【作者】 廖建雄；

【导师】 闵永安；

【作者基本信息】 上海大学 ， 材料加工工程， 2011， 硕士

【摘要】 长期以来,我国国家标准中只有唯一一种专用渗氮钢38CrMoAl, 38CrMoAl存在以下不足:强度和热稳定性满足不了高要求基体支撑强度要求;淬透性不高,热处理温度相对较高;由于含铝铁素体稳定性高,不易溶入奥氏体中,所以保温时间要比一般合金结构钢长0.5倍左右等,远不能满足高标准零件的要求。而欧标EN10085中就包含了9种专用渗氮钢,新型高性能专用渗氮钢在我国亟待开发。本文在对比国内外学者开发研究渗氮钢的合金成分特点后,提高Si、Mn含量,以提升渗氮钢淬透性及耐热性能,Cr、Mo为主要合金化元素,V、Nb作为微合金元素加入,控制钢中S、P含量和有害元素含量,以提高渗氮件的力学性能和渗氮工艺性能,发挥微合金元素的作用,研发出细晶粒新型渗氮钢。深入研究了新材料的相变特性、宏观力学性能、热加工性能及渗氮性能,利用相图软件进行模拟计算,并使用OM、SEM、TEM、EDS、3DAP等微观分析手段,对新型渗氮钢BTHJ钢的性能和微观机理进行了深入的分析研究,得出了以下结论:1、BTHJ钢在冷速为0.1～0.3℃/s时得到粒状贝氏体,马氏体临界冷速为3℃/s,在0.1℃/s以下是珠光体与贝氏体的混合组织。利用JMA公式求得其贝氏体转变激活能为54.5KJ/mol。等温转变时,贝氏体转变表现出更加明显的不完全转变特征,BTHJ钢的贝氏体转变温度在425℃～Ms之间。珠光体等温转变动力学可用Johnson-Mehl公式x = 1-exp[-K （T ）tⁿ /d^m]表示。2、BTHJ钢淬透性较高,在880℃～960℃内均能淬硬,并且在900℃～960℃其淬透半径超过了100mm;晶粒度测试表明,BTHJ钢在880℃～940℃淬火时,其晶粒度在9级以上;与38CrMoAl相比,BTHJ钢的淬透性有大幅度的提升,并且在较宽淬火温度内,晶粒还是非常细小的。3、BTHJ钢具有较好强度、塑性、韧性,其综合力学性能明显优于38CrMoAl钢。经光学显微镜和扫描电镜观察,BTHJ钢调质后组织均匀,碳化物细小弥散,在个别处发现少量未溶Nb的碳化物;通过透射电镜观察,BTHJ钢调质后的马氏体板条宽度在150nm左右,在基体上可以观察到多种形态的碳化物,这些碳化物主要含Cr、Mn、V等元素而含Mo碳化物很少,这表明大部分Mo溶入基体,提高了材料的抗回火软化能力。3DAP研究结果知,淬火态的BTHJ钢合金元素基本固溶在基体中,在回火后,Cr、Mn、V等合金元素形成M2C型细小碳化物,而Mo、Nb元素还是基本固溶在基体中且分布均匀,有效起到了固溶强化作用,提高其热强性。4、经过550℃保温60小时后,BTHJ钢硬度从41.4HRC降至37.6HRC,表现出良好的热稳定性,由光学显微镜和扫描电镜观察可知,在热稳定后基体发生了一定程度的回复再结晶,基体中析出碳化物明显增多,由碳化物萃取结果可知,BTHJ钢热稳定前后碳化物类型并没有发生明显的变化,主要有M23C6、M7C3、M3C型三种碳化物,结合透射电镜观察可知,材料硬度的下降主要由碳化物的粗化以及Mo元素的析出引起的。5、BTHJ钢在1100℃,0.01 s^-1的变形条件下发生了明显的动态再结晶,随着变形温度的降低,变形速率的增加,变形抗力逐渐增加,动态再结晶越来越不明显;通过计算得到BTHJ钢的热变形方程为: （ε|&） = 2. 42e⁵⁴ [sinh（0.0065σ）]^7.26exp（-591570/RT）;利用加工硬化率确定了BTHJ钢的在各变形条件下的动态再结晶临界条件,通过计算得出临界应变随lnZ的增大而增大,对其进行线性拟合得到他们拟合函数关系为ε_c = 0.01366lnZ-0.53.6、BTHJ钢渗氮后,表面最高硬度在950HV以上,离子渗氮试验表明,BTHJ钢能在较短时间内得到较深渗层,而气体渗氮表明,在较长时间渗氮后,BTHJ钢和38CrMoAl钢渗层深度相差不大,但是由于BTHJ钢晶粒细小,在渗氮后并不会出现脉状组织等缺陷,离子渗氮和气体渗氮均表明,BTHJ钢渗层硬度梯度平缓,从而能提高残余应力层的稳定性,提高其疲劳寿命。综合来说,BTHJ钢具有更佳的渗氮性能。更多还原

