Characterization of inclusions in clean steels: a review including the statistics of extremes methods

The application of new secondary refining techniques and non-metallic inclusion reduction techniques in steel production processes has greatly reduced the size and amount of non-metallic inclusions remaining in molten steels and steel products. This makes the inspection of inclusions difficult. Here the main methods used for the characterization of inclusions in clean steels are reviewed. The influences of inclusions on the properties of steels are discussed. Statistical methods for the prediction of the maximum inclusion size in a large volume of steel are introduced. Methods based on the statistics of extremes are described in detail. The methodology for the practical application of these methods is described and the factors affecting the precision of the estimation are discussed.     

Investigation of nonmetallic inclusions in continuously cast carbon steel by dissolution of the ferritic matrix

One of the main steel plant's problems has been the occurrence of inclusions throughout the process of steel making. In this sense, it is very important to detect nonmetallic inclusions in the steel, to determine their origin and to control the formation of such inclusions, in order to generate a final product of high quality. The aim of this work is to present a characterization method for nonmetallic inclusions which uses the expedient of dissolving the ferritic matrix in hydrochloric acid (HCl). Scanning electron microscopy connected to an energy-dispersive spectrometer (EDS) system is used to obtain the morphology, size and chemical composition of such inclusions. This analysis allows a better understanding about the nature of the inclusions, their incidence and distribution along the process of steel manufacturing, providing subsidies to formulate corrective actions that minimize the occurrence of nonmetallic inclusions.     

Thermocapillary phoresis of inclusions with interphase surfactant

The motion of a spherical inclusion (gaseous, liquid or solid) submerged in an infinite (continuous) viscous media with a constant temperature gradient is considered. The analysis is carried out by considering negligible values of the Reynolds and Marangoni (Peclet) numbers and some other certain assumptions. The formulae for the thermocapillary phoresis velocity of the inclusions with the surfactant films are derived and can be applied for emulsions containing spherical drops, systems of liquids with solid or gaseous inclusions, e.g. at flotation, metal foaming, steel refining from the inclusions.     

热轧带肋钢筋断裂分析

为进一步提高产品质量,针对螺纹钢出现的裂纹和断裂事故进行了系统的生产工序分析.采用光谱分析仪分析了断裂钢筋的化学组成,利用光学显微镜、电子能谱仪分析断裂钢筋组织结构和断裂钢筋中非金属夹杂物的类型、数量、大小、组成及分布,并讨论了目前生产工艺中非金属夹杂物的来源.研究表明,钢筋中非金属夹杂物的产生是钢筋裂纹和断裂的根源,夹杂物主要是由炼钢脱氧和耐火材料腐蚀的铝硅酸盐组成.     

电渣重熔马氏体时效钢中的非金属夹杂物研究

本文应用电子扫描法(SEM+EDS)和金相法,对真空感应熔炼(VIM)及随后进行电渣重熔(ESR)的改进型18NiNb马氏体时效钢中的非金属夹杂物进行了定性和评级分析。结果表明,电渣重熔对去除钢中的硫化物和尺寸较大的单颗粒夹杂效果明显,电渣重熔后钢中的夹杂物主要为细小的氧化物夹杂,数量明显减少、且呈弥散分布,进一步提高了18NiNb合金钢的钢锭质量。     

In situ studies of the agglomeration phenomena for calcium–alumina inclusions at liquid steel–liquid slag interface and in the slag

Studies of inclusion behavior at the metal–slag interface are of great importance for the steel industry in order to obtain better control of the size and of inclusions as well as improving the steel quality and casting process. In this work the agglomeration of liquid Al₂O₃–CaO particles at the liquid steel–liquid slag interfaces are studied with a confocal scanning laser microscope. In addition, the agglomeration of liquid Al₂O₃–CaO inclusions already transferred to the slag is investigated. It is found that agglomeration of the liquid inclusions at the steel–slag interface could only take place when the inclusions were forced towards each other, while the agglomeration of liquid particles was seen to be noticeably enhanced when the particles were already in the slag.     

