18.8%MnTRIP/TWIP钢拉伸应变硬化行为

对锰含量为18.8%的TRIP/TwIP钢进行单轴拉伸实验,研究了这种钢的应变硬化行为.结果表明:这种高锰TRIP/TWIP钢的真应力应变曲线不完全遵循Holliomon的线性关系,在不同变形阶段强化机制不同.在塑性变形的开始阶段TRIP效应比较明显,且应变硬化指数n是恒定的;而真应变在O.14-0.35之间时二阶导数d2σ/dε2>0,应变硬化指数n随着应变量的增加而增加,其微观机制是形成大量的形变孪晶,并有孪晶和位错的交互作用,TWIP效应在该阶段占主导作用.真应变大于0.35后有少量TRIP效应,此时两相均发生变形.     

Strain hardening behavior of a Fe–18Mn–0.6C–1.5Al TWIP steel

The strain hardening behavior of a Fe–18Mn–0.6C–1.5Al TWIP steel was investigated through the modified Crussard–Jaoul (C–J) analysis and microstructural observations. The strain hardening rate obtained by modified C–J analysis was high up to the critical strain of 37% and then greatly decreased with further strain. The electron backscatter diffraction (EBSD) observation showed that the deformation twinning rate is greatly decreased beyond about 34% strain, indicating that the reduced strain hardening rate at the large strain region is attributed to the deceleration of deformation twinning rate. The volume fraction of twinned region was increased with tensile strain due to the increase in the number of deformation twins not to the lateral growth of each deformation twin.     

Deformation behaviour of a low carbon high Mn TWIP/TRIP steel

TWIP and TRIP phenomena have been observed in Fe–20Mn–2.5Si–0.3Al–0.06C (wt-%) steel during cold deformation (CD). Mostly austenite, annealing twins and stacking faults are observed in hot rolled solution treated (HRACST) samples. Cold deformation results in γ→?→α′ transformation due to its low stacking fault energy (18?mJ?m^?2). 50CD (50% CD) sample reveals strong Goss, Brass and weak Copper Twin texture components due to slipping and micro-twinning. Maximum ductility of 62% with lower tensile strength is perceived in HRACST sample, whereas, 30CD sample shows excellent tensile strength (1039?MPa) with a lower ductility (23%). Premature failure in 50CD samples is related to the formation of α′-martensite (≈35%) and deformation twins along with a higher strain hardening due to low Al content.     

Characterization of Tensile Strain Hardening Behaviors for 32Mn-7Cr-1Mo-0.3N Cryogenic Austenitic Steel

The strain hardening behaviors of 32Mn-TCr-1Mo-0.3N austenitic steel were characterized by a simple and effective method. The results show that Hollomon relationship is not applicable during total uniform deformation stage. The flow equation was proposed, Inσ=αexp（lnε/b）＋c. The variation rates of strain hardening exponents with true strain at 77 K are obviously higher than that at other temperatures and the value of d^2σ/dε^2 becomes positive during high strain stage. The characters of this variation are principal reasons for increasing elongation at 77 K. The forming of mechanical twin as well as ε-martensite leads to a high elongation at 77 K.     

Crystallographic texture evolution and martensite transformation in the strain hardening process of a ferrite-based low density steel

The strain-induced martensite transformation is of great importance in the strain hardening process of ferrite based low-density steel.Based on the microstructure analysis,the texture evolution and martensite transformation behavior in the strain hardening process were studied.The results show that martensite transformation accompanied by TWIP effect and high density dislocations maintains the con-tinuous hardening stage.As the strain increases,the texture of retained austenite evolves towards the F orientation{111}〈112〉,which is not conducive to martensite transformation.After the strain of 5％,the number of austenite grains with high Schmid factor orientations is gradually increased,and then signif-icantly reduced when the strain is over 10％due to the occurrence of martensitic transformation,which results in a high martensitic transformation rate.However,the unfavorable orientation and the reduced grain size of austenite slow down the martensite transformation at the final hardening stage.Moreover,because of the coordination deformation of austenite grains,strain preferentially spreads between adja-cent austenite grains.Consequently,the martensite transformation rate in strain hardening process is dependent on the orientation and grain size evolution of austenite,leading to a differential contribution to each strain hardening stage.     

