A method for improving the surface quality of hot-rolled plain carbon steel coil

Abstract

A method for improving the surface quality of hot-rolled plain carbon steel coils, comprising the following processes: heating in a casting furnace, high-pressure water descaling, five-pass reversible reciprocating rough rolling, high-temperature steam cooling of an intermediate billet, flying shearing, seven-pass continuous finishing rolling, laminar cooling, and coiling, wherein the intermediate billet high-temperature steam cooling process is to spray high-temperature steam onto the surface of the intermediate billet through a large number of fine nozzles, and control the surface temperature of the intermediate billet to be 900°C-1000°C, and the steam temperature to be 200°C-400°C. The beneficial effects of the present invention are: by short-term cooling treatment of the intermediate billet with high-temperature steam, the uniformity of the intermediate billet oxide scale is improved, the thickness, morphology, performance and structure of the intermediate billet surface oxide scale are optimized, the high-temperature plasticity of the oxide scale is used to avoid the formation of press-in type oxide scale, and the descaling process before finishing rolling is saved, thereby improving production efficiency and reducing costs.

Description

Technical Field

[0001] The invention relates to a steel rolling method, in particular to a method for improving the surface quality of a hot-rolled plain carbon steel coil. Background Art

[0002] Hot-rolled steel strips and coils are one of the most widely used and largest-volume varieties of all rolled steel products. With the vigorous development of steel enterprises, the number of hot-rolled steel strips and coils production lines is large and the products are highly homogenized, resulting in increasingly fierce market competition for hot-rolled steel strips. The excellent surface quality of hot-rolled steel strips and coils is an important basis for customers to choose products.

[0003] As a product of the reaction between the steel matrix and oxygen, scale is a very common surface material in hot-rolled plate products. Its structure, thickness and morphology determine the surface quality of hot-rolled steel products. Usually, the furnace scale generated during the heating and insulation stage of the heating furnace is called primary scale, which is thicker and is generally removed by descaling. In the subsequent rolling process, the newly generated scale due to the high temperature of the plate is collectively referred to as secondary scale, which is thinner and brittle. During the rolling process, accompanied by the rolling force, the scale is likely to break and be pressed into the surface of the steel plate, forming a pressed scale defect. Therefore, the plasticity of the scale itself during the rolling process will affect the surface quality of the hot-rolled plate. Pressed scale defects are very common in hot-rolled plate products and are surface quality defects that have a significant impact on the use of downstream users. If the degree of pressed scale defects is very serious, even the pickling process cannot completely remove it, and the downstream deep processing processes such as stamping and coating are also significantly affected. Therefore, how to control and reduce scale defects has become a problem that hot-rolled plate manufacturers attach great importance to.

[0004] At present, although there are many studies on the removal of pressed iron oxide scale, most of them focus on physical descaling methods, such as descaling with high-pressure water before finishing rolling, descaling by rolling method and descaling by mechanical method. However, the structure of surface iron oxide scale and its bonding strength with the matrix are different due to the different chemical composition of plain carbon steel. In particular, steel materials often contain Si elements, which will generate Fe at the interface between the steel material matrix and the iron oxide scale. 2 SiO 4 (Ringwoodite structure), Fe 2 SiO 4 When the temperature is above 1177℃, it will change from solid to liquid, promoting the aggregation and growth of pores in the iron oxide scale and accelerating surface oxidation. 2 SiO 4 When it re-solidifies from the molten state, it will surround the FeO grains and form FeO / Fe 2 SiO 4The eutectic product greatly reduces the complete removal of FeO during descaling, so that the residual iron oxide scale forms pressed iron oxide scale defects during the rolling process. Summary of the invention

[0005] The present invention aims to solve the problems of the prior art and provides a method for improving the surface quality of a hot-rolled plain carbon steel coil. The method optimizes the structure of secondary iron oxide scale by adding a short-time steam cooling process for an intermediate billet and matching it with a heating and rolling process, thereby avoiding the formation of pressed iron oxide scale defects.

