JP4266341B2 - Steel for omitting spheroidizing annealing with excellent cold forgeability and anti-roughening properties during case hardening, and method for producing the same - Google Patents

Description

  The present invention relates to a case hardening steel useful for producing a cold forged part whose surface is subjected to case hardening (carburizing treatment, carbonitriding treatment, etc.), and more specifically, without performing spheroidizing annealing. The present invention relates to a case hardening steel having excellent cold forgeability and excellent ability to suppress coarsening of crystal grains during case hardening.

  As parts manufactured by cold forging and case hardening, for example, power transmission parts (pulleys, gears, shafts, etc.) used in machinery such as automobiles are known. These power transmission parts are subjected to chemical surface hardening heat treatment such as carburizing and carbonitriding for the purpose of improving surface strength, wear resistance, and pitting resistance after cold forging into parts. .

  However, when heat treatment such as carburizing is performed, the crystal grains become coarse, and the toughness and fatigue strength of the component are liable to decrease. In addition, when the crystal grains become coarse, variations in carburizing and quenching occur, heat treatment distortion occurs, and dimensional accuracy decreases, which may cause vibration and noise. Accordingly, there is a demand for steel capable of suppressing the coarsening of crystal grains during case baking.

  Moreover, the said cold forging components are conventionally manufactured by cold forging the steel softened by spheroidizing annealing. However, in recent years, for the purpose of reducing the cost by omitting the process, steels that can ensure cold forgeability even if spheroidizing annealing is omitted have been aimed.

  Non-Patent Document 1 describes that N-rich steel with Al / N ≦ 1.9 is excellent in anti-coarse graining characteristics (900 ° C.), and the anti-coarse grain resistance characteristics are further improved by adding Nb. It is described to do. However, in this document, by carrying out the partial rolling at a high temperature (1250 ° C.), the precipitation of carbonitrides is suppressed, and the above-mentioned coarsening characteristics are exhibited by finely depositing carbonitrides during spheroidizing annealing. Therefore, it is impossible to cope with the case where the spheroidizing annealing is omitted. Moreover, this nonpatent literature 1 has shown the examination result of Ti-less steel.

  Patent Document 1 discloses steel for case hardening with s-Al: 0.010 to 0.060% and N: 0.008 to 0.040%, and Al is combined with nitrogen in the steel. It is described that AlN is produced to improve the coarsening characteristics. According to Patent Document 1, regarding Ti, if Ti is made 0.005% or more, TiN increases and AlN decreases, so Ti needs to be regulated to less than 0.005%.

  Patent Document 2 discloses a case-hardened steel with Nb: 0.005 to 0.100% and Ti ≦ 0.10%, and teaches that Nb has an effect of refining austenite crystal grains. Yes. In addition, in this patent document 2, in order to ensure the hardenability improvement effect of B added simultaneously, Ti is added 3.6 times or more with respect to N.

  Patent Document 3 contains one or more of Ti, Zr, and Hf so that Ti + 0.53Zr + 0.27Hf = 0.01 to 0.05%, and N is 0.29Ti + 0.15Zr + 0.08Hf to 0.015%. Steel for structural machine for forging contained in the range of, and teaches that Ti, Zr, and Hf have an effect of suppressing grain coarsening. In Patent Document 3, steel is cold forged after spheroidizing and then carburized. And about the grain coarsening inhibitory property (crystal grain coarsening temperature), the steel [steel type (6)] added with Ti in combination with Zr rather than the steel added with Ti alone [steel type (2)]. The amount of N is specifically suppressed to a low level of 0.005 to 0.010% in the column of the examples.

