Steel parts and method for manufacturing the same

Abstract

To provide a steel component and a method of producing the same for achieving a high yield ratio of 0.60 or more in a non-heat treated steel component having a bainitic structure without performing refining after hot forging.SOLUTION: A steel component contains a prescribed component in a range where a value of F1 obtained by a formula (1) F1=C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 14 is 0.65% or more, having a component composition with a balance of Fe and inevitable impurities and a structure in which an area ratio of a bainitic structure is 93% or more and an area ratio of retained austenite is 5% or less. Crystal particles of the bainitic structure have an average diameter of 8 μm or more and 25 μm or less, an average aspect ratio of 0.40 or more, and an average ratio of lengths of crystal particle boundaries to circumferences of the crystal particles of 60% or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to steel parts, such as automotive and truck undercarriages, and more specifically, steel parts used for knuckles and front axles, and a method for manufacturing the same.

Background Art

[0002] In recent years, due to concerns about global warming, there have been increasing demands in the industrial world to suppress CO2 emissions. Under such demands, in the automotive industry, not only is the suppression of CO2 emissions as exhaust gas from the vehicle itself being promoted, but also various studies are being conducted on suppressing CO2 emissions in the manufacturing process of parts. As one technology for suppressing CO2 emissions in the manufacturing process of such automotive parts, the development of non-quenched and tempered parts that omit heat treatment, which is the process of building in part strength, that is, omit the steel quenching and tempering process, is active. The reason for enabling this omission of heat treatment is, for example, that in the manufacturing process of parts, a device is provided that enables the building in of part strength during the cooling of parts after the hot forging process.

[0003] Non-quenched and tempered parts are roughly classified into two types according to the type of their metal structure. That is, those whose main metal structure consists of two phases of so-called ferrite and pearlite, or those whose main metal structure consists of bainite. In the former non-quenched and tempered steel consisting of ferrite and pearlite, precipitation strengthening by vanadium alone is used. That is, in the cooling process after hot forging of parts, vanadium carbide precipitates finely to strengthen the structure, so that part strength at the same level as that obtained by performing a quenching and tempering treatment in the state after hot forging can be obtained. On the other hand, the strengthening mechanism of non-quenched and tempered parts with a bainite structure is transformation strengthening. That is, by causing bainite transformation in the cooling process after hot forging, a structure excellent in both strength and toughness is created. Which non-quenched and tempered steel is used depends on the required characteristics of the parts, but in general, non-quenched and tempered steel with a bainite structure tends to be used for parts that require both strength and toughness.

[0004] The non-quenched and tempered steel having a bainite structure has a good balance between strength and toughness as described above. Generally, however, the yield stress is low, and the ratio of the yield stress to the tensile strength, which is the maximum strength of the steel, so-called yield ratio, tends to be low. Further, this yield stress has a large variation (dispersion) compared with the tensile strength. In the design of steel for mechanical structures, it goes without saying that the yield stress, which is the strength at which plastic deformation starts, is more important than the maximum strength of the steel. From such a design perspective, even if the steel has good tensile strength and toughness, it is difficult to use steel with a low yield stress. This is a problem of steel having a bainite structure.

[0005] As a technique for solving such problems, Patent Document 1 has been proposed. Patent Document 1 proposes a technique for increasing the yield stress by controlling the cooling rate after hot working and annealing at 200°C to 600°C after cooling to room temperature. However, this technique requires an annealing process after hot forging, which greatly impairs the merits of non-quenching and tempering. Therefore, the pursuit of a technique that satisfies the required characteristics without annealing has also been carried out.

[0006] For example, Patent Document 2 proposes a technique for improving the yield stress by performing slow cooling at a specified cooling rate between 200°C and 500°C during cooling when cooling after hot forging. However, in order to perform slow cooling from a specific temperature range, capital investment for modifying existing cooling equipment is indispensable. In recent years, assembly factories for automobiles and trucks themselves have been built in countries such as Southeast Asia outside Japan, and along with this, the local procurement of hot forging parts has also advanced. In factories built in countries such as Southeast Asian countries, forged products after hot forging are often stacked in steel boxes. In this case, it is assumed that there is a significant difference in the cooling rate between the forged products near the bottom of the box and the forged products stacked on the upper part of the box. Further, the box is almost always cubic, and there are also differences in the cooling rate among the forged products near the four corners of the box, the forged products near the side surface of the box, and the forged products near the center of the box. Since the strength building process of non-quenched and tempered steel mainly occurs during the cooling process after hot forging, control of the cooling rate during cooling is important. The problem in local procurement is that there is a possibility that the control of this cooling rate may not be sufficient.

[0007] In order to address this problem, when introducing the technology according to Patent Document 2 described above, it is extremely difficult to introduce dedicated equipment for slow cooling after hot forging in the factories of these countries. Therefore, there is a demand for non-quenched and tempered steel that can achieve a predetermined strength without precisely controlling the cooling rate after hot forging. That is, there is a demand for non-quenched and tempered steel whose mechanical properties do not vary much even if the cooling rate varies slightly.

[0008] In addition, Patent Document 3 specifies the organizational factor that impairs the yield ratio and endurance ratio as an island-shaped martensite-retained austenite mixed structure, and a technique of reducing the addition amount of Si is proposed as a method for reducing this. However, in order to realize steel with Si reduced to the level specified in Patent Document 3 in an actual iron-making process, it must be manufactured by a refining method that does not use Si in the steel-making process, resulting in high manufacturing costs and being unfavorable from an economic perspective.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0010] The present invention has been invented in view of the above situation. That is, an object of the present invention is to provide a means for realizing a high yield ratio, specifically a yield ratio of 0.60 or more, in a component made of so-called non-quenched and tempered steel, particularly non-quenched and tempered steel having a bainite structure, without performing a quenching and tempering treatment after hot forging. Note that the target value of the yield ratio is set to 0.60 or more from the viewpoint of suppressing variations in mechanical properties. That is, in the mechanical properties of steel having a bainite structure as the main structure, when comparing the cooling rate dependence, the yield stress is more sensitive than the tensile stress. In other words, the yield strength is more greatly reflected in the change of mechanical properties due to the difference in cooling rate than the tensile strength. Here, the tensile stress is the highest stress value in the nominal stress-nominal strain curve obtained in a tensile test. Further, the yield stress is the 0.2% offset stress from the elastic region in the same curve. That is, for a steel in which the yield ratio, which is the ratio of the yield stress to the tensile stress, is a certain value or more, even if it is a non-quenched and tempered steel having a bainite-based structure, it can be regarded as a steel in which variations in the yield stress are suppressed. From such a viewpoint, if the yield ratio of non-quenched and tempered steel having a bainite structure is 0.60 or more, it can be considered that the sensitivity of the steel to the cooling rate is low.

