steel material

Abstract

This invention provides a steel with excellent machinability, capable of suppressing cracks during hot working and melt cracks during high-frequency quenching, and exhibiting excellent fatigue strength when used as a component in mechanical structures. The steel contains, by mass percent, C: 0.20–0.50%, Si: 0.01–0.80%, Mn: 0.50–2.00%, P: less than 0.030%, S: 0.010–0.095%, Cr: 0.01–1.30%, V: greater than 0.200% and less than or equal to 0.300%, Bi: 0.0051–0.1500%, and N: 0.0030–0.0200%, and satisfies the following formula (1). In the steel, the number density of fine Bi particles is 80–8000 particles / mm. 2 The number density of coarse Bi particles is 10 particles / mm. 2 Below. 0.80≤C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V≤1.50(1).

Description

Technical Field

[0001] This invention relates to steel, and more specifically, to steel used as blanks for mechanical structural components. Background Technology

[0002] High fatigue strength is required for mechanical structural components used in automobiles and construction vehicles, such as running gear and axles.

[0003] Furthermore, in the manufacturing process of mechanical structural components, the steel used as raw material for these components is sometimes machined. Therefore, high machinability is required for the steel used as raw material for mechanical structural components.

[0004] Steel used as blanks for mechanical structural components is disclosed, for example, in Japanese Patent Application Publication No. 57-19366 (Patent Document 1), Japanese Patent Application Publication No. 2004-18879 (Patent Document 2), and Japanese Patent Application Publication No. 2008-169411 (Patent Document 3).

[0005] Patent Document 1 describes the following: The steel disclosed in Patent Document 1 contains 0.001–0.05% Ca, 0.02–0.15% Pb and Bi (either alone or in combination), limits S to below 0.005%, and sets the inclusions as CaS-CaO, Pb, Bi-based inclusions, while suppressing Al2O3 inclusions to less than 0.001%. In this document, a large amount of Ca is continuously added to the molten steel, causing dissolved S to be converted into CaS. Furthermore, Al2O3 is eliminated or reduced to a very low level through a Ca-based reduction reaction. Therefore, the inclusions become CaS-CaO-based inclusions. Subsequently, a small amount of one or both of Pb and Bi are added to the molten steel to generate individual Pb or Bi inclusions. This improves the machinability of the steel.

[0006] Patent Document 2 discloses the following: The steel disclosed in Patent Document 2 contains, by mass percent, 0.001–0.010% B, 0.002–0.010% N, and 0.005–0.10% Bi. In this cold-forging steel, a total of 15 or more BN and B-containing Bi precipitates with a diameter of 0.7 μm or more are present in every 0.5 mm × 0.5 mm field of view of the cross-section. In this steel, by fixing N as BN, the amount of N dissolved in solid solution is reduced, thereby reducing work hardening. Furthermore, the formation of B-containing Bi precipitates improves chip handling properties.

[0007] Patent Document 3 describes the following: The steel disclosed in Patent Document 3 contains, by mass percent, C: 0.15–0.55%, Si: 0.01–2.0%, Mn: 0.01–2.5%, Cu: 0.01–2.0%, Ni: 0.01–2.0%, Cr: 0.01–2.5%, Mo: 0.01–3.0%, and the total amount of at least one element selected from the group consisting of V and W: 0.01–1.0%, with the balance consisting of Fe and unavoidable impurities. The steel is homogenized at 1010°C–1050°C, then cooled to 500°C–550°C at a cooling rate of 200°C / min or higher, then cooled to below 150°C at a cooling rate of 100°C / min or higher, and then heated in a temperature range of 550°C–700°C. The steel subjected to these heat and cooling treatments has a maximum LMP (lowest molecular weight) of 17.66 or higher, which imparts a maximum HRC hardness at room temperature. In this steel, the LMX (lowest molecular weight) is 17.66 or higher, thus increasing resistance to softening and improving thermal fatigue strength.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 57-19366

[0011] Patent Document 2: Japanese Patent Application Publication No. 2004-18879

[0012] Patent Document 3: Japanese Patent Application Publication No. 2008-169411 Summary of the Invention

[0013] The problem the invention aims to solve

[0014] Another example of the manufacturing process for a mechanical structural component using steel as raw material is as follows: The steel raw material is hot-worked to produce an intermediate product with a rough shape for the mechanical structural component. Hot working is, for example, hot forging. The intermediate product is then machined (cutting) to achieve a specified shape. The machined intermediate product is then quenched and tempered. Through these manufacturing processes, the mechanical structural component is produced.

[0015] In the quenching process described above, there are cases where the intermediate product (steel) is subjected to high-frequency quenching to improve the strength of localized parts of mechanical structural components. In this case, the area of ​​the intermediate product (steel) whose strength is to be improved is subjected to high-frequency induction heating, followed by rapid cooling (quenching).

[0016] However, during high-frequency induction heating, there is a possibility that the steel may be locally overheated due to the shape of the intermediate product (steel). Furthermore, localized melting and cracking may occur on the surface and inside of the steel. In this specification, such cracks are referred to as "melt cracks." When high-frequency quenching is performed in the manufacturing process of mechanical structural components, it is necessary to suppress melt cracks in the steel.

[0017] Furthermore, hot working (e.g., hot rolling, hot forging, etc.) is performed during the steel manufacturing process and the manufacturing process of mechanical structural components using this steel. Therefore, for steel used as blanks for mechanical structural components, it is required not only to suppress melting cracks but also to suppress cracks formed during hot working. Here, in this specification, cracks formed during hot working are also referred to as "hot working cracks." Therefore, for steel used as blanks for mechanical structural components, it is required not only to have excellent machinability and high fatigue strength when used as a mechanical structural component, but also to suppress both hot working cracks and melting cracks.

[0018] Among the aforementioned patent documents 1 to 3, at least none of them have studied the suppression of hot working cracks and the suppression of melting cracks.

[0019] The object of the present invention is to provide steel with excellent machinability, capable of suppressing cracks during hot working, capable of suppressing melting cracks during high-frequency quenching, and capable of obtaining excellent fatigue strength when used as a component for mechanical construction.

[0020] Solution for solving the problem

[0021] This invention provides a type of steel, wherein,

[0022] Its chemical composition, expressed in % by mass, contains

[0023] C: 0.20-0.50%

[0024] Si: 0.01~0.80%

[0025] Mn: 0.50~2.00%

[0026] P: below 0.030%

[0027] S: 0.010~0.095%

[0028] Cr: 0.01~1.30%

[0029] V: Greater than 0.200% and less than or equal to 0.300%

[0030] Bi: 0.0051~0.1500%

[0031] N: 0.0030~0.0200%,

[0032] The balance consists of Fe and impurities.

[0033] The chemical composition satisfies formula (1).

[0034] In the steel, the number density of fine Bi particles with a spherical equivalent diameter of 0.1–1.0 μm is 80–8000 particles / mm. 2 The number density of coarse Bi particles with a spherical equivalent diameter of 10.0 μm or more is 10 particles / mm. 2 the following,

[0035] 0.80≤C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V≤1.50(1)

[0036] In this formula, the content of each element symbol is substituted into the mass percentage.

[0037] The effects of the invention

[0038] The steel of the present invention has excellent machinability, can suppress cracks during hot working, can suppress melting cracks during high-frequency quenching, and can achieve excellent fatigue strength when used as a component for mechanical structure. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the microstructure of steel outside the scope of this invention heated to 1370°C at a heating rate of 100°C / second, held at that temperature for 15 seconds, and then water-cooled.

[0040] Figure 2 This is a schematic diagram of the microstructure of steel within the scope of this invention heated to 1370°C at a heating rate of 100°C / second and held at that temperature for 15 seconds, followed by water cooling.

[0041] Figure 3 This is a side view of the rotational bending fatigue test piece used in the embodiment. Detailed Implementation

[0042] The inventors first investigated the chemical composition of steels with excellent machinability and excellent fatigue strength when used as components in mechanical structures. As a result, the inventors concluded that steels with the following chemical composition (in mass percent): C: 0.20–0.50%, Si: 0.01–0.80%, Mn: 0.50–2.00%, P: 0.030% or less, S: 0.010–0.095%, Cr: 0.01–1.30%, V: greater than 0.200% and less than or equal to 0.300%, N: 0.0030–0.0200%, Al: 0–0.060%, Mg: 0–0.01%. 0.00%, Ti: 0-0.0200%, Nb: 0-0.0200%, W: 0-0.4000%, Zr: 0-0.2000%, Ca: 0-0.0030%, Te: 0-0.0100%, B: 0-0.0050%, Sn: 0-0.0100%, Rare Earth Elements: 0-0.0070%, Co: 0-0.0100%, Se: 0-0.0100%, Sb: 0-0.0100%, In: 0-0.0100%, Mo: 0-0.20%, Cu: 0-0.20%, Ni: 0-0.20%, and the balance consists of Fe and impurities.

[0043] However, simply setting the steel to the aforementioned chemical composition does not necessarily guarantee excellent machinability, nor does it guarantee excellent fatigue strength when used as a component in a mechanical structure. Even if the content of each element in the chemical composition is within the aforementioned range, if the steel's hardness is high, its machinability will decrease. On the other hand, even if the content of each element in the chemical composition is within the aforementioned range, if the steel's hardness is low, the fatigue strength of the mechanical structure component made from that steel will decrease. Therefore, setting the hardness of the steel used as the raw material for the mechanical structure component within an appropriate range is effective in balancing the fatigue strength of the component and the machinability of the steel.

