Hot-forged non-quenched and tempered steel and manufacturing method thereof

Abstract

Provided are: a hot-forged non-quenched and tempered steel which does not undergo tempering after hot forging, has high strength and high toughness, and has good high-frequency hardenability; and a method for producing the hot-forged non-quenched and tempered steel. The non-quenched and tempered steel has a prescribed component composition in which the value of Ceq (mass%) represented by formula (1) is 0.750-0.870, the component composition satisfies formula (2), and the non-quenched and tempered steel further has a microstructure in which the area ratio of a bainite structure is 90% or more. Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) 0.200 < = C-Mn / 18-Cr / 24-Mo / 3 < = 0.350 (2) The element symbols in the formulae (1) and (2) refer to the content of the element in the steel, and are 0 when the element is not contained in the steel.

Description

Technical Field

[0001] The present invention relates to a hot-forged non-quenched and tempered steel and a manufacturing method thereof. Background Art

[0002] Heretofore, high strength and high toughness have been required for chassis components of automobiles such as steering knuckles and upper arms, or for hydraulic components of construction machinery such as rod ends. Therefore, in these components, S43C, S45C, S48C, etc., which belong to carbon steels for mechanical structures, are used as raw materials. After hot forging and forming, heat treatment such as quenching and tempering (hereinafter, also referred to as "quenching and tempering") is carried out, and induction hardening is performed on the surface according to the type of component to ensure the required characteristics.

[0003] However, these quenching and tempering processes require a large amount of energy. Therefore, in order to meet the current social requirements for energy conservation, in recent years, non-quenched and tempered steels (hot-forged non-quenched and tempered steels) that do not require the above-mentioned quenching and tempering and have the required characteristics of steel in the formed state have been actively developed through hot forging and forming.

[0004] Heretofore, the design of hot-forged non-quenched and tempered steels (hot-forged non-quenched and tempered steels) to improve strength by precipitation strengthening of V carbonitrides added in the ferrite-pearlite structure has become mainstream. However, in the design aiming for higher strength, if the carbon content and V addition amount are simply increased, the toughness may be significantly impaired. Therefore, hot-forged non-quenched and tempered steels having a bainite structure have been developed with attention.

[0005] For example, in Patent Document 1, a non-quenched and tempered steel in which 0.05 to 0.50% by mass of V is added to a low-carbon steel containing about 0.10 to 0.30% by mass of C is proposed. This non-quenched and tempered steel does not require heat treatment after hot forging and has excellent strength and toughness by natural air cooling.

[0006] In Patent Document 2, a non-quenched and tempered steel containing more than 0.30% and 0.60% or less of C, more than 1.60% and 3.00% or less of Mn, and having a bainite structure in the hot-forged state by adding V or Nb is proposed.

[0007] In Patent Document 3, a non-quenched and tempered steel in which C is 0.10 to 0.35% and V is 0.30 to 0.70% and a bainite structure can be obtained without performing quenching and tempering treatment after hot forging is proposed.

[0008] In Patent Document 4, a non-quenched and tempered steel in which C is 0.25 to 0.38%, Mn is 1.51 to 2.2%, V is added, and a small amount of Mo is contained, and the structure after hot forging is mainly a bainite structure is proposed.

[0009] Prior Art Documents

[0010] Patent Documents

[0011] Patent Document 1: Japanese Patent Publication No. 2743116

[0012] Patent Document 2: Japanese Patent Publication No. 3196006

[0013] Patent Document 3: Japanese Patent Publication No. 3241897

[0014] Patent Document 4: Japanese Patent Publication No. 6390685 Summary of the Invention

[0015] However, the non-quenched and tempered steel described in Patent Document 1 has a problem that high strength cannot be obtained due to low carbon.

[0016] In addition, in the composition design of the precipitates described in Patent Document 2, there are problems of large performance deviations caused by differences in cooling rates due to forging conditions (heating temperature, processing temperature, etc.) and component wall thickness, and there are also concerns about material deterioration (reduction in toughness and fatigue limit) caused by segregation of Mn due to an increase in the Mn content.