[0006] The technical solution adopted in the present invention is as follows:

[0007] A method for improving the surface quality of a hot-rolled plain carbon steel coil comprises the following steps: heating in a casting furnace - high-pressure water descaling - five-pass reversible reciprocating rough rolling - intermediate billet high-temperature steam cooling - flying shearing - seven-pass continuous finishing rolling - laminar cooling - coiling, wherein the intermediate billet high-temperature steam cooling step is to spray high-temperature steam onto the surface of the intermediate billet through a large number of fine nozzles, and the surface temperature of the intermediate billet is controlled to be 900°C-1000°C, and the steam temperature is 200°C-400°C.

[0008] In the above-mentioned method for improving the surface quality of hot-rolled plain carbon steel coil, a steam spray device is arranged between the roughing mill and the flying shear, and the steam spray device includes a steam generating box and a vacuum furnace. The steam generating box is provided with a heating element, and a water inlet pipe is arranged at the lower part of one side of the steam generating box, and a steam delivery pipe is arranged at the upper part of the other side; the vacuum furnace is provided with an inner sleeve, and the outlet of the steam delivery pipe is connected to the inner sleeve. A steam pipe is arranged on the top of the vacuum furnace, and the outlet of the steam pipe is connected to a steam nozzle, and the steam nozzle corresponds to the upper and lower surfaces of the intermediate billet.

[0009] In the above method for improving the surface quality of hot-rolled plain carbon steel coil, the high-temperature steam cooling time of the intermediate billet is controlled within 20-25 seconds; the rough rolling temperature is controlled within 1000-1100° C., and the rolling rate range is controlled within 3.0-3.9 m / .

[0010] In the above method for improving the surface quality of hot-rolled plain carbon steel coil, the heating temperature of the slab in the heating process of the slab heating furnace is controlled in the range of 1100°C-1150°C, and the time in the furnace is controlled in 2-3 hours, preferably 2 hours.

[0011] In the above method for improving the surface quality of hot-rolled plain carbon steel coil, the descaling water pressure in the high-pressure water descaling process is ≥18MPa, and a side water spray is installed after the descaling box.

[0012] The above method for improving the surface quality of hot-rolled plain carbon steel coil, the chemical composition of the plain carbon steel coil is: C0.01-0.10%, Si≤0.15%, Mn 0.05-1.50%, P≤0.03%, S≤0.03%, Alt≤0.08%, and the rest is Fe, micro-alloying elements and impurities.

[0013] The above method for improving the surface quality of hot-rolled plain carbon steel coils comprises a five-pass reversible reciprocating rolling process, and a descaling treatment is performed in the odd-numbered passes.

[0014] The above method for improving the surface quality of hot-rolled plain carbon steel coil adopts side water spray cooling at the entrance of each finishing mill in the finishing process; the rolling temperature adopts a low-temperature rolling system, the temperature range is controlled at 840℃-1000℃, and the rolling rate range is controlled at 10.0-11.3m / s.

[0015] In the above method for improving the surface quality of hot-rolled plain carbon steel coil, the coiling temperature in the coiling process is controlled at 560°C-640°C.

[0016] The present invention takes targeted measures based on the high temperature plasticity of the oxide scale on the surface of plain carbon steel, combined with the generation and structural evolution of the oxide scale in the heating and rolling stages during the preparation of thin plates, and takes the intermediate billet steam short-time cooling as the characteristic, and proposes a specific implementation and solution. The present invention is simple and practical, and has important practical significance for improving the surface pressed oxide scale defect of plain carbon steel thin plates.

