Combined effects of high-temperature rolling and ultrafast cooling on the mechanical properties of low-carbon steel

The role of ultrafast cooling (UFC) on the grain refinement of ferrite, the precipitation behavior of cementite particles and the mechanical properties of a mild steel (Q235 grade) was evaluated by applying laminar cooling and UFC and varying the finish cooling temperature ranges during UFC after hot rolling. While UFC refined the ferrite grains, it accumulated the degeneration of pearlite, resulting in complete disappearance of the laminar pearlite at relatively low finish cooling temperatures. The minimum mean size of spheroidized cementite particles reached ~110?nm. Meanwhile, the enhancement of UFC on tensile strengths of mild steels mainly resulted from the grain refinement of ferrite and the precipitation strengthening of cementite particles; however, the contribution varied with the finish cooling temperature of UFC. A modified Ashby–Orowan model was also used for evaluating the yield strength increment of medium plates. This work will provide a theoretical basis for the diversity control of microstructure and for developing stronger and tougher mild steels by introducing UFC technology after high-temperature rolling.     

Effect of high magnetic field on the processing of pearlitic steels

This paper compares data from various sources concerning the impact of high magnetic field (HMF) on changes that occur in pearlite with respect to microstructure, phase transition, and mechanical properties. HMF raises the transformation temperatures of both ferrite and pearlite. This effect can be enhanced by increasing the carbon content. Other alloying elements may influence austenite decomposition temperature, Curie temperature, and magnetic moment, thus either increasing or decreasing the effect of HMF on phase transformation temperature. By altering the transformation thermodynamics, HMF increases the volume fraction of proeutectoid ferrite, decreases that of cementite, and decreases the lamellar spacing. HMF introduces the microstructure anisotropy and aligns proeutectoid ferrite grains parallel to the direction of the HMF. This effect becomes smaller when the cooling rate is higher. By affecting both phase transformation and the alignment of grains, HMF affects the morphology and microstructure of proeutectic ferrite and pearlite, and, consequently, their mechanical properties and corrosion resistance.     

Influence of cementite spheroidization on relieving the micro-galvanic effect of ferrite-pearlite steel in acidic chloride environment

The corrosion behavior of the as-received steel and the spheroidized steel in acidic chloride environment was investigated. The results indicate the corrosion mode and corrosion rate of two steels are diverse due to their difference in microstructure. For as-received steel with ferrite-pearlite microstructure, severe localized corrosion happens on the pearlite regions, and plenty of cathodic cementite remains in the pits, further strengthening the micro-galvanic effect and accelerating the corrosion rate. While for spheroidized steel with tempered martensite microstructure, the nanosized cementite particles evenly distributed on the ferrite substrate are easy to fall off, which can significantly reduce the cementite accumulation on the steel surface, relieving the acceleration effect of micro-galvanic corrosion.     

Suppression of hydrogen-induced blisters in SK4 carbon steel alloy by friction stir processing

The feasibility of using friction stir-processing technique as a microstructural refining method for preventing hydrogen-induced blisters of SK4 carbon steel alloy containing microstructure of hard cementite particles in a ductile ferrite matrix was examined. Amount and size of hydrogen-induced blisters decreased in the stirred layers with increasing rotation speed and completely disappeared when rotation speed attained 400 rpm because of the formation of a hard and fine microstructure consisting of pearlite, martensite, and retained austenite instead of that containing hard spheroidized cementite particles in a soft ferrite matrix. Interfaces between ferrite matrix and cementite particles were the preferential sites for the hydrogen blisters initiation.     

Microstructural development in non-oriented lamination steels

The isothermal decomposition of austenite in two commercial low carbon (0.04 w/o) steels has been examined using scanning electron microscopy and transmission electron microscopy. Particular emphasis has been placed on analysing the pearlite reaction and the development of massive films of cementite at pro-eutectoid ferrite/pearlite interfaces. Similarly, grain boundary precipitation of cementite has been investigated. The results strongly support the contention that films of cementite at ferrite/pearlite interfaces form predominantly by a coarsening process. In addition, it is shown that grain boundary precipitation of cementite can occur from super-saturated ferrite or from the decomposition of austenite. Examination of the early stages of the pearlite reaction has provided evidence that multiple nucleation of cementite can be a necessary precursor to the development of a pearlite colony.     

