STEEL BAR

Abstract

The invention proposes a steel bar with superior cold forging capability after spheroidization annealing. According to the scheme, for a steel bar containing specified amounts of Cr, Ni, and Cu, in a cross-section perpendicular to the length, when the shape of the cementite grains observed in m observation fields closely resembles ellipses, where m is an integer of 2 or greater, for the cementite grains theoretically having an aspect ratio of 2.0 or less, a principal axis length of 10.0 mm or less in all m observation fields, and the variation in Cu and Ni content measured in m observation fields is small.According to another scheme, for a steel bar containing a specified amount of Cr, Ni, and Cu, in a cross-section perpendicular to the length, among the circular equivalent diameters of cementite particles observed in n observation fields, where n is an integer of 2 or greater, the maximum value of the circular equivalent diameter in any observation field is 15.0 mm or less, and the variation at the maximum values ​​is small.

Description

Bar steel

[0001] The present invention relates to a steel bar having excellent cold forgeability.

[0002] In fields such as construction machinery, industrial machinery, and automobiles, structural machine parts are widely used, which are made by cutting bar steel with a circular or square cross section and then forging it into shapes such as bolts, nuts, and screws. The mechanical strength required for these structural machine parts is ensured by optimizing the conditions for heat treatment, such as quenching, which is carried out after processing. Furthermore, by using bar steel to which alloy elements have been added in advance, high mechanical strength can be stably achieved even if the heat treatment conditions vary slightly.

[0003] In recent years, in the manufacture of mechanical structural parts, the application of cold forging has been expanding, replacing the conventional hot forging, from the perspective of production efficiency, etc. When cold forging steel materials containing a large amount of alloying elements, the deformation resistance is higher than that of steel materials containing a small amount of alloying elements. This poses issues such as a reduced lifespan of the dies used in cold forging and a tendency for cracks to occur in the product. To reduce the deformation resistance and improve cold forgeability, it is effective to spheroidize the shape of the cementite contained in the metal structure of the steel material before cold forging. Various proposals have been made regarding cementite spheroidization technologies.

[0004] For example, Patent Document 1 describes an invention of a low-alloy steel material and a manufacturing method thereof in which the shape of cementite contained in the steel material after hot rolling is spheroidized by adjusting the hot rolling conditions and the cooling conditions after hot rolling, thereby improving cold forgeability. Patent Document 2 describes an invention of a steel wire material and a manufacturing method thereof in which the cold workability is improved by adjusting the number per unit volume of spheroidal cementite having a specific average particle size or less. Patent Document 3 describes an invention of a cold forging steel in which the cold workability is improved by adding a large amount of Cr to control the average value and standard deviation of the distance between carbides dispersed in ferrite grains.

[0005] JP 2004-100038 A JP 2009-275250 A JP 2023-21615 A

[0006] The technology described in Patent Document 1 is characterized by improving cold forgeability without performing spheroidizing annealing, and the cold forgeability itself is not significantly improved compared to when conventional spheroidizing annealing is performed. The technologies described in Patent Documents 2 and 3 improve cold forgeability compared to spheroidizing annealing in conventional technologies, but have a problem in that the shape of the cementite is not necessarily optimized because an average value is used as a representative value representing the particle diameter or interparticle distance of the spheroidized cementite.

[0007] The present invention has been developed in view of the above problems, and proposes a steel bar having excellent cold forgeability after spheroidizing annealing.

[0008] In order to improve cold forgeability, the present inventors investigated the relationship between the size of cementite particles observed in the cross section of a steel section after spheroidizing annealing and the cold forgeability. As a result, they found that excellent cold forgeability can be obtained in steel sections in which the equivalent circle diameters of cementite particles measured in multiple observation fields on the cross section are 15.0 μm or less and the variation in the equivalent circle diameters is small.

[0009] Next, the present inventors conducted a detailed investigation into what properties the steel section before spheroidizing annealing must have in order for the size of cementite particles after spheroidizing annealing to satisfy the above-mentioned condition. As a result, they found that the size of cementite particles after spheroidizing annealing satisfies the above-mentioned condition, if spheroidizing annealing is performed under normal annealing conditions for a steel section containing, by mass percentage, 0.02 to 0.30% Cu and 0.02 to 0.25% Ni, in which the Cu and Ni contents measured at multiple observation fields of the cross section of the steel section after hot rolling and before spheroidizing annealing have little variation, and in which the cementite particles observed at the multiple observation fields of the cross section have an aspect ratio of 2.0 or less when approximated as ellipses, and in which the maximum major axis value of the cementite particles is 10.0 μm or less.

[0010] The present invention is based on the above findings, and has the following gist and configuration.

[0011] [1] A steel sheet having a component composition containing, by mass percentage, C: 0.12 to 0.44%, Si: 0.15 to 0.35%, Mn: 0.30 to 0.95%, P: 0.001 to 0.030%, S: 0.001 to 0.030%, Cr: 0.85 to 1.50%, Cu: 0.02 to 0.30%, Ni: 0.02 to 0.25%, and N: 0.0020 to 0.0250%, with the balance being Fe and unavoidable impurities, wherein the length of the major axis when the shapes of all cementite particles observed in m observation fields (where m is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction are approximated as an ellipse is d. Ｌ , the length of the minor axis is d Ｓ and the length of the major axis is d Ｌ The length of the minor axis d Ｓ The length d of the major axis of a specific cementite particle having an aspect ratio of 2.0 or less obtained by dividing Ｌ is 10.0 μm or less in all of the m observation fields, and the maximum value of the mass percentage of the Cu content measured in the m observation fields is [Cu] ｍａｘ , the minimum value is [Cu] ｍｉｎ The maximum value of the mass percentage of the Ni content measured in the m observation fields is defined as [Ni] ｍａｘ , the minimum value is [Ni] ｍｉｎ and wherein the following formula (1) is satisfied:

[0012]

[0013] [2] The section steel according to the above [1], wherein the m observation fields include one observation field located in the center of the cross section and one or more observation fields located in the periphery of the cross section.

[0014] [3] The section steel according to the above [2], wherein the observation field located in the peripheral part is located at the midpoint of a line segment connecting the observation field located in the central part and the outer periphery of the cross section.

