Ultra-thin ultra-wide low-temperature steel plate and production method thereof

Abstract

The application provides an ultrathin ultrawide low-temperature steel plate and a production method thereof. The chemical composition of the steel plate comprises, in percentage by mass, C: 0.06%-0.09%, Si: 0.15%-0.25%, Mn: 1.10%-1.20%, P≤0.012%, S≤0.003%, Cr: 0.10%-0.20%, Ni: 0.20%-0.35%, Nb: 0.015%-0.025%, Ti: 0.005%-0.015%, Alt: 0.02%-0.05%, T.O≤0.002%, N≤0.005%, H≤0.00015%, and the balance is iron and inevitable impurities. The continuous casting billet with the aforementioned chemical composition of the steel plate is subjected to the processes of heating, descaling, rolling, cooling and straightening to obtain the ultrathin ultrawide low-temperature steel plate. The inclusion size of the continuous casting billet is less than or equal to 35 microns, and the center segregation grade is better than the B class 1.0 grade.

Description

Ultra-thin and ultra-wide low-temperature steel plates and their production methods Technical Field

[0001] This application relates to the field of low-temperature steel plate technology, and in particular to an ultra-thin and ultra-wide low-temperature steel plate and its production method. Background Technology

[0002] Liquefied petroleum gas (LPG) carriers are crucial tools for transporting important energy sources such as propane, propylene, butane, butene, and liquid ammonia, playing a vital role in ensuring energy supply and promoting economic development. Among these, Type A cargo tanks in LPG carriers offer advantages such as high structural efficiency, low construction and maintenance costs, and strong applicability, making them the mainstream choice for LPG carriers.

[0003] However, the design of Type A bulkheads places extremely high demands on the steel plates, mainly in the following aspects: 1. Size requirements: steel plate thickness is 8~35mm (of which thickness ≤10mm accounts for more than 20%), and width is ≤4300mm, in order to reduce the number of welds and welding deformation; 2. Plate shape requirements: straightness ≤3mm / m, to meet the assembly accuracy and laser cutting requirements of large-size bulkheads; 3. Performance requirements: yield strength ratio ≤0.85, yield strength ≤440MPa, to accommodate the transportation of corrosive media such as liquid ammonia.

[0004] The existing low-temperature steel plate production technology has the following technical bottlenecks: insufficient coordinated control of size, shape and performance: existing technologies are difficult to simultaneously meet the size requirements of ultra-thin (≤10mm) and ultra-wide (3800~4300mm), the shape requirements of high flatness (≤3mm / m), and the performance requirements of low yield strength ratio (≤0.85), narrow yield strength range (≤440MPa), and high toughness (-80℃). Summary of the Invention

[0005] The purpose of this application is to provide an ultra-thin and ultra-wide low-temperature steel plate, optimize the steel plate composition ratio and production process, and achieve coordinated control of low-temperature steel size, shape and performance. This solves the problem that it is difficult to simultaneously meet the requirements of ultra-thin and ultra-wide size, high flatness shape, low yield strength ratio, narrow yield strength range and high toughness performance in the prior art.

[0006] To achieve one of the aforementioned objectives, one embodiment of this application provides a method for producing an ultra-thin and ultra-wide low-temperature steel plate. The chemical composition of the steel plate, by mass percentage, includes: C: 0.06%~0.09%, Si: 0.15%~0.25%, Mn: 1.10%~1.20%, P≤0.012%, S≤0.003%, Cr: 0.10%~0.20%, Ni: 0.20%~0.35%, Nb: 0.015%~0.025%, Ti: 0.005%~0.015%, Alt: 0.02%~0.05%, TO≤0.002%, N≤0.005%, H≤0.00015%, with the balance being iron and unavoidable impurities.

[0007] A continuously cast billet with the aforementioned chemical composition is subjected to heating, descaling, rolling, cooling, and straightening processes to obtain an ultra-thin and ultra-wide low-temperature steel plate. The inclusion size of the continuously cast billet is ≤35μm, and the center segregation level is better than Class B 1.0.

[0008] In the heating process, when the target steel plate thickness is >10mm, the temperature of the soaking zone is controlled at T. NbC +90℃≤Heating Zone Temperature≤T NbCN +40℃, control the furnace time of the heating process to be st+70min≤furnace time≤st+220min; when the target steel plate thickness is ≤10mm, control the temperature of the soaking zone to T. NbC +150℃≤Panic section temperature≤T NbCN +60℃, the furnace time for the heating process is controlled to be st+100min≤furnace time≤st+220min; where T NbC It is the dissolution temperature of NbC, T NbC =6770 / [2.26-lg([Nb]×[C])]-273,T NbCN T is the dissolution temperature of Nb(C,N). NbCN =7900 / [3.42-lg([Nb]×[C] 0.7 ×[N] 0.3 [Nb], [C], and [N] are 100 times the mass percentages of Nb, C, and N elements in the steel plate, respectively, and st is the thickness of the continuously cast billet in mm;

[0009] In the rolling process, the reduction during rolling first increases and then decreases. The reduction rate for the last five passes is set as: A = 7.07 + 4.85 × B ± a, where a is the error, and a = 0.86 + 1.34 × B - 0.14 × B 2 A represents the reduction rate in percentage, B represents the number of reverse rolling passes; the final rolling temperature is Ar3-3.5t+0.04t. 2 +40℃~Ar3-3.5t+0.04t2 +60℃, where Ar3 = 907-310[C]-80[Mn]-15[Cr]-55[Ni]+116[Nb]+0.35t, t is the target steel plate thickness in mm, and [C], [Mn], [Cr], [Ni], and [Nb] are 100 times the mass percentage of C, Mn, Cr, Ni, and Nb elements in the steel plate, respectively.

