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金属学报, 2025, 61(10): 1485-1501 DOI: 10.11900/0412.1961.2024.00178

综述

综述：中间包钢液二次氧化对钢中非金属夹杂物的影响

段生朝1, 刘珍童2, 康君3, 白乘风3, 文健3, 刘刚3, 张立峰,1

1 北方工业大学 机械与材料工程学院 北京 100144

2 燕山大学 机械工程学院 秦皇岛 066004

3 金海不锈钢有限公司 梧州 543002

Review: Effect of Reoxidation on the Non-Metallic Inclusion in Molten Steels in Tundish

DUAN Shengchao1, LIU Zhentong2, KANG Jun3, BAI Chengfeng3, WEN Jian3, LIU Gang3, ZHANG Lifeng,1

1 School of Mechanical and Materials Engineering, North China University of Technology, Beijing 100144, China

2 School of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China

3 Jinhai Stainless Steel Co. Ltd., Wuzhou 543002, China

通讯作者: 张立峰，zhanglifeng@ncut.edu.cn，主要从事高品质钢制备和钢中非金属夹杂物控制的研究

责任编辑: 肖素红

收稿日期: 2024-05-23   修回日期: 2024-10-08  

基金资助: 国家重点研发计划项目(2023YFB3709901)
国家自然科学基金项目(52404335)

Corresponding authors: ZHANG Lifeng, professor, Tel:(010)88802114, E-mail:zhanglifeng@ncut.edu.cn

Received: 2024-05-23   Revised: 2024-10-08  

Fund supported: National Key Research and Development Program of China(2023YFB3709901)
National Natural Science Foundation of China(52404335)

作者简介 About authors

段生朝，男，1990年生，博士

摘要

中间包是钢液流过的最后一个冶金反应器，它对钢铁产品的质量有着重要的影响。随着对高纯净钢需求的日益增加，中间包冶金的作用也越来越受到人们的重视。本文系统地总结了非稳态浇注过程中的钢液吸气、稳态浇注过程中的中间包覆盖剂和中间包耐火材料对不同钢种二次氧化的影响，钢液二次氧化对非金属夹杂物的影响，以及减轻中间包内钢液二次氧化的措施。造成钢液二次氧化的3个因素是同时发生的，但是在非稳态浇注阶段，钢液吸气是造成钢液二次氧化的主要原因；在稳态浇注阶段，中间包覆盖剂和耐火材料则成为主要原因。当钢液吸气发生二次氧化时，不同成分钢液的吸气速率并不相同。在钢液稳定浇注过程中，中间包双层覆盖剂顶层碳化稻壳灰中高含量的SiO2会逐渐向底层高碱度覆盖剂溶解，且在渣/钢界面发生自溶解反应(SiO2) = [Si] + 2[O]，造成钢液中的Al、Ti和Mn元素的损失，Si元素和总氧(T.O)含量的增加，以及夹杂物成分、尺寸和数量的变化。Al2O3-SiO2-C质水口耐火材料也会发生碳热反应生成氧化性CO气体，这是造成超低碳含Ti铝镇静钢发生二次氧化，导致水口堵塞的主要原因。中间包喷补料和钢包引流砂中含有的不稳定氧化物(如Cr2O3、MnO、SiO2和FeO)也会造成钢液严重的二次氧化。通过设计新型中间包覆盖剂替代碳化稻壳灰，能够减少渣中SiO2向底层覆盖剂中的溶解，从而减轻钢液的二次氧化。对于水口材料，可以通过使用氮化物以减少氧化性气体CO的释放，从而抑制水口结瘤和堵塞。微孔镁质耐材具有良好的抗渣和隔热性能，微封闭微孔可以吸收热应力并减少耐材内部裂纹的萌生和扩展，因此微孔镁质耐材用于中间包内衬材料具有较好的应用前景。

关键词： 中间包; 二次氧化; 耐火材料; 非金属夹杂物; 热力学

Abstract

The tundish is the final metallurgical reactor through which molten steel flows, and it significantly affects the quality of steel products. With the increasing demand for high-purity steel, the role of tundish metallurgy has attracted greater attention. This study systemically examines the causes of reoxidation in molten steel, the effects of air absorption during nonsteady-state teeming, tundish cover flux interactions, and refractory materials during the steady-state teeming process. These factors were analyzed for their influence on the chemical and inclusion composition of different steel grades. In addition, measures to mitigate the reoxidation of molten steel in tundish were analyzed. The results demonstrate that the three factors causing the reoxidation of molten steel occur simultaneously. However, in the nonsteady-state teeming stage, air absorption in the molten steel is the primary cause of reoxidation. Conversely, in the steady-state teeming stage, tundish cover flux and refractory materials are the main reasons. When reoxidation occurs due to gas absorption by molten steel, the gas absorption rates varies for different steel compositions. In the stable teeming of molten steel, the high content of SiO2 in the rice husk ash (RHA) in the top layer of the double-layer cover flux gradually dissolves in the high-basicity cover agent in the bottom layer. At the slag-steel interface, the self-dissolution reaction (SiO2) = [Si] + 2[O] occurs, resulting in the loss of Al, Ti, and Mn elements in the molten steel, whereas the Si content, total oxygen (T.O) content increase, and the composition, size, and number density of the inclusions change. Carbothermal reactions between Al2O3-SiO2-C refractories and molten steel can generate oxidizing CO gas, which is the main cause of the reoxidation of ultra-low carbon Ti added Al-killed steel. In addition, unstable oxides such as Cr2O3, MnO, SiO2, and FeO present in the gunning material and ladle filler sand can cause serious steel reoxidation. The reoxidation of steel and dissolution of SiO2 in the underlying cover agent can be mitigated by designing a new type of tundish cover flux to replace the RHA. Nitrides can be used in the nozzle material to reduce the release of the oxidizing gas CO, preventing nozzle clogging. Microporous magnesia-refractory materials provide strong heat insulation and slag resistance.It can absorb thermal stress and reduce the initiation and expansion of cracks in refractory materials. Therefore, microporous magnesia refractories have good application prospects as tundish lining materials.

Keywords： tundish; reoxidation; refractory material; non-metallic inclusion; thermodynamics

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本文引用格式

段生朝, 刘珍童, 康君, 白乘风, 文健, 刘刚, 张立峰. 综述：中间包钢液二次氧化对钢中非金属夹杂物的影响[J]. 金属学报, 2025, 61(10): 1485-1501 DOI:10.11900/0412.1961.2024.00178

DUAN Shengchao, LIU Zhentong, KANG Jun, BAI Chengfeng, WEN Jian, LIU Gang, ZHANG Lifeng. Review: Effect of Reoxidation on the Non-Metallic Inclusion in Molten Steels in Tundish[J]. Acta Metallurgica Sinica, 2025, 61(10): 1485-1501 DOI:10.11900/0412.1961.2024.00178

钢铁工业是国民经济的主要基础产业，在世界各国的经济发展中发挥着巨大的作用。我国钢铁工业为我国国民经济的快速发展作出了重大贡献，也为世界钢铁工业的发展起到积极的促进作用[1]。2022年中国粗钢产量约为10.1 × 108 t，约占世界粗钢产量的55.2%，持续保持世界首位。从20世纪50年代以来，现代钢铁生产流程已发展成为炼钢→精炼→连铸→成型工艺的优化组合，其中连铸作为承上启下的重要环节，对高品质钢的生产效率、质量和经济性至关重要[2]。2022年，中国连铸钢产量占粗钢产量的98.4%。在连铸过程中，中间包是衔接钢包和结晶器的一个过渡容器，它的主要功能有：分流作用；连浇作用；降低中间包内钢液的静压力，减少对结晶器中初生坯壳的冲刷；夹杂物上浮和去除[3]。随着交通运输、能源化工和机械制造等重点行业对钢铁材料需求的不断增加，对钢铁材料纯净度的要求也越来越高[4]。中间包作为钢液进入结晶器前所经过的最后一个具有耐火材料的反应器，末端冶炼对钢铁产品的质量起到至关重要的作用。钢液流经中间包发生的二次氧化现象会对钢中非金属夹杂物的成分、尺寸和数量密度产生影响，并且可能堵塞水口，最终对钢铁产品的性能产生不利的影响[5~7]。造成钢液二次氧化的主要原因来自大气、中间包覆盖剂和耐火材料(图1[8])，为了更加明确钢液流经中间包过程中发生的二次氧化对钢液的影响机理，本文系统地总结了不同钢种中间包内钢液二次氧化的影响因素及二次氧化对钢液成分和非金属夹杂物特征的影响规律，分析了控制中间包中钢液二次氧化的措施，同时对建立数学模型研究中间包内钢液二次氧化行为对非金属夹杂物的影响进行了展望。

