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.     

Influence of Pre‐Straining and Bake Hardening on the Strain Rate Sensitivity of Automotive Sheet Steels

The present investigation deals with the influence of pre‐straining with or without bake hardening on the strain rate sensitivity of automotive sheet steels in typical crash conditions. The strain rate sensitivity m has been determined by means of dynamic tensile tests in the strain rate range 0.005‐1000 s^?1 and in the temperature range 233‐373K. A bake hardening heat treatment at 170 °C for 20 min without pre‐straining does not influence the m‐value in comparison to the base material condition. A small pre‐straining near plane strain condition, as commonly found in outer door panels, or a 10% uniaxial, plane strain and biaxial pre‐straining, as typically used in formed automotive crash components, without bake hardening does not affect the m‐value of sheet steels in comparison to the base material condition. Uniaxial 2% to 10% pre‐straining, longitudinal or transverse to rolling direction with subsequent bake hardening, does not clearly change the m‐value in comparison to the base material condition either. Small differences in the strain rate sensitivity behaviour are rather attributed to experimental scattering without real physical background.     

应变速率对管线钢在碱性溶液中应力腐蚀行为的影响

主要研究了不同应变速率对X70管线钢在库尔勒土壤模拟溶液中应力腐蚀行为的影响.采用慢应变速率拉伸实验(SSRT)研究了不同应变速率下X70管线钢在模拟溶液中应力腐蚀开裂行为,并利用扫描电镜分析了不同应变速率下的断面形貌.结果表明,应变速率(特别是局部应变速率)对X70管线钢在库尔勒土壤模拟溶液中应力腐蚀行为有重要影响,应变速率在5×10-7～5×10-6s-1之间有较强的应力腐蚀开裂敏感性,在裂纹扩展不同阶段氢和阳极溶解交互作用相对贡献大小不同.     

Strain Rate Effect on Material Behavior of TRIP‐Steel/Zirconia Honeycomb Structures

A pure TRIP‐steel alloy and a novel zirconia reinforced TRIP‐steel matrix composite were implemented in a 2D square‐celled honeycomb structure fabricated by a paste extrusion method, respectively. In terms of a series of compression tests in out‐of‐plane loading direction the buckling and the pronounced strain hardening behavior of the honeycomb structures are described with regard to different material compositions and varied nominal strain rates. Both the compressive flow behavior and the microstructure evolution in the crushed zones are controlled by the rate of formation of strain‐induced martensite and the ceramic particle/steel matrix interactions. The insertion of magnesia partially‐stabilized zirconia (Mg‐PSZ) particles in the austenitic steel matrix cause an increased yield strength and higher compression stresses up to certain deformations degrees. The limited ductility of the composite materials is a consequence of the rearrangement and fracture of zirconia particles initiating cracks and shear bands during deformation. Consistently, the visible strain rate effects on the mechanical responses of the honeycomb structures are similar to AISI 304L austenitic stainless steel specimens in the form of compact rods. However, at high local strain rates generated in drop weight impact tests a micro‐inertia factor support the failure behavior of the cellular structures.     

The Effects of Strain Rate and Temperature on the Deformation Behavior of Cold‐Rolled TRIP800 Steel

In the present study, the influences of temperature and strain rate on the deformation behavior of cold‐rolled TRIP800 steel were investigated. Microstructural observation and tensile tests were performed and volume fractions of retained austenite were measured at various temperatures and strain rates. The results reveal that both temperature and strain rate affect the volume fractions of retained austenite that transforms into martensite. The strain‐induced transformation of retained austenite is retarded with increasing temperature and the retained austenite becomes more stable against straining. The amount of retained austenite that transforms into martensite is not influenced significantly by strain rate. The variation in mechanical properties with temperature and strain rate was related to the effects of dynamic strain aging, tempering of banite, high temperature softening, and the volume fractions of retained austenite.     

Strain Rate Sensitivity of Pre‐Strained AISI 301LN2B Metastable Austenitic Stainless Steel

The dynamic behavior of AISI 301LN2B (EN 1.4318) metastable austenitic steel grade has been investigated at 296 K by means of servohydraulic tensile and split Hopkinson bar testing in the strain rate range 0.005–1000 s^?1. As delivered, as well as 10% uniaxial, biaxial, and plane strain pre‐strained conditions, without subsequent heat treatment have been tested. A negative strain rate sensitivity is observed in the low strain rate range between 10^?4 and 1–10 s^?1. Pre‐straining reduces the magnitude of the adiabatic tensile strength softening, especially in the plane strain condition with higher triaxility. The thermal activation related dynamic flow stress increase is not dependent on pre‐straining. The γ → α′ induced additional flow stress increase, however, is highly strain rate and pre‐straining sensitive. The amount of pre‐straining determines the overall ductility at fracture, and therefore the adiabatic temperature increase. The pre‐straining stress state influences the amount of α′‐martensite formed before dynamic testing, and consequently the maximum intensity of the TRIP induced flow stress increase by subsequent dynamic testing.     

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.     

In‐situ Synchrotron XRD Analysis of the Effect of Static Strain Aging on the Elasto‐plastic Transition in CMnSi TRIP Steel

In situ synchrotron X‐ray diffraction was used to investigate the martensitic transformation kinetics, lattice straining and diffraction peak broadening in cold‐rolled TRIP steel during tensile testing. Direct evidence of stress‐strain partitioning between different phases, dislocation pinning and differences in yielding behaviour of the different phases were clearly observed. The TRIP steel was subjected to a bake‐hardening treatment and a pronounced static strain aging effect was observed. In the present work, the martensitic transformation kinetics and the elastic micro‐strain evolution for both ferrite and retained austenite during the elasto‐plastic transition are reported with an emphasis on bake‐hardening with and without pre‐straining.