JPS5916949A - 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 its face hardened layer, by making said 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%, 0.06-1.30% Mn, Cr <0.20%, V >=0.05% but below 0.20%, soluble Al <=0.10%, 0.006-0.015% N and the balance Fe and inevitable impurities. In said composition, the addition amounts of Cr and Al are remarkably limited below those of conventional soft-nitriding steel, to obtain proper face hardness (Hv 500-600) and to remarkably improve the ductility of a hardened surface layer at the same time. In addition, the reduction of case depth caused by limiting the addition amount of Cr is compensated by adding the effective amount of V, and fatigue strength and pitching resistance are also improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a steel for soft nitriding, and particularly to a steel for soft nitriding that has excellent toughness in the core portion after soft nitriding treatment and excellent ductility in the hardened surface layer.

The soft nitriding treatment is performed at a temperature below the AI transformation point, generally around 570°C, for example, in a cyanide compound salt bath, +-t
A type of surface hardening method in which the workpiece is treated with gas (endothermic denatured gas) or NX gas (exothermic denatured gas), and some carbon along with nitrogen penetrates into the steel to harden the surface layer. It is.

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. The development of steel for soft nitriding as a steel type suitable for this purpose has not yet been sufficiently developed, and no steel has yet been found that can obtain the desired properties through long-term soft nitriding treatment.

For example, JIS steel for soft nitriding, which is commonly used
-SCM, 435 (0,35C-0,75Mn-
1,ICr-0,2Mo) and SACM 645 (
0,45C-0,4St-1,5Cr-0,2Mo
-1, OAA). In addition, 2.3 steel for soft nitriding, which is made by adding ■ to Cr-based case hardening steel, has been proposed, and foreign standards (
AISI 6118) 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 workpieces. 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 means that conventional steel has a C content of 0.2% or more.
Since r is added, and especially in KSACM645, a large amount of M is added, a very hard and brittle compound layer containing Cr nitride and A7 nitride is formed on the outermost surface layer, which improves wear resistance. This is because although the ductility of the surface layer improves, the ductility of the surface layer deteriorates significantly.

In addition, in the conventional case-hardened steel with ■addition of Cr, the amount of ■addition is 0.
05% or more, usually 0.1% or more, coarse carbonitrides are generated in the core, resulting in poor toughness of the core, and there are also economical problems. '? In the case of the above-mentioned SACM645 containing a large amount of AA', in addition to the above-mentioned problem of crack generation during straightening, the hardness gradient from the surface to the core becomes too steep after soft-nitriding treatment, making it difficult to handle under high loads. In gears, bearings, etc. used in steel, peeling occurs easily near the boundary between the hardened surface part and the core part, and there are also problems in terms of fatigue strength, pitting resistance, and spalling resistance. This steep hardness gradient is due to the fact that the surface hardness is extremely high at HV800 to 1100 due to the high content of Cr and Al, whereas the effective hardening depth (distance from the surface corresponding to Vickers hardness HV = 400) This is due to the fact that it is as small as about 0.15n at most.

Therefore, the object of the present invention is to provide excellent wear resistance, fatigue strength, and pitting resistance by ensuring appropriate surface hardness and hardening depth, and to improve cold straightening after soft nitriding by improving the ductility of the hardened layer. To provide a steel for nitrocarburizing that is less likely to generate cracks during processing and has good toughness in both the core and the core.

As a result of continuing research on steel for soft nitriding for the above purpose, the present inventors found that when the addition amount of Cr and A/ is significantly limited compared to the level of conventional copper for soft nitriding, moderate surface hardness Hv
500 to 600), and at the same time, the ductility of the hardened surface layer was found to be significantly improved. This is because Cr and A precipitate fine Cr carbonitrides and Al nitrides in the surface layer during the soft-nitriding process, resulting in a significant hardness improvement of the surface layer, while Cr nitrides and Al nitrides are present in the outermost layer. It is believed that limiting the amount of these elements produces the above results because a very brittle compound layer containing MN oxides forms, severely degrading the ductility of the hardened surface layer. on the other hand.

The reduction in hardening depth that often occurs due to the limitation of the amount of Cr added can be compensated for by adding V, which is effective in increasing the hardening depth. ．．
It has also been found that an amount of less than 0.05% V is sufficiently effective, and that such an amount can avoid the rapid decrease in core toughness that is observed when V is added in an amount of 0.05% or more. ■Since carbonitrides precipitate in large quantities in areas slightly inside the outermost layer, the surface hardness does not increase significantly. Therefore, a large hardening depth can be obtained with the V addition force 11 hardly deteriorating the ductility of the hardened surface layer. As a result, the hardness gradient from the surface becomes gentle, and fatigue strength and pitting resistance are also improved.

In addition, when particularly high fatigue strength is required, hardenability can be increased by adding B or MO, and processing (hot rolling, hot forging) or heat treatment (
It has also been found that fatigue strength can be further improved by changing the structure after normalizing (such as normalizing) to fine pearlite or iinite and increasing the core hardness.

