高锰合金钢的SME、TRIP、TWIP效应

 由于锰的价格低廉以及在材料中的重要作用而成为钢铁工业常用的合金元素。锰含量高时,可使Fe Mn合金形成的奥氏体在较低温度下存在。加入Si、Al元素可对合金中奥氏体的稳定性产生不同程度的影响,从而使材料在承受外界载荷时呈现出不同的反应。研究表明：Si可降低奥氏体层错能,有利于A→ε M相变,从而使合金易产生形状记忆效应。加大变形量,由于大量的奥氏体转变为α′ M时体积膨胀,在使材料伸长率提高的同时,强度也得到提高(相变诱发塑性效应),因此可用作高性能结构件。Al和Mn是提高奥氏体层错能的合金元素。对于Al、Mn含量高的钢,在外力作用下则可通过孪生诱发塑性变形产生孪晶诱发塑性效应,因而材料在具有较高强度的前提下,还具有60%~80%的伸长率。     

TRIP-相变诱发塑性钢的研究进展

相变诱发塑性钢是一种汽车用钢，通过相变诱发塑性(TRIP)效应使钢板中残余奥氏体在塑性变形作用下诱发马氏体生核和形成，并产生局部硬化，继而变形不再集中在局部，使相变均匀扩散到整个材料以提高钢板的强度和塑性。典型TRIP钢c含量为0．2％，Mn 1％～2％，Si 1％～2％，通过热轧变形热处理或冷轧 热处理，TRIP钢的组织由50％～60％铁素体，25％～40％贝氏体或少量马氏体和5％-15％残余奥氏体组成。TRIP钢的强度和韧性高于双相钢和微合金钢。介绍了TRIP钢的生产工艺和性能，残余奥氏体、合金元素、热处理对TRIP效应的影响和TRIP钢研究趋势。     

新型TRIP钢热处理工艺初探

新型TRIP复相钢仅含C、Si、Mn等合金元素,采用临界区等温淬火热处理工艺,获得铁素体、贝氏体和残余奥氏体三相组织。该钢在Ms-Md温度之间菜变,应变诱导相变,相变诱发塑性（TRIP）,其力学性能指标特别是伸长率大幅度提高。     

经济型铁素体/贝氏体高扩孔钢的组织与性能

 通过TMCP工艺实验,研究了Si、Mn含量对低碳Si Mn钢显微组织、力学及成形性能的影响,探讨了铁素体/贝氏体双相钢(FB钢)在扩孔过程中的裂纹形成及扩展行为。研究结果表明,增加Si含量,实验钢中等轴铁素体的体积分数增加,扩孔性能得到改善;而增加Mn含量,实验钢的强度和韧性显著提高,但塑性和扩孔性能有所下降。FB钢中的裂纹扩展主要是以微孔聚集机制进行,当遇到贝氏体时,裂纹通过铁素体 贝氏体相界面并剪断铁素体进行扩展。合理选择Si、Mn含量和TMCP工艺参数,可以获得690 MPa级的经济型热轧FB高扩孔钢,扩孔率达到了95%,综合性能较好。     

抗剥离车轮钢的合金设计及Si元素对抗剥离性能的影响

 利用热力学计算软件Thermo Calc和Gleeble物理模拟技术辅助进行抗剥离车轮钢的合金设计。热力学计算结果显示,从成分设计的角度出发,提高Si含量和降低C含量会提高车轮钢的相变点,可有效抑制车轮钢的制动剥离现象。动态升温模拟试验也说明,提高Si含量可改善车轮钢的抗剥离性能。但是,降低C含量也带来车轮钢耐磨性的降低,而提高Si含量则在基本不损害塑性的情况下,显著提高车轮钢的强度。     

硅锰含量对907钢综合性能的影响

为改善舰船用907钢的塑性和韧性,本文研究了硅、锰含量对907钢综合性能的影响。结果表明,硅含量从0.95％降低到0.40～0.70％,锰含量从0.65％提高到1.30％,钢的强度性能指标满足σ≥390 MPa的设计要求,而塑性和韧性显著提高,冲击韧性—脆性转变温度降低约50℃,在低温下,能够表征材料抗脆断能力之一的裂纹扩展功显著提高。 通过降Si提Mn调整907钢的化学成分,明显提高了钢的综合性能,为提高我国舰船用907钢的质量提出了一个改进的方向。     

