Steel bar and method for producing same

Abstract

Provided is a steel bar excellent in deformation anisotropy in compression processing. In this steel bar, an arbitrary cross-section is set orthogonal to the longitudinal direction of the steel bar, and, on a straight line passing through the center of the cross-section, a maximum hardness A (HRB), which is the maximum value of the hardness on a first straight line extending from the center to the surface of the steel bar in one direction, a maximum hardness B (HRB), which is the maximum value of the hardness on a second straight line extending from the center to the surface of the steel bar in a direction opposite to the first straight line, and a hardness C (HRB) at the center satisfy formula (1): ｜(C-A)-(C-B)｜≦4.0 HRB (1), and the average hardness in the arbitrary cross-section is 93 HRB or less.

Description

Steel bar and its manufacturing method

[0001] The present disclosure relates to a rod-shaped alloy steel material for mechanical structures, which is used as a material for mechanical parts such as gears and shafts in transportation equipment such as automobiles and industrial machinery such as construction machinery.

[0002] To impart strength, wear resistance, and the like to mechanical parts used in transportation equipment and industrial machinery, bar-shaped steel materials are processed into desired shapes by plastic processing or the like, and then subjected to heat treatments such as carburizing, nitriding, and carbonitriding to harden the surface. These mechanical parts are typically made of alloy steel materials for mechanical structures, such as chromium steel (SCr), chromium-molybdenum steel (SCM), and nickel-chromium-molybdenum steel (SNCM). Furthermore, machining processes such as forging and cutting are used in the forming process, which is a pre-processing step before the heat treatment. In particular, in forging, a process of annealing the material to reduce its deformation resistance is added before processing in order to improve the material's compressive deformability.

[0003] However, in recent years, efforts to achieve carbon neutrality and reduce manufacturing costs have led to attempts to omit the pre-processing annealing. Furthermore, as mentioned above, the main methods for forming materials into desired shapes are forging and cutting, which apply compressive stress to the material. However, even in these forming processes, process elimination is desired to further achieve carbon neutrality and reduce manufacturing costs. In particular, cutting is performed on processing areas that could not be processed to the required dimensional accuracy in forging, but if the dimensional accuracy in the forging process can be improved, the cutting process can be omitted. Therefore, low deformation anisotropy of the material when compressive stress, which is the main component of forging, is also a desired characteristic of steel.

[0004] To meet the above demands, materials have been proposed in which the material's structure, average hardness, standard deviation of hardness distribution, difference between maximum and minimum hardness values, etc. are specified. For example, Patent Document 1 (Japanese Patent No. 4500246) proposes a steel pipe-shaped material having a mixed structure of ferrite and pearlite, in which the average hardness value and the difference between maximum and minimum hardness values ​​are specified. Furthermore, Patent Document 2 (Japanese Patent No. 4448047), Patent Document 3 (Japanese Patent No. 4464862), and Patent Document 4 (Japanese Patent No. 4464864) propose steel materials in which 80% or more of the entire structure is a mixed structure of ferrite and pearlite, and the average hardness value and standard deviation of the hardness distribution are specified.

[0005] Patent No. 4500246 Patent No. 4448047 Patent No. 4464862 Patent No. 4464864

[0006] The materials disclosed in Patent Documents 1 to 4 have a mixed metal structure of ferrite and pearlite, which reduces the average hardness, making it possible to omit annealing before forging. However, they are unable to reduce the deformation anisotropy of the material when compressive stress is applied during forging. Patent Document 1 only specifies the maximum and minimum hardness values, but does not specify the directionality of the hardness distribution. Furthermore, Patent Documents 2 to 4 specify the standard deviation, which is the variation in hardness distribution, but do not specify the directionality of the variation, making it impossible to control the deformation anisotropy when compressive stress is applied.

[0007] Therefore, an object of the present invention is to provide a steel bar material that further reduces the anisotropy of deformation when a compressive stress is applied.