Means for Solving the Problems

[0011] The inventors, without using the technology of Patent Document 3 described above, for example, in order to contain 0.10% or more of Si, the ratio of the island-like martensite-retained austenite mixed structure is high, and the cross-sectional area is 3 μm 2 or more sulfide-based inclusions are less than 200 pieces / mm 2 Even so, intensive studies were conducted to develop a non-quenched and tempered steel having a yield ratio of 0.60 or more without performing a tempering treatment.

[0012] Now, the understanding of the current situation at the microscale of the yielding phenomenon of metals when stress is applied is the beginning of large-scale dislocation movement. That is, when the stress applied to the dislocation source exceeds a certain threshold, dislocations are generated and propagated from that dislocation source, and the dislocations thus generated move according to the stress field and accumulate at the grain boundaries, generating stress at the grain boundaries. When the stress at the grain boundaries thus generated exceeds a certain threshold, the dislocation accumulates not only in the grains inside the grain boundaries but also in the grains adjacent to this grain, involving a large number of grains to simultaneously undergo large-scale deformation macroscopically, and macroscopic deformation begins. This is the yielding phenomenon.

[0013] Looking at the above-described elementary processes of yielding, in order to increase the yield stress, the following methods can be considered: (i) reducing the dislocation density, (ii) reducing the dislocation sources, (iii) making it difficult for dislocations to be generated from the dislocation sources, and (iv) increasing the deformation threshold of the grains with dislocations accumulated inside. However, in the case of bainite-structured non-quenched and tempered steels, it is difficult to control (i), (ii), and (iii) for the sake of the yield ratio. This is because in bainite-structured non-quenched and tempered steels, when cooled to room temperature after hot forging, the dislocation density generated along with the bainite transformation is higher than that of ferrite and pearlite structures. Moreover, bainite also has a large amount of fine cementite that serves as dislocation sources. In fact, the dislocation strengthening obtained during transformation and the precipitation strengthening by fine cementite are the strengthening mechanisms of bainite. If the types, shapes, and distributions of the precipitates are determined, the threshold for dislocations to be generated from these dislocation sources will also be determined accordingly. Increasing the yield stress by controlling the dislocation sources contradicts the concept itself of the development of the bainite-structured non-quenched and tempered steel. Then, it is necessary to make grains in which dislocations are difficult to deform even if they accumulate, while allowing the generation, propagation, and accumulation of dislocations at the initial stage of stress loading. That is, from the perspective of (iv) above, in order to obtain a high yield ratio in steels mainly composed of bainite structure, this perspective is important.

[0014] From the perspective of (iv) above, there are two methods to make grains in which dislocations are difficult to deform even if they accumulate. The first is to make the crystal grain size fine. If the crystal grain size is fine, since the distance from the transfer source to the crystal grain boundary is short, the amount of transferred material deposited is limited, and the stress caused by the transferred material deposition can be suppressed to a correspondingly low level. That is, basically, the finer the crystal grain size, the better.

[0015] That is, in the non-quenched and tempered steel with a bainite structure, it is necessary that the average diameter of the crystal grains surrounded by crystal grain boundaries with an inclination of the angle difference of the crystal orientations of adjacent crystal grains of 15° or more is 25 μm or less. On the other hand, if the crystal grains become too fine, the crystal grain boundaries themselves will act as transfer sources, so a lower limit is required. This lower limit is 8 μm in terms of the above average diameter. Regarding the crystal grain size, specifying the crystal grains surrounded by crystal grain boundaries with an inclination of the angle difference of the crystal orientations of adjacent crystal grains of 15° or more is because transfer easily passes through crystal grain boundaries with an angle less than this, that is, sufficient transfer does not accumulate.

[0016] The second is the shape of the crystal grains. The closer the shape of the crystal grains is to a perfect sphere, the less likely they are to deform. That is, the aspect ratio (minor axis / major axis) of the crystal grains should be as close to 1 as possible. Also, the length of the crystal grain boundaries should be as close to that of a perfect circle as possible. In the case of non-quenched and tempered steel with a bainite structure, it is necessary that the aspect ratio (short side / long side) of the crystal grains is 0.40 or more, and the ratio of the length of the crystal grain boundaries to the circumference of the crystal grains is 60 or less on average.

[0017] Note that the circumference of a crystal grain should originally be the circumference of the cut surface (true circle) passing through the center of the sphere when the crystal grain is assumed to be a perfect sphere. However, to obtain such a circumference, it is necessary to know the true volume of the crystal grain. However, it requires mathematical processing to calculate this volume from the microscopic structure observation results of the cut surface as performed in current metal microstructure observations, and it is extremely difficult to accurately calculate this in structures where the presence of a collective structure is expected, such as in non-quenched and tempered steel. Therefore, as a compromise measure, for the circumference of the crystal grain, when the crystal grain in question is assumed to be a perfect sphere, the circumference obtained from the area of an arbitrary cut surface of the crystal grain is used. That is, this is the circumference of the true circle based on the area of the crystal grain when the so-called area of the crystal grain observed in metal microstructure observations, etc., is assumed to be a true circle.

[0018] Finally, it is the amount of retained austenite present in the bainite structure, which is set to 5% or less in terms of area fraction. The crystal structure of retained austenite is face-centered cubic lattice, while ferrite, which is the matrix structure of bainite, is body-centered cubic lattice, and this is more prone to deformation than the face-centered cubic lattice. That is, the retained austenite phase in ferrite is not only fine but also more difficult to deform than ferrite and serves as a source of transformation, so the smaller its amount, the better.

[0019] The inventors, having obtained the knowledge of the influence of the microstructure on the yield stress as described above, variously examined the combination of the alloy component balance of steel and the cooling rate after hot forging that can obtain such a bainite structure, narrowed down each range, and completed the present invention.

[0020] That is, the gist configuration of the present invention is as follows. 1. By mass% C: 0.36 - 0.45%, Si: 0.11 - 0.30%, Mn: 0.40 - 1.10%, P: 0.005 - 0.025%, S: 0.035 - 0.060%, Cr: 0.90 - 1.80%, Al: 0.010 to 0.070%, Ti: 0.005 to 0.020%, V: 0.05 to 0.15% and N: 0.0080 to 0.0150% including in the range where the value of F1 obtained by the following formula (1) is 0.65% or more, with the balance being Fe and inevitable impurities, and having a component composition and a structure in which the area ratio of the bainite structure is 93% or more and the area ratio of the retained austenite is 5% or less, The steel part in which the crystal grains of the bainite structure have an average diameter of 8 μm or more and 25 μm or less, an average aspect ratio of 0.40 or more, and an average ratio of the length of the grain boundary to the circumference of the crystal grain of 60 or less. F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14…(1) Here, each element symbol in formula (1) represents the content (mass%) of the element, and for elements not contained, F1 is calculated assuming 0.

