不同应变方式下TRIP钢中残余奥氏体的体积分数随应变量的变化

对TRIP钢板在单向拉伸、双向拉伸和平面应变3种应变方式下残余奥氏体的体积分数随应变量变化的规律进行了实验研究。结果表明：在变形过程中，残余奥氏体的体积分数随应变量的增加而减小；该变化量不仅与应变量有关，还与应变方式有关；平面应变时变化最大，双向拉伸次之，单向托伸下变化最小。还对同一零件不同区域残余奥氏体的含量对零件形状精度的影响进行了讨论。     

Analysis of lattice parameter changes following deformation of a 0.19C-1.63Si-1.59Mn transformation-induced plasticity sheet steel

Austenite and ferrite lattice parameters were monitored using X-ray diffraction subsequent to deformation in uniaxial and biaxial tension and plane straining of a 0.19C-1.63Si-1.59Mn transformation-induced plasticity (TRIP) sheet steel. Details from peak position results suggest the presence of stacking faults in the austenite phase, especially after deformation in uniaxial tension. The results also indicate residual stress or composition effects (through changes in the average carbon concentration due to selective transformation of lower carbon regions of austenite). Compressive residual stresses in the ferrite matrix were measured, and found to increase with increasing effective strain in specimens tested in biaxial tension and plane strain. Strain partitioning between softer ferrite and harder austenite (and possibly bainite or martensite) may be responsible for these residual compressive stresses in the ferrite, although volume expansion from the γ → α′ transformation and texture gradients through the sheet thickness are also possible contributors.     

Analysis of the Strain‐Induced Martensitic Transformation of Retained Austenite in Cold Rolled Micro‐Alloyed TRIP Steel

The stability of retained austenite and the kinetics of the strain‐induced martensitic transformation in micro‐alloyed TRIP‐aided steel were obtained from interrupted tensile tests and saturation magnetization measurements. Tensile tests with single specimens and at variable temperature were carried out to determine the influence of the micro‐alloying on the M_s^σ temperature of the retained austenite. Although model calculations show that the addition of the micro‐alloying elements influences a number of stabilizing factors, the results indicate that the stability of retained austenite in the micro‐alloyed TRIP‐aided steels is not significantly influenced by the micro‐alloying. The kinetics of the strain‐induced martensitic transformation was also not significantly influenced by addition of the micro‐alloying elements. The addition of micro‐alloying elements slows down the autocatalytic propagation of the strain‐induced martensite due to the increase of the yield strength of retained austenite. The lower uniform elongation of micro‐alloyed TRIP‐aided steel is very likely due to the presence of numerous precipitates in the microstructure and the pronounced ferrite grain size refinement.     

Properties and application of TRIP‐steel in sheet metal forming

A further development of dual‐phase‐steels are represented by TRIP (transformation induced plasticity) ‐steels. TRIP‐steels contain austenite, which is metastable at room temperature. It transforms to martensite during straining (TRIP effect). This process improves the strength‐ductility balance of these steels. Two types of TRIP‐steels, low alloyed (L‐TRIP) and high alloyed (H‐TRIP), can be applied in sheet forming processes and exhibit different forming characteristics. Basing on results of uniaxial tensile tests and the evaluation of Young's modulus the forming limits in deep drawing processes and the component properties of deep drawn parts are discussed. The Young's modulus decreases significantly with increasing pre‐strain, especially demonstrated for the L‐TRIP material TRIP700. Forming limit curves determined at different forming temperatures indicate its influence on the forming limits. Martensite transformation is suppressed at a temperature of approximately T = 200 °C and therefore the major strain ?₁ decreases significantly. For the investigated stainless steel AISI304 (H‐TRIP) different lubricant types in comparison to chlorinated paraffins have been tested. Lubricants consisting of sulphur additives led to good forming conditions in forming processes, even better than lubricants based on chlorinated paraffins. The evaluation of component properties, compared between L‐TRIP and H‐TRIP, was done based on the analysis of springback and dent resistance. The L‐TRIP material TRIP700 shows higher springback angles than AISI304 resulting from higher yield strength and decreased Young's modulus, resulting from the forming process. The dent resistance of TRIP‐steel was exemplarily demonstrated for AISI304. Uniaxial pre‐strained sheet specimen were analysed to show the dent resistance depending on dent depth. During elastic denting pre‐strain has no influence on dent resistance. Further increasing dent depth lead to increased dent forces for pre‐strained specimens.     

