A heat treatment method for 450MPa grade extra-thick steel plates suitable for low-temperature environments

Abstract

This invention discloses a heat treatment method for 450MPa grade extra-thick steel plates suitable for low-temperature environments, belonging to the field of extra-thick steel plate manufacturing technology in metallurgical engineering. This invention, through a low-cost composition system design of low C + appropriate amount of Ni + Nb, combined with precise control of the internal quality of continuously cast billets, innovatively adopts a four-step synergistic heat treatment process of high-temperature quenching → tempering → low-temperature quenching → low-temperature tempering. This solves the industry pain points of uneven core structure, insufficient low-temperature toughness, residual internal stress leading to cracking, and high manufacturing costs in extra-thick steel plates. The chemical composition of the obtained steel plate by mass percentage is: C≤0.12%, Si=0.15~0.50%, Mn=1.0~1.3%, P≤0.006%, S≤0.002%, Ni=0.40~0.60%, Alt=0.020~0.040%, Nb=0.015~0.040%, with the balance being Fe and unavoidable impurities; the finished steel plate thickness is 120~150mm, and the mechanical properties meet the following requirements: yield strength 260~400MPa, tensile strength 450~585MPa, elongation after fracture ≥22%, and V-notch impact energy at -75℃ ≥100J.

Description

Technical Field

[0001] This invention relates to the field of extra-thick steel plate manufacturing technology in metallurgical engineering, specifically to a heat treatment method for 450MPa-level extra-thick steel plates suitable for low-temperature environments. This steel plate can be widely used in fields with stringent requirements for low-temperature toughness and structural strength, such as low-temperature storage tanks, polar transport ships, large bridges in cold regions, and engineering machinery. Background Technology

[0002] With the global energy development extending to the polar deep sea and the rapid development of polar shipping and infrastructure construction in cold regions, the demand for extra-thick steel plates with a thickness of ≥100mm is increasing. These steel plates must simultaneously meet the dual core requirements of high strength (yield strength 260-400MPa, tensile strength 450-585MPa) and excellent low-temperature toughness (impact energy ≥100J at -75℃).

[0003] However, existing technologies for producing extra-thick low-temperature steel plates face three major industry bottlenecks that cannot be overcome: First, the internal quality defects of continuously cast billets are difficult to solve. Due to their large thickness, extra-thick steel plates are prone to central segregation caused by the enrichment of elements such as C and Mn during the solidification process of continuously cast billets, as well as central porosity caused by solidification shrinkage. Existing technologies mostly control the central segregation of continuously cast billets to Class B 0.5 or even higher, which cannot meet the stringent requirements of extra-thick low-temperature steel plates. Ultimately, this leads to uneven internal structure of the rolled steel plate, and heat treatment cannot completely eliminate this defect, directly causing the core to fail the low-temperature impact performance test.

[0004] Secondly, the heat treatment process is poorly compatible with extra-thick plates. Traditional extra-thick steel plates mostly adopt the conventional quenching and tempering process of "one quenching + one tempering". Although it can ensure the surface strength of the steel plate, it has three fatal defects: First, it cannot fully eliminate the segregation in the core of the steel plate, and the impact performance of the core of the extra-thick plate cannot meet the standards; second, after high-temperature quenching, the austenite grains tend to be coarse, which directly leads to a decrease in the low-temperature toughness of the steel plate; third, the cooling rate of the core of the thick plate is slow, which easily forms hard and brittle martensite or bainite structures. After tempering, the internal stress is not completely eliminated, and subsequent processing is prone to cracking.

[0005] Third, improving low-temperature toughness relies on high-Ni precious alloys, which keep costs high. To achieve low-temperature toughness at -70℃ and below, existing technologies generally adopt the approach of increasing Ni content (>0.8%, or even 3.5%-5.5%) or adding precious alloys such as Mo and Cr, which significantly increases raw material costs and is not conducive to large-scale industrial application.

[0006] Therefore, there is an urgent need for a technical solution that can achieve a balance between high strength and ultra-low temperature toughness in 450MPa grade ultra-thick steel plates at low cost by optimizing chemical composition design, precisely controlling the internal quality of continuously cast billets, and innovating heat treatment processes. Summary of the Invention

[0007] The technical problem to be solved by this invention is to provide a heat treatment method for 450MPa grade extra-thick steel plates suitable for low-temperature environments, which is in contrast to the above-mentioned prior art. By precisely controlling the chemical composition, the internal quality of the continuously cast billet, and an innovative four-step heat treatment process, the core problems of severe core segregation, insufficient low-temperature toughness, high internal stress, and high cost of extra-thick steel plates are completely solved. This ensures that the steel plate stably meets the core performance indicators of yield strength 260-400MPa, tensile strength 450-585MPa, impact energy at -75℃ ≥100J, and elongation after fracture ≥22%.

