CN114918263A - A method for improving the surface quality of hot-rolled plain carbon steel coil - Google Patents

Abstract

A method for improving the surface quality of a hot-rolled ordinary carbon steel coil, comprising the following steps: heating in a billet heating furnace--high-pressure water descaling--five-pass reversible reciprocating rough rolling--intermediate billet high-temperature steam cooling--flying shear- ‑Seven passes of continuous finishing rolling‑‑laminar cooling‑‑coiling, the high temperature steam cooling process of the intermediate billet is to spray high temperature steam on the surface of the intermediate billet through a large number of small nozzles, and control the surface temperature of the intermediate billet to 900℃‑1000℃ , the steam temperature is 200℃‑400℃. The beneficial effects of the present invention are: through the short-term cooling treatment of the high-temperature steam of the intermediate blank, the uniformity of the iron oxide scale of the intermediate blank is improved, and the thickness, morphology, performance and structure of the iron oxide scale on the surface of the intermediate blank are optimized. High-temperature plasticity avoids the formation of pressed-in scale, and saves the descaling process before finishing rolling, improving production efficiency and reducing costs.

Description

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

technical field

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 technique

Hot-rolled steel strip is one of the most widely used and largest-scale products of all rolled steel products. With the vigorous development of iron and steel enterprises, the number of hot-rolled steel strip and coil production lines is large, and the product homogeneity is serious, resulting in increasingly fierce market competition for hot-rolled strip. The excellent surface quality of hot-rolled steel strip is an important basis for customers to choose products.

As the product of the reaction between the steel matrix and oxygen, iron oxide scale is a very common surface material in hot-rolled sheet products, and its structure, thickness and morphology determine the surface quality of hot-rolled steel products. Usually, the furnace raw iron oxide scale generated in the heating and holding stage of the heating furnace is called primary iron oxide scale, and its thickness is relatively thick, which is generally removed by the descaling process. In the subsequent rolling process, the newly formed iron oxide scale due to the high temperature of the sheet is collectively referred to as secondary iron oxide scale, which is thin and brittle. During the rolling process, with the action of the rolling force, the iron oxide scale is likely to be broken and pressed into the surface of the steel plate, forming a pressed iron oxide scale defect. Therefore, the plasticity of the iron oxide scale itself during the rolling process will affect the surface quality of the hot-rolled sheet. Indented scale defects are very common in hot-rolled sheet products and are surface quality defects that have a significant impact on downstream users. If the defects of the pressed iron oxide scale are very serious, even the pickling process cannot be completely removed, and the production of downstream deep processing processes such as stamping and coating will also be significantly affected. Therefore, how to control and reduce the defects of iron oxide scale has become a very important issue for hot-rolled sheet manufacturers.

At present, although there are many studies on the elimination of indented iron oxide scale, most of them focus on physical descaling methods, such as descaling with high pressure water, descaling by rolling and mechanical descaling before finishing rolling. However, the chemical composition of ordinary carbon steel is different, and the structure of the surface iron oxide scale and its bonding force with the matrix are also different. Especially iron and steel materials often contain Si element, which will generate Fe ₂ SiO ₄ (spinel olivine structure) at the interface between the iron and steel material matrix and iron oxide scale. Fe ₂ SiO ₄ will change from solid phase to liquid phase when it is above 1177°C Promote the aggregation and growth of pores of iron oxide scale and accelerate surface oxidation. When Fe ₂ SiO ₄ is re-solidified from the molten state, it will surround FeO grains and form a FeO/Fe ₂ SiO ₄ eutectoid product, which greatly reduces the complete removal of FeO in descaling, so that the residual oxidation In the rolling process, the iron scale forms indented iron oxide scale defects.

SUMMARY OF THE INVENTION

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 structure of the secondary iron oxide scale is optimized to avoid the formation of defects in the pressed iron oxide scale.

The technical scheme adopted in the present invention is as follows:

A method for improving the surface quality of a hot-rolled plain carbon steel coil, comprising the following steps: heating in a slab heating furnace--high-pressure water descaling--five-pass reversible reciprocating rough rolling--intermediate billet high-temperature steam cooling--flying shear- -Seven passes of continuous finishing rolling--laminar cooling--coiling, the high-temperature steam cooling process of the intermediate billet is to spray high-temperature steam on the surface of the intermediate billet through a large number of small nozzles, and control the surface temperature of the intermediate billet to 900℃-1000℃ , the steam temperature is 200℃-400℃.

