Bolt steel and its manufacturing method

Abstract

The present invention discloses a steel for bolts, which contains the following chemical elements in mass percent in addition to Fe and unavoidable impurities: C: 0.37-0.45%, Si: 0.01-0.08%, Mn: 0.45-0.80%, Cr: 0.90-1.30%, Mo: 0.20-0.45%, Ni: 0.10-0.30%, V: 0.15-0.30%, and Al: 0.015-0.035%. The present invention further discloses a method for producing the steel for bolts, which includes the following steps: (1) smelting; (2) casting; (3) rough rolling; (4) high speed wire rolling; (5) Stelmor controlled cooling; and (6) heat treatment, where the holding temperature of the spheroidizing heat treatment is 760-790°C, and the holding time is 4-12h, followed by a slow cooling process at a cooling rate lower than 40°C / h. The drawing reduction rate of the coil rod is controlled at 5-30% during drawing. The heating temperature of the quenching and tempering heat treatment is 850-950°C. The tempering temperature is 500-600°C. The bolt steel disclosed in the present invention has uniform structure and performance, low production cost, and has high strength and good delayed fracture resistance properties.

Description

[Technical Field]

[0001] Technical Field The present invention relates to a metallic material and a method for producing the same, and more particularly to a steel grade and a method for producing the same. [Background technology]

[0002] background Fasteners, known as the "rice of industry," are a general term for the types of mechanical parts used to fasten and connect two or more parts (or components) together, and are the most widely and numerously used basic mechanical components in various sectors of the national economy. Bolt fastener connections have the advantages of simplicity, convenience, multiple disassembly and reassembly, high standardization, and low cost. A great number of different fasteners are used in various machinery and equipment, vehicles and ships, aircraft and satellites, railways and bridges, buildings, tools and instruments, electrical appliances and devices, daily necessities, etc.

[0003] Currently, fasteners are mainly used in the automotive industry, electronics industry, and construction and maintenance industries, with fasteners in the automotive industry being the most widely used. Commonly used materials for automotive fasteners include titanium alloys, steel, copper, aluminum, nylon, and other metals, nonmetals, etc. Steels for automotive fasteners can be classified into the following four categories according to the product characteristics: (1) non-heat-treatable steels: mainly including low-carbon steel series, where the grade of the processed finished products is 3.6-5.8; (2) quenched and tempered steels: mainly including medium-carbon steel, low-alloy steel, and alloy steel series; (3) surface-hardened steels: represented by ML18Mn, ML22Mn, and ML20Cr, where the main processed finished products are self-tapping screws, self-tapping self-drilling screws, and other products requiring surface carburizing; and (4) non-hardened and tempered steels: also known as alloy non-hardened and tempered steels, where the non-hardened and tempered steels for fasteners are mainly cold-work hardened steels, and their steel grade numbers are often preceded by the letters "LF".

[0004] Currently, there are four performance grades of high-strength fasteners widely used in automobiles: 8.8, 9.8, 10.9, and 12.9. Most high-strength bolts of grade 8.8 or higher are made of carbon steel or medium-carbon alloy steel because they are required to withstand large loads, have very complex stress states, and must be quenched and tempered to ensure sufficient strength and yield-to-tensile ratio.

[0005] High-strength fasteners commonly used in automobile engines typically include several categories: cylinder head bolts, connecting rod bolts, flywheel bolts, bearing cap bolts, pulley bolts, etc., which are among the most demanding automotive fasteners. Because the engine operates at high speeds during vehicle operation, engine fastener bolts are subjected to repeated tensile stress and are prone to fatigue failure. Engine fastener bolts are classified as high-strength fasteners of grade 8.8 or higher. In recent years, bolt strength has been continuously improved with the development of compact and small engines. However, such high-strength bolts are prone to hydrogen embrittlement delayed fracture. Once a problem occurs in an engine fastener bolt, the normal operation of the automobile engine is affected, resulting in significant damage. It is clear that uniformity issues in fastener bolts, such as engine cylinder head bolts, have a significant impact on engine reliability and fuel efficiency.

[0006] Therefore, to ensure the reliability and fuel efficiency of automobile engines, the focus of engine bolt research has always been on improving resistance to hydrogen embrittlement delayed fracture and improving the material uniformity of high-strength bolts such as automobile engine bolts, thereby ensuring consistency in bolt performance.

[0007] For example, Chinese Patent Publication No. CN111621714A, published on September 4, 2020, and titled "Round Bar for Bolts with Excellent Corrosion Resistance and Delayed Fracture Resistance and Method for Producing the Same," discloses a round bar for bolts with excellent corrosion resistance and delayed fracture resistance, which has a carbon content of 0.55-0.60%, a large amount of Si added up to 1.80-2.00%, while 0.20-0.35% Cu is also added. With this technical solution, it is difficult to control decarburization and cracking during the hot rolling and heat treatment of the alloy, and the performance of the processed bolts is poor.

