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A study of microstructure and phase transformations of CMnAlSi TRIP steel

Gajda B., Lis A. K.

Journal of Achievements in Materials and Manufacturing Engineering

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2008

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Vol. 31, nr 2

646-653

EN

Purpose: Purpose was to obtain the TRIP-type microstructure in the CMnAlSi steel. Heat treatment consisted of the partial austenitization at 900*C/60s and continuous cooling with rates: 0.5-40*C/s, was examined. Also the effect of Al and Si on Ac1 and Ac3 temperatures, and the volume fractions of austenite in CMnSi, CmnAl and CMnAlSi steels was investigated. Design/methodology/approach: The effect of alloying elements on Ac1 and Ac3 temperatures was investigated using Thermo-calc program. The influence of cooling rates on phase transformations and microstructures of samples austenitized at 900*C/60s was examined using dilatometer, light optical microscopy and scanning electron microscopy. X-ray diffraction technique was used to calculate the amount of retained austenite. Quantitative analyses of phases were done using Image pro Plus 3.0 program. The mechanical properties and Vickers hardness (HV10) measurements were also investigated. Findings: The TRIP-aided microstructure consisted of ferrite matrix, bainitic ferrite and metastable retained austenite can be obtained for the CMnAlSi steel through intercritical annealing at 900*C/60s and continuous cooling with the rate 20*C/s to the R.T. Isothermal holding at bainitic temperature range (600-400*C) during cooling is not necessary, because of the Al and Si additions to the steel. Practical implications: The CMn steel with addition of 1% Al and Si is well-suited for production of TRIP steel sheets in a large range of temperatures: 800-900*C. The advisable cooling rates are in the range from 10 to 40*C/s. Originality/value: In the TRIP steels the amount of residual austenite in structure at the R. T. strongly depends on the heat treatment parameters such as annealing temperature, cooling rates and amounts of added alloying elements. It is very important to determine the optimal annealing parameters for each TRIP steel grade to obtain the steel with the best mechanical properties and microstructure.

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Thermal processing of CMnAlSi steel at (α+γ) temperature range

Gajda B., Lis A. K.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 18, nr 1-2

355--358

EN

Purpose: Investigations of microstructure changes in the modern high-strength CMnAlSi steel after austenitization at (α+γ) temperature 900°C/60s were presented in order to determine the influence of the cooling rate on the phase transformations and obtaining multiphase TRIP-aided microstructure. Also the effect of alloying elements on the Ac1 and A c3 temperatures and the volume fractions of austenite in various (α+γ) austenitization temperatures for the investigated steel were presented. Design/methodology/approach: Thermo-calc program was used in order to determine influence of alloying elements such as Al, Si on Ac1 and A c3 temperatures. Dilatometric experiments of the CMnAlSi steel were done for the temperature 900°C from (α+γ) temperature range. Microstructures were investigated by light optical microscopy and scanning electron microscopy. The amount of retained austenite in the obtained microstructures was investigated with X-ray diffraction technique. The quantitative analysis of phases in microstructure were done using Image pro Plus computer program. Mechanical properties of investigated steel were examined. Findings: The multiphase microstructure containing about 10% retained austenite can be obtained in steel of 0.15 % C, 1.55 % Mn, 1% Si and 1% Al through continuous cooling from 900°C/60s to the room temperaturę without isothermal holding at bainitic transformation temperature range. Practical implications: Steel CMnAlSi is well suited for production of TRIP grade in a large range of temperatures from 800°C to 900°C at the cooling rates of about 10°C /s to 40°C/s. The amount of 50 % austenite at temperature 900°C allows for production of TRIP microstructure with stable retained austenite. Originality/value: The TRIP steels can be processed only if annealing parameters are perfectly adjusted to the chemical composition of the steel. The A c1 and A c3 temperatures differ for the various chemical compositions and they strongly depend on the C, Si and Al contents. It is hard to match the same annealing temperature for every TRIP steel grade. Also the cooing rate has an important role in obtaining the proper multiphase TRIP microstructure with stable retained austenite.

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Modelling of the DP and TRIP micro-structure in the CMnAlSi automotive steel

Lis A. K., Gajda B.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 15, nr 1-2

127--134

EN

Purpose: The CMnAlSi steel is a new grade of TRIP steels with 1wt % of Al and Si. It is important to determine the usability of the CMnAlSi for production of sheets for automotive applications. Design/methodology/approach: The effect of cooling rate and austenitization temperature on phase transformations was investigated. The dilatometric experiments of the steel were done for the full austenitization temperature 1200°C, and for (α+γ) temperature ranges: 1100°C, 1000°C, 900°C and 800°C. Steel was also processed to achieve TRIP grade by continuous annealing with modeled vertical hot dip galvanizing line. The microstructures were investigated by light optical microscopy and SEM with EDX attachment. The amount of retained austenite in the obtained microstructures was investigated with X-ray diffraction. Findings: There is possibility to produce “dual-phase” CMnAlSi steel grade with controlled rolling at finishing temperature below 900°C to 800°C and fast cooling. Steel CMnAlSi is well suited for production of TRIP grade via heating cycle which correspond to vertical hot dip galvanizing process. Practical implications: This steel is suitable for production of automotive applications. Originality/value: The new procedure of control rolling from the (α+γ) temperature range of CMnAlSi steel was presented.