Multi‐scale Finite Element Cellular Automata Simulation of Multi‐step Cold Forging Operations

The application of new materials to produce forged connecting parts is presented in this work. Particular attention is put on modern bainitic steels due to their increased ductile and strength properties, which influence the behaviour of final products under further exploitation conditions. Bainitic steels do not require a series of thermo‐mechanical operations to obtain these elevated properties, which is one of the advantages of this material. Experimental analysis and numerical simulations of steel behaviour during multi‐step cold forging operations are described in this paper. Since it is one of the possible fracture initiation mechanisms, strain localization development during cold forging is investigated in detail. Conventional constitutive models used in finite element programs have limitations in modeling stochastic and discontinuous phenomena that are responsible for strain localization. The cellular automata model is used as constitutive law in this work to overcome these difficulties and investigate material flow during multi‐stage cold forging operations. Connection of the cellular automata (CA) and finite element (FE) methods creates a so‐called multi‐scale CAFE model. The main aspects of the model are described briefly in this paper. The experimental part of this work supports the numerical investigation. Comparison of the parameters measured and predicted by the CAFE model is presented and discussed as well.     

Effects of Carbon on the Corrosion Behaviour in Fe‐18Cr‐10Mn‐N‐C Stainless Steels

The effects of carbon on general and pitting corrosion behaviour of Fe‐18Cr‐10Mn‐(0.33～0.44)N‐(0～0.38)C alloys were investigated using potentiodynamic tests. Carbon made the nitrogen‐bearing alloys inert and thus promoted general corrosion resistance. These results were supported by experimental findings, such as elevated corrosion potential, reduced active dissolution rate, lowered passive potential and accelerated hydrogen evolution rate in sulphuric acid solution. The resistance to pitting corrosion in chloride solution was also enhanced by the addition of carbon, which was attributed to the improvement of the stability of the passive film. XPS analysis revealed that the cationic fraction of chromium in the passive film was increased and hence the protection ability of the film was improved by the carbon addition.     

Metallurgical Aspects of Ultra Fast Cooling in front of the Down‐Coiler

The Ultra Fast Cooling technology developed by CRM has been implemented on the hot strip mill of Carlam (Arcelor group), just in front of the down‐coilers (late UFC). The results have demonstrated the controllability of the system and the great efficiency (≥ 3.5 MWm^?2, 300°C/s on 4 mm thick strip) of the cooling unit. Increasing the cooling rate in a controllable way makes the production of high strength steels easier and cheaper. The combination of laminar and UFC technologies opens the way for new cooling schedules. Multiphase microstructures are therefore easily generated by controlling the intermediate temperature ‐ between laminar cooling and UFC ‐ and the coiling temperature.     

Grain Refinement of TRIP‐Assisted Multiphase Steels through Strain‐Induced Phase Transformation

To develop high performance steels for automotive applications, enhanced strengthening mechanisms are required. This study aims at assessing the critical parameters leading to the refinement of the strain‐induced ferrite matrix of thermomechanically processed multiphase steels. Hot rolling simulations allowed the definition of the temperature, strain and cooling rate conditions bringing about the formation of strain‐induced ferrite with a reduced grain size. The relationship between the deformation and the concurrent or subsequent phase transformations is highlighted thanks to a thorough characterisation of the generated microstructures. It is also shown that the prior austenite grain size influences the distribution of the second phases within the finely grained ferrite matrix.     

A Method for the Calculation of Retained Austenite Evolution during Heat‐Treatment of Low‐Alloy TRIP‐Assisted Steels

