本发明涉及一种在负压条件下气相渗氮治炼COST-FB2钢的方法，先在高真空度条件下进行碳深脱氧，待钢液表面稳定气泡不再涌出时，脱氧完成。然后加入促进氮溶解的合金元素并完全熔化后，开始气相渗氮。气相渗氮时用热力学计算软件Factsage计算气相渗氮时的渗氮压力，精确控制钢液温度，由钢液的表面积、钢液体积和氮平衡含量推算确定渗氮时间。在气相渗氮快要结束时，加入合金元素硼和易挥发的锰，再利用充氩加压至高压力、保持氮分压不变的条件下进行加压浇铸，在凝固过程中保持带压状态，完全凝固后破真空，制得N含量在0.015~0.03%，0含量≤0.0035%，成分均匀、组织致密的COST-FB2钢铸锭。

The invention relates to a method for treating and refining cost-fb2 steel by gas-phase Nitriding under negative pressure. Firstly, carbon deep deoxidation is carried out under the condition of high vacuum, and the deoxidation is completed when the surface of liquid steel is stable and bubbles no longer gush out. Then, the alloying elements promoting nitrogen dissolution are added and completely melted, and the gas-phase nitriding is started. During gas-phase nitriding, the nitriding pressure during gas-phase nitriding is calculated with the thermodynamic calculation software factage, the liquid steel temperature is accurately controlled, and the nitriding time is calculated and determined from the surface area, liquid steel volume and nitrogen balance content of liquid steel. At the end of gas-phase nitriding, the alloying element boron and volatile manganese are added, and then pressurized to high pressure by argon filling, and the pressure casting is carried out under the condition of keeping the nitrogen partial pressure unchanged. During the solidification process, the pressurized state is maintained, and the vacuum is broken after complete solidification. Cost-fb2 steel ingot with uniform composition and compact structure with N content of 0.015 ~ 0.03% and 0 content ≤ 0.0035% is prepared.