JPS6131184B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention provides a soft-nitrided article, particularly a soft-nitrided article that has a large hardening depth and provides a hardening curve with a gentle hardness gradient from the surface to the core. Concerning the manufacturing method of articles. Soft-nitriding treatment is carried out at a temperature below the _A1 transformation point, generally around 570°C, for example in a cyanide compound salt bath, RX
A type of surface hardening method in which the workpiece is treated with gas (endothermic modified gas) or N gas (exothermic modified gas), etc., and some carbon along with nitrogen penetrates into the steel to harden the surface layer. It is. Unlike the carburizing-quenching method, this method does not cause distortion in the workpiece, and unlike nitriding, it does not require a long time, so it is suitable for mass production of mechanical parts. The development of steel for nitrocarburizing as a steel suitable for nitrocarburizing has not been sufficiently developed, and so far no steel has been found that can obtain the desired properties with a short nitrocarburizing treatment. For example, conventionally, JIS
SCM435 (0.35C―0.75Mn―1.1Cr―0.2Mo)
SACM645 (0.45C-0.4Si-1.5Cr-0.2Mo) is often used, but in the case of SCM435 steel, the effective hardening depth (micro-Vickers hardness Hv
= distance from the surface corresponding to 500) is at most
It was about 0.10 mm, and the surface hardness (microscopic Vickers hardness at 25 μm below the surface) was not higher than Hv650, so it was not satisfactory in terms of fatigue strength and wear resistance. In addition, in the case of SACM645, which has a large amount of Al and Cr added to improve the nitriding properties by improving the nitriding properties, the soft nitriding treatment increases the surface hardness to Hv800-1100, which is very high, but it is not effective. Since the hardening depth is small at about 0.15 mm in height, the hardness gradient from the surface to the core becomes too steep. Therefore, products such as gears and bearings that are operated under high loads tend to peel off near the boundary between the hardened surface part and the core, resulting in poor pitting resistance or spalling resistance. According to the findings of the present inventors, fatigue strength and wear resistance can be improved by setting the surface hardness to Hv650 or more and the effective hardening depth to 0.2 mm or more under normal soft-nitriding treatment conditions. On the other hand, by simultaneously limiting the surface hardness to Hv750 or less and increasing the core hardness, the hardness gradient from the surface to the core can be made gentler, improving pitting resistance. and spalling resistance is significantly improved. In view of this situation, as a result of intensive research, the present inventors succeeded in obtaining a steel for soft nitriding that has excellent fatigue strength and wear resistance, as well as pitting resistance and spalling resistance. (Unexamined Japanese Patent Publication 1983-
71357, 58-71358, and 58-71359), and further research revealed that the optimal steel structure for soft nitriding is a ferrite + pearlite structure near the surface. They discovered this and completed the present invention. That is, in the present invention, in order to obtain the desired surface hardness, the amounts of Cr and Al added are adjusted, and in order to make the hardness gradient from the surface gentle, first of all, the hardening depth is increased. Add effective V to
At the same time, a steel article with a combination of a steel composition with a specified amount of N in the steel that is effective in improving the hardness of the core due to its coexistence with V and a ferrite + pearlite structure near the surface is treated with soft nitriding. The essential feature is to do the following. Here, the present invention includes C: 0.15 to 0.35%, Si:
0.35% or less, Mn: 0.60 to 1.30%, Cr: more than 0.70% and 1.50% or less, V: 0.05 to 0.50%, sol.Al: 0.02 to
0.10%, N: 0.006-0.020%, and if necessary, S: 0.04-0.13%, Pb: 0.03-0.35% and
A processing material containing one or more of 0.0010% to 0.0100%, with the balance consisting of Fe and unavoidable impurities, is heated to 800℃ after being processed at ₃ or more hot temperatures and then cooled. and 500°C at a surface cooling rate of 2°C/second or less, followed by soft nitriding. When processing a workpiece material having a steel composition appropriately and then heating it to ₃ points or more for normalizing, the surface cooling rate is 2 between 800℃ and 500℃.
