An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside as-cast steel billets

Abstract

An ultrasonic flaw detection method for slag inclusions, cracks, and intergranular cracks near the surface inside an as-cast cast steel billet, which belongs to the technical field of as-cast cast steel billet detection. In this invention, a 100-150 mm wide sub-generatrix is ground on the surface of the as-cast cast steel billet using a portable grinding wheel to perform surface treatment on the surface of the as-cast cast steel billet; a high-precision portable ultrasonic digital flaw detector is selected, the surface of the treated as-cast cast steel billet is coated with a coupling agent, and then ultrasonic flaw detection is carried out using the high-precision portable ultrasonic digital flaw detector selected in step 2; after multiple flaw detection positioning, quantification, and sampling of the as-cast cast steel billet after detection, the obtained samples are subjected to macroetching acid pickling inspection and analysis to qualitatively and grade the defects of the as-cast cast steel billet, and to judge the situation of slag inclusions, cracks, and intergranular cracks near the surface inside the as-cast cast steel billet. This invention performs ultrasonic flaw detection on the as-cast cast steel billet, ensuring the quality of the as-cast cast steel billet for continuous casting when leaving the factory.

Description

Technical Field

[0001] The present invention belongs to the technical field of detection of as-cast steel billets; specifically, it relates to an ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast steel billet. Background Art

[0002] Internal quality problems of as-cast steel billets often lead to quality incidents. The external loss amount of as-cast steel billets can often reach as high as several million yuan. Usually, after the user forges the as-cast steel billet into a forged material finished product, internal defects are found when ultrasonic flaw detection is performed on the forged material. Due to the characteristics of the internal structure of as-cast steel billets, there is only one method of macro-examination in the national quality inspection standards. Due to the limitations of sampling verification in macro-examination, macro-examination is a destructive test; macro-examination can only represent the local quality situation and cannot reasonably analyze the true internal quality of the entire as-cast steel billet, resulting in an increase in the loss of defective billets processed into finished materials. Summary of the Invention

[0003] The object of the present invention is to provide an ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast steel billet with high detection accuracy.

[0004] The present invention is achieved through the following technical solutions:

[0005] An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast steel billet includes the following steps:

[0006] Step 1: Take the as-cast steel billet to be detected, use a portable grinder on the surface of the as-cast steel billet to grind out a sub-generatrix 100 - 150 mm wide, perform surface treatment on the surface of the as-cast steel billet, and remove the oxide scale on the surface of the as-cast steel billet.

[0007] Step 2: Select a high-precision portable ultrasonic digital flaw detector, and the probe of the high-precision portable ultrasonic digital flaw detector uses a large-size wafer probe with a low frequency of 0.5 - 1.25 MHz and a diameter of 30 mm.

[0008] Step 3: Coat the surface of the as-cast steel billet processed in Step 1 with a coupling agent, and then use the high-precision portable ultrasonic digital flaw detector selected in Step 2 for ultrasonic flaw detection. Move the large-size wafer probe on the surface of the as-cast steel billet with a certain pressure. First, detect the sound velocity of the as-cast steel billet. After determining the sound velocity of the as-cast steel billet, adjust the vertical linearity, horizontal linearity, and sensitivity margin of the high-precision portable ultrasonic digital flaw detector so that the peak value of the pulse waveform reaches 100%, and the pulse waveform is clearly displayed, obtaining the detection pulse spectrum of the as-cast steel billet.

[0009] Step 4: After performing multiple flaw detections for positioning, quantification, and sampling on the as-cast steel casting blank after the detection in Step 3, the obtained samples are subjected to macroetching inspection and analysis to qualitatively and classify the defects of the as-cast steel casting blank.

[0010] Step 5: Based on the qualitative and classification results of the defects of the as-cast steel casting blank obtained in Step 4 corresponding to the detection pulse spectrum of the as-cast steel casting blank obtained in Step 3, judge the internal near-surface crack and intergranular crack conditions of the as-cast steel casting blank.

[0011] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel casting blank according to the present invention, the surface roughness of the as-cast steel casting blank to be detected in Step 1 is controlled to be 12.5 microns.

[0012] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel casting blank according to the present invention, the frequency of the probe of the high-precision portable ultrasonic digital flaw detector in Step 2 is 1 MHz.

