Large-thickness high-homogeneity polar region ship low-temperature steel and preparation method thereof

Abstract

The invention relates to large-thickness high-homogeneity polar region ship low-temperature steel and a preparation method thereof. The steel plate comprises the following chemical components in percentage by weight: 0.06%-0.12% of C, 0.15%-0.35% of Si, 1.10%-1.40% of Mn, less than or equal to 0.008% of P, less than or equal to 0.008% of S, 0.020%-0.050% of Nb, 0.020%-0.040% of Al, 0.010%-0.020% of Ti, 0.04%-0.08% of residual element Mo and the balance of Fe and inevitable impurity elements. The carbon equivalent CEV is less than or equal to 0.38%, and the cold crack sensitivity index Pcm is less than or equal to 0.20%. The manufacturing process of the steel plate comprises the steps of KR, BOF smelting, LF, RH, slab continuous casting, slab slow cooling, heating, rolling, steel plate pile slow cooling and heat treatment, the matrix structure of the obtained steel plate is a uniform and fine ferrite and tempered pearlite mixed structure, and the grain size ranges from 8 micrometers to 15 micrometers. The yield strength of the transverse tensile property at the 1 / 4 position of the steel plate ranges from 376 MPa to 392 MPa, the tensile strength ranges from 528 MPa to 553 MPa, the ductility is larger than or equal to 27.0%, the yield strength of the transverse tensile property at the 1 / 2 position ranges from 359 MPa to 375 MPa, the tensile strength ranges from 524 MPa to 537 MPa, the ductility is larger than or equal to 26.5%, and the steel plate is excellent in performance, excellent in whole plate uniformity, good in plate shape and easy to weld.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and specifically relates to a high-thickness and high-homogeneity polar ship low-temperature steel and a preparation method thereof. Background Art

[0002] The deep sea contains rich energy and strategic resources such as oil, natural gas, and rare metal minerals. It is a treasure trove and the future of human scientific exploration and resource needs. Marine engineering equipment and technology have ushered in major development opportunities.

[0003] At present, IACS UR stipulates the materials for ship structures in ice zones of all levels, with the highest grade being 50 mm FH high-strength steel. As the potential of energy and trade routes in the Arctic region has received more and more attention, the demand and development of large-scale high-tech polar transport icebreakers have been promoted, and higher requirements have been put forward for low-temperature steel for polar ships that meet polar service conditions. The development trend is to use high-strength grade and thicker low-temperature steel for polar ships with excellent low-temperature toughness and easy weldability.

[0004] Traditional low-temperature steel for polar ships is produced by TMCP process, and TMCP steel generally has the characteristics of poor performance uniformity. On the one hand, the original structure caused by the solidification and cooling process of the ingot itself is coarse and uneven, which cannot be completely eliminated by the secondary heating of the ingot alone. On the other hand, the TMCP process causes obvious gradient structure in the thickness direction of the steel plate. The existence of its internal stress will cause the shape control of the steel plate to fail to meet expectations and affect the welding performance. The steel plate involved in the patent of this invention effectively controls the low-multiple mass of the continuous casting ingot through the design of low-carbon equivalent and low crack sensitivity index components, combined with the continuous casting dynamic light reduction technology, and greatly reduces the defects such as cracks, looseness, and porosity inside the ingot. Then match the single-pass large reduction rolling process in the rough rolling stage to bridge the remaining defects in the core of the ingot. Finally, through the rapid cooling process after normalizing, the banded structure is completely eliminated, and the ferrite and pearlite are fully refined and dispersed, which ensures the strength of the steel plate while having excellent -70℃ low-temperature impact toughness.

