Steel bar and its manufacturing method

Abstract

To provide a bar-shaped steel material excellent in deformation anisotropy in compression processing.SOLUTION: On a straight line passing through a center of an optional cross section orthogonal to a longer direction of a bar-shaped steel material, maximum hardness A (HRB) as a maximum value of the hardness on a first straight line extending from the center to the steel material surface in one direction, maximum hardness B (HRB) as a maximum value of the hardness on a second straight line extending from the center to the steel material surface in a direction opposite to the first straight line, and hardness C (HRB) at the center satisfy a relation in a following formula (1), and average hardness in the optional cross section is 93 HRB or less: |(C-A)-(C-B)|≤4.0 HRB (1).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a rod-shaped alloy steel material for machine structures used as a raw material for machine parts such as gears and shafts in transportation equipment such as automobiles and industrial machinery such as construction machinery. [Background technology]

[0002] To impart strength, wear resistance, and other properties to mechanical parts used in transportation equipment and industrial machinery, bar-shaped steel materials are processed into desired shapes by plastic processing or other methods, followed by heat treatments to harden the surface, such as carburizing, nitriding, or carbonitriding. These mechanical parts are typically made of alloy steels 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, forging processes require an additional step of annealing the material to reduce its deformation resistance, in order to improve its 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] In order to meet the above demands, Patent Documents 1 to 4 propose materials in which the material's structure, average hardness, standard deviation of hardness distribution, and difference between maximum and minimum hardness values ​​are specified. Of these, Patent Document 4 (JP Patent No. 4500246) proposes a steel pipe-shaped material with 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 Documents 1 to 3 (JP Patent Nos. 4448047, 4464862, and 4464864) propose steel materials in which 80% or more of the entire structure is a mixed structure of ferrite and pearlite, in which the average hardness value and standard deviation of the hardness distribution are specified. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4448047 [Patent Document 2] Patent No. 4464862 [Patent Document 3] Patent No. 4464864 [Patent Document 4] Patent No. 4500246 Summary of the Invention [Problem to be solved by the invention]

[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 4 only specifies the maximum and minimum hardness values, but does not specify the directionality of the hardness distribution. Furthermore, Patent Documents 1 to 3 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, the present invention is characterized by providing a steel bar material in which the deformation anisotropy when a compressive stress is applied is further reduced. [Means for solving the problem]

[0008] The present invention relates to a bar-shaped steel material that was completed after extensive investigations to solve the above-mentioned problems, and the details thereof are as follows.

[0009] (1) A bar-shaped steel material characterized in that, on a line passing through the center of any cross section perpendicular to the longitudinal direction of the bar-shaped steel material, a maximum hardness A (HRB) which is the maximum value of hardness on a first line extending from the center in one direction to the surface of the 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 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), and the average hardness in the any cross section is 93 HRB or less. |(CA)-(CB)|≦4.0HRB (1)

[0010] (2) The bar-shaped steel material according to (1) above, characterized in that the grain size of ferrite 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.

[0011] (3) The steel bar according to (1) above, wherein the arbitrary cross-sectional shape is circular or rectangular.

[0012] (4) The steel bar according to (1) above, characterized in that the steel material is any one of SCr, SCM, and Nb-added alloy steel with improved hardenability.

[0013] (5) The bar-shaped steel material according to (1) above, characterized in that the metal structure in the arbitrary cross section has a ratio of ferrite and pearlite structures of 80% or more.

[0014] (6) A method for producing a steel bar according to (1) above, A method for manufacturing a steel bar, comprising rolling a steel billet in a rolling mill to form an intermediate product, and introducing the intermediate product, which has a surface temperature of 1000°C or less, into a finishing rolling mill to perform finish rolling. [Effects of the Invention]

[0015] The present invention can provide a steel bar having reduced deformation anisotropy when compressive stress is applied. Furthermore, since the steel bar of the present invention has excellent deformation anisotropy when compressive stress is applied, cutting after forging can be omitted. Furthermore, since the annealing step before forging can also be omitted, machine parts can be obtained with reduced CO2 emissions and low manufacturing costs. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing a steel bar according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view taken along line aa in FIG. [Figure 3] FIG. 1 is a diagram showing the shapes of test samples cut from bar-shaped steel before and after compressive stress processing. [Figure 4] FIG. 10 is a diagram showing a steel bar according to another embodiment. [Figure 5] FIG. 5 is a cross-sectional view taken along line bb in FIG. 4. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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 aa at an arbitrary position in the longitudinal direction. FIG. 2 is an aa cross section and is a diagram illustrating 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 in 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.

