Steel material having crack resistance and method for producing same

Abstract

The purpose of the present invention is to provide a steel material in which it is possible to further reduce deformation resistance and the probability of cracks occurring when compressive stress is applied during cold working, and / or to provide a low-cost steel material for which it is possible to omit spheroidizing annealing prior to cold forging due to suppression of occurrences of cracks, low deformation resistance, and excellent workability. In order to achieve said purpose, the present invention provides a steel material that is for cold forging working and that comprises machine structural steel, wherein when cold compression working is performed on the steel material from a compression rate X% to a compression rate of Y%, the ratio ΔP / ΔL of the change amount ΔP (MPa) of the compressive stress to the compressive deformation amount ΔL (mm) is 30 MPa / mm or less.

Description

Crack-resistant steel material and method for manufacturing the same

[0001] The present invention relates to steel materials such as steel bars, which are suitable for cold forging and are made of steel materials with excellent workability, such as steel for machine structural use, and to a method for manufacturing said steel materials.

[0002] Patent Document 1 describes a steel that satisfies the following mass%, C: 0.1-0.35%, Si: 0.03-1.0%, Mn: 0.2-20%, S: 0.1% or less (including 0%), Nb: 0.025-0.20%, Ti: 0.025-0.12%, N: 0.020% or less (including 0%), Al: 0.13% or less (including 0%), with the remainder being substantially Fe, and contains 2.0 × 10 carbides and / or carbonitrides in its cross-section that satisfy the formula: (Ti) / (Nb) ≥ 0.05 [wherein (Ti) and (Nb) represent the respective content (mass%) of Ti and Nb in the carbides and / or carbonitrides]. ７ pieces / mm ２ Disclosed is a case-hardening steel that has excellent resistance to grain coarsening and cold workability, and can be used without softening annealing, characterized by having the above properties, an average Vickers hardness value of 180 or less in the cross-section, and a maximum standard deviation of Vickers hardness of 5 or less.

[0003] Patent Document 2 states that, in mass%, C: 0.05 to 0.35%, Si: 0.3% or less, Mn: 0.15 to 1.8%, P: 0.015% or less (including 0%), S: 0.02% or less (excluding 0%), Cr: 0.01 to 0.5%, sol. Disclosed is a linear or bar-shaped steel that satisfies the following conditions: Al: 0.01-0.06%, N: 0.0005-0.006%, B: 0.0003-0.0015%, Ti: 0.003-0.030%, where N, B, and Ti satisfy the formula: -0.0060 ≤ [N] - 1.3 × [B] - 0.29 × [Ti] ≤ 0.0020, with the remainder being iron and unavoidable impurities. The steel has a mixed structure of ferrite and pearlite, and when the diameter of the steel is D, the grain size number of the ferrite present in the range from the center of the steel to the D / 8 position is 6 to 12. This linear or bar-shaped steel does not require spheroidizing annealing.

[0004] Patent Document 3 discloses a wire that can be manufactured without softening heat treatment, characterized in that, by mass%, it consists of C: 0.2-0.45%, Si: 0.02-0.4%, Mn: 0.3-1.5%, Cr: 0.3-1.5%, Al: 0.02-0.05%, Mo: 0.01-0.5%, N: 0.01% or less, with the remainder being Fe and other unavoidable impurities, and has a microstructure in which, by area%, the protoprecipitation ferrite fraction is 40% or more of the equilibrium phase, the regenerated ferrite and bainite fraction is 40% or more, and the martensite fraction is 20% or less, and the average size of pearlite colonies in the region from 2 / 5 to 3 / 5 of the diameter from the surface is 5 μm or less.

[0005] Japanese Patent Publication No. 2006-307271, Japanese Patent Publication No. 2009-242916, Japanese Patent Publication No. 2022-512514

[0006] Patent Document 1 focuses on achieving both grain coarsening prevention and cold workability, but it lacks accuracy in evaluating crack initiation and microstructure uniformity, making it insufficient as an evaluation of workability.

