1 rolling equipment and technology
A rolling rod shop 22 mill thick, pre-finishing of 6 (18), finishing 4 the production specifications of φ20 ~~ 90; online tank 4 sets each tank There are three different inner diameter of the pipe, depending on the production specifications can use or combination use. Water cooling method using high-pressure water tank nozzle, # 1, # 2, water tanks, each lasting 7 m 9 nozzle (6 blowing two blowback blowing); 3A, 3B tank, each lasting 5 m (4 is blowing two blowback, a blowing). Water pressure 1.5 ~ 1.8 MPa, the maximum water consumption per hour 1180 L.
Analysis and modeling of ultra-rapid cooling after the 2 bearing steel rolling
2.1 after rolling fast cooling analysis
Controlled Cooling of process heat transfer process generally includes the following two stages: the first stage quench zone, steel leaving the finishing mill finish rolling temperature as soon as possible into the device of rapid cooling, rapid cooling. This phase heat transfer to boiling fish turbulence forced convection by forced convective heat transfer in two ways. Steel after entering the cooler, because the steel surface temperature is considerably higher than the saturation temperature of the water, the water temperature surge to the boiling point and vaporized, the the dynamic vapor film is formed in the steel wall. At this time shot water to ≥ 1.0 MPa pressure shocks steel surface, fully broken vapor film, the movement of the outer wall of the steel and promote the conduct of the boiling process, this heat transfer process having a very high heat flow, so that the heat transfer coefficient surge. The second stage is a slow cooling stage, the steel by means of the rapid cooling, the natural convection cooling in the air, re-distribution of heat in the cross section of this stage of steel. Both processes belong to the heat exchange between the rolling member and the external environment (phase change heat excluded). Internal rolling, regardless rolling at which stage they are heat conduction process. That is, the metal interior of the heat through the heat transmitted to the metal surface, and then by the metal surface is passed by way of radiation and convection to the surrounding medium, thereby causing the change of the rolling temperature throughout.
Rapid cooling after rolling cooling heat conduction process, mainly in the following two ways:
(1) Water-cooled: Strong convective heat transfer;
(2) air cooling: the equivalent thermal radiation and contact heat transfer.
2.2 control simulation of cooling after rolling
Using ANSYS finite Tools software, in the use of finite element solving rolled Controlled cold temperature field simulation calculation of the process, need thermal conductivity, different temperature than the heat capacity, the ambient temperature, the temperature of the cooling water, material density, thermal conductivity than the heat capacity is not constant, with the organization of the status and temperature of the material varies and, therefore, also vary over time. According to the relevant literature and field parameters, control the selection of bearing steel GCr15, material density 7810 kg/m3 cooling simulation; air temperature of 25 ° C; cooling water temperature of 20 ° C.
Continuous cooling process of the rolling member of the transient analysis, simulating a different cooling temperature change, taking into account the minimum temperature of the rear surface of the steel wear water can not be less than 350 ° C, so as not to produce martensite, and therefore the water flow should be controlled simulation analysis once to wear water with quite different secondary wear water, shown in Table 2.
Take time to wear water cooling bed back to red, the final maximum temperature greater than 750 ° C.
Take the second through the water after cooling bed back to red, the final maximum temperature is below 700 ° C.
         2.3 simulation to verify the results
φ40 bearing steel intermittent rapid cooling of the control two-stage cooling process from the finish rolling temperature of 950 ~ 1000 ℃ into the # 2 tank, the tank after a steel temperature back to red to 700 ~ 750 ℃, and then enter 3 # B tank secondary cooling, 3 # B tank steel surface temperature of 350 ~ 450 ℃, the surface temperature of 600 to 700 ° C on cooling bed, an increase of 10 to 30 ℃ maximum temperature on the cooling bed after steel reddening.
The comparison shows that the predicted temperature and the measured values ??from the measurement points, basic prediction deviation less than 20 ℃, shows that use of the numerical simulation study bearing steel round bar temperature field during cooling is feasible, a true reflection of the round steel sectional temperature change trend and the law.
2.4 bearing steel rolling the cooling temperature field analysis
Simulation results show that the bearing steel using a fast cold reddening after the temperature interval will generate coarse pearlite and serious mesh carbide. The large number of previous test results show that the precipitation of carbides mesh temperature between 700 ~ 900 ℃, and when back to red temperature less than 700 ° C, basically eliminating the network carbide in the original tissue, and therefore should speed up this a temperature of the cooling rate of steel. Bearing steel, the use of one-time rapid cooling to the end of the cold temperature, the martensitic transformation will occur due to the large steel cross-section, the steel surface, the cross-sectional temperature increase, resulting in the organization from the surface to the center, the center of the high temperature generated coarse pearlite carbide network and more serious. Several intermittent fast cooling process is appropriate should be used to prevent the emergence of uneven tissue. In this paper, the production line of the plant actually considered a secondary cooling process, through the process of adjustment, using ANSYS simulation and control back to red temperature between 600 ~ 680 ℃.
3 after rolling the ultra-rapid cooling organizational analysis
φ40 bearing steel rolling wear water organization time to wear water surface and center organization, visible surface tissue pearlite and intermittent network carbide, at the center of the pearlite and continuous thick carbide network carbide network is clear. Open # 2 and 3B tank, back red highest control below 700 ℃ surface tissue to pearlite, no carbide network, the center there is a small amount of fine mesh carbide surface.
φ40 bearing steel rolling speed of 3.4m / s, and immediately after rolling into the vicinity of 2 # tank quenched to 400 ~ 500 ℃, the cooling rate is controlled at 150 to 260 ° C / s, this temperature range should occur recrystallized grains grew been adequately suppressed. Then after short reddening, and then enters 3B tank, then air-cooled on the cooling bed, the control maximum reddening below 700 ℃, the total time required is 30 seconds, the average cooling rate in the interval of 10 to 20 ° C / s , to avoid the precipitation of carbides of the mesh.
4 Conclusion
(1) rapid cooling after rolling through ANSYSY finite element software simulation can better match the actual cooling conditions, and provide help for the development of the cooling process.
(2) ANSYSY finite element simulation, you can clearly analyze the surface, center temperature changes, and to develop a better cooling process, to get the ideal results.
(3) bar ultra fast cooling can be better suppressed mesh precipitation of carbides during the cooling process, and provides an effective means for improving the level of the mesh of the bearing steel.
(4) on-site production practices by ultra-rapid cooling after rolling control mesh level not greater than 2, in accordance with the new standard of GB/T18254-2002 annealing carbide network-level pass rate of 100%.
 



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