Cold forging steel

Abstract

To provide steel which can be suppressed in cracking occurring when the steel is cold-forged.SOLUTION: Steel for cold forging is provided, containing, by mass%, 0.14-0.33% C, 0.20-0.80% Si, 0.10-0.60% Mn, 1.40-2.70% Cr, 0.020-0.100% Al, 0.0040-0.0300% N, and the balance comprised of Fe with inevitable impurities, wherein P and S as the inevitable impurities are 0.030% or less, A1 point is 750°C or more, and in a spheroidized and annealed state, an area ratio of ferrite grains where a carbide is not precipitated is 6.0% or less, an area ratio of remaining pearlite and bainite is 5.0% or less, an area ratio of a carbide having an aspect ratio of 5.0 or more in all the carbides is 7.0% or less, and (area ratio of spheroidized carbide which is precipitated in grains and has aspect ratio of less than 5.0) / (area ratio of grain boundary carbide) is 3.5 or more.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to steel for cold forging which is hypoeutectoid steel. In particular, the present invention relates to a steel having excellent cold forgeability because spheroidized carbides are uniformly dispersed throughout the steel by spheroidizing annealing when component forming by cold forging is selected.

Background Art

[0002] When hypoeutectoid steel, which is a low-carbon steel, is processed and formed into parts or the like, cold forging is sometimes selected from the viewpoints of improving yield and production efficiency. Usually, in cold forging, spheroidizing annealing is performed to reduce deformation resistance.

[0003] In the spheroidizing annealing of hypoeutectoid steel, the following process is usually employed. (Step 1) Heat the steel to a temperature above the A1 point. (Step 2) Hold the temperature above the A1 point to obtain a structure consisting of two phases: a ferrite phase and an austenite phase. (Step 3) Thereafter, in order to suppress the austenite-to-pearlite transformation, gradually cool the steel from directly above the A1 point to directly below the A1 point. At this time, spherical cementite precipitates at the austenite / ferrite interface of the two-phase structure, and after the gradual cooling to directly below the A1 point is completed, a structure in which spheroidized cementite precipitates in ferrite grains is obtained.

[0004] Now, in the spheroidized annealed structure of hypoeutectoid steel, non-uniform distribution of spheroidized cementite after gradual cooling may inevitably occur due to the holding step in the two-phase region consisting of the ferrite phase and the austenite phase in (Step 2). Such non-uniform distribution of cementite promotes non-uniform deformation during cold forging and becomes one cause of crack generation, so it is undesirable from the viewpoint of workability.

[0005] Among conventional steel materials excellent in cold forgeability, for example, a case hardening steel material has been proposed that attempts to improve cold forgeability by appropriately controlling the area fractions of ferrite, bainite, and pearlite before spheroidizing annealing, thereby balancing the shape, density, and dispersion state of spheroidized carbides after spheroidizing annealing (see Patent Document 1).

[0006] Furthermore, a steel wire rod and bar for surface hardening have been proposed in which, by ensuring an appropriate amount of bainite area before spheroidizing annealing, fine carbides caused by bainite are dispersed after spheroidizing annealing, thereby improving cracking (see Patent Document 2).

[0007] Furthermore, a cold forging case-hardening steel has been proposed that aims to ensure cold forgeability by shortening the spheroidizing annealing time and ensuring uniform dispersion of cementite, by reducing the size of the ferrite grains and pearlite grains in the microstructure consisting of two phases, a ferrite phase and a pearlite phase, thereby increasing the ferrite / pearlite grain boundaries and promoting cementite precipitation at the interface (see Patent Document 3). [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2014-185389 [Patent Document 2] Japanese Patent Publication No. 2005-220377 [Patent Document 3] Japanese Patent Application Publication No. 11-012684 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] In the spheroidized annealed structure of hypoeutectoid steel, the process of holding the material in a two-phase region consisting of the ferrite phase and the austenite phase as described in (Step 2) above can inevitably lead to non-uniformity in the distribution of spheroidized cementite after slow cooling. Such non-uniformity in cementite distribution promotes non-uniform deformation during cold forging, which contributes to crack formation. Therefore, from the viewpoint of workability, it is desirable to have a uniform cementite distribution.