图1

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图1   钢液流经中间包所发生的物理化学反应示意图[8]

Fig.1   Schematic of the physicochemical reactions occurring in the molten steel flow through the tundish[8]


1 中间包内钢液二次氧化的来源

1.1 大气对中间包中钢液二次氧化的影响

在钢液初始浇注和稳定浇注阶段，钢液在中间包中的二次氧化行为并不一致[9]。在初始浇注阶段，中间包入口处出现液面裸露，也称“红眼”(open eye)[10]，导致钢液严重吸气，进而发生二次氧化，造成钢液的纯净度变差；在稳定浇注阶段，钢液表层的空气和中间包覆盖剂是造成钢液二次氧化的主要因素。Tanak等[11,12]分析了大气环境下中间包中钢液的二次氧化行为，指出中间包中钢液二次氧化的最主要原因来自大气，而随着浇注过程的进行，中间包覆盖剂对钢液造成的二次氧化则成为主要原因。Sasai和Mizukami[9]得出了类似的结论，指出钢液开始浇注的过程中，钢液表面大气、钢包内衬填料、中间包入口处和中间包覆盖剂对钢液造成的二次氧化反应都是同时进行的，如图2[9]所示，其中在中间包入口处造成的二次氧化占66%。在稳定浇注过程中，中间包中钢液的二次氧化主要受到钢液表面大气和中间包覆盖剂的影响。Sasai和Mizukami[13]还研究了中间包内钢液的搅动对二次氧化的影响，发现静态条件下铝镇静钢的二次氧化反应的限制性环节为O2在氧化膜中的扩散，而在搅动状态下则转变为O2在气相中的传质。钢液流经中间包内时，为了抑制由于吸气造成的钢液二次氧化现象，特别是钢液从钢包经过长水口进入中间包的过程，通常向中间包内吹入惰性气体，如Ar气，以隔绝大气中的O2与钢液的反应。但是一般情况下，受到中间包容量和实际操作的影响，使用Ar气难以实现对中间包的完全封闭。在这种情况下，O2也可以伴随着Ar射流进入中间包内部，进而使钢液产生二次氧化[14]。钢液成分也会影响钢液对大气中O2的吸收速率，Sasai和Matsuzawa[15]发现，在实际浇注过程中，在搅动的情况下(氧势PO2 > 10 kPa)，Ti和Ti-Al脱氧钢的氧化速率要大于铝脱氧钢；而在较低氧势情况下，3种钢的氧化速率基本相同。为解决上述问题，Li等[16]给出了中间包内O2的体积分数与Ar气吹入时间的关系，指出随Ar气吹入时间延长，中间包内O2的体积分数逐渐降低，并且要使O2体积分数降低到1%以下，空包状态时Ar气的吹入速率要大于220 N·m3/h，在浇注状态下要大于80 N·m3/h。在浇注过程中难以避免钢液产生温降，通过等离子体对中间包内钢液加热，实现钢液低过热度和恒温稳定浇注，有利于生产无缺陷的连铸坯[17]，但是使用等离子体对中间包内钢液加热时，也会带入O2从而影响钢液的纯净度。

图2

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图2   造成中间包内钢液二次氧化的原因[9]

Fig.2   Factors responsible for the reoxidation of molten steel in tundish[9] (Δ[O] represents the amount of oxidation of the molten steel by the factors in the initial teeming stage and the stable casting stage)

(a) oxidation by air at the ladle shroud

(b) oxidation by air from the surface

(c) oxidation by ladle well-packing material

(d) oxidation by tundish cover powder


1.2 中间包覆盖剂对钢液二次氧化的影响

中间包覆盖剂最初的作用只是防止钢液在浇注过程中温降过大，但随着对高品质钢的需求越来越大，对中间包覆盖剂冶金功能的要求也随之增加，如钢液保温减少热量损失、隔绝钢液与大气避免钢液二次氧化、吸附和溶解钢液中非金属夹杂物、不与钢液发生化学反应避免污染钢液以及减少对中间包耐火材料的侵蚀[18,19]。因此，在实际操作过程中，双层渣的应用较为广泛[6,20,21]，即底层为熔点较低的碱性覆盖剂，顶层为碳化稻壳灰(rice husk ash，RHA)。在浇注稳定阶段，由于钢液热量的传输，根据中间包内双层渣的物理状态可以将渣层分为3个区域：靠近温度较高的钢液一侧为完全液相渣区域、顶层温度较低且含有大量高熔点碳化稻壳灰的固相渣区域、处于完全固相渣和液相渣中间区域的固-液相渣共存的区域。底层的液相渣区域主要起到吸收和溶解钢中非金属夹杂物的作用，而顶层的固相渣以及固-液相渣共存区域则起到保温和隔绝大气的作用。但是，在实际操作过程中发现，顶层固相碳化稻壳灰中的SiO2不断地向下层熔渣中溶解和扩散，致使在渣-钢界面反应区域熔渣中SiO2的含量不断升高，造成钢液的二次氧化和夹杂物的生成。底层熔渣的黏度往往是影响SiO2溶解速率和机理的主要因素[22]。本课题组[23]最近提出了渣的夹杂物容量(inclusion capacity of slag)概念，即Zh指数，用来预报非金属夹杂物在渣中的溶解速率，其表达式如 式(1)所示：

Zh=g⋅ρslag2⋅(Ci, saturation-Ci)⋅dp, 03ηslag2

式中，Ci 和Ci, saturation分别表示渣中含有夹杂物相i的质量分数和渣中该物相的饱和质量分数(%)，dp, 0为夹杂物初始直径(m)，ρslag和ηslag分别代表熔渣的密度(kg/m3)和动力学黏度(kg/(m·s))，g为重力常数(m/s2)。从 式(1)可以看出，当SiO2颗粒较大时，CaO-Al2O3-SiO2渣的黏度越小，SiO2溶解进CaO-Al2O3-SiO2渣的速率就越快。该结果也意味着RHA中SiO2颗粒向碱性覆盖剂溶解速率较快，当钢中含有易氧化元素(如Al、Ti和Mn)时，会导致较为严重的钢液二次氧化。

为了明确中间包覆盖剂中SiO2对铝镇静钢的二次氧化反应行为的影响，Basu等[20]在浇注含Ti铝镇静超低碳钢时，在中间包使用底层碱性覆盖剂成分为(46~50)CaO-(42~45)Al2O3-(0~3)SiO2-(0~2)Fe2O3-(1.8~2.5)TiO2 (质量分数，%，下同)和顶层为碳化稻壳灰(SiO2 > 94%)的双层渣，发现在底层与钢液接触的熔渣中SiO2含量增加，并与镇静钢中Al和Ti元素发生氧化还原反应，生成Al2O3和TiOx -Al2O3夹杂物，如 式(2)所示，同时发现在中间包中钢液Si含量的平均增加量为0.002%~0.004%。Yan等[24]利用CaO-SiO2-Al2O3渣和Deng等[25]利用CaO-SiO2-FeO-MnO渣与超低碳钢之间的反应，模拟钢液的二次氧化，得到了类似的结论。

