Ultrafine-grained Low-carbon Steel Research Articles

Recovery of Ductility in Ultrafine-Grained Low Carbon Steel Processed by Electropulsing

Recovery of Ductility in Ultrafine-Grained Low Carbon Steel Processed by Electropulsing

Improvement of strength, toughness and ductility in ultrafine-grained low-carbon steel processed by warm bi-axial rolling

A 0.15%C-0.3%Si-1.5%Mn steel bar with ultrafine elongated grain structures of transverse grain size 1.2 μm was fabricated via multi-pass bi-axial rolling process at a warm working temperature. For comparison, conventionally quenched and tempered 0.29%C steel and 1.03%C steel with a martensitic structure and 0.15% low-carbon steel with a ferrite/pearlite structure were also prepared. The full-size Charpy V-notch impact and tensile tests were conducted at a temperature range of −196 °C to 200 °C, and the relationship between microstructures, yield stress, reduction in area and impact energy was studied. In the developed steel bar, the main orientation in the microstructure changed in the cross-sectional plane, and it was dominated by {001}<100> cube orientation at the center, {111}<110> at the quarter and rolling direction//<110> at the surface. Crack branching started to occur with decreasing temperature in the Charpy test. The fracture surfaces are very complicated at temperatures below −100 °C and the specimen did not separate into two pieces even at a low temperature of −196 °C. The strength–toughness balance of the developed steel was significantly improved compared with conventional steels. This advantage in the developed steel was also seen in the strength–reduction in area balance. As a result, the steel fabricated by warm bi-axial rolling was best balance in correlation between strength and ductility and between strength and toughness at all temperatures.

Development of single phase bimodal microstructure in bulk ultrafine-grained low carbon steel

The low carbon steel (LCS) rods were plastically deformed at room temperature up to a very high strain level by equal channel angular pressing (ECAP). ECAPed samples were further cryo-rolled at subzero temperature up to 94% area reduction. After plastic deformation, samples were annealed for short period at 475–675 °C temperature range. Optical microscopy and transmission electron microscopy are used to characterize primary microstructure and fine microstructural details respectively. Hardness of the deformed and annealed samples is measured by Vickers hardness testing. Results show that the grains are refined from 65 µm to 200 nm by ECAP at equivalent strain of 16.8, further grains are refined to <78 nm by cryo-rolling at −50 °C with increase in dislocation density. The cryo-rolling of ultrafine grained (UFG) LCS increases the hardness due to further grain refinement and the increased dislocation density. Hardness of the UFG LCS produced by ECAP is more than three times of coarse grained LCS. Flash annealing (at a range of temperature) of cryo-rolled UFG LCS is produced the single phase bimodal microstructure. Bimodal microstructure is the consequence of secondary recrystallization during flash annealing. Even after flash annealing of ECAP-16.8-CRR-94 deformed sample, hardness is more than 200% of as-received LCS and it decreases with increase in flash annealing temperature.In this study recrystallization, secondary recrystallization and hierarchy in grain growth with increasing annealing temperature is presented. Microstructure and mechanical properties correlation is demonstrated.

Development of Texture in Ultrafine-Grained Low-Carbon Steel Processed through Equal-Channel Angular Pressing

Developments of microstructures and texture are investigated for a low-carbon steel deformed by equal-channel angular pressing (ECAP) at room temperature for equivalent strain (evm) of 16.8. ECAP at evm = 0.6 reduces grain size by two orders of magnitude. With increasing strain, grain size reduces further and high-angle grain boundary (HAGB) fraction improves by grain subdivision and its change in orientation. ECAP of low-carbon steel up to evm = 16.8 can produce ultrafine grains of 0.2 µm size with HAGB fraction of 82%. The as-received steel has low texture index. Microstructural changes strongly influence texture development. It increases with equivalent strain as grain subdivision continues. Texture index fluctuates at intermediate strain range due to randomization of initial texture. Texture index reaches saturation at the high equivalent strain. On ECAP at low to intermediate strain evm = 0.6-6, components of both α (Fθ, Jθ, and $$ \overline{J} $$ θ) and γ (D1θ, D2θ, and Eθ) fibers are developed though all of them deviate from their ideal positions. At higher strain (evm = 9-16.8), the γ fiber components (D1θ, D2θ, Eθ) are strengthened, and the Eθ component reaches the highest intensity of ≈ 17 at evm = 12. There is no texture symmetry at low to high strain level. The presence of pearlite in low-carbon steel enhances strongly the deviation of texture components from their ideal positions in Euler’s space as well as it increases the intensities of the components more than that of IF steel for a similar equivalent strain.

