New parameter Y for static recrystallisation in hot deformed austenite

Abstract

The influence of hot deformation conditions on static and dynamic recrystallisation behaviour of 18%Ni maraging steel was studied. Using the Zener–Hollomon parameter in the dynamic recrystallisation diagram for reference, a new parameter Y (=tZ^mexp(?Q_rec/RT)) is recommended and a static recrystallisation diagram proposed. The occurrence of static recrystallisation of hot deformed austenite in 18%Ni maraging steel was determined as a function of the parameter Y (the holding time modified by temperature and strain rate). The structural changes after deformation in hot deformed austenite may be described by the static recrystallisation diagram (Y–? diagram). The procedure for constructing the static recrystallisation diagram may be simplified by introducing the parameter Y.

MST/699     

Influence of strain rate on recrystallisation– precipitation interaction in V,Nb, and V–Ti microalloyed steels

Abstract

A study has been made of the recrystallisation–precipitation interaction in three microalloyed steels, containing respectively V, Nb, and V–Ti, applying two different strain rates. Recrystallised fraction v.time curves were determined and used to draw recrystallisation–precipitation–time–temperature (RPTT) diagrams. The influence of strain rate has been shown to be similar in the three steels. On the basis of the results the value of }}SB0·19 has been found for the exponent of the strain rate, following Dutta and Sellars'model for the parameter t _0·05 , which differs from the value }}SB0·5 proposed by these authors. Simultaneously, the influence of strain rate on the static recrystallis ation critical temperature has been determined, it being observed that an increase in the former leads to a drop in the latter. Furthermore, strain rate is shown to have an influence on the recrystallisation–precipitation interaction, acting on those parameters that best contribute to defining RPTT diagrams In this sense, it was found that an increase in the strain rate led to a drop in the curve nose temperature T _N and a reduction in the time necessary for precipitation to finalise t _0·95 , as well as an increase in the recrystallisation rate.     

Hot deformation behaviour of precipitation hardening stainless steel

Abstract

The behaviour of 17-4 precipitation hardening (PH) stainless steel was studied using the hot compression test at temperatures of 950–1150°C with strain rates of 0·001–10 s^?1. The stress–strain curves were plotted by considering the effect of friction. The work hardening rate versus stress curves were used to reveal whether or not dynamic recrystallisation (DRX) occurred. Using the constitutive equations, the activation energy of hot working for 17-4 PH stainless steel was determined as 337 kJ mol^?1. The effect of Zener–Hollomon parameter Z on the peak stress and strain was studied using the power law relation. The normalised critical stress and strain for initiation of DRX were found to be 0·89 and 0·47 respectively. Moreover, these behaviours were compared to other steels.     

Dynamic recrystallisation of as cast austenite in 18–8 stainless steel

Abstract

Dynamic recrystallisation behaviour of an as cast 0Cr18Ni9Ti stainless steel during hot deformation was investigated by hot compression test at a temperature range of 950–1200°C and strain rate of 5 × 10^-3–1 × 10^-1 s^-1. Change of austenite grain size owing to dynamic recrystallisation was also studied by microstructural observation. The experimental results showed that the hot deformation conditions, such as temperature, strain, and strain rate determine the dynamic recrystallisation behaviour for the as cast stainless steel, and the dynamically recrystallised grain size is determined by the deformation conditions and is independent of the strain.     

Effect of nish rolling temperature on static recrystallisation in hot bands of electrical steel containing 1·3% silicon

Abstract

The effect of the finish rolling temperature (FRT) on recrystallisation behaviour in hot bands of an electrical steel containing 1·3%Si was investigated. Four sequential passes of hot rolling were carried out on the 1·3%Si electrical steel, with finish rolling temperatures ranging from 980 to 700°C, followed by isothermal annealing at 720°C. The experimental results showed that when Ar ₁ <FRT <Ar ₃, fine equiaxed subgrains formed at the boundaries between deformed and non-deformed grains in a necklacelike arrangement, and strain induced boundary migration (SIBM) was the main mechanism corresponding to the formation of recrystallisation nuclei for steels finish rolled below Ar ₁. However, the study also demonstrated that when FRT <(Ar ₁­100 K), a second nucleation mechanism, i.e. subgrain growth, became active in recrystallisation, this resulted in an increase of nucleus density. Steels in which SIBM was the dominant mechanism of recrystallisation possessed the largest grain size, and strongest textures with major component {100}〈110〉.     