Summary (abstract) : Despite the critical effect of heat‐treatment, and in particular of the isothermal bainitic treatment stage, on the amount and stability of retained austenite in the microstructure of low‐alloy TRIP‐assisted steels, determination of optimum heat‐treatment conditions is still largely empirical and experiment‐dependent. This work proposes a method by which it is possible to calculate the vol. fraction of retained austenite in the microstructure as a function of intercritical annealing temperature and isothermal bainitic treatment temperature and holding time. The method assumes diffusionless lengthening of bainitic ferrite (α_B) plates in austenite (γ), and subsequent thickness‐wise C rejection from the α_B plates to the adjacent γ layers. The relative thickness of α_B plates and adjacent γ layers is determined by the T_o line of the transforming system at any given bainitic transformation temperature. The C‐concentration profiles in γ are calculated with respect to a local time‐scale, referring to any random section of any random α_B plate. Determination of the variation of C‐concentration profiles with local time in γ, together with the use of a simple austenite‐retention criterion, allows the calculation of vol. fraction retained austenite (γ_R) as a function of transformation temperature and local time. Transition from local (calculation) time to actual heat treatment time is performed by introducing a time‐scale factor, which depends on transformation temperature and initial C‐content of α_B. The calculated behaviour of vol. fraction γ_R vs. bainitic holding time conforms to the well established, experimentally observed one: vol. fraction γ_R initially increases with holding time, reaches a maximum and decreases at longer holding times. According to calculated results, the decrease is attributed to the gradual homogenization of C inside the γ layers, which leads to inadequate stabilization and transformation of γ to martensite on quenching. As regards quantitative comparison with available experimental data, calculations show reasonable agreement. Certain refinements of the method, which are underway, are reported, in order to further improve quantitative results. Nevertheless, the method in its present form provides a calculational tool, by which the effect of different heat‐treatment conditions or steel compositions can be examined and compared to each other, which can be a useful step towards optimizing alloy compositions and heat‐treating processes.     

Weldability Evaluation and Microstructure Analysis of Resistance‐Spot‐Welded High‐Mn Steel in Automotive Application

In this paper, resistance spot weldability of high‐Mn steels were investigated in order to get high reliability in welded joints of automotive components. Microstructural characterizations, cross‐tensile test (CTT), microhardness tests of spot welded parts were conducted. The effects of weld current on the microstructural characteristics, mechanical properties, and fracture modes were investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The hardness in the weld nugget was observed to be lower than that in the base metal (BM). In CTT, the failure initiation was observed to occur at the boundary of the weld nugget. Also welding imperfections of welded parts were investigated. Liquation cracking in heat affected zone (HAZ), porosity, and shrinkage cavity were found most common welding defects in welded parts. Furthermore, the effects of welding imperfections on weld quality and failure criteria were identified and discussed.     

Evaluation of the Static Stress‐Strain Behaviour of Phosphorus Alloyed and Titanium Micro‐alloyed TRIP Steels

A considerable research effort has been done in the field of cold rolled TRIP steels submitted to a two‐step annealing cycle. After annealing, these steels contain retained austenite, which offers them superior mechanical properties required for specific applications in automotive industry. In the present work, a physically based microstructural model has been applied to describe the static stress‐strain behaviour of phosphorus alloyed TRIP steel. The impact of the TiC precipitation on the static stress‐strain behaviour for a Ti micro‐alloyed TRIP steel was simulated. The model calculations were compared with experimental stress‐strain curves. An excellent agreement between simulation and experimental data was demonstrated.     

On Restricting Aspects in the Production of Nonmagnetic Cr‐Mn‐N‐alloyed Steels

Cr‐Mn steel grades with high nitrogen contents are becoming increasingly important in the field of austenitic stainless steels. Industrial production facilities allow to use two different strategies to reach a high nitrogen content. The first involves taking advantage of the pressurised‐electroslag remelting process, which is operated at elevated nitrogen partial pressure; the second consists of adding elements which increase the nitrogen solubility of the melt so that high nitrogen contents can be achieved at atmospheric pressure. This paper focuses on nitrogen solubility and austenite stability. These have been observed as important and in some cases restricting for the successful implementation and production of high alloyed Cr‐Mn austenitic steels. The precondition for a stable austenitic microstructure can be predicted with the help of equations using chromium and nickel equivalents. Different formulae were tested and their results compared to the microstructure of the alloys. The nitrogen solubility in the melt is particularly important for the steel grades cast under atmospheric conditions. It has been found feasible to produce steel grades up to 0.9 mass percent nitrogen at atmospheric pressure on an industrial scale. Several theoretical approaches for calculating the nitrogen solubility in the melt were tested for atmospheric conditions and compared to the chemical analyses of conventionally cast Cr‐Mn steel grades.