Method for improving impact toughness of alloy steel bar

Abstract

The invention relates to a method for improving impact toughness of an alloy steel bar, and belongs to the technical field of metal materials. The process route of the method is as follows: component design of an alloy steel bar, smelting in a converter or an electric furnace, LF refining, casting, rolling, annealing, forging, machining and heat treatment. Through fine component design, homogenization design in the casting and rolling process and fine design of heat treatment, the technical problem of how to effectively improve the impact toughness while the strength of the alloy steel bar is improved is effectively solved.

Description

Technical Field

[0001] This application relates to the field of metallic materials technology, and in particular to a method for improving the impact toughness of alloy steel bars. Background Technology

[0002] In the field of steel technology, alloy steel bars, as a key material, are widely used in large-scale equipment, high-speed steel applications in new energy vehicles, wind power equipment, and high-power locomotives with power shift transmissions. These applications not only require alloy steel bars to possess high strength to withstand complex and variable loads, but also place extremely high demands on their impact toughness. Impact toughness is the ability of a material to absorb energy through plastic deformation and fracture processes under instantaneous impact loads. It directly reflects the sensitivity of the material to minute internal defects and its tendency to resist brittle fracture.

[0003] In recent years, with the continuous advancement of industrial technology and increasingly demanding application scenarios, the impact toughness requirements for alloy steel bars, especially high-alloy bars with Cr and Ni contents exceeding 1%, have become increasingly stringent. While traditional processes can improve the impact toughness of materials to some extent by adjusting alloy composition and optimizing heat treatment processes, they are still insufficient to meet the high toughness requirements under extreme conditions. Particularly in high-risk, high-reliability applications such as new energy, wind power, and heavy-duty steel bars, further improving the impact toughness of alloy steel bars has become a current research hotspot and challenge. Summary of the Invention

[0004] This application provides a method for improving the impact toughness of alloy steel bars to solve the following technical problem: how to solve the problem of insufficient impact toughness of alloy steel bars during the strength improvement process.

[0005] This application provides a method for improving the impact toughness of alloy steel bars, and the process route of the method is as follows:

[0006] The process involves: composition design of alloy steel bars → smelting in a converter or electric furnace → LF refining → casting → rolling → annealing → forging → machining → heat treatment.

[0007] Optionally, the chemical composition of the alloy steel bar, by mass fraction, includes: Cr: 1.0%–2.0%, Ni: 1.0%–2.0%, Mn: 0.5%–1.5%, Mn+Cr+Ni>4%, where Mn+Cr+Ni represents the sum of the mass fractions of Mn, Cr, and Ni.

[0008] Optionally, the chemical composition of the alloy steel bar, by mass fraction, includes: P ≤ 0.01%, S: 0.015% to 0.020% or S ≤ 0.01%.

[0009] Optionally, after rolling, a hot-rolled alloy steel bar is obtained, wherein the mass fraction fluctuation range of C, Mn, Cr and Ni at 1 / 4 circumference position of the hot-rolled alloy steel bar is ≤0.03%.

[0010] Optionally, the cross-sectional grain size difference of the hot-rolled alloy steel bar is ≤2 grade.

[0011] Optionally, the superheat of the molten steel being cast is 20℃~30℃, and the casting speed is uniform, ranging from 0.8m / min to 2.0m / min.

[0012] Optionally, the adjustment of the pulling speed shall follow the following specifications: the single adjustment amplitude of the normal pulling speed fluctuation shall be ≤0.01m / min, and the single adjustment time interval of the normal pulling speed fluctuation shall be >3s;

[0013] The single adjustment amplitude for adapting to changes in superheat is ≤0.05m / min, and the single adjustment time interval for adapting to changes in superheat is >3min.

[0014] Optionally, the heating temperature for rolling is 1100℃~1200℃, the initial rolling temperature is 1000℃~1100℃, and the final rolling temperature is 850℃~950℃.

[0015] Optionally, the heat treatment includes normalizing, quenching, and tempering.

[0016] Optionally, the normalizing temperature is 900℃~1000℃, and the normalizing time is 20min~60min.

[0017] Optionally, the quenching temperature is 800℃~900℃, the quenching adopts a segmented cooling process, and the quenching cooling medium is conventional quenching oil.

[0018] Optionally, the tempering temperature is 150℃~400℃.

[0019] Optionally, the segmented cooling process includes a first cooling section and a second cooling section. The temperature of the first cooling section is 900℃~550℃, and the cooling rate of the first cooling section is 50℃ / s~100℃ / s. The temperature of the second cooling section is 550℃~100℃, and the cooling rate of the second cooling section is 30℃ / s~70℃ / s.