[0008] In another example: Chinese Patent Publication No. CN108754303A, published on December 6, 2018, and entitled "High-Strength Bolt with Excellent Atmospheric Corrosion Resistance and Delayed Fracture Resistance," discloses a high-strength bolt with excellent atmospheric corrosion resistance and delayed fracture resistance, which requires the addition of 0.30-1.20% Ni and 0.20-0.60% Cu, as well as the addition of 0.005-0.030% Re rare earth element, resulting in high alloy costs and significant difficulties in smelting control.

[0009] In yet another example, Chinese Patent Publication No. CN110791715A, published on February 14, 2020, and entitled "14.9-grade high-strength bolt steel containing niobium and titanium and having atmospheric corrosion resistance and a method for producing the same," discloses a 14.9-grade high-strength bolt steel containing niobium and titanium and having atmospheric corrosion resistance, which requires the addition of 0.80-1.00% Mo and large amounts of V, Nb, Ti, Cr, and Cu. Such alloys in this solution are difficult to produce and have high costs, and the performance stability of the processed bolts cannot be guaranteed.

[0010] Many solutions in the field still have many drawbacks, and in recent years, with increasing legal requirements for vehicle energy conservation and emission reduction effects, the market and consumers are also putting higher demands on automobiles, which can be seen to require automobiles that improve engine efficiency while satisfying the requirements for light weight. Therefore, there is an urgent need for homogeneous high-strength, durable bolts that have high strength, excellent material uniformity, and delayed fracture resistance to ensure that automobile engines have good reliability and fuel efficiency while satisfying the requirements for light weight and compactness.

[0011] Based on this, in order to solve the above problems, the present invention aims to provide a bolt steel and a manufacturing method thereof, where the bolt steel has uniform structure and performance, good performance consistency and stability, low production cost, high strength, and good resistance to delayed fracture. The bolt steel can be used to manufacture homogeneous, high-strength, durable bolts, and the manufactured homogeneous, high-strength, durable bolts are beneficial for improving the stability of engine tightening force, thereby realizing engine miniaturization and high combustion efficiency, achieving the goals of energy saving and emission reduction, which has great economic and social benefits. Summary of the Invention [Means for solving the problem]

[0012] summary One of the objectives of the present invention is to provide a bolt steel with uniform structure and performance, as well as good delayed fracture resistance. This bolt steel can be used to produce uniform, high-strength, durable bolts that can be effectively applied to applications with high tightening force requirements, such as automobile engines and high-performance precision machinery. This significantly improves engine efficiency and machine machining accuracy, and therefore has broad market application prospects and very good economic and social benefits.

[0013] To achieve the above object, the present invention provides a steel for bolts which contains, in addition to Fe and unavoidable impurities, the following chemical elements in mass percent: C: 0.37-0.45% C; Si: 0.01-0.08% Si; Mn: 0.45-0.80% Mn; Cr: 0.90-1.30% Cr; Mo: 0.20-0.45% Mo; Ni: 0.10-0.30% Ni; V: 0.15-0.30% V; and Al: 0.015-0.035%.

[0014] Preferably, the bolt steel according to the invention consists of the following chemical elements in mass percentages: C: 0.37-0.45% C; Si: 0.01-0.08% Si; Mn: 0.45-0.80% Mn; Cr: 0.90-1.30% Cr; Mo: 0.20-0.45% Mo; Ni: 0.10-0.30% Ni; V: 0.15-0.30% V; Al: 0.015-0.035%, and the balance is Fe and unavoidable impurities.

[0015] In the bolt steel according to the present invention, the design principles of each chemical element are specifically described as follows:

[0016] C: In the bolt steel of the present invention, carbon is a necessary chemical component for ensuring the high strength of the bolt steel. The carbon content determines the number of carbides precipitated in the coil rod and finished bolt after quenching and tempering, which greatly affects the hardness and strength of the alloy. Therefore, to ensure the quality of the steel, the carbon content in the steel of the present invention must be controlled to 0.37% or more. However, it should be noted that the carbon content in the steel should not be too high. Designing with too high a carbon content will result in excessive precipitation and an increase in the size of carbides in the material, which will reduce the material's plasticity and toughness and result in a deterioration of delayed fracture resistance. Therefore, the carbon content must be controlled to 0.45% or less. Based on this, the mass content of carbon in the bolt steel of the present invention is controlled to 0.37-0.45%.

[0017] Si: In the bolt steel of the present invention, Si is often added to steel as a deoxidizer during smelting. The dissolved Si in the ferrite phase significantly improves the strength of the steel. However, the Si content in the steel should not be too high. It should be noted that if the Si content in the steel is too high, the cold heading formability of the material will decrease, further deteriorating the delayed fracture resistance of the material. Therefore, to ensure the quality of the steel, the Si content in the steel of the present invention must be controlled to 0.08% or less. At the same time, to lower the melting point of inclusions in the steel and eliminate large, non-deformable inclusions, the Si content must be controlled to 0.01% or more. Based on this, the mass content of Si in the bolt steel of the present invention is controlled to 0.01-0.08%.