[0010] In the cooling process, the steel plate is water-cooled, and the starting temperature of the cooling plate is controlled to be ≤Ar3-18+0.003t-0.01t. 2 ℃, final cooling temperature ≤300℃, cooling rate is 40 - 0.72t + 0.003t 2 ±b℃ / s, where b is the error, b = 24 - 1.14t + 0.016t 2 t represents the target steel plate thickness in mm.

[0011] In one embodiment of this application, the straightening process is performed by using a hot straightening machine and a warm straightening machine in sequence. The hot straightening temperature is 250~300℃, and the straightening is performed in 2~3 passes. The warm straightening temperature is <200℃, and the straightening is performed in 3~5 passes.

[0012] In one embodiment of this application, during the cooling process, the head blocking length is 6m and the tail blocking length is 5m along the length of the steel plate to reduce the amount of water in the blocking part. Within the blocking length, the amount of water from the end to the inside of the steel plate linearly recovers from 50% to 100%.

[0013] In one embodiment of this application, when the target steel plate thickness is ≤10mm, the heating time in the soaking zone is ≥50min, the furnace pressure of the heating furnace is controlled to be 10Pa~20Pa greater than atmospheric pressure, and the temperature difference between the continuous casting billet and the slab after exiting the furnace is ≤15℃.

[0014] In one embodiment of this application, when the target steel plate thickness is ≤10mm, no warming is required during rolling;

[0015] When the target steel plate thickness is greater than 10 mm, the intermediate billet thickness is greater than or equal to 2.5 t, where t is the target steel plate thickness in mm. The intermediate billet is cooled in air.

[0016] In one embodiment of this application, when the target steel plate thickness is ≤10mm, the continuous casting billet thickness is ≤220mm; when the target steel plate thickness is ≤10mm and the width is ≥3200mm, the continuous casting billet width is ≥2100mm.

[0017] In one embodiment of this application, the continuously cast billet is obtained through KR desulfurization, converter smelting, LF refining, RH refining, and continuous casting, wherein...

[0018] In the LF refining process, the tapping temperature is controlled as follows: T 液相线 +85℃≤Boiler Temperature≤T 液相线 +95℃, T 液相线 +75℃≤Continuous Casting Furnace Temperature≤T 液相线 +85℃, where T 液相线 =1536.6-90[C]-8[Si]-5[Mn]-30[P]-25[S]-1.55[Cr]-4[Ni]-18[Ti]-3[Al]-80[N], where [C], [Si], [Mn], [P], [S], [Cr], [Ni], [Ti], [Al], and [N] are 100 times the mass percentages of C, Si, Mn, P, S, Cr, Ni, Ti, Al, and N elements in the steel plate.

[0019] In one embodiment of this application, the tapping temperature in the RH refining process is controlled as follows: T 液相线 +55℃≤Boiler Temperature≤T 液相线 +65℃, T 液相线 +45℃≤Continuous Casting Furnace Temperature≤T 液相线 +55℃.

[0020] In one embodiment of this application, a long sprue nozzle, tundish covering agent, submerged entry nozzle, and argon sealing are used for fully protected casting. The long sprue nozzle is immersed in the molten steel to a depth of 200mm~300mm, and the submerged entry nozzle is immersed in the molten steel to a depth of 120mm~180mm. The tundish temperature is controlled at: T 液相线 +20℃≤Tundish Temperature≤T 液相线 At +30℃, during the casting process, the liquid level in the crystallizer fluctuates within a range of 2mm.

[0021] An embodiment of this application also provides an ultra-thin and ultra-wide low-temperature steel plate, prepared by the aforementioned production method. The chemical composition of the steel plate, by mass percentage, includes: C: 0.06%~0.09%, Si: 0.15%~0.25%, Mn: 1.10%~1.20%, P≤0.012%, S≤0.003%, Cr: 0.10%~0.20%, Ni: 0.20%~0.35%, Nb: 0.015%~0.025%, Ti: 0.005%~0.015%, Alt: 0.02%~0.05%, TO≤0.002%, N≤0.005%, H≤0.00015%, with the balance being iron and unavoidable impurities.

[0022] In one embodiment of this application, the microstructure of the steel plate is ferrite and bainite, with ferrite accounting for 75% to 90%, bainite accounting for 10% to 25%, and the average grain size being 6.5 μm to 7.5 μm.

[0023] In one embodiment of this application, the steel plate has a yield strength of 355MPa~440MPa, a tensile strength of 490MPa~650MPa, a yield strength ratio ≤0.80, an elongation after fracture ≥22%, an impact energy at -80℃ ≥200J, a ductile-brittle transition temperature ≤-100℃, and a CTOD at -60℃ ≥1.0mm.

[0024] In one embodiment of this application, after the steel plate is welded by submerged arc welding with a heat input of 15kJ / cm to 50kJ / cm, the impact energy of the heat-affected zone at -70℃ is ≥100J, and the CTOD of the coarse grain zone at -60℃ is ≥0.6mm.

