Hot-rolled Specimens Research Articles

Effect of hot rolling process on the microstructure and mechanical properties of a high-strength strip casting microalloyed steel

A novel Nb–V–Mo microalloyed steel with 1 GPa yield strength and high ductility was developed by strip casting. The effects of hot rolling on the microstructure and mechanical properties of the Nb–V–Mo microalloyed steel were investigated using scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, atom probe tomography and tensile tests. Both the as-cast and hot-rolled specimens exhibit a microstructure characterized by lath-like bainite and (Nb, V, Mo)C clusters, while the bainite lath was refined after hot rolling. Tensile test results show obvious improvements in the yield strength, tensile strength, and elongation of the hot-rolled specimen (1008 MPa, 1075 MPa, 20.1 %, respectively), compared to that of the as-cast ones (879 MPa, 978 MPa, 19.0 %, respectively). Additionally, the hot-rolled specimen displays a similar dislocation multiplication and storage capacity to the as-cast specimen, resulting in a comparable work-hardening capability during plastic deformation.

Investigations on the influence of hot rolling on the creep characteristics of additively manufactured Mg-5Ag alloy

The requirement for lightweight materials that can survive increasingly severe conditions for aerospace components has fueled the research on magnesium alloys. The current work investigates the influence of hot rolling on the creep characteristics of the Mg-5Ag alloy that is fabricated through a selective laser melting process. The creep properties of parent material (PM) and hot-rolled specimens (HRA) are determined by indentation creep tests at 250 °C, 275 °C, and 300 °C. The microstructural evolution, phase transformation kinetics, and texture evolution were deduced using instrumental analysis, including SEM, EBSD, TEM, and XRD. The specimen PM features α-Mg grains with intermetallic Mg-Ag particles along grain boundaries and random grain orientations with low-angle boundaries. However, specimen HRA has a more fraction of Mg-Ag precipitates, a dominant (0001) grain orientation, and higher peak intensity from the (002) plane. The HRA specimen also demonstrates higher apparent activation energies, attributed to microstructural changes from hot rolling. The results show that the creep resistance of the hot rolled Mg-5Ag alloy is superior to as-fabricated Mg-5Ag alloy based on the creep velocity, stress exponent, and apparent activation energy.

Combined effects of corrosion and fire on load-carrying response of hot-rolled steel reinforcement

PurposeAlthough separate studies on the influence of corrosion and fire exposure on the constitutive relationship of concrete and steel have been done, there is still a gap in knowledge on the influence of corrosion-temperature superimposition as nonlinear phenomenon. The current study is focused to investigate the response of hot-rolled steel bars subjected to corrosion-temperature superimposition.Design/methodology/approachUsing the accelerated corrosion-impressed-current technique, hot-rolled specimens with different levels of corrosion were obtained. The hot-rolled rebars were first corroded to target levels such as (6, 12, 18, 24, 30 and 36%) and subsequently subjected to target temperatures (250 °C, 400 °C, 550 °C, 800 °C and 950 °C), before tensile tests were carried out to evaluate the residual mechanical response.FindingsThe outcomes showed a significant decline in the parameters governing the mechanical properties of steel reinforcement due to the combined damage due to corrosion and fire. Corroded reinforcement still showed ductile failure after exposure to fire. Moreover, the combined loss of load-bearing characteristics due to corrosion and fire has little influence on the modulus of elasticity. The outcomes of this investigation provide a theoretical database for the assessment of aged structural elements exposed to combination after exposure to fire.Originality/valueThe information concerning structural material's response to corrosion-temperature combined damage is still limited. The cover of the reinforcement is designed to safeguard the reinforcing bars from foreign agencies but is often damaged and spalled off due to corrosion, rendering the reinforcing bars directly exposed. The study aims at the experimental production of fire conditions in a corrosion-damaged infrastructure to cover the aforementioned research gap. The effects of corrosion being superimposed by exposure to elevated temperatures on key parameters affecting mechanical behavior were examined.HighlightsInfluence of corrosion-temperature superimposition on the mechanical properties of hot-rolled rebars.Influence of corrosion-temperature superimposition on the macro and microstructure properties of hot-rolled rebars.Influence of corrosion-temperature superimposition on stress-strain curves of hot-rolled rebars.Influence of corrosion-temperature superimposition on tensile strength, modulus of elasticity and elongation of hot-rolled rebars.

