Steel materials and methods for manufacturing the same

Abstract

Provided is a steel material having excellent cold forgeability. This steel material has a predetermined component composition and a structure in which the area ratio of martensite is 10.0% or less with the remainder comprising at least bainite, wherein the number density of sulfide-based inclusions having an equivalent circle radius of 1-20 μm is at least 10.0 / mm2, and the number density of sulfide-based inclusions having an equivalent circle radius of at least 1 mm is at most 0.10 / mm2.

Description

[Technical Field]

[0001] The present invention relates to a steel material with excellent cold forging properties and a method for producing the same. [Background technology]

[0002] Cold-forged parts used in automotive applications and other fields are made by first performing preliminary processing such as wire drawing on hot-rolled steel, and then cold-forging them into the desired part shape. Further machining and heat treatment are performed as needed to create the final product.

[0003] Parts manufactured by cold forging become the final product either in their as-cold-worked state or after undergoing heat treatment (such as quenching and tempering, or high-frequency induction hardening and tempering) to adjust their strength, depending on the required strength.

[0004] In recent years, there has been a growing demand for so-called non-heat-treated steel, which omits the heat treatment performed after cold forging to adjust its strength. In this case, since the steel must satisfy a specified strength after cold forging, it is necessary to cold forge a steel material with higher strength compared to when heat treatment is performed to adjust the strength.

[0005] Steel used in cold forging is required to be crack-free even when subjected to large plastic strains during the forging process. Since cracks during cold forging mainly originate from inclusions in the steel, reducing coarse inclusions in the steel as much as possible is effective in suppressing cracks during cold forging.

[0006] On the other hand, parts that have undergone cold forging may also be subjected to further finishing processes such as machining. In this case, the machinability of the steel material becomes an important characteristic. A common method for improving the machinability of steel is to utilize MnS inclusions. However, if the MnS inclusions generated during casting are too coarse, these inclusions can easily become the starting point for cracks during cold forging.

[0007] As described above, to prevent cracking during cold forging, it is desirable to minimize the amount of coarse MnS inclusions.

[0008] In this context, prior art, for example, Patent Document 1, discloses a steel wire rod in which the size and number density of sulfides or sulfide-based composite compounds present in a longitudinal cross-section parallel to the central axis of the steel material are defined. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2000-204440 [Overview of the project] [Problems that the invention aims to solve]

[0010] However, Patent Document 1 specifies steel wire rods that are cold-forged after spheroidizing annealing, and does not take into account the omission of heat treatment for strength adjustment. Furthermore, Patent Document 1 lacks specific descriptions of manufacturing methods for controlling the size and number density of sulfides or sulfide-based composite compounds to predetermined values.

[0011] The present invention has been made in view of the above circumstances, and aims to suppress crack occurrence during cold forging, that is, to provide a steel material with excellent cold forging properties and a method for manufacturing the same. Furthermore, the present invention aims to provide a steel material with excellent cold forging properties, which is also useful as a non-heat-treated steel, and a method for manufacturing the same. [Means for solving the problem]

[0012] To solve the above problems, the inventor focused on controlling the morphology of sulfide-based inclusions by adding Cu. For example, pure MnS inclusions, a type of sulfide inclusion, have high thermal stability, making them prone to crystallization as coarse MnS inclusions during the casting of steel materials. These coarse MnS inclusions formed during casting do not dissolve in the steel even when the material is heated before hot rolling, and therefore remain in the steel as coarse inclusions after hot rolling, inducing cracking during cold forging.

[0013] In contrast, when Cu is added to steel in addition to Mn and S, a composite sulfide (Mn,Cu)S containing Cu is formed. This composite sulfide has a lower melting point compared to pure MnS inclusions. Therefore, it dissolves in the steel even at temperatures equivalent to the heating of the raw material before hot rolling, and re-precipitations in the steel as relatively fine inclusions during cooling after hot rolling. In other words, we have found that by adding Cu appropriately, it is possible to obtain steel in which the number density of coarse sulfide inclusions, which are the initiation points of cracks during cold forging, is reduced to a desired level.

[0014] Furthermore, the inventors conducted further studies on the component composition of steel materials with excellent cold forging properties, as well as the manufacturing conditions for steel materials. As a result, they further discovered that it is necessary to appropriately control the maximum heating temperature during hot rolling and the time the material remains in the heating furnace, while also appropriately controlling the component composition of the steel material.