[0017] The beneficial effects of the present invention are as follows: by subjecting the intermediate billet to a short-time high-temperature steam cooling treatment, the uniformity of the intermediate billet iron oxide scale is improved, the thickness, morphology, performance and structure of the intermediate billet surface iron oxide scale are optimized, the high-temperature plasticity of the iron oxide scale is used to avoid the formation of press-in type iron oxide scale, and the descaling process before finishing rolling is saved, thereby improving production efficiency and reducing costs; by controlling the heating system, the primary iron oxide scale on the surface of the steel plate does not have Fe 2 SiO 4 The melting and solidification of FeO are improved, and the proportion of FeO in the oxide scale is increased, which ensures that the oxide scale can be basically removed in one rough descaling treatment, and greatly reduces the incidence of oxide scale defects on the surface of the steel plate; by implementing low-temperature rolling and high-speed rolling schemes in the rough rolling and finish rolling stages, the formation and growth of oxide scale during the rolling process is effectively reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the process of the present invention;

[0019] Figure 2 is a schematic diagram of a steam spray device.

[0020] The numbers in the figure are: 1. heating furnace, 2. descaling machine, 3. roughing mill, 4. spray device, 5. flying shear, 6. finishing mill, 7. laminar cooling; 8. coiler, 9. steam generator box, 10. heating element, 11. water inlet pipe, 12. steam delivery pipe, 13. vacuum furnace, 14. inner sleeve, 15. argon gas pipeline, 16. steam pipeline, 17. steam nozzle, 18. intermediate billet, 19. vacuum machine, 20. temperature controller, 21. valve, 22. support frame. DETAILED DESCRIPTION

[0021] The design concept of the present invention is to control and optimize the structure of the scale when the intermediate billet is at high temperature, effectively improve the plasticity of the scale itself, thereby ensuring the integrity of the scale during rolling and avoiding the formation of pressed scale defects. The present invention is based on short-term high-temperature steam cooling of the intermediate billet, which matches the heating and rolling process system. It uses the good plasticity of the scale at high temperature to control the scale during the rolling process, avoid the control of scale on ordinary carbon steel thin plates, and achieve excellent surface quality of the thin plates. The overall feature of the present invention is the process self-consistency of combining short-term high-temperature steam cooling of the intermediate billet with low-temperature heating and low-temperature high-speed rolling.

[0022] Since the high-temperature oxidation of steel is sensitive to temperature, the implementation of a temperature control system can effectively control the high-temperature oxidation behavior of plain carbon steel, thereby optimizing the surface scale structure. The intermediate billet steam short-term cooling is a short-term high-temperature steam cooling of the intermediate billet after rough rolling. After the surface temperature of the intermediate billet is reduced, the finishing rolling is continued. Its mechanism of action is as follows: First, when the high-temperature steam covers the surface of the ordinary carbon steel intermediate billet, water vapor is formed due to the temperature difference, which effectively isolates the contact between the ordinary carbon steel matrix and oxygen, and reduces the amount of scale generated on the surface of the intermediate billet; secondly, the high-temperature characteristics of the steam reduce the temperature difference on the surface of the ordinary carbon steel, reduce the tendency of surface cracking, reduce the unevenness of local scale, and improve the uniformity of the overall scale of the intermediate billet, avoiding the scale defects caused by uneven scale; thirdly, the short-term cooling process of the intermediate billet can control the surface temperature of the plate, which is helpful to accurately control the finishing temperature, ensure the scale structure at high temperature and its good high-temperature plasticity, and achieve the purpose of optimizing the secondary scale structure. Finally, the intermediate billet is cooled by steam for a short time to optimize the structure of the secondary iron oxide scale, and the high-temperature plasticity of the iron oxide scale is used for direct finishing rolling, which saves the descaling process in the finishing rolling stage of the traditional process and improves production efficiency. The thickness of the iron oxide scale is suppressed in the steam environment, and the thickness of the iron oxide scale on the surface of the finished product is controlled below 10μm. The surface is smooth and there is no iron oxide scale indentation defect, which can meet the needs of pipe making, pickling, cold rolling, etc.