原板夹杂物诱发镀锡板点腐蚀缺陷分析

原板高强减薄和锡量降低是镀锡板发展的重要趋势,直接对原板夹杂物和镀锡板表面缺陷控制提出了更为严格的技术要求。通过对夹杂物含量异常的钢坯进行全流程加工得到的1.1/1.1 g/m2规格镀锡成品卷的研究,发现经过10 d在库测试,镀锡板表面产生点腐蚀缺陷。利用光学显微镜、扫描电镜和电子能谱等方法就镀锡板点腐蚀缺陷进行了深入分析,结果发现黄褐色腐蚀产物元素成分为O、Fe,源于基体腐蚀,并伴有原板夹杂物存在。通过金相切片确认典型点腐蚀缺陷样品夹杂物尺寸为96.82μm×50.00μm×8.86μm,属于原板表面嵌入型夹杂,其成分特征为主要含Al、O元素。夹杂物的存在改变了局部区域的原板轧制纹理和镀层形貌,产生近似椭圆白点缺陷,锡层沉积量由缺陷中心外延逐步增加,整体在11.02%～28.08%范围内波动。根据夹杂物伴随点腐蚀缺陷特征,其诱发镀锡板腐蚀行为与夹杂物形成的原板晶体结构缺陷、锡层差异沉积与夹杂物导电性和硬度直接相关。通过研究,清晰地描述了原板表面夹杂物对后续镀锡板成品的表面质量影响关系,为更好控制原板夹杂物提升镀锡板产品表面质量提供了实际数据支撑。     

Effects of electropulsing on inclusion size and corrosion-resistance of 304 stainless steel

Electropulsing has been imposed on molten AISI 304 stainless steel and its effects on the inclusion size and corrosion properties have been investigated. The average size of inclusions in electropulsing-treated samples was finer than that in untreated ones. A theoretical analysis shows that the mobility of solute atoms increases with electropulsing treatment. This promotes the nucleation of inclusions. Owing to a smaller inclusion size, the electrochemical experiments reveal that the pitting corrosion resistance of the electropulsing-treated stainless steel is significantly improved. The beneficial effects of electropulsing on the refinement of inclusions and the resistance to pitting corrosion increase with an increase in current density. Electropulsing treatment, therefore, provides a promising means to improve pitting corrosion resistance.     

Investigation of the chemical composition of nonmetallic inclusions utilizing ternary phase diagrams

Nonmetallic inclusions are normally present in steel products, as they originate from reactions that occur during the steelmaking process. Inclusions, which consist of stable nonmetallic phases depending on their chemical composition, can strongly affect the final quality of the metallurgical product. In this sense, the influence of inclusions in the steel is determined by their chemical composition. The aim of this work is to present the evolution of the chemical composition of nonmetallic inclusions utilizing ternary phase diagrams by means of the analysis of steel samples collected during the steel (aluminum-killed steel, SAE 1015) manufacturing process. A computational program was developed to furnish the positions of the inclusions on appropriate ternary phase diagrams by using as input data the chemical composition of the inclusions, determined with a scanning electron microscope (SEM) connected to an energy-dispersive spectrometer (EDS).     

Separation of electrically neutral non-metallic inclusions from molten steel by pulsed electric current

ABSTRACT

Effect of a pulsed electric current on the distribution of Al₂O₃ inclusions in liquid steel is explored; these inclusions ranged in size from micrometres to nanometres. When no electric current was applied, the inclusions were randomly distributed in the steel. However, when an electric current was applied, the inclusions were found in highly populated regions near the various interfaces. Moreover, this process applies to a wide range of inclusion diameters, in contrast to conventional inclusion removal methods which tend to apply to larger (>20?μm) particles only. Consequently, the application of pulsed electric current provides a method of removing inclusions from the steel and thereby improving the mechanical, physical and corrosion resistance properties of the steel.

This paper is part of a themed issue on Materials in External Fields.     

微量稀土元素对Q235B钢组织和性能的影响

为研究微量稀土元素对Q235B钢的夹杂物形态转化和细晶化及钢材强韧性能的影响,用真空感应炉熔化、精炼、制备了不同微稀土质量分数的钢样,用成分、OM、SEM、EDS和图像分析仪等方法,分析研究了微量稀土元素对Q235B钢微观组织和力学性能的影响.结果表明：在本研究条件下,随稀土量的增加,铁素体晶粒由24μm减小至12μm...     

Effect of non-metallic inclusions on butterfly wing initiation,crack formation,and spall geometry in bearing steels

Non-metallic inclusions such as sulfides and oxides are byproducts of the bearing steel manufacturing process. Stress concentrations due to such inclusions can originate cracks that lead to final failure. This paper proposes a model to simulate subsurface crack formation in bearing steel from butterfly-wing origination around non-metallic inclusions until final failure. A 2D finite element model was developed to obtain the stress distribution in a domain subjected to Hertzian loading with an embedded non-metallic inclusion. Continuum Damage Mechanics (CDM) was used to introduce a new variable called Butterfly Formation Index (BFI) that manifests the dependence of wing formation on depth. The value of critical damage inside the butterfly wings was obtained experimentally and was used to simulate damage evolution. Voronoi tessellation was used to develop the FEM domains to capture the effect of microstructural randomness on butterfly wing formation, crack initiation and crack propagation. Then, the effects of different inclusion characteristics such as size, depth, and stiffness on RCF life are studied. The results show that stiffness of an inclusion and its location have a significant effect on the RCF life: stiffer inclusions and inclusions located at the depth of maximum shear stress reversal are more detrimental to the RCF life. Stress concentrations are not significantly affected by inclusion size for the cases investigated; however, a stereology study showed that larger inclusions have a higher chance to be located at the critical depth and cause failure. Crack maps were recorded and compared to spall geometries observed experimentally. The results show that crack initiation locations and final spall shapes are similar to what has been observed in failed bearings.     

