JP2008057036A - Method for manufacturing soft-nitriding non-normalizing machine part - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a soft-nitriding non-normalizing machine part with which even the normalizing treatment is not applied after forging, but the soft-nitriding is applied, a high fatigue strength and a bending-correction can be obtained. <P>SOLUTION: A steel containing 0.30-0.45% C, 0.1-0.5% Si, 0.6-1.0% Mn, ≤0.05% P, ≤0.1% S, ≤.005% Al, 0.010-0.030 N, 0.005-0.05% Ti, 0.01-0.5% Mo and 0.0001-0.005% Ca, and the balance Fe with impurities, is heated to 1100-1250°C and a forging is started at ≤1100°C and finished at ≤800°C, and after air-cooling, without performing the normalizing treatment, the soft-nitriding is performed. Then, in the case of containing this steel to 0.35-0.45% C, 0.1-0.35% Si, 0.01-0.35 Mo, this heating can be performed at 900-1250°C. Further, according to the necessity in this steel, one or two elements of ≤0.05% Nb and ≤0.005% B can be contained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a “soft-nitriding non-normalized mechanical part”. More specifically, after performing forging into a desired shape, soft nitriding is performed without performing a normalizing treatment, and high fatigue strength and excellent resistance such as crankshafts and connecting rods used in automobiles, industrial machinery, construction machinery, etc. The present invention relates to a method for producing a soft nitriding non-tempered mechanical part that requires high bending straightness.

  Conventionally, mechanical parts such as crankshafts and connecting rods used in automobiles, industrial machinery, construction machinery, etc. are subjected to normalization after hot forging steel materials such as carbon steel and alloy steel for machine structures into a desired shape. The structure has been manufactured by soft nitriding mainly for the purpose of homogenizing and refining the structure and then further increasing the fatigue strength.

  However, in recent years, in the manufacture of machine parts, in order to reduce manufacturing costs and save energy, it is desirable to perform soft nitriding by omitting the normalizing process after hot forging. It has become.

  However, if the normalization process is omitted, the inhomogeneous structure generated during hot forging tends to remain, and the crystal grains grown and coarsened during heating of the workpiece before the start of hot forging remain as it is. It remains in the machine part after forging, resulting in a decrease in mechanical properties. That is, when the normalizing treatment after hot forging is not performed, the crystal grains remain uniformly coarsened, or an inhomogeneous structure in which some crystal grains grow abnormally remains. Or For this reason, when the normalizing process is omitted after hot forging, even if soft nitriding is performed, the machine part cannot be provided with a desired high fatigue strength.

  It should be noted that the nitrocarburizing causes distortion in the mechanical part, and the distortion impairs the dimensional accuracy of the mechanical part. Therefore, the bending correction process is often performed after nitrocarburizing. Therefore, in addition to high fatigue strength, nitrocarburized mechanical parts are required to have excellent bending straightness, that is, no cracks are formed on the surface until a large amount of bending displacement is reached.

  However, if the normalizing treatment after hot forging is omitted, coarse crystal grain structure and heterogeneous structure are generated as described above. Therefore, machine parts after soft nitriding have a bending straightening property in addition to a decrease in fatigue strength. Often a significant drop occurs.

  For this reason, even when the normalizing treatment after hot forging is omitted, there is a great demand for providing machine parts after soft nitriding with high fatigue strength and excellent bend straightening.

  Therefore, in order to meet the above-described demand, for example, Patent Document 1 and Patent Document 2 propose a technique that avoids the coarsening of the structure by hot forging as much as possible while keeping the microstructure of the steel in ferrite and pearlite. . Patent Document 3 and Patent Document 4 propose a technique for changing the microstructure of steel to bainite or a mixed structure with bainite.

  Specifically, Patent Document 1 states that “alloy element content is mass%, C: 0.15 to 0.40%, Si: 0.50% or less, Mn: 0.20 to 1.50%. Cr: 0.05 to 0.50%, if necessary, <1> Ni: 0.50% or less, Mo: 0.5% or less of one or two, <2> N: 0.005 to 0.030%, V: 0.3% or less, Nb: 0.3% or less, Ti: 0.2% or less, Zr: 0.1% or less, Ta: 0.2% or less 1 type or 2 or more types, <3> S: 0.01 to 0.30%, <4> Pb: 0.3% or less, Ca: 0.05% or less, Bi: 0.2% or less Te: containing at least one element selected from the group of four elements of one or more of 0.05% or less, the balance consisting of Fe and inevitable impurities, A nitrided steel in which the structure after the cold working is substantially a ferrite-pearlite structure, the area ratio of ferrite is 30% or more, the grain size number of ferrite is 5 or more, and the average dimension of pearlite is 50 μm or less. Is disclosed.

  Patent Document 2 states that “a nitriding component obtained by nitriding steel, in which the steel is in mass% as an alloy component, C: 0.15 to 0.40%, Si: 0.50% or less, Mn: 0.20 to 1.50%, Cr: 0.05 to 0.50%, if necessary, <1> Ni: 0.50% or less, Mo: 0.50% or less One or two of them, <2> N: 0.005 to 0.030% and V: 0.3% or less, Nb: 0.3% or less, Ti: 0.2% or less, Zr: 0.00. 2% or less, Ta: one or more of 0.2% or less, <3> S: 0.01 to 0.3%, Pb: 0.3% or less, Ca: 0.05% or less Bi: 0.2% or less, Te: 0.05% or less, including at least one element selected from at least one of the three element groups of the following: the balance is F And the steel has a mixed structure composed of ferrite and pearlite as it is hot-worked, and the average grain size of the ferrite is 50 μm or less, and the pearlite crystal grains The nitriding treatment part is disclosed in which the average dimension of is not more than 50 μm, the average hardening depth by the nitriding treatment is 0.3 mm or more, and the variation of the hardening depth is within 0.1 mm.

  Patent Document 3 states that “in mass%, C: less than 0.1 to 0.3%, Si: 0.01 to 1.0%, Mn: 1.5 to 3.0%, Cr: 0.01 to 0.5%, Mo: 0.1-1.0%, acid-soluble Al: 0.01-0.045%, N: 0.005-0.025%, if necessary, Including one or more of S: 0.20% or less, Ca: 0.01% or less, Pb: 0.30% or less, Bi: 0.30% or less, and the balance from Fe and inevitable impurities “Non-tempered steel for soft nitriding”.

  Patent Document 4 states that "in mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0.005 to 0". 0.1% and N: 0.015 to 0.030%, and if necessary, <1> Nb: 0.003 to 0.1%, Mo: 0.01 to 1.0%, Cu : 0.01 to 1.0%, Ni: 0.01 to 1.0% and B: one or more selected from 0.001 to 0.005%, <2> S: 0.01 to 0 1% and Ca: one or two elements out of 0.0001 to 0.005%, an element selected from at least one element group of two element groups, with the balance being Fe and impurities, bainite And a mixed structure composed of ferrite or bainite, a mixed structure composed of ferrite and pearlite, and a bainite in the mixed structure Rate is soft-nitriding for non-heat treated steel "is disclosed which is 5 to 90%.

