Steel bar

Abstract

Provided is a steel bar having excellent cold forgeability after spheroidizing annealing. One embodiment of the present invention relates to a steel bar containing prescribed amounts of Cr, Ni, and Cu, the length of the major axis of a specific cementite particle, which has an aspect ratio of 2.0 or less when the shape of the cementite particle observed in m observation fields (where m is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction is approximately elliptical, is 10.0 [mu] m or less in all the m observation fields. And the variation between the Cu content and the Ni content measured in the m observation fields is small. In other embodiments, among equivalent circle diameters of cementite particles observed in n observation fields (where n is an integer of 2 or more) in a cross-section perpendicular to the longitudinal direction, the maximum value of the equivalent circle diameter in one observation field is 15.0 [mu] m or less, and the variation of the maximum value is small.

Description

TECHNICAL FIELD

[0001] The present application relates to a bar steel excellent in cold forging property. BACKGROUND

[0002] In the fields of construction machines, industrial machines, and automobiles, etc., a bar steel having a circular or quadrangular cross section is widely used to be cut and processed into a mechanical structural member in the shape of a bolt, nut, screw, etc. by forging. The mechanical strength required for these mechanical structural members is ensured by appropriately conditioning a heat treatment such as quenching treatment performed after the processing. In addition, by using a bar steel to which an alloy element is added in advance, even in the case where the conditions of heat treatment are more or less varied, a high mechanical strength can be stably achieved.

[0003] In recent years, in the production of mechanical structural members, from the viewpoint of production efficiency, etc., the application of cold forging is expanding instead of the hot forging which has been used conventionally. In the case where a steel material containing a large amount of alloy elements is subjected to cold forging, the deformation resistance becomes higher compared with a steel material having a small amount of alloy elements. Therefore, there is a problem that the life of a die for cold forging is reduced or that a product is easily cracked. In order to reduce the deformation resistance and improve the cold forging property, it is effective to spheroidize the shape of cementite contained in the metal structure of a steel material before cold forging. As to the spheroidizing technique of cementite, various proposals have been made.

[0004] For example, in Patent Literature 1, an invention of a low-alloy steel material and a method for producing the same, in which the shape of cementite contained in a hot-rolled steel material is spheroidized by adjusting the conditions of hot-rolling and the cooling conditions after hot-rolling, thereby improving the cold forging property, is described. In Patent Literature 2, an invention of a steel wire and a method for producing the same, in which the number of spheroidized cementite per unit volume below a specific average particle diameter is adjusted, thereby improving the cold workability, is described. In Patent Literature 3, an invention of a cold forging steel, in which the average value and the standard deviation of the distance between carbides dispersed in ferrite grains are controlled by adding a large amount of Cr, thereby improving the cold workability, is described.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Laid-Open No. 2004-100038

[0008] Patent Literature 2: Japanese Patent Application Laid-Open No. 2009-275250

[0009] Patent Literature 3: Japanese Patent Application Laid-Open No. 2023-21615 SUMMARY

[0010] The technology described in Patent Literature 1 is characterized in that cold workability is improved without performing spheroidizing annealing, and the cold workability itself is not significantly improved compared to the case where spheroidizing annealing is performed as in the past. With the technologies described in Patent Literature 2 and Patent Literature 3, although the cold workability is improved compared to spheroidizing annealing of the past technology, since the average value is used as a representative value indicating the particle diameter or inter-particle distance of the cementite after spheroidizing, there is a problem in that it cannot be asserted that the shape of the cementite is certainly optimized.

[0011] The present application was developed in view of the above-described problems, and proposes a bar steel having excellent cold workability after spheroidizing annealing is performed.

[0012] In order to achieve improvement in cold workability, the present inventors investigated the relationship between the size of the cementite particles observed in the cross section of the bar steel after spheroidizing annealing is performed and the cold workability. As a result, it was found that excellent cold workability is obtained in a bar steel in which the equivalent circle diameter of the cementite particles measured in a plurality of observation fields of the cross section is 15.0 μm or less and the deviation of the equivalent circle diameter is small.

[0013] Next, in order to satisfy the above-described condition for the size of the cementite particles after spheroidizing annealing is performed, the present inventors investigated in detail what kind of properties the bar steel before spheroidizing annealing needs to have. As a result, it was found that if spheroidizing annealing is performed under normal annealing conditions on a bar steel containing 0.02 to 0.30% of Cu and 0.02 to 0.25% of Ni in terms of mass percentage, the deviation of the contents of Cu and Ni measured in a plurality of observation fields of the cross section of the bar steel after hot rolling before spheroidizing annealing is small, and when the cementite particles observed in a plurality of observation fields of the above-described cross section are approximated as ellipses, for the cementite particles having an aspect ratio of 2.0 or less, the maximum value of the major axis is 10.0 μm or less, the size of the cementite particles after spheroidizing annealing is performed satisfies the above-described condition.

[0014] The present application was completed based on the above-described insight, and the gist thereof constitutes as follows.

[0015] [1] A bar steel characterized by having a composition consisting of, in terms of mass percentage,

[0016] C: 0.12 to 0.44%,

[0017] Si: 0.15 to 0.35%,

[0018] Mn: 0.30 to 0.95%,

[0019] P: 0.001 to 0.030%,

[0020] S: 0.001 to 0.030%,

[0021] Cr: 0.85 to 1.50%,

[0022] Cu: 0.02 to 0.30%,

[0023] Ni: 0.02 to 0.25%, and

[0024] N: 0.0020 to 0.0250%,

[0025] the remainder consisting of Fe and unavoidable impurities,

[0026] the length of the long axis of the shape of all cementite particles observed in m observation fields of a cross section perpendicular to the length direction (where m is an integer of 2 or more) is set as d L the length of the short axis is set as d S the length d of the long axis is set as d L the length d of the short axis is set as d S the aspect ratio of the specific cementite particle obtained is 2.0 or less, the length d of the long axis is set as d L is 10.0 μm or less in all of the m observation fields,

[0027] the maximum value of the mass percentage of Cu content measured in the m observation fields is set as [Cu] max , and the minimum value is set as [Cu] min the maximum value of the mass percentage of Ni content measured in the m observation fields is set as [Ni] max , and the minimum value is set as [Ni] min the following formula (1) is satisfied.

