High-strength high-toughness Nb-containing steel and nano-phase controlled rolling and controlled cooling preparation method thereof

Abstract

This invention discloses a high-strength, high-toughness Nb-containing steel and its nano-phase controlled rolling and cooling preparation method, belonging to the field of iron and steel metallurgy and rolling technology. The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.12~0.20%, Si: 0.55~0.65%, Mn: 1.40~1.50%, Nb: 0.030~0.080%, N: 0.0080~0.0110%, Al: 0.020~0.050%, P≤0.030%, S≤0.025%, with the balance being Fe and unavoidable impurities; and the Nb / N mass ratio is controlled at 3.6~10.0, and the carbon equivalent C≤0.45%. The preparation method includes smelting, refining, continuous casting, heating, two-stage controlled rolling, and segmented controlled cooling steps. Through precise Nb / N ratio combined with controlled rolling and controlled cooling processes, nanoscale Nb(C,N) precipitates with a size of 20~80nm are induced, with a volume fraction ≥0.2%. The resulting steel plate has a microstructure of ferrite + bainite, a tensile strength ≥600MPa, a lower yield strength ≥450MPa, an elongation after fracture ≥18%, an impact absorption energy of ≥40J at -20℃, and a crack incidence rate ≤0.3%. This invention achieves a synergistic improvement in high strength, high toughness, and excellent crack resistance.

Description

Technical Field

[0001] This invention relates to the fields of iron and steel metallurgy and steel rolling technology, specifically to a high-strength, high-toughness Nb-containing steel. Background Technology

[0002] Niobium (Nb) microalloyed high-strength steel is widely used in key structural fields such as bridges, ships, pressure vessels, and marine engineering due to its excellent comprehensive mechanical properties. Its strengthening and toughening mechanism mainly relies on the nanoscale carbonitride precipitates formed by the microalloying element niobium during thermomechanical processing. Through the synergistic effect of grain refinement strengthening and precipitation strengthening, it maintains good toughness and weldability while improving strength.

[0003] For a long time, researchers have conducted extensive work on the microalloying design of Nb-containing steels. Studies have shown that the strain-induced precipitation behavior of niobium carbonitrides (such as Nb(C,N)) during austenite deformation plays a crucial role in suppressing recrystallization and refining the phase transformation microstructure. Through controlled rolling and cooling (TMCP) technology, the size, distribution, and volume fraction of the precipitated phases can be controlled, thereby optimizing the steel's properties. Furthermore, through rational compositional design, such as controlling the carbon equivalent and the proportion of microalloying elements, the weldability and crack resistance of the steel can be further improved.

[0004] However, existing technologies still have the following shortcomings: (1) First, in terms of precipitate control, although traditional Nb-containing steel can form a certain amount of carbonitrides, the size distribution of the precipitates is often wide. The presence of coarse precipitates (>100nm) not only weakens the precipitation strengthening effect, but also easily becomes the source of crack initiation, reducing the toughness and crack resistance of the material. Especially during continuous casting and welding thermal cycling, the interface between coarse precipitates and the matrix is ​​prone to stress concentration, inducing microcrack propagation.

[0005] (2) Secondly, in terms of crack control, Nb-containing high-strength steel faces multiple risks during production and use, including hot cracking, cold cracking, and delayed cracking. Although existing processes attempt to reduce crack sensitivity by controlling heating temperature, rolling deformation, and cooling rate, they often suffer from trade-offs—excessive strengthening of precipitates may lead to a decrease in toughness, while excessive pursuit of toughness makes it difficult to meet high strength requirements. In addition, if the Nb / N matching relationship is not properly designed, such as if the Nb / N ratio deviates from a reasonable range, brittle NbN phases may be formed or the number of precipitates may be insufficient, further deteriorating the crack resistance of the steel.

