JPH10152752A - Case hardening steel excellent in machinability and grain coarsening resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide case hardening steel excellent in machinability and in which the coarsening of austenitic grains can be suppressed even if high temp. short time treatment is executed at the time of carbo-nitriding. SOLUTION: This steel contains, by weight, 0.1 to 0.3% C, <=0.03% P, 0.02 to 0.15% S, 0.0525 to 0.400% Ti and <=0.008% N or contains 0.1 to 0.3% C, <=0.03% P, 0.02 to 0.15% S, 0.1 to 2% Mn and 0.005 to 0.1% Nb.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a case hardening steel having excellent machinability and resistance to coarsening.

[0002]

2. Description of the Related Art Case-hardened steel is frequently used for mechanical parts such as gears and bearings. The method of manufacturing these components can be broadly divided into the following two methods.

[0003] First, the rolling material is spheroidized and annealed, and after cold drawing, a part is formed by cold forging and cutting.
This is a step of carburizing or carbonitriding.

[0004] The second is a process in which a rolled material is first formed into a rough shape by hot forging, normalized or annealed, and then a part is formed by cutting, followed by carburizing or carbonitriding. Common to both manufacturing processes is a cutting process and a surface hardening process of carburizing or carbonitriding (hereinafter collectively referred to as carburizing), both of which occupy a part of the product manufacturing cost. Therefore, in order to reduce the manufacturing cost and the productivity, it is important to improve the machinability of the material and to shorten the surface hardening treatment, and various steel types and processing methods have been conventionally proposed.

In order to improve the machinability of a material, conventionally, free-cutting elements such as Pb and S have been added.

It is the Pb-added steel that contributes to the improvement of the machinability is the Pb particles dispersed in the steel, and the S-added steel is Mn.
Regarding S, there was a problem that all the particles were coarsely dispersed. Such coarse particles may be a starting point of fatigue fracture or impact fracture of a member, and when dispersed in a large amount that emphasizes improvement in machinability, fatigue strength and toughness are significantly reduced. In addition, the coarse particles lower the critical compressibility in the cold, and hinder the cold forgeability.

[0007] To shorten the surface hardening treatment time, it is most effective to raise the treatment temperature. Carburizing or carbonitriding is C
However, the diffusion speed of these elements can be remarkably increased by increasing the processing temperature.
However, when such high temperature treatment is performed,
There is a problem that austenite grains are coarsened and distortion of parts during quenching increases.

In order to solve this problem, a method of finely dispersing carbonitrides such as Nb and V has been proposed.
There is no help in improving the machinability described above, and machinability deteriorates when a large amount is added in order to exhibit a sufficient effect of suppressing coarsening by high-temperature treatment.

Japanese Unexamined Patent Publication (Kokai) No. Sho 60-262,941 discloses a case hardening steel containing Ti and S and in which the crystal grains are not coarsened even by carburizing. However, N is 0.00
Since it is contained in a large amount of 9 to 0.03%, TiN is generated and the machinability is poor.

[0010]

An object of the present invention is to provide a skin which is excellent in machinability and can suppress austenite grains from becoming coarse even if a high-temperature and short-time treatment is performed during carburization. Is to provide hardened steel.

[0011]

Means for Solving the Problems The inventors of the present invention conducted various experiments and studies on chemical components in order to develop a case hardening steel capable of preventing austenite grains from being coarsened even when carburizing treatment was performed at a high temperature. Obtained knowledge.

(A) If Ti is added to case hardening steel to which S is added as a machinability improving element, Ti sulfide can be finely dispersed (compared to MnS) in the steel, and Ti sulfide can be dispersed. The material exerts a lubricating effect and contributes to the improvement of machinability, and the coarsening of austenite grains can be suppressed even by performing carbonitriding at a high temperature. When Ti is added, the machinability deteriorates unless the N content is reduced.