Enhancing Wear Resistance in Functionally Graded Metallic Components: Insights from Nanoindentation and Mechanical Analysis.

To manufacture metallic components with high wear resistance, treatments such as nitriding and carburising followed by quenching and tempering (NQT and CQT, respectively) are applied to various types of steel to increase the hardness (H) of the friction surface. However, the wear mechanism of the resulting functionally graded materials has not been fully understood. In this study, specimens of industrial 99.82% pure iron treated with NQT at 913 and 1033 K, and CQT at 1203 K, as well as hot-rolled sheets without heat treatment were examined by performing nanoindentation tests to measure changes in their H, reduced Young's moduli (Er), elastic deformation energies (We), and plastic deformation energies (Wp) along the depth direction. The relationship between Wp/We and the elastic strain resistance (H/Er) can be expressed for all specimens via the equation Wp/We = -1.0 + 0.16 (H/Er)-1. Furthermore, the obtained H/Er av measured at 5 µm intervals based on the specimen profile and wear volume has a good correlation depending to the sliding distance, as confirmed by the results of the ring-on-plate sliding tests conducted for the carbon-treated, nitrogen-treated, and hot-rolled specimens. This study provides a new approach, using H/Er parameters to identify the dominant factors affecting wear resistance at the initial stage of wear that may contribute to the development of wear-resistant materials.

A Study of a Cryogenic CuAlMn Shape Memory Alloy

In extreme temperature environments, a newly emerging engineering application involves both the active and passive control of structures using cryogenic shape memory alloys, which are smart materials able to recover high deformation below the freezing point. With the objective of carrying out new advances in this area, the present work aims to investigate the Cu-7.5Al-13.5Mn (wt.%) shape memory alloy. Thermal, microstructural, and thermomechanical analyses of as-cast and hot-rolled specimens were performed, taking into account the effects of annealing and solubilization. It was observed that the phase transition occurs at temperatures below 120 K and changes according to the thermo-mechanical path. Specifically, hot-rolling lowers the phase transition temperature range with respect to the as-cast condition–from 34 K to 23 K for Mf, and from 89 K to 80 K for Af. Additionally, when the annealing temperature rises, the phase transformation temperature increases as well, and the alloy loses its cryogenic features when heat treated above 473 K. Finally, loss factors of 0.06 and 0.088, which were respectively found in dynamic and static settings, validate the material’s good damping response.

Influence of boron addition on microstructure and mechanical properties of medium-Mn advanced high-strength steel

This study investigates the effects of 0.12 wt% boron addition on the microstructure and mechanical properties of a Fe-0.08C-4.5Mn-2.5Cr–1Si medium-Mn steel in as-rolled conditions and after the intercritical annealing process. In the as-rolled conditions, boron addition increased the strength and reduced the alloy ductility. In as-hot rolled conditions, an ultimate tensile strength (UTS)/elongation of 1195 MPa/5.5% and 1008 MPa/7% was achieved in the boron-added and boron-free alloys, respectively. However, the effect of boron on the intercritically annealed specimens was diverse. The intercritical annealing had no tangible effect on the mechanical properties of the hot-rolled specimen. However, the intercritical annealing at 700 °C for 600 s led to a UTS/elongation of 1029 MPa/19.5% in the boron-added alloy, the highest UTS/elongation combination among all specimens. The formation of carbo-boride precipitates on lath boundaries of the boron-added alloy possibly locked the boundaries thereby a temper-resistant martensitic structure was attained. The lath structure of the annealed alloy caused an enhanced strength in the boron-added specimen. Nonetheless, in the boron-free alloy, martensite was normally tempered and a considerable reduction in the strength was observed.

