【作者】 张健；

【导师】 李艳梅；

【作者基本信息】 东北大学 ， 材料工程（专业学位）， 2018， 硕士

【摘要】 本文依托国家自然科学基金重点项目“超快冷条件下低碳微合金钢中纳米碳化物析出控制及综合强化机理”,通过一种微合金钢中厚板的热轧和热模拟实验重点研究了中厚板在厚度方向不同位置处的力学性能和微观组织结构。在此基础上,又通过单一变量的对照实验探究了冷却工艺和成分含量对微观组织及力学性能的影响,进而探讨了超快冷工艺下的低碳微合金钢中厚板的强韧化机理,为中厚板的成分设计和实际轧制中的冷却工艺制定提供了理论依据。经过热轧以及热模拟实验可知,低碳微合金钢中厚板经超快冷后,厚度方向不同位置处水冷阶段冷速不同,心部最慢而表面最快,除心部位置外均存在返温现象,越靠近表面返温时间越长且返温幅度越大。通常心部得到块状的多边形铁素体以及大的块状珠光体组织,视冷却工艺可能出现一定比例的粒状贝氏体。由心部到表面珠光体很快消失,多边形铁素体逐渐变为针状或粒状铁素体甚至完全消失,粒状贝氏体体积分数迅速上升,多在距表面1/4h处开始大量出现。表面处贝氏体体积分数最大,视冷却工艺可能出现板条状贝氏体甚至马氏体。贝氏体质硬而脆,铁素体与珠光体质软而韧,因此越靠近表面相变强化效果越强。同时越靠近表面晶粒尺寸越小,且析出物体积分数越小但析出物尺寸显著细化。析出物尺寸对析出强化的贡献最大,因此越靠近表面细晶强化和析出强化效果也越强。在多种强化机制的综合作用下,中厚板从心部至表面强度逐渐增大,屈服强度可从500MPa级别上升至650MPa级别。中厚板不同厚度位置的韧性由晶粒尺寸和各组织体积分数等因素共同决定,随冷却工艺的不同变化范围较大,0℃冲击功可在30J级别到140J级别间变化。终冷温度较高时,冷却速度过大将导致返温时间过长且返温幅度很大,使晶粒长大从而使冲击韧性大幅下降,1/4h处-40℃冲击功从138J降至80J;当终冷温度适中或较低时,冷却速度过大将导致多边形铁素体体积分数下降从而影响钢材的冲击韧性,1/4h处-40℃冲击功分别从149J降至130J、111J降至17J。因此为了获得更优异的综合力学性能,中厚板在轧后控制冷却阶段冷速不宜过大。冷却速度较为合适时,返红温度过高将导致晶粒尺寸很大,从而导致中厚板力学性能下降,1/4h处屈服强度仅535MPa,-40℃冲击功138J;返红温度过低会使各厚度处组织几乎全为粒状或板条状贝氏体,使中厚板冲击韧性大幅下降,1/4h处-40℃冲击功仅为111J。因此,为了获得好的冲击韧性,应尽可能提高返红温度,但应保持在一定范围内,可使1/4h处屈服强度达到550MPa级别,同时-40℃冲击功达到149J;如果想获得高的强度,应尽可能降低返红温度,但不应低于极限值,可使1/4h处屈服强度达到600Mpa级别,抗拉强度达到750MPa级别。微合金元素通过包含细晶强化、析出强化和相变强化等的综合强化机制使中厚板的强度提升,1/4h处屈服强度可由459MPa提升至629MPa,但大量析出物和铁素体体积分数下降会造成钢材塑韧性下降,1/4h处-40℃冲击功由144J降低至80J;降低含碳量可以提升钢材的塑韧性,但会降低强度;而镍在提高钢强度的同时,对钢塑韧性的损害较小,合理利用可以同时提高钢材的各项力学性能。更多还原

【Abstract】 Based on the key project of NSFC(Control of nano-carbide precipitates in low carbon steels under ultrafast cooling and comprehensive strengthening mechanism),the mechanical properties and microstructures of plate in different position along thickness direction were emphatically studied by hot rolling and thermal simulation experiments.The effects of cooling process and component on microstructure and mechanical properties were investigated through a controlled experiment of single variable.Furthermore,the strengthening mechanism of low carbon microalloyed steel plate produced by UFC process was discussed,which provides a theoretical basis for the composition design and the cooling process determination of plate.According to the results of hot-rolling and thermal simulation experiments,the cooling rate of different position along thickness direction is different after ultra fast cooling of the low carbon microalloyed steel plate.The cooling rate is the slowest in the core and the fastest at the surface.As the position is close to the surface,the temperature equalization time is getting longer.In general,the microstructure of the plate center consists of polygonal ferrite and blocky pearlite.The granular bainite may occur at certain cooling process.From the corn to surface,pearlite quickly disappears and the polygonal ferrite gradually evolves into acicular or granular ferrite and even disappears completely,and the volume fraction of granular bainite rises rapidly and reaches a maximum at the surface.The lath bainite and martensite may also appear at certain cooling process,so the phase transformation strengthening effect continues to enhance as the position approaches to the surface.Furthermore,the closer to the plate surface,the smaller the grain size and the volume fraction and the size of the precipitate particles.The contribution of precipitate size to the precipitation strengthening is greater than the precipitation volume fraction,so the closer to the surface,the stronger the fine grain strengthening and the second phase strengthening effect.Under the synthetic effect of various strengthening mechanisms,the yield strength of the plate increases from 500MPa to 650MPa from its center position to the surface.The toughness at different thickness of the plate is determined by the grain size and the volume fraction of the different phases,and the impact energy at 0℃ can vary from 30J to 140J under the condition of different cooling process.When the final cooling temperature is high,the high cooling rate will cause the re-reddening time too long and the re-reddening temperature range very large and result in large grain size,which will greatly reduce the impact power from 138J to 80J of the 1/4h at-40℃.When the final cold temperature is moderate or low,the high cooling rate leads to the decrease of the volume fraction of the polygonal ferrite,which affects the impact toughness of the steel,and the 1/4h-40℃ impact power is reduced from 149J to 130J and 111J to 17J,respectively.Therefore,in order to obtain more excellent comprehensive mechanical property,the cooling speed of the medium and heavy plate should not be too large after the control rolling.When the cooling speed is suitable,the high re-reddening temperature will lead to large grain size and decrease the mechanical properties of the plate,and the yield strength of 1/4h is only 535MPa.The lower re-reddening temperature will cause the microstructure of each thickness to be nearly all granular or lath bainite,and thus reduce the plate impact toughness.The impact power of 1/4h at-40℃ is only 111 J at this condition.Therefore,in order to obtain good impact toughness,the re-reddening temperature should be improved properly,which can ensure the yield strength reaching 550MPa and the impact power at-40℃ reaching 149J of 1/4h.Furthermore,600MPa yield strength and 750MPa tensile strength of 1/4h can be reached by reducing the re-reddening temperature,which is not below the limit value.The micro-alloying element can enhance the strength of the medium-thick plate by comprehensive strengthening mechanism including the precipitation enhancement and phase transformation enhancement,and the yield strength of the 1/4h increases from 459MPa to 629MPa,but the precipitation and the decrease of the ferrite volume fraction will result in the decrease of the impact power of 1/4h of the steel from 144J to 80J at-40℃.The reduction of C content can enhance the ductility and reduce the strength of the steel.Ni is good for strength and has little harm to the toughness,so the rational use of Ni can improve the mechanical properties of the steel at the same time.更多还原