A high-strength steel part with refined structure and a manufacturing method thereof

Abstract

The application discloses a kind of high-strength steel parts of organization refinement, it contains Fe and inevitable impurities, it also contains the following chemical elements with mass percentage as follows: C:0.37-0.43%;Si:0.03-0.12%;Mn:0.30-0.50%;Cr:0.90-1.15%;Mo:0.90-1.10%;Ni:0.15-0.50%;V:0.10-0.30%;Ti:0.006-0.012%;Al:0.055-0.090%;N:0.0055-0.0120%.In addition, the application also discloses the manufacturing method of the above-mentioned high-strength steel parts, which comprises the following steps: (1) smelting and off-oven refining: high-vacuum degassing time is greater than 25min;(2) with big billet continuous casting machine pours billet: carbon segregation in the core of blank is less than 1.08;(3) initial rolling cogging;(4) heating;(5) high-speed wire rod rolling of wire rod;(6) Stelmor fan cooling;(7) spheroidizing heat treatment: holding temperature is 750-850 DEG C, holding time is 4-28h, after holding slow cooling, cooling rate is less than 25 DEG C / h;(8) drawing or straightening;(9) quenching and tempering heat treatment: control quenching temperature is 850-1000 DEG C, then water cooling or oil cooling is carried out;Controlled tempering temperature is 480-680 DEG C.

Description

Technical Field

[0001] This invention relates to a metallic material and its manufacturing method, and more particularly to a high-strength steel part and its manufacturing method. Background Technology

[0002] Fasteners are a general term for mechanical parts used to secure two or more parts (or components) together into a single unit. They are the most widely used and numerous basic mechanical components in all sectors of the national economy. Fastener connections have advantages such as simplicity and convenience, the ability to be disassembled and reassembled multiple times, high standardization, and low cost. A wide variety and considerable quantity of fasteners are used in various mechanical equipment, vehicles, ships, aircraft, satellites, railway bridges, building structures, tools, instruments, and daily necessities.

[0003] Currently, fasteners are mainly used in the automotive, electronics, and construction and repair industries. In the automotive industry, high-strength fasteners are used in four performance grades: 8.8, 9.8, 10.9, and 12.9. High-strength (grade 8.8 and above) bolts need to withstand large loads and have very complex stress states; therefore, these bolts are mostly made of medium carbon or medium carbon alloy steel and undergo quenching and tempering (quenching followed by high-temperature tempering) to ensure sufficient strength and yield strength ratio. Meanwhile, pins, connecting rods, and shafts are also widely used in the automotive, machinery, and construction and repair industries. These components are mostly made of high-strength steel and also require high strength to reduce part size and improve connection strength.

[0004] Generally speaking, increasing the strength of fasteners, pins, connecting rods, shafts, and other parts by 100 MPa can reduce their weight by approximately 10%, which helps reduce the amount of steel used. Simultaneously, increasing steel strength allows for smaller part dimensions, contributing to a reduction in overall structural weight. For example, using higher-strength bolts in automotive engines facilitates engine miniaturization, improves combustion efficiency, and further promotes energy conservation and emission reduction. Therefore, whether in the automotive, machinery, construction and repair, or electronics industries, there is a desire to increase the strength of materials used in these parts to meet specific application requirements.

[0005] Furthermore, it is important to note that fasteners, pins, connecting rods, shafts, and other components are often subjected to alternating loads, operating in extremely harsh environments. Abnormal fractures can easily lead to major accidents or significant economic losses. For example, during use, fasteners are highly susceptible to corrosion and hydrogen permeation when in contact with the atmosphere, water, or corrosive liquids. This can result in unpredictable, delayed-destructive fractures, leading to structural collapse. Such failures are particularly prone to occur when the strength of the steel increases.

[0006] Therefore, in the actual design of fastener materials, in addition to high strength requirements, the market also places extremely high demands on the comprehensive mechanical properties, fatigue resistance and reliability of such high-strength steel in order to meet the long-term safe use requirements of these fasteners.

[0007] In current technologies, the strength of most fasteners, pins, connecting rods, and shafts used is below 1200 MPa. For example, the highest strength fastener commonly used in the automotive and construction industries is grade 12.9, and their durability and fatigue resistance often hinder the application of higher strength materials. Therefore, in order to meet the needs of high strength and miniaturization of components, there is an urgent need to develop a high-strength steel with high strength, high plasticity and toughness, and excellent fatigue resistance.

