On the Selection of the Optimal Intercritical Annealing Temperature for Medium Mn TRIP Steel

A model is proposed to predict the room temperature austenite volume fraction as a function of the intercritical annealing temperature for medium Mn transformation-induced plasticity steel. The model takes into account the influence of the austenite composition on the martensite transformation kinetics and the influence of the intercritical annealing temperature dependence of the austenite grain size on the martensite start temperature. A maximum room temperature austenite volume fraction was obtained at a specific intercritical annealing temperature T _M. Ultrafine-grained ferrite and austenite were observed in samples intercritically annealed below the T _M temperature. The microstructure contained a large volume fraction of athermal martensite in samples annealed at an intercritical temperature higher than the T _M temperature.     

Microstructure and Mechanical Properties of High Manganese TRIP Steel

 Abstract： Microstructure evolution and mechanical properties of newly designed 01C-6Mn-05Si-1Al TRIP-aided steels under different annealing conditions and the effects of matrix microstructure before intercritical annealing on the final microstructure were studied by means of X-ray diffraction (XRD), scanning electron microcopy (SEM), dilatometric simulation, optical microstructure (OM) and tensile testing in this work. The experimental results indicate that the TRIP steel with Mn of 6% could form a considerable amount of retained austenite with good TRIP effect after a simple intercritical annealing treatment, and the matrix microstructure before intercritical annealing treatment can greatly affect the final microstructure. The original microstructure of the ferritic matrix steel was eliminated, while annealed martensite was remained from the martensite matrix steel under the same intercritical annealing conditions.     

Influence of intercritical annealing temperature on microstructure and mechanical properties of medium Mn TRIP steel

The transformation, microstructure and mechanical properties of the 0. 2C- 5Mn TRIP steel after intercritical annealing were investigated using dilatometer, scanning electronic microscopy (SEM), transmission electron microscopy(TEM), X- ray diffraction (XRD), and tensile testing machine. The phase transformation thermodynamics of the investigated steel after intercritical annealing was calculated by Factsage software and the characteristics of the transformation were discussed. The results show that the reversed austenite content increases with the increasing of the intercritical annealing temperature, the carbon content in reversed austenite firstly increases and then decreases, manganese content in reversed austenite decreases, which results in the decreasing of the thermal stability of reversed austenite. When the intercritical annealing temperature is 700??, an obvious martensitic transformation occurs during the cooling process. With the increasing of intercritical annealing temperature, cementite is gradually dissolved, but it cannot be completely dissolved due to the short transformation time. When the intercritical annealing temperature is 600-675??, the microstructure after intercritical annealing consists of ferrite, cementite and retained austenite. When the intercritical annealing temperature is 700??, the microstructure after intercritical annealing consists of ferrite, retained austenite, martensite and a small amount of undissolved cementite. The engineering stress and strain curves of the investigated steel are significantly changed with increasing intercritical annealing temperature. At the same time, the optimal mechanical properties with tensile strength of 1138MPa and total elongation of 23% can be obtained after annealed at 675?? for 3min.     

连续退火参数对980 MPa级冷轧双相钢组织和性能的影响

采用带钢连续退火模拟试验机,研究了连续退火过程中加热速率、两相区保温温度和过时效温度对冷轧双相钢DP980组织和性能的影响规律。研究结果表明,适当提高加热速率有利于马氏体晶粒的细化和带状组织的改善,当加热速率达到45℃/s时可获得较高的强度和塑性。退火温度直接决定了硬质第二相的体积分数、分布和形貌,在800℃左右进行退火保温可以获得良好的综合性能,保温温度过低或过高都会导致强塑性匹配较差。随着过时效温度的降低,强度升高,伸长率下降,试验钢退火后加工硬化系数明显增大。     

