JPS5916948A - Soft-nitriding steel - Google Patents

Abstract

PURPOSE:To obtain the soft-nitriding steel esp. excellent in the toughness of its core part after being soft-nitrided and the ductility of a face hardened layer and having sufficient fatigue strength, by making the steel contain the predetermined ratio of each of C, Si, Mn, Cr, V, soluble Al and N. CONSTITUTION:The soft-nitriding steel comprising, by wt%, 0.15-0.50% C, Si <=1.20%, 0.60-1.30% Mn, Cr <0.20%, 0.05-0.20% V, soluble Al <=0.10%, 0.006- 0.020% N and the balance Fe and inevitable impurities. In said composition, the addition amounts of Cr and Al are remarkably limited below the levels of conventional soft-nitriding steel, to obtain proper face hardness (Hv 500-600) and to remarkably improve the ductility of a face-hardened layer at the same time. The reduction of hardness depth caused by limiting the addition amount of Cr is sufficiently compensated by adding a relatively large amount of V which exhibits the effect to improve fatigue strength, too.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a steel for soft nitriding, and particularly to a steel for soft nitriding which has excellent core toughness after softening treatment, excellent ductility of a hardened surface layer, and sufficient fatigue strength.

Soft-nitriding treatment is carried out at a temperature below the transformation point, generally around 570°C, for example, using a salt bath of a nitrogen compound, RX gas (endothermic modified gas) or NX gas (exothermic modified gas), etc. This is a type of surface hardening method in which a treated material is treated to allow some carbon to enter the steel along with nitrogen to harden the surface layer.

This method is suitable for mass production of mechanical parts, etc., because it does not cause large distortions in the workpiece like the carburizing-quenching method, nor does it require a long time like the nitriding method. Steel for soft nitriding as a steel type suitable for this purpose has not yet been sufficiently developed, and so far no steel has been found that can obtain the desired properties with short-time soft nitriding treatment.

For example, JIS steel for soft nitriding, which is commonly used
-3CM435 (0,35C-0,75Mn-1,1
Cr-0,2Mo) and SACM 645 (0,45C
-0,45i-1,5Cr -0,2Mo -1,OA
7). In addition, nitrocarburizing steels made by adding V to Cr-based case hardening steels have been proposed, and foreign standards (Al5I6
118) is also available.

Because nitrocarburizing is a low-temperature process, the heat treatment strain is similar to that of carburizing.
Although the amount of distortion is very small compared to hardening methods, it is impossible to completely eliminate this distortion, and considerable distortion occurs especially in asymmetrically shaped processed materials. For this reason, the material to be treated is usually subjected to a cold straightening process that involves slight plastic deformation after the nitrocarburizing treatment, but when this process is applied to the conventional steel described above, fine racks are produced on the surface. This is a halo to conventional steel. 2% or more C
r is added, and especially in SACM 645, a large amount of A7= is added, so C is added to the outermost layer.
This is because a very hard and brittle compound layer containing r-nitride or A nitride is generated, and although the wear resistance is improved, the ductility of the surface layer is seriously deteriorated.

On the other hand, if soft nitriding is applied to carbon steel that does not contain Cr, cracks are less likely to occur during cold straightening, but the surface hardness increases and the Vickers hardness (at a point 0.025111ffl below the surface) is Hv450. Since the results were only as follows, the desired improvements in wear resistance and fatigue strength could not be expected.

In addition, the above-mentioned SACM 6 containing a large amount of Cr and At
In the case of No. 45, in addition to the above-mentioned problem of cracks occurring during straightening, the hardness gradient from the surface to the core becomes too steep after the soft nitriding treatment, causing problems such as gears and bearings used under high loads. In this case, peeling phenomenon easily occurs near the boundary between the hardened surface part and the core part, and the fatigue strength If.

There were also problems in pitting resistance or spalling resistance. This sharp hardness gradient is due to the fact that the surface hardness becomes extremely high (Hv800-1100) due to the high content of Cr and At, while the effective hardening depth and Vickers hardness Hv
= distance from the surface corresponding to 400) is at most 15
Depends on the degree of blasphemy and smallness.