[0008] The present disclosure relates to a bar-shaped steel material that was completed after extensive investigations to solve the above-mentioned problems, and specifically provides the following aspects: [Aspect 1] A bar-shaped steel material, wherein, on a line that passes through the center of an arbitrary cross section that is perpendicular to the longitudinal direction of the bar-shaped steel material, a maximum hardness A (HRB) that is the maximum value of hardness on a first line that extends from the center in one direction to the surface of the bar-shaped steel material, a maximum hardness B (HRB) that is the maximum value of hardness on a second line that extends from the center in a direction opposite to the first line to the surface of the bar-shaped steel material, and a hardness C (HRB) at the center satisfy the relationship of the following formula (1): |(C-A)-(C-B)|≦4.0 HRB (1), and the average hardness in the arbitrary cross section is 93 HRB or less. [Aspect 2] The steel bar according to Aspect 1, wherein the ferrite grain size in the metal structure of the arbitrary cross section is 8.0 or more in terms of average grain size number measured in accordance with JIS G 0551:2020. [Aspect 3] The steel bar according to Aspect 1 or 2, wherein the shape of the arbitrary cross section is circular or rectangular. [Aspect 4] The steel bar according to any one of Aspects 1 to 3, wherein the steel bar is made of any one selected from the group consisting of chromium steel (SCr), chromium-molybdenum steel (SCM), and Nb-added hardenability improved alloy steel. [Aspect 5] The steel bar according to any one of Aspects 1 to 4, wherein the metal structure in the arbitrary cross section has an occupancy ratio of ferrite and pearlite structures of 80% or more. [Aspect 6] A method for producing a steel bar according to any one of Aspects 1 to 5, comprising: rolling a steel slab with a rolling mill to form an intermediate product; and introducing the intermediate product, having a surface temperature of 1000°C or less, into a finishing rolling mill to perform finish rolling.

[0009] According to the present invention, it is possible to provide a steel bar having reduced deformation anisotropy when compressive stress is applied. Since the steel bar of the present invention has excellent deformation anisotropy when compressive stress is applied, cutting after forging can be omitted. In addition, since the annealing step before forging can also be omitted, it is possible to reduce the CO ２ It is possible to obtain machine parts with reduced emissions and low manufacturing costs.

[0010] Fig. 4 is a diagram showing a steel bar according to an embodiment. Fig. 5 is a diagram showing a cross section along the line a-a in Fig. 1. Fig. 6 is a diagram showing the shapes of a test sample cut from a steel bar before and after compressive stress processing. Fig. 7 is a diagram showing a steel bar according to another embodiment. Fig. 8 is a diagram showing a cross section along the line bb in Fig. 4.

[0011] This embodiment will be described with reference to the drawings. FIG. 1 shows one embodiment of a steel bar 1, which is a bar-shaped steel material. The steel bar 1 has a long shape, and FIG. 2 shows a cross section 2 taken along line a-a at an arbitrary position in the longitudinal direction. FIG. 2 is an a-a cross section and illustrates the position at which hardness of the cross section is measured in this embodiment. The cross section 2 is a cross section perpendicular to the longitudinal direction of the steel bar 1. A line AL, which is a first line, and a line BL, which is a second line, shown in the cross section 2 of FIG. 2 are on an arbitrary line passing through the center 3 of the circular cross section 2, and extend in opposite directions from the center 3 toward the surface 4 of the steel bar 1.

[0012] In the bar-shaped steel material 1 of this embodiment, hardness measurements are performed at the center 3 of an arbitrary cross section 2 perpendicular to the longitudinal direction of the bar-shaped steel material 1, and at arbitrary positions on the line AL and the line BL, and the value obtained by performing a predetermined calculation based on the measured values ​​satisfies the condition of the following formula (1): |(C-A)-(C-B)|≦4.0HRB (1).