[0044] Therefore, the inventors investigated the content of elements that affect the hardness of steel with chemical compositions within the aforementioned range. C, Si, Mn, Cr, and V, among the elements in the aforementioned chemical composition, particularly increase the internal hardness of mechanical structural components made from steel blanks, resulting in improved fatigue strength. On the other hand, S decreases internal hardness. Therefore, the inventors believed that by setting the content of these elements within an appropriate range, it is possible to balance the improvement of machinability of the steel with the improvement of fatigue strength of mechanical structural components made from steel blanks. Further research revealed that, in steel with chemical compositions within the aforementioned range, if equation (1) is satisfied, excellent machinability can be obtained in the steel, and excellent fatigue strength can be obtained when used as a mechanical structural component.

[0045] 0.80≤C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V≤1.50 (1)

[0046] In this formula, the content of each element symbol is substituted into the mass percentage.

[0047] Next, the inventors investigated a method for suppressing melt cracking during high-frequency quenching in steel with the content of each element in its chemical composition within the aforementioned range and satisfying formula (1). First, to determine the cause of melt cracking in the steel during high-frequency quenching, the inventors observed the microstructure of the areas where melt cracking occurred. The result was that no decarburization occurred at the locations where melt cracking occurred. On the other hand, no melt cracking occurred at the locations where decarburization occurred.

[0048] Based on these results, the inventors believe that the carbon content affects the formation of melt cracks in steel during high-frequency quenching. Specifically, melt cracks are more likely to occur due to carbon segregation at grain boundaries. Therefore, the inventors investigated methods to suppress carbon segregation at grain boundaries.

[0049] The inventors discovered that by further including 0.0051 to 0.1500% Bi in the aforementioned chemical composition, melting cracks in steel during high-frequency quenching can be suppressed. The reason for this is as follows: If Bi is present in an appropriate amount, it exists as an inclusion in the steel. Hereinafter, inclusions containing Bi will be referred to as Bi particles. Bi particles suppress the coarsening of austenite grains in the steel during high-frequency quenching through a pinning effect. If the Bi particles are fine, the pinning effect is enhanced. During high-frequency quenching, if the austenite grains remain fine, the grain boundary area of ​​the austenite grains increases. If the grain boundary area increases, the concentration of C segregated per unit area at the austenite grain boundaries decreases. As a result, the generation of melting cracks can be suppressed.

[0050] As described above, by including an appropriate amount of Bi, the generation of melting cracks during high-frequency quenching can be suppressed. However, it has been found that cracks sometimes occur when hot working processes are performed on steel. Hot working processes, such as hot rolling performed in the steel manufacturing process or hot forging performed in the manufacturing process of mechanical components, are examples of such processes. Therefore, the causes of cracks during hot working were investigated. As a result, the inventors obtained the following new insights.

[0051] When Bi is present in steel to suppress melt cracking, fine Bi particles (Bi inclusions) with a circumscapular equivalent diameter of less than 1.0 μm and coarse Bi particles with a circumscapular equivalent diameter of more than 10.0 μm may sometimes form in the steel. Coarse Bi particles are prone to becoming the initiation point for cracks during hot working. Therefore, if the number density of coarse Bi particles is too high, cracks (hot working cracks) are easily generated during hot working.

[0052] As mentioned above, while Bi-containing steels are relatively easy to suppress melt cracking during high-frequency quenching, they are also prone to hot-working cracking caused by coarse Bi particles. If the Bi particles in the steel are fine, melt cracking during high-frequency quenching can be suppressed. On the other hand, if the Bi particles in the steel are coarse, hot-working cracking is more likely to occur.

[0053] Based on the above research results, the inventors believe that by ensuring the number density of fine Bi particles in the steel to a certain extent while suppressing the number density of coarse Bi particles in the steel, it is possible to suppress melting cracks during high-frequency quenching and also suppress hot working cracks. Therefore, the number density of fine Bi particles and the number density of coarse Bi particles that can fully exert these effects were further investigated and studied. As a result, the inventors found that in the steel with the above chemical composition, under the premise of satisfying the above formula (1), if the number density of fine Bi particles with a circular equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm. 2 Furthermore, the number density of coarse Bi particles with a spherical equivalent diameter of 10.0 μm or more is 10 particles / mm. 2 The following methods can suppress melting cracks during high-frequency quenching and also suppress hot working cracks.

[0054] Based on the above insights, the steel in this embodiment has the following structure.

[0055] [1] A type of steel, wherein,

[0056] Its chemical composition, expressed in % by mass, contains

[0057] C: 0.20-0.50%

[0058] Si: 0.01~0.80%

[0059] Mn: 0.50~2.00%

[0060] P: below 0.030%

[0061] S: 0.010~0.095%

[0062] Cr: 0.01~1.30%

[0063] V: Greater than 0.200% and less than or equal to 0.300%

[0064] Bi: 0.0051~0.1500%

[0065] N: 0.0030~0.0200%,

[0066] The balance consists of Fe and impurities.

[0067] The chemical composition satisfies formula (1).

[0068] In the steel, the number density of fine Bi particles with a spherical equivalent diameter of 0.1–1.0 μm is 80–8000 particles / mm. 2 ,

[0069] The number density of coarse Bi particles with a spherical equivalent diameter of 10.0 μm or more is 10 particles / mm. 2 the following,

[0070] 0.80≤C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V≤1.50(1)

[0071] In this formula, the content of each element symbol is substituted into the mass percentage.

[0072] [2] According to the steel described in [1], wherein,

[0073] The chemical composition further contains one or more elements selected from the group consisting of the following elements to replace a portion of Fe.

[0074] Al: below 0.060%

[0075] Mg: less than 0.0100%

[0076] Ti: below 0.0200%

[0077] Nb: below 0.0200%

[0078] W: Below 0.4000%

[0079] Zr: below 0.2000%

[0080] Ca: below 0.0030%

[0081] Te: below 0.0100%

[0082] B: Below 0.0050%

[0083] Sn: less than 0.0100%

[0084] Rare earth elements: below 0.0070%

[0085] Co: less than 0.0100%

[0086] Se: less than 0.0100%

[0087] Sb: below 0.0100%

[0088] In: less than 0.0100%

[0089] Mo: 0.20% or less,

[0090] Cu: less than 0.20%, and

[0091] Ni: below 0.20%.

[0092] The steel used in this embodiment will now be described in detail. Unless otherwise specified, "%" for elements refers to mass percentage.

[0093] [Technical characteristics of the steel in this embodiment]

[0094] The steel used in this embodiment satisfies the following technical features 1 to 4.

[0095] (Technical Feature 1)

[0096] The chemical composition, by mass%, contains: C: 0.20–0.50%, Si: 0.01–0.80%, Mn: 0.50–2.00%, P: less than 0.030%, S: 0.010–0.095%, Cr: 0.01–1.30%, V: greater than 0.200 and less than or equal to 0.300%, Bi: 0.0051–0.1500%, N: 0.0030–0.0200%, with the balance consisting of Fe and impurities.

[0097] (Technical Feature 2)

[0098] Assuming that the content of each element is within the range of technical feature 1, equation (1) is satisfied.

[0099] 0.80≤C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V≤1.50(1)

[0100] (Technical Feature 3)

[0101] In steel, the number density of fine Bi particles with a spherical equivalent diameter of 0.1–1.0 μm is 80–8000 particles / mm. 2 .

[0102] (Technical Feature 4)

[0103] In steel, the number density of coarse Bi particles with a spherical equivalent diameter of 10.0 μm or more is 10 particles / mm. 2 the following.

[0104] The following describes each of the technical features 1 to 4.

[0105] [(Technical Feature 1) Regarding Chemical Composition]

[0106] The steel in this embodiment has the following chemical composition.

[0107] C: 0.20~0.50%

[0108] Carbon (C) increases the hardness of mechanical structural components made from steel blanks and improves the fatigue strength of these components. If the C content is less than 0.20%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0109] On the other hand, if the carbon content is greater than 0.50%, carbon will segregate at the grain boundaries even if the contents of other elements are within the range of this embodiment. In this case, the carbon concentration at the grain boundaries becomes high. If the carbon concentration is high, the melting point decreases. Therefore, melting cracks are more likely to occur during high-frequency quenching.

[0110] Therefore, the C content is 0.20–0.50%.

[0111] The preferred lower limit for the C content is 0.21%, more preferably 0.22%, and even more preferably 0.23%.

[0112] The preferred upper limit for the C content is 0.49%, more preferably 0.48%, and even more preferably 0.47%.

[0113] Si: 0.01~0.80%

[0114] Silicon (Si) deoxidizes steel during the steelmaking process. Si also increases the hardness and fatigue strength of mechanical components. If the Si content is less than 0.01%, the aforementioned effects cannot be sufficiently achieved even with the content of other elements within the range specified in this embodiment.

[0115] On the other hand, Si has a weak affinity for C. Therefore, if the Si content is greater than 0.80%, even if the contents of other elements are within the range of this embodiment, C tends to segregate at grain boundaries during heating, compared to remaining within grains containing dissolved Si. As a result, melting cracks are more likely to occur during high-frequency quenching.

[0116] Therefore, the Si content is 0.01–0.80%. The preferred lower limit of the Si content is 0.02%, more preferably 0.05%, and even more preferably 0.08%.

[0117] The preferred upper limit for the Si content is 0.75%, more preferably 0.70%, more preferably 0.65%, and even more preferably 0.60%.