[0021] 2. The steel part according to 1 above, wherein the ratio ((Hv1 - Hv2) / Hv1)×100 of the maximum value Hv1 of the difference between the maximum value Hv1 and the minimum value Hv2 of the Vickers hardness at 1 mm below the surface of the steel part is 10% or less.

[0022] 3. The steel of the component composition further contains, in mass% Cu: 0.25% or less, Ni: 0.25% or less, Mo: 0.20% or less and Nb: 0.030% or less The steel part according to 1 or 2 above containing any one or more of the above.

[0023] 4. In mass% C: 0.36 to 0.45%, Si: 0.11 to 0.30%, Mn: 0.40 to 1.10%, P: 0.005 to 0.025%, S: 0.035 to 0.060%, Cr: 0.90 to 1.80%, Al: 0.010 to 0.070%, Ti: 0.005 to 0.020%, V: 0.05 to 0.15% and N: 0.0080 to 0.0150% A steel material having a component composition containing the above in a range where the value of F1 obtained by the following formula (1) is 0.65% or more, with the balance being Fe and inevitable impurities, is heated and held at 1150°C or higher and then hot forged, and then cooled at a cooling rate of 0.8°C / s or more and 3.0°C / s or less from 900°C to 600°C and 0.10°C / s or more and 1.00°C / s or less from 600°C to 300°C. A method for manufacturing a steel component. F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14…(1) Here, each element symbol in formula (1) represents the content (% by mass) of the element, and for elements not contained, F1 is calculated with 0.

[0024] 5. The cooling from 600°C to 300°C is performed such that the ratio ((V1 - V2) / V1)×100 of the maximum value V1 of the difference between the maximum value V1 and the minimum value V2 of the cooling rate distribution in the hot forged material is 25% or less. The method for manufacturing a steel component according to 4 above.

[0025] 6. The component composition further contains, in mass%, Cu: 0.25% or less, Ni: 0.25% or less, Mo: 0.20% or less and Nb: 0.030% or less The method for manufacturing a steel component according to 4 or 5 above, containing any one or more of the above.

Advantages of the Invention

[0026] In the steel component of the present invention, since 93% or more of a bainite structure satisfying desired conditions can be obtained during cooling after hot forging, the ratio of the yield stress to the tensile strength (yield ratio) can be 0.60 or more without performing tempering.

Embodiments for Carrying Out the Invention

[0027] Hereinafter, the present invention will be specifically described. First, the reasons for limiting the amounts of the respective elements in the component composition will be explained. In the component composition, the “%” indication means “mass %” unless otherwise specified.

[0028] C: 0.36 to 0.45% C (carbon) dissolves in the steel and increases the yield stress of the steel by acting to make the bainite structure after transformation approach a spherical (perfect circle) shape. It is also a beneficial element that forms carbides to improve the strength of the steel and further contributes to obtaining a bainite structure by improving the hardenability of the steel. Therefore, C needs to be added in an amount of 0.36% or more. On the other hand, if added in excess, the amount of retained austenite increases and the yield stress cannot be maintained. For this reason, the upper limit of the addition amount is set to 0.45%. Preferably, it is 0.39 to 0.44%.

[0029] Si: 0.11 to 0.30% Si (silicon) dissolves in the steel and is a beneficial element that increases the strength of the steel, improves the hardenability, and increases the bainite area ratio at the same time. In order to obtain these effects, an addition of 0.11% or more is required. On the other hand, Si also has a harmful effect of forming a thick film during the preheating before hot forging for hot forging, deteriorating the scale peelability before hot forging. In order to avoid this, an addition exceeding 0.30% must be avoided. Therefore, Si is set to 0.30% or less.

[0030] Mn: 0.40 to 1.10% Mn (manganese) dissolves in the steel and improves the yield stress of the steel and the hardenability of the steel by acting to make the bainite structure after transformation approach a spherical (perfect circle) shape. It is also an important element having various beneficial effects such as combining with S to form sulfides and improving the machinability of the steel. In order to obtain these effects, an addition of 0.40% or more is required. On the other hand, if Mn is added in excess, the amount of retained austenite becomes too large and the yield stress cannot be maintained. For this reason, the upper limit of the addition amount is 1.10%. Preferably, it is 0.50 to 1.00%.

[0031] P: 0.005 - 0.025% P (phosphorus) is a beneficial element that increases the yield stress of steel by acting to make the bainite structure after transformation in the steel approach a spherical (perfect circle) shape when dissolved in the steel. To obtain this effect, an addition of 0.005% or more is required. On the other hand, P segregates at the grain boundaries of austenite after hot forging, resulting in a decrease in the toughness of the steel and, as a result, an increase in the energy transition temperature. To avoid this, the P content should be 0.025% or less. Preferably, it is 0.007 - 0.020%.

[0032] S: 0.035 - 0.060% S (sulfur) is a beneficial element that increases the yield stress of steel by acting to make the bainite structure after transformation in the steel approach a spherical (perfect circle) shape when dissolved in the steel. To obtain this effect, an addition of 0.035% or more is required. Preferably, it is 0.040 or more. On the other hand, if S is added in excess, a large amount of MnS is formed, reducing the amount of Mn dissolved in Fe, not only diminishing the beneficial effects of Mn such as strength improvement and hardenability improvement, but also causing MnS to act as a transformation source, resulting in a decrease in the yield stress of the steel. To avoid this, the addition amount of S is capped at 0.060%. More preferably, it is 0.055% or less.

[0033] Cr: 0.90 - 1.80% Cr (chromium) is an important element that has various beneficial effects such as increasing the yield stress and hardenability of steel by acting to make the bainite structure after transformation in the steel approach a spherical (perfect circle) shape when dissolved in the steel. To obtain these effects, an addition of 0.90% or more is required. On the other hand, if Cr is added in excess, the amount of retained austenite becomes too large, making it impossible to maintain a high yield stress. For this reason, the upper limit of the addition amount is 1.80%. Preferably, it is 1.00 - 1.70%.

[0034] Al: 0.010 - 0.070% Al (aluminum) is a beneficial element that combines with oxygen inevitably entering the molten steel from the air during steel refining and casting to render it harmless. If deoxidation by Al is insufficient, the excess oxygen in the steel combines with Ti, and the effects of Ti described below cannot be fully obtained. For this purpose, an addition of 0.010% or more is required. On the other hand, if the addition exceeds 0.070%, conversely, a large amount of aluminum oxide itself will be contained in the steel, deteriorating the toughness of the steel. Therefore, additions exceeding this should be avoided. Preferably, it is 0.020 - 0.060%.