[0005] After retrieval, Chinese Patent CN103725959A is about a 130mm low-alloy thick plate with low-temperature toughness and its production method. Its rolling process adopts TMCP. Although the post-rolling cooling can significantly refine the grains, its drawback is that there will be obvious gradient structures in the thickness direction, resulting in differences in phase transformation driving forces during the normalizing heating and cooling processes, and ultimately leading to non-uniform grains. Even when using the grain refinement element combination of Nb+V+Ti, it is difficult to improve. On the other hand, V exists in the steel in the form of carbides, which can improve the strength but damages the low-temperature impact toughness of the steel. Chinese Patent CN105803175B is about a low-compression-ratio extra-thick EH36 ship plate steel and its preparation method, which adopts the process of TMCP+rapid cooling after normalizing+ tempering to obtain a tempered bainite structure. The production process is complex. And its composition contains 0.14%C and 1.55%Mn. Both C and Mn are strongly segregating elements, which will affect the central segregation of the continuous casting billet, thus damaging the core impact toughness of the steel plate. Even when adding 0.3%Ni to lower the critical transformation temperature of the steel, it is difficult to ensure the -60°C impact toughness of the core of the steel plate. At the same time, the internal stress generated by the rapid cooling after normalizing requires an additional tempering process to eliminate. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a large-thickness, highly homogeneous cryogenic steel for polar ships and its preparation method in view of the above-mentioned prior art, and to produce a highly homogeneous cryogenic steel FH36 for polar ships with a maximum thickness of 100mm, which has excellent low-temperature impact toughness, good resistance to lamellar tearing, is easy to weld.

[0007] The technical solution adopted by the present invention to solve the above problems is as follows: A large-thickness, highly homogeneous cryogenic steel FH36 for polar ships, the chemical composition of the steel is calculated by weight percentage as follows: C: 0.06 - 0.12%, Si: 0.15 - 0.35%, Mn: 1.10 - 1.40%, P: ≤0.008%, S: ≤0.008%, Nb: 0.020 - 0.050%, Al: 0.020 - 0.040%, Ti: 0.010 - 0.020%, residual element Mo: 0.04 - 0.08%, and the rest is Fe and inevitable impurity elements.

[0008] The technical solution adopted by the present invention to solve the above problems is a large-thickness, highly homogeneous cryogenic steel FH36 for polar ships, whose carbon equivalent CEV ≤ 0.38% and cold crack sensitivity index Pcm ≤ 0.20% to ensure excellent welding performance of the steel plate, avoid preheating before welding of the steel plate, and improve the efficiency of offshore operations. Among them, the carbon equivalent CEV and cold crack sensitivity index Pcm are calculated by the following formulas from the composition of the melting analysis. CEV (%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1) Pcm (%) = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B (2) Preferably, the chemical composition of the steel by weight percentage is C: 0.09%, Si: 0.25%, Mn: 1.35%, P: ≤0.005%, S: ≤0.002%, Nb: 0.035%, Al: 0.030%, Ti: 0.015%, residual element Mo: 0.06%, CEV: ≤0.38%, Pcm: ≤0.20%, and the balance is Fe and inevitable impurity elements.

[0009] Design principle of the chemical composition of the steel of the present invention: C: The main function is to improve the strength of the steel, and it can also improve the hardenability of thick steel plates. At the same time, C is a strongly segregating element. Excessive amount will affect the low-temperature impact toughness and ductility of the core of the steel plate, and is not conducive to welding performance. In the present invention, the carbon content is controlled within the range of 0.06 - 0.12%.

[0010] Si: It is mainly used for deoxidation, and at the same time can improve the strength and corrosion resistance of the steel, reduce the critical cooling rate of the steel, thereby improving the hardenability of the steel. However, if added in excess, it will damage the toughness and welding performance of the steel, and it is easy to form scale on the surface after rolling, affecting the surface quality of the steel. Therefore, its range is controlled at 0.15 - 0.35%.

[0011] Mn: It helps to delay the transformation from austenite to ferrite, refine ferrite, and improve the strength and toughness of the steel. However, too high Mn will damage the welding performance of the material and reduce the corrosion resistance of the material. In addition, Mn has a high segregation tendency and is prone to central segregation, damaging the impact toughness of the core of the steel plate. In the present invention, the manganese content is specified to be within the range of 1.10 - 1.40%.

[0012] Nb: Its functions include precipitation strengthening and grain refinement. During the heating process, Nb combines with carbon and nitrogen to form fine and dispersed Nb[C, N] compounds, which hinder the growth of austenite grains. During the rolling process, it has a strong inhibitory effect on recrystallization and refinement, improving the strength and toughness of the steel. However, too high Nb is not conducive to welding performance. Therefore, in the present invention, the niobium content is specified to be within the range of 0.020 - 0.050%.

