Non-recrystallization Temperature Research Articles

Assessment of structure-property relationships at the micrometer length scale in dual phase steels by electron microscopy and nanoindentation

This work presents a correlative investigation using nanoindentation and electron microscopy to characterize Advanced High Strength Steels (AHSS) that consist of a ferrite matrix along with a strengthening secondary phase/constituent such as pearlite, bainite and martensite. DP600 grade steel was hot rolled above the austenite non-recrystallization temperature (TnR) followed by coiling at different transformation temperatures to obtain ferrite-pearlite, ferrite-bainite and ferrite-martensite microstructures with identical nominal carbon content. Unlike the macroscale mechanical testing on dual phase steels which provides a composite response, in this work the variation in local mechanical properties as a function of processing was captured by high speed nanoindentation mapping at the length scale of the microstructure. Excellent agreement between the microstructure and hardness was observed for different dual phase microstructures (ferrite-pearlite (FP), ferrite-bainite (FB) and ferrite-martensite (FM)). High speed nanoindentation mapping data was deconvoluted using a clustering algorithm to obtain the hardness of the constituents. The hardness of ferrite in FP, FB and FM samples was found to be 3.26 GPa, 3.33 GPa and 4 GPa, respectively and the corresponding second phase / constituent was found to be 4.6 GPa, 4.43 GPa and 4.93 GPa, respectively. In spite of having identical nominal composition, the three different dual phase microstructures show differences in area fraction and hardness of ferrite and also the secondary constituents. These experimental observations are reconciled based on processing conditions and the length scale of the microstructure.

In situ measurement of austenite grain growth and recrystallization using laser ultrasonics

The development of next generation process models and advanced high-strength steel products for thin slab casting and direct rolling requires quantification of microstructure evolution during thermomechanical processing. Laser ultrasonics is a non-contact in-situ method to record grain growth, recrystallization and phase transformations in metals and alloys. Here, we will present an improved experimental design that facilitates a continuous microstructure measurement through the various stages of simulated hot rolling from reheating to runout table cooling using a Gleeble thermomechanical simulator equipped with a laser ultrasonics for metallurgy (LUMet) system. Austenite grain growth and static recrystallization after hot deformation are quantified based on attenuation of the ultrasound waves whereas austenite decomposition can be recorded with the changes in ultrasound velocity during the phase transformation. Further, the LUMet results for a microalloyed low carbon steel are validated with conventional techniques including optical and electron microscopy as well as double-hit tests. These experimental studies demonstrate the capabilities of laser ultrasonics in the identification of both normal and abnormal grain growth, non-recrystallization temperature, recrystallization, and austenite decomposition kinetics in a single test for a given processing path, as well as its potential for accelerated optimization of process control under industrial rolling conditions.

Evolution of austenite microstructure and microalloy precipitation state during double-twist torsion testing on Nb-Ti-bearing steels

Double-twist torsion testing was applied to two Nb-Ti-bearing microalloyed steels to develop the relationship between the mechanical response and the resulting microstructure. The double-twist torsion test applies pairs of isothermal deformation passes to a single specimen at progressively lower temperatures. The test captures the effect of multiple deformation and recrystallization events and allows changes in recrystallization behavior to be assessed with fractional softening calculations. Conventional methods for determining fractional softening were applied to the torsional data to infer changes in recrystallization behavior and estimate the temperature regime where partial recrystallization occurs. The non-recrystallization temperature (Tnr) was determined with multi-step hot torsion testing and compared to the partial softening regime from the double-twist torsion test. The traditional concept of Tnr as a single temperature was thereby assessed with respect to the fractional softening calculations and corresponding microstructural changes at the micro- and nano- scales. Tnr was within the partial softening regime assessed with double-twist torsion testing, suggesting partial recrystallization near Tnr. Measurements of grain aspect ratio matched the trends observed in the fractional softening data and confirmed that strain accumulation occurred throughout the partial softening regime. Higher Nb contents corresponded to greater strain accumulation at lower temperatures within the partial softening regime. The precipitation of Nb(C,N) occurred at high temperatures in the form of caps precipitating on pre-existing Ti-rich cores, while fine-scale precipitation occurred at lower temperatures. Increasing the Nb-content increased both the extent of cap formation and the extent of fine-scale precipitation.

