Lath Morphology Research Articles

Achieving balanced mechanical properties in Fe–0.16C plain low-carbon steel by trimodal microstructure and fine cementite

In the present study, for the first time, a trimodal microstructure (including coarse, fine, and ultrafine ferrite grains) was created in plain low-carbon steel by simple thermomechanical processing including intercritical annealing (at 770 °C, 800 °C, and 830 °C), 75% cold rolling, and finally heat treatment at 550 °C. The results showed that martensite with two different morphologies consisting of lath (low-carbon) and plate (high-carbon) were created in the microstructures of 800-75% and 830-75% sheets due to the non-homogeneous distribution of carbon atoms as a result of not using the homogenization treatment. After the final heat treatment, the samples exhibited trimodal grain size distribution of ferrite including coarse grains (larger than 5 micrometers), fine grains (between 1 and 5 micrometers), and ultrafine grains (UFGs) (smaller than 1 micrometer). The formation of ultrafine grains was ascribed to the presence of martensite with lath morphology in the 800-75% and 830-75% samples. The size of cementite particles in the 770-75%-550 sample was larger than the other two samples (800-75%-550 and 830-75%-550) owing to the presence of martensite phase only with plate morphology in 770-75%-550 steel. Unlike the 800-75%-550 sheet, the entire microstructure of the 770-75%-550 and 830-75%-550 samples had not undergone complete recrystallization and several deformed ferrite grains were still visible in the microstructures. The strength and hardness of the heat-treated sheets was larger than that of the as-received steel due to the presence of fine spherical cementite particles and fine ferrite grains in the microstructure of the heat-treated samples. Based on the obtained results, the 800-75%-550 sample revealed the best combination of strength-ductility-toughness due to trimodal microstructure along with the formation of fine spherical cementite particles. The failure mode of all heat-treated samples was ductile and there were both fine and coarse dimples in the fracture surfaces.

Link between b.c.c.-f.c.c. orientation relationship and austenite morphology in CF8M stainless steel.

Slow-cooled CF8M duplex stainless steel is used for critical parts of the primary coolant pipes of nuclear reactors. This steel can endure severe service conditions, but it tends to become more brittle upon very long-term aging (tens of years). Therefore, it is essential to understand its specific microstructure and temporal evolution. As revealed by electron backscatter diffraction (EBSD) analyses, the microstructure consists of millimetre-scale ferritic grains within which austenite lath packets have grown with preferred crystallographic orientations concerning the parent ferritic phase far from the ferrite grain boundaries. In these lath packets where the austenite phase is nucleated, the lath morphology and crystal orientation accommodate the two ferrite orientations. Globally, the Pitsch orientation relationship appears to display the best agreement with the experimental data compared with other classical relationships. The austenite lath packets are parallel plate-shaped laths, characterized by their normal n. A novel methodology is introduced to elucidate the expected relationship between n and the crystallographic orientation given the coarse interfaces, even though n is only partly known from the observation surface, in contrast to the 3D crystal orientations measured by EBSD. The distribution of retrieved normals n is shown to be concentrated over a set of discrete orientations. Assuming that the ferrite and austenite obey the Pitsch orientation relationship, the determined lath normals are close to an invariant direction of the parent phase given by the same orientation relationship.

Microstructural and elemental analysis of flow lips of heavy haul curved rails affected by deformation driven phase transformation

A combination of optical imaging (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) was used to examine the white etching layer (WEL) and brown etching layer (BEL) formed in the flow lip region of a heavy haul curved rail. These flow lips eventually serve as crack initiating regions, leading to reverse detail fractures. OM distinctly identified the WEL/BEL region based on light contrast, while SEM micrographs identified a fine-grained structure within the WEL with intermittent patches of a deformed pearlitic structure. SEM images of the WEL/BEL indicate that the fraction of cementite patches was higher in the BEL region than in the WEL. TEM investigations revealed the presence of martensite/nano-crystalline ferrite along with newly formed austenite and cementite particles in the WEL and the BEL. The ferrite/martensite grain size was much finer in the WEL than in the BEL. The lath morphology of martensite was observed in the BEL, whereas a mixed morphology of lath and twinned martensite was observed in the WEL. APT studies show no Mn/Si partitioning in WEL/BEL along with C-concentrations ranging from 10 to 15 at.% and up to 20 at.% in the WEL and BEL respectively. The synergistic effect of elevated wheel-rail contact temperatures and severe plastic deformation dictated the overall microstructural evolution of the BELs and WELs on the outer rail surface in the flow lip.