[0008] The technical solution adopted by this invention to solve the above problems is as follows: a heat treatment method for 450MPa grade extra-thick steel plates suitable for low-temperature environments, employing the following technical solution: 1. Chemical composition system design (mass percentage) C≤0.12%: Low-carbon design is adopted to reduce cementite precipitation, reduce the low-temperature brittleness of steel plate, and avoid the risk of welding cold cracking caused by excessive C content, thus ensuring the weldability of steel plate; Si=0.15~0.50%: As a deoxidizer in steelmaking, it also enhances the strength of steel plates through solid solution strengthening. Controlling the content within this range can avoid the decrease in steel plate toughness caused by excessive Si. Mn=1.0~1.3%: The core solid solution strengthening element, which can improve the stability of austenite, expand the austenite phase region, and ensure that the strength and toughness of the steel plate meet the requirements; P≤0.006%, S≤0.002%: Strictly control the content of harmful elements. P easily leads to cold brittleness of steel, and S easily forms sulfide inclusions. Controlling the content of both at ultra-low levels can significantly improve the low-temperature impact toughness of steel plates. Ni = 0.40~0.60%: The core low-temperature toughness element, which can effectively reduce the ductile-brittle transition temperature of steel. Compared with the traditional Ni content of >0.8%, this solution can significantly reduce raw material costs while ensuring impact energy at -75℃. Alt=0.020~0.040%: Effective deoxidizing element in steel, which can refine austenite grains, inhibit austenite grain growth, and significantly improve the strength and toughness of steel plates; Nb=0.015~0.040%: The core element of microalloying, which can form Nb (C,N) precipitates, hinder dislocation movement, achieve precipitation strengthening, refine austenite recrystallization grains, and optimize the uniformity of microstructure in the thickness direction of the steel plate; The balance is Fe and unavoidable impurities.

[0009] 2. Continuous casting and rolling process control To address internal quality defects in extra-thick steel plates, direct rolling of continuously cast billets is employed, and precise control standards are proposed for both the continuous casting and rolling processes. Continuous casting billet quality: Center segregation ≤ Class C 0.5 grade, which reduces element enrichment by optimizing the continuous casting secondary cooling weak cooling regime and dynamic light reduction process (reduction amount 2.5-4.0mm); Center porosity ≤ 0.5 grade, which ensures full solidification of the billet by controlling the continuous casting speed 0.8-1.0m / min. Rolling process: Multi-pass reversible two-stage controlled rolling is adopted. The roughing rolling temperature is 1050-1150℃, the single-pass reduction rate is ≥15%, and the cumulative reduction rate is ≥60%, which fully breaks the as-cast structure and refines the austenite grains. The finishing rolling temperature is 850-900℃ to ensure that deformation penetrates to the core of the steel plate and improves the uniformity of the structure in the thickness direction of the steel plate. The nominal thickness of the steel plate after rolling is 120~150mm.

[0010] 3. Four-step synergistic heat treatment process This invention innovatively employs a four-step process: "high-temperature quenching → tempering → low-temperature quenching → low-temperature tempering." Through the synergistic effect of two quenching processes and two tempering processes, it achieves microstructure refinement and gradual elimination of internal stress, completely solving the problem of uneven microstructure in the core of extra-thick plates. The specific steps are as follows: ① High-temperature quenching: The steel plate is heated to its austenitizing temperature of 870–910℃ and held at this temperature with a coefficient of 1.5 min / mm for the plate thickness. After holding, it is then subjected to high-pressure water quenching to room temperature. The surface quenching cooling rate is 30–35℃ / s. The core function of this step is to fully austenitize the steel plate, promote the complete dissolution of precipitates such as Nb (C,N), and form a fine lath bainite structure through rapid water cooling. This initially refines the core structure and grains of the steel plate, preparing the microstructure for subsequent tempering.

[0011] ② Tempering: Heat the high-temperature quenched steel plate to 620-660℃, hold it at this temperature with a coefficient of 3.0 min / mm for the steel plate thickness, and then air cool it to room temperature. The core function of this step is to eliminate the internal stress generated by high-temperature quenching, promote the precipitation of fine carbonitrides, improve the toughness of the steel plate, and at the same time avoid excessive reduction in the hardness of the steel plate, thus ensuring the basic strength.

