Investigation of phase transformations in high-strength low-alloyed steel.

Hauserova, Daniela ; Jirkova, Hana ; Masek, Bohuslav 等

1. INTRODUCTION

The current demand for high mechanical properties in steel at low production costs spurs rapid development of new types of steels and their further optimization. Properties of these new types of steels strongly depend on their multiphase structure. This is why individual parameters of their processing should be carefully observed. In particular, new steels are often characterized with narrow temperature intervals for preparation of adequate microstructures. For this reason, it is very important to know temperatures of individual phase transformations and effects of various cooling rates on formation of microstructures. A design of a processing procedure should be based on correct and experimentally verified transformation diagrams.

2. TRANSFORMATION DIAGRAMS OF STEELS

Transformation diagrams for austenite decomposition provide description of temperature-time relationship in transformation of undercooled austenite. They comprise two groups: diagrams for isothermal and continuous cooling tranformations. Time-temperature-transformation diagrams (TTT) show times for austenite decomposition under isothermal conditions. Continuous-cooling transformation diagrams (CCT) contain these times for different cooling rates. Special deformation CCT diagrams take into account effects of deformation before cooling.

CCT and DCCT diagrams in this paper were constructed on the basis of dilatometric monitoring of dimensional changes related to changes in crystal lattice. Data for construction of these diagrams was derived from the shape of expansion--time curves. Phase transformations in the sample were indicated by breakpoints, at which temperature and, to some extent, the nature of transformation can be identified.

3. EXPERIMENTAL

The main purpose of the experiment was to construct CCT and DCCT diagrams for 42SiCr low-alloyed steel. Initial blanks were hot formed and air-cooled. The as-received microstructure consisted of ferrite and pearlite. Chemical composition was measured by spectral analysis (Table 1).

3.1 Dilatometric Experiment

Phase transformation temperatures were determined by experimental methods in inert atmosphere in Bahr dilatometer. Temperature was measured by thermocouples welded onto the testing body with the diameter of 5 mm and length of 10 mm.

Several cooling rates were used in order to cover as wide as possible cooling range and to represent all available heat and thermomechanical treatment processes.

The temperature cycle comprised heating to 950[degrees]C in 60 s, 30-second hold, cooling down to 910[degrees]C in 15 seconds and subsequent cooling to room temperature at various rates. The dilatometric cycle providing data for construction of a transformation diagram involved introduction of strain of tp = 0.7 at the rate of 10s-1 before cooling to ambient temperature.

Nine cooling times for the 910--20[degrees]C interval were selected for the experiment: 10; 30; 100; 150; 300; 500; 1,000; 3,000 and 10,000 seconds.

Dilatometric cooling curves were evaluated in WinTA 6.2 software. In order to facilitate correct interpretation of transformations represented by dilatometric curves, metallographic examination of individual structures and Vickers hardness measuring were performed.

3.2 Construction of CCT and DCCT Transformation Diagrams

CCT (Fig.1.) and DCCT (Fig.2.) diagrams were constructed with results of dilatometric, metallographic and hardness tests.

Comparison between diagrams showed that deformation caused a shift of the ferrite nose towards higher cooling rates. Formation of ferrite was observed in the DCCT diagram at as low a rate as 9[degrees]C/s. In the CCT diagram, ferrite particles first occurred at the cooling rate of 6[degrees]C/s (Santofimia, 2008).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

3.3 Phase Transformation Temperatures Derived from Phenomenological Models

In some cases it is not possible to perform time-consuming dilatometric experiments to determine temperatures of phase transformations. It is, however, possible to quickly obtain their rough values using so-called phenomenological models described in literature. Typically, these models are accurate enough only in a restricted interval of validity. The limits of the interval are given by the steel grade and type of processing for which they were built. The limitation stems from a number of parameters: e.g. chemical composition, cooling rate, thickness of material, etc. For instance, the Andrews phenomenological model [M.sub.s] = 539-423C-30.4Mn-17.7Ni-12.1Cr-11Si-7Mo is only valid for steels with carbon content lower than 0.6% (Andrews, 1965).

3.4 Calculcation of Phase Transformation Temperatures Using JMatPro Software

Another way to rapid obtaining of information on transformation behaviour is calculation in JMatPro. This is a multi-platform software for calculation of properties and behaviour of multi-component alloys. Its input includes the chemical composition of material. Variable parameters include austenitizing temperature, grain size and lowest volume fraction percentage of the phase detected.

