Steel for non-quenched and tempered forging, non-quenched and tempered forged steel, and non-quenched and tempered forged component

Abstract

A non-quenched and tempered steel for forging, which has a prescribed chemical composition of C: 0.40 to 0.60 mass%, Si: 0.10 to 0.40 mass%, Mn: 0.30 to 0.80 mass%, P: 0.007 to 0.050 mass%, S: 0.010 to 0.070 mass%, Cr: 0.30 to 0.80 mass%, V: more than 0.30 mass% but not more than 0.38 mass%, Al: more than 0 mass% but not more than 0.050 mass%, N: more than 0 mass% but not more than 0.0080 mass%, and Ca: 0.0002 to 0.0050 mass%, and wherein X represented by formula (1) is 1.32 to 1.50. X = C + 0.28 Mn-1.03 S + 0.323 Cr + 1.69 V (1) (In the formula, C, Mn, S, Cr, and V respectively represent the content of C, Mn, S, Cr, and V in the steel in mass%, and the element not included is zero.

Description

Technical Field

[0001] The present invention relates to non-quenched and tempered forging steel, non-quenched and tempered forging steel and non-quenched and tempered forging parts. Background Art

[0002] Against the backdrop of increasingly severe global warming issues, the trend toward improving automobile fuel efficiency is accelerating. Lightening the vehicle body is effective in improving fuel efficiency, and to achieve this, automotive parts are required to have higher strength. Furthermore, forged parts are used for automotive parts such as connecting rods. However, for these automotive forged parts, non-tempered forged parts that are not heat-treated after forging are required to exhibit high strength from the perspectives of cost reduction and manufacturing efficiency.

[0003] As for non-quenched and tempered forged parts, Patent Document 1, for example, states that by satisfying predetermined composition ranges, predetermined formulas (1) to (3), and a predetermined structure, non-quenched and tempered forged parts can be obtained that achieve both high strength and excellent manufacturability, particularly preventing surface cracks during continuous casting. Furthermore, a non-quenched and tempered forging steel useful for producing such non-quenched and tempered forged parts is disclosed.

[0004] Patent Document 2 states that by satisfying specified component ranges, specified formulas (1) and (2), and a specified steel metallographic structure, a fracture-separation type connecting rod formed part can be obtained, which can suppress the occurrence of defects during fracture separation and achieve both improved strength and improved fit. Furthermore, a connecting rod using this fracture-separation type forming part is disclosed.

[0005] Patent Document 3 discloses that by satisfying parameters such as predetermined component ranges, a predetermined formula (1), a predetermined structure, and a combination of the value of formula (1) and the Vickers hardness, a fracture-separable connecting rod formed part can be obtained, which achieves both strength of the connecting rod portion and machinability of both the large and small ends. Furthermore, a connecting rod using the formed connecting rod part and a method for manufacturing the connecting rod are disclosed.

[0006] Patent Document 4 discloses that a rolled material suitable for manufacturing a connecting rod having a through-hole portion fracture-separated into a substantially semicircular shape for attachment to a crankshaft can be obtained by satisfying the prescribed ranges of components, the combination parameters of components and structure (1), the prescribed formula (2), a prescribed structure, and the prescribed form of sulfide-based inclusions. Furthermore, a hot-forged part obtained using the rolled material and a fracture-separation type connecting rod obtained using the hot-forged part are disclosed.

[0007] Patent Document 5 discloses a method for producing a high-strength connecting rod forging that effectively strengthens only the connecting portion, which requires high yield strength and fatigue strength, and easily fractures and separates, without requiring significant additional energy, processing, material, and mold costs. This method discloses that a rough connecting rod body is formed by hot forging using ferrite / pearlite non-quenched and tempered steel within specified composition ranges. During cooling, strain is not applied to the large end portion. Instead, the connecting portion of this rough body is subjected to a press working process combining shape correction and strain aging in a hot range of 200-700°C at a processing rate of 3-40%, thereby strengthening the connecting portion.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2019-143236

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2017-179476

[0012] Patent Document 3: Japanese Patent Application Laid-Open No. 2017-179475

[0013] Patent Document 4: Japanese Patent Application Laid-Open No. 2007-277705

[0014] Patent Document 5: Japanese Patent Application Laid-Open No. 2006-052432 Summary of the Invention

[0015] Problems to be solved by the invention

[0016] Non-tempered forging parts can be obtained by subjecting the non-tempered forging steel obtained by hot forging non-tempered forging steel to machining such as cutting and / or fracture separation. In the case where the non-tempered forging part is a connecting rod, from the perspective of reducing the manufacturing cost of the part, after the non-tempered forging steel is machined, it is fracture separated in a cold state so that the large end portion becomes two approximate semicircles. In addition, during the fracture separation in the cold state, poor segmentation such as deformation will not occur, and parts with high fit can be obtained, which is called "excellent fracture separation". Therefore, non-tempered forging parts such as connecting rods require high strength and excellent fracture separation. However, the problem that arises with the high strength of steel is that during fracture separation, part of the fracture surface is damaged or deformed, making it difficult to achieve both high strength and excellent fracture separation.

[0017] Patent Documents 1 to 3 do not investigate fracture splitting properties, and further improvements are considered necessary to ensure excellent fracture splitting properties. Patent Document 4 investigates a technique for suppressing deformation during fracture splitting, but further improvements are considered necessary to ensure even better fracture splitting properties. Furthermore, Patent Document 5 also attempts to improve fracture splitting properties, but further improvements are considered necessary to ensure even better fracture splitting properties.

[0018] The present invention has been proposed in view of the above situation, and its purpose is to provide a non-tempered forged part showing high strength such as an engine part such as a connecting rod on an automobile, a suspension component, etc., a non-tempered forged steel showing high strength and excellent fracture splitting properties useful for the manufacture of the non-tempered forged part, and a steel for non-tempered forging.

