Effect of prestrain and deformation temperature on the recrystallization behavior of steels microalloyed with niobium

The evolution of microstructure during the hot working of steels microalloyed with Nb is governed by the recrystallization kinetics of austenite and the recrystallization-precipitation interaction. The present study focuses on the effects of prestrain and deformation temperature on the rectrystallization behavior in these steels. The extent of recrystallization is characterized by a softening parameter calculated from a series of interrupted plane strain compression tests carried out at different deformation temperatures and strain levels. The results indicate that at low temperatures, softening is caused by static recovery, while at higher temperatures, static recrystallization is the predominant mechanism. The recrystallization-stop temperature (T _5pct) and the recrystallization-limit temperature (T _95pct), marking the beginning and end of recrystallization, respectively, are determined as a function of strain. In order to achieve a homogeneous microstructure, finish rolling should be carried out outside the window of partial recrystallization (T _5pct<T<T _95pct), as determined in this study. The Nb(CN) precipitation kinetics have been calculated using a model proposed in an earlier work, and these results are used to estimate the precipitate pinning force under the given processing conditions. Based on these estimations, a criterion has been proposed to predict the onset of recrystallization. The predicted results are found to be in reasonably good agreement with the experimental measurements.     

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 alloying elements on metadynamic recrystallization in HSLA steels

By means of interrupted torsion tests, the kinetics of metadynamic recrystallization (MDRX) were studied in a Mo, a Nb, and a Ti microalloyed steel at temperatures ranging from 850 °C to 1000 °C and strain rates from 0.02 to 2 s¹. Quenches were also performed after full MDRX. In contrast to the case of static recrystallization (SRX), the kinetics of MDRX are shown to be highly sensitive to a change of an order of magnitude in strain rate and are relatively insensitive to temperature changes within the range of values applicable to industrial hot-rolling practice. A similar algebraic dependence of the MDRX grain size on strain rate and temperature was found in the three steels. The kinetics of MDRX were slower in the Nb than in the Mo steel, and those of the Ti steel were slower than in the Nb and Mo steels. Above 900 °C and 950 °C, the retardation of MDRX in the Nb and Ti steels, respectively, is due to solute drag. Models predicting the start time for Nb and Ti carbonitride precipitation showed that MDRX is delayed below these temperatures by this mechanism. Comparison of the MDRX and precipitation start times in the Nb steel indicated that a temperature of “no-MDRX” could not be defined, in contrast to the well-definedT _nr (no recrystallization temperature) of SRX. By means of torsion simulations composed of multiple interruptions, it is shown that MDRX is retarded decreasingly as the accumulated strain is increased. This appears to be due to the promotion of precipitate coarsening by the continuing deformation.     

Structure forming processes during hot deformation of a C-Mn-V-N steel

Medium carbon V-N treated steel grade applied for long products and small drop forged parts investigated in a state of incomplete solution of V(C,N) precipitates. Activation energy changes investigated in the 850–1200°C temperature range and 0.6–30 s^?1 strain rate range is revealed to be temperature dependent. Calculated for σ_p flow stress value this energy in some cases expresses a combination of various processes. It is found that DRX starts well below ?_p. Softening during a short interpass time is caused by dynamic recrystallization DRX during forming and is followed by metadynamic recrystallization MDRX and static recrystallization SRX. At longer interpass times softening progress is hindered by precipitation.     

Effect of test variables on apparent activation energy for hot working and critical recrystallization temperatures of V-microalloyed steel

The effect of test variables on the apparent activation energy for hot working of a medium carbon V-microalloyed steel, Q_HW, and the critical recrystallization temperature, T_nr, have been studied by means of isothermal continuous and anisothermal multipass torsion test. The Q_HW is found to be temperature dependent. Above the T_nr, the Q_HW^U is close to that of austenite self diffusion, Q_SD, irrespective of the type of test or test variables. Below the T_nr, the Q_HW^L is not only considerably higher, but that based on multipass flow curve exibits a strong interpass time dependence. With increasing interpass time from 1.8 to 10 s the Q_HW^L and the T_nr increase and decrease respectively. The former is assumed to be controlled by solute drag, while the latter both by solute drag and precipitation pinning. Beyond 10 s, the two parameters exhibit the same trend, and the precipitation pinning is assumed to be the only recrystallization inhibiting mechanism.     

