Multi-phase microstructure design of a low-alloy TRIP-assisted steel through a combined computational and experimental methodology

The multiphase constitution of a transformation-induced plasticity (TRIP)-assisted steel with a nominal composition of Fe–1.5Mn–1.5Si–0.3C (wt.%) was designed, utilizing a combination of computational methods and experimental validation, in order to achieve significant improvements in both strength and ductility. In this study, it was hypothesized that a microstructure with maximized ferrite and retained austenite volume fractions would optimize the strain hardening and ductility of multiphase TRIP-assisted steels. Computational thermodynamics and kinetics calculations were used to develop a predictive methodology to determine the processing parameters in order to reach maximum possible ferrite and retained austenite fractions during conventional two-stage heat treatment, i.e. intercritical annealing followed by bainitic isothermal transformation. Experiments were utilized to validate and refine the design methodology. Equal channel angular pressing was employed at a high temperature (950 °C) on the as-cast ingots as the initial processing step in order to form a homogenized microstructure with uniform grain/phase size. Using the predicted heat treatment parameters, a multiphase microstructure including ferrite, bainite, martensite and retained austenite was successfully obtained. The resulting material demonstrated a significant improvement in the true ultimate tensile strength (～1300 MPa) with good uniform elongation (～23%), as compared to conventional TRIP steels. This provided a mechanical property combination that has not been exhibited before by low-alloy first-generation high-strength steels. The developed computational framework for the selection of heat treatment parameters can also be utilized for other TRIP-assisted steels and help design new microstructures for advanced high-strength steels, minimizing the need for cumbersome experimental optimization.     

Local laser heat treatment of ultra high strength steels to improve formability

Ultra high strength steels are of enormous interest especially in the automotive industry due to their potential in realising light weight structures and improving the crash behaviour. However the poor formability of these steels limits their application for many parts in the car body. A solution to this limitation can be a local heat treatment using a laser beam to soften the material where a high formability is needed. Laser treatment was performed using a Nd:YAG laser with 3 kW maximum power. The output power was temperature controlled to achieve a constant temperature level during the heat treatment. Large areas are treated by scanning the surface with the laser beam. The materials under investigation are dual phase (DP), retained austenite (RA) and martensitic (MS) steels with a tensile strength of 600–1,200 MPa. The microstructure of DP steels consists of martensite and ferrite. RA steels contain ferrite, martensite and additionally a certain amount of retained austenite which transforms into martensite during plastic flow. MS steels are fully martensitic which gives them the highest yield strength of all UHS steels. The various steels were provided as not galvanised sheets (hot or cold rolled) with a thickness of 1.5 mm. Depending on material and process parameters tempering of martensite, formation of ferrite and transformation of retained austenite to martensite are observed as a result of the heat treatment. Tensile tests of DP 600, DP 1000 and MS-W 1200 reveal a significant reduction in yield and tensile strength and an increase in elongation after LHT. The effect is due to tempering of martensite and in the case of MS-W also due to an increase in volume fraction of ferrite. Tensile tests of RA-K 700 reveal a minor reduction in yield and tensile strength and a decrease in elongation. This can be attributed to the lower content of retained austenite which has transformed into martensite during LHT. From this result it can be expected that LHT is not beneficial for high deformation degrees. Deep drawing of a mock-up geometry using MS-W 1200 and RA-K 700 showed a significant decrease in slide force (~20%) compared to the initial condition. This results show that laser heat treatment has the potential to improve formability of UHS steels.     