6+x2(SiO2)+4[Al]+[Ti]=
6+x2[Si]+(Al2O3)inclusion+(TiOx-Al2O3)inclusion

Goto和Miyazawa[26]对比了从钢包至中间包连铸过程中，渣中SiO2对非脱氧钢和铝脱氧钢二次氧化行为的影响，发现非脱氧钢中的Si和溶解氧含量均增加，铝脱氧钢中只有Si含量增加而溶解氧与Al反应生成Al2O3夹杂物，夹杂物上浮去除使得总氧(T.O)含量变化不大。Bessho等[27]发现，钢液中Al还原中间包SiO2-CaO-Al2O3渣中SiO2的反应速率取决于渣中SiO2活度(aSiO2)，当aSiO2 < 0.008时钢液的二次氧化现象可以得到抑制。Kim等[28]建立了“耐火材料-熔渣-金属-夹杂物”(refractory-slag-metal-inclusion，ReSMI)复合模型，研究了改变CaO-Al2O3渣(CA-Flux)中RHA的含量(表达为CaO-Al2O3渣的质量与CaO-Al2O3渣和RHA的总质量之比(RCA = [CA-Flux] / [CA-Flux + RHA]))对铝镇静Fe-C-Si钢二次氧化行为的影响，发现当RCA = 0.5时，钢液中的溶解氧含量增加量最大且伴随着钢液中Al含量的降低，说明此时钢液的二次氧化现象最为严重。当RCA超过0.8时(80CA-Flux + 20RHA，质量分数，%)钢液的二次氧化得到抑制。这是由于CaO-Al2O3渣的加入，使得CaO-Al2O3渣和RHA混合渣系中的SiO2活度降低所致。考虑钢包下渣的情况，Kim等[8]研究了在钢包CaO-Al2O3-SiO2-MgO-CaF2渣中加入不同量的RHA时对铝镇静Fe-C-Si钢二次氧化行为的影响，得到了类似的结论。Wang等[29]在实验室研究了MgO-CaO-SiO2-Al2O3-Cr2O3中间包覆盖剂中Cr2O3含量对铝镇静YT01钢二次氧化行为的影响，指出覆盖剂中Cr2O3的含量为5%时会造成钢液T.O含量和Al含量的损失。以上研究均表明，由于RHA中的SiO2向碱性中间包覆盖剂中扩散，导致钢液严重二次氧化反应的主要原因是渣中SiO2发生自溶解反应，如 式(3)所示。铝镇静钢钢液二次氧化机理可以分为以下步骤：随着渣中SiO2的自溶解，在渣/钢界面钢液一侧边界层内溶解氧和Si含量增加；随后钢液中的Al从钢液内部向渣/钢界面扩散，与增加的溶解氧反应生成Al2O3，随后向渣中扩散；渣/钢界面处增加的Si向钢液内部扩散，化学反应如 式(4)所示。中间包覆盖剂中SiO2含量增加导致铝镇静钢发生二次氧化的机理示意图如图3[30]所示。表1[8,20,21,24~29,31~34]为中间包覆盖剂造成钢液二次氧化的研究总结。

图3

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图3   化学反应作用下钢/渣界面动态行为的机理示意图[30]

Fig.3   Mechanism illustrations of the dynamic behavior at a steel/slag interface under the effect of chemical reactions[30]


表1   中间包覆盖剂造成钢液二次氧化的研究总结[8,20,21,24~29,31~34]

Table 1  Summaries of previous studies on the reoxidation of molten steel by tundish flux[8,20,21,24-29,31-34]

Tundish cover powderSteel compositionExperimental methodRef.
(39.5-62.5)CaO-(2.7-22.2)Al2O3-(2.6-47.4)SiO2

Ultra-low carbon

steel

The industrial trials were carried out at the No.5 continu-ous caster at Mizushima Works in Kawasaki Steel Corp-oration

[27]

52CaO-35Al2O3-13SiO2

50CaO-50SiO2-100SiO2

Al-killed steel

Reoxidation of Al-killed steel by slag and air was investigated in laboratory experiments and industrial trials in 85 t tundish at Sumitomo Metal Industries

[31]

*Non-killed and Al-killed steelsThe industrial trials were conducted in 60 t tundish at Nippon Steel Corporation[26]

RHA + (46-50)CaO-(42-45)Al2O3-3SiO2-2Fe2O3-

(1.8-2.5)TiO2

Ti-bearing Al-killed ultra-low carbon steel

The industrial trials were conducted at Tata Steel

[20]

26.8CaO-42.3Al2O3-

27.6SiO2

Ti-stabilized ultra-low carbon steel45 g steel covered with 40 g tundish slag was held in an alumina crucible at 1823 K[24]

RHA + 45CaO-35Al2O3-

10SiO2-5MgO-5CaF2

Al-killed Fe-C-Si steel500 g of steel and 50 g of slag were placed in a MgO crucible and heated to 1823 K (R = 0.14, 0.26, 0.38, and 1.0)

[8]

RHA + 55CaO-35Al2O3-

4SiO2-4MgO

Al-killed Fe-C-Si steel500 g of steel and 50 g of slag were placed in a MgO crucible and heated to 1823 K (RCA = 0.5, 0.75, 0.83, 0.87, and 0.90)

[28]

50CaO-30Al2O3-10SiO2-

10MgO-(5, 10)Cr2O3

Al-killed YT01 steel

The Al-killed steel was held in a MgO crucible at 1923 K for 30 min and then 20 g of slag with various Cr2O3 contents was added

[29]

73CaO-25SiO2-(1-15)FeO-1MnOUltra-low carbon steelThe slag-metal reactions between 30 g pre-melted slag and 100 g of sample were conducted in a zirconia crucible at 1853 K

[25]

44.2CaO-44.1Al2O3-

6.0SiO2-2.68MgO-1.17TiO2

Ti-bearing Al-killed ultra-low carbon steel400 kg of the basic tundish flux was added to the surface of the molten steel in the 70 t tundish, and then 60-80 kg of rice hull ball was added to the tundish flux

[21]

RHA + 45.65CaO-22.88Al2O3-20.75MgO-2.84SiO2Si-Mn killed SAE 1055 steel150 g steel and 20 g tundish slag (5 g RHA) were acco-mmodated in an Al2O3 crucible at 1853 K[32]
53.5CaO-41.5Al2O3-5MgO and 47.5CaO-47.5SiO2-5MgOSi-killed 304 stainless steel600 g of 304 stainless steel and 50 g of slag were cont-ained in a MgO crucible at 1773 K[33]

RHA, RHA + 51.1CaO-

43.3SiO2, and RHA +

52.6CaO-40.7Al2O3

Si-killed 316L stainless steel600 g of 316L stainless steel was placed in a MgO crucible at 1773 K and then 45 g of tundish flux was quickly added to the surface of the molten steel

[34]

Note: * indicates the composition of the tundish flux is not reported. RHA—rice husk ash. R = [RHA] / [RHA + Slag]. RCA = [CA-Flux] / [CA-Flux + RHA], in which CA-Flux represents CaO-Al2O3 slag

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(SiO2)=[Si]+2[O]
4[Al]+3(SiO2)=3[Si]+2(Al2O3)

Duan等[34]报道了不同种类的中间包覆盖剂，如RHA、RHA + CaO-SiO2 (RHA + Flux A)和RHA + CaO-Al2O3 (RHA + Flux B)，对硅脱氧316L不锈钢二次氧化行为的影响。结果表明，当使用RHA和RHA + Flux A覆盖剂时，钢液发生严重的二次氧化，且随着反应时间延长，钢中Si和Mn含量分别增加和降低，反应如 式(5)所示。而使用RHA + Flux B覆盖剂时，钢液的二次氧化则被抑制。假设钢液中Mn的传质为限制性环节，则Mn在钢液中的传质系数(km, Mn)可以由 式(6)获得。

2[Mn]+(SiO2)=[Si]+2(MnO)
-ln[%Mn]t-[%Mn]e[%Mn]0-[%Mn]e=
km, Mn⋅AVm⋅[%Mn]0[%Mn]0-[%Mn]e⋅t

式中，[%Mn]0和[%Mn]t 分别表示钢液中初始Mn含量和任意反应时间t时钢液中的Mn含量(质量分数，%)，[%Mn]e为反应(5)达到平衡时钢液中的Mn含量(质量分数，%)，A和Vm分别代表渣/钢反应界面的面积(m2)和钢液体积(m3)。根据Duan等[34]研究可以得到km,Mn = 6.4 × 10-6 m/s (1773 K)。

Alves等[32]比较了CaO-Al2O3基覆盖剂和RHA + CaO-Al2O3双层覆盖剂对Si-Mn镇静钢二次氧化行为的影响，指出仅使用CaO-Al2O3基覆盖剂就可以提高钢液的纯净度。Kim等[33]分析了53.5CaO-41.5Al2O3-5MgO和47.5CaO-47.5SiO2-5MgO覆盖剂对硅脱氧304不锈钢纯净度的影响，发现CaO-Al2O3-MgO能够增加304不锈钢的纯净度，CaO-SiO2-MgO则对不锈钢的纯净度基本没有影响。可以发现，虽然顶层为RHA和底层为碱性覆盖剂的双层渣可以在一定程度上或短时间内防止钢液在中间包中热量的耗散并起到吸收夹杂物的作用，但是顶层的RHA中的SiO2会向底层渣中扩散，造成底层渣中SiO2含量增高，导致钢液中的易氧化元素(如Al、Ti和Mn)与渣中SiO2发生二次反应(如式(2)、(4)和(5))，生成二次氧化夹杂物且T.O含量增加。若中间包使用CaO-Al2O3基覆盖剂则可以起到抑制钢液二次氧化的作用。