[0019] Means of solving the problem

[0020] A first embodiment of the present invention is a non-quenched and tempered forging steel containing

[0021] C: 0.40-0.60% by mass,

[0022] Si: 0.10-0.40 mass%,

[0023] Mn: 0.30-0.80 mass%,

[0024] P: 0.007-0.050% by mass

[0025] S: 0.010-0.070 mass%,

[0026] Cr: 0.30-0.80 mass%,

[0027] V: more than 0.30 mass% and not more than 0.38 mass%,

[0028] Al: more than 0 mass% and not more than 0.050 mass%,

[0029] N: more than 0 mass% and not more than 0.0080 mass%,

[0030] Ca: 0.0002-0.0050 mass%,

[0031] Mo: 0-0.05% by mass

[0032] Cu: 0-0.20 mass%,

[0033] Ni: 0-0.20 mass%,

[0034] Ti: 0-0.030 mass%,

[0035] Pb: 0-0.1% by mass

[0036] Bi: 0-0.1% by mass

[0037] Sb: 0-0.1% by mass

[0038] Mg: 0-0.005% by mass

[0039] Zr: 0-0.005% by mass

[0040] Te: 0-0.1 mass%, and

[0041] REM: 0-0.02% by mass

[0042] The balance contains Fe and unavoidable impurities,

[0043] X represented by the following formula (1) is 1.32 to 1.50.

[0044] X=C+0.28Mn-1.03S+0.323Cr+1.69V…(1)

[0045] Here, C, Mn, S, Cr, and V respectively represent the contents of C, Mn, S, Cr, and V in steel in mass %, and elements not contained are represented by zero.

[0046] A second embodiment of the present invention is a non-quenched and tempered forging steel containing

[0047] C: 0.40-0.60% by mass,

[0048] Si: 0.10-0.40 mass%,

[0049] Mn: 0.30-0.80 mass%,

[0050] P: 0.007-0.050% by mass

[0051] S: 0.010-0.070 mass%,

[0052] Cr: 0.30-0.80 mass%,

[0053] V: more than 0.30 mass% and not more than 0.38 mass%,

[0054] Al: more than 0 mass% and not more than 0.050 mass%,

[0055] N: more than 0 mass% and not more than 0.0080 mass%,

[0056] Ca: 0.0002-0.0050 mass%,

[0057] Mo: 0-0.05% by mass

[0058] Cu: 0-0.20 mass%,

[0059] Ni: 0-0.20 mass%,

[0060] Ti: 0-0.030 mass%,

[0061] Pb: 0-0.1% by mass

[0062] Bi: 0-0.1% by mass

[0063] Sb: 0-0.1% by mass

[0064] Mg: 0-0.005% by mass

[0065] Zr: 0-0.005% by mass

[0066] Te: 0-0.1 mass%, and

[0067] REM: 0-0.02% by mass

[0068] The balance contains Fe and unavoidable impurities,

[0069] X represented by the following formula (1) is 1.32 to 1.50.

[0070] X=C+0.28Mn-1.03S+0.323Cr+1.69V…(1)

[0071] Here, C, Mn, S, Cr, and V respectively represent the contents of C, Mn, S, Cr, and V in steel in mass %, and elements not contained are represented by zero.

[0072] A third aspect of the present invention is a non-tempered forged part produced using the non-tempered forged steel described in the second aspect.

[0073] Effects of the Invention

[0074] According to the present invention, it is possible to provide a non-heat-refined forged part exhibiting high strength, a non-heat-refined forged steel exhibiting high strength and excellent fracture splitting properties useful for the production of the non-heat-refined forged part, and a steel for non-heat-refined forging. DETAILED DESCRIPTION

[0075] 1. Non-quenched and tempered forging steel

[0076] As mentioned above, existing non-tempered forged steels have the problem of being difficult to achieve both high strength and excellent fracture separation properties. In particular, when non-tempered forged steel is used as forged steel for connecting rods, for example, if the non-tempered forged steel is fractured and separated to obtain a connecting rod, if the toughness of the non-tempered forged steel is high, deformation will occur during the fracture separation, resulting in a high number of defective products and a reduced yield rate. In addition, if the deformation is large even though the product is not defective, the inner diameter needs to be rough-machined before finishing, or it will become a factor in increasing the allowance for finishing, resulting in increased manufacturing costs. Based on the above situation, the inventors of the present invention have focused on the fact that in order to suppress deformation during fracture separation, it is important to reduce the toughness of the non-tempered forged steel used for fracture separation. With the goal of obtaining non-tempered forged parts such as connecting rods with high strength, they have conducted in-depth research to achieve both the high strength of the non-tempered forged steel used for the manufacture of the non-tempered forged parts and the reduction of toughness required to achieve excellent fracture separation properties.

[0077] As a result, it was discovered that in order to achieve both high strength and reduced toughness for achieving excellent fracture splitting properties in non-tempered forged steel used in the manufacture of non-tempered forged parts, it is necessary to control the chemical composition of the non-tempered forging steel used as the raw material for the non-tempered forging steel and the parameter X within appropriate ranges. In particular, in order to achieve reduced toughness, it is necessary to simultaneously satisfy the following (I) to (IV). Non-tempered forging steel that satisfies the following (I) to (IV) exhibits high strength and excellent fracture splitting properties after forging. In other words, the non-tempered forging steel obtained by hot forging the non-tempered forging steel of this embodiment has high strength and exhibits excellent fracture splitting properties. In addition, the use of this non-tempered forging steel enables good fracture splitting, and high-strength non-tempered forged parts can be obtained. When the non-tempered forging part is, for example, a connecting rod, a connecting rod with high strength and high fit of the fracture surface can be obtained.

