A thick high-performance steel plate for pressure-bearing equipment and its preparation method

Abstract

This invention relates to a thick, high-performance steel plate for pressure equipment and its preparation method. The chemical composition and weight percentage content of the steel plate are as follows: C: 0.12%–0.2%, Si: 0.14%–0.34%, Mn: 0.42%–0.84%, P: ≤0.015%, S: ≤0.005%, Mo: 0.36%–0.44%, Nb: 0.11%–0.32%, RE: 0.018%–0.029%, with the balance being Fe and unavoidable inclusions. The preparation method mainly employs a two-stage billet heating process + a two-stage controlled rolling process using TMCP + a two-stage heat treatment process to produce steel plates for pressure equipment with a thickness of 65–150 mm. The produced steel plates have a uniform microstructure, good strength and toughness matching, excellent low-temperature and high-temperature service performance, and good resistance to hydrogen-induced cracking.

Description

Technical Field

[0001] This invention relates to the field of metal materials technology, and in particular to a thick, high-performance steel plate for pressure-bearing equipment and its preparation method. Background Technology

[0002] Steel plates for pressure equipment are widely used in industries such as chemical, machinery, metallurgy, petrochemical, shipbuilding, and nuclear power. They come in a wide variety of types and specifications and are used in complex environments.

[0003] To ensure safety, large pressure vessels or key components of pressure vessels in pressure-bearing equipment mainly use thick steel plates for pressure-bearing equipment, which imposes more stringent technical requirements on the weldability, thickness uniformity, strength and toughness of the steel plates.

[0004] For steel plates used in pressure equipment produced by continuous casting, the compression ratio gradually decreases as the thickness of the steel plate increases, and the uniformity and density of the microstructure in the thickness direction of the steel plate deteriorate, resulting in a decline in mechanical properties.

[0005] Chinese patent application No. 201210064306.4 discloses a steel for nuclear power pressure vessels and its manufacturing method, including the following steps: 1) smelting and casting slabs, wherein the steel composition by weight percentage is: C 0.05~0.20%, Si 0.10~0.40%, Mn 0.75~1.6%, Cr 0.15~0.6%, Nb 0.010~0.04%, Ti 0.008~0.03%, Alt 0.030~0.050%, Ca 0.0010~0.0050%, N 0.003~0.012%, S≤0.010%, P≤0.012%, Sn≤0.003wt.%, Sb≤0.002%, As≤0. 0.003%, the remainder being Fe and unavoidable impurities, with Alt / N ≥ 2; 2) Hot rolling, slab heating temperature 1100~1250℃, first stage rolling temperature 950~1020℃, reduction rate ≥ 80%, second stage rolling temperature 780~900℃, reduction rate ≥ 60%; 3) Cooling and coiling, cooling rate 4.0~15℃ / s, coiling temperature 590~680℃, through controlled rolling and controlled cooling process, the microstructure of the steel plate for nuclear power pressure vessels is obtained as fine ferrite + pearlite structure. This patent is mainly for the production of conventional thickness specifications (2.5~16mm). Summary of the Invention

[0006] To overcome the shortcomings of the prior art, the technical problem solved by the present invention is to provide a steel plate for high-performance pressure equipment with large thickness and a method for preparing the same. The present invention can prepare steel plates with a thickness of 65 to 150 mm, and the thick steel plate has advantages such as good strength, low-temperature toughness and high-temperature service performance.

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

[0008] A thick, high-performance steel plate for pressure equipment, wherein the chemical composition and weight percentage content of the steel plate are as follows:

[0009] C: 0.12%–0.2%, Si: 0.14%–0.34%, Mn: 0.42%–0.84%, P: ≤0.015%, S: ≤0.005%, Mo: 0.36%–0.44%, Nb: 0.11%–0.32%, RE: 0.018%–0.029%, with the balance being Fe and unavoidable inclusions.

[0010] The mechanical properties of the steel plate are as follows: at room temperature, 310MPa≤Rel≤420MPa, 490MPa≤Rm≤640MPa, and KV2≥115J at -20℃.

[0011] The reasons for selecting the chemical elements and composition ranges for the steel plate of this invention are briefly explained below:

[0012] Carbon (C) is the most important element for increasing the strength of steel. Its combination with strong carbide alloying elements in steel plays a role in precipitation strengthening. However, excessively high carbon content can affect the machinability of steel, and the formation of supersaturated carbides can become the source of hydrogen-induced cracking (HIC), affecting the steel plate's resistance to hydrogen-induced cracking. Therefore, this invention sets the C content range to 0.12% to 0.2%.