Effect of carbon content on strength and toughness of deposited metal for 1000 MPa-grade high-strength welding wire

The microstructure and mechanical properties of the deposited metal of 1000 MPa-grade marine steel welding wire were observed and tested using characterization methods such as OM, SEM, TEM, XRD, and EBSD. The influence of carbon content (0.061–0.080 wt%) on the structure of the deposited metal was examined, with the strengthening and toughening mechanisms of the deposited metal elucidated. The results indicated that the deposited metal structure primarily consisted of lath bainite, martensite, and a small amount of retained austenite as the carbon content increased from 0.061 wt% to 0.080 wt%; the proportion of martensite increases, while bainite and retained austenite decrease. Additionally, the lath morphology changes from “interwoven” to “parallel.” The yield strength of the deposited metal increases from 915 MPa to 1007 MPa, while the impact energy at −50 ℃ decreases from 120 J to 59 J. The increase in martensite proportion contributed to the enhanced strength of the deposited metal. Solid solution and dislocation strengthening are the primary strengthening methods. The observation of the impact fracture section revealed that the bainite was segmented and refined. In contrast, the “intertwined” morphology of plate-striped bainite/martensite and high-angle grain boundaries hinders crack propagation, thereby improving the low-temperature impact toughness.

Gradient microstructure in Ni-Cr-Mo ultra-heavy steel plate after tempering

This paper studied through-thickness gradient microstructure of an 80mm ultra-heavy steel plate by multi-dimensional characterizations, meanwhile the cause of microstructure variability as well as its effect on strength was analyzed. Results show major lath-structure in the whole steel plate, while lath morphology and grain boundary distribution are different along the thickness direction. During phase transformation, the higher cooling rate in the surface enhances the forming ability of V1/V4 variant pairs and corresponding 5°-10° boundaries, meanwhile the higher cooling rate and smaller original austenite grain contribute to more nucleation sites and stronger strain incompatibility, which is corresponding to more low-angle boundaries, so the surface layer shows higher low-angle boundaries; Compared with the surface layer, the moderate cooling rate is beneficial for V1/V2 variant pairs, and Σ3 boundaries between the pairs are obtained, leading to higher high-angle boundary density with decreased low-angle grain boundary density. This boundary characteristic corresponds to the thinner and longer lath morphology. With respect to the relationship between mechanical properties and microstructure, more low-angle boundaries in the surface layer and more high-angle boundaries in inner layers contribute to stronger dislocation strengthening and grain boundary strengthening respectively, and realizes the coordinated through-thickness property.

Effect of Different Quenching Methods on the Microstructure and Mechanical Properties of 30MnB5NbTi

The effects of different quenching methods on the microstructure and mechanical properties of 30MnB5NbTi hot stamping steel are investigated, and the quenched microstructure is characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The mechanical properties are evaluated by uniaxial tensile test. The results reveal that, in comparison to die quenching, oil quenching, or air cooling, water‐quenched samples (tempered after water quenching, die quenching and oil quenching) exhibit the highest ultimate tensile strength (UTS) and yield strength (YS), reaching 2052 and 1422 MPa, respectively. Various quenching methods enable multi‐level strength control for the same steel grade, achieving a strength control range of ≈1000 MPa. The microstructure of samples quenched by water, die, and oil is fully martensite, and martensite exhibited block and lath morphology. With the increase of cooling rate, martensite laths become finer and the prior austenite size decreased. The microstructure of air‐cooled samples is martensite, ferrite, and retained austenite. The strengthening mechanisms of different quenched samples are calculated. The results show that dislocation strengthening and precipitation strengthening are the two dominant strengthening mechanisms in 30MnB5NbTi hot stamping steel. Therefore, the properties of hot stamping steel can be improved by changing the cooling rate by designing physical dies.

