STEEL MATERIALS AND METHODS OF MANUFACTURING THEM

Abstract

The invention proposes a steel material with superior cold forging capabilities. The steel material comprises: a predetermined chemical composition; and a microstructure in which the area proportion of martensite is 10.0% or less, with the remainder consisting of at least bainite, in which the density of sulfide radical inclusions with a radius of 1 µm to 20 µm is 10.0 / mm2 or greater, and the density of sulfide radical inclusions with a radius of 1 mm or greater is 0.10 / mm2 or less.

Description

Steel material and its manufacturing method

[0001] The present invention relates to a steel material having excellent cold forgeability and a method for producing the same.

[0002] Cold forged parts used in automobiles and other applications are made by cold forging hot rolled steel material after preliminary processing such as wire drawing, followed by cutting and heat treatment as necessary to produce the final product.

[0003] Depending on the required strength, parts manufactured by cold forging are either left as cold worked or undergo heat treatment for strength adjustment (quenching and tempering heat treatment, induction hardening and tempering heat treatment, etc.) before becoming the final product.

[0004] In recent years, there has been a growing need for so-called non-tempered steel, which does not require heat treatment for strength adjustment after cold forging. In this case, since it is necessary to satisfy a predetermined strength after cold forging, it is necessary to cold forge steel with higher strength than when heat treatment for strength adjustment is performed.

[0005] Steel materials used in cold forging are required to be free from cracks even when subjected to large plastic strains during the forging process. Because cracks during cold forging originate primarily 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, there are cases where parts after cold forging are further subjected to finishing processes such as cutting. In this case, the machinability of the steel material is also an important characteristic. A common method for improving the machinability of steel is to utilize MnS inclusions. However, if the MnS inclusions formed during casting are too coarse, cracks are likely to occur during cold forging, originating from these MnS inclusions.

[0007] As described above, in order to prevent cracks during cold forging, it is desirable to reduce the amount of coarse MnS inclusions as much as possible.

[0008] Here, as a conventional technique, for example, Patent Document 1 discloses a steel wire rod in which the size and number density of sulfides or composite compounds mainly composed of sulfides present in a longitudinal cross section parallel to the central axis of the steel rod are specified.

[0009] Japanese Patent Application Laid-Open No. 2000-204440

[0010] However, Patent Document 1 specifies a steel wire rod that is cold forged after spheroidizing annealing, and does not take into consideration omission of heat treatment for the purpose of adjusting strength. Furthermore, Patent Document 1 does not provide much description regarding a specific manufacturing method 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 an object of the present invention is to provide a steel material that suppresses the occurrence of cracks during cold forging, i.e., a steel material that is excellent in cold forgeability, and a method for manufacturing the same.Furthermore, an object of the present invention is to provide a steel material that is excellent in cold forgeability and is preferably useful as a non-heat treated steel, and a method for manufacturing the same.

[0012] In order to solve the above problems, the present inventors focused on controlling the morphology of sulfide-based inclusions by adding Cu. For example, pure MnS inclusions, which are an example of sulfide-based inclusions, have high thermal stability, and therefore tend to crystallize as coarse MnS inclusions during the casting of a steel material. The 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 material as coarse inclusions even after hot rolling, inducing cracking during cold forging.

[0013] In contrast, when Cu is further 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 than pure MnS inclusions. Therefore, it dissolves in the steel even at temperatures equivalent to the heating temperature of the material before hot rolling, and reprecipitates in the steel as relatively fine inclusions during cooling after hot rolling. In other words, it has been found that by adding an appropriate amount of Cu, it is possible to obtain a steel in which the number density of coarse sulfide-based inclusions, which serve as the starting point for cracks during cold forging, is reduced to a desired level.

[0014] The present inventors have further investigated the chemical composition of a steel material having excellent cold forgeability and the manufacturing conditions of the steel material, and as a result have found that it is necessary to appropriately control the maximum heating temperature during hot rolling and the residence time of the material in a heating furnace while appropriately controlling the chemical 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. A steel material having a chemical composition, in mass %, of 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%, with the balance being Fe and unavoidable impurities, having a structure in which the area ratio of martensite is 10.0% or less and the balance containing at least bainite, containing sulfide-based inclusions, and the number density of the sulfide-based inclusions having a circle-equivalent radius of 1 to 20 μm is 10.0 pieces / mm ２ or more, and the number density of inclusions with a circle equivalent radius of 1 mm or more is 0.10 pieces / mm ２ The following is steel.

