Analysis of the metallographic structure of the outer layer of cast high-speed steel rolls

The application of high-speed steel as a roll material began in Japan in 1988. Due to its high wear resistance and hardenability, especially its red hardness at high temperatures, high-speed steel is very suitable for manufacturing rolls. High-speed steel rolls are generally used in the front section of finish rolling of hot strip continuous rolling mills. Now they are also used in the back section of finish rolling and rough rolling stands. According to statistical analysis, high-speed steel rolls can be rolled for at least 3 cycles after normal grinding. In this way, during use, This reduces the frequency of roll replacement, increases the number of roll uses, and indirectly reduces roll consumption and maintenance costs.

Cast high-speed steel rolls use high-speed steel as the working side and ductile iron that meets the toughness requirements as the core material of the roll. Since the composition of the outer layer material and the core material are very different, it is easy to form a large amount of coarse carbides in the bonding layer. , and reduce the strength of the bonding area. For this purpose, three layers of composite casting are used, and the intermediate transition layer is graphite steel. This can make the composition gap not very large and the bonding layer position has sufficient strength. The three materials are cast together by centrifugal casting, which not only meets the high hardness and wear resistance of the outer layer but also makes the roll neck and core have good strength and toughness.

 

high-speed steel rolls, hot-rolled strip production

In this test, 4 working layer samples of cast high-speed steel were randomly selected, numbered 1#, 2#, 3#, and 4# respectively. Each sample was uniformly intercepted from the surface to the inside along the transverse direction of the roll. The length is 55mm. After grinding and polishing, marks are made every 5mm from the outside to the inside. After being corroded with 4% nitric acid alcohol solution, the measurements were carried out using the Oupot metallographic microscope, Leika image analyzer and microhardness tester. Metallographic structure analysis, carbide quantification, microhardness measurement, and finally the Shore hardness measurement at each mark. When quantifying carbides, 10 fields of view are selected for each marking position and the average value is taken. When measuring the microhardness value and Shore hardness, measure 3 points at each position and take the average.

1. Metallurgical structure and carbide morphology of the outer layer of the roll

After the cast high-speed steel rolls were differentially quenched and high-temperature tempered, the metallographic phases of the four samples were detected. The metallographic structure of high-speed steel is mainly martensite, retained austenite and carbide, but fine pearlite appears at the rear end of sample #3 (40-55mm mark). Analyzing the reason, the main reason is that the internal cooling rate of the working layer of the 3# roll is not enough during quenching and cooling, so pearlite appears in the rear section of the sample, and no pearlite is found in the outer layer of the sample.

The carbide forms of high-speed steel rolls are mainly block, chrysanthemum and strip. The block carbide is distributed inside the matrix structure, the strip carbide is mainly distributed at the boundary of the matrix, and the chrysanthemum carbide is aggregated. Among them, the strip carbides have a tendency to link into a network, and the strip carbides are obviously more than the other two carbides.

The reason is believed to be that the high-speed steel used in the roll contains more carbon, vanadium and molybdenum elements in order to obtain higher hardness and wear resistance. Combined with previous research results, it can be seen that when the vanadium content is high, a large amount of steel will be forced. Molybdenum element forms M2C carbide, and according to electron microscope analysis, the strip carbides are mainly M2C carbides, so the carbides are mostly distributed in strips.

2. Carbide content

The content of carbides is an important indicator of roll performance. Therefore, the test uses an image analyzer to quantify the carbides at each marked position and organize the obtained data. From this, it can be seen that the changing trend of carbides is determined by the external The content of the inner layer first increases and then decreases. The outer layer of the centrifugally cast high-speed steel roll contains more alloy elements. These alloy elements that form carbides will have a segregation distribution during the centrifugal casting process, that is, the outer layer alloy content is more than The content of the inner layer causes the carbide content of the outer layer to be greater than that of the inner layer. However, because the cooling rate of the outermost layer is too fast, the carbides in the austenite have no time to precipitate, resulting in less carbide content.

3. Shore hardness and microhardness measurement

Use a hardness tester to measure the marked sample, and the test results obtained show that the Shore hardness value of the 0-55mm outer layer of the roll is basically maintained between HS75-82, with a small hardness difference and stability. Better, the overall trend is decreasing from the outside to the inside. The analysis believes that it is because the inner layer of the roll cools slowly and the outer layer cools faster, causing the core to still have a higher temperature after the outer layer of the roll is cooled so that the core plays an annealing role on the cooled outside.

The microhardness of the matrix is mostly concentrated between HV550-750. This is because the matrix structure is mainly martensite. The hardness of martensite is higher, generally above HV550. Compare the matrix hardness of four samples. It was found that the hardness value of sample 4# is low. This is mainly because the matrix structure contains a large amount of retained austenite, and the hardness tester indenter does not press on the complete martensite structure, resulting in a low overall hardness.

A comprehensive analysis of the measured values ​​of the two hardnesses found that samples with high matrix microhardness also have high Shore hardness values. The two are almost proportional to each other. The hardness values of the four samples are ordered from high to low: 3# >1#>2#>4#. Looking at the distribution of carbides in the four samples, the higher the carbide content, the higher the hardness value. It can be concluded that the macro hardness of the roll is related to the carbide and matrix hardness. When the carbide content of cast high-speed steel is high and the residual austenite content in the matrix is small, its macroscopic performance is that the hardness value is relatively high.

First, the outer layer structure of the cast high-speed steel roll is generally martensite, retained austenite and carbide. If the cooling rate of the inner layer is not enough, fine pearlite will appear on the matrix.

Second, the carbide content of cast high-speed steel rolls gradually decreases from the outer layer to the inner layer. The carbide shapes are strip-shaped, chrysanthemum-shaped and block-shaped, especially strip-shaped.

Third, the Shore hardness of the roll shows a decreasing trend from outside to inside, but the hardness difference is small. The microhardness of the matrix martensite is concentrated in HV550-750. When the residual austenite content in the matrix is large, the microhardness will decrease.

Fourth, the macro hardness of the roll is related to the carbide and matrix hardness. When the carbide content of cast high-speed steel is high and the content of retained austenite in the matrix is small, its macroscopic performance is characterized by a high hardness value.

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