[0118] Mn: 0.50~2.00%

[0119] Manganese (Mn) deoxidizes steel during the steelmaking process. Furthermore, Mn has a strong affinity for carbon (C). Therefore, during heating, C remains within the grains containing dissolved Mn. Consequently, C segregation towards grain boundaries is suppressed, thus inhibiting the formation of melt cracks during high-frequency quenching. If the Mn content is less than 0.50%, even with the contents of other elements within the range specified in this embodiment, the aforementioned effects cannot be sufficiently achieved.

[0120] On the other hand, if the Mn content is greater than 2.00%, the hardness of the steel will become excessively high, even if the contents of other elements are within the range of this embodiment. As a result, the machinability of the steel is reduced.

[0121] Therefore, the Mn content is 0.50–2.00%.

[0122] The preferred lower limit for Mn content is 0.52%, more preferably 0.55%, more preferably 0.57%, and more preferably 0.60%.

[0123] The preferred upper limit for Mn content is 1.98%, more preferably 1.95%, more preferably 1.93%, and more preferably 1.90%.

[0124] P: below 0.030%

[0125] Phosphorus (P) is an impurity. P segregates at grain boundaries. Therefore, P lowers the melting point of steel. Consequently, it easily causes melting cracks during high-frequency quenching.

[0126] Therefore, the P content is below 0.030%.

[0127] Preferably, the phosphorus (P) content is as low as possible. However, excessively low P content can increase manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit for P content is greater than 0%, more preferably 0.001%, and even more preferably 0.002%.

[0128] The preferred upper limit for the P content is 0.028%, more preferably 0.026%, more preferably 0.023%, and more preferably 0.020%.

[0129] S: 0.010~0.095%

[0130] Sulfur (S) forms sulfide inclusions, improving the machinability of steel. If the S content is less than 0.010%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0131] On the other hand, sulfur (S) lowers the melting point of steel. Therefore, if the sulfur content is greater than 0.095%, melting cracks are likely to occur during high-frequency quenching, even if the contents of other elements are within the range of this embodiment.

[0132] Therefore, the sulfur content is 0.010–0.095%.

[0133] The preferred lower limit for the sulfur content is 0.012%, more preferably 0.015%, more preferably 0.018%, and more preferably 0.020%. The preferred upper limit for the sulfur content is 0.080%, more preferably 0.075%, and more preferably 0.070%.

[0134] Cr: 0.01~1.30%

[0135] Chromium (Cr) improves the hardenability of steel. Therefore, the internal hardness of mechanical components increases. As a result, the fatigue strength of mechanical components increases. Furthermore, Cr has a strong affinity for C. Therefore, during heating, C remains within the grains containing Cr in solid solution. Thus, C segregation towards grain boundaries is suppressed, and the formation of melting cracks during high-frequency quenching can be inhibited. Cr further combines with S to form Cr sulfides. In this case, the formation of coarse FeS is suppressed. As a result, the ductility of the steel during hot working is improved, and hot working cracks are suppressed. If the Cr content is less than 0.01%, even with the contents of other elements within the range of this embodiment, the above-mentioned effects cannot be sufficiently obtained.

[0136] On the other hand, if the Cr content is greater than 1.30%, the hardness of the steel will become excessively high, even if the contents of other elements are within the range of this embodiment. As a result, the machinability of the steel is reduced.

[0137] Therefore, the Cr content is 0.01–1.30%.

[0138] The preferred lower limit for Cr content is 0.02%, more preferably 0.04%, more preferably 0.06%, and more preferably 0.08%.

[0139] The preferred upper limit for Cr content is 1.28%, more preferably 1.26%, and even more preferably 1.24%.

[0140] V: Greater than 0.200 and less than or equal to 0.300%

[0141] Vanadium (V) precipitates as V precipitates in the ferrite of steel during the cooling process following hot working in the manufacturing process of mechanical structural components. The V precipitates increase the internal hardness of the mechanical structural components, resulting in increased fatigue strength. Furthermore, V bonds with carbon, fixing carbon within the γ grains. Therefore, V suppresses the formation of melting cracks during high-frequency quenching. If the V content is 0.200% or less, even with the content of other elements within the range of this embodiment, the above-mentioned effects cannot be sufficiently obtained.

[0142] On the other hand, if the V content is greater than 0.300%, the hardness of the steel will become excessively high, even if the contents of other elements are within the range of this embodiment. As a result, the machinability of the steel decreases. Furthermore, if the V content is greater than 0.300%, the above effects saturate, and the manufacturing cost increases.

[0143] Therefore, the V content is greater than 0.200 and less than or equal to 0.300%.

[0144] The preferred lower limit for the V content is 0.205%, more preferably 0.210%, more preferably 0.215%, more preferably 0.220%, more preferably 0.225%, and more preferably 0.230%.

[0145] The preferred upper limit for the V content is 0.295%, more preferably 0.290%, and even more preferably 0.285%.

[0146] Bi: 0.0051~0.1500%

[0147] Bismuth (Bi) forms inclusions (Bi particles) in the steel. Therefore, melting cracks during high-frequency quenching are suppressed. Bi further improves the machinability of the steel. If the Bi content is less than 0.0051%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0148] On the other hand, if the Bi content is greater than 0.1500%, coarse Bi particles will be generated even if the contents of other elements are within the range of this embodiment. Coarse Bi particles are prone to becoming the starting point of cracks during hot working in the steel manufacturing process or during hot working in the manufacturing process of mechanical structural parts made from steel blanks. Therefore, hot working cracks are easily generated.

[0149] Therefore, the Bi content is 0.0051–0.1500%.

[0150] The preferred lower limit for Bi content is 0.0080%, more preferably 0.0100%, more preferably 0.0120%, more preferably 0.0140%, and more preferably 0.0160%.

[0151] The preferred upper limit for Bi content is 0.1400%, more preferably 0.1350%, and even more preferably 0.1300%.

[0152] N: 0.0030~0.0200%

[0153] Nitrogen (N) forms nitrides and / or carbonitrides during the cooling process after hot working in the manufacturing process of mechanical structural components, thereby strengthening the steel through precipitation. As a result, the fatigue strength of the mechanical structural components increases. If the N content is less than 0.0030%, even if the contents of other elements are within the range of this embodiment, the above-mentioned effect cannot be sufficiently obtained.

[0154] On the other hand, if the N content is greater than 0.0200%, the hot workability of the steel will decrease even if the contents of other elements are within the range of this embodiment.

[0155] Therefore, the nitrogen content is 0.0030–0.0200%.

[0156] The preferred lower limit for the nitrogen content is 0.0032%, more preferably 0.0034%, and even more preferably 0.0036%.

[0157] The preferred upper limit for the nitrogen content is 0.0190%, more preferably 0.0170%, more preferably 0.0150%, more preferably 0.0130%, and more preferably 0.0100%.

[0158] The remaining chemical composition of the steel in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from the ore, waste, or manufacturing environment used as raw materials during the industrial manufacturing of steel, and are permissible within the range that do not adversely affect the steel of this embodiment.

[0159] As impurities, all elements other than the aforementioned impurities (P, S) can be listed. An impurity can be only one type or two or more. Other impurities besides those listed above include, for example, O: less than 0.0050%, Ta and Zn: 0–0.01% combined, and Pb: less than 0.09%.

[0160] [Optional Elements]

[0161] The chemical composition of the steel in this embodiment may further contain one or more elements selected from the group consisting of groups 1 to 5 below. These elements are all arbitrary elements.

[0162] [Group 1]

[0163] Al: below 0.060%

[0164] Mg: less than 0.0100%

[0165] [Group 2]

[0166] Ti: below 0.0200%

[0167] Nb: below 0.0200%

[0168] W: Below 0.4000%

[0169] Zr: below 0.2000%

[0170] [Group 3]

[0171] Ca: below 0.0030%

[0172] Te: below 0.0100%

[0173] B: Below 0.0050%

[0174] Sn: below 0.0100%

[0175] Rare earth elements: below 0.0070%

[0176] [Group 4]

[0177] Co: less than 0.0100%

[0178] Se: below 0.0100%

[0179] Sb: below 0.0100%

[0180] In: below 0.0100%

[0181] [Group 5]

[0182] Mo: 0.20% or less

[0183] Cu: less than 0.20%

[0184] Ni: below 0.20%

[0185] The following explains these arbitrary elements.

[0186] [Group 1: Al and Mg]

[0187] The steel in this embodiment may also contain one or more elements selected from the group consisting of Al and Mg to replace a portion of the Fe. These elements are arbitrary and all deoxidize the steel.

[0188] Al: below 0.060%

[0189] Aluminum (Al) can be any element, or it can be absent. That is to say, the Al content can be 0%.

[0190] In the presence of Al, Al deoxidizes steel. Even a small amount of Al can achieve this effect to some extent.

[0191] However, if the Al content is greater than 0.060%, even if the contents of other elements are within the range of this embodiment, Al will form coarse oxides. Coarse oxides will reduce the fatigue strength of mechanical structural components.

[0192] Therefore, the Al content is 0 to 0.060%, and in the case of Al, the Al content is less than 0.060%.

[0193] The preferred lower limit for Al content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.010%.

[0194] The preferred upper limit for the Al content is 0.055%, more preferably 0.050%, and even more preferably 0.045%.

[0195] Mg: less than 0.0100%

[0196] Magnesium (Mg) can be any element, or it can be absent. In other words, the Mg content can be 0%.

