APLICATION OF THE UNCONVENTIONAL THERMOMECHANICAL TREATMENTON LOW-ALLOYED HIGH-STRENGTH STEEL

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1 APLICATION OF THE UNCONVENTIONAL THERMOMECHANICAL TREATMENTON LOW-ALLOYED HIGH-STRENGTH STEEL Danuše LANGMAJEROVÁ a, Hana JIRKOVÁ a, Šárka MIKMEKOVÁ b, Bohuslav MAŠEK a a University of West Bohemia in Pilsen, Pilsen, Czech Republic, EU d.klauberova@seznam.cz, h.jirkova@ .cz, masekb@kmm.zcu.cz b Institute of the Scientific Instruments of the ASCR, v.v.i., Brno, Czech Republic, EU, sarka@isibrno.cz Abstract The demands are increasing on material properties, lowering the mass of components, and lowering production costs are all strong drivers for the development of low-alloyed steels with high strength, which are low cost owing to their low content of alloying elements. Modern advanced low-alloyed steels processed in unconventional ways of heat or thermomechanical treatment can reach substantially better mechanical properties when compared to conventional treatments. Experiments were carried out to test several strategies of thermomechanical treatment of a newly designed low-alloyed high-strength steel 42SiCr with about 0.4% C and 2% Si. The aim of the experiment was to develop and optimize a new type of heat treatment based on the Q&P process (Quenching & Partitioning) which should be capable of achieving strengths of up to 2000 MPa with ductility of 10%. During the optimization process, the main focus was on the influence of various intensities of incremental deformations and the speed of cooling on the development of the structure. By modelling the treatment, a microstructure was obtained formed of a martensite matrix, bainite and finely diffused retained austenite. The influence of the technological process parameters on the structure development was analyzed using several microscopic methods and the resulting mechanical properties were measured by means of a tensile test. The volume fraction of the retained austenite was established by X-ray diffraction phase analysis. Keywords: unconventional thermomechanical treatment, Q&P process, low-alloyed steel, retained austenite, mechanical properties 1. INTRODUCTION The trend today, especially in the automotive industry, is the development of new types of materials e.g. high strength low alloyed steels. They are relatively cheap steels because of the low content of alloying elements. It is necessary to find an optimal processing which would guarantee good mechanical properties of these steels, to make it possible to introduce them into regular production. When processing in the conventional way, high hardness but at the same time a decrease of ductility are achieved. For this reason new special procedures of heat or thermo-mechanical treatment are developed which enable a better combination of mechanical properties of high strength low alloyed steels to be achieved. To achieve these results, an unconventional heat treatment, the Q&P process, was combined with integrated incremental deformation. Using this unconventional treatment it is possible to gain a microstructure that is created by martensitic matrix with finely distributed retained austenite between martensitic needles. This new method of processing uses the positive influence of retained austenite on mechanical properties and enables an increase of the product hardness and the ductility of the steel. As a result of the correct alloying concept in this kind of processing the elimination of carbide is prevented and carbon is used for chemical stabilisation of retained austenite. [1]

2 2. THERMOMECHANICAL TREATMENT To facilitate and quicken the optimisation of thermo-mechanical treatment, a high-speed thermo-mechanical simulator was used. The simulator was developed in The Research Centre of Forming Technology FORTECH and enables very quick changes of parameters and exact imitation of the conditions of a real process. The top speed of a deformation element is up to 3 m.s -1. By using high-frequency heating combining the inductive and resistant methods of heating it is possible to achieve temperature gradients of over 250 C/s during heating and 350 C/s during cooling. Obviosity is also an exact monitoring of the process. This method, which was used also for the further mentioned research, enables optimization of corresponding parameters of the real process on a small quantity of material Q&P process The principle of the Q&P process (Fig. 1) is a quick quenching of material from temperature above A c3 between temperatures M s and M f to prevent both the martensitic transformation in the whole content of the material and follow-up tempering at the temperature M s, as well as cooling to room temperature. During the tempering the carbon is redistributed from supersaturated martensitic phase to untransformed austenite, which causes considerable stabilisation of retained austenite and its maintaining in steel under low temperature. Steels after the Q&P processing are composed of a mixture of martensite and retained austenite. [2] During the optimization of Q&P processing it is necessary to find out the influence of individual parameters to stabilise a sufficient amount of retained austenite and thus to ensure excellent mechanical properties. To refine the structure, a deformation during the cooling phase was used. The incremental deformation was integrated into the process in order to refine the original austenite grains as well as to refine martensitic structures. With the collective effect of plastic deformation, hardening and phase transitions in progress it is possible to achieve extremely high hardness [4]. 3. EXPERIMENT Fig. 1 Diagram of Q&P process A model procedure of thermo-mechanical treatment with Q&P process was used on low-alloyed experimental steel 42SiCr with content of carbon 0.4%. Carbon and silicon 2.04%, that prevents production of carbide during the degradation of martensite, were used as main alloying elements. Next alloying elements are manganese 0.58% and chromium, which is used to harden the solid solution. The overall low content of alloying elements guarantees the economic advantage of this steel. The initial structure of the material was created by a ferritepearlitic structure. The value of tensile strength was R m = 981 MPa with ductility about 32% and value of hardness was 295 HV10. A goal of the Q&P process is to achieve very fine martensitic structure with foil type of retained austenite. To achieve such a type of structure, it is necessary to optimise individual parameters of the thermomechanical treatment. From dilatometric measurement there were ascertained important temperatures for a plan of the process A r3 = 844 o C, M s = 289 C and M f = 156 C. Based on this heating the temperature of 900 C was chosen with 100s holding time. Various optimisations of processing with help of technological modelling followed afterwards.