[0078] (I) The value of X, represented by the formula (1): X = C + 0.28Mn - 1.03S + 0.323Cr + 1.69V, is controlled within the range of 1.32 to 1.50.

[0079] (II) P content is 0.007 mass % or more,

[0080] (III) Ca content of 0.0002 mass % or more,

[0081] (IV) The amount of C is increased to 0.40 mass % or more, the amount of V is increased to more than 0.30 mass %, and the amount of N is suppressed to 0.0080 mass % or less.

[0082] Hereinafter, the parameter X and the ranges of each component will be described in detail.

[0083] [X represented by the following formula (1) is 1.32 to 1.50

[0084] X=C+0.28Mn-1.03S+0.323Cr+1.69V…(1)

[0085] Here, C, Mn, S, Cr, and V respectively represent the contents of C, Mn, S, Cr, and V in the steel in mass %, and elements not contained are represented as zero.]

[0086] The microstructure of non-tempered forging steel obtained by hot forging is a ferrite-pearlite structure with pearlite as the main component. To reduce the toughness of this non-tempered forging steel, it is important to increase the pearlite fraction and reduce the ferrite fraction, thereby reducing the toughness of both the pearlite and ferrite.

[0087] As elements that affect the above-mentioned fractions of pearlite and ferrite, and the toughness of pearlite and ferrite, attention is focused on C, Mn, S, Cr, and V. C is an element that increases the fraction of pearlite and contributes to reducing the fraction of ferrite. Mn is an element that dissolves in ferrite and contributes to reducing the toughness of ferrite. Cr, like Mn, is an element that dissolves in ferrite and contributes to reducing the toughness of ferrite. Cr also contributes to reducing the toughness of pearlite. In addition to the effect of increasing the fraction of pearlite, V also generates fine V carbides when hot forging non-tempered forging steel under normal conditions to produce it, which particularly contributes to reducing the toughness of ferrite. On the other hand, S combines with Mn to form sulfides, which weakens the above-mentioned effects brought about by Mn and also has the effect of increasing the fraction of ferrite.

[0088] The present inventors studied the degree of influence of the above-mentioned elements on the toughness of steel and found the following equation (1) as an expression for the toughness of steel. As mentioned above, S has a negative coefficient because it has an adverse effect on the toughness of steel.

[0089] Furthermore, the absorption energy measured in the examples described later showed an absorption energy of 8 J / cm 2 The range of X for obtaining excellent fracture splitting properties was studied to obtain low toughness below 1.32. As a result, it was found that X needs to be in the range of 1.32 to 1.50. If X is lower than 1.32, when hot forging is performed under normal conditions to produce non-quenched and tempered forging steel, the ferrite fraction increases and the ferrite becomes more ductile, the pearlite fraction decreases and the pearlite becomes more ductile, and the toughness of the steel increases. Therefore, as mentioned above, X is set to 1.32 or more. X is preferably 1.33 or more, more preferably 1.34 or more. On the other hand, if X is higher than 1.50, when hot forging is performed under normal conditions to produce non-quenched and tempered forging steel, supercooled structures such as bainite are generated, and the toughness increases instead. From this point of view, X is below 1.50 as mentioned above. X is preferably below 1.48, more preferably below 1.45.

[0090] [C: 0.40-0.60 mass %]

[0091] C is an element required to reduce toughness. If there is too little C, the toughness becomes high. In addition, the strength is also reduced. From this point of view, the C content needs to be 0.40% by mass or more. The C content is preferably 0.45% by mass or more, and more preferably 0.47% by mass or more. However, if the C content is excessive, when non-quenched and tempered forging steel is produced by conventional hot forging, supercooled structures such as bainite are generated, and the toughness is increased instead. From this point of view, the C content needs to be 0.60% by mass or less. The C content is preferably 0.58% by mass or less, and more preferably 0.56% by mass or less.

[0092] [Si: 0.10-0.40 mass %]

[0093] Si is useful as a deoxidizing element during steel smelting, and when non-tempered forged steel is produced by conventional hot forging, it dissolves in ferrite and is a useful element for reducing the toughness of the steel. To achieve this effect, the Si content needs to be 0.10% by mass or more. The Si content is preferably 0.13% by mass or more, more preferably 0.15% by mass or more. However, if the Si content is excessive, the oxide scale after rolling and forging increases, causing tool wear. Therefore, the Si content needs to be 0.40% by mass or less. The Si content is preferably 0.37% by mass or less, more preferably 0.35% by mass or less.

[0094] [Mn: 0.30-0.80 mass%]

[0095] Mn forms sulfides, which become stress concentration sources and contribute to lower toughness. In addition, the yield strength of the parts can be ensured by solid solution strengthening. In order to achieve these effects, the Mn content is 0.30% by mass or more. The Mn content is preferably 0.35% by mass or more, more preferably 0.40% by mass or more. However, if the Mn content is excessive, when non-quenched and tempered forging steel is produced by conventional hot forging, supercooled structures such as bainite are generated, and toughness increases. From this point of view, the Mn content needs to be 0.80% by mass or less. The Mn content is preferably 0.70% by mass or less, more preferably 0.65% by mass or less.

[0096] [P: 0.007-0.050 mass%]

[0097] In this embodiment, phosphorus (P) is an element necessary to reduce the toughness of steel. To achieve this effect, the P content must be 0.007% by mass or greater. The P content is preferably 0.008% by mass or greater, and more preferably 0.009% by mass or greater. However, excessive P content can induce casting defects such as cracks during continuous casting. From this perspective, the P content must be 0.050% by mass or less. The P content is preferably 0.045% by mass or less, and more preferably 0.040% by mass or less.

