A. Grajcar

Thermo-mechanical processing of high-manganese austenitic TWIP-type steels

The high-manganese austenitic steels are an answer for new demands of automotive industry concerning the safety of passengers by the use of materials absorbing high values of energy during collisions. The chemical compositions of two high-manganese austenitic steels containing various Al and Si concentrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control the grain growth under hot-working conditions. The influence of hot-working conditions on recrystallization behaviour was investigated. On the basis of initial investigations realized by hot upsetting the thermo-mechanical conditions resulting in a fine-grained structure were designed. The σ-e curves and identification of thermally activated processes controlling work-hardening by the use of the Gleeble simulator were determined. It was found that the thermo-mechanical treatment conditions influence a phase composition of the investigated steels after solution heat treatment.

The influence of hot-working conditions on a structure of high-manganese steel

The high-manganese steels with the austenitic structure belong to a group of modern steels predicted to use in the automotive industry. The chemical composition of the steel containing 25% Mn, solution hardened by Si and Al was developed. Microadditions of Nb and Ti introduced into the steel creating stable nitrides and carbides should act by precipitation hardening and inhibit a grain growth of recrystallized austenite. The aim of the work was to determine the influence of various hot-working conditions on a structure of the investigated steel. The processes controlling work-hardening and removing strengthening after finishing the hot-working were identified. The preliminary upset forging by the use of eccentric press with a degree of deformation in a range of 20 to 60% and at temperatures of 850 and 1000 °C was carried out. On the basis of determined conditions the multi-stage axial compression tests ensuring the fine-grained austenite structure were performed.

Phases and Structure Characteristics of the Near Eutectic Al-Sl-Cu Alloy Using Derivative Thermo Analysis

For determining of the micro-structural changes taking place in a near eutectic Al-Si-Cu aluminium cast alloy during heating and cooling process the UMSA device (Universal Metallurgical Simulator and Analyzer) was used. In this work the dependence between the regulated cooling speed and structure on the basis of the thermo-analysis was carried out. The thermal analysis was performed at a cooling rate in a range of 0,2 °C to 1,25 °C. The changes were examined and evaluated qualitatively by optical and electron scanning microscopy methods and the EDS microanalysis. During the investigation the formation of aluminium reach (α-Al) dendrites was revealed and also the occurrence of the α+β eutectic, the ternary eutectic α+Al2Cu+β, as well a iron and manganese containing phase was confirmed. The performed investigation are discussed for the reason of an possible improvement of thermal and structural properties of the alloy. The achieved results can be used for liquid metal processing in science and industry – for example foundry for developing and obtaining of a required alloy microstructure and properties influenced by a proper production conditions.

Softening Kinetics in Nb-Microalloyed TRIP Steels with Increased Mn Content

Two 5Mn-1.5Al TRIP steels with and without Nb microaddition were developed in the present study. The steels contain bainite, martensite, interlath retained austenite and martensite- austenite islands. The paper presents the results of the compression tests carried out at various temperatures using the Gleeble simulator. To analyze the kinetics of static recrystallization in these steels, a softening kinetics were determined in a double-hit compression test. It was found that the dynamic recovery is a main thermally activated process occurring during hot deformation. The Nb microalloyed steel has higher flow stresses and peak strains than the Nb-free steel. A solute drag effect of Nb results in a slower softening kinetics of Nb containing steel. The effects of Mn on the retardation of Nb(C,N) precipitation and hot deformation characteristics are also discussed.

Microstructure Evolution of C-Mn-Si-Al-Nb High-Manganese Steel during the Thermomechanical Processing

The aim of the paper is to determine the influence of hot deformation conditions on σ-ε curves and microstructure evolution of new-developed high-manganese C-Mn-Si-Al-Nb austenitic steel. The force-energetic parameters of hot-working were determined in continuous and multi-stage compression tests performed in a temperature range of 850 to 1100°C by the use of the Gleeble 3800 thermomechanical simulator. Evaluation of processes controlling work-hardening were identified by microstructure observations of the specimens water-quenched after various conditions of plastic deformation. Multi-stage compression tests with true strain of 0.29 permit to use the dynamic and metadynamic recrystallization for forming the fine-grained, austenite microstructure of steel in the whole range of deformation temperature.