In Patent Document 4, Ti: 0.005 to 0.015 wt%, N: Ti / 3.42 + 0.01 to 0.02 wt%, Al / (N-Ti / 3.42) = 1.5 to 2.8. Ti, N, and Al are controlled so that And Ti combines with N to become TiN, and has the effect of suppressing the coarsening of austenite crystal grains when heated to a high temperature such as hot forging, except that N combines with Ti to become TiN. It also binds to Al added in a large amount to prevent the coarsening of austenite crystal grains during carburization, and Al is 1.5 of the remaining N (= N—Ti / 3.42) bonded to Ti. It teaches that it is effective when it becomes 2.8 times.
Yasuyuki Mino and two others, “Effects of Nb, Al, N and cold working on coarsening properties of high-strength steel”, Sumitomo Metal Technical Report, Sumitomo Metal Industries, Ltd., October 1989, Vol. 41 , No. 4, pp. 35-41 JP-A-8-199316 (Claim 1, paragraph 0007, paragraph 0008) JP-A-7-310118 (Claim 1, paragraphs 0011, 0013) Japanese Patent Publication No. 61-54844 (Claim 1, page 3, left column, lines 15-22, Tables 2, 3) Japanese Patent No. 2546045 (Claim 1, page 2, right column, line 47 to page 3, left column, line 15)

  An object of the present invention is to provide a steel that can ensure cold forgeability even when spheroidizing annealing is omitted, and that is excellent in crystal grain coarsening characteristics during a skin baking treatment, and a method for producing the same. .

  As a result of intensive research to solve the above problems, the inventors of the present invention decided to effectively use TiN in a specific steel not containing Zr, Nb, etc., and increased N instead of Ti. It has been found that if Al is added so that it is excessively large with respect to N that has not been combined with the steel, excellent cold forgeability can be secured without spheroidizing annealing and the coarsening resistance can be improved. The present invention has been completed. If Nb, Zr, or the like is added in accordance with the teachings of Patent Documents 2 and 3, cold forgeability is reduced when spheroidizing annealing is omitted. Even when Al is suppressed in accordance with the teaching of Patent Document 4, cold forgeability is reduced when spheroidizing annealing is omitted.

That is, the steel for case hardening excellent in the cold forgeability and the coarse graining resistance property during case hardening of the present invention that can achieve the above object is C: 0.05 to 0.3% (meaning mass%). The same shall apply hereinafter), Si: 0.01 to 0.35%, Mn: 0.2 to 2%, P: 0.001 to 0.02%, S: 0.001 to 0.02%, hardenability Element (Ni: 0.01-2%, Cr: 0.01-2%, Mo: at least one selected from 0.01-0.5%), Ti: 0.005-0.02%, Al: 0.020 to 0.1%, and N: 0.005 to 0.02%, the balance is Fe and inevitable impurities,
Moreover, the above Ti, Al, and N have a gist in that they further satisfy the following formula (1). The case hardening steel is also a steel for omitting spheroidizing annealing.

Al / (N-Ti / 3.4) ≧ 4 (1)
The hardened steel is normally precipitates having an average particle size of 5 to 50 nm, 0.25 [mu] m ² per contains 13 or more, average particle diameter 50nm than precipitates, 0.25 [mu] m ² per three It is as follows. Moreover, when the ferrite area ratio and the ferrite grain size number are measured uniformly over the entire cross section, the average value of the ferrite area ratio is usually 50% or more, and the average grain size number is 9.0 to 12.2. No. 5. The tensile strength is desirably 400 to 750 N / mm ² and the aperture is desirably 60% or more. When the carbon equivalent Ceq defined by the following formula (2) is 0.7 or less and / or when the ideal critical diameter DI value defined by the following formula (3) is 10 or more, the intensity control is simple. Become.

Ceq = [C] + [Si] / 7 + [Mn] / 5 + [Cr] / 9 + [Mo] / 2 + [Ni] / 6 (2)
(In the formula, [C], [Si], [Mn], [Cr], [Mo], [Ni] are the contents (mass%) of C, Si, Mn, Cr, Mo, or Ni, respectively. Indicate)

(In the formula, γGS means austenite grain size, and its value is 10. [C], [Si], [Mn], [Cr], [Mo], and [Ni] are C, Si, respectively. , Mn, Cr, Mo, or Ni content (% by mass))
The steel for case hardening is C: 0.05 to 0.3%, Si: 0.01 to 0.35%, Mn: 0.2 to 2%, P: 0.001 to 0.02%, S : 0.001-0.02%, hardenable element (Ni: 0.01-2%, Cr: 0.01-2%, Mo: at least one selected from 0.01-0.5% ), Ti: 0.005-0.02%, Al: 0.020-0.1%, and N: 0.005-0.02%, the balance being Fe and inevitable impurities, The steel ingot, cast slab or steel slab in which Ti, Al, and N further satisfy the above formula (1) can be manufactured by hot rolling under the conditions shown below and cooling.