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.     

Optimisation of hot rolling schedule for direct charging of thin slabs of Nb–V microalloyed steel

Abstract

A recently developed continuous casting simulator and the ‘Wumsi’ hot deformation simulator have been used to carry out laboratory simulation tests to determine the as cast microstructure and the recrystallisation behaviour of a Nb–V microalloyed steel during the process of direct charging. By variation of the initial specimen thickness (between 25 and 60 mm) different values of total strain Φ_Σ could be imposed to improve the coarse as cast microstructure. For a series of deformation schedules the total strain was divided systematically into two components: an austenite grain refining strain Φ_γGF (above the recrystallisation stop temperature T_RS) and an austenite strengthening strain Φ_γS (below T_RS). After hot deformation slow and accelerated cooling with simulated coiling were employed. It was found that a total strain Φ_Σ>1·4 is required to ensure mechanical properties that were comparable or even superior to those found using the conventional cold charging process. The coarse as cast austenite microstructure can be refined significantly when Φ_γGF=0·3–0·6. The austenite strengthening strain Φ_γS represents the dominant component of the total strain if a satisfactory toughness is to be achieved. Strength properties are less sensitive to the applied strain.

MST/1872     

Formation of precipitates in multiple microalloyed pipeline steels

Abstract

An investigation has been carried out to identify the precipitates in multiple microalloyed steels. The microalloying elements and interstitials included aluminium, niobium, titanium, vanadium, carbon, and nitrogen. It was found that the precipitates are complex in nature and they were rationalised on the basis of mutual solubility probably enhanced by non-stoichiometry. The precipitate morphologies were interpreted mainly in terms of steel compositions. Steels quenched from 1250°C contained titanium rich precipitates accompanied by the evolution of new niobium rich precipitates after hot rolling and quenching. A parameter K₁ indicative of solute participation in the precipitation phenomenon was established and showed excellent correlation between steel and precipitate analyses. A sequence of precipitation in multiple microalloyed steels was achieved using solubility relationships as a premise.

MST/803     

Interaction between precipitation,normal grain growth,and secondary recrystallisation in austenitic stainless steel containing particles

Abstract

With the aid of various complementary methods of microstructural analysis, the precipitation, grain growth, and secondary recrystallisation behaviour of an 15Cr–15Ni–1·2Mo–Ti–B (wt-%) austenitic stainless steel were studied over prolonged periods of time in the temperature range 600–1300°C. The experimental results showed that several types of precipitates were present in the material, and that the dissolution temperatures of each of these correlated with the type and extent of grain growth which was observed. It was, therefore, concluded that in the present study secondary recrystallisation was caused directly by the interaction of precipitates with grain boundaries. Furthermore, secondary recrystallisation produced a strong, predominantly {122{ 〈012〉 texture which has not previously been reported.     

Study on hot deformation behaviour of 316LN austenitic stainless steel based on hot processing map

Abstract

316LN is a type of austenitic stainless steel whose grain refinement only depends on hot deformation. The true stress–strain curves of 316LN were obtained by means of hot compression experiments conducted at a temperature range of 900–1200°C and at a strain rate range of 0·001–10 s^?1. The influence of deformation parameters on the microstructure of 316LN was analysed. Both the constitutive equation for 316LN and the model of grain size after dynamic recrystallisation were established, and the effect of different deformation conditions on the microstructure was analysed. The results show that the suitable working region is the one with a relatively higher deformation temperature and a lower strain rate, in which the dynamic recrystallisation is finely conducted. Moreover, the working region that should be avoided during hot deformation was indicated.     

Microstructural evolution in silicon alloyed γ-Ti–Al alloys after thermomechanical processing

Abstract

The microstructures of silicon alloyed γ-Ti–Al alloys containing silicide particles have been studied after thermomechanical treatments to investigate microstructural evolution. Important parameters including temperature, forging strain, and sequence of thermomechanical treatments were systematically studied. Isothermal forging below the eutectoid temperature resulted in inhomogeneous dynamic recrystallisation with fine equiaxed grains in recrystallised areas and residual α₂ + γ lamellae elsewhere. Eutectic silicides play an important role in destruction of the as cast structure by promoting dynamic recrystallisation during deformation and static recrystallisation on subsequent annealing. There is evidence that silicon, in solution, also enhances recrystallisation. The presence of fine silicides produced by precipitation in the solid state restricts the size of grains produced by both dynamic and static recrystallisation. Silicon also alters significantly the phase equilibrium between the α and γ phases.     

