JPH06264178A - Soft-nitriding steel - Google Patents

Abstract

PURPOSE:To obtain an excellent steel increased in effective hardening depth and improved in surface hardness by applying soft-nitriding treatment to a steel. CONSTITUTION:This steel has a composition which consists of 0.15-0.6% C, 0.03-0.5% Si, 0.4-2.0% Mn, >1.00-3.0% Cr, 0.03-0.4% V, >0.5-1.2% Al, 0.005-0.02% N, and the balance Fe with inevitable impurities and into which these elements are incorporated so that two characteristic inequalities 0<7.2-7.5Si-6.9Cr+3.8Mo+23.8V and 0<-4.78-1.25C-0.38Si+0.19Mn+1.10Cr+3.98V+5.27Al are satisfied and the content of oxygen among the inevitable impurities is regulated to <=0.002%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a soft nitriding steel which can be soft nitrided to obtain a steel having a surface hardness equal to or higher than that of a carburized material. The soft nitriding steel of the present invention can be used for the production of precision parts, such as gears, joints, and shafts, which are averse to the occurrence of heat treatment distortion, among industrial machine parts in which carburized materials have been conventionally used. .

[0002]

2. Description of the Related Art Conventionally, carburizing materials have been often used for parts for industrial machines, but a large amount of strain is generated during carburizing and quenching in carburizing. Therefore, as disclosed in JP-B-53-41611, A ₁
A nitriding method has been performed in which the surface hardening treatment is performed at a temperature below the transformation point.

The nitriding method is a method of obtaining extremely high surface hardness by infiltrating nitrogen into the steel surface generally in an atmosphere of ammonia gas or nitrogen gas at 500 to 525 ° C. However, since this nitriding method requires a long time treatment of 50 hours or more, a soft nitriding method has been developed as a method capable of completing nitriding in a short time.

The soft nitriding method uses a molten cyanide salt bath (about 570
℃) and blowing air into this salt bath, or RX without using such a cyanide compound.
Gas _{(e.g., CO: 20%, H 2} : 40%, N 2: 4
Nitriding is performed in an atmosphere in which 0% composition gas) and NH ₃ gas are mixed at a ratio of 50:50.

As the nitrided steel, there is SACM645 standardized in JIS. In addition, JIS standard steel SCM435 or the like for machine structural alloy steel or S45C or the like for machine structural carbon steel is used after being subjected to nitriding treatment. J like this
When IS standard steel is soft-nitrided, a carbonitride compound layer is generated in the surface layer of the steel, and carbonitride precipitates inside the steel.
Also, a diffusion layer in which nitrogen is dissolved in a high concentration occurs, and a soft nitrided steel having high surface hardness is obtained.

[0006]

In order to obtain a surface strength similar to that of a carburized material, the effective hardened layer depth, that is, the thickness (distance from the surface) of the hardened layer corresponding to the minute Vickers hardness HV420 is increased. There is a need. However, when SACM645 or SCM435 is subjected to soft nitriding treatment as described above, the effective hardened layer depth is as shallow as about 0.2 mm. There is a problem that peeling is likely to occur at the boundary with the core portion, resulting in poor spalling resistance. Further, with respect to bending fatigue, the conventional soft nitrided steel has a problem that it is easily broken from the inside.

The present invention has been made to solve the above problems, and is a machine having a deep effective hardened layer and a high surface hardness, that is, a surface strength equal to or higher than that of a carburized material. A soft nitriding steel for obtaining a structural soft nitriding steel is provided.

[0008]

The soft nitriding steel of the present invention which has achieved the above object is C: 0.15 to 0.6% by weight (hereinafter, simply referred to as%), Si: 0.03 to. 0.5%, M
n: 0.4 to 2.0%, Cr: more than 1.00 to 3.0%,
V: 0.03 to 0.4%, Al: more than 0.5 to 1.2%,
N: 0.005 to 0.02%, and each of the above elements is contained so as to satisfy the following characteristic equations 1 and 2, and the balance is Fe and inevitable impurities, and oxygen in the inevitable impurities is It is 0.002% or less. Characteristic formula 1: 0 <7.2-7.5Si-6.9Cr + 3.
8Mo + 23.8V Characteristic formula 2: 0 <-4.78-1.25C-0.38Si
+ 0.19Mn + 1.10Cr + 3.98V + 5.27
Al

In some cases, it is desirable for the nitrocarburizing steel of the present invention to contain Mo, Ni, Ti, Nb or the like. Furthermore, S, Pb, Bi, Te, Se, Z
It may be preferable to appropriately contain r and Ca.

[0010]

The action of each component and the reason for limiting the component range will be described below. C is for ensuring the strength of the core part, and 0.15% or more is necessary to effectively develop it, and if it exceeds 0.6%, the toughness and ductility of the core part is deteriorated and the machinability is reduced. Since the cold workability deteriorates, the upper limit was made 0.6%. Si acts as a deoxidizer and is a necessary element,
If it becomes excessive, the workability such as cold forgeability will deteriorate, so
It was set to 0.03% or more and 0.5% or less.

