淬火温度对高Ti低合金耐磨钢组织及力学性能的影响

研究了淬火温度对高Ti低合金耐磨钢组织转变、析出相和力学性能的影响,并分析了组织演变和力学性能变化的原因。结果表明:试验钢经不同温度淬火和200 ℃回火后的组织均为高位错密度板条马氏体;析出相尺寸主要为微米-亚微米-纳米三种尺度,微米级析出相呈杆棒状,亚微米以及纳米析出相呈球状,马氏体板条上分布着细小的(Ti, Mo)C析出相。随着淬火温度的升高,试验钢的屈服强度、抗拉强度和维氏硬度均先升高后降低,均在920 ℃时有最大值,分别为1248 MPa、1535 MPa和434 HV,此时伸长率为10.0%。随淬火温度升高,纳米级析出相逐渐回溶,数量减少且尺寸逐渐长大,沿轧制方向被压扁拉长的原奥氏体晶粒尺寸以及马氏体板条块尺寸略有增大,但马氏体板条宽度却无明显长大。大量的弥散分布的5～10 nm的(Ti, Mo)C粒子是促进耐磨钢硬度升高的主要因素。细小的(Ti, Mo)C析出相逐渐长大以及原奥氏体晶粒的增大都不利于耐磨钢硬度的提高。     

三种热处理工艺对低合金耐磨钢组织和磨损性能的影响

采用扫描电镜、透射电镜等研究了低合金耐磨钢经低温回火、循环热处理、一步配分热处理后的显微组织,采用磨粒磨损试验机测试其磨损质量.结果表明:试验钢经低温回火后的组织为板条马氏体加少量析出相;循环热处理的试验钢的马氏体板条消失,在原奥氏体晶界上和基体处均有碳化物析出相;淬火配分热处理的试验钢中的马氏体板条比较明显,并有少量的残留奥氏体.能谱成分分析可知,不同热处理工艺后试验钢中的微米尺寸的析出相主要是(Ti,Nb)C,球形与椭球形纳米尺寸析出相是(Ti,Nb,V,Mo)C.淬火加200℃低温回火处理的试验钢的硬度为46.5 HRC,循环热处理的试验钢的硬度最低,为31.48 HRC,淬火加一步配分热处理的试验钢的硬度为44.84 HRC.磨粒磨损实验结果表明,淬火加200℃低温回火处理后的试验钢的耐磨损性最佳,淬火加配分处理的试验钢的磨粒磨损性能与淬火加低温回火的试验钢相差不大,循环热处理的试验钢的磨粒磨损性能较差.     

700MPa级高塑低碳低合金钢的多相组织调控及性能

通过临界退火、临界回火以及回火的多步热处理方式,研究了低碳低合金钢的组织演变与力学性能.结果表明,临界退火后的组织为板条状的临界铁素体及贝氏体/马氏体的双相组织.经临界回火后,为临界铁素体、回火贝氏体/马氏体以及残余奥氏体的多相组织.残余奥氏体呈粒状和条状,分布在铁素体/贝氏体(马氏体)相界面及贝氏体/马氏体板条之间,含量高达29%,并在回火后保持稳定,主要通过C,Mn,Ni和Cu在逆转奥氏体中的富集来稳定.临界退火及回火过程中,Nb C在铁素体及贝氏体/马氏体中析出,呈球状、椭圆形或不规则形状,平均尺寸为10 nm;富Cu的析出相在临界回火及回火过程中形成,呈球状分布于铁素体及残余奥氏体中,尺寸在10~30 nm之间.通过残余奥氏体的应变诱导塑性(TRIP)效应及纳米析出相的析出强化作用,实验钢具有优异的力学性能:屈服强度高于700 MPa,抗拉强度高于900 MPa,均匀延伸率高于20%,总延伸率高于30%.     

