5-6mm normalizing rolled steel plate for low temperature pressure vessel and production method thereof

Abstract

The application relates to the technical field of metallurgy, and particularly discloses a 5-6mm normalized rolled steel plate for low-temperature pressure containers and a production method, which is produced by adopting a coiling-rolling controlled rolling and controlled cooling normalized rolling process to replace a hot rolling and heat treatment normalized process; according to the heating, rolling and cooling process characteristics of the 5-6mm thin specification coiling rolling, the chemical composition of the steel plate is as follows in percentage by weight: C: 0.05-0.10%, Si: 0.10-0.30%, Mn: 1.40-1.60%, P<=0.015%, S<=0.006%, Nb: 0.020-0.040%, Cr: 0.10-0.20%, Als: 0.015-0.040%, Ti: 0.015-0.020%, and the rest is Fe and residual elements; the mechanical properties of the 5-6mm thickness specification low-temperature pressure container steel plate produced by the application meet the strict requirements of low-temperature impact toughness at-40 DEG C.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a 5-6mm normalized rolled steel plate for low-temperature pressure vessels and its production method. Background Technology

[0002] C-Mn steel used under low-temperature conditions has a wide range of applications. For example, 16MnDR is widely used in pressure equipment in various countries and has been in use for a long time. Currently, the production of this type of low-temperature pressure vessel steel plate adopts the conventional production method of rolling, heat treatment, and normalizing. This method has a long production cycle, high energy consumption, and consumes heat treatment resources. Most steel mills in my country have the capability to produce this type of steel.

[0003] For example, a domestic steel mill uses 0.13% C and 1.42% Mn, with trace amounts of Nb and V elements. This utilizes the dispersed precipitates of carbonitrides formed by the strong carbides Nb and V elements to refine the original austenite grains, thereby improving the strength and toughness of the steel plate. In terms of rolling, a rough rolling process is used in the austenite recrystallization zone, followed by a finish rolling process in the non-recrystallization zone, and then normalizing to produce 16MnDR steel plates. Another example is a method for producing ultra-thin 16MnDR steel plates for LNG storage tanks, disclosed in patent application number CN2019107006335.8, which uses coil rolling to produce thin-gauge 16MnDR steel plates. Both of these technologies produce steel plates using a hot-rolled, heated, and normalized process. However, producing thin-gauge steel plates using a roller hearth furnace consumes significant heat treatment resources and has low production efficiency.

[0004] The corresponding grades of 16MnDR steel plates in foreign and domestic markets include EN10028-3 P355NL2 and ASME SA662C. Both EN10028-3 P355NL2 and 16MnDR steel plates are delivered in a normalized state. The main difference is that P355NL2 has a minimum KV2 transverse impact strength of 30J at -40℃; ASME SA662C is also delivered in a normalized state, with a minimum KV2 transverse impact strength of 27J at -40℃. Comparing these two standards, it can be found that 16MnDR conforming to GB / T 3531 has the most stringent requirements in terms of composition and performance, and the actual product even exceeds the national standard, but it is still delivered in a heat-treated normalized state.

[0005] Coil rolling offers significant advantages over traditional medium-thick plate mills in producing 5-6mm thin plates. However, it also presents a significant temperature difference (60-80℃) between the beginning and end of the plate and the middle section. This results in the beginning and end of the plate meeting the required low-temperature impact toughness, while the middle section, due to the higher final rolling temperature, fails to meet the requirements, necessitating normalizing to improve its low-temperature impact toughness. However, controlling the normalizing profile for 5-6mm thin plates is challenging, requiring cold straightening for correction, which increases operating costs and impacts order delivery efficiency.

[0006] Therefore, it is necessary to design the process and composition based on the characteristics of 5-6mm thin-gauge rolling and the requirements for impact toughness and strength at -40℃. This ensures rolling stability while meeting the requirements for toughness and strength at -40℃, thus realizing the replacement of rolling plus normalizing process with normalizing process. Summary of the Invention

[0007] In view of the problems existing in the prior art, the purpose of this invention is to provide a 5-6mm normalized rolled steel plate for low-temperature pressure vessels and a production method thereof.