[0016] 2. The steel material according to 1 above, wherein the chemical composition further contains, 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. When a steel material is heated in a heating furnace and hot-rolled, the material has a chemical composition, in mass%, of 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, the material further contains, 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 the maximum heating temperature of the material is 1000°C or higher and 1200°C or lower, and the residence time of the material in the heating furnace is t determined by the following formula (1) depending on the Cu concentration (mass%: [Cu]) in the material: １ (unit: min) or more. １ =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.

[0019] In the present invention, fine sulfide-based inclusions are present in an appropriate amount to ensure machinability, while the number density of coarse sulfide-based inclusions is reduced to suppress cracking during cold forging. Therefore, according to the present invention, it is possible to suppress cracking during cold forging while ensuring machinability. As a result, a steel material with excellent cold forgeability can be provided together with a manufacturing method thereof. Furthermore, according to the present invention, a steel material with excellent cold forgeability that is also useful as a non-heat treated steel can be provided together with a manufacturing method thereof.

[0020] The present invention will be described in detail below. Note that "%" representing the content of the following component elements means "% by mass" unless otherwise specified. Furthermore, any numerical range expressed using "to" in this specification means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively. The remainder of the component composition in the present invention is Fe and unavoidable impurities.

[0021] <Steel Material> The steel material of the present invention has a predetermined chemical 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 a predetermined range. 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 exhibits good cold forgeability and is still useful as a non-heat treated steel. The steel material of the present invention can be obtained, for example, according to the manufacturing method of the present invention.

[0022] [Component Composition] C: 0.05-0.60% C is added to ensure the strength and hardness of the steel. If the C content is less than 0.05%, the necessary strength and hardness cannot be ensured. 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 deteriorates. Therefore, the C content is set to a range of 0.05% to 0.60%. The C content is preferably 0.10% or more. Furthermore, the C content is preferably 0.55% or less.

[0023] Si: 0.01 to 1.00% Si is a deoxidizing element during refining and also improves the strength, hardness, and hardenability of steel. Addition of less than 0.01% Si does not provide these effects. On the other hand, addition of more than 1.00% Si results in excessively high hardenability, resulting in a structure containing hard martensite. As a result, hardness increases excessively and cold forgeability decreases. Therefore, the Si content is set to a range of 0.01% to 1.00%. The Si content is preferably 0.80% or less, and more preferably 0.50% or less.

[0024] Mn: 0.01 to 1.50% Mn is an element that improves the strength, hardness, and hardenability of steel. Furthermore, Mn combines with S in steel to form MnS inclusions and / or (Mn, Cu)S inclusions, thereby improving the machinability of the steel. Addition of less than 0.01% Mn does not achieve this effect. On the other hand, addition of more than 1.50% Mn results in excessively high hardenability, resulting in a structure containing hard martensite. As a result, hardness increases excessively and cold forgeability deteriorates. Furthermore, large amounts of coarse MnS inclusions precipitate during casting of the material, making it prone to cracking during cold forging. Therefore, the Mn content is limited to a range of 0.01% to 1.50%. The Mn content is preferably 1.20% or less, and more preferably 1.00% or less. The Mn content is preferably 0.05% or more, and more preferably 0.10% or more.

[0025] S: 0.001 to 0.100% S combines with Mn and Cu in steel to form sulfide-based inclusions such as MnS inclusions and / or (Mn, Cu)S inclusions, thereby improving the machinability of the steel. This effect cannot be achieved if the S content is less than 0.001%. On the other hand, if the S content exceeds 0.100%, coarse MnS inclusions are formed during casting of the material, making it more susceptible to cracking during cold forging. Therefore, the S content is set to 0.001% or more and 0.100% or less. The S content is preferably 0.070% or less, more preferably 0.050% or less. The S content is also preferably greater than 0.025%, more preferably 0.030% or more.

