Annealing-free direct rolling method for low-cost 430 stainless steel

Abstract

The 430-series stainless steel has good corrosion resistance and oxidation resistance, and is widely applied to the fields of buildings, decoration, household appliances, kitchen ware and the like due to the excellent performance and relatively low price of the 430-series stainless steel. As cover annealing is not carried out on 430, and the 430 is directly subjected to acid pickling and cold rolling through air cooling after hot rolling, adverse factors which influence cold rolling, such as edge cracking, high hardness of heads and tails, high strength and the like, are easy to occur in cold rolling. According to the annealing-free direct rolling method for the low-cost 430 stainless steel, slow cooling and heat preservation are conducted on the hot-rolled 430 stainless steel, the microscopic structure of the 430 stainless steel has the obvious grain recovery characteristic, the yield strength and tensile strength are reduced, the hardness is reduced, the plasticity is improved, the problems that the low-cost 430 stainless steel is high in hardness and poor in plasticity due to the fact that annealing is not conducted can be remarkably solved, cold rolling is better facilitated, and the service life of the low-cost 430 stainless steel is prolonged. And after slow cooling treatment, the problems of edge cracking, difficult rolling and the like do not occur in the cold rolling process of 430. The slow cooling heat preservation cover has an improvement effect on solving the problem of low-cost rolling 430 in the cold rolling process, and development of a low-cost cover-free annealing process and trial production of new products are facilitated.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment and relates to a method for direct rolling of 430 stainless steel without annealing at low cost. Background Art

[0002] 430 stainless steel has broad market prospects in the home appliance, kitchen and bathroom, hardware, elevator and other industries due to its low price and excellent performance. Most of the 430 produced by Taigang without bell annealing is used as low-cost re-rolling raw material, mainly for users to use for re-rolling. This variety mainly reduces production costs by reducing the bell furnace annealing process, but the hot-rolled raw material coils that are not annealed in the bell furnace show the disadvantages of high head and tail hardness and high strength during downstream cold rolling, and the strip head, middle and tail structure are uneven. These deficiencies lead to increased mill load during the rolling process, and the strip thickness accuracy and plate shape are difficult to control. There is a deviation between the actual thickness of the finished product and the set thickness, resulting in poor cold plate shape and other problems. The existence of the above problems has pushed up the production cost of users and affected their product competitiveness. Therefore, it is very necessary to develop a new type of bell-free annealing technology to obtain hot-rolled raw materials with excellent cold working mechanical properties.

[0003] The present invention provides a low-cost annealing-free direct rolling method for 430 stainless steel. The hot-rolled strip steel controls the strip steel temperature and time at each stage, and utilizes the residual heat of the steel coil itself to be kept warm in a slow cooling and heat preservation cover. The process does not require energy consumption and is a green, low-carbon, pollution-free advanced technology. Slow cooling and heat preservation after hot rolling allow the recrystallized grains and banded structures in the strip steel structure to have sufficient time to recover and grow, and slow cooling and heat preservation is conducive to the precipitation of carbides in the structure within the grains and at the grain boundaries, thereby obtaining good comprehensive mechanical properties and cold working properties. Summary of the invention

[0004] The purpose of the present invention is to provide a low-cost annealing-free direct rolling method for 430 stainless steel in view of the above problems.

[0005] The object of the present invention is achieved in this way: a low-cost 430 stainless steel annealing-free direct rolling method, comprising the following steps: step 1: the temperature of the steel coil in the hot rolling heating furnace is controlled at 1160-1240°C, the rough rolling outlet temperature is 1040-1080°C, the finishing rolling outlet temperature is 850-980°C, and the coiling temperature is 750-800°C; step 2: the distance from the end of coiling to the loading of the steel coil into the heat preservation cover is 50-80 meters, the required time does not exceed 60 minutes, and the unit hourly temperature drop in the cover is 1.5-3.0°C / hour; step 3: after the steel coil enters the heat preservation cover, it is double-stacked, and the temperature is required to be 6 50-700℃, the holding time is 48-72 hours or the temperature of the steel coil drops to below 300-400℃ before leaving the hood; Step 4: Hot wire pickling after leaving the hood, the scale breaking rate control range: 0.8%-2.4%, the elongation control range: 1.0%-3.0%, the shot blasting speed control range: 1900-2300 rpm; Step 5: Cold rolling after hot wire pickling, the finished product thickness range is 0.8-3.0mm, the deformation control range is 50%-80%, and the rolling speed is 90-100m / min; the rolling passes are 7 or 9, and the deformation control range of each pass is: 10%-15%.