In the above method for improving the surface quality of hot-rolled plain carbon steel coils, a steam spray device is arranged between the rough rolling mill and the flying shear. The steam spray device includes a steam generation box and a vacuum furnace. The steam generation box is provided with heating elements, and one side of the steam generation box is provided The lower part is provided with a water inlet pipe and the upper part of the other side is provided with a steam conveying pipe; the vacuum furnace is provided with an inner sleeve, the outlet of the steam conveying pipe is connected to the inner sleeve, the top of the vacuum furnace is provided with a steam pipe, and the outlet of the steam pipe is connected to the steam nozzle, and the steam nozzle corresponds to the intermediate blank the upper and lower surfaces.

In the above method for improving the surface quality of the hot-rolled plain carbon steel coil, the cooling time of the high temperature steam of the intermediate billet is controlled at 20-25 seconds;

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

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 greater than or equal to 18MPa, and the side water spray is installed after the descaling box.

The above-mentioned method for improving the surface quality of the 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%, the rest are Fe, microalloying elements and impurities.

The above-mentioned method for improving the surface quality of a hot-rolled plain carbon steel coil includes five-pass reversible reciprocating rolling processes, and descaling treatment is performed at odd-numbered passes.

In the above-mentioned method for improving the surface quality of hot-rolled plain carbon steel coil, side water spray cooling is adopted at the entrance of each finishing mill in the finishing rolling process; The rolling rate range is controlled at 10.0-11.3m/s.

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

According to the high-temperature plasticity of the iron oxide scale on the surface of ordinary carbon steel, combined with the formation and structural evolution of the iron oxide scale in the heating and rolling stages in the preparation process of the thin plate, and taking the short-term cooling of the intermediate billet steam as the characteristic, the invention proposes specific measures. Implementation and Solutions. The invention is simple and practical, and has important practical significance for improving the defects of iron oxide scale pressed into the surface of the plain carbon steel sheet.

The beneficial effects of the present invention are: through the short-time cooling treatment of the high-temperature steam of the intermediate billet, the uniformity of the iron oxide scale of the intermediate billet is improved, and the thickness, morphology, performance and structure of the iron oxide scale on the surface of the intermediate billet are optimized. High temperature plasticity avoids the formation of press-in type iron oxide scale, and saves the descaling process before finishing rolling, improves production efficiency and reduces costs; through the control of heating system, the primary iron oxide scale on the surface of the steel plate does not appear Fe ₂ SiO ₄ It melts and solidifies, and increases the proportion of FeO in the iron oxide scale, which ensures that the iron oxide scale can be basically removed during the rough descaling treatment, and greatly reduces the incidence of defects in the iron oxide scale pressed into the surface of the steel plate; Low-temperature rolling and high-speed rolling are implemented in the rolling stage, which effectively reduces the formation and growth of iron oxide scale during the rolling process.

Description of drawings

Fig. 1 is the technical process schematic diagram of the present invention;

Figure 2 is a schematic diagram of a vapor spray device.

The labels in the figure are: 1. Heating furnace, 2. Descaler, 3. Rough rolling mill, 4. Spray device, 5. Flying shear, 6. Finishing rolling unit, 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 vent, 18, Intermediate blank, 19, vacuum machine, 20, thermostat, 21, valve, 22, support frame.

Detailed ways

The design concept of the present invention is as follows: when the intermediate billet is high temperature, the structure of the scale is adjusted and optimized to effectively improve the self-plasticity of the scale, thereby ensuring the integrity of the scale during the rolling process and avoiding the generation of defects of the pressed scale. The invention is based on the short-time cooling of the high-temperature steam of the intermediate billet, which matches the heating and rolling process system, and is characterized by utilizing the good plasticity of the high-temperature iron oxide scale to control the scale of the rolling process, so as to avoid the occurrence of the scale of the ordinary carbon steel sheet with the scale. control, thereby achieving an excellent surface quality of the thin strip. The overall feature of the present invention is that the process of combining high-temperature steam and short-time cooling of the intermediate billet with low-temperature heating and low-temperature high-speed rolling is suitable.

