Fine Ferrite Grains Research Articles

Impact of boron and titanium on the mechanical properties and recrystallization behaviour of low carbon cold rolled and batch annealed HSLA steels

This study investigates the impact of boron (B) on the mechanical properties and recrystallization behaviour of cold rolled and batch-annealed titanium (Ti) microalloyed high-strength low-alloy (HSLA) steels. HSLA steels are essential in industries such as automotive and construction due to their superior mechanical properties and cost-effectiveness. The addition of microalloying elements, particularly Ti, has shown significant promise in enhancing these properties through mechanisms like precipitation strengthening and controlled recrystallization. However, the role of B in this context remains underexplored. Here, three alloy compositions were systematically examined: a reference low carbon steel base alloy with no Ti or B (Ref), the base alloy with added Ti (Ref + Ti), and the base alloy with both Ti and B additions (Ref + Ti + B). The findings showed that microalloying with Ti + B can provide excellent strength improvements in the hot rolled strip, mainly due to increased hardenability that promotes the formation of finer ferrite grains with a high dislocation density. However, the recovery and recrystallization behaviour is such that it is not possible to reproduce these benefits after cold rolling and batch annealing, highlighting the complexity of optimizing microalloying strategies for cold-rolled and batch annealed products. The insights gained from this study provide a deeper understanding of the mechanisms governing the recrystallization of Ti and B microalloyed HSLA steels, paving the way for the development of advanced materials with tailored properties for critical applications.

Effects of microstructure and phosphorus segregation on tensile properties of friction stir welded high phosphorus weathering steel

Defect-free sound joints of high-phosphorus (0.094 wt%) weathering steel (SPA-H) were successfully fabricated using friction stir welding (FSW). This study examines the microstructural evolution and mechanical properties of joints produced at different FSW parameters. Tensile tests of the joints revealed that fractures occurred in the base metal (BM) region, indicating almost 100 % welding efficiency. Furthermore, the stir zones (SZ) demonstrated a marked improvement in tensile properties. Particularly, at the rotation rate of 80 rpm and axial load of 45 kN condition (below A1), the microstructure featured ultra-fine ferrite and cementite, resulting in high hardness (270 HV) and tensile strength (704 MPa) in steel with just 0.08 wt% C, while maintaining nearly 80 % of the total elongation of BM. However, the SZ at 80 rpm exhibited an unusual decrease in local elongation despite the fine ferrite grain size and cementite presence. Electron probe microanalysis (EPMA) and nano-hardness revealed pronounced phosphorus segregation in the ferrite region and significant localized hardness disparities induced by the segregation behavior. The strain concentration at the interfaces between these regions during the tensile process leads to crack initiation and rapid propagation. The negative factor caused by the phosphorus segregation accelerates the failure of the specimen during the necking stage and ultimately shows decreased local elongation.

Electrochemical Characteristics and Corrosion Mechanisms of High-Strength Corrosion-Resistant Steel Reinforcement under Simulated Service Conditions

Long-term steel reinforcement corrosion greatly impacts reinforced concrete structures, particularly in marine and coastal settings. Concrete failure leads to human casualties, requiring extensive demolition and maintenance, which represents an inefficient use of energy and resources. This study utilizes microscopic observation, atomic force microscopy (SKPM), electrochemical experiments, and XPS analysis to investigate the corrosion behavior of 500CE and 500E under identical conditions. We compared 500E with 500CE, supplemented with 0.94% Cr, 0.46% Mo, 0.37% Ni, and 0.51% Cu through alloying element regulation to obtain a finer ferrite grain and lower pearlitic content. The results indicate that 500CE maintains a stable potential, whereas 500E exhibits larger grain sizes and significant surface potential fluctuations, which may predispose it to corrosion. In addition, despite its more uniform microstructure and stable electrochemical activity, 500E shows inferior corrosion resistance under prolonged exposure. The electrochemical corrosion rate of 500CE in both the pristine and passivated states and for various passivation durations is slower than that of 500E, indicating superior corrosion performance. Notably, there is a significant increase in the corrosion rate of 500E after 144 h of exposure. This study provides valuable insights into the chloride corrosion phenomena of low-alloy corrosion-resistant steel reinforcement in service, potentially enhancing the longevity of reinforced concrete structures.