[0006] The pickling speed of step 4 is: 55-65m / min, acid temperature: 80-100°C, sulfuric acid concentration: 280-350g / L, and the hydrofluoric acid concentration control range is 2-5g / L.

[0007] The beneficial effects of the present invention are as follows: the present invention is adopted to prevent and save the low-cost 430 hood annealing process by utilizing slow cooling after rolling, simplify the production process, reduce the production cost of low-cost 430 rolling, and can significantly improve the problems of high head and tail strength and high hardness in the 430 cold rolling process after slow cooling and heat preservation, reduce the adverse effects of hot-rolled raw materials on cold rolling due to uneven tissue, significantly improve the production efficiency of the next cold rolling process and downstream users, and reduce the incidence of surface quality defects. DETAILED DESCRIPTION

[0008] In order to meet the process requirements of annealing-free direct rolling, a slow cooling and insulation device is installed in the hot rolling workshop. The device does not require energy supply and can withstand a maximum furnace temperature of 950°C. The maximum temperature of the feeding workpiece does not exceed 900°C. The insulation time can be adjusted according to the actual temperature of the strip and controlled at 48-72 hours. The temperature drop in the insulation cover is controlled within -3°C / hour. The overall temperature drop should not be too fast. The same batch of steel coils are fed and discharged uniformly to ensure the temperature uniformity between the strips.

[0009] The strip finishing exit temperature and coiling temperature are one of the important indicators that affect ferritic stainless steel. If the finishing exit temperature and coiling temperature are too low, it will be deformed during processing, and coarsened after slow cooling recovery and recrystallization annealing, resulting in an uneven mixed crystal structure, which makes the strength and toughness worse. In addition, too low a temperature will also produce residual stress inside the strip, making the steel plate hard, which not only makes coiling and cold rolling difficult, but also makes it difficult to meet the requirements for thickness and performance; but if the finishing exit temperature and coiling temperature are too high, the grains will be coarse and ribbon-shaped, which is easy to produce bad directionality and deteriorate the mechanical properties. At the same time, it also affects the grain size of the cold plate, making the cold rolling plasticity worse and the deep drawing performance decreased. Therefore, the appropriate finishing exit temperature and coiling temperature are particularly important for the organization and performance of the hot-rolled coil raw materials of the hood-free direct rolling process.

[0010] After the hot-rolled strip is coiled, it needs to be transported to the vicinity of the insulation cover by the transport chain, and then hoisted into the insulation cover by the overhead crane. During this process, the temperature drop of the steel coil is about 100-150℃, and the temperature drop is greater the closer to the outside of the steel coil, and the temperature drop is smaller the closer to the core of the steel coil; the higher the temperature entering the insulation cover, the longer the steel coil stays in the high temperature range, and the more opportunities for the grains to recover and grow in this range. Therefore, it is also very important to shorten the time from coiling to loading into the insulation cover as much as possible.

[0011] In addition, since the steel coils in the insulation device are all hot-loaded, they are mainly used to ensure process insulation, so they have greatly improved the workshop environment and saved energy and reduced consumption.

[0012] The 430 hot coil that is not annealed in a bell furnace after hot rolling has problems such as uneven structure, high strength, and high hardness at the head and tail at room temperature, which causes excessive equipment load for downstream users during the cold rolling process, resulting in a deviation between the actual thickness of the cold-rolled finished product and the target thickness. In addition, the thickness accuracy and shape of the cold plate are difficult to control, resulting in poor shape and surface quality of the cold plate.