[0007] Patent Document 2 focuses on the small variation in hardness, but it does not accurately evaluate crack initiation and structural uniformity, and is therefore insufficient as an evaluation of machinability.

[0008] Patent Document 3 optimizes the average size of pearlite colonies and the microstructure before spheroidizing annealing, but it does not allow for the omission of spheroidizing annealing, nor does it consider crack resistance.

[0009] Therefore, an object of the present invention is to provide steel materials such as steel bars made from machine structural steel, for example, steel bars made from chromium steel (SCr), chromium-molybdenum steel (SCM), and hardenability-enhancing alloy steel with added Nb, which can further reduce deformation resistance and crack occurrence probability when compressive stress is applied during cold working, as well as a method for manufacturing said steel materials. Another object of the present invention is to provide low-cost steel materials that have suppressed crack occurrence, low deformation resistance, and excellent workability, thereby eliminating the need for spheroidizing annealing treatment before cold forging, as well as a method for manufacturing said steel materials.

[0010] To solve the above problems, the present invention provides the following invention: [1] A steel material for cold forging made of machine structural steel, wherein when the steel material is cold compressed from a compressibility of X% to a compressibility of Y%, the ratio ΔP / ΔL of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm) is 30 MPa / mm or less, and X and Y are arbitrary values ​​that satisfy equations (1) and (2): 0 ≤ X < Y ... (1) 10 ≤ Y - X ≤ 60 ... (2) [2] The steel material is a steel bar having a diameter D, wherein the total area fraction A of ferrite and pearlite at the D / 4 position ４ The total area fraction A of ferrite and pearlite at position D / 2 ２ Ratio A ４ / A ２ [1] A steel material according to [1], wherein the ratio is 0.80 to 1.10. [3] A steel material according to [1], wherein the average ferrite particle size is 10 to 40 μm. [4] A steel material according to [2], wherein the average ferrite particle size is 10 to 40 μm. [5] A method for manufacturing a steel material according to any one of [1] to [4], comprising the following steps: rolling a steel billet made of machine structural steel in 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 and performing finishing rolling.

[0011] The steel materials described in any one of items [1] to [4] are steel materials such as steel bars that are suitable for cold forging.

[0012] In the manufacturing method described in [5], the surface temperature of the intermediate product introduced into the finishing rolling mill is preferably 850 to 1000°C, more preferably 850 to 980°C, and even more preferably 850 to 960°C. That is, the manufacturing method described in [5] is a method for manufacturing steel according to any one of [1] to [4], comprising the following steps: a step of rolling a steel billet made of machine structural steel in a rolling mill to form an intermediate product; and a step of introducing the intermediate product having a surface temperature of preferably 850 to 1000°C, more preferably 850 to 980°C, and even more preferably 850 to 960°C into a finishing rolling mill and performing finishing rolling.

[0013] In one embodiment, the steel material produced by the manufacturing method described in [5] is the steel material described in [1]. In another embodiment, the steel material produced by the manufacturing method described in [5] is the steel material described in [2]. In yet another embodiment, the steel material produced by the manufacturing method described in [5] is the steel material described in [3]. In yet another embodiment, the steel material produced by the manufacturing method described in [5] is the steel material described in [4]. These steel materials are steel bars and the like, suitable for cold forging.

[0014] The steel material of the present invention is a steel bar suitable for cold forging, which further reduces deformation resistance and crack occurrence probability when compressive stress is applied during cold working. Because the steel material of the present invention has excellent cold workability, it can be suitably applied to cold working without spheroidizing annealing or cold working without normalizing. Therefore, if the steel material of the present invention is used, for example, in drivetrain components of automobiles manufactured by cold working, it is possible to reduce manufacturing costs.

[0015] The manufacturing method of the present invention is suitable for manufacturing the steel materials of the present invention. According to the manufacturing method of the present invention, steel materials such as steel bars with excellent cold workability can be obtained, and the obtained steel materials can be suitably applied to cold working without spheroidizing annealing treatment or cold working without normalizing treatment. For this reason, steel materials obtained by the manufacturing method of the present invention are suitable for cold forging.

