A rare earth microalloyed cold heading steel wire rod for 14.9 grade ultra-high strength fastener and a preparation method thereof

Abstract

The present application relates to the technical field of metallurgical materials, and particularly relates to a rare earth micro-alloyed cold heading steel wire rod for 14.9-grade ultra-high strength fasteners and a preparation method thereof. Through reasonable alloy component design, synergistic micro-alloying of rare earth elements and precise control of process parameters, the plasticity and delayed fracture resistance of the fastener are significantly improved while meeting the ultra-high strength requirement of the 14.9-grade fastener, and the stability of rare earth addition and the industrial applicability of the process are realized. After the fastener prepared by the present application is subjected to a quenching and tempering heat treatment, the tensile strength is greater than or equal to 1400 MPa, the yield strength is greater than or equal to 1260 MPa, the elongation after fracture is greater than or equal to 9%, the reduction of area is greater than or equal to 48%, the core hardness is HRC 44-49, and the mechanical performance requirement of the 14.9-grade fastener is fully met.

Description

Technical Field

[0001] This invention relates to the field of metallurgical materials technology, and more particularly to a rare earth microalloyed cold heading steel wire rod for 14.9 grade ultra-high strength fasteners and its preparation method. Background Technology

[0002] With the rapid development of industries such as aviation, automobiles, high-speed rail, shipbuilding, wind power, military, steel structures, and bridges, the demand for high-strength fasteners is increasing, driving further improvements in fastener strength grades. Taking the automotive industry as an example, with increasing pressure for energy conservation and emission reduction, lightweighting has become a crucial development direction. Using high-strength fasteners can reduce weight and increase installation space by reducing their size under the same clamping force, achieving overall weight reduction and performance optimization for equipment. Currently, the high-strength fasteners industrially produced in China include four grades: 8.8, 9.8, 10.9, and 12.9. Fasteners of grade 14.9 and above are referred to as ultra-high-strength fasteners, with grade 14.9 being the most widely and urgently needed grade among ultra-high-strength fasteners.

[0003] The mechanical property requirements for Grade 14.9 high-strength fasteners are extremely stringent. Products must meet comprehensive mechanical property indicators including tensile strength exceeding 1400 MPa, yield strength exceeding 1260 MPa, elongation after fracture exceeding 9%, reduction of area exceeding 48%, and core hardness reaching HRC 44-49. While meeting ultra-high strength requirements, fasteners must also possess sufficient plasticity and toughness to withstand cold heading, and exhibit excellent resistance to delayed fracture and fatigue performance. Ultra-high strength fastener steels face a sharp contradiction between strength and plasticity, as well as a significantly increased risk of delayed fracture (hydrogen embrittlement) with increasing strength.

[0004] In existing technologies, hot-rolled wire rods used to manufacture grade 14.9 fasteners typically employ medium-carbon steel with a carbon content of approximately 0.40%, with higher levels of Cr and Mo alloying elements, and a certain amount of microalloying elements such as V. However, existing technologies still suffer from the following technical problems: First, delayed fracture (hydrogen embrittlement) is a prominent issue. For high-strength fasteners with tensile strength exceeding 1200 MPa, delayed fracture (hydrogen embrittlement) is the most significant cause of service failure. Existing 14.9 grade fasteners made from cold-heading steel wire rods, while achieving ultra-high strength, often pose a risk of hydrogen-induced delayed fracture. Under ultra-high strength conditions, harmful elements (such as P and S) easily accumulate at grain boundaries in fasteners. Simultaneously, tempered carbides coarsen at the grain boundaries, providing favorable sites for hydrogen accumulation and crack initiation, leading to sudden, unpredictable fracture during long-term service. When ordinary steel experiences delayed fracture, a distinct grain boundary fracture surface is visible, indicating that grain boundaries are the weakest link in delayed fracture.

[0005] Secondly, grain boundary carbide coarsening is difficult to control. Grade 14.9 fasteners typically employ a quenching and tempering process to obtain tempered martensite or tempered sorbite. During tempering, carbides easily precipitate at the original austenite grain boundaries and gradually coarsen, which not only reduces the material's strength and toughness but also creates conditions for hydrogen capture and delayed fracture initiation. Currently, the main approach is to add elements such as Mo and V to form fine, dispersed alloy carbides to pin the grain boundaries. However, Mo and V are expensive alloying elements, and adding large amounts significantly increases production costs.