[0013] Here, A is the maximum hardness on the line AL, B is the maximum hardness on the line BL, and C is the hardness at the center 3. The hardnesses A, B, and C are all Rockwell hardness (HRB) (JIS Z2245). The maximum hardness A of AL and the maximum hardness B of BL can be determined by measuring the hardness at any multiple locations on each linear portion (excluding the position of the center 3) and selecting the maximum hardness from the measured values. In this embodiment, hardness is measured at four locations and the maximum hardness is extracted. The difference (C-A) between the maximum hardness A and the hardness C and the difference (C-B) between the maximum hardness B and the hardness C are calculated using the above formula (1), and the deformation anisotropy during compressive deformation of the bar-shaped steel material 1 can be evaluated by calculating the absolute value of the difference between these differences.

[0014] 3 shows the shapes of a test sample 5, which is a sample obtained by cutting the steel bar 1 of FIG. 1 to an arbitrary length, before a compressive stress 6 is applied to a stress-applying surface 7 along the longitudinal direction of the steel bar 1, and a post-compression sample 9 after the test sample 5 has been compressed and deformed by the compressive stress 6. In the post-compression sample 9, the surface 4 has been deformed into a drum-like shape due to the application of the compressive stress 6. In the post-compression sample 9, the post-processing stress-applied surface 10 to which the compressive stress 6 has been applied and the post-processing stress-receiving surface 11 that has received the compressive stress 6 have different shapes after processing due to the deformation anisotropy for which formula (1) is an index.

[0015] The smaller the deformation anisotropy of the steel material, the closer to similar the shapes of the post-processing stress-applied surface 10 and post-processing stress-receiving surface 11 to which the compressive stress 6 has been applied are to the shapes of the stress-applied surface 7 and the stress-receiving surface 8 of the test sample 5 before processing, respectively. On the other hand, the larger the deformation anisotropy of the steel material, the more partially non-similar the shapes of the post-processing stress-applied surface 10 and post-processing stress-receiving surface 11 of the compressively processed sample 9 become compared with the stress-applied surface 7 and the stress-receiving surface 8 of the test sample 5, respectively. In other words, when the deformation anisotropy is larger, if the cross-sectional shape of the test sample 5 is circular, the post-processing stress-applied surface 10 and post-processing stress-receiving surface 11 of the compressively processed sample 9 will become shapes non-similar to a circle (for example, elliptical).

[0016] In this way, if the cross-sectional shape changes to a non-similar shape before and after the application of compressive stress 6, the dimensional accuracy will deteriorate when the machine part is formed by forging, and cutting will be necessary after the forging process, making it difficult to omit the cutting process. In contrast, the steel material of this embodiment, which has a value of 4.0 HRB or less, which is an index of deformation anisotropy, and satisfies the conditions of formula (1), can maintain the desired dimensional accuracy even after the application of compressive stress, as will be described later in the examples. Therefore, it is possible to omit the cutting process after the forging process.

[0017] Furthermore, the average hardness of the cross section 2 of the bar-shaped steel material 1 of this embodiment is 93 HRB or less in Rockwell hardness. By setting the average hardness of the cross section 2 to 93 HRB or less, the annealing step before forging can be omitted. The average hardness is calculated by measuring hardness at any position on the cross section 2. Specifically, in any randomly determined cross section 2 perpendicular to the longitudinal direction of the bar-shaped steel material 1 (for example, this may be the same surface as the cross section 2 evaluated by Equation (1) or a different surface), where the diameter is D, the hardness (HRB) is measured at four points located at D / 4 from the surface 4. The four points are four D / 4 positions on two perpendicular straight lines passing through the center 3. The arithmetic mean of the measured values ​​at these four points is the average hardness of the cross section of this embodiment. Note that the lower limit of the average hardness of the cross section 2 is not particularly limited, but it may be 80 HRB or more.

[0018] In the above embodiment, the case of a steel bar 1 having a circular cross section perpendicular to the longitudinal direction has been described, but steel bars having other cross-sectional shapes may also be used. Figure 4 shows a steel bar 12 according to another embodiment. Figure 5 shows a cross section 13 of the steel bar 12 taken along line b-b shown in Figure 4. The cross section 13 is a cross section perpendicular to the longitudinal direction of the steel bar 12, and has a rectangular shape. The position of the center 3 in such a rectangular cross section 13 can be the point where diagonals 14 intersect.

