Technological operation method for improving low-temperature impact toughness of weather-resistant structural steel of medium-thickness plate bridge
In the production process of weather-resistant structural steel of medium-thick plate bridges, slab casting, heating, rolling and cooling processes are adopted, combined with high-temperature burr-downline and stacking insulation methods, the problem of low-temperature impact performance fluctuations in weather-resistant structural steel of medium-thick plate bridges is solved, and qualified products are efficiently produced, reducing production costs and delivery time.
Patent Information
- Application Number
- CN202510272599.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-03-10
- Publication Date
- 2025-04-29
AI Technical Summary
The weather-resistant structural steel of medium-thick plate bridge fluctuates in low-temperature impact performance, especially in high-strength steel grades such as Q420qENH and Q500qENH, resulting in a low product pass rate and increased production costs and lead time.
The slab pouring, heating, rolling and cooling processes are adopted, including controlling the temperature during the opening and re-rolling process, followed by high-temperature burr downline and stacking insulation to achieve high-temperature aging treatment.
In medium and thick plate factories that do not have the conditions for online heat treatment, qualified bridge weathering steel can be produced, which improves the low-temperature impact toughness and pass rate of the product, reduces production costs, and ensures efficient delivery of orders.
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Abstract
Description
Technical Field
[0001] The invention belongs to the technical field of plate rolling and relates to a process operation method for improving the low-temperature impact toughness of medium-thick plate bridge weather-resistant structural steel. Background Art
[0002] Weathering structural steel for bridges (e.g. Q345qENH, Q420qENH, Q500qENH) is widely used in key manufacturing fields such as bridges, vehicles and towers because of its excellent atmospheric corrosion resistance and durability. Among them, Q420qENH plays a more important role in modern bridge construction with its high strength and good low-temperature toughness. In the future, with the continuous development of my country's transportation infrastructure, the application of Q420qENH steel will become more and more extensive. However, in order to ensure a certain weathering index, the weathering steel for bridges adds high alloy elements such as Cr (not less than 0.4%), Ni (not less than 0.3%), and Cu (not less than 0.25%) to its composition, which significantly increases the hardenability of this steel in the production process. In addition, the different cooling processes after rolling lead to a more complex microstructure distribution at room temperature, which brings great difficulties to the mechanical properties of steel, especially the low-temperature impact performance (usually requires a temperature of -40°C). The national standard requires that the arithmetic mean of the impact energy (Akv) of a group of three samples of bridge weathering steel should not be less than 120J. It is allowed that the single value of one sample is lower than the specified value, but it shall not be lower than 70% of the specified value. However, the bridge weathering steel products rolled off the medium and thick plate production line have the phenomenon of unqualified low-temperature impact performance while meeting the strength requirements, that is, the higher the strength, the greater the fluctuation of low-temperature impact performance, especially high-strength Q420qENH and Q500qENH are more common. Therefore, in order to improve the low-temperature toughness of this type of steel and increase the qualified rate of products, all major steel mills currently use online heat treatment processes for their bridge weathering steel products to ensure that the low-temperature impact performance of this type of steel is qualified. For example, Nanjing Medium and Thick Plate Plant uses the "quenching + online tempering" process to produce qualified products.
[0003] However, for some existing steel mills in my country, the medium and thick plate production lines do not have the conditions for heat treatment, so the online heat treatment process cannot be implemented, resulting in the products off the rolling line needing to be transported to other heat treatment lines for heat treatment. This operation will further increase production costs for production units and will greatly slow down the fulfillment rate of orders, resulting in long delivery times. Summary of the invention
[0004] The purpose of the present invention is to provide a process operation method for improving the low-temperature impact toughness of medium-thick plate bridge weather-resistant structural steel in view of the problems existing in the prior art.
[0005] To this end, the present invention adopts the following technical solutions: A process operation method for improving the low-temperature impact toughness of medium-thick plate bridge weathering structural steel comprises the following steps: (1) Slab casting, casting molten steel into slabs; (2) Heating the slab to 1100-1150°C; (3) Slab rolling, rolling the slab into steel plate. During the rolling process, the starting rolling temperature is 1080-1100°C, the re-rolling temperature is 820-840°C, and the final rolling temperature is 720-740°C; (4) Steel plate cooling: The rolled steel plate is cooled on a cooling bed to 350°C~420°C, and then stacked for insulation for more than 15 hours.
