A high ductility, delayed fracture resistant, ultra-high strength fastener steel wire rod and method of making same

Abstract

The application relates to a manufacturing method of a high-plasticity delay fracture resistant super-high-strength fastener steel wire rod, the production diameter of the wire rod is 5.5-25 mm, and the production process flow is converter or electric furnace smelting, LF refining, RH or VD degassing, continuous casting, intermediate blank cogging, intermediate blank surface treatment, wire rod rolling and wire spitting, and controlled cooling. After subsequent annealing and cold upsetting, the produced high-strength fastener wire rod is subjected to isothermal heat treatment according to the isothermal heat treatment process provided by the application, lower bainite organization is obtained, the mechanical properties can meet the requirements of the 14.9-grade fastener, and excellent delay fracture resistance is achieved.

Description

Technical Field

[0001] This invention relates to the field of manufacturing wire rods for bainitic fasteners of grade 14.9 and above, and specifically to a manufacturing method applicable to hot-rolled wire rods. Background Technology

[0002] Fasteners are often referred to as the "rice of industry." my country produces a large quantity of fasteners, but high-strength fasteners account for a small percentage; 14.9 grade ultra-high-strength fasteners and raw materials still rely on imports. With the development requirements of industries such as automotive lightweighting, wind power, and machinery, fasteners are evolving from the common 8.8 grade to 10.9, 12.9, and even higher strength levels. High-strength fasteners provide better fastening force and better fatigue resistance; ultra-high strength fasteners have become a trend.

[0003] Fasteners and steel raw materials of grades 10.9 and 12.9 are becoming increasingly mature, but ultra-high strength fasteners of grade 14.9 and above are still in the research and development stage, and there are currently no publicly available standards and performance indicators for reference. With the increase in the tensile strength of fasteners, the problem of delayed fracture becomes very prominent. It is particularly worth mentioning that fasteners themselves have notches, making their susceptibility to delayed fracture several times that of other components.

[0004] All currently disclosed patents for heat treatment processes of ultra-high strength fasteners of grade 14.9 and above employ quenching and tempering, resulting in a tempered martensitic microstructure. Tempered martensitic microstructure generally exhibits poor resistance to delayed fracture due to the precipitation of carbides at the martensite lath boundaries, significantly reducing the fastener's resistance to delayed fracture. To ensure this resistance, Nb and Ti microalloying elements are added to the steel to form carbides, which act as hydrogen traps. However, the addition of microalloying elements easily leads to the formation of large-particle inclusions of NbN and TiN, which can impair the fastener's fatigue strength. Furthermore, to compensate for the strength loss during tempering, large amounts of alloying elements such as Mo, Cu, Ni, and Cu are added to the steel, which deteriorates the steel's formability and hinders cold heading. Summary of the Invention

[0005] The purpose of this invention is to provide a high-plasticity, delayed-fracture-resistant ultra-high-strength steel wire rod for fasteners and its manufacturing method, including a heat treatment process. By designing the alloy composition of the wire rod for fasteners, and combining it with billet rolling, intermediate billet surface treatment, wire rod rolling and cooling processes, a high-plasticity, delayed-fracture-resistant ultra-high-strength wire rod for fasteners is obtained. After cold heading, isothermal heat treatment is performed to obtain a bainitic structure, thereby achieving excellent mechanical properties that meet the requirements of ultra-high strength grade 14.9 mechanical properties, and exhibiting excellent resistance to delayed fracture.

[0006] One of the key technologies of this application is to obtain bainitic microstructure through isothermal heat treatment to manufacture fasteners with a strength of 14.9 or higher. The lower bainite in the steel is a combination of shear and diffusion phase transformations, with fine and dispersed carbide precipitation within the grains, providing high resistance to hydrogen-delayed fracture while also maintaining plasticity and excellent cold heading performance.

[0007] The technical solution adopted by this invention to solve the above problems is: a high-plasticity, delayed-fracture resistant, ultra-high-strength fastener steel wire rod and its manufacturing method, comprising:

[0008] Step 1, Elemental composition design: by mass percentage: C: 0.36-0.44%, Si: 0.15-0.35%, Mn: 0.40-0.80%, P: ≤0.015%, S: ≤0.015%, Cr: 0.90-1.20%, Mo: 0.50-0.75%, V: 0.20-0.35%, Al: 0.005-0.015%, Ni: 0.10-0.30%, N: 0.0060%-0.0080%, with the balance being Fe and unavoidable impurity elements.

