Microstructural evolution during thermomechanical processing of a Ti-Nb interstitial-free steel just below the Ar 3 temperature

Laboratory thermomechanical processing (TMP) experiments have been carried out to study the austenite transformation characteristics, precipitation behavior, and recrystallization of deformed ferrite for an interstitial-free (IF) steel in the temperature range just below Ar ₃. For cooling rates in the range 0.1 °C s⁻¹ to 130 °C s⁻¹, austenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically with cooling rate and austenite condition. There is indirect evidence that the transformation rates for both PF and MF are decreased by the presence of substitutional solute atoms and precipitate particles. When unstable austenite is deformed at 850 °C, it transforms to an extremely fine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb) _x S _y precipitates form at 850 °C as coprecipitates on pre-existing (Ti,Nb)N particles and as discrete precipitates within PF grains. Pre-existing intragranular (Ti,Nb) _x S _y precipitates retard recrystallization and grain coarsening of PF deformed at 850 °C and result in a stable, recovered subgrain structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels.     

Precipitation and Recrystallization in Some Vanadium and Vanadium-Niobium Microalloyed Steels

Static precipitation and recrystallization following hot compression of austenite and the interactions between the two processes have been studied in a set of aluminum-killed HSLA steels containing 0.1 pct carbon, [0.016 - 0.026] pct nitrogen and 0.1 or 0.2 pct vanadium. Two steels containing both vanadium (0.1 and 0.2 pct) and niobium (0.03 pct) were included for purposes of comparison. The compression and the static tests were all carried out isothermally at temperatures between 800 and 900 °C. The course of recrystallization was followed by measurements of the rate of softening and by optical metallography of specimens quenched from the test temperature after different times. Precipitation was studied by measurements of the rate of hardening, by transmission electron microscopy of thin foils, carbon and aluminum extraction replicas, and by X-ray dispersion and energy-loss spectroscopy from individual precipitates. The temperature of the nose of theC-curve for precipitation in vanadium steels is much lower than that in niobium steels, as is the temperature, T_R, below which no recrystallization occurs in short times. Precipitation occurs both at austenite grain boundaries and in the grains (matrix precipitation). The former starts early and the precipitates grow rapidly to an approximately constant size; the matrix precipitates grow more slowly and are responsible for the observed hardening of the austenite. The relevance of various models proposed for the retardation and arrest of recrystallization of austenite are discussed. In the steels containing vanadium and niobium the precipitates contain both heavy elements: (V,Nb) (C,N). The Nb/V ratio in the matrix precipitates is different than in the parent austenite. The grain-boundary precipitates, however, contain the same Nb/V ratio as the parent austenite. The rate of hardening exhibits a reverseC-curve behavior, being more rapid than in the corresponding vanadium steels at higher temperatures and about the same at lower temperatures. Formerly Research Associate at MIT     

Austenite recrystallization and carbonitride precipitation in niobium microalloyed steels

The response of austenite to thermomechanical treatment is investigated in two series of niobium microalloyed steels. Optical and electron metallographic techniques were used to follow the austenite recrystallization and carbonitride precipitation reactions in these steels. The first series of steels contained a constant level of 0.05Nb, with carbon levels varying from 0.008 to 0.25 pct. It was found that a lower carbon concentration results in faster austenite recrystallization, due to a smaller carbonitride supersaturation, which leads to a reduced precipitate nucleation rate. The second series of steels was designed with a constant carbonitride supersaturation, by simultaneously varying the Nb and C concentrations while maintaining a constant solubility product. In these steels, the recrystallization kinetics increase as the volume fraction of Nb(C, N) is reduced and/or as the precipitate coarsening rate is increased. The volume fraction of carbonitrides increases as the Nb: (C+12/14 N) ratio approaches the stoichiometric ratio of approximately 8:1. The precipitate coarsening rate was shown to increase with increasing amounts of niobium remaining in solution in the austenite (i. e., “excess” Nb after precipitation). As expected, recrystallization proceeds more slowly at lower temperatures and after a reduced amount of deformation. An experiment to determine whether Nb atoms dissolved in the austenite could exert a significant solute-drag effect on the recrystallization reaction indicated that 0.20Nb in solution could reduce the rate of recrystallization compared to a Nb-free C-Mn steel. However, this solute effect was smaller than the retarding effect which 0.01Nb can have when it is precipitated in the form of carbonitrides on the austenite substructure after rolling.     

