51kg grade bca2 crack arrest steel plate and manufacturing method thereof

Abstract

The application provides a 51 Kg grade BCA2 crack arrest steel plate and a manufacturing method thereof, the steel plate comprises the following components in percentage by weight: C: 0.03%-0.07%, Si: 0.20%-0.40%, Mn: 1.65%-2.00%, P: 0.003%-0.012%, S: less than or equal to 0.005%, Al: 0.01%-0.04%, Nb: 0.005%-0.02%, Ti: 0.005%-0.015%, Ni: 0.55%-0.80%, Cu: 0.15%-0.35%, Ca: 0.010%-0.050%, N: less than or equal to 0.0050%, and the balance is Fe and inevitable impurities. The manufacturing method comprises smelting, continuous casting, heating, controlled rolling and controlled cooling, and stack cooling; the application obtains a BCA2 type crack arrest ductile steel with super-high strength grade and low carbon equivalent, meets weldability, and meets the requirements of the grade and crack arrest grade of steel for super-large container ships.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and in particular relates to a 51Kg grade BCA2 crack-arresting steel plate and its manufacturing method. It is suitable for key parts such as the side rails and upper deck keels of ultra-large container ships, and can also be used for large welded structural components with crack-arresting toughness requirements. Background Technology

[0002] The trend towards larger ships has led to increased transport capacity, lower transport costs, and reduced carbon emissions. However, this increase in ship size places higher demands on the overall performance of steel plates used in shipbuilding. This is particularly true for critical components such as the side decks, upper decks, and keels. These components must withstand repeated alternating loads from surges and container collisions during ship operation. Therefore, thick, high-strength steel plates with excellent toughness, crack arrest properties, and good weldability are required to prevent brittle fracture and ensure ship safety. To address this, the International Federation of Classification Societies (IACS) issued mandatory regulations, which came into effect on January 1, 2014, requiring all new ships with a capacity of 14,000 TEU or more to use high-crack-arrest toughness steel plates in these critical components.

[0003] Currently, leading steel companies both domestically and internationally regard marine crack-arresting steel as a strategic product.

[0004] With the increase in transport volume, new requirements have been placed on the steel grade and crack arrest toughness level of steel plates. In 2020, IACS lowered the crack arrest toughness level to BCA2 (Kca≥8000N / mm²). 3 / 2 (A mandatory evaluation index for crack-arresting steel with a thickness of 80mm or more, to meet the safety requirements of larger ship structures.)

[0005] The invention patent with publication number CN112899551A, entitled "Low-cost, high-crack-arresting and high-weldability YP355MPa grade extra-thick steel plate and its manufacturing method", provides a low-strength grade crack-arresting steel with a weight of 36kg. Due to the low steel grade, when strength is used as an important design parameter for the fracture resistance of key parts of the hatch coaming structure, it is necessary to increase the thickness of the steel plate to ensure that the strength meets the design requirements. This leads to multiple serious consequences, such as increased ship weight, poor uniformity in the manufacture of thick steel plates, increased ship operating costs, and even reduced ship safety.

[0006] The patent with publication number CN112746219A, entitled "Low Yield-to-Tear Ratio, High Toughness, and High Weldability YP500MPa Grade Steel Plate and Its Manufacturing Method," proposes a steel plate for structures such as offshore wind power and cryogenic pressure vessels, achieving a brittle fracture toughness of 6000-7000 N / mm². 3 / 2 It cannot meet the International Association of Classification Societies (IACS) standard for ultra-large container ships with a Kca ≥ 8000 N / mm². 3 / 2 Key technical requirements.

[0007] The patent with publication number CN112746158A, entitled "Low-cost, high crack arrest and high weldability YP460MPa grade thick steel plate and its manufacturing method," proposes a steel plate with a yield strength of 460MPa, whose Kca only meets the requirement of 6000N / mm². 3 / 2 The BCA1 grade cannot meet the service requirements of higher crack arrest toughness, ultra-high strength structural verification, and high fracture resistance in terms of strength and crack arrest toughness.

