Carbon and oxygen behavior in the RH degasser with carbon powder addition

For ultra-low-carbon(ULC)steel production,the higher oxygen content before Ruhrstahl-Heraeus(RH)decarburization(de-C)treatment could shorten the de-C time in the RH degasser.However,this would lead to oxidation rates being exceeded by molten steel production,affecting ULC steel surface quality.In this work,a carbon powder addition(CPA)process was proposed to reduce the dissolved oxygen content at the end of RH de-C through addition of carbon powder to molten steel in the vacuum vessel.Carbon and oxygen behavior during the CPA and conventional process was then studied.The results demonstrated that the de-C rate with CPA was lower compared to the conventional process,but the carbon content at the end of de-C presented no difference.The de-C reaction for CPA process took place in the four reaction sites:(1)within the bulk steel where the spontaneous CO bubbles form;(2)splashing area on the liquid steel surface;(3)Ar bubble surface;(4)molten steel surface.The CPA process could significantly reduce the dissolved oxygen content at the end of de-C,the sum content of FeO and MnO in the slag,the aluminum consumption,and the defect rate of rolled products.This was beneficial in improving ULC steel cleanliness.     

Physical model of fluid flow characteristics in RH-TOP vacuum refining process

To understand the characteristic of circulation flow rate in 250-t RH-TOP vacuum refining process, the l:4 water model test was established through the bubble behavior and gas holdup in the up-leg to investigate the effects of different processes and equipment parameters on the RH circulation flow rate. With the increases of lifting gas flow rate, lifting bubble travel, and the internal diameter of the up-leg, and the decrease of nozzle diameter, the work done by bubble floatage and the circulation flow rate increase. The expression of circulation flow rate was derived from the regression analysis of experiment data. Meanwhile, the influences of vacuum chamber pressure and nozzle blockage situation on the circulation flow rate were discussed in detail by the bubble behavior and gas holdup in the up-leg. It is necessary to maintain a certain vacuum chamber liquid level in the molten steel circulation flow. Compared with a nozzle with symmetrical blockage in the up-leg, when a nozzle with non-symmetrical blockage is applied, the lifting gas distribution is non-uniform, causing a great effect on the molten steel circulation flow and making the circulation flow drop largely.     

Mathematical Model of Decarburization of Ultra Low Carbon Steel during RH Treatment

According to the balance of carbon and oxygen, a decarburization model for the RH treatment has been developed. in which the influence of the mass transfer of carbon and oxygen in the liquid steel and the stirring energy (ε) in the vacuum vessel on decarburization rate has been considered. The conclusion that the volumetric coefficients of the mass transfer of carbon is proportional to ε1.5 is drawn. Industrical experiment proves this model is reliable. The influence of some factors on decarburization rate has been obtained. which can provide directions for RH treatment The decarburization behavior of steel with RH-OB treatment is also studied. The OB-or-not curve, the optimized OB time and OB amount are discussed.     

Decarburization rate of RH refining for ultra low carbon steel

The decarburization behaviors of ultra low carbon steel in a 210-t RH vacuum degasser were investigated under practical operating conditions. According to the apparent decarburization rate constant (K_C) calculated by the carbon content in the samples taken from the hot melt in a ladle at an interval of 1–2 min, it is observed that the total decarburization reaction period in RH can be divided into the quick decarburization period and the stagnant decarburization period, which is quite different from the traditional one with three stages. In this study, the average apparent decarburization rate constant during the quick decarburization period is 0.306 min-1, and that of the stagnant period is 0.072 min-1. Increasing the initial carbon content and enhancing the exhausting capacity can increase the apparent decarburization rate constant in the quick decarburization period. The decarburization reaction comes into the stagnant decarburization period when the carbon content in molten steel is less than 14×10-6 after 10 min of decarburization.     

