JP4956146B2 - Case-hardened steel excellent in forgeability and prevention of grain coarsening, its manufacturing method, and carburized parts - Google Patents

Description

  The present invention relates to a case-hardened steel that is a material for machine parts used by carburizing treatment in transport equipment such as automobiles, construction machines and other industrial machines, and a method for manufacturing the same, and carburized parts. When carburized and used as a material for bearings, CVT pulleys, shafts, gears, shaft gears, etc., characteristics such that crystal grains do not become coarse even when carburized at a relatively high temperature ( Hereinafter, the case-hardening steel that exhibits excellent forgeability that is excellent in “crystal grain coarsening prevention characteristics” and that does not generate cracks during forging, and a method for producing the same, and carburizing the case-hardening steel. It relates to carburized parts.

  In machine parts used for automobiles, construction machinery, and other various industrial machines, surface hardening heat treatments such as carburizing, nitriding and carbonitriding (case hardening) have been conventionally applied to parts that require particularly high wear resistance and high fatigue strength. Processing). For these applications, case-hardened steels defined by JIS standards such as SCr, SCM, SNCM, etc. are usually used. After forming into the desired part shape by machining such as forging and cutting, carburizing, carbonitriding, etc. It is manufactured through a finishing process such as polishing.

  In recent years, it has been desired to reduce the manufacturing cost of parts used in automobiles, construction machines, industrial machines, and the like, and efforts are being made to reduce the cost of machining such as forging and cutting. Specifically, there is a tendency to change from cutting to forging, or from hot forging, even forging, to warm forging and cold forging that can reduce cutting costs after forging with high dimensional accuracy. There is.

  In the carburizing process, vacuum carburizing is used in combination with conventional gas carburizing. Although this vacuum carburizing has the advantage that the carburizing time can be shortened and an abnormal carburizing layer is hardly generated on the surface of the carburized part, there is a problem that austenite (γ) crystal grains are likely to be coarsened.

  For these reasons, there is a demand for carburizing steel (skin-hardened steel) that is suitable for warm forging and cold forging and that can also be applied to vacuum carburizing. Various techniques have been proposed so far to prevent coarsening of the case-hardened steel during carburizing. By adding elements such as Al, Nb and Ti, AlN, Nb (CN ), A technique for finely dispersing precipitates such as TiC is widely used.

  As such a technique, for example, Patent Document 1 proposes a technique for preventing coarsening of a crystal by using a Nb precipitate and a precipitate composed of a composite composition of Nb and Al. In this technique, it is also shown that Ti is added at the same time, but this is because N in the steel binds to B to precipitate a BN compound, and the effect of improving the hardenability by B tends to be reduced. By capturing N with Ti, the effect of B is ensured.

  Patent Document 2 proposes a technique of adding Ti under the restriction of the amount of N in order to positively add Nb to prevent grain coarsening and to suppress precipitation of the BN compound. ing.

  In these techniques, Ti is not actively added to prevent crystal grains from coarsening, but is intended to trap N in order not to reduce the effect of improving hardenability by B.

On the other hand, Patent Document 3 proposes that Ti carbide and Ti charcoal / nitride are finely precipitated by positively containing Ti in the range of more than 0.1 to 0.2%. However, when Ti is positively contained, workability (especially forgeability) may be deteriorated, and this technique has not recognized such inconvenience, and has both forgeability and prevention of grain coarsening. It is difficult to secure.
Japanese Patent No. 3480630 Japanese Patent No. 3551573 Japanese Patent Laid-Open No. 10-81938

  The present invention has been made paying attention to the circumstances as described above, and its purpose is to show good forgeability even when forging or cold forging is performed, and to improve the crystal grains by heating for carburizing treatment. Case-hardened steel capable of effectively suppressing coarsening, a useful method for producing such case-hardened steel excellent in forgeability and grain coarsening prevention properties, and carburizing using the case-hardened steel Is to provide carburized parts.

The case-hardened steel according to the present invention that has solved the above problems is: C: 0.05 to 0.30% (meaning “mass%”, the same shall apply hereinafter), Si: 2.0% or less (0% Mn: 1.0% or less (excluding 0%), P: 0.03% or less (including 0%), S: 0.03% or less (including 0%), Cr: 2 0.0% or less (excluding 0%), Al: 0.1% or less (not including 0%), Nb: 0.05 to 0.30%, Ti: 0.05 to 0.10%, N : 0.0080% or less (excluding 0%), O: 0.0020% or less (including 0%), the balance being composed of iron and inevitable impurities, and the composite nitriding containing Nb and Ti in the steel with a maximum particle size of the object is 20μm or less, the particle size is 1μm or more, the nitride is 20μm or less be present in 1 mm ² mean 50 or less And it has a gist point.

  In the case-hardened steel of the present invention, if necessary, (a) Cu: 1.0% or less (not including 0%) and / or Ni: 3.0% or less (not including 0%), (b ) Mo: 1.0% or less (not including 0%), (c) B: 0.0005 to 0.0030%, (d) Ca: 0.010% or less (not including 0%), (e ) Pb: 0.1% or less (not including 0%) and / or Bi: 0.1% or less (not including 0%), (f) V: 0.5% or less (not including 0%) , Zr: 0.5% or less (not including 0%) and W: 0.5% or less (not including 0%), or one or more elements selected from the group consisting of Is effective, and the characteristics of the case-hardened steel are further improved according to the type of element to be contained.

