The effect of YAG laser cutting on stretch-flangeability of ultra high strength TRIP-aided steel sheets with bainitic ferrite matrix (TBF steel sheet) austempered at 375 or 450°C, was investigated for automotive applications. Holes of 5 mm diameter for hole-expanding test were produced by YAG laser cutting and mechanical punching and the stretch-flangeability was evaluated by measurement of the hole-expanding ratio (l). In TBF steel sheet, laser cut specimens show higher stretch-flangeability than mechanically punched specimens. The hole-expanding ratio (l) of TBF steel sheet austempered at 375°C in the case of laser cutting at powers between 50 and 100 W was higher than those austempered at 450°C. Furthermore, the strength-stretch-flangeability balance (TSϫl) of TBF steel sheet austempered at 375°C showed the highest value after laser cutting at 100 W. Compared to mechanical punching, YAG laser cutting contributed to the improvement of the TSϫl to 1 100 MPa with TBF steel sheet possessing fine bainitic ferrite matrix.

To improve the machinability of SUS304 (Type 304) austenitic stainless steels, specimens were prepared containing 0.016 mass% boron and 0.2 mass% nitrogen, and hexagonal boron nitride (h-BN) particles with a diameter of 1 to 5 μm were precipitated. Precipitation of h-BN reduced the cutting force and tool wear during lathe turning with a cemented carbide tool insert, especially at cutting speeds of 40 m/min and higher. The reduction in cutting force appeared attributable to internal lubrication by h-BN in the chip shear region and the deformation flow layer, as well as to lubrication between the chip and carbide tool. Improved chip disposability and tool wear suppression were also achieved by h-BN precipitation. SUS304 steel with precipitated h-BN was found to exhibit good machinability in drilling and sawing operations with high-speed steel tools.

The authors have reported that the machinability (lathe turning, drilling and sawing) of SUS304 (Type 304) austenitic stainless steel is improved by precipitating hexagonal boron nitride (h-BN). In this study, we investigated the surface roughness after lathe turning in the h-BN precipitated SUS304 steels, where the nitrogen content was fixed at 0.2% and the boron content was changed as 0, 0.03, 0.05, and 0.1% (mass%). Surface roughness increased as the boron content increased, especially at a lower cutting speed of 22 m/min. However, SUS304 steel with boron content of up to 0.05 mass% had similar or smaller surface roughness compared with a commercial SUS304 steel, and the SUS304 steel containing 0.1 mass% boron had smaller surface roughness compared with a SUS303 (Type 303) sulfur-added free-cutting steel. Precipitation of h-BN increased the size of the build-up edge and increased the surface roughness. On the other hand, solute nitrogen increased the Vickers hardness, leading to decreased separation size of the build-up edge and decreased surfaced roughness.

In order to improve the machinability of Type 304 austenitic stainless steels, 0.016 mass % boron and 0.2 mass % nitrogen were added, and hexagonal boron nitride (h-BN) particles (1~5 μm diameter) were precipitated. During turning with cemented carbide insert, cutting force and tool wear were reduced by h-BN precipitation, especially at higher cutting speed of more than 40 m/min. The reduction of cutting force seems to be due to the internal lubrication effect of h-BN in chip shear region and deformation flow layer, along with the lubrication between chip and carbide tool. Better disposability of chip and suppression of tool wear were also achieved by h-BN precipitation. Good machinability of h-BN precipitated Type 304 steel was also found in drilling and sawing with high-speed steel tools.