Fatigue crack size evaluation using acoustic emission signals for wire and arc additive manufactured material

As one of the promising additive remanufacturing techniques, the wire and arc additive manufacturing has shown great potential in automated repair and restoration. However, the layer-by-layer fabrication process may lead to an uneven heating and cooling rates, which has a significant effect on the microstructure and mechanical behavior, as compared to conventionally processed materials. In this study, the fatigue performance of the low-carbon steel components made by wire and arc additive manufacturing is studied systematically. The components made by conventional hot-rolled method is used for comparison purposes. By leveraging the advanced causal network and graph neural networks techniques, a novel method for online fatigue crack size evaluations is proposed using acoustic emission signals. Low dimensional latent variables are extracted from the acoustic emission signal through a one-dimensional convolutional autoencoder (1D-CAE), and the data-driven causal discovery algorithm is adopted to construct the causal network of these latent variables, which characterizes their inherent cause-effect relations. The graph attention network (GAT) is adopted to aggregate a node information with its parent nodes as the node embedding in a causal network. Fully connected layers are constructed between the node embeddings and fatigue crack size label. For online scenarios where limited data is available, the above GAT model previously trained is transferred with different strategies for crack size evaluation. Experimental data with three different scenarios: specimens with same direction, specimens with different direction and hot-rolled specimens are employed to validate the proposed online fatigue crack size evaluation method.

Effects of hot isostatic processing and hot rolling on direct energy deposited CoCrNi medium entropy alloy – Microstructural heterogeneity, wear behaviour and corrosion characteristics

Metal additive manufacturing, particularly the direct energy deposition (DED) method, has rapidly evolved over the last decade and become a prime technique to revolutionize the fabrication sector, particularly for aerospace, automotive, and biomedical industries. In this study, the DED-printed CoCrNi medium entropy alloy has been subjected to two alternative processing methods: hot rolling and hot isostatic pressing. The effects of processing routes on microstructural evolution, defect minimization such as porosities, dislocations, and their cumulative impact on mechanical properties and wear response have been studied. The diverse responses of the material surface parallel and perpendicular to the build direction were rationalized based on the repetitive and complex heating of selective surfaces during deposition. Compared to the hot isostatic processed specimen, the hot rolled specimen illustrates a ∼ 40% increase in hardness and subsequent ∼21% reduction in wear rate for the material parallel to the build direction, whereas ∼34% increment in hardness and ∼ 19% reduction in wear rate for the material perpendicular to the build direction. Further, the effect of processing routes on the corrosion behaviour was recorded in 3.5 wt% NaCl solution using potentiodynamic polarization test. Hot isostatic pressed specimen demonstrates better corrosion resistance (∼37%) than hot rolled specimen due to the lesser number of grain boundaries or corrosion nucleation sites.

Structure-property relationship in vanadium micro-alloyed TRIP steel subjected to the isothermal bainite transformation process

Structure-property relationship in vanadium micro-alloyed TRIP steel is investigated focusing on austenite transformation, precipitation behavior and mechanical performance. Microstructural evolution of the steel during processing was characterized mainly using EBSD, TEM and APT techniques; and its influence on mechanical properties was discussed. The TRIP steel exhibits complex microstructure including intercritical ferrite (IF), bainitic ferrite (BF), retained austenite (RA) and martensite. After identical intercritical annealing (IA), IF fraction keeps constant, BF and RA fractions increase while martensite fraction decreases with decreasing isothermal bainite transformation (IBT) temperature from 470 to 380 °C. The initially precipitated vanadium carbides (VC) in the hot-rolled specimen undergo coarsening during IA process, accompanied with some newly precipitated VC in either ferrite or in austenite, confirmed by the co-existence of small-sized and coarsened VC after IA process, as well as occurrence of K–S orientation relationship between VC and the α-Fe matrix. Ultra-fine average grain size of less than 1 μm in the TRIP heat-treated steel is achieved by the refining effect of these nano-scale VC precipitates, which can inhibit phase boundary (PB) migration during IA process and promote bainite transformation during subsequent IBT process. The higher volume fraction of RA accompanied with higher carbon enrichment intensifies sustainable strain hardening and finally increased ductility of the specimens treated under lower IBT temperature. In the IBT-380 specimen with the highest RA and BF fractions, the occurrence of micro-yielding phenomenon is a result of the Cottrell atmosphere in soft domain BF and stress relaxation due to RA to martensite transformation, during which the soft domain IF/RA/BF and hard domain martensite yields step by step. An excellent mechanical performance is thus achieved in the IBT-380 specimen, meeting the requirements for the 3rd generation advanced auto-steel.

