Influence of intercritical annealing on strength and impact behaviour of niobium containing steels

Abstract

The influence of inter critical annealing at 730°C on the impact properties and strength of C–Mn–Al–Nb steels has been examined. For low Mn (0·56%), Nb steels, intercritical annealing resulted in improved impact performance and the impact transition temperature (ITT) was reduced by as much as 35 K with no change in strength. The improvement in impact performance is considered to be due to Mn segregating to the α/γ boundaries leading to refinement of the grain boundary carbides. This refinement increased with holding time at 730°C in accordance with an increased grain boundary segregation of Mn. Strength was not influenced because grain size remained unchanged on intercritical annealing. The improvement in impact behaviour was greater the longer the holding time at 730°C but was significant even after 15 min. Improvements occurred both on cooling from the austenitising temperature (9·20°C) to 730°C and on heating from room temperature to 730°C, the latter heat treatment being the more beneficial. For higher Mn (1·4%), Nb steels, improvements in impact performance resulting from intercritical annealing depended on cooling rate. Again, the Mn build-up in the y increases with time of intercritical annealing. Owing to the initial overall higher Mn level and finer grain size, the steels were susceptible to martensite formation if the cooling rate was too high. At a cooling rate of 40 K min - 1, improvements in impact behaviour occurred only after short intercritical annealing times (30 min) when only a small amount of martensite had formed. Long times caused a serious deterioration in impact behaviour due to the presence of high volume fractions of martensite. Slow cooling (1 K min^?1), however, ensured ferrite–pearlite structures and significant improvements in impact behaviour (20–60 K reductions in ITT) were noted on intercritical annealing with no change in strength. The short holding times required to achieve an improvement in impact behaviour in these fine grained steels are encouraging for the possible commercial exploitation of this heat treatment.

MST/1382     

Effect of intercritical annealing on the Lüders strains of medium Mn transformation-induced plasticity steels

The present work systematically investigates the effects of starting microstructure and intercritical annealing temperature on the tensile properties and Lüders strain of transformation-induced plasticity steels containing 5 wt.% Mn. It is found that higher intercritical annealing temperature leads to smaller Lüders strain, lower yield strength and higher ultimate tensile strength. The starting cold-rolled microstructure produces much coarser microstructural constituents and larger Lüders strain than the martensitic one. It is concluded that the fraction and size of austenite grains and the amount of carbide formed during intercritical annealing are the most important microstructural factors to determine the Lüders strain rather than the fraction of retained austenite and the grain size of the ferritic phase.     

Effects of carbon content on mechanical properties of 5%Mn steels exhibiting transformation induced plasticity

Abstract

A series of highly ductile, high strength steels exhibiting transformation induced plasticity due to retained austenite was developed by varying the carbon content in the range 0·01–0·4 wt-% in 5 wt-%Mn based steel. For up to 0·l%C the mechanical properties are insensitive to cooling rate after intercritical heating, but afurther increase in carbon content causes a large sensitivity to the cooling rate, owing to carbide precipitation occurring during slow cooling. By suppressing this carbide precipitation with water quenching after the intercritical holding, an excellent combination of tensile strength (1580 MN m^?2) and uniform elongation (21%) was attained at 0·3%C in this series.

MST/1964     

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.     

Ferrite recrystallisation and intercritical annealing of cold-rolled low alloy medium carbon steel

Static recrystallisation of cold-rolled AISI 4130 medium carbon steel in the ferritic regime and its response to intercritical annealing treatment are studied. A fine and recrystallised microstructure with improved mechanical properties is obtained via subcritical annealing of cold-rolled sheet, where the subsequent intercritical annealing results in the enhancement of tensile strength via the formation of dual-phase microstructure. Intercritical annealing of the cold-rolled sheet is characterised by an initial drop in hardness due to recrystallisation and subsequent rise in hardness as a result of austenitisation. It is revealed that continuous martensite phase can result in a higher deformation resistance. Finally, the effects of intercritical annealing at temperatures below pearlite dissolution finish temperature (Ac_1f) are discussed.     

