Steel material, blank material, and method for manufacturing component

Abstract

[Al] and [N] satisfy a predetermined relationship, where [Al] is the Al content in mass% and [N] is the N content in mass%, and d / 2 is the length of a line segment connecting the center of gravity of the cross-section and the surface of the cross-section closest to the center of gravity in a cross-section perpendicular to the rolling direction, and d / 2 is the length of a line segment connecting the center of gravity of the cross-section and the surface of the cross-section closest to the center of gravity of the cross-section. When a position at a distance of d / 2 from the surface in the direction of the center of gravity is a d / 2 part and a position at a distance of d / 4 from the surface in the direction of the center of gravity is a d / 4 part, the amount of solid-solution Al is 0.018 mass% or more and the amount of solid-solution N is 0.009 mass% or more in either the d / 2 part or the d / 4 part, the number density of MnS having an area of 1.0-10.0 [mu] m2 in the d / 4 part is 70.0 / mm2 or less, and the number density of MnS having an area of 1.0-10.0 [mu] m2 in the d / 4 part is 70.0 particles / mm2 or less. And the number density of MnS in an area exceeding 10.0 [mu] m2 is 4.0 / mm2 or less.

Description

Technical Field

[0001] This invention relates to steel, blanks obtained using the steel, and methods for manufacturing components using the blanks.

[0002] This application claims priority based on Japanese Patent Application No. 2023-175737, filed in Japan on October 11, 2023, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, the miniaturization of drive units for electric vehicles, construction vehicles, and other applications has led to a demand for higher strength mechanical drive components such as gears.

[0004] Furthermore, in gears, bending loads are applied to the teeth during meshing. Therefore, gear teeth are required to have bending fatigue strength. Moreover, in gears, the tooth surfaces slide against each other with relatively short cycles. Therefore, it is necessary to suppress pitting corrosion on the tooth surfaces. That is, in mechanical components, such as gears used in automobiles and construction vehicles, not only bending fatigue strength but also surface fatigue strength (pitting characteristics) is required. It is known that carburizing treatment (carburizing and quenching) is very effective in improving the surface fatigue strength of mechanical components.

[0005] Therefore, in general, mechanical parts such as gears are mostly manufactured in the following way: steel bars or other steel materials are processed into blanks by forging or other methods, the blanks are cut into the shape of the parts as needed, and then carburizing treatment (carburizing and quenching) is carried out.

[0006] As a carburizing process, vacuum carburizing, which reduces CO2 emissions, is used instead of the previously more common gas carburizing process. Gas carburizing is a surface treatment performed by heating the workpiece to the carburizing temperature in a carburizing gas. Vacuum carburizing, on the other hand, involves heating the workpiece to the carburizing temperature under vacuum, then introducing a carburizing gas at low pressure into the atmosphere for a carburizing period, and finally holding the workpiece in the vacuum atmosphere for a diffusion period to allow carbon diffusion, all in one or more steps. In vacuum carburizing, hydrocarbon gases are used as the carbon supply source. Therefore, vacuum carburizing does not produce CO2 during the carburizing reaction. Thus, vacuum carburizing allows for the manufacture of parts in an environmentally friendly process.

[0007] On the other hand, during carburizing, the material is heated to the carburizing temperature, which can cause abnormal grain growth in the steel, sometimes leading to a decrease in fatigue properties. This is especially true in vacuum carburizing, where surface denitrification occurs simultaneously with heating to the carburizing temperature, making abnormal grain growth even more likely.

[0008] To address the issue of suppressing abnormal grain growth during carburizing, Patent Document 1 discloses a surface-hardening rolled bar steel with excellent anti-grain coarsening properties during high-temperature carburizing. This steel is composed of steel containing, by mass%, 0.05-0.25% C, 0.05-2.0% Si, 0.01-1.5% Mn, 0.005-0.2% S, 0.4-1.5% Cr, 0.0085-0.0219% N, 0.058-0.062% Al, 0.038-0.100% Nb, and 0.008-0.012% Ti, with the remainder consisting of Fe and unavoidable impurities. The steel contains 0.5 × 10⁻⁶ carbides, nitrides, and carbonitrides, or two or more of these deposits or composite precipitates with an equivalent circle diameter of 100 nm or more, selected from at least one of Al, Nb, and Ti. 12 pcs / m 2 the following.

[0009] In addition, Patent Document 2 discloses a high-temperature carburizing steel that can prevent the coarsening and abnormal growth of austenite grains during carburizing treatment. The steel contains Nb: 0.001~0.10% (meaning mass %), Al: 0.01~0.15%, and N: 0.01~0.03%, and the N content [N] satisfies equation (1), and the amount of Al dissolved after hot working, as expressed by equation (2), is 0.01~0.10%. Patent Document 2 discloses that by containing an appropriate amount of Al in a dissolved state without Al precipitation after hot working, fine AlN, NbC, or NbCN composite precipitates with AlN can be obtained during subsequent heat treatment. These precipitates exert a pinning effect during high-temperature carburizing and can suppress grain coarsening at high temperatures.

[0010] Furthermore, regarding grain coarsening during vacuum carburizing, for example, Patent Document 3 discloses a vacuum carburizing billet capable of suppressing abnormal grain growth in the treated surface portion during vacuum carburizing. This billet has undergone final hot rolling and is vacuum carburized at a processing temperature of 980°C or higher, lower than T1 (°C) expressed by Formula (2), and lower than T2 (°C) expressed by Formula (3). This vacuum carburizing billet has a ferrite-pearlite structure, and the number of AlN precipitates with an equivalent circle diameter of 100 nm or more in the cross-section is 1.5 per 100 μm. 2 the following.

[0011] Existing technical documents

[0012] Patent documents

[0013] Patent Document 1: Japanese Patent No. 4807949

[0014] Patent Document 2: Japanese Patent Application Publication Nos. 2001-20038

[0015] Patent Document 3: Japanese Patent No. 7010320 Summary of the Invention

[0016] The technical problem that the invention aims to solve

[0017] The following issues exist in the technologies described in Patent Documents 1 to 3.

[0018] In the technology of Patent Document 1, a large amount of Ti is required. These elements have a certain effect on preventing grain coarsening, but if they are present, they may become a cause of defects during rolling or reduce machinability.

[0019] Furthermore, neither Patent Document 1 nor Patent Document 2 discloses the ability to prevent grain coarsening during vacuum carburizing. The inventors conducted research and found that while gas carburizing in Patent Documents 1 and 2 can suppress abnormal grain growth, vacuum carburizing, which is a more stringent condition, may not adequately suppress grain coarsening (abnormal grain growth).

[0020] Furthermore, in Patent Document 3, when regions with grains coarser than three grades than the original grain size account for more than 20%, it is judged as "mixed grains," indicating abnormal grain growth. However, the inventors' research results show that while Patent Document 3 suppresses abnormal grain growth by causing the finely precipitated AlN dissolved during forging to precipitate during vacuum carburizing, in this case, the fine AlN dissolves due to denitrification during vacuum carburizing, failing to sufficiently suppress abnormal grain growth. Specifically, it is clear that even if abnormal grain growth is judged to be absent according to the criteria of Patent Document 3, the characteristics are insufficient from the Charpy impact value perspective, and there is room for improvement in suppressing abnormal grain growth.

[0021] As mentioned above, there has been no steel or billet that does not require the presence of Ti and can suppress abnormal grain growth during carburizing even in vacuum carburizing (possessing resistance to coarsening).

[0022] Therefore, the objective of this invention is to provide steel and billets that have been carburized into components, wherein the steel and billets suppress abnormal grain growth during carburizing, and a method for manufacturing components using the billets.

[0023] Technical solutions for solving technical problems

[0024] The inventors have investigated a method for improving the resistance to coarsening during carburizing in steels that are substantially free of Ti (allowed to be present as an impurity).

[0025] As a result, the following insights were obtained.

[0026] (a) In the stage of hot-rolled steel, by reducing the amount of AlN precipitation and increasing the amount of dissolved Al, AlN is precipitated during subsequent forging and heat treatment (AlN is precipitated before carburizing begins), thereby improving the resistance to coarsening during carburizing.

[0027] (b) In the case of increasing the amount of Al dissolved in hot-rolled steel, it is effective to control the chemical composition and the precipitation state of MnS.

[0028] (c) In order to suppress denitrification during vacuum carburizing and to suppress the decrease in pinning force of AlN in carburizing caused by Ostwald growth, controlling the ratio of Al and N content is effective.

[0029] (d) When controlling the precipitation state of MnS, it is effective to carry out the secondary refining, slag separation, billet rolling and hot rolling processes as a whole under the specified conditions.

[0030] This invention is based on the above-mentioned insights. The main points of this invention are as follows.

