A steel plate for mobile containers having a yield strength of 690mpa and a method of manufacturing the same

Abstract

The present application provides a yield strength 690MPa mobile container steel plate and its manufacturing method, the composition of the steel plate is as follows in percentage by weight: C: 0.01%~0.05%, Si: 0.10%~0.30%, Mn: 1.00%~2.00%, P: ≤0.010%, S: ≤0.003%, Nb: 0.10%~0.50%, Ti: 0.10%~0.50%, Mo: 0.10%~0.50%, Ni: 0.50%~1.00%, Al: 0.0025%~0.0045%, the balance is Fe and inevitable impurities. The manufacturing method comprises smelting, continuous casting, heating, rolling, heat treatment; the steel plate produced by the application has a thickness specification of 8-30mm, a yield strength R eL ≥690MPa, a tensile strength R m : 800~920MPa, A ≥ 18%, -40℃ KV2 ≥ 100J, ReL / Rm ≤ 0.90. The performance of post-weld stress relief heat treatment has a yield strength R eL ≥690MPa, a tensile strength R m : 800~920MPa, A ≥ 18%, -40℃ KV2 ≥ 100J, ReL / Rm ≤ 0.90. eL ≥690MPa, a tensile strength R m : 800~920MPa, A ≥ 18%, -40℃ KV2 ≥ 100J, ReL / Rm ≤ 0.90.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and particularly relates to a steel plate for mobile containers with a yield strength of 690 MPa and a method for manufacturing the same. Background Technology

[0002] Mobile pressure vessels mainly refer to pressure vessels used for filling and transporting gases. Mobile containers do not have fixed locations, their operating environments frequently change, and they are susceptible to additional loads during transportation, making them more dangerous than stationary containers. Some mobile containers transport media such as liquefied petroleum gas, liquid ammonia, liquid chlorine, sulfur dioxide, propane, and propylene, which need to be pressurized to a liquid state at around -40℃. Therefore, mobile containers must ensure good impact toughness at temperatures below -40℃. In recent years, with the rapid development of the petroleum, petrochemical, chemical, and energy sectors, there has been a trend towards lightweight design for horizontal tanks and trolleys in vehicles. Mobile containers manufactured in my country are also gradually developing towards safety, energy conservation, and lightweighting. Therefore, higher strength and lower operating temperatures are required for the steel plates used in mobile containers, and the safety performance requirements for mobile containers are also increasing accordingly. Consequently, my country's GB / T 19905-2017 standard for liquefied gas tank trucks stipulates that the minimum standard tensile strength of steel plates should not be less than 570MPa, and the operating environment temperature of low-alloy steel plates should not exceed -40℃. Therefore, in addition to lightweight design and reduced transportation costs, mobile containers require steel plates that not only possess high strength and good mechanical properties after post-weld stress relief heat treatment during pressure vessel manufacturing, but also excellent weldability to meet the subsequent manufacturing requirements of mobile containers. Thus, researching a steel plate for mobile containers with a yield strength of 690 MPa has become an urgent task for the Chinese steel industry.

[0003] Steel plates for mobile containers are typically heat-treated with quenching and tempering to achieve high strength, good low-temperature impact toughness down to -40℃, and maintain satisfactory mechanical properties even after post-weld stress-relief heat treatment. Therefore, there is an urgent need to develop a steel plate for mobile containers with a yield strength of 690 MPa and its manufacturing method to meet the needs of the steel industry.

[0004] The patent document "A Production Method of Steel for Low-Temperature Mobile Containers" (CN113186453A) discloses a container steel with the following chemical composition: C 0.17-0.20%, Si 0.10-0.60%, Mn 1.00-1.70%, P≤0.008%, S≤0.003%, Alt 0.020-0.050%, Nb 0.020-0.050%, V≤0.2%, Ni 0.25-0.80%, Cu 0.1-0.3%, Ti 0.010-0.030%, Mo≤0.1%, Cr≤0.3%. After heat treatment, the microstructure is ferrite + pearlite with a grain size of grade 10 or higher. The yield strength is ≥460MPa, the tensile strength is 630-725MPa, and the yield strength is lower than 690MPa. However, the carbon equivalent of the designed composition is too high, which makes it prone to cracking during manufacturing.

