Plasticity Steel Research Articles

Mechanical properties and deformation behaviour of TWIP steel at different strain rates

The mechanical properties of TWIP at different strain rates ranging from 5 s−1 to 500 s−1 were investigated by a VHS 160/100-20 high-speed tensile testing machine. The Johnson-Cook (J-C) dynamic constitutive model of TWIP steel was established based on the test results and verified by numerical simulation. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to analyse the microstructures of TWIP steel before and after tensile deformation. The findings demonstrate that under the tested strain rate, both the tensile strength and yield strength of TWIP steel increase with increasing strain rate, whereas the elongation decreases first and then increases. The strain rate of 100 s−1 is the turning point of the mechanical properties. The stack fault energy (SFE) is 26 mJ m−2 at room temperature, and the microstructure of TWIP steel before and after deformation is composed of single austenite, which demonstrates that only TWIP effect occurs. Dislocations accumulate along grain boundaries, forming structures such as dislocation cells and dislocation walls, causing stress concentration and promoting the formation of microcracks. With increasing strain rate, the dislocation cells are more obvious, and the thickness of the deformation twins decreases, resulting in an increase in the stress required for dislocations to cross the twin boundaries and material strengthening. The formation of nanoscale twins can not only effectively hinder the dislocation motion, but also provide space for dislocation slip and change the grain orientation, so that the slip system which is not conducive to deformation has a new orientation, resulting in a synergistic effect on strength and plasticity of TWIP steel. This work elucidates the relationship between the microscopic deformation mechanism and the macroscopic mechanical properties of TWIP steel at a specific strain rate, and a dynamic constitutive model is established, which can provide a reference for the application of TWIP steel in the design of anti-explosion and anti-impact structures.

Martensite plasticity and damage competition in dual-phase steel: A micromechanical experimental–numerical study

Martensite damage in Dual-Phase (DP) steel has been studied extensively, yet, the exact deformation mechanisms that trigger or inhibit damage initiation remain mostly unexplored. Whereas generally assumed to be hard and brittle, lath martensite in fact deforms in a highly anisotropic manner, showing large strains under favorable habit plane orientations, which is attributed both to the lath morphology and to so-called ’substructure boundary sliding’. Yet, the correlation (or interplay) between plasticity and damage in lath martensite has not received much attention. Therefore, we raise the question whether these soft martensite plasticity mechanisms can delay or even inhibit damage initiation. We analyze several ’damage-sensitive’ martensite notches, i.e. thin contractions of two martensite islands, by combining several state-of-the-art experimental and analysis methods. Deformations are tracked in-situ at the nanoscale, aligned to detailed microstructure maps, and categorized, for each martensite variant, into habit plane or out-of-habit-plane slip. In these experiments, strong plasticity (>70%) is observed in martensite notches, enabled by slip along a favorably oriented habit plane, whereas damaged notches have unfavorably oriented habit planes, showing limited pre-damage strains (<10%), carried by out-of-habit-plane slip. Additionally, one-to-one experimentally based Crystal Plasticity (CP) simulations are performed in parallel, employing a recently introduced Enriched CP approach which models a soft plasticity mechanism on the variants’ habit plane. The Enriched CP simulations show considerably lower hydrostatic stresses in non-damaged and plastically deforming notches, thereby revealing that the soft habit plane mechanism is key for introducing the high plastic anisotropy that can lead to the inhibition of martensite damage in highly strained martensite notches. Finally, recommendations for improved damage inhibiting steels are proposed.

