the controlled rolling simulation of a...

18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU

THE CONTROLLED ROLLING SIMULATION OF A STRUCTURAL MICROALLOYED VANADIUM STEEL INTO BARS

Tomáš GAJDZICAa, Milan KOTASa, Kamil DROZDb, Jaromír HORSINKAb, Jiří KLIBERb a TŘINECKÉ ŽELEZÁRNY, a. s., Průmyslová 1000, 73970 Třinec, ČR,

[email protected], [email protected]

b FMMI, VŠB-TU Ostrava, 17. listopadu 15, 708 33 Ostrava - Poruba, ČR, [email protected], [email protected], [email protected]

Abstract

Present technical and technological possibilities of advanced continuous light rolling mills allow for thermo mechanical (controlled) rolling of microalloyed bars utilizing continuous cooling of rolled items in water boxes. It offers the possibility to monitor quality parameters of bar steel during the controlled forming process (under various temperatures, cooling speeds, etc.). Plastometric experiments are increasingly used for better understanding of the issue of dependency of forming conditions, i.e. temperature, deformation and strain rate.

The forming itself as well as further treatments of steels microalloyed using vanadium, columbium or titanium requires extensive knowledge of structural composition and behaviour of individual microalloying elements during the forming. Recrystallization and precipitation are very important for appropriate structure and mechanical properties of these steels; it is therefore necessary to maintain the temperature modes during rolling and cooling of the microalloyed steels. Therefore, we deal with modelling and detailed descriptions of the given developments in order to achieve optimal rolling conditions. It is therefore very necessary to manage modern rolling methods so that the required properties of bars are achieved. Research of new types of steels and testing of new treatment technologies are being performed continuously.

Advanced computer programs and plastometric equipments allowing simulation of forming process including controlled cooling may be used for the research. As an example, there is plastometric equipment Gleeble at IMŽ Gliwice, which has been used to obtain data for this article. The knowledge obtain in such theoretical method may contribute to development of bar production knowledge. Based on results obtained from the performed plastometric tests, the achieved microstructures were evaluated and discussed and stress – strain curves were generated.

Keywords: controlled rolling, plastometric simulation, microstructure analysis, bar, stress - strain curve

1. INTRODUCTION

Controlled rolling of machine steel grades in bars currently provides considerable potential for an increase in product quality and associated savings of both production as well as processing costs of metallurgical plants. In current conditions of continuous light section mills the controlled or thermomechanical forming can be realized in only one mode, i.e. heating of material to standard temperatures of conventional rolling (i.e. austenitization), forming highly above Ar3, cooling of material before final forming operation closely above Ar3 or also below it and subsequent final forming. After the last deformation the material can be further cooled both freely in the air or hardened and subsequently cooled to the ambient temperature.

Thanks to an extensive investment in 2003 continuous light rolling mill Trinecke zelezarny, a.s. (sign. KJT) allows for rolling in both conventional as well as standardizing mode and for smaller diameter also in

18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU controlled mode by means of a complex rolling system under predefined temperatures of rolled metal pieces and drafts in the individual passes. KJT diagram is shown in Fig. 1. [1].

Fig. 1. Scheme of continuous rolling light mill - right section

By means of achievement of the required microstructure immediately after bar rolling (i.e. smaller grain than in conventional rolling), which brings about higher values of mechanical properties it is possible to leave out some of the above-mentioned operations (controlled cooling), or reduce their times (refining). It therefore always brings about financial savings resulting from the limited time demandingness of the processing as well as especially the potential for future possible decrease of weight of forged pieces used for the needs of the automotive industry.

In direct link to the still increasing complexity of the customer requirements for supplies in rolled steel bars microalloyed with vanadium (especially in the drop forging industry) the optimization of technology of controlled forming for operating conditions of KJT makes use of simulation of this process in laboratory conditions of research institutes, which is followed by verification of results achieved in practice [2].