[0098] [S: 0.010-0.070 mass %]

[0099] S hardly dissolves in steel, but forms sulfides, which have the effect of promoting low toughness by utilizing the effect of stress concentration. In addition, it also has the effect of improving machinability that may be required during part manufacturing. In order to achieve this effect, the S content needs to be 0.010% by mass or more. The S content is preferably 0.015% by mass or more, and more preferably 0.020% by mass or more. On the other hand, excess S forms coarse and elongated sulfides, which hinder the progress of destructive cracks, thereby causing high toughness. In addition, it is also the cause of continuous casting cracks, cracks and defects during hot forging, and even the cause of reduced fatigue strength. Therefore, the S content needs to be 0.070% by mass or less. The S content is preferably 0.060% by mass or less, and more preferably 0.055% by mass or less.

[0100] [Cr: 0.30-0.80 mass%]

[0101] Cr is an element required for toughness reduction. In addition, it is also an element that can ensure the yield strength of parts by solid solution strengthening. In order to achieve these effects, the Cr content is set to 0.30% by mass or more. The Cr content is preferably 0.35% by mass or more, more preferably 0.40% by mass or more. However, if the Cr content is excessive, when non-quenched and tempered forging steel is manufactured by conventional hot forging, supercooled structures such as bainite are generated, and toughness increases instead. From this viewpoint, the Cr content needs to be 0.80% by mass or less. The Cr content is preferably 0.75% by mass or less, more preferably 0.70% by mass or less.

[0102] [V: higher than 0.30 mass % and lower than 0.38 mass %]

[0103] When non-quenched and tempered forged steel is manufactured by conventional hot forging, V forms fine V carbides in ferrite, which is an element that helps reduce the toughness of non-quenched and tempered forged steel. It was previously believed that even if a large amount of V was added, this effect would be saturated. However, as in the present embodiment, it was found that by adding a large amount of V on the basis of increasing the amount of C and reducing the amount of N, the toughness can be further reduced. Compared with Patent Document 4, because the amount of V is increased to a certain extent, it is possible to ensure better fracture separation properties than the technology of Patent Document 4. In order to achieve the above effect, it is necessary to make the V content more than 0.30% by mass. The V content is preferably 0.31% by mass or more, and more preferably 0.32% by mass or more. In the present embodiment, as mentioned above, it is a ferrite-pearlite steel with pearlite as the main body, and bainite is suppressed as much as possible so that it does not contain bainite in substance. As described above, the non-tempered forging steel of this embodiment sets the components including V and the parameter X within the prescribed ranges, thereby reducing the toughness of ferrite and pearlite and pursuing further improvement in fracture splitting properties, unlike conventional structures.

[0104] On the other hand, if the V content is excessive, during conventional hot forging of non-quenched and tempered forged steel, supercooled structures such as bainite are formed, which in turn increases toughness. Furthermore, continuous casting performance (resistance to surface cracking) is also reduced. From this perspective, the V content should be 0.38% by mass or less. The V content is preferably 0.37% by mass or less, and more preferably 0.36% by mass or less.

[0105] [Al: more than 0 mass % and not more than 0.050 mass %]

[0106] Al is useful as a deoxidizing element during smelting. Furthermore, it is an element with stronger deoxidizing power than Ca. By including an appropriate amount of Al, the formation of CaO can be suppressed. As a result, Ca is actively dissolved in the sulfide, making the sulfide spherical. These spherical sulfides become stress concentration sources, which can reduce toughness. Therefore, the Al content can be higher than 0 mass%. To fully realize the above effects, the Al content can be 0.001 mass% or higher. Al below 0.001 mass% can be included as an unavoidable impurity. However, if Al is included in excess, AlN is generated, resulting in grain refinement and increased toughness. Furthermore, the generation of AlN can lead to a decrease in continuous casting performance (resistance to surface cracking). From this perspective, the Al content needs to be 0.050 mass% or less. The Al content is preferably 0.040 mass% or less, and more preferably 0.020 mass% or less.

[0107] [N: higher than 0 mass % and not more than 0.0080 mass %]

[0108] Nitrogen is an unavoidable impurity, and has traditionally been incorporated at approximately 0.0100 mass%. However, excessive N content results in the formation of coarse V nitrides, preventing the aforementioned toughness-reducing effect from V. Furthermore, continuous casting performance (resistance to surface cracking) deteriorates. Therefore, in this embodiment, the N content is suppressed to 0.0080 mass% or less. The N content is preferably 0.0070 mass% or less, and more preferably 0.0060 mass% or less. The lower the N content, the better, but the lower limit of the N content can be approximately 0.0010 mass%.

[0109] [Ca: 0.0002-0.0050 mass %]

[0110] In the present invention, Ca dissolves in sulfides, spheroidizing them and promoting embrittlement. Furthermore, it is an element that improves machinability and, through effects such as the formation of Belag (a protective film on the tool), is effective in ensuring the machinability that may be required when manufacturing parts. To achieve these effects, the Ca content needs to be 0.0002% by mass or more. The Ca content is preferably 0.0004% by mass or more, and more preferably 0.0006% by mass or more. However, excessive inclusion of Ca saturates the aforementioned effects and leads to increased costs. From this perspective, the Ca content is 0.0050% by mass or less, preferably 0.0040% by mass or less, and more preferably 0.0030% by mass or less.

