Low-crack-sensitivity round steel with yield strength of 460MPa and preparation method of low-crack-sensitivity round steel

Abstract

The invention relates to the technical field of ferrous metallurgy, and discloses low-crack-sensitivity round steel with the yield strength of 460 MPa and a preparation method of the low-crack-sensitivity round steel. The chemical components of the round steel adopt low-aluminum and high-nitrogen matched vanadium-titanium microalloying design, acid-soluble aluminum Als is controlled to be smaller than or equal to 0.010%, the nitrogen content is controlled to be 0.0100%-0.0150%, a microscopic structure is free of a grain boundary AlN film and contains dispersed and precipitated (Ti, V) (C, N) particles, and the preparation method comprises the steps of converter silicomanganese deoxidation, RH vacuum nitrogen circulation and wire feeding double nitrogen increase, continuous casting, heating rolling, slow cooling and the like. In the continuous casting process, special casting powder with the alkalinity of 1.00-1.20 is adopted to be matched with a weak cooling process, and the heat transfer uniformity of a blank shell is improved; and 15-20 MPa low-pressure descaling before rolling and a casting blank and round steel double slow cooling system are matched. Through cooperation of component optimization and a whole-process technology, surface and internal cracks of peritectic steel are effectively inhibited, the yield strength of the obtained round steel is larger than or equal to 490 MPa, the ballistic work at the temperature of-20 DEG C is larger than or equal to 80 J, and the round steel has excellent comprehensive mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, specifically to a low-crack-sensitivity round steel with a yield strength of 460 MPa and its preparation method. Background Technology

[0002] With the development of industries such as construction machinery, mining equipment, and hydraulic supports, higher requirements have been placed on the strength and toughness of round steel for structural applications. Round steel with a yield strength of 460 MPa is widely used in these fields due to its high load-bearing capacity and weight reduction potential. To meet strength requirements, the carbon content of this grade of round steel is typically designed to be between 0.12% and 0.20%, falling within the typical peritectic or semi-peritectic steel range.

[0003] During continuous casting, steel with this carbon content range undergoes a peritectic phase transformation during solidification, accompanied by significant volume shrinkage. This volume shrinkage leads to uneven air gaps between the primary billet shell and the copper plate of the crystallizer, resulting in uneven heat transfer and making it highly susceptible to inducing longitudinal cracks on the billet surface. Furthermore, to achieve the 460 MPa strength standard, current technologies typically employ microalloying methods using vanadium, niobium, and titanium, combined with high nitrogen content to utilize precipitation strengthening mechanisms. However, traditional smelting processes often use aluminum as the primary deoxidizer and microalloying element. Under high nitrogen content conditions, a large amount of aluminum in the steel readily combines with nitrogen, precipitating thin films of aluminum nitride (AlN) at the austenite grain boundaries. These grain boundary precipitates significantly reduce the high-temperature plasticity of the steel, causing corner transverse cracks or network cracks to form in the billet during straightening.

[0004] Besides the surface quality issues of continuously cast billets, high-strength round steel is also quite sensitive to hydrogen-induced cracking (white spots) and thermal stress. In existing production processes, if the physicochemical properties of the continuous casting protective slag do not match the solidification characteristics of peritectic steel well, or if the cooling regime after rolling is not properly controlled, it often leads to microcracks or excessive residual stress inside the round steel, which seriously affects the fatigue life and low-temperature impact performance of the material.

[0005] Therefore, how to effectively solve the crack sensitivity problem in the continuous casting process of peritectic steel while ensuring that the yield strength reaches the level of 460MPa, and at the same time take into account the low-temperature toughness of the material, is a technical problem that the metallurgical industry urgently needs to solve. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a low-crack-sensitivity round steel bar with a yield strength of 460 MPa and its preparation method, which solves the problem that Q460 grade peritectic steel round steel bars are prone to surface cracks during continuous casting, and that it is difficult to achieve both high strength and good low-temperature toughness.

[0007] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a round steel bar with low crack sensitivity and a yield strength of 460 MPa, using the following technical solution: A low-crack-sensitivity round steel with a yield strength of 460 MPa has the following chemical composition by weight percentage: C: 0.12%–0.20%; Si: 0.30%–0.50%; Mn: 1.30%–1.60%; P: ≤0.025%; S: ≤0.015%; Als: ≤0.010%; Ti: 0.01%–0.03%; V: 0.07%–0.12%; N: 0.0100%–0.0150%; the remainder being Fe and unavoidable impurities.

[0008] By adopting the above technical solution, and using a low-aluminum, high-nitrogen composition system combined with vanadium-titanium microalloying, the following effects are achieved: Suppressing the precipitation of grain boundary embrittlement phases: By controlling the acid-soluble aluminum (Als) content below 0.010%, the amount of AlN inclusions formed by the combination of aluminum and nitrogen in the steel is reduced. During the continuous casting cooling process, AlN can be prevented from precipitating as a thin film along the austenite grain boundaries, thereby maintaining grain boundary bonding, improving the high-temperature plasticity of the steel, and reducing the surface crack sensitivity of the continuously cast billet.

[0009] Precipitation strengthening and grain refinement mechanism: In a low-alumina matrix, a relatively high content of nitrogen (0.0100%–0.0150%) and vanadium (0.07%–0.12%) is added. Utilizing the chemical affinity between vanadium and nitrogen, (Ti,V)(C,N) carbonitride particles are dispersed and precipitated during the austenite-to-ferrite transformation and within the ferrite region. These second-phase particles improve yield strength through a precipitation strengthening mechanism, compensating for the strength loss that may be caused by the low aluminum content. Simultaneously, the precipitated phases act as grain boundary pinning agents, refining the grain structure of the finished steel and improving both strength and low-temperature toughness.

[0010] Peritectic reaction control: The C content is controlled in the range of 0.12% to 0.20%, and the Mn and Si contents are optimized. Combined with the above-mentioned precipitate control and subsequent processes, the influence of volume shrinkage stress during the peritectic phase transformation on the grain boundaries is reduced.

