Why is 10B21 material a commonly used grade for fasteners?

　　10B21 is a typical cold heading steel with good plasticity, moderate strength, and significant heat treatment effect. This material is widely used in the manufacture of high-grade fasteners, such as bolts, nuts, etc.

　　10B21 production process: wire laying → rough drawing → spheroidizing annealing → acid washing → phosphating → medium drawing → heading → grinding → heat treatment → surface treatment → packaging

　　This article will start with chemical composition analysis to explore its influence on heat treatment performance and cold heading performance, and systematically study it in conjunction with production processes.

　　◆ Heat treatment properties

　　10B21 wire rod is a superior low-carbon alloy steel wire that complies with the American standard SAE J403. According to domestic Baosteel standards, its chemical composition is shown in the following table:

　　Compare the chemical composition limits of steel for each performance grade of fasteners specified in the national standards GB/T3098.1/GB/T3098.2 (table below).

　　It can be seen that 10B21 fully meets the composition requirements in the standard. Moreover, the addition of trace amounts of boron can significantly improve the hardenability of steel, with no or minimal impact on other properties.

　　This can to some extent replace Ni, Cr, Mo, and B dissolves in solid solutions, increasing the lattice size and strength. Boron in grain boundaries has a preventing effect on recrystallization diffusion, which can increase the thermal strength of steel.

　　◆ Cold heading capability

　　① Carbon C, Silicon Si

　　The strength of C element, as the main element determining mechanical properties, increases with the increase of C content, while the cold heading performance also decreases.

　　Si element can improve the strength and hardness of steel, while reducing its plasticity and toughness. The cold heading performance of cold heading steel decreases with the increase of silicon content.

　　10B21， In the chemical composition system, the C content should be between 0.18% and 0.23%, and the Si content should be below 0.10%. Make the material have characteristics such as low hardness, good plasticity, and easy deformation.

　　② Manganese Mn

　　Mn element plays a role in solid solution strengthening in steel. For cold heading steel, excessive Mn content will significantly increase the strength of the steel, which is not conducive to the cold forming of the steel; The Mn content of 10B21 is below 1.00%; Appropriate Mn content can not only improve the morphology and distribution in steel, but also enhance the cold formability of steel.

　　In addition, in order to better adapt the material to the cold heading process, there are certain requirements for grain size and form, distribution of non-metallic inclusions, etc.

　　◆ Weldability

　　The main factor affecting the weldability of steel is its chemical composition. Among various elements, the influence of carbon is significant, and the influence of other elements can be converted into the influence of carbon. Therefore, the carbon content method can be used to estimate the weldability of the welded steel.

　　As the carbon content in steel increases, the tendency to harden increases and the plasticity decreases, making it easier to produce welding cracks. The relationship between carbon content and weldability can be seen from the table below.

　　10B21， As superior low-carbon steel, it has good welding performance.

　　◆ Summary

　　Based on the above analysis, 10B21 has improved hardenability and good heat treatment effect by adding B element. At the same time, it also takes into account the cold heading and weldability of low carbon steel. Therefore, 10B21 can be widely used in the production of fasteners such as 8.8/9.8 grade bolts, nuts, and welded parts.

　　(Source: GAF Screw Master)