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Strain‒induced precipitation is a characteristic physical‒metallurgical phenomenon during hot‒rolling in microalloyed steel production that strongly affects the overallthermomechanical control process. In this study, the strain‒induced precipitation behavior in titanium‒molybdenum microalloyed steel was comprehensively investigated, and its complex effects on the austenite/ferrite transformation during continuous cooling were analyzed for the first time, based on stress relaxation and multi‒aspect characterization methods. The stress relaxation results revealed that the fastest strain‒induced precipitation occurred at 900 °C. The precipitates were identified as FCC structured (Ti, Mo)C particles with a coherent or semi‒coherent cubic‒cubic orientation relationship to the austenite matrix. The strain‒induced precipitation proved to increase the ferrite transformation temperature and proportion, significantly refine and homogenize the transformed grains. The intermittent quenching at 0.5 C/s further revealed that the (Ti, Mo)C particles with cubic‒cubic orientation relationship to austenite matrix exerted a dual pinning effect: by pinning dislocations, these particles facilitated diffusion‒controlled ferrite nucleation and growth both at austenite grain boundaries and within grains; by pinning migrating phase interfaces, the particles inhibited the coarsening of ferrite grains. Coupled with compressive testing and strengthening contribution analysis, the strain‒induced precipitation was shown to weaken precipitation strengthening but enhance grain refinement strengthening, thereby providing a novel approach to achieving an optimal balance between microstructural homogeneity and mechanical properties.

Abstract This study explores the hot deformation behavior of Al–Zn–Mg–Cu alloy through uniaxial hot compression (200 °C–450°C) using the Gleeble-1500. True stress–strain curves were corrected, and three models were established: the Arrhenius model, strain compensated (SC) Arrhenius model, and strain compensated recrystallization temperature (RT) segmentation-based (TS-SC) Arrhenius model. Comparative analysis revealed the limited predictive accuracy of the SC Arrhenius model, with a 25.12% average absolute relative error (AARE), while the TS-SC Arrhenius model exhibited a significantly improved to 9.901% AARE. Material parameter calculations displayed variations across the temperature range. The SC Arrhenius model, utilizing an average slope method for parameter computation, failed to consider temperature-induced disparities, limiting its predictive capability. Hot processing map, utilizing the Murty improved Dynamic Materials Model (DMM), indicated optimal conditions for stable forming of the Al–Zn–Mg–Cu alloy. Microstructural analysis revealed MgZn2 precipitation induced by hot deformation, with crystallographic defects enhancing nucleation rates and precipitate refinement.

This study explores the hot deformation behavior of Al–Zn–Mg–Cu alloy through uniaxial hot compression (200 °C–450°C) using the Gleeble-1500. True stress–strain curves were corrected, and three models were established: the Arrhenius model, strain compensated (SC) Arrhenius model, and strain compensated recrystallization temperature (RT) segmentation-based (TS-SC) Arrhenius model. Comparative analysis revealed the limited predictive accuracy of the SC Arrhenius model, with a 25.12% average absolute relative error (AARE), while the TS-SC Arrhenius model exhibited a significantly improved to 9.901% AARE. Material parameter calculations displayed variations across the temperature range. The SC Arrhenius model, utilizing an average slope method for parameter computation, failed to consider temperature-induced disparities, limiting its predictive capability. Hot processing map, utilizing the Murty improved Dynamic Materials Model (DMM), indicated optimal conditions for stable forming of the Al–Zn–Mg–Cu alloy. Microstructural analysis revealed MgZn2 precipitation induced by hot deformation, with crystallographic defects enhancing nucleation rates and precipitate refinement. [ABSTRACT FROM AUTHOR]

Strain-induced precipitation (SIP) behaviors of 7Mo super-austenitic stainless steel (SASS) were studied by strain relaxation test at 850 °C. Our results reveal that sigma (σ) phases are the predominant SIP of 7Mo SASS, with the chemical formula of (FeNi) (CrMo). SIP first precipitates in granular shape at HAGBs, such as deformed grain boundaries (SIP-GBs), interfaces between deformation twin layers and matrix (SIP-TW), and HAGBs inside deformed grains (SIP-HAGBs). This is attributed to the ability of HAGBs for providing high deformation stored energy, nucleation sites, and efficient element diffusion channels. When the nucleation sites at HAGBs become saturated with increasing holding time, needle-like SIPs (SIP-NL) are precipitated along closely packed planes of γ-Fe matrix. The specific orientation relationships between SIP-NL and matrix, namely (−111)γ-Fe//(00–1)σ, [0–11]γ-Fe//[-110]σ or (-1-1-1)γ-Fe//(00–1)σ, [0–11]γ-Fe//[-110]σ, and the needle-like shape are contribute to minimizing interfacial energy and elastic strain energy of SIP, respectively, thereby promoting SIP-NL. The absence of SIP on dislocations can be attributed to the large critical nucleus size of σ phase (about 4.87 nm), which poses challenges for SIP nucleation through dislocation distortion energy. Mo is the controlling element for the nucleation and growth of SIP. SIP can promote cDRX through particle-induced nucleation (PSN) mechanism during the deformation process, while residual high-density dislocations around SIP can also promote cSRX during the holding process. During holding process, the boundaries of recrystallized grains can also serve as nucleation sites for SIP, thereby promoting SIP, and finally form a multi-layer structure of “σ phase/Recrystallization/σ phase”.

