Effect of Strengthening Phase on Deformation Behaviour during Uniaxial Tension of Hot-rolled Dual Phase Steel

Two kinds of C-Si-Mn-Cr series tested steels were designed to obtain dual phase microstructures of ferrite （F） ＋martcnsite （M） or ferrite （F）-bainite （B） with different mechanical properties. Effects of strengthening phase on yielding and fracture behaviours during uniaxial tension of dual phase steel were discussed. Compared with hot-rolled martensite dual phase steel, ferrite-bainite dual phase steel has high ratio of yield strength to tensile strength （YS/TS） and low elongation. During necking process of uniaxial tension, microvoids of ferrite-martensite steel are generated by fracture of ferrite/martensite boundary or martensite islands with irregular shape. But ferrite matrix elongated remarkably along deformation direction, and strengthening phase also coordinated with ferrite matrix. Compatible de formation between ferrite and bainite is distinct. Ferrite-bainite dual phase steel has fine and less microvoid, and phase boundary of ferrite and bainite is beneficial for restraining generation and extending of microvoid.     

Nondestructive evaluation of steels using acoustic and magnetic barkhausen signals—I. Effect of carbide precipitation and hardness

The effect of microstructures on acoustic and magnetic Barkhausen signals has been investigated in a quenched and tempered steel and spheroidized steels with various carbon contents. Many different features between the acoustic and magnetic Barkhausen signals were observed. A major peak of the acoustic Barkhausen signal was induced when a magnetic field was increased from zero to the saturation state. A minor peak of the acoustic signal and a single peak of the magnetic signal appeared during the decreasing field. It was found in all the steels that the peak value of the acoustic Barkhausen signal shows a linear dependence on the sweep rate of a magnetic field while that of the magnetic Barkhausen signal does a nonlinear one. The increasing tempering temperature which gives rise to a decrease in hardness and an increase in carbide size and spacing caused that acoustic and magnetic Barkhausen peak voltages to increase precipitously and gradually, respectively. In the spheroidized steels, the acoustic peak voltage monotonically decreased with increasing carbon content from 0.17 to 0.96 wt% and the magnetic peak voltage was greatest when the carbon content was 0.46 wt%. The source mechanisms of the two types of the Barkhausen signals which are affected by the nucleation and growth processes of domains in a magnetic field are discussed to explain the effect of the change in carbide precipitate morphology and hardness.     

Bauschinger effect and residual phase stresses in two ductile-phase steels: Part II. The effect of microstructure and mechanical properties of the constituent phases on Bauschinger effect and residual phase stresses

A finite-element method has been employed to calculate the effect of the prestrain, particle size, volume fraction, and yield strength ratio of the constituent phases on the Bauschinger effect (BE) and residual phase stresses for ferrite-martensite two-phase steels. The relationships be-tween the BE parameters and residual phase stresses are given, and the basics of inelastic un-loading and the effect of reverse deformation on the BE and residual phase stresses are also discussed. Based on the decaying of the residual phase stresses (commonly called “back stresses”) during reverse loading, the relationship between back stresses and permanent softening has been elucidated. Some of the calculated results are compared with experimental ones, and good agree-ment between them is found.     

高伸长率冷轧双相钢DP980显微组织及性能

摘要：为了进一步提高冷轧双相钢DP980的强塑性,采用低C-Si-Mn-Nb-Cr成分,通过调整连续退火工艺参数,系统研究了工艺组织性能的关系,利用OM、SEM、EBSD分析了不同退火温度条件下各相的比例、尺寸、形貌、分布,同时利用力学拉伸试验手段研究了连退两相区退火温度对强塑性的影响。结果表明,通过优化调整连续退火工艺,不仅可以在冷轧铁素体和马氏体双相钢的组织上获得少量的残余奥氏体,也能细化再结晶晶粒,最终获得ReL/Rm≤0.5、高伸长率A50≥15%的冷轧DP980,提高强塑性的同时改善了成型性能。     

Nondestructive evaluation of steels using acoustic and magnetic barkhausen signals—II. Effect of intergranular impurity segregation