[0008] Currently, some researchers both domestically and internationally have conducted in-depth research on this need and have achieved certain research results:

[0009] For example, Chinese patent document CN110468340A, published on November 19, 2019, entitled "A 14.9 Grade High-Strength Fastener Steel Wire Rod and Its Preparation Method," discloses a 14.9 grade high-strength fastener steel wire rod and its preparation method. In its chemical composition design, the carbon content is controlled within the range of 0.44-0.48%, while various alloying elements such as V, Nb, and Ti are added, with the Ti content controlled within the range of 0.03-0.06%. The steel designed by this technical solution has a high carbon content and a large amount of alloying elements, making its actual production quite difficult.

[0010] For example, Chinese patent document CN110791715A, published on February 14, 2020, entitled "A Niobium-Titanium Containing High-Strength Bolt Steel of Grade 14.9 with Atmospheric Corrosion Resistance and Its Production Method," discloses a production method for a niobium-titanium containing high-strength bolt steel of grade 14.9 with atmospheric corrosion resistance. Its chemical composition is C: 0.45%–0.50%, Si ≤ 0.10%, Mn: 0.40%–0.70%, Cr… The alloy steel contains 1.20%–1.50% Mo, 0.80%–1.00% V, 0.20%–0.40% Nb, 0.02%–0.07% Ti, 0.030–0.050% Ni, 0.20%–0.30% Cu, 0.15%–0.35% Alt, and 0.015%–0.040%, etc. After heat treatment, its mechanical properties Rm ≥ 1400 MPa and yield strength ratio RP0.2 / Rm ≥ 0.9. In this technical solution, the alloy steel also adopts a high-carbon composition design, with a carbon content higher than 0.45%, and simultaneously adds multiple alloying elements such as V, Nb, Ti, and Cu. This design results in higher alloy costs, greater difficulty in controlling the microstructure during production, and a higher susceptibility to cracking during rolling.

[0011] For example, Chinese patent document CN113322410A, published on August 31, 2021, entitled "A High-Strength Bolt Steel with Excellent Resistance to Delayed Fracture and Its Preparation Method," discloses a high-strength bolt steel with excellent resistance to delayed fracture and its preparation method. Its chemical composition is: carbon: 0.35–0.50%, manganese: 0.20–0.70%, chromium: 0.75–1.35%, molybdenum: 0.55–1.45%, vanadium: 0.10–0.50%, aluminum: 0.005–0.10%, sulfur: 0–0.005%, silicon: 0.02–0.05%, titanium: 0.005–0.15%, etc., wherein the titanium content / nitrogen content ratio is ≥3.5; the sum of the contents of chromium, molybdenum, and vanadium is ≥1.75%; and the sum of the contents of sulfur, oxygen, and nitrogen is ≤0.01%. In this technical solution, the microstructure of the high-strength bolt steel is ferrite + lamellar pearlite + bainite, and the high strength and high toughness requirements are mainly achieved by controlling the size of chromium, molybdenum and vanadium precipitates.

[0012] Unlike the existing technical solutions mentioned above, the inventors adopted a completely new design and aimed to provide a new type of high-strength steel that can be used to process high-strength fasteners, connecting rods, pins, shafts and other parts with a tensile strength of over 1400MPa. This high-strength steel has high strength, excellent plasticity, toughness and fatigue resistance, and the parts processed from it can effectively meet the current needs of the automotive, machinery, construction and electronics industries. Summary of the Invention

[0013] One of the objectives of this invention is to provide a high-strength steel part with refined microstructure. Through reasonable chemical composition design and combined with microstructure and steel purity design and control, this high-strength steel part can achieve high strength while possessing excellent plasticity, toughness and fatigue resistance, thereby significantly improving the fatigue resistance and delayed fracture resistance of the high-strength steel part.

[0014] These high-strength steel parts can serve safely and for a long time, effectively meeting the needs of the automotive, machinery, construction, and electronics industries, and have excellent prospects for promotion and application value.

[0015] To achieve the above objectives, the present invention provides a high-strength steel part with a refined microstructure, containing Fe and unavoidable impurities, and further containing the following chemical elements in the following mass percentages:

[0016] C: 0.37-0.43%;

[0017] Si: 0.03-0.12%;

[0018] Mn: 0.30-0.50%;

[0019] Cr: 0.90-1.15%;

[0020] Mo: 0.90-1.10%;

[0021] Ni: 0.15-0.50%;

[0022] V: 0.10-0.30%;

[0023] Ti: 0.006-0.012%;

[0024] Al: 0.055-0.090%;

[0025] N: 0.0055-0.0120%.

