Microalloyed Steel Research Articles

Hot deformation behavior and processing map of Nb-V-Ti micro-alloyed steel

Optimization of process parameters during plate rolling of micro-alloyed steel plays an important role in achieving the stipulated mechanical properties. The hot deformation behaviour of Nb-V-Timicro-alloyed steel was studied to determine safe region during plate rolling and to achieve high strength (>450 MPa) defect free plates. The hot flow behaviour of experimental steel was studied with a Gleeble-3500 thermo-mechanical simulator, at 850–1100°C, 1–100 s-1. The constitutive equations of the experimental steel was established using Arrhenius equations and material constants were evaluated at different strains. Model too predict the flow stress was developed by establishing its relation with strain, strain rate and deformation temperature using multivariable optimization technique based on parameter estimation methodology. The processing map based on Dynamic Materials Model (DMM) was constructed to evaluate the hot workability of experimental micro-alloyed steel and identify the safe region for hot deformation. The processing maps of the experimental steel show a region of instability at temperature < 900 °C and when strain rate is > 1 s−1 and this domain needs to be avoided during hot deformation.

Mechanical behaviour of vanadium microalloyed steel under control environment compression

Microalloyed vanadium steels are high strength low alloy steel. Although by precise addition of nitrogen in the alloy extract the vanadium from the solid solution and can form vanadium based carbonitrides. In current work prechosen composition of Vanadium-Nitrogen based micro alloy steel was austenitized at 1000 °C. A set of experimental steels are subjected to hot compression using Thermo-Mechanical Simulator (Gleeble 3800®) at 800 °C under the control environment and then cooled to room temperature within the simulator, at a cooling rate of 80 °C/s. Microstructural characterization was done using optical and Scanning microscope. Mechanical testing are done using Instron 1186 testing machine. The characterization study reveals the acicular type of transformation product. That indicates the formation of precipitate of vanadium carbonitrides (V(C,N)). The tensile test result of austenitized sample shows increased ultimate tensile strength due to strain induced precipitate.

Study of Features of Phase Precipitate Formation, Structural State, and Properties of Nb, V-Microalloyed Low Carbon Ferritic Steels

Contemporary methods of optical and transmission electron microscopy, and mechanical property tests are used to study features of phase precipitate formation, structural state, and the properties of lowcarbon V, Nb micro-alloyed steels. The effect of nano-sized carbide (carbonitride) V, Nb precipitates formed in austenite during FCC → BCC-phase transformation of steel on the structural state and implementation of different mechanisms is evaluated. Characteristics of similar Ti and Mo precipitates in micro-alloyed ferritic steels are compared.

Stress relaxation behaviour of hot deformed austenite in high strength micro-alloyed pipeline steel

The objective of the present study is to investigate the static restoration of CMn(V-Nb-Ti) pipeline steel utilising comparative study of the stress relaxation and micro-alloying precipitates behaviour. The hot compression tests in temperature range of 850°C to 1,000°C, strains of 0.04, 0.1, 0.25, 0.4 and strain rates of 0.1 and 0.5 s−1 have been carried out and the hot flow stress of austenite has been evaluated by stress relaxation technique at annealing time intervals of 0.05 to 1000 s. The results indicate that at the specific deformation temperature of 850°C, the strain induced precipitation starts with the beginning of annealing time which retards recrystallisation. At higher temperatures than 900°C and strains more than 0.25 and 0.4, first recrystallisation occurs and then the plateau appears. The electron backscattering diffraction (EBSD) image also confirms the partial recrystallisation in a sample quenched at the end of the plateau.

Stress relaxation behaviour of hot deformed austenite in high strength micro-alloyed pipeline steel

The objective of the present study is to investigate the static restoration of CMn(V-Nb-Ti) pipeline steel utilising comparative study of the stress relaxation and micro-alloying precipitates behaviour. The hot compression tests in temperature range of 850°C to 1,000°C, strains of 0.04, 0.1, 0.25, 0.4 and strain rates of 0.1 and 0.5 s−1 have been carried out and the hot flow stress of austenite has been evaluated by stress relaxation technique at annealing time intervals of 0.05 to 1000 s. The results indicate that at the specific deformation temperature of 850°C, the strain induced precipitation starts with the beginning of annealing time which retards recrystallisation. At higher temperatures than 900°C and strains more than 0.25 and 0.4, first recrystallisation occurs and then the plateau appears. The electron backscattering diffraction (EBSD) image also confirms the partial recrystallisation in a sample quenched at the end of the plateau.