两相区退火处理含铝中锰钢的组织和力学性能

 为了研究两相区退火处理对冷轧含铝中锰钢(0.2C-0.6Si-5Mn-1.2Al)(质量分数,%)微观组织和力学性能的影响规律,利用SEM、XRD及单轴拉伸等试验方法表征了不同工艺状态后的微观组织及测试了拉伸性能。结果表明,冷轧试验钢在退火过程中组织发生奥氏体逆转变,在退火温度为670 ℃、退火时间为10 min时可获得较佳的力学性能,即抗拉强度达到1 276 MPa,总伸长率达到51.8%,强塑积高达66.1 GPa·%。随着退火温度升高,残余奥氏体组织逐渐粗化且向马氏体组织转变,机械稳定性逐渐降低。残余奥氏体机械稳定性主要受残余奥氏体中碳质量分数及其晶粒尺寸的影响,而残余奥氏体中锰质量分数对其影响较小。     

Comparison Between Different Processing Schedules for the Development of Ultrafine-Grained Dual-Phase Steel

A comparative study was carried out on the development of ultrafine-grained dual-phase (DP) (ferrite–martensite) structures in a low-carbon microalloyed steel processed using two thermomechanical processing routes, (i) intercritical deformation and (ii) warm-deformation and intercritical annealing. The samples were deformed using Gleeble3500^® simulator, maintaining a constant total strain (ε = 1) and strain rate ( $ \dot \varepsilon $  = 1/s). Evolution of microstructure and micro-texture was investigated by SEM, TEM, and EBSD. Ultrafine-grained DP structures could be formed by careful selection of deformation temperature, T _def (for intercritical deformation) or annealing temperature, T _anneal (for warm-deformation and annealing). Overall, the ferrite grain sizes ranged from 1.5 to 4.0 μm, and the sizes and fractions of the uniformly distributed fine-martensitic islands ranged from 1.5 to 3.0 μm and 15 to 45 pct, respectively. Dynamic strain-induced austenite-to-ferrite transformation followed by continuous (dynamic) recrystallization of the ferrite dictated the grain refinement during intercritical deformation, while, continuous (static) recrystallization by pronounced recovery dictated the grain refinement during the warm-deformation and the annealing. Regarding intercritical deformation, the samples cooled to T _def indicated finer grain size compared with the samples heated to T _def, which are explained in terms of the effects of strain partitioning on the ferrite and the heating during deformation. Alpha-fiber components dominated the texture in all the samples, and the fraction of high-angle boundaries (with >15 deg misorientation) increased with the increasing T _def or T _anneal, depending on the processing schedule. Fine carbide particles, microalloyed precipitates and austenitic islands played important roles in defining the mechanism of grain refinement that involved retarding conventional ferrite recrystallization and ferrite grain growth. With regard to the intercritical deformation, warm-deformation followed by annealing is a simpler process to control in the rolling mill; however, the need for high-power rolling mill and controlled annealing facility imposes industrial challenges.     

低碳中锰热轧TRIP钢退火工艺及组织演变

研究了第三代高强度高塑性TRIP钢的退火工艺对性能的影响和组织演变规律.热轧后形成的原始马氏体与临界退火时形成的残余奥氏体使TRIP钢具有良好的强度和塑性.结果表明:实验用钢可获得1000MPa以上的抗拉强度和30%以上的断后延伸率,且强塑积>30 Gpa·%;退火温度和保温时间对钢的力学性能具有显著影响,热轧TRIP钢临界退火温度为630℃,保温时间18 h时,实验用钢能获得最佳的综合力学性能.      

Benefits of Intercritical Annealing in Quenching and Partitioning Steel

Compared to the quenching and partitioning (Q&P) steel produced by full austenization annealing, the Q&P steel produced by the intercritical annealing shows a similar ultimate tensile stress but a larger tensile ductility. This property is attributable to the higher volume fraction and the better mechanical stability of the retained austenite after the intercritical annealing. Moreover, intercritical annealing produces more ferrite and fewer martensite phases in the microstructure, making an additional contribution to a higher work hardening rate and therefore a better tensile ductility.     

