With the automation of automobile gear processing line, in order to continuously improve production efficiency, many countries are studying the use of lead containing free cutting steel for automobile gears. Japan has been developing lead containing steel refining technology based on new ladle refining and vacuum degassing treatment, so as to make the lead evenly distributed while steadily reducing the oxygen content. In French and German standards, many steel grades have lower limit requirements for sulfur, and its sulfur content is generally only 0.020% ～ 0.035%, rather than the idea that the lower the sulfur in the original concept, the better. The sulfur content of these steels is much lower than that of the Chinese national standard gb8731-88 free cutting structural steel.
It is still difficult to improve the free cutting property only by conventional smelting methods. It is necessary to improve the shape and distribution of sulfide through appropriate smelting process. In addition, the air cooling treatment after steel forging and rolling can prevent the appearance of granular bainite and improve the metallographic structure, which is also an effective way to improve the cutting performance.
For automobile gear steel, in addition to the above characteristic requirements, the banded structure should also be reduced or eliminated. For example, in the smelting process, the content of harmful elements, gases and impurities in the steel should be reduced as much as possible, low pouring temperature should be adopted in the pouring process, and the formation of equiaxed crystals should be increased to ensure uniform properties. At present, reducing the Si content in the steel is the trend of carburized automobile gear steel.