Influence of austenitization temperature and number of incremental steps on structure development of TRIP-steel.

Jirkova, Hana ; Rezek, Martin ; Meyer, Lothar Werner 等

1. INTRODUCTION

Mechanical properties of the new types of steels are influenced by the final microstructure to a large extent. For the formation of a suitable structure, it is therefore necessary to apply an appropriate kind of heat or thermomechanical treatment. TRIP steels represent one of the interesting groups of low-alloyed high strength steels.

TRIP steels are multiphase steels whose structure consists of ferrite, bainite and small amount of retained austenite (Bleck, 2002). Because of their high capacity of energy absorption and good fatigue limit they have been used in the automotive industry for building-safety components recently. These include, for example, seat structures, cross-members, long post reinforcements, aprons and fender reinforcements (www.arcelorauto.com, 2002; Fischer et al., 1996). They feature a good combination of strength and ductility ensured by the TRIP effect caused by the deformation induced martensitic transformation (Lee et al., 2004; Zou et al., 2002). All the TRIP steel components are produced via metal sheet cold drawing at present. Other newly examined possibilities combining multiple technologies should be taken into consideration. These include thermomechanical treatment or just heat treatment in connection with incremental bulk cold forming. This research aims to explore the behavior of materials under specific conditions of selected unconventional TMT applications and to determine suitable technological parameters for their development.

2. EXPERIMENT

Low alloyed SiMn TRIP steel (Tab. 1) was investigated in this experiment. Alloying elements in this low cost steel significantly influence the transformation processes and the stabilization of the retained austenite.

Si promotes proeutectoid ferrite formation during cooling and--as an element which is not soluble in cementite--prevents or decelerates the carbide precipitation during bainite growth. At the same time Si supports carbon diffusion into the retained austenite. Mn enhances carbon solubility in austenite. Pearlite formation is retarded due to the influence of Mn, therefore the cooling time can be prolonged during thermomechanical processing. Both Mn and Si improve the strength of material through solid solution strengthening (Bleck, 2002).

A thermomechanical simulator with resistance heating of samples was used for the modelling and development of the process. The temperature during model treatment was measured by thermocouples welded on the surface of samples. Both light and scanning electron microscopy analysis were performed on experimental material after controlled thermomechanical processing.

The experiment was divided into two phases. In the first phase the influence of the number of deformation steps on the structure development was examined. The second one was devoted to the optimisation of the heating conditions as the austenitization temperature and time.

2.1 Influence of Various Deformation Intensity on Structure Development

Deformation in the intercritical region (730-800[degrees]C) supports ferrite formation, ferrite grain refinement and reduction of the bainite fraction. The influence of the number of deformation steps on the final microstructure was therefore investigated in the first step of the experiment. The austenitization temperature was 1050[degrees]C with 5s holding time. In the temperature interval between 950-720[degrees]C deformations consisting of four, eight and twelve steps took place (Tab. 2). Each deformation step represents a combination of tension and compression. The temperature of bainite transformation was equal to 425[degrees]C for all the strategies designed.

The four step deformation represented the true strain of [phi] [][]= 0.3. The structure composed of ferrite and course bainite blocks with the size of up to 50 [micro]m. The bainite fraction was determined to be 60%. The eight step deformation led to the formation of a fine structure with the ferrite grain size from 8 to 11 [micro]m. The higher number of deformation steps caused the refinement of the bainite formations, which reached the size of about 20-30 [micro]m with the volume fraction of 57%. Increasing the number of deformations steps to 12 representing the true strain of 3.5 led to the reduction of the bainite fraction and the refinement of its formations.

The experiment showed that the increase of the number of deformation steps in the temperature interval between 950-720[degrees]C retards bainite growth and refines its formations significantly.

2.2 Influence of Austenitization Temperature

The influence of the heating conditions and austenitization was tested in a series of examinations with various temperatures and holding times (Tab. 3).

Three austenitization holding times: 5s, 20s, 100s were tested for the initial austenitization temperature of 1050[degrees]C. The deformation was carried out in the temperature interval of 950-720[degrees]C. The bainitic transformation took place at the temeperature of 425[degrees]C with the holding time of 600 s. The finest bainite morphology with the average block size of 20 [micro]m was achieved with the shorter austenitization holding times. Prolonging the holding time caused microstructure coarsening and the growth of the bainite fraction, which implies that short austenitization time results in an incomplete austenitization. With longer austenitization times the proportion of austenite increased and subsequent bainite transformation then yielded greater bainite fractions. Decreasing the austenitization temperature by 75[degrees]C did not induce any considerable changes to the microstructure.

