WO2025069708A1 - Steel bar - Google Patents

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

Bars

The present invention relates to bar steel with excellent cold forgeability.

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

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

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

JP 2004-100038 A JP 2009-275250 A JP 2023-21615 A

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

The present invention was developed in consideration of the above problems, and proposes a bar steel that has excellent cold forgeability after spheroidizing annealing.

In order to improve cold forgeability, the inventors investigated the relationship between the size of cementite particles observed in the cross section of bar steel after spheroidizing annealing and cold forgeability. As a result, they found that bar steel in which the equivalent circle diameter of cementite particles measured in multiple observation fields of the cross section is 15.0 μm or less and the variation in the equivalent circle diameter is small can achieve excellent cold forgeability.

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

The present invention is based on the above findings and has the following gist:

[1] In 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 to 0.25%, and N: 0.0020 to 0.0250%
The balance is composed of Fe and unavoidable impurities,
When the shapes of all cementite particles observed in m observation fields (where m is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction are approximated as an ellipse, the length of the major axis is _dL and the length of the minor axis is _dS , and an aspect ratio obtained by dividing the length of the major axis _dL by the length of the minor axis _dS for a specific cementite particle is 2.0 or less, and the length of the major axis _dL is 10.0 μm or less in all of the m observation fields,
a maximum value of the mass percentage of the Cu content measured in the m observation fields is defined as [Cu] _max and the minimum value of the Cu content measured in the m observation fields is defined as [Cu] _min , and a maximum value of the mass percentage of the Ni content measured in the m observation fields is defined as [Ni] _max and the minimum value of the mass percentage of the Ni content measured in the m observation fields is defined as [Ni] _min , the following formula (1) is satisfied:

[2] 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 portion of the cross section.
The bar steel according to the above [1].

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

[4] The m observation fields of view include one observation field of view located at the center of the cross section and four observation fields of view located at the periphery of the cross section, and an angle between adjacent four line segments passing through the four observation fields located at the periphery of the cross section is 90 degrees.
The bar steel according to the above [3].

[5] The component composition further comprises, 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.
The bar steel according to any one of [1] to [4] above.

[6] The cross-sectional shape is any one of a circle, an ellipse, a square, and a rectangle.
The bar steel according to any one of [1] to [5] above.
[7] In 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 to 0.25%, and N: 0.0020 to 0.0250%
The balance is composed of Fe and unavoidable impurities,
A steel bar characterized in that, when the maximum value of the equivalent circle diameters in one observation field of view among all cementite particles observed in n observation fields of view (n is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction is _d1 , _d2 , ..., _dn , the maximum value of the equivalent circle diameters in one observation field of view _{is dmax} and the minimum value of _d1 to _dn is _dmin , the following formulas (2) and (3) are satisfied:

[8] 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 portion of the cross section.
The bar steel according to the above [7].

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

[10] The n observation fields include one observation field located at the center of the cross section and four observation fields located at the periphery of the cross section, and an angle between adjacent four line segments passing through the four observation fields located at the periphery of the cross section is 90 degrees.
The bar steel according to [9] above.

[11] The component composition further comprises, 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.
The bar steel according to any one of [7] to [10] above.

[12] The cross-sectional shape is any one of a circle, an ellipse, a square, and a rectangle.
The steel section according to any one of [7] to [11] above.

According to the present invention, it is possible to obtain excellent cold forgeability for section steel after spheroidizing annealing. This reduces damage to dies during cold forging and increases productivity. Furthermore, according to the present invention, the preferred metal structure of section steel before spheroidizing annealing is revealed, improving the production yield of steel material.

FIG. 1 is a diagram showing the position of the observation field for observing the properties in the cross section of a bar having a cross section shape of (a) a circle, (b) an ellipse, (c) a square, or (d) a rectangle. 1A is a top view of a test piece for measuring a critical upsetting ratio in an example, and FIG. 1C is a cross-sectional view showing an enlarged cutout portion. 1 is a graph showing the relationship between the maximum equivalent circle diameter of cementite particles and the critical upsetting ratio in Examples.

The following provides a detailed explanation of how to implement the present invention.

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

The chemical composition of the above bar steel corresponds to that of an alloy steel called chromium steel, which is carbon steel with approximately 1% Cr added. More specifically, it is similar to the chemical composition of materials with material symbols SCr415 to SCr440 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 bar steel according to the present invention and the range of the chemical composition of each element are not completely identical to that specified in the same standard.

