Growth of Austenite in the Intercritical Annealing of Fe-C-Mn Dual Phase Steels

Abstract

The growth of austenite during the intercritical annealing of a ternary Fe-C-Mn alloy is analysed, using a local equilibrium model. It is shown that austenite growth is first controlled by the diffusion of carbon. These initial stages are usually complete within a few seconds and are followed by a period of slow manganese-diffusion-controlled growth. The analysis predicts the partitioning of manganese, such that both the austenite and ferrite phases become non-uniform in Mn in relatively short times. Electron probe analyses are presented to support this conclusion.

The change in austenite volume fraction on cooling from intercritical annealing temperatures is discussed. It is shown that manganese partitioning, developed during isothermal processing can act to “lock” the ferrite-austenite interfaces, so that the high-temperature austenite precipitate size is retained on cooling.

Résumé

La croissance de l'austénite pendant le revenu intercritique d'un alliage ternaire Fe-C-Mn est analysée en utilisant un modéle de l'équilibre local. Nous avons montré que la croissance de l'austenite est tout d'abord contrôlée par la diffusion de carbone. Ces étapes initiales sont habituellement complétées en dedans de quelques secondes et sont suivies par une période de croissance lente contrôlée par la diffusion du manganèse. L'analyse predit la répartition du manganèse, de telle façon que les phases austénitique et ferritique deviennent non-uniformes en Mn en des temps relativement courts. Des analyses à la microsonde sont présentées qui supportent cette conclusion.

Le changement de la fraction volumique d'austénite au refroidissement à partir des températures de revenu intercritique est discuté. Nous avons montré que la ségrégation du manganèse développée pendant le traitement isotherme peut agir pour bloquer les interfaces ferrite-austénite de telle façon que la taille du précipité d'austenite obtenu à haute température est conservée au refroidissement.     

Effects of Silicon Content and Intercritical Annealing on Manganese Partitioning in Dual Phase Steels

Steels of constant manganese and carbon contents with silicon content of 0.34%-2.26% were cast.The as-cast steels were then hot rolled at 1100 ℃ in five passes to reduce the cast ingot thickness from 80 to 4 mm, air cooled to room temperature and cold rolled to 2 mm in thickness. Dual phase microstructures with different volume fraction of martensite were obtained through the intercritical annealing of the steels at different temperatures for 15 min followed by water quenching. In addition to intercritical annealing temperature, silicon content also altered the volume fraction of martensite in dual phase steels. The partitioning of manganese in dual phase silicon steels was investigated using energy-dispersive spectrometer (EDS). The partitioning coefficient, defined as the ratio of the amounts of alloying element in the austenite to that in the adjacent ferrite, for manganese increased with increasing intercritical annealing temperature and silicon content of steels. It was also found that the solubility of manganese in ferrite and austenite decreased with increasing intercritical temperature. The results were discussed by the diffusivity and the solubility of manganese in ferrite and austenite existed in dual phase silicon steels.     

Effects of Process Parameters prior to Annealing on the Formability of Two Cold Rolled Dual Phase Steels

This study concerns the effects of coiling temperature after hot rolling and the degree of reduction during cold rolling on formability‐related properties of high strength cold rolled dual phase (DP) steels. The effect of coiling temperature on the final structure and properties of two cold rolled and annealed DP‐steels is investigated. Further, the effect of cold rolling reduction and its impact on the final properties of the material is studied. Aspects of the impact of the different process parameters on the ferrite to austenite and austenite to martensite transformation are discussed based on results from production scale experiments, tensile testing and metallographic examinations of the materials.     

马氏体体积分数对双相钢变形行为的影响

 通过Crussard Jaoul （C J）分析和n值的分段分析对不同马氏体含量双相钢的变形行为进行了研究。C J分析时采用连续数据绘制C J曲线,避免了取点拟合过程中的不连续性,n值的分段分析基于微分原理,直观给出了不同应变量下的n值。结合对两者的分析,认为只有较低马氏体含量的双相钢才具有双屈服特性,并且双相钢的第二阶段硬化的拐点只有在一定量的马氏体发生变形时才出现。同时在小应变量下双相钢中的应变硬化速率随马氏体含量的增加而升高,而在低马氏体含量下n值会出现最大值。     

关键退火参数对590MPa级热镀锌双相钢力学性能的影响

 采用多功能退火模拟器,按照首钢1号CGL(连续镀锌生产线)配置研究了带速、加热温度及快冷终止温度对590MPa级热镀锌双相钢力学性能的影响。研究结果表明,提高带速可以提高冷却速度和减少带钢在均衡段停留时间,有利于保证双相钢的性能。要获得良好性能的双相钢,大于90m/min的带速是必要的。加热温度过低会导致双相钢强度不足,过高则会导致伸长率和硬化指数降低。800℃的加热温度可以获得较好的双相钢综合力学性能。快冷终止温度升高,双相钢的抗拉强度和加工硬化指数都显著降低。在均衡段电加热有能力保证镀锌温度的情况下,降低快冷终止温度有利于获得良好的双相钢力学性能。     

