A general perspective of Fe–Mn–Al–C steels

During the last years, the scientific and industrial community has focused on the astonishing properties of Fe–Mn–Al–C steels. These high advanced steels allow high-density reductions about ~?18% lighter than conventional steels, high corrosion resistance, high strength (ultimate tensile strength ~?1 Gpa), and at the same time ductility above 60%. The increase in the tensile or yield strength and the ductility at the same time is almost a special feature of this kind of new steels, which makes them so interesting for many applications such as in the automotive, armor, and mining industry. The control of these properties depends on a complex relationship between the chemical composition of the steel, the test temperature, the external loads, and the processing parameters of the steel. This review has been conceived to elucidate these complex relations and gather the most important aspects of Fe–Mn–Al–C steels developed so far.     

High strength low carbon hot rolled δ-ferritic steel: microstructure and mechanical properties

Abstract

The present study concerns the development of high strength low carbon hot rolled bainitic and martensitic δ transformation induced plasticity steels. Equilibrium and para-equilibrium phase evolution have been examined by carrying out thermodynamic calculation using MT-DATA software. Microanalysis demonstrates that both manganese and aluminium partition between liquid and solid phases. Isothermal treatment and tempering at 350°C for bainitic and martensitic microstructures respectively have yielded the best combination of strength and ductility. All the steels have exhibited the continuous yielding behaviour and favourable ratio of yield and tensile strength, which are desirable for formability. The annealed steel has yielded a high level of tensile strength with the static toughness value in between the conventional transformation induced plasticity assisted and dual phase steels.     

新型纳米强化超高强度钢的研究与进展

随着资源、能源和环境压力日益加大,超高强度钢的开发越来越受到世界各国的极大重视。传统的超高强度钢大都是依赖提高碳含量或合金元素含量而获得较高强度的马氏体或贝氏体钢,此种钢存在着焊接性能差、塑韧性低、钢材尺寸受限制和成本昂贵等问题,严重制约了经济的快速发展和现代国防的建设,因此,开发综合性能良好、成本低廉的新型超高强度钢刻不容缓。结合当前纳米科技的发展,介绍了新型纳米强化超高强度钢的设计理念,阐述了以纳米相析出强化为主、多种强化方式结合的强韧化理论,并总结了纳米析出强化超高强度钢在合金设计和工艺优化等方面的初步研究进展,最后探讨了新型纳米强化超高强度钢亟待解决的问题。     

Acoustic emissions during fracture toughness tests of steels exhibiting varying ductility

This investigation is primarily aimed at examining steels with varying ductility using characteristics of acoustic emission (AE). Four steels (AISI 1060, AISI 1080, SA333 grade 6 and AISI 304LN) were selected and their structure property relations were characterized using standard metallographic examinations, hardness and tensile properties. Fracture toughness of these steels was determined as per the guidelines of ASTM standard E1820 with simultaneous recording of AE signals. The results of these investigations have been used to demonstrate that: (a) nature of the variation of AE cumulative counts with time is different for linear and non-linear load–displacement plots, (b) synergistic analysis of the rate of change of cumulative energy, cumulative counts and intensity of AE signals provide the point of crack initiation in a material, and (c) fracture toughness of a material estimated using AE parameters is lower compared to that obtained by ASTM standard procedure.     

Influence of cold work on mechanical properties of Fe–Ti–N alloys

Abstract

High-strength steels offer fuel savings in automotive components through weight reduction and when high strength is achieved in low-alloy steels, an economical material is produced, suitable for a wide range of applications. ‘Formable and strengthenable’ (FS) steels have been developed as an alternative to present high-strength low-alloy (HSLA) steels, and are produced in a manner different from the conventional alloys. The FS steel is formed in the soft, fully annealed condition, and the final strength is developed by a nitriding heat treatment. The present paper describes the microstructure and properties of an FS steel containing titanium as the nitride-forming element. Annealed and nitrided Fe–Ti alloys are extremely hard and brittle but cold working before nitriding greatly improves ductility. The replacement of boundaries on cold working by a fine subcell structure eliminates embrittlement, and added strength is obtained by the stabilization of the cell structure by Ti–N clusters or TiN particles. Cold worked and nitrided alloys have a microstructure which varies with depth from the nitrided surface into the component, producing a complex structure which prevents detailed analysis of the strengthening process. However, the present work demonstrates the value of FS steels as a useful addition to the range of HSLA steels.