[0015] The present invention is a steel material developed based on the above findings, and its gist is as follows. 1. The composition is as follows, by mass%, C: 0.05~0.60%, Si: 0.01~1.00%, Mn: 0.01~1.50%, S: 0.001~0.100%, Cu: 0.010~1.000%, Cr: 0.01~2.00%, Mo: 0.01~1.00%, and N: 0.0020~0.0250%, with the remainder being Fe and unavoidable impurities. The area ratio of martensite is 10.0% or less, and the remainder has a structure containing at least bainite. The material contains sulfide-based inclusions, and among these sulfide-based inclusions, the number density of inclusions with an equivalent circular radius of 1 to 20 μm is 10.0 inclusions / mm². 2The above, wherein the number density of inclusions having an equivalent circle radius of 1 mm or more is 0.10 pieces / mm 2 or less, which is a steel material.

[0016] 2. The steel material according to 1 above, wherein the component composition further comprises, in mass%, one or more elements selected from the group consisting of Ni: 0.01 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, B: 0.0005 to 0.0050%, and Sb: 0.0010 to 0.0300%.

[0017] 3. A method for producing a steel material, comprising heating a raw material for a steel material in a heating furnace and performing hot rolling, wherein the raw material has a component composition comprising, in mass%, C: 0.05 to 0.60%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.50%, S: 0.001 to 0.100%, Cu: 0.010 to 1.000%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, and N: 0.0020 to 0.0250%, optionally further comprises, in mass%, one or more elements selected from the group consisting of Ni: 0.01 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, B: 0.0005 to 0.0050%, and Sb: 0.0010 to 0.0300%, with the balance being Fe and unavoidable impurities, and a maximum heating temperature of the raw material is set to 1000°C or higher and 1200°C or lower, and a holding time of the raw material in the heating furnace is set to t1 (unit: minutes) or more determined by the following formula (1) according to the Cu concentration (mass%: [Cu]) in the raw material. t1=60-10[Cu] ···(1)

[0018] 4. The method for producing a steel material according to 3 above, wherein the steel material obtained after the hot rolling is cooled from 800°C to 700°C at an average cooling rate of less than 25°C / s. [Advantageous Effects of Invention]

[0019] In the present invention, an appropriate amount of fine sulfide-based inclusions is allowed to exist to secure the machinability effect, while the number density of coarse sulfide-based inclusions is reduced to suppress the occurrence of cracking during cold forging. Therefore, according to the present invention, the occurrence of cracking during cold forging can be suppressed while securing machinability. As a result, a steel material excellent in cold forgeability can be provided together with a method for producing the same. Furthermore, according to the present invention, a steel material which is also useful as a non-heat-treated steel and is excellent in cold forgeability can be provided together with a method for producing the same.

Mode for Carrying Out the Invention

[0020] Hereinafter, the present invention will be described in detail. Note that "%" representing the content of component elements below means "% by mass" unless otherwise specified. In addition, any numerical range expressed using "~" in the present specification means a range including the numerical values described before and after "~" as the lower limit and the upper limit, respectively. The balance of the component composition in the present invention is Fe and unavoidable impurities in all cases.

[0021] <Steel Material> The steel material of the present invention has a predetermined component composition and a predetermined structure, and the number density of relatively coarse sulfide-based inclusions and the number density of relatively fine sulfide-based inclusions are each within predetermined ranges. When the steel material of the present invention satisfies the above requirements, cold forgeability can be improved without impairing machinability. The steel material of the present invention is still useful as a non-heat-treated steel while exhibiting good cold forgeability. The steel material of the present invention can be obtained, for example, according to the production method of the present invention.

[0022] [Component Composition] C: 0.05 to 0.60% Carbon (C) is added to ensure the strength and hardness of the steel. If the C content is less than 0.05%, the required strength and hardness cannot be achieved. On the other hand, if the C content exceeds 0.60%, the hardenability becomes too high, resulting in a microstructure containing hard martensite. As a result, the hardness becomes excessively high, and the cold forgeability decreases. Therefore, the C content should be in the range of 0.05% to 0.60%. The C content is preferably 0.10% or more. Also, the C content is preferably 0.55% or less.

[0023] Si: 0.01~1.00% Si is a deoxidizing element during refining and also improves the strength, hardness, and hardenability of steel. These effects cannot be obtained with Si content below 0.01%. On the other hand, if Si content exceeds 1.00%, the hardenability becomes too high, resulting in a structure containing hard martensite. As a result, the hardness becomes excessively high, and the cold forgeability decreases. Therefore, the Si content should be in the range of 0.01% to 1.00%. Preferably, the Si content is 0.80% or less, and more preferably 0.50% or less.