(B) When Nd is added to case hardening steel to which S has been added as a machinability improving element, sulfides of Nd are finely and largely dispersed in the steel, and become MnS precipitation nuclei. The austenite grains can be suppressed from becoming coarse even if carburizing treatment is performed at a high temperature.

The present invention has been made based on such findings, and the gist is as follows.

(1) By weight%, C: 0.1-0.3%,
P: 0.03% or less, S: 0.02 to 0.15%, T
Case hardening steel excellent in machinability and coarse-graining resistance characterized by containing i: 0.025 to 0.4% and N: 0.008% or less.

(2) C: 0.1-0.3% by weight,
P: 0.03% or less, S: 0.02 to 0.15%, M
A case hardening steel excellent in machinability and coarse-graining resistance, characterized by containing n: 0.1 to 2% and Nd: 0.005 to 0.1%.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for defining the chemical components of the Ti-containing steel and the Nd-containing steel, which are the case hardening steels of the present invention, will be described in detail below. In addition, all "%" show weight%.

1. Ti-containing case-hardened steel C: 0.1 to 0.3% Case-hardened steel is used after the surface layer is made high in C and N by carburizing, so that the toughness of the entire member is ensured. In order to achieve this, it is fundamental to reduce the C of the base material. However, if the amount of the base material C is too low, the toughness can be secured, but the strength of the entire member is insufficient. Therefore, the lower limit of C is set to 0.
1%. On the other hand, if the amount of the base material C exceeds 0.3%, not only the toughness is reduced but also the machinability is deteriorated, so the upper limit was made 0.3%. Desirable content of C is 0.15
0.23%.

P: not more than 0.03% P has an effect of deteriorating the toughness of the member after carburization. 0.
If the content exceeds 03%, this effect becomes significant, so the upper limit was made 0.03%. Desirable content of P is 0.025%
It is as follows.

S: 0.02 to 0.15% S forms a sulfide of Ti and has an effect of improving machinability and resistance to coarsening. If it is less than 0.02%, a sufficient amount of sulfide cannot be obtained, and neither the machinability nor the coarse graining resistance is improved, so the lower limit was made 0.02%.

On the other hand, Ti-based sulfide serves as a starting point of bending fatigue and rolling fatigue, and has an effect of reducing the fatigue strength of the member. In addition, it also has the effect of deteriorating the toughness and cold forgeability of the member. Since the Ti-based sulfide is finely dispersed as compared with MnS usually present in steel, the effect is small, but if S is added in excess of 0.15%, this effect cannot be ignored. Therefore, the upper limit of S is set to 0.15%. Desirable content of S is 0.03% to 0.10%
It is.

Ti: 0.025% to 0.4% Ti forms a sulfide and is stable at a high temperature. Therefore, Ti has an effect of suppressing coarsening by carburizing at a high temperature. 0.025
%, A sufficient amount of sulfide cannot be obtained, and neither machinability nor resistance to coarsening is improved, so the lower limit was made 0.025%.

On the other hand, sulfides act as starting points for bending fatigue and rolling fatigue, and have the effect of reducing the fatigue strength of members. In addition, it also has the effect of deteriorating the toughness and cold forgeability of the member.
Ti-based sulfides disperse finely compared to MnS normally present in steel, so this effect is small.
%, The effect cannot be ignored. Therefore, the upper limit of Ti is set to 0.4%. T
Desirable content of i is 0.050% to 0.3%.

N: 0.008% or less N forms nitrides of Ti and improves coarsening resistance, but when added in a large amount, has an effect of increasing nitrides and deteriorating machinability. If N is added in excess of 0.008%, the machinability deteriorates, so the upper limit was made 0.008%.
In addition, since coarse graining resistance can be ensured by the sulfide of Ti, it is desirable to reduce Ti nitride which deteriorates machinability. Therefore, it is desirable that the addition of N be as low as possible. A desirable upper limit of N is 0.006%.