Japanese Patent Laid-Open No. 9-291339 JP-A-9-324258 JP 2000-309846 A WO2005 / 021816

  The technique disclosed in Patent Document 1 described above stabilizes the nitrided layer during nitriding by making the microstructure after forging of a steel having a specific chemical composition a fine ferrite / pearlite structure, This is a technique that makes it possible to obtain excellent bending characteristics and fatigue characteristics after nitriding even if the normalizing process is omitted. However, as disclosed in this Patent Document 1, the microstructure in which the ferrite area ratio is 30% or more, the ferrite grain size number is 5 or more, and the average size of pearlite is 50 μm or less is as follows. However, it is not always obtained simply by hot forging a steel having a chemical composition range as disclosed therein, and a method for forming such a microstructure is not sufficiently disclosed. It was difficult to make products with good yield.

  The technique disclosed in Patent Document 2 is easy to correct after nitriding treatment by adjusting the chemical composition of steel, metal structure and nitriding hardening depth, even if the normalizing treatment after hot forging is omitted. In addition, this is a technique for obtaining a nitrided part having high fatigue strength. However, similarly to the technique disclosed in Patent Document 1, the technique disclosed in Patent Document 2 is a mixed structure composed of ferrite and pearlite, and the average size of ferrite crystal grains is 50 μm or less. A microstructure in which the average grain size is 50 μm or less is not always obtained by simply hot forging a steel having a chemical composition range, as disclosed therein. Since the method of putting in is not fully disclosed, it has been difficult to make a product with a high yield. In addition, it is disclosed that it is effective to set the average hardening depth after the nitriding treatment to 0.3 mm or more and to change the hardening depth within 0.1 mm. However, since there is no disclosure of a method for forming such a hardened layer structure, it has been difficult to produce a product with a high yield.

  The technology disclosed in Patent Document 3 is a technology that provides non-tempered steel for soft nitriding that can easily make the steel structure a high-strength bainite by air cooling after hot working. The bainite structure contains more lattice defects than the ferrite and pearlite structure of conventional non-tempered steel, so that the diffusion of nitrogen is fast and it is easy to obtain a deep nitrocarburized layer. -Since the toughness is higher than that of the pearlite structure and breakage in the bending straightening process hardly occurs, the non-tempered steel for soft nitriding has excellent bending straightening after soft nitriding. However, in the technique disclosed in Patent Document 3, the C content of steel is less than 0.3% at the maximum, and when applied to, for example, a crankshaft, the hardness of the material is not sufficient. And the Mn content is as low as 1.5% at the minimum, so from the concern of shaft wear, if the C content is set high, the hardenability of the steel is remarkably increased and the machinability is improved. The problem was that harmful martensite was easily generated.

  The technique disclosed in Patent Document 4 proposed by the present inventors suppresses the coarsening of crystal grains during hot working, and achieves a fine structure by using a mixed structure containing bainite. This is a technique that can give excellent fatigue strength and bend straightening to parts after nitrocarburizing treatment by using solid solution strengthening in GaN and precipitation strengthening by iron nitride generated during soft nitriding. However, when the bainite structure is not mixed due to some influence, a relatively coarse ferrite and pearlite structure is likely to be formed. Therefore, even if the bainite structure is not mixed due to some influence, the structure is fine. It was also necessary to study a method for producing such soft nitrided parts.

  The utilization of the bainite structure proposed in Patent Document 3 and Patent Document 4 described above is to suppress the formation of coarse pearlite colonies. In that sense, pearlite and ferrite proposed in Patent Document 1 and Patent Document 2 are used. This is consistent with the reduction of the average particle size.

  However, what transformation product the microstructure is composed of and how large the grain size of the transformation product depends strongly on the chemical composition of the steel and the conditions of the hot working process. Here, “the conditions of the hot working process” are, for example, the heating temperature, the working temperature, the cooling rate after the working and the working rate, and the so-called “working heat treatment” that controls these is widely used industrially. ing.

  Attempts to control the conditions of hot forging in order to omit the normalization after hot forging have been studied, for example, in the manufacturing method of connecting rods. Even when the hot working process is controlled and the normalizing process after hot forging is omitted, no consideration has been given to providing both nitrocarburized machine parts with both high fatigue strength and excellent bend straightening. .

  Therefore, an object of the present invention is to provide a soft nitrided part with high fatigue strength and bending straightening even when soft nitriding is performed without performing a normalizing treatment after forging into a desired shape. It is to provide a “method for producing a soft nitriding non-tempered mechanical part”.

  In order to solve the above-mentioned problems, the present inventors have investigated the microstructure after forging by changing hot forging conditions in various ways using steels having various chemical compositions. As a result, the following findings (a) and (b) were obtained.

  (A) The heating temperature of the raw material (the material to be forged) in hot forging has been usually 1100 to 1250 ° C., but even when heated at 1250 ° C., it is once cooled to 1100 ° C. or lower before being heated. When the forging is started, the structure obtained when the desired amount of processing is given and then cooled to room temperature is refined. However, in order to make the structure finer, pinning particles that effectively suppress the growth and coarsening of austenite grains must be dispersed in the steel, and Ti precipitates are effective as the pinning particles. is there.

  (B) Hot forging is generally performed in a plurality of processes, and at that time, processing heat is generated. For this reason, when forging and cooling, compared with the case where it cools without forging, since temperature fall becomes loose and the time kept at high temperature becomes long, forging completion temperature becomes high. And as the forging end temperature becomes higher, the structure obtained when cooling to room temperature after giving a predetermined processing amount becomes coarser. However, if the forging end temperature is set to 800 ° C. or lower, the coarsening of the structure can be considerably suppressed.

  Therefore, next, various conditions of hot forging were changed, and soft nitriding was performed after forging without performing normalizing treatment. Correlation between microstructure before soft nitriding and fatigue strength and bending straightening after soft nitriding Was investigated. As a result, the following knowledge (c) was obtained.

  (C) When hot forging is performed by heating the material to 1100 to 1250 ° C., the temperature at which forging is started is set to 1100 ° C. or less regardless of the heating temperature, and forging is started from the heating temperature of the hot forging. In order to effectively suppress the growth and coarsening of austenite grains until the temperature is reached, Ti precipitates are dispersed as pinning particles, and forging is completed at a temperature of 800 ° C. or lower. If it is air-cooled, both high fatigue strength and excellent bend straightening can be achieved even if nitrocarburizing without performing a normalizing treatment after forging.

