JP2007031787A - Case-hardened steel having superior grain coarsening resistance, fatigue characteristic and machinability, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a case-hardened steel which has all of superior grain coarsening resistance, fatigue characteristics and excellent machinability, and to provide a preferable manufacturing method therefor. <P>SOLUTION: The case-hardened steel comprises, by mass%, 0.10-0.35% C, 0.03-1.0% Si, 0.20-2.0% Mn, 0.030% or less P, 0.010-0.10% S, 0.2% or less Al, 0.03-0.30% Ti, 0.020% or less N, and the balance Fe with unavoidable impurities; has Ti-based carbonitrides finely dispersed in the steel; and contains Ti-based sulfides with an average diameter of 1.0 to 5.0 μm existing at a density of 10 pieces/mm<SP>2</SP>or more, and Ti-based sulfides with an average diameter of larger than 5.0 μm existing at a density of 10 pieces/mm<SP>2</SP>or less. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a case-hardened steel excellent in crystal grain coarsening characteristics and fatigue characteristics, and also having excellent machinability, and a method for producing the same.

Some mechanical structural components, such as gears, harden the surface of the component, and case-hardened steel is used as a material for such components. The case-hardened steel is formed into an intermediate part that approximates the target shape by hot forging, cold forging, and the like, and is further cut to be manufactured into a target-shaped part. This part is subjected to a surface hardening process such as a carburizing process or a carbonitriding process, and is finished as necessary to be a finished product.
In this way, the case hardening steel is subjected to surface hardening treatment such as carburizing and carbonitriding in the part manufacturing process, but these treatments involve heating at high temperatures, resulting in coarsening of crystal grains, fatigue characteristics, etc. There is a tendency for the mechanical properties of to decrease.

  In order to improve the grain coarsening of such case-hardened steel, Japanese Patent Application Laid-Open No. 10-813838 (Patent Document 1) and Japanese Patent Application Laid-Open No. 10-130720 (Patent Document 2) describe a skin to which Ti is added. Hardened boron steel has been proposed. By adding more than 0.1% to 0.2% Ti, this steel fixes N dissolved in the steel, and finely precipitates Ti carbide, Ti-containing composite carbide, and Ti nitride. By doing so, coarsening of austenite crystal grains during carburization is suppressed. Furthermore, in Japanese Patent Laid-Open No. 11-92824 (Patent Document 3) and Japanese Patent Laid-Open No. 11-92863 (Patent Document 4), the Ti carbide is finely dispersed to suppress the structural change during rolling fatigue, Case-hardened steel with improved rolling fatigue life has also been proposed.

  On the other hand, as described above, the case-hardened steel is formed into a shape approximate to a part and then subjected to various cutting processes, so that machinability is also required. JP-A-10-152756 (Patent Document 5) and JP-A-11-236646 (Patent Document 6) disclose case-hardened steel with improved machinability while suppressing the coarsening of austenite grains due to high-temperature treatment. Discloses a case-hardened steel in which TiS, NdS, Ti carbosulfide, and Zr carbosulfide are dispersed in the steel. JP 2003-34843 A (Patent Document 7) discloses an element such as S, Pb, Bi, etc. in case-hardened steel that is re-quenched and ultra-finely refined prior austenite grains after carburizing. It is disclosed that an appropriate amount is added to improve machinability.

Japanese Patent Laid-Open No. 10-81938 JP-A-10-130720 JP-A-11-92824 JP-A-11-92863 Japanese Patent Laid-Open No. 10-152752 Japanese Patent Laid-Open No. 11-236646 JP 2003-34843 A

Although the case-hardened steels of Patent Documents 1 to 4 have improved fatigue characteristics and rolling fatigue characteristics, it cannot be said that they have sufficient machinability, while those of Patent Documents 5 to 7 are machined. However, the crystal grain coarsening characteristics are insufficient, and sufficient fatigue characteristics and rolling fatigue characteristics are not obtained.
The present invention has been made in view of such problems, and an object thereof is to provide a case-hardened steel excellent in crystal grain coarsening characteristics and fatigue characteristics, and also having excellent machinability, and a suitable manufacturing method thereof. To do.

