Effect of Ce on inclusions and mechanical properties of 20MnTiB cold heading steel

Abstract

Industrialized tests are conducted to produce 20MnTiB cold heading steel with varying Ce contents. A Vickers hardness tester, tensile tester, impact tester, optical microscope, and scanning electron microscope are used to study the deterioration of inclusions in the steel and observe the changes in microstructure and mechanical properties of the hot-rolled wire rod after Ce addition. The application mechanism of Ce is also analyzed. The results show that the S content in the molten steel decreases, and the cleanliness is significantly improved after the addition of 0.0025% Ce. The inclusions in the wire rod transform from large-sized and elongated Al2O3·MgO·CaO·CaS composite inclusions to small-sized and spherical CeAlO3·MgO·CaO·CaS composite inclusions. Concurrently, the long strips of MnS inclusions disappear. Thermodynamic calculations indicate that at 1839 K, the order of precipitation of different Ce inclusions is as follows: CeAlO3 > Ce2O3 > Ce2O2S > CeO2 > Ce3S4 > Ce2S3 > CeS. This suggests that with a Ce mass fraction of 0.0025%, the most probable inclusions are CeAlO3. Considering that 20MnTiB cold heading steel contains B, Ti, and other hardenability elements, improper process control during hot rolling can easily lead to the formation of a bainite structure in the wire rod. This causes the mechanical strength of the wire rod to be higher than desired, leading to occasional cracking during late cold heading and significant wear on the cold heading mold. After the addition of Ce, the microstructure of the wire rod is refined, with an increased proportion of ferrite and a reduction in both the presence and size of granular bainite. Ferrite is a soft and tough phase, while bainite is a reinforcing phase. The reduction in granular bainite and the increase in ferrite contribute to a decrease in strength and hardness. Lower hardness and strength are beneficial for improving the cold heading performance of the wire rod. After Ce addition, the cold heading performance of the wire rod is improved to a certain extent. Additionally, the ambient-temperature impact toughness of the wire rod significantly increases from 31.7 to 52.3 J with the addition of Ce, an increase of 65.0%. This substantial increase in impact performance further enhances the cold heading performance of the wire rod. The reduction in hardness and mechanical strength, combined with the significant increase in impact properties, makes the rare-earth microalloyed hot-rolled wire rod more suitable for cold heading applications. These research results provide technical and theoretical support for the further development of new rare-earth microalloyed cold heading steels.

Keywords

• rare earth
• inclusion
• 20mntib cold heading steel
• granular bainite
• impact properties