[0019] In the case of a steel bar 12 having a rectangular cross section 13, the line used to calculate the hardness in Equation (1) may be any line that passes through the center 3 where the diagonal line 14 intersects and extends to the surface of the steel. The arbitrary line may be, for example, a diagonal line passing through the center 3, or a line other than a diagonal line passing through the center 3. Figure 5 shows an example in which the arbitrary line is the diagonal line 14. The hardness of this line is measured in the same manner as for steel having a circular cross section. That is, the hardness is measured at the center 3, on a line AL extending from the center 3 to the surface, and on a line BL extending to the surface in the opposite direction to the line AL. For the portions of the lines AL and BL, the hardness is measured at multiple arbitrary locations, and the maximum hardness A on AL and the maximum hardness B on BL are calculated. The average hardness of the cross section 13 may also be calculated in the same manner as for the circular cross section 2. Specifically, when the length of the diagonal in Fig. 5 is D, the hardness is measured at four points on the two diagonal lines, which are D / 4 from each vertex, and the average value is calculated. The cross-sectional shape of the steel bar may be a polygon other than a rectangle.

[0020] The steel bars 1 and 12 are made of carbon steel or alloy steel, preferably mechanical structural alloy steel. Examples of mechanical structural alloy steel include chromium steel (SCr) and chromium-molybdenum steel (SCM). A preferred example of an alloy steel with improved hardenability is an improved hardenability alloy steel with added Nb or the like.

[0021] Chromium steel (SCr) contains 0.12% by mass or more and 0.48% by mass or less of C, 0.15% by mass or more and 0.35% by mass or less of Si, 0.55% by mass or more and 0.95% by mass or less of Mn, 0% by mass or more and 0.030% by mass or less of P, 0% by mass or more and 0.030% by mass or less of S, 0% by mass or more and 0.25% by mass or less of Ni, and 0.85% by mass or more and 1.25% by mass or less of Cr, with the remainder being Fe and unavoidable impurities.

[0022] Chromium-molybdenum steel (SCM) contains 0.12 mass% to 0.49 mass% C, 0.15 mass% to 0.35 mass% Si, 0.30 mass% to 1.00 mass% Mn, 0 mass% to 0.030 mass% P, 0 mass% to 0.030 mass% S, 0 mass% to 0.25 mass% Ni, 0.85 mass% to 1.50 mass% Cr, and 0.15 mass% to 0.45 mass% Mo, with the remainder being Fe and unavoidable impurities.

[0023] The hardenability improved alloy steel contains 0.14% by mass or more and 0.25% by mass or less of C, 0.25% by mass or more and 0.55% by mass or less of Si, 0.25% by mass or more and 1.55% by mass or less of Mn, 0% by mass or more and 0.020% by mass or less of P, 0% by mass or more and 0.020% by mass or less of S, 0% by mass or more and 0.20% by mass or less of Ni, 1.50% by mass or more and 3.00% by mass or less of Cr, and 0% by mass or more and 0.070% by mass or less of Nb, with the remainder being Fe and unavoidable impurities.

[0024] The impurities in the chemical composition are components that are mixed in from components contained in raw materials or the manufacturing environment when steel is industrially manufactured, and are components that are acceptable within a range that does not adversely affect the steel according to this embodiment.

[0025] The metal structure of the steel bars 1 and 12 contains ferrite and pearlite structures (ferrite + pearlite structures), and it is preferable that the occupancy rate of the ferrite and pearlite structures in the cross-sectional area of ​​any cross section of the steel material is 80% or more. This is because if the occupancy rate is 80% or more, the hardened structure of the metal structure can be reduced, and the hardness of the steel bars 1 and 12 can be reduced. For example, if the occupancy rate of the ferrite and pearlite structures is 80% or more and the hardness of the steel material cross section is 93 HRB or less, the annealing step before forging can be omitted.