[0006] Furthermore, in step (3), if the thickness of the finished steel plate is 25-45 mm, the thickness of the slab to be heated during rolling is 2.8-3 times the thickness of the finished steel plate; if the thickness of the finished steel plate is less than 25 mm, the thickness of the slab to be heated during rolling is 2.5-2.8 times the thickness of the finished steel plate. Furthermore, in the step (4), the stacking and insulation is to stack the finished steel plates into a pile and place steel plates with temperature steel plates on the top and bottom, and no shims can be placed between the finished steel plates.
[0007] The beneficial effects of the present invention are: (1) For medium and thick plate mills without online heat treatment lines, they can produce qualified products of high-grade bridge weathering steel without online heat treatment; (2) High-temperature aging treatment is carried out by stacking and insulating the raw edges off the production line at high temperature, which not only saves energy but also effectively improves the product qualification rate and ensures the efficient fulfillment of orders. DETAILED DESCRIPTION
[0008] The present invention is described in detail below in conjunction with embodiments: A process operation method for improving the low-temperature impact toughness of medium-thick plate bridge weathering structural steel comprises the following steps: (1) Slab casting: casting molten steel into slabs. The composition and percentage of each substance in the slabs are consistent with those of existing conventional Q345qENH.
[0009] (2) Heating the slab: The slab is heated to 1100-1150° C. in a conventional manner. In this embodiment, the slab is heated to 1100° C.
[0010] (3) Slab rolling, rolling the slab into steel plate. During the rolling process, the starting rolling temperature is 1080-1100°C, and in this embodiment, it is heated to 1090°C. If the thickness of the finished steel plate is 25-45 mm, the waiting thickness of the slab during the starting rolling is 2.8-3 times the thickness of the finished steel plate. If the thickness of the finished steel plate is less than 25 mm, the waiting thickness of the slab during the starting rolling is 2.5-2.8 times the thickness of the finished steel plate. After the starting rolling, the recrystallization and coarsening of high-temperature grains are avoided. The re-rolling temperature is 820-840°C. The re-rolling adopts a multi-pass low-temperature large-reduction method to crush the grains to the greatest extent. In this embodiment, it is 830°C. The final rolling temperature is 720-740°C, and in this embodiment, it is 730°C. After the final rolling, the finished steel plate is obtained.
[0011] (4) Steel plate cooling: The rolled steel plates are cooled on a cooling bed to 350°C to 420°C. In this embodiment, the rolled steel plates are cooled to 360°C. The high-temperature burrs are then quickly removed from the production line. Finally, the steel plates are stacked for heat preservation. Specifically, the stacking and heat preservation process involves stacking the finished steel plates and placing heated steel plates on the top and bottom. Specifically, the temperature of the steel plates is 350°C to 420°C. No iron pads can be placed between the stacked finished steel plates to ensure good heat preservation. The heat preservation time is greater than 15 hours.
[0012] The properties of the finished steel plate are shown in Table 1: Table 1 It can be seen from Table 1 that the steel plates prepared by this method have all indicators meeting the corresponding requirements.
Claims
1. A process operation method for improving the low temperature impact toughness of medium and thick plate bridge weathering structural steel, characterized in that: The following steps are involved: (1) Slab casting, casting molten steel into slabs; (2) Heating the slab to 1100-1150°C; (3) Slab rolling, rolling the slab into steel plate. During the rolling process, the starting rolling temperature is 1080-1100°C, the re-rolling temperature is 820-840°C, and the final rolling temperature is 720-740°C; (4) Steel plate cooling: The rolled steel plate is cooled on a cooling bed to 350°C~420°C, and then stacked for insulation for more than 15 hours.
2. A process operation method for improving the low temperature impact toughness of medium and thick plate bridge weathering structural steel according to claim 1, characterized in that: In the step (3), if the thickness of the finished steel plate is 25-45 mm, the thickness of the slab to be heated during rolling is 2.8-3 times the thickness of the finished steel plate; if the thickness of the finished steel plate is less than 25 mm, the thickness of the slab to be heated during rolling is 2.5-2.8 times the thickness of the finished steel plate. According to claim 1, a process operation method for improving the low temperature impact toughness of medium and thick plate bridge weathering structural steel is characterized in that: In the step (4), the stacking and insulation is to stack the finished steel plates into a pile and place steel plates with temperature steel plates on the top and bottom, and no iron pads can be placed between the finished steel plates.