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

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

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

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

[S:0.001-0.030%]
S is an element that is effective in improving the machinability of steel by combining with Mn in the steel to form MnS. If the S content is 0.001% or more, the above effect can be sufficiently obtained. Also, if the S content is 0.030% or less, a large amount of MnS, which is the starting point of cracks during cold forging, is not formed, so there is no adverse effect on cold forgeability. For this reason, the S content is set to 0.001 to 0.030%.

[Cr:0.85-1.50%]
Cr is an effective element for imparting the necessary strength to steel by improving solid solution strengthening and hardenability. If the Cr content is 0.85% or more, the above effects can be sufficiently obtained. Also, if the Cr content is 1.50% or less, the hardness of the steel material increases and does not adversely affect the cold forgeability. For this reason, the Cr content is set to 0.85 to 1.50%. The upper limit of the Cr content is preferably 1.35%, and more preferably 1.20%.

[Cu:0.02-0.30%]
Cu is an element that effectively acts in controlling the size of cementite particles. That is, Cu suppresses the growth of cementite particles during spheroidizing annealing. If the Cu content is 0.02% or more, the above effect can be sufficiently obtained. Furthermore, if the Cu content is 0.30% or less, surface defects are unlikely to occur during steel production. 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%.

[Ni:0.02-0.25%]
Ni, like Cu, is an element that effectively acts in controlling the size of cementite particles. That is, Ni suppresses the growth of cementite particles during spheroidizing annealing. If the Ni content is 0.02% or more, the above effect can be sufficiently obtained. Furthermore, if the Ni content is 0.25% or less, surface defects are unlikely to occur during steel production. 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%.

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

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

Elements equivalent to unavoidable impurities include, for example, O (oxygen), Ca, Bi, Sb, etc. Even if the section steel according to the present invention unavoidably contains, by mass percentage, 0.0100% or less of O (oxygen), 0.01% or less of Ca, 0.01% or less of Bi, and 0.03% or less of Sb, the contents of each are so small that they do not affect the cold forgeability. It is permissible in the present invention for the section steel to contain trace amounts of elements other than the exemplified O (oxygen), Ca, Bi, and Sb, within a range that does not affect the effects of the present invention.

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

[Mo: 0.30% or less]
Mo is an element that can greatly improve the hardenability of steel material by adding a small amount, and is effective in improving the strength of steel. If the Mo content is 0.30% or less, it is possible to prevent the hardenability from becoming excessive and the cold forgeability from decreasing. Therefore, the Mo content in a preferred embodiment is 0.30% or less. If the Mo content is 0.15% or more, the above effect can be sufficiently obtained.

[Al: 0.100% or less]
Al is an element that promotes deoxidation in the melting process, and is also an element that is effective in promoting the refinement of ferrite grains and increasing the strength of bar steel by combining with N in the steel to form nitrides. If the Al content is 0.100% or less, it is possible to prevent a large amount of Al oxide from being generated in the steel, which makes it easier for cracks to occur during cold forging. For this reason, the Al content in a preferred embodiment is 0.100% or less. If the Al content is 0.001% or more, the above effects can be sufficiently obtained.

[Ti: 0.100% or less]
Ti, like Al, is an element that is effective in refining ferrite grains by combining with N in steel to form nitrides. If the Ti content is 0.100% or less, it is possible to prevent a large amount of Ti-based inclusions from being generated in the steel, which makes it easier for cracks to occur during cold forging. For this reason, the Ti content in a preferred embodiment is 0.100% or less. If the Ti content is 0.001% or more, the above effects can be sufficiently obtained.

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

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

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

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

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

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

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

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

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

[Properties before spheroidizing annealing]
Next, the properties of the section steel according to the present invention before spheroidizing annealing will be described. In one embodiment, the section steel according to the present invention satisfies the following formula (1) when the shapes of all cementite particles observed in m observation fields (where m is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction of the section steel before spheroidizing annealing obtained by hot rolling a steel ingot are approximated as an ellipse, the length of the major axis is _dL , the length of the minor axis is _dS , and the aspect ratio obtained by dividing the length of the major axis _dL by the length of the minor axis _dS of a specific cementite particle is 2.0 or less, and the length of the major axis _dL is 10.0 μm or less in all of the m observation fields, the maximum value of the mass percentage of the Cu content measured in the m observation fields is [Cu] _max , the minimum value is [Cu] _min , and the maximum value of the mass percentage of the Ni content measured in the m observation fields is [Ni] _max , and the minimum value is [Ni] _min .