铌微合金化高强塑积冷轧DP780钢的I&Q&P工艺

基于亚稳奥氏体形变诱导相变理论,在实验室采用盐浴炉对800 MPa级冷轧双相钢DP780的I&Q&P(临界退火与淬火配分)工艺进行了探讨,并采用光学显微镜、扫描电镜、拉伸试验机与XRD对不同工艺下试验钢的组织性能进行了研究。结果表明,在I&Q&P工艺试验条件下,试验钢的显微组织由铁素体、马氏体与残留奥氏体组成;830 ℃退火时铁素体晶粒尺寸以＞5 μm为主,860 ℃退火下其晶粒尺寸以＜5 μm为主。830 ℃退火时试验钢的力学性能随淬火温度的变化波动较大,860 ℃退火时试验钢的力学性能随淬火温度的变化波动较小。860 ℃退火+260 ℃淬火时,试验钢的综合力学性能最佳,其抗拉强度、伸长率与强塑积分别为802 MPa、26.8%与21.5 GPa·%,钢中残留奥氏体含量高达13.89%。     

低碳-硅-锰系TRIP钢的组织演变规律研究

采用彩色金相、SEM、TEM和X射线衍射技术研究了低碳-硅-锰TRJP钢在单向拉伸状态下的组织演变规律.结果表明,TRIP钢变形前的组织为F、B和残余奥氏体,经拉伸变形后部分残余奥氏体在应变作用下转变为孪晶结构的马氏体,提高了钢的强度;TRIP钢的断裂为韧性断裂,位于F晶界处的残余奥氏体发生相变从而松弛了应力,延缓了断裂的产生,使TRIP钢板获得高塑性.     

Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel

Phase transformation from austenite to ferrite is an important process to control the microstructures of steels. To obtain finer ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static ferrite transformation have been carried out for austenite phase. This article reviews the dynamic transformation (DT), in which ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress–true strain curve analysis. This analysis predicted the formation of ferrite grains even above the Ae₃ temperature (ortho-equilibrium transformation temperature between austenite and ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae₃ was experimentally confirmed. Moreover, the change in ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of ferrite.     

The Structure Stability of Carbide-Free Bainite Wheel Steel

Metallographic structures of carbide-free bainite steel wheel rim are mainly composed of supersaturated lath ferrite and retained austenite film among bainitic ferrite laths. It is suspected that supersaturated ferrite and retained austenite are likely to decompose under the influence of temperature change and mechanical stress. Stability of wheel rim structure is studied by means of x-ray diffraction, dye microscopy, and micro-hardness test. When the samples are tempered in the range of 150-350 °C, the retained austenite films are at the state of relative stability. Fifty percent of retained austenite is decomposed when the sample is tempered at 400 °C. Microhardness increases when the sample is tempered at 150 °C. The decrease in hardness is mild when the samples are tempered from 200 to 500 °C. The mechanical stability of retained austenite film is studied with tensile sample under the effect of tensile stress. The retained austenite appears to be stable in low and middle degree of deformation, and decomposition occurs at great amount of deformation. Diffraction peak of carbide is not found in all above experiments. The steel enriched silicon prevents the carbide precipitation during the transformation. It indicates the carbide-free bainite wheel steels have an excellent thermal and mechanical stability.     

Effect of silicon content and annealing temperature on formation of retained austenite and mechanical properties in multi-phase steels

Cold-rolled and annealed ultra-high strength sheet steels with good ductility accompanied by TRIP of retained austenite have received considerable attention in recent years. This paper discusses the effect of silicon content and annealing temperature on the formation of retained austenite and the mechanical properties in Fe-0.34%C-1.7% Mn steels whose structure consists of ferrite, bainite and retained austenite. Silicon inhibited the cementite formation in bainite during isothermal holding and partitioned carbon from bainite to austenite, resulting in an increase in retained austenite content. When the silicon content was increased to 1.0 wt.% or higher, the amount of retained austenite markedly increased leading to good mechanical properties. 0.34%C-1.03%Si-1.7%Mn steel showed a high tensile strength of 1,030 MPa and a total elongation of 34.5% when annealed at 780°C for 5 min followed by isothermal holding at 400°C for 5 min. In this case, the amount of retained austenite was about 25%. The variation in tensile strength-elongation combination had good correlation with that in the amount of retained austenite with both annealing temperature and silicon content. The most retained austenite was obtained in the steel annealed at just above A_C1 temperature. The annealing temperature which gives the most retained austenite was decreased with decreasing the silicon content.     

