ZG13Cr9Mo1VNbN Stainless Steel Casting for High Pressure Cylinder of Supercritical Turbine

Casting steel of supercritical unit steam turbine block is one of the important parts of thermal power unit. High-pressure inner cylinder is a casting steel which works at high temperature and bears pressure. Its material is fundamentally different from that of subcritical unit block.Current cast steel materials for sub-critical steam turbine block, such as ZG15Cr2Mo1, ZG15Cr1Mo1V, etc., are not enough to emphasize high-temperature creep fracture under steam conditions of 566 C or higher. Therefore, a material with high creep fracture strength, good toughness, excellent welding performance and good casting performance, namely ZG13Cr9Mo1VNbN cast steel material, is used to adapt to supercritical condition.Requirements for high pressure cylinders of steam turbines.

Based on the experience of producing high alloy special steels such as nuclear power, hydrogenation, rotor, supporting roll and ultra-low carbon stainless steel, the smelting and pouring process of ultra-pure stainless steel casting for high pressure inner cylinder of supercritical steam turbine is studied.

1. Technical Requirements

Chemical composition requirements of stainless steel ZG13Cr9Mo1VNbN for supercritical steam turbine high pressure cylinder produced in this production.Requirements for residual elements and hydrogen and oxygen content.

2. Formulation of Smelting Process Plan

The chemical composition of this material requires more stringent residual elements such as Cu, As and Sn. During the batching process, it is necessary to select high-quality returned scrap steel and ferroalloys with lower residual elements.Due to the high content of Cr element in chemical composition, adjusting Cr element and adding large amount of ferrochromium alloy during refining will increase the time of molten steel smelting. However, the content of C element in this material is low and long-term smelting in LF furnace will result in carbon increase in molten steel.In order to reduce the time of molten steel smelting and improve production efficiency, the combination of electric arc furnace and medium frequency induction furnace is used to provide 40 t rough steel water by electric arc furnace and medium frequency induction furnace respectively. High quality waste steel and low carbon ferrochromium alloy are added to the medium frequency induction furnace to melt the molten steel in advance.

For the strict control of H and O elements, vacuum treatment is used to remove gas H from molten steel, and precipitation deoxidization of ferrosilicon and ferromanganese is combined with diffusion deoxidization of ferrosilicon and carbon powder.Material has certain content requirements for N element, while N element can only be adjusted by using ferrous nitrogen alloy or nitrogen after vacuum. If the former is used, when ferroalloy is added, refining slag will be drawn into the molten steel and not easily separated, thus polluting the molten steel.In order to obtain high purity steel water, nitrogen is used to adjust the N content.Finally, the smelting process plan is drawn up as follows: EBT roughing + medium frequency furnace smelting LF refining vacuum degassing nitrogen treatment pouring.

3. Implementation of Smelting Process

3.1 Control of Roughening Link

According to the requirement of chemical composition and molten steel quantity in product smelting, 35.5 t high-quality low-carbon high-chromium waste steel and 5.5 t low-carbon ferrochromium alloy are added in Medium-frequency furnace smelting, which requires that elements C, Nb and residual elements P, S, Cu, As and Sn be melted to meet the requirements of Table 3.The electric arc furnace is equipped with 9 t pig iron and 35 t scrap steel, and slag-making materials such as lime and fluorite are added into the furnace. After power supply, the large material is blown red by oxygen gun to make foam slag by spraying carbon. At the later stage of melting, P element is removed by slag-setting and new slag-making to meet the requirements of Table 3.When both platforms have finished steel roughing, add a certain amount of ferrosilicon and ferromanganese alloys to the bottom of the refining package, pour all the molten steel in the medium frequency furnace into the refining package, and blend the molten steel in the electric arc furnace with it by means of backing and slag trapping. Some elements of the molten steel are added after the blending.

3.2 Key Control Points of Off-furnace Refining

In LF furnace, activated lime and fluorite slag are added in proportion of 4:1, and powdered deoxidizer ferrosilicon powder and carbon powder are added for diffusion deoxidization. Because the deoxidization reaction of powdered deoxidizer only takes place at the interface of slag steel, oxygen diffusion to the reaction interface is slow, refining time is longer, which is unfavorable for deoxidization and desulfurization. Therefore, argon is added to the bottom of the ladle and argon is used to stir steel liquid to accelerate deoxidization and deoxidization.Sulfur also facilitates uniform temperature and composition of molten steel.After the slag color is whitened by deoxidization, alloying is carried out. After all elements except N are adjusted into the internal control value, the temperature of molten steel is raised to 1650~1660 C, and VD vacuum degassing is carried out.Keep 20 minutes under high vacuum and stir argon more strongly during this period so that H, O, N and other gases float together with argon bubbles. In addition, inclusions will be adsorbed during the gas floatation process to purify the molten steel.After the vacuum is over, gas sample is taken to analyze the content of H, O and N in the liquid steel, and argon is changed to nitrogen. Nitrogen is used instead of argon to stir the liquid steel weakly. The stirring time of nitrogen is controlled by analyzing the content of N element in the gas sample until N element is brought into the internal control value.Melting analysis of molten steel components, electrical lifting of molten steel temperature tapping pouring.

4. Control of pouring process

Remove hot-air stove 1 hour before molten steel pouring, fill the cavity with argon when the molten steel crosses, and use argon shield over each gate to protect it. Place aluminium cap and paper cap over the argon shield to prevent secondary pollution of molten steel caused by oxygen products entering the cavity. Drain water from each ladle before pouring.

ZG13Cr9Mo1VNbN stainless steel water is used for smelting high-pressure inner cylinder of supercritical steam turbine. High-quality scrap steel, ferroalloy and pig iron are used for proportioning. Rational furnace arrangement not only improves the smelting efficiency of crude steel, but also ensures that residual elements such as Cu, As and Sn meet the requirements of smelting chemical composition.During the off-furnace refining process, the molten steel is vacuum degassed, which reduces the content of harmful gases H and O in the molten steel. After vacuum treatment, nitrogen is used to stir and adjust the content of N element. At the same time, the inclusions in the molten steel float off the molten steel and are adsorbed by the refined slag, thus further improving the purity of molten steel and laying a solid foundation for improving the internal quality of products.