Experimental Steel Research Articles

Effect of Toughness and Ductility on the Cavitation Erosion of Martensitic Stainless Steel

Martensitic stainless steel containing 13% Cr–4% Ni suffers cavitation erosion (CE) as the common material of hydro turbine impellers. Two 13% Cr–4% Ni stainless steel samples were obtained by different melting and heating processes. One was of relatively low toughness but high ductility (LTHD), and the other was of relatively high toughness but low ductility (HTLD). This paper is to clarify the relationship between the mechanical properties and the CE resistance of the experimental steel samples. The CE of the two materials was studied using an ultrasonic vibration cavitation erosion rig. Mass loss, morphological observation, nanoindentation characterization, and tensile tests were employed to clarify the erosion mechanism. The results showed that LTHD stainless steel had slightly higher ductility, but lower toughness than HTLD material. The mass loss method verified that the CE resistance of LTHD material was higher than that for the HTLD material. In addition, both materials had an incubation stage of 2 h in the distilled water. The SEM revealed that material removal was preferentially initiated from the grain boundaries and slip zone after the incubation period. The ductility could delay the fracture of the material, which contributed more to cavitation erosion resistance than the toughness of the materials. The hardness test showed few relationships with the CE resistance.

Dry sliding wear behaviour of hot-rolled air-cooled medium manganese martensitic steel

The microstructure, mechanical properties and dry sliding wear behaviour of hot-rolled air-cooled medium manganese martensitic steel at different finish rolling temperatures (FRT) were investigated. The fully martensitic structure with precipitation of acicular carbides in the matrix was obtained in the air-cooled process after hot rolling. With the decrease of FRT, the mechanical properties and sliding wear resistance of experimental steels were significantly improved. The worn surface morphology was mainly furrows, and the wear mechanism was micro-cutting. Both the width and depth of the furrows increased with the increasing hardness of the experimental steels. The mass loss rates of FRT-700, FRT-900 and Hardox450 samples were 4.53 × 10-4 g/m, 4.82 × 10-4 g/m and 5.14 × 10-4 g/m, respectively.

DIE STEEL 4Х4Н5М4Ф2 (WITHOUT FORGING TREATMENT) FOR HOT DEFORMATION OF ALUMINUM ALLOY

Purpose. Production of steel with controlled austenitic transformation during operation of the 4Х4Н5М4Ф2 grade (without forging treatment) using the technology of electroslag remelting and establishing optimal modes of heat treatment (annealing, quenching and tempering). Production of tools (matrices) from 4Х4Н5М4Ф2 steel (without forging treatment) for hot deformation of aluminum alloy at operating temperatures below the critical point A1.
 Research methods. Metallographic analysis of experimental die steels 4Х4Н5М4Ф2 (without forging treatment) and forged 4Х5МФ1С: study of metal structure; determination of specific resistance; determination of tensile strength; determination of hardness and impact strength.
 Results. The results of research on the optimization of heat treatment modes (quenching and tempering) of 4Х4Н5М4Ф2 steel (without forging treatment) are given. Adjusted chemical composition (4Х4Н5М4Ф2) and optimized mode of heat treatment of steel, which allowed to increase heat resistance up to a temperature of 650 °С. The tempering temperature of steel (475±25 °С) at which irreversible temper brittleness occurs is established. Experimental and industrial tests of steel 4Х4Н5М4Ф2 (without forging treatment) for hot deformation of AK7ч aluminum alloy are presented. It is shown that compared to forged steel H13 (an analogue of 4Х5МФ1С), which is used at one of the Chinese enterprises, the experimental steel has practically the same resistance .
 Scientific novelty. In the process of partial recrystallization (incomplete annealing at a temperature of 750±20 °C) of 4Х4Н5М4Ф2 steel (without forging treatment), a spheroidized carbide component is formed, which leads to an improvement in the mechanical processing of the workpiece for the manufacture of matrices.
 Practical value. The possibility of using steel with an adjustable austenitic transformation during operation of the 4Х4Н5М4Ф2 grade (without forging treatment) is shown for a wide range of operating temperatures of hot deformation: copper-nickel (at an operating temperature above the critical point A3) and aluminum alloy (at an operating temperature below the critical point A1) with increased service life compared to forged steel 4Х5МФ1С.

