Design And Research On The Non-quenched Prehardened Plastic Mould Steel

With the rapid development of the plastic industry, dimension of plastic products increases gradually, and manufacturing capability of the plastic mould is improved continuously. It is necessary to develop the non-quenched and tempered prehardened(NQP) plastic mould steel with its thickness over 300 mm, hardness between 36~42 HRC. Therefore, the thesis focuses on the requirement of plastic mould steel with large section and high hardness, and systematically develops study from temperature field, chemical composition design, microstructure control and pilot realization of test steel. The main results are the followings:Combined the temperature history of the 300 mm×300 mm×300 mm quenched and tempered prehardened(QP) plastic mould steel 3Cr2MnNiMo block under air cooling and fan cooling, and calculation result by the numerical simulation, the boundary condition range is acquired when the large section plastic mould steel block cools after forging. Subsequently, the temperature field of 10 ton round billets with diameter of 300~800 mm is calculated when they are air cooled and fan cooled, which lays the foundation for the chemical composition of bainitic NQP plastic mould steel.The relation between chemical composition of alloyed steel and its hardness under continuous cooling is built by Back Neural Network(BP) method. The hardness of alloyed steel under 0.5, 0.3, 0.05 and 0.03℃/s could be predicted, and chemical composition ofΦ300~800 mm round billet with whole bainite could be guided by BP model. 6 steels are chosen by BP model, the validity of which is proved by the continuous cooling transformation(CCT).The critical cooling rate of the 300 mm×1000 mm×3800 mm block and 500 mm×1000 mm×2300 mm block is acquired by numerical simulation. The SDFT1(0.20C-0.6Si-2Mn-1Cr- 0.3Mo-0.1V) steel fit for the 300 mm×1000 mm×3800 mm block and SDFT2(0.25C-0.4Si-2Mn- 1Cr-0.45Mo-0.1V) steel fit for the 500 mm×1000 mm×2300 mm block are developed by the test steels above. Tests of mechanical properties, machinability, polishing and corrosion resistance show that both steels satisfy the requirement of plastic mould steel.Both steels air cooled and sand cooled are bainite, and the start transformation temperatures of their bainite"C"curve are close between the cooling rate range of 0.03～0.5℃/s for SDFT1 steel and 0.02～0.3℃/s for SDFT2 steel. Fine microstructures observed by transmission electronic microscopy(TEM) are mainly composed of bainitic ferrite laths and martensite/austenite (M/A) islands, and cementite and/or residual austenite exist in bainitic ferrite laths. Considering CCT of the SDFT steel, their small hardness variation between air cooled and sand cooled is caused by their high bainitic hardenability. Moreover, differences of lath widths and dislocation density in bainitic ferrite lath strongly correlating to bainite transformation temperature make theirs hardness a little higher in air cooled than that in sand cooled.Yield strength and impact toughness of the SDFT steels without tempering are lower, but they improve with the tempering increasing, and reach the peak values at about 350℃, and then their yield strength decreases slowly, while their impact toughness decreases rapidly at 400～450℃, and reaches valley value at about 450℃. Combined the X-ray diffraction and TEM analysis, above results mainly caused from transition of residual austenite and decomposition of M/A islands.Based on the development of the SDFT steels, Baosteel company has produced the NQP plastic mould steel(the SWFT steel) with thickness at 300～600 mm, width at 800～1200 mm, and hardness at 36～42HRC. The SWFT steel block with sectional dimension of 460 mm×800 mm is researched by comparison with the QP plastic mould steel block(3Cr2MnNiMo) with that of 500 mm×800 mm by tests of mechanical properties, processability and application properties. Results show that microstructures of the SWFT steel both at core and at surface are bainite, fine microstructure of which is composed of bainitic ferrite laths with high dislocation density and interlath cementite and/or residual austenite, and width of bainitic ferrite laths at surface is narrower than that at surface; while microstructure of 3Cr2MnNiMo steel at surface is tempered martensite, and which at core is tempered martensite and bainite. Furthermore, the SWFT steel satisfies the requirement of the QP plastic mould steel in addition to simplified process and reduction of energy.Research results could directly instruct naming of the NQP plastic mould steel with large section and commercial production.