[0026] Cu: 0.010 to 1.000% Cu is a useful element that improves the hardenability of steel. Furthermore, adding Cu to steel containing Mn and S produces the complex sulfide (Mn, Cu)S. This complex sulfide has a lower melting point than pure MnS and is easily dissolved in steel even at the temperature at which the material is heated before hot rolling. Furthermore, since it reprecipitates as finer inclusions during cooling after hot rolling, it is possible to reduce the number density of coarse sulfide-based inclusions that cause cracking during cold forging while ensuring high machinability. Here, if the Cu content is less than 0.010%, complex sulfides containing Cu are not effectively formed, and the above-mentioned effects cannot be achieved. 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, which reduces cold forgeability. Therefore, the Cu content is set to a range of 0.010% to 1.000%. The Cu content is preferably 0.500% or less, more preferably 0.350% or less, and even more preferably 0.300% or less. The Cu content is also preferably 0.030% or more, and more preferably 0.050% or more.

[0027] Cr: 0.01 to 2.00% Cr is an element that improves the hardenability of steel. If the Cr content is less than 0.01%, this effect cannot be obtained. On the other hand, if Cr is added in an amount exceeding 2.00%, the hardenability of the steel becomes excessive, and a structure containing hard martensite is formed, resulting in a decrease in cold forgeability. Therefore, the Cr content is set to a 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 to 1.00% Mo is a useful element that significantly improves the hardenability of steel materials with a small amount of addition. If the Mo content is less than 0.01%, this effect cannot be obtained. On the other hand, if Mo is added in an amount exceeding 1.00%, the hardenability of the steel material becomes excessive, and a structure containing hard martensite is exhibited, resulting in a decrease in cold forgeability. Therefore, the Mo content is set to a range of 0.01% to 1.00%. The Mo content is preferably 0.50% or less, and more preferably 0.30% or less.

[0029] N: 0.0020 to 0.0250% N combines with nitride-forming elements in the steel to form nitrides, which act as grain boundary pinning particles, preventing grain coarsening. This effect cannot be achieved with an N content of less than 0.0020%. On the other hand, adding more than 0.0250% N may result in the formation of blowholes in the steel. Furthermore, solute N in the steel may undergo dynamic strain aging, making it more susceptible to cracking during cold forging. Therefore, the N content is set to a range of 0.0020% to 0.0250%. The N content is preferably 0.0200% or less, more preferably 0.0180% or less. The N content is preferably 0.0025% or more, more preferably 0.0030% or more.

[0030] Furthermore, the steel material of the present invention may further contain the following elements as necessary. Ni: 0.01 to 1.00% Ni is an element that improves the hardenability and toughness of the steel material and can be added. Addition of less than 0.01% Ni does not provide these effects. On the other hand, addition of more than 1.00% Ni excessively increases the hardenability of the steel material, resulting in a structure containing hard martensite and reduced cold forgeability. Therefore, when Ni is added, it is limited to a 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 to 0.100% Al can be added because it is a deoxidizing element and also has the effect of combining with N in the steel to form nitrides, thereby reducing the amount of solute N in the steel. If the Al content is less than 0.001%, this effect cannot be obtained. On the other hand, if the Al content exceeds 0.100%, a large amount of oxide-based inclusions is generated, making cracks more likely to occur during cold forging. Therefore, when Al is added, it should be in the range of 0.001% to 0.100%. The Al content is preferably 0.080% or less, and more preferably 0.050% or less.

[0032] Ti: 0.001 to 0.100% Like Al, Ti can be added because it is an element that combines with N in steel to form nitrides and has the effect of reducing the amount of solute N in steel. If the Ti content is less than 0.001%, this effect cannot be obtained. On the other hand, if Ti is added in an amount exceeding 0.100%, a large amount of Ti-based inclusions is generated in the steel, reducing cold forgeability. Therefore, when Ti is added, it is limited to a range of 0.001% to 0.100%. The Ti content is preferably 0.080% or less, and more preferably 0.050% or less.