Effect of Trace Rare-Earth Element Ce on the Microstructure and Properties of Cold-Rolled Medium Manganese Steel

Rare-earth elements have been widely used in the field of functional materials, but their effects on the cold-stamping formability of high-strength automotive steels have rarely been studied. In this paper, the effect of the trace rare-earth element Ce on the microstructure and properties of cold-rolled medium manganese steel after ART (austenite-reverted transformation, ART) annealing was studied. The microstructure of the experimental steel was observed using SEM, and the mechanical properties were tested using a universal tensile testing machine. The volume fraction of the retained austenite and the texture of the steel were measured using XRD. The results showed that the original austenite grain size of the experimental steel was smaller after adding the trace rare-earth element Ce. After ART annealing, the grain size distribution of the experimental steel with rare-earth Ce was more uniform, and the comprehensive mechanical properties were better. Under the conditions of quenching at 800 °C for 5 min and annealing at 645 °C for 15 min, the maximum product of tensile strength and elongation was 28.47 GPa·%.

Study of Transition Areas in Press-Hardened Steels in a Combined Tool for Hot and Cold Forming.

Press-hardening, also known as hot stamping, is a manufacturing process for producing car body parts that must meet the high demands of their mechanical properties and safety parameters. Moreover, these components often require different mechanical properties in different parts of the component. This work presents the press-hardening process in a special combined tool where one half of the tool is heated and the other half is cooled. The cooled part has been 3D printed due to the complexity of the internal cooling channels. The aim of this work is to investigate the variation of the microstructures in the sheet metal and the mechanical properties in relation to the cooling process in the tool and to determine the transition area where these properties cross over. Two steels were chosen for the experiment. The most commonly used steel 22MnB5, and an experimental high-strength steel with 0.2% C alloyed with manganese and aluminium. A temperature of 425 °C was set in the heated part of the tool, and different holding times in the tool were tested. In the heated part of the tool, a bainitic structure with a fraction of ferrite and retained austenite was formed, while in the quenched part of the tool, a martensitic transformation was promoted due to rapid cooling. In addition to microscopic analyses, mechanical tests and hardness measurements were also performed.

Development of the chemical composition and modes of heat treatment of steel for railway wheels with increased wear resistance at high axle loads

One of the directions of the development of railway transport is increasing the load on the axle, while it is necessary to create new, more wear-resistant materials to increase the durability of railway wheels. At the same time, in many cases, a high level of hardness and strength must be combined with no less high values of plasticity and viscosity indicators. Purpose: to investigate the effect of the developed chemical composition and processing modes on the mechanical properties of laboratory-produced steels for railway wheels with increased wear resistance, which work in light braking conditions with high axleloads. Material and methodology. Research was carried out on steels of the experimental composition of the laboratory manufacturing method. Metallographic studies of experimental and industrial steels were performed on a light microscope "Neophot 32" and "Axiovert 200 M MAT" of the company "Carl Zeiss". Grinds were made mechanically. The microstructure was revealed by etching in a 2—3 % alcoholic solution of nitric acid (HNO3). Mechanical tensile tests at room temperature were carried out according to standard methods. Results. Based on the results of analytical studies and calculation models that allow predicting the mechanical properties of steels depending on the content of chemical elements in the steel, the chemical composition of small-volume experimental ingots for smelting in laboratory conditions was developed. Smelting of experimental melts has been completed. The possibility of obtaining a full set of mechanical properties and microstructure, standardized by the requirements of AAR M-107/M-208 for railway wheels of class D from developed steel, has been established.

