JPH0625797A - Steel for soft-nitriding - Google Patents

Abstract

PURPOSE:To provide steel for soft-nitriding having excellent surface hardness after soft-nitriding, effective hardening depth and core hardness as well as high static bending strength and fatigue strength. CONSTITUTION:This steel contains, by weight, 0.15 to 0.45% C, <=0.50% Si, 0.50 to 2.00% Mn, 0.50 to 2.00% Cr, 0.10 to 1.00% Mo, 0.02 to 0.50% V, <=0.030% Al, 0.0080 to 0.0200% N and <=0.0020% O and furthermore contains, at need, one or >=two kinds among <=0.30% S, <=0.30% Pb and 0.0005 to 0.0050% Ca, and the balance Fe with impurity elements.

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 used for machine structural parts, and more specifically, to a desired hardness obtained by a short soft nitriding treatment, and a static bending strength after soft nitriding. This relates to steel for soft nitriding, which has excellent fatigue strength.

[0002]

2. Description of the Related Art One of the surface hardening methods for mechanical structure parts which has a small heat treatment distortion is soft nitriding. In this method, a material to be treated is treated in a mixed gas of NH ₃ and RX gas at a temperature below the A _{1 1} transformation point, carbon is diffused into steel together with nitrogen, and the surface layer portion is hardened.

[0003]

In the above method, unlike the carburizing treatment, the treatment temperature is not high, and since quenching is not required, heat treatment strain is less likely to occur in the treated material. However, S45C and SCM43 have been used as conventional soft nitriding steels.
Although 5 and the like are often used, there is a problem in obtaining a desired hardness, and it is not suitable for soft nitriding steel for improving fatigue resistance. Therefore, the present invention improves the fatigue strength of mechanical structural parts after soft nitriding, and can be applied to structural members that require strain correction after soft nitriding, and can be applied to static bending. A steel for soft nitriding having excellent strength is provided.

[0004]

Means for Solving the Problems The inventor has found that the static bending strength, the fatigue strength, the surface hardness and the effective hardening depth after soft nitriding (distance corresponding to micro micro Vickers hardness Hv = 400) are determined. We have earnestly studied the relationship between alloying elements. As a result, by adding Cr and V in combination, the surface hardness becomes H
v600 or more and 750 or less, and the effective hardening depth is set to 0.
By keeping the thickness to 2 mm or more, holding Al within the range intended for deoxidizing agent and grain refinement, and adding N, the precipitation of aluminum nitride during soft nitriding is suppressed and the static bending strength after soft nitriding is suppressed. And new knowledge that fatigue strength can be improved was obtained.

The present invention has been completed based on the above findings, and the soft nitriding steel of the first invention has a weight ratio of C:
0.15 to 0.45%, Si: 0.50% or less, Mn:
0.50 to 2.0%, Cr: 0.50 to 2.0%,
Mo: 0.10 to 1.00%, V: 0.02 to 0.50
%, Al: 0.030% or less, N: 0.0080 to 0.
0200%, O: 0.0020% or less, balance F
The gist is to consist of e and an impurity element. The second invention is the same as the first invention, with S: 0.30% or less, Pb: 0.
30% or less and Ca: 0.0005 to 0.0050%
The gist is that one or more of these are contained and the balance is Fe and impurity elements.

Next, the reasons for limiting the components of the steel for soft nitriding of the present invention will be explained. C: 0.15 to 0.45% C is an element necessary to secure the strength of steel, and in order to secure this effect, it is necessary to contain at least 0.15% or more. However, if the content is more than 0.45%, the hardness after soft nitriding is lowered, and further the workability and toughness are lowered, so the upper limit was made 0.45%.

Si: 0.50% or less Si is usually added as a deoxidizing agent. However, if it exceeds 0.50%, the soft nitriding property is deteriorated and the workability is deteriorated. It was set to 50%.

Mn: 0.50 to 2.00% Mn is an element necessary for securing the strength of steel, and 0.50% or more is necessary to obtain the above effects. However, if the content exceeds 2.00%, the workability deteriorates, and the upper limit was made 2.00%.

Cr: 0.50 to 2.00% Cr is an element necessary for increasing the surface hardness after soft nitriding and increasing the hardening depth, and 0.50% for obtaining the effect.
The above contents are required. However, when the content is 2.00% or more, the surface hardness increases and the static bending strength decreases, so the upper limit was made 2.00%.

Mo: 0.10 to 1.00% Mo suppresses the decrease in toughness due to the soft nitriding treatment and increases the resistance to temper softening, and therefore improves the fatigue strength after the soft nitriding treatment. Therefore, it is 0.10 to obtain the effect.
% Or more is required. However, if the content exceeds 1.00%, the effect is saturated and the workability is lowered, so the upper limit was made 1.00%.

V: 0.02 to 0.50% V is an element necessary for increasing the surface hardness and effective hardening depth like Cr, and since it increases the resistance to temper softening, it is soft-nitrided. Improves fatigue strength afterwards. However,
If the content exceeds 0.50%, the effect is saturated and the workability is deteriorated, so the upper limit was made 0.50%.