[0009] C: Carbon is one of the main strengthening elements of steel. If the carbon content is too low, the strength and hardness of the cold-heading steel will be too low after heat treatment. If the carbon content is too high, while the strength and hardness increase after heat treatment, the hot-rolled strength will also increase, but the plasticity will decrease, greatly reducing the life of the cold-heading die. Therefore, this invention controls the carbon content at 0.36-0.44%.

[0010] Silicon (Si) is a ferrite solid solution strengthening element and also a deoxidizing element. Silicon effectively strengthens ferrite, and high silicon content increases the hardening coefficient, hindering the plastic deformation of cold-heading steel. However, to reduce steelmaking costs and achieve effective deoxidation, this invention controls the silicon content in the steel to 0.15-0.35%.

[0011] Mn: Manganese is an element that improves the hardenability of steel and plays a role in solid solution strengthening, thereby increasing the strength of steel. Too low a manganese content will reduce the steel's toughness and plasticity, and also decrease the Mn / S ratio in the steel, easily leading to segregation in continuously cast billets. Therefore, to achieve a balance between strength and plasticity after heat treatment of cold-heading steel, this invention controls the manganese content in the steel to be between 0.40% and 0.80%.

[0012] Cr: Chromium is an element that enhances the hardenability of steel. Adding an appropriate amount can effectively increase the hardenability of low- and medium-carbon steel and improve the strength of cold-heading steel after isothermal heat treatment. However, excessive chromium content will increase hot-rolled strength and hardness after annealing, while reducing the steel's formability. Therefore, this invention controls the chromium content in the steel to 0.90-1.20%.

[0013] Mo: Molybdenum is a strong carbide-forming element. During isothermal heat treatment, it forms fine Mo2C particles, resulting in secondary strengthening. Simultaneously, it acts as a hydrogen trap to improve resistance to delayed fracture. The molybdenum content in this invention patent is controlled at 0.50-0.75%.

[0014] Vanadium is a microalloying element. When added to steel, it precipitates as nanoscale carbides during isothermal heat treatment, strengthening the matrix and acting as a hydrogen trap to improve resistance to delayed fracture. Therefore, this invention controls the vanadium content to 0.20-0.35%.

[0015] Aluminum (Al) primarily functions as a deoxidizer and nitrogen fixation agent. As a conventional deoxidizer added to steel, aluminum effectively reduces the oxygen content. However, aluminum is also a ferrite-forming element; excessively high aluminum content can lead to the formation of soft ferrite phases in segregation regions during isothermal heat treatment, compromising the material's fatigue performance. This invention controls the aluminum content of the steel to 0.005-0.015%.

[0016] Ni: Nickel can improve the toughness of steel, and a small amount of nickel can improve the stability of the isothermal heat treatment phase transformation of steel, while also enabling the isothermal heat treatment phase transformation to produce fine carbide particles. This invention controls the nickel content of the steel to 0.1-0.3%;

[0017] P, S: In principle, the lower the phosphorus and sulfur content, the better. However, excessively low content will undoubtedly increase smelting costs. Under the condition that quality allows, this invention controls the phosphorus and sulfur content to be below 0.015%.

[0018] Nitrogen (N) can form nitrides with microalloys in steel, and also form AlN particles with Al, which can be used to pin austenite grain boundaries and refine grains. Excessive nitrogen can cause strain aging in steel during cold heading, impairing its cold workability. This invention controls the nitrogen content of the steel to 0.0060-0.0080%.

[0019] Step Two: Steel Smelting: Steel is smelted according to the elemental composition determined in Step One. The smelting process involves converter primary refining, refining, and vacuum degassing. The converter primary refining process requires control of the final carbon content, generally maintained at ≥0.20%, to prevent over-oxidation of the molten steel. Pure aluminum granules are added to the steel during tapping, at a rate of 0.5-1 kg / t. No further aluminum granules are allowed to be added during the refining process after this initial addition. Slag and alloy blocks are added along the steel stream. Refining is LF refining, using only SiC diffusion deoxidation. During refining, N2 is blown into the ladle from the bottom to replenish the molten steel. Nitrogen is used as a lifting gas or low-pressure gas during vacuum degassing, with a high-vacuum degassing time preferably not less than 15 minutes.