The present paper has studied the influence of intergranular impurity segregation on acoustic and magnetic Barkhausen signals induced during magnetization in an undoped nickel-chromium steel and doped steels with antimony, tin or phosphorus. While the intergranular impurity segregation did not affect the magnetic hysteresis loop at all, it caused the characteristics of the acoustic and magnetic Barkhausen signals to change in a different manner. The impurity-doped steels produced a higher magnitude of the acoustic signal than the undoped one. However, the acoustic signal intensity of the impurity-doped steels remained the same independent of the quantity and type of segregants. With increasing quantity of segregants, the magnitude of the magnetic signal initially decreased and then increased above 0.078 atomic fraction. The magnetic signal also did not depend on the type of segregants. The effect of segregated impurities on the grain boundary energy and the magnetostatic energy at grain boundaries which change the nucleation and growth processes of domains is discussed.     

Microstructure and Tensile Properties in Dual Phase and Trip Steels

Advanced high‐strength steels, like dual phase and TRIP steels, have gained much interest for automotive application. The complex microstructures in dual phase steels, and even more critical, the metastable microstructure in TRIP steels, do not follow the well‐established traditional microstructure‐property relationships for deep drawing steels. The volume fraction of the different phases, the phase distribution, and the stability of metastable phases influence significantly the forming potential. This paper discusses the correlation between different microstructural features and the mechanical properties. The tensile test properties of dual phase steels are governed by the martensite volume fraction, the martensite hardness and to a much smaller extent the martensite island diameter. Both in dual phase and more pronounced in TRIP steels the retained austenite content plays a vital role in determining the formability. The stability of the retained austenite can be described by different methods, it needs to be adjusted according to the forming temperature and the type and amount of strain. In general, multiphase steels require a very strict microstructure control in order to develop predictable forming behaviour.     

Role of Niobium in Advanced High Strength Steels for Automotive Applications

Two major drivers for the use of newer steels in the automotive industry is fuel efficiency and increased safety performance.Fuel efficiency is mainly a function of weight of steel parts,which in turn,is controlled by gauge and design.Safety is determined by the energy absorbing capacity of the steel used to make the part.All of these factors are incentives for the U.S.automakers to use Advanced High Strength Steels (AHSS) to replace the conventional steels used to manufacture automotive parts in the past.AHSS is a general term used to describe various families of steels.The most common AHSS is the dual-phase steel that consists of a ferrite-martensite microstructure.These steels are characterized by high strength,good ductility,low tensile to yield strength ratio and high bake-hardenability.Another class of AHSS is the multi-phase steel which have a complex microstructure consisting of various phase constituents and a high yield to tensile strength ratio.Transformation Induced Plasticity (TRIP) steels is the latest class of AHSS steels finding interest among the U.S.automakers.These steels consist of a ferrite-bainite microstructure with significant amount of retained austenite phase and show the highest combination of strength and elongation,so far,among the AHSS in use.High level of energy absorbing capacity combined with a sustained level of high n value up to the limit of uniform elongation as well as high bake hardenability make these steels particularly attractive for safety critical parts and parts needing complex forming.Finally,martensitic steels with very high strengths are also in use for certain parts.The role of Niobium in all of the above families of advanced steels for the automotive industry will be discussed in this paper.     

Sheet metal forming behaviour and mechanical properties of TRIP steels

Recently various kinds of high-strength sheet steels have been developed to meet the requirements of the automotive industry such as passive safety, weight reduction and saving energy. Usually the main problem of high-strength steels is their inferior ductility. Multiphase steels however show a very good combination of strength and formability so that the applicable region of high-strength steels has been widely enlarged. Multiphase steels have been developed for various purposes because of their ability to tailor properties by adjusting the type, the amount, and the distribution of different phases. Especially new developed triple-phase steels which make use of the TRIP effect (transformation induced plasticity) can further improve formability as well as strength due to the transformation of retained austenite to martensite during the deformation. In this work the transformation behaviour and the mechanical properties of low alloyed TRIP steels were investigated. The influence of the annealing parameters on transformation behaviour and on the amount of retained austenite were determined. In addition the temperature dependence of the mechanical properties and the effect of testing speed on the formability were studied. The investigation was carried out on seven different TRIP steels with different chemical compositions, especially the influence of the microalloying element niobium was considered. For reasons of comparison various mild and high-strength steels were tested parallel to the TRIP steels. It was found that the investigated TRIP steels offer very attractive combinations of elongation and strength values. An interesting temperature dependence of the mechanical properties can be observed, in such a way that the elongation values of the TRIP steels possess a maximum between +50 and +100°C. Due to its effect on grain size and on precipitation behaviour the addition of niobium leads to higher strength values without a strong decrease in ductility. In general, the mechanical properties are strongly affected by the type and the distribution of the different phases. The most important parameters, however, to influence the mechanical behaviour are the amount and the stability of the retained austenite, which are mainly controlled by the heat treatment and the chemical composition.     