Strain-induced precipitation (SIP) behaviors of 7Mo super-austenitic stainless steel (SASS) under various deformation conditions were studied by stress relaxation tests. The research demonstrates that sigma phases are the primary SIP phases of 7Mo SASS. Generally, SIP is mainly distributed in granular shape at the boundaries of deformed grains or recrystallized grains, as well as around the deformed microstructure, such as deformation twin layers/matrix interfaces. The variation of deformation parameters can lead to changes in microstructure, therefore influencing the distribution of SIP. For instance, with the temperature increases, the SIP distribution gradually evolves from deformed grain boundaries to recrystallized grain boundaries. The average size of SIP increases with increasing temperature and strain, as well as decreasing strain rate. The SIP content also increases with increasing strain and decreasing strain rate, while exhibiting an initial rise followed by a decline with increasing temperature, reaching its maximum value at 850 °C. The presence of SIP can promote recrystallization by particle-induced nucleation (PSN) mechanism during the hot deformation process. Moreover, the boundaries of these recrystallized grains can also serve as nucleation sites for SIP, therefore promoting SIP. This process can be simplified as S I P → P S N R e c r y s t a l l i z a t i o n → N u c l e a t i o n   s i t e s S I P . With the increase in holding time and the consumption of stored energy, the process gradually slows down, leading to the formation of a multi-layer structure, namely SIPs/Recrystallized grains/SIPs structure. Moreover, SIP at recrystallized grain boundaries can hinder the growth of recrystallized grains. Through this study, a comprehensive understanding of the SIP behaviors in 7Mo SASS under different deformation conditions has been achieved, as well as the interaction between SIP and recrystallization. This finding provides valuable insights for effective control or regulation of SIP and optimizing the hot working processes of 7Mo SASS. [ABSTRACT FROM AUTHOR]

The austenite grain growth and recrystallization behaviors of three pipeline steels with different Nb contents were investigated through reheating and thermal simulation compression experiments. The initiation conditions for dynamic and sub-dynamic recrystallization of austenite were analyzed, and sub-dynamic recrystallization equations in Avrami form were established. The influences of Nb content and deformation conditions on the evolution of grain size during austenite recrystallization was examined. The findings indicate that the austenite grain size of the three steels increases gradually with higher reheating temperatures, while the average grain size decreases with increasing Nb content. Sub-dynamic recrystallization initiation temperatures for the B150-steel, B145-steel, and 73-steel were found to be 920 °C for 10 s, 940 °C for 30 s, and 960 °C for 30 s, respectively. During high-temperature deformation, Nb in solid solution hindered recrystallization by impeding grain boundary and dislocation movement. At lower deformation temperatures, Nb(C, N) precipitation pinned grain boundaries and dislocations and consumed substantial free energy, thus competing with recrystallization. As Nb content increased, strain-induced precipitation became more pronounced, resulting in more effective inhibition of recrystallized grain growth.

It is not realistic to optimize the roll pass design of profile rolling mills, which typically roll hundreds of profiles, using physical modelling or operational rolling. The use of reliable models of microstructure evolution is preferable here. Based on the mathematical equations describing the microstructure evolution during hot rolling, a modified microstructure evolution model was presented that better accounts for the influence of strain-induced precipitation (SIP) on the kinetics of static recrystallization. The time required for half of the structure to soften, t0.5, by static recrystallization was calculated separately for both situations in which strain-induced precipitation occurred or did not occur. On this basis, the resulting model was more sensitive to the description of grain coarsening in the high-rolling-temperature region, which is a consequence of the rapid progress of static recrystallization and the larger interpass times during rolling on cross-country and continuous mills. The modified model was verified using a plain strain compression test (PSCT) simulation of rolling a 100-mm-diameter round bar performed on the Hydrawedge II hot deformation simulator (HDS-20). Four variants of simulations were performed, differing in the rolling temperature in the last four passes. For comparison with the outputs of the modified model, an analysis of the austenite grain size after rolling was performed using optical metallography. For indirect comparison with the model outputs, the SIP initiation time was determined based on the NbX precipitate size distribution obtained by TEM. Using the PSCT and the outputs from the modified microstructure evolution model, it was found that during conventional rolling, strain-induced precipitation occurs after the last pass and thus does not affect the austenite grain size. By lowering the rolling temperature, it was possible to reduce the grain size by up to 56 μm, while increasing the mean flow stress by a maximum of 74%. The resulting grain size for all four modes was consistent with the operating results. [ABSTRACT FROM AUTHOR]

