超低氧添硫特殊钢中复合延性夹杂物的生成与演变机理

High melting point inclusions composed of CaO-Al2O3(-CaS), which can be frequently observed in ultra low oxygen resulfurized special steel, are extremely detrimental to fatigue resistance property and the caused problem is still remained to be solved. Quite different from previous works, present study focused on how to avoid CaO-Al2O3(-CaS) inclusions but not on how to control them after their formation. As a result, a new strategy was proposed towards the control of high melting point inclusions in this kind of special steel, which can be described as following: (i) reactions between slag and steel must be limited to prevent the formation of CaO-Al2O3 oxides; (ii) as a result, oxide inclusions in steel would be Al2O3 and MgO-Al2O3; (iii) as medium or high level of sulfur is contained in steel, a lot of MnS incluisons can be expected in steel which are soft and plastic; (iv) then,soft MnS inclusions can be utilized to wrap the much harder Al2O3 and MgO-Al2O3 to produce complex ductile inclusions to diminish their negative effects. A series of exploring experiments have been finished by present author and a proper method has been found to effectively prihibit slag-steel reaction. The obtained results indicated that no CaO-Al2O3(-CaS) inclusions were formed in steel. Hence, following experiments would be carried out to elucidate formation mechanisms of complex ductile inclusions, such as the dissolution behaviors of MnS during heating, effects of the size and number density of oxides and influence of cooling rates during solidification on the formation of complex ductile inclusion. Moreover, eolution of complex ductile inclusions in solid steel during heating was also of great interest in present work, including the influence of heating rate, heating temperature and soaking time on the dissolution of MnS layer, separation and spheroidization of elongated MnS of complex ductile inclusions. Thermodynamic models would be established to explain the evolution mechanisms of complex ductile inclusions duing heating by thermodynamic calculation and couple diffusions experiments, considering the diffusion between solid steel and inclusions.

超低氧添硫特殊钢中CaO-Al2O3(-CaS)等大尺寸、高熔点夹杂物是其疲劳失效的主要原因，如何对其进行有效控制是未解决的技术难题。本研究提出不同于以往的夹杂物控制策略，即：超低氧冶炼时通过抑制钢渣反应避免CaO-Al2O3(-CaS)生成，将夹杂物控制为Al2O3或MgO-Al2O3；利用钢中MnS包裹Al2O3、MgO-Al2O3，形成"软"包"硬"型复合延性夹杂物，提高疲劳寿命。目前已在实验室完成前期试验并掌握抑制钢渣反应的方法，杜绝了CaO-Al2O3(-CaS)类夹杂物。因此，拟通过本项目继续研究复合延性夹杂物的生成机理及其在轧制再加热过程中的演变规律。生成机理包括：复合延性夹杂物的形成时机、氧化物夹杂数量与尺寸的影响、凝固速率的影响等；演变规律包括：加热速率、保温温度、保温时间等因素对复合夹杂物中MnS的脱溶、分裂、球化等行为的影响，建立并验证再加热时夹杂转变的固相扩散模型。

本研究对超低氧添硫特殊钢中夹杂物提控制提出了新策略，即利用MnS包裹高熔点氧化物质点形成“软”包“硬”型复相夹杂物，提高切削性能和疲劳寿命。系统研究了复相夹杂物形成时机、氧化物夹杂数量与尺寸的影响以及加热速率、保温温度、保温时间等因素的影响。结果表明：.（1）形成MnS+oxide复相夹杂物有利于降低MnS在轧制方向的长度，当钢中[S]含量分别为120ppm、30~40ppm之间时，MnS+Oxide复相夹杂物、MnS夹杂物在横截面方向的尺寸（粗细）平均值分别为2.5μm、3.6μm或分别为2.1μm、2.3μm，复相夹杂物、MnS夹杂物的长宽比分别控制在5.6、9.3与4.2、8.7。即：形成复相夹杂物大幅降低了[S]含量增加对MnS夹杂物评级的影响。另外，夹杂物中Al含量增加、Ca含量降低有利于增加其作为MnS核心的能力；.（2）MnS夹杂物在钢液凝固时析出生成，并且观察到MnS形成的关键时期是在钢液完全凝固时的极短时间内快速地在树枝晶主干之间析出。超低氧条件下尺寸5µm以下的细小氧化物夹杂物易被凝固时的固-液相界面捕捉并在界面处发生碰撞、聚集、长大而成为尺寸超过12μm的夹杂物，不利于MnS在其上异质形核。.（3）耐火材料对氧化物夹杂物的形成有重要影响：MgO质耐火材料的使用是高熔点夹杂物MgO∙Al2O3生成的重要原因；ZrO2坩埚条件下则易形成高CaO的低熔点夹杂物，不利于形成MnS+Oxide复相夹杂物。精炼炉渣中MgO初始含量提高至10%时，有利于形成高熔点夹杂物。.（4）加热制度对钢中夹杂物的数量密度有明显影响。保温时间增加、保温温度增加不利于复相夹杂物的控制。