Steel bars

Abstract

Provided is a steel bar having excellent cold forgeability after being subjected to spheroidizing annealing. In one embodiment, in a steel bar containing predetermined amounts of Cr, Ni, and Cu, the length of the major axis of a specific cementite particle having an aspect ratio of 2.0 or less when the shape of cementite particles observed in m observation fields (m being an integer of 2 or more) in a cross section perpendicular to the length direction is elliptically approximated is 10.0 μm or less in all of the m observation fields, and variations in the Cu content and the Ni content measured in the m observation fields are small. In another embodiment, in a steel bar containing predetermined amounts of Cr, Ni, and Cu, among the equivalent circle diameters of the cementite particles observed in n observation fields (n being an integer of 2 or more) in a cross section perpendicular to the length direction, the maximum value of the equivalent circle diameter is 15.0 μm or less in every observation field, and the variation in the maximum value is small.

Description

[Technical Field]

[0001] This invention relates to steel bars with excellent cold forging properties. [Background technology]

[0002] In fields such as construction machinery, industrial machinery, and automobiles, structural components such as bolts, nuts, and screws are widely used. These components are made by cutting steel bars with a circular or square cross-section and then forging them into various shapes. The mechanical strength required for these structural components is ensured by optimizing the conditions of heat treatment, such as quenching, performed after processing. Furthermore, by using steel bars with alloying elements added beforehand, high mechanical strength can be stably achieved even if the heat treatment conditions fluctuate somewhat.

[0003] In recent years, in the manufacturing of mechanical structural parts, the application of cold forging has been expanding as an alternative to the conventionally used hot forging, due to considerations such as production efficiency. When cold forging steel materials containing a large amount of alloying elements, the deformation resistance is higher compared to steel materials with a low alloying element content. This leads to challenges such as a reduced lifespan of the molds used in cold forging and an increased likelihood of cracking in the finished product. To reduce deformation resistance and improve cold forgeability, it is effective to spheroidize the cementite contained in the metal structure of the steel material before cold forging. Various proposals have been made regarding cementite spheroidization technology.

[0004] For example, Patent Document 1 describes an invention of a low-alloy steel material and a method for manufacturing the same, in which the shape of cementite contained in the hot-rolled steel material is made spheroidal 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 rod and a method for manufacturing the same, in which cold workability is improved by adjusting the number of spherical cementite particles per unit volume that are below a specific average particle size. Patent Document 3 describes an invention of a cold-forging steel in which cold workability is improved by controlling the average value and standard deviation of the distance between carbides dispersed in ferrite grains by adding a large amount of Cr.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] The technique described in Patent Document 1 is characterized by improving cold forging properties without performing spheroidizing annealing. Compared with the case of performing the conventionally performed spheroidizing annealing, the cold forging properties themselves have not been greatly improved. Although the cold forging properties of the techniques described in Patent Document 2 and Patent Document 3 are improved compared with the spheroidizing annealing in the prior art, there is a problem in that the shape of the cementite is not necessarily optimized because an average value is adopted as a representative value representing the particle diameter or the interparticle distance of the spheroidized cementite.

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

Means for Solving the Problems

[0008] The present inventor investigated the relationship between the size of cementite particles observed in the cross section of bar steel after performing spheroidizing annealing and the cold forging properties in order to realize the improvement of cold forging properties. As a result, it was found that excellent cold forging properties can be obtained in bar steel in which the equivalent circle diameter of cementite particles measured in a plurality of observation fields of the cross section is 15.0 μm or less and the variation in the equivalent circle diameter is small.

[0009] Next, the inventors conducted a detailed investigation into the properties of steel bars before spheroidizing annealing in order for the size of cementite particles after spheroidizing annealing to satisfy the above conditions. As a result, they found that for steel bars containing 0.02-0.30% Cu and 0.02-0.25% Ni by mass percentage, with little variation in the Cu and Ni content measured in multiple observation fields of the cross-section of the steel bar after hot rolling and before spheroidizing annealing, and with an aspect ratio of 2.0 or less when the cementite particles observed in the multiple observation fields of the above cross-section are approximated as an ellipse, and with a maximum major axis of 10.0 μm or less, spheroidizing annealing under normal annealing conditions will satisfy the above conditions for the size of cementite particles after spheroidizing annealing.

[0010] The present invention is based on the above findings, and its gist is as follows.

[0011] [1] In terms of 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% It contains and has a component composition consisting of Fe and unavoidable impurities as the remainder, When the shape of all cementite particles observed in m observation fields (where m is an integer greater than or equal to 2) in a cross section perpendicular to the length direction is approximated as an ellipse, the length of the major axis is d. L , the length of the minor axis is d S Let the length of the major axis be d L The length of the short axis d S The length of the long axis d of a specific cementite particle whose aspect ratio obtained by dividing by is 2.0 or less Lis 10.0 μm or less in all of the m observation fields, Let the maximum value among the mass percentages of the Cu content measured in the m observation fields be [Cu] max , and the minimum value be [Cu] min , and let the maximum value among the mass percentages of the Ni content measured in the m observation fields be [Ni] max , and the minimum value be [Ni] min When this is the case, it satisfies the following formula (1) A steel bar characterized by this.