[0017] Moreover, in the technology described in Patent Document 3, when obtaining strength with a relatively low C content, a large amount of V needs to be added, but in this case, there is a problem of deteriorating toughness.

[0018] In addition to the above, the technology described in Patent Document 4 has problems of material deterioration (reduction in toughness and fatigue limit) caused by segregation of Mn due to a high Mn content. In addition, if V precipitates, there is a concern about deteriorating toughness.

[0019] The present invention has been developed to solve the above problems of existing non-quenched and tempered steels, and an object thereof is to provide a non-quenched and tempered steel having high strength, high toughness, and good high-frequency hardenability even without tempering after hot forging, and a favorable manufacturing method therefor.

[0020] For the above object, the inventors have conducted in-depth research on hot-forged non-quenched and tempered steels repeatedly, and as a result, obtained the following insights.

[0021] (1) Regarding the C content, a certain level is required to ensure the hardness of high-frequency quenching, but if it is too high, the amount of retained austenite in the steel structure will increase, and the toughness will deteriorate rapidly.

[0022] (2) If the C content of the low-carbon bainitic steel is simply increased, the strength becomes high, and the toughness decreases accordingly. However, by achieving a balance of Mn, Mo, and Cr to match the increase in the C content, the bainite structure can be maintained while increasing the strength. In addition, the decrease in toughness is suppressed due to the presence of the above bainite structure. In particular, if the addition amount of V is suppressed, the above effects can be obtained more effectively.

[0023] (3) When V carbonitrides precipitate, they are easily affected by forging conditions (heating temperature, processing conditions, cooling rate). Therefore, greatly reducing V can stabilize the material.

[0024] (4) When ferrite is not mixed in the structure of the steel, the depth of induction hardening becomes uniform within the cross-section of the steel, and the hardness deviation of the hardened layer can also be suppressed.

[0025] The present invention is based on the above insights.

[0026] That is, the gist of the present invention is as follows.

[0027] 1. A hot-forged non-quenched and tempered steel, which is a steel in a hot-forged state, having the following composition by mass%: C: 0.35% to 0.48%, Si: 0.05% to 0.35%, Mn: 0.50% to 1.20%, P: 0.005% to 0.020%, S: 0.030% to 0.070%, Al: 0.015% to 0.050%, Cr: 0.60% to 1.50%, Mo: 0.05% to 0.25%, V: less than 0.050%, B: 0.0005% or less, and N: 0.0030% to 0.0200%. The Ceq (mass%) represented by the following formula (1) is 0.750 to 0.870, satisfying the following formula (2). The balance is composed of Fe and inevitable impurities, and the area ratio of the bainite structure in the microstructure is 90% or more.

[0028] Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ··· (1)

[0029] 0.200 ≤ C - Mn / 18 - Cr / 24 - Mo / 3 ≤ 0.350 ··· (2)

[0030] The element symbols in the above formulas (1) and (2) refer to the content of the element in the steel, and are 0 when the element is not contained in the steel.

[0031] 2. The hot-forged non-quenched and tempered steel according to the above 1, wherein the above composition further contains by mass% one or more selected from Cu: 0.01% to 0.30%, Ni: 0.01% to 0.30%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.025%, Pb: 0.05% to 0.30%, Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, Bi: 0.05% to 0.30%, and Sb: 0.0015% to 0.0100%.

[0032] 3. The hot-forged non-quenched and tempered steel according to 1 or 2 above, wherein the above composition further contains Sn: 0.0010% to 0.030% by mass.

[0033] 4. A method for manufacturing a hot-forged non-quenched and tempered steel, in which a steel billet is heated to 1100 to 1300 °C and then hot-forged, and cooled at an average cooling rate of 0.10 to 3.00 °C / s between 950 °C and 350 °C. The above steel billet has the following composition, containing by mass: C: 0.35% to 0.48%, Si: 0.05% to 0.35%, Mn: 0.50% to 1.20%, P: 0.005% to 0.020%, S: 0.030% to 0.070%, Al: 0.015% to 0.050%, Cr: 0.60% to 1.50%, Mo: 0.05% to 0.25%, V: less than 0.050%, B: 0.0005% or less, and N: 0.0030% to 0.0200%. Ceq (mass%) shown in the following formula (1) is 0.750 to 0.870, satisfying the following formula (2), and the balance is composed of Fe and unavoidable impurities.