MST/379     

Microstructure and dynamic strain aging phenomena in two structural steels

Abstract

Two steels with bainitic and martensitic microstructures have been investigated with respect to the changes occurring in their uniaxial tensile properties in the temperature regime where they display dynamic strain aging (DSA) behaviour. The variations in yield strength, ultimate tensile strength, ductility and strain rate sensitivity in the case of martensitic steel have been found to vary in an embrittling manner as compared with those observed in the case of the bainitic steel within the DSA temperature regime. An attempt has been made to provide an insight into the difference in manifestations of DSA on the uniaxial behaviour of the two steels.     

Mn-Si-Cr系无碳化物贝氏体/马氏体复相高强钢的研究进展

无碳化物贝氏体/马氏体复相高强钢具有比同等强度马氏体钢更优异的韧性和塑性,被广泛应用到轨道交通、机械、建筑等领域。文章概述了低成本Mn-Si-Cr系无碳化物贝氏体/马氏体复相钢近年来在合金化设计、工艺设计、微观组织、强韧化机理、强塑化机理、延迟断裂及疲劳性能等方面取得的研究成果。特别介绍了近年来笔者在BQP工艺处理CFB/M复相钢方面的工作进展,经过BQP处理之后,CFB/M复相钢显示了更优异的强度、塑性、韧性和疲劳性能的匹配。最后简单介绍了Mn-Si-Cr系无碳化物贝氏体/马氏体复相钢在不同领域的应用情况,特别是其在重载高速铁路领域的应用现状和前景。     

Critical Assessment 15: Science of deformation and failure mechanisms in twinning induced plasticity steels

Manganese rich austenitic twinning induced plasticity steels with high strength and high ductility have been developed in 1990s as promising candidates for automotive applications. Tremendous efforts have therefore been made to explore the unusual deformation and failure mechanisms of these alloys. We provide here a critical assessment of the recent progress in understanding their deformation and failure mechanisms and discuss some scientific challenges that remain unresolved, for example, a physically based twinning kinetics model.     

The multiaxial creep ductility of austenitic stainless steels

Calculations of creep damage under conditions of strain control are often carried out using either a time fraction approach or a ductility exhaustion approach. In practice, calculations of creep damage are further complicated by the presence of multiaxial states of stress. In the case of the time fraction approach, there are a number of models that can be used to predict the effect of state of stress on creep rupture strength. In particular, Huddleston developed a model from data on stainless steels. The R5 procedure uses a ductility exhaustion approach to calculate creep damage and includes a model for use under triaxial states of stress. The aim of this paper is to describe the development of this model, which is based on considerations of cavity nucleation and growth and was developed from multiaxial creep data on Type 304 and 316 steels.     

ANN Optimization Method for High Strength High Fracture Toughness Steels

The two-target optimization problem for high strength high fracture toughness steels has been investigated. An effective method for two-target optimization of multi-variable non-linear complicated system is developed by combining simulated annealing algorithm with artificial neural network     