[0008] The patent CN112126759B, entitled "Method for Improving Impact Toughness of TMCP Steel for Shipbuilding by Utilizing Texture Control," proposes a method to improve toughness based on EH47 steel plate by controlling the deformation distribution of intermediate billets to improve texture configuration. However, this rolling process employs a four-stage cooling method (air cooling + water cooling + air cooling + water cooling), which significantly prolongs the online production time of the steel plate, greatly reduces rolling efficiency, and places extremely complex requirements on the cooling system and rolling line layout.

[0009] In November 2020, a paper titled "Microstructure and Cleavage Fracture Resistance of TMCP Extra-Thick Crack-Arresting Steel Plate" was published in the *Journal of Materials Heat Treatment*. The paper mentioned that an 80mm thick crack-arresting steel plate with a yield strength of 500MPa had a Kca index of only 7000 N / mm². 3 / 2 Its shortcoming is that it cannot meet the crack arrest toughness requirements of BCA2 level.

[0010] In summary, compared with the aforementioned publicly disclosed patents and related technologies, this invention patent has significant novelty and inventiveness. The complete and inventive manufacturing method it forms can meet the needs of stable mass production and is economically practical. Summary of the Invention

[0011] The purpose of this invention is to overcome the above-mentioned problems and deficiencies and provide a solution to the problem of deterioration of crack arrest toughness in brittle fracture caused by plane strain in thick materials, as well as the high standard requirements for multiple key material properties under mandatory specifications. This solution achieves a combination of comprehensive properties such as strength and toughness, high crack arrest performance of BCA2 type and weldability, and meets the needs of upgrading key materials for ultra-large container ships.

[0012] The objective of this invention is achieved as follows:

[0013] A 51kg grade BCA2 crack-arresting steel plate, the composition of which is as follows by weight percentage: C: 0.03%~0.07%, Si: 0.20%~0.40%, Mn: 1.65%~2.00%, P: 0.003%~0.012%, S≤0.005%, Al: 0.01%~0.04%, Nb: 0.005%~0.02%, Ti: 0.005%~0.015%, Ni: 0.55%~0.80%, Cu: 0.15%~0.35%, Ca: 0.010%~0.050%, N≤0.0050%, with the balance being Fe and unavoidable impurities.

[0014] Furthermore, the steel plate also contains one or more of the following: Cr: 0.15% to 0.35%, Mo: 0.08% to 0.25%, and RE: 0.02% to 0.08%.

[0015] Furthermore, the Mn content in the steel plate is 1.65% to 1.75%.

[0016] Furthermore, the Ca / S ratio in the steel plate is ≥5.

[0017] Furthermore, the Ceq in the steel plate is ≤0.52.

[0018] The microstructure of the steel plate is composed of acicular ferrite, fine lath bainite, and polygonal ferrite, with polygonal ferrite accounting for 15% to 20%, acicular ferrite accounting for 35% to 45%, and the remainder being fine lath bainite. The average grain size of the steel plate microstructure is 5 to 7.5 μm, and the number of large-angle grain boundaries accounts for ≥30%. Large-angle grain boundaries refer to grain boundaries with an orientation difference of >15° between adjacent grains.

[0019] The steel plate has a thickness of 50mm to 120mm; yield strength ReH: 520 to 600MPa, tensile strength Rm: 630 to 750MPa, elongation after fracture ≥23.5%; low-temperature impact energy at -60℃ ≥130J; after strain aging with 5% deformation and 1 hour of holding at 250℃, the steel plate has a low-temperature impact energy at -40℃ ≥150J; Kca at -10℃ ≥8100N / mm 3 / 2 .

[0020] The rationale for the design of the components in this invention is as follows:

[0021] Carbon (C) is an essential element for ensuring the strength of steel, especially for TMCP steel plates, where its content should be above 0.02%. However, when the content exceeds a certain level, it significantly worsens the material's low-temperature toughness, crack-arresting toughness, and weldability. Furthermore, increasing the C content also increases the tendency for retained austenite to form during cooling, further deteriorating weldability and low-temperature toughness. Therefore, the upper limit is 0.07%. The preferred C content is controlled between 0.03% and 0.07%.