[0026] The grain size of the ferrite in the structure preferably has an average grain size number of 8.0 or more as measured in accordance with JIS G 0551:2020. The average grain size number is calculated based on the measurement results by polishing the cross section of the steel bar 1 or 12, then subjecting it to Nital etching, and measuring the grain size through observation with an optical microscope. A grain size of the ferrite in the metal structure with an average grain size number of 8.0 or more and a smaller grain size is preferable because it reduces the deformation anisotropy of the steel bar 1 or 12. A larger average grain size number is preferable because it reduces the influence of the hardened structure in the microstructure and reduces the deformation anisotropy, so it is more preferable for the grain size of the ferrite in the metal structure to have an average grain size number of 9.0 or more.

[0027] The steel bars 1 and 12 are produced by rolling, typically by hot rolling. In this production method, steel billets are obtained through processes such as refining, ingot making, and blooming. These billets are heated in a heating furnace and subjected to continuous rolling by a roughing mill. This rolling process elongates the billets and reduces their diameter, resulting in intermediate products. These intermediate products are then continuously rolled by an intermediate mill and a finishing mill (finishing mill). These intermediate products are air-cooled to produce the steel bars 1 and 12.

[0028] The metal structure of the steel bars 1 and 12 is largely dependent on the temperature of the intermediate product in the finishing train rolling mill after rolling by a rolling mill such as a roughing mill. By setting this temperature relatively low, steel bars 1 and 12 having a large average grain size number and small anisotropy in hardness distribution can be obtained. Therefore, the surface temperature of the intermediate product when (immediately before) it is introduced into the finishing train rolling mill is preferably 1000°C or lower, more preferably 980°C or lower, and particularly preferably 960°C or lower. The lower limit of the surface temperature of the intermediate product when (immediately before) it is introduced into the finishing train rolling mill is not particularly limited, but is typically 800°C or higher, more typically 850°C or higher.

[0029] According to the present embodiment, the hardness of the cross section of the steel bar satisfies the condition of the above formula (1) and the average hardness of the cross section is equal to or less than the predetermined hardness, so that a steel bar having smaller deformation anisotropy when compressive stress is applied can be provided. Therefore, by using the steel bar of this embodiment, it is possible to omit cutting after forging.

[0030] Hereinafter, the embodiments will be described in more detail with reference to examples, but the scope of the disclosure of this specification should not be construed as being limited based on the description of these examples.

[0031] First, as an example, SCr420, SCM420, or hardenability improved alloy steel A (steel material A) having the compositions shown in Table 1 was melted, and two steel billets were obtained for each steel type. These steel billets were subjected to continuous rolling using a roughing train rolling mill, an intermediate train rolling mill, and a finishing train rolling mill to obtain intermediate products. The surface temperature of one of the intermediate products immediately before being introduced into the finishing train rolling mill was 1000°C for one of the steel billets ((a) for each steel type in Table 2 below) and 960°C for the other steel billet ((b) for each steel type in Table 2 below). These intermediate products were air-cooled, and rod-shaped steel materials 1 having a circular cross section and a diameter Di of 30 mm were obtained for each steel type.

[0032] As a comparative example, two steel billets were prepared for each steel type in the same manner as in the examples, and finish rolling was performed by setting the surface temperature of the intermediate product immediately before being introduced into the finishing train rolling mill to 1100°C for one steel billet ((a) for each steel type in Table 2) and 1150°C for the other steel billet ((b) for each steel type in Table 2), but the rest was done in the same manner as in Example 1 to obtain steel bar 1.

