A method for preparing hot-rolled strip steel for nickel-free container

Abstract

The present invention discloses a method for preparing hot-rolled strip steel for nickel-free containers, and belongs to the field of surface control technology. The preparation method comprises the steps of preparing raw materials of weathering steel, smelting into molten steel, then sequentially carrying out molten iron desulfurization, converter smelting, LF refining, continuous casting, slab reheating, rolling, cooling, and obtaining hot-rolled strip steel for containers; the rolling is rough rolling and finishing rolling, the rough rolling passes through 2 units in sequence, each unit performs 3 rolling passes, the finishing rolling passes through 7 units in sequence, each unit performs one rolling pass, and the rough rolling and finishing rolling are sequentially descaled after the first rolling. The present invention can achieve the purpose of corrosion resistance and good mechanical properties without adding Ni by optimizing the alloy composition. In addition, the present invention improves the rolling process, and performs a descaling process after the first rolling of rough rolling and finishing rolling to avoid copper brittle defects on the surface of the strip steel.

Description

Technical Field

[0001] The invention belongs to the technical field of surface control, and in particular relates to a method for preparing hot-rolled strip steel for a nickel-free container. Background Art

[0002] In the container manufacturing industry, the steel types widely used by users include SPA-H in the Japanese Industrial Standard JIS G3125 "High Atmospheric Corrosion Resistance Steel". The standard stipulates that certain alloy elements such as silicon, manganese, phosphorus, copper, and chromium should be added to the steel to obtain the required mechanical properties and corrosion resistance. Users generally use the main processes such as sandblasting → roller pressing (bending) forming processing → box welding → painting to manufacture container products for shipping various types of goods. Containers require steel plates to have good processing and forming properties, good surface quality and good plate quality, as well as excellent atmospheric corrosion resistance. Since the standard stipulates the addition of a variety of relatively expensive alloy elements, its production cost is relatively high. At the same time, in order to obtain better surface quality and control the occurrence of surface copper brittle defects, most manufacturers add nickel alloys with a copper content of about 1 / 3 to the steel, which further increases the production cost.

[0003] If nickel alloy is lacking in production, the melting point of copper is about 1083℃, while the heating temperature in box plate production is generally 1150~1280℃, which is much higher than this temperature. The relevant research results show that when the slab is heated, due to selective oxidation, elements such as silicon, manganese and iron are oxidized first, and the copper element has a weak affinity with oxygen at high temperature and is not easy to form oxides. When its diffusion rate in austenite is lower than the oxidation rate, it begins to enrich in the matrix under the oxide scale, forming a copper-rich phase that causes copper brittleness, making it difficult to obtain a good surface quality of the steel plate to meet the user's requirements.

[0004] Low-cost nickel-free weathering steel for containers has a large market demand, many manufacturers, and relatively mature and stable production technology. In recent years, due to the downturn in the container industry, demand has dropped sharply, the steel market has been seriously oversupplied, the variety benefits have seriously deteriorated, and profitability has become negative. Therefore, in order to improve the net profit and competitiveness of nickel-free weathering steel for containers, efforts have been made to optimize the manufacturing process, while maintaining product quality and performance to meet user requirements, and reduce the manufacturing cost of box plate products. Summary of the invention

[0005] In view of the above-mentioned shortcomings of the prior art, the present invention provides a method for preparing hot-rolled strip steel for nickel-free containers.

[0006] To achieve the above object, the technical solution adopted by the present invention is: a method for preparing hot-rolled strip steel for container, comprising the steps of preparing raw materials of weathering steel, smelting into molten steel, and then sequentially carrying out molten iron desulfurization, converter smelting, LF refining, continuous casting, slab reheating, rolling, and cooling to obtain hot-rolled strip steel for container;

[0007] The rolling process includes rough rolling and finish rolling. The steel material after reheating the slab is subjected to rough rolling and finish rolling in sequence. The rough rolling uses R1 and R2 units and is rolled in a 3+3 pass mode. The first rolling temperature of R1 is higher than 1200°C. Descaling is started after the first rolling. The descaling water pressure is above 17MPa. Descaling is not started in the remaining 5 rolling passes. The thinning deformation after each rolling pass in the rough rolling is less than 30%, and the total thinning deformation of the rough rolling is 81%-88%.

