A pressure vessel steel plate for mobile tank car and a manufacturing method thereof

Abstract

The present application belongs to the field of smelting technology, and particularly relates to a kind of pressure vessel steel plates for mobile tank cars, the chemical composition mass percentage is: C: 0.15%~0.19%, Si: 0.32%~0.54%, Mn: 1.31%~1.68%, P≤0.02%, S≤0.01%, Ni: 0.46%~0.59%, Nb: 0.05%~0.12%, Ti: 0.014%~0.026%, the balance is Fe and inevitable inclusions. The steel plate normal temperature: 690MPa≤tensile strength≤810MPa; 480MPa≤yield strength≤620MPa; -60℃, transverse impact energy≥100J. The present application obtains refined and pure microstructure, uniform and fine dispersed distribution of second phase particles, grain size in 8~9 levels, high homogeneity of inclusions level sum below 1.0, which ensures the strength and plasticity of the steel plate, and has good hydrogen-induced cracking resistance and processing performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of smelting, and particularly relates to a pressure vessel steel plate for mobile tank cars and a manufacturing method. BACKGROUND

[0002] The application relates to the field of metal materials, and particularly relates to a pressure vessel steel plate for mobile tank cars with excellent uniform structure performance and a manufacturing method. The service environment of the pressure vessel steel plate is relatively complex, such as high temperature, high pressure, hydrogen, and acid and alkali environments, and therefore, when the main material steel is selected, the smelting, chemical composition, mechanical properties, metallographic structure and welding process of the steel are strictly controlled. It is known that the performance of a metal material is closely related to the structure, and the uniformity of the structure is a powerful guarantee for high performance of the material. The refined and uniform structure can improve the strength and toughness of the steel plate, and further improve the comprehensive service performance of the steel plate.

[0003] With the upgrading of domestic pressure-bearing manufacturing equipment, domestic petrochemical projects are developing towards large-scale and high-performance. The performance of some traditional pressure-bearing equipment steels and the corresponding manufacturing methods cannot meet the requirements of this development trend, and some grades can meet the design requirements at the present stage, but there is still a need for upgrading in the long run. In addition, with the proposal of the "double carbon" requirement, high-performance pressure-bearing equipment steel manufacturing has further attracted attention. Therefore, it is urgent to develop a high-strength key material for pressure-bearing equipment that meets the requirements of normal temperature strength, forming performance and adaptation to complex service environments (such as high temperature and high pressure), to support the development needs of large-scale new energy equipment or equipment in China.

[0004] The published patent "A hydrogen corrosion resistant normalized mobile tank car low alloy steel and a preparation method thereof (CN106756536A)" discloses that the steel plate is composed of the following components by weight percentage: C 0.13-0.20%, Si 0.20-0.50%, Mn 1.20-1.70%, P≤0.030%, S≤0.010%, Ni 0.10-0.45%, Nb 0.010-0.050%, V 0.010-0.20%, 30ppm≤N≤50ppm, H≤2ppm, 8≤w(V) / w(N)≤15, and the balance is Fe and inevitable impurities. The above components are subjected to normalizing heat treatment, and a steel plate with a thickness specification of 6-25 mm is produced. The steel plate with a thickness specification greater than 25 mm is not studied, and the related content of the uniformity control and the low-temperature impact at less than-46 DEG C is not involved, and the application range is narrow. SUMMARY

[0005] The technical problem solved by the present application is to provide a pressure vessel steel plate for mobile tank cars and a manufacturing method, which produces a steel plate with uniform and pure structure, good strength and toughness matching, and excellent hydrogen-induced cracking resistance.

[0006] To achieve the above object, the present application adopts the following technical solutions:

[0007] A pressure vessel steel plate for mobile tank cars, the chemical composition in mass percentage is: C: 0.15% to 0.19%, Si: 0.32% to 0.54%, Mn: 1.31% to 1.68%, P≤0.02%, S≤0.01%, Ni: 0.46% to 0.59%, Nb: 0.05% to 0.12%, Ti: 0.014% to 0.026%, and the balance is Fe and unavoidable inclusions.

[0008] A pressure vessel steel plate for mobile tank cars, the performance index is: the tensile strength of the steel plate at room temperature is 690MPa≤, the yield strength is 480MPa≤, the transverse impact energy at-60℃ is ≥100J.

[0009] The reasons for limiting the amount of each chemical element in the steel plate are described as follows:

[0010] C is the most important element in steel to ensure the strength of the steel, and a low content cannot guarantee the performance of the steel plate after heat treatment. However, a high carbon content will affect the machining performance of the steel, and the supersaturated carbide will become the source of fracture cracks, affecting the low-temperature performance of the steel plate and the welding performance in the subsequent use process. Therefore, the C content range is set to 0.15% to 0.19%.

