Fubin Liu

Mechanical Properties of Nickel Free High Nitrogen Austenitic Stainless Steels

Abstract The nickel free Fe-Cr-Mn-Mo-N series high nitrogen austenitic stainless steels have been developed. The mechanical properties of high nitrogen austenitic stainless steels with 0.82% (A1) and 0.96% (A2) nitrogen content are investigated. The tensile deformation of A1 and A2 steels with different crosshead speed is performed at room temperature(RT), and their flow behavior is modeled. The tensile tests of A1 steel is carried out with the temperature from −196°C to 500°C. The A1 and A2 steels exhibit high strength and good ductility at RT. With increasing the crosshead speed, the tensile strength and elongation of A1 and A2 steel decrease, but their yield strength increases. The flow behavior of A1 and A2 steels is well described by the modified Ludwik equation. The flow parameters n 1 decrease with increasing the crosshead speed, which implies that the strain hardening of high nitrogen steels is more active under low crosshead speed. The strength factor K 1 true yield stress exp( K 2 ) and ɛ L increases, but the flow parameter - n 2 decreases with increasing nitrogen in the steel investigated. The ductility is improved with increasing test temperature in the range of −196°C to 300 °C. Above 300°C, the elongation of A1 steel decreases. Because transgranular cracking along {111} planes occurs, the elongation of A1 steel at −196°C decreases down to only about 5%.

Mathematical Modeling of ESR Process for Hollow Ingot with Current Supplying Mould

With increasing demand for large cylindrical forgings, a new technology—electroslag remelting (ESR) for direct manufacture of hollow ingots rather than solid ingots has been developed. The main features of the process include a T-shaped current supplying mould (CSM), double power supply, an ingot withdrawing system, a metal level automatic control system based on a level sensor using the electromagnetic eddy current method, and the exchange of a consumable multi-electrode. ANSYS software was used to calculate the fluid flow and heat transfer in the slag bath and metal pool of this ESR hollow ingot process with its T-shaped CSM. The mathematical model was verified by measuring the geometry of the liquid metal pool as observed in the macrostructure of ϕ650 mm (external diameter)/ϕ450 mm (internal diameter) hollow ingots by sulphur print method: the observed shape and depth of the slag bath were consistent with the simulated results. Simulation of the ESR process can improve understanding of the process and allow better operating parameters to be selected.

Production of High Quality Die Steels from Large ESR Slab Ingots

With the rapid development of manufacture industry in China, die steels are in great need of large slab ingot of high quality and large tonnage, such as P20, WSM718R and so on. Solidification structure and size of large slab ingots produced with conventional methods are not satisfied. However, large slab ingots manufactured by ESR process have a good solidification structure and enough section size. In the present research, the new slab ESR process was used to produce the die steels large slab ingots with the maximum size of 980×2000×3200mm. The compact and sound ingot can be manufactured by the slab ESR process. The ultra-heavy plates with the maximum thickness of 410 mm can be obtained after rolling the 49 tons ingots. Due to reducing the cogging and forging process, the ESR for large slab ingots process can increase greatly the yield and production efficiency, and evidently cut off product costs.

A Study of Slag and Steel Leakage Influence Factors during Electroslag Remelting Withdrawing Process

The electroslag remelting withdrawing (ESRW) process overcomes the shortcomings of the traditional electroslag remelting (ESR) process such as the low production efficiency and high cost, and so on. The main features of ESRW compared to traditional ESR are a continuous relative movement between ingot and mold, and electrode exchange with several times and ingot withdrawing. The key technology for ESRW is how to avoid the leakage of liquid slag and steel from the mold during ingot withdrawing process. It is found that the slag properties including shrinkage, high temperature strength and plasticity, lubrication and friction of slag skin on slag-metal interface, ingot solidification shrinkage behavior, process coordination control are the key factors which influence the leakage of liquid slag and steel in ESRW process. In addition, a multi-taper mold present in this paper is beneficial to obtain good surface quality of ingot based on mathematical modeling of ESRW process.

Effect of Fluoride Containning Slag on Oxide Inclusions in Electroslag Ingot

Besides controlling homogeneous composition and compact solidification structure, removal of non-metallic inclusions is an important characteristic for electroslag remelting process. Many factors influence the non-metallic inclusions in steel including gas and inclusions original content in consumable electrode, atmosphere, slag amount and its composition, power input, melting rate, filling ratio and so on. Fluoride containing slag, which influences the non-metallic inclusions to a great extent, has been widely used for electroslag remelting process. The present paper focuses on the effect of fluoride containing slag on the inclusions in electroslag ingot based on the interaction of slag-metal interface. In this work, steel grade MC5 and several slags have been employed for investigating the effect of slag on inclusions. These experiments had been carried out in an electrical resistance furnace under argon atmosphere in order to eliminate the effect of ambient oxygen. Some specimens had been taken at different times for analyzing the content, dimensions, and type of non-metallic inclusions. Quantitative metallographic analysis method has been adopted for observing and examining the inclusions. SEM-EDS analysis has been used to investigate the composition of non-metallic inclusions of specimens at different time for investigating the modification behavior of inclusions. All the results obtained will be comparison to the original state inclusions in steel, which will be in favor of choose of slag for electroslag remelting process.

Hot Test and Simulation of ESR Hollow Ingots Formation in Current Supplying Mould with Electrodes Change

A new electroslag remelting (ESR) technology for hollow ingot manufacture was recently commercialized due to the cooperation between Northeastern University of China and Elmet-Roll engineering company (Ukraine). The usage of T-shape current supplying mould (CSM) and ingot withdrawing system as well as the change of consumable multi electrode (electrodes) are the main features of performed process. Using the new process the 20t ESR hollow ingots (hollows) with perfect quality of surfaces and defect-free macrostructure were manufactured. Any visible defects in the external and internal surface of hollow ingot including all areas of electrodes changes were not found. It becomes to be possible due to non-stop supply of electric current via CSM from second transformer. Both the high cleanness and fine as-cast microstructure of hollow ingots were proved. Simulation of slag flow and heat transfer at ESR process was developed by using the ANSYS software. The model has been verified by measuring of geometry of liquid metal pool was revealed in macrostructure of 650/450mm hollow ingots.

Mathematical Model of Solidification during Electroslag Casting of Pilger Roll

A mathematical model for describing the interaction of multiple physical fields in slag bath and solidification process in ingot during pilger roll casting with variable cross-section which is produced by the electroslag casting (ESC) process was developed. The commercial software ANSYS was applied to calculate the electromagnetic field, magnetic driven fluid flow, buoyancy-driven flow and heat transfer. The transportation phenomenon in slag bath and solidification characteristic of ingots are analyzed for variable cross-section with variable input power under the conditions of 9Cr3NiMo steel and 70%CaF2 − 30%Al2O3 slag system. The calculated results show that characteristic of current density distribution, velocity patterns and temperature profiles in the slag bath and metal pool profiles in ingot have distinct difference at variable cross-sections due to difference of input power and cooling condition. The pool shape and the local solidification time (LST) during Pilger roll ESC process are analyzed.