Search Results (18)

For investigating the effect of temperature and strain rate on martensitic transformation and establishing the corresponding kinetic model for newly TRIP (transformation-induced plasticity) aided duplex stainless steel (DSS), the tensile tests are conducted at temperatures of 20–150 °C and strain rates of 0.0001–150 s⁻¹. The stepped cross-section tensile specimen is proposed and designed for obtaining microstructure at specific strain during dynamic tensile testing. The results demonstrate that the deformation mechanism of austenite in TRIP-aided DSS is highly sensitive to temperature and strain rate. As the deformation temperature increases, strain-induced martensitic transformation is inhibited, and the deformation mechanism transforms from martensitic transformation to the co-occurrence of martensitic transformation and twinning, and finally, twinning is the main deformation mechanism. This leads to reduced strength with an initial increase followed by a decrease in elongation. As the strain rate increases, martensitic transformation is inhibited, resulting in a reduction in strength and plasticity during quasi-static tensile testing, while during dynamic tensile testing, strength increases due to enhanced resistance to dislocation motion, and plasticity displays no significant variation because of the combination of adiabatic softening and martensitic transformation suppression. Moreover, during tensile deformation, a plastic temperature rise model is established for newly developed DSSs. Based on this model, the Ludwigson–Berger model for martensitic transformation was modified to couple the effect of temperature and strain rate by considering the non-uniform distribution of temperature rise within the material and its variation with strain rate, as well as the suppression of dynamic strain rate on martensitic transformation. This new model could accurately describe the characteristics of martensitic transformation in newly developed DSSs at different deformation temperatures and strain rates. Full article

In this experiment, a novel designed Mn-N-bearing, nearly Ni-free, TRIP-aided economical 19Cr (Fe-18.9Cr-10.1Mn-0.3Ni-0.26N-0.03C) duplex stainless steel (DSS) was prepared, and it exhibited a good combination of strength and toughness after suitable solution treatment, showing good application potential. The deformation mechanisms of ferrite and austenite are different during tensile deformation at room temperature: the ferrite phase was deformed by a dislocation slip mechanism and formed a cell structure due to its higher stacking fault energy; the lower stacking fault energy of austenite resulted in a strain-induced martensite phase transformation mechanism. With an increase in aging time from 1 h to 7 h at 750 °C in air, the σ phase precipitates in the ferrite triple grain boundary junction, which leads to an increase in ultimate tensile strength, acts as an obstacle to the dislocation motion and decreases the ductility, deteriorating the pitting corrosion resistance in 3.5 wt.% NaCl solution at the same time. The σ phase precipitation behavior does not alter the deformation mechanism of the phases of the solution-treated TRIP-aided economical DSS. Full article

This study addresses the characterization of the particular microstructural constituents of multiphase transformation-induced plasticity (TRIP)-aided steels belonging to the first and third generations of Advanced High Strength Steels (AHSS) to explore the possibilities of the EBSD method. Complex microstructures composed of ferrite, bainite, retained austenite and martensite were qualitatively and quantitatively assessed. Microstructural constituents with the same crystal structure were distinguished using characteristic EBSD parameters like confidence index (CI), image quality (IQ), kernel average misorientation (KAM) and specific crystallographic orientation relationships. A detailed linear analysis of the IQ parameter and misorientation angles was also performed. These tools are very helpful in linking different symmetric or asymmetric features of metallic alloys with a type of their structure and morphology details. Two types of samples were investigated: thermomechanically processed and subjected to 10% tensile strain to study the microstructural changes caused by plastic deformation. Full article

The damage to the shear-punched surface layers such as strain-hardening, strain-induced martensite transformation, and micro-void initiation behaviors was evaluated in the third-generation low-carbon advanced ultrahigh-strength TRIP-aided bainitic ferrite (TBF), bainitic ferrite–martensite (TBM), and martensite (TM) steels. In addition, the surface layer damage was related to (1) the mean normal stress generated during shear-punching and (2) microstructural properties such as the matrix structure, retained austenite characteristics, and second-phase properties. The shear-punched surface layer damage was produced under the mean normal stress between zero and negative in all the steels. The TBM and TM steels achieved relatively small surface layer damage. The small surface layer damage resulted in excellent cold stretch-flangeability, with a high crack-propagation/void-connection resistance on hole expansion. Full article