Microstructure and mechanical properties of S30432 steel modified by trace TiC/TiB2 nano-particles

In this work, S30432 steel containing trace TiC/TiB2 nano-particles was manufactured, and the effects of TiC/TiB2 nano-particles on the microstructure and room and high-temperature mechanical properties of the S30432 steel were investigated. The addition of trace TiC/TiB2 nano-particles can reduce the grain size from 19 ± 2.3 μm to 13 ± 4.5 μm. This is because the nano-particles provide efficient nucleation sites and constrain dendrite growth. Trace nano-particles can simultaneously enhance the strength and plasticity of the S30432 steel at room temperature. At high temperature of 600 °C, the S30432 steel with trace TiC/TiB2 nano-particles can still maintain high strength and plasticity. However, as the ambient temperature increases, the strength increment of the S30432 steel with trace TiC/TiB2 nano-particles decreases gradually, while the elongation increment significantly increases. The dynamic recrystallization process is promoted by the addition of nano-particles, causing an increased softening degree. Trace TiC/TiB2 nano-particles exhibit good plasticizing effect.

Characterization of Retained Austenite in Advanced High-Strength Steel

The retained austenite (RA) in advanced high-strength steels directly affects their plasticity. It is very important for the accurate characterization of their content and types. This paper prepared three specimens with three different Mn contents (1.0%, 1.4%, and 1.7%) that are used to obtain high-strength steel by ultrafast cooling heat treatment. The volume content and distribution of the RA were analysed by an X-ray Debye ring measurement system, electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). In addition, the mechanical tensile test provided the tensile properties and elongation of three specimens. It was finally concluded that when the content of Mn increased, the island-type and thin film-type RA both increased, which may effectively improve the plasticity of the martensitic steels.

Laser shock peening rounds influencing microstructural and mechanical properties of 300M steel

Laser shock peeing (LSP) was introduced to produce surface gradient structures on 300M steel to improve its performance, service life and related workpiece safety. The influences of LSP rounds on microstructural and mechanical properties were investigated to optimise the LSP processing. Smoother surface, surface nanocystallization, and gradient structure, hardness, compressive residue stress was observed LSP. Strength and plasticity of 300M steel improved significantly due to the surface changes induced by one round of LSP. However, excessive LSP rounds tend to deteriorate the surface roughness and mechanical properties despite increased surface nanocrystallization, hardening zone depth and compressive residual stress.

Effect of Mn Content on the Toughness and Plasticity of Hot-Rolled High-Carbon Medium Manganese Steel

The tensile and impact deformation behavior of three different Mn content test steels, xMn-1.0C-0.25V-1.5Cr-0.3Mo (5, 8 and 13 wt%), were investigated using mechanical properties testing, SEM-EBSD and TEM. The elongation and -20 °C impact energy of the three types of Mn content test steels increased as the Mn content increased. The room temperature tensile elongation was 9%, 23% and 81%, and the -20 °C impact energy was 9 J, 99 J and 241 J, respectively. The fracture morphologies of 5 Mn and 8 Mn were found to be cleavage fractures with secondary cracks and micro-voids. The 13 Mn fracture morphology was a plastic fracture with many coarse dimples. Transverse cracks perpendicular to the tensile direction occurred on the surface of the gauge area of 5 Mn and 8 Mn tensile specimens, reducing plasticity dramatically. This was mainly related to the martensitic transformation produced by stress. We characterized the martensite near the tensile fracture and speculated the main mode of crack propagation. Furthermore, a little amount of sharp-shaped BCC phase was found in the 5 Mn, which was determined to be a hard phase relative to the austenite matrix by nanoindentation test. These steels have stacking fault energies ranging from ~15 to ~29 mJ/m2 with increasing Mn content 13 Mn has high stacking fault energy (SFE) and austenite stability. Twin-induced plasticity (TWIP) was the deformation mechanism.

Compelling Evidence for the Role of Retained Austenite in the Formation of Low Cycle Fatigue Extrusions in a 9Ni Steel

The 9Ni martensitic steels have a martensitic microstructure which contains retained austenite after solution heat treatment and water quenching. Under low cycle fatigue, extrusions formed at the surface of the material and were very close to martensite lath boundaries. The presence of retained austenite at martensite laths has been highly suspected to impact the cyclic plasticity. However, the nano-size of the austenitic phase makes it difficult to obtain clear evidence of its role. The paper focuses on the precise determination of these extrusions and the link with the retained austenite. The paper also emphasizes the innovative and promising use of magnetic force microscopy (MFM) to document cyclic plasticity of a 9Ni steel. It is shown that electron microscopies, even the most advanced ones, may be unsuccessful in reaching this goal, while magnetic force microscopy (MFM) overcame the difficulty. This technique has allowed imaging of both the extrusion and the retained austenite. These analyses confirm that the fatigue extrusions originated from a local displacement of martensite lath. The proposed mechanism, in which the retained austenitic film acts as a lubricant film or greasy film promoting a flowing of martensite along the interfaces, is unambiguously demonstrated.

