Pct Mn Steel Research Articles

Characterization of Inclusions in 3rd Generation Advanced High-Strength Steels

Samples taken from laboratory-produced 3rd generation advanced high-strength steels, solidified at a low cooling rate, have been investigated to study the characteristics of non-metallic inclusions. Two steels, containing 2 and 5 pct Mn content, were produced for this purpose. A higher number of total inclusions were observed in 5 pct Mn steel. The four main types of inclusions observed were Al2O3, AlN, MnS, and AlSiMn-oxide. These classes were divided into subclasses according to variations in their chemistry. The major subclasses of AlN inclusions are either plate-like or regular in shape and have different size distributions. Thermodynamic calculations suggest that plate-like AlN inclusions are formed at the initial stage of solidification, while faceted/regular-shaped inclusions are precipitated toward the end of solidification. Moreover, it was found that the size of nitride inclusions is related to their N content. This phenomenon is discussed from the viewpoint of nucleation theory.

Effect of Solute Nb on Grain Growth in Fe-30 Pct Mn Steel

Niobium is known to segregate strongly to grain boundaries in steel. The strong interaction of Nb with grain boundaries results in an important reduction of the grain boundary mobility and strong retardation of recrystallization and grain growth. In this study, the effect of Nb on the mobility of grain boundaries in a second-generation Fe-30 pct Mn TWIP steel is investigated. Grain growth kinetics was measured in a series of Nb-containing Fe-30 pct Mn model alloys. An estimate of the grain boundary mobility was obtained for various temperatures and niobium contents. It was found that Nb slows down the mobility of random high-angle grain boundary segments that are not twin related. The effect of solute Nb on grain growth, in the presence of annealing twins, was modeled using Cahn’s theory of solute drag coupled with a recently formulated twin-inhibited grain growth model. The results are shown to be in very good agreement with the experimentally determined growth kinetics.

Identification of Inverse Bainite in Fe-0.84C-1Cr-1Mn Hypereutectoid Low Alloy Steel

A unique dilatation trend is observed for isothermal bainite transformation in Fe-0.84 pct C-1 pct Cr-1 pct Mn steel. The dilatation is found to occur in two stages with volumetric contraction dominating the first stage, followed by volumetric expansion dominating the second stage. Through electron microscopic characterization, bainitic microstructure is identified as inverse bainite with cementite (Fe3C) nucleating first from supersaturated austenite followed by the transformation of ferrite and secondary carbides (Fe3C, Fe2C, and Fe5C2) from carbon-depleted austenite.

Phase Transformation Behavior of Medium Manganese Steels with 3 Wt Pct Aluminum and 3 Wt Pct Silicon During Intercritical Annealing

Medium manganese steels alloyed with sufficient aluminum and silicon amounts contain high fractions of retained austenite adjustable to various transformation-induced plasticity/twinning-induced plasticity effects, in addition to a reduced density suitable for lightweight vehicle body-in-white assemblies. Two hot rolled medium manganese steels containing 3 wt pct aluminum and 3 wt pct silicon were subjected to different annealing treatments in the present study. The evolution of the microstructure in terms of austenite transformation upon reheating and the subsequent austenite decomposition during quenching was investigated. Manganese content of the steels prevailed the microstructural response. The microstructure of the leaner alloy with 7 wt pct Mn (7Mn) was substantially influenced by the annealing temperature, including the variation of phase constituents, the morphology and composition of intercritical austenite, the Ms temperature and the retained austenite fraction. In contrast, the richer variant 10 wt pct Mn steel (10Mn) exhibited a substantially stable ferrite-austenite duplex phase microstructure containing a fixed amount of retained austenite which was found to be independent of the variations of intercritical annealing temperature. Austenite formation from hot band ferrite-pearlite/bainite mixtures was very rapid during annealing at 1273 K (1000 °C), regardless of Mn contents. Austenite growth was believed to be controlled at early stages by carbon diffusion following pearlite/bainite dissolution. The redistribution of Mn in ferrite and particularly in austenite at later stages was too subtle to result in a measureable change in austenite fraction. Further, the hot band microstructure of both steels contained a large fraction of coarse-grained δ-ferrite, which remained almost unchanged during intercritical annealing. A recently developed thermodynamic database was evaluated using the experimental data. The new database achieved a better agreement with the experimental results for the 7Mn steel compared with the existing commercial TCFE database; however, some discrepancy in the predicted phase fractions and compositions still existed. The phase transformation behavior of the two steels during annealing and its implication on the design of high aluminum-silicon medium manganese steels were discussed in detail.