The shape of cementite particles before spheroidizing annealing can be evaluated by observing the metal structure in the cross section of a sample cut perpendicular to the length direction from a semi-finished bar steel product obtained by hot rolling a steel ingot. Specifically, the cross section of the sample is polished, and the polished surface is then etched with acid to make the cementite phase visible. A scanning electron microscope is preferably used to observe the metal structure. In observing the metal structure, the length of the major axis of the cementite particles is evaluated in the observation field. For this purpose, the number m of observation fields is an integer of 2 or more. To keep the observation conditions consistent, 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.

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

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

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

In one embodiment, when spheroidizing annealing is performed under known conditions on a semi-finished bar product before spheroidizing annealing that has the composition according to the present invention and satisfies the above-mentioned properties, the maximum circle equivalent diameter of the cementite particles after spheroidizing annealing is 15.0 μm or less, and the variation in circle equivalent diameter is reduced, as described below. As a result, a finished bar product with excellent cold forgeability can be obtained.

In one embodiment, the reason why a finished bar having excellent cold forgeability can be obtained by controlling the properties of the semi-finished bar before spheroidizing annealing is not clear in detail, but it is believed to be due to the following reason. First, it is believed that specific cementite particles having an aspect ratio of 2.0 or less before spheroidizing annealing are changed and grown into a sphere by Ostwald ripening without being divided by the subsequent spheroidizing annealing. Therefore, it is believed that the circle equivalent diameter of the cementite particles after spheroidizing annealing can be made 15.0 μm or less by controlling the length _dL of the major axis of the cementite particles before spheroidizing annealing to 10.0 μm or less.

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

In a preferred embodiment, the m observation fields include one observation field located in the center of the cross section and one or more observation fields located in the peripheral portion of the cross section. In a semi-finished product of a bar obtained by hot rolling a steel ingot, the metal structure in the peripheral portion of the cross section is generally susceptible to plastic deformation caused by the rolling rolls. In contrast, the metal structure in the center of the cross section is less susceptible to plastic deformation. For this reason, the length _dL of the major axis and the length _dS of the minor axis of the cementite particles may be significantly different between one observation field located in the center of the cross section and one or more observation fields located in the peripheral portion of the cross section. Therefore, in a preferred embodiment, the m observation fields are selected from both the center and the peripheral portions of the cross section. This allows the properties of the cementite particles to be correctly evaluated for the entire cross section of the sample.

In a more preferred embodiment, the observation field located in the peripheral portion is located at the midpoint of a line segment connecting the observation field located in the central portion and the outer periphery of the cross section. In a semi-finished bar product obtained by hot rolling a steel ingot, the metal structure of the peripheral portion of the cross section, particularly near the outer periphery, is generally susceptible to strong plastic deformation caused by the rolling rolls. For this reason, it is not preferable to use an observation field selected from this portion as a representative of the entire sample. Therefore, in a more preferred embodiment, the position of the peripheral portion is selected at the midpoint of a line segment connecting the observation field located in the central portion and the outer periphery of the cross section. This allows the peripheral portion of the cross section, particularly near the outer periphery, to be excluded from the observation target, making it possible to more accurately evaluate the properties of the cementite particles for the entire cross section of the sample.

In a further preferred embodiment, the m observation fields are composed 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 exemplified in FIG. 1, and the angle between adjacent lines passing through the four observation fields located in 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, it is not possible to observe the entire sample, and if it is too large, it takes a long time to measure. Therefore, in a further preferred embodiment, observation is performed using a total of five observation fields, including one central observation field and four peripheral observation fields. Also, the angle between adjacent lines connecting the one central observation field to the outer periphery of the cross section and the four line segments passing through the four observation fields located in the periphery of the cross section is 90 degrees. This makes it possible to uniformly determine the observation fields according to the shape of the cross section of the sample.

In a further preferred embodiment, the angle between the four adjacent line segments needs to be 90 degrees, and the direction of the line segments is not particularly limited. However, in a further preferred embodiment, if it is desired to specify the position of the observation field more precisely, as exemplified in FIG. 1, when the shape of the cross section perpendicular to the longitudinal direction of the bar steel 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 sides.