Analysis of Recrystallization Kinetics Concerning the Experimental, Computational, and Empirical Evaluation of Critical Temperatures for Static Recrystallization in Nb, Ti, and V Microalloyed Steels

Recrystallization kinetics and two critical temperatures—the non-recrystallization temperature TNR and the static recrystallization critical temperature TSRCT—of five Nb, Ti, and V microalloyed steel grades are evaluated. The experimental examination is realized by employing isothermal double-hit compression tests and continuous hot torsion tests, both performed on a Gleeble® 3800 thermo-mechanical simulator. The experimental results are used for the critical assessment of predicted TNR using four empirical equations from the literature, and for the validation of simulated TNR and TSRCT. The thermokinetic computer simulations are realized using the mean-field microstructure modeling software MatCalc. Analysis shows that higher microalloying contents increase both critical temperatures, TNR and TSRCT, whereby the effect of recrystallization retardation of Nb is more pronounced than that of Ti or V. The most accurate reproduction of the experimental recrystallization behavior of the five examined steel grades is realized by the employed physics-based simulation approach.

A Two‐Step Thermomechanical Process for Improving High‐Angle Grain Boundary Proportion and Achieving Delamination Toughening in Hot‐Rolled Steel with a Limited Slab‐to‐Plate Ratio

A Two‐Step Thermomechanical Process for Improving High‐Angle Grain Boundary Proportion and Achieving Delamination Toughening in Hot‐Rolled Steel with a Limited Slab‐to‐Plate Ratio

Strain-rate dependent workability and plastic flow instability of a (Nb+V) stabilized microalloyed steel

In the current work, the strain-rate dependent workability and flow instability has been investigated in a (Nb+V) stabilized microalloyed steel. Uniaxial compression tests were conducted in intercritical and single phase austenitic temperature domain (700–1100 °C) at several strain-rates (0.01–10 s−1), using a thermo-mechanical simulator (Gleeble®−3800). The results show that the flow stress increases at higher strain-rates. A good sample processing window arises at medium strain-rates (0.1–1 s−1); whereas, flow instability (serrations) at high (10 s−1) and low (0.01 s−1) strain-rate plastic deformations. After a detailed sample characterization, it appears that the reasons for flow instability during the hot and warm deformations are the formation of micro-cracks or void nucleation at low (0.01 s−1) strain-rate; whereas, flow localization and shear banding by adiabatic heating at a higher strain-rate (10 s−1). In both cases (0.01 and 10 s−1), the serration arises at periodic intervals with negative strain-rate sensitivity. At a higher strain-rate (10 s−1), the flow instability dominates till 1100 °C, because of relieving the deformation-induced stored energy mainly by dynamic recovery. At lower deformation temperatures (i.e., 700–800 °C), fine ferrite grains nucleate around shear bands by the diffusional transformation of austenite. The dynamic recrystallization is either absent or incomplete in agreement with the experimental determination of Tnr (non-recrystallization temperature) and texture analysis. The overall flow instability characteristics in terms of temperature, strain and strain-rate substantiate well with the dynamic materials model, by the superposition of flow instability and power dissipation efficiency, on revisiting the processing map.

Controlled Rolling of X80 Pipeline Steel in the Austenite Recrystallization Temperature Region and Its Effect on the Microstructure and Mechanical properties

In recent years, the improvement in the strength and toughness of X80 pipeline steel by only controlled rolling in the nonrecrystallization temperature region has been limited, it is difficult to meet higher application demands. Therefore, to further improve the effect of controlled rolling on the strength and toughness, the controlled rolling of X80 pipeline steel in the austenite recrystallization temperature region and its effect on the microstructure and mechanical properties after final rolling in the nonrecrystalization temperature region are investigated by thermal simulation experiments and hot‐rolling experiments. An increase in the last‐pass rough‐rolling deformation in the austenite recrystallization temperature region refines the austenite grain size after rough rolling and finish rolling, making the austenite grain size smaller and more uniform and improving the strength and low‐temperature impact toughness. The mechanism of austenite evolution based on the last‐pass deformation in the austenite recrystallization temperature region is revealed. The finer and more uniform austenite grain size accelerates the refinement of the granular bainite microstructure at room temperature. The main strengthening mechanisms are dislocation strengthening and grain refinement strengthening. The increase in toughness is due to the presence of a high percentage of high‐angle grain boundaries, which significantly impede crack propagation.