A review of chamfer technology in continuous casting process

Chamfer technology, with funnel-shaped curved surface on the narrow side of the mold, is a novel technology that can effectively improve the quality of continuously cast products. This study reviews the available literature on the theoretical and applied research in chamfer technology to provide an in depth analysis of the employed approaches and the obtained results. According to theoretical research results, combined with the working conditions of slab caster, patented technologies and related equipment were developed. The research has broken the technology bottlenecks of the industrial application, while ensuring a long-life operation of the chamfer mold. In recent years, chamfer technology, which is used in slab casting processes, can prevent transverse corner cracks to form at the slab surface of micro-alloyed steel completely. Chamfer technology, which is used in thin slab casting processes, can reduce the occurrence of longitudinal surface crack of the slabs and straight edge seam defects of the rolled strip. In addition, chamfer technology, which is used in a continuous casting bloom, can reduce the risk of internal cracks and avoid the off-corner cracks occurring in as-cast bloom.

Modification of MnS inclusion by tellurium in 38MnVS6 micro-alloyed steel

In 38MnVS6 steel, the morphology of sulfide inclusion has a strong influence on the fatigue life and machinability of the steel. In most cases, the MnS inclusions show strip morphology after rolling, which significantly affects the steel quality. Usually, the MnS inclusion with a spherical morphology is the best morphology for the steel quality. In the present work, tellurium was applied to 38MnVS6 micro-alloyed steel to control the MnS inclusion. Trace tellurium was added into 38MnVS6 steel and the effect of Te on the morphology, composition, size and distribution of MnS inclusions were investigated. Experimental results show that with the increase of Te content, the equivalent diameter and the aspect ratio of inclusion decrease strikingly, and the number of inclusions with small aspect ratio increases. The inclusions are dissociated and spherized. The SEM-EDS analysis indicates that the trace Te mainly dissolves in MnS inclusion. Once the MnS is saturated with Te, MnTe starts to generate and wraps MnS. The critical Te/S value for the formation of MnTe in the 38MnV6 steel is determined to be approximately 0.075. With the increase of Te/S ratio, the aspect ratio of MnS inclusion decreases and gradually reaches a constant level. The Te/S value in the 38MnVS6 steel corresponding to the change of aspect ratio from decreasing to constant ranges from 0.096 to 0.255. This is most likely to be caused by the saturation of Te in the MnS inclusion. After adding Te in the steel, rod-like MnS inclusion is modified to small inclusion and the smaller the MnS inclusion, the lower the aspect ratio.

Crack propagation in 3PB specimen made from welded joint

Crack propagation in 3PB specimen has been investigated experimentally by using Digital Image Correlation (DIC) technique. For this purpose, specimens were taken from butt welded plates, made of P460NL1 micro-alloyed steel. Fatigue pre-crack of 3PB specimens was introduced in the weld metal. Specimens were then statically loaded and analyzed using digital image correlation software ARAMIS to assess strain fields and crack propagation paths. Bifurcation of cracks was noticed as the main characteristics of crack prop-agation in the analyzed case.

Optimization of Refining Slag Composition and the Deoxidation Practice of Liquid Steel in BOF–CAS–CC Process

The cleanliness level in the slabs of micro-alloy structure steel produced in the BOF–CAS–CC process was studied by optimizing the compositions of molten steel and refining slag in trial heats. When the slag composition was controlled in the range of CaO = 45–53 wt%, Al2O3 = 28–32 wt%, SiO2 = 6–11 wt% and MgO = 5–10 wt% in CAS, the steel composition should be in the range of [Als] = 0.025–0.035 wt% and a[O] = 0.8–1.5 ppm. After CAS treatment without feeding calcium, the results showed that the removal ratio of total oxygen content increased to 61% and the nitrogen content decreased from 34 to 13 ppm in the improved process. The residual micro-inclusions in slabs mainly consisted of SiO2–Al2O3–TiO2 and sulfides, and the particle size was predominately below 5 μm.