This is a method for producing a soft-nitrided article with a large hardening depth, characterized by controlled cooling at a rate of 0.degree. C./second or less, followed by soft-nitriding. In the present invention, as described above, near the surface,
For example, the structure within about 1 mm below the surface is made into a ferrite + pearlite structure with good soft-nitriding properties, and for this purpose, according to the present invention, after appropriate hot working such as hot rolling or forging, or after A ₃ points or more The cooling rate of the surface of an article after normalizing is heated to
Limit the temperature to 2°C/sec or less between 800°C and 500°C. Due to the steel structure thus obtained, together with the specified steel composition, the article soft-nitrided by the method according to the invention exhibits excellent soft-nitriding properties. As can be easily understood from the purpose of the present invention as described above, the above-mentioned "processed material" includes not only general plate steel or steel bars, but also all materials that are processed into the final shape. The term "soft-nitrided article" is not limited to gears, bearings, etc., but includes general steel products that are ultimately used after being soft-nitrided. Next, the reason why the composition of the steel used in the method according to the present invention was limited within the above range will be described. C: C is a basic component for ensuring strength, and a minimum of 0.15% is required to ensure core strength. However, if it exceeds 0.35%, the ductility and toughness of the core decrease, machinability, cold workability, and weldability decrease, and the surface hardness and hardening depth after nitrocarburizing also significantly decrease. Therefore, the lower limit of the amount of C in the present invention is set to 0.15% and the upper limit is set to 0.35%. Si: Si is usually added as a deoxidizing agent, but it is an element that inhibits carburization, so even in soft nitriding, where carburization and nitriding proceed simultaneously, the less Si there is, the better the hardening properties are, and the deeper the hardening depth. can get. In particular, the effect becomes greater when Si becomes 0.35% or less, so in the present invention, the upper limit of Si is set to 0.35%.
And so. Furthermore, Si is a harmful element to weldability, toughness of the weld heat affected zone, and cold workability, and in particular, when it exceeds 0.10%, these properties begin to deteriorate rapidly. Therefore, in the present invention, the upper limit of the amount of Si is preferably 0.10%. 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 must be present at a minimum of 0.60% to ensure the performance of soft-nitrided products.
is necessary. 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 increases the surface hardness and increases the hardening depth by combining with the N intruded by nitrocarburizing. In order to fully demonstrate its effect, a Cr content exceeding 0.70% is required.
If it exceeds 1.50%, the surface hardness will be Hv750 or higher after soft-nitriding under normal soft-nitriding treatment conditions, so the upper limit should be set.
It was set at 1.50%. V: V improves surface hardness and surface depth by combining with intruding N and intruding C by soft nitriding to precipitate fine vanadium carbonitrides.
In particular, compared to Cr, V has a relatively small contribution to increasing surface hardness, but has a large contribution to increasing hardening depth. Further, since V increases the hardness of the core through precipitation hardening, it is an extremely effective element for obtaining a hardening curve with a deep hardening depth and a gentle hardness gradient from the surface to the core. In order to fully demonstrate its effect, at least
0.05% is necessary, but if it exceeds 0.50%, the effect will be saturated or even begin to decline, so the lower limit should be set.
0.05%, with an upper limit of 0.50%. sol.Al: Like Cr, Al also combines with intruding N to increase surface hardness, but is not very effective in increasing hardening depth. Especially when combined with V, 0.10% or more
Addition of Al actually decreases the hardening depth. However, even a small amount of addition is effective for surface hardness, at least 0.02% to ensure Hv650 or higher.
Since it is necessary, the lower limit was set to 0.02% and the upper limit was set to 0.10%. N: N refines the grain size, thereby improving the toughness of the core, and also causes precipitation hardening by forming a compound with V in the coexistence with V, thereby improving the hardness of the core. At least 0.006 to produce such precipitation hardening.
% is required. However, if it exceeds 0.020%,
Since an excessive amount of nitride is generated, cold workability and core toughness are rapidly deteriorated, so in the present invention, the lower limit of the N amount is set to 0.006% and the upper limit is set to 0.020%. S, Pb, Ca: These components are effective in improving machinability before cutting 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 components have no effect on the curing properties. The minimum addition amounts necessary to improve the machinability of structural steel are S: 0.04%, Pb: 0.03%, Ca: 0.0010%.