[0012]

Number

[0013] [2] The m observation fields include one observation field located at the central part of the cross section and one or more observation fields located at the peripheral part of the cross section. The steel bar according to [1] above.

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

[0015] [4] The m observation fields consist of one observation field located at the central part of the cross section and four observation fields located at the peripheral part of the cross section. For the four line segments passing through the four observation fields located at the peripheral part of the cross section, the angle formed by adjacent line segments is 90 degrees. The steel bar according to [3] above.

[0016] [5] The component composition is further, in mass percentage, 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 It contains one or more selected from the group consisting of, A steel bar as described in any of the above [1] to [4].

[0017] [6] The shape of the cross-section is one of a circle, an ellipse, a square, or a rectangle. A steel bar as described in any of the above [1] to [5]. [7] In terms of 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% It contains and has a component composition consisting of Fe and unavoidable impurities as the remainder, Among the equivalent circular diameters of all cementite particles observed in n observation fields (where n is an integer of 2 or more) in a cross section perpendicular to the length direction, the maximum value of the equivalent circular diameter in one observation field is d1, d2, ..., d n Let d1 to d n The maximum value up to d max , the minimum value is d min When this is done, the following equations (2) and (3) are satisfied. A steel bar characterized by the following features.

[0018]

number

[0019]

number

[0020] [8] The n observation fields include one observation field located in the central part of the cross section and one or more observation fields located in the peripheral part of the cross section. The steel bars described in [7] above.

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

[0022]

[10] 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 part of the cross section, and for the four line segments passing through the four observation fields located in the peripheral part of the cross section, the angle between adjacent line segments is 90 degrees. The steel bars described in [9] above.

[0023]

[11] The above component composition is further expressed by mass percentage as follows: 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 It contains one or more selected from the group consisting of, The steel bars described in any of the above [7] to

[10] .

[0024]

[12] The shape of the cross-section is one of a circle, an ellipse, a square, or a rectangle. The steel bars described in any of the above [7] through

[11] . [Effects of the Invention]

[0025] According to the present invention, excellent cold forgeability can be obtained for steel bars after spheroidizing annealing. This reduces damage to the die during cold forging and increases productivity. Furthermore, according to the present invention, the preferred microstructure of steel bars before spheroidizing annealing is revealed, improving the production yield of steel materials. [Brief explanation of the drawing]

[0026] [Figure 1] This diagram shows the position of the observation field for observing the properties of a cross-section of a steel bar whose cross-sectional shape is (a) a circle, (b) an ellipse, (c) a square, or (d) a rectangle. [Figure 2] This figure shows the shape of the test specimen used to measure the limit settling rate in the embodiment. (a) is a top view, (b) is a cross-sectional view from the side, and (c) is an enlarged cross-sectional view of the notched portion. [Figure 3] This graph shows the relationship between the maximum equivalent diameter of cementite particles and the limiting upsetting rate in the example. [Modes for carrying out the invention]

[0027] The embodiments for carrying out the present invention will be described in detail below.

[0028] [Component composition] In one embodiment, the steel bar according to the present invention has a composition in which, 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 remainder being Fe and unavoidable impurities. In this specification, unless otherwise specified, all compositional values ​​are expressed in mass percentage.

[0029] The component composition of the steel bars described above corresponds to the component composition of an alloy steel called chromium steel, which is carbon steel with approximately 1% Cr added. More specifically, it is similar to the component composition of material symbols SCr415 to SCr440 as specified in the Japanese Industrial Standard JIS G 4053:2008 "Alloy Steel Materials for Machine Structures". However, it should be noted that the types of elements contained in the steel bars according to the present invention and the range of component composition of each element are not exactly the same as those in the said standard.

[0030] [C:0.12~0.44%] Carbon (C) is added to ensure the strength of the component as a machine part. A C content of 0.12% or more is sufficient to ensure the necessary strength for a machine structural part. Furthermore, a C content of 0.44% or less does not adversely affect cold forgeability. For this reason, the C content is set to 0.12-0.44%. 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: 0.15~0.35%] Si is an essential element for deoxidation during the melting process, and is also effective in imparting the necessary strength to steel through solid solution strengthening and improved hardenability. A Si content of 0.15% or more is sufficient to achieve these effects. Furthermore, a Si content of 0.35% or less does not adversely affect cold forgeability. Therefore, the Si content is set to 0.15-0.35%. 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: 0.30~0.95%] Mn is an essential element for deoxidation during the melting process and is also effective in imparting the necessary strength to steel by improving its hardenability. A Mn content of 0.30% or more is sufficient to achieve these effects. Furthermore, a Mn content of 0.95% or less does not adversely affect cold forgeability. Therefore, the Mn content is set between 0.30% and 0.95%. The lower limit of the Mn content is preferably 0.45%, and more preferably 0.60%.

[0033] [P:0.001~0.030%] P is an effective element for increasing the strength of steel. A P content of 0.001% or more is sufficient to achieve the above effect. Furthermore, if the P content is 0.030% or less, P will not segregate at grain boundaries, thus preventing a decrease in the toughness of the steel. Therefore, the P content should be between 0.001% and 0.030%.