[0010] The aforementioned proposals in the prior art attempt to optimize the shape and dispersion state of spheroidized cementite after spheroidizing annealing, but they all remain merely proposals to appropriately control the structure before spheroidizing annealing. In other words, when considering the ferrite grains in the two-phase structure consisting of the ferrite phase and austenite phase in (step 2) of a normal spheroidizing annealing process, it is not possible to precipitate spheroidized cementite, and therefore, the non-uniform distribution of spheroidized cementite has not been adequately addressed.

[0011] Therefore, the problem that the present invention aims to solve is to provide a steel that can suppress cracking during cold forging by uniformly dispersing spheroidized carbides throughout the entire structure. [Means for solving the problem]

[0012] The inventors of this invention have diligently studied the carbon diffusion behavior just below the austenitization temperature. In conventional hypoeutectoid steel, just below the austenitization temperature, dissolved carbon within ferrite grains stably precipitates as M3C-type carbides (cementite). Therefore, carbon exists as M3C-type carbides in both pearlite grains (structure composed of ferrite and lamellar cementite) and bainite grains (structure composed of ferrite and cementite), and in ferrite grains (where cementite stably precipitates). As a result, it is difficult to create a difference in carbon diffusion behavior, and sufficient driving force for carbon diffusion from pearlite and bainite grains to ferrite grains cannot be obtained.

[0013] On the other hand, by increasing the Cr content, the solid-solution carbon within the ferrite grains precipitates stably as M7C3 type carbides (M = mixed components of Fe and Cr) just below the austenitization temperature. In other words, the difference in carbon diffusion behavior between pearlite grains (structure composed of ferrite and lamellar cementite) and bainite grains (structure composed of ferrite and cementite) and ferrite grains (where M7C3 carbides are stable) becomes significant. As a result, we found that carbon diffusion from pearlite and bainite grains to ferrite grains is sufficiently promoted.

[0014] Therefore, by adjusting the composition of the steel, we obtained the finding that by making the stable carbides within the ferrite grains at just below the austenitization temperature into the M7C3 type, the spheroidal carbides become more uniformly dispersed.

[0015] Thus, the inventors of the present invention have found that by subjecting steel with a predetermined component composition to a predetermined spheroidizing annealing process, spheroidized carbides are uniformly dispersed throughout the entire structure, resulting in steel that can suppress cracking during cold forging.

[0016] As a result, when the microstructure before heat treatment consists of two phases, a ferrite phase and a pearlite phase, a ferrite phase and a bainite phase, or a ferrite phase, a pearlite phase and a bainite phase, (a) during holding at just below the austenitization temperature, the lamellar cementite constituting the pearlite and the cementite constituting the bainite undergo solid solution and spheroidization, (b) solid solution carbon flows into the ferrite grains, (c) M7C3 type carbides precipitate within the ferrite grains, and (d) these grow into spheroidized carbides, thereby obtaining steel in which spheroidized carbides are uniformly dispersed throughout. Here, the M7C3 type carbide is a mixed component of M=Fe and Cr.

[0017] From the above, the first means for solving the problems of the present invention is a material composed of, by mass%, C: 0.14~0.33%, Si: 0.20~0.80%, Mn: 0.10~0.60%, Cr: 1.40~2.70%, Al: 0.020~0.100%, N: 0.0040~0.0300%, with the remainder being Fe and unavoidable impurities, wherein the unavoidable impurities P and S are P: 0.030% or less and S: 0.030% or less, and the A1 point is 750°C. The above describes a cold forging steel in a spheroidized annealed state, where the area ratio of ferrite grains without precipitated carbides is 6.0% or less, the area ratio of residual pearlite and bainite is 5.0% or less, the area ratio of carbides with an aspect ratio of 5.0 or more relative to the total carbides is 7.0% or less, and the ratio of (area ratio of intragranularly precipitated spheroidized carbides with an aspect ratio of less than 5.0) / (area ratio of grain boundary carbides) is 3.5 or more.