[0006] Third, the purity requirements for molten steel are extremely high and difficult to achieve consistently. Ultra-high strength fasteners have extremely strict requirements on the size, shape, and distribution of non-metallic inclusions in the steel, and conventional smelting processes cannot control inclusions to an ideal level.

[0007] Fourth, the application of rare earth microalloying in high-strength fasteners has not yet achieved a breakthrough. Rare earth elements have purifying, inclusion-modifying, and microalloying effects on molten steel, especially improving grain boundary characteristics, inhibiting the coarsening of tempering carbides, and enhancing resistance to hydrogen embrittlement and delayed fracture. Existing research shows that rare earth microalloying can significantly increase the room temperature impact toughness of 20MnTiB fasteners from 31.7J to 52.3J, an increase of 65%. Rare earth elements can transform the hydrogen-induced intergranular fracture of high-strength bolt steel in acidic environments into transgranular and dimple fracture, converting typical intergranular brittle fracture into ductile fracture, effectively improving stress corrosion resistance. However, due to the high reactivity and easy oxidation of rare earth elements, coupled with the complex alloying elements in ultra-high-strength steel, the addition of rare earth elements easily leads to technical bottlenecks such as nodule formation in continuous casting nozzles, low rare earth yield, and large performance fluctuations. Therefore, the application of rare earth microalloying in 14.9 grade ultra-high-strength fasteners has not yet achieved a substantial breakthrough. Summary of the Invention

[0008] The present invention aims to provide a rare-earth microalloyed cold heading steel wire rod for 14.9 grade ultra-high strength fasteners and its preparation method. Through rational alloy composition design, synergistic microalloying of rare earth elements, and precise control of process parameters, the invention significantly improves plasticity and resistance to delayed fracture while meeting the ultra-high strength requirements of 14.9 grade fasteners, and simultaneously achieves the stability of rare earth addition and the industrial applicability of the process. To achieve the above objectives, the technical solution of the present invention is as follows: The first aspect of this invention provides a rare earth microalloyed cold heading steel wire rod for 14.9 grade ultra-high strength fasteners, the wire rod comprising the following chemical composition by mass percentage: C: 0.45%~0.55%, Si: 0.10%~0.30%, Mn: 0.60%~0.90%, P: ≤0.010%, S: ≤0.005%, Cr: 0.90%~1.20%, Mo: 0.15%~0.50%, V: 0.05%~0.15%, Ti: 0.03%~0.06%, B: 0.0005%~0.0030%, Al: 0.010%~0.050%, O: ≤0.0010%, rare earth element RE: 0.0030%~0.0100%, with the balance being Fe and unavoidable impurities.

[0009] In the above technical solution, RE is further defined as Ce or La.

[0010] The selection of the amount of each of the above elements and their functions are explained below: C: C is the element with the most obvious solid solution strengthening effect in steel. As the C content increases, the strength and hardness of steel increase, while the plasticity and toughness decrease. The role of C in cold heading steel wire rod is to improve strength and hardness. In this invention, C is selected to be 0.45%~0.55%.

[0011] Si: Si is a ferrite solid solution strengthening element that can effectively inhibit grain growth, thereby improving the hardness and strength of steel. For cold heading steel wire rod, Si can improve the tempering stability of standard parts. In this invention, the Si range is selected from 0.10% to 0.30%.

[0012] Mn plays a role in solid solution strengthening and grain refinement strengthening, and delays the transformation of pearlite and ferrite, thereby improving the strength and work hardening properties of steel. For cold heading steel wire rod, Mn can ensure the hardenability of quenching and tempering. The Mn range selected in this invention is 0.60%~0.90%.

[0013] Cr: Cr can improve the strength and toughness of steel and enhance its resistance to atmospheric corrosion. It also effectively improves hardenability, ensuring the strength and hardness of the material after quenching. The Cr content in this invention ranges from 0.90% to 1.20%.

[0014] Mo: Mo can significantly improve the strength and hardness of steel, enhance its corrosion resistance, and eliminate temper brittleness. For cold heading steel wire rod, Mo can improve the high-temperature strength of standard parts while ensuring toughness. The Mo content range selected in this invention is 0.15%~0.50%.

[0015] V: V can combine with N and C to form fine precipitates, pinning grain boundaries and refining grains; at the same time, VN formed by the combination of V and N can pin dislocations and improve the strength of steel. The V content range in this invention is selected to be 0.05%~0.15%.