Evaluation of Railroad Wheel Steel with Lamellar Duplex Microstructures Using Diffuse Ultrasonic Backscatter

The effects of lamellar duplex microstructure within grains that contain alternating phases of cementite and ferrite on ultrasonic scattering in railroad wheel steel are evaluated using a diffuse ultrasonic backscatter technique. A new singly scattered response (SSR) model that considers the lamellar duplex microstructure within grains is developed based on a previous SSR model. The results show that the amplitude of ultrasonic scattering decreases with decreasing lamellar space. Corresponding experiments are performed with 10 MHz and 15 MHz focused transducers by scanning both unquenched and quenched wheels. The experimental results show that the ultrasonic scattering amplitudes drop dramatically near the quenched tread surface, a result which is attributed to the creation of duplex microstructure (pearlite phase) within grains due to the quenching process. The lamellar spacing within grains increases progressively from the tread surface to the deeper locations due to the non-uniform cooling rate. The distribution of lamellar spacing within grains as a function of depth is quantified with the modified SSR model. Good agreement with optical microscopy is observed. The diffuse ultrasonic backscatter technique exhibits strong sensitivity to microstructure changes, an outcome that may be applicable for quality control during manufacturing.     

GCr15钢温加工变形中组织结构的变化

研究了GCr15钢片状珠光体温变形过程中组织结构的变化。对变形中发生的变形应力随变形量增加而降低的动态软化现象进行了分析。结果表明,在温变形中,渗碳体片发生了明显的断裂,断裂的方式主要为剪切断裂和弯曲断裂,铁索体发生了动态回复,渗碳体中出现了位错的规则排列。动态软化现象主要是由于变形过程中片状渗碳体发生了断裂所造成的。     

Study on the initial electrodeposition behavior of Ni–P alloys

The initial electrodeposition behavior of Ni–P alloys on carbon steel has been studied in this paper and was found to be greatly influenced by the morphology of the substrate. Preferred nucleation of Ni–P alloy coatings takes place at the crystal boundaries and the surface of cementite. Preferred nucleation and growth of Ni–P coatings also takes place at pearlite because pearlite contains cementite phase and many crystal boundaries. The coatings exhibit a polycrystalline structure composed of gathering nano-sized grains. With a continued electroplating time prolonged, the grains not only present two-dimensional sideward growth, but also display three-dimensional growth on them to form a new polycrystalline layer.     

Analysis of Nodular Cast Iron Microstructures for Micromechanical Model Development

Abstract:  One of the main factors in determining the different grades of ductile iron is the matrix structure. In the as-cast condition, the matrix will consist of varying proportions of pearlite and ferrite, and as the amount of pearlite increases, the strength and hardness of the iron also increase. Three different nodular cast irons are here considered and their microstructure characterised in detail using metallographic methods. Then micromechanics models based on the unit cell approach and the finite element method are introduced to describe the actual constitutive response of the materials and the predicted behaviours are compared with experiments.     

Microstructure and microhardness of pearlitic steel after laser shock processing and annealing

The microstructure evolution of the high carbon pearlitic steel after laser shock processing (LSP) with different laser pulse energy and high temperature annealing was investigated. After LSP, the cementite lamella were bent, fractured and broken into granules. Fragmentation and dissolution of the cementite lamella were enhanced by increasing the laser pulse energy. Results show that the ferrite lattice parameter increased due to carbon atom dissolution in the ferrite matrix, and the corresponding ferrite X-ray diffraction peaks shifted significantly towards the smaller diffraction angles. After annealing at 650°C for 30?min, an ultrafine duplex microstructure (ferrite+cementite) was formed on the surface. After LSP with a high energy, equiaxed ferrite grains were refined to 400?nm and the cementite lamella were fully spheroidised with the particle diameter of ～150?nm. The corresponding grain size of ferrite and cementite under low pulse energy was 500 and 300?nm respectively. After annealing, the ferrite peaks significantly shifted towards the higher diffraction angles, and the ferrite lattice parameter decreased. The microhardness initially increases after LSP and then slightly decreases after subsequent annealing but remained higher than without LSP.     