The present inventor has focused on the particle size distribution of Ti-based sulfides in Ti-added case-hardened steel to which S is added in an appropriate amount, and as a result of earnest research on this, as a result of excessive Ti-based sulfides that deteriorate fatigue characteristics, By controlling the number of Ti-based sulfides with a size that improves machinability without degrading fatigue properties and optimizing the particle size distribution of Ti-based sulfides in steel, The inventors have found that fatigue characteristics and machinability are improved without impairing the grain coarsening characteristics, and have completed the present invention.
That is, the case-hardened steel of the present invention has a chemical composition of mass% (hereinafter simply referred to as “%”), C: 0.10 to 0.35%, Si: 0.03 to 1. 0%, Mn: 0.20 to 2.0%, P: 0.030% or less, S: 0.010 to 0.10%, Al: 0.2% or less, Ti: 0.03 to 0.30 %, N: 0.020% or less, the balance being Fe and inevitable impurities, Ti carbonitride is finely dispersed in the steel, and the average diameter is 1.0 to 5.0 μm Ti sulfide Is 10 pieces / mm ² or more, and Ti-based sulfides having an average diameter exceeding 5.0 μm contain 10 pieces / mm ² or less.

The case-hardened steel may further contain one or more elements from the following element groups (1) to (3) in addition to the chemical components.
(1) Cu: 3% or less, Ni: 3% or less, Cr: 2% or less, Mo: 2% or less, B: 0.005% or less
(2) Nb: 0.2% or less, V: 0.3% or less, Zr: 0.3% or less
(3) REM (rare earth element): 0.03% or less, Ca: 0.03% or less, Mg: 0.03% or less, Pb: 0.3% or less, Bi: 0.3% or less, Te: 0 .3% or less, Se: 0.3% or less, Sn: 0.3% or less

In addition, the method for producing the case-hardened steel according to the present invention was obtained by melting steel having the above chemical components, subjecting the slab to soaking treatment, and then performing rough plastic working with a processing rate of 30% or more. After re-homogenizing heat treatment for holding the steel slab at 1250 ° C. or higher, finish plastic working is performed at an end temperature of 700 to 1000 ° C. The processing rate means a ratio (percentage) represented by the following formula, where A is the cross-sectional area after processing, and A ₀ is the cross-sectional area before processing.
Processing rate (%) = (1−A / A ₀ ) × 100

According to the case hardening steel of the present invention, S is 0.010 to 0.10%, Ti is 0.03 to 0.30%, C is 0.10 to 0.35%, and N is 0.020% or less. Since the Ti-based carbonitride has a specific chemical component and is finely dispersed in the steel, it has excellent crystal grain coarsening characteristics, and the particle size distribution of the Ti-based sulfide has an average particle size of 1. the average particle size with 0~5.0μm of ones and 10 / mm ² or more was those 1.0 to 5.0 m 10 pieces / mm ² or more, the machinability and excellent fatigue properties Prepare. Moreover, according to the manufacturing method of this invention, the said case hardening steel can be manufactured easily.

  The chemical components of the case hardening steel of the present invention are: C: 0.10 to 0.35%, Si: 0.03 to 1.0%, Mn: 0.20 to 2.0%, P: 0.030% Hereinafter, S: 0.010 to 0.10%, Al: 0.2% or less, Ti: 0.03 to 0.30%, N: 0.020% or less are contained, and the balance is Fe and inevitable impurities. . Hereinafter, the reason for component limitation will be described.

C: 0.10 to 0.35%
C is an element necessary for ensuring the strength of the core after carburizing treatment of case-hardened steel. If it is less than 0.10%, sufficient strength cannot be obtained, and if it exceeds 0.35%, the toughness of the core decreases. In addition, cold workability is reduced. For this reason, the lower limit of the C amount is 0.10%, and the upper limit is 0.35%.

Si: 0.03-1.0%
Si is necessary as a deoxidizing element, and if it is less than 0.03%, a sufficient deoxidation effect cannot be obtained. If it exceeds 1.0%, deformation resistance during cold working increases and grain boundary oxidation during carburizing occurs. Promotes layer formation and reduces fatigue properties. For this reason, the lower limit of the Si amount is 0.03%, and the upper limit is 1.0%.