Contact Fatigue Resistance of Gun Barrel Steels

Structural steels can be used to produce barrels, due to their good properties. During the working cycle, steels are exposed to aggressive conditions such as thermal stress, fatigue, and wear. In this paper, two steels were selected, namely 30CrNiMo8 and 42CrMo4, which were abraded with a G40 ball bearing. All measurements were performed on a UMT TriboLab universal tribometer. The main goal of the experiment was to analyze and compare the wear of tool steels in contact with a bearing ball with a diameter of 4.76 mm. Experimental materials will be processed for three types of roughness. Both types of experimental steels were measured after the final heat treatment (hardening + tempering). The main goal of the paper was to determine how different technological surface treatment of the material will affect the abrasion resistance, and thus the life of these main materials at the point of contact with the projectile. The experiment shows that 30CrNiMo8 steel has at least the same coefficient of friction (COF) with respect to all measurement parameters. In contrast, the second investigated steel, 42CrMo4, showed a decreasing COF due to the increasing roughness of the measurement. The friction mechanism was subjected to adhesive wear on both steels. The wear of both steels at roughnesses Sa12.5 and Sa 6.3 showed the same values. With a finer roughness of Sa 3.6, 42CrMo4 steel showed 50% lower wear values.

High-stress abrasive wear characteristics of ultra-high strength press-hardening steel

Ultra-high strength steels are widely utilized in many applications operating in harsh abrasive wear conditions. For instance, the machineries used in mining and mineral handling or in agricultural sector require robust, but cost-effective wear-resistant materials. Steels provide excellent combination of mechanical properties and usability. This study encompasses mechanical and wear testing of an experimental medium-carbon press-hardening steel. The as-received material was austenitized at two different temperatures and quenched in water. Additionally, low-temperature tempering was applied for one variant. In total, three variants of the press-hardening steel were produced. Microstructural characterization and mechanical testing were conducted for the steel samples. The wear testing was carried out with high-stress abrasive method, in which the samples were rotated inside a crushed granite bed. A commercial 400 HB grade wear-resistant steel was included in the wear testing as a reference. The experimental steel showed very high mechanical properties reaching tensile strength up to 2600 MPa with hardness of 750 HV10. Wear testing resulted in only minimal differences between the three variants indicating that the improved impact toughness by tempering did not significantly affect the wear resistance. The reference steel had nearly two times greater mass loss compared to the higher hardness press-hardening steels. Microhardness measurements on the worn surface showed drastic increase in hardness for the deformed structure for all samples. It was concluded that even the high-hardness martensitic steels exhibit notable wear surface work-hardening. Therefore, hardness was determined to be the most significant factor affecting the wear performance of studied steels.

UNCONVENTIONAL METHODS OF MANUFACTURING THIN WIRES FOR APPLICATION AS INPUT MATERIAL IN ADDITIVE MANUFACTURING. PART 1: PREPARATION OF INPUT MATERIAL FOR WIRE DRAWING

Part 1 of the article presents the technological path of producing semi-finished products for wires constituting input material in additive technologies. On the basis of the developed chemical compositions of experimental Fe-based alloys, laboratory ingots with a square section of 120×120 mm / 130×130 mm were produced, which were then hot rolled into flat bars. In order to select the physical parameters of the drawing tests, numerical modelling of the process was performed. As a result of the simulations, it was found that the calculated drawing force exceeds the capabilities of the experimental drawing machine and for this reason, hydrostatic extrusion was used to produce bars intended for drawing wires. The hydrostatic extrusion method was used to make bars with a diameter of 5 mm from three tested materials, while three experimental steels showed insufficient susceptibility to extrusion at high pressure and cracked at various strain values. An analysis of possible causes of bar breakage during extrusion was carried out on the basis of the results of microstructure examination.