Al: 0.030% or less Al is an element necessary for improving the hardness after the soft nitriding treatment, but in particular, it improves the surface hardness and therefore reduces the static bending strength and fatigue strength. . Therefore, the upper limit is 0.0
It was set to 30% or less.

N: 0.0080 to 0.0200% N is an element necessary for improving the strength of steel, and is used for suppressing surface hardness after soft nitriding and refining crystal grains. The physical strength and fatigue strength. Therefore, in order to obtain the effect, it is necessary to contain 0.0080% or more. Further, even if the content exceeds 0.0200%, the effect is saturated and the workability is deteriorated, so the upper limit was made 0.0200% or less.

O: 0.0020% or less O combines with Al to form Al ₂ O ₃ , which becomes a starting point of fatigue and causes a decrease in fatigue strength. It is necessary to reduce as much as possible, and the upper limit is 0.0020%. Below.

S: 0.30% or less, Pb: 0.30% or less S and Pb are elements necessary for improving the machinability of steel. However, if the content exceeds 0.30%, the machinability improves, but the hot workability and fatigue strength decrease, so the upper limits were made 0.30% or less.

Ca: 0.0005 to 0.0050% Ca is an element that improves machinability like S and Pb, and 0.0005% or more is necessary to obtain the effect. However, if the content exceeds 0.0050%, the manufacturability and fatigue strength decrease, so the upper limit was made 0.0050% or less.

[0017]

EXAMPLES Next, the characteristics of the steel of the present invention will be clarified by examples compared with the conventional steel and the comparative steel. Tables 1 and 2 show the chemical composition of these test materials. In Tables 1 and 2, 1 to 5 steels are 1st invention steels, 6 to 10 steels are 2nd invention steels,
11 to 16 steels are comparative steels, 11 steel is a comparative steel having a high Cr content, 12 steel is a comparative steel having a high Al content, 13 steel is a comparative steel having a low Cr content, and 14 steel has a Mo content. Low comparative steel, 15 steel is a comparative steel with low Mn content, 16 steel is S
It is a conventional steel equivalent to CM435H.

After quenching and tempering these test materials, a three-point bending test piece (ρ = 3.
0, α = 1.60) and a Ono-type rotary bending test piece (ρ = 1.0, α = 1.78) for fatigue test, 5
The sample was subjected to a gas soft nitriding treatment at 80 ° C. for 4 hours and then provided. The static strength was defined as the static bending stress, which was the stress until the initial cracking. In addition, the surface hardness (mHv), effective hardening depth (mm), and core hardness (Hv) were measured at the parallel part of the three-point bending test piece after the test. Table 3 shows the measured results.

As is clear from Table 3, since Comparative Steel 11 and Comparative Steel 12 both have high surface hardness after soft nitriding treatment, the static bending strength is low, and Comparative Steel 13 has surface hardness after soft nitriding treatment. The comparative steels 14 and 15 have low core hardness, and therefore have low fatigue strength, and the conventional steel, 16 steel, has low surface hardness after nitrocarburizing. For,
Fatigue strength is extremely low. On the other hand, the steel of the present invention is 1
The surface hardness of each of the No. 10 to 10 steels is Hv63 after the soft nitriding treatment.
0 to 740, effective hardening depth of 0.20 mm or more, and core hardness of 300 to 370 are high, so fatigue strength is 58 to 80 Kgf / mm ² superior to conventional steel and static bending strength. It was also revealed that it was excellent at 115 to 145 Kgf / mm ² .

[0020]

As described above, the soft nitriding steel of the present invention has a surface hardness after soft nitriding of Hv 600 to 750 and an effective effect depth of 0. 0 by optimizing Cr, V, Al and N.
It is a steel adjusted to 20 mm or more to improve static strength and fatigue strength. Therefore, it is extremely useful for obtaining mechanical structural parts that are excellent in strain correction and fatigue strength after nitrocarburizing.

[Table 1]

[Table 2]

[Table 3]

Claims (2)

[Claims] 1. A weight ratio of C: 0.15 to 0.45.
%, Si: 0.50% or less, Mn: 0.50 to 2.00
%, Cr: 0.50 to 2.00%, Mo: 0.10 to
1.00%, V: 0.02-0.50%, Al: 0.0
30% or less, N: 0.0080 to 0.0200%, O:
A steel for soft nitriding, containing 0.0020% or less, and the balance being Fe and impurity elements. 2. A weight ratio of C: 0.15 to 0.45.
%, Si: 0.50% or less, Mn: 0.50 to 2.00
%, Cr: 0.50 to 2.00%, Mo: 0.10 to
1.00%, V: 0.02-0.50%, Al: 0.0
30% or less, N: 0.0080 to 0.0200%, O:
Contains 0.0020% or less, and further S: 0.30%
Below, Pb: 0.30% or less and Ca: 0.0005
A steel for soft nitriding, containing one or more of 0.0050%, and the balance being Fe and impurity elements.