[Properties after spheroidizing annealing]
Next, the properties of the section steel according to the present invention after spheroidizing annealing will be described. In another embodiment, the section steel according to the present invention, after the section steel obtained by hot rolling a steel ingot is further subjected to spheroidizing annealing, satisfies the following formulas (2) and (3) when, among the circle-equivalent 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 longitudinal direction, the maximum value of the circle-equivalent diameters in one observation field is defined as _d1 , d2, ..., dn, the maximum value from _d1 to _dn is defined as _dmax , and the minimum value is defined as _dmin .

The shape of the cementite particles after spheroidizing annealing can be evaluated by observing the metal structure in the cross section of a sample cut perpendicular to the length direction of the finished bar steel after spheroidizing annealing, in the same manner as the evaluation before spheroidizing annealing. Specifically, the cross section of the sample is polished, and the polished surface is then etched with acid to make the cementite phase visible. A scanning electron microscope is preferably used for observing the metal structure. In observing the metal structure, the variation in the size of the cementite particles depending on the observation field is evaluated. For this purpose, the number n of observation fields to be observed is an integer of 2 or more. In order to accurately evaluate the variation, it is preferable that the shape and size of the observation field to be observed are constant. The shape of the observation field is not particularly limited, and may be a circle, ellipse, rectangle, or square.

The circle-equivalent diameter of the cementite particles is determined based on the shape of the cementite particles observed in one observation field. Specifically, the image data of a metal structure photograph of one observation field is processed by image processing software to calculate the circle-equivalent diameter of each cementite particle. In this specification, the "circle-equivalent diameter of a cementite particle" refers to the diameter of a circle having the same area as the cross-sectional area of one cementite particle when the cross-sectional area of the observation surface of the cementite particle is calculated. The maximum value of the calculated circle-equivalent diameters in one observation field is designated as _d1 . This operation is repeated for n observation fields to determine the maximum values _d1 , _d2 , ..., _dn in each observation field.

In another embodiment, when the maximum value of _d1 to _dn obtained by the above procedure for the section steel after spheroidizing annealing according to the present invention is _dmax and the minimum value is _dmin , the maximum value _dmax satisfies the above formula (2). That is, the maximum value _dmax of the equivalent circle diameter of the cementite particles observed in n observation fields is equal to or smaller than 15.0 μm. This means that no cementite particles having an equivalent circle diameter exceeding 15.0 μm are present in the n observation fields. Note that the lower limit of _dmax is not particularly limited, but in one embodiment, _dmax may be 1.0 μm or more.

In addition, the maximum values d ₁ , d ₂ , ..., d _n of the circle equivalent diameters in the n observation fields of view satisfy the above formula (3). That is, for each of the maximum values d ₁ , d ₂ , ..., d _n of the circle equivalent diameters, it is equal to or smaller than the minimum value d _min among them multiplied by 1.25. This means that the maximum value d _max among d ₁ , d ₂ , ..., d _n is equal to or smaller than the minimum value d _min increased by 25%. In other words, the values of d ₁ , d ₂ , ..., d _n have small variations. Note that the lower limit of the values of d ₁ , d ₂ , ..., d _n is not particularly limited, but in one embodiment, the values of d ₁ , d ₂ , ..., d _n may be 1.00 μm or more.

In another embodiment, the reason why a finished bar steel product 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 clear in detail, but it is probably due to the following reason. Cementite is harder than ferrite. For this reason, the form in which cementite is present in the finished bar steel product affects the cold forgeability. By spheroidizing annealing the semi-finished bar steel product, the cementite contained in the ferrite and pearlite takes the form of spherical particles and is dispersed within the matrix. This makes it less likely that the presence of cementite will hinder plastic deformation during cold forging.

However, in conventional technology, some cementite particles had an equivalent circle diameter exceeding 15.0 μm. Furthermore, there was a large variation in the equivalent circle diameter depending on the observation position. Because cementite particles with a large equivalent circle diameter exist in part of the cross section of the bar, this part hinders plastic deformation in cold forging, reducing cold forgeability. In the present invention, the size and variation of the equivalent circle diameter of cementite particles after spheroidizing annealing are controlled within a specified range, so it is believed that a finished bar product with excellent cold forgeability can be obtained.

