Method for detecting gas composition of bubbles in a continuous casting billet
By combining industrial CT, cryogenic freezing, and FIB minimally invasive windowing with TOF-SIMS, the problem of precise positioning and highly sensitive analysis of gas composition in microbubbles inside continuously cast billets was solved, enabling accurate detection of bubble composition and process optimization.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- INST OF RES OF IRON & STEEL JIANGSU PROVINCE
- Filing Date
- 2026-05-15
- Publication Date
- 2026-06-19
Smart Images
Figure CN122238028A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of continuous casting billet bubble composition detection technology, specifically to a method for detecting the gas composition of bubbles inside a continuous casting billet. Background Technology
[0002] Continuously cast billets often contain air bubbles beneath the surface, typically ranging in size from hundreds of micrometers to several millimeters. During subsequent rolling processes, these defects can easily lead to quality problems such as scale, cracks, and folds on the finished product surface, ultimately affecting the product's mechanical properties and service reliability.
[0003] The main sources of subsurface bubbles in continuously cast billets can be summarized into four categories: 1) external gases (mainly N2 and O2) entrained by poorly sealed submerged entry nozzles; 2) internal gas products generated by incomplete deoxidation of molten steel (mainly CO and CO2); 3) gas products generated by the decomposition of moisture in the raw materials at high temperatures; and 4) inert gases (mainly Ar) introduced during the continuous casting protective casting process. Currently, there are still many shortcomings in the gas composition detection technology for subsurface bubbles in continuously cast billets: First, for specific target bubbles located under the surface, there is a lack of effective non-destructive positioning methods, making it difficult to avoid interference from non-target areas during sampling; second, using large-scale destructive mechanical breaking methods will cause trace amounts of gas within the bubbles to escape instantaneously and mix with ambient gases, making effective collection and detection difficult; third, the gas content of a single bubble is extremely low, often below the detection limits of conventional gas chromatography and mass spectrometry, placing extremely high demands on the sensitivity of analytical methods.
[0004] Currently, those skilled in the art mainly rely on the morphology and size of bubbles to indirectly infer their possible gas types and formulate corresponding process improvement plans accordingly. However, this method lacks direct compositional evidence and cannot accurately determine the root cause of the bubbles, resulting in insufficient targeting and low efficiency of process improvement measures, making it difficult to fundamentally eliminate such defects.
[0005] Although existing technologies have attempted to detect bubbles in continuously cast billets, these methods suffer from fundamental flaws, failing to achieve accurate and reliable analysis of the gas composition of tiny subcutaneous bubbles. Firstly, the energy dispersive spectroscopy (EDS) technique has a sensitivity limit. EDS detection limits are typically around 0.1%, suitable for macroscopic segregation or inclusions. However, for gaseous products adsorbed on the inner walls of bubbles, their thickness is only a few molecular layers to the nanometer scale, resulting in extremely small absolute amounts. Compared to the large background signal provided by the iron matrix, the signal intensity of trace elements on the inner walls of bubbles is far below the detection limit of EDS, making it impossible to produce discernible "abnormal elemental fluctuations." Therefore, claims that abnormal fluctuations in C, O, and N can be detected by EDS line scanning are ineffective for bubbles with diameters of only a few hundred micrometers and gas content as low as 1.4 × 10⁻⁶. -5The conclusion that the microbubbles at the mL level are technically impossible to achieve lacks reliable data support.
[0006] Secondly, the solvent etching process is inherently destructive and uncontrollable. While the aim is to expose the bubbles through etching, the process itself completely destroys their original state. Corrosive liquids rapidly penetrate the bubble's interior through micropores or grain boundaries, undergoing uncontrollable chemical reactions with the inner wall, leading to the escape of the original gas and severe alteration of the inner wall morphology and chemical state. Even subsequent cleaning cannot guarantee the complete cessation of the etching reaction, and residual acid continues to corrode the sample. This "destroy first, analyze later" approach completely fails to preserve the original composition of the gas within the bubble and the original interfacial chemical information of the inner wall, rendering subsequent composition determination meaningless. Essentially, this method remains at the level of observing the macroscopic morphology of the bubble and cannot provide any valuable chemical evidence for gas composition diagnosis.
[0007] In summary, existing technologies either cannot accurately locate tiny targets, or, due to their destructive nature and low detection sensitivity, cannot obtain the original composition information of the gas inside the bubbles. Therefore, there is an urgent need for a novel detection method capable of accurately locating, non-destructively fixing, minimally invasively exposing, and in-situ highly sensitively analyzing tiny bubbles inside continuously cast billets. This is precisely the technical problem that this invention aims to solve and the technical effects it can achieve. Summary of the Invention
[0008] The purpose of this invention is to provide a method for detecting the gas composition of bubbles inside continuously cast billets. This method constructs a sampling and analysis process of "precise positioning - state fixation - minimally invasive venting - in-situ analysis": First, industrial CT is used to perform three-dimensional non-destructive positioning of the bubbles; then, low-temperature freezing technology is used to solidify the contents of the bubbles to prevent gas escape; subsequently, under frozen conditions, FIB is used to perform precise minimally invasive venting of the target bubbles, exposing the original inner wall interface of the bubbles; finally, TOF-SIMS is used to perform in-situ scanning of the exposed interface to obtain information on the composition and spatial distribution of adsorbed gas molecules and elements. This method ultimately achieves precise analysis of the composition of trace bubbles.