[0020] Step 3, Casting: Using continuous casting, the molten steel from Step 2 is transferred to the tundish and cast into a continuous casting billet. The superheat of the molten steel is preferably controlled between 15-40℃. Argon gas protection is used throughout the continuous casting process to prevent the molten steel from coming into contact with oxygen. The billet size is preferably 390mm × 510mm or larger. After the billet is removed from the line, it is preferred to place it in a pit for slow cooling, with a slow cooling time of ≥48 hours.

[0021] Step 4: Billet Rolling: Roll the continuously cast billet into small-section square or round billets with a cross-sectional compression ratio ≥3. The billet rolling process involves heating and rolling. Ideally, the billet should be held at a soaking temperature of 1150-1250℃ for at least 6 hours, with a total heating time of at least 10 hours. After rolling, the billet should be promptly placed in a slow cooling pit for slow cooling, with an exit temperature ≥650℃ and a slow cooling time ≥48 hours.

[0022] Step 5: Surface Treatment of Billet: The intermediate billet after blanking needs to undergo surface flaw detection. Infrared flaw detection can be used, with a detection accuracy within 0.1mm. Surface flaws need to be repaired by grinding and then peeled off. The amount of surface peeling should preferably be controlled above 1.5mm.

[0023] Step Six: Wire Rod Rolling: This step involves heating and rolling processes. A walking beam furnace is preferred for heating. The temperature in the preheating section of the furnace is controlled at ≤650℃, and the temperature in the homogenization section is controlled at 1050-1150℃. The total heating time is controlled at 120-180 minutes. After heating, the billet undergoes high-pressure water descaling, followed by rolling. Rolling includes roughing, secondary water descaling, intermediate rolling, pre-finishing rolling, and finishing rolling. The roughing temperature is controlled at 900-1000℃. After roughing, secondary water descaling is performed to remove the secondary oxide scale generated during roughing. The intermediate rolling and pre-finishing rolling temperatures are controlled at 900-950℃, and the wire drawing temperature is controlled at 900-950℃. The wire rod specifications are φ5.5-25mm.

[0024] Step 7, Post-rolling cooling: The rolled wire rod is cooled on the Stellmore air-cooling line. It is first air-cooled through the high-temperature zone, then the insulation hood is opened, and the temperature drops to 800-850℃ before it enters the insulation hood for slow cooling. Before slow cooling, the microstructure of the wire rod should still be in a fully austenitic state and has not decomposed. The slow cooling rate is 0.1℃ / s-0.5℃ / s. It is slowly cooled to below 500℃ before exiting the hood. After exiting the hood, the wire rod is collected and naturally cooled to obtain a metallographic structure with bainite as the main component, a small amount of martensite, and a small amount of ferrite.

[0025] The above methods yield high-plasticity, delayed-fracture resistant, ultra-high-strength fastener steel wire rods. After spheroidizing annealing, these wire rods can meet the requirements of 1 / 4 or even 1 / 5 cold upsetting. They are suitable for fasteners with large cold upsetting ratios and complex head shapes.

[0026] This invention also provides an isothermal heat treatment method for ultra-high strength fasteners:

[0027] The aforementioned wire rod is heated in a box furnace or a continuous furnace with atmosphere protection to the austenitizing temperature of 880-950℃, held for 120-150 minutes, and then rapidly quenched into molten salt for isothermal heat treatment. The recommended isothermal heat treatment time is 100-150 minutes, and the salt temperature is controlled at 290-350℃. The heat treatment salt can be composed of potassium nitrate and sodium nitrate mixed in a 1:1 ratio. Afterwards, it is air-cooled or cooled with tap water. Fasteners obtained using the above wire rod and heat treatment method have a tensile strength ≥1400MPa, yield strength ≥1200MPa, elongation ≥10%, and reduction of area ≥45%. Attached Figure Description

[0028] Figure 1 The microstructure of the hot-rolled wire rod in this embodiment of the invention is shown in the figure. The microstructure is bainite + a small amount of martensite + a small amount of ferrite.