[0013] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel casting blank according to the present invention, the coupling agent in Step 3 is one or a mixture of two of yellow glycerin, machine oil, cellulose paste, and glass water.

[0014] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel casting blank according to the present invention, when the ultrasonic wave propagation direction emitted by the large-size wafer probe is perpendicular to the intergranular crack surface in Step 3, the detection pulse spectrum of the as-cast steel casting blank shows a wide wave bottom, a sharp wave peak in a cluster shape. As the sensitivity of the high-precision portable ultrasonic digital flaw detector decreases, the flaw wave disappears slowly. When the large-size wafer probe moves from the detection surface of the as-cast steel casting blank to both sides respectively, the amplitude of the flaw wave gradually decreases, and changes from a single main flaw beam to a series of sharp and independent small flaw waves.

[0015] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel casting blank according to the present invention, when the ultrasonic wave propagation direction emitted by the large-size wafer probe is perpendicular to the dendritic crack surface in Step 3, the detection pulse spectrum of the as-cast steel casting blank shows that the flaw wave of longitudinal wave flaw detection is in a beam shape, with a main flaw wave having a relatively high amplitude, accompanied by the secondary reflection of the main flaw wave. The flaw wave is a strong pulse in the center part. When the large-size wafer probe is moved circumferentially, the amplitude change of the flaw wave is significant, the waveform is sometimes strong and sometimes weak, and the number of bottom waves is very small or the bottom wave disappears.

[0016] An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to the present invention. In step 4, for the macroetching inspection and analysis, electrolytic corrosion is carried out using industrial hydrochloric acid with a concentration of 30%. The 30% industrial hydrochloric acid is heated to 65°C - 70°C, and the sample is corroded for 30 - 40 minutes. Then, the sample piece is taken out, rinsed clean with clear water and dried with strong wind. The morphology of the defects on the sample piece is observed macroscopically or with a 20-fold microscope, and the defects of the as-cast cast steel billet are qualitatively analyzed and classified according to the morphology of the defects.

[0017] An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to the present invention. Ultrasonic flaw detection is carried out on the as-cast cast steel billet, and the flaw detection result is used as one of the bases for judging the internal quality, setting a precedent in the industry for using ultrasonic flaw detection as a detection means for as-cast cast steel billets, and ensuring the quality of the continuous cast as-cast cast steel billets leaving the factory.

[0018] The accuracy of the detection of an ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to the present invention is 98%. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the mother and son lines of an ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to the present invention;

[0020] Figure 2 It is a test schematic diagram of intergranular cracks of an ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to the present invention;

[0021] Figure 3 It is a photo of intergranular cracks of an as-cast cast steel billet of an ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to the present invention;

[0022] Figure 4 It is a test schematic diagram of dendritic cracks of an ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to the present invention;

[0023] Figure 5 It is a photo of dendritic cracks of an as-cast cast steel billet of an ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to the present invention. DETAILED DESCRIPTION OF THE INVENTION Detailed Embodiment 1:

[0025] An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet includes the following steps:

[0026] Step 1: Take the as-cast steel casting blank to be detected, and use a portable grinder to grind a sub-generatrix with a width of 100 - 150 mm on the surface of the as-cast steel casting blank. Perform surface treatment on the surface of the as-cast steel casting blank to remove the oxide scale on the surface of the as-cast steel casting blank.

[0027] Step 2: Select a high-precision portable ultrasonic digital flaw detector. The probe of the high-precision portable ultrasonic digital flaw detector uses a large-size wafer probe with a low frequency of 0.5 - 1.25 MHz and a diameter of 30 mm.

[0028] Step 3: Coat the surface of the as-cast steel casting blank processed in Step 1 with a coupling agent, and then use the high-precision portable ultrasonic digital flaw detector selected in Step 2 for ultrasonic flaw detection. Move the large-size wafer probe on the surface of the as-cast steel casting blank with a certain pressure. First, detect the sound velocity of the as-cast steel casting blank. After determining the sound velocity of the as-cast steel casting blank, adjust the vertical linearity, horizontal linearity, and sensitivity margin of the high-precision portable ultrasonic digital flaw detector so that the peak value of the pulse waveform reaches 100%, and the pulse waveform is clearly displayed to obtain the detection pulse spectrum of the as-cast steel casting blank.