等温温度对低硅含铝热轧TRIP钢组织性能的影响

以低Si含Al热轧TRIP钢为研究对象,采用扫描电子显微镜、拉伸试验、X射线衍射仪和电子探针等试验方法,研究了不同等温温度对试验钢组织性能的影响。结果表明,试验钢的显微组织主要由多边形铁素体、贝氏体铁素体和残余奥氏体组成,随着等温温度的升高,残余奥氏体分解为新生成铁素体和碳化物;当等温温度为450 ℃时,试验钢的力学性能最佳,其抗拉强度为732.25 MPa,断后伸长率为36%,强塑积为26.36 GPa·%;残余奥氏体的体积分数先升高后降低,而C含量逐渐降低,等温温度为450 ℃时试验钢表现出较强的加工硬化行为。     

Microstructures and Mechanical Properties of Bearing Steels Modified for Preparing Nanostructured Bainite

Mo containing high-C-Cr bearing steel was modified with Si (0.8–1.5 wt.%) and 0.8Si–1.0Al to prepare nanostructured bainite by low-temperature isothermal heat treatment. The modified steels were isothermal held at 220 to 240 °C after partial austenitization in an intercritical gamma+carbide region, and the resultant microstructure and mechanical properties were studied. Carbide-free nanostructured bainite with plate thickness below 100 nm and film retained austenite, as well as a small amount of undissolved carbide particles, was obtained in the modified steels except in 0.8Si steel, in which carbides precipitated in bainitic ferrite. As Si content increased, the mean thickness of bainitic ferrite plates modestly decreased, whereas the fraction of retained austenite markedly increased. The thickness of bainitic ferrite plate and the fraction of retained austenite in Si-Al-modified steel were smaller than those in Si-modified steels. The hardness and elongation of the Si-Al-modified steel were lower than those of Si-modified steels. The yield strength of Si-Al-modified steel was superior to that of Si-modified steels. Mid-level ultimate tensile strength and impact toughness were achieved in Si-Al-modified steel. For bearing applications, Si-modified steels could provide higher hardness and toughness but lower dimensional stability. Meanwhile, Si-Al-modified steel could offer higher dimensional stability but lower hardness and toughness.     

A quantitative atom probe study of the Nb excess at prior austenite grain boundaries in a Nb microalloyed strip-cast steel

Most modern HSLA steels rely on the effect of Nb in steels to achieve the properties desired for a specific application. While the role of Nb in forming precipitates has been well characterized, its role in a solid solution is less well understood due to the difficulty of obtaining quantitative experimental data. In the current work, site-specific atom probe tomography was used to quantify the amount of Nb present at prior austenite grain boundaries in a commercial strip-cast steel, produced via the Castrip^® process. This was compared to the amount of Nb found at ferrite–ferrite grain boundaries that had formed during the transformation from austenite to ferrite. With the interfacial excess Nb measured, thermodynamic calculations were carried out and compared to the change in transformation temperature obtained by dilatometry, with reference to a comparable Nb free, strip-cast steel.     

Effect of strain rate on deformation behavior of TRIP steels

Tensile deformation behavior of Si–Mn TRIP (TRansformation Induced Plasticity) steel with vanadium and without vanadium and the DP (Dual Phase) steel of the same composition were studied in a large range of strain rate (0.001–2000 s^?1) by routine material testing machine, rotation disk bar–bar tensile impact apparatus and high-speed material testing machine of servo-hydraulic type. In situ measurement of the transformation of retained austenite was performed by means of X-ray stress apparatus in order to have detailed knowledge about the transformation of retained austenite at quasi-static tensile. Microstructure of steels before and after tensile were observed by means of optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). It is shown that there is no yield plateau observed on the stress–strain curve at quasi-static condition for TRIP steel containing vanadium because the vanadium carbide suppress the formation of Cottrell atmosphere in matrix. Retained austenite of Si–Mn TRIP steel containing vanadium transforms to martensite at loading stress of 502 MPa (its yielding strength is 486 MPa), while the transformation of retained austenite in matrix of Si–Mn TRIP steel without vanadium happens when its yielding process is finished at quasi-static tensile. It is confirmed that phase transformation of retained austenite in TRIP steel is strain induced phase transformation. It is noted that tensile elongation of TRIP steel at dynamic tensile is always lower than that at quasi-static tensile. That is because gradually strain induced phase transformation of retained austenite in TRIP steel is suppressed by deformation localization at dynamic tensile.     