[0011] Si acts as a reducing agent and deoxidizer during steelmaking. When the silicon content is too high, hard phase compounds are easily formed, increasing the brittleness of the steel plate and causing cracks during later processing. Therefore, the Si content range is set to 0.32% to 0.54%.

[0012] Mn can be infinitely solid-solved with Fe, and is an element that strongly stabilizes austenite and strengthens pearlite structure. The effect of Mn and C improves the strength of the steel while having relatively small influence on plasticity, and reduces the lower critical point of the steel, increasing the supercooling degree of austenite cooling, thereby refining the structure and improving the mechanical properties of the steel plate. In addition, the price is relatively low. However, a high Mn content will increase the tendency of hard phase MnS inclusions in the steel. Therefore, the Mn content range is set to 1.31% to 1.68%.

[0013] S, P as harmful elements in steel, in order to ensure the purity of steel and plastic toughness must be strictly controlled, known to produce MnS in steel will affect the strength and toughness of steel plate, the presence of P will increase the brittleness of the steel plate, the negative impact on the welding performance, in order to strictly control the harmful inclusion in the steel plate, therefore, the present invention is limited to P≤0.02%, S≤0.01%.

[0014] Ni can form an infinite solid solution with γ Fe Therefore, the hardenability of the steel plate can be improved. The addition of a certain amount of Ni element can strengthen the cast iron ferrite while increasing the number of pearlite and reducing the size. The steel plate containing Ni is generally not easy to overheat, so it can prevent the growth of grains at high temperature, and maintain the grain size during high temperature heating. Ni can improve the strength of the steel while maintaining good plasticity and toughness, especially low temperature toughness. However, since nickel is a relatively scarce resource, the content of Ni is controlled at 0.46% to 0.61% in consideration of the comprehensive cost control.

[0015] Nb can form NbC or NbN in steel. In the recrystallization process, due to the pinning effect of NbC and NbN on dislocations and the inhibition of grain growth, the recrystallization time is greatly increased. Above the critical temperature, the effect of niobium on recrystallization is shown as solute drag mechanism, while below the critical temperature, it is shown as precipitation pinning mechanism. In addition, Nb is the most effective alloying element for grain refinement, and the strengthening effect is significant. In the process of controlled rolling and normalizing, it has a very strong effect on delaying austenite recrystallization and refining grains, and a relatively low niobium content can have a significant effect. However, when the content is too high, the toughness and plasticity of the steel plate will deteriorate, so the content of Nb is controlled at 0.05% to 0.12%.

[0016] Ti is a strong carbide forming element, which forms stable Ti(C, N) carbide particles in steel. When it exists in the form of solid solution, it can significantly improve the hardenability of the steel plate, prevent grain growth, and improve the strength of the steel. After normalizing, the small size carbide formed can significantly improve the plasticity and impact toughness of the steel. Ti also has high affinity with O, which can significantly increase the A1 and A3 temperatures of the steel. However, when the Ti content in the steel is too high, large size second phase particles will be generated, which will increase the brittleness of the steel plate, therefore, the content of Ti is controlled at 0.014% to 0.026% in the present invention.

[0017] A method for manufacturing a mobile tank pressure vessel steel plate, the process flow is molten iron inoculation treatment - converter smelting - secondary refining - vacuum degassing - continuous casting - slab heating - controlled rolling and controlled cooling - heat treatment - flaw detection - inspection, comprising the following specific steps:

[0018] 1) Inoculation treatment of molten iron: Molten iron and scrap steel are added to the converter. The weight percentage of molten iron is controlled at 76-81%. Inoculation agent is added to the molten iron for inoculation treatment. The height-to-diameter ratio of the molten iron column should be between 1.3 and 1.4.

[0019] 2) Smelting process: Dephosphorization oxygen blowing is controlled at 7-10 min, decarburization oxygen blowing is controlled at 6-11 min, and the P mass fraction is reduced to within 0.015%; deep desulfurization treatment is carried out in the LF refining furnace to control the sulfur content below 0.005%; degassing is completed in the VD furnace with a net circulation time of 9-12 min and a settling time of 4-9 min before casting.