Mn is an element that has a deoxidizing action and is necessary for ensuring hardenability. To effectively act this element, Mn must be 0.4% or more, but exceeds 2.0%. Since the machinability and cold workability are deteriorated, the upper limit was made 2.0%.
Cr acts extremely effectively to form Cr nitrides during soft nitriding treatment to increase the surface hardness and to secure the hardening depth. However, when it becomes excessive, the effective hardened layer depth is rather decreased and the machinability is further reduced. Therefore, the lower limit of the addition amount is 1.0 in order to achieve the target surface hardness and hardening depth.
It was over 0 and the upper limit was 3.0%.

[0012] V effectively acts to form V nitrides during soft nitriding treatment to deepen the effective hardened layer depth and increase the surface hardness, but if added in excess, the effect saturates and cutting is performed. Since the property deteriorates, the lower limit of the addition amount is 0.03%,
The upper limit was 0.4%.

Al forms a compound with N invading during the soft nitriding treatment, and acts extremely effectively for increasing the surface hardness. However, there is no effect on the effective hardened layer depth, and the excessive presence of Al rather reduces the effective hardened layer depth. Conventionally, with respect to Al, if the addition amount exceeds 0.5%, the cleanliness of the steel is impaired, but according to the experiments of the present inventors, the oxygen content in the steel was 0.0025%.
It was confirmed that high cleanliness can be obtained by the following. Therefore, the lower limit of the amount of Al added is more than 0.5%, and the upper limit is 1.2%. The oxygen content was 0.002% or less.

In the present invention, the improvement of the surface hardness is considered to be an important issue, and the relationship between the Al amount and the characteristic formula 2 is defined on the assumption that 1.0% or more of Cr, which is an element for improving the surface hardness, is added. This is aimed at improving the surface hardness.

N has a great effect in forming a compound with Al and greatly contributing to the refinement of crystal grains, and also forming a compound with V and increasing the hardness of the core. At least 0.005% or more is necessary in order to sufficiently bring out these effects, but if it is present more than necessary, excessive nitrides are formed and the toughness is lowered, so the upper limit was made 0.02%.

Mo works to enhance the hardenability, but if it is contained in excess, the machinability deteriorates, so Mo was made less than 0.5%. Ni is an element effective for enhancing the toughness and ductility of the core part, but if it is excessively present, the machinability sharply decreases, so the upper limit was made 2%.

Ti is an element that agglomerates cementite and reduces the pearlite area ratio, thereby lowering the hardness and improving the machinability. In order to exert such an effect, it is necessary to add at least 0.005% or more, but if added excessively, the nitridability is impaired, so the upper limit is 1.0.
% Or less.

Nb is effective in increasing the toughness and ductility because it makes the crystal grains finer. To exert these effects, at least 0.005% or more is necessary. However, even if added excessively, the effect saturates and sometimes decreases, so the upper limit was made 0.30%.

S, Pb, Bi, Te, Se, Zr, Ca
Is an element effective in improving machinability, but if added in excess, it adversely affects hot workability and fatigue properties. Therefore, each element is set within the above range.

However, as a result of further research, it was found that a satisfactory soft nitriding steel cannot always be obtained by focusing only on the action of each element and limiting the component range as described above. Therefore, the present inventors consider that it is necessary to take into consideration the mutual action of each element, and to evaluate the hardened layer depth and the surface hardness by using some characteristic formula that includes the interaction of each element in the evaluation. , Various experiments were conducted.

The present inventors conducted experiments on various steels having different compositions, obtained multiple regression equations, and obtained the characteristic equations 1 and 2.
Among the characteristic formulas for adjusting the addition amount of each element, the characteristic formula 1 relates to the effective hardened layer depth, and the characteristic formula 2
Relates to surface hardness.

In the obtained characteristic expression 1, it is unexpected that Si acts in the negative direction and Mo acts in the positive direction, let alone the absolute value of the coefficient given to Si is the same as the coefficient given to Mo. It was unexpected that it would be about twice the absolute value. Although it was possible to predict that V acts in the positive direction and Cr acts in the negative direction, it was unexpected that V was about 3.5 times Cr in absolute value for each coefficient.

In the obtained characteristic equation 2, it is unexpected that C acts in the negative direction, Si acts in the negative direction, and Mn acts in the positive direction, and C which is said to act particularly in the direction of hardening the steel is negative. It was totally unexpected to have a coefficient of.

As described above, if the steel for soft nitriding satisfies not only the above respective component ranges but also the above characteristic expressions 1 and 2, the effective hardened layer depth after the soft nitriding treatment is deep, As a result of the research conducted by the present inventors, it was found that a steel having a hard surface can be obtained. In the present invention, in determining the blending amount of alloying elements, especially Cr, A which are elements for forming carbonitrides.
By adjusting the addition amount of l and V, it becomes possible to obtain steel having a high surface hardness of HV 800 or more. Above all, by adjusting the addition amount of V, the effective hardened layer depth is 0.25 mm or more. The steel can be obtained.