[0005] The patent document "A Production Method of Steel for Low-Temperature Pressure Vessels and Steel for Low-Temperature Pressure Vessels" (CN116623092A) discloses a container steel with the following chemical composition: C 0.14-0.16%, Si 0.20-0.40%, Mn 1.40-1.60%, P≤0.015%, S≤0.008%, Nb 0.020-0.030%, Als 0.020-0.040%, with the remainder being Fe and unavoidable inclusions. However, controlling the cooling by pouring water will increase the hardness of the steel plate and reduce its toughness, affecting subsequent steel plate manufacturing.

[0006] The chemical composition of the container steel disclosed in the patent document "A 35-50mm thick Q420R high-strength steel for low-temperature pressure vessels and its production method" (CN108165892A) is as follows: C 0.12-0.15%, Si 0.20-0.40%, Mn 1.45-1.55%, P≤0.015%, S≤0.005%, Ni 0.30-0.40%, Cr 0.22-0.27%, Nb 0.020-0.030%, V 0.05-0.06%, Als 0.015-0.035%, Ti 0.008-0.018%, with the balance being Fe and other elements. After sub-temperature quenching, the microstructure is prone to mixed crystal formation, and according to the heritability of the microstructure, the banded structure is easily severe after heat treatment, affecting the performance of the steel plate.

[0007] Therefore, in view of the above situation, it is urgent to develop a new type of high-strength steel by adjusting the manufacturing method and designing new components, and to develop a steel plate for mobile containers with a yield strength of 690MPa and its manufacturing method. Summary of the Invention

[0008] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a steel plate for mobile containers with excellent strength and toughness, low yield strength ratio and yield strength of 690 MPa, and a method for manufacturing the same.

[0009] The objective of this invention is achieved as follows:

[0010] A steel plate for mobile containers with a yield strength of 690 MPa, the steel plate having the following composition by weight percentage: C: 0.01%–0.05%, Si: 0.10%–0.30%, Mn: 1.00%–2.00%, P: ≤0.010%, S: ≤0.003%, Nb: 0.10%–0.50%, Ti: 0.10%–0.50%, Mo: 0.10%–0.50%, Ni: 0.50%–1.00%, Al: 0.0025%–0.0045%, with the balance being Fe and unavoidable impurities.

[0011] The steel plate has an Nb / C ratio of ≥5.

[0012] The microstructure of the steel plate is tempered sorbite with a very small amount of ferrite, wherein, by volume percentage, the tempered sorbite is 93% to 98% and the ferrite is 2% to 7%, and the carbide size in the microstructure is 1 to 4 μm.

[0013] The steel plate has a thickness of 8–30 mm and a yield strength R. eL ≥690MPa, tensile strength R m 800~920MPa, elongation A≥18%, KV2≥100J at -40℃, R eL / R m ≤0.90.

[0014] The rationale for the design of the components in this invention is as follows:

[0015] In steel, carbon (C) forms various carbides with alloying elements, playing a strengthening role and directly increasing the strength of steel plates. This invention reduces the formation of banded structures by lowering the C content, ensuring a sufficiently low CEV (Cellular Energy Efficiency) to meet excellent weldability requirements; therefore, the C content is limited to 0.01%–0.05%.

[0016] Si strengthens steel through solid solution treatment, increasing the yield strength of steel plates and acting as a deoxidizer during steelmaking. However, excessive Si content can negatively impact the low-temperature impact toughness and weldability of steel plates. Therefore, the Si content is limited to 0.10%–0.30%.

[0017] Mn can form a solid solution with Fe in steel, improving the strength and hardenability of the steel plate. Mn can also increase the stability of the austenite structure, delay the transformation of ferrite and pearlite in steel, and improve the stability of the tempered structure after heat treatment. However, if the content is too high, it is easy to form segregated MnS in the steel, which is not conducive to the low-temperature toughness of the steel. Therefore, Mn is limited to 1.00% to 2.00%.

[0018] Phosphorus (P) is a harmful element in steel, significantly impacting low-temperature impact toughness and being highly prone to segregation. Therefore, its content should be kept low during steelmaking. Thus, the P content is controlled below 0.010%.

