Cand.Tech.Sci. Gubanov O. M., Dr.Tech.Sci. Cheglov A.E., Kamysheva D.Y.

Novolipetsk Steel OJSC, Russia, Lipetsk State Technical University, Russia.

A Technique of Assessment of Microstructure of Steels with Pronounced Variations in Grain Size

Change of various indicators during the process of annealing allows one to assess development of recrystallization processes and structural state change dynamics. This can be a change in average or maximum size of grain, coefficient of grain size variation or surface area taken by grains of various size in sample section. The technique of measurement of segments of section area taken by grains of various size is most suitable, when a material with a significant grain size variation (d_max/d_cp > 1.9) is to be assessed, or in cases, where grain size variations occur and develop during processing, while it is necessary to quantitatively describe the changes during processing.

This technique consists in the following:

1. For each sample a number of microphotographs is made with a magnification sufficient for distinguishing of separate finest grains and measurement of their size.

2. A few areas containing preferably not less than 40 grains each are outlined in the microphotographs in random order.

3. I-size grains in an outlined area (n_i) is counted. For counting convenience grains are distributed to size groups with a certain interval (the authors most often use division into groups with interval if 10 microns).

Similarly, to reduce measurement error, a number of measurements is made for each sample with a total amount of grains not less than 500.

4. The nominal surface area taken by i-size grains is calculated.

S_{i ном}=N_i π r_i² , (1)

where r - radius of i-size grains.

5. The percent of section surface area taken by i-size grains is calculated

S_i= S_{i ном}/∑ S_{i ном} × 100% (2)

As is shown below, the technique of measurement of section segment surface area taken by grains of various size is an effective tool to study and analyze structures with pronounced grain size variations. It also helps to most clearly and accurately describe structural state change dynamics during recrystallization.

The authors used the technique to assess the dynamics of recrystallization in cold rolled isotropic electrotechnical alloyed steels during fast heating to various temperatures. Speed of heating was over 230 °C / sec. Steels from all principal alloy classes were selected for the research with content of 0.081 -3.055% Si and 0.011-0.26% P.

Not less than 10 measurements in steel samples for each temperature of heating in all studied alloy classes were made. The measuring inaccuracy was less than 5%. Practical application of the technique resulted in construction of curves that describe the kinetics of recrystallization in a steel prone to grain size variation during heat treatment.

Fig. 1. The kinetics of the recrystallization process during fast heating.

The data obtained allow not only description of the character of the processes occurring during recrystallization, but also forecasting changes in microstructure for selection of new or adjusting existing heat treatment conditions taking into account chemical composition.

The technique of measurement of section area segments taken by grains of various size is an effective method of assessment of microstructure of steels with pronounced grain size variation. It allows one to develop optimum steel heat treatment conditions to obtain tailored finished goods.

References:

1. Gubanov O. M. Influence of Silicon and Phosphorus on Microstructure and Texture Formation in Isotropic Electrotechnical Steel During Fast Heating [Text] / O. M. Gubanov, A.Y. Cheglov, D.Y. Kamysheva//Proizvodstvo Prokata. - 2015. - No. 1. - Pages 8-13.