[0029] Figure 2 The microstructure of hot-rolled wire rod after isothermal heat treatment in this embodiment of the invention is shown in the figure as lower bainite. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0031] Example 1

[0032] This embodiment relates to the production of high-plasticity, delayed-fracture resistant, ultra-high-strength steel wire rods for fasteners, with the following composition and weight percentages: C: 0.40%, Si: 0.22%, Mn: 0.68%, P: ≤0.010%, S: ≤0.006%, Cr: 1.05%, Mo: 0.60%, V: 0.28%, Al: 0.010%, Ni: 0.12%, N: 0.0068%, with the balance being Fe and unavoidable impurities. The wire rod diameter is 18mm. The process flow is as follows: the main raw materials are sequentially smelted in a converter; 80kg of aluminum granules are added during converter tapping; LF+RH ladle refining is employed, using SiC diffusion deoxidation during refining, RH vacuum degassing with nitrogen as the lifting gas, and high vacuum treatment time of 18 minutes. Molten steel is cast into continuously cast billets using a low superheat of 15–30℃ under argon protection throughout the process. The billets need to be heated in a billet heating furnace, with a soaking temperature of 1200℃ in the soaking section for 7 hours, for a total heating time of 11 hours. The billet size is 200mm × 200mm square billets. After billet preparation, the billets are immediately placed in a slow cooling pit for slow cooling at a line temperature of 750℃ for 50 hours. After slow cooling, the intermediate billets undergo surface treatment to remove surface defects, with a surface peeling depth of 1.7mm. After treatment, the billets are heated in a heating furnace to 1130℃ for a total heating time of 140 minutes before being taken out of the furnace. The roughing rolling temperature is 980℃, the intermediate rolling temperature is 920℃, the pre-finishing rolling temperature is 930℃, the finishing rolling temperature is 930℃, and the wire drawing temperature is 910℃. The Stellmore cooling process is performed with the roller conveyor speed set to 0.15 m / s. The four front insulation covers are open, while the rear insulation covers are all closed. The wire rod temperature entering the insulation cover is 820°C, and the temperature exiting the insulation cover is 480°C. After the Stellmore cooling process, the wire rod is collected and air-cooled.

[0033] Example 2

[0034] This embodiment relates to the production of high-plasticity, delayed-fracture resistant, ultra-high-strength steel wire rods for fasteners, with the following composition and weight percentages: C: 0.41%, Si: 0.20%, Mn: 0.70%, P: ≤0.010%, S: ≤0.010%, Cr: 1.00%, Mo: 0.63%, V: 0.30%, Al: 0.008%, Ni: 0.12%, N: 0.0073%, with the balance being Fe and unavoidable impurities. The wire rod diameter is 12mm. The process flow is as follows: the main raw materials are sequentially smelted in a converter; 100kg of aluminum granules are added during converter tapping; LF+RH ladle refining is employed, with SiC diffusion deoxidation used during refining, RH vacuum degassing using nitrogen as the lifting gas, and a high-vacuum treatment time of 16 minutes. Molten steel is cast into continuously cast billets using a low superheat of 15–30℃ under argon protection throughout the process. The billets need to be heated in a billet heating furnace, with a soaking temperature of 1220℃ in the soaking section for 6 hours, for a total heating time of 12 hours. The billet size is a φ170mm round billet. After billeting, it is immediately placed in a slow cooling pit for slow cooling at a line temperature of 720℃ for 52 hours. After slow cooling, the intermediate billets undergo surface treatment to remove surface defects, with a surface peeling depth of 2.0mm. After treatment, the billets are heated in a heating furnace to 1100℃ for a total heating time of 130 minutes before being taken out of the furnace. The rough rolling temperature is 970℃, the intermediate rolling temperature is 910℃, the pre-finish rolling temperature is 930℃, the finish rolling temperature is 940℃, and the wire drawing temperature is 920℃. The Stellmore cooling process is performed with the roller conveyor speed set to 0.20 m / s. The first six insulation covers are open, while the last insulation cover is completely closed. The wire rod temperature entering the insulation cover is 810℃, and the temperature exiting the insulation cover is 450℃. After Stellmore cooling, the wire rod is collected and air-cooled.

[0035] Table 1 shows the mechanical properties of wire rods from Examples 1 and 2 after isothermal heat treatment.

[0036]

[0037] Note: Isothermal heat treatment process, austenitizing process: heating temperature 900℃, holding time 130min, salt bath temperature 330℃, salt bath isothermal time 120min.