Determination of Non-Recrystallization Temperature for Niobium Microalloyed Steel.

In the present investigation, the non-recrystallization temperature (TNR) of niobium-microalloyed steel is determined to plan rolling schedules for obtaining the desired properties of steel. The value of TNR is based on both alloying elements and deformation parameters. In the literature, TNR equations have been developed and utilized. However, each equation has certain limitations which constrain its applicability. This study was completed using laboratory-grade low-carbon Nb-microalloyed steels designed to meet the API X-70 specification. Nb- microalloyed steel is processed by the melting and casting process, and the composition is found by optical emission spectroscopy (OES). Multiple-hit deformation tests were carried out on a Gleeble® 3500 system in the standard pocket-jaw configuration to determine TNR. Cuboidal specimens (10 (L) × 20 (W) × 20 (T) mm3) were taken for compression test (multiple-hit deformation tests) in gleeble. Microstructure evolutions were carried out by using OM (optical microscopy) and SEM (scanning electron microscopy). The value of TNR determined for 0.1 wt.% niobium bearing microalloyed steel is ~ 951 °C. Nb- microalloyed steel rolled at TNR produce partially recrystallized grain with ferrite nucleation. Hence, to verify the TNR value, a rolling process is applied with the finishing rolling temperature near TNR (~951 °C). The microstructure is also revealed in the pancake shape, which confirms TNR.

Microstructural refinement mechanism and its effect on toughness in the nugget zone of high-strength pipeline steel by friction stir welding

High-strength pipeline steel was subjected to friction stir welding (FSW) at rotation rates of 400–700 rpm, and the grain refinement mechanism of the nugget zone (NZ) was determined. The thermo-mechanical process during FSW in the NZ was simulated by multi-pass thermal compression, thereby achieving the austenitic non-recrystallization temperature (Tnr). The austenitic non-recrystallization in the NZ at the lowest rotation rate of 400 rpm caused a significant grain refinement. Furthermore, the reduced rotation rate also resulted in the formation of a high ratio of island-like martensite-austenite (M-A) constituent. The toughness of the NZs was enhanced as the rotation rate decreased, which is attributed to the fine effective grains and homogeneously distributed fine M-A constituents dramatically inhibiting crack initiation and propagation.

Effect of post-rolling cooling on the hot-rolled microstructure and texture of a new medium-carbon steel

Thermomechanical processing consisted of hot-rolling in the austenitic region with deformation both above and below the non-recrystallization temperature. Immediately after rolling, specimens were directly quenched in water to three different temperatures of 560 °C, 420 °C and room temperature. The first two samples, which were quenched to 560° C and 420°C, immediately cooled slowly in the furnace. The microstructure of samples quenched to 560 °C mostly comprised of upper bainite whereas the samples quenched to 420 °C mainly consisted of lath-type bainite. The microstructure of sample direct-quenched to room temperature was mainly martensite. The transformation texture of all samples at the midthickness position showed the same general texture, consisted of mainly ᵅ and ᵞ fibers components with high intensities close to the transformed copper {112}<110>ᵅ, transformed brass {111}<112>ᵅ and rotated cube {001}<110>ᵅ components. However, the direct-quenched sample produced slightly more intense texture compared to the bainitic samples. The local texture of samples near the rolled surface owing to sever deformation showed the shear features consisting of major components of {112}<111>ᵅ, {110}<112>ᵅ and {110}<111>ᵅ.