Multi-scale hybrid reinforced super duplex stainless steel matrix composites with high strength and ductility via laser powder bed fusion and an in-situ synthesis strategy

The emergence of nanoparticle-reinforced metal matrix composites represents a promising method for efficiently enhancing material strength without significant compromise in ductility. Nevertheless, the homogeneous distribution of nanoparticles in matrix remains a technical challenge. To address this issue, this study designed an advanced method to create a multi-scale hybrid to reinforce super duplex stainless steels (SDSSs) by coupling laser powder bed fusion (LPBF) technique with an in-situ synthesis strategy and optimizing post heat treatment. For as-built composites, both TiCxNy and M23C6 nanoparticles were generated in-situ by the introduction of micron-sized TiC particles into SDSSs, and their microstructure was mainly composed of fine ferrite grains. The post quenching treatment enabled heat-treated composites to possess micro-duplex matrix grains with a multi-scale hybrid, including micron-sized TiC, in-situ sub-micron M23C6 particles, and in-situ TiCxNy nanoparticles. Particularly, the obvious agglomeration phenomenon of in-situ TiCxNy nanoparticles was not observed. In comparison with heat-treated SDSSs (ultimate tensile strength (UTS): ∼839 MPa; uniform elongation (UE): ∼20.6 %), heat-treated composites processed by 1090 °C obtained a higher UTS (∼992 MPa) with only 0.8 % reduction in UE. The great ductility of heat-treated composites was primarily attributed to their relatively high strain hardening rate, facilitated by grain refinement and the formed multi-scale hybrid. The successful creation of a multi-scale hybrid sets the stage for producing high-performance metallic components via LPBF and in-situ strategy.

Thermomechanical Processing of V-N Structural Plate Steels in Low Temperature Austenite

Austenite restoration during thermomechanical (TM) rolling of typical vanadium-microalloyed structural steels was studied to optimize strength in the as-rolled and air-cooled condition. Multi-pass plate rolling simulations were performed on V-N microalloyed and CMn steels to compare recrystallisation behaviour in various temperature regions. Included were a conventional schedule ending at high temperature and two TM schedules with mill exit temperatures in the intermediate and low austenite regions. Increasing delay periods after roughing enhance the suppression of recrystallisation after the start of finishing thereby increasing both nucleation site density and nucleation rate for ferrite formation and refinement in grain size. Good agreement was found between microstructures after industrial TM rolling and those obtained from laboratory simulations. Although precipitation of vanadium carbonitrides is an effective strengthening mechanism, appreciable gains in yield strength due to grain refinement can be achieved by rolling in the lower austenite region. Low nitrogen contents in V steels produce coarser final ferrite grain sizes and lower strengths probably due to a larger precipitate size. V-N steels display similar flow behaviour to CMn grades down to approximately 825°C at low to intermediate strain rates but may experience alternate regions of work hardening and dynamic softening at lower temperatures in austenite.

Effect of Cr–Mo Addition on the Microstructure and Mechanical Properties of Medium‐Carbon Steel

Herein, the effects of Chromium–Molybdenum (Cr–Mo) addition on the microstructural evolution and mechanical properties of medium‐carbon steel after spheroidization annealing are systematically studied through scanning electron microscopy, electron backscatter diffraction, and tensile testing. Cr–Mo addition hinders the proeutectoid ferrite + pearlite transformation, thereby promoting the bainite transformation. Moreover, it refines the pearlite lamellar spacing as well as decreases the average carbide diameter, increases the number of carbides per unit area, and hinders ferrite recrystallization. Compared with those in the B1 steel annealed for 8 h, the size of carbides and their number per unit area in the CM1 steel are 30% lower and 2.2‐fold higher, respectively. Due to finer ferrite grains, smaller carbides, and a higher amount of carbides, the strength of steel improves, and the plasticity slightly reduces after Cr–Mo addition. After 2 h of annealing, the yield strengths of Cr–Mo steels are 77.5–109.5 MPa higher than those of base steels; the elongations are above 20%. The contributions of the strengthening mechanism of steel to the yield strength are as follows (from high to low): grain boundary, precipitation, solid solution, and dislocation strengthening.