The case-hardened steel as described above exhibits good forgeability even when warm forging or cold forging is performed, and even if carburizing treatment is performed after forging, it can prevent coarsening of crystal grains. In the carburized parts subjected to the treatment, 3.0 / μm ² or more of composite carbonitride containing Nb and Ti and having a size of 10 to 50 nm is present on the surface layer from the surface to a depth of 100 μm. .

On the other hand, the production method of the present invention that can achieve the above-mentioned object is: C: 0.05 to 0.30%, Si: 2.0% or less (not including 0%), Mn: 1.0% or less (Excluding 0%), P: 0.03% or less (including 0%), S: 0.03% or less (including 0%), Cr: 2.0% or less (not including 0%) Al: 0.1% or less (excluding 0%), Nb: 0.05 to 0.30%, Ti: 0.05 to 0.10%, N: 0.0080% or less (including 0%) no), O: 0.0020% or less met (including 0%), after the balance is slabbing a steel consisting of iron and inevitable impurities, was reheated continued to a temperature of 800 to 1050 ° C., hot working It has a gist in terms of performing.
In the partial rolling, heating is performed at a heating temperature T1 (° C.) and a heating time t (seconds) satisfying the relationship of the following formulas (1) and (2), or
It heats for 10 to 60 minutes at the heating temperature T1 (° C.) satisfying the relationship of the following formula (2) and selected from the range of 1200 to 1300 ° C.
4000 ≦ (T1 + 273) × log ₁₀ (t) ≦ 6000 (1)
T1-1030 × [Nb] −600 × [Ti] + 2052 × [N] −1150 ≧ 0 (2)
However, [Nb], [Ti] and [N] indicate the contents (% by mass) of Nb, Ti and N, respectively.

  In this manufacturing method, it is also useful to include various elements as described above in the target steel material, and the characteristics of the case-hardened steel are further improved according to the type of element to be included.

  According to the present invention, by specifying the chemical composition of steel and specifying the form and number of composite nitrides containing Nb and Ti, good forgeability is exhibited even when hot forging or cold forging is performed. In addition, a case-hardened steel capable of effectively suppressing the coarsening of crystal grains due to heating for carburizing treatment can be realized, and such a case-hardened steel is useful as a material for various machine parts.

  Under the prior art as described above, the present inventors mainly have a focus on the component composition of steel and the presence form of precipitates that affect their performance in order to further improve the grain coarsening prevention characteristics and forgeability. I have been doing research. As a result, if the component composition of the steel is specified, and the form and the number of the composite nitride containing Nb and Ti are specified, the case hardening steel having stable and excellent grain coarsening prevention characteristics and forgeability. Was found and the present invention was completed.

  In the present invention, Nb and Ti are added in combination, and a large amount of a composite nitride containing Nb and Ti (hereinafter sometimes simply referred to as “precipitate”) is dispersed and precipitated, thereby suppressing the effect of grain coarsening. It is an enhanced one. According to the study by the present inventors, in order to prevent the coarsening of the crystal grains of the warm / cold forged product, a larger amount of precipitates than before is generated, and for that purpose, Nb and Ti. It has been found that the amount added needs to be optimized. Further, in the vacuum carburizing treatment, precipitates tend to decrease in the surface layer portion, and coarsening of crystal grains is likely to occur in that portion. Therefore, by adding Ti in addition to Nb, precipitates (Nb The composite nitride containing Ti and Ti) was stabilized, and a good effect of preventing grain coarsening was obtained.

  In the case-hardened steel of the present invention, the basic components need to be appropriately adjusted. First, the reasons for limiting the range of chemical components of the steel are as follows.

[C: 0.05-0.30%]
C is an important element for securing the core hardness necessary for a part, and if it is less than 0.05%, the static strength as a part tends to be insufficient due to insufficient hardness. However, if the C content is too large, the hardness becomes excessively high and the forgeability and machinability deteriorate, so it is necessary to keep it to 0.30% or less. The minimum with preferable C content is 0.15%, and a preferable upper limit is 0.26%.

[Si: 2.0% or less (excluding 0%)]
Si is an element effective for suppressing the hardness reduction during the tempering process and ensuring the surface layer hardness of the carburized component. Such an effect increases as the content increases. However, if the Si content is too large, the deformation resistance of the material increases and the forgeability deteriorates. For these reasons, the Si content needs to be 2.0% or less. The upper limit with preferable Si content is 0.35%, More preferably, it is good to set it as 0.15% or less.

[Mn: 1.0% or less (excluding 0%)]
Mn acts as a deoxidizer and has the effect of reducing the amount of oxide inclusions to increase the internal quality of the steel material, and also has the effect of significantly increasing the hardenability during carburizing and quenching. However, as the Mn content increases, striped segregation becomes prominent, resulting in large variations in material, resulting in an adverse effect on cold workability. For these reasons, the Mn content needs to be 1.0% or less. In addition, the upper limit with preferable Mn is 0.6%, More preferably, it is good to set it as 0.5% or less.

