Steel bar for bolts

Abstract

This invention provides a steel bar suitable as a material for large-diameter, high-strength, and high-toughness bolts, a large-diameter, high-strength, and high-toughness bolt, and a method for manufacturing the bolt. The steel bar 1 has the following chemical composition (by mass%): C: 0.35% or more and 0.45% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.030% or less, S: 0.030% or less, Ni: 1.00% or more and 3.00% or less, Cr: 0.50% or more and 2.00% or less, and Mo: 0.10% or more and 0.50% or less, with the remainder being Fe and impurities. It has a DI value of 420 or more and a diameter of 65 mm or more.

Description

Technical Field

[0001] This invention relates to a steel bar, a bolt, and a method for manufacturing the bolt. Prior Technology

[0002] Patent Document 1 discloses a method for manufacturing large-diameter bolts. In this method, bolts of M25 to M40 are cold-forged from steel wire containing, by weight percent: C: 0.28% to 0.38%, Si: less than 0.10%, Mn: 0.60% to 1.20%, Cr: 0.20% to 0.60%, and whose calculated value according to a prescribed formula satisfies 30 to 45, including residual iron and impurities. The bolts are then heated, water-quenched, and tempered. This method allows for direct cold forging of M25 to M40 bolts without softening annealing, and provides sufficient hardenability, enabling the inexpensive manufacture of bolts with a tensile strength of 75 kgf / mm² or higher after quenching and tempering without cracking during bolt processing. [Existing Technical Documents] [Patent Literature]

[0003] Patent Document 1: Japanese Patent Application Publication No. 02-166231 Summary of the Invention

[0004] [The problem that the invention aims to solve] As disclosed in Patent Document 1, bolts undergo quenching and tempering treatments during manufacturing to achieve the desired strength. However, with the increasing size of bolts, it is difficult to obtain a quenched structure inside the bolt. In the bolts described in Patent Document 1, as mentioned above, the maximum diameter is 40 mm (M40). Furthermore, high toughness is also required for the bolts. Therefore, there is a need for steel bars suitable as materials for large-diameter, high-strength, and high-toughness bolts, large-diameter, high-strength, and high-toughness bolts, and a method for manufacturing such bolts.

[0005] The present invention is made in view of the above-mentioned actual situation, and its object is to provide a steel bar suitable as a material for a large-diameter, high-strength and high-toughness bolt, a large-diameter, high-strength and high-toughness bolt, and a method for manufacturing the bolt. [Methods for solving problems]

[0006] To achieve the aforementioned objectives, the steel bar, bolt, and bolt manufacturing method of the present invention are described below.

[0007] 1. A steel bar, wherein the chemical composition, in mass percent, is as follows: C: Above 0.35% and below 0.45% Si: 0.10% or more and 0.50% or less Mn: ≥0.50% and ≤2.00% P: below 0.030% S: Below 0.030% Ni: 1.00% or more and 3.00% or less Cr: ≥0.50% and ≤2.00% Mo: ≥0.10% and ≤0.50%, and The remainder consists of Fe and impurities. The hardenability index (DI value) represented by the following formula (1) is 420 or higher. The diameter is 65 mm or more. DI=(-23.7×[C] 2+38.3×[C]+3.54)×(1+3.3×[Mn])×(1+0.7×[Si])×(1+2.16×[Cr])×(1+3×[Mo])×(1+0.36×[Ni])･･･(1) Here, [C], [Mn], [Si], [Cr], [Mo] and [Ni] are the contents of C, Mn, Si, Cr, Mo and Ni expressed in mass% respectively.

[0008] 2. A bolt, which uses a steel bar as described in 1, wherein, The bolt has a shaft portion. On the cross-section perpendicular to the length direction of the shaft portion, the old γ grain size is between 4 and 10. In the metallic microstructure of the cross-section, the area fraction of tempered ramie iron is over 80%.