Ӧ��ȶ�TRIP�ֲ��������ת������Ӱ��

Forming limit diagram (FLD) of cold- rolled TRIP steel was established by experiments. The microstructures of samples before and after deformation were examined by metalloscopy and scanning electron microscopy and at the same time the contents of retained austenite after different strain ratios were measured by X- ray diffraction. The results show that the ultimate strain under plane strain state(FLD0) is 0. 397. With the strain ratio increasing, strain path changes from uniaxial stretching to plane strain and then biaxial stretching and the transformation amount of residual austenite increases gradually. Compared with dual- phase steel, the higher FLD0of TRIP steel is ascribed to TRIP effect and necking area is wider during deformation.     

Effect of changing strain paths on

The effect of changing strain paths on forming limits of aluminum alloy 2008-T4 has been investigated by determining forming limit diagrams (FLDs) after prestraining. Sheets were pre- strained to several levels in uniaxial, biaxial, and plane-strain tension parallel and perpendicular to the prior rolling direction (RD). Abrupt changes in the strain path can be used to increase the forming limits. Prestraining in biaxial tension generally lowers the forming limits for the entire FLD, whereas prestraining in uniaxial tension raises the limits on the right-hand side of the FLD without much effect on the left-hand side. Prestraining in plane-strain tension raises both sides away from the minimum. Finally, it was found that after prestraining, the amount of the additional plane-strain deformation possible depends on the effective strain during prestrain.     

Effects of Niobium Addition on Microstructures and Formability in Si‐Al‐Mn TRIP Cold‐rolled Steels

The effects of Nb addition on microstructures and formability in Si‐Al‐Mn TRIP cold‐rolled steels were investigated. These steels were intercritical annealed at 770 °C for 5 min, and isothermally treated at 400 °C for 3 min. Microstructural observation, tensile tests and forming limit diagram (FLD) tests were conducted, and the changes of retained austenite volume fraction as a function of tensile strain were measured by using an X‐ray diffractometer. The results showed that Nb addition makes grain size refined, the volume fraction of ferrite increase and that of bainite decrease, however, obviously it does not affect the volume fraction and carbon content of retained austenite. The Nb addition increased the stability of retained austenite owing to grain refinement. With Nb addition, increase in strength, ductility, strain hardening exponent and formability could be achieved simultaneously. These findings indicate that Nb addition can be a new direction of microalloying design for the low carbon TRIP steels with excellent formability and high stability of retained austenite.     

Transformation Textures in As‐hot rolled TRIP Steels

The microstructures of TRIP steels finish‐rolled above and below the recrystallization‐stop temperature (T_nr) are compared. Here, the retained austenite grains are equiaxed or elongated, respectively, according to whether final rolling was carried out above or below the T_nr. The recrystallized austenite did not contain a sharp texture, the best defined component of which was the cube. The bainite that formed in this case was characterized by weak concentrations of the Goss and rotated Goss and a fairly strong concentration of the rotated cube. It also displayed the transformation products of a retained rolling fibre in the prior austenite. The deformed austenite contained the typical fcc rolling texture, where the copper is considerably more intense than the brass under these conditions. After transformation to bainite, the presence of a strong transformed copper component is evident, together with somewhat less intense contributions from the three transformed brass components. The data indicate that strong variant selection took place in the deformed austenite and that it was also present in the recrystallized material, but to a lesser extent. The latter displayed evidence of incomplete recrystallization in that the transformation texture included components obtained from both “recrystallized” and “deformed” austenite.     

Low‐alloy TRIP steels: a correlation between mechanical properties and the retained austenite stability

In recent years the technology of low‐alloy TRIP steels has considerably advanced. The mechanical properties are characterised by a combination of high yield strength and high uniform elongation as well as enhanced formability. In the present work an effort to correlate mechanical properties with the retained austenite stability was made. Two low‐alloy TRIP steels were investigated. The first of them represents a typical composition of the low‐alloy TRIP steels, while the other one contains aluminum as alloying element. The influence of the heat treatment on the mechanical properties and especially on the amount and stability of the retained austenite was determined. The retained austenite stability was measured with a single specimen technique, in which a tensile specimen was used to determine the M^σ_S temperature with a loading‐unloading procedure. The results showed that there is a strong influence of the stability of the retained austenite on the mechanical properties. Increased stability combined with a high amount of retained austenite, exhibited an increase in both, yield strength and uniform elongation while increased amount of retained austenite with low stability did not show the same good combination of mechanical properties. The results clearly indicate that in order to get the maximum TRIP effect, a good combination of austenite stability and amount is required.     