[0008] The technical solution adopted by this invention to solve its technical problem is: a 5-6mm normalized rolled steel plate for low-temperature pressure vessels and a production method thereof, characterized in that the production process specifically includes the following steps:

[0009] 1) Heating process: Controlling heating temperature and heating time;

[0010] 2) Rolling process: The normalizing rolling process with coil rolling control is adopted to control the reduction rate, final rolling temperature and final rolling speed;

[0011] 3) Controlled cooling process: Control the final cooling temperature. Specifications that require controlled cooling should be sent to the rapid cooling equipment as quickly as possible after rolling for controlled cooling.

[0012] Specifically, the low-temperature pressure vessel plate with a steel plate thickness of 5-6mm is produced by using a 150mm continuous casting billet on a single-stand 3500mm hot rolling mill, employing a normalizing rolling process, and then using a weak water cooling method after rolling.

[0013] Specifically, in the heating process of step 1), the temperature of thin steel plates drops rapidly and the rolling is difficult. Not only does the billet need a high heating temperature, but it is also necessary to ensure that the billet is thoroughly heated. The temperature of the second heating section is controlled at 1240-1260℃, the temperature of the soaking section is controlled at 1220-1240℃, and the heating time is 10min / cm.

[0014] Specifically, in step 2), the roughing passes in the rolling process employ a large reduction. When the thickness is less than or equal to 75 mm, the maximum reduction rate per pass is not less than 20%. During the coiling stage, the coiling furnace serves to maintain the temperature of the steel plate and ensure its uniformity. The temperature is set at 900-960℃. The initial coiling thickness is reasonably set, and the reduction rate for the first two passes in the coiling stage is maintained at not less than 20% to maximize grain refinement and dislocation generation. The reduction rates for the last two passes are 15-17% and 11-13%, respectively.

[0015] Specifically, the final rolling speed in step 2) is 1.8 m / s, and the final rolling temperature at the head of the steel plate is 830 ± 10℃.

[0016] Specifically, the final cooling temperature in step 3) is set to 620°C.

[0017] Specifically, the steel plate of the container plate has a yield strength Rel ≥ 345 MPa in both the transverse and longitudinal directions, a tensile strength Rm of 510-640 MPa, an elongation A ≥ 21%, and an impact absorption energy of -40℃ ≥ 21 J.

[0018] Specifically, to meet the requirements of normalizing rolling process and -40℃ low-temperature impact toughness and strength of the low-temperature pressure vessel plate, the chemical composition of the billet is designed by weight percentage as follows: C: 0.05-0.10%, Si: 0.10-0.30%, Mn: 1.40-1.60%, P≤0.015%, S≤0.006%, Nb: 0.020-0.040%, Cr: 0.10-0.20%, Als: 0.015-0.040%, Ti: 0.015-0.020%, with the remainder being Fe and residual elements.

[0019] The present invention has the following beneficial effects:

[0020] This invention relates to a single-stand 3500mm hot roll mill for producing 5-6mm normalized low-temperature pressure vessel plates and its manufacturing method.

[0021] 1. The mechanical and technological properties of the 5-6mm thick steel plates produced can meet the stringent requirements for mechanical properties and impact toughness at -40℃.

[0022] 2. The low-C design can greatly improve the low-temperature impact toughness of the high-temperature part of the rolled steel sheet;

[0023] 3. Based on the strengthening elements Mn, Cr, and Si, appropriate amounts of Nb and Ti elements are added to produce fine grain strengthening and precipitation strengthening. Adding appropriate amounts of Cr elements can improve the strength and hardenability of the steel, and enhance the cooling intensity during the controlled cooling process after rolling.

[0024] 4. Adding an appropriate amount of Ti increases the austenite roughening temperature to about 1250℃, which increases the heating temperature and ensures the temperature uniformity of the billet, which is beneficial for thin-gauge rolling.

[0025] 5. In the production of 3500mm hot coil mill, the hot coil mill has significant advantages in producing 5-6mm thin specifications; using the coiling furnace to keep the thin steel plates warm not only ensures the temperature of the thin steel plates during rolling, but also improves the stability and temperature uniformity of the steel plate rolling.