[0011] Preferably, the chemical composition of the round steel is composed of the following components by weight percentage: C: 0.14%–0.18%; Si: 0.35%–0.45%; Mn: 1.35%–1.55%; P: ≤0.020%; S: ≤0.012%; Als: ≤0.009%; Ti: 0.015%–0.025%; V: 0.08%–0.11%; N: 0.0110%–0.0140%; the remainder being Fe and unavoidable impurities.

[0012] By adopting the above technical solution, the proportions of each element are further optimized, the Ti / N and V / N ratios are adjusted, and the precipitation of (Ti,V)(C,N) particles is promoted, so that the yield strength of the round steel is maintained above 490MPa and the impact energy at -20℃ is maintained above 80J.

[0013] Preferably, the microstructure of the round steel contains dispersed (Ti,V)(C,N) particles, and there is no thin film AlN precipitation at the austenite grain boundaries.

[0014] By adopting the above technical solutions, the material can be guaranteed to have both macroscopic mechanical properties and crack resistance at the microstructure level.

[0015] Secondly, the present invention provides a method for preparing low-crack-susceptibility round steel with a yield strength of 460 MPa, using the following technical solution: A method for preparing low-crack-susceptibility round steel with a yield strength of 460 MPa includes the following steps: S1. Converter smelting: Scrap steel and molten iron are added to the converter for smelting. During the tapping process, ferrosilicon and ferromanganese are added for deoxidation and alloying to control the Al content of the molten steel. S2 and LF refining: fine-tuning the composition of molten steel; S3, RH vacuum refining and nitrogen enrichment: Nitrogen gas is used as the circulating gas under vacuum conditions, and nitrogen-containing cored wire is fed in for nitrogen enrichment microalloying; S4. Large billet continuous casting: High-basicity protective slag is added to the crystallizer for continuous casting, and argon gas protection is used during the continuous casting process; S5. Slow cooling of the billet: After the billet is cut, it is placed in a slow cooling pit for the first slow cooling. S6. Billet heating and descaling: The slowly cooled billet is heated and descaled by high-pressure water after being taken out of the furnace. S7. Rolling: Rolling the cast billet into round steel; S8. Slow cooling of round steel: After the round steel is rolled, it is placed in a slow cooling pit for a second slow cooling, and then removed from the pit and cooled to room temperature.

[0016] By adopting the above technical solution, the synergistic effect of each step enables control over chemical composition, coagulation structure, and internal quality: Specific deoxidation process: During the converter tapping and refining stages, ferrosilicon and ferromanganese are used as the main deoxidizers, and the amount of aluminum-iron alloy added is controlled. This step removes dissolved oxygen while keeping the residual acid-soluble aluminum content at a low level (Als≤0.010%), thus inhibiting the formation of AlN inclusions.

[0017] Dual Nitrogen-Enriching Microalloying: During the RH vacuum refining stage, impurity gases are removed using the vacuum environment, and nitrogen is enriched through a combination of "nitrogen circulation + nitrogen-containing cored wire". Nitrogen gas serves as the circulating gas, providing the basic nitrogen source, while the nitrogen-containing cored wire is used to adjust the final nitrogen content. This method improves the nitrogen yield and distribution uniformity, providing a nitrogen source for subsequent (Ti,V)(C,N) precipitation.

[0018] Full-process crack control: The continuous casting stage is combined with specific protective slag and weak cooling process to improve the uniformity of the initial billet shell growth; the dual slow cooling process before and after rolling uses residual heat to promote the diffusion and escape of hydrogen atoms, reduce the risk of white spots and hydrogen embrittlement, and eliminate the thermal stress and structural stress generated during continuous casting and rolling, thereby reducing the occurrence of delayed cracks.

[0019] Preferably, in step S1, the tapping temperature is controlled at 1600–1630℃, a silicon-manganese combined deoxidation process is adopted, and the amount of aluminum and iron added is limited to control the finished product Als ≤ 0.010%; in step S3, the vacuum degree is controlled at ≤ 267 Pa, and the vacuum treatment time is ≥ 25 min; the flow rate of the circulating gas nitrogen is 70–90 Nm³ / h; the nitrogen-containing cored wire is chromium nitride cored wire, and its feeding amount is 100–250 m based on 130 tons of molten steel.

[0020] By adopting the above technical solutions, the thermodynamic and kinetic conditions of the smelting process are controlled. The tapping temperature of 1600-1630℃ ensures the temperature drop space for subsequent refining; high vacuum and processing time are used for dehydrogenation and denitrification; specific nitrogen flow rate and chromium nitride feed rate are used to adjust the nitrogen content, so that the nitrogen content of the finished product is controlled in the range of 0.0100%-0.0150%.

[0021] Preferably, in step S4, the chemical composition of the high-alkalinity protective slag, by weight percentage, includes: CaO: 31.5%–36.6%; SiO2: 30.5%–31.5%; Al2O3: 3.0%–6.0%; Na2O: 5.0%–6.5%; F: 3.0%–5.0%; TC: 14.5%–18.0%; MgO: 2.5%–4.0%; unavoidable impurities: ≤1.5%; the binary basicity CaO / SiO2 of the high-alkalinity protective slag is 1.00–1.20, the melting point is 1155–1195℃, and the viscosity at 1300℃ is 0.62–0.75 Pa·s.

[0022] By adopting the above technical solution, the high-alkalinity protective slag with this specific ratio plays a role in the continuous casting mold: Heat transfer control: Higher alkalinity (1.00-1.20) increases the slag film crystallization temperature and crystallization rate, increases the proportion of solid slag film and heat transfer resistance between the copper plate and the billet shell in the crystallizer, reduces the heat flux density of the crystallizer, slows down the cooling rate of the initial billet shell, and improves the uniformity of the billet shell temperature distribution.

[0023] Adapting to volume shrinkage: With a suitable viscosity (0.62~0.75Pa·s), the protective slag has flow and lubrication properties at the meniscus, filling the air gaps caused by the volume shrinkage of the peritectic reaction, preventing local separation of the billet shell from the crystallizer wall, and reducing longitudinal cracks caused by uneven heat transfer.