[0035] Ti: 0.005 - 0.020% Ti (titanium) is a beneficial element that forms extremely fine precipitates in the steel and prevents deterioration of toughness by suppressing coarsening of austenite grains before and after hot forging. To obtain such an effect, an addition of 0.005% or more is required. On the other hand, if the addition exceeds 0.020%, the precipitates coarsen during heating before hot forging, losing the effect of suppressing coarsening of austenite grains and causing coarsening of austenite grains. Therefore, it is necessary to avoid additions exceeding 0.020%. Preferably, it is 0.006 - 0.017%.

[0036] V: 0.05 - 0.15% V (vanadium) is a beneficial element that dissolves in the steel to solid-solution strengthen the steel and increases the hardenability of the steel to obtain a sufficient bainite area ratio. To obtain such an effect, an addition of 0.05% or more is required. On the other hand, V also has an effect of deteriorating the toughness of the steel by combining with C to form precipitates. To avoid this, the addition amount of V should be 0.15% or less. Preferably, it is 0.06 - 0.14%.

[0037] N: 0.0080 - 0.0150% N (nitrogen) is a beneficial element that mainly combines with Ti and V to prevent the coarsening of austenite grains before and after hot forging, thereby preventing the deterioration of toughness. To obtain this effect, an addition of 0.0080% or more is required. On the other hand, if the addition exceeds 0.0150%, strain aging occurs at room temperature (the effect that N segregates around the transformation to form a Cottrell atmosphere, significantly hindering the mobility of the transformation), and the energy transition temperature rises significantly. To avoid this, the upper limit of the N addition amount is set to 0.0150%. Preferably, it is 0.0085 - 0.0140%.

[0038] F1: 0.65% or more F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 …(1) Here, each element symbol in formula (1) represents the content (mass%) of the corresponding element. For elements not contained, F1 is calculated with 0. The minimum component control for obtaining a bainite structure is as described above, which is carried out in terms of the type of components and the addition range for each component. However, in order to enhance the robustness of the mechanical properties obtained when there is a range in the cooling rate during the actual cooling after hot forging, it is necessary to define the balance of the added alloying elements according to formula (1) represented by F1. When the formula for determining this balance is defined as F1, its value needs to be 0.65% or more. That is, if the F1 value is less than 0.65%, even if a structure mainly composed of bainite is obtained, it becomes difficult to constantly obtain steel with a yield ratio of 0.60 or more.

[0039] It contains the above elements, and the balance is Fe and inevitable impurities. Here, elements considered as inevitable impurities include O (oxygen), B (boron), Mg (magnesium), Ca (calcium), REM (rare earth metal), etc. For any of these elements, the content is less than 0.0015%.

[0040] Furthermore, if necessary, one or more of Cu, Ni, Mo, and Nb can be added in addition to the above component composition. Cu: 0.25% or less Cu (copper) is an element that dissolves in steel and solid-solution strengthens the steel, and can preferably be added at 0.03% or more to ensure strength. However, if Cu is added excessively, the amount of retained austenite increases and the yield stress decreases, resulting in an inability to obtain a predetermined yield ratio. To avoid this, the upper limit of its addition is set at 0.25%, more preferably 0.20%.

[0041] Ni: 0.25% or less Ni (nickel) is an element that dissolves in steel and solid-solution strengthens the steel, and can preferably be added at 0.03% or more to ensure strength. However, if Ni is added excessively, the amount of retained austenite increases and the yield stress decreases, resulting in an inability to obtain a predetermined yield ratio. To avoid this, the upper limit of its addition is set at 0.25%, more preferably 0.20%.

[0042] Mo: 0.20% or less Mo (molybdenum) is an element that dissolves in steel and has various beneficial effects such as enhancing the strength and hardenability of the steel, and can preferably be added at 0.05% or more. However, if Mo is added excessively, the amount of retained austenite becomes too large and the yield stress cannot be maintained. Therefore, the upper limit when adding is 0.20%.

[0043] Nb: 0.030% or less Nb (niobium) is a beneficial element that forms extremely fine precipitates in steel and prevents deterioration of toughness by suppressing coarsening of austenite grains before and after hot forging, and can preferably be added at 0.005% or more. However, if Nb is added excessively, surface defects tend to occur frequently during hot rolling. Therefore, the upper limit when adding is 0.030%.

[0044] Next, the requirements for the microstructure will be specifically described. Area ratio of bainite microstructure: 93% or more Since the bainite structure has a good balance between strength and toughness, it is suitable as the structure of non-quenched and tempered steel. Specifically, for steel with a bainite structure, the energy transition temperature obtained by the Charpy impact test defined in JIS Z2242 is less than 228K. That is, if the energy transition temperature is less than 228K, sufficient toughness can be ensured at the service temperature assumed in the environment where humans use heavy-duty vehicles such as automobiles, trucks, and trailers on the earth. To obtain such characteristics, a bainite structure with an area ratio of 93% or more is required. Preferably, it is 95% or more.

[0045] In addition, the structure other than the bainite structure is not particularly limited and may be any of pearlite, ferrite, etc., but the retained austenite needs to have an area ratio of 5% or less. Area ratio of retained austenite: 5% or less Since the retained austenite serves as a transformation source and reduces the yield stress, the lower its area ratio, the better, and its upper limit is 5%. Of course, it may be 0%.

[0046] Average diameter of bainite structure crystal grains: 8μm or more and 25μm or less The crystal grain size of the bainite structure is a very important factor for making crystal grains difficult to deform. The crystal grains here refer to the crystal grains surrounded by crystal grain boundaries with an inclination of the angle difference of the crystal orientations of adjacent crystal grains of 15° or more at the crystal grain boundaries. The average diameter refers to the weighted average value of the diameters of all crystal grains in the field of view where 20 or more such crystal grains are included in an arbitrary observation plane of the test piece. The diameter of the crystal grain refers to the diameter obtained from the area of the crystal grain assuming that the shape of the crystal grain observed in the cross-section of the microstructure observation is a perfect circle.

[0047] That is, in order to obtain crystal grains that are difficult to deform, the average diameter of the above crystal grains needs to be 25 μm or less. This is because the distance from the transfer source to the grain boundary is short, so the amount of transfer that can be deposited is limited, and the stress due to transfer deposition is also suppressed accordingly, resulting in the suppression of crystal grain deformation. More preferably, it is 23 μm or less. On the other hand, if it becomes too fine, the grain boundary itself will act as a transfer source, so the lower limit is set to 8 μm. Preferably, it is 9 μm or more.