  As a result of further investigation, the following findings (d) to (g) were obtained.

  (D) By using Ti precipitates as pinning particles in the heating and holding stage before hot forging, growth and coarsening of austenite grains can be suppressed, and the hot forging start temperature is 1100 ° C. The release of processing strain applied to the austenite grains can be suppressed by making the following, and by lowering the forging end temperature to 800 ° C. or less, processing to the austenite grains up to the stage immediately before the ferrite transformation starts. Since strain can be accumulated, it is possible to refine the pro-eutectoid ferrite based on the increase in nucleation sites.

  (E) If proeutectoid ferrite precipitates in the grain using the processing strain accumulated in the austenite grains as a precipitation site, the pearlite grains are divided, and the formation of coarse pearlite colonies can be suppressed.

  (F) If steel contains appropriate amounts of Ti and Mo, solid solution Ti and solid solution Mo contribute to the bainite formation of the structure.

  (G) By suppressing the growth and coarsening of the austenite grains of (d) above and refining proeutectoid ferrite, by suppressing the formation of coarse pearlite colonies of (e) and by bainite of the structure of (f), Even when soft nitriding is performed without performing a semi-treatment, high fatigue strength and excellent bend straightening can be achieved.

  The present inventors further clarify the component system in which a sufficiently uniform structure can be obtained even when the heating temperature before hot forging is lower than the normal lower limit temperature of 1100 ° C. Then, the hot forging conditions were changed in various ways using steels with various chemical compositions, and the microstructure after forging was investigated. As a result, the following knowledge (h) was obtained.

(H) C contained in the steel, by appropriately adjusting the amount of Si and Mo, because A ₃ point of the steel is lowered, if the 900 ° C. or higher even below the heating temperature before hot forging to 1100 ° C. Since the austenite single phase state is obtained during forging, the structure after forging becomes uniform.

Therefore, next, C contained in the steel, with a material that by adjusting the amount of Si and Mo lowers the _three points A steel, forged heating temperature before hot forging as below 1100 ° C., baked after forging Soft nitriding was performed without performing quasi-treatment, and the correlation with fatigue strength and bend straightening was investigated. As a result, the following knowledge (i) was obtained.

(I) C, to adjust the content of Si and Mo lowers the _three points A steel, if made to be single-phase austenite state during the forging, without performing the normalizing treatment after forging in hot Even in the case of soft nitriding, high fatigue strength and excellent bend straightening can be provided as in the case of heating to 1100 ° C. or higher before normal forging.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in a method for producing a soft nitriding non-tempered mechanical part shown in the following (1) to (4).

  (1) A method for producing a nitrocarburized mechanical component, in mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0% , P: 0.05% or less, S: 0.1% or less, Al: 0.05% or less, N: 0.010 to 0.030%, Ti: 0.005 to 0.05%, Mo: 0 .01% to 0.5% and Ca: 0.0001% to 0.005%, with the balance being steel and Fe and impurities are heated to 1100 to 1250 ° C., and forging is started at a temperature of 1100 ° C. or lower. A method for producing a nitrocarburizing non-tempered mechanical part, characterized in that after ending at a temperature of 800 ° C. or less and air cooling, soft nitriding is performed without performing a normalizing treatment.

  (2) A method for producing a nitrocarburized mechanical component, in mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0% , P: 0.05% or less, S: 0.1% or less, Al: 0.05% or less, N: 0.010 to 0.030%, Ti: 0.005 to 0.05%, Mo: 0 .01-0.5% and Ca: 0.0001-0.005%, and Nb: 0.05% or less and B: 0.005% or less, and the balance is Fe and impurities The steel is heated to 1100 to 1250 ° C., forged at a temperature of 1100 ° C. or less, finished at a temperature of 800 ° C. or less and air-cooled, and then soft nitrided without performing a normalizing treatment. A method of manufacturing a soft nitriding non-tempered mechanical part.

  (3) A method for producing a soft nitriding mechanical component, in mass%, C: 0.35 to 0.45%, Si: 0.1 to 0.35%, Mn: 0.6 to 1.0% , P: 0.05% or less, S: 0.1% or less, Al: 0.05% or less, N: 0.010 to 0.030%, Ti: 0.005 to 0.05%, Mo: 0 .01 to 0.35% and Ca: 0.0001 to 0.005%, the balance being Fe and impurities steel is heated to 900 to 1250 ° C., and forging is started at a temperature of 1100 ° C. or less. A method for producing a nitrocarburizing non-tempered mechanical part, characterized in that after ending at a temperature of 800 ° C. or less and air cooling, soft nitriding is performed without performing a normalizing treatment.

  (4) A method for producing a soft nitriding machine component, in mass%, C: 0.35 to 0.45%, Si: 0.1 to 0.35%, Mn: 0.6 to 1.0% , P: 0.05% or less, S: 0.1% or less, Al: 0.05% or less, N: 0.010 to 0.030%, Ti: 0.005 to 0.05%, Mo: 0 .01 to 0.35% and Ca: 0.0001 to 0.005%, and Nb: 0.05% or less and B: 0.005% or less, and the balance is Fe and impurities The steel is heated to 900 to 1250 ° C., forged at a temperature of 1100 ° C. or less, finished at a temperature of 800 ° C. or less and air-cooled, and then soft-nitrided without performing a normalizing treatment. A method of manufacturing a soft nitriding non-tempered mechanical part.

  In the present invention, “air cooling” refers to cooling in the atmosphere or forced air cooling using a fan.

  Hereinafter, the inventions related to the method for producing a soft nitrided non-tempered mechanical part of (1) to (4) above are referred to as “present invention (1)” to “present invention (4)”, respectively. Also, it may be collectively referred to as “the present invention”.

  According to the method of the present invention, a nitrocarburized mechanical component having high fatigue strength and bend straightening can be obtained even when soft nitriding is performed without performing normalization after forging into a desired shape. Therefore, reduction in manufacturing cost and energy saving can be achieved.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) Chemical composition C: 0.30 to 0.45% (in the case of the present invention (1) and the present invention (2)) or C: 0.35 to 0.45% (the present invention (3) and the present invention) (In the case of (4))
C is an indispensable element for bearing the strength and toughness of nitrocarburized non-tempered mechanical parts. It stabilizes austenite during hot forging and ensures wear resistance of products (soft-nitrided non-tempered mechanical parts). From the viewpoint, a content of 0.30% or more is necessary. On the other hand, if the content exceeds 0.45%, the hardenability is excessively increased and the formation of hard martensite that impairs the machinability is easily caused.

Incidentally, C is an element which lowers the _three points A steel. Therefore, by increasing the content of C, in particular, the content of C as above 0.35%, if lowering the _three-point A, the heating temperature before forging used in conventional hot forging Even when the temperature falls below 1100 ° C. as the lower limit of the temperature range, an austenite single phase state can be obtained during forging.