The effects of mean normal stress on the deformation properties such as the strain-hardening, strain-induced martensite transformation, and micro-void initiation behaviors of low-carbon ultrahigh-strength TRIP-aided bainitic ferrite (TBF), bainitic ferrite/martensite (TBM), and martensite (TM) steels were investigated to evaluate the various cold formabilities. In addition, the deformation properties were related to the microstructural properties such as the matrix structure, retained austenite characteristics, and second-phase properties. Positive mean normal stress considerably promoted strain-induced martensite transformation and micro-void initiation, with an increased strain-hardening rate in an early strain range in all steels. In TM steel, the primary martensite matrix structure suppressed the micro-void initiation through high uniformity of a primary martensite matrix structure and a low strength ratio, although the strain-induced transformation was promoted, and a large amount of martensite/austenite constituent or phase was contained. A mixed matrix structure of bainitic ferrite/primary martensite in TBM steel also suppressed the micro-void initiation because of the refined microstructure and relatively stable retained austenite. Promoted micro-void initiation of TBF steel was mainly promoted by a high strength ratio. Full article

The damage properties in the shear-punched surface layer, such as the strain-hardening increment, strain-induced martensite fraction, and initiated micro-crack/void characteristics at the shear and break sections, were experimentally evaluated to relate to the stretch-flangeability in three types of low-carbon high-strength TRIP-aided steel with different matrix structures. In addition, the surface layer damage properties were related to the mean normal stress developed on shear-punching and microstructural properties. The shear-punched surface damage of these steels was experimentally confirmed to be produced under the mean normal stress of negative to 0 MPa. TRIP-aided bainitic ferrite (TBF) steel had the smallest surface layer damage, featuring a significantly suppressed micro-crack/void initiation. This was due to the fine bainitic ferrite lath matrix structure, a low strength ratio of the second phase to the matrix structure, and the high mechanical stability of the retained austenite. On the other hand, the surface layer damage of TRIP-aided annealed martensite (TAM) steel was suppressed next to TBF steel and was smaller than that of TRIP-aided polygonal ferrite (TPF) steel. The surface layer damage was also characterized by a large plastic strain, a large amount of strain-induced martensite transformation, and a relatively suppressed micro-crack/void formation, which resulted from an annealed martensite matrix and a large quantity of retained austenite. The excellent stretch-flangeability of TBF steel might be caused by the suppressed micro-crack/void formation and high crack propagation/void connection resistance. The next high stretch-flangeability of TAM steel was associated with a small-sized micro-crack/void initiation and high crack growth/void connection resistance. Full article

The effects of hydrogen on the tensile properties, fatigue life, and tensile and fatigue fracture morphologies of nitrogen-added ultrahigh-strength transformation-induced plasticity (TRIP)-aided martensitic (TM) steels were investigated. The total elongation and number of cycles to failure (N_f) of the hydrogen-charged TM steels decreased with the addition of nitrogen; in particular, adding 100 ppm of nitrogen decreased the total elongation and N_f of the TM steels. The quasi-cleavage cracking around the AlN occurred near the sample surface, which is the crack propagation region, although dimples appeared at the center of the fracture surface in the tensile samples. The initial fatigue crack initiated at the AlN precipitate or matrix/AlN interface, located at the notch root. During crack propagation, new cracks were initiated at the AlN precipitates or matrix/AlN interfaces, while quasi-cleavage crack regions were observed around the AlN precipitates. The decrease in the total elongation and N_f of the hydrogen-charged TM steel with 100 ppm of added nitrogen might be attributable to the crack initiation around the AlN precipitates formed by a large amount of hydrogen trapped at the AlN precipitates and matrix/AlN interfaces, and to the dense distribution of AlN, which promoted crack linkage. Full article