[0020] The technical solutions provided in this application have the following advantages compared with the prior art:

[0021] This application provides a method for improving the impact toughness of alloy steel bars. The process route is as follows: composition design of the alloy steel bar → converter or electric furnace smelting → LF refining → casting → rolling → annealing → forging → machining → heat treatment. Through precise composition design, the proportions of alloying elements are controlled, particularly S is controlled within the range of ≤0.01% or 0.015% to 0.020%, to ensure material performance. By controlling parameters such as superheat and casting speed during the casting process, and through a strict casting speed adjustment strategy, the internal structure of the billet is ensured to be uniform, reducing segregation and laying a good foundation for subsequent rolling and heat treatment. During the rolling process, by controlling parameters such as heating temperature, initial rolling temperature, and final rolling temperature, the homogeneity of the billet in composition and structure is ensured, reducing the impact of cross-sectional segregation on performance. Finally, by precisely controlling the parameters of heat treatment processes such as normalizing, quenching, and tempering, the microstructure of the material is optimized, especially the tempered martensite structure, which has a good balance of strength and toughness. This improves the impact toughness of the alloy steel bar, meeting the urgent need for impact toughness of alloy steel bars with Cr, Ni and other alloy contents exceeding 1%. Attached Figure Description

[0022] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0023] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a flowchart illustrating a method for improving the impact toughness of alloy steel bars, as provided in an embodiment of this application. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0026] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1 to 6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "comprise" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0027] Figure 1 This is a flowchart illustrating a method for improving the impact toughness of alloy steel bars, as provided in an embodiment of this application.

[0028] Please see Figure 1 This application provides a method for improving the impact toughness of alloy steel bars. The process route of the method is as follows: composition design of alloy steel bars → smelting in converter or electric furnace → LF refining → casting → rolling → annealing → forging → machining → heat treatment.

[0029] After the LF refining step, VD or RH vacuum degassing can be performed. The casting can be continuous casting of square billets, continuous casting of rectangular billets, or die casting. The rolling can be two-stage rolling or one-stage rolling.

[0030] In some embodiments, the chemical composition of the alloy steel bar, by mass fraction, includes: Cr: 1.0%–2.0%, Ni: 1.0%–2.0%, Mn: 0.5%–1.5%, Mn+Cr+Ni>4%, wherein Mn+Cr+Ni represents the sum of the mass fractions of Mn, Cr, and Ni.

[0031] The method provided in this application has been shown to significantly improve the impact toughness of alloy steel bars with Cr and Ni contents both ≥1%. Particularly noteworthy is the fact that this method is particularly effective for alloy bars with Mn+Cr+Ni contents exceeding 4%.

[0032] In some embodiments, the chemical composition of the alloy steel bar, by mass fraction, includes: P ≤ 0.01%, S: 0.015% to 0.020% or S ≤ 0.01%.

[0033] Through a series of carefully designed orthogonal experiments, the effect of sulfur content on the impact toughness of alloy steel bars under tempered martensitic microstructure conditions was investigated in depth. The experimental results show that within a specific sulfur content range, i.e., when the mass fraction of sulfur is controlled at ≤0.01% or 0.015% to 0.020%, the impact toughness of alloy steel bars with a sulfur mass fraction of 0.015% to 0.035% can be increased by more than 40J to 70J.

[0034] In some embodiments, the chemical composition of the alloy steel bar, by mass fraction, includes: C: 0.12%–0.50%, Mn: 0.40%–2.00%, Cr: 0.80%–2.0%, and Ni: 0.5%–2.0%.

[0035] By precisely controlling the content of alloying elements such as C, Mn, Cr, and Ni, and optimizing the alloy composition, we can ensure that while improving strength, we do not excessively sacrifice toughness.

[0036] In some embodiments, the superheat of the molten steel being cast is 20°C to 30°C, and the casting speed is uniform, ranging from 0.8 m / min to 2.0 m / min.

