Warm Rolling Process Research Articles

Analysis of medium manganese steel through cold-rolling and intercritical annealing or warm-rolling

ABSTRACTMedium manganese steel is typically fabricated from hot-rolling followed by cold-rolling and intercritical annealing processes. However, a singular process, warm-rolling, is an appealing prospect. The microstructure, mechanical properties and texture of a 8 wt-% Mn–0.08 wt-% C steel was investigated by cold-rolling followed by intercritical annealing or warm-rolling. The product from both processes exhibited a highly refined microstructure as well as a large volume fraction of austenite. However, a slightly larger volume fraction as well as a more heterogeneous austenite morphology resulted from the warm-rolling process, associated with a higher work-hardening rate. Microstructural analysis revealed intense α-fibre components due to a higher strain following the warm-rolling process as well as a considerably high-angle grain boundary number which was associated with dislocation accumulation.This paper is part of a Thematic Issue on Medium Manganese Steels.

Improving Tensile Properties of Room-Temperature Quenching and Partitioning Steel by Dislocation Engineering

A new strong and ductile steel is developed by combining the dislocation engineering concept with the room-temperature quenching and partitioning process. The dislocations generated during the warm rolling process are inherited by both martensite and austenite grains during water quenching. The warm rolling process refines the martensite block size and increases the austenite volume fraction, owing to the dislocation stabilization mechanism, resulting in improvement of the ultimate tensile strength and uniform elongation simultaneously.

Super-high-strength and formable medium Mn steel manufactured by warm rolling process

In this paper, we demonstrate the design and manufacture of super-high strength medium Mn steel with good ductility, via the combination of warm rolling and V alloying. This new strategy could produce not only the extensive precipitation of fine VC particles in both ferrite and austenite but also the bimodal size distribution of retained austenite (RA) grains. A substantial increase of yield strength up to 650 MPa was achieved after the intercritical annealing of this V-alloyed medium Mn steel without deteriorating ductility; whereas the calculated precipitation strengthening using the classical Ashby–Orowan theory is no more than 400 MPa. Quantifying possible strengthening mechanisms revealed that dislocation strengthening should make an additional contribution because recovery in austenite could be strongly retarded by the nanosized precipitates. In this V-alloyed steel, the coarse RA grains firstly transformed to martensite during yielding at the much higher stress threshold, followed by a significant work hardening due to dislocations multiplication, twinning-induced-plasticity (TWIP) and transformation-induced-plasticity (TRIP) in the ultrafine RA grains. The best combination of strength and ductility included 1.5 GPa ultimate tensile strength and 28% total elongation. This was achieved due to the sustainable TRIP and TWIP effects in the later straining stage resulting from the ultrafine austenite grains, the latter were reversely transformed from the recrystallized ferrite grains.

Ultrahigh strength-ductile medium-Mn steel auto-parts combining warm stamping and quenching & partitioning

ABSTRACTA novel application of quenching and partitioning (Q&P) treatment to warm stamping of a warm-rolled medium Mn steel was investigated. The results show that Q&P could improve yield strength of auto-parts from the formation of carbides and twinned martensite, and reduce the yield point elongation for carbon partitioning during Q&P process and a higher dislocation density. Regardless of stamping temperature either the single austenite or dual-phase region, an extraordinary product of strength and ductility (≥24 GPa·%) was achieved, which is two or three times higher than those of hot-stamped boron steels. The combined warm rolling and warm stamping process with Q&P treatment may have potential to implement the application of medium Mn steels for ultrahigh strength-ductile auto-parts.