Localized Deformation in Multiphase,Ultra-Fine-Grained 6 Pct Mn Transformation-Induced Plasticity Steel

Multiphase, ultra-fine-grained transformation-induced plasticity (MP UFG TRIP) steel containing 6 mass pct Mn was obtained by cold rolling and intercritical annealing of an initially fully martensitic microstructure. UFG microstructures with an average grain size less than 300 nm were obtained. The amount of austenite in the microstructures, speculated to be formed by diffusionless transformation, was controlled by changing the intercritical temperature. The tensile properties were strongly influenced by the volume amount and the stability of the reversely transformed austenite. The MP UFG TRIP steel was characterized by pronounced localization of the deformation. The deformation band properties were analyzed in detail.     

Enhanced mechanical properties of dual-phase steel by repetitive intercritical annealing

The effect of repetitive intercritical annealing on the mechanical properties and work-hardening response of a typical C–Mn dual-phase (DP) steel was studied by consideration of different pre-intercritical annealing microstructures and martensite volume fractions. It was revealed that the DP steels produced from the initial martensitic microstructure have much better mechanical properties compared with those originated from the ferritic–pearlitic banded microstructure. The repetitive intercritical annealing of the initial martensitic microstructure was effective for the enhancement of strength–ductility balance, where at a comparable ductility; a twofold increase in the tensile strength was obtained. Finally, these results were discussed based on the Crussard–Jaoul work-hardening rate analysis.     

Effect of Intercritical Annealing Time on the Microstructure and Mechanical Properties of Dual-Phase Steel Processed by Large-Strain Asymmetric Cold-Rolling

Herein, the effect of intercritical annealing time on the microstructure and mechanical properties of dual-phase steel processed by large-strain asymmetric cold-rolling is studied. It is observed that the martensite islands are uniformly distributed in the ferrite phase in the microstructures of dual-phase steels due to performing the asymmetric cold-rolling before intercritical annealing treatment. As the intercritical annealing time increases up to 10 min, the fraction of martensite increases. By increasing the holding time and fraction of martensite, the carbon content of the martensite phase is decreased. The short-term intercritical annealing eliminates the yield point phenomenon. However, intercritical annealing at 860 °C for 20 min leads to the reoccurrence of a yield point phenomenon. Increasing the intercritical annealing time to 10 min improves the yield strength to 505 MPa and ultimate tensile strength to 834 MPa. However, the strength decreases sharply after the holding time of 20 min. There is a perfect linear relationship between the mechanical properties and the fraction of martensite. Ductile failure is observed at the center of the fracture surfaces of dual-phase steels while shear failure occurs at the edges of the fracture surfaces.     

Ultrafine Grained Duplex Structure Developed by ART-annealing in Cold Rolled Medium-Mn Steels简

The microstructural evolutions of the cold rolled Fe-0.1C-5Mn steel during intercritical annealing were ex- amined using combined advanced techniques. It was demonstrated that intercritical annealing results in an ultrafine granular ferrite and austenite duplex structure in cold rolled 0.1C-5Mn steel. The strong partitioning of manganese and carbon elements from ferrite to austenite was found during intercritical annealing by scanning transmission elec- tron microscopy （STEM） and X-ray diffraction （XRD）. Strong effects of boundary characters on the austenite for- mation were indicated by austenite fast nucleation and growth in the high angle boundaries but sluggish nucleation and growth in the low angle boundaries. The ultrafine grained duplex structure in 0.1C-5Mn was resulted from the the sluggish Mn-diffusion and the extra high Gibbs free energy of ferrite phase. Based on the analysis of the micro- structure evolution, it was pointed out that the intercritical annealing of the medium Mn steels could be applied to fabricate an ultrafine duplex grained microstructure, which would be a promising approach to develop the 3rd genera- tion austomobile steels with excellent combination of strength and ductility.     