回火时间对高Ti微合金化淬火马氏体钢组织及力学性能的影响

利用TEM,XRD和Vickers硬度计等研究了回火时间对高Ti微合金化马氏体钢组织及力学性能的影响,阐明了高Ti微合金化马氏体钢在回火过程中析出强化和组织软化之间的交互作用规律.结果表明,高Ti钢在600℃不同时间回火,硬度表现出不同的趋势.10~300 s回火,硬度不断升高,是由于Ti C的析出强化作用远大于基体回复而导致的软化作用;300 s~10 h回火,硬度保持长时间的平台,是由于细小Ti C粒子的不断析出,且5 nm以下的粒子所占比例提高,不断增加的细小Ti C粒子所产生的强化抵消了由于基体组织软化导致的硬度下降;10~20 h回火,硬度快速降低,且降低速率高于不含Ti钢,Ti C粒子的平均尺寸由10 h的2.76 nm粗化到20 h的3.15 nm.计算表明,Ti C粒子的粗化引起硬度降低11.94 HV,基体软化引起硬度降低24.56 HV,表明基体软化是硬度降低的主要因素,而Ti C粒子的粗化加速了高Ti钢硬度的降低,是导致硬度降低的又一重要因素.     

一种低碳低合金钢的纳米压痕表征

对具有两种不同组织状态的一种低碳低合金钢进行了纳米压痕表征．结果表明,在双相组织试样中,马氏体的硬度高于铁索体的70％以上．在纳米压痕实验过程中,由于马氏体相的尺寸较小并被软的铁素体基体所包围,当压痕深度超过40nm时,纳米压痕硬度呈现出明显的基底效应．由于在铁索体一奥氏体两相区加工过程中发生C元素向奥氏体的分配,双相组织试样中的马氏体中富集了数倍于钢的名义含量的C元素．结果导致双相组织试样中马氏体的平均纳米压痕硬度比同一钢的全马氏体组织试样高出30％以上．此外,还讨论了C的富集分配对马氏体Poisson比和Young’s模量的可能影响．     

Nb和Nb-Ti系微合金耐候钢的连续冷却转变与形变热模拟

在Gleeble-3500热模拟试验机上对Nb和Nb-Ti微合金耐候钢进行连续冷却转变与形变热模拟试验,观察比较两类钢显微组织、析出相与力学性能;利用TEM观察第二相粒子析出物状态。结果表明:随着冷却速度的提高,组织将由珠光体向贝氏体到马氏体转变。当冷却速度达到5℃/s时Nb微合金钢就发生马氏体转变;而Nb-Ti微合金钢在10℃/s才获得马氏体组织,即马氏体转变延迟。形变热模拟试验中,Nb-Ti微合金钢获得的铁素体更加细小,提高了材料的低温韧性;在TEM观察下,Nb C和(Nb、Ti) C粒子在晶内与晶界上是随机分布的,其中Nb C粒子尺寸大约20 nm,体积分数约为0. 6%,(Nb、Ti) C粒子尺寸大约75 nm,体积分数约为1. 33%; Nb-Ti微合金钢的硬度比Nb微合金钢的硬度值更高,说明粒子体积分数比粒子尺寸对硬度的贡献更大。     

(Nb,Ti)C在轧后卷取中的析出及对铁素体相微观力学特征的影响

以复合添加Nb和Ti的微合金钢为研究对象,采用热模拟、显微硬度、透射电镜以及纳米压痕技术等方法对实验钢在连续冷却条件下和卷取过程中的冷却速率对组织演变及显微硬度的影响进行观察和分析,研究了(Nb,Ti)C在卷取中的析出规律及其对铁素体相微观力学性能的影响.结果表明,连续冷却和卷取过程中的冷却速率的增加都能促进Nb-Ti实验钢从铁素体+珠光体组织向贝氏体组织转变,细化铁素体晶粒.在连续冷却条件下,实验钢的显微硬度随着冷却速率的增加逐渐升高,而在卷取过程中由于较小冷却速率能够促进(Nb,Ti)C在铁素体中的形核和长大使得铁素体中存在大量均匀弥散分布的纳米析出物,提高了基体的强度,因此随着卷取过程中冷却速率的增加实验钢的显微硬度呈现降低的趋势.Nb-Ti实验钢中铁素体相的纳米硬度为4.13 GPa,Young's模量为249.3 GPa,普通C-Si-Mn钢铁素体相的纳米硬度为2.64 GPa,Young's模量为237.4 GPa,纳米析出物对铁素体相的纳米硬度的贡献达到1.49 GPa.     

