Hot-rolled Microstructure Research Articles

Enhancing mechanical properties of medium Mn steel by warm rolling based on laminated elemental segregation

The hot-rolled microstructure of medium Mn steel has coarse grains and severe elemental segregation, resulting in low strength and plasticity. Constructing a multiphase structure, refining the microstructure, and regulating elemental segregation enhance the mechanical properties. In this study, liquid nitrogen treatment created a layered distribution of austenite and martensite. Warm rolling was then used to reduce layer thickness and refine grain structure. After liquid nitrogen and warm rolling treatments, the strength and plasticity of medium Mn steel increased to 1270 MPa and 23.3%, respectively, far exceeding the hot-rolled state (724 MPa, 12.8%). Warm rolling also triggers austenite reverted transformation (ART) and introduces high-density dislocations, further improving austenite stability. This strengthening effect is higher than that from intercritical annealing alone. Improved austenite stability delays the transformation induced plasticity (TRIP) effect, preventing brittle fracture and enhancing deformation coordination between layers, significantly increasing the plastic deformation capacity of medium Mn steel.

Unveiling the kinetics of interphase and random quaternary Nb[sbnd]Ti precipitations, and strengthening mechanism of HSLA steel

Traditional methods for analyzing multi-element precipitation are often time-consuming, labor-intensive, and expensive. Particularly, the initial nucleation stages of such precipitations remain poorly understood. A revised precipitation theory was employed to elucidate the nucleation and growth kinetic mechanisms of quaternary (Ti, Nb)(C, N) precipitation with two modes: interphase precipitation (IP) along the migrating α/γ interface and random precipitation (RP) along dislocations. The findings indicated that TiN preferentially nucleates at the ferrite side compared to that at the austenite side. The nucleation occurring at α/γ interface leads to a precipitation particle angle of approximately 5.3° within a single IP sheet. The interface provided more favorable nucleation sites than dislocations, significantly reducing the relative nucleation time. Additionally, the initial yielding behavior and strengthening mechanism were analyzed based on the two factors of the hot-rolling microstructure and precipitation. A favorable slip system was along the RD-ND plane as the texture index of α-{110} approximately 3.74, compared to 2.20 along the TD-ND plane, resulting in the lower upper yield strength. Once the number density of mobile dislocations increased during plastic deformation, the tensile stress decreased, leading to the local plastic instability. Finally, (Ti, Nb)(C, N) contributed to a strengthening increment of about 154 MPa. This study provides critical insights into the precipitation behavior in HSLA steels, offering significant implications for the design and development of advanced materials with enhanced mechanical properties.

Improvement in Grain Size Distribution Uniformity for Nuclear-Grade Austenitic Stainless Steel through Thermomechanical Treatment.

In this work, thermomechanical treatment (single-pass rolling at 800 °C and solution treatment) was applied to nuclear-grade hot-rolled austenitic stainless steel to eliminate the mixed grain induced by the uneven hot-rolled microstructure. By employing high-temperature laser scanning confocal microscopy, microstructure evolution during solution treatment was observed in situ, and the effect of single-pass rolling reduction on it was investigated. In uneven hot-rolled microstructure, the millimeter-grade elongated grains (MEGs) possessed an extremely large size and a high Schmid factor for slip compared to the fine grains, which led to greater plastic deformation and increased dislocation density and deformation energy storage during single-pass rolling. During subsequent solution treatment, there were fewer nucleation sites for the new grain, and the grain boundary (GB) was the main nucleation site in MEGs at a lower rolling reduction. In contrast, at a higher reduction, increased uniformly distributed rolling deformation and more nucleation sites were developed in MEGs. As the reduction increased, the number of in-grain nucleation sites gradually exceeded that of GB nucleation sites, and in-grain nucleation preferentially occurred. This was beneficial for promoting the refinement of new recrystallized grains and a reduction in the size difference of new grains during recrystallization. The single-pass rolling reduction of 15-20% can effectively increase the nucleation sites and improve the uniformity of rolling deformation distribution in the MEGs, promote in-grain nucleation, and finally refine the abnormally coarse elongated grain, and eliminate the mixed-grain structure after solution treatment.