[0018] The second method is a cold forging steel that, in addition to the chemical components described in the first method, contains one or more of Ni: 0.02-2.00% and Mo: 0.02-2.00%, with the remainder being Fe and unavoidable impurities, wherein the unavoidable impurities P and S are P: 0.030% or less and S: 0.030% or less, the A1 point is 750°C or higher, it is in a spheroidized annealed state, the area ratio of ferrite grains without precipitated carbides is 6.0% or less, the area ratio of remaining pearlite and bainite is 5.0% or less, the area ratio of carbides with an aspect ratio of 5.0 or more relative to the total carbides is 7.0% or less, and the ratio of (area ratio of spheroidized carbides with an aspect ratio of less than 5.0 precipitated within grains) / (area ratio of grain boundary carbides) is 3.5 or more.

[0019] The third method, in addition to the chemical components described in the first or second method, contains one or more of the following: Nb: 0.02-0.10%, Ti: 0.020-0.200%, B: 0.0010-0.0050%, V: 0.010-0.500%, with the remainder being Fe and unavoidable impurities, wherein the unavoidable impurities P and S are P: 0.030% or less and S: 0.030% or less, and the A1 point is 750°C or higher. The steel is a cold forging steel that has undergone spheroidal annealing treatment, in which the area ratio of ferrite grains without precipitated carbides is 6.0% or less, the area ratio of residual pearlite and bainite is 5.0% or less, the area ratio of carbides with an aspect ratio of 5.0 or more relative to the total carbides is 7.0% or less, and the ratio of (area ratio of spheroidal carbides with an aspect ratio of less than 5.0 precipitated within grains) / (area ratio of grain boundary carbides) is 3.5 or more. [Effects of the Invention]

[0020] The steel for cold forging of the present invention has a structure in which spheroidized carbides are uniformly dispersed throughout. As a result of measuring the area ratio of ferrite grains on which no spheroidized carbides are precipitated, the ratio is within 6.0% of the entire structure, and the area ratio of residual pearlite grains and bainite grains after spheroidizing annealing is also 5.0% or less, so that the spheroidization of carbides has proceeded sufficiently. As a result, the steel does not crack even under high compression during cold forging. Therefore, the steel for cold forging of the present invention has a hardness of 82 HRB or less, and does not crack even when cold forged at a cold upsetting ratio of 77%, allowing cold working. Thus, the steel for cold forging of the present invention exhibits excellent cold forgeability. [Brief Description of the Drawings]

[0021] [Figure 1] It is a schematic diagram illustrating the holding temperature T (°C), holding time (hr) and air cooling temperature history of spheroidizing annealing conditions. [Figure 2] For Invention Steel No. 6, Invention Steel No. 19 and Comparative Steel No. 6, optical microscope images after spheroidizing annealing, the area ratio of ferrite grains on which no spheroidized carbides are precipitated, and the area ratio of residual pearlite grains and bainite grains are shown. [Mode for Carrying Out the Invention]

[0022] Prior to describing the embodiments of the steel according to the present invention, the reasons for defining the chemical composition of the steel, the reason for defining the lower limit of the A1 point, the reason for defining the area ratio of ferrite grains with no precipitated carbides, the reason for defining the area ratio of residual pearlite and bainite, and the reasons for defining the distribution, shape, and intragranule / grain boundary ratio of carbides will be described first. Note that % in the following component compositions is % by mass.

