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.     

A Method for the Calculation of Retained Austenite Evolution during Heat‐Treatment of Low‐Alloy TRIP‐Assisted Steels

Summary (abstract) : Despite the critical effect of heat‐treatment, and in particular of the isothermal bainitic treatment stage, on the amount and stability of retained austenite in the microstructure of low‐alloy TRIP‐assisted steels, determination of optimum heat‐treatment conditions is still largely empirical and experiment‐dependent. This work proposes a method by which it is possible to calculate the vol. fraction of retained austenite in the microstructure as a function of intercritical annealing temperature and isothermal bainitic treatment temperature and holding time. The method assumes diffusionless lengthening of bainitic ferrite (α_B) plates in austenite (γ), and subsequent thickness‐wise C rejection from the α_B plates to the adjacent γ layers. The relative thickness of α_B plates and adjacent γ layers is determined by the T_o line of the transforming system at any given bainitic transformation temperature. The C‐concentration profiles in γ are calculated with respect to a local time‐scale, referring to any random section of any random α_B plate. Determination of the variation of C‐concentration profiles with local time in γ, together with the use of a simple austenite‐retention criterion, allows the calculation of vol. fraction retained austenite (γ_R) as a function of transformation temperature and local time. Transition from local (calculation) time to actual heat treatment time is performed by introducing a time‐scale factor, which depends on transformation temperature and initial C‐content of α_B. The calculated behaviour of vol. fraction γ_R vs. bainitic holding time conforms to the well established, experimentally observed one: vol. fraction γ_R initially increases with holding time, reaches a maximum and decreases at longer holding times. According to calculated results, the decrease is attributed to the gradual homogenization of C inside the γ layers, which leads to inadequate stabilization and transformation of γ to martensite on quenching. As regards quantitative comparison with available experimental data, calculations show reasonable agreement. Certain refinements of the method, which are underway, are reported, in order to further improve quantitative results. Nevertheless, the method in its present form provides a calculational tool, by which the effect of different heat‐treatment conditions or steel compositions can be examined and compared to each other, which can be a useful step towards optimizing alloy compositions and heat‐treating processes.     

Forming response of retained austenite in a C‐Si‐Mn high strength TRIP sheet steel

TRIP sheet steels typically consist of ferrite, bainite, retained austenite, and martensite. The retained austenite is of particular importance because its deformation‐induced transformation to martensite contributes to excellent combinations of strength and ductility. While information is available regarding austenite response in uniaxial tension, less information is available for TRIP steels with respect to the forming response of retained austenite in complex strain states. Therefore, the purpose of this work was to study the austenite transformation behaviour in different strain paths by determining the amount of retained austenite before and after forming. Forming experiments were performed on a high strength 0.19C‐1.63Si‐1.59Mn TRIP sheet steel 1.2 mm in thickness in two different strain conditions, uniaxial tension (ε₁ = ‐2ε₂) and balanced biaxial stretching (ε₁ = ε₂). Specimens were formed to strains ranging from zero to approximately 0.2 effective (von Mises) strain. Specimens were tested both longitudinally and transverse to the rolling direction in uniaxial tension, and subtle mechanical property differences were found. The volume fraction of austenite, determined with X‐ray diffraction subsequent to forming, was found to decrease with increasing strain for both forming modes. Some modification in the crystallographic texture of the ferrite was observed with increasing strain, in specimens tested in the balanced biaxial stretch condition. This trend was not evident in the uniaxial tensile test results. Slight differences were found in the transformation behaviour of the austenite when formed in different strain conditions. More austenite transformed in specimens tested parallel to the rolling direction than transverse to the rolling direction in uniaxial tension. The amount of austenite transformed during biaxial stretching was determined to be greater than the amount transformed in uniaxial tension for specimens tested transverse to the rolling direction at an equivalent von Mises strain. The amount of austenite that transformed in biaxial tension, however, was comparable to the amount of austenite that transformed in specimens tested longitudinal to the rolling direction in uniaxial tension.     

Effect of the Charging Temperature on the Hot Ductility of Nb‐Containing Steel in the Simulated Hot Charge Process

In order to develop a comprehensive understanding of the effect of hot charging temperature on the hot ductility of a Nb‐containing steel, direct hot charging process was simulated by using a Gleeble thermo stress/strain machine. Three kinds of thermal histories were introduced to assess the hot ductility of the steel during continuously cast, hot charging, and cold charging process by means of hot tensile test in relation to surface cracking of hot charging processed steel slabs. The ductility of the specimens charged at the temperature within the range of ferrite/austenite two‐phase region and charged at the temperature just below the A_r1 of the steel is largely reduced. These results can be ascribed to the retained ferrite films at the boundaries of austenite encouraging voiding at the boundaries and these voids gradually link up to give failure around 750°C, and the combination of inhogeneous austenite grain size and precipitations aggravating the ductility trough by encouraging grain boundary sliding at 950°C. The steel via the conventional cold charge process experienced a complete phase transformation from austenite to ferrite and pearlite structure during the cooling to the ambient temperature. This steel can be charged into a reheating furnace and rolled without experiencing hot embrittlement due to the recrystallization and the precipitates are trapped inside a newly formed grain of austenite. In comparison with the hot ductility results, the hot tensile strength is only slight influenced by the charging temperature.     