热处理中高铬钢激光熔凝层的组织转变

采用激光熔凝处理方法对高铬钢进行表面强化,然后在300～650℃区间回火处理,利用SEM、XRD和TEM等手段分析热处理对激光熔凝层组织的影响.结果表明,高铬钢激光熔凝处理后,得到的奥氏体组织中合金元素固溶度较高且晶粒细小,具有较高的回火稳定性.激光熔凝层450℃回火后硬度开始升高,560℃时达到最大值(672 HV0.2),回火温度高达650℃时硬度迅速降低.450℃回火后细小M23C6碳化物优先从过饱和奥氏体中析出,同时少量马氏体的生成使熔凝层硬度略有增加.560 ℃回火后由于M,C,和M23C6碳化物的析出、大量高硬度马氏体的生成以及位错强化的共同作用使硬度达到峰值,同时,马氏体组织中有少量的M,C渗碳体析出.650℃回火后基体完全转变为铁索体,析出大量层片状M3C渗碳体,硬度显著降低.     

长期时效对铸造ZG1Cr11Ni2WMoV马氏体耐热钢组织的影响

采用扫描电镜、透射电镜和金相显微镜研究了580℃长期时效对ZG1Cr11Ni2WMoV马氏体耐热钢显微组织的影响。研究结果表明:在1 050℃×1 h空冷淬火+580℃×2 h空冷回火后,存在纳米尺寸的M6C型碳化物弥散分布在回火马氏体板条上;580℃长期时效处理1 000 h,原奥氏体晶界和马氏体板条界上析出M23C6型碳化物,δ-铁素体中析出M23C6型碳化物和Laves相。     

S30432耐热钢650℃时效的组织和力学性能

研究了S30432耐热钢在650 ℃时效时的微观组织和力学性能,特别探讨了S30432钢时效过程中析出相的变化对力学性能的影响.结果表明,实验合金时效初期ε-Cu和M23C6大量析出,随后逐渐长大,其中M23C6尺寸较大且粗化较快,但ε-Cu长期时效后尺寸依然细小.时效初期硬度升高的主要原因是ε-Cu的析出,长期时效过程中ε-Cu尺寸保持细小是硬度稳定在较高水平的主要原因.此外,时效初期由于M23C6大量析出冲击韧性急剧下降,随后M23C6的粗化是冲击韧性继续下降的主要原因.     

Phase field simulation of the carbon redistribution during the quenching and partitioning process in a low-carbon steel

Phase-field modelling is used to simulate the quenching and partitioning process in a low-carbon transformation-induced plasticity (TRIP) steel, in order to understand the carbon redistribution in the microstructure during the heat treatment. The simulations show that, depending on local characteristics of the microstructure, including phase distributions and carbon-concentration gradients, different features in the carbon evolution during the partitioning step occur that are physically and practically relevant, but are not accessible for experimental observation. The overall carbon partitioning from martensite to austenite occurs not only by direct diffusion from martensite to austenite, but also through the bulk ferrite grains. The simulations also show interface migration driven by the free-energy difference between austenite and martensite, which affects the fractions of phases and the dimensions of the austenite grains. The carbon content of individual austenite, martensite and ferrite grains as well as average values are analysed, showing that the carbon concentration within the austenite grains is strongly inhomogeneous at short partitioning times, which contributes to a variable mechanical stability of individual austenite grains, affecting the occurrence of TRIP.     

淬火-配分处理对二次淬火时钢中残留奥氏体分解转变的影响

对0.26C-1.72Si-1.56Mn钢进行了不同碳配分时间的淬火-配分(Q-P)处理,并研究了其组织,特别是二次淬火中奥氏体的分解转变。结果表明:Q-P处理后都形成了板条马氏体+二次淬火组织,且二次淬火组织中都存在孪晶马氏体;碳配分时间在10~300 s范围内,Q-P处理后残留奥氏体中的C含量均高于1.0wt%,残留奥氏体的含量不低于11%(体积分数),有利于钢韧性的改善;初次淬火后未转变奥氏体的形态和尺寸是影响其稳定性的关键因素,初次马氏体板条界膜状奥氏体容易形成残留奥氏体;相对于块状未转变奥氏体,条状未转变奥氏体容易形成二次淬火马氏体及片状残留奥氏体。     

