含磷TRIP钢的CCT图

为了探索合金元素在TRIP钢相变过程中的重要作用,利用金相、显微硬度等方法研究了四种不同合金成分C-Mn-Al-PTRIP钢的CCT图.结果表明:Al元素强烈地缩小奥氏体相区,提高Ac₃与Ms;Al元素促使CCT图左移和上移.P元素能够阻碍碳化物生成,当钢中P质量分数达到0.14%时,能显著地将CCT图中的珠光体区与贝氏体区右移;P元素对铁素体相变和马氏体相变没有显著的影响.结果还显示出随着冷却速率的增加,材料的显微硬度随之增加.对于每一种成分,超过其临界冷却速率时,将得到完全的马氏体组织.     

Stress-assisted isothermal martensitic transformation: Application to TRIP steels

Low-temperature plastic flow in TRIP steels has been found to be controlled by stress-assisted isothermal martensitic transformation. For these conditions, the thermodynamics and kinetic theory of martensitic transformations leads directly to constitutive relations predicting the dependence of flow stress on temperature, strain, strain-rate, and stress-state, consistent with the observed behavior of TRIP steels. Guidelines are obtained for the control of temperature sensitivity, σ -ɛ curve shape, and stress-state effects to achieve novel mechanical properties.     

含Nb低碳Si-Mn系TRIP钢的连续冷却转变

利用Gleeble-1500热应力/应变模拟机研究了Nb对低碳Si-Mn系TRIP钢连续冷却转变的影响,测定了含nb和不含Nb两种低碳Si-Mn系TRIP钢的连续冷却转变(CCT)曲线,结果表明:Nb的加入使得静态CCT曲线上移;动态CCT曲线下移.试验结果可为TRIP钢的TMCP(Thermo-Mechnical Control Processing)工艺提供理论依据.     

On the toughness increment associated with the austenite to martensite phase transformation in TRIP steels

The fracture toughness of a high carbon TRIP alloy, deformed approximately 75 pct at 460°C was investigated over a range of temperatures from ?196°C to 200°C. Two distinct temperature regimes were present: a low temperature regime where martensite formed during fracturing and a high temperature regime where no martensite formed. The toughness values of the low temperature regime were higher than the extrapolated values of the high temperature regime indicating that the transformation makes a positive contribution to the fracture toughness of TRIP alloys. For the alloys used in this investigation the room temperature plane strain fracture toughnessK _IC was on the order of 95 ksi \(\sqrt {in} \) or in terms of the crack extension forceG _IC, 274 in.-lb per sq in. The fracture mode was cleavage and the extraordinary toughness for this mode of crack extension is attributed to the energy absorbed by the simultaneous phase transformation. The contribution due to the phase transformation was determined to be in the range 37 to 57 \(\sqrt {in} \) in terms of stress intensity or 168 to 232 in.-lb. per sq in. in terms of crack extension force using the extrapolation technique. The results obtained using the extrapolation technique represent the first experimental determination of the toughness contribution associated with the austenite to martensite phase transformation in TRIP alloys. An expression for the toughness associated with the transformation was derived using fundamental fracture mechanics relations. This expression, which contains easily measured parameters, was used to calculate the toughness contribution due to the phase transformation and the results were in good agreement with the experimental values.     