[0012] ③ Low-temperature quenching: The tempered steel plate is heated to the austenitizing temperature of 840-870℃ and held at this temperature with a coefficient of 1.5-2.0 min / mm based on the steel plate thickness. After holding, it is then water-quenched under high pressure to room temperature, with a surface quenching cooling rate of 30-35℃ / s. This step, by lowering the austenitizing temperature, can further refine the austenite grains by 20-30% compared to a single high-temperature quenching, forming a more uniform bainite structure and a small amount of ferrite structure, significantly improving the difference in microstructure between the core and surface of the steel plate. Extending the holding time promotes the homogenization of austenite in the steel plate, precisely controls the grain size, avoids the hard and brittle M / A island phases generated by a single quenching + single tempering process, and effectively improves the low-temperature impact toughness of the steel plate core.

[0013] ④ Low-temperature tempering: The steel plate after low-temperature quenching is heated to 580-620℃ and held at that temperature with a coefficient of 3.0 min / mm based on the nominal thickness of the steel plate. After holding, it is air-cooled to room temperature. The core function of this step is to completely eliminate the internal stress generated by low-temperature quenching, stabilize the microstructure of the steel plate, and ultimately obtain a composite microstructure of fine-grained tempered bainite + a small amount of ferrite, ensuring a perfect balance between toughness and strength at -75℃.

[0014] 4. Finished Product Performance Indicators The steel plates prepared using the above technical solutions must stably meet the following performance requirements: Yield strength: 260MPa~400MPa Tensile strength: 450MPa~585MPa Elongation after fracture: ≥22% -75℃ V-notch impact energy: ≥100J.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention achieves a perfect balance between high strength and ultra-low temperature toughness through a low C + appropriate amount of Ni + Nb composition design and a four-step synergistic heat treatment process. The impact energy of the steel plate at -75℃ can reach 100-180J, which is far higher than the average level of similar steel grades, and can stably meet the usage requirements of extreme low temperature environments at -75℃.

[0016] 2. This invention effectively solves the industry problem of uneven microstructure in the core of 120-150mm extra-thick steel plates by controlling the center segregation and porosity of the continuously cast billet to below level 0.5 and combining it with a two-quenching process to homogenize the microstructure. The steel plate exhibits small fluctuations in properties in the thickness direction and no obvious differences in properties between the surface and the core.

[0017] 3. The Ni content of this invention is precisely controlled at 0.40-0.60%, which reduces raw material costs by 15-20% compared to the traditional high Ni solution (Ni>0.8%), and there is no need to add precious alloys such as Mo and Cr; moreover, the four-step heat treatment process can be realized by a continuous heat treatment furnace, without the need for additional special equipment, making it suitable for industrial mass production.

[0018] 4. The steel plate prepared by this invention has excellent low-temperature toughness, strength, weldability and processing performance, which can meet the needs of various scenarios such as low-temperature storage tanks, polar ships, large bridges in cold regions, and engineering machinery. It is especially suitable for pressure equipment in extreme low-temperature environments of -75℃. Attached Figure Description

[0019] Figure 1 This is a metallographic diagram of a steel plate with a thickness of 1 / 4 in Example 1 of the present invention.

[0020] Figure 2 This is a metallographic diagram of the steel plate at 1 / 2 thickness in Embodiment 1 of the present invention.

[0021] Figure 3 This is a graph showing the performance distribution of the steel plate in the thickness direction in Embodiment 1 of the present invention.

[0022] Figure 4 This is a graph showing the four-step heat treatment process in an embodiment of the present invention.

[0023] Figure 5 This is a morphology diagram of the impact fracture surface of the steel plate at -75℃ in Embodiment 1 of the present invention. Detailed Implementation

[0024] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention. Example 1

[0025] Chemical composition (mass percentage): C=0.06%, Si=0.40%, Mn=1.1%, P=0.005%, S=0.001%, Ni=0.50%, Alt=0.030%, Nb=0.025%, balance Fe and unavoidable impurities.

[0026] Continuous casting and rolling: Production is carried out using continuously cast billets with a casting speed of 0.9 m / min, a dynamic light reduction of 3.0 mm, and a low-magnification center segregation grade C of 0.5 and a center porosity grade of 0.5. Two-stage controlled rolling is adopted, with a roughing rolling start temperature of 1150℃ and a cumulative reduction rate of 65%, and a finishing rolling finish temperature of 880℃, resulting in a steel plate thickness of 120 mm after rolling.