[P: 0.03% or less (including 0%)]
P is an element (impurity) inevitably contained in the steel material, and segregates at the grain boundaries to lower the impact characteristics of the component. Therefore, it is preferable to reduce P as much as possible. From such a viewpoint, the upper limit of the content of P is set to 0.03%. The upper limit with preferable P content is 0.02%, More preferably, it is good to set it as 0.015% or less.

[S: 0.03% or less (including 0%)]
S reacts with Mn to form MnS inclusions and lowers the fatigue strength and impact strength of the part, so it is preferable to reduce it as much as possible, but conversely the machinability is improved so that its content is within the above range. It is necessary to adjust accordingly. In ordinary mechanical structural steel, the S content is preferably suppressed to 0.03% or less from the viewpoint of fatigue strength and impact strength. The upper limit with preferable S content is 0.02%, More preferably, it is good to set it as 0.015% or less.

[Cr: 2.0% or less (excluding 0%)]
Cr is effective in improving the wear resistance because it has the effect of improving the hardness of the carbide by dissolving in the carbide. Moreover, the effect which remarkably improves the hardenability at the time of carburizing quenching similarly to Mn is exhibited. In particular, it is preferable to contain an appropriate amount in sliding parts such as gears and bearings. However, if the Cr content is excessive, the strength of the steel material becomes too high and the machinability and forgeability deteriorate, so it should be 2.0% or less. In order to exert such effects, the Cr content is preferably 0.9% or more. Moreover, the minimum with preferable Cr content is 1.2%.

[Al: 0.1% or less (not including 0%)]
It is preferable that Al be contained in an appropriate amount that effectively acts as a deoxidizer and reduces the amount of oxide inclusions to enhance the internal quality of the steel material. However, if the Al content is excessive, coarse and hard non-metallic inclusions (Al ₂ O ₃ ) are generated and the fatigue characteristics are deteriorated, so it should be suppressed to 0.1% or less. A preferable upper limit of Al is 0.07%, and more preferably 0.05% or less.

[Nb: 0.05 to 0.30%]
Nb combines with Ti in steel to form (Nb, Ti) C, (Nb, Ti) N or (Nb, Ti) CN, and exerts the action of suppressing the coarsening of γ crystal grains during carburizing To do. If the Nb content is less than 0.05%, a sufficient amount of precipitates cannot be obtained, and a satisfactory crystal grain coarsening preventing effect cannot be obtained. However, if the Nb content exceeds 0.30%, coarse Nb charcoal / nitride is generated during steel casting, and there is a concern that the impact strength and rolling fatigue strength may be deteriorated. The upper limit with preferable Nb content is 0.20%, More preferably, it is good to set it as 0.10% or less.

[Ti: 0.05-0.10%]
Ti combines with Nb in steel to form (Nb, Ti) C, (Nb, Ti) N or (Nb, Ti) CN, and has the effect of suppressing the coarsening of γ crystal grains during carburizing. Demonstrate. If the Ti content is less than 0.05%, a sufficient amount of precipitates cannot be obtained, and a satisfactory effect of preventing coarsening of crystal grains cannot be obtained. However, if the Ti content exceeds 0.10%, coarse TiN inclusions are generated, and there is a concern that the machinability and the rolling fatigue strength may be reduced. The upper limit with preferable Ti content is 0.09%, More preferably, it is good to set it as 0.08% or less.

[N: 0.0080% or less (excluding 0%)]
N is an impurity element that is preferably reduced as much as possible. When the N content is excessive, coarse TiN inclusions are generated to reduce the machinability and rolling fatigue strength, and the hardness and deformation resistance of the steel material are increased to reduce the forgeability. From such a viewpoint, the N content is preferably suppressed to 0.0080% or less. Preferably it is 0.0060% or less, more preferably 0.0040% or less.

[O: 0.0020% or less (including 0%)]
O is an element inevitably contained in the steel material, and if it is excessively contained, coarse oxide inclusions are generated and the fatigue characteristics of the steel material are lowered. From such a viewpoint, the O content is preferably suppressed to 0.0020% or less. Preferably it is 0.0015% or less, and more preferably 0.0010% or less.

  The essential constituent elements in the case-hardened steel of the present invention are as described above, and the balance is substantially Fe, but inevitable brought into the steel material depending on the situation of raw materials, materials, manufacturing equipment, etc. in addition to those described above. Impurities are allowed to enter.

  Further, in the case-hardened steel of the present invention, in addition to the above elements, if necessary, (a) Cu: 1.0% or less (not including 0%) and / or Ni: 3.0% or less ( (B) Mo: 1.0% or less (excluding 0%), (c) B: 0.0005 to 0.0030%, (d) Ca: 0.010% or less (excluding 0%) (E) Pb: 0.1% or less (not including 0%) and / or Bi: 0.1% or less (not including 0%), (f) V: 0.5 % Or less (not including 0%), Zr: 0.5% or less (not including 0%) and W: 0.5% or less (not including 0%) It is also effective to contain the above elements and the like, and the characteristics of the case-hardened steel are further improved according to the kind of elements to be contained. The reasons for limiting the ranges of these components are as follows.

