Nv4-4 steel sheet having high uniform elongation and good low temperature fracture toughness and method of manufacturing

Abstract

The present application relates to a kind of NV4-4 steel plate of high uniform elongation and low temperature fracture toughness good and manufacturing method, the chemical composition of steel plate is C:0.05~0.15%;Si:0.06~0.25%;Mn:1.10~1.75%;S:≤0.002%;P:≤0.008%;Als:0.015~0.045%;N:0.003~0.015%;Nb:0.02~0.06%;Cu:0.10~0.30%;Cr:0.10~0.30%;Ni:0.20~0.80%;Mo:0.05~0.20%;Ti:0.008~0.018%; the balance is Fe and impurity.Product steel plate has good structure performance uniformity, tear resistance and low temperature fracture resistance and other characteristics, can satisfy the technical requirement of marine harsh, demanding environment to ship steel plate.

Description

Technical Field

[0001] This invention relates to the field of shipbuilding steel preparation technology, and in particular to a TMCP-state NV4-4 high-strength low-temperature steel plate with high uniform elongation and good low-temperature fracture toughness, and its manufacturing method. Background Technology

[0002] Fully refrigerated liquefied petroleum gas (LPG) carriers can carry various types of LPG at temperatures below -50°C. They are the largest-capacity LPG carrier family and are recognized internationally as highly complex and technologically advanced vessels, facing the greatest design and construction challenges. LPG carriers are the mainstay for long-distance transportation of bulk LPG products, boasting advantages such as large carrying capacity, high safety, standardized operating procedures, and high added value, thus exhibiting promising development prospects.

[0003] With the increase in global energy trade and shale gas exports, the demand for liquefied petroleum gas (LPG) carriers has further increased. The ultra-large, fully refrigerated LPG carriers independently developed, designed, and built in China primarily utilize cryogenic steel plates, with each ship requiring approximately 10,000 tons of these plates. The high comprehensive performance requirements of the steel plates (requiring not only high strength but also high toughness while ensuring excellent weldability, and meeting low-temperature impact toughness requirements below -50°C) significantly increase the manufacturing difficulty of cryogenic steel. In particular, the NV2-4 cryogenic steel, required by downstream users to ensure excellent low-temperature toughness while also meeting the requirements for high uniform elongation and low-temperature fracture indicators, necessitates strict control over the billet heating temperature, rolling temperature, and post-rolling rapid cooling process, making production extremely challenging.

[0004] Chinese invention patent application number CN 202110031192.2 discloses "a wear-resistant steel with high uniform elongation and its manufacturing method". The chemical elements in the steel are as follows: 0.75%≤C≤2.2%, 0<Si≤0.2%, 0<Mn≤0.2%, 0.1%≤Al≤0.5%. The matrix of the wear-resistant steel is ferrite, and cementite particles are uniformly distributed within the grains and at the grain boundaries of the ferrite. The manufacturing method of the wear-resistant steel includes the following steps: (1) smelting and casting; (2) heating; (3) rolling; (4) cooling: cooling to room temperature at a cooling rate of ≤5℃ / s. It belongs to medium and high carbon steel, which is fundamentally different from the low carbon medium and heavy plate products involved in this invention.

[0005] Chinese invention patent application number CN 202110086416.X discloses "a hot-dip galvanized steel and its manufacturing method", the composition of which is designed as follows: C: 0.15-0.25%, Mn: 1.8-2.4%, Si: 0.3-0.9%, Al: 0.03-0.40%, and also includes at least one of the following chemical components: Ti: 0.005-0.1%, Nb: 0.005-0.1%, Cr: 0.01-0.3%, Mo: 0.02-0.2%, with the balance being Fe and unavoidable impurities. The contents of Si, Al and Cr satisfy 0.5% ≤ Si + Al + Cr ≤ 1.1%, and the contents of C and Si satisfy 4C + Si ≤ 2.0%. It belongs to the hot-dip galvanized cold-rolled series products, which are completely different from the medium and heavy plate products involved in this invention.

[0006] Chinese invention patent application number CN 201210327206.6 discloses a “deformation-resistant X80-X100 pipeline steel plate and its manufacturing method”. The steel composition is designed as follows: C: 0.04%~0.09%, Si: 0.10%~0.50%, Mn: 1.00%~2.00%, P≤0.015%, S≤0.005%, Nb: 0.05%~0.11%, Ti: 0.010%~0.025%, Mo≤0.30%, Cu≤0.40%, Ni≤0.50%, Cr≤0.40%, with the balance being Fe and unavoidable impurities. However, the maximum thickness of the finished steel plate is only 26.4mm, and there is no CTOD fracture evaluation, which cannot meet the actual requirements of marine equipment for low-temperature toughness, fracture toughness, and thickness of carbon-manganese systems.

