Delayed fracture resistant high strength bolt steel material and method of manufacturing a bolt

Abstract

This invention provides a high-strength bolt steel material resistant to delayed fracture and a bolt manufacturing method, belonging to the field of alloy steel technology. The effective components and contents of the steel material are as follows: C: 0.36-0.43%; Si: 0.15-0.35%; Mn: 0.40-1.00%; P: P≤0.012%; S: S≤0.008%; Cr: 0.40-1.00%; Mo: 0.15-0.35%; W: 0.15-0.35%; V: 0.05-0.15%; Nb: 0.02-0.07%; Cu: 0.20-0.50%, with the balance being Fe. The V, Mo, and W elements satisfy the relationship with the strengthening parameter θ: θ=V(%)+0.5Mo(%)+0.75W(%), 0.35≤θ≤0.42. This invention improves the strength of steel materials. By controlling the reasonable strengthening parameter θ and performing appropriate deformation direct quenching followed by high-temperature tempering, it promotes the large-scale dispersion precipitation of nano-sized MC and M2C carbides. While achieving a strength level of 1400MPa, it also improves the resistance to delayed fracture, enabling the production of 14.9 grade high-strength bolts. It eliminates the need for re-austenitizing heating and quenching after bolt forming, resulting in simplified processes, energy saving, and reduced consumption, which aligns with the green and low-carbon development trend.

Description

Technical Field

[0001] This invention relates to the field of alloy steel technology, specifically to a high-strength bolt steel material resistant to delayed fracture and a bolt manufacturing method. Background Technology

[0002] High-strength bolts are multi-notch components with high notch sensitivity, making them prone to hydrogen-induced delayed fracture at notch concentration points, such as the thread root or the transition between the shank and head. With the rapid development of industries such as automobiles, wind power, and steel structure construction, increasingly higher strength requirements are being placed on bolts to improve connection load-bearing capacity and efficiency. However, when bolt strength levels reach approximately 1200 MPa and above, the problem of hydrogen-induced delayed fracture becomes more prominent, significantly limiting its application. Therefore, in recent years, both domestic and international efforts have focused on developing high-strength bolt steels resistant to delayed fracture. Chinese invention patent application CN202111089001 discloses a method for producing ultra-strong bolt steel resistant to delayed fracture. The chemical composition (wt.%) of the steel is: C 0.38~0.42, Si 0.17~0.25, Mn 0.60~0.70, P≤0.010, S≤0.005, Cr 0.95~1.05, Mo 0.55~0.62, V 0.28~0.34, Al 0.010~0.020, with the balance being Fe and unavoidable impurities. After being processed in a converter—LF+VD—continuous casting—grinding—rolling, the wire rod, after quenching and tempering heat treatment, has a tensile strength greater than 1400MPa and can be used to manufacture grade 14.9 high-strength bolts. However, the high content of relatively expensive elements Mo and V in this steel significantly increases production costs, and the degree of improvement in delayed fracture resistance is limited. Chinese invention patent application CN202080086532 also discloses a wire rod with excellent resistance to hydrogen embrittlement for high-strength cold heading steel and its manufacturing method. Its composition (wt.%) is: C 0.3–0.5%, Si 0.01–0.3%, Mn 0.3–1.0%, Cr 0.5–1.5%, Mo 0.5–1.5%, V 0.01–0.4%, with the remainder being Fe and other impurity elements. This steel can achieve a strength level of 1400 MPa after heat treatment; however, the excessively high levels of Mo and V result in high production costs, affecting its industrial mass application, and the degree of improvement in delayed fracture resistance is limited. Summary of the Invention

[0003] The purpose of this invention is to provide a high-strength bolt steel material of grade 14.9 with excellent resistance to delayed fracture (above 1400 MPa) and a bolt manufacturing method, so as to solve at least one of the technical problems in the background art.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] On one hand, the present invention provides a high-strength bolt steel material resistant to delayed fracture, wherein the effective components and contents of the steel material are as follows: C: 0.36-0.43%; Si: 0.15-0.35%; Mn: 0.40-1.00%; P: P≤0.012%; S: S≤0.008%; Cr: 0.40-1.00%; Mo: 0.15-0.35%; W: 0.15-0.35%; V: 0.05-0.15%; Nb: 0.02-0.07%; Cu: 0.20-0.50%, with the balance being Fe.

[0006] Furthermore, the steel material also includes 0.02 to 0.07% Zr.

[0007] Furthermore, the V, Mo, and W elements satisfy the following relationship with the strengthening parameter θ: θ = V(%) + 0.5Mo(%) + 0.75W(%), 0.35 ≤ θ ≤ 0.42.