MnS Precipitation Behavior of High-Sulfur Microalloyed Steel Under Sub-rapid Solidification Process

A typical high-sulfur microalloyed steel was investigated by a sub-rapid solidification process for grain refinement of the as-cast microstructure. The size and distribution characteristics of the MnS precipitates were analyzed. The variations in the dendrite morphology and secondary dendrite arm spacing (SDAS) under different cooling rates have been studied, which strongly influence the precipitation behavior of MnS. The 3D-morphology of MnS precipitates was revealed by a novel saturated picric acid deep-etching method. Most MnS precipitates with a length smaller than 5 μm were columnar or equiaxed in the corresponding dendrite zones under sub-rapid solidification conditions at cooling rates of 261 to 2484 K/s. Furthermore, an area scan analysis of the precipitates showed the number of small MnS per square millimeter with lengths lower than 3 μm decrease from 200,537 to 110,067. The percentage of large MnS with a length over 5 μm increased from 2.6 to 6.2 pct as the solidification condition changed from sub-rapid to air cooling. In addition, the size of MnS precipitate was found to depend linearly on the SDAS.

Физико-химические свойства тройных металлических систем Fe-Si-Nb и Fe-Al-Nb

The Russian Federation has a sufficient number of promising deposits of niobium raw materials which can satisfy the niobium and tantalum demands of Russian metallurgical enterprises for many decades. Ferroalloy technologists are faced with the difficult tasks of developing from various types of ore raw materials not only effective processes for its processing but also new acceptable rational compositions of niobium-containing ferroalloys. The chemical composition of niobium ferroalloy should, on the one hand, correspond to the product obtained by benefication (concentrate) and, on the other hand, satisfy the requirements of steelmakers for ferroalloys intended for microalloying niobium steel. To develop rational compositions of new niobium-containing ferroalloys in this work the physicochemical characteristics (which include crystallization temperature and density) of alloys containing 10-50% Nb, 10-40% Si, and 5-30% Al were studied. Two-component Fe-Nb metal alloys have a rational crystallization onset temperature (&lt;1400 °С) only when the niobium content is not more than 10%. To achieve rational crystallization onset temperatures it is necessary to use complex alloys with silicon and aluminum. Studies have shown that a decrease in the crystallization onset temperature of complex niobium alloys occurs when the niobium content decreases with an increase in the concentration of silicon or aluminum. Three-component alloys Fe-Si-Nb and Fe-Al-Nb with a content of 15-20% Nb, 32-40 Si% or 12-30% Al belong to the category of low-melting ferroalloys. To achieve rational density values light metals such as silicon or aluminum must be introduced into a two-component system. The studied three-component alloys with a content of 25-40% Si or 15-30% Al have rational density values both from the point of view of their production and application to the processing of steel melt. The best physicochemical characteristics providing high service properties are possessed by complex niobium (15-20% Nb) FeNbSi alloys with 32-40% Si and FeNbAl with 15-30% Al which are recommended for widespread use in ladle microalloying of steels.

Converging criteria to characterize crack susceptibility in a micro-alloyed steel during continuous casting

The ductility drop and decrease in strength that lead to crack formation during continuous casting of steel is typically investigated by means of the hot ductility test. In this study, hot ductility tests are performed by using a thermo-mechanical Gleeble system to simulate the deformation of steels at high temperatures and low deformation rates similar to those during continuous casting. Thus, temperature was varied between 600 and 1000°C while strain rates covered a range from 0.001 to 0.1s−1. Tests are carried out to identify the temperature range at which the steel is susceptible to crack formation as well as the effect of strain rate. Characterization of fractured surfaces and phase transformation after thermo-mechanical tests are conducted in the SEM and Optical Microscope. The combination of these techniques makes possible to formulate cracking mechanisms during hot processing which show critical strain for failure at temperatures between 700 and 900°C based on the convergence of three different criteria: I) Reduction of area, II) True fracture strength-ductility and III) True total energy. This approach provides a better understanding of crack formation in steels at the high temperatures experienced during continuous casting. This information is key to productivity losses and avoid defect formation in the final cast products.

