Austenite Grain Refinement by Reverse α′→γ Transformation in Metastable Austenitic Manganese Steel

Microstructure of metastable austenitic manganese steel after reverse transformation treatment was investigated using optical microscopy,X-ray diffraction(XRD),electrical resistivity and hardness testing.Austenite grain refinement was successfully achieved by a two-step heat treatment.First,martensite was produced by cooling the solution-treated samples to-196 ℃.Then,the deep cryogenic treated samples were heated to 850 ℃ upon slow or rapid heating.The mean size of original austenite grain was about 400μm.But the mean size of equiaxed reversion austenite was refined to 50μm.Microstructure evolution and electrical resistivity change showed that martensite plates underwent tempering action upon slow heating,and the residual austenite was decomposed,resulting in the formation of pearlite nodules at the austenite grains boundaries.The refinement mechanism upon slow heating is the diffusion-controlled nucleation and growth of austenite.However,the reverse transformation upon rapid heating was predominated by displacive manner.The residual austenite was not decomposed.The plateα-phase was carbon-supersaturated until the starting of reverse transformation.The reverse transformation was accompanied by surface effect,resulting in the formation of plate austenite with high density dislocations.The refinement mechanism upon rapid heating is the recrystallization of displacive reversed austenite.     

等径角挤压变形奥氏体不锈钢的组织演变

 用实验方法研究了奥氏体不锈钢在等径角挤压冷变形(路径RC)过程中组织变化。实验结果表明：当剪切方向与孪晶带方向成一定角度时，在剪切力的作用下，孪晶逐渐由大块孪晶→由剪切带分割的孪晶(楼梯状)→小块状→奥氏体亚晶或马氏体晶粒；部分孪晶在剪切力作用下，剪切带可直接碎化成具有大角度位向差的细小晶粒(奥氏体亚晶+马氏体晶粒)，可发生马氏体相变；当剪切方向与孪晶带方向相同时，孪晶带区域也可发生马氏体转变；3道次变形后，具有明显特征的孪晶已很少，此后继续进行剪切变形，孪晶碎化组织(含马氏体)和奥氏体剪切滑移带(含碎化晶粒)的变形以剪切滑移方式进行，当奥氏体的滑移遇到阻力时，可局部形成局部形变孪晶来协调变形；随变形道次的增加，马氏体转变也越多，在多次剪切以及道次中的交叉滑移作用下，马氏体板条逐渐被高密度位错墙分割而碎化成细小的晶粒；8道次变形后，可获得60~230 nm的等轴晶粒。     

Effect of Cold Rolling on Microstructure and Mechanical Properties of AISI 301LN Metastable Austenitic Stainless Steels

 The microstructure and mechanical properties evolution of AISI 301LN metastable austenitic stainless steels during cold rolling were investigated. A wide range of cold thickness reduction (10%-80%) was carried out in a four-high rolling mill at ambient temperature. The X-ray and Feritscope MP30 were used to identify the strain-induced α′-martensite phase and its volume fraction, respectively. The microstructure was observed by optical micrograph and the mechanical properties were determined by tensile tests and microhardness. The results show that the strain-induced α′-martensite nucleated at the shear bands intersections and the growth of α′-martensite occurred by the repeated nucleation of new embryos. The volume fraction of strain-induced α′-martensite increased with increasing the cold rolling reduction. In addition, the percentage increased in the tensile strength is the same as that of hardness. The ratio between the average tensile strength and the average microhardness was found to range between 2.82 and 3.17.     

Effect of Texture and Temperature on Strain-Induced Martensitic Transformation in 304 Austenitic Stainless Steel

The austenitic stainless steel's remarkable mechanical properties are caused by twinning-induced plasticity and transformation-induced plasticity mechanisms. Numerous studies focus on stacking fault energy's effect, which is affected by various factors, to interpret and control these mechanisms. However, crystallographic orientation is also an important parameter for mechanical properties in metals. This study compares the mechanical properties and microstructural features of 304 austenitic stainless steel, focusing on the effect of initial texture and deformation temperature. Microstructural characterization is identified by an interrupted tensile test based on strain, tensile direction, and temperature conditions, and X-ray diffraction and electron back-scattered diffraction analysis are performed. The results show that the mechanical features and strain-induced martensitic transformation rate depend on the tensile directions. In addition, this trend is maintained irrespective of the temperature conditions. The attribute reason is that the difference in the Taylor factor and the formation rate of the deformed band structure is induced by the initial crystallographic orientations. Moreover, a decrease in temperature significantly increases the dislocation densities and abundant twins and transformed martensites formation. Furthermore, the yield and tensile strengths are enhanced while the elongation decreased with the tensile strains.     