[0025] One or more technical solutions provided in this application have at least the following technical effects or advantages:

[0026] This application optimizes the steel plate composition ratio and production process to achieve coordinated control of low-temperature steel dimensions, shape, and performance. It produces low-temperature steel plates with a thickness ≤40mm, width ≤4500mm, and straightness ≤2mm / m. This significantly reduces the upper limit of the steel plate's yield strength (≤440MPa), narrows the yield strength range (355MPa~440MPa), lowers the yield-to-tensile ratio (≤0.80), and reduces the ductile-brittle transition temperature (≤-100℃). It also improves weldability and fracture toughness, providing a reliable material basis for the manufacture of Type A tanks for liquefied petroleum gas (LPG) carriers. This meets the technical requirements for LPG carriers to be compatible with liquid ammonia transportation and enhances the safety of LPG carriers. Attached Figure Description

[0027] Figure 1 is a metallographic diagram of the steel plate at 1 / 2 thickness in Embodiment 1 of this application. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] This application provides a method for producing ultra-thin and ultra-wide low-temperature steel plates. The chemical composition of the steel plate, by mass percentage, includes: C: 0.06%~0.09%, Si: 0.15%~0.25%, Mn: 1.10%~1.20%, P≤0.012%, S≤0.003%, Cr: 0.10%~0.20%, Ni: 0.20%~0.35%, Nb: 0.015%~0.025%, Ti: 0.005%~0.015%, Alt: 0.02%~0.05%, TO≤0.002%, N≤0.005%, H≤0.00015%, with the balance being iron and unavoidable impurities.

[0030] The following explains the function of each chemical component in the steel plate described in this invention and the selection of their dosage:

[0031] C is a key strengthening element, but excessively high content will impair the low-temperature toughness and weldability of steel plates. When the C content is <0.04%, the difficulty of steelmaking increases significantly, and special processes are required to reduce C content. It is also not conducive to the control of yield strength ratio. Therefore, this application controls the C content at 0.06%~0.09%.

[0032] Si is a deoxidizing and solid solution strengthening element. Higher Si content leads to an increase in the M / A ratio, which in turn negatively impacts low-temperature toughness and weldability. This application controls the Si content to be between 0.15% and 0.25%.

[0033] Mn is a solid solution strengthening and grain refinement strengthening element, but it is also prone to segregation and formation of inclusions, which affects the low-temperature toughness of the steel plate core. This application controls the Mn content at 1.10%~1.20%, which ensures the strength of the steel plate and reduces segregation by controlling impurity elements such as P and S, and avoids the damage of MnS inclusions to the low-temperature toughness of the steel plate core.

[0034] Cr: As a solid solution strengthening element, it can promote ferrite transformation, thereby reducing the yield strength ratio. When the content is too high, it is easy to generate large residual stress during rapid cooling, which is not conducive to plate shape control. In this application, the Cr content is controlled at 0.10%~0.20%.

[0035] Ni (Ni): It promotes smooth and dispersed cementite edges and facilitates dislocation slip at low temperatures, making it an effective element for improving the low-temperature toughness of steel plates. However, the alloy cost is relatively high. This application controls the Ni content to be between 0.20% and 0.35%.

[0036] Nitrogen (Nb) is a fine-graining element that can suppress the coarsening of the original austenite grains during long-term high-temperature heating of continuously cast billets. However, excessive content can hinder the control of the yield strength ratio. In this application, the Nb content is controlled at 0.015%~0.025%.

[0037] Ti: It is a nitrogen-fixing and deoxidizing element. When it is in excess, it is easy to form large Ti (C, N) particles in the core of the steel plate, which affects the low-temperature toughness at 1 / 2 thickness. In this application, the Ti content is controlled at 0.005%~0.015%.

[0038] Al: It is a deoxidizing and fine-graining element. If the content is too high, it will increase Al2O3 inclusions and affect low-temperature toughness. In this application, Alt is controlled at 0.02%~0.05%.

[0039] P, S, O, N, and H are unavoidable impurity elements in steel. P tends to segregate in the center of the steel plate, S tends to combine with Mn to form MnS inclusions, O tends to form various oxide inclusions, N tends to combine with elements such as Al and Ti to form brittle inclusions with sharp edges, and H, as an interstitial atom, can cause lattice distortion and tends to accumulate at defects, inducing microcracks. These elements all disrupt the continuity of the steel plate structure, leading to uneven stress distribution, and consequently deteriorating low-temperature toughness and plate straightness. In production, the content of these impurity elements should be reduced as much as possible, but overly strict control will significantly increase the difficulty and cost of steelmaking. This application controls P ≤ 0.012%, S ≤ 0.003%, TO ≤ 0.002%, N ≤ 0.005%, and H ≤ 0.00015%. Combined with the overall chemical composition design and process control scheme, this ensures that the steel plate has excellent low-temperature toughness and plate shape while avoiding the problem of significantly increased production difficulty and cost due to overly strict composition control.

[0040] The continuously cast billet with the aforementioned chemical composition is subjected to heating, descaling, rolling, cooling, and straightening processes to obtain an ultra-thin and ultra-wide low-temperature steel plate. The inclusion size of the continuously cast billet is ≤35μm, and the center segregation level is better than Class B 1.0.