1.3 中间包耐火材料对钢液二次氧化的影响

水口包括长水口(ladle shroud)和浸入式水口(submerged entry nozzle，SEN)，是分别连接钢包和中间包以及中间包和结晶器并起到保护性浇注作用的重要耐火材料，可以起到防止钢液飞溅和卷渣等多重作用[35]。超低碳铝镇静钢在浇注时会产生严重的水口结瘤和堵塞[5]，其中一个原因就是，在浇注温度下(1803~1823 K)，水口材料中的氧化物会发生碳热还原反应生成CO和SiO气体[36~38]，并与钢液中的Al和Ti反应生成二次氧化产物，进而恶化钢液的可浇性[39]。Fukuda等[37]在1873 K下使用旋转柱侵蚀法将直径为30 mm的Al2O3-C耐火材料浸入钢液中4 h，发现在耐火材料表面有网状的Al2O3夹杂物生成，这种网状夹杂物的生成是由SiO与Al2O气体和钢中Al反应造成的。Sasai和Mizukami[40]计算了浸入式水口中SiO2与Al2O3发生碳热反应生成气体SiO、Al2O和AlO的平衡氧分压，发现不同氧化性气体的平衡氧分压的大小排序为pSiO > pAl2O > pAlO，意味着碳热反应生成的主要氧化性气体为SiO。Tsujino等[41]和Basu等[20]发现，除了SiO2与C发生碳热反应生成SiO和CO之外，水口材料中的添加剂ZrO2和Na2O也会分别与C反应生成ZrO和CO。以上结果并没有明确地揭示浇注低碳铝镇静钢时，由于水口耐火材料发生碳热反应造成结瘤的反应机理。Lee等[42]将不同种类的水口材料和SiO2-C加热至1873 K，并使用射频质谱仪原位分析挥发气体的成分，发现挥发气体中只存在CO，均未发现SiO。他们使用FactSageTM热力学软件(FactPS和FTOxid数据库)计算了标准状态下不同碳热反应标准Gibbs自由能与温度的关系，通过对比发现，反应(7)的标准Gibbs自由能最负，说明水口材料中的SiO2易与C反应生成以CO为主的氧化性气体。生成的CO气体穿过内部孔隙到达水口材料内壁与钢液接触，CO气体氧化钢液中的Al形成网状的Al2O3 (反应(8))并附着在水口内壁，同时伴随着低碳铝脱氧钢中固态Al2O3夹杂物向耐火材料表面黏附沉积，导致水口结瘤，反应机理如图4a[43]所示。

SiO2(s)+3C(s)=SiC(s)+2CO(g)
3CO(g)+2[Al]=Al2O3+3[C]

图4

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图4   低碳铝镇静钢[43]和超低碳含Ti铝镇静钢[44]的浇注过程中早期堵塞沉积物生长反应机理

(a) low carbon Al-killed steel[43] (AG refractory—alumina graphite refractory)

(b) ultra-low carbon Al-killed Ti-added steel[44] (Ti-ULC—Ti added ultra-low carbon)

Fig.4   Reaction mechanism causing the early-stage clog deposit growth


Lee等[44]在1833 K下使用CO气体(0.5 L/min)喷吹钢液表面30 min，用来模拟含Ti铝镇静钢在浇注过程中水口材料发生碳热反应产生的CO对钢液二次氧化行为的影响，反应机理如图4b[44]所示。在浇注含Ti铝脱氧超低碳钢时，水口耐火材料发生碳热反应生成的CO (反应(7))扩散至钢液与耐火材料界面，CO与钢液中Al、Ti和Fe同时反应，生成液态Fet O-Al2O3-TiOx (反应(9))和固态Al2O3夹杂物(反应(8))并附着在水口耐火材料内壁，液态Fet O-Al2O3-TiOx 中的Fet O被钢液中的Al或者水口耐火材料中的C还原，最终水口耐火材料的沉积物转变为还原的Fe液滴和Al2O3-TiOx 复合氧化物[44]。Lee和Kang[45]还报道了当水口耐火材料中含有CaO、ZrO2和SiO2时，它们会较快地溶解进生成的Fet O-Al2O3-TiOx 液态沉积物中，最终转变为Fe液滴和CaO-Al2O3-TiOx -ZrO2-SiO2水口沉积物。

CO(g)+[M](M=Fe, Al, Ti)→
(FetO-Al2O3-TiOx)deposit

可见，在超低碳铝镇静钢和超低碳含Ti铝镇静钢的浇注过程中，在水口内壁分别形成网状的Al2O3和液态Fet O-Al2O3-TiOx，可以作为钢中高熔点非金属夹杂物的沉积聚集位置，逐渐导致水口内钢液流速变慢，钢液热量损失加大，在水口内壁附着的夹杂物与凝固钢液的共同作用下导致水口堵塞[5]。由于以上钢种在浇注过程中造成的钢液二次氧化和水口结瘤的原因之一是水口耐火材料中碳热反应生成的CO，因此降低CO气体的生成可以有效抑制由于钢液二次氧化造成的水口结瘤问题。Lee等[42]设计了一种新的长水口耐火材料AlON-AlN-BN，这种耐火材料与钢液进行高温反应时只释放N2，可以减轻水口堵塞的问题。

在连铸过程中，中间包工作面与熔渣和钢液接触并承受着熔渣的化学侵蚀和钢液的物理冲刷等作用，会导致中间包耐火材料(主要成分为MgO-SiO2)出现局部损毁，进而影响钢液的纯净度和中间包的正常使用[46,47]。采用喷补技术对中间包耐火材料损毁处进行维护，可以延长中间包使用寿命[32,48]。中间包喷补料(gunning material，GM)主要分为MgO和Al2O3基耐火材料，杂质成分主要含有SiO2和FeO。Yan等[49]比较了3种喷补料MgO、Al2O3以及MgO + MgO·SiO2对钢液二次氧化行为的影响。通过电子探针(EPMA)对耐火材料与钢液界面的观察发现：(1) 钢液会渗入进喷补料中；(2) 耐火材料与钢液界面处发现喷补料剥落形成的大尺寸颗粒状外来夹杂物；(3) 钢液发生二次氧化现象(式(10)和(11))，且钢液界面处形成5~30 μm厚的(Mg, Fe)O氧化层，这是由于耐火材料的氧势(耐材内部的空气、FeO和SiO2)远高于钢液(PO2 = 10-11 Pa)所造成的，如图5[49]所示。

[Al, Ti]+SiO2→[Si]+(Al, Ti)Ox
[Al, Ti, Si, Mn]+FeO→[Fe]+(Al, Ti, Si, Mn)Oy

图5

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图5   钛稳定超低碳钢和耐火材料界面显微组织[49]

Fig.5   Microstructure of the interface between Ti-stabilized ultra-low carbon steel and MgO-based gunning material[49] (PO2—oxygen potential)

(a) oxygen from refractory (trapped air in pores, reducible FeOx and SiO2 oxides)

(b) molten steel with a low oxygen content (around 10-11 Pa)

(c) FeOx at the refractory/steel interface


程礼梅等[50]发现钢液渗入喷补料深度的排序为MgO > MgO + MgO·SiO2 > Al2O3，这主要是由于钢液与上述耐火材料的接触角不同所导致的。Yan等[24,49]和Liu等[51]得到类似结论，与MgO基喷补料相比，使用Al2O3基喷补料有利于提高钢液的纯净度。表2[24,32,49,51~55]总结了关于中间包耐火材料导致钢液二次氧化的研究。

表2   中间包耐火材料导致钢液二次氧化的研究总结[24,32,49,51~55]

Table 2  Summaries of reoxidation of molten steel caused by refractory material of tundish[24,32,49,51-55]

Tundish refractory

Steel composition

(mass fraction / %)

Main conclusionRef.