Influence of Initial Structures on the Mechanical Properties in Cold‐Rolled Transformation‐Induced Plasticity Steel

A novel transformation‐induced plasticity steel is analyzed to study the effects of initial structures and the intercritical annealing temperatures on austenization behavior, strain distribution, and the characteristics of retained austenite (RA). The resulting microstructure and its relationship with mechanical properties are measured using dilatometry, scanning electron microscopy, electron backscatter diffraction, X‐ray diffraction, and tensile testing. The results show that the initial structure is composed of martensite (M) or tempered martensite (TM) and can significantly refine the final microstructure and its strain distribution is more homogeneous compared to the ferrite + perlite (F + P) initial structure samples. However, it is found that the larger size of the RA mainly distributes at the ferrite grain boundaries while the smaller size mainly distributes in the bainite matrix and outside martensite. Meanwhile, most of the RA in the bainite matrix are films and the carbon concentration is high. In addition, the initial M samples have many homogeneous lath interfaces and the ferrite has a strong carbon discharge ability during high‐temperature annealing, so the fraction of RA is the highest at 23.1% when annealed at 820 °C, and the corresponding carbon concentration is 1.16 wt%, so the final combination of strength and ductility is up to 32.1 GPa%.

Improvement of Mechanical Properties and Wear Resistance of Direct‐Quenched Wear‐Resistant Steel by Deformed Austenite

Herein, thermomechanically controlled processing (TMCP) and direct‐quenching (DQ) process are investigated to improve the mechanical and wear properties of wear‐resistant steel, compared to the reheating–quenching (RQ) process. Scanning electron microscope, electron backscatter diffraction, transmission electron microscope, and X‐ray diffraction are employed to characterize the microstructures of the DQ and RQ specimens, and the mechanical and wear properties are investigated using the Vickers hardness, impact, tensile, and stirring wear tests for both processes. The results show that DQ steel exhibits strong plasticity, impact toughness, and wear resistance; the DQ process also retains the deformed austenite formed by rolling in the nonrecrystallization region. The compressed austenite reduces the size of the martensite lath and block structure, increases the density and proportion of the high‐angle grain boundaries, and improves the plasticity and toughness of DQ steel. Meanwhile, DQ steel also inherits the high‐density dislocations created during the rolling process, which is its primary strengthening mechanism. The deformed grains in DQ steel reduce the Schmid factor, improve resistance to wear deformation, and enhance its wear performance.

C-modified stacking-fault networks inducing the excellent strength- plasticity combinations of medium manganese steel by simple two-stage warm rolling without annealing

The medium manganese steel with ultrahigh yield strength of 1421 MPa, a tensile strength of 1575 MPa, and an excellent total elongation of 28.4% was designed by simple two-stage warm rolling without annealing. The improvement of yield strength is mainly attributed to the extensive strengthening defects in the lamellar dual-phase microstructures, which including the high-density dislocations in ferrites and high-density dislocations, nanoscale VC precipitates, stacking faults and Lomer-Cottrell (L-C) locks in austenite (22 Vol.%). In addition, the abnormally enhanced strain-hardening ability benefits from the C-modified stacking-fault networks, whose stability and C-segregated characteristics are precisely clarified by the advanced in-situ heating HRTEM and APT analysis. This special strengthening and plasticizing effects of stacking faults induced by simple two-stage warm rolling can effectively break through the trade-off between strength and plasticity of ultra-strong medium manganese steel.

