Overview of HSS Steel Grades Development and Study of Reheating Condition Effects on Austenite Grain Size Changes

This review paper concerns the development of the chemical compositions and controlled processes of rolling and cooling steels to increase their mechanical properties and reduce weight and production costs. The paper analyzes the basic differences among high-strength steel (HSS), advanced high-strength steel (AHSS) and ultra-high-strength steel (UHSS) depending on differences in their final microstructural components, chemical composition, alloying elements and strengthening contributions to determine strength and mechanical properties. HSS is characterized by a final single-phase structure with reduced perlite content, while AHSS has a final structure of two-phase to multiphase. UHSS is characterized by a single-phase or multiphase structure. The yield strength of the steels have the following value intervals: HSS, 180–550 MPa; AHSS, 260–900 MPa; UHSS, 600–960 MPa. In addition to strength properties, the ductility of these steel grades is also an important parameter. AHSS steel has the best ductility, followed by HSS and UHSS. Within the HSS steel group, high-strength low-alloy (HSLA) steel represents a special subgroup characterized by the use of microalloying elements for special strength and plastic properties. An important parameter determining the strength properties of these steels is the grain-size diameter of the final structure, which depends on the processing conditions of the previous austenitic structure. The influence of reheating temperatures (T_Reh) and the holding time at the reheating temperature (t_Reh) of C–Mn–Nb–V HSLA steel was investigated in detail. Mathematical equations describing changes in the diameter of austenite grain size (d_γ), depending on reheating temperature and holding time, were derived by the authors. The coordinates of the point where normal grain growth turned abnormal was determined. These coordinates for testing steel are the reheating conditions T_Reh = 1060 °C, t_Reh = 1800 s at the diameter of austenite grain size d_γ = 100 μm.     

Mechanical Behaviors of Microalloyed TRIP-Assisted Annealed Martensitic Steels under Hydrogen Charging

Transformation Induced Plasticity (TRIP)-assisted annealed martensitic (TAM) steel sheets with various microalloying additions such as niobium, vanadium, or titanium were prepared on laboratory scale and subjected to a double-quenching and austempering heat treatment cycle. Slow strain rate tensile (SSRT) was tested on the investigated TAM steels with and without hydrogen charging to reveal their tensile behaviors and hydrogen induced embrittlement effects. Microstructure observations by scanning electron microscope (SEM) are composed of a principal annealed martensitic matrix and 11.0–13.0% volume fraction of retained austenite, depending on the type of microalloying addition in the different steels. SSRT results show that these TRIP-assisted annealed martensitic steels under air media conditions combine high tensile strength (>1000 MPa) and good ductility (~25%), while under hydrogen charging condition, both tensile strength and ductility decrease where tensile strength ranges between 680 and 760 MPa, down from 1000–1100 MPa, and ductility loss ratio is between 78.8% and 91.1%, along with a total elongation of less than 5%. Hydrogen charged into steel matrix leads to the appearance of cleavage fractures, implying the occurrence of hydrogen induced embrittlement effect in TAM steels. Thermal hydrogen desorption results show that there are double-peak hydrogen desorption temperature ranges for these microalloyed steels, where the first peak corresponds to a high-density dislocation trapping effect, and the second peak corresponds to a hydrogen trapping effect exerted by microalloying precipitates. Thermal desorption analysis (TDS) in combination with SSRT results demonstrate that microalloying precipitates act as irreversible traps to fix hydrogen and, thus, retard diffusive hydrogen motion towards defects, such as grain boundaries and dislocations in microstructure matrix, and eventually reduce the hydrogen induced embrittlement tendency.     

