Surface Quality Of Steel Products Research Articles

Novel Terahertz Nondestructive Method for Measuring the Thickness of Thin Oxide Scale Using Different Hybrid Machine Learning Models

Effective control of the thickness of the hot-rolled oxide scale on the surface of the steel strip is very vital to ensure the surface quality of steel products. Hence, terahertz nondestructive technology was proposed to measure the thickness of thin oxide scale. The finite difference time domain (FDTD) numerical simulation method was employed to obtain the terahertz time-domain simulation data of oxide scale with various thickness (0–15 μm). Added Gaussian white noise with a Signal Nosie Reduction (SNR) of 10 dB was used when simulating real test signals, using four wavelet denoising methods to reduce noise and to compare their effectiveness. Two machine learning algorithms were adopted to set up models to achieve this goal, including the classical back-propagation (BP) neural network algorithm and the novel extreme learning machine (ELM) algorithm. The principal component analysis (PCA) algorithm and particle swarm optimization (PSO) algorithm were combined to reduce the dimensions of the terahertz time-domain data and improve the robustness of the machine learning model. It could be clearly seen that the novel hybrid PCA-PSO-ELM model possessed excellent prediction performance. Finally, this work proposed a novel, convenient, online, nondestructive, noncontact, safety and high-precision thin oxide scale thickness measuring method that could be employed to improve the surface quality of iron and steel products.

Dynamic Wetting of High-Al Steel by CaO-SiO2- and CaO-Al2O3-Based Mold Fluxes

Wetting of steel by mold fluxes affects the surface quality of steel products. Reaction between [Al] and mold fluxes in the continuous casting of high-Al steel leads to a dynamic interfacial phenomenon. This work investigated the wetting of the high-Al steel substrate by CaO-SiO2-based and CaO-Al2O3-based mold fluxes. The interfacial tensions were estimated and related to their high-temperature structures. The interface morphology revealed that CaO-SiO2-based mold flux and CaO-Al2O3-based mold flux with CaO/Al2O3 = 1 exhibited deep reaction zones in the steel substrates and irregular flux/steel interfaces, while CaO-Al2O3-based mold fluxes with CaO/Al2O3 = 2 and 3 showed smooth interfaces and superficial reaction zones. The sedation of dynamic wetting behavior in Samples 3 and 4 was related to the high viscosity in the flux droplet after the initial composition change, which led to the stabilization of the interface.

Defect detection of hot rolled steels with a new object detection framework called classification priority network

The defect detection is extremely crucial to analyze the surface quality of steel products, it not only affects the subsequent production, but also affects the corrosion and wear resistance of the end products. However, the low classification and detection rates of the conventional algorithms cannot satisfy the demands of some steel production lines, such as hot rolled plates. To improve the accuracy of defect inspection, we present a new object detection framework, classification priority network (CPN), and a new classification network, multi-group convolutional neural network (MG-CNN), to inspect the defects of steel surface. In CPN, the image is first classified by MG-CNN, which trains different groups of convolution kernels separately to extract the feature map groups of different types defects. Then, according to the classification result, the feature map groups that may contain defects is separately input into another neural network, which is build based on yolo, to regress the bounding boxes of the corresponding defects. Extensive experiments are carried out on samples from different steel production lines indicate that the proposed method had yielded best performances compared with prior methods used in defect inspection. The average detection rates of surface defects in hot rolled plates and strips are higher than 94%, and the classification rates of in above production lines are above 96%.