Effect of Annealing on Microstructure and Mechanical Properties of Ultrafine-Grained Low-Carbon Medium-Manganese Steel Produced by Heavy Warm Rolling

An ultrafine-grained (UFG) low-carbon medium-manganese steel was fabricated by the heavily warm rolling (HWR) and subsequent quenching, and the effects of annealing temperatures on microstructure and mechanical properties of the UFG HWRed steel were investigated. The results show that the HWRed steel exhibits simultaneous improvements in strength, uniform elongation and work hardening, which is mainly attributed to the refinement of martensitic microstructures. The HWRed steels comprise only α-phase when annealing at lower temperatures below to 550 °C and at higher temperatures above to 700 °C. Whereas, UFG γ-austenite is formed by reverse transformation when the HWRed steel was annealed at intermediate temperatures from 550 to 700 °C and the volume fraction increases with increasing annealing temperatures, consequently resulting in a dramatic increase in ductility of the annealed HWRed steels. It was found that the transformed UFG austenite and ferrite remained ~ 500 nm and ~ 800 nm in size when the HWRed steel was annealed at 650 and 700 °C for 1 h, respectively, showing an excellent thermal stability. Moreover, the HWRed steel annealed at 650 °C exhibits high strength-ductility combinations with a yield strength of 906 MPa, ultimate tensile strength (UTS) of 1011 MPa, total elongation (TEL) of 51% and product of strength and elongation (PSE: UTS × TEL) of 52 GPa%. It is believed that these excellent comprehensive mechanical properties are closely associated with the UFG austenite formation by reverse transformation and principally attributed to the transformation-induced plasticity (TRIP) effect.

Effect of Rolling Reduction on Microstructure and Property of Ultrafine Grained Low-Carbon Steel Processed by Cryorolling Martensite

A novel method of cryorolling martensite for fabricating ultrafine grained low-carbon steel with attractive strength was proposed. The results indicate that ultrafine-grain structured steel could be manufactured by cryorolling and the subsequent annealing of martensite. The mean ferrite size of 132.0 nm and the tensile strength of 978.1 MPa were obtained in a specimen with a reduction of 70% in thickness. There were peak value and valley value in the strength and grain size of ferrite with the increase of reduction from 50% to 80%, respectively. The further growth of ferrite grain at 80% reduction is attributed to the heavier distortion energy at large reduction, which activates the secondary recrystallization of ferrite. Furthermore, the distribution of ferrite grains became more uniform with increasing of reduction from 50% to 70%. Additionally, the amount of lamellar dislocation cell substructure increased with the reduction at liquid nitrogen temperature.

Development of High-Strength Bulk Ultrafine-Grained Low Carbon Steel Produced by Equal-Channel Angular Pressing