[0007] Chinese invention patent application CN 202110392421.3 discloses a "VL4-4L marine steel plate with good low-temperature toughness and its manufacturing method," with the following composition: C: 0.04%–0.09%; Si: 0.05%–0.14%; Mn: 1.10%–1.45%; S: ≤0.002%; P: ≤0.008%; Als: 0.015%–0.045%; N: 0.003 %~0.015%; Nb: 0.01%~0.03%; Cu: 0.05%~0.15%; Cr: 0.05%~0.20%; Ni: 0.25%~0.50%; V: 0%~0.04%; Ti: 0.008%~0.014%; the balance is Fe and unavoidable impurities; however, it only focuses on low-temperature toughness and does not evaluate uniform elongation or CTOD fracture toughness, which is fundamentally different from the present invention.

[0008] Chinese invention patent application number CN 202210404964.7 discloses a "500MPa grade crack-arresting steel plate and its production method," with the following composition: C: 0.04-0.08%, Si: 0.1-0.2%, Mn: 1.5-1.6%, Cr: 0.2-0.3%, Ni: 0.5-0.7%, Mo: 0.2-0.3%, Cu: 0.35-0.50%, Nb: 0. It contains 0.04-0.06% V, 0.01-0.03% Al, 0.02-0.05% Ti, 0.01-0.02% B, Ti / N ≥ 3.4, with residual iron and impurities; its carbon equivalent is 0.46-0.51%, and its cold crack sensitivity index is ≤ 0.22, but its low temperature resistance is only -60℃, and it has no product uniform elongation index, which is fundamentally different from the present invention. Summary of the Invention

[0009] This invention provides an NV4-4 steel plate with high uniform elongation and good low-temperature fracture toughness, as well as a manufacturing method. Through reasonable composition design and combination with a novel TMCP rolling process, the obtained steel plate has high uniform elongation and excellent low-temperature fracture performance across the entire thickness section. The prepared high-strength, low-temperature resistant, uniformly elongated steel plate with a maximum thickness of 60mm has good uniformity of microstructure, tear resistance, and resistance to low-temperature fracture, which can meet the technical requirements of marine steel plates in harsh and demanding marine environments.

[0010] To achieve the above objectives, the present invention employs the following technical solution:

[0011] NV4-4 steel plate with high uniform elongation and good low-temperature fracture toughness. The chemical composition of the steel plate, by mass percentage, is: C: 0.05%~0.15%; Si: 0.06%~0.25%; Mn: 1.10%~1.75%; S: ≤0.002%; P: ≤0.008%; Als: 0.015%~0.045%; N: 0.003%~0.015%; Nb: 0.02%~0.06%; Cu: 0.10%~0.30%; Cr: 0.10%~0.30%; Ni: 0.20%~0.80%; Mo: 0.05%~0.20%; Ti: 0.008%~0.018%; with the balance being Fe and unavoidable impurities.

[0012] Furthermore, the microstructure of the finished steel plate is ferrite + bainite, with the volume percentage of ferrite being 60% to 75%.

[0013] Furthermore, the finished steel plate has a yield strength ≥335MPa, tensile strength ≥490MPa, impact energy at -90℃ ≥200J, uniform elongation ≥10%, and a low-temperature fracture CTOD characteristic value of the base material and weld heat-affected zone at -30℃~-70℃ >0.2mm.

[0014] Furthermore, after simulated welding and heat treatment at 580-600℃, the finished steel plates meet the following mechanical properties: yield strength ≥335MPa, tensile strength ≥490MPa, uniform elongation ≥10%, and impact energy at -90℃ ≥180J.

[0015] Furthermore, the thickness of the finished steel plate is 40–60 mm.

