【作者】 王勇围；

【导师】 白秉哲；

【作者基本信息】 清华大学 ， 材料科学与工程， 2009， 博士

【摘要】 本文针对碳含量范围为0.07%～0.30%的一系列低碳Mn系贝氏体钢,总结了一整套强韧性优化的途径。具体研究了不同C含量和不同Si含量的钢在奥氏体化后不同冷速下的组织类型,经空冷、系统回火后组织性能的变化规律,详细讨论了粒状组织的相变过程及M/A小岛形态。对于碳含量为0.08%左右的Mn系贝氏体钢,通过Gleeble热力模拟试验,研究了未微合金化及Nb微合金化后经不同温度热变形对组织的影响,在实验轧机上控轧后的性能数据较好的验证了研究结果。得到了以下主要结论:1. Si对碳含量为0.07～0.08%的Mn系贝氏体钢空冷后的组织类型几乎不产生影响,但对碳含量为0.17%的钢能明显提高淬透性。2.低碳Mn、Si钢的粒状组织中M/A小岛具有无规则排列型和平行排列型两种形态,当高温铁素体以台阶机制长大时,容易生成平行排列的小岛,区分粒状贝氏体和粒状组织不能以小岛是否平行排列为标准, Cr能有效抑制低碳Mn、Si钢中的粒状组织转变,使高温先共析铁素体中不出现小岛。3. Mn系贝氏体钢空冷后存在较大的宏观残余应力,屈服强度较低。低温回火能有效消除残余应力,使屈服强度增高,400℃左右回火后屈服强度达到最高。将Mn系贝氏体钢提高Si含量后经低温回火能获得很好的强韧性配合,对于低Si钢,高温回火后韧性值显著提高。4.随着变形温度的降低,低碳Mn系贝氏体钢中铁素体量增多,晶粒细化,韧性提高,加Nb之后相同形变工艺下组织显著细化,终轧温度降至760℃后,铁素体的量不发生明显的增多,而进一步发生细化,铁素体相变开始以形核为主。含Nb的低碳Mn系贝氏体钢性能比不含Nb的对于终轧温度更敏感,降低终轧温度能更有效提高含Nb钢的性能。5.采用粒状贝氏体/马氏体(Bg/M)复相组织,成功研制出工艺简单、低成本的930MPa级超高强螺纹钢筋,已实现工业化生产。对原有低碳Mn系贝氏体非调质中厚钢板进行了性能优化,优化后组织类型为仿晶界铁素体、晶内铁素体、粒状贝氏体复相,控轧+空冷后,R0.2﹥600MPa, Rm﹥800MPa,0℃的V型冲击韧性值大于150J。更多还原

【Abstract】 Mn-series banitic steels with carbon content 0.07wt%-0.30wt% have been selected as the test materials in this paper. The methods of optimization on the strenghth and toughness of the steel have been systematically studied. The microstructures of Mn-series banitic steels of different carbon content and Si content and the changing of properties of the steels after air cooling and tempering have been studied. Phase transformation and the morphology of islands of granular structure were studied. The steels with the carbon content of about 0.08% were tested on Gleeble thermal-mechanical simulator machine, and the microstructures of non micro-alloyed and micro-alloyed steels were investigated after deformation at different temperature. The researching results were verified well by means of control rolling on testing rolling mills. The main conclusions of the paper are drawn as follows:1. Si almost has no influence on the microstructures of the steels with the carbon content 0.07～0.08%, but can improve the hardenbility of the steels with the carbon content 0.17%.2. The morphologies of M/A islands in the granular structure are two types: irregular and regular. The parallel islands are sensitive to form when the matrix grow in ledgewise mechanism. The islands parallelly or irregularly distributing are not the criterion of granular bainite and granular structure. The transformation of granular structure can be efficiently inhibited by Cr, so there are no islands in the proeutcoid ferrite.3. The huge macro residual stress will be remained in Mn-series bainitic steels after air cooling, result in low yield strength, and the stress will be efficiently eliminated after tempering at low temperature and the optimum temperature is about 400℃. The strength and toughness of the high Si content of the steels can match better after tempering at low temperature.4. The content of ferrite in low carbon Mn-series increases with the deformation temperature decreased, and the grains and toughness are refined and improved. The microstructures of the Nb micro-alloyed steel are more refined on the same deformation process, and the ferrites keeps a relatively stable content when the deformation temperature decrease to about 760℃. However the grain refined further and the transformation of ferrites turns to nucleation dominated process. The strength and toughness of Nb Micro-alloyed Mn Series Bainitic Steels are more sensitive to the finishing rolling temperature, and the properties of Nb Micro-alloyed steels can be improved more effectively with decreasing the finishing rolling temperature.5. The reinforced bar with the yeild strength 930MPa has been developed and the microstructure is B/M dual phase, the process is simple and cost is low, and successfully industrialized. The non-moderating medium plate with high strength and toughness has been developed, and the microstructure includes FGBA、intragranular ferrite、granular bainite, the properties are: R0.2﹥600MPa, Rm﹥800MPa, AKV(0℃)﹥150J after control rolling and air cooling。更多还原