Strain-induced precipitation (SIP) behaviors of 7Mo super-austenitic stainless steel (SASS) under various deformation conditions were studied by stress relaxation tests. The research demonstrates that sigma phases are the primary SIP phases of 7Mo SASS. Generally, SIP is mainly distributed in granular shape at the boundaries of deformed grains or recrystallized grains, as well as around the deformed microstructure, such as deformation twin layers/matrix interfaces. The variation of deformation parameters can lead to changes in microstructure, therefore influencing the distribution of SIP. For instance, with the temperature increases, the SIP distribution gradually evolves from deformed grain boundaries to recrystallized grain boundaries. The average size of SIP increases with increasing temperature and strain, as well as decreasing strain rate. The SIP content also increases with increasing strain and decreasing strain rate, while exhibiting an initial rise followed by a decline with increasing temperature, reaching its maximum value at 850 °C. The presence of SIP can promote recrystallization by particle-induced nucleation (PSN) mechanism during the hot deformation process. Moreover, the boundaries of these recrystallized grains can also serve as nucleation sites for SIP, therefore promoting SIP. This process can be simplified as SIP→PSNRecrystallization→Nucleation sitesSIP. With the increase in holding time and the consumption of stored energy, the process gradually slows down, leading to the formation of a multi-layer structure, namely SIPs/Recrystallized grains/SIPs structure. Moreover, SIP at recrystallized grain boundaries can hinder the growth of recrystallized grains. Through this study, a comprehensive understanding of the SIP behaviors in 7Mo SASS under different deformation conditions has been achieved, as well as the interaction between SIP and recrystallization. This finding provides valuable insights for effective control or regulation of SIP and optimizing the hot working processes of 7Mo SASS.

This study investigated the correlation between strain-induced precipitation (SIP) and static recrystallisation (SRX) in Ti microalloyed steel during stress relaxation after controlled compression. The final compression temperature strongly influenced the order of SIP and SRX and thus the evolution of the austenite structure. Precipitation-time-temperature (PTT) curve obtained for the experimental steel exhibited an inverted "S" shape. A recrystallisation kinetics model revealed that SRX, which occurs preferentially above 940 °C, resulted in delayed subsequent SIP, thus causing deviation in the PTT curve from the typical 'C' shape. Below 940 °C, the fastest nose temperature for precipitation was located at 900 °C, and the precipitate was constituted by TiC particles with a NaCl-type FCC structure. The dynamic competition between SIP and SRX processes were evaluated by comparing the relative magnitude of the recrystallisation driving force and precipitation pinning force during stress relaxation, combined with the evolution of precipitate and austenitic structure. The results indicated that the plateau period occurred because of the precipitation pinning effect inhibited recrystallisation-induced austenite softening. However, the non-uniform distribution of SIP restricted the mobility of the boundaries to a portion of the austenite grains, resulting in abnormal grain growth during the plateau period. [ABSTRACT FROM AUTHOR]

Static recrystallization is a method of tailoring the microstructure and mechanical properties of steels, which is important for microalloyed forging steels as the hot deformation process significantly affects their mechanical properties. In this paper, the static recrystallization behavior of a low-carbon Nb-V-microalloyed forging steel was investigated by double-pass hot compression tests at deformation temperature of 800–1100 °C and interruption time of 1–1000 s. The static recrystallization fractions were determined using the 2% offset method. The static recrystallization activation energy and the static recrystallization critical temperature (SRCT) of the experimental steel were determined. When the deformation temperature was higher than the SRCT, the recrystallization fraction curve conformed to the Avrami equation. When the deformation temperature was below the SRCT, the recrystallization curve appeared to plateau, which was caused by strain-induced precipitation. Before and after the plateau, the static recrystallization kinetics still obeyed the Avrami equation. [ABSTRACT FROM AUTHOR]

Two steels with a base composition of Fe-0.2C-0.8Mn-1.2Cr (wt%) but with different niobium (Nb) contents (0.02 and 0.03 wt%) were employed to study the effect of precipitate evolution on the softening resistance in the austenite region under elevated temperature deformation. The thermomechanical procedure was executed by a deformation-dilatometer and involved double deformation processes with 25% strain at a 0.25 s−1 strain rate at 900, 925, 950, and 1000 °C. The softening ratios, reflecting the competition between recrystallization and precipitation, were evaluated. The results indicated that both steels showed better softening resistance at 900 °C than at other temperatures. However, the softening ratio of 0.03 wt% Nb-containing steel (Steel 3N) rose after 100 s at 900 °C, while 0.02 wt% Nb-containing steel (Steel 2N) maintained a low softening ratio within 300 s at 900 °C, indicating that Steel 3N was relatively non-durable. A microstructural characterization showed that in the Steel 3N sample deformed at 900 °C, recrystallization occurred more strongly than for Steel 2N after a 1000 s holding time. A follow-up analysis then showed that Steel 3N treated at 900 °C revealed a faster coarsening of the carbides than Steel 2N even in the early stage of precipitation, evidencing that Steel 2N exhibited a lower softening resistance at 900 °C. [ABSTRACT FROM AUTHOR]

The paper studies the influence of 0.5–1.8 wt.% Ni alloying on the superplasticity, microstructural evolution, and dynamic grain growth effect in a temperature range of 625–775 °C and room temperature mechanical properties of two-phase Ti-Al-Mo-V alloys. Due to a decrease in β transus and an enhancement in the alloy diffusivity, an increase in Ni content significantly improved superplasticity. The Ni-modified alloys exhibited 1.5–3-fold lower flow stress, a 2.5–3-fold greater elongation to failure, and 1.4–1.7-fold higher strain rate sensitivity m coefficient compared to the Ni-free alloy. An intermetallic Ti2Ni compound precipitated in the 1.8 wt.% Ni-modified alloy during low-temperature deformation at 700 °C and decreased superplastic properties. The Ti-4Al-3Mo-1V-0.1B alloy with 0.9 wt.% Ni exhibited a good combination of the superplastic behavior and room-temperature mechanical properties: an elongation to failure of 500–900% at a low-temperature range of 625–775 °C and constant strain rate of 1 × 10−3 s−1 and a yield strength of 885 MPa and ultimate tensile strength of 1020 MPa after pre-straining for 100% in a superplastic regime and strengthening heat treatment. [ABSTRACT FROM AUTHOR]