Influence of 40% Cold Working and Annealing on Precipitation in AISI 316L Austenitic Stainless Steel

Intergranular corrosion is one of the most important processes affecting the behaviour of austenitic stainless steels. Factors such as steel chemical composition, the degree of prior deformation and the exposure temperature affect the degree of sensitisation. AISI 316L (0% CW) steel was annealed at 650 °C for 5, 10, 30, 100, 300 and 1000 h to analyse the influence of isothermal annealing on the precipitation of secondary phases. AISI 316L steel after 40% cold working and subsequent annealing at 650 °C for 1, 1.5, 2, 5 and 10 h was investigated. Time–temperature sensitisation (TTS) diagrams were created based on corrosion test (ASTM A 262, practice A) results. In the case of AISI 316L (0% CW), M₂₃C₆, chi and sigma phases precipitated at grain boundaries, and the Laves phase was mainly inside of the grains. In the case of AISI 316L (40% CW), sigma, chi, Laves and M₂₃C₆ were identified and precipitated mainly along the grain boundaries as well as on the shear bands within different annealing times. It was confirmed that the increase in the annealing time caused an increase in the amounts of secondary phases. Secondary phases in the equilibrium state were calculated using Thermo-Calc software.     

Effect of Oxygen Content on Microstructure and Tensile Properties of a 22Cr-5Al ODS Steel

The high tensile strength and irradiation resistance of oxide dispersion strengthened (ODS) ferritic steels is attributed to the ultrafine and dispersed oxides within the matrix. The high content of oxygen and yttrium is critical for the formation of dense Y-rich oxides. However, only few studies have reported the effect of oxygen content on the microstructure and mechanical properties of ODS steels. Herein, we employed gas atomization reactive synthesis to prepare pre-alloy powders and then hot isostatic pressing (HIP) to consolidate two 22Cr-5Al ODS steels with different oxygen content. Our results showed Y-rich precipitates at and near grain boundaries of the as-HIPed alloys. Moreover, with the oxygen content increasing from 0.04 to 0.16 wt%, more precipitates precipitated in the as-HIPed alloy, and the ultimate tensile strength of the alloy was improved. However, increasing the oxygen content to 0.16 wt% led to formation of stripe and chain precipitates at and near grain boundaries, which caused a partial intergranular fracture of the as-HIPed alloy.     

Gas Atomization of Duplex Stainless Steel Powder for Laser Powder Bed Fusion

Duplex stainless steel powders for laser additive manufacturing have not been developed extensively. In this study, the melts of a super duplex stainless steel X2CrNiMoCuWN25-7-4 (AISI F55, 1.4501) were atomized with different process gases (Ar or N₂) at different atomization gas temperatures. The process gas N₂ in the melting chamber leads to a higher nitrogen dissolution in the steel and a higher nitrogen content of the atomized powders. The argon-atomized powders have more gas porosity inside the particles than the nitrogen-atomized powders. In addition, the higher the atomization gas temperature, the finer the powder particles. The duplex stainless steel powders showed good processability during PBF-LB/M (Laser powder bed fusion). The gas entrapment in the powder particles, regardless of the gas chemistry and the gas content, appears to have a negligible effect on the porosity of the as-built parts.     

Effect of Phosphorus Content on Magnetic and Mechanical Properties of Non-Oriented Electrical Steel

The effect of target phosphorus (P) content on the precipitates, microstructure, texture, magnetic properties, and mechanical properties of low-carbon (C) and low-silicon (Si) non-oriented electrical steel (NOES) was investigated and the influence mechanism was clarified. The results indicate that the precipitates in the steels are mainly aluminum (Al)-manganese (Mn)-Si-bearing complex nitrides ((Al,Si,Mn)_xN_y) and P-bearing complex nitrides ((Al,Si,Mn)_xN_y-P). Increasing target phosphorus content in the steels decreases (Al,Si,Mn)_xN_y, and increases (Al,Si,Mn)_xN_y-P. The number density of the precipitates is the lowest, and the average size of the precipitates and grain size of the finished steel is the largest in the samples with target P content at the 0.14% level (0.14%P-targeted). The average grain size and microstructure homogeneity of the steels are influenced by the addition of phosphorus. The content of the {111}<112> component decreases, and the favorable texture increases after phosphorus is added to the steel. The magnetic induction of the steel is improved. Grain refinement and microstructure inhomogeneity lead to an iron loss increase after target phosphorus content increases in the steel. The best magnetic induction B₅₀ is 1.765 T in the 0.14%P-targeted samples. The tensile strength and yield strength are improved owing to solid solution strengthening and the grain refinement effect of phosphorus added to the steels.     