2. Nd-containing case-hardened steel C: 0.1-0.3% For the same reason as the above-mentioned Ti-containing steel, the content was 0.1-0.3%.

P: not more than 0.03% P has an effect of deteriorating the toughness of the member after carburizing. 0.
If the content exceeds 03%, this effect becomes significant, so the upper limit was made 0.03%. Desirable content of P is 0.025%
It is as follows.

S: 0.02% to 0.15% S forms MnS and has an effect of improving machinability and resistance to coarsening. If it is less than 0.02%, a sufficient amount of sulfide cannot be obtained, and neither the machinability nor the coarse graining resistance is improved, so the lower limit was made 0.02%.

On the other hand, MnS serves as a starting point of bending fatigue and rolling fatigue, and has an effect of reducing the fatigue strength of the member.
In addition, it also has the effect of deteriorating the toughness and cold forgeability of the member. Since MnS is finely dispersed by the addition of Nd, the above-mentioned effect is reduced. However, if S is added in excess of 0.15%, this effect cannot be ignored. Therefore, the upper limit of S is set to 0.15%. The desirable content of S is 0.03% to 0.1%.

Mn: 0.1-2% Mn has the effect of increasing the hardenability of steel. In the case of case hardened steel, quenching is performed after carburizing or carbonitriding, so it is necessary to ensure hardenability. If the content exceeds 2%, Si
Similarly to the above, the cold forgeability is deteriorated, so the upper limit is set to 2%.

Mn combines with S to form MnS and also has the effect of improving machinability. If it is less than 0.1%, a sufficient amount of MnS cannot be generated, so the lower limit was made 0.1%. Desirable content of Mn is 0.3% or more
1.5% or less.

Nd: 0.005 to 0.1% Nd forms a sulfide serving as a precipitation nucleus of MnS, and MnS
Has the effect of precipitating finely. If the content is less than 0.005%, a sufficient amount of sulfide cannot be obtained, and the effect of miniaturizing MnS is lost, and a large amount of MnS precipitates, so the lower limit is made 0.005%. .

On the other hand, even if the content of Nd exceeds 0.1%, the effect of miniaturizing MnS hardly changes. Therefore, the upper limit of Nd is set to 0.1%. The desirable content of Nd is 0.01% to 0.06%.

The preferred other chemical components of the two case hardening steels of the present invention are as follows, but are not particularly limited.

Cr: 2% or less Cr has the effect of increasing the hardenability of steel. In the case of case hardened steel, quenching is performed after carburizing, so that quenching properties must be ensured. Cr can be contained according to the size of the member and the coolant used for quenching. If the content exceeds 2%, the cold forgeability is deteriorated like Si and Mn. Therefore, the upper limit is preferably set to 2%. The desirable content of Cr is 1.5% or less.

Si: 2% or less Case-hardened steel is carburized and used for gears and bearings. Gears and bearings often require rolling fatigue life. Si
Has an effect of improving the rolling fatigue life, and therefore can be appropriately contained. If the content exceeds 2%, the cold forgeability and machinability of the steel itself are significantly deteriorated.
It is better to do the following. Desirable content of Si is 0.2% or more
0.8%.

Ni, Mo, B: If it is necessary to improve the toughness of the steel after carburizing, Ni: 0.5 to 3.5% or less, Mo: 0.15 to 1% or less, B: 0 .001 to 0.
It is preferable to contain at least one of 005% or less.

If the addition amount exceeds the above amount, the effect of improving the toughness is saturated, and in the case of Ni and Mo, the machinability of the member is deteriorated. Desirable upper limits of Ni, Mo, and B are 2.5%, 0.8%, and 0.004%, respectively.

Nb, V: When it is necessary to further improve the coarse graining resistance, Nb: 0.01 to 0.1%, V:
It is preferred to contain one or two of 0.05 to 0.3%. Nb and V form a carbonitride and have an effect of improving the coarsening resistance. In the case of Ti-containing case hardening steel, if these carbonitrides are used in combination with Ti sulfide, the resistance to coarsening can be further improved. If the content exceeds the above-mentioned content, the effect of improving the coarse graining resistance is saturated, and conversely, the machinability of the member is deteriorated.

