JPH08176733A - Steel for soft nitriding - Google Patents

Abstract

PURPOSE: To provide a steel for low-strain soft nitriding having as-hot-rolled or as-cast machinability and an excellent soft nitriding characteristic. CONSTITUTION: The steel having a compsn. contg. 0.15 to 0.40% C, <=1.20% Si, 0.60 to 1.80% Mn, 0.20 to 2.00% Cr, 0.02 to 0.10% Al, 0.006 to 0.020% N, 0.05 to 0.20% V, further, contg. >=1 kinds among 0.05 to 0.40% Mo, 0.005 to 0.020% Ti, 0.0005 to 0.0050% B, 0.005 to 0.060% S, 0.02 to 0.20% Pb, 0.005 to 0.010% Ca and having the conditions of 0.60<=C+0.1Si+0.2Mn+0.25Cr+1.65V +0.55Mo+0.20Ti+8B<=1.35, and 0.25Cr+1.15Mo+2V+1.5Ti<=0.85 and the balance Fe and inevitable impurities is used. The steel is cooled after hot rolling or hot forging, by which the steel having core part hardness Hv of 200 to 300 is obtd. without a heat treatment. The steel having the structure consisting of the structure of ferrite + pearlite or the mixed structures of ferrite + pearlite (+ bainite) of a bainite fraction of <20% is obtd.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a soft nitriding steel, especially a civil engineering machine, which is capable of achieving a high surface hardness, a deep hardening depth, and a reduction in heat treatment strain by performing a soft nitriding treatment.
Wear resistance, fatigue resistance, used in industrial machinery, etc.
The present invention relates to a soft nitriding steel for machine structural parts, which has excellent machinability and heat treatment strain characteristics.

[0002]

2. Description of the Related Art Carburizing treatment and nitriding treatment are typical representative surface hardening treatment methods that have been put to practical use. The carburizing process penetrates and diffuses C in the high temperature γ range, so that a deep hardening depth can be obtained, but since heat treatment of quenching and tempering is required after carburizing, it is a machine such as a gear that requires strict dimensional accuracy. For structural parts, heat treatment distortion was a problem, and seizure resistance and galling resistance were also problems.

On the other hand, the nitriding treatment is performed at a temperature of 500 to below the A ₁ transformation point.
This is a treatment that penetrates and diffuses N in the temperature range around 550 ° C to improve high surface hardness and wear resistance and seizure resistance. Since this method does not involve phase transformation due to quenching, there is no problem of heat treatment distortion, but usually the treatment time is extremely long at 50 to 100 hr, and it is necessary to remove the brittle compound layer on the surface even after the treatment. I had a problem with.

On the other hand, the soft nitriding treatment is generally performed at 500
N and C are simultaneously penetrated and diffused in the temperature range of ~ 600 ℃,
This is a treatment for surface hardening, which requires about half the treatment time compared to nitriding treatment and has less heat treatment distortion.
In recent years, it has been rapidly spreading to machine structural parts and the like.

However, sufficient surface hardness and hardening depth have not been obtained in the structural carbon steels and low alloy steels that have been used for nitrocarburizing at present, so that pitting resistance, spalling resistance, fatigue It is a big problem because the sex is not enough. Further, in recent years, there has been a strong demand for omitting steps, and it has been demanded to omit annealing and tempering, which have been performed for soft-nitriding steel to improve machinability before soft-nitriding.

In order to solve such a problem, some means have been proposed so far. For example, in JP-A-63-216950, the surface hardness and the hardening depth are limited by the limitation of the chemical composition. The quality is improved and the quality is improved.

However, the above method has a wide composition range and it is difficult to achieve its purpose, and the problem of heat treatment strain after soft nitriding treatment occurs, and it cannot be applied to a member requiring high precision.

[0008]

The object of the present invention is to solve the problems of the prior art as described above, and to achieve excellent machinability and hot nitriding characteristics in the as-hot-rolled or forged state (the step is omitted). The purpose is to provide a steel for strain soft nitriding.

