JP2000319748A - High strength and long life steel for induction hardening excellent in delayed fracture resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce steel for induction hardening having high strength and excellent in delayed fracture resistance and rolling fatigue life characteristics, and to provide a method for producing the same. SOLUTION: This steel for induction hardening having high strength and excellent in delayed fracture resistance and rolling fatigue life characteristics contains, by weight, 0.30 to 0.80% C, 0.05 to 0.50% Si, 0.2 to 2.0% Mn, 0.05 to 0.20% Ti, 0.010 to 0.050% Al, <=0.0120% N and <=12 ppm O, and the balance Fe with inevitable impurities, or moreover contains one or >= two kinds selected from 0.1 to 2.0% Ni, 0.20 to 2.0% Cr and 0.05 to 1.0% Mo to the above steel components, and Ti carbides and Ti carbonitrides of <=70 nm dimensions are finely dispersed into the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shaft or a constant velocity joint part which is used after being surface-hardened by induction hardening and tempering and which is subjected to rolling fatigue from rolling elements such as gears, balls, rollers, etc. It has excellent resistance to delayed fracture caused by hydrogen in the original steel material and hydrogen entering from the environment during heat treatment or use, etc., for mechanical parts such as linear motion shafts, The present invention relates to a high-strength and long-life steel for induction hardening and a method for producing the steel.

[0002]

2. Description of the Related Art Induction quenching is widely used to heat and harden a surface of a steel part by induction heating and then harden the steel part to improve fatigue strength and wear resistance. Since a higher quenching hardness is obtained as the carbon content of the material increases, a medium / high carbon steel having a carbon content of 0.30% or more is generally used for induction hardening. Among high-frequency hardened parts, mechanical parts such as shafts, constant velocity joint parts, and linear drive shafts that receive rolling fatigue from rolling elements such as gears, balls, rollers, etc. Rolling fatigue life characteristics that withstand repeated high surface pressure loads are required.

It is widely known that delayed fracture is generated from hydrogen invading from an external use environment or hydrogen existing in steel material in the presence of tensile stress, such as a structure such as a bolt or a bridge. In general, if the strength exceeds 1200 MPa (38 HRC in terms of hardness), the delayed fracture strength sharply decreases. Therefore, in these fields, from the material side, the hardness of the material, that is, the adjustment of the strength level, the regulation of alloy elements and impurities, and the like. In many cases, steel with excellent resistance to delayed fracture is being developed for bolts and steel bars.

[0004] In the case of induction hardened parts, delayed fracture may be a problem in parts having a threaded portion. However, shafts or the like which are subject to rolling fatigue from rolling elements such as gears, balls, rollers, etc., due to rolling or sliding. For mechanical parts such as speed joint parts and linear drive shafts, delayed fracture has rarely been a problem.

[0005]

In the case of induction hardening,
Infiltration of hydrogen from the atmosphere during the heat treatment was hardly a problem, and has not been discussed so far.
The inventors have found from many studies that even in the case of the induction hardened material, not only the strength but also the rolling fatigue life is significantly reduced in an environment where hydrogen enters. The present invention has led to the development of a material that is highly resistant to this.

The problem to be solved by the present invention is that such a shaft, a constant velocity joint component, a linear motion shaft, etc., which are subject to rolling fatigue from rolling elements such as gears, balls, rollers, etc. Providing low-cost, high-strength induction hardening steel with long life and excellent delayed fracture resistance suitable for machine parts, etc. by optimizing chemical components and developing manufacturing methods that enable it It is assumed that.

[0007]

Means for Solving the Problems The inventors of the present invention have studied the above problems, and have found that hydrogen in steel, which causes delayed fracture, is trapped by fine Ti-based precipitates.
It has been found that the resistance to delayed fracture is increased by disabling and the life is prolonged. Further, in that case, it has been found that the smaller the particle diameter of the precipitate, the stronger the force for trapping hydrogen, and it is effective in improving the characteristics to achieve uniform and fine dispersion as much as possible. However, it has been found that merely increasing the amount of addition only coarsens the precipitate, does not increase the amount of fine precipitate effective as a trap site, and weakens the hydrogen trapping power as a whole. In addition, coarsened precipitates become a source of stress concentration and consequently reduce fatigue strength and life characteristics, and it is important to optimize the amount of addition, and to secure finer precipitates. Found that it was necessary to optimize the steel production conditions.

That is, according to the first aspect of the present invention, there is provided a method of the present invention, wherein C: 0.30 to 0.3% by weight.
0.80%, Si: 0.05 to 0.50%, Mn: 0.
2 to 2.0%, Ti: 0.05 to 0.20%, Al:
0.010 to 0.050%, N: 0.0120% or less,
O: high-strength and delayed fracture resistance characterized by containing 12 ppm or less, the balance being Fe and unavoidable impurities, and finely dispersing Ti carbide and Ti carbonitride having a size of 70 nm or less in steel. Induction hardened steel with excellent rolling fatigue life characteristics.

