Eh40 crack arrest steel plate for container ship and manufacturing method therefor

Abstract

The present invention belongs to the field of ferrous metallurgy, and relates to an EH40 crack arrest steel plate for a container ship and a manufacturing method therefor. The steel plate comprises the following chemical components in percentages by mass: C: 0.03-0.06%, Si: 0.15-0.30%, Mn: 1.10-1.30%, P≤0.0080%, S≤0.0030%, Nb: 0.010-0.020%, V: 0.035-0.055%, Ti: 0.008-0.020%, Ni: 0.45-0.65%, Cu: 0.10-0.30%, Ca: 0.0005-0.0020%, H≤0.00015%, Ceq≤0.42%, and the balance of Fe and inevitable impurity elements. The steel plate has a yield strength of 409-440 MPa, a tensile strength ranging from 595 MPa to 625 MPa, a yield ratio of 0.84-0.88, and a longitudinal impact toughness value at -60ºC of greater than or equal to 250 J; and during instrumented impact testing, the energy E3 for crack arrest / unstable propagation and the absorbed energy E4 after crack arrest / unstable propagation account for more than or equal to 20% of the total impact absorption energy, and therefore the steel plate has good crack arrest performance.

Description

EH40 crack arrest steel plate for container ship and manufacturing method thereof Technical Field

[0001] The invention belongs to the field of steel metallurgy, and in particular relates to EH40 crack arrest steel for container ships and a manufacturing method thereof. Background Art

[0002] To reduce shipping costs, container ships are rapidly becoming larger. Large container ships of 7,000 TEU and above are already widely used, and the construction of ultra-large container ships of 18,000-24,000 TEU has become the norm. The increasing size and ultra-largeness of container ships requires the use of high-strength thick plates of EH36 and above during construction to ensure the strength and rigidity of the hull structure. As strength and thickness increase, the brittle fracture tendency of the steel plate increases, and once brittle fracture occurs, it often leads to catastrophic consequences. In order to prevent fracture accidents, in 2013, the International Association of Classification Societies (IACS) and some classification societies drew on Japan's development experience and formulated container ship steel plate standards specifically for key parts of large container ships. They also listed brittle fracture tests such as ESSO tests and double tensile tests (currently replaced by small-scale tests such as drop hammers in actual deliveries) as methods for testing the crack-arresting performance of steel plates, and incorporated them into classification society specifications, requiring the use of high-strength steel plates with crack-arresting properties in some key parts of container ships, such as the main deck, side top plating, hatch coamings, etc.

[0003] After searching, it was found that there are many patents involving crack arrest steel for container ships, most of which are produced using the TMCP process:

[0004] Chinese patent CN101307412A discloses a "steel plate with excellent brittle crack propagation arresting properties and toughness in the center of the plate thickness and its manufacturing method". This method adds special elements such as Mo, REM, B, etc. to the steel in addition to conventional alloying elements, and performs TMCP production to obtain EH47 grade crack arresting steel. Chinese patent CN101341269A "High-strength thick steel plate with excellent crack arresting properties" mainly controls the differences in the structure, grain boundary density and texture of the steel plate in the thickness direction, so that when the steel plate cracks, the crack propagation direction at different thickness positions will be different, thereby increasing the crack propagation resistance and improving the crack arresting performance of the steel plate. However, the essence is that the cooling capacity of the thick plate produced by it is insufficient, and the grain size of the structure at 1 / 2 of the plate thickness is coarse. This structure is not conducive to strength and toughness, and also has a negative impact on the crack arresting properties of the steel plate. This method does not fundamentally improve the toughness structure at the 1 / 2 position. Chinese patent CN102994874A, "500Ma Yield Strength High Crack Arrest Toughness Steel Plate and Production Method Thereof," discloses a method for refining grains and preventing crack propagation by rough rolling followed by rapid cooling to near the Ar3 phase transition point for finish rolling. Chinese patent CN 112501504 discloses "A BCA2 Grade Crack Arrest Steel Plate for Container Ships and Its Manufacturing Method," which also utilizes the TMCP process to adjust microstructure, increase crack initiation energy, and enhance crack propagation paths, achieving high crack arrestability.

