A production method for high surface quality ship plate steel

Abstract

A method for producing high-surface-quality ship plate steel belongs to the technical field of structural steel for ships and marine engineering, and includes the processes of slab heating, rough descaling, rolling, cooling, and straightening. Rough descaling is performed once the slab leaves the heating furnace. A single-stand rolling mill is used for rolling. A two-stage rolling process is adopted for A and B general-strength ship plate steels, and four fine descalings are performed in the first and second passes of cogging and the first and second passes after steel transfer. A two-stage rolling process is adopted for A32 and A36 high-strength ship plate steels. The first rolling process performs 4-5 fine descalings in the first and second passes of cogging and the first and third passes after steel transfer, and no descaling is performed in the second rolling process. Different cooling systems are adopted according to the thickness of the ship plate steel. The ship plate steel produced by the present invention has a thickness of 8 to 20 mm, and the iron oxide scale on its surface is evenly distributed, thin, and has no visible spots. After shot blasting, the iron oxide scale on the surface can be cleaned without defects such as pits, meeting the high surface coating quality requirements of ship plates.

Description

Technical Field

[0001] The invention belongs to the technical field of structural steel for ships and marine engineering, and particularly relates to a production method of high-surface-quality ship plate steel. Background Art

[0002] Ship plate steel is used in the manufacture of ship hull structures, docks, and terminal facilities. Because it requires painting during manufacturing, extremely stringent surface requirements are imposed. In today's fiercely competitive market, surface quality is a key quality indicator for ship plate. Shipyards use methods such as strong light exposure to inspect the surface of ship plates for defects such as mottling and pitting. These defects are caused by incomplete descaling of the steel billets after leaving the furnace or during the rolling process. During rolling, iron oxide scale is pressed into the ship plate in the form of flakes or blocks. Before and after shot peening, these spots or pits appear in varying sizes, shapes, and depths, directly affecting the surface quality after painting. Since ship plates are shipped in a rolled state and covered with iron oxide scale, and surface inspection is performed after shot blasting at the shipyard, the high painting requirements pose a technical bottleneck for shipyards, especially small and medium-sized ones.

[0003] When steel plates come into contact with air and water during hot rolling, scale forms. FeO is closest to the iron layer, followed by Fe₃O₄ and Fe₂O₃ outward. This scale is categorized as primary and secondary, with primary scale formed during the heating process and secondary scale during the rolling process. High-pressure water descaling systems are commonly used to remove this scale. The current trend in plate production equipment is toward larger-scale, automated production, and products are focused on high surface quality and strength. Therefore, timely descaling before and during rolling is crucial for surface quality. To achieve effective descaling, water pressures as high as 23 MPa are sometimes required. However, on older plate production lines, descaling water pressures are typically only 15-16 MPa. Using low-temperature heating to prevent the formation of Fe₂SiO₄, a material with poor scalability, would increase the number of rolling passes due to mill torque limitations. The application of traditional steel rolling technology makes it difficult to take into account many factors in the production process and cannot achieve large-scale production of high-surface-quality steel plates. How to produce high-surface-quality steel plates on such a production line has become a technical problem.

[0004] Chinese patent CN103506382B (A production method for improving the surface quality of thick ship plates) describes a method for producing ship plates 18 to 50 mm thick without surface defects such as roughness by adjusting rolling process parameters such as the heating furnace atmosphere, descaling pressure, descaling method, and controlled rolling temperature. Chinese patent CN109719146A (A high-efficiency descaling method for high-surface-quality medium and thick plate products) utilizes an aerosol spray device for descaling.

[0005] This innovative approach leverages the existing capabilities of older equipment, as it's difficult to completely remove the oxide scale from the surface of rolled steel plates. The resulting surface oxide scale, produced through this innovative process, is evenly distributed, thin, and free of visible mottle. Shot blasting completely removes the surface oxide scale without defects like pits, meeting the high-quality surface coating requirements for ship plates. This approach differs from existing approaches and technical approaches to removing oxide scale in existing technologies. Summary of the Invention

[0006] To solve the above technical problems, the present invention provides a method for producing high-surface-quality ship plate steel, which is suitable for the production of ship plate steel with a thickness of 8 to 20 mm and grades A and B for general strength and A32 and A36 for high strength. The technical solution adopted by the present invention is:

[0007] A method for producing high-surface-quality ship plate steel includes the steps of slab heating, rough descaling, rolling, cooling, and straightening;

[0008] After the slab leaves the heating furnace, it is roughly descaled in the descaling box. The descaling water pressure is 15-16Mpa and the flow rate is 3150±50L / min.

[0009] The rolling process adopts a single-stand rolling mill;

[0010] (1) For A and B general strength ship plate steel, a two-stage rolling process is adopted, with an initial rolling temperature of 1100±20℃, four times of fine descaling in the first and second passes of billet opening and the first and second passes after steel transfer, and no descaling in the second rolling process. The initial rolling temperature is 1020±20℃, and the final rolling temperature is 920±20℃.

