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1.
The resistance of austenite to plastic deformation (austenite flow stress) was measured using a high temperature tensile apparatus.
The flow stress was then correlated with the Ms temperature as determined magnetically during subsequent cooling. In one part of the study, the flow stress of the austenite
was varied only by work hardening the austenite, allowing the austenite composition, which is known to affect Ms, to be held constant. A decrease in Ms temperature with increasing austenite flow stress was observed. This observation was supported by the observation of a decrease
in the amount of austenite transformed at 25°C. In the other part of the study, a series of alloy steels of different chemical
compositions was tested. A decrease in Ms temperature with increasing austenite flow stress was again observed. Strengthening of austenite by plastic deformation was
shown not to change the chemical driving force for transformation. The effect of deformation on Ms temperature thus results from its influence on either the nucleation or the growth process. While the effect of austenite
deformation on martensite nucleation is uncertain, specific nucleation models can account for only approximately one-third
of the nonchemical free energy change which accompanies transformation. A proposal, consistent with the observations, was
made that the energy expended for the deformation of austenite during martensite plate growth could reasonably account for
a substantial part of the nonchemical free energy change. 相似文献
2.
Multiphase TRIP steels are a relatively new class of steels exhibiting excellent combinations of strength and cold formability, a fact that renders them particularly attractive for automotive applications. The present work reports models regarding the prediction of the stability of retained austenite, the optimisation of the heat‐treatment stages necessary for austenite stabilization in the microstructure, as well as the mechanical behaviour of these steels under deformation. Austenite stability against mechanically‐induced transformation to martensite depends on chemical composition, austenite particle size, strength of the matrix and stress state. The stability of retained austenite is characterized by the MσS temperature, which can be expressed as a function of the aforementioned parameters by an appropriate model presented in this work. Besides stability, the mechanical behaviour of TRIP steels also depends on the amount of retained austenite present in the microstructure. This amount is determined by the combinations of temperature and temporal duration of the heat‐treatment stages undergone by the steel. Maximum amounts of retained austenite require optimisation of the heat‐treatment conditions. A physical model is presented in this work, which is based on the interactions between bainite and austenite during the heat‐treatment of multiphase TRIP steels, and which allows for the selection of treatment conditions leading to the maximization of retained austenite in the final microstructure. Finally, a constitutive micromechanical model is presented, which describes the mechanical behaviour of multiphase TRIP steels under deformation, taking into account the different plastic behaviour of the individual phases, as well as the evolution of the microstructure itself during plastic deformation. This constitutive micromechanical model is subsequently used for the calculation of forming limit diagrams (FLD) for these complex steels, an issue of great practical importance for the optimisation of stretch‐forming and deep‐drawing operations. 相似文献
3.
Warm deformation tests were performed using a kind of tubby heater. The microstructures and mechanical properties of an Fe-C-Mn-Si multiphase steel resulting from different warm deformation temperatures were investigated by using LOM (light optical microscopy), SEM and XRD. The results indicated that the microstructure containing polygonal ferrite, granular bainite and a significant amount of the stable retained austenite can be obtained through hot deformation and subsequent austempering. Warm deformation temperature affects the mechanical properties of the hot rolled TRIP steels. Ultimate tensile strength balance reached maximum (881 MPa) when the specimen was deformed at 250 ℃, and the total elongation and strength-ductility reached maximum (38% and 28614 MPa·%, respectively) at deforming temperature of 100 ℃. Martensite could nucleate when austenite was deformed above Ms, because mechanical driving force compensates the decrease of chemical driving force. The TRIP effect occurs in the Fe-C-Mn-Si multiphase steel at deforming temperature ranging from 15 to 350 ℃. The results of the effects of warm deformation on the mechanical properties of the Fe-C-Mn-Si multiphase steel can provide theoretical basis for the applications and the warm working of the hot rolled TRIP sheet steels in industrial manufacturing. 相似文献
4.