[0018] Mn: In the bolt steel of the present invention, Mn is also often added to the steel as a deoxidizer during the steelmaking process. At the same time, Mn easily combines with the harmful element S in the steel to form MnS, thereby reducing the harmful effects of S. Furthermore, Mn is also a commonly used strengthening element in steel and plays a major role in solid-solution strengthening, resulting in higher strength of the resulting alloy cementite. Therefore, the Mn content in the alloy needs to be controlled to be 0.45% or more. However, it should be noted that the Mn content should not be too high. If the Mn content in the steel is too high, the tendency for grain coarsening during heating of the material increases, and the difficulty of controlling the controlled cooling structure increases. Mn tends to promote the segregation of residual elements. Therefore, the Mn content in the steel needs to be controlled to be 0.80% or less. Based on this, the mass content of Mn in the bolt steel of the present invention is controlled to be 0.45-0.80%.

[0019] Cr: In the bolt steel of the present invention, the addition of Cr improves the hardenability of the alloy, refines the structure during the bolt quenching and tempering process, and improves the strength of cementite, which improves both the strength and plasticity of the material. At the same time, Cr improves the corrosion resistance of the material and reduces its susceptibility to hydrogen embrittlement. Therefore, to ensure the quality of the steel, the Cr content in the steel must be controlled to be 0.90% or more. Furthermore, to prevent the occurrence of abnormal martensitic structures and reduce the difficulty of controlling the structure of the coil rod, the Cr content in the steel should not be too high and should be controlled to be 1.30% or less. Therefore, in the bolt steel of the present invention, the Cr mass content is controlled to be 0.90-1.30%.

[0020] Mo: In the bolt steel of the present invention, the addition of Mo is beneficial for refining the structure, improving the tempering stability of the material, improving the strength and hardness of the material under high-temperature tempering, and improving the delayed fracture resistance of the material. However, it should be noted that the content of Mo in the steel should not be too high, and if too much Mo is added to the steel, it becomes more difficult to control the material structure and the cost of the alloy increases. Therefore, in the bolt steel of the present invention, the mass content of Mo is controlled to be 0.20-0.45%.

[0021] Ni: In the bolt steel of the present invention, Ni is an austenite-forming element and can dissolve in the ferrite phase, which is beneficial to improving the strength of the material. At the same time, Ni can also effectively improve the hardenability of the material, thereby improving the structural uniformity and refining the structure during the bolt quenching and tempering process. However, it should be noted that the Ni content in the steel should not be too high. An excessively high Ni content can cause the development of an abnormal martensitic structure during the material production process and also affect the cost of the alloy. Therefore, in the bolt steel of the present invention, the Ni mass content is controlled to be 0.10-0.30%.

[0022] V: In the bolt steel of the present invention, V can easily react with C and N in the steel to precipitate carbonitrides. These nanoscale precipitates greatly enhance the strength and plasticity of the material. At the same time, V carbonitriding can act as a hydrogen trap, combining with free hydrogen in the steel and reducing its harmful effects. By controlling the number and size of V carbonitrides in the steel, the delayed fracture resistance of the material can be improved, while the consistency of material properties can also be improved. To achieve the promoting effect of V, the V content in the steel is designed to be 0.15% or more. However, adding an excessive amount of V causes the carbonitrides to increase in size, degrading the plasticity, toughness, and formability of the material. Therefore, the V content in the steel must be controlled to 0.30% or less. Based on this, the mass content of V in the bolt steel of the present invention is controlled to be 0.15-0.30%.

[0023] Al: In the bolt steel of the present invention, Al is the most effective deoxidizing element in the steelmaking process and exerts its deoxidizing effect. However, during deoxidation, Al tends to generate Al2O3 particles with sharp edges and corners, which significantly affect the fatigue life, durability, and delayed fracture resistance of the finished bolt, especially when the oxygen content in the steel is too high. Therefore, to ensure the performance of high-strength bolts and prevent the occurrence of large, brittle inclusions to improve the quality of the steel, the mass content of Al in the bolt steel of the present invention can be controlled to be 0.015-0.035%.

[0024] Preferably, in the steel for bolts according to the invention, the contents of impurity elements satisfy the following in mass percent: Cu≦0.05%; P≦0.01%; S≦0.010%; O≦0.001%; ​​and N≦0.005%.

[0025] In the above technical solutions, Cu, P, S, O and N elements are all impurity elements in steel. In order to obtain steel with better performance and better quality, the content of impurity elements in steel should be reduced as much as possible when technical conditions permit.