[0031] [1] A type of steel according to one aspect of the present invention, having, by mass%, the following composition: C: 0.10~0.30%, Si: 0.03~1.50%, Mn: 0.35~1.50%, P: 0.002~0.020%, S: 0.002~0.050%, Cr: 0.05~2.30%, Al: 0.020~0.060%, N: 0.011~0.030%, Ti: less than 0.007%, Nb: less than 0.100%, Mo: 0~0.40%, O: 0~0.0050%, V: 0~0.15%, B: 0~0.0050%, Cu: 0~0.40%, Ni: 0~0.30%, Sn: 0~0.100%, Ca: 0~0.0050%, Mg: 0~0.0 0.50% and the remainder: The chemical composition consisting of Fe and impurities, when the Al content in mass% is set as [Al] and the N content is set as [N], the [Al] and the [N] satisfy the following formulas (1) and (2), in a cross section perpendicular to the rolling direction, when the length of the line segment connecting the centroid of the cross section to the surface of the cross section closest to the centroid is set as d / 2, when the position d / 2 away from the surface in the direction of the centroid is set as d / 2 part, and the position d / 4 away from the surface in the direction of the centroid is set as d / 4 part, in either d / 2 part or d / 4 part, the amount of dissolved Al is 0.018% by mass or more, and the amount of dissolved N is 0.009% by mass or more, and in d / 4 part, the area is 1.0~10.0 μm. 2 The number density of MnS is 70.0 particles / mm².2 Below, areas exceeding 10.0 μm 2 The number density of MnS is 4.0 cells / mm². 2 the following.

[0032] 1.50≤[Al] / ([N]-(-0.0013×[Al] / [N]+0.007))≤5.00 (1)

[0033] 1.50≤[Al]×([N]-(-0.0013×[Al] / [N]+0.007))×10000≤10.00 (2)

[0034] [2] In the steel described in [1], the chemical composition may contain one or more of the following by mass%: Ti: more than 0% and less than 0.007%, Nb: more than 0% and less than 0.100%, Mo: 0.03 to 0.40%, O: 0.0001 to 0.0050%, V: 0.01 to 0.15%, B: 0.003 to 0.0050%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Sn: 0.010 to 0.100%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

[0035] [3] The billet of another embodiment of the present invention, by mass%, has the following composition: C: 0.10~0.30%, Si: 0.03~1.50%, Mn: 0.35~1.50%, P: 0.002~0.020%, S: 0.002~0.050%, Cr: 0.05~2.30%, Al: 0.020~0.060%, N: 0.011~0.030%, Ti: less than 0.007%, Nb: less than 0.100%, Mo: 0~0.40%, O: 0~0.0050%, V: 0~0. The chemical composition consists of 15% B, 0~0.0050% Cu, 0~0.40% Ni, 0~0.30% Sn, 0~0.100% Ca, 0~0.0050% Mg, and the remainder: Fe and impurities. When the Al content in the chemical composition is set as [Al] and the N content is set as [N], the [Al] and the [N] satisfy the following formulas (1) and (2), and the area is 1.0~10.0 μm at a position 5 mm inward from the surface in a direction perpendicular to the surface. 2 The number density of MnS is 70.0 particles / mm². 2 Below, areas exceeding 10.0 μm 2 The number density of MnS is 4.0 cells / mm². 2The amount of Al precipitated is above 0.012% by mass, and the average size of AlN is below 90 nm.

[0036] 1.50≤[Al] / ([N]-(-0.0013×[Al] / [N]+0.007))≤5.00 (1)

[0037] 1.50≤[Al]×([N]-(-0.0013×[Al] / [N]+0.007))×10000≤10.00 (2)

[0038] [4] In the billet described in [3], the chemical composition may contain one or more of the following by mass%: Ti: more than 0% and less than 0.007%, Nb: more than 0% and less than 0.100%, Mo: 0.03~0.40%, O: 0.0001~0.0050%, V: 0.01~0.15%, B: 0.003~0.0050%, Cu: 0.01~0.40%, Ni: 0.01~0.30%, Sn: 0.010~0.100%, Ca: 0.0001~0.0050%, and Mg: 0.0001~0.0050%.

[0039] [5] Another aspect of the present invention describes a method for manufacturing a component, comprising a vacuum carburizing process in which the blank described in [3] is subjected to vacuum carburizing in a temperature range of 900~1000°C.

[0040] Invention Effects

[0041] According to the above-described method of the present invention, it is possible to obtain steel that suppresses abnormal grain growth during carburizing (even if carburizing is vacuum carburizing) and a billet obtained using the steel. Furthermore, according to the above-described method of the present invention, it is also possible to provide a method for manufacturing a component using the billet. Attached Figure Description

[0042] Figure 1 This is a diagram showing an example of the shape of the steel (with a circular cross-section) in this embodiment.

[0043] Figure 2 This is a diagram showing an example of the shape of the steel (with a square cross-section) in this embodiment.

[0044] Figure 3 This is a diagram showing the shape of the test piece used in the Charpy test in the embodiment. Detailed Implementation

[0045] The present invention describes the steel (steel of this embodiment), the billet (bill of this embodiment), the manufacturing method thereof, and the manufacturing method of the component (carburized component) using the billet according to one embodiment of the present invention.

[0046] In this specification, the range of values ​​indicated by "~" refers to the range including the values ​​on both sides. That is, if it is A~B, it means that it is above A and below B.

[0047] <Steel>

[0048] The steel 1 in this embodiment is, for example, as shown in the example... Figure 1 As shown, this is a steel bar with a diameter d and a cross-section perpendicular to the rolling direction RD, or as... Figure 2 The image shows a square steel bar with a side length of d, perpendicular to the rolling direction RD. It is a steel bar or wire.

[0049] Hereinafter, in a section perpendicular to the rolling direction, if the length of the line segment connecting the centroid of the section and the surface of the section closest to the centroid is set to d / 2, the position d / 2 from the surface in the direction of the centroid is set as part d / 2, and the position d / 4 from the surface in the direction of the centroid is set as part d / 4. Figure 1 When the cross-section is circular, the position at a distance of d / 2 from the surface of the steel towards the center of the circle is d / 2 part (= center part) O, and the position at a distance of d / 4 from the surface towards the center of the circle is d / 4 part S. When the cross-section is square, the position at a distance of d / 2 from the surface of the center part C of TD in the width direction of one side towards the thickness direction is d / 2 part O, and the position at a distance of d / 4 from the surface of the center part C of TD in the width direction of one side towards the thickness direction is d / 4 part S.

[0050] In this embodiment, the description focuses on the case where the cross-section is a circle or a square, but the cross-section can also be an ellipse or a deformed circle, or a polygon other than a rectangle or quadrilateral with different side lengths.

[0051] The steel 1 of this embodiment has the following chemical composition: by mass%, it consists of C: 0.10~0.30%, Si: 0.03~1.50%, Mn: 0.35~1.50%, P: 0.002~0.020%, S: 0.002~0.050%, Cr: 0.05~2.30%, Al: 0.020~0.060%, N: 0.011~0.030%, Ti: less than 0.007%, Nb: less than 0.100%, Mo: 0~0.40%, O: 0~0.0050%, V: 0~0.15%, B: 0~0.0050%, Cu: 0~0.40%, Ni: 0~0.30%, Sn: 0~0.100%, Ca: 0~0.0050%, Mg: 0~0.0050%, and the remainder: Fe and impurities. In addition, the Al content and N content meet the specified relationship.

[0052] Furthermore, in the steel 1 of this embodiment, the amount of dissolved Al in d / 2 part O and d / 4 part S is 0.018% by mass or more, and the amount of dissolved N is 0.009% by mass or more. Additionally, in the steel 1 of this embodiment, the area in d / 4 part S is 1.0 to 10.0 μm. 2 The number density of MnS is 70.0 particles / mm². 2 Below, areas exceeding 10.0 μm 2 The number density of MnS is 4.0 cells / mm². 2 the following.

[0053] The reasons for these limitations will be explained in detail below.

[0054] [shape]

[0055] As described above, the steel 1 in this embodiment is, for example, like... Figure 1 , Figure 2 The figure shows a steel bar with a circular cross-section of diameter d perpendicular to the rolling direction RD, or a steel bar with a square cross-section of side length d perpendicular to the rolling direction RD.

[0056] In the case of steel used as blanks for gears, etc., bar steel with a circular cross-section is preferred.

[0057] The diameter d or the length of one side is not limited, but when considering applications such as gears, it is preferably 15mm to 120mm. In the case of gears made of bar steel, the area around d / 4 of the bar steel mostly becomes the surface portion of the gear.

[0058] Steel can also be coiled into a coil shape (so-called bar coil or wire).

[0059] [Chemical Composition]

[0060] The content of each element in the chemical composition of the steel constituting this embodiment and the reasons thereof will be explained. Unless otherwise specified, the percentage of element content (%) is by mass (%).

[0061] C: 0.10~0.30%C

[0062] Carbon is an element that improves the hardenability and hardness of steel, and also enhances the bending fatigue strength after carburizing. When the carbon content is less than 0.10%, even if the contents of other elements are within the range of this embodiment, the above-mentioned effects cannot be fully obtained. Therefore, the carbon content is 0.10% or more. The carbon content is preferably 0.13% or more, and more preferably 0.15% or more.

[0063] On the other hand, when the carbon content exceeds 0.30%, the machinability decreases. Therefore, the carbon content is 0.30% or less. Preferably, the carbon content is 0.28% or less, and more preferably 0.25% or less.

[0064] Si: 0.03~1.50%

[0065] Silicon (Si) is an element that deoxidizes steel. Additionally, Si improves the hardenability of steel and, consequently, enhances its strength through solid solution strengthening.