利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、电子探针(EPMA)、X射线衍射仪(XRD)、室温拉伸等手段, 通过两相区保温-淬火(IQ)、两相区形变后保温-淬火(DIQ)、两相区保温-淬火-配分-贝氏体区等温(IQ&PB)及两相区形变后保温-淬火-配分-贝氏体区等温(DIQ&PB)热处理工艺, 研究高温形变对室温组织、性能、残余奥氏体稳定性的综合影响作用.结果表明, 经15%的压缩形变后铁素体中位错密度由0.290×1014增加至1.286×1014 m-2, 马氏体(原奥氏体)中C、Cu元素富集浓度提高, 高温形变产生位错增殖对元素配分有明显促进作用.DIQ&PB工艺下, 形变后贝氏体板条尺寸变短且宽度增加0.1 μm左右, 贝氏体转变量较未变形时增加14%, 多边形铁素体尺寸明显减小.力学性能方面, 两相区形变热处理后抗拉强度增加132.85 MPa, 断后伸长率增加7%, 强塑积可达25435 MPa·%.形变后残余奥氏体体积分数由7.8%提高到8.99%, 残余奥氏体中碳质量分数由1.05%提高到1.31%. 相似文献
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A model for the stability of dispersed austenite in low alloy triple-phase steels has been developed. The model was based on the dislocation dissociation model for classical heterogeneous martensitic nucleation by considering stress effects on the nucleation site potency distribution. The driving force for martensitic transformation has been calculated with the aid of computational thermodynamics. The model allows for the effects of chemical composition of austenite, mean austenite particle size, yield strength of the steel and stress state on austenite stability. Chemical enrichment in C and Mn, as well as size refinement of the austenite particles lead to stabilization. On the contrary, the increase in the yield strength of the steel and triaxiality of the stress state lead to destabilization. The model can be used to determine the microstructural characteristics of the austenite dispersion, i.e. chemical composition and size, for optimum transformation plasticity interactions at the particular stress state of interest and can then be useful in the design of low-alloy triple-phase steels. 相似文献
8.
P. Jacques F. Delannay X. Cornet Ph. Harlet J. Ladriere 《Metallurgical and Materials Transactions A》1998,29(9):2383-2393
Dual-phase and transformation-induced plasticity (TRIP)-assisted multiphase steels are related families of high-strength formable
steels exhibiting excellent mechanical characteristics. This study shows how a ferrite-bainite-martensite microstructure containing
retained austenite can improve the mechanical properties of a cold-rolled low-carbon, low-silicon steel. Such a multiphased
microstructure is obtained by a heat treatment involving intercritical annealing followed by a bainite transformation tempering.
Depending on the heat-treatment parameters, the samples present a variety of microstructures. Due to the presence of retained
austenite, some samples exhibit a TRIP effect not anticipated with such a low silicon content. A composite strengthening effect
also results from the simultaneous presence of a ductile ferrite matrix with bainite and martensite as hard second phases.
A true stress at maximum load of 800 MPa and a true uniform strain of 0.18 can be obtained by forming a ferrite-bainite-martensite
microstructure containing up to 10 pct of retained austenite. These properties correspond to a favorable evolution of work
hardening during plastic deformation. 相似文献
9.
Dong Woo Suh Joo Hyun Ryu Min Sung Joo Hong Seok Yang Kyooyoung Lee H. K. D. H. Bhadeshia 《Metallurgical and Materials Transactions A》2013,44(1):286-293
The manganese concentration of steels which rely on transformation-induced plasticity is generally less than 2 wt pct. Recent work has highlighted the potential for strong and ductile alloys containing some 6 wt pct of manganese, but with aluminum additions in order to permit heat treatments which are amenable to rapid production. However, large concentrations of aluminum also cause difficulties during continuous casting. Alloy design calculations have been carried out in an effort to balance these conflicting requirements, while maintaining the amount of retained austenite and transformation kinetics. The results indicate that it is possible by adjusting the carbon and manganese concentrations to reduce the aluminum concentration, without compromising the mechanical properties or transformation kinetics. The deformation-induced transformation of retained austenite is explained quantitatively, for a range of alloys, in terms of a driving force which takes into account the very fine state of the retained austenite. 相似文献
10.