Strain-induced precipitation kinetics and precipitates' characteristics in a Ti microalloyed steel subjected to a two-stage controlled rolling process, simulating rough and finish rolling process of industrial production, were quantitatively investigated by stress relaxation experiments and transmission electron microscopy (TEM), as well as inductively coupled plasma-atomic emission spectroscopy (ICP-AES). In the present work, precipitation during the cooling stage after rough rolling was taken into consideration for the first time. It was found that a small amount of Ti-bearing precipitates, consuming about 10% of the elemental Ti, were formed on dislocations after rough rolling and cooling to 900 °C. The obtained precipitation-time-temperature (PTT) curves exhibited a classic C shape with a nose temperature of 900 °C and the shortest incubation time of 60 s. The PTT curves moved to the lower left by the introduction of 20% deformation at 1050 °C, which was attributed to more nucleation sites for strain-induced precipitation and the decrease of Ti concentration in deformed austenite. During the stress relaxation stage, strain-induced precipitates preferentially nucleated on dislocations and sub-boundaries and were identified as TiC particles. The mean size of precipitated TiC particles increased from 10.2 ± 2.1 to 25.2 ± 2.8 nm, as the holding time was increased from 100 to 1800 s at 900 °C. When the holding time exceeded 600 s the migration of austenite grain boundaries could not be completely inhibited due to the coarsening of precipitates. Isothermal treatment at 900 °C with a holding time of 60–100 s is suggested here as a viable combination for the processing of the 0.05C-0.21Si-1.05Mn-0.13Ti (wt. %) steel due to the shortest incubation time period and effective pinning of grain boundaries by strain-induced precipitates.

It is not realistic to optimize the roll pass design of profile rolling mills, which typically roll hundreds of profiles, using physical modelling or operational rolling. The use of reliable models of microstructure evolution is preferable here. Based on the mathematical equations describing the microstructure evolution during hot rolling, a modified microstructure evolution model was presented that better accounts for the influence of strain-induced precipitation (SIP) on the kinetics of static recrystallization. The time required for half of the structure to soften, t0.5, by static recrystallization was calculated separately for both situations in which strain-induced precipitation occurred or did not occur. On this basis, the resulting model was more sensitive to the description of grain coarsening in the high-rolling-temperature region, which is a consequence of the rapid progress of static recrystallization and the larger interpass times during rolling on cross-country and continuous mills. The modified model was verified using a plain strain compression test (PSCT) simulation of rolling a 100-mm-diameter round bar performed on the Hydrawedge II hot deformation simulator (HDS-20). Four variants of simulations were performed, differing in the rolling temperature in the last four passes. For comparison with the outputs of the modified model, an analysis of the austenite grain size after rolling was performed using optical metallography. For indirect comparison with the model outputs, the SIP initiation time was determined based on the NbX precipitate size distribution obtained by TEM. Using the PSCT and the outputs from the modified microstructure evolution model, it was found that during conventional rolling, strain-induced precipitation occurs after the last pass and thus does not affect the austenite grain size. By lowering the rolling temperature, it was possible to reduce the grain size by up to 56 μm, while increasing the mean flow stress by a maximum of 74%. The resulting grain size for all four modes was consistent with the operating results.

This study investigated the correlation between strain-induced precipitation (SIP) and static recrystallisation (SRX) in Ti microalloyed steel during stress relaxation after controlled compression. The final compression temperature strongly influenced the order of SIP and SRX and thus the evolution of the austenite structure. Precipitation-time-temperature (PTT) curve obtained for the experimental steel exhibited an inverted “S” shape. A recrystallisation kinetics model revealed that SRX, which occurs preferentially above 940 °C, resulted in delayed subsequent SIP, thus causing deviation in the PTT curve from the typical ‘C’ shape. Below 940 °C, the fastest nose temperature for precipitation was located at 900 °C, and the precipitate was constituted by TiC particles with a NaCl-type FCC structure. The dynamic competition between SIP and SRX processes were evaluated by comparing the relative magnitude of the recrystallisation driving force and precipitation pinning force during stress relaxation, combined with the evolution of precipitate and austenitic structure. The results indicated that the plateau period occurred because of the precipitation pinning effect inhibited recrystallisation-induced austenite softening. However, the non-uniform distribution of SIP restricted the mobility of the boundaries to a portion of the austenite grains, resulting in abnormal grain growth during the plateau period.

Static recrystallization is a method of tailoring the microstructure and mechanical properties of steels, which is important for microalloyed forging steels as the hot deformation process significantly affects their mechanical properties. In this paper, the static recrystallization behavior of a low-carbon Nb-V-microalloyed forging steel was investigated by double-pass hot compression tests at deformation temperature of 800–1100 °C and interruption time of 1–1000 s. The static recrystallization fractions were determined using the 2% offset method. The static recrystallization activation energy and the static recrystallization critical temperature (SRCT) of the experimental steel were determined. When the deformation temperature was higher than the SRCT, the recrystallization fraction curve conformed to the Avrami equation. When the deformation temperature was below the SRCT, the recrystallization curve appeared to plateau, which was caused by strain-induced precipitation. Before and after the plateau, the static recrystallization kinetics still obeyed the Avrami equation.