Al2O3-SiO2

Ni-Fe alloy

The substitute of mullite to SiO2 mullite of the refractory bonding matrix or the use of alumina bricks can avoid the reoxidation of the melt and intense inclusion formation

[52]

GM, MgO boards, and dry powder0.45C-3.1Si-0.4Mn-8.5Cr-0.25NiAn oxidized steel layer can be formed at the steel/refractory lining interface[53]

MgO, Al2O3, MgO + 2MgO·SiO2 GMs

Ti-stabilized ultra-low carbon steel

The large difference in oxygen potential between refractory and steel phase leads to the formation of (Mg, Fe)O layer (PO2 = 10-11 Pa); The thicker layer was formed using MgO-based GM compared with Al2O3-based GM

[49]

MgO- and Al2O3-based GMsTi-stabilized ultra-low carbon steelThe oxidation capacity of MgO GM with 10SiO2-6FeO was higher than that of Al2O3 GM with 3.3SiO2 + 2FeO[24]
MgO-CaO and MgO-based GMsSi-Mn-killed SAE 1055 steelThe GM with the reducible oxides SiO2 and FeO was responsible for providing oxygen and causing reoxidation of the molten steel

[32]

MgO- and Al2O3-based GMsTi-stabilized ultra-low carbon steelThe MgO GM represented a stronger oxidizing capacity, while Al2O3 can improve the cleanliness of the molten steel[51]
High-silica tundish refractory (66MgO-27SiO2-4FeO-3CaO)

Fe-2Al alloy

The content of the oxidizing oxides in the refractory should be reduced to avoid the loss of Al in the alloy

[54,55]

Note: GM—gunning material

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钢包引流砂(ladle filler sand，LFS)作为滑动水口的添加剂起到钢包自动开浇的重要作用，常用的引流砂成分为硅质、铬质、锆质和镁质。高锰高铝钢中的Mn和Al会还原引流砂(29.4Cr2O3-4.93MgO-10.7Al2O3-16.96Fe2O3-34.29SiO2)中的不稳定氧化物Cr2O3、MnO和FeO，造成钢液二次氧化，并且氧化产物使引流砂产生更多以MnO-Al2O3-FeO-SiO2为主的液相，恶化钢包引流砂的烧结性能，进而导致钢包无法自开[56]。引流砂中的Cr2O3、MnO和Fe2O3会造成轴承钢中Al元素的烧损并生成大量尺寸大于5 μm的Al2O3夹杂物，使得轴承钢中初始夹杂物类型从MgAl2O4尖晶石转变为富Al2O3尖晶石[57]。同时，某些MnO/FeO大型夹杂物也被证明来自铬质引流砂(铬铁矿相和石英相)及其烧结产物，一旦引流砂进入钢液中，想在中间包内完全去除是十分困难的[58]。钢包下渣也会造成中间包内钢液的二次氧化，应当采取相应的监控措施或优化钢包底部结构，进而减少氧化性精炼渣对钢液的二次氧化[7,59]。

2 钢液二次氧化对钢中非金属夹杂物的影响

钢液的二次氧化不仅会造成钢液成分的波动，还会造成钢中非金属夹杂物形貌、成分和数量密度的变化。Yang等[60]在调研铝脱氧钙处理API-X70管线钢的连铸过程时发现，当连铸至70 t时，钢液吸气造成钢液的二次氧化，钢液T.O含量和N含量分别从RH炉(Ruhrstahl-Heraeus degasser)出站后的14 × 10-6和31 × 10-6增加至21 × 10-6和35 × 10-6。钢液二次氧化的夹杂物成分的变化如图6[60]所示。可以看出，经过钙处理后钢中夹杂物主要成分为液态钙铝酸盐(CaO / Al2O3含量比值 = 0.86)，但是钢液发生二次氧化后中间包钢液夹杂物则转变为固态钙铝酸盐CaO·2Al2O3和CaO·6Al2O3 (CaO / Al2O3含量比值 = 0.16)。由于富Al2O3的钙铝酸盐夹杂物与钢液有较强的界面能[61]，因此与纯Al2O3夹杂物相比，CaO·2Al2O3和CaO·6Al2O3夹杂物更容易碰撞聚集形成较大颗粒的夹杂物，将会造成严重的水口结瘤[62,63]。诸多学者[64~67]也都报道了类似的结果，并发现铝脱氧钙处理钢发生二次氧化后，钢中夹杂物类型的转变同时受钢中S和Ca含量的影响。当钢中总钙(T.Ca)含量为10 × 10-6时，夹杂物的转变行为为CaS→液态夹杂物→CaO·2Al2O3→CaO·6Al2O3→Al2O3；而当钢中T.Ca = 30 × 10-6时，钢中CaS夹杂物全部通过反应(12)和(13)[68]分解并转变为固态钙铝酸盐夹杂物(CaO·2Al2O3 + CaO·6Al2O3)。

(CaS)inclusion+[O]=(CaO)in calcium aluminate
2[Al]+3[O]=(Al2O3)in calcium aluminate

图6

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图6   钢中夹杂物成分分布的变化[60]

Fig.6   Changes of inclusion composition in steel[60]

(a) Ruhrstahl-Heraeus degasser (RH furnace) out (b) tundish (c) slab (d) plate


Wang和Liu[69]发现，在含稀土Ce的铝镇静低合金钢的浇注过程中，钢液在浸入式水口位置吸气导致钢液发生二次氧化，导致钢液中的T.O含量和总氮(T.N)含量分别从11 × 10-6和41 × 10-6增加至72 × 10-6和169 × 10-6，而钢液中Ce含量从0.196%降低至0.190%。钢液二次氧化致使钢中细小的球形Ce2O2S夹杂物通过 式(14)转变为大尺寸的团簇状CeAlO3夹杂物，如图7[69]所示，或者直接由 式(15)生成CeAlO3夹杂物。他们指出，控制钢中[Ce] / [O]含量之比大于2.12时，可以避免由于钢液二次氧化造成的Ce2O2S夹杂物的转变。表3[8,28,34,44,60,64,69~81]总结了钢液二次氧化导致的不同钢种中夹杂物成分转变的研究。

Ce2O2S+[O]+[Al]=CeAlO3+[Ce]+[S]  
[Ce]+3[O]+[Al]=CeAlO3

图7

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图7   钢液发生二次氧化后板坯样品缺陷处典型团簇状CeAlO3簇的特征[69]

Fig.7   Characteristics of typical CeAlO3 clusters at the defect of slab samples with reoxidation[69]


表3   钢液二次氧化导致的不同钢种中夹杂物成分转变的研究总结[8,28,34,44,60,64,69~81]

Table 3  Summaries of inclusion transformation caused by molten steel reoxidation[8,28,34,44,60,64,69-81]

Steel compositionInitial inclusionInclusion composition after reoxidation of molten steelRef.

Al-killed and Ca-treated

API-X70 pipeline steel

Liquid calcium-aluminate inclusionAl2O3-rich inclusion[60]
Al-killed and Ca-treated 42CrMo steelLiquid CaO-MgO-Al2O3Solid CaO-MgO-Al2O3 and MgO-Al2O3[70]
Al-killed Fe-0.2C-0.1Si steelLiquid CaO-MgO-Al2O3Al2O3-rich inclusion[8,28]
Al-killed molten steelCaO-CaS (Al2O3)CaO-Al2O3 (CaS)[64]
Al-killed GCr15 bearing steelCaO-Al2O3-MgO-CaSThe increase in CaO and Al2O3 contents and decrease in MgO and CaS contents of the inclusion[71]
Al-killed low alloy steel containing rare earth CeSpherical Ce2O2S inclusionClustered-like CeAlO3[69]

Al-killed and Ca-treated

AH36 steel

Liquid CaO-Al2O3-MgOCaO-2Al2O3 and MgO-Al2O3[72]
Ca-treated carbon-manganese steelLiquid inclusion + CaSSemi-solid 60.4Al2O3-25.3CaO-7.2 SiO2-7.1MgO[73]
Al-killed and Ca-treated stainless steelCaO-Al2O3-SiO2-MgOMnO-Al2O3-SiO2-CaO[74]
GCr15 bearing steelLiquid calcium-aluminate inclusionCaO-6Al2O3 and Al2O3[75]
Fe-Al-Ti-O meltAl2O3 and Al2TiO5

Ti3O5 (high oxygen potential)

Al-Ti complex oxide (low oxygen potential)

[76]
Al-killed and Ti-alloyed IF steelAl2O3Al2O3-TiOx[77]
Ti-added ultra-low carbon steelAl2O3Liquid Fet O-Al2O3-TiOx[44]

Al-killed stainless steel

(20Cr-0.15Al-0.2Ti)

CaO-Al2O3-MgOSolid Al2O3-Cr2O3-TiO2[78]

Si-killed stainless steel

(18Cr-8Ni-0.48Si-1.06Mn)