Influence of Ni and Cu on Oxide-Scale Structure of Steel

In steel products, high surface quality is required in addition to characteristics corresponding to intended use. For improving surface quality of steel products, it is important to control structure of oxide scales. In hot rolling process, heterogeneous structure of oxide scales, such as uneven scale/steel interface, causes surface defects. For steels containing Ni, it has been reported that structure of oxide scales at the scale/steel interface is characteristic and complicated. Consequently, in this report, Ni and Cu, which are nobler than Fe, are focused on, and the effects of these elements on structure of oxide scales of steels are investigated. Fe-10mass%Ni and Fe-10mass%Cu alloys were oxidized at 1000°C or 1200°C in an atmosphere of 10%O2+20%H2O+70%N2 for 1800s. In all cases, uneven scale/steel interfaces are observed (see Fig.1). In case of Fe-Ni alloy, Ni-enriched metal phase of complicated form is observed in oxide scales. Concentration of Ni in metal phase increases as it approaches the surface. On the other hand, in case of Fe-Cu alloy, precipitation of Cu phase is observed. Cu precipitates as particles in oxide scale at 1000°C, while it precipitates as a layer at the scale/steel interface under oxide scale at 1200°C. With regard to the uneven scale/steel interface, it should be caused by the property that both Ni and Cu are nobler than Fe. The mechanism can be explained as follows: first, selective oxidation of Fe proceeds, and Ni or Cu enriches at the scale/steel interface. Then, internal oxides of Fe precipitate and Ni or Cu enriches around them. Finally, with proceeding of such heterogeneous oxidation, internal oxides of Fe and those locally Ni- or Cu-enriched area are simultaneously occluded into the scale/steel interface, and therefore uneven scale/steel interface forms. With regard to the different structure of oxide scales at the scale/steel interface in Fe-Ni and Fe-Cu alloys, that is, enrichment of Ni and precipitation of Cu, it should be caused by the difference of the solid solubility limit of Ni and Cu to austenite. According to the phase diagram of Fe-Ni alloy, Ni dissolves in austenite completely at both 1000°C and 1200°C. On the other hand, according to the phase diagram of Fe-Cu alloy, Cu dissolves in austenite partly at both 1000°C and 1200°C. Because the solid solubility limit of Cu to austenite is just about 10% at 1200°C, Cu which exceeds its solid solubility limit with proceeding of oxidation of Fe should precipitate as liquid phase at the scale/steel interface. Although both Ni and Cu are nobler than Fe, different structure of oxide scales forms at the scale/steel interface in Fe-Ni and Fe-Cu alloys corresponding to the solid solubility limit to austenite. Figure 1

Influence of Ni and Cu on Oxide-Scale Structure of Steel

Oxide scales exert important effects on the surface quality of steel products. In hot rolling processes in the steel industry, the heterogeneous structure of oxide scales, such as an uneven scale/steel interface, causes surface defects. For steels containing Ni, the structure of oxide scales at the scale/steel interface is characteristic and complicated. Consequently, this study focuses on Ni and Cu, which are nobler than Fe, and investigates their effects on the structure of oxide scales of steels. Fe-10mass%Ni and Fe-10mass%Cu alloys were oxidized at 1000 or 1200°C in an atmosphere of 10%O2+20%H2O+70%N2 for 1800 s. In all cases, uneven scale/steel interfaces were observed because both Ni and Cu are nobler than Fe. Furthermore, different structure of oxide scales was formed at the scale/steel interface between the Fe-Ni and Fe-Cu alloys depending on the alloys’ solid solubility limit to austenite.

Crucial Mechanism to the Eutectoid Transformation of Wüstite Scale on Low Carbon Steel

It is important to realize the transformation behavior of wüstite, because it greatly affects the final structure of the oxide layer and the surface quality of steel products. In the present study, the transformation behaviors of the wüstite layer are examined under nearly‐oxygen‐free conditions in order to simulate the cooling processes after the hot rolled strip is coiled. As the single phase wüstite is prepared at 950 °C, the 460 °C transformed oxide layer is composed of a mixture of iron and magnetite formed through eutectoid reactions. For the 750 °C‐fabricated wüstite, only magnetite is observed after transformation, without iron precipitates, and residual wüstite. It is speculated that the unusual transformation behavior of the low‐temperature‐made wüstite results from the pseudo‐structural intermediate phase transformation between wüstite and magnetite. This pseudo‐structure is a pre‐transformed wüstite and contains various concentrations of ferrous ion, which is determined by the fabrication conditions. During the hot strip mill process, wüstite is produced continuously from finish mill to laminar flow sections and culminated at 570 °C. Consequently, the final eutectoid transformation below 570 °C is dominantly controlled by the surface temperature ranging from 750 to 950 °C for low carbon steel.

横型遠心鋳造と縦型遠心鋳造の液相挙動の違い

A centrifugal casting method is a key technology for producing high quality rolls which are used in a rolling process in steel industry. Because the surface quality of steel products highly depends on the roll quality and it is affected by liquid motion in the centrifugal casting method, control of the liquid motion is essential for supply of defect free rolls to the steel industry. In this research, liquid flow in the centrifugal casting method has been numerically calculated for clarification of difference of liquid motion in a horizontal type centrifugal casting method and a vertical one. The mainly obtained results are as follows. The center of the gravity periodically oscillates and its cycle is longer than the rotation cycle of the mold in the both cases of the horizontal and vertical types. Its time average in the horizontal type locates above the center axis of the mold while that in the vertical type is the same with the center axis. Increase in apparent viscosity in the vicinity of the mold wall accelerates the damping of the oscillating motion of the center of the gravity.