[0009] 3. A bolt manufacturing method comprising cutting a steel bar as described in 1, and subjecting the cut steel bar to hot forging, thread rolling, quenching, and tempering to manufacture a bolt having a shaft portion and a bolt head, wherein in the bolt manufacturing method, In the hot forging process, the bolt head is formed. In the thread rolling process, thread ridges are formed on the shaft portion. In the quenching process, the heating temperature is set to above 820°C and below 1150°C, and the average cooling rate from 800°C to 300°C is set to above 0.5°C / s. [The effects of the invention]

[0010] The present invention provides a steel bar suitable as a material for large-diameter, high-strength and high-toughness bolts, a large-diameter, high-strength and high-toughness bolt, and a method for manufacturing the bolt. Simple Explanation of the Diagram

[0011] Figure 1 is a diagram showing an example of a steel bar in this embodiment. Figure 2 is a diagram showing an example of a bolt in this embodiment. Figure 3 is a flowchart illustrating an example of the bolt manufacturing steps. Figure 4 is a flowchart illustrating an example of the bolt manufacturing steps. Implementation

[0012] First, a summary of the steel bar, bolt, and bolt manufacturing method of this embodiment will be given.

[0013] The chemical composition of the steel bar in this embodiment, expressed as a percentage by mass, is: C: Above 0.35% and below 0.45% Si: 0.10% or more and 0.50% or less Mn: ≥0.50% and ≤2.00% P: below 0.030% S: Below 0.030% Ni: 1.00% or more and 3.00% or less Cr: ≥0.50% and ≤2.00% Mo: ≥0.10% and ≤0.50%, and The remainder consists of Fe and impurities.

[0014] Furthermore, the DI value represented by the following formula (1) for the steel bar of this embodiment is 420 or more. Moreover, in the following formula (1), [C], [Mn], [Si], [Cr], [Mo], and [Ni] are, in order, the contents of C, Mn, Si, Cr, Mo, and Ni expressed as mass% respectively. Additionally, the diameter of the steel bar is 65 mm or more.

[0015] DI=(-23.7×[C] 2+38.3×[C]+3.54)×(1+3.3×[Mn])×(1+0.7×[Si])×(1+2.16×[Cr])×(1+3×[Mo])×(1+0.36×[Ni])･･･(1)

[0016] The steel bar is suitable as a material for manufacturing large-diameter bolts. The bolt of this embodiment uses the steel bar and has a shaft portion in which, on a cross-section perpendicular to the length direction, the original γ grain size is 4 or more and 10 or less, and the area ratio of tempered slag in the metal structure of the cross-section is 80% or more. An example of the bolt manufacturing method is a bolt manufacturing method in which the steel bar is subjected to hot forging to form a bolt head, thread rolling to form the thread teeth of the shaft portion, and quenching and tempering to produce a bolt having a bolt head and a shaft portion. Furthermore, in the quenching, the heating temperature is set to 820°C or more and 1150°C or less, and the average cooling rate from 800°C to 300°C is set to 0.5°C / s or more.

[0017] In Figure 1, a steel bar 1 is shown as an example of a steel bar in this embodiment. In Figure 2, a bolt 2 with a bolt head 21 and a threaded shaft portion 22 is shown as an example of a bolt in this embodiment. The bolt 2 is formed by hot forging one end of the steel bar 1 after it has been cut to a specified length, and by rolling the threaded shaft portion 22 at the other end, thereby forming the shape shown in the figure. Furthermore, in Figure 2, the threaded shaft portion 22 is formed entirely, but in this embodiment, it is sufficient for the threaded shaft portion to be formed only in at least a portion of the shaft portion.

[0018] The steel bar will now be described in detail. Furthermore, in the following explanation, when only "%" is used, it means "mass %" unless otherwise specified.

[0019] The steel bar in this embodiment is a rod-shaped steel bar. From the viewpoint of making a steel bar suitable for bolts, the steel bar can, for example, be made into a cylindrical steel bar.

[0020] From the viewpoint of making a steel bar suitable for large-diameter bolts, the steel bar of the present invention has a diameter of 65 mm or more. On the other hand, there is no upper limit to the diameter, but it is preferably 100 mm or less.

[0021] As described above, the steel bar of this embodiment contains, as a chemical composition, C (carbon), Si (silicon), Mn (manganese), P (phosphorus), S (sulfur), Ni (nickel), Cr (chromium), and Mo (molybdenum), with the remainder being Fe (iron) and impurities. Impurities refer to elements that are permissible within a range that do not adversely affect the effects of the present invention. These impurities include so-called unavoidable impurities that inevitably mix in during the industrial manufacturing of the steel bar from the ore used as raw material, waste, or the manufacturing environment.

[0022] The following is a detailed description of the chemical composition of the steel bar (the content of each element in the steel bar).

[0023] The carbon (C) content is 0.35% or more and 0.45% or less. C is an element that affects the hardenability and tensile strength of the steel bar. When the C content is less than 0.35%, the tensile strength of the steel bar is insufficient. Therefore, the C content is 0.35% or more, preferably 0.38% or more. On the other hand, when the C content exceeds 0.45%, the toughness of the steel bar decreases. Therefore, the C content is 0.45% or less, preferably 0.43% or less.