[0037] Superheat refers to the process parameter of continuous casting or ingot casting of alloy steel bars, specifically the difference between the temperature of the molten steel and the liquidus temperature. Superheat directly affects the temperature gradient at the solidification front of the molten steel. A superheat above 30°C can lead to excessive growth of columnar crystals, increasing the risk of central segregation, while a superheat below 20°C may cause surface cracks or subcutaneous bubbles in the billet. Maintaining a superheat of 20°C to 30°C balances these two aspects, ensuring the internal quality of the billet. For example, the superheat of the molten steel used for casting can be 20°C, 22°C, 24°C, 26°C, 28°C, or 30°C. During casting, the casting speed must be strictly controlled according to the superheat. After normal initial casting, the casting speed should be increased to the target speed as soon as possible and maintained at a constant speed. A constant casting speed helps reduce compositional segregation and internal cracks, which is crucial for the uniformity of subsequent rolling. For example, the pulling speed can be 0.8m / min, 1.0m / min, 1.2m / min, 1.4m / min, 1.6m / min, 1.8m / min, 2.0m / min, etc.

[0038] In some implementations, the adjustment of the pulling speed shall follow the following specifications: the single adjustment amplitude of the normal pulling speed fluctuation shall be ≤0.01m / min, and the single adjustment time interval of the normal pulling speed fluctuation shall be >3s;

[0039] The single adjustment range for adapting to changes in superheat is ≤0.05m / min, and the single adjustment time interval for adapting to changes in superheat is >3min.

[0040] To ensure the stability and adaptability of casting speed, fine-tuning of speed fluctuations during the casting process is crucial. Specifically, if unexpected fluctuations occur in the casting speed, immediate fine-tuning is necessary to restore stability. This adjustment should adhere to strict control parameters: the adjustment range should not exceed 0.01 m / min per instance to ensure accuracy and minimize disruption to the casting process; simultaneously, the time interval between adjustments should be greater than 3 seconds to avoid system instability caused by frequent adjustments. This control mode, through discrete small-step adjustments, effectively avoids mechanical system overload and ensures the stability of the billet surface quality. Furthermore, when actively adjusting the casting speed to adapt to changes in molten steel superheat, a rigorous adjustment strategy must also be followed. Given the significant impact of superheat on the casting process, the adjustment range can be appropriately relaxed, but the single adjustment range should still be controlled within 0.05 m / min to prevent excessive adjustments from adversely affecting billet quality. At the same time, to ensure the effectiveness of the adjustment and the continuity of the casting process, the time interval between adjustments should be set to greater than 3 minutes to allow the system to fully respond and stabilize to the new casting speed. In summary, by employing a precise adjustment strategy to control casting speed fluctuations and overheating changes, the stability of the casting process and the consistency of billet quality can be effectively ensured, thereby improving the impact toughness of alloy steel bars.

[0041] By optimizing multiple factors such as superheat during casting, casting speed, single adjustment range of casting speed, and interval time, the influence of cross-sectional segregation on performance is controlled, ultimately resulting in a significant improvement of 10J to 20J in impact toughness.

[0042] In some embodiments, the heating temperature for rolling is 1100℃~1200℃, the initial rolling temperature is 1000℃~1100℃, and the final rolling temperature is 850℃~950℃.

[0043] The positive effects of limiting the rolling heating temperature to 1100℃~1200℃ are: ensuring sufficient austenitization of the billet, eliminating internal microstructure inhomogeneity, and providing a uniform basis for subsequent plastic deformation. Temperatures below 1100℃ may lead to insufficient diffusion of alloying elements, while temperatures above 1200℃ can easily cause grain coarsening. For example, the rolling heating temperature can be 1100℃, 1120℃, 1140℃, 1160℃, 1180℃, 1200℃, etc.

[0044] The positive effects of limiting the initial rolling temperature to 1000℃~1100℃ are: rolling in the austenitic single-phase region reduces deformation resistance and avoids rolling cracks. For example, the initial rolling temperature can be 1000℃, 1020℃, 1040℃, 1060℃, 1080℃, 1100℃, etc.

[0045] The positive effects of limiting the final rolling temperature to 850℃~950℃ include: refining grains through dynamic recrystallization while avoiding mixed-grain structures (coexistence of large grains and non-recrystallized regions). A final rolling temperature above 950℃ reduces the grain refinement effect, while a temperature below 850℃ may lead to a sharp increase in rolling force. For example, the final rolling temperature can be 850℃, 870℃, 890℃, 910℃, 930℃, 950℃, etc.

[0046] In some embodiments, after rolling, a hot-rolled alloy steel bar is obtained, wherein the mass fractions of C, Mn, Cr, and Ni at the 1 / 4 circumference position of the hot-rolled alloy steel bar all fluctuate within the range of ≤0.03%.