[0024] Mn: 0.01~1.50% Mn is an element that improves the strength, hardness, and hardenability of steel. Furthermore, Mn combines with sulfur in steel to form MnS inclusions and / or (Mn,Cu)S inclusions, improving the machinability of the steel. These effects cannot be obtained with an Mn content of less than 0.01%. On the other hand, if the Mn content exceeds 1.50%, the hardenability becomes too high, resulting in a structure containing hard martensite. As a result, the hardness becomes excessively high, and the cold forgeability decreases. Moreover, a large amount of coarse MnS inclusions precipitate during casting, making the material prone to cracking during cold forging. Therefore, the Mn content should be in the range of 0.01% to 1.50%. The Mn content is preferably 1.20% or less, more preferably 1.00% or less. Also, the Mn content is preferably 0.05% or more, more preferably 0.10% or more.

[0025] S: 0.001~0.100% S combines with Mn and Cu in the steel to form sulfide-based inclusions such as MnS inclusions and / or (Mn,Cu)S inclusions, which improve the machinability of the steel. This effect cannot be obtained if the amount of S is less than 0.001%. On the other hand, if the amount of S exceeds 0.100%, coarse MnS inclusions are formed during the casting of the material, and cracks are more likely to occur during cold forging. Therefore, the amount of S should be in the range of 0.001% to 0.100%. The amount of S is preferably 0.070% or less, and more preferably 0.050% or less. Furthermore, the amount of S is preferably more than 0.025%, and more preferably 0.030% or more.

[0026] Cu: 0.010~1.000% Cu is a useful element that improves the hardenability of steel. Furthermore, adding Cu to steel containing Mn and S generates the compound sulfide (Mn,Cu)S. This compound sulfide has a lower melting point than pure MnS and readily dissolves in the steel even at the heating temperature before hot rolling. During cooling after hot rolling, it re-precipitations as finer inclusions, thus ensuring high machinability while reducing the number density of coarse sulfide inclusions that cause cracking during cold forging. However, if the Cu content is less than 0.010%, the Cu-containing compound sulfide is not effectively formed, and the above-mentioned effects are not obtained. On the other hand, if the Cu content exceeds 1.000%, the hardenability of the steel becomes too high, resulting in a structure containing hard martensite and reduced cold forgeability. Therefore, the Cu content should be in the range of 0.010% to 1.000%. The amount of Cu is preferably 0.500% or less, more preferably 0.350% or less, and even more preferably 0.300% or less. Furthermore, the amount of Cu is preferably 0.030% or more, and more preferably 0.050% or more.

[0027] Cr: 0.01~2.00% Cr is an element that improves the hardenability of steel. This effect cannot be obtained if the Cr content is less than 0.01%. On the other hand, if the Cr content exceeds 2.00%, the hardenability of the steel becomes excessive, resulting in a structure containing hard martensite and a decrease in cold forgeability. Therefore, the Cr content should be in the range of 0.01% to 2.00%. The Cr content is preferably 1.80% or less, and more preferably 1.50% or less.

[0028] Mo: 0.01~1.00% Mo is a useful element that significantly improves the hardenability of steel materials with only a small amount of addition. This effect cannot be obtained if the amount of Mo is less than 0.01%. On the other hand, if the amount of Mo exceeds 1.00%, the hardenability of the steel material becomes excessive, resulting in a structure containing hard martensite and a decrease in cold forgeability. Therefore, the amount of Mo should be in the range of 0.01% to 1.00%. The amount of Mo is preferably 0.50% or less, and more preferably 0.30% or less.

[0029] N: 0.0020~0.0250% Nitrogen (N) combines with nitride-forming elements in steel to form nitrides and acts as a grain boundary pinning particle, thereby preventing grain coarsening. This effect cannot be obtained if the N content is less than 0.0020%. On the other hand, if the N content exceeds 0.0250%, there is a risk of blowhole formation in the steel. In addition, dissolved N in the steel causes dynamic strain aging, making it easier for cracks to occur during cold forging. Therefore, the N content should be in the range of 0.0020% to 0.0250%. Preferably, the N content is 0.0200% or less, and more preferably 0.0180% or less. Furthermore, preferably, the N content is 0.0025% or more, and more preferably 0.0030% or more.

[0030] Furthermore, the steel material of the present invention may further contain the following elements as needed. Ni: 0.01~1.00% Ni is an element that enhances the hardenability and toughness of steel and can be added. Adding less than 0.01% Ni does not produce these effects. On the other hand, adding more than 1.00% Ni increases the hardenability of the steel excessively, resulting in a structure containing hard martensite and reduced cold forgeability. Therefore, when adding Ni, the amount should be in the range of 0.01% to 1.00%. The Ni content is preferably 0.80% or less, and more preferably 0.60% or less.

[0031] Al: 0.001~0.100% Al is a deoxidizing element and can be added because it combines with N in steel to form nitrides, thereby reducing the amount of dissolved N in the steel. This effect cannot be obtained if the amount of Al is less than 0.001%. On the other hand, if the amount of Al exceeds 0.100%, a large amount of oxide-based inclusions are generated, making the steel more prone to cracking during cold forging. Therefore, when adding Al, the amount should be in the range of 0.001% to 0.100%. The amount of Al is preferably 0.080% or less, and more preferably 0.050% or less.