Impact Toughness of 0.2 Pct C-1.5 Pct Si-(1.5 to 5) Pct Mn Transformation-Induced Plasticity-Aided Steels with an Annealed Martensite Matrix

The impact properties of 0.2 pct C-1.5 pct Si-(1.5 to 5) pct Mn transformation-induced plasticity (TRIP)-aided steels with an annealed martensite matrix which had been subjected to isothermal transformation after inter-critical annealing were investigated for potential automotive applications. The impact properties are related to the retained austenite characteristics of the steels. The products of tensile strength (TS) and Charpy impact absorbed value (CIAV) were the same for the 1.5 and 5 pct Mn steels, although the ductile–brittle transition temperature was higher for the latter. The impact properties of the 3 pct Mn steel were worse than these two steels. The high TS × CIAV value for the 5 pct Mn steel at 293 K (25 °C) was mainly caused by the TRIP effect of a larger amount of retained austenite (36 vol pct) and the hardened matrix structure; low retained austenite stability and/or a hard martensite–austenite phase reduced this value. The higher ductile–brittle transition temperature of the 5 pct Mn steel was associated with Mn segregation, a large amount of unstable retained austenite on prior austenitic grain boundaries, and decreased cleavage fracture stress owing to the high Mn content.

Effect of the Intercritical Annealing Temperature on the Mechanical Properties of 10 Pct Mn Multi-phase Steel

Intercritically annealed 10 pct Mn steel has been shown to exhibit an excellent combination of strength and ductility due to the plasticity-enhancing mechanisms of mechanical twinning and strain-induced martensite transformation occurring in sequence. This mechanical behavior is only achieved for a multi-phase microstructure obtained after annealing within a specific intercritical temperature range. A model for the selection of the optimal intercritical annealing temperature was developed to achieve a high strength-ductility balance for 10 pct Mn multi-phase steel. The model considers the room temperature stacking fault energy and the thermodynamic stability of the retained austenite.

Tensile Behavior of Intercritically Annealed 10 pct Mn Multi-phase Steel

The exceptional elongation obtained during tensile testing of intercritically annealed 10 pct Mn steel, with a two phase ferrite–austenite microstructure at room temperature, was investigated. The austenite phase exhibited deformation-twinning and strain-induced transformation to martensite. These two plasticity-enhancing mechanisms occurred in succession, resulting in a high rate of work hardening and a total elongation of 65 pct for a tensile strength of 1443 MPa. A constitutive model for the tensile behavior of the 10 pct Mn steel was developed using the Kocks–Mecking hardening model.

Transmission Electron Microscopy Analysis of Yielding in Ultrafine-Grained Medium Mn Transformation-Induced Plasticity Steel

The microstructural changes occurring during yielding of ultrafine-grained medium Mn transformation-induced plasticity steel intercritically annealed at 873 K and 923 K (600 °C and 650 °C) were analyzed by transmission electron microscopy. The tensile properties and the work hardening behavior of the 7 wt pct Mn steel were found to have a pronounced dependence on the intercritical annealing temperature. A low intercritical annealing temperature of 873 K (600 °C) resulted in a martensite-free ultrafine-grained two-phase microstructure consisting of ferrite and austenite. After annealing at 923 K (650 °C), the ultrafine microstructure contained ferrite, low stability retained austenite, and athermal martensite. Yielding of the steel annealed at 873 K (600 °C) was associated with the initiation and propagation of Lüders deformation bands, whereas yielding of the steel annealed at 923 K (650 °C) was initiated at a lower tensile stress by stress-induced austenite transformation. The progression of the martensite transformation during yielding strongly suggests that the yielding and strain hardening of ultrafine-grained medium Mn transformation-induced steel are controlled by the stability of the ultrafine-grained austenite phase. Suppression of plastic flow localization is achieved when the UFG steel has an initially high rate of strain hardening resulting from transformation-initiated yielding.

Fatigue Behavior of Four High-Mn Twinning Induced Plasticity Effect Steels

Fatigue behavior of four high-Mn (18 to 22 wt pct Mn) steels, after cold rolling and soft annealing, were investigated in reversed plane bending in a high-cycle regime. The surfaces of fatigued specimens were examined using an optical microscope, a scanning electron microscope (SEM), and an atom force microscope (AFM). It was discovered that the chemical composition of the steel had only a minor influence on fatigue behavior, and the fatigue limit (FL), i.e., the stress amplitude leading to more than 2·106 cycles to failure, of all steels is about 400 MPa. This is about 42 to 48 pct of their tensile strength whose value is comparable to that of austenitic stainless steel and 780 MPa transformation induced plasticity (TRIP) steel. Mechanical twinning does not occur in the course of cycling; however, intense slip bands are formed. Fatigue cracks tend to nucleate at quite an early stage of fatigue life and most commonly on grain and annealing twin boundaries due to intersections of slip bands and boundaries.