[0006] (3) Furthermore, in terms of process integration, existing technologies mostly focus on optimizing a single step, such as adjusting the rolling process or cooling regime, and lack a systematic and coordinated technical solution that integrates alloy composition design, nanophase precipitation control and stress release. This makes it difficult for products to meet the increasingly stringent engineering application requirements in terms of comprehensive performance such as strength and toughness matching and crack incidence rate.

[0007] Therefore, how to achieve precise chemical composition ratio and coordinated control of the entire process to regulate the size and distribution of nanoscale precipitates, thereby significantly reducing the crack incidence while ensuring high strength and toughness, remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to address the shortcomings of the prior art by providing a high-strength and high-toughness Nb-containing steel with high heat exchange efficiency, stable operation, and low modification cost, and a method for preparing the same by nano-phase controlled rolling and controlled cooling.

[0009] The technical problem to be solved by this invention is achieved through the following technical solution: a high-strength, high-toughness Nb-containing steel, the chemical composition of which, by mass percentage, is as follows: C: 0.12~0.20%, Si: 0.55~0.65%, Mn: 1.40~1.50%, Nb: 0.030~0.080%, N: 0.0080~0.0110%, P≤0.030%, S≤0.025%, Al: 0.020~0.050%, with the remainder being Fe and unavoidable impurities; among which, the Nb / N mass ratio is controlled at 3.6~10.0, and the carbon equivalent C≤0.45%.

[0010] Preferably, the microstructure of the steel is ferrite + bainite, wherein the volume fraction of ferrite is 60-70%; nano-sized Nb(C,N) precipitates are dispersed in the matrix, wherein the volume fraction of the nano-sized Nb(C,N) precipitates is ≥0.2% and the average particle size is 20-80 nm; the tensile strength R≥600MPa, the lower yield strength R≥450MPa, the elongation after fracture A≥18%, and the impact energy absorbed at -20℃≥40J; and the crack incidence rate of the steel during preparation and use is ≤0.3%, wherein the cracks include hot cracks, cold cracks and delayed cracks.

[0011] Preferably, the process includes the following steps: smelting, refining, continuous casting, heating, two-stage controlled rolling, and segmented controlled cooling; The two-stage controlled rolling and segmented controlled cooling work together to regulate the precipitation behavior and size distribution of nanoscale Nb(C,N) precipitates.

[0012] Preferably, the smelting step adopts converter smelting, and the endpoint is controlled with C≤0.08%, P≤0.030%, and S≤0.025%. When the steel is tapped to 1 / 2 to 2 / 3, FeSi, FeMn, Al deoxidizer, ferroniobium FeNb, and manganese nitride MnN are added sequentially for alloying. The amount of FeNb added per ton of steel is controlled to be 3.0 to 8.0 kg, and the amount of MnN added per ton of steel is 0.8 to 1.2 kg, to ensure that the Nb and N content and the Nb / N mass ratio in the molten steel are 3.6 to 10.0.

[0013] Preferably, the refining step is carried out in an LF refining furnace, with argon gas introduced for stirring, and the stirring intensity controlled at 0.3~0.5m / s; in the later stage of refining, the temperature is raised to 1560~1600℃ and held for 20~30min.

[0014] Preferably, the continuous casting step adopts billet continuous casting, the continuous casting temperature is controlled at 1500~1540℃, and the casting speed is 1.0~1.4m / min; the crystallizer adopts electromagnetic stirring, and the cooling water volume is controlled at 120~150m³ / h to obtain continuously cast billets.

[0015] Preferably, the heating step adopts a low-temperature preheating and high-temperature homogenization mode: the preheating temperature is 800~900℃, the heating temperature is 1160~1220℃, the homogenization temperature is 1180~1200℃, and the total heating time is 120~180min.

[0016] Preferably, the two-stage controlled rolling includes: Rough rolling stage: initial rolling temperature is 1080~1200℃, single-pass deformation is 15~20%, and cumulative deformation is 40~50%; Finishing stage: The initial rolling temperature is 880~980℃, the deformation per pass is 8~12%, and the cumulative deformation is 30~40%; after finishing rolling, the controlled cooling process is started immediately.