[0016] B: B can significantly improve the hardenability of steel, and even a very small amount of B (0.0005%~0.0030%) can effectively improve hardenability.

[0017] Ti: In boron steel, Ti can combine with nitrogen (N) to form TiN, protecting boron (B) from being combined with nitrogen and ensuring the hardenability of B. The Ti content range in this invention is selected as 0.03%~0.06%.

[0018] Al: Al is a deoxidizer for cold heading steel wire rod, which can significantly reduce the oxygen content in the steel; at the same time, Al combines with N in the steel to form AlN, which can eliminate strain aging. The Al content range in this invention is selected to be 0.010%~0.050%.

[0019] O: O plays a detrimental role in high-strength fasteners, primarily by forming brittle inclusions that cause cracking; oxide inclusions affect the fatigue life of fasteners; and it oxidizes rare earth elements, reducing rare earth yield. Therefore, the O content in steel must be strictly controlled. In this invention, the O content range is selected to be ≤0.0010%.

[0020] RE (Ce or La): RE is a rare earth element that can effectively remove inclusions in steel, improve the purity of the steel, and thus improve the plasticity and resistance to delayed fracture of the steel. The RE composition range selected in this invention is 0.0030%~0.0100%.

[0021] A second aspect of the present invention provides a method for preparing the above-mentioned rare-earth microalloyed cold heading steel wire rod for 14.9 grade ultra-high strength fasteners, comprising the following steps: Step 1: Pretreatment of molten iron The molten iron is desulfurized to ensure that the sulfur content is ≤0.005%. Step 2: Converter smelting The final carbon content of the converter smelting is controlled at 0.05%~0.12%, the final temperature is 1620~1680℃, and the basicity of the final slag is controlled at ≥3.5. During the tapping process of the converter, deoxidizers and alloys are added with the steel stream for deoxidation and alloying. Step 3: LF furnace refining Molten steel is fed into an LF furnace for refining. The refining time is ≥40 min, the white slag holding time is ≥20 min, the slag basicity is controlled at 3.0~5.0, and the molten steel temperature is controlled at 1580~1650℃. During the refining process, CaSi wire is fed in for calcium treatment, and the Al content in the steel is controlled at 0.015%~0.045%. Step 4: RH Vacuum Refining The molten steel after LF refining is sent to RH vacuum refining furnace for degassing and composition fine-tuning. The vacuum degree is ≤100Pa, the vacuum treatment time is ≥20min, and the circulation degassing time is ≥15min to further reduce the gas content and inclusions in the steel. After RH vacuum refining treatment, rare earth elements are added to the molten steel by cored wire feeding. After the rare earth cored wire is added, bottom blowing argon gas is used for soft stirring for ≥10min. Step 5: Continuous casting and rolling Molten steel treated with rare earth elements is cast into a continuous casting billet, which is then rolled. Step Six: Cooling Control After rolling, Stellmore controlled cooling is used, with the cooling rate controlled at 0.5~3.0℃ / s and the coiling temperature at 600~700℃.

[0022] In the above technical solution, the deoxidizer is a silicon-aluminum-calcium-barium alloy, and the addition amount is 2.0~4.0 kg / t steel.

[0023] In the above technical solution, further, according to the target composition, the alloy added in the deoxidation alloying is one or more of ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, ferrovanadium, and ferrotitanium.

[0024] In the above technical solution, the feed rate of the CaSi wire is further 1.5~3.0m / t steel.

[0025] In the above technical solution, the amount of rare earth cored wire added is 0.5~3.0 kg / t steel; the rare earth cored wire is RE-Fe alloy cored wire, wherein the rare earth element RE content is 10%~30%.

[0026] In the above technical solution, the continuous casting process is further controlled as follows: the superheat of molten steel in the tundish is 15~35℃, the casting speed is 0.7~1.0m / min, electromagnetic stirring is used in the crystallizer, the current is 300~600A, the frequency is 2~4Hz, dynamic light reduction technology is used, the total reduction is 2~6mm, and the cross-sectional size of the continuously cast billet is (300~410)mm×(300~410)mm square billet.

[0027] In the above technical solution, the rolling process is further controlled as follows: after the continuously cast billet is heated, it is rolled into a billet at a temperature of 1150~1250℃. The cross-sectional dimensions of the billet are (130~160) mm × (130~160) mm square billet. The billet after rolling is inspected and ground to remove surface defects. The ground billet is heated to 1150~1250℃ and held in the furnace for 120~240 min. Then it is rolled into a billet at a rolling temperature of 1050~1150℃, a finishing rolling temperature of 900~1000℃, and a wire drawing temperature of 820~880℃.