[0034] Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ··· (1)

[0035] 0.200 ≤ C - Mn / 18 - Cr / 24 - Mo / 3 ≤ 0.350 ··· (2)

[0036] The element symbols in the above formulas (1) and (2) refer to the content of the element in the steel, and are 0 when the element is not contained in the steel.

[0037] 5. The method for manufacturing a hot-forged non-quenched and tempered steel according to 4 above, wherein the above composition further contains one or more selected from Cu: 0.01% to 0.30%, Ni: 0.01% to 0.30%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.025%, Pb: 0.05% to 0.30%, Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, Bi: 0.05% to 0.30%, and Sb: 0.0015% to 0.0100% by mass.

[0038] 6. The method for manufacturing a hot-forged non-quenched and tempered steel according to 4 or 5 above, wherein the above composition further contains Sn: 0.0010% to 0.030% by mass.

[0039] According to the present invention, a non-quenched and tempered steel having high strength, high toughness, and excellent high-frequency hardenability can be obtained without performing heat treatment (quenching and tempering) after hot forging. Detailed implementation mode

[0040] The present invention will be specifically described below.

[0041] [Hot forging non-quenched and tempered steel]

[0042] First, for the non-quenched and tempered steel of the present invention, the reasons for limiting the component composition of the steel to the above range will be described. It should be noted that in the following description, the content (%) of each element within the range without special description refers to mass%.

[0043] C: 0.35 - 0.48%

[0044] C is an element necessary to ensure strength and has the effect of increasing surface hardness during high-frequency quenching. To achieve the above effects, it is necessary to contain 0.35% or more. Preferably 0.37% or more, more preferably 0.39% or more. On the other hand, if the C content exceeds 0.48%, the amount of retained austenite increases too much and the toughness decreases. Therefore, the upper limit of the C content is 0.48%. Preferably 0.45% or less, more preferably 0.43% or less.

[0045] Si: 0.05 - 0.35%

[0046] Si is useful as a deoxidizer during the melting of steel, that is, in the steelmaking process, and must contain 0.05% or more. Preferably 0.06% or more, more preferably 0.09% or more. On the other hand, if the Si content exceeds 0.35%, the toughness decreases. Therefore, the upper limit of the Si content is 0.35%. Preferably 0.30% or less, more preferably 0.28% or less.

[0047] Mn: 0.50 - 1.20%

[0048] Mn is an element useful for improving the hardenability of steel and making the structure bainitic. However, if the Mn content is less than 0.50%, the hardenability is insufficient, the amount of bainite structure formed is small, and sufficient strength and toughness cannot be obtained. Therefore, Mn is contained at 0.50% or more. Preferably 0.55% or more, more preferably 0.60% or more. On the other hand, if the Mn content exceeds 1.20%, the hardenability becomes too high and the formation of retained austenite amount is promoted. As a result, not only the toughness decreases, but the fatigue limit also decreases. Therefore, the upper limit of the Mn content is 1.20%. Preferably 1.15% or less, more preferably 1.10% or less.

[0049] P: 0.005 - 0.020%

[0050] P is an element that segregates at prior austenite grain boundaries and reduces toughness. If its content exceeds 0.020%, the adverse effect on toughness is significant, so 0.020% is set as the upper limit. It is preferably 0.018% or less, more preferably 0.016% or less. On the other hand, the less P, the better the toughness, but the refining cost will increase, so the lower limit is 0.005%.

[0051] S: 0.030 - 0.070%

[0052] S is an element useful for improving machinability. To obtain this effect, it needs to contain 0.030% or more, so 0.030% is set as the lower limit. It is preferably 0.035% or more. On the other hand, excessive addition exceeding 0.070% makes the generated MnS a fracture initiation point and reduces toughness. Therefore, 0.070% is set as the upper limit. It is preferably 0.065% or less, further preferably 0.060% or less.