[0034] [S:0.001~0.030%] S (S) is an effective element for improving the machinability of steel by combining with Mn (Mn) in steel to form MnS. A S content of 0.001% or more is sufficient to achieve the above effect. Furthermore, if the S content is 0.030% or less, a large amount of MnS, which can be the initiation point for cracks during cold forging, will not be formed, thus not adversely affecting cold forgeability. Therefore, the S content should be between 0.001% and 0.030%.

[0035] [Cr: 0.85~1.50%] Cr is an effective element for imparting the necessary strength to steel through solid solution strengthening and improved hardenability. A Cr content of 0.85% or more is sufficient to achieve these effects. Furthermore, a Cr content of 1.50% or less does not increase the hardness of the steel, thus avoiding adverse effects on cold forgeability. Therefore, the Cr content is set between 0.85% and 1.50%. The upper limit of the Cr content is preferably 1.35%, and more preferably 1.20%.

[0036] [Cu: 0.02~0.30%] Cu is an element that effectively controls the size of cementite particles. Specifically, Cu suppresses the growth of cementite particles during spheroidizing annealing. The above effect can be sufficiently obtained if the Cu content is 0.02% or more. Furthermore, if the Cu content is 0.30% or less, surface defects are less likely to occur during steel manufacturing. For this reason, 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: 0.02~0.25%] Ni, like Cu, is an element that effectively controls the size of cementite particles. In other words, Ni suppresses the growth of cementite particles during spheroidizing annealing. The above effect can be sufficiently obtained if the Ni content is 0.02% or more. Furthermore, if the Ni content is 0.25% or less, surface defects are less likely to occur during steel manufacturing. For this reason, 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: 0.0020~0.0250%] Nitrogen (N) combines with nitride-forming elements in steel to form nitrides. These nitrides act as pinning particles that hinder the movement of austenite and ferrite grain boundaries. This prevents grain coarsening of ferrite in steel bars, thereby improving strength. The above effect can be sufficiently obtained with a N content of 0.0020% or more. Furthermore, if the N content is 0.0250% or less, dynamic strain aging caused by dissolved nitrogen in the steel can be prevented from causing cracks during cold working. For this reason, the N content should be between 0.0020% and 0.0250%.

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

[0040] Elements that constitute unavoidable impurities include, for example, O (oxygen), Ca, Bi, and Sb. Even if the steel bar according to the present invention inevitably contains 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 by mass percentage, the amount of each is so small that it does not affect its cold forgeability. It is permissible in the present invention for the steel bar to contain trace elements other than the O (oxygen), Ca, Bi, and Sb exemplified herein, as long as they do not affect the effects of the present invention.

[0041] In a preferred embodiment, the steel bar according to the present invention further contains one or more elements selected by mass percentage 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 (Mo) is an effective element for improving the strength of steel, as even a small amount can significantly improve its hardenability. A Mo content of 0.30% or less prevents excessive hardenability from reducing cold forgeability. Therefore, the preferred Mo content in the embodiment is 0.30% or less. A Mo content of 0.15% or more can sufficiently achieve the above effects.

[0043] [Al:0.100% or less] Al is an element that promotes deoxidation in the melting process and, by combining with N in the steel to form nitrides, promotes the refinement of ferrite crystal grains, thereby effectively increasing the strength of steel bars. If the Al content is 0.100% or less, it is possible to prevent cracking that would easily occur during cold forging due to the generation of a large amount of Al oxide in the steel. For this reason, the Al content in the preferred embodiment is 0.100% or less. If the Al content is 0.001% or more, the above effects can be fully obtained.

[0044] [Ti:0.100% or less] Like aluminum, titanium (Ti) is an effective element for refining ferrite grains by bonding with nitrogen (N) in steel to form nitrides. A Ti content of 0.100% or less can prevent the formation of large amounts of Ti-based inclusions in the steel, which can easily lead to cracking during cold forging. Therefore, the preferred Ti content in the embodiment is 0.100% or less. A Ti content of 0.001% or more can sufficiently achieve the above effects.

[0045] [V:0.300% or less] V, like Al and Ti, is an element that effectively refines the grain size of ferrite by bonding with N in steel to form nitrides. If the V content is 0.300% or less, it is possible to prevent cracking during cold forging, which can occur due to the precipitation of large amounts of V-based precipitates. For this reason, the V content in the preferred embodiment is 0.300% or less. If the V content is 0.001% or more, the above effects can be fully obtained.

[0046] [Nb:0.100% or less] Nb is an element that is effective in refining the crystal grains of ferrite by bonding with C in steel to form carbides. If the Nb content is 0.100% or less, it is possible to prevent cracking that is likely to occur during cold forging due to the generation of a large amount of Nb-based carbides. For this reason, the Nb content in the preferred embodiment is 0.100% or less. If the Nb content is 0.001% or more, the above effect can be sufficiently obtained.

[0047] [B:0.0100% or less] B is an element that can significantly improve the hardenability of steel materials with even 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 the addition. For this reason, the B content in the preferred embodiment is 0.0100% or less. If the B content is 0.0005% or more, the above effects can be sufficiently obtained.