Cleanliness of Ti-bearing Al-killed ultra-low-carbon steel during different heating processes

During the production of Ti-bearing Al-killed ultra-low-carbon (ULC) steel, two different heating processes were used when the converter tapping temperature or the molten steel temperature in the Ruhrstahl-Heraeus (RH) process was low:heating by Al addition during the RH decarburization process and final deoxidation at the end of the RH decarburization process (process-I), and increasing the oxygen content at the end of RH decarburization, heating and final deoxidation by one-time Al addition (process-Ⅱ). Temperature increases of 10℃ by different processes were studied; the results showed that the two heating processes could achieve the same heating effect. The T.[O] content in the slab and the refining process was better controlled by process-I than by process-Ⅱ. Statistical analysis of inclusions showed that the numbers of inclusions in the slab obtained by process-I were substantially less than those in the slab obtained by process-Ⅱ. For process-I, the Al₂O₃ inclusions produced by Al added to induce heating were substantially removed at the end of decarburization. The amounts of inclusions were substantially greater for process-Ⅱ than for process-I at different refining stages because of the higher dissolved oxygen concentration in process-Ⅱ. Industrial test results showed that process-I was more beneficial for improving the cleanliness of molten steel.     

Fluid Flow in Tundish Due to Different Type Arrangement of Weir and Dam

Tundish is an important metallurgical reactor in the continuous casting process. In order to control the fluid flow in tundish and thus take full advantage of the residencetime available for the removal of inclusions from molten steel, the effect of weir and dam on the fluid flow has been studied in a water model based on the characteristic number Froude and Reynold number similarity criteria. The residence time distribution curves of the flow were measured by SG800. The optimum arrangement of darn and weir and the nonstationary flow in tundish were discussed. The results show that the combination of weir and dam is benefit for the flow pattern in tundish, weir can prevent the upper recirculating flow, dam can cut off the bottom flow and turn to upwards, it is advantageous to separate the nonmetallic inclusions. Furthermore, it is important to exceed the critical depth of bath during exchange ladles, not only for the inclusion floatation but also for avoiding tundish slag drainage earlier.     

Coordinated control of carbon and oxygen for ultra-low-carbon interstitial-free steel in a smelting process

Low residual-free-oxygen before final de-oxidation was beneficial to improving the cleanness of ultra-low-carbon steel. For ul-tra-low-carbon steel production, the coordinated control of carbon and oxygen is a precondition for achieving low residual oxygen during the Ruhrstahl Heraeus (RH) decarburization process. In this work, we studied the coordinated control of carbon and oxygen for ultra-low-carbon steel during the basic oxygen furnace (BOF) endpoint and RH process using data statistics, multiple linear regressions, and thermodynamics computations. The results showed that the aluminum yield decreased linearly with increasing residual oxygen in liquid steel. When the mass ratio of free oxygen and carbon ([O]/[C]) in liquid steel before RH decarburization was maintained between 1.5 and 2.0 and the carbon range was from 0.030wt%to 0.040wt%, the residual oxygen after RH natural decarburization was low and easily controlled. To satisfy the re-quirement for RH decarburization, the carbon and free oxygen at the BOF endpoint should be controlled to be between 297 × 10?6 and 400 × 10?6 and between 574 × 10?6 and 775 × 10?6, respectively, with a temperature of 1695 to 1715°C and a furnace campaign of 1000 to 5000 heats.     

A Mathematical Model on Vacuum Chamber of a Multifunctional RH-degasser

A 3D-mathematical model was developed for the simulation of gas flow, combustion and heat convection in RH vacuum chamber. Under different conditions, the influences of the Kawasaki Oxygen Top Blowing system (KTB) on the gas flow, chemical reactions and temperature distributions were calculated based on the model. The optimum position of the lance is suggested for the RH-KTB degassing process.     