[0026] In the present invention, it should be noted that the impurity element Cu may cause hot embrittlement of high-strength steel, and at the same time, the uneven distribution of excess Cu element in the material will increase the retained austenite content in the material and reduce performance stability, and therefore, in the bolt steel according to the present invention, the content of Cu element needs to be controlled to be not more than 0.05%.

[0027] Therefore, in the present invention, too high contents of P and S elements in steel increase the brittleness of the steel, especially when segregation occurs, and therefore, in the steel for bolts according to the present invention, the contents of P and S elements need to be controlled so that P≦0.01% and S≦0.010%.

[0028] Furthermore, in the present invention, N element also causes an increase in the material, while too high contents of N and C elements cause an increase in the size of micro-alloy precipitates in the bolt steel, reducing the delayed fracture resistance properties of the material, and therefore, in the bolt steel according to the present invention, the content of N element needs to be controlled to be not more than 0.005%.

[0029] Furthermore, in the present invention, the content of O element as an impurity element can be controlled to 0.001% or less.

[0030] Preferably, in the steel for bolts according to the invention, the ratio between the content of the element Al and the content of the element O, in percent by mass, satisfies: Al / O>20.

[0031] In the technical solution according to the present invention, in order to obtain a steel with better performance and better quality and performance, when controlling the content of a single chemical element, preferably the ratio between the content of Al element and the content of O element can also be controlled to satisfy Al / O>20, where each element in the formula is replaced by the corresponding mass content of the element.

[0032] In the present invention, controlling Al / O>20 is beneficial to reduce the oxygen content in the steel while avoiding the generation of too many coarse single particle inclusions, which can ensure that the generated large particle inclusions have a size of less than 38 μm, thereby preventing damage to the plasticity and toughness of the material and the delayed fracture resistance properties.

[0033] Preferably, in the steel for bolts according to the present invention, the contents of the elements V, C and N satisfy the following in mass percent: V×(C+N)≦1 / 8.

[0034] In the technical solution according to the present invention, while controlling the content of a single chemical element, the contents of V, C and N elements can also be controlled to satisfy V×(C+N)≦1 / 8, where each element in the formula is replaced by the numerical value before the percentage symbol of the corresponding mass content of the element.

[0035] In the present invention, it is preferable to further control V×(C+N)≦1 / 8 so that the proportion of V carbonitride precipitates having a size of 5-50 nm in the finished bolt is higher than 90%, thereby controlling the content of V element in the steel. The presence of such nano-sized carbides advantageously increases the strength, plasticity, and toughness of the material, and at the same time acts as a hydrogen trap, reducing the tendency of the bolt to hydrogen embrittlement.

[0036] Preferably, the bolt steel according to the invention has a microstructure comprising tempered sorbite.

[0037] Preferably, the microstructure of the bolt steel according to the present invention further comprises V carbonitride precipitates, wherein the number proportion of V carbonitride precipitates having a size of 5-50 nm is higher than 90%.

[0038] As used herein, "size" with respect to V carbonitride precipitates and inclusions refers to the size of a single precipitate or inclusion, specifically the length of the longest line segment passing through the center of the precipitate or inclusion, such as the diameter (if spherical or approximately spherical) or major axis (if elliptical or approximately elliptical) or other length (if other shape).

[0039] Preferably, the inclusions in the bolt steel according to the invention have a size of less than 38 μm.

[0040] Preferably, the bolt steel according to the present invention satisfies the following properties: tensile strength ≥ 1200 MPa, yield to tension ratio > 0.9, tensile strength loss in hydrogen charging and slow strain rate tests ≤ 10%, bolt tightening and torsion variation ≤ 8%, and bolt fatigue life > 75000 cycles. "Yield to tension ratio" refers to the ratio between yield strength and tensile strength.

[0041] In this specification, the mechanical properties of steel are determined in accordance with GB / T 228.1-2010, "Metallic Materials - Tensile Testing." "Bolt Fatigue Life" refers to the number of tests at which the bolt breaks and is determined in accordance with GBT 13682-1992, "Axial Load Fatigue Test for Threaded Fasteners." "Tensile Strength Loss in Hydrogen Charging and Slow Strain Rate Testing" is calculated as (Tensile Strength Under Atmospheric Conditions) - (Tensile Strength Under Hydrogen Corrosion Conditions) / (Tensile Strength Under Atmospheric Conditions) and is determined in accordance with GB / T 15970.7-2017, "Corrosion of Metals and Alloys - Stress Corrosion Testing Part 7: Slow Strain Rate Testing." "Bolt Tightening and Torsion Variation" is calculated as (Actual Torque Applied During Bolt Tightening) - (Target Torque) / (Target Torque), where a smaller value indicates better material uniformity.

