Control cooling technology is the production of rolling a very important technology, which after rolling of steel by controlling the cooling temperature and speed to improve the organization and performance of steel and, ultimately, good mechanical properties. With the control of cooling technology, it has been used in more mature sheet, strip, wire and other fields.
In order to control the cooling after rolling and applied technology to better promote the seamless pipe production, there have been a number of steel companies and steel research institutes have carried out the experiment. Initial results show that: the low C Low Mn, seamless steel tube rolling surface obtained after rapid cooling of high-strength, high toughness of acicular ferrite and tempered sorbite, grain refinement significantly, the overall integrated steel mechanical properties has increased significantly; for carbon steel pipe and alloy steel pipe, rolling fast cooling after the adoption of appropriate technology, in the rapid cooling and tempering steel dual role of their own under the heat, get the surface with high strength, high toughness of the back Fire sorbite, acicular ferrite, lower bainite and other organizations at the same time, internal ferrite network of reduced grain refinement, the overall strength and toughness of steel mechanical properties while improving, and not in the elongation diminished greatly improved toughness, improved the impact toughness of low rolling-state status. Rapid cooling process after rolling thick-walled seamless steel pipe thickness direction of the organization will make composite, the overall benefit of improved overall performance. In order to apply the results of the initial study production practices, jointly developed a controlled cooling after hot rolling seamless system to improve the overall performance improvement of the product.
After hot-rolled steel, mainly through radiation, conduction and convection heat in three ways. Among them, the steel surface and the convection heat transfer between the cooling medium is the main form of heat. Using a mixture of water and air as a cooling medium, "the aerosol cooling." Air-cooled, the steel surface heat faster than steel itself much slower thermal conductivity, temperature more even thickness direction of steel; aerosol cooling, heat pipe surface greatly improved the speed, direction of the thickness of the pipe near the surface there is a certain temperature gradient. About 950 ℃ hot-rolled seamless pipe from the finishing temperature to room temperature in the process, according to the cooling conditions, the austenite will be broken down into different tissue types of the product. Therefore, the thickness direction of the aerosol cooling pipe surface temperature near the difference in the thickness direction of the organization will inevitably lead to the diversification of steel; as steel internal rely on thermal conduction heat transfer, it is continuous throughout the temperature changes, so even if the formation of different organizations, organizations also exist between the transition zone, that is: different microstructure of a coexistence between the mixing zone, like steel from the composite layers of different types of organizations formed. Organizational diversity of the mechanical properties of steel change, showing the strength and toughness of the new combination of mechanical properties; organizational continuity of the pipe between the different types of organizations through the "transition area" closely, that no "layering" phenomenon; organization to make steel in the compound at different thickness, showing a different mechanical properties, composite materials play features and benefits.
As we all know, steel composite material has many unique advantages, such as: grain refinement of the surface can make the overall fatigue performance of steel in the construction sector, the steel surface fine-grained uniform refinement with full-face steel compared to have almost the same ability to suppress fatigue crack growth; surface high-intensity round or spherical structure, with a higher resistance to external pressure capability is commonly referred to as "eggshell theory." Seamless surface of high strength, high toughness will bring more benefits to its performance.
For high-quality carbon steel 20ф, 45ф structure pipe, test the use of air-cooled after rolling process to produce a more uniform equiaxed ferrite + pearlite; when using aerosol cooling process after rolling, although subsequent air cooling the steel in the slight temperature rise, due to the cooling pipe surface by aerosol and the subsequent role of alternating air-cooled, the surface temperature rise after the first drop, while the middle and inner wall temperature is relatively slow diffuse. This is bound to pipe wall thickness direction of the organization's diversity. 20 # steel pipe, ф140mm × 13mm size aerosol cooling after rolling microstructure, for example. In the steel surface to generate a 0.9mm thickness of bainite, and 0.9mm to 35mm thickness of the transition zone composite tissue. As is usually acicular ferrite dislocation density is higher than the equiaxed ferrite dislocation density, therefore, from the point of view of dislocation strengthening, acicular ferrite yield strength corresponding to equiaxed ferrite is greater than the corresponding yield strength. In addition, the aerosol cooling after rolling tube 20ф structure point of view, the equiaxed ferrite grain size is smaller than obtained by air-cooled equiaxed ferrite grain, fine grain strengthening from the point of view, the mechanical properties significantly improved.
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