Improved Mechanical Properties in Carbon Martensitic Steel Achieved by Continuous Carbon Gradient and Multilayered Structure

Increasing carbon content in martensite enhances the strength of carbon steel but reduces ductility and toughness. In this study, a multilayered carbon gradient steel was developed to overcome this trade-off by stacking high-carbon (1 wt%) and low-carbon (0.2 wt%) steel plates through preliminary diffusion and multi-pass hot rolling. The resulting microstructure showed a continuous gradient from high-carbon martensite to low-carbon martensite. After low-temperature tempering, the tempered samples exhibited hardness fluctuations along the normal direction, with a maximum value of approximately 700 HV or more in high-carbon regions and a lower value of 500 HV or less in low-carbon regions. Compared to low-carbon steel, the sample tempered at 200 °C showed significant improvements in both strength and ductility, with 1880 MPa ultimate tensile strength and 4.7% uniform elongation. This larger uniform elongation than that of the plain low-carbon steel can be attributed to the greater strain hardening rate in high-carbon regions with a high carbon solid solution strengthening. Simultaneously, it is believed that more slip systems in high-carbon regions could be activated under the multiaxial stress around the layer interface, then showing a better ductility than that of the plain high-carbon steel. Additionally, the gradient structure between different regions effectively helped to avoid abrupt stress and deliver multiaxial stress at any location along the normal direction. The stepped path of the cracks under uniaxial tensile stress suggested a higher fracture toughness.