A comprehensive characterization method for black scale of stainless steel hot-rolled black scale

Abstract

The application firstly solves the detection precision problem of the rough surface of hot-rolled black skin through a special sample preparation process of metallographic cold inlaying + gradient polishing to Ra≤0.02 μm, and on this basis, the conventional method of SEM-EDS "area scanning, auxiliary qualitative" is abandoned, and multi-zone accurate point scanning of the iron oxide scale defect area, normal area and matrix interface area is created; a standardized element-oxide quantitative conversion method is established, and line scanning along the thickness direction of the iron oxide scale is carried out to accurately capture the composition gradient; thus, a complete and accurate industrial characterization method is formed, a characterization system for industrial batch application is realized, and a complete application closed loop of "characterization-process optimization" is constructed.

Description

Technical Field

[0001] This invention relates to the field of stainless steel smelting, and in particular to a comprehensive characterization method for oxide scale on the surface of hot-rolled black stainless steel. Background Technology

[0002] During the hot rolling process of stainless steel, after the slab is heated at high temperature, rolled in multiple passes, and descaled with high-pressure water, iron oxide scale is easily formed during the finishing rolling and low-temperature stages after finishing rolling. Uncontrolled iron oxide scale can easily be pressed into the surface of the steel plate during rolling, forming defects. This not only reduces the surface quality of hot-rolled products but also adversely affects subsequent deep processing steps such as pickling and cold rolling, increasing production losses. Due to its material characteristics, nickel-saving austenitic stainless steel, represented by BN1D4, exhibits significant oxidation reactions during hot rolling. The iron oxide scale is mainly FeO with no obvious Fe3O4 layer, making the iron oxide scale defect problem particularly prominent and a key technical issue restricting the production of high-quality hot-rolled stainless steel.

[0003] Existing methods for characterizing iron oxide scale are mostly designed for cold-rolled stainless steel sheets. They are only suitable for detecting the planar elemental distribution of thin iron oxide scale and cannot capture the compositional gradient changes in the thickness direction of hot-rolled black iron oxide scale. Furthermore, they are not suitable for the observation accuracy requirements of the rough surface of hot-rolled black scale.

[0004] The inventor published an article titled "Study on Iron Oxide Scale of Hot-Rolled Stainless Steel Coils Using Backscatter Imaging Technology" in the December 2024 issue of *Shanxi Metallurgy*. The article focuses on black-coiled hot-rolled coils. Samples were taken from the finished BN1D4 coil after hot rolling, and 30mm × 30mm samples were cut using a metallographic abrasive wheel cutter. After mounting and polishing, samples suitable for observing the surface oxide layer were prepared. Conventional backscatter imaging and conventional energy dispersive spectroscopy (EDS) point / line scanning were then used to conduct basic morphological and compositional observations of the hot-rolled iron oxide scale. However, this article only presents basic observations and does not yet establish a complete and precise industrial characterization method, making it unsuitable for direct industrial batch testing and hot-rolling production process guidance. Summary of the Invention

[0005] The purpose of this invention is to provide a comprehensive characterization method for oxide scale on the surface of hot-rolled black stainless steel.