Discontinuous dynamic recrystallization behavior and texture evolution of large-size pure titanium flat ingot via electron beam cold hearth melting during short process direct hot rolling

Large-size pure titanium ingots melted by single electron beam cold hearth melting (EBCHM)) could be rolled into large-width and heavy-weight titanium strip coils using short process without forging. It is crucial to study the large-span refinement mechanism from highly coarse grains, whose sizes are in the order of ∼cm, to micronmeter. In this study, the recrystallization mechanism and texture evolution of pure titanium with a size of 1570 × 200 × > 8000 mm was investigated by multi-pass simulated rolling and experimental rolling. The results indicated the hot deformation can result in marked reduction in grain size of the as-cast grain, from 3 to 5 cm to 17–23 μm. The discontinuous dynamic recrystallization (DDRX) mechanism of a pure titanium ingot during direct multi-pass hot deformation causes the reconstruction of as-cast microstructure and refinement of hot rolling microstructure. Furthermore, DDRX occurred, during hot deformation, and was accompanied with weakening of deformation texture. Raising the initial hot deformation temperature results in a modification of the texture type of the {0001} crystal plane. The strength of the texture experiences a decrease initially and then eventually an increase. This alteration in strength is inversely related to the degree of dynamic recrystallization. Based on the simulated rolling experiment, the results of experimental rolling also found the coarse as-cast grains was broken and reconstructed. It has strength (average yield strength: 334 MPa, ultimate tensile strength: 371 MPa), ductility (total elongation: 30 %). Such a combination of mechanical properties makes the hot rolling plate a potential choice for sheet practical application.

Effects of Al additions on precipitation, recrystallization and mechanical properties of hot-rolled ultra-super ferritic stainless steels

In this study, 1.0 wt%Al element was added into the traditional 26.8Cr-3.7Mo–2Ni super-ferritic stainless steel to tailor the brittle phase precipitation, recrystallization microstructure and mechanical property via solution annealing at temperatures of 950–1150 °C. The experimental results demonstrated that hot-rolled microstructure for 1.0Al sample consisted of surface fine grain region and core elongated grains together with TiN, NbC and Laves phase, while none of these fine grains appeared in 0Al samples. After annealing below 1050 °C, both σ-phase and Laves phase were formed in Al-free samples, while only abundant Laves phase precipitated in 1.0Al sample indicating the inhibition effect of Al on the σ-phase and the promotion effect on Laves phase. When annealing above 1050 °C, recrystallization was completed and neither σ-phase nor Laves phase were observed. Relatively stable mean grain size was produced due to the Laves phase pinning effect during annealing at 1050 °C, while the Nb/Al solute drag effect bore responsibility for the situation at 1050–1150 °C. Sudden grain coarsening phenomenon appeared after annealing above 1150 °C. Al additions gave a solution strengthening and intensive Laves phase precipitation strengthening for the designed alloys, and good comprehensive mechanical properties were obtained demonstrating the probability of developing of the Al-modified ultra-super stainless steels.

Development of Desirable Fine Ferrite Grain Size and Random Second Phase Dual-Phase Steel Microstructures Using Composition and/or Processing Modifications