Effect of rolling temperature on microstructure and mechanical properties of medium manganese steel

The effect of rolling temperature on the incomplete recrystallization behavior and the relationship to mechanical properties in 0.06C–4Mn-1.2Cr (wt.%) steel was investigated. Three plates of experimental steel were produced by hot rolling at different temperatures and intercritical annealing processes in this study. The microstructure of experimental steels was examined by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and transmission electron microscope (TEM). The mechanical properties of test steel were examined by impact test and tensile test. Austenite reversion and cementite precipitation-dissolution was numerically simulated under the assumption of local equilibrium. With the decrease of hot rolling temperature from 1000 °C to 800 °C, the uniform equiaxed prior austenite grains microstructures changed to heterogeneous microstructures of laminated and ultrafine prior austenite grains arranged alternatively. The tensile and impact tests revealed that the medium manganese steels rolling at 800 °C had an excellent combination of mechanical properties. Elongation had a great relationship with retained austenite fraction and stability. The fractography of impact specimens was examined to explore the toughening mechanism of the micro-laminated steels. The steel with uniform equiaxed grain structure exhibited a relatively low tensile strength of 750 MPa and low elongation of ∼20% and an extremely poor low-temperature toughness of ∼40 J at −40 °C. An enhanced low-temperature toughness (∼233 J at −40 °C), as well as an improved tensile strength (823 MPa) and higher elongation of ∼40%, was obtained in the steel with heterogeneous microstructures. The content of austenite in heterogeneous microstructure is twice that in uniform equiaxed grain structure. And cementite dissolved more thoroughly in the heterogeneous microstructure. The high austenite content of heterogeneous microstructures enabled the steel to be significantly stronger and ductile, which contributed to the high tensile strength and elongation to some extent. It is concluded that the effect of interface decohesion resulted in the occurrence of delamination. And the high austenite content and delamination greatly improved the low-temperature toughness of experimental steel.

Effects of Rare Earth Elements on Microstructure and Corrosion Resistance of Low Alloy Steel Based on Ultra-Thin Cast Strip Process

This laboratory study aimed to reveal the inner connects between the microstructure and corrosion properties of a RE microalloyed ultra-thin cast strip (UCS) steel. The microstructure mainly consisted of homogeneous polygonal ferrite (PF) with a small amount of pearlite (P), while adding multiple alloying elements led to the appearance of granular bainite (GB) and bainitic ferrite (BF). RE elements strongly promoted the homogenization and refinement of microstructure by segregating towards the solid–liquid interface. Potentiodynamic polarization, EIS, and weight loss curves under wet–dry immersion test confirmed that the corrosion behavior was significantly improved by RE, while the addition of RE had no obvious change on tensile strength. The corrosion resistance of the homogeneous single-phase microstructure was proved to be better than that of multiphase microstructure. Hence, RE had a remarkable influence on improving corrosion resistance when the experimental steels processed single-phase microstructures.

Characteristic Flow Behavior and Processing Map of a Novel Lean Si Spring Steel for Automotive Stabilizer Bars

The spring steel for automotive stabilizer bars has a great responsibility in that its quality directly affects the stability, safety, and comfort of vehicle operation. The isothermal thermal compression behavior of a novel lean Si spring steel that was used to manufacture an anti-roll bar was investigated with a DIL805A/D quenching thermal dilatometer in this research. A hyperbolic sine type of constitutive model was established, and hot processing maps were produced to evaluate the experimental steel’s hot workability properties. The experimental results suggest that dynamic recrystallization (DRX) preferentially occurs at a low strain rate and high thermal processing temperature, while the processing maps of the experimental steel are susceptible to strain. The instability regions increase as the strain increases. The processing maps’ stable and instable domains should be decided upon comprehensive analysis of the instability criterion, power dissipation efficiency, and strain rate sensitivity index. The optimum parameters of hot processing for the experimental steel at various strains are that the deformation temperature of 1000–1150 °C and the strain rate of 0.1, approximately.