[0006] The technical solution to achieve the objective of this invention is: a comprehensive characterization method for oxide scale on the surface of hot-rolled stainless steel black skin, comprising the following steps: 1) Sample preparation: Block-shaped hot-rolled black stainless steel coils were cut, subjected to metallographic cold inlay treatment, and then finely polished to a mirror state with a surface roughness Ra≤0.02μm after gradient grinding with silicon carbide sandpaper. 2) Sample cleaning: Place the prepared sample in anhydrous ethanol, clean it with ultrasonic waves, and then blow it dry in the same direction to obtain the sample to be tested. 3) Place the test sample obtained in step 2) into the scanning electron microscope sample stage and evacuate it. Use backscattered electron imaging mode to observe the interface bonding state and layered structure between the iron oxide scale and the substrate. 4) Using a scanning electron microscope with an energy dispersive spectroscopy (EDS) instrument, surface point scanning and thickness direction line scanning were performed on the defective area, normal area, and interface area of ​​the iron oxide scale. The elemental content data of O, Fe, Cr, and Mn were collected by point scanning and the average value was taken. The composition of Cr2O3, Mn2O3, Fe2O3, and FeO was obtained by data conversion and calculation. The elemental content gradient was obtained by thickness direction line scanning to analyze the layered structure and phase composition of the iron oxide scale. The data conversion method is based on the mass fractions of O, Fe, Cr, and Mn elements measured by scanning electron microscopy energy dispersive spectroscopy, combined with the molar mass of oxides and stoichiometric ratios, and the distribution of Fe valence states through the molar equilibrium of oxygen. The specific steps are as follows: (1) Based on the mass fraction w and molar mass M of each element, calculate the number of moles of each element using the formula n = w / M to obtain n(O), n(Fe), n(Cr), and n(Mn); (2) Calculate the number of moles of oxygen consumed by Cr and Mn oxides: n(O)Cr = 3 / 2 × n(Cr), n(O)Mn = 3 / 2 × n(Mn), and the total oxygen consumption is n(O)CrMn = n(O)Cr + n(O)Mn; (3) After deducting the oxygen consumed by Cr and Mn, calculate the remaining number of moles of oxygen that can combine with Fe: n(O)Fe = n(O) -n(O)CrMn; (4) Establish the Fe valence equilibrium equation: Let Fe 2+ The number of moles is n (Fe 2+ ), Fe 3+ The number of moles is n(Fe) 3+ Based on the conservation of total Fe and the balance of oxygen, a system of equations is established: n(Fe) = n(Fe 2+ )+n(Fe 3+ ) n(O)Fe = n(Fe 2+ ) +3 / 2× n(Fe 3+ ) Solving for n (Fe) 2+ ) and n (Fe 3+ ); (5) Calculate the molar number and mass fraction of each oxide: n(FeO) = n(Fe 2+ n(Fe2O3) = 1 / 2 × n(Fe 3+ ) After calculating the preliminary content based on the molar mass of each oxide, the final mass fractions of Cr2O3, Mn2O3, Fe2O3, and FeO were obtained through normalization. 5) Integrate the microstructure obtained in step 3) with the energy dispersive spectroscopy data obtained in step 4) to clarify the cause of iron oxide scale defects and complete the comprehensive characterization of iron oxide scale on the surface of hot-rolled stainless steel black skin.

[0007] Furthermore, the ultrasonic cleaning time in step 2) is 2 to 3 minutes.

[0008] This invention first solves the problem of detection accuracy for the rough surface of hot-rolled black iron oxide scale by using a proprietary sample preparation process of metallographic cold mounting and gradient polishing to Ra≤0.02μm. Based on this, it abandons the conventional use of SEM-EDS for "surface scanning and auxiliary qualitative analysis" and pioneers multi-zone precise point scanning of the defect area, normal area and matrix interface area of ​​the iron oxide scale. It establishes a standardized element-oxide quantitative conversion method and conducts line scanning along the thickness direction of the iron oxide scale to accurately capture the composition gradient. Thus, a complete and precise industrial characterization method is formed, realizing a characterization system for industrial mass application and constructing a complete application closed loop of "characterization-process optimization". Attached Figure Description

[0009] Figures 1-2 This is a backscattered electron microstructure image of the characteristic region of the hot-rolled black oxide scale of BN1D4 stainless steel described in the embodiment. Figures 3 to 10 The above are EDS point scan energy spectrum and composition ratio diagram of characteristic sites of hot-rolled black iron oxide scale of BN1D4 stainless steel described in the example. Figures 11-12 The image shows the energy dispersive spectral analysis of the iron oxide scale on the cross-section of the hot-rolled black skin of BN1D4 stainless steel described in the example. Detailed Implementation

[0010] The preferred embodiments of the present invention will be described in detail below.