  Therefore, the content of C in the nitrocarburizing non-tempered mechanical parts according to the present invention (1) and the present invention (2) in which the heating temperature before forging is in the temperature range of 1100 ° C. or higher is the same as in normal hot forging. The content of C in the nitrocarburized non-tempered mechanical parts according to the present invention (3) and the present invention (4) including 0.30 to 0.45% and the case where the heating temperature before forging is lower than 1100 ° C. The amount was 0.35 to 0.45%.

Si: 0.1 to 0.5% (in the case of the present invention (1) and the present invention (2)) or Si: 0.1 to 0.35% (in the case of the present invention (3) and the present invention (4)) )
Si is added as a deoxidizer in the steel making process, but it is also effective for strengthening the solid solution of ferrite, so a content of 0.1% or more is necessary. On the other hand, if the Si content exceeds 0.5%, the hot deformation resistance of steel is increased, and the toughness and machinability are deteriorated.

Incidentally, Si is an element to increase the _three points A steel. Therefore, by lowering the content of Si, in particular, the content of Si as 0.35% or less, if lowering the _three-point A, the heating temperature before forging used in conventional hot forging Even when the temperature falls below 1100 ° C. as the lower limit of the temperature range, an austenite single phase state can be obtained during forging.

  Therefore, the content of Si in the nitrocarburized non-tempered mechanical parts according to the present invention (1) and the present invention (2), in which the heating temperature before forging is a temperature range of 1100 ° C. or higher similar to that in normal hot forging, The content of Si in the non-tempered nitrocarburized machine parts according to the present invention (3) and the present invention (4) including 0.1 to 0.5% and the case where the heating temperature before forging is lower than 1100 ° C. The amount was 0.1 to 0.35%.

Mn: 0.6 to 1.0%
Mn is added in the steel making process as a deoxidizing agent similarly to Si. Moreover, austenite is stabilized and hardenability is improved. Furthermore, Mn combines with S in steel to form MnS, which is effective in improving machinability. For this purpose, a Mn content of 0.6% or more is necessary. On the other hand, when the content of Mn exceeds 1.0%, the hardenability is excessively increased, and martensite that is harmful to the machinability is easily generated. Therefore, the Mn content is set to 0.6 to 1.0%.

P: 0.05% or less P is an impurity contained in steel and segregates at grain boundaries to promote grain boundary embrittlement cracking. In particular, when the content exceeds 0.05%, grain boundary embrittlement occurs. The occurrence of cracking becomes remarkable. Therefore, the content of P is set to 0.05% or less.

S: 0.1% or less S has the effect | action which improves the machinability of steel materials. However, when S is contained excessively, segregation defects occur in the steel slab or hot workability is deteriorated. In particular, when the S content exceeds 0.1%, segregation in the steel slab is caused. Defects are generated and hot workability is significantly reduced. Therefore, the content of S is set to 0.1% or less. In addition, in order to acquire the machinability improvement effect of steel, it is preferable that content of S shall be 0.02% or more.

Al: 0.05% or less Al is usually added as a deoxidizer at the time of melting, but remains in the steel as alumina particles or forms NIN by combining with N. Among these, alumina is an oxide-based inclusion with high hardness, and shortens the life of a tool used for cutting. In addition, AlN precipitates near the surface during soft nitriding or promotes the growth of the surface compound layer, thereby significantly increasing the hardness of the surface layer and lowering the bending straightness. Furthermore, since AlN dissolves at the hot forging temperature, it cannot be expected to function as pinning particles, and is hardly useful for refining crystal grains. Therefore, it is better that the Al content is low, but minimizing the lower limit of the Al content causes a restriction in the deoxidation step and leads to an increase in cost. Therefore, soft nitriding can be performed without performing a normalization treatment. When applied, the bend straightening property is set to 0.05% or less which does not hinder. The Al content is preferably 0.005% or less.

N: 0.010 to 0.030%
N stabilizes austenite, constitutes pinning particles to suppress grain coarsening, forms Fe nitride to contribute to precipitation strengthening, and further contributes to solid solution strengthening as solid solution nitrogen Thus, it has an effect of increasing the strength of the base material, so it is positively contained. In addition, even if consumed as pinning particles, N needs to be contained in an amount of 0.010% or more in order to obtain the effect of solid solution strengthening. On the other hand, if the content of N exceeds 0.030%, bubble defects may be generated in the ingot and the material may be damaged. Therefore, the N content is set to 0.010 to 0.030%. The desirable content of N is 0.015 to 0.025%.

Ti: 0.005 to 0.05%
Ti is an essential element for forming pinning particles for suppressing crystal grain coarsening during hot forging. Pinning particles include Ti nitrides, carbides, and carbonitrides. In order to generate pinning particles having a sufficient distribution density, the content must be 0.005% or more. On the other hand, even if the Ti content exceeds 0.05%, the above effects are saturated and the hardenability becomes excessive. Therefore, the Ti content is set to 0.005 to 0.05%.

Mo: 0.01 to 0.5% (in the case of the present invention (1) and the present invention (2)) or Mo: 0.01 to 0.35% (in the case of the present invention (3) and the present invention (4)) )
Mo is an element that increases the hardenability of steel and contributes to high strength, and is also effective in improving toughness. Moreover, when it contains Mo, it will become easy to produce | generate bainite and there also exists an effect which suppresses the production | generation of a coarse pearlite colony. In order to obtain such an effect, the content needs to be 0.01% or more. On the other hand, if the Mo content exceeds 0.5%, the hardenability increases, so the formation of martensite is promoted, and the bending straightening and toughness after the soft nitriding treatment are reduced.

Incidentally, Mo is an element to increase the _three points A steel. Therefore, by lowering the content of Mo, in particular, the content of Mo as 0.35% or less, if lowering the _three-point A, the heating temperature before forging used in conventional hot forging Even when the temperature falls below 1100 ° C. as the lower limit of the temperature range, an austenite single phase state can be obtained during forging.

  Therefore, the content of Mo in the nitrocarburizing non-tempered mechanical parts according to the present invention (1) and the present invention (2) in which the heating temperature before forging is in the temperature range of 1100 ° C. or higher is the same as in normal hot forging. Inclusion of Mo in the nitrocarburized non-tempered mechanical parts according to the present invention (3) and the present invention (4), including 0.01 to 0.5% and a case where the heating temperature before forging is lower than 1100 ° C. The amount was 0.01 to 0.35%. In addition, also in the case of the nitrocarburizing non-tempered mechanical parts according to any of the present invention (1) to the present invention (4), the desirable content of Mo is 0.05 to 0.3%.

