Annealing Line Research Articles

PSO-FDM (Particle Swarm Optimization-Finite Difference Method)-Based Simulation Model of Temperature and Velocity of Full-Scale Continuous Annealing Furnace

Improving the accuracy of the temperature field prediction model for continuous annealing line strips and enhancing the model’s adaptability to full-size strips are key technical challenges in continuous annealing lines. This paper developed a continuous annealing temperature prediction model based on a variable step-size strategy for the heating section, even-heat section, slow-cooling section, and fast-cooling section of the continuous annealing line. To improve the prediction accuracy for different strip sizes, the PSO optimization algorithm was employed to determine the optimal heat transfer coefficient for each strip size. Additionally, due to the limited production of certain strip gauges, providing insufficient data for optimization, this study introduces a combined file approach to address gauge vacancies. The experimental results indicate that the optimized model with variable step size can control the absolute prediction error to less than 4 °C, improving prediction accuracy by 61.9% and prediction speed by 26.8% compared to the traditional equal-step prediction model. This study verified that the merger method is effective for addressing side gauge vacancies, while the proposed method is suitable for resolving middle gauge vacancies. The main technical contribution of this study is the establishment of a high-precision prediction model for continuous annealing temperature of variable step length strips, ensuring high temperature control accuracy for full-gauge strips when passing through the continuous annealing production line.

Experimental Measurement of Emissivity of Polished Steel Strips from a Continuous Annealing Line.

The long-term use of steel strip in various industries makes it an important semi-finished product, which makes it necessary to improve its chemical composition and mechanical properties, reduce its thickness and weight, expand the range of new types of steel strip and increase its production. This entails a large number of technological operations dependent on precise temperature measurement and control. In some industrial plants, the steel strip is in continuous motion, which makes the use of contact measuring devices impossible. When using non-contact measuring devices such as pyrometers or thermal imaging cameras, the emissivity of the materials being measured is a problematic parameter, as setting an incorrect emissivity value to the measuring device results in inaccurate temperature readings. The essence of this research was to establish a measurement method and to perform experimental measurements of the emissivity of a polished steel strip used in a continuous annealing line, the subsequent processing of the data from these measurements and their evaluation. The emissivity measurements were carried out for 5 types of steel strip of different parameters, while the measurement itself was carried out in the long wavelength range of 7.5-14 µm and at strip temperatures of 100-300 °C. Depending on the type of steel strip, the mean emissivity values ranged from 0.0835- to 0.1143. The emissivity of the steel strip increased with increasing strip temperature, and it was not a linear dependence. The emissivity values determined in this research could be applied to measuring equipment in actual production, which could improve the accuracy of temperature measurement in the heat treatment of polished steel strip. Thermal camera measurements in the long wavelength range, taking thermal images and their processing and determining the emissivity value of polished steel strips are the parts of this research that make it different from other already published research.

Investigating the sensitivity of dual-phase steel microstructure to continuous annealing line parameters

The mechanical properties of dual-phase (DP) steels are influenced by ferrite grain size, martensite distribution/morphology, and strength differences between ferrite/martensite phases. A sensitivity study was conducted on a continuous annealing line cycle of DP steel manufacturing process to understand the effect of individual segments on the final microstructural features. From the study results, an optimised cycle was developed to obtain a DP steel microstructure with reduced martensite banding, fine ferrite grain size and less untempered martensite. The main modifications are austenite soaking temperature above Ac3 temperature, higher cooling rate of the slow cooling stage, increased soak time at ferrite soaking temperature, and higher over-aging temperature. The optimised cycle produced DP steel microstructure with reduced martensite banding at a similar phase fraction (35%) and decreased ferrite grain size from 8.4 to 5.1 µm. This improved the mechanical properties, increasing tensile strength from 789 ± 21 to 831 ± 14 MPa and total elongation from 15 ± 3 to 20 ± 2%.