[0197] In the presence of Mg, Mg deoxidizes steel. Even a small amount of Mg can achieve this effect to some extent.

[0198] However, if the Mg content is greater than 0.0100%, Mg will form coarse oxides even if the contents of other elements are within the range of this embodiment. Coarse oxides will reduce the fatigue strength of mechanical structural components.

[0199] Therefore, the Mg content is 0 to 0.0100%, and in the case of Mg, the Mg content is below 0.0100%.

[0200] The preferred lower limit for Mg content is greater than 0%, more preferably 0.0001%, more preferably 0.0003%, and more preferably 0.0005%.

[0201] The preferred upper limit for Mg content is 0.0050%, more preferably 0.0045%, and even more preferably 0.0040%.

[0202] [Group 2: Ti, Nb, W, and Zr]

[0203] The chemical composition of the steel in this embodiment may further include one or more elements selected from the group consisting of Ti, Nb, W, and Zr to replace a portion of the Fe. These elements are arbitrary and all form precipitates, improving the toughness of mechanical structural components.

[0204] Ti: below 0.0200%

[0205] Titanium (Ti) can be any element, or it can be absent. That is, the Ti content can be 0%.

[0206] In the presence of Ti, Ti forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of mechanical structural components, thereby refining the grain size. This results in increased toughness of the mechanical structural components. The above effects can be achieved to a certain extent even with only a small amount of Ti.

[0207] However, if the Ti content is greater than 0.0200%, the above-mentioned effects will saturate even if the contents of other elements are within the range of this embodiment. Furthermore, manufacturing costs will increase.

[0208] Therefore, the Ti content is 0 to 0.0200%, and when Ti is present, the Ti content is below 0.0200%.

[0209] The preferred lower limit for Ti content is greater than 0%, more preferably 0.0001%, more preferably 0.0010%, more preferably 0.0050%, and more preferably 0.0080%.

[0210] The preferred upper limit for the Ti content is 0.0180%, more preferably 0.0170%, and even more preferably 0.0150%.

[0211] Nb: below 0.0200%

[0212] Niobium (Nb) can be any element, or it can be absent. That is, the Nb content can be 0%.

[0213] In the presence of Nb, Nb forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of mechanical structural components, thereby refining the grain size. This results in increased toughness of the mechanical structural components. The above-mentioned effects can be achieved to a certain extent even with only a small amount of Nb.

[0214] However, if the Nb content is greater than 0.0200%, the above-mentioned effects will saturate even if the contents of other elements are within the range of this embodiment. Furthermore, manufacturing costs will increase.

[0215] Therefore, the Nb content is 0 to 0.0200%, and in the case of Nb, the Nb content is below 0.0200%.

[0216] The preferred lower limit for Nb content is greater than 0%, more preferably 0.0001%, more preferably 0.0010%, more preferably 0.0050%, and more preferably 0.0080%.

[0217] The preferred upper limit for Nb content is 0.0180%, more preferably 0.0170%, and even more preferably 0.0150%.

[0218] W: Below 0.4000%

[0219] Tungsten (W) can be any element, or it can be absent. That is to say, the W content can be 0%.

[0220] In the presence of W, W forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of mechanical structural components, thereby refining the grain size. As a result, the toughness of the mechanical structural components increases. The above-mentioned effects can be obtained to a certain extent even with only a small amount of W.

[0221] However, if the W content is greater than 0.4000%, the above-mentioned effects will saturate even if the contents of other elements are within the range of this embodiment. Furthermore, manufacturing costs will increase.

[0222] Therefore, the W content is 0 to 0.4000%, and when W is present, the W content is below 0.4000%.

[0223] The preferred lower limit for W content is greater than 0%, more preferably 0.0001%, more preferably 0.0050%, and more preferably 0.0500%.

[0224] The preferred upper limit for W content is 0.3500%, more preferably 0.3000%, and even more preferably 0.2000%.

[0225] Zr: below 0.2000%

[0226] Zirconium (Zr) can be any element, or it can be absent. That is, the Zr content can be 0%.

[0227] In the presence of Zr, Zr forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of mechanical structural components, thereby refining the grain size. As a result, the toughness of the mechanical structural components increases. The above-mentioned effects can be achieved to a certain extent even with only a small amount of Zr.

[0228] However, if the Zr content is greater than 0.2000%, the above-mentioned effects will saturate even if the contents of other elements are within the range of this embodiment. Furthermore, manufacturing costs will increase.

[0229] Therefore, the Zr content is 0 to 0.2000%, and in the case of Zr, the Zr content is below 0.2000%.

[0230] The preferred lower limit for Zr content is greater than 0%, more preferably 0.0001%, more preferably 0.0010%, more preferably 0.0020%, and more preferably 0.0050%.

[0231] The preferred upper limit for Zr content is 0.1500%, more preferably 0.1000%, more preferably 0.0500%, and more preferably 0.0100%.

[0232] [Group 3: Ca, Te, B, Sn and rare earth elements]

[0233] The chemical composition of the steel in this embodiment may further include one or more elements selected from the group consisting of Ca, Te, B, Sn, and rare earth elements (REM) to replace a portion of the Fe. These elements are arbitrary and all improve the machinability of the steel.

[0234] Ca: below 0.0030%

[0235] Calcium (Ca) can be any element, or it may not contain calcium (Ca). That is to say, the Ca content can be 0%.

[0236] In the presence of calcium (Ca), Ca improves the machinability of steel. Even a small amount of Ca can achieve this effect to some extent.

[0237] However, if the Ca content is greater than 0.0030%, coarse oxides will form even if the contents of other elements are within the range of this embodiment. In this case, the fatigue strength of the mechanical components decreases.

[0238] Therefore, the Ca content is 0 to 0.0030%, and in the case of Ca, the Ca content is below 0.0030%.

[0239] The preferred lower limit for Ca content is greater than 0%, more preferably 0.0001%, more preferably 0.0010%, and more preferably 0.0015%.

[0240] The preferred upper limit for Ca content is 0.0025%, more preferably 0.0023%, and even more preferably 0.0020%.

[0241] Te: below 0.0100%

[0242] Tellurium (Te) can be any element, or the container can be free of tellurium (Te). In other words, the Te content can be 0%.

[0243] In the presence of Te, Te improves the machinability of steel. Even a small amount of Te can achieve this effect to some extent.

[0244] However, if the Te content is greater than 0.0100%, hot working cracks are likely to occur in the steel even if the contents of other elements are within the range of this embodiment.

[0245] Therefore, the Te content is 0 to 0.0100%, and when Te is present, the Te content is less than 0.0100%.

[0246] The preferred lower limit for Te content is greater than 0%, more preferably 0.0001%, more preferably 0.0003%, and more preferably 0.0010%.

[0247] The preferred upper limit for the Te content is 0.0090%, more preferably 0.0085%, and even more preferably 0.0080%.

[0248] B: Below 0.0050%

[0249] Boron (B) can be any element, or the element may be absent. In other words, the B content can be 0%.

[0250] In the presence of boron (B), B improves the machinability of steel. Even a small amount of B can achieve this effect to some extent.

[0251] However, if the B content is greater than 0.0050%, hot working cracks are likely to occur in the steel even if the contents of other elements are within the range of this embodiment.

[0252] Therefore, the B content is 0 to 0.0050%, and in the case of B, the B content is less than 0.0050%.

[0253] The preferred lower limit for the B content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0010%.

[0254] The preferred upper limit for the B content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0255] Sn: below 0.0100%

[0256] Tin (Sn) can be any element, or it can be absent. That is, the Sn content can be 0%.

[0257] In the presence of Sn, Sn improves the machinability of steel. Even a small amount of Sn can achieve this effect to some extent.

[0258] However, if the Sn content is greater than 0.0100%, hot working cracks are likely to occur in the steel even if the contents of other elements are within the range of this embodiment.

[0259] Therefore, the Sn content is 0 to 0.0100%, and in the case of Sn, the Sn content is below 0.0100%.

[0260] The preferred lower limit for Sn content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0010%.

[0261] The preferred upper limit for the Sn content is 0.0095%, more preferably 0.0090%, more preferably 0.0085%, and even more preferably 0.0080%.

[0262] Rare earth elements: below 0.0070%

[0263] Rare earth elements (REM) can be any element, or the room may not contain any rare earth elements (REM). In other words, the REM content can be 0%.

[0264] When REM is present, it improves the machinability of steel. Even a small amount of REM can achieve this effect to some extent.

[0265] However, if the REM content is greater than 0.0070%, hot working cracks are likely to occur in the steel even if the contents of other elements are within the range of this embodiment.

[0266] Therefore, the REM content is 0 to 0.0070%, and in the case of REM, the REM content is below 0.0070%.

[0267] The preferred lower limit for REM content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0010%.

[0268] The preferred upper limit for REM content is 0.0065%, more preferably 0.0060%, and even more preferably 0.0055%.

[0269] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. Furthermore, the REM content in this specification refers to the total content of these elements.

[0270] [Group 4: Co, Se, Sb, and In]

[0271] The chemical composition of the steel in this embodiment may further contain one or more elements selected from the group consisting of Co, Se, Sb, and In to replace a portion of the Fe. These elements are arbitrary and all inhibit decarburization of the steel.

[0272] Co: less than 0.0100%

[0273] Cobalt (Co) can be any element, or it can be absent. That is to say, the Co content can be 0%.

[0274] In the presence of Co, Co inhibits decarburization of steel during hot working. Even a small amount of Co can achieve this effect to some extent.