Two steels with a base composition of Fe-0.2C-0.8Mn-1.2Cr (wt%) but with different niobium (Nb) contents (0.02 and 0.03 wt%) were employed to study the effect of precipitate evolution on the softening resistance in the austenite region under elevated temperature deformation. The thermomechanical procedure was executed by a deformation-dilatometer and involved double deformation processes with 25% strain at a 0.25 s−1 strain rate at 900, 925, 950, and 1000 °C. The softening ratios, reflecting the competition between recrystallization and precipitation, were evaluated. The results indicated that both steels showed better softening resistance at 900 °C than at other temperatures. However, the softening ratio of 0.03 wt% Nb-containing steel (Steel 3N) rose after 100 s at 900 °C, while 0.02 wt% Nb-containing steel (Steel 2N) maintained a low softening ratio within 300 s at 900 °C, indicating that Steel 3N was relatively non-durable. A microstructural characterization showed that in the Steel 3N sample deformed at 900 °C, recrystallization occurred more strongly than for Steel 2N after a 1000 s holding time. A follow-up analysis then showed that Steel 3N treated at 900 °C revealed a faster coarsening of the carbides than Steel 2N even in the early stage of precipitation, evidencing that Steel 2N exhibited a lower softening resistance at 900 °C.

The paper studies the influence of 0.5–1.8 wt.% Ni alloying on the superplasticity, microstructural evolution, and dynamic grain growth effect in a temperature range of 625–775 °C and room temperature mechanical properties of two-phase Ti-Al-Mo-V alloys. Due to a decrease in β transus and an enhancement in the alloy diffusivity, an increase in Ni content significantly improved superplasticity. The Ni-modified alloys exhibited 1.5–3-fold lower flow stress, a 2.5–3-fold greater elongation to failure, and 1.4–1.7-fold higher strain rate sensitivity m coefficient compared to the Ni-free alloy. An intermetallic Ti2Ni compound precipitated in the 1.8 wt.% Ni-modified alloy during low-temperature deformation at 700 °C and decreased superplastic properties. The Ti-4Al-3Mo-1V-0.1B alloy with 0.9 wt.% Ni exhibited a good combination of the superplastic behavior and room-temperature mechanical properties: an elongation to failure of 500–900% at a low-temperature range of 625–775 °C and constant strain rate of 1 × 10−3 s−1 and a yield strength of 885 MPa and ultimate tensile strength of 1020 MPa after pre-straining for 100% in a superplastic regime and strengthening heat treatment.

In the current study, the effect of deformation in the non-recrystallization region on the characteristics of interphase precipitation was extensively studied in a Ti-Mo steel. The present observation revealed a novel mechanism affecting the interphase precipitation characteristics during an austenite-to-ferrite transformation, as a result of the thermomechanical processing. The hot deformation primarily led to the fragmentation of an austenite grain interior through the formation of elongated dislocation walls, locally deviating the orientation within a grain. Relatively coarse strain-induced precipitates were initially formed on the dislocation walls upon cooling. During the isothermal austenite to ferrite transformation, the interphase precipitation took place on the advancing austenite-ferrite interface front. This was similar to the strain-free condition, though the alignment of planar rows was abruptly altered while encountering the microband walls due to local orientation change at either side of a given microband. In addition, the deformation reduced the planar spacing of interphase precipitates up to a given strain (i.e., ∼0.3), beyond which it became nearly constant. This was due to the saturation of dislocation density within austenite above a certain strain and/or the consumption of alloying elements through the strain-induced precipitation formed on the microbands prior to the austenite to ferrite transformation. Image 1 [ABSTRACT FROM AUTHOR]

This work has focused on the study of hot working behavior of Ti-Nb microalloyed steels with high Ti contents (> 0.05%). The role of Nb during the hot deformation of low carbon steels is well known: it mainly retards austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, to design rolling schedules that result in properly conditioned austenite microstructures, it is necessary to develop models that take into account the effect of high Ti concentrations on the microstructural evolution of austenite. To that end, in this work torsion tests were performed to investigate the microstructural evolution during hot deformation of steels microalloyed with 0.03% Nb and different high Ti concentrations (0.05%, 0.1%, 0.15%). It was observed that the 0.1% and 0.15% Ti additions resulted in retarded softening kinetics at all the temperatures. This retardation can be mainly attributed to the solute drag effect exerted by Ti in solid solution. The precipitation state of the steels after reheating and after deformation was characterized and the applicability of existing microstructural evolution models was also evaluated. Determined recrystallization kinetics and recrystallized grain sizes reasonably agree with those predicted by equations previously developed for Nb-Ti microalloyed steels with lower Ti concentrations (

The interaction between strain-induced precipitation, recovery and recrystallization governs the properties of the particle strengthened Mo-Hf-C alloy MHC. Complex, multi-staged thermomechanical processes have become the key approach for further improvement of the high temperature properties of MHC products within recent years. An investigation of those processes, including a wide field of parameters, usually turns out to be both difficult and time consuming. With multi-pass compression tests it is possible to analyze complex thermomechanical processes on small scale samples in a straightforward manner. The evaluation of the gathered information is demonstrated and the results are compared to microstructural investigations, conventional mechanical tests as well as relevant literature. Furthermore, the influence of different particle size distributions in MHC is interpreted. Precipitation between individual deformation steps resulted in minor additional hardening of the final material. Intermediate recrystallization during the thermomechanical processing showed high softening and ductilization of the final condition. [ABSTRACT FROM AUTHOR]