Current state of Fe-Mn-Al-C low density steels

Fe-Mn-Al-C steels, previously developed in the 1950s for replacing Fe-Cr-Ni steels, are currently generating a lot of interest with potential applications for structural parts in the automotive industry because they are lighter. This paper provides a review on the physical metallurgy, processing strategies, strengthening mechanisms and mechanical properties of Fe-Mn-Al-C steels from the published literature over a period of many years, and suggests avenues for future applications of these alloys in the automotive sector.The addition of Al to Fe-C steels leads to a reduction in both density and Young’s modulus. A 1.3% reduction in density and a 2% reduction in Young’s modulus are obtained per 1 wt% addition of Al. Due to the addition of the high amounts of Al, together with Mn and C, the physical metallurgy, general processing, microstructural evolutions and deformation mechanisms of these steels are largely different from those of the conventional steels.The addition of Al to high-Mn austenitic steels brings two other important effects: increasing the stacking fault energy (SFE) and producing short-range ordering (SRO) and/or κ′-carbide precipitation. Plastic deformation of low-density Fe-Mn-Al-C steels with a high SFE, which involves SRO, is dominated by planar glide. New deformation mechanisms such as the microband induced plasticity (MBIP), the dynamic slip band refinement (DSBR) and the shear band induced plasticity (SIP) are introduced to describe plastic deformation of Fe-Mn-Al-C austenitic steels in addition to the transformation-induced plasticity (TRIP) and the twinning-induced plasticity (TWIP), which are often observed in Mn TWIP steels. These new deformation mechanisms are related to the formation and uniform arrangement of the SRO or nano-sized κ′-carbides which are coherent with the austenitic matrix. The κ′-carbide precipitation is a unique strengthening mechanism in the austenitic Fe-Mn-Al-C steels bearing high amounts of Al and C.The lightweight Fe-Mn–Al-C alloys can produce a variety of microstructures and achieve a wide range of properties. These alloys can be classified into four categories: ferritic steels, ferrite based duplex steels, austenite based duplex steels and austenitic steels. The austenitic steels are the most promising in terms of properties and processing. The tensile properties of the austenitic lightweight steels are similar to those of high Mn TWIP steels. The impact toughness of these steels in the solution treated condition is slightly lower than that of Cr-Ni stainless steels but is higher than that of the conventional high strength steels. The energy absorption at high strain rate is similar to that of high Mn TWIP steels and higher than that of conventional deep drawing steels. The ferrite based duplex low-density steels is another promising alternative. A bimodal microstructure can be obtained here through process control for steels with lower alloying contents, in which the plastic deformation of the ferrite and the TRIP and/or TWIP effects from the retained austenite can be profitably used. This type of Fe-Mn-Al-C steels exhibits an improved combination of strength and ductility compared with the first generation advanced high strength steels. The ferritic Fe-Al steels have tensile properties comparable with HSLA steels of 400–500 MPa strength level. The corrosion behaviour of Fe-Mn-Al-C steels is not improved in comparison with the conventional high strength steels. The application properties such as the fatigue behaviour and formability of Fe-Mn-Al-C steels cannot be properly understood at this stage, because of the limited experimental results so far. Some other application aspects such as weldability, coatability are not well documented.The applications of the Fe-Mn-Al-C steels in the automobiles is still not prevalent due to the lack of knowledge related to application properties so far. Above all, the reduced Young’s modulus of these steels and the processing problems as a result of the high Al and high Mn contents are the main issues. The future developments will therefore have to concentrate on the alloying and processing strategies and also on the methods to increase the Young's modulus. An improved processing strategy and a high value for the Young’s modulus will go a long way towards upscaling these steels to real automotive applications.     

Critical assessment 29: TRIP-aided bainitic ferrite steels

Transformation-induced plasticity (TRIP)-aided bainitic ferrite steels developed for automotive applications have attractive mechanical properties such as ductility, formability, toughness, fatigue strength and delayed fracture strength. These mechanical properties are principally associated with a ductile lath-structure matrix and the strain-induced transformation of the metastable-retained austenite films of 3–20 vol.%. In this paper, data on the microstructural and mechanical properties of the low-carbon TRIP-aided bainitic ferrite steels are critically assessed, as well as their deformation mechanism.     

贝/马复相钢超高周疲劳行为及非夹杂起裂

贝/马复相钢具有较低的夹杂物敏感性,组织因素对其超高周疲劳性能具有显著影响。组织因素引起的"非夹杂起裂"成为贝/马复相钢重要的裂纹萌生方式,贝/马复相组织的类型、形态、均匀性、细化程度等都对钢的超高周疲劳性能具有显著影响。讨论了组织纯净化、组织细化和残余奥氏体对贝/马复相钢超高周疲劳性能及其裂纹萌生机制的影响,在合理控制夹杂物水平的基础上,调控复相组织,可以在1 600MPa级别的贝/马复相钢中,获得超高周(循环周次大于108)疲劳强度达到900MPa的优异性能。同时对非夹杂起裂机理进行了初步探讨。     

Controlled Phase Transformation Simulations to Design Microstructure for Tailored Mechanical Properties in Steel

Advanced materials with light weight but having high strength and ductility are required to decrease the weight of automobiles. Hereby, advanced high-strength steels like dual-phase steels and multiphase steels are vital as they possess good strength in conjunction with good formability. Several methods have been used for the processing of multiphase steels but very limited research has been reported on their processing through controlled cooling and by using a lean composition. The present research reports on methods of production of dual-phase/ multiphase microstructures in a steel of lean chemical composition (0.11C, 1.8Mn, and 0.325Si), making the steel multifunctional. Annealing parameters were determined using Thermo-Calc and JMat-Pro software. Software predictions were validated through experiments in a muffle furnace, followed by actual annealing experiments in an annealing simulator. Multiphase microstructures were obtained from the initial ferrite-pearlite structure by inter-critical annealing involving controlled cooling in the annealing simulator. Dual-phase structures were produced by inter-critical annealing, followed by rapid cooling to room temperature, whereas multiphase microstructures were produced by holding in the bainitic and martensitic ranges, respectively. The steel exhibited good combinations of strength–ductility, with tensile strength and ductility in the range of 550–705 MPa and 11–33%, respectively.     