It is. In addition, if S exceeds 0.13% and Pb exceeds 0.35%, the strength and toughness will deteriorate significantly.On the other hand, it is difficult to add more than 0.0100% of Ca in melting process, so S
For Pb, the lower limit was 0.04% and the upper limit was 0.13%, for Pb the lower limit was 0.03% and the upper limit was 0.35%, and for Ca the lower limit was 0.0010% and the upper limit was 0.0100%. According to the present invention, a processed material having such a steel composition is heated to A ₃ points or higher and then subjected to appropriate hot working such as hot forging or bending, or heated to A ₃ points or higher after appropriate processing. Normalizing is carried out, and at that time, controlled cooling is carried out by cooling between 800° C. and 500° C. at a surface cooling rate of 2° C./sec or less. In the present invention, in the steel composition specified as described above, a ferrite+pearlite structure suitable for nitrocarburizing is formed near the surface by the controlled cooling. If the above-mentioned surface cooling rate exceeds 2°C/sec and the surface is rapidly cooled, a martensitic structure or a bainite structure will be formed, and the hardening depth obtained by soft nitriding is insufficient to achieve satisfactory fatigue. Since strength and pitting resistance cannot be obtained, the surface cooling rate is limited to 2° C./second or less in the present invention. During cooling, the cooling rate is fastest near the surface, and therefore bainite or martensitic structures are likely to occur. In addition, water may be poured after hot working or heat treatment to secure the shape and accelerate cooling, and bainite may be formed on some surfaces. Therefore, in order to prevent the formation of such bainite or martensite, in the present invention, cooling after hot rolling is controlled by the surface cooling rate of the product. Note that the fact that the vicinity of the surface has a ferrite + pearlite structure naturally means that the interior also has a ferrite + pearlite structure, but in the present invention, as mentioned above, the appropriate amount of V and N is added. We aim to ensure the required core strength by adjusting the alloy composition, including oxidation. Thus, according to the present invention, it is possible to obtain a product (article) to be soft-nitrided, such as a gear, in which a ferrite+pearlite structure with excellent soft-nitriding characteristics appears uniformly near the surface. Such products are then subjected to further cold working (e.g., forging, drawing, cutting, etc.) as necessary to achieve the final shape, and then nitrocarburizing, but the previous structure is not the same as the hot working or sintering. It is determined by the cooling rate during adjustment cooling after conditioning. Therefore, according to the present invention, by suitably processing the processed material of the steel composition to the final shape and then nitrocarburizing it, a nitrocarburizing product exhibiting the excellent nitrocarburizing properties as described above can be obtained. be. Note that the soft nitriding treatment employed in the present invention is not particularly limited, and may be any commonly performed treatment, but preferably, for example, ammonia gas +
550 ~ using a mixed gas of RX gas (volume ratio 1:1)
This method involves processing at 600°C for 1 to 6 hours. Next, the present invention will be further explained with reference to Examples. Example 100Kgf of each steel type with the steel composition shown in Table 1
The steel was heated to 1250°C and hot rolled to obtain a 100 mm square billet. Using this as a processed material, the following two types of processing and controlled cooling were performed. (1) This example shows an as-hot-forged example, in which the processed material was heated to 1250°C and hot-forged into a round bar with a diameter of 25 mm at a finishing temperature of 1000°C. When cooling from this finishing temperature, the surface cooling rate is adjusted, and cooling within the insulation cover, air cooling, wind cooling, and spray cooling are performed to reduce the temperature range from 800℃ to 500℃ to 1.0 to 5.0℃. Cooling was performed at a surface cooling rate of /sec. It was then cold machined to a diameter of 24.5 mm. (2) This example shows an example of normalizing after hot working, in which the above processed material is heated to 1250°C and hot forged into a round bar with a diameter of 25 mm at a finishing temperature of 1000°C.
Forged with. This finishing temperature was allowed to cool to room temperature in the atmosphere. Next, it was heated to 900°C for normalizing. During this normalizing process, the surface cooling rate is adjusted when cooling from the above normalizing temperature, and cooling within a heat insulation cover, air cooling, air cooling, and spray cooling are performed to 800℃~ 500
℃ temperature range was cooled at a surface cooling rate of 1.0 to 5.0℃/sec. It was then cold machined to a diameter of 24.5 mm. Furthermore, among the steel types shown in Table 1, steel types No. 1 to 13
has a steel composition within the scope of the present invention, while steel grade No.
Nos. 14 to 20 have steel compositions outside the scope of the present invention, and steel types No. 21 and 22 are steels corresponding to JIS-SCM435 and JIS-SACM645, respectively. Next, the steel bar with a diameter of 24.5 mm thus obtained was corroded with a 2% nital corrosive solution to observe the metallographic structure of the steel bar surface, and the steel bar was heated at 570°C for 4 hours.