Therefore, the object of the present invention is to provide a soft-nitriding material that has excellent wear resistance, fatigue strength, and pitting resistance, has improved ductility of the hardened layer, and is less prone to cracking during cold straightening after soft-nitriding treatment. It is to provide steel.

As a result of continuing research on steel for soft nitriding for the above purpose, the present inventors found that by significantly limiting the amount of Cr and kL added compared to the level of conventional steel for soft nitriding, a suitable surface hardness can be achieved.
It was found that the ductility of the hardened surface layer was significantly improved at the same time as the hardened surface layer was obtained. This is Cr
and M precipitates fine Cr carbonitrides and At nitrides in the surface layer during the soft-nitriding process, resulting in a significant improvement in the hardness of the surface layer, while the outermost layer contains Cr nitrides and M nitrides. It is believed that limiting the amount of these elements produces the above results because a very brittle compound layer is formed, severely degrading the ductility of the hardened surface layer. On the other hand, the decrease in hardening depth caused by limiting the amount of Cr added can be sufficiently compensated for by adding more than 0.05% V, which is effective in increasing the hardening depth, and can also improve fatigue strength. It was also found to be sufficiently effective. ■Carbide precipitates in large quantities in the outermost layer and in areas a little deeper into the interior; therefore, by adding carbide, a large hardening depth can be obtained without deteriorating the ductility of the hardened surface layer. be able to. As a result, the hardness gradient from the surface becomes gentler, and fatigue strength and pitting resistance are also improved.

In addition, when particularly high fatigue strength is required, the hardenability can be improved by adding B or Mo, and the processing (hot rolling, hot forging) or heat treatment (normalizing) before nitrocarburizing can be performed. It has also been found that fatigue strength can be further improved by making the subsequent structure into fine pearlite or bainite and increasing the core hardness.

Furthermore, when cutting is performed before soft-nitriding treatment, it is preferable to add s, pb, or Ca, which are effective in improving machinability.

Here, the present invention includes C: (1.15 to 0.50%, Si:
1.20% or less.

Mn: 0.60-1.30%, Cr: 0.2
Less than 0%.

V: 0.05-0.20%, sol, A
t: 0.10% or less.

N: 0. (X) 6-0.020%.

Furthermore, if necessary, B: 0.0005-0.00
50 CH and] VIo: 0.05-0.25% of one or two kinds, and/or S: 0.04-0.
13 pieces, Pb: 0.03-0.35% and Ca:
The steel for soft nitriding contains one or more of 0.0010 to 0.0100%, with the balance being Fe and unavoidable impurities.

The reason why the composition of the steel according to the present invention is limited within the above range will be explained next.

C: C is a basic component for ensuring strength, and a minimum content of 0.15% is required to ensure core strength.

However, if it exceeds 0.50%, the ductility and toughness of the core decrease, machinability and cold workability decrease, and the surface hardness and hardening depth after nitrocarburizing begin to decrease rapidly. Therefore, the lower limit of the amount of C in the present invention is 0.15 cm, and the upper limit is 5 cm.
It was set to 0.

Sl: St is usually added as a deoxidizing agent, but it is also effective in improving solid solution strengthening and temper softening resistance, and as a result increases the core hardness after soft nitriding treatment. Therefore, the higher the amount added, the better, but if it exceeds 1.20%, the soft-nitriding properties begin to deteriorate. In particular, since the decrease in surface hardness becomes significant and also harms cold workability and weldability, the upper limit was set at 1.20%.

Mn: Mxi is essential as a deoxidizing agent during steel manufacturing, and is also effective in improving the strength and toughness of the core, and is the minimum required to ensure the performance of soft-nitrided products. , 6° is necessary. However, if it exceeds 1.30%, the machinability begins to deteriorate significantly, so the lower limit is set at 0.60% and the upper limit is set at 1.30%.
And so.

Cr: Cr is an extremely effective element that combines with N intruded by nitrocarburizing to increase surface hardness and hardening depth. Therefore, in order to improve wear resistance and fatigue strength, it is desirable to add multiple amounts of Ni, but if it exceeds 0.20%, the ductility of the hardened surface layer begins to deteriorate rapidly. Therefore, during cold straightening, which is usually performed to remove the heat treatment strain generated during nitrocarburizing, cracks are likely to occur on the surface due to elastic deformation and plastic deformation associated with processing. , less than 20%.

