1. Performance Requirements of Steel for Automotive Fasteners

Steel for automotive fasteners shall satisfy the following requirements:

• Excellent surface quality, high dimensional precision, low levels of non-metallic inclusions and segregation;
• High tensile strength and superior cold heading performance;
• High fatigue resistance and resistance to repeated impact tension;
• Sufficiently low susceptibility to delayed fracture and low ductile-brittle transition temperature.

2. Smelting and Rolling Characteristics of Steel for Automotive Fasteners

2.1 Smelting Technology

Secondary refining and continuous casting with electromagnetic stirring are adopted. The main objective is to control the main elements such as C, Si, Mn, Cr and Mo within narrow ranges, and minimize impurities including P, S, O and N in steel. This achieves high steel cleanliness, low non-metallic inclusions and low segregation grades, so as to improve the cold heading performance and surface quality of steel.

2.2 Rolling Technology

High-speed and high-precision rolling process with controlled rolling and controlled cooling functions is applied to produce rolled and heat-treated wire rods, which feature high dimensional accuracy and minimal surface defects.

3. Current Status and Development Direction of Steel for Automotive Fasteners

3.1 Clean Bolt Steel

To improve cold heading performance and steel quality, impurity element content in steel shall be reduced as much as possible. Reducing S content improves the deformability of steel, decreases non-metallic inclusions and enhances toughness and plasticity. Reducing P content lowers the deformation resistance of steel. Meanwhile, lowering both P and S can reduce their segregation at grain boundaries that aggravates grain boundary embrittlement, and improve the delayed fracture resistance of steel. Reducing oxygen content in steel effectively decreases oxide inclusions and improves the cold working deformability of steel.

The Central Iron and Steel Research Institute has conducted studies on the threshold stress intensity factor for stress corrosion cracking (\(K_{ISCC}\)) of high-cleanliness ML42CrMo steel. The \(K_{ISCC}\) of high-cleanliness steel is substantially higher than that of commercial steel. Therefore, to guarantee excellent comprehensive properties after high-strength strengthening, the P and S contents in high-strength bolt steel shall be further reduced.