[Cu: 1.0% or less (not including 0%) and / or Ni: 3.0% or less (not including 0%)]
Since Cu is an element that is less likely to be oxidized than Fe, it is used as an element that improves the corrosion resistance of steel. Therefore, when corrosion resistance is required, it is preferable to make it contain in 1.0% or less of range. However, if the Cu content exceeds 1.0%, the hot ductility of the steel material is lowered, and problems such as cracking tend to occur. The upper limit with more preferable Cu content is 0.3%, More preferably, it is good to set it as 0.1% or less.

  Ni is an element effective for improving the corrosion resistance of a steel material like Cu. Ni also acts effectively to improve the impact resistance of the steel material. However, if the Ni content is excessive, the cost increases, so the upper limit is preferably set to 3.0%. The preferable lower limit of the Ni content is 0.1%, more preferably 0.3% or more, and the more preferable upper limit of the Ni content is 2.0%, more preferably 1.5%. % Or less is good.

[Mo: 1.0% or less (excluding 0%)]
In addition to having the effect of significantly improving the hardenability during carburizing and quenching, Mo is effective in improving the impact strength and is contained as necessary. However, if the Mo content is excessive, the material hardness increases and the machinability decreases, so the content is preferably 1.0% or less. More preferably, it is 0.35% or less, but more preferably less than 0.15%, which is less than JIS case hardened steel (SCM 420: Mo content 0.15 to 0.30%).

[B: 0.0005 to 0.0030%]
B has the effect of significantly increasing the hardenability of the steel material in a small amount, and also has the effect of enhancing the impact characteristics by strengthening the grain boundaries. Such an effect is effectively exhibited by adding 0.0005% or more. However, these effects are saturated when the content exceeds 0.0030%. On the other hand, if the B content exceeds 0.0030% and excessive, B nitrides are likely to be produced, and if this is produced, cold and hot working will be worsened. A more preferable lower limit of the B content is 0.0008%, and more preferably 0.0010% or more. A more preferable upper limit of the B content is 0.0025%, and more preferably 0.0020% or less.

[Ca: 0.010% or less (excluding 0%)]
Ca has the effect of improving the forgeability by suppressing the expansion of sulfide in the steel material and improving the impact characteristics, and suppressing the formation of coarse Ti sulfide. However, if the Ca content becomes excessive and exceeds 0.010%, a coarse oxide is generated and the material strength is reduced. A preferable lower limit of the Ca content is 0.0005%, and more preferably 0.0008% or more. Moreover, the upper limit with more preferable Ca content is 0.0030%, More preferably, it is good to set it as 0.0020% or less.

[Pb: 0.1% or less (not including 0%) and / or Bi: 0.1% or less (not including 0%)]
Pb and Bi are both effective elements for improving the machinability of the steel material, and are contained if necessary. However, since the material strength is lowered when excessively contained, it is preferable that the content is 0.1% or less. The lower limit is preferably 0.02%, more preferably 0.03% or more. Moreover, a more preferable upper limit is 0.08%, and further preferably 0.06% or less.

[V: 0.5% or less (not including 0%), Zr: 0.5% or less (not including 0%) and W: 0.5% or less (not including 0%) 1 type or 2 types or more]
V, Zr and W are all active elements such as carbon and nitrogen, and by forming fine precipitates, the crystal grain coarsening prevention property can be improved, and therefore all of 0.5% or less You may make it contain in the range of. A more preferable upper limit of these elements is 0.3%, and further preferably 0.1% or less.

  By the way, when Nb and Ti are contained, precipitates such as carbides, nitrides, and carbonitrides containing these elements in a composite form are formed. Among them, it is important to pay attention to the composite nitride containing Nb and Ti, and to define the form and number of the composite nitrides. The reasons for specifying these are as follows.

[Maximum particle size of composite nitride containing Nb and Ti is 20 μm or less]
The composite nitride containing Nb and Ti is an inclusion that is inevitably generated when N of molten steel is combined with Nb and Ti during solidification of the steel. Of these inclusions, coarse inclusions (composite nitrides) reduce the workability (deformability) of the steel material, and are therefore preferably generated as finely as possible. For these reasons, the maximum diameter of the target composite nitride was set to 20 μm or less. The maximum diameter of the precipitates was measured using an optical microscope, and a field area of 10 mm ² was examined at a magnification of 100 times, and 20 samples were extracted in descending order of inclusions. It is a representation.

[An average of 50 or less of the nitride having a particle size of 1 μm or more and 20 μm or less exists in 1 mm ² ]
The number of inclusions is preferably small from the viewpoint of workability of the steel material, and the average number is 50 or less in 1 mm ² . In addition, the number of fields is measured by using a sample with a field of view area of 10 mm ² , selecting 20 fields of 1 mm × 1 mm randomly with an optical microscope at a magnification of 100 times, measuring the number of fields, and calculating the average per field of view. The number is measured. In addition, the particle size of the composite nitride to be measured being 1 μm or more means a minimum size that can be identified with an optical microscope.