[0041] During the heating process, when the target steel plate thickness is >10mm, the temperature of the soaking zone is controlled at T. NbC +90℃≤Heating Zone Temperature≤T NbCN +40℃, control the furnace time of the heating process to be st+70min≤furnace time≤st+220min; when the target steel plate thickness is ≤10mm, control the temperature of the soaking zone to T. NbC +150℃≤Panic section temperature≤T NbCN +60℃, the furnace time for the heating process is controlled to be st+100min≤furnace time≤st+220min; where T NbC It is the dissolution temperature of NbC, T NbC =6770 / [2.26-lg([Nb]×[C])]-273,T NbCN T is the dissolution temperature of Nb(C,N). NbCN =7900 / [3.42-lg([Nb]×[C] 0.7 ×[N]0.3 [Nb], [C], and [N] are 100 times the mass percentage of Nb, C, and N elements in the steel plate (i.e., the values ​​without % in the aforementioned chemical composition, the same below), and st is the thickness of the continuously cast billet in mm.

[0042] In this application, limiting the heating temperature by the dissolution temperatures of NbC and Nb(C,N) allows for complete solid solution of alloying elements while preventing excessive coarsening of the original austenite grains, which is beneficial for improving the low-temperature toughness of the finished steel plate. Ensuring sufficient heating time ensures that the temperature of the continuously cast billet is sufficiently homogenized, while preventing deformation of the billet and providing a uniform temperature field and a flat initial plate shape for the rolling process, which is beneficial for plate shape control during the rolling process.

[0043] In the rolling process, the reduction during rolling first increases and then decreases. The reduction rate for the last five passes is set as: A = 7.07 + 4.85 × B ± a, where a is the error, and a = 0.86 + 1.34 × B - 0.14 × B 2 A represents the reduction rate in percentage, B represents the number of reverse rolling passes; the final rolling temperature is Ar3-3.5t+0.04t. 2 +40℃~Ar3-3.5t+0.04t 2 +60℃, where Ar3=907-310[C]-80[Mn]-15[Cr]-55[Ni]+116[Nb]+0.35t, t is the target steel plate thickness in mm, and [C], [Mn], [Cr], [Ni], and [Nb] are 100 times the mass percentages of C, Mn, Cr, Ni, and Nb elements in the steel plate, respectively.

[0044] Where B represents the inverse number of rolling passes, specifically counting from the last pass. For example, B=1 for the penultimate pass and B=2 for the penultimate pass. Gradually decreasing the reduction rate in the last five passes effectively reduces the accumulation of residual stress during rolling, preventing the residual stress from exceeding the critical stress for steel plate deformation, and resulting in a smooth plate shape after rolling.

[0045] Ar3 is the temperature at which austenite begins to transform into ferrite. By adjusting the final rolling temperature according to the target steel plate thickness, the influence of the temperature difference between the surface and core of the thicker steel plate on the control of phase transformation is reduced, ultimately resulting in a refined steel plate structure and ensuring the low-temperature toughness of the steel plate.

[0046] Rolling can be performed using a single-stand or double-stand four-high reversible rolling mill.

[0047] Preferably, in the rolling process, high-pressure descaling is not performed in the last four passes. By the time the steel plate has reached the target thickness, contact with descaling water at this point can easily lead to uneven temperature distribution of the steel plate due to uneven descaling water, ultimately resulting in poor plate shape.

[0048] During the cooling process, the steel plate is water-cooled, and the starting temperature of the cooling plate is controlled to be ≤Ar3-18+0.003t-0.01t. 2 ℃, final cooling temperature ≤300℃, cooling rate is 40 - 0.72t + 0.003t 2 ±b℃ / s, where b is the error, b = 24 - 1.14t + 0.016t 2 t represents the target steel plate thickness in mm.

[0049] Before accelerated cooling, a portion of proeutectoid ferrite forms in the steel plate, reducing the yield strength ratio; during accelerated cooling, a bainitic hard phase structure forms, further reducing the yield strength ratio. A multi-functional intermittent cooling system can be used for water cooling of the steel plate.

[0050] It should be noted that in the formulas involving the target steel plate thickness and the continuous casting billet thickness in this application, only the values ​​of the target steel plate thickness and the continuous casting billet thickness in mm are used in the calculation.

[0051] In some embodiments of this application, in the straightening process, a hot straightening machine and a warm straightening machine are used sequentially for straightening. The hot straightening temperature is 250~300℃, and the straightening is performed in 2~3 passes; the warm straightening temperature is <200℃, and the straightening is performed in 3~5 passes.

[0052] To ensure that the flatness of the ultra-thin and ultra-wide low-temperature steel plate in this application is low, a hot straightening machine is first used to straighten the plate, which initially releases the phase transformation stress and thermal stress, making the plate flat. Then, a warm straightening machine is used to straighten the plate again, which releases the thermal stress again, making the stress distribution uniform and ensuring that the steel plate maintains its shape stability during subsequent cutting.

[0053] In this application, the hot straightener has 11 rollers, with 5 upper straightening rollers and 6 lower straightening rollers, and is located after the multi-functional intermittent cooling system; the warm straightener has 9 rollers, with 4 upper straightening rollers and 5 lower straightening rollers, and is located after the cooling bed.