Al2O3-SiO2-CaO-MnOAl2O3-SiO2-CaO-MnO with high SiO2-MnO content[79]
Si-killed spring steel

CaO-SiO2 and

Al2O3-SiO2-CaO

Al2O3-SiO2-MnO and SiO2-MnO[80]
Si-killed stainless steelCaO-SiO2-MgO-Al2O3Liquid + Mg[Al,Ti]2O4 composite inclusions[81]
Si-killed 316 stainless steelSiO2-MnOSiO2-rich inclusion[34]

Note: IF steel—interstitial-free steel

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含Ti超低碳钢液中Al和Ti含量约在0.01%~0.1%的范围之内，如图8[82]所示，此时钢中溶解氧含量较低且可以稳定存在的夹杂物为Al2O3 (Region A)。若钢液暴露在任何可能存在的氧化性环境下，如大气或者水口耐火材料释放的CO (CO(g) = [C] + [O])，造成钢液中溶液氧含量增加，夹杂物成分则由固态Al2O3 (Region A)转变为液态Fet O-Al2O3-TiOx 夹杂物(Region B)。Lee等[44]在感应炉中使用CO气体与Fe-(0.0125, 0.025, 0.05)Al-0.05Ti合金的反应(1560 ℃)来模拟含Ti超低碳不锈钢在浇注过程中由于水口材料释放CO造成钢液二次氧化从而形成水口结瘤的现象，通过理论模拟和实验结果得出，水口耐火材料和钢液的溶解氧含量最高可以达到约500 × 10-6 (pCO = 105 Pa，1540 ℃)，约是钢液内部溶解氧含量(约20 × 10-6)的20倍。从图8[82]可以看出，在此高溶解氧含量条件下，钢中稳定的夹杂物为液态的Fet O-Al2O3-TiOx，而该液态夹杂物与水口耐火材料具有良好的润湿性，因此Fet O-Al2O3-TiOx 和Al2O3的存在是含Ti铝镇静钢在连铸过程中发生水口结瘤的主要原因[45,83]。Kim等[78]指出，不锈钢(Cr含量 > 20%)中同时含有较高含量的Al (> 0.15%)和Ti (> 0.2%)时，钢中溶解氧含量会降低至约3 × 10-6。但钢液由于吸气或者钢包引流砂(Fe2O3-Cr2O3-SiO2)发生二次氧化，使得浇注过程的pO2大于10-3 Pa (1520 ℃)，导致钢液中生成Al2O3-Cr2O3-TiO2夹杂物，这类固相夹杂物将会使不锈钢卷产品产生严重的质量缺陷。

图8

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图8   1540 ℃下Fe-Al-Ti-O系氧化物优势区图[82]

Fig.8   Oxide stability diagram of Fe-Al-Ti-O system at 1540 oC[82]


Li等[79]发现，在开始浇注硅脱氧不锈钢(18Cr-8Ni-0.48Si-1.06Mn)至稳定浇注前，钢液吸气造成的二次氧化使得钢液局部溶解氧含量较高而Al和Ca含量很低，钢液中夹杂物的MnO和SiO2含量增加的速率高于Al2O3和CaO含量增加的速率，由于钢中Mn含量较高，形成了直径为1~2 μm的富MnO的小尺寸颗粒状夹杂物(2[Mn] + O2 = 2(MnO))。随着钢液浇注进入稳定阶段，钢液中的小尺寸颗粒状富MnO的夹杂物作为中间产物被钢液中Ca和Al还原，伴随着夹杂物的碰撞和长大，最终钢中富MnO夹杂物消失。Lyu等[80]报道了类似的结果，发现在硅脱氧0.55C-1.32Si-0.68Mn-0.61Cr弹簧钢的浇注过程中，由于中间包内钢液的搅动吸气造成钢液二次氧化，钢液中的Si和Mn与大气中的O2反应生成MnO-SiO2夹杂物，如 式(16)所示，之后形成的MnO-SiO2夹杂物被钢液中的Al还原，生成MnO-SiO2-Al2O3夹杂物，如 式(17)所示。

[Si]+[Mn]+1.5O2→(SiO2-MnO)inclusion
[Al]+(SiO2-MnO)inclusion→
(MnO-SiO2-Al2O3)inclusion+[Mn]

在中间包覆盖剂中，RHA中过高的SiO2含量同样会造成硅镇静316L不锈钢在浇注过程中夹杂物成分发生变化。Duan等[34]使用RHA、RHA + Flux A和RHA + Flux B分别研究了中间包覆盖剂对316L不锈钢二次氧化行为的影响，实验结果如图9[34]所示。可以看出，当使用RHA和RHA + Flux A时，由于钢液发生较为严重的二次氧化(式(3))，导致当反应时间为30 min时钢中夹杂物均转变为富SiO2夹杂物，而当使用RHA + Flux B覆盖剂时，钢中富SiO2夹杂物的比例大幅减小。同时可以看出，随时间延长，仅使用RHA时单位面积的夹杂物数量持续增加，说明RHA中的SiO2不断地发生自溶解反应，向钢液内传输溶解氧，造成钢液二次氧化(图9a[34])；使用RHA + Flux A时，单位面积的夹杂物数量基本不发生变化，意味着由于钢液二次氧化产生夹杂物的能力与熔渣吸收夹杂物的能力基本相等，与仅使用RHA相比，Flux A中的CaO可以降低渣中SiO2的活度，同时降低熔渣的熔化温度和黏度，一定程度上有利于降低覆盖剂的氧化性和增加吸收夹杂物的能力(图9b[34])；使用RHA + Flux B时，覆盖剂中SiO2的活度降至最低，覆盖剂吸收夹杂物的能力最大，因此有利于提高316L不锈钢在浇注时的纯净度(图9c[34])。由以上研究结果可知，钢液成分和造成钢液二次氧化原因的不同均会导致非金属夹杂物转变行为的不同，这对于连铸过程中钢液纯净度控制不利。

图9

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图9   加入不同渣后钢中夹杂物数量密度的变化[34]

Fig.9   Changes in number density of inclusions in the steel with reaction time after various flux additions[34]

(a) RHA only

(b) RHA + CaO-SiO2 (RHA + Flux A)

(c) RHA + CaO-Al2O3 (RHA + Flux B)


3 钢液二次氧化控制的研究进展

3.1 中间包结构优化

一个浇次非稳态浇注过程主要包括第一个钢包的开浇、换包、更换水口和本浇次最后一个钢包浇注结束。开始浇注时，钢液经过钢包长水口以较高的浇注速率首先射流到湍流抑制器底部，然后反向冲击到熔渣液面形成“渣眼”，此时钢液将会暴露在大气下造成钢液二次氧化[84,85]。考虑到冶金过程的高温特性，很难通过实验观察到钢液内部流场和温度场的分布情况[86]，因此诸多学者通过数值模拟和物理模拟的方法研究利用控流装置、水口结构、水口浸入深度以及吹氩流量对由于“渣眼”形成而造成的中间包钢液二次氧化进行控制。陈宏亮[87]研究了控流装置和工艺参数对中间包内钢液多相流动的影响，指出带檐的湍流抑制器、合适的长水口浸入深度以及喇叭型长水口的结构能有效降低射流对钢液面的冲击、主流区钢液面的湍动能并预防卷渣。Wang等[85]建立了耦合钢液-渣-空气多相流动和传热的数学模型，模拟了钢液非稳定浇注阶段(钢液开浇和换包过程)钢液中溶解氧含量变化情况，结果如图10[85]所示。可以看出，初始钢液的溶解氧含量较低，但是由于非稳定浇注过程中“渣眼”打开，使钢液中溶解氧含量增加，并逐渐向中间包的两侧扩散。当钢水撞击到侧壁则向下运动，导致中间包出口处溶解氧含量较高。需要指出的是，由于建立的钢液-空气-渣-夹杂物-耐火材料和传热耦合数学模型还不完善，对整个浇注过程中钢液发生二次氧化以及对非金属夹杂物成分变化的影响还需进一步研究。

图10

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图10   非稳定浇注过程中钢水中溶解氧的分散情况[85]

Fig.10   Dispersions of the dissolved oxygen in the molten steel during unsteady processes[85] (t—time)