[0048] Average aspect ratio of crystal grains: 0.40 or more In order to obtain crystal grains that are difficult to deform, the shape of the crystal grains should be closer to a perfect sphere, and its aspect ratio needs to be 0.40 or more. Preferably, it is 0.45 or more. Here, the aspect ratio of the crystal grains is obtained as follows. First, the centroid position of the crystal grains obtained by observation is determined. Two arbitrary lines perpendicular to each other at this centroid position are drawn on the crystal grains. When the centroid position is not within the crystal grain plane like a C shape, the point on the grain boundary closest to the determined centroid position is used as this point. With the combination of the intersections of the two arbitrary lines drawn in this way and the crystal grains, when the difference between the intersection distance of one line and the intersection distance of the other line is the largest, the respective distances are taken as the longest side and the shortest side. The aspect ratio of the crystal grains is the value obtained by dividing the shortest side by the longest side. Also, the average is the arithmetic mean value of the aspect ratios of all the crystal grains within the field of view in a range where 20 or more crystal grains as described above are included in an arbitrary observation plane of the test piece.

[0049] Average ratio of the length of the grain boundary to the circumference of the crystal grains: 60 or less The ratio of the circumference of the crystal grains to the length of the grain boundaries obtained from the diameter of the above crystal grains needs to be 60 or less on average. Here, the length of the grain boundaries refers to the length of the grain boundaries obtained by converting the total number of pixels determined to be grain boundaries in the crystal grains measured by EBSD described later into length according to the observation magnification. Hereinafter, the ratio of the length of the grain boundaries to the circumference of the crystal grains is referred to as the grain boundary length ratio. The averaging of this grain boundary length ratio is also the weighted average of the grain boundary length ratios of all the crystal grains within the field of view in a field of view where 20 or more crystal grains are included in an arbitrary observation plane of the test piece. The average ratio of the length of the grain boundaries to the circumference of the above crystal grains (hereinafter, also referred to as the average grain boundary length ratio) can also be obtained by EBSD. And when this average grain boundary length ratio exceeds 60, the area of the grain boundaries serving as the dislocation emission source becomes too large, making it difficult to obtain a sufficient yield ratio. A more preferable average grain boundary length ratio is 55 or less.

[0050] Furthermore, it is preferable that the ratio ((Hv1 - Hv2) / Hv1)×100 of the maximum value Hv1 to the difference between the maximum value Hv1 and the minimum value Hv2 of the Vickers hardness at 1 mm below the surface of the steel part is 10% or less. [((Hv1 - Hv2) / Hv1)×100: 10% or less] Steel parts that satisfy the above conditions will have desired mechanical properties. The mechanical properties thus obtained are preferably uniform in each steel part. For this purpose, it is preferable to make the difference in hardness ((Hv1 - Hv2) / Hv1)×100 (hereinafter, also referred to as the subsurface hardness difference) below the part surface 10% or less. That is, since the deformation of the part is rate-limited by the surface hardness, if the subsurface hardness difference is 10% or less, the performance of the part will be more uniform. Note that the subsurface hardness difference can be obtained according to the measurement method in the examples described later.

[0051] Next, the conditions for manufacturing the above-described steel parts will be described. That is, after melting steel according to the above-described component composition and casting it into a steel material, for example, a steel ingot, the steel ingot is heated and held at a temperature of 1150°C or higher, and then hot forged into a desired part shape. Further, after this hot forging, it is important to cool at a cooling rate of 0.8°C / s or more and 3.0°C / s or less from 900°C to 600°C, and 0.10°C / s or more and 1.00°C / s or less from 600°C to 300°C.

[0052] [Steel material heating temperature: 1150°C or higher] First, the steel material is heated to 1150°C or higher. This is because in order to ensure forgeability during hot forging and dissolve carbides and nitrides, which are precipitates that deteriorate forgeability, it is necessary to heat to 1150°C or higher. Although the upper limit does not particularly need to be limited, from the viewpoint of deterioration of the yield due to the surface oxidation film, it is preferably 1300°C or lower.

[0053] After heating and holding the steel material at a temperature of 1150°C or higher, it is hot forged into a desired part shape. However, the conditions for hot forging are not particularly limited, and it may follow the general conditions for hot forging for forming each part. After this hot forging, it is necessary to perform the following two-stage cooling. Incidentally, the two-stage cooling is particularly advantageously suitable for the post-hot-forging stacking in factories in various foreign countries described above. That is, the cooling rate from the end of hot forging to 600°C, which is the first-stage cooling, conforms to the regulations at the stage until the forged product is put into the box. And the cooling rate after 600°C, which is the second-stage cooling, conforms to the regulations at the stage from after the forged product is put into the box until the start of transformation.

[0054] [From 900°C to 600°C at 0.8°C / s or more and 3.0°C / s or less] The process of imparting the desired strength to the non-quenched and tempered steel parts is that the cooling rate is very important during the cooling process of the parts after hot forging. Here, since the shapes of the hot forged parts are not uniform and various, there are parts with a slow cooling rate and conversely parts with a fast cooling rate in the parts during cooling. When such a variation in the cooling rate occurs within one part, in order to obtain a certain degree of uniform characteristics throughout the part during the cooling process, the non-quenched and tempered steel used as the material must have robustness against the cooling rate. From such a perspective, when the cooling rate of the steel within the component range defined in the present invention is less than 0.8 °C / s from 900 °C to 600 °C, the prior austenite grains before the bainite transformation become coarse, and the energy transition temperature obtained in the Charpy test increases. Therefore, the cooling rate from 900 °C to 600 °C is set to 0.8 °C / s or more. Preferably, it is 0.9 °C / s or more, more preferably 1.0 °C / s or more.

[0055] On the other hand, when the cooling rate in the above temperature range exceeds 3.0 °C / s, the prior austenite grains before the bainite transformation become too fine, and as a result, the average crystal grain size of the bainite becomes less than 8 μm. Therefore, the cooling rate from 900 °C to 600 °C is set to 3.0 °C / s or less. Preferably, it is 2.7 °C / s or less, more preferably 2.5 °C / s or less.

[0056] [Cooling rate from 600 °C to 300 °C: 0.10 °C / s or more and 1.00 °C / s or less] As described above, from the perspective of imparting robustness against the cooling rate to the non-quenched and tempered steel, the range from 900 °C to 600 °C is controlled within the range of 0.8 °C / s or more and 3.0 °C / s or less. However, even when this condition is satisfied, it is necessary to stipulate the cooling rate in the subsequent temperature range from 600 °C to 300 °C. That is, when the cooling rate from 600 °C to 300 °C is less than 0.10 °C / s, the average ratio of the above-described grain boundary lengths will exceed 60. Therefore, the cooling rate from 600 °C to 300 °C is set to 0.10 °C / s or more. Preferably, it is 0.12 °C / s or more.

[0057] On the other hand, even if the cooling rate from 900°C to 600°C is within the above-specified range, if the cooling rate from 600°C to 300°C exceeds 1.00°C / s, the average aspect ratio of the above-described crystal grains will be less than 0.40. Therefore, the cooling rate from 600°C to 300°C is set to 1.00°C / s or less. Preferably, it is 0.90°C / s or less.