Low carbon high manganese bainitic steel

Abstract

The present study concerns the mechanical properties of low carbon (0·05 wt-%) high Mn bainitic steel. The continuous cooling transformation diagram exhibited bainitic transformation without any prior diffusive transformation of austenite even for a cooling rate as low as 0·5°C/s. The bainitic steels have shown continuous elongation behaviour with attractive combination of strength (>1200 MPa) and elongation (>14%). The bainitic microstructure obtained after annealing treatment has yielded excellent combination of strength, uniform elongation, yield ratio and static toughness value.     

Microstructure and tensile properties of a 0.20C–4.86Mn steel after short intercritical-annealing times

This work studies the microstructure and tensile properties of a cold-rolled Fe–0.20C–4.86Mn (mass %) steel after short intercritical annealing (IA) times using scanning and transmission electron microscopy, and uniaxial tensile tests. The short IA time is applied to represent the process characteristics of the industrial continuous annealing line. The experimental results show that IA temperature has a strong influence on the final microstructure and tensile properties while IA time has less. The fractions of retained austenite are much higher after IA at 650 and 675°C than the other IA temperatures, and thus improving elongation. Simulations using the DICTRA software and constitutive modelling are further performed to assist the understanding of the microstructure evolution and stress–strain curves.     

Analysis of medium manganese steel through cold-rolling and intercritical annealing or warm-rolling

ABSTRACT

Medium manganese steel is typically fabricated from hot-rolling followed by cold-rolling and intercritical annealing processes. However, a singular process, warm-rolling, is an appealing prospect. The microstructure, mechanical properties and texture of a 8?wt-% Mn–0.08?wt-% C steel was investigated by cold-rolling followed by intercritical annealing or warm-rolling. The product from both processes exhibited a highly refined microstructure as well as a large volume fraction of austenite. However, a slightly larger volume fraction as well as a more heterogeneous austenite morphology resulted from the warm-rolling process, associated with a higher work-hardening rate. Microstructural analysis revealed intense α-fibre components due to a higher strain following the warm-rolling process as well as a considerably high-angle grain boundary number which was associated with dislocation accumulation.

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

Impact and tensile properties of ferrite–martensite dual‐phase steels

The effect of martensite morphology on the impact and tensile properties of dual phase steels with a 0.25 volume fraction of martensite (V_m) under different heat treatments was investigated. These treatments are direct quenching (DQ) and step quenching (SQ) that result in different microstructures and mechanical properties. To process dual phase steels, a low carbon manganese steel was used. At first the banding present in the initial steel was eliminated, then the two different heat treatments were applied. To reach a 0.25 volume fraction of martensite a variation of intercritical annealing temperatures was adopted for both treatments that allowed the evolution of different volume fraction of martensite. Phase analysis showed that an intercritical temperature of 725 °C (between A₃, A₁) gives the desired 0.25 V_m of martensite. A comparison of impact, tensile and ductile–brittle transition temperature (DBTT) indicates that the microstructure of the direct treatment has a better toughness. The DBTT for the DQ and SQ treatment is ?49 and ?6 °C, respectively.     

Design of an effective heat treatment involving intercritical hardening for high-strength–high elongation of 0.2C–1.5Al–(6–8.5)Mn-Fe TRIP steels: microstructural evolution and deformation behaviour

ABSTRACT

We propose an effective heat treatment involving a combination of intercritical hardening and tempering to obtain high strength–high ductility in hot-rolled 0.2C–1.5Al–(6–8.5)Mn–Fe transformation-induced plasticity (TRIP) steels. An excellent combination of high ultimate tensile strength of 1045–1380?MPa and total elongation of 34–39% was obtained when the steels were subjected to intercritical hardening at 630–650?°C and tempered at 200?°C. Intercritical hardening impacted the co-existence of austenite, ferrite and martensite, such that the deformation behaviour varied with the Mn content. The excellent properties of the steels were attributed to cumulative contribution of enhanced TRIP effect of austenite and ferrite and martensite constituents. The discontinuous TRIP e?ect during tensile deformation involves stress relaxation and led to consequent enhancement of ductility.     