[0028] [Mathematical Formula 1]

[0029]

[0030] [2] The bar steel according to the above [1], wherein

[0031] the m observation fields include one observation field located at a central portion of the cross section and one or more observation fields located at a peripheral portion of the cross section.

[0032] [3] The bar steel according to the above [2], wherein

[0033] the observation field located at the peripheral portion is at a midpoint of a line segment connecting the observation field located at the central portion and an outer periphery of the cross section.

[0034] [4] The bar steel according to the above [3], wherein

[0035] The m observation fields of view are composed of one observation field of view at the center of the section and four observation fields of view at the periphery of the section, and the angle between adjacent line segments of the four line segments of the four observation fields of view at the periphery of the section is 90 degrees.

[0036] [5] The steel strip according to any one of the above [1] to [4], wherein

[0037] the component composition further contains, in mass percent, one or two or more selected from

[0038] Mo: 0.30% or less,

[0039] Al: 0.100% or less,

[0040] Ti: 0.100% or less,

[0041] V: 0.300% or less,

[0042] Nb: 0.100% or less,

[0043] B: 0.0100% or less, and

[0044] Sn: 0.100% or less

[0045]

[0046] [6] The steel strip according to any one of the above [1] to [5], wherein the shape of the section is any one of a circle, an ellipse, a square, and a rectangle.

[0047] [7] A steel strip characterized by having a component composition containing, in mass percent,

[0048] C: 0.12 to 0.44%,

[0049] Si: 0.15 to 0.35%,

[0050] Mn: 0.30 to 0.95%,

[0051] P: 0.001 to 0.030%,

[0052] S: 0.001 to 0.030%,

[0053] Cr: 0.85 to 1.50%,

[0054] Cu: 0.02 to 0.30%,

[0055] Ni: 0.02 to 0.25%, and

[0056] ​N: 0.0020 to 0.0250%,

[0057] the remainder consisting of Fe and inevitable impurities,

[0058] of all the cementite particles observed in n observation fields of view (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 of view is set as d1, d2, , d n , the maximum value from d1 to d n is set as d max , and the minimum value is set as d min , the following formulas (2) and (3) are satisfied.

[0059] [Mathematical Formula 2]

[0060]

[0061] [Mathematical Formula 3]

[0062]

[0063] [8] The bar steel according to the above [7], wherein

[0064] The n observation fields of view include one observation field of view located in a central portion of the cross section and one or more observation fields of view located in a peripheral portion of the cross section.

[0065] [9] The bar steel according to the above [8], wherein

[0066] The observation field of view located in the peripheral portion is at a midpoint of a line segment connecting the observation field of view located in the central portion and an outer periphery of the cross section.

[0067]

[10] The bar steel according to the above [9], wherein

[0068] The n observation fields of view include one observation field of view located in a central portion of the cross section and four observation fields of view located in a peripheral portion of the cross section, and with respect to four line segments of the observation fields of view located in the peripheral portion of the cross section, the angle formed by adjacent line segments is 90 degrees.

[0069]

[11] The bar steel according to any one of the above [7] to

[10] , wherein the component composition further contains, in mass percentage, one or more selected from

[0070] Mo: 0.30% or less,

[0071] Al: 0.100% or less,

[0072] Ti: 0.100% or less,

[0073] V: 0.300% or less,

[0074] Nb: 0.100% or less,

[0075] B: 0.0100% or less, and

[0076] Sn: 0.100% or less

[0077] one or two or more of them.

[0078]

[12] The bar steel according to any one of the above-mentioned [7] to

[11] , wherein

[0079] the shape of the above-mentioned cross section is any one of a circle, an ellipse, a square, and a rectangle.

[0080] According to the present application, for the bar steel after spheroidizing annealing, excellent cold workability can be obtained. Thus, damage to a die in cold forging can be reduced, and productivity can be improved. In addition, according to the present application, the preferable metal structure of the bar steel before spheroidizing annealing becomes clear, and the production yield of the steel material is improved. BRIEF DESCRIPTION OF DRAWINGS

[0081] Figure 1 is a diagram showing the position of an observation field of view which indicates the appearance observed in the cross section of the bar steel whose cross section shape is (a) a circle, (b) an ellipse, (c) a square, or (d) a rectangle.

[0082] Figure 2 is a diagram showing the shape of a test piece for measuring the limit upset ratio in the example. (a) is a plan view, (b) is a cross-sectional view as viewed from the side, and (c) is a cross-sectional view in which a notch portion is enlarged.

[0083] Figure 3 is a graph showing the relationship between the maximum value of the equivalent circle diameter of cementite particles and the limit upset ratio in the example. DETAILED DESCRIPTION

[0084] Hereinafter, a mode for carrying out the present application will be described in detail.

[0085] [Component Composition]

[0086] In one embodiment, the steel strip of the present application has a composition consisting of, in mass percent, 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 remainder consisting of Fe and unavoidable impurities. In the present specification, the marks of the composition are all expressed in mass percent unless otherwise specified.

[0087] The composition of the above steel strip corresponds to that of an alloy steel called chromium steel in which approximately 1% of Cr is added to carbon steel. More specifically, it is similar to the composition of the material quality symbols SCr415 to SCr440 prescribed in Japanese Industrial Standards JIS G 4053:2008 "Alloy steels for mechanical structures". However, it should be noted that the types of elements contained in the steel strip of the present application and the ranges of the composition of each element are not completely the same as the specifications.

[0088] [C: 0.12 to 0.44%]

[0089] C is added in order to ensure strength as a mechanical member. If the content of C is 0.12% or more, the strength required as a mechanical structural member can be ensured. In addition, if the content of C is 0.44% or less, no negative influence on cold workability is caused. Therefore, the content of C is 0.12 to 0.44%. The lower limit value of the content of C is preferably 0.22%, more preferably 0.32%. The upper limit value of the content of C is preferably 0.42%, more preferably 0.39%.

[0090] [Si: 0.15 to 0.35%]

[0091] Si is an element required for deoxidation in the dissolving process, and is also an element effective for imparting the required strength to the steel by solid solution strengthening and improving hardenability. If the content of Si is 0.15% or more, the above effects can be sufficiently obtained. In addition, if the content of Si is 0.35% or less, no negative influence on cold workability is caused. Therefore, the content of Si is 0.15 to 0.35%. The lower limit value of the content of Si is preferably 0.17%, more preferably 0.19%. The upper limit value of the content of Si is preferably 0.33%, more preferably 0.31%.

