JP2010229508A - Case hardening steel having excellent size-reduction property of maximum crystal grain - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a case hardening steel in which crystal grains are highly controlled, and which is useful for producing a steel component used by being subjected to surface hardening treatment such as carburizing treatment. <P>SOLUTION: The case hardening steel has a chemical component composition comprising, by mass, 0.10 to 0.25% C, ≤0.45% (not including 0%) Si, ≤0.60% (not including 0%) Mn, ≤2.5% (not including 0%) Cr and 0.010 to 0.060% Ti, and the balance iron with inevitable impurities. The total area ratio of Ti based sulfides and/or Ti based carbosulfides with an area of ≥10 μm<SP>2</SP>is 1×10<SP>-5</SP>to 1.0×10<SP>-4</SP>%, and the following inequality (1) is satisfied. In the hardening steel, the size-reduction properties of the maximum crystal grains are excellent: A/[Ti]≤0.080.. (1) (wherein, A denotes the total area ratio (%) of the Ti based carbides, carbonitrides, nitrides, sulfides and carbosulfides with an area of ≥10 μm<SP>2</SP>, and [Ti] denotes the Ti content (mass%) in the steel). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a case-hardened steel useful for producing steel parts used for surface hardening treatment such as carburizing treatment in automobiles, construction machines, and other industrial machines.

  In automobiles, construction machines, and other industrial machines, case-hardened steels defined by JIS standards such as SCr, SCM, SNCM, etc. are usually used for mechanical structural parts that require high material strength. These case-hardened steels are formed into a desired shape by cutting, and then subjected to surface hardening treatment such as carburizing or carbonitriding, and then steel parts are manufactured through a process such as polishing.

  In recent years, steel parts used in automobiles, construction machinery, industrial machinery, etc. have been desired to improve production efficiency and reduce production costs. By changing the forming method from cutting to forging, machining can be performed. Efforts are being made to reduce such costs. However, since it is difficult to manufacture parts that require high accuracy only by hot forging, it is desired to manufacture high-precision parts only by cold forging.

  In addition, when a surface hardening treatment is performed after cold forging, austenite crystal grains become coarse, so case hardening steel that performs cold forging requires crystal grain coarsening prevention characteristics in addition to cold forgeability. .

  As techniques for preventing the coarsening of crystal grains, for example, Patent Documents 1 to 4 disclose techniques that use fine Ti-based carbides and carbonitrides. Patent Documents 1 and 2 are characterized in that they are sufficiently heated at the time of rolling in order to form fine TiC and Ti-based carbonitride. For example, Patent Document 1 describes that when heating for rolling is insufficient, TiC becomes coarse and Ti-based carbon sulfide is formed, so that fine TiC cannot be sufficiently secured. In Patent Document 2, a coarse Ti-based carbonitride is once completely dissolved in austenite in order to precipitate Ti-based carbonitride uniformly and finely, and is heated at 1300 ° C. for about 3 hours, for example.

  On the other hand, Patent Documents 3 to 4 are characterized in that both short-time heating and long-time heating are included. In Patent Document 3, the heating conditions before rolling are 1200 to 1350 ° C. for about 10 to 60 minutes (Example), and in Patent Document 4, the heating conditions before rolling are 1200 ° C. and 10 minutes or more. (Example). Accordingly, in Patent Documents 3 to 4, in the case of heating for a long time, similarly to Patent Documents 1 and 2, Ti-based carbides / carbonitrides are completely precipitated and then finely precipitated. On the other hand, Ti-based carbides and carbonitrides may not completely dissolve in short-time heating.

JP 2008-81841 A JP 2007-31787 A JP 2007-162128 A JP-A-2005-240175

  As in Patent Documents 1 to 4 described above, the use of fine precipitates can improve crystal grain coarsening prevention characteristics. The object of the present invention is to improve the prior art and to control the crystal grains to a higher degree.