[0005] These technologies primarily involve the TMCP process for steel plates with yield strengths of 460MPa and 500MPa, thicknesses ≤100mm, and the addition of sufficient alloying elements to compensate for microstructural variations across the TMCP thickness. These include the addition of elements such as Mo, REM, and B, which are not yet permitted by classification society standards. Studies have shown that thinner steel plates, higher strength, higher carbon equivalent, and more uniform microstructure across the thickness of the plate are more conducive to improving the crack arrestability assessment value, Kca, in ESSO testing or double tensile testing. However, existing technologies rarely address EH40 crack arrest steel, which has a lower strength grade of 400MPa and a low carbon equivalent limit (plate thickness ≥100mm, Ceq ≤0.45%), for thicknesses greater than 100mm.

[0006] Therefore, existing technology is still unable to meet the production requirements of EH40 crack arrest steel with a maximum thickness of 120mm. To reduce costs and ensure the performance stability and safety of the steel plate during use, it is necessary to conduct reasonable supporting research and analysis considering the relationship between the strength, plasticity, low-temperature toughness, crack arrest performance of EH40 steel plate and its composition and processing, so as to develop an EH40 crack arrest steel for container ships.

[0007] Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide an EH40 crack arrest steel for container ships with a maximum thickness of 120 mm and a manufacturing method thereof, in view of the above-mentioned prior art. The steel plate has high low-temperature toughness, brittle fracture resistance and excellent crack arrest performance.

[0009] The technical solution adopted by the present invention to solve the above problems is: an EH40 crack arrest steel for container ships, the chemical composition of the steel plate is as follows by mass: C: 0.03-0.06%, Si: 0.15-0.30%, Mn: 1.10-1.30%, P: ≤0.0080%, S: ≤0.0030%, Nb: 0.010-0.020%, V: 0.035-0.055%, Ti: 0.008-0.020%, Ni: 0.45-0.65%, Cu: 0.10-0.30%, Ca: 0.0005-0.0020%, H: ≤0.00015%, Ceq ≤0.42%, and the balance is Fe and unavoidable impurity elements.

[0010] The reasons for limiting the steel composition in the present invention are as follows:

[0011] C can significantly improve the strength of thick plate steel, but excessive C content is detrimental to the steel's low-temperature impact resistance, low-temperature strain aging performance, weldability, and crack arrestability. Considering that the strength of the present invention is moderate, and toughness and crack arrestability are the difficulties of the present invention, the present invention adopts low carbon control, and the C content is controlled between 0.03 and 0.06%.

[0012] Si is mainly used for deoxidation. Although its addition amount is determined according to different smelting methods, it must be above 0.15% to obtain good steel plate performance. However, if it exceeds 0.30%, it will cause segregation at 1 / 2 of the thickness and affect welding performance. Therefore, its upper limit is set at 0.30%.

[0013] Mn in the steel delays the transformation of austenite to ferrite, contributing to ferrite refinement and improved strength and toughness. However, at low manganese levels, this effect is less pronounced, resulting in lower strength and toughness in the steel. Excessive manganese levels can lead to segregation in the continuous casting, poor toughness, and reduced weldability. Therefore, the present invention specifies a manganese addition level of 1.10-1.30%, taking into account classification society carbon equivalent and cold crack sensitivity index limits, the low-carbon content of the present invention, and the production of extra-thick plate.

[0014] The solute drag effect of Nb and the pinning effect of Nb(C,N) on austenite grain boundaries both inhibit recrystallization of deformed austenite, expand the non-recrystallization range of austenite, and reduce the waiting time during the production of extra-thick plate. It also forms precipitates during cooling, thereby improving both strength and toughness, and also enhancing the corrosion resistance of the steel. Typically, the addition amount is no less than 0.010%, considering the addition of V, another grain-refining element, and reducing costs. Therefore, the present invention specifies a niobium content within the range of 0.010-0.020%.

[0015] Vanadium is an excellent deoxidizer for steel and an effective grain-refining element, improving its strength and toughness. It forms precipitates during cooling or tempering, effectively refining the grains. Therefore, the present invention specifies that the vanadium content be within the range of 0.035% to 0.055%.