Forming condition and control strategy of ferrite decarburization in 60Si2MnA spring steel wires for automotive suspensions

The ferrite decarburization behavior of 60Si2MnA spring steel wires for automotive suspensions, including the forming condition and the influence of heating time and cooling rate after hot rolling, was investigated comprehensively. Also, a control strategy during the reheating process and cooling process after rolling was put forward to protect against ferrite decarburization. The results show that ferrite decarburization, which has the strong temperature dependence due to phase transformation, is produced between 675 and 875℃. The maximum depth is found at 750℃. Heating time and cooling rate after rolling have an important influence on decarburization. Reasonable preheating temperature in the billet reheating process and austenitizing temperature in the heat-treatment process are suggested to protect against ferrite decarburization.     

Mn evaporation and denitrification behaviors of molten Mn steel in the vacuum refining with slag process

Considering the precise composition control on the vacuum refining of high-Mn steel, the behaviors of both Mn evaporation and nitrogen removal from molten Mn steel were investigated via vacuum slag refining in a vacuum induction furnace. It was found that the reaction interfaces of denitrification and Mn evaporation tend to migrate from the surface of slag layer to the surface of molten steel with the gradual exposure of molten steel during the vacuum slag refining process. Significantly, compared with the experimental group without slag addition, the addition of slag into steel can result in a lower Mn evaporation rate constant of 0.0192 cm·min~(-1) at 370 Pa, while the denitrification rate is almost not affected. Besides, the slag has a stronger inhibitory effect on Mn evaporation than the reduced vacuum pressure. Moreover, the inhibitory effect of the slag layer on Mn evaporation can be weakened with the increase of the initial Mn content in molten steel. The slag layer can work as an inhibitory layer to reduce the Mn evaporation from molten steel, the evaporation reaction of Mn mainly proceeds on the surface of the molten steel. This may be attributed to the Mn mass transfer coefficient for one of reaction at steel/slag interface, mass transfer in molten slag, and evaporation reaction at slag/gas interface is lower than that of evaporation reaction at steel/gas interface. The introduction of slag is proposed for both denitrification and manganese control during the vacuum refining process of Mn steels.     

侧底复吹RH真空精炼脱碳过程研究

基于RH内流场,结合冶金反应热力学及动力学,通过建立数学模型研究了侧底复吹RH真空脱碳过程.数值结果表明计算结果与试验结果符合良好.在总吹气量相同条件下,侧底复吹RH前20 min的脱碳速率高于传统RH的脱碳速率.对于传统RH脱碳,前3 s以熔池内CO本体脱碳为主,3~1 000 s以氩气泡表面脱碳为主;对于侧底复吹RH脱碳,前1 000s以氩气泡表面脱碳为主,并且氩气泡表面脱碳速率约为熔池内CO本体脱碳速率的两倍;提高RH处理后期的脱碳速率可提高超低碳钢生产效率.     

RH冶炼超低碳钢内部脱碳机理及控制工艺

为了研究RH真空处理过程脱碳反应速率及其影响因素,并有效地控制超低碳钢在RH真空处理过程中碳含量的变化,根据热力学、动力学原理建立了RH真空处理脱碳数学模型,通过RH真空处理脱碳数学模型研究了内部脱碳反应深度和脱碳速率之间的关系.模型计算结果表明,反应深度的变化和内部脱碳的反应速率是相对应的,采取预真空操作,提升了反应深度,淡化了前期脱碳转折点的影响,加速了前期的脱碳反应,并在RH处理后期找到了内部脱碳向表面脱碳转变的时间临界点.     

RH脱碳过程中极低氧钢水的碳氧反应机理

BOF+LF+RH+CC工艺路线生产IF钢,在RH脱碳前,钢水经脱氧和LF精炼后,钢中自由氧达到极低水平.根据表观脱碳速率常数的不同,这种极低氧钢水的RH脱碳可以划分为四个阶段.与传统三个阶段的RH脱碳不同的是在低速脱碳阶段和快速脱碳阶段存在一个脱碳速率介于两者之间的过渡阶段.在正规溶液模型的基础上,建立了能够准确预报钢液氧含量及顶渣FeO含量的RH脱碳模型.结果表明:在RH吹氧前,极低氧含量的钢液与顶渣之间基本不传氧;吹氧之后,钢液氧含量呈线性增加,当钢液氧势大于顶渣氧势后,钢液向顶渣传氧,渣中FeO含量上升;RH处理结束FeO含量较处理初始有所回升,但是仍处于极低水平,能够有效降低顶渣对钢液的二次氧化.     

