Temperature Dependent Deformation Mechanisms of a High Nitrogen‐Manganese Austenitic Stainless Steel

The influence of temperature on the deformation behaviour of a Fe‐16.5Cr‐8Mn‐3Ni‐2Si‐1Cu‐0.25N (wt%) austenitic stainless steel alloy was investigated using transmission electron microscopy and X‐ray diffraction measurements. Recrystallized samples were deformed under tension at ?75°C, 20°C, and 200°C and the microstructures were characterized after 5% strain and after testing to failure. Deformation to failure at ?75°C resulted in extensive transformation induced plasticity (TRIP) with over 90% α′‐martensite. The sample deformed to 5% strain at ?75°C shows that the austenite transformed first to ?‐martensite which served to nucleate the α′‐martensite. Transformation induced martensite prohibits localized necking providing total elongation to failure of over 70%. At room temperature, in addition to some TRIP behaviour, the majority of the deformation is accommodated by dislocation slip in the austenite. Some deformation induced twinning (TWIP) was also observed, although mechanical twinning provides only a small contribution to the total deformation at room temperature. Finally, dislocation slip is the dominant deformation mechanism at 200°C with a corresponding decrease in total elongation to failure. These changes in deformation behaviour are related to the temperature dependence on the relative stability of austenite and martensite as well as the changes in stacking fault energy (SFE) as a function of temperature.     

高锰合金钢的SME、TRIP、TWIP效应

 由于锰的价格低廉以及在材料中的重要作用而成为钢铁工业常用的合金元素。锰含量高时,可使Fe Mn合金形成的奥氏体在较低温度下存在。加入Si、Al元素可对合金中奥氏体的稳定性产生不同程度的影响,从而使材料在承受外界载荷时呈现出不同的反应。研究表明：Si可降低奥氏体层错能,有利于A→ε M相变,从而使合金易产生形状记忆效应。加大变形量,由于大量的奥氏体转变为α′ M时体积膨胀,在使材料伸长率提高的同时,强度也得到提高(相变诱发塑性效应),因此可用作高性能结构件。Al和Mn是提高奥氏体层错能的合金元素。对于Al、Mn含量高的钢,在外力作用下则可通过孪生诱发塑性变形产生孪晶诱发塑性效应,因而材料在具有较高强度的前提下,还具有60%~80%的伸长率。     

Determination of stacking fault energy of austenite in a duplex stainless steel

A determination of stacking fault energy (SFE) of the austenite phase of a duplex stainless steel, material no. 1.4462, has been carried out using transmission electron microscopy (TEM). Furthermore, cold rolling tests and microstructural analysis have been realized in order to allow a detailed discussion of the obtained SFE-values. The results of this Investigation indicate that the stacking fault energy of the austenite phase within the duplex stainless steel Is lower than those of single-phase austenitic stainless steels. This is justified by the chemical composition; mainly by the Cr and Ni alloying contents. Nevertheless, work hardening of the austenite during cold deformation is not as accentuated as expected by the low SFE-values, because at higher deformation levels the deformation mainly occurs within the ferrite phase.     

Strain Rate Dependent Flow Stress and Energy Absorption Behaviour of Cast CrMnNi TRIP/TWIP Steels

Modern steel developments often use additional deformation mechanisms like the deformation induced martensitic transformation (TRIP‐effect) and mechanical twinning (TWIP‐effect) to enhance elongation and strength. Three high‐alloyed cast CrMnNi‐steels with different austenite stabilities were examined. Dependent on the austenite stability, TRIP‐effect and TWIP‐effect were found. A low austenite stability causes a distinctive formation of deformation induced α'‐martensite and therefore a strong strain hardening. The increase of strain rate leads to an increase in yield strength and flow stress, but also to a counteractive adiabatic heating of the specimen. Dependent on the degree of deformation, low austenite stabilities and high strain rates lead to excellent values in specific energy absorption.     