[0048] [Sn:0.100% or less] Sn (Tin) is an effective element for improving machinability by moderately embrittlementing ferrite. If the Sn content is 0.100% or less, it is possible to prevent a decrease in cold forgeability due to excessive embrittlement. Therefore, the Sn content in the preferred embodiment is 0.100% or less. The above effects can be fully obtained if the Sn content is 0.001% or more.

[0049] [Steel bars] The present invention relates to steel bars. In this specification, "steel bars" refers to rolled steel materials that are not flat in shape, and includes steel bars and wires. The shape of the cross-section perpendicular to the length direction of the steel bars according to the present invention is not particularly limited and may be any shape. In a preferred embodiment, the steel bars according to the present invention have a cross-sectional shape of a circle (a), an ellipse (b), a square (c), or a rectangle (d), as illustrated in Figure 1. These cross-sectional shapes are preferred because they have high symmetry, making it easy to homogenize the metal structure, as described later. The cross-sectional shape of the steel bars may also be, for example, a hexagon, or a circle with protrusions, as in the case of deformed reinforcing bars.

[0050] [casting] Next, the method for manufacturing steel bars according to the present invention will be described. In the normal manufacturing process of steel bars, three steps are carried out in this order: casting, hot rolling, and spheroidizing annealing. Casting is a process in which molten steel having a predetermined component composition is poured in and cooled to obtain a steel ingot. In casting, first, molten steel having a predetermined component composition is prepared. The component composition of the molten steel is adjusted so that the component composition of the steel bar is the component composition described above. A batch-type electric furnace or a continuous-type blast furnace can be used to manufacture the molten steel. Electric furnaces are suitable for the manufacture of small quantities of many types of steel materials because it is easy to adjust the component composition. However, the preparation of molten steel in the present invention is not limited to the method using an electric furnace.

[0051] Next, the prepared molten steel is poured into a mold to perform casting and obtain a steel ingot. Preferably, the temperature of the molten steel during pouring is above the melting point of the molten steel with the given composition, and no more than 100°C higher than the melting point. The molten steel may be poured into the mold by continuous casting or by using a batch-type mold. In the case of continuous casting, the pouring speed, that is, the speed at which the cooled steel ingot descends in the mold, 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 dispersed evenly in the steel ingot.

[0052] [Hot rolling] Hot rolling is a process in which a steel ingot produced in the casting process is heated and rolled at a high temperature to form the shape of a predetermined steel bar. A heating furnace can be used to heat the steel ingot. The temperature at which the steel ingot is heated is preferably between 1000°C and 1250°C. Rolling rolls can be used for rolling. The finishing temperature of the hot rolling is preferably 750°C or higher, followed by cooling. By satisfying these hot rolling conditions, the shape of the cementite particles, which will be described later, can be controlled. In this specification, a steel bar that has been formed into a predetermined cross-sectional shape by hot rolling and has not undergone spheroidizing annealing may be referred to as a "semi-finished steel bar."

[0053] [Spheroidizing annealing] Spheroidizing annealing is a process in which a semi-finished steel bar obtained in the hot rolling process is annealed to spheroidize the cementite particles contained in the metal structure of the steel bar. An annealing furnace can be used for spheroidizing annealing. Spheroidizing annealing can be carried out 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 metal structure is obtained in the semi-finished steel bar before spheroidizing annealing by satisfying the above casting and hot rolling conditions, a steel bar with excellent cold forgeability can be obtained by performing spheroidizing annealing under known conditions. Note that the component composition of the steel bar hardly changes before and after spheroidizing annealing. In this specification, the product obtained by subjecting a semi-finished steel bar to spheroidizing annealing may be referred to as a "finished steel bar."

[0054] [Properties before spheroidizing annealing] Next, the properties of the steel strip according to the present invention before spheroidizing annealing will be described. In one embodiment, for the steel strip according to the present invention obtained by hot rolling a steel ingot before spheroidizing annealing, the length of the major axis of the elliptic approximation of the shape 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 length direction is d L , the length of the minor axis is d S Let the length of the major axis be d L The length of the short axis dS The length of the long axis d of a specific cementite particle whose aspect ratio obtained by dividing by is 2.0 or less L However, in all m observation fields, the size is 10.0 μm or less, and the maximum value among the mass percentages of Cu content measured in the m observation fields is [Cu] max , the minimum value is [Cu] min The maximum value among the mass percentages of Ni content measured in the m observation fields is [Ni] max , the minimum value is [Ni] min When this is done, the following equation (1) is satisfied.

[0055]

number

[0056] The shape of cementite particles before spheroidizing annealing can be evaluated by observing the microstructure in a cross-section of a sample of a semi-finished steel bar obtained by hot-rolling a steel ingot, with the end cut perpendicular to the length. Specifically, the cementite phase is made visible by polishing the cross-section of the sample and then etching the polished surface with acid. A scanning electron microscope is preferably used for observing the microstructure. In observing the microstructure, the length of the major axis of the cementite particles is evaluated using the observation field. For this purpose, the number of m in the observation field is set to an integer of 2 or more. To standardize the 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 a circle, ellipse, rectangle, 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 d. L , the length of the minor axis is d S Specifically, by processing the image data of a metallographic photograph taken from one observation field using image processing software, the length of the long axis of each cementite particle is determined to be d. L and the length of the minor axis d SThis calculates the d of the particle. In this specification, "ellipse approximation" means finding the shape of a virtual ellipse that has the same area, major axis, minor axis direction, and center position as the contour shape of the cementite particle of interest. For example, when using the open-source image processing software ImageJ, you can specify Fit Ellipse as the value to be measured in the Analyze Particles command for the image data of the particle of interest, and then calculate the d of that particle. L and d S It is possible to calculate this.