[0032] Ti: 0.001~0.100% Like aluminum, titanium (Ti) can be added to steel because it combines with nitrogen (N) to form nitrides, thereby reducing the amount of dissolved nitrogen in the steel. However, this effect cannot be obtained if the amount of Ti is less than 0.001%. On the other hand, if the amount of Ti exceeds 0.100%, a large amount of Ti-based inclusions will be generated in the steel, reducing its cold forgeability. Therefore, when adding Ti, the amount should be in the range of 0.001% to 0.100%. Preferably, the amount of Ti is 0.080% or less, and more preferably 0.050% or less.

[0033] V: 0.001~0.300% V, like Al and Ti, can be added because it combines with N in steel to form nitrides, thereby reducing the amount of dissolved N in the steel. This effect cannot be obtained if the amount of V is less than 0.001%. On the other hand, if the amount of V exceeds 0.300%, the amount of V-based precipitates in the steel becomes excessive, making it prone to cracking during cold forging. Therefore, when adding V, the amount should be in the range of 0.001% to 0.300%. The amount of V is preferably 0.200% or less, and more preferably 0.150% or less.

[0034] Nb: 0.001~0.100% Nb can be added because it combines with carbon in steel to form carbides, contributing to grain refinement. This effect cannot be obtained if the Nb amount is less than 0.001%. On the other hand, if the Nb amount exceeds 0.100%, a large amount of coarse Nb-based carbides are generated, reducing cold forgeability. Therefore, when adding Nb, the amount should be in the range of 0.001% to 0.100%. The Nb amount is preferably 0.050% or less, and more preferably 0.030% or less.

[0035] B: 0.0005~0.0050% B is an element that significantly improves the hardenability of steel materials with only a small amount added, and can therefore be added. This effect cannot be obtained if the amount of B is less than 0.0005%. On the other hand, if the amount of B exceeds 0.0050%, the effect of improving hardenability saturates. Therefore, when adding B, the amount should be in the range of 0.0005% to 0.0050%. The amount of B is preferably 0.0040% or less, and more preferably 0.0030% or less.

[0036] Sb: 0.0010~0.0300% Sb is an element that readily segregates on the surface of steel and can be added because it has the effect of suppressing the decarburization reaction on the surface of the steel. This effect cannot be obtained if the amount of Sb is less than 0.0010%. On the other hand, if the amount of Sb added exceeds 0.0300%, the amount of Sb that segregates on the surface becomes excessive, degrading the surface properties of the steel. Therefore, when adding Sn, the amount should be in the range of 0.0010% to 0.0300%. The amount of Sb is preferably 0.0200% or less, and more preferably 0.0150% or less.

[0037] [Organization] Area percentage of martensite: 10.0% or less, with the remainder containing at least bainite. When steel contains a large amount of hard martensitic structure, its hardness becomes excessively high. As a result, cracking during cold forging becomes more likely, and its cold forgeability tends to be poor. Therefore, it is necessary to reduce the area ratio of martensite in the steel as much as possible. Accordingly, the area ratio of martensite in the steel of the present invention is limited to 10.0% or less. Preferably, the area ratio of martensite is 5.0% or less. The lower limit of the area ratio of martensite is not particularly limited and may be 0.0% (structure without martensite). Furthermore, it is essential that the steel material of the present invention contains at least bainite as the remainder of the structure. By including bainite (with a bainite area ratio greater than 0%), the strength of the steel material can be increased, making it useful for applications requiring high strength after cold forging. In other words, even if heat treatment for strength adjustment is omitted after cold forging the steel material of the present invention, high strength can be obtained, making it usable as an excellent non-heat-treated steel. The area ratio of such bainite is preferably 30% or more, more preferably 33% or more, and even more preferably 35% or more. If the bainite area ratio is above the lower limit, the strength and hardness of the steel material can be further increased, resulting in an even better non-heat-treated steel. Also, the area ratio of bainite is preferably 90% or less, and more preferably 85% or less. If the bainite area ratio is below the upper limit, it is easier to achieve both good cold forgeability and machinability without excessively increasing strength and hardness. The area ratio of bainite is preferably in the range of 30 to 90%.

[0038] The remaining microstructure other than martensite and bainite may be known microstructures for cold forging steels, such as ferrite and pearlite. The remaining microstructure can be 0% by area percentage, preferably 5% or more, preferably 20% or less, more preferably 15% or less, and preferably in the range of 5 to 20%. By setting the remaining microstructure above the lower limit, sufficient carbon can be dissolved in the bainite, improving hardenability. Conversely, by setting the remaining microstructure below the upper limit, sufficient bainite can be formed, achieving both excellent strength and cold forgeability. Here, the area ratio of martensite, bainite, and other residual structures refers to the area ratio relative to the cross-sectional area of ​​the steel material, and can be specifically measured by cross-sectional observation according to the examples described later.