[0017] Preferably, the segmented cooling system is a three-stage cooling system consisting of rapid cooling, slow cooling, and air cooling. The first stage of rapid cooling: the cooling rate is 8~12℃ / s, cooling to 680~720℃; The second stage is slow cooling: the cooling rate is 2~4℃ / s, cooling to 500~550℃; Third stage: Air cooling to room temperature.

[0018] Preferably, the precise ratio of Nb and N elements is achieved through the smelting and refining steps, the in-situ precipitation of nano-sized Nb (C, N) is induced by the two-stage controlled rolling, and the size of the precipitated phase is controlled and residual stress is released by the segmented controlled cooling. The three elements work together to construct a low-stress crack-resistant microstructure system based on fine grain strengthening and nano-phase precipitation strengthening.

[0019] Compared with the prior art, the beneficial technical effects of the present invention are: (1) The Nb content is controlled at 0.030~0.080%, the N content at 0.0080~0.0110%, and the Nb / N mass ratio is limited to 3.6~10.0. This design ensures the formation of nanoscale Nb(C,N) precipitates with a size of 20~80nm in the steel, with a volume fraction of over 0.2%. On the one hand, it avoids the formation of coarse and brittle NbN phases due to excessive N, eliminating crack initiation sources; on the other hand, it prevents insufficient precipitate quantity due to insufficient Nb, ensuring the full utilization of precipitation strengthening effect. Compared with Nb-containing steels in the prior art with a wide distribution of precipitate size (some coarse phases >100nm), this invention achieves precise nanoscale control of precipitates, significantly improving strengthening efficiency.

[0020] (2) Through the synergistic design of the basic elements C, Si, and Mn (C: 0.12~0.20%, Si: 0.55~0.65%, Mn: 1.40~1.50%), combined with the grain refinement and precipitation strengthening effects of nanoscale precipitates, the steel achieves excellent comprehensive mechanical properties: tensile strength R≥600MPa, lower yield strength R≥450MPa, elongation after fracture A≥18%, and impact absorption energy at -20℃≥40J. In particular, by controlling the carbon equivalent C≤0.45%, the hardening tendency is effectively reduced while ensuring the strength base, avoiding the technical problem of high strength and high toughness being mutually exclusive.

[0021] (3) Through the coordinated control of the entire process of smelting-refining-continuous casting-heating-controlled rolling-controlled cooling, the steel cracking rate is controlled to within 0.3%, and no hot cracks or delayed cracks are generated. Specifically: During the smelting stage, the P and S contents are strictly controlled (P≤0.030%, S≤0.025%) to avoid grain boundary weakening caused by brittle inclusions such as FeS and FeP. Two-stage controlled rolling induces in-situ precipitation of nanoscale Nb (C, N), pinning austenite grain boundaries and refining the final microstructure; The rapid cooling, slow cooling, and air cooling processes in the segmented controlled cooling, especially the second stage slow cooling at 2~4℃ / s to 500~550℃, effectively released the residual stress after rolling and avoided cold cracks and delayed cracks caused by stress concentration.

[0022] (4) The segmented controlled cooling process of rapid cooling, slow cooling and air cooling adopted in this invention has a dual synergistic effect: the first stage of rapid cooling (8~12℃ / s to 680~720℃) promotes the rapid precipitation of nano-sized Nb (C, N), inhibits the coarsening of pearlite, and refines the microstructure; the second stage of slow cooling (2~4℃ / s to 500~550℃) effectively releases the residual stress accumulated during rolling while ensuring sufficient microstructure transformation. This process breaks through the limitations of traditional controlled cooling technology in that it is difficult to simultaneously control the size of precipitated phases and release stress, and achieves the unity of microstructure refinement and low stress state.