To analyze various types of cold formability in TRIP-aided polygonal ferrite (TPF), annealed martensite (TAM), and bainitic ferrite (TBF) steels, the effects of the mean normal stress on the strain-hardening, strain-induced martensite transformation, and void-formation behaviors were investigated. The strain-hardening behavior was influenced by positive mean normal stress and was hardly influenced by zero and negative mean normal stresses in all steels. Positive mean normal stress promoted the strain-induced martensitic transformation behavior, especially in TBF steel due to the high mechanical stability of the retained austenite. The void-formation behavior was also promoted by positive mean normal stress, especially in TPF steel. These behaviors were also related to the microstructural properties, such as the matrix structure, retained austenite characteristics, and second phase. Full article

The effects of partial replacement of Si by Al on the microstructure, tensile properties, and Charpy impact toughness were investigated using 0.2%C-Si/Al-Mn-Cr-B TRIP-aided martensitic steels to promote the application of galvanized third-generation ultrahigh- and high-strength steels. The impact toughness was related to the microstructural and mechanical properties. The partial replacement decreased the volume fraction of retained austenite and increased the mechanical stability, accompanied by softening and an increase in the volume fraction of the primary martensite. Resultantly, the partial replacement decreased strength and ductility. The impact absorbed energy (value) at 25 °C was slightly increased by the partial replacement. The increased impact absorbed energy was mainly caused by high crack/void propagation energy due to the softened primary martensite and a small contribution of the stabilized retained austenite. The 50% shear fracture ductile-to-brittle transition temperature was marginally raised by the partial replacement. The raised transition temperature was mainly associated with an increase in a unit crack path of quasi-cleavage/cleavage fracture. Full article

Production of high-quality maraging steel is dependent not only on the production technology but also on the alloying design and heat treatment. In this work, cobalt-free, low nickel, molybdenum-containing maraging steel was produced by melting the raw materials in a vacuum induction melting furnace and then refining with a shielding gas electroslag remelting unit. The critical transformation temperatures of the investigated steel samples were determined experimentally by differential scanning calorimetry (DSC) analysis and theoretically aiding Thermo-Calc software. Types and chemical composition plus volume fraction and starting precipitation temperature of suggested constituents calculated with the aid of Thermo-Calc software. The microstructures of forged steel specimens that were heat-treated under several conditions were evaluated by X-ray diffraction (XRD), optical microscopy (OP), scanning electron microscopy (SEM), and electron backscattering (EBSD), in addition to transmission electron microscopy (TEM). The mechanical properties of the investigated steel specimens were evaluated by measuring the tensile strength properties and micro-hardness, furthermore, estimating their fracture surface using scanning electron microscopy at lower magnification. The metallographic results show that the microstructure of steel in aged conditions includes high-alloyed martensite and nickel-rich phase, in addition to the low-alloyed-retained-austenite, intermetallic compounds, and lavas-phase (MoCr). Furthermore, TEM and EBSD studies emphasized that the produced steel has high dislocation density with nano-sized precipitate with an average size of ~19 ± 1 nm. Moreover, the metallographic results show that the mentioned microstructure enhances the tensile properties by precipitation strengthening and the TRIP phenomenon. The tensile strength results show that the n-value of investigated steel passes two stages and is comparable with the n-value of TRIP-steel. Steel characterized by 2100 MPa ultimate tensile strength and uniform elongation of more than 7% can be produced by the investigated production routine and optimum heat treatment conditions. Full article

Cold stretch-formability and stretch-flangeability of 0.2%C-1.5%Si-5.0%Mn (in mass%) martensite-type medium Mn steel were investigated for automotive applications. High stretch-formability and stretch-flangeability were obtained in the steel subjected to an isothermal transformation process at temperatures between M_s and M_f − 100 °C. Both formabilities of the steel decreased compared with those of 0.2%C-1.5%Si-1.5Mn and -3Mn steels (equivalent to TRIP-aided martensitic steels), despite a larger or the same uniform and total elongations, especially in the stretch-flangeability. The decreases were mainly caused by the presence of a large amount of martensite/austenite phase, although a large amount of metastable retained austenite made a positive contribution to the formabilities. High Mn content contributed to increasing the stretch-formability. Full article