The non-equilibrium segregation of boron during the recrystalization of Nb-treated HSLA steels

The effect of boron segregation on recrystallization was investigated in two Nb and two Nb-free steels. The results indicate that the addition of B to a Nb steel retards the recrystallization of austenite after high temperature deformation to a significant degree. By contrast, the effect of B when added alone is not of commercial importance. At deformation temperatures above 950°C, the influence of boron is mainly due to grain boundary segregation and solute drag; below 900°C, the retardation results from the precipitation of Nb carbonitride. During isothermal holding, there are two types of non-equilibrium segregation on boundaries. Strain-induced segregation occurs on the original boundaries; it increases and attains a maximum rapidly, then decreases and disappears after 60 s of holding at 1000°C. A second type of non-equilibrium segregation occurs on newly formed boundaries during recrystallization. It develops quickly as the new boundaries are formed and begin to move, and can persist until grain impingement occurs and the motion is halted. The enhanced effect of B in the presence of Nb is attributed to the formation of (Nb, B) complexes, which appear to increase the solute drag force and decrease the boundary velocity.     

Dynamic Recrystallization and Precipitation in 13Cr Super-Martensitic Stainless Steels

The influence of precipitation on the kinetics of static and dynamic recrystallization (DRX) was investigated in AISI 403 and 403Nb martensitic stainless steels. Hot compression tests were performed in the temperature range of 1073 K to 1473 K (800 °C to 1200 °C) and strain rates of 0.001 and 0.1 s^?1 to study DRX and precipitation behaviors. In parallel, stress relaxation tests were conducted with pre-strains of 0.1, 0.15, 0.2, and 0.25, a strain rate of 0.1 s^?1, and in the 1073 K to 1473 K (800 °C to 1200 °C) temperature range to study the kinetics of precipitation and recrystallization. Samples of hot compression and stress relaxation tests were quenched and the evolution of the microstructure was examined using optical and scanning electron microscopy. The results indicated that DRX interacts with dynamic precipitation (DP) over the temperature range of 1173 K to 1273 K (900 °C to 1000 °C). Hot compression testing results, confirmed by EBSD analysis, indicated that partial DRX occurs before precipitation in 403Nb, at 1073 K (800 °C). By contrast, no DRX was observed in 403 steel. At higher temperatures, i.e., over 1273 K (1000 °C), DRX preceded DP in both steels. Increasing the strain rate raised the temperature range of interaction between DRX and DP up to 1373 K (1100 °C). Strain-induced precipitation (SIP) was observed over the entire range of investigated test temperatures. Static recrystallization (SRX) took place predominantly in the temperature range of 1173 K to 1373 K (900 °C to 1100 °C), at which SIP significantly delayed the SRX finishing time. The results are analyzed in the framework of the classical nucleation theory and the underlying mechanisms are identified.     

Recrystallization kinetics of microalloyed steels deformed in the intercritical region

A plain carbon and two microalloyed steels were tested under interrupted loading conditions. The base steel contained 0.06 pct C and 1.31 pct Mn, and the other alloys contained single additions of 0.29 pct Mo and 0.04 pct Nb. Double-hit compression tests were performed on cylindrical specimens of the three steels at 820 °C, 780 °C, and 740 °C within the α + γ field. A’softening curve was determined at each temperature by the offset method. In parallel, the progress of ferrite recrystallization was followed on quenched specimens of the three steels by means of quantitative metallography. It was observed that, in the base steel, a recrystallizes more slowly thany. The addition of Mo retards recrystallization and has a greater influence on γ than on α recrystallization. This effect is in agreement with calculations based on the Cahn theory of solute drag. Niobium addition has an even greater effect on the recrystallization of the two phases. In this steel, the recrystallization of ferrite was incomplete at the three intercritical temperatures. Furthermore, the austenite remained completely unrecrystallized up to the maximum time involved in the experiments (1 hour). The metallographic results indicate that the nucleation of recrystallization occurs heterogeneously in the microstructure, the interface between ferrite and austenite being the preferred site for nucleation.     

