【作者】 王佳慧；

【导师】 赵宇光； 夏振佳；

【作者基本信息】 吉林大学 ， 材料工程（专业学位）， 2016， 硕士

【摘要】 在当今社会,汽车工业发展迅猛,对汽车材料轻量化的要求程度越来越高,传统的钢铁材料已经不再能够满足现代汽车的需求,因此钢铁材料的改良势在必行。到目前为止,先进高强钢已经发展至第三代,从最初的仅要求提高强度和韧性,上升到既要能满足高强韧又要能满足低成本,同时兼顾性能和经济两方面指标。本文正是基于此,以第三代高强钢为基础,依据各合金元素在钢中的作用以及市场价格,最终确定了碳、硅、锰、铬、硼以及其它一些微量元素作为添加元素,熔制出一种低合金、低成本、高强韧、非调质的铸造用钢,牌号定为ZG24Si Mn2Cr B。在本文实验中,主要探讨了ZG24Si Mn2Cr B实验钢的强韧化热处理工艺及参数的优化,试图提高实验钢的强韧性,使所设计的合金钢同时满足高强韧和低成本的要求。首先,是对亚温处理工艺中两相区保温温度的探讨。在本实验条件下,对实验钢进行900°C风冷预处理后,分别将其在770°C、790°C、810°C、830°C、850°C风冷后,再在340°C回火,研究两相区保温温度对实验钢微观组织的影响。通过观察可以发现,经不同两相区温度保温后实验钢在室温下均为由铁素体、贝氏体、马氏体以及少量残余奥氏体组成的混相组织。随着两相区保温温度的升高,在室温下得到的铁素体量减少,贝氏体和马氏体的含量增加。因此可以通过调节两相区的保温温度控制各相所占的比例,从而得到不同综合力学性能的钢。其次,是对热处理工艺中两相区保温后的冷却方式的探讨。在本实验条件下,进行900°C风冷预处理+810°C保温后,分别经空冷、风冷、雾冷、NJ介质冷却至室温,随后在340°C回火,研究两相区保温后的冷却方式对实验钢微观组织和力学性能的影响。结果表明,经不同冷却方式冷却后实验钢的室温组织均为由不同比例的铁素体、贝氏体、马氏体和少量残余奥氏体组成的多相组织。在风冷的条件下,实验钢具有最优的力学性能,抗拉强度可达到1567.4 MPa,延伸率为16.3%,强塑积为25549 MPa·%,硬度为38.62 HRC。经NJ介质中冷却后,实验钢的室温组织中含有较多的贝氏体,因此证实了其高温快冷、中温缓冷的特性。最后,是对热处理工艺中回火温度的探讨。在本实验条件下,进行900°C风冷预处理+810°C后,在上述四种冷却方式下,分别经300°C、340°C、380°C、420°C回火,研究回火温度对实验钢微观组织和力学性能的影响。通过观察可以发现,在相同的冷却方式下,随着回火温度的升高,实验钢室温组织下得到的贝氏体和马氏体的含量降低。在不同的冷却方式下,其最佳的回火温度不同,空冷后在380°C回火,风冷、雾冷、NJ介质冷却后在340°C回火。而空冷、风冷后在340°C回火,雾冷、NJ介质冷却后在300°C回火,即可获得较高的强塑积,能够满足最初对实验钢的设计要求,即σ≥1200 MPa,δ≥10%。这也说明了随着冷却速度的加快,在较低的回火温度回火,即可获得较高的力学性能。更多还原

【Abstract】 Nowadays, with the rapidly development of automobile industry, the demand for lightweight auto-materials is more and more increasing. Traditional steels can no longer meet the need of automobile, so it is no time to improve the steels. So far, Advanced High-Strength Steel(AHSS) has developed to the third generation, which has high strength and ductility at low cost. The third generation of AHSS meets both property and economy well. Based on the third generation of AHSS, the aim of the paper is to design a kind of cast steels with low content alloys at low cost. Having a known about the function and the price of alloys in the steel, carbon, silicon, manganese, chromium and other alloys with few amount are put into the steel, whose name is ZG24 Si Mn2Cr B.In this paper, the heat treatment parameter of the test steel has been optimized, so that we can acquire a kind of steel with high strength and ductility.First, intercritical temperature has been discussed. At this experimental condition, the test steel is heated to 770 °C、790 °C、810 °C、830 °C、850 °C after pre-heat treatment, and then cooled by wind. The steel has to be tempered at 340 °C right now. What has been observed in the steel are ferrite, bainite, martensite and retained austenite. Along with the increasing of the intercritical temperature, the amount of ferrite declines, and the amount of bainite and martensite has been improved. In a word, we can adjust the intercritical temperature to control the proportion of each phase, so that we can acquire the steels with different comprehensive mechanical properties.Second, the ways of cooling has been discussed. At this experimental condition, the test steels are cooled down to the room’s temperature though air, wind, spray and NJ. The steel cooled by wind has the best mechanical properties that (7≥ 1500 (6, ≥16%,Hardness ≥ 38 HRC. The microstructure of the steel cooled by NJ obtains more bainite than three other ways of cooling, so the way of NJ cooling has the property of cooling fast in high temperature and cooling slow in medium temperature.Finally, the tempering temperature has been discussed. At this experimental condition, the test steels are tempered by 300 °C, 340 °C, 380 °C, 420 °C after every way of cooling. And we can obtain it that with the rising of tempering temperature, whatever way of cooling, the content of bainite and martensite is increasing in the microstructure of test steel at room temperature. The test steel has the best property at 380 °C when it is cooled by air. When the test steel is cooled by wind spray and NJ, the tempering temperature that the test steel has the best property is 340 °C. When the test steel is cooled by air and wind at 340 °C and by spray and NJ at 300 °C, it all can have a optimum combination of temperature and cooling way, which meets the need of property. Therefore, with the improvement of the cooling rate, at the lower tempering temperature, the test steel can obtain higher mechanical properties更多还原