[0029] Step 4: After performing multiple flaw detection positioning, quantification, and sampling on the as-cast steel casting blank detected in Step 3, conduct macroetching inspection and analysis on the obtained samples, and qualitatively and grade the defects of the as-cast steel casting blank.

[0030] Step 5: According to the qualitative and grading results of the defects of the as-cast steel casting blank obtained in Step 4 corresponding to the detection pulse spectrum of the as-cast steel casting blank obtained in Step 3, judge the internal near-surface crack and intergranular crack conditions of the as-cast steel casting blank.

[0031] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel casting blank described in this embodiment, the surface roughness of the as-cast steel casting blank to be detected in Step 1 is controlled to be 12.5 microns.

[0032] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel casting blank described in this embodiment, the frequency of the probe of the high-precision portable ultrasonic digital flaw detector in Step 2 is 1 MHz.

[0033] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel casting blank described in this embodiment, the coupling agent in Step 3 is a mixture of yellow glycerin and cellulose paste, and the mass ratio of yellow glycerin to cellulose paste is 9:1.

[0034] An ultrasonic flaw detection method for near-surface cracks and intergranular cracks inside an as-cast cast steel billet. In step 3, when the ultrasonic wave emitted by the large-size wafer probe propagates perpendicular to the intergranular crack surface, the detection pulse spectrum of the as-cast cast steel billet shows a wide wave bottom, a sharp wave peak in a clustered shape. As the sensitivity of the high-precision portable ultrasonic digital flaw detector decreases, the flaw wave disappears slowly. When the large-size wafer probe moves from the detection surface of the as-cast cast steel billet to both sides respectively, the amplitude of the flaw wave gradually decreases, and changes from a single main flaw beam to a series of sharp and independent small flaw waves.

[0035] An ultrasonic flaw detection method for near-surface cracks and intergranular cracks inside an as-cast cast steel billet. In step 4, for the macroetching inspection and analysis, electrolytic corrosion is carried out using industrial hydrochloric acid with a concentration of 30%. Heat the industrial hydrochloric acid with a concentration of 30% to 65 - 70 degrees Celsius, corrode the sample for 30 - 40 minutes, then take out the sample piece, rinse it with clean water and dry it with a strong wind. Observe the morphology of the defects on the sample piece macroscopically or under a 20-fold microscope, and qualitatively and grade the defects of the as-cast cast steel billet according to the morphology of the defects.

[0036] An ultrasonic flaw detection method for near-surface cracks and intergranular cracks inside an as-cast cast steel billet. The test results are as Figure 2 and Figure 3 shown. During the test, the flaw waves in longitudinal wave flaw detection are in a bundled and grass-like shape (the grass-like wave is related to coarse grains). Before and after the main flaw wave, there are often some small flaw waves with lower amplitudes. As the probe moves, these small flaw waves appear and disappear quickly, changing rapidly, while the main flaw wave moves erratically with the movement of the probe, sometimes fast and sometimes slow, continuously moving back and forth. At this time, the waveform of the flaw wave conforms to the characteristics of the basic waveform of intergranular cracks.

[0037] An ultrasonic flaw detection method for near-surface cracks and intergranular cracks inside an as-cast cast steel billet. Conduct ultrasonic flaw detection on the as-cast cast steel billet, and use the flaw detection result as one of the bases for judging the internal quality, creating a precedent in the industry for using ultrasonic flaw detection as a detection means for as-cast cast steel billets, and ensuring the quality of the continuous cast as-cast cast steel billets leaving the factory. Specific embodiment two:

[0039] An ultrasonic flaw detection method for near-surface cracks and intergranular cracks inside an as-cast cast steel billet, including the following steps:

[0040] Step 1: Take the as-cast cast steel billet to be detected, use a portable grinding wheel to grind a 100 - 150 mm wide sub-generatrix on the surface of the as-cast cast steel billet, perform surface treatment on the surface of the as-cast cast steel billet, and remove the oxide scale on the surface of the as-cast cast steel billet.

