Dongping Zhan

Aging Precipitation Behavior of 18Cr-16Mn-2Mo-1. 1N High Nitrogen Austenitic Stainless Steel and Its Influences on Mechanical Properties

The solution-treated (ST) condition and aging precipitation behavior of 18Cr-16Mn-2Mo-1. 1N high nitrogen austenitic stainless steel (HNS) were investigated by optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that the ST condition of 18Cr-16Mn-2Mo-1. 1N HNS with wN above 1% is identified as 1 100 °C for 90 min, followed by water quenching to make sure the secondary phases completely dissolve into austenitic matrix and prevent the grains coarsening too much. Initial time-temperature-precipitation (TTP) curve of aged 18Cr-16Mn-2Mo-1. 1N HNS which starts with precipitation of 0.05% in volume fraction is defined and the “nose” temperature of precipitation is found to be 850 °C with an incubation period of 1 min. Hexagonal intergranular and cellular Cr2N with a = 0.478 nm and c = 0.444 nm precipitates gradually increase in the isothermal aging treatment. The matrix nitrogen depletion due to the intergranular and a few cellular Cr2N precipitates induces the decay of Vickers hardness, and the increment of cellular Cr2N causes the increase in the values. Impact toughness presents a monotonie decrease and SEM morphologies show the leading brittle intergranular fracture. The ultimate tensile strength (UTS), yield strength (YS) and elongation (El) deteriorate obviously. Stress concentration occurs when the matrix dislocations pile up at the interfaces of precipitation and matrix, and the interfacial dislocations may become precursors to the misfit dislocations, which can form small cleavage facets and accelerate the formation of cracks.

Effects of yttrium on microstructure and properties of reduced activation ferritic-martensitic steel

ABSTRACT This study investigates the inclusions, microstructure, tensile properties, and impact toughness of reduced activation ferritic/martensitic (RAFM) steels with different Y contents (0.006, 0.034 and 0.071 wt-%). Owing to the pinning of the grain boundary to the Y inclusions (which refines the original austenite grain size) and an existing Fe–Cr–Ta–Y–S–O phase, the tensile strength at both room and high temperatures increased with increasing Y content (below 0.071%). Increasing the Y content to 0.034 wt-% decreased the ductile-to-brittle transition temperature (DBTT). However, when the Y content reached 0.071 wt-%, the DBTT increased as the Y precipitated into blocky yttrium-rich inclusions. The microstructure, tensile properties, and impact toughness of the RAFM steel were optimised at a Y content of approximately 0.034 wt-%.

Optimisation of the MgO/Al2O3 ratio of high-alumina BF slag based on MOPSO algorithm

ABSTRACT To reduce the content of MgO in the slag, which helps in lowering production costs, the chemical components of a SiO2–CaO–MgO–Al2O3–TiO2 blast furnace (BF) slag system with high Al2O3 content were optimised using the multi-objective particle swarm optimisation (MOPSO) algorithm. First, models for predicting the metallurgical properties of BF slag were established and verified, based on mixture experimental designs and the ion and molecule coexistence theory. Second, the problem was solved by MOPSO using the MATLAB software package. Optimisation results show that the appropriate content of MgO in the slag should be 4–7%, and the MgO/Al2O3 ratio should be 0.2–0.6. Finally, optimised solutions were used in the 3200 m3 BF of a steel company in China. MgO in the slag was reduced to less than 5%, the MgO/Al2O3 ratio was controlled at 0.3–0.4, and the fuel ratio remained at 515 kg t−1.

Chromium Recycling from Argon-oxygen Decarburization Dust in Hot Metal Pre-dephosphorization Process

The chromium recycling from the stainless steel dust of an argon-oxygen decarburization (AOD) furnace during a hot metal pretreatment process was investigated. Experiments at different temperatures or with different amounts of AOD dusts were carried out in a laboratory furnace equipped with MoSi2 heating elements and subsequently industrial experiments were conducted in a stainless steelmaking factory, in order to study the thermodynamic mechanism of C and Si reacted with Cr2 O3 to get Cr from AOD dust. The results showed that the reaction between C and Cr2 O3 occurred below 1572. 18 K. Although its reducing ability was weaker than that of Si, C enabled the Cr in Cr2 O3 in the AOD dust to be recycled using the Si in the slag. By combining the AOD dust added in an earlier stage, the hot metal pretreatment slag added in a later stage could not only recycle Cr from AOD dust but also remove Si, S, and P. Higher hot metal temperatures resulted in higher Cr content and lower remained P concentration in the molten iron.

Effects of AIMnCa and AIMnFe Alloys on Deoxidization of Low Carbon and Low Silicon Aluminum Killed Steels

To confirm the effects of AlMnCa and AlMnFe alloys on the deoxidization and modification of Al2O3 inclusions, experiments of 4-heat low carbon and low silicon aluminum killed steels deoxidized by AlMnCa and AlMnFe alloys were done in a MoSi2 furnace at 1 873 K. It is found that the 1# AlMnCa alloy has the best ability of deoxidization and modification of Al2O3 inclusions than 2# AlMnCa and AlMnFe alloys. Steel A deoxidized by 1# AlMnCa alloy has the lowest total oxygen content in the terminal steel, which is 37 × 10−6. Most of the inclusions in the steel deoxidized by 1# AlMnCa alloy are spherical CaO-containing compound inclusions, and 89.1 % of them are smaller than 10 µm. The diameter of the inclusion bigger than 50 µm is not found in the final steels deoxidized by AlMnCa alloys. Whereas, for the steels deoxidized by AlMnFe alloys, most inclusions in the terminal steel are Al2O3 or Al2O3-MnO inclusions, and a few of them are spherical, and only 76.8% of them are smaller than 10 µm. Some inclusions bigger than 50 µm are found in the steel D deoxidized by AlMnFe alloy.