Fig. 1 Schematic diagram of the manufacture of multilayered carbon gradient steel, including the acid pickling, preliminary diffusion in vacuum hot-pressing furnace a and hot rolling followed by low-temperature tempering b. RD, ND, and TD represent the rolling direction, normal direction, and transverse direction, respectively

Fig. 2 Content of a carbon, b manganese, c nickel, along normal direction between low-carbon and high-carbon regions

Fig. 3 Microstructure of the quenched and tempered samples: A diagram and optical image of quenched samples in RD-ND profile; B SEM images of quenched sample in the b1, b2, b3 and b4 locations marked in A, which is located in low-carbon, medium-carbon and high-carbon regions; C and D SEM images of tempered samples at 200 °C and 300 °C, respectively. The locations of c1 d1, c2 d2, c3 d3 and c4 d4 correspond with those in B

Fig. 4 Phase distribution from the EBSD results of quenched a and tempered samples at 200 °C b, 300 °C c. Retained austenite and prior carbides are marked by pink and green respectively. The element maps of carbon and manganese are included to mark the demarcation between high-carbon and low-carbon regions

Fig. 5 Hardness along normal direction of the quenched a, tempered samples at 200 °C b, 300 °C c

Fig. 6 Tensile stress-strain curves of quenched a, tempered MLCGS samples at 200 °C b, 300 °C c. Note that tensile stress-strain curves of high-carbon steel, low-carbon steel and stainless steel (304ss) are added for comparison

Fig. 7 Strain hardening rate of the tempered MLCGS samples at 200 °C a, 300 °C b, in comparison with the low-carbon steel, high-carbon steel and stainless steel (304ss)

Fig. 8 Comparison of results with short-time diffusion sample after tempering at 200 °C: a, b optical microscopy images, c comparison of the microhardness, d comparison of the tensile stress-strain curves

Fig. 9 Strain distribution in RD-ND plane after tensile test with the plastic strain approximately 6%, for the multilayered sample tempered at 200 °C: a, b optical images before and after tensile test; c, d strain mappings along the TD and ND after tensile test, respectively; e detailed mean value of plastic strain along the TD and ND, ΔεTD and ΔεND

Fig. 10 Images of tensile specimen a, fracture mode b, fracture surface morphology c of the tempered MLCGS sample at 200 °C. The 3D morphology of fracture surface is added in c