Elucidating the role of stress-induced martensite on the tensile behaviour of metastable β Ti-10V-2Fe-3Al alloy

Stress-induced martensitic transformation (SIMT) during loading in metastable alloy systems result in strength-ductility enhancement via transformation-induced plasticity (TRIP). The present study aims to investigate the influence of stress-induced martensites (SIM) on the tensile behaviour of metastable β Ti-10 V-2Fe-3Al (Ti-1023) alloy. The solutionized, and thermo-mechanically processed (i.e., cold rolling and annealing at 820 °C for 5 to 30 min) condition of Ti-1023, with single β phase microstructure, has been considered. The β grain size of solutionized Ti-1023 was ~625 μm, while the thermo-mechanically processed Ti-1023 was in the range of ~48 μm to ~106 μm as function of annealing time. The room temperature tensile behaviour of considered samples show SIMT, which results in strength enhancement (~7.5%), along with an increase in the trigger stress (~30%) as a function of β grain size (i.e., ~48 μm to ~625 μm). However, the % strain to failure show a significant decrease. A detailed microstructural characterization indicates the relation of SIM formation capability with β grain size. For finer β grain size, SIM displays a pronounced embrittling effect on the Ti-1023 tensile behaviour. Fracture surfaces show distinct regions with lath morphologies, which are further confirmed by surface as well as cross-sectional transmission electron microscopy (TEM) of the tensile fractured surfaces, illustrating the embrittling effects induced by SIM. In comparison, the dislocation-induced plasticity regions in the surrounding remnant β matrix result in dimple morphology. Thus, the coupling of enhanced martensite transformability, with reduced grain size, and the eventual decrease in martensitic laths spacing (i.e., from ~48 μm to ~7 μm), increase the density of martensite/matrix interfaces and the embrittling action. This results in pronounced dislocation pileups and formation of multiple martensitic variants facilitating premature failure by micro-void nucleation (at the martensite/matrix interface), growth and coalescence mechanism. The observations suggest that grain boundary engineering be optimized in order to fully exploit SIM's potential in enhancing the strength-ductility relation in Ti-1023 alloy.

A laser additive manufactured metastable Ti-10V-2Fe-3Al β-titanium alloy: Microstructure, mechanical properties, and deformation mechanisms

In the present work, the deformation mechanism is investigated by tailoring the microstructure of a laser powder bed fusion (L-PBF) processed β-type Ti-10V-2Fe-3Al (Ti1023) alloy. The microstructure analysis shows columnar β grains with <001>//BD texture. The α phase appeared lath morphology after using laser power of 160 W and presented fine and intersected morphology at 200 W, and the shape of ω clusters transformed from bulk to intersected acicular with increasing laser power. The Ti1023 alloy exhibits excellent mechanical properties with a total elongation of 18 % and high yield strength above 828 MPa. The existence of α″ phase as well as {332}<113> and {112}<111> deformed twins were found in fractured microstructure at 160W, confirming the multiple deformation mechanisms consisting of dislocation slip, transformation-induced plasticity (TRIP), and twinning-induced plasticity (TWIP). By increasing the laser power, the deformation mechanisms are limited to dislocation slip. At last, the finite element analysis of the temperature and temperature gradient change reveals the underlying mechanism for microstructure evolution based on thermal history. This study provides a pathway to regulate L-PBF Ti1023 strengthening and work-hardening behavior by controlling the process parameters during the fabrication of the material.

Mechanical properties and microstructure of the heterogeneous DZ2 axle steel under high-strain-rate compression at ambient temperature

This study aims to clarify the mechanical properties and deformation characteristics of the newly-developed DZ27CrNiMo (DZ2) axle steel for the China standard electric multiple units under dynamic loadings along the diameter direction of the axle at room temperature. The DZ2 axle steel has a heterogeneous band structure consisting of equiaxed grain and lath morphology tempered martensite. When being impacted at the strain rate range from 500 s−1 to 2440 s−1, its yield strength, compressive strength, and plastic strain increase. The flow stress upturn after yielding mainly depended on the strain rate effect rather than the work hardening due to the suppressed dislocation cross-slip. Meanwhile, the steady plasticity was consistent with the presence of uniformly distributed dislocations and microbands within the heterogeneous laminates. Although the nanoscale adiabatic shear bands (ASBs) and micro-cracks were found to initiate at the high strain rate of 2440 s−1, their propagation could be inhibited by the hard-tempered sorbate laths. As a consequence, an excellent resistance for impact-induced fracture was realized in DZ2 axle steel. Taking the thermal activation damage evolution into account, a modified Zerilli-Armstrong (ZA) constitutive model was proposed to describe the dynamic mechanical behavior of DZ2 axle steel. The modeling results were consistent with the experimental data, indicating a high prediction accuracy and good application availability in the operation and maintenance of DZ2 axle.