Hot Deformation Behavior of As-Cast 2101 Grade Lean Duplex Stainless Steel and the Associated Changes in Microstructure and Crystallographic Texture

The hot deformation behavior of 2101 grade lean duplex stainless steel (DSS, containing ~5 wt pct Mn, ~0.2 wt pct N, and ~1.4 wt pct Ni) and associated microstructural changes within δ-ferrite and austenite (γ) phases were investigated by hot-compression testing in a GLEEBLE 3500 simulator over a range of deformation temperatures, T _def [1073 K to 1373 K (800 °C to 1100 °C)], and applied strains, ε (0.25 to 0.80), at a constant true strain rate of 1/s. The microstructural softening inside γ was dictated by discontinuous dynamic recrystallization (DDRX) at a higher T _def [1273 K to 1373 K (1000 °C to 1100 °C)], while the same was dictated by continuous dynamic recrystallization (CDRX) at a lower T _def (1173 K (900 °C)]. Dynamic recovery (DRV) and CDRX dominated the softening inside δ-ferrite at T _def ≥ 1173 K (900 °C). The dynamic recrystallization (DRX) inside δ and γ could not take place upon deformation at 1073 K (800 °C). The average flow stress level increased 2 to 3 times as the T _def dropped from 1273 to 1173 K (1000 °C to 900 °C) and finally to 1073 K (800 °C). The average microhardness values taken from δ-ferrite and γ regions of the deformed samples showed a different trend. At T _def of 1373 K (1100 °C), microhardness decreased with the increase in strain, while at T _def of 1173 K (900 °C), microhardness increased with the increase in strain. The microstructural changes and hardness variation within individual phases of hot-deformed samples are explained in view of the chemical composition of the steel and deformation parameters (T _def and ε).

     

Hot workability of M2 type high-speed steel

The hot workability of an M2 type HSS in cast, forged and rolled condition has been studied by means of a torsion test and metallography. Continuous tests were used to determine the temperature and strain-rate dependence of the flow stress and temperature dependence of the strain to fracture, while multistage tests were used to determine the extent of interpass softening. Temperature and strain dependence of the flow stress is described by a relation of the form: in which the activation energy for hot working (Q_HW) is found to be temperature dependent. The hot ductility of the cast steel is not only lower but is also little affected by temperature in comparison to worked steels, in spite of the fact that critical strain for dynamic recrystallization is smaller, and the extent of metadynamic and static recrystallization during interpass interval is more extensive in the former. This is related to a large volume fraction of carbides, which give rise to a high stored energy and enhanced recrystallization on the one hand, and to the suppression of recrystallization within the continuous network of carbides on the other hand. Carbides give rise to an easy nucleation, growth and coalescence of cracks.     

Tensile failure behavior of plain carbon steels at elevated temperatures

The onset of tensile instability and the occurrence of fracture in plain carbon steels containing up to 1.89C has been examined in the temperature range 500 to 1300 °C and the strain-rate range 6 X lO^-6 to 2 × 10⁻² s⁻¹. In the ferrite-plus-pearlite mixtures at temperatures below the eutectoid temperature, the work-hardening exponent decreases with increasing amount of pearlite, and there is a corresponding decrease in the Considére strain. However, the onset of necking is delayed to well beyond the Considére strain, and these mixtures are inherently ductile even at the eutectoid composition. In the austenite region, the general intrusion of dynamic recrystallization compctes with intergranular embrittlement at temperatures below about 1050 °C. The embrittlement is related to precipitation which takes place either during cooling (MnS) or at the deformation temperature [AIN, Nb (CN),etc.]. In hypereutectoid steels, the ductility of austenite-plus-cementite and pearlite-plus-cementite mixtures diminishes drastically with decreasing temperature and increasing amount of cementite. The areas of possible fracture modes are mapped in temperature-strain rate and temperature-carbon content space.     

