In the expansive fields of metalworking and materials science, 55SiMnMo tool steel stands out for its excellent hardenability, toughness, and high fatigue strength, making it the material of choice for manufacturing light-duty rock drilling tools. During the rolling process, cracks often appear on the surface of the steel. It affects the product quality and appearance and leads to significant economic losses and processing challenges for manufacturers. Crack formation is a complex process that involves multiple factors, including material composition, production processes, equipment conditions, and environmental temperatures. To effectively reduce cracking in 55SiMnMo steel rolled products, it is essential to understand the root causes of cracking and implement corresponding process improvements. This article will explore how to address this issue and propose effective solutions to help manufacturers improve product quality and production efficiency.
Causes of Cracking Issues
- Excessive Superheating: In the continuous casting process, if the steel melt temperature is too high, it can prevent the liquid slag from effectively penetrating the interface between the mold wall and the cast billet, hindering uniform billet growth. Excessive superheating can also increase internal thermal stress, raising the risk of cracks.
- Improper Cooling: The temperature and flow rate of the cooling water during rolling significantly impact the material’s microstructure and performance. If cooling is too rapid or uneven, it can generate large thermal stresses on the surface of the steel, which can lead to cracking.
- Inadequate Chemical Composition Control: The content of trace elements such as B and Ti in the steel significantly affects its hardenability and susceptibility to thermal cracking. Improper control of these elements increases the likelihood of cracks.
- Unreasonable Process Parameters: Process parameters such as drawing speed and secondary cooling water flow, if set incorrectly, can negatively impact the quality of the steel.
Process Improvements to Reduce Cracking Issues
Optimize Continuous Casting Process
- Control Superheating: Maintain the steel melt’s superheating within a reasonable range (e.g., 20–30°C) to promote the transformation from columnar crystals to equiaxed crystals, thereby improving the uniformity of the billet structure.
- Improve Slag Performance: Select suitable slag for 55SiMnMo steel to ensure it effectively penetrates the interface between the mold and billet, reducing thermal stress and improving lubrication.
Refine Cooling Process Control
- Optimize Cooling Water Parameters: Adjust the cooling water temperature and flow rate based on the steel’s specifications and rolling speed to ensure even and rapid cooling of the steel surface.
- Enhance Cooling Water Recirculation: Increase the utilization rate of cooling water to reduce waste while ensuring the water’s quality and stability.
Strict Control of Chemical Composition
- Precise Control of B and Ti Content: Control the content of boron (B) (e.g., ≤0.0008%) and titanium (Ti) (e.g., 0.015%–0.025%) to reduce the steel’s sensitivity to thermal cracking.
- Strengthen Raw Material Testing: Perform strict quality checks on raw materials to ensure their chemical composition meets production requirements.
Optimize Rolling Process Parameters
- Reasonably Set Drawing Speed: Adjust the drawing speed (e.g., 0.41 m/min) according to the steel’s specifications and material properties to minimize thermal and mechanical stresses during the rolling process.
- Adjust Secondary Cooling Water Flow: Adjust the secondary cooling water flow (e.g., 0.17 L/kg) to control the cooling speed and the microstructure of the steel.
Strengthen Quality Testing and Process Control
- Establish a Strict Quality Control System: Implement rigorous quality checks at every stage, from raw materials to finished products, to ensure they meet customer requirements.
- Enhance Process Monitoring: Use real-time monitoring and data analysis to identify and address production issues promptly, improving efficiency and product quality.
Conclusion
To effectively reduce the cracking problem of 55SiMnMo steel rolled products, it is necessary to carry out complex considerations and process optimization from several aspects. On the one hand, by precisely controlling key production parameters such as steel melt superheating, secondary cooling conditions, drawing speed, and alloy element content, the occurrence of cracks can be significantly reduced. On the other hand, detailed adjustments to heat treatment processes, such as normalizing and tempering temperatures, can help achieve the desired microstructure and mechanical properties, avoiding excessive tempering that could lead to reduced carbon-rich austenite and increased brittleness.
Furthermore, raw material quality control is essential, especially in minimizing inclusions, which are often the source of crack initiation. By optimizing the smelting and casting processes, such as raising steel liquid temperature to promote the flotation of inclusions and using advanced production techniques to reduce non-metallic inclusion content, steel purity and strength can be further enhanced. Additionally, necessary corrosion protection treatments, such as phosphating and waxing, not only prevent internal bubble corrosion and delay fatigue crack initiation but also improve the service life and aesthetics of the tool rods.
In conclusion, reducing cracking in 55SiMnMo tool steel rolled products is a systemic project that requires the integration of advanced production processes, stringent quality control, and scientific material design. By employing these strategies, manufacturers can significantly enhance product quality and reliability.
