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Third generation of AHSS with increased fraction of retained austenite for the automotive industry

Grajcar A., Kuziak R., Zalecki W.

Archives of Civil and Mechanical Engineering

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2012

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Vol. 12, no. 3

334--341

EN

Third generation of advanced high-strength steels for the automotive industry contains a high volume fraction of fine-grained ferrite, carbide-free bainite, martensite and retained austenite. The level of strength and ductility is highly dependent on the fraction and mechanical stability of austenitic phase. One of the methods to obtain an increased fraction of γ phase is trough its chemical stabilization by Mn. Two 0.17C–3Mn–1.5Al–0.2Si–0.2Mo steels with and without Nb microaddition were developed in the present study. The steels were subjected to the thermomechanical processing designed on the basis of the DCCT diagram (deformation – continuous cooling transformation). The paper presents the results of the multi-stage compression tests and multiphase microstructures obtained as a result of the controlled multi-stage cooling. It was found that the hot workability of a new generation of AHSS is very challenging due to high values of flow stresses required. However, the thermomechanical processing enables to obtain very fine-grained bainite-based microstructures with a fraction of retained austenite up to 20%. The highest fraction of fine grains and interlath austenite was obtained for the temperature range between 400 and 450 °C. The effect of Nb results in higher flow stresses and better distribution of all the microstructural constituents.

2

Effect of hot-working in the gamma+alpha range on a retained austenite fraction in TRIP-aided steel

Grajcar A.

Journal of Achievements in Materials and Manufacturing Engineering

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2007

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Vol. 22, nr 2

79-82

EN

Purpose: The aim of the paper is to determine the influence of thermomechanical processing conditions on an austenite stability in a TRIP-aided microalloyed steel. Design/methodology/approach: The heat treatment and thermomechanical processing in a two-phase region to obtain ferritic-bainitic structures with the retained austenite in a low-carbon microalloyed steel were conducted. The heat treatment consisted of austenitizing of specimens at 750 degrees centigrade, oil cooling, isothermal holding in a temperature range from 300 to 500 degrees centigrade and slow cooling to room temperature. A part of the specimens was forged with a degree of deformation of 28% before cooling. Optical and transmission electron microscopy were employed for structure observations. The retained austenite amount was determined by X-ray diffraction method. Findings: It was found that hot-working in the gamma+alpha range contributes to a considerable refinement of a ferritic matrix. The grain size of the alpha phase is about 4 micrometres and its volume fraction equals from 60 to 68%. The optimum structure containing 10% fraction of retained austenite was obtained for the specimen forged in the two-phase region and isothermally held at a temperature of 300 degrees centigrade. Research limitations/implications: To determine with more detail the relationship between hot-working conditions and the stability of retained austenite investigations in a wider deformation temperature range should be carried out. Practical implications: The proposed thermomechanical treatment route can be useful in a development of the technology for TRIP-aided low-carbon microalloyed steels with a reduced silicon content. Originality/value: The design thermomechanical treatment conditions made for obtaining the 10% fraction of retained austenite in a steel containing 0.5% Si only in comparison to 1.5% Si concentration used in TRIP-aided steels usually.

3

Hot-working in the γ + α region of TRIP-aided microalloyed steel

Grajcar A.

Archives of Materials Science and Engineering

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2007

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Vol. 28, nr 12

743-750

EN

Purpose: The aim of the paper is to investigate the influence of hot-working in the γ + α range and isothermal holding temperature in a bainitic range on a stability of retained austenite in a TRIP-aided microalloyed steel. Design/methodology/approach: The thermomechanical processing in the γ + α range to obtain multiphase structures with the retained austenite in a microalloyed steel was realized. It consisted of plastic deformation of specimens at 750°C or 780°C, oil cooling and isothermal holding in a bainitic region. Degree of deformation was 28 or 50%. To reveal the multiphase structure optical and transmission electron microscopy were used. X-ray diffraction method was employed to determine a volume fraction of retained austenite. Findings: It was found that hot-working in the two-phase region influences essentially a multiphase structure of investigated steel. The ferrite fraction is comparable for heat-treated and thermo-mechanically processed specimens but the ferrite grain size is twice smaller in a case of plastically-deformed specimens. The optimum isothermal holding temperature in a bainitic range is 300°C, independent on austenitizing temperature. The specimens forged in the γ + α range and isothermally held at this temperature made it possible to obtain about 10% of retained austenite. Research limitations/implications: Investigations concerning the influence of isothermal holding time in a bainitic range on the stability of retained austenite should be carried out. Practical implications: The established conditions of the thermomechanical processing can be useful in a development of the hot-rolling technology for TRIP-aided microalloyed steels. Originality/value: The realized thermomechanical processing enabled to obtain about 10% fraction of retained austenite in a steel containing 0.5% Si.