[0015] [4] The section steel according to the above [3], wherein the m observation fields consist of one observation field located in the center of the cross section and four observation fields located in the peripheral parts of the cross section, and the angle between adjacent four line segments passing through the four observation fields located in the peripheral parts of the cross section is 90 degrees.

[0016] [5] The steel section according to any one of [1] to [4] above, wherein the chemical composition further contains, by mass percentage, one or more elements selected from the group consisting of Mo: 0.30% or less, Al: 0.100% or less, Ti: 0.100% or less, V: 0.300% or less, Nb: 0.100% or less, B: 0.0100% or less, and Sn: 0.100% or less.

[0017] [6] The steel bar according to any one of [1] to [5] above, wherein the cross-sectional shape is any one of a circle, an ellipse, a square, and a rectangle. [7] A steel sheet having a composition containing, by mass percentage, C: 0.12 to 0.44%, Si: 0.15 to 0.35%, Mn: 0.30 to 0.95%, P: 0.001 to 0.030%, S: 0.001 to 0.030%, Cr: 0.85 to 1.50%, Cu: 0.02 to 0.30%, Ni: 0.02 to 0.25%, and N: 0.0020 to 0.0250%, with the balance being Fe and unavoidable impurities, wherein the maximum value of the circle-equivalent diameter in one observation field of view among the circle-equivalent diameters of all cementite particles observed in n observation fields (n is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction is defined as d １ , d ２ , ..., d ｎ Let d １ From d ｎ The maximum value up to d ｍａｘ , the minimum value is d ｍｉｎ and (3) satisfies the following formulas (2) and (3):

[0018]

[0019]

[0020] [8] The section steel according to [7] above, wherein the n observation fields include one observation field located in the center of the cross section and one or more observation fields located in the periphery of the cross section.

[0021] [9] The section steel according to the above [8], wherein the observation field located in the peripheral part is located at the midpoint of a line segment connecting the observation field located in the central part and the outer periphery of the cross section.

[0022]

[10] The section steel according to [9] above, wherein the n observation fields consist of one observation field located in the center of the cross section and four observation fields located in the peripheral parts of the cross section, and the angle between adjacent four line segments passing through the four observation fields located in the peripheral parts of the cross section is 90 degrees.

[0023]

[11] The steel section according to any one of [7] to

[10] above, wherein the chemical composition further contains, in mass percentage, one or more elements selected from the group consisting of Mo: 0.30% or less, Al: 0.100% or less, Ti: 0.100% or less, V: 0.300% or less, Nb: 0.100% or less, B: 0.0100% or less, and Sn: 0.100% or less.

[0024]

[12] The steel bar according to any one of [7] to

[11] above, wherein the cross-sectional shape is any one of a circle, an ellipse, a square, and a rectangle.

[0025] According to the present invention, excellent cold forgeability can be obtained for section steel after spheroidizing annealing. This reduces damage to dies during cold forging and increases productivity. Furthermore, according to the present invention, a preferable metal structure of section steel before spheroidizing annealing is revealed, thereby improving the production yield of steel materials.

[0026] 1 is a diagram showing the position of an observation field for observing properties in a cross section of a section steel having a cross section shape of (a) a circle, (b) an ellipse, (c) a square, or (d) a rectangle. 2 is a diagram showing the shape of a test piece for measuring the critical upsetting ratio in an example. 3 is a diagram showing (a) a top view, (b) a cross section seen from the side, and (c) a cross section with an enlarged cutout portion. 4 is a graph showing the relationship between the maximum value of the circle-equivalent diameter of cementite particles and the critical upsetting ratio in an example.

[0027] Hereinafter, embodiments of the present invention will be described in detail.

[0028] [Component Composition] In one embodiment, the section steel according to the present invention has a component composition containing, by mass percentage, 0.12 to 0.44% C, 0.15 to 0.35% Si, 0.30 to 0.95% Mn, 0.001 to 0.030% P, 0.001 to 0.030% S, 0.85 to 1.50% Cr, 0.02 to 0.30% Cu, 0.02 to 0.25% Ni, and 0.0020 to 0.0250% N, with the balance being Fe and unavoidable impurities. In this specification, component compositions are all expressed in mass percentage unless otherwise specified.

[0029] The chemical composition of the above-mentioned section steel corresponds to that of an alloy steel called chromium steel, which is carbon steel with approximately 1% Cr added. More specifically, it is similar to the chemical composition of materials designated SCr415 to SCr440 as specified in Japanese Industrial Standard JIS G 4053:2008 "Alloy Steels for Machine Structural Use." However, it should be noted that the types of elements contained in the section steel according to the present invention and the range of the chemical composition of each element are not completely identical to those specified in the standard.

[0030] [C: 0.12 to 0.44%] C is added to ensure the strength required for machine components. If the C content is 0.12% or more, the strength required for machine structural components can be ensured. Furthermore, if the C content is 0.44% or less, there is no adverse effect on cold forgeability. Therefore, the C content is set to 0.12 to 0.44%. The lower limit of the C content is preferably 0.22%, more preferably 0.32%. The upper limit of the C content is preferably 0.42%, more preferably 0.39%.

[0031] [Si: 0.15 to 0.35%] Si is an element necessary for deoxidation during the melting process, and is also effective in imparting the necessary strength to steel by improving solid solution strengthening and hardenability. If the Si content is 0.15% or more, the above effects can be sufficiently obtained. Furthermore, if the Si content is 0.35% or less, there is no adverse effect on cold forgeability. Therefore, the Si content is set to 0.15 to 0.35%. The lower limit of the Si content is preferably 0.17%, more preferably 0.19%. The upper limit of the Si content is preferably 0.33%, more preferably 0.31%.

[0032] [Mn: 0.30 to 0.95%] Mn is an element necessary for deoxidation in the melting process, and is also an element effective in imparting the necessary strength to steel by improving hardenability. The above-mentioned effects can be sufficiently obtained if the Mn content is 0.30% or more. Furthermore, if the Mn content is 0.95% or less, there is no adverse effect on cold forgeability. Therefore, the Mn content is set to 0.30 to 0.95%. The lower limit of the Mn content is preferably 0.45%, more preferably 0.60%.