[0053] Al: 0.015 - 0.050%

[0054] Al is an element with a strong deoxidation effect. However, if the content is less than 0.015%, the sufficient deoxidation effect cannot be obtained, so the lower limit of the Al content is 0.015%. It is preferably 0.019% or more. On the other hand, if it contains more than 0.050% of Al, not only does its addition effect saturate, but the fatigue limit also decreases due to excessive inclusions. Therefore, the upper limit of the Al content is 0.050%. It is preferably 0.045% or less, more preferably 0.040% or less.

[0055] Cr: 0.60 - 1.50%

[0056] Cr, like Mn, is an element required for bainitic transformation of the structure. However, if the Cr content is less than 0.60%, the manifestation of this effect is insufficient. On the other hand, if the Cr content exceeds 1.50%, it promotes the formation of the amount of retained austenite and reduces the fatigue limit. Therefore, the Cr content is in the range of 0.60 - 1.50%. It is preferably 0.65% or more, more preferably 0.70% or more. In addition, it is preferably 1.40% or less, more preferably 1.30% or less.

[0057] Mo: 0.05 - 0.25%

[0058] Mo is an element required to suppress the transformation of ferrite and pearlite, bainitize the structure, refine bainite laths to improve toughness, and suppress the amount of retained austenite. However, if the content of Mo is less than 0.05%, the manifestation of the above effects becomes insufficient. Therefore, the lower limit of the Mo content is 0.05%. It is preferably 0.07% or more, and more preferably 0.10% or more. On the other hand, if the content of Mo exceeds 0.25%, the cost becomes high and the formation of the amount of retained austenite is suppressed, and the impact value and fatigue limit decrease. Therefore, the upper limit of the Mo content is 0.25%. It is preferably 0.20% or less, and more preferably 0.18% or less.

[0059] V: less than 0.050%

[0060] V has a strong affinity for C and N and precipitates as carbonitrides in steel. However, in the bainite structure, this effect is small and instead reduces toughness and the like. Therefore, the V content needs to be controlled to less than 0.050%. To improve low-temperature toughness, it is preferably 0.040% or less, and more preferably 0.035% or less. On the other hand, the lower limit of the V content is not particularly limited, but due to the mixing of impurities in the raw materials, it is preferably about 0.001%.

[0061] B: 0.0005% or less

[0062] B is an element that improves hardenability and strength, but the hardenability depends greatly on the cooling rate, and strength deviation occurs in the raw material after hot forging. Therefore, the upper limit is set to 0.0005%. It is preferably 0.0003% or less. On the other hand, the lower limit of the B content is not particularly limited, but due to the mixing of impurities in the raw materials, it is preferably about 0.00005%.

[0063] N: 0.0030% - 0.0200%

[0064] N forms nitrides in steel and has the effect of suppressing the coarsening of crystal grain size during heating. To obtain this effect, at least 0.0030% or more needs to be added. It is preferably 0.0050% or more. More preferably 0.0060% or more. On the other hand, since excessive addition will promote crack defects in the raw materials and cause a decrease in toughness, the upper limit is set to 0.0200%. It is preferably 0.0190% or less. More preferably 0.0180% or less.

[0065] Moreover, in the present invention, the component composition needs to satisfy the following formulas (1) and (2).

[0066] 0.750 ≤ Ceq(=C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14) ≤ 0.870 ···(1)

[0067] 0.200 ≤ C - Mn / 18 - Cr / 24 - Mo / 3 ≤ 0.350 ··· (2)

[0068] The element symbols in the above formulas (1) and (2) refer to the content of the element in the steel, and are 0 when the element is not contained in the steel.

[0069] In addition, the above formulas (1) and (2) are both indexes for controlling the bainite structure. By converging these elements within the index range, the specified strength and excellent toughness in the bainite structure can be obtained. The Ceq (mass %) shown in the above formula (1) is preferably 0.770 - 0.850.