[0016] Figure 1 is a schematic diagram illustrating the cross-section perpendicular to the rolling direction, the cross-section parallel to the rolling direction, the D / 4 position, and the D / 2 position, using a steel bar as an example.

[0017] The present invention will be described below.

[0018] The steel material of the present invention is a steel material for cold forging made of machine structural steel, wherein when the steel material is cold compressed from a compressibility of X% to a compressibility of Y%, the ratio of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm) ΔP / ΔL is 30 MPa / mm or less, and X and Y are arbitrary values ​​that satisfy equations (1) and (2): 0 ≤ X < Y ... (1) 10 ≤ Y - X ≤ 60 ... (2). The steel material of the present invention can be suitably used in cold forging.

[0019] In one embodiment, the steel material of the present invention is a steel bar (a bar-shaped steel material) having a diameter D. The steel bar having a diameter D is a steel material in which the shape of its cross-section (a cross-section perpendicular to the rolling direction) is circular with a diameter D.

[0020] The steel material of the present invention is made of structural steel for machinery. By manufacturing the steel material using structural steel for machinery according to a predetermined procedure, a steel material having a ΔP / ΔL of 30 MPa / mm or less can be obtained. Examples of structural steel for machinery include alloy steel for machinery described in JIS (Japanese Industrial Standards) G4053:2023. Examples of alloy steel for machinery that are described in JIS include SCr steel and SCM steel. In the present invention, SCr420, SCM420, etc. can be suitably used as structural steel for machinery. The present invention is not limited to steels described in JIS, as long as it satisfies the characteristics of structural steel for machinery, and alloy steels with improved hardenability, as described later, can also be used.

[0021] SCr steel will be explained below. The unit "%" used to indicate the content of each element means mass percent.

[0022] Examples of SCr steel include steels with the following composition by mass%, C: 0.12% to 0.48%, Si: 0.15% to 0.35%, Mn: 0.55% to 0.95%, Cr: 0.85% to 1.25%, Al: 0% to 0.050%, N: 0% to 0.030%, and the remainder being Fe and unavoidable impurities.

[0023] C, Si, Mn, and Cr are essential elements. Al and N are optional elements. SCr steel may contain one or more elements selected from Al and N. The remainder, other than the essential and optional elements, consists of Fe and unavoidable impurities. "Unavoidable impurities" refer to components that are introduced from the raw materials of the steel (e.g., ore, scrap, etc.), the environment of the manufacturing process, etc., and are not components that are intentionally included in the steel. Examples of unavoidable impurities include P, S, Ni, Cu, etc. The unavoidable impurities may contain one or more elements selected from P, S, Ni, and Cu.

[0024] The content rate of P is preferably 0.030% or less. The content rate of P may be 0% or may exceed 0%. The content rate of P may be, for example, 0.005% or more.

[0025] The content rate of S is preferably 0.030% or less. The content rate of S may be 0% or may exceed 0%. The content rate of S may be, for example, 0.005% or more.

[0026] The content rate of Ni is preferably 0.25% or less. The content rate of Ni may be 0% or may exceed 0%. The content rate of Ni may be, for example, 0.03% or more.

[0027] The content rate of Cu is preferably 0.30% or less. The content rate of Cu may be 0% or may exceed 0%. The content rate of Cu may be, for example, 0.03% or more.

[0028] The content rate of Al may be 0% or may exceed 0%. The content rate of Al may be, for example, 0.01% or more.

[0029] The content rate of N may be 0% or may exceed 0%. The content rate of N may be, for example, 0.01% or more.

[0030] Regarding SCM steel or less, SCM steel will be described below. "%" used as the unit of the content rate of each element means mass%.

