Enhanced grain refinement and mechanical properties in a severely plastic deformed Ni-30Cr alloy

The present study was carried out to evaluate the microstructures and mechanical properties of severely deformed Ni-30Cr alloys. Cross-roll rolling (CRR) was introduced as a severe plastic deformation (SPD) process. Ni-30Cr alloy sheets were cold rolled to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the recrystallized microstructure. Electron back-scattered diffraction (EBSD) was introduced to analyze grain boundary character distributions (GBCDs). The application of CRR to the Ni-30Cr alloy effectively enhanced grain refinement through heat treatment; consequently, the average grain size was significantly refined from 33 μm in the initial material to 0.6 μm. This grain refinement directly improved the mechanical properties, in which yield and tensile strengths significantly increased relative to those of the initial material. We systematically discuss the grain refinement and accompanying improvement in mechanical properties in terms of the effective strain imposed by CRR relative to conventional rolling (CR).     

Effect of asymmetrical rolling and annealing the mechanical response of an 1050-o sheet

The asymmetrical rolling process has been studied as a way to promote intense shear deformations across the sheet thickness. These shear deformations may lead, given the proper conditions, to the development of shear texture components ({001}<110>, {111}<110> and {111}<112>) and also grain refinement. In this work, a 1050-O sheet is asymmetrically rolled and annealed. Conventional rolling is also performed, for comparison purposes. Shear texture components are obtained for the asymmetrically rolled specimens, and seem to be retained after annealing. Differences in mechanical response between asymmetrical and conventionally rolled specimens, as well as texture evolution after heat treatment processing are inferred based on experimental tensile and shear tests. Numerical simulations are used to help explain the differences found on experimental tests. It is proven that it is difficult to spread shear texture through the entire sheet thickness from a general asymmetric rolling process. Based on the fact, future research is discussed at closure.     

高性能镁合金板材轧制技术的研究

综述了铗合金板材轧制技术的研究进展,阐述了交叉辊轧制、大应变轧制、交叉轧制、非对称轧制技术的原理,分析了不同轧制技术对应板材的组织特点和晶粒细化机制,以及织构特点和形成原因,指出了当前镬合金板材轧制存在的主要问题及今后的发展方向.     

初始晶粒尺寸对高温交叉轧制镁合金板材组织和力学性能的影响

目的 揭示晶粒尺寸对多道次高温交叉轧制AZ31镁合金板材组织和力学性能的影响规律及机制.方法 通过对不同初始晶粒尺寸的镁合金板材进行高温交叉轧制变形及热处理,获得不同状态的镁合金板材,采用金相显微分析、X射线衍射(XRD)分析及室温拉伸实验等手段研究镁合金板材的晶粒组织(形态、尺寸、取向)及力学性能.结果 经过多道次交...     

Formation of textures and microstructures in asymmetrically cold rolled and subsequently annealed aluminum alloy 1100 sheets

The formation of textures and microstructures in asymmetrically cold rolled and subsequently annealed AA 1100 sheets was investigated. The asymmetrical rolling procedure in this experiment was performed in a rolling mill with different roll velocities (roll velocity ratio of 1.5/1.0). In order to enhance the shear deformation, asymmetrical rolling was performed by a large reduction per pass and without lubrication. Asymmetrical rolling led to the formation of strong shear textures. The evolution of asymmetrically cold rolled textures was analyzed by FEM simulations. After recrystallization annealing, pronounced {111}//ND orientations prevailed in all thickness layers. Intensified shear deformations by asymmetrical rolling also led to the formation of ultra-fine grains after recrystallization annealing.     

CSP生产低碳钢的组织演变和析出物研究

为了阐明EAF-LF-CSP工艺生产的低碳钢组织细化机理,在薄板坯和不同道次变形后的同一轧件上取样,利用金相、SEM、TEM、XEDS等技术研究了连轧过程中显微组织演变和钢中第二相析出物.结果表明:与普通连铸板坯相比薄板坯的凝固组织更加细小;随轧制道次增加,薄板坯表面和心部的组织差异逐渐减小,轧后室温组织细化;CSP生产的低碳钢中存在大量纳米尺寸的氧化物和硫化物,起到细化晶粒的作用.CSP生产中采用快速冷却和凝固工艺、单道次大压下连轧工艺和层流冷却工艺,是成品组织细化的主要原因.     