Super-High-Strength and Formable Medium Mn Steel Manufactured by Warm Rolling Process

In this paper, we demonstrate the design and manufacture of super-high strength medium Mn steel with good ductility, via the combination of warm rolling and V alloying. This new strategy produced not only the extensive precipitation of fine VC particles in both ferrite and austenite but also the bimodal size distribution of retained austenite (RA) grains. A substantial increase of yield strength up to 650 MPa was achieved after the intercritical annealing (IA) of this V-alloyed medium Mn steel without deteriorating ductility; whereas the calculated precipitation strengthening using the classical Ashby-Orowan theory is no more than 400 MPa. Quantifying possible strengthening mechanisms revealed that dislocation strengthening should make an additional contribution because recovery in austenite could be strongly retarded by the nanosized precipitates. In this V-alloyed steel, the coarse RA grains firstly transformed to martensite during yielding at the much higher stress threshold, followed by a significant work hardening during successive deformation due to dislocations multiplication, twinning-induced-plasticity (TWIP) and transformation-induced-plasticity (TRIP) in fine RA grains. The best achieved combination of tensile properties included 1.8 GPa ultimate tensile strength, 1.0 GPa yield strength and 18% total elongation.

Effect Of Warm Rolling On Microstructure, Porosity And Hardness Of Spray Formed LM25 Alloy

Abstract LM25 alloy was obtained by the consolidated spray forming method. By using power saw the central portion was cut down into 20mmX20mm and then warm rolled with 20%,40%, 60% and 80 % at 1000c.Samples were prepared as per standards to see the microstructure by using the optical microscope. The results show that fine microstructure was observed in LM25 spray deposited Al alloy after warm rolling process due to the expediting cooling and freezing rate of spray deposit. Porosity and Mechanical properties of the warm rolled LM 25 substrate was improved than the spray casted and cold rolled.

Enhancing the Mechanical Properties of Al‐Containing Medium Mn Steel Through Warm Rolling and Intercritical Annealing

This investigation attempts to further enhance the mechanical properties of an Al‐containing medium Mn steel, Fe‐0.25C‐8.5Mn‐0.5Si‐2.5Al (wt%), through warm rolling (WR) and subsequent intercritical annealing (IA) treatments. Two schedules of WR processes were applied with a total thickness reduction of ≈60%. The results show that an ultrafine‐grained (UFG) lamellar structure was well developed after WR, while subsequent IA led to a coarsening of the microstructure as well as the formation of bimodal‐morphological ferrite and retained austenite (RA) grains, i.e., lamellar and equiaxed. Compared with warm rolled samples with clear discontinuous yielding and large yield point elongation, the IA treatment after WR could eliminate this phenomenon mainly due to microstructure coarsening. A significantly enhanced combination of strength and ductility (the product of the ultimate tensile strength and total elongation) as high as ≈75 GPa% was obtained for the WR + IA sample. This enhancement is notably superior to that of previously investigated medium Mn steels with similar Mn contents and is similar or even better than that of twinning‐induced plasticity (TWIP) steels with high‐Mn content. It is thus suggested that WR plus subsequent IA is a promising and feasible approach to further enhance the mechanical properties of medium Mn steel.

Evaluation of fracture toughness and rupture energy absorption capacity of as-rolled LZ71 and LZ91 Mg alloy sheet