[0275] However, if the Co content is greater than 0.0100%, hot working cracks are likely to occur in the steel even if the contents of other elements are within the range of this embodiment.

[0276] Therefore, the Co content is 0 to 0.0100%, and in the case of Co, the Co content is below 0.0100%.

[0277] The preferred lower limit for the Co content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0010%.

[0278] The preferred upper limit for the Co content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0279] Se: below 0.0100%

[0280] Selenium (Se) can be any element, or it can be absent. That is to say, the Se content can be 0%.

[0281] In the presence of selenium (Se), Se inhibits decarburization of steel during hot working. Even a small amount of Se can achieve this effect to some extent.

[0282] However, if the Se content is greater than 0.0100%, hot working cracks are likely to occur in the steel even if the contents of other elements are within the range of this embodiment.

[0283] Therefore, the Se content is 0 to 0.0100%, and in the case of Se, the Se content is less than 0.0100%.

[0284] The preferred lower limit for Se content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0010%.

[0285] The preferred upper limit for the Se content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0286] Sb: below 0.0100%

[0287] Antimony (Sb) can be any element, or it can be absent. That is to say, the Sb content can be 0%.

[0288] In the presence of Sb, that is, when the Sb content is greater than 0%, Sb inhibits decarburization of steel during hot working. Even a small amount of Sb can achieve this effect to some extent.

[0289] However, if the Sb content is greater than 0.0100%, hot working cracks are likely to occur in the steel even if the contents of other elements are within the range of this embodiment.

[0290] Therefore, the Sb content is 0 to 0.0100%, and in the case of Sb, the Sb content is below 0.0100%.

[0291] The preferred lower limit for Sb content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0010%.

[0292] The preferred upper limit for Sb content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0293] In: below 0.0100%

[0294] Indium (In) can be any element, or it can be absent. That is to say, the In content can be 0%.

[0295] In the presence of in, in inhibits decarburization of steel during hot working. Even a small amount of in can achieve this effect to some extent.

[0296] However, if the In content is greater than 0.0100%, hot working cracks are likely to occur in the steel even if the contents of other elements are within the range of this embodiment.

[0297] Therefore, the In content is 0 to 0.0100%, and in the case of In, the In content is less than 0.0100%.

[0298] The preferred lower limit for the In content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, and more preferably 0.0010%.

[0299] The preferred upper limit for the In content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0300] [Group 5: Mo, Cu, and Ni]

[0301] The chemical composition of the steel in this embodiment may further include one or more elements selected from the group consisting of Mo, Cu, and Ni to replace a portion of the Fe. These elements are arbitrary and all improve the fatigue strength of mechanical structural components.

[0302] Mo: 0.20% or less

[0303] Molybdenum (Mo) can be any element, or it can be absent. In other words, the Mo content can be 0%.

[0304] In the presence of Mo, Mo improves the fatigue strength of mechanical structural components. This effect can be achieved to some extent even with only a small amount of Mo.

[0305] However, if the Mo content is greater than 0.20%, the hardness of the steel will become excessively high, even if the contents of other elements are within the range of this embodiment. As a result, hot workability is reduced.

[0306] Therefore, the Mo content is 0 to 0.20%, and in the case of Mo, the Mo content is less than 0.20%.

[0307] The preferred lower limit for Mo content is greater than 0%, more preferably 0.01%, more preferably 0.05%, and more preferably 0.10%.

[0308] The preferred upper limit for the Mo content is 0.19%, more preferably 0.17%, and even more preferably 0.15%.

[0309] Cu: less than 0.20%

[0310] Copper (Cu) can be any element, or it can be absent from the container. In other words, the Cu content can be 0%.

[0311] In the presence of Cu, Cu improves the fatigue strength of mechanical structural components. This effect can be achieved to some extent even with only a small amount of Cu.

[0312] However, similar to Si, Cu promotes the formation of melt cracks during high-frequency quenching. Therefore, if the Cu content is greater than 0.20%, melt cracks are easily generated during high-frequency quenching even if the contents of other elements are within the range of this embodiment.

[0313] Therefore, the Cu content is 0 to 0.20%, and in the case of Cu, the Cu content is less than 0.20%.

[0314] The preferred lower limit for Cu content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.03%.

[0315] The preferred upper limit for the Cu content is 0.15%, more preferably 0.13%, and even more preferably 0.10%.

[0316] Ni: below 0.20%

[0317] Nickel (Ni) can be any element, or it can be absent. That is to say, the Ni content can be 0%.

[0318] In the presence of Ni, Ni improves the fatigue strength of mechanical structural components. Even a small amount of Ni can achieve this effect to some extent.

[0319] However, similar to Si and Cu, Ni promotes the formation of melt cracks during high-frequency quenching. Therefore, if the Ni content is greater than 0.20%, melt cracks are easily generated during high-frequency quenching even if the contents of other elements are within the range of this embodiment.

[0320] Therefore, the Ni content is 0 to 0.20%, and in the case of Ni, the Ni content is less than 0.20%.

[0321] The preferred lower limit for Ni content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.03%.

[0322] The preferred upper limit for Ni content is 0.15%, more preferably 0.13%, and even more preferably 0.10%.

[0323] [(Technical Feature 2) Regarding Equation (1)]

[0324] The steel in this embodiment is further provided that the content of each element is within the above-mentioned range, that is, it satisfies formula (1) on the premise of satisfying technical feature 1.

[0325] 0.80≤C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V≤1.50 (1)

[0326] In this formula, the content of each element symbol is substituted into the mass percentage.

[0327] Defined as fn1 = C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V. fn1 is a hardness index of steel. C, Si, Mn, Cr, and V increase the internal hardness of mechanical structural components made from steel. On the other hand, S decreases the internal hardness of mechanical structural components.

[0328] Even if the content of each element in the steel is within the range of this embodiment, if fn1 is less than 0.80, the internal hardness of the mechanical component will be excessively reduced. As a result, the fatigue strength of the mechanical component will decrease. On the other hand, even if the content of each element in the steel is within the range of this embodiment, if fn1 is greater than 1.50, the hardness of the steel will be excessively increased. As a result, the machinability of the steel will decrease.

[0329] Therefore, fn1 is 0.80 to 1.50.

[0330] The preferred lower limit for fn1 is 0.81, more preferably 0.82, and even more preferably 0.85.

[0331] The preferred upper limit for fn1 is 1.48, further preferably 1.45, and even more preferably 1.43.

[0332] [(Technical Feature 3) Regarding Fine Bi Particles]

[0333] In the steel of this embodiment, provided that the content of each element is within the above range and satisfies formula (1), the number density of fine Bi particles (hereinafter also referred to as fine Bi particles) with a circular equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm. 2 If the number density of fine Bi particles is 80–8000 particles / mm 2 This can suppress the generation of melting cracks during high-frequency quenching.

[0334] Bi exists in steel as Bi monomer particles or as particles containing high concentrations of Bi. In this specification, Bi monomer particles or particles containing high concentrations of Bi are collectively defined as Bi particles. In this specification, fine Bi particles refer to Bi particles with a spherical equivalent diameter of 0.1 to 1.0 μm. Bi is a heavy element; therefore, in reflected electron microscopy, Bi particles exhibit a high brightness compared to their surroundings. Fine Bi particles can also exist independently in steel without contact with other particles (precipitates or inclusions). Alternatively, fine Bi particles can exist in steel by adhering to or contacting other particles.

[0335] As described above, during high-frequency induction heating, Bi particles pin the austenite grain boundaries. If the equivalent diameter of the Bi particles is 0.1–1.0 μm, the pinning effect on the austenite grain boundaries is enhanced. During high-frequency quenching, if the austenite grains remain fine, the grain boundary area of ​​the austenite grains increases. If the grain boundary area increases, the concentration of C segregated at the grain boundaries decreases. As a result, the generation of melt cracks can be suppressed. Even if the content of each element in the chemical composition of the steel is within the range of this embodiment and satisfies formula (1), and the number of coarse Bi particles with an equivalent diameter of 10.0 μm or more is 10 / mm. 2 In the following cases, if the fine Bi particles are less than 80 per mm 2 Otherwise, the above effects cannot be fully achieved.

[0336] On the other hand, even if the content of each element in the chemical composition of the steel is within the range of this embodiment and satisfies formula (1), and the number of coarse Bi particles with a spherical equivalent diameter of 10.0 μm or more is 10 / mm. 2 In the following cases, if the number density of fine Bi particles is greater than 8000 particles / mm², 2 If this happens, the above effects will saturate, and manufacturing costs will increase.

[0337] Therefore, in the steel of this embodiment, the number density of fine Bi particles with a spherical equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm. 2 .

[0338] The preferred lower limit for the number density of fine Bi particles is 90 particles / mm. 2 The further preferred lower limit is 95 pieces / mm. 2 The further preferred lower limit is 100 pieces / mm. 2 .

[0339] The preferred upper limit for the number density of fine Bi particles is 7900 particles / mm. 2 Further optimized to 6000 pieces / mm 2 Further optimized to 3000 pieces / mm 2 Further optimized to 1000 pieces / mm 2 Further optimized to 900 pieces / mm 2 Further optimized to 800 pieces / mm 2 .

