According to the statistics of China Automobile Industry Association in January 2010, the production and sales of automobiles in China in 2009 were 13.7910 million and 13.648 million respectively, up 48% and 46% year on year respectively. The rapid development of the automobile industry has increased the demand for steel materials. Although China has been the world's largest steel producer for many years, the steel used for fasteners of automobile engines and key components is mainly imported due to strength limitations. At the same time, with the development trend of high-performance and lightweight automobiles, mold manufacturers have put forward higher requirements for the design stress and lightweight of automobile fasteners, The high strength of fastener steel is an effective way to solve this problem.
1. Performance Requirements of Steel for Automobile Fasteners
Requirements for steel used for automobile fasteners:
With good surface quality, high level of dimensional accuracy and low level of non-metallic inclusions and segregation;
It has high tensile strength and good cold heading performance;
It has high fatigue resistance and multiple impact tensile resistance;
It has sufficiently low delayed fracture sensitivity and low ductile brittle transition temperature.
2. Smelting and Rolling Characteristics of Steel for Automobile Fasteners
2.1 Smelting technology
The continuous casting process with external refining and electromagnetic cable mixing is adopted. It is mainly to control the composition of C, Si, Mn, Cr, Mo and other major elements in the steel in a small range, and reduce the content of P, S, 0, N and other impurities in the steel as much as possible, so as to achieve the goal of high purity of steel, low non-metallic inclusions, and low segregation level, thereby improving the cold heading performance of steel and improving the surface quality of steel.
2.2 Rolling technology
The high speed and high precision rolling process with the functions of controlled emulsion and controlled cooling is adopted to obtain the rolled heat treated wire rod with good dimensional accuracy and as few surface defects as possible.
3. Current Situation and Development Trend of Steel for Automobile Fasteners
3.1 Clean bolt steel
In terms of improving the cold heading property and quality of steel, it is necessary to reduce the content of impurities in steel as much as possible. Reducing S content can improve the deformation ability of steel, reduce non-metallic inclusions in steel, and improve toughness and plasticity; The reduction of P content can reduce the deformation resistance of steel, and the reduction of P, S content can also reduce its segregation at grain boundaries, reduce grain boundary embrittlement, and improve the delayed fracture resistance of steel; Reducing the content of 0 in steel can effectively reduce oxide inclusions, thus improving the cold working deformability of steel.
The General Institute of Iron and Steel Research has studied the stress corrosion threshold stress intensity factor KISCC of ML42CrMo steel with high cleanliness. The KISCC of steel with high cleanliness is significantly higher than that of commercial steel. Therefore, in order to ensure that the steel still has high comprehensive performance after high strength, the content of P and S in high-strength bolt steel should be further reduced.
3.2 Economical high-strength bolt steel
3.2.1 Microalloy non quenched and tempered steel
Using microalloyed non quenched and tempered steel to make bolts can omit spheroidizing annealing before cold drawing and quenching and tempering treatment after bolt forming, reduce decarburization tendency of thread threads, and improve the yield of bolts, with obvious economic benefits.