An elastoplastic phase-field study of the precipitation behaviors of Mg17Al12 phase in Mg-Al-based alloys: Part II. Precipitation under various loadings

An elastoplastic phase-field model, developed in Part I, was employed to study stress-assisted diffusional phase transformation, taking plastic deformation into consideration. Different loading conditions were applied to a single crystal of Mg-Al-based alloys during precipitation, aiming at investigating the effects of elastoplastic deformation associated with both phase transformation and external loadings. The simulation results show that even when external loadings are below the materials’ yield limits, plastic deformation may occur due to stress accumulation triggered by the volume expansion of β-Mg17Al12 precipitates and the lattice mismatch between the precipitate and the matrix. Under both axial and shear loading conditions, plasticity plays a significant role in the morphological evolution of β-Mg17Al12 precipitate with Burgers orientation. In contrast to the lath morphology observed in the absence of external loadings, a rhombic-like precipitate is formed when applying an external tensile loading of 30 MPa along [112̅0]α direction, while a mirror-symmetrical-parallelogram-like morphology emerges under a shear loading of + 30 MPa. Additionally, plasticity plays a similar dual role during precipitation under different loading conditions, which would promote the growth of the precipitate at the initial stage but tend to retard it afterward. These findings illustrate that the plastic strain generated under external loadings has a great impact on the precipitate microstructure, which provides guidelines for designing the precipitate microstructure to obtain better mechanical properties.

Martensite plasticity and damage competition in dual-phase steel: A micromechanical experimental–numerical study

Martensite damage in Dual-Phase (DP) steel has been studied extensively, yet, the exact deformation mechanisms that trigger or inhibit damage initiation remain mostly unexplored. Whereas generally assumed to be hard and brittle, lath martensite in fact deforms in a highly anisotropic manner, showing large strains under favorable habit plane orientations, which is attributed both to the lath morphology and to so-called ’substructure boundary sliding’. Yet, the correlation (or interplay) between plasticity and damage in lath martensite has not received much attention. Therefore, we raise the question whether these soft martensite plasticity mechanisms can delay or even inhibit damage initiation. We analyze several ’damage-sensitive’ martensite notches, i.e. thin contractions of two martensite islands, by combining several state-of-the-art experimental and analysis methods. Deformations are tracked in-situ at the nanoscale, aligned to detailed microstructure maps, and categorized, for each martensite variant, into habit plane or out-of-habit-plane slip. In these experiments, strong plasticity (>70%) is observed in martensite notches, enabled by slip along a favorably oriented habit plane, whereas damaged notches have unfavorably oriented habit planes, showing limited pre-damage strains (<10%), carried by out-of-habit-plane slip. Additionally, one-to-one experimentally based Crystal Plasticity (CP) simulations are performed in parallel, employing a recently introduced Enriched CP approach which models a soft plasticity mechanism on the variants’ habit plane. The Enriched CP simulations show considerably lower hydrostatic stresses in non-damaged and plastically deforming notches, thereby revealing that the soft habit plane mechanism is key for introducing the high plastic anisotropy that can lead to the inhibition of martensite damage in highly strained martensite notches. Finally, recommendations for improved damage inhibiting steels are proposed.