[0013] The present invention shortens the circulation time of hot-rolled coils entering the insulation hood through the "rapid temperature rolling" method, while increasing the hot rolling finish rolling outlet temperature (FDT) and the coiling temperature (CT), and quickly loads the 430 steel coils still in the high temperature range (≥650℃) into the insulation hood. After a certain period of slow cooling and heat preservation, the slow cooling grain recovery makes the steel coil structure more uniform, the strength and hardness are reduced, the plasticity is improved, and better cold processing performance is obtained. The detailed invention technical scheme is: The present invention aims to develop a 430 stainless steel annealing-free direct rolling process, which is different from the ordinary hood furnace annealing process and does not require a heat source supply, so as to solve the problem of poor performance of 430 stainless steel hot-rolled coils without hood annealing. Grain recovery is achieved by adjusting the temperature of the steel strip and the slow cooling and heat preservation time, and the room temperature mechanical properties of the steel strip are effectively improved.

[0014] 1. The furnace temperature of the steel coil in the hot rolling heating furnace is controlled at 1200-1240℃, the outlet temperature of the rough rolling is 1040-1080℃, the outlet temperature of the finishing rolling is 940-980℃, and the coiling temperature is 750-800℃.

[0015] 2. The distance from the end of coiling to the loading into the insulation cover is 50-80 meters, and the time required is no more than 60 minutes. The temperature drop per hour in the cover is 1.5-3.0℃ / hour.

[0016] 3. After the steel coils enter the insulation cover, they are stacked in double layers. The temperature when entering the cover is required to be 650-700℃, and the insulation time is 48-72 hours or the temperature of the steel coils drops below 300-400℃ before leaving the cover.

[0017] 4. The control range of hot wire scale breaking elongation is 1.0%-3.0%, the control range of shot blasting speed is 1900-2300 rpm, the pickling speed is 55-65m / min, the acid temperature is 80-100℃, the concentration of sulfuric acid is 280-350g / L, and the concentration of hydrofluoric acid is 2-5g / L.

[0018] 5. The thickness of cold-rolled finished products ranges from 0.8 to 3.0 mm, the deformation control range is 50% to 80%, and the rolling speed is 90 to 100 m / min. Example 1

[0019] The chemical composition of 430 ferritic stainless steel coil contains C: 0.045%; Si: 0.34%; Mn: 0.32%; P: 0.028%; S: 0.001%, Cr: 16.7%; N: 0.041%; the rest is Fe.

[0020] 1. The process flow of converter → AOD → LF → continuous casting is adopted to obtain a 200mm thick continuous casting billet, and then hot-rolled stainless steel coils with a thickness of 3.0-6.0mm are obtained through hot continuous rolling.

[0021] 2. The furnace temperature of the steel coil in the hot rolling heating furnace is controlled at 1230℃, the rough rolling outlet temperature is 1070℃, the finishing rolling outlet temperature is 960℃, and the coiling temperature is 750℃.

[0022] 3. The distance from the end of coiling to the loading into the insulation cover is 60 meters, and the time required is no more than 60 minutes. The temperature drop per hour in the cover is 1.8℃.

[0023] 4. After the steel coils enter the insulation cover, they are stacked in double layers. The temperature when entering the cover is required to be 650℃, and the insulation time is 64 hours or the steel coil temperature drops below 350℃ before leaving the cover.

[0024] 5. Hot wire scale breaking elongation control range: 1.0%-3.0%, shot blasting speed control range: 1900-2300 rpm, pickling speed: 55-65m / min, acid temperature: 100℃, sulfuric acid concentration: 350g / L, hydrofluoric acid concentration control range is 2g / L.

[0025] 6. The thickness of cold-rolled finished products ranges from 0.8 to 3.0 mm, the deformation control range is 50% to 80%, and the rolling speed is 90 to 100 m / min.

[0026] By adopting the design of the present invention, the 430 hot-rolled steel coil is subjected to slow cooling and heat preservation, and the mechanical properties are tested. The yield strength of the steel coil without slow cooling and heat preservation is 385 MPa, the tensile strength is 651 MPa, the elongation is 20%, the Rockwell hardness HRB 91, and the grain size is 5.5. The yield strength of the steel coil after 64 hours of slow cooling and heat preservation is 389 MPa, the tensile strength is 559 MPa, the elongation is 25%, the Rockwell hardness HRB 82, and the grain size is 6.5. By comparison, it can be seen that the present invention effectively improves the hot-rolled 430 mechanical properties and cold rolling processing performance. Example 2

[0027] The chemical composition of 430R ferritic stainless steel coil contains C: 0.022%; Si: 0.34%; Mn: 0.35%; P: 0.028%; S: 0.003%, Cr: 16.4%; N: 0.041%; the rest is Fe.