[0027] Heat treatment process: ① High-temperature quenching: Heat to 870℃, hold at 1.5 min / mm, and then quench under high pressure with water to room temperature; ② Tempering: Heat to 660℃, hold at 3.0 min / mm, and air cool to room temperature; ③ Low-temperature quenching: Heat to 860℃, hold at 2.0 min / mm, and then quench under high pressure with water to room temperature; ④ Low-temperature tempering: Heat to 620℃, hold at 3.0 min / mm, and air cool to room temperature. Example 2

[0028] Chemical composition (mass percentage): C=0.07%, Si=0.40%, Mn=1.2%, P=0.004%, S=0.001%, Ni=0.55%, Alt=0.030%, Nb=0.030%, balance Fe and unavoidable impurities.

[0029] Continuous casting and rolling: Production is carried out using continuously cast billets with a casting speed of 0.85 m / min, a dynamic light reduction of 3.5 mm, and a low-magnification center segregation grade C of 0.5 and a center porosity grade of 0.5. Two-stage controlled rolling is adopted, with a roughing rolling start temperature of 1100℃ and a cumulative reduction rate of 62%, and a finishing rolling finish temperature of 870℃, resulting in a steel plate thickness of 130 mm after rolling.

[0030] Heat treatment process: ① High-temperature quenching: Heat to 890℃, hold at 1.5 min / mm, and then quench under high pressure with water to room temperature; ② Tempering: Heat to 650℃, hold at 3.0 min / mm, and air cool to room temperature; ③ Low-temperature quenching: Heat to 850℃, hold at 2.0 min / mm, and then quench under high pressure with water to room temperature; ④ Low-temperature tempering: Heat to 610℃, hold at 3.0 min / mm, and air cool to room temperature. Example 3

[0031] Chemical composition (mass percentage): C=0.08%, Si=0.40%, Mn=1.3%, P=0.004%, S=0.001%, Ni=0.60%, Alt=0.030%, Nb=0.035%, balance Fe and unavoidable impurities.

[0032] Continuous casting and rolling: Production is carried out using continuously cast billets with a casting speed of 0.8 m / min, a dynamic light reduction of 4.0 mm, and a low-magnification center segregation grade C of 0.5 and a center porosity grade of 0.5. Two-stage controlled rolling is adopted, with a roughing rolling start temperature of 1050℃ and a cumulative reduction rate of 60%, and a finishing rolling finish temperature of 860℃, resulting in a steel plate thickness of 150 mm after rolling.

[0033] Heat treatment process: ① High-temperature quenching: Heat to 900℃, hold at 1.5 min / mm, and then quench under high pressure with water to room temperature; ② Tempering: Heat to 640℃, hold at 3.0 min / mm, and air cool to room temperature; ③ Low-temperature quenching: Heat to 840℃, hold at 2.0 min / mm, and then quench under high pressure with water to room temperature; ④ Low-temperature tempering: Heat to 590℃, hold at 3.0 min / mm, and air cool to room temperature.

[0034] Performance test results For the steel plates prepared in the above three embodiments, samples were taken at 1 / 2 of the plate thickness for mechanical property testing. The impact test temperature was -75℃. The test results are shown in the table below: Table 1 Mechanical properties of the steel plates produced in the examples

[0035] Test results show that the steel plates prepared in the three embodiments of the present invention have stable performance that meets the design requirements. The impact energy at -75℃ is much higher than the requirement of ≥100J. Even the extra-thick steel plate with a thickness of 150mm still has excellent low-temperature toughness in the core.

[0036] Comparative Example 1 (Conventional one-quench-one-return process) The same chemical composition, continuous casting and rolling process as in Example 1 were used. The heat treatment adopted the conventional "one-time quenching + one-time tempering" process: quenching temperature 880℃, holding temperature 1.5min / mm, water cooling to room temperature; tempering temperature 630℃, holding temperature 3.0min / mm, air cooling to room temperature.

[0037] Performance test results: Yield strength 360MPa, tensile strength 530MPa, elongation after fracture 25%, and impact energy at -75℃ only 45, 52, and 48J, respectively, which are far below the requirements of this invention. The low-temperature toughness of the core is unqualified.

[0038] Comparative Example 2 (High Ni content system) The chemical composition was adjusted to Ni=1.0%, and the remaining components were exactly the same as in Example 1. The continuous casting, rolling, and heat treatment processes were exactly the same as in Example 1.

[0039] Performance test results: Yield strength 355MPa, tensile strength 525MPa, elongation after fracture 29%, impact energy at -75℃ 148, 155, 152J. The performance is comparable to that of Example 1, but the Ni content is increased by 1 times, the raw material cost is increased by 18%, and the economic efficiency is far lower than that of this invention.