Rolling start temperature: 800-950 ° C
Rolling temperature (excluding finish rolling): 780-900 ° C
Finish rolling temperature: 750-850 ° C
Cooling start temperature: 750-800 ° C
Cooling rate in the temperature range of 750 to 550 ° C .: 0.05 to 0.3 ° C./second

  The steel of the present invention balances strength and cold forgeability by controlling the components within an appropriate range. And by using TiN effectively without using Zr, Nb, etc., the coarse grain resistance is enhanced while ensuring the cold forgeability. In addition, the amount of N, not Ti, is increased, and a sufficient amount of Al is added to N that did not bind to Ti. Therefore, even if steel for omitting spheroidizing annealing is used, reasonably fine TiN, AlN It is possible to cause precipitation, and it is possible to reliably improve the coarse graining resistance property while ensuring cold forgeability.

  The steel of the present invention has C: 0.05 to 0.3% (meaning mass%, the same shall apply hereinafter), Si: 0.01 to 0.35%, Mn: 0.2 to 2%, P: 0.00. 001-0.02%, S: 0.001-0.02%, hardenable elements (Ni: 0.01-2%, Cr: 0.01-2%, Mo: 0.01-0.5 %), Ti: 0.005 to 0.02%, Al: 0.020 to 0.1%, N: 0.005 to 0.02%, the balance being Fe and Inevitable impurities. The Ti, Al, and N further satisfy the following formula (1).

Al / (N-Ti / 3.4) ≧ 4 (1)
Hereinafter, the reason for limitation of each component is demonstrated.

C: 0.05-0.3%
C is an essential element for ensuring strength. Therefore, the C content is 0.05% or more, preferably 0.08% or more, more preferably 0.10% or more. On the other hand, when C is excessive, the ferrite fraction (area ratio) of the steel tends to decrease, and the deformability during cold forging tends to decrease. Accordingly, the C content is 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.

Si: 0.01 to 0.35%
Si is useful not only for deoxidation but also for securing a predetermined strength by solid solution strengthening. Accordingly, the Si content is 0.01% or more, preferably 0.5% or more, and more preferably 1.0% or more. On the other hand, when Si becomes excessive, cold forgeability will fall. Therefore, the Si content is 0.35% or less, preferably 0.30% or less, and more preferably 0.2% or less.

Mn: 0.2-2%
Mn is also useful for enhancing the hardenability of the steel and ensuring a predetermined strength. Therefore, the amount of Mn is 0.2% or more, preferably 0.3% or more, more preferably 0.4% or more. On the other hand, when Mn is excessive, the ferrite fraction of steel tends to decrease, and the deformability during cold forging tends to decrease. Therefore, the amount of Mn is 2% or less, preferably 1.7% or less, and more preferably 1.5% or less.

P: 0.001 to 0.02%
It is desirable to reduce P as much as possible to promote work hardening during cold forging and to reduce deformability. Therefore, the P content is 0.02% or less, preferably 0.015% or less, more preferably 0.012% or less. It is difficult or expensive to set P to 0%. Therefore, the P content is 0.001% or more, preferably 0.003% or more, more preferably 0.005% or more.

S: 0.001 to 0.02%
S forms sulfide inclusions and reduces the deformability during cold forging. Therefore, the P content is 0.02% or less, preferably 0.015% or less, more preferably 0.012% or less. Note that it is difficult or expensive to set S to 0%. Therefore, the S amount is 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more.

Hardenable elements (Ni: 0.01-2%, Cr: 0.01-2%, Mo: 0.01-0.5%)
These hardenable elements are useful for ensuring strength. Therefore, the Ni content is 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more. The Cr content is 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more. The amount of Mo is 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, when these hardenable elements are added excessively, the ferrite fraction becomes high and the deformability at the time of cold forging tends to decrease. Therefore, the Ni content is 2% or less, preferably 1.7% or less, and more preferably 1.5% or less. The Cr content is 2% or less, preferably 1.7% or less, and more preferably 1.5% or less. The amount of Mo is 0.5% or less, preferably 0.4% or less, and more preferably 0.3% or less.