[0009] To achieve the above objectives, the present invention proposes the following technical solution:
[0010] A method for detecting the composition of gas bubbles inside a continuously cast billet includes the following steps:
[0011] Three-dimensional non-destructive scanning was performed on the continuous casting billet sample with bubble defect area to identify the size and three-dimensional morphology of the target bubble and mark the target bubble mark point;
[0012] The marked continuous casting billet sample is subjected to freezing treatment to solidify the gaseous substances inside the target bubble;
[0013] Based on the target bubble markers, the coordinate system of the three-dimensional non-destructive scanning system is associated and mapped with the coordinate system of the focused ion beam scanning system to accurately guide and position the target bubble in the focused ion beam scanning system.
[0014] In the focused ion beam scanning system, a predetermined window area is positioned above the target bubble, and a conductive protective layer is deposited by focusing ion beam induction.
[0015] A focused ion beam is used to create a micro-invasive window on the protective layer above the target bubble and the continuous casting billet substrate to expose the inner wall surface of the target bubble.
[0016] Surface mass spectrometry was used to perform in-situ scanning analysis on the inner wall surface of the exposed target bubble. By analyzing the mass-to-charge ratio of the excited secondary ions, the types of key element ions contained in the inner wall surface were determined, and their spatial distribution information was obtained.
[0017] As a preferred embodiment of the present invention, the three-dimensional non-destructive scanning is industrial X-ray computed tomography, with a spatial resolution of 3–20 μm.
[0018] As a preferred embodiment of the present invention, the protective layer is a platinum protective layer or a carbon protective layer, with a thickness of 0.1 to 2 μm.
[0019] As a preferred embodiment of the present invention, the step of performing three-dimensional non-destructive scanning on the continuously cast billet sample with bubble defect areas to identify the size and three-dimensional morphology of the target bubbles and mark the target bubble markers includes:
[0020] The continuous casting billet sample with bubble defect area is subjected to the first surface treatment, and then three-dimensional non-destructive scanning technology is used to scan and construct its three-dimensional image to identify the size, three-dimensional morphology and spatial location of the target bubble;
[0021] Based on the spatial position information displayed in the three-dimensional image, a second surface treatment is performed on the continuous casting billet sample to make the vertical distance between the top of the target bubble and the polished surface of the continuous casting billet sample less than 50 μm. Then, the position of the target bubble is marked on the continuous casting billet sample to obtain the target bubble marking point.
[0022] As a preferred embodiment of the present invention, the observation window formed by the minimally invasive fenestration has an opening area of less than or equal to 20 μm. 2 .
[0023] As a preferred embodiment of the present invention, the surface mass spectrometry analysis is time-of-flight secondary ion mass spectrometry analysis;
[0024] The key element ion is at least one of carbon, oxygen, hydrogen, nitrogen, and argon, or its molecular ion.
[0025] As a preferred embodiment of the present invention, the equivalent sphere diameter of the target bubble is less than or equal to 500 μm.
[0026] As a preferred embodiment of the present invention, the freezing temperature of the freezing process is below -150°C, and the freezing time is 25 to 35 minutes.
[0027] As a preferred embodiment of the present invention, after performing in-situ scanning component analysis, the method further includes diagnosing the cause of the bubbles:
[0028] After exposing the inner wall surface of the target bubble, the microstructure information of the inner wall surface is obtained by scanning with a focused ion beam scanning system;
[0029] The size and three-dimensional morphology of the target bubble, the microscopic morphology information of the inner wall surface, and the types and spatial distribution information of key element ions contained in the inner wall surface are correlated and fused to form a comprehensive feature spectrum.
[0030] The comprehensive feature spectrum is compared with a pre-stored bubble formation diagnosis database to diagnose the cause of the bubbles.
[0031] As a preferred embodiment of the present invention, the construction of the bubble formation diagnosis database includes the following steps:
[0032] Collect standard samples of bubbles in continuously cast billets from known sources;
[0033] The detection method described above is applied to the standard sample to obtain standard feature data including three-dimensional morphological features, size range and micro-morphological features of the inner wall surface, as well as the chemical features of the inner wall surface;
[0034] The standard feature data is correlated with its corresponding bubble formation causes to form the bubble formation cause diagnosis database.
[0035] The causes of bubble formation include poor deoxygenation, gas entrapment, and water decomposition.
[0036] As can be seen from the above technical solutions, the technical solution of the present invention provides a method for detecting the gas composition of bubbles inside continuously cast billets, which has the following advantages compared with the prior art:
[0037] (1) Precise positioning and non-destructive screening of microbubbles were achieved. Industrial X-ray computed tomography (CT) technology was used to perform three-dimensional imaging of the interior of the continuously cast billet with a high spatial resolution of 3-20 μm. This enabled precise identification of microbubbles ranging from hundreds of micrometers to several millimeters in size under the surface, and determination of their spatial location, size, and morphology. This positioning process was completely non-destructive, avoiding interference from non-target areas introduced by blind sampling in traditional flaw detection or mechanical crushing methods, and providing a reliable spatial coordinate basis for subsequent point-based analysis.
[0038] (2) The pioneering "low-temperature freezing + minimally invasive windowing" technique effectively prevents gas escape and sample contamination. By cooling the sample to below -150℃, the trace amounts of gas inside the bubble are solidified or their vapor pressure is greatly reduced, fundamentally solving the problem of instantaneous gas escape and inability to collect caused by traditional mechanical crushing. Based on this, a focused ion beam (FIB) is used to perform step milling on the substrate above the bubble, precisely exposing the original inner wall of the bubble in a "minimally invasive windowing" manner. At the same time, a protective layer is deposited using ion beam induction to avoid processing damage. This process preserves the original interface state of the bubble's inner wall and eliminates the mixing and contamination of environmental gases, creating the prerequisites for high-sensitivity in-situ analysis.