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 ordinary carbon steel, thereby optimizing the surface scale structure. The short-term steam cooling of the intermediate billet is a short-term high-temperature steam cooling of the intermediate billet after rough rolling, and the finishing rolling is continued after the surface temperature of the intermediate billet is reduced. The mechanism of action is as follows: first, when the high-temperature steam covers the surface of the intermediate blank of ordinary carbon steel, 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 oxide scale on the surface of the intermediate blank; secondly, the high temperature of the steam The characteristics reduce the temperature difference on the surface of ordinary carbon steel, reduce the tendency of surface cracking, reduce the unevenness of local iron oxide scale, improve the overall iron oxide scale uniformity of the intermediate billet, and avoid the occurrence of iron oxide scale caused by uneven iron scale. In addition, the short-term cooling process of the intermediate billet can control the surface temperature of the sheet, which is helpful to accurately control the finishing temperature, ensure the structure of the iron oxide scale and its good plasticity at high temperature, and achieve the purpose of optimizing the structure of the secondary iron oxide scale. Finally, the short-term cooling of the intermediate billet steam optimizes 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 the production efficiency. In the steam environment, the formation thickness of the iron oxide scale is suppressed, and the thickness of the iron oxide scale on the surface of the finished product is controlled below 10 μm.

The control of the present invention is embodied in two stages of heating and rolling. The implementation of low-temperature heating control at 1100℃-1150℃ is firstly beneficial to reduce the generation of raw iron oxide scale in the heating process, and secondly, it inhibits the aggregation of Fe ₂ SiO ₄ at the interface between iron oxide scale and steel matrix, combined with FeO during long-term heat preservation. The increase of the proportion in the iron oxide scale ensures the removal effect of the rough descaling process conditions on the raw iron oxide scale of the furnace. The low-temperature rolling process used in the rolling process not only reduces the amount of iron oxide scale formed during the rolling process, but also optimizes the surface scale structure, reduces the proportion of loose FeO in the scale, and retains the high-temperature plasticity of the scale. Enhance the adhesion between the iron oxide scale and the carbon steel substrate, avoid the peeling of the iron oxide scale, and reduce the generation of press-in type 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 scale structure.

Referring to Fig. 1, the present invention includes the following steps: the slab is heated in a heating furnace 1--the heated slab is subjected to high-pressure water descaling by a descaling machine 2--the slab is subjected to five-pass reversible reciprocating rough rolling in a rough rolling mill 3 --The intermediate billet after rough rolling is cooled by the steam spray device--Flying shear 5 cuts the intermediate billet--The intermediate billet is subjected to seven-pass continuous finishing rolling through the finishing rolling unit 6--Laminar cooling 7 The strip is carried out Cooling - the cooled strip is coiled in the coiler 8 . Among them, the short-term cooling process of the intermediate billet through the steam spray device is to set up a water mist spray device between the rough rolling mill and the flying shears, and spray high-temperature water vapor through a large number of small nozzles in the form of water mist to cover the surface of the rough rolled intermediate billet. The steam temperature is 200℃-400℃, and the surface temperature of the intermediate blank is controlled at 900℃-1000℃. The reasons for the temperature control of the intermediate billet are as follows: (a) Controlling this temperature ensures the temperature at the exit of the finishing rolling, thereby reducing the rolling pressure of the finishing rolling group and ensuring the starting temperature of laminar cooling; (b) Controlling this temperature can ensure that the surface of the intermediate billet is oxidized. High temperature and good plasticity to prevent the formation of iron oxide scale intrusion defects during the finishing rolling process; (c) Controlling this temperature is conducive to the formation of lower thickness of the surface secondary iron scale before the intermediate billet enters the finishing mill, thereby reducing the indentation of iron oxide scale. Defect generation probability. 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.

Referring to FIG. 2 , the steam spraying 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 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. In order to ensure that the liquid level of the water in the steam generation box is lower than the steam transmission pipe, the steam generation box is provided with an overflow pipe. The vacuum furnace 13 is provided with an inner jacket 14, and the outlet of the steam output pipe communicates with the inner jacket. The top of the vacuum furnace is provided with a steam pipe 16 , and the outlet of the steam pipe is connected to a 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 small nozzles, so as to spray the steam mist evenly on the upper and lower surfaces of the intermediate blank. The steam line 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 spraying device is as follows. The water is injected into the steam generating box from the water inlet pipe, and the water in the steam generating box is heated by several heating elements to generate steam. The steam then enters the inner jacket of the vacuum furnace, and the vacuum furnace device passes through The argon gas input port and the vacuum machine realize the effect of the vacuum atmosphere, and the vacuum furnace heats the water vapor. The inner sleeve, the steam pipe and the inner wall of the nozzle are all made of high temperature resistant and high strength ceramic materials, so as 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 pipeline to achieve the purpose of short-term cooling of the surface of the intermediate blank. A support frame 22 for supporting the steam jets is arranged beside the roller table to ensure the stability of the steam jets.

Specific examples are provided below:

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 are Fe, microalloying elements and impurities.