[0041] Step 2: Select a high-precision portable ultrasonic digital flaw detector. The probe of the high-precision portable ultrasonic digital flaw detector uses a large-size wafer probe with a low frequency of 0.5 - 1.25 MHz and a diameter of 30 mm.

[0042] Step 3: Coat the surface of the as-cast steel billet processed in Step 1 with a coupling agent, and then use the high-precision portable ultrasonic digital flaw detector selected in Step 2 for ultrasonic flaw detection. Move the large-size wafer probe on the surface of the as-cast steel billet with a certain pressure. First, detect the sound velocity of the as-cast steel billet. After determining the sound velocity of the as-cast steel billet, adjust the vertical linearity, horizontal linearity, and sensitivity margin of the high-precision portable ultrasonic digital flaw detector so that the peak value of the pulse waveform reaches 100%, and the pulse waveform is clearly displayed to obtain the detection pulse spectrum of the as-cast steel billet.

[0043] Step 4: After performing multiple flaw detection positioning, quantification, and sampling on the as-cast steel billet detected in Step 3, conduct low-power acid pickling inspection and analysis on the obtained samples, and qualitatively and grade the defects of the as-cast steel billet.

[0044] Step 5: Based on the qualitative and grading results of the defects of the as-cast steel billet obtained in Step 4 corresponding to the detection pulse spectrum of the as-cast steel billet obtained in Step 3, judge the internal near-surface crack and intergranular crack conditions of the as-cast steel billet.

[0045] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel billet described in this embodiment, the surface roughness of the as-cast steel billet to be detected in Step 1 is controlled to be 12.5 microns.

[0046] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel billet described in this embodiment, the frequency of the probe of the high-precision portable ultrasonic digital flaw detector in Step 2 is 1 MHz.

[0047] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel billet described in this embodiment, the coupling agent in Step 3 is a mixture of engine oil and windshield washer fluid, and the mass ratio of engine oil to windshield washer fluid is 8:2.

[0048] For the ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel billet described in this embodiment, when the propagation direction of the ultrasonic wave emitted by the large-size wafer probe is perpendicular to the dendritic crack surface, the detection pulse spectrum of the as-cast steel billet shows that the longitudinal wave flaw detection wave is in a beam shape, with a main flaw wave having a relatively high amplitude, accompanied by the secondary reflection of the main flaw wave. The flaw wave is a strong pulse at the center, and the amplitude of the flaw wave changes significantly when the large-size wafer probe is moved in the circumferential direction. The waveform is sometimes strong and sometimes weak, and the number of bottom waves is very small or the bottom wave disappears.

[0049] An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to this embodiment. In step 4, for the macroetching inspection and analysis, electrolytic corrosion is carried out using industrial hydrochloric acid with a concentration of 30%. Heat the industrial hydrochloric acid with a concentration of 30% to 65°C - 70°C, corrode the sample for 30 - 40 minutes, then take out the sample piece, rinse it with clean water and dry it with a strong wind. Observe the morphology of the defects on the sample piece macroscopically or under a 20-fold microscope, and qualitatively and grade the defects of the as-cast cast steel billet according to the morphology of the defects.

[0050] An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to this embodiment. The test results are as Figure 4 and Figure 5 shown. When detecting a continuous casting round billet, the defect waveform of intermittent cracks on the whole billet is displayed. Through flaw detection positioning and quantitative sampling at typical defect locations and macroetching inspection, it is confirmed that there are dendritic crack defects in this continuous casting round billet.

[0051] An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet according to this embodiment. Carry out ultrasonic flaw detection on the as-cast cast steel billet, and use the flaw detection result as one of the bases for judging the internal quality, creating a precedent in the industry for using ultrasonic flaw detection as a detection means for as-cast cast steel billets, and ensuring the quality of the continuous casting as-cast cast steel billets leaving the factory. Specific Embodiment 3:

[0053] An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet, comprising the following steps:

[0054] Step 1: Take the as-cast cast steel billet to be detected, use a portable grinding wheel to grind a 100 - 150 mm wide generatrix on the surface of the as-cast cast steel billet, perform surface treatment on the surface of the as-cast cast steel billet to remove the oxide scale on the surface of the as-cast cast steel billet.