[0016] A method for manufacturing NV4-4 steel plate with high uniform elongation and good low-temperature fracture toughness, the production process includes steel smelting and casting, billet heating, multi-stage controlled rolling, controlled cooling, and slow cooling by stacking; wherein the following processes are controlled:

[0017] 1) Heating of steel billets;

[0018] Heating temperature 1150~1250℃, heat spread temperature 1130~1230℃, heat spread time 60~90min;

[0019] 2) Two-stage controlled rolling;

[0020] A two-stage controlled rolling process is adopted; the initial rolling temperature of the first stage is 1120-1180℃, and the deformation rate per pass is ≥15%; the thickness of the intermediate billet is 1.8-2.5 times the thickness of the finished steel plate; the initial rolling temperature of the second stage is 800-900℃, and the deformation rate per pass is ≥12%; the final rolling temperature is 780-880℃.

[0021] 3) Control cooling;

[0022] The average cooling rate is 10-20℃ / s, the starting cooling temperature is 720-780℃, and the final cooling temperature is 430-500℃.

[0023] 4) Stacking and slow cooling: The steel plates after controlled cooling are stacked and slow cooled for a time of ≥36 hours to obtain finished steel plates.

[0024] Furthermore, during the steel smelting and casting process, the processing time for both LF refining and RH refining is 20-40 minutes, and the superheat of the molten steel in the tundish is ≤35℃; the continuously cast slab undergoes slow cooling treatment by stacking, with a processing time ≥24 hours.

[0025] Furthermore, during the steel smelting and casting process, the molten steel is protected during the entire casting process, and the continuous casting billet forming process adopts light pressure and electromagnetic stirring.

[0026] Furthermore, during the steel smelting and casting process, the inclusions in the refined steel meet the following requirements: Class A inclusions ≤ 0.5 grade, Class B inclusions ≤ 1.0 grade, Class C inclusions ≤ 1.0 grade, and Class D inclusions ≤ 0.5 grade.

[0027] Furthermore, in step 3), the cooling is controlled using the ACC laminar flow fully automatic cooling mode, with the head and tail of the steel plate being shielded throughout the process.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] 1) The chemical composition is designed reasonably. By adding appropriate amounts of alloys and using low P and S pure steel for smelting, the purity of the steel is improved, ensuring that the NV4-4 high-strength steel plate has high uniform elongation and excellent low-temperature fracture performance.

[0030] 2) By rationally designing the homogenization temperature and the holding time during the homogenization stage when heating the billet, the grain size of the original austenite structure of the steel plate can be refined, ensuring the smooth rolling of the steel plate and the uniformity of the structure, thus providing a structural basis for improving the strength and toughness of the steel plate.

[0031] 3) By combining two-stage controlled rolling + water cooling (TMCP) process, the NV4-4 high-strength steel plate with high uniform elongation and excellent low-temperature fracture resistance is strengthened and toughened, the phase transformation structure is made more uniform and refined, the morphology and grain size are controlled, and the dimensional accuracy and surface quality are controlled to achieve higher.

[0032] 4) The prepared steel plate has high strength (yield strength ≥335MPa, tensile strength ≥490MPa), low temperature resistance (impact energy ≥200J at -90℃), uniform elongation ≥10%, maximum thickness 60mm, and low temperature fracture CTOD (crack tip opening displacement) characteristic value of the base metal and weld heat-affected zone (-30℃, -50℃, -70℃) >0.2mm. The mechanical properties of the steel plate after simulated welding and heat treatment at 580~600℃ still meet the requirements of yield strength ≥335MPa, tensile strength ≥490MPa, uniform elongation ≥10%, and impact energy ≥180J at -90℃. Its good uniformity of microstructure, tear resistance and low temperature fracture resistance can meet the technical requirements of marine steel plates for harsh marine environments. Attached Figure Description

[0033] Figure 1 This is a metallographic photograph of the finished steel plate prepared in Example 1 of the present invention.

[0034] Figure 2 This is a photograph of the non-metallic inclusions in the finished steel plate prepared in Example 1 of the present invention. Detailed Implementation

[0035] The present invention discloses an NV4-4 steel plate with high uniform elongation and good low-temperature fracture toughness. The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.05%–0.15%; Si: 0.06%–0.25%; Mn: 1.10%–1.75%; S: ≤0.002%; P: ≤0.008%; Als: 0.015%–0.045%; N: 0.003%–0.015%; Nb: 0.02%–0.06%; Cu: 0.10%–0.30%; Cr: 0.10%–0.30%; Ni: 0.20%–0.80%; Mo: 0.05%–0.20%; Ti: 0.008%–0.018%; with the balance being Fe and unavoidable impurities.

[0036] The microstructure of the finished steel plate is ferrite + bainite, with the volume percentage of ferrite being 60% to 75%.