The preferred embodiment for determining n observation fields for the finished section steel product after spheroidizing annealing is similar to the preferred embodiment for determining m observation fields for the semi-finished section steel product before spheroidizing annealing, so a description thereof will be omitted here.

[Evaluation of cold forgeability]
The effect of the present invention can be evaluated by the cold forgeability of the finished bar. For example, when the cross-sectional shape of the 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 provided on the side of the cylinder, and a grooved cylindrical test piece 4 having the shape shown in FIG. 2 is prepared.

Next, the test piece 4 is set in a compression tester and compressed in the height direction at a strain rate of 10 s ^-1 under the condition that the compressed surface 4b is restrained. First, the test piece is pressed down by 0.3 mm, and the bottom of the V-shaped groove 4a on the side of the test piece is visually observed to see if a crack occurs, and if a crack occurs, its length is measured. Next, the test piece is pressed down by 0.3 mm at a time until the length of the crack is 0.5 mm or more. Such a compression test is performed on six samples, and the cumulative reduction rate at the time when a crack of 0.5 mm or more is confirmed in three of the six test pieces is defined as the limit upsetting rate. In this specification, the "cumulative reduction rate" refers to the percentage of the final height of the test piece divided by the original height of 21 mm. Under the above test conditions, it can be said that a test piece with a limit upsetting rate of 55% or more has excellent cold forgeability.

Below, examples of the present invention are described. Note that the embodiments of the present invention are not limited to the following examples, and the embodiments of the present invention can be modified as desired without departing from the gist of the present invention.

[Example of the invention]
Molten steel having the composition of steel No. 1 to 35 in Table 1 was prepared using an electric furnace, and a steel ingot was cast by continuous casting. The molten steel temperature at the time of pouring was 1450°C, and the pouring speed was 1.0 m/min. The steel ingot was heated to 1100°C using a heating furnace, and then hot rolled into a round bar having a diameter of 15 mm using a rolling roll. The finishing temperature of the hot rolling was 800°C. Samples were taken from the semi-finished products of the hot-rolled bar (round bar) obtained from No. 1 to 35, and the cross section perpendicular to the longitudinal direction was polished, and the observation surface was corroded using a 3% aqueous solution of picric acid.

Next, the maximum value of the length _dL of the major axis of cementite particles having an aspect ratio of 2.0 or less was obtained in the five observation fields shown in FIG. 1(a) using a scanning electron microscope and image processing software (ImageJ Ver. 1.53c). The magnification of the scanning electron microscope was 5000 times, and the diameter of the smallest cementite particle observable at this magnification was 60 nm. The size of one observation field at this time was 24 μm in width and 18 μm in length. In addition, the Cu content and Ni content were measured in the same observation field using an electron beam 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, No. 1 having the component composition according to the present invention was measured. In samples No. 1 to No. 35, the length _dL of the major axis of a 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 Ni content measured in the five observation fields satisfied formula (1).

[Comparative Example]
On the other hand, in the case of samples No. 36 to 47 in Table 2 and samples No. 50 and 51, semi-finished products of bar (round bar) were produced from molten steel having the composition of steel No. 36 to 47 in Table 1, which is outside the scope of the present invention. In the case of sample No. 48 in Table 2, semi-finished products of bar (round bar) were produced from molten steel having the composition of steel No. 11 in Table 1, but the casting conditions were different from the above conditions, with the molten steel temperature at the time of pouring being 1530°C and the pouring speed being 3.5 m/min. In the case of sample No. 49 in Table 2, semi-finished products of bar (round bar) were produced from molten steel having the composition of steel No. 6 in Table 1, but the hot rolling conditions were different from the above conditions, with the heating temperature being 1270°C and the finishing temperature being 740°C. As a result, in the case of sample No. 48, the Cu content and Ni content did not satisfy formula (1), and the results of the results of the test of sample No. In 49 samples, the maximum value of _dL of specific cementite grains having an aspect ratio of 2.0 or less exceeded 10.0 μm.

[Examples of the invention and comparative examples]
Next, for both the invention example and the comparative example, the round bars were subjected to spheroidizing annealing in an annealing furnace. The annealing temperature was 690° C., and the temperature holding time during annealing was 15 hours. Samples were taken from the finished bar products (round bars) obtained after spheroidizing annealing, and the cross sections perpendicular to the longitudinal direction were polished, and then the observation surfaces were etched using a 3% aqueous solution of picric acid.