[0018] In the present embodiment, the hardness of the bar-shaped steel material 1 is measured 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. The value obtained by performing a predetermined calculation based on the measured values ​​satisfies the condition of the following formula (1). |(CA)-(CB)|≦4.0HRB (1)

[0019] Here, A is the maximum hardness on the straight line AL, B is the maximum hardness on the straight 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 straight line (excluding the position of the center 3) and selecting the maximum hardness from the measured values. In this embodiment, the hardness is measured at four locations and the maximum hardness is extracted. The difference (CA) between the maximum hardness A and the hardness C and the difference (CB) 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.

[0020] Figure 3 shows the shapes of test sample 5, which is a sample obtained by cutting the bar-shaped steel material 1 in Figure 1 to an arbitrary length, before compressive stress 6 is applied to stress-applied surface 7 along the longitudinal direction of bar-shaped steel material 1, and post-compression processed sample 9 after test sample 5 is compressively deformed by compressive stress 6. In post-compression processed sample 9, surface 4 is deformed into a drum shape due to the application of compressive stress 6. In post-compression processed sample 9, post-processing stress-applied surface 10 to which compressive stress 6 is applied and post-processing stress-receiving surface 11 to which compressive stress 6 is applied differ in shape after processing due to the deformation anisotropy for which formula (1) is an index.

[0021] 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 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 dissimilar 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 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 dissimilar to a circle (for example, elliptical).

[0022] In this way, if the cross-sectional shape changes non-similarly 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 as 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.

[0023] Furthermore, the average hardness of the cross section 2 of the steel bar 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 steel bar 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. The lower limit of the average hardness of the cross section 2 is not particularly limited, but it may be 80 HRB or more.

[0024] 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. Fig. 4 shows a steel bar 12 according to another embodiment. Fig. 5 shows a cross section 13 of the steel bar 12 taken along line bb shown in Fig. 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.

[0025] 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 on 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 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 on the surface in the opposite direction to line AL. The hardness is measured at multiple arbitrary locations on the lines AL and BL, 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, at a distance of D / 4 from each vertex, and the average value is calculated. The cross-sectional shape of the bar-shaped steel material may be a polygon other than a rectangle.

[0026] The steel bars 1 and 12 are made of carbon steel or alloy steel, and preferably made of alloy steel for mechanical structures. Examples of alloy steel for mechanical structures include SCr and SCM. Furthermore, alloy steel with improved hardenability, which contains Nb or the like, is also a preferred material for improving hardenability.

[0027] SCr contains 0.12 mass% or more and 0.48 mass% or less of C, 0.15 mass% or more and 0.35 mass% or less of Si, 0.55 mass% or more and 0.95 mass% or less of Mn, 0.030 mass% or less of P, 0.030 mass% or less of S, 0.25 mass% or less of Ni, 0.85 mass% or more and 1.25 mass% or less of Cr, and the remainder being Fe and unavoidable impurities.

[0028] The SCM contains 0.12 mass% or more and 0.49 mass% or less of C, 0.15 mass% or more and 0.35 mass% or less of Si, 0.30 mass% or more and 1.00 mass% or less of Mn, 0.030 mass% or less of P, 0.030 mass% or less of S, 0.25 mass% or less of Ni, 0.85 mass% or more and 1.50 mass% or less of Cr, and the remainder being Fe and unavoidable impurities.

[0029] The hardenability improved alloy steel contains 0.14 mass% or more and 0.25 mass% or less of C, 0.25 mass% or more and 0.55 mass% or less of Si, 0.25 mass% or more and 1.55 mass% or less of Mn, 0.020 mass% or less (including 0.00 mass%) of P, 0.020 mass% or less of S, 0.20 mass% or less (including 0.00 mass%) of Ni, 1.50 mass% or more and 3.00 mass% or less of Cr, 0.070 mass% or less (including 0.00 mass%) of Nb, and the balance being Fe and unavoidable impurities.

[0030] 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.

[0031] The metal structure of the steel bar 1 or 12 contains a ferrite + pearlite structure, and it is preferable that the ferrite + pearlite structure occupy 80% or more of the cross-sectional area of ​​any cross section of the steel. If it is 80% or more, the hardened structure of the metal structure can be reduced, and the hardness of the steel bar 1 or 12 can be reduced. For example, if the occupancy rate of the ferrite + pearlite structure is 80% or more and the hardness of the steel cross section is 93 HRB or less, the annealing step before forging can be omitted.

[0032] 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. The average grain size number is calculated based on the measurement results, by polishing the cross section of the steel bar 1 or 12, subjecting it to nital etching, and measuring the grain size by observation with an optical microscope. If the grain size of the ferrite in the metal structure has an average grain size number of 8.0 or more and a smaller grain size, the deformation anisotropy of the steel bar 1 or 12 becomes smaller, which is preferable. The larger the average grain size number, the smaller the influence of the hardened structure in the microstructure and the smaller the deformation anisotropy, which is preferable, so the grain size of the ferrite in the metal structure more preferably has an average grain size number of 9.0 or more.