The case-hardened steel of the present invention uses a composite nitride containing Nb and Ti for the purpose of preventing the coarsening of crystal grains generated in the vacuum carburized surface layer portion. Reflecting this, the composite carbonitride [(Nb, Ti) (C, N)] containing Nb and Ti in the surface layer portion seems to have 3.0 precipitates / μm ² or more having a size of 10 to 50 nm. Form. The surface layer portion of the part means a region from the surface to a depth of 100 μm, and the composite carbonitride can be measured by a transmission electron microscope (TEM) at about 50,000 times.

Regarding the conditions for producing the case-hardened steel having the above-mentioned characteristics, the steel satisfying the above-mentioned chemical composition is melted, and casting, soaking (solution treatment), hot working (for example, according to a conventional method) , may be hot rolled) but, particularly if Takamere the cooling rate from solidification start to the solidification ended 2.5 ° C. / min or more, Ti-based precipitates crystallized during cooling are fine, the coarse precipitates Since generation is suppressed, cold workability (particularly, cold forgeability) can be improved. The cooling rate is preferably 5 ° C./min or more, more preferably 7.5 ° C./min or more, in order to suppress coarse Ti-based inclusions. As a means for increasing the cooling rate, an improvement in casting speed and an improvement in water cooling ability can be mentioned. In the case of producing a slab by continuous casting, for example, the casting speed is preferably set to 1.05 m / min or more, and the amount of cooling water in the mold is set to, for example, 0.15 ton / hour or more per nozzle.

  Although it does not limit about other conditions, For example, it is preferable that the soaking | uniform-heating conditions (solution treatment conditions) before the lump rolling after casting shall be 5 minutes or more in the temperature range of 1200-1350 degreeC. By performing such a treatment, the composite carbonitride is solid-dissolved and later easily finely precipitated, which is useful for improving the crystal grain coarsening prevention characteristics. In addition, since the effect is saturated even if the treatment is performed for an excessively long time, 10 hours or less is preferable from the viewpoint of productivity.

The present inventors have studied a useful method for producing the case-hardened steel as described above. As a result, using the steel material having the chemical composition as described above, after heating at the heating temperature T1 (° C.) and the heating time t ( seconds ) satisfying the relationship of the following formulas (1) and (2), If it is rolled and subsequently reheated to a temperature of 800 to 1050 ° C. and then hot-worked, a case-hardened steel having the above properties can be obtained. It has been found that it has excellent coarsening prevention properties. The reason why the relationship between the following expressions (1) and (2) is specified will be described.
4000 ≦ (T1 + 273) × log ₁₀ (t) ≦ 6000 (1)
T1-1030 × [Nb] −600 × [Ti] + 2052 × [N] −1150 ≧ 0 (2)
However, [Nb], [Ti] and [N] indicate the contents (% by mass) of Nb, Ti and N, respectively.

  The solution treatment described above is necessary to temporarily dissolve the coarse precipitates produced during casting (manufactured by continuous casting or ingot making) and to produce fine precipitates that are effective in preventing grain coarsening in the subsequent process. However, if the heating temperature and the heating time are insufficient, the coarse precipitates are not sufficiently dissolved in the substrate, and the effect of preventing the coarsening of crystal grains cannot be obtained. Conversely, when the heating temperature and the heating time are excessive, a large number of very fine precipitates are generated, the hardness is increased, and the cold forgeability is lowered. From these viewpoints, the present inventors examined the influence of the heating temperature T1 and the heating time t (minute) during the solution treatment on the coarsening of the crystal grains.

  FIG. 1 shows that the same steel type is used, the heating temperature T1 is kept constant, and the heating time t when the heating time t is changed is a fine precipitate (composite carbonitride containing Nb and Ti and having a size of 10 to 50 nm. Is a graph showing the influence on the number of the test samples (test Nos. 56 to 61 of Example 2 described later). As is clear from this result, it can be seen that the number of fine precipitates increases logarithmically with heating time.

  FIG. 2 shows that the same steel type is used, the heating time t is constant, and the heating temperature T1 when the heating temperature T1 is changed is a fine precipitate (composite carbonitride containing Nb and Ti and having a size of 10 to 50 nm. (No. 59, 62 to 64 in Example 2 described later). As is apparent from this result, it can be seen that the number of fine precipitates increases in direct proportion to the heating time.

  Based on these results, the effects of the heating temperature T1 and the heating time t in the solution treatment on the number of fine precipitates are summarized in terms of data. When the relationship of the above formula (1) is satisfied, It has been found that the precipitates are appropriately dispersed to improve the forgeability and achieve the effect of preventing grain coarsening.

The relationship of the above formula (1) is obtained from the fact that the number of fine precipitates increases linearly with the heating temperature T1 and logarithmically with the heating time, but (T1 + 273) × log ₁₀ (t) When the value (hereinafter referred to as “A value”) exceeds 6000, the number of fine precipitates increases, the hardness of the steel material increases, and the forgeability deteriorates. On the other hand, when the A value is less than 4000, the effect of preventing coarsening of crystal grains due to fine precipitates cannot be achieved. In addition, the realistic ranges of the heating temperature T1 and the heating time are appropriate ranges such as the heating temperature T1: 1200 to 1350 ° C. and the heating time: about 5 minutes to 10 hours, as described above. The formula will be selected within the range of these heating temperatures and heating times.