[0028] 1. The process flow of converter → AOD → LF → continuous casting is adopted to obtain a 200mm thick continuous casting billet, and then hot-rolled stainless steel coils with a thickness of 3.0-6.0mm are obtained through hot continuous rolling.

[0029] 2. The furnace temperature of the steel coil in the hot rolling heating furnace is controlled at 1160℃, the rough rolling outlet temperature is 1050℃, the finishing rolling outlet temperature is 880℃, and the coiling temperature is 800℃.

[0030] 3. The distance from the end of coiling to the loading into the insulation cover is 60 meters, and the time required is no more than 60 minutes. The temperature drop per hour in the cover is 1.8℃.

[0031] 4. After the steel coils enter the insulation cover, they are stacked in double layers. The temperature when entering the cover is required to be 700℃, and the insulation time is 48 hours before leaving the cover.

[0032] 5. Hot wire scale breaking elongation: 2.3%, shot blasting speed: 2100 rpm, pickling speed: 65m / min, acid temperature: 100℃, sulfuric acid concentration: 280g / L, hydrofluoric acid concentration control range is 2g / L.

[0033] 6. The thickness of cold-rolled finished products ranges from 2.0 to 4.0 mm, the deformation control range is 40% to 60%, and the rolling speed is 90 to 100 m / min.

[0034] By adopting the design of the present invention, the 430R hot-rolled steel coil is subjected to slow cooling and heat preservation, and the mechanical properties are tested. The yield strength of the steel coil without slow cooling and heat preservation is 394 MPa, the tensile strength is 671 MPa, the elongation is 23%, the Rockwell hardness HRB 91, and the grain size is 6.5. The yield strength of the steel coil after 48 hours of slow cooling and heat preservation is 354 MPa, the tensile strength is 563 MPa, the elongation is 26%, the Rockwell hardness HRB 81, and the grain size is 6.0. By comparison, it can be seen that the present invention effectively improves the hot-rolled 430 mechanical properties and cold rolling processing performance.

[0035] The above description is only a specific embodiment of the present invention, but the structural features of the protection scope of the present invention are not limited thereto. Any changes or modifications made by any technician in the field of the present invention are all covered by the patent scope of the present invention.

Claims

1. A method for direct rolling of 430 stainless steel without annealing at low cost, characterized in that: The following steps are involved: Step 1: The furnace temperature of the steel coil in the hot rolling heating furnace is controlled at 1160-1240°C, the rough rolling outlet temperature is 1040-1080°C, the finishing rolling outlet temperature is 850-980°C, and the coiling temperature is 750-800°C; Step 2: The distance from the coiling end to the loading into the insulation cover is 50-80 meters, and the time required is no more than 60 minutes. The temperature drop per hour in the cover is 1.5-3.0℃ / hour; Step 3: After the steel coils enter the heat preservation cover, they are stacked in double layers. The temperature when entering the cover is required to be 650-700℃. The heat preservation time is 48-72 hours or the temperature of the steel coils drops below 300-400℃ before leaving the cover. Step 4: After exiting the hood, hot wire pickling is performed, the scale breaking rate control range is: 0.8%-2.4%, the elongation control range is: 1.0%-3.0%, and the shot blasting speed control range is: 1900-2300 rpm; Step 5: Cold rolling after hot wire pickling, the finished product thickness range is 0.8-3.0mm, the deformation control range is 50%-80%, and the rolling speed is 90-100m / min; the rolling passes are 7 or 9, and the deformation control range of each pass is: 10%-15%.

2. A low-cost annealing-free direct rolling method for 430 stainless steel according to claim 1, characterized in that: The pickling speed of step 4 is: 55-65m / min, acid temperature: 80-100°C, sulfuric acid concentration: 280-350g / L, and the hydrofluoric acid concentration control range is 2-5g / L.