[0013] Ti: Fixes N element, prevents the coarsening of austenite grains during billet heating, rolling, especially during welding, and improves the low-temperature toughness of the base metal and the heat-affected zone of the weld. In the present invention, the Ti content is specified to be within the range of 0.010 - 0.020%.

[0014] Al: As a deoxidizer, it can reduce the inclusion content in the steel and has the effect of refining grains. However, excessive Al will damage the surface quality of the billet. Therefore, the total Al content should be controlled at 0.020 - 0.040%.

[0015] Mo: The core of the present invention is to control the high content of residual element Mo. Mo can improve the hardenability of steel. At the same time, Mo can be completely dissolved in ferrite, improving the strength of ferrite, shifting the C curve to the right, reducing the pearlite transformation temperature, and refining pearlite. In addition, Mo also has the effect of increasing the tempering stability. Therefore, the present invention controls the residual Mo content between 0.04% and 0.08%.

[0016] P, S: Both P and S are harmful elements in steel, reducing the physical and mechanical properties of steel. At the same time, they are strong segregation elements. Therefore, it is better to control the contents of P and S as low as possible.

[0017] To avoid center segregation and improve the uniformity of properties in the thickness direction of the steel plate, a low carbon equivalent and low crack sensitivity index composition design is adopted. On this premise, the difficulty lies in how to improve the strength of the steel plate and take into account good low-temperature impact toughness. The present invention uses high-purity steel smelting technology to greatly remove harmful elements such as P, S, H, O, and N in the steel. Add Al, Nb, and Ti, and fully refine the ferrite + pearlite grains during the two-stage rolling process. And further refine the grains through subsequent normalizing + rapid cooling process. So that the steel plate has excellent strength and toughness matching.

[0018] The present invention further provides a large-thickness high-homogeneity polar ship low-temperature steel FH36 and its preparation method as described above. The specific process is as follows: 1. Smelting process Select high-quality scrap steel containing Mo, control the LF smelting time above 30 min; the RH high-vacuum degassing time is 20 - 40 min; after breaking the vacuum, the hydrogen content [H] ≤ 1.0 ppm, and analyze the N content of the finished product sample, and control the [N] content below 50 ppm; then carry out wire feeding (calcium silicate wire), and blow argon after the wire feeding is completed.

[0019] 2. Continuous casting process Before casting, the tundish preheating time ≥ 5 h, the superheat during casting is controlled at 15 - 30 °C, and the soft reduction technology is applied at the end of billet solidification to reduce the center segregation of the continuous casting billet.

[0020] 3. Slow cooling of continuous casting billet After the casting billet is taken off the production line, it is stacked and slowly cooled at a temperature above 600 °C. Then, the surface defects are cleaned at a temperature above 200 °C. Prevent the surface quality of the steel plate from being affected during the rolling process.

[0021] 4. Rolling and cooling The blank is heated to above 1100°C, and the time in the high-temperature section + soaking section is ensured to be ≥200 min. On the one hand, the micro-alloying elements in the steel are fully solid-solved, and on the other hand, the temperature uniformity in the thickness direction of the blank is ensured. After the blank is taken out of the furnace, it is descaled by high-pressure water to remove the oxide on the surface of the blank. Subsequently, two-stage rolling is carried out. In the rough rolling stage, large reduction rolling is carried out, and the rough rolling starting temperature is 1070°C ± 2°C, so that the deformation penetrates into the core of the continuous casting billet, thereby bridging the defects such as internal cracks, porosity, and voids in the casting billet. Subsequently, the intermediate billet is water-cooled to avoid the growth of austenite grains. When the blank temperature reaches 780 - 820°C, finish rolling is carried out. The reduction in the last three passes of finish rolling is reduced, the reduction is ≥100 mm, and the finish rolling temperature is ≤760°C to control the rolled plate shape. After rolling, air cooling is carried out. The steel plate is taken off the production line at <650°C and then subjected to hydrogen-expanded stacking and slow cooling.