  In addition, said Ni, Cr, and Mo may be added individually, respectively, and may be added in combination as appropriate.

Ti: 0.005-0.02%
Ti forms TiN during ingot forming or continuous casting. Since this TiN suppresses the austenite grains from coarsening due to the pinning action when the obtained steel is case-hardened, Ti is useful for enhancing the anti-coarse graining characteristics. Therefore, the Ti amount is set to 0.005 or more. In addition, it is good also as 0.01% or more, for example, 0.013% or more, especially 0.015% or more. In addition, since Al is also added in the present invention, AlN exhibits a pinning action when Ti is small, but AlN is coarsened when Ti is small, and its pinning effect (anti-granulation property) is insufficient. It has become. Therefore, in the present invention, it is important to add a predetermined amount or more of Ti. On the other hand, when Ti becomes excessive, TiN becomes coarse and the pinning action becomes insufficient. Further, in the present invention, as will be described later, although precipitation of fine TiC is prevented by controlled rolling, excess Ti tends to precipitate as fine TiC when Ti is excessive, and the tensile strength of steel is high. As a result, the deformation resistance during cold forging becomes high. More precisely, although fixing C with Ti is useful for improving cold forgeability in terms of preventing hardening due to strain aging caused by the Cottrell atmosphere, TiC precipitates very finely, so steel It is disadvantageous for improving the cold forgeability in that the tensile strength of the steel is excessively increased. When the amount of Ti is excessive, the influence of TiC precipitation is strong and the deformation resistance is increased. Therefore, the Ti amount is 0.02% or less. Preferably it is 0.018% or less, More preferably, it is 0.015% or less. Since TiN is not as fine as TiC, it does not reduce cold forgeability.

N: 0.005 to 0.02%
N forms TiN and AlN and is useful for enhancing the coarsening resistance by pinning. In particular, in the present invention, not Ti but N is added so as to be excessive (N-Ti / 3.4 is, for example, more than 0, preferably 0.002 or more, more preferably 0.003 or more). If N is added excessively, unlike the case where Ti is added excessively, the coarsening of TiN can be suppressed, and the pinning effect of TiN can be effectively exhibited.

Al: 0.020 to 0.1%
Al is useful not only for deoxidation, but also for forming AlN and enhancing the anti-granulation properties by pinning. In particular, when Ti is contained in a predetermined amount or more, since AlN is appropriately refined, the pinning action of AlN can be used effectively. Therefore, the Al content is 0.020% or more, preferably 0.023% or more, more preferably 0.025% or more. On the other hand, if Al is added too much, oxide inclusions increase and other characteristics (such as fatigue resistance) may be reduced, so the upper limit was made 0.1%. A preferred upper limit is 0.05%, especially 0.045%.

Al / (N-Ti / 3.4) ≧ 4 (1)
In the present invention, Al is 4 times or more, preferably 4.5 times or more, more preferably 4 times or more with respect to surplus N (= N-Ti / 3.4) which did not form TiN during ingot forming or continuous casting. Is added 5.0 times or more. In the steel of the present invention, since spheroidizing annealing is omitted, surplus N cannot be changed to AlN in the spheroidizing annealing step, and the steel production stage (at the time of ingot forming, at the time of continuous casting, at the time of block rolling, etc. It is necessary to ensure that the excess N is AlN. Accordingly, Al is added in an amount sufficient for the excess N. In addition, by ensuring that the surplus N is AlN, the pinning effect of AlN can be used, and the coarsening resistance can be improved. Further, if the surplus N is AlN, strain aging can be prevented and it is useful for securing cold forgeability.

  As is clear from the above formula (1), if N-Ti / 3.4 is negative, Al / (N-Ti / 3.4) cannot be 4 or more. It is assumed that Ti / 3.4 is positive (addition of N in excess of Ti).

