The accidental birth of free-cutting steel 80 years ago was related to the urgent need for war (World War I) and the shoddy steel (high-sulfur and high-phosphorus steel). Today, 80 years later, sulfur easy-cut steel, lead free-cutting steel, calcium free-cutting steel and composite free-cutting steel are widely used in various industrial sectors. As the degree of processing automation increases, the machinability requirements for materials increase. The cost of cutting processing accounts for about 50% of the cost of parts. Therefore, reducing cutting processing costs, enabling free-cutting steel to adapt to automated production lines, and increasing labor productivity are urgent needs of steel material users. The embrittlement of phosphorus on the ferrite matrix and the internal notch effect of the sulfide are the roots of the high machinability of the sulfur-based free-cutting steel. To this day, sulfur free-cutting steel is still the most widely used and largest free-cutting steel. Lead-based free-cutting steel has been produced for more than 50 years. In recent years, the lead-based free-cutting steel produced by ladle refining, argon blowing treatment and continuous casting method has stable composition, uniform distribution of lead particles, and reduced chemical composition segregation. Lead added to free-cutting steel exists in a spherical form in steel, so lead hardly affects the mechanical properties of steel. However, in the past 10 years, although steel mills have used protective devices to solve the problem of lead poisoning, it is still difficult to meet the increasingly demanding requirements of the environment. Therefore, the development of multi-purpose free-cutting steel with low sulfur and controlled sulfide form is an important development direction. The sulfur content of low-sulfur free-cutting steel is generally ≤0.04%, the maximum is 0.10%, and calcium is added to improve the sulfide form. As a special deoxidizer, calcium forms a complex calcium aluminum oxide with the original alumina. On this basis, a softer calcium-manganese sulfide is coated, which is beneficial to improve cutting performance and mechanical properties. Therefore, calcium is generally selected as a composite additive element. In addition, zirconium, titanium and rare earth elements can also be used to control the sulfide form.
Low-sulfur composite free-cutting steel is currently used to produce gears, steering racks, spring blocks, connecting rods, crankshafts, etc. In addition to low-sulfur non-tempered steel and gear steel, Japan produces and applies SUM, SC, alloy and non-quenched steels containing lead, sulfur, calcium and composites according to different needs. Elemental free-cutting steel, widely used in the production of various automotive parts. The annual output of Japanese free-cutting steel exceeds 1 million tons, of which SC series lead-free free-cutting steel has the largest output, about 350,000 tons. The annual output of free-cutting steel in China is less than 20,000 tons.
The research and production trends of free-cutting steel are: developing an economical, reliable and simple new method for sulfide morphology control; studying the relationship between metallurgy and metallurgical factors of cutting steel and cutting performance; developing new and no composite Cutting steel; developing new free-cutting steel suitable for various tool processing; combining cutting materials, tools and processing machinery to improve the cutting performance of steel.
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