[0058] Furthermore, the cooling from 600°C to 300°C is preferably performed such that the ratio ((V1 - V2) / V1)×100 of the maximum value V1 of the difference between the maximum value V1 and the minimum value V2 of the cooling rate distribution in the hot-forged material having the part shape given by hot forging to the maximum value V1 is 25% or less. [((V1 - V2) / V1)×100 is 25% or less] As described above, the mechanical properties are preferably uniform in each steel part, and particularly, as the performance of the part, it is preferable that the structure and hardness are uniform. Therefore, even if the part shape is complex, it is advantageous to keep the cooling rate within a certain range. For this purpose, it is preferable that the cooling rate difference (V1 - V2) / V1 (hereinafter, also referred to as the cooling rate difference) in the hot-forged material is 25% or less. If the cooling rate difference is 25% or less, the difference in structure and hardness is small, and the mechanical performance of the part becomes more uniform. Note that the cooling rate difference can be obtained according to the measurement method in the examples described later.

Examples

[0059] Hereinafter, the present invention will be described in more detail with reference to examples. However, each inventive example in the examples is merely an example, and the present invention is not limited to the following examples.

[0060] Steel having the components shown in Table 1 was melted in a vacuum melting furnace, and a 50 kg steel ingot (steel material) was cast. The steel ingot (ingot) thus obtained was hot-worked into a cylinder with a diameter of 32 mm at a temperature of 1150°C or higher. Next, immediately after holding the cylinder with a diameter of 32 mm obtained in this way at 1200°C for 1 hour, hot swaging (hot forging) was performed to obtain a round bar with a diameter of 32 mm. A plurality of round bars made of the same steel type under the same manufacturing conditions were prepared.

[0061] Note that the starting temperature of the hot swaging process is 1100°C or higher. When the starting temperature of the hot swaging process is controlled in this way, the finishing temperature of the swaging process was approximately 1000°C or higher. For the round bar with a diameter of 32 mm after the swaging process, cooling was performed in the range of 0.8°C / s to 3.0°C / s from 900°C to 600°C and in the range of 0.10°C / s to 1.00°C / s from 600°C to 300°C.

[0062] In the cooling treatment after the above swaging process, using a thermoviewer, the time change of the temperature at five locations separated by a 50 mm pitch in the axial direction from the tip of the round bar was measured, and the average cooling rate at each measurement location was obtained. The cooling rates from 900°C to 600°C and from 600°C to 300°C shown in Table 2 are the average values of the average cooling rates calculated at each of the five locations. Further, in the above cooling treatment, the cooling rate difference ((V1 - V2) / V1)×100 in the temperature range from 600°C to 300°C was obtained from the maximum value V1 and the minimum value V2 among the average cooling rates from 600°C to 300°C at the above five locations.

[0063] Furthermore, for the round bar (part) that had undergone the measurement of the cooling rate, its subsurface hardness was investigated. The subsurface hardness was measured with a Vickers hardness tester at two arbitrary circumferential locations at a position 1 mm in the radial direction from the surface in each of the radial cross-sections (so-called C-sections) of the above five cooling rate measurement positions, and a total of 10 locations on the entire round bar. Using the maximum value Hv1 and the minimum value Hv2 of this measurement result, ((Hv1 - Hv2) / Hv1)×100 was calculated and taken as the subsurface hardness difference. The obtained subsurface hardness difference is shown in Table 2 as the maximum value at the five cooling rate measurement positions.

[0064] From the round bar (part) with a diameter of 32 mm obtained by cooling after hot forging in this way, among the five positions corresponding to the above five cooling rate measurement positions (hereinafter referred to as measurement corresponding positions), from the D / 2 positions on one end side and the other end side of the round bar with the third measurement corresponding position as the boundary, two No. 4 tensile test pieces specified in JIS Z2241 were collected, one each. Therefore, one of the test pieces includes the above first and second measurement corresponding positions, and the other test piece includes the above fourth and fifth measurement corresponding positions.

[0065] Furthermore, from the D / 4 position of the above round bar (steel part), which is different from the round bar from which the tensile test piece was collected, five U-notch test pieces for Charpy impact test specified in JIS Z2242 were collected, one each at the above five measurement corresponding positions. The notch depth of this Charpy impact test piece is 5 mm. The mechanical properties and microstructure were investigated from the test pieces obtained in this way.

[0066] First, the tensile test was carried out in accordance with JIS Z2241 at a tensile speed of 0.167 mm / s. By this tensile test, the yield stress (the strength obtained by offsetting 0.2% from the elastic limit of the straight line obtained from the slope during elastic deformation, so-called 0.2% proof stress) and the ultimate strength reached (the ultimate strength in the nominal stress - nominal strain curve obtained by the tensile test, so-called tensile strength) were obtained. The obtained yield stress and tensile strength were taken as the average values for the two test pieces.

[0067] In the Charpy impact test, the energy transition temperature specified in Appendix D of JIS Z2242 was obtained. The energy transition temperature was obtained by conducting tests at multiple temperatures and drawing an approximate curve. If this energy transition temperature is 228 K (Kelvin) or lower, it can be said that the steel part has excellent toughness.

[0068] Next, regarding the microstructure, the following values were measured with the radial cross-section at the above temperature measurement corresponding position as the observation surface from the round bar (part) for which the surface hardness was measured. That is, the bainite area ratio was determined by the point method from an optical micrograph of the observation surface moderately etched with a nital solution. The point method is an area ratio measurement method for obtaining the ratio of the points on the tissue for which the area ratio is to be determined out of the total number of points appropriately arranged on the optical micrograph. There are no particularly preferable conditions for the size and arrangement method of the points, but generally, it is common to use so-called lattice points, which are the points where a plurality of lines arranged at equal intervals and orthogonally intersect on the tissue photograph. There is no regulation on the line thickness, but for example, it is common to form lattice points with lines having a thickness of 0.5 to 0.75 pt on the slide of the Microsoft PowerPoint application. The ratio of the total number of lattice points that overlap on the tissue photograph and the number of lattice points on the tissue for which the area ratio is to be measured as obtained above to the total number of lattice points can be regarded as the area ratio or volume ratio. When the lattice point overlaps on the boundary of the tissue, it may be counted as 0.5 lattice points. Then, the value obtained by subtracting the retained austenite area ratio determined by EBSD described later from the bainite area ratio thus obtained was taken as the bainite area ratio of the steel part.