[0047] The 1 / 4 circumference position of a hot-rolled alloy steel bar refers to the location on the cross-section of the hot-rolled alloy steel bar that is 1 / 4 of the diameter from the surface. This position is prone to compositional segregation, which can lead to grain boundary embrittlement, localized stress concentration, and reduced U-notch impact toughness. In the alloy steel of this application embodiment, the high Mn / Cr / Ni content makes segregation likely. This indicator ensures the uniformity of the material's internal composition by limiting elemental fluctuations, reducing localized brittle phases or structural defects. A fluctuation range of ≤0.03% indicates that the content fluctuations of carbon, manganese, chromium, and nickel are strictly controlled and do not exceed 0.03%. The purpose of this indicator is to ensure the uniformity of the material composition and avoid performance inhomogeneity caused by segregation.

[0048] In some embodiments, the cross-sectional grain size difference of the hot-rolled alloy steel bar is ≤2 grade.

[0049] Grain size range refers to the ASTM grade difference corresponding to the maximum grain size difference in different regions of a material. During hot rolling, due to the influence of various factors such as temperature, deformation, and cooling rate, the cross-section of steel may exhibit uneven grain size. This unevenness affects the mechanical properties of the steel, especially its impact toughness. A range ≤ 2 indicates that the grain size difference in various parts of the material cross-section is strictly limited to within 2 ASTM grades. This uniformity can reduce local stress concentration and improve the energy absorption capacity under dynamic loads. In the embodiments of this application, the grain size range of the hot-rolled alloy steel bar cross-section is ≤ 2 grades, which can be understood as a microstructure achieved through specific rolling processes and subsequent treatments. In this state, the grain size of the steel cross-section is relatively uniform, and the grain size range is controlled within a small range (≤ 2 grades), thereby helping to improve the impact toughness of the steel.

[0050] The embodiments of this application can achieve an impact energy increase of 2J to 5J by controlling the grain size difference of the cross section through multiple factors such as the rolling process.

[0051] In some embodiments, the heat treatment includes normalizing, quenching, and tempering.

[0052] Normalizing is the first step in heat treatment. Its main purpose is to eliminate residual stress after rolling or forging, refine the grains, homogenize the microstructure, and provide a uniform austenitic matrix for subsequent quenching. During quenching, alloy steel bars are rapidly cooled to a low temperature (usually below the martensitic transformation temperature) to form a martensitic structure, which has high strength and hardness but relatively low toughness. Tempering is an important step in the heat treatment process, usually occurring after quenching. Quenching rapidly cools the steel, forming a hard and brittle structure such as martensite. However, while this structure has high hardness, it is also brittle and prone to fracture during use. Therefore, tempering is needed to eliminate or reduce this brittleness while maintaining the hardness of the steel.

[0053] In some embodiments, the normalizing temperature is 900°C to 1000°C, and the normalizing time is 20 min to 60 min.

[0054] The normalizing temperature can be between 900℃ and 1000℃. 900℃ is the lower limit of the austenitization initiation temperature, which ensures sufficient solid solution of alloying elements; the upper limit of 1000℃ avoids excessive grain growth and maintains a fine-grained strengthening effect; the 900℃ to 1000℃ range corresponds to 50℃ to 150℃ above the Ac3 phase transformation point of the material, which is beneficial for eliminating banded structures during rolling. For example, the normalizing temperature can be 900℃, 920℃, 940℃, 960℃, 980℃, 1000℃, etc.

[0055] The normalizing time is 20 to 60 minutes, with 20 minutes being the minimum holding time to ensure uniform cross-sectional temperature and complete phase transformation; the upper limit of 60 minutes prevents excessive oxide scale buildup due to prolonged high-temperature exposure. For example, the normalizing time can be 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, etc.

[0056] In some embodiments, the quenching temperature is 800℃~900℃, the quenching adopts a segmented cooling process, and the quenching cooling medium is conventional quenching oil.

[0057] Using conventional quenching oil as the cooling medium offers moderate cooling capacity compared to water-based or rapid quenching oils, ensuring adequate microstructure transformation while reducing the risk of cracking. The selection of the quenching temperature range is based on the phase transformation critical points of the alloying elements (e.g., Ac3 point). Quenching temperatures below 800℃ may prevent complete austenitization of the alloy steel bar, meaning the ferrite and cementite in the material cannot fully transform into austenite. This will affect the formation of martensite during subsequent cooling, thus reducing the material's hardness and impact toughness. Quenching temperatures above 900℃ may lead to coarse grains, conversely reducing the material's toughness. The 800℃–900℃ temperature range ensures complete austenitization while preventing excessive grain growth. Quenching at 800℃–900℃ results in a matrix structure dominated by fine lath martensite, providing an ideal microstructure basis for subsequent tempering. Examples of quenching temperatures include 800℃, 820℃, 840℃, 860℃, 880℃, and 900℃.