In this study, for the first time, fracture behavior and rupture energy absorption of Mg-Li alloys are evaluated. At the first, dual-phase Mg LZ71 and LZ91 alloys were melted at the temperature of 770 °C under argon atmosphere. After this, high plastic deformation was applied during a warm rolling process to prepare thin sheet from the initial ingot, for the present investigation. Fracture behavior and rupture energy absorption were evaluated through plane stress fracture toughness tests, extracted R-curve and calculated the area below force-displacement curves. Plane stress fracture test and compact tension (CT) were performed according to ASTM E-561 and E-399. Also, microstructure, mechanical properties, and fractography are evaluated using x-ray Diffraction (XRD), Optical Microscopy (OM), microhardness measurement, uniaxial tensile test and Scanning Electron Microscopy (SEM). The optical images and XRD analysis demonstrated that Mg-Li alloys possess a dual-phase microstructure containing β-Mg-Li matrix with Body Centered Cubic (BCC) and partitioned α-Mg phase with Hexagonal Closest Packed (HCP) structures. According to the OM images, HCP phase of as-cast specimens has been observed in lath shape and after rolling process, the α–Mg phase was elongated and arranged in the rolling direction. The results showed that increasing of the lithium mass portion from 7% to 9%, not only did not increase mechanical properties, absorbed energy and fracture toughness, but also all of them decreased. Moreover, results of XRD analysis depict that adding more Li into the alloys chemical composition did not cause to the formation of any new phases. Hence, the ultimate tensile strength, microhardness, absorbed energy and fracture toughness of Mg LZ71 were achieved 1.07, 1.06, 1.91 and 1.51 times higher than LZ91 respectively. Due to the dependence of fracture toughness on two significant parameters, namely strength and ductility, adding lithium into the composition will increase the value of fracture toughness with compared to AZ alloys, because of the fact that a significant increase in ductility compensates the reduced strength. Furthermore, SEM photographs of tensile fracture surfaces of as-rolled LZ71 and LZ91 samples displayed dimples and microvoids which are characterized as a ductile rupture mode. Thus, Mg-Li alloys had ductile fracture, which were consistent with the results of the tensile test.

Mechanical Properties and Microstructural Evolution of Bulk UFG Al 2014 Alloy Processed Through Cryorolling and Warm Rolling

Tensile properties, microstructural evolution and fracture toughness of Al 2014 alloy subjected to cryorolling followed by warm rolling (CR + WR) have been investigated in the present study. The solution-treated (ST) Al 2014 alloy is cryorolled followed by warm rolling process at different temperatures (110, 170 and 210 °C). The mechanical properties and microstructural features of deformed and undeformed Al 2014 alloys were characterised by optical microscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The CR + WR samples at 170 °C showed an improved hardness (179 HV), tensile (UTS 499 MPa, YS 457 MPa) and fracture toughness (KQ = 37.49 MPa $$\sqrt {\text{m}}$$ , Kee = 37.39 MPa $$\sqrt {\text{m}}$$ and J integral = 33.25 kJ/mm2) with respect to ST alloy as measured from the tensile and fracture toughness test. The improved mechanical properties of CR + WR alloy are attributed to grain boundary strengthening, combined recovery and recrystallisation, precipitation hardening and dynamic ageing effect during the deformation. The precipitation of metastable spherical phase Al2Cu was responsible for the improved tensile and fracture properties of fine-grained Al 2014 alloy observed in the present work.

Impact of Warm Rolling Process Parameters on Crystallographic Textures, Microstructure and Mechanical Properties of Low-Carbon Boron-Bearing Steels

The effect of the warm rolling process on the microstructure and the mechanical properties of low-carbon high-boron steels are studied in this work. To investigate these effects, boron-bearing low-carbon steel, subjected to roll at three different temperatures, was studied symmetrically and asymmetrically. The results of the experimental trials achieved by mechanical and microstructural analysis revealed that the use of warm rolling can represent a favorable method to suppress strain aging completely, and to eliminate the fluting effect and yield point elongation. In addition, the tensile and elongation properties are modified with the formation of boron nitride precipitates and dislocations in different thermomechanical conditions.

The Influence of Warm Rolling Reduction on Microstructure Evolution, Tensile Deformation Mechanism and Mechanical Properties of an Fe-30Mn-4Si-2Al TRIP/TWIP Steel