Nano grained AZ31 alloy achieved by equal channel angular rolling process

Equal channel angular rolling (ECAR) is a severe plastic deformation process which is carried out on large, thin sheets. The grain size could be significantly decreased by this process. The main purpose of this study is to investigate the possibility of grain refinement of AZ31 magnesium alloy sheet by this process to nanometer. The effect of the number of ECAR passes on texture evolution of AZ31 magnesium alloy was investigated. ECAR temperature was controlled to maximize the grain refinement efficiency along with preventing cracking. The initial microstructure of as-received AZ31 sheet showed an average grain size of about 21 μm. The amount of grain refinement increased with increasing the pass number. After 10 passes of the process, significant grain refinement occurred and the field emission scanning electron microscopic (FESEM) micrographs showed that the size of grains were decreased significantly to about 14-70 nm. These grains were formed at the grain boundaries and inside some of the previous larger micrometer grains. Observation of optical microstructures and X-ray diffraction patterns (XRD) showed the formation of twins after ECAR process. Micro-hardness of material was studied at room temperature. There was a continuous enhancement of hardness by increasing the pass number of ECAR process. At the 8th pass, hardness values increased by 53%. At final passes hardness reduced slightly, which was attributed to saturation of strain in high number of passes.     

终轧温度对Ti-V-Mo复合微合金钢组织演变和硬度的影响

采用Gleeble3800热模拟试验机、OM、EBSD、TEM及Vickers硬度计等研究终轧温度对Ti-V-Mo复合微合金钢的组织转变、析出相和硬度的影响,并阐明了组织演变和硬度变化的原因。结果表明,不同终轧温度的Ti-V-Mo钢其组织均为多边形铁素体;随着终轧温度由1000℃降低到800℃,Ti-V-Mo钢的硬度由400HV提高到427HV;铁素体晶粒的平均尺寸由3.44μm减小到3.05μm;(Ti, V, Mo)C粒子的析出数量增加,其平均尺寸由8.38 nm减小到6.25 nm。随着终轧温度的降低,铁素体平均晶粒尺寸的减小和纳米级(Ti, V, Mo)C粒子的增多及细化是硬度增大的主要因素。在980℃以下,降低终轧温度(Ti, V, Mo)C在奥氏体中的形核率不断减小,使得其在铁素体中析出的10 nm以下的(Ti, V, Mo)C粒子不断增多,促进了硬度的提高。     

The important role of titanium microalloying in refining austenite grain of heavy-duty H-beam steel during rough rolling produced by a new technology

A new technology of austenite grain refinement, fine austenite enhanced ferrite transformation, is proposed for heavy-duty hot-rolled H-beam steels in this work. Titanium microalloying is very important and necessary for the new technology. The effect of titanium on the prior austenite grain size of steels during simulated rough rolling was investigated. The results show that the prior austenite grain sizes of specimens with titanium and niobium elements are much finer than those of specimens with niobium but without titanium deformed at the same parameters. For the alloying composition of studied steels, titanium nitride particle maybe precipitated in specimens with titanium at above 1,200 °C, however, niobium carbide particles can't form in specimens without titanium at above 1,150 °C. The thermodynamically stable titanium nitride particles can impede the grain growth at high temperature for example furnace heating before rough rolling and bring the epitaxial growth of niobium carbide on pre-existing themselves which induces a large number of titanium nitride-niobium carbide composite precipitates. These fine precipitates can pin austenite grain boundaries effectively and ensure austenite grain refinement.     

高压扭转致纯铜晶粒细化及与应变的关系

设计并加工了在材料试验机上使用的高压扭转夹持装置,并用装置获得了不同扭转变形量的纯铜试样.使用电子背散射衍射技术测量了处理后试样晶粒尺寸沿径向的分布.对比分析了晶粒细化程度和应变量的关系,提出了一个描述晶粒细化的简单剪切球模型.结果表明,晶粒细化程度随着应变量的增大而增强,平均晶粒尺寸为原始平均尺寸的1/50.当剪切应变小于10时,晶粒尺寸随着剪切应变的增大迅速变小;当剪切应变大于10时,晶粒尺寸的变化趋于缓慢,晶粒细化程度与Stuwe等效应变之间有幂函数关系.依据简单剪切球形晶粒的模型,得到了晶粒细化程度随剪切应变增大而增强的规律,并与实验数据的变化趋势一致.     