[0058] In some embodiments, the segmented cooling process includes a first cooling section and a second cooling section. The temperature of the first cooling section is 900℃~550℃, and the cooling rate of the first cooling section is 50℃ / s~100℃ / s. The temperature of the second cooling section is 550℃~100℃, and the cooling rate of the second cooling section is 30℃ / s~70℃ / s.

[0059] The segmented cooling design, by controlling the phase transformation kinetics, enables the material to achieve high strength while maintaining good toughness. Combined with a tempering process (150℃~400℃), it can effectively eliminate quenching stress, optimize dislocation structure, and ultimately improve impact toughness.

[0060] In the high-temperature range of 900℃ to 550℃, the cooling rate is strictly controlled at 50℃ / s to 100℃ / s. Rapid cooling is necessary at this stage to avoid the pearlite transformation zone, inhibit the formation of ferrite-pearlite microstructure, and ensure complete austenite-martensite transformation. Simultaneously, a cooling rate of 50℃ / s to 100℃ / s helps refine the martensite lath bundles, improving material strength. In the bainite / martensite transformation temperature range of 550℃ to 100℃, the cooling rate is adjusted to 30℃ / s to 70℃ / s. This cooling rate slows down the martensite transformation rate, reduces microstructural stress, allows some retained austenite to transform into bainite, increasing toughness reserves, and prevents microcrack initiation due to uneven cooling. Controlling this stage is particularly crucial for improving impact toughness; a cooling rate faster than 70℃ / s may cause stress concentration in the microstructure.

[0061] In some embodiments, the tempering temperature is 150°C to 400°C.

[0062] The purpose of tempering is to eliminate quenching stress and adjust the microstructure and properties. Different temperature ranges lead to different microstructural transformations. Within the tempering temperature range of 150℃ to 400℃, alloy steel bars undergo a series of microstructural transformations. As the temperature increases, brittle structures such as martensite gradually decompose, forming more stable tempered martensite and other structures. These structures not only have good hardness but also good toughness and plasticity, meeting the requirements for use of alloy steel bars under complex stress states. Furthermore, the selection of the tempering temperature also needs to consider the specific composition of the alloy steel bar. High contents of elements such as Cr and Ni will affect the phase transformation point, requiring higher tempering temperatures to ensure sufficient diffusion and carbide precipitation, thereby optimizing performance. Therefore, setting the tempering temperature within the range of 150℃ to 400℃ is to optimize the microstructure and properties of the alloy steel bar through microstructural transformations during heat treatment, thereby improving its impact toughness. For example, tempering temperatures can be 150℃, 200℃, 250℃, 300℃, 350℃, 400℃, etc.

[0063] Through orthogonal experimental design of variables such as normalizing temperature and time, quenching temperature, quenching medium and cooling rate, and tempering temperature, the influence of each factor on impact toughness was investigated. Impact statistics show that in high alloy steel with Cr and Ni contents both exceeding 1%, the optimized parameter combination in the embodiments of this application increases the impact energy by 10J to 30J or more.

[0064] The contributions of the various factors to the impact energy proposed in this application's embodiments exhibit non-linear superposition characteristics, and their comprehensive improvement effect stems from the synergistic effect of multiple process steps and the precise coupling of technical parameters. The specific mechanism is explained below:

[0065] Multi-factor synergistic mechanism:

[0066] Sulfur content control contributes 40J to 70J improvement; heat treatment parameter optimization contributes 10J to 30J improvement by adjusting the tempering martensite morphology; casting process control suppresses segregation defects, contributing 10J to 20J improvement; rolling process optimization improves grain size uniformity, contributing 2J to 5J improvement; the above process parameters are verified to have nonlinear coupling effects through orthogonal experiments.

[0067] In the service environment where the microstructure is tempered martensitic, this application solves the U-shaped impact toughness problem by combining material design and process design. This results in an increase in impact toughness value of 10J to 100J under the corresponding process and demand conditions, which can meet the urgent needs of alloy steel bars with Cr or Ni content exceeding 1% (especially suitable for alloys with Mn+Cr+Ni≥4%).

[0068] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0069] Design the composition of alloy steel bars for three furnaces:

[0070] Alloy steel bar composition 1: The constituent elements and mass percentages are: C 0.17%, Si 0.23%, Mn 0.60%, P 0.008%, S 0.0030%, Cr 1.60%, Ni 1.6%, Al 0.025%, N 0.0075%, with the balance being Fe and unavoidable impurities.

