Controlled cooling and rapid balling of the hottes

2022-08-23
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Controlled cooling and rapid spheroidization process after rolling of high carbon chromium bearing steel bar

1 test method

controlled cooling and rapid spheroidization process after rolling of high carbon chromium bearing steel bar

the test is carried out under normal production rolling conditions. The test material is GCr15 bearing steel, chemical composition

note: the test steel is Φ 55mm round steel

under normal rolling conditions, Φ 50mm GCr15 bearing round bar is cut to length and then moved horizontally, which is rapidly cooled by primary and secondary water coolers. When the steel starts cooling, the minimum temperature is 860 ~ 870 ℃, and the maximum temperature is about 920 ℃. The section temperature curve of round steel after twice rapid cooling is shown in Figure 1. When the surface temperature of the round steel is 920 ℃, it enters the 1# water cooler, and the surface temperature of the outlet water cooler decreases to 400 ~ 500 ℃. After a certain time, the steel temperature turns red to 600 ~ 700 ℃, and enters the 2# water cooler for secondary cooling. The steel surface temperature of the outlet 2# water cooler is generally 400 ~ 460 ℃, and the steel temperature rises to 550 ~ 650 ℃ after passing through the roller table to the collection table, and then the steel temperature decreases. The test results show that the steel surface temperature should not be lower than 300 ℃ every time when it is rapidly cooled in the water cooler, so as to prevent the formation of martensite on the steel surface

the air-cooled and controlled cooling materials rolled under the same rolling process are annealed in a continuous annealing furnace or simulated continuous annealing system for spheroidizing annealing. The microstructure and properties were compared

2 test results and analysis

2.1 effect of cooling water pressure and steel reddening temperature on Microstructure

Φ The relationship between the maximum surface reddening temperature of 50mm round bar after water cooling and the water pressure of cooling water is shown in Figure 2 (water cooling starts at 885 ℃). When the steel feeding speed is constant, with the increase of cooling water pressure, the steel reddening temperature decreases and the cooling capacity increases

the test results show that when the steel temperature is 885 ℃ and starts rapid cooling, only one rapid cooling is used, and the reddening temperature of the steel reaches 780 ℃. Through the examination of metallography and electron microscope, it can be seen that flake and abnormal pearlite and a small amount of carbide are obtained at the cut-off frequency part and 1/4 diameter of the low-pass filter on the side of the circular section, while the center part is fine flake pearlite and carbide. The maximum reddening temperature of bearing steel bar after secondary rapid cooling is 630 ~ 660 ℃, and its edge and 1/4 of its diameter are abnormal sorbite and some spherical or hemispherical carbides. There are very fine and thin carbides in some places. The central structure is intermittent fine flake pearlite, sorbite and a small amount of fine carbide

2.2 effect of cooling water pressure on bar like carbide of bearing steel

Φ Taking 55mm bearing steel round bar as an example, the grade of metallographic sample like carbide decreases with the increase of water pressure at 870 ℃ and changes little. The grade of carbide like is less than grade 2.5 (yjz-84), and the carbide like generally appears in the heart

2.3 effect of initial cooling temperature on bar like carbides

the test shows that with the increase of initial cooling temperature, the grade of bar like carbides decreases. At a temperature above 875 ℃, the effect of initial cooling temperature on the precipitation of bar like carbides is not obvious, because the precipitation temperature of medium like carbides in bearing steel is between 960 ~ 700 ℃ under deformation conditions, and the precipitation amount is relatively small at high temperature, up to the temperature range of 700 ~ 750 ℃, Carbide precipitation is the most intense. If the steel is cooled quickly from a higher temperature after rolling, the precipitation of carbides in this temperature range can be restrained

therefore, if the bar is cooled to above 800 ℃ by water cooling immediately after rolling, the grain growth can be prevented and the deformed austenite grain can be further refined. Due to deformation, the carbide precipitation temperature ARCM increases, and after rapid cooling, the ARCM temperature decreases, reducing the amount of carbide precipitation. At the same time, due to the refinement of austenite grain, the carbide precipitation is dispersed and thinned. When the secondary rapid cooling is carried out, the steel temperature can be reduced to below 650 ℃, and the further precipitation of carbide can be prevented

if the reddening temperature after a rapid cooling is in the temperature range of intense carbide precipitation, and there is no immediate secondary cooling, so that the steel bar is slowly cooled in this temperature range, a coarse carbide structure will be obtained