High strength, ductility and sheet formability by normalizing and quenching of low carbon microalloyed dual-phase steel

This work investigates the possibility of developing high strength, ductility, and normal-anisotropy in cold rolled (CR, ∼84%) 0.05 wt% carbon microalloyed dual-phase (DP) steel sheets by normalizing (N, 920°C, 30 min) followed by intercritical annealing and quenching (IA&Q, 80 0°C, 3 min). CR + N produces ferrite with ∼6.9μm grain size and ∼0.23 bainite fraction with ∼5.3μm packet size. A high engg. (true) strength of ∼605(∼798)MPa; uniform engg. (true) strain eU≈0.32 (εU≈0.28); engg. fracture strain, ef≈0.48; with high normal average anisotropy (r‾) of ∼1.84 and low planar anisotropy, Δr of ∼0.30 could be obtained. Further, IA&Q produced ferrite with ∼6.3μm average grain size, ∼0.25 martensite fraction (Ceq≈0.16) with ∼3.2μm island size. A high engg. (true) strength of 690(864)MPa; engg. (true) uniform strain of ∼0.25 (∼0.22); engg. fracture strain ∼0.39; and high r‾ of ∼1.51, low Δr of ∼0.31, suitable for sheet metal formability, were obtained. The high strength could be attributed to (a) fine ferrite grain size, (b) secondary phase fraction (∼0.23B/∼0.25M), packet size and island morphology, and (c) geometrically necessary boundaries (GNB). High ductility could be due to (a) fine ferrite grains with secondary phase morphology, (b) GNB, coincident site lattice boundaries (CSLB), and random high angle grain boundary (RHAGB) networking. Better anisotropic properties could be associated with strong γ-fiber, and suppression of α-fiber and θ-fiber texture. Loci in the stress and strain space indicate that the hydraulic bulge and plane-strain biaxial strength-ductility improves to 950MPa-0.41 and 1030MPa-0.67 after CR + N, and to 968MPa-0.30 and 1070MPa-0.58 after CR + N + IA&Q.

Microstructure and mechanical properties of AISI 410S ferritic stainless steel narrow gap weld using flux strip constraining arc welding

The achievement of high comprehensive mechanical properties in welded joints of ferritic stainless steels has attracted considerable attention in the manufacturing industry. Here, a low heat input flux strip constrained arc welding (FSCAW) was performed on a hot-rolled AISI 410S sheet. Defect free joints with a small HAZ width were obtained by one side welding both sides formation. The microstructure of HAZ consists of fine ferrite grains and a small amount of M23C6-decorated martensite due to short high-temperature residence time and rapid cooling rate, resulting in high toughness and hardness. Influenced by the stirring effect of the constraining arc and the promotion of heterogeneous grain nucleation by Ti-rich MX particles originating from the transition of alloying elements in the flux strip, the fusion zone (FZ) exhibits a unique precipitates-strengthened martensite+austensite uniform microstructure with weak martensitic texture. This translates into a simultaneous increase in impact toughness and hardness of FSCAW FZ compared to the MAG joint with a non-uniform mixed microstructure, strong martensitic texture, and large grain size. This work establishes the potential for using FSCAW welding to join ferrite stainless steels.