[0013] In steelmaking, silicon (Si) acts as a reducing agent and deoxidizer. Si is also an inexpensive alloying element; adding an appropriate amount of Si to steel, which dissolves in austenite, can improve the steel's hardness and strength. However, excessively high silicon content can easily form hard phase compounds, affecting the plasticity and toughness of the steel plate and making it prone to cracking during later processing. Therefore, this invention sets the Si content range to 0.14%–0.34%.

[0014] Mn is infinitely soluble in Fe and is a strong austenite stabilizer, playing a role in fixing austenite in steel and replacing some of the role of nickel. While increasing the strength of steel, Mn has a relatively small impact on plasticity, lowers the lower critical point of steel, increases the undercooling during austenite cooling, thereby refining the microstructure and improving the mechanical properties of the steel plate. Furthermore, it is relatively inexpensive. However, excessively high Mn content increases the tendency for the formation of hard MnS inclusions in the steel; therefore, this invention sets the Mn content range to 0.42%–0.84%.

[0015] S and P are harmful elements in steel, and must be strictly controlled to ensure the purity and toughness of steel. It is known that MnS generated in steel will have a negative impact on the strength, toughness and resistance to hydrogen-induced cracking of steel plates. In order to strictly control the generation of MnS inclusions in steel plates, this invention limits P ≤0.015% and S ≤0.005%.

[0016] Mo is a weak solid solution strengthening element. Its main role in steel is to increase the supercooling capacity of austenite, thereby refining the microstructure. It also has a positive influence on impact toughness and the brittle-to-ductile transition temperature. Furthermore, the addition of molybdenum reduces the tendency of steel plates to pit corrosion caused by chloride ions, increasing the corrosion resistance of the steel plates. However, excessive Mo has an adverse effect on the weldability of the steel plates. Therefore, this invention sets the Mo content range to 0.36%–0.44%.

[0017] In steel, nitrogen (Nb) plays a crucial role. When dissolved in solid solution, it primarily strengthens the steel through solid solution, significantly improving its hardenability. Furthermore, due to its high affinity for carbon (C) and oxygen (O), when present as carbide and oxide particles, it refines the grain size, reduces hardenability, improves impact toughness, lowers the brittle transition temperature, and maintains strength. However, when Nb content reaches a certain level, segregation occurs, leading to significant microstructural differences within the grains and affecting the alloy's mechanical and wear-resistant properties. Therefore, this invention sets the Nb content range to 0.11%–0.32%.

[0018] RE can improve the plasticity, impact toughness, oxidation resistance, and corrosion resistance of steel. Its oxidation resistance surpasses that of elements such as silicon, aluminum, and titanium. It improves the fluidity of steel, reduces non-metallic inclusions, and makes the steel structure denser and purer. Due to its good affinity with sulfur and oxygen in steel, adding a certain amount of RE can effectively deoxidize and desulfurize the steel, improve impact toughness (especially low-temperature toughness), and enhance resistance to high-temperature corrosion (HIC). RE is also stable at high temperatures, increasing the oxidation resistance of the steel plate, maintaining fine grains at high temperatures, and improving high-temperature strength, thereby ensuring the high-temperature service performance of the steel plate. However, considering manufacturing costs and the feasibility of mass production, this invention sets the RE content range to 0.018%–0.029%.

[0019] This invention also provides a method for preparing thick, high-performance steel plates for pressure equipment. The method mainly employs a combination of a two-stage billet heating process, a two-stage controlled rolling process using TMCP, and a two-stage heat treatment process to produce steel plates for pressure equipment with a thickness of 65–150 mm. The specific method is as follows:

[0020] (1) Slab continuous casting: After steel smelting, slab continuous casting machine is used for casting. The tundish steel casting temperature is 1526~1558℃, the superheat is set to 25~34℃, and the billet pulling speed during casting is 1.0~1.4m / min.

[0021] (2) Stacking and slow cooling: The billet is stacked and slow cooled after it leaves the production line. The stacking and slow cooling time is 36 to 48 hours.