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.     

In situ observations on the austenite stability in TRIP‐steel during tensile testing

In‐situ deformation tests have been performed on a steel displaying the transformation‐induced plasticity (TRIP) effect, while monitoring the phase transformation by means of X‐ray diffraction. A tensile stress is applied to 0.4 mm thick samples of this steel with mass contents of 0.26 % Si, 1.5 % Mn, and 1.8 % Al in a transmission geometry for a synchrotron‐radiation beam of 25 μm · 25 μm. On the diffraction patterns every grain appears as a discrete spot. The austenite {200} reflections are analysed during this investigation. The diffraction patterns are treated like a powder pattern for five different η‐angles, with η representing the angle between the tensile direction and the normal direction of the diffracting {200} planes. The results of the analysis show that η = 0° and η = 90° are the preferential orientations for the transformation to martensite. The Ludwigson and Burger model [9] is used to gain more information about the stress dependence of the deformation induced martensite formation. The microdiffraction patterns also reveal the changes in carbon concentration in austenite at each retained austenite fraction.     

Effect of alloying elements on the microstructure‐property relationship in thermomechanically processed C‐Mn‐Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe‐C‐Mn‐Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X‐ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise ~ 50 % of polygonal ferrite, 7 ‐12 % of retained austenite, non‐carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure‐property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C‐Si‐Mn‐Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb‐Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C‐Si‐Mn‐Nb steel leads to a good combination of strength (~ 940 MPa) and elongation (~ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ~7 ‐ 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb‐Mo‐Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb‐Mo‐Al steel appears to be more granular than in the Nb‐Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.     

Energetics of Plastic Deformation of Metastable Austenitic Stainless Steel

The change in the internal energy during uniaxial tensile deformation of austenitic stainless steels EN 1.4301 (AISI 304) and EN 1.4318 (AISI 301LN) was determined by measuring the extent of γ→α'‐martensite transformation and the temperature increase of the samples. From the results the fraction of the stored energy of cold work and the free energy change related to the strain‐induced γ→α'‐martensite transformation were determined. The fraction of stored energy varied around 0.4. With the metastable steel grades the free energy change related to the γ→α'‐martensite transformation was found to vary between ‐98 MJ/m³ and ‐206 MJ/m³ depending on the austenite stability of the steel. Furthermore, the magnitude of the mechanical driving force was estimated by comparing the results with the free energy change of thermally induced transformation.     

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.     

Deformation Induced Martensite Formation and its Effect on Transformation Induced Plasticity (TRIP)

The knowledge of the stress‐ and deformation‐induced martensite formation in metastable austenitic steels including the formation temperatures and amounts formed is of considerable importance for the understanding of the transformation induced plasticity. For this purpose a stress‐temperature‐transformation (STT) and a deformation‐temperature‐transformation (DTT) diagram have been developed for the steel X5CrNi 18 10 (1.4301, AISI 304). It is shown that the M_d‐temperature for γ→?, ?→α', γ→?→α’ and γ→α’ martensite formation is defined by two stress‐temperature curves which show a different temperature dependence. They specify the beginning and the end of the deformation‐induced martensite formation in the range of uniform elongation. The intersection point defines the corresponding M_d‐temperature. The stress difference which results from the stresses for the end and the beginning of the martensite formation shows positive values below the M_d‐temperature. It defines the amount of martensite being formed. When the M_d^γ→? temperature is reached and the formation of the first deformation‐induced amount of ?‐martensite appears, an anomalous temperature dependence of the maximum uniform elongation starts. The highest values of the maximum uniform elongation are registered for the tested steel in the immediate vicinity of the M_d^γ→α' or the M_d^γ→?→α' temperature ‐ similar as in other metastable austenitic CrNi steels. At this temperature the highest amount of deformation‐induced ?‐phase exists. The transformation plasticity in the test steel is considerably caused by the deformation‐induced ? and α’ martensite formation. Using the new evaluation method, the increase of plasticity ΔA (TRIP‐effect) and strength ΔR can be quantified.     

