Electropulsed steels

Abstract

With appropriate electropulsing parameters, the microstructure of steels can be manipulated towards a state that possesses favourable physical, mechanical and chemical properties. This work demonstrates the application of designed electropulsing to pearlitic steel, transformation induced plasticity steel, austenite stainless steel and clean steel in order to generate novel microstructures that are difficult to achieve by other conventional thermomechanical processes. The principles for the design of electropulsing processing have been discussed. The method is particularly suitable for fabrication of the advanced multiphase and multicomponent alloys.     

Comparative study of precipitation effects during aging in superaustenitic and superferritic stainless steels

Abstract

The concept of designing a steel that would match high corrosion resistance coupled with improved mechanical properties when exposed at elevated temperatures has always been a challenge for a metallurgist. The present paper relates the results of microstructural analysis of two commercial superaustenitic stainless steel grades and a novel experimental grade of superferritic stainless steel. A study of the microstructural stability and attendant mechanical property evolution has been carried out, employing a variety of aging experiments. Following prolonged aging up to 1000 h in the temperature range between 650 and 950°C, microstructure evolution was determined, while the mechanical properties were preliminary assessed via hardness testing. The present study helped clarify the effect of high temperature exposure on the precipitation sequences of both superaustenitic and novel superferritic stainless steels. The heat treatments performed indicate a number of precipitation sequences of intermetallic phases taking place, such as σ phase formation, clearly specifying a time–temperature regime where care must be taken during the fabrication and use of these materials.     

Comparative study on microstructures and properties of Mn-containing and Mn-free bainitic steels

ABSTRACT

The effects of Mn on the microstructure and impact-abrasion wear resistance of bainitic steel were studied. Results showed that the Mn-containing steel possessed finer microstructure and higher volume fraction of retained austenite, in comparison with the Mn-free steel. This was caused by lower transformation temperature and higher strength of undercooled austenite. The weight loss of Mn-free steel varying with the impact load was larger than that of Mn-containing steel. High strength, hardness and toughness of Mn-containing steel were conducive to improving wear resistance. More retained austenite in Mn-containing steel played an active role in work hardening and hindering crack propagation. However, the portion of retained austenite that induced martensitic transformation was the same with increasing impact-wear load.     

Development of rapid heating and cooling (flash processing) process to produce advanced high strength steel microstructures

Abstract

Flash processing of an AISI8620 steel sheet, which involves rapid heating and cooling with an overall process duration of <10 s, produced a steel microstructure with a high tensile strength and good ductility similar to that of advanced high strength steels. Flash processed steel [ultimate tensile strength (UTS): 1694 MPa, elongation: 7·1%], showed at least 7% higher UTS and 30% greater elongation than published results on martensitic advanced high strength steel (UTS: 1585 MPa, elongation: 5·1%). The underlying microstructure was characterised with optical, scanning electron, transmission electron microscopy as well as hardness mapping. A complex distribution of bainitic and martensite microstructures with carbides was observed. A mechanism for the above microstructure evolution is proposed.     

High-strength medium Mn quenching and partitioning steel with low yield ratio

ABSTRACT

A new microstructural design is proposed to develop a strong and ductile quenching and partitioning (Q&P) steel with low yield ratio. This steel has a heterogeneous dual phase microstructure which is developed by varying austenite thermal stability through Mn segregation. The heterogeneous microstructure contains large austenite grains which contribute to the low yield strength. The ultra-high tensile strength and good ductility are ascribed to the enhanced strain hardening behaviour resulted from the continuous transformation-induced plasticity (TRIP) effect. The present microstructural design enables a conventional medium Mn steel with high tensile strength, good ductility and low yield ratio, which promises easy forming and potential applications in automotive industries.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Finite element analysis and experimental study of microstructure evolution of nitrogen alloyed ultralow carbon stainless steel during hot deformation

Abstract

Hot compression experiments of a nitrogen alloyed ultralow carbon stainless steel were performed in the temperature range of 1223–1423 K, at strain rates of 0.001–1 s^?1, and with deformation amounts of 30–70% on a Gleeble-3500 thermal-simulator. Based on the results from thermo-physical simulation experiments and metallographic analyses, a physically-based constitutive model and a dynamic recrystallisation (DRX) model of the studied steel were derived, and the developed models were further embedded into a finite element method (FEM) software. The microstructure evolution of the studied steel under various hot deformation conditions was simulated by FEM, and the effects of deformation amount, strain rate and temperature on the microstructure evolution were clarified. The results obtained from the finite element analysis were verified by the experiments. The finding confirms that the thermal-mechanical FEM coupled with the developed constitutive model and DRX model can be used to accurately predict the microstructure evolution of the studied steel during hot deformation.     

Precipitation,microstructure and mechanical properties of low nickel maraging steel

Abstract

A maraging steel with a composition of Fe–12·94Ni–1·61Al–1·01Mo–0·23Nb (wt-%) was investigated. Optical, scanning electron and transmission electron microscopy and X-ray diffraction analysis were employed to study the microstructure of the steel after different aging periods at temperatures of 450–600°C. Hardness and Charpy impact toughness of the steel were measured. The study of microstructure and mechanical properties showed that nanosized precipitates were formed homogeneously during the aging process, which resulted in high hardness. As the aging time is prolonged, precipitates grow and hardness increases. Fractography of the as forged steel has shown mixed ductile and brittle fracture and has indicated that the steel has good toughness. Relationships among heat treatment, microstructure and mechanical properties are discussed. Further experiments using tensile testing and impact testing for aged steel were carried out.     