[0008] Secondly, the present invention provides a method for manufacturing high-strength bolts resistant to delayed fracture using the high-strength bolt steel material for delayed fracture as described above, employing a coupling method of austenitization and bolt hot forming, comprising: induction heating of round steel bar to 920-950°C, holding at the temperature and then hot forging at 850-700°C, and immediately quenching and cooling to room temperature using the residual heat after forging; subsequently heating the bolt to 550-650°C and holding at the temperature before air cooling.

[0009] The beneficial effects of this invention are: it improves the strength of steel materials. By controlling the reasonable strengthening parameter θ and after appropriate deformation direct quenching and high-temperature tempering treatment, it promotes the large-scale dispersion precipitation of nano-sized MC and M2C carbides. While achieving a strength level of 1400MPa, it also improves the resistance to delayed fracture. It can be used to manufacture 14.9 grade high-strength bolts, eliminating the need for re-austenitizing heating and quenching after bolt forming. It has the effects of simplifying the process, saving energy and reducing consumption, and conforms to the green and low-carbon development trend.

[0010] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and will become apparent from the description or may be learned by practice of the invention. Detailed Implementation

[0011] The embodiments of the present invention are described in detail below. The described embodiments are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0012] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0013] It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as here.

[0014] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this specification means the presence of the stated features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, and / or groups thereof.

[0015] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of those different embodiments or examples.

[0016] In the description of this specification, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0017] Unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "setting" should be interpreted broadly. For example, they can refer to a fixed connection or setting, a detachable connection or setting, or an integral connection or setting. Those skilled in the art can understand the specific meaning of these terms in this art according to the specific circumstances.

[0018] In one specific embodiment, a high-strength bolt steel material with a tensile strength of 1400 MPa or higher and excellent resistance to delayed fracture is provided, which can be used to manufacture grade 14.9 high-strength bolts with a tensile strength of 1400 MPa or higher.

[0019] In this embodiment, (1) Mo+W+V composite alloying is used to significantly improve the tempering resistance of the steel. The secondary hardening effect of the dispersed fine MC+M2C carbides and other precipitates ensures that the required strength level (tensile strength greater than 1400MPa) is obtained after quenching and high-temperature tempering. At the same time, the higher tempering temperature can make the thin film cementite at the grain boundary spheroidize, thereby reducing the segregation of hydrogen at the grain boundary and improving the grain boundary strength; (2) An appropriate amount of microalloying element Nb is added to increase the non-recrystallization temperature of the steel, so as to ensure deformation at a higher temperature and minimize the deformation resistance of the material. At the same time, it can also refine the grains and precipitate MC-type carbides to improve the steel's resistance to delayed fracture. 3) During the high-strength bolt forming process, the temperature is first raised to a relatively high temperature (920-950℃) to allow as many Mo, W, and V carbides as possible to dissolve into the austenite. This ensures that a large number of nano-sized MC and M2C carbides precipitate during the high-temperature tempering process. Subsequently, the bolt is formed at a lower temperature (850-700℃) and then directly quenched after forming to obtain a fine martensite structure with a high density of dislocations. This accelerates the precipitation of a large number of finely dispersed MC and M2C carbides during the subsequent high-temperature tempering process. These carbides not only act as hydrogen traps to neutralize hydrogen and provide precipitation strengthening, but also improve the utilization rate of microalloying elements or reduce their addition amount.

[0020] Unlike traditional hot forging processes that only deform the bolt head, the process in this embodiment involves low-temperature large deformation forming of both the bolt shank and head in the non-recrystallized austenite region (below 850°C, with the non-recrystallization temperature controlled by appropriately adding microalloying element Nb).

[0021] The specific chemical composition (wt%) of the steel material described in this embodiment is as follows: C 0.36–0.43%, Si 0.15–0.35%, Mn 0.40–1.00%, P ≤ 0.012%, S ≤ 0.008%, Cr 0.40–1.00%, Mo 0.15–0.35%, W 0.15–0.35%, V 0.05–0.15%, Nb 0.02–0.07%, Cu 0.20–0.50%, with the balance being Fe. If necessary, 0.02–0.07% Zr may also be added to the specific chemical composition (wt%) of this steel. Simultaneously, the V, Mo, and W elements must satisfy the strengthening parameter θ relationship: θ = V(%) + 0.5Mo(%) + 0.75W(%), 0.35 ≤ θ ≤ 0.42.

[0022] The roles and proportions of each element are as follows:

[0023] C: In order to obtain the required strength level after quenching and high-temperature tempering, the C content must be above 0.36%; however, increasing the C content will greatly impair the ductility, toughness, resistance to delayed fracture, and formability of the steel, so the C content should be controlled below 0.43%.

[0024] Si: A commonly used deoxidizer in steel; in addition, Si significantly deteriorates the cold working properties of steel, and also promotes the grain boundary segregation of impurity elements P and S, which has a deteriorating effect on the resistance to delayed fracture. Therefore, its content is controlled at 0.15-0.35%.