[0024] Preferably, the high-alkalinity protective slag is prepared by a method comprising the following steps: Step 1, pre-melting of base material: weigh raw materials other than TC according to the proportion, mix them, pre-melt them at 1350-1400℃ for 1-2 hours, cool them with water, crush and sieve them to obtain pre-melted base material powder; Step 2, slurry preparation: add carbonaceous materials and binders to the pre-melted base material powder, add water and mix to prepare slurry; Step 3, granulation and sieving: spray granulate the slurry, dry it and sieve it to obtain hollow particles with a particle size of 0.1-0.8 mm.

[0025] By adopting the above technical solutions, the pre-melting treatment allows oxides and fluorides to react and form a uniform mineral phase, reducing the influence of high-melting-point monomers in the raw materials on melting performance and ensuring the melting stability and compositional uniformity of the protective slag; the hollow particles produced by spray granulation improve fluidity and spreadability, forming a uniform covering layer on the surface of the molten steel in the crystallizer.

[0026] Preferably, in step S5, the temperature of the billet entering the pit is 600-650℃, and the slow cooling time is 48-52 hours; in step S8, the temperature of the round steel entering the pit is 500-550℃, and the slow cooling time is 48-55 hours.

[0027] By adopting the above technical solution, a dual slow cooling system is implemented. Slow cooling during the billet stage releases solidification stress and initially diffuses hydrogen; slow cooling during the round steel stage targets phase transformation stress and residual hydrogen after rolling. This process ensures a reduction in diffusible hydrogen content in the steel, minimizing hydrogen-induced cracking and white spot defects.

[0028] Preferably, in step S6, the temperature of the soaking zone is 1180-1220℃, and the descaling water pressure is controlled at 15-20MPa; in step S7, the rough rolling start temperature is 1050-1100℃, and the final rolling temperature is 800-950℃.

[0029] By adopting the above technical solutions, the temperature of the soaking zone is guaranteed to ensure solid solution of carbonitrides; the descaling water with a pressure of 15-20MPa is used, which reduces the pressure compared with traditional high-pressure descaling, avoids thermal shock cracks caused by rapid temperature drop on the surface of the billet during the descaling process, and reduces mechanical damage to the peritectic steel surface by the water jet.

[0030] Preferably, in step S4, the continuous casting speed is controlled at 0.5–0.7 m / min, the water flow rate in the crystallizer is controlled at 3400–3600 L / min, and the superheat of the continuous casting furnace is controlled at 25–40 °C.

[0031] By adopting the above technical solution, a continuous casting process with low casting speed and weak cooling is used. The lower casting speed and moderate cooling water volume increase the growth time of the billet shell in the mold, increase the thickness and strength of the billet shell exiting the mold, resist bulging and deformation caused by the static pressure of molten steel, and inhibit crack formation.

[0032] This invention provides a low-crack-susceptibility round steel bar with a yield strength of 460 MPa and its preparation method. It has the following beneficial effects: 1. This invention, by controlling the acid-soluble aluminum content to ≤0.010% and matching it with a nitrogen content of 0.0100% to 0.0150% and vanadium microalloying, suppresses the precipitation of thin-film AlN at the austenite grain boundaries, reducing the tendency for grain boundary embrittlement. At the same time, the high nitrogen content promotes the dispersed precipitation of (Ti,V)(C,N) particles, resulting in precipitation strengthening and grain refinement. This composition design solves the contradiction between the strength improvement and crack sensitivity of traditional peritectic steel, enabling round steel to maintain good low-temperature toughness and crack resistance while achieving a yield strength of 460MPa.

[0033] 2. This invention employs a specially formulated high-alkalinity protective slag in conjunction with a weak-cold continuous casting process. This protective slag forms a highly crystallizable slag film within the crystallizer, increasing the thermal resistance, reducing the heat flux density, and improving the heat transfer uniformity of the initial billet shell. Simultaneously, the appropriate viscosity ensures the slag film's ability to fill the gaps caused by the volume shrinkage during peritectic reaction. This alleviates the uneven shrinkage stress during the solidification process of peritectic steel, reduces the incidence of longitudinal cracks on the billet surface, and improves the surface quality of the continuously cast billet.

[0034] 3. This invention employs a fully coordinated control process, including RH vacuum nitrogen circulation and dual nitrogen enrichment during wire feeding, low-pressure descaling, and a dual slow cooling system for the billet and round steel. The precise nitrogen enrichment process ensures a nitrogen source for precipitation strengthening; the low-pressure descaling at 15-20 MPa reduces surface thermal shock damage; and the strict dual slow cooling process promotes hydrogen diffusion and releases thermal stress in the structure. These measures work together to reduce the risk of white spots and hydrogen-induced cracks, ensuring the internal quality stability of the round steel. Attached Figure Description

[0035] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Please see the appendix Figure 1 This invention provides a low-crack-sensitivity round steel bar with a yield strength of 460 MPa and its preparation method.

[0038] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0039] The scrap steel, molten iron, and argon and nitrogen used in the smelting process are all conventional industrial-grade raw materials in the iron and steel smelting industry.

[0040] Ferrosilicon alloy (CAS No. 8049-17-0, Si≥75%), low-carbon ferromanganese alloy (CAS No. 12604-53-4, Mn≥80%), ferrovanadium alloy (CAS No. 12604-58-9), ferrotitanium alloy (CAS No. 12756-71-1), and ferroaluminum alloy (CAS No. 12003-78-0) are all commercially available industrial-grade metallurgical raw materials; chromium nitride powder (CAS No. 24094-93-7, N≥20%) is a commercially available high-purity powder.

[0041] The limestone (main component CaCO3, CAS No. 471-34-1), quartz sand (main component SiO2, CAS No. 14808-60-7), bauxite (main component Al2O3, CAS No. 1344-28-1), soda ash (Na2CO3, CAS No. 497-19-8), fluorite (main component CaF2, CAS No. 7789-75-5), and magnesia (main component MgO, CAS No. 1309-48-4) used to prepare the protective slag are all commercially available industrial-grade raw materials; earthy graphite (CAS No. 7782-42-5) and carbon black (CAS No. 1333-86-4) are commercially available carbonaceous materials; dextrin (CAS No. 9004-53-9) and carboxymethyl cellulose (CAS No. 9000-11-7) are commercially available binders.

