Application of low-magnification macroscopic inspection in continuous casting production

Abstract: Low-magnification inspection plays a vital role in steel inspection, especially continuous casting billet inspection. By examining the solidification structure of continuous casting slabs, the qualitative and quantitative relationship between the solidification structure and solidification conditions can be grasped. In this way, the solidification state of continuous casting molten steel can be purposefully adjusted and controlled to reduce casting defects and obtain a sound solidification structure. Therefore, analyzing the process information such as superheat degree and secondary cooling water intensity in continuous casting production is helpful to adjust the process level and maintain the normal operation of the equipment. It is of great significance to guide the production process and reduce the occurrence of low-magnification defects in finished materials.

Keywords: continuous casting billet; solidification structure; inspection method

In recent years, with the gradual transformation of my country from a big steel country to a powerful country, efficient continuous casting production has basically replaced the traditional die casting production process. How to ensure the quality of continuous casting billets to meet the requirements of rolling high-quality steel products in the subsequent processes has also This has become a matter of great concern to continuous casting workers. For different enterprises, due to different product structures and steel rolling levels, the quality requirements for continuous casting billets are also different. Through macro inspection and defect analysis of the solidification structure of continuous casting billets, guidance is provided on the formulation of subsequent processing techniques. During production, a large number of low-magnification samples are cut from various steel types and different slab sections, and acid etching low-magnification inspection technology is fully applied to detect the quality of the slabs to improve the quality of the slabs.

Application of low-magnification inspection in continuous casting production

The purpose of producing cast slabs is to obtain a high-quality low-magnification structure of relatively developed equiaxed crystals and columnar crystals whose growth is suppressed. And it is allowed to have qualities such as dispersed looseness, lighter macrosegregation, as few or no internal and surface cracks as possible, and as few and relatively evenly distributed inclusions as possible. The low-magnification acid etching inspection of continuous cast billets is used to determine the type, shape and distribution of internal cracks in the cross-section or longitudinal section of the billet, the macro-distribution of inclusions, the porosity and segregation conditions and the cross-sectional shape of the low-magnification structure. Evaluate based on the specific defect types, shapes and locations reflected in the low-magnification tissue. Through its relationship with solidification conditions, steel type, and solidification characteristics of cross-section, problems existing in continuous casting process parameters, pouring operations, and equipment operating conditions can be judged in a timely manner. Corresponding measures should be taken to improve and enhance the quality of cast slabs, and at the same time provide a basis for the development of new varieties of continuous casting processes.

The application of low-magnification inspection in continuous casting production mainly includes the following aspects:

1. Determine the internal quality of continuous casting billet

The shape of defects and the degree of defects in continuous casting billet are determined. This is a commonly used method now. This is because the low-magnification inspection method is simple, fast and accurate. The low-magnification inspection method is used to determine various internal defects of the slab, such as cracks, shrinkage cavities, etc. Various defect levels are determined according to the low-magnification inspection standards of continuous casting billets, and the material specifications and scrap standards of continuous casting billets are determined based on these defect levels.

2. Low-magnification inspection can quickly provide important information on processes and equipment in continuous casting production.

Since continuous casting production is a relatively complex process, the quality of the cast slab is affected by many factors, such as the continuous casting process, equipment, and the purity of the molten steel. The analysis of continuous casting billet quality problems is a more complex issue. Sometimes a defect is caused by multiple factors. In actual production, defects in cast billets are the result of a combination of multiple factors, such as the superheat of molten steel, secondary cooling water, Pull speed and so on. Therefore, it is difficult to correctly analyze the causes of defects by only looking at low-magnification samples. In actual production, the shape and location of low-magnification defects are often combined with a comprehensive analysis of continuous casting machine equipment and processes. Through the accumulation of large amounts of data, the causes of various defects can be accurately analyzed. On the one hand, the defect morphology is accurately displayed through low-magnification samples, and at the same time, the mechanism of various defects generated during the continuous casting process is understood, providing conditions for accurate defect determination.

3.The low-magnification inspection method also plays an important role. A specific low-magnification inspection method is used to display the solidification crystal structure of the continuous casting billet. As we all know, in the production of continuous cast slabs, the heat transfer, operation, cooling, solidification and crystallization processes of the slabs are all carried out according to certain laws, and the final crystallization form must be determined by factors that affect this law. Different continuous casting billet structures have different effects on the quality of the cast billet. For example, it is generally believed that maintaining a certain proportion of equiaxed crystals in the center of the cast slab can reduce the formation of shrinkage cavities and central segregation, and that developed columnar crystals increase the tendency of central shrinkage cavities and internal cracks. Therefore, the crystallographic structure of the slab can be controlled by adjusting various process parameters during the continuous casting process. Due to different cooling conditions of different types of continuous casting machines, there are certain differences in the structure of the slab. It is very meaningful to study these structures to control the cooling conditions. Controlling the structure of the continuous casting billet and its influence on the quality of the rolled material is an important topic in the development of continuous casting technology. The application of low-magnification inspection to reveal the formation mechanism of structural defects in the billet is an important link in this research.

Low magnification inspection of continuous casting billet

Traditional macroscopic inspection of cast slabs usually includes: acid etching inspection method, sulfur print inspection method, etc. Acid etching inspection methods include: hot acid etching inspection method and cold acid etching inspection method. The macroscopic inspection methods for the solidification structure and defects of continuous cast steel mainly use low-magnification hot acid etching inspection method, sulfur print inspection method and cold acid etching inspection method. The test surface to be inspected is generally the cross section of the slab.