Hot-rolled Al-added medium Mn steel (Fe-8Mn-2.85Al-1Si-0.2C): Microstructural evolution and tensile behavior

In this study, a low Al (<3%) added medium manganese steel has been developed through melting casting route in an open-air induction furnace. The cast steel ingot has been intercritical annealed at 720 °C after hot deformation, that includes hot forging and hot rolling treatment, respectively. Microstructure characterization coupled with X-ray diffraction (XRD) has been performed after each processing step to understand the extent of austenite formation and its subsequent transformation to ferrite/martensite. The processed steel possesses the ultimate tensile strength, yield strength and ductility of 1000±20 MPa,700±10 MPa and 18.5 ± 0.5, respectively. To understand the possible cause of enhanced strength and deformation mode rietveld analysis has been carried out. It reveals that developed (final processed) steel possesses ∼18% (volume%) of austenite that subsequently reduced to ∼6.139% after tensile testing, indicating significant transformation of austenite to martensite (continuous and enhanced TRIP effect). The morphological characteristic of martensite in developed steel has been investigated using transmission electron microscopy (TEM). It reveals lath morphology of martensite due to low carbon content. The TEM and 3D atom probe analysis reveals the precipitation of intergranular type kappa (k) carbide in intercritically annealed specimens due to probable segregation of C and Al. However, volume fraction of the same is low. The SFE of the steel has been found to be ∼16 mJ m− 2, calculated by the modified Olson and Cohen method that also concludes TRIP is the preferred deformation mechanism in the developed steel.

Use of miniature tensile specimens for measuring mechanical properties in the steady-state and transient zones of Ti–6Al–4V wire-fed electron beam deposits

Recent developments in additive manufacturing (AM) have shown the importance of small-scale mechanical testing due to material availability issues, the microstructural variance within the complex build geometries, or thin-type features such as lattices. The current approach in AM is to test standard-sized coupons and postulate that a component with complex geometry has similar mechanical properties even though their thermal histories are completely different in many cases. In this research, small-scale tensile testing methodologies of AM parts made by electron beam wire-fed (EB-WF) deposited Ti–6Al–4V were investigated. Specifically, specimens extracted from well-established Ti–6Al–4V microstructural zones of transient and steady-state regions were compared to understand the effect of local microstructure on mechanical properties, which were evaluated through microhardness and miniature tensile testing. The average microhardness in the transient and steady-state zones are 321 Hv and 306 Hv, respectively. Although distinct microstructural features were observed in these zones, their tensile properties were similar. Both deposited zones had tensile yield and ultimate strength values ranging between 825-832 MPa and 880–907 MPa, respectively. The measured properties of the miniature specimens were comparable with E8 standard-sized specimens, higher than the ASTM F1108 cast and lower than the AMS 4911P wrought Ti–6Al–4V standards. Deformation and fractography analysis were used to correlate microstructure and property relationships for different α lath morphologies. Basket-weave morphology in the transient zone specimen showed profound grain splitting by basal slip which led to microstructural refinement and hindered softening during plastic flow.

Microstructural evolution and stability during strain-controlled fatigue in a multiphase microalloyed steel

Multiphase ferrite-bainite-martensite (F-B-M) microstructure processed through rolling and two-step cooling in a V-microalloyed steel exhibited excellent stability under strain-controlled fatigue. The steel exhibited a steady state response and even cyclic hardening up to a total strain amplitude of 0.6% and displayed a cyclic softening response at total strain amplitudes >0.7%. To understand and determine the degradation mechanisms, a detailed transmission electron microscopic investigation was carried out in fatigue tested samples as a function of total strain amplitude. At low strain amplitudes (<0.4%), microstructural constituents such as the bainite/martensite colonies, and the inter-lath films of retained austenite remained stable. The deformation occurred only in the polygonal ferrite and the lamellar retained austenite, which was sheared into fine, smaller strips. Only some boundaries of bainite/martensite colonies were subjected to shearing. At an intermediate strain amplitude (0.6%), all the coarse bainite/martensite colonies exhibited shear deformation with very fine retained austenite segments distributed over large regions. The bainite/martensite lath interiors were filled with dislocation substructures and sheared inter-lath austenite layers. Austenite grain rotation was noticed, and the grain interior exhibited sheared or segmented layers. Selective inter-lath austenite layers transformed into acicular martensite. At higher amplitudes (0.8%), the F-B-M microstructure displayed homogeneous deformation and cyclic softening. Carbides and lath debris were distributed throughout the matrix and consisted of decomposed bainite and martensite. Formation of acicular and twinned martensite was present due to the TRIP effect. The F-B-M microstructure comprising small colonies of bainite/martensite with fine lath morphology, polygonal ferrite (∼20%), retained austenite with lamellar morphology and inter-lath films (∼5%) along with a fine dispersion of carbonitride precipitates in the matrix appears to be highly resistant to fatigue damage. Hot rolling processes aimed at promoting these microstructural features in the steel, which is also accompanied by texture strengthening, seem to be effective against cyclic softening in fatigue.