Influence of interface mobility on the evolution of austenite–martensite grain assemblies during annealing

The quenching and partitioning (Q&P) process is a new heat treatment for the creation of advanced high-strength steels. This treatment consists of an initial partial or full austenitization, followed by a quench to form a controlled amount of martensite and an annealing step to partition carbon atoms from the martensite to the austenite. In this work, the microstructural evolution during annealing of martensite–austenite grain assemblies has been analyzed by means of a modeling approach that considers the influence of martensite–austenite interface migration on the kinetics of carbon partitioning. Carbide precipitation is precluded in the model, and three different assumptions about interface mobility are considered, ranging from a completely immobile interface to the relatively high mobility of an incoherent ferrite–austenite interface. Simulations indicate that different interface mobilities lead to profound differences in the evolution of microstructure that is predicted during annealing.     

Q&P钢残留奥氏体含量的热动力学计算

基于CALPHAD方法建立了Q&P钢的配分扩散模型,并建立了一套特定成分在特定QP工艺下的组织转变计算任务流,通过计算QP钢一次淬火过程的马氏体/残留奥氏体含量和配分过程中残留奥氏体的碳富集量,并结合Thermo-Calc软件内置的基于吉布斯自由能的马氏体相变本构模型,预测稳定保留至室温的残留奥氏体含量。利用该模型计算文献钢种(Fe-0.2C-1.28Mn-0.37Si-0.0018B, wt%)的室温残留奥氏体含量,结果显示计算马氏体转变温度比试验数据高60 ℃,计算室温残留奥氏体含量为4.41%,与试验数据基本吻合,从而验证了该计算模型的半定量性。利用该模型进一步计算分析了碳、锰元素含量和热处理制度对AQT980和AQT1180钢一次残留奥氏体含量的影响规律,计算结果显示碳、锰元素含量的增加可使钢中相变点(A₃、Ms、Mf)温度下降;在固定淬火温度下,钢中的碳含量和锰含量增加可使一次残留奥氏体含量大幅增加;当碳、锰元素含量一定时,一次淬火温度的上升会使一次残奥含量大幅增加。     

Mechanical properties of high-Si plate steel produced by the quenching and partitioning process

The microstructures and mechanical properties of a high-Si (1.5 wt.%) steel produced by a novel process of quenching and partitioning (Q & P) were compared with those obtained using traditional heat treatments (i.e. austempering, intercritical annealing for dual phase, quench and tempering). Plate steel was included for exploration of the Q & P process in applications requiring strength and toughness (such as an API line pipe), where retained austenite may contribute to the overall toughness via the TRIP phenomenon at a crack top. The Q & P process is based on the partial transformation of austenite to martensite, followed by partitioning of carbon from martensite into austenite, which leads to an untypical microstructure. Retained austenite amounts up to 6 vol.% with a carbon content of up to 0.88 wt.% were achieved in 0.1% carbon steel using Q & P. Superior impact toughness at higher yield strength levels was found after Q & P compared to other traditional heat treatments with equivalent partitioning, austempering or tempering conditions.     

Q&P工艺对22MnB5钢微观组织及力学性能的影响

利用光学显微镜、拉伸试验机、扫描电镜、XRD和EBSD等手段对22MnB5钢的微观组织及力学性能进行了表征,并重点分析了一步法Q&P工艺处理后的22MnB5钢中残留奥氏体含量及残留奥氏体中碳含量与力学性能的关系。结果表明：采用一步法Q&P工艺,可以获得抗拉强度超过1400 MPa,伸长率超过15%的超高强度22MnB5钢板。随着淬火温度从240 ℃升高至300 ℃,22MnB5钢的组织由马氏体转变为马氏体+残留奥氏体复相组织,试样中的残留奥氏体含量逐渐增加。相同配分温度延长配分时间,残留奥氏体含量呈现先增加后降低趋势。不同热处理工艺下残留奥氏体中的平均碳含量为1.49wt%。采用一步法Q&P热处理工艺可以使残留奥氏体中富集碳,提高残留奥氏体稳定性,强塑积可以达到22.14 GPa·%。     

