Analysis on the causes of breakage of backup rolls in 3500 mm Steckel rolling mill

Abstract: Low-magnification pickling, metallographic examination, scanning electron microscopy, energy spectrum analysis and other methods were used to analyze the fracture surface of the 3500 mm Steckel Mill backup roll that suffered initial fracture. The results show that the large residual stress and the accumulation and distribution of various inclusions and carbides are the main reasons for the fracture of the extra large support roller.

Keywords: backup rolls; fracture; inclusion; carbide aggregation

The accident roller was a support roller of a 3500 mm Steckel rolling mill. It was installed on the machine for the second time and was installed in the lower roller position. During the debugging process of rolling mill bouncing, a loud abnormal noise suddenly occurred, and the inspection found that the lower support roller was broken. The rolling force on each side is 26.36 MN when it breaks (the required value for rolling mill bounce debugging is 29.4 MN on each side). The bouncing operation process of this rolling mill is a fully automatic operation procedure, and there are no abnormalities in the procedure.

The back-up roll material is 3% CrMoV, the roller body size is 1950mm×3450 mm, the finished product weighs 115.25 t, and the steel ingot weighs 227 t. The finished product passed the ultrasonic test. In order to find out the cause of the accident roller breakage, an anatomical analysis was conducted on the remaining roller.

1 Macroscopic fracture observation of the accident roller

The backup roller broke at about 1315 mm from the end face of the roller body on the operating side (non-marking end). The fracture surface is nearly perpendicular to the axis of the roller body, and the fracture is relatively straight, as shown in Figure 1. The initial fracture source is about 305 mm from the center, see Figure 2.

2 Accident roller production process

The actual production process of the backup roller is: electric furnace roughing → vacuum refining → vacuum casting → forging → post-forging heat treatment → rough processing and ultrasonic testing → final heat treatment → finishing and ultrasonic testing → packaging and shipping.

Figure 1 Fracture morphology

Figure 2 The distance between the original fracture source and the center of the circle

3 Anatomy test analysis of accident roller

3.1 Low-magnification inspection of cross-section

Use the section of one of the broken rollers to process it into a low-magnification test piece with a thickness of 50 mm for low-magnification inspection. Three obvious defects were found, respectively located in areas A (and surrounding adjacent areas), B (and surrounding adjacent areas), and C in Figure 3.

Figure 3 Low magnification defect distribution map

Area A consists of dark point-like segregation distributed within a range of 80 mm to 180 mm from the outer circle (not limited to the wire frame marked A), see Figure 4.

Figure 4 Dark punctate segregation in area A

Area B is a light-colored point-like segregation distributed within a range of 200 mm to 510 mm from the center of the circle (not limited to the wire frame marked B), see Figure 5.

Figure 5 Light-colored point-like segregation in area B

Area C is a light-colored network of defects distributed within a range of 250 mm from the center of the circle (basically distributed within the C-marked box), see Figure 6.

Figure 6 Light-colored network defects in area C

3.1.1 Low magnification inspection results of area A

Energy spectrum analysis shows that in addition to the uneven distribution of granular carbides rich in Cr, Mo, and V, there are many types of non-metallic inclusions in the low-magnification sample in area A. Among them are massive titanium nitride, manganese sulfide, silicon oxide, alumina, and complex oxide slag rich in C, Al, Mg, Ca, Si, Cr, Na, Cl, Mn, S, etc. There is little difference in alloy matrix composition. Combining tissue and energy spectrum analysis, the black spots in the low-magnification sample in area A are non-metallic inclusions rich in C, S and Al, Mg, Ca, Si, Cr, Na, Cl, and Mn.

3.1.2 Low magnification inspection results of area B

Through low-magnification detection, metallographic microscope, scanning electron microscope and energy spectrum analysis. White spot-like segregation is an aggregate of small granular carbides rich in Cr, Mo, and V and large granular oxides rich in Cr, Si, S, and C centered on bubbles or holes. Higher O content was detected inside carbides and bubbles and in their matrix structures.

