Multipass Deformation Research Articles

Effect of Dissolved and Precipitated Niobium in Microalloyed Steel on Deformation Induced Ferrite Transformation (DIFT)

Hot deformation processing was designed to study the effects of niobium (Nb) on DIFT. A prestrain of 0.51 at 880 °C for different isothermal time was used for adjusting the deformed austenite constitution and Nb existing state, followed by a secondary heavy deformation at 780 °C for inducing the ferrite transformation. The volume fraction and grain size of deformation induced ferrite (DIF) obtained at different isothermal time between double hits were investigated. It was found that Nb dissolved in austenite is adverse to DIFT; however, the precipitation of Nb is beneficial to DIFT. As Nb plays the role in the conventional TMCP, Nb retards the recrystallization of deformed austenite and enhances the deformation stored energy in the multipass deformation, and in result, Nb promotes DIFT.

Investigation of Retained Strains during Multipass Deformation

Double-hit compression tests were carried out at different temperatures and strain rates for a nickel based alloy and a stainless steel. Using microhardness measurements the retained strains after the first and second pass were investigated as a function of the amount of deformation, temperatures as well as strain rates and dwell durations. In general, the retained strain decreases with increasing dwell durations. It is shown that at a given total amount of deformation, the retained strain is reduced for the as deformed grains that have not been recrystallized yet, but increased for the recrystallized grains, when comparing double hit with single hit compression tests.

Formation of Ultrafine Ferrite Grains during Interrupted Multi-Pass Deformation under Hot Torsion

Strain induced transformation (SIT) of austenite into ferrite has been frequently used as a powerful ferrite grain refinement mechanism. Ordinarily ferrite grain sizes of the order of 1-3μm are achieved via mechanical testing such as compression and torsion. Nonetheless, most of the work done so far employed continuous deformation in the range of 0.8 for compression experiments and in excess of this for torsion. SIT is a promising technique which may be used during actual hot rolling processing. However, in this case, not only deformations are applied with time interrupts between them but also the amount of total deformation allowable is relatively low, in order to attend to flatness and final gauges requirements. This work explores the consequences on SIT microstructure of deformation given in multiple passes as opposite to the usual continuous deformation presented in the literature. Multiple pass deformation at high temperature led to partial dynamic recrystallization and to a mixture of coarse and fine ferrite grains. Multiple pass deformation at the vicinity of Ar3 produced, on the hand, finer ferrite grains indicating that SIT took place. In this case, ferrite grains in the range of 1-3μm were produced and a much more homogeneous distribution of these grains was present.

The Effect of Multiple Deformations on the Formation of Ultrafine Grained Steels

A C-Mn-Nb-Ti steel was deformed by hot torsion to study ultrafine ferrite formation through dynamic strain-induced transformation (DSIT) in conjunction with air cooling. A systematic study was carried out first to evaluate the effect of deformation temperature and prior austenite grain size on the critical strain for ultrafine ferrite formation (e C,UFF) through single-pass deformation. Then, multiple deformations in the nonrecrystallization region were used to study the effect of thermomechanical parameters (i.e., strain, deformation temperature, etc.) on e C,UFF. The multiple deformations in the nonrecrystallization region significantly reduced e C,UFF, although the total equivalent strain for a given thermomechanical condition was higher than that required in single-pass deformation. The current study on a Ni-30Fe austenitic model alloy revealed that laminar microband structures were the key intragranular defects in the austenite for nucleation of ferrite during the hot torsion test. The microbands were refined and overall misorientation angle distribution increased with a decrease in the deformation temperature for a given thermomechanical processing condition. For nonisothermal multipass deformation, there was some contribution to the formation of high-angle microband boundaries from strains at higher temperature, although the strains were not completely additive.

Effect of multipass deformation with an intermediate heat treatment and natural aging on the flow curves of aluminum alloys

The characteristics of plastic-deformation resistance and the limiting strains of aluminum alloys are simulated at any stage of multipass plastic forming with an intermediate heat treatment and natural aging. The model proposed is experimentally tested on various anisotropic thermally hardenable sheet alloys.