[0028] The beneficial effects of this invention are as follows: 1. This invention marks the first industrial application of rare earth microalloying in cold-heading steel wire rod for 14.9 grade ultra-high strength fasteners. In existing technologies, cold-heading steel wire rod for 14.9 grade ultra-high strength fasteners mainly employs a high-carbon composition with added precious alloying elements such as Cr, Mo, and V. The application of rare earth microalloying in ultra-high strength cold-heading steel wire rod has not yet seen substantial breakthroughs. This invention overcomes the technical bottleneck of uneven rare earth distribution by precisely controlling the amount of rare earth added (0.0030%~0.0100%) and using a cored wire feeding method after RH refining, achieving a rare earth recovery rate of over 60%. The preparation method of this invention enables stable addition of rare earth elements, obtaining cold-heading steel hot-rolled wire rod with uniform structure and stable performance, providing reliable raw materials for the industrial production of 14.9 grade ultra-high strength fasteners.

[0029] 2. Significantly Improves Resistance to Delayed Fracture (Hydrogen Embrittlement). The greatest service risk faced by 14.9 grade ultra-high strength fasteners is delayed fracture. This invention, through the addition of trace amounts of rare earth elements, forms rare earth-rich nanoclusters with sizes ranging from 1 to 50 nm in the steel. These nanoclusters have the same crystal structure as the matrix, which can inhibit the segregation of harmful elements such as P, S, and As at grain boundaries, significantly improving the steel's resistance to delayed fracture. Rare earth elements can transform the hydrogen-induced intergranular fracture of bolt steel in acidic environments into transgranular and dimple fracture, converting typical intergranular brittle fracture into ductile fracture. Simultaneously, rare earth elements can inhibit the coarsening of temper carbides at grain boundaries, further reducing the nucleation sites for stress corrosion crack initiation and hindering crack propagation, thereby effectively improving resistance to delayed fracture. Experiments show that the fasteners made from cold-headed steel wire rods of the present invention do not experience delayed fracture within 100 hours under an acidic solution with pH=3.5 and an applied stress of 80% of the yield strength, and the fracture time is ≥150h, which is far superior to existing products.

[0030] 3. Achieving efficient modification of inclusions and a significant improvement in steel purity. This invention strictly controls P≤0.010% and S≤0.005%, and through the strong purification effect of rare earth elements, transforms large-sized, irregularly shaped Al2O3 and other composite inclusions in steel into fine, spherical rare earth composite inclusions, while significantly reducing elongated MnS inclusions. The inclusion size is refined from the conventional 10~20μm to ≤3μm, effectively reducing the cutting effect of inclusions on the steel matrix and significantly improving the plasticity and toughness of the steel.

[0031] 4. Reduce the amount of precious alloying elements used, saving production costs. This invention, through the micro-alloying effect of rare earth elements, can appropriately reduce the amount of precious alloying elements such as Mo and V while meeting the 14.9 grade ultra-high strength requirement. Rare earth elements synergistically enhance the strengthening effect of alloying elements by refining grains, improving grain boundary characteristics, and influencing phase transformation processes, achieving a balance between ultra-high strength and good ductility and toughness, thus reducing raw material costs.

[0032] 5. Excellent comprehensive mechanical properties and good cold heading performance. The fasteners produced by cold heading steel wire rod processing of this invention, after quenching and tempering heat treatment, have a tensile strength ≥1400MPa, yield strength ≥1260MPa, elongation after fracture ≥9%, reduction of area ≥48%, and core hardness HRC 44~49, fully meeting the mechanical performance requirements of grade 14.9 fasteners. Detailed Implementation

[0033] The following examples are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0034] Example 1 This embodiment provides a rare earth microalloyed cold heading steel wire rod for 14.9 grade ultra-high strength fasteners, the chemical composition of which is as follows (by mass percentage): C: 0.53%, Si: 0.25%, Mn: 0.80%, P: 0.008%, S: 0.003%, Cr: 1.10%, Mo: 0.45%, V: 0.13%, Ti: 0.035%, Al: 0.028%, B: 0.0022%, O: 0.0008%, Ce: 0.0055%, balance Fe and unavoidable impurities.