[0054] In some embodiments of this application, during the cooling process, the head blocking length is 6m and the tail blocking length is 5m along the length of the steel plate to reduce the amount of water in the blocked portion. Within the blocking length, the amount of water from the end to the inside of the steel plate linearly recovers from 50% to 100%.

[0055] The two ends of the steel plate are blocked by 6m and 5m respectively along the length of the steel plate to avoid the performance fluctuations caused by the excessive cooling of the beginning and end of the steel plate, and to avoid the poor plate shape caused by the inconsistent cooling temperature of the beginning, middle and end.

[0056] Of course, as mentioned above, the blocking does not mean completely blocking out water cooling. Instead, the water volume is gradually reduced linearly towards both ends until it reaches 50% of the water volume in the middle. The water volume at the positions 6m from the head and 5m from the tail is the same as the water volume in the middle, which is 100%.

[0057] In some embodiments of this application, when the target steel plate thickness is ≤10mm, the heating time of the soaking zone is ≥50min, the furnace pressure of the heating furnace is controlled to be greater than atmospheric pressure by 10Pa~20Pa, and the temperature difference between the continuous casting billet and the slab after exiting the furnace is ≤15℃.

[0058] When the target steel plate thickness is ≤10mm, the temperature difference between the surface temperature and the core temperature of the steel plate has a greater impact on the plate shape than the temperature difference between the surface temperature and the core temperature of thicker steel plates. Therefore, during the heating stage, the heating time of the soaking zone is controlled so that the temperature difference between the continuous casting billet and the slab after exiting the furnace is ≤15℃, ensuring uniform temperature inside and outside the steel plate, thereby reducing the impact of the internal and external temperature difference on the plate shape.

[0059] The furnace pressure of the heating furnace is kept relatively high to prevent cold air from being drawn in by the furnace head, which would cause uneven temperature of the continuously cast billet.

[0060] In some embodiments of this application, when the target steel plate thickness is ≤10mm, the rolling process does not require waiting for the temperature to rise; instead, after the steel plate is descaled, it is fed to the rolling mill at a uniform speed to begin rolling, without waiting for the temperature to rise during the rolling process. This avoids aggravating the temperature non-uniformity of the intermediate billet and provides good temperature field conditions for plate shape control.

[0061] When the target steel plate thickness is greater than 10 mm, the intermediate billet thickness is greater than or equal to 2.5 t, where t is the target steel plate thickness in mm. The intermediate billet is cooled in air.

[0062] In some embodiments of this application, when the target steel plate thickness is ≤10mm, the continuous casting billet thickness is ≤220mm to avoid excessive compression ratio leading to the accumulation of residual rolling stress, which in turn affects the plate shape quality. When the target steel plate thickness is ≤10mm and the width is ≥3200mm, the continuous casting billet width is ≥2100mm to prevent excessive width ratio from causing the accumulation of residual rolling stress, which in turn affects the plate shape quality.

[0063] In some embodiments of this application, the continuously cast billet is obtained through KR desulfurization, converter smelting, LF refining, RH refining, and continuous casting, wherein,

[0064] In the LF refining process, the tapping temperature is controlled as follows: T 液相线 +85℃≤Boiler Temperature≤T 液相线 +95℃, T 液相线 +75℃≤Continuous Casting Furnace Temperature≤T 液相线 +85℃, where T 液相线=1536.6-90[C]-8[Si]-5[Mn]-30[P]-25[S]-1.55[Cr]-4[Ni]-18[Ti]-3[Al]-80[N], where [C], [Si], [Mn], [P], [S], [Cr], [Ni], [Ti], [Al], and [N] are 100 times the mass percentages of C, Si, Mn, P, S, Cr, Ni, Ti, Al, and N elements in the steel plate.

[0065] The start-up furnace temperature is the temperature required for LF refining (or RH refining) of molten steel when the tundish is used for the first time in the continuous casting process, and the continuous casting furnace temperature is the temperature required for LF refining (or RH refining) of molten steel when the tundish is used for the second to the last time.

[0066] It should be noted that, since it takes time for the molten steel to enter the tundish after the LF / RH refining process, while the tapping temperature of the LF / RH refining process is controlled at the furnace start-up temperature, the tundish of the continuous casting process has not yet been replaced with a new tundish. Instead, the old tundish is used for the last few times. When the molten steel that was tapped at the LF / RH refining process starts to enter the tundish for continuous casting at the furnace start-up temperature, a new tundish is replaced.

[0067] The optimal time between the end of the LF furnace tapping and the start of the RH treatment is 10 minutes. This improves the morphology of inclusions, promotes their flotation and removal, and reduces the formation of large inclusions. For every minute the tapping time is advanced or delayed, the tapping temperature decreases or increases by 0.4°C.

[0068] In the LF refining process, pure calcium wire is fed at a rate of 0.6 m / t to 0.75 m / t for 2 to 3 minutes before tapping to modify the high-melting-point Al2O3 inclusions into low-melting-point calcium-aluminum inclusions. The feeding speed is 1.0 m / s to 2.0 m / s. Here, m / t is the length of pure calcium wire added per ton of molten steel (in meters).

[0069] Furthermore, in the RH refining process, the tapping temperature is controlled as follows: T 液相线 +55℃≤Boiler Temperature≤T 液相线 +65℃, T 液相线 +45℃≤Continuous Casting Furnace Temperature≤T 液相线 +55℃.