[0023] The control of the present invention is embodied in two stages: heating and rolling. The implementation of 1100℃-1150℃ low-temperature heating control is firstly beneficial to reduce the amount of furnace oxide scale generated during the heating process, and secondly to inhibit Fe 2 SiO 4 The aggregation at the interface of iron oxide scale and steel matrix, combined with the increase in the proportion of FeO in iron oxide scale during long-term heat preservation, ensures the removal effect of the rough descaling process conditions on the furnace-generated iron oxide scale. The low-temperature rolling process used in the rolling process not only reduces the amount of iron oxide scale generated during the rolling process, but also optimizes the surface iron oxide scale structure, reduces the proportion of loose FeO in the iron oxide scale, and retains the high-temperature plasticity of the iron oxide scale, enhances the adhesion of the iron oxide scale to the plain carbon steel matrix, avoids the shedding of the iron oxide scale, and reduces the generation of pressed-in iron oxide scale defects. High-speed rolling has a similar effect to low-temperature rolling, both of which can reduce the amount of secondary iron oxide scale generated during the rolling stage and optimize the iron oxide scale structure.

[0024] See also Figure 1 The present invention includes the following steps: the billet is heated in a heating furnace 1 - the heated billet is descaled by high-pressure water in a descaling machine 2 - the billet is subjected to five reversible reciprocating rough rolling in a rough rolling mill 3 - the intermediate billet after rough rolling is cooled by a steam spray device - the intermediate billet is sheared by a flying shear 5 - the intermediate billet is subjected to seven continuous finishing rollings by a finishing mill 6 - the strip is cooled by a laminar cooling 7 - the cooled strip is coiled by a coiler 8. Among them, the process of short-time cooling of the intermediate billet by the steam spray device is to set a water mist spray device between the rough rolling mill and the flying shear, spray high-temperature water vapor in the form of water mist through a large number of fine nozzles to cover the surface of the rough rolled intermediate billet, the steam temperature is 200℃-400℃, and the surface temperature of the intermediate billet is controlled to be 900℃-1000℃. The reasons for controlling the temperature of the intermediate billet are as follows: (a) Controlling this temperature ensures the temperature at the exit of the finishing rolling mill, thereby reducing the rolling pressure of the finishing mill and ensuring the starting temperature of laminar cooling; (b) Controlling this temperature can ensure the high-temperature good plasticity of the oxide scale on the surface of the intermediate billet to prevent the formation of oxide scale indentation defects during the finishing rolling process; (c) Controlling this temperature is conducive to the formation of a lower thickness of secondary iron scale on the surface of the intermediate billet before entering the finishing mill, thereby reducing the probability of the formation of oxide scale indentation defects. The high-temperature steam cooling time of the intermediate billet is controlled at 20-25 seconds, and the time for the intermediate billet to pass through the steam spray device in the running state can be controlled by controlling the transmission speed of the roller table between the roughing mill and the finishing mill.

[0025] See also Figure 2The steam spray device includes a steam generating box 9 and a vacuum furnace 13. The steam generating box is provided with several heating elements 10, a water inlet pipe 11 is provided at the lower part of one side of the steam generating box, and a steam delivery pipe is provided at the upper part of the other side. In order to ensure that the liquid level of water in the steam generating box is lower than the steam delivery pipe, the steam generating box is provided with an overflow pipe, the high position of the overflow pipe is lower than the steam delivery pipe, and the low position of the overflow pipe is connected to the water inlet pipe. The vacuum furnace 13 is provided with an inner sleeve 14, and the outlet of the steam output pipe is connected to the inner sleeve. The top of the vacuum furnace is provided with a steam pipe 16, and the outlet of the steam pipe is connected to the steam nozzle 17, and the two steam nozzles correspond to the upper and lower surfaces of the intermediate blank 18. Each steam nozzle is provided with a large number of fine nozzles so that the steam mist can be evenly sprayed on the upper and lower surfaces of the intermediate blank. The steam pipe is provided with a valve 21. The upper part of the vacuum furnace is provided with an argon gas pipeline, and the lower part of the vacuum furnace is provided with a vacuum machine 19 and a temperature controller 20. The working process of the steam spray device is as follows: water is injected into the steam generator box through the water inlet pipe, and the water in the steam generator box is heated by several heating elements to generate water vapor. The water vapor then enters the inner sleeve of the vacuum furnace. The vacuum furnace device realizes its vacuum atmosphere effect through the argon gas inlet and the vacuum machine, and the vacuum furnace heats the water vapor. The inner sleeve, steam pipe and the inner wall of the nozzle are all made of high-temperature resistant and high-strength ceramic materials to cope with the high-pressure environment during the heating process of water vapor. After the water vapor temperature rises to the set temperature, the valve 21 is opened, and the high-temperature water vapor is ejected from the nozzle through the steam pipe to achieve the purpose of short-term cooling of the intermediate blank surface. A support frame 22 for supporting the steam nozzle is set on the side of the roller to ensure the stability of the steam nozzle.