Comparative experiment of steel bar corrosion at concrete construction joints

Introduction: Construction joint is common and even inevitable in most of the reinforcement concrete structures. This study was to assess the effect of construction joints on chloride-induced corrosion of reinforcing steel in concrete.Methods: Test parameters included two environmental conditions (salt solution immersion condition and cyclic wet-dry condition), two forms of construction joint (direct wet joint and roughened wet joint) and four types of steel bar (mild steel bar, ferritic stainless-steel bar, austenitic-ferritic stainless-steel bar and epoxy-coated steel bar). The corrosion test of 90 specimens was carried out by electrochemical accelerated corrosion method. The weight loss of each steel bar and steel bar section in specimens was measured. An influence coefficient (k_j) of construction joint on local weigh loss of steel bars was defined.Results: Except for epoxy-coated steel bars, the most severe corrosion of experimental steel bars in concrete specimens all occurred at the joints, while the corrosion in non-joint sections of steel bars was relatively uniform and less. The weight loss rate of specimens has the range of 1.18% to 15.73% with an average value of 6.22%. The average k_j of mild steel bars, S11203 stainless steel bars, and S23043 stainless steel bars are 1.38, 1.92, and 1.97, respectively. The average k_j of specimens in immersion condition and cyclic wet-dry condition are 1.44 and 2.07. The corrosion of epoxy-coated steel bars mainly occurred at the damage locations of epoxy coating, not mainly at the joints.Conclusion: Chloride-induced corrosion of steel bars at construction joints was always more severe than at non-joints, especially in cyclic wet-dry environments, even for stainless-steel bar, but epoxy-coated steel bars were excluded.

Effect of Complex Strengthening on the Continuous Cooling Transformation Behavior of High-Strength Rebar.

The effects of niobium and composite strengthening on the phase transformation characteristics and precipitation behavior of continuous cooling transformation of high-strength rebar during thermal deformation and subsequent cooling were investigated. The results show that when the cooling rate was within 0.3-5 °C/s, ferrite transformation and pearlite transformation occurred in the experimental steels. The Nb content increased to 0.062 wt.%, and the starting temperature of the ferrite transformation decreased. Meanwhile, the ferrite phase transformation zone gradually expanded, and the pearlite phase transformation zone gradually narrowed with the increase in the cooling rate. When the cooling rate was 1 °C/s, bainite transformation began to occur, and the amount of transformation increased with the increase in the cooling rate. It was found that the main precipitates in the experimental steels were (Nb, Ti, V)C, with an average particle size of about 10-50 nm. When the Nb content was increased to 0.062 wt.% and the cooling rate was increased to 5 °C/s, the ferrite grain size was reduced from 19.5 to 7.5 μm, and the particle size of the precipitate (Nb, Ti, V)C could be effectively reduced. The strength of the steel was significantly improved, but the elongation of the steel was reduced. However, the comprehensive mechanical properties of 0.062 wt.% Nb experimental steel was significantly improved at a cooling rate of 5 °C/s.

Strain rate response of mechanical behaviors for Fe-8Mn-6Al-0.4C duplex low-density steels with different austenite stabilities

Duplex low-density steels exhibit excellent mechanical properties under quasi-static tensile tests with the help of the transformation-induced plastic (TRIP) effect produced by the deformation-induced martensitic transformation (DIMT), which is sensitive to the strain rate. Therefore, adjusting the microstructure to accommodate the deformation at different strain rates is crucial for the optimal design of high-performance duplex low-density steels. In this study, Fe-8Mn-6Al-0.4C low-density steel was subjected to intercritical annealing (IA) treatment to obtain duplex microstructures with an inhomogeneous distribution of austenite grain sizes and tensile tests were performed at two strain rates. The response of the mechanical behavior to the strain rate was investigated, and the mechanical stability of austenite was evaluated using a method based on the grain size. The results show that the diverse mechanical behaviors of experimental steel depend mainly on the mechanical stability of austenite rather than the stacking fault energy (SFE). The mechanical stability of austenite increased with decreasing annealing temperature and was further enhanced by adiabatic heating at a higher strain rate. Accordingly, the DIMT mode changed from a stress-induced transformation to a strain-induced transformation until the DIMT was inhibited by adiabatic heating. The improvement in the mechanical properties of the experimental steel is attributed to the sustainable DIMT with a reasonable mechanical stability of austenite.

