Large-thickness normalized S355G8 + N maritime work steel plate and production method thereof

Abstract

The invention relates to the technical field of ferrous metallurgy, in particular to a large-thickness normalized S355G8 + N maritime work steel plate and a production method thereof.The steel plate comprises, by mass, 0.07%-0.12% of C, 0.15%-0.35% of Si, 1.45%-1.65% of Mn, smaller than or equal to 0.012% of P, smaller than or equal to 0.005% of S, 0.025%-0.045% of Alt, 0.015%-0.035% of Nb, 0.030%-0.045% of V, 0.35%-0.50% of Ni, 0.08%-0.15% of Cr, smaller than or equal to 50 ppm of N, smaller than or equal to 2 ppm of H and the balance Fe and inevitable impurities. According to the production method of the large-thickness normalized S355G8 + N maritime work steel plate, through internal quality control, low-temperature controlled rolling and normalizing process control, the low-temperature toughness of the full thickness of the thick plate at the temperature of minus 40 DEG C is improved, and technical support is provided for development and product application of maritime work steel in a special low-temperature complex service environment.

Description

Technical Field

[0001] The invention relates to the technical field of iron and steel metallurgy, and in particular to a large-thickness normalized S355G8+N marine engineering steel plate and a production method thereof. Background Art

[0002] Thick normalized offshore steel plates are widely used in offshore oil and gas platform structure construction. Due to the complex service environment of offshore projects such as low temperature, high strength, and large span, technical requirements such as large thickness, high toughness, and easy welding are put forward for materials. Generally, plates below 80 mm thick can be refined by high compression ratio and core penetration to improve the stability of impact performance. However, for plates above 80 mm, normalized offshore steel must meet both strength requirements and excellent low-temperature impact performance, which are contradictory. When the compression ratio is constant, the penetration of the rolled core is reduced. How to ensure the large thickness normalizing conditions while increasing the thickness and improving the core impact performance has become the key to the normalized offshore steel thick plate process.

[0003] European standard 10225 marine steel for S355G8 series marine steel with thickness greater than 40mm has proposed the impact requirements of near surface + core at -40℃. For thick normalized steel, it is equivalent to ensuring the impact performance of the steel plate near the surface and from 1 / 4 position to the core at the same time, and the performance uniformity of thick plates is demanding. With the large-scale marine equipment and complex service environment requirements, large thickness and high toughness of marine steel have gradually become the development trend of products. Therefore, the development of thick, high-toughness and high-homogeneity normalized marine steel provides strong technical support for oil and gas marine projects. Summary of the invention

[0004] In view of the above technical problems, the present invention overcomes the shortcomings of the prior art and provides a large-thickness normalized S355G8+N marine engineering steel plate and a production method thereof. The low-temperature toughness of the thick plate at -40°C is improved through internal quality control, low-temperature controlled rolling and normalizing process control, providing technical support for the development and product application of marine engineering steel in special low-temperature and complex service environments.

[0005] In the first aspect, the present invention provides a thick normalized S355G8+N marine steel plate, the chemical composition and mass percentage of the steel plate 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, and the rest are Fe and unavoidable impurities.

[0006] Furthermore, the thickness of the steel plate is 80-100mm, and the mechanical properties meet the following requirements: ReH≥325Mpa; Rm: 470-630Mpa, A≥22%, and yield strength ratio≤0.85%; the -40°C transverse impact value of the steel plate: near the surface ≥200J, 1 / 2 ≥200J, section shrinkage ≥35%, and 5% time-effect strain impact value ≥150J.

[0007] In a second aspect, the present invention further provides a method for producing the large-thickness normalized S355G8+N marine engineering steel plate described in any scheme of the first aspect, which specifically includes a smelting process, a heating process, a rolling process and a heat treatment process.

[0008] Furthermore, the smelting process specifically includes: adopting top and bottom combined blowing converter + LF refining + RH vacuum + continuous casting process; the oxygen of the converter steel is controlled to be ≤480ppm, the LF refining furnace produces white slag, the calcium-aluminum ratio is controlled to be 1.5-1.7, and the purity of the molten steel is improved; 370mm billet is used for continuous casting, weak cooling system is used for cooling, and the dynamic reduction is controlled to be 8-15mm to improve the center segregation of the billet.

[0009] Furthermore, the heating process specifically includes: the billet cooling time is ≥48 hours, continuous furnace segmented heating is adopted, the total heating time is 9-13min / cm, the soaking section is kept warm for 50-70min, the soaking temperature is 1080-1100°C, so as to prevent the grains on the billet surface from overburning and growing.

[0010] Furthermore, the rolling process specifically includes: 7-11 rough rolling passes, the intermediate billet thickness is 130-180mm, the total reduction rate of the last three rough rolling passes is ≥35%; 7-9 finishing rolling passes, the reduction rate of the first finishing pass is ≥10%, the starting rolling temperature is 800-840℃, and the final cooling temperature is 700-740℃. The single-pass reduction of rough rolling and finishing rolling improves the uniformity of the thick plate in the thickness direction, 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°C, heating + holding time of 170-230min, air cooling of the steel plate after it leaves the furnace, fan cooling on the cooling bed, and offline temperature below 150°C.