Microstructural morphology is known to have a significant impact on the mechanical properties of dual-phase steels. A fine ferrite grain size and random distribution of small second phase islands are desirable to provide superior isotropic properties compared to the banded second phase distribution that is typical for this type of steel. A rapid alloy prototyping (RAP) facility has been used to investigate three different DP 800 variants by systematically varying the compositions and/or process parameters compared to the ‘standard’ DP800 composition and processing that gives a banded microstructure. For Variant 1, the heating rate during the annealing cycle after cold rolling varied between 0.65 and 30 °C/s for the 45%, 60% and 75% cold reduction samples. It was found that a cold reduction of 75% and heating rate of 15 °C/s resulted in the microstructure that can give the best combination of strength and ductility because of the fine grain size and high martensite volume fraction. For Variant 2, the effect of changing the hot rolled (HR) microstructure (ferrite–pearlite, ferrite–bainite or martensite) on the final microstructure was investigated. Both the ferrite–50% bainite and fully martensite/bainite HR materials for all cold reductions resulted in annealed microstructures with necklace martensite morphology and finer ferrite grains compared to the ferrite–pearlite HR material, which gave a typical banded ferrite–martensite microstructure with a coarser ferrite grain size. For Variant 3, the Mn content was reduced, and increased Nb was used to achieve higher pancaking during the hot rolling stage, which refined ferrite grains in the HR condition with the same hardness. After annealing with the standard parameters only the 45% cold-reduced material produced a finer ferrite grain size than the standard material, whereas the 60% and 75% cold-reduced samples required a higher heating rate to achieve finer ferrite grain sizes due to rapid recrystallisation and growth kinetics.

Tribology behaviors of Ti–Ni51.5 at% shape memory alloy with different microstructures and textures

The microstructure characteristics of TiNi alloy were investigated to elucidate the active deformation mechanisms during hot rolling and the corresponding tribology performance. The deformation was mainly dominated by a combination of mechanical twinning involving the {114}B2 and {112}B2 twins, dislocations, and stacking faults during the manufacturing process. Moreover, various heat treatments were performed to differentiate the discrepancies in tribological behaviors of hot-rolled and heat-treated alloys. Hot-rolled microstructures with high defect density approximate to nanoscale precipitates through aging as the wear resistance of the alloy improves, albeit it is softer compared to the aged one. As demonstrated by the X-ray diffraction (XRD) results of the wear tracks, microstructures undergo the stress-induced martensitic (SIM) transformation during dry sliding. From differential scanning calorimetry (DSC) analysis, the martensitic transformation temperature is lowered by defects in the austenite phase. The activation of the SIM transformation can be a sluggish process owing to the effect of twinning, where both primary and secondary twins are involved. Furthermore, a reversible transformation that is related to pseudoelasticity causes a decrease in wear volume at room temperature, which is higher than the austenite finish temperature. Meanwhile, abrasive wear advances toward adhesion. Additionally, the hot-rolled sheet with γ fiber-{111}//ND (normal direction) exhibits better wear resistance compared to the sheet with α fiber-{100}//ND.

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.

The significance of hot rolled microstructure controlled by fine-tuning Al content to texture evolution and magnetic properties of low silicon non-oriented electrical steels

In non-oriented electrical steels, the initial hot rolled microstructure usually had important effects on the successive microstructure and texture development during cold rolling and annealing and the final magnetic properties. In this paper, we reported that the hot rolled microstructure of a 0.4 wt% Si steel could be controlled by designing Ar1 (γ/α phase transformation temperature), which was susceptive to the content of Al. Three steel slabs with different Al contents (0.41, 0.30 and 0.21 wt%) were produced and then subjected to the same hot rolling, cold rolling and annealing treatment in an industrial production line. Interestingly, it was found that, the heavily elongated deformed grains were reduced or even avoided in the hot rolled sheets by decreasing Ar1 with a slight decline of Al. As a result, after cold rolling and annealing, weak γ-fiber (〈111〉//ND) texture and bigger grains were obtained. Hence, the magnetic properties were significantly improved, i.e. high magnetic induction (B50) together with low iron loss (P1.5/50). The microstructure and texture evolutions of these three steels throughout the whole processing route were comparatively investigated in detail, and the effects on magnetic properties were discussed in terms of the classical loss separation theory. This work also provides a promising method to optimize the microstructure, texture and magnetic properties for other low silicon non-oriented electrical steels.