In recent years, the development of nuclear grade FeCrAl-based alloys with enhanced accident tolerance has been carried out for light water reactor (LWR) fuel cladding to serve as a substitute for zirconium-based alloys. To achieve excellent microstructure stability and mechanical properties, the control of precipitation particles is critical for application of FeCrAl-based alloys. In this paper, the effect of thermomechanical processing on the microstructure and precipitation behavior of hot-rolled FeCrAl alloy plates was examined. After hot rolling, the FeCrAl alloy plates had typical deformation textures. The rolling direction (RD) orientation gradually rotated from to along with increasing reduction. Shear bands and cell structures were formed in the matrix, and the former acted as preferable nucleation sites for crystallization. Improved deformation helped to produce strain-induced precipitation. The plate with 83% reduction had the most homogeneous and finest precipitation particles. Identification results by TEM indicated that the Laves precipitation was of the Fe2Nb-type crystal structure type, with impurities including Mo, Cr, and Si. The plate with uniform Laves particles displayed favorable heat stability after a long period of aging at 800 °C. The microstructure evolution of the aged sample was also observed. The deformation microstructure and the strain-induced precipitation mechanism of FeCrAl alloys are discussed.

The powder metallurgically processed molybdenum base alloy MHC (Mo–Hf–C), with a nominal content of 0.65 at.% Hf and 0.65 at.% C, is of considerable interest for high temperature applications. After uniaxial deformation and subsequent aging of the as-sintered material, plate-like, nm-sized hafnium carbides are formed by heterogeneous nucleation at dislocations. The main focus of this work was to study the evolution of the size distribution of these small hafnium carbides and to gain statistically reliable data from a large volume regarding the morphology and the volume fractions using small-angle neutron scattering (SANS) experiments for samples aged at 1600 °C for different times. Additionally, complementary methods were used to support the SANS results. Hardness testing, two-stage interrupted compression tests, optical light microscopy, electron backscatter diffraction, scanning and transmission electron microscopy were used for further characterization of the aged MHC microstructures and as complementary methods to the SANS measurements. The small-angle neutron scattering investigations revealed anisotropic 2D patterns in relation to the loading direction of the deformed samples which were assigned to not fully compressed pores. It was possible to analyze the thickness distribution of the fine hafnium carbides due to streak formation of the scattering patterns caused by an orientation relationship between face centered cubic hafnium carbides and the body centered cubic molybdenum matrix. The evolution of the microstructure and hardness are in good agreement with the results of the small-angle neutron scattering measurements. The highest hardness was revealed after aging for 1 min at 1600 °C exhibiting small hafnium carbides with a mean diameter of ∼13 nm, a mean plate-thickness of ∼3 nm and a volume fraction of 0.05 vol%. Although, the volume fraction of these carbides was increasing with progressing aging time the hardness dropped due to recovery, recrystallization as well as increasing particle size. [ABSTRACT FROM AUTHOR]

The coupled effect of strain-induced precipitation and stabilized substructure on the elevated-temperature strength of an Al–Mn–Si alloy and its thermal stability have been investigated. Prestrain significantly promotes the nucleation of nano-sized dispersoids, and strain-induced precipitation suppresses recrystallization, stabilizing substructure at elevated temperatures. Compared with the dispersoids formed during the heat treatment of as-cast alloy, substructure does not increase the coarsening rate of strain-induced precipitates. The strain-induced precipitation and stabilized substructure profoundly strengthen the aluminum alloy at the elevated temperature (300 °C).

A new thermomechanical processing route is described for a microalloyed steel, with roughing deformation below the recrystallisation-stop temperature (T 5% ), followed by a rapid reheat to 1200 °C for 10s, and then finish deformation at the same temperature as the rough deformation. The new route focused on optimising the kinetics of strain-induced precipitation (SIP) and the formation of deformation-induced ferrite transformation (DITF). For comparative purposes, two experimental 0.06 wt% C steels were studied: one with 0.03 wt% Nb (Nb steel), and a second with both 0.03 wt% Nb and 0.02 wt% Ti (Nb Ti steel). Two processing routes were studied. The first was a conventional route, which consisted of a simulated rough deformation schedule with the final roughing pass taking place at 850 °C, which produced fully unrecrystallised austenite grains during deformation with no strain-induced ferrite formation. The second, new, thermomechanical processing route used the same roughing step, after which the steels were reheated at 10 °C/s to a temperature of 1200 °C, isothermally held for 10s allowing for precipitate dissolution, prior to air cooling to a finishing deformation temperature of 850 °C. This route resulted in DIFT primarily on the prior-austenite grain boundaries. The precipitate solution during the reheat treatment increased the supersaturation of Nb and Ti in the austenite matrix on subsequent cooling, which therefore increased the undercooling due to the increased A e3 . The observation of nanoscale cementite in the DIFT supports the view that it formed through a massive transformation mechanism. The volume fraction of SIP after finish deformation was influenced by the supersaturation of microalloy elements in solution during heat treatment. The new process route led to a significant refinement of the final ferrite grain size. [ABSTRACT FROM AUTHOR]