Effect of air cooling time on temperature and microstructure uniformity of 960 MPa high strength steel by direct quenching

AbstractIn order to improve the temperature and microstructure uniformity in the thickness direction of online‐quenching 960 MPa high strength steel, the finite element software ADINA and the experiments have been used to analyze the effect of internal air cooling on temperature variation in the thickness direction during the online quenching process. The results showed that the interval air cooling could effectively improve the uniformity of temperature, microstructure and hardness in the direction of thickness of steel plate during the online quenching process. When air cooled for 3.2 s, the temperature difference between the surface and the core of the steel plate could be reduced by 90.13 %, and the cooling rate in the core could be reduced by about 85.20 % compared with continuous water cooling condition. The hardness difference between the surface and the core could reduce from 23 HV 5 to 5 HV 5. The internal air cooling process had an influence on the matensite lath morphology compared with continuous water cooling process. The microstructures with the different cooling conditions were martensite and the properties could still meet the requirements of 960 MPa high strength steel.

Increasing the resistance to hydrogen embrittlement in martensitic steel by partial phase transformation and tempering

This paper presents a method to increase the hydrogen-embrittlement (HE) resistance of martensite steel using by partial phase transformation and tempering. Dual-tempered martensitic (DTM) steel was produced by two-step quenching and tempering after austenitization: (1) quenching between Ms and Mf temperatures and tempering at 600 °C, then (2) water quenching and tempering at 300 °C. The resulting martensite was composed of two different lath morphologies: one that contained fine cementite precipitates, and the other that contained fine vanadium carbide precipitates. Compared to the conventional tempered martensite (TM), DTM had much higher HE resistance, especially at low hydrogen content. Moreover, the process did not significantly decrease the tensile strength of DTM. Analysis using a multi-trap diffusion model attributed the increase of HE resistance in DTM mainly to strong trapping of hydrogen by vanadium carbide precipitates produced during the first quenching and tempering process. Our results suggest a new approach that can simultaneously increase the HE resistance and strength of martensitic steel.

Microstructural Development of Ti-6Al-4V Alloy via Powder Metallurgy and Laser Powder Bed Fusion

A detailed study was carried out to gain a better understanding of the microstructural differences between Ti-6Al-4V parts fabricated via the conventional powder metallurgy (PM) and the laser powder bed fusion (L-PBF) 3D printing routes. The parts were compared in terms of the constituent phases in the microstructure and their effects on the micro- and nano-hardness. In L-PBF parts, the microstructure has a single phase of martensitic α′ with hcp crystal structure and acicular laths morphology, transformed from prior parent phase β formed upon solidification of the melt pool. However, for the sintered parts via powder metallurgy, two phases of α and β are noticeable and the microstructure is composed of α grains and α + β Lamellae. The microhardness of L-PBF processed Ti-6Al-4V samples is remarkably higher than that of the PM samples but, surprisingly, the nano-hardness of the bulk martensitic phase α′ (6.3 GPa) is almost the same as α (i.e., 6.2 GPa) in PM samples. This confirms the rapid cooling of the β phase does not have any effect on the hardening of the bulk martensitic hcp α′. The high microhardness of L-PBF parts is due to the fine lath morphology of α′, with a large concentration of low angle boundaries of α′. Furthermore, it is revealed that for the α phase in PM samples, a higher level of vanadium concentration lowers the nano-hardness of the α phase. In addition, as expected, the compacting pressure and sintering temperature during the PM process led to variations in the porosity level as well as the microstructural morphology of the fabricated specimens, which will in turn have a significant effect on the mechanical properties.

The Influence of Annealing Temperature on the Morphology of Structures and the Mechanical Properties of Prequenching-Quenching and Partitioning Steel.