Mn: 0.20 to 2.0%
Mn is an element necessary for ensuring hardenability. Therefore, addition of 0.20% or more is necessary, but excessive addition exceeding 2.0% increases deformation resistance during cold working. At the same time, it promotes the formation of a grain boundary oxide layer during carburizing and reduces fatigue properties. For this reason, the lower limit of the amount of Mn is 0.20%, and the upper limit is 2.0%.

P: 0.030% or less P is segregated at the grain boundary to cause a decrease in grain boundary strength.

S: 0.010 to 0.10%
S is an important element for improving the machinability in the present invention, and improves the machinability by forming a Ti-based sulfide or MnS. If the amount of S is less than 0.010%, the effect is too small. On the other hand, if it exceeds 0.10%, the cold workability deteriorates. For this reason, the lower limit of the amount of S is 0.010%, preferably 0. 0.015%, and the upper limit is 0.10%, preferably 0.05%.

Al: 0.2% or less Al is necessary as a deoxidizing element. However, if it exceeds 0.2%, alumina-based oxides increase and fatigue characteristics and cold workability are deteriorated. Stop below 2%.

Ti: 0.03-0.30%
Ti is an element necessary for fixing solid solution N and suppressing the coarsening of austenite crystal grains during carburization by finely depositing Ti carbide, composite carbide containing Ti, and Ti nitride. At the same time, it is an element necessary for precipitating Ti sulfide and improving machinability. If it is less than 0.03%, such an action is insufficient. On the other hand, if it exceeds 0.30%, the amount of precipitates becomes excessive and cold workability deteriorates. For this reason, the lower limit of the Ti amount is 0.03%, preferably 0.05%, and the upper limit is 0.30%, preferably 0.28%.

N: 0.020% or less N combines with Ti to form TiN, and reduces fatigue strength. If added over 0.020%, the amount of TiN increases and the effect becomes significant. For this reason, the upper limit of the N amount is 0.020%, preferably 0.010%, and more preferably 0.008%.

The case-hardened steel of the present invention is typically formed with the balance of Fe and unavoidable impurities in addition to the basic components described above, but instead of part of Fe, any one of the following groups (1) to (3) One or two or more selected elements can be added alone or in combination. As a result, the mechanical properties of the case hardening steel can be further improved. Hereinafter, the reasons for limiting these characteristic improving elements will be described.
(1) Cu: 3% or less, Ni: 3% or less, Cr: 2% or less, Mo: 2% or less, B: 0.005% or less
(2) Nb: 0.2% or less, V: 0.3% or less, Zr: 0.3% or less
(3) REM: 0.03% or less, Ca: 0.03% or less, Mg: 0.03% or less, Pb: 0.3% or less, Bi: 0.3% or less, Te: 0.3% or less , Se: 0.3% or less, Sn: 0.3% or less

(1) Group Cu: 3% or less Cu is an element that improves hardenability and corrosion resistance. For this reason, it is preferable to add 0.2% or more, but if the addition exceeds 3%, the effect is saturated, so the upper limit of the amount of Cu is made 3%. If Cu is contained alone, the hot workability of the steel tends to deteriorate. Therefore, in order to avoid this adverse effect, it is desirable to use Ni having an effect of improving the hot workability.

Ni: 3% or less Ni is an element that improves hardenability and toughness, and is preferably added in an amount of 0.2% or more, but if added over 3%, the effect is saturated, so the amount of Ni Is 3%.

Cr: 2% or less Cr has an effect of improving hardenability and improving carburization. For this reason, it is preferable to add 0.2% or more, but if added over 2%, the toughness decreases, so the upper limit of the Cr content is made 3%.

Mo: 2% or less Mo is an element that improves hardenability and improves toughness. For this reason, it is preferable to add 0.05% or more, but if added over 2%, the workability is lowered and the toughness is also lowered, so the upper limit of the Mo amount is made 2%.