High strength-ductility combination by quenching and partitioning of a low carbon microalloyed dual-phase steel

This work investigates the quenching and partitioning of hot and cold rolled (HR & CR) low carbon micro-alloyed (Fe-0.05C-1.3Mn-0.4Si-0.6Cr) dual-phase (DP) steel. The quenching (Q) step involved intercritical annealing (IA) at 840°C for 60min and water quenching (200°Cs−1). This is followed by partitioning (P) treatment comprising of below critical annealing at 600°C for 15min and water quenching. The final microstructure after Q and Q&P contains ∼33% martensite within fine ferrite grains with lath for HR and island type martensite morphology for CR sheets. A high strength, ductility and isotropy combination of ∼544±4MPa UTS; 44%(n=0.30) &49% (n=0.32) and Lankford parameter (r‾)=∼1, respectively were obtained for the HR and CR conditions. The strength could be attributed to the equiaxed fine ferrite grain structure (4.3−6.5μm avg. grain size), secondary martensite phase, and dislocation density. The ductility could be a combined effect of the fine ferrite grain size, low dislocation density, geometrically necessary boundaries networking inside the grains, increased co-incidence site lattice boundaries (CSLB), TRIP effect, and texture weakening. ‘P’ treatment successfully stabilized the retained austenite in this lean alloy and induced the TRIP effect. The martensitic transformation during quenching led to ∑3, ∑11, ∑19b, ∑25b, ∑31b and ∑33c CSLB formation (predominantly ∑3), and partitioning treatment could further increase the CSLB density. The presence of CSLB led to the development of weak and partial α-, β-, γ- and θ- fiber texture in the final microstructure. The weak texture led to isotropy (r‾∼1).

Development of Desirable Fine Ferrite Grain Size and Random Second Phase Dual-Phase Steel Microstructures Using Composition and/or Processing Modifications

Microstructural morphology is known to have a significant impact on the mechanical properties of dual-phase steels. A fine ferrite grain size and random distribution of small second phase islands are desirable to provide superior isotropic properties compared to the banded second phase distribution that is typical for this type of steel. A rapid alloy prototyping (RAP) facility has been used to investigate three different DP 800 variants by systematically varying the compositions and/or process parameters compared to the ‘standard’ DP800 composition and processing that gives a banded microstructure. For Variant 1, the heating rate during the annealing cycle after cold rolling varied between 0.65 and 30 °C/s for the 45%, 60% and 75% cold reduction samples. It was found that a cold reduction of 75% and heating rate of 15 °C/s resulted in the microstructure that can give the best combination of strength and ductility because of the fine grain size and high martensite volume fraction. For Variant 2, the effect of changing the hot rolled (HR) microstructure (ferrite–pearlite, ferrite–bainite or martensite) on the final microstructure was investigated. Both the ferrite–50% bainite and fully martensite/bainite HR materials for all cold reductions resulted in annealed microstructures with necklace martensite morphology and finer ferrite grains compared to the ferrite–pearlite HR material, which gave a typical banded ferrite–martensite microstructure with a coarser ferrite grain size. For Variant 3, the Mn content was reduced, and increased Nb was used to achieve higher pancaking during the hot rolling stage, which refined ferrite grains in the HR condition with the same hardness. After annealing with the standard parameters only the 45% cold-reduced material produced a finer ferrite grain size than the standard material, whereas the 60% and 75% cold-reduced samples required a higher heating rate to achieve finer ferrite grain sizes due to rapid recrystallisation and growth kinetics.