[0055] Step 2: Select a high-precision portable ultrasonic digital flaw detector. The probe of the high-precision portable ultrasonic digital flaw detector uses a large-size wafer probe with a low frequency of 0.5 - 1.25 MHz and a diameter of 30 mm.

[0056] Step 3: Coat the surface of the as-cast cast steel billet processed in step 1 with a coupling agent, and then use the high-precision portable ultrasonic digital flaw detector selected in step 2 for ultrasonic flaw detection. Move the large-size wafer probe on the surface of the as-cast cast steel billet with a certain pressure. First, detect the sound velocity of the as-cast cast steel billet. After determining the sound velocity of the as-cast cast steel billet, adjust the vertical linearity, horizontal linearity, and sensitivity margin of the high-precision portable ultrasonic digital flaw detector so that the peak value of the pulse waveform reaches 100% and the pulse waveform is clearly displayed, obtaining the detection pulse spectrum of the as-cast cast steel billet.

[0057] Step 4: After performing multiple flaw detection positioning, quantification, and sampling on the as-cast steel-casting blank after the detection in Step 3, the obtained samples are subjected to macroetching inspection and analysis, and the defects of the as-cast steel-casting blank are qualitatively determined and graded.

[0058] Step 5: Based on the qualitative determination and grading results of the defects of the as-cast steel-casting blank obtained in Step 4 and corresponding to the detection pulse spectrum of the as-cast steel-casting blank obtained in Step 3, judge the internal near-surface crack and intergranular crack conditions of the as-cast steel-casting blank. Specific Embodiment Four:

[0060] For an ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel-casting blank according to Specific Embodiment Three, the surface roughness of the as-cast steel-casting blank to be detected in Step 1 is controlled to be 12.5 micrometers. Specific Embodiment Five:

[0062] For an ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel-casting blank according to Specific Embodiment Three, the frequency of the probe of the high-precision portable ultrasonic digital flaw detector in Step 2 is 1 MHz. Specific Embodiment Six:

[0064] For an ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel-casting blank according to Specific Embodiment Three, the coupling agent in Step 3 is one or a mixture of two of yellow glycerin, machine oil, cellulose paste, and glass water. Specific Embodiment Seven:

[0066] For an ultrasonic flaw detection method for internal near-surface cracks and intergranular cracks of an as-cast steel-casting blank according to Specific Embodiment Three, when the ultrasonic wave propagation direction emitted by the large-size wafer probe is perpendicular to the intergranular crack surface, the detection pulse spectrum of the as-cast steel-casting blank shows a wide wave bottom, a sharp wave peak in a cluster shape. As the sensitivity of the high-precision portable ultrasonic digital flaw detector decreases, the flaw wave disappears slowly. When the large-size wafer probe moves from the detection surface of the as-cast steel-casting blank to both sides respectively, the amplitude of the flaw wave gradually decreases and changes from a single main flaw beam to a series of sharp and independent small flaw waves. Specific Embodiment Eight:

[0068] An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel blank as described in Embodiment 3. In step 3, when the ultrasonic wave emitted by the large-size wafer probe propagates perpendicular to the dendritic crack surface, the detection pulse spectrum of the as-cast cast steel blank shows that the longitudinal wave flaw detection wave is in a beam shape, with a main flaw wave having a relatively high amplitude, accompanied by a secondary reflection of the main flaw wave. The flaw wave is a strong pulse at the center. When the large-size wafer probe is moved in the circumferential direction, the amplitude of the flaw wave changes significantly, the waveform is sometimes strong and sometimes weak, and the number of bottom waves is very small or the bottom waves disappear. Embodiment 9:

[0070] An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel blank as described in Embodiment 3. In step 4, for the low-magnification acid pickling inspection and analysis, electrolytic corrosion is carried out using industrial hydrochloric acid with a concentration of 30%. The 30% industrial hydrochloric acid is heated to 65 to 70 degrees Celsius, the sample is corroded for 30 - 40 minutes, then the sample piece is taken out, rinsed clean with clear water and dried with a strong wind. The morphology of the defects on the sample piece is observed macroscopically or with a 20-fold microscope, and the defects of the as-cast cast steel blank are qualitatively and classified according to the morphology of the defects.