固溶前冷轧压下率对Cu-Ni-Si-Mg合金组织与性能的影响

为了优化Cu-Ni-Si-Mg合金的轧制工艺参数,利用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)、能谱(EDS)和X射线衍射(XRD)等手段,对固溶前不同冷轧压下率下的Cu-Ni-Si-Mg合金时效态的组织性能及拉伸断口进行了分析.结果表明:随着冷轧压下率的增加,Cu-Ni-Si-Mg合金的平均晶粒尺寸逐渐增加,屈服强度、硬度和延伸率先增大后减小,抗拉强度表现为先增大,后在710 MPa左右波动,再减小的趋势;压下率从94%增加到96%,晶粒尺寸从12μm增加到52μm,而抗拉强度变化较小,归因于低压下时Cu-Ni-Si-Mg合金的细晶强化和时效强化占主导,而大压下时第二相粒子均匀的时效析出弥补了因晶粒粗大而造成的抗拉强度损失;压下率为93%时,合金的断裂为单一的韧性断裂,压下率为97%时,合金的断裂由韧性断裂和沿晶脆性断裂组成.     

Microstructure and mechanical properties of twin-roll cast Mg–4.5Al–1.0Zn sheets processed by differential speed rolling

The microstructure and mechanical properties of the twin-roll cast (TRC) Mg–4.5Al–1.0Zn alloy sheets produced by differential speed rolling (DSR) were investigated by optical microscopy, transmission electron microscope and electron backscattered diffraction. The results are compared with those of the sheet processed by equal speed rolling (ESR). It is shown that twining played an important role at the initial stage of rolling. With the increase of the rolling reduction, the microstructures in the processed sheets become more homogeneous and they are more refined by DSR than by ESR. After annealing, the sheet processed by DSR shows a higher elongation and slightly lower yield strength than those of the ESR-processed sheet, which could be attributed to grain refinement and texture weakening. These results suggest that, in comparison with conventional casting and rolling, the combined technology of TRC and DSR is a more effective way to process magnesium sheets with enhanced formability after final annealing.     

Achievements of New Generation Steel Project in China

Major achievements of the national project, Fundamental Research on New Generation of Iron and Steel Materials in China (NG Steel). are reviewed in the paper. Uhrafine grained steel technology, based on deformarion induced ferrite trartsformation (DIFT)and successivechanges, is illustrated tor grain refinement in both plain low carbon steel and mieroalloyed steel. Delayed fracture resistance of alloy structure steel can be improved through prior austenite grain refinement. It is shown by results that nano scale precipitates play an important role to grain refinement in thin slab casting and rolling (TSCR)process. Progresses on super cleanliness, high homogeneity, welding and metallurgical process simulation are also briefly introduced.     

Achievements of New Generation Steel Project in China

Major achievements of the national project, Fundamental Research on New Generation of Iron and Steel Materials in China (NG Steel), are reviewed in the paper. Ultrafine grained steel technology, based on deformation induced ferrite transformation (DIFT)and successive microstructure changes, is illustrated for grain refinement in both plain low carbon steel and microalloyed steel. Delayed fracture resistance of alloy structure steel can be improved through prior austenite grain refinement. It is shown by results that nano scale precipitates play an important role to grain refinement in thin slab casting and rolling (TSCR)process. Progresses on super cleanliness, high homogeneity, welding and metallurgical process simulation are also briefly introduced.     

Effect of grain refinement on the hydrogen embrittlement of 304 austenitic stainless steel

The effect of grain size (in the range from 4 μm to 12 μm) on the hydrogen embrittlement (HE) of 304 austenitic stainless steel (ASS) was studied. HE susceptibility result shows that HE resistance increases with grain refinement. Electron backscattered diffraction kernel average misorientation (EBSD-KAM) mapping shows that the strain localization can be mitigated by grain refinement. Hence, strain localization sites which act as highways for hydrogen diffusion and preferred crack initiation sites can be reduced along with grain refinement, leading to a high HE resistance. Meanwhile, grain size shows no influence on the strain induced martensite (SIM) transformation during the hydrogen charging slow strain tensile test (SSRT). Hence, the SIM formed during hydrogen charging SSRT is not responsible for the different HE resistance of 304 ASSs with various grain sizes. Hydrogen diffusion is supposed to be controlled by a competition between short-circuit diffusion along random grain boundary (RGB) and hydrogen trapping at dislocations, leading to a maximum hydrogen diffusion coefficient in the 304 ASS with an average grain size of 8 μm.     

Grain refinement of high purity aluminium by asymmetric rolling

Abstract

For the purpose of grain refinement, development of the microstructure of coarse grain, high purity aluminium during cold asymmetric rolling has been studied by electron backscatter pattern (EBSP) analysis, as well as optical and transmission electron microscopy, and compared with that developed during conventional rolling. In 91.3% asymmetrically rolled sheet, new fine equiaxed grains with an average size of ～2 µm are evolved almost uniformly throughout the thickness. On the other hand, in conventionally rolled sheet, the coarse fibrous structure is predominant. A change of grain boundary misorientation distribution with an increase in reduction shows that the fraction of sub-boundaries below 10° decreases linearly, and that of the high angle boundaries above 15° increases linearly. The fine grain evolution during asymmetric rolling seems to result from the development of sub-boundaries into high angle boundaries promoted by a simultaneous action of two deformation modes, namely compression and additional shear deformation. Fine grains evolved during asymmetric rolling are stable at temperatures below 423 K. Annealing at temperatures above 473 K results in remarkable grain growth.     