  In each of the long-time heating techniques of Patent Documents 1 and 2 and Patent Documents 3 and 4, Ti-based carbides and carbonitrides are sufficiently solid-dissolved to finely precipitate them. However, as a result of further studies by the present inventors, in the case where Ti-based carbides / carbonitrides are sufficiently dissolved, Ti-based sulfides / carbonitrides are also dissolved, and in that case coarsely after rolling. Ti-based inclusions (particularly carbides and carbonitrides) are increasing, and most of the crystal grains are prevented from becoming coarse, but there are only a few large grains (mixed grains). Became clear. In addition, when the solid solution time becomes longer, some Ti-based carbides / carbonitrides are Ostwald-growth and the surrounding fine Ti-based inclusions are absorbed, which seems to be a cause of mixed grains.

  On the other hand, it has been clarified that mixed grains are produced even by the short-time heating technique disclosed in Patent Documents 3 to 4. As a result of further investigation on the cause, Patent Documents 3 to 4 show that a relatively large amount of Ti is added, so that coarse Ti-based inclusions remain in short-time heating, and coarse Ti-based inclusions. It was found that when the material remained, as a result of the Ostwald growth by absorbing the surrounding fine Ti-based inclusions during the heat treatment in the austenite region, the crystal grains specifically increased in a very small part.

  Therefore, further investigation was advanced with the aim of a technique (a technique capable of preventing mixed grains) capable of reducing the largest crystal grains while dispersing a large number of fine Ti-based inclusions to prevent the crystal grains from becoming coarse. As a result, if the addition amount of Ti is suppressed and coarse Ti-based inclusions in the casting stage are suppressed, the heating time before rolling is shortened to secure an appropriate amount of Ti-based sulfide / carbon sulfide. Ti-based inclusions (carbides, nitrides, carbonitrides, sulfides, carbosulfides) can be prevented, resulting in prevention of Ostwald growth of Ti-based inclusions, and Ti-based sulfides / carbonides The present inventors have found that the pinning effect due to can be utilized, and that the crystal grains can be refined without generating mixed grains, thereby completing the present invention.

That is, the case-hardened steel according to the present invention has C: 0.10 to 0.25% (meaning mass%, hereinafter the same for the chemical composition), Si: 0.45% or less (excluding 0%). , Mn: 0.60% or less (not including 0%), Cr: 2.5% or less (not including 0%), Ti: 0.010 to 0.060%, the balance being iron and inevitable The total area ratio of Ti-based sulfide and / or Ti-based carbon sulfide having an area of 10 μm ² or more is 1 × 10 ^{−5 to} 1.0 × 10 ⁻⁴ %, and the following formula (1) Is pleased. Such a case-hardened steel is excellent in the reduction characteristics of the maximum crystal grains.
A / [Ti] ≦ 0.080 (1)
(In the formula, A indicates the total area ratio (%) of Ti-based carbides, carbonitrides, nitrides, sulfides, and carbonsulfides having an area of 10 μm ² or more. [Ti] is the Ti content in the steel. (Mass%) is shown.)

  The case-hardened steel further includes B: 0.0005 to 0.0050%, Mo: 2.0% or less (not including 0%), Cu: 0.10% or less (not including 0%), Ni : 0.10% or less (not including 0%) may be added as appropriate. The inevitable impurities are, for example, P: 0.03% or less (not including 0%), S: 0.030% or less (not including 0%), Al: 0.1% or less (including 0%) N), N: 0.025% or less (not including 0%).

  The present invention also includes a steel material that has been subjected to cold working on the above case-hardened steel and then subjected to surface hardening treatment.

  According to the present invention, since the addition amount of Ti is suppressed, and an appropriate amount of Ti-based sulfide / carbon sulfide is ensured, coarse Ti-based inclusions can be suppressed, and the maximum crystal of case-hardened steel Grain shrinkage characteristics can be enhanced.

