High-strength Low-carbon Steel Research Articles

Crystallographic Features of Phase Transformations in High-Strength Low-Carbon Pipe Steel

Crystallographic Features of Phase Transformations in High-Strength Low-Carbon Pipe Steel

The influence of carbon content gradient and carbide precipitation on the microstructure evolution during carburizing-quenching-tempering of 20MnCr5 bevel gear

Due to the mechanism of microstructural transition, high-strength low-carbon alloy steel gears exhibit high strength and surface hardness after Carburizing-Quenching-Tempering (CQT) combined heat treatment, while maintaining good toughness in the core. The carbon potential gradient and the precipitation of carbides are the main factors affecting the final microstructure, yet there is a shortage of related research. To address this gap, this study developed a multi-physics coupled model considering the impact of carbon content to simulate microstructural transformation. Using finite element analysis, we simulated the microstructural evolution during the CQT heat treatment of 20MnCr5 bevel gear. By comparing the simulation results with the calculations from the commercial heat treatment software DANTE®, the accuracy of this model in predicting microstructural distribution and hardness has been validated. Furthermore, the study established a carbide precipitation kinetics model that takes into account the influence of grain size, to investigate the precipitation behavior of carbides within the matrix during the tempering stage. The cellular automaton method was applied to demonstrate the evolution process of the quenched microstructure during tempering. The study shows that during tempering, carbides primarily precipitate in areas with higher carbon content, preferentially at the boundaries of fine grains with a high density of lattice defects. As the carbon content increases, the number density of precipitates rises, while their average radius decreases. The reduction in matrix supersaturation due to carbide precipitation is one of the causes for the reduction of tooth surface hardness during tempering. The model constructed in this study provides a new perspective for understanding the laws governing material microstructural evolution and offers a solid theoretical foundation for the study of stress and deformation during the heat treatment process.

The influence of annealing temperature on the microstructure and mechanical properties of Fe-0.52 C-11Mn-5.14Al-1Cr lightweight steel

Steel used in the automotive industry must strike a delicate balance between strength and ductility to maintain the structural integrity of vehicles and achieve intricate designs. Traditional methods of reducing weight by using high-strength low-carbon steels have limitations. As a result, a new dual-phase lightweight steel with an austenite-ferrite structure and the composition Fe-0.52 C-11Mn-5.14Al-1Cr was developed. The relationship between the microstructure and mechanical properties of this lightweight steel, which underwent various annealing processes (700°C-1000°C) after cold rolling, was examined using specialized equipment like a universal testing machine, electron microscopy, and X-ray diffractometer. The findings reveal that the cold-rolled test steel (CR) exhibits high tensile strength (1500 MPa) but low elongation at break (3.9 %). The performance of the experimental steel post-annealing treatment has shown significant improvement, with the average grain size increasing with higher annealing temperatures. The γ phase generally shows an increasing trend, with finer grains post-annealing, and the distribution of the α phase displays a consistent pattern along γ boundaries. Additionally, a substantial number of annealing twins appear in the matrix. These twin boundaries absorb some dislocations, and slip leads to the movement of twin boundaries, effectively reducing stress concentration from deformation and enhancing the ability of twin boundaries to accommodate plastic strain, creating twin dislocation interaction. The interplay among dislocations, stacking faults, and twins regulates the balance between strength and plasticity. Following treatment at 850°C × 1 min + 500°C × 3 min, the steel displays excellent mechanical properties, including ultimate tensile strength (836 MPa), elongation (53 %), and strength-ductility product (43,723 MPa·%). This study identifies a specific heat treatment parameter that achieves a harmonious equilibrium between outstanding strength and plasticity (strength-ductility product) within the designed composition.

Evaluation of Shear-Punched Surface Layer Damage in Three Types of High-Strength TRIP-Aided Steel

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.

Effects of Lath Boundary Segregation and Reversed Austenite on Toughness of a High-Strength Low-Carbon Steel

Effects of Lath Boundary Segregation and Reversed Austenite on Toughness of a High-Strength Low-Carbon Steel

Geometry of dimples and its correlation with mechanical properties in high-strength bainitic steel

ABSTRACT The experimental measurements of 2 D micro/nanolayered dimples' dimensions (both geometry and pattern) on the published tensile fractographs of a series of austempered high-strength low-carbon bainitic steel are carried out and interpreted with the corresponding initial microstructural features and tensile mechanical responses as a function of austempering temperature.

