Thick normalized s355g8+n offshore steel plate and production method therefor

Abstract

The present invention relates to the technical field of ferrous metallurgy, and particularly to a thick normalized S355G8+N offshore steel plate and a production method therefor. The chemical composition of the steel plate comprises, in mass percentage: C: 0.07-0.12%, Si: 0.15-0.35%, Mn: 1.45-1.65%, P≤0.012%, S≤0.005%, Alt: 0.025-0.045%, Nb: 0.015-0.035%, V: 0.030-0.045, Ni: 0.35-0.50%, Cr: 0.08-0.15%, N≤50 ppm, H≤2 ppm, with the balance being Fe and unavoidable impurities. The production method for the thick normalized S355G8+N offshore steel plate of the present invention achieves an improvement in through-thickness low-temperature toughness at -40°C by controlling internal quality, low-temperature controlled rolling, and a normalizing process, thereby providing technical support for the development and application of offshore steel plates used in special low-temperature and complex service environments.

Description

A thick normalized S355G8+N marine engineering steel plate and its production method Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, and in particular to a thick normalized S355G8+N marine engineering steel plate and its production method. Background Technology

[0002] Thick, normally heated marine engineering steel plates are widely used in the construction of offshore oil and gas platform structures. Due to the complex service environment of offshore projects, characterized by low temperatures, high strength, and large spans, technical requirements for materials include large thickness, high toughness, and ease of welding. Generally, plates thicker than 80mm can have their grains refined and impact performance improved through high compression ratios and core penetration. However, for plates thicker than 80mm, normally heated marine engineering steel must meet both strength requirements and excellent low-temperature impact performance, which presents a contradiction. At a fixed compression ratio, rolling core penetration decreases. Therefore, the key to the normalizing process for thick marine engineering steel plates lies in how to improve core impact performance while increasing thickness under normalizing conditions.

[0003] The European standard 10225 for marine engineering steel specifies near-surface and core impact resistance requirements for S355G8 series marine engineering steels thicker than 40mm. For thick normalized steel, this means simultaneously ensuring impact resistance from the near-surface, the first quarter of the way down, to the core, placing stringent demands on the uniformity of performance in thick plates. With the increasing size of marine equipment and the demands of complex service environments, greater thickness and higher toughness in marine engineering steel have gradually become the development trend. Therefore, developing thick, high-toughness, and highly homogeneous normalized marine engineering steel to provide strong technical support for oil and gas marine engineering projects is of great significance to the national energy strategy. Summary of the Invention

[0004] This invention addresses the aforementioned technical problems and overcomes the shortcomings of existing technologies by providing a thick normalized S355G8+N marine engineering steel plate and its production method. Through internal quality control, low-temperature controlled rolling, and control of the normalizing process, the toughness of the thick plate at -40℃ is improved, providing technical support for the development and application of marine engineering steel for special low-temperature and complex service environments.

[0005] In a first aspect, the present invention provides a thick normalized S355G8+N marine engineering steel plate, the chemical composition and mass percentage of which are: C: 0.07-0.12%, Si: 0.15-0.35%, Mn: 1.45-1.65%, P≤0.012%, S≤0.005%, Alt: 0.025-0.045%, Nb: 0.015-0.035%, V: 0.030-0.045%, Ni: 0.35-0.50%, Cr: 0.08-0.15%, N≤50ppm, H≤2ppm, with the remainder being Fe and unavoidable impurities.

[0006] Furthermore, the steel plate has a thickness of 80-100mm and mechanical properties that meet the following requirements: ReH≥325Mpa; Rm: 470-630Mpa, A≥22%, yield strength ratio≤0.85%; the steel plate has the following transverse impact values ​​at -40℃: near the surface ≥200J, 1 / 2 point ≥200J, reduction of area ≥35%, and 5% time-effect deformation impact value ≥150J.

[0007] Secondly, the present invention also provides a method for producing thick normalized S355G8+N marine engineering steel plates as described in any of the embodiments of the first aspect, specifically including smelting process, heating process, rolling process and heat treatment process.

[0008] Furthermore, the smelting process specifically includes: adopting a top-and-bottom combined blowing converter + LF refining + RH vacuum + continuous casting process; controlling the oxygen content of the converter tapping steel to ≤480ppm, producing white slag in the LF refining furnace, controlling the calcium-aluminum ratio to 1.5-1.7, and improving the purity of the molten steel; using 370mm billets in continuous casting, adopting a weak cooling regime, and controlling the dynamic reduction to 8-15mm to improve the segregation at the center of the billet.

[0009] Furthermore, the heating process specifically includes: billet stacking cooling time ≥ 48 hours, continuous furnace segmented heating, total heating time 9-13 min / cm, soaking zone holding for 50-70 min, soaking temperature 1080-1100℃, to prevent overheating and growth of grains on the billet surface.