[0009]

Here, the present inventors
In order to achieve the above-mentioned object, various studies have been repeated and the following findings have been obtained to complete the present invention.

Measures for improving soft nitriding characteristics By adding Cr, V, and Al in optimum amounts, the surface hardness and the hardening depth can be improved, and the soft nitriding treatment time can be shortened.

Measures for Improving Heat Treatment Strain Characteristic Before heat treatment after soft nitriding treatment, the heat treatment strain is very small, which is a structure of ferrite + pearlite or a structure of ferrite + pearlite + bainite with a bainite fraction of less than 20%.
The structure can be controlled by adjusting r, Mo, V, Ti, etc. As a result, it becomes possible to omit steps such as polishing after the heat treatment that has been performed for improving the dimensional accuracy.

Process abbreviation Cooling after hot rolling or hot forging by adjusting C, Si, Mn, Cr, V, Mo, B, etc., without heat treatment, good balance of mechanical properties / cuttability Core hardness Hv200-300
The above state is obtained, and the steps of annealing and tempering that have been performed for hardness adjustment can be omitted, contributing to cost reduction. Further, addition of S, Pb, and Ca is effective for further improving the machinability. From these measures, according to the present invention, it is possible to shorten the soft nitriding treatment time and reduce the cost.

Here, the gist of the present invention is that, in weight%, C: 0.15 to 0.40%, Si: 1.20% or less, M
n: 0.60 to 1.80%, Cr: 0.20 to 2.00%, Al: 0.02 to 0.1
0%, N: 0.006 to 0.020%, V: 0.05 to 0.20%, Mo: 0
~ 0.40%, Ti: 0 ~ 0.020%, B: 0 ~ 0.0050%, S: 0
~ 0.060%, Pb: 0 ~ 0.20%, Ca: 0 ~ 0.010%, and 0.60≤C + 0.1Si + 0.2Mn + 0.25Cr + 1.65V + 0.55Mo + 0.20Ti + 8B≤1.35 ... (1) and 0.25Cr + 1. 15Mo + 2V + 1.5 Ti ≦ 0.85 ・ ・ ・ (2) Having a steel consisting of balance Fe and unavoidable impurities, hot rolling or hot forging followed by cooling, without heat treatment, core hardness Hv200-300, Ferrite + pearlite with a structure of ferrite + pearlite or bainite fraction less than 20%
It is a steel for soft nitriding characterized by having a mixed structure of (+ bainite) and having a high surface hardness, a deep hardening depth, and a low heat treatment strain characteristic by subjecting it to a soft nitriding treatment.

Thus, according to the present invention, it is possible to shorten the soft nitriding treatment time and reduce the cost. According to a preferred embodiment of the present invention, it is as follows.

(1) C: 0.15 to 0.40% by weight, S
i: 1.20% or less, Mn: 0.60 to 1.80%, Cr: 0.20 to 2.0
0%, Al: 0.02-0.10%, N: 0.006-0.020%,
V: steel containing 0.05 to 0.20%, balance Fe and unavoidable impurities, and 0.60 ≦ C + 0.1 Si + 0.2 Mn + 0.25Cr + 1.65 V ≦ 1.
35 and 0.25Cr + 2V ≦ 0.85 steel is used, hot rolling or hot forging is followed by cooling, without heat treatment, core hardness is Hv200-300, structure is ferrite + pearlite or bainite fraction Is 20
% Of ferrite + pearlite (+ bainite) mixed structure, and by subjecting it to soft nitriding treatment, it has high surface hardness, deep hardening depth, and low heat treatment strain characteristics. steel.

(2) The steel composition, in% by weight, Mo: 0.05-
0.40%, Ti: 0.005 to 0.020%, B: 0.0005 to 0.0050%
0.60 ≦ C + 0.1 Si + 0.2 Mn + 0.25Cr + 1.65V + 0.
55Mo ＋ 0.20Ti ＋ 8B ≦ 1.35, and 0.25Cr ＋ 1.15Mo ＋ 2V
The steel for soft nitriding according to (1) above, which has a condition of + 1.5Ti ≤ 0.85.