INVESTIGATION OF THE AUSTENITE DECAY KINETICS DURING CONTINUOUS COOLING OF STEEL WITH 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Са FOR RAILWAY RAILS OF THE NEW GENERATION

Purpose. Investigation of the supercooled austenite decay kinetics during continuous cooling of steel with 0.84 % C, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca for railway rails of a new generation with increased operational properties to develop parameters of differential cooling. Method. The investigation of the austenite decay kinetics was carried out using differential thermal analysis. Samples were cooled at different cooling rates, temperature control was performed using chromel-aluminum thermocouples installed in the center of the sample. Microstructural investigations are performed using optical (AxioVert 200M mat) and electronic (SEM-106) microscopes. The interplate distance in pearlite is determined by the linear method, the diagonals are located perpendicular to the plate packages. Vickers hardness at a load of 10 kg. The number of structural components is recognized on microstructural images using the imageJ software complex. Results. The TKD analysis showed that at cooling rates of 0,06...5,96 °C/s, the structure of the investigated steel consists of pearlite. As the cooling rate increases, the morphology changes and the dispersion of pearlite increases: from medium pearlite to sorbitol-like. It should be noted that in the cooling rate range of 0,06...0,07°С/s, tertiary cementite is formed from ferrite. The diagram shows that at a cooling rate of 5.96 °С/s the hardness is 468 НВ (432 НВ), and the structure does not contain bainite. At a cooling rate of 1.47 °С/s, the hardness is 356 НV (345 НВ). This is slightly below the minimum allowable value (370 HV), but by interpolation it can be determined that the cooling rate must be at least 2.5 °C/s to achieve a hardness of at least 393 HV (370 HV). That is, when the rolling surface of the rail is cooled at a rate of 5.96 °C/s, the central volumes of the rail head at a rate of at least 2.5 °C/s, high-strength rails of the following categories can be manufactured: higher (DSTU (National Standard of Ukraine) 4344:2004) and R400HT (EN 13674-1:2011) from steel with 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca. Scientific novelty. Constructed thermokinetic diagram of steel austenite decay with 0.84 % С, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Са. Practical value. It was established that with the use of experimental steel with 0.84 % C, 0.44 % Si, 0.95 % Mn, 0.01 % B, 0.0006 % Ca it is possible to achieve a hardness above 400NV without the formation of needle-like structures, therefore this is recommended chemical composition for the manufacture of high-strength rails of the R400NT category according to EN 13674-1-2011.

Experimental Study on Self-Compacting Concrete-Filled Thin-Walled Steel Tube Columns

Concrete-filled steel tubes present excellent structural and constructional performances because they ideally combine the advantage of concrete and steel tube. Thus, they are widely used in civil infrastructures. However, they inevitably suffer from either hard compacting or high costs. Thus, convenient and rapid construction for compacting concrete and cost saving are the urgent challenges for concrete-filled steel tubes. Therefore, this study investigates filling a thin-walled steel tube with self-compacting concrete to solve the challenges presented by traditional concrete-filled steel tube columns, such as poor compacting performance and high costs. This experimental study tests self-compacting concrete-filled thin-walled steel tube (SCCFTST) columns under concentric compression from loading to failure. Effects of wall thickness of the thin-walled steel tube on the failure modes, load-deformation behaviors, and the ultimate loads of the SCCFTST columns are comprehensively investigated. The ultimate loads between experiments and their calculated values in terms of design codes are also compared. The results suggest that buckling on thin-walled steel tube surface is the typical failure mode. The amount of local buckling increases with decreasing wall thickness, and the decreasing rate of the load-deformation curves in the descending branch decreases by increasing the wall thickness, as well as the ultimate load increasing with increases wall thickness. The ACI and CECS are the most conservative and accurate design codes, respectively, for predicting the ultimate load. Therefore, the SCCFTST columns can be used as structural components in civil infrastructures and their peak loads can be calculated using design codes for conventional concrete-filled steel tube columns. However, modification measures must be taken while predicting the ultimate loads of the SCCFTST columns by design codes. The experimental results of this paper can contribute towards the application of SCCFTST columns in practice.

