Manufacturing method of large heat input welding thick steel plate with stable low-temperature toughness

Abstract

The invention relates to a manufacturing method of a large heat input welding thick steel plate with stable low-temperature toughness. The manufacturing method comprises the processes of converter smelting, LF refining and calcium treatment, continuous casting, slab heating and controlled rolling, controlled cooling after rolling and the like. By introducing multi-element synergistic deoxidation and narrow window process control, the size uniformity and distribution uniformity of oxide particles are remarkably improved, it is ensured that after large heat input welding is conducted on a thick steel plate product, the HAZ low-temperature toughness (especially impact energy at the temperature ranging from-40 DEG C to-60 DEG C) of the thick steel plate product is excellent, intra-batch and inter-batch fluctuation is extremely small, and the welding quality of the thick steel plate product is greatly improved. And the harsh requirement of high-end equipment on the material reliability is completely met.

Description

Technical Field

[0001] This invention relates to the field of steel plate production technology, and in particular to a method for manufacturing a high heat input welded thick steel plate with stable low-temperature toughness. Background Technology

[0002] With the development of marine engineering, construction, and energy equipment towards larger and more efficient construction, the thickness of steel plates used exceeds 50mm or even 100mm. To achieve efficient construction, these thick steel plates are typically welded using high heat input welding (usually ≥100kJ / cm, or even as high as 400-500kJ / cm), which can significantly reduce the number of welding passes and improve construction efficiency. However, high heat input causes the weld heat-affected zone (HAZ) to undergo prolonged high-temperature austenitization, leading to a sharp coarsening of austenite grains. During the subsequent cooling process, coarse side-lamellar ferrite and upper bainite, among other brittle structures, are formed on the coarse grain boundaries, significantly deteriorating the low-temperature toughness of the HAZ and becoming a potential hazard to structural safety.

[0003] To address the severe degradation of the heat-affected zone (HAZ) properties during high-heat-input welding, oxide metallurgy technology emerged. Its core idea is to introduce fine, dispersed, and thermally stable oxide inclusions into steel. These inclusions act as particle pinning points to austenite grain boundaries, inhibiting grain growth and coarsening in the HAZ, and also serve as nucleation sites for intragranular ferrite (IGF), inducing IGF nucleation, refining the microstructure, and thus improving toughness. By "designing" the microscopic inclusions in steel, oxide metallurgy endows steel with the "superpower" to maintain excellent toughness even after undergoing harsh welding thermal cycles. This not only solves a bottleneck problem in engineering manufacturing but also promotes the application of high-strength steel in more critical and demanding environments, representing a remarkable advancement in modern metallurgical technology.

[0004] In view of the current problems in oxide metallurgy technology, such as lack of systematic design, poor control over the type and size of oxides, and unclear process windows, Benxi Steel Plate Co., Ltd. has developed "A Method for Manufacturing High Heat Input Welded Steel with Full-Process Oxide Metallurgy" (patent application filed on the same day). This method can achieve precise control over the type, size, distribution, and quantity of oxides in steel, ensuring the toughness of the high heat input welded HAZ (heat-affected zone) meets the requirements. However, after the industrial application of this technology, especially in low-temperature environments of -40℃ to -60℃, it is still necessary to further ensure the stability and consistency of product performance.

[0005] In conventional production processes, the oxide metallurgy process window is relatively wide. Influenced by factors such as raw material fluctuations and changes in alloy yield, the effective oxide particle number density in each heat of steel can fluctuate by more than ±30%, directly increasing the dispersion of the HAZ impact energy. On the one hand, although the average toughness value can meet the standard, the -40℃ impact energy between different plates in the same batch may fluctuate from 70J to 130J. This instability poses a potential risk to structures with extremely high safety requirements, such as LNG storage tanks and polar vessels. On the other hand, the difficulty of controlling microstructure uniformity increases. During the solidification and rolling process of thick steel plates, the distribution of oxide particles is prone to macroscopic and microscopic inhomogeneities, leading to a localized "weakest link" effect in the weld heat-affected zone.

