CC BY
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Filippov Yulian Kirillovich, doctor of technical sciences, professor, kiod@mami.ru, Russia, Moscow, Umech,
Molodov Andrey Viktorovich, candidate of technical science, docent, kiod@mami., Russia, Moscow, Umech,
Zaycev Anton Gennadievich, postgraduate, kiod@mami.ru, Russia, Moscow,
Umech,
Evsicov Roman Aleksandrovich, postgraduate, kiod@mami.ru, Russia, Moscow,
Umech
УДК 621.77
PLASTOMETRIC TESTING OF 20MnB4 AND 30MnB4 MICROADDITION COLD UPSETTING STEELS AND C45 AND C70
HIGH-CARBON-STEELS
S. Sawicki, H. Dyja, A. Kawalek
The paper presents a method for determining the real steel work-hardening curves based on the cylindrical specimen compression test. The subject of testing were steels with micro-additions intended for cold upsetting (20MnB4, 30MnB4) and selected high-carbon steels of a carbon content from 0,45 to 0,73 % (C42D - C76D). The tests were carried out using the physical simulator of metallurgical processes GLEEBLE 3800 for the temperature range of 700.1 200 °C and the strain rate range of 0,1.50,0s1. Based on plastic deformation parameters recorded during the experiment, mathematical processing, that is the digital filtration and approximation of the obtained testing results, will be performed. Then, using the inverse method, the actual values of the coefficients of the numerical models for the rheologi-cal properties of the tested materials will be determined.
Key words: steels 20MnB4, 30MnB4, C45, C70, plastometric testing.
1. Introduction. A basis for carrying our the proper simulation and design of technological processes is the knowledge of characteristics describing the rheological properties of steel. For plastic working processes, the basic feature characterizing material to be plastically formed is the yield stress sp. Determining the sp value of the examined steel is very important when designing hot rolling processes [1 - 10]. The correct determination of the steel properties in the form of stress-strain diagrams ensures the subsequent enhancement of calculation accuracy when using empirical formulas, as well as during numerical computations.
Plastometric tests were carried out on the Gleeble 3800 simulator (Fig. 1). An overall view of the specimen in testing and the working chamber of the device is shown in Fig. 2.
Fig. 1. Gleeble 3800physical simulator
Fig. 2. A specimen in test: 1 - specimen, 2 - anvils, 3 - K-type thermocouples, 4 - tantalum or graphite with graphite grease
Coupling a PC with servomotors and sensors in the deformation chamber enabled continuous recording of examined parameters during plastometric testing. The yield stress was determined using software supporting the device. By programming a specific range of temperatures, strains and speeds, the course of the real rolling process can be simulated. Due to the relatively high strain rates used during rolling in a continuous rolling mill, but not attainable in the laboratory, with the use of appropriate relationships it is possible to transpose the conversion of plastometer test results to the conditions found in industrial practice. The yield stress, as dependent on the rolling process parameters, was determined by the hot compression test. The uniaxial compression test (Fig. 3) involved compressing cylindrical specimens between two well lubricated planes.
The advantage of the uniaxial compression test at elevated temperature is the fact that the information on the actual stress against the actual strain can be obtained for a much wider range of strains compared to those examined in the tensile test.
Fig. 3. Schematic of the compression test
2. Material - testing methods. The steel properties were determined from tests carried out at high temperatures corresponding to the real rolling conditions. The tests were conducted in a vacuum chamber at the constant temperature of the specimen being deformed. For plastometric tests, D = 10 mm and l = 12 mm cylindrical specimens were used. To minimize the effect of the friction phenomenon (specimen "barrelling"), tantalum and graphite washers and special graphite-based grease were put between the specimen faces and the tool surfaces. For the recording and monitoring of temperature variations, two K-type (NiCr - NiAl) thermocouples were connected to the lateral specimen surface. The cylindrical specimens were resistance heated using the working anvils.
For cooling, an air blow or non-aggressive liquid medium spray was used. Thanks to employing such cooling, the material after deformation did not have any oxide layer on its surface. To reduce the influence of occurring scale, testing was conducted in a vacuum. The aim of the study was to establish the effect of strain, strain rate and temperature on the magnitude of yield stress in the temperature range of hot plastic working. Chemical composition of the steels is given in Table.
Chemical composition of the stee<
Steel Grade C Mn Si P max. S max. Cr max. Ni max. Mo max. Cu max. Al max.
C45 0,40-0,45 0,50-0,80 0,10-0,30 0,035 0,035 0,20 0,25 0,05 0,30 0,01
C70 0,68-0,73 0,50-0,80 0,10-0,30 0,035 0,035 0,15 0,20 0,05 0,25 0,01
20MnB4 0,18-0,23 0,90-1,20 max. 0,30 0,025 0,025 0,30 - - 0,25 -
30MnB4 0,27-0,32 0,80-1,10 max. 0,30 0,025 0,025 max. 0,30 - - 0,25 -
s /wt%
The tests in the Gleeble simulator were planned so that it would be possible later to determine the yield stress function and its coefficients during hot
rolling:
■ temperature: 700, 800, 900, 1 000, 1 100 and 1 200 °C;
■ strain rate: 0,1; 1,0; 10,0; 50,0 s-1;
strain: from 0 to 1,3 - e = ln
H 0
H1
Round specimens of a diameter of 10 mm were heated at a constant heating rate up to the temperature of the upper plastic working limit, that is 1 250 °C, then austenitized at this temperature and then cooled at a constant cooling rate down to selected temperature values, at which specimen deformation took place.