[0022] 5. Heat treatment: Adopt the process of rapid cooling after normalizing (NAC).

[0023] Normalizing process: The quenching temperature is 850 - 880°C, and the time in the furnace is 2.0 - 2.4 min / mm. The holding time is controlled above 20 min. The process of low-temperature normalizing + long-time holding is adopted. On the one hand, it prevents the growth of austenite grains, and on the other hand, it makes the temperature of the steel plate fully uniform.

[0024] Cooling process: It is divided into two-stage cooling. The first stage is that the steel plate quickly enters the quenching machine after leaving the furnace, with weak water cooling and a cooling rate of 0.5 - 2°C / s. The second stage is high-pressure strong water cooling with a cooling rate of 3 - 9°C / s. Control the ratio of upper and lower water in the quenching machine: 0.5 - 0.6 to ensure uniform cooling of the upper and lower surfaces of the steel plate. Immediately after the steel plate exits the water, high-pressure purging is carried out to avoid water accumulation on the surface of the steel plate, which affects temperature uniformity. The temperature of the steel plate is measured 1 - 2 min after it exits the water, and the return red temperature is 550 - 600°C.

[0025] Cooling: After the steel plate is measured for temperature, it is placed on the cooling bed for air cooling, which can form a short-time self-tempering phenomenon, eliminate the internal stress of the steel plate, thereby ensuring the plate shape of the steel plate, and the flatness can be controlled within 2 mm / m and 5 mm / whole plate.

[0026] The steel plate has a fine ferrite + tempered pearlite mixed structure in the thickness direction, and the average grain size is 6 - 15 μm.

[0027] Compared with the prior art, the advantages of the present invention are: 1) The composition design with low carbon equivalent and low crack sensitivity index is adopted, combined with the dynamic soft reduction technology of continuous casting, effectively controlling the macrostructure quality of continuous casting billets, reducing the production of banded structure, and greatly reducing defects such as internal cracks, porosity, and voids in the billets. Then, matching with the single-pass large reduction rolling process in the rough rolling stage to bridge the remaining defects in the core of the billet, and finally, through the rapid cooling process after normalizing, the banded structure is completely eliminated, making ferrite and pearlite disperse and distribute, ensuring the uniformity of the steel plate properties.

[0028] 2) The low carbon equivalent composition design is beneficial to the welding performance of the steel plate, but faces the problem of insufficient strength. Through a series of measures such as the combination of grain refinement elements of Al+Nb+Ti, suppressing grain growth during the rolling process of the steel plate, and rapid cooling of NAC after low-temperature normalizing, the ferrite + pearlite grains are fully refined, greatly increasing the grain boundary area, which not only hinders the passage of dislocations but also strengthens the resistance to crack propagation. Thus, the strength and low-temperature impact toughness of the steel plate are improved.

[0029] 3) The plastic deformation of low-carbon steel usually occurs throughout the entire tensile stage, so there is often no yield plateau, and the presence of the yield plateau has nothing to do with the texture and grain size of the material. In this patent, by controlling the high content of the residual element Mo, Mo can not only improve the tempering stability of the steel. Most importantly, Mo is completely dissolved in ferrite, causing lattice distortion, increasing the dislocation density formed by the aggregation of supersaturated vacancies in the grains. Mo can shift the C-curve to the right, increasing the proportion of pearlite and reducing the lamellar spacing, thus generating a yield plateau. The yield plateau can significantly reduce the plastic deformation of the steel when stressed, effectively limiting the plastic deformation, and improving the rigidity of the material to a certain extent, ensuring the stability and reliability of the steel. Brief Description of the Drawings

[0030] Figure 1 It is the metallographic structure at 1 / 4 and 1 / 2 thicknesses of the steel plate in Example 1 of the present invention, which is uniform and fine ferrite + tempered pearlite; Figure 2 It is the metallographic structure at 1 / 4 and 1 / 2 thicknesses of the steel plate in Example 2 of the present invention. The matrix structure is also uniform and fine ferrite + tempered pearlite. The difference is that the proportion of pearlite is lower than that in Example 1, resulting in no yield plateau. Detailed Description of the Invention

[0031] The technical solution of the present invention will be described in more detail in combination with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred embodiments of the present invention and cannot impose any limitation on the scope of the present invention.