[0031] Examples of SCM steel include, in mass%, C: 0.12% or more and 0.49% or less, Si: 0.15% or more and 0.35% or less, Mn: 0.30% or more and 1.00% or less, Cr: 0.85% or more and 1.50% or less, Mo: 0.15% or more and 0.45% or less, Al: 0% or more and 0.050% or less, N: 0% or more and 0.030% or less, and the balance: steel consisting of Fe and inevitable impurities.

[0032] C, Si, Mn, Cr, and Mo are essential elements. Al and N are optional elements. SCM steel may contain one or two elements selected from Al and N. The remainder, other than the essential and optional elements, consists of Fe and unavoidable impurities. The meaning of "unavoidable impurities" is as described above. Examples of unavoidable impurities include P, S, Ni, and Cu. The unavoidable impurities may contain one or more elements selected from P, S, Ni, and Cu.

[0033] The P content is preferably 0.030% or less. The P content may be 0% or greater than 0%. For example, the P content may be 0.005% or more.

[0034] The sulfur content is preferably 0.030% or less. The sulfur content may be 0% or greater than 0%. For example, the sulfur content may be 0.005% or more.

[0035] The Ni content is preferably 0.25% or less. The Ni content may be 0% or greater than 0%. For example, the Ni content may be 0.03% or more.

[0036] The Cu content is preferably 0.30% or less. The Cu content may be 0% or greater than 0%. For example, the Cu content may be 0.03% or more.

[0037] The Al content may be 0% or greater than 0%. For example, the Al content may be 0.01% or more.

[0038] The N content may be 0% or greater than 0%. For example, the N content may be 0.01% or more.

[0039] The following section describes alloy steels with improved hardenability. The unit "%" used to indicate the content of each element refers to mass percentage.

[0040] Examples of alloy steels with improved hardenability include steels with the following composition by mass%, C: 0.14% to 0.25%, Si: 0.25% to 0.60%, Mn: 0.20% to 1.55%, Cr: 1.30% to 3.00%, Al: 0% to 0.050%, N: 0% to 0.030%, Nb: 0% to 0.070%, and the remainder being Fe and unavoidable impurities.

[0041] C, Si, Mn, and Cr are essential elements. Al, N, and Nb are optional elements. Alloy steel with improved hardenability may contain one or more elements selected from Al, N, and Nb. The remainder, other than the essential and optional elements, consists of Fe and unavoidable impurities. The meaning of "unavoidable impurities" is as described above. Examples of unavoidable impurities include P, S, Ni, and Cu. The unavoidable impurities may contain one or more elements selected from P, S, Ni, and Cu.

[0042] The P content is preferably 0.020% or less. The P content may be 0% or greater than 0%. For example, the P content may be 0.005% or more.

[0043] The sulfur content is preferably 0.020% or less. The sulfur content may be 0% or greater than 0%. For example, the sulfur content may be 0.005% or more.

[0044] The Ni content is preferably 0.20% or less. The Ni content may be 0% or greater than 0%. For example, the Ni content may be 0.03% or more.

[0045] The Cu content is preferably 0.30% or less. The Cu content may be 0% or greater than 0%. For example, the Cu content may be 0.03% or more.

[0046] The Al content may be 0% or greater than 0%. For example, the Al content may be 0.01% or more.

[0047] The N content may be 0% or greater than 0%. For example, the N content may be 0.01% or more.

[0048] The Nb content may be 0% or greater than 0%. For example, the Nb content may be 0.01% or more.

[0049] ΔP / ΔL: 30 MPa / mm or less. ΔP / ΔL is the ratio of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm) when a steel material is cold-compressed from a compressibility of X% to a compressibility of Y%.

[0050] X and Y are any numbers that satisfy equations (1) and (2): 0 ≤ X < Y ... (1) 10 ≤ Y - X ≤ 60 ... (2).

[0051] It is preferable that X and Y are any numerical values ​​that further satisfy equation (3): Y ≤ 78 ... (3).

[0052] Cold compression can be performed by statically cold compressing a steel material in the longitudinal direction while the end faces are fully constrained. Cold compression can be performed, for example, using a cylindrical test piece (e.g., 14 mm in diameter x 21 mm in length) made from the center of the steel material.