[0092] [Mn: 0.30 to 0.95%]

[0093] Mn is an element required for deoxidation in the dissolving process, and is an element effective for imparting a required strength to the steel by improving the hardenability. If the content of Mn is 0.30% or more, the above-mentioned effects can be sufficiently obtained. Further, if the content of Mn is 0.95% or less, the cold workability is not adversely affected. Therefore, the content of Mn is 0.30 to 0.95%. The lower limit value of the content of Mn is preferably 0.45%, more preferably 0.60%.

[0094] [P: 0.001 to 0.030%]

[0095] P is an element effective for increasing the strength of the steel. If the content of P is 0.001% or more, the above-mentioned effects can be sufficiently obtained. Further, if the content of P is 0.030% or less, the toughness of the steel is not decreased because P is segregated at the grain boundaries. Therefore, the content of P is 0.001 to 0.030%.

[0096] [S: 0.001 to 0.030%]

[0097] S is an element effective for improving the machinability of the steel by forming MnS by combining with Mn in the steel. If the content of S is 0.001% or more, the above-mentioned effects can be sufficiently obtained. Further, if the content of S is 0.030% or less, MnS which is a starting point of cracks at the time of cold forging is not formed in a large amount, and therefore the cold workability is not adversely affected. Therefore, the content of S is 0.001 to 0.030%.

[0098] [Cr: 0.85 to 1.50%]

[0099] Cr is an element effective for imparting a required strength to the steel by solid solution strengthening and improving the hardenability. If the content of Cr is 0.85% or more, the above-mentioned effects can be sufficiently obtained. Further, if the content of Cr is 1.50% or less, the hardness of the steel is increased, and the cold workability is not adversely affected. Therefore, the content of Cr is 0.85 to 1.50%. The upper limit value of the content of Cr is preferably 1.35%, more preferably 1.20%.

[0100] [Cu: 0.02 to 0.30%]

[0101] Cu is an element that effectively functions in controlling the size of cementite particles. That is, Cu suppresses the growth of cementite particles in spheroidizing annealing. If the content of Cu is 0.02% or more, the above-mentioned effect can be sufficiently obtained. In addition, if the content of Cu is 0.30% or less, surface defects are not easily generated in the production of steel. Therefore, the content of Cu is 0.02 to 0.30%. The lower limit value of the content of Cu is preferably 0.04%, more preferably 0.06%. The upper limit value of the content of Cu is preferably 0.25%, more preferably 0.20%.

[0102] [Ni: 0.02 to 0.25%]

[0103] Ni is an element that effectively functions in controlling the size of cementite particles, like Cu. That is, Ni suppresses the growth of cementite particles in spheroidizing annealing. If the content of Ni is 0.02% or more, the above-mentioned effect can be sufficiently obtained. In addition, if the content of Ni is 0.25% or less, surface defects are not easily generated in the production of steel. Therefore, the content of Ni is 0.02 to 0.25%. The lower limit value of the content of Ni is preferably 0.04%, more preferably 0.06%. The upper limit value of the content of Ni is preferably 0.20%, more preferably 0.15%.

[0104] [N: 0.0020 to 0.0250%]

[0105] N combines with a nitride-forming element in steel to form a nitride. The formed nitride functions as a pinning particle that hinders the movement of the grain boundaries of austenite and ferrite. Thereby, the coarsening of the ferrite grains in the bar steel is prevented, and the strength is improved. If the content of N is 0.0020% or more, the above-mentioned effect can be sufficiently obtained. In addition, if the content of N is 0.0250% or less, cracks at the time of cold working due to dynamic strain aging of solid-solution nitrogen in steel can be prevented in advance. Therefore, the content of N is 0.0020 to 0.0250%.

[0106] [Fe and inevitable impurities]

[0107] In one embodiment, the bar steel of the present application has a composition consisting of the above-mentioned elements, the remainder consisting of Fe and inevitable impurities. Fe is the main component of the bar steel of the present application. In the present specification, "inevitable impurities" means substances that are generally present in raw materials in metal products or inevitably mixed in the manufacturing process, and are substances that are not originally required, but can be allowed impurities since they are in a trace amount and do not cause an influence on the characteristics of the metal product.

[0108] Among the elements equivalent to unavoidable impurities, for example, 0 (oxygen), Ca, Bi, Sb, and the like can be given. Even if the bar steel of the present application contains 0.0100% or less of 0 (oxygen), 0.01% or less of Ca, 0.01% or less of Bi, and 0.03% or less of Sb in terms of mass%, the contents are all in trace amounts, and thus do not affect the cold workability. In the present application, the bar steel is allowed to contain trace amounts of elements other than 0 (oxygen), Ca, Bi, and Sb exemplified here, within a range that does not affect the effects of the present application.

[0109] In the preferred embodiment, the bar steel of the present application further contains, in terms of mass%, one or two or more 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.

[0110] [Mo: 0.30% or less]

[0111] Mo can significantly improve the hardenability of the steel material with a small amount of addition, and is an element effective for improving the strength of the steel. If the content of Mo is 0.30% or less, it is possible to prevent in advance the excess of the hardenability and the decrease in the cold workability. Therefore, the content of Mo in the preferred embodiment is 0.30% or less. If the content of Mo is 0.15% or more, the above-mentioned effects can be sufficiently obtained.

[0112] [Al: 0.100% or less]

[0113] Al is an element that promotes deoxidation in the dissolving process, and is also an element effective for improving the strength of the bar steel by forming nitrides by combining with N in the steel, thereby promoting the refinement of the grains of ferrite. If the content of Al is 0.100% or less, it is possible to prevent in advance the easy occurrence of cracks at the time of cold working due to the generation of a large amount of Al oxides in the steel. Therefore, the content of Al in the preferred embodiment is 0.100% or less. If the content of Al is 0.001% or more, the above-mentioned effects can be sufficiently obtained.