[0033] [P: 0.001 to 0.030%] P is an element effective in increasing the strength of steel. If the P content is 0.001% or more, the above effect can be sufficiently obtained. Furthermore, if the P content is 0.030% or less, P does not segregate at grain boundaries and thereby reduce the toughness of the steel. For this reason, the P content is set to 0.001 to 0.030%.

[0034] [S: 0.001 to 0.030%] S is an element that effectively improves the machinability of steel by combining with Mn in the steel to form MnS. The above effect can be fully achieved if the S content is 0.001% or more. Furthermore, if the S content is 0.030% or less, a large amount of MnS, which can serve as the starting point for cracks during cold forging, is not formed, and therefore there is no adverse effect on cold forgeability. Therefore, the S content is set to 0.001 to 0.030%.

[0035] [Cr: 0.85 to 1.50%] Cr is an element effective in imparting the necessary strength to steel by improving solid solution strengthening and hardenability. A Cr content of 0.85% or more can sufficiently achieve the above effects. Furthermore, a Cr content of 1.50% or less prevents the hardness of the steel from increasing and adversely affecting cold forgeability. Therefore, the Cr content is set to 0.85 to 1.50%. The upper limit of the Cr content is preferably 1.35%, more preferably 1.20%.

[0036] [Cu: 0.02 to 0.30%] Cu is an element that effectively controls the size of cementite particles. That is, Cu suppresses the growth of cementite particles during spheroidizing annealing. If the Cu content is 0.02% or more, the above effect can be sufficiently obtained. Furthermore, if the Cu content is 0.30% or less, surface defects are less likely to occur during steel production. Therefore, the Cu content is set to 0.02 to 0.30%. The lower limit of the Cu content is preferably 0.04%, more preferably 0.06%. The upper limit of the Cu content is preferably 0.25%, more preferably 0.20%.

[0037] [Ni: 0.02 to 0.25%] Ni, like Cu, is an element that effectively controls the size of cementite particles. That is, Ni suppresses the growth of cementite particles during spheroidizing annealing. When the Ni content is 0.02% or more, the above effect can be sufficiently obtained. Furthermore, when the Ni content is 0.25% or less, surface defects are less likely to occur during steel production. Therefore, the Ni content is set to 0.02 to 0.25%. The lower limit of the Ni content is preferably 0.04%, more preferably 0.06%. The upper limit of the Ni content is preferably 0.20%, more preferably 0.15%.

[0038] [N: 0.0020 to 0.0250%] N combines with nitride-forming elements in the steel to form nitrides. The formed nitrides act as pinning particles that prevent the movement of austenite and ferrite grain boundaries. This prevents the coarsening of ferrite grains in the steel bar, improving strength. If the N content is 0.0020% or more, the above effects can be sufficiently obtained. Furthermore, if the N content is 0.0250% or less, dynamic strain aging caused by solute nitrogen in the steel can be prevented, preventing cracks from occurring during cold working. For this reason, the N content is set to 0.0020 to 0.0250%.

[0039] [Fe and inevitable impurities] In one embodiment, the section steel according to the present invention has a composition containing the elements described above, with the balance being Fe and inevitable impurities. Fe is the main component of the section steel according to the present invention. In this specification, the term "unavoidable impurities" generally refers to impurities that are present in raw materials for metal products or that are inevitably mixed in during the manufacturing process, and that are essentially unnecessary but are tolerated because they are present in trace amounts and do not affect the properties of the metal products.

[0040] Examples of elements corresponding to inevitable impurities include O (oxygen), Ca, Bi, Sb, etc. Even if the section steel according to the present invention inevitably contains, by mass percentage, 0.0100% or less of O (oxygen), 0.01% or less of Ca, 0.01% or less of Bi, and 0.03% or less of Sb, the contents of these elements are so small that they do not affect the cold forgeability. It is permissible in the present invention for the section steel to contain trace amounts of elements other than the exemplified O (oxygen), Ca, Bi, and Sb, as long as the effects of the present invention are not affected.

[0041] In a preferred embodiment, the section steel according to the present invention further contains, by mass percentage, one or more elements selected from the group consisting of Mo: 0.30% or less, Al: 0.100% or less, Ti: 0.100% or less, V: 0.300% or less, Nb: 0.100% or less, B: 0.0100% or less, and Sn: 0.100% or less.

[0042] [Mo: 0.30% or less] Mo is an element that can significantly improve the hardenability of steel material with a small amount of addition, and is effective in improving the strength of steel. If the Mo content is 0.30% or less, excessive hardenability and a decrease in cold forgeability can be prevented in advance. Therefore, in a preferred embodiment, the Mo content is 0.30% or less. If the Mo content is 0.15% or more, the above-mentioned effects can be sufficiently obtained.

[0043] [Al: 0.100% or less] Al is an element that promotes deoxidation during the melting process, and also combines with N in the steel to form nitrides, thereby promoting the refinement of ferrite grains and effectively increasing the strength of the bar steel. If the Al content is 0.100% or less, it is possible to prevent cracks from easily occurring during cold forging due to the generation of a large amount of Al oxide in the steel. Therefore, in a preferred embodiment, the Al content is 0.100% or less. If the Al content is 0.001% or more, the above-mentioned effects can be sufficiently obtained.

[0044] [Ti: 0.100% or less] Like Al, Ti is an element that combines with N in steel to form nitrides, thereby effectively refining ferrite grains. If the Ti content is 0.100% or less, it is possible to prevent cracks from easily occurring during cold forging due to the formation of a large amount of Ti-based inclusions in the steel. Therefore, in a preferred embodiment, the Ti content is 0.100% or less. If the Ti content is 0.001% or more, the above-mentioned effects can be sufficiently obtained.

[0045] [V: 0.300% or less] Like Al and Ti, V is an element that combines with N in steel to form nitrides, thereby effectively refining ferrite grains. If the V content is 0.300% or less, it is possible to prevent cracks from easily occurring during cold forging due to the precipitation of a large amount of V-based precipitates. Therefore, in a preferred embodiment, the V content is 0.300% or less. If the V content is 0.001% or more, the above-mentioned effects can be sufficiently obtained.