[0010] Furthermore, the rolling process specifically includes: 7-11 roughing passes, intermediate billet thickness of 130-180mm, and total reduction rate of the last three roughing passes ≥35%; 7-9 finishing passes, reduction rate of the first finishing pass ≥10%, initial rolling temperature of 800-840℃, and final cooling temperature of 700-740℃. The single-pass reduction in roughing and finishing improves the uniformity of the thickness direction of the thick plate, refines the grains, and promotes the transformation of fine ferrite and grains.

[0011] Furthermore, the heat treatment process specifically includes: heat treatment temperature of 890℃, heating and holding time of 170-230min, air cooling after the steel plate is taken out of the furnace, cooling by a fan on the cooling bed, and the offline temperature is below 150℃.

[0012] Furthermore, the composition of the molten iron in the smelting process is controlled as follows: As≤0.020%, Sb≤0.010%, Sn≤0.020%, Pb≤0.010%, Bi≤0.010%, Ca≤0.0025%, B≤0.0005%.

[0013] The beneficial effects of this invention are:

[0014] (1) The production method of thick normalized S355G8+N marine engineering steel plate provided by the present invention adopts peritectic composition design, low temperature heating and strong controlled rolling process to improve the core penetration of thick plate, and efficient heat treatment process to achieve steel plate structure control, realize the stable production of high homogeneous thick plate, the product meets the -40℃ impact requirement near the surface and core, has excellent welding performance and cold deformation performance, and the product can meet the technical requirements of marine engineering steel in special, low temperature and complex service environment;

[0015] (2) The mechanical properties of the 80-100mm thick normalized S355G8+N marine steel plate obtained by the present invention meet the following requirements: ReH≥325Mpa; Rm:470-630Mpa, A≥22%, yield strength ratio≤0.85%, transverse impact value at -40℃: near the surface in the thickness direction≥200J, 1 / 2 point≥200J, section reduction rate≥35%, and time-effect deformation impact value at 5%≥150J. Attached Figure Description

[0016] Figure 1 is a near-surface metallographic diagram of the 100mm thick S355G8+N marine steel plate in Embodiment 2 of the present invention;

[0017] Figure 2 is a metallographic diagram of the 1 / 4 position of the 100mm thick S355G8+N marine steel plate in Embodiment 2 of the present invention;

[0018] Figure 3 is a metallographic diagram of the core area of ​​a 100mm thick S355G8+N marine steel plate in Embodiment 2 of the present invention. Detailed Implementation

[0019] Embodiment 1 of the present invention is specifically exemplified by an 80mm thick S355G8+N marine engineering steel plate and its production method.

[0020] (1) The composition control of Example 1 is shown in Table 1. High steel purity and internal quality are ensured through smelting and continuous casting processes. The process of top and bottom blowing converter + LF refining + RH vacuum + continuous casting is adopted. The oxygen in the converter is 450ppm, the white slag is produced in the LF refining furnace, and the calcium-aluminum ratio is controlled at 1.6. 370mm billet is used for continuous casting. A weak cooling secondary cooling system is adopted, and the dynamic reduction is controlled at 10mm. The main reduction sections are in the 5th and 6th sections to improve the center segregation.

[0021] (2) The heating process: the billet cooling time is ≥48 hours, continuous furnace segmented heating is adopted, the total heating time is 12 min / cm, the heat soaking section is kept for 60 min, the heat soaking temperature is 1085℃, to prevent excessive growth of surface grains.

[0022] (3) The rolling process: the steel plate is 80mm thick and 3200mm wide, with 12 roughing passes, 135mm thick intermediate billet, and a total reduction rate of 35% for the last three roughing passes; 7 finishing passes, with a reduction rate of 12% for the first finishing pass, an initial rolling temperature of 805℃, and a final cooling temperature of 715℃.

[0023] (4) The heat treatment process: heat treatment temperature 890℃, heating + holding time 195min, air cooling after steel plate is taken out of furnace, fan cooling on cooling bed, and offline temperature 110℃.

[0024] (5) The mechanical properties of the 80mm thick S355G8+N marine steel in this embodiment are shown in Table 2. The CTOD characteristic values ​​of the base metal at -10℃ are 1.005mm, 1.076mm, and 1.113mm, respectively. The steel plates were welded, and the impact values ​​at -40℃ at different positions of the weld joint were as follows: weld center WM: 118 / 114 / 98, fusion line FL: 189 / 216 / 221, fusion line FL+2mm: 255 / 257 / 242, fusion line FL+5mm: 256 / 251 / 247. The weld heat-affected zone has excellent impact performance.

[0025] Embodiment 2 of the present invention specifically illustrates a 100mm thick S355G8+N marine engineering steel plate and its production method.