Transformation behavior of TRIP steels

True-stress (σ), true-strain (ε) and volume fraction martensite(f) were measured during both uniform and localized flow as a function of temperature on TRIP steels in both the solution-treated and warm-rolled conditions. The transformation curves(f vs ε) of materials in both conditions have a sigmoidal shape at temperatures above M_s ^σ (maximum temperature at which transformation is induced by elastic stress) but approach initially linear behavior at temperatures below M_s ^σ where the flow is controlled by transformation plasticity. The martensite which forms spontaneously on cooling or by stress-assisted transformation below M_s ^σ exhibits a plate morphology. Additional martensite units produced by strain-induced nucleation at shear-band intersections become important above M_s ^σ. Comparison of σ-ε andf-ε curves indicate that a “rule of mixtures” relation based on the “static” strengthening effect of the transformation product describes the plastic flow behavior reasonably well above M_s ^σ, but there is also a dynamic “transformation softening” contribution which becomes dominant below M_s ^σ due to the operation of transformation plasticity as a deformation mechanism. Temperature sensitivity of the transformation kinetics and associated flow behavior is greatest above M_s ^σ. Less temperature-sensitive TRIP steels could be obtained by designing alloys to operate with optimum mechanical properties below M_s ^σ.     

2种含铝TRIP钢连续冷却转变动力学分析

用热膨胀法,通过对含1.5%w（Al）和含w（Al）1.0%＋w（Si）0.5%的2种TRIP钢的显微组织观察,绘制了2种不同w（Al）、w（Si）的TRIP钢的静态CCT曲线,研究了含Al TRIP钢在冷却过程中的相转变,分析了冷却速率及不同w（Al）对微观组织和贝氏体转变动力学的影响。结果表明,Al元素加速TRIP钢冷却时铁素体转变和贝氏体转变,但是推迟珠光体转变。     

合金元素和工艺参数对热轧TRIP钢动态相变的影响

利用Gleeble热模拟试验机进行单轴压缩试验,研究了C-Mn-Si TRIP钢和C-Mn-Al-Si TRIP钢过冷奥氏体形变过程的组织演变,分析了合金元素和工艺参数对过冷奥氏体动态相变的影响.与等温相变相比,C-Mn-Si钢和C-MnAl-Si钢动态相变动力学明显加快.与C-Mn-Si钢相比,用质量分数约1%的Al替代Si后,C-Mn-Al-Si钢的A₃温度明显提高,在相同变形工艺条件下C-Mn-Al-Si钢过冷奥氏体动态相变较易发生,而C-Mn-Si钢动态相变得到的铁素体晶粒比较细小.减小动态相变前奥氏体晶粒尺寸,有利于过冷奥氏体动态相变的进行.提高过冷奥氏体形变时的变形温度或应变速率均对动态相变产生一定的阻碍作用,但影响不显著.     

相变诱导塑性汽车用钢的发展现状与趋势

论述了相变诱导塑性(TRIP)钢的发展现状及其在汽车工业上的应用,重点讨论了 TRIP 效应的机理及TRIP钢性能的影响因素。介绍了2种采用新型工艺(低温贝氏体转变和淬火-碳分配工艺)的 TRIP 钢,并且通过对TRIP钢研究的最新数据,对比了2种工艺下TRIP钢的高速拉伸性能;最后对汽车用 TRIP 钢的研究方向进行了展望。     

低碳Si-Mn系TRIP钢的动态拉伸性能

在气动式间接杆杆型冲击拉伸实验机上对工业生产的两种低碳Si-Mn系TRIP钢不同应变率下的高速冲击拉伸性能进行了研究,并和静态拉伸性能进行了比较.结果表明,两种钢的室温拉伸性能随应变率变化具有相同趋势,但动态下的应变率敏感性比静态下的要高得多.由于TRIP钢组织中残余奥氏体的变形诱发向马氏体的转变显著改善了材料的塑性.     

Experimental determination of the stability of retained austenite in low alloy TRIP steels

The stability of retained austenite is the most important parameter controlling the transformation plasticity effects in multiphase low alloy TRIP steels. In this work the thermodynamic stability of the retained austenite has been determined experimentally by measuring the M^σ_s temperature as a function of bainite isothermal transformation (BIT) temperature and time in two low alloy TRIP steels. A single-specimen temperature-variable tension test technique (SS-TV-TT) has been employed, which allowed to link the appearance of yield points in the stress-strain curve with the mechanically-induced martensitic transformation of the retained austenite. The results indicated that the M^σ_S temperature varies with BIT temperature and time. Higher austenite stability is associated with a BIT temperature of 400°C rather than 375°C. In addition, the chemical stabilization of the retained austenite associated with carbon enrichment from the growing bainite is lowered at short BIT times. This stability drop is due to carbide precipitation and comes earlier in the Nb-containing steel. At longer BIT times the retained austenite dispersion becomes finer and its stability rises due to size stabilization. The experimental results are in good agreement with model predictions within the range of anticipated carbon enrichment of the retained austenite and measured austenite particle size.     