[0026] The following provides specific embodiments:

[0027] Example 1: The chemical composition of the hot-rolled plain carbon steel coil is SPHC steel, and the chemical composition is: C, 0.042%; Si, 0.008%; Mn, 0.15%; P, 0.018%; S, 0.018%; Alt, 0.035%, and the rest is Fe, micro-alloy elements and impurities.

[0028] The thickness of the ingot used is 300mm, and the thickness of the finished product is 4.5mm. The actual temperature of the heating furnace is 1120.74℃. The pressure of high-pressure water descaling is 18MPa, and the descaling temperature is 1007.39℃. The rough rolling process is five-pass reversible rolling, and the rolling temperatures of the odd passes are: Pass 1 1046.63℃; Pass 3 1048.58℃; Pass 5 1053.03℃, and the rolling speed is controlled at 3.6m / s. The intermediate billet is steam cooled, the steam temperature is 250℃, the cooling treatment time is 20s, and the intermediate billet temperature is 978℃. The finishing rolling process is seven continuous rolling passes, and the reduction rate, inlet and outlet temperatures of each pass are shown in Table 1. The rolling speed is controlled at 10.5m / s.

[0029] Table 1 Finishing rolling process parameters of Example 1

[0030] Finishing pass Reduction rate (%) Inlet temperature(℃) Outlet temperature(℃) 1 0.43 959.36 957.02 2 0.42 936.35 932.62 3 0.34 927.94 924.09 4 0.31 919.62 915.86 5 0.29 910.57 907.00 6 0.25 901.18 897.89 7 0.16 891.16 888.09

[0031] After finishing rolling, laminar cooling is performed and the coiling temperature is 620℃.

[0032] The steel coil of Example 1 has excellent surface quality and high smoothness, and no iron oxide scale or pitting defects are found. The thickness of the surface iron oxide scale is 8.8 μm.

[0033] Example 2

[0034] The chemical composition of Example 2 is Q195 steel, and the chemical composition is: C, 0.057%; Si, 0.03%; Mn, 0.10; P, 0.014%; S, 0.014%; Alt, 0.038%, and the rest is Fe, micro-alloy elements and impurities.

[0035] The thickness of the ingot is 300mm and the thickness of the finished product is 3mm.

[0036] The actual outlet temperature of the heating furnace is 1139.54℃.

[0037] The water pressure of high-pressure water descaling is 18MPa and the descaling temperature is 1023.46℃.

[0038] The rough rolling process is a five-pass reversible rolling process. The rolling temperatures for the odd-numbered passes are: Pass 1 1087.05°C; Pass 3 1082.33°C; Pass 5 1089.25°C. The rolling speed is controlled at 3.3 m / s.

[0039] The intermediate billet is steam cooled for a short time, the steam temperature is 280℃, the cooling treatment time is 23s, and the intermediate billet temperature is 998℃.

[0040] The finishing rolling process of Example 2 is seven continuous rolling passes, and the reduction rate, inlet and outlet temperatures of each pass are shown in Table 2. The rolling speed is controlled at 10.7 m / s.