TRIP‐assisted multiphase steels have been thoroughly studied in the cold‐rolled and annealed state. The effects of hot‐rolling conditions on these steels are much less studied even though these are of major importance for industrial practice. This study was carried out in order to understand the effect of the hot deformation of austenite on the tensile properties of TRIP‐assisted multiphase steels. Two different compositions and microstructures are investigated. The first one is a low‐carbon steel (mass content of 0.15 %) with a microstructure consisting of an intercritical ferritic matrix, bainite and retained austenite. The second one is a medium‐carbon steel (mass content of 0.4 %) that consists of bainite and retained austenite. Both steels were deformed to various strain levels below the non‐recrystallisation temperature of austenite. The medium carbon steel was deformed in the fully austenitic temperature range whereas the low‐carbon steel was deformed in the intercritical temperature range. In both cases, the prior hot deformation of austenite brings about a large enhancement of the work‐hardening capabilities. In the case of the medium‐carbon steel, this effect can be attributed to a much larger TRIP effect taking place during straining. In the case of the low‐carbon steel, the improvement of the work‐hardening behaviour was attributed to an Interaction between the martensitic transformation and the dislocations already present within the surrounding ferrite matrix. 相似文献
11.
A mesoscale study on the thermodynamic effect of stress on martensitic transformation 总被引:1,自引:0,他引:1
The effect of stress on martensitic transformation (MT) is addressed with special emphasis on the mechanical driving force
(MDF) for triaxial stress states. The mechanical driving force appears to be additional to the chemical driving force in thermodynamic
transformation con-ditions derived from a Gibbs free energy formulation for stressed solids undergoing MT. The thermodynamic
criterion of Patel and Cohen predicts the change in martensite start temperature if MT occurs in a stress field. This criterion
is extended from uniaxial to triaxial stress states and is discussed in the light of emerging microstresses. As a source of
microstress, the elastic anisotropy of single crystals is taken into account. Its influence on the martensite start temper-ature
is investigated by mesomechanical finite-element modeling. The critical stress for the start of transformation occurring in
a stress field is calculated from a transformation condition and compared with results based on statistical theories for stress-assisted
nucleation. In the context of martensitic transformation and mechanical effects, the MDF accounts for the Magee effect. The
range of temperature for which the Magee effect has an influence on the macroscopic deformation behavior of a specimen is
determined in dependence of the level of uniaxial applied stress. Finally, a constitutive equation in incremental form for
transformation-induced plasticity (TRIP) derived within the continuum-thermodynamics framework is suggested. 相似文献
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The strength, annealing behavior, and microstructure of reverted austenite has been measured in an Fe-31 pct Ni-0.03 pct C
alloy that was plastically deformed in the martensitic state prior to the reversion to austenite. Mechanical properties of
reverted austenite (e.g., austenite formed by the reverse martensite shear transformation) were measured as a function of the amount of prior deformation,
heating and cooling rates to the reversion temperature, austenitizing temperature and time, repetitive cycling from martensite
to reverted austenite, and prereversion heat treatments. The results showed that 80 pet prior deformation increases the yield
strength of reverted austenite about 30 pct. Along with this strengthening, the dislocation configuration changes from a plate-like
fine structure with a random array of tangled dislocations in reverted samples without prior deformation to a equiaxed fine
structure with a high density of tangled dislocations within the fine structure in samples with 80 pct deformation prior to
reversion. Although smaller amounts of prior deformation (20 pct) have only a small effect on the strength of the reverted
austenite, this amount of prior deformation significantly increases the driving force for recrystallization. The results are
explained on the basis that the prior deformation and the reversion process produce separate components to the strength and
annealing behavior.