[0069] In addition, the retained austenite area ratio, the average crystal grain size of the bainite structure, the average aspect ratio of the crystal grains, and the average ratio of the crystal grain boundary length were measured using EBSD (Electron Backscatter Diffraction). Also for these measurements by EBSD, each of the above items was measured in 5 randomly extracted fields, and the average value at 5 temperature measurement corresponding positions was taken as the value for the steel part. Regarding the definition of crystal grains in the tissue investigation by this EBSD, as described above, crystal grains surrounded by crystal grain boundaries with an angular difference of 15° or more in the crystal orientation between adjacent crystal grains were used. The results of these tests are also shown in Table 2.

[0070]

Table 1

[0071]

Table 2

[0072] In Tables 1 and 2, steels No. 1 to 38 are examples that satisfy the chemical composition of the present invention. Steel parts Nos. 1 to 38 and 1A made of these steels No. 1 to 38 and cooled within the cooling rate range according to the present invention after hot forging have the structure and excellent mechanical properties specified in the present invention.

[0073] In the comparative example of steel part No. 39, the C is below the specification of the present invention, and therefore the average aspect ratio of the crystal grains and the average ratio of the actual grain boundary length to the circumference of the crystal grain deviate from the specification of the present invention, so that sufficient yield stress cannot be obtained and the steel part has a low yield ratio.

[0074] In the comparative example of steel part No. 40, since the C content exceeds the limit of the present invention, sufficient yield stress cannot be obtained, resulting in a steel part with a low yield ratio.

[0075] In the comparative example of steel part No. 41, the Si content deviates from the specification of the present invention, so a sufficient bainite area ratio cannot be obtained, and as a result, the energy transition temperature of the steel part is 228K or higher.

[0076] The comparative example of steel part No. 42 is a steel in which the Si content is outside the range specified in the present invention, and a strong oxide film is formed during preheating for hot forging, so that the oxide film gets caught between the dies for hot forging, causing the equipment to not work properly and making it impossible to complete the hot forging. As a result, the properties of the part could not be evaluated.

[0077] In the comparative example of steel part No. 43, the Mn content is below the specification of the present invention, and therefore the average aspect ratio of the crystal grains and the average ratio of the actual grain boundary length to the circumference of the crystal grain deviate from the specification of the present invention, so that sufficient yield stress cannot be obtained and the steel part has a low yield ratio.

[0078] In the comparative example of steel part No. 44, since Mn deviates from the provisions of the present invention, the retained austenite area ratio becomes extremely high, resulting in insufficient yield stress and a low yield ratio for the steel part. Also, as a result of excessive retained austenite, the bainite area ratio has fallen below the specified value, so the steel part has an energy transition temperature of 228 K or higher.

[0079] In the comparative example of steel part No. 45, since P is below the provisions of the present invention, the average aspect ratio of the crystal grains and the average ratio of the actual grain boundary length to the circumference of the crystal grains deviate from the provisions of the present invention, resulting in insufficient yield stress and a low yield ratio for the steel part.

[0080] In the comparative example of steel part No. 46, since P deviates from the provisions of the present invention, the steel part has an energy transition temperature of 228 K or higher.

[0081] In the comparative example of steel part No. 47, since S is below the provisions of the present invention, the average aspect ratio of the crystal grains and the average ratio of the actual grain boundary length to the circumference of the crystal grains deviate from the provisions of the present invention, resulting in insufficient yield stress and a low yield ratio for the steel part.

[0082] In the comparative example of steel part No. 48, since S exceeds the provisions of the present invention, a large amount of MnS is formed. As a result, the effect of Mn cannot be obtained, and a sufficient bainite structure cannot be obtained. Consequently, insufficient yield stress is obtained, the yield ratio is low, and the steel part has an energy transition temperature of 228 K or higher.

[0083] In the comparative example of steel part No. 49, since Cu exceeds the provisions of the present invention, the retained austenite area ratio becomes extremely high, resulting in insufficient yield stress and a low yield ratio for the steel part. Also, as a result of excessive retained austenite, the bainite area ratio has fallen below the specified value, so the steel part has an energy transition temperature of 228 K or higher.

[0084] In the comparative example of steel part No. 50, since Ni exceeds the requirements of the present invention, the retained austenite area ratio becomes extremely high, resulting in insufficient yield stress and a low yield ratio for the steel part. Also, due to the excessive amount of retained austenite, the bainite area ratio falls below the requirements, so the steel part has an energy transition temperature of 228 K or higher.

[0085] In the comparative example of steel part No. 51, since Cr is below the requirements of the present invention, the average aspect ratio of the crystal grains and the average ratio of the actual grain boundary length to the circumference of the crystal grains deviate from the requirements of the present invention, resulting in insufficient yield stress and a low yield ratio for the steel part.

[0086] In the comparative example of steel part No. 52, since Cr exceeds the requirements of the present invention, the retained austenite area ratio becomes extremely high, resulting in insufficient yield stress and a low yield ratio for the steel part. Also, due to the excessive amount of retained austenite, the bainite area ratio falls below the requirements, so the steel part has an energy transition temperature of 228 K or higher.

[0087] In the comparative example of steel part No. 53, since Mo exceeds the requirements of the present invention, the retained austenite area ratio becomes extremely high, resulting in insufficient yield stress and a low yield ratio for the steel part. Also, due to the excessive amount of retained austenite, the bainite area ratio falls below the requirements, so the steel part has an energy transition temperature of 228 K or higher.

[0088] In the comparative example of steel part No. 54, since Al is below the requirements of the present invention, the effects of Ti as described above are not fully obtained. As a result, the austenite grains before transformation become coarsened, and the average crystal grain size exceeds the specified value, resulting in insufficient yield stress and a low yield ratio for the steel part.

[0089] In the comparative example of steel part No. 55, since Al exceeds the requirements of the present invention, the steel part has an energy transition temperature of 228 K or higher.

[0090] In the comparative example of steel part No. 56, since Ti is below the specified value of the present invention, as a result of the coarsening of the austenite grains before transformation, the steel part has an energy transition temperature of 228 K or higher.

[0091] In the comparative example of steel part No. 57, since Ti exceeds the specified value of the present invention, as a result of the coarsening of the austenite grains before transformation, the steel part has an energy transition temperature of 228 K or higher.

[0092] In the comparative example of steel part No. 58, since V is below the specified value of the present invention, a sufficient bainite area ratio cannot be obtained. As a result, the steel part has an energy transition temperature of 228 K or higher.

[0093] In the comparative example of steel part No. 59, since V exceeds the specified value of the present invention, excessive VC is precipitated, resulting in a decrease in the toughness of the steel. As a result, the steel part has an energy transition temperature of 228 K or higher.

[0094] In the comparative example of steel part No. 60, since Nb exceeds the specified value of the present invention, a large amount of cracks were formed in the material to be hot forged during sizing for hot forging. As a result, it was difficult to secure a sufficient volume for test piece processing, so the properties of the steel could not be investigated. Since such steel cannot be actually manufactured, it was used as a comparative example.