[0114] [Ti: 0.100% or less]

[0115] Ti is also an element effective for the refinement of the grains of ferrite by forming nitrides by combining with N in the steel, like Al. If the content of Ti is 0.100% or less, it is possible to prevent in advance the easy occurrence of cracks at the time of cold working due to the generation of a large amount of Ti-based inclusions in the steel. Therefore, the content of Ti in the preferred embodiment is 0.100% or less. If the content of Ti is 0.001% or more, the above-mentioned effects can be sufficiently obtained.

[0116] [V: 0.300% or less]

[0117] V is an element effective for the refinement of the grain of ferrite by forming a nitride by combining with N in the steel. If the content of V is 0.300% or less, it is possible to prevent in advance the easy generation of cracks at cold forging due to the precipitation of a large amount of V-based precipitates. Therefore, the content of V in the preferred embodiment is 0.300% or less. If the content of V is 0.001% or more, the above-mentioned effects can be sufficiently obtained.

[0118] [Nb: 0.100% or less]

[0119] Nb is an element effective for the refinement of the grain of ferrite by forming a carbide by combining with C in the steel. If the content of Nb is 0.100% or less, it is possible to prevent in advance the easy generation of cracks at cold forging due to the generation of a large amount of Nb-based carbides. Therefore, the content of Nb in the preferred embodiment is 0.100% or less. If the content of Nb is 0.001% or more, the above-mentioned effects can be sufficiently obtained.

[0120] [B: 0.0100% or less]

[0121] B can remarkably improve the hardenability of the steel material with a small amount of addition and is an element effective for the improvement of the strength of the steel. If the content of B is 0.0100% or less, it is possible to prevent unnecessary addition due to the saturation of the effect of the addition. Therefore, the content of B in the preferred embodiment is 0.0100% or less. If the content of B is 0.0005% or more, the above-mentioned effects can be sufficiently obtained.

[0122] [Sn: 0.100% or less]

[0123] Sn can moderately embrittle ferrite and is an element effective for the improvement of machinability. If the content of Sn is 0.100% or less, it is possible to prevent in advance the decrease in cold workability due to excessive embrittlement. Therefore, the content of Sn in the preferred embodiment is 0.100% or less. If the content of Sn is 0.001% or more, the above-mentioned effects can be sufficiently obtained.

[0124] [Bar Steel]

[0125] The object of the present application is bar steel. In the present specification, "bar steel" means a rolled steel material having an uneven shape, and includes a rod steel and a wire. The shape of the cross section of the bar steel of the present application perpendicular to the length direction is not particularly limited and can be an arbitrary shape. In the preferred embodiment, as shown in Figure 1The shape of the cross section of the steel bar of the present application is exemplified as any one of a circle (a), an ellipse (b), a square (c), or a rectangle (d). These shapes of the cross section are high in symmetry, and thus are preferable in terms of easiness of homogenization of the metal structure as described later. In addition to these, the shape of the cross section of the steel bar can also be a hexagon, or a circle with a protrusion as in the case of deformed steel bars.

[0126] [CASTING]

[0127] Next, the manufacturing method of the steel bar of the present application will be described. In the usual manufacturing process of the steel bar, three processes of casting, hot rolling, and spheroidizing annealing are sequentially performed. The casting is a process of obtaining an ingot by casting molten steel having a prescribed composition and cooling. In the casting, first, molten steel having a prescribed composition is prepared. The composition of the molten steel is adjusted so that the composition of the steel bar becomes the composition described in the above. As for the manufacturing of the molten steel, a batch-type electric furnace or a continuous blast furnace can be used. The electric furnace is easy to adjust the composition, and thus is suitable for the manufacturing of small quantities of various steel materials. However, the preparation of the molten steel in the present application is not limited to the method using the electric furnace.

[0128] Next, the prepared molten steel is cast into a mold to perform the casting, and an ingot is obtained. The temperature of the molten steel at the time of casting is preferably a temperature of 100°C or less higher than the melting point of the molten steel of this composition and above the melting point. The casting of the molten steel into the mold can be performed by continuous casting, or can be performed using a batch-type mold. The casting speed in the case of continuous casting, that is, the speed at which the ingot descends after the cooling of the mold is preferably 0.1 m / minute to 3.0 m / minute. By satisfying these conditions of the casting, Cu and Ni contained in the molten steel can be dispersed without bias in the ingot.

[0129] [ROLLING]

[0130] The hot rolling is a process of forming the shape of the ingot produced in the casting process into a prescribed cross-sectional shape of the steel bar by heating the ingot and performing rolling at a high temperature. As for the heating of the ingot, a heating furnace can be used. The temperature of the heated ingot is preferably 1000°C to 1250°C. In addition, a rolling roll can be used in the rolling. It is preferable that the final temperature of the hot rolling be 750°C or higher, and then cooling be performed. By satisfying these conditions of the hot rolling, the shape of the cementite particles as described later can be controlled. In the present specification, the product before the spheroidizing annealing of the steel bar formed into a prescribed cross-sectional shape by the hot rolling is sometimes referred to as a "semifinished product of the steel bar".

[0131] [Spheroidizing Annealing]

[0132] Spheroidizing annealing is a process of annealing a semi-finished bar steel product obtained from a hot rolling process to spheroidize the cementite particles contained in the bar steel's microstructure. 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 to 700°C. Furthermore, the temperature holding time during annealing is preferably 12 to 20 hours. If the desired microstructure before spheroidizing annealing can be obtained for the semi-finished bar steel product by satisfying the above-mentioned casting and hot rolling conditions, then bar steel with excellent cold forging properties can be obtained by performing spheroidizing annealing under known conditions. It should be noted that the composition of the bar steel remains almost unchanged before and after spheroidizing annealing. In this specification, the product obtained by performing spheroidizing annealing on the semi-finished bar steel product is sometimes referred to as a "finished bar steel product."

[0133] [Properties before spheroidizing annealing]

[0134] Next, the properties of the bar steel of the present invention before spheroidizing annealing will be described. In one embodiment, for the bar steel of the present invention obtained by hot rolling of steel ingots before spheroidizing annealing, the length of the major axis is set as d when the shape of all cementite particles observed in m observation fields (where m is an integer greater than or equal to 2) of a section perpendicular to the length direction is approximately elliptical. L Let the length of the minor axis be d. S And the length d of the aforementioned major axis L Divide by the length d of the minor axis mentioned above S The length d of the aforementioned major axis of the specific cementite particles with an aspect ratio of 2.0 or less is obtained. L For all m observation fields mentioned above, the maximum value among the mass percentages of Cu content measured in these m observation fields that are below 10.0 μm will be set as [Cu]. max Set the minimum value to [Cu]. min The maximum value among the mass percentages of Ni content measured in the above m observation fields is set as [Ni]. max Set the minimum value to [Ni]. min When, the following equation (1) is satisfied.