[0039] (3) This method overcomes the sensitivity bottleneck of trace gas detection and does not rely on gas volume. It directly uses time-of-flight secondary ion mass spectrometry (TOF-SIMS) to perform in-situ scanning of the exposed inner wall of the bubble, detecting positive and negative secondary ions sputtered from the surface to obtain spatial distribution information of elements and molecules. This method does not rely on collecting free gas within the bubble, but rather infers the gas composition by analyzing substances adsorbed or reacted on the inner wall surface. Therefore, even if the gas content of a single bubble is far below the detection limits of conventional equipment such as gas chromatography and mass spectrometry, accurate detection can still be achieved, completely solving the technical problem of the inability to effectively analyze trace gases.
[0040] (4) Obtaining interfacial chemical information enables accurate diagnosis of bubble formation. TOF-SIMS can not only detect characteristic elements such as C, O, N, Ar, and H, but also identify their combined states (such as CO). + OH - By combining the three-dimensional morphology (shape, inner wall smoothness, subcutaneous depth) obtained from CT scans, a "morphology-chemistry" multimodal feature spectrum is constructed. Comparison with a pre-established etiological diagnostic database clearly distinguishes bubbles from different sources, such as poor deoxidation type (CO, CO2 enrichment), gas entrapment type (N2, O2, Ar enrichment), and moisture decomposition type (H2, H2O enrichment). This provides direct and reliable evidence for targeted improvements to the continuous casting process, overcoming the subjectivity and uncertainty of traditional morphology-based inferences.
[0041] (5) A standardized and reproducible analytical process has been established, with promising prospects for industrial application. This invention integrates "CT three-dimensional positioning → cryo-fixation → FIB minimally invasive fenestration → TOF-SIMS in-situ analysis → database diagnosis" into a complete operational chain. The parameters for each step are clearly defined (such as scanning resolution, freezing temperature, ion beam current, protective layer thickness, etc.), making it easy to reproduce in different laboratories or production sites. This method can be extended to the compositional detection of subcutaneous bubbles and other micro-porosity defects in various iron-based metal continuous casting billets, providing direct technical support for steel enterprises to optimize deoxidation processes, improve protective casting, and eliminate bubble defects, with significant economic benefits and practical value.
[0042] (6) It overcomes technical bias and achieves a leap from non-metallic materials to metallic materials. Traditional methods for detecting bubbles in non-metallic materials such as glass (such as heating and stretching, mechanical puncture, etc.) cannot be directly applied due to the high density, opacity, grain boundary scattering, and easy gas escape at high temperatures of iron-based metals. This invention redesigns the entire technical route based on the special physical properties of metallic materials, successfully solving the problem of in-situ composition analysis of micro-nano-scale bubbles in metallic matrices, and filling the technical gap in this field.
[0043] It should be understood that all combinations of the foregoing concepts and the additional concepts described in more detail below can be considered part of the inventive subject matter of this disclosure, provided that such concepts do not contradict each other.
[0044] The foregoing and other aspects, embodiments, and features of the teachings of the present invention will be more fully understood from the following description in conjunction with the accompanying drawings. Other additional aspects of the invention, such as features and / or beneficial effects of exemplary embodiments, will become apparent from the following description or may be learned through practice of specific embodiments according to the teachings of the present invention. Attached Figure Description
[0045] The accompanying drawings are not drawn to scale. In the drawings, each identical or nearly identical component shown in the various figures may be denoted by the same reference numeral. For clarity, not every component is labeled in each figure. Embodiments of various aspects of the invention will now be described by way of example and with reference to the accompanying drawings, wherein:
[0046] Figure 1 This is a cross-sectional photograph of the continuously cast billet sample in Example 1;
[0047] Figure 2 This is a schematic diagram of the sample after thinning in Example 1;
[0048] Figure 3 This is a schematic diagram of the sample after the protective layer was deposited in Example 1;
[0049] Figure 4This is a schematic diagram showing the removal of the substrate above the bubbles in Example 1;
[0050] Figure 5 This is a microscopic morphology diagram of the inner wall surface of the bubble in Example 1. Detailed Implementation
[0051] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art to which this invention pertains.
[0052] The terms "first," "second," and similar words used in the specification and claims of this patent application do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, unless the context clearly indicates otherwise, the singular forms of "an," "a," or "the," etc., do not indicate a quantity limitation, but rather indicate the presence of at least one. Terms such as "comprising" or "including" mean that the element or object preceding "comprising" encompasses the features, wholes, steps, operations, elements, and / or components listed following "comprising" or "including," and do not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components, and / or collections thereof. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.
[0053] Existing conventional detection methods, such as flaw detection, have poor resolution, making it difficult to accurately locate minute defects like subcutaneous bubbles in continuous castings. Sampling is also susceptible to interference from other areas. Extensive destructive mechanical breakage can cause trace amounts of gas within the bubble to escape instantaneously, making collection and further analysis difficult. Furthermore, the bubble's size and the volume of gas it contains are extremely small, falling below the detection limits of common gas chromatographs and mass spectrometers. For example, the detection limits of existing detection equipment are usually expressed in concentration units (ppm, ppb, ppt) or absolute mass units. To achieve effective and stable detection with conventional equipment, a gas sample volume of tens to thousands of milliliters is typically required. However, a bubble with a diameter of approximately 300 μm in a continuously cast billet typically has a gas volume of only 1.4 × 10⁻⁶ under standard conditions. -5The volume of gas in the continuously cast billet is far below the required amount, thus existing traditional detection methods cannot achieve effective analysis. Therefore, this invention provides a method for detecting the gas composition of bubbles inside the billet. This method constructs a detection approach of "precise positioning - fixed state - minimally invasive windowing - in-situ analysis". First, X-ray computed tomography (CT) technology is used to perform high-resolution three-dimensional positioning of the bubbles inside the continuously cast billet. Then, a sample freezing stage is used to freeze the gas inside the bubbles. Next, a focused ion beam (FIB) is used to create a minimally invasive opening on the sample surface. Finally, time-of-flight secondary ion mass spectrometry (TOF-SIMS) is used to perform precise component analysis, thereby solving the problem that existing technologies cannot analyze the gas composition inside bubbles in continuously cast billets.
