Brief analysis of ultrasonic flaw detection of continuous casting billet

Based on the defects produced by continuous casting billets, it is proposed to use the direct probe contact method to detect internal defects, use the sound pressure pulse formula to quantify the defects, use a dual-crystal probe to detect near-surface defects, and use the test block comparison method to determine the size of the defects. The results are compared with low-magnification samples to determine the quality of the slab.

Keywords: continuous casting billet; ultrasonic wave; flaw detection; comparison test block; defect quantification; defect characterization


The cross-sectional dimensions of the continuous casting billets used in the medium-section line rolled bars of Nanjing Iron and Steel Co., Ltd. are 320 mm × 480 mm and 180 mm × 240 mm. It is mainly used for the development of new varieties of steel, especially the development of some high-standard varieties of steel, which places strict requirements on the surface quality of continuous casting billets. Because units that provide continuous cast billets generally use YB/T153-1999 “Low-magnification Structure and Defect Rating Chart of High-Quality Carbon Structural Steel and Alloy Structural Steel Continuous Casting Billets” for inspection and rating. However, the properties of the continuous casting billet itself are difficult to inspect furnace by furnace, especially in the early stages of production, which directly affects the rolling quality. Therefore, the use of ultrasonic inspection and re-inspection has become a practical and feasible method, and also provides a reliable basis for the division of responsibilities for some quality problems. However, there are currently no relevant standards for ultrasonic testing of continuously cast billets. In addition, the characteristics of continuous cast billets cannot be discarded as soon as excessive defects are found like bars. Instead, a comprehensive evaluation should be carried out based on the location, size and hazard of the defects. , providing units with quality information for billet, and more importantly, preparing for the reasonable arrangement of production plans for the steel rolling process.

Flaw detection methods and flaw detection technology

As shown in Figure 1, L=320mm, H=480mm, the cast structure distribution of smelting metal is divided into three parts from the outside to the inside, namely the fine grain area (thickness is about 2 to 3 mm), the middle columnar grain area, and the center Isometric area. In the figure, the positions of the centers of the I, II and III probes are (1/16)L, (1/4)L and (1/2)L respectively. The I position detects subcutaneous cracks, subcutaneous bubbles and corner cracks, the II position detects middle cracks, and the III position detects defects such as middle cracks, central segregation, shrinkage cavities, and slag inclusions. The method used on the H side is exactly the same as the L side.

The selection of the probe has a direct impact on the detection results. Due to the rough surface of the continuous casting billet, coarse grains, uneven structure and other reasons, the continuous casting billet does not use a relatively high frequency probe like forgings and welds, so it should be selected. Probes with lower frequencies, larger probe diameters, and relatively higher signal-to-noise ratios. Although some foreign probes have high signal-to-noise ratios and relatively small clutter, they are expensive and easy to damage, so domestic probes have become the first choice. Therefore, we choose CTS-4020 ultrasonic flaw detector, and the internal defects are φ20 mm, A 1.25MHZ straight probe, and for near-surface defects such as subcutaneous pores or subcutaneous slag inclusions, use an F=10mm, 2.5MH dual-element probe.

For the bottom wave reflection near the side wall, the minimum distance to avoid side wall interference is d>2√αλ. For steel, this value is approximately equal to 50 mm (α is the blank section length; λ probe frequency). The short-range defect quantitative incident field is not The conditions affected are:


H—Sectional length of continuous casting billet

R—Probe lens size radius;

f – probe frequency;

C—The speed of ultrasonic sound in steel.

The sound energy loss of the upper and lower surfaces: The measured energy coupling loss of the upper and lower surfaces is 14~17dB, the energy reflection loss of the lower surface is 3~5 dB, and the total average is 20 dB. The sidewall has little influence on short-range defect quantification. If the flaw detection clutter is large, a 1 MHZ probe can be used for detection. In order to ensure that the flaw detector has sufficient sensitivity margin, the attenuator should be above 110 dB. The iron oxide scale on the flaw detection surface must be polished with a sand wheel to reduce the surface roughness to the maximum Reduce coupling losses. Check from two adjacent sides.

Figure 1 Schematic diagram of continuous casting billet detection position

Comparative test blocks

In order to adjust the sensitivity of the dual-element probe and quantify defects as accurately as possible, make your own comparison test block (of course it is best to buy it from a professional manufacturer). The comparison test block is shown in Figure 2. Take a defect-free blank sample of the same material as the blank to be inspected and process it into a comparison test block with a thickness of 30 mm and 5 holes φ4 (the hole size reference is based on the GB/T4162-2008 Class C standard). The centers of holes 1, 2, and 3 are at 1/16, 1/2, and 3/4 of the diagonal. Hole 4 is 10 mm from the surface, and hole 5 is 5 mm from the surface. The center positioning accuracy of hole 4 and hole 5 is ±0.5 mm. The curve part in the figure shows the original surface state of the unprocessed continuous casting billet, which is used to measure the coupling loss.

Figure 2 Continuous casting billet comparison test block

Qualitative analysis of defects

Some defects can be characterized based on the relationship between the location of the defect and the waveform. For example, if the probe is in the I position, the sound path of the defect is between 10 and 20 mm, the defect waveform is sharp, the amplitude of the damage wave gradually decreases from the middle to both sides when the probe is moved, and the bottom wave decreases significantly or there is no bottom wave, so detect from the adjacent surface. The waveforms are almost the same. This is a typical crack waveform feature, as shown in Figure 3.