[0022] Si is the main deoxidizing component in steelmaking. To achieve sufficient deoxidation, it must contain more than 0.10%. However, if it exceeds the upper limit, it will reduce the toughness of the base material and the weld. Si in solid solution form can increase strength and also increase the ductile-brittle transition temperature. Therefore, the Si content should be 0.20-0.40%.

[0023] Mn (manganese) is an essential element for ensuring the strength and toughness of steel. Mn combines with sulfur (S) to form MnS, preventing hot cracking caused by FeS formation at grain boundaries. Mn is also a good deoxidizer. While manganese is a low-cost strengthening and toughening element, its content is too low to guarantee material strength. However, when the Mn content exceeds 2.00%, it exacerbates segregation in the cast billet and worsens the low-temperature toughness of the coarse-grained heat-affected zone (CGHAZ). Therefore, the Mn content should be controlled between 1.65% and 2.00%, with 1.65% to 1.75% being the preferred range.

[0024] P (phosphorus): An unavoidable impurity element in steel, it deteriorates the steel's toughness and weldability. Studies have shown that when the P content is higher than 0.015%, its embrittlement properties are significantly improved; therefore, the upper limit is preferably 0.012%.

[0025] If the sulfur (S) content exceeds 0.01%, it will form a large number of MnS inclusions in the steel. The formation of MnS inclusions and the resulting anisotropy severely reduce the toughness, plasticity, and weldability of the steel plate. Simultaneously, increasing the S content will increase the tendency of hot-rolled steel plates to hot crack. Therefore, measures are taken during the smelting process to minimize the S content in the steel. In this invention, the upper limit of the S content is determined to be 0.005%.

[0026] Al: As a deoxidizing and grain-refining element, its content is generally above 0.01%. However, when it exceeds 0.05%, it promotes the precipitation of AlN particles at grain boundaries, causing transverse cracks. Moreover, it can also lead to crack formation when straightening the billet within the brittle temperature range (900–700°C). Therefore, the upper limit of Al content is 0.04%, and the preferred content range is 0.01–0.04%.

[0027] Ni: As an austenite stabilizing element, increasing Ni can improve the solid solution strengthening effect. Nickel can lower the ferrite phase transformation temperature, refine the ferrite grain size, and play a role in grain refinement strengthening. In addition, it can promote the formation of acicular ferrite. While generating phase transformation strengthening, its large-angle grain boundary characteristics can significantly improve the fracture toughness and crack arrest ability of the material. Therefore, the preferred Ni content is between 0.55% and 0.80%.

[0028] Cu can significantly improve the hardenability and corrosion resistance of steel plates. It is also an austenite stabilizing element in steel. Appropriate addition can refine the microstructure of TMCP steel plates and improve low-temperature toughness. However, excessive addition will cause a tendency for "copper embrittlement", making it easy for cracks to appear on the surface and inside of the billet, reducing the mechanical properties of the rolled steel plate, and reducing toughness, causing the steel plate to become embrittled. Therefore, the Cu content in this invention is controlled at 0.15% to 0.35%.

[0029] Cr: Chromium is a weak carbide-forming element. Adding a certain amount of Cr can improve the hardenability of steel plates and promote the formation of strengthening structures represented by bainite. It can replace some of the strengthening elements such as C and Mn, reducing the toughness deterioration caused by the former's increased strength. However, adding too much Cr can also lead to a deterioration in weldability. Therefore, under the TMCP process, the preferred Cr content range is between 0.15% and 0.35%.

[0030] Mo: As an element that significantly improves hardenability and bainitrification tendency, appropriate addition of Mo to steel can inhibit the formation of pearlite. Its effect on improving the strengthening of steel is similar to that of Cr, which improves the strength of steel through the formation of carbides. However, excessive addition can also cause deterioration of weldability and low-temperature toughness. Therefore, the preferred Mo content range is between 0.08% and 0.25%.