The influence of elevated deformation temperatures on the relationships between the microstructure and mechanical properties in a hot-rolled Si-Al-alloyed transformation-induced plasticity (TRIP)-aided steel was studied in a static tensile test. The morphological features of specimens deformed at the different temperatures were characterized by different microstructural techniques: optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). An increase in the deformation temperature from 20 to 200 °C resulted in the reduced effectiveness of the TRIP effect, due to the increasing mechanical stability of the γ phase. The gradual transformation of retained austenite into martensite expressed by a progressive increase in the work hardening exponent (n) led to a beneficial balance of strength, uniform elongation and total elongation. The best product of UTS × TEl = 17,805 MPa% showed the sample deformed at 20 °C with a peak n value amounting to 0.3. Full article

Ultrahigh-strength ferrous steels, related to the strain-induced martensite transformation (or transformation-induced plasticity: TRIP) of metastable retained austenite, such as TRIP-aided bainite/martensite steels, quenching and partitioning steels, nanostructured bainitic steels (or carbide free bainitic steels) and medium manganese steels, are currently receiving a great deal of attention from both academic and industry sectors, due to their excellent formability and mechanical properties [...] Full article

To develop ultra high-strength cold stamping steels for automobile frame parts, the effects of alloying elements on hydrogen embrittlement properties of ultra high-strength low alloy transformation induced plasticity (TRIP)-aided steels with a martensite matrix (TM steels) were investigated using the four-point bending test and conventional strain rate tensile test (CSRT). Hydrogen embrittlement properties of the TM steels were improved by the alloying addition. Particularly, 1.0 mass% chromium added TM steel indicated excellent hydrogen embrittlement resistance. This effect was attributed to (1) the decrease in the diffusible hydrogen concentration at the uniform and fine prior austenite grain and packet, block, and lath boundaries; (2) the suppression of hydrogen trapping at martensite matrix/cementite interfaces owing to the suppression of precipitation of cementite at the coarse martensite lath matrix; and (3) the suppression of the hydrogen diffusion to the crack initiation sites owing to the high stability of retained austenite because of the existence of retained austenite in a large amount of the martensite–austenite constituent (M–A) phase in the TM steels containing 1.0 mass% chromium. Full article

The quenching and partitioning (Q&P) process of lean medium Mn steels is a novel approach for producing ultra-high strength and good formable steels. First, the steel is fully austenitized, followed by quenching to a specific quenching temperature (T_Q) in order to adjust an appropriate amount of initial martensite (α’_initial). Subsequently, the steel is reheated to a partitioning temperature (T_P) in order to ensure C-partitioning from α’_initial to remaining austenite (γ_remain) and thus retained austenite (RA) stabilization. After isothermal holding, the steel is quenched to room temperature (RT), in order to achieve a martensitic-austenitic microstructure, where the meta-stable RA undergoes the strain-induced martensitic transformation by the so-called transformation induced plasticity (TRIP) effect. This paper systematically investigates the influence of the Q&P process on the isothermal bainitic transformation (IBT) kinetics in a 0.2C-4.5Mn-1.3Al lean medium Mn steel by means of dilatometry. Therefore, the Q&P annealing approach was precisely compared to the TRIP-aided bainitic ferrite (TBF) process, where the samples were directly quenched to the temperature of the IBT after full austenitization. The results indicated an accelerated IBT for the Q&P samples, caused by the formation of α’_initial during quenching below the martensite start (M_S) temperature. Furthermore, a significant influence of the annealing parameters, such as T_Q and T_P, was observed with regard to the transformation behavior. For further characterization, light optical microscopy (LOM) and scanning electron microscopy (SEM) were applied, showing a microstructure consisting of a martensitic-bainitic matrix with finely distributed RA islands. Saturation magnetization method (SMM) was used to determine the amount of RA, which was primarily depending on T_Q. Furthermore, the hardness according to Vickers revealed a remarkable impact of the annealing parameters, such as T_Q and T_P, on the predicted mechanical properties. Full article