2.4 effect of stop temperature of rapid cooling after rolling on the grade of carbide

stop temperature of rapid cooling after rolling is a very important process parameter, which determines the self reddening temperature of steels with different section sizes after cooling. In rolling Φ It is suitable for 34 ~ 55mm bearing steel bar to be quickly cooled to a surface temperature of 450 ~ 500 ℃ after rolling. After reddening, the steel temperature can be controlled in the range of 550 ~ 650 ℃, and then air cooling, a relatively ideal structure can be obtained

round steel with large section must be cooled for many times. At the same time, a certain time should be separated between the two water cooling to achieve the purpose of steel surface reddening and prepare for the next cooling. The reddening temperature depends on the required control cooling process system

e.g. rolling Φ 55mm GCr15 bearing steel bar, when the initial cooling temperature is 893 ℃, the steel reddening temperature after primary cooling is 690 ℃, and the reddening temperature after secondary cooling is 640 ℃. The cooling speed of steel is faster between 893 ~ 700 ℃, and the internal and external temperature difference is small, which inhibits the precipitation of carbide. The carbide grade of the rolled sample is 2.5, and the inside and outside are relatively uniform. The initial cooling temperature of the other rolled piece is 925 ℃, the reddening temperature after the first water cooling is 760 ℃, and the maximum reddening temperature after the second water cooling is 680 ℃. Because it stays at 700 ~ 760 ℃ for a certain time from the first water cooling to the second open cooling, it is at the sharp precipitation temperature of carbides, so the carbides along the fracture surface reach grade 4

2.5 control the influence of cooling process on spheroidizing annealing process

the purpose of rapid cooling immediately after rolling is to prevent the growth of deformed austenite grains and the formation of coarse pearlite pellets after phase transformation. At the same time, increase the undercooling and reduce the temperatures of ARCM and AR1, so as to reduce the lamellar spacing size of pearlite, and form degenerated pearlite and degenerated sorbite, which is conducive to rapid spheroidization, Shorten spheroidizing annealing time

the spheroidizing annealing process of GCr15 bearing steel controlled cooling material and air cooling material was studied and compared

the controlled cooling material used for spheroidizing annealing is steel under different controlled cooling conditions as the test material and air cooling test material. The controlled cooling process is as follows:

(1) after secondary water cooling, the reddening temperature is 630 ℃, 640 ℃ and 660 ℃ respectively

(2) after one-time water cooling, the reddening temperature is 780 ℃ for water-cooled materials

(3) air cooling materials

the rolling process of the above three types of steel is the same. The spheroidizing annealing test is carried out in a 98m long continuous annealing furnace at different steel walking speeds. In order to change the temperature system of each section of spheroidizing annealing, a box electric heating furnace is used to simulate the spheroidizing annealing process of continuous furnace according to the temperature system of each section of continuous heating furnace and different steel walking speed. The steel walking speed in the furnace is 3m/h, 4m/h, 5m/h, 6m/h and 7m/h respectively

the test results show that the qualified spheroidized structure can not be obtained by spheroidizing annealing at the steel walking speed of more than 4m/h. Using the controlled cooling material with the reddening temperature of 630 ℃, the steel walking speed is from 5m/h to 7m/h, but the qualified spheroidized structure (grade 2 ~ 2.5) can be obtained by spheroidizing annealing. On the contrary, if the steel after water cooling is annealed at the steel walking speed of 3m/h, its spheroidizing structure will coarsen to grade 3.5 ~ 6. When the reddening temperature after water cooling is higher than 640 ℃, the steel speed uses low-cost technology to improve the performance of plastic packaging materials by more than 5m/h, and the spheroidized structure after spheroidization is unqualified (less than grade 2). The test shows that the spheroidizing annealing time of water-cooled materials is significantly shorter than that of air-cooled materials, which ensures that the whole machine has high resolution and excellent repeatability