Improved tensile properties of partially recrystallized submicron grained TWIP steel

The effects of cold rolling reduction and annealing temperature on the mechanical properties of twinning induced plasticity (TWIP) steel have been investigated. The results indicated that the strengthening effect of unrecrystallized areas with a high density of nano-scale mechanical twins increased with increasing cold rolling reduction. In addition, the ductility also increased with increasing annealing temperature. Therefore, utilization of large cold rolling reduction and subsequently annealing treatment in the partial recrystallization region was suggested as an effective method to obtain submicron grained TWIP steel with an excellent combination of strength and ductility.     

A novel high manganese austenitic steel with higher work hardening capacity and much lower impact deformation than Hadfield manganese steel

To tackle the problem of poor work hardening capacity and high initial deformation under low load in Hadfield manganese steel, the deformation behavior and microstructures under tensile and impact were investigated in a new high manganese austenitic steel Fe18Mn5Si0.35C (wt.%). The results show that this new steel has higher work hardening capacity at low and high strains than Hadfield manganese steel. Its impact deformation is much lower than that of Hadfield manganese steel. The easy occurrence and rapid increase of the amount of stress-induced ε martensitic transformation account for this unique properties in Fe18Mn5Si0.35C steel. The results indirectly confirm that the formation of distorted deformation twin leads to the anomalous work hardening in Hadfield manganese steel.     

Predictive modeling of multi-track laser hardening of AISI 4140 steel

Laser hardening provides benefits over the conventional hardening processes, including minimum distortion in the parts and the absence of a quenchant. This process is also faster than conventional hardening processes and can be used for selective hardening of specific areas of components. One known problem with laser hardening in steels, however, is back tempering when a large area is hardened by multiple, overlapping passes. This study focused on the development of a numerical model to predict the back tempering in multi-track laser hardening. A tempering model was combined with existing models of thermal behavior and phase change kinetics, which were developed earlier in the authors’ group, to predict three-dimensional hardness profiles after multiple track laser hardening. The combined model was first validated through multi-track laser hardening tests and then used to predict and optimize the laser hardened case depth in multi-track laser hardening of AISI 4140 steel. The predictions and parameters optimized to obtain maximum case depth with the least variation along width of the hardened zone were experimentally verified. Case depths up to 2 mm were obtained with 5 mm overlapping of laser tracks.     

Application of nanoengineering to research and development and production of high strength steel sheets

Abstract

The design concepts and properties of three unique high strength steel sheets developed utilising nanoengineering are reviewed. The first steel is developed by optimising the distribution of nanoprecipitates and exhibits low yielding ratio in spite of being strengthened by grain refinement and precipitation hardening. The second is the ferrite single phase tensile strength 780 MPa grade advanced high strength steel sheet utilising the thermally stable nanosized precipitates, which possesses significantly well balanced elongation and stretch flangeability. These two were already commercialised. The last is the ultrahigh strength steel, of which the formability is enhanced by optimising the combination of hard phases. The steel consists of bainite, retained austenite and tempered martensite and exhibits 35% of elongation with 1470 MPa of tensile strength. Although further optimisation of the composition and the processing are needed to produce and commercialise the steel, the results indicate that the approach has the potential to improve formability dramatically.     