[0042] Therefore, another object of the present invention is to provide a method for manufacturing steel for bolts. The manufacturing method is simple to operate, and the steel for bolts manufactured by the method has uniform structure and performance. The steel has a tensile strength of 1200 MPa or more, a yield-to-tensile ratio of greater than 0.9, a tensile strength loss of ≤10% in hydrogen charging and slow strain rate tests, a bolt tightening and torsion variation of ≤8%, and a bolt fatigue life of more than 75,000 cycles. The steel can be used to manufacture bolts that can be effectively applied in application scenarios with high requirements for tightening force, such as automobile engines and high-performance precision machinery, and therefore has very good economic and social benefits.

[0043] In order to achieve the above object, the present invention provides a method for producing steel for bolts, which includes the following steps: (1) The process of smelting molten steel; (2) Casting the smelted molten steel to produce billets; (3) rough rolling the billet; (4) high speed wire rolling to produce coil rod; (5) providing Stelmor controlled cooling to the coil rod; and (6) Heat treatment process: The coil rod is successively subjected to spheroidizing heat treatment, wire drawing, and quenching and tempering heat treatment, wherein the holding temperature of the spheroidizing heat treatment is 760-790°C, the holding time is 4-12 hours, and the holding is followed by a slow cooling process at a cooling rate lower than 40°C / h; wherein the drawing reduction rate of the coil rod is controlled at 5-30% during wire drawing; wherein the heating temperature of the quenching and tempering heat treatment is 850-950°C, and the tempering temperature is 500-600°C.

[0044] Preferably, the method for manufacturing steel for bolts according to the present invention may further include other steps that are commonly performed in the manufacturing process of steel for bolts in the art, such as cold heading after wire drawing.

[0045] By the method for manufacturing a steel for bolts according to the present invention, a coil rod of the steel for bolts according to the present invention having the above-described excellent properties can be efficiently manufactured.

[0046] In the technical solution of the present invention, in the smelting process of step (1), molten steel can be smelted using an electric furnace or a converter and then subjected to external refining. Specifically, in the external refining, a ladle furnace (LF) and a vacuum degassing (VD) or Ruhrstahl Heraeus (RH) degassing process can be used; the composition and amount of synthetic slag added can be adjusted during the smelting process to control the content of impurity elements in the steel, and the vacuum degassing time can be controlled to be longer than 15 minutes. In the smelting process, the content of impurity elements P and S in the steel can be controlled to be lower than 0.010%, the vacuum degassing time can be required to be longer than 15 minutes, the end point of the O content can be controlled to be lower than 0.0010%, the end point of the N content can be controlled to be lower than 0.0050%, and the end point of the H content can be controlled to be lower than 2 ppm.

[0047] Therefore, in step (2) of the manufacturing method according to the present invention, a bloom caster can be used to cast a square bloom during the casting process, argon protection can be used during the casting process, the size of the square bloom can be controlled to be 300-450 mm, and the carbon segregation in the core of the billet can be controlled to be lower than 1.10 by adjusting the drawing speed, as well as the cooling parameters and soft end reduction parameters during the continuous casting process.

[0048] As used herein, "carbon segregation in the core" refers to the ratio of the carbon content of the core of a cast slab to the average carbon content of the cast slab, where the carbon content may be determined according to standard GB / T 20123.

[0049] In step (3) of the manufacturing method of the present invention, i.e., the rough rolling step, the continuously cast slab is bloomed at a temperature of 1050-1250°C and then formed into 150-250mm square billets using a two-heat production process. After the square billets undergo eddy current inspection, magnetic particle inspection, grinding wheel trimming, auxiliary magnetic particle inspection, and trimming, the processed square billets are heated in a heating furnace. During the billet heating process, the heating temperature can be controlled to be 960-1150°C, and the holding time can be controlled to be 1.5-3.0 hours.

[0050] Furthermore, in step (4) of the manufacturing method according to the present invention, in the high-speed wire rolling process, the rolling speed can be controlled to be 8-90 m / s. Obviously, in some preferred embodiments, in order to achieve better performance, the inlet temperature of the finishing rolling unit in the online temperature can be preferably controlled to be 850-970°C, the inlet temperature of the reduction and sizing unit can be controlled to be 800-950°C, and the laying temperature can be controlled to be 750-900°C.

[0051] Furthermore, in the present invention, the size specification of the coil rod obtained by rolling can be Φ6-26 mm, and the structural transformation of the coil rod can be controlled by adjusting the air volume of the Stelmor wire fan to optimize the structure of the coil rod during the Stelmor controlled cooling process in step (5) of the present invention.

[0052] Therefore, in step (6) of the manufacturing method of the present invention, in the heat treatment step, the manufactured coil rod can be subjected to spheroidizing heat treatment, where the holding temperature of the spheroidizing heat treatment can be controlled to be 760-790°C, the holding time can be controlled to be 4-12 hours, and the holding can be followed by a slow cooling process at a cooling rate lower than 40°C / h. The area reduction rate of the coil rod can be controlled to be 5-30%, the heating temperature of the quenching and tempering heat treatment can be controlled to be 850-950°C, and the tempering temperature can be controlled to be 500-600°C.