Effects of Fine Precipitates on Austenite Grain Refinement of Micro-alloyed Steel during Cyclic Heat Treatment

Effects of Fine Precipitates on Austenite Grain Refinement of Micro-alloyed Steel during Cyclic Heat Treatment

Flow stress characteristics and design of innovative 3-steps multiphase control thermomechanical processing to produce ultrafine grained bulk steels

In the present study, at first flow behavior of Nb–Ti microalloyed and interstitial-free (IF) steels was investigated to know the effects of processing parameters on their microstructural evolution. Then, innovative 3-steps multiphase control rolling schedules have been designed to yield submicron size uniform grains structure and successfully achieved ultrafine ferrite+martensite (0.69–0.78 μm) and ferritic structure (0.83–0.88 μm), respectively, in microalloyed and IF steels. The good combination of yield strength and ductility was achieved for the microalloyed (924 MPa, 13.6% elongation) and IF steel (621 MPa, 19.4% elongation) after rolling as per the designed 3-steps multiphase control deformation schedules. Deformation induced ferrite transformation followed by continuous dynamic recrystallization of the dynamically transformed ferrite is found to be the key mechanism for the formation of the ultrafine grained structure. Due to application of high amount of strains specifically within α+γ phase regime, the α-phase subdivided into several subgrains. These α-subgrains are strongly pinned by the γ/α grain boundaries and thereby restrict the dynamic recovery of the ferrite through reknitting and unravelling subgrain boundaries. On the application of further straining, the misorientation angle between these subgrain boundaries increases continuously through accumulation of the dislocations and finally, ultrafine ferrite grain structure is developed through continuous dynamic recrystallization.

Mechanism Understanding of the Role of Rare Earth Inclusions in the Initial Marine Corrosion Process of Microalloyed Steels.

In this study, the corrosion behavior of rare earth (RE) microalloyed steels was first evaluated through potentiodynamic polarization tests and corrosion weight loss experiments, and then the corrosion morphologies were observed by scanning electron microscope (SEM). After immersion in a NaCl solution, the sulfides (or oxygen sulfides) dissolved preferentially, followed by corrosion at the boundary between the Fe matrix and oxides. Afterwards, the inclusions fell off as a whole, which promoted pitting nucleation. The first principle modeling demonstrated that the work functions of various kinds of inclusions descended in the following order: La2Zr2O7 > LaAlO3 > (La2O3 ≈ Fe ≈ La2O2S) > La2S3, which provided a theoretical explanation to the dissolution behaviors of inclusions. That is, inclusions containing sulfur tend to dissolve preferentially, whereas the oxides do not dissolve easily. Subsequently, the surface current distributions were detected by the scanning vibrating electrode technique (SVET), which provided more microscopic insight into the role of inclusions in the corrosion propagation. Results showed that the active sites of pitting nucleation accelerated the transverse propagation of corrosion. Finally, local corrosion spread to the whole surface as uniform corrosion.

A Novel Cyclic-Quenching-ART for Stabilizing Austenite in Nb–Mo Micro-Alloyed Medium-Mn Steel

In the context of obtaining an excellent elongation and tensile-strength combination in the third generation of advanced high strength steel, we emphasized the practical significance of adjusting the retained austenite fraction and stability in medium-Mn steel to obtain better mechanical properties. A novel cyclic quenching and austenite reverse transformation (CQ-ART) was used to obtain a large retained austenite content in Fe-0.25C-3.98Mn-1.22Al-0.20Si-0.19Mo-0.03Nb (wt.%) Nb–Mo micro-alloyed medium-Mn steel. The results show that after twice cyclic quenching and ART, the alloy exhibited optimum comprehensive properties, characterized by an ultimate tensile strength of 838 MPa, a total elongation of 90.8%, a product of strength and elongation of 76.1 GPa%, and the volume fraction of austenite of approximately 62 vol.%. The stability of retained austenite was significantly improved with the increasing of the number of cyclic quenching. Moreover, the effects of CQ-ART on the microstructure evolution, mechanical properties, C/Mn partitioning behavior, and austenite stability were investigated. Further, the strengthening effect of microalloying elements Nb–Mo was also discussed.