(3) The steel composition, in wt%, S: 0.005 to
0.060%, Pb: 0.02 to 0.20%, Ca: 0.005 to 0.010% One or more of the above (1) or (2)
Steel for nitrocarburizing.

[0018]

Next, the reason for limiting the steel composition and structure in the present invention as described above will be described in detail together with its operation. In the present specification, unless otherwise specified, “%”
Is "% by weight".

C: 0.15 to 0.40%: C is an element necessary to secure hardenability and form a solid solution in the matrix to secure core strength. For this purpose, it is necessary to contain 0.15% or more, but if it exceeds 0.40%, the hardenability increases and the toughness decreases, and the machinability decreases significantly. Therefore, the range is 0.15 to 0.40%.

Si: 1.20% or less: Si is added as a deoxidizing agent, but it shows solid solution strengthening, and if it exceeds 1.20%, toughness and machinability are deteriorated, so it is set to 1.20% or less. The lower limit is not particularly limited, but is preferably 0.20% or more. More preferably, Si: 0.20 to 1.00%.

Mn: 0.60 to 1.80%: Mn is indispensable as a deoxidizing agent, and is an element effective in ensuring the strength of the core,
0.60% in relation to other elements to secure core hardness
The above is necessary. Further, if over 1.80%, the workability and machinability are impaired, so the range was made 0.60 to 1.80%.

Cr: 0.20 to 2.00%: Cr improves the strength of the core and, with regard to soft nitriding, it is an element that increases the surface hardness / hardening depth as more is added, but if it is less than 0.20%, it is soft. It is not possible to obtain the effect of improving the nitriding property and the core strength,
On the other hand, if it exceeds 2.00%, a strong soft nitriding layer is formed on the surface, so that the hardening depth decreases. Therefore, 0.20 to 2.00
The range is%. Preferably, it is more than 0.50% and not more than 1.50%.

Al: 0.02 to 0.10%: Al is used as a deoxidizer during melting, and is an element effective for increasing the surface hardness by combining with penetrating N by soft nitriding and deepening the hardening depth. .
In order to exert this effect, the content must be 0.02% or more, while if it exceeds 0.10%, a strong soft nitriding layer is formed on the surface, and conversely the hardening depth decreases. Therefore, 0.02 ~
The range was 0.10%. It is preferably 0.02 to 0.08%.

N: 0.006 to 0.020%: N refines the crystal grain size and improves the toughness of the core. For this, 0.
It is necessary to contain 006% or more. If it exceeds 0.02%, the formation of V-nitride becomes remarkable and the toughness starts to deteriorate. Therefore, the range is 0.006 to 0.020%. Preferably, it is 0.006 to 0.018%.

V: 0.05 to 0.20%: V improves the hardenability and also improves the surface hardness and the surface depth by combining with N and C during soft nitriding to precipitate fine V carbonitrides. . In particular, since it greatly contributes to the increase in the hardening depth, it is extremely effective in fatigue resistance and the like. In order to obtain this effect, 0.05% or more is necessary, but if it exceeds 0.20%, it is combined with the contained N and coarse V nitrides are precipitated to deteriorate the core toughness. Therefore, the range is 0.05 to 0.20%. It is preferably 0.07 to 0.20%.

The nitrocarburized steel according to the present invention contains the above elements as essential components, but if necessary, one or more selected from the group consisting of Mo, Ti, and B in the following addition amounts, and / Or one or more selected from the group consisting of S, Pb, and Ca may be contained.

Mo: 0.05 to 0.40%: Mo is an element effective for ensuring good hardenability and at the same time improving toughness. In order to further improve the hardenability of the steel of the present invention, Mo
Is preferably contained at 0.05% or more. However, 0.40%
If it exceeds 1.0, the hardenability will rather deteriorate.
The range was 0.40%. Preferably, it is 0.05 to 0.35%.

Ti: 0.005 to 0.020%: Ti has the effect of improving the hardenability and improving the hardenability effect of B when B is added. For this purpose, it is necessary to contain 0.005% or more, but if it exceeds 0.020%, the effect is saturated and the machinability deteriorates. Therefore, the range was made 0.005 to 0.020%.