[0066] This effect cannot be obtained when the Si content is below 0.03%. Therefore, the Si content is 0.03% or more. Preferably, the Si content is 0.04% or more, and more preferably 0.05% or more.

[0067] On the other hand, when the Si content exceeds 1.50%, the machinability and cold workability of the steel used as a raw material decrease. Therefore, the Si content is 1.50% or less. The Si content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.30% or less.

[0068] Mn: 0.35~1.50%

[0069] Manganese (Mn) is an element that deoxidizes steel. Furthermore, Mn improves the hardenability and strength of steel, thereby increasing the hardness of the core and the low-cycle fatigue strength of carburized components. This effect is not achieved when the Mn content is below 0.35%. Therefore, the Mn content is 0.35% or more. Preferably, the Mn content is 0.40% or more, more preferably 0.50% or more, and even more preferably 0.65% or more.

[0070] On the other hand, when the Mn content exceeds 1.50%, the machinability and cold workability decrease. Therefore, the Mn content is 1.50% or less. The Mn content is preferably 1.20% or less, and more preferably 1.00% or less.

[0071] P: 0.002~0.020%

[0072] Phosphorus (P) is an impurity. P is an element that segregates at the austenite grain boundaries during carburizing, thus reducing the grain boundary strength of the carburized layer. If the P content exceeds 0.020%, its adverse effects become significant; therefore, the P content is set to 0.020% or less. The P content is preferably 0.018% or less, more preferably 0.015% or less, and even more preferably 0.012% or less.

[0073] On the other hand, while a low phosphorus (P) content is preferred, excessively reducing P can increase the cost of P removal. Therefore, considering the economics of refining, a P content of 0.002% or higher is desired. The P content can be 0.005% or higher, or 0.008% or higher.

[0074] S: 0.002~0.050%

[0075] Sulfur (S) is an impurity. S is an element that remains at grain boundaries and reduces the grain boundary strength of the carburized layer. S is also an element that further forms MnS as precipitation sites for AlN. When the S content exceeds 0.050%, the aforementioned adverse effects become significant, therefore the S content is 0.050% or less. The S content is preferably 0.043% or less, more preferably 0.040% or less, and even more preferably 0.035% or less.

[0076] On the other hand, while it is preferable to keep the sulfur content as low as possible, excessively reducing the sulfur content will increase the cost of desulfurization. Therefore, considering the economics of refining, the sulfur content is kept at 0.002% or higher. The sulfur content can be 0.005% or higher, or 0.008% or higher.

[0077] Cr: 0.05~2.30%

[0078] Chromium (Cr) is an element that improves the hardenability of steel and increases core hardness, thereby enhancing its low-cycle fatigue strength. This effect cannot be fully achieved when the Cr content is less than 0.05%. Therefore, the Cr content is 0.05% or more. Preferably, the Cr content is 0.08% or more, more preferably 0.60% or more, and even more preferably 0.70% or more.

[0079] On the other hand, when the Cr content exceeds 2.30%, the machinability and cold workability decrease. Therefore, the Cr content is 2.30% or less. The Cr content is preferably 1.80% or less, more preferably 1.65% or less, even more preferably 1.50% or less, and even more preferably 1.35% or less.

[0080] Al: 0.020~0.060%

[0081] Al (aluminum) is an element that deoxidizes steel. Furthermore, Al combines with nitrogen (N) in steel to form AlN, which inhibits the coarsening (coarsening) of austenite grains during carburizing. To achieve these effects, the Al content is set to 0.020% or more. The Al content is preferably 0.025% or more, more preferably 0.028% or more, and even more preferably 0.030% or more.

[0082] On the other hand, if the Al content is higher than 0.060%, the coarsening of inclusions may prevent the suppression of austenite grain coarsening. Therefore, the Al content is 0.060% or less. The Al content is preferably 0.058% or less, and more preferably 0.055% or less.

[0083] However, in addition to setting the Al content within the range mentioned above, it needs to be set to satisfy the ranges of equations (1) and (2) described later.

[0084] In addition, the Al content here refers to the acid-soluble Al (sol.Al) content.

[0085] N: 0.011~0.030%

[0086] Nitrogen (N) is an element that combines with Al in steel to form nitrides, thereby inhibiting the coarsening of austenite grains during carburizing. To achieve this effect, the N content is 0.011% or more. The N content is preferably 0.013% or more, and more preferably 0.014% or more.

[0087] On the other hand, the above-mentioned effects saturate when the nitrogen content exceeds 0.030%. Therefore, the nitrogen content is 0.030% or less. The nitrogen content is preferably 0.028% or less, and more preferably 0.025% or less.

[0088] However, the N content, in addition to being set within the range mentioned above, needs to be set to satisfy the ranges of equations (1) and (2) described later.

[0089] Ti: below 0.007%

[0090] Titanium (Ti) is an element that combines with nitrogen (N) in steel to form nitrides (TiN). If TiN forms, the dissolved nitrogen content decreases, and even when using the steel described in this embodiment, sufficient AlN cannot precipitate before carburizing. Furthermore, this becomes a cause of defects during rolling. Therefore, a low Ti content is preferable. While Ti nitrides have some resistance to grain coarsening, they are coarser than AlN, thus their effect on grain coarsening resistance is minimal.

[0091] If the Ti content is below 0.007%, its impact is small; therefore, the Ti content is below 0.007%. The Ti content is preferably below 0.006%, more preferably below 0.005%, and even more preferably below 0.004%.

[0092] Ti sometimes gets mixed in as an impurity from scrap and other materials. In the steel of this embodiment, the Ti content is sufficiently reduced in the secondary refining process and the slag separation process, but it is difficult to make the Ti content 0%. Therefore, from the viewpoint of manufacturing cost, the Ti content is preferably more than 0%, and more preferably 0.001% or more.

[0093] Nb: below 0.100%

[0094] Niobium (Nb) is an element that combines with nitrogen (N) in steel to form nitrides (NbN) and carbonitrides (Nb(C,N)). Therefore, it may be present in steel.

[0095] However, if these nitrides and carbonitrides form, the dissolved nitrogen decreases, and sufficient AlN cannot precipitate before carburizing. Therefore, to ensure sufficient AlN, the Nb content is set to below 0.100%. NbN is coarser than AlN, and therefore has less effect on resisting coarsening.

[0096] The Nb content is preferably 0.080% or less, more preferably 0.050% or less, and even more preferably 0.010% or less.

[0097] Nitrogen (Nb) can sometimes be introduced as an impurity from waste materials. Therefore, in terms of manufacturing costs, the Nb content is preferably greater than 0%, and more preferably greater than 0.001%.

[0098] Mo: 0~0.40%

[0099] Mo (Mo) is an element that improves the low-cycle fatigue strength of carburized components by enhancing the hardenability of steel and increasing core hardness. Mo also improves the toughness of the carburized layer. Therefore, Mo can be included to achieve these effects. When these effects are achieved, the Mo content is preferably 0.03% or more.

[0100] On the other hand, if the Mo content exceeds 0.40%, these effects become saturated, and the raw material cost increases. Therefore, the Mo content is 0.40% or less. Preferably, the Mo content is 0.30% or less.

[0101] O: 0~0.0050%

[0102] Oxygen (O) is an unavoidable element that segregates at grain boundaries and easily causes grain boundary embrittlement. Furthermore, O easily forms hard oxide inclusions in steel, contributing to brittle fracture. To prevent such grain boundary embrittlement and brittle fracture, the O content is set to 0.0050% or less. The O content is preferably 0.0030% or less. While the O content is preferably as low as possible, it can also be 0.0001% or more, taking into account manufacturing costs.

[0103] V: 0~0.15%

[0104] Vanadium (V) is an element that combines with carbon and nitrogen in steel to form V carbonitrides (V(CN)), thereby suppressing the coarsening of austenite grains during carburizing. Therefore, it may be included in the composition of vanadium. To achieve the above-mentioned effect, the V content is preferably 0.01% or more, more preferably 0.05% or more.

[0105] On the other hand, V has a lower affinity for N compared to Al, but if the V content exceeds 0.15%, the amount of N dissolved in the solid decreases, and sufficient AlN may not precipitate during carburizing. Therefore, the V content is 0.15% or less. The V content is preferably 0.10% or less.

[0106] B: 0~0.0050%

[0107] Boron (B) is an element that improves the low-cycle fatigue strength of carburized components by enhancing the hardenability of steel and increasing core hardness. Therefore, it may be included in the composition of carburized components. To achieve the aforementioned effects, the B content is preferably 0.0003% or more. More preferably, the B content is 0.0010% or more.

[0108] On the other hand, if the B content exceeds 0.0050%, these effects become saturated. Therefore, the B content is 0.0050% or less. Preferably, the B content is 0.0030% or less.

[0109] Cu: 0~0.40%

[0110] Cu (copper) is an element that improves the low-cycle fatigue strength of carburized components by enhancing the hardenability of steel and increasing core hardness. Therefore, it may be included in the composition of carburized components. To achieve the above effects, the Cu content is preferably 0.01% or more.

[0111] On the other hand, if the Cu content exceeds 0.40%, the hot workability decreases. Therefore, the Cu content is 0.40% or less. Preferably, the Cu content is 0.35% or less.