The effects of warm rolling reduction ratio ranging from 20% to 55% on microstructure evolution, the tensile deformation mechanism, and the associated mechanical properties of an Fe-30Mn-4Si-2Al TRIP/TWIP steel were studied. The warm rolling process resulted in the formation and proliferation of sub-structure, comprising dislocations, deformation twins as well as shear bands, and the densities of dislocation and twins were raised along with the increase in rolling reduction. The investigated steel, with a fully recrystallized state, exhibited a single ε-TRIP effect during the room temperature tensile deformation, on top of dislocation glide. However, the formation and growth of twin lamellae and ε-martensite were detected simultaneously during tensile deformation of the warm rolled specimen with rolling reduction of 35%, leading to a good balance between high yield strength of 785 MPa, good total ductility of 44%, and high work hardening rate. As the rolling reduction increased to 55%, the specimen revealed a relatively low work hardening rate, due to the high dislocation density, and dislocation glide was the main deformation mechanism. As a result, a tensile deformation mechanism that started from a single ε-martensitic transformation moved to a bi-mode of ε-martensitic transformation accompanied with deformation twinning, and finally to dislocation glide with the increasing warm rolling reduction was proposed.

In situ neutron diffraction in quantifying deformation behaviors of nano-sized carbide strengthened UFG ferritic steel

The microstructures and mechanical properties of a low-alloy medium-carbon steel with a duplex microstructure composed of nanoscale spheroidized carbides in ultrafine-grained (UFG) ferritic steel are examined. The average grain size of the studied steel is ~ 430 nm, and these grains coexist with numerous carbides. Neutron diffraction reveals that the intensity of (011) and (022) peaks for the UFG sample is significantly enhanced, suggesting that the (011)//RD texture is a result of the warm rolling process. The lattice parameter of UFG steel is smaller than that of a martensitic steel (M steel) counterpart, indicating a lower carbon concentration in the lattice. The estimated dislocation densities for M steel and UFG steel are 2.59 × 1014 cm−2 and 1.76 × 1012 cm−2, respectively. The UFG steel reveals a nearly isotropic lattice strain response under initial tension from 0 to 450 MPa, where the lattice strains of the (110), (002), and (112) planes are identical. The increase of lattice strain of the (110) plane becomes smaller than that of the (002) and (112) planes as the stress exceeds 450 MPa, suggesting that the nanosized carbides contribute to the hardening ability by promoting the accumulation of geometrically necessary dislocations around the particles, and the (110) lattice becomes harder compared to the other two planes.

Investigation of the bonding strength of the stainless steel 316L/polyurethane/stainless steel 316L tri-layer composite produced by the warm rolling process

In this study, a metal/polymer tri-layer composite was produced by direct adhesion (without adhesive), and mechanical locks were created using the warm rolling process. The effect of the process parameters including preheating temperature, rolling speed, thickness reduction, surface roughness, and the orientation of the surface scratches on the bond strength between layers was investigated. The results indicated that the suitable polymer fluidity and penetration, to provide stronger mechanical locks and higher bond strengths, could be achieved at an optimum preheating temperature and a rolling speed of 240°C and 36 r/min, respectively. In addition, the most appropriate surface pretreatment was obtained in the wire brush in the rolling direction mode with the surface roughness of 0.65 µm, so that the failure mechanism in this case was cohesive and the optimum thickness reduction was achieved at 40%. Furthermore, the mechanical properties of the sandwich sheet with highest bonding strength were evaluated.

Superdense microbands strengthening of textured low alloy ferritic steel

Microstructure evolution of Ti-Mo microalloyed ferritic steel was studied via warm rolling process. Results revealed that rolling at 500 °C (R3) mainly facilitated the formation of superdense microbands (∼200 nm in spacing), especially in the γ textured region. Average spacing of microbands (MBs) increased with temperature and it turned to be about 850 nm when rolling at 650 °C (R2). As rolling temperature further increased to 700 °C (R1), average ferritic grain size was refined to near 1.9 μm whereas no MBs appeared. Additionally, due to the preexisting superdense MBs, ultimate tensile stress of R3 increased to 1041 MPa and the elongation kept high at 22.9% simultaneously. MBs are characterized by high density dislocation walls and formed through the activation of {112}<111> slip systems, which is induced by the pile up of dislocations during grain boundary sliding. In that process steps appear in the grain boundary and gradually heighten with the extension of MBs. Large dynamic recovery rate could slack the dislocation pile up rate and increase the spacing of MBs. Moreover, preexisting of superdense MBs sheds new insights on the development of high strength ferritic steels with excellent combination of strength and ductility.