Effect of Annealing Prior to Cold Rolling on the Microstructure Evolution and Energy Losses of Low-Si, Ultra-Low-C Hot-Rolled Electrical Steel

The microstructure of a low Si, ultra-low-C, hot-rolled electrical steel strip is modified by annealing at T &lt; To, the α→γ transformation temperature. This heat treatment causes the abnormal anisotropic growth of surface grains which consumes the original hot-rolled microstructure. The growth of the surface grains first takes place parallel to the rolling direction and then in a columnar form parallel to the normal direction until grains growing in opposite directions from the surfaces impinge at the center of the strip. It is shown that cold rolling and a short annealing treatment at temperatures between 700 and 800 °C leads to microstructures which result in iron energy losses that can be as much as 30% lower than those observed in the same material not subjected to the annealing prior to cold rolling. The magnitude of the reduction in energy losses depends on strip thickness and processing parameters. The major effect is observed in material annealed at 710 °C and the relative effect (with respect to material that is not annealed prior to cold rolling) decreases as the strip thickness decreases. It is shown that these effects can be attributed to the effect of the processing conditions on texture and grain size. The maximum reduction in energy losses is observed when the final microstructure consists of ferrite grains ~1.5 times larger than those obtained if the material is not annealed prior to cold rolling.

The effects of precipitates on microstructure and β-fiber texture in an Al-Cu-Li alloy during hot rolling

The hot rolling microstructures and textures were strongly influenced by the prior heat treatments. Various prior heat treatments were utilized to investigate the effects of precipitates on microstructure and β-fiber texture after hot rolling. For the microstructure, the extent of planar slips strongly affects the hot rolling microstructure. Shear events of δ′ particles resulted in massive shear bands, which were the consequence of extensive planar slips. T1 platelets and T2 rods would homogenize the dislocation slips, resulting in a homogenous microstructure. The abnormal microstructure in overaged specimens was ascribed to successive work softening in the precipitation free zones (PFZs). With respect to hot rolling texture, the kind of precipitates in the matrix affected the intensities and locations of the β-fiber tube in Euler space, especially in the section between Copper and S orientation.

Dynamic recrystallization control in hot rolling

Recrystallization is important for steel processing because it modifies microstructure along the process route and thus determines the final properties of steel products. Loads in various deformation operations and consistency of dimensions and shape of deformed steels are also influenced by recrystallization and its homogeneity. These factors are especially important in hot rolling that aims at uniform refined hot rolled microstructure. This can be achieved by avoiding dynamic recrystallization (DRX) of austenite during hot rolling passes and static recrystallization between them and before cooling. Alternatively, microstructure can be refined by stimulating recrystallization. Experimental data and constitutive analysis show that for a given steel there exist two limiting values of Zener-Hollomon parameter Z that delineate three temperature – strain ranges for DRX: no-DRX range (high Z) where DRX cannot be initiated irrespective of the applied strain, full DRX range (low Z) and partial DRX range (intermediate Z). Avoiding DRX in finishing rolling sequence implies setting up rolling speeds and temperatures in no-DRX range of Z. Alternatively, for roughing sequence the mill settings should be such as to keep rolling speeds and temperatures in the full DRX range. In any case it is imperative to avoid partial DRX, i.e., to avoid rolling speeds and temperatures that correspond to intermediate Z range. This approach can be directly utilized to control DRX in hot strip mills and plate mills based on the experimental data.

Dynamic precipitation and recrystallization in Al-12.5 wt%Si-0.6 wt%Mg-0.1 wt%Ti alloy during hot-rolling and their impacts on mechanical properties

Dynamic precipitation and recrystallization behaviors in Al-12.5 wt%Si-0.6 wt%Mg-0.1 wt%Ti alloy during hot-rolling and their impacts on mechanical properties were investigated by microstructure characterization and tension test. The results show that higher temperature suppresses the dynamic precipitation of Mg2Si phase to a great extent, however, larger reduction ratio enhances this behavior. Phase distribution maps by EBSD strongly indicate that rolling above 500 °C leads to significant dynamic recrystallization. Increasing either the rolling temperature or reduction ratio favors dynamic recrystallization and refines the recrystallized grains. The reduction ratio has more distinguished effect than the temperature. Though dynamic recrystallization occurs remarkably, the deformation textures are dominant in the hot-rolled microstructure. Increasing either the temperature or reduction ratio make the texture intensity enhance. After rolling, the yield strength (YS) of all samples is improved compared with as-homogenized state except for the sample rolled at 500 °C with 33% reduction. At the same rolling reductions, the increment in strength by increasing the rolling temperature is resulted from the combination of diminished Mg2Si precipitation and refined recrystallized grains. At the same rolling temperatures, the increment in strength by increasing the rolling reduction is mainly attributed to the high intensity of deformation texture and high density of dislocations retained in recrystallized grains.