ODS ferritic steel engineered with bimodal grain size for high strength and ductility

An attractive way to enhance the ductility of ODS ferritic steels is to develop an alloy with a bimodal grain size distribution, in which the micron-sized coarse grains provide high ductility. The nanometer-sized fine grains enhance the tensile strength. The microstructures were obtained by blending the gas-atomized powders and mechanical alloyed powders, followed by hot forging and annealing. The homogeneously distributed nanometer-sized oxide nanoparticles can also be detected. Mechanical properties tests revealed a great improvement in ductility in comparison with other ODS ferritic steels, and high strength over the whole range of test temperatures, owing to the fine grains and oxide nanoparticles. The combination of high ductility and high strength makes this ODS ferritic steel much promising in high-temperature application.     

Prevention of the overestimation of long-term creep rupture life by multiregion analysis in strength enhanced high Cr ferritic steels

Long-term creep rupture life is often evaluated from short-term data by a time–temperature parameter (TTP) method. However the conventional TTP methods sometimes fail in understanding creep rupture behavior of strength enhanced high Cr ferritic steels and overestimate creep rupture life in long-term creep. In the present paper, creep rupture data of seven kinds of heat resistant steels with different W and Cr concentrations have been analyzed. The conventional TTP method like Orr–Sherby–Dorn analysis evaluates long-term creep rupture life assuming a unique value of activation energy for all the creep rupture data. This analysis is called single region analysis in this paper. The single region analysis can represent well the creep rupture data of steels containing less than 8% Cr. The creep rupture analyses of steels containing more than 8% Cr exhibit that apparent activation energy changes from a high value in short-term creep region to a low value in long-term creep region. In each case a creep data was divided into several data sets, and then the conventional single region analysis was applied to each divided data set. This analysis is referred to as multiregion analysis. The multiregion analysis describes very well all the data points, whereas regression curves of the single region analysis deviate from the data points, resulting in overestimation of long-term rupture life. The difference between the two activation energies of short-term and long-term creep increases with increasing Cr concentration. Therefore, the overestimation due to singles region analysis is expected to be more serious at higher Cr concentration.     

Application of a Solidification Mathematical Model and a Genetic Algorithm in the Optimization of Strand Thermal Profile Along the Continuous Casting of Steel

This work presents an optimization method based on a genetic algorithm applied to continuous casting process. A simple genetic algorithm was developed, which works linked to a mathematical model permitting the determination of optimum values for the water flow rates in the secondary cooling zones. First, experimental data (industrial) were compared with simulated results obtained by the solidification mathematical model, to determine the metal/cooling heat transfer coefficients along the machine by the inverse heat conduction problem method. The industrial data concerning surface strand temperature were obtained by using infrared pyrometers along a continuous caster machine during casting of both SAE 1007 and 1025 steels. In a second step, these results were used by a numerical code based on a genetic algorithm for determining optimum settings of water flow rates in the different sprays zones, which are conducive to the best quality of the solidified strand. The simulations were carried out by analyzing the solidification process during continuous casting to attain metallurgical restrictions concerning the reheating of strand surface temperature and metallurgical length.     

Nickel-free austenitic stainless steels for medical applications

Abstract

The adverse effects of nickel ions being released into the human body have prompted the development of high-nitrogen nickel-free austenitic stainless steels for medical applications. Nitrogen not only replaces nickel for austenitic structure stability but also much improves steel properties. Here we review the harmful effects associated with nickel in medical stainless steels, the advantages of nitrogen in stainless steels, and emphatically, the development of high-nitrogen nickel-free stainless steels for medical applications. By combining the benefits of stable austenitic structure, high strength and good plasticity, better corrosion and wear resistances, and superior biocompatibility compared to the currently used 316L stainless steel, the newly developed high-nitrogen nickel-free stainless steel is a reliable substitute for the conventional medical stainless steels.     

Current opinion in medium manganese steel

Abstract

Medium Mn steels have been actively investigated due to their excellent balance between material cost and mechanical properties. The steels possess a single α′ martensite phase in hot and cold rolled states and multiphases after intercritical annealing. Many studies have focused on investigating the influences of chemical composition and annealing conditions on the microstructure, particularly the grain size and retained γ (γ_R), and on the tensile properties. The steels exhibit high strength and good ductility due to transformation induced plasticity occurring in γ_R, whose volume fraction is approximately 0·2–0·4. The present review summarises the important results of previous studies about the effects of both intercritical annealing conditions and alloying elements on the microstructure and tensile properties of medium Mn steels.