[0112] Ni: 0~0.30%

[0113] Ni (Ni) is an element that improves the hardenability and hardness of steel, thereby enhancing its flexural fatigue strength after carburizing. Ni also improves the toughness of the carburized layer. Therefore, it may be included in the composition of Ni. To achieve the above effects, it is preferable that the Ni content be 0.01% or more.

[0114] On the other hand, if the Ni content exceeds 0.30%, the amount of retained austenite increases, the surface hardness decreases, and the flexural fatigue strength after carburizing decreases. Therefore, the Ni content is 0.30% or less. Preferably, the Ni content is 0.25% or less.

[0115] Sn: 0~0.100%

[0116] Sn (tin) is an element that improves the corrosion resistance of steel. Therefore, it can be included in steel. To achieve the above-mentioned effects, it is preferable to have a Sn content of 0.010% or more.

[0117] On the other hand, if the Sn content is higher than 0.100%, forging cracks are more likely to occur. Therefore, the Sn content is kept below 0.100%. The Sn content is preferably below 0.080%.

[0118] Ca: 0~0.0050%

[0119] Mg: 0~0.0050%

[0120] Calcium (Ca) and magnesium (Mg) are elements that improve the cold forgeability of steel by controlling the morphology of inclusions. Therefore, they may be included. To achieve the above-mentioned effects, it is preferable to contain 0.0001% or more of one or both of Ca and Mg in total. More preferably, it is 0.0010% or more.

[0121] On the other hand, if there is an excessive amount of Ca and / or Mg, excessive oxides of Ca and / or Mg will be generated. These oxides become the starting point for bending fatigue and surface fatigue. Therefore, the Ca content and Mg content are both 0.0050% or less. Preferably, the Ca content and Mg content are both 0.0040% or less.

[0122] Remaining components: Fe and impurities

[0123] The remaining chemical composition of the steel in this embodiment consists of Fe and impurities. Here, impurities refer to substances that may be introduced into the steel during industrial manufacturing from raw materials such as ore, waste, or the manufacturing environment, and are permissible within a range that does not adversely affect the steel of this embodiment.

[0124] In addition, in the steel of this embodiment, when the content of each element is set to the above range, and the Al content is set to [Al] and the N content is set to [N] in mass%, the [Al] and the [N] need to satisfy the following formulas (1) and (2).

[0125] 1.50≤[Al] / ([N]-(-0.0013×[Al] / [N]+0.007))≤5.00 (1)

[0126] 1.50≤[Al]×([N]-(-0.0013×[Al] / [N]+0.007))×10000≤10.00 (2)

[0127] By satisfying equations (1) and (2), coarsening (abnormal grain growth) in carburizing can be suppressed.

[0128] When [Al] / ([N]-(-0.0013×[Al] / [N]+0.007)) in Equation (1) is less than 1.50, the amount of denitrification on the surface increases significantly during carburizing, AlN dissolution occurs, and the coarsening resistance deteriorates.

[0129] On the other hand, when [Al] / ([N]-(-0.0013×[Al] / [N]+0.007)) exceeds 5.00, AlN grains grow during carburizing, pinning force decreases, and resistance to coarsening deteriorates.

[0130] In addition, when [Al]×([N]-(-0.0013×[Al] / [N]+0.007))×10000 in equation (2) is less than 1.50, sufficient AlN precipitation cannot be guaranteed, and the coarsening resistance is deteriorated.

[0131] On the other hand, when [Al]×([N]-(-0.0013×[Al] / [N]+0.007))×10000 in Equation (2) exceeds 10.00, AlN is not dissolved during forging, and AlN will not precipitate finely during carburizing, thus the resistance to coarsening is deteriorated.

[0132] The chemical composition of the aforementioned steel can be determined using common analytical methods. For example, it can be determined using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S, which are difficult to determine using ICP-AES, can be determined using the combustion-infrared absorption method; N can be determined using the inert gas melting-thermal conductivity method; and O can be determined using the inert gas melting-non-dispersive infrared absorption method.

[0133] Regarding the content of each element, based on the significant figures specified in this embodiment, the measured values ​​are rounded to the nearest whole number up to the minimum value of each element specified in this embodiment.

[0134] <Soluble Al content>

[0135] <Solidated Ni content>

[0136] In the steel of this embodiment, the amount of dissolved Al in both d / 2 and d / 4 sections is 0.018% by mass or more, and the amount of dissolved N in both sections is 0.009% by mass or more.

[0137] Previously, steel was incorporating elements such as Nb and Ti to suppress coarsening during carburizing through the pinning effect of their nitrides and carbides. However, nitrides such as NbN and TiN are large in size, resulting in insufficient pinning effect and sometimes inadequate resistance to coarsening. Furthermore, while carbides such as NbC and TiC are small in size, they can still cause defects during rolling.

[0138] Therefore, in the steel of this embodiment, a chemical composition is formed in which the amounts of Ti and Nb are adjusted at a trace level, and AlN is precipitated before carburizing by subsequent heat treatment, thereby improving the resistance to coarsening through the pinning effect of the precipitated AlN.

[0139] The rationale for dissolving Al and N in hot-rolled steel is that AlN precipitated in hot-rolled steel (during casting or rolling) is typically coarse, thus contributing little to improving resistance to coarsening. In contrast, AlN precipitates finely (e.g., with an average size of less than 90 nm) during heat treatments such as normalizing after forging, prior to carburizing. In other words, it is important to ensure that Al and N are in a solid solution state as much as possible during the steel production stage, and to precipitate them during the heat treatment after forging (ensuring a certain level of precipitation in the billet stage).

[0140] Therefore, in the steel of this embodiment, the amount of dissolved Al in both d / 2 and d / 4 sections is 0.018% by mass or more, and the amount of dissolved N in both sections is 0.009% by mass or more. There is no upper limit, and all Al and N contained can be in a solid solution state. That is, the amount of dissolved Al can be 0.060% by mass or less, and the amount of dissolved N can be 0.030% by mass or less.

[0141] When steel is used as a blank, it is subjected to conventional forging. During forging, the steel is heated, causing some of the AlN precipitated in the steel to melt. However, in practice, it is sometimes difficult to achieve an equilibrium state. Under normal forging conditions, in most cases, the heating temperature and heating time are insufficient, and most of the AlN precipitated in the steel does not dissolve. According to the inventors' research, even when hot forging is performed at a high temperature of 1200°C, AlN will still dissolve and remain.

[0142] Therefore, in the steel of this embodiment, Al and N are in a state of complete solid solution.

[0143] In the steel of this embodiment, the solid solution amounts of the d / 2 and d / 4 portions are specified because the area near the d / 4 portion of steel such as bar steel is the surface portion that mostly becomes gears. In the d / 2 portion, the temperature is difficult to rise during heating, and compared with other positions, it is a position where the solid solution of AlN is more likely to become insufficient.

[0144] The amount of dissolved Al in the d / 2 and d / 4 portions of the steel is obtained by subtracting comb.-Al from Total-Al at each location. That is, it can be calculated as: Dissolved Al amount = (Total-Al) - (comb.-Al).

[0145] Total-Al and comb.-Al are obtained by the following method.

[0146] [Total Al content]

[0147] The powdered sample collected from the designated location (d / 4 or d / 2 section) is decomposed with a mixed acid (hydrochloric acid:nitric acid:water = 1:1:2 by volume), separated using filter paper (5 types of C), and the main solution and washings are stored in volumetric flasks. The residue, along with the filter paper, is placed in a platinum crucible for drying. The residue and filter paper are then ashed together and decomposed by alkaline fusion. The melt obtained from alkaline fusion is dissolved in hydrochloric acid and combined with the remaining main solution in the volumetric flask. The Total Al (mass %) is determined by ICP-OES analysis of the solution.

[0148] [comb.-Al (Al content in electrolytic residue)]

[0149] Centered on the d / 4 or d / 2 section of the steel, cut out test pieces measuring 10mm × 10mm × 10mm (length direction is the rolling direction). For the test pieces, use a 10% AA-based electrolyte (10% vol% acetylacetone - 1% wt% tetramethylammonium chloride - methanol solution) at 0.5A for approximately 20-30 minutes for pre-electrolysis, followed by constant current electrolysis using the same 10% AA-based electrolyte (10% vol% acetylacetone - 1% wt% tetramethylammonium chloride - methanol solution) at 0.5A for 2 hours.

[0150] Then, the electrolytic residue was filtered using a polycarbonate filter membrane with a diameter of 47 mm and a pore size of 0.2 μm.

[0151] The filter residue was ashed along with the filter and decomposed by alkali fusion. The decomposition solution was measured using ICP-OES, and the comb-Al was determined from this analysis.

[0152] In addition, the amount of dissolved nitrogen is obtained by subtracting the amount of nitrogen in the electrolytic residue (comb.-N) from the total nitrogen content (Total-N). That is, it can be calculated as: Dissolved nitrogen = (Total-N) - (comb.-N).

[0153] The total nitrogen content (Total-N) and the nitrogen content in the electrolytic residue (comb.-N) can be determined using the following methods.

[0154] [Total-N (Total N Content)]

[0155] The determination can be performed using common analytical methods. For example, the inert gas melting-thermal conductivity method can be used.