[0032] The chemical compositions of the FH36 steel plates corresponding to each embodiment are shown in Table 1. The data in the table are the mass percentage contents of each element, and the rest are Fe and inevitable impurity elements.

[0033] Table 1 Chemical composition (wt%) of a large-thickness and high-homogeneity cryogenic steel FH36 for polar ships in Example 1

[0034] Its production process is as follows: (1) Smelting: After the molten steel is deeply desulfurized by KR, it is smelted in a converter, then refined in an LF furnace and subjected to RH vacuum degassing treatment. After breaking the vacuum, hydrogen determination is carried out.

[0035] (2) Continuous casting: The molten steel is cast into a continuous casting billet with a cross-section of 450 mm x 2500 mm by a continuous caster. The casting temperature is controlled at 15 - 30 °C above the liquidus. The dynamic soft reduction technology is adopted to ensure that the central segregation of the continuous casting billet is ≤ C0.5 level.

[0036] (3) Billet slow cooling and hydrogen diffusion: After the continuous casting billet is taken offline, it is stack-cooled at a temperature above 600 °C, and then the surface defects are cleaned at a temperature above 200 °C.

[0037] (4) Rolling: The billet is reheated to above 1100 °C for the second time, and the time in the high-temperature section + soaking section is ensured to be ≥ 200 min to make the alloying elements in the steel fully dissolve, and at the same time ensure the temperature uniformity in the thickness direction of the billet. After the billet is taken out of the furnace, it is descaled by high-pressure water to avoid affecting the surface quality of the steel plate due to the pressing-in of oxides during rolling. Subsequently, two-stage rolling is carried out. In the rough rolling stage, large reduction rolling is carried out to make the deformation penetrate into the core of the continuous casting billet, thereby bridging defects such as internal cracks, porosity, and voids in the casting billet. Then, the intermediate billet is cooled to avoid the growth of austenite grains. When the billet temperature reaches 780 - 820 °C, finish rolling is carried out. The reduction amount is reduced in the last three passes of finish rolling to control the rolled plate shape. Air cooling is carried out after rolling.

[0038] (5) Steel plate slow cooling and hydrogen diffusion: The steel plate is stack-cooled after being taken offline at a temperature < 650 °C.

[0039] (6) Heat treatment: The cooled steel plate is normalized. The normalizing temperature is 850 - 880 °C, the time in the furnace is 2.0 - 2.4 min / mm, and the holding time is controlled above 20 min. The low-temperature normalizing + long-time holding process is adopted. On the one hand, it can prevent the growth of austenite grains at high temperatures, and on the other hand, it can make the temperature of the steel plate fully uniform.

[0040] After the steel plate is taken out of the furnace, it quickly enters the quenching machine. First, it is cooled by weak water with a cooling rate of 0.5 - 2 °C / s. When the steel plate is completely inside the quenching machine, high-pressure water cooling is carried out with a cooling rate of 3 - 9 °C / s. The ratio of the upper and lower water in the quenching machine is controlled at 0.5 - 0.6 to ensure uniform cooling of the upper and lower surfaces of the steel plate. Immediately after the steel plate exits the water, high-pressure purging is carried out to prevent water accumulation on the surface of the steel plate, which may affect the temperature uniformity. The temperature of the steel plate is measured 1 - 2 minutes after it exits the water, and the return red temperature is 550 - 600 °C. After the temperature of the steel plate is measured, it is placed on the cooling bed for air cooling, which can form a short-term self-tempering effect, eliminate the internal stress of the steel plate, and thus ensure the plate shape of the steel plate. The flatness can be controlled within 2 mm / m and 6 mm for the whole plate.

[0041] (7)Analyze the uniformity of the whole steel plate according to the ABS ship rules. Conduct transverse tensile tests, Charpy longitudinal impact tests on the near-surface, 1 / 4 and 1 / 2 thickness positions, and metallographic structure observations at the head corners, 1 / 4 and 1 / 2 widths, tail corners, 1 / 4 and 1 / 2 widths of the steel plate at the 1 / 4 and 1 / 2 thickness positions.