[0071] Composition 2 of alloy steel bar: The constituent elements and mass percentages are: C 0.17%, Si 0.23%, Mn 0.60%, P 0.008%, S 0.0150%, Cr 1.60%, Ni 1.6%, Al 0.025%, N 0.0075%, with the balance being Fe and unavoidable impurities.

[0072] Composition 3 of alloy steel bar: The constituent elements and mass percentages are: C 0.17%, Si 0.23%, Mn 0.60%, P 0.008%, S 0.035%, Cr 1.60%, Ni 1.6%, Al 0.025%, N 0.0075%, with the balance being Fe and unavoidable impurities.

[0073] Example 1

[0074] Alloy steel bars of composition 1 and composition 2 are produced using the following processes. During casting, the casting speed is strictly controlled according to the superheat level: at 25°C, the casting speed is 0.8 m / min. After normal casting begins, the casting speed is increased to the target speed as quickly as possible and maintained at a constant speed throughout the process. Each adjustment to the casting speed fluctuation is 0.01 m / min, with an interval of at least 3 seconds. For adjustments to the casting speed due to changes in superheat, each adjustment is within a range of 0.05 m / min, with an interval of at least 3 minutes between adjustments. The elemental fluctuation range at 1 / 4 circumference of hot-rolled alloy steel bars is as follows: △C: 0.03%, △Mn: 0.03%, △Cr: 0.03%, △Ni: 0.03%; heating temperature: 1160℃, initial rolling temperature: 1050℃, final rolling temperature: 900℃; normalizing temperature: 950℃, time: 20min; quenching temperature: 860℃, tempering temperature: 170℃; cooling medium: conventional quenching oil; cooling rate: 70℃ / s in the high-temperature range of 900-550℃, and 40℃ / s in the low-temperature range of 550-100℃ (bainite or martensite transformation temperature range).

[0075] Example 2

[0076] Alloy steel bars of composition 1 and composition 2 are produced using the following processes respectively. During casting, the casting speed is strictly controlled according to the superheat level: at 20°C, the casting speed is 0.9 m / min. After normal casting begins, the casting speed is increased to the target speed as quickly as possible and maintained at a constant speed throughout the process. Each adjustment to the casting speed fluctuation is 0.01 m / min, with an interval of at least 3 seconds. For adjustments to the casting speed due to changes in superheat, each adjustment is within a range of 0.05 m / min, with an interval of at least 3 minutes between adjustments. The elemental fluctuation range at 1 / 4 circumference of hot-rolled alloy steel bars is as follows: △C: 0.02%, △Mn: 0.02%, △Cr: 0.02%, △Ni: 0.02%; heating temperature: 1170℃, initial rolling temperature: 1060℃, final rolling temperature: 910℃; normalizing temperature: 950℃, time: 22min; quenching temperature: 860℃, tempering temperature: 170℃; cooling medium is conventional quenching oil; cooling rate is 70℃ / s in the high-temperature range of 900-550℃ and 40℃ / s in the low-temperature range of 550-100℃ (bainite or martensite transformation temperature range).

[0077] Example 3:

[0078] Alloy steel bars of composition 1 and composition 2 are produced using the following processes respectively. During casting, the casting speed is strictly controlled according to the superheat level: at 20°C, the casting speed is 0.8 m / min. After normal casting begins, the casting speed is increased to the target speed as quickly as possible and maintained at a constant speed throughout the process. Each adjustment to the casting speed fluctuation is 0.01 m / min, with an interval of at least 3 seconds. For adjustments to the casting speed due to changes in superheat, each adjustment is within a range of 0.04 m / min, with an interval of at least 3 minutes between adjustments. The elemental fluctuation range at 1 / 4 circumference of hot-rolled alloy steel bars is as follows: △C: 0.02%, △Mn: 0.03%, △Cr: 0.02%, △Ni: 0.02%; heating temperature: 1170℃, initial rolling temperature: 1040℃, final rolling temperature: 930℃; normalizing temperature: 950℃, time: 20min; quenching temperature: 860℃, tempering temperature: 170℃; cooling medium is conventional quenching oil; cooling rate is 70℃ / s in the high-temperature range of 900-550℃ and 40℃ / s in the low-temperature range of 550-100℃ (bainite or martensite transformation temperature range).