B: 0.0005 to 0.0050%: When a small amount of B is added, the hardenability is improved and the core strength is improved. In order to further improve the hardenability of the steel of the present invention, at least
0.0005% or more is required, but if it exceeds 0.0050%, the hardenability rather deteriorates, so the range was made 0.0005 to 0.0050%. Preferably, it is 0.0005 to 0.0030%.

S: 0.005 to 0.060%, Pb: 0.02 to 0.20
%, Ca: 0.0050 to 0.010%: S, Pb, and Ca are all elements for improving machinability. Further, in order to improve the machinability of the steel of the present invention, these elements should be at least S:
0.005%, Pb: 0.02%, Ca: 0.005% or more, at least 1
It is preferable to contain a seed. However, even if added in excess of the above upper limit, no significant improvement effect on machinability is recognized, which rather reduces toughness. Therefore, S: 0.005 to
0.060%, Pb: 0.02-0.20%, Ca: 0.0050-0.010%.

Formula (1) or (2): 0.60 ≦ C + 0.1 Si + 0.2 Mn + 0.25Cr + 1.65V (+ 0.55Mo + 0.20Ti + 8B) ≦ 1.35・ ・ ・ [ Machinability improvement index] After hot rolling or hot rolling The hardness is adjusted so that cutting can be performed after forging, and the structure is adjusted so that the soft nitriding property is good.
Therefore, when the above-mentioned machinability improvement index is less than 0.60, mechanical properties are deteriorated due to insufficient hardness. Also, 1.35
If it exceeds the limit, the machinability will decrease due to the increase in hardness, and the strain characteristics will deteriorate, so 0.60 ≦ C + 0.1 Si +
0.2 Mn ＋ 0.25Cr ＋ 1.65V (＋ 0.55Mo ＋ 20Ti ＋ 8B) ≦ 1.
The range was 35.

0.25Cr + 2V (+ 1.15Mo + 1.5Ti) ≦ 0.85 ... [Structural control index] Cr, Mo, V, and Ti are elements having a strong ability to form carbides, but these elements are often retained in the grain boundaries. Since they are deposited and form carbides during cooling, the movement of the C element necessary for transformation is delayed, and CC
It has the feature that the ferrite + pearlite nose on the T curve is shifted to the long side. Therefore, the CCT curve has a shape in which the bainite nose pops out to the short side, and as a result, it affects the ratio of ferrite, pearlite, and bainite. The structure capable of reducing the strain after hot rolling or hot forging is ferrite + pearlite or ferrite + pearlite + bainite <less than 20%>, so it is necessary to adjust to have that structure. Therefore, if it exceeds 0.85 in the above formula, the bainite fraction increases, the target structure cannot be obtained, and the strain characteristics deteriorate, so 0.25Cr + 1.15Mo + 2V + 1.5Ti ≤
The range was 0.85.

Hv 200 to 300 : Machine structural parts (for example, in the case of gears) are cut after hot rolling or hot forging. Therefore, when Hv is 300 or more, it becomes difficult to perform cutting and it takes a long time. , Increase costs. Further, when the hardness for securing the core strength is Hv 200 or less, the core strength becomes low, which causes a decrease in fatigue strength. Therefore, Hv
The range was 200-300.

Ferrite + perlite or ferrite
+ Pearlite (+ bainite <20%>) When high-precision members (such as gears) are soft-nitrided, the amount of strain after processing greatly differs depending on the anterior structure. In the case of ferrite + pearlite or ferrite + pearlite + bainite <less than 20%>, residual stress is remarkably small due to reduction of supersaturated solid solution element, stabilization of structure, etc., and heat treatment strain due to residual stress release after soft nitriding treatment is small. for that reason,
It is possible to reduce the noise of the sliding part and to simplify the process such as polishing correction after soft nitriding treatment. Therefore, ferrite + perlite or ferrite + perlite (+ bainite <20
% Less than>). Here, the bainite fraction means the area ratio on the cross section.