Onset of recrystallization during the tensile deformation of austenitic iron at intermediate strain rates

The onset of recrystallization during the tensile deformation of austenitic iron has been fully documented for the temperature range 950 to 1350°C and strain-rate range 2.8 x 10^-5 to 2.3 x 10^-2 s^-1. Representative materials are zone-refined iron, electrolytic iron, Fe-0.05 C and Fe-5.2 Mn. In general, the strain at the onset of recrystallization decreases with increasing temperature of deformation and decreasing strain rate. The postponement of recrystallization is favored by prior annealing at temperatures above 1200°C and is greatest for the Fe-5.2 Mn alloy; however, for the range of strain rates used, it is difficult to completely eliminate recrystallization. The effects of test conditions on the onset of recrystallization are discussed in terms of a nucleation process that requires a critical amount of stored energy.     

Determination of residual stress and critical rolling temperatures in a microalloyed steel with low carbon and niobium contents

Using torsion tests, residual stress (Δσ) and critical rolling temperatures (T_nr, A_r3, A_r1) have been determined for a low Nb content microalloyed steel by means of simulation of rolling cycles and subsequent representation of mean flow stress versus the inverse of the temperature. The above magnitudes were determined as a function of interpass time for two strains applied in each pass (0.20, 0.35), respectively. Among the results found, it is notable that Δσ decreases with longer interpass times until it reaches zero, and is greater the smaller the strain applied. With regard to the cooling transformation temperatures A_r3 and A_r1, these were found to be practically independent of the interpass time and were higher for smaller applied strains. Temperatures A_r3 and A_r1 were also determined by dilatometry, and comparison of these values showed that both methods yield similar results, except in the value of A_r1.     

High-temperature deformation properties of austenitic Fe-Mn alloys

The influence of the Mn content on the hot deformation properties of austenitic binary Fe-Mn alloys containing 1 to 20 mass pct Mn has been investigated for the first time. The influence of the Mn content on the constitutive equations was determined for the temperature range of 950 °C to 1250 °C and the strain rate range of 0.1 to 2 s⁻¹. The activation energy for hot working increased with increasing Mn content, and dynamic recrystallization was observed for all the Fe-Mn binary alloys. The Mn was found to delay dynamic recrystallization. An increase in the Mn content resulted in an increase of both the peak stress σ _p and the corresponding peak strain ε _p , thus revealing the pronounced influence of Mn on the hot deformation process.     

Onset of recrystallization during the tensile deformation of austenitic iron at intermediate strain rates

The onset of recrystallization during the tensile deformation of austenitic iron has been fully documented for the temperature range 950 to 1350°C and strain-rate range 2.8 × 10^-5 to 2.3 × 10^-2 s^-1. Representative materials are zone-refined iron, electrolytic iron, Fe−0.05 C and Fe−5.2 Mn. In general, the strain at the onset of recrystallization decreases with increasing temperature of deformation and decreasing strain rate. The postponement of recrystallization is favored by prior annealing at temperatures above 1200°C and is greatest for the Fe−5.2 Mn alloy; however, for the range of strain rates used, it is difficult to completely eliminate recrystallization. The effects of test conditions on the onset of recrystallization are discussed in terms of a nucleation process that requires a critical amount of stored energy.     

Hot workability of three grades of tool steel

Three tool steels, a cold-work air-hardening grade, a hot-work die grade, and a high-speed type, were deformed by torsion in the range of 900 to 1100 °C at rates of 0.1 to 5 s^•1. In a series of continuous deformation tests the flow stress and ductility were determined. The exponent of the flow stress was proportional to the strain rate and to the temperature in a reciprocal Arrhenius relationship. In general the flow stress for a given deformation condition, the activation energy, and the strain for the start of dynamic recrystallization increased for the steels in the order listed above; however, the ductility of the hot-work grade is superior to the other two grades. Multistage tests were carried out on each steel to determine its softening behavior during intervals between passes. Each test was carried out under isothermal conditions with constant strain rate, pass strain, and interval duration. Softening occurred by both recovery and recrystallization with the amount increasing with temperature, strain rate, pass strain, and accumulated strain. The first two steels were similar in behavior having extensive softening at 1000 °C, whereas the high-speed steel experienced considerably less softening.     