[0135] [Mathematical Formula 4]

[0136]

[0137] Evaluation of the shape of the cementite particles before spheroidizing annealing can be performed by observing the metal structure in a cross section of a sample cut perpendicularly to the length direction of a semi-finished product of a bar steel obtained by hot-rolling a steel ingot. Specifically, after polishing the cross section of the sample, the polished surface is etched with acid, whereby the cementite phase can be visually recognized. In the observation of the metal structure, it is preferable to use a scanning electron microscope. In the observation of the metal structure, the length of the long axis of the cementite particles based on the observation field is evaluated. For this purpose, the number m of observation fields is an integer of 2 or more. In order to make the conditions of observation uniform, it is preferable that the shape and size of the observation field be constant. The shape of the observation field is not particularly limited and can be a circle, an ellipse, a rectangle, or a square.

[0138] According to the shape of the cementite particles observed in one observation field, the length of the long axis when the shape of the cementite particles is approximated to an ellipse is set as d L , and the length of the short axis is set as d S . Specifically, the image data of the metal structure photograph in which one observation field is captured is processed using image processing software, whereby the length d L of the long axis and the length d S of the short axis are calculated for each cementite particle. In this specification, "approximated to an ellipse" means that the shape of an imaginary ellipse in which the area, the direction of the major and minor axes, and the position of the center are the same as those of the outline of the cementite particle of interest is found. For example, in the case where ImageJ, which is open source software, is used as the image processing software, d L and d S may be calculated by specifying Fit Ellipse as the value to be measured in the instruction called Analyze Particles for the image data of the particle of interest.

[0139] Next, a specific cementite particle having an aspect ratio of d L divided by d S of 2.0 or less is selected. This operation is performed with the aim of removing the cementite in the pearlite remaining in the semi-finished product of the bar steel before spheroidizing annealing from the evaluation target. The cementite in the pearlite forms a lamellar structure together with ferrite, and thus the aspect ratio thereof is usually more than 2.0. In one embodiment, the d L of the specific cementite particle having an aspect ratio of 2.0 or less is 10.0 μm or less in all m observation fields. This means that there is no cementite particle having a size of the length d L of the long axis exceeding 10.0 μm and an aspect ratio of 2.0 or less in all m observation fields. The selection of the specific cementite particle having an aspect ratio of 2.0 or less and the length dL The maximum value of the maximum value of the length of the long axis of the cementite particle in the m observation fields can be confirmed using the image processing software described above. Note that the maximum value of the length of the long axis of the cementite particle in the m observation fields can be confirmed by using the image processing software described above. L The lower limit of the maximum value of the length of the long axis of the cementite particle in the m observation fields is not particularly limited, but in one embodiment, the maximum value can be 1.0 μm or more.

[0140] Next, in the m observation fields described above, the mass percentage of the Cu content and the mass percentage of the Ni content are each measured. For the measurement of the contents of Cu and Ni, it is preferable to use an electron probe microanalyzer. The measurement is sufficient if it is performed at one position in one observation field. The maximum value of the mass percentage of the Cu content measured in the m observation fields is set as [Cu]max, and the minimum value is set as [Cu]min. The maximum value of the mass percentage of the Ni content measured in the m observation fields is set as [Ni]max, and the minimum value is set as [Ni]min. max min max min When these values satisfy the above-described formula (1), the sum of [Cu]maxand [Ni]maxis divided by the sum of [Cu]minand [Ni]min, and the value obtained thereby is equal to 1.15 or less. This means that the deviation of the Ni content and the 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) can be 1.00 or more. max max min min 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) can be 1.00 or more.

[0141] In one embodiment, if the semi-finished product of the bar steel having the composition of the present application and satisfying the properties described above is subjected to spheroidizing annealing under publicly known conditions, the maximum value of the equivalent circular diameter of the cementite particle after spheroidizing annealing is 15.0 μm or less, and the deviation of the equivalent circular diameter becomes small as described later. As a result, a finished product of the bar steel having excellent cold workability can be obtained.

[0142] In one embodiment, the reason for obtaining a finished product of the bar steel having excellent cold workability by controlling the properties of the semi-finished product of the bar steel before spheroidizing annealing is not explicitly clear, but it is considered to be based on the following reasons. First, it is considered that the specific cementite particle having a length-width ratio of 2.0 or less before spheroidizing annealing is not divided by the subsequent spheroidizing annealing, but is changed by Ostwald ripening to grow into a spheroid. Therefore, by controlling the length d of the long axis of the cementite particle before spheroidizing annealing to be 10.0 μm or less, the equivalent circular diameter of the cementite particle after spheroidizing annealing can be made to be 15.0 μm or less. L ​​​​​​​

[0143] In addition, as described above, Cu and Ni have an effect of inhibiting the growth of cementite particles in spheroidizing annealing. The bar steel of the present application contains Cu and Ni in prescribed amounts. However, if the variation in the Cu content and the Ni content in the semi-product of the bar steel is large, the growth of cementite particles cannot be inhibited in the places where the contents are insufficient, and sometimes the maximum value of the equivalent circle diameter exceeds the prescribed value. In one embodiment, it is considered that by reducing the variation in the Cu content and the Ni content, the growth of cementite particles can be inhibited without fail.

[0144] In a preferred embodiment, the m observation fields include one observation field located at the central portion of the cross section and one or more observation fields located at the peripheral portion of the cross section. In the semi-product of the bar steel obtained by hot rolling the ingot, generally, the metal structure of the peripheral portion of the cross section is easily affected by plastic deformation caused by the rolling rolls. In contrast, the metal structure of the central portion of the cross section is not easily affected by plastic deformation. Therefore, in the one observation field located at the central portion of the cross section and the one or more observation fields located at the peripheral portion of the cross section, the length d L of the long axis of the cementite particles and the length d S are sometimes largely different. Therefore, in the preferred embodiment, the m observation fields are selected from both the central portion and the peripheral portion of the cross section. Thereby, the properties of the cementite particles can be correctly evaluated for the entire cross section of the test piece.