[0111] [Ballast: Fe and unavoidable impurities]

[0112] In a preferred embodiment, the balance is Fe and inevitable impurities. As inevitable impurities, trace elements (e.g., As, Sb, Sn, etc.) introduced due to the conditions of raw materials, materials, and manufacturing equipment are permitted. Furthermore, for example, N is generally considered an inevitable impurity because the lower the content, the better. However, there are elements whose composition ranges are separately defined as described above. In this specification, the term "inevitable impurities" constituting the balance of the aforementioned "balance: Fe and inevitable impurities" is a concept that excludes elements (N) whose composition ranges are separately defined, Al: less than 0.001 mass%, and any elements with the following inevitable impurity levels. Mo≤0.01 mass%, Cu≤0.02 mass%, Ni≤0.02 mass%, Ti≤0.007 mass%, Pb≤0.00002 mass%, Bi≤0.0001 mass%, Sb≤0.0004 mass%, Mg≤0.0001 mass%, Zr≤0.0002 mass%, Te≤0.0001 mass%, REM≤0.0001 mass%.

[0113] The chemical composition of the present embodiment may not include any of the elements described below. As long as the desired properties can be maintained, any other elements may be further included. Containing any of the elements described below as needed can further improve strength, etc.

[0114] [Mo: 0-0.05 mass %]

[0115] [Cu: 0-0.20 mass%]

[0116] [Ni: 0-0.20 mass%]

[0117] [Ti: 0 to 0.030 mass%]

[0118] Mo, Cu, Ni, and Ti may not be contained. Alternatively, for example, within the above-mentioned amount range, one or more elements selected from the group consisting of Mo, Cu, Ni, and Ti may be contained. The inclusion of these elements can contribute to further reducing the toughness of the steel. In addition, these elements are also elements that contribute to high strength through solid solution strengthening. To achieve this effect, the Mo content can be higher than 0.01% by mass, the Cu content can be higher than 0.02% by mass, the Ni content can be higher than 0.02% by mass, and the Ti content can be higher than 0.007% by mass. However, if the content of these elements is excessive, an undercooled structure such as bainite is generated when non-quenched and tempered forging steel is produced by conventional hot forging, and the toughness is increased instead. From this point of view, the Mo content is 0.05% by mass or less. The Mo content is preferably 0.04% by mass or less, and more preferably 0.03% by mass or less. The Cu and Ni contents are each 0.20% by mass or less. The Cu and Ni contents are each preferably 0.18% by mass or less, and more preferably 0.15% by mass or less. The Ti content is 0.030% by mass or less. The Ti content is preferably 0.025 mass % or less, more preferably 0.020 mass % or less.

[0119] [Pb: 0-0.1 mass%]

[0120] [Bi: 0-0.1 mass %]

[0121] [Sb: 0-0.1 mass %]

[0122] It may not contain Pb, Bi, or Sb. Alternatively, it may contain one or more elements selected from the group consisting of Pb, Bi, and Sb, for example, within the above-mentioned amount range. When these elements are contained, the toughness of the steel may be further reduced. In addition, it also helps to ensure the machinability that may be required during part manufacturing. In order to achieve this effect, the Pb content may be higher than 0.00002 mass%, the Bi content may be higher than 0.0001 mass%, and the Sb content may be higher than 0.0004 mass%. However, if the content of these elements is excessive, the continuous casting performance (resistance to surface cracks) deteriorates. From this point of view, the content of Pb, Bi, and Sb is preferably less than 0.1 mass%. The content of Pb, Bi, and Sb is preferably less than 0.05 mass%, and more preferably less than 0.03 mass%.

[0123] [Mg: 0 to 0.005 mass %]

[0124] [Zr: 0 to 0.005 mass%]

[0125] [Te: 0-0.1 mass %]

[0126] [REM: 0-0.02 mass %]

[0127] Mg, Zr, Te, and REM may not be present. Alternatively, one or more elements selected from the group consisting of Mg, Zr, Te, and REM may be contained within the above-mentioned ranges. When these elements are contained, they exhibit the effect of being fixed in sulfides, spheroidizing the sulfides, and promoting embrittlement. To achieve this effect, the Mg content may be higher than 0.0001% by mass, the Zr content may be higher than 0.0002% by mass, the Te content may be higher than 0.0001% by mass, and the REM content may be higher than 0.0001% by mass. However, excessive contents of these elements increase the cost of the steel. From this perspective, the Mg and Zr contents are each 0.005% by mass or less. The Mg and Zr contents are each preferably 0.004% by mass or less, and more preferably 0.003% by mass or less.

[0128] The Te content is 0.1% by mass or less. The Te content is preferably 0.05% by mass or less, and more preferably 0.03% by mass or less. The REM content is 0.02% by mass or less. The REM content is preferably 0.01% by mass or less, and more preferably 0.005% by mass or less. The term REM refers to elements including lanthanides (the 15 elements from La to Lu), Sc (scandium), and Y (yttrium).

[0129] (Manufacturing method of non-quenched and tempered forging steel)

[0130] The non-tempered forging steel of this embodiment can be manufactured by the following method. First, steel satisfying the aforementioned chemical composition and the range of the parameter X is melted and cast. The casting method is not particularly limited, and a commonly used method can be used. For example, an ingot casting method and a continuous casting method can be used. After casting, hot slab rolling can be performed as needed. Slab rolling can include a heat treatment before slab rolling. The slab rolling conditions are not particularly limited, and commonly used conditions can be used. For example, slab rolling can be performed at 1000°C to 1250°C. The conditions for hot rolling performed after slab rolling are also not particularly limited, and commonly used conditions can be used. For example, hot rolling can be performed at 850 to 1200°C. In addition, the processing performed by hot rolling can also be performed by forging and stretching instead of hot rolling.

[0131] 2. Non-quenched and tempered forging steel

[0132] The non-tempered forging steel of this embodiment has the same chemical composition and parameter X range as the non-tempered forging steel described above. Since the chemical composition and parameter X of the non-tempered forging steel of this embodiment are within the specified ranges, it is possible to achieve both high strength and excellent fracture splitting properties.