[0026] 6. The normalizing rolling process is adopted to meet the requirements of low-temperature impact toughness and strength; the heat treatment normalizing process is eliminated, which greatly improves the utilization rate of heat treatment resources and production efficiency, significantly reduces the company's operating costs, reduces energy consumption, and achieves low-carbon and green development. Attached Figure Description

[0027] Figure 1 This is a metallographic diagram of the container steel plate in Example 1.

[0028] Figure 2 This is a metallographic diagram of the container steel plate in Example 2. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0030] A method for producing 5-6mm normalized low-temperature pressure vessel plates using a single-stand 3500mm hot-rolling mill and its manufacturing process is disclosed. This method overcomes the limitations of traditional hot-rolling and heating treatment methods, offering advantages such as faster production speed, reduced heat treatment resources, and lower energy consumption. By employing a suitable chemical composition design and a combined hot-rolling and controlled-cooling process, dislocation strengthening, grain refinement strengthening, precipitation strengthening, and phase transformation strengthening are achieved, ensuring both strength and impact toughness at -40℃, while maintaining a uniform microstructure.

[0031] To meet the above requirements for low-temperature pressure vessel steel plates, normalized rolling is used for delivery. All properties must meet the normalized state standard: yield strength Rel ≥ 345 MPa for both transverse and longitudinal specimens, tensile strength Rm: 510-640 MPa, elongation A ≥ 21%, and impact absorption energy at -40℃ ≥ 21 J. To ensure the -40℃ low-temperature impact energy and weldability under the coil-and-normalized rolling process, the product in this invention adopts a low-carbon design. Based on Mn, Cr, and Si strengthening elements, appropriate amounts of Nb and Ti elements are added to produce fine-grain strengthening and precipitation strengthening. Adding appropriate amounts of Cr elements increases strength while also improving the hardenability of the steel and enhancing the cooling intensity during the controlled cooling process after rolling. This invention uses a normalized rolling process to replace the hot-rolled heating treatment normalized process, significantly improving the production efficiency of thin-gauge low-temperature pressure vessel steel, significantly reducing company operating costs, lowering energy consumption, and achieving low-carbon, green development.

[0032] The technical solution of this invention involves component design, heating process, controlled rolling process of hot-rolled coil, and controlled cooling process, to achieve the production of thin-gauge low-temperature pressure vessel plates by normalizing rolling. The specific production process is as follows:

[0033] The process control method is as follows: KR → converter smelting → LF ladle refining → RH vacuum treatment → CCM continuous casting under light pressure. After the continuously cast billet is cut, it is slowly cooled for 48 hours to promote the diffusion of hydrogen. After the slow cooling is completed, it enters the hot working stage, and the specific process is as follows:

[0034] It is produced using 150mm continuous casting billets on a 3500mm hot roll mill.

[0035] (1) Heating process: To ensure low-temperature impact performance, sufficient heating temperature and time can ensure the full dissolution of Nb carbonitrides, which will play a role in expanding the non-recrystallized austenite zone and washing-out strengthening in subsequent controlled rolling and cooling. To avoid excessive grain growth and ensure low-temperature impact performance, a trace amount of Ti can raise the austenite coarsening temperature to about 1250℃. At the same time, thin-gauge steel plates cool down quickly and are difficult to roll, so it is necessary to ensure that the billet is thoroughly heated; therefore, the temperature of the second heating section is controlled at 1240-1260℃, the temperature of the soaking section is controlled at 1220-1240℃, and the heating time is 10min / cm.

[0036] (2) Rolling process: The rolling mode adopted is shown in Table 1. The roughing pass achieves a large reduction, which is beneficial to grain refinement. When the thickness is less than or equal to 75 mm, the maximum reduction rate of the pass is not less than 20%, which improves the penetration of deformation into the core structure, makes the structure more uniform, and obtains a good internal structure foundation. In the coiling stage, the coiling furnace plays a role in heat preservation of the steel plate and can also keep the temperature of the steel plate uniform. The temperature is set at 900-960℃. The initial coiling thickness is reasonably set, and the reduction rate of the first two passes in the coiling stage is not less than 20%, which refines the grains and generates dislocations to the greatest extent, increases the ferrite nucleation points, and obtains fine ferrite grains. The reduction rates of the last two passes are 15-17% and 11-13% respectively to ensure the plate shape.