The balance is Fe and inevitable impurities, that is, no precipitation strengthening elements such as Zr, Nb, W, and V are added in the present invention. When these precipitation strengthening elements are added, TiN cannot be sufficiently formed. Although Nb is known to be Nb (CN) and exert a pinning effect, Nb (CN) is extremely fine and increases deformation resistance during cold forging. Therefore, in the present invention, TiN is effectively used instead of Nb (CN).

  The steel of the present invention is cold forged without spheroidizing annealing. The steel for omitting the spheroidizing annealing is not only effective in reducing the cost by omitting the process, but also can prevent the AlN from coarsening during the spheroidizing annealing and effectively exhibit the pinning effect. It is also useful.

  The above steel balances strength and cold forgeability by controlling the components within an appropriate range. In addition, by using TiN effectively without using Zr, Nb, etc., the coarse grain resistance is enhanced while ensuring cold forgeability. In addition, the amount of N, not Ti, is increased, and a sufficient amount of Al is added to N that could not be combined with Ti. AlN can be precipitated, and the coarsening resistance can be reliably improved while ensuring cold forgeability.

The steel of the present invention usually has properties as shown in the following (1) to (4).
(1) Precipitates having an average particle diameter of 5 to 50 nm: 13 or more per 0.25 μm ² Precipitates having an average particle diameter of more than 50 nm: 3 or less per 0.25 μm ² That is, in the present invention, as described above, Since the balance of Ti, Al, and N is appropriately controlled and no precipitation strengthening element such as Nb is added, coarse precipitates with an average particle size exceeding 50 nm (nitriding of AlN, TiN, etc.) Etc.) and moderately sized precipitates (nitrides such as AlN and TiN) having an average particle size of about 5 to 50 nm are increasing. A moderately sized precipitate (such as nitrides such as AlN and TiN) is useful for achieving both cold forgeability and coarse grain resistance. In order to control the size and number of precipitates, controlled rolling described later is also effective.

The number of precipitates having an average particle diameter of 5 to 50 nm is, for example, 13 or more, preferably 15 or more, and more preferably 17 or more per 0.25 μm ² . The number of precipitates having an average particle size exceeding 50 nm is, for example, 3 or less, preferably 2 or less, more preferably 1 or less (including 0) per 0.25 μm ² .
(2) The ferrite area ratio is 50% or more The ferrite grain size number is 9.0 to 12.5 The larger the ferrite area ratio and the larger the ferrite grain size number, the lower the cooling rate when spheroidizing annealing is omitted. It is useful for securing the forgeability. It is also useful for increasing the drawing of steel (particularly wire).

  The ferrite area ratio is, for example, 50% or more, preferably 70% or more, and more preferably 85% or more. The crystal grain size number is, for example, about 9.0 to 12.5, preferably about 9.0 to 12.0, and more preferably about 9.5 to 11.0.

  The ferrite area ratio and the grain size number are average values when they are measured evenly over the entire cross section of the steel (for example, when 3 fields of view or more, preferably 5 fields of view or more, especially 10 fields of view or more are measured). Show.

The ferrite area ratio and the grain size number can be adjusted by controlling the chemical components within the above ranges and controlling the cooling conditions after controlled rolling as described below.
(3) Tensile strength: 400 to 750 N / mm ²
Drawing: 60% or more The tensile strength is preferably as high as possible from the viewpoint of securing the strength, but it is desirable as low as possible from the viewpoint of ensuring the cold forgeability. Therefore, it is desirable to control it within a predetermined range. Also, the higher the drawing, the better the cold forgeability is desirable.

Tensile strength, for example, 400 N / mm ² or more (preferably 420N / mm ² or more, more preferably 430N / mm ² or more), 750 N / mm ² or less (preferably 600N / mm ² or less, more preferably 500 N / mm ² or less). The aperture is, for example, 60% or more, preferably 65% or more, and more preferably 70% or more. The upper limit of the aperture is not particularly limited, but is usually about 90% or less (particularly 85% or less).