[0145] In a more preferred embodiment, the observation field located at the peripheral portion is at the midpoint of the line segment connecting the observation field located at the central portion and the outer periphery of the cross section. In the semi-product of the bar steel obtained by hot rolling the ingot, generally, the metal structure of the portion of the peripheral portion of the cross section that is particularly close to the outer periphery is easily strongly affected by plastic deformation caused by the rolling rolls. Therefore, it is not preferable to select the observation field from this portion as representative of the entire test piece. Therefore, in the more preferred embodiment, the position of the peripheral portion is selected at the midpoint of the line segment connecting the observation field located at the central portion and the outer periphery of the cross section. Thereby, the portion of the peripheral portion of the cross section that is particularly close to the outer periphery is excluded from the observation target, and the properties of the cementite particles can be further accurately evaluated for the entire cross section of the test piece.

[0146] In a further preferred embodiment, as Figure 1As illustrated, the aforementioned m observation fields consist of one observation field located at the center of the cross-section and four observation fields located at the periphery of the cross-section. The angle between adjacent line segments passing through the four observation fields at the periphery of the cross-section is 90 degrees. If the number m of observation fields for observing the properties of cementite particles is too small, the entire sample cannot be observed; if it is too large, the measurement takes too long. Therefore, in a further preferred embodiment, observation is performed in a total of five observation fields, in addition to the one observation field at the center. Furthermore, the angle between adjacent line segments connecting the one observation field at the center to the outer periphery of the cross-section and passing through the four observation fields at the periphery of the cross-section is 90 degrees. Thus, the observation fields can be uniformly determined according to the shape of the sample's cross-section.

[0147] In a further preferred embodiment, it is sufficient that the angle formed by four adjacent line segments is 90 degrees, and the direction of the line segments is not particularly limited. However, in a further preferred embodiment, if it is desired to more strictly determine the position of the observation field of view, such as... Figure 1 As illustrated, when the shape of the cross section perpendicular to the length direction of the bar is an ellipse (b), the direction of the line segment can be set as the direction of the major and minor axes of the ellipse. When the shape of the cross section is a square (c) or a rectangle (d), the direction of the line segment can be set as the direction perpendicular to the side.

[0148] [Properties after spheroidizing annealing]

[0149] Next, the properties of the bar steel after spheroidizing annealing according to the present invention will be described. In other embodiments, regarding the bar steel of the present invention, after further spheroidizing annealing of the bar steel obtained by hot rolling of steel ingot, among all the equivalent circle diameters of cementite particles observed in n observation fields (where n is an integer of 2 or more) in a section perpendicular to the length direction, the maximum value of the above-mentioned equivalent circle diameter in one observation field is set as d1, d2, ... d n From d1 to d n The maximum value is set to d. max Set the minimum value to d min When, it satisfies the following equations (2) and (3).

[0150] [Mathematical Formula 5]

[0151]

[0152] [Mathematical Formula 6]

[0153]

[0154] Evaluation of the shape of the cementite particles after spheroidizing annealing is performed in the same manner as before spheroidizing annealing by observing the metal structure in a cross section of a sample cut perpendicularly to the finished product of the bar steel after spheroidizing annealing. Specifically, after polishing the cross section of the sample, the polished surface is etched with acid, whereby the cementite phase can be visually recognized. In the observation of the metal structure, it is preferable to use a scanning electron microscope. In the observation of the metal structure, the deviation in the size of the cementite particles based on the observation field is evaluated. For this purpose, the number n of observation fields observed is an integer of 2 or more. In order to correctly evaluate the deviation, it is preferable that the shape and size of the observation field observed be constant. The shape of the observation field is not particularly limited and can be a circle, an ellipse, a rectangle, or a square.

[0155] From the shape of the cementite particles observed in one observation field, the equivalent circle diameter of the cementite particles is calculated. Specifically, the image data of the photograph of the metal structure of one observation field is processed using image processing software, whereby the equivalent circle diameter of each cementite particle is calculated. In the present specification, the "equivalent circle diameter of the cementite particle" refers to the diameter of a circle having the same area as the cross-sectional area of the observation surface of one cementite particle. The maximum value among the calculated equivalent circle diameters in one observation field is set as d1. This operation is repeated for n observation fields, and the maximum value d1, d2, d3,..., dn in each observation field is calculated. , d n .

[0156] In other embodiments, with respect to the bar steel after spheroidizing annealing of the present application, when the maximum value from d1 to d n calculated in the above-described steps is set as d max , and the minimum value is set as d min , the maximum value d max satisfies the above-described formula (2). That is, the maximum value d max of the equivalent circle diameters of the cementite particles observed in n observation fields is equal to 15.0 μm or less. This means that there are no cementite particles having a size of an equivalent circle diameter exceeding 15.0 μm in n observation fields. Note that the lower limit of d max is not particularly limited, but in one embodiment, d max may be 1.0 μm or more.

[0157] In addition, the maximum value d1, d2, d3,..., dn of the equivalent circle diameters in n observation fields is calculated in the same manner as described above. When the maximum value d , d nsatisfies the above (3). That is, the maximum value d1, d2, , d n any one of them is equal to or less than the value obtained by multiplying the minimum value d min by 1.25. This means that the maximum value d n of d1, d2, , d max is equal to or less than the value obtained by increasing the minimum value d min by 25%. That is, the values of d1, d2, , d n deviate little. It should be noted that the lower limit of the values of d1, d2, , d n is not particularly limited, but in one embodiment, the values of d1, d2, , d n may be 1.00 μm or more.

[0158] In other embodiments, the reason why a finished product of a bar steel having excellent cold workability is obtained by limiting the size and deviation of the equivalent circle diameters of cementite particles in n observation fields after spheroidizing annealing in a prescribed range is not clear, but it is considered to be based on the following reason. Cementite has a high hardness compared to ferrite. Therefore, the existence form of cementite in a finished product of a bar steel has an influence on cold workability. By spheroidizing annealing of a semi-finished product of a bar steel, the cementite contained in ferrite and pearlite becomes a spherical particle form and is dispersed in the matrix. In this way, the existence of cementite does not easily become an obstacle to plastic deformation in cold work.