[0340] [(Technical Feature 4) Regarding coarse Bi particles]

[0341] In the steel of this embodiment, the number density of coarse Bi particles (hereinafter also referred to as coarse Bi particles) with a circumference equivalent diameter of 10.0 μm or more is 10 particles / mm. 2 The following applies if the number density of coarse Bi particles is 10 particles / mm. 2 The following methods can suppress cracks (hot working cracks) that occur during hot working in the manufacturing process of steel or during hot working in the manufacturing process of mechanical structural components made from steel blanks. Hot working includes, for example, hot rolling and hot forging.

[0342] In this specification, coarse Bi particles refer to Bi particles with a circumscaping equivalent diameter of 10.0 μm or more. In the number density measurement method for coarse Bi particles described later, if a particle has a circumscaping equivalent diameter of 10.0 μm or more and is observed to exhibit a high brightness higher than its surroundings, then the particle is identified as a coarse Bi particle. Coarse Bi particles may also exist independently in steel without contact with other particles (precipitates or inclusions). Alternatively, coarse Bi particles may exist in steel by adhering to or contacting other particles. There is no particular upper limit to the circumscaping equivalent diameter of coarse Bi particles, but in the case of the chemical composition of this embodiment, the upper limit for the circumscaping equivalent diameter of coarse Bi particles is 50.0 μm.

[0343] As mentioned above, fine Bi particles in steel suppress melting cracks during high-frequency quenching. However, Bi in steel sometimes forms coarse Bi particles instead of fine ones. These coarse Bi particles can become the initiation point for hot-working cracks in steel.

[0344] Even though the content of each element in the chemical composition of the steel is within the range of this embodiment, and satisfies formula (1), and the fine Bi particles are 80 to 8000 per mm. 2 In the case where the number of coarse Bi particles is greater than 10 / mm 2 Sometimes, hot working cracks may occur in the steel.

[0345] Therefore, in the steel of this embodiment, the number density of coarse Bi particles with a spherical equivalent diameter of 10.0 μm or more is 10 particles / mm. 2 the following.

[0346] The preferred upper limit for the number density of coarse Bi particles is 8 particles / mm. 2 Further preferred is 7 per mm 2 Further preferred is 6 per mm 2 Further preferred is 5 per mm 2 .

[0347] Preferably, the number density of coarse Bi particles is as low as possible. That is, the number density of coarse Bi particles is preferably 0 particles / mm. 2 However, excessively reducing the number density of coarse Bi particles would increase manufacturing costs. Therefore, considering typical industrial productivity, the preferred lower limit for the number density of coarse Bi particles is 1 particle / mm. 2 Further preferably two / mm 2 .

[0348] Furthermore, in the steel of this embodiment, not only the aforementioned fine Bi particles and coarse Bi particles are sometimes present, but also intermediate Bi particles (hereinafter also referred to as intermediate Bi particles) with a size greater than 1.0 μm and less than 10.0 μm. However, intermediate Bi particles are unlikely to affect hot working cracks and melt cracks during high-frequency quenching. Therefore, intermediate Bi particles can be disregarded in the suppression of hot working cracks and melt cracks.

[0349] [Methods for measuring the number density of fine and coarse Bi particles]

[0350] The number density of fine Bi particles and coarse Bi particles can be measured by the following method.

[0351] A test piece containing steel (bar) is collected from a cross-section perpendicular to the axial direction (rolling direction), specifically the R / 2 portion. Here, R / 2 refers to the central portion of the cross-section perpendicular to the axial direction of the steel. The surface of the collected test piece corresponding to the cross-section perpendicular to the axial direction of the steel is used as the observation surface.

[0352] The observation surface was mirror-polished. Using a scanning electron microscope (SEM), the R / 2 portion of the mirror-polished observation surface was observed in 20 fields of view at 1000x magnification. The area of ​​each field of view was set to 100μm × 120μm.

[0353] Based on reflected electron images obtained from various fields of view through SEM observation, the number density of coarse and fine Bi particles was investigated using known image analysis particle analysis methods. Specifically, the particles in the steel were determined based on the interface between the parent phase and the particles. Here, particles are referred to as inclusions or precipitates. Image analysis was performed to determine the circular equivalent diameter of the determined particles. Specifically, the area of ​​each determined particle was determined. The diameter of the circle with the same area as the determined area was set as the circular equivalent diameter (μm) of that particle.

[0354] As mentioned above, Bi is a heavy element, and therefore, it exhibits high brightness in electron reflectance images. Therefore, particles with a circumspherical equivalent diameter of 0.1–1.0 μm observed in electron reflectance images obtained through SEM observation, and which exhibit high brightness compared to their surroundings, are identified as fine Bi particles. Conversely, particles with a circumspherical equivalent diameter of 10.0 μm or more observed in electron reflectance images obtained through SEM observation, and which exhibit high brightness compared to their surroundings, are identified as coarse Bi particles.

[0355] Furthermore, in the embodiments described later, the chemical composition of the fine and coarse Bi particles determined by the above method was analyzed using an energy dispersive X-ray spectroscopy (EDX) apparatus of a SEM. The results confirmed that they were all Bi particles. Additionally, the EDX beam diameter used for confirmation was 0.1–1.0 μm.

[0356] Fine and coarse Bi particles were determined using the method described above. The total number of fine Bi particles identified in each field of view and the total area of ​​the 20 fields of view (0.24 mm) were used as the basis for the determination. 2 ), calculate the number of fine Bi particles per unit area (particles / mm). 2Additionally, based on the total number of coarse Bi particles determined in each field of view and the total area of ​​the 20 fields of view (0.24 mm), 2 ), calculate the number of coarse Bi particles per unit area (particles / mm). 2 ).

[0357] [The effect of the steel in this embodiment]

[0358] As described above, the steel of this embodiment satisfies technical features 1 to 4. Therefore, the steel of this embodiment exhibits excellent machinability, can suppress cracks during hot working and melt cracks during high-frequency quenching, and possesses excellent fatigue strength when used as a component for mechanical structures. These effects will be explained below.

[0359] [Preferred uses of the steel in this embodiment]

[0360] The steel of this embodiment can be widely used, for example, as a blank for mechanical structural components. The steel of this embodiment is particularly suitable for applications where high-frequency quenching is performed during the manufacturing process of mechanical structural components. However, even without high-frequency quenching, the steel of this embodiment can be used as a blank for mechanical structural components.

[0361] [Manufacturing Method]

[0362] An example of the steel manufacturing method according to this embodiment is as follows. The steel manufacturing method according to this embodiment includes a refining process, a casting process, and a hot working process. The hot working process is optional and may be omitted. Each process will be described below.

[0363] An example of the steel manufacturing method of this embodiment includes the following steps. Furthermore, step 3 is arbitrary and may be omitted.

[0364] (Process 1) Refining Process

[0365] (Process 2) Casting process

[0366] (Process 3) Hot working process

[0367] The following describes each process.

[0368] [(Process 1) Refining Process]

[0369] In the refining process, molten steel with a chemical composition that satisfies technical characteristics 1 and 2 described above is produced. The refining process includes a primary refining process and a secondary refining process.

[0370] In a primary refining process, molten iron produced by known methods is refined in a converter. In a secondary refining process, alloying elements are added to the molten steel to ensure that its chemical composition meets technical characteristics 1 and 2. Specifically, in the secondary refining process, the composition of the molten steel, except for Bi, is adjusted while stirring it using known refining methods. Then, while stirring the molten steel, Bi is added to it via a wire to adjust the Bi composition.

[0371] In the secondary refining process, the following conditions must be met.

[0372] (condition)

[0373] The time T from the addition of Bi to the molten steel to the end of stirring in the secondary refining process is set to be greater than 15 minutes and less than 60 minutes.

[0374] In the secondary refining process, the time from the addition of Bi to the end of the stirring in the secondary refining process is greater than 15 minutes and less than 60 minutes.

[0375] If the time between adding Bi and the end of stirring in the secondary refining process is less than 15 minutes, Bi does not diffuse sufficiently in the molten steel. In this case, excessive coarse Bi particles will be generated in the steel.

[0376] When the time between adding Bi and the end of stirring in the secondary refining process is more than 60 minutes, fine Bi particles tend to aggregate. Therefore, the number density of fine Bi particles decreases.

[0377] In the secondary refining process, if the time from the addition of Bi to the end of stirring in the secondary refining process is greater than 15 minutes, Bi diffuses sufficiently in the molten steel. Therefore, fine Bi particles are sufficiently generated in the steel. Furthermore, if the time from the addition of Bi to the end of stirring in the secondary refining process is less than 60 minutes, the aggregation of fine Bi particles is effectively suppressed. Therefore, the number density of fine Bi particles is 80 particles / mm². 2 The number density of coarse Bi particles is 10 particles / mm. 2 the following.

[0378] The preferred upper limit of the time from the addition of Bi to the molten steel until the end of stirring in the secondary refining process is 50 minutes, more preferably 40 minutes. The preferred lower limit of the time from the addition of Bi to the end of stirring in the secondary refining process is 20 minutes, more preferably 30 minutes.

[0379] In addition, the temperature of the molten steel from the addition of Bi until the end of stirring in the secondary refining process is 1510 to 1560°C.

[0380] [(Process 2) Casting Process]

[0381] In the casting process, molten steel is used to manufacture billets (slabs or large billets) or ingots (cast ingots) using known casting methods. Examples of casting methods include continuous casting and ingot casting.

[0382] [(Process 3) Hot working process]

[0383] The heat treatment process is arbitrary. That is to say, a heat treatment process may or may not be performed.