[0026] Furthermore, in the high-strength steel parts with refined microstructure described in this invention, the mass percentage content of each chemical element is as follows:

[0027] C: 0.37-0.43%;

[0028] Si: 0.03-0.12%;

[0029] Mn: 0.30-0.50%;

[0030] Cr: 0.90-1.15%;

[0031] Mo: 0.90-1.10%;

[0032] Ni: 0.15-0.50%;

[0033] V: 0.10-0.30%;

[0034] Ti: 0.006-0.012%;

[0035] Al: 0.055-0.090%;

[0036] N: 0.0055-0.0120%;

[0037] The balance is Fe and unavoidable impurities.

[0038] In the high-strength steel parts with refined microstructure described in this invention, the design principles of each chemical element are as follows:

[0039] C: In the high-strength steel parts with refined microstructure described in this invention, carbon (C) is an essential chemical component of steel. The C content directly determines the amount of carbides precipitated after quenching and tempering heat treatment of high-strength steel wire rods and finished parts, which greatly affects the hardness and strength of the alloy. Therefore, to ensure the performance of high-strength steel parts, the C content in the steel must be controlled to be higher than 0.37%.

[0040] However, it is important to note that the carbon (C) content should not be too high. Excessive C content will lead to an excessive number and size of carbide precipitates in the material, reducing its ductility and toughness, and deteriorating its fatigue resistance and resistance to delayed fracture. Furthermore, excessive C content can easily cause segregation during continuous casting; therefore, the C content must be controlled below 0.43%. Based on this, in the high-strength steel parts with refined microstructure described in this invention, the mass percentage of C is controlled between 0.37% and 0.43%.

[0041] Si: In the high-strength steel parts with refined microstructure described in this invention, Si is often added to the steel as a deoxidizer during the smelting process, where it plays a deoxidizing role. Simultaneously, Si dissolved in the alloy ferrite phase significantly improves the material's strength. However, the Si content in the steel should not be too high, as excessive Si content will reduce the material's plasticity, adversely affecting its resistance to delayed fracture. Therefore, to maximize the beneficial effects of Si, the mass percentage of Si in the high-strength steel parts with refined microstructure described in this invention is controlled between 0.03% and 0.12%.

[0042] Mn: In the high-strength steel parts with refined microstructure described in this invention, Mn is often added as a deoxidizer during the steelmaking process. Mn also readily combines with sulfur (S) in steel to form MnS, thereby reducing the harmful effects of S. Furthermore, Mn is a commonly used strengthening element in steel, playing a role in solid solution strengthening, and the resulting alloy cementite has higher strength. Therefore, to maximize the beneficial effects of Mn, the Mn content in the steel should be controlled to be higher than 0.30%.

[0043] However, the Mn content in steel should not be too high. When the Mn content is too high, it will increase the tendency for grain coarsening during the heating process. In addition, Mn can also promote the segregation of residual elements. Therefore, the amount of Mn added should be controlled below 0.50%. Based on this, in the high-strength steel parts with refined microstructure described in this invention, the mass percentage of Mn is controlled between 0.30% and 0.50%.

[0044] Cr: In the high-strength steel parts with refined microstructure described in this invention, the addition of Cr element is beneficial to improving the hardenability of the material. During the quenching and tempering process, it helps to refine the microstructure and increase the strength of cementite, thereby improving the strength and plasticity of the material. Simultaneously, Cr element is beneficial to improving the corrosion resistance of the material and can reduce hydrogen embrittlement sensitivity. Therefore, to maximize the beneficial effects of Cr element, the Cr content in the steel needs to be controlled to be higher than 0.9%. Correspondingly, to prevent the formation of abnormal martensitic microstructure, reduce the difficulty of controlling the wire rod microstructure, and prevent coarse carbonitride size, the Cr content in the steel needs to be lower than 1.15%. Based on this, in the high-strength steel parts with refined microstructure described in this invention, the mass percentage of Cr element is controlled between 0.90% and 1.15%.

[0045] Mo: In the high-strength steel parts with refined microstructure described in this invention, the addition of Mo not only improves the tempering stability of the material, but also enhances its strength and hardness under high-temperature tempering. When the Mo content is increased to above 0.9%, it helps to refine the tempered martensite structure, refine the size of M (N, C) and M23C6 precipitates, and improve the material's fatigue performance and resistance to delayed fracture. However, the Mo content in the steel should not be too high. Excessive Mo addition will increase the difficulty of controlling the material's microstructure and lead to increased material costs. Therefore, considering both cost factors and the beneficial effects of Mo, the mass percentage of Mo in the high-strength steel parts with refined microstructure described in this invention is controlled between 0.90% and 1.10%.