[0054] The method for detecting the gas composition of bubbles inside a continuously cast billet according to the present invention specifically includes the following steps:
[0055] S100, Defect area localization: High-resolution three-dimensional spatial localization based on X-ray computed tomography.
[0056] S101. Pre-inspection of continuously cast billet samples: Non-destructive testing methods such as ultrasonic or magnetic particle testing are used to initially screen the continuously cast billet, locating one or more areas suspected of having bubble defects. Subsequently, the areas suspected of having bubble defects are completely cut from the continuously cast billet matrix using cold working methods that do not generate cutting heat, such as wire cutting or diamond saw blade cutting, to prepare initial samples. Using cold working methods can avoid the high-temperature heat effects of traditional hot cutting (such as flame cutting, plasma cutting, etc.), which can cause the gas inside the bubbles to expand, escape, or change in composition, thus ensuring that the subsequent analysis results can truly reflect the original state of the bubbles.
[0057] S102. Pre-processing of defect areas: The initial sample containing defect areas cut out in S101 is ground and polished to eliminate macroscopic unevenness and stress layers caused during cutting, and to avoid artifact interference in subsequent X-ray scanning imaging. Specifically, silicon carbide sandpaper of progressively finer grit (e.g., 180 grit, 400 grit, 800 grit, 1200 grit, 2000 grit) is used for grinding. After grinding, diamond polishing with gradually decreasing grit is used to finally obtain a mirror effect. The surface roughness Ra of the initial sample is <0.1 μm.
[0058] S103. Place the polished initial sample in an industrial X-ray CT system for scanning. Set the spatial resolution to 3-20 μm and the voltage to 30-220 kV. Complete the 360-degree rotation scan to obtain the two-dimensional projection data of the initial sample.
[0059] S104. The two-dimensional projection of the initial sample collected in S103 is reconstructed into three dimensions to obtain three-dimensional volume data of the steel matrix (high absorption) and internal cavity (low absorption) of the initial sample. A three-dimensional image of the sample is constructed based on the three-dimensional volume data, and the location region of the target bubble is identified based on the three-dimensional image. Preferably, the location region of the target bubble can be further cut and separated from the initial sample to obtain a smaller sample block, which is beneficial for subsequent marking, milling, etc.
[0060] S105. Based on the three-dimensional image of the sample in S104, further refine and polish the sample surface to ensure that the vertical distance between the tip of the target bubble and the newly polished surface is <50μm. This distance is a crucial prerequisite for the efficient and controllable subsequent FIB minimally invasive window opening. If the distance is too large, the milling time will be too long, the cost will be high, and the bubble is prone to deviation; if the distance is too small or the bubble is already exposed, the gas may have escaped before processing. The grinding and polishing methods can be the same as those in S102.
[0061] S106. Based on the three-dimensional image of the sample in S104, mark the area corresponding to the target bubble on the surface of the sample after it has been polished again, and obtain the target bubble marking point.
[0062] S200, solidifies the sample state based on low-temperature freezing technology.
[0063] S201. Place the sample after marking the target bubble on the frozen sample stage, and transfer the sample together with the frozen sample stage to the focused ion beam-scanning electron microscope-time-of-flight secondary ion mass spectrometry system. In subsequent operations, the focused ion beam-scanning electron microscope (FIB-SEM) system is used to perform ion beam bombardment milling to achieve minimally invasive windowing, and the secondary electron image of the inner wall surface of the target bubble is obtained by observing through the window using electron beam imaging mode.
[0064] S202. Set the sample stage temperature to below -150℃ and freeze continuously for a certain period of time, preferably 25 to 35 minutes, so that the temperature of the sample is reduced to a sufficiently low temperature to ensure that the gas in the target bubble inside the sample reaches a deep-freezing state, so as to avoid the gas inside the bubble from escaping during milling, which is beneficial to the subsequent analysis of key element ion composition.
[0065] S300, a minimally invasive fenestration based on focused ion beam (FIB).
[0066] S301. In the FIB-SEM system, locate the target bubble marker point at S106 and accurately correlate the three-dimensional coordinates of the bubble obtained by the CT system with the coordinate system of the FIB-SEM cryogenic sample stage.
[0067] Specifically, firstly, in the 3D image obtained in S104, the CT system simultaneously records the center coordinates of the target bubble and the coordinates of the target bubble marker. After transferring the sample to the FIB-SEM system, the target bubble marker is located in the electron beam image, and its coordinates in the new coordinate system are recorded. Using the coordinate values of the same marker in both coordinate systems, the system software (e.g., using the least squares method) can automatically calculate the spatial transformation relationship between the two coordinate systems. Using this transformation relationship, the system can automatically convert the CT coordinates of the bubble to the FIB-SEM working coordinates and guide the ion beam / electron beam to the approximate area of the target bubble. Subsequently, the operator uses the visually visible marker as an absolute reference and, based on the precise position of the bubble relative to the marker stored in the CT image (e.g., 30 μm directly below), makes a final fine-tuning of the beam spot position to ensure the absolute accuracy of the windowing position.