[0053] Preferably, in the production method of the present invention, in step (1), the vacuum degassing time is controlled to be longer than 15 minutes during smelting.

[0054] Preferably, in the manufacturing method of the present invention, in step (2), the carbon segregation in the core of the billet is controlled to be lower than 1.10 during casting.

[0055] Preferably, in the manufacturing method of the present invention, in step (3), during the billet heating process, the heating temperature is controlled to be 960-1150°C, and the holding time is controlled to be 1.5-3.0 hours.

[0056] Preferably, in the manufacturing method of the present invention, in step (4), the rolling speed is controlled to be 8-90 m / s.

[0057] Preferably, in step (4) of the production method of the present invention, the inlet temperature of the finishing rolling unit is controlled to be 850-970°C, the inlet temperature of the reduction and sizing unit is controlled to be 800-950°C, and the laying temperature is controlled to be 750-900°C.

[0058] Preferably, in the manufacturing method according to the present invention, in step (5), the Stelmor controlled cooling uses at least 14 fans, wherein the airflow of fans F1-F5 is not more than 80%, the airflow of fans F6-F12 is not more than 50%, and the airflow of fans F13-F14 is not more than 45%.

[0059] In this specification, the term "air volume" with the unit % refers to the percentage of air volume of each fan, where the air volume of each fan is 200,000 m 3 For example, "The airflow rate of fans F1-F5 is 80% or less" means that the airflow rate of each of fans F1-F5 is 80% or less, that is, the airflow rate of each of fans F1-F5 is 160,000 m 3 / h or less; "the air volume of fans F6-F12 is 50% or less" means that the proportion of the air volume of each of fans F6-F12 is 50% or less, i.e., the air volume of each of fans F6-F12 is 100,000 m 3 / h or less; and "the air volume of fans F13-F14 is 45% or less" means that the air volume of each of fans F13-F14 is 45% or less, i.e., the air volume of each of fans F13-F14 is 90,000 m 3 / h or less.

[0060] In the above technical solution of the present invention, the Stelmor controlled cooling is carried out by using at least 14 fans, and the coil rod of bolt steel obtained after the Stelmor controlled cooling can obtain good plasticity and toughness.

[0061] Compared with the prior art, the bolt steel and its manufacturing method according to the present invention have the following advantages and beneficial effects:

[0062] The bolt steel of the present invention has uniform structure and performance, good performance consistency and stability, low production cost, high strength, and good resistance to delayed fracture. It can be used to manufacture uniform, high-strength, durable bolts, which can be effectively applied to applications with high tightening force requirements, such as automobile engines and high-performance precision machinery, which can significantly improve engine efficiency and machine machining accuracy. It has broad market application prospects and very good economic and social benefits.

[0063] The bolt steel according to the present invention has a refined tempered sorbite structure after quenching and tempering heat treatment, and has uniform structure and performance. In some preferred embodiments, the number percentage of V carbonitride precipitates with a size of 5-50 nm is higher than 90%. The inclusions have a size of less than 38 μm. The tensile strength can reach 1200 MPa or more.

[0064] The fatigue life and delayed fracture resistance of the finished high-strength bolts manufactured from the bolt steel of the present invention are more than twice as long as those of conventional materials, and at the same time, the bolt tightening and torsion fluctuations are less than 8%, so that the uniformity of the bolt tightening force can be significantly improved, thereby realizing engine downsizing and high combustion efficiency, and achieving the goals of energy saving and emission reduction. DETAILED DESCRIPTION OF THE INVENTION

[0065] Detailed Description Next, the bolt steel and its manufacturing method according to the present invention will be further described and illustrated with reference to specific examples, but this description and illustration do not constitute an undue limitation of the technical solution of the present invention.

[0066] Examples 1-10 and Comparative Examples 1-4 All bolt steels in Examples 1-10 are manufactured by the following process: (1) Smelting molten steel according to the chemical composition shown in Table 1-1 and Table 1-2: the molten steel is smelted by an electric furnace or a converter, and then subjected to external refining, where the external refining uses a ladle furnace (LF) and a vacuum degassing (VD) or Ruhrstahll Heraeus (RH) degassing process, the composition and added amount of synthetic slag are adjusted during the smelting process, and the vacuum degassing time is controlled to be longer than 15 minutes during the smelting process.

[0067] (2) Casting the smelted molten steel to produce billets: A bloom caster is used to cast square blooms, where argon protection can be used during casting, and the size of the square blooms can be controlled to be 300-450mm. By adjusting the drawing speed, as well as the cooling parameters and soft end reduction parameters during the continuous casting process, the carbon segregation in the core of the billet can be controlled to be lower than 1.10.