[0053] The compression ratio (%) can be calculated using the formula: Compression ratio = ((Length of steel material before cold compression) - (Length of steel material after cold compression)) / (Length of steel material before cold compression) × 100.

[0054] The change in compressive stress ΔP (MPa) when steel is cold-compressed from a compressibility of X% to a compressibility of Y% can be calculated using the formula: ΔP = (compressive stress at a compressibility of Y%) - (compressive stress at a compressibility of X%). Note that the compressive stress at a compressibility of 0% is 0 MPa.

[0055] The amount of compressive deformation ΔL (mm) when steel is cold-compressed from a compressibility of X% to a compressibility of Y% can be calculated using the formula: ΔL = (length of the steel at a compressibility of X%) - (length of the steel at a compressibility of Y%).

[0056] When cold working is performed using a steel material with ΔP / ΔL exceeding 30 MPa / mm, the probability of crack generation during cold working increases, and the load on the die also increases. Therefore, ΔP / ΔL of the steel material of the present invention is set to 30 MPa / mm or less. That is, when the steel material of the present invention is cold-compressed from a compression ratio of X% to a compression ratio of Y%, for any X and Y satisfying formulas (1) and (2) (preferably formulas (1) to (3)), ΔP / ΔL ≤ 30 MPa / mm is satisfied. ΔP / ΔL is preferably 29 MPa / mm or less. The lower limit value of ΔP / ΔL can be adjusted as appropriate. ΔP / ΔL may be, for example, 25 MPa / m or more. The above lower limit value may be combined with any of the above upper limit values.

[0057] When X is a numerical value A, the fact that ΔP / ΔL when the steel material is cold-compressed from a compression ratio of X% to a compression ratio of Y% is 30 MPa / mm or less means that the maximum value of ΔP / ΔL when Y is changed in the range of (10 + A) to (60 + A) is 30 MPa / mm or less. For example, when X is 0, the fact that ΔP / ΔL when the steel material is cold-compressed from a compression ratio of X% to a compression ratio of Y% is 30 MPa / mm or less means that the maximum value of ΔP / ΔL when Y is changed in the range of 10 to 60 is 30 MPa / mm or less.

[0058] A ４ / A ２ : 0.80 to 1.10 A ４ / A ２ is an index for evaluating the probability of crack generation due to the non-uniformity of the structure. A ４ / A ２ is the ratio of the total area fraction A ４ (%) of ferrite and pearlite at the D / 4 position (the middle part, that is, the part between the surface and the center) to the total area fraction A ２ (%) of ferrite and pearlite at the D / 2 position (the center part).

[0059] "D" refers to the diameter of the steel material. If the cross-section of the steel material is circular, "Diameter" refers to the diameter of that circle. If the cross-section of the steel material is elliptical, "Diameter" refers to the major or minor axis of that ellipse. If the cross-section of the steel material is quadrilateral (square or rectangle), "Diameter" refers to the diameter of the circle circumscribing that quadrilateral. "Cross-section of the steel material" refers to the cross-section perpendicular to the rolling direction of the steel material.

[0060] "D / 4 position" refers to the part of the steel material that is D / 4 depth from the surface. "D / 2 position" refers to the part of the steel material that is D / 2 depth from the surface.

[0061] In one embodiment, the steel material of the present invention is a steel bar having a diameter D. In this embodiment, the cross-sectional shape of the steel material of the present invention is a circle, and "diameter" refers to the diameter of that circle. For the cross-section perpendicular to the rolling direction, the cross-section parallel to the rolling direction, the D / 4 position, and the D / 2 position when the steel material of the present invention is a steel bar, please refer to Figure 1.