Nucleation and Growth Kinetics of MgO in Molten Steel

The size,number,morphology and type of inclusion particles are the key factors to estimate the quality of steel product.Although considerable efforts have been made in the mathematical modeling of inclusion growth,few papers were involved in inclusion s nucleation and collision-growth,and all the existing researches about the behaviors of magnesia inclusion were based on the experiments.Thus,a mathematical model was developed to investigate the nucleation,Ostwald ripening and collision-growth of magnesia inclusion in the molten steel.Numerical results showed that the predicted particle size distributions are consistent with the previous experimental data.For the magnesia inclusions smaller than 100 nm,Brownian collision is the main collision modes.For the inclusions ranging from 0.1 to 10 μm,Brownian collision and turbulent collision are the main collision modes.For the inclusions ranging from 10 to 100 μm,turbulent collision and Stokes collision are the main collision modes.Thus,the strong turbulent flow can decrease the peak-value diameter of the magnesia inclusion effectively.     

Insight into the triggering effect of(Al,Mg, Ca,Mn)-oxy-sulfide inclusions on localized corrosion of weathering steel

The surface properties of weathering steel(WS)is very important for its service performance and safety,and the localized corrosion induced by inclusions is closely related to the surface properties of WS and its application.In the current work,a common spherical(Al,Mg,Ca,Mn)-oxy-sulfide inclusion was selected to investigate the corrosion evolution of complex inclusion and its effect on localized corrosion on WS surface.The results indicate the inclusion in WS consists of(Ca,Mn)sulfides part and(Ca,Al,Mg)oxides part with complex core-shell structure.Locally preferential dissolution occurs in(Ca,Mn)sulfides part as well as metal matrix around the inclusions.Furthermore,both parts of the inclusions with poor conductivity and high-density dislocation at metal matrix around the inclusions was found,which suggests that traditional micro-galvanic corrosion cell may not be the cause of inclusion-induced localized corrosion on WS surface at initial stage of corrosion.The variation in maximum and average depth around the inclusion or selected region with immersion time indicates that localized corrosion induced by inclusions is overwhelmed by uniform corrosion of WS in the latter stage of immersion,then the rust formed on WS surface consists of two layers.     

Al-Ti-Mg复合脱氧钢研究进展

钢铁冶炼过程中脱氧问题是决定钢材质量的关键环节。在对氧化物冶金可利用的脱氧产物的研究中,Ti、Mg对钢的复合脱氧形成的氧化物夹杂更加分散,具有很好的脱氧效果。针对Al-Ti-Mg复合脱氧钢,从合金的制造、夹杂物形成机理、晶粒结构、力学性能、夹杂物对铁素体形核的影响等方面的研究进展进行了综述,提出了当前Al-Ti-Mg复合脱氧技术还处于未完善阶段,仍存在许多问题亟待解决,并为未来冶金领域研究工作者进一步探索与研究提供了方向。     

Effects of inclusions on formation of acicular ferrite and propagation of crack in high strength low alloy steel weld metal

Abstract

High strength low alloy steel was welded by gas shielded arc welding process without preheating. Microstructural characteristics of the weld metal, morphology of inclusions and crack propagation paths were investigated by means of optical microscopy and scanning electron microscopy. The chemical composition of the inclusion and element distribution across the inclusion were analysed via energy dispersive spectroscopy system. Results indicated relatively large inclusions with diameters of about 0·6–0·8 μm are much more effective in providing nucleation sites for acicular ferrite transformation and refining the microstructure within austenite grain than small ones with diameters of about 0·3–0·5 μm. When the main crack tip encountered inclusion, more crack paths would be initiated from the interface between inclusion and acicular ferrite plates.     