Scheduling of parallel continuous annealing lines with alternative processing modes to optimize efficiency under tardiness constraints

Continuous annealing is a core process in steel cold-rolling facilities. It is used to set desired material properties by passing coils of flat steel through a high-temperature furnace in a precisely controlled manner. High efficiency is achieved by feeding the steel through the furnace in the form of a continuous strand. For this purpose, the coils are welded together before being fed into the furnace. Whenever two consecutive coils are incompatible, a special dummy coil called stringer is used to connect them, which reduces efficiency and adds costs and emissions. We consider the scheduling of coils with specific due dates and alternative order-specific processing modes on parallel heterogeneous lines. The problem is to simultaneously assign coils to lines and sequence them on these lines with a defined processing mode with adherence to tardiness constraints while minimizing the number of stringers needed. To address this problem, we formulate a mixed-integer linear program based on a model from the literature and propose a two-phase heuristic solution procedure. The procedure combines an opening phase using shortest path algorithms and local search with an improvement phase based on problem-specific decompositions according to the principles of an Fix-and-Optimize procedure. The results show that our heuristic outperforms a state-of-the-art commercial solver in finding good solutions in short computation time. Based on a sensitivity analysis for several industry-inspired data sets, we observe a trade-off between efficiency and tardiness which strongly depends on the operating conditions.

A Numerical and Experimental Sensitivity Analysis on Electrochemical Neutral Pickling for Stainless Steel Using Secondary Current Distribution

Electrochemical neutral pickling (ENP) is an extended process used in the stainless steel industry to remove the oxide surface layer formed during the annealing process. The absence of strong acids such as nitric acid results in a minimum loss of base material, improve operators working conditions, and reduces the pickling environmental impact, while keeping relatively low operation and maintenance costs. However, despite its multiple benefits, ENP may become the bottleneck for integrated annealing and pickling line, especially when processing thin austenitic and duplex steels. Fortunately, there is room for improvement as the total Faradaic efficiency of the process in typical operation is in general low (around 20 – 30 % for austenitic materials) [1]–[3], while estimations of the energy efficiency are as low as to 1 - 2 % [4].This work provides guidelines to the design and operation of ENP cells to increase the oxide removal rate. We focus on the effect of the geometry and some operational parameters, using secondary current distribution. The simulations are validated against experimental, and a sensitivity analysis of the oxide removal rate with respect to the geometry and some key operational parameters is performed. The electrode-to-strip and the inter-electrode distances were found to have a determining effect on the total cell resistance and on its so-called geometric efficiency (relative current reaching the strip). Moreover, an optimal range for the ratio of these distances is proposed. In addition, the strip thickness and width were found to significantly affect the cell performance, especially for the thinnest gauges.

The Application of Physical Simulation in Hot and Cold Rolling of Steels

Physical simulation is a well-known tool for cloning industrial rolling conditions and generating large quantities of useful data. Physical simulation allows not only considerable time and resources savings in the development process but also product quality improvement. Additionally, physical simulation enables risk-free experiments and at negligible cost when compared to full-scale trials. In this paper, three physical simulation cases are presented. All cases are applicable to steel hot rolling or post cold rolling: (1) a strip rolling simulation where the roll forces are predicted, (2) a plate rolling simulation aiming improvement in mechanical properties and (3) a continuous annealing line case where the increase in the line speed generated a large raise in profit and productivity.

Inverse parameter identifications and forward strip temperature simulations of the continuous annealing line with physics-informed neural network and operation big data

The continuous annealing line is a key equipment in industrial metal heat treatment. It is a large-thermal-inertia cascade thermal system, and has minute-level time hysteresis between adjustable control parameters and outlet strip temperature, which is challenging for real operation. Large strip temperature deviation from targets will greatly damage the mechanical properties and the surface coating quality of the strip. Traditional methods such as energy balance method, computational fluid dynamics and end-to-end data-driven model are hard to solve the problem due to complex parameter settings, large computing costs and low interpretability, respectively. Here, a novel way seamlessly coupling physical model and data based on physics-informed neural network is used to solve the problem. The mathematical-physical model of the continuous annealing line is developed and a physics-informed neural network model is built to solve inverse problem to identify the heat transfer coefficient of the continuous annealing line. The strip temperature is numerically simulated both with physics-informed neural network model and computational fluid dynamics model. The simulated results are compared with the measured data. The simulated outlet strip temperatures of the two models agree well with the measured data and the accuracy of the identified heat transfer coefficient is verified, where the computational fluid dynamics model has higher accuracy. The physics-informed neural network models developed here will benefit intelligent feedforward cascade control of the continuous annealing line and improve the strip quality in real productions.

