1. Introduction
In view of the problem of edge cracking of the chain plate when the 40Mn steel strip is pressed, the chemical composition and metallographic structure of the chain plate with edge cracking are analyzed.
The results show that the main reason for the edge crack of the 40Mn chain plate is that there are a large number of oversized non-metallic inclusions in the steel, the structure of the steel strip is irregular, and there is band segregation.
As a medium-carbon and low-alloy steel, 40Mn has high hardness and wear resistance after quenching, making it an important raw material for chain plate processing.
Recently, during the production of a certain type of chain, our company discovered that chain parts stamped from 40Mn steel had a certain proportion of cracks on the edges of the chain plate (see Fig. 1).
Fig. 1 Macromorphology of the crack at the edge of the chain plate
A large number of products are scrapped.
Therefore, the cracked chain plate is inspected and analyzed to solve the problem of edge cracking when the chain plate is pressed.
2. Chemical composition analysis
The chemical composition of the edge crack chain plate is analyzed and the results are shown in Table 1.
Table 1 Chemical composition of 40Mn steel edge crack current plate (mass fraction) (%)
| Element | Measured values | Standard value |
| W | 0.4 | 0.37~0.44 |
| Yes | 0.19 | 0.17~0.37 |
| Mn | 0.85 | 0.70~1.00 |
| Cr | 0.21 | ≤0.25 |
| P | 0.02 | ≤0.035 |
| s | 0.007 | ≤0.035 |
| Ass | 0.13 | ≤0.25 |
According to the test results, the chemical composition of the edge crack current plate meets the standard requirements.
3. Macromorphological analysis
It can be seen from Fig. 1 that the shear surface of the chain plate is relatively smooth, with no obvious traces of improper processing.
The crack position is located at 1/3 of the thickness of the chain plate, and the crack severity is different.
4. Metallographic structure analysis
Metallographic analysis was carried out on several plates with crack chains on the edges.
After sampling the crack and encrustation and grinding, it was observed that the crack depth was 1.0-2.0mm, as shown in Figure 2.
The crack extends obliquely inward from the surface and then extends inward parallel to the surface of the chain plate.
It is the split end of a split chain plate (see Fig. 3).
There are a large number of oversized sulfide inclusions (see Fig. 4) and silicate inclusions (see Fig. 5) around it.
It is evaluated according to the microscopic inspection method of GB/T 10561-2005 standard classification chart for determining the non-metallic inclusion content in steel.
The inclusion grade of cracked chain plate is A3.0 and C3.0.
After etching with 4% nitric acid alcohol, the chain plate edge structure has obvious deformation on one side due to the impact of stamping processing.
The cracking direction of the crack is consistent with the deformation direction of the structure.
There is no decarburization on either side of the crack.
The structure is point spheroidite and a small amount of pearlite + banded ferrite, as shown in Fig.
There is an obvious black stripe-shaped structure in the crack of a side chain plate, and the crack opening is only located in the black stripe-shaped structure, as shown in Fig.
Therefore, the structure of the chain plate is not uniform, and the crack is particularly obvious, showing segregation of the strip-shaped structure, which shows that the crack is a strip-shaped high-carbon pearlite structure.
Through the micro Vickers hardness test at the black band position and the normal position in Fig. 7, it is found that the average hardness of the black band position is 274HV0.5, while the average hardness of the normal position is only 220HV0, 5, and the hardness of the black band position is significantly higher than the normal value.
It is confirmed that the black band is located in a large area of accumulation of C elements.
5. Cause analysis
Through the above analysis, it can be seen that there are two main reasons for the cracking of the stamping edge of 40Mn steel chain plate.
(1) There are many oversized non-metallic inclusions on the chain plate.
On the one hand, non-metallic inclusions destroy the continuity of the material and seriously reduce its plasticity.
Under the action of an external force, plastic deformation occurs around the non-metallic inclusion due to stress concentration, resulting in a large number of displacements around the non-metallic inclusion;
When the displacement reaches the interface between the non-metallic inclusion and the matrix under the impact force, the interface separates to form micropores, and the micropores quickly aggregate and expand under the impact force, resulting in the cracking of the plate edge of the current.
On the other hand, silicate inclusions belong to fragile and immutable inclusions, which have a great difference with the thermal deformation capacity of the matrix.
During rolling, the steel strip can easily form pores or cracks at the interface between the non-metallic inclusions and the steel matrix.
(2) The structure of the material is not uniform and the band segregation of element C is serious.
Band segregation causes the existence of high hardness bands in the steel strip, resulting in uneven transverse performance of the material.
Under the impact load, the stress concentration phenomenon is easy to occur in the area where element C meets and at the junction.
At the same time, under the instantaneous action of impact load, it is easy to initiate microcracks and expand rapidly, resulting in edge cracks in the steel strip stamping process.
6. Conclusion
1) A large number of non-metallic inclusions in the 40Mn steel strip is one of the main reasons for the cracking of the stamping edge of the chain plate.
The non-metallic inclusions in the steel strip are mainly class A and class C inclusions.
2) The irregular structure of the 40Mn steel strip and the serious band segregation of the C element are also one of the main reasons for the cracking at the stamping edge of the chain plate.
Band segregation causes the existence of high hardness bands in the steel strip, resulting in uneven transverse performance of the steel strip.