B: 0.005% or less B is an element that improves hardenability by adding a small amount. For this reason, it is preferable to add 0.0003% or more, but if added over 0.005%, the hardenability is lowered, so the upper limit of the amount of B is made 3%.

(2) Group Nb, V, and Zr form carbides and nitrides and have an effect of preventing coarsening of austenite crystal grains. For this reason, it is preferable to add 0.005% or more for each, but if Nb: 0.2%, V: 0.3%, Zr: 0.3% is added, the amount of precipitates becomes excessive, and the processing Therefore, the upper limit of the amount of Nb is set to 2%, and the upper limit of V or Zr is set to 0.3%.

(3) Group REM, Ca, Mg, Pb, Bi, Te, Se, and Sn are elements that improve machinability. For this reason, it is preferable to add 0.005% or more of each, but REM: 0.03%, Ca: 0.03%, Mg: 0.03%, Pb: 0.3%, Bi: 0.3% , Te: 0.3%, Se: 0.3%, Sn: If added over 0.3%, the toughness decreases, so the upper limit of the amount of REM, Ca or Mg added is 0.03%, Pb , Bi, Te, Se, SnV or Sn is added to 0.3%.

The case-hardened steel of the present invention has the above chemical components, Ti-based carbonitride is finely dispersed in the steel, and Ti-based sulfides having an average diameter of 1.0 to 5.0 μm are 10 pieces / mm ² or more. The Ti-based sulfide having an average diameter exceeding 5.0 μm contains 10 pieces / mm ² or less.
As a result of investigating the size of the Ti-based sulfide that works effectively on the machinability by the present inventor, the size of the Ti-based sulfide that works effectively on the machinability is 1.0 μm or more in average diameter. Was discovered. On the other hand, it was found that Ti-based sulfides having an average diameter exceeding 5.0 μm adversely affect fatigue characteristics. Therefore, in the present invention, the number of Ti-based sulfides having an average diameter of 1.0 to 5.0 μm is defined from the viewpoint of ensuring machinability. If the number of Ti-based sulfides is less than 10 pieces / mm ^2, it is not effective for improving machinability, so the number is 10 pieces / mm ² or more, preferably 15 pieces / mm ² or more. Although the upper limit is not particularly defined, it is practically about 300 / mm ² at the most. On the other hand, if the number of Ti-based sulfides having an average diameter exceeding 5.0 μm exceeds 10 pieces / mm ² , the fatigue characteristics deteriorate significantly, so the upper limit is 10 pieces / mm ² , preferably 5 pieces / mm ^2. And

Next, the suitable manufacturing method of the case hardening steel of this invention is demonstrated.
The case-hardened steel of the present invention is obtained by melting steel having the above chemical components, and applying a soaking treatment (homogenization treatment) A that holds the cast slab at about 1250 to 1400 ° C. as shown in FIG. After completely dissolving the carbide, after performing the rough plastic processing B with a processing rate of 30% or more, after performing the re-uniform heat treatment C for holding the obtained steel piece at 1250 ° C. or higher, the end temperature is set to 700 to Manufactured by applying finish plastic processing D of 1000 ° C. and cooling.

As the rough plastic working, hot working with good workability, for example, hot forging or hot rolling is usually applied. By this rough plastic working, the Ti-based sulfide is stretched and divided to prepare a predetermined size. When the processing rate of the rough plastic working is less than 30%, the Ti-based sulfide is insufficiently expanded and divided, and 10 Ti / sulfide having an average diameter of 1.0 to 5.0 μm is 10 pieces / mm. ^You won't get more than ² . For this reason, a rough plastic working with a processing rate of 30% or more, preferably 40% or more, is performed before reheating.