[0384] In the case of implementing a hot working process, the steel of this embodiment is manufactured by hot working the billet or ingot produced by the above-described casting process. The steel of this embodiment is, for example, bar steel. The hot working process can be, for example, hot rolling or hot forging.

[0385] When hot rolling is performed in a hot working process, it can be, for example, only a roughing rolling process, or a roughing rolling process and a finishing rolling process. The roughing rolling process is, for example, a primary rolling. The finishing rolling process is, for example, a finishing mill using a continuous rolling mill. In a continuous rolling mill, for example, a horizontal mill with a pair of horizontal rolls and a vertical mill with a pair of vertical rolls are arranged alternately in a row. The heating temperature in the roughing and finishing rolling processes is, for example, 1000–1300°C.

[0386] The steel of this embodiment is manufactured through the above manufacturing processes. As described above, the hot working process can also be omitted in this manufacturing method. That is, the steel of this embodiment can be a casting (billet or ingot). Alternatively, a hot working process can be performed to manufacture the steel of this embodiment.

[0387] [Manufacturing methods for mechanical structural components]

[0388] As described above, the steel used in this embodiment serves as a blank for mechanical structural components. These mechanical structural components are, for example, components used in automobiles. Examples of mechanical structural components include running gear, axles, and crankshafts.

[0389] Mechanical structural components using the steel of this embodiment as a blank are manufactured, for example, by the following known manufacturing method.

[0390] First, the steel used in this embodiment is hot-processed to produce an intermediate product with a rough shape for a mechanical component. Hot processing, for example, is hot forging. The intermediate product is then machined into a predetermined shape. The machined intermediate product is then subjected to high-frequency quenching and tempering. Through these processes, the mechanical component is manufactured.

[0391] In the steel of this embodiment, the content of each element in the chemical composition is within the range of this embodiment and satisfies equation (1). Furthermore, the number density of fine Bi particles with a spherical equivalent diameter of 0.1–1.0 μm is 80–8000 particles / mm. 2 The number density of coarse Bi particles with a spherical equivalent diameter of 10.0 μm or more is 10 particles / mm. 2 In other words, the steel of this embodiment satisfies technical features 1 to 4. Therefore, excellent machinability can be obtained in the steel of this embodiment. Furthermore, excellent fatigue strength can be obtained in mechanical structural components manufactured using the steel of this embodiment as a blank. Moreover, hot working cracks can be suppressed during the manufacturing process of the steel or the mechanical structural component. Furthermore, when manufacturing mechanical structural components using the steel of this embodiment as a blank, even when high-frequency quenching is performed, melting cracks can be suppressed.

[0392] Example

[0393] The effects of the steel in this embodiment are further illustrated through examples. The conditions in the following examples are examples used to confirm the feasibility and effectiveness of the steel in this embodiment. Therefore, the steel in this embodiment is not limited to this single example.

[0394] Steels with the chemical compositions shown in Tables 1 to 4 were manufactured using the following manufacturing processes. Furthermore, in all test numbers, the O (oxygen) content was 0.0050% or less by mass. Additionally, the total Ta and Zn content was 0 to 0.01% by mass. Furthermore, the Pb content was 0.09% or less by mass.

[0395] [Table 1]

[0396] sheet

[0397]

[0398] [Table 2]

[0399] Table 2

[0400]

[0401] [Table 3]

[0402] sheet

[0403]

[0404] [Table 4]

[0405] sheet

[0406]

[0407] The "-" part in Tables 1 to 4 indicates that the corresponding element content is 0% in the significant figures (up to the least significant digit) specified in the embodiments. In other words, it means that the corresponding element content is 0% after rounding the last digit of the significant figures (up to the least significant digit) specified in the above embodiments.

[0408] For example, the Mo content specified in this embodiment is defined as a value to two decimal places. Therefore, test number 1 in Table 2 refers to a measured Mo content of 0% after rounding to the third decimal place.

[0409] Furthermore, the Mg content specified in this embodiment is defined as a value to four decimal places. Therefore, test number 1 in Table 1 refers to a measured Mg content of 0% after rounding to the fifth decimal place.

[0410] In addition, rounding means that if the next digit after the least specified digit (the last digit) is less than 5, it is discarded; if it is 5 or more, it is carried over.

[0411] Molten steel was used, and a 70-ton converter was used to perform refining processes (primary refining and secondary refining). In the primary refining process, molten iron produced using known methods was refined in a converter. In the secondary refining process, alloying elements were added to produce molten steel with the chemical composition of the steel of this embodiment. Specifically, secondary refining was performed using known methods, and the composition of the molten steel, except for Bi, was adjusted while stirring the molten steel. Subsequently, Bi was added using a wire while stirring the molten steel, and the Bi composition was adjusted. The time T (minutes) from the addition of Bi in the secondary refining process to the end of stirring in the secondary refining process is shown in Tables 5 and 6. Furthermore, the temperature of the molten steel from the addition of Bi to the end of stirring was 1510–1560°C. A billet with a cross-section of 300 mm × 400 mm (large square billet) was produced by continuous casting.

[0412] After heating the cast billet, it is first rolled to produce a small square billet with a cross-section of 180mm × 180mm. After heating the small square billet to 1250℃, it is hot forged to produce steel bars (bars) with a diameter of 80mm and the chemical composition shown in Table 1.

[0413] Table 5

[0414]

[0415] Table 6

[0416] Table 6

[0417]

[0418] [Manufacturing of intermediate parts for simulating mechanical structures]

[0419] A simulated heat treatment process was implemented for hot forging in the manufacturing process of mechanical structural components made from steel blanks. Specifically, the steel was heated to 1100°C and held at that temperature for 30 minutes. Afterward, the steel was allowed to cool naturally in the atmosphere, producing intermediate products for the simulated mechanical structural components. The intermediate products for each test number of the simulated mechanical structural components were steel bars with a diameter of 80 mm.

[0420] [Evaluation Test]

[0421] The following evaluation tests were conducted on the steel for each test number and the intermediate products of the simulated mechanical structure components for each test number.

[0422] (Experiment 1) Evaluation Test of Hot Working Cracks

[0423] (Experiment 2) Number density measurement of fine Bi particles and coarse Bi particles

[0424] (Experiment 3) Evaluation Test of Melt Crack

[0425] (Experiment 4) Cutting performance test (drill bit life test)

[0426] (Experiment 5) Fatigue Strength Evaluation Test (Rotational Bending Fatigue Test)

[0427] The following describes Experiments 1 through 5.

[0428] [(Experiment 1) Evaluation Test for Hot Working Cracks]

[0429] The surface of the manufactured steel was visually inspected. Based on the results of the visual inspection, if no more than 3 obvious cracks were observed on the surface of the steel along the axial direction (rolling direction), it was judged that hot working cracks were sufficiently suppressed (marked as "E" (Excellent) in the "Hot Working Cracks" column in Tables 5 and 6).

[0430] On the other hand, if visual observation results in more than three distinct cracks per 1m along the axial direction (rolling direction) of the steel, it is judged that hot working cracks have not been adequately suppressed (marked as "NA" (Not Accepted) in the "Hot Working Cracks" column of Tables 5 and 6).

[0431] Furthermore, if hot working cracks are not adequately suppressed in the hot working crack evaluation test, tests 3 to 5 will not be performed (marked as "-" in the "Melting Crack", "Machinability", and "Fatigue Strength" columns of Tables 5 and 6).

[0432] [(Experiment 2) Number density measurement of fine and coarse Bi particles]

[0433] Based on the method described above [Method for measuring the number density of fine Bi particles and coarse Bi particles], the number density (particles / mm²) of fine Bi particles in the steel for each test number was calculated. 2 ) and the number density of coarse Bi particles (numbers / mm) 2 In addition, test pieces were collected from steel (bar steel) prior to the heat treatment performed under simulated hot forging. The results of the obtained fine Bi particle number density are shown in Tables 5 and 6, “Fine Bi Particle Number Density (particles / mm²)”. 2 The number density of coarse Bi particles is shown in Tables 5 and 6 under the column “Number Density of Coarse Bi Particles (numbers / mm)”. 2 In the column “)”.

[0434] Furthermore, regarding the intermediate products for simulating mechanical structure components, simulated hot forging heat treatment was performed on steel with a diameter of 80 mm. When only simulated hot forging heat treatment at 1100℃ was performed, the number density of fine and coarse Bi particles in the steel was not affected. Therefore, the number density of fine and coarse Bi particles in the intermediate products for simulating mechanical structure components is substantially the same as that in the steel with a diameter of 80 mm. Based on the method described above [Method for measuring the number density of coarse and fine Bi particles], the number density (particles / mm²) of fine Bi particles in the intermediate products for simulating mechanical structure components for each test number was determined. 2 ) and the number density of coarse Bi particles (numbers / mm) 2 The result was the number density (particles / mm²) of fine Bi particles in the intermediate products of the simulated mechanical components for each test number. 2 ) and the number density of coarse Bi particles (numbers / mm) 2 The number density (particles / mm) of fine Bi particles in the steel corresponding to the test number. 2 ) and the number density of coarse Bi particles (numbers / mm) 2 They are roughly the same.

[0435] [(Experiment 3) Evaluation Test of Melt Cracks]

[0436] A test piece with a width of 10 mm, a thickness of 3 mm, and a length of 10 mm is collected from the R / 2 section of the intermediate part of the simulated mechanical structure component, which contains each test number and is perpendicular to the axial direction (rolling direction). The length direction of the test piece is parallel to the axial direction (rolling direction) of the intermediate part of the simulated mechanical structure component. In addition, the central axis parallel to the length direction of the test piece coincides with the R / 2 section.