一次淬火马氏体对Q&P钢组织和性能的影响

 研究了一次淬火马氏体对低合金钢经淬火和配分(Quenching and Partitioning,Q&P)工艺后微观组织和单轴拉伸性能的影响,用扫描电镜进行微观组织表征,用X射线法测量残留奥氏体量。试验结果表明,随着一次淬火马氏体比例的增加,二次淬火马氏体的尺寸和数量逐渐减少,残留奥氏体体积分数呈先增加后减少的趋势,一次淬火马氏体体积分数为40%时获得最大残留奥氏体体积分数为16.92%。一次淬火马氏体体积分数为30%～70%时试验钢获得了较高的塑性和强塑积,马氏体基体为钢提供了高强度,残留奥氏体在变形过程中的TRIP效应提高了钢的塑性。     

Modeling tensile deformation of dual-phase steel

An analytical model has been developed which can describe the tensile deformation behavior of dualphase steel containing retained austenite which transforms to martensite during deformation. The model takes into account the internal back stresses created in the material as a result of the deformation. The influence of various metallurgical factors, such as the amounts of the secondary phases (martensite and retained austenite), strength ratio of the phases, work hardening coefficients, and the stability of retained austenite with respect to the strain-induced transformation, was analyzed. The strongest influence on both strength and ductility was found to result from a large work hardening coefficient of the martensite. Increasing the stability of retained austenite to strain-induced transformation improved the ductility remarkably. The model developed was used to predict the tensile deformation behavior of a commercial dual phase steel fairly accurately.     

Stress‐Temperature‐Transformation and Deformation‐Temperature‐Transformation Diagrams for an Austenitic CrMnNi as‐cast Steel

Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are well‐suited to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown in the DTT diagram. The microstructure evolution of a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel during deformation was investigated at various temperatures using static tensile tests, optical microscopy and the magnetic scale for the detection of ferromagnetic phase fraction. At the temperatures above 250 °C the steel only deforms by glide deformation of the austenite. Strain‐induced twinning replaces the glide deformation at temperatures below 250 °C with increasing strain. Below 100 °C, the strain‐induced martensite formation becomes more pronounced. The kinetics of the α'‐martensite formation is described according to stress and deformation temperatures. The STT and DTT diagrams, enhanced with the kinetics of the martensite formation, are presented in this paper.     

Development of thermo-mechanical treatments of a maraging steel for yield strengths above 3 GPa

Hot-rolling of austenite and cold-rolling of martensite were combined with aging treatments to obtain new microstructures in a maraging steel with a high Mo-content. Purpose of the investigation is to acquire an optimum combination of ultra high yield stress (σ_r>3000 MPa) and ductility (or toughness). The best results were obtained by treatments consisting of high amounts of plastic deformation between 1000°C and 600°C (ausforming) and aging of the martensite which had formed from the deformed austenite. Ausaging around 550°C induced pronounced intercrystalline embrittlement. Intercritical heat treatment at 700°C provide considerable ductility (uniform elongation) however, insufficient strength (<2000 MPa). The most favourable thermo-mechanical treatments are discussed in context with fabrication methods for the investigated steel.     

Effect of Bake‐Hardening on the Structure‐Property Relationship of Multiphase Steels for the Automotive Industry

The effect of a bake‐hardening (BH) treatment on the microstructure and mechanical properties has been studied in C‐Mn‐Si TRansformation Induced Plasticity (TRIP) and Dual Phase (DP) steels after: (i) thermomechanical processing (TMP) and (ii) intercritical annealing (IA). The steels were characterized using X‐ray diffraction, transmission electron microscopy (TEM) and three‐dimensional atom probe tomography (APT). All steels showed high BH response. However, the DP and TRIP steels after IA/BH showed the appearance of upper and lower yield points, while the stress‐strain behavior of the TRIP steel after TMP/BH was still continuous. This was due to the higher volume fraction of bainite and more stable retained austenite in the TMP/BH steel, the formation of plastic deformation zones with high dislocation density around the “as‐quenched” martensite and “TRIP” martensite in the IA/BH DP steel and IA/BH TRIP steel, respectively.     

Strong and Ductile Martensitic Steels for Automotive Applications

The limits of strength and ductility of a medium‐carbon silicon chromium spring steel are investigated for the case of conventional heat treatment including austenitization, quenching and tempering. The effect of phosphorus and austenite deformation prior to quenching was studied by measuring mechanical properties after quenching and tempering and by microstructural investigation. Strong influence of phosphorus on the ductility is observed for the quenched and tempered martensite without prior austenite deformation. The minimum in ductility found after tempering at 350°C is explained by the formation of cementite and grain boundary segregation of phosphorus. Two thermomechanical treatments were tested involving different austenite conditions produced by variation of the deformation temperature. The deformed conditions, recrystallized or work‐hardened, exhibit higher ductility at all tempering temperatures tested. A combined thermomechanical treatment is proposed that provides the highest ductility after tempering at 300°C independent of the phosphorus content. All thermomechanical treatments described in this study refine or eliminate carbide films at prior austenite grain boundaries. It was found possible to increase the tensile strength and the fatigue limit by deformation of austenite prior to quenching while maintaining or increasing the ductility level.     