Fine structure in the inter-critical heat-affected zone of HQ130 super-high strength steel

The microstructure in the intercritical heat-affected zone (ICHAZ) of HQ130 steel, has been investigated by thermo-simulation test, SEM and TEM. The problem of toughness decrease in the ICHAZ (T _p = 800°C) as well as the effect of M-A constituent and carbide precipitation on brittleness was analysed. The test results indicated that the microstructure in the ICHAZ of HQ130 steel was mostly a mixture of lath martensite (ML) and granular bainite (Bg) with a fine but nonuniform grain structure. The cause of brittleness in the ICHAZ was related to production of the M-A constituent in the local region and carbide precipitation. By controlling the welding heat input carbide precipitation and the formation of the M-A constituent can be avoided or decreased.     

Hydrogen-induced change in microstructure and properties of steels: 18Cr10Mn–0.4N vis-à-vis 18Cr10Ni

ABSTRACT

Cr–Mn–N stainless steels have a cost and strength advantage over conventional Cr–Ni stainless steels. In this study microstructure and mechanical property of hydrogen-charged 18Cr10Mn-0.4N was compared with 18Cr10Ni austenitic stainless steel. This is the first such study for 18Cr10Mn–0.4N austenitic stainless steel. Electron microscopy was used to compare the deformed microstructure of the uncharged and hydrogen-charged specimens. The results are discussed in view of the current knowledge on hydrogen embrittlement. The 18Cr10Mn–0.4N steel suffered higher embrittlement mainly because it absorbed moref hydrogen.     

Microstructure and tensile behaviour of 15–24 wt-%Mn TWIP steels

Abstract

The present study examines the microstructure and tensile behaviour of twinning induced plasticity steel with different Mn contents. The results show that reducing the Mn content from 24 to 15 wt-% is beneficial for the improvement of the tensile strength and fracture strain of the steels. The effect of Mn content on the microstructure and workhardening behaviour as well as the effect of strain rate on the microstructure and tensile behaviour is discussed.     

Ultrahigh strength steel wires processed by severe plastic deformation for ultrafine grained microstructure

Abstract

A comprehensive review of recent literature on high strength, fine grained steels has been conducted. While relevant technologies in alloy design, processing and heat treating are included in the present review, the emphasis has been on high carbon steel wire processing technology that can be achieved with ‘conventional’ wire rolling and drawing processes. The thermomechanical processing of a pearlitic microstructure, followed by cold drawing, is recommended as the process of choice to efficiently produce an ultrafine grained ferrite–cementite microstructure for ultrahigh strength, ultrahigh carbon steel wires.     

Development of an ultrahigh strength low alloy steel for armour applications

Abstract

The present paper describes a steel with yield strength exceeding 1900 MPa and fracture toughness in the range of 40–50 MPa?m^1/2, in its optimum heat treated condition. Its strength is similar to that of 18 Ni (300) grade of maraging steel with good fracture toughness. When tempered at 300°C, it shows tempered martensite along with a small amount of retained austenite phase. The steel shows nearly 25% reduction in weight over typical rolled homogeneous armour (RHA) steel against high velocity hard steel core projectiles. The processing, microstructure, mechanical and ballistic properties of the steel are demonstrated.     

Continuous cooling transformation behaviour of Si–Mn and Al–Mn transformation induced plasticity steels

Abstract

The continuous cooling transformation (CCT) behaviour of two transformation induced plasticity (TRIP) steels was investigated using quench dilatometry. One was an established steel grade with a composition (wt-%) of Fe–0·2C–2Si–1·5Mn while the other steel was a novel composition where 2 wt-% Al replaced the silicon in the former grade. Characteristics of the α→γ transformation during reheating and the subsequent decomposition of austenite during continuous cooling were studied by dilatometry, and CCT diagrams were constructed for both steels. The effects of accelerated cooling and steel composition on γ transformation start temperature Ar ₃, phase transformation kinetics, and microhardness were investigated. The results showed that the Al–Mn steel had a much wider α→γ transformation range during reheating, compared with the Si–Mn steel. Furthermore, the Al–Mn steel exhibited no significant change in the rate of expansion during α→γ transformation. On the other hand, during continuous cooling, the Al–Mn steel exhibited higher Ar ₃, faster transformation kinetics, a higher volume fraction of polygonal ferrite in the microstructure, and lower hardness, compared with the Si–Mn steel. The addition of aluminium was found to have a significant effect on the products of phase transformation, kinetics, and form of the CCT diagram. For both steels, an increase in cooling rate lowered the Ar ₃ temperature, decreased the time of transformation, and increased the hardness.     