V: V combines with the intruded N and C introduced by nitrocarburizing to precipitate fine vanadium carbides, thereby improving surface hardness, surface hardness, and surface depth.

In particular, compared to Cr, V has a relatively small contribution to increasing the surface hardness and a large contribution to increasing the hardening depth. The decrease in is small. Make full use of this effect,
To expect an improvement in fatigue strength, an amount of V greater than 05 cm is required. However, even if added in excess of 0.20%,
Not only does the effect not improve any further, but the ductility of the hardened surface layer also begins to deteriorate, so if Vi is 0.05 inch or more, 0.
It was set to 20 inches or less.

sol, At: Like Cr, AL is also an element effective in bonding with intruding N to increase surface hardness. In particular, when added in combination with (2), an interaction between V and At occurs, promoting the effect of increasing the effective hardening depth. That is, the moderate surface hardness improving effect of At and the hardening depth improving effect of (2) are superimposed, and the hardening depth is further increased. However, 0.1
If it exceeds 0%, the hardening depth will actually begin to decrease, and the ductility of the hardened surface layer will also rapidly deteriorate, making it easier for cracks to occur on the surface during straightening, so the upper limit should be set at 0.10%.
%.

N: N refines the grain size and improves the toughness of the core. For this purpose, 0.006% or more is required, but if it exceeds 0.020%, the formation of V nitride in the core will become noticeable, and the toughness of the core will begin to deteriorate, so the lower limit should be set. 0.006%, with an upper limit of 0.020%.

B: Adding a small amount of B improves hardenability, so the hardness increases after processing (hot rolling, hot forging) or heat treatment (normalizing, etc.) before soft-nitriding.

Become. Therefore, when this is subjected to a softening treatment, the core hardness is improved as a result, and the fatigue strength is improved. Therefore, the joint B is effective when particularly high fatigue strength is required. When B is added, an amount of at least 0.0005 h is required to obtain the above improvement, but an amount of 0.0005 h is required to obtain the above improvement.
When it exceeds 0050chi, the effect begins to saturate, so
The lower limit was set to 0.0005%, and the upper limit was set to 0.0050%.

Mo: Like B, Mo also improves hardenability, increases the hardness after hot working or heat treatment before soft nitriding treatment,
This is effective in further improving fatigue strength. For this purpose, when adding Iori, it is necessary to add at least 0.05%, but if it is added in excess of 0.25'%, the hardenability will be too high and the machinability will deteriorate. Since it is economically disadvantageous, the lower limit was set at 0.05% and the upper limit was set at 25%.

Note that both B and Mo have little effect on surface hardness. Further, if the required level of fatigue strength is not particularly high, B and MO may not be added.

S, Pb, Ca: These components are effective in improving machinability when performing cutting before nitrocarburizing. When deep hole drilling, heavy cutting, high-speed cutting, etc. are performed before soft-nitriding treatment, one or more of these elements can be included depending on the degree of machinability required. . These elements have no effect on the hardening properties.

The minimum amount added to improve the machinability of structural steel is:
S: 0.04%, Pb: 0.03%, Ca:
It is 0.0010chi. Also, S is 0.13% and Pb is 0.
．． If it exceeds 35%, the strength and toughness will be seriously reduced.On the other hand, it is difficult to add more than 0.0100% of Ca in the melting process, so for S, the lower limit is 04 and the upper limit is 0.
．． 13%, for pb, lower limit 'ff: 0.03%, upper limit is 0.35%, for Ca, lower limit is o, o o i
o%, the upper limit was set to o, o i o o%.

Next, the present invention will be explained by examples.

Examples Steel having the composition shown in Table 1 is melted in the atmosphere in a high-frequency melting furnace to form a steel ingot, heated to 1250'C, hot forged into a round bar with a diameter of 30 m, and the as-forged material and further For each of the materials normalized at 950'C for 1 hour, a disk-shaped test piece with a diameter of 25 cm and a thickness of 19 cm was prepared. In addition, for normalizing only, a round rod-shaped static image test piece with a diameter of 25 mm and a length of 300 m++ was also prepared.