[0016] Ti is used to fix nitrogen in steel. Under appropriate conditions, titanium and nitrogen form titanium nitride, which prevents grain growth during heating / rolling / welding of the steel billet and improves the toughness of the base material and the heat-affected zone of the weld. When titanium is less than 0.008%, the nitrogen fixation effect is poor. When it exceeds 0.020%, the nitrogen fixation effect reaches saturation. Excess titanium will deteriorate the toughness of the steel. When the atomic ratio of Ti to N in the steel is 1:1, the TiN particles are the finest and most dispersed, and the refining effect on high-temperature austenite grains is the strongest, which not only achieves excellent toughness but also enables high-energy-wire welding of more than 30 kJ / cm. At this time, the weight ratio of Ti to N is 3.42. The trace addition of Ti can also improve the corrosion resistance of the steel. The present invention stipulates that the minimum is 0.008% and the upper limit is 0.020%.

[0017] Nickel (Ni) is an element that increases steel strength and is the most commonly used element to effectively improve steel's low-temperature toughness. Furthermore, its combined action with the copper and phosphorus in steel contributes to its corrosion resistance. However, excessive nickel addition significantly increases steel cost. Therefore, the nickel content in this specification is specified to be between 0.45% and 0.65%.

[0018] Cu can inhibit the formation of polygonal ferrite and pearlite, promote low-temperature microstructural transformation, and increase steel strength. However, excessive Cu content can easily cause surface cracks and significantly increase costs. Therefore, the copper content in the present invention is controlled to 0.10-0.30%.

[0019] Although P can improve corrosion resistance, it will reduce low-temperature toughness and hinder weldability, which is inappropriate for structural steel. The present invention stipulates that its content is controlled below 0.0080%.

[0020] S forms MnS inclusions, which also lead to central segregation and have an adverse effect on corrosion resistance. The present invention stipulates that it is controlled below 0.0030%.

[0021] H: A harmful gas element. High H content can easily produce white spots, reduce the steel's low-temperature toughness and crack arrest properties, and seriously damage its performance. To improve the overall performance of the steel, the present invention strictly controls the H content to within 0.00015%.

[0022] Ceq: Carbon equivalent should be calculated based on the heat analysis using the following formula,

[0023] The present invention is EH40 steel, which further limits Ceq to 0.42% based on the requirements of classification society specifications, ensuring excellent weldability of extra-thick plate EH40.

[0024] The present invention further provides a method for preparing the above-mentioned EH40 crack arrest steel for container ships, the specific process of which is as follows:

[0025] (1) Smelting and continuous casting process: The preferred melting method is to smelt the raw materials in sequence through BOF smelting, LF refining, RH refining, and slab continuous casting machine to produce continuous casting billets. The billet thickness is 370mm or above, and the casting temperature is controlled at 5-25°C above the liquidus line. Dynamic soft reduction is implemented during the casting process. The continuous casting billets are slowly cooled, the surface is cleaned, and longer continuous casting billets are flame cut into preset sizes;

[0026] (2) Slab slow cooling and cleaning and grinding process: After the slab comes off the production line, it must be slow cooled. Preferably, the hot slab is stacked with a cover or placed in a pit for slow cooling. The slow cooling starting temperature must be no less than 600°C and the cooling time must be no less than 72 hours.

[0027] (3) Heating process: The elements are fully dissolved and homogenized. Preferably, the ingot is heated in a walking beam furnace at a heating rate of 10 to 12 min / cm. The maximum temperature of the preheating section is ≤ 650°C, the preheating section lasts 3 hours, the first heating section is at a temperature of 1000 to 1180°C, the first heating section is at a temperature of 2 hours, the second heating section is at a temperature of 1220 to 1240°C, the soaking section is at a temperature of 1200 to 1220°C, and the total time of the second heating section and the soaking section is 2 to 4 hours.

[0028] (4) Rolling process: After the billet is removed from the furnace, it is descaled with high-pressure water (the descaling system pressure is not less than 20 MPa), and then two-stage rolling is carried out; the first stage of rolling is rough rolling, the starting rolling temperature is in the range of 1080-1150℃, the target thickness of rough rolling is ≥1.7t, t is the thickness of the finished steel plate, and the single-pass reduction is between 40-60mm; then the second stage of rolling (finishing rolling) is carried out, the starting rolling temperature is in the range of 860-900℃, and the single-pass reduction rate is ≥10% for at least 3 passes;

[0029] (5) ACC cooling process: After rolling, the steel plate is cooled at a cooling rate of 3-5℃ / s, and the final cooling temperature of the steel plate is 660-700℃, followed by stacking cooling.