Modelling Strain Induced Precipitation of Niobium Carbonitride during Multipass Deformation of Austenite

A new model of strain induced precipitation in niobium microalloyed austenite is proposed. This is based on the experimental observation of formation of microbands during hot deformation of iron – 30% nickel, which remains austenitic to room temperature. Precipitates are preferentially nucleated on nodes in the dislocation network in the microbands. This geometry enables features of earlier models to be simply explained, and facilitates extension of the model to multipass deformation. It is shown that the model captures all the essential features of previous experimental observations on microalloyed C – Mn steels. Currently, sensitivity analysis of the model and systematic experimental work are being undertaken to quantitatively validate the model.

Hot working and multipass deformation of a 41Cr4 steel

The hot working response of a 41Cr4 steel has been investigated by means of torsion tests at temperatures of 850, 950 and 1050 °C, in order to simulate the rolling schedule for this material. The equivalent strain rates used in this part of the study ranged between 5 and 0.05 s −1. The peak stresses obtained from the flow curves were related to strain rate ( ε) and temperature ( T) by means of the well known equation ε ̇ exp Q/RT=A( sinh ασ) n where Q was the activation energy for hot working, n was the stress exponent, and A and α were material parameters. A least square analysis of the data led to n=4.3, α=0.0096 MPa −1 and Q=295 kJ mol −1. A subsequent analysis was aimed at simulating rolling practice; the strain and strain rate for each single pass were determined on the basis of the real rolling schedule, and a series of multi-step torsion tests with different finishing temperatures (1050, 800 and 750 °C) were carried out. The microstructures of the samples after deformation were then analysed by means of light microscopy, and the differences in term of morphology, distribution and amount of the various constituents are discussed.

Deformation Behaviour of 7075Al/SiCp Composite during Multi-Pass Deformation at High Temperatures

Deformation Behaviour of 7075Al/SiCp Composite during Multi-Pass Deformation at High Temperatures

Flow stress behaviour, static recrystallisation and precipitation kinetics in a Nb-microalloyed steel after a strain reversal

In the present work, a Nb microalloyed steel has been hot worked in torsion mode following multipass deformation sequences. Some of these sequences involve a reversion of the strain. The effect of this reversion on the mechanical response of the steel on reloading after the pre-straining has been analysed. For pre-strains within the strain hardening deformation range, after a transient, the normal flow curve shape corresponding to austenite exhibiting a single peak dynamic recrystallisation is reached. However, the new position of the peak indicates that the dynamic recrystallisation is clearly delayed by the reversal. Deformation sequences including holding times after different strain paths have been used to study the effect of the reversal on the static softening and the strain induced Nb(C,N) precipitation. A complex relationship between the kinetics of both processes and the strain path is observed.

Feasibility study of partial recrystallisation in multi-pass hot deformation process and application to calculation of mean flow stress

A feasibility study for handling the partial recrystallisation in multi-pass hot deformation where the heterogeneity of microstructure of deformed austenite is inherently accompanied is presented. The proposed model is based on modification of the conventional model in which the microstructure of deformed austenite at each pass is simply taken as being homogeneous during the multi-pass deformation. The usefulness of the modified model has been demonstrated by applying it to a four-pass oval–round (or round–oval) rod rolling sequence. The recrystallised fraction, austenite grain size (AGS) and mean flow stress at each pass computed from the modified model has been compared with those from the conventional model. The result showed that the recrystallisation behaviour and evolution of AGS at a given pass were dependent on the modelling method of the partial recrystallisation in the multi-pass rolling for the case studied. As the rolling speed increased, the difference between the mean flow stresses calculated by the conventional model and the proposed model was gradually larger in accordance with the contribution of partial recrystallisation.

Flow stress equation for multipass hot-rolling of aluminum alloys

A series of simple axisymmetric compression tests were carried out on the computer servo-controlled Gleeble 1 500 machine when strain rates ranged between 0.05–25 s−1 and deformation temperature 300–500 °C. The results show that flow stress is related to the Zener-Hollonom parameter Z and strain, as well as the static recrystallization fraction between passes during multipass hot deformation of 5182 aluminum alloy. Hence, a modified exponential flow stress equation was presented by considering the values of InA and β as functions of strain, and by using the uniform softening method and incorporating the static recrystallization fraction between passes to consider the effects of residual strain during multipass hot-rolling of 5182 aluminum alloy. The validity of the equation was examined by a typical non-isothermal multipass deformation test.