[0039] Number density of sulfide-based inclusions with an equivalent circular radius of 1-20 μm is 10.0 inclusions / mm². 2 Furthermore, the number density of sulfide-based inclusions with an equivalent circular radius of 1 mm or more is 0.10 inclusions / mm². 2 below Sulfide inclusions are effective for improving the machinability of steel materials. On the other hand, coarse sulfide inclusions serve as initiation points for cracks during cold forging. Therefore, it is important to disperse a large amount of fine sulfide inclusions while reducing the number density of coarse sulfide inclusions in steel. In order to disperse a large amount of fine sulfide inclusions, a predetermined amount of Cu is added as described above. Then, the number density of fine sulfide inclusions having an equivalent circle radius of 1 µm or more and 20 µm or less is adjusted to 10.0 pieces / mm 2 or more, preferably 20.0 pieces / mm 2 or more. If the number density of the above fine sulfide inclusions is not less than the above lower limit, machinability can be improved without impairing cold forgeability. Although the upper limit of the number density of the fine sulfide inclusions is not particularly limited, from the viewpoint of production cost, it can be 50.0 pieces / mm 2 or less. In addition, the number density of coarse sulfide inclusions having an equivalent circle radius of 1 µm or more needs to be limited to 0.10 pieces / mm 2 or less, preferably 0.05 pieces / mm 2 or less. If the number density of the above coarse sulfide inclusions is not more than the above upper limit, cracking during cold forging is suppressed, and good cold forgeability can be obtained. The lower limit of the number density of coarse sulfide inclusions having an equivalent circle radius of 1 µm or more is not particularly limited, and may be 0.00 pieces / mm 2 (not contained).

[0040] As used herein, the term "sulfide inclusions" refers to sulfides with a single element (e.g., MnS), composite sulfides with multiple elements (e.g., (Mn,Cu)S), or composite compounds mainly composed of sulfides (those obtained by compounding oxides, carbides, nitrides, etc. with sulfides), and these may exist alone or as a mixture. Sulfide inclusions also include those in which other elements such as Fe are solid-dissolved in sulfides. As the sulfide inclusions, MnS and (Mn,Cu)S are preferable, and (Mn,Cu)S is more preferable. In addition, the number density of sulfide inclusions refers to the number per unit area in the cross-sectional area of a steel material, and specifically can be measured by cross-sectional observation according to the examples described later.

[0041] As mentioned above, if steel is excessively hard, it is prone to cracking during cold forging and tends to have poor cold forgeability. Therefore, the Vickers hardness of steel is preferably 400 Hv or less, and more preferably 350 Hv or less. On the other hand, for use as a cold forged part, for example, it is desirable for the steel to be somewhat hard. Therefore, the Vickers hardness of steel is preferably 150 Hv or more, and more preferably 200 Hv or more. The hardness of steel can be measured according to the method described in the examples below.

[0042] <Methods for manufacturing steel materials> The steel manufacturing method of the present invention requires not only adjusting the component composition of the steel material but also paying attention to the conditions during hot rolling of the material, particularly the maximum heating temperature and furnace time. By satisfying the above requirements with the manufacturing method of the present invention, it is possible to obtain steel with improved cold forgeability without impairing machinability. The steel obtained by the manufacturing method of the present invention exhibits good cold forgeability and remains useful as unheat-treated steel. Here, the component composition of the material for steel is the same as that of steel, as described above.

[0043] [Heating conditions during hot rolling] The maximum heating temperature of the material is 1000-1200℃ If the maximum heating temperature during hot rolling of steel materials is too low, the solid solution of coarse sulfide inclusions does not proceed sufficiently, and coarse inclusions remain in the steel. As a result, cracks are more likely to occur during cold forging of the steel, originating from these coarse inclusions. On the other hand, if the heating temperature is too high, although it is advantageous for dissolving coarse sulfide inclusions in the matrix, the austenite grains coarse during heating, increasing the hardenability. Consequently, martensite is formed after hot rolling, and the cold forgeability is reduced. Therefore, when hot-rolling steel materials, the maximum heating temperature should be in the range of 1000°C to 1200°C. Preferably, this maximum heating temperature is 1030°C or higher. Also, preferably, this maximum heating temperature is 1150°C or lower.

