Effects of morphology and strength of martensite on cyclic deformation behaviour in dual-phase steels

Abstract

Tension–compression cyclic deformation behaviour in dual-phase steels has been studied. Three different ferrite (α)–martensite (α′) microstructures, i.e. isolated α′-colonies dispersed in α-matrix (I), continuous α′ (C), and laminated α–α′ (L), were prepared by appropriate heat treatments, keeping the α′ volume fraction at ~0·3. The work hardening and the Bauschinger effect are found to be greater in microstructure C or L than in I when they are compared at an arbitrary forward (tension) prestrain level. An increase in the hardness of α′ enhances the Bauschinger effect and then narrows the stress–strain hysteresis loop. The stress evolved as a result of the Bauschinger stress (defined as the difference between forward prestress and backward (compression) 0·1% proof stress) is found to be almost independent of microstructure and hardness when it is compared at an arbitrarily fixed prestress level. The slip lines are very fine and relatively straight in microstructure C, but wavy in microstructure I. These findings are discussed from the standpoints of the accumulation of the average internal stress resulting from inhomogeneous plastic flow between two constituent phases and the plastic relaxation.

MST/382     

Development of heavy steel plate for Mayflower Resolution,special purpose vessel for erection of offshore wind towers

Abstract

Special problems necessitate special solutions! Installation vessels for the erection of offshore wind towers are subject to extremely demanding design and structural specifications. Such projects are made possible only by the use of high strength, fine grained structural steels possessing good toughness properties even at extremely low temperatures; in addition, such steels must also offer good workability. Such steel plate material exhibits mechanical properties greatly superior to those possessed by conventional shipbuilding plate. This article focuses on the material for such an installation vessel and the underlying steel development work performed at AG der Dillinger Hüttenwerke.     

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     

Development of structural steels with re resistant microstructures

Abstract

This paper is concerned with the design and characterisation of fire resistant steels for building construction. Steel design considerations are discussed. Issues raised include controlling the grain size, properties of substitutional elements, and processing. New experimental fire resistant steels microalloyed with molybdenum and niobium, or tungsten, titanium, and boron have been made and their microstructures and tensile properties characterised. The steels possess satisfactory high temperature strength, owing partly to their relatively large grain sizes compared with conventional steels. The nature of equilibrium precipitation has been calculated using Thermo Calc. Optical microscopy, SEM, TEM, and differential scanning calorimetry have been used to determine the physical characteristics. The strengthening mechanisms observed on the experimental steels of this study could be attributed to secondary formation of fine precipitates, in line with previous observations.     

Development of de-alloyed zones during oxidation: effects on microstructure and spallation behaviour

Abstract

The development of de-alloyed zones during oxidation of martensitic and austenitic steels and Ni based superalloys has been reviewed and the influence of de-alloying on local creep strength has been assessed. The de-alloyed zones in martensitic steels have similar, possibly higher, strength than the bulk material, whereas in Ni based superalloys the de-alloyed zone is significantly weaker than the bulk alloy. The effect in austenitic steels varies according to the strengthening phases present in the alloys: the de-alloyed zone is weaker in alloys strengthened by chromium carbides and/or γ′ but has similar strength in alloys strengthened by niobium carbide.     

Acicular ferrite microstructure in titanium bearing low carbon steels

Abstract

Different microstructures having acicular ferrite as the major phase but with various types and amounts of microphases were obtained by applying different cooling processes to C–Mn steels containing fine non-metallic inclusions. Optical and electron microscopy were carried out to identify the various microphases in the acicular ferrite microstructure, and their mechanical properties were measured and compared to study the effect of the microphases on the microstructure–properties relationship in C–Mn wrought steels. The existence and increase of the fraction of small isolated martensite between the acicular ferrite laths were found to play an important role in determining the tensile strength and low temperature impact toughness of the steels. However, the elongation and room temperature impact toughness were rather insensitive to the microphases. This may be attributed to the uniform distribution and isolation of relatively small martensite due to the fine interlocking character of the acicular ferrite microstructure.     

Interface migration during partitioning of Q&P steel

Abstract

The quenching and partitioning (Q&P) process for heat treatment of steel has previously been shown yield to good combinations of strength and ductility owing to the presence of martensite and austenite. Interface mobility has been discussed in previous literature, mostly related to local driving forces. The present work considers the migration of the martensite/austenite interface in two steels (containing CMnSiMo or CNiSiMo). Experimental data show clear evidence of interface migration in the CMnSiMo steel during partitioning treatments at temperatures between 200 and 400°C for times ranging from 30 to 1000 s; conversely, the interface in the CNiSiMo steel was stationary during the same partitioning treatments. The different behaviours observed are considered in the context of differences in interface mobility and driving forces, and it appears that interface crystallography differences could influence the partitioning behaviour of Q&P steels.     