[0025]

EXAMPLES Examples 1 to 19 relating to the steel for soft nitriding of the present invention are shown in Table 1 below for the chemical components contained therein. Further, for comparison with the present embodiment, Comparative Examples 30 to 38, carburizing standard steel SCr420 (No. 39), JIS standard steel SACM6.
45 (No. 40 to 43), JIS Standard Steel SCM435
The chemical components contained in (No. 44, 45) are shown in Table 2 below. No. 40 shows the lower limit value of the SACM645 component standard range, and No. 41 shows the upper limit value.
o.42 and No.44 are those that conform to the characteristic formula 1 in the component standard ranges of the SACM645 and SCM453, respectively.
Reference numerals 43 and 45 show those which meet the characteristic formula 2.

[0026]

[Table 1]

[0027]

[Table 2]

As shown in Table 2, the chemical compositions of Comparative Examples and the like fall within the scope of the present invention, but one or both of the values (characteristic values) of the characteristic formulas 1 and 2 are satisfied. Absent. Next, a method of soft nitriding the test materials (steel for soft nitriding) having the chemical components shown in Tables 1 and 2 will be described.

First, the above-mentioned test materials excluding carburizing steel (No. 39) were hot forged into a round bar having a predetermined size, and the temperature was increased to 0.5 at 900 ° C.
Each test piece was prepared by carrying out normalizing for time and air cooling. Each test piece is α for notch rotation bending fatigue test.
= 2, a notch bottom diameter of 0.8 mmR, for a thrust type rolling fatigue test φ50 mm × 5 mm thickness, finished surface roughness 0.
A 2S test piece having a diameter of 10 mm was prepared as a soft nitriding property test piece.

Then, a soft nitriding treatment was performed at 570 ° C. for 10 hours in a gas atmosphere of NH ₃ : RX = 50: 50,
Air cooled.

For carburizing steel (No. 39), each test piece was subjected to carburizing treatment, oil quenching, and then 180 ° C.
It was tempered for 2 hours and then air-cooled. The carburizing treatment was performed at 925 ° C for the notched rotary bending fatigue test piece.
The thrust type rolling contact fatigue test piece was conducted at 930 ° C. for 8 hours.

As a heat treatment property test for nitrocarburizing of each of the treated steels, the effective hardened layer depth and the surface layer hardness were measured, and the rotary bending fatigue strength was measured using an Ono rotary bending fatigue tester. Then, the rolling fatigue strength was measured at a surface pressure of 5165 N / mm ² using a thrust type rolling fatigue tester. The Al ₂ O ₃ oxide cleanliness was measured by the JIS method. The measurement results are shown in Tables 3 and 4 below.

[0033]

[Table 3]

[0034]

[Table 4]

As can be seen from Tables 3 and 4, unlike the comparative example which does not satisfy the characteristic formula, a good soft nitrided steel was obtained in this example. That is, a mechanical structure having an effective hardened layer depth (depth of HV420 obtained) of 0.25 mm or more and a surface hardness of HV800 or more after soft nitriding treatment for about 10 hours and having a strength equal to or higher than that of a carburized material. Soft nitriding steel was obtained.

[0036]

As described above, in the nitrocarburizing steel of the present invention, since each chemical component is within the above range and satisfies the above characteristic equations 1 and 2, the nitrocarburizing steel is softened. When the nitriding treatment is performed, a steel having a deep effective hardened layer depth and a hard surface hardness is obtained. Therefore, in the mechanical structural parts that conventionally used the carburized material in terms of hardness, the thermal strain of the present invention The effect is that less nitrocarburized steel can be used.

Claims (4)

[Claims] 1. C: 0.15-0.6% by weight, Si:
0.03-0.5 wt%, Mn: 0.4-2.0 wt%, Cr: more than 1.00-3.0 wt%, V: 0.03-
0.4% by weight, Al: more than 0.5 to 1.2% by weight, N:
0.005 to 0.02% by weight, the balance consisting of Fe and inevitable impurities, oxygen in the inevitable impurities is 0.002% by weight or less, and the following characteristic expressions 1 and 2 are satisfied. A steel for soft nitriding characterized by: Characteristic formula 1: 0 <7.2-7.5Si-6.9Cr + 3.
8Mo + 23.8V Characteristic formula 2: 0 <-4.78-1.25C-0.38Si
+ 0.19Mn + 1.10Cr + 3.98V + 5.27
Al 2. Further, Mo: less than 0.5% by weight, Ni: 2
The steel for soft nitriding according to claim 1, which contains any one or more of the components by weight or less. 3. Further, Ti: 0.005 to 1.0% by weight,
The steel for soft nitriding according to claim 1 or 2, which contains any one or more of Nb: 0.005 to 0.3% by weight. 4. Further, S: 0.07 wt% or less, Pb:
0.15 wt% or less, Bi: 0.15 wt% or less, T
e: 0.06 wt% or less, Se: 0.06 wt% or less,
Zr: 0.12 wt% or less, Ca: 0.0005 to 0.
The nitrocarburizing steel according to any one of claims 1 to 3, which contains at least one selected from the group consisting of 01 wt%.