Double twist torsion testing to determine the non recrystallization temperature

A double-twist torsion testing technique has been developed using a 316 stainless steel as an exemplar material to experimentally assess recrystallization behavior and determine the non-recrystallization temperature (Tnr). This new method was compared to the traditional methods of double-hit compression and multi-step hot torsion testing. The double-twist torsion test allows Tnr to be related to the extent of austenite recrystallization through measurements of fractional softening while accommodating multiple deformation and recrystallization steps with a single specimen. The double-twist torsion test resulted in average Tnr values similar to those determined with multi-step hot torsion, and a partially recrystallized microstructure was observed in the vicinity of the calculated Tnr for all three methods. The ability of the double-twist torsion test to relate the experimental Tnr to the evolution of austenite recrystallization via fractional softening measurements while incorporating effects of multiple deformation steps offers an advantage over traditional methods for quantifying changes in austenite recrystallization during thermomechanical processing.

Flow Stress Behaviour and Static Recrystallization Characteristics of Hot Deformed Austenite in Microalloyed Medium-Carbon Bainitic Steels

In the past decade, efforts have been focused on developing very fine, medium-carbon bainitic steels via the low-temperature (typically 300–400 °C) ausforming process, which not only enables shorter isothermal holding times for bainitic transformation at low temperatures, but also offers significantly improved strength. This paper describes static recrystallization (SRX) characteristics of austenite in four medium-carbon 2%Mn-1.3%Si-0.7%Cr steels with and without microalloying intended for the development of these steels. The stress-relaxation method on a Gleeble simulator resulted in recrystallization times over a wide range of temperatures, strains and strain rates. Also, the occurrence of precipitation was revealed. Powers of strain (−1.7 to −2.7) and strain rate (−0.21 to −0.28) as well as the apparent activation energies (225–269 kJ/mol) were in the ranges reported in the literature for C-Mn and microalloyed steels with lower Mn and Si contents. The new regression equations established for estimating times for 50% SRX revealed the retardation effects of microalloying and Mo addition showing reasonable fits with the experimental data, whereas the previous model suggested for ordinary microalloyed steels tended to predict clearly shorter times on average than the experimental values for the present coarse-grained steels. The Boratto equation to estimate the non-recrystallization temperature was successfully modified to include the effect of Mo alloying and high silicon concentrations.

Simultaneously Improving Mechanical Properties and Stress Corrosion Cracking Resistance of High-Strength Low-Alloy Steel via Finish Rolling within Non-recrystallization Temperature

The effect of hot rolling process on microstructure evolution, mechanical properties and stress corrosion cracking (SCC) resistance of high-strength low-alloy (HSLA) steels was investigated by varying the finish rolling temperature (FRT) and total rolling reduction. The results revealed granular bainite with large equiaxed grains was obtained by a total rolling reduction of 60% with the FRT of 950 °C (within recrystallization temperature Tr). The larger grain size and much less grain boundaries should account for the relatively lower strength and SCC resistance. A larger rolling reduction of 80% under the same FRT resulted in the formation of massive martensite–austenite (M/A) constituents and resultant low ductility and SCC resistance. In contrast, a good combination of strength, ductility and SCC resistance was obtained via 80% rolling reduction with the FRT of 860 °C (within non-recrystallization temperature Tnr), probably because of the fine grain size and M/A constituents, as well as a high density of grain boundary network.

Effect of rolling reduction below γ non-recrystallization temperature on pancaked γ, microstructure, texture and low-temperature toughness for hot rolled steel