According to the second aspect of the present invention, C: 0.
30 to 0.80%, Si: 0.05 to 0.50%, M
n: 0.2 to 2.0%, Ti: 0.05 to 0.20%,
Al: 0.010 to 0.050%, N: 0.0120%
Hereinafter, O: contains 12 ppm or less, and further contains Ni: 0.1 ppm.
1 to 2.0%, Cr: 0.20 to 2.0%, Mo: 0.
One or two or more selected from the group consisting of 0.05 to 1.0%, the balance being Fe and unavoidable impurities.
The steel for induction hardening has a high strength and is excellent in delayed fracture resistance and rolling fatigue life characteristics, characterized by finely dispersing Ti carbide and Ti carbonitride of not more than m in steel.

[0010] According to the third aspect of the present invention, C: 0.
30 to 0.80%, Si: 0.05 to 0.50%, M
n: 0.2 to 2.0%, Ti: 0.05 to 0.20%,
Al: 0.010 to 0.050%, N: 0.0120%
Hereinafter, a steel material containing O: 12 ppm or less, the balance being Fe and unavoidable impurities is heated and rolled to a temperature range of 1200 to 1350 ° C.
By rolling or forging a predetermined steel material or part at a temperature of 50 ° C., Ti carbide and Ti carbonitride having a size of 70 nm or less are finely dispersed in steel in a subsequent heat treatment step. It is a method for producing induction hardening steel having excellent delayed fracture resistance and rolling fatigue life characteristics.

According to the fourth aspect of the present invention, C: 0.
30 to 0.80%, Si: 0.05 to 0.50%, M
n: 0.2 to 2.0%, Ti: 0.05 to 0.20%,
Al: 0.010 to 0.050%, N: 0.0120%
Hereinafter, O: contains 12 ppm or less, and further contains Ni:
0.1-2.0%, Cr: 0.20-2.0%, Mo:
A steel material containing one or more kinds selected from 0.05 to 1.0%, the balance being Fe and inevitable impurities is 120
By heating and rolling to a temperature range of 0 to 1350 ° C., and further rolling or forging a predetermined steel material or part at the same temperature to a temperature of 800 to 1050 ° C., a Ti carbide having a size of 70 nm or less in a subsequent heat treatment step; This is a method for producing a steel for induction hardening, which is characterized by finely dispersing Ti carbonitride in steel, and which is high in strength and excellent in delayed fracture resistance and rolling fatigue life characteristics.

The reasons for limitation in practicing the present invention will be described below. First, the reasons for limiting the chemical components will be described.

C: 0.30 to 0.80% C is an essential element for securing hardenability and ensuring the strength of parts, but if less than 0.30%, induction hardenability is insufficient. Since sufficient surface hardness cannot be obtained and good rolling fatigue life characteristics cannot be obtained, the lower limit is set to 0.30%. Also,
If the content exceeds 0.80%, the quenching hardness is saturated, and the susceptibility to quenching cracking increases, and under normal rolling or forging heating conditions, it becomes difficult to obtain the following effective fine Ti carbides. The upper limit is set to 0.80%.

Si: 0.05 to 0.50% Si is added for deoxidizing and improving hardenability.
If it is less than 05%, the deoxidizing effect is not sufficient, and if it exceeds 0.50%, the processability decreases. Therefore, the lower limit is set to 0.05% and the upper limit is set to 0.50%.

Mn: 0.2 to 2.0% Mn is also added to improve the deoxidizing property and hardenability similarly to Si, but if it is less than 0.20%, sufficient hardenability during induction hardening is obtained. When the content exceeds 2.0%, the workability is remarkably reduced and the susceptibility to quenching cracking is increased.
0% and the upper limit is 2.0%.

Ti: 0.05 to 0.20% Ti is finely dispersed in steel as Ti carbide or Ti carbonitride to reduce fatigue characteristics, life characteristics, and crystal grain size, and to reduce hydrogen trap sites. It is added to act as. If it is less than 0.05%, the effect is not sufficient, and if it exceeds 0.20%, Ti carbide or T
Since i carbonitride is coarsened and does not show a sufficient effect, the lower limit is set to 0.05% and the upper limit is set to 0.20%.

Al: 0.005 to 0.050% Al is added as a deoxidizing agent. If it is less than 0.005%, its effect is not obtained. If it exceeds 0.050%, alumina-based oxide increases. In order to reduce fatigue strength and workability, the lower limit is set to 0.005% and the upper limit is set to 0.050%.