Microstructure—Properties Correlation of Dual Phase Steels Produced by Controlled Rolling Process

The purpose of this research is to quantify the effects of compositional and processing parameters on the microstructure and properties of dual phase steel produced directly by hot rolling and rapid cooling.Steels with the base composition of 0.1%C,1.4%Si,and 1.0%Mn with additions of 0.5%Cr to influence hardenability,0.04%Nb to retard recrystallization in the latter stages of rolling,or 0.02% Ti to inhibit grain growth during and after reheating were investigated.Investigation was made to predict microstructure evolution and to correlate microstructure with processing parameters.The effects of the important microstructure parameters such as ferrite grain size,martensite volume fraction (VM) and morphology (polygonal or fibrous) on the tensile and impact properties are discussed.Multiple linear regression analysis of the ultimate tensile strength has shown that,increasing VM and martensite microhardness and grain refinement of ferrite are the major contributions to increase the strength of the steel.It was found that the dual-phase steel produced by controlled rolling process,with a microstructure which consisted of fine grained ferrite(4μm) and 35%-40% fibrous martensite,presented optimum tensile and impact properties because of enhanced resistance to crack propagation.     

Microstructural and mechanical properties evolution of magnesium AZ61 alloy processed through a combination of extrusion and thermomechanical processes

In this work, the microstructures and tensile properties of a commercial magnesium alloy “AZ61” processed by a combination of hot extrusion and thermomechanical processing (TMP) were investigated. The TMP was consisting of two or three hot rolling steps with large reductions per pass, thus allowing significant grain refinement. The microstructural evolution has been studied by means of optical and scanning electron microscopes, as well as X-ray diffraction analysis. The as-cast material is extruded in the form of a cylinder with initial diameter of 250 mm to a final diameter of 110 mm (80% reduction in cross-sectional area). Then hot rolling regimes were performed at 300 °C with different percentage of strain per pass. Tensile and hardness tests were performed in the samples (as-cast, extruded, and rolled) at room temperature in order to evaluate the mechanical properties of the material. The results of experiments demonstrated that fine grain size might be achieved in magnesium alloy AZ61 by using a two-step processing route involving an initial extrusion step followed by thermomechanical processing with large reduction in thickness per pass. This two-step process, designed to achieve average grain sizes of 10–20 μm.     

Microstructure evolution of medium carbon steel during manufacture process for non-quenched and tempered oil well tubes

The microstructure evolution of medium carbon microalloyed steel during the manufacture sub-processes of N80 grade hot rolling seamless oil well tubes was investigated by compression simulation on Gleblee 1500 thermal simulator and industrial experiments of seamless tubes production. The results show that the tested steel has good anti-coarsening ability at equalization sub-procedure. The quantitative analysis of austenite grain size has illustrated that the average austenite grain size in the steel would almost always continuously increase in the tube manufacture process till just before the stretch-reduction-diameter deformation. The most effective microstructure refinement in industrial experiment did not occur in austenite state, but mainly in austenite decomposition following the stretch-reduction-diameter process. The intra-granular ferrite formation should be very useful for effective refinement of the whole microstructure. The variation tendencies of austenite grain size in simulated and industrial experiments are nearly the same, while the optical microstructures and precipitation characterization are quite different.     

Development of ultrafine-grained Al 6063 alloy by cryorolling with the optimized initial heat treatment conditions

The effects of rolling temperature and rolling strain on the microstructural refinement of Al 6063 alloy are investigated in the present work by employing electron back scattered diffraction (EBSD) analysis, transmission electron microscope (TEM) investigations, and X-ray diffraction (XRD) analysis. The solution treated bulk Al 6063 alloy samples are subjected to cryorolling and room temperature rolling to produce sheets with different strain levels such as 0.4, 2.3 and 3.8, respectively. Prior to cryorolling and room temperature rolling, the initial conditions such as solution treatment temperature, and sample immersion duration time, in liquid nitrogen, before cryorolling are optimized by using EBSD analysis, TEM investigations, hardness test, and tensile test. It is observed that the formation of recrystallized ultrafine-grains with the high angle grain boundaries occurs at the strain value of 3.8. However, in case of room temperature rolled samples, the sub-grains are not recrystallized even up to the strain value of 3.8.