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TRIP steel topography examined by AFM (Atomic Force Microscopy)

Dryja M., Lis A., Wieczorek P.

Inżynieria Materiałowa

2014

Vol. 35, nr 2

106--108

The paper presents test of the surface topography carried out on samples made of CMnAlSi steel after austenitizing treatment at 850°C, and then intercritical annealing at 500°C for 600 s and 300 s and cooling down from 800°C with cooling rate of 5°C/s. The analysis of surface topography was performed by the half-contact method which gave details of height, amplitude and phase imaging. The results of the measurements showed that, after etching in nital, the ferrite is stronger etched than other structural components, and is visible as the undermost dark areas (Figures 2, 3 a÷c). Conducted linear analysis and determination of the average surface roughness parameters Ra of the steel for the sample cooled with V = 5°C/s revealed the presence of ferrite, bainite and retained austenite (Fig. 6).

W pracy przedstawiono badania topografii powierzchni przeprowadzone na próbkach ze stali CMnAlSi poddanej austenityzowaniu w 850°C, a następnie: wytrzymaniu izotermicznemu w temperaturze 500°C przez 600 s, 300 s oraz chłodzeniu z temperatury 800°C z szybkością 5°C/s. Analizę topografii powierzchni przeprowadzono metodą półkontaktu, uzyskując dane z wysokości, amplitudy drgań oraz obrazowania fazowego. W wyniku pomiarów wykazano, że trawienie w nitalu powoduje, iż ferryt trawi się najmocniej w porównaniu z pozostałymi składnikami strukturalnymi i jest widoczny jako ciemne obszary najniżej położone (rys. 2, 3a÷c). Przeprowadzona analiza liniowa oraz wyznaczenie parametru chropowatości Ra i średniej wysokości w stali dla próbki chłodzonej z V = 5°C/s wykazała występowanie ferrytu, bainitu oraz austenitu szczątkowego (rys. 6).

Intercritical annealing with isothermal holding of TRIP CMnAlSi steel

Gajda B., Lis A. K.

Journal of Achievements in Materials and Manufacturing Engineering

2007

Vol. 20, nr 1-2

439-442

Purpose: The purpose was to obtain the TRIP microstructure in the modern CMnAlSi steel, after cooling from the temperature 850 degrees centigrade. Samples were continuously cooled with rate 20 degrees centigrade/s to the R.T. or isothermally annealed in the bainitic transformation range during 600s at 500, 450 and 400 degrees centigrade. The influence of cooling type on microstructure and amount of retained austenite were investigated. Design/methodology/approach: Dilatometric experiments of the CMnAlSi steel were done. Microstructure were investigated by light optical microscopy OM and scanning electron microscopy SEM. The amounts of retained austenite were investigated with X-ray diffraction technique and LePera color etching method. Quantitative analyses of phases were done using Image pro Plus program. Vickers hardness HV10 measurements were done. Findings: The TRIP type microstructure can be obtained for the investigated steel through continuous cooling from 850 degrees centigrade/60s or after cooling with the isothermal holding at the bainitic transformation temperatures range. The highest amount of austenite (-12%) was observed in samples isothermally annealed at the temperatures 450/600s and 400 degrees centigrade/600s. Practical implications: Steel CMnAlSi is well suited for production of TRIP grade in a large range of isothermal holding temperatures from 500 to 400 degrees centigrade and also after continuous cooling with the rate 20 degrees centigrade/s to the R.T. The amount of 39% austenite at temperature 850 degrees centigrade allows for production of TRIP microstructure with stable retained austenite. Originality/value: The additions of Al and/or Si to the CMn steels have the influence on Ac1 and Ac3 temperatures, morphology of bainite and on stability of retained austenite in the TRIP steels. Therefore it is important to determine the heat treatment parameters for each grade of TRIP steels to obtain optimal microstructure with good mechanical properties for such applications as car bodies and car parts.