[0133] (Manufacturing method of non-quenched and tempered forged steel)

[0134] The non-tempered forging steel of this embodiment can be obtained by hot forging the non-tempered forging steel described above. The microstructure of non-tempered forging steel is generally influenced primarily by the heating temperature before hot forging and the cooling rate after hot forging. However, the non-tempered forging steel of this embodiment, by appropriately controlling the steel composition and the aforementioned X value, can achieve the aforementioned pearlite-based ferrite-pearlite structure simply by hot forging under standard conditions. This structure exhibits low toughness in both pearlite and ferrite. For example, the heating temperature before hot forging can be set to 1150-1350°C, and the cooling rate after hot forging, for example, an average cooling rate from 800°C to 500°C, can be set to 0.8-3.0°C / sec.

[0135] (characteristic)

[0136] The non-tempered forged steel of this embodiment exhibits high strength, and non-tempered forged parts obtained by machining this non-tempered forged steel also have sufficient strength as parts. In this embodiment, the non-tempered forged steel being high-strength and "having sufficient strength as parts" means that the 0.2% yield strength, as evaluated in the Examples described below, is 880 MPa or higher. In this embodiment, this 0.2% yield strength is preferably 900 MPa or higher, more preferably 930 MPa or higher, and even more preferably 950 MPa or higher.

[0137] The non-tempered forged steel of this embodiment has excellent fracture splitting properties as described above. In this embodiment, the term "excellent fracture splitting properties" means that the absorbed energy measured in the examples described below is 8 J / cm 2 The following low toughness.

[0138] 3. Non-quenched and tempered forging parts

[0139] The present invention also includes non-quenched and tempered forged parts. The chemical composition of the non-quenched and tempered forged parts and the range of the parameter X are the same as those of the non-quenched and tempered forging steel described above and also the same as those of the non-quenched and tempered forging steel.

[0140] Specific examples of the non-heat-refined forged parts according to the present embodiment include forged parts such as connecting rods, lower arms, and crankshafts used in engines and suspension systems of transportation equipment such as automobiles and ships.

[0141] (Manufacturing method of non-quenched and tempered forging parts)

[0142] The non-tempered forged part of this embodiment can be obtained using the non-tempered forged steel. The non-tempered forged part of this embodiment can be obtained by subjecting the non-tempered forged steel to one or more of mechanical processing and fracture separation (also known as "cleavage"). The non-tempered forged part of this embodiment can be obtained by fracture separation of the non-tempered forged steel, or by mechanical processing followed by fracture separation, or by mechanical processing followed by fracture separation. When the non-tempered forged steel of this embodiment is subjected to the fracture separation to produce, for example, a fracture-separation connecting rod as a non-tempered forged part, it exhibits excellent fracture separation properties (also known as "cleavage properties"). Specifically, when the non-tempered forged steel of this embodiment is used in the manufacture of a fracture-separation connecting rod, for example, the aforementioned problems are avoided, and excellent fracture separation is achieved, resulting in a fracture-separation connecting rod with high strength, minimal deformation at the fracture surface, and high fit at the fracture surface. Therefore, the non-heat-refined forged steel of this embodiment is suitable for the production of fracture-separation type connecting rods.

[0143] Example

[0144] The present invention is not limited to the following examples, and may be implemented with appropriate modifications within the scope of the aforementioned and subsequent descriptions, all of which fall within the technical scope of the present invention.

[0145] Next, evaluation methods including sample preparation methods are described based on characteristics.

[0146] 1. Tensile strength (tensile test)

[0147] 1-1. Preparation of specimens for tensile testing

[0148] Steels No. 1 and No. 4 in Table 1 were melted using a conventional melting method using an actual machine and then cast to obtain the chemical compositions and X values ​​shown in Table 1. In Table 1, the "-" for Al content in No. 2 and No. 3 indicates an unavoidable impurity level of less than 0.001% by mass, and the "-" for Ca content in No. 5 indicates an unavoidable impurity level of less than 0.0001% by mass. Furthermore, the "unavoidable impurities" shown in Table 1 and Table 2 described below may include any element within the aforementioned unavoidable impurity levels. Underlined values ​​in Table 1 indicate that the composition, etc., are outside the ranges specified in this embodiment.

[0149] The steel sheets were flaked and rolled at a temperature between 1100°C and 1250°C. After flaking and rolling, they were hot rolled at a temperature between 850°C and 1200°C. In No. 1, a round bar with a diameter of 36 mm, equivalent to non-quenched and tempered forging steel, was obtained. In No. 4, hot rolling was performed under the same conditions to obtain a round bar with a diameter of 80 mm. This round bar was then forged and elongated at a heating temperature of 1200°C to obtain a square bar with a cross section perpendicular to the longitudinal direction and a side length of 16 mm.

[0150] In Table 1, Nos. 2, 3, 5, 6, and 7 were melted and cast using a small melting furnace (capacity 150 kg / ch) according to conventional melting methods to obtain steel sheets having the chemical compositions and X values ​​shown in Table 1. These steel sheets were then subjected to forging and elongation processing at a heating temperature of 1200°C to obtain square bars having a cross section perpendicular to the longitudinal direction and a side length of 20 mm, equivalent to non-quenched and tempered forging steel.

[0151] In Table 2, No. 8 to 11, a small melting furnace (capacity 150 kg / ch) was used to melt and cast according to the usual melting method to obtain steel sheets with the chemical composition and X value shown in Table 2. In Table 2, "-" for selected elements means that they were not intentionally added. In addition, the underlined values ​​in Table 2 indicate that they are outside the range of the composition, etc. specified in this embodiment. Using the above steel sheets, forging and stretching were performed at a heating temperature of 1200°C to obtain square bars with a side of 20 mm in a cross section perpendicular to the longitudinal direction, which are equivalent to non-tempered forging steel.