2. DESCRIPTION OF EXPERIMENT

The basic task of the given experiment was to make a laboratory simulation of temperature – deformation conditions of controlled rolling of machine steel microalloyed with vanadium in bars. We opted of a simulation on the plastometer GLEEBLE in IMZ Gliwice, which was supposed to simulate the temperature - deformation conditions dependant on time in the process of rod rolling in KJT.

2.1 GLEEBLE Plastometer

GLEEBLE 3800 is an integrated, fully digital thermo-mechanical plastometric device that works under applications of Windows. It is able to provide for heating of the testing sample in the required speed and maintain precisely selected volume-balanced temperature. Highly heat conducting jaws clamping the sample allow for using of sharp cooling speed. Precise temperature regulation of the measured sample is ensured by the thermocouple or an infrared pyrometer. The plastometer is equipped with a high-speed heating system. The system records and provides all the measured values necessary for correct progress of the therm-mechanical tests and concurrently controls the progress of the test with its servo-drive system [3].

18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU 2.2 Parameters of the realized experiment

Within the optimization of the technology of controlled forming for operational conditions of KJT, in the course of assignment of individual parameters for the above mentioned laboratory simulation of the rolling process, we measured especially the variations of the second deformation e2 (0.45), which simulates the process of forming in the finishing block of KJT and speed of cooling to the temperature of T2 (400 °C), i.e. temperature of rolled pieces on the cooling bed.

Three schemes were proposed for plastometric simulation in temperature of second deformations of 930, 900 and 870 °C in the range of cooling speed to T2 0.1 - 0.6 °C/s. The linear pressure deformation on the GLEEBLE plastometer (Plain Strain Compression Test – PSCT) was applied to 18 samples, i.e. for the temperature of second deformation of 900 °C 6 samples were compressed with the speeds of cooling to T2 0.1 - 0.6 °C/s. Other simulation parameters like temperature, speed, heat-up time or dimension of the performed deformations was identical for all the schemes.

The input material for taking samples for plastometric experiments was cut off from the rolled piece of microalloyed steels with vanadium content of 0.1%, which originated by through-rolling of billet with a square of 150 mm after four passes to round bar diameter of 100 mm. Surface layer of 10 mm was machined off from it. The originated round bar was drilled into with a drill bit of 60 mm in diameter, whose result was a tube with inner diameter of 60 mm and with external diameter of 80 mm, from which samples were made by cutting. The samples were taken at a distance of approximately 700 – 2000 mm from the face of the rolled piece.

2.3 Evaluation of the structure of samples after press test with linear deformation

Based on the performed plastometric deformation according to the above-mentioned conditions of forming there were metallographic examination specimens were made from the re-formed samples and metallographic analysis was made in the testing house of Trinecke zelezarny, a.s. The samples were etched in 2% nital solution. Phase share analysis was also performed in TŽ, a.s. on the samples and grain size was specified. The evaluation was performed in the place of sample after plastometric test identified in Fig. 2.

Areas of sample after forming:

1 – Non-deformed area,

2 – Surface of deformed area,

3 – Centre of the deformed area.

Comparison of microstructures in point 1

The resulting differences in microstructures in point 1 (non-deformed area) for various speeds of aftercooling in samples with second deformation at temperatures of 900 and 930 °C are minimum (almost none), the specified example is the sample 1P (second deformation at 930 °C and speed of cooling to T2 0.1 °C/s), see Fig. 3. and Fig. 4., where the grain size is 8-9 according to ASTM standard, the share of ferrite is 30 %, perlite 70 % and martensite 0 %. In samples with second deformation at 870 °C the grain size changed to the value of 9-10 and share of ferrite to 25 % and perlite to 75 %, see sample 13P (second deformation at 870 °C and cooling speed to T2 0.1 °C/s) Fig. 5. and Fig. 6.