[0020] 3) Continuous casting process: After vacuum breaking, the slab is cast using a trolley on a ladle and a continuous casting machine. The temperature of the molten steel flowing into the crystallizer, i.e., the casting temperature, is controlled at 1469-1496℃, and the superheat is set at 8-15℃. The billet pulling speed during casting is 1.2-1.4m / min. A light reduction process for the continuous casting billet is adopted, and the light reduction rate is controlled at 5-7%. The billet is placed in a stack for slow cooling after leaving the line. The slow cooling time in the stack is not less than 24 hours, and the cooling rate is controlled at 19-24℃ / min.

[0021] 4) Heating process: The continuously cast slab is sent to the heating furnace for heating. The heating temperature range is 1195~1235℃, and the slab soaking time is controlled at 1.5~2.1h.

[0022] 5) Rolling process: Rolling adopts a two-stage controlled rolling and cooling process. The finishing temperature of the recrystallization zone is 1100-1145℃, with 3-5 passes and a single-pass reduction rate controlled at 15-19%. The starting temperature of the non-recrystallization zone is 965-989℃, and the finishing temperature is 870-890℃, with 8-10 passes and a total reduction rate controlled at 35-46%. Controlled cooling is used, with the first cooling section starting at 742-760℃, the cooling water pressure controlled between 0.8-1.2 MPa, and the cooling rate controlled at 65-84℃ / s. The second cooling section starts at 452-478℃, and the cooling rate is controlled at 21-30℃ / s.

[0023] 6) Heat treatment process: normalizing treatment + high temperature homogenization treatment. The first stage of normalizing treatment: control the temperature at 862~915℃ and hold for 20~35min. The second stage adopts the homogenization treatment process: control the temperature at 560~600℃ and hold for 1.3~2.1min / mm.

[0024] In step 1), the particle size of the inoculant is 5-10 mm.

[0025] Compared with existing technologies, the beneficial effects of this invention are:

[0026] This invention, based on strengthening elements such as C, Si, and Mn, adds appropriate amounts of alloying elements such as Ni, Nb, and Ti, while strictly controlling the content of harmful elements P and S. It employs an advanced combination of iron inoculation treatment, billet heating process, two-stage rolling, and efficient heat treatment (normalizing treatment - high-temperature homogenization treatment) to produce pressure vessel steel plates with a thickness of 5–35 mm. This results in a refined and pure microstructure, with uniformly fine and dispersed second-phase particles, a grain size of 8–9, and a high homogeneity with a total inclusion grade below 1.0. This ensures the steel plate's strength and plasticity while also exhibiting good resistance to hydrogen-induced cracking and excellent machinability. The steel plate's tensile strength at room temperature is 690 MPa ≤ tensile strength ≤ 810 MPa, yield strength is 480 MPa ≤ yield strength ≤ 620 MPa, and transverse impact energy at -60℃ is ≥ 100 J. According to GB / T 8650-2006 "Evaluation Method for Hydrogen-Induced Cracking Resistance of Pipeline Steel and Pressure Vessel Steel", the crack length ratio (CLR), crack thickness ratio (CTR), and crack sensitivity ratio (CSR) were all 0% in hydrogen sulfide corrosion resistance tests in solutions A and B. This meets the manufacturing and application requirements for steel used in high-performance pressure vessel mobile tank trucks. Detailed Implementation

[0027] It should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein in the specification is for the purpose of describing particular embodiments only and is not intended to limit the scope of the application. The reference to "embodiment" herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0028] A pressure vessel steel plate for mobile tank trucks has the following chemical composition by mass percentage: C: 0.15%–0.19%, Si: 0.32%–0.54%, Mn: 1.31%–1.68%, P≤0.02%, S≤0.01%, Ni: 0.46%–0.59%, Nb: 0.05%–0.12%, Ti: 0.014%–0.026%, with the balance being Fe and unavoidable inclusions.

[0029] A method for manufacturing pressure vessel steel plates for mobile tank trucks, comprising the following process steps: hot metal inoculation treatment—converter smelting—ladle refining (LF)—vacuum degassing (VD)—continuous casting—slab heating—controlled rolling and cooling—heat treatment (normalizing treatment-homogenization treatment)—flaw detection—inspection:

[0030] 1) Inoculation Treatment of Molten Iron: To improve smelting efficiency, steelmaking is carried out in a converter. To reduce production costs, high-quality scrap steel and molten iron are used as raw materials, with the molten iron content controlled at 76-81%. To promote nucleation and inhibit growth, achieving the goal of refining steel grains, the molten iron is pre-inoculated. The inoculant is added within 15-20 minutes, and the inoculant particle size is controlled at 5-10 mm. To reduce steel plate loss during production and prevent molten iron splashing, the height-to-diameter ratio of the molten iron column should be between 1.3 and 1.4.