  The relationship of the above formula (1) defines the heating temperature T1 and the heating time t necessary for once dissolving a coarse precipitate. However, depending on the contents of Ti, Nb and N, the precipitate becomes stable at a high temperature, and the precipitate does not form a solid solution even if heat treatment (solution treatment) is performed under the condition satisfying the relationship of the above formula (1). There is a case. For these reasons, in a steel material containing a relatively large amount of Ti, Nb, and N, it is necessary to increase the heating temperature T1 in accordance with these contents. In addition, since the affinity between Ti and N is strong, when the N content is large, the generation of TiN preferentially proceeds, and the amount of Ti forming the composite nitride containing Ti and Nb is reduced, so that the composite nitriding The stability of things will be reduced. Considering these phenomena, it is necessary to set an appropriate heating temperature T1. The present inventors have confirmed by experiments based on such an idea, and may set the heating temperature T1 according to the contents of Ti, Nb and N so as to satisfy the relationship of the above formula (2). It has been found.

  The above equation (2) is set in consideration of the influence of each element (Nb, Ti, N) on the high-temperature stability of fine precipitates, and is obtained based on multiple regression analysis. . When the value (hereinafter referred to as “B value”) of (T1-1030 × [Nb] −600 × [Ti] + 2052 × [N] −1105) is less than 0, the heating temperature T1 becomes an appropriate temperature range. Thus, the coarse precipitates deposited at the time of casting form a solid solution, and fine precipitates are formed in the subsequent steps. For example, even if the heating temperature T1 and the heating time t are the same, the number of fine precipitates changes according to the components of the steel material (the number of precipitates increases as the B value increases). 63, 66, 68, 70).

  After heating so as to satisfy the relationship of the above formulas (1) and (2), it is necessary to perform block rolling and subsequently perform hot working (for example, hot rolling) to form a steel bar or wire. is there. In performing this hot working, it is necessary to reheat to a temperature of 850 to 1050 ° C. This step (reheating step) is to prevent coarse precipitation of Nb and Ti that are in a solid solution state by the heat treatment during hot working.

  The reheating temperature is preferably as low as possible from the viewpoint of not coarsening the fine precipitates, that is, not reducing the crystal grain coarsening prevention property. However, when it is less than 850 ° C., the two-phase region of ferrite + austenite Thus, the structure after hot working becomes a mixed grain, and the prevention of crystal grain coarsening is reduced.

  On the other hand, when the reheating temperature exceeds 1050 ° C., Nb and Ti in a solid solution state are coarsely and sparsely precipitated, and it is difficult to achieve the effect of preventing the coarsening of crystal grains. The preferable lower limit of the reheating temperature is about 900 ° C., and the preferable upper limit is about 1025 ° C.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and is suitable as long as it can meet the purpose described above and below. It is also possible to carry out the invention with modifications, and these are all included in the technical scope of the present invention.

[Example 1]
Steel materials having the chemical compositions shown in Tables 1 and 2 were melted in a melting furnace, cast at different cooling rates during casting, subsequently heated to 1200 ° C., hot forged into a 50 mm diameter steel bar, 1280 ° C. Solution treatment for 60 minutes. The cooling rate during casting was varied by using different size molds. Then, after performing normalizing treatment at 900 ° C. by simulating actual rolling, spheroidizing treatment was performed, and a circle of φ8 mm × 12 mm from the position of D / 4 (D represents the diameter of the bar steel) of the cross-section of the forged material A columnar specimen was prepared.

  A compression test was performed with a press using the cylindrical test piece, and the cold forgeability was evaluated by measuring the variable resistance and crack limit. At this time, the deformation resistance was determined from the load when 70% compression was applied. In addition, the crack limit was determined by applying a compression of 2.5% stepwise after applying 50% compression, and determining the processing rate until cracking occurred. Each test was performed with end face restraint, the deformation resistance measurement was performed 3 times (n = 3), and the crack limit was performed 8 times (n = 8), and the average value was obtained.

For the grain coarsening test, the cylindrical specimen was compressed at a processing rate of 70% and then carburized at 900 ° C. or 1000 ° C. in a vacuum carburizing furnace [soaking time: 70 minutes, carburizing / diffusion time: 80 minutes, carburizing gas: acetylene (C ₂ H ₂ )], assuming a heat treatment, heating at 880 ° C. for 40 minutes, oil cooling to 60 ° C., and then setting the austenite grain size to JIS G 0551 Measured according to the austenite grain size test method and evaluated by the area ratio of coarse grains having a grain size number of 5 or less [exceeding 5%: defective (×), 5% or less: good (◯)] .