N: 0.0120% or less In the case of ordinary case hardening steel, N is positively added to obtain the effect of reducing the grain size by AlN, but in the steel of the present invention, conversely, as the N content increases, , As coarse Ti nitrides and Ti carbonitrides. 0.0 in relation to manufacturing conditions
120% or less.

O: 12 ppm or less O is present in the steel as an oxide-based inclusion and may also serve as a trap site for hydrogen. However, it is more likely that coarse precipitates play a large role in lowering the fatigue strength, and the smaller the amount, the better. desirable. Therefore, the upper limit is set to 12 ppm.

[0020] In addition to the above components, the steel of the present invention includes Ni, C
r and Mo can be contained alone or in combination. These actions are as follows.

Ni: 0.1 to 2.0% Ni improves hardenability and improves fatigue strength and toughness. If the content is less than 0.1%, the effect is not sufficient.
If it exceeds 0%, the softening of the material becomes difficult, the workability remarkably decreases, and the susceptibility to quenching cracking increases.
%.

Cr: 0.20 to 2.0% Cr improves hardenability and spheroidization of carbides, and improves fatigue strength and toughness. If it is less than 0.2%, the effect is not sufficient, and if it exceeds 2.0%, the softening of the material becomes difficult and the workability is significantly reduced, so the upper limit is made 2.0%.

Mo: 0.05 to 1.0% Mo improves hardenability and improves fatigue strength, toughness and the like. If less than 0.05%, the effect is not enough,
If it exceeds 1.0%, it becomes difficult to soften the raw material and the workability is remarkably reduced, and the above-mentioned effects are saturated and the cost is disadvantageous. Therefore, the upper limit is made 1.0%.

When a steel material having the above chemical composition is manufactured under the following conditions, the most effective fine Ti-based precipitate can be obtained.
It exhibits excellent delayed fracture characteristics.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the embodiment of the present invention, the weight%
, C: 0.30-0.80%, Si: 0.05-0.
50%, Mn: 0.2 to 2.0%, Ti: 0.05 to
0.20%, Al: 0.010 to 0.050%, N:
It contains 0.0120% or less and O: 12ppm or less, and is defined as a steel component of the balance of Fe and unavoidable impurities, or Ni: 0.1 to 2.0% and Cr: 0.1 to 2.0%.
Rolling a steel obtained by smelting a steel component containing one or more selected from 20 to 2.0% and Mo: 0.05 to 1.0% into a billet or a steel material. . In this case 120
The material is heated and rolled to a temperature range of 0 to 1350 ° C., and is further rolled or forged into parts of a predetermined size in the same temperature range or a lower temperature range of 800 to 1050 ° C. By performing the above steps, Ti carbide and Ti carbonitride having a size of 70 nm or less can be finely dispersed in steel after the final carburizing quenching and tempering, and high strength and long life having excellent delayed fracture resistance can be achieved. Induction hardening steel can be obtained.
Hereinafter, an embodiment will be described in detail.

[0026]

EXAMPLE 100 kg of a test material having the chemical components shown in Table 1 was used.
Melt in a vacuum melting furnace.

In Table 1, invention steel 1, invention steel 2, invention steel 4, invention steel 5 and invention steel 6 are claimed in claim 2 or claim 4.
1 shows the components of one embodiment of the present invention. Invention Steel 3 is a component of the first embodiment of the present invention. The steel billet obtained by the above smelting is hot-rolled in a temperature range of 1200 to 1350 ° C, and a part is in the same temperature range and another part is in a temperature range of 800 to 1050 ° C and is 65 mmφ.
And forged to 25 mmφ. The obtained steel material is 850 ° C x
After holding for 0.5 h, quenching with oil cooling, 600 ° C x 1
After holding for h, the sample was air-cooled and tempered, processed into various test pieces, and subjected to induction hardening and tempering under the heat treatment conditions shown in Table 2.

[0028]

[Table 1]

[0029]

[Table 2]

A thrust test is performed with a thrust life tester having the structure shown in FIG. In FIG. 1, a thrust test piece 2 is set on a holding frame 1, a 9.525 mmφ steel ball 4 is held by a holder 3, and an upper race 5 covering the upper portion of the steel ball 4 is rotated by a rotation shaft 6. Perform a thrust test. In this case, a lubricating oil 7 is put in the holding frame 1 and placed. That is, length 60mm
The test piece 2 having a width of 40 mm and a thickness of 8 mm was subjected to the heat treatment shown in Table 2 and then subjected to finish polishing and lapping. Under lubricating oil with 5% pure water added,
ERZ surface pressure: 5292 MPa, stress load speed: 1800
subjected to a thrust life test in cpm, it was determined with 10% probability life (B ₁₀ life).