[0156] [comb.-N (N content in electrolytic residue)]

[0157] Test pieces of 10mm×10mm×10mm (with the length direction being the rolling direction) are cut out with the d / 4 or d / 2 part of the steel as the center.

[0158] For the test sample, a 10% AA-based electrolyte (10% acetylacetone - 1% tetramethylammonium chloride - methanol solution) was used for constant current electrolysis at 0.5 A for 2 hours. The electrolytic residue was then filtered using a 47 mm diameter, 0.2 μm pore size polycarbonate membrane. The residue was decomposed along with the filter using sulfuric acid, copper sulfate, and potassium sulfate. The resulting decomposed solution was distilled and the nitrogen content was determined spectrophotometrically, which was then used as comb.-N.

[0159] [MnS number density in d / 4 part]

[0160] The state of MnS in steel significantly affects the precipitation amount of AlN. This is because MnS acts as a nucleus for AlN precipitation. Specifically, if MnS is present, AlN will coarsely precipitate from MnS at undesirable temperature ranges, reducing the amount of dissolved Al. Therefore, a nucleus with an area of ​​1.0 μm is required. 2 The above describes the presence density (number density) of MnS. Additionally, areas exceeding 10.0 μm... 2 MnS contributes to the decrease in fatigue strength. Therefore, it is necessary to specifically specify the area exceeding 10.0 μm. 2 The number density of MnS.

[0161] Specifically, in the d / 4 section, the area is 1.0~10.0 μm. 2 The number density of MnS is 70.0 particles / mm². 2 Below, areas exceeding 10.0 μm 2 The number density of MnS (sometimes referred to as coarse MnS) is 4.0 particles / mm². 2 The following applies. When the size of MnS is large, the adverse effects are greater; therefore, an area exceeding 15.0 μm is preferred. 2 The number density of MnS is 1.5 cells / mm. 2 The following is a further preferred option: an area exceeding 10.0 μm. 2 The number density of MnS is less than 0.5 molecules / mm². 2 The number density of MnS can be 0.0 particles / mm. 2 .

[0162] Area less than 1.0 μm 2 MnS does not become a large AlN precipitation site, so its number density is not limited.

[0163] In part d / 2, compared to part d / 4, the parts are mostly located far from the surface after being processed into components, and are less prone to coarsening. Therefore, the MnS number density is not required to meet the above conditions.

[0164] The number density of MnS was determined by the following method.

[0165] The steel was cut parallel to the axial direction (rolling direction), and a test piece with an observation surface of 10mm (length direction) × 6mm (width direction) was prepared, with the d / 4 section as the center of the observation surface. After embedding the test piece in resin, the cut surface (observation surface) was mirror-polished. The polished surface was then 60mm thick. 2 As the observation area, scanning electron microscopy was used for observation at 400x magnification (visual observation), and MnS was determined based on the contrast of the observed reflected electron images and EDS. The observed area was displayed as a grayscale image in the reflected electron images. Specifically, for precipitates with contrasts different from the parent phase observed in the reflected electron images, compositional analysis was performed using EDS. Among all elements determined by EDS, Mn had the highest mass proportion excluding Fe, and the presence of S was also detected simultaneously (specifically, quantification was performed after removing Fe from all elements in the obtained X-ray energy dispersive spectroscopy, resulting in the detection of S exceeding 1% by mass), which was then identified as MnS.

[0166] Furthermore, through image analysis, the area (where is the area of ​​the observed surface and is the cross-sectional area of ​​the observed surface of the precipitate) within the field of view identified as MnS was determined to be 1.0~10.0 μm. 2 The precipitate has an area (the area of ​​the observation surface, where is the cross-sectional area of ​​the precipitate's observation surface) exceeding 10.0 μm. 2 The precipitate has an area (the area of ​​the observation surface is denoted by ) exceeding 15.0 μm. 2 The number of precipitates is divided by the area of ​​the field of view (60 mm²). 2 The number density of each is thus determined. In this embodiment, the number density obtained by the above method is defined as the number density of MnS in the d / 4 part.

[0167] However, in the above measurements, MnS with an equivalent circle diameter (meaning the same below) of 1.0 μm or larger was used. This is because 1.0 μm or larger is the range of particle size and composition that can be statistically processed in commonly used equipment in reality.

[0168] MnS mainly exists as MnS, but in addition to NbC, NbN, and NbCN, it sometimes exists as a composite inclusion with AlN. In the steel of this embodiment, the amount of MnS existing as a composite inclusion is less than that of MnS existing alone. When quantitatively evaluated by removing Fe from all elements in the obtained X-ray energy dispersive spectroscopy, the large amount of MnS contained more than 30% by mass of Mn.

[0169] <Burnt Material>

[0170] The blank in this embodiment is an intermediate product obtained by forging and heat treating the steel in this embodiment before the carburizing process when obtaining the part (carburized part) from the steel in this embodiment.

[0171] The shape of the blank in this embodiment is not limited. The blank in this embodiment has excellent resistance to coarsening during carburizing, and is therefore suitable as a blank for gears and shafts that are made into parts after carburizing. It is acceptable as long as the shape takes into account the application of these parts.

[0172] [Chemical Composition]

[0173] As described above, the billet of this embodiment is obtained by forging and heat treating the steel of this embodiment. During forging and heat treatment, the chemical composition (the content of each element) remains unchanged; therefore, the element content of the billet of this embodiment is the same as the element content of the steel of this embodiment. Therefore, further explanation is omitted here.

[0174] [MnS number density at a position 5 mm from the surface]

[0175] In the blank of this embodiment, at a position 5 mm inward from the surface in a direction perpendicular to the surface, the area (area in the observation plane, cross-sectional area of ​​the precipitate in the observation plane) is 1.0~10.0 μm. 2 The number density of MnS is 70.0 particles / mm². 2 Below, areas exceeding 10.0 μm 2 The number density of MnS is 4.0 cells / mm². 2 the following.

[0176] The billet in this embodiment is obtained by forging and heat treating the steel of this embodiment, but the MnS number density does not change significantly during forging and heat treatment. Therefore, it has the same MnS number density as the steel used as the billet. However, when the billet is obtained from the steel, its shape changes due to forging. Regarding the d / 4 portion of the steel, in the billet, it is usually about 5 mm from the surface. Considering the amount of surface grinding when manufacturing the part, the position about 5 mm from the surface of the billet usually becomes the surface of the part. Therefore, in the billet of this embodiment, the MnS number density at the position 5 mm from the surface is specified.

[0177] The MnS number density at a position 5 mm from the surface can be determined as follows: A test piece with dimensions of 10 mm (length direction) × 6 mm (width direction) is cut parallel to the axial direction (rolling direction) of the billet, with the observation surface centered at a position 5 mm from the surface. The measurement is then performed using the same method as for steel. In this embodiment, the number density obtained by the above method is defined as the MnS number density at a position 5 mm from the surface.

[0178] [Amount of Al precipitation at a location 5 mm from the surface]

[0179] To suppress abnormal grain growth during carburizing, especially vacuum carburizing, a specified amount of fine AlN precipitation is required during heat treatment, rather than the specified amount of AlN precipitation during hot rolling. In the billet of this embodiment, the amount of precipitated Al is 0.012% by mass or more. If the amount of precipitated Al is low, sufficient resistance to coarsening cannot be obtained. There is no upper limit to the amount of precipitated Al, and it can be 0.060% by mass or less. Preferably, it is 0.050% by mass or less.

[0180] Regarding the Al deposition at a position 5 mm from the surface, test pieces of 4 mm × 4 mm × 10 mm were cut with the position 5 mm from the surface of the billet as the center. Electrolytic extraction was performed, and the obtained particles were analyzed by ICP to calculate the Al deposition amount. During electrolytic extraction, a so-called "10% AA-based electrolyte," namely 10% acetylacetone - 1% tetramethylammonium chloride - methanol solution, was used at a current density of 250~350 A / m³. 2 Extraction (electrolysis) is performed, and the extracted solution is filtered using a filter with a mesh size of 0.05 μm. In this embodiment, the amount of Al deposited by the above method is defined as the amount of Al deposited at a position 5 mm from the surface.

[0181] [Average size of AlN at a position 5mm from the surface]

[0182] In the billet of this embodiment, AlN is precipitated in a fine manner during heat treatment such as normalizing after forging before carburizing. Specifically, the AlN precipitates with an average size (average equivalent circle diameter) of 90 nm or less. This AlN improves the resistance to coarsening during carburizing. If the average size of AlN is large, its contribution to the resistance to coarsening is small.

[0183] The average size of AlN is observed as follows.

[0184] A transmission electron microscope (TEM) specimen is prepared centered at a position 5 mm from the surface of the blank. For example, after preparing a thin film specimen using conventional methods, a thin film TEM sample is prepared by electrolytic grinding. In preparing the thin film TEM sample, a 1 mm thin film specimen is cut out. The cut sheet is then ground on both sides to a thickness of 50 μm, punched with a disc punch at a diameter (φ) of 3 mm, thinned by electrolytic grinding, cleaned with methanol, and dried.

