas and their absolute values are comparable, ranging from 5 to 9 MPa. a) b) c) Fig. 4. Distribution of mean stresses areas during forging process for stock slenderness: a) 2.7; b) 1.5; c).9-4 -

5 3. MODELLING OF THE STRETCH-FORGING PROCESS Computations were performed for a forging ingot weighing 21 Mg, with five forging reductions (1,6; 2,35; 3,5; 5,1; 7,4) assumed; each of them obtained in three drafts sequence, imposed by means of flat dies, in the way letting obtain square cross-section [5]. For the metal flow pattern simulation, the finite element method was employed, based on assumption of visco-plastic model of the deformed material. Figure 5 shows the calculated mean values of effective stress in 1/3 of the distance from the centre forging line, at the cross sections of subsequent degrees of forging, respectively. For the first three cases (1,6; 2,35; 3,5), numerical values of forging degree and effective stress are comparable. Relative increase of the calculated total effective stress, for the following degrees of forging (5,1; 7,4) is less than for the previous ones. k, ei k ei The following degree of forging Fig. 5. Comparison between amount forging reduction ratio (k) and effective strain (e i ) of the forging stretch-forged in flat dies From absolute values of degree of forging, derived from initial to final cross-section area ratios and effective strain, one may conjecture their mutual dependence. For the analysed stretch forging process, dependence of degree of forging k on effective strain ( i may be described k = a ε i, where a = 1,4. For the upset forging process of axi-symmetrical parts, above mentioned relationship may be established basing on investigation of the mechanical properties of forgings made of ingot 4. SUMMARY Elimination of surface defects from continuously cast steel, achievable nowadays in many steelworks, enables direct using of it for die forgings, provided, required amount of deformation is maintained. In case of axi-symmetrical die forgings, where upsetting is the dominating part of the forming process, numerically estimated effective strain value, may be regarded as a measure of amount of deformation

6 Using stock of proper slenderness (h/d>1), required amount of deformation is achieved, which assures demanded operational properties of parts forged from continuously cast slab. Acknowledgements Finacial assistance of Mining and Metallurgy University, Faculty of Metallurgy and Materials Science, Found No. AGH is acknowledged. REFERENCES 1. Grabelus J., Mazur A., Bulkowski L.: Możliwości ELSTAL Łabędy w zakresie technologii otrzymywania wlewków ciągłych o jakości wymaganej do kucia. Mat. Sem. Nowoczesne półwyroby do przeróbki plastycznej, Gliwice 1999, s Madej W., Kusiak J., Sińczak J.: Symulacja komputerowa procesu spęczania przedkuwki do walcowania obręczy profilowych w walcarce promieniowo-osiowej. Materiały V konferencji: Zastosowanie komputerów w zakładach przetwórstwa metali Bukowina Tatrzańska, Edytorzy: Pietrzyk M., Kusiak J., Piela A.,1998, s Sińczak J., Madej W: Wpływ stopnia przerobu na własności mechaniczne wyrobów z wsadu z procesu ciągłego odlewania stali. Mat. Konf. Walcownictwo 99. Edytorzy: Nowakowski A., Turczyn S., Kuźmiński Z., Madej W., Ustroń, 1999, s Sińczak J., Cybuka L., Kowalski B., Żurek W.: Wpływ stopnia przekucia na własności mechaniczne i plastyczne odkuwek. Hutnik Wiad. Hutnicze, 1998, 6, s Sińczak J., Majta J., Głowacki M., Pietrzyk M.: Prediction of mechanical properties of heavy forgings. J. Mat. Proc. Tech., 1998, 8-81, s