FIG. 1 is a diagram showing a heat pattern when the steel material obtained in the example is annealed. FIG. 2 is a diagram showing the shape of a test piece used in the examples. FIG. 3 is a view showing a heat pattern when gas carburizing the steel material obtained in the example. FIG. 4 is a diagram showing the relationship between the left side (A / [Ti]) of the formula (1) and the grain size number of the largest crystal grain.

  The present invention is directed to a steel material suitable for surface hardening after being processed into a part shape, that is, case-hardened steel. And Ti is added in order to prevent a crystal grain coarsening at the time of the heating in a surface hardening process. Ti produces fine Ti (C, N) in the steel and suppresses coarsening of crystal grains during surface hardening (such as carburizing). On the other hand, when the amount of Ti becomes excessive, coarse Ti-based inclusions remain, and the Ostwald growth occurs during the heat treatment in the austenite region, so that the crystal grains specifically increase in a small part. Moreover, the manufacturing cost of steel materials increases, and cold forgeability and impact strength decrease due to the generation of coarse Ti-based inclusions. Therefore, the Ti amount is 0.010 to 0.060%, preferably 0.015 to 0.050%, more preferably 0.025 to 0.045%.

  The case-hardened steel of the present invention further includes C: 0.10 to 0.25%, Si: 0.45% or less (not including 0%), Mn: 0.60% or less (not including 0%), Cr: 2.5% or less (not including 0%) is contained as an essential element. The reason for adding each element is as follows.

C: 0.10 to 0.25%
C is an important element for securing the core hardness necessary for a component. If the amount of C is insufficient, the static strength as a component is insufficient due to insufficient hardness after carburizing. On the other hand, when the amount of C is excessive, it becomes too hard and the forgeability decreases. Therefore, the C content is 0.10 to 0.25%, preferably 0.12 to 0.23%, more preferably 0.14 to 0.20%.

Si: 0.45% or less (excluding 0%)
Si is an effective element for ensuring the hardness of the surface layer of the surface-hardened treated part in order to suppress the hardness reduction during the tempering process. However, as the addition amount increases, the deformation resistance of the material increases and the forgeability decreases. Therefore, Si is 0.45% or less, preferably 0.02 to 0.35%, more preferably 0.05 to 0.15%.

Mn: 0.60% or less (excluding 0%)
Mn acts as a deoxidizer, reduces the amount of oxide inclusions and enhances the internal quality of the steel, and significantly improves the hardenability during quenching after surface hardening (such as carburizing). Have. However, as the Mn increases, striped segregation becomes prominent, resulting in large variations in material and adversely affecting cold workability. Therefore, the amount of Mn is 0.60% or less, preferably 0.10 to 0.55%, more preferably 0.20 to 0.50%.

Cr: 2.5% or less (excluding 0%)
Since Cr has the effect of improving the hardness of the carbide by dissolving in the carbide, it is effective for improving the wear resistance. Further, like Mn, it has an effect of remarkably improving the hardenability at the time of quenching after the surface hardening treatment (carburization, etc.). On the other hand, if the amount of Cr is excessive, the hardness of the material becomes too high and the forgeability becomes poor. Therefore, the Cr content is 2.5% or less, preferably 0.5 to 2.3%, more preferably 0.8 to 2.0%.

  The case hardening steel of this invention may contain the other component as needed. Examples of other components include B, Mo, Cu, and Ni, and these may be added alone or in appropriate combination. Preferred addition amounts and reasons for addition of B, Mo, Cu, Ni, etc. are as follows.

B: 0.0005 to 0.0050%
B may be added in a small amount because it has the effect of significantly improving the hardenability of the steel material and has the effect of strengthening the grain boundaries and increasing the impact strength. However, if it is added excessively, nitrides are likely to be formed, and cold and hot workability are reduced. Therefore, the B content is preferably 0.0005 to 0.0050%, more preferably 0.0010 to 0.0040%, and still more preferably 0.0015 to 0.0030%.