Microstructural Analysis and Mechanical Behaviour of Copper CDA 101/AISI-SAE 1010 Dissimilar Metal Welds Processed by Friction Stir Welding

In this work, low-carbon steel AISI-SAE grade 1010 with copper grade CDA 101 was joined by friction stir welding (FSW) using a tapered pin profiled tool. The rotational speed of the tool is 900 rpm, a traverse rate of 30 mm/min, and an axial force of 5 kN were used to produce the joints. The microstructural analysis and mechanical properties of the weld joints have been successfully examined. The optical microscopy, scanning electron microscopy, and X-ray diffraction (XRD) techniques were performed to examine the macropatterns and micropatterns of the welded joints. The tensile and hardness test was performed to evaluate the mechanical behaviours of the FSW joints. The fine ferrite grain features with uniform size were obtained in the microstructure of the nugget zone (stir zone). It is purely influenced by the alternating dynamic rearrangement (recrystallization) mechanism. High hardness was identified in the stir zone, even as the slightest stability was established in the heat-affected zone. The tensile investigation proposed that all the joints explored just lesser unbending nature than the parent material. The tensile strength of 181.5 MPa, the hardness of 144 VHN, and elongation of 14.03% were observed for the welded samples. The better properties for the weld joints were attained at 900 rpm spindle speed and tool traverse speed of 30 mm/min. The FSW is an attractive material joining process for both similar and dissimilar materials compared to other conventional types of joining processes, such as aerospace, marine engineering, shipbuilding, and industrial sector applications.

Study on Microstructure and Properties of Welded Joints with Low Strength Backing Layer for WELDOX960

The welding of WELDOX960 ultra-high strength steel must consider not only the strength but also the toughness of the welding zone. In this paper, a new welding process with low strength matching backing layer is studied, that is, we choose ER50-6 wire for backing welding, use T union gm120 wire for MAG welding filling, and complete the cover welding. We prepared two groups of welding samples of ER50-6 wire backing welding and T union gm120 wire backing welding. Then we test the samples by optical metallography, tensile test, impact test and hardness test. The results show that the properties of the two kinds of backing weld can meet the requirements. The basic structure of the weld outside the backing layer of the two welding methods is similar, which are acicular ferrite and carbide. Using ER50-6 welding wire as backing, the microstructure of the weld is uniform and fine ferrite grain and a small amount of pearlite. Using T union GM120 high strength steel as backing, the microstructure of the weld is acicular ferrite and carbide. The toughness of ER50-6 is higher than that of T union GM120, and the hardness is lower than that of T union GM120.The research results have been successfully applied to the welding of large tonnage truck crane boom, and the enterprise has achieved high economic benefits.

Mechanical properties and precipitation behavior of high strength hot-rolled ferritic steel containing Nb and V

The mechanical properties and precipitation behavior of hot-rolled microalloyed steels with varied Nb and V additions were investigated. The steels had a predominantly ferritic microstructure with a very good combination of strength ( σ U T S = 800–1000 MPa), ductility (total elongation = 16–19%) and hole expansion ratio (32–34%). The strength increased with increasing Nb and V content. The stretch-flangeability of the steels did not deteriorate with increasing strength. The favorable hole expansion behavior of the steels is attributed to the fine ferrite grain size (1.6–1.8 μm) and uniformity of the microstructure, as well as the fine precipitates of Nb and V carbonitrides in the matrix. Nb carbonitrides formed during hot rolling and contributed to the grain refinement through their interaction with recrystallization. The co-precipitation of Nb and V carbonitrides during coiling was confirmed and characterized by 3D atom probe tomography. The formation of these fine precipitates contributed to further increase in the strength of the V-containing steel allowing the steel to achieve strength >1000 MPa.

Concurrent Enhancement of Ductility and Efficiency for Annealing High‐Speed Steel via Tuning Cooling Rates

High‐speed steel usually uses very slow cooling rates for complete annealing to guarantee good ductility at the expense of sacrificing the production efficiency. Herein, the aim is to probe the full potential of elevating cooling rates in simultaneous enhancement of ductility and efficiency. The results show that higher cooling rates produce finer ferrite grains and larger‐fractioned eutectoid carbides, which evolve from granular M6C to rod‐shaped M23C6. The precipitate evolution originates from a transition of eutectoid transformation mechanism from a divorced mode to a lamellar manner due to upgraded undercoolings. Granular carbides display an excellent strain‐hardening ability compared with the rod‐shaped counterparts. Highly dispersive granular precipitates and fine ferrite grains formed under appropriate cooling conditions may contribute to larger work hardening rates and higher ductility. For the investigated M42 steel, cooling rates in the range of 2–5 °C min−1 can enhance the annealing efficiency while promising good ductility.