[0046] [Nb: 0.100% or less] Nb is an element that is effective in refining ferrite grains by combining with C in steel to form carbides. If the Nb content is 0.100% or less, it is possible to prevent cracks from easily occurring during cold forging due to the generation of a large amount of Nb-based carbides. Therefore, in a preferred embodiment, the Nb content is 0.100% or less. If the Nb content is 0.001% or more, the above-mentioned effects can be sufficiently obtained.

[0047] [B: 0.0100% or less] B is an element that can significantly improve the hardenability of steel material with a small amount of addition, and is effective in improving the strength of steel. If the B content is 0.0100% or less, it is possible to prevent unnecessary addition due to saturation of the effect of addition. Therefore, in a preferred embodiment, the B content is 0.0100% or less. If the B content is 0.0005% or more, the above-mentioned effect can be sufficiently obtained.

[0048] [Sn: 0.100% or less] Sn is an element that can moderately embrittle ferrite and is effective in improving machinability. If the Sn content is 0.100% or less, it is possible to prevent a decrease in cold forgeability due to excessive embrittlement. Therefore, in a preferred embodiment, the Sn content is 0.100% or less. If the Sn content is 0.001% or more, the above-mentioned effects can be sufficiently obtained.

[0049] [Steel Sections] The subject of the present invention is steel sections. In this specification, "steel sections" refers to rolled steel material that is not flat, and includes steel bars and wire rods. The cross-sectional shape perpendicular to the longitudinal direction of the steel sections according to the present invention is not particularly limited, and any shape may be used. In a preferred embodiment, the steel sections according to the present invention have a cross-sectional shape that is either a circle (a), an ellipse (b), a square (c), or a rectangle (d), as exemplified in FIG. 1 . These cross-sectional shapes are preferred in that they are highly symmetrical and therefore facilitate homogenization of the metal structure, as described below. The cross-sectional shape of the steel sections may also be, for example, a hexagon, or a circle with protrusions, as in the case of deformed reinforcing bars.

[0050] [Casting] Next, a method for producing section steel according to the present invention will be described. In a typical process for producing section steel, three steps, namely, casting, hot rolling, and spheroidizing annealing, are carried out in this order. Casting is a process in which molten steel having a predetermined chemical composition is poured and cooled to obtain a steel ingot. In casting, first, molten steel having a predetermined chemical composition is prepared. The chemical composition of the molten steel is adjusted so that the chemical composition of the section steel becomes the chemical composition described above. A batch-type electric furnace or a continuous blast furnace can be used to produce molten steel. Electric furnaces are suitable for producing a wide variety of steel products in small quantities because the chemical composition can be easily adjusted. However, the preparation of molten steel in the present invention is not limited to a method using an electric furnace.

[0051] Next, the prepared molten steel is poured into a mold for casting to obtain a steel ingot. The temperature of the molten steel during pouring is preferably equal to or higher than the melting point of the molten steel of the relevant chemical composition, but not higher than a temperature that is 100°C higher than the melting point. Pouring of the molten steel into the mold may be performed by continuous casting, or may be performed using a batch-type mold. In the case of continuous casting, the pouring speed, i.e., the speed at which the steel ingot cooled in the mold descends, is preferably 0.1 m / min or more and 3.0 m / min or less. By satisfying these casting conditions, the Cu and Ni contained in the molten steel can be evenly dispersed throughout the steel ingot.

[0052] [Hot rolling] Hot rolling is a process in which a steel ingot produced in a casting process is heated and rolled at a high temperature to form the steel ingot into a predetermined cross-sectional shape of section steel. A heating furnace can be used to heat the steel ingot. The temperature to which the steel ingot is heated is preferably 1000°C or higher and 1250°C or lower. Furthermore, reduction rolls can be used for rolling. The finishing temperature for hot rolling is preferably 750°C or higher, followed by cooling. By satisfying these hot rolling conditions, the shape of cementite particles, which will be described later, can be controlled. In this specification, section steel formed into a predetermined cross-sectional shape by hot rolling and before spheroidizing annealing, may be referred to as a "semi-finished section steel."

[0053] [Spheroidizing Annealing] Spheroidizing annealing is a process of annealing a semi-finished section steel product obtained in the hot rolling process to spheroidize cementite particles contained in the metallographic structure of the section steel. An annealing furnace can be used for spheroidizing annealing. Spheroidizing annealing can be performed under known conditions. Specifically, the annealing temperature is preferably 680°C or higher and 700°C or lower. Furthermore, the temperature holding time during annealing is preferably 12 hours or higher and 20 hours or lower. If the desired metallographic structure of the semi-finished section steel product before spheroidizing annealing is obtained by satisfying the above-mentioned casting and hot rolling conditions, then spheroidizing annealing under known conditions can produce a section steel product with excellent cold forgeability. Note that the chemical composition of the section steel product remains almost unchanged before and after spheroidizing annealing. In this specification, a product obtained by subjecting a semi-finished section steel product to spheroidizing annealing may be referred to as a "finished section steel product."

[0054] [Properties before spheroidizing annealing] Next, the properties of the section steel according to the present invention before spheroidizing annealing will be described. In one embodiment, the section steel according to the present invention is obtained by hot rolling a steel ingot before spheroidizing annealing. The length of the major axis of all cementite particles observed in m observation fields (where m is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction of the section steel before spheroidizing annealing is d Ｌ , the length of the minor axis is d Ｓ and the length of the major axis is d Ｌ The length of the minor axis d Ｓ The length d of the major axis of a specific cementite particle having an aspect ratio of 2.0 or less obtained by dividing Ｌ is 10.0 μm or less in all of the m observation fields, and the maximum value of the mass percentage of the Cu content measured in the m observation fields is [Cu] ｍａｘ , the minimum value is [Cu] ｍｉｎ The maximum value of the mass percentage of the Ni content measured in the m observation fields is defined as [Ni] ｍａｘ , the minimum value is [Ni] ｍｉｎ When this is the case, the following formula (1) is satisfied.