Factors influencing the resistance of dual-phase steels to edge cracking

The key microstructural factors affecting the resistance to edge cracking of two cold-rolled dual-phase (DP) steels, a DP600 and a DP800 DH, were investigated. The resistance to edge cracking was characterised by the hole-expansion ratio, λ, which was measured according to ISO 16630:2017. It was shown that for both steels λ could be greatly improved by adjusting the temperatures in the continuous annealing line. However, the adjustments also resulted in changes in the mechanical properties of the materials, in particular the yield strength, Rp0.2. For both materials the increase in λ correlated with a decrease in the proportion of martensite at the stamped edge, which was approximated by the total proportion of martensite and retained austenite in the bulk microstructure, and an increase in the proportion of bainite. There was no correlation between λ and the hardness of the ferrite and martensite in either steel. For the DP800 DH an increase in λ correlated with (i) a decrease in the intensity of the crystallographic texture, characterised by maximum orientation distribution function, f(g)max, and the proportions of the microstructure corresponding to the α and γ fibres, and (ii) a decrease in grain size resulting from a more complete austenitisation of the microstructure. For the DP600 there was no correlation between the crystallographic texture and λ.

The effect of scrap originating trace elements on the properties of low alloyed steels

The present intention to reach fossil-free steel manufacturing will inevitably result in an increase in the use of steel scrap as a raw material for steel production. Consequently, the amounts of elements, seen as impurities, will increase in steels. This has already been seen in electric arc furnace (EAF) processed steels, where the Cu and Sn levels have doubled in some cases after 1980’s. This may cause problems, as it is well-known, that some impurity elements have harmful effects on the properties of steel. This has been widely studied in low-alloy steels containing chromium and molybdenum which are widely used in components for the petroleum and electrical power generation applications. However, limited number of studies have been performed on formable steel grades, and the published reports/articles have mostly concentrated on the effects of P and B. Thus, there is still a need to understand the roles of other impurity elements. In the present study, a formable C-Mn steels containing additions (either individually or in combination) of Cu and Sn is investigated. The samples were cold rolled and annealed following typical time-temperature profiles of modern continuous annealing lines. Mechanical and forming properties (incl. bending and cupping tests) are determined as well as elemental profile analysis is conducted. The results identify that minor additions of impurity elements, in this case Cu and Sn, does not affect the mechanical and forming properties of low alloyed formable steel grades considerably.

Production of transverse magnetic field annealed brass band and how the production process influences the mechanical properties, as well as their correlation and compliance

This paper examines the effect of the thickness h, the deformation reached during the last cold rolling operation ε and the final normalized annealing mode v on the mechanical properties of brass band. The paper looks at the correlation between the ultimate strength σв, elongation and hardness HV in terms of their meeting the specification. The production of L68 brass band involved semi-continuous casting, hot rolling to the thickness of 6 mm and cold rolling to h = ~0.56–0.96 mm with intermediate annealing at H = 1.5–2.7 mm ensuring the deformation ε within 62 to 72%. The intermediate and final continuous annealing lines are equipped with three-phase TFIH (Transverse Flux Induction Heating) units. An intermediate annealing mode has been established that ensures the grain size of 40–80 μm. The strip travel speed was varied as follows during final annealing: ν = 30–46 m/min. The authors determined the stochastic dependences of the properties on ν , h and ε, as well as the annealing modes that are necessary to achieve each property. Actual correlations HV(σв), δ(σв) within the specified ranges were considered versus the properties of the band produced by deformation. It was established that the cold rolling process, together with the heat treatment mode, has an effect on the properties of annealed band and, vice versa, on the heat treatment mode that is necessary to achieve the required properties. With hardness being the same, the induction heat annealed brass band has a higher ultimate strength в than the band produced by normalized final deformation. As a result, the linear relationship σв = f(HV) of the rolled band produced by normalized final annealing is shifted in relation to the optimum position of such relationship within the specified limits adopted in the period when the rolled band was exclusively produced by final deformation. Consequently, the final annealing mode ranges that are necessary for achieving certain properties within the specified ranges, do not match, thus making the task of having the resulting properties hit the above ranges more challenging. Thus, it is proposed to use online control and annealing control to eliminate rejects, and to use the annealing mode-required property relationships obtained from the given data for setting up the initial process mode.