冷轧双相钢连续退火组织的转变

采用光学显微镜与扫描电镜观察分析了实验钢冷轧组织在连续退火过程中的再结晶与相变规律,研究了过时效回火对双相钢显微组织的影响.实验表明,在连续退火初期的加热过程中,在600～720℃大量进行再结晶.加热速度对再结晶行为有较大影响,以10℃/s加热,再结晶将持续到双相区.珠光体在低于720℃的加热过程中变化不明显,而铁素体晶界与晶内出现球状碳化物颗粒.双相区退火过程中,奥氏体首先在珠光体处形成,原铁素体晶界与晶内的碳化物颗粒也形成奥氏体岛.800℃保温后缓慢冷却至630～680℃可以得到合理比例的双相钢组织.当过时效温度大于300℃,马氏体分解,碳化物颗粒析出,将对双相钢性能产生不良影响.     

罩式退火工艺对无间隙原子钢织构和性能的影响

在试验室对典型成分的Ｔｉ－ＩＦ钢和Ｔｉ＋Ｎｂ－ＩＦ钢的罩式退火工艺进行了模拟试验，找到了罩式退火工艺（主要是退火温度和保温时间）对ＩＦ钢性能的影响规律，为优化ＩＦ钢罩式退火工艺提供了依据。     

铬对C-Si-Mn-Nb铁素体贝氏体双相钢相变规律的影响

通过比较C-Si-Mn-Nb和C-Si-Mn-Cr-Nb两种实验钢热变形后的CCT曲线,分析了合金元素Cr对铁素体贝氏体双相钢连续冷却过程相变规律和组织演变的影响.结果表明,Cr可轻微抑制实验钢的铁索体转变,细化铁素体晶粒,但对铁素体显微硬度没有影响.在铁素体转变量很少或未转变的情况下,Cr可增强亚稳奥氏体的稳定性和淬透性,细化贝氏体产物中的铁素体板条和MA岛,提高贝氏体的显微硬度.     

热处理对冷轧Inconel718合金组织与性能的影响

研究结果得出：Inconel718合金经1040℃固溶处理720℃8h，55℃／h炉冷，620℃8h时效后，存在缺口敏感性。该合金经970℃固溶处理和时效后没有缺口敏感性，并且屈服强度和抗拉强度随冷轧变形量的增加而提高。在1040℃＋850℃（中间处理）并时效后，可以消除缺口敏感性，但合金强度随之显著降低。     

Effect of Hot Rolling Parameters on Excess Phase Precipitation in Two-Phase Ferritic-Martensitic Steels

Metallurgist - Results are given for studying the effect of hot rolling parameters on morphological features of the structure and precipitation in hot rolled products destined for further cold...     

固溶和时效处理时间对IN706合金组织与性能的影响

研究了增加标准的980℃1h固溶-825℃ 2.5 h稳定化-720℃8 h+620℃8 h时效的处理时间对IN706合金(/%:0.03C,41Ni,16Cr,1.6Ti,2.9Nb)组织和性能的影响。结果表明,固溶处理时间(1～3 h)的增加,主要影响合金的晶粒尺寸,从而影响合金的性能;随稳定化处理时间(2.5～5 h)的增加,η相析出增多,导致强化相形成元素减少,合金性能亦有所下降,并对合金塑性和韧性产生不良影响;增加一级(8～18 h)和二级(8～36 h)时效处理的时间,会促进γ和γ’相的析出,并且其形态也会发生变化,进而对合金的性能产生影响,但固溶1～3 h,稳定化2.5～5 h,以及一级时效8～18 h,二级时效8～36 h时合金的力学性能指标均满足标准要求。     

Comparison of Work Hardening Behaviour of Ferritic‐Bainitic and Ferritic‐Martensitic Dual Phase Steels

In this research, the stress‐strain curves of two types of dual phase steels, namely ferritic‐bainitic and ferritic‐martensitic steels with 0.16%C and 1.2% Mn have been obtained using tensile tests. Both steels were intercritically annealed under different conditions and the ferritic bainitic steels subsequently quenched in a salt bath, while the ferritic martensitic steels were water quenched. The stress‐strain data of the specimens were checked using Hollomon's equation. The results showed that both types of dual phase steels had two stages of work hardening and each stage had a different work hardening exponent. The effects of volume fraction of hard phases (bainite and martensite) on ultimate tensile strength, total elongation and work hardening exponent were also investigated. The results indicated that with increasing volume fraction of hard phase the UTS was increased whereas the work hardening exponent and total elongation were decreased.