The thermomechanical processing of high strength low allow (HSLA) steels during low-temperature roughing, followed by rapid reheating to higher temperatures was investigated to better understand the Nb dissolution kinetics in austenite, and the subsequent precipitation behaviour during the final finishing passes. For comparative purposes, two experimental 0.06 wt% C steels were studied, one containing 0.03 wt% Nb (Nb steel), and the second containing both 0.03 wt% Nb and 0.02 wt% Ti (Nb–Ti steel). Processing of these steels consisted of a simulated roughing schedule, with the final roughing pass taking place at 850 °C. The strain-induced precipitation intensity in the steels subsequently quenched where characterised using transmission electron microscopy. Following this, the steels were rapidly reheated at a rate of 10 °C/s to a temperature of 1200 °C, held at temperature for various times, and water quenched to room temperature so that both the precipitate dissolution kinetics, together with the austenite grain coarsening kinetics could be established. [ABSTRACT FROM AUTHOR]

Double-hit torsion tests were performed in order to study the effect of high Al levels (up to 2 wt.%) and Nb microalloying (up to 0.07 wt.%) on the static softening kinetics of 0.2%C-2%Mn steels. The addition of 1%Al leads to a delay in the softening kinetics due to solute-drag effect, equivalent to that exerted by 0.027%Nb. For the 2%Al steels, at temperatures below 1000 °C, γ → α phase transformation occurs after deformation, resulting in a larger retardation of the softening kinetics. At temperatures higher than 1000 °C, Nb in solid solution also contributes to the retardation of the static softening kinetics, and at lower temperatures NbC strain-induced precipitation leads to incomplete softening for the 1%Al steel, and to a complex interaction between softening, phase transformation, and NbC strain-induced precipitation for the 2%Al-Nb steels. The effect of Al on the static softening kinetics was quantified and introduced in a model developed in previous works for the prediction of the austenite microstructural evolution. In order to validate the results of the model, multipass torsion tests were carried out at conditions representative of hot strip and plate rolling mills. Model predictions show reasonable agreement with the results obtained at different deformation conditions. [ABSTRACT FROM AUTHOR]

It is not realistic to optimize the roll pass design of profile rolling mills, which typically roll hundreds of profiles, using physical modelling or operational rolling. The use of reliable models of microstructure evolution is preferable here. Based on the mathematical equations describing the microstructure evolution during hot rolling, a modified microstructure evolution model was presented that better accounts for the influence of strain-induced precipitation (SIP) on the kinetics of static recrystallization. The time required for half of the structure to soften, t , by static recrystallization was calculated separately for both situations in which strain-induced precipitation occurred or did not occur. On this basis, the resulting model was more sensitive to the description of grain coarsening in the high-rolling-temperature region, which is a consequence of the rapid progress of static recrystallization and the larger interpass times during rolling on cross-country and continuous mills. The modified model was verified using a plain strain compression test (PSCT) simulation of rolling a 100-mm-diameter round bar performed on the Hydrawedge II hot deformation simulator (HDS-20). Four variants of simulations were performed, differing in the rolling temperature in the last four passes. For comparison with the outputs of the modified model, an analysis of the austenite grain size after rolling was performed using optical metallography. For indirect comparison with the model outputs, the SIP initiation time was determined based on the NbX precipitate size distribution obtained by TEM. Using the PSCT and the outputs from the modified microstructure evolution model, it was found that during conventional rolling, strain-induced precipitation occurs after the last pass and thus does not affect the austenite grain size. By lowering the rolling temperature, it was possible to reduce the grain size by up to 56 μm, while increasing the mean flow stress by a maximum of 74%. The resulting grain size for all four modes was consistent with the operating results.0.5 , by static recrystallization was calculated separately for both situations in which strain-induced precipitation occurred or did not occur. On this basis, the resulting model was more sensitive to the description of grain coarsening in the high-rolling-temperature region, which is a consequence of the rapid progress of static recrystallization and the larger interpass times during rolling on cross-country and continuous mills. The modified model was verified using a plain strain compression test (PSCT) simulation of rolling a 100-mm-diameter round bar performed on the Hydrawedge II hot deformation simulator (HDS-20). Four variants of simulations were performed, differing in the rolling temperature in the last four passes. For comparison with the outputs of the modified model, an analysis of the austenite grain size after rolling was performed using optical metallography. For indirect comparison with the model outputs, the SIP initiation time was determined based on the NbX precipitate size distribution obtained by TEM. Using the PSCT and the outputs from the modified microstructure evolution model, it was found that during conventional rolling, strain-induced precipitation occurs after the last pass and thus does not affect the austenite grain size. By lowering the rolling temperature, it was possible to reduce the grain size by up to 56 μm, while increasing the mean flow stress by a maximum of 74%. The resulting grain size for all four modes was consistent with the operating results.