[0044] Furnace duration t1 or longer During hot rolling, it is necessary to ensure sufficient heating time to dissolve the coarse sulfide inclusions generated during casting. The inventors focused on the fact that the more Cu added to the material, the lower the melting point of the Cu-containing composite sulfide, and thus the earlier the solid solution of the sulfide inclusions is completed. In other words, they focused on the fact that the more Cu there is, the shorter the required heating time. The inventors then diligently investigated the effect of the amount of Cu added and the heating time on the precipitation state of sulfide inclusions. As a result, they found that t1 (unit: minutes), which is determined by the following formula (1) according to the Cu concentration (unit: mass%, also written as [Cu]), is the minimum furnace time required for the solid solution of coarse sulfide inclusions. t1 = 60 - 10[Cu] ... (1) Thus, the minimum time spent in the heating furnace is determined according to the Cu concentration ([Cu]) and must be at least t1 (minutes). By setting the furnace time to at least the minimum, the presence of coarse sulfide inclusions can be reduced, improving the cold forgeability of the steel. Furthermore, increasing the amount of fine sulfide inclusions can improve machinability. From the above viewpoint, the furnace time is preferably 1.1 × t1 or more, more preferably 1.3 × t1 or more, and even more preferably 1.5 × t1 or more.

[0045] On the other hand, longer furnace dwell times are advantageous in terms of solid solution of coarse sulfide inclusions, so there is no need to specify an upper limit. However, if the furnace dwell time is too long, the amount of scale increases, which can worsen the yield and reduce productivity. Therefore, a furnace dwell time of 6.0 × t1 or less is preferable, and 5.0 × t1 or less is more preferable.

[0046] Furthermore, when performing a cooling process after hot rolling, it is preferable to cool the resulting steel material in a temperature range of 800°C to 700°C at an average cooling rate of less than 25°C / s. If the average cooling rate is 25°C / s or higher, the transformation to bainite may be insufficient, the area ratio of martensite may increase, and the cold forgeability may decrease. More preferably, the average cooling rate is less than 20°C / s. Furthermore, the average cooling rate is preferably 3°C / s or higher. By setting the average cooling rate to 3°C / s or higher, the transformation to ferrite is suppressed, the transformation to bainite is promoted, and the strength and hardness of the steel can be further increased.

[0047] Furthermore, in the steel materials and methods for manufacturing steel materials according to the present invention, any items not described herein can be handled using the provisions and conventional methods applicable to known steel materials. [Examples]

[0048] The following examples illustrate the structure and effects of the present invention in detail. However, the present invention is not limited by the following examples, and modifications can be made as appropriate within the scope of the spirit of the invention; all such modifications fall within the technical scope of the present invention.

[0049] A 160mm square billet (material for steel) having the component composition listed in Table 1, with the remainder being Fe and unavoidable impurities, was used. This material was heated in a heating furnace under the conditions described in Table 2 below, and then hot-rolled into a wire rod with a diameter of 15mm to obtain hot-rolled wire rod as steel. Steels No. 1 to 9 in Table 1 are comparative steels that fall outside the range of component composition of the present invention, while steels No. 10 to 33 are suitable steels that fall within the range of component composition of the present invention.

[0050] [Table 1]

[0051] To confirm the microstructure of the steel material, the obtained hot-rolled wire rods were cut and microstructural observations were performed. These observations were conducted on a cross section (C section (circular)) perpendicular to the rolling direction and right to the central axis of the wire rod. Furthermore, these observations were performed under conditions of 5 randomly selected fields of view and an observation magnification of 100x, with an observation area of ​​600 μm × 800 μm per field of view. After taking photographs of the tissue observed in each field of view, the area ratio of martensite and bainite within the observation field was calculated using image analysis, and the average value for the five fields of view was then calculated. ImageJ, an image analysis software, was used to calculate the area ratio of martensite and bainite.

[0052] Next, the size of sulfide inclusions in the hot-rolled wire was measured. The hot-rolled wire was cut to a length of 20 mm, and then further cut so that the cross section parallel to the central axis of the wire (longitudinal section) would serve as the observation surface. In this way, three inclusion observation samples with an observation area of ​​15 mm × 20 mm were taken for each type of steel. After mirror polishing these inclusion observation samples, the entire observation surface was observed with a scanning electron microscope (SEM) to obtain SEM images of all inclusions within the field of view. The observation magnification during SEM image acquisition was appropriately varied within the range of 50 to 2000x so that the entire inclusion was contained within the imaging field. Furthermore, to identify the type of inclusion, an energy-dispersive X-ray spectrometer (SEM-EDX) attached to the SEM was used, and inclusions in which Mn and S, and Mn, Cu and S were detected were considered to be sulfide inclusions. Furthermore, sulfide inclusions in the SEM images were traced and binarized, and their cross-sectional areas were determined through image analysis and converted to equivalent circular radii. ImageJ, an image analysis software, was used to calculate the cross-sectional area and equivalent circular radii of the inclusions. Of the above sulfide-based inclusions, the number of those with an equivalent circular radius of 1 μm or more and 20 μm or less, and those with an equivalent circular radius of 1 mm or more were counted, and the observed cross-sectional area (= 300 mm²) was measured. 2 The number density of sulfide inclusions in each observation surface was calculated by dividing by ( ). Then, the average number density of sulfide inclusions across the three fields of view of that observation surface was calculated.