[0031] 2) Converter smelting process: To ensure the purity of the steel, effectively control the content of harmful element P, and ensure the efficiency of decarburization, dephosphorization and decarburization are carried out separately in the converter. The oxygen blowing for dephosphorization is controlled at 7-10 minutes, and the oxygen blowing for decarburization is controlled at 6-11 minutes, ultimately reducing the P mass fraction to below 0.015%. Deep desulfurization treatment is carried out in the LF refining furnace, and the sulfur content is strictly controlled to below 0.005%. Degassing is completed in the VD furnace, with a net circulation time of 9-12 minutes and a settling time of 4-9 minutes before casting.

[0032] 3) Continuous casting process:

[0033] After vacuum breaking, the molten steel is cast using a slab continuous casting machine via a ladle turret. The temperature of the molten steel flowing into the crystallizer, i.e., the casting temperature, is controlled at 1469–1496℃, with a superheat set at 8–15℃. The casting speed during pouring is 1.2–1.4 m / min. This ensures a suitable casting speed for low-temperature casting, improving the grain size of the original as-cast microstructure and guaranteeing the quality of the cast billet. To further reduce center segregation and porosity in the continuously cast billet, a light reduction process is adopted, with the reduction rate controlled at 5–7%. The billet is then stacked for slow cooling after leaving the casting line, with a stacking time of not less than 24 hours and a cooling rate controlled at 19–24℃ / min.

[0034] 4) Heating Process: The continuously cast slab is sent to a heating furnace for heating. To ensure the internal quality of the steel plate while improving heating efficiency, the heating temperature range is 1195–1235℃. The slab soaking time is controlled between 1.5 and 2.1 hours. When the heating temperature is below 1195℃, the coarse precipitates in the continuously cast slab cannot dissolve, 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 above 1235℃ and the heating time in the high-temperature section is too long, the fine precipitates in the continuously cast slab are prone to re-dissolved and excessive grain growth can occur.

[0035] 5) Rolling Process: Rolling adopts a two-stage controlled rolling and cooling process. In the recrystallization zone, the rolling end temperature is 1100–1145℃, with 3–5 passes and a single-pass reduction controlled at 15–19%. In the non-recrystallization zone, the rolling start temperature is 965–989℃, and the final rolling temperature is 870–890℃, with 8–10 passes and a total reduction controlled at 35–46%. At this stage, the austenite grains are further flattened and elongated, and the grains are sufficiently refined as the grain boundary area increases. Cooling is controlled, with the first cooling stage starting at 742–760℃, the cooling water pressure controlled between 0.8–1.2 MPa, and the cooling rate between 65–84℃ / s. The second cooling stage starts at 452–478℃, with a cooling rate controlled at 21–30℃ / s.

[0036] 6) Heat Treatment Process: Due to the addition of elements such as C, Si, Mn, P, S, Ni, Nb, and Ti to the steel, the rolled steel plate immediately yields a ferrite + pearlite microstructure with excellent strength and toughness. However, the uneven grain size distribution of the steel plate leads to concentrations of structural and thermal stress, making it prone to delayed cracking during flame cutting. Therefore, timely heat treatment is necessary for microstructure homogenization, softening, and stress relief. To further control the internal microstructure of the steel plate while ensuring high production efficiency, this invention employs normalizing treatment followed by high-temperature homogenization treatment to ensure that the steel plate does not lose strength while possessing suitable plasticity, toughness, resistance to hydrogen-induced cracking, and good processing performance. Therefore, the first stage of normalizing treatment involves controlling the temperature at 862–915℃ and holding for 20–35 min; the second stage employs a homogenization treatment process, controlling the temperature at 560–600℃ and holding for 1.3–2.1 min / mm.

[0037] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0038] Table 1 shows the chemical composition of the embodiments of the present invention; Table 2 shows the pretreatment and smelting process parameters of the steel in the embodiments; Table 3 shows the heating and rolling process parameters of the continuously cast billet; Table 4 shows the heat treatment parameters of the steel in the embodiments; Table 5 shows the final mechanical properties of the embodiments; Table 6 shows the test results of the hydrogen-induced cracking resistance of the embodiments; Table 7 shows the test results of the grain size and inclusion grade evaluation of the steel plate microstructure of the embodiments.