[0011] Taking the iron oxide scale on the surface of hot-rolled black stainless steel BN1D4 as an example, a comprehensive characterization method for the oxide scale on the surface of hot-rolled black stainless steel includes the following steps: 1) Sample preparation: Block-shaped hot-rolled black stainless steel coils were cut, subjected to metallographic cold inlay treatment, and then successively ground with 180#, 600# and 1200# silicon carbide sandpaper in a gradient, and finely polished to a mirror state with a surface roughness Ra≤0.02μm. 2) Sample cleaning: Place the prepared sample in anhydrous ethanol, ultrasonically clean for 2-3 minutes, blow dry in the same direction to remove surface oil, dust and polishing residue, and obtain the sample to be tested. 3) Place the test sample obtained in step 2) into the scanning electron microscope sample stage and evacuate it. Use the secondary electron imaging mode to observe the microstructure and defect distribution of the sample surface in sequence, and then switch to the backscattered electron imaging mode to observe the interface bonding state and layered structure between the iron oxide scale and the substrate. The scanning electron microscope (SEM) parameters were: accelerating voltage 15 kV, working distance 8.5 mm, and magnification 5000x. Backscattered electron imaging was used for cross-sectional observation, as shown in Figure 1. Figure 2 As shown, the cross-section of the iron oxide scale exhibits a continuous and dense layered structure with a clearly distinguishable interface with the substrate. At defective locations, the iron oxide scale is clearly separated from the substrate and exhibits micro-gap characteristics.

[0012] 4) Using a scanning electron microscope with an energy dispersive spectroscopy (EDS) instrument, point scanning and thickness direction line scanning were performed on the defect area, normal area, and interface area of ​​the iron oxide scale. The elemental content data of O, Fe, Cr, and Mn were collected by point scanning and averaged. The composition of Cr2O3, Mn2O3, Fe2O3, and FeO was calculated through data conversion. The mass fraction of elements obtained by point scanning is shown in Table 1. Table 1. Energy dispersive spectral density (EDS) analysis of BN1D4 hot-rolled black iron oxide scale, elemental mass fraction (%)

[0013] Based on the mass fractions of O, Fe, Cr, and Mn in Table 1, and combining the molar mass of oxides with the elemental ratio, the number of moles of each element is calculated using n = w / M, with the molar distribution of oxygen and Fe in equilibrium. 2+ / Fe 3+ The mass fractions of Cr2O3, Mn2O3, Fe2O3, and FeO were calculated as follows: Cr2O3 content 12.70%–17.03%, Mn2O3 content 4.93%–7.44%, Fe2O3 content 11.24%–27.74%, and FeO content 50.14%–71.13%. Note: Spectra 14 and 15 are the matrix region, where the oxygen content is significantly reduced, which is a normal detection result.

[0014] Depend on Figures 3 to 10 As can be seen, the energy dispersive spectroscopy (EDS) spot scanning method described in this application can accurately collect the characteristic peaks of core elements such as O, Fe, Cr, and Mn at characteristic sites of iron oxide scale. The characteristic peaks of the elements are complete in shape, stable in signal intensity, and free from interference from impurity peaks, and the mass fraction of each element can be accurately obtained. After the data conversion method is specific to this application, the specific contents of Cr2O3, Mn2O3, Fe2O3, and FeO can be directly quantified, realizing the accurate analysis of the phase composition of iron oxide scale, which is different from the limitation of conventional EDS which can only perform qualitative analysis. The elemental content gradient was obtained by thickness-direction line scanning, and the energy dispersive spectroscopy (EDS) analysis results of the iron oxide scale are shown in Table 2. Table 2. Energy dispersive spectroscopy (EDS) analysis results of iron oxide scale (mass fraction, %)

[0015] Based on Table 2 Figure 11 (11a, 11b) and Figure 12 It can be seen that from the outer layer of iron oxide scale to the substrate, Cr2O3 and Fe2O3 gradually decrease, while FeO gradually increases. There is no obvious Fe3O4 layer, and it shows a transitional structure from Fe2O3 to FeO.

[0016] 5) Integrate microstructure and energy dispersive spectroscopy data to clarify the causes of iron oxide scale defects and provide a basis for hot-rolled iron oxide scale defect control schemes.

[0017] 5.1) Result judgment: In this embodiment, the BN1D4 hot-rolled black iron oxide scale is mainly composed of iron oxide, and Fe oxidation is the main cause of defects. The high FeO content makes subsequent pickling difficult.

[0018] 5.2) Process optimization: shorten the high-temperature period from finishing rolling to laminar flow cooling, reduce the laminar flow cooling temperature, and increase the pressure and frequency of high-pressure descaling water in finishing rolling.