  Further, a carburized impact test piece shown in FIG. 3 was prepared for the steel material subjected to the above normalizing treatment. The test piece was subjected to carburizing treatment at 1000 ° C. in a vacuum carburizing furnace (soaking time: 70 minutes, carburizing / diffusion time: 80 minutes), heated at 880 ° C. for 40 minutes, and then oil-cooled to 60 ° C. Thereafter, a tempering treatment at 170 ° C. for 2 hours was performed, a Charpy impact test defined by JIS Z 2242 was performed, and the impact strength after carburization was evaluated by measuring the absorbed energy measured at that time. The results of these tests are the cooling rate during casting, the size (maximum particle size) / number of nitrides (1-20 μm) measured by the above method, the number of carbonitrides (10-50 nm), etc. Together with these, Tables 3 and 4 below collectively show.

  From these results, it can be considered as follows (hereinafter, “No.” indicates the test No.). First, no. Nos. 1 to 5 (Nos. 1 and 5 are the same) are examples that satisfy all of the requirements defined in the present invention, and both the grain coarsening resistance and cold forgeability are good. I understand that.

  No. Those of 2 to 4, 37 to 53, 55 lack any of the requirements defined in the present invention, and any of the characteristics is deteriorated, and the object of the invention cannot be achieved.

[Example 2]
A steel material (steel grades A to G) having a chemical composition shown in Table 5 was produced by casting a 150 kg slab using a vacuum induction melting furnace, and subjected to solution treatment and hot working under conditions assuming a lump pressure in an actual machine. The billet shape was × 155 mm × about 500 mm. The solution treatment conditions at this time are as shown in Table 6 below. The conditions at this time were changed such that the heating temperature was 1200 to 1300 ° C. and the heating time was 5 to 300 minutes. Test No. Nos. 56 to 61 are examples in which the heating time t during the solution treatment was changed using the steel type A shown in Table 5 (FIG. 2).

  Subsequently, the billet-shaped prototype material was welded to the dummy material to form a billet of 155 mm × 155 mm × about 10 m, held at a predetermined heating temperature, and then rolled into a φ46 mm bar in a rolling line. The heating temperature at this time is as shown in Table 6 below. Test No. 59 and 62-64 change the heating temperature T1 at the time of solution treatment using the steel type A (refer said FIG. 1).

  Thereafter, the rolled steel bar was cut into a length of about 300 mm, and then spheroidizing annealing was performed in a heat treatment furnace. The annealing conditions at this time were maintained at 760 ° C. for 5 hours, and then cooled from that temperature to 680 ° C. at an average cooling rate of 10 ° C./hour (8 hours), and then cooled in the furnace. The hardness after spheroidizing annealing was measured at 3 points with a load of 10 kgf (98 N) using a Vickers hardness tester at a D / 4 (D represents the diameter of the steel bar) position of a φ46 mm steel bar. Asked.

  Next, a cylindrical test piece (test piece for cold forging) of φ15 mm × 22.5 mm was produced from the position of D / 4 (D represents the diameter of the steel bar) of the steel bar after spheroidizing annealing. Using the cylindrical test piece, a compression test was performed with a press, and the cold forgeability was evaluated by measuring variable resistance. At this time, the cold resistance value of the steel material was measured over a processing rate of 10 to 70%. The deformation resistance was obtained from the load when 20% compression was applied. Other conditions are the same as in the first embodiment.

For the crystal grain coarsening test, after compressing the cylindrical specimen at a processing rate of 70%, carburizing treatment at 900 to 1050 ° C. in a vacuum carburizing furnace [soaking time: 70 minutes, carburizing time: 38 minutes, Diffusion time: 42 minutes, carburizing gas: acetylene (C ₂ H ₂ )], assuming heat treatment, heated at 880 ° C. for 40 minutes, and then oil-cooled to 60 ° C. for observation of crystal grains Thus, the grain coarsening situation was judged. The carburizing temperature at this time was set in steps of 25 ° C. from 900 ° C. to 1050 ° C. In the grain coarsening test, the cross section of the test piece is observed, the austenite grain size is measured according to the austenite grain size test method defined in JIS G 0551, and coarse grains with a grain size number of 5 or less are recognized. (The coarsening temperature is good when the temperature is 1000 ° C. or higher).

  These test results are shown as solution treatment conditions (heating temperature T1, heating time t), values of equations (1) and (2), and the size of nitrides (1 to 20 μm) measured by the above method ( Table 7 below collectively shows the maximum particle size), the number, the number of composite carbonitrides (with a size of 10 to 50 nm), and the like.

  From these results, it can be considered as follows. First, test no. Examples 57 to 59, 63 to 65, 67 to 69, and 75 are examples that satisfy all of the requirements defined in the present invention, and that both the grain coarsening resistance property and the cold forgeability are good. I understand.

  No. 56, 60 to 62, 66, 70 to 74, and 76 to 79 lack any of the requirements defined in the present invention, and any of the characteristics has deteriorated, and the object of the invention can be achieved. Not.

It is the graph which showed the influence which the heating time t has on the number of fine precipitates. It is the graph which showed the influence which heating temperature T1 has on the number of fine precipitates. It is a figure which shows the test piece for impact test evaluation employ | adopted in experiment.