[0070] The above is an explanation of the basic components of the present invention. However, the remaining part of the hot-forged non-quenched and tempered steel of the present invention other than the above components is Fe and inevitable impurities. However, the present invention can further appropriately add the following components as needed.

[0071] Specifically, it is one or more selected from Cu: 0.01% - 0.30%, Ni: 0.01% - 0.30%, Nb: 0.005% - 0.050%, Ti: 0.005% - 0.025%, Pb: 0.05% - 0.30%, Ca: 0.0005% - 0.0050%, Mg: 0.0005% - 0.0050%, Bi: 0.05% - 0.30%, Sb: 0.0015% - 0.0100%, and Sn: 0.0001 - 0.030%.

[0072] Cu, Ni, and Nb are elements effective for improving strength. To obtain this effect, when adding Cu, Ni, and Nb, it is necessary to add Cu: 0.01% or more, Ni: 0.01% or more, and Nb: 0.005% or more, respectively. On the other hand, excessive addition of Cu, Ni, and Nb will lead to a decrease in surface properties, an increase in manufacturing cost, and a decrease in toughness. Therefore, the upper limit of each addition is set to Cu: 0.30%, Ni: 0.30%, and Nb: 0.050%.

[0073] Ti forms TiN, etc., and has the effect of suppressing the coarsening of crystal grain size during heating and improving toughness. To obtain this effect, it is necessary to add 0.005% or more. On the other hand, excessive addition will lead to a decrease in toughness and fatigue strength due to the formation of coarse precipitates. Therefore, the upper limit of addition is set to 0.025%.

[0074] Pb, Ca, Mg, and Bi are all elements effective in improving machinability. To achieve this effect, when adding Pb, Ca, Mg, and Bi, the respective addition amounts need to be Pb: 0.05% or more, Ca: 0.0005% or more, Mg: 0.0005% or more, and Bi: 0.05% or more. On the other hand, even if added in large amounts, not only will this effect saturate, but it will also reduce toughness. Therefore, the upper limits of the respective additions are set to Pb: 0.30%, Ca: 0.0050%, Mg: 0.0050%, and Bi: 0.30%.

[0075] Sb has the effect of suppressing decarburization of the surface layer during high-temperature heating and improving fatigue strength. Since this effect appears at 0.0015% or more, the lower limit is set to 0.0015%. On the other hand, since excessive addition will reduce toughness, the upper limit of addition is set to 0.0100%.

[0076] Sn has the effect of suppressing decarburization of the surface layer during high-temperature heating and improving fatigue strength. Since this effect appears at 0.0010% or more, the lower limit is set to 0.0010%. On the other hand, excessive addition will reduce toughness, so the upper limit of addition is set to 0.030%.

[0077] In the present invention, the steel structure is specified as follows.

[0078] Area ratio of bainite structure: 90% or more

[0079] To obtain specified strength and high toughness, etc., the hot-forged non-quenched and tempered steel of the present invention requires the above steel composition, and the bainite structure in the microstructure must be 90% or more in terms of area ratio. Also, in the surface hardening treatment by high-frequency heat treatment, compared with the ferrite-pearlite structure in which carbon is unevenly distributed, the bainite structure with uniform carbon distribution also has the advantage of smaller hardness non-uniformity. The above area ratio is preferably 93% or more, more preferably 95% or more, and can also be 100%.

[0080] The above steel structure can be obtained by adjusting the heating temperature before hot forging and the average cooling rate after hot forging when the steel billet is hot forged into a component shape. Specifically, the two manufacturing conditions of the heating temperature before hot forging: 1100 - 1300 °C and the average cooling rate after hot forging: 0.10 - 3.00 °C / s need to be satisfied.

[0081] Heating temperature: 1100 - 1300 °C

[0082] In hot forging, if the heating temperature is low, the above-described steel structure cannot be obtained. That is, if the heating temperature before hot forging is lower than 1100 °C, ferrite is likely to be generated, and it is impossible to achieve an area ratio of bainite structure in the microstructure of 90% or more. On the other hand, the higher the heating temperature, the easier it is to obtain a bainite structure, but it will cause adverse conditions such as a decrease in toughness due to coarsening of the structure, a decrease in the yield rate due to scale loss, and an increase in energy costs. Therefore, the upper limit of the heating temperature is 1300 °C. The heating temperature is preferably 1150 °C to 1250 °C. This heating temperature is the central temperature of the steel billet.

