A 60-100mm thick SA537CL1 steel plate with excellent performance of submerged arc welding and a manufacturing method thereof

Abstract

This invention relates to a 60-100mm thick SA537CL1 steel plate with excellent die-welding performance and its manufacturing method, belonging to the field of metallurgical technology. The chemical composition of the steel plate (wt%) is: C: 0.06-0.09%, Si: 0.25-0.50%, Mn: 1.40-1.60%, P≤0.010%, S≤0.003%, Ni: ≤0.10%, Mo: 0.05-0.08%, Nb: 0.010-0.030%, Ti: 0.010-0.030%, with the balance being Fe and unavoidable impurities, and a carbon equivalent (Ceq) ≤0.36%. The surface microstructure is bainite, the microstructure at 1 / 4 thickness position is acicular ferrite + pearlite, and the core microstructure is polygonal ferrite + pearlite + acicular ferrite. The manufacturing process is as follows: oxygen converter smelting → ladle furnace refining → vacuum treatment → thick slab continuous casting → billet cooling → billet heating → controlled rolling → controlled cooling → stacking and slow cooling.

Description

Technical Field

[0001] This invention belongs to the field of metal materials and their smelting technology, and relates to a low alloy steel plate with excellent performance after mold welding and its manufacturing method. Background Technology

[0002] Extra-thick steel plates are typically over 60mm thick and are mainly used in large-scale facilities and components requiring high steel performance, such as offshore platforms, boilers, hydropower plants, and high-rise buildings. With the rapid development of energy development and infrastructure construction in my country, the demand for extra-thick steel plates has increased significantly, especially for high-performance extra-thick steel plates with a thickness of ≥80mm. To reduce the adverse effects of thickness on the mechanical properties of the steel plates, production often involves increasing the amount of alloying elements or adding heat treatment. However, this leads to insufficient production capacity, long delivery cycles, high material costs, and a significant reduction in product market competitiveness.

[0003] Large equipment and components are placing increasingly higher demands on the performance of extra-thick steel plates. Besides requiring the base material to possess high strength (tensile strength: 485~620MPa), high toughness (transverse impact energy KV2≥47J at -50℃ at 1 / 4 of plate thickness), excellent weldability (Ceq≤0.38%), and resistance to lamellar tearing (Z... Z In addition to ≥35%, strict requirements were also put forward on the strength and toughness indicators of the steel plate mold after welding.

[0004] Patent document CN 106957988 A discloses a high-toughness steel plate with a yield strength of 345MPa. It adopts a C-Si-Mn composition design system, has a low alloy cost, and good impact toughness at -40℃. However, the maximum production thickness of the steel plate that meets the performance requirements is only 80mm, which cannot meet the needs of the development of pressure vessel equipment towards larger sizes.

[0005] Patent document CN 108486471 A discloses a normalized A537CL1 steel plate for pressure vessels and its production method. It adopts a normalizing heat treatment process, which results in a longer production process for the steel plate. Moreover, the low-temperature toughness of the product is only suitable for low-temperature environments of -20℃, which severely limits the loading medium or applicable working conditions.

[0006] Patent document CN 112501500 B discloses a 100mm extra-thick earthquake-resistant and fire-resistant steel plate with uniform internal quality and a microstructure mainly composed of bainite. It has the characteristics of low yield strength ratio of the base material, high temperature strength and excellent low temperature toughness at -40℃. However, the mechanical properties of the steel plate after mold welding heat treatment are not designed.

[0007] Patent document CN 113046627 B discloses a 345MPa grade weathering bridge steel and its manufacturing method. Through a reasonable controlled rolling and cooling process, a microstructure mainly composed of acicular ferrite is formed, which has good strength and toughness matching and weather resistance. However, it has added precious metals such as Cu, Ni, Cr and magnesium-aluminum alloys, which significantly increases the production cost.

