Influence of initial microstructures on intercritical annealing behaviour in a medium Mn steel

ABSTRACT

The present work reports the effect of different initial microstructures on reverse transformation kinetics and morphologies of austenite formed during intercritical annealing in Fe-0.14C-7Mn-1Si (wt-%) medium Mn steel. Three different initial microstructures were produced by cold-rolling and cold-rolling followed by austenitisation at 820°C and 900°C. The specimen austenitised at higher temperature shows lath-type austenite after intercritical annealing. The difference in austenitisation temperature leads to different Mn distribution in martensitic initial microstructures, thereby leading to a difference in morphology of austenite. The inhomogeneous Mn profiles in initial microstructures also affect reverse transformation kinetics of austenite upon intercritical annealing. The presence of Mn-enriched regions accelerates austenite growth at an early stage of intercritical annealing but retards the transformation kinetics afterwards.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Relative effect of C and Mn on strength-toughness of medium Mn steels

In order to find an alternative choice for structural load-bearing components, an effort was made to improve the impact toughness of medium Mn steel through inter-critical annealing treatment without significant reduction in strength. Besides, the relative effect of C and Mn contents on microstructure and mechanical properties of medium Mn steels was also studied. Fibrous microstructures with fine, alternate films of ferrite and martensite with retained austenite were obtained. The low-C, high-Mn steel showed a superior combination of yield strength, ductility and impact toughness due to finer microstructure and higher retained austenite fraction, as compared to high-C, low-Mn steel. Thus, the beneficial effect of Mn enrichment in stabilising retained austenite and improving mechanical properties by transformation induced plasticity effect becomes evident.     

1.0 GPa low carbon medium Mn heavy steel plate with excellent ductility

ABSTRACT

A vanadium-bearing medium Mn heavy steel plate was designed, and the yield strength reaches around 1.0?GPa and the total elongation reaches around 18% after the annealing treatments. Meanwhile, the steel also possesses relatively good cryogenic impact toughness. Although the dislocation density of ～2.65?×?10¹⁴?m^?2 in the hot-rolled steel decreases to ～1.13?×?10¹⁴?m^?2 in both annealed steels and the volume fractions of retained austenite reach ～7.9% and ～13.5% for the steels annealed at 620°C for 1 and 2?h, there is no or small decrease in yield strength owing to numerous nanometer-sized carbides precipitated in martensite matrix.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Current opinion in medium manganese steel

Abstract

Medium Mn steels have been actively investigated due to their excellent balance between material cost and mechanical properties. The steels possess a single α′ martensite phase in hot and cold rolled states and multiphases after intercritical annealing. Many studies have focused on investigating the influences of chemical composition and annealing conditions on the microstructure, particularly the grain size and retained γ (γ_R), and on the tensile properties. The steels exhibit high strength and good ductility due to transformation induced plasticity occurring in γ_R, whose volume fraction is approximately 0·2–0·4. The present review summarises the important results of previous studies about the effects of both intercritical annealing conditions and alloying elements on the microstructure and tensile properties of medium Mn steels.     

High-strength medium Mn quenching and partitioning steel with low yield ratio

ABSTRACT

A new microstructural design is proposed to develop a strong and ductile quenching and partitioning (Q&P) steel with low yield ratio. This steel has a heterogeneous dual phase microstructure which is developed by varying austenite thermal stability through Mn segregation. The heterogeneous microstructure contains large austenite grains which contribute to the low yield strength. The ultra-high tensile strength and good ductility are ascribed to the enhanced strain hardening behaviour resulted from the continuous transformation-induced plasticity (TRIP) effect. The present microstructural design enables a conventional medium Mn steel with high tensile strength, good ductility and low yield ratio, which promises easy forming and potential applications in automotive industries.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Tuning austenite stability in a medium Mn steel and relationship to structure and mechanical properties