[0025] Mn is an effective element for deoxidation and desulfurization, and it can also improve the hardenability and strength of steel. However, when quenched steel is tempered at high temperature, Mn and P have a strong tendency to co-segregate at grain boundaries, which promotes temper brittleness. Therefore, the Mn content should be controlled between 0.40 and 1.00%.

[0026] P: P can form micro-segregation when molten steel solidifies, and then agglomerate at the grain boundaries when heated at high temperature, which significantly increases the brittleness of steel and thus significantly increases the delayed fracture sensitivity of steel. In addition, reducing the P content can reduce the deformation resistance of steel, so the P content should be controlled below 0.012%.

[0027] S: An unavoidable impurity, it forms MnS inclusions and segregates at grain boundaries, which deteriorates the cold working properties and resistance to delayed fracture of steel. Reducing the S content in steel can improve the deformation capacity of steel and reduce the number of non-metallic inclusions in steel. At the same time, it can also reduce the segregation of S at grain boundaries, thereby alleviating grain boundary embrittlement and improving the cold working properties, toughness, and resistance to delayed fracture of steel. Therefore, its content should be controlled below 0.008%.

[0028] Cr: It can effectively improve the hardenability and tempering resistance of steel to obtain the required high strength; at the same time, when added in combination with Cu, it can significantly improve the weather resistance of steel. A content of less than 0.40% is unlikely to have the above effects, but a content of more than 1.00% will deteriorate the toughness and cold workability of steel.

[0029] Mo: The combined addition of Mo and Cr can significantly improve the hardenability of steel, inhibit the formation of proeutectoid ferrite, and facilitate deformation at lower temperatures. At the same time, Mo can also improve tempering resistance and strengthen grain boundaries, thereby improving the steel's resistance to delayed fracture. A content of less than 0.15% is unlikely to achieve the above effects, but a content exceeding 0.35% saturates the above effects and significantly increases costs.

[0030] W: Its effect is similar to that of Mo. Since the atomic weight of W (184) is nearly twice that of Mo (96), its grain boundary strengthening effect is higher than that of Mo. Furthermore, W segregated at grain boundaries can hinder hydrogen diffusion along or across grain boundaries, which makes its effect on improving the steel’s resistance to delayed fracture significantly higher than that of Mo. Therefore, the W content should be controlled between 0.15% and 0.35%.

[0031] Cu: Cu can significantly improve the weather resistance of steel. Its effect is more obvious when the content is above 0.20%, but the effect is saturated when the content exceeds 0.50%, and it will reduce the high-temperature plasticity of steel, making it prone to cracking during hot working. Therefore, the Cu content should be controlled between 0.20 and 0.50%.

[0032] Vanadium carbonitride (V): V can refine grains. During tempering at higher temperatures, the precipitated vanadium carbonitride not only produces secondary hardening, further increasing the strength of the steel, but also, due to its strong trapping energy, can capture hydrogen and disperse it uniformly within the grains, inhibiting hydrogen diffusion and grain boundary segregation, thereby improving the steel's resistance to delayed fracture. A V content of less than 0.05% is insufficient to achieve these effects, while a content exceeding 0.15% results in saturation and increases costs.

[0033] Nitrogen (Nb) significantly increases the non-recrystallization temperature of steel, allowing for deformation at higher temperatures with lower deformation resistance to obtain a larger amount of non-recrystallized microstructure. This is beneficial for obtaining an ultrafine, strip-shaped microstructure with high dislocation density after direct quenching. Nb also forms MC-type carbides, refining the grain size and improving the toughness of the steel. Furthermore, its carbides act as strong hydrogen traps, further improving the steel's resistance to delayed fracture. A content less than 0.02% does not produce these effects, but the effect saturates when the content exceeds 0.07%.

[0034] Zr: It acts similarly to Nb.

[0035] Furthermore, to obtain excellent resistance to delayed fracture, extensive research and analysis have revealed that elements such as Mo, W, and V require appropriate composite addition, meaning their content must satisfy the parameter θ relationship: 0.35 ≤ V (%) + 0.5 Mo (%) + 0.75 W (%) ≤ 0.42. When θ is less than 0.35, even though the content of individual Mo, W, and V elements may be within the above optimal range, excellent resistance to delayed fracture cannot be obtained; when θ is greater than 0.42, the effect is saturated, and the cost of steel increases.

[0036] The steel of this invention can be smelted in an electric arc furnace or a converter with ladle refining, cast into steel ingots or continuously cast into billets, and then rolled into products such as bars and wire rods.