[0037] Table 1 Rolling Process

[0038]

[0039] (3) Controlled cooling process: The final cooling temperature is set at 620℃; specifications requiring controlled cooling are rapidly cooled after rolling using a rapid cooling device to suppress the growth of ferrite grains after transformation. This lowers the austenite-to-ferrite phase transformation temperature; the increased undercooling increases the nucleation rate of the ferrite phase. Simultaneously, the increased cooling rate prevents premature precipitation of carbides and nitrides, allowing the second phase to precipitate and disperse, thus improving the strength and toughness of the steel plate. A phase transformation then occurs, controlling the final microstructure to be ferrite plus pearlite plus granular bainite.

[0040] Chemical composition (by weight percentage): C: 0.05-0.10%, Si: 0.10-0.30%, Mn: 1.40-1.60%, P≤0.015%, S≤0.006%, Nb: 0.020-0.040%, Cr: 0.10-0.20%, Als: 0.015-0.040%, Ti: 0.015-0.020%, with the remainder being Fe and residual elements.

[0041] The selection and dosage of key elements in this invention are explained below:

[0042] C element: It improves the strength of steel by forming solid solutions and carbide structures, but excessive C content has an adverse effect on low-temperature impact toughness and weldability; in this invention, the C content is controlled at 0.05-0.10%.

[0043] Si element: It acts as a reducing agent and deoxidizer in the steelmaking process; it improves the strength of steel through solid solution, but excessive content will affect the low-temperature impact toughness; in this invention, the Si content is controlled at 0.10-0.30%.

[0044] Mn element: It is the main element for improving strength; however, excessive content will affect welding performance; in this invention, the Mn content is controlled at 1.40-1.60%.

[0045] Nitrogen (Nb) element: Its strengthening effects on steel are mainly grain refinement and precipitation strengthening. During controlled rolling and controlled cooling, it reacts with carbon and nitrogen in the steel to form stable carbides and carbonitrides, which pin dislocations and inhibit subgrain boundary migration, thus suppressing grain growth and prolonging recrystallization time, thereby refining the grains and achieving precipitation strengthening. In this invention, the Nb content is controlled at 0.020-0.040%.

[0046] Cr element: The addition of this element can increase hardenability and improve the strength of steel. However, excessive addition will affect the low-temperature impact toughness of the steel; in this invention, the Cr content is controlled at 0.10-0.20%.

[0047] Al element: mainly plays a role in deoxidation and controlling grain size; in this invention, the Al content is controlled at 0.015-0.040%.

[0048] Ti element: Its carbonitrides have a high precipitation temperature, which can refine austenite grains; at the same time, Ti nitrides can also improve the distribution and morphology of sulfides and improve the anisotropy of the mechanical properties of steel plates; in this invention, the Cr content is controlled at 0.015-0.020%.

[0049] Implementation 1

[0050] 1. Chemical composition: C: 0.08%, Si: 0.325%, Mn: 1.55%, P: 0.008%, S: 0.002%, Nb: 0.036%, Als: 0.033%, Cr: 0.17%, Ti: 0.018%, with the remainder being Fe and residual elements.

[0051] 2. Heating process: The temperature of the second heating section is controlled at 1230-1250℃, the temperature of the soaking section is controlled at 1210-1230℃, the furnace exit temperature is 1220℃, and the heating time is 10min / cm.

[0052] 3. Hot Rolling Mill Rolling Process: The initial roughing temperature is controlled at 1020-1050℃, employing a two-stage rolling process of roughing and coiling, with 7+4 passes (7 passes for roughing and 4 passes for coiling). The coiling furnace temperature is 920±20℃. The initial thickness for coiling is set at 16mm. The final pass reduction is 11-13%. The final rolling temperature is 830±10℃. The final thickness is 6mm.

[0053] 4. Water cooling process: The final cooling temperature is controlled at 620℃.