[0068] S302. In the predetermined window area above the target bubble, an ion-beam induced deposition is performed to form a protective layer with good conductivity and relative stability under ion beam bombardment, preferably a platinum or carbon protective layer, with a thickness of 0.1–2 μm. By employing a protective layer with good conductivity and stability against ion beam bombardment, during subsequent high-energy particle beam rough milling, this protective layer first withstands the bombardment and implantation of the ion beam, effectively protecting the sensitive original inner wall interface of the bubble below from ion beam-induced lattice damage, amorphization, and gallium ion contamination, ensuring that the finally exposed inner wall surface represents its original physicochemical state. This protective layer itself will be partially or completely removed during subsequent fine milling.
[0069] S303. Step-by-step milling using an ion beam is used to remove the steel substrate above the target bubble layer by layer, with a window area of less than or equal to 20 μm. 2 The progress is monitored in real time using an electron beam imaging system until the inner wall of the target bubble's dome is just exposed. After the minimally invasive window is completed, a secondary electron image of the target bubble's inner wall surface is obtained through the window using ion beam / electron beam imaging mode. This secondary electron image displays the physical morphology and geometry of the sample surface, such as the appearance of the bubble's inner wall surface, its high and low points, and whether it is smooth or rough, similar to a high-precision black and white topographic map. Figure 5 As shown.
[0070] In some embodiments of the present invention, when performing step milling with an ion beam, the area of the window can be further adjusted according to actual detection needs. Especially in certain specific cases, when the condensation temperature of the gas inside the bubble to be tested is low, or the local cooling effect is insufficient, only a micro-hole with a size much smaller than the conventional window area can be milled, for example, exposing only a single venting point. This micro-hole allows the gas inside the bubble to escape outward in a controlled manner. The escaping gas is ionized under the action of the ion beam, and the generated ions can be directly received and analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS), thereby achieving effective detection of the gas composition inside the bubble. This embodiment avoids interference or rapid gas loss that may be introduced by large-area windowing while still ensuring effective gas ionization and signal collection.
[0071] S400, in-situ component imaging analysis based on time-of-flight secondary ion mass spectrometry (TOF-SIMS).
[0072] S401. After completing the minimally invasive windowing, switch the operating mode of the focused ion beam-scanning electron microscope-time-of-flight secondary ion mass spectrometry (TOF-SIMS) system from "ion beam / electron beam imaging mode" to "TOF-SIMS analysis mode." Use the ion beam to scan and image the inner surface of the exposed, deeply frozen target bubble. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is an extremely high-resolution measurement technique that uses primary ions to excite the sample surface, generating extremely small amounts of secondary ions. The mass of the ions is determined by the different travel times to the detector due to their varying masses. It can analyze all elements, including hydrogen, carbon, nitrogen, and oxygen, and also provides isotopic information, analyzes compound components and molecular structure, and offers high sensitivity, high mass resolution, a wide measurable molecular weight range, and can perform micro-area component imaging and depth profile analysis. In analysis mode, TOF-SIMS typically operates in static mode, using an extremely low-dose pulsed ion beam to ensure that each analysis consumes only a very small portion of the molecules from the top layer of the sample (usually less than 1%), maintaining the integrity of the original surface state, thus enabling the acquisition of high-resolution mass spectra and two-dimensional chemical images. Two-dimensional chemical imaging maps show the spatial distribution of specific chemical components on the surface of a sample. For example, they show which areas water molecules mainly gather, and are a type of chemical composition map that indicates the location of different substances.
[0073] S402. Detect positive and negative secondary ions sputtered from the surface. A time-of-flight mass analyzer performs mass-to-charge ratio (m / z) analysis on these ions at a microsecond-level time resolution, obtaining a high-resolution mass spectrum and a two-dimensional chemical imaging map reflecting the molecular and elemental distribution on the inner surface of the target bubble. From this mass spectrum, characteristic mass-to-charge ratio peak positions and their relative intensity ratios of at least one element selected from carbon, oxygen, hydrogen, nitrogen, and argon are extracted to characterize the source of the bubble gas composition. Specifically, the TOF-SIMS system uses a focused primary ion beam to stably bombard the sample. The primary ions transfer some of their energy to lattice atoms, causing some atoms to move towards the surface and emit secondary ions (sputtered particles). The sputtered secondary ions are accelerated to the same kinetic energy and fly in a flight tube without an electromagnetic field. Depending on the mass-to-charge ratio (m / z), lighter ions have shorter flight times, and heavier ions have longer flight times, thus achieving ion separation and identification, and obtaining a mass spectrum. By analyzing the specific mass-to-charge ratio peak positions and their intensities in this mass spectrum, carbon (C) can be identified. + m / z=12), oxygen (O + (m / z=16), hydrogen (H) + (m / z=1), nitrogen (N) + m / z=14), Argon (Ar + Element ions and their molecular ions (such as CO, m / z=40) + m / z=28; OH - (m / z=17). Based on this information, and combined with two-dimensional chemical imaging, a characteristic ion fingerprint spectrum of the inner wall surface of the target bubble is constructed. This characteristic ion fingerprint spectrum includes the types of characteristic ions, their relative intensity ratios, and two-dimensional ion imaging distribution information.
[0074] S500: Construct a multimodal data fusion and causal diagnosis database.
[0075] The core of this step is to transform the raw data obtained by different detection methods into characteristic parameters with clear physical or chemical significance, and to establish a correlation model between these characteristic parameters and the cause of bubble formation.