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.     

Recrystallization, Precipitation Behaviors, and Refinement of Austenite Grains in High Mn, High Nb Steel

Through a series of experiments conducted on three kinds of high Mn steels with different Nb content, including stress relaxation tests, physical metallurgical modeling, and observation of prior austenite grains and precipitates, the effect of Nb on recrystallization and precipitation behaviors were investigated. The results indicate the existence of a novel deformation temperature range for grain refinement resulting from complete static recrystallization (SRX) in high Mn, high Nb steel, whereas slow SRX kinetics can be accelerated by a finer initial grain size. In this deformation temperature range, the effect of precipitation is too weak to prohibit SRX nucleation efficiently, but solute drag is still large enough to slow down growth rate. As a consequence, shorter incubation and homogeneous recrystallized nucleation can be realized at relative low temperature, and the coarsening rate of grains is much slower because of the high solute drag effect in the rolling of low C high Mn, high Nb line pipe steel.     

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.     

Transformation during the isothermal deformation of low-carbon Nb-B steels

The transformation behavior during isothermal deformation of four steels containing different microalloying additions was investigated by means of the “strain-rate change” technique. The flow curves obtained at temperatures ranging from 620 °C to 850 °C, and the associated microstructures, indicate that the transformation in the Mo-Nb-B and Mo-B steels is of the austenite-to-bainite type. Here, dramatic increases in flow stress are observed at lower temperatures. By contrast, the transformation in the Nb-15B and Nb-64B steels is basically of the austenite-to-ferrite type; in these two grades, the flow stress increases observed are attributable to strengthening by NbC precipitation. Large intergranular and intragranular Fe₂₃(C,B)₆ particles were found in the Nb-64B steel samples deformed to ɛ=0.1 after holding for 60 seconds at 800°C. These large precipitates are considered to be responsible for accelerating the transformation in the Nb-64B steel by reducing the concentration of boron atoms available for boundary segregation and by acting as nucleation sites for the formation of polygonal ferrite. The flow curves of the Mo-Nb-B steel exhibit distinct serrations, indicating that a displacive mechanism is involved in the γ-to-B transformation.     

Influence of boron on the decomposition of austenite in low carbon alloyed steels

The influence of boron on the isothermal decomposition of Fe-Ni₆-C_0.12 (wt pct) steels has been investigated. The isothermal γ→ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C_0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history. Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied. Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates were identified as Fe₂₃(B, C)₆ (FCC structure with a ≈ 10.6Å). The grain boundary Fe₂₃(B, C)₆ were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe₂₃(B, C)₆ precipitates with respect to the γ → proeutectoid ferrite reaction is discussed.     

Interactions between recrystallization and precipitation in hot-deformed microalloyed steels

The static recrystallization and precipitation characteristics of low alloy steels containing Nb were studied following high temperature compression in the austenite range. The recrystallization kinetics in the Nb-steels were slower than those in the plain carbon steels by approximately an order of magnitude when Nb was in solution. However, much greater retardation of recrystallization was observed in the Nb-steels when precipitates were formed. The initiation of both recrystallization and precipitation appeared to be localized in the early stage of their nucleation. Recrystallization initiated predominantely at the prior austenite grain boundaries. Boundaries were also the preferential nucleation sites for NbCN precipitation. Hence, the NbCN particles were distributed in a highly localized fashion, sometimes delineating what may have been the prior austenite grain or subgrain boundaries. The heterogeneous nature of the nucleation process for recrystallization and precipitation suggested that the critical factor which determined the retardation of recrystallization was the local pinning effect of precipitation. The local pinning force was estimated from the local distribution of precipitation and was found to be of magnitude comparable to that of the driving force for recrystallization. The precipitation pinning force increased in the beginning of precipitation, showed a peak in the intermediate stage, and finally decreased as particles coarsened and were distributed more uniformly.     