[0042] Preparation Example 1: This preparation example provides a high-basicity mold flux for continuous casting of low-crack-sensitivity round steel. Its chemical composition by weight percentage is: CaO 31.5%, SiO2 31.5%, Al2O3 6.0%, Na2O 6.5%, F 5.0%, T.C 14.5%, MgO 4.0%, with the remainder being unavoidable impurities (total approximately 1.0%). The high-basicity mold flux has a binary basicity R (CaO / SiO2) of 1.00, a melting point of 1155℃, and a viscosity of 0.62 Pa·s at 1300℃.

[0043] The preparation method of high-alkalinity crystallizer protective slag includes the following steps: Step 1 (Pre-melting of base material): According to the proportion of oxides and fluorides other than TC (total carbon) in the above chemical composition, weigh limestone, quartz sand, bauxite, soda ash, fluorite and magnesia, mix them evenly and send them into a rotary kiln for pre-melting treatment at 1350℃. Hold the temperature for 1 hour to allow the material to completely melt and react. Then, quench it with water, dry it and mechanically crush it and ball mill it through a 200-mesh sieve to obtain pre-melted base material powder. Step 2 (Slurry Preparation): Take the above pre-melted base powder, add the required amount of earthy graphite and carbon black (both of which provide TC as carbonaceous materials), and add 0.5% of dextrin as a binder. Mix evenly in a high-speed mixer, and add an appropriate amount of water to make a slurry with a solid content of 60%. Step 3 (granulation and screening): The slurry is pumped into the spray drying tower for spray granulation. The inlet air temperature is controlled at 320℃ and the outlet air temperature at 120℃. The moisture is removed to obtain hollow granular protective slag. The slag is then screened by a vibrating screen, and particles with a particle size between 0.1mm and 0.8mm are selected as the finished product.

[0044] Preparation Example 2: This preparation example provides a high-basicity mold flux for continuous casting of low-crack-sensitivity round steel. Its chemical composition by weight percentage is: CaO 34.1%, SiO2 31.0%, Al2O 34.5%, Na2O 6.0%, F 4.0%, T.C 16.0%, MgO 3.2%, with the remainder being unavoidable impurities (total about 1.2%). The high-basicity mold flux has a binary basicity R (CaO / SiO2) of 1.10, a melting point of 1175℃, and a viscosity of 0.69 Pa·s at 1300℃.

[0045] The preparation method of high-alkalinity crystallizer protective slag includes the following steps: Step 1 (Pre-melting of base material): According to the proportion of oxides and fluorides other than TC in the above chemical composition, weigh limestone, quartz sand, bauxite, soda ash, fluorite and magnesia, mix them evenly and send them into a rotary kiln for pre-melting treatment at 1380℃ for 1.5 hours. Then, quench them with water, dry them and mechanically crush them and ball mill them through a 200-mesh sieve to obtain pre-melted base material powder. Step 2 (Slurry Preparation): Take the above pre-melted base powder, add the required amount of earthy graphite and carbon black, and add 0.8% of carboxymethyl cellulose as a binder. Mix evenly in a high-speed mixer, and add an appropriate amount of water to make a slurry with a solid content of 65%. Step 3 (granulation and screening): The slurry is pumped into the spray drying tower for spray granulation. The inlet air temperature is controlled at 330℃ and the outlet air temperature at 125℃ to obtain hollow granular protective slag. The slag is then screened by a vibrating screen, and particles with a diameter between 0.1mm and 0.8mm are selected as the finished product.

[0046] Preparation Example 3: This preparation example provides a high-basicity mold flux for continuous casting of low-crack-sensitivity round steel. Its chemical composition by weight percentage is: CaO 36.6%, SiO2 30.5%, Al2O3 3.0%, Na2O 5.0%, F 3.0%, T.C 18.0%, MgO 2.5%, with the remainder being unavoidable impurities (total about 1.4%). The high-basicity mold flux has a binary basicity R (CaO / SiO2) of 1.20, a melting point of 1195℃, and a viscosity of 0.75 Pa·s at 1300℃.

[0047] The preparation method of high-alkalinity crystallizer protective slag includes the following steps: Step 1 (Pre-melting of base material): According to the proportion of oxides and fluorides other than TC in the above chemical composition, weigh limestone, quartz sand, bauxite, soda ash, fluorite and magnesia, mix them evenly and send them into a rotary kiln for pre-melting treatment at 1400℃ for 2 hours. Then, quench them with water, dry them and mechanically crush them and ball mill them through a 200-mesh sieve to obtain pre-melted base material powder. Step 2 (Slurry Preparation): Take the above pre-melted base powder, add the required amount of earthy graphite and carbon black, and add 1.0% of dextrin as a binder as the total weight of the powder. Mix evenly in a high-speed mixer, and add an appropriate amount of water to make a slurry with a solid content of 70%. Step 3 (granulation and screening): The slurry is pumped into the spray drying tower for spray granulation. The inlet air temperature is controlled at 350℃ and the outlet air temperature at 130℃ to obtain hollow granular protective slag. The slag is then screened by a vibrating screen, and particles with a diameter between 0.1mm and 0.8mm are selected as the finished product. Example 1

[0048] This embodiment provides a low crack sensitivity round steel with a yield strength of 460MPa and its preparation method.

[0049] The chemical composition of the round steel, by weight percentage, is: C 0.12%, Si 0.30%, Mn 1.60%, P 0.012%, S 0.005%, Als 0.004%, Ti 0.01%, V 0.12%, N 0.0150%, with the remainder being Fe and unavoidable impurities.

