Composition optimization of high-strength ship plate AH36/DH36

Using methods such as slab low-magnification inspection, in-situ analysis, and scanning electron microscopy, the low-magnification, segregation, and tensile fractures of the free-strength ship plate before and after optimization were compared and analyzed. The test results show that the low power and element segregation of the optimized high-strength ship slab are significantly better than those before optimization, and the tensile fracture delamination is also significantly improved, and the pass rate is significantly improved.
Key words: fracture segregation low magnification in-situ statistical analysis

With the rapid development of shipbuilding technology, marine steel plates are required not only to ensure that the tensile properties, weldability and surface quality of the product meet the relevant standards of the country or classification society, but also to meet increasingly stringent ship inspection requirements. During the production of high-strength ship plates, Anyang Iron and Steel Group Company found that part of the fracture surface of tensile samples had delamination, which seriously affected the pass rate of high-strength ship plates; The optimization was carried out, and the effect of high-strength ship plate composition before and after optimization was compared and analyzed by means of low-magnification detection of cast slab, scanning electron microscope microscopic analysis and in-situ analysis.
1 Composition optimization test plan
The production process of high-strength ship plate of Anyang Iron and Steel Group Company is : 100 t converter, nitrogen blowing station , LF refining , continuous casting of 200 mm slab, billet cutting, heating, descaling, 2800 mm medium plate rolling mill, straightening, cooling and shearing All collected into storage.
According to the solidification theory, the pure part of the molten steel is preferentially crystallized during the solidification process of the slab, while the part rich in C , P , S , Mn and their compounds is often squeezed to the center of the slab and finally solidifies. The solidification is the part with severe segregation, which affects the continuity of the metal structure and becomes one of the main factors of steel performance problems; the unqualified fracture of high-strength ship plate is mainly caused by central segregation, especially violent segregation. In the actual production, the content of C, P and S elements in the high-strength ship plate is relatively low. Therefore, the center segregation caused by element segregation can be reduced by reducing the manganese content, and the fracture morphology can be improved to improve the fracture performance on the premise of ensuring the strength performance . rate is the main direction.
2 Analysis of test results
2.1 Low power analysis
The low-magnification samples of the billet before and after the composition optimization were taken respectively. The steel types of the samples taken were AH36/DH36 , and the numbers were respectively : 1# sample is the sample before reducing the manganese content , and 2 # sample is the sample before reducing the manganese content The composition of the final sample is shown in Table 1 . The 1/4 and 1/2 places where the 1# and 2# samples are located are longitudinally sectioned, polished, etched with nitric acid and then observed.
Table 1 Composition of high-strength ship plate AH36/DH36 samples
Table 1 Chemical compositions of samples of high strength ship plates AH36/DH36 %

serial number	Grade	C	Si	mn	P S Nb Alt
1# , 3# 2# , 4#	AH36/DH36 AH36/DH36	0.18 0.18	0.36 0.32	1.48 1.25	0.011 0.012 0.018 0.025 0.012 0.004 0.019 0.035

2.2 In situ analysis
Metal in-situ analysis ( Original Position Statistic Distribution Analysis, OPA) system adopts OPA- 100 manufactured by Beijing Nanoco Analytical Instrument Co. The speed is 1 mm/s, the excitation frequency is 480 Hz, the excitation capacitance is 7.0 uF, the excitation resistance is 6.0 Q, and the spark gap is 2.0 mm . The signal acquisition speed is 100 kHz, and the production of the working curve and the processing of the analysis results are processed by OPA-100 software.
in high - strength ship plates , in -situ analysis was carried out on the casting failure before the manganese content was reduced. The scanning range is shown in Fig . See Table 1 for the specific composition of the sample after reducing the content of tungsten . From the three-dimensional distribution diagram (see Figure 4) and the two-dimensional contour map (see Figure 5) of the manganese element composition in the slab before and after reducing the manganese content , it can be seen that when the manganese content of the high-strength ship plate AH36/DH36 is 1.48% , the casting There is an obvious segregation zone in the center of the slab, and when the manganese content of the high-strength ship plate AH36/DH36 is reduced to 1.25% , the segregation zone in the center of the slab is obviously improved. The statistical results of the in-situ analysis of the high-strength ship plate elements in the sample before and after the drop in temperature are shown in Table 2o

Table 3 Fracture qualified rate before and after falling
Table 3 Percent of pass of layered fracture before and after

reducing manganese content, %
time	Pre-check pass rate	positive inspection pass rate
Before dropping the chain ( 08.3-08.10)	93.57	90.1
After the fall (08.11 -09.4)	98.61	95.4

Analysis of advantages and disadvantages of various steel pipe flaw detection methods
Ultrasonic inspection has a high detection rate for internal and external surface defects of steel pipes, but the inspection speed is limited. In the steel pipe water immersion method flaw detection, the sound wave propagates and attenuates seriously in the pipe wall, because the acoustic impedance of water is much smaller than that of steel, so the reciprocating transmission rate of sound wave from water to steel and steel to water is very small. Secondly, each reflection of the sound wave in the pipe wall is accompanied by a wave mode conversion, and when the transverse wave is transmitted to the water, it must be completely absorbed by the water, so the ultrasonic attenuation of the water immersion probe is more significantly increased, causing the sound wave to travel along the pipe. The wall propagation distance is small.
Eddy current testing does not require contact or couplant, so the detection speed is high, and it can also be tested at high temperatures, but the object of eddy current testing must be conductive materials, and due to electromagnetic induction, it is very sensitive to surface and near-surface defects, but It is not suitable for detecting deep internal defects, and the specific location cannot be determined. Eddy current testing is still in the stage of equivalent comparison detection, and accurate qualitative and quantitative judgments on defects are yet to be developed.
The detection of defects on the inner wall of steel pipe involves more complicated acoustic problems, so the design of electromagnetic ultrasonic ( EMAT) is also more complicated and fine. If the EMAT is properly designed, a higher signal-to-noise ratio can be obtained for both inner and outer wall defects. Using EMAT technology, it has many incomparable advantages for steel pipe flaw detection. For example: convenient debugging, no interference wave caused by water coupling, no need (or only low speed) rotating steel pipe or probe, more importantly, it has a high detection rate for natural defects such as inner and outer folds; and it can be used with EMAT at the same time Strip damage, hole damage, folded natural damage and man-made damage are detected. The performance of steel pipes detected by EMAT can fully meet the requirements of GB5777-96 .
in conclusion
Through the comparison of ultrasonic, eddy current, and electromagnetic ultrasonic flaw detection methods, it can be seen that these flaw detection methods have their own advantages and disadvantages. However, the electromagnetic ultrasonic (EMAT) method can not only detect artificial defects stipulated by various standards, but also detect a variety of natural defects. Therefore, electromagnetic ultrasound has incomparable advantages and broad application prospects in other non-destructive testing.

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