The effect of coiling temperature and continuous annealing on the properties of bake hardenable IF steels

Five bake hardenable IF steels were investigated. The transformation and recrystallization behaviour, the precipitation and the mechanical properties were studied. The coiling temperature was varied between 580 and 720°C. Soaking was carried out in a temperature range from 700 to 900°C and the soaking time ranged from 60 to 240 s. It was found that boron significantly retards transformation and recrystallization. The bake hardening is increased but deep drawability is lowered by boron additions. In the Ti-containing steels, TiS, Ti₄C₂S₂ and MnS precipitate competitively during the hot rolling process after reheating to 1250°C. To obtain a reasonable bake hardening effect, the amount of carbide forming elements should be substoichiometrical related to C.     

Influence of boron on the decomposition of austenite in low carbon alloyed steels

The influence of boron on the isothermal decomposition of Fe-Ni₆-C_0.12 (wt pct) steels has been investigated. The isothermal γ→ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C_0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history. Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied. Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates were identified as Fe₂₃(B, C)₆ (FCC structure with a ≈ 10.6?). The grain boundary Fe₂₃(B, C)₆ were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe₂₃(B, C)₆ precipitates with respect to the γ → proeutectoid ferrite reaction is discussed.     

A stress relaxation method for following carbonitride precipitation in austenite at hot working temperatures

Stress relaxation measurements were carried out on a plain carbon and four solution-treated Ti steels over the temperature range 850 to 1050 °C. The results show that the stress relaxation of plain carbon austenite after a 5 pct prestrain (i.e., in the absence of precipitation) can be described by the relation σ = σ₀ -α ln(l + βt). By contrast, in the solution-treated Ti steels, relaxation is arrested at the start of precipitation and is resumed when precipitation is completed. As a result, this mechanical method is particularly suitable for following carbonitride precipitation in microalloyed austenite at hot working temperatures. A new model regarding the effect of precipitation on dislocation motion is proposed, on the basis of which, the phenomenology of the stress relaxation technique is clarified. Precipitation-time-temperature (PTT) diagrams were determined for the Ti bearing steels containing 0.05, 0.11, 0.18, and 0.25 pct Ti. The PTT curves obtained are C-shaped for all the steels. The upper parts of these curves are shifted to significantly longer times as the Ti and C concentrations are reduced. By contrast, the positions of the lower arms of the curves are relatively independent of the compositions of the steels tested.     

Continuous-cooling-precipitation kinetics of Nb(CN) in high-strength low-alloy steels

Although isothermal precipitation has been frequently studied with respect to industrial hot deformation processing, the temperature decreases continuously under these conditions so that isothermal data cannot be applied directly to predict the precipitation kinetics. This study therefore was concerned with the continuous-cooling-precipitation (CCP) behavior of Nb carbonitride in austenite. In the present work, the Liu-Jonas (L-J) model was used to calculate the precipitation start (P _s) time at a given temperature from experimental data. A new calculation method for predicting the precipitation finish (P _f) time, based on reaction kinetics and classical nucleation and growth theory, was also developed. The additivity rule was then used to calculate the extent of precipitation during continuous cooling. Isothermal precipitation rates for 0.04 pct Nb steels were measured experimentally by the stress relaxation method. The CCP behavior was then calculated from the model, and the accuracy of the predictions was evaluated by carrying out continuous-cooling tests using a deformation dilatometer. The precipitate size distributions were determined by the transmission electron microscopy of specimens quenched after increasing intervals of cooling at various cooling rates. TheP _s andP _f times estimated from the particle size data show good agreement with the calculated CCP behavior.     

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.     