[0008] In summary, existing steel grades similar to SA537CL1 have problems such as high alloy content, long production process and thin maximum thickness. At the same time, they lack mechanical property data of steel plates after simulated post-weld heat treatment, which cannot adapt to the future development trend of low-carbon and high-efficiency manufacturing of pressure vessels. Summary of the Invention

[0009] This invention provides a 60-100mm thick SA537CL1 steel plate with excellent die-welding performance and its manufacturing method, which addresses the above-mentioned prior art. The steel plate is delivered in a controlled-rolled state (without heat treatment), significantly reducing the carbon equivalent and the dependence on alloying elements. The steel plate simultaneously meets the requirements of high toughness, easy welding and excellent post-die-welding performance.

[0010] The technical solution adopted by this invention to solve the above problems is as follows: a 60-100mm thick SA537CL1 steel plate with excellent die welding performance, the chemical composition by mass percentage is: C: 0.06-0.09%, Si: 0.25-0.50%, Mn: 1.40-1.60%, P≤0.010%, S≤0.003%, Ni: ≤0.10%, Mo: 0.05-0.08%, Nb: 0.010-0.030%, Ti: 0.010-0.030%, with the balance being Fe and unavoidable impurities, and the content of the corresponding elements satisfies Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15≤0.36%.

[0011] The surface microstructure of the steel plate in this application is bainite, the microstructure at 1 / 4 of the thickness is acicular ferrite + pearlite, and the core microstructure is polygonal ferrite + pearlite + acicular ferrite. Further, the thickness of the bainite microstructure on the surface of the steel plate is 10-20 mm, and its thickness does not exceed 1 / 4 of the steel plate thickness.

[0012] The steel plate product of this application has a yield strength ≥355MPa, tensile strength: 510~620MPa, elongation ≥30%, Zz≥35%, and transverse impact energy KV8≥100J at -50℃ at 1 / 4 of the plate thickness. After mold welding, the performance is the same as the base material. One of the mold welding processes is: heating to 620℃, holding for 3 hours, heating and cooling rate ≤100℃ / h above 300℃, and air cooling after taking it out of the furnace below 300℃, and repeating the aforementioned thermal simulation cycle 3 times.

[0013] Chemical composition is a major factor affecting the performance of extra-thick steel plates. To ensure that the extra-thick steel plates of this invention achieve excellent comprehensive mechanical properties, especially the properties after mold welding, the main elements of this invention are specified, and the main principles are as follows:

[0014] Carbon: C is the most effective element for improving the strength of steel plates. When C exists in steel as interstitial atoms, it improves the strength and plasticity of steel by hindering dislocation slip; on the other hand, a decrease in C content promotes the formation of ferrite in steel, thereby significantly improving the low-temperature toughness and weldability of the steel. Therefore, in order to ensure that the steel has a good balance of strength and toughness and weldability, the C content of this invention is controlled at 0.06% to 0.09%.

[0015] Silicon (Si) can improve the strength of steel, and its solid solution in austenite strengthens the microstructure. In low-alloy steel, an increase of 0.10% Si can increase the tensile strength of hot-rolled steel by approximately 8 MPa; however, when the Si content exceeds 0.50%, it causes a decrease in impact toughness and reduction of area, while also affecting the surface quality of the steel sheet. Therefore, the Si content in this invention is controlled at 0.25–0.50%.

[0016] Manganese (Mn) increases the strength of steel by forming a solid solution within austenite, and also improves the low-temperature toughness of steel. However, excessive Mn can easily lead to severe segregation and hot cracking in steel plates. Therefore, the Mn content in this invention is controlled at 1.40–1.60%.

[0017] Molybdenum (Mo) can improve the tempering stability and refine the grain of steel, especially improving the mechanical properties of the steel plate after die welding and preventing a significant drop in strength. Appropriate amounts of Mo can also improve the toughness of the welded joint. However, as a precious metal, excessive addition of Mo will significantly increase the cost of the steel. Therefore, the Mo content in the steel of this invention is 0.05–0.08%.