[0083] Average cooling rate after hot forging: 0.10 to 3.00 °C / s

[0084] If the average cooling rate in the specified temperature range after hot forging is less than 0.10 °C / s, the steel structure becomes a ferrite - pearlite structure and the strength decreases. On the other hand, if this average cooling rate exceeds 3.00 °C / s, the hardness of the obtained steel becomes too high and the toughness decreases significantly. This average cooling rate is preferably 0.30 °C / s to 2.80 °C / s, more preferably 0.50 °C / s to 2.50 °C / s. This average cooling rate is the cooling rate at the surface temperature, and the above-specified temperature range refers to the range where the surface temperature is from 950 to 350 °C.

[0085] Furthermore, manufacturing conditions other than the above for the preferred manufacturing method of the hot-forged non-quenched and tempered steel according to the present invention will be described.

[0086] The molten steel having the above composition is melted by a usual smelting method such as a converter or an electric furnace, and formed into a steel billet by a usual continuous casting and blooming method. Then, the steel billet is heated as needed and hot-rolled into a bar steel by hot rolling such as sheet rolling or bar and wire rolling. The above heating and rolling conditions are not particularly limited, but can be appropriately determined according to the required material. For example, in order to be advantageous for forging, machining, etc. for subsequent component forming, structure control such as MnS control for improving machinability can be performed.

[0087] In addition, the content of each element in the steel can be determined by methods such as spark discharge atomic emission spectrometry, X-ray fluorescence spectrometry, ICP emission spectrometry, ICP mass spectrometry, and combustion method.

[0088] It should be noted that other manufacturing conditions not described in this specification can follow the general manufacturing method of steel.

[0089] Examples

[0090] Next, examples of the present invention will be described. It should be noted that the following examples are examples shown for more specifically explaining the present invention, and the present invention is not limited only to the scope of the following examples.

[0091] [Example 1]

[0092] Ingots with the composition shown in Table 1 (the balance, Fe and inevitable impurities) were hot-rolled into round bars with a diameter of 36 mm. After heating them to 1250 °C, they were hot-forged into round bars with a diameter of 25 mm and air-cooled to 600 °C, and then slowly cooled to 100 °C at a rate of 0.15 °C / s. The average cooling rate in the temperature range from 950 °C to 600 °C was 0.50 °C / s. Therefore, the average cooling rate in the temperature range from 950 °C to 350 °C was 0.35 °C / s.

[0093] The round bars after air-cooling and slow cooling were used as test materials, and the microstructure, tensile strength, fatigue limit, and impact value were measured for each test material (one for each test condition) using the method described below. In addition, the high-frequency hardenability was evaluated using the method described below.

[0094] The methods for measuring the microstructure, tensile strength, fatigue limit, and impact value are as follows.

[0095] (1) For the microstructure, the cross-section of the steel was ground and etched with nital, and the exposed cross-section was observed with an optical microscope and photographed. The bainite fraction (bainite area ratio) was obtained by image processing of the resulting image.

[0096] Specifically, three fields of view were photographed at a magnification of 400 times, and the total area measured was 105600 μm 2 (35200 μm for each field of view 2 ). In addition, the bainite phase was specified as a composite phase of ferrite laths and carbides other than ferrite, pearlite, and residual γ phase, and the above bainite fraction was derived by image analysis using the software ImageJ.

[0097] (2) For the tensile strength, JIS No. 4 tensile test pieces were collected from the round bars, and a tensile test was carried out at a tensile speed of 1 mm / s in accordance with JIS Z 2241 for measurement.

[0098] (3) For the fatigue limit, smooth test pieces with a diameter of 8 mm were collected, and the maximum stress at which no fracture occurred up to 10 7 cycles was determined by the Ono-type rotating bending fatigue test in accordance with JIS Z 2274.