Mo: 2.0% or less (excluding 0%)
Mo is effective for improving the impact strength in addition to the effect of remarkably improving the hardenability at the time of quenching after surface hardening treatment (such as carburizing), and may be added. However, if added excessively, the material becomes hard and machinability is lowered. Therefore, the Mo amount is preferably 2.0% or less, more preferably 0.01 to 1.0%, and still more preferably 0.05 to 0.9%.

Cu: 0.10% or less (excluding 0%)
Since Cu is an element that is less likely to be oxidized than Fe, it improves the corrosion resistance of the steel material. However, when the amount of Cu becomes excessive, the hot ductility of the steel material decreases. Therefore, the Cu content is preferably 0.10% or less, more preferably 0.01 to 0.05%, and still more preferably 0.01 to 0.03%.

Ni: 0.10% or less (excluding 0%)
Ni is an element that improves the corrosion resistance of the steel together with Cu, and may be added alone, but it is desirable to add it in combination with Cu. Ni also has the effect of improving the impact resistance of the steel material. However, excessive addition causes an increase in the cost of the steel material. Therefore, the Ni content is preferably 0.10% or less, more preferably 0.01 to 0.05%, and still more preferably 0.01 to 0.03%.

  In the case-hardened steel of the present invention, the components other than the above (remainder) are usually iron and inevitable impurities. The inevitable impurities mean impurities mixed from raw materials (main raw materials, auxiliary raw materials, etc.) and manufacturing equipment, and examples thereof include P, S, Al, N, and the like. Preferred amounts of P, S, Al, N, etc. are as follows.

P: 0.03% or less (excluding 0%)
Since P is an element that segregates at the grain boundaries and reduces the impact characteristics of the part, it is preferably reduced as much as possible. The amount of P is 0.03% or less, preferably 0.02% or less, more preferably 0.015% or less.

S: 0.030% or less (excluding 0%)
Since S combines with Mn to generate MnS inclusions and lowers the fatigue strength and impact strength of the part, it is preferably reduced as much as possible. Therefore, the S content is 0.030% or less, preferably 0.025% or less, more preferably 0.023% or less. In addition, S may contribute to improvement of machinability. Accordingly, the S amount may be, for example, 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more.

Al: 0.1% or less (excluding 0%)
Al acts as a deoxidizing material and exhibits an effect of reducing the oxide inclusions and improving the internal quality of the steel material. On the other hand, when the amount of Al is excessive, coarse and hard non-metallic inclusions (Al ₂ O ₃ ) are generated, and fatigue characteristics are deteriorated. Therefore, the Al content is 0.1% or less, preferably 0.01 to 0.05%.

N: 0.025% or less (excluding 0%)
When the amount of N is large, coarse Ti-based inclusions are generated to lower the impact strength, and the hardness and deformation resistance of the steel material are increased to reduce the forgeability. Therefore, the N amount is 0.025% or less, preferably 0.020% or less, more preferably 0.018% or less.

In the case-hardened steel of the present invention, as described above, the amount of Ti added is suppressed to suppress the formation of coarse Ti-based inclusions at the casting stage. Therefore, the Ostwald growth in the heat treatment in the austenite region can be suppressed, and the coarsening of Ti-based inclusions can be suppressed. Further, by setting the heating time before the partial rolling to a short time, an appropriate amount of Ti-based sulfide / carbon sulfide can be secured, and coarse Ti-based inclusions after rolling can be suppressed. If coarsening of Ti inclusions is suppressed, the maximum crystal grains can be reduced and mixed grains can be prevented. Furthermore, the cold forgeability can be improved. Since the amount of coarse Ti-based inclusions decreases as the amount of Ti decreases, the allowable amount of coarse Ti-based inclusions is determined according to the amount of Ti added. That is, the case-hardened steel of the present invention satisfies the formula (1).
A / [Ti] ≦ 0.080 (1)
(In the formula, A indicates the total area ratio (%) of Ti-based carbides, carbonitrides, nitrides, sulfides, and carbonsulfides having an area of 10 μm ² or more. [Ti] is the Ti content in the steel. (Mass%) is shown.)