[0008] Finishing rolling passes through 7 units in sequence, which are F1, F2, F3, F4, F5, F6, and F7. Each unit adopts one-pass rolling. The first rolling temperature of F1 rolling is 1000-1090℃, and the thinning deformation exceeds 40%. Descaling is turned on after the first rolling, and the descaling water pressure is 20-23MPa. Descaling is not turned on for the remaining 6 rolling passes. The total thinning deformation of the strip after finishing rolling is 91%-98%;

[0009] Weathering steel contains the following percentages of elements: C: 0.06-0.10%, Si: 0.30-0.40%, Mn: 0.40-0.50%, P: 0.07-0.11%, S: 0-0.0125%, Cu: 0.25-0.33%, Ni: 0-0.09%, Cr: 0.30-0.40%, Ti: 0.020-0.035%, B: 0.0025-0.0050%, and the balance is Fe.

[0010] The hot-rolled strip steel for containers prepared by the method for preparing the hot-rolled strip steel for containers of the present invention has the advantages of corrosion resistance and excellent mechanical properties, mainly based on the following principles:

[0011] (1) Alloy composition: The present invention reduces the C content, which is beneficial to reduce the formation of pearlite and obtain ferrite with stronger corrosion resistance; reducing the S content can reduce the formation of sulfur-containing inclusions and reduce the expansion of pitting during the corrosion process; increasing the Si content refines the rust layer grains and slows down the pitting rate on the strip surface. The combined adjustment of all elements contained in the weathering steel plays a significant role in improving the corrosion resistance of weathering steel for nickel-free containers. In addition, the Si element in the weathering steel causes the alloy to form more internal oxidation products 2FeO·SiO2 after oxidation, which changes the oxide layer structure of the copper-containing weathering steel and prevents copper from forming a liquid copper phase at the interface between the matrix and the oxide layer, which has a significant effect on inhibiting the segregation of copper-rich elements on the surface of the matrix after low-temperature oxidation. On the one hand, the Ti element alleviates the oxidation process of the alloy to a certain extent and can prevent the diffusion of the copper-rich phase to the matrix grain boundary. On the other hand, the oxide layer formed by the alloy containing the Ti element is relatively dense, which inhibits the diffusion of the copper element to the outside of the oxide layer. Therefore, more copper elements appear in the oxide layer. However, it is generally believed that the copper-rich phase segregation formed inside the oxide layer can be well removed with the high-pressure water descaling of the slab surface, so the effect on the surface quality of the hot-rolled slab is small. The addition of the Ti element is beneficial to slow down the "copper brittleness" tendency of copper-containing weathering steel. The B element prevents the penetration of the copper-rich liquid phase to the austenite grain boundary during the high-temperature oxidation process by segregating at the austenite grain boundary, thereby inhibiting the formation of copper-brittle hot cracks. The composition design route of increasing Si, Ti, and B to overcome the formation of copper-induced thermal cracks and reducing C and S to improve the corrosion resistance of the slab can obtain good surface quality and product performance, and reduce alloy costs.

[0012] (2) Preparation process: During the heating process, weathering steel slabs for containers should avoid long-term heating at 1100-1200℃ as much as possible, and adopt "high temperature fast burning" and reducing atmosphere process. The first rolling temperature of the rough rolling is above 1200℃, which is much higher than the melting point of copper, 1083℃. The copper-rich phase gradually dissolves into the iron matrix, resulting in a decrease in the number of surface cracks and microcracks, shortening the length and shallowing the depth. At this time, the thinning deformation is less than 30%. Due to the reduction of the sample deformation and the surface rolling force, no obvious surface copper brittle hot cracks appear, thereby improving the material's handling performance. The temperature of the first pass of finishing rolling is 1000-1090℃. When rolling is carried out at a temperature lower than the melting point of copper, the copper element will be enriched in the grain boundaries of the shallow surface of the matrix, resulting in increased brittleness of the grain boundaries. Although the deformation is large, the high-pressure descaling after the first thinning of finishing rolling is started in time, and the copper-containing iron sheet on the surface is broken and descaled in time, which reduces the stress concentration caused by the pressure of the iron sheet and avoids the cracking of the surface austenite grain boundaries at the stress concentration position to form copper brittle defects. The high-temperature fast burning and reducing atmosphere processes of the heating furnace are developed. At the same time, the deformation of the first rolling pass is reduced at high temperature, and the descaling strength of the slab surface is increased at low temperature, so as to obtain stable surface quality.