Microstructure and Properties of 1.0C–1.5Cr Bearing Steel in Processes of Hot Rolling, Spheroidization, Quenching, and Tempering

A novel online subcritical spheroidization annealing technology is proposed. To verify the validity and advantage, microstructure, and properties of 1.0C–1.5Cr bearing steel in three process of hot rolling, spheroidization, quenching, and tempering are investigated. It is demonstrated that the refining of hot rolled microstructure is beneficial to obtaining finer spheroidized microstructure, which accelerates cementite dissolution during the austenitization process of subsequent quenching treatment. When the ferrite grain size of spheroidized microstructure decreases from 8.3 to 1.7 µm, and mean diameter of spheroidized cementite decreases from 0.38 to 0.22 µm, the prior austenite grain size of the microstructure quenched from 850 °C decreases by about 27%, and mean diameter of undissolved cementite decreases by at least 20%. The fatigue limit of the tested steel after quenching and tempering is measured. It is increased from 789 to 986 MPa due to the comprehensive effect of the refining of both undissolved cementite and prior austenite grain, and the increase of retained austenite content in the martensite matrix.

Effect of Ti on the Microstructure and Mechanical Properties of a Hot Rolled Advanced High Strength Steel Strip

The addition of titanium is a well-known microalloying concept for hot rolled structural steels. Concerning advanced high strength steels for the automotive sector, the use of Ti microalloying (usually with Nb-V) has been an active research area. However, Ti addition has not been explored in depth. For the current contribution, a laboratory hot rolled 0.2C-2.4Mn-1.5Si steel with and without Ti addition was studied. Mechanical testing of the hot strip revealed a very high UTS (1GPa) for the Ti added steel, whilst for the unalloyed chemistry the UTS was some 300 MPa lower. Observation of the hot rolled microstructures via optical microscopy showed a significantly higher hardenability for the Ti added steel. Moreover, X-ray diffraction analysis indicated a significant amount of retained austenite in the Ti added strip, which transformed completely to martensite after the tensile test. Further analysis via TEM and chemical extraction indicated that Ti was present both as Ti (C,N) precipitates and in solution. Finally, in light of these observations, the possible mechanisms leading to the enhanced hardenability observed for the Ti added hot rolled strip steel were discussed.

Exploring the influence of Al content on the hot deformation behavior of Fe-Mn-Al-C steels through 3D processing map

The effect of Al content on the hot deformation behavior of high-Mn low density FeMnAlC steels was investigated by the 3D processing map at the temperatures of 850–1050 °C and the strain rates of 0.001–10s−1. The high-Al steel showed a higher flow stress and a greater activation energy (443 kJ/mol) in contrast to the low-Al steel (394 kJ/mol). The microstructures of 8Al steel and 10Al steel depends on the deformation parameters. The initial hot rolling microstructure has been displaced by the recrystallized structure and substructures for both steels, while the unstable zone has deformation bands and flow localization. As the Z content increases with increasing Al content, this leads to a significant inhibition of the DRX process, resulting in an unstable domain with deformation zones and flow localization. For high-Al steel, the morphology and distribution of ferrite transform from the continuous band ferrite to discontinuous granular ferrite, moreover, the flow localization features cannot be observed with the decrease of strain rate to 0.001s−1. Furthermore, the proportion of instability region for each of the strains increases with the increasing of the Al content, the high Al content weakens the workability of the FeMnAlC steels.