Microstructure and Mechanical Properties of Martensitic Stainless Steel 6Crl5MoV

 Martensitic stainless steel containing Cr of 12% to 18% (mass percent) are common utilized in quenching and tempering processes for knife and cutlery steel. The properties obtained in these materials are significantly influenced by matrix composition after heat treatment, especially as Cr and C content. Comprehensive considered the hardness and corrosion resistance, a new type martensitic stainless steel 6Cr15MoV has been developed. The effect of heat treatment processes on microstructure and mechanical properties of 6Cr15MoV martensitic stainless steel is emphatically researched. Thermo-Calc software has been carried out to thermodynamic calculation; OM, SEM and TEM have been carried out to microstructure observation; hardness and impact toughness test have been carried out to evaluate the mechanical properties. Results show that the equilibrium carbide in 6Cr15MoV steel is M23C6 carbide, and the M23C6 carbides finely distributed in annealed microstructure. 6Cr15MoV martensitic stainless steel has a wider quenching temperature range, the hardness value of steel 6Cr15MoV can reach to HRC 608 to HRC 616 when quenched at 1060 to 1100 ℃. Finely distributed carbides will exist in quenched microstructure, and effectively inhabit the growth of austenite grain. With the increasing of quenching temperature, the volume fraction of undissolved carbides will decrease. The excellent comprehensive mechanical properties can be obtained by quenched at 1060 to 1100 ℃ with tempered at 100 to 150 ℃, and it is mainly due to the high carbon martensite and fine grain size. At these temperature ranges, the hardness will retain about HRC 592 to HRC 616 and the Charpy U-notch impact toughness will retain about 173 to 20 J. A lot of M23C6 carbides precipitated from martensite matrix, at the same time along the boundaries of martensite lathes which leading to the decrease of impact toughness when tempered at 500 to 540 ℃. The M3C precipitants also existed in the martensite matrix of test steel after tempered at 500 ℃, and the mean size of M3C precipitates is bigger than that of M23C6 precipitates.     

不同冷轧压下量对亚稳态奥氏体不锈钢00Cr17Ni7织构的影响

 采用X射线衍射技术(XRD)研究了不同冷轧压下量对亚稳态奥氏体不锈钢00Cr17Ni7织构的影响,分析了亚稳态奥氏体不锈钢00Cr17Ni7中马氏体相和奥氏体相的织构变化情况。研究结果表明,不同冷轧压下量下,00Cr17Ni7中的奥氏体相织构主要由Brass{110}<112>、Goss{110}<001>和少量的Copper{112}<111>、S{123}<634>组成,并且随着压下量的增加Brass和Goss织构强度显著提高;同时马氏体相织构主要以{115}<110>、{112}<110> 、{111}<112>、{332}<113>组成,织构的形成主要归因于“Kurdjumov-Sachs取向关系”和“体心立方金属轧制织构类型演变的特点”共同作用的结果。     

304奥氏体不锈钢带氧化铁皮直接冷轧技术的研究

结合现场生产实际,通过在试验室对奥氏体不锈钢304黑皮卷直接进行压下率分别为10%,20%,30%的冷轧然后退火酸洗的试验,证明在退火酸洗工艺相同的情况下,通过在热轧后进行一定压下率的直接轧制,可以获得与传统No.1产品相比晶粒尺寸等级相同、表面粗糙度更低、力学性能和耐蚀性相近的2E产品,并且获得更大的热轧产品厚度范围,降低冷轧一个轧程后的产品厚度.因此根据不同客户的要求,可以用2E产品替代No.1产品.     

奥氏体不锈钢的组织超细化技术

在试验的基础上研究了获得奥氏体不锈钢超细组织的方法,研究结果表明,原始晶粒尺寸为100 μm的304N不锈钢经1道次等径角挤压变形 退火工艺处理后,晶粒尺寸可显著细化到2～7 μm,强度可提高50%以上,而塑性并不降低;增加等径角挤压变形的道次,经退火后可获得更细小、均匀的再结晶晶粒.     

Energetics of Plastic Deformation of Metastable Austenitic Stainless Steel

The change in the internal energy during uniaxial tensile deformation of austenitic stainless steels EN 1.4301 (AISI 304) and EN 1.4318 (AISI 301LN) was determined by measuring the extent of γ→α'‐martensite transformation and the temperature increase of the samples. From the results the fraction of the stored energy of cold work and the free energy change related to the strain‐induced γ→α'‐martensite transformation were determined. The fraction of stored energy varied around 0.4. With the metastable steel grades the free energy change related to the γ→α'‐martensite transformation was found to vary between ‐98 MJ/m³ and ‐206 MJ/m³ depending on the austenite stability of the steel. Furthermore, the magnitude of the mechanical driving force was estimated by comparing the results with the free energy change of thermally induced transformation.     

Effect of Cu on Surface Microcrack of Austenitic Stainless Cold Rolled Coil

In order to find out the cause of surface microcrack on 304 austenitic stainless cold rolled coils which is produced in a steel plant of China, lots of studies have been carried out. The results indicated that the copper guide of steekle mill used in hot rolling process contacts directly with the hot rolled coil, so parts of copper melt and glued to the surface of the stainless steel plates due to a higher temperature of stainless steel plates than the copper melting temperature, which leads to deterioration of austenitic grain boundaries. Shear stress produced in the process of repeat-rolling on finishing mill induces the surface microcracks and promotes it. After changing the copper guide to the cast steel one, such kinds of surface microcracks have never appeared.     

