Analysis on the Causes of Tensile Fracture Cracks of AH32 Ship Plate Steel
reasons for tensile fracture cracking of AH32 ship plate steel were analyzed by means of optical microscope and in-situ analyzer .
With the substantial increase of my country's shipbuilding volume, the demand for ship plate steel continues to grow, and the variety is gradually increasing. In the inspection of ship plate steel, fracture cracking of tensile specimens occasionally occurs, but the classification society has strict regulations on marine steel, requiring that the material should be uniform, free of defects such as delamination and cracks, and segregation and non-segregation in the steel. Metal inclusions should be minimized or eliminated as much as possible.
of AH32 ship plate steel was inspected and analyzed, the reason of fracture cracking of tensile specimen was found out, and improvement measures were put forward.
Cracking occurs in the tensile fracture of AH32 ship plate steel. Abnormal fracture structures appear in the central area of the fracture thickness direction, showing white and bright color stratification. Cracks and white and bright abnormal structures coexist. However, the normal tensile fracture has no white and bright fracture structure, and there is no crack, and the entire fracture area presents better uniformity.
Chemical Composition Analysis of AH32 Ship Plate Steel
AH3 2 ship plate steel samples were taken for fracture cracked tensile specimens for chemical composition analysis. It can be seen that the chemical composition of the sample meets the requirements of China Classification Society's "Materials and Welding Rules".
Table 1 Chemical composition (mass fraction) of tensile fracture cracking specimens
T"b 1 Chemical compositions of the (racture
cracking tensile sample) mass) %
|
Top H |
|
mn |
S |
P |
Si |
als |
|
Measured fi |
0. 16 |
1.2-1 |
0. ()10 |
().<)16 |
0. 29 |
0. ()35 |
|
standard value |
O. 18 |
().9(- |
0.035 |
()35 |
5( ) |
>0.015 |
AH3 2 Tensile test of ship plate steel
AH3 ship plate steel , it can be seen that although the tensile properties of the sample meet the requirements of the "Materials and Welding Specifications" of China Classification Society, due to the cracking phenomenon in the fracture of the tensile sample, the ship plate steel Tensile test failed.
Table 2 Mechanical properties of abnormal tensile fracture specimens
T ; ib. 2 Mechanical properties of the fracture cracking tensile sample
|
|
|
|
A/% |
|
Measured value |
511 |
366 |
26.5 |
|
standard value |
440 ~ 57() |
2313 |
222.() |
AH32 ship plate steel is the result of the combined action of central segregation, non-metallic inclusions and abnormal structural stress.
The results show that: AH3 ship plate steel tensile fracture cracking is closely related to factors such as central segregation, more non-metallic inclusions and abnormal structure, and corresponding improvement measures are given.
1.4 Metallographic examination
1. 1. I Microstructure inspection
The microstructure of the tensile fracture cracked sample was observed under an optical microscope. It was found that there was segregation in the central area of the sample thickness. The microstructure of the central area of the sample was quite different from other areas. The content of pearlite in the central segregation area was significantly higher than that of Matrix. Bainite structure exists at the same time, and 9 cracks in bainite coexist, as shown in Figure 2 and Figure 3 .
I.4.2 Non-metallic inclusion inspection
According to the actual inspection method A in GB/T 10561 200 5
Fig.2 Microstructure of central segregation zone
big. 2 Microstructure of the central segregation zone
Fig. 3 Microstructure of the base metal
The content of non-metallic inclusions is evaluated, and the results are: A2 , BO , CO , D0. 5, as shown in Figure 4 . The sulfide inclusions are mainly located in the central area. It can be seen that the inclusions are unevenly distributed, and most of them are concentrated in the central segregation area.
I 1()( )
Figure 4 Morphology of non-metallic inclusions
Fig. 1 Morphology of the non-nietallic inclusions
h4.3 Microhardness test
The microhardness test was carried out on the central area of the thickness of the tensile fracture cracked sample. The results are as follows: the average microhardness of ferrite is 221 HV0.1 , the average microhardness of pearlite is 292 HV0.1 , and the abnormal structure The average value of microhardness is 431 HV0.1 . From the microhardness and microstructure, it can be judged that the abnormal structure in the central area is bainite.