[0037] Compared with existing technologies, the steel of this invention not only has a high strength level, but more importantly, by controlling the reasonable strengthening parameter θ and undergoing appropriate deformation direct quenching + high temperature tempering treatment, it promotes the large-scale dispersion precipitation of nano-sized MC and M2C carbides. While achieving a strength level of 1400MPa, it also has excellent resistance to delayed fracture. It can be used to manufacture 14.9 grade high-strength bolts, and can eliminate the need for re-austenitizing heating and quenching after bolt forming. It also has the effects of simplifying the process, saving energy and reducing consumption, which is in line with the green and low-carbon development trend.

[0038] Based on the designed chemical composition range, five heats of the steel material described in the embodiments of this invention and four heats of comparative steel were smelted in a 50kg vacuum induction furnace. The specific chemical compositions are shown in Table 1. Heat numbers 1-5# are the steel of this invention, and heat numbers 6-9# are the comparative steel. Molten steel was cast into ingots, then forged into round bars, and slowly cooled after forging. The round bars were induction heated to 920-950℃, held at that temperature, and then hot-forged at 850-700℃. The residual heat after forging was immediately used for quenching and cooling to room temperature. Subsequently, the bolts were heated to 550-650℃, held, and then air-cooled. For comparison, some round bars were directly subjected to quenching and tempering treatment, i.e., heated to 920-950℃, held, quenched, and cooled to room temperature, then reheated to 575-625℃, held, and then air-cooled.

[0039] Table 1

[0040]

[0041] The heat-treated bolts and round bars were sampled and machined into standard room temperature tensile specimens (l0 = 5d0, d0 = 5mm) and notched tensile delayed fracture specimens (diameter d = 5mm, notched d0 = 5mm). N =3mm, notch 60°±2° / 0.15R±0.025). The specimens were subjected to conventional tensile and constant-load delayed fracture tests at room temperature. The delayed fracture test used notched tensile specimens, and the solution was Walpole corrosion inhibitor (16.4 g anhydrous sodium acetate + 15.4 mL primary concentrated hydrochloric acid + 1000 mL deionized water or distilled water) with pH = 3.5±0.5. (e.g., σ) f The minimum stress at which fracture occurs, σ n The critical tensile stress σ is defined as the maximum stress that prevents fracture within a specified deadline of 100 hours. c For: σ c =1 / 2(σ f +σ n To ensure that the measured value differs from the actual value by less than 10%, σ is required to... f -σ n ≤0.2σ c To avoid random errors in the experiments, at least 10 parallel specimens were prepared for each group of experiments. A higher notched tensile critical stress (resistance to delayed fracture) indicates better resistance to delayed fracture in the material. The results are listed in Table 2.

[0042] Table 2

[0043]

[0044] As shown in Table 2, the steel of this invention, after conventional quenching and high-temperature tempering, can achieve a tensile strength of over 1400 MPa, a notched tensile critical stress of over 1700 MPa, and good plasticity. Further, after bolt forming and direct quenching and high-temperature tempering, the strength level and notched tensile critical stress are significantly improved while maintaining good plasticity, both meeting the strength and plasticity requirements of grade 14.9 bolts. Compared with the comparative steel, the steel of this invention not only has a high strength level and good plasticity but also excellent resistance to delayed fracture, making it suitable for manufacturing grade 14.9 high-strength bolts resistant to delayed fracture. Since the high-strength bolts are directly quenched using residual heat after low-temperature forming, it also simplifies the process, saves energy, and reduces consumption, aligning with the green and low-carbon development trend.

[0045] While the specific embodiments of the present invention have been described above, they are not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that, based on the technical solutions disclosed in the present invention, various modifications or variations that can be made by those skilled in the art without creative effort should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing high-strength bolts resistant to delayed fracture using high-strength bolt steel material resistant to delayed fracture, characterized in that, The austenitizing and bolt hot forming coupling method includes: induction heating of round steel bars to 920~950 ℃, holding at that temperature, and then hot forging at 850~700 ℃; quenching and cooling to room temperature immediately using the residual heat after forging; subsequently heating the bolts to 550~650 ℃, holding at that temperature, and then air cooling; the weight percentage composition of the high-strength bolt steel material with delayed fracture resistance is: C: 0.36~0.43%; Si: 0.15~0.35%; Mn: 0.40~1.00%; P: P≤0.012%; S: S≤0.008%; Cr: 0.40~1.00%; Mo: 0.15~0.35%; W: 0.15~0.35%; V: 0.05~0.15%; Nb: 0.02~0.07%; Cu: 0.20~0.50%; with the balance being Fe. Among them, the V, Mo, and W elements satisfy the following relationship with the strengthening parameter θ: θ = V(%) + 0.5Mo(%) + 0.75W(%), 0.35 ≤ θ ≤ 0.

42.

2. The method for manufacturing high-strength bolts resistant to delayed fracture according to claim 1, characterized in that, The steel material also includes 0.02% to 0.07% Zr.

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