J. Majta

Modelling of the influence of thermomechanical processing of Nb-microalloyed steel on the resulting mechanical properties

Abstract Two-stage compression tests were performed at various temperatures and reductions. The influence of finish deformation temperature in the range of 650–820°C and strain on microstructure and final mechanical properties of Nb steel is investigated. Starting from detailed simulation of microstructure development, i.e. recrystallization, precipitation and grain growth kinetics, the material behaviour was modelled. An empirical model of yield strength, developed in previous work, is employed to predict mechanical properties of microalloying steel during hot deformation above and below the γ–α transformation temperature. The comparison of simulated and measured results shows that the real thermomechanical behaviour as well as the microstructure development of Nb-steel has been correctly implemented. The results provided by both finite element analyses and microstructure modelling were compared with earlier similar investigations of C–Mn steel.

Modelling and measurements of mechanical behaviour in multi-pass drawing process

Abstract The paper focuses on the theoretical considerations as well as experimental investigations concerning cylindrical bar drawing processes with different drawing schedules. The single- and multi-pass drawing technologies were analysed. The finite element method calculations resulted in obtaining the distributions of strain rate tensor components, effective strain rate, effective strain and temperature. The deformation of the grid of coordinates was also determined experimentally in order to describe the strain state in the die deformation zone. The experiments included the measurements of strains and mechanical properties distributions on the cross sections of drawn products. The obtained results describing theoretical distributions of effective strain were compared with the experimental data. The changes of mechanical properties of final products drawn in single- and multi-pass technologies, applying various drawing velocities, were analysed. As a result of investigations which have been carried out the answer was obtained to the question: whether and to what extent the properties of the products obtained in multi-stage technologies of deformation are connected with the parameters used in individual single operations and what conditions have to be fulfilled in order to obtain the final product of proper quality. The materials being investigated were stainless steel and copper.

Modeling of ferrite structure after deformation in the two-phase region

Abstract This study presents some modeling aspects of microstructure development under intercritical (austenite–ferrite two-phase region) forming conditions in microalloyed and low carbon steels. The employed integrated computer model effectively links the advanced finite-element approach simulating metal flow and heat transfer during hot plastic deformation with the submodels describing complex microstructure development. It is proposed that a correct quantitative description of the microstructure allows the selection of processing parameters to control the microstructure inhomogeneity. The effective strain accumulated in the ferrite phase and temperature distributions were combined with the empirical formulas that enable calculation of ferrite grain size with separation between the two mechanisms of refinement process, i.e., transformation from hardened austenite and ferrite recrystallization. The predicted ferrite structures show reasonable agreement with those obtained in various experiments. Hence, the distribution of the mechanical properties will naturally be predicted based on the initial process conditions and using the proposed modeling procedure.

Use of the computer simulation to predict mechanical properties of C-Mn steel, after thermomechanical processing

Abstract The objective of the paper is to demonstrate the ability of a computer simulation to analyse the development of microstructure and finally to the predict mechanical properties of C-Mn steels. The microstructure of ferrite is the main parameter which controls the mechanical properties of steels after hot deformation. This microstructure depends on the grain size and morphology of austenite just before the transformation. However, when the last deformation takes place below the γ-α transformation temperature, the mechanisms connected with substructure and dislocation forest becomes a significant part of the strengthening process. The microstructural model makes it possible to separate the relative contributions of solid solution, ferrite microstructure, substructure and dislocation strengthening. The change in microstructure at lower temperatures requires an improvement in the model which will allow to account for the substructure and dislocation density. In the present work hot deformation conditions were simulated using Finite Element Method. The investigation was focused on the case when the last deformation takes place in the two phase or ferrite region. The two stage constant strain rate compression tests were conducted, in continuous cooling condition, at the temperature range 1050 – 650 °C. The material after deformation was investigated to obtain the microstructure and verify the model of substructure and dislocation strengthening mechanisms. The experimental results were used to validate and improve the empirical equations that were employed to the general model. The computer simulation suggested in the work can be used to predict mechanical properties, including all the events that occur under industrial processing conditions which cannot be reproduced in the laboratory.

Mechanical behaviour of hot and warm formed microalloyed steels

Abstract The aim of this work is to investigate the possibility of producing hot and warm formed steels with multiphase structures with high strength and good workability. As an example a niobium-microalloyed steel was investigated. For studying the microstructure development products that were formed in hot and inter-critical conditions, continuous and interrupted compression tests were performed. As a result the detailed indications for controlling the microstructure evolution and inhomogeneity of mechanical properties in products after hot forming were acquired. A number of direct results and conclusions were incorporated in the integrated model, considering the required distributions as well as average values of mechanical properties of deformed product. The models employed here link advanced, finite element approaches simulating metal flow and heat transfer during hot-plastic deformation with constitutive equations describing microstructural processes, phase transformation and mechanical properties. The results establish a methodology for the possible development of structural steels with both controlled microstructure and texture. These provide materials with significant increases in yield strength and a great potential value for the steel industry. The formulated conclusions may find their application in the analysis of more complex metal forming processes and grades of steels.