[0046] Ni: In the high-strength steel parts with refined microstructure described in this invention, Ni is an austenite-forming element that can dissolve in the ferrite phase and effectively improve the strength of the material. Simultaneously, Ni can also improve the hardenability of the material, refine the microstructure, and during the precipitation of carbonitrides in the alloy, the enrichment of Ni at the interface will help improve the interfacial bonding force, enhancing the toughness and fatigue resistance of the material. However, it should be noted that the Ni content in the steel should not be too high. Excessive Ni content not only makes the material more prone to martensitic abnormal structures during production, but also affects the alloy cost. Therefore, in the high-strength steel parts with refined microstructure described in this invention, the mass percentage of Ni is controlled between 0.15% and 0.50%.

[0047] V and Ti: In the high-strength steel parts with refined microstructure described in this invention, V and Ti elements readily react with C and N elements in the high-strength steel to precipitate carbonitrides. The carbonitride formation of V and Ti can act as hydrogen traps. Especially when both are added in combination, the lattice mismatch between the precipitates and the ferrite in the steel increases, resulting in a greater lattice distortion energy and a stronger fixation effect on hydrogen atoms, effectively reducing their harmful effects. Therefore, to achieve the above effects, in this invention, the mass percentage of V element is controlled between 0.10% and 0.30%.

[0048] Accordingly, in this invention, to maximize the beneficial effects of Ti, the Ti content in the steel needs to be controlled to be higher than 0.006%. However, the Ti content in the steel should not be too high, as Ti readily combines with N and precipitates during steel smelting. When the Ti content in the steel is too high, it easily forms a large number of coarse-sized nitrides, which are detrimental to the material's ductility, toughness, and fatigue performance. Therefore, the Ti content in the steel needs to be controlled to be no higher than 0.012%. Based on this, in the high-strength steel parts with refined microstructure described in this invention, the mass percentage of Ti is controlled between 0.006% and 0.012%.

[0049] Al and N: In the high-strength steel parts with refined microstructure described in this invention, Al is the most effective deoxidizing element in the steelmaking process. However, Al easily produces Al2O3 particles during deoxidation, which have sharp edges. In particular, when the oxygen content in the steel is too high, it will have a great impact on the fatigue life, durability, and resistance to delayed fracture of the finished parts. In order to ensure the performance of high-strength parts, it is necessary to control the oxygen content in the material to below 0.0010%, improve the purity of the steel, and at the same time increase the content of Al in the steel.

[0050] In this invention, Al can combine with N to form nanoscale AlN. The precipitation of nanoscale AlN can refine the grain size and tempering structure of the material. Therefore, in this invention, the Al content in the steel must be controlled to be no less than 0.055%. However, it should be noted that the Al content in the steel should not be too high, as excessive Al content will lead to the formation of too many alumina inclusions and promote the growth of precipitates. Therefore, the Al content in the steel must be controlled to be no higher than 0.090%. Based on this, in this invention, the mass percentage of Al is controlled between 0.055% and 0.090%.

[0051] Similarly, nitrogen (N) can combine with al to form nanoscale AlN. However, excessively high N content will lead to an increase in the size of microalloyed precipitates in the steel, which will reduce the material's resistance to delayed fracture. Therefore, in order to ensure sufficient AlN precipitation, the N content in the high-strength steel parts with refined microstructure described in this invention is controlled within the range of 0.0055-0.0120%.

[0052] Furthermore, in the high-strength steel parts with refined microstructure described in this invention, among other unavoidable impurities: P≤0.01%, S≤0.009%, O≤0.0010%, H≤0.0002%.

[0053] In the above-described technical solution of this invention, P, S, H and O are all impurity elements in the high-strength steel parts described in this invention. When technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in high-strength steel parts with finer microstructure should be reduced as much as possible.

[0054] It should be noted that excessively high levels of impurity elements P and S will increase the brittleness of the material. In particular, when segregation occurs, it has a significant impact on the performance of the steel. Therefore, the content of P and S elements in the steel must be strictly controlled, with P controlled to P≤0.01% and S controlled to S≤0.009%.

[0055] O and Al readily combine to form Al2O3 particles, which have sharp edges. This is particularly problematic when the oxygen content in steel is too high, as it significantly impacts the fatigue life, durability, and resistance to delayed fracture of finished parts. Therefore, to ensure the performance of high-strength parts, the oxygen content is controlled to ≤0.0010%.