[0035] The above-mentioned method for preparing wire rod includes the following steps: (1) Hot metal pretreatment: The hot metal is desulfurized and the sulfur content is controlled to be less than 0.005%; (2) Converter smelting: control the final C content to 0.08%, the final temperature to 1645℃, the final slag basicity R1=3.8, the converter charge to 120 tons / furnace, and add 3.0 kg / t of silicon-aluminum-calcium-barium alloy with the steel stream during the converter tapping process to deoxidize the steel. Add ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, ferrovanadium, fertitanium and ferroboron according to the target composition for alloying. (3) LF furnace refining: Molten steel enters the LF furnace for refining. The LF refining time is 45 min, the white slag holding time is 22 min, the slag basicity R2=4.0, the molten steel temperature is 1620℃, and CaSi wire is fed in for calcium treatment during the refining process. The wire feeding amount is 2.5 m / t steel, and the Al content in the molten steel is controlled to be 0.025%. (4) RH vacuum refining: The molten steel after LF refining is sent into the RH vacuum refining furnace for degassing and composition fine adjustment. The vacuum degree is ≤100Pa, the vacuum treatment time is 25min, and the circulation degassing time is 18min. After RH vacuum refining treatment, rare earth elements are added to the molten steel by cored wire feeding. The rare earth cored wire is Ce-Fe alloy cored wire (Ce content 20%). The feeding amount is 1.8kg / t steel. After the steel is added, bottom blowing argon gas is used for soft stirring. The soft stirring time is ≥10min. (5) Continuous casting: The superheat of molten steel in the tundish is 22℃, the casting speed is 0.95m / min, the electromagnetic stirring current of the crystallizer is 450A, the frequency is 3Hz, the total reduction under dynamic light pressure is 4mm, and the cross-section of the continuous casting billet is 320mm×410mm square billet. (6) Rolling: The continuous casting billet is heated to 1200℃. After heating, the continuous casting billet is rolled into a billet with a billet temperature of 1180℃ and a billet cross section of 140mm×140mm square billet. The billet after billet rolling is subjected to ultrasonic flaw detection and surface grinding. After grinding, the billet is heated to 1200℃ and in the furnace for 180min before rolling. The initial rolling temperature is 1120℃, the finishing rolling temperature is 950℃, and the wire drawing temperature is 850℃. (7) Cooling control: Stellmore control cooling is adopted, with a cooling rate of 1.5℃ / s and a winding temperature of 650℃ to obtain Φ14mm wire rod.

[0036] Example 2 This embodiment prepares a 14.9 grade ultra-high strength rare earth microalloyed cold heading steel wire rod, the chemical composition of which, by mass percentage, is as follows: C: 0.48%, Si: 0.22%, Mn: 0.70%, P: 0.006%, S: 0.002%, Cr: 0.9%, Mo: 0.50%, V: 0.12%, Ti: 0.040%, Al: 0.030%, B: 0.0025%, O: 0.0006%, La: 0.0065%, balance Fe and unavoidable impurities.

[0037] The above-mentioned method for preparing wire rod includes the following steps: (1) Hot metal pretreatment: The hot metal is desulfurized and the sulfur content is controlled to be less than 0.005%; (2) Converter smelting: control the final C content to 0.12%, the final temperature to 1680℃, the final slag basicity R1=4.5, the converter charge to 120 tons / furnace, add 4.0 kg / t of silicon-aluminum-calcium-barium alloy with the steel stream during the converter tapping process to deoxidize the steel, and add ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, ferrovanadium, fertitanium and ferroboron according to the target composition for alloying; (3) LF furnace refining: Molten steel enters the LF furnace for refining. The LF refining time is 42 min, the white slag holding time is 22 min, the slag basicity R2=5.0, the molten steel temperature is 1650℃, and CaSi wire is fed in for calcium treatment during the refining process. The wire feeding rate is 3.0 m / t steel, and the Al content in the molten steel is controlled to be 0.040%. (4) RH vacuum refining: The molten steel after LF refining is sent to the RH vacuum refining furnace for degassing and composition fine adjustment. The vacuum degree is ≤100Pa, the vacuum treatment time is 23min, and the circulation degassing time is 20min. After RH vacuum refining treatment, rare earth elements are added to the molten steel by cored wire feeding. The rare earth cored wire is Ce-Fe alloy cored wire (Ce content 30%). The feeding amount is 1.5kg / t steel. After the steel is added, bottom blowing argon gas is used for soft stirring. The soft stirring time is ≥10min. (5) Continuous casting: The molten steel in the tundish is superheated to 30℃, the casting speed is 0.85m / min, the electromagnetic stirring current of the crystallizer is 450A, the frequency is 3Hz, the total reduction under dynamic light pressure is 6mm, and the cross-section of the continuous casting billet is 320mm×410mm square billet. (6) Rolling: The continuous casting billet is heated to 1200℃. After heating, the continuous casting billet is rolled into a billet with a billet temperature of 1180℃ and a billet cross section of 160mm×160mm square billet. The billet after billet rolling is subjected to ultrasonic flaw detection and surface grinding. After grinding, the billet is heated to 1150℃ and in the furnace for 170min before rolling. The initial rolling temperature is 1100℃, the finishing rolling temperature is 940℃, and the wire drawing temperature is 860℃. (7) Cooling control: Stellmore control cooling is adopted, with a cooling rate of 2.0℃ / s and a winding temperature of 620℃ to obtain Φ14mm wire rod.