[0053] To evaluate the cold forgeability of hot-rolled wire rods, the limit upset rate of each steel was determined using the method described in the literature "Cold Upset Test Method" (Edited by the Materials Research Committee, Cold Forging Division, Japan Society for Technology of Plasticity: Plasticity and Processing, 22 (1981), pp. 139-144). Specifically, after completely removing the scale from the surface of the hot-rolled wire rod by pickling, it was drawn to obtain a wire rod with a diameter of φ:14 mm. After cutting the drawn wire rod to a height of 21 mm, a notch (notch bottom: R=0.15 mm, notch depth: 0.8 mm, notch angle: 30°) was added to the side by machining to obtain a test specimen for cold forging. This test specimen corresponds to "Test Specimen No. 2" described in the above literature. Ten cold forging test specimens were taken from each steel material under the conditions shown in Table 2 and used for subsequent tests. The cold forging specimens collected as described above were subjected to sequential compression tests under end-face restraint conditions, and the critical upset ratio was measured. The initial compression was 15% in the height direction, and subsequent compression tests were performed in 0.5 mm increments. After compression, the bottom of the notch in the specimen was observed to check for the presence of cracks with a length of 0.5 mm or more. Compression, unloading, and observation of the notch bottom were repeated until cracks with a length of 0.5 mm or more occurred in all specimens. The cumulative compression ratio at which cracks with a length of 0.5 mm or more occurred in half of the specimens (5 specimens) was defined as the critical upset ratio for that steel type. Steel grades with a limit uplift ratio of 40% or higher were judged to be acceptable (excellent cold forging properties).

[0054] The method for determining the hardness (Hv) of steel is as follows: The cross-section (a section perpendicular to the central axis of the wire) of a hot-rolled wire with a diameter of 15 mm was mirror-polished, and then measured using a Vickers hardness tester (load: 10 kgf). Measurements were taken at a total of five points: one point at the center of the cross-section and four points at intermediate positions (midpoints on the radius between the center of the cross-section and the circumference). The average hardness of these five points was taken as the hardness value (Hv) of the steel.

[0055] The evaluation results are summarized in Table 2.

[0056] [Table 2]

[0057] Examples No. 1 and 2 are cases where the C and Si content exceeded the specified range, respectively. These steels exhibited a microstructure containing more than 10.0% martensite by area due to their excessively high hardenability. As a result, the limit uptake rate was low, less than 40%, and they had poor cold forgeability.

[0058] Case No. 3 is an example where the Mn content exceeded the specified range. This steel exhibited a microstructure containing more than 10.0% martensite by area due to its excessively high hardenability. Furthermore, the high Mn content resulted in a large number of coarse sulfide inclusions with an equivalent radius of 1 mm or more. Consequently, the limit upset rate was low at less than 40%, resulting in poor cold forgeability.

[0059] Case No. 4 is an example where the sulfur content exceeded the specified range. This steel contained a large amount of coarse sulfide inclusions, resulting in a low limit upset ratio of less than 40% and poor cold forgeability.

[0060] Case No. 5 is an example where the amount of Cu fell below the specified range. In this steel, due to the low amount of Cu added, solid solution of coarse sulfide inclusions did not proceed during heating before hot rolling, and Ostwald growth of sulfide inclusions occurred during heating, reducing the number density of fine sulfide inclusions with an equivalent radius of 1 to 20 μm. Also, for the same reason, many coarse sulfide inclusions remained in the hot-rolled material. As a result, the limit upsetting rate was low, less than 40%, and the cold forgeability was poor. Furthermore, because there were not enough fine sulfide inclusions in the hot-rolled material, the machinability was poor.

[0061] Examples No. 6-8 are cases where the Cu, Cr, and Mo content exceeded the specified ranges, respectively. These steels exhibited a microstructure containing more than 10.0% martensite by area due to their excessively high hardenability. As a result, the limit upset rate was low, less than 40%, and they had poor cold forgeability.

[0062] Case No. 9 is an example where the amount of nitrogen exceeds the specified range. In this steel, the amount of dissolved nitrogen in the steel is high, and due to the effects of dynamic strain aging, the critical upsetting rate is low at less than 40%, resulting in poor cold forgeability.