Low carbon steel (LCS) workpieces have been deformed by equal-channel angular pressing (ECAP) at a large equivalent strain of 16.8 at room temperature. The mechanisms of microstructural refinement, strengthening, hardening, and fracture behavior are investigated. LCS becomes refined by a sequence of mechanisms of elongation of grains, splitting of elongated grains to bands at low strain, subdivision of bands to cells at intermediate strain, elongation of bands to ribbon grains, and breaking of ribbons to near-equiaxed grains at a high strain level. ECAP of LCS at e vm = 16.8 refines the material to near-equiaxed grains of size 0.2 µm having a high-angle grain boundary fraction of 82.4 pct and average misorientation angle of 40.8 deg. The ultrafine-grained (UFG) LCS contains a dislocation density of 1.7 × 1015 m2. In the initial passes of ECAP, the yield and tensile strengths increase rapidly due to rapid grain refinement, reduction in domain size, and increase in dislocation density. At high strain levels, strengthening can be attributed to a combination of grain refinement, dissolution of cementite in the ferrite matrix, and increase in misorientation angle. At e vm = 16.8, the ultimate tensile strength (UTS) reaches >1000 MPa with a consequent drop in ductility to ≈10.6 pct. Reduction in ductility is found to be due to high dislocation density, high stored energy in the matrix, and occurrence of nonequilibrium grain boundaries. The LCS at low equivalent strain fails by ductile fracture. The dimple size and its volume fraction decrease, but their number density and stored energy increase with increasing equivalent strain. Beyond a critical equivalent strain of 9, the material fails by ductile-brittle fracture. At e vm = 16.8, equal-channel angular pressed UFG LCS fails mainly by cleavage fracture.

Effect of Annealing on the Microstructure and Mechanical Properties of a Low-Carbon Steel with Ultrafine Grains

The microstructures and properties of the ultrafine-grained low-carbon steel were investigated. Martensite microstructure was obtained by quenching a low-carbon steel, followed by 50% strain cold rolling and then annealing at 500–650 °C for 2 and 30 min, respectively. Microstructures were observed, and tensile properties were measured for the specimens treated with cold rolling and annealing. The effects of annealing parameters on the microstructure and mechanical properties were analyzed. It shows that the microstructure of specimen annealed at 550 °C for 30 min consists of ferrite grains with an average size of 330 nm. The ultrafine-grained low-carbon steel exhibits not only high tensile strength (867 MPa), but also good elongation (16.7%).

Recovery of Ductility in Ultrafine-Grained Low-Carbon Steel Processed Through Equal-Channel Angular Pressing Followed by Cold Rolling and Flash Annealing

The low-carbon steel workpieces are deformed by equal-channel angular pressing at 293 K (20 °C) up to an equivalent strain of ~12 using route Bc, which results in the bulk ultrafine-grained (UFG) structure with high dislocation density and partial dissolution of cementite. The yield strength (YS) is enhanced from 208 (as-received) to 872 MPa and the tensile strength is increased from 362 to 996 MPa, but the material loses total elongation (TE) from 36.2 to 2.9 pct. Cold rolling of equal-channel angular pressed steel produces the refined structure of grain size 0.11 μm. The YS increases further to 924 MPa with a marginal gain in ductility due to the reappearance of the γ fiber component. Flash annealing the samples, which were equal-channel angular pressed followed by cold rolling, at 873 K (600 °C) results in 27 pct of micron-sized (9 µm) ferrite grains in submicron-sized (<1 µm) matrix with a reduced defect density and small amount of precipitation of cementite. TE increases from 2.9 to 23.3 pct. The material retains a YS of 484 MPa and tensile strength of 517 MPa, which are higher than those of the as-received material. The UFG grains are failed by cleavage, but the micron-sized grains display ductile fracture. The ductility of the flash-annealed material is recovered significantly due to bimodal grain size distribution in ferrite and the development of a good amount of γ fiber texture components. The major contribution toward recovery of ductility comes from the bimodal grain size distribution in ferrite rather the precipitation of cementite.

Annealing Characteristics of Ultrafine Grained Low-Carbon Steel Processed by Differential Speed Rolling Method

The annealing behavior of ultrafine grained ferrite in low-carbon steel (0.18 wt pct C) fabricated using a differential speed rolling (DSR) process was examined by observing the microstructural changes by electron backscatter diffraction and transmission electron microscopy. For this purpose, the samples processed by 4-pass DSR at a roll speed ratio of 1:4 for the lower and upper rolls, respectively, were annealed isochronally at temperatures ranging from 698 K to 898 K (425 °C to 625 °C) for 1 hour. The deformed samples exhibited a complex microstructure in the ferrite phase consisting of an equiaxed structure with a mean grain size of ~0.4 µm and a lamellar structure with a mean lamellar width of ~0.35 µm. The texture evolved during deformation was characterized by the rolling and shear components with specific orientations. After annealing at temperatures lower than 798 K (525 °C), the aspect ratio of the deformed grains tended to shift toward a unit corresponding to the equiaxed shape, whereas the grain size remained unchanged as the annealing temperature increased. At temperatures above 798 K (525 °C), however, some grains with a low dislocation density began to appear, suggesting that the starting temperature of static recrystallization in the severely deformed ferrite grains was 798 K (525 °C). The annealing texture of the present sample after heat treatment showed a uniform fiber texture consisting of α- and γ-components.

