Probing the Characteristic Deformation Behaviors of Transformation-Induced Plasticity Steels

The characteristic micromechanical behaviors of contrasting transformation-induced plasticity (TRIP) steels were investigated under tensile loading by in-situ neutron diffraction and transmission electron microscopy in detail. As demonstrated by the lattice strain development from the neutron diffraction, in the TRIP steel with ∼10 pct RA, microyielding of soft ferrite was responsible for the first stress partition, but a second stress sharing was caused by effective martensitic transformation. In the TRIP steel with less than 5 pct RA, where the contribution from the martensitic transformation was minor, stress partition took place virtually between the ferrite and bainite phase. Probing with systematic transmission electron microscopy (TEM) observations, we pin down the inherent correlation between the microstructural evolutions and the stress partition mechanism. Based on the experimental observations, the factors influencing the work-hardening behavior of TRIP steels are discussed. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

Influences of Alloying Elements on Oxidation Behavior of Steels and Microstructure of Oxide Scales

In order to figure out the oxidation behavior of steels during heating,five micro-alloyed steels were subjected to continuous and isothermal oxidation using the thermo gravimetric analyzer and the Gleeble-3500thermo-mechanical simulator.The microstructure of oxide scales,especially the thickness fractions of Fe2O3,Fe3O4 and FeO layers,was analyzed using the scanning electron microscope(SEM),electron probe microanalyzer(EPMA)and electron backscattered diffraction(EBSD)techniques.The micro-alloyed steels containing alloying elements(Si,Cr,Ni and Cu)show a higher oxidation resistance compared with the low carbon steel.It is found that alloying elements accumulated at scale/substrate interface during high temperature oxidation.Alloying elements function in two ways in the oxidation of steels:one is enhancing the scale/substrate interface and consequently suppressing the blister of scales;and the other is impeding the outward diffusion of iron cations from substrate to scales,resulting in the decrease of oxidation rate.As the diffusion of iron cations is impeded,the thickness fractions of Fe2O3 and Fe3O4of micro-alloyed steels are more than those of low carbon steels.     

Si对中碳弹簧钢氧化脱碳行为的影响

试验研究了成分(%)为0.49C-0.29Si-1.65Mn、n53C-0.78Si-1.56Mn和0.54C-1.74Si-0.71Mn(60Si2MnA)3种中碳弹簧钢850℃5～120 min水冷和780～1000℃60 min水冷热处理后的脱碳敏感性和氧化失重行为。结果表明,随钢中硅含量增加,钢的脱碳层深度增加,氧化失重量减少,说明Si能增加钢中碳的扩散系数,促进钢的脱碳。     

Effect of Alloying Elements on Thermal Wear of Cast Hot-Forging Die Steels

The effect of main alloying elements on thermal wear of cast hot-forging die steels was studied. The wear mechanism was discussed. The results show that alloying elements have significant influences on the thermal wear of cast hot-forging die steels. The wear rates decrease with an increase in chromium content from 3% to 4% and molybdenum content from 2% to 3%, respectively. With further increase of chromium and molybdenum contents, chromium slightly reduces the wear resistance and molybdenum severely deteriorates the wear resistance with high wear rate. Lower vanadium/carbon ratio （1.5-2.5） leads to a lower wear resistance with higher wear rate. With an increase in vanadium/carbon ratio, the wear resistance of the cast steel substantially increases. When vanadium/carbon ratio is 3, the wear rate reaches the lowest value. The predominant mechanism of thermal wear of cast hot-forging die steels are oxidation wear and fatigue delamination. The Fe2O3 and Fe3O4 or lumps of brittle wear debris are formed on the wear surface.     

TRIP钢中合金元素的作用和处理工艺的研究进展

 为了给TRIP钢的试制提供参考,对各种合金元素在TRIP钢中的作用进行了描述,并介绍了热轧和冷轧TRIP钢的处理工艺。认为TRIP钢的研究、生产、应用与双相钢相似,能生产双相钢的生产线即可生产TRIP钢。为生产符合我国国情的TRIP钢,应加强微合金元素钒、钛在TRIP钢中作用的基础研究。     

Simultaneous Increase of Both Strength and Ductility of Medium Mn Transformation-Induced Plasticity Steel by Vanadium Alloying

In general, microalloying such as vanadium (V) is considered to deteriorate the mechanical stability of austenite grains in medium Mn steel due to the consumption of C content. In this paper, we show that the mechanical stability of austenite grains could be optimized by V-alloying. This is because the V-alloying will refine the austenite grain size and tends to stabilize the austenite grains. The competition between the grain refinement and the reduced C content results in proper mechanical stability of austenite grains, providing continuous transformation-induced plasticity (TRIP) effect. In addition, the V-alloying suppresses the formation of intergranular cracks, leading to a ductile fracture morphology and a large non-uniform elongation.     