Influence of Oxidation Temperature and Cr Content on the Adhesion and Microstructure of Scale on Low Cr Steels

Descalability of steels during hot rolling processes is essential in order to maintain the surface quality of steel products, because the remaining scale can be impressed into the steel, producing surface defects. Therefore, a quantitative evaluation of scale adhesion at high temperature is needed in order to find suitable heating conditions for hot rolling processes that promote the formation of scale that easily detach. Round bar shape specimens of low Cr steels were oxidized in a fuel combustion environment, and then compressed at high temperature to detach the scale. The area fraction of the remaining scale was evaluated by image analysis. Under a thick outer oxide scale, a thin continuous inner oxide layer, which consisted of Cr and Si oxides, strongly adhered to the substrate, resulting in poor descalability. On the other hand, if this inner scale changed into a discontinuous layer with a porous FeO layer beneath, the descalability improved. It was found that scales with greater descalability formed on steels containing less Cr and for oxidation at higher temperatures.

Motion of Fine Particles under Interfacial Tension Gradient in Relation to Solidification of Steel

It is important to understand the behavior of argon gas bubbles and non-metallic inclusions in front of the solidifying shell in order to improve the surface quality of steel products, in the steelmaking industry. Based on a series of fundamental researches on the motion of fine particles under the interfacial tension gradient, Mukai and Lin [1] proposed a new mechanism wherein the solute concentration gradient in front of the solidifying shell, which is the interfacial tension gradient between liquid steel and particles, plays an important role in the entrapment of argon gas bubbles and non-metallic inclusions by the solidifying shell in continuous casting of steel. Developing this idea, this paper proposes Mukai-value M for use in estimating the dependence of the interfacial gradient force required to entrap the particles on solute concentrations of steel. Use of this index enables rapid quantitative prediction of the behavior of argon gas bubbles and non-metallic inclusions in the continuous casting of steel.

Inhibition of hot cracking in grain oriented silicon steel containing a small amount of molybdenum.

In order to elucidate the mechanism by which the surface quality of steel products containing a small amount o f Mo is noticeably improved, hot cracking in the grain oriented silicon steels containing 0.013% Mo and in steel containing no Mo has been investigated by the impact experiment. In the impact experiment after slab soaking, and under elevated temperatures the intergranular fracturing of silicon steel containing a small amount of Mo is found to be less than that of Mo free steel.In Electron Probe Micro Analysis (EPMA) and Glow Discharge Spectrometry (GDS) measurements after slab soaking, Mo segregates preferentially in the vicinity of the surface in the silicon steel containing Mo.It is concluded that, because of the preferential segregation of Mo in the vicinity of the surface of silicon steel containing a small amount of Mo during slab soaking or hot rolling, intergranular fracturing is inhibited, and, consequently the surface quality of final products is noticeably improved.

Molybdenum compounds formed on the surface of silicon steel slab.

In an attempt to elucidate the mechanism by which the surface quality of steel products containing a small amount of Mo is noticeably improved, the precipitates formed on the surface of silicon steel slabs after coating with Mo compounds and the subsequent annealing under various atmospheres of N2, Ar and H2 gases have been investigated. Many fine spherical precipitates of O.05-10μm are formed on the surface of silicon steel slab after annealing under a H2 atmosphere, whereas agglomerated and complex precipitates are formed during slab annealing under N2 and Ar atmospheres. In the analyses of the precipitates by Electron Probe Micro Analysis (EPMA) and Energy Dispersive X-ray (EDX), Mo is detected together with the elements S, P, Se and Sb, which have very low solid solubilities in α-iron and the tendency to segregate to iron surface or to grain boundary. As Mo enhances the formation of precipitates consisting of Mo, S, P, Se and Sb on the surface of silicon steel slab and thereby scavenges the ferrite matrix and strengthens the grain boundary, it is considered that the slab annealing treatment after coating with Mo compounds will noticeably inhibit intergranular fracturing.