[0037] The finished steel plate has a yield strength ≥335MPa, a tensile strength ≥490MPa, an impact energy of -90℃ ≥200J, a uniform elongation ≥10%, and a low-temperature fracture CTOD characteristic value of the base material and weld heat-affected zone of -30℃~-70℃ >0.2mm.

[0038] After simulated welding and heat treatment at 580-600℃, the finished steel plate meets the following mechanical properties: yield strength ≥335MPa, tensile strength ≥490MPa, uniform elongation ≥10%, and impact energy at -90℃ ≥180J.

[0039] The thickness of the finished steel plate is 40-60mm.

[0040] The present invention discloses a method for manufacturing NV4-4 steel plate with high uniform elongation and good low-temperature fracture toughness. The production process includes steel smelting and casting, billet heating, multi-stage controlled rolling, controlled cooling, and slow cooling in a stacked manner; wherein the controlled processes are as follows:

[0041] 1) Steel smelting and casting;

[0042] The processing time for both LF refining and RH refining is 20–40 min, and the superheat of the molten steel in the ladle is ≤35℃. The continuously cast slabs undergo slow cooling in a stack for ≥24 h. The molten steel is poured under full protection, and the continuous casting slab forming process uses light reduction and electromagnetic stirring. The inclusions in the refined steel meet the following requirements: Class A inclusions ≤0.5 grade, Class B inclusions ≤1.0 grade, Class C inclusions ≤1.0 grade, and Class D inclusions ≤0.5 grade.

[0043] 2) Heating the steel billet;

[0044] Heating temperature 1150~1250℃, heat spread temperature 1130~1230℃, heat spread time 60~90min;

[0045] 3) Two-stage controlled rolling;

[0046] A two-stage controlled rolling process is adopted; the initial rolling temperature of the first stage is 1120-1180℃, and the deformation rate per pass is ≥15%; the thickness of the intermediate billet is 1.8-2.5 times the thickness of the finished steel plate; the initial rolling temperature of the second stage is 800-900℃, and the deformation rate per pass is ≥12%; the final rolling temperature is 780-880℃.

[0047] 4) Control cooling;

[0048] The average cooling rate is 10-20℃ / s, the starting cooling temperature is 720-780℃, and the final cooling temperature is 430-500℃. The cooling is controlled by an ACC laminar flow fully automatic cooling mode, with the steel plate completely shielded at both ends.

[0049] 5) Stacking and slow cooling: The steel plates after controlled cooling are stacked and slow cooled for a time of ≥36 hours to obtain finished steel plates.

[0050] This invention addresses various aspects, including alloy element screening and proportioning, steel cleanliness control, heating process improvement, rolling process optimization, cooling process control, and microstructure strength-toughness matching, ultimately determining a composition system and manufacturing process that meets the objectives of this invention. A novel low-temperature steel plate design is achieved using appropriate carbon + manganese content and a low-alloy composition system. The production process employs a novel TMCP controlled rolling process, utilizing 230–360 mm cross-section continuous casting billets to prepare TMCP-condition NV2-4L marine steel plates with a maximum thickness of 60 mm. These plates exhibit excellent strength-toughness matching across the entire thickness cross-section, along with high uniform elongation and good low-temperature CTOD and NDT fracture properties. This results in excellent low-temperature fracture and tear resistance, ensuring the technical requirements for low-temperature steel plates under different service environments are met.

[0051] The mechanism of action of each alloy component in the NV4-4 steel plate with high uniform elongation and good low-temperature fracture toughness described in this invention is as follows:

[0052] C is an inexpensive element that effectively improves the strength of steel plates. However, as the carbon content increases, the plasticity, low-temperature toughness and resistance to weld cracks of the steel plate will be significantly reduced. An appropriate amount of carbon can be used in conjunction with cooling control to ensure the strength and toughness of the steel plate. From the perspective of improving the product's resistance to low-temperature fracture, this invention controls the C content to 0.05% to 0.15%.

[0053] Si (Si) is a major deoxidizing component in steelmaking. It acts as both a deoxidizer and a reducing agent, contributing to increased steel plate strength. However, when its content exceeds 0.5%, it promotes the formation of Mao islands, impairing weldability and low-temperature toughness. Appropriate Si addition can improve the elastic limit and yield strength of steel, as well as its oxidation resistance at high temperatures. For marine steel plates, a lower Si content can also improve the surface quality. This invention aims to achieve products with excellent resistance to low-temperature fracture; therefore, the Si content should not be too high. This invention controls the Si content to be between 0.06% and 0.25%.