Development in fundamental research on TWIP steel used in automobile industry

Abstract

This paper tracks the progress in research regarding the use of twinning induced plasticity (TWIP) steel in the automobile industry. The chemical composition of TWIP steel ensures that it has stable austenite and proper stacking fault energy at room temperature, allowing the main deformation mechanism (twinning) to work. The effects of alloying elements on the microstructure and deformation mechanism of TWIP steel are explained in detail, and their properties deformed under static and dynamic conditions are examined. The TWIP steel deformed at a low strain rate shows higher total elongation and strength. When the TWIP steel deforms under dynamic strain conditions, the stress, microhardness and the work hardening rate, all increase along with the increase in strain and the strain rate. The twin characteristics of TWIP steels deformed at various strain rates vary also and the twins generated under a high strain rate exhibit thinner widths and smaller interspaces compared with those formed under a low strain rate. It has also been observed that multisystem twins are able to generate and develop together. The mechanisms of toughening and strengthening in TWIP steels are noted. Finally, some potential application fields have been found for the promising material.     

Effect of the Intercritical Annealing Temperature on the Mechanical Properties of 10 Pct Mn Multi-phase Steel

Intercritically annealed 10 pct Mn steel has been shown to exhibit an excellent combination of strength and ductility due to the plasticity-enhancing mechanisms of mechanical twinning and strain-induced martensite transformation occurring in sequence. This mechanical behavior is only achieved for a multi-phase microstructure obtained after annealing within a specific intercritical temperature range. A model for the selection of the optimal intercritical annealing temperature was developed to achieve a high strength-ductility balance for 10 pct Mn multi-phase steel. The model considers the room temperature stacking fault energy and the thermodynamic stability of the retained austenite.     

Effect of Cold Deformation and Multiphase Treatment Conditions on Low‐Carbon,Low‐Silicon Multiphase Steel

This paper presents multiphase (MP) treatments of a low‐C, low‐Si cold rolled steel. Despite the much lower content of Si compared to a typical TRIP steel, up to about 8 pct of retained austenite (γ_r) with 1.2 % carbon content can be obtained. Increasing prior cold deformation (i.e. decrease of parent austenite grain size) accelerates the transformation to bainite resulting in a decrease of the volume fraction of residual austenite (γ_r + martensite). Tensile strength of MP steel intercritically annealed at high temperature increases with higher cold reduction degree due to the smaller grain size of the present phases. On the contrary, the ductility and strength‐ductility balance deteriorate because the banded structure becomes more pronounced and the γ_r volume fraction diminishes. Decreasing intercritical annealing temperature results in an increasing γ_r fraction and a uniform distribution of second phases. Hence, the ductility and strength‐ductility balance are improved. Crystallographic preferred orientation is evident in the ferrite and martensite and its extent increases with higher cold deformation.     

Austenite stabilization in Cr 13% Ni 4–6% steels

The present article deals with the study of the effects of concentration changes taking place in the matrix during partial austenitization of Cr 13% Ni 4–6% steel, while it is tempered, in the range of intercritical temperatures, on the structural stability of forming austenite. In the investigation, the authors made use of some special techniques of the potential polarization method and electron microanalysis. Some geometrical factors, such as the morphology and distribution of austenite areas were also studied. Increased austenite content in the matrix provides for an increased level of plastic properties and for improved technological characteristics. The stability of austenite areas depends on their morphology, distribution and, above all, on their dimensions - their thickness should not exceed 0.15 to 0.20 μm. The increase in nickel content observed in these areas was as much as 100% compared with the nominal nickel content of this steel.     