Interaction between Recrystallization and Precipitation during Multipass Rolling in a Low Carbon Niobium Microalloyed Steel.

The interaction between recrystallization and precipitation processes during multipass deformation has been analysed in terms of the non recrystallization temperature (T nr ). Multipass torsion tests performed at continuous cooling conditions have been used for thermomechanical simulation. The effect of the pass-strain and the initial austenite microstructure on the T nr value have been investigated. Conventionally reheated austenite (T soak = 1 200-1 250°C) and as-cast austenite, the latter simulated in the laboratory by reheating at very high temperatures (T soak = 1 400°C), have been considered. It was observed that at low pass-strains the retardation on recrystallization was mainly due to solute drag effects, while increasing the strain, precipitation induced by deformation was allowed to occur during the interpass interval leading to a drastic reduction of the softening reached between passes at temperatures below the T nr . The increment of the reheating temperature led to higher T nr values, being this effect related to higher austenite supersaturation levels prior to deformation.

Effect of Interpass Time and Cooling Rate on Apparent Activation Energy for Hot Working and Critical Recrystallization Temperature of Nb-microalloyed Steel.

Two-stage linear ln[sinh(ασ)] vs. 1/T relations indicative of double QHW behavior, are obtained from anisothermal multipass flow curves generated by testing Nb/Ti microalloyed steel in torsion (in the range 1250-800oC), except for the case of the high cooling rate-short interpass time tests, which gave rise to single-stage plots, indicative of single QHW behavior. Above the Tnr, the QHWU (apparent activation energy for hot working corresponding to the upper temperature range) is little affected by test variables (interpass time and cooling rate) and type of deformation (isothermal continuous and anisothermal multipass deformation give virtually equivalent QHWU, values above the Tnr). However, below the Tnr, the QHWL, corresponding to the lower temperature range, becomes sensitive to test variables, and may show considerable deviation from QHWL obtained in continuous tests. Therefore, the temperature dependence of the flow stress, below the Tnr, relevant to a multipass hot working operation, can be well described only by the QHWL obtained from the multipass flow curves, because it is interpass time dependent. The interpass time dependence of both the QHWL and the Tnr can be divided into three regions. Within the short interpass time region (1.8 to 10 sec), the two parameters show opposite trends, while beyond 10 sec they correlate well. Within the 1.8 to 10 sec region the Tnr is assumed to be controlled by Nb in solid solution, while the QHWL is controlled by both solute and precipitation effect. Beyond 10 sec precipitation is the only controlling mechanism. In addition to the Tnr derived from Mean Flow Stress (MFS) and ln[sinh(ασ)] vs. 1/T plots, the recrystallization limit, Trl, which correlates well with both Tnr's, and the recrystallization stop, Trs, temperatures are derived from % Fractional Softening (FS) vs. 1/T plots.

Combined Effect of Nb and Ti on the Recrystallisation Behaviour of Some HSLA Steels

The recrystallisation behaviour of several Nb-Ti bearing high-strength low-alloy steels was investigated during multipass deformation by torsion under continuous cooling conditions. The effect of the strain on the no-recrystallisation temperature (T nr ), the temperature at which recrystallisation is no longer complete, was determined. It was observed that the T nr decreases with increasing strain. It has been observed that for similar Nb concentrations, increasing the Ti content increases the T nr , but this effect seems to saturate at the higher Ti-concentrations and over a given Ti content there is no additional increase of the T nr . This effect can be related to the presence in the austenite of an increased amount of undissolved (Ti,Nb)(C,N) particles when the Ti/N ratio is increased by the addition of titanium. Because of this, the supersaturation level is reduced leading to a low driving force for precipitation during deformation. Additionally these undissolved particles may favour heterogeneous precipitation of carbonitrides and in consequence the precipitation on dislocations, which is the responsible for the retardation of recrystallisation, can be seriously impaired. Both effects can significantly reduce the effectiveness of the titanium addition in increasing the T nr .