[0041] Table 2 Finishing rolling process parameters of Example 2

[0042] Finishing pass Reduction rate (%) Inlet temperature(℃) Outlet temperature(℃) 1 0.43 988.72 985.97 2 0.40 975.84 972.96 3 0.34 963.26 960.21 4 0.30 950.47 947.35 5 0.29 937.18 934.17 6 0.26 923.79 921.19 7 0.17 908.13 906.36

[0043] The cooling process is laminar cooling and the coiling temperature is 620°C.

[0044] The surface quality of the steel coil in Example 2 is excellent, with high smoothness, and no iron oxide scale or pitting defects are found. The thickness of the surface iron oxide scale is 9.2 μm.

Claims

1. A method for improving the surface quality of hot-rolled plain carbon steel coil, characterized in that: The process includes the following steps: heating in a casting furnace - high-pressure water descaling - five-pass reversible reciprocating rough rolling - intermediate billet high-temperature steam cooling - flying shear - seven-pass continuous finishing rolling - laminar cooling - coiling. The intermediate billet high-temperature steam cooling process is to spray high-temperature steam onto the surface of the intermediate billet through a large number of fine nozzles, control the surface temperature of the intermediate billet to 900°C-1000°C, the steam temperature to 200°C-400°C, and the intermediate billet high-temperature steam cooling time to 20-25 seconds. A steam spray device is arranged between the rough rolling mill and the flying shear, and the steam spray device comprises a steam generating box and a vacuum furnace. The steam generating box is provided with a heating element, a water inlet pipe is arranged at the lower part of one side of the steam generating box, and a steam conveying pipe is arranged at the upper part of the other side; the vacuum furnace is provided with an inner sleeve, and the outlet of the steam conveying pipe is connected to the inner sleeve. A steam pipeline is arranged on the top of the vacuum furnace, and the outlet of the steam pipeline is connected to the steam nozzle. The steam nozzle corresponds to the upper and lower surfaces of the intermediate billet, and high-temperature water vapor is sprayed in the form of water mist through a large number of fine nozzles to cover the surface of the rough-rolled intermediate billet.

2. The method for improving the surface quality of hot-rolled plain carbon steel coil according to claim 1, characterized in that: The rough rolling temperature is controlled at 1000-1100°C, and the rolling rate range is controlled at 3.0-3.9 m / S.

3. The method for improving the surface quality of hot-rolled plain carbon steel coil according to claim 2, characterized in that: In the heating process of the billet heating furnace, the billet heating temperature is controlled in the range of 1100°C-1150°C, and the time in the furnace is controlled in 2-3 hours.

4. The method for improving the surface quality of hot-rolled plain carbon steel coil according to claim 3, characterized in that: In the high-pressure water descaling process, the descaling water pressure is ≥18MPa, and a side water sprayer is installed behind the descaling box.

5. The method for improving the surface quality of hot-rolled plain carbon steel coil according to claim 4, characterized in that: The chemical composition of plain carbon steel coil is: C 0.01-0.10%, Si ≤0.15%, Mn 0.05-1.50%, P ≤0.03%, S≤0.03%, Alt ≤0.08%, and the rest are Fe, micro-alloying elements and impurities.

6. The method for improving the surface quality of hot-rolled plain carbon steel coil according to claim 5, characterized in that: Five-pass reversible rough rolling process, with descaling in odd-numbered passes.

7. The method for improving the surface quality of hot-rolled plain carbon steel coil according to claim 6, characterized in that: In the finishing process, side water spray cooling is adopted at the entrance of each finishing mill; the rolling temperature adopts the low-temperature rolling system, the temperature range is controlled at 840℃-1000℃, and the rolling rate range is controlled at 10.0-11.3m / s.

8. The method for improving the surface quality of hot-rolled plain carbon steel coil according to claim 7, characterized in that: The coiling temperature of the coiling process is controlled at 560°C-640°C.

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