[0033] V: 0.001 to 0.300% Like Al and Ti, V can be added because it combines with N in the steel to form nitrides, reducing the amount of solute N in the steel. This effect cannot be achieved if the V content is less than 0.001%. On the other hand, if V is added in an amount exceeding 0.300%, the amount of V-based precipitates in the steel becomes excessive, making it more likely to crack during cold forging. Therefore, when V is added, it should be in the range of 0.001% to 0.300%. The V content is preferably 0.200% or less, and more preferably 0.150% or less.

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

[0035] B: 0.0005 to 0.0050% B is an element that can greatly improve the hardenability of steel with a small amount, and can be added. If the B content is less than 0.0005%, this effect cannot be obtained. On the other hand, if B is added in an amount exceeding 0.0050%, the effect of improving hardenability saturates. Therefore, when B is added, it should be in the range of 0.0005% to 0.0050%. The B content is preferably 0.0040% or less, and more preferably 0.0030% or less.

[0036] Sb: 0.0010 to 0.0300% Sb is an element that easily segregates in the surface layer of a steel material and has the effect of suppressing decarburization reactions on the surface of the steel material, so it can be added. If the Sb content is less than 0.0010%, this effect cannot be obtained. On the other hand, if Sb is added in an amount exceeding 0.0300%, the amount of Sb segregating in the surface layer becomes excessive, deteriorating the surface properties of the steel material. Therefore, when Sn is added, it is set to a range of 0.0010% to 0.0300%. The Sb content is preferably 0.0200% or less, and more preferably 0.0150% or less.

[0037] [Structure] Area fraction of martensite: 10.0% or less, with at least bainite in the balance. When a steel material contains a large amount of hard martensite, the hardness increases excessively. As a result, cracks tend to occur during cold forging, resulting in poor cold forgeability. For this reason, it is necessary to minimize the area fraction of martensite in the steel material. Therefore, the area fraction of martensite in the steel material of the present invention is limited to 10.0% or less. The area fraction of martensite is preferably 5.0% or less. The lower limit of the area fraction of martensite is not particularly limited and may be 0.0% (a structure not containing martensite). It is also essential that the steel material of the present invention contains at least bainite as the balance of the structure. By including bainite (with an area fraction of bainite exceeding 0%), the strength of the steel material can be increased, making it useful for applications requiring strength after cold forging. In other words, the steel material of the present invention can achieve high strength even if heat treatment for strength adjustment is omitted after cold forging, and therefore can be used as an excellent non-tempered steel. The area fraction of such bainite is preferably 30% or more, more preferably 33% or more, and even more preferably 35% or more. If the area fraction of bainite is equal to or greater than the above lower limit, the strength and hardness of the steel can be further increased, resulting in an even better non-heat treated steel. Furthermore, the area fraction of bainite is preferably 90% or less, more preferably 85% or less. If the area fraction of bainite is equal to or less than the above upper limit, strength and hardness are not excessively increased, and it is easy to achieve both good cold forgeability and machinability. The area fraction of bainite is preferably in the range of about 30 to 90%.

[0038] The remaining structure other than martensite and bainite may be a structure known in steel materials for cold forging, such as ferrite or pearlite. The area ratio of the remaining structure can be 0%, preferably 5% or more, preferably 20% or less, more preferably 15% or less, and preferably in the range of about 5 to 20%. By setting the other remaining structure at or above the lower limit, C can be sufficiently dissolved in bainite, thereby improving hardenability. Furthermore, by setting the other remaining structure at or below the upper limit, bainite can be sufficiently formed, thereby achieving both excellent strength and cold forgeability. Here, the area ratio of martensite, bainite, and other remaining structure refers to the area ratio of the cross-sectional area of ​​the steel material, and specifically, can be measured by cross-sectional observation according to the examples described below.