[0019] Sulfur (S) readily forms MnS in steel, which can easily become crack initiation points during subsequent processing and significantly affect the toughness of the steel. Therefore, the S content should be controlled below 0.003%.

[0020] Mo, as the main element in this invention, dissolves in austenite and ferrite through solid solution strengthening to improve the strength and toughness of steel. It can also reduce the critical cooling rate of steel to improve the hardenability of the steel plate. During the two-stage rolling process, it shrinks the austenite phase region, promotes bainite transformation, and generates Mo-containing carbides, which can also suppress temper brittleness. After quenching and tempering heat treatment, Mo can improve the low-temperature toughness of the steel plate. Therefore, the Mo content is between 0.10% and 0.50%.

[0021] Ni, as the main element in this invention, is a key element for stabilizing austenite, enhancing the strength of steel through solid solution strengthening and refining grains. Ni can also improve the low-temperature toughness and elongation of steel plates after quenching and tempering heat treatment. However, adding too much Ni will increase Ceq, significantly increasing costs. Therefore, the Ni content is limited to between 0.50% and 1.00%.

[0022] Nitrogen (Nb), as the main element in this invention, can suppress austenite recrystallization during the rolling process in steel. It allows for controlled rolling of the non-recrystallized region to increase the dislocation density within the austenite body, promote grain refinement, improve the strength and toughness of the tempered steel plate, and reduce temper brittleness. Therefore, the Nb content is limited to 0.10%–0.50%. With increasing carbon content, more Nb-containing carbides precipitate in the steel, and the carbide size increases, leading to a decrease in strength. Therefore, the Nb / C ratio is limited to ≥5.

[0023] Ti, as the main element in this invention, can significantly refine grains. Ti has a strong affinity for C and N, and the resulting carbides, nitrides, or carbonitrides have very high dissolution temperatures. During heating, undissolved carbonitride particles increase the nucleation sites for austenite and hinder austenite grain growth at high temperatures. The fine nitrides greatly suppress austenite grain coarsening at high welding temperatures, and during cooling, they refine the microstructure of the weld and heat-affected zone, improving toughness. Therefore, the Ti content is limited to 0.10%–0.50%.

[0024] The main functions of Al in steel are deoxidation and nitrogen fixation. The AlN formed can refine the grain structure and improve the toughness of the steel. However, excessive Al content can lead to the formation of larger AlN particles, reducing the low-temperature impact toughness of the steel plate. Therefore, the Al content is limited to 0.0025%–0.0045%.

[0025] The second technical solution of the present invention is to provide a method for manufacturing a steel plate for a mobile container with a yield strength of 690 MPa, including smelting, continuous casting, heating, rolling and heat treatment;

[0026] (1) Smelting: Iron pretreatment is adopted, with iron temperature of 1370~1470℃; during refining treatment, LF time is 40~50min, RH time is 30~45min, and vacuum degree is ≤5.0mbar.

[0027] (2) Continuous casting: The tundish steel pouring temperature is 1390-1440℃, and electromagnetic stirring technology is adopted. The preferred electromagnetic stirring frequency is 3-6Hz, the billet speed is 0.65-0.95m / min, the secondary cooling water is 5400-5900L / min, the superheat is 23-31℃, and the straightening temperature is 980-1100℃. The use of electromagnetic stirring technology can expand the equiaxed crystal zone, promote the uniform temperature distribution of molten steel, significantly reduce center segregation, and improve the surface and internal quality of the billet.

[0028] (3) Heating: The heating of the continuously cast billet is divided into a preheating section, a heating section and a soaking section; the temperature range of the heating section is 1180~1210℃; the temperature range of the soaking section is 1160~1200℃; the total time in the furnace is 130~160min. By heating the billet in three stages, the core temperature of the billet reaches 1160~1200℃, ensuring that the billet structure is completely austenitic.