[0006] This invention, based on solving the problem of achieving the required toughness in welded thick steel plates with high heat input, carries out advanced research and development to ensure the stable and consistent performance of welded thick steel plates with high heat input. Summary of the Invention

[0007] This invention provides a manufacturing method for high heat input welded thick steel plates with stable low-temperature toughness. By introducing multi-element synergistic deoxidation and narrow window process control, the size uniformity and distribution uniformity of oxide particles are significantly improved. This ensures that after high heat input welding, the low-temperature toughness of the HAZ (especially the impact energy at -40℃ to -60℃) of the thick steel plate products not only performs excellently, but also exhibits minimal intra-batch and inter-batch fluctuations, fully meeting the stringent requirements of high-end equipment for material reliability.

[0008] To achieve the above objectives, the present invention employs the following technical solution: A method for manufacturing a high-heat-input welded thick steel plate with stable low-temperature toughness includes the following steps: 1) Converter smelting: The final carbon content was controlled within the range of 0.03% to 0.08%, and the tapping temperature was 1600 to 1650℃; the composition of [Als] in the molten steel was 0.015% to 0.035%, [Ti] was 0.005% to 0.02%, [Mg] was 0.0008% to 0.0015%, and [B] was 0.0008% to 0.0015%. 2) LF refining and calcium treatment: The LF refining time is controlled within 20–40 min; after LF refining, soft blowing argon stirring is used for ≥10 min; after calcium treatment, the calcium-aluminum mass ratio [Ca] / [Als] in the molten steel is within the range of 0.08–0.15. 3) Continuous casting: Fully protected casting is adopted, and the superheat of molten steel in the tundish is controlled within the range of 15℃ to 20℃; the casting speed is controlled at 0.7 to 0.9 m / min; and the volume percentage of oxide particles with a size of 0.2 to 0.6 μm in the steel is ≥80%. 4) Slab heating and controlled rolling: The slab is heated to 1180–1220℃ and held for 35–45 minutes; a two-stage controlled rolling process is adopted, with the initial rolling temperature in the recrystallization zone ≥1050℃ and the final rolling temperature in the non-recrystallization zone 810–850℃, and the cumulative reduction rate in the non-recrystallization zone ≥65%; 5) Post-rolling controlled cooling: A two-stage accelerated cooling process is adopted. The first stage of cooling is to immediately cool to 550-620℃ at a cooling rate of 12-18℃ / s after rolling. The second stage of cooling is to cool to the final cooling temperature of 450-550℃ at a cooling rate of 5-8℃ / s.

[0009] During the converter smelting process, the final carbon content is controlled between 0.04% and 0.06%.

[0010] During the steel tapping process in the converter, a composite deoxidizer is used for deoxidation and alloying. The composite deoxidizer is composed of aluminum particles, ferrotitanium and metallic magnesium.

[0011] After converter smelting, the molten steel contains [Als]: 0.020%–0.030%, [Ti]: 0.008%–0.015%, [Mg]: 0.0010%–0.0012%, and [B]: 0.0010%–0.0013%.

[0012] After LF refining, calcium treatment is carried out by feeding calcium-iron wire or silicon-calcium wire; after calcium treatment, the calcium-aluminum mass ratio [Ca] / [Als] in the molten steel is in the range of 0.09 to 0.12.

[0013] The thickness of the finished steel plate is 50-80mm.

[0014] After the finished steel plates underwent welding thermal simulation tests with a simulated heat input of 300-400 kJ / cm, the Charpy impact energy at -40℃ in the HAZ was in the range of 120 J ± 10 J; and the Charpy impact energy at -60℃ in the HAZ was in the range of 100 J ± 10 J.

[0015] The number density of effective oxide particles with a particle size of 0.2 to 0.6 μm in the finished steel plate is 2200 ± 200 particles / mm², and the distribution uniformity index is ≥ 0.85.

[0016] The Pcm value of the finished steel plate is 0.18% to 0.22%.

[0017] Compared with the prior art, the beneficial effects of the present invention are: (1) Significantly improved low-temperature toughness and stability: After welding heat simulation test (simulated heat input 300-400kJ / cm), the Charpy impact energy of HAZ at -40℃ can be stabilized within the range of 120J±10J, and the batch fluctuation coefficient is ≤8%, which is far superior to the thick steel plates produced by conventional methods (fluctuation range of more than ±30J).

[0018] (2) Excellent uniformity of oxide particles: The number density of effective oxide particles (particle size 0.2~0.6μm) in the finished steel plate is stable at 2200±200 particles / mm², and the distribution uniformity index is ≥0.85.