[0019] Implementation Results: Surface quality testing was conducted according to GB / T 4237-2015 "Hot-rolled Stainless Steel Plates and Strips". After optimization, the defect rate of iron oxide scale pressing, peeling, and delamination in BN1D4 hot-rolled black coils decreased from 8.2% before optimization to below 1.5%, a reduction of 81.7%. The proportion of high-melting-point Fe2O3 in the iron oxide scale decreased, while the proportion of FeO remained stable and controllable. Pickling efficiency increased by more than 35%, pickling time was shortened by 20%, and the product surface quality rate increased to 99.2%. The surface quality and compatibility with subsequent deep processing were significantly improved.

[0020] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent process transformations made using the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A comprehensive characterization method for oxide scale on the surface of hot-rolled stainless steel black sheet, characterized in that: It includes the following steps: 1) Sample preparation: Block-shaped hot-rolled black stainless steel coils were cut, subjected to metallographic cold inlay treatment, and then finely polished to a mirror state with a surface roughness Ra≤0.02μm after gradient grinding with silicon carbide sandpaper. 2) Sample cleaning: Place the prepared sample in anhydrous ethanol, clean it with ultrasonic waves, and then blow it dry in the same direction to obtain the sample to be tested. 3) Place the test sample obtained in step 2) into the scanning electron microscope sample stage and evacuate it. Use backscattered electron imaging mode to observe the interface bonding state and layered structure between the iron oxide scale and the substrate. 4) Using a scanning electron microscope with an energy dispersive spectroscopy (EDS) instrument, surface point scanning and thickness direction line scanning were performed on the defective area, normal area, and interface area of ​​the iron oxide scale. The elemental content data of O, Fe, Cr, and Mn were collected by point scanning and the average value was taken. The composition of Cr2O3, Mn2O3, Fe2O3, and FeO was obtained by data conversion and calculation. The elemental content gradient was obtained by thickness direction line scanning to analyze the layered structure and phase composition of the iron oxide scale. The data conversion method is based on the mass fractions of O, Fe, Cr, and Mn elements measured by scanning electron microscopy energy dispersive spectroscopy, combined with the molar mass of oxides and stoichiometric ratios, and the distribution of Fe valence states through the molar equilibrium of oxygen. The specific steps are as follows: (1) Based on the mass fraction w and molar mass M of each element, calculate the number of moles of each element using the formula n = w / M to obtain n(O), n(Fe), n(Cr), and n(Mn); (2) Calculate the number of moles of oxygen consumed by Cr and Mn oxides: n(O)Cr = 3 / 2 × n(Cr), n(O)Mn = 3 / 2 × n(Mn), and the total oxygen consumption is n(O)CrMn = n(O)Cr + n(O)Mn; (3) After deducting the oxygen consumed by Cr and Mn, calculate the remaining number of moles of oxygen that can combine with Fe: n(O)Fe = n(O) -n(O)CrMn; (4) Establish the Fe valence equilibrium equation: Let Fe 2+ The number of moles is n (Fe 2+ ), Fe 3+ The number of moles is n(Fe) 3+ Based on the conservation of total Fe and the balance of oxygen, a system of equations is established: n(Fe) = n(Fe 2+ )+n(Fe 3+ ) n(O)Fe = n(Fe 2+ ) +3 / 2× n(Fe 3+ )Fe Solving for n (Fe) 2+ ) and n (Fe 3+ ); (5) Calculate the molar number and mass fraction of each oxide: n(FeO) = n(Fe 2+ )、n(Fe2O3) =1 / 2× n(Fe 3+ ) After calculating the preliminary content based on the molar mass of each oxide, the final mass fractions of Cr2O3, Mn2O3, Fe2O3, and FeO were obtained through normalization. 5) Integrate the microstructure obtained in step 3) with the energy dispersive spectroscopy data obtained in step 4) to clarify the cause of iron oxide scale defects and complete the comprehensive characterization of iron oxide scale on the surface of hot-rolled stainless steel black skin.

2. The comprehensive characterization method for oxide scale on the surface of hot-rolled stainless steel black skin according to claim 1, characterized in that: The ultrasonic cleaning time in step 2) is 2 to 3 minutes.

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