Mesoscale Modeling of Microstructure and Texture Evolution During Deformation Processing of Metals

The application of the finite element method to model the deformation of metals at the mesoscale to study the microstructure and texture evolution is described. The finite element discretization is applied directly to the various grains, and crystal plasticity is used as the constitutive basis to model the plastic deformation by crystallographic slip, and to evolve the slip system strength and crystal lattice orientation of the material. Applications of the methodology to detailed studies of the non‐uniform deformations of individual grains, and effects of grain interactions on the distributions of deformation and stress in the microstructure are discussed.     

ECAP变形对20MnSi钢组织与性能的影响

采用C方式等径弯曲通道变形(Equal Channel Angular Pressing,简称ECAP)对20MnSi钢进行了4道次室温变形,研究了变形道次对显微组织和力学性能的影响.结果表明,铁素体组织随变形道次的增加逐渐演变为等轴状大角度晶界的亚微晶组织;试验用钢的硬度和强度随变形道次的增加而增加,4道次后强度有所降低;在本试验条件下,珠光体组织中的片状渗碳体表现出很强的塑性变形能力.     

Three-dimensional periodic Voronoi grain models and micromechanical FE-simulations of a two-phase steel

A three-dimensional model is proposed for modeling of microstructures. The model is based on the finite element method with periodic boundary conditions. The Voronoi algorithm is used to generate the geometrical model, which has a periodic grain structure that follows the original boundaries of the Voronoi cells. As an application, the model is used to model a two-phase ferrite/pearlite steel. It is shown that periodic cells with only five grains generate representative stress–strain curves.     

超快冷对碳素钢中渗碳体析出强化行为的影响*

用超快速冷却技术并控制轧后冷却温度, 研究了3种碳含量不同的碳素钢热轧后组织中渗碳体的析出行为和强化机制。结果表明, 在超快速冷却条件下0.04%C和0.5%C(质量分数, 下同)实验钢的主要强化方式分别是细化晶粒和细化珠光体片层间距, 没有纳米级渗碳体颗粒析出, 而在0.17%C实验钢的组织中则有大量弥散的纳米级渗碳体析出, 颗粒直径范围为10-100 nm, 通过超快速冷却技术实现了在不添加微合金元素的条件下纳米级渗碳体的析出。随着超快速冷却终冷温度的降低纳米渗碳体的析出强化作用使0.17%C钢的屈服强度提高110 MPa, 强化效果明显。在超快速冷却的工艺基础上若继续采用形变热处理工艺, 可进一步提高0.17%C实验钢的位错密度, 促进渗碳体均匀形核, 实现纳米级渗碳体颗粒在整个组织中更加均匀弥散的分布, 达到更好的均匀强化效果。在超快速冷却和形变热处理工艺条件下0.17%C钢的屈服强度可达到650 MPa以上, 强化效果提高300 MPa以上。     

Mechanical properties of ultrafine grained ferritic steel sheets fabricated by rolling and annealing of duplex microstructure

A new route to fabricate ultrafine grained (UFG) ferritic steel sheets without severe plastic deformation is proposed in this article. A low-carbon steel sheet with a duplex microstructure composed of ferrite and martensite was cold-rolled to a reduction of 91% in thickness, and then annealed at 620–700 °C. The microstructure obtained through the process with annealing temperatures below 700 °C was the UFG ferrite including fine cementite particles homogenously dispersed. The grain size of ferrite matrix changed from 0.49 to 1.0 μm depending on the annealing temperature. Dynamic tensile properties of the produced UFG steels were investigated. The obtained UFG ferrite–cementite steels without martensite phase showed high strain rate sensitivity in flow stress. The UFG ferritic steels are expected to have high potential to absorb crash energy when applied to automobile body.     

Effect of Nb-Mo additions on precipitation behaviour in V microalloyed TRIP-assisted steels

The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation-induced plasticity-assisted steels with an acicular/bainitic ferrite matrix were investigated by a heat treatment up to the austenite formation range. It was found that during the heating stage the acicular/bainitic ferrite matrix resisted recrystallisation, while cementite and martensite were decomposed and austenite was formed in the acicular/bainitic ferrite. Both Nb-V-Mo and V containing steels after the heat treatment showed a microstructure consisting of a polygonal ferrite matrix with small islands of pearlite. During these transformations, the microscopy observations showed that 0.04 wt% Nb and 0.08 wt% Mo additions to the 0.16 wt% V microalloyed steel considerably reduced the growth-coarsening of microalloy precipitates.     