[0095] In the comparative example of steel part No. 61, since N is below the specified value of the present invention, the effect of suppressing the coarsening of austenite grains by precipitates such as Ti was not sufficiently obtained. As a result, the austenite grains coarsened, and the steel part has an energy transition temperature of 228 K or higher.

[0096] In the comparative example of steel part No. 62, since N exceeds the specified value of the present invention, strain aging occurs, and the steel part has an energy transition temperature of 228 K or higher.

[0097] The comparative example of steel part No. 63 has a chemical composition within the specified range of the present invention, but since the value of F1 is outside the specified range of the present invention, although the cooling rate in each temperature range was within the specified range of this patent, the average aspect ratio of the crystal grains deviated from the specification of this patent, resulting in a steel part where a sufficient yield ratio could not be obtained.

Example

[0098] Next, for a steel having the component composition of steel No. 29 shown in Table 1, it was melted in a vacuum melting furnace, a 50 kg steel ingot (steel material) was cast, and for a 25 mm diameter round bar obtained by subjecting it to swaging under the same conditions as in Example 1, after hot swaging, it was cooled under various cooling rate conditions shown in Table 3 to obtain steel parts. Note that for some round bars (steel parts No. 76, 77, 78), the length of the tip 150 mm portion was made 30 mm in diameter. That is, to simulate the case where the part has a complex shape and investigate the influence of the cooling rate difference on the characteristics, the cooling rate difference was intentionally imparted. The results of investigating the steel structure and mechanical properties of the thus obtained steel parts in the same manner as in Example 1 are also shown in Table 3.

[0099]

Table 3

[0100] Steel parts No. 65, 66, 68, 69, 71, 72, 74, 75, 76, 77, 78 shown in Table 3 are inventive examples that satisfy the component composition of the present invention and the cooling rate conditions after hot forging. On the other hand, steel part No. 64 is a comparative example where the cooling rate from 900 °C to 600 °C is below the specification of the present invention, and due to the coarsening of the prior austenite crystal grains, the energy transition temperature has become greater than 228 K.

[0101] Also, for steel part No. 67, as a result of the cooling rate from 900 °C to 600 °C exceeding the specification of the present invention, the average of the crystal grain sizes of bainite is less than 8 μm, which is the lower limit of the range specified in the present invention, and it is a comparative example where a yield ratio of 0.60 or more could not be obtained.

[0102] Steel part No. 70 is an example in which the average ratio of the actual grain length to the circumferential length when the grain is made into a perfect circle exceeds the upper limit of 60 of the range specified in the present invention as a result of the cooling rate from 600°C to 300°C being lower than the specification of the present invention, and it is a comparative example in which a yield ratio of 0.60 or more could not be obtained.

[0103] Steel part No. 73 is an example in which the average aspect ratio of the crystal grains becomes smaller than the lower limit of 0.40 of the range specified in the present invention as a result of the cooling rate from 600°C to 300°C exceeding the specification of the present invention, and it is a comparative example in which a yield ratio of 0.60 or more could not be obtained.

[0104] For steel parts No. 76, 77, and 78, part of them has a diameter of 30 mm. Since the cooling rate difference from 600°C to 300°C exceeds 25%, this is a case where the surface hardness difference in the parts exceeds 10%.

Claims

1. by mass percentage C: 0.36 to 0.45%, Si: 0.11 to 0.30%, Mn: 0.40 to 1.10%, P: 0.005 to 0.025%, S: 0.035 to 0.060%, Cr: 0.90 to 1.80%, Al: 0.010 to 0.070%, Ti: 0.005 to 0.020%, V: 0.05 to 0.15% and N: 0.0080 to 0.0150% are included in a range where the value of F1 obtained by the following formula (1) is 0.65% or more, and the balance is Fe and inevitable impurities, and has a structure in which the area ratio of the bainite structure is 93% or more and the area ratio of the retained austenite is 5% or less. The crystal grains of the bainite structure have an average diameter of 8 μm or more and 25 μm or less, an average aspect ratio of 0.40 or more, and an average ratio of the length of the grain boundary to the circumference of the crystal grain of 60 or less, a steel part. F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14…(1) Here, each element symbol in formula (1) is the content (mass%) of the element, and for elements not contained, F1 is calculated with 0.

2. The maximum value Hv of the Vickers hardness at 1 mm below the surface of the steel component １ and the minimum value Hv ２ and the maximum value Hv of the difference therebetween １ The ratio ((Hv １ −Hv ２ ) / Hv １ ) × 100 is 10% or less. The steel component according to claim 1

3. The steel having the above composition further contains, by mass percentage Cu: 0.25% or less, Ni: 0.25% or less, Mo: 0.20% or less and Nb: 0.030% or less The steel part according to claim 1 or 2 containing any one or more of the above.

4. by mass percentage C: 0.36 to 0.45%, Si: 0.11 to 0.30%, Mn: 0.40 to 1.10%, P: 0.005 to 0.025%, S: 0.035 to 0.060%, Cr: 0.90 to 1.80%, Al: 0.010 to 0.070%, Ti: 0.005 to 0.020%, V: 0.05 to 0.15% and N: 0.0080 to 0.0150% A steel material having a component composition containing, in a range where the value of F1 obtained by the following formula (1) is 0.65% or more, with the balance being Fe and inevitable impurities, is heated and held at 1150°C or higher and then subjected to hot forging. Subsequently, it is cooled at a cooling rate of 0.8°C / s or more and 3.0°C / s or less from 900°C to 600°C, and 0.10°C / s or more and 1.00°C / s or less from 600°C to 300°C. It has a structure with an area ratio of bainite structure of 93% or more and an area ratio of retained austenite of 5% or less, and the crystal grains of the bainite structure have an average diameter of 8 μm or more and 25 μm or less, an average aspect ratio of 0.40 or more, and an average ratio of the length of the grain boundary to the circumference of the crystal grain of 60 or less. A method for manufacturing a steel part. F1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14…(1) Here, each element symbol in formula (1) represents the content (mass%) of the element, and for elements not contained, F1 is calculated with 0.

5. The cooling from 600 ° C to 300 ° C is the maximum value V of the cooling rate distribution in the hot forged material subjected to the hot forging. １ and the minimum value V ２ The maximum difference between １ Ratio to ((V １ -V ２ ) / V １ 5. The method of claim 4, wherein the ratio of the strain rate to the strained state is 25% or less.

6. The component composition is further in mass% Cu: 0.25% or less, Ni: 0.25% or less, Mo: 0.20% or less and Nb: 0.030% or less The method for manufacturing a steel part according to claim 4 or 5, containing any one or more of the above.

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