The tensile strength and the drawing can be adjusted by controlling the chemical component within the above range and controlling the ferrite area ratio and the crystal grain size number within the above range. If the following carbon equivalent Ceq and ideal critical diameter DI value are controlled, intensity control becomes easier.
(4) Carbon equivalent Ceq: 0.7 or less Ideal critical diameter DI value: 10 or more Carbon equivalent Ceq is defined by the following formula (2), and ideal critical diameter DI value is defined by the following formula (3). Is.

Ceq = [C] + [Si] / 7 + [Mn] / 5 + [Cr] / 9 + [Mo] / 2 + [Ni] / 6 (2)
(In the formula, [C], [Si], [Mn], [Cr], [Mo], [Ni] are the contents (mass%) of C, Si, Mn, Cr, Mo, or Ni, respectively. Indicate)

(In the formula, γGS means austenite grain size, and its value is 10. [C], [Si], [Mn], [Cr], [Mo], and [Ni] are C, Si, respectively. , Mn, Cr, Mo, or Ni content (% by mass))
Since Ceq is an index of strength and DI value is an index of hardenability, the strength and hardenability of steel can be easily controlled by adjusting these values within a predetermined range.

  Ceq is, for example, 0.7 or less, preferably 0.65 or less, and more preferably 0.6 or less. The lower limit of Ceq is usually about 0.3 (particularly about 0.4). The DI value is, for example, 10 or more, preferably 15 or more, and more preferably 20 or more. The upper limit of the DI value is usually about 60 (particularly about 45).

  The steel of the present invention is preferably produced as follows. That is, a steel ingot obtained by melting steel having C, Si, Mn, P, S, hardenable elements (Ni, Cr, Mo, etc.), Ti, Al, N, etc. adjusted to the above predetermined range, The slab or steel slab is hot-rolled (controlled rolling) under the following conditions and cooled.

Rolling start temperature: 800-950 ° C (preferably 820-900 ° C)
Rolling temperature (excluding finish rolling): 780 to 900 ° C. (preferably 790 to 850 ° C.)
Finish rolling temperature: 750-850 ° C. (preferably 775-825 ° C.)
Cooling start temperature: 750-850 ° C. (preferably 775-825 ° C.)
Cooling rate in a temperature range of 750 to 550 ° C. (final cooling): 0.05 to 0.3 ° C./second The above rolling temperature means the temperature of rolling other than finish rolling, for example, rough rolling and intermediate rolling. The rolling temperature and finish rolling temperature can be controlled by adopting intermediate water cooling, adjusting the rolling speed, and the like. After rolling, cooling (final cooling) may be started immediately, or cooling (final cooling) may be started after cooling to a cooling start temperature by water cooling or the like as necessary.

  The above conditions are characterized in that the rolling start temperature, rolling temperature, finish rolling temperature, etc. are kept low. By keeping these low, it is possible to suppress the precipitation of AlN, TiN that has already been precipitated, and newly precipitated AlN by reheating, thereby increasing the precipitation of TiC and the coarseness of AlN and TiN. Can be prevented. Moreover, since re-dissolution of C and N can be prevented, strain aging during cold forging can be suppressed. In addition, since the austenite grains during rolling can be made fine, it is also useful in terms of refining ferrite grains and securing strength.

  Another feature is that the cooling rate is set low. The ferrite fraction (area ratio) can be increased by slowing the cooling rate.

  In order to satisfy all of the above requirements (1) precipitate particle size, (2) ferrite area ratio / particle size number, and (3) tensile strength / drawing, it is desirable to manufacture under the above-mentioned manufacturing conditions. When the requirement is removed, the manufacturing conditions may be changed as appropriate. In addition, (4) when the carbon equivalent Ceq and the ideal critical diameter DI value are out of the above ranges, the production conditions may be appropriately changed.

  The steel of the present invention obtained as described above is excellent in cold forgeability even without spheroidizing annealing, and even if it is subsequently case-hardened (carburizing, carbonitriding, etc.), the grains become coarse Is suppressed. Therefore, it is useful for increasing the toughness, fatigue strength, dimensional accuracy, etc. of a part, although it can be processed into a part at low cost.

  The steel of the present invention is particularly useful for producing parts produced by cold forging and case hardening, for example, power transmission parts (pulleys, gears, shafts, etc.) used in machinery such as automobiles.