Comprehensive Deformation Analysis of a Newly Designed Ni-Free Duplex Stainless Steel with Enhanced Plasticity by Optimizing Austenite Stability

A new metastable Ni-free duplex stainless steel has been designed with superior plasticity by optimizing austenite stability using thermodynamic calculations of stacking fault energy and with reference to literature findings. Several characterization methods comprising optical microscopy, magnetic phase measurements, X-ray diffraction (XRD) and electron backscattered diffraction were employed to study the plastic deformation behavior and to identify the operating plasticity mechanisms. The results obtained show that the newly designed duplex alloy exhibits some extraordinary mechanical properties, including an ultimate tensile strength of ~900 MPa and elongation to fracture of ~94 pct due to the synergistic effects of transformation-induced plasticity and twinning-induced plasticity. The deformation mechanism of austenite is complex and includes deformation banding, strain-induced martensite formation, and deformation-induced twinning, while the ferrite phase mainly deforms by dislocation slip. Texture analysis indicates that the Copper and Rotated Brass textures in austenite (FCC phase) and {001}〈110〉 texture in ferrite and martensite (BCC phases) are the main active components during tensile deformation. The predominance of these components is logically related to the strain-induced martensite and/or twin formation.     

冲击载荷对10Mn钢磨损性能的影响及耐磨机制分析

通过水韧处理配合450℃时效热处理工艺,研究10Mn钢在2和5 J冲击载荷下耐磨性能.结果表明,10Mni钢在5J冲击载荷下失重量更少,加工硬化的速率更快,并且在不同冲击载荷下磨损失重量都出现了周期性变化.磨损表面SEM结果表明,表面存在犁削磨损、凿削磨损、裂纹.通过XRD数据对亚表面奥氏体和马氏体体积分数进行定量计算...     

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.     

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.     

Martensitic Transformation During Cold Rolling Deformation of an Austenitic Fe-26Mn-0.14C Alloy

A high-Mn austenitic steel was deformed in cold rolling to study the martensitic transformation and microstructure using X-ray diffraction and electron backscatter diffraction. Despite heavy deformation of 70 pct reduction (1.2 true strain), α′-martensite could not be induced in this alloy, but about 90 pct of the austenite transformed to ε-martensite. However, a small fraction (~4 pct) of α′-martensite could be observed when the same alloy was subjected to low strain compression tests in a Gleeble simulator. The stability of ε-martensite was attributed to the increase in stacking fault energy of the steel, expected to be more than 20 mJ/m² because of the increase in temperature during the cold rolling deformation.     

Development and characterization of high strength impact resistant Fe-Mn-(Al-, Si) TRIP/TWIP steels

Iron manganese steels with Mn mass contents of 15 to 30 % exhibit microstructural related superior ductility and extraordinary strengthening behaviour during plastic deformation, which strongly depends on the Mn content. This influences the austenite stability and stacking fault energy γ_fcc and shows a great impact on the microstructure to be developed under certain stress state or during severe plastic deformation. At medium Mn mass contents (15 to 20 %) the martensitic γ-ε-ά phase transformation plays an important role in the deformation mechanisms of the TRIP effect in addition to dislocation glide. With Increasing Mn mass content large elongation is favoured by intensive twinning formation. The mechanical properties of plain iron manganese alloys are strongly influenced by the alloying elements, Al and Si. Alloying with Al Increases the stacking fault energy and therefore strongly suppresses the martensitic γ-ε transformation, while Si sustains the γ-ε transformation by decreasing the stacking fault energy γ_fcc. The γ-ε phase transformation takes place in Fe-Mn-X alloys with γ_fcc ≤ 20 mJm⁻². The developed light weight high manganese TRIP and TWIP (twinning induced plasticity) steels exhibit high ultimate tensile strength (600 to 1100 MPa) and extremely large elongation of 60 to 95 % even at high strain rates of έ = 10³ s⁻¹. Particularly due to the advanced specific energy absorption of TRIP and TWIP steels compared to conventional deep drawing steels high dynamic tensile and compression tests were carried out in order to investigate the change in the microstructure under near crash conditions. Tensile and compression tests of iron manganese alloys with varying Mn content were performed at different temperatures and strain rates. The resulting formation of γ twins, ά- and ε martensite by plastic deformation was analysed by optical microscopy and X-ray diffraction. The deep drawing and stretch forming behaviour at varying deformation rates were determined by performing cupping tests and digitalised stress-strain-analysis.     