Claims

1. An ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside an as-cast cast steel billet, characterized in that, The steps are as follows: Step 1: Take the as-cast steel casting blank to be detected, and use a portable grinding wheel to grind a sub-generatrix with a width of 100 - 150 mm on the surface of the as-cast steel casting blank. Perform surface treatment on the surface of the as-cast steel casting blank to remove the oxide scale on the surface of the as-cast steel casting blank; Step 2: Select a high-precision portable ultrasonic digital flaw detector. The probe of the high-precision portable ultrasonic digital flaw detector uses a large-size wafer probe with a low frequency of 0.5 - 1.25 MHz and a diameter of 30 mm; Step 3: Coat the surface of the as-cast steel casting blank processed in Step 1 with a coupling agent, and then use the high-precision portable ultrasonic digital flaw detector selected in Step 2 for ultrasonic flaw detection. Move the large-size wafer probe on the surface of the as-cast steel casting blank with a certain pressure. First, detect the sound velocity of the as-cast steel casting blank. After determining the sound velocity of the as-cast steel casting blank, adjust the vertical linearity, horizontal linearity, and sensitivity margin of the high-precision portable ultrasonic digital flaw detector so that the peak value of the pulse waveform reaches 100% and the pulse waveform is clearly displayed, and obtain the detection pulse spectrum of the as-cast steel casting blank; In Step 3, when the propagation direction of the ultrasonic wave emitted by the large-size wafer probe is perpendicular to the intergranular crack surface, the detection pulse spectrum of the as-cast steel casting blank shows a wide wave bottom, a sharp wave peak in a cluster shape. As the sensitivity of the high-precision portable ultrasonic digital flaw detector decreases, the flaw wave disappears slowly. When the large-size wafer probe moves from the detection surface of the as-cast steel casting blank to both sides respectively, the amplitude of the flaw wave gradually decreases, and changes from a single main flaw beam to a series of sharp and independent small flaw waves; or in Step 3, when the propagation direction of the ultrasonic wave emitted by the large-size wafer probe is perpendicular to the dendritic crack surface, the detection pulse spectrum of the as-cast steel casting blank shows that the longitudinal wave flaw detection wave is in a beam shape, with a main flaw wave, accompanied by a secondary reflection of the main flaw wave. The flaw wave is a strong pulse in the center part. When the large-size wafer probe is moved circumferentially, the amplitude of the flaw wave changes significantly, the waveform is sometimes strong and sometimes weak, and the bottom wave disappears; Step 4: After performing multiple flaw detection positioning, quantification, and sampling on the as-cast steel casting blank detected in Step 3, conduct low-power acid pickling inspection and analysis on the obtained samples, and qualitatively and classify the defects of the as-cast steel casting blank; Step 5: According to the qualitative and classification results of the defects of the as-cast steel casting blank obtained in Step 4 corresponding to the detection pulse spectrum of the as-cast steel casting blank obtained in Step 3, judge the crack and intergranular crack conditions near the surface inside the as-cast steel casting blank.

2. The ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside the as-cast steel billet according to claim 1, characterized in that, In Step 1, the surface roughness of the as-cast steel casting blank to be detected is controlled to be 12.5 microns.

3. The ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside the as-cast steel casting blank according to claim 2, wherein, In Step 2, the frequency of the probe of the high-precision portable ultrasonic digital flaw detector is 1 MHz.

4. The ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside the as-cast steel billet according to claim 3, characterized in that, In Step 3, the coupling agent is one or a mixture of two of yellow glycerin, machine oil, cellulose paste, and glass water.

5. The ultrasonic flaw detection method for cracks and intergranular cracks near the surface inside the as-cast steel billet according to claim 4, characterized in that, In step 4, for the low-magnification acid leaching inspection and analysis, electrolytic corrosion is carried out using industrial hydrochloric acid with a concentration of 30%. Heat the industrial hydrochloric acid with a concentration of 30% to 65 - 70 degrees Celsius, corrode the sample for 30 - 40 minutes, then take out the sample piece, rinse it with clean water and dry it with a strong wind. Observe the morphology of the defects on the sample piece macroscopically or under a 20-fold microscope, and qualitatively and classify the defects of the as-cast steel billet according to the morphology of the defects.

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