Comparison of penetrated MnS grains by steel matrix in variously loaded free-cutting steels with/without Pb as one of the workability parameters

摘要

The free-cutting steel is one of the essential structural steels in electrical appliance industry and automotive industry. Most of the steel contains Pb, as solid lubricant, chip breaker, tool edge stabilizer and tool life extender. MnS grains also play the same role of Pb. However, Pb is toxic so that it will be prohibited to add to the steel in near future. Steel industry has tried to replace Pb with other element(s) and/or compound(s). The trials are still going on. This study proposes a candidate of the workability parameters for free-cutting steels with/without Pb. Investigated steels were loaded quasi-statically (strain rate: 1x10(-3) s(-1)) and dynamically (1x10(3) s(-1)) up to fracture. Half of the specimens were pre-fatigued to form easier slip situation in steel matrix. After fracture, thin steel matrix penetrated into plenty of MnS grains. For the steel with Pb, the ratios of the number of such penetrations to that of total grains maintained almost teh same value for all loading conditions. The ratios were varied with the loading conditions for the steel without Pb. These facts meant that Pb was a key element for easier slip of steel matrix. Therefore, this ratio had a feasibility to be one of the workability parameters for the free-cutting steel.