[0050] The preparation method includes the following steps: Step 1, Converter smelting: 20% scrap steel and 80% molten iron are added to a 130-ton top and bottom blown converter. The blowing temperature is controlled at 1600℃ and the carbon content of the steel is 0.06%. Ferrosilicon and low-carbon ferromanganese are added during the tapping process for preliminary alloying, and a very small amount of ferroaluminum is added for deoxidation. Step 2, LF refining: The molten steel is sent into the LF refining furnace, and ferrovanadium, ferrotitanium, ferrosilicon and low carbon ferromanganese are added to fine-tune the composition. The refining time is 35 minutes. Step 3, RH vacuum refining and nitrogen addition: The molten steel is fed into the RH furnace, and the high vacuum treatment time (vacuum degree ≤267Pa) is controlled at 25min; nitrogen is used as the circulating gas, and the flow rate is controlled at 70Nm³ / h; after the vacuum treatment is completed, 250m of chromium nitride cored wire is fed into the molten steel to adjust the N content in the steel to the target value. Step 4, large billet continuous casting: the continuous casting speed is controlled at 0.5 m / min; the high basicity protective slag prepared in Preparation Example 1 above is used in the crystallizer; the water flow rate in the crystallizer is controlled at 3400 L / min; the superheat of the start-up furnace is controlled at 45℃ and the superheat of the continuous casting furnace is controlled at 40℃; argon gas protection is used throughout the tundish and nozzle system. Step 5, Slow Cooling of Billet: The billet is immediately hoisted into the slow cooling pit after cutting. The temperature in the pit is 650℃, and the slow cooling time is 52 hours. Step 6, Billet Heating and Descaling: The billet is sent into the heating furnace, the temperature of the soaking zone is controlled at 1180℃, and the total heating time is 260min; after exiting the furnace, the descaling water pressure is controlled at 15MPa. Step 7, Rolling: The roughing rolling temperature is 1050℃, and the rolling passes are 8; the final rolling temperature is 800℃, and the rolled round steel is sent to the cooling bed. Step 8, slow cooling of round steel: The round steel is sent from the cooling bed into the slow cooling pit at a temperature of 550°C. It is covered with an insulation cover and the slow cooling time is 50 hours. Then it is taken out of the pit and cooled to room temperature. Step 9, Round Steel Flaw Detection: Straighten, chamfer, and perform surface magnetic flux leakage flaw detection on the round steel. The flaw detection standard is 0.3mm for artificial flaw depth. After passing the test, the steel is put into storage. Example 2

[0051] This embodiment provides a low crack sensitivity round steel with a yield strength of 460MPa and its preparation method.

[0052] The chemical composition of the round steel, by weight percentage, is: C 0.16%, Si 0.40%, Mn 1.45%, P 0.018%, S 0.008%, Als 0.007%, Ti 0.02%, V 0.09%, N 0.0125%, with the remainder being Fe and unavoidable impurities.

[0053] The preparation method includes the following steps: Step 1, Converter smelting: Add 20% scrap steel and 80% molten iron to a 130-ton top and bottom blown converter, control the tapping temperature at 1620℃, and the carbon content of the tapping steel is 0.08%; add ferrosilicon and low-carbon ferromanganese alloying during the tapping process, and control the amount of aluminum ferromanganese added. Step 2, LF refining: The molten steel is sent into the LF refining furnace, and ferrovanadium, ferrotitanium, ferrosilicon and low carbon ferromanganese are added to fine-tune the composition. The refining time is 40 minutes. Step 3, RH vacuum refining and nitrogen addition: The molten steel is fed into the RH furnace, and the high vacuum treatment time (vacuum degree ≤267Pa) is controlled at 30min; nitrogen is used as the circulating gas, and the flow rate is controlled at 80Nm³ / h; after the vacuum treatment is completed, 180m of chromium nitride cored wire is fed into the molten steel to adjust the N content in the steel. Step 4, Large billet continuous casting: The continuous casting speed is controlled at 0.6 m / min; the high basicity protective slag prepared in Preparation Example 2 above is used in the crystallizer; the water flow rate in the crystallizer is controlled at 3500 L / min; the superheat of the start-up furnace is controlled at 40℃ and the superheat of the continuous casting furnace is controlled at 32℃; argon gas protection is used throughout the process. Step 5, Slow Cooling of Billet: After the billet is cut, it is hoisted into the slow cooling pit at a temperature of 620℃ for 50 hours. Step 6, Billet Heating and Descaling: The billet is sent into the heating furnace, the temperature of the soaking zone is controlled at 1200℃, and the total heating time is 280min; after exiting the furnace, the descaling water pressure is controlled at 18MPa. Step 7, Rolling: The initial rolling temperature of the roughing mill is 1080℃, and the rolling passes are 8; the final rolling temperature is 880℃, and the rolled round steel is sent to the cooling bed. Step 8, slow cooling of round steel: The round steel is sent from the cooling bed into the slow cooling pit at a temperature of 520°C. It is covered with an insulation cover and the slow cooling time is 55 hours. Then it is taken out of the pit and cooled to room temperature. Step 9, Round Steel Flaw Detection: Straighten, chamfer, and perform surface magnetic flux leakage flaw detection on the round steel. The flaw detection standard is 0.3mm for artificial flaw depth. After passing the test, the steel is put into storage. Example 3

[0054] This embodiment provides a low crack sensitivity round steel with a yield strength of 460MPa and its preparation method.

[0055] The chemical composition of the round steel, by weight percentage, is: C 0.20%, Si 0.50%, Mn 1.30%, P 0.025%, S 0.015%, Als 0.010%, Ti 0.03%, V 0.07%, N 0.0100%, with the remainder being Fe and unavoidable impurities.