不同热处理工艺对Nb-Mo中锰钢组织和力学性能的影响

摘要：探讨了不同热处理工艺对Fe-0.25C-3.98Mn-1.22Al-0.20Si-0.19Mo-0.03Nb中锰钢组织演变与力学性能的影响。研究发现,与临界退火和淬火配分（IA & QP）工艺相比,奥氏体逆转变（ART）工艺处理后的实验钢获得了更为优异的力学性能;采用ART工艺经过680℃临界退火5h后的实验钢展现出了最佳的力学性能,即抗拉强度为830MPa,伸长率为48.9%,强度与塑性的乘积达到40.6GPa·%;ART工艺实验钢因长时间退火而促进了Mn向奥氏体中富集,有利于奥氏体含量及稳定性增加,在应变中后期可展现更为广泛的TRIP效应,更有利于获得优异的力学性能。另外,微合金元素Nb和Mo在中锰钢主要起析出强化和细晶强化的作用。     

逆相变退火对连铸坯组织和力学性能的影响

 研究了逆相变退火温度对0.1C5Mn钢连铸坯的组织结构和力学性能的影响规律,采用SEM进行组织结构的表征,利用XRD技术分析连铸坯退火后奥氏体含量,并测试了退火试样的力学拉伸性能。试验结果表明,连铸坯退火过程中发生奥氏体逆转变且在较低退火温度下有少量碳化物析出,随着退火温度升高,奥氏体含量先增加后减少,析出物逐渐溶解消失。提高退火温度可以显著提高试验钢的抗拉强度但却降低它的屈服强度,另外随退火温度升高,断后伸长率和强塑积先增高后降低。在625~650℃退火,可以获得20%~25%的伸长率。研究结果说明利用逆转变退火可以大幅度提高中锰钢铸坯的力学性能。     

Tensile Properties of Medium Mn Steel with a Bimodal UFG α + γ and Coarse δ-Ferrite Microstructure

While the tensile strength and elongation obtained for medium Mn steel would appear to make it a candidate material in applications which require formable ultra-high strength materials, many secondary aspects of the microstructure–properties relationships have not yet been given enough attention. In this contribution, the microstructural and tensile properties of medium Mn steel with a bimodal microstructure consisting of an ultra-fine grained ferrite + austenite constituent and coarse-grained delta-ferrite are therefore reviewed in detail. The tensile properties of ultra-fine-grained intercritically annealed medium Mn steel reveal a complex dependence on the intercritical annealing temperature. This dependence is related to the influence of the intercritical annealing temperature on the activation of the plasticity-enhancing mechanisms in the microstructure. The kinetics of deformation twinning and strain-induced transformation in the ultra-fine grained austenite play a prominent role in determining the strain hardening of medium Mn steel. While excellent strength–ductility combinations are obtained when deformation twinning and strain-induced transformation occur gradually and in sequence, large elongations are also observed when strain-induced transformation plasticity is not activated. In addition, the localization of plastic flow is observed to occur in samples after intercritical annealing at intermediate temperatures, suggesting that both strain hardening and strain rate sensitivity are influenced by the properties of the ultra-fine-grained austenite.     

Effect of intercritical temperature and cold-deformation on the kinetics of austenite formation during the intercritical annealing of dual-phase steels

The objective of this investigation was to study the effect of the intercritical temperature and percentage of cold-deformation on the kinetics auf austenite formation during the intercritical annealing in the alpha + gammy (α + γ) phase field of the iron-carbon phase diagram. This investigation was carried out on an Fe–0.11 C–1.58Mn–0.4 Si ferritic-pearlitic alloy with different structures of 0% (hot-rolled), 25% and 50% cold-deformation. The intercritical annealing temperatures were 735, 750°C and the intercritical annealing time ranged from 15 to 1815 s. It has been observed that recrystallization of the deformed ferrite was completed before any austenite formation. Surprisingly, it was noted that the recrystallized ferrite grain size was independent of percentage cold-deformation. Furthermore, it was expected that cold-deformation accelerates the kinetics of austenite formation. Nevertheless, the amounts of austenite formed from pearlite dissolution were mostly equal, irrespective of the starting condition. As has been previously reported, increasing the intercritical annealing temperature was found to increase the amount of austenite.     