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 2 Tensile properties of extruded and Conformed gas atomised powders

Abstract

The effects of aging treatments on the tensile properties and microstructure of Al–Cr–Zr–Mn powder metallurgy aluminium alloys prepared from high pressure gas atomised powders were investigated. The alloy compositions were designed to give powders with or without Al₁₃Cr₂ intermetallics in the <45 μm size fraction. The Al–5·2Cr–1·4Zr–1·3Mn alloy is typical of the former (concentrated alloy) and the Al–3·3Cr–0·7Zr–0·7Mn alloy of the latter (dilute alloy). The alloys were prepared using a canning/degassing/extrusion sequence or the Conform consolidation process. Measurements of micro hardness and electron microscopy were used to correlate the microstructure with the tensile properties. The extruded powders of both alloys exhibited better properties than those of the Conformed powders. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The dilute alloys had consistently better ductility. Neither alloy retained its strength after prolonged aging at 400°C, but the results indicate that a service temperature of 300°C may be possible.

MST/1247b     

Enhanced tensile properties of a reversion annealed 6.5Mn-TRIP alloy via tailoring initial microstructure and cold rolling reduction

The feasibility of improving the overall performance of medium Mn steels was demonstrated via tailoring the initial microstructure and cold rolling reduction.The combined effects of cooling patterns after hot rolling(HR) and cold rolling(CR) reductions show:(1) as the cooling pattern varied from furnace cooling(FC) to oil quenching(OQ),the intercritically annealed microstructure was dramatically refined and the fraction of recrystallized ferrite dropped,regardless of CR reductions.This resulted in both high yield/ultimate tensile strengths(YS/UTS) but low total elongation to fracture(El);(2) as the CR reduction increased from 50% to 75%,the OQ-samples after annealing exhibited a more refined microstructure with relatively higher fractions of retained austenite and sub-structure,leading to higher YS and UTS but lower El; whereas the FC samples appeared to exhibit little difference in overall tensile properties in both cases.The differences in microstructural evolution with cooling patterns and CR reductions were explained by the calculated accumulated effective strain(εAES),which was considered to be related to degrees of recovery and recrystallization of the deformed martensite(α').The optimal tensile properties of ~1 GPa YS and ~40 GPa·% UTS×El were achieved in the OQ-50%CR annealed samples at 650?C for 1 h.This was quite beneficial to large-scale production of ultra-high strength steels,owing to its serious springback during heavy cold working.     

Effect of intercritical quenching on microstructure and tensile properties of steels 15 and 15Mn2Nb

The microstructure and tensile properties of steels 15 and 15Mn2Nb after quenching from intercritical (γ + α) temperatures were studied. It was shown that steel 15 has an “island-type” dual-phase structure (ferrite plus martensite) after intercritical quenching, while the steel 15Mn2Nb has the “lamellar” structure with alternatively arranged ferrite and martensite needles. Such a lamellar dual-phase structure obviously has improved strength and ductility and is suggested as one of the effective methods in developing high strength dual-phase steels.     

Formation of ultrafine grained ferrite during thermomechanical treatment in microalloyed steel

Abstract

In the present work, the formation of ultrafine grained ferrite has been studied by applying suitable thermomechanical treatment. A high amount of deformation (～80%) at varying strain rates (0·01–10 s^?1) was applied in the temperature range of Ar₃ to Ac₃ followed by water quenching. This treatment resulted in a two-phase ferrite–martensite microstructure as compared to fully martensite structure after quenching without deformation. The formation of ultrafine ferrite (?3 μm) during deformation was favourable at a lower temperature and a slower strain rate. A maximum ～50% ferrite formed during deformation at 780°C with a strain rate of 0·01 s^?1. Experimental rolling with a high strain (～1·3) with finish rolling temperature just above Ar₃ (～750°C) resulted in fine ferrite–pearlite of ?3 μm, and the properties showed a high value of strength as compared to steels rolled in a conventional way. Dual phase microstructure (ferrite and martensite) was produced after partial austenisation to 780°C followed by quenching in water, and this resulted in an excellent combination of properties (high ultimate tensile strength, low yield strength/ultimate tensile strength, high elongation and high n values).     