The soft nitriding steel according to the present invention described above is directly cut into a predetermined shape and then subjected to a conventional soft nitriding treatment. The soft nitriding method performed on the steel according to the present invention is not particularly limited.

[0036]

【Example】

(Example 1) Sample steel Nos. 1 to 42 having the chemical compositions shown in Tables 1 and 2 were melted, and each was made into a 160 mm square piece, which was heated to 1100 ° C, and the finishing temperature After hot forging at 950 ° C to form a round bar with a diameter of 30 mm, it was allowed to cool. Conventional steels 41 and 42 were also water-cooled and tempered at 650 ° C for 1 hr.

After cooling each round bar, a JIS No. 4 tensile test piece, JI
A S3 Charpy impact test piece and a machinability test piece (diameter 30 mm x length 35 mm) were sampled, and a tensile test, a Charpy impact test and a machinability test were conducted to investigate the properties after forging. Regarding the Charpy impact test piece, the hardness of the base material (hardness of the core) was measured after the impact test was performed, and the microstructure was also observed.

In the machinability test, a machinability test using a drill was performed. The tool uses a Ti coating straight drill with a diameter of 10 mm, performs dry cutting with a feed rate of 0.15 mm / rev,
The life was judged by the cutting depth at which cutting was impossible.

Further, in order to investigate the characteristics after the soft nitriding treatment, a hardness measuring test piece (diameter 30 mm × length 35 mm), a JIS No. 2 rotating bending fatigue test piece, and a strain test piece (outer diameter 25 mm <inner diameter 20 mm>
× Thickness 5 mm ··· Fig. 1) was sampled and NH ₃ gas: RX gas =
570 ° C. × 4 hr → oil-cooled gas soft nitriding treatment was performed in a 1: 1 mixed gas, and hardness measurement, fatigue test, and strain amount measurement were performed.

The measurement results after forging are shown in Table 3. As is clear from the results shown in Table 3, the invention steels of sample steel Nos. 1 to 18 satisfy the target values for the core hardness, microstructure, tensile strength, absorbed energy, and tool life. There is. This is the same property as conventional steel Nos. 41 and 42 (tempered).

On the other hand, test steel Nos. 19 to 40 and conventional steel No. 41,
Of the 42 (cooled) comparative steels, both the machinability improvement index and the microstructure control index are satisfied, and the C, Mn, Cr, and V are less than specified (No. 19, 22, 24, 26) and conventional steels. (No.41, 42)
Has a ferrite + pearlite structure, but the core hardness is greatly reduced, and the tensile strength is also outside the target.

Steel containing more than C, Si, Mn, Cr, V, Mo
In (No. 20, 21, 23, 25, 27, 32), the microstructure is a bainite structure, but the core hardness greatly increases and deviates from the target hardness. These steels whose core hardness has greatly increased do not meet the targets for absorbed energy and tool life.

Steel containing more than Al and N (No. 29, 31, 3)
In 3), the core hardness, microstructure, and tool life are satisfied, but the Charpy absorbed energy is greatly reduced, and deterioration of toughness is observed.

Further, in the steels (No. 35, 37) which satisfy the chemical composition but have the machinability improvement index lower than the regulation, the structure becomes ferrite + pearlite, but the core hardness greatly decreases,
You miss your goal.

In the steel (No. 36) having a machinability index higher than the standard, the microstructure also becomes bainite, the hardness of the core is greatly increased, and the absorbed energy and tool life do not meet the target.

Table 4 shows the measurement results after the soft nitriding treatment. As is clear from the results shown in Table 4, the sample steels of the present invention Nos. 1 to 18 satisfy the target values for surface hardness, hardening depth, fatigue strength, and strain amount. This is conventional steel No. 38, 39 (tempered)
A remarkable improvement in the characteristics was recognized as compared with.

On the other hand, test steel Nos. 19 to 40 and conventional steel No. 41,
Among the 42 (cooled) comparative steels, steels with less Cr, V, and Al than specified (No. 24, 26, 28, 33) and conventional steels (No. 41, 4)
In 2), the surface hardness, the hardening depth, and the fatigue strength were all low, which was outside the target. In addition, steels with more Cr, V, and Al than specified (No. 25, 27, 29, 34) have a high surface hardness, but the hardening depth is shallow and the fatigue strength is low, so the target values cannot be satisfied. .

