X-Ray Diffraction Study on the Strain Anisotropy and Dislocation Structure of Deformed Lath Martensite

18Ni (300) maraging steel possessing lath martensite structure was deformed by four passes of equal-channel angular pressing (ECAP) at ambient temperature. Line profile analysis (LPA) of X-ray diffraction (XRD) patterns identified strong strain anisotropy and remarkable increases in the relative fraction of screw dislocations after ECAP. The strain anisotropy was reasonably accounted for by the anisotropy of elastic constants. Domination of screw dislocations in the deformed structure was attributed to the preferred annihilation of edge dislocations in the early stages of deformation along with the difficulties for annihilation of screw dislocations by cross slipping. Cobalt addition was mainly assumed to make cross slipping difficult by reducing stacking-fault energy and favoring short-range ordering.     

Correlation of Tensile Properties and Strain Hardening Behavior with Martensite Volume Fraction in Dual-Phase Steels

In the present study, tensile properties, strain hardening and fracture behavior of dual-phase (DP) steels were correlated with martensite volume fraction (V _M ). A series of DP steels with different amounts of V _M (28–50 %) were produced by cold rolling and subsequent intercritical annealing of a ferrite-pearlite starting structure. Hardness and tensile tests results of DP steels showed that variation of hardness, uniform elongation and total elongation with V _M was linear and obeyed the rule of mixtures, whereas yield strength and ultimate tensile strength exhibited a nonlinear variation with V _M . Analysis of strain hardening behavior of DP steels by the Hollomon analysis showed two stages of strain hardening corresponding to ferrite deformation and co-deformation of ferrite and martensite, respectively. The strain hardening exponent of first stage (n _I ) increased with increasing V _M , while the strain hardening exponent of second stage (n _II ) as well as transition strain between the deformation stages decreased.     

Model of Precipitation Kinetics Induced by Strain for Microalloyed Steels

In microalloyed steels, static recrystallisation is temporarily inhibited by precipitation which is occurring at the same time. A high number of microalloyed steels containing various combinations of carbon, nitrogen and precipitate forming elements like V, Nb and Ti were recrystallised at different temperatures and strain rates. From these results recrystallisation‐precipitation–time‐temperature (RPTT) diagrams were established. The influence of grain size and strain rate on the RPTT diagrams was studied. The precipitation kinetics were mathematically described for isothermal conditions and converted to cooling conditions, which enables an application to hot rolling. Under cooling conditions, completion of recrystallisation is prevented, especially for Nb alloyed steels.     

Influence of Strain Rate,Temperature, Plastic Strain,and Microstructure on the Strain Rate Sensitivity of Automotive Sheet Steels

This work identifies the influence of strain rate, temperature, plastic strain, and microstructure on the strain rate sensitivity of automotive sheet steel grades in crash conditions. The strain rate sensitivity m has been determined by means of dynamic tensile tests in the strain rate range 10^?3–200 s^?1 and in the temperature range 233–373 K. The dynamic flow curves have been tested by means of servohydraulic tensile testing. The strain rate sensitivity decreases with increasing plastic strain due to a gradual exhausting of work hardening potential combined with adiabatic softening effects. The strain rate sensitivity is improved with decreasing temperature and increasing strain rate, according to the thermally activated deformation mechanism. The m‐value is reduced with increasing strength level, this decrease being most pronounced for steels with a yield strength below 400 MPa. Solid solution alloying with manganese, silicon, and especially phosphorous elements lowers the strain rate sensitivity significantly. Second phase hardening with bainite and martensite as the second constituent in a ferritic matrix reduces the strain rate sensitivity of automotive sheet steels. A statistical modeling is proposed to correlate the m‐value with the corresponding quasistatic tensile flow stress.     

Influence of the Martensitic Transformation on the Microscale Plastic Strain Heterogeneities in a Duplex Stainless Steel

The influence of the martensitic transformation on microscale plastic strain heterogeneity of a duplex stainless steel has been investigated. Microscale strain heterogeneities were measured by digital image correlation during an in situ tensile test within the SEM. The martensitic transformation was monitored in situ during tensile testing by high-energy synchrotron X-ray diffraction. A clear correlation is shown between the plasticity-induced transformation of austenite to martensite and the development of plastic strain heterogeneities at the phase level.     

Strain Hardening Behavior of High Performance FBDP,TRIP and TWIP Steels

Based on n‐value differential equation and microstructural observation, strain hardening behaviors of FBDP, TRIP, and TWIP steels during uniaxial tension were investigated. TRIP steel exhibits both superior strength and ductility than FBDP steel, and TWIP steel displays much higher total and uniform elongations in comparison to FBDP and TRIP steels. The instantaneous n values of FBDP and TRIP steels increase at small strains, reach a maximum value, smoothly decrease at higher strains, and then rapidly drop up to the specimen rupture. The strain hardening of TRIP steel persists at higher strains where that of FBDP steel begins to diminish. TWIP steel exhibits gradually increased instantaneous n values over the whole uniform plastic deformation, implying that TWIP steel shows a much larger strain hardening capability than FBDP and TRIP steels.     