Further lowering of the austenitization temperature to 900[degrees]C required shifting of the deformation temperature to the range between 900-720[degrees]C. With respect to the expected slowdown of the transformation processes, the austenitization holding times were prolonged to 20, 100 and 450s. Coarser bainite blocks have only formed with the holding time of 450 s.

The most suitable microstructure with bainite block sizes below 10 [micro]m and ferrite grain sizes below 5 [micro]m was formed in the range of holding times between 20 and 100 s. Above the austenitization temperature of 900[degrees]C, the microstructure contained more globular bainite rather than the lath bainite typical for lower austenitization temperatures and the bainite microstructure is much finer and more uniformly distributed throughout the ferrite matrix. Prolonging the holding times at lower austenitization temperatures leads to the formation of lath bainite as well.

The effects of austenitization times between 20 and 450s were investigated for the lowest austenitization temperature of 850[degrees]C with the deformation being performed between 850 and 720[degrees]C.

In this case no effects of the holding time were observed. All microstructures showed similar characteristics with fine grains and uniform distribution of phases (Fig. 1). The microstructure of the sample whose holding time was 20s contained fine ferrite grains with grain size of about 5 [micro]m and globular bainite.

[FIGURE 1 OMITTED]

Due to lowering the austenitization temperature below the [A.sub.c3] temperature only partial austenitization occurred and the fraction of bainite dropped. Even with the holding time of 450s no significant coarsening of bainite blocks was observed.

Mechanical properties were investigated via the tensile test. In all testing regimes the tensile strength over 1000 MPa was reached.

3. CONCLUSION

The model thermomechanical treatment was tested using the experimental CMnSi TRIP steel. It was determined that deformation in the intercritical region causes an intensive ferrite grain refinement and the reduction of the bainite fraction. The second phase of the experiment implies that the structure development does not require high austenitization temperatures. This fact is very important from both the economical as well as the technological precision point of view. If the technological material plasticity suffices, the [A.sub.c3] temperature does not need to be exceeded during heating. At the same time the lower temperature affects the structure refinement and homogeneity. Shifting the technological temperature interval to lower values leads to changes in the bainite morphology from lath to granular without influencing the mechanical properties.

4. ACKNOWLEDGEMENTS

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

5. REFERENCES

Bleck, W. (2002). Using the TRIP efekt--the down of a promising group of cold formable steels, Proccedings of International Conference on TRIP--Aided High Strength Ferrous Alloys, De Cooman B. C. (Ed), pp 13-23, ISBN 90-76019-17-7, Belgium, 2002, Wissenschaftsverlag Mainz GmbH, Aachen www.arcelorauto.com/ v_ang/produits/fiches/trip3.html, 2002

Fischer, F. D.; Sun, Q.-P. & Tahala, K. (1996). Transformation-induced plasticity, Appl. Mech. Rev., Vol. 49, No. 6, June 1996, pp 317-322, ISSN 0003-6900

Lee, Ch. G.; Kim, S. J.; Lee, T. H. & Lee, S. (2004). Effects of volume fraction and stability of retained austenite on formability in a 0,1C-1,5Si-1,5Mn-0,5Cu TRIP-aided cold-rolled steel sheet. Materials Science and Engineering A, Vol. 371, No. 1-2, April 2004, pp 16-23, ISSN 0921-5093

Zou, H. H. et al. (2002). Effect of retained austenite stability of Si-Mn TRIP steel on the product of strength and duktility. Proccedings of International Conference on TRIP--Aided High Strength Ferrous Alloys, De Cooman B. C. (Ed), pp 317-321, ISBN 90-76019-17-7, Belgium, 2002, Wissenschaftsverlag Mainz GmbH, Aachen

Tab. 1. Chemical compositon of C-Mn-Si TRIP steel [%]

 C Mn Si P S Cr Ni Cu Nb

0.19 1.45 1.9 0.02 0.07 0.07 0.03 0.04 0.003

Tab. 2. Series of model treatment with different numbers
of incremental deformation steps

[T.sub.A] [t.sub.A] Bainite
[[degrees]C] [sec] Def. [phi] [-] fraction [%]

1050 5 8x 1.4 57
1050 5 4x 0.3 60
1050 5 12x 3.5 48

Tab. 3. Series of model treatment with various temperature
of austenitization

[T.sub.A] [t.sub.A] Deformation [R.sub.m] Bainite
[[degrees]C] [s] [[degrees]C] RA [%] [MPa] [%]

1050 5 950--720 14 1000 57
1050 20 950--720 7,8 1037 68
1050 100 950--720 8,2 1027 68
975 100 950--720 12,4 1014 67
900 20 900--700 -- 1048 --
900 100 900--700 13,1 1027 70
900 450 900--700 -- 1018 --
850 20 850--700 -- 1044 --
850 100 850--700 10,6 1001 67
850 450 850--700 12,6 1060 62