1.4.4 In situ analysis
In-situ analysis has the characteristics of intuition and accurate positioning for the macro-segregation analysis of materials. When analyzing element segregation, the segregation degree is used to characterize the segregation degree of elements, and the maximum segregation degree of each element and its exact position are detected by the instrument. The segregation degree of the maximum segregation point is the ratio of the content value at the point with the highest element content in the analysis area to the average value of the element content in the analysis area.
The in-situ analysis was carried out on the central area of the tensile fracture cracked sample. From the in-situ analysis results, it can be found that the central segregation is relatively serious, and the carbon, sulfur and manganese elements have relatively serious segregation. The maximum segregation degree of carbon element in the central area of the sample is 1.141, the maximum segregation degree of sulfur element is 2.206, and the maximum segregation degree of element is 1.14 ().Ml shown in Figures 5 to 7 .
2 Reason analysis of fracture cracking
AH32 ship plate steel is produced during the tensile test. The research shows that the fracture cracking of the tensile specimen is the result of the three-dimensional stress in the specimen by the weak interface inside the specimen when the specimen is deformed by force in the tensile test⑴.
2.1 Influence of center segregation
to the in-situ analysis results, the element segregation in the center area of the tensile fracture cracked AH32 ship plate steel is relatively serious, especially the segregation of carbon, sulfur and manganese elements is very serious. This has a greater impact on the microstructure and purity of the rolled steel plate. big.
2.2 Influence of non-metallic inclusions and abnormal structure
segregation area can reach 2.206 times that of the normal area ; it can be seen from the detection results of non-metallic inclusions that the sulfide inclusions are mainly concentrated in the central area .
The segregation of carbon makes the carbon content in the central segregation area reach 1.141 times that of the normal area . Combined with the chemical composition analysis results, it can be seen that the carbon content at the maximum segregation position has exceeded the upper limit required by the ship plate steel standard. At the same time, the aggregation of carbon promotes a large amount of pearlite. form.
The content of manganese in the central area can reach 1.140 times that of the normal area . The segregation of alloying elements such as manganese makes the ( 'CT curve of AH32 ship plate steel move to the right. It increases the stability of austenite and leads to the direct occurrence of austenite. In the medium temperature transformation, the supercooled bainite structure that should not exist appears in the central segregation area (4).
The bainite structure has poor plasticity, which will hinder the deformation of ferrite during deformation, resulting in stress concentration in bainite, thus forming grooves along the rolling direction and surrounding unrolled joints distributed along the rolling direction at the bainite. The loose pores and microcracks caused by sulfide inclusions will produce a notch effect. Cracks are easy to form on the center plane of the plate thickness parallel to the rolling surface. As the stress increases, new cracks continue to increase. And the cracks gradually expand , Aggregation. Thus forming a macro-visible fracture cracking phenomenon.
3 Improvement measures
There are severe central segregation and inclusions in the slab, which cause the ship plate steel to break [ I - I : crack. Central segregation is the internal defect of cast slab, steel
709 liquid composition, superheat, slab fracture size, continuous casting machine working conditions, casting speed, etc. will all affect segregation, and the selection of casting speed is related to the efficiency of heat discharge in solidified metal, tundish superheat, cross section, It is related to the secondary cooling system and crystallizer specifications.
Combined with the production situation, the following measures should be taken to reduce the cracking rate of tensile fracture: reduce the superheat of molten steel, select the best working casting speed, maintain the casting machine in time to ensure the accuracy of the spoke and opening, properly cast large-section billets, and strictly control The content of carbon, sulfur and manganese should be increased appropriately to improve the purity of molten steel.
4 Conclusion
AH32 ship plate steel is the result of the combined action of central segregation, non-metallic inclusions and abnormal structural stress.
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