Physical Metallurgy of Multi‐Phase Steel for Improved Passenger Car Crash‐Worthiness

The dynamic testing of high strength automotive steel grades is of great practical importance if their crash‐worthiness is to be evaluated. During forming operations, steels are processed in a controlled dynamic manner. In collisions, the deformation is different in the sense that the deformation is not controlled, i.e. both strain and strain rate are not pre‐determined. No clear standard testing procedures are currently available to test high strength steels dynamically, in order to evaluate their performance during car crashes. High tensile strength TRIP‐aided steels have been developed by the steel industry because of their promising high strain rate performance. The present contribution focuses on the effect of the strain rate and temperature on the mechanical behaviour of the low alloy high strength TRIP steel. The tests were carried out on the separated phases in order to determine their specific high strain rate deformation response. The temperature‐dependence of the transformation rate of the retained austenite is presented. It is argued that the adiabatic conditions present during high strain rate deformations have a beneficial effect on the behaviour of TRIP steel.     

Special features of transformation kinetics of austenite in heat treatment of stamped and welded components made of 10G2FBYu steel

The results of investigations of the special features of austenite transformation kinetics during heat treatment of welded joints in components made of 10G2FBYu steel are used to determine the conditions for producing stamped and welded connecting components with strengths similar to that of the parent metal.     

使用θ-θ/Ψ测角仪测定钢中残余奥氏体的原理与方法

介绍了X射线衍射法测定钢中残余奥氏体含量的原理。给出了使用θ-θ/Ψ测角仪测定钢中残余奥氏体的方法,并通过实例阐述了测试技术要点和注意事项。     

Dilatometric Analysis of Irreversible Volume Change during Phase Transformation in Pure Iron

One assumption underlying the conventional dilatometric analysis based on the lever rule is that the volume of the specimen changes isotropically during phase transformation,which conflicts with the irreversible length change shown in actual measurements.The contribution of this irreversible effect to the dilation data of pure iron up-on heating and cooling was respectively quantified via conversion equations based on lattice parameters.A model con-sidering the elastic strain and creep deformation was established for both the interpretation of the irreversible volume change and the discrepancy between the results measured by a dilatometer and a micrometer.     

Compression Stability of Reversed Austenite in 9Ni Steel

The effect of compressive stress on the stability of reversed austenite in 9Ni steel was investigated by uniaxial and hydrostatic compression.It was found that the uniaxial compressive pressure promoted the γ→α transformation,while the hydrostatic pressure suppressed the γ→α transformation.The pressure dependent transformation behavior can be explained according to thermodynamic analysis.     

Eddy Current Detection of Retained Austenite in Ni-Hard4 Cast Iron

In this article, the nondestructive eddy current (EC) method was used for microstructural evaluation of Ni-hard4 cast iron (NiHCI). Different destabilizing heat treatments were used to produce different microstructures and hardness. The microstructural analysis and hardness measurement were done for conventional characterization of the reference blocks. Then, the EC techniques were applied to evaluate the hardness and microstructural changes by the detection of EC parameters at different frequencies. Metallurgical investigations showed that increasing the destabilizing soaking time and temperature decreases the amount of retained austenite in NiHCIs. The nondestructive evaluation method (NDT) revealed that it can be good correlated between retained austenite amount and hardness with EC parameters. This shows that the EC technique can detect NiHCIs products according to the metallurgical characterization with good accuracy.     

Critical assessment 29: TRIP-aided bainitic ferrite steels

Transformation-induced plasticity (TRIP)-aided bainitic ferrite steels developed for automotive applications have attractive mechanical properties such as ductility, formability, toughness, fatigue strength and delayed fracture strength. These mechanical properties are principally associated with a ductile lath-structure matrix and the strain-induced transformation of the metastable-retained austenite films of 3–20 vol.%. In this paper, data on the microstructural and mechanical properties of the low-carbon TRIP-aided bainitic ferrite steels are critically assessed, as well as their deformation mechanism.     

Multiphase microstructure and property of low carbon bainite/ferrite steel by quenching and carbon partitioning

The multiphase microstructure and properties of low carbon steel by IQ&PB process under different partitioning temperatures and time were studied by means of SEM, EPMA, XRD and tensile testing. Results show that the microstructures of experimental steel consist of ferrite, bainite and retained austenite. With the increase of the holding time in two- phase region, the C and Mn contents apparently increase in martensite which transformed from austenite, and C and Mn content are 1. 47 times and 1. 16 times than average value of the base. With the decrease of the quenching and partitioning temperature, the volume fraction of the bainite increases, the microstructure is refined, and the content of M/A islands increases. With the temperature and time increased, the volume fraction of retained austenite at room temperature increases, the tensile strength decreases, the total elongation increases, and work hardening behavior occurrs continuously. When the partitioning time is 30min and the partitioning temperature is 400??, the tensile strength of the steel is 1107MPa, the elongation is 24%, the product of strength and elongation reaches above 26568MPa??%.     

The Effect of Size and Shape of Austenite Grains on the Mechanical Properties of a Low‐Alloyed TRIP Steel

The effects of size and shape of austenite grains on the extraordinary hardening of steels with transformation induced plasticity (TRIP) have been studied. The deformation and transformation of austenite was followed by interrupted ex situ bending tests using electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). A finite element model (FEM) was used to relate the EBSD based results obtained in the bending experiments to the hardening behavior obtained from tensile experiments. The results are interpreted using a simple rule of mixture for stress partitioning and a short fiber reinforced composite model. It is found that both, the martensite transformation rate and the flow stress difference between austenite and martensite significantly influence the hardening rate. At the initial stage of deformation mainly larger grains deform, however, they do not reach the same strain level as the smaller grains because they transform into martensite at an early stage of deformation. A composite model was used to investigate the effect of grain shape on load partitioning. The results of the composite model show that higher stresses develop in more elongated grains. These grains tend to transform earlier as it is confirmed by the EBSD observations.