[0058] Next, d L d S Select specific cementite particles whose aspect ratio obtained by dividing by 2.0 or less is 2.0 or less. This operation is performed to exclude from evaluation cementite in pearlite remaining in the semi-finished steel bars before spheroidizing annealing. Since cementite in pearlite forms a layered lamellar structure together with ferrite, its aspect ratio is usually greater than 2.0. In one embodiment, d of specific cementite particles with an aspect ratio of 2.0 or less L However, in all m observation fields, the size is 10.0 μm or less. This means that within all m observation fields, there are specific cementite particles with an aspect ratio of 2.0 or less, and with a major axis length d L This means that no cementite particles larger than 10.0 μm exist. The selection of specific cementite particles with an aspect ratio of 2.0 or less and the length of their long axis d L The maximum value can be confirmed using the image processing software described above. Furthermore, for m observation fields, the d of specific cementite particles with an aspect ratio of 2.0 or less can be determined. 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 Cu content and the mass percentage of Ni content are measured in the m observation fields. It is preferable to use an electron probe microanalyzer to measure the Cu and Ni content. Measurement only needs to be performed at one location in each observation field. The maximum value among the mass percentages of Cu content measured in the m observation fields is [Cu]. max , the minimum value is [Cu] min The maximum value among the mass percentages of Ni content measured in m observation fields is [Ni] max , the minimum value is [Ni] min When this is the case, these values ​​satisfy equation (1) above. That is, [Cu] max and [Ni] max The sum of [Cu] min and [Ni] min The value obtained by dividing by the sum of is equal to or less than 1.15. This means that the variation in Ni and Cu content in m observation fields is small. Note that there is no particular lower limit to the numerical value on the left side of equation (1), but in one embodiment, the numerical value on the left side of equation (1) may be 1.00 or greater.

[0060] In one embodiment, if a semi-finished steel bar having the component composition according to the present invention and satisfying the above properties is subjected to spheroidizing annealing under known conditions, as described later, the maximum equivalent circle diameter of the cementite particles after spheroidizing annealing becomes 15.0 μm or less, and the variation in the equivalent circle diameter is reduced. As a result, a finished steel bar with excellent cold forging properties can be obtained.

[0061] In one embodiment, the reason why a finished steel bar with excellent cold forging properties can be obtained by controlling the properties of the semi-finished steel bar before spheroidizing annealing is not fully understood, but it is likely due to the following reasons. First, certain cementite particles with an aspect ratio of 2.0 or less before spheroidizing annealing are thought to change and grow spherically by Ostwald growth without being divided during the subsequent spheroidizing annealing. Therefore, the length of the long axis d of the cementite particles before spheroidizing annealing LBy controlling this to 10.0 μm or less, it is thought that the equivalent circular diameter of cementite particles after spheroidizing annealing can be reduced to 15.0 μm or less.

[0062] Furthermore, as described above, Cu and Ni have the effect of suppressing the growth of cementite particles during spheroidizing annealing. The steel bar according to the present invention contains predetermined amounts of Cu and Ni. However, if there is a large variation in the Cu and Ni content within the semi-finished steel bar, the growth of cementite particles will not be suppressed in areas where the content is insufficient, and the maximum value of the equivalent circle diameter may exceed a predetermined value. In one embodiment, it is considered that the growth of cementite particles can be suppressed uniformly by reducing the variation in the Cu 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 periphery of the cross-section. In a semi-finished steel bar obtained by hot rolling a steel ingot, the microstructure in the periphery of the cross-section is generally susceptible to plastic deformation caused by the rolling rolls. In contrast, the microstructure in the center of the cross-section is less susceptible to plastic deformation. Therefore, in the 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, the length d of the long axis of the cementite grains is considered. L and the length of the minor axis d S These can vary significantly. Therefore, in a preferred embodiment, m observation fields are selected from both the central and peripheral parts of the cross-section. This allows for accurate evaluation of the properties of cementite particles across the entire cross-section of the sample.

[0064] In a more preferred embodiment, the observation field located in the peripheral area is positioned at the midpoint of the line segment connecting the observation field located in the central area and the outer periphery of the cross-section. In semi-finished steel bars obtained by hot rolling a steel ingot, the microstructure of the peripheral area of ​​the cross-section, particularly the part closer to the outer periphery, is generally strongly affected by plastic deformation caused by the rolling rolls. For this reason, it is not desirable to use an observation field selected from this area as representative of the entire sample. Therefore, in a more preferred embodiment, the position of the peripheral area is selected at the midpoint of the line segment connecting the observation field located in the central area and the outer periphery of the cross-section. This allows for the exclusion of the peripheral area of ​​the cross-section, particularly the part closer to the outer periphery, from the object of observation, and enables a more accurate evaluation of the properties of cementite particles across the entire cross-section of the sample.