The influence of dynamic strain aging on resistance to strain reversal as assessed through the Bauschinger effect

The Bauschinger effect of three commercially produced medium carbon bar steels representing different microstructural classes with similar tensile strengths and substantially different yielding and work-hardening behaviors at low-strain was evaluated at room temperature and in situ at temperatures up to 361 °C. The influence of deformation at dynamic strain aging temperatures as a means to produce a more stable dislocation structure was evaluated by measuring the resistance to strain reversal during in situ Bauschinger effect tests. It was shown that the three medium carbon steels exhibited substantial increases in strength at dynamic strain aging temperatures with the peak in flow stress occurring at a test temperature of 260 °C for an engineering strain rate of 10⁻⁴ s⁻¹. Compressive flow stress data following tensile plastic prestrain levels of 0.01, 0.02 and 0.03 increased with an increase in temperature to a range between 260 °C and 309 °C, the temperature range where dynamic strain aging was shown to be most effective. The increased resistance to flow on strain reversal at elevated temperature was attributed to the generation of more stable dislocation structures during prestrain. It is suggested that Bauschinger effect measurements can be used to assess the potential performance of materials in fatigue loading conditions and to identify temperature ranges for processing in applications that utilize non-uniform plastic deformation (e.g. shot peening, deep rolling, etc.) to induce controlled residual stress fields stabilized by the processing at temperatures where dynamic strain aging is active.     

Effect of porosity and tension–compression asymmetry on the Bauschinger effect in porous sintered steels

The Bauschinger effect is a common phenomenon in metallic materials. In this paper, the Bauschinger effect in Fe–0.85Mo–2Ni powder metallurgy (PM) steel was investigated for different porosities and as a function of loading sequence (compression–tension versus tension–compression). Both the porosity and loading sequence had a significant effect on the magnitude and asymmetry of Bauschinger effect. Compression followed by tensile loading lead to a higher Bauschinger effect than tension followed by compression. This asymmetry of Bauschinger effect was more significant for higher porosity. Crack formation and propagation, observed in this study, were the main factors influencing the asymmetry in Bauschinger effect. Finite element analysis, based on the actual microstructure of the steels, yielded good agreement with the experimental stress-strain behavior. FEM showed that both the Bauschinger effect in the steel matrix and porosity contribute to the global Bauschinger effect of the PM steels.     

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.     

Processing of steel for ultrafine ferrite grain structures

Abstract

The attainment of ultrafine ferrite grain structures in low carbon, low alloy steels is of interest because of the improvement in yield strength and Charpy impact transition temperature predicted by extrapolation of known data to very fine grain sizes. This paper presents a summary of research aimed at producing ultrafine ferrite in a niobium microalloyed, low carbon steel by three processing routes. Transformational grain refinement (TGR), in which extrafine austenite is hot rolled and cooled rapidly, has been shown to be capable of producing grain sizes of <1 µm in a surface layer, and 1.5 µm in the centre of 3 mm thick plate. Dynamic recrystallisation of ferrite during multipass warm rolling was shown to be neither complete nor uniform within the cross-section of the plate. Nevertheless, a partly recrystallised, partly recovered grain structure with an average grain size of 1.5 µm was obtained in the centre of 3 mm thick plate. Cold rolling and recrystallisation of ferrite that had been previously refined by TGR to an intermediate grain size was shown to produce an ultrafine grain microstructure (<1 µm grain size) throughout the section of 1 mm thick strip. The hardness of ultrafine ferrite was shown to obey a linear relationship with the inverse square root of grain size, but with a lower slope than expected from the Petch relationship for yield strength.     

Effect of temper rolling on tensile properties of C-Mn steels

Abstract

The tensile properties of two C-Mn steels TR1 (Fe-0.135C-0.66Mn) and TR2 (Fe-0.019C-0.18Mn) under different temper rolling conditions were investigated. It was found that the lower yield strength and ultimate tensile strength of steels TR1 and TR2 which were temper rolled at different reductions can be expressed by the following formula, σ(MPa)=σ⁰+K?_eq(%), where σ is the strength after temper rolling; σ⁰ is the strength without temper rolling; ?_eq is the equivalent strain of the temper rolling reduction; K is a constant. The uniform elongation and the total elongation of these two steels which have been temper rolled at different reductions (or equivalent plastic strains) are those of samples without temper rolling subtracted from their equivalent plastic strains. The work hardening exponents of temper rolled samples can be predicted using the tensile curves of the samples which have not been temper rolled. Very good agreement between the experimental results and the calculated data was obtained.     

Tensile deformation of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

Two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~400–250°C) have been deformed in tension. The bainitic microstructures in these steels contain appreciable amounts of retained austenite (instead of interlath cementite), and the behaviour of this phase during tensile deformation, and its apparent influence on the mechanical properties, has been examined. In particular, it is shown that provided the retained austenite exists in an interlath, thin-film morphology it has appreciable mechanical stability. Larger volumes of retained austenite have less mechanical and thermal stability, forming plate martensite structures and also undergoing deformation twinning. The effects of these variations on tensile strength and ductility are discussed.