[0030] (6) Quenching heat treatment process: After rolling, the steel plate is subjected to offline quenching treatment. The quenching temperature is 900±(0-10)℃, and the furnace time is 1.8~2.0min / mm. To ensure the uniformity of the steel plate, the temperature control accuracy is ±10℃.

[0031] (7) Tempering heat treatment process: After quenching, the steel plate is tempered at 640±(0-10)℃ and kept in the furnace for 2.8~3.5min / mn. This allows enough time for the carbon in the quenched martensite to fully diffuse and form ferrite. During the subsequent long-term holding process, the ferrite lamellae grow slightly, and the supersaturated carbon in the original martensite precipitates to form cementite films and cementite particles. This achieves the best match between the strength and toughness of the steel plate and achieves the purpose of the steel plate's engineering application. Due to the inevitable segregation of carbon and manganese at 1 / 2 of the steel plate's thickness, some degenerate pearlite exists. Since the degenerate pearlite occupies a small proportion of the cross-section in the entire thickness direction, it has no significant adverse effect on the performance of the steel plate.

[0032] This invention utilizes a low-cost composition design method and the TMCP+QT process to produce an EH40 crack-arrest steel for container ships. The structure is primarily composed of ferrite lamellae and cementite films or fine carbide particles. Except for unavoidable carbon and manganese segregation at 1 / 2 the thickness and some degenerate pearlite, pearlite is absent elsewhere. The present invention utilizes a low carbon content, appropriate additions of alloying elements such as Mn and Ni, and increased Cu content to improve the strength and toughness of the steel plate. The addition of Ni significantly increases the toughness of the steel and refines the grain size. The refined grains, through the increased number of grain boundaries, increase the difficulty of crack propagation and enhance the crack arrestability of the steel plate. Not adding Cr is to prevent Cr from forming carbides, reduce the C content of the matrix during quenching, and result in insufficient hardenability of the steel plate. In order to refine the final grains, the present invention, in addition to taking measures to increase effective refining elements, further controls the process, which are manifested in: 1) a large rough rolling pass reduction rate, giving the steel billet a greater penetration force in the thickness direction; 2) a higher finishing temperature, which allows the finishing pass reduction rate to be increased during rolling, reducing the total number of rolling passes, ensuring that the storage energy of non-recrystallized austenite is increased and segregation is reduced within the allowable range of the rolling mill load; 3) quenching treatment is performed after rolling, and the austenite is homogenized again, followed by appropriate tempering treatment, which ensures that the finished steel plate obtains ideal strength and toughness, and overcomes the inevitable structural gradient in the thickness direction of the TMCP steel plate through strong cooling after rolling; 4) Usually, the production of EH47 and EH40 crack arrest steels by TMCP requires obtaining fine needle-shaped ferrite, and the cooling rate and final cooling temperature range for forming this target structure are relatively small. To achieve the desired effect, the water volume often needs to be adjusted according to the thickness, width, and length of the steel plate, which greatly complicates on-site production operations. Furthermore, the process is subject to fluctuations in weather and water temperature, leading to poor batch production stability and inconsistent steel plate performance. The QT process of the present invention offers a wide production window, is easy to operate, and ensures batch production stability.

[0033] The extra-thick plate obtained by the present invention not only meets the conventional mechanical property EH40 level, but also maintains excellent strength and impact properties at 1 / 2 thickness, and still has good low-temperature toughness and crack arrest performance at -60°C. Specific performance is: under the transverse tensile test, the yield strength is between 506 and 535 MPa, the tensile strength is between 595 and 624 MPa, the yield strength ratio is between 0.84 and 0.88, and the elongation is moderate. The tensile performance at 1 / 2 thickness of the steel plate is not much different from the performance at 1 / 4 thickness of the steel plate, and the strength difference is less than 20 MPa. The -60°C longitudinal impact toughness value is ≥250J.