[0022] (3) Two-stage billet heating: The continuously cast slab is sent to the heating furnace for heating; the billet is heated in two stages. The first stage heating temperature range is 995~1016℃, and the holding time is 1.7~2.3h; the second stage heating temperature range is 1215~1232℃, and the billet soaking time is controlled at 1.5~2.2h.

[0023] (4) TMCP two-stage controlled rolling: the rolling end temperature in the recrystallization zone is 1145~1160℃, the total deformation rate is controlled above 55%, and large deformation rapid rolling is adopted; the rolling start temperature in the non-recrystallization zone is 915~930℃, the final rolling temperature is 840~880℃, the reduction per pass is controlled at 5~7%, and small deformation multi-pass rapid rolling is adopted.

[0024] (5) Heat treatment: The rolled steel plate undergoes two-stage heat treatment; the first stage is normalizing with weak cooling, and the second stage is short-time normalizing.

[0025] In step (1), steelmaking is carried out in a converter, and dephosphorization and decarburization are carried out separately in the converter; deep desulfurization is carried out in the LF refining furnace, and the sulfur content is strictly controlled to be below 0.002%; degassing is completed in the VD furnace, with a net circulation time of 9 to 12 minutes and a pre-casting settling time of 5 to 7 minutes.

[0026] The oxygen blowing time for dephosphorization is controlled at 9-12 minutes, and the oxygen blowing time for decarbonization is controlled at 7-10 minutes.

[0027] In step (1), in order to control the center segregation and porosity of the continuous casting billet, electromagnetic stirring or light reduction process of continuous casting billet is adopted, wherein the light reduction rate is controlled at 5-7%.

[0028] In step (5), the first stage of normalizing and weak cooling is: the initial temperature is controlled at 890-900℃, the final temperature is controlled at 720-750℃, and the cooling rate is controlled at 11-16 min / mm; the second stage of short-time normalizing is: the temperature is controlled at 480-510℃, the temperature is held for 1.1-1.5h, and the furnace is air-cooled to room temperature.

[0029] Compared with the prior art, the beneficial effects of the present invention are:

[0030] 1) Based on the strengthening elements C, Si, and Mn, this invention adds appropriate amounts of Mo, Nb, and RE alloying elements, while strictly controlling the content of harmful elements P and S, and combines the production process to obtain steel plates with a thickness of 65-150mm. The steel plates have a refined microstructure and uniformly fine and dispersed second-phase particles. Therefore, the steel plates have strong plasticity and low-temperature toughness, as well as good high-temperature service performance, resistance to HIC, and processing performance.

[0031] 2) The thick steel plate for pressure equipment prepared by the method of the present invention exhibits the following mechanical properties: 310MPa≤Rm≤420MPa at room temperature, 490MPa≤Rel≤640MPa, and KV2≥115J at -20℃. It also has excellent high-temperature service performance and HIC resistance. Detailed Implementation

[0032] The specific embodiments of the present invention will be further described below:

[0033] This invention provides a thick, high-performance steel plate for pressure-bearing equipment, wherein the chemical composition and weight percentage content of the steel plate are as follows:

[0034] C: 0.12%–0.2%, Si: 0.14%–0.34%, Mn: 0.42%–0.84%, P: ≤0.015%, S: ≤0.005%, Mo: 0.36%–0.44%, Nb: 0.11%–0.32%, RE: 0.018%–0.029%, with the balance being Fe and unavoidable inclusions.

[0035] The mechanical properties of the steel plate are as follows: at room temperature, 310MPa≤Rm≤420MPa, 490MPa≤Rel≤640MPa, and KV2≥115J at -20℃.

[0036] This invention also provides a method for preparing thick, high-performance steel plates for pressure equipment. The method mainly employs a two-stage billet heating process combined with a two-stage controlled rolling process using TMCP and a two-stage heat treatment process to produce steel plates for pressure equipment with a thickness of 65–150 mm. The produced steel plates have a uniform microstructure, good strength-toughness ratio, excellent low-temperature and high-temperature performance, and good resistance to hydrogen-induced cracking. The specific method is as follows:

[0037] (1) Slab continuous casting: After breaking the vacuum, slab continuous casting machine is used for casting. The key is to control the casting temperature. The casting temperature of molten steel in the tundish is 1526~1558℃, the superheat is set to 25~34℃, and the billet pulling speed during casting is 1.0~1.4m / min. Low temperature casting is better to refine the original as-cast structure.

[0038] (2) Stacking and slow cooling: The billet is stacked and slow cooled after it leaves the production line. The stacking and slow cooling time is 36 to 48 hours.