Microstructure and tensile behavior of duplex low-density steel containing 5 mass% aluminum

The microstructure and tensile behavior of low-density steels containing 5 mass% Al were investigated. An alloy obtained under a specific heat treatment condition showed deformation-induced martensitic transformation, which yielded excellent mechanical properties of a high tensile strength of >900 MPa and a high total elongation of >50%. The volume fraction and grain size of the austenite depended on the annealing temperature, which resulted in a transition from stable to metastable behavior of the austenite. The effects of solute content on grains and of austenite grain size on stability were discussed.     

Dependency of Deformation Behavior of Retained Austenite in TRIP Steels on Microstructural and Chemical Homogeneity

The room-temperature stability of the retained austenite against strain-induced martensitic transformation, its deformation behavior, the response to the bainitic isothermal treatment, the appearance of yield point elongation and other peculiarities of plastic flow, and the mechanical properties of transformation-induced plasticity(TRIP) steel were tailored based on the chemical homogeneity and the relative distribution of the retained austenite, bainite, and ferrite in the microstructure. The presence of ferritic-pearlitic banded structure in the initial microstructure resulted in an inhomogeneous TRIP microstructure, in which the retained austenite and bainite were confined to some bands and it was found to be responsible for the resultant inferior mechanical properties. The appearance of discontinuous yielding for the chemically inhomogeneous material was related to the martensitic transformation of unstable retained austenite at the initial stage of tensile deformation. These results are essential for better understanding of the behavior of advanced high-strength steels and their applications.     

Residual Ferrite Control of 9Cr ODS Steels by Tailoring Reverse Austenite Transformation

By tailoring the reverse austenite transformation behavior of 9 Cr oxide dispersion strengthened(ODS) ferritic/martensitic steels, the residual ferrite in ODS steels can be controlled. The reverse austenite transformation behavior of ODS steels is closely related to the initial microstructure conditions prior to austenite transformation. For the spark plasma sintered steels, both the amount and size of residual ferrite decrease with increasing heating rate. Nevertheless, high heating rate will increase the amount and size of residual ferrite in annealed ODS steels. As an isothermal treatment is performed at temperatures above Ac 1, lower isothermal temperature has a more evident effect on the ferrite distribution in spark plasma sintered steels than that in annealed ones.     

贝氏体铁素体的形核

采用60Si2CrV、20CrMo等工业用钢,在贝氏体转变开始线附近进行短时间的贝氏体等温淬火,获得少量贝氏体,然后水冷,得到少量贝氏体＋马氏体＋残留奥氏体组织。应用QUANTA-400环扫电镜对贝氏体相变的形核进行了观察,发现贝氏体铁素体在晶界、相界面、晶内等处形核。研究了形核机制和形核时的能量变化,计算了晶核的临界尺寸、形核功,获得贝氏体临界晶核尺寸a＊=16.7 nm（b=25 nm）,形核功△G＊=2.7×102 J.mol-1。     