[0039] Equivalent circle radius: Number density of sulfide-based inclusions of 1 to 20 μm is 10.0 pieces / mm ２ or more, and the number density of sulfide-based inclusions with a circle equivalent radius of 1 mm or more is 0.10 pieces / mm ２ Sulfide-based inclusions are effective in improving the machinability of steel materials. On the other hand, coarse sulfide-based inclusions become the starting point for cracks during cold forging. Therefore, it is important to disperse a large amount of fine sulfide-based inclusions while reducing the number density of coarse sulfide-based inclusions in steel. In order to disperse a large amount of sulfide-based inclusions finely, a predetermined amount of Cu is added as described above. Then, the number density of fine sulfide-based inclusions with a circle-equivalent radius of 1 μm or more and 20 μm or less is reduced to 10.0 pieces / mm ２ More than 20.0 pieces / mm ２ If the number density of the fine sulfide-based inclusions is equal to or greater than the above lower limit, machinability can be improved without impairing cold forgeability. There is no particular upper limit to the number density of the fine sulfide-based inclusions, but from the viewpoint of production costs, it is preferable to set the upper limit to 50.0 pieces / mm ２ In addition, the number density of coarse sulfide-based inclusions with a circle equivalent radius of 1 mm or more can be 0.10 pieces / mm ２ It is necessary to limit the number of particles to 0.05 or less per mm. ２If the number density of the coarse sulfide-based inclusions is equal to or less than the above upper limit, cracking during cold forging is suppressed, and good cold forgeability is obtained. The lower limit of the number density of coarse sulfide-based inclusions with a circle-equivalent radius of 1 mm or more is not particularly limited, and is 0.00 pieces / mm ２ (or not contained).

[0040] In this specification, the term "sulfide-based 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 (sulfides combined with oxides, carbides, nitrides, etc.), and these may exist alone or in combination. The sulfide-based inclusions also include sulfides in which other elements such as Fe are dissolved. Preferred sulfide-based inclusions are MnS and (Mn, Cu)S, with (Mn, Cu)S being more preferred. The number density of sulfide-based inclusions refers to the number per area in the cross-sectional area of ​​a steel material, and can be measured, specifically, by cross-sectional observation according to the examples described below.

[0041] As mentioned above, if a steel material is too hard, it is likely to crack during cold forging, and its cold forgeability is likely to be poor. Therefore, the Vickers hardness of the steel material is preferably 400 Hv or less, and more preferably 350 Hv or less. On the other hand, in order to be used well as a cold forged part, for example, it is desirable that the steel material be hard to a certain extent. Therefore, the Vickers hardness of the steel material is preferably 150 Hv or more, and more preferably 200 Hv or more. The hardness of the steel material can be measured according to the method of the examples described below.

[0042] <Method for manufacturing steel material> In the method for manufacturing steel material of the present invention, it is necessary not only to adjust the chemical composition of the raw material for steel material, but also to pay attention to the conditions when hot rolling the raw material, particularly the maximum heating temperature and dwell time of the raw material. By satisfying the above requirements, the manufacturing method of the present invention can obtain steel material with improved cold forgeability without impairing machinability. The steel material obtained by the manufacturing method of the present invention exhibits good cold forgeability and is still useful as a non-tempered steel. Here, the chemical composition of the raw material for steel material is the same as the chemical composition described above for the steel material.

[0043] [Heating Conditions During Hot Rolling] Maximum Heating Temperature of Material: 1000-1200°C. If the maximum heating temperature during hot rolling of a steel material is too low, the solid solution of coarse sulfide-based inclusions does not proceed sufficiently, leaving the coarse inclusions in the steel. Therefore, cracks originating from these coarse inclusions are likely to occur during cold forging of the steel. On the other hand, if the heating temperature is too high, although this is advantageous for dissolving the coarse sulfide-based inclusions in the matrix, the austenite grains become coarse during heating, increasing hardenability. As a result, martensite is formed after hot rolling, resulting in reduced cold forgeability. Therefore, when hot rolling a steel material, the maximum heating temperature is set to a range of 1000°C or higher and 1200°C or lower. This maximum heating temperature is preferably 1030°C or higher. Furthermore, this maximum heating temperature is preferably 1150°C or lower.