[0159] However, in the conventional technology, there are particles having an equivalent circle diameter exceeding 15.0 μm among the cementite particles. In addition, the deviation of the equivalent circle diameters is large depending on the position of observation. There are cementite particles having a large equivalent circle diameter in a part of the cross section of a bar steel, and therefore this part becomes an obstacle to plastic deformation in cold work, and the cold workability is reduced. In the present application, it is considered that the size and deviation of the equivalent circle diameters of the cementite particles after spheroidizing annealing are controlled in a prescribed range, and therefore a finished product of a bar steel having excellent cold workability is obtained.

[0160] The preferred embodiment of determining n observation fields for the finished product of the spheroidization-annealed strip steel is the same as that of determining m observation fields for the semi-finished product of the strip steel before spheroidization annealing, and thus the description is omitted here.

[0161] [evaluation of cold forgeability]

[0162] The effect of the present application can be evaluated by the cold forgeability of the finished product of the strip steel. For example, in the case where the shape of the cross section of the strip steel is a circle with a diameter of 15 mm, the cold forgeability can be evaluated in the following manner. First, an 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 so as to have a cylindrical shape with a length of 21 mm. A test piece 4 having a grooved cylindrical shape shown in Fig. 1 is prepared by providing a V-shaped groove with a groove front end curvature R of 0.15 mm, a groove depth of 0.8 mm, and a groove angle of 30 degrees on the side surface of the cylinder. Figure 2

[0163] Next, the test piece 4 is set in a compression testing machine at a strain speed of 10 s －1 The compression is performed in the height direction under the condition that the compressed surface 4b is restrained. Initially, only 0.3 mm is pressed down, and whether or not a crack is generated at the bottom of the V-shaped groove 4a of the side surface of the test piece is observed by visual observation, and in the case where a crack is generated, the length thereof is measured. Next, each 0.3 mm of pressing down is performed successively until the length of the crack is 0.5 mm or more. Such a compression test is performed with six test pieces, and the cumulative pressing down rate at the time when the crack of 0.5 mm or more is confirmed in three test pieces out of the six is taken as the limit upset ratio. In the present specification, the "cumulative pressing down rate" means the percentage of the value obtained by dividing the final height of the test piece by the original height, i.e., 21 mm. In the test conditions described above, the test piece in which the limit upset ratio is 55% or more can be said to have excellent cold forgeability.

[0164] Example

[0165] Hereinafter, examples of the present application will be described. It should be noted that the embodiments of the present application are not limited to the following examples, and the embodiments of the present application can be arbitrarily changed within the scope of the gist of the present application.

[0166] [Inventive Example]

[0167] ​A molten steel having the composition of Steel Nos. 1 to 35 of Table 1 was prepared using an electric furnace, and an ingot was cast by continuous casting. The temperature of the molten steel at the time of pouring was 1450°C, and the casting speed was 1.0 m / min. After the ingot was heated to 1100°C using a heating furnace, it was hot-rolled into a round bar having a diameter of 15 mm using a rolling mill. The final temperature of the hot-rolling was 800°C. From the obtained semi-product of the hot-rolled bar (round bar) of Nos. 1 to 35, a test piece was taken, and after polishing a cross section perpendicular to the longitudinal direction, the observation surface was etched using a 3% aqueous solution of picric acid.

[0168] Next, in the five observation fields shown in (a), the length d Figure 1 of the long axis of the cementite particle having an aspect ratio of 2.0 or less was measured using a scanning electron microscope and image processing software (ImageJ Ver. 1.53c). L The magnification of the scanning electron microscope was 5000x, and the diameter of the smallest cementite particle that could be observed in this magnification was 60 nm. The size of one observation field at this time was 24 μm in the horizontal direction and 18 μm in the vertical direction. In addition, in the same observation field, the Cu content and the Ni content were measured using an electron probe microanalyzer. 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 the test pieces of Nos. 1 to 35 having the composition of the present application, the maximum value of the length d L of the long axis of the specific cementite particle having an aspect ratio of 2.0 or less was 10.0 μm or less in all five observation fields, and the Cu content and the Ni content measured in the five observation fields satisfied the formula (1).

[0169] [Comparative Examples]

[0170] On the other hand, in the test pieces of Nos. 36 to 47 and the test pieces of Nos. 50 and 51 of Table 2, a semi-product of a bar (round bar) was manufactured from a molten steel having the composition of Steel Nos. 36 to 47 of Table 1, which was outside the range of the present application. In addition, in the test piece of No. 48 of Table 2, a semi-product of a bar (round bar) was manufactured from a molten steel having the composition of Steel No. 11 of Table 1, but the casting conditions were different from the above-described conditions, and the temperature of the molten steel at the time of pouring was set to 1530°C, and the casting speed was set to 3.5 m / min. In the test piece of No. 49 of Table 2, a semi-product of a bar (round bar) was manufactured from a molten steel having the composition of Steel No. 6 of Table 1, but the conditions of the hot-rolling were different from the above-described conditions, and the heating temperature was set to 1270°C, and the final temperature was set to 740°C. As a result, in the test piece of No. 48, the Cu content and the Ni content did not satisfy the formula (1), and in the test piece of No. 49, the maximum value of d L of the specific cementite particle having an aspect ratio of 2.0 or less exceeded 10.0 μm.

[0171]

[0172] [Invention Example [Comparative Example]

[0173] Next, the Invention Example The comparative example was subjected to spheroidizing annealing of the round bar using an annealing furnace. The annealing temperature was 690°C, and the temperature retention time at the time of annealing was 15 hours. From the finished product of the spheroidizing-annealed strip steel (round bar) obtained, a test piece was taken, and after polishing the cross section perpendicular to the long side direction, the observation surface was corroded using a 3% aqueous solution of picric acid.

[0174] Next, in Figure 1 (a) of the five observation fields, the equivalent circular diameter of the cementite particles was found using a scanning electron microscope and image processing software (ImageJ Ver. 1.53c), and the maximum values d1, d2, d3, d4, and d5 were obtained. In addition, from these maximum values, the maximum value d max and the minimum value d min were determined at the same time, and it was determined whether or not the formula (3) was satisfied. The evaluation results obtained are shown in Table 2.