Effect of Delamination and Grain Refinement on Fracture Energy of Ultrafine-Grained Steel Determined Using an Instrumented Charpy Impact Test

Improving the balance of strength and toughness in structural materials is an ongoing challenge. Delamination and grain refinement are some of the methods used to do this. In this paper, two different steels, 0.15% C–0.3% Si–1.5% Mn–Fe and 0.4% C–2% Si–1% Cr–1% Mo–Fe (mass %), were prepared. Two steel bars with an ultrafine elongated grain (UFEG) structure were fabricated via multipass warm caliber rolling. The UFEG steels were characterized by a strong <110>//rolling-direction fiber texture. The transverse grain size, d_t, was 1.0 µm for the low-carbon steel and 0.26 µm for the medium-carbon steel. For comparison, conventional heat-treated steels were also fabricated. An instrumented Charpy impact test was performed, and the impact load (P) and deflection (u) during the test were recorded. The P–u relations at the test temperature at which delamination fracture occurred exhibited a unique curve. Delamination effectively enhances the low-temperature toughness, and this was characterized by a plateau region of constant load in the P–u curve. Assuming no delamination, two routes in the P–u curves, the ductile route and the brittle route, were proposed. The results showed that the proposed methods can be predicted by an energy curve for ultrafine grained steels. Delamination is a more effective method of enhancing toughness for ultra-high-strength steels.     

Performance Comparison of Al–Ti Master Alloys with Different Microstructures in Grain Refinement of Commercial Purity Aluminum

Three types of Al–5Ti master alloys were synthesized by a method of thermal explosion reaction in pure molten aluminum. Performance comparison of Al–5Ti master alloy in grain refinement of commercial purity Al with different additions (0.6%, 1.0%, 1.6%, 2.0%, and 3.0%) and holding time (10, 30, 60 and 120 min) were investigated. The results show that Al–5Ti master alloy with blocky TiAl₃ particles clearly has better refining efficiency than the master alloy with mixed TiAl₃ particles and the master alloy with needle-like TiAl₃ particles. The structures of master alloys, differing by sizes, morphologies and quantities of TiAl₃ crystals, were found to affect the pattern of the grain refining properties with the holding time. The grain refinement effect was revealed to reduce markedly for master alloys with needle–like TiAl₃ crystals and to show the further significant improvement at a longer holding time for the master alloy containing both larger needle–like and blocky TiAl₃ particles. For the master alloy with finer blocky particles, the grain refining effect did not obviously decrease during the whole studied range of the holding time.     

Effects of Ti and Cu Addition on Inclusion Modification and Corrosion Behavior in Simulated Coarse-Grained Heat-Affected Zone of Low-Alloy Steels

In this paper, the effects of Ti and Cu addition on inclusion modification and corrosion behavior in the simulated coarse-grained heat-affected zone (CGHAZ) of low-alloy steels were investigated by using in-situ scanning vibration electrode technique (SVET), scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDS), and electrochemical workstation. The results demonstrated that the complex inclusions formed in Cu-bearing steel were (Ti, Al, Mn)-O_x-MnS, which was similar to that in base steel. Hence, localized corrosion was initiated by the dissolution of MnS. However, the main inclusions in Ti-bearing steels were modified into TiN-Al₂O₃/TiN, and the localized corrosion was initiated by the dissolution of high deformation region at inclusion/matrix interface. With increased interface density of inclusions in steels, the corrosion rate increased in the following order: Base steel ≈ Cu-bearing steel < Ti-bearing steel. Owing to the existence of Cu-enriched rust layer, the Cu-bearing steel shows a similar corrosion resistance with base steel.     