[0023] (5) This invention organically combines alloy composition design (precise Nb / N ratio), heating regime (low temperature preheating + high temperature homogenization to ensure complete Nb (C, N) solid solution), two-stage controlled rolling (inducing in-situ precipitation of nano phases) and segmented controlled cooling (size control + stress release) to form a systematic anti-cracking technology solution from molten steel to finished steel. Compared with the existing technology that only optimizes a single link, this invention achieves global optimal control of nano phase precipitation behavior and residual stress state through multi-process collaboration, which significantly improves the comprehensive performance and service reliability of the product. Attached Figure Description

[0024] Figure 1 The image shows the TEM morphology of the steel obtained in Example 1 of this invention after controlled rolling. Figure 2 This is a metallographic diagram of the steel obtained in Example 1 of the present invention; Figure 3 This is a SEM image of the pearlite morphology of the steel obtained in Example 1 of the present invention; Figure 4 The image shows the TEM morphology of the Nb(C,N) precipitates in steel obtained in Example 1 of this invention. Detailed Implementation

[0025] The specific technical solutions of the present invention will be further described below with reference to the accompanying drawings, so as to enable those skilled in the art to further understand the present invention, without constituting a limitation on its rights.

[0026] Example 1, referring to Figure 1-4 This embodiment provides a high-strength, high-toughness Nb-containing steel plate and its nano-phase controlled rolling and controlled cooling preparation method.

[0027] 1. Chemical Composition: The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.15%, Si: 0.35%, Mn: 1.45%, Nb: 0.050%, N: 0.0090%, Al: 0.035%, P: 0.022%, S: 0.018%, with the balance being Fe and unavoidable impurities. The calculated Nb / N mass ratio is 5.56, and the carbon equivalent C = 0.40%.

[0028] 2. Preparation process: (1) Smelting: A converter is used for smelting, and the final control is C=0.06%, P=0.022%, S=0.018%. When the steel is tapped to 2 / 3, FeSi, FeMn, Al deoxidizer, ferroniobium FeNb and manganese nitride MnN are added in sequence for alloying. The amount of FeNb added per ton of steel is controlled to be 5.0 kg and the amount of MnN added per ton of steel is 1.0 kg, so that the content of Nb and N elements in the molten steel and the mass ratio Nb / N reach the range defined in claim 1.

[0029] (2) Refining: Refining is carried out in an LF refining furnace at a refining temperature of 1580℃. Argon gas is introduced for stirring, and the stirring intensity is controlled at 0.4m / s. The temperature is maintained for 25min to ensure the uniformity of the composition and the full solid solution of Nb and N.

[0030] (3) Continuous casting: The billet is continuously cast at a temperature of 1520℃ and a casting speed of 1.2m / min. The crystallizer is electromagnetically stirred and the cooling water volume is controlled at 135m³ / h to obtain a 165mm×165mm billet.

[0031] (4) Heating: The continuous casting billet is fed into a walking beam furnace and a low-temperature preheating + high-temperature homogenization mode is adopted: preheating temperature 850℃, heating temperature 1180℃, homogenization temperature 1190℃, and total heating time 150min, to ensure that Nb (C, N) is completely dissolved and to avoid austenite grain coarsening.

[0032] (5) Two-stage controlled rolling: Rough rolling stage: initial rolling temperature 1120℃, single-pass deformation 18%, cumulative deformation 45%; Finishing stage: initial rolling temperature 880℃, single-pass deformation 10%, cumulative deformation 35%; immediately after finishing rolling, it enters the controlled cooling process.

[0033] (6) Segmented cooling: First stage (rapid cooling): Cooling rate 10℃ / s, cooling to 700℃; Second stage (slow cooling): Cooling rate 3℃ / s, cooling to 520℃; Third stage: Air cooling to room temperature.

[0034] 3. Performance Testing: The mechanical properties and microstructure of the steel plate obtained in this embodiment were tested, and the results are as follows: Mechanical properties: tensile strength R=630MPa, lower yield strength R=480MPa, elongation after fracture A=20%, impact energy absorbed at -20℃ KV=48J; Microstructure: The microstructure consists of ferrite and bainite, with ferrite accounting for approximately 65% ​​by volume; nanoscale Nb (C, N) precipitates are dispersed in the matrix, with an average size of 45 nm and a volume fraction of 0.25%. Crack resistance: Crack incidence rate is 0.2%, with no hot cracks or delayed cracks.