(a) t = 10.2 s (b) t = 40.2 s (c) t = 70.2 s


3.2 覆盖剂成分优化

当浇注进入稳定状态，使用碳化稻壳灰的中间包覆盖剂对钢液的二次氧化成为主要影响因素。Xu等[21]设计了一种中间包覆盖剂(27.5 ± 7.5)CaO-(12.5 ± 5.0)SiO2-(22.5 ± 7.5)Al2O3-(12.5 ± 7.5)MgO-(7.5 ± 2.5)(Na2O-K2O)作为覆盖剂的上层渣以替换碳化稻壳灰，下层则为原先的CaO-Al2O3基覆盖剂。新设计的双层中间包覆盖剂可以抑制由碳化稻壳灰中高SiO2含量造成的钢中Al和Ti元素的损失和Si元素的增加，并降低钢液T.O含量进而提高钢液的纯净度。Kim等[33]发现，使用CaO-Al2O3基覆盖剂(53.5CaO-41.5Al2O3-5MgO)可以使硅镇静304不锈钢(18Cr-8Ni)的T.O含量比使用CaO-SiO2基覆盖剂(47.5CaO-47.5SiO2-5MgO)降得更低，但是使用CaO-Al2O3基覆盖剂可以使钢中Al含量增加，通过 式(18)和(19)使钢中SiO2-MnO夹杂物逐渐转变为MgO-Al2O3尖晶石。但在使用CaO-SiO2基覆盖剂时，并未对钢中非金属夹杂物成分造成影响。

3(MnO)inclusion+2[Al]=3[Mn]+(Al2O3)inclusion
3(SiO2)inclusion+4[Al]=3[Si]+2(Al2O3)inclusion

Yu等[88]向CaO-Al2O3-SiO2-MgO (CaO / (Al2O3 + SiO2)含量比值 = 1.45)覆盖剂中添加碱金属氧化物(如Li2O、Na2O和K2O)，研究了321不锈钢浇注过程中覆盖剂对夹杂物的吸收能力。结果表明，K2O的添加可以降低不锈钢中夹杂物的平均尺寸和数量密度，从而提高不锈钢的纯净度。Kekkonen等[89]设计了5种底层覆盖剂成分，其中上层覆盖剂为碳化稻壳灰，并在工业实验中研究底层覆盖剂成分对浇注16MnCrS5钢纯净度的影响，发现高CaO含量的70CaO-10MgO-20Al2O3和60CaO-10MgO-20Al2O3-10CaF2覆盖剂具有较强的吸收钢中Al2O3和MgO-Al2O3尖晶石类夹杂物的能力，以保证钢液浇注的连续性。以上结果通过设计中间包覆盖剂的成分提升了钢液的纯净度，如CaO-Al2O3基覆盖剂，但该覆盖剂会造成MgO基中间包耐火材料的严重侵蚀。因此，合适的覆盖剂成分和中间包耐火材料的设计是保证钢液末端冶炼纯净度应当同时考虑的。

3.3 耐火材料的选择

在浇注过程中，覆盖剂对耐火材料的侵蚀不仅是钢液中外来夹杂物的来源，而且可以改变钢中夹杂物的成分。通过设计中间包耐火材料的成分或结构可以有效地抑制钢液浇注过程中耐火材料的侵蚀，提高中间包耐火材料的使用寿命和钢液的纯净度。熔渣对耐火材料的侵蚀行为主要包括[90]：(1) 耐火材料组元直接或者间接溶解进熔渣，如形成(MgO·Al2O3)或(Mg, Fe)O/(Mg, Fe, Mn)O层[91,92]；(2) 熔渣渗入耐火材料内部，造成耐火材料的抗热震性能和力学性能降低[47,93]；(3) 渗入的熔渣在耐火材料内部形成新相(液相或固相)，影响耐火材料的侵蚀行为[94,95]。如前文所述，高碱度中间包覆盖剂具有较高的吸收夹杂物的能力，可提高浇注过程中钢液的纯净度，此外高碱度熔渣还可以降低MgO在熔渣中的饱和溶解度，以降低覆盖剂对中间包耐火材料的化学侵蚀[96]。Zhang等[95]比较研究了CaO-MgO-SiO2渣碱度(CaO / SiO2 = 1.5~2.0)对MgO耐火材料侵蚀行为的影响，发现当熔渣碱度大于1.5时，在熔渣和耐火材料界面形成3CaO·SiO2和2CaO·SiO2高熔点固相产物层，阻碍了熔渣通过毛细作用进一步渗入到MgO耐火材料内部，同时平均晶粒尺寸为1 μm的MgO耐火材料具有更强的抗渣性。Lao等[97]指出，CaO-Al2O3-SiO2-MgO熔渣对耐火材料的侵蚀受到熔渣物理化学性质(如表面张力(γ)、黏度(η)和接触角(θ))的影响：

l2=rγcosθ2ηt

式中，l为熔渣浸入深度，r为耐材微孔尺寸，t为时间。Lao等[97]发现，当增加熔渣碱度和MgO含量时，微孔镁质耐火材料的抗渣性能要高于电熔镁质耐火材料，改变熔渣碱度对熔渣的γ和η的影响要大于θ，改变MgO含量则反之。Tan等[98]得到类似的结论，指出高碱度覆盖剂(CaO / SiO2含量比值 = 9.0)渗入到电熔镁质耐火材料内部并破坏耐火材料内部结构，导致MgO晶粒向熔渣中剥落和溶解。虽然微孔镁质耐火材料内部孔洞可以为熔渣提供渗入的通道，但可以使熔渣内MgO快速饱和，从而抑制微孔镁质耐火材料进一步侵蚀。他们同时向微孔镁质耐火材料内部添加ZrO2纳米颗粒，发现在MgO晶界处形成的CaZrO3-ZrO2相可以阻止熔渣向耐火材料内部渗入。Zhang等[99]研究了将微孔镁质耐火材料用作中间包过滤器对无间隙原子钢(IF钢)纯净度的影响，结果表明，耐火材料可以吸收IF钢中的Al2O3和TiO2夹杂物，并且耐火材料中的MgO与钢液中的Al和O发生反应，在界面处形成的MgO·Al2O3具有降低熔渣对耐火材料的侵蚀和降低钢液T.O含量的作用。由于微孔镁质耐火材料具有良好的隔热性能，微封闭微孔可以吸收热应力并减少耐火材料内部裂纹的萌生和扩展，因此微孔镁质耐火材料用于中间包具有良好的应用前景[47,100]。

4 结论与展望

(1) 在钢液连续浇注过程中，造成钢液二次氧化的主要原因有：钢液吸气、中间包覆盖剂和耐火材料。在非稳定浇注阶段，钢液吸气是造成钢液二次氧化的主要原因；在稳定浇注阶段，钢液表层的空气和中间包覆盖剂则成为主要原因。

(2) 目前双层中间包覆盖剂结构被广泛使用，即上层为碳化稻壳灰，下层为熔点较低的碱性覆盖剂，以使钢液保温减少热量损失、隔绝钢液与大气避免钢液二次氧化、吸附和溶解钢液中非金属夹杂物。但是碳化稻壳灰中高含量的SiO2向下层熔渣中不断地溶解和扩散，导致下层覆盖剂中SiO2活度增加并在渣/钢界面发生自溶解反应((SiO2) = [Si] + 2[O])造成钢液发生严重的二次氧化，致使钢液中的易氧化元素损失、Si元素含量增加。

(3) 中间包耐火材料和钢包引流砂中含有的不稳定氧化物FeO、SiO2、MnO和Cr2O3可以导致钢液发生二次氧化。在钢液浇注过程中，Al2O3-SiO2-C质水口材料发生碳热反应生成CO氧化性气体，并穿过内部空隙到达水口内壁与钢液接触，CO与钢液中的Al、Ti和Fe等元素同时反应生成结瘤物，这是造成水口堵塞的主要原因。

(4) 由于钢液吸气造成二次氧化，钙处理铝镇静钢的夹杂物成分由液态的钙铝酸盐夹杂物转变为固态的CaO·2Al2O3和CaO·6Al2O3或Al2O3夹杂物(取决于钢中的T.Ca含量)；硅脱氧钢夹杂物中的SiO2-MnO含量增加；铈处理钢中的夹杂物由细小的球形Ce2O2S夹杂物转变为团簇状CeAlO3夹杂物。由于水口耐火材料碳热反应释放的氧化性气体CO造成耐火材料内壁和钢液界面溶解氧含量增加，超低碳含Ti铝镇静钢中夹杂物由纯Al2O3夹杂物转变为液态Fet O-Al2O3-TiOx 夹杂物；由于中间包上层覆盖剂碳化稻壳灰中的SiO2含量高，造成硅脱氧316L不锈钢中液态SiO2-MnO夹杂物转变为纯SiO2夹杂物。

