摘要
本文研究工作之初,作者认为“快速、大量析出的铁素体晶粒抑制了中温区剩余奥氏体向贝氏体的转变,从而在低温区(Ms以下)形成马氏体”；而“C-Mn钢中铁素体晶粒快速、大量析出”是CSP生产流程的重要特点。因而,采用连续冷却工艺,在包钢CSP线上进行540MPa级C-Mn双相钢的开发试验,取得令人满意的结果。虽然如此,但猜测性的机理仍需从逻辑或理论上加以论证。因此,本文选题为“C-Mn双相钢的组织形成机理研究与开发”,确切地说,是对连续冷却条件下过冷奥氏体依次形成铁素体、马氏体而不产生贝氏体(或只产生微量贝氏体)的相变动力学过程的研究。
通过推导连续冷却条件下“膨胀曲线”与“相变动力学曲线”之间的数学关系,获得连续冷却条件下的表观意义上的相变动力学曲线。以连续冷却相变动力学曲线为基础,建立不同类型相变之间以及同类相变不同阶段之间的临界温度和对应转变分数的分析方法,命名为"Austin-Rickett指数法”。利用Austin-Rickett指数法,研究连续冷却条件下奥氏体晶粒尺寸、冷却速度、C含量对先共析转变过程的影响,解释连续冷却条件下铁素体快速、大量析出的原因。通过研究快速、大量析出铁素体的先共析转变对剩余奥氏体后续相变的影响,分析讨论了CCT图中贝氏体区分离的可能性,建立连续冷却相变过程中铁素体和马氏体双相组织的形成机理,并利用补充实验验证其正确性。
利用上述“双相组织形成机理”,重新解释“在CSP线,以普通C-Mn钢为原料,采用连续冷却工艺生产出540MPa级C-Mn双相钢”的原因,分析CSP工艺对于生产热轧C-Mn双相钢的适应性。根据“双相组织形成机理”,在包钢CSP线安装“超快速冷却”装置,扩大双相钢生产规格,实现低成本C-Mn热轧双相钢的商业化生产。
在研究过程中建立的“获得连续冷却转变动力学曲线的方法”以及"Austin-Rickett指数法”,是一种研究连续冷却条件下相变规律的新思路；贝氏体区“分离”的CCT图,是采用连续冷却工艺开发热轧C-Mn系双相钢的理论基础。
Before the present work, a prediction was given by the author, according to a conjecture mechanics, which is that the dual phase steel could be made of C-Mn steel by using one-stage type continuous cooling on Compact Strip Production (CSP) line. Simply, the conjecture mechanics consider that the fast & largely nucleation ferrite grain suppress the remainder austenite-bainite transition in the middle temperature scale and make the remainder austenite change to martensite under the Ms point. Ferrite grain nucleation fast & largely is the important feature of the CSP line. During the subsequent industrial experiments, a 540MPa grade C-Mn dual-phase steel strip was developed successfully by using continuous cooling process on Baotou CSP line, based on the prediction of the author. The result of the prediction was verified certainly. However, the conjecture mechanics need detecting in logic or in theory by no means. Therefore, the topic of the paper is that the research on the structures-formation mechanics in the hot rolled C-Mn dual-phase steel produced by using continuous cooling process, i.e., the research on the over-cooling austenite transformation to the ferrite and martensite, but no bainite or only microscale, on the condition of continuous cooling.
Firstly, a method for obtaining the transition kinetics curve was established, based on the mathematics relationship between the dilatation curve and the transition kinetics curve. Also, based on the transition kinetics curve, a new method, named Austin-Rickett formula Exponent (ARE) method, was constructed, which is used for analyzing or determining the critical temperature and the corresponding transition fraction among the different transformation course or the different stage in the same transformation course. By using the ARE method, the effects of the austenite grain size, cooling rate and carbon content on the pro-eutectoid transition were researched respectively. The phenomenon of the ferrite liberating quickly and massively was interpreted. The effect of the pro-eutectoid with the ferrite liberating quickly and massively on the subsequent transformation of the residue austenite was researched. Based on the CCT with the bainite zone'segregation', the dual-phase structures-formation mechanics in the hot rolled C-Mn dual-phase steel produced by using continuous cooling process was established. The validity of the mechanics was certified by a supplementary experiment.
The reason why the dual phase steel was made of C-Mn steel by using continuous cooling on Baotou CSP line was explained by using the mechanics. The adaptability of the CSP line for producing dual-phase steels was analyzed. According to the mechanics, the High Rate Cooling (HRC) device had been installed on the CSP line. It is used for increasing the thickness specification of dual-phase steel and producing low cast C-Mn dual phase steel commercially.
The method for obtaining the transition kinetics curve and the ARE method in the present work are the new means for studying the transition regularity on the condition of the continuous cooling. Also, the CCT with bainite zone 'segregation' is the theoretical basis on the aspect of transition kinetics for the development of C-Mn dual-phase steels using continuous cooling process.
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