[0038] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing a high-plasticity, delayed-fracture resistant, ultra-high-strength fastener steel wire rod, characterized in that: Includes the following steps: Step 1, Elemental Composition Design: By mass percentage: C: 0.36-0.44%, Si: 0.15-0.35%, Mn: 0.40-0.80%, P: ≤0.015%, S: ≤0.015%, Cr: 0.90-1.20%, Mo: 0.50-0.75%, V: 0.20-0.35%, Al: 0.005-0.015%, Ni: 0.10-0.30%, N: 0.0060%-0.0080%, with the balance being Fe and unavoidable impurity elements; Step Two, Steel Smelting: Smelt steel according to the elemental composition in Step One. The smelting process involves converter primary refining, refining, and vacuum degassing. The carbon content at the end of the converter primary refining process is controlled to be ≥0.20%. Pure aluminum granules are added to the steel output at the time of tapping, with an addition rate of 0.5-1 kg / t. After the addition of aluminum granules, no more aluminum granules should be added during the entire refining process. At the same time, slag and alloy blocks are added along the steel stream. The refining process is LF refining, and only SiC diffusion deoxidation is used during the refining process. N2 is blown into the steel from the bottom of the ladle during the refining process to replenish nitrogen to the molten steel. Nitrogen is used as the lifting gas or low-pressure gas during vacuum degassing, and the high vacuum degassing time is not less than 15 minutes. Step 3, Casting: The molten steel from Step 2 is transferred to the tundish and cast into a continuous casting billet using a continuous casting process. The billet is then cooled slowly after it leaves the production line. Step 4: Billet rolling: Roll the continuously cast billet into small cross-section square or round billets with a cross-sectional compression ratio ≥3. Step 5: Surface treatment of billet: After the billet is cut, the intermediate billet is subjected to surface flaw detection using infrared flaw detection with a flaw detection accuracy of less than 0.1mm. Surface flaws are repaired by grinding and peeling. Step 6, Wire Rod Rolling: The process is divided into heating and rolling. After the billet is heated, it is descaled by high-pressure water, and then rolled. The rolling process is divided into rough rolling, secondary water descaling, intermediate rolling, pre-finishing rolling and finishing rolling. The rough rolling temperature is controlled at 900-1000℃. Secondary water descaling is to blow away the secondary oxide scale generated during the rough rolling process. The intermediate rolling temperature and pre-finishing rolling temperature are controlled at 900-950℃, and the wire drawing temperature is controlled at 900-950℃. The wire rod specifications are φ5.5-25mm. Step 7, Post-rolling cooling: The rolled wire rod is cooled on the Stellmore air-cooling line. It is first air-cooled through the high-temperature zone, and then the insulation hood is opened. After the wire rod temperature drops to 800-850℃, it is slowly cooled in the insulation hood. Before the slow cooling, the wire rod is still in a fully austenitic state and has not decomposed. The cooling rate in the insulation hood is controlled at 0.1℃ / s-0.5℃ / s. It is slowly cooled to below 500℃ and then removed from the hood. After the wire rod is removed from the hood, it is coiled and collected for natural cooling to obtain a metallographic structure mainly composed of bainite. Step 8: Isothermal heat treatment: The wire rod is heated in a box furnace or a continuous furnace with atmosphere protection to the austenitizing temperature of 880-950℃. After holding at this temperature for 120-150 minutes, it is quickly quenched into molten salt for isothermal treatment. The isothermal heat treatment time is 100-150 minutes, and the salt temperature is 290-350℃. After cooling, the lower bainite structure is obtained.

2. The method according to claim 1, characterized in that: In step three, the superheat of the molten steel is controlled at 15-40℃. Argon gas is used for protection throughout the continuous casting process to prevent the molten steel from coming into contact with oxygen. The billet size is above 390mm×510mm. After the billet is removed from the line, it is placed in the pit for slow cooling for more than 48 hours.

3. The method according to claim 1, characterized in that: In step four, the continuous casting billet opening process includes heating and rolling processes. The billet is kept at a temperature of 1150-1250℃ in the soaking zone for more than 6 hours, with a total heating time of more than 10 hours. After opening, the billet is promptly placed in a slow cooling pit for slow cooling, with an offline temperature of ≥650℃ and a slow cooling time of ≥48 hours.

4. The method according to claim 1, characterized in that: In step five, the amount of surface peeling is more than 1.5mm.

5. The method according to claim 1, characterized in that: In step six, a walking beam furnace is used for heating. The temperature of the preheating section of the furnace is controlled at ≤650℃, the temperature of the heating uniform section is controlled at 1050-1150℃, and the total heating time is controlled at 120-180min.

6. The method according to claim 1, characterized in that: The salt used for isothermal heat treatment is composed of potassium nitrate and sodium nitrate mixed in a 1:1 weight ratio; the wire rod is air-cooled or water-cooled after isothermal heat treatment.

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