[0044] The furnace time is t １ As described above, during hot rolling, it is necessary to ensure a heating time sufficient to dissolve the coarse sulfide-based inclusions formed during casting. Here, the present inventors have noticed that the greater the amount of Cu added to the material, the lower the melting point of Cu-containing composite sulfides, and accordingly, the earlier the solid solution of the sulfide-based inclusions is completed. In other words, the greater the amount of Cu, the shorter the required heating time. The present inventors have then conducted extensive research into the effects of the amount of Cu added and the heating time on the precipitation state of the sulfide-based inclusions. As a result, it has been found that the t, which is determined by the following formula (1) depending on the Cu concentration (unit: mass%, also referred to as [Cu]), １ It was found that t (unit: minutes) is the minimum residence time required for dissolving coarse sulfide-based inclusions. １ = 60 - 10 [Cu] (1) In this way, the lower limit of the residence time in the heating furnace is determined according to the Cu concentration ([Cu]), and the above t １ (minutes) or more. By making the residence time equal to or greater than the above lower limit, the presence of coarse sulfide-based inclusions can be reduced, and the cold forgeability of the steel can be improved. Furthermore, by increasing the number of fine sulfide-based inclusions, the machinability can be improved. From the above viewpoints, the residence time is set to 1.1 × t １ It is preferable that the value is 1.3×t or more. １More preferably, it is 1.5×t or more. １ More preferably, it is equal to or greater than this.

[0045] On the other hand, since a longer residence time is advantageous in terms of dissolving coarse sulfide-based inclusions, there is no need to specify an upper limit. However, if the residence time is too long, the amount of scale increases, which may result in a decrease in yield and a decrease in productivity. Therefore, the residence time is preferably 6.0 × t1 or less, and more preferably 5.0 × t1 or less.

[0046] Furthermore, when a cooling step is performed after hot rolling, it is preferable to cool the obtained steel material in a temperature range of 800°C to 700°C at an average cooling rate of less than 25°C / s. If such an average cooling rate is 25°C / s or more, transformation to bainite becomes insufficient, the area ratio of martensite increases, and cold forgeability may decrease. Such an average cooling rate is more preferably less than 20°C / s. Furthermore, such an average cooling rate is preferably 3°C / s or more. By setting the average cooling rate to 3°C / s or more, transformation to ferrite is suppressed and transformation to bainite is promoted, thereby making it possible to further increase the strength and hardness of the steel material.

[0047] In the steel material and the method for manufacturing the steel material according to the present invention, any items not described in this specification can be made in accordance with the known standards and conventional methods for steel materials.

[0048] The following examples are provided to specifically explain the configuration and effects of the present invention. The present invention is not limited to the following examples, and can be modified as appropriate within the scope of the present invention, and all such modifications are within the technical scope of the present invention.

[0049] A 160 mm square billet (steel material) was used, having the chemical composition shown in Table 1, with the balance being Fe and unavoidable impurities. This material was heated in a heating furnace under the conditions shown in Table 2 below, and then hot-rolled into a wire rod having a diameter of 15 mm, to obtain a hot-rolled wire rod as a steel material. Note that Steel Nos. 1 to 9 in Table 1 are comparative steels that fall outside the range of the chemical composition of the present invention, and Steel Nos. 10 to 33 are suitable steels that fall within the range of the chemical composition of the present invention.

[0050]

[0051] In order to confirm the structure of the steel material, the obtained hot-rolled wire rod was cut and subjected to microstructural observation. The microstructural observation was performed on a cross section (C-section (circular)) perpendicular to the rolling direction and perpendicular to the central axis of the wire rod. The microstructural observation was performed on five randomly selected fields at a magnification of 100x, with the observation area per field being 600 µm x 800 µm. After taking a photograph of the structure observed in each field, the area ratios of martensite and bainite in the area of ​​the observed field were calculated by image analysis, and the average value of the five fields was calculated. The area ratios of martensite and bainite were calculated using image analysis software ImageJ.

