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The H13 die steel was prepared by 2 t electroslag remelting furnace. Combined with thermodynamic calculation， experimental observation and statistical analysis， the type， size， morphology and distribution of inclusions in H13 die steel in electroslag ingot under a melting rate of 0.12 kg/s were systematically studied. The results show that the main inclusions in H13 die steel are MgO·Al2O3 and spherical CaO·MgO·Al2O3. During the remelting process of electric slag， MgO·Al2O3 inclusion evolves into MgO·Al2O3+CaO·Al2O3 mixed type inclusion or class CaO·MgO·Al2O3 inclusion. At the same time， a small amount of S will be precipitated with various inclusions as nucleation particles. The precipitated component is mainly CaS， forming CaO·MgO·Al2O3 inclusions wrapped by CaS. The size of inclusions in the electroslag ingot is mainly less than 5 μm， 5-10 μm inclusions are less， and only a small amount of inclusions larger than 10 μm exist. The distribution is similar to that of the electrode， but the overall number of inclusions is reduced. The radial distribution shows a trend of increasing first and then decreasing， and the enrichment of inclusions occurs near half of the radius.

In order to achieve better strength and toughness matching for the newly developed Fe-C-Cr-Ni-Mo-Co-Al co-precipitation hardening ultra-high strength steel， this study deeply discussed the mechanism of dual-aging process on the microstructure evolution and mechanical properties of experimental steel， and determined the optimal aging process parameters to achieve excellent strength and toughness. The results show that the experimental steel could achieve higher ultimate tensile strength （1924 MPa） under normal aging treatment， but lower yield strength and toughness. The dual-aging technology （570 ℃×0.5 h+480 ℃×20 h） could achieve superior combination of strength and toughness. After the optimal dual-aging treatment， the yield strength has been increased by 224 MPa and the impact work has been increased by 77% while the ultimate tensile strength was merely decreased by 42 MPa， with an error range of ±0.5 J for the impact toughness.The microstructure characterization and theoretical calculation results show that the content of precipitation-hardening phases and reversed austenite play decisive role in the strength and toughness of the experimental steel. The higher pre-aging temperature is benefit to the nucleation and growth of the reversed austenite and the secondary hardening precipitates in the experimental steel. The formation of reversed austenite is helpful to increase the impact energy of the test steel， but high austenite content would significantly reduce the strength of the steel. When continuously aged at 610 ℃， precipitates coarsening and martensite microstructure recovery would lead to a decrease in strength and toughness.

The continuous cooling transition curve （CCT curve） and impact energy curve of QDH of the new hot work die steel with 0.8% Ni and without Ni were determined by thermal expansion phase change instrument and impact testing machine， and the influence of Ni on the continuous transformation law and impact energy of QDH steel were analyzed by combining the microstructure and hardness. The test results of thermal expansion curve show that Ni element can significantly reduce the critical phase transition point （Ac1， Ac3 and Ms） of QDH steel， expand the austenite phase region， shift the CCT curve to the right as a whole， reduce the critical cooling rate of pearlite from 0.03 ℃/s to less than 0.02 ℃/s， and reduce the critical cooling rate of martensitic from 0.4 ℃/s to 0.2 ℃/s， which improves the hardenability of steel. The test results of impact energy curves show that the first type of tempering brittleness and the second type of tempering brittleness of the two experimental steels are in the tempering temperature range of 350 ℃-500 ℃ and 550 ℃-620 ℃， respectively， and the impact energy of Ni-containing steel in the whole tempering temperature range is higher than that of Ni-free steel， indicating that Ni element can improve the hardenability of QDH steel and improve its toughness.

The high-end equipment industries in the fields of aerospace， energy， petrochemical， shipbuilding， rail transportation， new energy vehicles， energy conservation and environmental protection， and electronic information have developed strongly， putting forward higher requirements for the quality and performance of special steel and special alloy materials， and the demand has surged. Therefore， in recent ten years， China's special metallurgy industry has been rapidly development. This paper first analyzes and summarizes the new requirements of ultra-high strength steel， supper alloy， corrosion resistant alloy， heat resistant steel， special stainless steel， high performance bearing steel， tool and die steel and precision alloy for the above-mentioned high-end equipment manufacturing. Secondly， the development status and trend of traditional special metallurgical processes and several new special metallurgical processes are analyzed. It is emphasized that the combination with basic oxygen furnace/electric arc furnace steelmaking process can provide high clean consumable electrode for electroslag remelting and vacuum arc remelting， and can also provide pure raw material for vacuum induction furnace. The short process of the electroslag remelting with continuous casting billet as the consumable electrode can significantly improve the production efficiency and reduce the production cost. At the same time， the duplex process of high nitrogen stainless steel smelting and the process flow of powder metallurgy and spray forming of tool and die steel are also briefly introduced. Third， China's special metallurgical industry development status， as well as the progress of new technology and new equipment have been summarized. Finally， suggestions and prospects for the technical development of special metallurgy in China in the next ten years are put forward.

Pressurized electroslag remelting （PESR） is currently the most effective production route for industrial preparation of high nitrogen steel and high nitrogen alloys， and is currently a research focus and the most advanced technology in the field of electroslag metallurgy. This article introduces the development of the integrated line equipments， process technology， and main product characteristics of PESR. A review is conducted on the core components of the nitrogen alloy feeding system and water-gas dynamic balance system in the current PESR furnace equipment. In response to the current technological difficulties in PESR process， this paper elaborates on the development of new technologies such as the mechanism and method of nitrogen addition and gas-phase nitrogen addition technology for nitriding alloys， development of a new high-efficiency heating refining remelting slag system， and precise element control. In recent years， with the application of series newly built domestically produced pilot scale PESR furnaces and the successful development of series of high nitrogen steel products， China has made a significant breakthroughs in the integrated line equipment independent research and technological development， and gradually stepped forward to the international advanced level.At the same time， deepening the independent research and development of industrial grade PESR equipment and process technology research and development， ensuring the preparation of key materials urgently needed for high-quality special steels and special alloys in China's aviation， aerospace， and military industries fields and so on， is an important direction for the development of modern electroslag metallurgy technology in China.