[0019] (3) Reliable welding performance: The Pcm value of the finished steel plate is controlled at 0.18% to 0.22%, and the sensitivity to cold cracking during welding is low. It is suitable for multi-layer, multi-pass welding of thick plates with high heat input.

[0020] (4) Strong process adaptability: It can be achieved without changing the existing equipment conditions, simply by optimizing the process parameters, without the need for additional major investment. Detailed Implementation

[0021] The present invention discloses a method for manufacturing a high heat input welded thick steel plate with stable low-temperature toughness, comprising the following steps: 1) Converter smelting: The endpoint carbon content is controlled within the range of 0.03% to 0.08% (preferably 0.04% to 0.06%) to avoid over-oxidation, reduce endogenous large inclusions, and provide a suitable oxygen potential for subsequent deoxidation.

[0022] The tapping temperature is controlled at 1600-1650℃ to ensure the temperature requirements of subsequent processes.

[0023] During the converter tapping process, a composite deoxidizer is used for deoxidation and alloying. The composite deoxidizer consists of aluminum granules, ferrotitanium, and metallic magnesium (existing technology). The final molten steel composition is controlled as follows: [Als]: 0.015%–0.035%, [Ti]: 0.005%–0.02%, [Mg]: 0.0008%–0.0015%, [B]: 0.0008%–0.0015%.

[0024] Preferably, the final molten steel contains [Als]: 0.020%–0.030%, [Ti]: 0.008%–0.015%, [Mg]: 0.0010%–0.0012%, and [B]: 0.0010%–0.0013%.

[0025] The focus of converter smelting is to form Al2O3 and TiO2. x The initial deoxidation products, with MgO as the core, lay the foundation for the subsequent formation of composite oxides.

[0026] 2) LF refining and calcium treatment: The LF refining time is controlled between 20 and 40 minutes to ensure uniform composition, precise temperature, and sufficient flotation to remove large particle inclusions.

[0027] After LF refining, soft blowing argon stirring is used for ≥10 min to promote the collision, growth and flotation of small inclusions.

[0028] Calcium treatment is performed by feeding calcium-iron wire or calcium-silicon wire, resulting in a calcium-to-aluminum mass ratio [Ca] / [Als] in the molten steel after calcium treatment, which is in the range of 0.09 to 0.12. Preferably, the calcium-to-aluminum mass ratio [Ca] / [Als] in the molten steel after calcium treatment is in the range of 0.08 to 0.15.

[0029] This invention modifies high-melting-point Al₂O₃ (melting point approximately 2050℃) into low-melting-point calcium aluminate (such as 12CaO·7Al₂O₃) by precisely controlling the Ca / Al ratio range. This calcium aluminate is then further combined with a large number of deoxidation products such as TiO₂ and MgO to form a dispersed secondary phase with MgO and TiO₂ as nucleation cores. The numerous dispersed secondary phases are uniformly distributed during subsequent solidification and rolling processes, serving as effective nucleation cores for intragranular ferrite.

[0030] 3) Continuous casting: Fully protective casting is adopted to prevent secondary oxidation of molten steel. The superheat of molten steel in the tundish is controlled within a narrow range of 15℃ to 20℃, and the casting speed is controlled at 0.7 to 0.9 m / min to match it.

[0031] This invention controls overheating and drawing speed within a narrow range to achieve a moderate and stable cooling rate. This avoids excessively rapid cooling that leads to overly fine oxide particles (which easily dissolve at the high welding temperatures), while also preventing excessively slow cooling that causes excessive oxide coarsening. This approach facilitates the formation of diffusely distributed oxide particles with a size of 0.2–0.6 μm, ensuring that the volume percentage of oxide particles with a size of 0.2–0.6 μm is ≥80%.

[0032] 4) Slab heating and controlled rolling: The continuously cast slab is fed into a heating furnace and heated to 1180–1220°C, and held for 35–45 minutes to ensure austenite homogenization and partial dissolution of coarse precipitates, while retaining the target oxide.

[0033] Two-stage controlled rolling is adopted: Recrystallization zone rolling: initial rolling temperature ≥1050℃, and the as-cast structure is fully broken through multiple rolling passes.

[0034] Rolling in the non-recrystallization zone: The final rolling temperature is controlled at 810-850℃, and the cumulative reduction rate is ≥65%.