3. Testing results. The graphs of the relationship of yield stress versus the actual strain for a temperature of 900 and 1 100 °C and a strain rate of 0,1 and 10 s-1, respectively, are shown in Fig. 4 - 11.
From the data in Fig. 4 - 11 it can be found that the magnitude of the yield stress of the examined steels is very strongly influenced by the strain rate and the deformed metal temperature, for the examined range of these variables. In the examined range of strain values (0 < e < 1,3 ) at a temperature of 900 °C, the highest magnitudes of yield stress s were obtained for strains in the range from 0,2 to 0,6 and a strain rate of 0,1 s for steels 20MnB4, 30MnB4 and C45, while the lowest stress magnitude was obtained for steel C70. With the further increase in preset strain, the yield stress magnitude either decreased or remained at a constant level.
250 ■ Strain rate
200 - TO CL ^
[1/sj
■ 0,1000 —1,0000 ♦ 10,000 0
150 ■ CO CD i_ w
50 0 1 0
0 0 3 0 6 0 Strain /- 9 1 2
Fig. 4. Strain-stress curves for the steel 20MnB4 at a temperature
of900 °C
Fig. 5. Strain-stress curves for the steel 20MnB4 at a temperature
of 1100 C
Fig. 6. Strain-stress curves for the steel 30MnB4 at a temperature
of900 C
Fig. 7. Strain-stress curves for the steel 30MnB4 at a temperature
of 1100 C
107
o
Fig. 8. Strain-stress curves for the steel C45 at a temperature of900 C
Fig. 9. Strain-stress curves for the steel C45 at a temperature of 1100 C
o
Fig. 10. Strain-stress curves for the steel C70 at a temperature of900 C
o
Fig. 11. Strain-stress curves for the steel C70 at a temperature of 1100 C
For a strain rate of 10 s-1 at a temperature of 1 100 °C, the greatest values of the sp stress were achieved for strains contained in the range from 0,2 to 0,4. In this case, the greatest yield stress value was obtained for steel 20MnB4. When examining the curves in Fig. 4 - 11, it can also be found that recovery processes occur in them during deforming these steels at a small rate. In these steels for the temperatures indicated in the Fig. 4 - 11, a distinct decrease in yield stress is observed with increasing strain. This suggests that dynamic re-crystallization occurs in the examined steels.
At a strain rate of 10 s-1 and a temperature of 900 °C, the greatest yield stress magnitudes were obtained for the actual strain contained in the range from 0,2 to 0,8 for the examined steels. Also in this case, with the continued increase in strain, the yield stress magnitude either decreased or remained at a constant level. With the increase in temperature, the stress magnitudes decreased, and at a temperature of 1 100 °C the highest stress values were obtained for the strain lying in the range from 0,2 to 0,6.
When examining the curves in Fig. 7 - 8, we observe that during deforming these steels at a strain rate of 10 s-1, dynamic recrystallization also occurs in the steels for the temperatures indicated in the Fig. Only for steel 20MnB4 (Fig. 4) at a temperature of 900 °C is the recovery process no longer so intensive, and the shape of the curve becomes flat after a certain yield stress value has been attained. Whereas, from Fig. 4, a strain hardening of the material can be noticed.
The majority of the curves for the steels investigated in the study have a similar behaviour for the investigated range of temperatures and strain rates. The increase in temperature would cause the magnitude of yield stress to decrease. The increase in strain rate, on the other hand, caused an increase in the yield stress value. It can be noticed that the yield stress magnitude is influenced by the temperature, at which deformation takes place, as well as by the strain rate and
the grade of the examined steel.
4. Summary
- The plastometric tests of micro-addition cold upsetting steels 20MnB4 and 30MnB4 and high-carbon steels C45 and C70 have shown that the magnitude of preset strain, temperature and strain rate have all a great effect on the magnitudes of yield stress.Plastometric tests enable the subsequent assessment of the structure and properties of the material after deformation under specific conditions.
- Based on the plastic deformation parameters recorded during the experiment, the mathematical processing, that is digital filtration and approximation of the obtained test results is possible. Then, using the inverse method, the actual values of the coefficients in the numerical models describing the rheological properties of the tested materials will be determined.
- Entering the rheological properties of steel, obtained from plastometric tests, to computer programs during the numerical examination of plastic working processes ensures a higher accuracy of the computed technological process parameters to be achieved.
Acknowledgement. This scientific study was financed from the resources of the National Research and Development Centre in the years 2013-2016 as Applied Research Project No. PBS2/A5/0/2013.
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Sawicki Sylwester Zdzistaw, Associate Professor, Institute of Plastic Working and Safety Engineering, sylsaw awip.pcz.pl, Poland, Czestochowa, Czestochowa University of Technology, Faculty of Production Engineering and Materials Technology,
Dyja Henryk Stanislaw, Professor, Director of Institute of Plastic Working and Safety Engineering, dviaawip.pcz.pl, Poland, Czestochowa, Czestochowa University of Technology, Faculty of Production Engineering and Materials Technology,
Laber Konrad Blazej, Assistant Professor, Institute of Plastic Working and Safety Engineering, labera wip.pcz.pl, Poland, Czestochowa, Czestochowa University of Technology, Faculty of Production Engineering and Materials Technology
