【正文】 respectively , two bars were first cut for 60 min at the cutting speed of m/ min , and then cut for 35 min at the cut ting speed of 34 m/ min in the experiment of the flank wear of tool. Table 1 Chemical position of test steels 2 Results and Discussion Simulation of the cooling rates of large section NQP The NQP steel for plastic mould is expected to be as large as possible , while its cooling rate slows down with its size increasing. For the block with size 460 mm 800 mm 3 200 mm (it is predesigned in the factory) , it is necessary to know it s temperature field , which can guide the alloy design with proper hardenability. Briefly speaking , if the core and surface cooling rate of air cooled block are known , and the corresponding microstructures of the test steel at both cooling rates are similar , the hardness of NQP steel for a large section mould will distribute uniformly. Curve B and curve C in show the estimated temperature at core and edge in the block separately by FEM simulation. These estimated cooling rates provide the reference data to select the suitable hardenability for the NQP steel. For physically simulating above the slowest and fastest cooling rates ,the temperature of test steels was measured by the Ktype thermocouple when cooled at cont rolled rate and air cooled after fing. The measured temperature is shown as curve A and curve D. As the latent heat is not considered in the FEM simulation , and the radiation emissivity coefficient of surface may vary in the actual cooling condition , the controlled cooling rate is slower than that of the estimated cooling rate of the core. Furthermore , the cooling rate of test steels air cooled after fing is faster than that of the surface of the block estimated. According to this , the scope of the controlled cooling rate and the air cooling rate is wider than that of the estimated cooling rate of the section in the block. Combined FEM simulation of 460 mm 800 mm 3 200 mm block and physical simulation of the test steels , microstructures , and properties at core and surface in the block can be pared by controlled cooled test steels and air cooled ones Chemical position design When the prehardened steel for plastic mould is manufactured into the mould cavity , its interior side bees the working surface , which means it should have a uniform level of hardness. Therefore , hardenability and hardness uniformity throughout the crosssection play an important part in prehardened steel[6 ] . Since conventional prehardened steel is cooled in quenchant during heat treatment , its hardness uniformity is closely correlated to the martensitic hardenability in several cases[7]. But for the NQP steel after fing , heat exchange with air of its surface is much slower than that with quenchant , and the NQP prehardened steel for large section mould is difficult to obtain martensite through the whole section. However , if only the polygonal ferrite transformation retards and the bainite transformation occurs through the whole section , the hardness of the NQP steel may differ slightly. The hardenability for the NQP steel is called bainitic hardenability in this article. Though carbon is an easy and conventional element to harden steel , excessive carbon obviously impairs weldability and may affect the to this , the carbon content must be around % %. Molybdenum can retard high temperature polygonal ferrite transformation efficiently while