[0076] S501. Extract the following morphological feature parameters of the target bubble from the three-dimensional volume data of the target bubble obtained in S104:
[0077] (1) Three-dimensional geometry: Through three-dimensional reconstruction fitting, the target bubble is qualitatively or quantitatively described as spherical, ellipsoidal or irregular.
[0078] (2) Dimensional parameters: Measure and record the equivalent sphere diameter, maximum length, width and depth of the target bubble.
[0079] (3) Spatial position parameters: Record the vertical distance between the top of the target bubble and the test surface of the sample, or the subcutaneous distance.
[0080] S502. Extract the surface micromorphological features of the inner wall of the target bubble from the secondary electronic image obtained in S303, such as the smoothness level of the inner wall, the presence of specific textures or attachments, etc.
[0081] S503. Extract the following chemical characteristic parameters from the characteristic ion fingerprint spectrum obtained in S402 on the inner wall surface of the target bubble:
[0082] (1) Characteristic peak identification: Record the prominent mass-to-charge ratio peaks, such as C + m / z=12, O + m / z=16, CO + m / z=28, etc., are used as chemical fingerprint type identifiers for diagnosing the source of air bubbles.
[0083] (2) Relative intensity ratio: Calculate the relative intensity ratio between key ion pairs, for example, C + / O + Strength ratio, CO + / C + Intensity ratio, etc., are used as quantitative characteristics.
[0084] (3) Spatial distribution pattern of characteristic ions: Based on the two-dimensional ion imaging distribution information, determine whether the distribution of key substances such as carbon, oxygen, nitrogen, and argon on the inner wall surface of the target bubble is uniform, locally enriched, or distributed in a ring.
[0085] S504. Construction of a database for diagnosing the causes of air bubbles.
[0086] Collect standard samples: Collect bubble samples from continuously cast billets of known origin.
[0087] For example, incomplete deoxidation type: through experimental simulation, incomplete deoxidation is intentionally controlled in molten steel to obtain bubble samples with CO and CO2 as the main causes.
[0088] Gas entrapment type: Simulating the immersion nozzle sealing failure condition, the bubble sample was obtained with N2, O2 and Ar as the main causes.
[0089] Moisture type: Adding trace amounts of water-containing substances to molten steel yielded bubble samples with H2 and H2O as the main causes.
[0090] Multimodal data acquisition: For each type of standard sample, the detection process from S100 to S400 was fully executed to obtain standard characteristic data for each type of bubble, including: morphological characteristics: shape (spherical, ellipsoidal, irregular), size distribution, location (subcutaneous depth), inner wall smoothness level, and presence of specific textures or attachments obtained from CT 3D reconstruction; chemical characteristics: characteristic ion fingerprint spectrum obtained from TOF-SIMS analysis. For example, the inner wall of CO-type bubbles is significantly enriched with C, O and their compound ions, while the inner wall of Ar-type bubbles has almost no or only extremely low ion signals.
[0091] Table 1. Correlation Indicators between Bubble Formation Types and Comprehensive Characteristic Spectra
[0092]
[0093] Establish association rules and database: Associate the above-mentioned "morphological-chemical" features with the corresponding causes of bubble formation (poor deoxygenation, gas entrainment, water decomposition, etc.), and use multivariate statistical methods (such as principal component analysis PCA) or machine learning algorithms (such as support vector machine SVM) to build a classification model, forming a searchable and comparable "bubble formation diagnosis database".
[0094] S600, Diagnosis of Unknown Bubble Causes (Application Stage).
[0095] S601. When detecting a new unknown bubble, perform steps S100 to S400 to obtain the measured comprehensive characteristic spectrum of the unknown bubble, including the bubble's morphological characteristics (e.g., spherical, diameter 300 μm, smooth / rough inner wall surface) and the characteristic ion fingerprint spectrum obtained from TOF-SIMS analysis (e.g., significant C). + m / z=12, O + m / z=16, CO + (m / z=28 peaks).
[0096] S602. Import the measured comprehensive feature spectrum into the bubble formation diagnosis database constructed in step S500 for comparison. The system automatically outputs the best-matching bubble formation diagnosis result by comparing the similarity or distance between the measured comprehensive feature spectrum and various standard feature spectra in the bubble formation diagnosis database.
[0097] In summary, the present invention uses FIB to perform "minimally invasive windowing" on the target bubble, exposing the solid interface of its inner wall, and then uses the ultra-high sensitivity "TOF-SIMS" for in-situ molecular-level scanning. This not only avoids dependence on the gas volume, enables the detection of trace gases, but also can obtain interface chemical molecular information that cannot be provided by traditional methods, providing direct evidence for accurately identifying the causes of bubbles. Finally, by correlating the three-dimensional morphology, microscopic images, and chemical composition of the bubbles obtained by the above technology, an accurate diagnostic model for the causes of bubbles is established, upgrading this method from laboratory characterization to a precise diagnostic tool that can directly guide the closed-loop optimization of the production process.
[0098] Example 1
[0099] This example provides a specific instance of using the above detection method to detect the gas composition of bubbles in a 70 steel continuous casting billet sample with subcutaneous bubble defects produced by a certain steel plant. Through this example, it aims to clearly demonstrate the actual operation process and technical effects of the entire set of processes of "precision positioning - state fixation - minimally invasive windowing - in-situ analysis". The specific steps are as follows:
[0100] (1) Three-dimensional spatial positioning of the defect area
[0101] ① For a 70 continuous casting billet sample with subcutaneous bubble defects produced by a certain steel plant, a suspected bubble-dense area is located by an ultrasonic flaw detector.