60Si2Mn弹簧钢的柔性轧制技术

通过调整终轧温度,研究了冷却速度对60Si2Mn钢相变组织及力学性能的影响。结果表明：终轧温度和冷却速度的变化对60Si2Mn钢显微组织和性能有显著的影响,在相同的终轧温度下,随冷却速度增大铁素体组织和珠光体片层间距得到细化,索氏体含量提高,硬度逐渐增加。     

等温淬火温度对C-Si-Mn系TRIP钢组织和力学性能的影响

利用金相显微镜和X射线衍射方法研究了0.11C-1.23Si-1.65Mn冷轧TRIP钢等温淬火温度对组织和力学性能的影响.结果表明,实验钢在840℃×180 s退火 420℃× 240 s等温处理后可得到6.55%的残余奥氏体,此时可获得较佳的相变诱发塑性和较好的强韧性配合,其强塑积可达到2.28×104MPa·%,提高或降低等温温度都会降低强塑积.在840℃退火,适当延长退火时间,可提高残余奥氏体体积分数及碳含量,有助于提高材料的综合性能.     

TRIP钢在分段冷却过程中的相变行为

 建立了热轧低碳Si Mn系TRIP钢相变动力学模型。将该模型用于模拟热轧TRIP钢分段冷却过程中显微组织的演变,并定量分析了热变形和冷却速率对热变形奥氏体相变行为的影响。结果表明,预测值与实测值符合较好;降低终轧温度、提高终轧变形量,延长两相区缓冷时间都能促进铁素体相变,从而有利于提高残余奥氏体中的碳含量。     

退火温度对热轧0.14C-5Mn钢组织和力学性能的影响

0.14C-5Mn钢(/%:0.14C、5.0Mn、0.008P、0.002S、0.0030N)由50 kg真空感应炉冶炼,轧成4 mm板材(终轧温度1000℃,空冷)用扫描电子显微镜,X射线衍射法和单轴拉伸试验研究了0.14C-5Mn钢热轧4 mm板550～650℃6 h退火后的组织和力学性能。结果表明,热轧0.14C-5Mn钢退火过程中产生了奥氏体(发生相变诱发塑性-TRIP效应),随退火温度的升高,奥氏体体积分数逐渐增加,钢的伸长率和强塑积(抗拉强度与断后伸长率的乘积)明显增加,650℃6 h退火时奥氏体体积分数达30.11%,该钢的抗拉强度为945 MPa,强塑积为37.3 GPa%     

Partial Inverse Grain Size Dependence of Strength in High Mn Steels Microalloyed With Nb

 Two steels without and with Nb addition were chosen to investigate the effects of Nb on the microstructures and the mechanical properties of Fe-Mn-Al-Si steels. The results revealed that Nb refined the grains markedly and both TRIP and TWIP effects occurred during deformation process. The Nb containing steel possesses higher yield strength and much lower tensile strength, the latter being explained by the suppression of TRIP effect due to the increase of stacking fault energy. This indicates grain refining is secondary for strengthening of steels when TRIP or TWIP effect exists during the deformation of low carbon and high Mn steels.     

Development and characterization of high strength impact resistant Fe-Mn-(Al-, Si) TRIP/TWIP steels

Iron manganese steels with Mn mass contents of 15 to 30 % exhibit microstructural related superior ductility and extraordinary strengthening behaviour during plastic deformation, which strongly depends on the Mn content. This influences the austenite stability and stacking fault energy γ_fcc and shows a great impact on the microstructure to be developed under certain stress state or during severe plastic deformation. At medium Mn mass contents (15 to 20 %) the martensitic γ-ε-ά phase transformation plays an important role in the deformation mechanisms of the TRIP effect in addition to dislocation glide. With Increasing Mn mass content large elongation is favoured by intensive twinning formation. The mechanical properties of plain iron manganese alloys are strongly influenced by the alloying elements, Al and Si. Alloying with Al Increases the stacking fault energy and therefore strongly suppresses the martensitic γ-ε transformation, while Si sustains the γ-ε transformation by decreasing the stacking fault energy γ_fcc. The γ-ε phase transformation takes place in Fe-Mn-X alloys with γ_fcc ≤ 20 mJm⁻². The developed light weight high manganese TRIP and TWIP (twinning induced plasticity) steels exhibit high ultimate tensile strength (600 to 1100 MPa) and extremely large elongation of 60 to 95 % even at high strain rates of έ = 10³ s⁻¹. Particularly due to the advanced specific energy absorption of TRIP and TWIP steels compared to conventional deep drawing steels high dynamic tensile and compression tests were carried out in order to investigate the change in the microstructure under near crash conditions. Tensile and compression tests of iron manganese alloys with varying Mn content were performed at different temperatures and strain rates. The resulting formation of γ twins, ά- and ε martensite by plastic deformation was analysed by optical microscopy and X-ray diffraction. The deep drawing and stretch forming behaviour at varying deformation rates were determined by performing cupping tests and digitalised stress-strain-analysis.     