Ca: 0.0001 to 0.005%
Ca has the effect of enhancing the machinability of the steel material, so 0.0001% or more is contained. However, when Ca is contained excessively, segregation defects occur in the steel slab, or hot workability is deteriorated. In particular, when the Ca content exceeds 0.005%, The occurrence of segregation defects and the decrease in hot workability become significant. Therefore, the content of Ca is set to 0.0001 to 0.005%. A desirable lower limit of the Ca content is 0.001%.

  For the above reasons, the soft nitriding non-tempered mechanical part according to the present invention (1) in which the heating temperature before forging is in the temperature range of 1100 ° C. or higher is the same as in normal hot forging. It is specified that the steel is composed of Fe and impurities.

  Further, the soft nitriding non-tempered mechanical part according to the present invention (3) including the case where the heating temperature before forging is lower than 1100 ° C. contains the elements from C to Ca in the above-mentioned range, with the balance being Fe and It was stipulated that steel made of impurities should be used as the material.

  In addition, in order to further increase the strength of the soft nitriding non-tempered mechanical part, which is a product, in addition to the above-described elements, Nb and One or more elements selected from B may be contained as optional elements.

  Hereinafter, Nb and B as the above optional elements will be described.

Nb: 0.05% or less Nb precipitates as fine carbonitride during cooling after finishing hot forging, and has the effect of increasing strength or increasing fatigue strength after soft nitriding. Nb is an element that can be used to form pinning particles for suppressing crystal grain coarsening during hot forging. Furthermore, a part of Nb has an effect of suppressing the grain growth of recrystallized grains by staying at the grain boundary while being in solid solution. In order to obtain such an effect, the Nb content is preferably 0.003% or more. On the other hand, even if the Nb content exceeds 0.05%, the above effect is saturated, and in addition to the increase in the content, the bend straightening after soft nitriding is lowered. Therefore, when Nb is contained, the Nb content is set to 0.05% or less. When Nb is contained, the Nb content is preferably 0.003 to 0.05%, and more preferably 0.005 to 0.03%.

B: 0.005% or less B has the effect of increasing the hardenability of the steel and increasing the strength. In order to obtain this effect, the B content is preferably 0.001% or more. On the other hand, if the B content exceeds 0.005%, the toughness of the steel is impaired. Therefore, the content of B when contained is set to 0.005% or less. In addition, when it contains, it is desirable for the content of B to be 0.001 to 0.005%.

  Said Nb and B can be contained only in any 1 type or 2 types of composite.

  For the reasons described above, the soft nitriding non-tempered mechanical part according to the present invention (2) in which the heating temperature before forging is a temperature range of 1100 ° C. or higher is the same as in normal hot forging. And the above elements, and Nb: not more than 0.05% and B: not more than 0.005%, and the balance is specified to be made of steel composed of Fe and impurities.

  In addition, the soft nitriding non-tempered mechanical part according to the present invention (4) including a case where the heating temperature before forging is lower than 1100 ° C. includes the elements from C to Ca in the above-described range, and the above Nb: 0. .05% or less and B: 0.005% or less of one or two types, with the balance being steel made of Fe and impurities.

  In addition, the elements described above are impurities in the steel that is the material of the soft nitriding non-tempered mechanical part according to the present invention, and are not intentionally added. However, it is preferable to set the allowable amounts of V, Cr, Cu, and Ni as follows, among impurities, for reasons such as avoiding unnecessarily high costs in the steelmaking process.

V:
The smaller the content of V as an impurity, the better. This is because V precipitates as a nitride, and in particular, the hardness of the layer near the surface of the soft nitriding mechanical component is remarkably increased to impair the bending straightness. The object of the present invention to obtain a nitrocarburized machine part having high fatigue strength and bend correction even when subjected to soft nitriding without performing a normalizing treatment after forging into a desired shape, Considering the refining cost and the purity of steel in the melting method other than the “blast furnace-converter method”, V up to 0.02% is acceptable as an impurity.

Cr:
The smaller the content of Cr as an impurity, the better. This is because Cr, as well as V, precipitates as nitrides, and in particular, the hardness of the layer near the surface of the nitrocarburized machine part is remarkably increased and the bending straightness is impaired. The object of the present invention to obtain a nitrocarburized machine part having high fatigue strength and bend correction even when subjected to soft nitriding without performing a normalizing treatment after forging into a desired shape, Taking into consideration the refining costs and the purity of steel in a melting method other than the “blast furnace-converter method”, up to 0.15% of Cr is acceptable as an impurity. In addition, the content of Cr as an impurity is more preferably 0.1% or less because it does not impair the bending straightness as much as possible.

Cu:
Unlike V and Cr described above, Cu hardly harms the bend straightening after soft nitriding, so as an impurity as long as the refining cost of the steel used as the material for the nitrocarburizing non-tempered mechanical parts does not increase excessively. It may be included. For example, when steel is used as a raw material for scrap, Cu is often contained up to about 0.2%, and this amount of Cu is subjected to a normalizing treatment after forging into a desired shape. Even when nitrocarburizing is performed without performing, nitrocarburizing machine parts having high fatigue strength and bend straightening are not affected. Therefore, up to 0.2% Cu is acceptable as an impurity.

Ni:
Ni, like Cu, unlike V and Cr, as described above, since it hardly harms the bend straightening after soft nitriding, so long as the refining cost of the steel used as the material for the soft nitriding non-tempered mechanical parts does not increase excessively It may be contained as an impurity. For example, when steel is used as a raw material for scrap, Ni is often contained up to about 0.2%, and this amount of Ni is subjected to normalization after forging into a desired shape. Even when nitrocarburizing is performed without performing, nitrocarburizing machine parts having high fatigue strength and bend straightening are not affected. Therefore, up to 0.2% Ni is acceptable as an impurity.

(B) Manufacturing method of nitrocarburizing non-tempered mechanical part according to the present invention The nitrocarburized non-normalized mechanical part according to the present invention (1) and the present invention (2) has the chemical composition described in the above section (A). The steel is heated to 1100 to 1250 ° C., forged at a temperature of 1100 ° C. or lower, finished at a temperature of 800 ° C. or lower and air-cooled, and then manufactured by soft nitriding without performing a normalizing treatment .

  Further, in the soft nitriding non-tempered mechanical part according to the present invention (3) and the present invention (4), the steel having the chemical composition described in the above section (A) is heated to 900 to 1250 ° C., and forging is performed to 1100. After starting at a temperature of ℃ or less, ending at a temperature of 800 ℃ or less and air-cooling, it is manufactured by soft nitriding without performing a normalization treatment.

  The above will be described below.