Fabrication of the Ultrafine Grained Low Carbon Steel by Cold Compression and Annealing of Martensite

The aim of this work was to produce ultrafine grained low carbon steel (0.13 wt% carbon) by cold compression and subsequent annealing of lath martensite and study its formation kinetics. In this regard, the martensitic specimens were first cold compressed by 65 % and then annealed at 723, 773 and 823 K for various times ranging between 0 and 120 min. Results showed that annealing of the cold-compressed martensitic specimen at 823 K for 90 min led to the formation of ferrite grains with a mean size of 1.8 μm and small precipitates of Fe3C with sizes less than 300 nm. By using the Johnson–Mehl–Avrami–Kolmogorov analysis to describe the kinetics of ferrite recrystallization, the average value of time exponent (n) was found to be 0.96. The activation energy for the ferrite recrystallization was calculated as 83 kJ mol−1, which was consistent with the activation energy value for carbon diffusion in bcc iron (81 kJ mol−1).

Micro end-milling of channels using ultrafine-grained low-carbon steel

During micromachining, the interaction between the cutting tool and the workpiece material may cause damages on the machined surface related to material deformation. It would be interesting that the workpiece microstructure suits the scale of the cutting parameters. Very little has been investigated on how a metallurgically modified material responds to microcutting. This research evaluated the effect of an ultrafine-grained material in the micromilling of grooves. Dual-phase low-carbon steel (ferrite-pearlite) was submitted to warm rolling for grain refinement (from 11- to 0.7-μm size of ferrite grains). The effect of tool cutting edge radius (re), feed per tooth (ft), tool diameter, and speed cutting upon surface roughness and burr formation during end-milling of the original material and the modified one was evaluated. The ultrafine-grained material showed better results of surface finishing and presence of burrs when compared to the original dual-phase material. The metallurgical modification of low-carbon steels by grain refinement favored for micromachining of grooves, making it possible to extend applications of this class of steels.

Effect of Ferrite Grain Size on Dynamic Tensile Properties of Ultrafine Grained Low Carbon Steels with Various Chemical Compositions

The effects of ferrite grain size on dynamic tensile properties of low carbon steels with various chemical compositions are shown. The strain rate dependence of flow stress, represented by the difference between flow stresses at high and low strain rates, ¦·, was the highest in the interstitial free (IF) steel having ferrite single-phase microstructure and the 0.1% C-steel having ferrite­cementite (FC) microstructure. The ¦· decreased when P and Mn were added. In all the steels used, the ¦· was almost constant or decreased only slightly when the ferrite grains were refined down to 2­4µm, and it decreased significantly by further grain refinement down to sub-micrometer region. The FC and ferrite singlephase microstructures strengthened mainly by grain refinement of ferrite showed higher dynamic absorbed energy compared with the other steels strengthened by alloy addition. [doi:10.2320/matertrans.MA201323]

Comparison of the monotonic and cyclic mechanical properties of ultrafine-grained low carbon steels processed by continuous and conventional equal channel angular pressing

In the current study the mechanical properties of ultra-fine grained low carbon steel processed by conventional equal channel angular pressing and a continuous equal channel angular pressing-Conform process were investigated. Both monotonic and cyclic properties were determined for the steel in either condition and found to be very similar. Microstructural analyses employing electron backscatter diffraction were used for comparison of the low carbon steels processed by either technique. Both steels feature very similar grain sizes and misorientation angle distributions. With respect to fatigue life the low carbon steel investigated shows properties similar to ultra-fine grained interstitial-free steel processed by conventional equal channel angular pressing, and thus, the general fatigue behavior can be addressed following the same routines as proposed for interstitial-free steel. In conclusion, the continuously processed material exhibits very promising properties, and thus, equal channel angular pressing-Conform is a promising tool for production of ultra-fine grained steels in a large quantity.