Plastic deformation of quenched and tempered 52100 bearing steel in compression

The compressive flow stress and rate of work hardening of quenched and tempered AISI 52100 steel were measured for a variety of heat treatments. Both the flow stress and the work hardening index,n, increase with decreasing tempering temperature. Flow stresses increase initially with increasing austenitizing temperature,T _a, then decrease with a further increase inT _a as the amount of retained austenite increases.n tends to increase asT _a increases. In specimens temperared to eliminate retained austenite,n decreases to near zero as the strain increases. This behavior appears to be characteristic of tempered martensite. When less than 10 pct retained austenite is present,n still decreases with increasing strain, but witn n ore than about 15 pct retained austenite,n increases with strain. Heat treatments which refine the primary carbides increase the flow stress forT _a≤840°C. Since fine primary carbides lead to more retained austenite at a givenT _a, n tends to be greater when primary carbides are refined. For one heat treatment, the retained austenite content was measured by an X-ray method as a function of plastic strain. From changes in the relative intensities of austenite reflections, it was found that austenite crystals most favorably oriented for deformation in compression transform most readily to martensite on straining.     

Prediction of steel flow stresses at high temperatures and strain rates

The flow behavior of steels during deformation in the roll gap was simulated by means of single hit compression tests performed in the temperature range 800 °C to 1200 °C. Strain rates of 0.2 to 50 s⁻¹ were employed on selected low-carbon steels containing various combinations of niobium, boron, and copper. The stress/strain curves determined at the higher strain rates were corrected for deformation heating so that constitutive equations pertaining to idealized isothermal conditions could be formulated. When dynamic recovery is the only softening mechanism, these involve a rate equation, consisting of a hyperbolic sine law, and an evolution equation with one internal variable, the latter being the dislocation density. When dynamic recrystallization takes place, the incorporation of the fractional softening by dynamic recrystallization in the evolution equation makes it possible to predict the flow stress after the peak. These expressions can be employed in computer models for on-line gage control during hot-rolling.     

The strain dependence of postdynamic recrystallization in 304 H stainless steel

Using double-hit hot compression tests, the softening behavior of 304 H stainless steel was studied during unloading. The prestrains used were associated with the initiation of dynamic recrystallization (DRX) (ε _c), the peak strain (ε _p), 1/2 (ε _c+ε _p), the strain at maximum softening rate (ε _i), and the onset of steady state flow (ε _s). The following conditions of deformation were used: T=1000 °C, 1050 °C, and 1100 °C, =0.01 and 0.1 s⁻¹, and delay times of 0.3 to 1000 seconds. To define the above important strains, single-hit hot compression tests were performed over a wider range of deformation conditions than the double-hit ones—i.e., 900 °C to 1100 °C and =0.01 to 1 s⁻¹. The results show that a transition strain (ε*) separates the strain-dependent range of postdynamic softening from the strain-independent range. At strains between ε _c and ε*, both metadynamic and static recrystallization contribute to interhit softening. The value of ε* obtained in this work was ε*=4/3 ε _p. It was also found that the strain hardening rate was identical at all the critical strains (ε*) and took the value −22 MPa.     

The influence of niobium supersaturation in austenite on the static recrystallization behavior of low carbon microalloyed steels

This work describes the effect of Nb supersaturation in austenite, as it applies to the strain-induced precipitation potential of Nb(CN), on the suppression of the static recrystallization of austenite during an isothermal holding period following deformation. Four low carbon steels, microalloyed with Nb, were used in this investigation. Three of the steels had variations in Nb levels at constant C and N concentrations. Two steels had different N levels at constant C and Nb concentrations. The results from the isothermal deformation experiments and the subsequent measurement of the solution behavior of Nb in austenite show that the recrystallization-stop temperature (T _RXN) increases with increasing Nb supersaturation in austenite. Quantitative transmission electron microscopy analysis revealed that the volume fraction of Nb(CN) at austenite grain boundaries or subgrain boundaries was 1.5 to 2 times larger than Nb(CN) volume fractions found within the grain interiors. This high, localized volume fraction of Nb(CN) subsequently led to high values for the precipitate pinning force (F _PIN). These values forF _PIN were much higher than what would have been predicted from equilibrium thermodynamics describing the solution behavior of Nb in austenite.     