The diagram was calculated for austenitizing temperature of 950[degrees]C, grain size of 10 [micro]m and lowest volume fraction of the phase of 0.1%. (Fig.3.) (Tab.2.).

[FIGURE 3 OMITTED]

3.5 Results and Comparison of Phase Transformation Temperatures

Comparison of phase transformation temperatures from various methods showed that application of suitable phenomenological models provides very good agreement with results of physical measuring. Temperatures obtained with JMatPro confirm these results (Tab. 2).

The [A.sub.r3] temperature in the Choquet model was compared with that determined in dilatometric experiments at the lowest cooling rate of 0.1[degrees]C/s. The difference was 33[degrees]C which may be due to the absence of chrome in the model used. JMatPro software provided an [A.sub.r3] temperature value which was different from the dilatometry result by about 20[degrees]C.

Nipon Steel 2 model applies to a cooling rate of 20[degrees]C/s. Results of this model showed a difference from measuring results of 72[degrees]C.

Calculated Ms temperature values and JMatPro results were compared with dilatometric data for the highest cooling rate of 90[degrees]C/s. Andrews' model takes into account the impact of all alloying elements in the steel. The difference was no more than 6[degrees]C. Setting identical austenitizing temperature and grain size in JMatPro program made it possible to arrive at the same temperature difference from results of dilatometric experiments and phenomenological models.

4. CONCLUSION

For effective design of heat treatment for 42SiCr steel, dilatometric and metallographic analyses were conducted and CCT and DCCT diagrams built. Results were compared with software calculation of phase transformation temperatures and results of parametric equations for selected phenomenological models. The results confirmed the assumption that accuracy of the models strongly depends on their validity intervals and on the response to effects of alloying elements.

The best match with the difference of no more than 6[degrees]C with all methods was found with the Ms temperature.

5. ACKNOWLEDGEMENTS

This paper includes results achieved within the project GACR 106/09/1968 Development of New Grades of High-Strength Low-Alloyed Steels with Improved Elongation Values.

6. REFERENCES

Andrews, K. W. (1965). Empirical fomulae for the calculation of some transformation temperatures, Journal of The Iron and Steel Institute, p. 721, 1965

Choquet, P. et al. (1985). Mathematical model for predictions of austenite and ferrite microstructures in hot rolling processes, IRSID Report, St. Germain-en-Laye, p.7, 1985

Krauss, G. (1990). Principles of heat treatment and processing of steels, ASM International, p. 43-87, 1990

Santofimia, M.J., Zhao, L., Petrov, R. & Sietsma, J.(2008). Characterization of the microstructure obtained by the quenching and partitioning process in a low-carbon steel, Materials Characterization, Vol. 59, (2008), 1758-1764, ISSN: 1044-5803

Steven, W. et al. (1956). Journal of the Iron and Steel Institute, 183, 1956, 349

***R&D (2003). Team of the Kimitsu Steelworks of Nippon Steel, 2003

Tab. 1. Chemical composition of 42SiCr experimental steel

 C[%] Si[%] Cr[%] Mn[%] Ni[%] P[%] S[%]

 0.43 2.03 1.33 0.59 0.07 0.009 0.004

 Cu[%] Nb[%] Mo[%] Sn[%] Al[%] N[%] Ti[%]

 0.07 0.035 0.03 0.013 0.008 0.0076 0.004

 V[%] B[%] Pb[%] As[%] Ca[%] Sb[%]

 0.004 0.0025 0.002 0.002 0.0015 0.001

Tab. 2. Results and comparison of phase transformation
temperatures (R&D Team, 2003) (Choquet et al, 1985) (Steven
et al, 1956) (Andrews, 1965) (Krauss, 1990)

[T.sub.t] Dilatometry JMatPro
 [[degrees]C] [[degrees]C]

[Ar.sub.3] 813 --
[Ar.sub.3] 844 822
[B.sub.s] -- 504
[M.sub.s] 305 299
[M.sub.s] 305 299

[T.sub.t] Model
 [[degrees]C]

[Ar.sub.3] 750[degrees]C--Nippon St. 2
[Ar.sub.3] 877[degrees]C--Choquet
[B.sub.s] 563[degrees]C--Steven
[M.sub.s] 299[degrees]C--Andrews
[M.sub.s] 321[degrees]C--Krauss