For the investigation of the delayed fracture characteristics,
The test was performed using a notched test piece shown in FIG. 2 from a forged material. That is, using the notched test piece shown in FIG. 2A, a delayed fracture test as a static characteristic was performed based on a breaking load in a tensile test after immersion in 5% hydrochloric acid. Also, FIG.
Using a notched specimen of φ8 mm shown in (1), the fatigue limit in the air and in pure water dripping was obtained as a fatigue property by a rotary bending fatigue test.

[0032]

[Table 3]

Table 3 shows the manufacturing conditions for each test material, the measurement results of the precipitate particle size of the induction hardened material, and the results of various tests. Here, those in which two conditions are entered in the rolling conditions are those in which forging was performed twice. As can be seen from Table 3, the steel according to the present invention has a fine particle diameter of the Ti-based precipitate, a small decrease in the life characteristics under a hydrogen intrusion environment, and a small decrease in the delayed fracture strength and the rotational bending fatigue strength. I understand.

[0034]

As described above, by the effective addition of Ti and the optimization of the production conditions, the steel according to the present invention can be used to form Ti-based precipitates suitable for mechanical parts such as shafts, constant velocity joint parts, and linear motion shafts. The particle size of the product is fine, and it has excellent resistance to delayed fracture caused by hydrogen entering from the environment during heat treatment or use, that is, the life characteristics under hydrogen intrusion environment High strength and long life induction hardening steel with excellent resistance to delayed fracture with little decrease.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing the structure of a thrust life tester.

FIG. 2 is a diagram showing shapes of test pieces in a delayed fracture test and a rotating bending fatigue test.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Test piece holding frame 2 Thrust test piece 3 Cage 4 Steel ball 5 Upper race 6 Rotation axis 7 Lubricating oil

Continued on the front page F term (reference) 4K032 AA01 AA02 AA05 AA06 AA11 AA12 AA14 AA16 AA19 AA21 AA23 AA24 AA26 AA27 AA29 AA31 AA35 BA02 CA03 CC03 CC04

Claims (4)

[Claims] (1) C: 0.30 to 0.80% by weight,
Si: 0.05 to 0.50%, Mn: 0.2 to 2.0
%, Ti: 0.05-0.20%, Al: 0.010-0
0.050%, N: 0.0120% or less, O: 12pp
m or less, the balance being Fe and unavoidable impurities,
Induction hardening steel having high strength and excellent delayed fracture resistance and rolling fatigue life characteristics, characterized in that Ti carbide and Ti carbonitride having a size of 70 nm or less are finely dispersed in the steel. 2. C .: 0.30 to 0.80% by weight,
Si: 0.05 to 0.50%, Mn: 0.2 to 2.0
%, Ti: 0.05-0.20%, Al: 0.010-0
0.050%, N: 0.0120% or less, O: 12pp
m or less, Ni: 0.1 to 2.0%, C
r: 0.20 to 2.0%, Mo: 0.05 to 1.0%, one or more selected from the group consisting of a balance of Fe and unavoidable impurities, a Ti carbide having a size of 70 nm or less; High frequency quenching steel having high strength and excellent delayed fracture resistance and rolling fatigue life characteristics characterized by finely dispersing Ti carbonitride in the steel. C .: 0.30 to 0.80% by weight,
Si: 0.05 to 0.50%, Mn: 0.2 to 2.0
%, Ti: 0.05-0.20%, Al: 0.010-0
0.050%, N: 0.0120% or less, O: 12pp
m, the steel material consisting of the balance Fe and unavoidable impurities is heated and rolled to a temperature range of 1200 to 1350 ° C.,
Further, by rolling or forging a predetermined steel material or part at the same temperature or a temperature of 800 to 1050 ° C., a Ti carbide or Ti having a size of 70 nm or less is formed in a subsequent heat treatment step.
A method for producing a steel for induction hardening, which is characterized by finely dispersing carbonitrides in the steel, and which has high strength and excellent delayed fracture resistance and rolling fatigue life characteristics. C: 0.30 to 0.80% by weight,
Si: 0.05 to 0.50%, Mn: 0.2 to 2.0
%, Ti: 0.05-0.20%, Al: 0.010-0
0.050%, N: 0.0120% or less, O: 12pp
m or less, Ni: 0.1 to 2.0%, C
r: 0.20 to 2.0%, Mo: 0.05 to 1.0%, one or two or more selected from the group consisting of a balance of Fe and inevitable impurities in a temperature range of 1200 to 1350 ° C. Heating and rolling to the same temperature or 800 ~ 1
By rolling or forging a predetermined steel material or part at a temperature of 050 ° C., a Ti carbide or Ti carbonitride having a size of 70 nm or less is finely dispersed in the steel in a subsequent heat treatment step. A method for producing induction hardened steel that is strong and has excellent delayed fracture resistance and rolling fatigue life characteristics.