[0013] The molten iron desulfurization, converter smelting, LF refining and continuous casting of the present invention adopt the technical solutions commonly used in the field.

[0014] As a preferred embodiment of the present invention, the first rolling temperature of the rough rolling R1 is higher than 1200°C, the second rolling temperature is higher than 1100°C, and the third rolling temperature is higher than 1000°C; the three rolling temperatures of the rough rolling R2 are all higher than 1000°C, and the R2 outlet temperature is higher than 1000°C.

[0015] As a preferred embodiment of the present invention, the slab reheating is carried out in three stages under a reducing atmosphere. In the preheating stage, the gas volume is controlled to be 1000-1500m 3 / h, heating temperature is 650-900℃, time is 23-37min; first heating section: temperature is below 1030℃, time is 34-52min; second heating and equalizing section: full flow, temperature is 1083℃-1290℃, time is 119-127min.

[0016] The heating time of the slab reheating is less than or equal to 350 minutes.

[0017] As a preferred embodiment of the present invention, the air excess coefficient under the reducing atmosphere is less than 1.0, and a more preferred air excess coefficient is 0.90-0.95.

[0018] As a preferred embodiment of the present invention, the cooling process is specifically as follows: the strip thickness after finish rolling is 1.5-1.62mm, the final rolling temperature is controlled to be 860±20℃, the coiling temperature is 550±20℃, the upper spraying is preferentially staged cooling, and the rolling thickness is 1.62-2.1mm; the final rolling temperature is controlled to be 870±20℃, the coiling temperature is 550±20℃, the upper spraying is preferentially staged cooling, and the rolling target thickness is 2.1-16.0mm; the final rolling temperature is controlled to be 840±20℃, the coiling temperature is 550±20℃, and the front-stage cooling is performed.

[0019] The present invention controls the final rolling temperature at 840±20°C, significantly reduces the area with larger grains, and reduces the width from 50mm to about 10mm. Increasing the final rolling temperature has a significant effect on reducing the growth of edge grains.

[0020] The present invention adopts front-stage cooling to reduce the coiling temperature, and rapidly cools in the shortest time after rolling, which is beneficial to grain refinement, reducing the level difference between large and small grains, and reducing the influence of mixed crystals on subsequent processing and forming performance. In addition, the front-stage cooling is adopted to reduce the layer cooling temperature, which is beneficial for the steel strip to quickly pass through the A1 line, reduce the formation of grain boundary cementite, and improve the processing and forming performance of steel.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention achieves the purpose of stably controlling the occurrence of copper embrittlement by optimizing the alloy composition, improving the heating process and the rolling process, and forms a nickel-free hot-rolled strip for containers with good corrosion resistance and mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The present invention is a process flow chart of the method for preparing hot-rolled strip steel for containers.

[0023] Figure 2 Surface morphology of the hot-rolled strip steel for containers prepared in Examples 1-4 of the present invention and the SPA-H material before corrosion in a full immersion test in a sulfuric acid solution, wherein from left to right are the SPA-H material, Example 1, Example 2, Example 3 and Example 4.

[0024] Figure 3 Surface morphology of the hot-rolled strip steel for containers prepared in Examples 1-4 of the present invention and the SPA-H material after full immersion corrosion in a sulfuric acid solution. From left to right in the figure are SPA-H material, Example 1, Example 2, Example 3 and Example 4.