[0034] In the oscillographic impact test, the impact absorption energy is divided into crack initiation energy (E1), steady-state propagation energy (E2), crack arrest / instability propagation energy (E3), and post-crack arrest / instability propagation energy (E4) based on the impact-load characteristic. The greater the crack arrest / instability propagation energy (E3) and post-crack arrest / instability propagation energy (E4), the better the crack arrest performance. The present invention achieves excellent crack arrest performance when the ratio of E3+E4 to the total impact absorption energy is ≥20%. This fully meets the comprehensive performance requirements of marine engineering steel, including high strength, high impact toughness, corrosion resistance, low-temperature resistance, and crack arrest. The production process is stable and easily adaptable to mass production.

[0035] The present invention has the following advantages:

[0036] 1. By controlling low C, P, and S, and relatively low Mn, we can avoid the negative effects of center segregation during the continuous casting process of the billet. By adjusting the high temperature rolling and tempering treatment, we can further homogenize the material and reduce the negative effects of segregation at 1 / 2 of the thickness.

[0037] 2. The addition of Ni content in steel effectively improves hardenability and toughness, making up for the insufficient strength caused by low C.

[0038] 3. Adding sufficient V can ensure precipitation of thick plates during cooling, forming a precipitation strengthening effect, hindering the growth of ferrite lamellae, changing the crack propagation path, and improving the crack arrest performance of the steel plate.

[0039] 4. Although the QT process adds a heat treatment process to the production of extra-thick plates, it is the first choice for high-quality steel. For EH40 crack arrest steel with a thickness of up to 120mm, QT is an ideal process to replace TMCP and ensure the stability of mass production.

[0040] 5. After the billet is taken out of the furnace, it is descaled by high-pressure water with a pressure of not less than 20MPa in the descaling system, which effectively avoids the influence of Ni and Nb on the surface quality of the steel plate.

[0041] 6. Current ESSO or dual tensile testing specimens are relatively large, measuring thickness x 500 mm x 500 mm. A set of three test plates weighs 0.65 tons, significantly reducing the yield rate of steel plates. This also leads to high testing costs, long test cycles, and significant labor and material resources. This invention uses an oscillographic impact test to evaluate the crack arrest performance of smaller specimens, reducing testing costs and improving the yield rate of steel plates. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 shows the microstructure of the steel plate at 1 / 4 of the thickness direction of Example 1; the original austenite grain size is small and uniform, the austenite grain size is between 10-20 μm, and the ferrite lamellae perpendicular to the austenite grain boundaries are even finer.

[0043] Figure 2 is a further magnified microstructure of the steel plate of Example 1 at 1 / 4 of the thickness direction under a scanning electron microscope. The slender cementite film is distributed between the fine ferrite lamellae. The average ferrite lamellae are about 1 μm, and extremely fine carbide particles are distributed at the triangular grain boundaries and within the ferrite lamellae.

[0044] Figure 3 shows the microstructure of the steel plate at 1 / 2 of the thickness direction of Example 1. At 1 / 2 of the thickness, the ferrite has grown significantly, but still maintains a certain orientation relationship. Cementite and other carbide particles are concentrated at the triangular grain boundaries, and there is a very small amount of degenerate pearlite.

[0045] Figure 4 shows the typical acicular ferrite morphology of EH47 crack arrest steel produced by TMCP. In comparison, the grains of acicular ferrite produced by TMCP are larger than the ferrite lamellae produced by quenching and tempering of the present invention, and the grain boundary density is also lower than that of EH40 steel produced by quenching and tempering of the present invention.

[0046] FIG5 is a load-displacement diagram of a typical oscillographic impact test in Example 4 of the present invention. The crack arrest / instability expansion work E3 and the absorption work after crack arrest / instability expansion E4 are relatively high, and E3+E4 accounts for 21.5% of the total impact absorption energy.

[0047] Figure 6 shows the non-crack arrest steel produced by quenching and tempering. The impact value at -60℃ is 232J. The crack arrest / instability expansion work E3 and the absorbed energy after crack arrest / instability expansion E4 account for a small proportion of only 11.67%. This steel has anti-brittle fracture performance, but poor crack arrest performance. DETAILED DESCRIPTION

[0048] The present invention is further described in detail below with reference to the examples. The examples are illustrative and intended to explain the present invention, but are not to be construed as limiting the present invention.

[0049] The production process of EH40 crack arrest steel for container ships in this embodiment is: converter or electric furnace steelmaking -> LF refining -> RH high vacuum degassing -> Ca treatment -> continuous casting -> slow cooling treatment of ingot -> ingot surface cleaning -> heating -> high temperature rolling (cooling) -> quenching -> tempering.