铸态304L奥氏体不锈钢等径角挤压变形研究

 研究了铸态304L奥氏体不锈钢在等径角挤压(ECAP)变形过程中显微组织的演变过程。结果表明,经4道次剪切变形后树枝晶破碎、原始粗大晶粒碎化。显微组织的变化过程可归纳为：原始粗晶粒→晶粒被滑移带分割→位错发展形成高密度位错墙,与滑移带共同作用形成胞块结构→应变增加形成层片状界面→形成大角度晶界的细小晶粒。表明铸态304L奥氏体不锈钢经ECAP变形后塑性变形机制主要由滑移完成。     

奥 氏 体 不 锈 钢 的 再 结 晶 动 力 学

 采用双道次压缩和应力松弛2种实验方法,研究了含铌奥氏体不锈钢347热变形后的再结晶动力学特征,确定了AVRAMI方程表达式,同时通过定量金相技术分析了对应2种实验方法的淬火试样中再结晶组织所占份数。比较不同实验方法所得结果可知：定量金相分析结果与应力松弛法结果较吻合,而与双道次压缩实验结果差异较大。仅进行1次应力松弛实验就可以得到一条再结晶率与时间的关系曲线,可大大减少实验量。     

Strain Rate Sensitivity of Pre‐Strained AISI 301LN2B Metastable Austenitic Stainless Steel

The dynamic behavior of AISI 301LN2B (EN 1.4318) metastable austenitic steel grade has been investigated at 296 K by means of servohydraulic tensile and split Hopkinson bar testing in the strain rate range 0.005–1000 s^?1. As delivered, as well as 10% uniaxial, biaxial, and plane strain pre‐strained conditions, without subsequent heat treatment have been tested. A negative strain rate sensitivity is observed in the low strain rate range between 10^?4 and 1–10 s^?1. Pre‐straining reduces the magnitude of the adiabatic tensile strength softening, especially in the plane strain condition with higher triaxility. The thermal activation related dynamic flow stress increase is not dependent on pre‐straining. The γ → α′ induced additional flow stress increase, however, is highly strain rate and pre‐straining sensitive. The amount of pre‐straining determines the overall ductility at fracture, and therefore the adiabatic temperature increase. The pre‐straining stress state influences the amount of α′‐martensite formed before dynamic testing, and consequently the maximum intensity of the TRIP induced flow stress increase by subsequent dynamic testing.     

301奥氏体不锈钢薄板拉伸强度与冷轧硬度之间的关系

 为对冷轧不锈钢薄板的产品硬度控制提供指导,尝试用一个新的方法来取代试轧,既达到控制冷轧板硬度的目的,又能降低成本、提高效率。对099mm厚的经过退火的301奥氏体不锈钢薄板进行冷轧减薄,并进行室温拉伸试验,测量其维氏硬度。通过观察金相和利用X射线衍射,验证了应变诱导马氏体相变是导致301奥氏体不锈钢冷轧和拉伸时产生加工硬化的主要原因。试验结果表明,冷轧和拉伸有着相似的加工硬化趋势,综合拉伸与轧制试验数据,确定了拉伸强度与冷轧硬度之间的关系,实现了通过拉伸强度来得到对应应变下的冷轧硬度,具有很好的预见性。冷轧可以提高301不锈钢的强度和硬度,显著改善其力学性能。     

Microstructures of Austenitic Stainless Steel Produced by Twin-Roll Strip Caster

 The microstructures of austenitic stainless steel strip were studied using color metallographic method and electron probe micro analysis (EPMA). In the cast strips, there are three kinds of solidification structures: fine cellular dendrite in the surface layer, equiaxed grains in the center and fine dendrite between them. The solidification mode in the surface layer is the primary austenite AF mode because of extremely high cooling rate, with the retained ferrite located around the primary cellular austenite. In the fine dendrite zone, the solidification mode of molten stainless steel changes to FA mode and the residual ferrite with fish-bone morphology is located at the core of the dendrite. The retained ferrite of equiaxed grains in the center is located in the center of broken primary ferrite dendrite with vermicular morphology.     

316L奥氏体不锈钢的氮合金化

采用金相显微镜、XRD、拉伸试验机及高低温冲击试验机等,并结合Thermo-calc软件计算研究了氮对316L奥氏体不锈钢微观组织、析出相、力学性能和耐点蚀性能的影响.结果表明:氮合金化能够抑制316L不锈钢中σ相和Chi相的析出,增加Cr2N的析出倾向,对奥氏体晶粒细化不明显;氮的添加能够提高316L不锈钢的室温强度和-100℃以上温度的夏比冲击功,降低-100℃以下的夏比冲击功,但对室温拉伸塑性影响不明显.此外,氮能够改善316L不锈钢的耐点蚀能力.