[0062] A ４ / A ２ When the ratio falls below 0.80, the probability of cracking due to structural non-uniformity increases. Therefore, the A of the steel material of the present invention ４ / A ２ It is preferable that it be 0.80 or higher, and more preferably 0.90 or higher. ４ / A ２ When this value exceeds 1.10, the probability of cracking due to structural heterogeneity increases. Therefore, A ４ / A ２ It is preferable that the value be 1.10 or less. The above upper limit may be combined with any of the above lower limits.

[0063] A ２ Preferably, it is 55% or more, more preferably 60% or more. The upper limit is 100% or less. The above upper limit may be combined with any of the above lower limits.

[0064] A ４ Preferably, it is 60% or more, more preferably 65% ​​or more. The upper limit is 100% or less. The above upper limit may be combined with any of the above lower limits.

[0065] A２ and A ４ This can be determined by the method described in the examples.

[0066] Average ferrite particle size: 10 to 40 μm If the average ferrite particle size in the steel material is less than 10 μm, the deformation resistance of the steel material will decrease, but the deformability of the steel material will decrease, so the workability will not be sufficient. On the other hand, if the average ferrite particle size in the steel material exceeds 40 μm, the deformation resistance of the steel material will become too high, and the probability of cracking will increase. Therefore, it is preferable that the average ferrite particle size in the steel material of the present invention be 10 to 40 μm. The average ferrite particle size is more preferably 20 to 35 μm. Each of the above lower limits may be combined with any of the above upper limits. The average ferrite particle size may be, for example, 10 to 35 μm or 20 to 40 μm.

[0067] The average ferrite grain size can be determined as the average value of 400 arbitrarily selected ferrite grain sizes (μm) obtained from an optical microscope image obtained by observing the cross-section of steel material with an optical microscope (at a magnification of, for example, 400x). Note that "ferrite grain size" refers to the equivalent circular diameter of the ferrite grain (the diameter of the circle corresponding to the area of ​​the ferrite grain).

[0068] The average ferrite particle size can be determined by the method described in the examples.

[0069] Steel manufacturing method The steel manufacturing method of the present invention includes the following steps: a step of rolling a steel billet made of structural steel in a rolling mill to form an intermediate product; and a step of introducing the intermediate product with a surface temperature of 1000°C or less into a finishing rolling mill and performing finishing rolling.

[0070] The surface temperature of the intermediate product introduced into the finishing mill is the surface temperature of the intermediate product immediately before it is introduced into the finishing mill.

[0071] If the surface temperature of the intermediate product introduced into the finishing rolling mill exceeds 1000°C, it becomes difficult to obtain steel with a ΔP / ΔL of 30 MPa / mm or less, and the probability of cracking increases. Therefore, the surface temperature of the intermediate product introduced into the finishing rolling mill is preferably 1000°C or less, more preferably 980°C or less, and even more preferably 960°C or less. If the surface temperature of the intermediate product introduced into the finishing rolling mill is less than 850°C, the ferrite grains become excessively fine, and the deformability decreases. Therefore, the surface temperature of the intermediate product introduced into the finishing rolling mill is preferably 850°C or higher. That is, the surface temperature of the intermediate product introduced into the finishing rolling mill is preferably 850 to 1000°C, more preferably 850 to 980°C, and even more preferably 850 to 960°C.

[0072] The steel material of the present invention will be described based on examples.

[0073] <Examples> 100 kg each of JIS SCr420, JIS SCM420, and steel grade A, consisting of the component compositions listed in Table 1 with the remainder being Fe and unavoidable impurities, were melted in a vacuum induction melting furnace to obtain two steel billets for each steel grade. Steel grade A is a structural alloy steel for machinery, designed with improved hardenability in mind. In Table 1, "*" indicates an unavoidable impurity.