[0055]

[0056] The shape of cementite particles before spheroidizing annealing can be evaluated by observing the metallographic structure of a cross-section of a sample obtained by cutting a semi-finished product of long steel obtained by hot rolling a steel ingot perpendicular to the longitudinal direction. Specifically, the cross-section of the sample is polished, and then the polished surface is etched with acid to make the cementite phase visible. A scanning electron microscope is preferably used to observe the metallographic structure. In observing the metallographic structure, the length of the major axis of the cementite particles is evaluated in the observation field. For this purpose, the number m of the observation fields is an integer of 2 or more. To ensure uniform observation conditions, it is preferable that the shape and size of the observation field are constant. The shape of the observation field is not particularly limited and may be circular, elliptical, rectangular, or square.

[0057] Based on the shape of the cementite particle observed in one observation field, the length of the major axis when the shape of the cementite particle is approximated as an ellipse is defined as d Ｌ , the length of the minor axis is d Ｓ Specifically, by processing the image data of a metal structure photograph taken from one observation field using image processing software, the length of the major axis of each cementite particle is calculated as d Ｌ and the length of the minor axis d Ｓ In this specification, "ellipse approximation" refers to obtaining a virtual ellipse shape that has the same area, major axis and minor axis directions, and center position as the outline shape of the cementite particle of interest. For example, when using the open source ImageJ as image processing software, the d of the particle can be calculated by specifying Fit Ellipse (ellipse approximation) as the value to be measured in a command called Analyze Particles (particle analysis) for image data of the particle of interest. Ｌ and d Ｓ can be calculated.

[0058] Next, d Ｌ d ＳSpecific cementite particles having an aspect ratio of 2.0 or less, obtained by dividing the d by the d of the specific cementite particles having an aspect ratio of 2.0 or less, are selected. This operation is performed for the purpose of excluding cementite in pearlite remaining in the semi-finished product of long steel before spheroidizing annealing from the evaluation target. Since cementite in pearlite forms a layered lamellar structure together with ferrite, its aspect ratio usually exceeds 2.0. In one embodiment, the d of the specific cementite particles having an aspect ratio of 2.0 or less is selected. Ｌ is 10.0 μm or less in all m observation fields. This means that in all m observation fields, there are specific cementite particles with an aspect ratio of 2.0 or less and a major axis length d Ｌ This means that there are no cementite particles with a size exceeding 10.0 μm. Selection of specific cementite particles with an aspect ratio of 2.0 or less and the length d of the major axis Ｌ The maximum value of d of specific cementite particles having an aspect ratio of 2.0 or less in m observation fields can be confirmed by using the image processing software described above. Ｌ The lower limit of the maximum value is not particularly limited, but in one embodiment, the maximum value may be 1.0 μm or more.

[0059] Next, the mass percentage of the Cu content and the mass percentage of the Ni content are measured in the m number of observation fields. An electron probe microanalyzer is preferably used to measure the Cu and Ni contents. Measurement at one location in one observation field is sufficient. The maximum value of the mass percentage of the Cu content measured in the m number of observation fields is defined as [Cu] ｍａｘ , the minimum value is [Cu] ｍｉｎ The maximum value of the mass percentage of Ni content measured in m observation fields is defined as [Ni] ｍａｘ , the minimum value is [Ni] ｍｉｎ When these values ​​satisfy the above formula (1), that is, [Cu] ｍａｘ and [Ni] ｍａｘ The sum of [Cu] ｍｉｎ and [Ni] ｍｉｎThe value obtained by dividing by the sum of m is equal to or smaller than 1.15. This means that the variation in the Ni content and Cu content in the m observation fields is small. Note that the lower limit of the value on the left side of formula (1) is not particularly limited, but in one embodiment, the value on the left side of formula (1) may be 1.00 or more.

[0060] In one embodiment, if a semi-finished product of a steel section before spheroidizing annealing, which has the chemical composition according to the present invention and satisfies the above-described properties, is subjected to spheroidizing annealing under known conditions, the maximum equivalent circle diameter of the cementite particles after spheroidizing annealing will be 15.0 μm or less, and the variation in the equivalent circle diameter will be small, as will be described later. As a result, a finished steel section having excellent cold forgeability can be obtained.

[0061] In one embodiment, the reason why a finished section steel product with excellent cold forgeability can be obtained by controlling the properties of the semi-finished section steel product before spheroidizing annealing is not clear in detail, but it is thought to be due to the following reason. First, it is thought that specific cementite particles having an aspect ratio of 2.0 or less before spheroidizing annealing change and grow into spheres by Ostwald ripening without being divided by the subsequent spheroidizing annealing. Therefore, the length d of the major axis of the cementite particle before spheroidizing annealing Ｌ It is believed that by controlling the diameter to 10.0 μm or less, the equivalent circle diameter of cementite particles after spheroidizing annealing can be made 15.0 μm or less.

[0062] As described above, Cu and Ni have the effect of suppressing the growth of cementite particles during spheroidizing annealing. The section steel according to the present invention contains predetermined amounts of Cu and Ni. However, if the Cu content and Ni content in the semi-finished section steel vary greatly, the growth of cementite particles may not be suppressed in areas where the contents are insufficient, and the maximum value of the equivalent circle diameter may exceed a predetermined value. In one embodiment, it is believed that the growth of cementite particles can be uniformly suppressed by reducing the variation in the Cu content and Ni content.

[0063] In a preferred embodiment, the m observation fields include one observation field located in the center of the cross section and one or more observation fields located in the peripheral parts of the cross section. In a semi-finished product of long steel obtained by hot rolling a steel ingot, the metal structure in the peripheral parts of the cross section is generally susceptible to plastic deformation by the rolling rolls. In contrast, the metal structure in the center of the cross section is less susceptible to plastic deformation. For this reason, in one observation field located in the center of the cross section and one or more observation fields located in the peripheral parts of the cross section, the length d of the major axis of the cementite grains is Ｌ and the length of the minor axis d Ｓ Therefore, in a preferred embodiment, m observation fields are selected from both the central and peripheral positions of the cross section, which allows the properties of cementite particles to be correctly evaluated over the entire cross section of the sample.