硅含量与淬火温度对Q&P钢组织及性能的影响

采用盐浴对两种硅含量不同的试验钢进行了淬火配分处理,并用金相显微镜、扫描电镜与拉伸试验机对不同淬火温度下试验钢组织及性能的转变规律展开了研究。结果表明,试验钢的显微组织由铁素体、马氏体、残留奥氏体与贝氏体组成;硅含量增加,有利于试验钢中残留奥氏体体积分数提高,抗拉强度和屈服强度显著提高,伸长率降低,强度随淬火温度变化的幅度减小;经260 ℃淬火、360 ℃配分后,2.13%(质量分数)Si钢在拥有高强度的同时保持了较好的伸长率,其抗拉强度为958.66 MPa,屈服强度为458.99 MPa,伸长率为15.35%,强塑积为14.66 GPa·%,综合力学性能最佳。     

980 MPa级热浸镀钢的新型淬火配分工艺及组织性能

针对980 MPa级热浸镀钢,在C-Mn-Si-Al系成分设计基础上,开发了一种以高淬火温度(Ms点以上)为特征的新型淬火配分工艺(High-quenching-temperature quenching and partitioning,HQ&P),并与传统的一步过时效工艺(Quenching and austempering,QAT)相比较,分析不同热处理工艺下的组织结构与力学性能变化规律。试验结果表明,试验钢组织为临界区铁素体、贝氏体和马奥岛复相结构。一步过时效工艺下,随退火温度的增加,铁素体含量逐渐减少,贝氏体含量逐渐增加;高温淬火后配分处理的两步工艺下,试验钢发生了两次贝氏体转变,最终贝氏体含量更高,组织更加均匀且含有少量的残留奥氏体。在HQ&P工艺下,试验钢获得最佳的力学性能,即抗拉强度1005 MPa,伸长率26.1%。     

In situ neutron diffraction investigation of the collaborative deformation–transformation mechanism in TRIP-assisted steels at room and elevated temperatures

The deformation behaviour of two transformation induced plasticity (TRIP)-assisted steels with slightly different microstructures due to different thermo-mechanically controlled processing (TMCP) was investigated by the in situ neutron diffraction technique during tensile straining at room temperature and two elevated (50 and 100 °C) temperatures. The essential feature of the TRIP deformation mechanism was found to be significant stress redistribution at the yield point. The applied tensile load is redistributed within the complex TRIP-steel microstructure in such a way that the retained austenite bears a significantly larger load than the ferrite–bainite α-matrix. The macroscopic yielding of the steel then takes place through the simultaneous cooperative activity of the austenite-to-martensite transformation in the austenite phase and plastic deformation in the α-matrix. It is concluded that, although its volume fraction is small, the martensitically transforming retained austenite phase dispersed within the α-matrix governs the plastic deformation of TRIP-assisted steels.     

淬火加热温度对超细晶Q&P钢组织性能的影响

研究了淬火加热温度对超细晶Q&P钢微观组织、元素分布、残留奥氏体体积分数和力学性能的影响。结果表明,当淬火加热温度升高时,铁素体含量逐渐减少,马氏体含量升高,残留奥氏体含量呈现先增加后减少的趋势,高淬火加热温度下C元素的扩散速率加快,残留奥氏体的机械稳定性更好。软相铁素体的存在为试验钢提供了良好的韧性。当淬火加热温度为820 ℃时,Q&P钢的综合力学性能最好,抗拉强度为863 MPa,伸长率为26.1%,强塑积为22.5 GPa·%。     

EFFECT OF AUSTEMPERING ON TRANSFORMATION INDUCED PLASTICITY OF HOT ROLLED MULTIPHASE STEELS

Effect of austempering on the transformation induced plasticity (TRIP) of hot rolled multiphase steel was investigated. Polygonal ferrite, granular bainite, and a large amount of stabilized retained austenite could be obtained in the hot rolled multiphase steel. Strain induced martensite transformation (SIMT) of retained austenite and TRIP effectively occur under straining owing to austempering after hot rolling, and mechanical properties of the present steel remain at a relatively high constant value for austempering at 400℃. The mechanical properties of the steel exhibited a good combination of tensile strength (791MPa) and total elongation (36%) because the stability of retained austenite is optimal when the steel is held for 20min.