[0029] (4) Rolling: After the billet exits the furnace, a two-stage controlled rolling and cooling technology is adopted. In the roughing stage, the initial rolling temperature is 1140-1180℃, the roll speed is 0.5-0.7m / s, and the single-pass reduction rate is 11%-15%; in the finishing stage, the initial rolling temperature is 840-890℃, the final rolling temperature is 750-800℃, the roll speed is 0.4-0.6m / s, and the single-pass reduction rate is 8%-13%; in the cooling stage, the initial cooling temperature is 720-770℃, and the final cooling temperature is 430-500℃; after cooling, the steel plates are stacked and slowly cooled at a temperature of 400-430℃ for 600-800min. Through two-stage rolling, using high reduction and slow roll speed rolling, the deformation storage energy in the microstructure is increased, the time for dynamic recrystallization is prolonged, and the grains are refined; by controlling the cooling, the ferrite transformation is suppressed and the bainite transformation is promoted.

[0030] (5) Heat treatment: After slow cooling in stacks, the steel plates undergo quenching and tempering heat treatment. The first quenching temperature is 940-960℃, and the holding time is 10-30 min; the second quenching temperature is 850-880℃, and the holding time is 5-10 min; the tempering temperature is 550-580℃, and the holding time is 60-100 min. Through the second quenching and tempering, the quenched structure is fully transformed and the large amount of ferrite in the structure is reduced. After tempering, the structure is tempered sorbite with a very small amount of ferrite, and the structure is uniformly distributed. The mechanical properties of the steel plate can be improved after heat treatment.

[0031] The beneficial effects of this invention are as follows:

[0032] 1. Based on traditional carbon steel, this invention significantly increases the strength and low-temperature toughness of the steel plate by adding sufficient amounts of Nb and Ti. Nb in the steel can inhibit austenite recrystallization during rolling, refine grain size, improve the strength and toughness of the quenched and tempered steel plate after heat treatment, improve temper resistance, and improve weldability. Ti can significantly refine grains, and the formed carbides, nitrides, or carbonitrides can inhibit the growth of austenite grains at high temperatures, refine the microstructure of the weld and heat-affected zone during cooling, and improve toughness. At the same time, by adding sufficient Mo and Ni, the hardenability and temper resistance of the steel plate are increased. Mo can inhibit the formation of grain boundary ferrite, promote bainite transformation, improve the strength and toughness of the steel, and inhibit temper brittleness. Ni can stabilize austenite and exist in austenite and ferrite in a solid solution with Fe, improving the strength of the steel, refining grains, and improving the low-temperature toughness of the quenched and tempered steel plate.

[0033] 2. The manufacturing method of this invention is reasonable. By adopting controlled rolling and controlled cooling combined with secondary quenching and tempering heat treatment, it is ensured that the microstructure of the finished product is tempered sorbite with a very small amount of ferrite. The carbide size in the microstructure is between 1 and 4 μm and is evenly distributed, which reduces the damage of ferrite to the strength of the steel plate. While ensuring that the steel plate has excellent strength and toughness, the yield strength ratio is relatively low. Moreover, it still has excellent strength and toughness after post-weld stress relief heat treatment.

[0034] 3. The steel plate of this invention has a thickness of 8-30mm and a yield strength R. eL ≥690MPa, tensile strength R m :800~920MPa, A≥18%, -40℃KV2≥100J, R eL / R m ≤0.90. Yield strength R of post-weld stress relief heat treatment. eL ≥690MPa, tensile strength R m :800~920MPa, A≥18%, -40℃KV2≥100J, ReL / Rm≤0.90. Detailed Implementation

[0035] The present invention will be further illustrated below through examples.

[0036] (1) Heating: The heating of the continuous casting billet is divided into a preheating section, a heating section and a soaking section; the temperature range of the heating section is 1180~1210℃; the temperature range of the soaking section is 1160~1200℃; the total time in the furnace is 130~160min.

[0037] (2) Rolling: After the billet exits the furnace, a two-stage controlled rolling and cooling technology is adopted. In the roughing stage, the initial rolling temperature is 1140-1180℃, the roll speed is 0.5-0.7m / s, and the single-pass reduction rate is 11%-15%; in the finishing stage, the initial rolling temperature is 840-890℃, the final rolling temperature is 750-800℃, the roll speed is 0.4-0.6m / s, and the single-pass reduction rate is 8-13%; in the cooling stage, the initial cooling temperature is 720-770℃, and the final cooling temperature is 430-500℃; after cooling, the steel plates are stacked and slowly cooled at a temperature of 400-430℃ for 600-800min.