(B-1) Hot forging and subsequent cooling As materials (materials for hot forging) used for hot forging, billets obtained by performing ingot rolling on ingots, billets obtained by performing ingot rolling on continuous cast materials, or heating these It may be a hot rolled steel bar, a hot forged round bar or the like, but its chemical composition must be as described in the above section (A).

  If the chemical composition of the steel is as described in the above section (A), the temperature range before starting hot forging (that is, 1100 to 1250 ° C. in the case of the present invention (1) and the present invention (2), Further, in the case of the present invention (3) and the present invention (4), the Ti precipitates are utilized as pinning particles that effectively suppress the growth and coarsening of austenite grains in the holding at 900 to 1250 ° C. Because you can.

  In the case of the present invention (1) and the present invention (2), the heating temperature of the hot forging material needs to be 1100 to 1250 ° C. This is because even if the heating temperature exceeds 1250 ° C., there is almost no effect of shortening the time in the austenite single phase, but rather the load on the equipment becomes more problematic, such as shortening the life of the furnace wall and heater. It is. On the other hand, if the heating temperature is lower than 1100 ° C., it takes a long time to make the austenite single phase, and sometimes cementite partially remains, and the austenite single phase may not be obtained. In the case of the present invention (1) and the present invention (2), the holding time at 1100 to 1250 ° C. is preferably 30 to 60 minutes.

On the other hand, in the case of this invention (3) and this invention (4), the minimum of the heating temperature of the raw material for hot forging may be 900 degreeC. This enhances the lower limit of the content and C, by lowering the upper limit of the content of Si and Mo, because it reduced the _three points A, heating temperature before forging used in conventional hot forging This is because even if the temperature falls below 1100 ° C. as the lower limit of the temperature range to be produced, if it is 900 ° C. or higher, an austenite single phase state can be obtained during forging. Conversely, to increase the lower limit of the content and C, by lowering the upper limit of the content of Si and Mo, also lowering the _three-point A, the heating temperature before forging is below 900 ° C., It takes a long time to make the austenite single phase, and sometimes cementite remains partially, and the austenite single phase cannot be obtained. In the case of the present invention (3) and the present invention (4), the holding time at 900 to 1250 ° C. is preferably 30 to 60 minutes.

  After heating to the above temperature range, forging must be started at a temperature of 1100 ° C. or lower in any of the present invention (1) to the present invention (4). This is because once the structure is cooled to 1100 ° C. and then hot forging is started, even when heated at 1250 ° C., the structure obtained when cooled to room temperature after giving the desired amount of processing It is because it refines.

  That is, with the precipitation of Ti as pinning particles, the growth and coarsening of austenite grains from the heating / holding state before hot forging to the temperature at which forging starts is effectively suppressed. Because forging is performed at a low temperature, the processing strain introduced into the austenite grains becomes difficult to be released, and as a result, the nucleation sites of ferrite transformation increase and the structure becomes finer.

  In the present invention (3) and the present invention (4), when the heating temperature of the hot forging material is 900 ° C. or higher and lower than 1100 ° C., “start forging at a temperature of 1100 ° C. or lower” It means that hot forging may be started immediately after cooling without cooling the material.

  In any case of the present invention (1) to the present invention (4), the lower limit of the hot forging start temperature is not particularly limited as long as it is a so-called “hot” temperature range. From the viewpoint of forgeability such as deformation resistance and cracking, it is preferably set to 850 ° C. or higher.

  Forging started at a temperature of 1100 ° C. or lower needs to be finished at a temperature of 800 ° C. or lower. The higher the forging end temperature is, the coarser the structure obtained when cooling to room temperature after giving a predetermined amount of processing, but if the forging end temperature is set to 800 ° C. or less, the austenite grains are strengthened. This is because the ferrite transformation is caused in a state where the processing strain is not lost after the processing, so that the nucleation of ferrite is accelerated and the coarsening of the structure can be suppressed considerably.

  The forging end temperature may be 800 ° C. or lower, and the lower limit is not particularly limited. However, if it is too low, the ferrite transformation has already started, that is, it is finished in a two-phase region of ferrite and austenite. Will suddenly increase and place an excessive load on the hot forging machine. For this reason, it is preferable to finish from 650 degreeC or more from an industrial manufacture viewpoint, in order to avoid the excessive increase in deformation resistance.

  In general, hot forging is performed in a plurality of processes. At this time, processing heat is generated to raise the temperature of the material to be forged. Even at this time, the temperature of the material to be forged exceeds 1100 ° C. For example, when the temperature of the forging material is severely increased, the time between the forging processes is adjusted to give time for the temperature of the forging material to decrease. Is good. The number of hot forging steps is not particularly limited and may be any as long as it can be finished into a desired shape.

  In addition, what is necessary is just to measure the temperature of the raw material (forging material) in forge molding, for example using a radiation thermometer. If the correlation curve is created by measuring the temperature of the material (the material to be forged) with respect to the holding time or elapsed time between each forging process and the deformation amount (processing amount) in each forging process, It is not necessary to measure the temperature every time.

  The cooling after hot forging described above needs to be air cooling, that is, cooling in the atmosphere or forced air cooling using a fan.

  This is because if the cooling rate after hot forging exceeds the range of air cooling, hard martensite that impairs machinability is generated.

On the other hand, if the cooling after hot forging is air cooling, the structure of the material to be forged
(I) a structure with a high ferrite fraction,
(Ii) a structure with small pearlite grains,
(Iii) a ferrite structure in which crystal grains are equiaxed (that is, having a small aspect ratio),
Any of the above three structures contributes to the improvement of the bending straightness.

  That is, since ferrite is a soft phase rich in ductility and easy to be plastically deformed, if the ferrite fraction becomes high, the ease of bending deformation, that is, the bending straightness improves. The structure with a high ferrite fraction is because the austenite grains become coarse and difficult to recrystallize during and after hot forging, resulting in an increase in ferrite nucleation sites. This is because processing is added in a low temperature austenite region, and thus the nose of the ferrite transformation in the continuous cooling transformation diagram (CCT curve diagram) shifts to a short time side, and the generation of ferrite is accelerated.

  In addition, reducing the size of pearlite grains is nothing but miniaturization of the grain structure, and this makes the dislocation slip due to stress load uniform, avoiding non-uniform deformation and local stress concentration, and bending. Contributes to the improvement of straightness. The size of the pearlite grains is reduced because the austenite grains become coarse and difficult to recrystallize during hot forging and after hot forging. In addition, the ferrite fraction increases as described above. This is because the formation of intragranular ferrite is promoted, and as a result, the austenite grains are fragmented, so that the size of pearlite grains is also reduced.

  Furthermore, the equiaxed shape of the ferrite grains is nothing but the refinement of the grain structure, which makes the dislocation slip due to the stress load uniform and avoids non-uniform deformation and local stress concentration. Contributes to improved bend straightening. The reason why the ferrite grains are equiaxed is that if the austenite grains are prevented from coarsening, the number of nucleation sites at the prior austenite grain boundaries increases and the formation of intragranular ferrite is also promoted.