Effect of CO2 and O2 Mixed Injection on the Decarburization and Manganese Retention in High-Mn Twinning-Induced Plasticity Steels

Metallurgical and Materials Transactions B - In order to achieve decarburization and manganese retention simultaneously, CO2 was introduced in the smelting process of high-Mn twinning-induced...     

Advanced Micromechanical Model for Transformation-Induced Plasticity Steels with Application of In-Situ High-Energy X-Ray Diffraction Method

Compared to other advanced high-strength steels, transformation-induced plasticity (TRIP) steels exhibit better ductility at a given strength level and can be used to produce complicated automotive parts. This enhanced formability comes from the transformation of retained austenite to martensite during plastic deformation. In this study, as a first step in predicting optimum processing parameters in TRIP steel productions, a micromechanical finite element model is developed based on the actual microstructure of a TRIP 800 steel. The method uses a microstructure-based representative volume element (RVE) to capture the complex deformation behavior of TRIP steels. The mechanical properties of the constituent phases of the TRIP 800 steel and the fitting parameters describing the martensite transformation kinetics are determined using the synchrotron-based in-situ high-energy X-ray diffraction (HEXRD) experiments performed under a uniaxial tensile deformation. The experimental results suggest that the HEXRD technique provides a powerful tool for characterizing the phase transformation behavior and the microstress developed due to the phase-to-phase interaction of TRIP steels during deformation. The computational results suggest that the response of the RVE well represents the overall macroscopic behavior of the TRIP 800 steel under deformation. The methodology described in this study may be extended for studying the effects of the various processing parameters on the macroscopic behaviors of TRIP steels. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

高Co—Ni二次硬化马氏体钢中合金元素对相变的影响

利用“固体与分子经验电子理论”（ＥＥＴ）分析了高Ｃｏ－Ｎｉ二次硬化马氏体钢中几种主要合金元素在不同状态下的价电子结构，认为Ｃｏ、Ｎｉ在奥氏体中与Ｃ相互作用较强，它们能降低Ｃ的扩散能力，推迟马氏体相变。在马氏体回火过程中，Ｃｏ 和Ｎｉ溶入渗碳体形成的合金渗碳体，在较高的温度下分解，使固溶于基体中的Ｍｏ 与位错充分结合。在渗碳体分解后，Ｍｏ 与Ｃ形成富Ｃ、Ｍｏ的溶质簇，或以Ｍ２Ｃ的形式析出，在基体内弥散分布，形成二次硬化。     

合金元素对中碳硅锰钼钒钢连续冷却转变及淬透性的影响

本文用顶端淬火法测定了40、40Si₂、40Mn₂、40Mo和40Si₂Mn₂钢的连续冷却转变曲线和淬透性曲线,同时研究了硅锰钼钒钢中不同硅或锰含量对钢的淬透性的影响,通过对试样中心为半马氏体时钢的淬透性乘子的计算,相对定量地说明了硅、锰对钢的淬透性的作用规律。
通过试验表明,硅使铁素体一珠光体的转变温度上升,使连续冷却转变曲线稍向右移,当钢中加入钼或锰后,使钢的转变温度和时间有较大地降低和延缓,尤其是锰钢。当钢中同时存在有硅和锰,则硅、锰的复合作用远比单一的锰或硅要大得多,使连续冷却转变曲线更往右下方移动。单一的合金元素对钢的淬透性影响最大的是锰,其次是钼,而硅的效果最小。但硅加到40Mn钢中,大大地提高了钢的淬透性,其复合作用不是简单地单个元素作用的叠加,而是乘子关系。为了保证40硅锰钼钒钢有足够的淬透性,必须加入大于1.5％的锰,硅保持在1.3％左右,当然还应加入适量的钼。     

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。     

Effect of Alloying Elements（Sb,B） on Recrystallization and Oxidation of Mn-Containing IF Steel

 The effects of minor alloying elements (antimony, boron) on the recrystallization and oxidation of Mn-containing interstitial free (IF) steels were investigated using confocal scanning laser microscope (CSLM) under controlled atmosphere of 95% Ar and 5% H2 (volume percent) at different temperatures. The results indicated that oxidation and recrystallization were primarily controlled by the grain boundaries, which moved due to release of the stored energy or acted as the fast path diffusion of alloying elements. It was found that the addition of antimony suppressed both surface oxidation and internal oxidation, whereas boron addition accelerated surface oxidation but decreased internal oxidation. The reasons caused were that the alloying elements of antimony or boron were known to segregate on the surfaces or grain boundaries to occupy the surface adsorption sites, which were expected to be less catalytic than bare iron on the transportation of alloying elements. The recrystallization was also retarded through adding minor antimony and boron elements. The oxidation kinetics of formation of grain boundary oxides were studied through calculating the areas along grain boundaries, and it was found that the areas parabolically increased with increasing time.