[0102] ② The bubble-dense area is cut off by wire cutting to make an initial sample block with dimensions of approximately 10 mm × 6 mm × 3 mm.
[0103] ③ The surface of the initial sample is polished with sandpapers of 180 mesh, 800 mesh, 1200 mesh, and 2000 mesh from coarse to fine, and polished with 5 μm diamond polishing agent to obtain a flat and smooth surface. The macroscopic photo is as Figure 1 shown, where the larger white frame contains millimeter-scale bubbles, and the smaller white frame contains micron-scale bubbles.
[0104] ④ The polished initial sample in ③ is fixed on the sample stage of an industrial X-ray computed tomography system. The scanning voltage is set to 180 kV, and the spatial resolution is selected as 5 μm. The sample stage is controlled to rotate 360 degrees to collect a complete two-dimensional projection sequence of the initial sample.
[0105] ⑤ The two-dimensional projection sequence data is three-dimensionally reconstructed using the software supporting the industrial X-ray computed tomography system to obtain three-dimensional volume data showing the steel matrix and internal cavities of the initial sample, and a three-dimensional image of the initial sample is constructed based on this three-dimensional volume data. An approximately spherical target bubble located approximately 0.6 mm below the skin with an equivalent spherical diameter of approximately 300 μm is identified according to this three-dimensional image.
[0106] ⑥ The initial sample is finely cut around the target bubble to form a small sample block approximately 3mm × 1mm × 1mm in size. The surface of this small sample block is then finely ground and polished according to the grinding and polishing method described in ③. The polished surface of this small sample block corresponds to the position directly above the target bubble, reducing the distance between the top of the target bubble and the final surface of the sample to approximately 30μm. A schematic diagram of the sample is shown below. Figure 2 As shown.
[0107] ⑦ On the surface of the polished small sample block, at the position directly above the target bubble, use a micro indenter to create a tiny positioning mark.
[0108] (2) Sample freezing state fixation
[0109] ① Transfer the small sample block with the completed positioning mark to a focused ion beam scanning electron microscope (FIB-SEM) equipped with a liquid nitrogen cryogenic sample stage.
[0110] ② Set the temperature of the sample stage to -175℃ and freeze for 30 minutes to allow the small sample block and the substance inside the target bubble to reach a thermodynamically stable state.
[0111] (3) Minimally invasive fenestration
[0112] ① In the FIB-SEM system, the secondary electron image of the sample surface is obtained by using its scanning electron microscope imaging function. By identifying the target bubble marker point ⑦ in (1) of the secondary electron image, the bubble position in the industrial CT coordinate system is accurately guided to the working coordinate of the FIB-SEM system, thereby completing the positioning.
[0113] ②The gallium ion beam (Ga + The accelerating voltage was set to 30 kV and the current to 50 pA. A platinum protective layer with a thickness of 1 μm and an area of 20 μm × 20 μm was deposited directly above the target bubble. A schematic diagram of the sample is shown below. Figure 3 As shown.
[0114] ③ The ion beam current was set to 5 nA. Using a stepped milling method, the platinum layer and the underlying steel substrate were removed layer by layer, ultimately forming a square observation window with a side length of approximately 15 μm, fully exposing the inner wall surface of the bubble. A schematic diagram of the sample is shown below. Figure 4 As shown; secondary electron images of the inner wall surface of the target bubble are obtained by observation through a window using ion beam / electron beam imaging mode. The secondary electron images of the inner wall surface of the target bubble are shown in the figure. Figure 5 As shown.
[0115] (4) In-situ component analysis
[0116] ① Switch the ion beam / electron beam imaging mode of the device to TOF-SIMS analysis mode. By setting the ion beam acceleration voltage to 30kV, scan and analyze the inner wall of the bubble to obtain mass spectra and two-dimensional chemical imaging images.
[0117] ②The specific mass-to-charge ratio peak positions and intensities of different ions are obtained through mass spectrometry analysis.
[0118] (5) Data fusion and causal diagnosis
[0119] ① In this embodiment, significant characteristic peaks appear at mass-to-charge ratios (m / z) of 12, 16, and 28 in the mass spectrum obtained by TOF-SIMS analysis, corresponding to C0, respectively. + O + and CO + / CO2 + Two-dimensional chemical imaging obtained using plasma-enhanced TOF-SIMS mode shows significant and relatively uniform enrichment of C and O characteristic signals on the inner wall of the bubble. Meanwhile, CT data indicates that the bubble is nearly spherical with an equivalent diameter of approximately 300 μm, and secondary electron imaging shows that its inner wall is relatively smooth.
[0120] ② The measured comprehensive characteristic spectrum was compared with the bubble formation diagnosis database. In this embodiment, the measured comprehensive characteristic spectrum had the highest matching degree with the standard characteristic spectrum of "poor deoxygenation type" in the database, so it was attributed to the poor deoxygenation type bubble. Among them, the bubble formation diagnosis database has stored the standard characteristic spectra of various types of bubbles with known causes, including: poor deoxygenation type: significant enrichment of C and O + spherical / ellipsoidal shape; gas entrainment type: enrichment of N and Ar + irregular morphology; water classification: enrichment of H and OH + rough inner wall.
