Enhanced mechanical stability of ultrafine grained steel through intercritical annealing cold rolled martensite

The ultrafine grained ferritic steels possess high strength but low ductility due to the shortage of work hardening.Fine grained ferrite-martensite dual phase microstructure was obtained in a microalloyed steel and low carbon steels through intercritical annealing of the cold rolled martensite.The dual phase microstructure was uniform and the ferrite grain size was smaller in the microalloyed steel resulted from the pinning effect of microalloyed precipitates.But ferrite grown apparently and the volume fraction of the martensite was much higher without the addition of microalloying elements.By introducing martensite into the fine grained ferrite,the work hardening was effectively improved,leading to better mechanical stability.As a result of the fine ferrite grain size as well as uniform distribution of the martensite,the work hardening was enhanced,showing better strength-ductility balance in the microalloyed dual phase steel.     

Plastic flow characteristics of ultrafine grained low carbon steel during tensile deformation

In order to explain steady-state plastic deformation, i.e. the absence of strain hardening in ultrafine grained low carbon steel during tensile deformation, steel of different ferrite grain sizes was prepared by intense plastic straining followed by static annealing and then tensile-tested at room temperature. A comparison between the ferrite grain size of ultrafine grained steel and the dislocation cell size of coarse grained steel formed during tensile deformation revealed that uniform dislocation distribution with high density and cell formation were unlikely to occur in this ultrafine grained steel. This is ascribed to the fact that the ultrafine grain size is comparable to or smaller than the cell size at the corresponding stress level. In addition, from a consideration of dynamic recovery, it was found that the characteristic time for trapped lattice dislocations to spread into the grain boundaries was so fast that the accumulation of lattice dislocation causing strain hardening could not occur under this ultrafine grain size condition. Therefore, the extremely low strain hardening rate of ultrafine grained low carbon steel during tensile deformation is attributed to the combined effects of the two main factors described above.     

A new and simple process to obtain nano-structured bulk low-carbon steel with superior mechanical property

A new process to obtain ultrafine grained bulk steel was developed. Plain low-carbon steel sheet with martensite starting microstructure was simply cold-rolled by 50% and annealed. The specimens annealed at intermediate temperatures such as 773 K revealed the multiphased nano-structure and showed superior mechanical properties.     

Producing the nano/ultrafine grained low carbon steel by martensite process using plane strain compression

In this study, a simple experimental method was developed to produce the nano/ultrafine grained structure in a low carbon steel. This method involved cold plane strain compression of martensite starting structure and subsequent annealing. The martensitic specimens were first plane strained to 75% reduction in thickness (equivalent strain of 1.5), followed by subsequent annealing at the temperature range of 400-550 °C for 0-180 min. The microstructures and mechanical properties of specimens were characterized. The microstructure of the specimen annealed at 500 °C for 65 min showed cementite precipitates in the ferritic microstructure with the grain size of about 145 nm. The results showed superior mechanical properties with the estimated values of ultimate strength of 1135 MPa and uniform elongation of 11.6% for the fully recrystallized specimen.     

400MPa级细晶粒钢电阻点焊接头组织分析

以智能型点焊机为试验平台,研究超级钢点焊接头的组织和显微硬度分布情况.结果表明,普通低碳钢的点焊工艺不适用于超级钢,需附加锻压力,以消除气孔等收缩性缺陷.熔核内部组织结构分别为等轴晶区和胞状晶区;熔核外部的热影响区包括过热区和重结晶区.接头区域的显微硬度明显高于母材组织,主要是由于点焊快冷条件下产生了马氏体组织;而热影响区的组织硬度稍低于熔核中心区域,应是残余应力及再生细晶相互作用的结果.     