[0031] Nitrogen (Nb) is one of the key elements for grain refinement and strengthening. Its grain refinement effect manifests in two aspects: firstly, it significantly delays austenite recrystallization, increasing the recrystallization temperature and preventing the growth of recrystallized austenite; secondly, as the rolling temperature decreases, Nb's C and N compounds disperse and precipitate before the austenite transforms into ferrite, becoming ferrite nucleation sites. This allows ferrite to form under low supercooling, making it less prone to growth and refining the ferrite grain size. As an element with an extended non-recrystallization temperature range, Nb can improve crack propagation paths during fracture by refining the grain structure, thereby increasing fracture absorption work and improving crack arrest toughness. This invention controls Nb levels to be 0.005%–0.02%.

[0032] Ti: Trace amounts of titanium combine with C and N in steel to form fine and stable C and N compound particles. During the slab heating process, this effectively prevents austenite grain coarsening, and during welding, it suppresses grain coarsening caused by welding heat, improving the matrix structure and the low-temperature toughness of the weld heat-affected zone. This invention controls Ti content to be 0.005%–0.015%.

[0033] Ca: Calcium treatment modifies inclusions. CaO combines with Al2O3 inclusions to form calcium aluminate, which floats into the slag. Simultaneously, Ca combines with S to form CaS, which can coat alumina into spherical shapes, reducing the amount of MnS in the steel and thus improving the transverse properties of the steel plate. The fine-sized CaO formed in the steel also refines the grains and improves the material's toughness. In this invention, the preferred Ca content is 0.010%–0.050%, and the Ca / S ratio is ≥5.

[0034] N can form fine precipitates with Nb, Ti, and V, which can strengthen and refine the grains, improving strength and toughness. However, excessive N content can deteriorate weldability and easily cause strain aging, which can worsen low-temperature toughness. It is advisable to control the N content to ≤0.0050%.

[0035] RE: Rare earth elements can significantly reduce the deteriorating behavior of low-melting-point elements in steel caused by grain boundary segregation. They improve strength and toughness by altering the morphology of inclusions and refining the steel's microstructure. They also change the distribution and morphology of sulfides in steel, particularly MnS inclusions which tend to form fine stripes during hot rolling, resulting in significant directionality and severely deteriorating transverse properties. Simultaneously, a certain amount of rare earth elements can also improve the weldability and corrosion resistance of steel; the preferred content is 0.02%–0.08%.

[0036] The second technical solution of the present invention is to provide a manufacturing method for 51Kg grade BCA2 crack-arresting steel plate, including smelting, continuous casting, heating, controlled rolling and controlled cooling, and stacking cooling;

[0037] Smelting:

[0038] The steel of this invention uses deep desulfurized molten iron with a sulfur content of ≤0.002%. After the molten iron arrives at the converter, it is smelted using a process that combines "double slag" dephosphorization with "slag removal" of the molten steel after the furnace. The final slag basicity is controlled at R=3.2~4.1. Through effective slag blocking operation, a large amount of slag is prevented from falling, and the steel discharge time is not less than 5 minutes.

[0039] The smelting process employs a high-pulling carbon single-point blowing method. Within the converter, the main elements of the steel are adjusted to the range specified in this invention, and other alloying components are added as required for smelting.

[0040] The molten steel removed from the converter undergoes secondary refining to further reduce the content of harmful impurities such as O, S, and non-metallic inclusions. During the LF refining process, aluminum granules, silicon carbide, and calcium carbide are used to adjust the slag, and the final slag basicity is controlled above 2.4. After LF, Ca treatment is performed, and wire is fed at a depth of 0.8–1.2 m per ton of molten steel.

[0041] Continuous casting:

[0042] During continuous casting, the superheat is ≤25℃. Light pressure and electromagnetic stirring at the end of the crystallizer are used. The secondary cooling adopts segmented cooling. The cooling rate of the strong cooling in the vertical section is 10.0~20.0℃ / s, and the cooling rate of the weak cooling in the bending section is 3.0~5.0℃ / s. The continuous casting billet pulling speed is 0.8~1.2m / min, and the thickness of the cast slab is 300~360mm.

