According to the statistics released by the China Association of Automobile Manufacturers in January 2010, China’s automobile production and sales in 2009 reached 13.791 million units and 13.6448 million units respectively, representing a year-on-year increase of 48% and 46%. The rapid development of the automotive industry has boosted the demand for steel materials. Although China has remained the world’s largest steel producer for many consecutive years, imported steel still dominates for fasteners used in automobile engines and other critical components due to strength limitations. Meanwhile, the trends toward higher performance and lightweighting of automobiles impose stricter requirements on the design stress and weight reduction of automotive fasteners. High-strength steel for fasteners serves as an effective solution to meet these requirements.

1. Performance Requirements of Steel for Automotive Fasteners

Steel for automotive fasteners shall satisfy the following requirements: (1) Good surface quality, high dimensional accuracy, and low levels of non-metallic inclusions and segregation; (2) High tensile strength and excellent cold heading performance; (3) High fatigue resistance and resistance to repeated impact tension; (4) 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 primary purpose is to control the main alloying elements such as C, Si, Mn, Cr and Mo within narrow ranges, and minimize impurity elements 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

A high-speed, high-precision rolling process with controlled rolling and controlled cooling functions is applied to produce rolled heat-treated wire rods (non-quenched and tempered or annealing-free). The wire rods feature favorable dimensional accuracy (no need for drawing treatment) and minimal surface defects (no peeling required).

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

3.1 Clean Bolt Steel

To improve cold heading performance and steel quality, the content of impurity elements in steel should be reduced as much as possible. Reducing sulfur (S) content improves the steel’s deformability, reduces non-metallic inclusions and enhances its toughness and plasticity. Reducing phosphorus (P) content lowers the deformation resistance of steel. Meanwhile, lowering both P and S content can reduce their segregation at grain boundaries to mitigate grain boundary embrittlement, and improve the steel’s resistance to delayed fracture. Reducing oxygen (O) content in steel effectively decreases oxide inclusions and improves the cold working deformability of steel.