Alloy design by dislocation engineering

Ultra-high strength alloys with good ductility are ideal materials for lightweight structural application in various industries. However, improving the strength of alloys frequently results in a reduction in ductility, which is known as the strength-ductility trade-off in metallic materials. Current alloy design strategies for improving the ductility of ultra-high strength alloys mainly focus on the selection of alloy composition (atomic length scale) or manipulating ultra-fine and nano-grained microstructure (grain length scale). The intermediate length scale between atomic and grain scales is the dislocation length scale. A new alloy design concept based on such dislocation length scale, namely dislocation engineering, is illustrated in the present work. This dislocation engineering concept has been successfully substantiated by the design and fabrication of a deformed and partitioned (D&P) steel with a yield strength of 2.2GPa and an uniform elongation of 16%. In this D&P steel, high dislocation density can not only increase strength but also improve ductility. High dislocation density is mainly responsible for the improved yield strength through dislocation forest hardening, whilst the improved ductility is achieved by the glide of intensive mobile dislocations and well-controlled transformation-induced plasticity (TRIP) effect, both of which are governed by the high dislocation density resulting from warm rolling and martensitic transformation during cold rolling. In addition, the present work proposes for the first time to apply such dislocation engineering concept to the quenching and partitioning (Q&P) steel by incorporating a warm rolling process prior to the quenching step, with an aim to improve simultaneously the strength and ductility of the Q&P steel. It is believed that dislocation engineering provides a new promising alloy design strategy for producing novel strong and ductile alloys.

Study on warm forming effects of the axial-pushed incremental rolling process of spline shaft with 42CrMo steel

The effects of the warm forming temperature on the axial-pushed incremental rolling process were investigated through finite element analysis and experimental studies. Firstly, the principle of warm axial-pushed incremental rolling process of spline shaft was introduced. Next, the material properties of 42CrMo steel at different forming temperatures were studied to discuss the effects of the warm forming temperature. Through finite element analysis, the simulations of the axial-pushed incremental rolling process were carried out to investigate the effects of the warm forming temperature on rolling forces. The results indicated that the axial and radial forces on the rolling dies were both reduced at the warm forming temperature. Finally, the experiment studies were carried out on a warm axial-pushed incremental rolling equipment. The dimensional precision, microstructure, and hardness of the formed spline shafts at warm temperature were compared with those of the formed spline shafts at room temperature. The results indicated that the spline shafts, which were formed at the warm temperature, possesses of a good dimensional precision and excellent performance. The results in this paper demonstrated that the warm forming temperature has the positive effects on the performance improvement of the axial-pushed incremental rolling process of spline shaft.

A strong and ductile 7Mn steel manufactured by warm rolling and exhibiting both transformation and twinning induced plasticity

Using a new design strategy for the metastable phase in transformation induced plasticity (TRIP) steels, we have successfully manufactured a 7 wt.% Mn containing steel with an excellent combination of mechanical properties (950 MPa ultimate tensile strength and 63% total elongation) via warm rolling and intercritical annealing (IA) processes. Compared with the cold rolled microstructure that is usually completely recrystallized during annealing, the warm rolled microstructure is just partially recrystallized due to the lower driving force that is accumulated. This leads to about 50 vol.% of the retained austenite grains having both lamellar and equiaxed morphologies after IA, with the latter having a wide and almost evenly spaced size distribution. In-situ synchrotron X-ray examination revealed that about 40 vol.% of the retained austenite grains could transform to martensite in a sustainable way during deformation because they had a range of mechanical stability owing to their different morphologies and wide size distributions. Moreover, the partitioning of the solute elements during IA leads to some austenite grains having proper values of stacking fault energy using which they can twin during deformation. Finally, the retained austenite grains in the medium-Mn steel can either transform to martensite gradually or twin during deformation, and contribute to large elongation at high strengths via both TRIP-assisted and TWIP effects.