[0035] The TEM image of the steel plate obtained in this embodiment after controlled rolling is shown below. Figure 1 As shown, the metallographic structure diagram is as follows: Figure 2 As shown in the SEM image of pearlite morphology Figure 3 As shown, the TEM morphology of the Nb(C,N) precipitated phase is as follows: Figure 4 As shown.

[0036] Example 2: This example provides a high-strength, high-toughness Nb-containing steel plate and its nano-phase controlled rolling and controlled cooling preparation method.

[0037] 1. Chemical Composition: The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.12%, Si: 0.20%, Mn: 1.40%, Nb: 0.030%, N: 0.0080%, Al: 0.020%, P: 0.025%, S: 0.020%, with the balance being Fe and unavoidable impurities. The calculated Nb / N mass ratio is 3.75, and the carbon equivalent C = 0.36%.

[0038] 2. Preparation process: (1) Smelting: A converter is used for smelting, and the final control is C=0.05%, P=0.025%, S=0.020%; when the steel is tapped to 1 / 2, FeSi, FeMn, Al deoxidizer, ferroniobium FeNb and manganese nitride MnN are added in sequence for alloying. The amount of FeNb added per ton of steel is controlled to be 3.0 kg and the amount of MnN added per ton of steel is 0.8 kg, so that the content of Nb and N elements in the molten steel and the mass ratio Nb / N reach the range defined in claim 1.

[0039] (2) Refining: Refining is carried out in an LF refining furnace at a refining temperature of 1560℃. Argon gas is introduced for stirring, and the stirring intensity is controlled at 0.3m / s. The temperature is maintained for 20min.

[0040] (3) Continuous casting: The billet is continuously cast at a temperature of 1500℃ and a casting speed of 1.0m / min. The crystallizer is electromagnetically stirred and the cooling water volume is controlled at 120m³ / h to obtain a 165mm×165mm billet.

[0041] (4) Heating: Preheating temperature 800℃, heating temperature 1150℃, uniform heating temperature 1180℃, total heating time 120min.

[0042] (5) Two-stage controlled rolling: Rough rolling stage: initial rolling temperature 1100℃, single-pass deformation 15%, cumulative deformation 40%; Finishing stage: initial rolling temperature 850℃, single-pass deformation 8%, cumulative deformation 30%; immediately after finishing rolling, it enters the controlled cooling process.

[0043] (6) Segmented cooling: First stage (rapid cooling): Cooling rate 8℃ / s, cooling to 680℃; Second stage (slow cooling): Cooling rate 2℃ / s, cooling to 500℃; Third stage: Air cooling to room temperature.

[0044] 3. Performance Testing: The mechanical properties and microstructure of the steel plate obtained in this embodiment were tested, and the results are as follows: Mechanical properties: tensile strength R=600MPa, lower yield strength R=450MPa, elongation after fracture A=18%, impact energy absorbed at -20℃ KV=40J; Microstructure: The microstructure consists of ferrite and bainite, with ferrite accounting for approximately 70% by volume; the average size of the nanoscale Nb (C, N) precipitates is 60 nm, with a volume fraction of 0.20%. Crack resistance: Crack incidence rate 0.3%, with one crack observed.

[0045] Example 3: This example provides a high-strength, high-toughness Nb-containing steel plate and its nano-phase controlled rolling and controlled cooling preparation method.

[0046] 1. Chemical Composition: The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.20%, Si: 0.56%, Mn: 1.50%, Nb: 0.080%, N: 0.0110%, Al: 0.050%, P: 0.028%, S: 0.023%, with the balance being Fe and unavoidable impurities. The calculated Nb / N mass ratio is 7.27, and the carbon equivalent C = 0.45%.