C和Mn元素配分行为对冷轧中锰TRIP钢组织性能的影响

采用冷轧+两相区温轧退火（CR+WR+IA）热处理工艺,研究了两相区退火时间对超细晶铁素体与奥氏体中组织形貌演变、C和Mn元素配分行为以及力学性能的影响。结果表明,冷轧试验钢经两相区形变退火处理后,获得了由铁素体、残余奥氏体或新生马氏体组成的超细晶复相组织。在645℃随退火时间的延长,形变马氏体向逆相变奥氏体配分的C、Mn元素增多,C、Mn元素富集位置增加,同时富Mn区形变马氏体回复再结晶现象明显;伴随少量碳化物溶解,试验钢的屈服强度由741持续降低到325MPa。两相区退火10min时,试验钢力学性能最佳,此时抗拉强度达到最大值1141MPa,断后伸长率及均匀伸长率分别为236%和181%,强塑积达到26928MPa·%。     

退火工艺对780 MPa冷轧多相钢组织及性能影响

为了研究退火工艺对780 MPa冷轧多相钢组织及性能影响,采用万能试验机、场发射扫描电镜(SEM)、综合成形试验机及电子探针(EPMA)对两种具有相同成分、抗拉强度均为780 MPa但具有不同屈强比和扩孔性能的多相钢进行了相关研究,结果表明热处理工艺对试验钢组织、力学性能及扩孔性能有着重要影响,采用两相区退火时马氏体岛...     

On the Effect of Manganese on Grain Size Stability and Hardenability in Ultrafine-Grained Ferrite/Martensite Dual-Phase Steels

Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM). The Mn distribution in ferrite and martensite is analyzed by high-resolution electron backscatter diffraction (EBSD) combined with energy dispersive X-ray spectroscopy (EDX). The experimental findings are supported by the calculated phase diagrams, equilibrium phase compositions, and the estimated diffusion distances using Thermo-Calc (Thermo-Calc Software, McMurray, PA) and Dictra (Thermo-Calc Software). Mn substantially enhances the grain size stability during intercritical annealing and the ability of austenite to undergo martensitic phase transformation. The first observation is explained in terms of the alteration of the phase transformation temperatures and the grain boundary mobility, while the second is a result of the Mn enrichment in cementite during large strain warm deformation, which is inherited by the newly formed austenite and increases its hardenability. The latter is the main reason why the ultrafine-grained material exhibits a hardenability that is comparable with the hardenability of the coarse-grained reference material.     

Contribution to the investigation of microsegregation processes in 13% Cr 4% Ni steel during long-term annealing at 400°C

The present paper deals with the study of the development of microsegregation processes in the 13% Cr 4% Ni martensiteaustenite stainless steel during long-term annealing. The long-term annealing of 13% Cr 4% Ni steel at 400°C is accompanied by the decrease in notch impact toughness values, which is associated with an increasing tendency to the occurrence of the brittle failures. The conditions for the transition from the transcrystalline brittle failure to the intercrystalline brittle failure depend on the initial heat treatment affecting the achieved microstructure of investigated steel. The higher frequency occurrence of intercrystalline failure on the fracture surfaces of notch impact toughness specimens is accompanied by an enrichment of the prior austenite grain boundaries by phosphorus and nitrogen. At the same time the enrichment of intercrystalline fracture surfaces by nickel, or chromium was also observed. The application of an additional intercritical annealing after quenching accelerates the formation of intercrystalline failure towards shorter times during the isothermal annealing at 400°C.