[0035] This invention, through the combination of final rolling temperature and reduction rate, not only refines the original austenite grains, but more importantly, through strong deformation-induced precipitation, promotes the precipitation of carbonitrides of microalloying elements (such as B and Ti) in the non-recrystallization region of austenite or during phase transformation. These carbonitrides then work synergistically with pre-existing composite oxides to further enhance the microstructure refinement effect.

[0036] 5) Post-rolling controlled cooling: A two-stage accelerated cooling (ACC) process is employed. First stage cooling: immediately after rolling, cool to 550-620℃ at a cooling rate of 12-18℃ / s; Second stage cooling: Then cool at a slower rate of 5-8℃ / s to the final cooling temperature (450-550℃).

[0037] The thickness of the finished steel plate is 50-80mm.

[0038] After welding thermal simulation tests with a simulated heat input of 300–400 kJ / cm, the Charpy impact energy at -40℃ in the HAZ was within the range of 120 J ± 10 J; the Charpy impact energy at -60℃ in the HAZ was within the range of 100 J ± 10 J. The effective oxide particle number density with a particle size of 0.2–0.6 μm in the finished steel plate was 2200 ± 200 particles / mm², and the distribution uniformity index was ≥ 0.85. The Pcm value of the finished steel plate was 0.18%–0.22%.

[0039] Unless otherwise specified, all contents in this invention are mass contents.

[0040] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0041]

Example 1

[0042] 2. LF refining and calcium treatment: LF refining time was 30 min, and soft blowing time was 15 min. After calcium treatment, [Ca] / [Als] = 0.115.

[0043] 3. Continuous casting: molten steel superheated to 15℃, casting speed 0.85m / min.

[0044] 4. Slab heating and controlled rolling: The slab is heated to 1200℃ and held for 40 minutes. The initial rolling temperature in the recrystallization zone is 1120℃, and the final rolling temperature in the non-recrystallization zone is 830℃, with a cumulative reduction of 70%.

[0045] 5. Post-rolling controlled cooling: In the first stage, the temperature is cooled to 580°C at a rate of 15°C / s, and in the second stage, it is cooled to 480°C at a rate of 7°C / s, followed by air cooling to room temperature.

[0046] Performance: The impact energy of the finished steel plate at -40℃ is ≥260J. After a welding thermal simulation test with a heat input of 400kJ / cm, the impact energy of the HAZ of the three groups of samples at -40℃ is 113J, 124J, and 118J, respectively.

[0047]

Example 2

[0048] 2. LF refining and calcium treatment: LF refining time was 33 min, and soft blowing time was 18 min. After calcium treatment, [Ca] / [Als] = 0.118.

[0049] 3. Continuous casting: molten steel superheated to 16℃, casting speed 0.8m / min.

[0050] 4. Slab heating and controlled rolling: The slab is heated to 1190℃ and held for 38 minutes. The initial rolling temperature in the recrystallization zone is 1150℃, and the final rolling temperature in the non-recrystallization zone is 820℃, with a cumulative reduction of 68%.

[0051] 5. Post-rolling controlled cooling: In the first stage, the temperature is cooled to 570°C at a rate of 14°C / s, and in the second stage, it is cooled to 470°C at a rate of 6°C / s, followed by air cooling to room temperature.

[0052] Performance: The impact energy of the finished steel plate at -60℃ is ≥180J. After a welding thermal simulation test with a heat input of 350kJ / cm, the impact energy of the three groups of samples at -60℃ in the HAZ is 99J, 103J, and 107J, respectively.

[0053]

Example 3

[0054] 2. LF refining and calcium treatment: LF refining time was 28 min, and soft blowing time was 12 min. After calcium treatment, [Ca] / [Als] = 0.112.

[0055] 3. Continuous casting: molten steel superheated to 16℃, casting speed 0.9m / min.

[0056] 4. Slab heating and controlled rolling: The slab is heated to 1210℃ and held for 35 minutes. The initial rolling temperature in the recrystallization zone is 1140℃, and the final rolling temperature in the non-recrystallization zone is 840℃, with a cumulative reduction of 72%.

[0057] 5. Post-rolling controlled cooling: In the first stage, the temperature is cooled to 590℃ at a rate of 16℃ / s, and in the second stage, it is cooled to 490℃ at a rate of 8℃ / s, and then air-cooled to room temperature.