[0047] 2. Preparation process: (1) Smelting: A converter is used for smelting, and the final control is C=0.08%, P=0.028%, S=0.023%. When the steel is tapped to 2 / 3, FeSi, FeMn, Al deoxidizer, ferroniobium FeNb and manganese nitride MnN are added in sequence for alloying. The amount of FeNb added per ton of steel is controlled to be 8.0 kg and the amount of MnN added per ton of steel is 1.2 kg, so that the content of Nb and N elements in the molten steel and the mass ratio Nb / N reach the range defined in claim 1.

[0048] (2) Refining: Refining is carried out in an LF refining furnace at a refining temperature of 1600℃. Argon gas is introduced for stirring, and the stirring intensity is controlled at 0.5m / s. The temperature is maintained for 30min.

[0049] (3) Continuous casting: Slab continuous casting is adopted, the continuous casting temperature is 1540℃, and the casting speed is 1.4m / min; the crystallizer adopts electromagnetic stirring, and the cooling water volume is controlled at 150m³ / h.

[0050] (4) Heating: Preheating temperature 900℃, heating temperature 1220℃, uniform heating temperature 1200℃, total heating time 180min.

[0051] (5) Two-stage controlled rolling: Rough rolling stage: initial rolling temperature 1150℃, single-pass deformation 20%, cumulative deformation 50%; Finishing stage: initial rolling temperature 900℃, single-pass deformation 12%, cumulative deformation 40%; immediately after finishing rolling, it enters the controlled cooling process.

[0052] (6) Segmented cooling: First stage (rapid cooling): Cooling rate 12℃ / s, cooling to 720℃; Second stage (slow cooling): Cooling rate 4℃ / s, cooling to 550℃; Third stage: Air cooling to room temperature.

[0053] 3. Performance Testing: The mechanical properties and microstructure of the steel plate obtained in this embodiment were tested, and the results are as follows: Mechanical properties: tensile strength R=660MPa, lower yield strength R=500MPa, elongation after fracture A=19%, impact energy absorbed at -20℃ KV=45J; Microstructure: The microstructure consists of ferrite and bainite, with ferrite accounting for approximately 60% by volume; the average size of the nanoscale Nb (C, N) precipitates is 35 nm, with a volume fraction of 0.30%. Crack resistance: Crack incidence rate 0.15%, no visible cracks.

[0054] Comparative Example 1: This comparative example provides a conventional Nb-containing steel and its preparation method to illustrate the superior effects of the technical solution of the present invention.

[0055] 1. Chemical Composition: The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.15%, Si: 0.35%, Mn: 1.45%, Nb: 0.050%, N: 0.0060% (below the lower limit of this invention), Al: 0.035%, P: 0.022%, S: 0.018%, with the balance being Fe and unavoidable impurities. The calculated Nb / N mass ratio is 8.33, and the carbon equivalent C = 0.40%.

[0056] 2. Preparation process: The preparation process is the same as in Example 1.

[0057] 3. Performance Testing: The mechanical properties and microstructure of the steel plate obtained in this comparative example were tested, and the results are as follows: Mechanical properties: tensile strength R=580MPa, lower yield strength R=420MPa, elongation after fracture A=20%, impact energy absorbed at -20℃ KV=35J; Microstructure: The microstructure consists of ferrite and pearlite, with a small amount of nanoscale Nb (C, N) precipitates, an average size of 85 nm (some are coarse), and a volume fraction of 0.12%. Crack resistance: Crack incidence rate is 0.8%, with a small number of microcracks appearing.

[0058] The results show that when the N content is lower than the range of the present invention, the number of precipitates is insufficient and the size is too large, resulting in lower strength, decreased low-temperature toughness, and increased crack incidence.

[0059] Comparative Example 2: This comparative example provides a conventional Nb-containing steel and its preparation method to illustrate the superior effects of the technical solution of the present invention.