Niobium carbonitride precipitation and austenite recrystallization in hot-rolled microalloyed steels

The response of austenites to thermomechanical treatments is studied in a series of niobium (columbium) HSLA steels. Interactions between composition, plastic deformation, strain-induced precipitation, and austenite recrystallization are described and related to previous work in the field. Niobium in solution prior to deformation leads to significant retardation of subsequent austenite recrystallization if Nb(C,N) precipitation takes place prior to or during the early stages of recrystallization. Such straininduced precipitation proceeds in two stages: initially at austenitic grain boundaries and deformation bands, and later on substructural features in the unrecrystallized austenite. The latter precipitation is accelerated only if it occurs in the unrecrystallized austenite; if recrystallization precedes Nb(C,N) precipitation, then the precipitation reaction is much slower. Thus, the Nb(C,N) precipitation and austenite recrystallization reactions are coupled phenomena. The conditions necessary for such an interaction are analyzed, and it is proposed that the level of supersaturation of Nb(C,N) in the austenite at the deformation temperature is a critical factor in determining whether or not an effective interaction will operate at that temperature. This paper is based on a presentation made at a symposium on “Precipitation Processes in Structural Steels” held at the annual meeting of the AIME, Denver, Colorado, February 27 to 28, 1978, under the sponsorship of the Ferrous Metallurgy Committee of The Metallurgical Society of AIME.     

Carbide precipitation and high-temperature strength of hot-rolled high-strength,low-alloy steels containing Nb and Mo

The effects of a Mo addition on both the precipitation kinetics and high-temperature strength of a Nb carbide have been investigated in the hot-rolled high-strength, low-alloy (HSLA) steels containing both Nb and Mo. These steels were fabricated by four-pass hot rolling and coiling at 650°C, 600°C, and 550°C. Microstructural analysis of the carbides has been performed using field-emission gun transmission electron microscopy (TEM) employing energy-dispersive X-ray spectroscopy (EDS). The steels containing both Nb and Mo exhibited a higher strength at high temperatures (∼600 °C) in comparison to the steel containing only Nb. The addition of Mo increased the hardenability and led to the refinement of the bainitic microstructure. The proportion of the bainitic phase increased with the increase of Mo content. The TEM observations revealed that the steels containing both Nb and Mo exhibited fine (<10 nm) and uniformly distributed metal carbide (MC)-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb only. The EDS analysis also indicated that the fine MC carbides contain both Nb and Mo, and the ratio of Mo/Nb was higher in the finer carbides. In addition, electron diffraction analysis revealed that most of the MC carbides had one variant of the B-N relationship ((100)_MC//(100)_ferrite, [011]_MC//[010]_ferrite) with the matrix, suggesting that they were formed in the ferrite region. That is, the addition of Mo increased the nucleation sites of MC carbides in addition to the bainitic transformation, which resulted in finer and denser MC carbides. It is, thus, believed that the enhanced high-temperature strength of the steels containing both Nb and Mo was attributed to both bainitic transformation hardening and the precipitation hardening caused by uniform distribution of fine MC particles.     

Grain-boundary segregation and precipitation of boron in 0.2 percent carbon steels

The segregation and precipitation of boron have been studied in two 0.2C−0.6Mn0.−5Mo steels containing (nominally) 10 and 50 ppm B. After heating to 1260 δC, samples were air-cooled to 870 °C and then held for times between 0 and 5000 seconds. Additional samples were heated to 1260 °C, air-cooled to 900 °C, reduced in thickness by 50 pct, and then held for various times at 870 °C, as above. The distributions of boron under these various conditions were investigated qualitatively by an autoradiographic technique. In both steels, segregation and/or precipitation of boron at austenite grain boundaries was detected under all conditions examined. Precipitation of iron borocarbide particles occurred along austenite grain boundaries in the 50 ppm B steel during cooling to the holding temperature of 870 °C, while in the 10 ppm B steel, such precipitation occurred only after long times at 870 °C. Mechanical properties of single-pass-rolled samples were measured after tempering to assess the effects of borocarbide precipitation on notch toughness. Such precipitation lowered the Charpy upper shelf energy and increased the transition temperature.