  In this way, the crystal grain structure close to that obtained by normalizing after hot forging by cooling after hot forging by air cooling, that is, cooling in the atmosphere or forced air cooling using a fan. The effect of miniaturization can be obtained.

  That is, a fine mixed structure of bainite and ferrite, a fine mixed structure of bainite, ferrite and pearlite, or a fine mixed structure of ferrite and pearlite can be obtained.

  Note that the volume fraction of each tissue in the above mixed tissue varies depending on the cooling rate and chemical composition, but as long as the above-described conditions are satisfied, the volume fraction of any tissue will not be 100%. , No martensite mixed.

  The adjustment of the cooling rate in the “air cooling” after the hot forging, that is, whether to cool in the air or forced air cooling using a fan, is a continuous cooling transformation diagram (CCT curve diagram) in advance. Is prepared, the cooling rate range passing through the bainite transformation region is obtained, and the obtained cooling rate range is adjusted.

(B-2) Soft nitriding In the present invention, after the hot forging and air cooling described in the above section (B-1), soft nitriding is performed without performing a normalizing process. In addition, what is necessary is just to select suitably the method of the cutting process in the case of cutting after hot forging and before soft nitriding and finishing to a desired soft nitriding part shape according to a part shape.

  For soft nitriding, a normal method such as gas soft nitriding, salt bath soft nitriding, or ion nitriding can be used.

  Whichever method is used, it is possible to uniformly form a compound layer (nitride layer) having a thickness of about 20 μm and a diffusion layer directly therebelow on the surface of the product (soft nitriding non-tempered mechanical part).

  In order to obtain a desired machine part by gas soft nitriding, for example, RX gas (“RX gas” is a trademark of endothermic modified gas) and ammonia gas are mixed in an atmosphere of 1: 1. What is necessary is just to perform a process for 1-2 hours at the heat temperature of 560-620 degreeC.

  For the above reasons, the manufacturing method of the soft nitriding non-tempered mechanical parts of the present invention (1) and the present invention (2) heats steel having the chemical composition described in the above (A) to 1100 to 1250 ° C. Then, forging was started at a temperature of 1100 ° C. or lower, finished at a temperature of 800 ° C. or lower and air-cooled, and then soft nitriding was performed without performing a normalizing treatment.

  Moreover, the manufacturing method of the nitrocarburizing non-tempered mechanical part of this invention (3) and this invention (4) heats the steel which has the chemical composition described in the said (A) term to 900-1250 degreeC, forging Was started at a temperature of 1100 ° C. or lower, ended at a temperature of 800 ° C. or lower and air-cooled, and then soft-nitrided without performing a normalizing treatment.

  Hereinafter, the present invention will be described in more detail with reference to examples.

[Example 1]
After steel 1-5 having the chemical composition shown in Table 1 was melted in a 180 kg vacuum melting furnace, the steel ingot was heated to 1250 ° C. and hot forged so that the steel material temperature did not fall below 1000 ° C., and one side was 75 mm. The square bar.

  A part of the square bar with a side of 75 mm obtained in this way was heated to 1250 ° C. and held for 30 minutes, and then removed from the furnace once and after the surface temperature of the square bar reached various temperatures, hot forging Furthermore, the end temperature of hot forging, that is, the end temperature of the final process in hot forging was also changed to process a round bar having a diameter of 50 mm.

  In addition, a part of a square bar with a side of 75 mm was heated to 1000 ° C. or 900 ° C. and held for 30 minutes, and then hot forging was started immediately after removal from the furnace, and a round bar with a diameter of 50 mm was processed. .

  Table 2 shows the details of the hot forging conditions. The cooling after hot forging was allowed to cool in the atmosphere, and the temperature of the forged material was measured with a radiation thermometer.

  Parallel to the forging direction (forging axis) from R / 2 part ("R" represents the radius of the round bar) of each round bar with a diameter of 50 mm obtained by cooling in the air after the above hot forging, Ono type rotating bending fatigue test piece shown in FIG. 1 (smooth round bar test piece with parallel part diameter and length of 10 mm and 18 mm, respectively) and test piece for bending straightness evaluation shown in FIG. 2 (depth is 5 mm) Then, a rectangular test piece having a length of 100 mm and a notch having a bottom curvature radius of 10 mm was collected. In addition, the unit of the dimension in the test piece shown in FIG.1 and FIG.2 is all "mm".

  Next, the Ono rotating bending fatigue test piece and the test piece for evaluation of bending straightness were gas softened under the condition of holding at 580 ° C. for 2 hours in an atmosphere in which RX gas and ammonia gas were mixed at 1: 1. Nitriding was performed, followed by cooling into oil at 100 ° C.

Using the soft-nitrided Ono-type rotating bending fatigue test piece, the Ono-type rotating bending fatigue test was conducted at room temperature, in the atmosphere, at a rotational speed of 3400 rpm, and the maximum stress that did not break at the stress addition repetition rate of 10 ⁷ times. Was evaluated as fatigue strength.

  In addition, the following three-point bending test was performed by attaching a strain gauge in the direction parallel to the longitudinal direction of the test piece at the center of the notch bottom of the soft nitrided test piece for bending straightening evaluation. The amount of strain was measured and the bending straightness was investigated.

  That is, two fulcrums are provided in the longitudinal direction of the surface including the notches so that the distance between the fulcrums is 70 mm with the notch as the center, and the indentation speed is 0.5 mm / min. In the relationship between the pushing force and the displacement measured with a strain gauge with a reference length of 2 mm, the moment when a phenomenon (so-called “pop-in”) in which the pushing force decreases despite an increase in the displacement is generated. Assuming that the surface of the test piece was cracked, the value obtained by dividing the displacement at this time by the reference length of 2 mm was evaluated as the maximum strain correctable strain (μ).

  Table 2 also shows the results of the above tests. Further, FIG. 3 shows contour lines based on the value of the maximum bending correctable strain at each data point with the forging start temperature on the horizontal axis and the forging end temperature on the vertical axis.

  From Table 2, in the manufacturing method of the present invention, the maximum bendable strain is larger than that of the comparative example, the fatigue strength is equal to or higher than that of the comparative example, and the bending straightness is further improved while maintaining the fatigue strength. Clearly it can be improved.

  In addition, it is clear from FIG. 3 that high bend straightening is obtained when manufactured under the conditions specified in the present invention.

[Example 2]
In order to verify the cause of the effects as shown in Example 1, the microstructure of the hot forging was investigated.