[0065] In a more 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 periphery of the cross-section, as illustrated in Figure 1, and the angle between adjacent line segments is 90 degrees for the four line segments passing through the four observation fields located in the periphery of the cross-section. If the number of observation fields m for observing the properties of cementite particles is too small, the entire sample cannot be observed, and if it is too large, the measurement takes too long. Therefore, in a more preferred embodiment, observation is performed using a total of five observation fields, consisting of one observation field in the center and four in the periphery. Furthermore, the angle between adjacent line segments is set to 90 degrees for the four line segments that pass through the four observation fields located in the periphery of the cross-section, connecting the one observation field located in the center to the outer periphery of the cross-section. 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 angle between four adjacent line segments is 90 degrees, and the direction of the line segments is not particularly limited. However, in a more preferred embodiment, if it is desired to more precisely specify the position of the observation field, as illustrated in Figure 1, when the shape of the cross section perpendicular to the length direction of the steel bar is an ellipse (b), the direction of the line segments can be the direction 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 direction of the line segments can be the direction perpendicular to the side.

[0067] [Properties after spheroidizing annealing] Next, the properties of the steel bar according to the present invention after spheroidizing annealing will be described. In another embodiment, the steel bar according to the present invention is obtained by hot rolling a steel ingot and then subjected to further spheroidizing annealing. Among the equivalent circle diameters of all cementite particles observed in n observation fields (where n is an integer of 2 or more) in a cross section perpendicular to the length direction, the maximum value of the equivalent circle diameter in one observation field is d1, d2, ..., d n Let d1 to d n The maximum value up to d max , the minimum value is d min When this is done, equations (2) and (3) below are satisfied.

[0068]

number

[0069]

number

[0070] The shape of cementite particles after spheroidizing annealing can be evaluated in the same way as before spheroidizing annealing, by observing the microstructure in the cross-section of a sample of the finished steel bar cut perpendicular to the length direction after spheroidizing annealing. Specifically, the cementite phase is made visible by polishing the cross-section of the sample and then etching the polished surface with acid. A scanning electron microscope is preferably used to observe the microstructure. In observing the microstructure, the variation in the size of cementite particles across the observation field is evaluated. For this purpose, the number of observation fields n is set to an integer of 2 or more. To accurately evaluate the variation, it is preferable that the shape and size of the observation fields be constant. The shape of the observation field is not particularly limited and may be a circle, ellipse, rectangle, or square.

[0071] Based on the shape of cementite particles observed in a single observation field, the equivalent circular diameter of the cementite particles is determined. Specifically, the equivalent circular diameter of each cementite particle is calculated by processing the image data of a metallographic photograph taken in a single observation field using image processing software. In this specification, "equivalent circular diameter of a cementite particle" refers to the diameter of a circle having the same area as the cross-sectional area calculated for the observation surface of a single cementite particle. The maximum value of the calculated equivalent circular diameters in a single observation field is denoted as d1. This operation is repeated for n observation fields, and the maximum values ​​d1, d2, ..., d in each observation field are determined. n We seek.

[0072] In another embodiment, for the steel bar after spheroidizing annealing according to the present invention, d1 to d obtained by the above procedure n The maximum value up to d max , the minimum value is d min When this is the case, the maximum value d max This satisfies equation (2) above. That is, the maximum value d of the equivalent circular diameter of cementite particles observed in n observation fields. max This is equal to or less than 15.0 μm. This means that there are no cementite particles with an equivalent circle diameter larger than 15.0 μm within the n observation fields. max The lower limit is not particularly limited, but in one embodiment, d max It may be 1.0 μm or larger.

[0073] Furthermore, the maximum values ​​of the equivalent circle diameters in the n observation fields are d1, d2, ..., d n The above equation (3) is satisfied. That is, the maximum values ​​of the equivalent diameter of the circle d1, d2, ..., d n For each of these, the minimum value d min It is equal to or less than the value obtained by multiplying by 1.25. This is d1, d2, ..., d n The maximum value d max The minimum value is d min This means that the value is equal to or less than the value obtained by increasing it by 25%. In other words, d1, d2, ..., d nThe values ​​of have little variation. Note that d1, d2, ..., d n The lower limit of the value of is not particularly limited, but in one embodiment, d1, d2, ..., d n The value may be 1.00 μm or larger.

[0074] In other embodiments, the reason why a finished steel bar with excellent cold forgeability can be obtained by limiting the size and variation of the equivalent circle diameter of cementite particles in n observation fields after spheroidizing annealing to a predetermined range is not fully understood, but it is likely due to the following reasons: Cementite has higher hardness than ferrite. Therefore, the form of cementite in the finished steel bar affects its cold forgeability. By spheroidizing annealing the semi-finished steel bar, the cementite contained in ferrite and pearlite disperses within the matrix in the form of spherical particles. As a result, the presence of cementite becomes less likely to hinder plastic deformation during cold forging.