[0068] (3) Rough rolling of the billet: The continuous cast slab is bloomed at 1100-1250°C and then turned into 150-250mm square billets using a two-heating process. The square billets undergo ultrasonic testing, magnetic particle testing, grinding wheel trimming, and auxiliary magnetic particle testing and trimming before being heated in a heating furnace. During the billet heating process, the heating temperature is controlled to 960-1150°C, and the holding time is controlled to 1.5-3.0 hours.

[0069] (4) High-speed wire rolling is carried out to produce coil rods with a size specification of Φ6-26mm, where the rolling speed is controlled to be 8-90m / s, the inlet temperature of the finishing rolling unit is controlled to be 850-970℃, the inlet temperature of the reduction and sizing unit is controlled to be 800-950℃, and the laying temperature is controlled to be 750-900℃.

[0070] (5) Stelmor controlled cooling is performed on the coil rod: Stelmor controlled cooling is performed on the coil rod by using 14 Stelmor line fans, where the air volume of fans F1-F5 is less than 80%, the air volume of fans F6-F12 is less than 50%, and the air volume of fans F13-F14 is less than 45%, and the structural transformation of the coil rod is controlled by adjusting the air volume of the Stelmor line fans to optimize the structure of the coil rod.

[0071] (6) Heat treatment: The coil rod is successively subjected to spheroidizing heat treatment, wire drawing, and quenching and tempering heat treatment, where the holding temperature of the spheroidizing heat treatment is controlled to be 760-790°C, the holding time is 4-12 hours, and the holding is followed by a slow cooling process at a cooling rate lower than 40°C / h, the wire drawing reduction rate of the coil rod is controlled to be 5-30%, the heating temperature of the quenching and tempering heat treatment is controlled to be 850-950°C, and the tempering temperature is 500-600°C.

[0072] It should be noted that the coil rods in Examples 1-10 of the present invention were manufactured using the above-described process and had chemical compositions and related process parameters that satisfied the design specification control requirements of the present invention. The comparative coil rods in Comparative Examples 1-4 were also manufactured using the processes of smelting, casting, rough rolling, high-speed wire rolling, Stelmor controlled cooling, and heat treatment, but some of the chemical compositions and related process parameters did not satisfy the design requirements of the present invention.

[0073] Table 1 lists the mass percent of each chemical element for the bolt steels in Examples 1-10 and the comparative steels in Comparative Examples 1-4.

[0074] [Table 1-1]

[0075] [Table 1-2]

[0076] Tables 2-1 and 2-2 list the specific process parameters for the bolt steels in Examples 1-10 and the comparative steels in Comparative Examples 1-4 in the above steps.

[0077] [Table 2-1]

[0078] [Table 2-2]

[0079] The finally produced steels for bolts in Examples 1-10 and comparative steels in Comparative Examples 1-4 were separately sampled, and the steel samples in the Examples and Comparative Examples were observed and analyzed to obtain the structures of the steels in the Examples and Comparative Examples. After the observations were completed, the steel samples in the Examples and Comparative Examples were subjected to mechanical property tests, and the results of the observations and mechanical property tests are shown in Tables 3 and 4, respectively.

[0080] The relevant mechanical property test methods are listed below: Tensile test: The test is carried out at room temperature in accordance with GB / T 228.1-2010 "Metallic materials - Tensile test".

[0081] Table 3 lists the structural observation results of the bolt steels in Examples 1-10 and the comparative steels in Comparative Examples 1-4.

[0082] [Table 3]

[0083] As shown in Table 3, it should be noted that in the present invention, the microstructure of the bolt steel in Examples 1-10 contains tempered sorbite.

[0084] Furthermore, it should be noted that in the bolt steels of Examples 1-10 of the present invention, the microstructure also has V carbonitride precipitates, where the proportion of V carbonitride precipitates having a size of 5-50 nm is higher than 90%.

[0085] Furthermore, in the bolt steels of Examples 1-10 of the present invention, the inclusions in the steel have a size of less than 38 μm.

[0086] Table 4 lists the results of mechanical property tests of the bolt steels in Examples 1-10 and the comparative steels in Comparative Examples 1-4.

[0087] [Table 4]

[0088] It should be noted that the sampled bolt steels in Examples 1-10 and the sampled comparative steels in Comparative Examples 1-4 are processed to obtain the corresponding bolts. The bolts obtained in Examples 1-10 and Comparative Examples 1-4 were tested for relevant properties, and the obtained property test results are listed in Table 5.

[0089] Table 5 lists the property test results of bolts obtained by processing the bolt steels of Examples 1-10 and the comparative steels of Comparative Examples 1-4.