[0024] The silicon (Si) content is between 0.10% and 0.50%. Si is an element that affects the hardenability and temper softening resistance of steel bars. When the Si content is less than 0.10%, hardenability is insufficient, and the tensile strength of the steel bar decreases. Therefore, the Si content is 0.10% or more, preferably 0.15% or more. On the other hand, when the Si content exceeds 0.50%, temper softening resistance increases, and the toughness of the steel bar decreases. Therefore, the Si content is 0.50% or less, preferably 0.35% or less.

[0025] The Mn content is 0.50% or more and 2.00% or less. Mn is an element that affects the hardenability of steel bars and the phase transformation temperature of ferrite. When the Mn content is less than 0.50%, the hardenability is insufficient, and the tensile strength of the steel bar decreases. Therefore, the Mn content is 0.50% or more, preferably 0.70% or more. On the other hand, when the Mn content exceeds 2.00%, the phase transformation temperature of ferrite decreases, and residual ferrite is easily generated, resulting in insufficient tensile strength of the steel bar. Therefore, the Mn content is 2.00% or less, preferably 1.00% or less.

[0026] The phosphorus (P) content is 0.030% or less. P is an element that segregates at grain boundaries and affects the toughness of the steel bar. When the P content exceeds 0.030%, the grain boundary strength decreases, and the toughness of the steel bar decreases. Therefore, the P content is 0.030% or less, preferably 0.020% or less. On the other hand, since the presence of P is usually unavoidable, excessively low P content can sometimes lead to increased refining time or increased refining costs. Therefore, the P content is preferably set at 0.003% by mass or more, and more preferably at 0.007% by mass or more.

[0027] The sulfur (S) content is 0.030% or less. S is an element that affects the tensile strength and toughness of steel bars by forming MnS through bonding with Mn. MnS is the starting point for failure in the steel structure. When the S content exceeds 0.030%, the toughness of the steel bar decreases. Therefore, the S content is 0.030% or less, preferably 0.015% or less. On the other hand, since the presence of S is usually unavoidable, excessively low S content can sometimes increase refining costs. Therefore, the S content is preferably set at 0.003% by mass or more, and more preferably at 0.007% by mass or more.

[0028] The Ni content is 1.00% or more and 3.00% or less. Ni is an element that affects the hardenability and toughness of steel bars. When the Ni content is less than 1.00%, hardenability and toughness decrease. Therefore, the Ni content is 1.00% or more, preferably 1.50% or more. On the other hand, when the Ni content exceeds 3.00%, the improvement effect on hardenability and toughness becomes saturated, while the cost increases. Therefore, the Ni content is 3.00% or less, preferably 2.00% or less.

[0029] The Cr content is 0.50% or more and 2.00% or less. Cr is an element that affects the hardenability of steel bars and the phase transformation temperature of slag. When the Cr content is less than 0.50%, the hardenability is insufficient and the tensile strength decreases. Therefore, the Cr content is 0.50% or more, preferably 0.70% or more. On the other hand, when the Cr content exceeds 2.00%, the effect of improving hardenability becomes saturated, while the cost increases. Therefore, the Cr content is 2.00% or less, preferably 1.00% or less.

[0030] The Mo content is between 0.10% and 0.50%. Mo is an element that affects the hardenability and tempering softening resistance of steel bars. When the Mo content is less than 0.10%, hardenability is insufficient and tensile strength decreases. Therefore, the Mo content is 0.10% or more. On the other hand, when the Mo content exceeds 0.50%, the effect of improving hardenability becomes saturated. Therefore, the Mo content is 0.50% or less.

[0031] The DI value represented by equation (1) serves as the benchmark for hardenability. When the DI value is low, hardenability is insufficient. Therefore, the DI value is set to 420 or higher, preferably 450, and even more preferably 500. On the other hand, there is no particular limitation on the upper limit of the DI value; for example, it can be 8000 or lower.

[0032] The bolts of this embodiment will be described in detail below.

[0033] The bolt in this embodiment uses the aforementioned steel bar. Furthermore, the bolt has a shaft portion. The outer diameter of the shaft portion is equivalent to the diameter of the steel bar used as the material. Therefore, the outer diameter of the shaft portion can be set to 65 mm or more. Alternatively, the outer diameter of the shaft portion is preferably 100 mm or less.