[0075] However, in conventional technology, some cementite particles had an equivalent diameter exceeding 15.0 μm. Furthermore, there was considerable variation in the equivalent diameter depending on the observation location. Because cementite particles with large equivalent diameters exist in a portion of the cross-section of the steel bar, that portion hinders plastic deformation during cold forging, resulting in reduced cold forgeability. In the present invention, the size and variation of the equivalent diameter of the cementite particles after spheroidizing annealing are controlled within a predetermined range, so it is expected that a finished steel bar with excellent cold forgeability can be obtained.

[0076] A preferred embodiment for determining n observation fields for a finished steel bar after spheroidizing annealing is the same as a preferred embodiment for determining m observation fields for a semi-finished steel bar before spheroidizing annealing, so a description is omitted here.

[0077] [Evaluation of cold forging properties] The effects of the present invention can be evaluated by the cold forgeability of the finished steel bar. For example, if the cross-sectional shape of the steel bar 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 made on the side surface of this cylinder, and a grooved cylindrical test piece 4 having the shape shown in Figure 2 is prepared.

[0078] Next, this test piece 4 is placed in the compression testing machine, and the strain rate is set to 10s. -1 The specimen is compressed in the height direction under conditions that constrain the compression surface 4b. First, it is reduced by 0.3 mm, and the bottom of the V-shaped groove 4a on the side of the specimen is visually inspected to see if a crack has formed. If a crack has formed, its length is measured. Next, the specimen is reduced by 0.3 mm increments until the length of the crack is 0.5 mm or more. This compression test is performed on 6 specimens, and the cumulative reduction rate at which cracks of 0.5 mm or more are confirmed in 3 of the 6 specimens is defined as the limit upsetting rate. In this specification, "cumulative reduction rate" refers to the percentage of the value obtained by dividing the final height of the specimen by its original height of 21 mm. Under the above test conditions, specimens with a limit upsetting rate of 55% or more are said to have excellent cold forging properties. [Examples]

[0079] The following describes embodiments of the present invention. However, the embodiments of the present invention are not limited to the following embodiments, and the embodiments of the present invention can be modified as appropriate without departing from the spirit of the invention.

[0080] [Example of invention] Molten steel with the component compositions of steels No. 1 to 35 shown in Table 1 was prepared using an electric furnace, and steel ingots were cast by continuous casting. The molten steel temperature during pouring was 1450°C, and the casting speed was 1.0 m / min. After heating the steel ingots to 1100°C using a heating furnace, they were hot-rolled into 15 mm diameter round bars using rolling mills. The finishing temperature of the hot rolling was 800°C. Samples were taken from the semi-finished hot-rolled steel bars (round bars) No. 1 to 35, and after polishing the cross-section perpendicular to the longitudinal direction, the observation surface was etched using a 3% aqueous solution of picric acid.

[0081] Next, in the five observation fields shown in Figure 1(a), the length of the long axis d of cementite particles with an aspect ratio of 2.0 or less was determined using a scanning electron microscope and image processing software (ImageJ Ver.1.53c). L The maximum value of was determined. The scanning electron microscope magnification was 5000x, and the smallest diameter of cementite particles observable at this magnification was 60 nm. The size of the observation field at this time was 24 μm horizontally and 18 μm vertically. In addition, the Cu and Ni content was measured using an electron probe microanalyzer in the same observation field. The beam diameter of the electron beam at this time 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 composition according to the present invention, the length of the long axis d of a specific cementite particle with an aspect ratio of 2.0 or less L However, the size was 10.0 μm or less in all five observation fields, and the Cu and Ni content measured in the five observation fields satisfied equation (1).

[0082] [Comparative Example] On the other hand, for samples No. 36 to 47 and No. 50 and 51 in Table 2, semi-finished bars (round bars) were produced from molten steel having component compositions outside the scope of the present invention, as shown in steels No. 36 to 47 in Table 1. Furthermore, for sample No. 48 in Table 2, semi-finished bars (round bars) were produced from molten steel having the component composition of steel No. 11 in Table 1, but the casting conditions differed from those above, with a molten steel temperature of 1530°C and a casting speed of 3.5 m / min during pouring. For sample No. 49 in Table 2, semi-finished bars (round bars) were produced from molten steel having the component composition of steel No. 6 in Table 1, but the hot rolling conditions differed from those above, with a heating temperature of 1270°C and a finishing temperature of 740°C. As a result, in sample No. 48, the Cu and Ni content did not satisfy equation (1), and in sample No. 49, the d of specific cementite particles with an aspect ratio of 2.0 or less L The maximum value exceeded 10.0 μm.

[0083] [Table 1]

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

[0085] Next, the equivalent circular diameter of cementite particles was determined using a scanning electron microscope and image processing software (ImageJ Ver.1.53c) in the five observation fields shown in Figure 1(a), and the maximum values ​​d1, d2, d3, d4, and d5 were obtained. Furthermore, the maximum value d was selected from among these maximum values. max and minimum value d min We determined the value and also checked whether equation (3) was satisfied. The evaluation results obtained are shown in Table 2.

[0086] Next, a test specimen 4, shown in Figure 2, was prepared from the finished steel bar (round bar) after spheroidizing annealing, and the limit upset ratio was measured using the method described above. The obtained measurement results are shown in Table 2. The maximum value d max The relationship between this and the maximum load factor is shown in Figure 3.

