Microalloyed steel

About: Microalloyed steel is a research topic. Over the lifetime, 2183 publications have been published within this topic receiving 33586 citations.

Evaluation of corrosion resistance of microalloyed reinforcing steel

Abstract: The corrosion resistance of three microalloyed steels and two conventional reinforcing steels in concrete was evaluated. The microalloyed steels contain concentrations of chromium, copper, and phosphorus that, while low, are significantly higher than used in conventional reinforcing steel. Two of the microalloyed steels contain amounts of phosphorus that exceed the amounts allowed in ASTM specifications (ASTM A 615), while the other microalloyed steel has normal amounts of phosphorus. One of the conventional steels and the three microalloyed steels are heat treated by the Thermex process, which includes quenching and tempering of the steel immediately after rolling, while the other conventional steel is hot-rolled. The study was undertaken because earlier tests on similar steels indicated that the Thermex-treated, microalloyed steel corrodes at only one-half the rate of conventional reinforcing steel. The relative corrosion rate dropped to one-tenth if both steels were epoxy-coated. In the current study, the reinforcing steels were tested using two rapid evaluation tests, the corrosion potential and corrosion macrocell tests, and three bench-scale tests, the Southern Exposure, cracked beam, and ASTM G 109 tests. The corrosion potential, corrosion rate, and mat-to-mat resistance are used to evaluate the steel. Tension and bending tests were performed to evaluate the effect of the microalloying and heat treatment on the mechanical properties of the reinforcing steel. Results show that the corrosion potential of the five steels is approximately the same, indicating that they have a similar tendency to corrode. The results from the rapid macrocell test showed that the five steels had similar corrosion rates, with no improved behavior for the microalloyed steels. The microalloyed steel with regular phosphorus content (CRT) exhibited consistently lower corrosion losses than conventional steel in the bench-scale tests. Although CRT appears to be much more corrosion resistant than conventional steel in the G 109 tests (64% less total corrosion loss after 70 weeks), its overall performance does not show such an advantage. In the cracked beam test after 70 weeks, it had only 4% less corrosion loss than conventional steel, which indicates that in cracked concrete the two steels behave in a similar manner. In the Southern Exposure test, CRT steel had an 11% lower corrosion loss than conventional steel after the same period. This improved behavior is not enough to use the steel without an epoxy coating or to justify continued research on the steel as a superior epoxy-coated material. The mechanical properties of the microalloyed steels were similar to those of conventional steel, indicating that the increased phosphorus content did not affect the mechanical properties.

Microstructure and texture evolution during tensile deformation of symmetric/asymmetric-rolled low carbon microalloyed steel

Abstract: The deformation and fracture mechanisms of a low carbon microalloyed steel processed by asymmetric rolling (AsR) and symmetric rolling (SR) were compared by microstructural and texture evolutions during uniaxial tensile deformation. A realistic microstructure-based micromechanical modeling was involved as well. AsR provides more effective grain refinement and beneficial shear textures, leading to higher ductility and extraordinary strain hardening with improved yield and ultimate tensile stresses as well as promoting the occurrence of ductile fracture. This was verified and further explained by means of the different fracture modes during quasi-static uniaxial deformation, the preferred void nucleation sites and crack propagation behavior, and the change in the dislocation density based on the kernel average misorientation (KAM) distribution. The equivalent strain/stress partitioning during tensile deformation of AsR and SR specimens was modeled based on a two-dimensional (2D) representative volume element (RVE) approach. The trend of strain/stress partitioning in the ferrite matrix agrees well with the experimental results.

Ultrafine Grained Austenite in a Low Carbon Vanadium Microalloyed Steel

Abstract: Ultrafine austenite grains with average size of 2 μm were successfully obtained by combining thermo-mechanical control process followed by reheating in a vanadium microalloyed steel. The mixed microstructure transformed from pancaked austenite formed during controlled rolling has a higher density of high angle boundaries, compared to that transformed from equiaxial austenite. It contributes to increasing nucleation density of austenite grain during the reheating process. A certain volume fraction of undissolved nano-sized (Ti, V)C particles, which are formed during the controlled rolling process and/or the reheating process, effectively inhibit austenite grain growth and consequently refine austenite grain size significantly. The critical grain size of austenite calculated by Gladman model agrees well with the experimental result.

Influence of AlN Precipitation on Thermodynamic Parameters in C-Al-V-N Microalloyed Steels

Abstract: An estimation of the equilibrium compositions of the austenite and carbonitride phases, as well as the mole fraction of each phase in C-Al-V-N microalloyed steels at different austenitising temperatures was made by calculations based on experimental data. Further, a comparison of the mole fraction was made from two thermodynamic models due to Adrian and Rios, with and without considering aluminium in the steels. The results indicate that both models produce very similar results and can be used to calculate the equilibrium parameters and predict the solution temperature of carbonitrides and aluminium nitride in the range 800–1300°C for an alloy system contained up to three microalloying elements and aluminium. Both models predict that most of the carbon remains in solution at the calculated temperature. When AlN precipitation is included in the calculation, it is seen that the mole fraction of the carbonitrides fP and the atomic fraction of carbon in the interstitial lattice of the carbonitrides fC decreases, while the atomic fraction of nitrogen in the interstitial lattice of carbonitride fN, increases. The effect of aluminium on these equilibrium parameters depends on the chemical composition of the steel. Increasing the contents of Al, N, C, and V together in the experimental steels has a more significant influence on these equilibrium parameters than changing only the contents of Al; Al and V or Al and N.

Formation and analysis of stack cracks in a pipeline steel

Abstract: Extensive cracking of the type known as stack cracking was demonstrated in a cathodically charged X65 microalloyed pipeline steel containing a weldment. It is shown that the formation and propagation of rolling-plane cracks, which constitute the primary stages of the stack cracking, is due to local concentration of hydrogen gas pressure and a lowering of the cohesive strength of a number of interfaces by hydrogen. The characteristic S-shape of individual cracks which occurred during the linking up of cracks was attributed to stress interactions at crack tips and cleavage cracking normal to the rolling plane. An explanation of hydrogen embrittlement fracture is given in terms of electronic state modifications of the steel, including charge polarization.