Influence of martensite composition and content on the properties of dual phase steels

A study has been made of the mechanical properties of dual phase (martensite plus ferrite) structures produced when Fe-Mn-C alloys are quenched from the austenite plus ferrite phase field, so as to give a series of alloys with constant ferrite and martensite compositions but varying percent martensites. It is found that the strength of a dual phase structure is dependent on the ferrite grain size and the volume fraction of martensite, and is independent of the composition and strength of the martensite. In agreement with previous work the ductility of these steels is superior to that for standard HSLA steels at the same tensile strength. As shown in a previous paper the strength and ductility as a function of percent martensite are in agreement with Mileiko’s theory of composites of two ductile phases. This theory and the results indicate that the superior ductility of dual phase steels is largely a consequence of the high strength (fine grained), highly ductile (low interstitial content) ferrite matrix.     

热轧双相钢DP600组织性能的研究

选择添加铬、钼合金元素的碳、锰、硅系的高强双相钢DP600为研究对象,选择Gleeble-3800热模拟试验机为研究方法测定了DP600动态CCT曲线,并模拟DP600双相钢热轧过程。采用金相组织观察、织构分析及力学性能测试等手段分析了不同工艺制度下双相钢组织及织构变化规律以及对性能的影响,从中获得最佳组织配比及优化的热轧工艺参数。根据优化的中试结果,进行了热轧双相钢DP600的工业试制。结果表明,试制样品的显微组织为铁素体及马氏体;屈强比均小于0.65,抗拉强度均在600 MPa以上;伸长率在24%以上;其拉伸曲线均为连续曲线,无屈服点伸长,具有典型的双相钢特征。     

Influence of Martensite Distribution on the Mechanical Properties of Dual Phase Steels: Experiments and Simulation

The application of multiphase steels in the automotive industry has been rapidly increased according to economic, environmental and safety reasons. To determine an optimal combination of high strength and good formability of multiphase steels by using the FE modelling, their complex microstructures have to be considered. Two‐dimensional Representative Volume Elements (RVEs) were currently developed based on real microstructures for dual phase (DP) steels. In general, the microstructure of DP steels contains hard martensite particles and a soft ferritic matrix. The strain hardening behaviour of the individual phases was described in the model taking the microstructural constituents and the carbon partitioning during intercritical annealing into account. Two dual phase microstructures with same martensitecontent but different martensite distributions were investigated in experiment as well as in FEM simulation by means of the RVE. The resulting mechanical properties of these steels are strongly influenced by the phase distribution and interaction. As validation, calculated flow curves were compared with the experimental results from quasi‐static tensile tests. In addition, the local stress and strain partitioning between both phases depending on the spatial phase distribution and morphology is discussed.     

Influence of Laves Phase Precipitation and Coarsening on High‐Temperature Strength of Ferritic Stainless Steels

Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the high‐temperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic Fe? Cr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms – precipitation–hardening, grain refinement, and solid‐solution strengthening – were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 900°C for up to 1440 h. It is shown that, the solid solution effect of Mo and W increases the high‐temperature strength about 40%, also after long‐term annealing. The contribution of the Laves phase precipitates on the high‐temperature strength is rather small due to their rapid coarsening.     