The present work investigated the evolution of strain-induced NbC precipitates in a model austenitic Fe–30Ni–Nb steel deformed at 925 °C to a strain of 0.2 during post-deformation holding between 3 and 1000 s and their effect on the reloading flow stress. The precipitate particles preferentially nucleated on the nodes of the periodic dislocation networks constituting microband walls. Holding for 10 s resulted in the formation of fine, largely coherent NbC particles with a mean diameter of ∼5 nm, which displayed a cube-on-cube orientation relationship with austenite and caused the maximum increase in the reloading steady-state flow stress. A further increase in the holding time from 30 to 1000 s led to the formation of semi-coherent, gradually coarser and more widely spaced particles with a mean diameter of 8 nm and above, which led to a gradual decrease in the reloading steady-state flow stress. The holding time increase resulted in progressive disintegration of the dislocation substructure and dislocation annihilation through static recovery processes, which was also reflected by the measured softening fractions. The precipitate particle shape changed during post-deformation annealing from elliptical to faceted octahedral and subsequently to tetra-kai-decahedral. [ABSTRACT FROM AUTHOR]

Abstract: A model for describing the strain-induced precipitation kinetics of vanadium carbonitride in the hot deformed austenite of precipitation-hardened Ferritic–Pearlitic (AFP – Ausscheidungshärtende Ferritisch–Perlitische) steels is presented. The model considers heterogeneous nucleation of precipitates on dislocation nodes and predicts the volume fraction, size and composition of the precipitates as well as the composition of the remaining austenite during the nucleation, growth and coarsening stages of the precipitation process. The effect of static recovery on the precipitation kinetics is also incorporated in the model. A fairly good agreement between the model predictions and experimental observations is demonstrated. [Copyright &y& Elsevier]

Strain-induced precipitation was systematically investigated in austenitic Inconel 617B superalloy (IN617B) after deformation of 0.2 true strain in its service temperature range (700–850 °C). The precipitation-time-temperature (PTT) curves obtained by stress relaxation experiments showed a classic C-shape with the nose temperature of 800 °C, and a minimum incubation period of 30 s, and the main strain-induced precipitate was M 23 C 6 carbide. After specimens deformed at 800 °C to a strain of 0.2 were held between 10 and 1000 s, they underwent a second-pass strain test, during which the M 23 C 6 precipitates evolved and influenced the flow stress; this behavior was investigated. During the nucleation, growth, and coarsening of the strain-induced precipitates, the morphology of the M 23 C 6 particles gradually changed from spherical to polygonal with increasing size, but the precipitates always maintained their cube-on-cube orientational relationship with the austenitic matrix. During the nucleation stage before 30 s, many pre-precipitation clusters and weak static recovery softening led to low second-deformation flow stress, which persisted at approximately 630 MPa. In the precipitation growth phase before 360 s, recovery softening balanced precipitation hardening. The strain-induced precipitates grew larger, and they were able to pin dislocations. In addition, some complex microstructures resulting from particle collisions were observed. This resulted in the maximum rate of increase of the second deformation flow stress, which quickly reached almost 700 MPa. When the holding time was extended to 1000 s, the recovery softening effect further increased, while the M 23 C 6 particles gradually coarsened; this weakened the increasing trend of the second deformation flow stress, which increased to only 715 MPa at 1000 s • The strain-induced precipitate in IN617B alloy is dominated by M 23 C 6 carbide in the temperature range of 700–850 °C. • Morphological evolution mechanism of the strain-induced precipitates M23C6 in IN617B alloy. • M 23 C 6 has different degrees of hardening effect on the matrix during nucleation, growth, and coarsening processes. [ABSTRACT FROM AUTHOR]

Efforts to produce more accurate through-process models for hot working need to take into account variations in the deformation conditions. In the present work, the effect of the reversal of the strain for a Nb-microalloyed steel is discussed. It is shown that, depending on the magnitude of the reversal, the static softening kinetics can be accelerated or delayed. The same is true for the case of the strain-induced Nb-carbonitride precipitation, leading to an interaction between both processes and the deformation path. Current models of recrystallization and precipitation have been used to analyze the obtained results. [Copyright &y& Elsevier]

This work has focused on the study of hot working behavior of Ti-Nb microalloyed steels with high Ti contents (> 0.05%). The role of Nb during the hot deformation of low carbon steels is well known: it mainly retards austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, to design rolling schedules that result in properly conditioned austenite microstructures, it is necessary to develop models that take into account the effect of high Ti concentrations on the microstructural evolution of austenite. To that end, in this work torsion tests were performed to investigate the microstructural evolution during hot deformation of steels microalloyed with 0.03% Nb and different high Ti concentrations (0.05%, 0.1%, 0.15%). It was observed that the 0.1% and 0.15% Ti additions resulted in retarded softening kinetics at all the temperatures. This retardation can be mainly attributed to the solute drag effect exerted by Ti in solid solution. The precipitation state of the steels after reheating and after deformation was characterized and the applicability of existing microstructural evolution models was also evaluated. Determined recrystallization kinetics and recrystallized grain sizes reasonably agree with those predicted by equations previously developed for Nb-Ti microalloyed steels with lower Ti concentrations (<0.05%). [ABSTRACT FROM AUTHOR]

The coupled effect of strain-induced precipitation and stabilized substructure on the elevated-temperature strength of an Al–Mn–Si alloy and its thermal stability have been investigated. Prestrain significantly promotes the nucleation of nano-sized dispersoids, and strain-induced precipitation suppresses recrystallization, stabilizing substructure at elevated temperatures. Compared with the dispersoids formed during the heat treatment of as-cast alloy, substructure does not increase the coarsening rate of strain-induced precipitates. The strain-induced precipitation and stabilized substructure profoundly strengthen the aluminum alloy at the elevated temperature (300 °C). [ABSTRACT FROM AUTHOR]