[0056] The preparation method includes the following steps: Step 1, Converter smelting: Add 20% scrap steel and 80% molten iron to a 130-ton top and bottom blown converter, control the tapping temperature at 1630℃, and the carbon content of the tapping steel is 0.10%; add ferrosilicon and low-carbon ferromanganese alloying during the tapping process, and control the amount of aluminum ferromanganese added. Step 2, LF refining: The molten steel is sent into the LF refining furnace, and ferrovanadium, ferrotitanium, ferrosilicon and low carbon ferromanganese are added to fine-tune the composition. The refining time is 45 minutes. Step 3, RH vacuum refining and nitrogen addition: The molten steel is fed into the RH furnace, and the high vacuum treatment time (vacuum degree ≤267Pa) is controlled at 35min; nitrogen is used as the circulating gas, and the flow rate is controlled at 90Nm³ / h; after the vacuum treatment is completed, 100m of chromium nitride cored wire is fed into the molten steel to adjust the N content in the steel. Step 4, large billet continuous casting: The continuous casting speed is controlled at 0.7 m / min; the high basicity protective slag prepared in Preparation Example 3 above is used in the crystallizer; the water flow rate in the crystallizer is controlled at 3600 L / min; the superheat of the start-up furnace is controlled at 35°C and the superheat of the continuous casting furnace is controlled at 25°C; argon gas protection is used throughout the process. Step 5, Slow Cooling of Billet: After the billet is cut, it is hoisted into the slow cooling pit at a temperature of 600℃ and a cooling time of 48 hours. Step 6, Billet Heating and Descaling: The billet is sent into the heating furnace, the temperature of the soaking zone is controlled at 1220℃, and the total heating time is 300min; after exiting the furnace, the descaling water pressure is controlled at 20MPa. Step 7, Rolling: The initial rolling temperature of the roughing mill is 1100℃, and the rolling passes are 8; the final rolling temperature is 950℃, and the rolled round bars are sent to the cooling bed. Step 8, slow cooling of round steel: The round steel is sent from the cooling bed into the slow cooling pit. The temperature in the pit is 500℃. It is covered with an insulation cover and the slow cooling time is 48 hours. Then it is taken out of the pit and cooled to room temperature. Step 9, Round Steel Flaw Detection: Straighten, chamfer, and perform surface magnetic flux leakage flaw detection on the round steel. The flaw detection standard is 0.3mm for artificial flaw depth. After passing the test, the steel is put into storage. Example 4

[0057] This embodiment provides a low crack sensitivity round steel with a yield strength of 460MPa and its preparation method.

[0058] The chemical composition of the round steel, by weight percentage, is: C 0.14%, Si 0.35%, Mn 1.35%, P 0.016%, S 0.010%, Als 0.006%, Ti 0.015%, V 0.08%, N 0.0110%, with the remainder being Fe and unavoidable impurities.

[0059] The preparation method is basically the same as in Example 2, with the only difference being the adjustment of process parameters: In step 3, the RH circulating gas flow rate is controlled at 75 Nm³ / h, and the chromium nitride cored wire feed rate is 150 m. In step 4, the high-basicity protective slag prepared in Preparation Example 2 above is used in the crystallizer, the continuous casting speed is controlled at 0.55 m / min, the water flow rate in the crystallizer is controlled at 3450 L / min, and the superheat of the continuous casting furnace is controlled at 38°C. In step 6, the temperature of the heat exchange zone is controlled at 1190℃, and the descaling water pressure is controlled at 16MPa. In step 7, the final rolling temperature is controlled at 850℃; In step 8, the temperature of the round steel entering the slow cooling pit is 530℃. Example 5

[0060] This embodiment provides a low crack sensitivity round steel with a yield strength of 460MPa and its preparation method.

[0061] The chemical composition of the round steel, by weight percentage, is: C 0.18%, Si 0.45%, Mn 1.55%, P 0.020%, S 0.012%, Al 0.009%, Ti 0.025%, V 0.11%, N 0.0140%, with the remainder being Fe and unavoidable impurities.

[0062] The preparation method is basically the same as in Example 2, with the only difference being the adjustment of process parameters: In step 3, the RH circulating gas flow rate is controlled at 85 Nm³ / h, and the chromium nitride cored wire feed rate is 200 m. In step 4, the high-basicity protective slag prepared in Preparation Example 2 above is used in the crystallizer, the continuous casting speed is controlled at 0.65 m / min, the water flow rate in the crystallizer is controlled at 3550 L / min, and the superheat of the continuous casting furnace is controlled at 28°C. In step 6, the temperature of the heat exchange zone is controlled at 1210℃, and the descaling water pressure is controlled at 19MPa. In step 7, the final rolling temperature is controlled at 920℃; In step 8, the temperature of the round steel entering the slow cooling pit is 510℃.

[0063] Comparative Example 1: This comparative example aims to verify the advantages of the "low aluminum + nitrogen-enriched vanadium-titanium microalloying" composition system in suppressing cracks compared to the traditional "high aluminum and low nitrogen" composition system.

[0064] The difference compared to Example 2 lies in the adjustment of chemical composition and alloying process: The chemical composition differs: the Al (acid-soluble aluminum) content in the round steel is controlled at 0.030%, the N content is not artificially added and is maintained at around 0.0050% (50ppm), and the V content is adjusted to 0.07%. The preparation process differs: In step 1, during the deoxidation of the steel tapped from the converter, the amount of aluminum-iron alloy added is increased to ensure that Als ≥ 0.040%; in step 3, during the RH vacuum treatment, only argon is used as the circulating gas (nitrogen is not used), and the operation of feeding chromium nitride cored wire is eliminated. The remaining steps and parameters are the same as in Example 2.

[0065] Comparative Example 2: This comparative example aims to verify the importance of the specially formulated "high basicity, high melting point" protective slag of this invention for the quality of peritectic steel continuous casting.

[0066] The difference between Example 2 and Example 3 lies in the mold flux used in the continuous casting step: In step 4 of this comparative example, a commercially available ordinary round steel continuous casting protective slag was used instead of the protective slag in Preparation Example 2. The physicochemical properties of this commercially available ordinary round steel continuous casting protective slag are as follows: binary basicity R = 0.70, melting point 1100℃, viscosity at 1300℃ 0.40 Pa·s, and the main components contain 9.0% Na2O and 7.0% F.

[0067] The remaining steps and parameters are the same as in Example 2.

[0068] Comparative Example 3: This comparative example aims to verify the necessity of the "weak cooling + high superheat" continuous casting process parameters for reducing surface cracks in the cast billet.

[0069] The difference compared to Example 2 lies in the adjustment of the continuous casting process parameters: In step 4, a strong cooling and low superheat mode is adopted: the flow rate of the cooling water in the crystallizer is set to 4200L / min (strong cooling), and the superheat of the molten steel in the continuous casting furnace is controlled at 15℃~20℃ (low superheat).

[0070] The remaining steps and parameters are the same as in Example 2.

[0071] Comparative Example 4: This comparative example aims to verify the effect of the "low-pressure descaling" process in preventing microcracks and indentation defects on the surface of round steel.