3.1.3 Low magnification inspection results of area C

Determined through low-magnification detection, metallographic microscope, scanning electron microscope and energy spectrum analysis. Network segregation is an aggregate composed of a large number of white granular carbides rich in Fe, Cr, and Mo and a small amount of large granular oxides rich in Fe, Cr, and C.

3.2 Analysis of origin and causes of fracture cracks

A test rod was taken from the original fracture source of the accident roller for experimental analysis.

The metallographic structure of the fracture specimens was analyzed. Under low magnification, it was found that there were unevenly distributed gray-white segregation areas in the matrix tissue. After magnification, it was found that there was a faintly visible line on the edge of one side of the sample. The gray-white tissue is fine and the matrix tissue is thick, as shown in Figure 7.

Figure 7 Low magnification metallographic structure of fracture surface

Observed under high magnification, the gray-white fine structure is troostite + pearlite matrix + a large number of granular carbides. Carbide may be aggregated or distributed in a network. The coarse matrix structure is troostite + sorbite + pearlite, and it is difficult to find large particles of carbides in the matrix. In addition, more non-metallic inclusions can be seen near both sides of the grain line, see Figure 8.

Figure 8 High magnification metallographic structure of fracture surface

Observing under a scanning electron microscope, we can more clearly see inclusions of different sizes, shapes and distributions, as well as dark grain boundaries. Dark grain boundaries represent the generation of overheated structures, see Figure 9. Observe inclusions at different times at different parts of the sample, see Figure 10.

Figure 9 Microstructure morphology of fracture surface under scanning electron microscope

Figure 10 Inclusion morphology under fracture surface scanning electron microscope

Under low magnification, it was found that there were unevenly distributed black spots and white spots on the surface of the sample. Magnifying it, the black spots are slag rich in C (65.05%), O (5.39%), Cr (1.82%), Na (1.35%), Cl (0.43%), and the rest is Fe. The white spots and the white spots in the C area belong to the same type of inclusions. According to energy spectrum analysis, they are rich in C (21.88%), O (38.81%), Zr (22.26%), Al (7.35%), S (3.54%), and Mn. (2.58%), Mg (1.01%), Fe (1.8%) oxide inclusions. At higher magnifications, you can also see smaller bright white inclusions that are aggregated and dispersed in local areas, which are rich in C (19.38%), Na (20.03%), Cl (4.10%), Cr (2.83%), Si (0.62%), the rest is Fe inclusions. As for the grain lines, there are more non-metallic inclusions gathered around them, which may be caused by skinning during ingot injection.

Take samples from the fracture specimen to obtain the fracture surfaces of the impact and tensile specimens. Visual inspection of the fracture surface revealed that the impact fracture surface had a uniform metallic luster, while the tensile fracture surface had an uneven distribution of metallic luster. Low-magnification scanning revealed that the tensile and impact fracture surfaces were uneven. High-power scanning revealed that they were all cleavage brittle fractures. In addition, large pieces of TiN inclusions were also found on the impact fracture surface, as shown in Figures 11 and 12.

Figure 11 Microstructure morphology of the fracture surface of the impact sample scanned

Figure 12 Microstructure morphology of tensile sample fracture surface scan

In summary, the structure of the fracture sample is a coarse matrix structure (troostite + sorbite + pearlite) with unevenly distributed carbides. The carbides are either aggregated or reticulated, and there are many non-metallic inclusions of uneven distribution, shapes and sizes.

The local carbide aggregation is due to the severe carbide segregation in the original ingot structure. At the same time, it was not broken during the forging process and was not fully annealed after forging. Therefore, the segregation of carbon and alloy elements in the raw material was not effectively improved. Especially for primary carbides, the carbides are not completely dissolved during the heating process of forging and post-forging heat treatment. It is precisely because of the presence of undissolved carbides that hinders the growth of austenite grains, making the structure grains in this region relatively fine. Network carbides are due to the high heating temperature and slow cooling rate of heat treatment. In areas with higher carbon content, carbides precipitate along the original austenite grain boundaries to form network carbides.

