退火气氛中CO_(2)含量及加热时间对钢坯氧化的影响

Effect of CO_(2) content in annealing atmosphere and heating time on billet oxidation

在热轧加热炉中,燃气热值的波动会带来退火气氛的变化,钢坯在炉停留时间也会受生产节奏变化的影响产生变化,从而造成钢坯氧化难以预测和控制。本工作在1250℃的加热温度及CO_(2)+N;的气氛下进行实验,研究了CO_(2)含量和加热时间对钢坯氧化行为的影响。结果表明,钢坯氧化动力学曲线分为线型和抛物线两个阶段,随着CO_(2)含量的增加,线型阶段缩短,该阶段的氧化增重也减少;在抛物线阶段,抛物线速率常数k_(p)与CO_(2)含量呈对数关系。钢坯表面氧化层分为致密层和疏松层,致密层位于氧化层外侧,由Fe_(3)O_(4)及其还原产物FeO组成;疏松层位于致密层/金属基体之间,由FeO及其先共析产物Fe_(3)O_(4)组成。疏松层存在孔洞,并且CO_(2)含量越高孔洞越多。

Gas-fired heating continues to be the main heating method in the steel processing industry.In the reheating furnace before hot rolling,the annealing atmosphere can vary significantly because of the fluctuation of the calorific value of gas.When the calorific value of gas is low,the residence time of billet in the furnace will be prolonged,which will affect the production rhythm.For the above reasons,the oxidation of steel billet in reheating furnace is difficult to predict and control.To achieve quantitative understanding and prediction of steel oxidation in a reheating furnace,an experiment was conducted to study the oxidation behavior of steel billet in simulated atmospheres at the temperature of a soaking zone of the heating furnace.The results of steel oxidation in high purity nitrogen with selected CO_(2)content at 1523 K(1250℃)are obtained.The result shows that oxidation kinetics curves of samples in different CO_(2)content are divided into a linear phase and a parabolic phase.In the linear phase,the time and weight gains of the linear phase decrease with CO_(2)content increase.In the parabolic stage,the parabolic rate constant has a logarithmic relationship with CO_(2)content.The oxide film on the surface of the sample is divided into a loose compact layer and a loose layer.The compact layer is located on the outside of oxide film consisting of Fe_(3)O_(4)and its reduction product FeO,while the loose layer is between compact layer and metal matrix and is mainly composed of FeO as well as a few proeutectoid Fe_(3)O_(4)during cooling.There are voids and cracks inside of the loose layer,and the higher the CO_(2)content is,the more voids there are.Condensation of vacancy was believed to be responsible for the formation of voids,while increased CO_(2)content can promote the process.By observing the surface morphology,it can be inferred that FeO in the compact layer might be generated by the reduction of Fe_(3)O_(4).