[0058] Performance: The impact energy of the finished steel plate at -40℃ is ≥240J. After a welding thermal simulation test with a heat input of 300kJ / cm, the impact energy of the HAZ of the three groups of samples at -40℃ is 127J, 121J, and 125J, respectively.

[0059] Conclusion: The steel plates produced in Examples 1-3 all meet the requirements of various performance indicators, proving that the method described in this invention is feasible.

[0060] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for manufacturing a high-heat-input welded thick steel plate with stable low-temperature toughness, characterized in that, The process includes the following: 1) Converter smelting: The final carbon content was controlled within the range of 0.03% to 0.08%, and the tapping temperature was 1600 to 1650℃; the composition of [Als] in the molten steel was 0.015% to 0.035%, [Ti] was 0.005% to 0.02%, [Mg] was 0.0008% to 0.0015%, and [B] was 0.0008% to 0.0015%. 2) LF refining and calcium treatment: The LF refining time is controlled within 20–40 min; after LF refining, soft blowing argon stirring is used for ≥10 min; after calcium treatment, the calcium-aluminum mass ratio [Ca] / [Als] in the molten steel is within the range of 0.08–0.

15. 3) Continuous casting: Fully protected casting is adopted, and the superheat of molten steel in the tundish is controlled within the range of 15℃ to 20℃; the casting speed is controlled at 0.7 to 0.9 m / min; and the volume percentage of oxide particles with a size of 0.2 to 0.6 μm in the steel is ≥80%. 4) Slab heating and controlled rolling: The slab is heated to 1180–1220℃ and held for 35–45 minutes; a two-stage controlled rolling process is adopted, with the initial rolling temperature in the recrystallization zone ≥1050℃ and the final rolling temperature in the non-recrystallization zone 810–850℃, and the cumulative reduction rate in the non-recrystallization zone ≥65%; 5) Post-rolling controlled cooling: A two-stage accelerated cooling process is adopted. The first stage of cooling is to immediately cool to 550-620℃ at a cooling rate of 12-18℃ / s after rolling. The second stage of cooling is to cool to the final cooling temperature of 450-550℃ at a cooling rate of 5-8℃ / s.

2. The method for manufacturing a high-heat-input welded thick steel plate with stable low-temperature toughness according to claim 1, characterized in that, During the converter smelting process, the final carbon content is controlled between 0.04% and 0.06%.

3. The method for manufacturing a high-heat-input welded thick steel plate with stable low-temperature toughness according to claim 1, characterized in that, During the steel tapping process in the converter, a composite deoxidizer is used for deoxidation and alloying. The composite deoxidizer is composed of aluminum particles, ferrotitanium and metallic magnesium.

4. The method for manufacturing a high-heat-input welded thick steel plate with stable low-temperature toughness according to claim 1, characterized in that, After converter smelting, the molten steel contains [Als]: 0.020%–0.030%, [Ti]: 0.008%–0.015%, [Mg]: 0.0010%–0.0012%, and [B]: 0.0010%–0.0013%.

5. The method for manufacturing a high-heat-input welded thick steel plate with stable low-temperature toughness according to claim 1, characterized in that, After LF refining, calcium treatment is carried out by feeding calcium-iron wire or silicon-calcium wire; after calcium treatment, the calcium-aluminum mass ratio [Ca] / [Als] in the molten steel is in the range of 0.09 to 0.

12.

6. The method for manufacturing a high-heat-input welded thick steel plate with stable low-temperature toughness according to claim 1, characterized in that, The thickness of the finished steel plate is 50-80mm.

7. The method for manufacturing a high-heat-input welded thick steel plate with stable low-temperature toughness according to claim 1, characterized in that, After the finished steel plates underwent welding thermal simulation tests with a simulated heat input of 300-400 kJ / cm, the Charpy impact energy at -40℃ in the HAZ was in the range of 120 J ± 10 J; and the Charpy impact energy at -60℃ in the HAZ was in the range of 100 J ± 10 J.

8. The method for manufacturing a high-heat-input welded thick steel plate with stable low-temperature toughness according to claim 1, characterized in that, The number density of effective oxide particles with a particle size of 0.2 to 0.6 μm in the finished steel plate is 2200 ± 200 particles / mm², and the distribution uniformity index is ≥ 0.

85.

9. The method for manufacturing a high-heat-input welded thick steel plate with stable low-temperature toughness according to claim 1, characterized in that, The Pcm value of the finished steel plate is 0.18% to 0.22%.