Monte Carlo Simulation of Dust Particles in a Protoplanetary Disk: Crystalline to Amorphous Silicate Ratio in Comets

Abstract The observationally inferred crystalline abundance in silicates in comets, which should have been formed in the outer region of a protoplanetary disk, is relatively high (∼10%–60%), although crystalline silicates would be formed by the annealing of amorphous precursors in the inner disk region. In order to quantitatively address this puzzle, we performed a Monte Carlo simulation of the advection/diffusion of silicate particles in a turbulent disk in a setting based on the pebble accretion model: pebbles consisting of many small amorphous silicates embedded in an icy mantle are formed in the outer disk region, silicate particles are released at the snow line, crystalline silicate particles are produced at the annealing line, silicate particles diffuse beyond the snow line, and they eventually stick to drifting pebbles to return to the snow line. In the simple case without sticking and with steady pebble flux, we show through the simulations and analytical arguments that the crystalline components in silicate materials beyond the snow line are robustly and uniformly ≃5%. On the other hand, in a more realistic case with sticking and with a decaying pebble flux, the crystalline abundance increases to ∼20%–25%, depending on the ratio of the decay to diffusion timescales. This abundance is consistent with the observations. In this investigation, we assume a simple steady-accretion disk. The simulations coupled with the disk evolution are needed for a more detailed comparison with observed data.

On online texture quality control and management of continuous processes for the production of non-ferrous sheets

The results of the studies of the connection of texture parameters and the quality characteristics of commercial batches of the finally rolled or finally annealed tape made of fcc- materials, which differ in the value of stacking fault energy, phase composition and processing technology are considered. Strips of brass L68 and L63 containing ~62.5 and ~64% of copper and aluminum alloy AMg2 were hot rolled from ingots that were cast in water-cooled and electromagnetic casting mould respectively. Cold rolling was carried out with intermediate annealing to the state of complete recrystallization in the resistance heating furnace (AMg2) or in the induction annealing line (brass) to the finite thickness: ~0.3 mm (AMg2) and 0.6–1.0 mm (brass), varying the degree of the finite plastic deformation. During the final annealing of the brass strip in the line of induction annealing and etching, the speed was varied. The tape was tested for breaking, Vickers hardness number, cutting-out and draw-forming of “glasses” in the combination die. In particular, we controlled the characteristics of the anisotropy of plastic properties, the magnitude of the relative earing and intensity 200 and 220 lines of the texture on the general purpose diffractometers in Fe Karadiation by the Schulz reflection method. The texture parameters T and Т, expressing the ratio of the components of the recrystallization texture and the rolling texture of fcc-materials, were determined. It was found that the accuracy of texture control during annealing of brass L63 rolled products using the same texture parameters is reduced in comparison with materials with high stacking fault energy due to the small fraction of {100} <hkl> recrystallization texture component and the dependence of the structure and phase ratio on small, 1.5% for copper, fluctuations in the chemical composition. At the same time, it remains high enough to control the process of organizing feedback when managing the process online. That is, texture control can be extended to annealing materials with very low stacking fault energy, which contain the second phase. At the same time, there are serious restrictions on the degree of deformation during rolling and on the annealing regimes, in which it is reasonable to use the texture control, as well as the absolute need to determine regression dependences of the properties on the texture when changing the technological processing scheme. In each individual case, the necessary condition for organizing the texture control is the presence of the “close” correlation between the texture parameter and the controlled characteristic. The proposed structural scheme of the texture control during rolling is discussed.The authors gratefully acknowledge S. G. Khayutin, Doctor of Technical Sciences, a founder of a texture control research area in Russia, globally pione ering in texture control of mechanical properties, taking part in acquiring some experimental data used in the research paper, and giving important recommendations at the initial stage of preparing the presented paper.

An Incremental Grey-Box Current Regression Model for Anomaly Detection of Resistance Mash Seam Welding in Steel Mills

Annealing and galvanization production lines in steel mills run continuously to maximize production throughput. As a part of this process, individual steel coils are joined end-to-end using mash seam welding. Weld breaks result in a production loss of multiple days, so non-destructive, data-driven techniques are used to detect and replace poor quality welds in real-time. Statistical models are commonly used to address this problem as they use data readily available from the welding machine and require no specialized equipment. While successful in finding anomalies, these statistical models do not provide insight into the underlying process and are slow to adapt to changes in the machine’s or material’s behavior. We combine knowledge-based and data-driven techniques to create an incremental grey-box welding current prediction model for detecting anomalous welds, resulting in a powerful and interpretable model. In this work, we detail our approach and show evaluation results on industrial welding data collected over a period of 15 months containing behavioral shifts attributed to machine maintenance. Due to its incremental nature, our model resulted in two-thirds fewer rejected welds compared to statistical models, thus greatly reducing production overhead. Grey-box modeling can be applied to other welding features or domains and results in models that are more desirable for the industry.

