Reason Analysis of Grain Coarsening in Surface Layer of Hot Continuous Rolled Coil

Analyzed the reasons for the abnormal coarsening of grains on the surface of hot-rolled 08F steel plate. The results show that the coarse ferrite grains are the result of the abnormal growth of the phase-transformed group after coiling at high temperature. This grain growth phenomenon is related to the off-temperature coiling and low finishing rolling temperature in the production process.
High temperature coiling in hot continuous rolling with grain growth

Several batches of 08 F hot continuous rolling steel coils were broken on the pickling line (the pickling conditions were normal). After sampling and inspection, it was found that there were abnormally coarse grains on the upper and lower surfaces of the steel plate, and even the entire thickness section of the steel plate, with a size of 40L800 "m. It is rare for such coarse grains to appear on hot-rolled sheets.
1 Steel coil process 81 and chemical composition
The broken strip coils analyzed are 08F steel produced by converter melting-continuous casting-hot rolling-coiling. The slab heating temperature is 1200X3, the final rolling temperature is 865X3, the coiling temperature is 690t, and the steel plate thickness is 2.5mm. The chemical composition is listed in Table 1 .
Table 1 Chemical Composition of Steel Coil (%)
2 test
Cut the full plate width sample from the broken strip steel coil, and take metallographic samples from the edge, 1/4 and 1/2 of the plate width respectively. Take the original fracture sample to observe the fracture morphology. 2.1 Metallographic observation
(1) The microstructure of the sample at the wide edge of the plate is a network of free cementite with a small amount of ferrite distributed along the grain boundary. The two surface layers of the plate thickness are coarse-grained ferrite, with grains of grade 6 to 6.5 , and the middle part is small grains of grade 9 , and the transition zone from the surface layer to the middle is a mixed crystal structure (see Figure 1) .
(2) The microstructure of the sample at 1/4 of the plate width is still ferrite + network and island free cementite. The two surface layers of plate thickness are coarse ferrite

 

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2.2 Fracture analysis
Observing the fracture sample at the broken zone, it can be seen macroscopically that the fracture is a coarse-grained brittle fracture with a shiny metallic luster, and the particle size is in the range of 0.5-1.0 mm , which corresponds to the large grain size observed by metallography (see Fig. 4) . When observed under the scanning electron microscope, the fracture morphology is a typical transgranular cleavage, the size of the cleavage plane is large, and the river pattern is obvious (see Figure 5).
2.3 Analysis and discussion

After the final rolling, the hot strip steel is cooled by laminar flow and enters the coiling unit for coiling. Generally, the coiling temperature is in the range of 600~650t . The normal microstructure of the steel plate is ferrite + pearlite. The steel coil process requires off-temperature coiling, and the coiling temperature is set in the range of 680~710t . Because the steel coil is large, it takes a long time to cool from the coiling degree to room temperature. The metallographic observation results show that there are abnormally coarse grains in the upper and lower surface layers of the steel plate. The thickness and grain size of the large grain layer are unevenly distributed in the width direction of the plate. The grain size of the surface layer of the edge sample is only grade 6 (average size 38~50 "m), as the sampling approaches the middle of the plate width, the large grain size becomes thicker and the grain size increases significantly ( grade 1~0 , average size 300gm)・The distribution of the large grain structure in the width direction of the strip reflects the influence of the temperature difference between the middle and edge of the steel coil, which is consistent with the cooling conditions of the steel coil after coiling.
In addition, there is no pearlite in the normal hot-rolled state in the microstructure of the steel plate, but ferrite+network and island-like free cementite (Figure 6 ) . This cementite morphology indicates that the steel coil after final rolling It took a long time in the temperature zone of 700t, because low carbon steel can only produce this cementite form when it is kept warm in such a temperature range . These microstructural signs indicate that high temperature coiling is the temperature condition for producing this kind of organization.
from figures 2 and 3 that the shape of the large grains is irregular, and the small grain area in the middle of the plate thickness in the crystal figure 6 has many twists and turns, such as a map shape. The concave surface of the curvature generally faces the small grains, showing the characteristics of grain boundary migration when the grains grow. The large grains gradually grow and advance from the steel plate to the middle of the plate thickness. When they "swallow" the small grains, the cementite on the boundary of the original small grains remains in the large grains and becomes island-shaped carbides. These characteristics indicate that the large grains on the surface of the steel plate are not the direct product of austenite transformation after finishing rolling, but the result of abnormal growth of ferrite grains after phase transformation.
In addition to the above-mentioned temperature conditions, two conditions must be met for the grains to grow so abnormally: ( 1) There is a significant difference in size between the surface grains and the middle grains of the steel plate after the phase transformation (that is, the grain size of the surface layer is larger) ; (2) Reserve a certain amount of cold work hardening (deformation energy storage) for the surface tissue. The first condition has been confirmed for the microstructure of the broadside specimens. The second condition can be explained by analyzing the rolling process of this batch of strip steel coils. The production process data shows that the final rolling temperature of the steel coil is 8659. The temperature of the steel measured by the laboratory is 866 1. It can be seen that when the final rolling temperature is close to the Ar, temperature, before the final rolling, the surface of the steel plate cools faster than the internal part, and a part of the Vf « transformation is bound to occur. The ferrite grain size formed at a higher temperature is smaller than that of the subsequent The ferrite transformed at low temperature is large, and at the same time retains a certain amount of work hardening after final rolling, and its deformation is close to the critical deformation. They grow up during the slow cooling process after high-temperature coiling, swallow the adjacent small grains and advance to the middle of the plate thickness. As the growth progresses, the difference between the grain size and the middle grain size becomes larger and larger, and the larger and longer, finally forming abnormally coarse grains.

3 details
The strip breakage of the hot-rolled steel coil analyzed in this paper is the wrist fracture caused by the abnormally coarse grain structure on the surface of the steel plate. Coarse ferrite grains are the result of abnormal growth of the phase-transformed structure after high-temperature coiling. This abnormal grain growth phenomenon is related to the high temperature coiling and lower finishing temperature in the production process.

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