[0018] Niobium and titanium have extremely strong bonding forces with C and N, forming corresponding stable carbides and nitrides. These provide nucleation sites for acicular ferrite, while simultaneously refining the grains and preventing the coarsening of the original austenite grains, thus improving the strength and toughness of the steel, especially its low-temperature toughness, and giving the steel good weldability. Therefore, in this invention, the Nb content is controlled at 0.010–0.030%, and the Ti content is controlled at 0.010–0.030%.

[0019] Phosphorus and sulfur: Both S and P are detrimental to the hot workability and mechanical properties of steel, especially for extra-thick steel plates requiring low-temperature toughness; the lower their content, the better. Therefore, in this invention, the P content is controlled at ≤0.010%, and the S content is controlled at ≤0.003%.

[0020] The main manufacturing processes for the 60-100mm thick extra-thick low-alloy steel of this invention are: oxygen converter smelting → ladle furnace refining → vacuum treatment → thick slab continuous casting → slab cooling → slab heating → controlled rolling → controlled cooling → stacking slow cooling → flaw detection → performance testing. Specific requirements are as follows:

[0021] (1) Steelmaking and continuous casting: Before entering the oxygen converter, the molten iron is guaranteed to have [S] ≤ 0.002%, and the tapping temperature of the converter is controlled at 1600~1660℃; the ladle furnace refining time is ≥ 35 minutes, aluminum wire deoxidation is used to ensure [O] ≤ 20ppm, and niobium-iron alloy is added; the vacuum treatment time is ≥ 15 minutes, after which titanium wire and calcium wire are fed for treatment, and soft blowing is performed for ≥ 10 minutes; during continuous casting, the casting speed is controlled at 0.45~0.55m / min, the superheat is strictly controlled at 10~30℃, and the casting is poured into a thick billet, which is then slowly cooled by a cover.

[0022] (2) The heating of the thick billet adopts the low temperature and long time mode, the heating temperature is 1150~1220℃, the total furnace time is ≥400min, and the soaking time is ≥100min; the reduction of the three passes after the initial rolling is 40~50mm, and the final rolling temperature is 980~1050℃; the thickness of the finishing rolling is 2.0H, where H is the thickness of the finished steel plate, the initial rolling temperature is 780~820℃, and it is rolled into a 60~100mm thick steel plate; after rolling, the surface cooling rate of the steel plate is controlled at 10~15℃ / s, the cooling rate at 1 / 4 thickness of the steel plate is 5~10℃ / s, the reddening temperature is 480~540℃, and the stack is slowly cooled to room temperature.

[0023] The microstructure of the steel plate in the delivery state designed in this application is as follows: the surface layer is mainly bainite, with acicular ferrite and a small amount of pearlite at 1 / 4 of the thickness, and the core microstructure is polygonal ferrite, pearlite, and a small amount of acicular ferrite. The surface bainite thickness is 10-20mm. When the chemical composition is constant, the surface bainite thickness is mainly affected by the cooling rate, and its thickness does not exceed 1 / 4 of the plate thickness. This ensures that after high-temperature and multiple die welding heat treatments, the steel plate has high strength near 1 / 4 of the plate thickness, while also exhibiting good low-temperature impact toughness due to the presence of acicular ferrite.

[0024] To meet performance design requirements, the metallographic structure of this application exhibits different structures at different thicknesses. The control of the structure at different thicknesses is related to the controlled cooling process: the surface layer of 5-10mm cools rapidly, forming lath bainite; the cooling rate begins to decrease at the 10-20mm thickness, but due to the addition of a certain amount of Mo, granular bainite is formed at this thickness; the cooling rate at 1 / 4 thickness is 10-15℃ / s, and the structure is mainly acicular ferrite; further towards the core of the steel plate, the cooling rate gradually decreases to less than 5℃ / s, forming polygonal ferrite + pearlite + a small amount of acicular ferrite.