We study here the underlying factors that govern the stability of austenite in a medium Mn (Fe–0.18C–11Mn–3.8Al) (wt-%) steel. In this regard, a novel heat treatment involving intercritical quenching and tempering was designed to obtain high total elongation (TEL) and high ultimate tensile strength (UTS) in the cold-rolled steel. And the UTS and TEL approached 920–1150?MPa and 35–65%, respectively. The product of TEL and UTS (PSE) exceeded 40?GPa%, with a maximum value of 60?GPa%. A detailed analysis of microstructure before and after tensile deformation revealed that the TRIP effect occurred and the stability of austenite was predominantly governed by the grain sizes of austenite rather than the orientation of austenite grains. The theoretical analysis of work hardening data suggested that the superior elongation of medium Mn TRIP steel is related to the high stability of austenite and the cooperative deformation of ferrite.     

Microstructure and tensile properties of a 0.20C–4.86Mn steel after short intercritical-annealing times

This work studies the microstructure and tensile properties of a cold-rolled Fe–0.20C–4.86Mn (mass %) steel after short intercritical annealing (IA) times using scanning and transmission electron microscopy, and uniaxial tensile tests. The short IA time is applied to represent the process characteristics of the industrial continuous annealing line. The experimental results show that IA temperature has a strong influence on the final microstructure and tensile properties while IA time has less. The fractions of retained austenite are much higher after IA at 650 and 675°C than the other IA temperatures, and thus improving elongation. Simulations using the DICTRA software and constitutive modelling are further performed to assist the understanding of the microstructure evolution and stress–strain curves.     

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.     

Enhanced strength-ductility of medium Mn steel by quenching,partitioning and tempering

ABSTRACT

A quench and partition (Q&P) process was combined with tempering in a medium Mn steel. The partitioning treatment enriched the austenite in carbon, and stabilised the austenite against transformation during cooling. The ductility of Q&P steel is significantly improved by tempering, with negligible loss in strength. The ductility was found to be determined by the martensite in the structure, rather than the austenite in the present case. The reason for the significant improvement in the ductility with tempering was suggested to be the reduction in the dislocation density after extended tempering treatment. The energy absorption of this alloy was increased to 28.5?GPa·% together with an ultrahigh tensile strength ～1400?MPa, which is one of the largest observations in this system.     

TRIP钢中奥氏体的力学稳定性研究

采用分阶段拉伸、XRD、EBSD、SEM、TEM等实验手段,研究了TRIP钢奥氏体的力学稳定性.结果表明:拉伸变形初期奥氏体转变较快,拉伸变形后期奥氏体转变较慢;奥氏体的含碳量不同,在相同的拉伸变形阶段奥氏体转化率的增加速率不同;处于铁素体、贝氏体晶界或者相界面1um以上大颗粒奥氏体几乎在变形初期就全部发生相变,而晶粒...     

应变速率对热冲压淬火-配分钢显微组织与力学性能的影响EI北大核心CSCD

采用分离式Hopkinson压杆对热冲压淬火-配分(HS-Q&P)钢在0~12000 s^(-1)应变速率范围内进行动态压缩实验,利用SEM,EBSD,XRD等分析表征手段探究动态压缩过程中试样的变形行为。结果表明:实验钢在不同速率下的变形行为基本相似且分为3个阶段,在平台处应力有小幅度增加,增幅更多体现在应变上。在压缩过程中出现的绝热升温会带来软化效应。残余奥氏体的存在会提高实验钢的强度和塑性变形能力。钢中残余奥氏体发生相变诱导塑性(transformation induced plasticity,TRIP)效应减少的体积分数与马氏体增加的体积分数基本一致,证明TRIP效应为钢中主要的强化机制。同时,通过SEM可观测到残余奥氏体发生TRIP效应转变成细小针状马氏体,随着应变速率增加,晶格畸变越来越严重,EBSD图像中可以观测到部分形变孪晶,在不同应变速率下,〈001〉取向的晶粒都会更容易产生形变孪晶。     

Enhancement of tensile properties by room-temperature quenching and partitioning of 0.2C–10Mn–2Al steel