[0063] Case No. 10 is an example where the component composition is within the specified range, but the maximum heating temperature during hot rolling exceeds the specified range. In this steel, the austenite grains coarsened during heating before hot rolling, resulting in excessively high hardenability of the steel and exhibiting a microstructure containing more than 10.0% martensite by area. As a result, the critical upsetting ratio was low at less than 40%, and the cold forgeability was poor.

[0064] Case No. 11 is an example where the component composition is within the specified range, but the maximum heating temperature during hot rolling falls below the specified range. Because the heating temperature was too low, the coarse sulfide inclusions could not be sufficiently dissolved during heating, and a large amount of coarse inclusions remained after hot rolling. As a result, the limit upset ratio was low, less than 40%, and the cold forgeability was poor.

[0065] Cases No. 12 and 13 are examples where the chemical composition and the maximum heating temperature during hot rolling were within the specified range, but the time spent in the heating furnace fell below the specified range. In these steel materials, the short heating time prevented sufficient solution treatment of coarse sulfide inclusions, resulting in a large amount of coarse inclusions remaining after hot rolling. As a result, the critical upsetting ratio was low, less than 40%, and the cold forgeability was poor.

[0066] On the other hand, examples No. 14 to 33 are cases where both the composition of the steel material and the heating conditions during hot rolling are within the specified range. In all of these steel materials, the area ratio of martensite was low, bainite was present in the steel, and the number density of coarse sulfide inclusions was reduced while ensuring the presence of fine sulfide inclusions. As a result, all examples showed high critical upsetting rates of 40% or more, demonstrating excellent cold forging properties.

[0067] Here, No. 14 represents a case where the composition of the steel material and the heating conditions during hot rolling are within a specified range, but the average cooling rate from 800 to 700°C after hot rolling is relatively high. In this steel material, because the average cooling rate was fast, there was little transformation to bainite, and as a result of the increased area ratio of martensite, the critical upsetting rate was relatively low.

[0068] Furthermore, it has been confirmed that the number density of sulfide inclusions in samples No. 14 to 33 does not impair the machinability-enhancing effect of sulfide inclusions. In addition, it has been confirmed that samples No. 14 to 33, which have a sufficient amount of bainite and martensite kept below a predetermined area ratio, possess sufficient hardness and strength to be used well as unheat-treated steel.

Claims

1. The component composition is expressed in mass percent. C: 0.05-0.60%, Si: 0.01-1.00%, Mn: 0.01 to 1.50%, S: 0.001-0.100%, Cu: 0.010-1.000%, Cr: 0.01-2.00%, Mo: 0.01–1.00% and N: Contains 0.0020 to 0.0250%, The remainder is Fe and unavoidable impurities, The structure has a martensite area ratio of 10.0% or less, an area ratio of other materials besides martensant and bainite of 20% or less, and the remainder being bainite. The material contains sulfide-based inclusions, and among these sulfide-based inclusions, the number density of inclusions with an equivalent circular radius of 1 to 20 μm is 10.0 inclusions / mm². ２ The above conditions are met, and the number density of inclusions with an equivalent circular radius of 1 mm or more is 0.10 inclusions / mm². ２ The following are the steel materials.

2. The above component composition is, in mass%, Ni: 0.01-1.00%, Al: 0.001-0.100%, Ti: 0.001 to 0.100%, V: 0.001-0.300%, Nb: 0.001 to 0.100%, B: 0.0005 to 0.0050% and Sb: 0.0010-0.0300% The steel material according to claim 1, further containing one or more elements selected from the group consisting of the following.

3. When heating steel materials in a heating furnace and hot rolling them, The aforementioned material has a component composition in mass%, C: 0.05-0.60%, Si: 0.01-1.00%, Mn: 0.01 to 1.50%, S: 0.001-0.100%, Cu: 0.010-1.000%, Cr: 0.01-2.00%, Mo: 0.01–1.00% and N: Contains 0.0020 to 0.0250%, Optionally, in mass percent, Ni: 0.01-1.00%, Al: 0.001-0.100%, Ti: 0.001 to 0.100%, V: 0.001-0.300%, Nb: 0.001 to 0.100%, B: 0.0005 to 0.0050% and Sb: 0.0010-0.0300% It further contains one or more elements selected from the group consisting of, The remainder is Fe and unavoidable impurities, The maximum heating temperature of the aforementioned material shall be 1000°C or higher and 1200°C or lower. The time the material remains in the heating furnace is determined by the following formula (1) according to the Cu concentration (mass %: [Cu]) in the material. １ A method for manufacturing steel according to claim 1 or 2, wherein the time is 1.1 times or more (unit: minutes). t １ =60-10[Cu] ・・・(1)

4. The method for manufacturing steel according to claim 3, wherein the steel obtained after the hot rolling is cooled from 800°C to 700°C at an average cooling rate of less than 25°C / s.

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