[0152] The round bars and square bars No. 1 to 7 were cut perpendicularly to the longitudinal direction to obtain round bar sheets and square bar sheets with a length of 100 mm. These round bar sheets and square bar sheets were hot forged as follows to obtain forged steel. Specifically, the round bar sheets and square bar sheets were kept at 1250°C for 10 minutes, then removed from the furnace and press forged to obtain a thickness of 10 mm for No. 1, 8 mm for No. 2, 3, 5, 6, and 7, and 6 mm for No. 4. After cooling to room temperature, tensile test specimens equivalent to non-quenched and tempered forged steel were obtained. The average cooling rate from 800°C to 500°C after press forging to cooling to room temperature was approximately 1.4 to 2.2°C / sec.

[0153] The square bars No. 8 to 11 were cut perpendicularly to the longitudinal direction to obtain square bar pieces with a length of 100 mm. These square bar pieces were hot forged as follows to obtain forged steel. Specifically, the square bar pieces were held at 1250°C for 10 minutes, removed from the furnace, press-forged to a thickness of 8 mm, and then cooled to room temperature to obtain tensile test specimens equivalent to non-quenched and tempered forged steel. The average cooling rate from 800°C to 500°C after press forging to room temperature was approximately 1.5 to 1.6°C / sec.

[0154] 1-2. Tensile test

[0155] Tensile tests were conducted using the aforementioned tensile test specimens. Specifically, the aforementioned tensile test specimens were cut to produce test pieces. All specimens conformed to JIS Z 2241 (2011). Specimen No. 1 was a 14A specimen with a parallel section diameter of 4 mm. Specimens Nos. 2, 3, 4, 5, 6, and 7 were 14B specimens with a parallel section thickness of 3 mm and a width of 10 mm. Specimens Nos. 8 to 11 also had 14B specimens with a parallel section thickness of 3 mm and a width of 10 mm. The longitudinal direction of the tensile test specimens coincided with the longitudinal direction of the round and square bars, and the load was applied parallel to the longitudinal direction. Tensile tests were conducted at room temperature in accordance with JIS Z 2241 (2011). A 0.2% yield strength of 880 MPa or greater was considered high strength. The results for specimens Nos. 1 to 7 are shown in Table 1, and the results for specimens Nos. 8 to 11 are shown in Table 2.

[0156] 2. Fracture separation (pendulum impact test)

[0157] 2-1. Preparation of specimens for pendulum impact test

[0158] In Table 1, Nos. 1 and 4 were melted using a conventional melting method using an actual machine and then cast to obtain steel slabs having the chemical compositions and X values ​​shown in Table 1. These steel slabs were then bloom rolled at a temperature between 1100 and 1250°C. After bloom rolling, they were hot rolled at a temperature between 850 and 1200°C to obtain round bars with a diameter of 50 mm, equivalent to non-tempered forging steel.

[0159] In Table 1, Nos. 2, 3, 5, 6, and 7 were melted and cast using a small melting furnace (150 kg / ch capacity) according to conventional melting methods to obtain steel slabs having the chemical compositions and X values ​​shown in Table 1. These steel slabs were then subjected to forging and elongation processing at a heating temperature of 1200°C to obtain round bars with a diameter of 50 mm, equivalent to non-tempered forging steel.

[0160] In Table 2, Nos. 8 to 11 were melted and cast using a small melting furnace (150 kg / ch capacity) according to conventional melting methods to obtain steel slabs having the chemical compositions and X values ​​shown in Table 2. These steel slabs were then subjected to forging and elongation processing at a heating temperature of 1200°C to obtain square bars having a cross section perpendicular to the longitudinal direction and a side of 35 mm, corresponding to non-tempered forging steel.

[0161] The round bars Nos. 1 to 7 were cut perpendicularly to their longitudinal direction to produce 70 mm long round bar pieces. These round bar pieces were then hot forged as follows. Specifically, the round bar pieces were held at 1200°C for 60 minutes, removed from the furnace, and subjected to press forging with a 50% compression. The pieces were then cooled to room temperature by blast cooling to obtain specimens for the Charpy impact test, equivalent to those of non-quenched and tempered forged steel. The average cooling rate from 800°C to 500°C after press forging to room temperature was approximately 1.0°C / sec.

[0162] The square bars Nos. 8-11 were cut perpendicularly to their longitudinal direction to produce 100 mm long square bar pieces. These square bar pieces were then hot forged as follows. Specifically, the square bar pieces were held at 1200°C for 60 minutes, removed from the furnace, press-forged to a thickness of 14 mm, and then cooled to room temperature to obtain specimens for pendulum impact testing equivalent to non-quenched and tempered forged steel. The average cooling rate from press forging to cooling to room temperature, from 800°C to 500°C, was approximately 0.8 to 1.3°C / sec.

[0163] 2-2. Pendulum impact test

[0164] As an evaluation index of fracture separation, a pendulum impact test is performed to evaluate the toughness of the steel. Specifically, in the above No. 1 to 7, the cutting of the specimens for the pendulum impact test was carried out, and the U-shaped notch test pieces shown in JIS Z 2242 (2018) were produced, and the notch depth and notch bottom radius of the test piece were 2 mm. The longitudinal direction of the test piece was consistent with the longitudinal direction of the round rod, and the direction of the impact was perpendicular to the longitudinal direction. In the above No. 8 to 11, the cutting of the specimens for the pendulum impact test was carried out, and the U-shaped notch test pieces shown in JIS Z 2242 (2018) were produced, and the notch depth and notch bottom radius of the test piece were 2 mm. The longitudinal direction of the test piece was consistent with the longitudinal direction of the square rod, and the direction of the impact was perpendicular to the longitudinal direction. The pendulum impact test was carried out at room temperature in accordance with JIS Z 2242 (2018). Then, the absorbed energy was 8 J / cm 2 The following cases were evaluated as having low toughness but excellent fracture splitting properties. The results of Nos. 1 to 7 are summarized in Table 1, and the results of Nos. 8 to 11 are summarized in Table 2.