[0034] Next, the microstructure of the bolt's shaft portion will be described. The microstructure of the bolt's shaft portion described below is the microstructure observed in a cross-section perpendicular to the length direction of the shaft portion.

[0035] In the aforementioned metal microstructure, the old γ grain size (also known as the old Wostian iron crystal grain size) has a grain size number of 4 or higher and 10 or lower. Furthermore, the grain size number in this embodiment is based on the Japanese Industrial Standard (JIS) G 0551 "Steel - Microscopic Test Method for Crystal Grain Size," and is measured at a distance of d / 4 from the radial center of the shaft. Furthermore, d refers to the outer diameter of the shaft. More specifically, the measurement is performed using the method described in the examples.

[0036] The old grit size affects the toughness of the steel bar. When the old grit size is smaller than grit size 4, the toughness of the bolt decreases. Therefore, the old grit size is grit size 4 or higher, preferably grit size 6 or higher. On the other hand, when the old grit size exceeds grit size 10, the tensile strength decreases. Therefore, the old grit size is grit size 10 or lower, preferably grit size 9 or lower.

[0037] Next, the area ratio of tempered ramed iron in the metal structure of the cross-section will be explained. Furthermore, the area ratio of tempered ramed iron is measured at the radial center of the shaft portion. More specifically, it is measured using the method described in the embodiments.

[0038] The area ratio of tempered ferrite affects the tensile strength and toughness of bolts. When the area ratio of tempered ferrite is low, the tensile strength and toughness of the bolt decrease. Therefore, the area ratio of tempered ferrite is 80% or more, preferably 85% or more. On the other hand, there is no upper limit to the area ratio of tempered ferrite, which can be 100%, and the metal structure may also include tempered ferrite. Furthermore, the structure of the remaining portion is not limited and can be selected from one or more structures chosen from the group consisting of ferrite, ferrite, ferritic iron, and residual ferrite.

[0039] By increasing the tensile strength of the shaft portion, bolts suitable for use can be manufactured. Therefore, the tensile strength of the shaft portion is preferably 1040 MPa or higher. Furthermore, the tensile strength of the shaft portion is measured based on JIS Z 2241 at a distance of d / 4 from the radial center outward. More specifically, the measurement is performed using the method described in the embodiments.

[0040] By improving the toughness of the shaft portion, bolts suitable for use can be manufactured. Therefore, the Charpy energy absorbed by the shaft portion is preferably 27 J or higher. Furthermore, the Charpy energy absorbed by the shaft portion is measured by the Charpy impact test specified in JIS Z 2242 at a distance of d / 4 from the radial center outward. More specifically, it is measured by the method described in the embodiments.

[0041] The bolt manufacturing method is described in detail below.

[0042] Figure 3 shows an example of the bolt manufacturing process. In the bolt manufacturing process, steel is used to manufacture the steel bar and bolt of this embodiment.

[0043] The bolt manufacturing process includes: a steel bar manufacturing step S1, which uses steel to manufacture a steel bar that will serve as the bolt material; and a bolt forming step S3, which processes the steel bar manufactured in the steel bar manufacturing step S1 into a bolt. This bolt manufacturing process may include an intermediate step S2, which transports the steel bar from the work site or office performing the steel bar manufacturing step S1 to the work site or office performing the bolt forming step S3.

[0044] In the steel bar manufacturing step S1, steel having the stated chemical composition and DI value is hot-rolled or hot-forged (rolling step S11) to produce steel bars of a specified diameter (e.g., steel bars with a diameter of 65 mm or more). The steel can be, for example, steel billets cast by continuous casting or steel ingots cast by ingot casting. The steel bars undergo specified quality inspections as required.

[0045] In intermediate step S2, the steel bars are shipped (shipping step S21) and transported (transportation step S22) to the work site or office where the bolt forming step S3 is performed, and received by the work site or office where the bolt forming step S3 is performed (receiving step S23).

[0046] In the bolt forming step S3, the received steel bar is cut to a specified size (cutting step S31). Then, the head of the bolt is formed by hot forging of the cut steel bar (hot forging step S32), and the thread teeth are formed by rolling (thread rolling step S33). After that, the bolt is quenched (quenching step S34) and tempered (tempering step S35) to obtain the bolt as a finished product.