汽车用高强度钢的高应变速率

0 前言 通过Split Hopkinson Bar(SHB:张力和压缩试验系统)试验,研究200～100000s-1范围内高应变速率材料的机械性能. 高应变速率是先进的高强度钢(AHSS)的重要特性之一,这是因为高强度钢大多用于易冲撞部位.Ispat Inland公司已经着手一项使一组AHSS钢产生高应变值的研究项目,其中包括双相钢和TRIP钢.由于对钢的高应变速率试验缺乏经验,而且没有标准的试验方法和可行的试验步骤,为此研究出了试验方法及其数据处理过程.本文对两种主要试验方法--伺服液压试验系统和Split Hopkinson Bar进行了比较,提供数据整理和曲线调整的过程,最后研讨了所试验的AHSS钢的高应变速率性能.     

Interpretation of Dynamic Strain Aging in an Intercritical Annealed Steel by Dislocation Multiplication Induced by Martensitic Transformation

Metallurgical and Materials Transactions A - To elucidate the dynamic strain aging (DSA) mechanism that causes serrated flow in the tensile curves of an intercritical annealed steel with retained...     

Thermodynamically Based Prediction of the Martensite Start Temperature for Commercial Steels

A thermodynamic method for predicting the martensite start temperature of commercial steels is developed. It is based mainly on information on M _s from binary Fe-X systems obtained from experiments with very rapid cooling, and M _s values for lath and plate martensite are treated separately. Comparison with the experimental M _s of several sets of commercial steels indicates that the predictive ability is comparable to models based on experimental information of M _s from commercial steels.     

Effects of Twin-Dislocation and Twin-Twin Interactions on the Strain Hardening Behavior of TWIP Steels

 In the present paper, tensile tests of Fe-30Mn-5Si -2Al steel were carried out for different strains of 0.05, 0.14, 0.26, and up to the strain-to-failure in order to observe the evolution of microstructure during deformation and investigate the strain hardening behavior. Three-stage strain hardening behavior was observed in this steel during tensile test. In stage I, planar dislocation structure was observed by TEM to be the main deformation mechanism, and low strain hardening rate exponent was exhibited. Primary deformation twinning occurred in stage II, and the strain hardening rate exponent increased due to the blockage of dislocations’ motion by twin boundaries. In stage III, the strain hardening rate exponent had increased to be higher than 0.5. The obstacle effect of twin boundaries and twin-twin interaction had been observed by TEM, and the interactions between primary and secondary twins were found to cause the additional hardening in addition to the obstacle effect on dislocations’ motion, which led to the twinning induced plasticity effect in the later stage of deformation.     

Characterization and Prediction of Flow Behavior in High-Manganese Twinning Induced Plasticity Steels: Part II. Jerky Flow and Instantaneous Strain Rate

The jerky and smooth flow curves in high-manganese twinning induced plasticity (TWIP) steels were investigated by comparing Fe-Mn-C and Fe-Mn-Al-C systems. The pronounced serrations on the flow curves of Fe-Mn-C TWIP steel, produced during tensile testing at 300 K (27 °C) and 373 K (100 °C), were shown to be the result of localized high-temperature Portevin Le-Chatelier (PLC) bands moving across the gage length throughout the deformation. The speed of the PLC bands and their temperature effects were found to be strongly dependent on the applied strain rate, which was controlled by adjusting the cross-head speed of the tensile testing machine. The localized temperature-dependent stacking fault energy (SFE) variations resulting from the PLC effect and adiabatic heating were analyzed and compared for both slow and fast deformation rates. The instabilities in the measured logarithmic strain values caused by jerky flow could cause the local strain rate to deviate systematically from the targeted (applied) strain rate. These instabilities are better observed by calculating the instantaneous strain rate (ISR) values for each instant of deformation along the entire gage length. Finally, a new type of diagram was developed by plotting the true stress against the ISR values. From the diagram, the onset of different mechanisms, such as deformation twinning, nonpronounced, and pronounced serrations, could be marked precisely.     

Numerical Investigation of Influence of the Martensite Volume Fraction on DP Steels Fracture Behavior on the Basis of Digital Material Representation Model

Development of the methodology for creating reliable digital material representation (DMR) models of dual-phase steels and investigation of influence of the martensite volume fraction on fracture behavior under tensile load are the main goals of the paper. First, an approach based on image processing algorithms for creating a DMR is described. Then, obtained digital microstructures are used as input for the numerical model of deformation, which takes into account mechanisms of ductile fracture. Ferrite and martensite material model parameters are evaluated on the basis of micropillar compression tests. Finally, the model is used to investigate the impact of the martensite volume fraction on the DP steel behavior under plastic deformation. Results of calculations are presented and discussed in the paper.     

The Role of Plasticity in the Transverse Lattice Strain Evolution of a Martensitic Steel

In this work, in-situ neutron diffraction measurements are performed for a martensitic steel in conjunction with crystal plasticity analysis. The results indicate that heterogeneous plastic contraction on transverse {200} grains is responsible for the observed nonlinear lattice strain evolution. The effect of slip properties on plastic contraction is computationally identified.