Fire resistance of ultra-high performance strain hardening cementitious composite: Residual mechanical properties and spalling resistance

Ultra high performance strain hardening cementitious composites (UHP-SHCC) is a special type of cement-based composite material with outstanding mechanical and protective performance at room temperature. But its fire performance is unknown and there is a lack of research in this aspect. This study presents an experimental program to study fire resistance of UHP-SHCC under two aspects, viz. high-temperature explosive spalling resistance and residual mechanical performance after a fire. Both compressive strength and tensile strength of UHP-SHCC were found to deteriorate with increasing exposure temperature. Tensile strain-hardening feature of UHP-SHCC would be lost at 200 °C and above. It was found that PE fibers are found not effective in mitigating explosive spalling, although they start to melt at 144 °C. FE-SEM (Field Emission Scanning Electron Microscopy) and EDX (Energy Dispersive X-ray) techniques were used to study the state of fiber, fiber/matrix interaction, and microcracks development. Microscopic study found that melted PE fibers were still present in the cementitious matrix, and the melting did not introduce more microcracks. Furthermore, it was difficult for melted PE fibers to diffuse through the matrix, thus providing the reason that PE fibers did not mitigate explosive spalling in UHP-SHCC.     

Influences of Thermal Martensites and Grain Orientations on Strain-induced Martensites in High Manganese TRIP/TWIP Steels

Strain-induced martensites in high manganese TRIP/TWIP steels were investigated in the presence of thermal martensites and under the influence of austenitic grain orientation by X-ray diffraction(XRD),scanning electron microscopy(SEM) and electron backscattered diffraction(EBSD).Before deformation,the morphology of α’-M depended mainly on the number of variants and growing period.Regardless of martensite morphologies and deformation,the Kurdjumov-Sachs(K-S) orientation relationships always maintained.The 6 α-M variants formed from a plate of ε-M were of 3 pairs of twins with a common axis <110> α’ parallel to the normal of {111} γ habit plane to minimize transformation strain.When α’-M could be formed only by deformation,it nucleated at the intersection of ε-M variants and grew mainly in thick ε-M plates.Thick ε plates promoted significantly the α’-M and weakened the influence of grain orientations.During tension,the transformation in <100>-oriented grains was observed to be slower than that in <111>-oriented grains.Deformation twins promoted ε-M formation slightly and had no apparent effect on α’-M.Deformation increased the number of ε-M variants,but reduced that of α’-M variants.     

Mechanical and Transformation Behaviors of a C-Mn-Si-Al-Cr TRIP Steel under Stress

Transformation induced plasticity （TRIP） steels combine high strength and excellent ductility, making them suited for application in crash-relevant parts in the automotive industry. However, the high Si contents in the conventional TRIP steel will generate surface defects on the hot rolled strip, which is difficult to process in continuous galvanizing lines. In order to solve the above problem the TRIP steel with the addition of Al replacing majority of Si was designed. In the present paper, the volume fraction of various phases in a C-Mn-Si-Al-Cr TRIP steel was determined by metallographic examination and X-ray diffraction analysis, and the multi-phase microstructures were characterized using an atomic force microscope based on their height difference. Tensile tests were performed at different temperatures ranging from -40℃ to 90℃. The results show that transition temperature Ms^σ in the present TRIP steel cannot be determined due to its lower volume fraction of retained austenite, different from the conventional TRIP steel. While the yield stress and tensile strength at different temperatures are higher than those of the conventional TRIP steel, which is attributed to the addition of Cr. In order to evaluate the effect of martensitic transformation on the total elongation, the sample without retained austenite obtained by quenching in liquid nitrogen was carried out under tensile test. The results indicate that the elongation of the original sample containing 9% retained austenite is about 20% higher than that of the sample quenched in liquid nitrogen, which demonstrates that the retained austenite plays an important role in improving the elongation of the TRIP steel.     

Cryogenic deformation microstructures of 32Mn–7Cr–1Mo–0.3N austenitic steels

The cryogenic deformation microstructures of impact and tensile specimens of 32Mn–7Cr–1Mo–0.3N austenitic steel were investigated using light microscopy and transmission electron microscopy. The results show that the deformation microstructures of the impact specimens are mainly composed of stacking faults, network dislocation, slip bands, and a few mechanical twins and -martensite. These microstructures cross with each other in a crystal angle. The deformation microstructures of the tensile specimens consist only of massive slip bands, in which a few mechanical twins and -martenite are located. Because of the larger plastic deformation the slip band traces become bent. All the deformation microstructures are formed on the {111} planes and along the <110> orientation.     