Characterization of hot forged P285NH steel by metallurgical investigation and reliability analysis

AbstractAn extensive study was carried out to investigate the effect of cooling rate after hot forging process and normalization step on the hardness, strength and impact toughness and microstructure of P285NH steel. Understanding of the combined effect of cooling rate and normalization on the mechanical and microstructural properties of the steel would help to select conditions required to achieve optimum mechanical properties. The results indicated that the microstructures of all forging and cooling conditions were dominated by ferrite and pearlite phases with different morphologies and grain sizes according to various cooling rates. Conveyor cooling led to a formation of relatively fine acicular ferrite and pearlite grains in comparison to batch cooling which presented coarse polygonal ferrite with pearlite. Based on the data fluctuation of Charpy tests, the normal distribution provided a statistical analysis method for assessing the reliability. Through the statistical analysis of the distribution function, it can be concluded that normalization step is necessary for higher reliability. Both batch cooling and conveyor cooling did not give the required reliability level for safety components due to heterogeneities in the microstructure.

Short thermal cycle treatment with laser of vanadium microalloyed steels

Abstract Improvement of crankshaft fatigue properties can be approached by altering its mechanical properties in the surface, such as laser surface treatment. Laser beam treatment offers efficient and precise surface hardening processing with possibility of reducing the production cost compared to the conventional hardening techniques. However, its characteristic of having short thermal cycle can be a challenge for the development of laser surface hardening techniques, such as inadequacy of literatures in phase transformation and resulting mechanical properties under rapid heating and cooling rate. Therefore, this work investigated the impact of short thermal cycles induced by the laser beam on the resulting microstructure and hardness properties in the surface of 38MnSiVS5 and 44MnSiVS6 microalloyed steels. Temperature cycles during the process were recorded and examined with the resulting microstructure along with microhardness values. 44MnSiVS6 microalloyed steel, which contains ca. double the amount of vanadium compared to 38MnSiVS5 steel, produces finer ferrite grains in the treated area for all investigated short thermal cycles. This fine-grained microstructure leads to steady hardness distributions in the treated area. The short thermal cycle was assumed to be unable to dissolve the vanadium precipitates that reside in the ferrite grains, which then initiate precipitation hardening.

Experimental Study of the Micromechanical Behavior of Ferrite in DP Steel Under Various Stress States

The low yield ratio and continuous yielding behavior of ferrite-martensite dual-phase (DP) steels are determined by their production process and microstructure. In this work, high-resolution in situ electron backscatter diffraction (EBSD) experiments were performed to study the deformation and hardening behavior of DP steel in uniaxial tensile, plane strain and shear stress states. In the EBSD experiments, we first collected the data for different deformation stages from the same area for each sample (loading was paused during EBSD data collection). Subsequently, we analyzed the strain contouring maps, geometrically necessary dislocation (GND) distribution and ferrite deformation of all the samples based on the postprocessing of the EBSD data. The results showed that (i) the volume expansion accompanying the austenite-to-martensitic transformation during annealing gives rise to ferrite plastic deformation, and the residual deformation after overaging of the fine ferrite grains near the martensite group is the largest; (ii) the high-density GND bands of the shear sample is the same as the shear stress/strain direction, and the high-density GND bands of the uniaxial tensile and plane strain samples are perpendicular to their maximum stress/strain directions; and (iii) in the shear state, the average strain of ferrite grains in each grain size range is similar; but the deformation tends to aggregate toward large-size grains in the uniaxial tensile and plane strain samples.