After the rough plastic working, it is necessary to reheat the steel slab at a temperature of 1250 ° C. or higher.
The reason is to first further divide the Ti-based sulfide. In SaiHitoshinetsu temperature is lower than 1250 ° C., be subjected to plastic working at the working ratio, the average diameter is not obtained Ti-based sulfides 1.0~5.0μm has 10 / mm ² or more, the average diameter This is because Ti sulfide exceeding 5.0 μm exceeds 10 pieces / mm ² . The second reason is that coarse Ti-based carbonitrides present in steel are dissolved in austenite and Ti-based carbonitrides are uniformly and finely precipitated in the subsequent steps. By finely dispersing Ti-based carbonitride in steel, it is possible to prevent coarsening of austenite crystal grains during carburizing treatment. Furthermore, by performing re-soaking at 1250 ° C. or higher, segregation can be reduced, and cold workability can be improved in combination with the reduction of coarse Ti-based carbonitrides. The reheating temperature should be 1250 ° C. or higher, and the higher it is, the higher it is up to 1500 ° C., but in practice it will be limited to about 1350 ° C. due to restrictions on equipment. In addition, the holding time during re-uniform heating is 10 min or more and less than 20 hr in order to promote the division of Ti sulfide while suppressing the number of coarse Ti carbonitrides exceeding 5.0 μm. preferable.

After reheating, once air-cooled to room temperature, heated to about 800-1350 ° C to secure the finishing temperature while suppressing the number of coarse Ti-based carbonitrides exceeding 5.0 µm. Alternatively, after reheating, the temperature is continuously cooled to the above temperature range, and finish plastic working (finish rolling or finish forging) is performed, followed by air cooling. The finishing temperature of finish plastic working is not particularly defined, but is preferably about 700 to 1200 ° C. This is because if the temperature is lower than 700 ° C., the deformation resistance becomes excessive, and if it exceeds 1200 ° C., the hardenability is excessively improved and cracks are likely to occur in the processed material.
The steel produced as described above has a structure in which Ti-based sulfides are dispersed in the steel with the particle size distribution and Ti-carbonitrides are finely dispersed.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not interpreted limitedly by this Example.

  Steel of chemical composition shown in Table 1 is melted in vacuum, 150 kg steel ingot is subjected to soaking treatment for 1300 ° C. and 3 hours, and then 155 mm square steel slab by hot forging at the processing rates shown in Table 2 and Table 3. Got. Subsequently, re-homogenization heat treatment was performed for 3 hours at the temperature shown in the same table, and air cooling was performed to room temperature. Thereafter, the reheated steel slab was heated to 1000 ° C., the finish rolling finish temperature was set to 900 ° C., and the hot rolling was finished to obtain a steel bar having a diameter of 65 mm.

Using the steel bar, the number of Ti-based sulfide inclusions (hereinafter simply referred to as “inclusions”) in the steel is measured per 1 mm ^{2 in the following} manner, and the crystal coarsening resistance and machining are also measured. The properties and fatigue properties were investigated. The results are also shown in Tables 2 and 3.

The number of inclusions in the steel is determined by detecting the image detected by the reflection detector of EPMA (JCMA-733 manufactured by JEOL Ltd.) using an image analysis device (NORAN VOYAGER) according to the following criteria. The size was measured, and the density (number per 1 mm ² ) of Ti-based sulfides having an equivalent circle diameter of 1 to 5 μm and an equivalent circle diameter of more than 5 μm was measured. The measurement area was 100 mm ² . To determine inclusions, first, chemical analysis of Al, Si, Mn, Ti, S is performed with the EDS apparatus attached to the apparatus, and Ti-based sulfides are determined as Ti> Al, Ti> Si, Ti from component analysis results. > Mn and S / (Al + Si + Mn + Ti)> 5 (element symbol indicates the concentration) were used as inclusions.

  The crystal coarsening resistance was obtained by cold forging the steel bar at a processing rate of 70%, holding at 1000 ° C. for 3 hours, quenching, examining the austenite crystal grains when heated at the temperature, and the grain size of 5 or less The area ratio of coarse grains was examined, and the area ratio of coarse grains passed 5% or less, and more than 5% was evaluated as failed. The heat treatment simulates a carburizing treatment.

  The machinability was evaluated based on the tool life by performing a turning test on a test piece obtained by holding the steel bar at 925 ° C. for 1 hour and then air cooling, using a carbide tool (P10). The said heat processing assumes the annealing process normally performed after hot forging. Turning conditions were a cutting speed of 200 m / min, a cutting amount of 1.5 mm, a feed amount of 0.25 mm / rev, and no cutting oil. The time when the flank wear amount (VB) of the cemented carbide tool was 0.2 mm was evaluated as pass when the time was 20 min or more, and rejected when the time was less than 20 min.