[0043] heating:

[0044] The heating temperature is 1130–1180℃; the continuously cast billet is heated to a temperature not lower than 1100℃ but not higher than 1180℃. This is because temperatures below 1100℃ are insufficient for the alloying elements to completely dissolve into the austenite, making it impossible to guarantee the final rolling temperature required for hot rolling. Temperatures above 1200℃ cause significant coarsening of the original austenite grains, which reduces the low-temperature toughness of the steel plate.

[0045] Controlled rolling and controlled cooling:

[0046] It includes a two-stage rolling process: roughing and finishing.

[0047] The roughing rolling start temperature is 950℃~1100℃, and the cumulative reduction rate during the roughing rolling stage is not less than 35%. The finishing rolling start temperature is 800~850℃, and the finishing rolling finish temperature is 750~780℃, with a cumulative reduction rate of not less than 45% during the finishing rolling stage. Preferably, the cumulative reduction rate during the finishing rolling stage is 45%~65%, the purpose of which is to fully flatten the austenite in the non-recrystallized region, accumulate dislocation and deformation energy, and refine the grains for subsequent phase transformation. When the cumulative reduction rate during the finishing rolling stage is <40%, the total reduction ratio is too low, the austenite is not sufficiently flattened, and its size is too large. When the cumulative reduction rate during the finishing rolling stage is >65%, the total reduction ratio during finishing rolling is large, leading to a decrease in the total reduction ratio during roughing rolling and insufficient recrystallization. It is necessary to balance the total reduction ratios of roughing and finishing rolling, and the number of finishing rolling passes will also increase, making temperature control during finishing rolling more difficult. Immediately after rolling, the steel plate enters a rapid cooling device with an initial cooling temperature between 680 and 750°C and a cooling rate controlled at 3 to 8°C / s, cooling the steel plate to below 580°C.

[0048] Stacking and slow cooling: After the steel plates come off the production line, they are stacked and slow cooled. The initial temperature of slow cooling is between 250 and 450°C, and the stacking time is not less than 48 hours.

[0049] The beneficial effects of this invention are as follows:

[0050] (1) The steel of this invention is based on the chemical composition system of the traditional 51kg steel grade of the classification society. The fine grain structure of the multiphase structure is mainly composed of acicular ferrite and lath bainite, which is a high strength and toughness structure and the proportion of high-angle grain boundaries is controlled at more than 30%. This results in BCA2 type crack-arresting tough steel with ultra-high strength and low carbon equivalent (Ceq≤0.52). Compared with the international classification society standard of Ceq≤0.54, it has a lower carbon equivalent, meets the requirements of easy weldability, and meets the requirements of ultra-large container ships for improved steel grade and crack-arresting grade.

[0051] (2) Add appropriate amounts of Ni, Cu and Mo to compensate for the insufficient strength caused by low carbon content, and avoid the deterioration of low temperature toughness, crack arrest performance and weldability.

[0052] (3) By adding appropriate amounts of toughening elements and using precise smelting and TMCP processes, the combined effects of multiple strengthening methods such as fine grains and phase transformation are achieved. The crack-arresting steel obtained by the composition design and production method of this invention has a microstructure of acicular ferrite + fine lath bainite + polygonal ferrite, with an average grain size of about 5 to 7.5 μm. Low-temperature heating and high cumulative reduction rate in multiple stages have effectively controlled the grain size of the original austenite grains and the microstructure after phase transformation, ensuring the acquisition of strength and toughness.

[0053] (5) The production process of the steel of the present invention is simple and feasible, suitable for industrial mass production, and can effectively ensure production efficiency. Attached Figure Description

[0054] Figure 1 This is a metallographic image of the microstructure of Embodiment 1 of the present invention. Detailed Implementation

[0055] The present invention will be further illustrated below through examples.

[0056] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, continuous casting, controlled rolling and cooling, and stacking cooling.

