Production method for high‑homogeneity and high‑toughness s355MLO thick offshore steel plate

Abstract

A production method for a high‑homogeneity and high‑toughness S355MLO thick offshore steel plate. The steel plate comprises the chemical components in mass percentage: C: 0.03-0.07%, Si: 0.15-0.35%, Mn: 1.45-1.65%, P≤0.015%, S≤0.005%, Alt: 0.025-0.045%, Nb: 0.025-0.040%, Ni: 0.25-0.50%, Cr: 0.10-0.25%, N≤50 ppm, and H ≤2 ppm, with the following impurity elements in mass percentage: As≤0.020%, Sb≤0.010%, Sn≤0.020%, Pb≤0.010%, Bi≤0.010%, Ca≤0.0025%, and B≤0.0005%, and with the balance being Fe and inevitable impurities. The present invention has the advantages of improving the full-thickness low‑temperature toughness at –50°C, the stability and the pass rate.

Description

A method for producing high-homogeneity, high-toughness S355MLO marine engineering steel thick plates Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and in particular relates to a method for producing high homogeneous and high toughness S355MLO marine steel thick plates. Background Technology

[0002] Offshore steel thick plates are widely used in the construction of oil and gas extraction platforms. Due to the complex service environment requirements, technical requirements such as low-temperature toughness, high uniformity, and excellent weldability are usually put forward for the materials. Generally, for plates thicker than 80mm, core penetration can be increased, grains can be refined, and performance uniformity can be improved through low-carbon equivalent design and rolling process optimization. However, for plates thicker than 80mm, it is difficult to effectively guarantee core penetration when the compression ratio is constant. How to improve the performance uniformity in the thickness direction while increasing the thickness has become the key to the TMCP thick plate process.

[0003] As the technical requirements for marine engineering services become increasingly stringent, higher technical demands are being placed on marine steel materials themselves. Impact toughness has been adjusted from -40℃ to -50℃, with requirements extending from the near-surface and quarter-thickness portions of the steel plate to the core. The performance uniformity requirements for thick plates are particularly stringent. Patent application CN202211408508.6 discloses an extra-thick S355MLO steel plate for fixing marine structures and its production method. This method innovatively obtains high-quality continuously cast billets through low-manganese composition and smelting methods, and then produces 100–120 mm thick S355MLO steel plates using rolling and heat treatment methods. The plates exhibit good low-temperature toughness at -40℃ near the surface and half-thickness portions. Patent application CN202211587741.5 discloses an extra-thick steel plate produced using the TMCP process and its manufacturing method. This method employs compositional design, two-stage low-temperature rolling, relaxation, and strong cooling processes to precisely control the post-rolling phase transformation microstructure, forming a fine multiphase structure to refine the original austenite grains. The low-temperature impact toughness at -40℃ in the 1 / 4 position and the core reaches over 100J. High-toughness steel plates requiring low-temperature impact at -40℃ or higher are produced using continuously cast billets. Furthermore, patent application CN202410168048.7 discloses a thick-gauge high-toughness marine engineering structural steel and its preparation method. Through economical compositional system design, matching heating and controlled rolling cooling processes, it stably produces thick-gauge high-strength, high-toughness marine engineering wind power steel plates, improving the -40℃ low-temperature impact toughness and reduction of area in the core.

[0004] However, the aforementioned patents proposed optimizations in composition and rolling process, with low-temperature impacts all at -40°C. The impact locations included the near-surface, 1 / 4 position, and core, with a focus on the core. The patents did not address the issue of low-temperature impacts at -50°C on the full thickness of the thick plate.

[0005] Summary of the Invention

[0006] The purpose of this invention is to solve the problems of insufficient toughness, poor stability, and low product qualification rate of existing marine engineering steel thick plates at -50℃. It provides a production method for high homogeneity and high toughness S355MLO marine engineering steel thick plates. By controlling the internal quality and rolling process, the impact performance of the thick plate is improved. By controlling the microstructure and grain size of the thick plate, the uniformity of the impact performance is improved, thereby effectively improving the performance qualification rate of TMCP thick plates. It is suitable for the production of TMCP marine engineering steel thick plates.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for producing high-homogeneity, high-toughness S355MLO marine engineering steel thick plates, wherein the chemical composition and mass percentage of the steel plate include: C: 0.03-0.07%, Si: 0.15-0.35%, Mn: 1.45-1.65%, P≤0.015%, S≤0.005%, Alt: 0.025-0.045%, Nb: 0.025-0.040%, Ni: 0.25-0.50%, Cr: 0.10-0.25%, N≤50ppm, H≤2ppm; wherein, the impurity elements include: As≤0.020%, Sb≤0.010%, Sn≤0.020%, Pb≤0.010%, Bi≤0.010%, Ca≤0.0025%, B≤0.0005%, with the remainder being Fe and unavoidable impurities.