[0099] (4) For the impact value, Charpy test pieces with a U-notch having a width of 3 mm and a depth of 5 mm were collected in a 10 mm square, and the Charpy impact test was carried out after cooling to -50 °C for measurement.

[0100] In addition, the high-frequency hardenability is evaluated as follows by the surface hardness after high-frequency quenching, the average hardened layer depth, and the standard deviation of the effective hardened layer depth.

[0101] At a frequency of 200 Hz, quenching conditions capable of obtaining an effective hardened layer depth exceeding 2.00 mm in a measurement in one direction within the cross-section were previously searched for and set in each steel. After quenching under these conditions, tempering was performed at 160°C for 1 hour.

[0102] The surface hardness is the lowest value when measured three times using Rockwell hardness (HRC). Then, a cross-section perpendicular to the height direction of the cylinder was cut out, and the hardened layer depth within the cross-section circle was measured using a Vickers hardness tester with a load of 2.94 N (300 gf) at 90° intervals. The above intervals were measured from three directions towards the center of the cross-section circle at intervals of 0.2 mm. Then, the average value of the length from the surface of the steel to the position where Hv400 is reached on the cross-section circle was calculated and taken as the average hardened layer depth.

[0103] In addition, for the deviation of the effective hardened layer depth in three directions, the standard deviation σ of the above average hardened layer depth was calculated.

[0104] The above test results and evaluation results are shown in Table 2.

[0105]

[0106]

[0107]

[0108]

[0109]

[0110]

[0111]

[0112]

[0113] As can be seen from Table 2, for the steels No. A to L, AO to AQ, AS, AU, AW, BA, BD, and BF of the inventive examples, the area ratio of the bainite structure in the microstructure is 90% or more, the tensile strength is 820 MPa or more, the impact value at -50°C is 25 J / cm 2 or more, and the fatigue strength is 550 N / mm 2The surface hardness after high-frequency treatment is HRC: 47 or more, the average hardened layer depth after high-frequency treatment is 2.00 mm or more, and the standard deviation σ of the effective hardened layer depth is 0.12 mm or less.

[0114] It should be noted that in the present invention, a case where the tensile strength is 820 MPa or more is evaluated as having excellent strength.

[0115] In the present invention, the impact value at -50°C is 25 J / cm 2 or more is evaluated as having excellent toughness.

[0116] In the present invention, a case where the fatigue strength is 550 N / mm 2 or more is evaluated as having excellent fatigue strength.

[0117] In addition, in the present invention, a case where the surface hardness after high-frequency treatment is HRC: 47 or more is evaluated as having excellent fatigue strength, wear resistance, and high-frequency hardenability.

[0118] In the present invention, a case where the average hardened layer depth after high-frequency treatment is 2.00 mm or more and the standard deviation σ of the effective hardened layer depth is 0.12 mm or less is evaluated as having excellent fatigue strength and high-frequency hardenability.

[0119] In contrast, the steels No. M to AN, AR, AT, AV, AX to AZ, BB, BC, BE, and BG to BJ of the comparative examples are inferior in one of the tensile strength, the impact value at -50°C, the fatigue strength, the surface hardness after high-frequency treatment, the average hardened layer depth, and the standard deviation σ of the effective hardened layer depth compared with the invention examples.

[0120] [Example 2]

[0121] Next, ingots composed of the steels shown by the steel Nos. A, B, and C in Table 1 above were hot-rolled into round bars with a diameter of 36 mm, and they were hot-forged into round bars with a diameter of 25 mm. The heating temperature before hot forging and the average cooling rate after hot forging were carried out under various conditions shown in Table 3.

[0122] The round bars thus obtained were measured for the microstructure, tensile strength, fatigue strength, and impact value at -50°C in the same manner as in Example 1 above, and the high-frequency hardenability was evaluated. These results are shown in Table 3 together.