Steel not satisfying the structure control index (No. 38,
39, 40) cannot satisfy the target of strain amount because the structure cannot satisfy ferrite + pearlite or ferrite + pearlite + bainite (less than 20%). In addition, although the steel satisfies the regulations, it is out of the composition standard (No. 20, 21, 2).
3), 25), 27), and 32), the target tissue cannot be obtained and the strain target value cannot be reached.

Example 2 Test Steel N shown in Tables 1 and 2
o. 1, 5 160 mm square steel slabs are heated to 1100 ° C and finished at a finishing temperature of 9
After hot forging at 50 ° C., cooling is performed in the heat patterns shown in FIGS. 2 (a) to 2 (d), water cooling is performed, and salt bath retention is performed. The properties after hot forging and the properties after nitrocarburizing were investigated.

Tables 5 and 6 collectively show the measurement results together with the heat patterns. Ferrite + pearlite + bainite with a bainite fraction exceeding 20% satisfies core hardness, Charpy impact absorption energy and tool life, but has low surface hardness and hardening depth after soft nitriding treatment, fatigue strength, The amount of distortion has not reached the target.

Martensite has a high hardness of the core portion, the tool life is deteriorated, and the strain amount cannot reach the target value.
If the core hardness is low even with ferrite + pearlite,
Low tensile strength and surface hardness / hardening depth after soft nitriding,
The fatigue strength cannot reach the target value.

However, if the core hardness of ferrite + pearlite or ferrite + pearlite + bainite (less than 20%) is satisfied, the tool life is long and the surface hardness, hardening depth and fatigue strength after nitrocarburizing treatment are long. Furthermore, the amount of distortion can be reduced.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

[Table 3]

[0056]

[Table 4]

[0057]

[Table 5]

[0058]

[Table 6]

[0059]

EFFECT OF THE INVENTION The soft nitriding steel according to the present invention does not require heat treatment such as annealing and tempering when it is processed as a machine structural part, and the processing cost is low. It has surface hardness / hardening depth and fatigue strength, and can further reduce heat treatment distortion. It aims to improve the performance of high-precision gears and lower costs.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a shape of a strain test piece used in an example.

2 (a) to 1 (d) are schematic explanatory views of each heat pattern of Example 2. FIG.

Claims (3)

[Claims] 1. By weight%, C: 0.15 to 0.40%, Si: 1.20% or less, Mn: 0.60
~ 1.80%, Cr: 0.20 ~ 2.00%, Al: 0.02 ~ 0.10%, N: 0.006
〜0.020%, V: 0.05〜0.20%, balance Fe and unavoidable impurities, and 0.60 ≦ C + 0.1Si + 0.2Mn + 0.25Cr + 1.65V ≦ 1.
35, and steel with the condition of 0.25Cr + 2V ≦ 0.85, hot rolling or hot forging and then cooling, without heat treatment, core hardness Hv200-300, structure ferrite + pearlite or bainite fraction Is 20
% Of ferrite + pearlite (+ bainite) mixed structure, and by subjecting it to soft nitriding treatment, it has high surface hardness, deep hardening depth, and low heat treatment strain characteristics. steel. 2. The steel composition, in% by weight, Mo: 0.05-0.
40%, Ti: 0.005 to 0.020%, B: 0.0005 to 0.0050%, and 0.60≤C + 0.1Si + 0.2Mn + 0.25Cr + 1.65V + 0.
55Mo ＋ 0.20Ti ＋ 8B ≦ 1.35, and 0.25Cr ＋ 1.15Mo ＋ 2V
The soft nitriding steel according to claim 1, wherein the condition is + 1.5Ti≤0.85. 3. The steel composition, in wt%, S: 0.005 to 0.
The steel for soft nitriding according to claim 1 or 2, which contains one or more of 060%, Pb: 0.02 to 0.20%, and Ca: 0.005 to 0.010%.