Twinning-Induced Plasticity Aided High Ductile Duplex Stainless Steel

Extended ductility over 70 pct was realized in duplex stainless steel by implementing twinning-induced plasticity (TWIP). The steel also exhibited the tensile strength over 800 MPa. The steel chemistry was designed so that the stacking fault energy of austenite was high enough to induce TWIP during deformation. After the initial decrease, the strain hardening rate increased at high tensile strains above ~30 pct. The microstructures of austenite at such high strains were manifested by well-developed primary twins and nanotwins between them, which effectively block dislocation motion. This observation ensures that extended ductility and high strength of a newly designed duplex stainless steel are originated from TWIP in austenite.     

Effect of hot‐rolling conditions on the tensile properties of multiphase steels exhibiting a TRIP effect

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.     

锆含量对高强韧Ti-Zr微合金化钢奥氏体组织的影响

为了满足各行业对钢铁材料提出的更高强度、更好韧性的要求,在钢中加入适量的微合金化元素并结合合理的控扎控冷工艺,有利于在高温轧制阶段获得细小均匀的奥氏体再结晶晶粒,这是提高钢材强度及韧性的有效途径之一。通过Gleeble-3800型热模拟试验机,对两种不同锆含量的低碳微合金Ti-Zr钢进行多道次压缩变形试验,模拟实际轧制情况,研究试验钢在不同锆含量和不同变形方式(等温变形和变温变形)下的热变形行为,并结合组织观察分析讨论了锆含量和变形条件对试验钢奥氏体组织细化行为和析出行为的影响。结果表明,变形温度的升高可以降低高锆钢各道次的流变应力。变温变形条件下,锆含量的升高会提高试验钢各道次的流变应力;奥氏体再结晶晶粒会随着锆含量的升高和变形温度的降低而发生细化,采用变温变形方式比等温变形方式更有利于得到细小的奥氏体晶粒,高锆钢在1 050℃→1 25℃→1 000℃变温变形后得到了最小的奥氏体平均晶粒尺寸(为8.2μm);锆含量升高会提高试验钢中析出相的数量,变形方式对析出相数量的影响不大,变形温度的升高会使析出相发生粗化。锆含量增加所导致的形变诱导析出相的增多,以及变温变形过程中温度的降低,起...     

Cyclic Deformation Behaviour of Three Austenitic Cast CrMnNi TRIP/TWIP Steels with Various Ni Content

This study presents the cyclic deformation behaviour of three high‐alloyed austenitic cast steels which are characterized by different chemical compositions leading to different austenite stabilities and stacking fault energies. Thus, depending on the chemical composition different deformation mechanisms arise which have a significant influence on the cyclic deformation behaviour and life time relations. The materials were characterized under total‐strain control. The fatigue life relations of Basquin and Manson‐Coffin are applied successfully for all steel variants. The cyclic stress‐strain response is described using the Ramberg‐Osgood relationship. It is shown that the parameters n' and K' depend strongly on the accumulated plastic strain λ_p. The mechanical properties are discussed together with microstructural investigations of deformation structures and martensitic transformations as well as twinning, respectively.     

An investigation of the effects of austenite strength and austenite stacking fault energy on the morphology of martensite in Fe-Ni-Cr-0.3C alloys

The relative effects of austenite stacking fault energy and austenite yield strength on martensite morphology have been investigated in a series of three Fe-Ni-Cr-C alloys. Carbon content (0.3 wt pct) andM ₆ temperature (− 15°) were held constant within the series. Austenite yield strength atM _s was measured by extrapolating elevated temperature tensile data. Austenite stacking fault energy was measured by the dislocation node technique. Martensite morphologies were characterized by transmission electron microscopy and electron diffraction techniques. A transition from plate to lath martensite occurred with decreasing austenite stacking fault energy. The austenite yield strength atM _s for the low SFE, lath-forming alloy was found to be higher than previously reported for lath-forming alloys. The relative effects of these variables on martensite morphologies in these alloys is discussed.     

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.