Hardenability Requirements
Hardenability is one of the key performance indicators for this type of steel, primarily ensuring the core hardness of the material, which is also beneficial for controlling heat treatment deformation. The hardenability and hardenability bandwidth of this steel primarily depend on the chemical composition and its uniformity. It is required that the steel composition remains stable and evenly distributed, with internal control indicators for chemical composition limited to a narrow range. Additionally, it is necessary to control segregation and reduce the grade of banded structures.
Grain Size Control
Grain size is an important indicator for this type of steel. Fine and uniform austenite grains in the steel result in fine martensite structures after quenching, significantly improving the fatigue performance of the material and reducing deformation after gear heat treatment. Grain refinement elements such as Al are added and combined with optimized heating and rolling processes to meet grain size requirements.
Production Process
The production process for 20CrNiMo includes: hot metal pretreatment → converter → LF → RH → continuous casting of rectangular billets → hot transfer → heating → continuous rolling line → holding → annealing → straightening → grinding → inspection.
Hot Metal Pretreatment
Before converter refining, the hot metal undergoes magnesium-based powder injection desulfurization treatment, followed by slag removal. The hot metal must be strictly selected.
Converter Refining
The steel is refined using full iron charging and single slag methods. The converter process employs argon blowing throughout. Early slag formation, thorough slag melting, and high temperature and alkalinity of the initial slag ensure good fluidity, facilitating early phosphorus and carbon removal. During mid-refining, the lance height is appropriately raised to control the decarburization rate. The strong stirring conditions in the molten pool enhance desulfurization efficiency.
LF + RH Refining
Upon reaching the treatment position, the ladle undergoes temperature and oxygen measurement. Based on the oxygen determination, aluminum balls are added to adjust A1s to 0.03-0.04%. Then, slag melting is conducted for 3-5 minutes, followed by sampling. Slag-making materials include active lime and refining slag. Aluminum balls and ferrosilicon powder are added during the slag-making process for diffusion deoxidation, producing “white slag.” After refining under white slag for a set period, micro-alloying adjustments and heating are carried out (with strong argon stirring during the heating and composition adjustment process). Aluminum wire is fed for final deoxidation, followed by mild argon blowing.
The RH refining process temperature is controlled based on continuous casting requirements. RH degassing is performed along with alloy adjustments. Pre-vacuum treatment in RH achieves a pressure below 100 Pa for no less than 15 minutes, with RH circulation time controlled at 20-25 minutes. After repressurization, mild argon stirring is performed for no less than 15 minutes based on the temperature.
Precise control of the steel’s chemical composition ensures the optimal range of hardenability, maintaining lower limits for carbon (C) and higher limits for alloying elements such as chromium (Cr) and manganese (Mn).
Post time: Jan-10-2025