Pb, Ca: Pb: 0.
One or two of 0.3 to 0.3% and Ca: 0.001 to 0.1% can be contained. Pb and Ca have an effect of improving machinability, and if they are added together with Ti sulfide, machinability can be further improved. Even if the content exceeds the above-mentioned content, the effect of improving machinability is saturated, and in the case of Pb, on the contrary, the fatigue strength after carburizing or carbonitriding of the member is deteriorated.

In the case of Ti-containing case hardened steel, Mn is 2
% Or less. Since hardening is performed after carburizing or carbonitriding, it is necessary to ensure hardenability.
Mn can be added according to the size of the member or the coolant used for quenching. If the content exceeds 2%, as in the case of Si, the cold forgeability deteriorates. Therefore, the upper limit is preferably set to 2%. The desirable content of Mn is 1.5% or less.

In the case of Nd-containing case hardening steel, sol.Al:
0.1% or less and N: 0.02% or less can be contained.

Sol.Al combines with N to form AlN,
This has the effect of improving the coarsening resistance. However, sol.Al
If the content exceeds 0.1%, the effect does not change. A desirable content is 0.02% to 0.04%. If N is added in excess of 0.02%, the cold forgeability deteriorates. Desirable content is 0.010% to 0.01
5%.

In using the steel of the present invention, carburizing treatment or carbonitriding treatment is premised. Carburizing and carbonitriding are performed by various methods, and can be roughly classified into solid carburizing by graphite, carburizing by salt bath, gas carburizing and ion carburizing.

Although these methods differ in the mechanism of penetration of C and N into the members, they are still high-temperature treatments, and the effects of the present invention are not lost in any treatment.

[0045]

【Example】

(Example 1) Steels of 37 components shown in Table 1 were melted in vacuum.

[0046]

[Table 1]

A series is case hardened steel of the present invention. A1
The steel is a steel obtained by adding Ti to JIS G4052 SCr420 steel and changing the amounts of S and N. The A2 to A12 steels are steels aiming at the upper and lower limits of C, P, S, Ti, and N defined in the present invention. A13 to A27 steels are Ni, Mo, B,
It is a steel containing V, Nb, Pb, and Ca. Among them, A13 and A20 steels are JIS SNC815, SNC
This is a steel obtained by adding Ti to M420 steel and changing the amounts of S and N.

The B series is a comparative steel and has S, Ti and N contents outside the range specified in the present invention. C series is conventional steel, C1 to C7 are A1, A13, A14, A
20, S24, A18, and A16 steels have S, Ti, and N levels at normal levels.

A 50 kg ingot of 37 steel grades was heated at 1250 ° C. and hot forged to a finish of 1000 ° C. to form a round bar having a diameter of 65 mm. Next, these round bars were heated at 925 ° C. for 1 hour, inserted into a furnace at 600 ° C., held for 1 hour, and allowed to cool in the air. This process simulates the annealing process usually performed after hot forging. Thereafter, a cylindrical machinability test piece having a diameter of 60 mm and a length of 45 mm, and a coarse-grained test piece having a diameter of 10 mm and a length of 20 mm were produced. The following tests were performed using these test pieces.

(1) Machinability test JIS G4403 Diameter of high speed steel SKH52: 10 m
Using a drill of m, diameter: 60 mm, length: 45 mm
A hole having a depth of 40 mm was formed in the test piece.

The drilling conditions were as follows: using a water-soluble lubricant, feed rate: 0.20 mm / rev, rotation speed: 980 rpm
And

(2) Coarse Graining Test A test piece having a diameter of 10 mm and a length of 20 mm was pseudo-carburized. The pseudo carburizing condition was heating at 1000 ° C. for 3 hours.

