In industries such as coal, metal, and rock mining, 55SiMnMo brazing steel is an indispensable material due to its excellent elastic limit, high yield strength, and good toughness and plasticity. However, some users have reported the presence of microcracks on the surface during production, which not only affects the appearance but could also pose a threat to its service life and safety. So, why does surface cracking occur in 55SiMnMo brazing steel?
Overview of 55SiMnMo Brazing Steel
55SiMnMo brazing steel is an alloy steel designed specifically for rock drilling tools, offering excellent mechanical properties and processing performance. It is widely used in industries such as coal, metal, and rock mining, making it an essential material. However, due to its high strength and toughness, strict requirements are placed on the chemical composition, non-metallic inclusions, segregation, and surface quality during production.
Visual Manifestation of Cracking
When producing and processing 55SiMnMo brazing steel, defects such as pitting, peeling, and scratches were found on the steel surface. After grinding and heating it to produce square steel, fine linear microcracks appeared on the surface. These cracks were distributed linearly along the rolling direction, varying in length with extremely fine widths, and their distribution lacked any obvious pattern. It created significant challenges for subsequent processing and usage.
Analysis of Crack Causes
To identify the cause of the cracks, researchers conducted an in-depth analysis. Using microscopic detection and analysis methods, they performed detailed observations of the cracks and surrounding tissue and combined this with a comprehensive analysis of the production process.
Impact of Continuous Casting Process
Continuous casting is a crucial stage in the production of 55SiMnMo brazing steel, as the control of the continuous casting process directly affects the surface quality of the steel billets. During continuous casting, improper control of parameters such as tundish overheating, withdrawal speed, and secondary cooling water flow can easily lead to surface cracks. For example, excessive tundish overheating can cause uneven solidification speed, resulting in internal stresses and cracking. Similarly, excessive withdrawal speed can exert excessive tensile stress on the billet during solidification, which can also lead to cracks.
Oxidation During Billet Heating
Before rolling, steel billets need to undergo high-temperature heating. However, if there are surface cracks, these cracks may partially oxidize during the heating process. Once the oxide scale falls off, deeper crack marks remain. Additionally, excessive heating temperature or prolonged heating times can further exacerbate the oxidation of the billet surface, worsening the crack situation.
Influence of Subsequent Processing
Although cracks mainly occur during the continuous casting stage, improper operations during subsequent processing may also exacerbate crack development. For example, uneven pressure during rolling and severe wear of the rolling mill rolls may cause new cracks to form on the surface of the steel billet or exacerbate the expansion of existing cracks.
Improvement Measures
Optimize Continuous Casting Process
By adjusting the continuous casting process control parameters, such as reducing the overheating temperature of the middle ladle, controlling the pulling speed, and adjusting the secondary cooling water ratio, it is possible to effectively reduce the occurrence of cracks on the surface of the steel ingot. At the same time, the use of multi-hole water nozzles and special protective slag further improves the surface quality of the continuous casting ingot.
Strengthen Billet Grinding
Grinding and polishing continuous casting billets to remove surface cracks and defects is an effective means of improving the surface quality of steel billets. Through grinding and polishing, the number of cracks on the surface of steel billets can be significantly reduced, thereby lowering the risk of cracks during subsequent processing.
Control Heating Temperature and Time
Before hot rolling, strictly control the heating temperature and time to avoid excessive oxidation of the steel billet surface. At the same time, strengthen the maintenance and management of the heating furnace to ensure temperature uniformity during the heating process and reduce internal stress caused by uneven temperatures.
Improve Rolling Process
Inspect and replace severely worn rollers to ensure uniform pressure during rolling. Moreover, optimizing rolling process parameters such as rolling speed and rolling force can reduce the occurrence of cracks caused by improper rolling.
After implementing these improvement measures, the surface quality of 55SiMnMo brazing steel was significantly improved. User feedback and actual testing showed a marked reduction in the number of cracks on the billet surface, meeting the users’ requirements.
Conclusion
The appearance of surface cracks in 55SiMnMo brazing steel is the result of multiple factors, including the continuous casting process, heating process, and rolling process. By analyzing the causes of cracking and taking targeted measures such as optimizing the continuous casting process, strengthening billet grinding, and improving heating and rolling processes, the steel’s surface quality and overall performance have been significantly enhanced. With technological advancements and ongoing process optimization, we believe that 55SiMnMo brazing steel will continue to play a vital role in heavy industry, contributing to the development of the industry with superior performance and quality.