Formula (1) means that Ti-based inclusions having an area of 10 μm ² or more are positioned as coarse inclusions, and the amount thereof is reduced. Of the Ti-based inclusions present in the steel, fine inclusions are called pinning particles and effectively work to prevent coarsening of crystal grains. On the other hand, coarse Ti-based inclusions absorb fine Ti-based inclusions at the time of heat treatment in the austenite region and cause Ostwald growth, so that pinning particles are reduced around coarse Ti-based inclusions, resulting in coarse crystal grains. The anti-oxidation effect cannot be exhibited effectively, causing large abnormal grains to be generated locally. Therefore, if coarse Ti-based inclusions are suppressed, the maximum crystal grains can be reduced and mixed grains can be prevented. Since Ostwald growth is rate-determined by the size of inclusions, the present invention uses an area of 10 μm ² or more as a reference as to whether or not Ostwald growth is performed.

  Furthermore, since coarse Ti-based inclusions are harder than steel substrates, they are less susceptible to deformation during cold forging, and local strain concentrates at the interface between the steel substrate and coarse Ti-based inclusions. It adversely affects cold forgeability, such as cracking. Therefore, by reducing the coarse Ti-based inclusions, in addition to reducing the maximum crystal grains, it is possible to ensure cold forgeability.

  Therefore, the value of the left side (A / [Ti]) of the preferable formula (1) is set to 0.080 or less, preferably 0.07 or less, more preferably 0.06 or less. Note that the lower limit of the value of the left side (A / [Ti]) of the formula (1) is not particularly limited. However, even if the value is too small, the control becomes difficult and the effect is saturated. Therefore, the lower limit may be, for example, 0.01 or more (particularly 0.02 or more).

As described above, the case-hardened steel of the present invention is also characterized in that an appropriate amount of Ti-based sulfide / carbosulfide is secured. Specifically, Ti-based sulfide having an area of 10 μm ² or more and / The total area ratio of the Ti-based carbon sulfide is 1 × 10 ^{−5 to} 1.0 × 10 ⁻⁴ %, preferably 2 × 10 ^{−5 to} 0.8 × 10 ⁻⁴ %, more preferably Is 3 × 10 ^{−5 to} 0.6 × 10 ⁻⁴ %.

  Ti-based sulfides and Ti-based carbon sulfides are coarse, and when precipitated, the effect of preventing crystal grain coarsening is reduced. However, Ti-based sulfides and Ti-based carbon sulfides have a pinning effect although they are weaker than Ti-based carbides and Ti-based carbonitrides. In addition, if the precipitation amount of Ti-based sulfide / Ti-based carbon sulfide is reduced to 0, Ti-based carbide / Ti-based carbonitride is coarsened. If it is a little, you should leave it.

  The case-hardened steel of the present invention increases the cooling rate during casting, increases the heating rate at the time of rolling to a heating temperature of 1200 ° C. or more, and extremely shortens the heating time, and the heating temperature before hot working It can be manufactured by lowering. For example, when manufacturing a wire rod (such as a steel bar), it is subjected to split rolling, hot rolling, etc. However, since the heating temperature at the time of split rolling is usually higher, this heating temperature is set to 1200 ° C. or higher, And it is recommended to heat quickly and for a short time. By increasing the cooling rate of casting and increasing the heating rate, it is possible to prevent the Ti inclusions from becoming coarse. Further, by shortening the heating time, Ostwald growth of Ti inclusions can be prevented, and disappearance of Ti sulfide and Ti carbon sulfide can be prevented. Further, by lowering the heating temperature before hot working, it is possible to prevent the disappearance of Ti-based sulfide and Ti-based carbon sulfide. Preferred production conditions for each step are as follows.