[0033] 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 lengthens the billets and reduces their diameter, resulting in intermediate products. These intermediate products are further rolled continuously by an intermediate mill and a finishing mill (finishing mill). These intermediate products are air-cooled to produce the steel bars 1 and 12.

[0034] The metal structure of the steel bars 1 and 12 is largely dependent on the temperature of the intermediate product in the finishing 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 rolling mill is preferably 1000°C or lower, more preferably 980°C or lower, and particularly preferably 960°C or lower.

[0035] 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. [Example]

[0036] 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.

[0037] First, as an example, SCr420, SCM420, and hardenability improved alloy steel A (steel material A) having the chemical compositions shown in Table 1 were melted, and two steel billets were obtained for each steel type. These steel billets were subjected to continuous rolling using a roughing mill, an intermediate mill, and a finishing mill to obtain intermediate products. The surface temperature of the intermediate products immediately before being introduced into the finishing 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.

[0038] 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).Otherwise, the same procedures as in Example 1 were carried out to obtain steel bar 1.

[0039] [Table 1]

[0040] 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. The Rockwell hardness (HRB) of the cross section of this test piece was measured at the center 3 shown in Figure 2. The Rockwell hardness (HRB) was also measured at four points on two arbitrary lines passing through the center 3, lines AL and BL. The maximum hardness A on line AL and the maximum hardness B on 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, as shown in Equation (1). The anisotropy was then verified from the results. Furthermore, assuming that the diameter of the cross section 2 of the test piece is D, the hardness (HRB) was measured at four points along two perpendicular lines passing through the center 3, from the surface 4 to a position D / 4. The arithmetic average of the hardness measurements 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%.

[0041] 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 steel test pieces were 4.0 HRB or less, satisfying the conditions, but in the comparative examples, all the results exceeded 4.0 HRB.

[0042] Furthermore, to confirm the actual deformation anisotropy due to stress application, the deformation amounts on the lines AL and BL after the test specimens were compressed were measured. Specifically, the lengths of each line were measured from the center position of the cross section of the test specimens after compression, and the difference F (= |length of line AL after compression - length of line BL after compression|) was calculated. The lines AL and BL used to calculate the difference F were the same lines as the lines AL and BL used to measure the hardness used in equation (1). As a result, as shown in Table 2, in the examples, the difference F between the lengths was 0.5 mm or less, which was within the normal dimensional tolerance for metal pressed products specified in JIS B 0408. However, in the comparative examples, some exceeded the dimensional tolerance of 1 mm, and even if they did not exceed 1 mm, the difference F was larger than that of the examples and closer to 1 mm.

[0043] Therefore, it was confirmed that in the Examples, the deformation anisotropy when compressive stress was applied was sufficiently reduced, making it possible to omit cutting after forging. On the other hand, in the Comparative Examples, the dimensional tolerance was large, 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 Examples, confirming that the annealing step before forging could be omitted. In all Comparative Examples, the average cross-sectional hardness was greater than 93 HRB.

[0044] [Table 2] [Industrial Applicability]

[0045] The steel bar of the present invention is an optimum steel material for various machine parts that achieve carbon neutrality and reduced manufacturing costs. [Explanation of symbols]

[0046] 1, 12 Steel bars 2. Cross section 3 Center (of bar steel cross section) 4. Surface (of steel bars) 5. Test Samples 6 Compressive stress 7 Stress application surface 8 Stress-bearing surface 9. Sample after compression processing 10 Stressed surface after processing 11 Stress-bearing surface after processing 13 Cross Section 14 Diagonal AL, BL: A straight line passing through the center of a cross section of a steel bar

Claims

1. The chemical composition is 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, 0.030% by mass or less of P, 0.030% by mass or less of S, 0.25% by mass or less of Ni, 0.85% by mass or more and 1.25% by mass or less of Cr, with the remainder being Fe and unavoidable impurities, or 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, 0.020% by mass or less (including 0.00% by mass), 0.020% by mass or less of S, 0.20% by mass or less (including 0.00% by mass) of Ni, 1.50% by mass or more and 3.00% by mass or less of Cr, 0.070% by mass or less of 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 steel material in one direction on a line passing through the center of any cross section perpendicular to the longitudinal direction of the rod-shaped steel material, the balance being Fe and unavoidable impurities; 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 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); the average hardness in the any cross section is 93 HRB or less; and the metal structure in the any cross section has an occupancy rate of ferrite and pearlite structures of 80% or more. |(C-A)-(C-B)|≦4.0HRB (1)

2. 2. The bar-shaped steel material for forging according to claim 1, wherein the grain size of ferrite 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.

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

4. A method for producing a steel bar for forging according to any one of claims 1 to 3, A method for producing a steel bar for forging, comprising rolling a steel slab in a rolling mill to form an intermediate product, and introducing the intermediate product, which has a surface temperature of 1000°C or less, into a finishing rolling mill to perform finish rolling.

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