[0185] For the obtained samples, continuous observation was performed using a transmission electron microscope at an appropriate magnification of 50,000 to 500,000. Simultaneously, EDS analysis was used to analyze the precipitates within the field of view. Precipitates with an Al and N detection intensity peak greater than 1.50 times the background were identified as AlN. For those identified as AlN, their equivalent circular diameter was measured. This measurement was repeated until 50 AlN samples were measured. The average equivalent circular diameter of the 50 AlN samples was taken as the average size of the AlN sample.

[0186] <Manufacturing Method>

[0187] The steel and billet of this embodiment can achieve their effects regardless of the manufacturing method, as long as they possess the aforementioned characteristics. Therefore, the steel and billet of this embodiment are not limited to any particular manufacturing method.

[0188] On the other hand, the steel of this embodiment can be stably manufactured according to a manufacturing method including the following steps.

[0189] (I) Secondary refining process, adjusting the chemical composition of molten steel; (II) Slag separation process, after the secondary refining process, separating the slag from the metal by letting the molten steel stand for more than 20 minutes; (III) Casting process, after the slag separation process, casting the molten steel while performing electromagnetic stirring in the mold at a casting speed of 0.5~1.0 m / min from the tundish to the mold to obtain a billet with a specified chemical composition; (IV) Draft rolling process, after heating the billet to 1200~1350°C and holding it for more than 1 hour, draft rolling is performed, and then cooling to below 500°C to obtain a steel billet; (V) Hot rolling process, after heating the steel billet after the draft rolling process to above 1200°C and holding it for more than 30 minutes, hot rolling the steel billet to a final rolling temperature of 950~1200°C, and cooling to below 500°C to obtain steel.

[0190] By controlling them as a series of processes, it is possible to control the amount of dissolved Al, the amount of dissolved N, and the number density of MnS of a specified size.

[0191] Furthermore, the blank of this embodiment can be obtained by using the steel of this embodiment obtained by the manufacturing method including the above-described process as the blank, and further by the manufacturing method including the following process.

[0192] (VI) A forging process in which steel is heated to a temperature above 1000°C, then forged, and cooled to below 300°C at an average cooling rate of 0.1~1.0°C / s.

[0193] (VII) A heat treatment process in which the steel after the forging process is heated to a temperature range of 900~1020°C, held for 10~100 minutes, and then cooled to below 300°C at an average cooling rate of 0.1~1.0°C / s.

[0194] Furthermore, by supplying the blank of this embodiment obtained using a manufacturing method including the above-described steps to the following steps, a component (carburized component) can be obtained.

[0195] (VIII) The blank of this embodiment is subjected to a vacuum carburizing process in a temperature range of 900~1000°C.

[0196] The following is a description of each process.

[0197] [Secondary refining process]

[0198] In the secondary refining process, the chemical composition of the molten steel is adjusted.

[0199] However, although the steel in this embodiment does not need to contain Ti (preferably in small amounts), Ti is present as an impurity. Therefore, in the slag separation process described later, such Ti needs to be separated as slag to separate Ti oxides such as TiO and TiO2, as well as TiS. In this embodiment, Ti is added in the secondary refining process so that the Ti content of the molten steel is 0.020% by mass or more. With such Ti addition, especially the large-scale generation of Ti oxides, the slag easily solidifies, small slag particles do not enter the molten steel, and separation in the next slag separation process becomes easier.

[0200] On the other hand, if the Ti content is too high, it cannot be discharged as slag, and the Ti content of the steel is excessive. Therefore, the added Ti is less than 1.0% by mass.

[0201] [Slag Separation Process]

[0202] In the slag separation process, after the secondary refining process, the slag is separated from the metal by letting the molten steel stand for more than 20 minutes.

[0203] After the secondary refining process, by allowing the mixture to stand for at least 20 minutes, Ti oxides and TiS are appropriately formed as slag and separated from the metal by gravity. This process reduces the Ti content that is mixed into the metal as an impurity. To prevent re-oxidation, the separated slag can be further removed. When the standing time is less than 20 minutes, the separation of Ti oxides and TiS is insufficient. Ti oxides and TiS affect the distribution of MnS; therefore, when the separation of Ti oxides and TiS is insufficient, coarse MnS is formed, and the required number density of MnS in the steel of this embodiment cannot be obtained within the specified size. Furthermore, coarse AlN is generated using the coarse MnS as a core; therefore, by suppressing the formation of coarse MnS, the formation of coarse AlN can be suppressed.

[0204] Here, the completion of the secondary refining process refers to the moment when all refining treatments are completed and the molten steel is no longer forcibly stirred. Additionally, "setting" refers to allowing the steel to settle.

[0205] There is no upper limit to the settling time, but considering productivity, the settling time can be set to less than 60 minutes.

[0206] There are no restrictions on the method for slag removal; any known method may be used. Furthermore, in the processes up to the secondary refining stage, the composition of the molten steel is adjusted to obtain a billet with a specified chemical composition, taking into account the changes in chemical composition during the slag separation process.

[0207] [Casting Process]

[0208] In the casting process, while the molten steel after the slag separation process is cast at a pouring speed of 0.5~1.0 m / min from the tundish to the mold and electromagnetic stirring is performed in the mold, a billet with a specified chemical composition is obtained.

[0209] Here, the formation of MnS is suppressed by adjusting the casting speed.

[0210] When the casting speed is below 0.5 m / min, productivity is poor. On the other hand, if the casting speed exceeds 1.0 m / min, the final solidification position of the billet along the axis is prone to change, and the segregation of internal impurities becomes greater.

[0211] Furthermore, electromagnetic stirring can reduce Mn segregation and suppress MnS precipitation. Without electromagnetic stirring, excessive MnS precipitation occurs. The conditions for electromagnetic stirring can be within a known range.

[0212] [Blank rolling process]

[0213] [Hot rolling process]

[0214] In the billet rolling process, the billet is heated to 1200~1350℃ and held for more than 1 hour before being rolled. Then, it is cooled to below 500℃ at an average cooling rate of 0.1~1.0℃ / s to produce a steel billet.

[0215] In the hot rolling process, the steel sheet after the billet rolling process is heated to above 1200°C, held for more than 30 minutes, and then hot rolled at a final rolling temperature of 950~1200°C, and cooled to below 500°C at an average cooling rate of 0.1~1.0°C / s.

[0216] Through these rolling processes, steel of a specified shape is obtained. When hot rolling is performed as bar rolling, bar steel is obtained as steel.

[0217] Furthermore, by rolling under the above conditions, even if AlN precipitates during casting, it can be dissolved to a certain extent, and the precipitation of AlN is suppressed. Therefore, steel with high solid solution Al content and solid solution N content can be obtained.

[0218] If the heating temperature in the billet rolling process is below 1200℃ or the holding time is less than 1 hour, the coarse AlN formed during casting cannot be dissolved.

[0219] On the other hand, when the heating temperature exceeds 1350°C, the furnace refractory is damaged, so it is set to below 1350°C.

[0220] Furthermore, productivity decreases when the average cooling rate falls below 0.1°C / s up to 500°C. On the other hand, the steel becomes excessively hard when the average cooling rate exceeds 1.0°C / s.

[0221] If the heating temperature in the hot rolling process is below 1200℃ or the holding time is below 30 minutes, the solid solution of AlN becomes insufficient.

[0222] When the final rolling temperature is below 950℃, the AlN precipitated during casting cannot be fully dissolved, resulting in insufficient dissolved Al and N content in the steel. On the other hand, when the temperature exceeds 1200℃, grain growth occurs, and γ grains become coarse, resulting in a coarse microstructure.

[0223] When the average cooling rate to below 500°C during the hot rolling process is less than 0.1°C / s, coarse AlN may precipitate. On the other hand, when the average cooling rate exceeds 1.0°C / s, the hardness of the steel becomes excessively high.

[0224] Through the above-described processes, the steel of this embodiment can be obtained.

[0225] [Forging process]

[0226] By forging this steel, a billet can be made. There are no restrictions on the forging conditions; for example, the steel can be heated to a temperature above 1000°C and then forged, and then cooled to below 300°C.

[0227] [Heat treatment process]

[0228] In the heat treatment process, the steel after the forging process is heated to a temperature range of 900~1020℃, held for 10~100 minutes, and then cooled to below 300℃ at an average cooling rate of 0.1~1.0℃ / s.

[0229] This allows AlN to precipitate.

[0230] When the heating temperature is below 900℃, AlN may not precipitate sufficiently. On the other hand, when the temperature exceeds 1020℃, the precipitated AlN may dissolve again, potentially resulting in insufficient precipitation. Additionally, the microstructure may become coarse.

[0231] If the holding time is less than 10 minutes, AlN may not be fully precipitated. On the other hand, if the holding time exceeds 100 minutes, AlN will undergo Ostwald ripening or the precipitates will grow into grains, reducing pinning force and potentially causing abnormal grain growth during vacuum carburizing.

[0232] Furthermore, productivity decreases when the average cooling rate is below 0.1°C / s. On the other hand, if it exceeds 1.0°C / s, the microstructure becomes coarse.

[0233] After the above processes, the blank of this embodiment can be obtained.

[0234] [Vacuum Carburizing Process] In addition, to obtain a specified shape, the blank is machined as needed and then carburized, thereby forming mechanical parts such as gears. The carburizing conditions are not limited, for example, vacuum carburizing at 900~1000°C.