[0023] C: 0.14 to 0.33% C is an element that improves the hardness of the raw material. When C is less than 0.14%, the core hardness after carburizing decreases, leading to insufficient strength. On the other hand, when C exceeds 0.33%, the hardness of the raw material increases excessively, which reduces workability, resulting in inferior machinability and cold forgeability. Therefore, C is set to 0.14-0.33%. Preferably, C is 0.14-0.27%.

[0024] Si: 0.20~0.80% Si is a useful element for deoxidation. If the Si content is less than 0.20%, there will be a shortage of deoxidizing agent. On the other hand, if the Si content exceeds 0.80%, the hardness of the material will increase too much, reducing workability, and carburizing inhibition will occur. Therefore, the Si content should be 0.20-0.80%. Preferably, the Si content should be 0.30-0.80%.

[0025] Mn: 0.10~0.60% Mn is an element that improves hardenability. If the Mn content is less than 0.10%, the hardening will be insufficient. On the other hand, if the Mn content exceeds 0.60%, the machinability will decrease. Therefore, Mn is 0.10-0.60%. Preferably, Mn is 0.15-0.50%.

[0026] Cr: 1.40~2.70% Cr is an element that stabilizes M7C3 type carbides. If the Cr content is less than 1.40%, M7C3 type carbides will not precipitate, resulting in insufficient carbon inflow from pearlite grains to ferrite grains during spheroidizing annealing. This leads to an uneven distribution of spheroidized carbides, resulting in reduced cold forgeability. Furthermore, hardenability will be insufficient. On the other hand, if the Cr content exceeds 2.70%, the hardness of the material increases too much, reducing workability. Carburizing inhibition also occurs. Therefore, the Cr content should be between 1.40% and 2.70%. Preferably, the Cr content is between 1.40% and 2.50%.

[0027] Al: 0.020~0.100% Al is a useful element for deoxidation. If the Al content is less than 0.020%, it will be insufficient as a deoxidizing agent. Furthermore, there will be a shortage of fine nitrides, resulting in a lack of precipitation nuclei for M7C3 type carbides, which will lead to grain coarsening and a decrease in toughness and fatigue properties. On the other hand, if the Al content exceeds 0.100%, coarse nitrides will be formed, which will decrease fatigue properties and workability. Therefore, the Al content should be between 0.020 and 0.100%. Preferably, the Al content is between 0.020 and 0.050%.

[0028] N: 0.0040~0.0300% N is the element that forms nitrides. If the N content is less than 0.0040%, there will be a lack of fine nitrides, resulting in a shortage of precipitation nuclei for M7C3, which leads to grain coarsening and a decrease in toughness and fatigue properties. On the other hand, if the N content exceeds 0.0300%, coarse carbonitrides will form, which will decrease fatigue properties and workability. Therefore, the N content should be between 0.0040 and 0.0300%. Preferably, the N content is between 0.0040 and 0.0200%.

[0029] P: Less than 0.030% as an unavoidable impurity. When the phosphorus (P) content exceeds 0.030%, toughness decreases due to grain boundary segregation. Therefore, even when P is included as an unavoidable impurity, its content should be kept below 0.030%.

[0030] S: Less than 0.030% as an unavoidable impurity. If the sulfur content exceeds 0.030%, toughness and fatigue strength decrease due to the formation of MnS. Therefore, even when sulfur is present as an unavoidable impurity, its content should be kept below 0.030%.

[0031] The following describes optional components that can be selectively added. Ni: 0.02~2.00% Ni is an element that improves hardenability and toughness. If the Ni content is less than 0.02%, the effect of improving hardenability and toughness is small. On the other hand, if Ni is added in amounts exceeding 2.00%, the cost increases, and the hardness of the material also increases, resulting in a decrease in workability. Therefore, when adding Ni, the amount should be between 0.02% and 2.00%. Preferably, the amount of Ni added should be between 0.02% and 1.80%.