Roles of pre-formed martensite in deformation behavior and strain partitioning of medium-carbon bainitic steel by quasi-in-situ tensile tests

Microstructural features play a major role in strain distribution and TRIP effect, influencing the plastic deformation behavior of multiphase steel. In this work, the bainitic multiphase microstructure with or without pre-formed martensite (PM) was obtained by the below-MS and above-MS austempering process. The evolution of strain partitioning and TRIP effect on plastic deformation were studied by combining micro digital image correlation (μ-DIC) analysis with in-situ tensile test in the multiphase microstructure. The changes of microstructure during deformation were systematically investigated by various characterization methods. The results show that the regions of strong strain partitioning in multiphase microstructure are different. Compared with the above-MS austempered sample without PM, the strain partitioning of the below-MS austempered sample containing PM is concentrated in the pre-formed martensite region while not in the blocky retained austenite (RA) region. The strain concentration in the PM region of the below-MS austempered sample is smaller than that in the blocky RA region of the above-MS austempered sample at the same engineering strain. The PM shares the stress load on RA during plastic deformation, thus can coordinate microstructure deformation and delay the early occurrence of the TRIP effect of the below-MS austempered sample. Moreover, the introduction of PM in the below-MS austempered sample increases the mechanical stability of RA, which is beneficial to produce an effective TRIP effect and delay necking to high strain. Consequently, the plasticity of the medium-carbon bainitic multiphase steel containing PM is enhanced by the effective TRIP effect and uniform strain partitioning.

Plastic deformation and ductile fracture of L907A ship steel at increasing strain rate and temperature

A combined theoretical, numerical, and experimental approach is adopted to systematically characterize the plastic deformation and ductile fracture behaviors of L907A low-alloy ship steel widely used in ship construction, with the effects of strain rate and thermal softening duly accounted for. A mixed Swift-Voce hardening model coupled with strain rate and temperature-dependent terms is proposed, while the Hosford-Coulomb model of ductile fracture is modified by introducing temperature-dependent term to account for the effect of thermal softening. To calibrate the parameters appearing in the constitutive models, dynamic uniaxial tensile tests at varying strain rates (0.001 ∼ 5200 s−1) and quasi-static uniaxial tensile tests at varying temperatures (20 ∼ 800 °C) are separately performed, with the coupling between strain rate and temperature effects ignored. The calibrated plasticity and ductile fracture models are numerically implemented with the method of finite elements (FE), and their applicability to practical applications is validated against experimental results of ballistic impacts. Normal and oblique impacts on L907A target plates with conical, hemispherical, and blunt projectiles are considered. Good agreement between FE simulation results of plastic deformation and ductile fracture modes and those measured experimentally is achieved, thus demonstrating the viability of the established plasticity and fracture models of L907A steel for applications in complex ballistic impact scenarios.

Износостойкость низколегированных сталей в абразивной среде

The import substitution of agricultural machinery implements is possible with the development of high quality steel and hardening technologies appropriate to the operating conditions of agricultural machinery. Along with high wear resistance in abrasive environment, steel must have high ductility and impact toughness. To investigate wear resistance of the developed low-alloy steels in the abrasive environment and estimate the possibility of their application for manufacturing high-resistance working tools of agricultural machinery the authors developed four batches of steel samples with additives. All samples were subject to heat treatment with heating to 900°C and quenching in water at 20°C. The second group of samples was additionally tempered at a temperature of 280°C for an hour, followed by air cooling. The wear resistance of samples was determined according to GOST 23.208‑79 with the test equipment. The least wear intensity of 0.0581 g/m was recorded for steel 0.43C‑1.60Si‑0.01Mn‑1.1Cr‑0.95Mo‑0.08V‑ 0.05Nb‑0.04Ti subjected to heat treatment with quenching. The high tensile strength of 2170 MPa was obtained through the formation of carbonitrides enriched with Nb. The wear resistance of this steel grade is 1.66 times higher than that of the 65G steel grade. The heat treatment of quenching with tempering yielded the relative wear resistance of the given steel by 10.7% lower, which can be explained by phase transformations reducing the ultimate strength to 2040 MPa, but increasing the plasticity of steel. Studies have shown a wide range of changes in wear resistance depending on the heat treatment mode. This makes it possible to modify the structure of steel in relatively small temperature intervals of quenching and tempering, thereby influencing the properties that provide the greatest wear resistance.