Ductile-to-Brittle Transition and Brittle Fracture Stress of Ultrafine-Grained Low-Carbon Steel

Ductile-to-brittle transition (DBT) temperature and brittle fracture stress, σ_F, are important toughness criteria for structural materials. In this paper, low-carbon steels with an ultrafine elongated grain (UFEG) structure (transverse grain size 1.2 μm) and with two ferrite (α)-pearlite structure with grain sizes 10 µm and 18 µm were prepared. The UFEG steel was fabricated using multipass warm biaxial rolling. The tensile tests with a cylindrical specimen and three-point bending tests with a single-edge-notched specimen were performed at −196 °C. The local stress near the notch was quantitatively calculated via finite element analysis (FEA). The σ_F for each sample was quantified based on the experimental results and FEA. The relationship between σ_F and d_α in the wide range of 1.0 μm to 138 μm was plotted, including data from past literature. Finally, the conditions of grain size and temperature that cause DBT fracture in low-carbon steel were shown via the stress−d^−1/2 map. The results quantitatively showed the superiority of α grain size for brittle fracture.     

Strengthening Effect of Nb on Ferrite Grain Boundary in X70 Pipeline Steel

Understanding the strengthening effect of niobium on ferrite grain boundaries from the perspective of valence electron structures will help to use niobium and other microalloying elements more effectively to improve the performance of steel materials. In this paper, the effect of niobium element on ferrite grain boundary strengthening is studied based on microstructure analysis at the nanometer scale. The enrichment of niobium in pipeline steel at ferrite boundary was observed by a three-dimensional atomic probe test. Segregation of Nb is observed in the ferrite grain boundaries of X70 steel, and its maximum concentration is 0.294–0.466 at.%. The charges in the occupancy of the Fe 3d state in grain and grain boundary were 7.23 and 7.37, respectively, based on quantitative analysis of electron energy loss spectra (EELS). The first-principle calculation suggests that the charges in the occupancy of 3d state for grain boundary iron are 6.57 and 6.68, respectively, before and after the Nb doping (with an increase of 1.67%), which reveals a similar trend to that of the EELS results. Through Nb alloying, the 3d valence electronic density of the state of Fe in grain boundary moves to a lower energy, which can reduce the total energy of the system and make the grain boundary more stable. Meanwhile, the charges in the occupancy of the 3d state for Fe in the grain boundary increases, providing more electrons for grain boundary bonding. These improve the strength and toughness of the material. This work provides a fundamental understanding for pipeline steel strengthening by element alloying.     

Effect of Yttrium on the Microstructure and Mechanical Properties of PH13-8Mo Stainless Steels Produced by Selective Laser Melting

In the present work, PH13-8Mo stainless steel parts without yttrium and with yttrium (Y) were manufactured by selective laser melting (SLM). The microstructure, phase composition and grain orientation of the stainless steels parts with Y and without Y were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The characterization results revealed that the addition of Y clearly refined the grain size of the PH13-8Mo steel formed part, resulting in more equiaxed massive grains and in a less anisotropic microstructure. PH13-8Mo stainless steel formed parts were mainly composed of martensite and retained austenite. The addition of Y could significantly increase the content of retained austenite and also generate nano-sized precipitates containing Y. The mechanical test results showed that both strength and toughness of the shaped parts containing Y were improved synergistically. The yield strength reached 1443 MPa, the elongation was 12.2%, and the room temperature impact energy reached 124.25 J/cm². The strengthening and toughening by Y of the formed parts were mainly attributed to grain refinement, higher volume fraction of the retained austenite and the formation of nano-sized precipitates containing Y.     

Synergistic Effect of La and TiB2 Particles on Grain Refinement in Aluminum Alloy

Synergistic effect of TiB₂ (in form of Al-5Ti-1B) and La on grain refining results in Al-2Cu alloy was investigated. α-Al grains are significantly refined by Al-5Ti-1B. When trace La is added to the melt, further refinement is exhibited. Average grain size and nucleation undercooling of α-Al reduce first and then almost remain unchanged with La addition. Satisfactory grain refining result achieves when La addition level reaches 600 ppm. When more than 600 ppm La is added to the melt, La-rich particles form and the effect of solute La left in matrix on the microstructure almost no longer changes. Theoretical calculation results demonstrate that solute La segregates to Al melt/TiB₂ particles interface along with Ti and Cu prior to α-Al nucleation and the synergistic effect of La and TiB₂ particles on grain refinement mainly attributes to the enhancement in the potency of TiB₂ particles to heterogeneously nucleate α-Al by trace La addition.     