Behaviour of low and medium carbon free cutting steels during deformation to large strains

Abstract

Low and medium carbon free cutting steels were deformed by cold rolling to reductions of up to 98%. The resultant microstructures were observed and characterised using optical microscopy, SEM, and TEM. Deformation of the pearlite grains and manganese sulphide inclusions was quantified in terms of their relative plasticity (compared to that of the steel). The evolution of the ferrite microstructure in the steels was seen to be dependent on the volume fraction of pearlite present. The ferrite grains in the low carbon steels underwent structural subdivision characterised by the formation of dense dislocation walls and microbands. At intermediate rolling deformations much of the strain was accommodated inhomogeneously in narrow bands of shear (S bands). Strain in the pro-eutectoid ferrite of the medium carbon steel occurred in a more homogeneous manner owing to the constraints imposed by the pearlite. The manganese sulphides and pearlite in the steels also acted as fiducial markers of the surrounding ferrite flow. Plasticity of the sulphides was generally found to be less than the overall rolling strain. However, within certain narrow strain ranges, sulphide plasticities greater than that of the steel were measured.     

Effect of gaseous hydrogen charging on the tensile properties of standard and medium Mn X70 pipeline steels

The tensile properties of two X70 steels with high (1.14 wt-%) and medium (0.5 wt-%) Mn contents have been investigated by testing at 25°C of tubular specimens charged with an internal gas pressure of 10?MPa of hydrogen or argon. The hydrogen-charged samples were additionally tested at 50 and 100°C. Tensile testing showed that the equiaxed ferrite–pearlite microstructure of higher Mn steel was most sensitive to hydrogen embrittlement and that the banded ferrite–pearlite microstructure of the higher Mn strip was more susceptible to hydrogen embrittlement than the medium Mn strip. The more highly banded ferrite–pearlite microstructure in the higher Mn steel provided numerous sites for concentration of hydrogen to levels that promoted crack initiation and growth. Test temperatures up to 100°C reduced the yield and tensile strengths, increased the total elongation and decreased the extent of hydrogen embrittlement because of enhanced dislocation mobility and less effective hydrogen trapping.     

Origin and mechanism of torsion fracture in cold-drawn pearlitic steel wires

The structural and mechanical factors that control the torsion fracture behavior of cold-drawn eutectoid steel wires are examined. Two types of the fracture are identified; namely, flat- and cleavage-type. Torsion cracks are found to initiate in ferrite and propagate along the ferrite/cementite interface. The shear stress distribution within the wires is affected not only by the applied torque, but also by the residual stress. The maximum shear stress occurs halfway from center to the surface, where the cracks initiated. The growth of torsion cracks is sensitive to the orientation of cementite lamellas in pearlite grains. The influence of thermal history on the occurrence of cleavage fracture is ascertained, with the assistance of atom probe. It shows that the cleavage fracture results from a decrease in dislocation mobility, caused by thermally activated diffusion of carbon atoms into ferrite.     

Simulation of hot-rolled dual-phase weathering steel 09CuPCrNi

The continuous cooling transformation diagram of deformed austenite for steel 09CuPCrNi was constructed by means of a combined method of dilatometry and metallography. The diagram exhibits an elongated polygonal ferrite C-curve with a delayed pearlite start and a metastable austenite gap between the polygonal ferrite and pearlite regions and between the ferrite and bainite regions. For this experimental steel, it is possible to obtain a dual-phase microstructure directly by hot rolling and appropriate cooling. Based on the diagram, the technical process of a hot-rolled dual-phase treatment was established and simulated using a thermal simulation testing machine. Dual-phase microstructures were obtained that show some bainite phase and are characterized by an irregular distribution of island-shaped martensite in a matrix of equiaxed ferrite grains. The morphology of the martensite phase is essentially of the lath type, with small areas of micro-twins appearing.