Evidence-Based and Explainable Smart Decision Support for Quality Improvement in Stainless Steel Manufacturing

This article demonstrates the use of data mining methods for evidence-based smart decision support in quality control. The data were collected in a measurement campaign which provided a new and potential quality measurement approach for manufacturing process planning and control. In this study, the machine learning prediction models and Explainable AI methods (XAI) serve as a base for the decision support system for smart manufacturing. The discovered information about the root causes behind the predicted failure can be used to improve the quality, and it also enables the definition of suitable security boundaries for better settings of the production parameters. The user’s need defines the given type of information. The developed method is applied to the monitoring of the surface roughness of the stainless steel strip, but the framework is not application dependent. The modeling analysis reveals that the parameters of the annealing and pickling line (RAP) have the best potential for real-time roughness improvement.

CFD investigation of a vertical annealing furnace for stainless steel and non-ferrous alloys strips – A comparative study on air-staged & MILD combustion

A 3D CFD modelling approach of a vertical high temperature furnace in a bright annealing line for stainless steel is presented in this study. The observed line has a maximum production rate of 4.5 tonnes per hour and an overall thermal input of 1.5 MW which is provided by gas burners. The muffle furnace is an indirect heating system equipped with flameless burners, which can operate at nominal power and in off-design. The burners are arranged in four heating zones, which can be controlled separately. Pure hydrogen is used inside the muffle as protection gas around the metal. The metal strip is modelled as a highly viscous fluid with temperature-dependent properties including the specific heat, thermal conductivity and emissivity. Good accordance with measurements from an industrial line was achieved with several values from furnace thermocouples and the transient heating characteristic of a selected strip dimension, thus, demonstrating the applicability of the CFD model for the present investigations. Further simulations were performed with air-staged combustion for an evaluation of combustion and furnace efficiency as well as the thermal load on the furnace’s components. It was found that the efficiency decreased by about 0.5% using flameless combustion. The results also showed a decreased peak temperature of about 1155 °C and a reduced heat flux of about 23.3 kW/m2 (−5%) on the muffle when Moderate or Intense Low oxygen Dilution (MILD) combustion is applied. The present methodology presents an efficient approach for investigation and optimization of indirect heated muffle type furnaces.

Understanding the effect of annealing temperature on the mechanical properties of thin strips made from CuNi12Zn24 alloy going through a continuous line

Alloys of copper with nickel (5–35%) and zinc (13–45%) are known, which are called nickel silver (translated from German Neusilber – “new silver”). The alloy CuNi15Zn20 is widely used. Industry uses materials similar in properties to the alloy CuNi15Zn20, but cheaper due to the reduction in the content of nickel and copper, for example, CuNi12Zn24. Under the conditions of Kirov Non-Ferrous Metals Processing Plant (JSC “KNFMP”), there is a technical possibility to obtain products from ingots to thin strip from these alloys. To obtain thin strips 0.7–0.043 mm thick in a soft state with a uniform distribution of mechanical properties along both length and width, as well as with a high surface quality, a continuous annealing line is used. The line is usually used for annealing thin strips of copper and brass of various alloy grades. The work was carried out on the selection of temperature and speed modes of the line for heat treatment of tapes from the alloy CuNi12Zn24, not typical of the line, in order to obtain products that meet consumer requirements. The effect of annealing temperature on the change in the mechanical properties of a cold-rolled strip 0.1 mm thick made of the alloy CuNi12Zn24 was studied. The graphs of changes in the yield strength (σt), tensile strength (σb) and elongation (δ) versus annealing temperature (t) of the strip after cold rolling with a relative compression of 83% on a four-roll reversible mill 160/650×710 are obtained. The regularities are established and the dependences of the main indicators of the mechanical properties on the temperature of annealing of strips are obtained. The research results made it possible to develop a rational heat treatment regime on the continuous annealing line of JSC “KNFMP”. Application of the developed annealing regime made it possible to manufacture and supply to the European consumer an experimental batch of tapes with a cross section of 0.1×400 mm in the soft state from the alloy CuNi12Zn24 according to the standard EN 1652-97 in the amount of 1000 kg. There were no comments from the consumer about quality of the strips; currently, orders for the production of these products continue to be received at JSC “KNFMP”.