Further, when cutting is performed before soft-nitriding treatment, it is preferable to add Ca across s and pb, which is effective for improving machinability.

Here, in the present invention, C: 0.15 to 0.50%! si: 1.20%
below.

Mn: OJO~1.30%, Cr: 0.2
Less than 0%.

V: 0.02% or more, less than 0.05%.

sol, AJ: 0.10% or less, N: 0.0
06-0.015%.

Furthermore, if necessary, B: 0.0005 to 0.005
0% and one or two of Mo 70.05-0.25% and/or S: 0.04-0.1
3%.

Pb: 0.03-0.35% and (:a: 0
．． 0010 to 0.0100%, and the remainder is 1i'e 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.

CTC is a basic component for ensuring strength, and a minimum of 0.15% is required to ensure core strength. but,'
When it exceeds 0.50%, the ductility and toughness of the core portion 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% and the upper limit is 0.50%.

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

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

Cr: Cr is an extremely effective element that combines with N intruded by nitrocarburizing to increase surface hardness and hardening depth. Therefore, it is desirable to add a large amount to improve wear resistance and fatigue strength, 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 heat treatment strain generated during nitrocarburizing, cracks are likely to occur on the surface due to elastic deformation and plastic deformation accompanying the processing. For the above reasons, Crtit was set to less than 0.20%.

■: V combines with intruded N and intruded C due to soft nitriding, and f
Surface hardness and surface depth are improved by precipitating vanadium carbonitride. In particular, V
#' Compared to iCr, as mentioned above, the contribution to the increase in surface hardness is relatively small, and the contribution to increase in hardening depth is large, so although the effect of improving fatigue strength is large, the hardened surface layer The decrease in ductility is small. Although the above hardening is achieved by adding a small amount of V, at least 0.
An amount of 0.02% is necessary. In addition, as long as it is less than 0.05%, the toughness of the core hardly changes, but 0.05%
If the temperature exceeds this level, coarse nitrides (75) will combine with the N content and precipitate in the core, and the toughness of the core will begin to deteriorate rapidly.
The range was 0.02% or more and less than 0.05%.

sol, Al: Like Cr, Al is also an element effective in bonding with intruding N to increase surface hardness. In particular, when combined with V, an interaction between ■ and Al occurs, promoting the effect of increasing the effective hardening depth. That is, the moderate surface hardness improving effect of Al and the hardening depth improving effect of (2) are superimposed, and the hardening depth is further increased. However, 0.
If it exceeds 10%, 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, thereby improving the toughness of the core. For this purpose, 0.006% or more is required, but if it exceeds 0.015%, the formation of nitrides in the core will become noticeable, and the toughness of the core will begin to deteriorate, so the lower limit should be set to 0. .006%, with an upper limit of 0.015%.

Adding tlB by weight improves hardenability, so processing (hot rolling, hot forging) or heat treatment (
The hardness increases after normalizing, etc.).

Therefore, when this is subjected to nitrocarburizing treatment, the core hardness is improved as a result, and the fatigue strength is improved. Therefore, the addition of B is effective when particularly high fatigue relaxation is required. When adding B, an amount of at least 0.0005% is required to obtain the above improvement, but 0.00% is required.
If it exceeds 50%, the effect begins to be saturated, so the lower limit was set at 0.0005% and the upper limit was set at 0.0050%.

Mo: Like B, Mo also improves hardenability, greatly reduces the hardness during hot working before nitrocarburizing or after heat treatment.
This is effective in further improving fatigue strength. For this, M
When adding O, it is necessary to add at least 0.05%, but if it is added in excess of 0.25%, the hardenability will increase too much, the machinability will deteriorate, and it is also economically disadvantageous. , the lower limit was set to 0.05%, and the upper limit was set to 0.25%.

It should be noted that neither B nor Mo has much influence on the surface hardness than dPl. 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 cutting is performed before soft-nitriding treatment. 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
: 0.0010% “C” Aro 4ft S
If it exceeds 13% for H o, 35% for Pb Vio, the strength and toughness will deteriorate significantly, while it is difficult to add more than 0.0100% for ca in melting, so for S, the lower limit is set. 0.04%, upper limit 0.13%
, 1] For b, the lower limit is 0.03% and the upper limit is 0.35
%, and for CaK, the lower limit is 0.0010% and the upper limit is 0.
It was set to 0100%.

Next, the present invention will be explained by examples.

Example Steel having the composition shown in Table 1 was melted in the atmosphere in a high-frequency melting furnace to form a steel ingot, which was then heated to 1250°C and made into a steel ingot with a diameter of 3
JIS No. 3 Charpy test pieces (25 mm diameter
10 questions) were created. These series of test pieces were subjected to gas soft nitriding treatment at 570° C. for 4 hours in a mixed gas of ammonia gas + RX gas (1:1). The obtained soft-nitrided test piece was heated to room temperature (20°C).
A Charpy impact test was conducted on the specimen, and the surface hardness (Vickers hardness at a position 0.025 mm from the surface) and hardening depth (distance from the surface corresponding to Vickers hardness Hv = 400) were measured. The results are also listed in Table 1.

