【作者】 罗毅；

【导师】 吴晓春；

【作者基本信息】 上海大学 ， 材料学， 2009， 博士

【摘要】 随着塑料工业的迅速发展,针对塑料件尺寸逐渐增大,塑料模具制造能力的要求不断提高,急需开发模块厚度大于300 mm,硬度在36~42 HRC的非调质预硬型塑料模具钢。因此,本文聚焦大截面高硬度非调质预硬型塑料模具用钢的需求现状,从温度场计算、成分设计、组织控制和实验钢中试等方面进行了系统研究,取得了如下成果:研究了300 mm×300 mm×300 mm的调质预硬型塑料模具钢3Cr2MnNiMo模块加热后于静止空气和流动空气中冷却时的温度场,数值模拟计算了模块在两种冷却条件下的温度场,获得了大截面模块锻造后冷却时的边界条件范围(空冷时辐射率为0.6,对流换热系数为7 W/(m2·℃);风冷时辐射率为0.6,对流换热系数为29 W/(m2·℃)),进而计算了直径为300~800 mm、重10 t的不同圆坯空冷和风冷的温度场,得到了临界冷却速率,为全贝氏体型非调质塑料模具钢的成分设计奠定基础。采用误差反向传播神经网络(BP神经网络)模型,建立了合金钢成分同其连续冷却时硬度之间的关系,预测了合金钢以0.5、0.3、0.05和0.03℃/s连续冷却时的硬度,进而指导Φ300~800 mm圆坯空冷时获得全贝氏体组织的成分设计,筛选出6组钢的成分。连续冷却相变实验研究表明,BP模型用于全贝氏体型非调质钢的成分设计是有效的。采用数值模拟计算获得了300 mm×1000 mm×3800 mm和500 mm×1000 mm×2300 mm预硬型塑料模具钢模块的临界冷却速率,并采用以上实验室设计的钢成分中试了上述尺寸的模块SDFT1(0.20C-0.6Si-2Mn-1Cr-0.3Mo-0.1V)钢和SDFT2(0.25C-0.4Si-2Mn-1Cr-0.45Mo -0.1V)钢。力学性能、车削加工性能、抛光性能和耐腐蚀性能研究表明,SDFT1和SDFT2钢均满足预硬型塑料模具钢的使用要求。空冷和砂冷的SDFT1和SDFT2钢组织均为贝氏体。两钢分别在0.03～0.5℃/s和0.02～0.3℃/s范围内连续冷却时贝氏体开始转变温度接近。透射电镜(TEM)观察表明,贝氏体组织的精细结构主要为贝氏体铁素体板条和少量马氏体/奥氏体(M/A)岛,贝氏体铁素体板条间有残余奥氏体或碳化物,空冷的贝氏体铁素体板条较窄,并且M/A岛尺寸也较小。贝氏体转变开始温度不同,位错强化效果有差别;冷却速率不同导致的贝氏体板条宽度变化,对两钢分别在空冷和砂冷的硬度影响较小。SDFT钢未回火时的屈服强度较低,随着回火温度的升高屈服强度开始迅速提高,约350℃达到峰值,随后缓慢下降;随着回火温度的升高冲击韧性开始提高,约350℃达到峰值,当回火温度在400～500℃之间继续升高时,试验钢的冲击韧性开始快速降低,约450℃冲击韧性降到最低,随后又开始升高。X射线衍射和TEM研究表明,SDFT钢的力学性能随回火温度变化的可能原因是残余奥氏体转变和马氏体岛的分解。以上述研究为基础,宝钢股份公司生产了厚度在300～600 mm、宽度在800～1200 mm的非调质塑料模具钢模块(SWFT钢),预硬化范围在36～42 HRC,硬度波动在±1.5 HRC内。对截面460 mm×800 mm的SWFT钢和500 mm×800 mm的3Cr2MnNiMo钢比较研究表明,SWFT钢在整个截面尺寸范围内均为贝氏体,其精细结构主要为贝氏体铁素体板条和少量M/A岛,贝氏体铁素体板条间有残余奥氏体或碳化物,表层的贝氏体铁素体板条窄于心部;3Cr2MnNiMo钢表层组织为回火马氏体,心部组织为回火马氏体和贝氏体。SWFT钢的力学性能、加工性能和使用性能均满足调质预硬型塑料模具钢的要求,而且减少了3Cr2MnNiMo钢锻造后再次奥氏体化淬火的热处理工序,缩短了生产流程,降低了能耗。研究成果对于大截面非调质预硬型塑料模具用钢的命名以及工业化生产均有指导意义。更多还原

【Abstract】 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.更多还原