Claims (16)

C: 0.05 to 0.30% (meaning “mass%”, the same shall apply hereinafter), Si: 0.21 % or less (not including 0%), Mn: 1.0% or less (not including 0%) ), P: 0.03% or less (including 0%), S: 0.03% or less (including 0%), Cr: 2.0% or less (not including 0%), Al: 0.1 % Or less (excluding 0%), Nb: 0.05 to 0.30%, Ti: 0.05 to 0.10%, N: 0.0080% or less (not including 0%), O: 0 .0020% or less (including 0%) is satisfied, the balance is composed of iron and inevitable impurities, and the composite nitride containing Nb and Ti in the steel material has a maximum particle size of 20 μm or less and a particle size of 1 μm or more. The forgeability and prevention of grain coarsening, characterized in that the average number of nitrides of 20 μm or less is 50 or less in 1 mm ² Case-hardened steel with excellent stopping properties.   The case hardening steel according to claim 1, further comprising Cu: 1.0% or less (excluding 0%) and / or Ni: 3.0% or less (not including 0%). Furthermore, the case hardening steel of Claim 1 or 2 containing Mo: less than 0.15% (it does not contain 0%).   Furthermore, B: 0.0005-0.0030% is contained, The case hardening steel in any one of Claims 1-3.   Furthermore, Ca: 0.010% or less (0% is not included) The case hardening steel in any one of Claims 1-4.   Further, Pb: 0.1% or less (not including 0%) and / or Bi: 0.1% or less (not including 0%), case hardening steel according to any one of claims 1 to 5 .   Further, from the group consisting of V: 0.5% or less (excluding 0%), Zr: 0.5% or less (not including 0%), and W: 0.5% or less (not including 0%) The case-hardened steel according to any one of claims 1 to 6, which contains one or two or more elements selected. Carburized using the case-hardened steel according to any one of claims 1 to 7, and a size of a composite carbonitride containing Nb and Ti on the surface layer from the surface of the part after carburizing to a depth of 100 µm. A carburized part having 10 to 50 nm of 3.0 pieces / μm ² or more. C: 0.05 to 0.30%, Si: 2.0% or less (not including 0%), Mn: 1.0% or less (not including 0%), P: 0.03% or less (0 %), S: 0.03% or less (including 0%), Cr: 2.0% or less (not including 0%), Al: 0.1% or less (not including 0%), Nb : 0.05 to 0.30%, Ti: 0.05 to 0.10%, N: 0.0080% or less (not including 0%), O: 0.0020% or less (including 0%) Satisfying, with the balance being steel and steel with inevitable impurities, heated at a heating temperature T1 (° C.) and a heating time t (seconds) satisfying the relationship of the following formulas (1) and (2), and then batch-rolled: Continuously reheating to a temperature of 850 to 1050 ° C., followed by hot working, a method for producing case-hardened steel excellent in forgeability and grain coarsening prevention characteristics .
4000 ≦ (T1 + 273) × log ₁₀ (t) ≦ 6000 (1)
T1-1030 × [Nb] −600 × [Ti] + 2052 × [N] −1105 ≧ 0 (2)
However, [Nb], [Ti] and [N] indicate the contents (% by mass) of Nb, Ti and N, respectively. C: 0.05 to 0.30%, Si: 2.0% or less (not including 0%), Mn: 1.0% or less (not including 0%), P: 0.03% or less (0 %), S: 0.03% or less (including 0%), Cr: 2.0% or less (not including 0%), Al: 0.1% or less (not including 0%), Nb : 0.05 to 0.30%, Ti: 0.05 to 0.10%, N: 0.0080% or less (not including 0%), O: 0.0020% or less (including 0%) After satisfying the relationship of the following formula (2) and heating at a heating temperature T1 (° C.) selected from the range of 1200 to 1300 ° C. for 10 to 60 minutes after the filling, the balance is rolled for 10 to 60 minutes. Then, after reheating to a temperature of 850 to 1050 ° C., it is excellent in forgeability and grain coarsening prevention characteristics, characterized by performing hot working. Method of manufacturing a hardened steel was.
T1-1030 × [Nb] −600 × [Ti] + 2052 × [N] −1105 ≧ 0 (2)
However, [Nb], [Ti] and [N] indicate the contents (% by mass) of Nb, Ti and N, respectively.   The skin hardening according to claim 9 or 10, wherein the steel material further contains Cu: 1.0% or less (not including 0%) and / or Ni: 3.0% or less (not including 0%). Steel manufacturing method.   The method for producing case-hardened steel according to any one of claims 9 to 11, wherein the steel material further includes Mo: 1.0% or less (not including 0%).   The method for producing case-hardened steel according to any one of claims 9 to 12, wherein the steel material further contains B: 0.0005 to 0.0030%.   The method for producing case-hardened steel according to any one of claims 9 to 13, wherein the steel material further contains Ca: 0.010% or less (not including 0%).   The steel material according to any one of claims 9 to 14, further comprising Pb: 0.1% or less (not including 0%) and / or Bi: 0.1% or less (not including 0%). A method for producing case-hardened steel. Furthermore, V: 0.5% or less (not including 0%), Zr: 0.5% or less (not including 0%)
The method for producing a case-hardened steel according to any one of claims 9 to 15, which contains one or two or more elements selected from the group consisting of W and 0.5% or less (not including 0%).

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