Effect of cerium sulphide particle dispersions on acicular ferrite microstructure development in steels

Abstract

Of the phases found in wrought steels, cerium sulphide particles are notable in that they can be both stable at liquid metal temperatures and exhibit good lattice coherency with α iron. An investigation has been carried out to determine the effectiveness of cerium sulphide particle dispersions in nucleating intragranular acicular ferrite microstructures in steels. Vacuum melts of 50 kg have been manufactured of appropriate base steel compositions with varying additions of cerium (0·04–0·18%) and sulphur (0·01–0·04%). The work has shown that 0·02–0·12% cerium and 70–340 ppm sulphur may be retained in steels after deoxidation and desulphurisation reactions while oxygen can be reduced to <20 ppm. Resulting inclusions are largely spheroidal in shape and consist of several crystalline compounds, notably CeS, Ce₃S₄ and Ce₂O₂S. The inclusion numbers are of the order of 0·68–6·12 × 10⁶ mm^?3 with mean diameters of 0·63–1·70 μm. The densities of these inclusion dispersions are approaching those in weld metals where acicular ferrite is the dominant microstructure constituent. Significant volume fractions of acicular ferrite (up to 65%) have been obtained in steels after thermally cycling in a dilatometer and cooling at rates simulating transformation conditions ranging from high heat input welding to air cooling of forgings and water cooling of plate. A potential beneficial effect of acicular ferrite on mechanical properties in high heat input welding (heat affected zone grain refinement) and in thermo-mechanically processed steels (relaxed schedules) has been highlighted. A pilot plant billet cast of steel has shown the feasibility of achieving the required particle dispersions and acicular ferrite microstructure in tonnage steelmaking.     

Development of hypoeutectoid graphitic steel for wires

Abstract

Hypoeutectoid graphitic steels with BN as prime nucleating site for graphite nucleation were developed for improvement of drawability of wires. Al₂O₃ also acted as nucleation site for graphite. The steel chemistry has played an important role for BN formation and thus dictated the behaviour of graphite formation and tensile properties after graphitising annealing. The steels are with a low yield ratio of ～0·5 while percentage elongation is in the range of 23–25% and percentage reduction in area is in the range of 45–51%.     

A medium-Mn steel processed by novel twin-roll strip casting route

A medium-Mn steel (Fe–0.3C–4Mn–1.82Al–0.6Si wt-%) was produced by a novel processing route involving twin-roll strip casting, hot rolling and intercritical annealing (IA). Macrosegregation was absent in the as-cast strip. The microstructure of the as-cast strip consisted of martensite and austenite (～10 vol.-%), and the solidification structure was characterised by dendritic structure. With an increase in IA temperature from 680 to 725 and to 755°C, austenite fraction in intercritically annealed steels was increased from 22 to 45% and then decreased to 27%. The 710°C intercritically annealed steel yielded excellent mechanical properties with a tensile strength of ～1007?MPa and total elongation of ～48%, achieved by a high volume fraction of austenite (～42%) with appropriate mechanical stability.     

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     

Effects of Cr,Mn, Si,Cu and Zr on microstructure and mechanical properties of high carbon Fe–16Al alloy

Abstract

Effects of alloying elements Cr, Mn, Si, Cu and Zr on the microstructure and mechanical properties of Fe₃Al (Fe–16Al) based alloy containing ～0·5 wt-%C have been investigated. Six alloys were prepared by a combination of air induction melting with flux cover and electroslag refining (ESR). ESR ingots were hot forged and hot rolled at 1373 K and were further characterised with respect to microstructure and mechanical properties. The base alloy and the alloys containing Cr, Mn, Si and Cu exhibit a two phase microstructure of Fe₃AlC_0·5 precipitates in Fe₃Al matrix whereas the alloy containing Zr exhibits a three phase microstructure, the additional phase being Zr rich carbide precipitates. Cr and Mn have high solubility in Fe₃AlC_0·5 precipitates as compared to Fe₃Al matrix whereas Cu and Si have very high solubility in Fe₃Al matrix compared to Fe₃AlC_0·5 precipitate and Zr has very low solubility in both Fe₃Al matrix and Fe₃AlC_0·5 precipitate. No significant improvement in room and high temperature (at 873 K) strengths was observed by addition of these alloying elements. Furthermore, it was observed that addition of these alloying elements has resulted in poor room and high temperature ductility. Addition of Cr, Mn, Si and Cu has resulted in marginal improvement in creep life, whereas Zr improved the creep life significantly from 22·3 to 117 h.