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

[0013] The beneficial effects of the present invention are:

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

[0015] (2) The mechanical properties of the 80-100 mm thick normalized S355G8+N marine engineering steel plate obtained by the present invention meet the following requirements: ReH ≥ 325 MPa; Rm: 470-630 MPa, A ≥ 22%, yield strength ratio ≤ 0.85%, -40°C transverse impact value: near the surface ≥ 200 J in the thickness direction, ≥ 200 J at 1 / 2, section shrinkage ≥ 35%, and 5% time-induced strain impact value ≥ 150 J. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0018] Figure 3 This is the metallographic structure diagram of the core of the 100 mm thick S355G8+N marine steel plate in Example 2 of the present invention. DETAILED DESCRIPTION

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

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

[0021] (2) The heating process: the billet cooling time is ≥ 48 hours, and continuous furnace is used for staged heating, with a total heating time of 12 min / cm, a soaking section for 60 min, and a soaking temperature of 1085°C to prevent excessive growth of surface grains.

[0022] (3) The rolling process is as follows: the thickness of the rolled steel plate is 80 mm, the width is 3200 mm, the rough rolling is performed for 12 passes, the intermediate billet thickness is 135 mm, the total reduction rate of the last three rough rolling passes is 35%, the finishing rolling is performed for 7 passes, the reduction rate of the first finishing rolling pass is 12%, the starting rolling temperature is 805°C, and the final cooling temperature is 715°C.

[0023] (4) The heat treatment process: the heat treatment temperature is 890°C, the heating + holding time is 195 minutes, the steel plate is air-cooled after leaving the furnace, the cooling bed is cooled by a fan, and the offline temperature is 110°C.

[0024] (5) The mechanical properties of the 80 mm thick S355G8+N marine engineering steel of this embodiment are shown in Table 2. The CTOD characteristic values ​​of the parent material at -10°C are 1.005 mm, 1.076 mm, and 1.113 mm, respectively. The steel plates are welded, and the -40°C impact values ​​of different positions of the weld joints are: 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 properties.

[0025] Embodiment 2 of the present invention is specifically exemplified by a 100 mm thick S355G8+N marine steel plate and a production method thereof.

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

[0027] (2) The heating process: the billet cooling time is ≥ 48 hours, and continuous furnace is used for staged heating, with a total heating time of 13 min / cm, a soaking section for 62 min, and a soaking temperature of 1095°C to avoid excessive growth of surface grains.

[0028] (3) The rolling process: the rolled steel plate has a thickness of 100 mm and a width of 3000 mm, 11 rough rolling passes, an intermediate billet thickness of 145 mm, and the reduction rates of the last three rough rolling passes are 11%, 12%, and 10% respectively; 8 finishing rolling passes, with a reduction rate of 11% in the first finishing rolling pass, a starting rolling temperature of 815°C, and a final cooling temperature of 720°C.

[0029] (4) The heat treatment process: the heat treatment temperature is 890°C, the heating + holding time is 210 minutes, the steel plate is air-cooled after leaving the furnace, the cooling bed is cooled by a fan, and the offline temperature is 120°C.

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

[0031] Table 1 Composition of each embodiment

[0032] Example C Si Mn P S Alt Ni Nb Cr V N H Fe and inevitable impurities 1 0.105 0.25 1.52 0.012 0.002 0.032 0.45 0.026 0.11 0.034 0.0035 0.00015 margin 2 0.098 0.25 1.54 0.010 0.001 0.031 0.42 0.028 0.13 0.035 0.0038 0.00010 margin

[0033] Table 2 Performance of each embodiment

[0034]

[0035] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A thick normalized S355G8+N marine steel plate, characterized in that: The chemical composition and mass percentage of the steel plate 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, and the rest are Fe and unavoidable impurities.

2. The thick normalized S355G8+N marine steel plate according to claim 1, characterized in that: The thickness of the steel plate is 80-100mm, and the mechanical properties meet the following requirements: ReH≥325Mpa; Rm: 470-630Mpa, A≥22%, and yield strength ratio≤0.85%; the -40°C transverse impact value of the steel plate is: near the surface ≥200J, 1 / 2 ≥200J, section shrinkage ≥35%, and 5% time-effect strain impact value ≥150J.

3. The method for producing a thick normalized S355G8+N marine steel plate as claimed in claim 1 or 2, characterized in that: Including smelting process, heating process, rolling process and heat treatment process.

4. The production method according to claim 3, characterized in that: The smelting process specifically includes: top and bottom combined blowing converter + LF refining + RH vacuum + continuous casting process; converter steel oxygen is controlled to be ≤480ppm, LF refining furnace produces white slag, and the calcium-aluminum ratio is controlled to be 1.5-1.7; continuous casting uses 370mm billet, cooling adopts weak cooling system, and dynamic reduction is controlled to be 8-15mm.

5. The production method according to claim 3, characterized in that: The heating process specifically includes: the billet cooling time is ≥48 hours, continuous furnace segmented heating is adopted, the total heating time is 9-13min / cm, the soaking section is kept warm for 50-70min, and the soaking temperature is 1080-1100°C.

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

7. The production method according to claim 3, characterized in that: The heat treatment process specifically includes: heat treatment temperature of 890°C, heating + holding time of 170-230min, air cooling of the steel plate after it leaves the furnace, fan cooling on the cooling bed, and offline temperature below 150°C.

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

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