3 3.1. Increasing of intensity of incremental deformation As first, various intensities of inserted incremental deformations were tested: 20 deformation steps (total logarithmic deformation φ=5), 40 deformation steps (φ=10.4) (Fig.3) or 60 deformation steps (φ=13. 4) taking 10s and running in a temperature interval of C (Fig. 2). Afterwards the sample was processed by heat regime simulating the procedures of the Q&P process. After the deformation the material was quenched at a rate of 19 C/s to a temperature of 200 C, which is a temperature far below the temperature Ms. After this the material was tempered for 600s at a temperature of 250 C. This temperature is just below the Ms temperature. When testing various intensities of deformations it was necessary to find out and describe the influence of deformation on mechanical properties, especially its intensity, size and interval of temperature, at which the deformation was performed. The correct choice of deformation parameters causes refinement of original austenite grains and also of the final martensitic structure, which positively influence the mechanical properties of the material. It will also be possible to stabilise an even higher amount of austenite in the structure, which will be more rapidly chemically stabilised in finer shapes. The fraction of retained austenite was determined by x-ray diffraction analysis and mechanical properties were measured by mini-tensile test. Hardness was determined according to Vickers. Fig. 2 Designed thermomechanical regimes optimisation of deformation intensity Fig. 3 Incremental deformation process with controlled cooling, 40 deformation steps φ=10.4 During the optimisation it was discovered that when increasing the number of incremental deformation steps, the values of tensile strength do not noticeably differ. An approximate tensile strength was achieved ranging between 1940 and 2050 MPa. Intensity of deformation in the tested range does not have a noticeable influence even on the values of ductility, which are in the range of 16.5% with 20 deformation steps to 15% with 60 deformation steps (Tab. 1). In terms of the fraction of retained austenite in the structure it was observed that with lower intensity of 20 deformation steps higher quotients of about 16.5% were achieved, while with higher intensity of 40 deformation steps only 11% were stabilised and with 60 deformation steps about 13%. The value of hardness was 590 HV10 with 20 deformation steps. By increasing deformation intensity the structure was refined and hardened and so values of hardness rose with all strategies by about 40 HV10, more precisely to the value of about 630 HV10. Tab. 1 Influence of increasing the number of incremental deformations on mechanical properties Heat treatment 900 C/100s 200 C/10s/ 250 C/600s Number of HV10 R p0,2 R m A 5mm R m xa 5mm RA deformation steps [-] [MPa] [MPa] [%] [-] [%] 20x x x Because of its ultra-fine structure the microstructure was examined using a Magellan 400 scanning electron microscope that enables resolution up to 0.8 nm. With a more detailed view it is possible to compare the degree of refinement of the structure. With 20 deformation steps (Fig. 4a) needles of martensite are coarser

4 and bigger than with 60 deformation steps (Fig. 4b). Using the scanning electron microscope an occurrence of lower bainite was also visible (Fig 5a). Under the maximum magnification that the microscope allows, white spots on the boundaries of martensitic needles were found which indicated the occurrence of foil type retained austenite (Fig. 5b). For this reason the structure was also observed using a transmission electron microscope. On carbon replicas it was discovered that the retained austenite occurs both in the form of a thin film along the martensitic needles in the 20 deformation steps mode (Fig. 6a) and also, for example, in the regime with 40 deformation steps in globular form. (Fig. 6b). Fig. 4 Mode of treatment 900 C/100s 200 C/10s C/600s scanning electron microscope a) 20 deformation steps, b) 60 deformation steps B Fig. 5 Mode of treatment 900 C/100s 200 C/10s C/600s, 40 deformation steps Scanning electron microscope: a) martensite microstructure with lower bainite, b) detail of martensite M RA RA Fig. 6 Mode of treatment 900 C/100s 200 C/10s C/600s - transmission electron microscope a) 20 deformation steps, b) 40 deformation steps