In the machinability test, the number of holes before drilling becomes impossible due to wear of the drill bit is used as an index of machinability, and 20
It was judged that the machinability was excellent if it was 0 or more. In the coarse-graining test, after quenching, the austenite crystal grain size was measured.

Table 2 shows the results.

[0055]

[Table 2]

The evaluation in the table was as follows.

：: excellent in machinability and resistance to coarsening,
Δ: Either machinability or coarse-graining resistance is inferior,
×: Both properties of machinability and resistance to coarsening are inferior.

The A1 to A12 steels are base steels and steels aiming at the upper and lower limits of C, P, S, Ti and N specified in the present invention. Have achieved. Conventionally, C1-C6 steels are A1 steel and A1 steel, respectively.
3 steel, A14 steel, A20 steel, A24 steel, A18 steel S
Although the steels had the Ti and N contents at normal levels, the machinability and the grain size did not reach the targets. From this, S, Ti,
It turns out that the amount of N is important. C7 steel is a steel in which the amounts of S, Ti and N of A16 steel are set to normal levels, but the grain size has reached the target grain size by adding Nb, but the machinability has been significantly deteriorated.

From the comparison between the A1 steel and the B1 steel, it can be seen that when S is below the lower limit of the specified amount of the present invention, the amount of sulfide decreases and the machinability deteriorates.

From the comparison between the A1 steel and the B2 steel, when Ti is less than the specified amount of the present invention, MnS is generated instead of Ti sulfide, so that the machinability is good but the grain size is deteriorated.

From the comparison between A1 steel and B3 steel, when N exceeds the upper limit specified in the present invention, TiN is formed and the grain size is good, but the machinability is deteriorated.

Ni, Mo, B for improving the toughness of case hardened steel
A13-A15 steel, A20 steel, A
From the comparison of No. 21 steel, the machinability is slightly deteriorated by the addition of these alloy elements, but all of them are within the allowable range.

In the steels to which Nb and V are added to refine the crystal grains, the machinability is made to be within the allowable range and the grain size is set by adding these alloy elements from comparison of A16 steel, A17 steel and A22 steel. It can be seen that further improvements can be made.

In steels to which Pb and Ca are added to improve machinability, a comparison between A18 steel, A19 steel and A23 steel shows that the addition of these elements makes the grain size within an allowable range.
It can be seen that the machinability can be further improved.

Similarly, toughness improving elements (Ni, Mo,
B), fine-graining elements (Nb, V), machinability improving elements (P
It can also be seen that the composite-added steels of b and Ca) are also excellent in both machinability and grain size within the specified range of the present invention.

(Example 2) Steels of 34 kinds of components shown in Table 3 were melted in vacuum.

[0067]

[Table 3]

The D series is the case hardened steel of the present invention. D1
The steel is a steel obtained by adding Nd to JIS SCr420 steel and changing the S content. D2 to D11 steels are steels aiming at the upper and lower limits of C, P, S, and Mn specified in the present invention. D
12-D26 steel is Ni, Mo, B, V, Nb, Pb,
It is a steel containing Ca. Among them, D12 and D19 steels are steels obtained by adding Nd to JIS G4052 SNC815 and SNCM420 steels and changing the S content.

The E series is a comparative steel, and the amounts of S and Nd are out of the range defined in the present invention. F series is conventional steel, and F1 to F6 are D1, D12, D13, D1 respectively.
9, D23 and D15 steels with S and Nd contents at normal levels.

A 50 kg ingot of 34 steel grades was heated to 1250 ° C. and hot forged to a finish of 1000 ° C. to form a round bar having a diameter of 65 mm. Next, these round bars were heated at 925 ° C. for 1 hour, inserted into a furnace at 600 ° C., held for 1 hour, and allowed to cool in air. This process simulates the annealing process usually performed after hot forging.