[0050] The production method of EH40 crack arrest steel for container ships of Examples 1-4 of the present invention comprises the following steps:

[0051] (1) Smelting: Select high-quality raw materials, smelt in a 150-ton converter, and then send them to the LF furnace for refining and air-breaking for Ca treatment, and then undergo RH vacuum degassing. The composition control is shown in Table 1.

[0052] (2) Continuous Casting: The molten steel is cast into 370mm thick continuous casting billets. The casting temperature is controlled at 5-25°C above the liquidus. Dynamic soft reduction is applied during the casting process.

[0053] (3) Slow cooling treatment of slabs: Continuous casting slabs require that the hot slabs be stacked with a cover or placed in a pit for slow cooling. The slow cooling starting temperature must be no less than 600°C and the slow cooling time must be ≥72h.

[0054] (4) Heating: Place the continuous casting billet obtained in step (3) into a walking beam heating furnace with a heating rate of 10 to 12 min / cm, a maximum temperature of the preheating section ≤ 650°C, a preheating section time of 3 hours, a heating section temperature of 1000 to 1180°C, a heating section time of 2 hours, a second heating section temperature of 1220 to 1240°C, a soaking section temperature of 1200 to 1220°C, and a total heating section time of 2 to 4 hours.

[0055] (6) Rolling: After the billet is removed from the furnace, it is descaled with high-pressure water at a system pressure of not less than 20 MPa, and then subjected to two-stage controlled rolling: rough rolling and finishing rolling. The first stage of rolling is rough rolling, with the starting rolling temperature in the range of 1080-1150°C, the target thickness of rough rolling ≥1.7t (t is the finished thickness of the steel plate), and the single-pass reduction between 40-60mm; then the second stage of rolling (finishing rolling) is carried out, with the starting rolling temperature in the range of 860-900°C, and at least 3 passes with a single-pass reduction rate of ≥10%. After rolling, the steel plate enters the ACC system for cooling, with a cooling rate of 3-5°C / s, and the final cooling temperature of the steel plate is 660-700°C, followed by stack cooling. The relevant process parameters are shown in Table 2.

[0056] (7) Quenching: The quenching temperature of the steel plate is 900±10℃, and the time in the furnace is 1.8~2.0min / mm.

[0057] (8) Tempering: The steel plate is tempered at a temperature of 640±10℃ and the furnace time is 2.8~3.5min / mm.

[0058] (9) After tempering, the steel plates were subjected to a transverse tensile test, a longitudinal impact test, an oscillographic impact test (only in Example 4), and a scanning test.

[0059] The tensile properties of the steel plates produced according to the manufacturing processes of the above-described embodiments are shown in Table 3. The tensile properties of the EH40 steel corresponding to the embodiments show that in transverse tensile tests, the yield strength of the steel plates ranged from 409 to 440 MPa, the tensile strength ranged from 595 to 625 MPa, the yield strength ratio ranged from 0.84 to 0.88, and the elongation was moderate. The tensile properties at half the thickness of the steel plates in the embodiments were comparable to those at quarter the thickness, with the strength difference less than 20 MPa.

[0060] The low-temperature impact toughness of the steel plates of the examples is shown in Table 3, and the longitudinal impact toughness value at -60°C is ≥250 J. The oscillographic impact test was performed on Example 4.

[0061] Table 1 Chemical composition of the examples (wt, %)

[0062] Table 2 Steel rolling process parameters of the embodiment

[0063] Table 3 Results of tensile, impact and oscillometric impact tests in the examples

Claims

1. An EH40 crack arrest steel plate for container ships, characterized by: The chemical composition of the steel plate is as follows by mass percentage: C: 0.03-0.06%, Si: 0.15-0.30%, Mn: 1.10-1.30%, P: ≤0.0080%, S: ≤0.0030%, Nb: 0.010-0.020%, V: 0.035-0.055%, Ti: 0.008-0.020%, Ni: 0.45-0.65%, Cu: 0.10-0.30%, Ca: 0.0005-0.0020%, H: ≤0.00015%, Ceq ≤0.42%, and the balance is Fe and unavoidable impurity elements.