Characterisation of dynamic recrystallisation in nickel using processing map for hot deformation

Abstract

The hot deformation behaviour of polycrystalline nickel has been characterised in the temperature range 750–1200°C and strain rate range 0·0003–100 s^?1 using processing maps developed on the basis of the dynamic materials model. The efficiency of power dissipation, given by [2m/(m+1)], where m is the strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. A domain of dynamic recrystallisation has been identified, with a peak efficiency of 31% occurring at 925°C and 1 s^?1. The published results are in agreement with the predictions of the processing map. The variations of efficiency of power dissipation with temperature and strain rate in the dynamic recrystallisation domain are identical to the corresponding variations of hot ductility. The stress–strain curves exhibited a single peak in the dynamic recrystallisation domain, whereas multiple peaks and ‘drooping’ stress–strain curves were observed at lower and higher strain rates, respectively. The results are explained on the basis of a simple model which considers dynamic recrystallisation in terms of rates of interface formation (nucleation) and migration (growth). It is shown that dynamic recrystallisation in nickel is controlled by the rate of nucleation, which is slower than the rate of migration. The rate of nucleation itself depends on the process of thermal recovery by climb, which in turn depends on self-diffusion.

MST/1524     

Effect of silicon on recrystallisation of V–Ti microalloyed steel

Abstract

The dynamic recrystallisation (DRX) and static recrystallisation (SRX) behaviours of three V–Ti microalloyed steels were studied by the analysis of the true stress–strain curves and the stress relaxation curves under different deformation conditions. The results of DRX showed that deformation activation energy Q_def, peak stress and peak strain increased, as a result of the solute strengthening and dragging effect due to Si. The results of SRX showed that Si increased the SRX activation energy Q_SRX. The solute retardation parameter for static recrystallisation of Si was calculated. Based on the SRX results, to quantify the drag effect of Si and V, a new model was proposed to describe the time for 50% recrystallisation (t_0·5), which was tested and verified by previously published data on similar steels. Precipitation during recrystallisation could lead to a lower value of the Avrami exponent.     

Flow curves up to peak strength of hot deformed D2 and W1 tool steels

Abstract

A high carbon, high chromium cold work die steel (D2) and a water hardenable carbon tool steel (W1) were hot deformed in torsion between 900 and 1150°C for the alloy steel and 900 and 1200°C for the carbon steel, at strain rates from 0·1 to 4 s^-1. The slope of the stress–strain curves, which represents strain hardening, decreased linearly then changed gradually to a slower linear decline before decreasing quickly to zero at the peak stress. On further strain, stress decreased to a steady state regime, indicative of dynamic recrystallisation. Kocks–Mecking analysis provided an activation enthalpy with an average comparable to the activation energy derived from a sinh modified Arrhenius analysis of peak stress. The alloy carbides of the D2 steel have the effect of raising its strength to over twice that of the carbon steel, accelerating the onset of dynamic recrystallisation but drastically lowering its ductility.     

Study of fractional softening of AZ31 magnesium alloy under multistage hot deformation

Abstract

In the present study flow softening behaviour of AZ31 magnesium alloy was investigated by double hit compression tests in the temperature range 250–400°C and strain rate range 10^–3–10^–1 s^–1. The tests were conducted with delay times varying 4–250 s after achieving the critical strain ?_c in each deformation condition. It has been found that static restoration processes (recovery and recrystallisation) were intensely depended on strain rate and deformation temperature. Fractional softening values increased with increasing strain rate and deformation temperature. Accordingly the softening curves were divided into three regions. The softening in the short interpass times was attributed to the occurrence of static recovery and followed by static recrystallisation and grain growth as dominant softening mechanisms for the second and third regions respectively. In addition static recrystallisation kinetics was interpreted by Avrami equation. Analysis of the results indicated that Avrami constant was changed by varying temperature.     