[0009] Furthermore, the thickness of the steel plate is 80–120 mm, and its mechanical properties meet the requirements of R. eH ≥355Mpa, R m The strength ranges from 470 to 630 MPa, with an A value ≥ 22% and a yield strength ratio ≤ 0.93%. The transverse impact values ​​at -50℃ include: ≥ 150 J near the surface, ≥ 200 J at 1 / 2 and 1 / 4 of the surface, a reduction of area ≥ 40%, and a time-dependent impact value of 5% ≥ 150 J.

[0010] Furthermore, the production method of the high homogeneous and high toughness S355MLO marine engineering steel thick plate includes a smelting process, a heating process, and a rolling process that work together. In the smelting process, a top-and-bottom blowing converter + LF refining + RH vacuum + continuous casting process is adopted. The oxygen content of the steel produced from the converter is controlled to be ≤500ppm. The LF refining furnace produces white slag and controls the calcium-aluminum ratio to be 1.6-1.8 to improve the purity of the molten steel. The continuous casting uses 460mm billets, and the cooling adopts a strong cooling system with the dynamic reduction controlled at 13-15mm to improve the center segregation.

[0011] Furthermore, in the heating process, the billet cooling time is ≥72h, continuous furnace segmented heating is adopted, the total heating time is 9~13min / cm, the soaking section is held for 60~80min, the soaking temperature is 1070~1100℃, to prevent excessive growth of surface grains.

[0012] Furthermore, in the rolling process, the roughing process consists of 7 to 11 passes, with an intermediate billet thickness of 130 to 180 mm and a total reduction rate of ≥30% for the last three passes of roughing; the finishing process consists of 7 to 9 passes, with a reduction rate of ≥10% for the first pass of finishing; the initial rolling temperature is 740 to 780°C, and the final cooling temperature is 400 to 450°C.

[0013] Furthermore, the chemical composition and mass percentage of the steel plate include: C: 0.052%, Si: 0.30%, Mn: 1.47%, P: 0.012%, S: 0.002%, Alt: 0.034%, Ni: 0.35%, Nb: 0.031%, Cr: 0.12%, N: 0.0040%, H: 0.00015%, with the remainder being Fe and unavoidable impurities.

[0014] Furthermore, the production method of the high homogeneous and high toughness S355MLO marine engineering steel thick plate includes:

[0015] (1) In the smelting process, high purity and internal quality of molten steel 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 content of the steel produced by the converter is 490ppm. The white slag is produced by the LF refining furnace and the calcium-aluminum ratio is controlled at 1.7. The continuous casting uses 460mm billet and adopts a strong cooling secondary cooling system. The dynamic reduction is controlled at 13mm. The main reduction sections are in the 6th and 7th sections to improve the center segregation.

[0016] (2) During the heating process, the billet cooling time is ≥72h. A continuous furnace is used for segmented heating with a total heating time of 12min / cm. The soaking section is kept at 70min and the soaking temperature is 1080℃ to prevent excessive growth of surface grains.

[0017] (3) In the rolling process, the thickness of the rolled steel plate is 100mm and the width is 3000mm. There are 11 rough rolling passes, the thickness of the intermediate billet is 155mm, and the total reduction rate of the last three rough rolling passes is 33%. There are 6 finishing rolling passes, the reduction rate of the first finishing rolling pass is 12%, the initial rolling temperature is 745℃, and the final cooling temperature is 405℃.

[0018] Furthermore, the chemical composition and mass percentage of the steel plate include: C: 0.045%, Si: 0.25%, Mn: 1.58%, P: 0.010%, S: 0.001%, Alt: 0.032%, Ni: 0.42%, Nb: 0.033%, Cr: 0.13%, N: 0.0039%, H: 0.00010%, with the remainder being Fe and unavoidable impurities.

[0019] Furthermore, the production method of the high homogeneous and high toughness S355MLO marine engineering steel thick plate includes:

[0020] (1) In the smelting process, high purity and internal quality of molten steel 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 content of the steel produced by the converter is 485ppm. The white slag is produced by the LF refining furnace and the calcium-aluminum ratio is controlled at 1.65. 460mm billet is used for continuous casting. The strong cooling secondary cooling system is adopted and the dynamic reduction is controlled at 14mm. The main reduction sections are in the 6th and 7th sections to improve the center segregation.