【Abstract】 For a long time, 38CrMoAl steel is the unique dedicated nitriding steel in Chinese standard. There are lots of deficiencies exist in 38CrMoAl steel:strength and heat resistance can not be meet the high demand of the substrate; low hardenability; relatively, the heat treatment temperature is too high. Since the stability of aluminum-containing ferritic is very high and the aluminum is difficult to dissolve in austenite, the holding time is longer than that of alloy steel for about 0.5 times, all of those make 38CrMoAl steel can not meet the demands of high-standard parts. While there are 9 kinds of nitriding steels in Euro standard, new high-performance dedicated nitriding steels urgently need to be developed in China.While contrast the characters of composition of nitriding steels developed by domestic and foreign scholars, a new fine grain nitriding steel was developed in this paper. The content of Si and Mn were increased to enhance the hardenability and heat resistance; Cr and Mo are the main alloy elements, V and Nb were added as micro-alloying elements in order to improve the mechanical properties and nitriding performance; the content of S, P and other harmful elements were controled. The phase transformation performance, mechanical properties, hot deformation property and nitriding performance of new steel were studied. Simulated by thermodynamic software-Jmatpro, OM, SEM, TEM, EDS, 3DAP and some other micro-analytical techniques were used to investigate the microstructures and micro-mechanism of new nitriding steel BTHJ. Main research results are listed as following:1. Granular bainite were obtained when cooling rate between 0.1and 0.3℃/s, and the critical cooling rate of mantensite is 3℃/ s, while cooling rate was under 0.1℃/s, the microstructures of BTHJ is bainite and pearlite. The bainite transformation activation enegy of BTHJ is 54.5KJ/mol caculated by JMA formula. The bainite transformation showed more obvious character of incomplete transformation while isothermal transformation, the bainite transformation temperature is 425℃-Ms. Isothermal transformation kinetics of pearlite can be expressed by Johnson-Mehl formula.2. The hardenibility of BTHJ is excellent, it can be hardend between 880℃and 960℃and the harden radius is more than 100mm while austenite temperatue between 900℃and 960℃. The grain size of BTHJ is over 9 grade while quenched at 880℃～960℃. Compared to 38CrMoAl steel, the hardenibility of BTHJ is improved significantly.3. The mechanical properties of BTHJ is better than that of 38CrMoAl for its excellent strength, ductility and toughness. The microstructure of BTHJ after quenching and tempering is quite fine, and fine carbides were dispersed in matrix investigated by OM and SEM, a small amount of undissolved MC carbide which contained Nb were found, too. The TEM results showed that the width of martensite lath of BTHJ is about 150nm and various forms of carbides which contain Cr, Mn and V dispersed in matrix, but the carbides contained Mo is rare, which suggests that most of Mo dissolved in matrix, thereby enhanced the tempring resistance. The 3DAP results showed that the most of alloy element dissolved in matrix after quenching, after tempering, M2C type fine carbides which contain Cr, Mn, V were precipitated, while most of Mo and Nb were still dissolved in matrix which played a role in the effect of solid solution strengthening and enhancing heat resistance.4. After holding at 550℃for 60 hours, the hardness of BTHJ steel droped from 41.4HRC to 37.6HRC, exhibited excellent thermal stability. OM and SEM investigation showed that the matrix recrystallized and the quantity of carbides increased significantly. The carbides extraction results suggested that the type of carbides did not changed before and after thermal stability test, the types of carbides were M23C6, M7C and M3C, the TEM results showed that of decreasing of hardness is mainly caused by the coarsening of carbides and the precipitation of Mo. 5. The obvious dynamic recrystallization were occurenced when the BTHJ steel deformed at 1100℃,0.01 s^-1, with the decreasement of deformation temprature and increasement of deformation rate, the deformation resistance increased and the dynamic recrystallization turned to be not obvious. The hot deformatin formula of BTHJ as following: （ε|&） = 2. 42e⁵⁴ [sinh（0.0065σ）]^7.26exp（-591570/RT）; The harden rate was used to determine the critical deformation condition, the critical strain increased with the increasement of lnZ, the relation between critical strain and lnZ can be express by the following function:ε_c = 0.01366lnZ-0.53.6. After nitriding, the highest surface microhardness was over 950HV, the results of plasma nitriding test showed that BTHJ steel can obtained deeper nitriding case than 38CrMoAl in short period of time, while gas nitriding showed that after long time nitriding, the depth of BTHJ and 38CrMoAl almost similar, but due to the fine grain, the nitriding case of BTHJ did not exist nervatied microstructure, and the hardness gradient of BTHJ was much more gentle, which can improve the stability of residual stress, thereby improve the fatigue life. Totally, the nitriding performance of BTHJ is much better.更多还原