Analysis method for corresponding relation between slab macroscopic center segregation and microcosmic precipitate

Abstract

The invention discloses a method for analyzing a corresponding relation between slab macroscopic center segregation and microscopic precipitates, and belongs to the technical field of metal material detection. The method comprises the following steps: firstly, carrying out low-power corrosion on a slab sample, and clearly displaying a macroscopic central segregation zone; then, according to the morphology and corrosion depth of a segregation zone, precisely cutting out thick tensile samples at specific positions representing different segregation degrees (no segregation, slight segregation and serious segregation); breaking the thick tensile sample on a tensile testing machine to obtain fractures containing different segregation degree information; and finally, observing and analyzing a specific area of the fracture by utilizing a field emission scanning electron microscope, and qualitatively and quantitatively identifying the type, size, distribution and quantity density of the precipitates. According to the method, the direct and in-situ corresponding relation between the macrosegregation and the microscopic precipitate is established, and an accurate and reliable experimental basis is provided for accurately evaluating the slab center quality and optimizing the continuous casting and rolling process.

Description

Technical Field

[0001] This invention belongs to the field of metal material testing and analysis technology, and relates to an analytical method for establishing a direct correspondence between macroscopic center segregation defects and microscopic precipitates in continuously cast slabs. Background Technology

[0002] In the continuous casting process of steel, center segregation is a common and serious defect inside the slab. It is caused by the accumulation and solidification of residual molten steel rich in solute elements such as C, Mn, P, and S in the center of the slab during the final stage of solidification. Macroscopic compositional inhomogeneity, i.e., segregation, directly leads to the formation of different types of precipitates at the microscopic scale during subsequent cooling or heat treatment, such as sulfides, oxides, and carbonitrides.

[0003] These microscopic precipitates have a crucial impact on the mechanical properties of steel, especially its toughness, plasticity, fatigue performance, and processing properties. For example, under stress, precipitates can become initiation sites for microcracks, leading to premature material failure. Currently, the analysis of segregation at the center of slabs is mostly limited to macroscopic low-magnification inspection or isolated microscopic composition analysis. Macroscopic low-magnification inspection can visually show the location and severity of segregation, but it cannot provide information on microscopic precipitates; while traditional microscopic analyses such as metallographic observation often involve random sampling locations, making it difficult to accurately correspond to a specific degree of segregation on a macroscopic scale.

[0004] Therefore, there is an urgent need for an analytical method that can accurately and in situ correlate macroscopic segregation morphology with microscopic precipitate characteristics in order to gain a deeper understanding of the influence mechanism of segregation on the microstructure of materials, thereby providing direct evidence for controlling and improving the quality of the slab center from the root. Summary of the Invention

[0005] To address the shortcomings of the existing technologies, this invention aims to provide an analytical method for the correspondence between macroscopic center segregation and microscopic precipitates in slabs. This method can achieve a seamless connection from macroscopic positioning to microscopic analysis, accurately revealing the characteristics of microscopic precipitates corresponding to regions with different degrees of segregation.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: An analytical method for the correspondence between macroscopic center segregation and microscopic precipitates in slabs includes the following steps: S1. Low-magnification corrosion test: Low-magnification samples are cut from the slab and processed, milled and cold pickled to expose the macroscopic central segregation morphology. S2. Positioning and Sampling: Based on the macroscopic segregation morphology shown in step S1, take full-thickness tensile specimens at the target positions corresponding to different degrees of segregation, along the direction perpendicular to the slab surface. S3. Tensile testing to obtain fracture surface: The thick tensile specimen obtained in step S2 is stretched on a tensile testing machine until it breaks, thereby obtaining a fracture surface containing information about the target segregation region. S4. Microscopic precipitate observation: The fracture surface obtained in step S3 is observed using a field emission scanning electron microscope, focusing on the analysis of precipitates on the fracture surface to obtain information on their composition, morphology, size and distribution.

[0007] Furthermore, in step S2, the different degrees of segregation include at least a region without segregation, a region with slight segregation, and a region with severe segregation.

[0008] Furthermore, in step S2, the thickness of the tensile specimen is the same as the slab thickness to ensure that the fracture surface retains sufficient information about the original segregation region.

[0009] Furthermore, step S4 also includes statistical analysis of the observed precipitates, including the type, average size, number density, and distribution of the precipitates per unit area, thereby establishing a quantitative relationship between different degrees of segregation and precipitate characteristics.

[0010] Compared with the prior art, the present invention has the following significant advantages: 1) Strong direct correspondence: Through macroscopic corrosion positioning, fracture surfaces for microscopic analysis can be directly prepared in specific segregation areas, ensuring a clear and unambiguous correspondence between the observed precipitates and the degree of macroscopic segregation, thus avoiding the uncertainty brought about by traditional random sampling.

[0011] 2) High information fidelity: By using the tensile fracture surface as the observation surface, the precipitates are directly exposed on the fracture surface, avoiding the detachment, dragging or deformation of small and brittle precipitates caused by sample preparation processes such as grinding and polishing, and can more realistically reflect the original morphology and distribution of the precipitates.

[0012] 3) Comprehensive and accurate analysis: Field emission scanning electron microscopy combined with energy dispersive spectroscopy can not only qualitatively identify the types of precipitates such as MnS, Al2O3, Nb(C,N), etc., but also perform semi-quantitative component analysis and precise quantitative statistics, thereby establishing a complete data chain from macroscopic segregation level to microscopic precipitate parameters including type, quantity and size.

[0013] 4) High practical value: This method provides metallurgists with a powerful analytical tool that can accurately evaluate the effects of different continuous casting and rolling processes on center segregation and precipitate control, thereby improving the quality of high-end steel products. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the overall process of the analytical method of the present invention.