Optimizing the hydrogen embrittlement resistance by Cu addition in a low carbon high strength steel

This study used the electrochemical hydrogen permeation test and the slow strain rate tensile (SSRT) test to examine the impact of Cu addition on hydrogen embrittlement (HE) in a low carbon high strength steel. The following method explains how Cu alloying affected HE resistance. (i) Cu alloying refined the prior austenite grain size, which increased the effective area of grain/packet/block/lath boundaries and mitigated the strain localization, and thus provided additional reversible hydrogen traps, homogenized the hydrogen distribution and suppressed local accumulation of hydrogen at boundaries. (ii) Cu alloying promoted the Cu-rich precipitates, which had strong binding energy with hydrogen. Also, the “incoherent” fcc Cu-rich precipitates contrary to the “slightly coherent” Cu-rich with an energy-poor trap strongly trapped hydrogen and enhanced the HE resistance.

A Simple Calibration Method to Consider Plastic Deformation Influence on X-ray Elastic Constant Based on Peak Width Variation

The sin²⁡ψ method is the general method for analyzing X-ray diffraction stress measurements. This method relies on the estimation of a parameter known as 12S2hkl, which is generally considered as a material constant. However, various studies have shown that this parameter can be affected by plastic deformation leading to proportional uncertainties in the estimation of stresses. In this paper, in situ X-ray diffraction measurements are performed during a tensile test with unloads on a low-carbon high-strength steel. The calibrated 12S2hkl parameter varies from 3.5×10−6 MPa−1 to 5.5 ×10−6 Mpa−1, depending on the surface condition and on the plastic strain state, leading to a maximum error on the stress level of 40% compared to reference handbook values. The results also show that plastic strain is responsible for 6 to 14% of the variation, depending on the initial surface sample condition. A method is then proposed to correct this variation based on the fit of the 12S2hkl evolution with respect to the peak diffraction width, the latter being an indication of the plasticity state. It is shown that the proposed methodology improves the applied stress increment prediction, although the absolute stress value still depends on pseudo-macrostresses that also vary with plastic strain.

Enhancement of the resistance to hydrogen embrittlement by tailoring grain boundary characteristics in a low carbon high strength steel

In this work, the feasibility of using “grain boundary engineering” to reduce the susceptibility to hydrogen embrittlement in a low carbon low alloy steel was examined. By adding Cu element, the fraction of random grain boundary (RGB) exhibited the highest value in the steel with 1%Cu, while the fraction of special grain boundary (SGB) showed a monotonical decline. The slow strain rate tensile (SSRT) test revealed that the elongation loss presented an increase with Cu addition increasing from 1 % to 3 %. This can be explained in terms of hydrogen diffusion, hydrogen trap and crack propagation. The steel with 1%Cu had a higher fraction of high angle grain boundary (HAGB), which contributed to a higher density of hydrogen traps and a lower hydrogen diffusion rate. Moreover, the steel with 1%Cu had the highest fraction of SGB (∑3, ∑5, ∑7 and ∑≥9), which was beneficial to the resistance to crack propagation. Under the combined effect of RGB and SGB, a higher resistance to hydrogen embrittlement was achieved in the steel with 1%Cu.

Life‐Cycle‐Assessment of purlin systems: the influence of the low carbon high strength steels in CO2eq. emissions

AbstractDue to the decarbonization of the energy sector, embodied carbon has been recognized as the dominant climate impact driver. In response, countries are currently accelerating their efforts to comply with climate change commitments and regulations, pressure grows for the construction sector to reduce its impact rapidly.In the present paper, the life cycle assessments (LCA) of purlin roof systems, used as secondary structural elements on single‐story industrial buildings, are compared. Three structural materials are considered: metallic‐coated structural steel, reinforced concrete, and structural timber. In addition, different steel grades are compared to highlight the environmental benefits of using high‐strength steels (HSS) allowing weight‐optimized design.The LCA focuses on the product stage (modules A1‐A3), transport (module A4), end‐of‐life (module C), and the benefits and loads beyond the system boundary (module D). The life cycle inventory (LCI) is composed of construction environmental product declarations (EPD) published according to EN 15804. All the LCA evaluations are performed using the commercial software One Click LCA following the recommendations of EN 15978.