Prediction model for crack sensitive temperature region and phase fractions of slab under continuous casting cooling rates based on finite number of experiments

Accurate prediction of the crack sensitive temperature region and phase fractions variation of slabs during continuous cooling is an important guide to avoid cracks and effectively control the quality. Based on finite number of measurements, at different cooling rates of the continuous casting process, a prediction model for characteristic temperatures of austenite decomposition, the variation of phase fractions with temperature, the crack sensitive temperature regions, and the final microstructural compositions of casting slabs at different cooling rates has been established and evaluated the accuracy. The results show that austenite decomposition temperature range moves toward the low temperature region as cooling rate increases, and the independent peak of ferrite transition become weaker. The characteristic temperatures of austenite decomposition can be quantitatively calculated by TC(CR) = A−exp(B + C/CR) at different cooling rates, which the maximum relative error for experimental steels is −2.2%. The ferrite and pearlite phase fractions increases with decreasing temperature during continuous casting cooling, which means that the ability of the billet to resist deformation and external force changes. Meanwhile, the final ferrite content of slabs for Steel B and Steel C at different cooling rates are 83.24620−exp(2.59364–13.72283/CR) and 85.07143−exp(1.71320–15.82244/CR), respectively. The crack sensitive temperature region Ae3 ∼ Tα40%(CR) calculated by the prediction model is in good agreement with the low ductility zone measured by experiment. Moreover, the critical temperatures Tα40%(CR) of the crack sensitive temperature regions are 890.35731−exp(2.99719–20.67781/CR), 745.87462−exp(4.83056–44.18511/CR) and 729.46168−exp(2.96621–12.21949/CR) for three experimental steels under different cooling rates.

Research on the design of new high-strength and high-toughness steel and its manufacturing parameters

The lightweight of the steel structure can be achieved by the high strength and high toughness of steel. In this paper, the composition design and optimal manufacturing process of the experimental steel were investigated. The fine and uniform microstructure of tempered sorbite can be obtained under the conditions of TMCP (Thermo Mechanical Control Process), 920 °C quenching, and 650 °C tempering for the steel by micro-alloying with Nb, V, and Ti. The steel has properties as follows: yield strength of 1015 MPa, tensile strength of 1076 MPa, elongation of 14.5%, and -40 °C AKV 81 J. The parameters of quenching and tempering heat treatment have a decisive influence on the size and distribution of ferrite grains and precipitates of tempered sorbite.

Microstructure Evolution and Mechanical Properties of Ferrite-Austenite Duplex Fe-Mn-Al-(Cu)-C Steel under Different Annealing Temperatures.

The effect of Cu addition and the intercritical annealing (IA) temperature on the microstructural evolution and mechanical properties of Fe-0.4C-7Mn-4Al (wt%) was investigated via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and nanoindentation tests. The results showed that the volume fraction and the average grain size of austenite, and the fraction of high angle grain boundaries, increased with IA temperature increase in the range of 650 °C to 710 °C. The addition of Cu facilitates the formation of Cu-rich nanoparticles, raises the volume fraction of austenite, and delays the recrystallization of austenite. As IA temperature increased, the yield strength (YS), ultimate tensile strength (UTS), and Lüders bands strain (LBS) decreased in both experimental steels. The Cu addition not only increases the YS of medium Mn steel but also benefits the decrease of LBS. The best comprehensive mechanical properties were obtained at the IA temperature of 690 °C in the studied steel, with Cu addition. According to nanoindentation experiments, the Cu addition raises the hardness of ferrite and austenite from 4.7 GPa to 6.3 GPa and 7.4 GPa to 8.5 GPa, respectively, contributing to the increase of YS of medium-Mn steel.