Microstructural and mechanical behavior of a duplex stainless steel under hot working conditions

In the hot deformation of the duplex stainless steels, the complexity of the microstructure evolution and mechanical response is increased as compared with those of single-phase ferritic or austenitic stainless steels. In the present work, plane strain compression and torsion deformation modes have been used to analyze the microstructural evolution and the mechanical behavior of a duplex stainless steel in as-cast and wrought conditions, as a function of spatial phase distribution, the nature of interface, and the relative mechanical properties of both phases. The law of mixtures has been used to explain the different flow curves obtained when changing the phase distribution and/or the deformation mode. On deforming as-cast microstructures, the deformation partitions vary heterogeneously between both phases and some austenite areas act as hard nondeforming particles. Cracks have been observed to occur at the interface of such regions, from relatively low strains, for which the initial Kurdjumov-Sachs orientation relationship between ferrite and austenite is still present.     

Effect of Microalloying Additions on the Microstructure and Mechanical Properties of Low Carbon Steel

Low carbon steels are characterized by good weldability,formability and fracture toughness properties.However,the low strength levels of these steel grades limit their wide applications.On the other hand,increasing the strength by increasing the carbon content and alloying elements deteriorates the other properties.In this study,the microalloying technique was used to examine the possibility of attaining low carbon steels with good combination of strength,ductility and impact properties.A low carbon steel microalloyed with single addition of vanadium and another one microalloyed with combined addition of vanadium and titanium were used in this investigation and their properties were compared with non-microalloyed low carbon steel having the same base composition.Furthermore,other two nonmicroalloyed and V-microalloyed steels with higher carbon,silicon and manganese contents were also investigated to reveal the effect of base composition.Tensile,hardness,room and zero temperature Charpy V-notch impact tests were conducted to evaluate the variations in the mechanical properties of low carbon hot forged steel containing vanadium and combinations of vanadium and titanium.In addition,the microstructures of the different investigated steels were observed using both optical microscope and scanning electron microscope.Furthermore,the hardness of the ferrite phase was also determined using micro-hardness technique.The results showed improvement of the mechanical properties of the investigated steels by both single V-and combined V + Ti-microadditions.Tensile,hardness and impact tests results indicated that good combinations of strength,ductility and impact properties can be achieved by V-microalloying addition.Steel with combination of V and Ti microaddition has much higher hardness,yield strength,ultimate tensile strength and impact energy at both room and zero temperatures compared with non-microalloyed and single Vmicroalloyed steels.Higher C,Si and Mn contents result in increasing the strength accompanied with decrea     

XRD Analysis of Local Strain Fields in Pearlitic Steels ‐ towards the Fast Examination of Microstructure after Hot Rolling

Analysis of the crystallographic anisotropy of the lattice strains, i.e. the analysis of the dependence of the lattice strain on the crystallographic direction, is discussed to be an efficient method for getting information about the mesoscopic local strains and microscopic local strain fields in dual‐phase materials. This technique is illustrated on the example of hot‐rolled pearlitic steels containing ferritic lamellae separated by cementite from each other. In these samples, the information about the local strain fields was further used to build a microstructure model that describes the interaction between crystallites of different phases on the microscopic scale. Such a microstructure model is quite appropriate for examination of the correlations between the structure and properties of the pearlitic steels, because it links the microstructure parameters obtained using X‐ray diffraction on the atomic level with the interaction between the crystallites or grains of different phases, which can more directly be related to the macroscopic mechanical properties of the materials. The second important result of this study was the detection and explanation of several correlations between individual microstructure parameters, which are obtained from X‐ray diffraction. This offers a possibility to use the X‐ray diffraction for a fast microstructure analysis of pearlitic steels, or generally for a fast microstructure analysis of dual‐phase steels, after or even during the rolling processes.     

Contrary Effects of Microstructure and Cleanliness on Tensile and Toughness Properties in Precipitation Hardening Stainless Steels

A detailed microstructural analysis on two precipitation hardening steels of similar chemical composition and of identical heat treatment but with different microstructures and mechanical properties was performed to reveal the constituents that are of major importance for strength and toughness. Light optical microscopy, magnetic saturation measurements, energy dispersive X‐ray analysis, scanning and transmission electron microscopy were used to identify and quantify the internal cleanliness, the secondary phases (δ‐ferrite, retained austenite), and the copper‐rich precipitates. The high purity and homogeneity of the steel's microstructure proved to be beneficial for upper shelf toughness and general isotropic behavior. In contrast, good elongation depended on local element segregation, allowing the stabilization of austenite to ambient temperature. The precipitation of copper‐enriched inter‐metallic phase was documented both in the martensitic matrix and in the δ‐ferrite; it ensured – for given heat treatment – sufficient yield strength independent of other microstructural constituents.     