[0038] Example 3 This embodiment prepares a 14.9 grade ultra-high strength rare earth microalloyed cold heading steel wire rod, the chemical composition of which is as follows (by mass percentage): C: 0.52%, Si: 0.30%, Mn: 0.85%, P: 0.007%, S: 0.0025%, Cr: 1.10%, Mo: 0.40%, V: 0.12%, Ti: 0.030%, Al: 0.025%, B: 0.0020%, O: 0.0005%, Ce: 0.0045%, balance Fe and unavoidable impurities.

[0039] The above-mentioned method for preparing wire rod includes the following steps: (1) Hot metal pretreatment: The hot metal is desulfurized and the sulfur content is controlled to be less than 0.005%; (2) Converter smelting: control the final C content to 0.010%, the final temperature to 1630℃, the final slag basicity R1=4.2, the converter charge to 120 tons / furnace, add 2.0 kg / t of silicon-aluminum-calcium-barium alloy with the steel stream during the converter tapping process to deoxidize the steel, and add ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, ferrovanadium, fertitanium and ferroboron according to the target composition for alloying; (3) LF furnace refining: Molten steel enters the LF furnace for refining. The LF refining time is 40 min, the white slag holding time is 20 min, the slag basicity R2=3.5, the molten steel temperature is 1600℃, and CaSi wire is fed in for calcium treatment during the refining process. The wire feeding amount is 3.0 m / t steel, and the Al content in the molten steel is controlled to be 0.020%. (4) RH vacuum refining: The molten steel after LF refining is sent into the RH vacuum refining furnace for degassing and composition fine adjustment. The vacuum degree is ≤100Pa, the vacuum treatment time is 23min, and the circulation degassing time is 17min. After RH vacuum refining treatment, rare earth elements are added to the molten steel by cored wire feeding. The rare earth cored wire is Ce-Fe alloy cored wire (Ce content 25%). The feeding amount is 2.0kg / t steel. After the steel is added, bottom blowing argon gas is used for soft stirring. The soft stirring time is ≥10min. (5) Continuous casting: The molten steel in the tundish is superheated to 30℃, the casting speed is 0.82m / min, the electromagnetic stirring current of the crystallizer is 400A, the frequency is 3Hz, the total reduction under dynamic light pressure is 6mm, and the cross-section of the continuous casting billet is 320mm×410mm square billet. (6) Rolling: The continuous casting billet is heated to 1230℃. After heating, the continuous casting billet is rolled into a billet with a billet temperature of 1190℃ and a billet cross section of 160mm×160mm square billet. The billet after billet rolling is subjected to ultrasonic flaw detection and surface grinding. After grinding, the billet is heated to 1130℃ and in the furnace for 180min before rolling. The initial rolling temperature is 1100℃, the finishing rolling temperature is 930℃, and the wire drawing temperature is 850℃. (7) Cooling control: Stellmore control cooling is adopted, with a cooling rate of 2.5℃ / s and a winding temperature of 600℃ to obtain Φ14mm wire rod.

[0040] Comparative Example 1 This comparative example uses conventional 14.9 grade cold heading steel wire rod (excluding rare earth elements), and its chemical composition by mass percentage is as follows: C: 0.60%, Si: 0.38%, Mn: 0.82%, P: 0.010%, S: 0.008%, Cr: 1.18%, Mo: 0.48%, V: 0.16%, Ti: 0.032%, Al: 0.026%, B: 0.0021%, contains no rare earth elements, the balance is Fe and unavoidable impurities.