[0054] Manganese (Mn) is an essential element for ensuring the strength and toughness of steel. Mn combines with sulfur (S) to form MnS, which prevents hot cracking caused by FeS formation at grain boundaries. Mn is also a good deoxidizer. Appropriate amounts of manganese can improve the strength and toughness of steel, but excessive content can lead to segregation in the cast billet, resulting in banded structures that are difficult to eliminate after rolling, reducing the transverse properties and resistance to lamellar tearing of the steel plate. To improve the strength-toughness balance of the product, this invention adds Mn along with nickel, which enhances the product's resistance to low-temperature fracture and low-temperature elongation. Therefore, this invention controls the Mn content to be 1.10%–1.75%.

[0055] P: It is an element that has an adverse effect on the impact value and can impair low-temperature toughness by segregation in the center of the slab and agglomeration at grain boundaries. In this invention, the P content is controlled to be no higher than 0.008%.

[0056] S: It is an element that has an adverse effect on the impact value and can form sulfide inclusions that become crack initiation sites. This invention controls the S content to be no higher than 0.002%.

[0057] Als: As a necessary deoxidizing and grain-refining element added in this invention, its content is above 0.01%, but when the content exceeds 0.08%, it is easy to cause hot cracking of the billet, and the toughness of the steel will decrease. Therefore, this invention controls its content to be 0.015% to 0.045%.

[0058] Nb: Adding Nb to steel can effectively refine the grain size of the steel and improve its strength and toughness. However, the effect is not obvious when the addition amount is less than 0.01%, so the present invention controls the Nb content to be between 0.02% and 0.06%.

[0059] Mo is a key element in expanding the γ-phase region, delaying the formation of ferrite during the γ→α phase transformation, and promoting the formation of acicular ferrite. It plays a crucial role in controlling the phase transformation microstructure, effectively improving material strength, lowering the phase transformation temperature, and reducing the critical cooling rate for bainite transformation. This facilitates the promotion of bainite transformation over a wider cooling rate range, giving thick steel plates better process adaptability. Simultaneously, it effectively improves the stability of the strength and toughness properties in the thickness direction of the steel plate. Therefore, this invention controls the Mo content to be between 0.05% and 0.20%.

[0060] Cu: Adding Cu to steel can improve its corrosion resistance, strength, weldability, formability, and machinability. Using Cu and Ni together can also prevent hot brittleness. In this invention, the Cu content is controlled at 0.10%–0.30%.

[0061] Cr: An important element for improving the hardenability of steel. For thick-gauge shipbuilding and offshore platform steel, adding a higher Cr content can effectively improve hardenability to compensate for the strength loss caused by thickness and improve the uniformity of performance in the thickness direction. Therefore, the Cr content in this invention is controlled at 0.10% to 0.30%.

[0062] Ni has a solid solution strengthening effect, which can promote the formation of a stable austenitic structure in alloy steel. It has the characteristics of minimizing the Ar3 point and the increase in carbon equivalent or cold crack sensitivity coefficient Pcm. It can improve the strength and toughness of steel and improve the hot brittleness caused by Cu in steel. Therefore, the present invention controls the Ni content at 0.20% to 0.80%.

[0063] Nitrogen (N) combines with elements such as Al, Ti, and Nb to form nitrides, which are elements that refine the microstructure of the base material. To achieve this effect, the N content needs to be above 0.002%; however, excessive dissolved N deteriorates the toughness of the HAZ (heat-affected zone). Reasonable control of the N content can refine the grains. Therefore, this invention controls the N content to be between 0.003% and 0.015%.

[0064] Ti: As a component added to improve the toughness of steel and the toughness of welded parts, it exists in the form of TiN and plays a role. However, when its content exceeds 0.04%, it is easy to form large TiN particles and lose its due effect. Therefore, the present invention controls the Ti content to be between 0.008% and 0.018%.