[0025] To obtain fine acicular ferrite, this application mainly controls the process from the following three aspects: 1. Low hardenability element composition design, especially low C content, ensures ferrite structure after cooling of the steel plate; 2. Controlling the cooling rate of the steel plate after rolling to 10-15℃ / s, thus avoiding the polygonal ferrite formation range at 1 / 4 of the plate thickness (typically cooling rate <5℃ / s); 3. By adding Nb and Ti, the combination of these two precipitating elements can begin to precipitate high-temperature particles below 1150℃, which has an earlier effect of refining austenite grains during rough rolling (initial rolling) and provides more nucleation particles for the formation of acicular ferrite during finish rolling. Ultimately, this achieves the production of 60-100mm thick SA537CL1 steel plates with excellent low-temperature toughness at -50℃ using TMCP instead of normalizing.

[0026] This invention adopts a low-cost composition design based on the C-Si-Mn system, supplemented with trace amounts of Nb, Ti, Mo, and other elements, controlling the carbon equivalent to within 0.36% to ensure excellent low-temperature toughness and weldability. It controls inclusions and gas content in molten steel through a double refining mode of ladle furnace + vacuum degassing, and strictly controls the superheat and casting speed of molten steel during continuous casting to improve defects such as center segregation and central porosity in the billet, further enhancing the intrinsic metallurgical quality of the billet. The billet is subjected to a low-temperature, long-time heating regime and a controlled rolling technology under high temperature and high pressure, followed by a rationally controlled cooling process to further refine the grains and obtain a specific microstructure that meets performance requirements: the microstructure at 1 / 4 of the thickness of the extra-thick steel plate is acicular ferrite with a small amount of pearlite.

[0027] Compared with existing production technologies and patents, the features and beneficial effects of this invention are as follows:

[0028] (1) Compared with traditional extra-thick pressure vessel steel plates with a yield strength of 345MPa and a thickness of 60-100mm, the production cost of the extra-thick steel plates of the present invention is significantly reduced, the production cycle is significantly shortened, and the competitiveness of medium and heavy plate products is greatly improved.

[0029] (2) This invention adopts a low-cost composition design based on the C-Si-Mn system, supplemented with trace amounts of Nb, Mo, Ti and other elements, and then develops 60-100mm thick extra-thick low alloy steel through advanced pure steel smelting technology, double refining mode of ladle furnace + vacuum, controlled rolling and controlled cooling processes, which has strong operability and applicability.

[0030] (3) The 60-100mm thick SA537CL1 steel plate of this invention has a carbon equivalent Ceq≤0.36%, and especially after mold welding, its performance meets the requirements of yield strength≥355MPa, tensile strength510-620MPa, elongation≥30%, Zz≥35%, and transverse impact energy KV8≥100J (or even ≥200J) at -50℃ at 1 / 4 of the plate thickness. It has the characteristics of high strength, excellent low temperature toughness, and excellent weldability, and has a wider range of applications. Attached Figure Description

[0031] Fig. 1 The surface metallographic structure of the 100mm thick steel plate in Example 1 is mainly bainite.

[0032] Fig. 2 The metallographic structure at 1 / 4 of the 100mm thick steel plate in Example 1 is mainly composed of acicular ferrite and a small amount of pearlite. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0034] Examples 1-2 and the comparative example are a kind of extra-thick low alloy steel plate. Its main production process is KR hot metal pretreatment → oxygen converter smelting → ladle furnace refining → vacuum treatment → thick slab continuous casting → slab cooling → slab heating → controlled rolling → controlled cooling → stacking slow cooling → flaw detection → performance inspection.

[0035] The smelting chemical composition of Examples 1-2 and the comparative examples is shown in Table 1:

[0036] Table 1. Chemical composition of the smelting process (wt%)

[0037]

[0038] The above embodiments and comparative examples are all smelted in a top and bottom combined blowing converter, then desulfurized and refined in a ladle furnace, and finally degassed in a vacuum furnace. Soft blowing is performed for a certain period of time to allow large inclusions to float up and be removed, and to ensure uniform composition. Then, the slabs are cast into continuous castings through light pressing and full-process protection.