[0041] The above-mentioned method for preparing wire rod includes the following steps: (1) Hot metal pretreatment: The hot metal is desulfurized and the sulfur content is controlled to be less than 0.005%; (2) Converter smelting: control the final C content to 0.11%, the final temperature to 1650℃, the final slag basicity R1=3.5, the converter charge to 120 tons / furnace, and add 2.0 kg / t of silicon-aluminum-calcium-barium alloy with the steel stream during the converter tapping process to deoxidize the steel. Add ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, ferrovanadium, fertitanium, ferroboron and other alloys according to the target composition. (3) LF furnace refining: Molten steel enters the LF furnace for refining. The LF refining time is 40 min, the white slag holding time is 20 min, the slag basicity R2=5.0, the molten steel temperature is 1640℃, and CaSi wire is fed in for calcium treatment during the refining process. The wire feeding amount is 1.5 m / t steel, and the Al content in the molten steel is controlled to be 0.025%. (4) RH vacuum refining: The molten steel after LF refining is sent into the RH vacuum refining furnace for degassing and composition fine adjustment. The vacuum degree is ≤100Pa, the vacuum treatment time is 25min, and the circulation degassing time is 20min. (5) Continuous casting: The molten steel in the tundish is superheated to 25℃, the casting speed is 0.90m / min, the electromagnetic stirring current of the crystallizer is 450A, the frequency is 3Hz, the total reduction under dynamic light pressure is 4mm, and the cross-section of the continuous casting billet is 320mm×410mm square billet. (6) Rolling: The continuous casting billet is heated to 1200℃. After heating, the continuous casting billet is rolled into a billet with a billet temperature of 1150℃ and a billet cross section of 160mm×160mm square billet. The billet after billet rolling is subjected to ultrasonic flaw detection and surface grinding. After grinding, the billet is heated to 1150℃ and in the furnace for 180min before rolling. The initial rolling temperature is 1100℃, the finishing rolling temperature is 930℃, and the wire drawing temperature is 850℃. (7) Cooling control: Stellmore control cooling is adopted, with a cooling rate of 1.0℃ / s and a winding temperature of 700℃ to obtain Φ14mm wire rod.

[0042] Performance testing and evaluation: The cold-heading steel wire rods prepared in Examples 1-3 and Comparative Example 1 were used to prepare fasteners using conventional methods, followed by quenching and tempering heat treatment (quenching: heating temperature 900℃, holding time 1 hour, oil cooling to room temperature; tempering: heating temperature 590℃, holding time 2 hours, natural cooling to room temperature). The mechanical properties and resistance to delayed fracture of the fasteners were then tested. The test methods were in accordance with GB / T 3098.1-2010 "Mechanical Properties of Fasteners - Bolts, Screws and Studs" standard, and the results are shown in Tables 1 and 2.

[0043] Table 1 Comparison of mechanical properties of Examples 1-3 and Comparative Example 1

[0044] As shown in Table 1, all embodiments fully meet the mechanical performance requirements of Grade 14.9 fasteners (Rm≥1400MPa, Rel≥1260MPa, A≥9%, Z≥48%, HRC 44~49). Compared with conventional Grade 14.9 cold heading steel wire rod without rare earth elements (Comparative Example 1), the rare earth microalloyed cold heading steel wire rod of the embodiments of the present invention maintains a comparable or slightly higher strength level while increasing the elongation after fracture by approximately 11%~22% and the reduction of area by approximately 6%~11%. The rare earth microalloying leads to a significant improvement in plasticity, which is crucial for the cold heading and service safety of ultra-high strength fasteners.

[0045] Table 2 Comparison of inclusion evaluation and delayed fracture resistance of Examples 1-3 and Comparative Example 1

[0046] As shown in Table 2, the maximum inclusion size in the embodiments of the present invention was reduced from 12.0 μm in the comparative example to 2.0~2.8 μm, demonstrating a significant inclusion refinement effect. More importantly, the elongated MnS inclusions in conventional steel completely disappeared in the steel of the present invention, replaced by fine, spherical rare earth composite inclusions, which greatly improved the ductility, toughness, and resistance to hydrogen embrittlement of the steel. Regarding delayed fracture resistance, the fasteners of Examples 1-3 of the present invention, under acidic solution at pH=3.5 and with 80% yield strength stress, achieved a fracture time of 156~185 hours, far superior to the 82 hours of Comparative Example 1. Fracture surface analysis showed that the fracture surface of the cold-heading steel fasteners of the present invention exhibited transgranular + dimple characteristics, i.e., a ductile fracture mode; while the fracture surface of the comparative example fasteners showed a typical intergranular brittle fracture mode. The addition of rare earth elements effectively suppressed hydrogen-induced intergranular cracking and significantly improved the resistance to delayed fracture.

