【作者】 万荣春；

【导师】 单爱党； 孙锋；

【作者基本信息】 上海交通大学 ， 材料学， 2012， 博士

【摘要】 随着市场需求的加大以及对建筑品质的要求越来越高，迫切需要加快研发具有高强度的低Mo系列低成本耐火钢，以期实现钢结构建筑的低成本、轻量化，达到节约原材料、节能减排的低碳经济目的。本文设计了一系列Mo含量为0-0.9wt.%的简化耐火钢成分的Fe-Mo-C模型钢，利用两种热处理工艺制度使Fe-Mo-C钢获得铁素体和铁素体+贝氏体两种组织，分离并定量解析Mo的各种高温强化机理，得出Mo的主要高温强化机理，并基于此提出利用Nb、V、Ti的沉淀强化和贝氏体相变强化共同强化低Mo高强耐火钢的设计思想。根据此设计思想，成功开发出Mo含量小于0.15wt.%，室温屈服强度为488.7-508.9MPa，达到Q500级别的低成本高强耐火钢。相对目前常规耐火钢，Mo含量降低70%以上，每吨钢合金成本降低约1550元，屈服强度提高42%以上。此外，还根据上述研究结果提出抗拉强度级别从490-790MPa的低成本耐火钢设计中Mo含量、贝氏体体积分数和微合金元素添加量与高温屈服强度的关系式。主要研究内容和成果如下。利用电子显微探针（EPMA）和透射电子显微镜（TEM）分析得出Mo元素在Fe-Mo-C钢不同组织中的分布。铁素体组织：Mo<0.3wt.%时，Mo基本固溶在铁素体中；Mo≥0.3wt.%时，Mo会以Mo2C的形式析出。珠光体和贝氏体组织：Mo首先固溶在珠光体和贝氏体组织的铁素体基体中；当铁素体基体达到饱和时，多余的Mo会与Fe、C形成复合的合金渗碳体。通过分离并定量解析Mo的固溶强化、沉淀强化和贝氏体相变强化对耐火钢高温强度的影响，得出Mo在耐火钢中主要高温强化机理为固溶强化和沉淀强化：Mo<0.3wt.%时，为固溶强化；Mo≥0.3wt.%时，为固溶强化和沉淀强化。Mo≤0.5wt.%时，其固溶强化和沉淀强化效果明显，每添加0.1wt.%的Mo，600oC的屈服强度增量为14.76MPa（σss+σppt=147.6CMo+54.76）；Mo>0.5wt.%后，其强化效果减弱。贝氏体相变强化对耐火钢的高温强度有重要影响，每增加1vol.%的贝氏体，600oC的屈服强度增量为1.655MPa（σBs=8.833+1.655fB）。铁素体晶粒尺寸对耐火钢高温强度的影响不明显。铁素体晶粒尺寸从18.2μm增加到26.3μm，600oC的屈服强度仅增加4.3MPa。低Mo（<0.3wt.%）高强耐火钢的设计思想为：通过Nb、V、Ti的沉淀强化和贝氏体相变强化替代Mo的固溶强化从而降低Mo含量。通过控制终轧后冷却速度和Nb、V、Ti微合金化，成功开发了Q345（Mo：约0.3wt.%）和Q500（Mo：约0.15wt.%）两种强度级别的低成本高强耐火钢。两种强度级别耐火钢具都有低的屈强比（<0.634）、较好的延伸率（>22.3%）与冲击韧性（0oC冲击功>35J）和高的强度，其中Q500级别耐火钢的室温屈服强度到达488.7-508.9MPa。两种强度级别耐火钢均有良好耐火性能，其YS(600oC)/YS(RT)均超过2/3。经验证，本文得出关于Mo的固溶强化和贝氏体相变强化对耐火钢的高温屈服强度强化效果的定量关系式是可靠的。由此还推导出了抗拉强度级别从490-790MPa的低成本耐火钢设计中Mo含量（CMo，wt.%）、贝氏体体积分数（fB，vol.%）和微合金元素添加量（析出相体积分数fppt，vol.%）与高温屈服强度（YS(600oC)）的关系式：YS(600oC)=σ0+147.6CMo+1.655fB+296.27f ppt。微合金元素Nb的沉淀强化对耐火钢高温强度的影响可以分成两个阶段：首先热轧过程中析出粒子的钉扎作用使耐火钢中的位错结和位错密度明显增加，强化软相铁素体基体；其次在拉伸过程中，特别是高温拉伸中，析出粒子可以阻碍位错攀移和位错的回复，从而可以有效提高耐火钢的室温和高温强度。更多还原