Hydrogen embrittlement characteristics in cold-drawn high-strength stainless steel wires

Hydrogen uptake and embrittlement characteristics of a cold-drawn austenitic stainless steel wire were investigated. Slow strain rate testing and fracture surface analysis were applied to determine the hydrogen embrittlement resistance, providing an apparent decrease in resistance to hydrogen embrittlement for a 50% degree of cold deformation. The hydrogen content was assessed by thermal desorption and laser-induced breakdown spectroscopy establishing a correlation between the total absorbed hydrogen and the intensity of near-surface hydrogen. The sub-surface hydrogen content of the hot-rolled specimen was determined to be 791 wt.ppm.

Study on the role of cryogenic treatment on corrosion and wear behaviors of high manganese austenitic steel

In this study, electrochemical corrosion and wear experiments were conducted to clarify the role of cryogenic treatment on the microstructural morphology and resistance to corrosion and wear of high-manganese austenitic steel. Heat treatment of the solution (950 °C) and annealing (500 °C and 600 °C) were performed on the hot-rolled specimens. Cryogenic treatment was applied to hot-rolled, hot-rolled + heat treated specimens. Cryogenic treatment is a practical technology for eliminating the strip morphology of hot-rolled specimens and improves the wear resistance of heat-treated specimens. However, it plays different role on the corrosion resistance of tested specimens related with production process. The wear mechanism differs due to the cryogenic treatment. A schematic model of the wear mechanism is proposed based on the experimental results. Cryogenic treatment coarsens the grain size regardless of the production process.

Effect of intermediate heat treatment during hot rolling on the texture and formability of annealed AZ31 Mg alloy sheets

In this study, we investigated the effect of intermediate heat treatment (IHT) on the texture evolution and stretch formability of annealed AZ31 Mg alloy sheets during different stages of hot rolling. Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) techniques were used to measure the crystallographic texture and to explain how IHT contributes to the texture evolution in hot-rolled specimens. The behavior of static recrystallization (SRX) during IHT was investigated to explain its role in weakening the basal texture in IHT specimens. Furthermore, t-EBSD was used to investigate the orientation distribution of recrystallized grains in the deformed matrix. Moreover, the effect that IHT can exert on the mechanical and Erichsen Index (IE) of annealed sheets was investigated at room temperature, and also at 100 and 200 °C. An analysis of the different IHT specimens indicated that the influence of IHT on the IE was significantly dependent on the measurement temperature.

Microstructural Evolution and Electrochemical Behavior of Solution Treated, Hot Rolled and Aged MgDyZnZr Alloy

In order to develop a potential route to fabricate plates and clips for orthopedic applications, a Mg–3.4Dy–0.2Zn–0.4Zr (wt.%) alloy was produced and analyzed in different conditions: solution treated at 525 °C for 3 h, hot rolled and hot rolled and aged at 250 °C. The aging behavior of the rolled alloy was investigated during isothermal aging at 250 °C, and a significant peak was observed at 10 h. The electrochemical behavior was evaluated in 0.9 wt.% NaCl solution at 37 ± 0.5 °C by potentiodynamic polarization and electrochemical impedance spectroscopy. The 525 °C-3 h and hot rolled specimens exhibited corrosion rates of 2.0 and 1.7 mm/year, respectively. The hot rolled and aged at 250 °C for 10 h specimen presented a grain size of 11.8 ± 1.7 μm with an intense macrotexture of the basal {0002} plane, hardness of 73 ± 3 HV and higher impedance modulus and obtained the highest corrosion resistance with a corrosion rate of 0.9 mm/year.