Effect of Finish Rolling Temperature on the Texture and Fracture Resistance of Low-Carbon High-Strength Pipe Steels during Thermomechanical Treatment

Effect of Finish Rolling Temperature on the Texture and Fracture Resistance of Low-Carbon High-Strength Pipe Steels during Thermomechanical Treatment

The Influence of Tempering Temperature on Retained Austenite and Ductility–Toughness of a High-Strength Low-Carbon Alloyed Steel

High-strength alloyed steel has been widely used in engineering equipment because of its exceptional strength and toughness, particularly at low temperatures. However, the performance of high-strength alloy steel has not been fully developed, and it is necessary to further optimize the microstructure and mechanical properties. Therefore, the focus of this study is on the phase transition and corresponding mechanical properties of high-strength low-carbon alloyed steels. Three experimental steels were austenitized at 900 °C for 1 h, followed by water quenching, and were then tempered at 570, 600, and 630 °C. They were denoted as QT570, QIT600, and QIT630, respectively. The results show that appropriate intercritical tempering of QIT600 steel significantly increases the proportion of retained austenite and promotes VC precipitation within tempered martensite in comparison to QT570 and QIT630 steels. The enrichment of multiple alloys improved the thermal stability of retained austenite, which was further demonstrated with low-temperature insulation tests. Meanwhile, QIT600 steel with 18 vol.% of retained austenite achieved a superior yield strength of 1025 MPa, an elongation of 21%, and a cryogenic impact energy of 1.25 MJ/m2. The plasticity induced by the transformation of the retained austenite significantly enhanced the strain-hardening rate and postponed necking, thereby increasing elongation. The retained austenite enhanced cryogenic toughness by significantly arresting crack growth and increasing the ability of plastic deformation.

Influence of the content of chromium in low-carbon bainitic-martensitic steels on the efficiency

The effect of chromium content on phase transformations in low-carbon high-strength steel for shipbuilding has been studied. Under industrial conditions, two rolled sheets 50 mm thick were manufactured from steel with a chromium content of 1.08% and 0.42% using hot rolling technology, followed by furnace hardening and high-temperature tempering, standard mechanical properties and performance characteristics were determined. With the help of optical and transmission electron microscopy, the structural features of steel were revealed.

Controllable selection of martensitic variant enables concurrent enhancement of strength and ductility in a low-carbon steel

A novel strategy through controlling the selection of martensitic variant in metallic materials is proposed to break through the trade-off between strength and ductility. Here we prepared a high-strength low-carbon steel by applying the two-stage mechanical processing combined with high-temperature large-reduction compression and the subsequent compression above the low critical point of austenitization. The steel has the lowest degree of static recrystallization and dynamic recrystallization after the two-stage mechanical processing, leading to the smallest prior austenite grains with an average diameter of 16.4 μm and the highest dislocation density of 10.21 × 1014 m−2. With the refinement of the prior austenite grains caused by the adjustment of the two-stage deformation process, the driving force required for martensitic transformation increases and variants selectivity becomes stronger, leading to a large number of fine martensitic laths and nanoscale twins after martensitic transformation. Consequently, the best combination of strength and ductility is obtained, with the yield strength of 871 MPa, tensile strength of 1054 MPa and the total elongation of 25%. The use of variant selection exploits the strengthening of both grain boundaries and dislocations, resulting in simultaneous enhancement of strength and ductility. These findings demonstrate how multiple deformation mechanisms can be deliberately activated via the controllable selection of martensitic variants.

The role of reverted transformation in hydrogen embrittlement of a Cu-containing low carbon high strength steel

This work presents an investigation on the role of reverted transformation in hydrogen embrittlement, aiming at revealing the influencing mechanism of austenite and grain boundary characteristics on hydrogen embrittlement (HE) susceptibility during reverted transformation. The results showed that HE susceptibility decreased with the annealing temperature increasing from 680 °C to 720 °C, which attributed to a combined effect of austenite and grain boundary distribution. Both the fractions of austenite and high angle grain boundary (HAGB) increased with the annealing temperature increasing from 680 °C to 700 °C, which resulted in a lower HE susceptibility at 700 °C due to strong H storage of austenite and HAGB. As the annealing temperature further increased to 720 °C, the fraction of austenite exhibited a decline, but the fraction of HAGB monotonously increased. As a result, a lower HE susceptibility was achieved at 720 °C. This indicated that grain boundary distribution played a determining role in the resistance to HE. We ascribed this to the variant selection and Bain/CP grouping.