[0056] Furthermore, in the high-strength steel parts with refined microstructure described in this invention, the N and Ti elements satisfy: 1000×(N-Ti / 3.4)≥3, where N and Ti are substituted with the values ​​before the percentage sign of the mass percentage content of each element.

[0057] Of course, in some preferred embodiments, while controlling the mass percentage content of a single chemical element, the mass percentage content of N and Ti can be further controlled to meet the following condition: 1000 × (N - Ti / 3.4) ≥ 3. Furthermore, the size of carbon and nitrogen precipitates of V and Al is controlled to be less than 80 nm. Combined with the optimized addition of Mo, this ensures that the martensite grade after tempering heat treatment is no higher than level 2, refining the microstructure. Precipitation strengthening and microstructure refinement enhance the material's strength, ensuring both high strength and good ductility and toughness.

[0058] Furthermore, in the high-strength steel parts with refined microstructure described in this invention, the microstructure consists of tempered martensite and precipitates.

[0059] Furthermore, in the high-strength steel parts with refined microstructure described in this invention, the grade of tempered martensite is no higher than level 2.

[0060] Furthermore, in the high-strength steel parts with refined microstructure described in this invention, the precipitates include carbonitriding precipitates of V and Al, and the size of the carbonitriding precipitates of V and Al is less than 80 nm.

[0061] Furthermore, in the high-strength steel parts with refined microstructure described in this invention, the size of individual particle inclusions is less than 27 μm.

[0062] Furthermore, in the high-strength steel parts with refined microstructure described in this invention, the tensile strength is ≥1400MPa, the reduction of area is ≥55%, and the fatigue life is ≥200,000 cycles.

[0063] Accordingly, another objective of the present invention is to provide a method for manufacturing high-strength steel parts with refined microstructure. This method is simple to operate, and the high-strength steel parts obtained by this method have excellent mechanical properties and fatigue resistance, enabling them to serve safely and for a long time, and effectively meeting the current needs of the automotive, machinery, construction, and electronics industries.

[0064] To achieve the above objectives, the present invention proposes a method for manufacturing high-strength steel parts with refined microstructure, comprising the following steps:

[0065] (1) Smelting and ladle refining: control the high vacuum degassing time to be greater than 25 min;

[0066] (2) Use a large billet continuous casting machine to cast billets: control the carbon segregation in the core of the billet to be below 1.08;

[0067] (3) Initial rolling and billet preparation;

[0068] (4) Heating;

[0069] (5) High-speed wire rod rolling;

[0070] (6) Stelmore fan cooling;

[0071] (7) Spheroidizing heat treatment: control the holding temperature to 750-850℃, the holding time to 4-28h, and then slowly cool down, with a cooling rate of less than 25℃ / h;

[0072] (8) Pulling or straightening;

[0073] (9) Quenching and tempering heat treatment: control the quenching temperature to 850-1000℃, and then perform water cooling or oil cooling; control the tempering temperature to 480-680℃.

[0074] This invention is based on a scientific and rational alloy composition design. Through alloy smelting and ladle refining, continuous casting of large billets, initial rolling, heating, and high-speed wire rod rolling, wire rod can be effectively produced. After rolling, the wire rod needs to be cooled by adjusting the Steyrmore fan's cooling capacity to control the transformation of the wire rod and optimize its microstructure. After Steyrmore cooling, the wire rod exhibits good plasticity and toughness.

[0075] After being rolled into wire rod and cooled by the Steyrmore fan, the wire rod undergoes a spheroidizing heat treatment process to optimize its microstructure and properties. This ensures the material's performance and prevents quality issues such as cracking and surface roughness during subsequent drawing or straightening processes. After completing the drawing or straightening steps, an optimized quenching and tempering heat treatment is required to obtain high-strength steel parts with a refined microstructure, achieving a tensile strength ≥1400MPa, a reduction of area ≥55%, and a fatigue life ≥200,000 cycles.

[0076] In step (1) above, when actually carrying out smelting and ladle refining, an electric furnace or converter can be used for smelting. After smelting is completed, ladle refining is carried out. Ladle refining can specifically adopt the LF furnace plus VD or RH degassing process.

[0077] It should be noted that in step (2) above, in some embodiments, when casting billets using a large billet continuous casting machine, an inert gas, such as argon, can be used for protection during the casting process. Operators can control the carbon segregation in the billet core to be below 1.08 by adjusting the casting speed, cooling, and end-of-cast light reduction parameters during the continuous casting process.