The influence of pearlite on Barkhausen noise generation in plain carbon steels

Barkhausen noise refers to the abrupt, discontinuous changes in magnetization which result from domain walls overcoming barriers to their movement. Barkhausen noise is sensitive to a number of parameters, notably stress level (for which it is being examined as a basis for non-destructive evaluation) and microstructure. The present study investigates the influence of pearlite content on the pulse height distribution of the magnetic Barkhausen noise signal in plain carbon steels. The responses of purely pearlitic and purely ferritic microstructures are determined to be quite different. Samples containing no pearlite display a narrow, symmetrical distribution of relatively small pulse heights. The behaviour of the pearlitic steel is quite different, however, exhibiting an asymmetrical pulse height distribution with a significant tail extending to comparatively large pulse heights (approximately four times that of the maximum for the ferrite signal). Steels containing both ferrite and pearlite produce signal distributions between the two extremes, but contain no significant tail and also appear to tend toward larger pulse heights than what would be expected for a purely “composite” distribution. Results are explained in terms of differences in domain wall pinning behaviour between the various microstructures.     

Formability of High Strength Dual‐phase Steels

The application of ferritic‐martensitic dual‐phase (DP) steels has become an increasing trend in the automotive industry due to the possibility to achieve significant weight reduction and fuel efficiency with improved crash performance while keeping the manufacturing costs at affordable levels. In order to meet the different design requirements of individual auto‐body components, a wide variety of DP grades exhibiting different strength and ductility levels is currently industrially produced. Despite the numerous studies on the relationship between the mechanical properties and the microstructural characteristics of DP steels over the last decades, it is still a challenge to increase their formability at a constant strength level (or equivalently increasing the strength while maintaining a high ductility). One of the possibilities to increase strength is grain refinement. Ultrafine‐grained ferritic‐martensitic microstructures were produced by intercritical annealing of a cold‐rolled, pre‐processed dual‐phase steel. Ferrite mean grain sizes in the order of ~ 1.5 μm were obtained. The mechanical properties of these steels are studied, revealing the beneficial effect of grain refinement. Ultimate tensile strength above 800 MPa is achievable, while reaching remarkable high uniform and total elongations, which are only slightly affected by the martensite volume fraction. Moreover, the yield to tensile strength ratio can be adjusted between 0.4 and 0.5. Light and electron microscopy investigations, fracture profile and fracture surface analyses, hole expansion tests and additional ultramicrohardness measurements are used for the interpretation of the results and for the correlation of the mechanical properties and the formability characteristics with the microstructure of the steel.     

1 500 MPa级高伸长率双相钢的组织性能分析

为了进一步提高双相钢的性能,通过合理的化学成分设计,在实验室研发了 1 500 MPa级Nb-Ti微合金化的高伸长率冷轧双相钢,并且利用连退模拟试验机、扫描电镜等设备,系统研究了退火温度和过时效温度对双相钢组织性能的影响.结果表明,抗拉强度随着退火温度的升高而增大,在840℃时可达到1 650 MPa.当温度继续升高时...     

Effect of Finishing Rolling Temperature on Microstructure and Mechanical Properties of Microalloyed TRIP Steels

Different samples of TRIP (transformation induced plasticity) steel obtained by two different hot-rolling schedules are investigated by using a SEM (scanning electron microscope). The microstructure is characterized by using an OM (optical microscope) for phase distribution and by EBSD (electron backscatter diffraction) for texture and phase mapping. ODF (orientation distribution function) graphs are used to investigate the effect of recrystallization behavior of the hot-deformed austenite on phase transformation during the controlled cooling process. The mechanical behavior is interpreted in terms of the strength of both hard and soft phases, in combination with the quantity, location and transformation kinetics of the mechanically induced martensite (TRIP effect). The results show that more austenite grains exist in the steels obtained at finishing rolling temperature (FRT) of 750 °C, which inherited the deformation structure after the hot-rolling process. The instantaneous n value (n_i) of those steels is kept high during a large range of strain before failure, while the tensile strength and total elongation of the steels with respect to the different finishing rolling temperatures do not show any significant differences.