ABSTRACT

Processing conditions better than those of conventional quenching and partitioning process are suggested for 0.2C–10Mn–2Al steel. The steel can retain 24% of austenite on quenching to room temperature and effectively partition carbon from martensite to austenite at 200°C. The resulting tensile properties were comparable to those produced by conventional quenching and partitioning. Moreover, the suggested processing condition resolves an issue of intercritically annealed medium Mn steels by improving the yield strength and eliminating yield point phenomenon as well as serrated flow.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

含磷和钒热轧TRIP钢组织控制及力学性能研究

为探讨含磷和钒热轧TRIP钢的组织控制和力学性能,采用不同变形温度（900和800℃）,研究其相变行为,并在此基础上进行热轧试验.研究表明：随着冷速增加,变形温度对铁素体相变开始温度（Ar3）的影响逐渐增大;相同冷速条件下,变形使贝氏体相变开始温度（Bs）升高;变形对贝氏体相变的促进作用,随着变形温度的降低而减弱.终轧...     

Q&P热处理工艺对含Cu TRIP钢组织和性能的影响

随着能源的短缺和环境污染的日益严重,汽车轻量化需求日益迫切,如何通过工艺及成分设计革新、获得兼具高强度和高塑性的钢板尤为重要.尝试将Cu作为合金元素加入TRIP钢中,采用淬火配分(QP)工艺对含Cu TRIP钢进行一步法和两步法热处理,通过拉伸试验、X射线衍射分析、扫描电镜、透射电镜等实验手段,对热处理后的组织及性能进行测试和观察,探究了不同热处理工艺对组织和性能的影响.研究结果表明:一步法处理后的显微组织为铁素体、马氏体和残余奥氏体,两步法处理后不仅包含上述3种组织,还含有贝氏体.一步法处理后,抗拉强度达2 200 MPa,拉伸延展率为15%,强塑积为33 GPa·%;两步法处理后综合力学性能优于一步法,在400℃等温5 min后,抗拉强度为1 300 MPa,拉伸延展率为43%,强塑积超过55 GPa·%.实验钢良好的综合力学性能得益于铁素体、马氏体/贝氏体和残余奥氏体的合理配比,变形过程中残余奥氏体的相变诱导塑性效应,以及马氏体位错与Cu粒子的交互作用.     

Effect of retained austenite stability on cyclic deformation behavior of low‐alloy transformation‐induced plasticity steels

The retained austenite (RA) characteristics of Al‐containing TRIP700 steels have been manipulated using varying bainitic isothermal transformation (BIT) processing. The microstructural evolution was investigated using optical microscopy and quantitative image analysis, while the amount of transformed RA was evaluated with the saturation magnetization (SM) technique. Cyclic behavior is found to depend on the applied strain amplitude and stability of RA. At strain amplitudes with comparable elastic and plastic strain components, cyclic softening prevails, facilitated by more stable RA microstructures and Low Cycle Fatigue (LCF) performance benefits from a lower RA stability, which controls the amount of cyclic softening rate. With increasing plastic strain component, a transition to cyclic hardening is observed, and the transition strain increases with increasing RA stability. LCF performance deteriorates because of excessive cyclic strain hardening promoting martensitic transformation. The effect is accompanied by a transition from mixed dimple/cleavage to cleavage‐type fracture characteristics.     

强塑积大于30 GPa%的热轧中碳TRIP钢组织及性能研究

为研究贝氏体相变温度对中碳热轧TRIP钢组织与性能的影响,采用扫描电镜(SEM)、X射线衍射(XRD)与高分辨透射电镜(HRTEM)对含Ti与Mo的中碳热轧TRIP钢进行了显微组织观察、残余奥氏体含量测定以及析出相的表征与分析.结果表明:在400℃贝氏体相变温度下,试验钢的残奥含量与强塑积均达到最大值,分别为28.2%和31.14 GPa·%;同时在钢中发现了呈块状、无规则形状以及片层状形貌分布的残余奥氏体,对衍射斑标定后显示,片层状残余奥氏体与铁素体基体同时满足kurdjumov-Sachs(K-S)与Nishiyama-Wasserman(N-W)位向关系;HRTEM分析显示,Mo可以溶入TiC而生成(Ti,Mo)C粒子,而纳米级的(Ti,Mo)C粒子可以显著提高钢的沉淀析出强化效果.     