[0060] 1. Chemical Composition: The chemical composition of the steel plate, by mass percentage, is as follows: C: 0.15%, Si: 0.35%, Mn: 1.45%, Nb: 0.050%, N: 0.0130% (higher than the upper limit of this invention), Al: 0.035%, P: 0.022%, S: 0.018%, with the balance being Fe and unavoidable impurities. The calculated Nb / N mass ratio is 3.85, and the carbon equivalent C = 0.40%.

[0061] 2. Preparation process: The preparation process is the same as in Example 1.

[0062] 3. Performance Testing: The mechanical properties and microstructure of the steel plate obtained in this comparative example were tested, and the results are as follows: Mechanical properties: tensile strength R=610MPa, lower yield strength R=440MPa, elongation after fracture A=16%, impact energy absorbed at -20℃ KV=28J; Microstructure: The microstructure is ferrite + bainite, but there are a small amount of coarse NbN brittle phase, and the size distribution of the precipitated phase is uneven (20~120nm). Crack resistance: Crack incidence rate 1.2%, with obvious hot cracks and delayed cracks.

[0063] The results show that when the N content is higher than the range of the present invention, coarse and brittle NbN phase is easily formed, which leads to a significant decrease in plasticity and toughness and a significant increase in crack sensitivity.

[0064] Comparative Example 3: This comparative example provides a Nb-containing steel plate using a conventional controlled cooling process and its preparation method, to illustrate the superior effect of the segmented controlled cooling process of the present invention.

[0065] 1. Chemical composition: The chemical composition is the same as in Example 1 (C: 0.15%, Nb: 0.050%, N: 0.0090%).

[0066] 2. Preparation process: The smelting, refining, continuous casting, heating, and two-stage controlled rolling processes are the same as in Example 1, but the controlled cooling process adopts a conventional one-stage rapid cooling: Cooling rate 10℃ / s, directly air-cooled to room temperature (without intermediate slow cooling).

[0067] 3. Performance Testing: The mechanical properties and microstructure of the steel plate obtained in this comparative example were tested, and the results are as follows: Mechanical properties: tensile strength R=625MPa, lower yield strength R=470MPa, elongation after fracture A=18%, impact energy absorbed at -20℃ KV=36J; Microstructure: The microstructure consists of ferrite and bainite, with an average size of 48 nm and a volume fraction of 0.24% for nanoscale Nb (C, N) precipitates. Crack resistance: Crack incidence rate is 0.7%, with a small number of delayed cracks appearing.

[0068] The results show that when the slow cooling process of the second stage of the present invention is not used, although similar precipitated phase characteristics can be obtained, the residual stress after rolling cannot be effectively released, resulting in a crack incidence rate that is significantly higher than that of Example 1 (0.2%). This demonstrates the unique role of the three-stage controlled cooling process of the present invention, which combines fast cooling, slow cooling, and air cooling, in stress release.

[0069]

[0070] As can be seen from the table above, by precisely controlling the chemical composition, especially the Nb / N ratio within the range of 3.6 to 10.0, and by adopting a nano-phase synergistic controlled rolling and cooling process, the steel plates produced in Examples 1-3 of this invention maintain high strength and high toughness while exhibiting a significantly lower crack incidence rate than the comparative examples, thus achieving a good match between strength, toughness, and crack resistance.

Claims

1. A high-strength, high-toughness Nb-containing steel, characterized in that, Its chemical composition, by mass percentage, is as follows: C: 0.12~0.20%, Si: 0.55~0.65%, Mn: 1.40~1.50%, Nb: 0.030~0.080%, N: 0.0080~0.0110%, P≤0.030%, S≤0.025%, Al: 0.020~0.050%, with the remainder being Fe and unavoidable impurities; among which, the Nb / N mass ratio is controlled at 3.6~10.0, and the carbon equivalent C≤0.45%.