Influence of multipass deformation onAr3for niobium bearing steel using torsion simulations

The continuous cooling transformation behaviour of a niobium microalloyed steel is studied by combining multipass deformation and continuous cooling torsion testing and by monitoring the variation of flow stress as afunction of temperature during cooling. Deformation above the non-recrystallisation temperature Tnr slightly increases the Ar3 temperature and so encourages the austenite to ferrite transformation. Further deformation below Tnr both enhances the Ar3 temperature and promotes the transformation significantly. Decreasing the interpass time and increasing the pass strain below Tnr increases the Ar3 temperature. A method is proposed to estimate the stored deformation energy from pass to pass as the deformation proceeds below Tnr based on the course of the mean flow stress curve and on the concept of shear deformation energy. The effect of the process variables, mainly the interpass time below Tnr on the stored deformation energy and consequently on the Ar3 temperature is discussed.MST/3198

Modelling of interaction between recrystallisation and precipitation during multipass rolling of niobium microalloyedsteels

The interaction between recrystallisation and precipitation during multipass deformation has been modelled by considering the non-recrystallisation temperature to be afunction of the process variables which include pass strain, strain rate, interpass time, pass temperature, and pass interval, etc. The method of applying the established isothermal models for recrystallisation and precipitation in modelling the non-recrystallisation temperature during a continuous hot rolling process is discussed in detail. Torsion tests have been used to verify the results of the modelling.MST/2090

Modelling the evolution of microstructure during hot torsion of niobium microalloyed steels

By considering the competition between recrystallization, precipitation, and interpass time during multi-pass deformation, a microstructural model is developed to simulate the microstructural evolution during multi-pass rolling based on the current isothermal equations for recrystallization, grain growth, and precipitation, in which the condition “t 0.05PS < t 0.95RX and t 0.05PS < interpass time” is suggested to replace “t 0.05PS < t 0.95RX” especially at short interpass time in modelling the T nr temperature for multi-pass deformation. The agreement between the austenite grain size obtained by hot torsion test and by calculation is satisfactory using this microstructural model for multi-pass deformation.

Measurement and modelling of the effects of precipitation on recrystallization under multipass deformation conditions

The effects of precipitation on austenite recrystallization during the multipass deformation of Nb-containing HSLA steels were investigated under continuous cooling conditions. For this purpose, the processing temperature at which recrystallization is no longer complete (the T nr) was first determined from rolling simulations. The changes in the size distributions of the freshly precipitated particles were then followed by transmission electron microscopy. Models were also developed for predicting the precipitation start and finish temperatures, T Ps and T Pf, during continuous cooling. The present results indicate that the recrystallization behavior of steels can be divided into three distinct regions: (i) when the interpass time t I is short and the cooling rate Ψ is rapid, the rate of recrystallization is solute-controlled and the T nr decreases when t I is increased or Ψ decreased; (ii) when t I and Ψ adopt intermediate values, precipitation is rate-limiting and the T nr increases with increasing t I or decreasing Ψ; (iii) finally, when t I is relatively long and Ψ relatively slow, particle coarsening weakens the effect of the precipitates so that the T nr tends to decrease again.

Effect of Deformation Parameters on the No-Recrystallization Temperature in Nb-Bearing Steels

The recrystallization behavior of three Nb-bearing high-strength low-alloy (HSLA) steels was investigated during multipass deformation under continuous cooling conditions. The niobium concentrations of these steels varied from 0.05 to 0.09 wt pct. The specimens were tested on a computerized torsion machine using a simulation schedule of 17 passes. Deformation tem-peratures of 1180 °C to 700 °C were employed, together with pass strains of 0.1 to 0.7, strain rates of 0.2 to 10 s-1, and interpass times of 5 to 200 seconds. By means of mean flow stressvs 1000/T diagrams, the effect of the deformation conditions on the no-recrystallization tem-perature (T nr ), the temperature at which recrystallization is no longer complete, was determined. It decreases with increasing strain and also decreases slightly with increasing strain rate. There is aT r minimum at times of about 12∼15 seconds, and both increases and decreases from this value raise this characteristic temperature. When the interpass times are short, solute atoms control the rate of recrystallization, the extent of which decreases as the time is decreased. When the interpass times are long, precipitation takes place and retards recrystallization, so that the extent of softening decreases.

Prediction of article quality with multipass deformation and intermediate heat treatment

Prediction of article quality with multipass deformation and intermediate heat treatment