[0078] Furthermore, in the manufacturing method described in this invention, in step (3), the continuous casting billet is initially rolled into a square billet at a temperature of 1050-1350℃ using a two-fire forming process.

[0079] In the initial rolling process of step (3) of the present invention, a two-fire forming process can be used to initially roll the continuously cast billet into a square billet at a temperature of 1050-1350℃. After the square billet undergoes eddy current testing, magnetic particle testing, grinding wheel mold repair, supplementary magnetic particle testing and mold repair, the square billet can be fed into the heating furnace for the heating process of the subsequent step (4).

[0080] Furthermore, in the manufacturing method described in this invention, in step (4), the heating temperature is 960-1150℃ and the holding time is 1.0-4.0h.

[0081] Furthermore, in the manufacturing method described in this invention, in step (5), the rolling speed is controlled to be 8-120 m / s, the inlet temperature of the finishing mill is 780-960℃, the inlet temperature of the reducing mill is 750-950℃, and the wire drawing temperature is 700-900℃.

[0082] Furthermore, in the manufacturing method described in this invention, in step (6), the component adjustment range of the 14 fans of the Steyrmo line is controlled as follows: the air volume of the F1-F5 fans is 0-60%, the air volume of the F6-F12 fans is 0-50%, and the air volume of the F13-F14 fans is 0-40%.

[0083] Furthermore, in the manufacturing method described in this invention, in step (8), when a drawing process is used, the drawing area reduction rate is controlled to be 5-35%.

[0084] The high-strength steel parts with refined microstructure and their manufacturing method described in this invention have the following advantages and beneficial effects compared to the prior art:

[0085] Compared with the fastener steel in the prior art, the high-strength steel parts with refined microstructure described in this invention have significant advantages in terms of alloy composition design, mechanical properties, and fatigue life.

[0086] The high-strength steel parts designed in this invention, after quenching and tempering heat treatment, exhibit refined tempered martensite microstructure and precipitates. The carbon and nitrogen precipitates are smaller than 80 nm in size, and the martensite grade after quenching and tempering is no higher than level 2. The microstructure is uniform and refined, and the size of single-particle inclusions in the steel is less than 27 μm, indicating high steel purity. These high-strength steel parts possess a tensile strength ≥1400 MPa, a reduction of area ≥55%, and a fatigue life ≥200,000 cycles. Their resistance to delayed fracture is significantly improved compared to conventional high-strength steel components.

[0087] Therefore, it can be seen that by optimizing and controlling the alloy composition, manufacturing process, microstructure, and steel purity, this invention can ensure that parts not only have high strength but also excellent fatigue resistance and delayed fracture resistance. This enables high-strength steel parts such as bolts, connecting rods, pins, and shafts to serve safely and for a long time during use. It can effectively meet the needs of the automotive, machinery, construction, and electronics industries and has a very good prospect for promotion and application value. Detailed Implementation

[0088] The following will provide further explanation and description of the high-strength steel parts with refined microstructure and their manufacturing method according to the present invention, with reference to specific embodiments. However, this explanation and description do not constitute an undue limitation on the technical solution of the present invention.

[0089] Examples 1-10 and Comparative Examples 1-4

[0090] The high-strength steel parts with refined microstructure described in Examples 1-10 of this invention are all prepared using the following steps:

[0091] Smelting and ladle refining were carried out according to the chemical composition shown in Tables 1-1 and 1-2: after smelting in an electric furnace or converter, LF furnace refining and VD treatment were carried out, and the high vacuum degassing time was controlled to be greater than 25 min; during the smelting process, the composition and amount of synthetic slag were adjusted to control the P and S content in the steel to be less than 0.01% and 0.009% respectively, the O content in the steel to be less than 0.001%, and the H content in the steel to be less than 2 ppm.

[0092] Large billet continuous casting machine is used to cast billets: Argon gas protection is used during the casting process, and the size of the large billet is controlled to be 240-550mm. By adjusting the casting speed, cooling and end light reduction parameters during the continuous casting process, the carbon segregation in the core of the billet is controlled to be less than 1.08.

[0093] Initial rolling: The continuous casting billet is initially rolled into a 135-240mm square billet at a temperature of 1050-1350℃ using a two-fire forming process. After eddy current testing, magnetic particle testing, grinding wheel mold repair, supplementary magnetic particle testing and mold repair, the square billet is fed into the heating furnace for heating.

[0094] Heating: Control the heating temperature to 960-1150℃ and the holding time to 1.0-4.0h.