[0032] Mo: 0.02~2.00% Mo is an element that improves hardenability. If the Mo content is less than 0.02%, the effect of improving hardenability is small. On the other hand, if the Mo content exceeds 2.00%, not only does the cost increase, but the hardness of the material also increases, resulting in a decrease in workability. Therefore, when adding Mo, the amount should be between 0.02% and 2.00%. Preferably, the amount of Mo added should be between 0.02% and 0.50%.

[0033] Nb: 0.02~0.10% Nb is an element that generates carbonitrides and suppresses grain coarsening. If the Nb content is less than 0.02%, there will be insufficient fine carbonitrides, resulting in a reduced grain coarsening effect and insufficient toughness and fatigue strength. On the other hand, if the Nb content exceeds 0.10%, the amount of carbonitrides becomes excessive, reducing workability. Therefore, when adding Nb, the amount should be between 0.02% and 0.10%. Preferably, the added Nb content should be between 0.02% and 0.08%.

[0034] Ti: 0.020~0.200% Ti is an element that forms carbonitrides and suppresses grain coarsening. If the Ti content is less than 0.020%, there will be insufficient fine nitrides. In addition, nitrogen will not be fixed, forming BN, which will reduce hardenability. Furthermore, the effect of suppressing grain coarsening will be reduced. On the other hand, if the Ti content exceeds 0.200%, the amount of carbonitrides becomes excessive, reducing processability. Therefore, when adding Ti, the concentration should be 0.020 to 0.200%. Preferably, the added Ti concentration should be 0.020 to 0.100%.

[0035] B: 0.0010~0.0050% B is an element that improves hardenability. If B is less than 0.0010%, the effect of improving hardenability is small. If B exceeds 0.0050%, the workability decreases due to the increase in material hardness. Therefore, when adding B, the amount should be between 0.0010% and 0.0050%. Preferably, the amount of B added should be between 0.0010% and 0.0030%.

[0036] V: 0.010~0.500% V is an element that generates carbonitrides and suppresses grain coarsening. If the V content is less than 0.010%, there will be insufficient fine carbonitrides, resulting in a weak effect in suppressing grain coarsening and thus insufficient toughness and fatigue strength. On the other hand, if the V content exceeds 0.500%, the amount of carbonitrides will be excessive, reducing workability. Therefore, when adding V, the amount should be between 0.010% and 0.500%. Preferably, the amount of V added should be between 0.010% and 0.400%.

[0037] A1 point: 750℃ or higher If point A1 is below 750°C, the holding temperature for spheroidizing annealing becomes low, resulting in insufficient spheroidization of the carbides that make up pearlite and bainite. Consequently, the cold workability decreases due to insufficient softening. Therefore, point A1 should be 750°C or higher. More preferably, point A1 should be 750 to 800°C. Furthermore, A1 in this invention can be calculated using Ac1 = 723℃ - 14Mn[%] + 22Si[%] - 14.4Ni[%] + 23.3Cr[%].

[0038] Area percentage of ferrite grains without carbide precipitation: 6.0% or less If the area ratio of ferrite grains without carbide precipitation exceeds 6.0%, the carbide precipitation distribution can be considered non-uniform. A non-uniform distribution of spheroidal carbide precipitation promotes non-uniform deformation during cold forging, making cracks more likely to occur in high-processing forging. Therefore, the area ratio of ferrite grains without carbide precipitation should be kept below 6.0%.

[0039] Area percentage of residual perlite and bainite: 5.0% or less If the area ratio of residual pearlite and bainite is excessive, it leads to insufficient softening of the material, reducing workability, and the precipitation distribution of spheroidal carbides becomes uneven. This promotes uneven deformation during cold forging, making cracks more likely to occur during high-processing forging. Therefore, the area ratio of residual pearlite and bainite should be 5.0% or less.

[0040] Area percentage of carbides with an aspect ratio of 5.0 or higher relative to total carbides: 7.0% or less Insufficient spheroidization of carbides promotes uneven deformation during cold forging, making cracks more likely to occur in high-processing forging. Therefore, the area ratio of carbides with an aspect ratio of 5.0 or higher relative to the total carbides should be 7.0% or less.