Precipitation enhanced ultragrain refinement of Ti-Mo microalloyed ferritic steel during warm rolling

Ultragrain refinement caused by continuous dynamic recrystallization (cDRX) was studied from microstructure, texture and precipitation. Compared with microstructures of deformation induced ferrite transition (DIFT), ferrite matrix refined through cDRX presented more substructures. Textures formed through the cDRX mainly consisted of (001)[11¯0] and (111)[1¯1¯2] and the intensity of (001)[11¯0] could increase with strain. Rolling in γ non-recrystallization region facilitated the strain induced precipitation, hence more subboundaries formed during the subsequent warm rolling process. With the consumption of alloying elements, stability of remaining austenite was reduced and this promoted the formation of high temperature proeutectoid ferrite. Additionally, stress fields induced by the nanoscale precipitates could suppress the formation of microshear bands and further promote the generation of high angle boundaries, especially in the γ-fiber textures. Grains were refined to an average size of ~1µm through cDRX and most refined grains transformed from γ-fiber textures presented an orientation of <110>//ND. Moreover, the refined matrix performed better mechanical properties along the rolling direction and the precipitation enhanced ultragrain refinement through cDRX showed greater potential on the refining of ferrite grains.

Microstructure and mechanical properties of the as-cast and warm rolled Mg-9Li-x(Al-Si)-yTi alloys with x = 1, 3, 5 and y = 0.05 wt.%

The as-cast Mg-9Li alloys with difference Al-Si addition were fabricated using induction casting method under the argon atmosphere and then warm rolling process was performed at 473 K. The Al-Si addition to Mg-9Li alloy leads to the formation of Mg2Si intermetallic compounds and microstructural evolutions such as increased volume fraction of α-Mg phases and changed morphology of α-Mg primary phase on the as-cast specimens. Moreover, the addition of 0.05 wt.% Ti elements on Mg-9Li-3(Al-Si) alloy raised the further microstructural evolutions, e.g. volume fraction of α-Mg phases, which bring about change in the microstructure after warm rolling process. The effects of this micro alloying influences the formation of fine microstructure after warm rolling process, which is the main factor in determining the mechanical properties. With these microstructural evolutions, tensile strength was increased from 101.71 MPa to 247.41 MPa by the minor alloying and warm rolling process. Moreover, the remarkably enhanced strength of Mg-9Li-3(Al-Si) alloy was the results of the addition of 0.05 wt.% Ti from 247 MPa to 412 MPa. Furthermore, the warm press-forming tests were carried out at 523 K to evaluate the applicability of this novel alloy for commercial products.

Finite element analysis on axial-pushed incremental warm rolling process of spline shaft with 42CrMo steel and relevant improvement

In this paper, we firstly discussed the principle and deformation characteristic of axial-pushed incremental rolling process of spline shaft, and then, an axial-pushed incremental warm rolling process of spline shaft with 42CrMo steel is introduced and studied to investigate the formation mechanism and analyze the effects of process parameters. In order to improve the accuracy of finite element simulation, the plastic flow behavior and constitutive characteristic of 42CrMo steel during warm forming process are studied through the isothermal warm compression tests, and then, the microstructure of tempered sorbite and high-accuracy constitutive equations of 42CrMo steel are obtained. Next, the effects of die angle and deformation temperature on forming load and tooth-filled quality during axial-pushed incremental warm rolling process are analyzed based on finite element analysis (FEA). The optimum die angle of the entrance section and warm forming temperature are determined as 8 and 700 °C, respectively. To verify the results of simulation, the corresponding experiments are carried out on the warm rolling equipment, which is designed by the authors. Finally, the relevant improvement by tooth divided flow method (TDFM) is raised to eliminate the negative effect of blank diameter on this process.