1000MPa级高延伸率Q&P钢的实验室研究

以C-Si-Mn系TRIP钢成分为基础,设计了四种不同Si和Mn含量的合金成分,并采用不同两相区奥氏体化温度的淬火—配分(QP)工艺进行处理,得到了兼具高强度和高塑性的QP钢。其中,当奥氏体化温度为820℃时,0.18C-1.8Si-2.2Mn(质量分数,%)钢和0.18C-1.8Si-2.5Mn钢在抗拉强度达到1 000 MPa以上的同时断后延伸率仍不低于20%,显示了极佳的强塑性结合。利用SEM和XRD等对热处理材料的显微组织进行了表征,结果显示,其显微组织为铁素体、板条马氏体和一定量的残余奥氏体,残余奥氏体多呈块状且被铁素体所包围,且奥氏体化温度为820℃时,材料中的残余奥氏体含量和平均碳浓度均较高。更多且稳定的残余奥氏体在变形过程中发生TRIP效应,可以在不显著降低材料强度的情况下更有效地改善材料的塑性,这也是四种试验用钢经820℃的QP工艺处理后显示出更佳强塑性结合的主要原因。     

Effect of Bake‐Hardening on the Structure‐Property Relationship of Multiphase Steels for the Automotive Industry

The effect of a bake‐hardening (BH) treatment on the microstructure and mechanical properties has been studied in C‐Mn‐Si TRansformation Induced Plasticity (TRIP) and Dual Phase (DP) steels after: (i) thermomechanical processing (TMP) and (ii) intercritical annealing (IA). The steels were characterized using X‐ray diffraction, transmission electron microscopy (TEM) and three‐dimensional atom probe tomography (APT). All steels showed high BH response. However, the DP and TRIP steels after IA/BH showed the appearance of upper and lower yield points, while the stress‐strain behavior of the TRIP steel after TMP/BH was still continuous. This was due to the higher volume fraction of bainite and more stable retained austenite in the TMP/BH steel, the formation of plastic deformation zones with high dislocation density around the “as‐quenched” martensite and “TRIP” martensite in the IA/BH DP steel and IA/BH TRIP steel, respectively.     

Study on Microstructures and Mechanical Properties of High Manganese Steels with TRIP/TWIP Effects

In the present work, advanced high strength and high ductility TRIP/TWIP steels with different manganese concentrations were studied. The microstructures of these steels were evaluated prior to and after deformation and the mechanical properties of these steels were determined. The microstructure analysis indicated that both TRIP and TWIP effects appeared in the steel with lower Mn content, while the TWIP effect was the dominant deformation mechanism in the steel with a higher Mn content, with many deformation twins formed during the deformation. In addition, the forming limit diagrams of these steels were recorded and the results showed an excellent formability.     

合金元素和工艺参数对热轧TRIP钢动态相变的影响

利用Gleeble热模拟试验机进行单轴压缩试验,研究了C-Mn-Si TRIP钢和C-Mn-Al-Si TRIP钢过冷奥氏体形变过程的组织演变,分析了合金元素和工艺参数对过冷奥氏体动态相变的影响.与等温相变相比,C-Mn-Si钢和C-MnAl-Si钢动态相变动力学明显加快.与C-Mn-Si钢相比,用质量分数约1%的Al替代Si后,C-Mn-Al-Si钢的A₃温度明显提高,在相同变形工艺条件下C-Mn-Al-Si钢过冷奥氏体动态相变较易发生,而C-Mn-Si钢动态相变得到的铁素体晶粒比较细小.减小动态相变前奥氏体晶粒尺寸,有利于过冷奥氏体动态相变的进行.提高过冷奥氏体形变时的变形温度或应变速率均对动态相变产生一定的阻碍作用,但影响不显著.      