[0070] The optimal time between the end of tapping and the start of continuous casting in the RH process is 10 minutes. Before the ladle is lifted, an insulating agent is added to maintain the temperature, covering the entire slag surface and spreading it evenly. This reduces the temperature drop of the molten steel, further removes inclusions, and improves the purity of the molten steel. Similarly, for every minute that the tapping time is advanced or delayed, the tapping temperature decreases or increases by 0.4℃.

[0071] Furthermore, a full protective casting process is employed, using a long sprue nozzle, tundish covering agent, submerged entry nozzle, and argon sealing. The long sprue nozzle is immersed in the molten steel to a depth of 200mm~300mm, and the submerged entry nozzle is immersed in the molten steel to a depth of 120mm~180mm. The tundish temperature is controlled at: T 液相线 +20℃≤Tundish Temperature≤T 液相线 At +30℃, during the casting process, the liquid level in the crystallizer fluctuates within a range of 2mm.

[0072] In the converter smelting process, during tapping, alloys and slag are added in the following order: aluminum → ferrosilicon → metallic manganese → lime. Argon is blown throughout the tapping process. Before tapping, the argon blowing pressure is controlled at 0.5MPa~0.6MPa, and the diameter of the bright ring on the molten steel surface is 250mm~400mm. After 75% of the steel has been tapped, the argon blowing pressure is controlled at 0.4MPa~0.5MPa, and the diameter of the bright ring on the molten steel surface is 200mm~350mm. This homogenizes the composition and temperature of the molten steel and adsorbs non-metallic inclusions in the molten steel, carrying them to the slag layer for removal.

[0073] When the bright ring on the surface of the molten steel is formed by bottom-blown argon, the argon gas agitates the molten steel, causing the slag to move around and thus forming a slag-free ring in the center of the molten steel surface.

[0074] Through the synergistic effect of the above steelmaking processes, inclusions can be fully modified and floated away, ensuring that the impurity elements in the steel are P≤0.012%, S≤0.003%, TO≤0.002%, N≤0.005%, and H≤0.00015%, thus guaranteeing the cleanliness of the molten steel. At the same time, the molten steel has good castability, ultimately yielding a continuously cast billet with a thickness of 150mm~320mm and a width of 1300mm~2700mm.

[0075] This application optimizes the steel plate composition ratio and production process to achieve coordinated control of low-temperature steel dimensions, shape, and performance. It produces low-temperature steel plates with a thickness ≤40mm, width ≤4500mm, and straightness ≤2mm / m. This significantly reduces the upper limit of the steel plate's yield strength (≤440MPa), narrows the yield strength range (355MPa~440MPa), lowers the yield-to-tensile ratio (≤0.80), and reduces the ductile-brittle transition temperature (≤-100℃). It also improves weldability and fracture toughness, providing a reliable material basis for the manufacture of Type A tanks for liquefied petroleum gas (LPG) carriers. This meets the technical requirements for LPG carriers to be compatible with liquid ammonia transportation and enhances the safety of LPG carriers.

[0076] This application provides an ultra-thin and ultra-wide low-temperature steel plate, the chemical composition of which, by mass percentage, includes: C: 0.06%~0.09%, Si: 0.15%~0.25%, Mn: 1.10%~1.20%, P≤0.012%, S≤0.003%, Cr: 0.10%~0.20%, Ni: 0.20%~0.35%, Nb: 0.015%~0.025%, Ti: 0.005%~0.015%, Alt: 0.02%~0.05%, TO≤0.002%, N≤0.005%, H≤0.00015%, with the balance being iron and unavoidable impurities.

[0077] Furthermore, the microstructure of the steel plate consists of ferrite and bainite, with ferrite accounting for 75%~90% and bainite accounting for 10%~25%, and the average grain size is 6.5μm~7.5μm.

[0078] Furthermore, the steel plate has a yield strength of 355MPa~440MPa, a tensile strength of 490MPa~650MPa, a yield-to-tensile ratio ≤0.80, an elongation after fracture ≥22%, an impact energy of -80℃ ≥200J, a ductile-brittle transition temperature ≤-100℃, and a CTOD of -60℃ ≥1.0mm.

[0079] After the aforementioned steel plates are welded by submerged arc welding with a heat input of 15kJ / cm~50kJ / cm, the impact energy of the heat-affected zone at -70℃ is ≥100J, and the CTOD of the coarse grain zone at -60℃ is ≥0.6mm.

[0080] The technical solution of this application will be further described below with reference to some specific embodiments.

[0081] The chemical composition of the steel plate is shown in Table 1. The steel plate is processed according to the procedures in Tables 2 to 8, and the microstructure and properties of the steel plate are shown in Tables 9 to 11.