[0065] The present invention discloses a method for manufacturing NV4-4 steel plate with high uniform elongation and good low-temperature fracture toughness. The production process includes: steel smelting and casting → billet heating → two-stage controlled rolling → controlled cooling → stacking and slow cooling. The key steps of its preparation process are as follows:

[0066] 1) Steel smelting and casting process: Select high-quality raw materials, primarily iron or high-quality recycled steel, with P and S content as low as possible. Control the smelting composition according to target values, strictly control residual element content, and avoid exceeding the carbon equivalent limit. LF refining and RH refining each require 20-40 minutes. The superheat of the molten steel in the ladle should be ≤35℃, and casting should be carried out under full protection. Inclusions of types A, B, C, and D in the steel must meet the following requirements: Type A inclusions ≤0.5 grade, Type B inclusions ≤1.0 grade, Type C inclusions ≤1.0 grade, and Type D inclusions ≤0.5 grade. The continuous casting billet forming process employs light reduction technology and incorporates electromagnetic stirring equipment, effectively addressing center segregation and grain size uniformity in the continuous casting billet. Continuously cast slabs require slow cooling through stacking for ≥24 hours.

[0067] 2) Steel billet heating process: A new heating system is adopted, with a heating temperature of 1150~1250℃, a heat soaking temperature of 1130~1230℃, and a heat soaking time of 60~90min.

[0068] 3) Two-stage controlled rolling process: A new two-stage controlled rolling technology is adopted. The first stage rolling temperature is 1120~1180℃, and the single-pass deformation rate is ≥15%; the intermediate billet thickness is 1.8~2.5 times the thickness of the finished steel plate; the second stage rolling temperature is 800~900℃, the single-pass deformation rate is ≥12%, and the final rolling temperature is 780~880℃.

[0069] This invention combines large deformation rolling process to refine the original microstructure, control and adjust the composition, uniformity and refinement of phase transformation microstructure and multiphase particle precipitation behavior, so as to ensure the uniformity of grain size on the full thickness section of the thick steel plate, and improve the uniform elongation and resistance to low temperature fracture (CTOD) of the steel grade.

[0070] 4) Controlled cooling process: The average cooling rate of the steel plate is 10℃~20℃ / S, the starting cooling temperature is 720~780℃, and the final cooling temperature is 430~500℃. The cooling process adopts ACC laminar flow fully automatic controlled cooling mode, and the steel plate is shielded at both ends throughout the process to ensure the uniformity and stability of the performance of different positions of the steel plate.

[0071] 5) Stacking and slow cooling process: After controlled cooling, the steel plates are stacked and slow cooled for ≥36 hours to obtain the finished steel plate.

[0072] The following embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0073]

Example

[0074] The chemical composition of the NV4-4 high-strength low-temperature marine steel plate (hereinafter referred to as steel plate) prepared in each embodiment is shown in Table 1, the steelmaking process parameters are shown in Table 2, the rolling and cooling process parameters of the steel plate are shown in Table 3, the conventional mechanical properties of the steel plate are shown in Table 4-1, the mechanical properties of the steel plate after simulated welding and heat treatment in each embodiment are shown in Table 4-2, and the low-temperature fracture properties of the steel plate are shown in Table 5.

[0075] Table 1. Chemical composition (mass percentage) of the steel in each embodiment.

[0076]

[0077] Table 2 Steelmaking process parameters for each embodiment

[0078]

[0079] Table 3. Steel plate rolling and cooling process parameters for each embodiment.

[0080]

[0081] Note: t is the thickness of the finished steel plate, in mm.

[0082] Table 4-1 Conventional mechanical properties of finished steel plates from various embodiments

[0083]

[0084]

[0085] Table 4-2 Mechanical properties of finished steel plates after simulated welding and heat treatment in each embodiment

[0086]

[0087] Table 5 Low-temperature fracture properties of finished steel plates from each embodiment

[0088]

[0089] Microstructure and inclusions of the steel plate prepared in Example 1 ( Figure 1 , Figure 2 As can be seen, after being processed by the novel TMCP two-stage controlled rolling process described in this invention, the steel plate structure is basically ferrite + bainite. The finished steel plate has clear grain boundaries, fine grains, and few inclusions, thus achieving the characteristics of high uniform elongation and good low-temperature fracture toughness of NV4-4 steel plate.