[0041] (Area ratio of spheroidal carbides with an aspect ratio of less than 5.0 that precipitate within grains) / (Area ratio of grain boundary carbides): 3.5 or higher If the proportion of intergranular carbides is high compared to intragranular carbides, the amount of spheroidal carbides precipitated within the grain will be insufficient, resulting in non-uniformity of spheroidal carbides. This promotes non-uniform deformation during cold forging, making cracks more likely to occur in high-processing forging. Therefore, the ratio of (area percentage of intragranular spheroidal carbides with an aspect ratio of less than 5.0) / (area percentage of intergranular carbides) should be set to 3.5 or higher.

[0042] In spheroidizing annealing, if carbon diffusion from pearlite and bainite grains to ferrite grains is insufficient, carbides cannot precipitate within the ferrite grains, resulting in an uneven carbide distribution and reduced cold forgeability. Therefore, in this invention, a high chromium (Cr) component is used to ensure the presence of M7C3 type carbides. As a result, the difference in carbon diffusion behavior between pearlite grains (structure composed of ferrite and lamellar cementite) and bainite grains (structure composed of ferrite and cementite) and ferrite grains (where M7C3 carbides are stable) becomes large. Consequently, carbon diffusion from pearlite and bainite grains to ferrite grains is sufficiently promoted, and spheroidized carbides become more uniformly dispersed, resulting in excellent cold forging properties.

[0043] (Conditions for spheroidizing annealing treatment) Figure 1 shows a schematic diagram illustrating the temperature history of the holding temperature T (°C) and holding time t (hr) for the spheroidizing annealing treatment. The conditions for the holding temperature T and holding time t for the spheroidizing annealing treatment in this invention are those that satisfy the following [Equations 1] to [Equations 3]. [Formula 1]: (A1 point -30℃)≦T≦(A1 point -5℃) [Formula 2]:t≧120 / (T-A1+50) [Formula 3]: A1=723℃-14Mn[%]+22Si[%]-14.4Ni[%]+23.3Cr[%]

[0044] (Manufacturing process of test specimens) Each of the steels described in Table 1, No. 1 to 20, and Comparative Examples No. 1 to 17, consisting of the chemical composition and the remainder being Fe, was melted in a 100 kg vacuum induction furnace (VIM) to produce steel ingots. These ingots were then forged to a diameter of 32 mm at 1250 °C, normalized at 925 °C for 1 hour, and then cut into 100 mm lengths to obtain the test specimens.

[0045] [Table 1]

[0046] (Spheroidizing annealing process) The test specimens prepared as described above were subjected to spheroidizing annealing using a Kanthal furnace according to the following procedure. The test specimens were placed in a furnace set to the holding temperature for spheroidizing annealing, as shown in the holding temperature T and holding time t in Table 2. The heating time for the specimens was allowed to rise for 30 minutes, and after being held for the specified time, they were air-cooled.

[0047] [Table 2]

[0048] (Measurement of aspect ratio and various area ratios of spheroidal carbides) For the specimens after spheroidizing annealing, a section was cut from the D / 4 position, embedded in resin, polished, and then etched with Nital etching solution. Three fields of view were observed using a 5000x electron microscope, and the area ratio was calculated by image analysis. Carburized particles with an equivalent circle diameter of less than 0.10 μm were excluded. Furthermore, the aspect ratio of a carbide can be determined from the ratio of its major axis to its minor axis.

[0049] The results of measuring the area percentage of ferrite grains without precipitated spheroidized carbides in the observation field after spheroidized annealing indicate that grains with an area percentage exceeding 6.0% of the total structure are either insufficiently spheroidized or not uniformly dispersed. The results are shown in Table 2. Note that carbides with an equivalent circle diameter of less than 0.10 μm were excluded, so "no precipitated spheroidized carbides" means that the material does not contain carbides with an equivalent circle diameter of 0.10 μm or larger.