[0026] (1) The composition control of Example 2 is shown in Table 1. High steel purity and internal quality are ensured by smelting and continuous casting processes. The process of top and bottom blowing converter + LF refining + RH vacuum + continuous casting is adopted. The oxygen in the converter is 465ppm. The LF refining furnace produces white slag and controls the calcium-aluminum ratio to 1.65. 370mm billet is used for continuous casting. Strong cooling secondary cooling system is adopted. The dynamic reduction is controlled at 11mm. The main reduction sections are in the 5th and 6th sections to improve the center segregation.

[0027] (2) The heating process: the billet cooling time is ≥48 hours, continuous furnace segmented heating is adopted, the total heating time is 13 min / cm, the heat soaking section is kept for 62 min, the heat soaking temperature is 1095℃, and the surface grains are avoided from growing excessively.

[0028] (3) The rolling process: the steel plate is 100mm thick and 3000mm wide, with 11 roughing passes, 145mm thick intermediate billet, and the reduction rates of the last three roughing passes are 11%, 12%, and 10%, respectively; the finishing passes are 8, with the reduction rate of the first finishing pass being 11%, the initial rolling temperature being 815℃, and the final cooling temperature being 720℃.

[0029] (4) The heat treatment process: heat treatment temperature 890℃, heating + holding time 210min, air cooling after the steel plate is taken out of the furnace, cooling by fan on the cooling bed, and the offline temperature 120℃.

[0030] (5) The mechanical properties of the 120mm thick S355G8+N marine steel in this embodiment are shown in Table 2. The CTOD characteristic values ​​of the base metal at -10℃ are 1.260mm, 1.047mm, and 1.022mm, respectively. The steel plate was welded, and the impact values ​​at -40℃ at different positions of the weld joint were as follows: weld center WM: 116 / 128 / 113, fusion line FL: 155 / 120 / 109, fusion line FL+2mm: 212 / 209 / 229, fusion line FL+5mm: 231 / 249 / 254. The weld heat-affected zone has excellent impact performance.

[0031] Table 1. Components of each embodiment

[0032] Table 2 Performance of Each Embodiment

[0033] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A thick, normalized S355G8+N marine engineering steel plate, characterized in that, The chemical composition and mass percentage of the steel plate are as follows: C: 0.07-0.12%, Si: 0.15-0.35%, Mn: 1.45-1.65%, P≤0.012%, S≤0.005%, Alt: 0.025-0.045%, Nb: 0.015-0.035%, V: 0.030-0.045%, Ni: 0.35-0.50%, Cr: 0.08-0.15%, N≤50ppm, H≤2ppm, with the remainder being Fe and unavoidable impurities.

2. The thick normalized S355G8+N marine engineering steel plate according to claim 1, characterized in that, The steel plate has a thickness of 80-100mm and its mechanical properties meet the following requirements: ReH≥325Mpa; Rm:470-630Mpa, A≥22%, yield strength ratio≤0.85%; the steel plate's transverse impact value at -40℃ is: near the surface ≥200J, 1 / 2 point ≥200J, reduction of area ≥35%, and 5% time-effect deformation impact value ≥150J.

3. The method for producing thick normalized S355G8+N marine engineering steel plate as described in claim 1 or 2, characterized in that, This includes smelting processes, heating processes, rolling processes, and heat treatment processes.

4. The production method according to claim 3, characterized in that, The smelting process specifically includes: using a top-and-bottom combined blowing converter + LF refining + RH vacuum + continuous casting process; controlling the oxygen content of the converter steel to ≤480ppm, producing white slag in the LF refining furnace, and controlling the calcium-aluminum ratio to 1.5-1.7; using 370mm billets for continuous casting, adopting a weak cooling regime, and controlling the dynamic reduction to 8-15mm.

5. The production method according to claim 3, characterized in that, The heating process specifically includes: billet cooling time ≥ 48 hours, continuous furnace segmented heating, total heating time 9-13 min / cm, soaking zone holding time 50-70 min, and soaking temperature 1080-1100℃.

6. The production method according to claim 3, characterized in that, The rolling process specifically includes: 7-11 roughing passes, intermediate billet thickness of 130-180mm, and total reduction rate of the last three roughing passes ≥35%; 7-9 finishing passes, reduction rate of the first finishing pass ≥10%, initial rolling temperature of 800-840℃, and final cooling temperature of 700-740℃.

7. The production method according to claim 3, characterized in that, The heat treatment process specifically includes: heat treatment temperature of 890℃, heating and holding time of 170-230min, air cooling after the steel plate is taken out of the furnace, cooling by a fan on the cooling bed, and the offline temperature is below 150℃.

8. The production method according to claim 3, characterized in that, The composition of the molten iron in the smelting process is controlled as follows: As≤0.020%, Sb≤0.010%, Sn≤0.020%, Pb≤0.010%, Bi≤0.010%, Ca≤0.0025%, B≤0.0005%.

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