【Abstract】 With the increasing market demand and the increasingly high demand for qualityconstruction, there is an urgent need to develop a low-cost fire-resistant steel with low Moaddition and excellent strength. In the present investigation, a series of simplifiedFe-Mo-C steels with Mo addition from0to0.9wt.%have been prepared. Two heattreatments were carried out on Fe-Mo-C steels for obtaining either ferrite microstructuresor ferrite-bainite microstructures to separate the high-temperature strengtheningmechanisms of Mo in fire-resistant steel. And quantitative analysis is made to reveal thedominant high-temperature strengthening mechanism of Mo addition. The results in thiswork proposed a new design of the low-Mo high-strength fire-resistant steels based on acombination of bainite strengthening and the precipitation strengthening of Nb, V, and Ti.Thus, a low-Mo (about0.15wt.%) fire-resistant steel with high strength (yield strength:488.7-508.9MPa) and low yield ratio (0.634) has been successfully developed. Comparewith conventional fire-resistant steel, the Mo content was reduced more than70%, thealloy cost was reduced about1550yuan per ton of steel, and the yield strength wasincreased more than42%. Moreover, the results also established a relation between Mocontent, bainite volume fraction, the additive amount of microalloying elements addition,and high-temperature yield strength in the development of a series of low-costfire-resistant steels with UTS from490to790MPa. The main achievements are expressedbelow.Mo distribution in different microstructures of Fe-Mo-C steels is identified by usingelectron probe micro-analysis (EPMA) and transmission electron microscope (TEM). Theresults show that almost all Mo atoms are in solid-solution in ferrite microstructure whenMo content is less than0.3wt.%, and the precipitates Mo2C were formed in ferritemicrostructure when Mo more than or equal to0.3wt.%. Moreover, in bainite and pearlite microstructures, Mo atoms are first in solid-solution in the ferrite matrix, and the alloycementites consisting of Mo, Fe, and C were formed when the ferrite matrix Mo-saturated.Quantitative analysis indicates that the dominant high-temperature strengtheningmechanisms of Mo in fire-resistant steel are solid-solution strengthening (dominant whenMo less than0.3wt.%) and precipitation strengthening (both dominant when Mo morethan or equal to0.3wt.%), and these strengthening effects improve the yield strength at600oC by a significant147.6MPa/wt.%when Mo less than or equal to0.5wt.%(σss+σppt=147.6CMo+54.76), but these strengthening effects become relatively weak whenMo content is more than or equal to0.5wt.%.Bainite strengthening plays an important role in improving the high-temperaturestrength of fire-resistant steel, and it improve the yield strength at600oC by a significant1.655MPa/vol.%(σBs=8.833+1.655fB).Ferrite grain size has less effect on high-temperature strength of fire-resistant steel.The yield strength at600oC only increases about4.3MPa with the variation of the ferritegrain size from18.2to26.3μm.Results also provide that a low-Mo (Mo less than0.3wt.%) fire-resistant steels withexcellent strength can be designed by bainite strengthening and the precipitationstrengthening of Nb, V, and Ti.Two low-cost high-strength (Q345and Q500) fire-resistant steels with two Moaddition levels (about0.15and0.3wt.%) have been successfully developed by controllingcooling rate and microalloying with Nb, V, and Ti in combination. Two low-costhigh-strength fire-resistant steels have low yield ratio (below0.634), good ductility(elongation: more than22.3%and the charpy energy at0oC: above35J), and highstrength. The room-temperature yield strength of Q500class fire-resistant steels withabout0.15wt.%Mo addition are488.7-508.9MPa. It is possible to obtain two-thirds ofroom-temperature yield strength at600oC in two low-cost high-strength fire-resistantsteels.The results in this work verified that the quantitative expressions relating Mo content,bainite volume fraction, and the high-temperature yield strength of fire-resistant steel arereliable. Moreover, the results also proposed an expression (YS(600oC)=σ0+147.6CMo+1.655fB+296.27f ppt) relating Mo content (CMo, wt.%), bainite volume fraction (fB, vol.%), the amount of microalloying elements addition (precipitate volume fraction fppt,vol.%), and high-temperature yield strength (YS(600oC)) in the development of a seriesof low-cost fire-resistant steels with ultimate tensile strength from490to790MPa.There exist two stages of precipitation strengthening by microalloying with Nb on thehigh-temperature strength of fire-resistant steel. First, the dislocation tangle anddislocation density of fire-resistant steel increases during hot rolling due to the pinningeffect of Nb(C, N) precipitates, and the strength of ferrite matrix remarkably improves.Second, the precipitates have effects on retarding the climb motion and recovery ofdislocation during tensile tests, especially during high-temperature tensile tests. Thus, thehigh-temperature strength of steels shows a remarkable improvement。更多还原