[0175] Next, from the finished product of the spheroidizing-annealed strip steel (round bar) Figure 2 Test piece 4 shown in FIG. 4 was produced, and the limit upset ratio was measured using the method described above. The measurement results obtained are shown in Table 2. In addition, the relationship between the maximum value d max and the limit upset ratio is shown in Figure 3 .

[0176]

[0177] From Tables 1 and 2, it was found that, regarding the specific cementite particles having a length of the long axis d L of 10.0 μm or less and the content of Cu and Ni satisfying the formula (1) in the strip steel satisfying the component composition of the present invention and having an aspect ratio of 2.0 or less before spheroidizing annealing, the equivalent circular diameter of the cementite particles after spheroidizing annealing satisfied the conditions of the formula (2) and the formula (3), and the limit upset ratio was greater than 55% in all of the No. 1 to 35 samples obtained by subjecting the samples to ordinary spheroidizing annealing.

[0178] On the other hand, in the No. 36 to 51 samples in which the component composition of the strip steel did not satisfy the component composition of the present invention or the conditions of casting or hot rolling were different from the above conditions, the equivalent circular diameter of the cementite particles after spheroidizing annealing did not satisfy the conditions of at least one of the formula (2) or the formula (3). As a result, the limit upset ratio of these samples all showed a value of less than 55%.

[0179] From Figure 3It is understood that, with respect to the examples of the application indicated by the white circles, which are the bar steels obtained by subjecting the semi-finished products of the bar steels of the application to spheroidizing treatment and satisfying the conditions of the finished products of the bar steels of the application, the limit upset ratio is greater than 55%. On the other hand, with respect to the comparative examples indicated by the black circles and black triangles, which do not satisfy the conditions of the finished products of the bar steels of the application, it is understood that the limit upset ratio shows a value less than 55%.

[0180] From these results, it is understood that, by controlling the properties of the semi-finished products of the bar steels before spheroidizing annealing, it is possible to make the equivalent circle diameter of the cementite particles in the finished products of the bar steels after spheroidizing annealing satisfy the conditions of equations (2) and (3), and it is possible to obtain bar steels having excellent cold workability. At the same time, from these results, it is understood that, in the case where the equivalent circle diameter of the cementite particles in the finished products of the bar steels after spheroidizing annealing satisfies the conditions of equations (2) and (3) at the same time, it is possible to obtain bar steels having excellent cold workability.

[0181] Explanation of symbols

[0182] 1 Cross section of bar steel

[0183] 2 Observation field

[0184] 3 Line segment

[0185] 4 Test piece

[0186] 4a V-shaped groove

[0187] 4b Compressed surface

Claims

1. A steel strip, characterized in that, having a component composition containing, in mass percent, C：0.12～0.44%、 Si: 0.15 to 0.35%, Mn: 0.30 to 0.95%, P：0.001～0.030%、 S：0.001～0.030%、 Cr：0.85～1.50%、 Cu: 0.02 to 0.30%, Ni: 0.02 to 0.25%, and N：0.0020～0.0250%， the remainder consisting of Fe and unavoidable impurities, The length of the long axis of all the cementite particles observed in m observation fields of view (where m is an integer of 2 or more) of a cross section perpendicular to the length direction, when the shape of the observed cementite particles is approximated to an ellipse, is set as d L The length of the short axis is set as d S The aspect ratio obtained by dividing the length d L of the long axis by the length d S of the short axis is 2.0 or less, and the length d L of the long axis in all the m observation fields of view is 10.0 μm or less, when the maximum value of the mass percentage of the Cu content measured in the m observation fields of view is set as [Cu] and the minimum value is set as [Cu] max when the maximum value of the mass percentage of the Ni content measured in the m observation fields of view is set as [Ni] and the minimum value is set as [Ni] min max min the following (1) formula is satisfied.​​ 2. The steel strip defined in claim 1 wherein the m observation fields include one observation field located at a central portion of the cross section and one or more observation fields located at a peripheral portion of the cross section.

3. The steel strip defined in claim 2 wherein, The observation fields located at the peripheral portion are at midpoints of line segments connecting the observation field located at the central portion and an outer periphery of the cross section.

4. The steel strip defined in claim 3 wherein, The m observation fields consist of one observation field located at a central portion of the cross section and four observation fields located at a peripheral portion of the cross section, and with respect to four of the line segments passing through the four observation fields located at the peripheral portion of the cross section, angles formed by adjacent line segments are 90 degrees.

5. The steel strip according to any one of claims 1 to 4, wherein the component composition further contains, in mass percent, one or two or more kinds selected from 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. The shape of the cross section is any one of a circle, an ellipse, a square, and a rectangle.

6. The steel strip defined in any one of claims 1 to 5 wherein, having a component composition containing, in mass percent, 7. A steel strip, characterized in that Si: 0.15 to 0.35%, C：0.12～0.44%、 Mn: 0.30 to 0.95%, Cu: 0.02 to 0.30%, P：0.001～0.030%、 S：0.001～0.030%、 Cr：0.85～1.50%、 Ni: 0.02 to 0.25%, and the remainder consisting of Fe and unavoidable impurities, N：0.0020～0.0250%， the n observation fields include one observation field located at a central portion of the cross section and one or more observation fields located at a peripheral portion of the cross section. In all equivalent circle diameters of all cementite particles observed in n observation fields of a cross section perpendicular to the length direction (where n is an integer of 2 or more), the maximum value of the equivalent circle diameter in one observation field is set as d1, d2, , d n , the maximum value from d1 to d n is set as d max , and the minimum value is set as d min , the following (2) and (3) are satisfied.

8. The steel strip defined in claim 7 wherein, The observation fields located at the peripheral portion are at midpoints of line segments connecting the observation field located at the central portion and an outer periphery of the cross section.

9. The steel strip defined in claim 8 wherein, The n observation fields consist of one observation field located at a central portion of the cross section and four observation fields located at a peripheral portion of the cross section, and with respect to four of the line segments passing through the four observation fields located at the peripheral portion of the cross section, angles formed by adjacent line segments are 90 degrees.

10. The steel strip defined in claim 9 wherein, The component composition further contains, in mass percent, one or two or more kinds selected from 11. The steel strip defined in any one of claims 7 to 10 wherein, 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. The shape of the cross section is any one of a circle, an ellipse, a square, and a rectangle. ​ 12. The steel strip defined in any one of claims 7 to 11 wherein, ​

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