[0025] Figure 4 The microstructure diagrams of the hot-rolled strip steel for containers prepared in Comparative Example 1 and Example 5 are shown in FIG. (a) and FIG. (b) show the microstructure of the hot-rolled strip steel for containers prepared in Comparative Example 1. Figure 1 Figure (c) shows the microstructure of the hot-rolled strip steel for container prepared in Comparative Example 1. Figure 2 Figure (b) is the microstructure of the hot-rolled strip steel for container prepared in Example 5 Figure 1 Figure (d) shows the microstructure of the hot-rolled strip steel for container prepared in Example 5. Figure 2 . DETAILED DESCRIPTION

[0026] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0027] Examples 1-5

[0028] A method for preparing hot-rolled steel strip for container, comprising the steps of preparing raw materials of weathering steel according to Examples 1-4 in Table 1, smelting into molten steel, and then sequentially performing molten iron desulfurization, converter smelting, LF refining, continuous casting, slab reheating, rolling, and cooling to obtain hot-rolled steel strip for container. The specific method is shown in Figure 1 .

[0029] Preferably, the rolling includes rough rolling and finish rolling, and the steel after reheating the slab is subjected to rough rolling and finish rolling in sequence. The rough rolling uses R1 and R2 units and is rolled in a 3+3 pass manner; the first rolling temperature of R1 is higher than 1200°C, and descaling is started after the first rolling, and the descaling water pressure is above 17MPa. Descaling is not started in the remaining 5 rolling passes. The thinning deformation after each rolling pass in the rough rolling is less than 30%, and the total thinning deformation of the rough rolling is 81%-88%.

[0030] In the present embodiments 1-5, the temperature of the rough rolling process is set according to the data in Table 3, and the measured values ​​of the rolling temperature of each pass are shown in Table 3.

[0031] Preferably, the finishing rolling passes through 7 units in sequence, which are F1, F2, F3, F4, F5, F6, and F7, and each unit adopts one-pass rolling; the first rolling temperature of F1 rolling is 1000-1090℃, the thinning deformation exceeds 40%, and descaling is started after the first rolling, and the descaling water pressure is 20-23MPa; descaling is not started in the remaining 6 rolling passes, and the total thinning deformation of the strip after finishing rolling is 91%-98%.

[0032] Preferably, the slab reheating is carried out in three stages under a reducing atmosphere, and the temperature and time of each stage are set according to Examples 1-5 in Table 2; the gas volume in the preheating stage is controlled to be 1000-1500m 3 / h, the air excess coefficient under the reducing atmosphere is less than 1.0, and the more preferred air excess coefficient is 0.90-0.95.

[0033] The cooling process is specifically as follows: the strip thickness after finish rolling is 1.5-1.62mm, the final rolling temperature is controlled to be 860±20℃, the coiling temperature is 550±20℃, the upper spraying is preferentially staged cooling, and the rolling thickness is 1.62-2.1mm; the final rolling temperature is controlled to be 870±20℃, the coiling temperature is 550±20℃, the upper spraying is preferentially staged cooling, and the rolling target thickness is 2.1-16.0mm; the final rolling temperature is controlled to be 840±20℃, the coiling temperature is 550±20℃, and the front-stage cooling is performed.

[0034] The molten iron desulfurization, converter smelting, LF refining and continuous casting adopt the technical solutions commonly used in the field.

[0035] 09CuPTiRe in Table 1 is the economical weathering steel in patent CN1800428A.

[0036] Table 1

[0037]

[0038] Table 2

[0039]

[0040]

[0041] Table 3

[0042]

[0043] Comparative Example 1

[0044] The only difference from Example 1 is that the raw materials of weathering steel are prepared according to Table 4.

[0045] Table 4

[0046]

[0047] Effect example

[0048] The hot-rolled strip steel for containers prepared in Examples 1-4 and the SPA-H material were subjected to corrosion tests and mechanical property tests. The results are shown in Tables 5 and 6.

[0049] Test samples: hot-rolled steel strips for containers prepared in Examples 1-4 and SPA-H materials.

[0050] Corrosion test: The test sample was placed in a 20 vol. % sulfuric acid corrosion solution at 30°C for 24 hours.