[0095] (5) High-speed wire rod rolling: The rolling speed is controlled at 8-120m / s, the inlet temperature of the finishing mill is 780-960℃, the inlet temperature of the sizing mill is 750-950℃, the wire drawing temperature is 700-900℃, and finally the wire rod with a size specification of Ф5.5-28mm is obtained.

[0096] (6) Cooling of Steilmo fans: There are 14 fans on the Steilmo line. The control range of the 14 fans on the Steilmo line is: the air volume of F1-F5 fans is 0-60%, the air volume of F6-F12 fans is 0-50%, and the air volume of F13-F14 fans is 0-40%.

[0097] (7) Spheroidizing heat treatment: control the holding temperature to 750-850℃, the holding time to 4-28h, and then slowly cool down after holding, controlling the cooling rate to be less than 25℃ / h.

[0098] (8) Drawing or straightening: When using the drawing process, control the drawing area reduction rate to 5-35%.

[0099] (9) Quenching and tempering heat treatment: control the quenching temperature to 850-1000℃, and then perform water cooling or oil cooling; control the tempering temperature to 480-680℃.

[0100] It should be noted that, in this invention, the chemical composition design and related processes of the high-strength steel parts with refined microstructure in Examples 1-10 all meet the design specifications of this invention. Correspondingly, the chemical composition design of the comparative steel parts in Comparative Examples 1-4 can be found in Tables 1-1 and 1-2 below.

[0101] Although the comparative steel parts of Comparative Examples 1-4 were also produced using the above-mentioned process: smelting and ladle refining, casting of billets using a large billet continuous casting machine, initial rolling, heating, high-speed wire rod rolling, Steyrmore fan cooling, spheroidizing heat treatment, drawing or straightening, and quenching and tempering heat treatment, their chemical composition and related process parameters all failed to meet the design requirements of this invention.

[0102] Table 1-1 below lists the mass percentage of each chemical element in the high-strength steel parts with refined microstructure of Examples 1-10 and the comparative steel parts of Comparative Examples 1-4.

[0103] Table 1-1. (wt%, balance Fe and other unavoidable impurities other than P, S, O and H)

[0104]

[0105] Table 1-2 lists the elemental synergistic ratios in the high-strength steel parts with refined microstructures of Examples 1-10 and the comparative steel parts of Comparative Examples 1-4.

[0106] Table 1-2.

[0107]

[0108] Note: In Table 1-2 above, N and Ti in the formula “1000×(N-Ti / 3.4)” should be replaced with the values ​​before the percentage sign of the mass percentage content of each element.

[0109] Tables 2-1 and 2-2 list the specific process parameters for the high-strength steel parts with refined microstructure in Examples 1-10 and the comparative steel parts in Comparative Examples 1-4 during the above-described process steps.

[0110] Table 2-1.

[0111]

[0112] Table 2-2.

[0113]

[0114] Samples were taken from the high-strength steel parts with refined microstructure obtained from Examples 1-10 and the comparative steel parts from Comparative Examples 1-4, obtained through the above process steps. The microstructure of the parts from each example and comparative example was observed and analyzed. The specific observation and analysis results are listed in Table 3 below:

[0115] Table 3.

[0116]

[0117] Accordingly, after completing the microstructure observation of each of the above embodiments and comparative examples, the refined high-strength steel parts of the finished products of Examples 1-10 and the comparative steel parts of Comparative Examples 1-4 can be collected again, and the mechanical properties and fatigue life of the refined high-strength steel parts of Examples 1-10 and the comparative steel parts of Comparative Examples 1-4 can be further tested to obtain the mechanical properties and fatigue life of the finished parts of each embodiment and comparative example. The results obtained from the performance test are listed in Table 4 below.

[0118] The relevant performance testing methods are as follows:

[0119] Tensile test: The tensile strength and reduction of area of ​​each embodiment and comparative part were tested according to GB / T 228.1—2010 Metallic materials - Tensile testing - Part 1: Test method at room temperature to obtain the tensile strength and reduction of area of ​​each embodiment and comparative part.

[0120] Fatigue life test: Under room temperature conditions, fatigue life tests were conducted on the parts of each embodiment and comparative example according to GB / T 13682-1992 Axial load fatigue test method for threaded fasteners to obtain the fatigue life of the parts of each embodiment and comparative example.

[0121] Table 4 lists the relevant performance test results of the high-strength steel parts with refined microstructure in Examples 1-10 and the comparative steel parts in Comparative Examples 1-4.

[0122] Table 4.