[0121] It is particularly important to note that the continuous casting billet matrix involved in Example 1 is an iron-based metal material, which differs fundamentally from non-metallic materials such as glass in terms of physicochemical properties, microstructure, and defect formation mechanisms. This determines that bubble detection methods from other fields (such as glass) cannot be directly applied. Specifically: From the perspective of material properties and microstructure, the continuous casting billet is a polycrystalline metallic material with characteristics such as high density, high conductivity, opacity, and anisotropy. Ultrasonic waves are easily scattered and attenuated at grain boundaries, significantly interfering with the positioning accuracy of tiny bubbles. In contrast, glass is an amorphous non-metallic material with light transmittance and isotropy, and its internal bubbles can be directly observed through optical methods. From the perspective of bubble formation mechanism and interface characteristics, bubbles in the continuous casting billet are formed during the rapid solidification process of high-temperature molten steel. Complex interfacial reactions may occur between the molten steel and the gas inside the bubbles. The gas inside the bubbles is in a high-pressure, micro-volume state, and it instantly dissipates once the matrix is destroyed. In contrast, bubbles in glass usually originate from physical inclusions during the melting process, and their interfacial bonding and gas dissipation behavior are completely different from those of the metallic matrix. The aforementioned fundamental differences mean that conventional methods used in the glass industry, such as heating and stretching bubbles or creating stress cuts to puncture and collect gas, not only fail to achieve accurate detection of gas composition when applied to continuously cast billets, but also introduce greater errors due to changes in metal structure or gas escape.
[0122] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention shall be determined by the claims.
Claims
1. A method for detecting the composition of gas bubbles inside a continuously cast billet, characterized in that, Includes the following steps: Three-dimensional non-destructive scanning was performed on the continuous casting billet sample with bubble defect area to identify the size and three-dimensional morphology of the target bubble and mark the target bubble mark point; The marked continuous casting billet sample is subjected to freezing treatment to solidify the gaseous material inside the target bubble; Based on the target bubble markers, the coordinate system of the three-dimensional non-destructive scanning system is associated and mapped with the coordinate system of the focused ion beam scanning system to accurately guide and position the target bubble in the focused ion beam scanning system. In the focused ion beam scanning system, a predetermined window area is positioned above the target bubble, and a conductive protective layer is deposited by focusing ion beam induction. A focused ion beam is used to create a micro-invasive window on the protective layer above the target bubble and the continuous casting billet substrate to expose the inner wall surface of the target bubble. Surface mass spectrometry was used to perform in-situ scanning analysis on the inner wall surface of the exposed target bubble. By analyzing the mass-to-charge ratio of the excited secondary ions, the types of key element ions contained in the inner wall surface were determined, and their spatial distribution information was obtained.
2. The method for detecting the gas composition of bubbles inside a continuously cast billet according to claim 1, characterized in that, The three-dimensional non-destructive scanning is industrial X-ray computed tomography, with a spatial resolution of 3–20 μm.
3. The method for detecting the gas composition of bubbles inside a continuously cast billet according to claim 1, characterized in that, The protective layer is a platinum protective layer or a carbon protective layer, with a thickness of 0.1–2 μm.
4. The method for detecting the gas composition of bubbles inside a continuously cast billet according to claim 1, characterized in that, The step of performing three-dimensional non-destructive scanning on the continuously cast billet sample with bubble defect areas to identify the size and three-dimensional morphology of the target bubbles and mark the target bubble markers includes: The continuous casting billet sample with bubble defect area is subjected to the first surface treatment, and then three-dimensional non-destructive scanning technology is used to scan and construct its three-dimensional image to identify the size, three-dimensional morphology and spatial location of the target bubble; Based on the spatial position information displayed in the three-dimensional image, a second surface treatment is performed on the continuous casting billet sample to make the vertical distance between the top of the target bubble and the polished surface of the continuous casting billet sample less than 50 μm. Then, the position of the target bubble is marked on the continuous casting billet sample to obtain the target bubble marking point.
5. The method for detecting the gas composition of bubbles inside a continuously cast billet according to claim 1, characterized in that, The observation window formed by the minimally invasive fenestration has an opening area of less than or equal to 20 μm. 2 .
6. The method for detecting the gas composition of bubbles inside a continuously cast billet according to claim 1, characterized in that, The surface mass spectrometry analysis was time-of-flight secondary ion mass spectrometry analysis. The key element ion is at least one of carbon, oxygen, hydrogen, nitrogen, and argon, or its molecular ion.
7. The method for detecting the gas composition of bubbles inside a continuously cast billet according to claim 1, characterized in that, The equivalent sphere diameter of the target bubble is less than or equal to 500 μm.
8. The method for detecting the gas composition of bubbles inside a continuously cast billet according to claim 1, characterized in that, The freezing temperature for the freezing process is below -150°C, and the freezing time is 25 to 35 minutes.
9. The method for detecting the gas composition of bubbles inside a continuously cast billet according to claim 1, characterized in that, It also includes diagnosing the causes of air bubbles: After exposing the inner wall surface of the target bubble, the microstructure information of the inner wall surface is obtained by scanning with a focused ion beam scanning system; The size and three-dimensional morphology of the target bubble, the microscopic morphology information of the inner wall surface, and the types and spatial distribution information of key element ions contained in the inner wall surface are correlated and fused to form a comprehensive feature spectrum. The comprehensive feature spectrum is compared with a pre-stored bubble formation diagnosis database to diagnose the cause of the bubbles.
10. The method for detecting the gas composition of bubbles inside a continuously cast billet according to claim 9, characterized in that, The construction of the bubble formation diagnosis database includes the following steps: Collect standard samples of bubbles in continuously cast billets from known sources; The standard sample is subjected to the detection method according to any one of claims 1 to 8 to obtain standard feature data including three-dimensional morphological features, size range and micro-morphological features of the inner wall surface, and chemical features of the inner wall surface; The standard feature data is correlated with its corresponding bubble formation causes to form the bubble formation cause diagnosis database. The causes of bubble formation include poor deoxygenation, gas entrapment, and water decomposition.