[0047] Figure 4 shows another example of the process flow for manufacturing a bolt. As shown in Figure 4, in the bolt forming step S3, rolling (thread rolling step S33) can also be performed after quenching treatment (quenching step S34) and tempering treatment (tempering step S35).

[0048] After bolt forming step S3, the bolts can also be treated with paint or plating.

[0049] The quenching treatment (quenching step S34) and tempering treatment (tempering step S35) are described in detail below. Regarding the bolt, the step conditions for obtaining the desired tensile strength and toughness are explained. Furthermore, in the description of the following steps, the temperature of the steel forming the bolt shape is represented by the temperature of the radial center of the shaft portion.

[0050] The quenching in step S34 is performed at a heating temperature of 820°C or higher and 1150°C or lower, and at an average cooling rate of 0.5°C / s or higher from 800°C to 300°C. If the heating temperature is lower than 820°C, it is impossible to achieve a single-phase structure of ferrosilicon in the shaft portion. Therefore, the heating temperature is 820°C or higher. On the other hand, if the heating temperature is excessively increased, it will lead to coarsening of the crystal grains and a decrease in toughness; therefore, the heating temperature is 1150°C or lower, preferably 900°C or lower.

[0051] By setting the cooling rate to an average cooling rate of 0.5°C / s or higher from 800°C to 300°C, phase transformation to structures other than ferrite or ferroalloys is suppressed. When the average cooling rate is slow, the desired microstructure cannot be obtained. Therefore, the average cooling rate from 800°C to 300°C is set to 0.5°C / s or higher. Furthermore, by setting the cooling stop temperature to 150°C or lower, the ferroalloy phase transformation can occur. Therefore, the cooling stop temperature is preferably 150°C or lower.

[0052] The tempering in step S35 is preferably performed by heating to above 500°C and holding for at least 30 minutes. That is, the holding temperature is preferably above 500°C, and the holding time is preferably at least 30 minutes. After holding, it is preferably cooled to room temperature. During this cooling process, it is ideal to accelerate the cooling rate as much as possible to avoid embrittlement caused by grain boundary segregation of P. [Example]

[0053] The following describes an example. In the following example, in order to evaluate the relationship between the characteristics of a steel bar and the characteristics of a bolt made from the steel bar, a specimen simulating the shaft portion of a bolt was prepared by quenching and tempering the steel bar without forming threads or a bolt head, and the specimen was used for evaluation.

[0054] Molten steel (steel No. A to steel No. V) with the composition (remaining portion being Fe and impurities) and DI value shown in Table 1 is produced by casting using a continuous casting machine to form a 300 mm × 400 mm casting sheet. Furthermore, in Table 1, the underlined values ​​indicate that the value is outside the range of composition or DI value specified in this embodiment.

[0055] [Table 1] Table 1 Steel No. C (wt%) Si (wt%) Mn (wt%) P (wt%) S (wt%) Ni (wt%) Cr (wt%) Mo (wt%) DI value Remark A 0.38 0.20 1.01 0.020 0.010 1.50 1.33 0.25 756 Suitable for steel B 0.35 0.15 0.88 0.015 0.010 1.88 1.28 0.30 727 Suitable for steel C 0.44 0.25 0.93 0.015 0.009 1.80 1.32 0.28 882 Suitable for steel D 0.43 0.48 1.95 0.025 0.010 2.87 1.97 0.48 4047 Suitable for steel E 0.35 0.11 0.87 0.007 0.008 1.05 0.54 0.48 426 Suitable for steel F 0.36 0.12 0.57 0.007 0.009 1.08 1.97 0.12 442 Suitable for steel G 0.35 0.49 1.49 0.008 0.008 1.05 0.51 0.11 430 Suitable for steel H 0.39 0.24 0.78 0.015 0.029 1.66 0.79 0.25 470 Suitable for steel I 0.41 0.26 0.75 0.028 0.007 1.77 0.84 0.31 557 Suitable for steel J 0.39 0.21 0.71 0.006 0.007 1.63 0.69 0.17

[0340] Comparison of steel K [0.47] 0.20 1.03 0.013 0.008 1.60 0.75 0.30 642 Comparison of steel L [0.33] 0.21 1.02 0.012 0.008 1.61 0.75 0.31 544 Comparison of steel M 0.38 [0.52] 1.01 0.012 0.008 1.59 0.72 0.31 672 Comparison of steel N 0.36 [0.08] 1.01 0.013 0.007 1.60 0.73 0.22 440 Comparison of steel O 0.37 0.20 [2.05] 0.012 0.007 1.62 0.73 0.29 977 Comparison of steel P 0.38 0.21 [0.47] 0.011 0.008 1.61 0.72 0.31