Improvement of stress-relief cracking resistance in coarse-grained heat-affected zone of T23 steel by refining sub-structure through second thermal cycle

Simulated coarse-grained heat-affected zone (CGHAZ), second reheated CGHAZ (UA-CGHAZ), super-critical reheated CGHAZ (SCR-CGHAZ) and inter-critical CGHAZ (ICR-CGHAZ) of T23 steel were produced via thermal simulation of welding. Their corresponding stress-relief cracking (SRC) susceptibility were assessed using isothermal slow strain rate tensile test. The fracture features and microstructures were characterized to reveal the cracking mechanisms. The simulated CGHAZ of T23 steel was highly susceptible to SRC at temperatures of 550–750°C and the fracture mode exhibited micro-void coalescence. M23C6 carbides precipitated at grain boundary may promote the micro-void nucleation and weaken the grain boundary, which resulted in grain boundary cracking preferentially. The strain was concentrated on the weakened grain boundary under tensile load, and the micro-voids gradually grew and coalesced into micro-crack, which ultimately propagated along grain boundary until complete fracture occurred. The SRC susceptibility of SCR-CGHAZ was reduced partially and the SRC susceptibility of ICR-CGHAZ was eliminated. After a second thermal cycle with a peak temperature slightly higher than Ac3 (SCR-CGHAZ), the grain size of CGHAZ was significantly refined and the crack resistance was elevated. When the peak temperature of the second thermal cycle was limited between Ac1 and Ac3, partial grains near grain boundaries were austenitized and transformed to fine ferrite grains along the prior austenite grain boundaries. The fine ferrite grains inhibited M23C6 precipitation and improve the resistance of cracking propagation. Thus, the ductility was improved and the SRC susceptibility was reduced.

Mechanical properties and microstructure of longitudinal submerged arc welded joint of a new C–Mn steel sheet cladding fine grained ferrite in surface layer for a long distance transport pipeline

A new type of C–Mn pipeline steel plates with the ult5ra-fine grained ferrite in surface layer and the fine-grained ferrite in the central layer has been developed successfully. In order to manufacture the long distance oil pipelines using this new ultra-fine grain pipeline steel sheet, the microstructure and mechanical properties of the welded joints by the Longitudinal Submerged Arc Welding (LSAW), especially the impact toughness, need to be evaluated. Tensile tests, impact tests, are used to study the mechanical properties in this paper according the Chinese National standards. The metallographic technique and SEM are adapted to analyze the microstructure and impact fracture morphology of the LSAW joint. The research results show that the HAZ near the fusion line is a weak part of impact toughness of LSAW joint. However, the base metal is of the fine-grained and ultra-fine grained microstructure itself, and the heat treatment effects between the post-process welding seam and pre-process welding seam on the multi-layer and multi-pass submerged arc welding process, the impact toughness of the HAZ is greatly improved. The whole welded joint is better than the performance of ordinary pipeline steel, which is satisfied with the welding performance requirements of steel sheet engineering application, and can be used in long distance pipeline manufacturing projects.

Influence of C content and annealing temperature on the microstructures and tensile properties of Fe–13Mn–8Al–(0.7, 1.2)C steels

In the present work, the Fe-13Mn-8Al-(0.7, 1.2)C steels were subjected to different post-cold rolling annealing treatments in a temperature range of 800–1000 °C for 15 min to investigate the microstructural evolution and the mechanical properties. Both steels annealed at 800 °C consisted of fine austenite and ferrite grains along with intergranular κ-carbides formed by eutectoid reaction, which contributed to the high strength but deteriorated ductility. With annealing temperature increasing, the volume fraction of ferrite and intergranular κ-carbides progressively decreased. Meanwhile, the precipitation of intragranular κ′-carbides enhanced the yield strength, whereas coarse austenite was harmful to both strength and elongation. The increase in C content increased the volume fraction of intergranular κ-carbides and austenite as well as retarded the austenite recrystallization, which further increased strength and strain hardening rate but deteriorate ductility. The deformation mechanism in austenite was massive planar slip. The microbands were widely observed in both steels at an annealing temperature above 900 °C.