[0011] (2) For A32 and A36 high-strength ship plate steel, a two-stage rolling process is adopted. The first rolling process: the starting rolling temperature is 1100±20℃, and 4 or 5 fine descaling is performed in the first and second passes of the blanking and the first, second and third passes after the steel is turned; the second rolling process: no descaling, the starting rolling temperature is 970±20℃, and the final rolling temperature is 870±30℃;

[0012] The water pressure of the fine descaling is 15-16 MPa, and the flow rate is 3150±50 L / min.

[0013] In the cooling process, when H≤14mm, air cooling is adopted; when 14<H≤20mm, ACC weak cooling process is adopted with a cooling rate of 5-8°C / s; H is the thickness of the finished ship plate steel.

[0014] In the slab heating process, the preheating section temperature is ≤850°C, the I heating section temperature is 1000±50°C, the II heating section temperature is 1250±30°C, the soaking section temperature is 1200±30°C, the total heating time is 150-210 minutes, and the residual oxygen content in the heating furnace is 2-5%.

[0015] The rolling process adopts a 3000mm single-stand rolling mill, and the slab stays on the roller table for no more than 10 seconds.

[0016] The rolling process, the temperature-waiting thickness in the two-stage rolling process is 2 to 3 times the thickness of the finished ship plate steel, and the rolling torque M max =1150KNm, rolling speed V max =4.2m / s.

[0017] The thickness of the finished ship plate steel is 8 to 20 mm.

[0018] This invention targets older medium and heavy plate production lines with dephosphorization water pressures of only 15-16 MPa. Through innovative rolling and cooling processes, descaling is omitted during the final pass or second rolling stage to prevent the generation of oxygen pressure from incomplete secondary scale removal. High-temperature controlled rolling ensures good scale plasticity, while rapid rolling reduces scale thickness. Different cooling regimes are employed based on ship plate thickness to compensate for strength losses associated with high-temperature final rolling. These process parameters ensure that the surface of the as-rolled ship plate produced is free of visible mottling, and the resulting scale is uniformly distributed, dense, and thin, allowing it to be completely removed by shot blasting, leaving no defects such as pits or wrinkles. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the surface image of the rolled ship plate steel of Example 2-1;

[0020] Figure 2 This is the surface image of the rolled ship plate steel of Example 4-2;

[0021] Figure 3 This is the surface image of the rolled ship plate steel of Example 4-3;

[0022] Figure 4 This is the surface image of the rolled ship plate steel after shot blasting in Example 2-1;

[0023] Figure 5 This is the surface image of the rolled ship plate steel in Example 4-2 after shot blasting.

[0024] Figure 6 This is the surface image of the rolled ship plate steel after shot blasting in Example 4-3;

[0025] Figure 7 This is the surface image of the rolled ship plate steel of Example 5-1;

[0026] Figure 8 This is the surface image of the rolled ship plate steel of Example 7-3;

[0027] Figure 9 This is the surface image of the rolled ship plate steel of Example 8-1;

[0028] Figure 10 This is the surface image of the as-rolled ship plate steel after shot blasting in Example 5-1;

[0029] Figure 11 This is the surface image of the rolled ship plate steel after shot blasting in Example 7-3;

[0030] Figure 12 This is the surface image of the rolled ship plate steel in Example 8-1 after shot blasting. DETAILED DESCRIPTION

[0031] A method for producing high-surface-quality shipbuilding steel plates is described. The method is suitable for producing A and B standard-strength shipbuilding steel plates, and A32 and A36 high-strength shipbuilding steel plates with thicknesses of 8 to 20 mm. The method includes slab heating, rough descaling, rolling, cooling, and straightening. The steps are as follows:

[0032] 1. Slab heating process: preheating section temperature ≤ 850℃, heating section I temperature 1000±50℃, heating section II temperature 1250±30℃, soaking section temperature 1200±30℃, total heating time 150~210min, residual oxygen content in the heating furnace 2~5%.

[0033] 2. Rough descaling process: After the slab leaves the heating furnace, rough descaling is carried out. The descaling water pressure is 15-16 MPa and the flow rate is 3150±50 L / min.

[0034] 3. Rolling process: 3000mm single-stand rolling mill is used for rolling, and the slab stays on the roller table for no more than 10 seconds;

[0035] (1) For A and B general strength ship plate steel, a two-stage rolling process is adopted, and the waiting thickness is 2 to 3 times the thickness of the finished ship plate steel. The starting rolling temperature is 1100±20℃, and four fine descaling passes are carried out in the first and second passes of the blanking and the first and second passes after the steel is turned. The second rolling process does not involve descaling. The starting rolling temperature is 1020±20℃, and the final rolling temperature is 920±20℃.