Effect of Strain-Induced Precipitation on the Austenite Non-recrystallization (Tnr) Behavior of a High Niobium Microalloyed Steel

The non-recrystallization temperature (Tnr) of high niobium microalloyed steel was determined from both multihit and double-hit compression tests obtained under plane strain condition. The Tnr was determined to be in the range of 985 °C to 1010 °C for multihit conditions. The double-hit tests carried out at an interpass time of 5 seconds gave very low Tnr in the range of 860 °C to 900 °C. In order to understand this, double-hit experiments were carried out for different interpass times (2, 15, 100 seconds) at three different temperatures (950 °C, 1050 °C, and 1150 °C). The negative softening behavior was observed at 950 °C for higher interpass times of 15 and 100 seconds. This implies that the Tnr of this steel is well above 950 °C. This was due to the high amount of strain-induced precipitation of niobium carbonitrides at higher interpass times as revealed from TEM investigation. The precipitate size evolution considering the Dutta and Sellars nucleation condition in TC-PRISMA agrees well with the experimentally observed precipitate sizes. However, the evolution of Zener pinning forces considering TC-PRISMA nucleation condition is similar to both reported and experimentally determined values. The evolution of tensile properties also correlates well with the observed austenite recrystallization softening behavior. Therefore, a minimum interpass time of 15 seconds is required during double-hit compression tests to effectively precipitate Nb(C, N) and delay the static recrystallization softening behavior of this steel. This lead to the determination of comparable Tnr values between double-hit and multihit methods.

Precipitation behavior and strengthening mechanism of Ti micro-alloyed steel via CSP process

Two types of micro-alloyed steel with different carbon content and coiling temperature were produced by compact strip production (CSP) process. The microstructure characteristics and precipitation behavior of the Ti particles during casting, soaking, rolling, coiling and cooling stages were discussed to reveal the strengthening mechanism. The samples were composed of ferrite bands and discontinuous pearlite/bainite at the grain boundaries. The average grain sizes of the sample with low carbon content and high coiling temperature was confirmed as 5.6 μm by quantitative metallography method, while that with high carbon content and low coiling temperature was confirmed as 5.2 μm. Moreover, the TiN precipitated before soaking stage was large in size because of the high undercooling temperature, which had nearly no strengthening effect. The strength was mainly attributed by cooling and coiling stages with the TiC precipitated at the inter-phase or within the ferrite. Taking Q345B steel with yield strength of 505 MPa produced by CSP as contrastive sample, the strength increments of fine-grain/solution/dislocation/precipitation strengthening for the low carbon sample were 65, 0, 22 and 113 MPa, while those for the high carbon sample were 74, 33, 28 and 57 MPa. Fine-grain and precipitation strengthening were the main strengthening mechanisms for the high strength CSP plates.

Significance of Finish Cooling Temperature to Microstructure and Property Relationship of Low-Carbon V-N-Cr Microalloyed High-Strength Steel

Herein, three final cooling temperatures were employed to achieve high strength and toughness communication in the V-N-Cr microalloyed steels. This study shows that V-N-Cr microalloyed steel primarily consisted of ferrite, granular bainite, acicular ferrite, and a small quantity of M/A phase at the final cooling temperature of 600 °C, while the polygonal ferrite, lamellar bainite, granular bainite, and a small fraction of acicular ferrite were obtained at 450 °C. The width of lamellar bainite was ~ 200-400 nm, and acicular ferrite was composed of fine non-parallel ferrite platelet structures with high dislocations densities. As the final cooling temperature decreased further, the microstructure included granular bainite and lath bainite at 350 °C. The nanoscale precipitates of V-N-Cr microalloyed steel were divided into two types, large-scale precipitates near the grain boundary that precipitated at high temperatures and small precipitates of diameter 5-10 nm at low temperatures, which significantly contributed toward strength. The hot rolled low-carbon V-N-Cr microalloy steel exhibited optimal mechanical properties at the final rolling temperature of 830 °C, as well the final cooling temperature was 450 °C. The highest yield strength and tensile strength of V-N-Cr microalloyed steel were 835 MPa and 989 MPa, respectively. The outstanding impact toughness of 161 J tested at − 60 °C was received. The transformation strengthening and precipitation hardening played a crucial role in affecting the thermo-mechanical properties.

X-Ray Radiography of Micro-alloyed Steel Joined by Submerged Arc Welding

In this study, micro-alloyed steels were welded by using submerged arc welding method. Different welding current values ​​in experimental studies were carried out according to Table 3. After welding process, the radiographic tests were examined by using X-Ray. The experimental results indicated that microalloyed steel could be joined by using the submerged arc welding technique with sufficient strength. The results of the radiographic tests indicated that with increasing welding current, the amount of deep penetration increased in all samples.