Figure 3 Crack waveform characteristics

Therefore, it can be initially determined to be a subcutaneous crack or a corner crack. If the sound path position of the defect is in zone II or III, it can be determined as a middle crack or a central crack.

The middle shrinkage cavity and the middle porosity waveform characteristics are related to the low magnification characteristics. The middle shrinkage cavity and the middle porosity have uneven inner walls, especially near the generally large shrinkage cavity, there are small shrinkage cavities or inclusions. Therefore, one of the characteristics of the waveform is: the injury wave is bundle-shaped, the bottom wave is wide, the wave crest is branched, and the main injury wave is often accompanied by many small injury waves. The central shrinkage cavity and the uneven inner wall of the loose center cause a large amount of diffuse reflection of sound waves, and there is residual gas inside, making it difficult for the ultrasonic beam to reach the bottom. Therefore, the second characteristic of the waveform is that the bottom wave drops greatly. In severe cases, the defect wave amplitude is not much different from the primary bottom wave, or even higher than the primary bottom wave, causing the primary bottom wave to be invisible. In addition, the existence of central porosity makes the defect wave have continuity. A typical picture is shown in Figure 4.

Figure 4 Typical characteristics of shrinkage cavities

Non-metallic inclusions or central segregation often see some inclusions or some relatively dense structures at low magnification, which makes the defects have certain sound transmission properties and good acoustic reflection properties, and the damage waves are independent and have high amplitude. It is similar to or even exceeds the amplitude of the first bottom wave, but has little effect on the reduction of the bottom wave. Due to the volumetric defects of subcutaneous slag inclusions and subcutaneous pores, coupled with the near-field area of the straight probe, a dual-element straight probe is generally selected for detection. The defect damage wave is sharp and the bottom wave is reduced, but the repeatability of the defect is poor and the positioning is difficult.

Quantification of defects

The size of the defect directly determines the quality of the blank and is also the most important basis for judgment. Therefore, ultrasonic quantification is particularly important. Therefore, the pulse sound pressure formula is used, as shown in formula (1):

In the formula, w is the center frequency of the probe, R is the radius of the element, and r is the time constant (defined as the time required for the mottle width pulse to drop to 1/10 replication, r=0.127△T).

Assume that the flat bottom hollow radius of the standard test block is R₁, the sound path Z1 is the measured decibel number dB, the blank defect equivalent radius is R₂, the sound path Z₂, and the sound path Z1 is the measured decibel number dB₂, then:

△B is the compensation decibel number and is the coupling loss. △B is determined by actual measurement. This quantitative method seems a bit cumbersome in actual operation, but it is necessary to accumulate experimental data more accurately in the early stages of production, especially during variety development.

Rating of Continuous Casting Billets

The low-magnification sample is compared according to the rating chart in YB/T153-1999. If it meets the standards for levels 1 to 2, it will be rated as level A. If it meets the standards for levels 3 to 4, it will be rated as level B. It can be treated as a downgrade or a downgrade. Basis for Purchase Fees.

Center crack, middle crack, shrinkage cavity

At positions Ⅱ and Ⅲ in the figure, according to the defect amplitude, Df ≤ 4mm, it is classified as a high-quality blank and rated as grade A. If a single defect or multiple defects appear at 4 mm ≤ Df ≤ 8 mm and are 6 Df apart from the single largest defect, and the number of defects at the same time is no more than 5, it will be rated as grade B. If any of the following conditions occurs, you must return to do acid etching at low magnification: ① The 6dB reduction is only a single defect above 50 mm and a single crack. ② The primary bottom wave is more than 12dB lower than the normal value.

Corner cracks and subcutaneous cracks

Subcutaneous cracks and corner cracks were measured at the I position. If the defect location is less than 2 mm from the surface, the crack length doubles. This is because the crack is too close to the surface and the crack defect is difficult to heal after rolling. If Df≤3mm is graded as A, if 3mm≤Df≤6 mm has a single defect or multiple defects are 6 Df apart from a single maximum defect, and the number of defects is no more than 3 at the same time, it will be graded as B. If any of the following conditions occurs, you must return to do acid etching at low magnification: ① The reduction of 6 dB is only a single defect above 50 mm and a single crack. ② The primary bottom wave is more than 12dB lower than the normal value.

Subcutaneous slag inclusions and subcutaneous pores

Such defects have a great impact on the surface and are difficult to repair, so they must be strictly controlled. For example, Df ≤ 3 mm is rated as grade A. If Df ≥ 3mm, return to low-magnification sample observation.

Central porosity and central segregation

These two defects are difficult to determine based on their location. The loose features are similar to shrinkage cavities. There are clutter on the upper and bottom of the waveform, and the roots are wider. If Df≤3mm, it is rated as grade A. If 3mm ≤ Df ≤ 6mm, it will be rated as Class B. If Df ≥ 10mm, it must be rated as low magnification.


In November 2008, we began to conduct targeted flaw detection on the continuous casting billets of some types of steel. So far, we have tested 20MnCr5, 48MnV, AISI4145H and other steel types. A total of 156 furnaces of flaw inspections were carried out, including 16 furnaces of Grade A billets, 138 furnaces of Grade B billets, and 2 furnaces of pickling samples were returned to the furnace for re-smelting. In addition, judging from the final rolling results, the quality control has achieved the expected results, and the steel sold after flaw detection has not had any quality objections so far. Therefore, it is feasible to use ultrasonic waves to detect defects in continuous casting slabs and control the quality of continuous casting slabs.

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