Effect of ultrafine grain structure on strain hardening of C–Mn steel warm rolled and subjected to intercritical annealing

Abstract

The behaviour during the work hardening of low carbon–manganese (0·15%C–1·39%Mn) steel with an ultrafine ferritic grain structure was investigated using Jaoul–Crussard analysis. This microstructure was produced through out quenching, warm rolling and intercritical annealing at 800°C. The steel exhibited a high strain hardening exponent and tensile strength.     

Surface carburised AISI 8620 steel with dual phase core microstructure

Abstract

In this study, the production of dual phase steel structure in the core of surface carburised AISI 8620 cementation steel and the effect of martensite volume fraction on tensile properties have been investigated. For these purposes, surface carburised (~0·8 wt-%C) specimens were oil quenched from 900°C to obtain a fully martensitic starting microstructure. Then specimens were oil quenched from intercritical annealing temperatures of 731 or 746°C to produce dual phase steel structure in the core of specimens with martensite fractions of ~25 or ~50 vol.-% and nearly wholly martensitic microstructure at the surface. Generally, specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strengths but superior ductility without sacrificing surface hardness than those specimens with fully martensitic microstructure in the core produced by using conventional heat treatment involving quenching from 850 to 950°C. Also tensile strength increased and ductility decreased with increasing martensite volume fraction.     

Progress in thermomechanical control of steel plates and their commercialization

Abstract

The water-cooled thermomechanical control process (TMCP) is a technology for improving the strength and toughness of water-cooled steel plates, while allowing control of the microstructure, phase transformation and rolling. This review describes metallurgical aspects of the microalloying of steel, such as niobium addition, and discusses advantages of TMCP, for example, in terms of weldability, which is reduced upon alloying. Other covered topics include the development of equipment, distortions in steel plates, peripheral technologies such as steel making and casting, and theoretical modeling, as well as the history of property control in steel plate production and some early TMCP technologies. We provide some of the latest examples of applications of TMCP steel in various industries such as shipbuilding, offshore structures, building construction, bridges, pipelines, penstocks and cryogenic tanks. This review also introduces high heat-affected-zone toughness technologies, wherein the microstructure of steel is improved by the addition of fine particles of magnesium-containing sulfides and magnesium- or calcium-containing oxides. We demonstrate that thanks to ongoing developments TMCP has the potential to meet the ever-increasing demands of steel plates.     

New generation ultrahigh strength boron steel for automotive hot stamping technologies

Abstract

The automotive industry is under increasing pressure to: (1) reduce the weight of vehicles and (2) improve crash performance. ‘Hot stamped’ mild carbon–manganese–boron steel 22MnB5 has become common place in the body structure of the European vehicle over the past decade. The ultrahigh strength martensitic microstructure resulting from hot stamping (proof strength ～1200 MPa and ultimate tensile strength ～1500 MPa) enables down gauging while not compromising crash performance. However, with demands for yet higher strength in the final component so to enable further down gauging, novel grades must be developed. In this paper, development of the novel grade 38MnB5 was reported. Following hot stamping, 38MnB5 demonstrated proof strength in excess of 1400 MPa and ultimate tensile strength in excess of 2000 MPa. Owing to the immense strength, the novel grade 38MnB5 was considered to offer significant down gauging and weight reduction opportunities to the automotive industry.     

Low density steel 1·2C–1·5Cr–5Al designed for bearings

Abstract

A novel alloy design, designated as 1·2C–1·5Cr–5Al, has been proposed with high aluminium(～5 wt-%) and more carbon(～1·2 wt-%) addition into the classical 1C–1·5Cr bearing steel for lowering density and improving performance simultaneously, which is approximate 8 wt-% lighter than convention. In order to understand preliminarily the suitability of the novel alloy for bearing application, the martensite starting temperature and hardness, related to microstructure evolution and mechanical properties, respectively, after partial austenitisation treatment with undissolved carbides have been investigated carefully. The martensite starting temperature is comparable with conventional 1C–1·5Cr alloy. The hardness of 860±3 HV20 achieved is much higher than convention.     

Acicular ferrite formation during hot plate rolling for pipeline steels

Abstract

The transformation of supercooled austenite in a commercial pipeline steel was investigated by means of continuous cooling transformation (CCT) and hot simulation experiments. Based on the obtained results, an improved thermomechanical control process (TMCP) was proposed, which could produce a mixed microstructure dominated by acicular ferrite. Results indicated that an increase in the cooling rate could improve the percentage of acicular ferrite in the final microstructure under the present experimental conditions. Furthermore, the acicular ferrite dominated microstructure could be obtained by a two stage controlled rolling in the austenite recrystallisation region plus the non-recrystallisation region and controlled cooling at a cooling rate of 30 K s^-1.     

Application of advanced analytical techniques to study structure–property relationship of hot rolled high strength low alloy steel

Abstract

The microstructure–property relationship in conventional high strength low alloy (HSLA) steel was evaluated using data obtained from transmission electron microscopy (TEM) and atom probe tomography (APT). Atom probe tomography allowed the characterisation of fine TiC particles with average radius of 3±1·2 nm that were not observed by TEM. The increase in the yield strength of steel due to the presence of fine precipitates was calculated to be 128 MPa.