Analysis of the tensile behavior of a TWIP steel based on the texture and microstructure evolutions

The texture and microstructure evolutions of a fine-grained TWIP steel subjected to tensile tests at room temperature were investigated in relation to the mechanical behavior. This steel combines both high ductility and strength owing to the TWIP effect. Also the steel exhibits a high strain hardening rate that evolves according to five stages, which are related to the microstructure and texture evolutions and characteristics. The formation of nano-twins in the initial stage of deformation leads to an increase in strain hardening rate. The development of the pronounced <1 1 1> fiber in the tensile direction sustains mechanical twinning and maintains the strain hardening rate on a high level. The resulting microstructure exhibits several types of twin configurations and sub-boundaries with high misorientations due to intense activities of dislocation glide. The twin volume fraction was estimated to be 9% at the final stage of tensile deformation. The new orientations generated by mechanical twinning do not change considerably the final texture.     

Microstructure and tensile properties of the laser welded TWIP steel and the deformation behavior of the fusion zone

Microstructure and tensile properties of the laser welded joint of Fe–18.8Mn–0.6C TWIP steel were investigated in this research. The microstructure of fusion zone (FZ) was characterized by means of X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). TEM and in-situ SEM observation were employed to investigate the microstructural evolution and strengthening mechanism of FZ during deformation. The welded joint with a fully austenitic structure was obtained by the laser welding. The granular divorced eutectic phases (Fe, Mn)₃C and inclusions formed in the interdendritic regions during the solidification of FZ. The fully austenitic structure and coarse dendrite grains were responsible for the fracture at the weld seam. The FZ exhibited a good combination of strength (e.g. tensile strength up to 1000 MPa) and ductility (e.g. total elongation up to 73%). The microstructural evolution revealed that dislocation slip was the main deformation mechanism at low strains of FZ, while at relatively high strains, mechanical twinning was the domain deformation mechanism and played an important role in improving the strength and ductility as well as the work-hardening effect of FZ.     

Transmission and scanning electron microscope study on the secondary cyclic hardening behavior of interstitial-free steel

Strain controlled fatigue experiment was employed to evaluate automotive grade interstitial-free ferrite steel. Hundreds of grains were examined by scanning electron microscope under electron channeling contrast image technique of backscattered electron image mode for comprehensive comparison of micrographs with those taken under transmission electron microscope. The cyclic stress responses clearly revealed that rapid hardening occurs at the early stage of cycling as a result of multiplication of dislocations to develop loop patches, dipolar walls and dislocation cells at various total strain amplitudes. After primary rapid hardening, stress responses varied from being saturated to further hardening according to dislocation structure evolution at various strain amplitudes. The fatigue failure was always accompanied with further hardening including secondary hardening. The corresponding dislocation structures with the three types of hardening behaviors are discussed. Once the secondary hardening starts, dislocation cells began to develop along grain boundaries in the low strain region and then extended into grain interiors as strain amplitudes increased and cycling went on. The secondary hardening rates were found to be directly proportional to their strain amplitudes.     

Material behaviour of a dual hardening steel under thermomechanical loading

Dual hardening steels are a group of metals, which reach their material properties through a combination of strengthening via carbides and intermetallic precipitates. Because of their combination of mechanical properties, dual hardening steels are a promising alloying concept for hot‐work applications. The applied materials for hot‐work applications have to meet certain requirements, such as high hardness, high thermal strength, thermal stability, and fracture toughness. In this paper, a dual hardening steel in different heat treatment conditions was tested under out‐of‐phase thermomechanical loading conditions. All tests were done under full reverse strain control and the minimum temperature was kept constant. In the thermomechanical fatigue tests, solution annealed samples reached higher lifetimes compared with aged specimens. The hardness measurements show that the starting procedure of the thermomechanical fatigue leads to an increase of the hardness approximate to the values of the specimens with the ageing heat treatment. Cyclic softening can be observed in the test with the highest maximum temperature of 600°C. An increase of the maximum temperature also causes a decrease of the lifetime.     