The thickness of the slab used is 300mm, and the thickness of the finished product is 4.5mm. The actual release temperature of the heating furnace is 1120.74°C. The pressure of high-pressure water descaling water 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-numbered passes are: 1046.63 °C for the first pass; 1048.58 °C for the third pass; 1053.03 °C for the fifth pass, and the rolling speed is controlled at 3.6m/s. The intermediate blank is steam cooled, the steam temperature is 250°C, the cooling treatment time is 20s, and the intermediate blank temperature is 978°C. The finishing rolling process consists of seven passes of continuous rolling, and the reduction ratio, inlet and outlet temperatures of each pass are shown in Table 1. The rolling speed is controlled at 10.5m/s.

The finishing rolling process parameter of table 1 example 1

Finishing passes 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

Laminar cooling after finishing rolling. The coiling temperature was 620°C.

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

Example 2

The chemical composition of Example 2 is Q195 steel, 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 are Fe, microalloying elements and impurities.

The thickness of the slab is 300mm, and the thickness of the finished product is 3mm.

The actual furnace temperature is 1139.54℃.

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

The rough rolling process is five-pass reversible rolling, and the rolling temperatures of the odd-numbered passes are: 1087.05 °C for the first pass; 1082.33 °C for the third pass; 1089.25 °C for the fifth pass, and the rolling speed is controlled at 3.3m/s.

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

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

The finishing rolling process parameter of table 2 example 2

Finishing passes 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

The cooling process is laminar cooling. The coiling temperature was 620°C.

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

Claims (9)

1. a method for improving the surface quality of hot-rolled plain carbon steel coil, is characterized in that: comprise the following operations: casting billet heating furnace heating--high-pressure water descaling--five times reversible reciprocating rough rolling--intermediate billet high temperature steam Cooling -- flying shear -- seven-pass continuous finishing rolling -- laminar cooling -- coiling, the high-temperature steam cooling process of the intermediate billet is to spray high-temperature steam on the surface of the intermediate billet through a large number of small nozzles to control the surface of the intermediate billet The temperature is 900℃-1000℃, and the steam temperature is 200℃-400℃. 2. the method for improving the surface quality of hot-rolled plain carbon steel coil according to claim 1, is characterized in that: steam spraying device is set between rough rolling mill and flying shear, steam spraying device comprises steam generating box and vacuum furnace, steam generating The box is provided with heating elements, the lower part of one side of the steam generating box is provided with a water inlet pipe, and the upper part of the other side is provided with a steam conveying pipe; the vacuum furnace is provided with an inner sleeve, the outlet of the steam conveying pipe is connected to the inner sleeve, and the top of the vacuum furnace is provided with a steam pipe, The outlet of the steam pipe is connected with the steam nozzle, and the steam nozzle corresponds to the upper and lower surfaces of the intermediate blank. 3. The method for improving the surface quality of a hot-rolled plain carbon steel coil according to claim 2, wherein the cooling time of the high temperature steam of the intermediate billet is controlled at 20-25 seconds; the rough rolling temperature is controlled at 1000-1100°C, The rolling rate range is controlled at 3.0-3.9m/. 4. The method for improving the surface quality of a hot-rolled plain carbon steel coil according to claim 3, characterized in that: in the heating process of the slab heating furnace, the slab heating temperature is controlled within the range of 1100°C-1150°C, and the furnace time is controlled at 2-3 hours, preferably 2 hours. 5. The method for improving the surface quality of a hot-rolled plain carbon steel coil according to claim 4, wherein the descaling water pressure is greater than or equal to 18 MPa in the high-pressure water descaling process, and a side spray is installed after the descaling box. 6. The method for improving the surface quality of a hot-rolled plain carbon steel coil according to claim 5, wherein the chemical composition of the 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, microalloying elements and impurities. 7 . The method for improving the surface quality of a hot-rolled plain carbon steel coil according to claim 6 , wherein the reversible reciprocating rolling process for five passes is performed, and descaling treatment is carried out in odd passes. 8 . 8. The method for improving the surface quality of a hot-rolled plain carbon steel coil according to claim 7, characterized in that: in the finishing rolling process, side spray cooling is adopted at the entrance of each finishing mill; the rolling temperature adopts low-temperature rolling system, the temperature range is controlled at 840°C-1000°C, and the rolling rate range is controlled at 10.0-11.3m/s. 9 . The method for improving the surface quality of a hot-rolled plain carbon steel coil according to claim 8 , wherein the coiling temperature in the coiling process is controlled at 560°C-640°C. 10 .

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