2. The high-strength, high-toughness Nb-containing steel according to claim 1, characterized in that: The microstructure of the steel is ferrite + bainite, with ferrite accounting for 60-70% by volume. Nanoscale Nb(C,N) precipitates are dispersed in the matrix, with a volume fraction ≥0.2% and an average particle size of 20-80 nm. The tensile strength R ≥600 MPa, the lower yield strength R ≥450 MPa, the elongation after fracture A ≥18%, and the impact energy absorbed at -20℃ ≥40 J. Furthermore, the crack incidence rate of the steel during preparation and use is ≤0.3%, including hot cracks, cold cracks, and delayed cracks.

3. A method for preparing high-strength, high-toughness Nb-containing steel according to any one of claims 1-2 using nanophase-controlled rolling and controlled cooling, characterized in that, Includes the following steps: Smelting, refining, continuous casting, heating, two-stage controlled rolling and segmented controlled cooling; The two-stage controlled rolling and segmented controlled cooling work together to regulate the precipitation behavior and size distribution of nanoscale Nb(C,N) precipitates.

4. The method for preparing nanophase-controlled rolling and controlled cooling according to claim 3, characterized in that, The smelting process employs converter smelting, with the final concentration controlled as C≤0.08%, P≤0.030%, and S≤0.025%. When the steel is tapped to 1 / 2 to 2 / 3 full, FeSi, FeMn, Al deoxidizer, ferroniobium (FeNb), and manganese nitride (MnN) are added sequentially for alloying. The amount of FeNb added per ton of steel is controlled to be 3.0 to 8.0 kg, and the amount of MnN added per ton of steel is 0.8 to 1.2 kg, ensuring that the Nb and N content and the Nb / N mass ratio in the molten steel are 3.6 to 10.

0.

5. The nano-phase-controlled rolling controlled cooling preparation method according to claim 3, characterized in that: The refining process is carried out in an LF refining furnace, with argon gas introduced for stirring, and the stirring intensity is controlled at 0.3~0.5m / s; in the later stage of refining, the temperature is raised to 1560~1600℃ and held for 20~30min.

6. The method for preparing nanophase-controlled rolling and controlled cooling according to claim 3, characterized in that: The continuous casting process involves casting square billets, with the casting temperature controlled at 1500~1540℃ and the casting speed at 1.0~1.4m / min. The crystallizer uses electromagnetic stirring, and the cooling water flow rate is controlled at 120~150m³ / h to obtain continuously cast billets.

7. The method for preparing nanophase-controlled rolling and controlled cooling according to claim 3, characterized in that, The heating process employs a low-temperature preheating and high-temperature homogenization mode: the preheating temperature is 800~900℃, the heating temperature is 1160~1220℃, the homogenization temperature is 1180~1200℃, and the total heating time is 120~180min.

8. The method for preparing nanophase-controlled rolling and controlled cooling according to claim 3, characterized in that, The two-stage controlled rolling process includes: Rough rolling stage: initial rolling temperature is 1080~1200℃, single-pass deformation is 15~20%, and cumulative deformation is 40~50%; Finishing stage: The initial rolling temperature is 880~980℃, the deformation per pass is 8~12%, and the cumulative deformation is 30~40%; after finishing rolling, the controlled cooling process is started immediately.

9. The method for preparing nanophase-controlled rolling and controlled cooling according to claim 3, characterized in that, The segmented cooling system refers to a three-stage cooling system consisting of rapid cooling, slow cooling, and air cooling. The first stage of rapid cooling: the cooling rate is 8~12℃ / s, cooling to 680~720℃; The second stage is slow cooling: the cooling rate is 2~4℃ / s, cooling to 500~550℃; Third stage: Air cooling to room temperature.

10. The nanophase-controlled rolling controlled cooling preparation method according to any one of claims 3-9, characterized in that: The precise ratio of Nb and N elements is achieved through the smelting and refining steps. The in-situ precipitation of nano-sized Nb (C, N) is induced by the two-stage controlled rolling. The size of the precipitated phase is controlled and residual stress is released by the segmented controlled cooling. The three elements work together to construct a low-stress crack-resistant microstructure system based on fine grain strengthening and nano-phase precipitation strengthening.

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