Effect of Grain Size on the Corrosion Behavior of Fe-3wt.%Si-1wt.%Al Electrical Steels in Pure Water Saturated with CO2

The corrosion behavior of two silicon steels with the same chemical composition but different grains sizes (i.e., average grain area of 115.6 and 4265.9 µm²) was investigated by metallographic microscope, gravimetric, electrochemical and surface analysis techniques. The gravimetric and electrochemical results showed that the corrosion rate increased with decreasing the grain size. The scanning electron microscopy/energy dispersive x-ray spectroscopy and X-ray photoelectron spectroscopyanalyses revealed formation of a more homogeneous and compact corrosion product layer on the coarse-grained steel compared to fine-grained material. The Volta potential analysis, carried out on both steels, revealed formation of micro-galvanic sites at the grain boundaries and triple junctions. The results indicated that the decrease in corrosion resistance in the fine-grained steel could be attributed to the higher density of grain boundaries (e.g., a higher number of active sites and defects) brought by the refinement. The higher density of active sites at grain boundaries promote the metal dissolution of the and decreased the stability of the corrosion product layerformed on the metal surface.     

Effect of Cr on Aqueous and Atmospheric Corrosion of Automotive Carbon Steel

This study investigated the effect of Cr alloying element on the corrosion properties of automotive carbon steel (0.1C, 0.5Si, 2.5Mn, Fe Bal., composition given in wt.%) in aqueous and atmospheric conditions using electrochemical measurement and cyclic corrosion tests. Three steels with 0, 0.3, and 0.5 wt.% Cr were studied by electrochemical impedance spectroscopy. Polarization resistance (R_p) of 0.3 Cr and 0.5 Cr steels was higher than that of 0 Cr steel, and the R_p also increased as the Cr content increased. Therefore, Cr increases the corrosion resistance of automotive carbon steel immersed in a chloride ion (Cl⁻)-containing aqueous solution. In the cyclic corrosion test results, Cl⁻ was concentrated at the metal/rust interface in all of the steels regardless of Cr content. The Cl⁻ was uniformly concentrated and distributed on the 0 Cr steel, but locally and non-uniformly concentrated on the Cr-added steels. The inner rust layer consisted of β-FeOOH containing Cl⁻ and Cr-goethite, while the outer rust layer was composed of amorphous iron oxyhydroxide mixed with various types of rust. FeCl₂ and CrCl₃ are formed from the Cl⁻ nest developed in the early stage, and the pitting at CrCl₃-formed regions are locally accelerated because Cr is strongly hydrolyzed to a very low pH.     

Effect of V Content and Heat Input on HAZ Softening of Deep-Sea Pipeline Steel

In this paper, the welding thermal cycle process of deep-sea pipeline steel was investigated by welding thermal simulation. The microstructure evolution, crystallology and second-phase precipitation behavior of the soft zone of the heat-affected zone (HAZ) were characterized and analyzed by combining scanning electron microscopy, electron back-scattered diffraction, transmission electron microscopy and hardness testing. The results show that HAZ softening appeared in the fine-grained zone with a peak temperature of 900–1000 °C for deep-sea pipeline steel, the base metal microstructure of which was the polygonal ferrite and acicular ferrite. Using V microalloying and low welding heat input could effectively decrease the softening of the HAZ fine-grained region, which was achieved by reducing the effective grain size, increasing the proportion of the dislocation substructures, and precipitating the nanoscale second-phase particles.     

Evolution of Precipitated Phases during Creep of G115/Sanicro25 Dissimilar Steel Welded Joints

This paper studies the evolution of the microstructure and microhardness in the G115 side of the G115/Sanicro25 dissimilar steel welded joint during the creep process. The joints were subjected to creep tests at 675 °C, 140 MPa, 120 MPa and 100 MPa. A scanning electron microscope equipped with an electron backscattering diffraction camera was used to observe the microstructure of the cross-section. The fracture position of the joint and the relationship between the cavity and the second phase were analyzed. The microstructure morphology of the fracture, the base metal and the thread end was compared and the composition and size of the Laves phase were statistically analyzed. The results show that the fracture locations are all located in the fine-grain heat-affected zone (FGHAZ) zone, and the microstructure near the fracture is tempered martensite. There are two kinds of cavity in the fracture section. Small cavities sprout adjacent to the Laves phase; while large cavities occupy the entire prior austenite grain, there are more precipitated phases around the cavities. The Laves phase nucleates at the boundary of the M₂₃C₆ carbide and gradually grows up by merging the M₂₃C₆ carbide. Creep accelerates the coarsening rate of the Laves phase; aging increases the content of W element in the Laves phase.     