[0082] Table 1. Chemical composition as a percentage by mass (%)

[0083]

[0084] Table 2 Converter Smelting Process

[0085]

[0086] Table 3 LF Refining Process

[0087]

[0088] Table 4 RH Refining Process

[0089]

[0090] Table 5 Continuous Casting Process

[0091]

[0092] Table 6 Heating Process

[0093]

[0094] Table 7 Rolling Process

[0095]

[0096] Table 8 Straightening Process

[0097]

[0098] Table 9. Flatness and microstructure of steel plates

[0099]

[0100] Table 10 Strength Properties of Steel Plates

[0101]

[0102] Table 11 Strength properties of steel plates after welding

[0103]

[0104] The flatness of the steel plate was tested according to the classification society's material and welding specifications, and the test results are shown in Table 9. Examples 1 and 2 used laser cutting, Example 3 used plasma cutting, and Example 4 used flame cutting. After cutting the steel plate into different shapes, the flatness was measured, and the test results are shown in Table 9. Samples were taken at 1 / 4 of the width of the steel plate head according to the classification society's material and welding specifications to test the metallographic structure, conventional mechanical properties, CTOD fracture toughness, and weldability. The specific test methods and results are as follows:

[0105] (1) Metallographic structure: 15cm×15cm samples were taken from the head of the steel plate and prepared as metallographic samples along the rolling direction. After mechanical polishing and etching with nitric acid and alcohol, the samples were observed under a metallographic microscope. The metallographic structures of Examples 1 to 4 were all ferrite + bainite. The metallographic structure of the steel plate in Example 1 is shown in Figure 1, which is ferrite + bainite, with ferrite accounting for 86.7% and bainite accounting for 13.3%, and the average grain size is 7.25μm. It can represent the metallographic structure of the steel plates in each embodiment. The metallographic structure images of the other embodiments are omitted. The structure, the proportion of each phase and the grain size are shown in Table 9.

[0106] (2) Conventional mechanical properties: The tensile and impact properties of the steel plates in the above embodiments were tested according to the material and welding specifications of the classification society. The test results are shown in Table 10.

[0107] (3) CTOD fracture toughness: CTOD fracture toughness test was performed on Example 4 at a test temperature of -60℃. The test results are shown in Table 11.

[0108] (4) Welding performance: Submerged arc welding was performed on Example 4 with heat input of 15 kJ / cm and 50 kJ / cm. The welding groove was a single-sided V-shaped groove. After welding, impact specimens were taken at FL, FL+2 and FL+5 to test the low-temperature toughness of the heat-affected zone. The test temperature was -70℃. CTOD specimens were taken at the coarse grain region of FL+1 to test the fracture toughness of the heat-affected zone. The test temperature was -60℃. The test results are shown in Table 11.

[0109] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0110] The detailed descriptions listed above are merely specific descriptions of feasible implementation methods of this application and are not intended to limit the scope of protection of this application. All equivalent implementation methods or modifications made without departing from the spirit of the art of this application should be included within the scope of protection of this application.

Claims

1. A method for producing ultra-thin and ultra-wide low-temperature steel plates, characterized in that, The chemical composition of the steel plate, by mass percentage, includes: C: 0.06%~0.09%, Si: 0.15%~0.25%, Mn: 1.10%~1.20%, P≤0.012%, S≤0.003%, Cr: 0.10%~0.20%, Ni: 0.20%~0.35%, Nb: 0.015%~0.025%, Ti: 0.005%~0.015%, Alt: 0.02%~0.09%. 5%, TO≤0.002%, N≤0.005%, H≤0.00015%, with the balance being iron and unavoidable impurities; a continuously cast billet with the aforementioned chemical composition is subjected to heating, descaling, rolling, cooling, and straightening processes to obtain an ultra-thin, ultra-wide low-temperature steel plate. The inclusion size of the continuously cast billet is ≤35μm, and the center segregation grade is better than Class B 1.

0. During the heating process, when the target steel plate thickness is >10mm, the soaking temperature is controlled at T. NbC +90℃≤Heating Zone Temperature≤T NbCN +40℃, control the furnace time of the heating process to be st+70min≤furnace time≤st+220min; when the target steel plate thickness is ≤10mm, control the temperature of the soaking zone to T. NbC +150℃≤Panic section temperature≤T NbCN +60℃, the furnace time for the heating process is controlled to be st+100min≤furnace time≤st+220min; where T NbC It is the dissolution temperature of NbC, T NbC =6770 / [2.26-lg([Nb]×[C])]-273,T NbCN T is the dissolution temperature of Nb(C,N). NbCN =7900 / [3.42-lg([Nb]×[C] 0.7 ×[N] 0.3 [Nb], [C], and [N] are 100 times the mass percentages of Nb, C, and N elements in the steel plate, respectively, and st is the thickness of the continuously cast billet in mm; in the rolling process, the reduction in rolling first increases and then decreases, and the reduction rate of the last five passes is set as: A = 7.07 + 4.85 × B ± a, where a is the error, and a = 0.86 + 1.34 × B - 0.14 × B 2 A represents the reduction rate in percentage, B represents the number of reverse rolling passes; the final rolling temperature is Ar3-3.5t+0.04t. 2 +40℃~Ar3-3.5t+0.04t 2 +60℃, where Ar3 = 907 - 310[C] - 80[Mn] - 15[Cr] - 55[Ni] + 116[Nb] + 0.35t, t is the target steel plate thickness in mm, and [C], [Mn], [Cr], [Ni], and [Nb] are 100 times the mass percentages of C, Mn, Cr, Ni, and Nb elements in the steel plate, respectively; in the cooling process, the steel plate is water-cooled, and the starting cooling temperature of the steel plate is controlled to be ≤Ar3 - 18 + 0.003t - 0.01t. 2 ℃, final cooling temperature ≤300℃, cooling rate is 40 - 0.72t + 0.003t 2 ±b℃ / s, where b is the error, b = 24 - 1.14t + 0.016t 2 t represents the target steel plate thickness in mm.