2.47 GPa grade ultra-strong 15Co-12Ni secondary hardening steel with superior ductility and fracture toughness

This study investigated the effect of multi-step heat treatment on the microstructure, mechanical properties and fracture behavior of thick 15 Co-12 Ni secondary hardening steel. As-quenched sample was found to have elongated prior austenite grain(PAG) and coarse lenticular martensitic structure. On the other hand, heat-treated sample was observed to have fine lenticular martensitic structure due to fine PAG size and a lot of nano-sized carbides. Also, after heat treatment, nano-scale reverted austenite film was formed at the martensite interfaces. The heat-treated sample showed 2.47 GPa superior tensile strength and superior elongation of about 12 %. The high strength was mainly due to fine block size and high number density of nano-sized carbides. The average value of plane strain fracture toughness(KIC) was 29.3 MPa m1/2, which indicated a good fracture toughness even with the high tensile strength. The tensile fracture surface was observed to have ductile fracture mode(cup-and-cone) and the formation of about ~1 μm ultra-fine dimples. In addition to this, nano-sized carbides were observed within the dimples.The findings suggested that the nano-sized carbide had a positive effect not only on the strength but also on the ductility of the alloy. The fractured surface after toughness test, also showed ductile fracture mode with a lot of dimples. Based on the above results, correlation among microstructural evolution,deformation and fracture mechanisms along the heat-treatment was also discussed.     

Mn含量对低碳中锰TRIP钢组织性能的影响

为研究连续退火工艺生产中锰TRIP钢汽车板的可行性,采用CCT-AY-Ⅱ型钢板连续退火机模拟分析了不同锰含量对中锰TRIP钢组织性能的影响规律.采用SEM、TEM和EBSD等微观分析方法观察不同锰含量中锰TRIP的微观组织,利用XRD法测量了残留奥氏体量,实验测量其力学性能.结果表明:试验钢在650℃保温3 min时,随着锰质量分数(4.8%≤w(Mn)≤8%)的增加,屈服强度先增加后降低,抗拉强度持续升高,断后延伸率则基本不变,维持在20%左右,残余奥氏体含量也随着锰含量的增加而增加;当锰质量分数超过6%(含6%)时,真实应力-应变曲线由于动态应变时效而呈锯齿状,且加工硬化指数远大于5Mn钢.试验钢的高塑性由亚稳奥氏体的TRIP效应和超细晶铁素体或马氏体共同提供.     

耐酸性高Mn奥氏体钢的实验研究

为适应特殊油气开采环境的复杂工况条件,提高设备的使用寿命和安全性,降低开采成本,不同于常规管线钢低C低Mn的合金设计思路,采用高C高Mn成分体系获得了综合性能优异的新型耐酸性奥氏体钢.通过拉伸实验、冲击试验以及氢致开裂实验等方法对其综合性能进行研究,并利用光学显微镜、扫描电镜、透射电镜等手段对高Mn奥氏体钢的组织进行了观察分析.研究结果表明:实验钢抗拉强度达到1 153 MPa,屈强比仅为0.46,伸长率高达50%,-40℃冲击功达到123 J,同时A溶液条件下经96 h浸泡未发现氢鼓泡及裂纹.实验钢显微组织为单相奥氏体组织,组织中存在大量位错、层错以及孪晶.与常规管线钢相比较,实验钢具有低屈强比、高均匀塑性变形的优点.此外,奥氏体组织的溶氢能力极强,本实验钢具有优良的抗氢致开裂腐蚀性能.