Thermal Stability Study of Ultrafine Grained 304L Stainless Steel Produced by Martensitic Process

An ultrafine grain 304L stainless steel with average grain size of about 650 nm was produced by martensitic process. 10 mm as-received sheets were 80% cold rolled in the temperature of ?15 °C and then annealed at 700 °C for 300 min to obtain ultrafine grained microstructure. The results showed that the ultrafine grained 304L steel has yield strength of 720 MPa, tensile strength of about 920 MPa, and total elongation of 47% which is about twice that of coarse grain structure. The effect of annealing temperature (750-900 °C) on the grain growth kinetics was modeled by isothermal kinetics equation which resulted in the grain growth exponent (n) and activation energy for grain growth of 4.8 and 455 KJ/mol, respectively. This activation energy was also compared with those for other austenitic steels to better understanding of the nature of grain growth and atoms mobility during annealing. It was found that activation energy for grain growth is about twice higher than self-diffusion activation energy of austenite that is related to the Zener pinning effects of the second phase particles.     

热处理对冷加工奥氏体不锈钢晶粒度的影响

本文研究了热处理对冷加工奥氏体不锈钢组织的影响，试验结果表明利用形变马氏体的逆转变可获得超细化不锈钢晶粒。     

A microstructural investigation of the surface of a drilled hole in carbon steels

The microstructure of the surface of drilled holes generated under different drilling conditions in carbon steels has been investigated. It is found that the surface microstructure depends strongly on the drilling parameters and the hardness of the matrix. White etching layers, composed of an equiaxed nanocrystalline structure layer with an average grain size of the order of several 10 nm and a submicron grained layer containing fresh martensite along the depth, formed on the hole surfaces during drilling at moderate to high cutting speed in carbon steels with high matrix hardness. The existence of a high content of austenite at the hole surface suggests that dynamic phase transformation (DPT) from body-centered cubic to face-centered cubic occurred during high-speed drilling. It is proposed that the ultrafine structure layer on the surface of a drilled hole is produced by severe plastic deformation-induced DPT together with a large strain gradient and high strain rate.     

异构片层结构304不锈钢的制备及其力学性能

采用形变诱导马氏体退火逆转变工艺制备了异构片层结构(HLS)的304奥氏体不锈钢。通过扫描电镜和X射线衍射仪分析了304奥氏体不锈钢的显微组织和物相组成,并采用室温拉伸试验研究了其力学性能。结果表明,通过变形量为34%的热轧、75%的冷轧以及700 ℃退火12 min后,试验钢中的马氏体相逆转变为奥氏体相,部分残留奥氏体发生再结晶,获得了由微米再结晶晶粒与超细晶/纳米晶晶粒以及残留奥氏体晶粒组成的异构片层结构,微米再结晶晶粒和残留奥氏体被超细晶/纳米晶晶粒所包围。异构片层结构304奥氏体不锈钢的屈服强度为940.1 MPa,断裂总延伸率为43.1%,获得了良好的强度-塑性匹配。     

异构纳米/超细晶钢的研究进展

纳米/超细晶钢在具备高强度的同时,往往存在屈强比高、加工硬化率低和拉伸伸长率低的问题。近年来广泛的实验研究证明,异构组织往往能通过不同结构区域的协同耦合来优化材料的力学性能,是改善纳米/超细晶钢强/塑性“倒置”问题的有效途经。综述了近年来国内外关于钢中纳米/超细晶组织调控与其性能特点的研究进展,包括钢中主要异构组织的种类、性能特点和制备加工技术;同时,对异构纳米/超细晶钢面临的一些基础科学问题和发展前景进行了讨论和展望。     

超细晶钢微观组织与力学性能的各向异性研究

通过温轧技术,成功制备了平均晶粒尺寸约1μm的超细晶钢。利用扫描电镜、电子背散射衍射和室温拉伸等检测手段,研究了超细晶钢板的微观组织与力学性能的各向异性。结果表明：制备的超细晶钢板不同方向的微观组织和力学性能具有明显的各向异性;纵截面表层和心部的晶粒大多呈现拉长的形态,同时表层的晶粒拟合椭圆长/短轴比相较于心部更小,形状更加接近于圆形,横截面的晶粒拉长状特征相较于纵截面减弱,更接近等轴态,晶粒在三维空间呈细长的扁梭形态;各向强度由高到低依次为轧向、横向、与轧向成45°方向;长梭形晶粒组成的微观组织形态和织构是导致超细晶钢板力学性能各向异性明显的主要原因。     