[0051] Table 5 Corrosivity data

[0052]

[0053]

[0054] Table 6 Mechanical properties data

[0055] Element Yield strength / MPa Tensile strength / MPa Yield-to-strength ratio Strength-to-yield ratio Elongation / % SPA-H 370 470 0.79 1.27 39 Example 1 425 540 0.79 1.27 36.5 Example 2 405 500 0.81 1.23 35 Example 3 390 495 0.79 1.27 39.5 Example 4 385 480 0.80 1.25 39

[0056] According to Table 5, the hot-rolled strip steel for container of the present invention has strong corrosion resistance, and according to Figure 2-3 It can be seen that the sulfuric acid corrosion resistance of the hot-rolled strip for container is much better than that of SPA-H. The surface of SPA-H is blackened, while the corrosion degree of the hot-rolled strip for container is relatively shallow. The corrosion weight loss of the hot-rolled strip for container is significantly less than that of weathering steel, indicating that the corrosion resistance of weathering steel containing Ni is not as good as that of the hot-rolled strip for container, indicating that the Ni element has no obvious effect on the corrosion resistance of the hot-rolled strip for container.

[0057] It can be seen from Table 6 that the hot-rolled strip steel for container of the present invention has stronger yield strength and tensile strength than SPA-H.

[0058] according to Figure 4It can be seen that the present invention explores the effect of Ti on copper embrittlement and finds that Ti has a relatively obvious inhibitory effect on the oxidation behavior of copper-containing steel during heating.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for preparing hot-rolled steel strip for container, comprising the steps of preparing weathering steel raw materials, smelting into molten steel, and then sequentially carrying out molten iron desulfurization, converter smelting, LF refining, continuous casting, slab reheating, rolling, and cooling to obtain hot-rolled steel strip for container; The rolling process includes rough rolling and finish rolling. The steel material after reheating the slab is subjected to rough rolling and finish rolling in sequence. The rough rolling uses R1 and R2 units and is rolled in a 3+3 pass mode. The first rolling temperature of R1 is higher than 1200°C. Descaling is started after the first rolling. The descaling water pressure is above 17MPa. Descaling is not started in the remaining 5 rolling passes. The thinning deformation after each rolling pass in the rough rolling is less than 30%, and the final thinning deformation of the rough rolling is 81%-88%. Finish rolling passes through 7 units in sequence, the 7 units are F1, F2, F3, F4, F5, F6, F7, each unit adopts one-pass rolling; the first rolling temperature of F1 rolling is 1000-1090℃, the thinning deformation exceeds 40%, and descaling is turned on after the first rolling, and the descaling water pressure is 20-23MPa; the remaining 6 rolling passes do not start descaling, and the final thinning deformation of the strip after finishing rolling is 91%-98%; Weathering steel contains the following percentages of elements: C: 0.06-0.10%, Si: 0.30-0.40%, Mn: 0.40-0.50%, P: 0.07-0.11%, S: 0-0.0125%, Cu: 0.25-0.33%, Ni: 0-0.09%, Cr: 0.30-0.40%, Ti: 0.020-0.035%, B: 0.0025-0.0050%, and the balance is Fe.

2. The method for preparing hot-rolled strip steel for container according to claim 1, characterized in that: The second rolling temperature of the rough rolling R1 is higher than 1100°C, and the third rolling temperature is higher than 1000°C; the three rolling temperatures of the rough rolling R2 are all higher than 1000°C, and the R2 outlet temperature is higher than 1000°C.

3. The method for preparing the hot-rolled strip steel for container according to claim 1, characterized in that: The slab reheating is carried out in three stages under a reducing atmosphere. In the preheating stage, the gas volume is controlled to be 1000-1500m 3 / h, heating temperature is 650-900℃, time is 23-37min; first heating section: temperature is below 1030℃, time is 34-52min; second heating and equalizing section: full flow, temperature is 1083℃-1290℃, time is 119-127min.

4. The method for preparing the hot-rolled strip steel for container according to claim 3, characterized in that: The air excess coefficient under the reducing atmosphere is less than 1.

0.

5. The method for preparing hot-rolled steel strip for container according to claim 1, characterized in that: The cooling process is specifically as follows: the strip thickness after finish rolling is 1.5-1.62mm, the final rolling temperature is controlled to be 860±20℃, the coiling temperature is 550±20℃, the upper spraying is preferentially staged cooling, and the rolling thickness is 1.62-2.1mm; the final rolling temperature is controlled to be 870±20℃, the coiling temperature is 550±20℃, the upper spraying is preferentially staged cooling, and the rolling target thickness is 2.1-16.0mm; the final rolling temperature is controlled to be 840±20℃, the coiling temperature is 550±20℃, and the front-stage cooling is performed.

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