Why does it crack during the stamping process?
When many purchasers or technicians encounter cracking in stamping processes, their first reaction is often "insufficient strength". However, for materials like SG295, which are used for deep drawing of gas cylinder heads, the situation is quite the opposite. Cracking is usually not due to the material being "too weak", but rather because it is "too brittle" or has "defects in its microstructure".
Detailed explanation
Elongation (Extension Rate) - The "compromise" ability of the materiald
Standard requirements vs. actual performance:
The JIS G3116 standard stipulates that the elongation rate of SG295 should be ≥ 26%.
The steel coils we offer have a content of 27-32%. This 4-6% margin is the first line of defense.
Why is elongation important?
During the stamping process, the steel plate needs to be subjected to intense stretching. For example, at the sealing head of a gas cylinder, the material will be elongated and thinned to fit the mold.
The higher the elongation rate, the greater the stretching capacity the material can withstand before breaking. If the elongation rate of your material is just at the 26% standard line, once the rolling direction of the raw material does not match the stamping stress direction, or there is a slight fluctuation in the mold gap, it is easy to reach the deformation limit of the material, resulting in cracking.
"Our material's typical value reaches 27-32%," which provides the customer's production process with a wider and safer "process window". Even if the mold is slightly worn or the lubrication is poor, the material can still "compromise" and deform without breaking.
Manganese (Mn) and Carbides - The Invisible "Starting Point" of Cracks
Manganese is a "double-edged sword":
The standard stipulates that Mn should be less than or equal to 1.00%. The main function of manganese is to enhance strength.
However, under certain conditions, manganese combines with carbon to form (Fe, Mn)₃C carbides (ledeburite) at the grain boundaries. These carbides are extremely hard and brittle.
The "form" of carbides determines life and death:
Filamentous carbides (most dangerous): If the production process (especially the rolling temperature and deoxidation method) is not properly controlled, carbides will form continuous or semi-continuous films along the grain boundaries. This is like mixing sharp glass shards into the cement (grain boundaries) between bricks (grain particles). When subjected to stamping and stretching, the stress will concentrate on this brittle film, and small cavities (Microvoids) are likely to form here. These cavities quickly connect, resulting in visible grain-boundary cracks that can be seen with the naked eye, leading to the destruction of the workpiece.
Granular carbides (safer option): By optimizing the smelting process (such as Al-killed steel) and increasing the rolling temperature, the carbides can be transformed from "film-like" to isolated granular or "block-like" forms. The fragmented effect on grain boundaries of granular carbides is much less, and the material still maintains good plasticity and formability.
Strict manganese content control (0.80 - 0.95%): We not only meet the upper limit requirements, but also keep the manganese content within a 'golden range' that can ensure strength (295 MPa+) while avoiding excessive formation of brittle carbides due to manganese segregation.
Optimizing the carbide morphology through process: Our material undergoes a processing similar to 'aluminum deoxidized steel', aiming to make the grain boundary carbides exist as harmless granular form rather than as film-like form that causes cracks. This is the metallurgical secret behind our material maintaining a high yield rate (as reported by customers at 98%) during deep stamping.
Conclusion
If you encounter cracking during the pressing process of SG295, apart from inspecting the mold, you might also consider the following three points from the perspective of the material:
Look at the elongation rate: Is it merely meeting the minimum standard? Choose materials with a larger elongation rate margin (such as 28% or more).
Check the manganese content: Is the manganese content close to the upper limit of 1.0%, or should it be kept within a more moderate range?
Check Certification: Can the supplier provide MTC (factory material certificate) and ensure the traceability of the furnace batch numbers? Because if the curling temperature control of a certain furnace run fluctuates, it may lead to the deterioration of the carbide morphology of this batch. Traceability is the lifeline for tracing the root cause of the problem.
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Through years of understanding and precipitation of steel, Yuxin Group has now become a mature steel processing and manufacturer, providing various products for many construction companies and steel structure companies around the world. Thanks to Yuxin Group's professionalism and integrity, it has become a world-renowned enterprise.