[0021] (2) During the heating process, the billet cooling time is ≥72h. A continuous furnace is used for segmented heating with a total heating time of 13min / cm. The heat soaking section is kept at 72min and the heat soaking temperature is 1075℃ to prevent excessive growth of surface grains.

[0022] (3) In the rolling process, the thickness of the rolled steel plate is 120mm and the width is 3000mm. There are 11 rough rolling passes, the thickness of the intermediate billet is 145mm, and the total reduction rate of the last three rough rolling passes is 32%. There are 8 finishing rolling passes, the reduction rate of the first finishing rolling pass is 11%, the initial rolling temperature is 745℃, and the final cooling temperature is 420℃.

[0023] The technical solution of this invention adopts a low-carbon composition design, low-temperature heating and high-pressure controlled rolling process, optimizes continuous casting cooling and dynamic reduction process, improves center segregation, utilizes large reduction and high-pressure controlled rolling process to enhance the core penetration of thick plates, and low-temperature heating to improve surface structure, thereby achieving high homogeneity and high toughness control of thick plate structure, achieving improved toughness at -50℃ across the entire thickness of the thick plate, effectively improving the performance stability and pass rate of marine engineering steel thick plates, the product meets the -50℃ impact performance requirements of near surface, 1 / 4 position and core, has excellent welding performance, and the product can meet the technical requirements of marine engineering steel in special, low-temperature and complex service environments. Attached Figure Description

[0024] Figure 1 is a photograph of the microstructure of 120mm thick S355MLO marine steel near the top 2mm of this embodiment;

[0025] Figure 2 is a photograph of the microstructure at 1 / 4 position of the 120mm thick S355MLO marine steel in this embodiment;

[0026] Figure 3 is a photograph of the microstructure of the 120mm thick S355MLO marine steel core in this embodiment;

[0027] Figure 4 shows the mechanical properties of 100mm thick S355MLO marine steel in Example 1 of this embodiment;

[0028] Figure 5 shows the mechanical properties of the 120mm thick S355MLO marine steel used in Example 2. Detailed Implementation

[0029] Example 1

[0030] To make the present invention clearer, the following description, in conjunction with the accompanying drawings, further illustrates a method for producing high-homogeneity, high-toughness S355MLO marine engineering steel thick plates. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0031] In this embodiment, taking a 100mm thick S355MLO plate as an example, the chemical composition and mass percentage of the steel plate include: C: 0.052%, Si: 0.30%, Mn: 1.47%, P: 0.012%, S: 0.002%, Alt: 0.034%, Ni: 0.35%, Nb: 0.031%, Cr: 0.12%, N: 0.0040%, H: 0.00015%, with the remainder being Fe and unavoidable impurities.

[0032] The specific production methods include:

[0033] (1) In accordance with the above composition control, 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 content of the converter steel is 490ppm, the white slag is produced in the LF refining furnace, and the calcium-aluminum ratio is controlled at 1.7. The continuous casting uses 460mm billet, adopts strong cooling secondary cooling system, and the dynamic reduction is controlled at 13mm. The main reduction sections are in the 6th and 7th sections to improve the center segregation.

[0034] (2) The heating process: the billet cooling time is ≥72h, the continuous furnace is used for segmented heating, the total heating time is 12min / cm, the heat soaking section is kept for 70min, the heat soaking temperature is 1080℃, and the surface grains are prevented from growing excessively.

[0035] (3) The rolling process: the steel plate is 100mm thick and 3000mm wide, with 11 roughing passes, 155mm thick intermediate billet, and a total reduction rate of 33% for the last three roughing passes; 6 finishing passes, with a reduction rate of 12% for the first finishing pass, an initial rolling temperature of 745℃, and a final cooling temperature of 405℃.

[0036] The mechanical properties of the 100mm thick S355MLO marine steel in this embodiment are shown in Figure 4. The CTOD characteristic values ​​of the base metal welded at -10℃ are 1.216mm, 1.117mm, and 1.123mm, respectively. Welding of the steel plate was performed, and the impact values ​​at -50℃ at different locations of the weld joint were as follows: weld center WM: 128 / 124 / 88, fusion line FL: 199 / 206 / 231, fusion line FL+2mm: 285 / 287 / 242, fusion line FL+5mm: 346 / 341 / 307. The weld heat-affected zone exhibits excellent impact performance.