  That is, in Example 1, for the observation of the microstructure from the R / 2 part (R is the radius of the round bar) of the round bar having a diameter of 50 mm of the steel 3 produced under the test conditions 13 to 18, 27 and 28. A sample was taken and the cross section perpendicular to the forging axis was mirror-polished, etched with nital, observed with an optical microscope at a magnification of 200, and photographed.

  The optical micrograph thus obtained was subjected to image analysis to determine the ferrite fraction, the pearlite particle size, and the ferrite grain aspect ratio.

  The pearlite particle diameter was defined as the pearlite particle diameter by visually determining a pearlite colony and surrounding it with a closed curve, and the diameter of a circle corresponding to the area, that is, the equivalent circle diameter.

  Similarly, the ferrite grain size was determined visually and surrounded by a closed curve, and the diameter of the circle corresponding to the area was defined as the ferrite grain size.

  The aspect ratio of the ferrite grains was calculated by the following procedure.

  First, the distance between two points, which is the maximum distance among the two points on the outer peripheral line of the region surrounded by the closed curve, was defined as the major axis. Next, when a set of two straight lines parallel to this is arranged so as to be in contact with the outer peripheral line so as to sandwich the ferrite grains, the distance between the two straight lines is taken as the short diameter, The value of the diameter ratio was defined as the aspect ratio of the ferrite grains.

  4-6 show the above survey results in an organized manner. In addition, the test number in Table 2 was written together in FIGS.

  FIG. 4 is a diagram showing the relationship between the ferrite fraction and the hot forging conditions. FIGS. 4 (a) and 4 (b) show the relationship between the ferrite fraction and the hot forging start temperature and the ferrite fraction, respectively. It is a figure which shows the relationship between hot forging completion temperature.

  FIG. 5 is a diagram showing the relationship between the pearlite particle size and the hot forging conditions, and FIGS. 5A and 5B show the relationship between the pearlite particle size and the hot forging start temperature and the pearlite, respectively. It is a figure which shows the relationship between a particle size and hot forging completion temperature.

  FIG. 6 is a diagram showing the relationship between the aspect ratio of the ferrite grains and the hot forging conditions. FIGS. 6 (a) and 6 (b) show the aspect ratio of the ferrite grains and the hot forging start temperature, respectively. It is a figure which shows the relationship between the aspect ratio of a ferrite grain, and the hot forging completion temperature.

  As apparent from FIGS. 4 to 6, when both the hot forging start temperature and the end temperature satisfy the conditions specified in the present invention, the ferrite fraction increases to 45% or more, and the pearlite grain size is 50 μm or less. And the aspect ratio of the ferrite grains is as small as 2.0 or less. And it is thought that all of these contribute to improving the bending straightness.

  According to the method of the present invention, a nitrocarburized mechanical component having high fatigue strength and bend straightening can be obtained even when soft nitriding is performed without performing normalization after forging into a desired shape. Therefore, reduction in manufacturing cost and energy saving can be achieved.

It is a figure which shows the shape of the Ono type | formula rotation bending fatigue test piece used in the Example. It is a figure which shows the shape of the test piece for bending straightness evaluation used in the Example. It is a figure which shows the influence of the start temperature of hot forging, and the end temperature which exerts on the maximum bendable strain amount in an Example. It is a figure which shows the relationship between the ferrite fraction of the steel 3 used in the Example, and hot forging conditions, (a) And (b) is the relationship between a ferrite fraction and hot forging start temperature, respectively, and a ferrite fraction. It is a figure which shows the relationship between hot forging completion temperature. It is a figure which shows the relationship between the pearlite particle size of the steel 3 used in the Example, and hot forging conditions, (a) And (b) is the relationship between the pearlite particle size and the hot forging start temperature, respectively, and the pearlite particle size. It is a figure which shows the relationship between hot forging completion temperature. It is a figure which shows the relationship between the aspect ratio of the ferrite grain of the steel 3 used in the Example, and hot forging conditions, (a) And (b) is the relationship between the aspect ratio of a ferrite grain and hot forging start temperature, respectively. It is a figure which shows the relationship between the aspect-ratio of a ferrite grain, and hot forging completion temperature.

Claims (4)

  A method for producing a nitrocarburized mechanical component, in mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, P: 0.05% or less, S: 0.1% or less, Al: 0.05% or less, N: 0.010 to 0.030%, Ti: 0.005 to 0.05%, Mo: 0.01 to 0.5% and Ca: 0.0001 to 0.005% contained, with the balance being steel and Fe and impurities heated to 1100 to 1250 ° C., forging started at a temperature of 1100 ° C. or less and 800 ° C. or less A method for producing a soft nitriding non-normalizing mechanical part, characterized by performing soft nitriding without performing a normalizing treatment after air cooling after finishing at the above temperature.   A method for producing a nitrocarburized mechanical component, in mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, P: 0.05% or less, S: 0.1% or less, Al: 0.05% or less, N: 0.010 to 0.030%, Ti: 0.005 to 0.05%, Mo: 0.01 to Steel containing 0.5% and Ca: 0.0001 to 0.005%, Nb: 0.05% or less and B: 0.005% or less, the balance being Fe and impurities Soft nitriding is performed without performing normalizing treatment after heating to 1100 to 1250 ° C., starting forging at a temperature of 1100 ° C. or less, finishing at 800 ° C. or less and air cooling. Manufacturing method for non-normalized machine parts.   A method for producing a nitrocarburized mechanical component, in mass%, C: 0.35 to 0.45%, Si: 0.1 to 0.35%, Mn: 0.6 to 1.0%, P: 0.05% or less, S: 0.1% or less, Al: 0.05% or less, N: 0.010 to 0.030%, Ti: 0.005 to 0.05%, Mo: 0.01 to 0.35% and Ca: 0.0001 to 0.005% contained, the balance being Fe and impurities steel is heated to 900 to 1250 ° C, and forging is started at a temperature of 1100 ° C or less and 800 ° C or less A method for producing a soft nitriding non-normalizing mechanical part, characterized by performing soft nitriding without performing a normalizing treatment after air cooling after finishing at the above temperature.   A method for producing a nitrocarburized mechanical component, in mass%, C: 0.35 to 0.45%, Si: 0.1 to 0.35%, Mn: 0.6 to 1.0%, P: 0.05% or less, S: 0.1% or less, Al: 0.05% or less, N: 0.010 to 0.030%, Ti: 0.005 to 0.05%, Mo: 0.01 to Steel containing 0.35% and Ca: 0.0001 to 0.005%, Nb: 0.05% or less and B: 0.005% or less, the balance being Fe and impurities Soft nitriding is carried out by heating to 900 to 1250 ° C., forging is started at a temperature of 1100 ° C. or less, finished at a temperature of 800 ° C. or less and air-cooled, and then subjected to soft nitriding without performing normalization treatment Manufacturing method for non-normalized machine parts.

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