  It is recommended to increase both the average cooling rate from the start of solidification to the end of solidification during casting and the average heating rate at the time of partial rolling in order to prevent the Ti-based inclusions from becoming coarse. The average cooling rate from the start of solidification to the end of solidification during casting is, for example, 150 ° C./h or more (more preferably 180 ° C./h or more, more preferably 200 ° C./h or more). The average heating rate is, for example, 250 ° C./h or more (more preferably 260 ° C./h or more, more preferably 270 ° C./h or more).

It is preferable that the heating temperature T1 (° C.) and the heating time t (min) (t ≦ 10) of the block rolling satisfy the expressions (2) and (3).
1200 ≦ T1 ≦ 1300 (2)
(T1 + 273) × log ₁₀ (t) <1100 (3)

  When the heating temperature T1 is less than 1200 ° C., Ostwald growth occurs slightly, but there is almost no dissolution of Ti-based inclusions in the iron base, and Ti-based sulfide and / or Ti-based carbon sulfide is dissolved. As a result, the total area ratio exceeds the above range, so that coarse Ti-based inclusions cannot be suppressed. On the other hand, when the heating temperature T1 exceeds 1300 ° C., solid solution of Ti carbide and Ti carbonitride progresses and fine inclusions decrease to increase the average crystal grain size, and Ti sulfide and / or As the solid solution of Ti-based carbon sulfide progresses, an appropriate amount of these cannot be ensured, and further, Ostwald growth of coarse Ti-based inclusions progresses, and coarse Ti-based inclusions cannot be suppressed. Therefore, it is desirable that the heating temperature T1 is 1200 ≦ T1 ≦ 1300. A more preferable range of T1 is 1230 ≦ T1 ≦ 1270.

When the heating time t of the block rolling is increased, the fine Ti-based inclusions are decreased, and Ti-based sulfides and / or Ti-based carbon sulfides are decreased, and Ostwald growth of coarse Ti-based inclusions proceeds. Therefore, it is recommended to shorten the heating time t of the block rolling as much as possible, and the left side of the equation (3) ((T1 + 273) × log ₁₀ (t)) is preferably less than 1100, and may be even smaller, It may be 500 or less, particularly 0 or less.

  The heating temperature of the hot rolling is preferably 850 to 1000 ° C. (more preferably 900 to 1000 ° C.). When the heating temperature of hot rolling exceeds 1000 ° C., Ti-based inclusions become coarse, and Ti-based sulfides and / or Ti-based carbon sulfides are dissolved, making it difficult to secure an appropriate amount. .

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  The steel adjusted to the chemical composition shown in Table 1 by casting in a converter was cast, and the resulting slab was heated to a predetermined temperature, and then subjected to block rolling to obtain a billet of 155 mm × 155 mm × 10 m. The obtained billet was reheated and hot-rolled (steel rolling) to produce a steel bar having a diameter of 23 mm. Table 2 shows the heating conditions before the block rolling and the heating conditions before the bar rolling. In addition, the average cooling rate at the time of casting and the average heating rate at the time of block rolling are shown in Tables 3-8.

Each characteristic of the obtained steel bar was evaluated as follows.
(1) Cold forgeability The steel bar was heated at 760 ° C. for 5 hours as shown in FIG. 1, then cooled to 680 ° C. over 8 hours, and then furnace-cooled (spheroidizing annealing or softening annealing). A cylindrical test piece having a diameter of 15 mm and a height of 22.5 mm was cut out from the annealed steel bar. A sample of the cross section of the test piece (cross section perpendicular to the axis) was taken, and the hardness at the D / 4 position (D is the diameter) was measured with a Vickers hardness meter (manufactured by Akashi Corporation, load 10 kg). The case where the Vickers hardness was 150 HV or less was evaluated as being excellent in cold forgeability.

