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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Strain aging behaviour of Cu-containing microalloyed low carbon seamless pipeline steel

The effect of strain aging on the microstructure, including copper precipitation, and mechanical properties of an industrially produced Cu-containing microalloyed low carbon pipeline steel has been investigated. The precipitation of round bcc-Cu particles with diameters of ～7?nm has been clearly observed in the microstructure after strain aging. The strength of pipeline steel significantly increased while the elongation and impact toughness did not apparently decrease. Unlike welded pipe, few carbon atoms in supersaturated solid solution diffuse to the mobile dislocations, forming Cottrell atmospheres and producing strain aging phenomenon in seamless pipe. This difference is attributed to the different pipe making technique: thermo mechanically controlled processed for welded pipe and traditional heat-treatment for seamless pipe.     

Molecular dynamics study of the mechanical behavior of nickel nanowire: Strain rate effects

We present the analysis of uniaxial deformation of nickel nanowires using molecular dynamics simulations, and address the strain rate effects on mechanical responses and deformation behavior. The applied strain rate is ranging from 1 × 10⁸ s⁻¹ to 1.4 × 10¹¹ s⁻¹. The results show that two critical strain rates, i.e., 5 × 10⁹ s⁻¹ and 8 × 10¹⁰ s⁻¹, are observed to play a pivotal role in switching between plastic deformation modes. At strain rate below 5 × 10⁹ s⁻¹, Ni nanowire maintains its crystalline structure with neck occurring at the end of loading, and the plastic deformation is characterized by {1 1 1} slippages associated with Shockley partial dislocations and rearrangements of atoms close to necking region. At strain rate above 8 × 10¹⁰ s⁻¹, Ni nanowire transforms from a fcc crystal into a completely amorphous state once beyond the yield point, and hereafter it deforms uniformly without obvious necking until the end of simulation. For strain rate between 5 × 10⁹ s⁻¹ and 8 × 10¹⁰ s⁻¹, only part of the nanowire exhibits amorphous state after yielding while the other part remains crystalline state. Both the {1 1 1} slippages in ordered region and homogenous deformation in amorphous region contribute to the plastic deformation.     

Microstructures and Mechanical Properties of Si-Al-Mn TRIP Steel with Niobium

Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP (transformation induced plasticity) steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10-2 to 10-4 s-1, the volume fraction of transformed martensite from retained austenite,as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.     

氮强化高锰奥氏体低温钢的拉伸应变硬化行为

采用低温拉伸、SEM和TEM等方法,对32Mn-7Cr-1Mo-0.3N奥氏体钢进行表征,研究了它的拉伸应变硬化行为.结果表明,32Mn-7Cr-1Mo-0.3N奥氏体钢的真应力与真应变不遵循Hollomon的线性关系,应变硬化指数n随着真应变的增大而提高,但当ε>0.2后,77 K下的dn/dε值明显高于其它温度的值.在77 K真应变ε>0.2后材料的d2σ/dε2变为正值.dn/dε与d2σ/dε2这一特殊变化趋势导致77 K下应变硬化率和延伸率的提高.其微观机制是,孪晶的形成速率以及孪晶与位错之间的相互作用与硬化率相协调,进而延迟了颈缩的产生,导致较高的均匀变形能力.     

Ageing behavior of a Cu-bearing ultrahigh strength steel

On ageing at different temperatures a various combination of properties has been obtained for this Cu-bearing ultrahigh strength steel. A substantial increase in strength has been obtained at 450 °C, accompanied by a drop in percentage elongation, percentage reduction in area and toughness. At 550 °C temperature extensive -Cu precipitates have been observed. The increased strength value retained in the temperature range of 450–600 °C and a secondary hardening peak obtained at 600 °C is probably due to the formation of fine Mo carbide precipitates. The decrease in strength at 650 °C along with an increase in percentage elongation, percentage reduction in area and toughness is due to the coarsening of Cu particles and a partial recovery of matrix. At 700 °C most of the Cu precipitates become rod shaped and formation of fresh martensite with a dark contrast is observed at the lath boundaries.     

Tensile deformation behavior of high manganese austenitic steel: The role of grain size

The tensile deformation behavior and microstructural evolutions of twinning induced plasticity (TWIP) steel with the chemical composition of Fe–31Mn–3Al–3Si and average grain sizes in the range of 2.1–72.6 μm have been analyzed. For each grain size, the Hollomon analysis and also the Crussard–Jaoul (C–J) analysis as an alternative method to describe the work hardening behavior were investigated. The results indicated that the optimum mechanical properties as a function of work hardening capacity can be obtained by changing the grain size. The microstructural observations showed that the pile-ups of planar dislocations are necessary for triggering the mechanical twinning and grain refinement suppresses the mechanical twinning in TWIP steel. Furthermore, the mechanical twinning increases with increasing applied strain. As a result, a high instantaneous work hardening due to the mechanical twin boundaries enhances the uniform elongation. The contribution from the strain of twinning and hardening due to an increase in the hardness of the twinned regions (i.e., the Basinski mechanism) may be also useful in achieving the high strength–ductility in TWIP steels.     

Simplification of heat treatment process in a tool steel by aluminium addition

A novel concept for simplification of heat treatment process in a tool steel by the addition of aluminium has been proposed in this research. The addition of 1.08?wt-% aluminium leads to an approximately fully pearlitic state, of which the cementite lamellae are largely spheroidised. Excessive addition of 1.58?wt-% aluminium would result in the formation of a large amount of δ-ferrite. These results are mainly attributed to the synthetic effect of aluminium on the driving force of pearlite transformation and the inter-spacing distance between the proeutectoid carbides. The mechanical properties’ analyses show that aluminium has promising potential to substantially simplify the processing method for developing a relative low-cost mould steel without the concomitant mechanical properties’ reductions.