[0057] Heating: Heating temperature 1130~1180℃;

[0058] Controlled rolling and controlled cooling: This includes two-stage rolling processes: roughing and finishing.

[0059] The roughing rolling temperature is 950℃~1100℃, and the cumulative reduction rate during the roughing rolling stage is not less than 35%; the finishing rolling temperature is 800~850℃, the finishing rolling temperature is 750~780℃, and the cumulative reduction rate during the finishing rolling stage is not less than 45%; immediately after rolling, the steel plate enters rapid cooling, with an initial cooling temperature of 680~750℃ and a cooling rate controlled at 3~8℃ / s, cooling the steel plate to below 580℃;

[0060] Stacking and slow cooling: After the steel plates come off the production line, they are stacked and slow cooled. The initial temperature of slow cooling is between 250 and 450°C, and the stacking time is not less than 48 hours.

[0061] Furthermore, the cumulative reduction rate during the finishing rolling stage is 45%–65%.

[0062] Further; smelting:

[0063] Deeply desulfurized molten iron with a sulfur content ≤0.002% is used. After the molten iron arrives at the converter, it is smelted using a process that combines "double slag" dephosphorization with "slag removal" of the molten steel after the furnace. The final slag basicity is controlled at R=3.2~4.1, and the steel charging time is not less than 5 minutes.

[0064] The smelting process employs a high-pulling carbon single-point blowing method for production;

[0065] The molten steel removed from the converter undergoes secondary refining; the basicity of the final slag after LF refining is controlled above 2.4; after LF, Ca treatment is carried out, and wire is fed 0.8 to 1.2m per ton of molten steel.

[0066] Furthermore, during continuous casting, the superheat is ≤25℃, light pressure and electromagnetic stirring at the end of the crystallizer are adopted, and the secondary cooling adopts segmented cooling. The cooling rate of the strong cooling in the vertical section is 10~20℃ / s, the cooling rate of the weak cooling in the bending section is 3~5℃ / s, the continuous casting billet pulling speed is 0.8~1.2m / min, and the thickness of the cast slab is 300~360mm.

[0067] The composition of the steel in this embodiment is shown in Table 1. The smelting process of the steel in this embodiment is shown in Table 2. The main process parameters of the steel in this embodiment are shown in Table 3. The properties of the steel in this embodiment are shown in Table 4. The microstructure of the steel in this embodiment is shown in Table 5.

[0068] Table 1. Composition (wt%) of steel in embodiments of the present invention

[0069]

[0070] Table 2 Steelmaking process in embodiments of the present invention

[0071]

[0072] Table 3 Main process parameters of steel in the embodiments of the present invention

[0073]

[0074] Table 4 Properties of steel in embodiments of the present invention

[0075]

[0076] Table 5. Microstructure of steel in the embodiments of the present invention

[0077]

[0078] As can be seen from the above, the 51kg grade BCA2 type crack-arresting steel plate produced using this invention has a thickness of 50mm to 120mm, a yield strength ReH of 520 to 600MPa, a tensile strength Rm of 630 to 750MPa, and an elongation after fracture ≥23.5%; a low-temperature impact energy of ≥130J at -60℃; after strain aging with 5% deformation and 1 hour of heat treatment at 250℃, the steel plate has a low-temperature impact energy of ≥150J at -40℃; and a Kca of ≥8100N / mm at -10℃. 3 / 2； It possesses excellent comprehensive mechanical properties, crack arrest properties, and weldability, meeting the 51kg strength index and crack arrest toughness requirements of the corresponding shipbuilding steel grade.