(2) Grain coarsening prevention characteristics and maximum grain reduction characteristics (average grain size number and largest grain size number)
A cylindrical test piece having a diameter of 15 mm and a height of 22.5 mm was cut out from the steel bar (FIG. 2), and the test piece was compressed in the height direction (compression ratio 70%). Thereafter, the carburizing conditions shown in FIG. 3 (the carburizing period conditions are temperature: 950 ° C., time: 70 minutes, carburizing gas: propane gas, carbon potential: 0.8. The diffusion period conditions are temperature: 850 ° C., time: 60 Min, carburizing gas: propane gas, carbon potential: 0.8, quenching conditions were oil-cooled to 80 ° C.).
The grain size number Gh of the prior austenite grains at a location where the equivalent strain is 1.1 was determined according to JIS G0551. More specifically, the average grain size number was determined by a counting method (Appendix 3). The maximum grain size number was determined by a comparative method to obtain the grain size number of the coarsest crystal grain in the observation field of 800 μm × 800 μm, and this was used as the maximum grain size number. When the maximum grain size number was larger than 5, it was evaluated that the maximum grain size could be reduced.

(3) Area ratio of Ti inclusions with an area of 10 μm ² or more
A sample of a longitudinal section (a section parallel to the axis) was cut and polished from the D / 4 position (D is the diameter) of the steel bar. The polished surface was measured with EPMA (Electron Probe Microanalyzer). The composition of inclusions having an area of 10 μm ² or more was examined. Inclusions having a Ti content of 5% by mass or more were made Ti-based inclusions, and those having an S content of 5% by mass or more were Ti-based sulfides or Ti The area ratio was calculated using a system carbon sulfide. The total area ratio of Ti carbide, Ti nitride, and Ti carbonitride was determined by subtracting the total area ratio of Ti sulfide and Ti carbon sulfide from the area ratio of all Ti inclusions. . The maximum particle size is measured for each Ti-based inclusion, the average value of the maximum particle size in Ti-based carbides / nitrides / carbonitrides, and the maximum particle size in Ti-based sulfides / carbonitrides. The average value was obtained and shown in Tables 3 to 8 as “average maximum particle size”. The measurement conditions of EPMA are as follows.
EPMA analyzer: JXA-8100 type electron probe microanalyzer (manufactured by NEC Corporation)
Analyzer (EDS): SystemSix (manufactured by Thermo Fisher Scientific)
Acceleration voltage: 15 kV
Operating current: 4nA
Measurement area: 100 mm ² or more Observation magnification: 200 times The results are shown in Tables 3-8.

  Moreover, the left side (A / [Ti]) of Formula (1) is calculated and shown in Tables 3-8. Furthermore, the relationship between this left side (A / [Ti]) and the grain size number of the largest crystal grain is shown in FIG.

  No. 1 to 15, 17 to 24, 26 to 29, 31 to 38, 40 to 55, 57 to 60, 62 to 65, 67 to 70, 72 to 83, 85 to 92, and 94 to 97 include an appropriate amount of Ti. The rolling conditions I to IV that can be added and can secure an appropriate amount of Ti-based sulfide and / or Ti-based carbon sulfide are employed. As a result, generation of coarse Ti-based inclusions can be suppressed, and A / [Ti] can be reduced, so that the maximum crystal grain can also be reduced. It also has excellent cold forgeability.

  On the other hand, no. Nos. 16, 25, 30, 39, 56, 61, 71, and 93 are examples in which the production condition V having a low ingot rolling temperature is employed. In these, the dissolution of Ti inclusions in the iron base hardly occurs, but since Ostwald growth occurs slightly, the value of A / [Ti] becomes large and the maximum crystal grain becomes large.

  No. 66 and 84 are examples in which production conditions VI having a high ingot rolling temperature are employed. Due to the high rolling temperature, all Ti-based sulfides and carbonitrides are dissolved, and Ti-based carbides, nitrides, and carbonitrides are Ostwald-grown, resulting in an increase in coarse Ti-based inclusions. As a result, the value of A / [Ti] is increased. Therefore, the maximum crystal grain is large.

  No. 98 to 121 are examples in which the component composition deviates from the scope of the present invention.