[0123]

[0124] As can be seen from Table 4, compared with the comparative steel parts of Comparative Examples 1-4, the high-strength steel parts with refined microstructure of Embodiments 1-10 of the present invention have significant advantages in terms of mechanical properties and fatigue life. In particular, in terms of fatigue life, the fatigue life of the high-strength steel parts of Embodiments 1-10 of the present application is much higher than that of the comparative steel parts of Comparative Examples 1-4.

[0125] As shown in Table 4 above, the tensile strength of the high-strength steel parts with refined microstructure in Examples 1-10 is between 1432 and 1485 MPa, their area reduction rate is between 57 and 59%, and their fatigue life is between 219063 and 285643 cycles.

[0126] Therefore, compared with the fastener steel in the prior art, the high-strength steel parts with refined microstructure in the embodiments of the present invention have significant advantages in terms of alloy composition design, mechanical properties, and fatigue life.

[0127] The steel designed in this invention can ensure that parts have high strength while also having excellent fatigue resistance and delayed fracture resistance. This allows high-strength steel parts such as bolts, connecting rods, pins, and shafts to be used safely and for a long time. It can effectively meet the needs of the automotive, machinery, construction, and electronics industries and has a very good prospect for promotion and application value.

[0128] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0129] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A microalloyed high strength steel part, characterized in that, The mass percentage of each chemical element is: C：0.37-0.43%； Si: 0.03-0.12%; Mn: 0.30-0.50%; Cr：0.90-1.15%； Mo: 0.90-1.10%; Ni: 0.15-0.50%; V：0.10-0.30%； Ti: 0.006-0.012%; Al：0.055-0.090%； N：0.0055-0.0120%； the balance being Fe and inevitable impurities; wherein the N and Ti elements satisfy: 1000×(N-Ti / 3.4)≥3, wherein N and Ti are substituted into the values before the mass percentage of each element.

2. The microalloyed high strength steel part with refined structure of claim 1, wherein, Among other inevitable impurities: P≤0.01%, S≤0.009%, O≤0.0010%, H≤0.0002%.

3. The microalloyed high strength steel part with refined structure of claim 1, wherein, The microstructure thereof is tempered martensite and precipitates.

4. The microalloyed high strength steel part with refined structure of claim 3, wherein, The grade of the tempered martensite is not higher than 2.

5. The microalloyed high strength steel part with refined structure of claim 3, wherein, The precipitates include V, Al carbonitride precipitates, the size of which is less than 80 nm.

6. The microalloyed high strength steel part with refined structure of claim 1, wherein, The size of single particle inclusions is less than 27 μm.

7. The microalloyed high strength steel part with refined structure of claim 1, wherein, The tensile strength thereof is ≥1400 MPa, the area reduction is ≥55%, and the fatigue life is ≥200,000 times.

8. The method of manufacturing a high-strength steel part with a refined structure according to any one of claims 1 to 7, characterized in that, It comprises the steps of: (1) smelting and secondary refining: control the high vacuum degassing time to be greater than 25 min; (2) using a bloom continuous casting machine to cast a bloom: control the carbon segregation in the core of the bloom to be lower than 1.08; (3) initial rolling and blooming; (4) heating; (5) high-speed wire rod rolling of the coil; (6) Stelmor fan cooling; (7) spheroidizing heat treatment: control the holding temperature to be 750-850℃, the holding time to be 4-28 h, and slowly cool after holding, the cooling speed being lower than 25℃ / h; (8) drawing or straightening; (9) quenching and tempering heat treatment: control the quenching temperature to be 850-1000℃, and then water or oil cooling; control the tempering temperature to be 480-680℃.

9. The production method according to claim 8, wherein In step (3), the continuous casting billet is initially rolled and bloomed into a bloom at a temperature of 1050-1350℃ using a two-heating material process.

10. The production method according to claim 8, wherein In step (4), the heating temperature is 960-1150℃, and the holding time is 1.0-4.0 h.

11. The production method according to claim 8, wherein In step (5), the rolling speed is controlled to be 8-120 m / s, the inlet temperature of the finishing rolling mill is 780-960℃, the inlet temperature of the reducing and sizing mill is 750-950℃, and the wire drawing temperature is 700-900℃.

12. The production method according to claim 8, wherein In step (6), the Stelmor wire 14 fan air volume adjustment range is controlled to be: the air volume of F1-F5 fans is 0-60%, the air volume of F6-F12 fans is 0-50%, and the air volume of F13-F14 fans is 0-40%.

13. The production method according to claim 8, wherein In step (8), when the drawing process is used, the drawing reduction is controlled to be 5-35%.

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