[0036] (2) For A32 and A36 high-strength ship plate steel, a two-stage rolling process is adopted, and the waiting thickness is 2 to 3 times the thickness of the finished ship plate steel. The first rolling process: the starting rolling temperature is 1100±20℃, and 4 or 5 fine descaling is carried out in the first and second passes of the blanking and the first, second and third passes after the steel is turned; the second rolling process: no descaling, the starting rolling temperature is 970±20℃, and the final rolling temperature is 870±30℃;

[0037] The water pressure of the fine descaling is 15-16 MPa, the flow rate is 3150±50 L / min; the rolling torque M max =1150KNm, rolling speed V max =4.2m / s.

[0038] 4. Cooling process: When H≤14mm, ACC cooling is not used and natural air cooling is used; when 14<H≤20mm, ACC weak cooling process is used with a cooling rate of 5-8℃ / s; H is the thickness of the finished ship plate.

[0039] 5. The steel plates are put into storage after straightening and shearing.

[0040] The chemical composition and mass percentage of the ship plate steel produced by the above method are shown in Tables 1 and 2, the process parameter settings of each embodiment are shown in Tables 3, 4, 5, and 6, and the mechanical properties of the ship plate steel are shown in Tables 7 and 8.

[0041] Table 1 Chemical composition and mass percentage of ship plate steel in various examples (%)

[0042]

[0043] Table 2 Chemical composition and mass percentage of ship plate steel in various examples (%)

[0044]

[0045] Table 3. Parameters of slab heating and rough descaling processes in Examples 1-4

[0046]

[0047] Table 4. Parameters of slab heating and rough descaling processes in Examples 5-8

[0048]

[0049] Table 5. Rolling and cooling process parameters of Examples 1-4

[0050]

[0051] Table 6, rolling and cooling process parameters of Examples 5-8

[0052]

[0053] Table 7 Mechanical properties of ship plate steel of Examples 1-4

[0054]

[0055] Table 8 Mechanical properties of ship plate steel of Examples 5-8

[0056]

[0057] Depend on Figure 1 、 2, 3, 7, 8, and 9 show that the surface of the ship plate steel produced by the present invention has no visible spots, and the generated iron oxide scale is evenly distributed, dense, and thin.

[0058] Depend on Figure 4 、 5 , 6, 10, 11, and 12 show that the surface of the ship plate steel produced by the present invention is clean after shot blasting, and there are no defects such as pits and wrinkles.

Claims

1. A method for producing high surface quality ship plate steel, characterized in that: It includes slab heating, rough descaling, rolling, cooling and straightening processes; In the rough descaling process, the slab is subjected to a rough descaling in the descaling box after leaving the heating furnace, and the descaling water pressure is 15-16 MPa and the flow rate is 3150±50 L / min; The rolling process adopts a 3000mm single-stand rolling mill, and the slab stays on the roller table for no more than 10 seconds; (1) For A and B general strength ship plate steel, a two-stage rolling process is adopted, with an initial rolling temperature of 1100±20℃, four times of fine descaling in the first and second passes of billet opening and the first and second passes after steel transfer, and no descaling in the second rolling process. The initial rolling temperature is 1020±20℃, and the final rolling temperature is 920±20℃. (2) For A32 and A36 high-strength ship plate steel, a two-stage rolling process is adopted. The first rolling process: the starting rolling temperature is 1100±20℃, and 4 or 5 fine descaling is performed in the first and second passes of the blanking and the first, second and third passes after the steel is turned; the second rolling process: no descaling, the starting rolling temperature is 970±20℃, and the final rolling temperature is 870±30℃; The water pressure of the fine descaling is 15-16 MPa, and the flow rate is 3150±50 L / min; In the cooling process, when H≤14mm, air cooling is adopted; when 14<H≤20mm, ACC weak cooling process is adopted with a cooling rate of 5-8°C / s; H is the thickness of the finished ship plate steel.

2. The method for producing high surface quality ship plate steel according to claim 1, characterized in that: In the slab heating process, the preheating section temperature is ≤850°C, the I heating section temperature is 1000±50°C, the II heating section temperature is 1250±30°C, the soaking section temperature is 1200±30°C, the total heating time is 150-210 minutes, and the residual oxygen content in the heating furnace is 2-5%.

3. The method for producing high surface quality ship plate steel according to claim 2, characterized in that: In the rolling process, the waiting thickness in the two-stage rolling process is 2 to 3 times the thickness of the finished ship plate steel.

4. The method for producing high surface quality ship plate steel according to claim 3, characterized in that: The rolling process, rolling torque M max =1150KNm, rolling speed V max =4.2m / s.

5. The method for producing high surface quality ship plate steel according to any one of claims 1 to 4, characterized in that: The thickness of the finished ship plate steel is 8 to 20 mm.

Images