Fig. 2. Evaluation points of the sample

18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU

Comparison of microstructures in point 2

In point 2 (slightly below the surface) similarly to the place of non-deformed area of samples we observed slight increase of perlitic structure, i.e. from 75% of perlite and 25% ferrite (for samples with second deformation at 900 and 930°C) to 80% perlite and 20% ferrite (for samples with second deformation at 870°C). The exception are however the samples 2P and 6P (second deformation at 930°C and speed of cooling to T2 0.1 and 0.6 °C/s) and 11P (second deformation at 900 °C and speed of cooling to T2 0.5 °C/s), where the shares of stages on one side of the surface changed by drop of the content of ferrite from 25 % to 5 % and increase of perlite up to 95 %, see Fig. 7. and Fig. 8.

Fig. 3. The sample 1P in point 1 (Zoom 100x) Fig. 4. The sample 1P in point 1 (Zoom 500x)


Fig. 8. The sample 2P in point 2 (Zoom 500x)

Fig. 7. The sample 2P in point 2 (Zoom 100x)

18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU Comparison of microstructures in point 3

In the place of 3 samples after plastometric test, which was in the centre of the deformed part of the material the used speeds did not have any influence on the change of share of stages, there were 30% of ferrite and 70% of perlite in all the samples. The specified example shows sample 1P (second deformation at 930 °C and speed of cooling to T2 0,1 °C/s) in Fig. 9. and Fig. 10.

2.4 Stress - strain curve

The obtained data for the individual schemes gradually implied the stress-strain curves. Fig. 11. shows intermittent tests performed according to Scheme 1, i.e. temperature at first deformation of 950 °C and at second deformation of 930 °C, when the first and the second deformation were constant and thus it is possible to theoretically estimate the same progress of the curves. Whereas in the first deformations this assumption was confirmed, in second deformations, mostly at temperature of 930 °C, quite significant variations of the progress of second curves were observed.

Fig. 11. Stress - strain curve


18. ‐ 20. 5. 2010, Roznov pod Radhostem, Czech Republic, EU 3. CONCLUSION

The realized experiment provides information about simulation of controlled rolling process with focus on the area of bar rolling from vanadium micro-alloyed steels for specific conditions of light section mill, i.e. description of resulting microstructures. Such theoretically obtained knowledge serves especially for optimization of technology of production of bars from these steels. The achieved microstructures and stress-strain curves were evaluated on the basis of results obtained from plastometric tests. Metallographic analysis was performed on all the supplied samples. The analysis of these samples revealed that the required (ferritic-perlitic) structure was achieved, but different speeds of aftercooling (that differed almost by one order) were supposed to ensure gradual change of phases in the way that at higher speed of aftercooling the share of perlitic stage was supposed to rise, which however did not happen, contrary to expectations. In exceptional cases the share of perlite in the thin surface layer increased up to 95 %, we explain this however by uneven deformation of the sample caused probably by different value of friction between the tested material and testing device jaws. Modular stress-strain show significant differences in the values of stress especially in samples with second deformation at a temperature of 930 °C and aftercooling speeds of 0.2 and 0.4 °C/s.

REFERENCES

[1] ČMIEL K. M., SCHINDLER I. Simulace řízeného válcování vybraných konstrukčních ocelí za různých teplotních podmínek. In METAL 2005: 14.mez. metal. konference: 24. - 26.05. 2005. Hradec nad Moravicí, Červený zámek, Česká republika [CD-ROM]. Ostrava: TANGER: Květen, 2005, No./Č. 198. ISBN 80-86840-13-1.

[2] KLIBER, J. Řízené tváření, HL č. 4, 7/2000. ročník LV. s. 86-91. ISSN 0018-8069.

[3] KOTAS, M. Plastometrická simulace termomechanického válcování oceli mikrolegované vanadem. In METAL 2007: 16.mez. metal. konference: 22. - 24.05. 2007. Hradec nad Moravicí, Červený zámek, Česká republika [CD-ROM]. Ostrava: TANGER: Květen, 2007, No./Č. 164. ISBN 80-86840-33-8.

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