[0079] Comparative Example 1

[0080] The alloy steel bar with composition 3 was produced using the following process. The difference between this comparative example and Example 1 is that the impact toughness improvement effect was compared between conditions with sulfur contents of 0.0030% and 0.0150% and a sulfur content of 0.035%.

[0081] During casting, the casting speed should be strictly controlled according to the superheat. When the superheat is 25℃, the casting speed should be 0.8m / min. After the casting starts normally, the casting speed should be increased to the target speed as soon as possible and kept constant throughout the process. The adjustment range for each fluctuation in casting speed should be 0.01m / min, with an interval of more than 3 seconds. For casting speed adjustments due to changes in superheat, the adjustment range should be 0.05m / min, with an interval of more than 3 minutes between adjustments. The elemental fluctuation range at 1 / 4 circumference of hot-rolled alloy steel bars is as follows: △C: 0.03%, △Mn: 0.03%, △Cr: 0.03%, △Ni: 0.03%; heating temperature: 1160℃, initial rolling temperature: 1050℃, final rolling temperature: 900℃; normalizing temperature: 950℃, time: 20min; quenching temperature: 860℃, tempering temperature: 170℃; cooling medium: conventional quenching oil; cooling rate: 70℃ / s in the high-temperature range of 900-550℃, and 40℃ / s in the low-temperature range of 550-100℃ (bainite or martensite transformation temperature range).

[0082] Comparative Example 2

[0083] The alloy steel bars of composition 1 were produced using the following process. The difference between this comparative example and Example 2 lies in the comparison of the effects of parameters such as superheat and casting speed during the casting process on performance.

[0084] During casting, the casting speed should be strictly controlled according to the superheat. When the superheat is 50℃, the casting speed should be 0.5m / min. After the casting starts normally, the casting speed should be increased to the target speed as soon as possible and kept constant throughout the process. The adjustment range for each fluctuation in casting speed should be 0.1m / min, with an interval of more than 3 seconds. For the adjustment of casting speed to accommodate changes in superheat, the adjustment range should be 0.2m / min, with an interval of more than 3 minutes between each adjustment. The elemental fluctuation range at 1 / 4 circumference of hot-rolled alloy steel bars is as follows: △C: 0.04%, △Mn: 0.04%, △Cr: 0.04%, △Ni: 0.04%; heating temperature: 1170℃, initial rolling temperature: 1060℃, final rolling temperature: 910℃; normalizing temperature: 950℃, time: 22min; quenching temperature: 860℃, tempering temperature: 170℃; cooling medium: conventional quenching oil; cooling rate: 70℃ / s in the high-temperature range of 900-550℃, and 40℃ / s in the low-temperature range of 550-100℃ (bainite or martensite transformation temperature range).

[0085] Comparative Example 3

[0086] Alloy steel bars of composition 1 were produced using the following process. The difference between this comparative example and Example 3 is that the influence of heat treatment process parameters on impact toughness was investigated through orthogonal experimental design of variables such as normalizing temperature and time, quenching temperature, quenching cooling rate, and tempering temperature.

[0087] During casting, the casting speed should be strictly controlled according to the superheat. When the superheat is 20°C, the casting speed should be 0.8 m / min. After the casting starts normally, the casting speed should be increased to the target speed as soon as possible and kept constant throughout the process. The adjustment range for each fluctuation in casting speed should be 0.01 m / min, with an interval of more than 3 seconds. For the adjustment of casting speed in response to changes in superheat, the adjustment range should be 0.04 m / min, with an interval of more than 3 minutes between each adjustment. The elemental fluctuation range at 1 / 4 circumference of hot-rolled alloy steel bars is ΔC: 0.02%, ΔMn: 0.03%, ΔCr: 0.02%, and ΔNi: 0.02%; heating temperature: 1170℃, initial rolling temperature: 1040℃, and final rolling temperature: 930℃; no normalizing process; quenching temperature: 860℃; tempering temperature: 140℃; cooling medium is conventional quenching oil; cooling rate is 120℃ / s in the high-temperature range of 900-550℃ and 10℃ / s in the low-temperature range of 550-100℃ (bainite or martensite transformation temperature range).

[0088] Comparative Example 4

[0089] Alloy steel bars of composition 3 were produced using the following processes. The difference between this comparative example and Example 1 lies in the combined influence of factors such as composition, continuous casting process parameters, and heat treatment process parameters.