[0033]

[0034] A steel bar 1 was machined to obtain a cylindrical test piece. This test piece had a diameter of 14 mm and a height of 21 mm. For the cross section of this test piece, the Rockwell hardness (HRB) was measured at the center 3 shown in FIG. 2 , and at four points on each of the lines AL and BL, which are arbitrary lines passing through the center 3. The maximum hardness A on the line AL and the maximum hardness B on the line BL were extracted from the measurements, and the absolute values ​​of the hardness distribution on each line were calculated based on the difference from the hardness C at the center 3 shown in Equation (1). Anisotropy was then verified from the results. Furthermore, when the diameter of the cross section 2 of the test piece was D, the hardness (HRB) was measured at four points at a position D / 4 from the surface 4 on two perpendicular lines passing through the center 3, and the arithmetic mean value of the hardness was calculated as the cross-sectional average hardness (HRB). Thereafter, the test piece was compressed in the height direction with its end faces restrained at an upsetting ratio (= [(height before processing) - (height after processing)] / (height before processing)) of 60%.

[0035] The Rockwell hardness was measured and the values ​​of formula (1) were calculated, and the results are shown in Table 2. In the examples, the values ​​of formula (1) for all the steel test pieces were 4.0 HRB or less, satisfying the conditions, but in the comparative examples, all the results exceeded 4.0 HRB.

[0036] Furthermore, to confirm the actual deformation anisotropy due to the application of stress, the deformation amounts on the straight lines AL and BL after the test specimens were compressed were confirmed. Specifically, the lengths of each straight line were measured from the center position of the cross section of the test specimens after compression, and the difference F (=|(length of straight line AL after processing) - (length of straight line BL after processing)|) was calculated. The straight lines AL and BL used to calculate the difference F are straight lines at the same positions as the straight lines AL and BL used to measure the hardness used in formula (1). As a result, as shown in Table 2, in the examples, the difference F between the respective lengths was 0.5 mm or less, which was within the normal dimensional tolerance for metal pressed products in JIS B 0408, while in the comparative examples, the difference F exceeded the dimensional tolerance of 1 mm, and even if it did not exceed 1 mm, it was larger than that of the examples and closer to 1 mm.

[0037] Therefore, it was confirmed that the deformation anisotropy when compressive stress was applied was sufficiently reduced in the examples, making it possible to omit cutting after forging. On the other hand, the dimensional tolerances were large in the comparative examples, making cutting necessary. Furthermore, the average grain size number of ferrite was 8.0 or more in the examples, but less than 8.0 in the comparative examples. Furthermore, the average cross-sectional hardness before compression was 93 HRB or less in all the examples, confirming that the annealing step before forging could be omitted. The average cross-sectional hardness of all the comparative examples was greater than 93 HRB.

[0038]

[0039] The steel bar of the present invention is an optimum steel material for various machine parts that achieve carbon neutrality and reduced manufacturing costs.

Claims

1. A bar-shaped steel material, in which a maximum hardness A (HRB) which is the maximum value of hardness on a first line extending from the center to the surface of the bar-shaped steel material in one direction on a line passing through the center of the cross section set in any cross section perpendicular to the longitudinal direction of the bar-shaped steel material, a maximum hardness B (HRB) which is the maximum value of hardness on a second line extending from the center to the surface of the bar-shaped steel material in the opposite direction to the first line, and a hardness C (HRB) at the center satisfy the relationship of the following formula (1): |(C-A)-(C-B)|≦4.0HRB (1), and the average hardness at the any cross section is 93 HRB or less.

2. The bar steel material according to claim 1, wherein the crystal grain size of ferrite in the metal structure of any cross section is 8.0 or more in terms of average grain size number measured in accordance with JIS G 0551:2020.

3. The bar steel material according to claim 1, wherein the arbitrary cross-sectional shape is circular or rectangular.

4. The bar-shaped steel material according to claim 1, wherein the bar-shaped steel material is made of any one selected from the group consisting of chromium steel (SCr), chromium molybdenum steel (SCM), and alloy steel with improved hardenability to which Nb has been added.

5. The bar steel material according to claim 1, wherein the metal structure in any cross section has a ratio of ferrite and pearlite structures of 80% or more.

6. A method for producing a steel bar according to any one of claims 1 to 5, comprising: rolling a steel slab with a rolling mill to form an intermediate product; and introducing the intermediate product, the surface temperature of which is 1000°C or less, into a finishing rolling mill to perform finish rolling.

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