For these series of test pieces, ammonia gas +f, tX
The specimens were subjected to a nitrocarburizing treatment at 570° C. for 4 hours in a mixed gas (1:1). After this nitrocarburizing treatment, the surface hardness of the disc-shaped test piece was increased from the surface corresponding to the Vickers hardness Hv at the 16' position of 0.025 (turn) and the hardening depth to the Vickers hardness Hv = 400. distance) was measured. Further, a static bending test as shown in FIG. 1 was carried out on the round bar-shaped test piece, and the deflection -4 when a crack appeared on the hardened surface was determined. These results are also listed in Table 1.

Steel type N [L 1 to 16 are steels according to the present invention, steel type N
α17-19 is steel type N120°2 in terms of Cr content.
1 is comparative steel in terms of content, 22 is a comparison steel that is outside the scope of the present invention in terms of sol and At, and the remaining steel types m23, 24 and 25 are JIS-8C.
M435. JIS-8ACM 645 and JIS
It is a conventional steel equivalent to -840C.

As can be seen from the results in Table 1, all of the steels of the present invention have moderate surface hardness within the range of Hv 500 to 600.
Moreover, the hardening depth is as large as 0.20W or more. In addition, the amount of deflection when a crack occurs in a static bending test is 3.
4 tnn or more.

On the other hand, the comparative steels N[117-19 and 21°22 have sufficient hardening depth, but the surface hardness is too high, and the amount of deflection when cracking occurs is 2.1 m.
It is very small as shown below. In addition, for steel types 20 and 23 to 25, the hardening depth is 0.18.
In addition to being small at less than w, Nα25 has a surface hardness of Hv442.
~445, which is insufficient, and conversely, the surface hardness of m24 is ■v
990 to 995, which is too high, and the amount of deflection when a crack occurs is extremely small, at 0.2 m+.

Figure 2 shows the results of a soft bending test after soft nitriding and treatment for normalized steel types t1α4.3, 19.18.17, which can be considered to have substantially the same composition except for the Cr content (when surface cracks occur). This is a graph plotting the amount of deflection) against the Cr content8.As is clear from this graph,
When the r content exceeds 0.2%, the amount of deflection begins to decrease rapidly. That is, the ductility of the hardened layer decreases rapidly.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing the procedure of the static bending test; and FIG. 2 is a graph showing the relationship between the amount of deflection at the time of crack generation and the Cr content in the static bending test.

Claims (1)

[Claims] (1) C: 0.15 to 0.50%, Si:
1.20% or less. Mn: 0.60-1.30%, Cr: 0
．． Less than 20%. V: 0.05-0.20%, sol, AA
: 0.10% or less. N: (1,006~0.020%. Steel for soft nitriding consisting of the balance Fe and inevitable impurities. f21c: 0.15~0.50%, S
i: 1.20% or less. Mn: 0.60-1.30%, Cr: 0.
Less than 20%. V: 0.05-0.20%, sol, At:
0-10% or less. N: 0.006-0.020%. Furthermore, B: 0.0005-0.0050% and M
O: A steel for soft nitriding containing 05 to 0.25% of one, two, or two of Q, and the balance being Fe and unavoidable impurities. (3) C: 0.15-0.50%, Si:
1.20% or less. Mn: 0.60-1.30%, Cr: 0.2
Less than 0%. V: 0.05NO, 20%, sol, A
T: 0.10% or less N: 0.006 to 0.02
0chi. Furthermore, S: 0.04-0.13%, Pb: 0.03
~0.35% and Ca: 0.0010-0.01
00%, and the remainder is Fe.
and unavoidable impurities. <4) C: 0.15-0.50chi, Sl: 1
．． Less than 20%. Mn: 0.60-1.30%, Cr: 0.2
Less than 0%. V: 0.05-0.20%, sol, At
: 0.10% or less. N: 0.006-0.020%. Furthermore, B: 0.0005-0.0050% and MO=
0.05-0.25% of one or two types and S20.0
4-0.13%, Pb: 0.03-0.35% and C
a: A steel for soft nitriding containing one or more of 0.0010-0.0100%, with the balance being Fe and unavoidable impurities.