Thereafter, the diameter: 60 mm and the length: 45 mm
Cylindrical machinability test piece, diameter: 10 mm, length: 2
A 0 mm coarse-grained test piece was prepared. The following tests were performed using these test pieces.

(1) Machinability test JIS G4403 Diameter of high speed steel SKH52: 10 m
Using a drill of m, diameter: 60 mm, length: 45 mm
A hole having a depth of 40 mm was formed in the test piece.

The drilling conditions were as follows: using a water-soluble lubricant, feed rate: 0.20 mm / rev, rotation speed: 980 rpm
And

(2) Coarse Graining Test A test piece having a diameter of 10 mm and a length of 20 mm was pseudo-carburized. The pseudo carburizing condition was heating at 1000 ° C. for 3 hours.

In the machinability test, the number of holes until drilling becomes impossible due to wear of the drill bit is used as an index of machinability.
It was judged that the machinability was excellent if it was 0 or more. In the coarse-graining test, after quenching, the austenite crystal grain size was measured.

Table 4 shows the results.

[0077]

[Table 4]

The evaluation in the table was as follows.

：: excellent in machinability and resistance to coarsening,
Δ: Either machinability or coarse-graining resistance is inferior,
×: Both properties of machinability and resistance to coarsening are inferior.

D1 to D11 steels are base steels and steels aiming at the upper and lower limits of C, Si, Mn, P and S specified in the present invention. Have achieved. Conventionally, F1 to F5 steels are D1 steel and D1 steel, respectively.
Steel No. 12, steel D13, steel D19, and steel D23 had the S and Nd amounts at normal levels, but the machinability and the grain size did not reach the targets. This indicates that the contents of S and Nd are important. The F6 steel is a steel in which the amounts of S and Nd of the D15 steel are at normal levels, and it can be seen that the grain size is achieved by adding Nb, but the machinability is significantly deteriorated.

From the comparison between the D1 steel and the E1 steel, when S falls below the lower limit specified in the present invention, the amount of fine MnS decreases.
It can be seen that the machinability deteriorated and the particles became coarse.

From the comparison between D1 steel and E2 steel, when Nd is below the lower limit specified in the present invention, coarse Mns increases and machinability is good, but the grain size is coarse.

Ni, Mo, B for improving the toughness of case hardened steel
, D12-D14 steel, D19 steel, D
From a comparison of 20 steels, the machinability is slightly deteriorated by the addition of these alloy elements, but all are within the allowable range.

In the steels to which Nb and V are added to refine the crystal grains, the machinability is set within the allowable range by adding these alloying elements, and the crystal grain size is determined by comparing D15 steel, D16 steel and D21 steel. Can be further improved.

In the case of steels to which Pb and Ca are added to improve machinability, comparison of D17 steel, D18 steel and D22 steel shows that the addition of these alloying elements makes the grain size within the allowable range, and the machinability is improved. Further improvements have been made.

Similarly, toughness improving elements (Ni, Mo,
B), fine-graining elements (Nb, V), machinability improving elements (P
It can be seen that even in the composite addition of b and Ca), the machinability and the particle size are both excellent within the specified range of the present invention.

[0087]

The case hardening steel of the present invention is excellent in machinability and does not cause austenite grains to be coarsened by carburizing at a high temperature. Alternatively, the processing time required for carbonitriding can be reduced,
Manufacturing costs can be reduced.

Claims (2)

[Claims] C .: 0.1 to 0.3% by weight, P:
0.03% or less, S: 0.02 to 0.15%, Ti:
A case hardening steel excellent in machinability and coarse-graining resistance characterized by containing 0.025 to 0.4% and N: 0.008% or less. 2. C: 0.1 to 0.3% by weight, P:
0.03% or less, S: 0.02 to 0.15%, Mn:
A case hardening steel having excellent machinability and coarse-graining resistance characterized by containing 0.1 to 2% and Nd: 0.005 to 0.1%.