To optimize rolling schedules for producing heavy plates with a limited slab-to-plate reduction ratio, we investigated the effect of rolling reduction below the non-recrystallization temperature (Tnr) of austenite (γ) on microstructure, texture, and low-temperature toughness for crack arrest heavy-gauge EH47 steel for shipbuilding. Four plates were hot rolled with 30%, 49%, 60% and 67% reduction below Tnr respectively, of which the total rolling reduction ratio was the same as 3.57. The microstructures and texture were characterized, and the low-temperature Charpy impact toughness was tested as well for each plate. Variations in toughness were significant among the steels, being highly correlated with the effective grain size (EGS), amount of high angle grain boundaries (HAGB), and texture. The steel with 60% reduction below Tnr showed the best toughness down to −80 °C with the impact absorbed energy of ~233 J for the minimum EGS of 3.4 ± 4.2 μm, the maximum HAGB proportion of 51.6%, {110} slip planes of 39.1%, and the strongest texture of {113}~{112}<110>, {112}<131>, {332}<113>. The theoretical calculation shows that the nucleation sites for austenite-ferrite transformation are mainly affected by the size of γ after rolling in the γ recrystallization region and the deformation substructure generated below Tnr. The changes of the pancaked γ can be quantified interpreted by the theory of effective interface area per unit volume. The relation of microstructures, texture, and low-temperature toughness to the rolling reduction below Tnr could be well interpreted by evaluating effective interfacial area per unit volume of austenite to ferrite during transformation.

Constitutive Analysis of the Mean Flow Stress of a Nb- and N-Bearing Austenitic Stainless Steel Biomaterial

The correlation between hardening and softening in an ASTM F-1586 stainless steel used as biomaterial was investigated by means of hot torsion simulations. Multi-pass deformation under continuous cooling was employed to simulate industrial hot rolling. Samples were subjected to 17 deformation passes of strains of 0.20 and 0.30, strain rate of 1.0 s−1 in a temperature range of 1250 to 930 °C. Interpass times (tp) of 5.0, 10, 20, 40, and 80 s were used. The obtained results showed direct dependence of the mean flow stress (MFS) and all applied thermomechanical parameters. The work hardening rate associated with the degree of stress accumulation (Δσ) inhibited the metadynamic recrystallization. This led to the variation of the non-recrystallization temperature (Tnr), associated with intense static recovery in the material. From the experimental data, a constitutive function of the mean flow stress (MFS) obtained through multiple nonlinear regression technique was proposed. The study aimed to have a better understanding of the physical metallurgy behind the thermomechanical behavior of the steel under a multi-pass process. The results allowed to monitor the recrystallized fraction (X) and the grain size (d) during the simulated conditions.

Development of thermo-mechanical processing to form high density of uniformly distributed nanosized carbides in austenitic stainless steels

Model alloys and thermomechanical processing (TMP) were developed to form a high density of uniformly distributed nanosized (<10 nm) carbides in an austenitic stainless steel matrix. The composition of the model alloys was determined to control the formation of precipitates during the TMP. The TMP consisted of homogenization at 1200 °C for 1 h, multiple hot-rolling passes above and below the non-recrystallization temperature (TNR), and precipitation heat treatment at 800 °C for 2 h. By proper division of the rolling passes above and below TNR, a microstructure with a homogeneous grain size and a well-developed dislocation structure was obtained. Then, subsequent precipitation heat treatment produced a microstructure containing a high density (~1.1 × 1022/m3) of uniformly distributed and coherent NbC precipitates in the austenitic matrix. The evolution of the nanosized NbC during the precipitation heat treatment was coupled with the dislocation characteristics, such as the density and distribution, which were controlled by the hot-rolling conditions.

Effect of Strain-Induced Precipitation on the Austenite Non-recrystallization (Tnr) Behavior of a High Niobium Microalloyed Steel

The non-recrystallization temperature (Tnr) of high niobium microalloyed steel was determined from both multihit and double-hit compression tests obtained under plane strain condition. The Tnr was determined to be in the range of 985 °C to 1010 °C for multihit conditions. The double-hit tests carried out at an interpass time of 5 seconds gave very low Tnr in the range of 860 °C to 900 °C. In order to understand this, double-hit experiments were carried out for different interpass times (2, 15, 100 seconds) at three different temperatures (950 °C, 1050 °C, and 1150 °C). The negative softening behavior was observed at 950 °C for higher interpass times of 15 and 100 seconds. This implies that the Tnr of this steel is well above 950 °C. This was due to the high amount of strain-induced precipitation of niobium carbonitrides at higher interpass times as revealed from TEM investigation. The precipitate size evolution considering the Dutta and Sellars nucleation condition in TC-PRISMA agrees well with the experimentally observed precipitate sizes. However, the evolution of Zener pinning forces considering TC-PRISMA nucleation condition is similar to both reported and experimentally determined values. The evolution of tensile properties also correlates well with the observed austenite recrystallization softening behavior. Therefore, a minimum interpass time of 15 seconds is required during double-hit compression tests to effectively precipitate Nb(C, N) and delay the static recrystallization softening behavior of this steel. This lead to the determination of comparable Tnr values between double-hit and multihit methods.

