Cause analysis of stamping cracking of HP295 steel plate

HP295 hot-rolled steel coils in the newly developed hot-tandem rolling mill has been progressing smoothly. However, starting from the second half of 1996 , several users reported that quality problems occurred during the use of HP295 steel coils, mainly due to punching and cracking. This paper finds out the reason of the phenomenon of fracturing through the analysis of the fracturing parts retrieved from the user's factory. At the same time, the reasons for the cracking of samples taken from the welding part of stamping parts for back bending test are also analyzed and studied.
2 Samples and test methods

HP295 steel coil is 50t converter smelting, continuous casting, and finally rolled into a 3.0mm thick steel coil in a hot rolling mill, and sent to the user for use.
A fracture and a metallographic sample were cut from the punched piece punched by the central user, and a fracture and a metallographic sample were cut from the cracked backbend test piece, and two samples were taken for chemical composition analysis, see Table 1 .
Table 1 Sample chemical composition ( wt%)

serial number	C	Si	mn	P	S	al
1	0.20	0.04	0. 77	0. 016	0.021	0.05
2	0. 17	0.06	0. 86	0.009	0.020	0.03

The metallographic microscope and scanning electron microscope were used to observe the structure and fracture of the above samples, and the LINK-ISIS microanalyzer was used to analyze the inclusions.
3 test results
3.1 Fracture observation
It is observed that the fracture of the larger deformation part of the punching part is a layered fracture, as shown in Figure 1 . Observation by scanning electron microscope and analysis by LINK-ISIS energy spectrometer, there are MnS inclusions along the cavity and edge of the cracked part.

3-2 Observation of inclusions
Observation of metallographic samples along the longitudinal section and cross section of the steel plate, there are many and very thin MnS inclusions, and the tensile along the rolling direction

	- - -	—		12
20-			|Fe i	!0	c J	严_
15-				8
			1	s		Mn
10				& 6		■i ■
	S	Mn		4	! { Fej
5		1' 1 \\ 1	1 Fe	2		， : Fe
0		11 J		0	, " shop Mr j w , - i - _
	2 keV 4	6			2	keV 4 6
	(MnS) of point A on Figure 3 and Figure 2				Figure 4	The composition of point E on Figure 2 ( MnS)

Long, shorter along the transverse direction, other inclusions are not obvious.

3- 3 Observation of backbending specimens
Observe the structure of the section of the cracked backbending specimen, see Figure 7 and Figure 8. Figure 8 is an enlargement of the upper right corner of the crack in Figure 7 . It can be seen from the structure that the backbend cracking occurs in the mother plate of the stamping. Many small cracks can be seen in the extension area of the front end of the crack and near the two sides of the crack. And some small cracks contain MnS. See Figure 9 .

The backbending specimen was broken along the crack, and MnS fragments can be seen on the fracture surface, as shown in Figure 10 .
4 Analysis and Discussion
All the punched parts retrieved from the user's factory cracked along the rolling direction of the steel plate, which shows that the transverse ductility of the steel plate is poor, and the main factor affecting the transverse ductility is the number and shape of inclusions. In particular, the impact of banded inclusions on the reduction of area is more prominent. With the increase of banded inclusions, the reduction of area in the transverse direction decreases significantly. This banded inclusion is mainly sulfide.
From the above analysis results, it can be seen that because there are many thinner MnS inclusions inside the stamping, the fracture starts from the microcracks generated at the interface between the band-shaped MnS inclusions and the matrix. As the cracks expand, the micro-cracks are connected to each other, which finally leads to the cracking of the stamping. However, it can be seen from Figure 5 and Figure 6 that the longitudinal length of MnS inclusions is much greater than the transverse length, so the cracks are all along the rolling direction.

The bending cracking of backbending specimens is also closely related to MnS inclusions. Relevant data mention that the sulfide inclusions in aluminum-deoxidized steel are mainly type I MnS. Due to the severe elongation during hot working, lamellar tearing will occur due to the heat influence of pressure vessel welding. The backbending problem we discussed is also different, it is brought about by the sample preparation method, the backbending sample is sampled from the welding part, the welding method is to shrink the punching part inside, and the non-shrinking punching part is outside. After welding, the Before the back bending test, the necking part should be ground off along the straight line in Fig. 11 , so that for the back bending specimen, the side of the necking punched part near the weld is not the rolled surface of the steel plate, but the transverse surface of the steel plate. Section, so that the strip-shaped MnS inclusions in the steel plate are exposed. It is approximately 15° from the surface of the test piece, see Figure 8 and Figure 12 . Therefore, when doing the bending test, they become the source of cracking, causing the cracking of the backbending specimen. Figure 8 is the morphology of MnS fragments on the back-bent tear surface .
It can also be seen from the above analysis that all cracking sources occur on the side of the necking punching, rather than in the weld and the side of the non-shrinking punching. In addition, the surface-bending sample does not have the problem of exposing the strip-shaped MnS inclusions, so no cracking occurs.

in conclusion
( 1) The main reason for the cracking of the stamping is that the MnS inclusions are distributed more and longer along the rolling direction, which reduces the transverse ductility of the steel plate.
( 2) Backbend cracking is due to improper processing of the sample, resulting in the exposure of strip-shaped MnS inclusions and becoming the source of cracks.
( 3) In order to prevent the occurrence of cracking, the steelmaking process management should be strengthened, the steel quality should be purified, and the formation of MnS inclusions should be controlled to improve the ductility of the steel plate, mainly the transverse ductility.

---