Carbide precipitates and mechanical properties of medium Mn steel joint with metal inert gas welding

Medium Mn steel was metal inert gas(MIG)welded with NiCrMo-3 and 307Si filler wires.The effect of filler wires on the microstructure and mechanical properties of joint was investigated,and the carbide precipitates were contrastively discussed.The results revealed that the microstructure of weld metal,heat-affected zone and base metal are austenite.Obvious grain coarsening occurred in the heat-affected zone(HAZ),and the maximum grain size grew up to 160 pm.In HAZ,C and Cr segregated at grain boundaries,the carbides was identified as Cr7C3.The dispersive(Nb,Mo)C phase was also found in weld metal with NiCrMo-3 filler wire.All the welded joints failed in HAZ during tensile tests.The tensile strength of welded joint with NiCrMo-3 filler wire was 675 MPa,which is much higher than that with 307Si filler wire.In comparison to base metal,higher microhardness and lower impact toughness were obtained in HAZ for these two welded joints,which was attributed to the precipitation of Cr7C3 phase and grain coarsening.The impact toughness around the fusion line is the worst for these two welded joints.     

Enhanced tensile properties of a reversion annealed 6.5Mn-TRIP alloy via tailoring initial microstructure and cold rolling reduction

The feasibility of improving the overall performance of medium Mn steels was demonstrated via tailoring the initial microstructure and cold rolling reduction.The combined effects of cooling patterns after hot rolling(HR) and cold rolling(CR) reductions show:(1) as the cooling pattern varied from furnace cooling(FC) to oil quenching(OQ),the intercritically annealed microstructure was dramatically refined and the fraction of recrystallized ferrite dropped,regardless of CR reductions.This resulted in both high yield/ultimate tensile strengths(YS/UTS) but low total elongation to fracture(El);(2) as the CR reduction increased from 50% to 75%,the OQ-samples after annealing exhibited a more refined microstructure with relatively higher fractions of retained austenite and sub-structure,leading to higher YS and UTS but lower El; whereas the FC samples appeared to exhibit little difference in overall tensile properties in both cases.The differences in microstructural evolution with cooling patterns and CR reductions were explained by the calculated accumulated effective strain(εAES),which was considered to be related to degrees of recovery and recrystallization of the deformed martensite(α').The optimal tensile properties of ~1 GPa YS and ~40 GPa·% UTS×El were achieved in the OQ-50%CR annealed samples at 650?C for 1 h.This was quite beneficial to large-scale production of ultra-high strength steels,owing to its serious springback during heavy cold working.     

连续退火对5% Mn超细晶TRIP钢组织性能的影响

为开发强度和塑性良好配合的第三代汽车钢,采用CCT-AY-Ⅱ型钢板连续退火机模拟分析了不同退火温度和时间对0.1C-5Mn中锰TRIP钢组织性能的影响规律.采用SEM和EBSD等微观分析方法观察不同工艺下制备的中锰TRIP的微观组织,利用XRD方法测量残余奥氏体体积分数,通过实验测量其力学性能.结果表明:实验的中锰TRIP钢在650℃保温3 min退火后获得最佳的综合力学性能,其抗拉强度为1 022 MPa,总延伸率为19.3%,强塑积为20 GPa·%;应变前期试验钢中大量残余奥氏体发生转变,超细晶间的协调机制对试验钢的塑性起主导作用.     

Development and characterisation of C-Mn-Al-Si-Nb TRIP aided steel

Performance of TRIP (transformation induced plasticity) aided steel depends on the amount and stability of retained austenite in the final microstructure. In the current work, a small sized C-Mn-Si-Al-Nb steel was made and suitably heat treated to produce multiphase TRIP microstructure. A combination of relevant transformation models and empirical artificial neural network based model was used to select the heat treatment parameters required to achieve the desired amount of retained austenite. Characterisation of microstructure carried out using a variety of techniques confirmed the predictions of these models. The alloy was found to have an attractive combination of strength and ductility.