[0064] In a more preferred embodiment, the observation field located in the peripheral region is located at the midpoint of a line segment connecting the observation field located in the central region and the outer periphery of the cross section. In a semi-finished product of long steel obtained by hot rolling a steel ingot, the metallographic structure of the peripheral region of the cross section, particularly that portion close to the outer periphery, is generally susceptible to plastic deformation by the rolling rolls. For this reason, it is not preferable to use an observation field selected from this portion as a representative of the entire sample. Therefore, in a more preferred embodiment, the position of the peripheral region is selected at the midpoint of a line segment connecting the observation field located in the central region and the outer periphery of the cross section. This eliminates the peripheral region of the cross section, particularly that portion close to the outer periphery, from the observation target, allowing for a more accurate evaluation of the properties of cementite particles throughout the entire cross section of the sample.

[0065] In a further preferred embodiment, the m observation fields consist of one observation field located in the center of the cross section and four observation fields located in the peripheral portion of the cross section, as exemplified in FIG. 1 , and the angle between adjacent line segments passing through the four observation fields located in the peripheral portion of the cross section is 90 degrees. If the number m of observation fields for observing the properties of cementite particles is too small, it will be impossible to observe the entire sample, and if it is too large, measurement will take a long time. Therefore, in a further preferred embodiment, a total of five observation fields are used for observation, including one central observation field and four peripheral observation fields. Furthermore, the angle between adjacent line segments connecting the central observation field to the periphery of the cross section and passing through the four peripheral observation fields is 90 degrees. This allows the observation fields to be uniformly determined according to the shape of the cross section of the sample.

[0066] In a more preferred embodiment, the angles formed by four adjacent line segments are only required to be 90 degrees, and the directions of the line segments are not particularly limited. However, in a more preferred embodiment, if it is desired to specify the position of the observation field more precisely, as exemplified in Figure 1, when the shape of the cross section perpendicular to the longitudinal direction of the section steel is an ellipse (b), the directions of the line segments can be set to the directions of the major and minor axes of the ellipse, and when the shape of the cross section is a square (c) or a rectangle (d), the directions of the line segments can be set to the directions perpendicular to the sides.

[0067] [Properties after spheroidizing annealing] Next, the properties of the section steel according to the present invention after spheroidizing annealing will be described. In another embodiment, the section steel according to the present invention is obtained by hot rolling a steel ingot, and then further performing spheroidizing annealing on the section steel. The section steel is obtained by observing the maximum circle-equivalent diameter in one observation field of n observation fields (n is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction of the steel. １ , d ２ , ..., d ｎ Let d １ From d ｎ The maximum value up to d ｍａｘ , the minimum value is d ｍｉｎ When this is set, the following expressions (2) and (3) are satisfied.

[0068]

[0069]

[0070] The evaluation of the shape of cementite particles after spheroidizing annealing can be carried out, similarly to the evaluation before spheroidizing annealing, by observing the metallographic structure of a cross-section of a sample obtained by cutting the finished long steel product after spheroidizing annealing perpendicular to the longitudinal direction. Specifically, the cross-section of the sample is polished, and then the polished surface is etched with acid to make the cementite phase visible. A scanning electron microscope is preferably used for observing the metallographic structure. In observing the metallographic structure, the variation in the size of cementite particles depending on the observation field is evaluated. For this purpose, the number n of observation fields to be observed is an integer of 2 or greater. To accurately evaluate the variation, it is preferable that the shape and size of the observation fields to be observed are constant. The shape of the observation field is not particularly limited and may be a circle, ellipse, rectangle, or square.

[0071] The circle-equivalent diameter of the cementite particles is determined based on the shape of the cementite particles observed in one observation field. Specifically, the circle-equivalent diameter of each cementite particle is calculated by processing image data of a metallographic photograph of one observation field using image processing software. In this specification, the "circle-equivalent diameter of a cementite particle" refers to the diameter of a circle having the same area as the cross-sectional area calculated on the observation surface of one cementite particle. The maximum value of the calculated circle-equivalent diameters in one observation field is defined as d １ This operation is repeated for n observation fields, and the maximum value d １ , d ２ , ..., d ｎ Ask for.

[0072] In another embodiment, the d value obtained by the above procedure for the section steel after spheroidizing annealing according to the present invention is １ From d ｎ The maximum value up to d ｍａｘ , the minimum value is d ｍｉｎ When ｍａｘsatisfies the above formula (2). That is, the maximum value d of the circle equivalent diameter of the cementite particles observed in n observation fields ｍａｘ is equal to or smaller than 15.0 μm. This means that there are no cementite particles with a circle equivalent diameter exceeding 15.0 μm in the n observation fields. ｍａｘ Although the lower limit of is not particularly limited, in one embodiment, ｍａｘ can be 1.0 μm or more.

[0073] In addition, the maximum value of the circle equivalent diameter in n observation fields d １ , d ２ , ..., d ｎ satisfies the above formula (3). That is, the maximum value of the circle equivalent diameter d １ , d ２ , ..., d ｎ For any of these, the minimum value d ｍｉｎ is equal to or less than 1.25 multiplied by d １ , d ２ , ..., d ｎ The maximum value d among ｍａｘ is the minimum value d ｍｉｎ This means that the value is equal to or less than 25% of the value of d １ , d ２ , ..., d ｎ The value of d has little variation. １ , d ２ , ..., d ｎ Although the lower limit of the value of d １ , d ２ , ..., d ｎ The value of may be 1.00 μm or greater.

[0074] In another embodiment, the reason why a finished section steel product with excellent cold forgeability can be obtained by limiting the size and variation of the equivalent circle diameters of cementite particles in n observation fields after spheroidizing annealing to a predetermined range is not clear in detail, but it is thought to be due to the following reason. Cementite has a higher hardness than ferrite. Therefore, the form of cementite present in the finished section steel product affects the cold forgeability. By spheroidizing annealing the semi-finished section steel, the cementite contained in ferrite and pearlite becomes spherical particles and is dispersed within the matrix. As a result, the presence of cementite is less likely to interfere with plastic deformation during cold forging.

[0075] However, in the prior art, some cementite particles had an equivalent circle diameter exceeding 15.0 μm. Furthermore, the equivalent circle diameter varied greatly depending on the observation position. Because cementite particles with a large equivalent circle diameter exist in a portion of the cross section of the steel section, this portion hinders plastic deformation during cold forging, resulting in reduced cold forgeability. In the present invention, the size and variation of the equivalent circle diameter of cementite particles after spheroidizing annealing are controlled within a predetermined range, and it is believed that a finished steel section with excellent cold forgeability can be obtained.

