Achieving an excellent combination of mechanical properties in multiphase steels by controlled development of microstructure.

Klauberova, Danuse ; Jirkova, Hana ; Malina, Jiri 等

1. INTRODUCTION

There is a wide range of materials on the market for which new optimal technologies and approaches to treatments must be found. One effective way of doing this is by material-technological modelling on a thermomechanical simulator. This equipment enables the behaviour of materials to be tested, and the establishment of frequently required ranges of parameters for real processes. It is also possible to very economically modify and optimize existing technologies, evolve new materials and alloying concepts, modify structures and optimize mechanical properties without operational risks.

2. THERMOMECHANICAL TREATMENT SIMULATOR

A thermomechanical simulator carries out precisely controlled thermal and deformation regimes, including rapid incremental deformations (Fig. 2). The present state of modelling enables precise temperature courses and deformation parameters to be set, just as required in the real process of developing new technologies or materials (Masek et al., 2006).

The existing equipment enables rapid changes to parameters and precise simulation of the process conditions. For steels, this means temperature gradients of over 100[degrees]C per second for heating and 250[degrees]C per second for cooling. The precise monitoring of the process is assured.

3. EXPERIMENT

The aim of the experiment was the thermomechanical treatment of low-alloyed steel to achieve high strength cca. 2000MPa whilst maintaining high ductility. One way of doing this is to integrate the Q-P process with thermomechanical treatment. After optimizing the parameters, this approach enables the required microstructures, which are composed of fine martensite and foliated retained austenite, to be created.

Low-alloyed steel CSiMnCr with 0.4% carbon and a significant amount of silica (cca. 2%) was used in the experiments. Higher concentrations of silica contribute to intensive stabilization of retained austenite and suppression of formation of carbides. The total low content of alloying elements ensures the economic benefits of this steel.

The initial structure is formed of a ferrite-pearlite mix (Fig. 1). The yield strength in tension reached 981 MPa with a hardness of 295 HV10.

3.1 Thermomechanical treatment

The applied Q&P process (Edmonds et al., 2006; Speer et al., 2005) relies on rapid quenching of the material to below temperature Ms to prevent the transformation of martensite to austenite. Subsequent heating to just below [M.sub.s] results in the release of martensite and the diffusion of excess carbon from the martensite to the retained austenite. The diffusion of carbon from the saturated martensite to the untransformed austenite increases the stability of the retained austenite during subsequent cooling to room temperature. Undesirable reactions, mainly the precipitation of carbides, are suppressed by suitable alloying. The aim of the Q&P process is to achieve a very fine martensitic structure with foliated retained austenite. To achieve this kind of structure, it is necessary to optimize individual parameters of the thermomechanical treatment. A temperature of [A.sub.c3] = 840[degrees]C was recorded by dilatometric measurement. On the basis of this, a heating temperature of 900[degrees]C with 100s thermal arrest was proposed. This was followed by various cooling strategies. The first strategy was comparative, with cooling to 300[degrees]C and with isothermic arrests at this temperature. The next two strategies involved cooling to 200[degrees]C, with or without 10s thermal arrest. This was followed by heating to 250[degrees]C, with isothermic holding for 600s at this temperature. Further strategies investigated the influence of preserving the previous parameters of the process. The temperature of the arrests was selected as 350[degrees]C with holding for 600 s.

The deformation process (Fig. 2) at temperature intervals 900-820[degrees]C lasted 10s and was composed of 20 incremental deformation steps of tension/compression with a total logarithmic deformation of [phi] = 5.