[0334] Comparison of steel Q 0.39 0.25 1.02 [0.031] 0.008 1.61 0.73 0.30 590 Comparison of steel R 0.38 0.25 1.01 0.010 [0.032] 1.60 0.73 0.30 576 Comparison of steel S 0.38 0.25 1.00 0.011 0.008 [0.95] 0.72 0.31 491 Comparison of steel T 0.37 0.25 1.01 0.012 0.008 1.65 [0.48] 0.31 462 Comparison of steel U 0.38 0.24 1.02 0.011 0.007 1.63 0.85 [0.09] 428 Comparison of steel V 0.38 0.24 0.89 0.011 0.007 1.64 0.80 0.14

[0416] Comparison of steel

[0056] Next, the casting is homogenized at 1250°C for 30 minutes, and then hot-rolled into a steel sheet with a rectangular cross-section of 140 mm on each side. Then, each steel sheet is hot-rolled to the diameters shown in Table 2 to produce steel bars No. 1 to No. 30 (raw material in the hot-rolled state). Here, steel sheets other than No. J are hot-rolled into steel bars with diameters of 65 mm to 100 mm. Steel sheets of No. J are hot-rolled into steel bars with a diameter of 40 mm. Steel bars No. 1 to No. 30 are cylindrical.

[0057] [Table 2] Table 2 steel bar No. steel bar Remark Steel No. diameter (mm) 1 A 65 Invention Examples 2 A 70 Invention Examples 3 A 75 Invention Examples 4 A 80 Invention Examples 5 A 100 Invention Examples 6 A 70 Invention Examples 7 B 70 Invention Examples 8 C 70 Invention Examples 9 D 70 Invention Examples 10 E 70 Invention Examples 11 F 70 Invention Examples 12 G 70 Invention Examples 13 H 70 Invention Examples 14 I 70 Invention Examples 15 [J]

[40] Reference Example 16 A 70 Invention Examples 17 A 70 Invention Examples 18 A 70 Invention Examples 19 [K] 70 Comparative example 20 [L] 70 Comparative example twenty one [M] 70 Comparative example twenty two [N] 70 Comparative example twenty three [O] 70 Comparative example twenty four [P] 70 Comparative example 25 [Q] 70 Comparative example 26 [R] 70 Comparative example 27 [S] 70 Comparative example 28 [T] 70 Comparative example 29 [U] 70 Comparative example 30 [V] 70 Comparative example

[0058] Next, the hot-rolled steel bar is quenched using the heating temperature, cooling method, and average cooling rate shown in Table 3. In Table 3, the "Heating Temperature" item refers to the temperature during quenching. Furthermore, "Cooling Method" refers to the cooling method performed after the heating used for quenching, and "Average Cooling Rate" indicates the average cooling rate from 800°C to 300°C. Additionally, in Table 3, underlines on the values ​​indicate that the value is outside the range of the manufacturing method conditions specified in this embodiment.

[0059] [Table 3] Table 3 Products No. steel bar Quenching conditions Metal structure Evaluation results Remark steel bar No. Heating temperature (°C) Cooling methods Average cooling rate (℃ / s) Tempering Asada Santetsu Area ratio of the organization (%) Remaining tissue Old γ particle size Tensile strength (MPa) absorb energy (J) 1 1 845 oil cooling 3.1 100 - 8 1105 68 Invention Examples 2 2 845 oil cooling 2.7 100 - 8 1112 72 Invention Examples 3 3 845 oil cooling 2.6 96 Fiber ductile iron 8 1082 76 Invention Examples 4 4 845 oil cooling 2.5 95 Fiber ductile iron 8 1043 65 Invention Examples 5 5 845 oil cooling 1.5 81 Fiber ductile iron 8 1041 63 Invention Examples 6 6 845 Water cooling 9.2 100 - 8 1195 70 Invention Examples 7 7 845 oil cooling 2.7 100 - 8 1073 78 Invention Examples 8 8 845 oil cooling 2.7 100 - 8 1168 60 Invention Examples 9 9 845 oil cooling 2.7 100 - 8 1201 58 Invention Examples 10 10 845 oil cooling 2.7 82 Fiber ductile iron 8 1040 71 Invention Examples 11 11 845 oil cooling 2.7 83 Fiber ductile iron 8 1047 73 Invention Examples 12 12 845 oil cooling 2.7 81 Fiber ductile iron 8 1053 75 Invention Examples 13 13 845 oil cooling 2.7 90 Fiber ductile iron 8 1073 71 Invention Examples 14 14 845 oil cooling 2.7 95 Fiber ductile iron 8 1121 67 Invention Examples 15