旋转磁场作用下真空精炼装置内脱碳过程的模拟研究

为了了解旋转磁场下真空精炼装置内的脱碳反应,对RH装置内钢液流场与脱碳过程进行了耦合计算;采用修正的均相流模型计算流场,避免了先前的一些不具推广性的方法,如简化计算区域或者预先规定含气率分布等.分别对有、无旋转磁场作用时的RH系统进行了模拟计算.模拟结果表明:流场和脱碳过程与现有文献中实验结果相符合;当旋转磁场作用后,磁感应强度为0.04 T时,初期脱碳速度比无旋流作用时增加了约15%,这是由于旋流增加了系统的搅拌能,提高了脱碳反应速率.脱碳反应进行到最后阶段,不同磁感应强度下的碳含量趋于同一稳定值.     

RH真空精炼脱碳速率的物理模拟研究

以某钢厂210 t RH装置为研究对象,利用水力模型对现场生产过程进行物理模拟,研究驱动气体流量、顶吹气体流量、枪位、浸入深度和真空度对脱碳速率的影响.结果表明,随顶吹气体流量的增大,脱碳速率明显增大;随插入管浸入深度的增大,脱碳速率略有增大;随真空度的增大、枪位的减小,脱碳速率逐渐增大;驱动气体流量对脱碳速率的影响很小.真空度为3 616 Pa、枪位为40 mm、插入管浸入深度为125 mm、驱动气体流量为4.0 m3/h和顶吹气体流量为4.8 m3/h时,脱碳速率最大.     

超低碳钢RH冶炼脱碳过程的数学模型

在充分考虑RH平衡碳氧浓度的前提下，建立脱碳反应数学模型．以210t超低碳钢RH冶炼工艺为背景，详细给出数学模型的建立原则与过程．将模拟结果与实际测量数据进行对比发现，数学模型与实际测量数据有很好的吻合度．碳元素在钢液内存在一定的不均匀性，真空室自由液面下降管上方碳元素质量分数最小，钢渣界面处上升管右侧碳元素质量分数最大，循环20min后，二者相差0．0025％左右．     

超低碳铝镇静钢冶炼过程氮含量的控制

针对企业冶炼超低碳铝镇静钢过程中增氮量高、波动大及控制不稳定的问题,采用工艺数据统计和现场取样的手段,系统梳理了冶炼过程钢液脱氮和增氮的主要环节和影响因素.转炉脱碳期和真空处理是脱氮的主要环节,碳氧期的总脱碳量高则终点氮含量低;转炉底吹N2/Ar切换点在吹炼70%以前对终点氮含量影响不大;VD在无氧条件下脱氮有利,RH则在有氧条件下脱氮有利.控制钢中溶解氧>200×10-6则出钢过程增氮可控制在5×10-6以下;炉料的氮带入是真空精炼环节增氮的重要因素,最高达11×10-6;采用密封垫+吹Ar的保护方式,增氮量最低为1×10-6.     

210 tRH精炼过程的混匀特性

以某钢厂210tRH真空精炼装置为原型,根据相似原理建立1﹕4水模型,研究了吹气量、浸入深度、真空度以及气孔堵塞对混匀时间的影响。结果表明,RH混匀时间随着吹气量的增加而呈现减小的趋势;随着浸入深度的增加先减小后增大,并存在最佳浸入深度480 mm;随真空室压力的减小而减小;随着吹气孔堵塞个数的增加先减小后增加。利用粒子成像测速技术( particle image velocimetry,PIV)测量了RH精炼过程钢包内二维流场,与数值模拟结果对比,发现钢包内的流体运动主要是从下降管到上升管的循环流动以及下降管周围的回流运动,不活跃区主要集中在渣-钢界面以下浸渍管浸入深度范围内。