马氏体温变形超细化过共析钢

利用Gleeble 1500热模拟试验机进行单轴热压缩实验, 研究了含Al和不含Al两种过共析钢马氏体温变形和等温回火过程中的组织超细球化演变及超细球化组织的力学性能. 结果表明: 与马氏体等温回火相比, 马氏体温变形加快马氏体的分解动力学, 在较短的时间 内即获得超细化 (α+θ)复相组织. 温变形过程中的组织超细化演变主要经历渗碳体粒子的析出与粗化及铁素体基体的动态回复和动态再结晶; 而在等温回火过程中, 铁素体主要发生静态回复和晶粒长大, 并没有再结晶现象发生. 合金元素Al的加入在等温回火和温变形过程中均抑制马氏体的分解, 阻碍渗碳体粒子的粗化和铁素体晶粒的长大, 导致复相组织的细化. 同时, Al的加入使马氏体温变形和等温回火后所得超细化 (α+θ) 复相组织在不降低总延伸率的前提下, 强度得以明显提高.     

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．     

400MPa级超细晶粒钢电弧焊焊接接头组织与性能

采用普通电弧焊方法（手工电弧焊、二氧化碳气体保护焊、埋弧焊）对400MPa级超细晶粒钢进行焊接,并对不同工艺条件下的焊接接头的组织与性能进行了分析。结果表明,400MPa级超细晶粒钢电弧焊焊接接头热影响区存在着明显的晶粒长大现象;手工电弧焊和埋弧焊焊接接头性能明显下降,而二筘化碳气体保护焊焊接接头性能下降不明显。根据试验结果,实际生产中可以利用二氧化碳气体保护焊方法焊接400MPa级超细晶粒钢。     

喷水冷却对400MPa级超细晶粒钢焊接接头组织和性能的影响

采用手工电弧焊焊接400MPa级超细晶粒钢，并对不同冷却条件下的焊接接头显微组织及力学性能进行分析。结果表明，400MPa级超细晶粒钢手工焊焊接接头热影响区存在着严重的晶粒长大和强度下降现象；焊接过程中采用喷水冷却可明显提高焊接接头强度，实际生产中可以应用手工电弧焊水冷处理方法焊接400MPa级超细晶粒钢。     

Role of tandem submerged arc welding thermal cycles on properties of the heat affected zone in X80 microalloyed pipe line steel

The influence of thermal cycles on the properties of the coarse grained heat affected zone in X80 microalloyed steel has been investigated. The thermal simulated involved heating the X80 steel specimens to the peak temperature of 1400 °C, with different cooling rates. The four-wire tandem submerged arc welding process, with different heat input values, was used to generate a welded microstructure. The martensite/austenite constituent appeared in the microstructure of the heat affected zone region for all the specimens along the prior-austenite grain boundaries and between the bainitic ferrite laths. The blocky-like and stringer martensite/austenite morphology were observed in the heat affected zone region. The fractional area of M/A particles due to different cooling rate was the main factor in increasing of the hardness values in the coarse grained heat affected zone. The Charpy absorbed energy of specimens was assessed using Charpy impact testing at −50 °C. The martensite/austenite constituent's size such as mean diameter and length are important factors influencing Charpy impact properties of coarse grained heat affected zone. The micro crack nucleation may occur from M/A particles at the intersection of prior-austenite grain boundaries. Similar crack initiation sites and growth mechanism were investigated for specimens welded with different heat input values.     