[0042] The properties of the example steel plates produced according to the above process flow are shown in Table 3.

[0043] Example 1: For the 100 mm thick FH36 steel plate, the yield strength of the transverse tensile property at 1 / 4 thickness is in the range of 376 - 392 MPa, the tensile strength is in the range of 528 - 553 MPa, and the elongation rate ≥ 27.0%. The yield strength of the transverse tensile property at 1 / 2 thickness is in the range of 359 - 375 MPa, the tensile strength is in the range of 524 - 537 MPa, and the elongation rate ≥ 26.5%. The Charpy longitudinal impact energy at 1 / 4 thickness at -60 °C ≥ 200 J, and at 1 / 2 thickness ≥ 125 J. The Charpy longitudinal impact energy at 1 / 4 thickness at -70 °C ≥ 150 J, and at 1 / 2 thickness ≥ 80 J.

[0044] Example 2: For the 100 mm thick FH36 steel plate, there is no yield platform in the tensile test. The tensile strength and elongation rate have no difference compared with Example 1. Although the impact toughness at -60 °C meets the requirements of the classification society specifications, the stability is poor.

[0045] The steel plate has excellent welding performance, which greatly improves the offshore operation efficiency. The plate shape of the steel plate is good, avoiding welding stress caused by the unevenness of the steel plate, which may lead to cracks in the weak area of the weld, and ensuring the structural safety of offshore engineering equipment. The flaw detection result of the whole steel plate meets the requirements of ASTM A578 / A578M Grade C. The surface quality of the steel plate is excellent.

[0046] Figure 1 、 2Typical microstructural photos of the examples are given. The microstructure of the finished steel plate is a mixed structure of uniformly fine ferrite + granular bainite, and the grain size is between 6 - 15 μm. The difference lies in the proportion of pearlite and the pearlite lamellar spacing, which ultimately leads to a significant difference in the strength of the steel plate. The reason for this difference is the effect brought by the residual element Mo.

[0047] Table 2 Rolling Process Control

[0048] Table 3 Tensile, Impact and Z - direction Properties of the Examples of the Present Invention

[0049]

[0050] *T is the thickness of the steel plate.

[0051] Although the preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various changes and modifications can be made to the present invention by those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A cryogenic steel for polar ships with large thickness and high homogeneity, characterized in that: The chemical composition of the steel by mass percentage is as follows: C: 0.06 - 0.12%, Si: 0.15 - 0.35%, Mn: 1.10 - 1.40%, P: ≤0.008%, S: ≤0.008%, Nb: 0.020 - 0.050%, Al: 0.020 - 0.040%, Ti: 0.010 - 0.020%, residual element Mo: 0.04 - 0.08%, and the rest is Fe and inevitable impurity elements.

2. A kind of cryogenic steel for large-thickness and high-homogeneity polar ships according to claim 1, characterized in that: The carbon equivalent CEV of the steel ≤0.38%, and the cold crack sensitivity index Pcm ≤0.20%. The carbon equivalent CEV and the cold crack sensitivity index Pcm are calculated by the following formulas using the melting analysis composition. CEV (%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 Pcm (%) = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B.

3. A kind of cryogenic steel for large-thickness and high-homogeneity polar ships according to claim 2, characterized in that: The chemical composition of the steel by weight percentage is C: 0.09%, Si: 0.25%, Mn: 1.35%, P: ≤0.005%, S: ≤0.002%, Nb: 0.030%, Al: 0.030%, Ti: 0.015%, residual Mo: 0.06%, CEV: ≤0.38%, Pcm: ≤0.20%, and the balance is Fe and inevitable impurity elements.

4. A kind of cryogenic steel for large-thickness and high-homogeneity polar ships according to claim 1, characterized in that: The matrix structure of the steel is a homogeneous and fine mixed structure of ferrite + tempered pearlite, the grain size is between 8 - 15 μm, and the maximum thickness of the steel plate reaches 100 mm.