[0039] The thick continuous casting billet is heated to 1150-1220℃, with a total furnace time of ≥400min and a soaking time of ≥100min; after the initial rolling, three passes are rolled with a reduction of 40-50mm per pass, and the final rolling temperature is 980-1050℃; the finishing rolling temperature is 780-820℃, and the billet is rolled into a 60-100mm thick steel plate; the cooling rate is controlled at 10-15℃ / s after rolling, the reheating temperature is 480-540℃, and the billet is stacked and slowly cooled to room temperature.

[0040] The rolling process parameters for Examples 1-2 and the comparative examples are shown in Table 2.

[0041] Table 2 Comparison of Main Rolling Process Parameters

[0042]

[0043] Mechanical property tests were conducted on samples of the steel plates, and the results are shown in Table 3.

[0044] Table 3 Comparison of mechanical properties of steel plate base materials in Examples 1-2 and Comparative Examples.

[0045]

[0046] As shown in Table 3, the 60-100mm thick SA537CL1 steel plate of this invention has good comprehensive performance, with a yield strength ≥355MPa, tensile strength ≥510-620MPa, and elongation A. 50 ≥30%, Zz≥35%, KV8 single value at -50℃ at 1 / 4 of plate thickness is above 100J, while the comparative strength is low, and the impact value at 1 / 4 of plate thickness fluctuates greatly.

[0047] The steel plates were subjected to mold welding (molding process: heating to 620℃, holding for 3 hours, controlling the heating and cooling rate ≤100℃ / h above 300℃, and air cooling after removal from the furnace below 300℃, repeated 3 times) and then tensile and impact properties were tested. The results are shown in Table 4.

[0048] Table 4 Comparison of mechanical properties of steel plates after welding in Examples 1-2 and the comparative example.

[0049]

[0050] As shown in Table 4, the tensile strength and impact energy of the steel plate in the embodiment decreased after mold welding, but still met the standard requirements and had a certain margin. However, the comparative embodiment could not meet the technical requirements.

[0051] Figs. 1-2 The microstructure of the surface layer and the 1 / 4 section of the 100mm thick steel plate are shown respectively. The microstructure at the 1 / 4 section consists of acicular ferrite with a small amount of pearlite. The ferrite grains are grade 8 to 9, and the grains are fine and uniform, which not only ensures that the steel of this invention has sufficient strength, but also gives it excellent low-temperature toughness.

[0052] The 60-100mm thick extra-thick low-alloy steel plate of this invention has excellent mechanical properties of the base material and after mold welding treatment, and can be widely used in hydropower, wind power, bridges, large steel structures, containers, marine engineering structures, etc. in low-temperature environments.