Effect of ultrasonic surface treatment of steel 40Kh13 on the microstructure of nitrided layer formed by high-intensity low-energy implantation with nitrogen ions

X-ray diffraction and transmission electron microscopy were used to study the microstructure and phase composition of surface layers of steel 40Kh13 subjected to irradiation with high-intensity low-energy ion beams, ultrasonic surface modification, and combined treatment including ultrasonic surface modification and ion implantation. It was found that the ultrasonic modification of steel surface leads to changes in the structure of tempered martensite and the formation of grain structure with a grain size of 0.3 μm and nanosized special carbides Cr₂₃C₆ in the martensite lamellae. The ion implantation into this steel results in the formation of a nitrided layer consisting of a nitride region, which represents a mixture of several phases (α-, γ′-, ?, and ultrafine chromium nitrides), and a zone of internal nitriding (α″ and nitrogen-containing martensite α_N). The preliminary ultrasonic modification causes an increase in the nanohardness and in the thickness of the nitrided layer, which is due to the more intense penetration of nitrogen atoms into the surface layer and an increase in the volume fraction of iron nitrides and density of ultrafine chromium nitrides in this layer.     

Effect of temperature and stress on creep behavior of ultrafine grained nanocrystalline Ni-3 at% Zr alloy

In this paper, molecular dynamics (MD) simulation based study of creep behavior for nanocrystalline (NC) Ni-3 at% Zr alloy having grain size ~ 6 nm has been performed using embedded atom method (EAM) potential to study the influence of variation of temperature (1220-1450 K) as well as change in stress (0.5-1.5 GPa) on creep behavior. All the simulated creep curves for this ultra-fine grained NC Ni-Zr alloy has extensive tertiary creep regime. Primary creep regime is very short and steady state creep part is almost absent. The effect of temperatures and stress is prominent on the nature of the simulated creep curves and corresponding atomic configurations. Additionally, mean square displacement calculation has been performed at 1220 K, 1250 K, 1350 K, and 1450 K temperatures to correlate the activation energy of atomic diffusion and creep. The activation energy of creep process found to be less compared to activation energies of self-diffusion for Ni and Zr in NC Ni-3 at% Zr alloy. Formation of martensite is identified during creep process by common neighbour analysis. Presence of dislocations is observed only in primary regime of creep curve up till 20 ps, as evident from calculated dislocation density through MD simulations. Coble creep is found to be main operative mechanism for creep deformation of ultrafine grained NC Ni-3 at% Zr alloy.     

Effect of austenization temperature on the microstructure evolution of the medium manganese steel （0.2C-5Mn） during ART-annealing

Microstructure evolution during ART-annealing (austenite reverted transformation annealing) of 0.2C-5Mn steel processed by austenitation at different temperatures was examined by SEM, TEM and XRD. It was demonstrated that the initial microstructures resulted from austenization at different temperatures strongly affect the microstructure evolution during followed ART-annealing, even the ultrafine grained ferrite/austenite duplex structure with about 30% austenite could be obtained after long time ART-annealing in all cases. Austenization in the intercritical region (between A c1 and A c3 ) gave a duplex structure after quenching, which was nearly not affected by followed annealing process. However, high temperature austenization (above A c3 ) resulted in a full martensite structure after quenching, which gradually transformed into a ferrite/austenite duplex structure during the following annealing process. Based on the analysis of austenite fraction and carbon concentrate, it was found that not only carbon partitioning but also manganese partitioning in the austenite affected the stability of austenite and even dominated the development of lamellar ferrite and austenite duplex structure during intercritical annealing with different times. At last an austenite lath nucleation and thickening model was proposed to describe the microstructure evolution of medium mangenese steel during ART-annealing.     

Development of a New Ultrafine/Nano Ferrite-Carbide Microstructure by Thermomechanical Processing

The steel with the new microstructure, bimodal submicrometer equiaxed ferrite grains with uniformly distributed nanosized cementite particles, was manufactured by a new approach utilizing simple cold-rolling and subsequent annealing of a dual phase ferrite-martensite starting structure. The mean ferrite grain size and carbide size range of the specimen after 80% cold-rolling and subsequent annealing at 600 °C for 20 min were 0.35 lm and 70–140 nm, respectively. A combination of bimodal ultrafine ferrite and nanoscale carbides used as a more effective method for achieving an excellent balance in strength-ductility. The strength of the steel with the new microstructure increased to about 880 MPa(nearly 60% higher than that of the as-received state, e.g., 540 MPa), without significant loss of ductility.