Deformation microstructures in titanium sheet metal

A metallographic study has been made of the microstructures produced by room temperature deformation of 0.6mm thick commercially pure titanium sheet metal in uniaxial, plane strain and biaxial tension. Deformation twinning becomes increasingly important as the deformation mode changes from uniaxial through plane strain to equibiaxial tension, and is more significant for strain transverse to the rolling direction than for strain in the longitudinal direction. In uniaxial tension, 1122 twins are dominant in longitudinal straining, while 1012 twins dominate in transverse straining. In plane strain and equibiaxial straining, 1012 twinning is suppressed and largely replaced by 1122 twinning. The observed changes in twin occurrence and type are attributed to the interaction of the imposed stress system and the crystallographic texture of the rolled sheet, which alters the distribution of the grain basal-plane poles with respect to the operative stress axes. In uniaxial tension parallel to the longitudinal direction, twins favored by ‘c’ axis compression are produced, while in the transverse direction twins favored by ‘c’ axis tension appear. In plane strain and biaxial tension the dominant stress is through-thickness compression, which produces twins favored by ‘c’ axis compression in nearly all cases. The alterations in twin orientation and numbers are associated with changes in stress-strain behavior. As twin volume fraction increases and twins are aligned more closely to the principal stress axis, the instantaneous work-hardening rate tends to stabilize at a nearly constant value over a large strain range. Formerly Chief Metallurgist, The APV Company.     

Work Hardening Behavior in Steel with Multiple TRIP Mechanisms

Transformation-induced plasticity (TRIP) behavior was studied in steel with the composition Fe-0.07C-2.85Si-15.3Mn-2.4Al-0.017N that exhibited two TRIP mechanisms. The initial microstructure consisted of both ε- and α-martensites with 27 pct retained austenite. TRIP behavior in the first 5 pct strain was predominately austenite transforming to ε-martensite (Stage I), but upon saturation of Stage I, the ε-martensite transformed to α-martensite (Stage II). Alloy segregation also affected the TRIP behavior with alloy-rich regions producing TRIP just prior to necking. This behavior was explained by first-principles calculations which revealed that aluminum significantly affected the stacking fault energy in Fe-Mn-Al-C steels by decreasing the unstable stacking fault energy and promoting easy nucleation of ε-martensite. The addition of aluminum also raised the intrinsic stacking fault energy and caused the ε-martensite to be unstable and transform to α-martensite under further deformation. The two-stage TRIP behavior produced a high strain hardening exponent of 1.4 and led to an ultimate tensile strength of 1165 MPa and elongation to failure of 35 pct.     

Bake Hardening of Hot Rolled Multiphase Steels under Biaxial Pre‐strained Conditions

Multiphase steels show a strong bake hardening effect being of importance for shaping of car body structural parts. The raised yield strength is exploited for improved crash resistance. Especially the automotive industry has a growing interest in using this effect. Normally the bake hardening effect is examined in tensile tests whereas under industrial conditions shaping of structural parts shows a wide spread of stress strain behaviour, from uniaxial conditions over plain strain to biaxial ones. So it is obvious that the bake hardening behaviour of a material cannot be described with results of the uniaxial tension test only. To give a first answer to this question, the dependence of the bake hardening effect on different biaxial prestrains was investigated for several hot rolled multiphase steels using various baking temperatures and holding times whereas the bake hardening effect under uniaxial prestrain had already been examined in [5]. Considering the choices to generate biaxial strain, a Marciniak forming tool with a diameter of 250 mm mounted on a 2500 kN hydraulic press was chosen. For control of plastic deformation and adjustment the non‐contact measuring system ARGUS, was used. To reduce the quantity of experiments “Design of Experiments” and statistical methods were applied for a martensitic steel, a dual phase steel, a complex phase steel, a ferrite‐bainite steel, and a retained austenite steel known as TRIP, all in hot rolled condition. As a result, a formula for yield stress, tensile strength and residual deformability was developed. Furthermore, a method was found to predict easily whether a steel under investigation is qualified for additional experiments in regard to bake hardening or more exactly its response to different baking temperatures and holding times.     