Analysis of the γ → α transformation in a C-Mn steel by phase-field modeling

This article deals with the austenite (γ) decomposition to ferrite (α) during cooling of a 0.10 wt pct C-0.49 wt pct Mn steel. A phase-field model is used to simulate this transformation. The model provides qualitative information on the microstructure that develops on cooling and quantitative data on both the ferrite fraction formed and the carbon concentration profile in the remaining austenite. The initial austenitic microstructure and the ferrite nucleation data, derived by metallographic examination and dilatometry, are set as input data of the model. The interface mobility is used as a fitting parameter to optimize the agreement between the simulated and experimental ferrite-fraction curve derived by dilatometry. A good agreement between the simulated α-γ microstructure and the actual α-pearlite microstructure observed after cooling is obtained. The derived carbon distribution in austenite during transformation provides comprehension of the nature of the transformation with respect to the interface-controlled or diffusion-controlled mode. It is found that, at the initial stage, the transformation is predominantly interface-controlled, but, gradually, a shift toward diffusion control takes place to a degree that depends on cooling rate.

A novel observation of strain-induced ferrite-to-austenite retransformation after intercritical deformation of C-Mn steel

It was found that a ferrite-to-austenite retransformation can take place during isothermal holding after deformation in 0.19 pct C-1.46 pct Mn steel at higher intercritical temperatures, but it was not observed at lower intercritical temperatures. This new finding was revealed by the combined techniques of optical observation and quantitative metallographic measurements. Also, two unexpected experimental observations on the quenched microstructure can be interpreted very well by the retransformation: one is that microhardness does not decrease at the end of relaxation as expected; the other is that an experimentally observed X-ray peak gets broader after long relaxation. In contrast, dilatometry measurements on the undeformed specimens show that the thermodynamically induced transformation during the isothermal holding should be from austenite to ferrite. Therefore, the ferrite-to-austenite retransformation after deformation should be strain induced. Such a retransformation can be explained quite well using a validated and quantitative model for the intercritical rolling and softening of both austenite and ferrite. The model presented is based on the fact that in intercritically deformed steel there is a strain distribution over the two phases with the more strain in the softer ferrite. The ferrite dynamically recovers, which leads to a shift in the relative amount of stored energy in the two phases. This phenomenon leads to a shift in the equilibrium fractions of ferrite and austenite.

Effects of laser-shock processing on the microstructure and surface mechanical properties of hadfield manganese steel

The effects of laser-shock processing (LSP) on the microstructure, hardness, and residual stress of Hadfield manganese (1 pct C and 14 pct Mn) steels were studied. Laser-shock processing was performed using a Nd: glass phosphate laser with 600 ps pulse width and up to 120 J/pulse energy at power density above 1012 W/cm2. The effects of cold rolling and shot peening were also studied for comparison. Laser-shock processing caused extensive formation of e hexagonal close-packed (hep) martensite (35 vol pct), producing up to a 130 pct increase of surface hardness. The surface hardness increase was 40 to 60 pct for the shot-peened specimen and about 60 pct for the cold-rolled specimen. The LSP strengthening effect on Hadfield steel was attributed to the combined effects of the partial dislocation/stacking fault arrays and the grain refinement due to the presence of the e-hcp martensite. For the cold-rolled and shot-peened specimens, the strengthening was a result of e-hcp martensite and twins with dislocation effects, respectively. Shot peening resulted in a relatively higher compressive residual stress throughout the specimen than LSP.

Simulation of the hot rolling and accelerated cooling of a C-Mn ferrite-bainite strip steel

By means of torsion testing, the microstructures and mechanical properties produced in a 0.14 Pct C-1.18 Pct Mn steel were investigated over a wide range of hot-rolling conditions, cooling rates, and simulated coiling temperatures. The austenite grain size present before accelerated cooling was varied from 10 to 150 μm by applying strains of 0 to 0.8 at temperatures of 850 °C to 1050 °C. Two cooling rates, 55 °C/s and 90 °C/s, were used. Cooling was interrupted at temperatures ranging from 550 °C to 300 °C. Optical microscopy and transmission electron microscopy (TEM) were employed to investigate the microstructures. The mechanical properties were studied by means of tensile testing. When a fine austenite grain size was present before cooling and a high cooling rate (90 °C/s) was used, the microstructure was composed of ferrite plus bainite and a mixture of ferrite and cementite, which may have formed by an interphase mechanism. The use of a lower cooling rate (55 °C/s) led to the presence of ferrite and fine pearlite. In both cases, the cooling interruption temperature and the amount of prior strain had little influence on the mechanical properties. Reheating at 1050 °C, which led to the presence of very coarse austenite, resulted in a stronger influence of the interruption temperature. A method developed at Institut de Recherche Siderurgique (IRSID, St. Germain-en-Laye, France) for deducing the Continuous-Cooling-Transformation (CCT) diagrams from the cooling data was adapted to the present apparatus and used successfully to interpret the observed influence of the process parameters.