Mechanical Properties of Ausformed Carbide-Free Bainite

In this study, the effect of ausforming temperature and strain on the bainite transformation in a model alloy containing 0.39 pct C, 2.0 pct Mn, 1.9 pct Si, and 0.5 pct Cr (mass percent) was investigated. Samples were deformed at 300 °C, followed by isothermal holding to monitor the phase transformation kinetics. Dilatometry results confirm the acceleration of transformation kinetics when the material is deformed to small strains (0.05 to 0.20). Tensile testing results show that, for long isothermal holding times, no improvement in strength or ductility is achieved as a result of ausforming in comparison with the non-deformed state. However, the benefit of ausforming lies in its ability to accelerate the bainite transformation kinetics and to reduce the isothermal holding time needed to avoid premature fracture due to the presence of a high volume-fracture of fresh martensite. Fracture analysis reveals ductile fracture, with cracks along martensitic regions within the microstructure. The distribution of martensite in the final microstructure was associated with bands of Mn segregation. This banding effect was less visible within the ausformed microstructure because bainite was able to nucleate within both the high- and low-Mn regions. A second advantage of ausforming is, therefore, to reduce the microstructure heterogeneity associated with compositional banding within the steel sheets and, by so doing, eliminate crack propagation within the bands. In spite of the fact that ausforming accelerates the bainite transformation kinetics, the required transformation times for the present steel are still too long for use within industrial continuous annealing lines.

The Fundamental Thermal Equation of Metal Strip Cooling

Abstract The cooling process of metallic strips in cooling section of continuous annealing lines can be calculated using an equation which is based purely on the fundamentals of engineering heat transfer and can be solved by simple numerical methods. It includes thermal conduction, convective cooling, radiative cooling and phase changes. Several issues of plant- and process engineering can be considered with this equation. The effects of different measures to increase the cooling rate by changing the fluid flow velocity, the ambient temperature, the temperature of the cooling fluid and the properties of the cooling fluid can be investigated directly using solutions of the presented equation. The risk of a premature start of the cooling due to heat conduction in the direction of strip flow can also be calculated. Numerical solutions for the equations are given for applications in process modelling or plant design.

Improvement of Radiant Heat Efficiency of the Radiant Tube Used for Continuous Annealing Line by Application of Additive Manufacturing Technology

This study presents the application of additive manufacturing (AM) technology to a W-type INCONEL radiant tube (RT) used to improve its radiant heat efficiency. Appropriate dimensions of honeycomb structure were determined from finite element (FE) analysis and the resulting increase in radiant heat was computed. The honeycomb patterns on the RT surfaces were printed using the directed energy deposition (DED) method. Radiant heat efficiency of a prototype RT with a honeycomb pattern printed was examined in a pilot furnace emulating the continuous annealing line (CAL). Finally, soundness of the prototype RT was tested on-site on the actual the CAL of No. 3 CGL in POSCO Gwangyang Steel Works. The results revealed that partial FE analysis, which predicts the amount of radiant heat by partially modeling the RT structure rather than modeling the entire RT structure, is suitable for overcoming the limitation of the computer memory capacity and calculating the design parameters of honeycomb patterns. The DED is suitable for printing honeycomb patterns on RT with large and curved surfaces. The average amount of gas consumed to maintain 780 °C and 880 °C for 1440 min was reduced by 10.42% and 12.31%, respectively. There were no cracks and no gas leaks on the RT surface in an annual inspection over three years.

Rarely Occurring Defects on Tinplates

This paper deals with rarely occurring defect on the surface of the tinplates produced in the conditions of U. S. Steel Košice, s.r.o. at the cleaning section of the continuous annealing line (CAL), which was caused by burnt. Random occurrence of such surface imperfections was observed within the short period of time (two months) for various continuous annealed tinplate grades, i.e. TH415, TH435 or TH550 with the thickness of the materials in the interval of 0.172 – 0.240 mm. Surface defects caused by burnt (thermal attack) manifested in different ways. Their appearance on the sheet surface was spot with regular or irregular circle shape, as well as line with the length of up to 2 mm oriented not directly in the rolling direction. By observing of the imperfections in scanning electron microscope (SEM) it was found that the surface of the steel substrate was always compact in the defective areas with clearly visible rolling lines. Foreign material, EDS analysis of which revealed that its chemical composition was on the basis of Fe and Cr, was deposited on the surface of the base material. The appearance of the foreign material shape indicated that it was in the liquid state at the time of incidence on the steel substrate surface. Metallographic analysis from the longitudinal metallographic sections through the defective areas revealed that the substrate under the spot defect with regular circle shape had thermally influenced microstructure. Similar manifestations of the microstructure influence were observed for the line defects. In the case of the spot defect with irregular shape, the microstructure of the material was not influenced, what indicated that the particle of liquid metal was solidified at the time of incidence on the plate surface. After revising technical conditions of the cleaning section of CAL, this type of imperfection was suppressed.