高锰TRIP/TWIP钢变形行为的研究进展

高强度高塑性是汽车用钢发展的主要趋势.Fe-Mn-Al-Si系TRIP/TWIP钢、Fe-Mn-C系TWIP钢和Fe-Mn-Al-C钢具有高的强度、优良的塑性和成形性,为新一代汽车材料.近年来,这些奥氏体汽车用钢的研究与开发受到了高度重视.本文对高锰TRIP/TWIP钢的组织性能、晶体学行为、强韧化机制、应变硬化行为和高速变形方面的研究工作进行了综述.     

Simulation of intercritical annealing in low‐alloy TRIP steels

A coupled thermodynamic/kinetic calculation of austenite formation during intercritical annealing of low‐alloy TRIP steels is presented. The simulation was performed with the use of Dictra computational kinetics software, which employs a procedure for the numerical solution of the coupled diffusion equations involved, as well as mobility databases for the retrieval of the appropriate kinetic data. Calculated results are compared with available experimental data, in order to evaluate the model. Simulation results, regarding the amount and composition of austenite, the rate of transformation and the effect of annealing temperature and heating conditions, are presented and discussed. It is concluded that the simulation can assist the design of the intercritical annealing in these steels.     

Tensile properties of 0.05 to 0.20 Pct C TRIP steels

The uniaxial tensile properties of a series of TRIP steels of varying carbon contents and processing histories were determined over a wide range of test temperatures. The yield strengths at room temperature varied both with the deformation temperature (over the range 250° to 550°C) and with the carbon content (0.05 to 0.20 pct). Possible reasons for these variations are advanced. For all steels, the −100°C yield strengths were substantially lower than the 100°C yield strengths. The minima and maxima in the yield strengths vs temperatures curves were especially pronounced for the steels processed at the lowest deformation temperatures. Both the rate of work hardening and the elongation were influenced by the strain-induced austenite-to-martensite transformation. The rate of strain hardening and the rate of production of strain-induced martensite (per unit strain) increased with decreasing temperature. Formerly Graduate Student, University of California, Berkeley, Calif.     

Prediction of transformation textures in steels

Both discrete and continuous mathematical formalisms are employed to simulate texture evolution during the γ-to-α transformation in steels. Five f.c.c. NiCo alloys with different stacking fault energies (SFE's) were previously cold rolled to four reductions (40, 70, 90 and 95%) and their textures characterized by the orientation distribution function (ODF) method. The corresponding b.c.c. transformation textures are calculated from these experimental textures according to three different orientation relationships. The ODF's derived from the Bain relation are much sharper than the ones deduced from the Kurdjumov-Sachs (KS) or the Nishiyama-Wassermann (NW) relations, although the general trends of the three families of textures are similar. Ferrite textures determined on controlled rolled steels, heavily deformed in the unrecrystallized γ region, agree reasonably well with the b.c.c. textures calculated, according to the KS relationship, from the NiCo alloy with similar SFE. The two major ferrite components, namely the {332} 〈113〉 and {113} 〈110〉, are shown to originate from the three main orientations of cold rolled f.c.c. material, i.e. the {112} 〈111〉 (Cu), {110} 〈112〉 (Bs) and {123} 〈634〉 (S). Such ferrite formation from heavily deformed austenite follows the KS relationship without any variant selection. By contrast, the texture of martensite produced from deformed austenite involves significant amounts of selection.     