  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.

Experimental example 1
After melting the steel of the component shown in the following Table 1 and dividing it, a wire rod having a diameter of 17 mm was obtained by controlled rolling and cooling under the conditions shown in the following Table 2. In Table 2 below, no. 6 was further subjected to spheroidizing annealing (condition: heating at a temperature of 760 ° C. for 7 hours, cooling to a temperature of 680 ° C. at a cooling rate of 15 ° C./hr, and air cooling).

  The structure and precipitates of the obtained wire were measured as follows.

[Organization]
The wire was cut, the cross section was polished and mirror finished, then corroded with nital, and observed using an optical microscope (magnification: 100 times). The area ratio was measured with the region observed with white contrast as ferrite. The average value of three fields of view selected evenly from the cross section was defined as the area ratio (fraction) of ferrite.

  Further, the ferrite grain size number of the cross section was measured based on the ferrite crystal grain size test method described in JIS G 0552. The average value for three fields of view selected evenly from the cross-section was used as the ferrite grain size number.

[Precipitate]
After cutting and polishing the wire, a field of view of 0.5 μm × 0.5 μm (0.25 μm ² ) was photographed with a transmission electron microscope (TEM). The obtained photograph was subjected to image processing, and the number and size of precipitates such as TiN and AlN were determined. The size of the precipitate was determined as the diameter of a circle having the same area after measuring the area of the precipitate. The average value of three fields of view selected evenly from the cross section was taken as the size of the precipitate.

  The results are shown in Tables 1-2. No. 1 (steel A), No. 1 5 (Steel E), No. 5 7 (Steel F), No. 7 8 (steel G), and A TEM photograph of 9 (steel H) is shown in FIGS.

  Various characteristics of the wire obtained as described above were examined as follows.

[Tensile properties]
The tensile strength of the wire was measured according to JIS Z 2201. In addition, an aperture (RA) was calculated from the cross-sectional area after fracture.

[Cold forgeability]
The wire was drawn to a diameter of 16.3 mm and cut to a height of 24.5 mm to obtain a test piece.

  The test piece was compressed to a height of 9.8 mm (compression rate = 60%), and the deformation resistance was measured.

  The test piece was compressed to a height of 4.9 mm (compression rate = 80%). The presence or absence of cracks after compression was visually confirmed, and the case where cracks occurred was defined as low deformability and the case where cracks did not occur was defined as high deformability.

[Carburization characteristics]
The wire (diameter: 17.0 mm) was drawn to a diameter of 16.3 mm, and further three steps of extrusion processing were performed ahead. The obtained processed product has a diameter of 4.7 mm (reduction rate from the wire = 70%), a diameter of 6.0 mm (reduction rate from the wire = 50%), a diameter of 7.1 mm (a wire) in order from the tip side. The area reduction ratio is 30%), and when this is combined with the root (diameter 16.3 mm), it has a four-stepped shape.

  The processed product was carburized at a temperature of 900 ° C. for 3 hours and then cooled with water. The tip (4.7 mm in diameter) was cut and the cross section was mirror finished, then corroded with nital, and observed with an optical microscope (magnification: 3.5 times). When the coarsened portion of the crystal grain was confirmed as a pattern, it was regarded as coarse, and when it was not confirmed, it was regarded as no coarseness.

  The carburizing temperature was changed to 925 ° C., 950 ° C., 975 ° C., 1000 ° C., or 1025 ° C., and the same test as described above was performed, and the maximum temperature (GG suppression possible temperature) at which no coarsening of crystal grains was confirmed was obtained. .

  The results are shown in Table 3.