In this study, we comparatively study the microstructures and mechanical properties of prequenching—quenching and partitioning (QQ&P) and traditional Q&P samples at different annealing temperatures (intercritical annealing temperatures). When the annealing temperature is 780 °C, the ferrite and retained austenite in QQ&P samples with lath and blocky morphologies. The lath retained austenite is mainly distributed along the lath ferrite. As the annealing temperature increases, the lath ferrite recrystallizes and gradually grows into the blocky (equiaxed) shape, leading to a decrease in the lath retained austenite content. When the annealing temperature increases to 870 °C, the ferrite content decreases significantly, and the retained austenite is mainly blocky and thin film, distributed at the boundaries of prior austenite grains and between martensite laths, respectively. Different from QQ&P samples, the ferrite and retained austenite in Q&P samples are mainly blocky when the annealing temperature is 780 °C or 810 °C. When the annealing temperature is increased to 870 °C, the microstructures of the Q&P sample are basically the same as that of the QQ&P sample. The 780 °C-QQ&P sample and the 810 °C-QQ&P sample have higher total elongation and product of strength and elongations (PSEs) than their counterpart Q&P samples due to the fact that lath ferrite and retained austenite are conducive to carbon diffusion and carbon homogenization in austenite grains, thereby improving the thermal stability and volume fraction of the retained austenite. In addition, the lath structures can release local stress concentration and delay the formation of voids and microcracks. The difference of mechanical properties between QQ&P samples and Q&P samples decreases with the increase in the annealing temperature. The results show that the low annealing temperature combined with prequenching—Q&P heat treatments can significantly improve the elongation and PSE of Q&P steel.

Structure formation during sheet steel production in a unit of continuous casting and deformation

The paper considers the main problem of improving quality of steel sheets for welded pipes obtained at plate rolling mills and the main reasons that reduce it. There is a possibility of improving quality of steel sheets using technological capabilities of the unit of combined continuous casting and deformation. Problem of determining stress state of metal in cyclic deformation zone is posed and solved. The results of obtaining 09G2S sheet steel in pilot unit of continuous casting and deformation are presented. Magnitude of stresses in the center of cyclic deformation during production of steel sheets was determined for welded pipes at the unit. The authors present results of experimental study of the process of steel sheets production at pilot unit of continuous casting and deformation. The article considers the results of microstructure analysis, microhardness determination and microanalysis of chemical composition of the studied steel in various zones of the strip cross section. Structural inhomogeneity development in ferritic-pearlitic steel is interpreted along the thickness of sheet under conditions of combined thermomechanical processing of metal solidified in the mold. Predominantly ferritic microstructure is generated in the central light interlayer of the strip compared to base metal, characterized by a sharp decrease in carbon concentration in it. There is a possibility of such structure genesis at the first stages of crystallization during forging of low-carbon steel, when the excess phase of high-temperature ferrite that solidifies first is located mainly in the strip center. The size of pearlite grains in the light interlayer covariates with the grain structure of pearlite in the strip base metal and does not form precipitates of lath morphology, which increase tendency of steel to brittle fracture.

Peculiarities of microstructure formation in the heat-affected zone of welded joints of pipes of x100 strength class

The features of the formation of the structure of coarse-grained heat-affected zone (CGHAZ) of a welded joint of pipe steel grade X100 are investigated depending on the cooling rate by simulating welding thermal cycles on thermo-mechanical Simulator &ndash; Gleeble 3800. As a result, a continuous cooling thermokinetic (CCT) diagram of austenite decomposition in the CGHAZ metal of pipe steel grade X100 was constructed, which makes it possible to study the structure and kinetics of phase transformations when the steel is exposed to the welding thermal cycle. It was found that the austenite decomposition is accompanied by bainitic and martensitic transformations in the range of cooling rates of 0.1&ndash;150 &ordm;C/s. With an increase in the cooling rate, the bainite structure changes its morphology from a globular and coarse acicular to a dispersed acicular with a pronounced packet organization and a lath morphology, islands of the martensite-austenitic (MA)-constituents, which have an acute-angled shape and are distributed at the boundaries between laths, packets and inside them, are gradually transformed into thin layers. The martensite structure appears at a cooling rate of more than 15 &ordm;C/s. Large blocks (crystals) of bainite with MA components can be observed in the structure despite an increase in the cooling rate. Hardness values change from 245&ndash;285 to 307&ndash;353 HV with the appearance and increase in the fraction of the martensite structure.