Sheet metal forming behaviour and mechanical properties of TRIP steels

Recently various kinds of high-strength sheet steels have been developed to meet the requirements of the automotive industry such as passive safety, weight reduction and saving energy. Usually the main problem of high-strength steels is their inferior ductility. Multiphase steels however show a very good combination of strength and formability so that the applicable region of high-strength steels has been widely enlarged. Multiphase steels have been developed for various purposes because of their ability to tailor properties by adjusting the type, the amount, and the distribution of different phases. Especially new developed triple-phase steels which make use of the TRIP effect (transformation induced plasticity) can further improve formability as well as strength due to the transformation of retained austenite to martensite during the deformation. In this work the transformation behaviour and the mechanical properties of low alloyed TRIP steels were investigated. The influence of the annealing parameters on transformation behaviour and on the amount of retained austenite were determined. In addition the temperature dependence of the mechanical properties and the effect of testing speed on the formability were studied. The investigation was carried out on seven different TRIP steels with different chemical compositions, especially the influence of the microalloying element niobium was considered. For reasons of comparison various mild and high-strength steels were tested parallel to the TRIP steels. It was found that the investigated TRIP steels offer very attractive combinations of elongation and strength values. An interesting temperature dependence of the mechanical properties can be observed, in such a way that the elongation values of the TRIP steels possess a maximum between +50 and +100°C. Due to its effect on grain size and on precipitation behaviour the addition of niobium leads to higher strength values without a strong decrease in ductility. In general, the mechanical properties are strongly affected by the type and the distribution of the different phases. The most important parameters, however, to influence the mechanical behaviour are the amount and the stability of the retained austenite, which are mainly controlled by the heat treatment and the chemical composition.     

Fatigue strength of high-alloy corrosion-resistant TRIP steels (review)

The changes in the structural state of high-alloy TRIP steels during cyclic deformation in low- and high-cycle fatigue regions and the cyclic cracking resistance characteristics are analyzed. The strain-induced martensite that forms during cyclic deformation is shown to substantially affect the cyclic strength characteristics of the TRIP steels.     

Bainite transformation temperatures in high-silicon steels

The bainite transformation temperatures of eight high-silicon steels were determined metallographically. The bainite start (B _s ) temperatures, which define the highest temperature at which bainite can form, all lay below the T ₀ loci, where ferrite and austenite of the same chemical compositions have identical free energy. The established method of calculating B _s temperatures gave reasonable agreement with the experimental results. Careful study of the isothermally reacted samples revealed that Widmanstätten ferrite and bainite could both be observed, even at the beginning of the transformation, at around the B _s temperature. On the other hand, the lower bainite start (LB _s ) temperatures of these steels were found to be very close to the martensite start (M _s ) temperatures. Silicon is considered to be responsible for depressing the LB _s temperature by retarding the formation of cementite. The coformation of upper and lower bainite near the LB _s temperature is also confirmed. The results indicate that the displacive formation mechanism of bainite is sustainable.     

Observations concerning transformation interfaces in steels

The transformation interfaces of pearlite, allotriomorphic cementite, M₂₃C₆, and Widmanstätten cementite plates in high-Mn high-C alloy steels have been studied by TEM. Linear striations in the interface have been analysed and related to intersections with stacking faults in the parent austenite phase. Emphasis is given to the pearlite interface where it is found that the striations at the interface increased as a result of thermomechanical treatment of the austenite prior to isothermal transformation, consistent with an increased density of planar defects. The effect of heat treatment, and Si alloying additions, are also considered. Both conventional and in situ TEM of the pearlite interface showed that the linear defects stretched across both ferrite and cementite phases at the pearlite interface, apparently without any deviation or change in image contrast. The results are compared with similar ones made of the static γ/α interphase boundaries in duplex stainless steel. The effect of prior deformation structure in the parent austenite on the growth and interface structure of Widmanstätten cementite plates has also been considered.