The effect of La treatment on the transformation of inclusions in H13 steel was investigated by subjecting H13 die steel to vacuum induction melting and subsequently treating the molten steel with La under an argon atmosphere protection.The results demonstrate that La has a beneficial effect on inclusions in H13 steel， the number of inclusions increases with the increase of La content， while the average size of inclusions initially decreases and then increases. The inclusions in H13 steel without the rare earth La additions are primarily Al₂O₃ and MnS. However， with a La content of 0.016% in H13 steel， the dominant inclusions shift to La2O2S. When the La content in H13 steel is 0.016%， the inclusions in the steel are mainly La2O2S；Furthermore， when the La content reaches 0.045% and 0.073%， the predominant inclusions in steel become LaP and La2O2S. Thermodynamic calculations show that the transformation process of La-treated inclusions is Al2O3→LaAlO3→La2O2S in molten steel at 1 600 ℃. During the solidification process， La2S3 is initially precipitated in the molten steel. As the temperature continues to decline， LaP begins to precipitate on the surface of La2S3 and La2O2S， ultimately forming the composite inclusions La2O2S-LaP and La2S3-LaP.

During the process of electric arc remelting， the content of gas components （such as H， O， N， etc.） in the steel is significantly influenced by the gas permeability of the slag system used. In order to explore a slag system with low hydrogen permeability suitable for nuclear-grade 316H stainless steel in electric arc remelting， the hydrogen permeability of five slag systems used in electric arc remelting was determined. The results showed that the newly developed 63%CaF2-30%Al2O3-7%MgO slag system had the lowest hydrogen permeability.meanwhile the hydrogen permeability of slag system， 65%CaF2-30%Al2O3-5%MgO， was similar to that of the 63%CaF2-30%Al2O3-7%MgO slag system， and hydrogen permeability of both slag systems exhibited a significant reduction compared to the original slag system used in the steel plant's electric arc production，declined from 6.58×10-6 mol/（cm·min） to 1.89×10-6 mol/（cm·min） and 2.18×10-6 mol/（cm·min） respectively.The higher the optical basicity of the slag was， the higher the hydrogen permeability was. CaO in the slag had a high affinity for water and can easily increase the hydrogen content in the steel ingot during electric arc remelting， while the addition of MgO in the slag system can significantly reduce hydrogen permeability. Under the research conditions， the influence of the slag system's constituents on its hydrogen permeability was more significant than the optical basicity.

A fully coupled flow heat mass model was established based on the phase field method to simulate the growth of columnar crystals in ingots during vacuum arc remelting （VAR） in a forced flow field. The influence mechanism of melt flow on the growth of columnar crystals was analyzed by comparing the growth characteristics of columnar crystals with and without flow field conditions.The simulation results indicated that when the velocity of flow field in the solidification front is 45 μm/s and the direction is perpendicular to the direction of dendrite growth， the secondary dendrites exhibit ‘counter-flow’ growth， and the dendrite trunks tilt downstream due to erosion. A small eddy current is formed between adjacent dendrites， and the solutes and the heat in the upstream of the dendrite trunks are carried into the downstream side with the melt. The boundary layer thickness of the solute and the temperature on the upstream side decreases， from 51 μm in the absence of a flow field to 31.5 μm. The large temperature gradient promotes the development of secondary dendrites. The concentration of solutes and heat in the downstream boundary layer is high， and the boundary layer thickness increases to 78 μm. The decrease in gradient significantly inhibits the development of secondary dendrites. The downstream solute concentration is higher than that in the absence of a flow field， and the slow growth of dendrites increases the risk of micro-segregation.

This study focuses on the investigation of the mechanical properties and corrosion resistance of 430 ferritic stainless steel with an addition of 0.30% Sn， by incorporating varying amounts of Cu into the experimental steel. The results of room temperature and high temperature tensile tests reveal that Cu atoms act as solid solution strengtheners in the matrix， enhancing the tensile strength of the stainless steel. However， the formation of a significant amount of Cu-rich phase at grain boundaries leads to a decrease in the steel's plasticity. The uniform corrosion resistance of the steel initially increases and then decreases with increasing Cu content， reaching the highest effectiveness at 0.45% Cu. Similarly， Cu exhibits a comparable pattern in its resistance to intergranular corrosion in the experimental steel， achieving the best outcomes at a content of 0.98% Cu.

Three parts of the experiment were mainly carried out in this research: the effect of Ca on inclusions in 38CrMoAl high aluminum steel under atmospheric pressure, the effect of pressure on the solubility of Ca in pure iron under pressurized conditions, and the effect of Ca on inclusions in 38CrMoAl high aluminum steel under pressurized conditions. The results indicated that Ca can significantly transform the inclusions such as Al2O3 and MgO·Al2O3 with high melting points into calcium aluminate inclusions with low melting points. It was found that the pressure condition can significantly improve the solubility of Ca in pure iron and molten steel, which provided reliable data reference and guidance for popularizing Ca treatment of steel under pressure conditions in the future. Finally, the mechanism of inclusion transformation after Ca treatment was put forward. In this study, the theoretical calculation of thermodynamics was closely compared with the experimental data. It was proposed that to modify the inclusions in high aluminum steel into calcium aluminate inclusions with low melting points, it is necessary to accurately control Ca in the target region.