Gas nitrocarburizing was performed using an endothermic modified gas (RX gas) under the condition of NH ₃ :RX = 1:1, and the surface hardness (fine Vickers hardness at a depth of 25 μm from the surface) and effective hardening depth were evaluated. (Micro-Vickers hardness
The distance from the surface corresponding to Hv500) was measured.
The results are summarized in Tables 2 and 3.

【table】

【table】

[Table] As is clear from the results shown in Tables 2 and 3, the steel has a composition within the range of the present invention and is cooled at a predetermined surface cooling rate after hot working or normalizing. All of the processed products have a ferrite + pearlite structure, and the effective hardening depth after soft-nitriding treatment is 0.2 mm or more, and the surface hardness shows a stable value of Hv650 to 750. The core hardness (Vickers hardness Hv at a depth of 1 mm from the surface) was 180 to 300, which caused no practical problems. On the other hand, even if the steel has a composition within the scope of the present invention, if the surface cooling rate exceeds 20°C/sec during controlled cooling after hot working or normalizing, bainite or martensitic structures may occur. It can be seen that the effective hardening depth after nitrocarburizing rapidly decreases and becomes less than 0.2 mm in all cases. In addition, regarding steel types No. 14 to 22 as comparative examples, even if the structure is a ferrite + pearlite structure, the effective hardening depth due to soft nitriding treatment is less than 0.2 mm or the surface hardness is less than 0.2 mm. The value may be outside the appropriate range of Hv650-750. Thus, with the present invention, for the first time, the effective hardening depth after nitrocarburizing is 0.2 mm or more, and the surface hardness is Hv650-750.
A soft-nitrided product exhibiting excellent soft-nitriding properties can be obtained.

Claims (1)

[Claims] 1 C: 0.15 to 0.35%, Si: 0.35% or less, Mn: 0.60 to 1.30%, Cr: more than 0.70% and 1.50%
Below, V: 0.05~0.50%, N: 0.006~0.020%,
sol.Al: 0.02 to 0.10%, the balance is Fe and unavoidable impurities when processing the processed material at ₃ or more hot temperatures and cooling it at 800°C and 500°C.
and controlled cooling at a surface cooling rate of 2° C./sec or less, followed by soft nitriding treatment,
A method for manufacturing nitrocarburized articles with a large hardening depth. 2 C: 0.15-0.35%, Si: 0.35% or less, Mn: 0.60-1.30%, Cr: over 0.70%, 1.50
% or less, V: 0.05~0.50%, N: 0.006~0.020%,
sol.Al: 0.02-0.10%, S: 0.04-0.13%, Pb: 0.03-0.35%
Obi Ca: 1 or 2 of 0.0010-0.0100%
When processing a processed material with a composition consisting of _A.
℃ at a surface cooling rate of 2℃/second or less, and then subjected to soft nitriding treatment,
A method for manufacturing nitrocarburized articles with a large hardening depth. 3 C: 0.15-0.35%, Si: 0.35% or less, Mn: 0.60-1.30%, Cr: over 0.70% and 1.50%
Below, V: 0.05~0.50%, N: 0.006~0.020%,
sol.Al: 0.02~0.10%, after processing the processed material with a composition consisting of Fe and unavoidable impurities.
A: When performing normalizing by heating to ₃ or more points, hardening is characterized by adjusting cooling between 800°C and 500°C at a surface cooling rate of 2°C/sec or less, and then performing nitrocarburizing treatment. A method for producing nitrocarburized products with a large depth. 4 C: 0.15-0.35%, Si: 0.35% or less, Mn: 0.60-1.30%, Cr: over 0.70% and 1.50%
Below, V: 0.05~0.50%, N: 0.006~0.020%,
sol.Al: 0.02-0.10%, S: 0.04-0.13%, Pb: 0.03-0.35%
After processing a processed material containing one or more of 0.0010 to 0.0100% of Ca and 0.0010% to 0.0100% of the material, with the balance consisting of Fe and unavoidable impurities.
A: When performing normalizing by heating to ₃ or more points, hardening is characterized by adjusting cooling between 800°C and 500°C at a surface cooling rate of 2°C/sec or less, and then performing nitrocarburizing treatment. A method for producing nitrocarburized products with a large depth.