The Optimization of Mechanical Alloying Conditions of Powder for the Preparation of a Fe-10Al-4Cr-4Y2O3 ODS Nanocomposite

Mechanical alloying (MA) of powders represents the first processing step in the production of oxide dispersion-strengthened (ODS) alloys. MA is a time and energy-consuming process also in the production of Fe-10Al-4Cr-4Y₂O₃ creep and oxidation-resistant ODS nanocomposite, denoted as the FeAlOY, and it deserves to be optimized. MA is performed at two different temperatures at different times. The powder after MA, as well as the microstructure and high-temperature strength of the final FeAlOY, are characterized and the optimal MA conditions are evaluated. The obtained results show that the size distribution of the powder particles, as well as the dissolution and homogenization of the Y₂O₃, becomes saturated quite soon, while the homogenization of the metallic components, such as Al and Cr, takes significantly more time. The high-temperature tensile tests and grain microstructures of the secondary recrystallized FeAlOY, however, indicate that the homogenization of the metallic components during MA does not influence the quality of the FeAlOY, as the matrix of the FeAlOY is sufficiently homogenized during recrystallization. Thus, the conditions of MA correspond to sufficient dissolution and homogenization of Y₂O₃ and can be considered the optimal ones.     

Effect of Material Anisotropy on the Mechanical Response of Automotive Steel under High Strain Rates

A constitutive model for automobile steel with high elongation needs to be established to predict the dynamic deformation behavior under hydroforming applications. In order to clarify the confusing discrepancy in the essential parameters of the classical Cowper-Symonds (C-S) model, a series of automobile structural steels have been employed to investigate the strain rate response by conducting tensile dynamic deformation. Metallographic microscopy and orientation distribution functions were used to characterize the microstructure and texture components of the steels. The microstructure observation discloses that the matrix of all steels is mainly of ferrite and the texture constituent provides a framework for steel to withstand external deformation. The C-S model can be applied to simulate the dynamic deformation with satisfied expectations. It is concluded that the essential parameters D and p in the model show a specific relationship with the steel grade, and the parameter D is proportional to the steel grade and related to material anisotropy, while the parameter p is inversely proportional to the steel grade and has close links with the grain boundary characteristics.     

Active Crack Obstruction Mechanisms in Crofer® 22H at 650 °C

Increased cyclic loading of components and materials in future thermal energy conversion systems necessitates novel materials of increased fatigue resistance. The widely used 9–12% Cr steels were developed for high creep strength and thus base load application at temperatures below 620 °C. At higher temperature, these materials present unstable grain structure, prone to polygonization under thermomechanical fatigue loading and limited resistance to steam oxidation. This seminal study compares thermomechanical fatigue resistance and long crack propagation of the advanced ferritic-martensitic steel grade 92 and Crofer^® 22H, a fully ferritic, high chromium (22 wt. %) stainless steel, strengthened by Laves phase precipitation. Crofer^® 22H features increased resistance to fatigue and steam oxidation resistance up to 650 °C. Both thermomechanical fatigue (crack initiation) and residual (crack propagation) lifetime of Crofer^® 22H exceeded that of grade 92. The main mechanisms for improved performance of Crofer^® 22H were increased stability of grain structure and “dynamic precipitation strengthening” (DPS). DPS, i.e., thermomechanically triggered precipitation of Laves phase particles and crack deflection at Laves phase-covered sub-grain boundaries, formed in front of crack tips, actively obstructed crack propagation in Crofer^® 22H. In addition, it is hypothesized that local strengthening may occur near the crack tip because of grain refinement, which in turn may be impacted by testing frequency.