2. The steel plate according to claim 1, wherein: Carbon equivalent Ceq≤0.42%.

3. The steel plate according to claim 1, wherein: In the transverse tensile test of the steel plate, the yield strength is 409-440MPa, the tensile strength is between 595-625MPa, and the yield strength ratio is 0.84-0.88; the difference in tensile strength between 1 / 2 thickness of the steel plate and 1 / 4 thickness of the steel plate is less than 20MPa; the longitudinal impact toughness value at -60℃ is ≥250J; in the oscillographic impact test, the crack arrest / instability expansion work E3 + the absorption work after crack arrest / instability expansion E4 accounts for ≥20% of the total impact absorption energy.

4. The steel plate according to claim 1, wherein: The maximum thickness of the steel plate is 120 mm.

5. The steel plate according to claim 1, wherein: The microstructure at 1 / 4 of the thickness of the steel plate: the original austenite grain size is small and uniform, the austenite grain size is between 10 and 20 μm, the ferrite lamellae perpendicular to the austenite grain boundary are close to 1 μm, cementite and other carbide particles are concentrated at the triangular grain boundaries, and only a very small amount of degenerate pearlite exists at 1 / 2 of the thickness of the steel plate.

6. A method for manufacturing the steel plate according to claim 1, characterized in that: Steps include (1) Smelting and continuous casting: prepare smelting raw materials according to the composition design, melt the smelting raw materials into molten steel, and cast the molten steel into ingots; (2) Slab slow cooling and cleaning and grinding: Slab is slowly cooled after it comes off the production line, and the surface of the slab is cleaned after slow cooling; (3) Heating: The billet is heated in the furnace, with the soaking zone temperature at 1200-1220°C to achieve solid solution of elements and homogenization of the structure; (4) Rolling: After the billet is removed from the furnace, it is descaled and then subjected to two-stage rolling; the first stage of rolling is rough rolling, with the starting rolling temperature at 1080-1150°C, the target thickness of rough rolling ≥1.7t, t is the thickness of the finished steel plate, and the single-pass reduction is between 40-60mm; followed by the second stage of finishing rolling, with the starting rolling temperature at 860-900°C, and at least three passes with a single-pass reduction rate of ≥10%; (5) Cooling: After rolling, the steel plate is cooled at a cooling rate of 3-5°C / s, and the final cooling temperature of the steel plate is 660-700°C, followed by stacking cooling; (6) Quenching heat treatment: After rolling, the steel plate is subjected to offline quenching treatment, the quenching temperature is 900±(0~10)℃, and the furnace time is 1.8~2.0min / mm; (7) Tempering heat treatment: After quenching, the steel plate is tempered at a tempering temperature of 640±(0-10)℃ and kept in the furnace for 2.8-3.5min / mm, so that the carbon in the quenched martensite can be fully diffused to obtain ferrite. In the subsequent long-term holding process, the ferrite lamellae grow and the supersaturated carbon in the original martensite precipitates to form cementite films and cementite particles.

7. The method according to claim 6, characterized in that: Step (1), the smelting raw materials are sequentially subjected to BOF smelting, LF refining, RH refining, and slab continuous casting machine to produce continuous casting billets, the billet thickness is 370 mm or more, and the casting temperature is controlled at 5 to 25°C above the liquidus line; dynamic soft pressure is implemented during the casting process, the continuous casting billets are slowly cooled, the surface is cleaned, and the longer continuous casting billets are flame cut into preset sizes.

8. The method according to claim 6, wherein: Step (2) requires that the hot billet be covered and stacked or placed in a pit for slow cooling. The slow cooling starting temperature shall not be lower than 600°C and the time shall not be less than 72 hours.

9. The method according to claim 6, wherein: Step (3), placing the billet into a walking beam heating furnace for heating, with a heating rate of 10 to 12 min / cm, a maximum temperature of the preheating section ≤ 650°C, a preheating section time of 3 hours, a heating section temperature of 1000 to 1180°C, a heating section time of 2 hours, a second heating section temperature of 1220 to 1240°C, a soaking section temperature of 1200 to 1220°C, and a total heating section and soaking section time of 2 to 4 hours.

10. The method according to claim 6, wherein: Step (5) uses the ACC cooling method to water-cool the rolled steel plate.

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