通过对中间包内钢液发生二次氧化的机理分析，今后可以从以下方向对控制钢液纯净度和减少钢液二次氧化进行研究。

(1) 为减少在整个浇注阶段由于钢液吸气造成的二次氧化，除了对中间包结构进行优化设计外，同时应当研究不同成分钢液的吸氧速率。对于浇注含有易与SiO2发生剧烈化学反应的合金元素(如Al、Ti、Mn和稀土元素)的钢种，可通过设计新型覆盖剂成分降低SiO2活度以提高熔渣对夹杂物的吸收作用并减轻对钢液的氧化作用，在设计覆盖剂成分时也应当考虑覆盖剂对中间包耐火材料的侵蚀。在中间包和水口耐火材料成分及结构设计时，应减少中间包耐火材料中不稳定氧化物含量和水口材料释放的CO气体，可以减轻钢液的二次氧化和延缓水口结瘤。

(2) 整个浇注过程涉及钢液的多相流动和温度的变化，建立钢液-空气-渣-夹杂物-耐火材料多相流动和传热耦合数学模型是模拟中间包内钢液二次氧化过程的关键，可以对整个浇注过程中元素和夹杂物成分的变化做到全面的分析。同时这一过程受到热力学和动力学等参数的共同影响，且非稳定和稳定浇注过程中钢液发生二次氧化的机理也不相同。目前的研究成果对中间包内二次氧化机理研究还不够深入，数学模型还不够完善，对中间包内钢液成分和夹杂物成分变化的准确预报还需要进一步研究。

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为了研究钢中MnO/FeO大型夹杂物的来源，基于工业试验和大样电解分析，对比了钢中的显微夹杂物和引流砂的烧结行为。研究结果表明，钢中MnO/FeO大型夹杂物与细小显微夹杂物没有明显关联，这些大型夹杂物并不是来源于钢中的细小夹杂物。依据热力学分析可知，这些夹杂物也不是二次氧化的产物。基于铬质引流砂烧结机理和大型夹杂物的化学成分，可以确定这些大型夹杂物为引流砂及其烧结产物。当引流砂进入钢液后，要在中间包内完全去除是十分困难的。在钢包开浇时，应尽可能移除引流砂，这对提升钢的质量具有重要意义。

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In order to study the effect of secondary oxidation on inclusions in bearing steel, an iron sampler was used to sample the bearing steel liquid after RH breaking. At the same time, different qualities of wellpacking sand or FeO powder were added to different iron samplers before sampling to simulate different degrees of oxidation of the steel liquid. The research results indicate that the inclusions in the steel during RH breaking are mainly calcium aluminate. When the steel undergoes slight oxidation, the total oxygen mass fraction increases to (0.3-1.0)×10-6, MgO·Al2O3 inclusions can be detected in steel; CaO·6Al2O3 and Al2O3 inclusions are observed with further oxidation. When the total oxygen mass fraction of molten steel increases by more than 6 ×10-6, the MgO content in the inclusions will significantly decrease with the formation of a large amount of Al2O3 inclusions, resulting in the detection of MgO·Al2O3 inclusions in the steel. The industrial results show that the total oxygen mass fraction of molten steel in the casting process increases by (0.3—1.5)×10-6 compared to that of the end of RH vacuum, and many MgO·Al2O3 inclusions can be detected in the tundish. The number of MgO·Al2O3 inclusions in the casting zone of the tundish is significantly higher than that in its impact zone, indicating that the oxidation that occurs during the flow of molten steel in the casting area is the main link in the oxidation of molten steel in the tundish..

徐建飞, 王昆鹏, 王 郢 等.

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为了研究二次氧化对轴承钢夹杂物影响，采用铁质取样器对RH破空后轴承钢钢水进行取样，同时取样前在不同铁质取样器中加入不同质量的铬质引流砂或FeO粉，以模拟钢水不同程度的氧化。研究结果表明，RH破空时钢中夹杂物以钙铝酸盐为主，当钢水发生轻微的氧化，总氧质量分数增加(0.3~1.0)×10-6，可以在钢中检测到许多MgO·Al2O3夹杂物；随着氧化程度的提高，还伴有CaO·6Al2O3和Al2O3夹杂物生成；当钢水总氧质量分数增加值超过6×10-6，夹杂物中MgO含量会随Al2O3夹杂物大量生成而显著降低，导致钢中检测不到MgO·Al2O3夹杂物。工业结果表明，浇铸过程中间包钢水总氧质量分数相比RH真空结束增加(0.3~1.5)×10-6，此时可以在中间包钢水中检测到许多MgO·Al2O3夹杂物，并且中间包浇铸区MgO·Al2O3夹杂物数量显著多于冲击区，说明浇铸区内钢水流动过程中发生的氧化是中间包钢水氧化的主要环节。

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对首钢京唐生产IF钢的同一浇次前2炉的RH精炼、镇静和中间包浇铸过程进行了系统取样，并利用Aspex自动扫描电子显微镜分析统计了钢中夹杂物的成分、尺寸等信息.研究发现，Al<sub>2</sub>O<sub>3</sub>-Ti<sub>○</sub>xO复合夹杂物在Ti合金化和二次氧化的情况下都会生成，并随着精炼的进行逐渐转变为Al<sub>2</sub>O<sub>3</sub>，这与热力学计算的结果一致；而Al<sub>2</sub>O<sub>3</sub>可以作为Al<sub>2</sub>O<sub>3</sub>-Ti<sub>○</sub>xO的形核核心，形成Al<sub>2</sub>O<sub>3</sub>-Ti<sub>○</sub>xO包裹Al<sub>2</sub>O<sub>3</sub>的夹杂物，并且在Al<sub>2</sub>O<sub>3</sub>-Ti<sub>○</sub>xO转变为Al<sub>2</sub>O<sub>3</sub>的过程中会导致钢滴进入夹杂物内部，从而形成Al<sub>2</sub>O<sub>3</sub>包裹钢滴的夹杂物.

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The metallurgical process involves complex phenomena comprising high temperature, the multiphase flow, and the physical and chemical reactions in the process reactors. Because of the complexity of the metallurgical process and the limitation conditions for the direct measuring and observation, numerical and physical simulations have become indispensable and effective tools to analyze and reproduce the transport phenomena and mechanisms occurring in the process. Transport phenomena of the gas-liquid two-phase flow plays a dominant role in process metallurgy since their respective movement laws govern the kinetics of the various physical phenomena in the metallurgical reactors. The gas-liquid two-phase flow has complex interface structures, and the accuracy of the interfacial momentum transfer models, including the interfacial forces, which is one of the keys to predicting the distribution of gas phase in the two-phase flow system successfully. This paper is aiming at reviewing the two-phase flow models based on the Euler-Euler system, the interfacial force model, and the turbulence model for gas-liquid two-phase flow. The use and extent of numerical and physical simulation for transport phenomena of two-phase flow in the steelmaking and casting processes are summarized and explored, including the basic oxygen furnace, electric arc furnace, refining, tundish, and molds. The methods and typical application in the numerical and physical simulation of gas-liquid two-phase flow will provide useful guides for the research.

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The ceramic filter in continuous casting tundish can effectively improve the cleanliness of high-performance steel by regulating tundish flow field to promote the removal of inclusions and adsorbing or blocking fine inclusions in the molten steel into the mold. The interaction between microporous magnesia refractories used as tundish filter and molten interstitial- free (IF) steel at 1873 K was investigated to reveal the formation mechanism of their interface layer and its effect on steel cleanliness by laboratory research and thermodynamic calculations. The results show that the magnesium–aluminum spinel layer at the interface between the molten IF steel and the microporous magnesia refractories is formed mainly by the reaction of MgO in the refractory with the [Al] and [O] in the molten steel, significantly reducing the total O content, the size and amount of inclusions of the molten steel. In addition, the interparticle phases of microporous magnesia refractories at high temperature can adsorb Al2O3 and TiO2 inclusions in the molten steel into interparticle channels of the refractories to form high melting point spinel, impeding the further penetration of the molten steel. As a result, the consecutive interface layer of high melting point spinel between microporous magnesia refractories and molten steel can improve the cleanliness of the molten steel by adsorbing inclusions in the molten steel and avoid the direct dissolution of refractories of the tundish ceramic filter immersed in the molten steel, increasing their service life.

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[本文引用: 1]

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