STUDIES OF THE INFLUENCE OF TECHNOLOGICAL PARAMETERS OF THE PRODUCTION OF GRINDING BALLS FROM THE REJECTION OF RAIL STEEL ON THEIR IMPACT RESISTANCE

Based on the conducted research, a list of technological parameters of steelmaking and rolling processing that have a significant impact on the impact resistance of grinding balls from the rejection of K76F grade rail steel in the conditions of JSC "Guryev Metallurgical Plant" has been determined. Using standard methods of statistical and metallographic analyses, the degree and nature of the influence of these parameters on the rejection of grinding balls based on the results of copra tests has been established. It is determined that an increase in the content of carbon, hydrogen and sulfur in steel leads to an increase in the rejection of grinding balls according to the results of impact resistance tests. At the same time, the influenceof carbon is associated with the formation of cementite–type carbides when the carbon content increases to over-eutectoid values, the influence of hydrogen is associated with the formation of floccenes, and the influence of sulfur is associated with the formation of non-plastic sulfides. According to the rolling conversion, a significant effect of an increase in the temperature of rolling balls on an increase in their impact resistance was established, due to an increase in the plasticity of K76F steel in the actual rolling temperature range (an increase in the probability of defects brewing during deformation). It is determined that the total relative degree of influence of the parameters of steelmaking and rolling processing on the rejection of balls during impact resistance tests is 71%, the remaining 29% relate to the influence of the parameters of the heat treatment of balls, which is confirmed by the results of metallographic studies (quenching cracks were detected in the fracture of the balls).

Local Instability Analysis for Axial Compressive Steel Columns

This study presents a nonlinear analysis method for steel columns considering local instability under axial compression. The stress-strain relationship of steel is developed while considering the effects of local instability utilizing the energy principles. The second-order effects are considered by stability functions, while the plasticity of steel is captured by a fiber finite element method. Nonlinear equilibrium equations are solved by the generalized displacement control method. As a result, the proposed formulations are effective, accurate, and reliable in predicting the load-carrying capacity and local instability of steel columns.

Effect of a new pretreatment-Q&P process on the microstructure and mechanical properties of light-weight Al-containing medium-Mn steels

This study examines the impact of a new pretreatment-Q&P process on the mechanical properties and microstructure of lightweight Al-containing medium Mn steels. The tested steel is pre-strained (0%, 1%, 2%, and 3%) before going through the Q&P process. The tested steel's microstructure and mechanical properties were examined. And the results revealed that the tested steel was composed of martensite, α-ferrite, δ-ferrite, and retained austenite. After the heat treatment process, the retained austenite in the microstructure is distributed in a dispersed distribution. According to research, the amount of elemental Mn required to maintain a stable presence at room temperature varies depending on the retained austenite morphology. Simultaneously, it was revealed that the process significantly improved martensite/retained austenite/ferrite to accept dislocations. This is because the increase in dislocation channels allows C/Mn/Al/Si elements to diffuse more easily, allowing the block, granular retained austenite to remain moderately stable at room temperature. This facilitates the TRIP effect, which enhances the plasticity of the medium manganese steels. At the same time, when the external stress reaches a certain degree, the sudden opening of dislocations in the microstructure and their mutual cancellation causes stress relaxation and redistribution in the medium manganese steel microstructure. The plasticity of medium manganese steels is greatly enhanced by the combined effect of the TRIP effect and the redistribution of stress between the microstructures.