[15] 845 oil cooling 6.7 100 - 8 1102 66 Reference Example 16 16

[1200] oil cooling 2.5 100 - [3] 1045 15 Comparative example 17 17

[0800] oil cooling 2.6

[75] Fiber ductile iron

[11] 945 twenty three Comparative example 18 18 845 air cooling [0.3] [0] Fiber ductile iron 8 912 twenty one Comparative example 19

[19] 845 oil cooling 2.7 100 - 8 1253 twenty four Comparative example 20

[20] 845 oil cooling 2.7 95 Fiber ductile iron 8 1012 81 Comparative example twenty one [twenty one] 845 oil cooling 2.7 100 - 8 1212 25 Comparative example twenty two [twenty two] 845 oil cooling 2.7

[77] Fiber ductile iron 8 953 twenty three Comparative example twenty three [twenty three] 845 oil cooling 2.7

[78] Residual Worsfield iron 8 920 68 Comparative example twenty four [twenty four] 845 oil cooling 2.7

[72] Fiber ductile iron 8 938 27 Comparative example 25

[25] 845 oil cooling 2.7 100 - 8 1097 twenty two Comparative example 26

[26] 845 oil cooling 2.7 100 - 8 1108 25 Comparative example 27

[27] 845 oil cooling 2.7 95 Fiber ductile iron 8 1082 20 Comparative example 28

[28] 845 oil cooling 2.7

[78] Fiber ductile iron 8 960 28 Comparative example 29

[29] 845 oil cooling 2.7

[78] Fiber ductile iron 8 937 29 Comparative example 30

[30] 845 oil cooling 2.7

[77] Fiber ductile iron 8 928 twenty one Comparative example

[0060] In the quenching process, firstly, the radial center of the steel bar is heated to the heating temperature shown in Table 3, held at this temperature for 30 minutes, and then cooled using the cooling method and average cooling rate shown in Table 3. Furthermore, the oil used for oil cooling is high-speed quench oil manufactured by Nippon Grease. The oil or water temperature during cooling is set to room temperature. Except for products No. 15 and No. 18, the average cooling rate from 800°C to 300°C is set to 1.5°C / s to 9.2°C / s. For products No. 15 and No. 18, the average cooling rates from 800°C to 300°C are set to 6.7°C / s and 0.3°C / s, respectively.

[0061] After quenching, the steel bar is heated to 540°C at the radial center, held at this temperature for 120 min, and then subjected to water-cooled tempering.

[0062] For the samples No.1 to No.30 of the quenched and tempered products obtained in the above manner, the samples were treated as the shaft of the bolt, and the microstructure of the cross section, the old γ grain size was measured, and the tensile test and Charpy impact test were performed.

[0063] Tissue observation was performed on a cross-section of the specimen perpendicular to its length. The cross-section was created by intersecting the specimen along its length. The cross-section was then etched with a 1% nitric acid etching solution and observed using an optical microscope.

[0064] The observation location was set within a 4 mm × 4 mm rectangle around the center (radial center of the sample) of the Asada sparse iron microstructure, where the cooling rate was slowest and it was difficult to obtain the microstructure. Three randomly selected fields of view were observed at 400x magnification to visually distinguish the tempered Asada sparse iron from other microstructures. The area fraction of the tempered Asada sparse iron microstructure was then calculated based on this discrimination result. The area fraction was calculated using the average of the three fields of view. The area fraction of the tempered Asada sparse iron is shown in Table 3. In Table 3, for samples where the area fraction of the tempered Asada sparse iron microstructure was not 100%, the microstructure observed outside the Asada sparse iron microstructure is shown in the "Remaining Microstructure" section.

[0065] The determination of the old γ grain size was performed on a cross-section perpendicular to the length direction, similar to the microstructural observation. Picric acid aqueous solution was used to expose the old γ grain boundaries on the cross-section, and observation was conducted using an optical microscope. The observation position was set at a distance of d / 4 from the center of the sample, with the sample diameter as d. Grain size designation was based on JIS G 0551, determined by comparison with a standard diagram. The results of the old γ grain size determination are shown in Table 3.