Effect of thermomechanical treatment on structure and mechanical properties of Mo-bearing dual phase steel

The effects of hot rolling of a dual phase steel in the (α + γ) range on microstructure and mechanical properties was investigated by using two thermomechanical (TMT) routes. The first consisted of heating A_c3, soaking, cooling to deformation temperature in the (α + γ) range. The second comprises heating to deformation temperature in the (α + γ) range, followed by rolling and quenching. Parameters varied were temperature (with the first route) and extent of deformation (with the second). The microstructures were characterised by optical and transmission electron microscopy. The results indicate a distinct difference in the final structure and properties due to the two different TMT routes. The first TMT route resulted in a greater amount of ferrite, finer lath width of martensite, finer ferrite grain size and increased density of dislocations. The strength properties decreased, the YS/UTS ratio decreased and ductility increased with the increase in the extent and temperature of deformation. However, TMT route 2 resulted in an increase in the amount of martensite, finer ferrite grain size, decrease in the martensite lath width and increased dislocation density. The strength properties increased, YS/UTS ratio increased and ductility decreased with increase in the extent and temperature of deformation.     

Steel Microstructure Effect on Mechanical Properties and Corrosion Behavior of High Strength Low Carbon Steel

Different thermomechanical treatments were applied to a high strength low carbon steel with a novel chemical composition. As a result, three different microstructures were produced with dissimilar mechanical and corrosion properties. Subsequently, a tempering heat treatment was applied to redistribute the phases in the steel. Microstructure A with 56 pct martensite and 32 pct bainite presented high strength but medium ductility; microstructure C with 95 pct ferrite and 3 pct martensite/austenite resulted in low strength and high ductility, and finally microstructure B with 98 pct bainite and 2 pct martensite/austenite resulted in high strength and ductility. Alternatively the corrosion behavior obtained by polarization curves was characterized in 0.1 M H₂SO₄, 3 M H₂SO₄, 3.5 wt pct NaCl, and NS4 solutions resulting in similar magnitudes, while the corrosion behavior acquired by electrochemical impedance spectroscopy had slightly differences in 3 M H₂SO₄.     

Microstructure and Mechanical Properties of a Low-Carbon Mn-Si Multiphase Steel Based on Dynamic Transformation of Undercooled Austenite

The microstructure evolution of 0.20C-2.00Mn-2.00Si steel treated by the thermomechanical process to manufacture hot-rolled, transformation-induced plasticity (TRIP) steel based on dynamic transformation of undercooled austenite was investigated using a Gleeble 1500 (Dynamic Systems, Inc., Poestenkill, NY) hot simulation test machine in combination with light microscope (LM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The mechanical properties of this steel with different multiphase microstructures were also analyzed using room-temperature tensile tests. The results indicated that the multi-phase microstructures consisting of fine-grained ferrite with a size of 1–3 μm, bainite packets, and retained austenite and martensite were formed for the used steel by a thermo-mechanical process involving dynamic transformation of undercooled austenite, controlled cooling, isothermal bainite treatment and water-quenching. With the increase in the strain of hot deformation of undercooled austenite, the fraction of ferrite increased, that of bainite decreased, and that of martensite increased. At the same time, the fraction of retained austenite (RA), as well as the carbon content of RA, first increased and then decreased. For the used steel treated by such process, the tensile strength is about 1200 MPa with a total elongation of about 20 pct, and the product of tensile strength and total elongation can be up to 25,000 MPa × pct.     

Mechanical Behavior of Deformation‐Induced α′‐Martensite and Flow Curve Modeling of a Cast CrMnNi TRIP‐Steel

For the modeling of the mechanical behavior of a two phase alloy with the rule of mixture (RM), the flow stress of both phases is needed. In order to obtain these information for the α′‐martensite in high alloyed TRIP‐steels, compression tests at cryogenic temperatures were performed to create a fully deformation‐induced martensitic microstructure. This martensitic material condition was subsequently tested under compressive loading at ?60, 20, and 100°C and at strain rates of 10^?3, 10⁰, and 10³ s^?1 to determine the mechanical properties. The α′‐martensite possesses high strength and surprisingly good ductility up to 60% of compressive strain. Using the flow stress behavior of the α′‐martensite and that of the stable austenitic steel AISI 316L, the flow stress behavior of the high alloyed CrMnNi TRIP‐steel is modeled successfully using a special RM proposed by Narutani et al.