Effect of distribution characters of ferrite/bainite on deformation compatibility in dual-phase pipeline steel: Experimental and numerical study

The morphology distribution of microstructure has an important influence on the match of the strength and plasticity of dual-phase pipeline steel. Bainite and PF dual-phase steels with layered and network morphology were obtained by combining the Thermomechanical Control Process (TMCP) and inter-critical isothermal heat treatment. A novel experimental-numerical methodology studied the effect of phase distribution characters on the deformability of PF and bainite dual-phase pipeline steel. The plastic damage features and microscopic strain and stress distribution in both network and layered microstructure were clarified. The obvious differences in the microstructure evolution for both samples were captured by a quasi-situ tensile test. The PF phase in the network morphology sample was believed the main deformation phase, but the deformation of the PF phase in the layered morphology sample was restrained. Two three-dimensional (3D) crystal plasticity finite element models (CPFEMs) were established based on the electron backscatter diffraction (EBSD) data. The simulation results indicated that the strain field in layered microstructure was more uniform than that in network microstructure, and the continuous internal stress relaxation (ISR) in the bainite phase led to plastic damage at low tensile strain level; The strain field in network microstructure mainly existed in the PF phase, the deformation of the PF phase can be fully utilized. When changing the tensile x-direction of the layered microstructure to the z-direction, the strain field showed an obvious anisotropy, and loading in the z-direction increased the risk of interlaminar crack formation. • Quasi-situ tensile test was used to study the deformation behavior • Strain/stress distribution characteristics were clarified using CPFEM method • Uniform strain distribution and ISR in layered microstructure decreased its plasticity • z-axis loading increased the risk of interlaminar crack for layered microstructure

Efficient stress integration algorithm of memory surface model with shrinking rule under plane stress field

Reliable nonlinear finite element analysis (FEA) of steel structures requires a constitutive model capable of predicting the cyclic plasticity of structural steels. Steel structure members are typically composed of multiple thin steel plates. The FEA of such structures has frequently used shell elements to capture the bending deformation and reduce the computational complexity, where its stress state can be regarded as the plane stress field. This study provides a constitutive model with a memory surface under a plane stress field for the cyclic plasticity of structural steels. A noble computationally efficient algorithm for stress integration and consistent tangent operator was proposed, where the constitutive equations were linearized after solving some unknown variables. The proposed model was implemented in the Abaqus FEA code. The residual-force convergence rate of the consistent tangent operator is investigated using a single-shell element under simple boundary conditions, where the consistent tangent operator achieves more than a quadratic convergence. The proposed model was validated by comparing the experimental results and FEA results with the proposed model for thin-walled steel members under cyclic loading. The average difference in peak forces between FEA and experimental results was 11.5 % for the local buckling tests and 8.4 % for the shear buckling tests at most. These results suggest that the proposed model provides an accurate prediction for other thin-walled steel structures under cyclic loading.

Influence of Carbon Content on Tensile Properties of Pure High Manganese Austenitic Steel

The tensile properties of high manganese austenitic steels with a carbon content ranging from 0.79 to 1.28 wt.% were tested. X-ray diffraction, electron backscattering diffraction, transmission electron microscopy, and optical microscopy were used to observe the microstructures after tensile deformation. Results showed that the strength and plasticity of these high manganese austenitic steels increased with increasing carbon content. The tensile strength and elongation of the 130Mn11 steel reached 941 MPa and 38.2%, respectively. The 0.79% carbon-containing steel (80Mn11) formed the most deformation twins at the same strain because of the low stacking fault energy, which resulted in a high strain hardening rate. However, this high strain hardening rate was unsustainable, and the tensile properties of the 80Mn11 steel were the worst, with its tensile strength nearly 200 MPa lower than that of the 130Mn11 steel. In the case of the 1.28% carbon-containing steel (130Mn11), the relatively low density of deformation twins, the large number of dislocations, and intensified DSA effect made the steel display a moderate strain hardening rate, which facilitated the sustainability of deformation, and an excellent combination of strength and plasticity were obtained.

Preface to the Special Issue on “Strength, Plasticity, and Fracture in Steels: Towards Quantitative Bridging of Experiment and Simulation”

Preface to the Special Issue on “Strength, Plasticity, and Fracture in Steels: Towards Quantitative Bridging of Experiment and Simulation”