[0079] 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 51 Kg grade BCA2 fracture arrest steel plate characterized in that, The steel plate has the following components in percentage by weight: C: 0.03%~0.07%, Si: 0.20%~0.40%, Mn: 1.65%~2.00%, P: 0.003%~0.012%, S≤0.005%, Al: 0.01%~0.04%, Nb: 0.005%~0.02%, Ti: 0.005%~0.015%, Ni: 0.55%~0.80%, Cu: 0.15%~0.35%, Ca: 0.010%~0.050%, N≤0.0050%, and the balance of Fe and inevitable impurities; The Ca / S in the steel plate is greater than or equal to 5.33; the microstructure of the steel plate is a microstructure of acicular ferrite + fine lath bainite + polygonal ferrite, wherein the proportion of the polygonal ferrite is 15%~20%, the proportion of the acicular ferrite is 35%~45%, and the rest is the fine lath bainite; the average grain size of the microstructure of the steel plate is 5~7.5μm, and the proportion of the number of high-angle grain boundaries is greater than or equal to 30%.

2. A 51 Kg grade BCA2 fracture arrest steel plate as claimed in claim 1, wherein, The steel plate further contains one or more than two of Cr: 0.15%~0.35%, Mo: 0.08%~0.25%, and RE: 0.02%~0.08%.

3. A 51 Kg grade BCA2 fracture arrest steel plate as claimed in claim 1, wherein, The Mn in the steel plate is 1.65%~1.75%.

4. A 51 Kg grade BCA2 arrestant steel sheet as claimed in claim 1, wherein, The thickness of the steel plate is 50mm~120mm; the yield strength ReH is 520~600MPa, the tensile strength Rm is 630~750MPa, and the elongation after fracture is greater than or equal to 23.5%; the low-temperature impact energy at-60℃ is greater than or equal to 130J; The low-temperature impact energy of the steel plate is ≥ 150 J at -40 °C after strain aging with 5% deformation and 1 hour holding at 250 °C; Kca≥ 8100 N / mm at -10 °C 3 / 2 . 5.A method for manufacturing the 51Kg grade BCA2 crack arrest steel plate according to any one of claims 1-4, comprising smelting, continuous casting, heating, controlled rolling and controlled cooling, and stack cooling; characterized in that: the heating temperature is 1130~1180℃; the controlled rolling and controlled cooling comprises two-stage rolling including rough rolling and finish rolling; the rough rolling temperature is 950℃~1100℃, and the cumulative rolling reduction in the rough rolling stage is not less than 35%; the finish rolling temperature is 800~850℃, the final rolling temperature is 750~780℃, and the cumulative rolling reduction in the finish rolling stage is not less than 45%; the steel plate is immediately subjected to rapid cooling after rolling, the cooling temperature is 680~750℃, and the cooling rate is controlled to be 3~8℃ / s, so that the steel plate is cooled to below 580℃; the stack slow cooling: the steel plate is subjected to stack slow cooling after being discharged, the slow cooling starting temperature is 250~450℃, and the stack time is not less than 48 hours.

6. The method of claim 5, wherein the 51 Kg grade BCA2 crack arrest steel plate is manufactured by the steps of: wherein, the cumulative rolling reduction in the finish rolling stage is 45%~65%. 7.A method for manufacturing the 51Kg grade BCA2 crack arrest steel plate according to claim 5, characterized in that: the smelting: deep desulfurization molten iron is used, and the sulfur content is less than or equal to 0.002%; after the molten iron is transferred to the converter, a process combining "double slag” dephosphorization with "skimming slag” after the molten steel in the converter is used, the final slag basicity is controlled to be R=3.2~4.1, and the tapping time is not less than 5min; the smelting process is produced by high carbon extraction one-point blowing; the molten steel moved out of the converter is subjected to secondary refining; the LF refining final slag basicity is controlled to be greater than or equal to 2.4; Ca treatment is performed after the LF ends, and 0.8~1.2m of wire is fed per ton of molten steel.

8. The method of claim 5, wherein the 51 Kg grade BCA2 crack arrest steel plate is manufactured by the steps of: In the continuous casting process, the superheat is ≤25℃, light press-down and electromagnetic stirring at the end of the crystallizer are adopted, the cooling rate of the vertical section is 10~20℃ / s, the cooling rate of the curved section is 3~5℃ / s, the casting speed of the continuous casting billet is 0.8~1.2m / min, and the thickness of the cast slab is 300~360mm. ​

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