E. GOLD, formerly with the Aeronutronic Division, Philco-Ford Corporation, Newport Beach, Calif. 相似文献
15.
Hermann Vetters 《国际钢铁研究》1996,67(10):408-411
The stress induced martensitic transformation in the upper metastable intermediate state of γ-α transformation in ferrous materials, structured as ferritic bainite, is discussed. The fibrous structured ferritic bainite consists of retained austenite and ferrite platelets growing in the [111]α//[101]γ direction. The ferrite growth Induces carbon enrichment of the adjacent austenite at the phase boundaries. Strengthening at high stress levels up to the yield point causes dislocation tangles in the ferrite fibre and the formation of shear bands crossing each other in the retained austenite. At lower carbon contents of the austenite, lath martensite precipitates at the shear band intersections and at high shear band densities martensite blocks are observed. In carbon enriched austenite martensite lenses formed by shear processes have been observed. At alternating loading conditions, exceeding the stress level for athermic martensite formation, various shear planes are activated forming characteristic patterns of plate martensite. 相似文献
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17.
S. Kajiwara 《Metallurgical and Materials Transactions A》1986,17(10):1693-1702
In order to elucidate roles of dislocations and grain boundaries in martensite nucleation, the transformation temperature
(Ms) of specimens austenitized at various temperatures and subjected to prestrain has been measured, using Fe-Ni, Fe-Ni-C, and
Fe-Cr-C alloys. It is concluded that the plastic accommodation, in austenite, of the shape strain of the transforming martensite
is a vital step in the nucleation event. Any factors impeding such plastic accommodation, such as the lack of dislocations,
work hardening, and grain refinement, suppress the transformation. Contrary to the general belief, dislocations themselves
do not act as favorable nucleation sites. Grain boundaries provide nucleation site, but only certain types of grain boundaries
are qualified to be potential nuclei. A quantitative analysis shows that the increasing difficulty for the plastic accommodation
with decreasing grain size is the main factor to depress Ms in fine-grained specimens. 相似文献
18.
The effect of Nb concentration on the transformation from austenite to bainitic ferrite has been examined under simulated strip casting conditions. Nb concentration was found to delay the nucleation of bainite, but accelerated its growth. It is suggested that the delay in nucleation increases the driving force for transformation, which results in an increase in the growth rate of the bainite. The bainite/austenite interfaces are proposed to move too quickly to suffer appreciable solute drag. 相似文献
19.
J. Van Slycken P. Verleysen J. Degrieck L. Samek B. C. de Cooman 《Metallurgical and Materials Transactions A》2006,37(5):1527-1539
High-strength, low-alloy transformation-induced plasticity (TRIP) steels are advanced multiphase steel grades that combine
high-strength levels with an excellent ductility, making them ideally suited for application in crash-relevant parts of automotive
car bodies. The enhanced plastic hardening and deformability are due to a complex interaction between the microstructural
phases and to the transformation of metastable austenite to martensite during plastic deformation. During high-strain-rate
loading, not only the material but also the transformation will be influenced by adiabatic heating. The impact-dynamic properties
of CMnAl- and CMnSi-TRIP steels were determined in the range of 500 to 2000 s−1 using a split Hopkinson tensile bar (SHTB) setup. Bake-hardening treatments were applied to study the effect of strain aging.
The experiments show that strain-rate hardening is superior to thermal softening: yield stresses, deformation, and energy
dissipation increase with the strain rate. Phenomenological material models were investigated to describe the strain-rate
and temperature-dependent behavior of TRIP steels. Both the Johnson-Cook model and an extended version of the Ludwig model
were found to give good agreement with the experimental data. 相似文献
20.
Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of ε’ (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shear-band intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fee-bee chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fee, bec, and hep entropy differences through proper compositional control. 相似文献