Various types of non-metallic inclusions are caused by the low purity of the alloy liquid, which fails to emerge from the surface of the alloy liquid during the cooling and crystallization process and remains in the alloy ingot, forming non-metallic inclusions. Or it may be due to improper operation during ingot pouring (pouring temperature is too low, pouring speed is uneven, ingot film quality is poor, etc.). The oxide scale floating on the surface of the molten steel will turn into the molten steel. If it cannot float, it will remain in the molten steel and form scale. The skin is a special form of non-metallic inclusions. The skin cannot be forged during forging. If the oxide is thick, it will separate from the matrix and form cracks. The lines observed under metallographic and scanning electron microscopy may be cracks formed by not forging the skin.

Except for the massive TiN inclusions found in the fracture surface of the impact sample, the granular second phase and other types of inclusions found in the observation of the metallographic structure did not appear. This shows that the inclusions in the metallographic structure sample may be caused by improper pouring, causing the inclusions to be involved, resulting in a higher content, more types, and concentrated distribution of local inclusions in the sample. This will inevitably seriously reduce the overall performance of the alloy, and will also cause greater stress concentration, becoming the origin of cracks.

Crystalline metallic luster and uneven surfaces can be seen on the fracture surfaces of both samples, indicating that the structure is coarse and uneven. This structural characteristic will reduce the plastic toughness of the alloy (which is confirmed by its cleavage and brittle fracture morphology).

The matrix structure has coarse grains, uneven distribution of carbides, and the presence of many non-metallic inclusions, which will reduce the strength and plasticity of the alloy. In particular, the presence of carbides, inclusion accumulation areas and large inclusions will cause stress concentration, which will reduce the fracture resistance and cause crack initiation. Coarse grains will also have an adverse effect on the strength and plastic toughness of the alloy and promote crack expansion. Under the action of stress, due to the poor plastic toughness of the matrix structure, cracks propagate and final fracture occurs.

Based on the above analysis, it can be seen that the fracture crack of the accident roll originated from various non-metallic inclusions. Due to the coarse matrix structure and uneven distribution of carbides, the plastic toughness of the alloy is reduced. Under the action of stress, the cracks are promoted and eventually fractured.

4 Analysis of the causes of accidental roller breakage

Based on the above test analysis, the accident roller breakage is mainly related to the following aspects:

(1) The carbides are unevenly distributed. It is mainly reflected in white spot-like segregation and network segregation under low magnification. This is the result of the original ingot being affected by the smelting quality, pouring conditions and cooling crystallization characteristics of the steel during the solidification process. There is serious uneven distribution of carbides in the original ingot structure, and the forging and post-forging annealing are insufficient, so the uneven carbide phenomenon cannot be effectively improved. It is precisely because of the uneven carbide that the grain size distribution of the matrix structure is uneven. In particular, the carbon-poor area is prone to overheating and forming a coarse structure.