[0072] Compared to Example 2, the difference lies in the descaling process parameters after heating: In step 6, the high-pressure water descaling pressure after the billet exits the furnace is set to 25 MPa (conventional high-pressure descaling).

[0073] The remaining steps and parameters are the same as in Example 2.

[0074] Comparative Example 5: This comparative example aims to verify the effect of the "full-process slow cooling" process on eliminating internal stress and preventing delayed cracking and white spot defects.

[0075] Compared to Example 2, the difference lies in the elimination of the slow cooling process: The slow cooling of the billet in step 5 is cancelled. After the continuous casting billet is cut, it is directly sent into the heating furnace (hot charging) or cooled naturally in the air. Step 8, slow cooling of the round steel, is omitted. After the round steel is rolled, it is directly air-cooled to room temperature on a cooling bed. The remaining steps and parameters are the same as in Example 2.

[0076] Test Example 1: Feasibility Verification (Chemical Composition Analysis) Experimental steps Round steel products prepared in Examples 1 to 5 and Comparative Example 1 were selected. Cross-sectional samples with a thickness of 20 mm were cut from any position on the round steel. Block samples were prepared by drilling or cutting at half the radius of the sample (R / 2 position). According to GB / T4336 "Determination of Multi-Element Content in Carbon Steel and Medium-Low Alloy Steel - Spark Discharge Atomic Emission Spectrometry (Conventional Method)", the contents of C, Si, Mn, P, S, Ti, V, Cr, and Als were determined using a direct-reading spark spectrometer. The sample surface was ground and polished. The arithmetic mean of three excitation points was taken for each sample. According to GB / T20124 "Determination of Nitrogen Content in Iron and Steel - Inert Gas Melting Thermal Conductivity Method (Conventional Method)", the contents of N and O were determined using an oxygen-nitrogen-hydrogen combined analyzer.

[0077] The chemical composition test results of the finished round steel products in each group are shown in Table 1.

[0078] Table 1. Chemical composition (wt%) of finished steel products from the examples and comparative examples.

[0079] Conclusion Analysis The finished steel test data from Examples 1 to 5 show that the acid-soluble aluminum (Als) content was controlled within the range of 0.0042% to 0.0095%, all meeting the design requirement of Als ≤ 0.010%; the nitrogen (N) content was controlled within the range of 0.0103% to 0.0148%, and the vanadium (V) content was controlled within the range of 0.072% to 0.118%. In contrast, Comparative Example 1, which did not employ specific deoxidation and nitrogen enrichment processes, had an Al content of 0.0285% and a N content of only 0.0052%.

[0080] The above data indicates that the process of using silicon-manganese combined deoxidation instead of strong aluminum deoxidation, and strictly limiting the amount of aluminum-iron alloy added, can achieve deep deoxidation of molten steel and control residual aluminum at a low level. The low aluminum content reduces the material basis for AlN precipitation at austenite grain boundaries. Simultaneously, the use of nitrogen circulation combined with chromium nitride cored wire feeding during the RH vacuum treatment stage achieves the expected nitrogen microalloying effect. This low-aluminum, high-vanadium-nitrogen composition system alters the precipitation behavior of second-phase particles in the steel, inhibiting the formation of thin-film AlN that easily leads to grain boundary embrittlement, and promoting the dispersed precipitation of (Ti,V)(C,N) particles at lower temperatures. The precipitation of (Ti,V)(C,N) particles plays a precipitation strengthening role and helps refine the grains, thereby reducing crack sensitivity while ensuring the material's strength indicators.

[0081] Test Example 2: Performance Comparison Test The performance of the round steel (Φ100mm) and the corresponding continuous casting billets prepared in Examples 1 to 5 and Comparative Examples 1 to 5 were tested. Tensile performance testing was conducted according to GB / T228.1 "Metallic materials, tensile testing—Part 1: Tests at room temperature." Standard tensile specimens were prepared by sampling at the R / 2 mark of the round steel and measured using a universal testing machine to determine the upper yield strength (ReH), tensile strength (Rm), and elongation after fracture (A). Impact performance testing was conducted according to GB / T229 "Metallic materials, Charpy impact test method." V-notch standard specimens (10mm×10mm×55mm) were prepared, and the Charpy impact absorbed energy (KV2) at -20℃ was measured. The arithmetic mean of three specimens was taken. Crack statistics on the surface of the continuously cast billet were performed after the continuous casting process. A section of the continuously cast billet was cut off, the surface was cleaned, and the number of longitudinal cracks, transverse cracks, and corner cracks was visually inspected and counted. The crack incidence rate was calculated. Magnetic flux leakage flaw detection on the surface of the round steel was performed automatically along the entire length according to GB / T 32547 and related standards. The standard artificial flaw depth was set to 0.3mm, and the first-pass yield rate was calculated.

[0082] The performance test results for each group are shown in Table 2.

[0083] Table 2. Test results of mechanical properties and surface quality of the examples and comparative examples.

[0084] Conclusion Analysis: The test results of Examples 1 to 5 show that the yield strength of the round steel is greater than 490 MPa, the tensile strength is greater than 635 MPa, the impact energy at -20℃ is greater than 80 J, the crack incidence rate of the cast billet is less than 0.20%, and the finished product flaw detection pass rate is greater than 97.5%. In contrast, the cast billet of Comparative Example 1 has a higher crack incidence rate and lower low-temperature impact energy. This is because the higher Al content in Comparative Example 1 leads to AlN film precipitation at the austenite grain boundaries, reducing high-temperature plasticity; at the same time, the lack of precipitation strengthening effect of vanadium nitride results in relatively coarse matrix grains, affecting low-temperature toughness. The examples, through low-aluminum control and vanadium-nitrogen microalloying, suppressed AlN formation and refined the grains.