[0066] The tensile test was conducted according to JIS Z 2241. The specimen for the tensile test was collected at a distance of d / 4. The specimen shape was JIS size 4 (diameter of the parallel section was 14 mm). The test was performed at a tensile speed of 3 mm / min. The tensile strength was calculated by dividing the maximum load shown in the tensile test by the cross-sectional area of ​​the specimen measured before the test.

[0067] To evaluate toughness, Charpy impact tests were performed based on JIS Z 2242. The specimens (cubic prisms with a square cross-section of 10 mm on each side) used for the Charpy impact tests were collected at a position where the center of the square cross-section of the specimen was located at a distance of d / 4. The Charpy impact specimens were collected with one side of the specimen perpendicular to the radial center of the specimen (the perpendicular line extending from the side of the specimen perpendicular to the radial center of the steel overlaps with the center of the specimen). The notch shape of the Charpy impact specimen was a 2 mm V (a V-shaped notch with a depth of 2 mm). The notch was applied to the face perpendicular to the radial center of the specimen at the time of specimen collection. The Charpy impact tests were performed at -40°C. The absorbed energy obtained from these Charpy impact tests is shown in Table 3.

[0068] Of the products No. 1 to No. 30 shown in Table 3, products No. 1 to No. 14 correspond to inventive examples. Products No. 16 to No. 30 do not meet the necessary conditions specified in this embodiment and are corresponding to comparative examples. Product No. 15 is a reference example using a steel bar with a diameter of 40 mm as the material.

[0069] For applications involving bolts, a tensile strength of 1040 MPa or higher is sufficient (i.e., qualified), and an energy absorption of 27 J or higher is sufficient (i.e., qualified).

[0070] As specified in Table 3, for the specimens of Articles No. 1 to No. 14, despite their large diameter of 65 mm or more, the tensile strength is 1040 MPa or more, and the absorbed energy is 27 J or more. Therefore, they possess the strength and toughness required for bolts. Specifically, the specimens of Articles No. 1 to No. 14 have the same tensile strength and absorbed energy as specimens of Article No. 15 with a diameter of 40 mm, which is comparable to that of the prior art. Furthermore, the specimens of Articles No. 1 to No. 14 have significantly higher tensile strength and absorbed energy compared to the specimens of Articles No. 15 to No. 30, which do not meet the necessary conditions specified in this embodiment.

[0071] That is, in order to manufacture bolts with high tensile strength and toughness, it is effective to use steel bars with the chemical composition and DI value described above. Furthermore, by using bolts made from these steel bars, the necessary conditions of the original γ grain size and the area ratio of the tempered slag can be met, thus achieving high tensile strength and toughness.

[0072] Furthermore, the structures disclosed in the embodiments (including other embodiments, the same below) can be combined with the structures disclosed in other embodiments as long as they do not cause contradictions. In addition, the embodiments disclosed in this specification are illustrative, and the embodiments of the present invention are not limited thereto, and can be appropriately modified within the scope of the purpose of the present invention. [Industry availability]

[0073] This invention can be applied to steel bars, bolts, and bolt manufacturing methods.

[0074] 1: Steel bar 2: Bolts 21: Bolt head 22: Shaft S1: Steel Bar Manufacturing Steps S11: Rolling Steps S2: Intermediate Steps S21: Shipping Procedures S22: Transfer Procedure S23: Receiving Steps S3: Bolt Formation Steps S31: Cutting Step S32: Hot forging steps S33: Thread rolling process S34: Quenching Steps S35: Tempering Step

Claims

1. A bolt steel bar, comprising 0.35% by mass and 0.45% by mass of C, 0.10% by mass and 0.50% by mass of Si, 0.93% by mass and 2.00% by mass of Mn, 0.030% by mass of P, 0.030% by mass of S, 1.00% by mass and 3.00% by mass of Ni, 0.50% by mass and 2.00% by mass of Cr, and 0.1% by mass of... The composition consists of Mo, Fe and impurities of 0% by mass and less than 0.50% by mass, with a DI value of 557 or more and a diameter of 65 mm or more as expressed in the following formula (1): DI = (-23.7×[C]2+38.3×[C]+3.54)×(1+3.3×[Mn])×(1+0.7×[Si])×(1+2.16×[Cr])×(1+3×[Mo])×(1+0.36×[Ni]). (1) Here, [C], [Mn], [Si], [Cr], [Mo] and [Ni] are the contents of C, Mn, Si, Cr, Mo and Ni expressed in mass% respectively.

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