5. A high-homogeneity cryogenic steel for large-thickness polar ships according to claim 1, characterized in that: The yield strength of the transverse tensile property at 1 / 4 of the steel plate thickness is in the range of 376 - 392 MPa, the tensile strength is in the range of 528 - 553 MPa, the elongation rate ≥27.0%. The yield strength of the transverse tensile property at 1 / 2 of the thickness is in the range of 359 - 375 MPa, the tensile strength is in the range of 524 - 537 MPa, the elongation rate ≥26.5%. The Charpy longitudinal impact energy at 1 / 4 of the thickness at -60°C ≥200 J, at 1 / 2 of the thickness ≥125 J. The Charpy longitudinal impact energy at 1 / 4 of the thickness at -70°C ≥150 J, at 1 / 2 of the thickness ≥80 J.

6. A preparation method of a large-thickness and high-homogeneity cryogenic steel for polar ships as described in claim 1, characterized in that: The method mainly includes the following steps: (1) Smelting Select high-quality Mo-containing scrap steel. After the molten steel is deeply desulfurized by KR, it is smelted in a converter, then refined in an LF furnace and subjected to RH vacuum degassing treatment. After breaking the vacuum, hydrogen determination is carried out. (2) Continuous casting Before casting, the tundish is preheated first. Low superheat protected casting is adopted, and the superheat is controlled at 15 - 30°C. The soft reduction technology is applied at the end of the billet solidification to reduce the center segregation of the continuous casting billet. (3) Billet slow cooling After the casting billet is taken offline, it is stacked and slowly cooled. Subsequently, surface defect cleaning is carried out at a temperature above 200°C to prevent the surface quality of the steel plate from being affected during the rolling process. (4) Rolling Heat the blank to above 1100°C and ensure that the time in the high-temperature section + soaking section ≥ 200 min. After the blank is taken out of the furnace, descale it with high-pressure water, and then carry out two-stage rolling. In the rough rolling stage, carry out large reduction rolling, and then cool the intermediate billet to avoid the growth of austenite grains. When the blank temperature reaches 780 - 820°C, carry out finish rolling. Reduce the reduction in the last three passes after finish rolling to control the rolled plate shape. (5)Cooling After rolling, carry out air cooling. After the steel plate is taken offline at < 650°C, carry out hydrogen-expansion stacking slow cooling. (6)Heat treatment: Normalizing process: The quenching temperature is 850 - 880°C, the time in the furnace is 2.0 - 2.4 min / mm, and the holding time is controlled above 20 min. (7)Cooling After the steel plate is taken out of the furnace, quickly enter the quenching machine. First, cool the steel plate with weak water. When the steel plate completely enters the quenching machine, carry out high-pressure strong water cooling. Immediately after the steel plate comes out of the water, carry out high-pressure blowing. Measure the temperature 1 - 2 min after the steel plate comes out of the water. The return red temperature is 550 - 600°C. After the steel plate is measured for temperature, place it on the cooling bed for air cooling, which can form a short-time self-tempering effect, eliminate the internal stress of the steel plate, and thus ensure the plate shape of the steel plate. The flatness is controlled within 2 mm / m and 6 mm for the whole plate.

7. The preparation method of a low-temperature steel for large-thickness and high-homogeneity polar ships according to claim 6, characterized in that: In step (1), the LF smelting time is controlled above 30 min; the RH high-vacuum degassing time is 20 - 40 min; after breaking the vacuum, the determined hydrogen [H] ≤ 1.0 ppm, and the N content of the finished product sample is analyzed. The [N] content is controlled below 50 ppm.

8. The preparation method of a large-thickness and high-homogeneity cryogenic steel for polar ships according to claim 6, characterized in that: In step (4), reduce the reduction in the last three passes after finish rolling. The reduction ≥ 100 mm, and the final rolling temperature ≤ 760°C.

9. The preparation method of a large-thickness and high-homogeneity cryogenic steel for polar ships according to claim 6, characterized in that: In step (7), the weak water cooling rate is 0.5 - 2°C / s, the strong water cooling rate is 3 - 9°C / s, and the upper and lower water ratio of the quenching machine is controlled at 0.5 - 0.6.

Images