[0076] A preferred embodiment for determining n observation fields for a finished section steel product after spheroidizing annealing is similar to the preferred embodiment for determining m observation fields for a semi-finished section steel product before spheroidizing annealing, and therefore will not be described here.

[0077] [Evaluation of Cold Forgeability] The effects of the present invention can be evaluated by the cold forgeability of the finished section steel. For example, when the cross-sectional shape of the section steel is a circle with a diameter of 15 mm, the cold forgeability can be evaluated by the following method. First, the oxide film formed on the surface of the round bar is removed by pickling. Next, the diameter of the round bar is reduced to 14 mm by wire drawing, and the round bar is cut into a cylindrical shape with a length of 21 mm. A V-shaped groove with a groove tip curvature R of 0.15 mm, a groove depth of 0.8 mm, and a groove angle of 30 degrees is formed on the side of this cylinder, and a grooved cylindrical test piece 4 having the shape shown in Figure 2 is prepared.

[0078] Next, the test piece 4 was set in a compression tester and subjected to a strain rate of 10 s －１ The specimen is compressed in the height direction under the condition that the compressed surface 4b is constrained by a pressure of 0.3 mm. First, the specimen is reduced by 0.3 mm, and the bottom of the V-shaped groove 4a on the side surface is visually inspected for cracks. If cracks are present, their length is measured. Next, reductions of 0.3 mm are performed sequentially until the crack length reaches 0.5 mm or more. This compression test is performed on six specimens, and the cumulative reduction rate at which cracks of 0.5 mm or more are confirmed in three of the six specimens is defined as the critical upsetting rate. In this specification, the "cumulative upsetting rate" refers to the percentage obtained by dividing the final height of the specimen by the original height of 21 mm. Under the above test conditions, a specimen with a critical upsetting rate of 55% or more can be said to have excellent cold forgeability.

[0079] Examples of the present invention will be described below. Note that the embodiments of the present invention are not limited to the following examples, and the embodiments of the present invention can be modified as desired without departing from the gist of the present invention.

[0080] [Examples of the Invention] Molten steels having the chemical compositions of Steel Nos. 1 to 35 in Table 1 were prepared using an electric furnace, and steel ingots were cast by continuous casting. The molten steel temperature at the time of pouring was 1,450°C, and the pouring speed was 1.0 m / min. The steel ingots were heated to 1,100°C using a heating furnace and then hot-rolled into round bars with a diameter of 15 mm using rolling mill rolls. The finishing temperature of the hot rolling was 800°C. Samples were taken from the semi-finished long bar products obtained after hot rolling for Steel Nos. 1 to 35, and the cross sections perpendicular to the longitudinal direction were polished. The observation surfaces were then etched using a 3% aqueous solution of picric acid.

[0081] Next, in the five observation fields shown in FIG. 1(a), the length d of the major axis of cementite particles with an aspect ratio of 2.0 or less was measured using a scanning electron microscope and image processing software (ImageJ Ver. 1.53c). ＬThe maximum value of the aspect ratio of the long axis of the specific cementite particles having an aspect ratio of 2.0 or less was determined. The magnification of the scanning electron microscope was 5000 times, and the diameter of the smallest cementite particle observable at this magnification was 60 nm. The size of one observation field was 24 μm horizontally and 18 μm vertically. The Cu content and Ni content were also measured in the same observation field using an electron probe microanalyzer. The beam diameter of the electron beam was approximately 10 μm. The obtained evaluation results are shown in Table 2. According to Table 2, in samples No. 1 to 35 having the component compositions according to the present invention, the length d of the long axis of the specific cementite particles having an aspect ratio of 2.0 or less was Ｌ However, the average particle size was 10.0 μm or less in all five observation fields, and the Cu content and Ni content measured in the five observation fields satisfied formula (1).

[0082] Comparative Examples On the other hand, for Samples No. 36 to 47 and Samples No. 50 and 51 in Table 2, semi-finished long steel bars (round bars) were produced from molten steel having compositions outside the range of the present invention, i.e., Steel Nos. 36 to 47 in Table 1. For Sample No. 48 in Table 2, a semi-finished long steel bar (round bar) was produced from molten steel having the composition of Steel No. 11 in Table 1, but the casting conditions were different from those described above, with the molten steel temperature at the time of pouring being 1530°C and the pouring speed being 3.5 m / min. For Sample No. 49 in Table 2, a semi-finished long steel bar (round bar) was produced from molten steel having the composition of Steel No. 6 in Table 1, but the hot rolling conditions were different from those described above, with the heating temperature being 1270°C and the finishing temperature being 740°C. As a result, Sample No. In the sample No. 48, the Cu content and Ni content did not satisfy the formula (1), and in the sample No. 49, the d of specific cementite particles having an aspect ratio of 2.0 or less Ｌ The maximum value exceeded 10.0 μm.

[0083]

[0084] [Inventive Examples and Comparative Examples] Next, for both the inventive examples and comparative examples, the round bars were subjected to spheroidizing annealing in an annealing furnace. The annealing temperature was 690°C, and the temperature holding time during annealing was 15 hours. Samples were taken from the resulting finished section steel (round bars) after spheroidizing annealing, and the cross section perpendicular to the longitudinal direction was polished, after which the observation surface was etched using a 3% aqueous solution of picric acid.

[0085] Next, the circle-equivalent diameters of the cementite particles were determined in the five observation fields shown in FIG. 1(a) using a scanning electron microscope and image processing software (ImageJ Ver. 1.53c), and the maximum value d １ , d ２ , d ３ , d ４ and d ５ Among these maximum values, the maximum value d ｍａｘ and the minimum value d ｍｉｎ The evaluation results are shown in Table 2.

[0086] Next, test pieces 4 shown in FIG. 2 were prepared from the finished steel bars (round bars) after spheroidizing annealing, and the critical upsetting ratio was measured by the method described above. The measurement results are shown in Table 2. ｍａｘ The relationship between the maximum upsetting rate and the maximum swaging rate is shown in Figure 3.