Achieving superior low-temperature toughness in high-strength low-carbon steel via controlling lath boundary segregation

The optimal aging temperatures of 400–550 °C for high strength steels falls into the dangerous temperature range of temper-embrittlement. To obtain a good low-temperature toughness, temperatures above 600 °C are usually selected for aging treatments, resulting in a huge loss of strength. In this work, the effects of aging treatments at 500 and 550 °C on the impact performance of a Cu precipitation-strengthened steel at a low temperature of −80 °C were systematically investigated. The solute segregation at lath boundaries is found to be the main factor controlling the low-temperature toughness. Excellent impact performance of ∼180 J at −80 °C along with a high yield strength of ∼1050 MPa and a total elongation of 19% can be obtained by controlling the segregation of solute elements, specifically Mo and Mn at the lath boundaries. The evolutions of matrix and precipitates during aging treatments were investigated. The strengthening and toughening mechanisms are also critically discussed.

Surface Topography Model of Ultra-High Strength Steel AF1410 Based on Dynamic Characteristics of Milling System

AF1410 is a low carbon high alloy ultra-high strength steel. It not only has high strength and high toughness, but also has a high stress corrosion resistance. However, due to the characteristics of hard quality and poor thermal conductivity, AF1410 is a difficult material to process. In the process of milling, the geometric factors of process parameters, the flexible deformation of milling cutter and the flutter of the process system all affect the surface roughness, which makes it difficult to predict the surface roughness of milling parts. In order to solve this problem, a prediction model for surface topography of ultrahigh strength steel AF1410 was studied. To solve this problem, this paper studies the formation of milling surface topography, considers the dynamic displacement of the milling system, proposes a modeling method of surface topography based on the dynamic characteristics of the milling system and forms a prediction model. On this basis, the surface topography of ultra-high strength steel is simulated and analyzed, and the accuracy of the model is verified by experiments. The study realizes the prediction of milling surface topography of AF1410 parts and reveals the formation mechanism of milling surface topography from geometric and physical perspectives.

The effects of hydrogen on dynamic fracture toughness of high-strength low-carbon medium manganese steel

The effect of retained austenite (RA) stability and dislocation density of martensite matrix on low-temperature dynamic fracture toughness and the hydrogen embrittlement (HE) susceptibility was investigated in a high-strength low-carbon medium manganese steel subjected to three different heat treatments, i.e., the inter-critical annealing at 630 °C (QHA), inter-critical annealing at 610 °C (QLA), and direct quenching (DQ). Dynamic cracked three-point bending tests were performed at 20 °C and −40 °C with and without hydrogen charging. The RA volume fraction and the martensite matrix's dislocation density significantly affected the dynamic fracture toughness and HE behavior. All QHA and QLA tested at 20 °C fractured in the ductile mode, but all DQ and QLA tested at −40 °C fractured in the brittle mode. Dynamic fracture toughness decreases with RA volume fraction under the same tested temperature. The HE mechanism in the ductile region is HELP, and that in the brittle region is HEDE.

Aspects of Changing the Structure and Hardness of Metal of Heat-Affected Zone During Welding of Low-Carbon High-Strength Steels for Pipes with Increased Deformability

Aspects of Changing the Structure and Hardness of Metal of Heat-Affected Zone During Welding of Low-Carbon High-Strength Steels for Pipes with Increased Deformability

Effect of Cu Content on Microstructure and Mechanical Properties for High-Strength Deposited Metals Strengthened by Nano-Precipitation

With the rapid development of low-carbon high strength steel, higher requirements are put forward for the matching welding consumables. The deposited metals with 0.62–2.32% Cu addition was prepared by tungsten inert gas welding via metal cored wire. The effect of Cu element on microstructure and mechanical properties of deposited metals were investigated. The multiphase microstructure of deposited metals consists of bainite, martensite, residual austenite, and martensite-austenite constituents. It is found that Cu decreases the start temperature of martensite (Ms) and enlarges the temperature range of bainite from 372 K to 416 K, improving the formation of bainite. With the increase of Cu content, the fraction of martensite decreases and the shape of M-A constituents changes from strip into granular. There are BCC and FCC Cu precipitates in deposited metals. The diameter of Cu precipitates is 14–28 nm, and the volume fraction of it increases with the increase of Cu content. Meanwhile, the deposited metals with 1.79% Cu can achieve a 10% enhancement in strength (yield strength, 873–961 MPa, ultimate tensile strength, 1173–1286 MPa) at little expense of impact toughness (64.56–56.39 J at −20 °C). Cu precipitation can effectively improve the strength of the deposited metals, but it degrades toughness because of lower crack initiation energy. The deposited metal with 1.79% Cu addition shows an excellent strength-toughness balance.