Influence of strain rate on interpass softening

Most laboratory simulations of hot rolling involve a scaling down of the strain rate from the much higher industrial levels. This leads to slower softening between each rolling pass, for which corrections must be made. In the present work, torsion testing simulations of “warm” rod rolling were conducted on a Ti-Nb interstitial-free (IF) steel at 840 °C and 770 °C(i.e., in the ferrite range). For this purpose, “strain rate corrected” interpass times were used, in addition to the more familiar corrections for the stress. The results are compared with those obtained from simulations using uncorrected industrial interpass times. At 840 °C, simulations using corrected interpass times led to high levels of softening between the stages of rolling, thus triggering the reinitiation of cycles of dynamic recrystallization. The initially high stress level at the start of these cycles was responsible for the large differences in the pass-to-pass mean flow stress behavior, compared with that observed when using uncorrected industrial interpass times, or continuous deformations. The differences were much less pronounced at 770 °C, where the rate of softening is much slower than at 840 °C. Predictions for softening based on the Avrami equation underestimated the softening observed using the continuous and uncorrected industrial interpass time schedules and overestimated it for the corrected ones. The former is due to the occurrence of recovery, which is not addressed by the Avrami relation, while the latter is due to the precipitation that takes place during the corrected (longer) interpass times. It was also found that simulations using continuous deformations are applicable only if the interpass softening that would be expected using the corrected interpass times does not exceed about 20 pct.     

Evaluation of the Austenite Recrystallization by Multideformation and Double Deformation Tests

A high amount of deformation below the non‐recrystallization temperature (T_nr) is a common industrial practice to achieve a good combination of toughness and strength in microalloyed steels. To combine the industrially relevant optimum combination of high productivity and product quality, an accurate knowledge of T_nr and the recrystallization kinetics is required. Although a lot of literature data is available on the recrystallization behaviour of microalloyed steels, correlations are often difficult to be made due to the effect of different experimental set‐ups and test schedules used to obtain this data. Although it would significantly improve the knowledge about these steels, so far, no systematic comparison has been presented in literature to correlate the different techniques one to another. In this study, different hot rolling simulation techniques and testing schedules were compared, within the experimental constraints of the used equipment, to determine the T_nr temperature of two microalloyed steels. Good agreement was found between the results from different test equipment. Furthermore, the results from the multideformation tests under continuous cooling conditions could be correlated with the results from isothermal double deformation tests.     

Effect of molybdenum,niobium, and vanadium on static recovery and recrystallization and on solute strengthening in microalloyed steels

The effect of molybdenum, niobium, and vanadium on the occurrence of static recovery and recrystallization after high temperature deformation was investigated in a series of microalloyed steels. The steels had a base composition of 0.05 pct C and 1.40 pct Mn. To this, single additions of 0.30 pct Mo, 0.035 pct Nb, and 0.115 pct V were made. Interrupted hot compression tests were performed at 900 and 1000 °C, and at a constant true strain rate of 2 s^-1. The load-free time was decreased from 5000 s to 50 ms, and the degree of static softening during this period was determined. Both graphite and glass were used as lubricants. Percent softeningvs delay time curves are presented and the retarding effect of molybdenum, niobium, and vanadium addition on the rate of static recovery and recrystallization is discussed. The greatest solute retardation of static recovery and recrystallization is produced by niobium addition, followed by that of molybdenum, vanadium leading to the smallest delay. Although the rank order of this effect is the same as found under dynamic softening conditions, the relative contribution of niobium is more profound for the static condition. The solute strengthening attributable to each element was also assessed, and found to follow the same order as for the recovery and recrystallization results. At 900 °C, the onset of the static precipitation of Nb (CN) was detected at approximately 10 seconds, somewhat earlier than previously reported. Formerly Graduate Student at McGill University, Montreal, Quebec.