[0437] A simulated high-frequency quenching test was conducted on the test piece using a thermal cycling test apparatus manufactured by Fuji Denpa Koki Co., Ltd. Specifically, the test piece was heated to 1370°C using a high-frequency coil at a heating rate of 100°C / second. The test piece was then held at 1370°C for 15 seconds. Afterward, the test piece was water-cooled.

[0438] The cross-section (observation surface) perpendicular to the length direction of the water-cooled test piece was mechanically ground. The mechanically ground observation surface was then etched using a picric acid-alcohol reagent. The field of view, corresponding to the R / 2 portion, within the etched observation surface was observed using a 400x optical microscope. The presence or absence of melt cracks was visually confirmed within the observed field of view. The area of ​​the field of view was 250 μm × 400 μm.

[0439] Even when a region (corrosion zone) with a width of 5 μm or more is observed at the grain boundary of the microstructure within the observed field of view, it is judged as insufficient suppression of melt cracking (marked as "E" in the "Melting Cracks" column of Tables 5 and 6). A corrosion zone with a width of 5 μm or more at the grain boundary is, for example... Figure 1 As shown, this refers to the etched region 10 at the grain boundary GB in the field of view, with a maximum width of 5 μm or more. On the other hand, as... Figure 2 In this case, if no corrosion zone is observed at the grain boundary GB, it is determined that the melt cracking has been sufficiently suppressed (marked as "NA" in the "Melting Cracks" column in Tables 5 and 6).

[0440] [(Experiment 4) Cutting performance evaluation test (drill bit life test)]

[0441] Machinability evaluation test pieces were collected from intermediate prototypes of simulated mechanical components of various test numbers. Specifically, drilling was performed on an intermediate prototype of a simulated mechanical component with a diameter of 80 mm at a depth of 21 mm from the outer surface of the steel in a section perpendicular to the longitudinal direction. A model SD3.0 drill bit manufactured by Nachi-Fujikoshi Co., Ltd. was used, with a feed rate of 0.25 mm / rev per revolution. The piercing depth of each hole was set to 9 mm. During piercing, water-soluble cutting oil was continuously supplied as a lubricant to the pierced area.

[0442] The machinability of steel was evaluated by drilling under the above conditions. The maximum cutting speed VL1000 (m / min) was used as the evaluation metric. The maximum cutting speed VL1000 refers to the fastest cutting speed of a drill bit capable of drilling a 1000mm long hole.

[0443] When the maximum cutting speed VL1000 is 15 m / min or higher, excellent machinability is determined (marked as "E" in the "Machinability" column of Tables 5 and 6). On the other hand, when the maximum cutting speed VL1000 is less than 15 m / min, excellent machinability is determined (marked as "NA" in the "Machinability" column of Tables 5 and 6).

[0444] [(Experiment 5) Fatigue Strength Evaluation Test (Rotational Bending Fatigue Test)]

[0445] Rotational bending fatigue test pieces were collected from intermediate parts of simulated mechanical structures. Figure 3 This is a side view of a rotating bending fatigue test piece collected from intermediate parts of various simulated mechanical structures. Figure 3 The value of "φ" in the figure refers to the diameter (mm) at that location.

[0446] The diameter of the parallel portion of the rotary bending fatigue test piece is 8 mm, and the diameter of the clamping portion is 12 mm. Specifically, the parallel portion is manufactured by machining on a lathe to a depth of 3.5 mm from the surface of the intermediate part simulating a mechanical structure component. Therefore, the surface of the parallel portion is at least within a depth of 5 mm from the surface of the bar steel. Furthermore, the length direction of the parallel portion of the rotary bending fatigue test piece is parallel to the length direction of the simulated mechanical structure component. The rotary bending fatigue test piece is envisioned as an intermediate part obtained by machining a hot-worked intermediate part during the manufacturing process of a mechanical structure component using steel. The parallel portion of the collected rotary bending fatigue test piece is finely ground to adjust the surface roughness. Specifically, the average surface roughness (Ra) along the centerline is set to within 3.0 μm, and the maximum height (Rmax) is set to within 9.0 μm.

[0447] Furthermore, if the fatigue strength is sufficiently high based on tests using rotating bending fatigue test pieces collected from intermediate samples of simulated mechanical components before high-frequency quenching, then excellent fatigue strength can be obtained even in mechanical components after high-frequency quenching, which is common knowledge to those skilled in the art. Therefore, using the aforementioned rotating bending fatigue test pieces, an Ono-style rotating bending fatigue test was conducted under alternating rotational speeds of 3600 rpm in an atmospheric atmosphere at room temperature (23°C). Fatigue tests were conducted by varying the stress applied to multiple test pieces, and results were obtained in 10... 7The highest stress that does not fracture after one cycle is taken as the fatigue strength (MPa).

[0448] If the obtained fatigue strength is 550 MPa or higher, it is judged as excellent fatigue strength (marked as "E" in the "Fatigue Strength" column of Tables 5 and 6). If the fatigue strength is less than 550 MPa, it is judged as not excellent fatigue strength (marked as "NA" in the "Fatigue Strength" column of Tables 5 and 6).

[0449] [Evaluation Results]

[0450] Referring to Tables 1 to 6, the steels tested (numbers 1 to 43) meet technical characteristics 1 to 4. Therefore, hot working cracks and melt cracks are effectively suppressed. Furthermore, in the machinability evaluation test, the maximum cutting speed VL1000 is 15 m / min or higher, resulting in excellent machinability. And in the fatigue strength evaluation test, the fatigue strength is 550 MPa or higher, demonstrating excellent fatigue strength.

[0451] On the other hand, in test number 44, the carbon content was too high. As a result, melt cracks occurred.

[0452] In test number 45, the C content was too low. Therefore, the fatigue strength was low.

[0453] In experiment number 46, the Si content was too high. As a result, melt cracks occurred.

[0454] In test number 47, the Mn content was too high. Therefore, the steel had low machinability.

[0455] In test number 48, the Mn content was too low. As a result, melt cracks occurred.

[0456] In test number 49, the phosphorus content was too high. As a result, melt cracks occurred.

[0457] In test number 50, the sulfur content was too high. As a result, melt cracks occurred.

[0458] In test number 51, the sulfur content was too low. Therefore, the steel had poor machinability.

[0459] In test number 52, the Cr content was too high. Therefore, the steel had low machinability.

[0460] In test number 53, the V content was too high. Therefore, the steel had low machinability.

[0461] In experiment number 54, the Bi content was too high. Therefore, the number density of coarse Bi particles was greater than 10 particles / mm². 2 Therefore, hot working cracks were generated.

[0462] In test number 55, the Bi content was too low. Therefore, the steel had poor machinability. Furthermore, the number density of fine Bi particles was less than 80 particles / mm². 2 As a result, melt cracks were generated.

[0463] In test number 56, the nitrogen content was too high. As a result, hot working cracks occurred.

[0464] In test number 57, the nitrogen content was too low. Therefore, the fatigue strength was low.

[0465] In tests 58 and 59, the value of fn1 was too high. That is to say, fn1 does not satisfy equation (1). Therefore, the machinability of the steel is low.

[0466] In tests 60 and 61, the value of fn1 was too low. That is, fn1 does not satisfy equation (1). Therefore, the fatigue strength is low.

[0467] In experiments 62-64, the time T (minutes) from the addition of Bi to the end of stirring during the refining process was too short. Therefore, the number density of coarse Bi particles was greater than 10 particles / mm². 2 Therefore, hot working cracks were generated.

[0468] In experiments 65 and 66, the time T (minutes) from the addition of Bi to the end of stirring during the refining process was too long. Therefore, the number density of fine Bi particles was less than 80 particles / mm². 2 This resulted in molten cracks.

[0469] The embodiments of the present invention have been described above. However, the above embodiments are merely illustrative examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and appropriate modifications can be made to the above embodiments without departing from its spirit.

Claims

1. A type of steel, wherein, Its chemical composition, expressed in % by mass, contains C：0.20～0.50％、 Si: 0.01~0.80% Mn: 0.50~2.00% P: below 0.030% S：0.010～0.095％、 Cr：0.01～1.30％、 V: Greater than 0.200% and less than or equal to 0.300% Bi: 0.0051~0.1500% N：0.0030～0.0200％， The balance consists of Fe and impurities. The chemical composition satisfies formula (1). In the steel, the number density of fine Bi particles with a spherical equivalent diameter of 0.1–1.0 μm is 80–8000 particles / mm. 2 The number density of coarse Bi particles with a spherical equivalent diameter of 10.0 μm or more is 10 particles / mm. 2 the following, 0.80≤C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V≤1.50(1) In this formula, the content of each element symbol is substituted into the mass percentage.

2. The steel according to claim 1, wherein, The chemical composition further contains one or more elements selected from the group consisting of the following elements to replace a portion of Fe. Al: below 0.060% Mg: less than 0.0100% Ti: below 0.0200% Nb: below 0.0200% W: Below 0.4000% Zr: less than 0.2000%, Ca: less than 0.0030%, Te: less than 0.0100%, B: less than 0.0050%, Sn: less than 0.0100%, rare earth elements: less than 0.0070%, Co: less than 0.0100%, Se: less than 0.0100%, Sb: less than 0.0100%, In: less than 0.0100%, Mo: less than 0.20%, Cu: less than 0.20%, and Ni: less than 0.20%.

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