Investigation of the Microstructure Dependence of Critical Pitting Temperature and Pitting Potential in a 2205 Duplex Stainless Steel

Dependence of 2205 duplex stainless steel pitting resistance on its microstructure was investigated using microstructural examinations and potentiodynamic and potentiostatic polarization techniques. An as-received hot-rolled alloy was subjected to different heat treatments to understand how microstructural characteristics individually affect the corrosion resistance in terms of critical pitting temperature (CPT), pitting potential (E pit), and metastable pitting characteristics. Results revealed that there is a transition temperature interval for the 2205 DSS. By promoting pit initiation and pit propagation, the Widmanstätten structure significantly deteriorated both CPT, by 10 °C, and E pit, compared to the as-received hot-rolled specimen. The σ phase appearance was also associated with a minor drop in CPT, but a considerable reduction in E pit as it increased the occurrence frequency of metastable pits. The specimen having equiaxed grains had the highest CPT, underwent stable pitting at 47 °C, but still it had a low E pit compared to the hot-rolled specimen due to its higher occurrence frequency of metastable pits. The formation of the Widmanstätten austenite needles weakened the ferrite phase in terms of the pitting resistance by reducing the contents of Cr and Mo in it and made it more susceptible to pit propagation.

Effect of Cold Rolling and Hot Rolling on Recrystallization Texture and Superelastic Properties of Ti–30Zr–10Nb–2Sn Shape Memory Alloy

A comparative study on the microstructure, texture evolution, and superelasticity of cold-rolled and hot-rolled Ti–30Zr–10Nb–2Sn (at.%) alloys was performed using electron back-scattered diffraction (EBSD), X-ray diffraction (XRD), and tensile test. The cold-rolled sheet was solution-treated at 1173 K after a 98.0% cold rolling at room temperature. Hot rolling was carried out at 1273 K. Both cold-rolled and hot-rolled sheets were solution-treated at 1173 K for 1.8 ks. Both solution-treated alloy specimens consisted of a single β-phase at room temperature. A very high (200)β peak was observed for the cold-rolled specimen, and a high (110)β peak was observed for the hot-rolled specimen after the solution treatment at 1173 K for 1.8 ks. The {001}β 〈110〉 β recrystallization texture was obtained in the 1173 K solution-treated alloy specimen owing to grain growth after recrystallization. The {001}β 〈110〉 β recrystallization texture was not observed in the hot-rolled specimen; instead, a (221)β[¯1¯14]β recrystallization texture was observed. The 1173 K solution-treated alloy specimen exhibited superelasticity with complete recovery resulting from the {001}β 〈110〉 β recrystallization texture. Superelasticity was also observed for the hot-rolled specimen, but was not complete, probably because of the low critical stress for slip.

Continuous Cooling Transformation of Under-Cooled Austenite of SXQ500/550DZ35 Hydropower Steel

The expansion curves of the continuous cooling transformation of undercooled austenite of SXQ500/550DZ35 hydropower steel at different heating temperatures and cooling rates were measured by use of a DIL805A dilatometer. Combined with metallography and Vickers hardness measurement, the continuous cooling transformation diagrams (CCT) of the studied steel under two different states were determined. The results show that in the first group of tests, after the hot-rolled specimens were austenitized at 920 °C, when the cooling rate was below 1 °C·s−1, the microstructure was composed of ferrite (F), pearlite (P) and bainite (B). With the cooling rates between 1 °C·s−1 and 5 °C·s−1, the microstructure was mainly bainite, and martensite (M) formed as the cooling rate reached 5 °C·s−1. When the cooling rate was up to 10 °C·s−1, the microstructure was completely martensite and the hardness value increased significantly. In the second group of tests, after the hot-rolled specimens were quenched at 920 °C and then heated at an intercritical temperature of 830 °C, in comparison with the first group of tests, and except for additional undissolved ferrites in each cooling rate range, the other microstructure types were basically the same. Due to the existence of undissolved ferrite, the microstructures of the specimens heated at intercritical temperatures were much finer, and the toughness values at low temperatures were better.

Effect of Deformation on Microstructure of 301L Stainless Steel

After hot rolling and multipass cold rolling experiment, the hot rolled sheet and the sheets with reduction ratios of 20% and 40% of 301L stainless steel were obtained. The effect of cold rolling reduction ratio on microstructure of 301L stainless steel was investigated by metallographic analysis, XRD analysis and TEM analysis. The research shows that carbides precipitate along the austenite phase interface in the hot rolled specimen. Crystalline grains are refined by cold rolling: their average grain size decreases from 91.9 to 17.3 nm as the reduction ratio increases, which is accompanied by the blocking of dislocations.

