To elucidate the phase transformation behaviors for 38MnB5Nb ultra high-strength steel, the continuous cooling transformation (CCT) and time-temperature-transformation (TTT) curves were determined by the thermal expansion method with different cooling rates and isothermal temperatures after complete austenization. To be more accurate, the microstructure was observed and the hardness was tested. The results showed that the starting and ending transformation temperatures of austenite during heating are 748 °C and 805 °C, respectively. Bainite’s start temperature is between 540 °C and 550 °C, while martensite’s start temperature is about 310 °C. The critical cooling transformation rate is between 10 °C/s and 15 °C/s. The results showed that the microstructures are severely related to the cooling rate during the continuous cooling process and are related to isothermal temperatures during the isothermal process. The relationship between hardness and the microstructure was investigated and hardness is severely related to the microstructure. Based on the results of CCT curves compared with the conventional 22MnB5 hot stamping steel, the studied 38MnB5Nb steel is more beneficial for selective cooling processes.

Tailored properties hot forming of 38MnB5Nb by partition cooling can be configured on-demand. A gradient distribution microstructural characteristic can be generated by undertaking an appropriate cooling-tempering process for the regions with different properties requirements before forming. A unified viscoplastic constitutive model coupled with the primary cooling temperature and related material constants is established based on genetic algorithm. Meanwhile, the use of the mixture of jet air and dry ice particles to cool the partition region is essential to achieve different primary cooling temperatures. In this paper, the inverse heat conduction problem is solved to obtain the relationship between the interfacial heat transfer coefficient and different cooling conditions in the partition cooling process. The U-shaped part is taken as an example to simulate the change of temperature, stress-strain, thickness, and spring-back in the process of partition cooling and tailored hot forming properties. The results show that the gradient microstructural characteristic formed by partition cooling has a great influence on the stress field distribution. The maximum stress of 345 MPa can be reached after complete pressure holding in the partition slow cooling tailored properties of the hot forming process. The maximum stress can reach 743 MPa in the partition fast cooling tailored properties of the hot forming process due to the relatively high deformation resistance of the tempered martensite at the bottom center. The maximum residual stress in the two processes after spring-back drops to 305 MPa and 545 MPa, respectively. The spring-back is small under the two processes, with a maximum spring back angle of no more than 1°.

To elucidate the hardness evolution behaviors for 38MnB5Nb ultra-high-strength hot stamping steel, a series of tempering processes with varying tempering temperatures and times were carried out with a dilatometer. Meanwhile, the hardness of each sample was measured after dilatometer experiments. The results indicated that the tempering process parameters (including the tempering temperature and time) play an important role in the hardness of the studied steel. The hardness of 38MnB5Nb ultra-high-strength hot stamping steel at the quenched state is about 580 Hv, while it is 240 Hv for the quasi-annealed state. As the tempering time extends, the hardness is decreased sharply at the initial stage; then, the hardness is decreased in a quasi-linear trend with a slight slope; finally, the hardness almost keeps a constant value, which depends on the tempering temperature. In addition, the tempering process has a big effect on the mechanical properties of 38MnB5Nb ultra-high-strength hot stamping steel by increasing the product of the strength and elongation by about 40%.