[0039] Table 1: Chemical composition of the examples (wt, %)

[0040]

[0041] Table 2: Process parameters for hot metal pretreatment, smelting, and continuous casting in the examples

[0042]

[0043]

[0044] Table 3: Process parameters for heating and rolling of continuously cast billets in the examples

[0045]

[0046] Table 4: Heat treatment process parameters for the steel in the examples

[0047]

[0048] Table 5: Final Mechanical Properties of Examples

[0049]

[0050]

[0051] Table 6: Results of Hydrogen-Induced Crack Resistance Tests for Examples

[0052]

[0053] Table 7: Test Results of Grain Size and Inclusion Grade Evaluation of Steel Plate Microstructure in Examples

[0054]

[0055]

[0056] Based on the above results, it can be concluded that the steel plate for mobile tank trucks with uniform microstructure and excellent properties with a thickness of 5-35mm provided by the present invention exhibits the following mechanical properties: at room temperature: 690MPa≤Rm≤820MPa, 480MPa≤Rel≤620MPa; at -60℃: KV2≥100J. It has a fine and uniform microstructure throughout the entire thickness, excellent plasticity and toughness, low-temperature performance, resistance to hydrogen-induced cracking, and good processing performance.

[0057] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for manufacturing pressure vessel steel plates for mobile tank trucks, characterized in that, The chemical composition of the pressure vessel steel plate, by mass percentage, is as follows: C: 0.15%–0.19%, Si: 0.32%–0.54%, Mn: 1.31%–1.68%, P≤0.02%, S≤0.01%, Ni: 0.46%–0.59%, Nb: 0.05%–0.12%, Ti: 0.014%–0.026%, with the balance being Fe and unavoidable inclusions; The manufacturing method and process flow are: molten iron inoculation treatment—converter smelting—ladle refining—vacuum degassing—continuous casting—slab heating—controlled rolling and cooling—heat treatment—flaw detection—inspection, including the following specific steps: 1) Inoculation treatment of molten iron: Molten iron and scrap steel are added to the converter. The weight percentage of molten iron is controlled at 76-81%. Inoculation agent is added to the molten iron for inoculation treatment. The height-to-diameter ratio of the molten iron column should be between 1.3 and 1.

4. 2) Smelting process: Dephosphorization oxygen blowing is controlled at 7-10 min, decarburization oxygen blowing is controlled at 6-11 min, and the P mass fraction is reduced to within 0.015%; deep desulfurization treatment is carried out in the LF refining furnace to control the sulfur content below 0.005%; degassing is completed in the VD furnace with a net circulation time of 9-12 min and a settling time of 4-9 min before casting. 3) Continuous casting process: After vacuum breaking, the slab is cast using a trolley on a ladle and a continuous casting machine. The temperature of the molten steel flowing into the crystallizer, i.e., the casting temperature, is controlled at 1469-1496℃, and the superheat is set at 8-15℃. The billet pulling speed during casting is 1.2-1.4m / min. A light reduction process for the continuous casting billet is adopted, and the light reduction rate is controlled at 5-7%. The billet is placed in a stack for slow cooling after leaving the line. The stacking and slow cooling time is not less than 24 hours, and the cooling rate is controlled at 19-24℃ / min. 4) Heating process: The continuously cast slab is sent to the heating furnace for heating. The heating temperature range is 1195~1235℃, and the slab soaking time is controlled at 1.5~2.1h. 5) Rolling process: Rolling adopts a two-stage controlled rolling and cooling process. The finishing temperature of the recrystallization zone is 1100-1145℃, with 3-5 passes and a single-pass reduction rate controlled at 15-19%. The starting temperature of the non-recrystallization zone is 965-989℃, and the finishing temperature is 870-890℃, with 8-10 passes and a total reduction rate controlled at 35-46%. Controlled cooling is used, with the first cooling section starting at 742-760℃, the cooling water pressure controlled between 0.8-1.2 MPa, and the cooling rate between 65-84℃ / s. The second cooling section starts at 452-478℃, and the cooling rate is controlled at 21-30℃ / s. 6) Heat treatment process: normalizing treatment + high temperature homogenization treatment. The first stage of normalizing treatment: control the temperature at 862~915℃ and hold for 20~35min. The second stage adopts the homogenization treatment process: control the temperature at 560~600℃ and hold for 1.3~2.1min / mm.

2. The pressure vessel steel plate for mobile tank trucks prepared according to claim 1, characterized in that, Performance indicators: Steel plate at room temperature: 690MPa≤Tensile strength≤810MPa; 480MPa≤Yield strength≤620MPa; Transverse impact energy at -60℃≥100J.

3. The method for manufacturing pressure vessel steel plates for mobile tank trucks according to claim 1, characterized in that, In step 1), the particle size of the inoculant is 5-10 mm.

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