[0074]

[0075] The steel bars (bar-shaped steel materials) of the example were obtained from two steel billets of each steel type as follows. First, the two steel billets of each steel type were subjected to continuous rolling using a roughing mill, an intermediate rolling mill, and a finishing rolling mill to obtain intermediate products. At that time, the surface temperature of the intermediate products immediately before being introduced into the finishing rolling mill was adjusted to 1000°C for one steel billet of each steel type (in the examples in Table 2, each steel type is marked with "(a)") and to 960°C for the other steel billet (in the examples in Table 2, each steel type is marked with "(b)"). Next, the two intermediate products of each steel type were air-cooled to obtain steel bars with a circular cross-section having a diameter D of 30 mm. Each of the obtained steel bars was cut to a length of 200 mm to obtain test specimens of each steel bar.

[0076] <Comparative Example> Two steel billets were obtained for each steel grade in the same manner as in the example. Except that the surface temperature of the intermediate products immediately before being introduced into the finishing row rolling mill was adjusted to 1150°C for one steel billet of each steel grade (in the comparative examples in Table 2, each steel grade is marked with "(a)") and to 1200°C for the other steel billet (in the comparative examples in Table 2, each steel grade is marked with "(b)"), comparative steel bars (bar-shaped steel materials) were obtained from the two steel billets obtained for each steel grade using the same procedure as in the example. Each obtained steel bar was cut to a length of 200 mm to obtain test specimens for each steel bar.

[0077] <Test Example> To evaluate the properties of each steel bar, (1) microstructural observation, (2) deformation resistance measurement, and (3) limit upsetting test were performed.

[0078] (1) Observation of microtissue [Ratio A ４ / A ２ Each test specimen was cut through its center with a plane parallel to the rolling direction, the cut surface was mirror-polished, and then etched with Nital solution. Subsequently, the D / 4 and D / 2 positions from the surface were observed with an optical microscope (100x magnification, 10 fields of view) to identify and distinguish between the ferrite and pearlite structures.

[0079] Each captured microscope image was processed using image processing software, and the total number of pixels in each microscope image, the number of pixels in the ferrite tissue in each microscope image, and the number of pixels in the pearlite tissue in each microscope image were counted.

[0080] For each optical microscope image taken at the D / 4 position, the ratio of the number of pixels of ferrite tissue to the total number of pixels (%) was calculated, and the average of these ratios was defined as the "area fraction of ferrite tissue at the D / 4 position (%)". For each optical microscope image taken at the D / 4 position, the ratio of the number of pixels of pearlite tissue to the total number of pixels (%) was calculated, and the average of these ratios was defined as the "area fraction of pearlite at the D / 4 position (%)". The area fraction of ferrite tissue (%) at the D / 4 position and the area fraction of pearlite (%) at the D / 4 position were added together to determine the total area fraction of ferrite and pearlite (%) at the D / 4 position.

[0081] For each optical microscope image taken at position D / 2, the ratio of the number of pixels of ferrite tissue to the total number of pixels (%) was calculated, and the average value of these ratios was defined as the "area fraction of ferrite tissue at position D / 2 (%)". For each optical microscope image taken at position D / 2, the ratio of the number of pixels of pearlite tissue to the total number of pixels (%) was calculated, and the average value of these ratios was defined as the "area fraction of pearlite at position D / 2 (%)". The area fraction of ferrite tissue (%) at position D / 2 and the area fraction of pearlite (%) at position D / 2 were added together to determine the total area fraction of ferrite and pearlite (%) at position D / 2.

[0082] The total area fraction of ferrite and pearlite at position D / 4 is "A ４ " and the total area fraction of ferrite and pearlite at position D / 2 is "A ２ " and ratio A ４ / A ２ We calculated the value. The results are shown in Table 2.

[0083] [Average Ferrite Grain Size] Each test specimen was cut through its center with a plane parallel to the rolling direction, the cut surface was mirror-polished, etched with Nital solution, and then observed with an optical microscope (magnification 400x). The ferrite grain size (μm) of 400 arbitrarily selected ferrite particles was measured in the optical microscope images, and the average value of these was defined as the "average ferrite grain size." "Ferrite grain size" refers to the equivalent circle diameter of the ferrite grain (the diameter of the circle corresponding to the area of ​​the ferrite grain). The results are shown in Table 2.