[0050] A percentage of remaining pearlite and bainite grains after spheroidizing annealing of 5.0% or less indicates that the spheroidization of carbides has progressed sufficiently. However, if the percentage of remaining pearlite and bainite grains exceeds 5.0%, it indicates that the spheroidization of carbides is insufficient, resulting in reduced cold forgeability due to insufficient softening. The results are shown in Table 2.

[0051] Figure 2 shows optical microscope images of invention steel No. 6, invention steel No. 19, and comparative steel No. 6 after spheroidizing annealing, as well as the area percentage of ferrite grains without precipitated spheroidized carbides, and the area percentages of remaining pearlite and bainite grains.

[0052] (Hardness measurement) The hardness (HRB) of a spheroidized annealed steel bar was measured using a Rockwell hardness tester at the midpoint between the center and the outer circumference (referred to as "D / 4," where D represents the diameter). When the spheroidized annealing hardness exceeds 82 HRB, the cold forging deformability decreases, and cracking becomes more likely when cold forging is performed at a high rate of deformation. Therefore, it is desirable to keep the spheroidized annealing hardness below 82 HRB. The results are shown in Table 2.

[0053] (Presence or absence of cracks after 77% cold upsetting) A cylindrical specimen measuring φ14 × 21 mm was prepared from the center of the test material (with the forging direction of the test material and the longitudinal direction of the specimen parallel). Cold upsetting tests were performed at room temperature using these cylindrical specimens. The upsetting speed was 10 mm / min, and the final upsetting rate (percentage reduction in specimen height) was set to 77%. The test was performed four times (n=4), and the presence or absence of surface cracks in the processed specimens was observed. The results are shown in Table 2.

[0054] Inventive steels 1 to 20 are within the range of chemical composition defined by the present invention, with an A1 point of 750°C or higher. M7C3 carbides are confirmed within the spheroidized annealed grains. When the area percentage of ferrite grains without precipitated spheroidized carbides is measured, it is within 6.0% of the total microstructure. The area percentage of remaining pearlite and bainite grains is also within 5.0%. The proportion of high aspect ratio carbides is low, and there are fewer grain boundary carbides compared to intragranular carbides. As a result, spheroidization is sufficiently advanced, and a uniformly dispersed cold forged steel is obtained. Furthermore, inventive steels 1 to 20 all have a hardness of 82 HRB or less, and did not exhibit cracking in a 77% cold upsetting test, confirming that inventive steels have excellent cold workability.

[0055] Comparative steels 1 and 2 had an excess of Mn and a deficiency of Cr, an A1 point below 750°C, a high area ratio of ferrite grains without intragranular spheroidal carbides, a high area ratio of residual pearlite and bainite, a high proportion of carbides with a high aspect ratio, and a relatively high ratio of intergranular carbides to intragranular carbides. Consequently, cracking was observed in the 77% cold upsetting test. Comparative steels 3-7 had low Cr content, an A1 point below 750°C, a high area ratio of ferrite grains without intragranular spheroidal carbide precipitation, a high area ratio of residual pearlite and bainite, a high proportion of carbides with high aspect ratios, and cracking was observed in the 77% cold upsetting test. Comparative Example 8 had an excessive amount of Cr, a high proportion of carbides with a high aspect ratio, and a high ratio of intergranular carbides to intragranular carbides. It was hard at 84 HRB and cracked in a 77% cold upsetting test. Comparative Example 9 had an excess of Mn and Cr, and a high proportion of carbides with a high aspect ratio. It was hard at 85 HRB, and cracking was observed in the 77% cold upsetting test. Comparative Example 10 had an excess of Si and a high proportion of grain boundary carbides. It was hard at 85 HRB and cracked during a 77% cold upsetting test. Comparative Example 11 had excessive amounts of C, Mo, and Al, and the ratio of grain boundary carbides to intragranular carbides was high. It was hard at 86 HRB and cracking was observed in the 77% cold upsetting test. Comparative Example 12 had an excessive amount of Cr, resulting in a higher ratio of intergranular carbides to intragranular carbides, and cracking was observed in the 77% cold upsetting test. Comparative Example 13 had an excess of Si and Mn, a high proportion of carbides with a high aspect ratio, and a high ratio of grain boundary carbides to intragranular carbides. Furthermore, cracking was observed in the 77% cold upsetting test. Comparative Example 14 had an excessive amount of Cr and Al, and a high proportion of carbides with a high aspect ratio. Cracking was observed in the 77% cold upsetting test. Comparative Example 15 had an excessive amount of carbon, and the ratio of intergranular carbides to intragranular carbides was also high. Furthermore, it was hard at 83 HRB, and cracking was observed in the 77% cold upsetting test. Comparative Example 16 had an excess of Si and Al, and the ratio of grain boundary carbides to carbides was also high. As a result, cracking was observed in the 77% cold upsetting test. Comparative Example 17 had excessive amounts of Mn, Cr, and Al, and a high proportion of carbides with a high aspect ratio. Cracking was observed in the 77% cold upsetting test.