Effect of Heat Input on Microstructure and Hardness Distribution of Laser Welded Si-Al TRIP-Type Steel

This study is concerned with issues related to laser welding of Si-Al type TRIP steels with Nb and Ti microadditions. The tests of laser welding of thermomechanically rolled sheet sections were carried out using keyhole welding and a solid-state laser. The tests carried out for various values of heat input were followed by macro- and microscopic metallographic investigations as well as by microhardness measurements of welded areas. A detailed microstructural analysis was carried out in the penetration area and in various areas of the heat affected zone (HAZ). Special attention was paid to the influence of cooling conditions on the stabilisation of retained austenite, the most characteristic structural component of TRIP steels. The tests made it possible to determine the maximum value of heat input preventing the excessive grain growth in HAZ and to identify the areas of the greatest hardness reaching 520 HV0.1.

Influence of Nb Microaddition on a Microstructure of Low-Alloyed Steels with Increased Manganese Content

The present study is a first step of a project to obtain thermo-mechanically processed fine-grained increased Mn content TRIP steels with large fractions of retained austenite. Two 0.17C-3Mn-1.6Al-0.2Si-0.2Mo steels with and without Nb microaddition were produced in a vacuum induction furnace. The influence of Nb microaddition on a macrostructure, a grain size and hot-working behavior were examined. The steels are characterized by a slight macrosegregation of Al in the as-cast state, minimized for a Nb-microalloyed steel. After hot forging refined bainitic-martensitic structures with large fractions of γ phase obtained. The steel microalloyed with Nb has finer granules of retained austenite at comparable fractions of this phase. The force-energetic parameters of hot-working were determined in an uniaxial hot-compression test at temperatures of 1150 and 950°C and strain rates from 0.1 to 10s-1. The Gleeble 3800 thermomechanical simulator was used. The hot-working behaviour of the investigated steels is challenging because of higher flow stresses and εmax strains compared to conventional TRIP steels with lower Mn contents.

Thermodynamic analysis of precipitation processes in Nb–Ti-microalloyed Si–Al TRIP steel

The work deals with the thermodynamic calculations of precipitation processes in austenite of the Nb–Ti-microalloyed steel with increased Si and Al content dedicated for the automotive industry. The analysis is based on the equilibrium precipitation of individual MX-type interstitial phases, as well as the effect of various Mn and Si additions is included. The solubility products and corresponding limits of the mutual solubility of microalloy and metalloid additions in austenite were calculated. The temperature sequence of the precipitation under equilibrium conditions was determined. The Dutta–Sellars model has been applied for determination of recrystallization stop temperature of austenite and time needed for Nb(C,N) precipitation. The calculations were verified by microstructure investigations including revealing prior austenite grain size as a function of austenitizing temperature and the identification of complex carbonitrides using transmission electron microscopy. The model calculations are in good agreement with experimental results.

Microstructural Analysis of a Thermomechanically Processed Si-Al TRIP Steel Characterized by EBSD and X-Ray Techniques

The work focuses on the analysis of microstructural features of retained austenite in a thermomechanically processed Si-Al TRIP-type steel microalloyed with Nb and Ti. Austenite amount was determined using XRD and EBSD. Combined methods of LM, SEM and EBSD were applied to reveal the morphology, grain size and distribution of structural constituents. It is possible to retain 14% of  phase enriched in C to about 1.14 wt.%. Retained austenite is uniformly located as blocky grains with a diameter up to 4.5 m in a fine-grained ferritic matrix or between bainitic ferrite laths as thin layers. Special crystallographic relationships between bainitic ferrite and retained austenite were identified on the basis of the analysis of misorientation angles and image quality values.

Microstructure–property relationships in TRIP aided medium-C bainitic steel with lamellar retained austenite

Abstract This work discusses the development of the microstructure and mechanical properties of medium-carbon steel that contains silicon, aluminium and microadditions of Nb and Ti. Two cooling strategies were designed based on the thermodynamic equilibrium calculations and continuous cooling transformation diagram, which was determined for plastically deformed austenite. The cooling paths enabled the production of ferrite based and bainite based steels. The specimens were obtained via the thermomechanical rolling process with isothermal holding of steel at 450°C. Microstructure investigations were performed using light, scanning and transmission microscopy methods. The distribution and amount of retained austenite were determined using the electron backscatter diffraction technique, whereas transmission electron microscopy allowed the identification of the morphology of the γ phase. The amount of austenite and its carbon content were assessed using X-ray diffraction. Relations between microstructure and mechanical properties were formulated based on the mechanical stability of the retained austenite.