[0084] (2) Measurement of deformation resistance A cylindrical test specimen (14 mm in diameter x 21 mm in length) was prepared from the center of each test material. Each obtained test specimen was statically cold-compressed in the longitudinal direction with the end faces fully constrained. The ratio ΔP / ΔL of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm) was determined when each test specimen was cold-compressed from a compressibility of X% to a compressibility of Y%. X was set to 0 (Test 1), 2 (Test 2), 4 (Test 3), 6 (Test 4), 8 (Test 5), 10 (Test 6), 15 (Test 7), and 18 (Test 8). In each test, Y was varied in the range of (10 + X) to (60 + X). For example, in Test 1 (X = 0), Y was varied in the range of 10 to 60. In each test, the maximum value of ΔP / ΔL was determined when Y was varied in the range of (10 + X) to (60 + X). Table 2 shows the maximum value of ΔP / ΔL obtained from all tests.

[0085] (3) Limit Uplift Test A cylindrical specimen (14 mm in diameter x 21 mm in length) was prepared from the center of each test material. In the limit uplift test, five specimens were cold-compressed in the longitudinal direction, and the compression ratio at which cracking began to occur was determined using a magnifying glass. The average of the determined compression ratios was defined as the "limit compressibility". The target value for the limit compressibility in cold compression was 65% or higher. The results are shown in Table 2. In Table 2, specimens with a limit compressibility of less than 65% are marked with the symbol "B" as unsuitable, and specimens with a limit compressibility of 65% or higher are marked with the symbol "A" as good.

[0086]

[0087] In the example, steel was obtained by finish rolling an intermediate product with a surface temperature of 960 to 1000°C. In the steel of the example, ΔP / ΔL was 30 MPa / mm or less, A ４ / A ２ The ferrite particle size was 1 to 1.09, and the average ferrite particle size was 24 to 33 μm. In the limit upsetting test of the steel material in the example, a limit compressibility of 65% or more was achieved.

[0088] In the comparative example, steel was obtained by finish rolling an intermediate product with a surface temperature of 1150 to 1200°C. In the steel of the comparative example, ΔP / ΔL was greater than 30 MPa / mm, A ４ / A ２The compression ratio was greater than 1.10, and the average ferrite particle size was 68-91 μm. In the limit upsetting test of the comparative steel material, a limit compressibility of 65% or more was not achieved.

[0089] 1. Cross section perpendicular to the rolling direction of the steel bar. 2. Center of the cross section perpendicular to the rolling direction of the steel bar. 3. Cross section parallel to the rolling direction of the steel bar. 4. Center position of the steel bar. 5. Position at a depth of D / 2 from the surface. 6. Position at a depth of D / 4 from the surface. D: Diameter of the steel material.

Claims

1. A steel material for cold forging made of machine structural steel, wherein when the steel material is cold compressed from a compressibility of X% to a compressibility of Y%, the ratio of the change in compressive stress ΔP (MPa) to the compressive deformation ΔL (mm) ΔP / ΔL is 30 MPa / mm or less, and X and Y are arbitrary values ​​that satisfy equations (1) and (2): 0 ≤ X < Y ... (1) 10 ≤ Y - X ≤ 60 ... (2).

2. The steel material is a steel bar having a diameter D, and the total area fraction A of ferrite and pearlite at the D / 4 position. ４ The total area fraction A of ferrite and pearlite at position D / 2 ２ Ratio A ４ / A ２ The steel material according to claim 1, wherein the ratio is 0.80 to 1.

10.

3. The steel material according to claim 1, wherein the average ferrite particle size is 10 to 40 μm.

4. The steel material according to claim 2, wherein the average ferrite particle size is 10 to 40 μm.

5. A method for manufacturing steel according to any one of claims 1 to 4, comprising the following steps: rolling a steel billet made of structural steel in a rolling mill to form an intermediate product; and introducing the intermediate product having a surface temperature of 850 to 1000°C into a finishing rolling mill to perform finishing rolling.

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