Effects of Widmanstätten ferrite on the mechanical properties of a 0.2 pct C-0.7 pct Mn steel

Laboratory melted and rolled C-Mn steel plates were austenitized at either 925 °C or 1150 °C to produce nominal austenite grain sizes of 60 and 200 µm, resspectively. The plates were then cooled at rates in the range of about 2 °C/min to 400 °C/min to produce mixed polygonal ferrite/Widmanstatten ferrite/pearlite microstructures. The percentage of WidmanstAtten structure (a Widmanstatten ferrite/pearlite aggregate) increases with increasing prior austenite grain size and cooling rate. Both yield strength and impact toughness increase with decreasing austenite grain size and increasing cooling rate. This simultaneous improvement in strength and toughness is attributed to overall refinement of both the polygonal ferrite and WidmanstAtten structure. Both yield and tensile strength increase with an increase in the volume fraction of WidmanstAtten ferrite and a reduction in ferrite grain size. In contrast, the toughness level achieved in these polygonal ferrite/WidmanstAtten ferrite/pearlite microstructures depends largely on the ferrite grain size; the finer the grain size, the better the toughness.

The relationship between toughness and microstructure in Fe-high Mn binary alloys

The influence of Mn content on the ductile-brittle transition in 16 to 36 wt pct Mn steels was investigated and interpreted in light of the evolving microstructure. It was found that when hcp e martensite is present in the as-quenched condition or forms during deformation, it lowers the toughness. In 25Mn steel, the stress concentrations at e plate intersections result in the formation of planar void sheets along the {111}γ planes. The deformation-induced α’ martensite in 16 to 20 pct Mn alloys enhances the toughness, but leads to a ductile-to-brittle transition at low temperatures that is due to the intrusion of an intergranular fracture mode. Binary alloys with greater than 31 pct Mn also fracture in an intergranular mode at 77 K although the impact energy remains quite high. Auger spectroscopy of the fracture surfaces shows no evidence of significant impurity segregation, which suggests the importance of slip heterogeneity in controlling intergranular fracture in these alloys.

Formation of austenite in 1.5 pct Mn steels

The formation of austenite from different microstructural conditions has been studied in a series of 1.5 pct Mn steels that had been heated in and above the intercritical (α+ γ) region of the phase diagram. The influence of variables such as cementite morphology, initial structural state of the ferrite and the carbon content has been assessed in terms of their respective effects on the kinetics of austenite formation and final microstructure. Austenite was found to form preferentially on ferrite-ferrite grain boundaries for all initial structures. The results of this study have shown that the 1.5 pct Mn has lowered both the AC3 and AC1, lines causing large amounts of austenite to form in low carbon steel. The kinetics of austenite formation at 725 °C were not only very slow but also were approximately independent of the amount formed. Austenite appeared to form slightly more rapidly from cold rolled ferrite than from recrystallized ferrite or ferrite-pearlite structures.

Effect of phosphorus on the mechanical properties of normalized 0.1 pct C, 1.0 pct Mn steels

The effect of phosphorus, in amounts up to 0.2 pct, on the stress-strain characteristics and strain-rate sensitivity of a series of laboratory-produced 0.1C–1.0Mn steels has been determined on normalized 10 mm-thick (0.38 in.) plate at temperatures from −196 to 400°C (−321 to 752°F). It was found that 0.05 pct phosphorus increased the yield strength of the 0.1C–1.0Mn steel about 24 MPa (3.5 ksi) at 22°C (72°F) and above without significantly affecting the Charpy V-notch shelf energy or fracture-appearance transition temperature. Additions of 0.1 and 0.2 pct phosphorus caused a greater increase in the yield strength (48 and 93 MPa or 7 and 13.5 ksi) but also increased the transition temperature. The strengthening effect of phosphorus in these steels is much the same as that found previously for phosphorus in iron, and analysis of the strain-rate-sensitivity data shows that the same deformation models are applicable. Strong elastic interactions between phosphorus atoms and dislocations are believed to be responsible for the observed deformation behavior. Comparison of the present results with those obtained previously on Fe−P alloys and with data in the literature indicates that the strengthening resulting from phosphorus additions to steel is expected to be additive to other strengthening mechanisms.