Influence of liquid nitriding on the properties of the electric steel 10

Liquid nitriding (carbonitriding) effect on hardness, wear resistance and magnetic properties of electric steel grade 10 is reviewed here, as well as on the electric contact resistance. It was found that hot-rolled steel 10 loses its hardness as a result of annealing. Surface carbonitriding of specimens generates an improved layer of higher hardness, high wear resistance and increased electrical resistance. The specimens carbonitrided have contact electric resistance higher than that of hot-rolled specimens after descaling. The magnetic permeability of ‘grade 10’ steel does not practically change after annealing and carbonitriding, while the magnetic permeability corresponds to the magnetic field strength in a very typical way with typical parameters for carbon steels. Traditional methods of processing electrical steels, carried out in order to achieve such a complex effect, combine heterogeneous processes, such as electroplating, vacuum, plasma, high - temperature annealing, coating-are very expensive and difficult to implement. Carbonitration is more accessible for practical use and has a significantly lower cost. The results obtained can be used in the design of magnetic circuits for various electric devices.

Effect of As-Cast Structure and Macrosegregation on Mechanical Properties in Direct-Quenched Low-Alloy Ultrahigh-Strength Steel

The effect of as-cast structure and macrosegregation on the mechanical properties of a direct-quenched low-alloy martensitic ultrahigh-strength aluminum killed and calcium treated steel cast at different superheats was studied. Samples from the castings were laboratory hot rolled with two different finishing rolling temperatures to distinguish the effect of hot rolling. Using optical emission spectrometry, the steel composition was analyzed as a function of slab thickness in order to detect the variations in steel chemistry due to macrosegregation. Further, hardness profiles, prior austenite grain sizes and tensile and impact toughness were determined for the hot-rolled specimens. It was found that interdendritic segregation was more intense at the higher superheat, which led to more pronounced positive segregation in the columnar-to-equiaxed transition (CET) zone, and negative segregation between CET and the centerline. These macrosegregation patterns were inherited by the hot-rolled samples causing local variations in hardness, which followed the variations in carbon content. However, altering the superheat had a minor effect on the nominal transformed microstructures and nominal prior austenite grain sizes. This occurred because of the interdendritic segregation induced composition variations both enlarged and decreased by turns the grain sizes. The CET also reduced measured impact toughness values.

Effect of multiaxial forging followed by hot rolling on non-basal planes and its influence on tensile and fracture toughness behaviour of Mg–4Zn–4Gd alloy

Large true strain (up to ~4.7) through hybrid severe plastic deformation (HSPD) technique was imparted to Mg–4Zn–4Gd alloy, to study its influence on microstructural evolution, fracture toughness and tensile properties in the present work. The tensile and fracture toughness of processed Mg alloy were compared with solutionized bulk Mg alloy. The tensile properties are found to be superior in multiaxially forged followed by repetitive rolled specimen at 723 K compared to ST sample. The key strengthening mechanisms such as microstructural bimodality, coherency of the twin boundaries, and precipitation are characterized in detail, to elucidate the enhancement in hardness (~61%), tensile (~62%), yield (~42%) and elastic-plastic fracture toughness (~40%) properties. These improvements are observed in the optimised thermo-mechanically processed condition “6 pass hot forged followed by 75% repetitive hot rolled (6PF-75R) specimen”. However, high dislocation density observed in 6PF-75R specimen leads to lower the ductility compared to other HSPDed conditions but still found higher than that of the solutionized specimens. The fracture phenomena were also explored through post-deformation processed samples through scanning electron microscopy based fractograph to understand the response of different processed specimens using tension and fracture tests. Various microstructural features such as microshear bands, nano-twinning, slip clusters and especially the effect of continuous dynamic recrystallization upon varying true strain are examined by using high-resolution transmission electron microscopy (HR-TEM) and low angle annular dark field scanning transmission electron microscopy (LAADF-STEM) techniques. Correlation of these features with tensile and fracture behaviour are made with the support of light microscopy and X-ray diffraction techniques.