Tensile and fatigue properties of ultrafine-grained low-carbon steel processed by equal channel angular pressing

High-strength ultrafine-grained low-carbon steel was processed by equal channel angular pressing (ECAP) for different passes and then heat-treated to achieve the tensile strength in the range of 1400–1500MPa. Then its microstructure and mechanical properties, especially fatigue properties were systemically investigated. It is found that the simultaneous enhancement of tensile strength and elongation can be obtained by only one ECAP pass; the optimum fatigue strength of 460MPa can be gained by two ECAP passes; while further increasing the number of ECAP passes made the fatigue properties worse. Therefore, combing with the effects of heat treatment and fatigue damage mechanism, it is suggested that few number of ECAP passes is enough to achieve the optimum fatigue properties for low carbon steel, which makes it easy, economical and efficient for the engineering application of ECAP.

The improvement of ductility in nano/ultrafine grained low carbon steels via high temperature short time annealing

A new method has been applied to fabricate bimodal grain structure in ultrafine-grained low carbon steels. This method is based on the plane strain compression of a martensitic starting structure and subsequent annealing at elevated temperature for short time. The annealing process was carried out at the temperature of 500 and 600°C for 1.5ks and 120s, respectively. The final microstructure after recrystallization is composed of ultrafine and coarse grains along with the precipitation of carbide particles (cementite) within the coarse grains. This has been considered to be the reason for the observed superior balance of ductility and strength.

OS0109 フェールセーフ設計された材料の強靭化 : 第1報 800MPaクラス低炭素鋼の衝撃特性

Bulk ultrafine-grained (UFG) low carbon steel bars were produced by warm caliber rolling, and the impact and tensile properties were investigated. The microstructures in the rolled bars consisted of an elongated UFG structure with a strong α-fiber texture. The rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain size of 0.8-1.0μm exhibited superior strength-ductility balance and impact properties. In the rolled bars, cracks during an impact test branched parallel to the longitudinal direction of the test samples as temperatures decreased. Delamination caused by such crack branching remarkably appeared near the ductile-to-brittle transition temperature. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation; in addition, delamination effectively enhances the low-temperature toughness.

A new process to fabricate ultrafine-grained low-carbon steel with high strength and high elongation

A new process to fabricate ultrafine-grained low-carbon steel was proposed. Low-carbon steel plates were intercritically quenched to obtain martensite–ferrite structure, and then warmly rolled at 700 °C. The processed samples revealed bimodal ultrafine ferritic grains and nanoscale carbide particles, and showed excellent mechanical properties.

Optimum Pass Design of Bar Rolling for Producing Bulk Ultrafine-Grained Steel by Numerical Simulation

The groove design for creating ultrafine-grained low-carbon steel through a caliber rolling process was studied from the viewpoint of the large strain accumulation and cross-sectional shape variation in a bar. A three-dimensional finite element analysis was employed for this purpose. The caliber rolling process of foval (flat-like-oval)/square type was proposed as a method to efficiently introduce a large strain in material. The relationship among the foval configuration, strain, and cross-sectional shape was examined in the caliber rolling. The influence of the equivalent strain distribution by 1st pass (foval rolling) depends strongly on the strain distribution and a cross-sectional shape by 2nd pass, and the foval configuration to accumulate a large strain efficiently was shown. The optimum pass schedule to fabricate a 13mm square bar of ultrafine-grained steel from a 24 mm square bar by caliber rolling at warm working temperatures was proposed.

High tensile ductility and high strength in ultrafine-grained low-carbon steel

High tensile ductility and high strength were simultaneously obtained in UFG low-carbon steel prepared by appropriate annealing of cold-rolled martensite–ferrite dual-phase structure. High tensile ductility and high strength are attributed to the coexistence of fully recrystallized ferrite grains with bimodal size distribution and dispersive nanoscale carbides.