Research on surface cracks of AH32 ship plate
High-strength AH32 ship plate is a variety with large production volume at Jinan Iron and Steel Co., but surface cracks occur from time to time . Due to the contingency of crack formation and the complexity of process factors, it is difficult to find the key factors that cause cracks . Recently, during mass production of AH32 ship plates, there were 19 billets in one steelmaking furnace, and surface cracks appeared in 18 billets after rolling . However, there were no steel plate cracks in adjacent heats of similar steel types . In order to find out the cause of the cracks, a special study was carried out on the steel plate with concentrated cracks and the remaining one continuous casting slab .
1 Production process and chemical composition
1 . 1 production process
Production process route: KR hot metal pretreatment — converter — CAS tLFtRHt continuous casting — hot charging — heating — rolling — air cooling — cutting to length and trimming — collection
1 . 2 chemical composition
1 shows the smelting chemical composition of the heats where the cracks on the surface of the steel plate are concentrated .
Table 1 The melting chemical composition wt/% of the furnaces that produced AH32 ship plate surface cracks
|
element |
C |
Si |
mn |
P |
S |
als |
Ti |
o |
N |
|
content |
0 . 15 |
0 . 18 |
1 . 51 |
0 . 014 |
0 . 003 |
0 . 030 |
0 . 015 |
0 . 001 9 |
0 . 006 0 |
2 crack observation
Macroscopic morphology of cracks on the surface of AH32 ship plate . The surface cracks are located in the center and 1/4 of the width direction, and have the characteristics of large area , continuity , and extending through the steel plate in the rolling direction .
2 . 1 metallographic observation
The metallographic morphology around the crack is shown in Figure 2 , and the depth generally extends to 100 ~ 300 ^ m . The matrix structure is ferrite and pearlite, there is no decarburization around the crack, and the banded structure is parallel to the crack propagation direction . From these signs, it can be inferred that cracks existed before rolling deformation .
2 . 2 Scanning electron microscope observation
cracked section of AH32 steel plate observed by scanning electron microscope . It can be clearly seen that there are a large number of cloudy black spots around the crack. Compositional analysis of these black dots was performed using an electron probe. The content of Fe , Mn , Si and O is relatively high. During the process of high temperature heating, because the crack gap is an oxygen-poor environment, a small amount of oxygen atoms diffuse from the crack and form oxidation dots with the oxygen-loving elements Si and Mn in the matrix. , indicating that cracks already existed during high-temperature heating.
In this paper, the cracks on the surface of AH32 ship plate appeared in the same steelmaking furnace .
Through a series of studies on cast slabs and steel plate samples, it is believed that because the low Ti/N ratio cast slab adopts hot delivery process, a large amount of C and N compounds are precipitated in the third brittle zone of the cast slab, and remain in the austenite grain boundary As a result, the grain boundary is brittle, and microcracks are formed under the action of stress, which expand into cracks in the AH32 ship plate during large deformation rolling . By taking a series of measures such as reducing N and increasing Ti and controlling the furnace temperature, the cracks on the surface of AH32 ship plate caused by low Ti/N ratio have been significantly improved .
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