The role carbon plays in the martensitic phase transformation of an Fe–Mn–Al alloy

We observed direct evidence that 18R martensite is induced by carbon atoms in the BCC grains of an Fe–27.0wt.%Mn–5.3wt.%Al–0.1wt.%C alloy via high-temperature quenching. A single BCC phase structure formed 18R martensite in the present study. The lowest carbon content found for the formation of 18R martensite is 0.035 wt.% in Fe–Mn–Al alloys.     

Q＆P工艺对0.3C-1.35Mn-1.30Si钢力学性能的影响

研究了不同Q＆P工艺参数对0.3C-1.35Mn-1.30Si钢力学性能的影响。结果表明：淬火温度主要影响马氏体的含量;配分温度与配分时间影响碳配分的程度,最终影响残留奥氏体的含量。微观组织的含量影响力学性能。伸长率的变化趋势与残留奥氏体量的变化趋势基本一致,Q＆P钢的塑性主要与残留奥氏体的含量有关,残留奥氏体中的含碳量为1.2%～1.3%。     

直接淬火-碳分配处理后高强度钢的组织与力学性能

采用一种中碳低合金高强度钢,在轧后进行直接淬火后再快速升温至400～600℃进行碳分配处理的直接淬火-碳分配(Quenching Partitioning)处理(DQP),研究DQP工艺对钢的组织与力学性能的影响。利用扫描电镜和透射电镜观察组织及析出物的变化,采用X射线衍射仪分析了钢中残留奥氏体体积分数。结果表明:DQP处理后,钢的组织为板条马氏体组织和残留奥氏体。马氏体板条宽150～250 nm;残留奥氏体位于马氏体板条间,随工艺参数不同,其体积分数在4%～8%。钢中析出物尺寸大多为20 nm左右。经过DQP处理后,钢的抗拉强度达到1200 MPa以上,伸长率15%～17%。-40℃冲击功达到30 J以上。合理的淬火终淬温度可以获得更多残留奥氏体,而升高分配温度会增加析出、并使析出物长大,这是提高钢的强度和韧性的主要原因。     

Control of Austenite Characteristics and Ferrite Formation Mechanism by Multiple-Cyclic Annealing in Quenching and Partitioning Steel

Quenching and partitioning (Q&P) treatment is a novel method to produce advanced high strength steel with excellent mechanical properties. In this study, combination of multiple-cyclic annealing and Q&P process was compared with traditional cold-rolled Q&P steel to investigate the microstructural characteristics and austenite retention. The results showed that retained austenite in traditional Q&P sample was principally located in the exterior of austenite transformation products, while those in multiple-cyclic annealing samples were mainly distributed inside the transformation products. With the increase in cyclic annealing number, both of austenite fraction and austenite carbon content increased, attributing to higher initial austenite carbon content and larger number of austenite/neighbored phase interface to act as carbon partitioning channel. In traditional Q&P sample, the deformed ferrite was recrystallized by sub-grain coalescence, while the austenite was newly nucleated and grew up during annealing process. As a comparison, the ferrite in multiple-cycle annealing samples was formed by means of three routes: tempered martensite that completely recovered with retention of interior martensite variant, epitaxial ferrite that formed on basis of tempered martensite, ferrite that newly nucleated and grew up during the final annealing process. Both of lath martensite and twin martensite were formed as initial martensite and then tempered during partitioning process to precipitate ε carbide with C enrichment, Mn enrichment and homogeneous Si distribution. Compared with the traditional cold-rolled Q&P steel, the Q&P specimens after multiple-cyclic annealing show smaller strength and much larger elongation, ascribing to the coarser microstructure and more efficient transformation induced plasticity (TRIP) effect deriving from retained austenite with high carbon content and larger volume fraction. The application of double annealing treatment can optimize the mechanical properties of Q&P steel to show a striking product of strength and elongation as about 29 GPa%, which efficiently exploit the potential of mechanical performance in low carbon steel.