in the test scheme, the heating temperature of spheroidizing annealing was reduced from 820 ℃ to 800 ℃, and the residence time of 690 ℃ and 660 ℃ at the end of the furnace was shortened from 1H and 37min at each stage to 39min. Speed up the cooling. When the steel walking speed is 5m/h, the total annealing time is 6h 38min; The steel walking speed is 6m/h, and the total annealing time is 5h 38min; The steel walking speed is 7m/h, and the total annealing time is 4H and 46min

the results show that the spheroidizing annealing microstructure of air-cooled materials at the speed of more than 5m/h is below grade 2, and the spheroidizing microstructure of water-cooled materials after primary cooling is only grade 2 at the speed of 5m/h. However, the qualified spheroidized structure can still be obtained by spheroidizing annealing at the speed of 6m/h for the water-cooled material after secondary cooling. The pre microstructure before annealing with the reddening temperature of 630 ℃ is better. Even if the steel is spheroidized at the speed of 7m/h, the spheroidized microstructure is also grade 2

because the microstructure of the controlled cooling material after rolling is abnormal sorbite and pearlite with relatively thin lamellar spacing. During spheroidizing annealing, the diffusion path of carbon atoms is short, and the carbides are easy to dissolve. After dissolution, there are many points of residual carbide, which provides more parts for carbide precipitation in the cooling process, so a faster cooling speed can be adopted. At the same time, due to the small spacing between the layers, the interface between the layers increases correspondingly, and the interface energy increases, which also plays a role in accelerating atomic diffusion and spheroidizing annealing. In the controlled cooling material after rolling, the cementite in pearlite is intermittent or even hemispherical, which is conducive to the spheroidization process and shortens the spheroidization time

2.6 effect of controlled cooling on the properties of bearing steel bars

the hardness of controlled cooling material and air cooling material after annealing increases with the increase of spheroidizing steel walking speed, as shown in Figure 3

under the same steel walking speed, the hardness of the controlled cooling material is lower than that of the air-cooled material, because with the increase of the steel walking speed, the annealing time is shortened, and the spheroidizing structure level of the steel is reduced, so the hardness of the steel is increased, and the controlled cooling material is easier to spheroidize than the air-cooled material. At the same annealing time, the spheroidizing structure level of the controlled cooling material after annealing is higher than that of the air-cooled material. The carbide particles are uniform in size and dispersed in distribution, so the hardness of the control cooling material is low

the contact fatigue life of the control cooling material is higher than that of the air cooling material, which is given in Table 2 Φ Comparison results of contact fatigue life of 50 mm GCr15 bearing steel cold and air-cooled annealed materials

due to Φ 50mm bearing molten steel workbench moves along the y-axis in a straight line. The carbide particles of cold retreating fire material are smaller and more uniform than those of air-cooled retreating fire material, and the former has a diameter of 0.53 μ m. The latter is 0.63 μ M and the average distance of carbide particles is 1.28 respectively μ M and 1.34 μ m。

3 conclusion

gcr15 bearing round steel, with a diameter of Φ Within the range of 34 ~ 55mm, the grade of carbide can be reduced to less than 2.5 by adopting appropriate post rolling rapid cooling process. Shortening the spheroidizing annealing time can obtain 2 ~ 3 grades of spheroidizing structure in line with yjz-84 standard. Therefore, the rapid cooling process after rolling and rapid spheroidizing annealing process for large section bearing steel bars show that it is an advanced rolling production process in theory and practice, which should be promoted

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