Estimation of Impact Toughness Transition Temperatures of As-Quenched Steels

High strength and sufficient toughness are key requirements for modern high-performance structural steels. In an attempt to develop a suitable estimation of impact toughness transition temperatures for as-quenched steels, we investigated the determiners of low-temperature toughness with a group of thermomechanically rolled direct-quenched steels with varying martensite contents. These were produced by altering chemical composition, finish rolling temperature and reduction below the non-recrystallization temperature, i.e. austenite pancaking, and characterised in terms of microstructural constituents, grain size distributions, texture and fractography. Provided the finish rolling temperature is high enough to avoid the formation of granular bainite on subsequent cooling, high levels of austenite pancaking yield the best combinations of low-temperature toughness and strength by effectively refining the size of the coarsest grains and randomizing the texture. While absolutely no direct correlation is found within as-quenched steels between the impact toughness transition temperatures and yield strength alone, T28J and T50 do closely follow a dynamic reference toughness, i.e. the opening stress intensity factor defined by yield strength and the size of the coarsest grains in the effective grain size distribution. This parameter reflects the transition temperatures – the lower the temperature, the lower the reference toughness needed to cause a local brittle fracture. Finally, we show that the impact toughness transition temperatures T28J and T50 of as-quenched steels can be accurately estimated, irrespective of the test specimen orientation, by utilizing just the dynamic reference toughness and the fraction of {100} cleavage planes within ± 15° of the specimen notch plane.

Through-Scale Analysis of Strain Path Effects in Microalloyed Austenite Subjected to Reverse Rolling

Thermomechanical hot rolling processes are often realized using reverse rolling stands, where the rolled stock is fed forward and backward through the rolling gap. During those processes, material undergoes several strain reversals that significantly alter microstructure evolution of austenite with respect to continuously rolled counterparts. In Nb-microalloyed steels, where precipitation hardening is usually expected, the effects of strain reversal are especially complex. When rolling direction is reversed, both static recrystallization (SRX) kinetics and strain-induced precipitation (SIP) processes are slowed down due to decreasing dislocation density. It affects the competition between driving force for SRX and pinning pressure for SIP and, in turn, changes the non-recrystallization temperature (Tnr), compared to the case where strain path is linear. In the present paper, detailed through-scale analysis of strain path effects in microalloyed austenite will be presented. Physical simulation and detailed microstructural analysis will be employed to study global and local effects in microalloyed austenite after complex deformation histories. Conclusions regarding the influence of strain path changes on the interactions between SRX and SIP will be drawn.

Effect of Mo, Nb and V on Hot Deformation Behaviour, Microstructure and Hardness of Microalloyed Steels

Three novel low carbon microalloyed steels with various additions of Mo, Nb and V were investigated after thermomechanical processing simulations designed to obtain ferrite-bainite microstructure. With the increase in microalloying element additions from the High V- to NbV- to MoNbV-microalloyed steel, the high temperature flow stresses increased. The MoNbV and NbV steels have shown a slightly higher non-recrystallization temperature (1000 °C) than the High V steel (975 °C) due to the solute drag from Nb and Mo atoms and austenite precipitation of Nb-rich particles. The ambient temperature microstructures of all steels consisted predominantly of polygonal ferrite with a small amount of granular bainite. Precipitation of Nb-and Mo-containing carbonitrides (&gt;20 nm size) was observed in the MoNbV and NbV steels, whereas only coarser (~40 nm) iron carbides were present in the High V steel. Finer grain size and larger granular bainite fraction resulted in a higher hardness of MoNbV steel (293 HV) compared to the NbV (265 HV) and High V (285 HV) steels.