Fracture Analysis of 65Mn Steel Coil

The 65Mn steel coil was fractured during peeling before cold rolling. The cause of fracture was analyzed by chemical composition analysis, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the fracture is caused by the presence of hard tissue segregation bands and more MnS inclusions in the core of the steel coil .
Key words : 65Mn steel coil; fracture; hard structure segregation zone; sulfide inclusion

Since November 2009 , a factory produced 1580 hot rolling 65Mn spring steel hot-rolled coils, the user has repeatedly broken strips when slitting and splitting the coils. Through understanding, the user entrusts the 65Mn hot-rolled coil to the cooperative unit to slitting, and then peels and pickles, anneals, and then cold-rolls. The process of strip breakage is peeling and pickling. In this process, the slit rolls will be flattened, deformed, peeled, pickled, and rolled. It is easy to break the tape when peeling and winding, as shown in Figure 1 . The fracture is staggered along the center line of the plate thickness, and there is a phenomenon of delamination, as shown in Figure 2 .
The rolling parameters of the test coils are: rough rolling temperature 1 200 °C , finish rolling temperature 989 ° C , finish rolling temperature 897 ° C , average cooling rate 25 C/s, coiling temperature 648 °C . The author extracted test steel coils with coil numbers R0 — 029094A010, R0 — 033746A010 (No. 1 and No. 2 steel coils respectively ), and carried out physical and chemical inspection and analysis of the reasons for the broken strips.

Physical and chemical test
Sampling was carried out on the test steel coil for chemical composition analysis. The results are shown in Table 1. It can be seen that its chemical composition is within the range required by GB/T 710-2008 .

Table 1 Chemical composition (mass fraction ) of test steel coil
Tab. 1 Chemical compositions of

the test steel rolls (mass)			%
condition	C Si Mn	S	p
measured value	0 67 0 25 1 01	0 003	0 02
standard value	0 64 ~ 0 680 20 ~ 0 35 0 95 ~ 1 15	< 0.010	< 0 020

1.2 Metallographic examination
from No. 2 steel coil to observe its microstructure. The matrix structure of the steel coil can be seen to be composed of pearlite and semi-reticular ferrite; the core structure can be seen to have white segregation bands, and there are fine sulfide inclusions in the segregation bands, and the position of the segregation bands is consistent with the fracture layer position match.
The micro-Vickers hardness of the matrix structure and the white segregation zone was tested, and the results were 310 HV and 470 HV respectively ; the sulfide in the steel coil inclusions was 15 , and the other inclusions were less.

1. Analysis and Discussion

The matrix structure of the steel coil is pearlite and a small amount of ferrite, which has low strength and hardness, but good plasticity and toughness; while the core is harder troostite, which has high strength and hardness, but poor plasticity and toughness. When the steel coil is stressed, the dislocations and slip bands produced by the matrix due to large-scale plastic deformation are hindered around the troostite in the hard tissue segregation zone, thereby increasing the strength of the material and reducing the toughness of the material [ 1 \ So the 65Mn steel coil is under the action of external force, the matrix is relatively soft, and slip dislocations are first generated, and the slip band is hindered when it encounters the relatively hard troostite structure, thereby initiating cracks and leading to fracture.
MnS inclusions is that it is most likely to cause central manganese element segregation, which is the main factor to form the hard structure segregation zone at the core of the steel coil. MnS inclusions are the birthplace of micro-cracks when the metal is deformed by force.

time	major	level	Fee/yuan
April 2012 _ _	Chemical analysis, spectral analysis, mechanical properties, metallographic examination	First and second level	Level 1 1500 Level 2 2000
August 2012 _ _	Chemical analysis, spectral analysis, mechanical properties, metallographic examination	First and second level	Level 1 1500 Level 2 2000
December 2012 _ _	Chemical analysis, spectral analysis, mechanical properties, metallographic examination	First and second level	Level 1 1500 Level 2 2000
August 2012 _ _	Mechanical Equipment Failure Analysis Technology	Secondary	2000
October 2012 _ _	Chemical analysis, mechanical properties, metallographic examination	Level three	2500

---