[0015] Figure 2This is a schematic diagram showing the macroscopic central segregation morphology of a slab sample after low-magnification etching. The sampling locations for different segregation patterns are marked in the figure.

[0016] Figure 3 This is a schematic diagram of thick tensile specimens taken from different segregation regions.

[0017] Figure 4 The image shows the morphology and energy spectrum of precipitates from the fracture surface of a severely segregated region as observed by field emission scanning electron microscopy.

[0018] In the figure: 1-low magnification sample; 2-central segregation zone; 3-no segregation zone; 4-slight segregation zone; 5-severe segregation zone; 6-thick tensile sample; 7-thick tensile fracture surface of no segregation zone; 8-thick tensile fracture surface of slight segregation zone; 9-thick tensile fracture surface of severe segregation zone; 10-precipitate; 11-energy dispersive spectroscopy point analysis. Detailed Implementation

[0019] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. Example 1

[0020] Taking the analysis of center segregation in a high-strength steel continuously cast slab as an example, the overall process is as follows: Figure 1 As shown, it includes the following steps: S1. Low-magnification corrosion test: Cut a 1-low-magnification sample with dimensions of 800mm × 100mm × 260mm (thickness direction) from the cross-section of the slab. Mill the sample surface, then remove oil stains, and then cold-pickle for about 5 minutes. After corrosion, wash and dry; a clear 2-center segregation band will be visible. Figure 2 As shown. Based on the depth and morphology of corrosion, the central region is divided into three zones: 1 - no segregation zone, 2 - slight segregation zone, and 3 - severe segregation zone.

[0021] S2. Positioning and Sampling: Using a saw, in Figure 2 Three full-thickness (6-mm) tensile specimens were taken from the center of each of the three regions shown: 3 (no segregation), 4 (slight segregation), and 5 (severe segregation). The tensile specimen dimensions were: 10mm diameter and 260mm length (in the thickness direction of the slab). Figure 3 As shown.

[0022] S3. Tensile testing to obtain fracture surfaces: On a universal tensile testing machine, at a tensile rate of 2 mm / min, thick-walled tensile specimens from three regions—3 (no segregation zone), 4 (slight segregation zone), and 5 (severe segregation zone)—are broken to obtain fracture surfaces from three regions: 7 (no segregation zone), 8 (slight segregation zone), and 9 (severe segregation zone). The broken specimens are carefully stored to prevent contamination or damage to the fracture surface.

[0023] S4. Observation of microscopic precipitates: a. Place the 9-severely segregated thick tensile fracture surface in the sample chamber of a field emission scanning electron microscope; b. Observe the entire fracture surface at low magnification (50~100x) and locate the region on the fracture surface corresponding to the macroscopic segregation zone; c. Switching to the backscattered electron mode at high magnification (1000x), a large number of bright, granular 10- precipitates of varying sizes were observed; d. Typical precipitates were subjected to 11-energy dispersive spectroscopy (EDS) analysis. The results are shown in Table 1. The main component is NbC. e. Using image analysis software, statistical analysis was performed on NbC precipitates in multiple fields of view. The results showed that in the 5-severe segregation region, the average size of the NbC precipitates was approximately 10 μm, with a number density as high as 1.2 × 10⁻⁶. 4 Pieces / mm²; f. In contrast, the same operation was performed on the fracture surface of region A (non-segregation region), and it was found that the number of MnS precipitates was sparse, with an average size of less than 15 μm and a number density of less than 3.8 × 10⁻⁶. 3 Pieces / mm²

[0024] Through the above steps, the embodiments of the present invention successfully and quantitatively revealed a significant positive correlation between the severity of macroscopic center segregation and the quantity and size of microscopic NbC precipitates, providing accurate data support for evaluating the center segregation control level of this steel grade and subsequent process improvements.

[0025] Table 1. Results of Energy Spectroscopy Analysis .

Claims

1. A method of analyzing a correspondence between macroscopic center segregation and microscopic precipitates of a slab, characterized by The method comprises the following steps: S1. Macro-etching inspection: a macro sample is cut from the slab, and is processed, milled and ground, and cold pickling etched to reveal the macro center segregation morphology; S2. Positioning and sampling: according to the macro segregation morphology shown in step S1, full-thickness tensile samples are cut at target positions corresponding to different segregation degrees in a direction perpendicular to the slab surface; S3. Tensile fracture preparation: the tensile sample obtained in step S2 is stretched on a tensile testing machine until it breaks, thereby obtaining a fracture containing information of the target segregation area; S4. Microscopic precipitate observation: the fracture surface obtained in step S3 is observed using a field emission scanning electron microscope, and the precipitates on the fracture are analyzed in detail to obtain their composition, morphology, size and distribution information.

2. The method of claim 1, wherein: the slab macroscopic center segregation and the microscopic precipitates are analyzed by using a relationship between the slab macroscopic center segregation and the microscopic precipitates. In step S2, the different segregation degrees at least include a non-segregation area, a slight segregation area and a severe segregation area.

3. The method of claim 1, wherein: the slab macroscopic center segregation and the microscopic precipitates are analyzed by using a macroscopic center segregation analysis method and a microscopic precipitate analysis method, respectively. In step S2, the thickness of the tensile sample is the thickness of the slab to ensure that the fracture can retain sufficient original segregation area information.

4. The method of claim 1, wherein: the slab macroscopic center segregation and the microscopic precipitates are analyzed by using a macroscopic center segregation analysis method and a microscopic precipitate analysis method, respectively. Step S4 further comprises statistics of the observed precipitates, and the statistical items include the type, average size, number density and distribution in unit area of the precipitates, thereby establishing a quantitative relationship between different segregation degrees and precipitate characteristics.

Images