[0038] (3) Heat treatment: After stacking and slow cooling, the steel plates are subjected to quenching and tempering heat treatment. The first quenching temperature is 940-960℃ and the holding time is 10-30min; the second quenching temperature is 850-880℃ and the holding time is 5-10min; the tempering temperature is 550-580℃ and the holding time is 60-100min.

[0039] Further smelting: hot metal pretreatment is adopted, with hot metal temperature of 1370~1470℃; during refining treatment, LF time is 40~50min, RH time is 30~45min, and vacuum degree is ≤5.0mbar.

[0040] Further; Continuous casting: tundish steel pouring temperature 1390~1440℃, billet pulling speed 0.65~0.95m / min, secondary cooling water 5400~5900L / min, superheat 23~31℃, straightening temperature 980~1100℃.

[0041] Furthermore, electromagnetic stirring technology is used in the continuous casting process, with an electromagnetic stirring frequency of 3–6 Hz.

[0042] According to the component ratio of the technical solution, the embodiments of the present invention involve smelting, continuous casting, heating, rolling, and heat treatment. The composition of the steel in the embodiments of the present invention is shown in Table 1. The main process parameters for smelting and casting of the steel in the embodiments of the present invention are shown in Table 2. The main process parameters for heating and rolling of the steel in the embodiments of the present invention are shown in Table 3. The process parameters for cooling, slow cooling, and heat treatment of the steel in the embodiments of the present invention are shown in Table 4. The properties of the steel in the embodiments of the present invention are shown in Table 5. The properties of the steel after post-weld stress relief heat treatment are shown in Table 6.

[0043] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0044] Example C Si Mn P S Nb Ti Mo Ni Al Nb / C 1 0.01 0.10 1.08 0.001 0.001 0.10 0.10 0.10 0.52 0.0025 10.0 2 0.05 0.27 1.54 0.001 0.001 0.35 0.10 0.12 0.57 0.0042 7.0 3 0.02 0.12 1.23 0.006 0.002 0.25 0.13 0.12 0.58 0.0031 12.5 4 0.05 0.24 1.75 0.008 0.003 0.30 0.15 0.15 0.59 0.0028 6.0 5 0.05 0.21 1.59 0.004 0.002 0.14 0.17 0.23 0.64 0.0045 2.8 6 0.04 0.30 1.98 0.006 0.003 0.39 0.20 0.25 0.68 0.0029 9.8 7 0.04 0.11 1.17 0.010 0.003 0.26 0.29 0.28 0.73 0.0037 6.5 8 0.04 0.18 1.42 0.007 0.003 0.50 0.31 0.32 0.74 0.0025 12.5 9 0.01 0.13 1.40 0.004 0.002 0.22 0.33 0.35 0.77 0.0033 22.0 10 0.05 0.19 1.49 0.003 0.001 0.43 0.33 0.36 0.81 0.0029 8.6 11 0.03 0.12 1.31 0.002 0.001 0.17 0.35 0.36 0.83 0.004 5.7 12 0.04 0.21 1.56 0.005 0.002 0.42 0.38 0.42 0.84 0.0028 14.0 13 0.01 0.22 1.65 0.009 0.003 0.15 0.41 0.43 0.86 0.0028 15.0 14 0.05 0.24 1.89 0.003 0.001 0.47 0.46 0.44 0.95 0.0029 9.4 15 0.04 0.20 1.90 0.002 0.001 0.44 0.50 0.48 0.98 0.0039 11.0

[0045] Table 2 Main process parameters for steel smelting and casting in the embodiments of the present invention.

[0046]

[0047] Table 3 Main process parameters for steel heating and rolling in the embodiments of the present invention.

[0048]

[0049] Table 4. Process parameters for steel cooling, slow cooling, and heat treatment in embodiments of the present invention.