[0040] In addition to the above embodiments, the present invention can also adjust parameters such as billet thickness and continuous casting process according to the production requirements of converters and electric furnaces of different tonnages. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A heat treatment method of a 450 MPa grade ultra-heavy steel plate suitable for low temperature environment, characterized in that, The following steps are performed sequentially: (1) Chemical composition configuration: The chemical composition of the steel plate, by mass percentage, is as follows: C≤0.12%, Si=0.15~0.50%, Mn=1.0~1.3%, P≤0.006%, S≤0.002%, Ni=0.40~0.60%, Alt=0.020~0.040%, Nb=0.015~0.040%, with the balance being Fe and unavoidable impurities; (2) Continuous casting and rolling: The continuous casting billet is rolled in multiple passes. The center segregation of the continuous casting billet is ≤ Class C 0.5 grade and the center porosity is ≤ 0.5 grade. The thickness of the steel plate after rolling is 120~150mm. (3) Four-step synergistic heat treatment: The rolled steel plate is subjected to high-temperature quenching, medium-temperature tempering, low-temperature quenching, and low-temperature tempering in sequence. The specific process is as follows: ① High-temperature quenching: Heat the steel plate to 870~910℃, hold it at a temperature of 1.5 min / mm based on the steel plate thickness, and then water cool it to room temperature after holding. ② Medium-temperature tempering: Heat the high-temperature quenched steel plate to 620~660℃, hold it at a coefficient of 3.0 min / mm for the steel plate thickness, and air cool it to room temperature after holding. ③ Low temperature quenching: Heat the steel plate after medium temperature tempering to 840~870℃, hold it at a coefficient of 1.5~2.0 min / mm for the steel plate thickness, and then water cool it to room temperature after holding. ④ Low-temperature tempering: Heat the steel plate after low-temperature quenching to 580~620℃, hold it at a coefficient of 3.0 min / mm for the steel plate thickness, and air cool it to room temperature after holding.

2. The heat treatment method of the 450 MPa grade extra thick steel plate for low temperature service according to claim 1, characterized in that, In step (1), the chemical composition, by mass percentage, is: C=0.06~0.08%, Si=0.30~0.40%, Mn=1.1~1.3%, P≤0.005%, S≤0.001%, Ni=0.50~0.60%, Alt=0.025~0.035%, Nb=0.025~0.035%.

3. The heat treatment method of the 450 MPa grade extra thick steel plate for low temperature service according to claim 1, characterized in that, In step (2), the continuous casting process adopts a constant casting speed of 0.8~1.0m / min, and the dynamic light reduction process controls the center segregation. The total reduction under dynamic light reduction is 2.5~4.0mm.

4. The heat treatment method for 450MPa grade extra-thick steel plates suitable for low-temperature environments according to claim 1, characterized in that, In step (2), the rolling process adopts a two-stage controlled rolling process, wherein the roughing rolling temperature is 1050~1150℃, the single-pass reduction rate is ≥15%, and the cumulative reduction rate is ≥60%; the finishing rolling temperature is 850~900℃.

5. The heat treatment method for 450MPa grade extra-thick steel plates suitable for low-temperature environments according to claim 1, characterized in that, In step (3), the heating temperature for high-temperature quenching is 870~900℃, the water cooling process adopts high-pressure water quenching, and the quenching cooling rate of the steel plate surface is 30~35℃ / s.

6. The heat treatment method for 450MPa grade extra-thick steel plates suitable for low-temperature environments according to claim 1, characterized in that, In step (3), the heating temperature for medium-temperature tempering is 640~660℃.

7. The heat treatment method for 450MPa grade extra-thick steel plates suitable for low-temperature environments according to claim 1, characterized in that, In step (3), the heating temperature for low-temperature quenching is 840~860℃, the heat preservation coefficient is 2.0min / mm, the water cooling process adopts high-pressure water quenching, and the quenching cooling rate of the steel plate surface is 30~35℃ / s.

8. The heat treatment method for 450MPa grade extra-thick steel plates suitable for low-temperature environments according to claim 1, characterized in that, In step (3), the heating temperature for low-temperature tempering is 590~620℃.

9. A heat treatment method for 450MPa grade extra-thick steel plates suitable for low-temperature environments according to claim 1, characterized in that, The mechanical properties of the obtained steel plate meet the following requirements: yield strength 260~400MPa, tensile strength 450~585MPa, elongation after fracture ≥22%, and V-notch impact energy at -75℃ ≥100J.

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