[0039] (3) Two-stage billet heating: The continuously cast slab is sent to the heating furnace for heating; a two-stage heating method is selected for the billet. The first stage heating temperature range is 995~1016℃, and the holding time is 1.7~2.3h; the second stage heating temperature range is 1215~1232℃, and the billet homogenization time is controlled at 1.5~2.2h; when the second stage heating temperature is lower than 1215℃, the coarse precipitates in the continuously cast billet cannot be dissolved, the austenitization of the steel plate is incomplete, and the final rolling temperature of the first stage cannot be guaranteed; when the heating temperature is higher than 1232℃, the fine precipitates in the continuously cast billet are easily re-dissolved and excessive grain growth is caused.

[0040] (4) TMCP two-stage controlled rolling: The rolling end temperature in the recrystallization zone is 1145-1160℃, which fully refines the original austenite structure. The total deformation rate is controlled above 55%, and large deformation rapid rolling is adopted. The rolling start temperature in the non-recrystallization zone is 915-930℃, and the final rolling temperature is 840-880℃. The reduction per pass is controlled at 5-7%, and small deformation multi-pass rapid rolling is adopted. At this time, the austenite grains are further flattened and elongated. With the increase of the grain boundary area, the grains are fully refined.

[0041] (5) Heat treatment: Since steel contains elements such as C, Si, Mn, P, S, Mo, Nb, and RE, the steel plate can obtain a ferrite + pearlite structure with excellent strength and toughness after rolling. However, the grain size distribution of the steel plate is uneven, and there is a concentration of structural stress and thermal stress, which can easily lead to delayed cracking during flame cutting. Therefore, heat treatment should be carried out in a timely manner to homogenize the structure, soften and eliminate stress. Therefore, the rolled steel plate undergoes two-stage heat treatment: the first stage is normalizing with weak cooling, and the second stage is short-time normalizing.

[0042] In step (1), steelmaking is carried out in a converter. To effectively control the content of harmful element P and ensure decarburization efficiency, dephosphorization and decarburization are carried out separately in the converter. To reduce production costs, high-quality scrap steel and molten iron are used as raw materials, and the molten iron content is controlled at 66-74%. Deep desulfurization is carried out in the LF refining furnace, and the sulfur content is strictly controlled below 0.002%. Degassing is completed in the VD furnace, with a net circulation time of 9-12 minutes and a pre-casting settling time of 5-7 minutes. Dephosphorization oxygen blowing is controlled at 9-12 minutes, and decarburization oxygen blowing is controlled at 7-10 minutes.

[0043] In step (1), in order to control the center segregation and porosity of the continuous casting billet, electromagnetic stirring or light reduction process of continuous casting billet is adopted, wherein the light reduction rate is controlled at 5-7%.

[0044] In step (5), the first stage of normalizing and weak cooling is: the initial temperature is controlled at 890-900℃, the final temperature is controlled at 720-750℃, and the cooling rate is controlled at 11-16 min / mm; the second stage of short-time normalizing is: the temperature is controlled at 480-510℃, the temperature is held for 1.1-1.5h, and the furnace is air-cooled to room temperature.

[0045] The specific values ​​of element content for each embodiment and comparative example are listed in Table 1. High-performance pressure-bearing equipment steel plates were prepared according to the above preparation method in each embodiment, and the smelting process parameters for each embodiment and comparative example are shown in Table 2. The heating process parameters for the continuously cast billets in each embodiment and comparative example are shown in Table 3. The rolling parameters for each embodiment and comparative example are shown in Table 4. The heat treatment process parameters for each embodiment and comparative example are shown in Table 5. The final mechanical properties of each embodiment and comparative example are shown in Table 6. The high-temperature tensile property test results (unit: MPa) for each embodiment and comparative example are shown in Table 7. The hydrogen-induced cracking resistance test results for each embodiment and comparative example are shown in Table 8.

[0046] Table 1 Chemical composition (wt, %)

[0047]

[0048] Table 2 Smelting process parameters

[0049]

[0050] Table 3 Heating process parameters for continuously cast billets

[0051]

[0052] Table 4 shows the rolling parameters.