Claims

1. In mass%, the composition is as follows: C: 0.14-0.33%, Si: 0.20-0.80%, Mn: 0.10-0.60%, Cr: 1.40-2.70%, Al: 0.020-0.100%, N: 0.0040-0.0300%, with the remainder being Fe and unavoidable impurities, where the unavoidable impurities P and S are P: 0.030% or less and S: 0.030% or less. A 1 Cold forging steel having a temperature of 750°C or higher, having undergone spheroidal annealing, with the area ratio of ferrite grains without precipitated carbides being 6.0% or less, the area ratio of residual pearlite and bainite being 5.0% or less, the area ratio of carbides with an aspect ratio of 5.0 or more relative to the total carbides being 7.0% or less, and the ratio of (area ratio of intragranularly precipitated spheroidal carbides with an aspect ratio of less than 5.0) / (area ratio of grain boundary carbides) being 3.5 or more.

2. In addition to the chemical components described in claim 1, it contains one or more of Ni: 0.02 to 2.00% and Mo: 0.02 to 2.00%, with the remainder being Fe and unavoidable impurities, wherein the unavoidable impurities P and S are P: 0.030% or less and S: 0.030% or less. A 1 Cold forging steel having a temperature of 750°C or higher, having undergone spheroidal annealing, with the area ratio of ferrite grains without precipitated carbides being 6.0% or less, the area ratio of residual pearlite and bainite being 5.0% or less, the area ratio of carbides with an aspect ratio of 5.0 or more relative to the total carbides being 7.0% or less, and the ratio of (area ratio of intragranularly precipitated spheroidal carbides with an aspect ratio of less than 5.0) / (area ratio of grain boundary carbides) being 3.5 or more.

3. In addition to the chemical components described in claim 1 or claim 2, the material contains one or more of the following: Nb: 0.02 to 0.10%, Ti: 0.020 to 0.200%, B: 0.0010 to 0.0050%, V: 0.010 to 0.500%, with the remainder being Fe and unavoidable impurities, wherein the unavoidable impurities P and S are P: 0.030% or less and S: 0.030% or less. A 1 Cold forging steel having a temperature of 750°C or higher, having undergone spheroidal annealing, with the area ratio of ferrite grains without precipitated carbides being 6.0% or less, the area ratio of residual pearlite and bainite being 5.0% or less, the area ratio of carbides with an aspect ratio of 5.0 or more relative to the total carbides being 7.0% or less, and the ratio of (area ratio of intragranularly precipitated spheroidal carbides with an aspect ratio of less than 5.0) / (area ratio of grain boundary carbides) being 3.5 or more.

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