2. The method for producing ultra-thin and ultra-wide low-temperature steel plates according to claim 1, characterized in that, In the straightening process, a hot straightening machine and a warm straightening machine are used in sequence for straightening. The hot straightening temperature is 250~300℃, and the straightening is performed in 2~3 passes; the warm straightening temperature is <200℃, and the straightening is performed in 3~5 passes.

3. The method for producing ultra-thin and ultra-wide low-temperature steel plates according to claim 1, characterized in that, During the cooling process, the head of the steel plate is blocked for 6m and the tail for 5m along its length to reduce the amount of water in the blocked section. Within the blocked length, the amount of water from the end to the inside of the steel plate linearly recovers from 50% to 100%.

4. The method for producing ultra-thin and ultra-wide low-temperature steel plates according to claim 1, characterized in that, When the target steel plate thickness is ≤10mm, the heating time in the soaking zone is ≥50min, the furnace pressure of the heating furnace is controlled to be 10Pa~20Pa greater than atmospheric pressure, and the temperature difference between the continuous casting billet and the slab after exiting the furnace is ≤15℃.

5. The method for producing ultra-thin and ultra-wide low-temperature steel plates according to claim 1, characterized in that, When the target steel plate thickness is ≤10mm, no warming is required during rolling; when the target steel plate thickness is >10mm, the intermediate billet thickness is ≥2.5t, where t is the target steel plate thickness in mm, and the intermediate billet is cooled in air.

6. The method for producing ultra-thin and ultra-wide low-temperature steel plates according to claim 1, characterized in that, When the target steel plate thickness is ≤10mm, the continuous casting billet thickness is ≤220mm; when the target steel plate thickness is ≤10mm and the width is ≥3200mm, the continuous casting billet width is ≥2100mm.

7. The method for producing ultra-thin and ultra-wide low-temperature steel plates according to claim 1, characterized in that, The continuously cast billet is obtained through KR desulfurization, converter smelting, LF refining, RH refining, and continuous casting. In the LF refining process, the tapping temperature is controlled at: T 液相线 +85℃≤Boiler Temperature≤T 液相线 +95℃, T 液相线 +75℃≤Continuous Casting Furnace Temperature≤T 液相线 +85℃, where T 液相线 =1536.6-90[C]-8[Si]-5[Mn]-30[P]-25[S]-1.55[Cr]-4[Ni]-18[Ti]-3[Al]-80[N], where [C], [Si], [Mn], [P], [S], [Cr], [Ni], [Ti], [Al], and [N] are 100 times the mass percentages of C, Si, Mn, P, S, Cr, Ni, Ti, Al, and N elements in the steel plate.

8. The method for producing ultra-thin and ultra-wide low-temperature steel plates according to claim 7, characterized in that, In the RH refining process, the tapping temperature is controlled as follows: T 液相线 +55℃≤Boiler Temperature≤T 液相线 +65℃, T 液相线 +45℃≤Continuous Casting Furnace Temperature≤T 液相线 +55℃.

9. The method for producing ultra-thin and ultra-wide low-temperature steel plates according to claim 8, characterized in that, Fully protected casting is achieved using a long sprue nozzle, tundish covering agent, submerged entry nozzle, and argon sealing. The long sprue nozzle is immersed in the molten steel to a depth of 200mm~300mm, and the submerged entry nozzle is immersed in the molten steel to a depth of 120mm~180mm. The tundish temperature is controlled at: T 液相线 +20℃≤Tundish Temperature≤T 液相线 At +30℃, during the casting process, the liquid level in the crystallizer fluctuates within a range of 2mm.

10. An ultra-thin, ultra-wide low-temperature steel plate, characterized in that, The steel plate, prepared by the production method according to claim 1, comprises, by mass percentage: C: 0.06%~0.09%, Si: 0.15%~0.25%, Mn: 1.10%~1.20%, P≤0.012%, S≤0.003%, Cr: 0.10%~0.20%, Ni: 0.20%~0.35%, Nb: 0.015%~0.025%, Ti: 0.005%~0.015%, Alt: 0.02%~0.05%, TO≤0.002%, N≤0.005%, H≤0.00015%, with the balance being iron and unavoidable impurities.

11. The ultra-thin and ultra-wide low-temperature steel plate according to claim 10, characterized in that, The microstructure of the steel plate consists of ferrite and bainite, with ferrite accounting for 75%~90% and bainite accounting for 10%~25%, and the average grain size is 6.5μm~7.5μm.

12. The ultra-thin and ultra-wide low-temperature steel plate according to claim 11, characterized in that, The steel plate has a yield strength of 355MPa~440MPa, a tensile strength of 490MPa~650MPa, a yield strength ratio ≤0.80, an elongation after fracture ≥22%, an impact energy of -80℃ ≥200J, a ductile-brittle transition temperature ≤-100℃, and a CTOD of -60℃ ≥1.0mm.

13. The ultra-thin and ultra-wide low-temperature steel plate according to claim 12, characterized in that, After the steel plate is welded by submerged arc welding with a heat input of 15kJ / cm~50kJ / cm, the impact energy of the heat-affected zone at -70℃ is ≥100J, and the CTOD of the coarse grain zone at -60℃ is ≥0.6mm.

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