[0090] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An NV4-4 steel sheet having high uniform elongation and good low-temperature fracture toughness, characterized by, The chemical composition of the steel plate, by mass percent, is C: 0.05% to 0.15%; Si: 0.06% to 0.25%; Mn: 1.10% to 1.75%; S: ≤0.002%; P: ≤0.008%; Als: 0.015% to 0.045%; N: 0.003% to 0.015%; Nb: 0.02% to 0.06%; Cu: 0.10% to 0.30%; Cr: 0.10% to 0.30%; Ni: 0.40% to 0.80%; Mo: 0.05% to 0.12%; Ti: 0.008% to 0.018%; the balance being Fe and unavoidable impurities; the yield strength of the finished steel plate is ≥335 MPa, the tensile strength is ≥490 MPa; the impact energy at -90 ℃ is ≥200 J; the uniform elongation is ≥10%; the CTOD characteristic value of the base metal at -30 ℃ is >1.52 mm, the CTOD characteristic value of the base metal at -50 ℃ is >1.01 mm, and the CTOD characteristic value of the base metal at -70 ℃ is >0.55 mm; the CTOD characteristic value of the welded heat-affected zone at -30 ℃ is >1.19 mm, the CTOD characteristic value of the welded heat-affected zone at -50 ℃ is >0.66 mm, and the CTOD characteristic value of the welded heat-affected zone at -70 ℃ is >0.32 mm.

2. The NV4-4 steel sheet having high uniform elongation and good low-temperature fracture toughness according to claim 1, characterized by, The microstructure of the finished steel plate is ferrite + bainite, and the volume percentage of ferrite is 60% to 75%.

3. The NV4-4 steel sheet having high uniform elongation and good low-temperature fracture toughness according to claim 1, characterized by, After the finished steel plate is subjected to simulated welding and heat treatment at 580 to 600 ℃, the mechanical properties meet the following requirements: the yield strength is ≥335 MPa, the tensile strength is ≥490 MPa, the uniform elongation is ≥10%, and the impact energy at -90 ℃ is ≥180 J.

4. The NV4-4 steel sheet having high uniform elongation and good low-temperature fracture toughness according to claim 1, characterized by, The thickness of the finished steel plate is 40 to 60 mm.

5. The method of producing a NV4-4 steel sheet having high uniform elongation and good low-temperature fracture toughness according to any one of claims 1 to 4, characterized by, The production process flow includes molten steel smelting and casting, billet heating, multi-stage controlled rolling, controlled cooling, and stacking and slow cooling processes; the following processes are controlled: 1) Billet heating; The heating temperature is 1150 to 1250 ℃, the soaking temperature is 1130 to 1230 ℃, and the soaking time is 60 to 90 min; 2) Two-stage controlled rolling; Two-stage controlled rolling is adopted; the opening rolling temperature of the first stage is 1120 to 1180 ℃, and the single pass deformation rate is ≥15%; the intermediate billet thickness is 1.8 to 2.5 times the thickness of the finished steel plate; the opening rolling temperature of the second stage is 800 to 900 ℃, and the single pass deformation rate is ≥12%; and the finish rolling temperature is 780 to 880 ℃; 3) Controlled cooling; The average cooling speed is 10 to 20 ℃ / s, the opening cooling temperature is 720 to 780 ℃, and the final cooling temperature is 430 to 500 ℃; 4) Stacking and slow cooling; the steel plate after controlled cooling is subjected to stacking and slow cooling treatment, and the treatment time is ≥36 h to obtain the finished steel plate.

6. The method of producing a NV4-4 steel sheet having high uniform elongation and good low-temperature fracture toughness according to claim 5, characterized by, In the molten steel smelting and casting process, the treatment time of LF refining and RH refining is 20 to 40 min, and the molten steel overheat degree in the tundish is ≤35 ℃; the continuous casting slab is subjected to stacking and slow cooling treatment, and the treatment time is ≥24 h.

7. The method of producing a NV4-4 steel sheet having high uniform elongation and good low-temperature fracture toughness according to claim 5, characterized by, In the molten steel smelting and casting process, the molten steel is fully protected during casting, and the continuous casting billet forming process adopts light press-down and electromagnetic stirring.

8. The method of producing a NV4-4 steel sheet having high uniform elongation and good low-temperature fracture toughness according to claim 5, characterized by, In the molten steel smelting and casting process, the inclusions in the refined steel satisfy the requirements that the A-type inclusions are less than or equal to 0.5 level, the B-type inclusions are less than or equal to 1.0 level, the C-type inclusions are less than or equal to 1.0 level, and the D-type inclusions are less than or equal to 0.5 level.

9. The method of producing a NV4-4 steel sheet having high uniform elongation and good low-temperature fracture toughness according to claim 5, characterized by, In the step 3), the controlled cooling adopts an ACC laminar flow full-automatic controlled cooling mode, and the head and tail of the steel plate are shielded throughout the whole process.

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