[0054] The steel plates produced using the above composition design and process meet the mechanical property requirements and have a uniform microstructure, as shown in Table 2 and... Figure 1 As shown:

[0055] Table 2 Mechanical properties of steel plates in Implementation 1

[0056]

[0057] Implementation 2

[0058] 1. Chemical composition: C: 0.08%, Si: 0.325%, Mn: 1.55%, P: 0.008%, S: 0.002%, Nb: 0.036%, Als: 0.033%, Cr: 0.17%, Ti: 0.018%, with the remainder being Fe and residual elements.

[0059] 2. Heating process: The temperature of the second heating section is controlled at 1240-1260℃, the temperature of the soaking section is controlled at 1230-1250℃, the furnace exit temperature is 1240℃, and the heating time is 10min / cm.

[0060] 3. Hot Rolling Mill Rolling Process: The initial roughing temperature is controlled at 1040-1060℃, employing a two-stage rolling process of roughing and coiling, with 6+5 passes (6 passes for roughing and 5 passes for coiling). The coiling furnace temperature is 940±20℃; the initial thickness for coiling is set at 18mm. The reduction in the final pass is no more than 12%. The final rolling temperature is 830±10℃. The final thickness is 5mm.

[0061] 4. Water cooling process: The final cooling temperature is controlled at 620℃.

[0062] The steel plates produced using the above composition design and process meet the mechanical property requirements and have a uniform microstructure, as shown in Table 3 and Figure 2 As shown:

[0063] Table 3 Mechanical properties of steel plates from Implementation 2

[0064]

[0065] This invention is not limited to the above-described embodiments. Anyone should know that any structural changes made under the guidance of this invention, and any technical solutions that are the same as or similar to this invention, fall within the protection scope of this invention.

[0066] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A 5-6mm normalized rolled steel plate for cryogenic pressure vessels and its production method, characterized in that, To meet the requirements of normalizing rolling process and impact toughness and strength at -40℃ for low-temperature pressure vessel plates, the chemical composition of the billet is designed as follows (by weight percentage): C: 0.05-0.10%, Si: 0.10-0.30%, Mn: 1.40-1.60%, P≤0.015%, S≤0.006%, Nb: 0.020-0.040%, Cr: 0.10-0.20%, Als: 0.015-0.040%, Ti: 0.015-0.020%, with the remainder being Fe and residual elements; The production process specifically includes the following steps: 1) Heating process: control the heating temperature and heating time; in the heating process, the temperature of thin steel plates drops quickly and the rolling is difficult. Not only does the billet need a high heating temperature, but it is also necessary to ensure that the billet is thoroughly heated; the temperature of the second heating section is controlled at 1240-1260℃, the temperature of the soaking section is controlled at 1220-1240℃, and the heating time is 10min / cm. 2) Rolling process: The normalizing rolling process with coil rolling control is adopted, controlling the reduction rate, final rolling temperature, and final rolling speed. Large reductions are used in the roughing passes, with a maximum reduction rate of not less than 20% when the thickness is less than or equal to 75mm. In the coiling stage, the coiling furnace serves to maintain the temperature of the steel plate and ensure temperature uniformity, with a temperature set at 900-960℃. The initial coiling thickness is reasonably set, and the reduction rate for the first two passes is maintained at not less than 20% to maximize grain refinement and dislocation generation. The reduction rates for the last two passes are 15-17% and 11-13% respectively. The final rolling speed is 1.8m / s, and the final rolling temperature at the head of the steel plate is 830±10℃. 3) Controlled cooling process: Control the final cooling temperature, which is set at 620℃. Specifications requiring controlled cooling should be fed into the rapid cooling equipment as quickly as possible after rolling to control the final microstructure as ferrite, pearlite and granular bainite.

2. The 5-6mm normalized rolled steel plate for cryogenic pressure vessels and its production method according to claim 1, characterized in that, The low-temperature pressure vessel plate with a steel plate thickness of 5-6mm is produced by normalizing rolling on a single-stand 3500mm hot rolling mill using a 150mm continuous casting billet, and is then cooled by weak water cooling.

3. The 5-6mm normalized rolled steel plate for cryogenic pressure vessels and its production method according to claim 2, characterized in that, The steel plate of the container plate has a yield strength Rel≥345MPa in the transverse and longitudinal directions, a tensile strength Rm: 510-640MPa, and an elongation A≥21%. Impact absorption energy at -40℃ ≥21J.

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