MST/527     

Low temperature mechanical properties of quenched and tempered 0·4C–Ni–Cr–Mo steel after controlled rolling

Abstract

The mechanical properties of a quenched and tempered 0·4C–Ni–Cr–Mo steel after controlled rolling (CRP steel) have been studied over the temperature range 77–293 K with the aim of developing a CRP steel for low temperature ultrahigh strength applications. The results obtained were compared with those of a conventional quenched and tempered 0·4C–Ni–Cr–Mo steel (CHT steel). The CRP process was found to improve greatly the strength, ductility, and fracture and impact toughness for tempers at and below 473 K, independent of test temperature, but there was some concomitant deterioration in the transverse properties. It is postulated that the fine subcell structure, introduced during the CRP, is mainly responsible for the improved mechanical properties. However, there is an abrupt reduction in fracture energy of fatigue precracked steels for tempers above 473 K, so above this temperature there is little difference in the properties of the CRP and CHT steels. This is attributed to fine carbide precipitation, which promotes shear localisation and dimple fracture. Despite this, it is demonstrated by the present work that the CRP steel is attractive for low temperature ultrahigh strength steel applications.

MST/734     

Fatigue strength of mash seam welded joints

Reversed bending fatigue tests have been conducted using four series of mash seam welded joints obtained from the coupling of two different steels and plate thicknesses. Fatigue strength was evaluated and the effects of material property changes resulting from welding were studied. Fatigue strength of all series of the welded joints decreased slightly compared with that of the base steel. Type of steel and plate thickness in the welded samples exerted very little influence on fatigue strength. In the welded joints between steels with the same plate thickness, fatigue failure took place at a location away from the weld zone in the plate with the lower strength, while in the welded joints between plates of different thickness, failure occurred at the shoulder between the thin and thick plate, i.e. at the weld zone. Regardless of the type of steel and the plate thicknesses joined, fatigue strengths of the mash seam welded joints were slightly higher than those of the laser welded butt joints.     

Fracture toughness of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

The fracture toughness of two experimental silicon-containing steels in the bainitic condition has been measured and related to the microstructural state of the steels. The optimum bainitic microstructure for high strength and high toughness combinations consists of bainitic ferrite and thin interwoven laths of retained austenite instead of cementite, this condition being achieved through the silicon addition to the steels. The thin films of retained austenite are thermally and mechanically stable and act to reduce the effective fracture grain size and also possibly help to blunt propagating microcracks; blockier volumes of retained austenite are unstable and hence not beneficial to toughness. The two experimental steels achieved strength and toughness values equal to, or better than, some commercial steels in the martensitic condition.

MST/528     

Role of silicon in influencing strength and impact behaviour of ferrite and its likely influence at ultrafine grain size

Abstract

Plates of fine grained Nb containing steels having Si contents of 0.03, 0.1, and 0.51% were austenitised at 920°C and cooled at 40 and 7.5 K min^-1 through the γ to αtransformation temperature range. Increasing the Si level from 0.03 to 0.1%resulted in a decrease in the yield strength and an improvement in impact performance for both cooling rates. Raising the Si level further to 0.51%Si caused the strength to increase and the impact behaviour to deteriorate. The results are explained in terms of Si segregating to the ferrite boundaries, increasing the activity of C and N, and thereby displacing these atoms from the boundary regions. In this way, although Si increases σ₀in the Hall–Petch relationship the K _y value is reduced. Consequently, at fine grain size, Si additions can actually reduce the strength of steels and, at ultrafine grain sizes (1 µm), this would be expected to result in very substantial softening.     

大口径高钢级螺旋埋弧焊管强度测试影响因素分析

针对西气东输二线用大口径高钢级螺旋埋弧焊管,采用两种试样进行拉伸试验.综合分析影响螺旋焊管强度测试的因素,如试样形式、试验方法等.研究结果表明:不同形式试样测定的螺旋焊管管体横向屈服强度值是包辛格效应、形变强化效应及组织等因素综合作用的结果;包辛格效应是影响其屈服强度测试结果的主要因素;测定屈服强度时采用的总伸长量应根据胀环试验测定的管体实际屈服强度值确定.     

Influence of thermomechanical processing and accelerated cooling on microstructures and mechanical properties of plain carbon and microalloyed steels

Abstract

A thermomechanical control process consisting of slab reheating, controlled rolling, and accelerated cooling has been adopted at the plate mill, Bhilai Steel Plant, India for achieving high strength and toughness in C–Mn and microalloyed steels while keeping the mechanical properties and the flatness constant. A mathematical model has been developed to predict the temperature distribution in the plate during accelerated cooling, taking into account the heat generation of the phase transformation. Effects of chemistry and mill parameters on ferrite grain refinement are explained in terms of nucleation and growth rate.