5 3.2. Optimization of cooling speed In the next phase of optimization the influence of the cooling rate from 900 C to 250 C on the development of the structure, especially on production of ferrite, pearlite or bainite was investigated. During the optimization a deformation with 20 incremental steps was used and cooling rates 19 C/s, 7 C/s and 5 C/s were tested (Fig. 7). Fig. 7 Optimisation of cooling rate from 900 o C to 250 o C The result of the highest rate of cooling 19 C/s from the temperature of 900 C to 200 C was a fine martensitic structure with lower bainite and 16.5% fraction of retained austenite (Fig. 5). Lowering the cooling rate caused a decrease of the fraction of retained austenite to 9% and in the structure there could be observed a bigger amount of ferrite with a low occurrence of pearlite (Fig. 8). Further lowering of the cooling rate on 5 C/s caused higher production of pearlite and ferrite in the structure (Fig. 9). A change of the structure led to the decrease of the amount of retained austenite to 8% and to a decrease of values of mechanical properties. Up to 400 MPa decrease of hardness, 9% lower ductility and 8.5% smaller amount of retained austenite were determined during the comparison of cooling rates of 19 C/s and 5 C/s (Fig. 2). The value of hardness also fell from 590 HV10 to 515 HV10. Tab. 2 Influence of cooling speed from 900 C to temperature 200 C Heat treatment 900 C/100s 200 C/10s/ 250 C/600s 20 x deformation steps Cooling speed [ o C/s] HV10 [-] R p0,2 [MPa] R m [MPa] A 5mm [%] R m xa 5mm [-] RA [%] Fig. 7 Mode of treatment 900 C/100s 200 C/10s C/600s, cooling speed 7 C/s, a) laser confocal microscopy, b) transmission electron microscope F

6 F P a) b) Fig. 8 Mode of treatment 900 C/100s 200 C/10s C/600s, Cooling speed 5 C/s, a) laser confocal microscopy, b) transmission electron microscope 4. CONCLUSION During the experimental program a thermo-mechanical treatment with incremental deformation and integrated Q&P process was tested on the low-alloyed steel 42SiCr. The results showed possibilities for effectively influencing the structure and thus the mechanical properties of the material with the help of individual parameters of processing. Various intensities of incremental deformations and cooling rates from a temperature of 900 C to 200 C were experimentally tested. During the optimization of intensities of deformations it was discovered that even increasing the number of incremental deformations does not noticeably influence the structure and mechanical properties. The best mechanical properties were achieved with the mode with 20 deformation steps, primarily ultimate strength of 2047 MPa with relatively high ductility A 5mm running to 16%. In the structure 16. 5% of retained austenite was stabilised. Afterwards the influence of cooling rate on development of microstructure and mechanical properties was discovered. Decreasing cooling rate leads to production of a distinct amount of ferrite and pearlite. This change of structure causes lowering of hardness and strength. With the lowest cooling rate of 5 C/s, tensile strength of about 1662 MPa with ductility of only 7% and the fraction of retained austenite of 8% was achieved. ACKNOWLEDGEMENTS This paper includes results created within the project P107/12/P960 Influence of a Structure Modification on Mechanical Properties of AHS Steel. The project is subsidised from specific resources of the state budget for research and development. LITERATURE [1] SPEER, J. G.. et al. The Quenching and partitioning process: Background and Recent Progress. Materials Research, 2005, Vol. 8, č. 4, s [2] EDMONDS, D.V. et al. Quenching and partitioning martensite A novel steel heat treatment. Materials Science and Engineering, 2006, A , s [3] MATLOCK, D.K. et al. Application of the quenching and partitioning (Q&P) process to a medium-carbon, high-si microalloyed bar steel. Proceedings of Thermec Uetikon-Zurich, Switzerland: 2003, s [4] MAŠEK, B. et al. The Effect of Mn and Si on the Properties of Advanced High Strength Steels Processed by Quenching and Partitioning, Materials Science Forum (2010)94-97.