Effect of Microstructure on Post-Rolling Induction Treatment in a Low C Ti-Mo Microalloyed Steel

Cost-effective advanced design concepts are becoming more common in the production of thick plates in order to meet demanding market requirements. Accordingly, precipitation strengthening mechanisms are extensively employed in thin strip products, because they enhance the final properties by using a coiling optimization strategy. Nevertheless, and specifically for thick plate production, the formation of effective precipitation during continuous cooling after hot rolling is more challenging. With the aim of gaining further knowledge about this strengthening mechanism, plate hot rolling conditions were reproduced in low carbon Ti-Mo microalloyed steel through laboratory simulation tests to generate different hot-rolled microstructures. Subsequently, a rapid heating process was applied in order to simulate induction heat treatment conditions. The results indicated that the nature of the matrix microstructure (i.e., ferrite, bainite) affects the achieved precipitation hardening, while the balance between strength and toughness depends on the hot-rolled microstructure.

A Study of the Optimum Quenching Temperature of Steels with Various Hot Rolling Microstructures after Cold Rolling, Quenching and Partitioning Treatment

Quenching and partitioning (Q&amp;P) processes were applied to a cold-rolled high strength steel (0.19C-1.26Si-2.82Mn-0.92Ni, wt %). The effects of the prior hot-rolled microstructure on the optimum quenching temperature of the studied steels were systematically investigated. The microstructure was analyzed by means of transmission electron microscope (TEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). Compared with the ferrite pearlite mixture matrix, the lower martensite start (Ms) temperature and smaller prior austenite grain size in the cold-rolled martensite matrix are the main reasons for the optimum quenching temperature shifting to a lower temperature in the Q&amp;P steels. We found that an empirical formula that only considers the influence of the alloy composition in the calculation of the Ms temperature will cause a certain interference to the pre-determined optimum quenching temperature of the Q&amp;P steel.

Microstructural Characterization of Metastable Beta Titanium Alloys in Hot Rolled and Solution Treated condition

This paper presents the microstructural characterization of metastable beta titanium alloys (Ti-5Al-5Mo-5V-3Cr (A1), Ti-5Al-3.5Mo-7.2V-3Cr (A2), Ti-5Al,-5Mo-8.6V-1.5Cr (A3) and Ti-5Al-3.5Mo-5V-3.9Cr (A4) in both hot rolled and solution treated conditions. The microstructures of the hot rolled alloys A1, A3 and A4 exhibit the presence of in-grain shear bands. The alloy A2 does not contain in-grain shear bands. The hot rolled microstructure consists of two phases namely, β (matrix) and α. Microstructures with similar equiaxed β grains with average size of ∼54±1 μm have been obtained with different combinations of solution treatment temperatures and time. The kinetics of phase transformation for obtaining equiaxed microstructure with similar grain size of all the four alloys is different.

Improvement of magnesium alloy edge cracks by multi-cross rolling

In order for the edge cracks during the AZ31 Mg alloy rolling to be reduced, the hot-rolling microstructure and texture were modified through multi-cross rolling (MCR) during the deformation. The AZ31B magnesium alloy sheets were hot rolled at a temperature ranging from 250 °C to 400 °C and at a rolling speed of 0.5 m/s. Four different multi-cross rolling (MCR) routes were selected in the test. The macroscopic morphology, microstructure and texture of the as-rolled AZ31B sheets were characterized to investigate the edge-crack behavior during the rolling with various rolling routes and temperatures. Through the various rolling routes comparison, the grain size, the texture and the twins were the main factors that affected the cracks. It was demonstrated that the grain refinement and weak basal textures obtained by RII where the rolling direction was changed by 90° between two adjacent passes, could significantly reduce the edge cracks during the rolling at an elevated temperature. When rolling with RII at 400 °C, no apparent edge cracks appear on the sheet rolled with four passes. The propagation mechanism of the crack tips was studied in details at the temperature of 350 °C, whereas the results demonstrated that a high-sized area of grain boundaries in-between the finer grains could increase the crack propagation resistance.