[0165]

surface

[0166]

[0167]

surface

[0168]

[0169] The following can be seen from Tables 1 and 2. No. 1 is an invention example that meets all the requirements of the embodiment of the present invention. That is, each component and X shown in formula (1) are within the specified range, so it has high strength and an absorption energy of 8 J / cm 2 Below, it shows low toughness and excellent fracture splitting properties.

[0170] In contrast, Nos. 2 to 7 do not meet any of the requirements of the embodiment of the present invention, resulting in poor results in at least one of strength and fracture splitting properties. Specifically, Nos. 2 and 3 have insufficient P content, resulting in high toughness but poor fracture splitting properties.

[0171] In particular, No. 4 has high toughness but poor fracture splitting properties because X represented by formula (1) is outside the prescribed range.

[0172] No. 5 does not contain the amount of Ca specified in the present embodiment, X shown in formula (1) is outside the specified range, and the N content is excessive. Therefore, despite high toughness, fracture splitting properties are poor.

[0173] No. 6 has a Ca content lower than the lower limit value specified in the present embodiment and an excessive N content, resulting in high toughness but poor fracture splitting properties.

[0174] No. 7 has high toughness and poor fracture splitting properties because X shown in formula (1) is outside the prescribed range and the N content is excessive.

[0175] No. 8, 9 and 11 are invention examples that meet all the requirements of the embodiment of the present invention. That is, each component and X shown in formula (1) are within the specified range, so they have high strength and the absorption energy is 8 J / cm 2 The following samples exhibit low toughness and excellent fracture splitting properties. Furthermore, No. 8, which contains Bi in its chemical composition, exhibits high strength and excellent fracture splitting properties. This suggests that even when Pb and Sb, which exhibit similar effects to Bi, are included as optional elements, they can exhibit high strength and excellent fracture splitting properties similar to those of No. 8. Furthermore, No. 9, which contains Ti in its chemical composition, and No. 11, which contains Ni in its chemical composition, both exhibit high strength and excellent fracture splitting properties. This suggests that even when Cu and Mo, which exhibit similar effects to Ti and Ni, are included as optional elements, they can exhibit high strength and excellent fracture splitting properties similar to those of Nos. 9 and 11.

[0176] No. 10 has various components within the prescribed ranges, but X shown in formula (1) is outside the prescribed range. Therefore, despite high toughness, fracture splitting properties are poor.

[0177] This application claims priority based on Japanese Patent Application No. 2023-030262, filed on February 28, 2023, and Japanese Patent Application No. 2023-170213, filed on September 29, 2023. Japanese Patent Application Nos. 2023-030262 and 2023-170213 are incorporated herein by reference.

Claims

1. A non-quenched and tempered forging steel containing C: 0.40-0.60% by mass, Si: 0.10-0.40 mass%, Mn: 0.30-0.80 mass%, P: 0.007-0.050% by mass S: 0.010-0.070 mass%, Cr: 0.30-0.80 mass%, V: more than 0.30 mass% and not more than 0.38 mass%, Al: more than 0 mass% and not more than 0.050 mass%, N: more than 0 mass% and not more than 0.0080 mass%, Ca: 0.0002-0.0050 mass%, Mo: 0-0.05% by mass Cu: 0-0.20 mass%, Ni: 0-0.20 mass%, Ti: 0-0.030 mass%, Pb: 0-0.1% by mass Bi: 0-0.1% by mass Sb: 0-0.1% by mass Mg: 0-0.005% by mass Zr: 0-0.005% by mass Te: 0-0.1 mass%, and REM: 0-0.02% by mass The balance contains Fe and unavoidable impurities, X represented by the following formula (1) is 1.32 to 1.50, X=C+0.28Mn-1.03S+0.323Cr+1.69V…(1) in, C, Mn, S, Cr, and V respectively represent the contents of C, Mn, S, Cr, and V in steel in mass %, and elements not contained are represented as zero.

2. A non-quenched and tempered forging steel containing C: 0.40-0.60% by mass, Si: 0.10-0.40 mass%, Mn: 0.30-0.80 mass%, P: 0.007-0.050% by mass S: 0.010-0.070 mass%, Cr: 0.30-0.80 mass%, V: more than 0.30 mass% and not more than 0.38 mass%, Al: more than 0 mass% and not more than 0.050 mass%, N: more than 0 mass% and not more than 0.0080 mass%, Ca: 0.0002-0.0050 mass%, Mo: 0-0.05% by mass Cu: 0-0.20 mass%, Ni: 0-0.20 mass%, Ti: 0-0.030 mass%, Pb: 0-0.1% by mass Bi: 0-0.1% by mass Sb: 0-0.1% by mass Mg: 0-0.005% by mass Zr: 0-0.005% by mass Te: 0-0.1 mass%, and REM: 0-0.02% by mass The balance contains Fe and unavoidable impurities, X represented by the following formula (1) is 1.32 to 1.50, X=C+0.28Mn-1.03S+0.323Cr+1.69V…(1) in, C, Mn, S, Cr, and V respectively represent the contents of C, Mn, S, Cr, and V in steel in mass %, and elements not contained are represented as zero.

3. A non-quenched and tempered forged part, made of the non-quenched and tempered forged steel according to claim 2.

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