Modeling Of Microstructure Evolution Of BCC Metals Subjected To Severe Plastic Deformation

Prediction of microstructure evolution and properties of ultrafine‐grained materials is one of the most significant, current problems in materials science. Several advanced methods of analysis can be applied for this issue: vertex models, phase field models, Monte Carlo Potts, finite element method (FEM) discrete element method (DEM) and finally cellular automata (CA). The main asset of the CA is ability for a close correlation of the microstructure with the mechanical properties in micro‐ and meso‐scale simulation. Joining CA with the DEM undoubtedly improves accuracy of modeling of coupled phenomena during the innovative forming processes in both micro‐ and macro‐scale. Deformation in micro‐scale shows anisotropy, which connected with that the polycrystalline material contains grains with different crystallographic orientation, and grain deformation is depended from configuration of directions of main stresses and axis of grain. Then, CA and DEM must be joint solutions of crystal plasticity theory. In t...

Analysis of inhomogeneity of mechanical properties in stainless steel rods after drawing

Abstract This paper presents the results of investigation of the inhomogeneity of mechanical properties in the 1H18N9T stainless steel rods. Sixteen drawing schedules of reduction ratios varying from 5.5% to 24.4% and die angles varying from 6° to 20° were examined. Local values of R0.2, Rm and A100 were determined from the tension tests carried on the mini-specimens. Distribution of the HV5 hardness on the transverse and longitudinal cross sections were evaluated. Experimentally found coefficient allows to determine the 0.2% offset stress and tensile strength from the hardness measurements.

Influence of Strain Path Changes on Microstructure Inhomogeneity and Mechanical Behavior of Wire Drawing Products

Cold drawn low carbon steel wires are widely used in several engineering applications where a proper combination of strength and ductility is of the paramount importance. In the present paper, the multi-pass angular accumulative drawing (AAD) is proposed as a new forming process where the high strain accumulation is used as a way to achieve much higher microstructure refinement level compared to the conventional wire drawing process. This process is characterized by a complex strain path history, being an effect of wire diameter reduction, bending, tension and torsion, what directly affects the microstructure changes in the final product. This process also evolves high inhomogeneity of microstructure, that if properly controlled, can lead to further properties improvement - what can be especially beneficial for alloys that are not characterized by complex compositions. In the present paper, special emphasis is given on the inhomogeneity of both deformation and microstructure and resulted mechanical properties. After drawing and annealing (at 500oC) mechanical properties measurements and microstructure analysis on the longitudinal sections of the wires were performed to assess the differences existing with respect to the conventional wire drawing process.

Characterization of UFG Microalloyed Steel Produced by Combined SPD Treatment

Studies of the effects of inhomogeneity of UFG (ultrafine-grained) microstructure evolution have been performed on severely deformed specimens produced by AAD (angular accumulative drawing), followed by wire drawing and wire flattening processes i.e. “top-down” systems of the grain refinement process. In this paper, deformation behavior and size effects are illustrated by means of UFG microalloyed steel with different combinations of microstructure length scale and deformation inhomogeneity. The refined and severely elongated structures were investigated by light microscopy, TEM and EBSD. Of particular importance was the understanding of the effects of strain path, microalloying elements and deformation inhomogeneity on grain refinement and dislocation substructure formation processes.

Mechanical Response of Microalloyed Steel Subjected to Nonlinear Deformation

Microalloyed steels have been the subject of theoretical and experimental studies revealing their exceptional mechanical response under nonlinear deformation conditions. In microalloyed steels, especially in multiphase steels, the mechanical properties are adjusted by combination of microstructure components with different levels of theirs mechanical responses, including hardness and ductility. A comprehensive studies have revealed that a transition from the development of usual bulk dislocation microstructures to more architecture ones occurs when the applied strain path allows to cumulate the deformation energy what is also strictly connected with the chemical and structural compositions of analyzed materials. The study presented here aims at understanding the complex strengthening mechanisms as well as microstructure evolution and to provide a link with the mechanical behaviour of investigated steels under nonlinear deformation conditions. The proper choice of the work hardening model for the cyclic plastic deformation is essential for predicting the inhomogeneities occurring during metal forming. Aim of the current work is to discuss the differences between various hardening models with respect to their capabilities in capturing complex deformation models and possibilities of their direct application to finite element modelling of such deformation processes. The results of experimental studies are integrated with computer modelling and dislocation theory to provide insight into the unprecedented combination of properties achieved in certain multiphase steels such as ultra-high flow strengths, good ductility and workability. Finally, based upon results obtained in performed computer simulations, conclusions regarding the possibilities of potential application of the work hardening models in the identification process parameters, trough the inverse analysis, are drawn.