[0087]

[0088] According to Tables 1 and 2, the length d of the major axis of the specific cementite particles having an aspect ratio of 2.0 or less before spheroidizing annealing satisfies the component composition of the section steel according to the present invention. Ｌ It can be seen that for Samples No. 1 to No. 35, which were samples in which the maximum value of 10.0 μm or less and the contents of Cu and Ni satisfied the formula (1) were subjected to normal spheroidizing annealing, the equivalent circle diameter of the cementite particles after spheroidizing annealing satisfied the conditions of the formulas (2) and (3), and the critical upsetting ratio was greater than 55% in each case.

[0089] On the other hand, in Samples No. 36 to 51 in which the composition of the section steel did not satisfy the composition according to the present invention or the casting or hot rolling conditions were different from the above-mentioned conditions, the equivalent circle diameter of the cementite particles after spheroidizing annealing did not satisfy at least one of the conditions of formula (2) or (3). As a result, the critical upsetting ratio of these samples all showed values ​​smaller than 55%.

[0090] 3, it can be seen that the inventive examples, which were obtained by subjecting semi-finished section steel products according to the present invention to spheroidizing treatment and which met the requirements for the finished section steel products according to the present invention and which are indicated by open circles, all had critical upsetting ratios greater than 55%. On the other hand, it can be seen that the comparative examples, which did not meet the requirements for the finished section steel products according to the present invention and which are indicated by closed circles and closed triangles, all had critical upsetting ratios less than 55%.

[0091] These results show that by controlling the properties of the semi-finished section steel before spheroidizing annealing, it is possible to make the equivalent circle diameter of cementite particles in the finished section steel after spheroidizing annealing satisfy the conditions of formulas (2) and (3), and to obtain section steel with excellent cold forgeability. At the same time, these results show that when the equivalent circle diameter of cementite particles in the finished section steel after spheroidizing annealing satisfies both the conditions of formulas (2) and (3), it is possible to obtain section steel with excellent cold forgeability.

[0092] 1 Cross section of steel bar 2 Observation field 3 Line segment 4 Test piece 4a V-shaped groove 4b Compressed surface

Claims

1. A steel sheet having a composition containing, by mass percentage, C: 0.12-0.44%, Si: 0.15-0.35%, Mn: 0.30-0.95%, P: 0.001-0.030%, S: 0.001-0.030%, Cr: 0.85-1.50%, Cu: 0.02-0.30%, Ni: 0.02-0.25%, and N: 0.0020-0.0250%, with the balance being Fe and unavoidable impurities, wherein the length of the major axis of all cementite particles observed in m observation fields (where m is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction is approximated as an ellipse is d. Ｌ , the length of the minor axis is d Ｓ and the length of the major axis is d Ｌ The length of the minor axis d Ｓ The length d of the long axis of a specific cementite particle having an aspect ratio of 2.0 or less obtained by dividing Ｌ is 10.0 μm or less in all of the m observation fields, and the maximum value of the mass percentage of the Cu content measured in the m observation fields is [Cu] ｍａｘ , the minimum value is [Cu] ｍｉｎ The maximum value of the mass percentage of the Ni content measured in the m observation fields is defined as [Ni] ｍａｘ , the minimum value is [Ni] ｍｉｎ The steel section is characterized in that, when 2. The section steel according to claim 1, wherein the m observation fields include one observation field located in a central portion of the cross section and one or more observation fields located in a peripheral portion of the cross section.

3. The bar according to claim 2, wherein the observation field located in the peripheral portion is located at the midpoint of a line segment connecting the observation field located in the central portion and the outer periphery of the cross section.

4. The section steel according to claim 3, wherein the m observation fields consist of one observation field located at the center of the cross section and four observation fields located on the periphery of the cross section, and the angle between adjacent four line segments passing through the four observation fields located on the periphery of the cross section is 90 degrees.

5. The bar steel according to any one of claims 1 to 4, wherein the chemical composition further contains, by mass percentage, one or more elements selected from the group consisting of Mo: 0.30% or less, Al: 0.100% or less, Ti: 0.100% or less, V: 0.300% or less, Nb: 0.100% or less, B: 0.0100% or less, and Sn: 0.100% or less.

6. The steel bar according to any one of claims 1 to 5, wherein the cross-sectional shape is any one of a circle, an ellipse, a square and a rectangle.

7. A steel sheet having a composition containing, by mass percentage, C: 0.12-0.44%, Si: 0.15-0.35%, Mn: 0.30-0.95%, P: 0.001-0.030%, S: 0.001-0.030%, Cr: 0.85-1.50%, Cu: 0.02-0.30%, Ni: 0.02-0.25%, and N: 0.0020-0.0250%, with the balance being Fe and unavoidable impurities; and among the circle equivalent diameters of all cementite particles observed in n observation fields (n is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction, the maximum circle equivalent diameter in one observation field is defined as d. １ , d ２ , ..., d ｎ Let d １ From d ｎ The maximum value up to d ｍａｘ , the minimum value is d ｍｉｎ and wherein the following formulas (2) and (3) are satisfied:

8. The section steel according to claim 7, wherein the n observation fields include one observation field located in a central portion of the cross section and one or more observation fields located in a peripheral portion of the cross section.

9. The section steel according to claim 8, wherein the observation field located in the peripheral portion is located at the midpoint of a line segment connecting the observation field located in the central portion and the outer periphery of the cross section.

10. The section steel according to claim 9, wherein the n observation fields consist of one observation field located at the center of the cross section and four observation fields located on the periphery of the cross section, and the angle between adjacent four line segments passing through the four observation fields located on the periphery of the cross section is 90 degrees.

11. The bar steel according to any one of claims 7 to 10, wherein the chemical composition further contains, by mass percentage, one or more elements selected from the group consisting of Mo: 0.30% or less, Al: 0.100% or less, Ti: 0.100% or less, V: 0.300% or less, Nb: 0.100% or less, B: 0.0100% or less, and Sn: 0.100% or less.

12. The steel bar according to any one of claims 7 to 11, wherein the cross-sectional shape is any one of a circle, an ellipse, a square and a rectangle.

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