[0052] Next, the size of sulfide-based 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 a cross section (longitudinal cross section) parallel to the central axis of the wire was the observation surface. In this way, three inclusion observation samples with an observation area of ​​15 mm x 20 mm were collected for each steel type. After mirror polishing, these inclusion observation samples were observed over the entire observation surface using a scanning electron microscope (SEM), and SEM images of all inclusions within the field of view were obtained. The observation magnification during SEM image acquisition was appropriately changed within the range of 50 to 2000 times so that the entire inclusions were within the field of view. In addition, to identify the inclusion types, an energy dispersive X-ray spectrometer (SEM-EDX) attached to the SEM was used, and inclusions in which Mn and S, as well as Mn, Cu, and S, were detected were considered to be sulfide-based inclusions. Furthermore, sulfide-based inclusions in the SEM images were traced and binarized, and the cross-sectional areas of the inclusions were determined by image analysis and converted into equivalent circle radii. ImageJ, an image analysis software, was used to calculate the cross-sectional areas and equivalent circle radii of the inclusions. Of the above sulfide-based inclusions, the number of inclusions with an equivalent circle radius of 1 μm or more and 20 μm or less and the number of inclusions with an equivalent circle radius of 1 mm or more were counted, and the observed cross-sectional area (= 300 mm ２ The number density of the sulfide-based inclusions in each observation surface was calculated by dividing the number density of the sulfide-based inclusions in each observation surface by the average value of the number density of the sulfide-based inclusions in the three fields of view of the observation surface.

[0053] To evaluate the cold forgeability of the hot-rolled wire rods, the critical upsetting ratio of each steel was determined using the method described in the literature "Cold Upsetting Property Test Method" (edited by the Materials Research Group of the Cold Forging Subcommittee of the Japan Society for Technology of Plasticity: Plasticity and Processing, 22 (1981), pp. 139-144). Specifically, the surface scale of the hot-rolled wire rod was completely removed by pickling, and then the wire was drawn to obtain a wire rod with a diameter of 14 mm. The drawn wire rod was cut to a height of 21 mm, and then a notch (notch base: R = 0.15 mm, notch depth: 0.8 mm, notch angle: 30°) was created on the side by cutting to obtain a test specimen for cold forging. This test specimen corresponds to the "No. 2 test specimen" described in the literature. Ten cold forging test specimens were taken from each steel material under the conditions shown in Table 2 and subjected to the following tests. The cold forging test pieces obtained above were subjected to sequential compression tests under end face restraint conditions to measure the critical upsetting ratio. The initial compression was 15% in the height direction, and subsequent compression tests were performed at 0.5 mm intervals. After compression, the notch base of the test piece was observed to check for the presence or absence of cracks with a length of 0.5 mm or more. Compression, unloading, and notch base observation were repeated until cracks with a length of 0.5 mm or more occurred in all test pieces. The cumulative compression ratio at which cracks with a length of 0.5 mm or more occurred in half of the test pieces (5 pieces) was defined as the critical upsetting ratio for that steel type. Steel types with a critical upsetting ratio of 40% or more were judged to be acceptable (excellent cold forgeability).

[0054] The hardness (Hv) of a steel material was determined as follows. After mirror-polishing a cross section (a cross section perpendicular to the central axis of the wire material) of a hot-rolled wire material with a diameter of 15 mm, the hardness was measured using a Vickers hardness tester (load: 10 kgf). The measurement was performed at five points in total: one point at the center of the cross section of the wire material and four points at intermediate positions (intermediate positions on the radius between the center of the cross section and the circumference). The average value of the hardness values ​​at these five points was taken as the hardness value (Hv) of the steel material.

[0055] The above evaluation results are shown in Table 2.

[0056]

[0057] Nos. 1 and 2 are examples in which the C and Si contents, respectively, exceeded the predetermined ranges. These steel materials had too high hardenability, resulting in a microstructure containing martensite at an area ratio of more than 10.0%. As a result, the critical upsetting ratio was low at less than 40%, and the cold forgeability was poor.

[0058] No. 3 is an example in which the Mn content exceeded the specified range. This steel had too high hardenability, resulting in a microstructure containing more than 10.0% martensite by area. Furthermore, because the Mn content was too high, a large amount of coarse sulfide-based inclusions with a circle-equivalent radius of 1 mm or more were present. As a result, the critical upsetting ratio was low at less than 40%, resulting in poor cold forgeability.

[0059] No. 4 is an example in which the amount of S exceeded the predetermined range. In this steel material, a large amount of coarse sulfide-based inclusions was present, so the limiting upsetting ratio was low at less than 40%, and the cold forgeability was poor.