Next, the circle equivalent diameters of the cementite particles were obtained in the five observation fields shown in Figure 1 (a) 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. Among these maximum values, the maximum value _dmax and the minimum value _dmin were determined, and it was determined whether or not they satisfied formula (3). The obtained evaluation results are shown in Table 2.

Next, a test piece 4 shown in Fig. 2 was prepared from the finished steel section (round bar) after spheroidizing annealing, and the critical upsetting ratio was measured by the method described above. The measurement results obtained are shown in Table 2. The relationship between the maximum value _dmax and the critical upsetting ratio is shown in Fig. 3.

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

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

As can be seen from Figure 3, for the invention examples, which were obtained by subjecting the semi-finished section steel products of the present invention to a spheroidizing treatment and which are indicated by open circle marks and which satisfy the conditions for the finished section steel products of the present invention, the limit upsetting ratios were all greater than 55%. On the other hand, for the comparative examples, which are indicated by filled circle and filled triangle marks and which do not satisfy the conditions for the finished section steel products of the present invention, the limit upsetting ratios were all less than 55%.

From these results, it is clear that by controlling the properties of the semi-finished long steel product before spheroidizing annealing, it is possible to make the equivalent circle diameter of cementite particles in the finished long steel product after spheroidizing annealing satisfy the conditions of formulas (2) and (3), and long steel with excellent cold forgeability can be obtained. At the same time, from these results, it is clear that when the equivalent circle diameter of cementite particles in the finished long steel product after spheroidizing annealing satisfies both the conditions of formulas (2) and (3), long steel with excellent cold forgeability can be obtained.

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

Claims (12)

In 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 to 0.25%, and N: 0.0020 to 0.0250%
The balance is composed of Fe and unavoidable impurities,
When the shapes of all cementite particles observed in m observation fields (where m is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction are approximated as an ellipse, the length of the major axis is _dL and the length of the minor axis is _dS , and an aspect ratio obtained by dividing the length of the major axis _dL by the length of the minor axis _dS for a specific cementite particle is 2.0 or less, and the length of the major axis _dL is 10.0 μm or less in all of the m observation fields,
a maximum value of the mass percentage of the Cu content measured in the m observation fields is defined as [Cu] _max and the minimum value of the Cu content measured in the m observation fields is defined as [Cu] _min , and a maximum value of the mass percentage of the Ni content measured in the m observation fields is defined as [Ni] _max and the minimum value of the mass percentage of the Ni content measured in the m observation fields is defined as [Ni] _min , the following formula (1) is satisfied:

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.
The steel section according to claim 1. the observation field located in the peripheral portion is located at the midpoint of a line segment connecting the observation field located in the central portion and the outer periphery of the cross section;
The steel section according to claim 2. the m observation fields of view are composed of one observation field of view located at the center of the cross section and four observation fields of view located at the periphery of the cross section, and an angle between adjacent four line segments passing through the four observation fields located at the periphery of the cross section is 90 degrees;
The steel section according to claim 3. The composition further comprises, 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.
The steel section according to any one of claims 1 to 4. The cross-sectional shape is any one of a circle, an ellipse, a square, and a rectangle.
The steel section according to any one of claims 1 to 5. In 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 to 0.25%, and N: 0.0020 to 0.0250%
The balance is composed of Fe and unavoidable impurities,
A steel bar characterized in that, when the maximum value of the equivalent circle diameters in one observation field of view among all cementite particles observed in n observation fields of view (n is an integer of 2 or more) in a cross section perpendicular to the longitudinal direction is _d1 , _d2 , ..., _dn , the maximum value of the equivalent circle diameters in one observation field of view _{is dmax} and the minimum value of _d1 to _dn is _dmin , the following formulas (2) and (3) are satisfied:

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.
The steel section according to claim 7. the observation field located in the peripheral portion is located at the midpoint of a line segment connecting the observation field located in the central portion and the outer periphery of the cross section;
The steel section according to claim 8. the n observation fields of view are comprised of one observation field of view located at the center of the cross section and four observation fields of view located at the periphery of the cross section, and an angle between adjacent four line segments passing through the four observation fields located at the periphery of the cross section is 90 degrees;
The steel section according to claim 9. The composition further comprises, 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.
The steel section according to any one of claims 7 to 10. The cross-sectional shape is any one of a circle, an ellipse, a square, and a rectangle.
The steel section according to any one of claims 7 to 11.

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