Claims

1. A 60-100mm thick SA537CL1 steel plate with excellent die-welding performance, characterized in that: The chemical composition, by mass percentage, is as follows: C: 0.06–0.09%, Si: 0.25–0.50%, Mn: 1.40–1.60%, P≤0.010%, S≤0.003%, Ni: ≤0.10%, Mo: 0.05–0.08%, Nb: 0.010–0.030%, Ti: 0.010–0.030%, with the balance being Fe and unavoidable impurities. The content of the corresponding elements satisfies Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15≤0.36%; The method for manufacturing 60-100mm thick SA537CL1 steel plates with excellent die-welding performance is as follows: The process involves oxygen converter smelting → ladle furnace refining → vacuum treatment → continuous casting of thick slabs → billet cooling → billet heating → controlled rolling → controlled cooling → stacking and slow cooling. Specific control processes are as follows. I. Steelmaking and Continuous Casting: Before entering the oxygen converter, the molten iron must have [S] ≤ 0.002%, and the tapping temperature of the converter must be controlled at 1600~1660℃; the ladle furnace refining time must be ≥ 35 minutes, aluminum wire deoxidation must be used to ensure [O] ≤ 20ppm, and niobium-iron alloy must be added; the vacuum treatment time must be ≥ 15 minutes, after which titanium wire and calcium wire are fed for treatment, and soft blowing must be performed for ≥ 10 minutes; during continuous casting, the casting speed must be controlled at 0.45~0.55m / min, the superheat must be strictly controlled at 10~30℃, and the casting must be cast into a billet, which must be slowly cooled by a cover. II. Hot Forming: The billet is heated in a low-temperature, long-time mode, with a heating temperature of 1150–1220℃, a total furnace time of ≥400 min, and a soaking time of ≥100 min. After exiting the furnace, it is rolled. The reduction in the first three passes after the initial rolling is 40–50 mm, and the initial and final rolling temperatures are 980–1050℃. The thickness for finishing rolling is 2.0H, where H is the thickness of the finished steel plate. The initial finishing rolling temperature is 780–820℃, and the steel plate is rolled to a thickness of 60–100 mm. After rolling, the surface cooling rate of the steel plate is controlled at 10–15℃ / s, and the cooling rate at 1 / 4 of the thickness of the steel plate is 5–10℃ / s. The cooling time is controlled to ensure that the steel plate reaches a red-hot temperature of 480–540℃, and the plates are stacked and slowly cooled to room temperature. The surface microstructure of the steel plate is bainite, the microstructure at 1 / 4 of the thickness is acicular ferrite + pearlite, and the core microstructure is polygonal ferrite + pearlite + acicular ferrite. The thickness of the bainitic structure on the surface of the steel plate is 10-20 mm, and its thickness does not exceed 1 / 4 of the thickness of the steel plate.

2. The SA537CL1 steel plate with excellent die-welding performance of 60-100mm thickness as described in claim 1, characterized in that: The steel plate has a yield strength ≥355MPa, tensile strength 510~620MPa, elongation ≥30%, Zz ≥35%, and transverse impact energy KV8 at 1 / 4 of the plate thickness at -50℃ ≥100J.

3. A method for manufacturing the 60-100mm thick SA537CL1 steel plate with excellent die-welding performance as described in claim 1, characterized in that: The process is as follows: oxygen converter smelting → ladle furnace refining → vacuum treatment → thick slab continuous casting → slab cooling under a hood → slab heating → controlled rolling → controlled cooling → stacking and slow cooling. The specific control processes are as follows. I. Steelmaking and Continuous Casting: Before entering the oxygen converter, the molten iron must have [S] ≤ 0.002%, and the tapping temperature of the converter must be controlled at 1600~1660℃; the ladle furnace refining time must be ≥ 35 minutes, aluminum wire deoxidation must be used to ensure [O] ≤ 20ppm, and niobium-iron alloy must be added; the vacuum treatment time must be ≥ 15 minutes, after which titanium wire and calcium wire are fed for treatment, and soft blowing must be performed for ≥ 10 minutes; during continuous casting, the casting speed must be controlled at 0.45~0.55m / min, the superheat must be strictly controlled at 10~30℃, and the casting must be cast into a billet, which must be slowly cooled by a cover. II. Hot Forming: The billet is heated in a low-temperature, long-time mode, with a heating temperature of 1150–1220℃, a total furnace time of ≥400 min, and a soaking time of ≥100 min. After exiting the furnace, it is rolled. The reduction of the billet in the first three passes is 40–50 mm, and the initial and final rolling temperatures are 980–1050℃. The thickness of the finishing roll is 2.0H, where H is the thickness of the finished steel plate. The starting temperature of the finishing roll is 780–820℃, and the steel plate is rolled into a thickness of 60–100 mm. After rolling, the surface cooling rate of the steel plate is controlled at 10–15℃ / s, and the cooling rate at 1 / 4 of the thickness of the steel plate is 5–10℃ / s. The cooling time is controlled to ensure that the steel plate reaches a red-hot temperature of 480–540℃, and the plates are stacked and slowly cooled to room temperature.

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