[0060] No. 5 is an example in which the Cu content was below the specified range. In this steel, the Cu content was low, so solid solution of coarse sulfide-based inclusions did not progress during heating before hot rolling. Furthermore, Ostwald ripening of the sulfide-based inclusions occurred during heating, resulting in a decrease in the number density of fine sulfide-based inclusions with a circle-equivalent radius of 1 to 20 μm. For the same reason, many coarse sulfide-based inclusions remained in the hot-rolled material. As a result, the critical upsetting ratio was low at less than 40%, resulting in poor cold forgeability. Furthermore, the hot-rolled material lacked sufficient fine sulfide-based inclusions, resulting in poor machinability.

[0061] Nos. 6 to 8 are examples in which the Cu content, Cr content, and Mo content exceeded the specified ranges, respectively. These steel materials had too high hardenability, resulting in a microstructure containing martensite at an area ratio of more than 10.0%. As a result, the limiting upsetting ratio was low at less than 40%, and the cold forgeability was poor.

[0062] No. 9 is an example in which the amount of N exceeded the predetermined range. In this steel material, the amount of dissolved N in the steel was large, and due to the influence of dynamic strain aging, the limiting upsetting ratio was low at less than 40%, resulting in poor cold forgeability.

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

[0064] No. 11 is an example in which the chemical composition was within the specified range, but the maximum heating temperature during hot rolling was below the specified range. Because the heating temperature was too low, the coarse sulfide-based inclusions could not be sufficiently dissolved during heating, and a large amount of coarse inclusions remained after hot rolling. As a result, the limiting upsetting ratio was low at less than 40%, and the cold forgeability was poor.

[0065] Nos. 12 and 13 are examples in which the chemical composition and the maximum heating temperature during hot rolling were within the specified range, but the residence time in the heating furnace was below the specified range. Because the heating time was short, the coarse sulfide-based inclusions were not sufficiently dissolved, and a large amount of coarse inclusions remained after hot rolling. As a result, the critical upsetting ratio was low at less than 40%, and the cold forgeability was poor.

[0066] On the other hand, Nos. 14 to 33 are examples in which the chemical composition of the steel material and the heating conditions during hot rolling are both within the specified ranges. In all of these steel materials, the area ratio of martensite is low, bainite is present in the steel material, and the number density of coarse sulfide-based inclusions is reduced while maintaining fine sulfide-based inclusions. As a result, all of the examples exhibited a high critical upsetting ratio of 40% or more, which indicated excellent cold forgeability.

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

[0068] It has been confirmed that the machinability-improving effect of the sulfide-based inclusions is not impaired if the number density of the sulfide-based inclusions is the same as that of Nos. 14 to 33. It has also been confirmed that Nos. 14 to 33, which have a sufficient amount of bainite and have martensite suppressed to a predetermined area ratio or less, have sufficient hardness and strength that allows them to be used favorably as non-heat-treated steel.

Claims

1. A steel sheet having a chemical composition, in mass%, of 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 balance being Fe and unavoidable impurities, having a structure in which the area ratio of martensite is 10.0% or less and the balance containing at least bainite, containing sulfide-based inclusions, and the number density of the sulfide-based inclusions having a circle equivalent radius of 1-20 μm is 10.0 pieces / mm ２ or more, and the number density of inclusions having a circle equivalent radius of 1 mm or more is 0.10 pieces / mm ２ The following are steel materials.

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

3. When a steel material is heated in a heating furnace and hot rolled, the material has a composition, in mass%, of 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%, and optionally, in mass%, Ni: 0.01-1.00%, Al: 0.001-0.100%, Ti: 0.001-0.100%, V: 0.001-0.300%, The material further contains one or more elements selected from the group consisting of 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, the maximum heating temperature of the material is 1000°C to 1200°C, and the residence time of the material in the heating furnace is t determined by the following formula (1) according to the Cu concentration (mass%: [Cu]) in the material. １ (unit: min) or more. １ =60-10[Cu]...(1) 4. The method for producing a steel material according to claim 3, 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.