The Effect of Annealing Temperature and Temper-Rolling on Microstructure,Mechanical Properties,and Lüders Strain of Fe–0.25C–5.9Mn–1.0Al–1.57Si Medium Mn Steel

This study investigates the effect of austenite reverted transformation (ART) annealing temperature and temper-rolling on the microstructure, mechanical properties, and deformation behaviors of cold-rolled Fe–0.25C–5.9Mn–1.0Al–1.57Si transformation-induced plasticity (TRIP) steel. The cold-rolled steel annealed at 700 °C demonstrates excellent mechanical properties. The ultimate tensile strength, total elongation, and product of strength and elongation are observed as 1212 MPa, 31.8%, and 38.6 GPa%, respectively. The excellent combination of strength and ductility is related to the discontinuous TRIP effect; still, an inhomogeneous deformation is observed during tensile deformation, known as the Lüders strain. Temper-rolling is used for the ART-annealed specimens at 700 and 720 °C, and yield point elongation decreases when temper-rolling reduction increases. When the temper-rolling reduction increases by 8%, the yield point elongation of the specimen annealed at 700 °C is noted at 1%, while the specimen annealed at 720 °C exhibits continuous yielding. The strain-induced martensite transformation and increased dislocation density in the ferritic matrix improve the early-stage strain hardening rate, thus suppressing the Lüders band's formation.     

Forming Limit Curve (FLC) and Fracture Mechanism of Newly Developed Low‐Carbon Low‐Silicon TRIP Steel

The Forming‐Limited Diagram (FLD) of intercritically annealed 0.11C‐1.65Mn‐0.62Si TRIP‐assisted steel was investigated. The high FLD₀ value of this new low carbon TRIP steel was indicative of a superior formability. The micro‐structural changes during deformation and fracture were studied in detail. The polygonal ferrite phase was found to plastically deform first and deformed most at larger strains. Fracture was initiated by micro‐voids nucleated at ferrite grain boundaries, within ferrite grains or at the interface between ferrite and the harder phases. Cracks were formed after micro‐voids grew, coalesced, and expanded in one direction. When crack tips reached the bainite phase or the martensite/austenite constituent, the cracks propagated along the boundary of these phases. Cracks reaching retained austenite islands caused stress‐induced martensite transformation at the crack tip. The direction of motion of the cracks also changed in this case.     

Towards an austenite decomposition model for TRIP steels

The current status of developing a fundamental model for describing the overall austenite decomposition kinetics to ferrite and carbide‐free bainite in low carbon TRIP steels alloyed with Mn and Si is reviewed. For ferrite growth, a model is proposed where both interface and carbon diffusion‐controlled ferrite formation are considered in a mixed‐mode approach. The kinetic model is coupled with Thermocalc to obtain necessary thermodynamic information. Spherical geometry with an outer ferrite shell is assumed to capture in a simple way the topological conditions for growth. The mixed‐mode modelling philosophy has been identified to permit a rigorous incorporation of the solute drag effect of substitutional alloying elements, in particular Mn. The Purdy‐Brechet solute drag theory is adopted to characterize the interaction of Mn with the moving austenite‐ferrite interface. The challenges of quantifying the required solute drag parameters are discussed with an emphasis on a potential solute drag interaction of Mn and Si. The model is extended to non‐isothermal processing paths to account for continuous and stepped cooling occurring on the run‐out table of a hot strip mill or on a continuous annealing line. The transformation start temperature during cooling is predicted with a model combining nucleation and early growth which had previously been validated for conventional low carbon steels. The overall model is evaluated by comparing the predictions with experimental data for the ferrite growth kinetics during continuous cooling of a classical TRIP steel with mass contents of 0.19 % C, 1.49 % Mn and 1.95 % Si. Extension of the model to include bainite formation remains a challenge. Both diffusional and displacive model approaches are discussed for the formation of carbide‐free bainite. It is suggested to develop a combined nucleation and growth model which would enable to capture a potential transition from a diffusional to a displacive transformation mode with decreasing temperature.