[0235] Example

[0236] The molten steel with the specified chemical composition undergoes secondary refining. The addition of Ti at this stage is described below.

[0237] (A) Add 0.025% by mass of Ti during secondary refining.

[0238] (B) No Ti addition

[0239] In addition, slag separation is performed on the molten steel after secondary refining under any of the following conditions.

[0240] (A) After the secondary refining process is completed, let the molten steel stand for 20 to 60 minutes.

[0241] (B) After the secondary refining process, let the molten steel stand for less than 20 minutes.

[0242] Then, casting is carried out under any of the following conditions.

[0243] (A) Equipped with electromagnetic stirring, casting speed 0.6 m / min

[0244] (B) No electromagnetic stirring, casting speed 1.2m / min

[0245] Thus, a cast billet with the chemical composition recorded in Table 1 is obtained.

[0246] In Table 1, column F1 contains the calculation result of [Al] / ([N]-(-0.0013×[Al] / [N]+0.007)). Column F2 contains the calculation result of [Al]×([N]-(-0.0013×[Al] / [N]+0.007))×10000.

[0247] Table 1 shows the values ​​of Ti and Nb, which need to be adjusted at minute levels.

[0248] The obtained billet is heated to the heating temperature recorded in Tables 2 and 3, held, and then rolled. After that, it is cooled to below 500°C at an average cooling rate of 0.1~1.0°C / s to obtain a steel billet.

[0249] The steel billet after the initial rolling is heated to the heating temperature recorded in Table 2, held, and then finished rolled at the final rolling temperature of Table 2. Then, it is cooled to below 500°C at an average cooling rate of 0.1~1.0°C / s.

[0250] Thus, a bar steel with a diameter of 30mm (φ30mm) is manufactured.

[0251] For the obtained bar steel, the amount of dissolved Al and dissolved N in the d / 2 and d / 4 sections, as well as the number density of MnS in the specified dimensions of the d / 4 section, are determined by the above method.

[0252] The MnS samples were quantitatively evaluated by removing Fe from all elements in the obtained X-ray energy spectra, and the results showed that they contained more than 30% by mass of Mn.

[0253] The results are shown in Tables 4 and 5.

[0254] Table 1

[0255]

[0256] Table 2

[0257]

[0258] Table 3

[0259]

[0260] Table 4

[0261]

[0262] Table 5

[0263]

[0264] The obtained bar steel is cut into sections perpendicular to the length direction to a length of 600 mm, heated at 1200°C for 20 minutes, hot-forged to a diameter of 25 mm, and then cooled to below 300°C.

[0265] Then, heat it to 925°C, hold it for 60 minutes, and then heat treat it by cooling it to below 300°C to obtain the billet.

[0266] For the obtained billet, the amount of Al precipitation, the number density of MnS of a specified size, and the average size of AlN were measured at a position 5 mm from the surface using the above method.

[0267] The results are shown in Table 6.

[0268] Table 6

[0269]

[0270] The obtained billet was machined into test pieces with a diameter (φ) of 20 mm and a rolling direction of 50 mm. The test pieces were then heated to 1000 °C under reduced pressure of 100 Pa, and after a 60-minute soaking time, vacuum carburizing was performed for 30 minutes followed by diffusion for 76 minutes. The temperature was then lowered to 880 °C and held for 30 minutes before quenching by immersion in oil at 80 °C. Finally, a low-temperature tempering process was performed, holding the sample at 180 °C for 120 minutes.

[0271] For blanks after vacuum carburizing (assuming they are carburized parts), the following methods are used to evaluate their resistance to coarsening.

[0272] The test piece was cut from the blank after vacuum carburizing so that a cross section perpendicular to the length direction (C section: a circular cross section with a diameter (φ) of 20 mm) could be observed at the center of the length direction (25 mm from the end of the length direction). The C section was then mirror-polished.

[0273] The polished surface was etched with a mixed solution of picric acid and ethanol (4g of picric acid per 100ml of ethanol) to bring out the old austenite grain boundaries.

[0274] Then, the surface and core of the polished surface were observed using an optical microscope, and the area fraction of old austenite (γ) grains with an equivalent circle diameter of 200 μm or more was measured within the observed area. For the surface, the entire circumference of the 500 μm region extending from the outer periphery of the cross-section towards the center was used as the observation range. For the core, an 8 mm × 8 mm region including half the cross-section (the center) was used as the observation range.

[0275] The observation results show that the presence of old austenite (γ) grains larger than 200 μm in diameter (equivalent to a circle) at both the surface and core locations, with a total area ratio exceeding 0.5%, is considered an abnormal grain. The presence of abnormal grains at any location is considered unacceptable, while the absence of abnormal grains exceeding 0.5% at any location is considered acceptable. Specifically, the area of ​​particles larger than 200 μm is calculated through image analysis, and the area ratio is determined by dividing this area by the total observed area.

[0276] The results are shown in Table 7.

[0277] Table 7

[0278]

[0279] As shown in Tables 1 to 7, when steel with a specified chemical composition, ensuring the solid solution content of Al and N, and controlling the number density of MnS of a specified size, such as No.1 to No.15, is used as raw material, and a billet with a specified amount of Al precipitation is produced by forging and heat treatment, the billet can suppress abnormal grain growth caused by vacuum carburizing (excellent resistance to coarsening).

[0280] In contrast, in manufacturing Nos. 16 to 23, the chemical composition is not within the scope of this invention, and the resistance to coarsening during vacuum carburizing is poor.

[0281] Specifically, in manufacturing No. 16, the sol.Al content of the steel was too high, and the N content did not satisfy equations (1) and (2). In addition, the amount of dissolved N in the steel was low, and the average size of AlN in the billet was coarse. As a result, the resistance to coarsening after vacuum carburizing of this billet was insufficient.

[0282] In Manufacturing No. 17, the sol.Al content of the steel was too low. Furthermore, the amount of dissolved Al in the steel was insufficient, resulting in a low amount of Al precipitated in the billet. Consequently, the resistance to coarsening after vacuum carburizing of this billet was inadequate.

[0283] In Manufacturing No. 18, the N content of the steel was too high, and the N content did not satisfy Equation (2). Furthermore, the average size of AlN in the billet was too large. As a result, the resistance to coarsening after vacuum carburizing of this billet was insufficient. In Manufacturing No. 19, the N content of the steel was too low. Furthermore, the amount of dissolved N in the steel was low, and the amount of Al precipitated in the billet was too small. As a result, the resistance to coarsening after vacuum carburizing of this billet was insufficient.

[0284] In manufacturing No. 20, the Ti content of the steel was too high. Additionally, the amount of dissolved N in the steel was low, resulting in insufficient Al precipitation in the billet. Consequently, the resistance to coarsening after vacuum carburizing of this billet was inadequate.

[0285] In manufacturing No. 21, the Ti content in the steel was too high. Additionally, the amount of dissolved N in the steel was low, and the amount of Al precipitated in the billet was insufficient. Furthermore, the MnS number density in both the steel and the billet was too high. As a result, the resistance to coarsening after vacuum carburizing of this billet was insufficient.

[0286] In manufacturing No. 22, equation (1) is not satisfied. Furthermore, the amount of dissolved Al in the steel is insufficient. Additionally, the amount of Al precipitated from the billet is too small. As a result, the resistance to coarsening after vacuum carburizing of the billet is inadequate.

[0287] In manufacturing No. 23, equation (2) is not satisfied. Furthermore, the amount of dissolved N in the d / 2 portion of the steel is insufficient, and the size of AlN in the billet is too large. As a result, the resistance to coarsening after vacuum carburizing of this billet is inadequate.

[0288] In manufacturing No. 24, equation (2) is not satisfied. Furthermore, the amount of dissolved Al and dissolved N in the d / 2 portion of the steel is low. Additionally, the amount of Al precipitated from the billet is too small. As a result, the resistance to coarsening after vacuum carburizing of the billet is insufficient.

[0289] Furthermore, in manufacturing methods Nos. 25-29, the manufacturing methods are not preferred, resulting in the following: in the steel, the amount of dissolved Al and N, or the number density of MnS of a specified size, deviates from the scope of this invention. Additionally, in the billet, the average size of AlN and the number density of MnS of a specified size deviate from the scope of this invention. Consequently, the resistance to coarsening during vacuum carburizing is poor.

[0290] Specifically, in manufacturing No. 25, Ti was not added during the secondary refining process. Therefore, the amount of Ni dissolved in the steel was excessive, and the area exceeding 10.0 μm in both the steel and the billet was significant. 2 The MnS number density is too high. As a result, the resistance to coarsening after vacuum carburizing of the billet is insufficient.

[0291] In manufacturing No. 26, insufficient settling (placement) was not performed during the slag separation process. Therefore, the amount of nitrogen dissolved in the steel was excessive, and the area within the steel was 1.0~10.0 μm. 2 The number density of MnS is too high, and its area exceeds 10.0 μm in steel and billets. 2 The MnS number density is too high. As a result, the resistance to coarsening after vacuum carburizing of the billet is insufficient.