Non-metallic inclusions and their relation to the J-integral, Ji, phys, at physical crack initiation for different steels and weld metals

For two types of steel, the heat-resistant 12% Cr–steel X 20 CrMoV 12 1 and the fine-grained pressure vessel steel 20 MnMoNi 5 5 (A 508 cl.3), a wide range of toughness values in the upper shelf was realized by different sulfur contents and the inclusion of similar weld metals. Additionally, the pressure vessel steel 22 NiMoCr 3 7 (A 508 cl.2) was also investigated. Tensile, fracture mechanics and Charpy V-notch impact tests, as well as detailed microstructural investigations with respect to the size distribution and density of non-metallic inclusions and precipitates, were carried out. In order to ensure ductile behaviour, a test temperature of 150 °C was chosen. The relevance of two quantitative relations available for the calculation of the J-integral, Ji, phys, at physical crack initiation using tensile test data and microstructural parameters, were examined by comparison with the corresponding experimental Ji, phys-values. Only one quantitative relation was able to give good agreement between calculated and experimental Ji, phys-values. This holds not only for the base materials but also for the weld metals. The importance of the size and density of the non-metallic inclusions became quantitatively obvious with the consequence that their size times density is a decisive parameter for toughness. Observations of void initiation, growth and coalescence illustrate the fracture process. © 1998 Chapman & Hall     

Improving the wear resistance of M41 steel by nitrogen alloying and ESR

AISI M41 high-speed steel (HSS) is considered as a super-hard tool steel due to high hardness level (65–70 HRC). Nitrogen alloying of AISI M41 HSS produces marked solid solution hardening and precipitation strengthening in addition to an improvement in pitting resistance. The mechanical properties in general, and wear resistance in particular, are strongly affected by the steel cleanliness and the status of non-metallic inclusions in steel. For this reason tool steel should be subjected to a secondary refining process. In this work, the wear characteristics of AISI M41 HSS were investigated. The effect of nitrogen alloying and electroslag refining (ESR) of this steel grade were considered. Both conventional and nitrogen alloyed grades were melted in open air induction furnaces and then remelted under three different compositions of calcium fluoride-based flux in an ESR machine. The wear behaviour of the resulting steels, for both conventional and nitrogen-alloyed grades before and after ESR, was monitored. The addition of nitrogen improves markedly the wear resistance of AISI M41 HSS. This improvement depends on the total nitrogen content and is independent on the form of the nitrogen constituent. The ESR process improves markedly the wear resistance of both conventional and nitrogen-alloyed grades.     

Modern computer assisted methods in metallurgy

Image analysis systems have been used in metallurgy for over two decades and play a vital role in quality assessment for the steel industry. One major application is the characterization of steel by counting and measuring the non-metallic inclusion content. Visual steel assessment, relying on the comparison with idealized ‘charts’ of varying inclusion content, for example as defined byastm andsep, is the most common manual method employed. However, this has proved to be most difficult to automate due to the degree of subjectivity associated with the manual methods. This has previously restricted the acceptance of image analysis systems for routine quality control. The increased software power of the more modern instruments has enabled the installation of systems which satisfactorily mimic the skilled observer in a reproducible manner. These work with an economically attractive throughput and produce results in the accepted standard format. A limitation of these systems is that they grade a field on the basis of the majority inclusion type present. This can lead to significant but lesser inclusions being ignored altogether and presenting a false picture of the sample. This paper describes a new method which separately identifies individual inclusions in a field. The grade number of a field can then be determined for each inclusion type. This is known as the mixed field method. This paper describes this method and its implementation on the Cambridge Instruments’quantimet 920 image analysis system.     

Detrimental effect of nitride and aluminium oxide inclusions on fatigue life in rotating bending of bearing steels

Abstract

Rotating bending fatigue tests were performed on hardened AISI type 52100 bearing steel. Fracture surfaces after testing at a stress amplitude of 950 MPa showed that the Ti(C,N) inclusions which caused fatigue failure were significantly smaller than the corresponding alumina inclusions. The smallest crack initiating Ti(C,N) inclusion had a size of 3 μm and the smallest alumina inclusion was 17 μm. It was also shown that fatigue life was significantly shorter for a steel which showed cracked alumina inclusions on the fracture surfaces than for a steel which had non-cracked inclusions. Finite element calculations were performed to determine the driving forces of short cracks at Ti(C,N) and alumina inclusions. Two configurations were studied in each case, based on both non-cracked and cracked inclusions. The calculations incorporated heat treatment simulation and cyclic loading with successive growth of cracks. It was found that the Ti(C,N) configurations gave the highest driving forces for crack growth. The alumina configuration with a non-cracked inclusion gave the lowest driving force. It was concluded based both on experimental evidence and theoretical considerations that Ti(C,N) inclusions are more detrimental to fatigue life than alumina inclusions of the same size. It is their shape and thermal properties which make Ti(C,N) inclusions more detrimental than alumina inclusions. Internal cracking of alumina inclusions leads to reduced fatigue life.