[0090] [Table 5]

[0091] As can be seen in relation to Tables 4 and 5, the comparative bolts in Comparative Examples 1-4 all performed significantly worse than the bolts in Examples 1-10. In the present invention, the bolt steels in Examples 1-10 all have good performance, with tensile strengths of 1200 MPa or more, yield-to-tensile ratios greater than 0.9, tensile strength losses of ≦10% in hydrogen charging and slow strain rate tests, bolt tightening and torsional variations of 8% or less, and bolt fatigue lives of more than 75,000 cycles.

[0092] The bolt steel according to the present invention can be used to manufacture homogeneous, high-strength, durable bolts, and the manufactured homogeneous, high-strength, durable bolts can be effectively applied to application scenarios with high requirements for tightening force, such as automobile engines and high-performance precision machinery, which can significantly improve the efficiency of the engine and the machining accuracy of the machine, and therefore has a wide market application prospect and very good economic and social benefits.

[0093] Furthermore, the combination of technical features in the present invention is not limited to the combination in the claims or specific embodiments of the present invention, and all technical features in the present invention can be freely combined or combined in any way as long as there is no contradiction between them.

[0094] It should be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above-mentioned embodiments, and similar variations or modifications made therewith can be directly obtained from the contents disclosed by the present invention or can be easily associated by those skilled in the art, and all fall within the protection scope of the present invention.

Claims

1. Steel for bolts consisting of the following chemical elements in mass percent: C：0.37-0.45%； Si: 0.01-0.08%; Mn: 0.45-0.80%; Cr：0.90-1.30%； Mo: 0.20-0.45%; Ni: 0.10-0.30%; V：0.15-0.30%； Al: 0.015-0.035%; and As unavoidable impurities, Cu ≤ 0.05%; P≦0.01%； S≦0.010%； O≦0.001%； N ≤ 0.005%; and The balance is Fe and other unavoidable impurities other than Cu, P, S, O and N.

2. 2. Steel for bolts according to claim 1, wherein the ratio between the content of the element Al and the content of the element O, in mass percent, satisfies: Al / O>20.

3. The steel for bolts according to claim 1, wherein the contents of the elements V, C and N satisfy the following condition in mass percent: V×(C+N)≦1 / 8.

4. 2. The bolt steel of claim 1, wherein the bolt steel has a microstructure comprising tempered sorbite.

5. 5. The steel for bolts according to claim 4, wherein the microstructure further comprises V carbonitride precipitates, wherein the number fraction of V carbonitride precipitates having a size of 5-50 nm is higher than 90%.

6. 2. A bolt steel according to claim 1, wherein the inclusions in the bolt steel have a size of less than 38 μm.

7. 2. The steel for bolts according to claim 1, wherein the steel for bolts satisfies the following properties: tensile strength ≥ 1200 MPa, yield to tension ratio > 0.9, tensile strength loss in hydrogen charging and slow strain rate tests ≤ 10%, bolt tightening and torsion variation ≤ 8%, and bolt fatigue life > 75,000 cycles.

8. A method for producing steel for bolts according to claim 1, comprising the following steps: (1) A process for smelting molten steel; (2) Casting the smelted molten steel to produce billets; (3) rough rolling the billet; (4) performing high speed wire rolling to produce coil rod; (5) providing Stelmor controlled cooling to the coil rod; and (6) Heat treatment process: The coil rod is successively subjected to spheroidizing heat treatment, wire drawing, and quenching and tempering heat treatment, wherein the holding temperature of the spheroidizing heat treatment is 760-790°C, the holding time is 4-12 hours, and the holding is followed by a slow cooling process at a cooling rate lower than 40°C / h; wherein the drawing reduction rate of the coil rod is controlled at 5-30% during wire drawing; wherein the heating temperature of the quenching and tempering heat treatment is 850-950°C, and the tempering temperature is 500-600°C.

9. 9. The method of claim 8, wherein in step (1), the vacuum degassing time is controlled to be longer than 15 minutes during smelting.

10. 9. The method of claim 8, wherein in step (2), carbon segregation in the core of the billet is controlled to be less than 1.10 during casting.

11. 9. The manufacturing method according to claim 8, wherein in step (3), the rough rolling includes heating the billet after blooming, wherein the heating temperature during the billet heating process is controlled to be 960-1150°C, and the holding time is controlled to be 1.5-3.0h.

12. 9. The method according to claim 8, wherein in step (4), the rolling speed is controlled to be 8-90 m / s.

13. 13. The manufacturing method according to claim 12, wherein in step (4), the inlet temperature of the finishing rolling unit is controlled to be 850-970°C, the inlet temperature of the reduction and sizing unit is controlled to be 800-950°C, and the laying temperature is controlled to be 750-900°C.

14. 9. The manufacturing method of claim 8, wherein in step (5), the Stelmor controlled cooling uses at least 14 fans, wherein fans F1-F5 have an airflow of 80% or less, fans F6-F12 have an airflow of 50% or less, and fans F13-F14 have an airflow of 45% or less.

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