Hot acid etching test method

The hot acid etching inspection method of continuous casting billet is to cut a 15~20mm thick cross section of the cast billet with a surface roughness of Ra≤1.6um, place it in a 1:1 hydrochloric acid aqueous solution, and add it to 60~80° for corrosion. The specific time depends on the low-magnification structural defects of the cast billet. The inspection standard used for the inspection of continuous cast steel spacing blanks is GB226-91 “Acid Etching Inspection Method for Low Magnification Structure and Defects of Steel”. YB/153-1999 “Low-magnification structural defect rating chart of high-quality carbon alloy structural steel continuous casting billet”. YB/4002-1991 “Low magnification structural defect rating chart of continuous cast steel billet”. The solidification structure defects of the continuous casting billet displayed by this method include: shrinkage cavities, center segregation, center porosity, center cracks, middle cracks, corner cracks, subcutaneous cracks, subcutaneous bubbles and inclusions, etc.

Sulfur print test method

The sulfur print test method of continuous casting billet is to cut a 15~20mm thick cross section of the cast billet with a roughness of Ra≤1.6um. The inspection standard adopted is GB/4236-1984 “Sulfur Print Test Method for Steel”. The sulfur print test method can be used to test the distribution of sulfides in steel. The principle is to use dilute sulfuric acid to react with the sulfide in the steel to release hydrogen sulfide gas, and then let the hydrogen sulfide gas interact with the silver bromide on the photographic paper to generate brown silver sulfide deposits. The brown marks on the photographic paper are where the sulfides are located. When photographic paper shows large, heavy brown marks, it indicates severe segregation and high levels of sulfur in the sample. The sulfur print test is a qualitative test, and the sulfur print test results cannot be used to estimate the sulfur content in steel. Sulfur in continuous casting slabs mainly exists in the form of manganese sulfide and iron sulfide.

Cold acid etching test method

The cold acid etching inspection method is a simple method to check the macrostructure and defects of steel. It uses acid etching at room temperature to display low-magnification macrostructure defects. Cold acid etching corrosives are provided in GB226-91 “Acid Etching Test Method for Low Magnification Structure and Defects of Steel“. This method consumes less energy, causes less pollution, and is in line with the economic and environmentally friendly steel technology development concept.

Low-magnification macroscopic defects of continuous casting billets and countermeasures

The shape of the solidification structure of the continuous cast slab is one of the important factors affecting the quality of the slab and the properties of the rolled product. The following introduces the causes of typical defects in continuous casting billets and strategies for mitigating defects. As shown in Table 1.

Table 1 Typical slab defect characteristics, causes and solutions

numberCast billet defectsDefect characteristicsCausepreventive solution
1shrinkage cavityThere are irregular holes in the center of the transverse acid leaching specimen.High degree of superheat and uneven cooling of secondary cold waterSuppress the growth of columnar crystals, reduce the superheat of the tundish liquid, and control the drawing speed; apply external force to the billet, press lightly and spray water for cooling.
2Corner cracksAt the corner of the acid-leached sample, there is a certain depth from the surface and is perpendicular to the surface, extending inward along the diagonal.Improper cooling of the copper mould tube, inappropriate taper, and uneven thickness of the shell result in diamond deformation and desquamation.Choose the appropriate copper mould tube to reduce billet deformation
3center crackIn the columnar area close to the center equiaxed crystal and perpendicular to the inner arc surface of the slab, in severe cases it can pass through the centerThe continuous casting billet is caused by the stress caused by the solidification and shrinkage of the core of the molten steel in the final stage of solidification; the continuous casting billet is bulging, the secondary cooling system is improper, and the straightening stress is too large.Reasonable secondary cold water cooling, no straightening with liquid center
4middle crackProduced in the columnar crystal area, at one quarter of the thickness of the slab, perpendicular to the sideUneven cooling, the surface temperature of the slab rises after exiting the secondary cooling zone, resulting in thermal stress, which is caused by excessive mechanical stress during billet drawing and straightening.Control the stress and deformation of the billet in the secondary cooling zone
5subcutaneous cracksSmall cracks at the junction between the chill layer and the columnar crystal at the edge of the test pieceIt is related to many factors such as mold deformation, excessive local friction, inaccurate arc alignment, uneven cooling of the mold and secondary cooling zone, bulging of the continuous casting billet, and excessive straightening stress.Choose the appropriate COPPER MOULD TUBE to reduce billet deformation
6loose centerGaps and dark spots in the centerThe loose central part of the tissue caused by volume shrinkage is caused by gas evolution and accumulation of impurities.Improve the purity of molten steel and the shape of the billet
7SlagBlocks or particles of different shapes and colors on the test pieceThe rapid low-position pouring of the tundish creates a vortex, and the mold powder is sucked into the mold without floating or the mold powder is involved in the mold powder.Choose the appropriate water inlet to extend the tundish metallurgical time

Conclusion

The macroscopic inspection method of continuous casting slabs has the advantages of wide observation range, accurate defect location, and fast inspection speed. It is an important means of quality analysis of continuous casting slabs. It has attracted great attention from domestic and foreign metallurgical peers, and its application scope has continued to expand.

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