[0037] Example 2

[0038] This embodiment takes a 120mm thick S355MLO plate as an example. The chemical composition and mass percentage of the steel plate include: C: 0.045%, Si: 0.25%, Mn: 1.58%, P: 0.010%, S: 0.001%, Alt: 0.032%, Ni: 0.42%, Nb: 0.033%, Cr: 0.13%, N: 0.0039%, H: 0.00010%, with the remainder being Fe and unavoidable impurities.

[0039] The specific production method is as follows:

[0040] (1) In accordance with the above composition control, 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 content of the converter steel is 485ppm, the white slag is produced in the LF refining furnace, and the calcium-aluminum ratio is controlled at 1.65. 460mm billet is used for continuous casting. A strong cooling secondary cooling system is adopted, and the dynamic reduction is controlled at 14mm. The main reduction sections are in sections 6 and 7 to improve the center segregation.

[0041] (2) The heating process: the billet cooling time is ≥72h, the continuous furnace is used for segmented heating, the total heating time is 13min / cm, the heat soaking section is kept warm for 72min, the heat soaking temperature is 1075℃, and the surface grains are prevented from growing excessively.

[0042] (3) The rolling process: the steel plate is 120mm thick and 3000mm wide, with 11 roughing passes, 145mm thick intermediate billet, and a total reduction rate of 32% for the last three roughing passes; 8 finishing passes, with a reduction rate of 11% for the first finishing pass, an initial rolling temperature of 745℃, and a final cooling temperature of 420℃.

[0043] In this embodiment, the mechanical properties of 120mm thick S355MLO marine steel are shown in Figure 5, and its full-thickness microstructure is shown in Figures 1, 2, and 3. The CTOD characteristic values ​​of the base metal at -10℃ are 1.160mm, 1.147mm, and 1.122mm, respectively. Welding was performed on the steel plate, and the impact values ​​at -50℃ at different locations of the weld joint were as follows: weld center WM: 146 / 158 / 163, fusion line FL: 195 / 170 / 170, fusion line FL+2mm: 222 / 219 / 209, fusion line FL+5mm: 301 / 329 / 324. The weld heat-affected zone exhibits excellent impact performance.

[0044] This invention improves the toughness of thick plates at -50℃ across their entire thickness by controlling internal quality, low-temperature heating, and high-pressure rolling processes. This effectively enhances the performance stability and pass rate of marine engineering steel thick plates, providing technical support for the development and application of marine engineering steel in special low-temperature and complex service environments.

[0045] 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 method for producing high-homogeneity, high-toughness S355MLO marine engineering steel thick plates, characterized in that: The chemical composition and mass percentage of the steel plate include: C: 0.03-0.07%, Si: 0.15-0.35%, Mn: 1.45-1.65%, P≤0.015%, S≤0.005%, Alt: 0.025-0.045%, Nb: 0.025-0.040%, Ni: 0.25-0.50%, Cr: 0.10-0.25%, N≤50ppm, H≤2ppm; wherein, the impurity elements include: As≤0.020%, Sb≤0.010%, Sn≤0.020%, Pb≤0.010%, Bi≤0.010%, Ca≤0.0025%, B≤0.0005%, with the remainder being Fe and unavoidable impurities.

2. The method for producing high-homogeneity, high-toughness S355MLO marine engineering steel thick plates according to claim 1, characterized in that: The steel plate has a thickness of 80–120 mm and its mechanical properties meet the requirements of R. eH ≥355Mpa, R m The strength ranges from 470 to 630 MPa, with an A value ≥ 22% and a yield strength ratio ≤ 0.93%. The transverse impact values ​​at -50℃ include: ≥ 150 J near the surface, ≥ 200 J at 1 / 2 and 1 / 4 of the surface, a reduction of area ≥ 40%, and a time-dependent impact value of 5% ≥ 150 J.

3. The method for producing high-homogeneity, high-toughness S355MLO marine steel thick plates according to claim 1 or 2, characterized in that: The production method of the high homogeneous and high toughness S355MLO marine engineering steel thick plate includes a smelting process, a heating process, and a rolling process that work together. In the smelting process, a top and bottom blowing converter + LF refining + RH vacuum + continuous casting process is adopted. The oxygen content of the steel produced from the converter is controlled to be ≤500ppm. The LF refining furnace produces white slag and controls the calcium-aluminum ratio to be 1.6-1.8 to improve the purity of the molten steel. The continuous casting uses 460mm billets, and the cooling adopts a strong cooling system with the dynamic reduction controlled at 13-15mm to improve the center segregation.