[0292] In manufacturing No. 27, the casting process involved high pouring speeds, and electromagnetic stirring was not employed. Therefore, the surface area in the steel and billet was 1.0~10.0 μm. 2 The MnS number density is too high, and the AlN size in the billet is too large. As a result, the resistance to coarsening after vacuum carburizing of the billet is insufficient.

[0293] In manufacturing No. 28, the heating temperature during the billet rolling process was low. Therefore, the amount of dissolved Al in the steel was too small, and the AlN size in the billet was too large. As a result, the resistance to coarsening after vacuum carburizing of the billet was insufficient.

[0294] In manufacturing No. 29, the heating temperature during the hot rolling process was low. Therefore, the dissolved Al and N content in the d / 2 section of the steel was too low, and the AlN size in the billet was too large. As a result, the resistance to coarsening after vacuum carburizing of the billet was insufficient.

[0295] For Manufacturing No. 22, unlike the above, the resistance to grain coarsening in conventional gas carburizing, which is less prone to abnormal grain growth compared to vacuum carburizing, was also evaluated.

[0296] Specifically, after carburizing at 1000°C for 166 minutes in an atmosphere with a carbon potential of 0.8, the temperature is lowered to 880°C while maintaining the carbon potential at 0.8. This temperature is held for 30 minutes, followed by quenching by immersion in oil at 80°C. Then, a low-temperature tempering process is performed, holding the temperature at 180°C for 120 minutes.

[0297] After low-temperature tempering, the coarsening resistance is evaluated based on whether the total area ratio of old austenite (γ) grains with an equivalent circle diameter of 200 μm or more is greater than 0.5%, using the same method as when vacuum carburizing was performed.

[0298] As a result, in gas carburizing, if the above-mentioned criteria are met, sufficient resistance to coarsening is exhibited.

[0299] The results show that No. 22 exhibits resistance to grain coarsening in gas carburizing, but its resistance to grain coarsening is insufficient in vacuum carburizing, which is the target of this invention. This comparative example also demonstrates that vacuum carburizing, compared to gas carburizing, presents much stricter conditions for suppressing abnormal grain growth.

[0300] This invention demonstrates excellent resistance to coarsening under very stringent conditions for vacuum carburizing, which suppresses abnormal grain growth, and thus it can be said that it also has sufficient resistance to coarsening under gas carburizing.

[0301] Additionally, for No.1 and No.19, test pieces measuring 12×14×59mm were taken from the billet. A 2mm R10 notch was machined at the center of the 12×59mm face along its length. These pieces were heated to 1000°C under reduced pressure (100Pa), and after a 60-minute soaking time, vacuum carburizing was performed for 30 minutes followed by diffusion for 76 minutes. Then, the temperature was lowered to 880°C, held for 30 minutes, and then quenched by immersion in oil at 80°C. Finally, a low-temperature tempering process was performed at 180°C for 120 minutes. After vacuum carburizing, to maintain the carburized state of the groove opening, the groove opening and its surface were not finished; the other five surfaces were finished by removing 2mm at a time. Figure 3 The Charpy test piece with an R10 notch is shown. For this test piece, a Charpy test was performed at room temperature according to JIS Z 2242:2023.

[0302] As a result, the Charpy impact value of No. 1 was 61 J / cm. 2 The Charpy impact value of No. 19 is 39 J / cm. 2 .

[0303] If the Charpy impact value is 50 J / cm2 Therefore, it can be determined that a sufficient Charpy impact value was obtained. That is, No.1 obtained a sufficient Charpy impact value, but No.19 did not obtain a sufficient Charpy impact value.

[0304] Specifically, in Patent Document 3 No. 19, the proportion of old austenite (γ) grains with an equivalent circle diameter of 200 μm or more is 0.5% or more in terms of total area, but the proportion of old austenite (γ) grains with an equivalent circle diameter of 125 μm or more in terms of total area is 15%. This result indicates that, based on the benchmark of Patent Document 3, the region with a grain size of 3 or more is less than 20%, the anomalous grain growth is excellent, but even if this benchmark is met, the Charpy impact value is insufficient.

[0305] Based on this result, it can be said that the steel of the present invention has superior resistance to coarsening compared to Patent Document 3.

[0306] Industrial availability

[0307] According to the present invention, a billet that suppresses abnormal grain growth during carburizing and a steel material that becomes the billet can be obtained. Furthermore, if a manufacturing method for a component using this billet is applied, a component in which abnormal grain growth is suppressed even after vacuum carburizing can be obtained. Since suppressing abnormal grain growth can suppress the reduction in fatigue properties, etc., the present invention has high industrial applicability as a billet for mechanical components that are carburized into components.

[0308] Explanation of reference numerals in the attached figures

[0309] 1: Steel (bar steel)

[0310] d: diameter

[0311] S:d / 4 parts

[0312] O: d / 2 (core)

[0313] RD: Rolling direction

[0314] TD: Width direction

Claims

1. A type of steel, characterized in that, by mass%, it has a chemical composition comprising, in weight percent: C: 0.10~0.30%, Si: 0.03~1.50%, Mn: 0.35~1.50%, P: 0.002~0.020%, S: 0.002~0.050%, Cr: 0.05~2.30%, Al: 0.020~0.060%, N: 0.011~0.030%, Ti: less than 0.007%, Nb: less than 0.100%, Mo: 0~0.40%, O: 0~0.0050%, V: 0~0.15%, B: 0~0.0050%, Cu: 0~0.40%, Ni: 0~0.30%, Sn: 0~0.100%, Ca: 0~0.0050%, Mg: 0~0.0050%, and the remainder being Fe and impurities. When the Al content (in mass%) is defined as [Al] and the N content is defined as [N], the [Al] and the [N] satisfy the following equations (1) and (2). In a cross-section perpendicular to the rolling direction, when the length of the line segment connecting the centroid of the cross-section to the surface of the cross-section closest to the centroid is defined as d / 2, the position at a distance of d / 2 from the surface towards the centroid is defined as part d / 2, and the position at a distance of d / 4 from the surface towards the centroid is defined as part d / 4. In either d / 2 or d / 4, the amount of Al dissolved in the solution is 0.018% by mass or more, and the amount of N dissolved in the solution is 0.009% by mass or more. In the d / 4 section, the area is 1.0~10.0 μm. 2 The number density of MnS is 70.0 particles / mm². 2 Below, areas exceeding 10.0 μm 2 The number density of MnS is 4.0 cells / mm². 2 the following, 1.50≤[Al] / ([N]-(-0.0013×[Al] / [N]+0.007))≤5.00 (1) 1.50≤[Al]×([N]-(-0.0013×[Al] / [N]+0.007))×10000≤10.00 (2).

2. The steel according to claim 1, characterized in that, The chemical composition contains one or more of the following by mass percent: Ti: more than 0% and less than 0.007%, Nb: more than 0% and less than 0.100%, Mo: 0.03~0.40%, O: 0.0001~0.0050%, V: 0.01~0.15%, B: 0.003~0.0050%, Cu: 0.01~0.40%, Ni: 0.01~0.30%, Sn: 0.010~0.100%, Ca: 0.0001~0.0050%, and Mg: 0.0001~0.0050%.

3. A billet, characterized in that, It has a chemical composition, by mass%, consisting of C: 0.10~0.30%, Si: 0.03~1.50%, Mn: 0.35~1.50%, P: 0.002~0.020%, S: 0.002~0.050%, Cr: 0.05~2.30%, Al: 0.020~0.060%, N: 0.011~0.030%, Ti: less than 0.007%, Nb: less than 0.100%, Mo: 0~0.40%, O: 0~0.0050%, V: 0~0.15%, B: 0~0.0050%, Cu: 0~0.40%, Ni: 0~0.30%, Sn: 0~0.100%, Ca: 0~0.0050%, Mg: 0~0.0050%, and the remainder being Fe and impurities. When the Al content in the chemical composition is set as [Al] and the N content is set as [N], the [Al] and the [N] satisfy the following equations (1) and (2). At a position 5 mm inward from the surface in a direction perpendicular to the surface, the area is 1.0~10.0 μm. 2 The number density of MnS is 70.0 particles / mm². 2 Below, areas exceeding 10.0 μm 2 The number density of MnS is 4.0 cells / mm². 2 the following, The amount of Al precipitated is greater than 0.012% by mass. The average size of AlN is below 90nm. 1.50≤[Al] / ([N]-(-0.0013×[Al] / [N]+0.007))≤5.00 (1) 1.50≤[Al]×([N]-(-0.0013×[Al] / [N]+0.007))×10000≤10.00 (2).

4. The billet according to claim 3, characterized in that, The chemical composition contains one or more of the following by mass percent: Ti: more than 0% and less than 0.007%, Nb: more than 0% and less than 0.100%, Mo: 0.03~0.40%, O: 0.0001~0.0050%, V: 0.01~0.15%, B: 0.003~0.0050%, Cu: 0.01~0.40%, Ni: 0.01~0.30%, Sn: 0.010~0.100%, Ca: 0.0001~0.0050%, and Mg: 0.0001~0.0050%.

5. A method for manufacturing a component, characterized in that, The process includes a vacuum carburizing process in which the blank described in claim 3 is subjected to vacuum carburizing in a temperature range of 900~1000°C.

Images