  No. Since 98 to 101 have a small amount of Ti, the value of A / [Ti] is increased, and as a result, the maximum crystal grain is increased. Further, since the amount of Ti is small, the amount of fine Ti-based inclusions is insufficient, and the average crystal grain is also coarsened.

  No. Nos. 102 to 105 are examples in which the amount of Ti is large. Since the amount of Ti is excessive, the Ti-based inclusions are coarsened, the value of A / [Ti] is increased, and the maximum crystal grain is increased. Further, since the amount of Ti is excessive, the Vickers hardness is increased.

  No. 106 to 109 are examples in which the amount of Mn is large, and it becomes easier to form carbides of Mn. As a result, the amount of C in the iron base decreases, and as a result, among Ti-based carbides, Ti-based carbonitrides, and Ti-based nitrides. Coarse Ti-based nitrides are easily formed, and the value of A / [Ti] is increased, so that the maximum crystal grain is increased. Further, since the amount of Mn is excessive, the Vickers hardness is increased.

  No. 110 to 113 are examples in which the amount of Mn and the amount of S were large. For the same reason as 106 to 109, the value of A / [Ti] is increased and the maximum crystal grain is increased.

  No. 114 to 117 are examples in which the amount of Si was large, and as a result of the decrease in the amount of C in the iron base because Si carbide is easily formed, among Ti-based carbide, Ti-based carbonitride, Ti-based nitride, Coarse Ti-based nitride is easily formed, the value of A / [Ti] is increased, and the maximum crystal grain is increased.

  No. 118 to 121 are examples in which the amount of Si and the amount of Mn were large, and as a result of reducing the amount of C in the iron base because Si carbide and Mn carbide are easily formed, Ti-based carbide, Ti-based carbonitride, Of the Ti-based nitrides, coarse Ti-based nitrides are easily formed, the value of A / [Ti] increases, and the maximum crystal grain becomes larger.

  The case-hardened steel of the present invention is excellent in workability (especially cold forgeability) and can prevent mixed grains after the surface hardening treatment. Therefore, steel parts (for example, gears, shafts) in automobiles, construction machinery, and other industrial machines. , Continuously variable transmission (CVT) pulley, constant velocity joint (CVJ), bearing, etc.).

Claims (6)

C: 0.10 to 0.25% (meaning mass%, hereinafter the same for chemical composition)
Si: 0.45% or less (excluding 0%),
Mn: 0.60% or less (excluding 0%),
Cr: 2.5% or less (excluding 0%),
Ti: 0.010 to 0.060%,
The balance is iron and inevitable impurities,
The total area ratio of Ti-based sulfide and / or Ti-based carbon sulfide having an area of 10 μm ² or more is 1 × 10 ^{−5 to} 1.0 × 10 ⁻⁴ %,
A case-hardening steel excellent in shrinking characteristics of maximum crystal grains characterized by satisfying the following formula (1).
A / [Ti] ≦ 0.080 (1)
(In the formula, A indicates the total area ratio (%) of Ti-based carbides, carbonitrides, nitrides, sulfides, and carbonsulfides having an area of 10 μm ² or more. [Ti] is the Ti content in the steel. (Mass%) is shown.) The case-hardened steel according to claim 1, wherein the inevitable impurities include P, S, Al, and N, and the contents thereof are as follows.
P: 0.03% or less (excluding 0%)
S: 0.030% or less (excluding 0%)
Al: 0.1% or less (excluding 0%)
N: 0.025% or less (excluding 0%)   Furthermore, the case hardening steel of Claim 1 or 2 containing B: 0.0005-0.0050%.   Furthermore, the case hardening steel in any one of Claims 1-3 containing Mo: 2.0% or less (0% is not included).   Further, Cu: 0.10% or less (excluding 0%) and / or Ni: 0.10% or less (not including 0%), case hardening steel according to any one of claims 1 to 4 .   A steel material subjected to a surface hardening treatment after cold working the case-hardened steel according to any one of claims 1 to 5.

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