4. The method for producing high-homogeneity, high-toughness S355MLO marine steel thick plates according to claim 3, characterized in that: In the heating process, the billet cooling time is ≥72h, and continuous furnace segmented heating is adopted. The total heating time is 9-13min / cm, the soaking section is held for 60-80min, and the soaking temperature is 1070-1100℃ to prevent excessive growth of surface grains.

5. The method for producing high-homogeneity, high-toughness S355MLO marine engineering steel thick plates according to claim 3, characterized in that: In the rolling process, there are 7 to 11 roughing passes, with an intermediate billet thickness of 130 to 180 mm and a total reduction rate of ≥30% for the last three roughing passes; and 7 to 9 finishing passes, with a reduction rate of ≥10% for the first finishing pass, an initial rolling temperature of 740 to 780°C, and a final cooling temperature of 400 to 450°C.

6. The method for producing high homogeneous and high toughness S355MLO marine steel thick plates according to claim 1 or 2, characterized in that: The chemical composition and mass percentage of the steel plate include: C: 0.052%, Si: 0.30%, Mn: 1.47%, P: 0.012%, S: 0.002%, Alt: 0.034%, Ni: 0.35%, Nb: 0.031%, Cr: 0.12%, N: 0.0040%, H: 0.00015%, with the remainder being Fe and unavoidable impurities.

7. The method for producing high-homogeneity, high-toughness S355MLO marine engineering steel thick plates according to claim 6, characterized in that: The production method of the high-homogeneity, high-toughness S355MLO marine engineering steel thick plate includes: (1) In the smelting process, high purity and internal quality of molten steel 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 content of the steel produced by the converter is 490ppm. The white slag is produced by the LF refining furnace and the calcium-aluminum ratio is controlled at 1.

7. The continuous casting uses 460mm billet and adopts a strong cooling secondary cooling system. The dynamic reduction is controlled at 13mm. The main reduction sections are in the 6th and 7th sections to improve the center segregation. (2) During the heating process, the billet cooling time is ≥72h. A continuous furnace is used for segmented heating with a total heating time of 12min / cm. The soaking section is kept at 70min and the soaking temperature is 1080℃ to prevent excessive growth of surface grains. (3) In the rolling process, the thickness of the rolled steel plate is 100mm and the width is 3000mm. There are 11 rough rolling passes, the thickness of the intermediate billet is 155mm, and the total reduction rate of the last three rough rolling passes is 33%. There are 6 finishing rolling passes, the reduction rate of the first finishing rolling pass is 12%, the initial rolling temperature is 745℃, and the final cooling temperature is 405℃.

8. The method for producing high homogeneous and high toughness S355MLO marine engineering steel thick plates according to claim 1 or 2, characterized in that: The chemical composition and mass percentage of the steel plate include: C: 0.045%, Si: 0.25%, Mn: 1.58%, P: 0.010%, S: 0.001%, Alt: 0.032%, Ni: 0.42%, Nb: 0.033%, Cr: 0.13%, N: 0.0039%, H: 0.00010%, with the remainder being Fe and unavoidable impurities.

9. The method for producing high-homogeneity, high-toughness S355MLO marine engineering steel thick plates according to claim 8, characterized in that: The production method of the high-homogeneity, high-toughness S355MLO marine engineering steel thick plate includes: (1) In the smelting process, high purity and internal quality of molten steel 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 content of the steel produced by the converter is 485ppm. The white slag is produced by the LF refining furnace and the calcium-aluminum ratio is controlled at 1.

65. 460mm billet is used for continuous casting. The strong cooling secondary cooling system is adopted and the dynamic reduction is controlled at 14mm. The main reduction sections are in the 6th and 7th sections to improve the center segregation. (2) During the heating process, the billet cooling time is ≥72h. A continuous furnace is used for segmented heating with a total heating time of 13min / cm. The heat soaking section is kept at 72min and the heat soaking temperature is 1075℃ to prevent excessive growth of surface grains. (3) In the rolling process, the thickness of the rolled steel plate is 120mm and the width is 3000mm. There are 11 rough rolling passes, the thickness of the intermediate billet is 145mm, and the total reduction rate of the last three rough rolling passes is 32%. There are 8 finishing rolling passes, the reduction rate of the first finishing rolling pass is 11%, the initial rolling temperature is 745℃, and the final cooling temperature is 420℃.

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