[0085] Comparative Example 2, using a low-basicity protective slag, resulted in an increased crack rate in the cast billet. The low-basicity protective slag has weak heat transfer control capabilities and cannot adapt to the volume shrinkage during the solidification process of peritectic steel, leading to uneven growth of the initial billet shell and resulting in longitudinal cracks. Comparative Example 3, employing a strong cooling and low superheat process, resulted in a large temperature gradient on the billet surface, increasing thermal stress and also leading to an increased crack rate. Comparative Example 4, using a higher descaling pressure, showed a lower pass rate for magnetic flux leakage testing of round steel, indicating that excessive pressure may cause rapid surface temperature drop or micro-damage. Comparative Example 5, eliminating the slow cooling process, significantly reduced elongation after fracture and impact energy, and resulted in a low pass rate for flaw detection. This was because diffusible hydrogen in the steel failed to escape sufficiently, and residual stress after rolling was not eliminated, leading to hydrogen embrittlement or internal defects in the material.

[0086] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A round steel of a grade of 460 MPa yield strength with low crack sensitivity, characterized in that, The chemical composition of the round steel bar consists of the following components by weight percentage: C：0.12%～0.20%； Si: 0.30%~0.50%; Mn: 1.30%~1.60%; P：≤0.025%； S：≤0.015%； Als: ≤0.010%; Ti: 0.01%~0.03%; V：0.07%～0.12%； N：0.0100%～0.0150%； The remainder consists of Fe and unavoidable impurities.

2. The round steel of a yield strength of 460 MPa grade with low crack sensitivity according to claim 1, characterized by, The chemical composition of the round steel bar consists of the following components by weight percentage: C：0.14%～0.18%； Si: 0.35%~0.45%; Mn: 1.35%~1.55%; P：≤0.020%； S：≤0.012%； Als: ≤0.009%; Ti: 0.015%~0.025%; V：0.08%～0.11%； N：0.0110%～0.0140%； The remainder consists of Fe and unavoidable impurities.

3. The low crack sensitivity round steel with a yield strength of 460 MPa according to claim 1, characterized in that, The microstructure of the round steel contains dispersed (Ti,V)(C,N) particles, and there is no thin film AlN precipitation at the austenite grain boundaries; the yield strength of the round steel is ≥490MPa, and the impact energy at -20℃ is ≥80J.

4. A method for preparing a low-crack-susceptibility round steel bar with a yield strength of 460 MPa according to any one of claims 1-3, characterized in that, Includes the following steps: S1. Converter smelting: Scrap steel and molten iron are added to the converter for smelting. During the tapping process, ferrosilicon and ferromanganese are added for deoxidation and alloying to control the Al content of the molten steel. S2 and LF refining: fine-tuning the composition of molten steel; S3, RH vacuum refining and nitrogen enrichment: Nitrogen gas is used as the circulating gas under vacuum conditions, and nitrogen-containing cored wire is fed in for nitrogen enrichment microalloying; S4. Large billet continuous casting: High-basicity protective slag is added to the crystallizer for continuous casting, and argon gas protection is used during the continuous casting process; S5. Slow cooling of the billet: After the billet is cut, it is placed in a slow cooling pit for the first slow cooling. S6. Billet heating and descaling: The slowly cooled billet is heated and descaled by high-pressure water after being taken out of the furnace. S7. Rolling: Rolling the cast billet into round steel; S8: After the round steel is rolled, it is placed in a slow cooling pit for a second slow cooling, and then removed from the pit and cooled to room temperature.

5. The method for preparing a low-crack-sensitivity round steel with a yield strength of 460 MPa according to claim 4, characterized in that, In step S1, the tapping temperature is controlled at 1600–1630℃, a silicon-manganese combined deoxidation process is adopted, and the amount of aluminum and iron added is limited to control the finished product Als ≤ 0.010%. In step S3, the vacuum degree is controlled to be ≤267Pa and the vacuum treatment time is ≥25min; the flow rate of the circulating gas nitrogen is 70~90Nm³ / h; the nitrogen-containing cored wire is chromium nitride cored wire, and the feed rate is 100~250m based on 130 tons of molten steel.

6. The method for preparing a low-crack-sensitivity round steel with a yield strength of 460 MPa according to claim 4, characterized in that, In step S4, the chemical composition of the high-alkalinity protective slag, by weight percentage, includes: CaO: 31.5%~36.6%; SiO2: 30.5%~31.5%; Al2O3: 3.0%~6.0%; Na2O: 5.0%~6.5%; F：3.0%～5.0%； TC: 14.5%~18.0%; MgO: 2.5%–4.0%; Unavoidable impurities: ≤1.5%; The high-alkalinity protective slag has a binary basicity of CaO / SiO2 of 1.00–1.20, a melting point of 1155–1195℃, and a viscosity of 0.62–0.75 Pa·s at 1300℃.

7. The method for preparing a low-crack-sensitivity round steel with a yield strength of 460 MPa according to claim 6, characterized in that, The high-alkalinity protective slag is prepared by a method including the following steps: Step 1: Base material pre-melting: Weigh the raw materials except TC according to the proportion, mix them, pre-melt them at 1350-1400℃ for 1-2 hours, cool them with water, crush and sieve them to obtain pre-melted base material powder; Step 2, Slurry preparation: Add carbonaceous materials and binders to the pre-melted base powder, and mix with water to prepare a slurry; Step 3: Granulation and sieving: The slurry is spray-granulated, dried, and sieved to obtain hollow particles with a particle size of 0.1 to 0.8 mm.

8. The method for preparing a low-crack-susceptibility round steel with a yield strength of 460 MPa according to claim 4, characterized in that, In step S5, the temperature of the billet entering the pit is 600-650℃, and the slow cooling time is 48-52 hours. In step S8, the temperature of the round steel entering the pit is 500-550℃, and the slow cooling time is 48-55 hours.

9. The method for preparing a low-crack-susceptibility round steel with a yield strength of 460 MPa according to claim 4, characterized in that, In step S6, the temperature of the heat soaking zone is 1180-1220℃, and the descaling water pressure is controlled at 15-20MPa. In step S7, the roughing rolling temperature is 1050–1100℃, and the final rolling temperature is 800–950℃.

10. The method for preparing a low-crack-susceptibility round steel with a yield strength of 460 MPa according to claim 4, characterized in that, In step S4, the continuous casting speed is controlled at 0.5–0.7 m / min, the water flow rate in the crystallizer is controlled at 3400–3600 L / min, and the superheat of the continuous casting furnace is controlled at 25–40 °C.

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