[0050]

[0051] Table 5 Properties of Steel in the Invention Embodiments

[0052]

[0053] Table 6 Performance of Post-Weld Stress Relief Heat Treatment of Steel in Embodiments of the Invention (2019)

[0054]

[0055] Note: The post-weld stress relief heat treatment temperature is 550-570℃, and the holding time is 60-90min.

[0056] The steel plate produced using this invention has a microstructure of tempered sorbite with a very small amount of ferrite. The steel plate has a thickness of 8–30 mm, and its properties and post-weld stress-relieving heat treatment performance meet the requirements of R… eL ≥690MPa, R m :800~920MPa, A≥18%, -40℃KV2≥100J, R eL / R m ≤0.90.

[0057] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A steel plate for mobile containers with a yield strength of 690 MPa, characterized in that, The composition of the steel plate by weight percentage is as follows: C: 0.01%~0.05%, Si: 0.10%~0.30%, Mn: 1.00%~2.00%, P: ≤0.010%, S: ≤0.003%, Nb: 0.10%~0.50%, Ti: 0.10%~0.50%, Mo: 0.10%~0.50%, Ni: 0.50%~1.00%, Al: 0.0025%~0.0045%, with the balance being Fe and unavoidable impurities; the microstructure of the steel plate is tempered sorbite + a small amount of ferrite, wherein, by volume percentage, the tempered sorbite is 93%~98%, the ferrite is 2%~7%, and the carbide size in the microstructure is 1~4μm.

2. The steel plate for mobile containers with a yield strength of 690 MPa according to claim 1, characterized in that, The steel plate has an Nb / C ratio of ≥5.

3. The steel plate for mobile containers with a yield strength of 690 MPa according to claim 1, characterized in that, The steel plate has a thickness of 8~30mm and a yield strength R. eL ≥690MPa, tensile strength R m 800~920MPa, elongation A ≥18%, -40℃ KV 2 ≥100J, R eL / R m ≤0.

90.

4. A method for manufacturing a steel plate for a mobile container with a yield strength of 690 MPa as described in any one of claims 1-3, comprising smelting, continuous casting, heating, rolling, and heat treatment; characterized in that: (1) Heating: The heating of the continuously cast billet is divided into a preheating section, a heating section and a soaking section; the temperature range of the heating section is 1180~1210℃; the temperature range of the soaking section is 1160~1200℃; the total time in the furnace is 130~160min. (2) Rolling: After the billet exits the furnace, a two-stage controlled rolling and cooling technology is adopted; the initial rolling temperature of the roughing stage is 1140~1180℃, the roll speed is 0.5~0.7m / s, and the single-pass reduction rate is 11%~15%; the initial rolling temperature of the finishing stage is 840~890℃, the final rolling temperature is 750~800℃, the roll speed is 0.4~0.6m / s, and the single-pass reduction rate is 8%~13%; the initial cooling temperature of the cooling stage is 720~770℃, and the final cooling temperature is 430~500℃; after cooling, the steel plates are stacked and slowly cooled at a temperature of 400~430℃ for 600~800min. (3) Heat treatment: After stacking and slow cooling, the steel plates are subjected to quenching and tempering heat treatment. The first quenching temperature is 940~960℃ and the holding time is 10~30min; the second quenching temperature is 850~880℃ and the holding time is 5~10min; the tempering temperature is 550~580℃ and the holding time is 60~100min.

5. The method for manufacturing a steel plate for a mobile container with a yield strength of 690 MPa according to claim 4, characterized in that: Smelting: Hot metal pretreatment is adopted, with hot metal temperature of 1370~1470℃; during refining treatment, LF time is 40~50min, RH time is 30~45min, and vacuum degree is ≤5.0mbar.

6. The method for manufacturing a steel plate for a mobile container with a yield strength of 690 MPa according to claim 4, characterized in that: The tundish steel casting temperature is 1390~1440℃, electromagnetic stirring technology is used, the billet pulling speed is 0.65~0.95m / min, the secondary cooling water is 5400~5900L / min, the superheat is 23~31℃, and the straightening temperature is 980~1100℃.

7. The method for manufacturing a steel plate for a mobile container with a yield strength of 690 MPa according to claim 6, characterized in that: The electromagnetic stirring frequency is 3~6Hz.

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