[0053]

[0054] Table 5 Heat treatment process parameters

[0055]

[0056] Table 6 Final Mechanical Properties

[0057]

[0058] Table 7 Results of High Temperature Tensile Properties Test

[0059]

[0060] Table 8 Test results of hydrogen-induced cracking resistance steel

[0061]

[0062] Based on the above results, it can be concluded that the (65-150) mm thick high-performance pressure-bearing equipment steel plate prepared by this invention has the following mechanical properties: at room temperature, 310MPa≤Rel≤420MPa, 490MPa≤Rm≤640MPa, and at -20℃, KV2≥115J. It has excellent plasticity and toughness, low-temperature performance, high-temperature service performance, HIC resistance, and good processing performance.

[0063] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, and these simple modifications all fall within the scope of protection of the present invention. It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner unless there is any contradiction. In order to avoid unnecessary repetition, the present invention will no longer describe various possible combinations separately. In addition, the various different embodiments of the present invention can also be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for preparing a thick, high-performance steel plate for pressure-bearing equipment, characterized in that, The chemical composition and weight percentage content of the steel plate are as follows: C: 0.12%–0.2%, Si: 0.14%–0.34%, Mn: 0.42%–0.84%, P: ≤0.015%, S: ≤0.005%, Mo: 0.36%–0.44%, Nb: 0.11%–0.32%, RE: 0.018%–0.029%, balance Fe and unavoidable inclusions; The preparation method mainly employs a combination of a two-stage billet heating process, a two-stage controlled rolling process using TMCP, and a two-stage heat treatment process to produce steel plates for pressure equipment with a thickness of 65–150 mm; the specific method is as follows: (1) Slab continuous casting: After steel smelting, slab continuous casting machine is used for casting. The tundish steel casting temperature is 1526~1558℃, the superheat is set to 25~34℃, and the billet pulling speed during casting is 1.0~1.4m / min. (2) Stacking and slow cooling: The billet is stacked and slow cooled after it leaves the production line. The stacking and slow cooling time is 36 to 48 hours. (3) Two-stage billet heating: The continuously cast slab is sent to the heating furnace for heating; the billet is heated in two stages. The first stage heating temperature range is 995~1016℃, and the holding time is 1.7~2.3h; the second stage heating temperature range is 1215~1232℃, and the billet soaking time is controlled at 1.5~2.2h. (4) TMCP two-stage controlled rolling: the rolling end temperature in the recrystallization zone is 1145~1160℃, the total deformation rate is controlled above 55%, and large deformation rapid rolling is adopted; the rolling start temperature in the non-recrystallization zone is 915~930℃, the final rolling temperature is 840~880℃, the reduction per pass is controlled at 5~7%, and small deformation multi-pass rapid rolling is adopted. (5) Heat treatment: The rolled steel plate undergoes two-stage heat treatment; the first stage is normalizing with weak cooling, and the second stage is short-time normalizing; In step (5), the first stage of normalizing and weak cooling is: the initial temperature is controlled at 890-900℃, the final temperature is controlled at 720-750℃, and the cooling rate is controlled at 11-16 min / mm; the second stage of short-time normalizing is: the temperature is controlled at 480-510℃, the temperature is held for 1.1-1.5h, and the furnace is air-cooled to room temperature.

2. The method for preparing a thick, high-performance pressure-bearing steel plate according to claim 1, characterized in that, The mechanical properties of the steel plate are as follows: at room temperature, 310MPa≤Rel≤420MPa, 490MPa≤Rm≤640MPa, and KV2≥115J at -20℃.

3. The method for preparing a thick, high-performance pressure-bearing steel plate according to claim 1, characterized in that, In step (1), steel smelting is carried out in a converter, and dephosphorization and decarburization are carried out separately in the converter; deep desulfurization is carried out in the LF refining furnace, and the sulfur content is strictly controlled to be below 0.002%; degassing is completed in the VD furnace, with a net circulation time of 9 to 12 minutes and a pre-casting calming time of 5 to 7 minutes.

4. The method for preparing a thick, high-performance pressure-bearing steel plate according to claim 3, characterized in that, The oxygen blowing time for dephosphorization is controlled at 9-12 minutes, and the oxygen blowing time for decarbonization is controlled at 7-10 minutes.

5. The method for preparing a thick, high-performance pressure-bearing steel plate according to claim 1, characterized in that, In step (1), in order to control the center segregation and porosity of the continuous casting billet, electromagnetic stirring or light reduction process of continuous casting billet is adopted, wherein the light reduction rate is controlled at 5-7%.

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