(2) Non-metallic inclusions. From the surface to the inside of the accident roller, there are many different types of non-metallic inclusions. Different types of inclusions exist in different ways. Based on the energy spectrum analysis of different samples, the main inclusion types inside the accident roller are: large pieces of TiN, TiC and long strips of MnS. Granular low melting point slag rich in C, O, Al, Mg, Mn, Zr, Ti, Ca, P, S, etc. Small particle oxides rich in Mg, Al, Na, Zr, Mn, etc.; smaller white particles rich in C, Na, Cl, Si, Cr are aggregated and included. Among them, large pieces of TiN, TiC and long strips of MnS are present in different parts of the accident roller, that is, from the surface to the inside. Except for MnS, which has a certain plasticity and exists in the form of long strips, TiN and TiC both exist in the form of large chunks with sharp edges; low melting point slag mainly exists in the form of aggregates of different particle sizes. Due to the different parts of the accident roller, its existence form is also different. In the outer layer of the accident roller, it is mainly accompanied by low melting point inclusions and exists in the form of granular aggregation of different sizes. Under low magnification, it appears as black spot-like segregation at the edge. The middle layer and core inside the accident roller are aggregated with the aggregation of granular carbides, and are mixed with granular carbides, oxides, etc. to form aggregates. Under low magnification, they appear as white spot-like segregation and network segregation. Most oxides such as Mg, Al, Na, Zr, Mn, etc. are attached to the surface of the slag and exist along with the slag. Some of them are independent small particle oxides that gather together with the aggregation of carbides, forming white spot-like segregation and network segregation. Particle inclusions rich in C, Na, Cl, Si, and Cr are only found in the fracture surface, and they exist in smaller aggregates. The different types and existence forms of the above inclusions are mainly related to the smelting quality, pouring conditions and crystallization characteristics of the raw material alloys. Especially for fracture specimens located at the fracture source, not only are the unevenness of carbides more severe, but there are also more various inclusions and their distribution is concentrated. This is mainly due to improper pouring during the alloy pouring process, which causes the oxides, low melting point inclusions, easily segregated solute elements, slag, molding slag and other inclusions on the alloy surface to be drawn into the molten steel and fail to float. It can be seen that the fracture of the accident roller originated from inclusions caused by turning over the skin.

Through the observation and analysis of different fracture surfaces, almost all fracture surfaces originate from the accumulation area of carbides and inclusions. That is, it originates from white spots and network segregation visible to the naked eye under low magnification. Since white spots and network segregation are aggregates composed of granular carbides, slag, low melting point inclusions, oxides, etc., they cause an extreme reduction in the plastic toughness of the alloy and produce stress concentration, which promotes the initiation of cracks. Especially the fracture specimen located in the fracture source area of the accident roller, because the ingot formed a skin during pouring, and the internal inclusions and carbide accumulation were very serious. The skin was not pressed together during the forging process, and the inclusions and carbide aggregation were not broken up to form a dispersed distribution during the forging and post-forging heat treatment processes, seriously severing the continuity of the metal matrix and reducing the strength and plasticity of the alloy. , and produce stress concentration, which leads to the occurrence of fracture sources in the accident roller.

Whether it is the crack source area or the crack expansion area, the fracture morphology is dominated by cleavage brittle fracture, indicating that the accident roller system is a typical brittle fracture. This is corroborated by its structural characteristics of coarse matrix structure, many inclusions and uneven distribution of carbides.

(3) Residual stress. Due to the large size of the accident roller, the temperature distribution on the surface and core is uneven during the heating and cooling process of heat treatment, resulting in thermal stress. After heat treatment, the thermal stress is not completely eliminated and is stored in the accident roller as residual stress. In addition, because the matrix structure of the accident roller is greatly different in different areas from the surface to the core, different types, sizes, and shapes of inclusions and carbides are aggregated and distributed, and the matrix structure grains are coarse and uneven, which will inevitably produce stress concentration. In particular, the stress concentration is the largest in areas where inclusions and carbides are seriously accumulated. The superposition of the residual thermal stress of the accident roller and the complex structural stress caused it to exceed the fracture resistance of the material, so cracks sprouted there and became the source of fracture.

Once a crack initiates, it propagates from the inside out. Due to the coarse matrix structure, aggregation of inclusions and carbides, different matrix grain sizes, and the existence of brittle structures, the strength and plastic toughness of the alloy will be seriously reduced, causing cracks to propagate and fracture in a cleavage-brittle manner.

5 Conclusion

The direct cause of the fracture of the backup roll of the Steckel Rolling Mill analyzed in this article is the large residual stress inside the backup roll. The main reason is the accumulation and distribution of various inclusions and carbides in the structure of the backup roll.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

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