[0123]

[0124] As can be seen from Table 3, when the heating temperature before hot forging and the average cooling rate after hot forging are within the scope of the present invention, the following are all the area ratios of the bainite structure of the microstructure: 90% or more, the tensile strength: 820 MPa or more, the impact value at -50°C: 25 J / cm 2 , the fatigue strength: 550 N / mm 2 or more, the surface hardness HRC after high-frequency treatment: 47 or more, the average hardened layer depth after high-frequency treatment: 2.00 mm or more and the standard deviation σ of the effective hardened layer depth is 0.12 mm or less.

[0125] In contrast, when the heating temperature and the average cooling rate after hot forging are outside the scope of the present invention, one of the tensile strength, the impact value at -50°C, the fatigue strength, the surface hardness after high-frequency treatment, the average hardened layer depth, and the standard deviation σ of the effective hardened layer depth does not meet the desired value and is poor.

Claims

1. A hot-forged non-quenched steel, which is a steel in a hot-forged state, has the following composition: containing C: 0.35% - 0.48%, Si: 0.05% - 0.35%, Mn: 0.50% - 1.20%, P: 0.005% - 0.020%, S: 0.030% - 0.070%, Al: 0.015% - 0.050%, Cr: 0.60% - 1.50%, Mo: 0.05% - 0.25%, V: less than 0.050%, B: 0.0005% or less, and N: 0.0030% - 0.0200% by mass, the Ceq shown in the following formula (1) is 0.750 - 0.870, satisfies the following formula (2), and the remaining part is composed of Fe and inevitable impurities, the area ratio of the bainite structure in the microstructure is 90% or more, Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ··· (1) 0.200 ≤ C - Mn / 18 - Cr / 24 - Mo / 3 ≤ 0.350 ··· (2) The element symbols in the formulas (1) and (2) refer to the content of the element in the steel, and are 0 when the element is not contained in the steel.

2. The hot-forged non-quenched and tempered steel according to claim 1, wherein, The composition further contains, by mass, one or more selected from Cu: 0.01% - 0.30%, Ni: 0.01% - 0.30%, Nb: 0.005% - 0.050%, Ti: 0.005% - 0.025%, Pb: 0.05% - 0.30%, Ca: 0.0005% - 0.0050%, Mg: 0.0005% - 0.0050%, Bi: 0.05% - 0.30%, and Sb: 0.0015% - 0.0100%.

3. The hot-forged non-quenched and tempered steel according to claim 1 or 2, wherein, The composition further contains Sn: 0.0010% - 0.030% by mass.

4. A manufacturing method of a hot-forged non-quenched steel, heating a steel billet to 1100 - 1300 °C and then performing hot forging, and further cooling at an average cooling rate of 0.10 - 3.00 °C / s between 950 °C and 350 °C, the steel billet has the following composition: containing C: 0.35% - 0.48%, Si: 0.05% - 0.35%, Mn: 0.50% - 1.20%, P: 0.005% - 0.020%, S: 0.030% - 0.070%, Al: 0.015% - 0.050%, Cr: 0.60% - 1.50%, Mo: 0.05% - 0.25%, V: less than 0.050%, B: 0.0005% or less, and N: 0.0030% - 0.0200% by mass, the Ceq shown in the following formula (1) is 0.750 - 0.870, satisfies the following formula (2), and the remaining part is composed of Fe and inevitable impurities, Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ··· (1) 0.200 ≤ C - Mn / 18 - Cr / 24 - Mo / 3 ≤ 0.350 ··· (2) The element symbols in the formulas (1) and (2) refer to the content of the element in the steel, and are 0 when the element is not contained in the steel.

5. The manufacturing method of the hot-forged non-quenched and tempered steel according to claim 4, wherein, The composition further contains, by mass%, one or more selected from Cu: 0.01% to 0.30%, Ni: 0.01% to 0.30%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.025%, Pb: 0.05% to 0.30%, Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, Bi: 0.05% to 0.30%, and Sb: 0.0015% to 0.0100%.

6. The manufacturing method of the hot-forged non-quenched and tempered steel according to claim 4 or 5, wherein, The composition further contains, by mass%, Sn: 0.0010% to 0.030%.

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