[0090] During casting, the casting speed should be strictly controlled according to the superheat. When the superheat is 50℃, the casting speed should be 0.5m / min. After the casting starts normally, the casting speed should be increased to the target speed as soon as possible and kept constant throughout the process. The adjustment range for each fluctuation in casting speed should be 0.1m / min, with an interval of more than 3 seconds. For the adjustment of casting speed to accommodate changes in superheat, the adjustment range should be 0.2m / min, with an interval of more than 3 minutes between each adjustment. The elemental fluctuation range at 1 / 4 circumference of hot-rolled alloy steel bars is ΔC: 0.04%, ΔMn: 0.04%, ΔCr: 0.04%, and ΔNi: 0.04%; heating temperature: 1170℃, initial rolling temperature: 1040℃, and final rolling temperature: 930℃; no normalizing process; quenching temperature: 860℃; tempering temperature: 140℃; cooling medium is conventional quenching oil; cooling rate is 120℃ / s in the high-temperature range of 900-550℃ and 10℃ / s in the low-temperature range of 550-100℃ (bainite or martensite transformation temperature range).

[0091] Impact statistics show:

[0092] Compared to Comparative Example 1, Example 1 showed an increase in impact toughness of 60J and 42J; Example 2 showed an increase in impact toughness of 20J and 10J compared to Comparative Example 2; and Example 3 showed an increase in impact toughness of 35J and 20J compared to Comparative Example 3.

[0093] The overall effect of Example 1 compared to Comparative Example 4 shows that the impact toughness is improved by 90J and 70J.

[0094] Furthermore, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0095] Suitable for high alloy content bars: The embodiments of the present invention are particularly suitable for alloy steel bars with a mass fraction of Cr and Ni alloys both exceeding 1%, especially alloy bars with Mn+Cr+Ni exceeding 4%, where the effect is most significant.

[0096] Industrial production verification: The embodiments of the present invention have been verified in the industrial production process. They are also effective for alloy steel bars with low content, showing good application prospects and promotion value.

[0097] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A method for improving the impact toughness of alloy steel bars, the process route of which is as follows: The process involves: composition design of alloy steel bars → smelting in a converter or electric furnace → LF refining → casting → rolling → annealing → forging → machining → heat treatment.

2. The method according to claim 1, characterized in that, The chemical composition of the alloy steel bar, by mass fraction, includes: Cr: 1.0%–2.0%, Ni: 1.0%–2.0%, Mn: 0.5%–1.5%, Mn+Cr+Ni>4%, where Mn+Cr+Ni represents the sum of the mass fractions of Mn, Cr, and Ni.

3. The method according to claim 1, characterized in that, The chemical composition of the alloy steel bar, by mass fraction, includes: P ≤ 0.01%, S: 0.015% to 0.020% or S ≤ 0.01%.

4. The method according to claim 1, characterized in that, After rolling, hot-rolled alloy steel bars are obtained. At the 1 / 4 circumference position of the hot-rolled alloy steel bars, the mass fraction fluctuation range of C, Mn, Cr and Ni is ≤0.03%.

5. The method according to claim 4, characterized in that, The cross-sectional grain size difference of the hot-rolled alloy steel bar is ≤2 grade.

6. The method according to claim 1, characterized in that, The superheat of the molten steel being cast is 20℃~30℃, and the casting speed is uniform, ranging from 0.8m / min to 2.0m / min.

7. The method according to claim 6, characterized in that, The adjustment of the pulling speed shall follow the following specifications: the single adjustment amplitude of the normal pulling speed fluctuation shall be ≤0.01m / min, and the single adjustment time interval of the normal pulling speed fluctuation shall be >3s; The single adjustment range for adapting to changes in superheat is ≤0.05m / min, and the single adjustment time interval for adapting to changes in superheat is >3min.

8. The method according to claim 1, characterized in that, The heating temperature for rolling is 1100℃~1200℃, the initial rolling temperature is 1000℃~1100℃, and the final rolling temperature is 850℃~950℃.

9. The method according to claim 1, characterized in that, The heat treatment includes normalizing, quenching, and tempering; and / or, The normalizing temperature is 900℃~1000℃, and the normalizing time is 20min~60min; and / or, The quenching temperature is 800℃~900℃, the quenching adopts a segmented cooling process, and the cooling medium for quenching is conventional quenching oil; and / or, The tempering temperature is 150℃~400℃.

10. The method according to claim 9, characterized in that, The segmented cooling process includes a first cooling section and a second cooling section. The temperature of the first cooling section is 900℃~550℃, and the cooling rate of the first cooling section is 50℃ / s~100℃ / s. The temperature of the second cooling section is 550℃~100℃, and the cooling rate of the second cooling section is 30℃ / s~70℃ / s.

Images