Regarding the normalized phase of some copper, a round bar with a diameter of 25μ and a length of 300 mm was created by turning, and after being subjected to soft nitriding treatment under the above conditions, it was processed as shown in Figure 1. We conducted a static test and measured the deflection when a crank occurs on the hardened surface, and the results are also listed in Table 1.

Steel types N1 to 17 are steels according to the present invention, and steel types 1'h1
8 to 20 in terms of Cr content, steel grade 1'% 21 to 23 in terms of ■ content, and steel grade Arashi 24 in terms of BOl, AA! These are comparative steels that are outside the scope of the present invention, and the remaining steel grades Arashi 25, 26, and 27 are JIS-SCM
435. JIS-SACM645 and JIS
- It is a conventional steel corresponding to S 4 QC.

As can be seen from the results in Table 1, all of the steels of the present invention have a moderate surface hardness within the HV range of 500 to 600 m, and a hardening depth of 0.15 m1 or more, compared to the conventional steel SCM 4.
35 and SACM 645.

In addition, the impact value of each song is over 4.7 Yu·m/word. Furthermore, in the static direction, the deflection at the time of cracking of the steel types N11 to 9 tested was all 3.5 or more.

On the other hand, the comparative steel type lV! 1118-20 has sufficient hardening depth and impact value, but has a surface hardness of 600 or more, a small amount of deflection at the time of cranking of 2.0 or less, and has a problem in surface ductility. In addition, the steel teeth 21 have a small hardening depth and a slow impact value of 2.4 U·m1.
It is smaller than cr&.

On the other hand, conventional steel grades 25 and 27 have a small hardening depth, and N[127 has a surface hardness of HV442 to 445.
and insufficient. On the other hand, steel type 1'426 has a surface hardness of H.
V is very high at 990 to 995-, and the amount of deflection when cracking occurs is extremely small at between 0.2.

Next, the Charpy impact values of normalized steels of steel types Nct21, 3, 4, and 22.23, which can be considered to have substantially the same composition except for the content and amount, are plotted against the content (), and the second The graph shown in the figure was obtained.As is clear from this graph, when the content (1) exceeds 0.05%, the impact value rapidly decreases and the toughness rapidly deteriorates.

Figure 3 shows steel type Il!, which can be considered to have substantially the same composition except for the Cr content. It is a graph in which the results of static bending tests (the amount of deflection when surface cracks occur) after soft-nitriding treatment for normalized materials Nos. 15, 4, 20, and 19.18 are plotted against the Cr content. As is clear from this graph, C
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 drawings]

Figure 1 is a graph showing the outline of the static bending test; Figure 2 is a graph showing the relationship between the impact value and content in the Charpy impact test; and Figure 3 is a graph showing the relationship between the impact value and content in the static bending test. It is a graph showing the relationship between the amount of slack and Cr content over time. Applicant's representative Patent attorney Akira Hirose-Yo/Tui Kaori 2 Figure V Kumanyo (ΦDou 2)

Claims (1)

[Claims] (1 N C: 0.15 to 0.50 9 et al.
Si2 1.20% or less. Mn: 0.60-1.30%, Cr: 0.
Less than 20%. V: 0.02% or more, less than 0.05%. sol, AJ: 0.10% or less, N2 0.006~
0.015%. A steel for soft nitriding consisting of the remainder Fe and unavoidable impurities. (2) C: 0.15-0.50%, St:
1.20% or less. Mn: 0.60-1.30%, Cr: 0.
Less than 20%. V: 0.02% or more, less than 0.05%. sol, A6: 0.10% or less, N: 0.0
06-0.015%. Furthermore, B: 0.0005-0.0050% and M
o: Contains 0.05 to 0.25% of one or two types,
A steel for soft nitriding consisting of the remainder Fe and unavoidable impurities. (3) C: 0.15-0.50%, Si:
1.20% or less. Mn: 0.60-1.30%, Cr: 0.
Less than 20%. V: 0.02% or more, less than 0.05%. gol, AJl! : 0.10% or less, N:
0.006-0.015%. Furthermore, S: 0.04-0.13%, FD: S, LJ
, j ~ 0.35% and Ca: 0.0010 ~ 0.01
A steel for soft nitriding, which contains one or more of the following: (4) C: 0.15-0.50%, Si:
1.20% or less. Mn: 0.60-1.30%, Cr: 0.
Less than 20%. V: 0.02% or more, less than 0.05%. sol, AJ: 0.10% or less, N: 0.
006-0.015%. Additionally B + 0.0005-0.0050% and M
One or two types of O = 0.05 to 0.25% and S: 0
．． 04-0.13%, Pb: 0.03-0.35
% and Ca: 0.0010 to 0.0100%, and the remainder is Fe and inevitable impurities.