Effect of Silicon and Manganese on Mechanical Properties of Low-Carbon Plain TRIP Steel

Inthelasttwodecadesconsiderableefforthas beenputonthedevelopmentofhighstrengthsteels fortheautomotiveindustry.Themainaimistore ducecarweightbyincreasingthesteelstrength. Nowmoreandmoreattentionhasbeenpaidtohigh strengthtransformation inducedplasticity(TR…     

Room Temperature Shear Band Development in Highly Twinned Wrought Magnesium AZ31B Sheet

Failure mechanisms were studied in wrought AZ31B magnesium alloy after forming under different strain paths. Optical micrographs were used to observe the shear band formation and regions of high twin density in samples strained under uniaxial, biaxial, and plane strain conditions. Interrupted testing at 4?pct effective strain increments, until failure, was used to observe the evolution of the microstructure. The results showed that shear bands, with a high percentage of twinned grains, appeared early in the samples strained under biaxial or plane strain tension. These bands are similar to those seen in uniaxial tension specimens just prior to failure where the uniaxial tensile ductility was much greater than that observed for plane strain or biaxial tension conditions. A forming limit diagram for AZ31B, which was developed from the strain data, showed that plane strain and biaxial tension had very similar limit strains; this contrasts with materials like steel or aluminum alloys, which typically have greater ductility in biaxial tension compared to plane strain tension.     

Metallurgical Phenomena during Processing of Cold Rolled TRIP Steel

Metallurgical phenomena taking place during processing of TRIP Steel are investigated and described with the aim of achieving better understanding of the microstructure development throughout the entire integrated processing routes. Different TRIP steel structure sizes were created by controlling the hot rolling process prior to cold rolling. After that the specimens were intercritically annealed under different conditions to obtain prescribed austenite fractions, and subsequently quenched in salt bath at the bainite transformation temperature. The microstructures had been investigated using light optical microscopy (LOM) and the amount of retained austenite was determined by magnetometry.     

<Emphasis Type="Italic">In Situ</Emphasis> Local Measurement of Austenite Mechanical Stability and Transformation Behavior in Third-Generation Advanced High-Strength Steels

Austenite mechanical stability, i.e., retained austenite volume fraction (RAVF) variation with strain, and transformation behavior were investigated for two third-generation advanced high-strength steels (3GAHSS) under quasi-static uniaxial tension: a 1200 grade, two-phase medium Mn (10 wt pct) TRIP steel, and a 980 grade, three-phase TRIP steel produced with a quenching and partitioning heat treatment. The medium Mn (10 wt pct) TRIP steel deforms inhomogeneously via propagative instabilities (Lüders and Portevin Le Châtelier-like bands), while the 980 grade TRIP steel deforms homogenously up to necking. The dramatically different deformation behaviors of these steels required the development of a new in situ experimental technique that couples volumetric synchrotron X-ray diffraction measurement of RAVF with surface strain measurement using stereo digital image correlation over the beam impingement area. Measurement results with the new technique are compared to those from a more conventional approach wherein strains are measured over the entire gage region, while RAVF measurement is the same as that in the new technique. A determination is made as to the appropriateness of the different measurement techniques in measuring the transformation behaviors for steels with homogeneous and inhomogeneous deformation behaviors. Extension of the new in situ technique to the measurement of austenite transformation under different deformation modes and to higher strain rates is discussed.     

Stability and Constitutive Modelling in Multiphase TRIP Steels

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 M^σ_S 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.     

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.     

Anisotropy of Retained Austenite Stability during Transformation to Martensite in a TRIP‐Assisted Steel

Retained austenite as a key constituent in final microstructure plays an important role in TRansformation Induced Plasticity (TRIP) steels. The volume fraction, carbon concentration, size, and morphology of this phase are the well‐known parameters which effects on the rate of transformation of retained austenite to martensite and the properties of steel, are studied by many researchers. Of the transformation of retained austenite to martensite under strain in a TRIP steel is studied in this paper. The experimental results show that the transformation rate of retained, austenite with similar characteristics, to martensite in differently processed TRIP steel samples, exhibits an anisotropic behavior. This phenomenon implies a kind of variant selection of martensitic reaction of retained austenite under strain and is explained by ferrite texture developed in steel.     

Forming Limit Curve (FLC) and Fracture Mechanism of Newly Developed Low‐Carbon Low‐Silicon TRIP Steel

The Forming‐Limited Diagram (FLD) of intercritically annealed 0.11C‐1.65Mn‐0.62Si TRIP‐assisted steel was investigated. The high FLD₀ value of this new low carbon TRIP steel was indicative of a superior formability. The micro‐structural changes during deformation and fracture were studied in detail. The polygonal ferrite phase was found to plastically deform first and deformed most at larger strains. Fracture was initiated by micro‐voids nucleated at ferrite grain boundaries, within ferrite grains or at the interface between ferrite and the harder phases. Cracks were formed after micro‐voids grew, coalesced, and expanded in one direction. When crack tips reached the bainite phase or the martensite/austenite constituent, the cracks propagated along the boundary of these phases. Cracks reaching retained austenite islands caused stress‐induced martensite transformation at the crack tip. The direction of motion of the cracks also changed in this case.