Fatigue characteristics were obtained by taking a disk-shaped test piece having a diameter of 60 mm and a thickness of 5 mm from the steel bar, holding the test piece at 930 ° C. for 6 hours, carburizing and quenching, tempering, and then lapping the test piece. The surface roughness (Ra) was 40 μm or less, a rolling fatigue test was performed under the following conditions, and the rolling fatigue life was evaluated. The test conditions were a surface pressure maximum hertz stress of 5.16 GPa, a rotation speed of 1800 rpm, a steel ball number of 6, turbine oil # 68 of lubricating oil Nippon Oil Co., Ltd., and a test number of 10. Transfer fatigue life (average) of 5 × 10 ⁶ times or more was regarded as acceptable as the life (L10 life) at which the failure probability was 10% or less.

From Tables 2 and 3, Sample Nos. 30 to 37 are comparative examples in which the chemical composition of the steel is outside the specified range of the present invention, and the manufacturing conditions are appropriate, but the size distribution of the Ti-based sulfide is within the range of the present invention. The other sample Nos. 32, 33, 35 and 36 do not have both machinability and rolling fatigue characteristics, and the sample Nos. 30, 31, 34 and 37 whose size distribution is within the scope of the present invention are also rolling. The fatigue characteristics are degraded. In Sample Nos. 30, 31, 33, and 36, the cold forgeability was not good as compared with the cold forging performed when evaluating the coarsening resistance characteristics.
In addition, even when the chemical composition is within the scope of the invention, Sample Nos. 3 to 5 and 38 with inadequate production conditions also have a Ti-based sulfide size distribution that is outside the scope of the present invention. It has no dynamic fatigue characteristics.
On the other hand, in the inventive examples of Sample Nos. 1, 2, 6 to 29 with appropriate chemical components and manufacturing conditions, the size distribution of the Ti-based sulfide satisfies the scope of the present invention, and the crystal coarsening resistance, A case-hardened steel having both machinability and rolling fatigue characteristics was obtained.

It is a thermomechanical diagram which shows the manufacturing process of the case hardening steel of this invention.

Claims (5)

mass%
C: 0.10 to 0.35%,
Si: 0.03-1.0%,
Mn: 0.20 to 2.0%,
P: 0.030% or less,
S: 0.010 to 0.10%,
Al: 0.2% or less,
Ti: 0.03 to 0.30%,
N: 0.020% or less, consisting of the balance Fe and inevitable impurities, Ti-based carbonitride is finely dispersed in steel, and Ti-based sulfide having an average diameter of 1.0 to 5.0 μm is 10 pieces / mm ² or more, Ti-based sulfide average diameter exceeds 5.0μm contains 10 / mm ² or less, resistance to grain growth characteristics, fatigue characteristics and machinability with excellent hardening steel.   The skin according to claim 1, further comprising one or more of Cu: 3% or less, Ni: 3% or less, Cr: 2% or less, Mo: 2% or less, and B: 0.005% or less. Burnt steel.   The case hardening steel according to claim 1 or 2, further comprising one or more of Nb: 0.2% or less, V: 0.3% or less, and Zr: 0.3% or less.   Furthermore, REM: 0.03% or less, Ca: 0.03% or less, Mg: 0.03% or less, Pb: 0.3% or less, Bi: 0.3% or less, Te: 0.3% or less, Se The case hardening steel according to any one of claims 1 to 3, containing one or more of 0.3% or less and Sn of 0.3% or less.   The steel according to any one of claims 1 to 4 is melted and subjected to soaking treatment on the slab, and then subjected to rough plastic working with a processing rate of 30% or more. A method for producing a case-hardening steel excellent in crystal grain coarsening properties, fatigue properties, and machinability, which is subjected to re-homogenization heat treatment that is maintained at or above C, and then subjected to finish plastic working and cooling.

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