  No. 6 (steel E) corresponds to the conventional method, and corresponds to the case of hot rolling under normal conditions and then processing after spheroidizing annealing. This No. In No. 6, since the rolling conditions are inappropriate, the crystal grains after rolling are coarsened. No. In No. 5 (steel E), although the rolling temperature is controlled within an appropriate range to refine the crystal grains, Al alone alone has a poor pinning effect, so that the coarsening resistance is insufficient. No. No. 9 (steel H) and 11 (steel J) are also No. Similarly to No. 5 (steel E), Ti is insufficient, so TiN inclusions of 50 nm or less are insufficient, and the coarsening resistance is insufficient. No. No. 10 (steel I) has excessive Ti and precipitates fine TiC, which increases deformation resistance during cold forging. No. In 12 (steel K), although Ti is appropriate, Al is insufficient, so fine TiC or the like is precipitated, or tensile strength and deformation resistance increase, resulting in insufficient cold forgeability. . No. 8 (steel G) contains Zr, although the balance of Ti, Al, N, etc. is appropriately controlled, and therefore lacks TiN-based inclusions of 50 nm or less and is resistant to roughening. The granulation characteristics are insufficient. On the other hand, no. In No. 7 (steel F), Nb increases the coarse graining resistance, but Nb increases the tensile strength, deformation resistance, etc., and decreases the cold forgeability.

  On the other hand, no. 1-4 (steel A to D) are balanced in the proper range of Al, Ti, N, and do not contain Nb, Zr, etc., so they have cold forgeability and resistance to coarsening. Both are compatible.

FIG. 1 is a transmission electron micrograph of 1 (steel A) (magnification: 60000 times). FIG. 5 is a transmission electron micrograph of 5 (steel E) (magnification: 60000 times). FIG. 8 is a transmission electron micrograph of 8 (steel G) (magnification: 60000 times). FIG. 9 is a transmission electron micrograph (magnification: 60000 times) of 9 (steel H). FIG. 7 is a transmission electron micrograph (magnification: 60000 times) of 7 (steel F).

Claims (3)

C: 0.05 to 0.3% (meaning mass%; the same applies hereinafter), Si: 0.01 to 0.35%, Mn: 0.2 to 2%, P: 0.001 to 0.02% , S: 0.001 to 0.02 %, Cr: 0.01 to 2%, Ti: 0.005 to 0.02%, Al: 0.020 to 0.1%, N: 0 0.005 to 0.02%,
The balance is Fe and inevitable impurities,
The Ti, Al, and N further satisfy the following formula (1) :
13 or more precipitates having an average particle diameter of 5 to 50 nm are contained per 0.25 μm ² , and precipitates having an average particle diameter of more than 50 nm are 3 or less per 0.25 μm ² ,
When the ferrite area ratio and the ferrite grain size number were measured uniformly over the entire cross section, the average ferrite area ratio was 50% or more, and the average grain size number was 9.0 to 12.5. Yes,
The tensile strength is 400 to 750 N / mm ² , the aperture is 60% or more,
The carbon equivalent Ceq defined by the following formula (2) is 0.7 or less,
The ideal critical diameter DI value defined by the following formula (3) is 10 or more, and the steel for spheroidizing and omitting spheroidizing annealing, which has excellent cold forgeability and anti-roughening properties during case hardening treatment .
Al / (N-Ti / 3.4) ≧ 4 (1)
Ceq = [C] + [Si] / 7 + [Mn] / 5 + [Cr] / 9 + [Mo] / 2 + [Ni] / 6 (2)
(In the formula, [C], [Si], [Mn], [Cr], [Mo], [Ni] are the contents (mass%) of C, Si, Mn, Cr, Mo, or Ni, respectively. Indicate)
(In the formula, γGS means austenite grain size, and its value is 10. [C], [Si], [Mn], [Cr], [Mo], and [Ni] are C, Si, respectively. , Mn, Cr, Mo, or Ni content (% by mass)) Furthermore, the steel for spheroidization-annealing skin hardening of Claim 1 containing Ni: 0.01-2% and / or Mo: 0.01-0.5%. The steel ingot, cast slab or steel slab having the chemical composition shown in claim 1 or 2 is hot-rolled and cooled under the following conditions, and is cold forgeable and resistant to coarsening during carburizing. A method for producing case-hardening steel with excellent properties.
Rolling start temperature: 800-950 ° C
Rolling temperature (excluding finish rolling): 780-900 ° C
Finish rolling temperature: 750-850 ° C
Cooling start temperature: 750-800 ° C
Cooling rate in the temperature range of 750 to 550 ° C .: 0.05 to 0.3 ° C./second