[0087] [Table 2]

[0088] According to Tables 1 and 2, the component composition of the steel bar according to the present invention is satisfied, and the length of the long axis of a specific cementite particle with an aspect ratio of 2.0 or less before spheroidizing annealing is d L For samples No. 1 to 35, which underwent normal spheroidizing annealing for samples where the maximum value was 10.0 μm or less and the Cu and Ni content satisfied equation (1), it can be seen that the equivalent circle diameter of the cementite particles after spheroidizing annealing satisfied the conditions of equations (2) and (3), and the limiting upsetting rate was greater than 55% in all cases.

[0089] On the other hand, in samples No. 36 to 51, where the component composition of the steel bar did not satisfy the component composition according to the present invention, or where the casting or hot rolling conditions differed from the above conditions, the equivalent circular diameter of the cementite particles after spheroidizing annealing did not satisfy at least one of the conditions of equation (2) or (3). As a result, the limit upsetting rate for all of these samples was less than 55%.

[0090] As shown in Figure 3, the examples of the invention, indicated by white circles, which are obtained by spheroidizing the semi-finished steel bars according to the present invention and satisfy the conditions for the finished steel bars according to the present invention, all had a limit upsetting ratio greater than 55%. On the other hand, the comparative examples, indicated by black circles and black triangles, which did not satisfy the conditions for the finished steel bars according to the present invention, all showed a limit upsetting ratio less than 55%.

[0091] These results show that by controlling the properties of the semi-finished steel bar before spheroidizing annealing, the equivalent circular diameter of the cementite particles in the finished steel bar after spheroidizing annealing can be made to satisfy the conditions of equations (2) and (3), thereby obtaining steel bars with excellent cold forgeability. At the same time, these results show that when the equivalent circular diameter of the cementite particles in the finished steel bar after spheroidizing annealing satisfies both the conditions of equations (2) and (3), steel bars with excellent cold forgeability can be obtained. [Explanation of symbols]

[0092] Cross-section of a single steel bar 2. Field of View 3 line segments 4 Test specimens 4a V-shaped groove 4b Compressed surface

Claims

1. In terms of 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% It contains, with the remainder being Fe and unavoidable impurities, and has a component composition. When the shape 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 length direction is approximated as an ellipse, the length of the major axis is d. Ｌ , the length of the minor axis is d Ｓ Let the length of the major axis be d Ｌ The length of the short axis d Ｓ The length of the long axis d of a specific cementite particle whose aspect ratio obtained by dividing by is 2.0 or less Ｌ However, in all of the aforementioned m observation fields, the size is 10.0 μm or less. Let the maximum value among the mass percentages of the Cu content measured in the m observation fields be [Cu] ｍａｘ , and the minimum value be [Cu] ｍｉｎ . Also, let the maximum value among the mass percentages of the Ni content measured in the m observation fields be [Ni] ｍａｘ , and the minimum value be [Ni] ｍｉｎ . When this is the case, the following formula (1) is satisfied, 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 part of the cross-section, wherein the observation field located in the peripheral part is located at the midpoint of the line segment connecting the observation field located in the center and the outer periphery of the cross-section. A steel bar characterized by the following features. [Math 1]

2. The m observation fields consist of one observation field located in the center of the cross-section and four observation fields located in the periphery of the cross-section, and for the four line segments passing through the four observation fields located in the periphery of the cross-section, the angle between adjacent line segments is 90 degrees. The steel bar according to claim 1.

3. The aforementioned component composition is further expressed by mass percentage as follows: 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 It contains one or more selected from the group consisting of, The steel bar according to claim 1 or 2.

4. The shape of the cross-section is one of a circle, an ellipse, a square, or a rectangle. The steel bar according to claim 1 or 2.

5. The shape of the cross-section is one of a circle, an ellipse, a square, or a rectangle. The steel bar according to claim 3.

6. In terms of mass percentage, C: 0.12 to 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% It contains, with the remainder being Fe and unavoidable impurities, and has a component composition. Of the equivalent circular diameters of all cementite particles observed in n observation fields (where n is an integer of 2 or more) in a cross-section perpendicular to the length direction, the maximum value of the equivalent circular diameter in one observation field is d. １ d ２ , , d ｎ to, d １ from d ｎ The maximum value up to d ｍａｘ , the minimum value is d ｍｉｎ When this is the case, equations (2) and (3) below are satisfied, 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 peripheral part of the cross-section, wherein the observation field located in the peripheral part is located at the midpoint of the line segment connecting the observation field located in the center and the outer periphery of the cross-section. A steel bar characterized by the following features. [Math 2] [Math 3]

7. The n observation fields consist of one observation field located in the center of the cross-section and four observation fields located in the periphery of the cross-section, and for the four line segments passing through the four observation fields located in the periphery of the cross-section, the angle between adjacent line segments is 90 degrees. The steel strip according to claim 6.

8. The aforementioned component composition is further expressed by mass percentage as follows: 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 It contains one or more selected from the group consisting of, The steel bar according to claim 6 or 7.

9. The shape of the cross-section is one of a circle, an ellipse, a square, or a rectangle. The steel bar according to claim 6 or 7.

10. The shape of the cross-section is one of a circle, an ellipse, a square, or a rectangle. The steel bar according to claim 8.

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