Effect of restoration on hot ductility of high alloy and duplex stainless steels

Abstract

The effect of restoration on the hot ductility of two high alloy austenitic stainless steels and one ferritic–austenitic stainless steel was investigated by means of hot rolling and stress relaxation testing. Cracking tendency was assessed on the basis of the length of the cracks formed. It was found that the recrystallisation kinetics of the high alloy steels is relatively slow, so only partial softening can occur between rolling passes. In the ferritic–austenitic steel the restoration is fairly fast, so softening can be completed between hot rolling passes. The cracking tendency of the steels in the as cast condition increases with increasing pass strain and temperature, but it is negligible in rolling of the steels in the as wrought condition and also minimal in rolling of the as cast steels when using a small strain of 0.1 in the first pass. On the basis of these results, it can be concluded that the cracking problems in these steels are present in the cast structure only. The hot ductility of even partially recrystallised material is perfectly adequate. Hot ductility improves nearly independently of the degree of static recrystallisation, which indicates that ductility is controlled mainly by the grain or phase sizes, not by recrystallisation itself.     

Processing parameters for the mechanical working of 9 Cr–1 Mo steel: processing maps approach

Abstract

Processing and instability maps using a dynamic materials model have been developed for 9Cr–1Mo steel in the temperature range 850 to 1200°C and strain rate range 0.001–100 s^–1 with a view to optimising its hot workability. The efficiency of power dissipation increased with increase in temperature and decrease in strain rate. The 9Cr–1Mo material exhibited two dynamic recrystallisation domains, one with a peak efficiency of 37% occurring at 950°C and 0.001 s^–1 and the other with a peak efficiency of 35% occurring at 1200°C and 0.1 s^–1. These results are in good agreement with those found in industry.     

Dynamic recrystallisation during hot working of Zr–2·5Nb: characterisation using processing maps

Abstract

The characteristics of the hot deformation of Zr–2·5Nb (wt-%) in the temperature range 650–950°C and in the strain rate range 0·001–100 s^?1 have been studied using hot compression testing. Two different preform microstructures: equiaxed (α+β) and β transformed, have been investigated. For this study, the approach of processing maps has been adopted and their interpretation carried out using the dynamic materials model. The efficiency of power dissipation given by [2m/(m+1)], where m is the strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. A domain of dynamic recrystallisation has been identified in the maps of equiaxed (α+β) and β transformed preforms. In the case of equiaxed (α+β), the stress–strain curves are steady state and the dynamic recrystallisation domain in the map occurs with a peak efficiency of 45% at 850°C and 0·001 s^?1. On the other hand, the β transformed preform exhibits stress–strain curves with continuous flow softening. The corresponding processing map shows a domain of dynamic recrystallisation occurring by the shearing of α platelets followed by globularisation with a peak efficiency of 54% at 750°C and 0·001 s^?1. The characteristics of dynamic recrystallisation are analysed on the basis of a simple model which considers the rates of nucleation and growth of recrystallised grains. Calculations show that these two rates are nearly equal and that the nucleation of dynamic recrystallisation is essentially controlled by mechanical recovery involving the cross-slip of screw dislocations. Analysis of flow instabilities using a continuum criterion revealed that Zr–2·5Nb exhibits flow localisation at temperatures lower than 700°C and strain rates higher than 1 s^?1.

MST/3103     

Effect of precipitation on hot ductility of niobium and aluminium microalloyed steels

Abstract

The precipitation reactions occurring in C–Mn–Al and C–Mn–Nb steels before and after hot deformation have been examined and their influence upon hot ductility is discussed. Precipitation has been studied at 850°C, when ductility is poor, and also at 1100°C, when the ductility is good. Rapid intergranular precipitation occurred at 1100°C, but the precipitation present before deformation did not prove to be detrimental to ductility and grain boundary mobility at this temperature. Although only a limited amount of precipitation occurred at 850°C before deformation, intergranular precipitation continued during deformation resulting in embrittlement of the steels. At this temperature, strain induced transgranular precipitation of Nb(CN) occurred in the C–Mn–Nb steel and this is thought to be a major cause of poor hot ductility in this steel. By holding the steels for 15 min at 800–850°C before reheating to 1100°C, the area fraction of intergranular precipitation at 1100°C was increased. This produced a decrease in ductility at this temperature in the C–Mn–Al steel but had a less marked effect in the C–Mn–Nb steel.

MST/107