[0047] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A rare-earth microalloyed cold-heading steel wire rod for 14.9 grade ultra-high strength fasteners, characterized in that, The wire rod comprises the following chemical composition by mass percentage: C: 0.45%~0.55%, Si: 0.10%~0.30%, Mn: 0.60%~0.90%, P: ≤0.010%, S: ≤0.005%, Cr: 0.90%~1.20%, Mo: 0.15%~0.50%, V: 0.05%~0.15%, Ti: 0.03%~0.06%, B: 0.0005%~0.0030%, Al: 0.010%~0.050%, O: ≤0.0010%, rare earth element RE: 0.0030%~0.0100%, with the balance being Fe and unavoidable impurities.

2. The rare earth microalloyed cold heading steel wire rod according to claim 1, characterized in that, The RE is either Ce or La.

3. A method for preparing rare-earth microalloyed cold heading steel wire rod for 14.9 grade ultra-high strength fasteners as described in any one of claims 1-2, characterized in that, Includes the following steps: Step 1: Pretreatment of molten iron The molten iron is desulfurized to ensure that the sulfur content is ≤0.005%. Step 2: Converter smelting The final carbon content of the converter smelting is controlled at 0.05%~0.12%, the final temperature is 1620~1680℃, and the basicity of the final slag is controlled at ≥3.

5. Deoxidizers and alloys are added during the converter tapping process for deoxidation and alloying. Step 3: LF furnace refining Molten steel is fed into an LF furnace for refining. The refining time is ≥40 min, the white slag holding time is ≥20 min, the slag basicity is controlled at 3.0~5.0, and the molten steel temperature is controlled at 1580~1650℃. During the refining process, CaSi wire is fed in for calcium treatment, and the Al content in the steel is controlled at 0.015%~0.045%. Step 4: RH Vacuum Refining Vacuum degree ≤100Pa, vacuum treatment time ≥20min, circulation degassing time ≥15min, rare earth cored wire is added after RH vacuum refining treatment, and bottom blowing argon gas is used for soft stirring after the rare earth cored wire is added, and the soft stirring time is ≥10min. Step 5: Continuous casting and rolling Molten steel treated with rare earth elements is cast into a continuous casting billet, which is then rolled. Step Six: Cooling Control After rolling, Stellmore controlled cooling is used, with the cooling rate controlled at 0.5~3.0℃ / s and the coiling temperature at 600~700℃.

4. The preparation method according to claim 3, characterized in that, The deoxidizer is a silicon-aluminum-calcium-barium alloy, and the addition amount is 2.0~4.0 kg / t steel.

5. The preparation method according to claim 3, characterized in that, According to the target composition, the alloy added during the deoxidation alloying process is one or more of ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, ferrovanadium, and ferrotitanium.

6. The preparation method according to claim 3, characterized in that, The feed rate of the CaSi wire is 1.5~3.0m / t steel.

7. The preparation method according to claim 3, characterized in that, The rare earth cored wire is added at a rate of 0.5~3.0 kg / t steel; the rare earth cored wire is an RE-Fe alloy cored wire, wherein the mass content of rare earth element RE is 10%~30%.

8. The preparation method according to claim 3, characterized in that, The continuous casting process is controlled as follows: the tundish molten steel superheat is 15~35℃, the casting speed is 0.7~1.0m / min, the crystallizer is electromagnetically stirred with a current of 300~600A and a frequency of 2~4Hz, and dynamic light reduction technology is adopted with a total reduction of 2~6mm.

9. The preparation method according to claim 3, characterized in that, The rolling process is controlled as follows: after the continuously cast billet is heated, it is rolled at a temperature of 1150~1250℃. The refurbished billet is then heated to 1150~1250℃ and held in the furnace for 120~240 minutes. Rolling is then carried out at a temperature of 1050~1150℃, a finishing rolling temperature of 900~1000℃, and a wire drawing temperature of 820~880℃.

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