[FIGURE 1 OMITTED]

3.2 Results of mechanical testing

It was found that all these approaches resulted in a very fine martensitic matrix with retained austenite (Fig. 3). Vickers hardness was measured. Mechanical properties were tested by mini-tension test and notch ductility (Tab. 1). The first strategy, with isothermic holding at 300[degrees]C for 600s, resulted in a martensitic structure with 10% retained austenite. Average hardness values were 602 HV. The Q&P process was applied to the other strategies. The strategy with cooling to 200[degrees]C, with or without 10s arrest, with subsequent heating to 250[degrees]C and 600s isothermic holding, gave values of hardness at tension above 2000 MPa with ductility 10 - 12 %. Both regimes resulted in a fracture surface characteristic of brittle fracture and notch ductility of 14.4 KCV. In contrast, the strategy with cooling to 200[degrees]C, with or without 10s arrests and heating to 350[degrees]C with 600s holding, resulted in reduction of the hardness values and yield strength in tension by approximately 500 MPa. On the other hand, an increase of ductility of 20-23% was measured. This corresponds with the higher values of notch ductility 27KCV. The fracture surface was documented using a laser confocal microscope Fig. 4.

4. CONCLUSION

The results proved the wide-ranging possibilities for influencing the development of structures by varying the cooling strategies and parameters of the Q&P process. Even quite small changes to the character of cooling and length of holding led to significant changes not only to the structure but also to mechanical properties. The best properties were achieved the specimen with parameters 900[degrees]C/100s-200[degrees]C/10s-350[degrees]C /600s, where the high strength 2080MPa was measured whilst maintaining of ductility in relation to the basic state. Measured value of notch ductility was 14,4KCV.

Further optimizing steps will lead to describing further influences of Q&P process on the development of structures.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

5. ACKNOWLEDGEMENTS

This paper includes results obtained within the project 1M06032 Research Centre of Forming Technology.

6. REFERENCES

Edmonds, D. V.; He K.; Rizzo F. C.; De Cooman B.C. & Matlock D.K. & Speer J.G. (2006). Quenching and partitioning martensite--A novel steel heat treatment, Available from: www.sciencedirect.com, Accessed: 2006-02-02

Masek, B.; Stankova, H.; Klauberova, D. & Skalova, L. (2008). The Most Recent Findings from Physical-Material Modelling of UHSS Structure Development on a Thermomechanical Simulator, 3rd International Conference on Thermo-mechanical Processing of Steel, Italy, September 2008

Nemecek, S.; Novy, Z.; Uhlif, J.; Kusy, M. & Janovec, J. (2008). Metallography of high strength steels, 11. konference Prinos metalografie pro resent vyrobnich problemu. 11th conference Methalografy for the solution of production problems, pp 159-162, ISBN 978-80-01-040393, Lazne Libverda, June 2008, CVUT, Praha

Santofimia, M.J.; Zhao, L.; Petrov, R. & Sietsma, J. (2008). Chareacterization of the microstructure obtained by the quenching and partitioning process in a low-carbon steel, Available from: www.sciencedirect.com, Accessed: 2008-08-11

Speer, J.; Matlock, D.K.; Cooman, B.C. & Schroth, J.G. (2003). Carbon paritioning into austenite after martensite transforma-tion, Available from: www.sciencedirect.com, 2003-01-30

Tab. 1. Results of mechanical testing

 Hardness
 [R.sub.p0.2] [R.sub.m]
Strategy HV10 HV30 [MPa] [MPa]

900[degrees]C/100s- 602 602 -- --
300[degrees]C/600s

900[degrees]C/100s- 574 590 2098 2101
200[degrees]C-
250[degrees]C /600s

900[degrees]C/100s- 564 568 2068 2081
200[degrees]C/10s
-250[degrees]C/600s

900[degrees]C/100s- 511 491 1625 1698
200[degrees]C-
350[degrees]C /600s

900[degrees]C/100s- 501 544 1652 1703
200[degrees]C/10s-
350[degrees]C/600s

 [A.sub.5mm] KCV
Strategy [%] [J-[cm.sup.-2]]

900[degrees]C/100s- -- --
300[degrees]C/600s

900[degrees]C/100s- 10,3 14,4
200[degrees]C-
250[degrees]C /600s

900[degrees]C/100s- 12,5 14,4
200[degrees]C/10s
-250[degrees]C/600s

900[degrees]C/100s- 23,3 27,7
200[degrees]C-
350[degrees]C /600s

900[degrees]C/100s- 20,6 26,7
200[degrees]C/10s-
350[degrees]C/600s