JPS60159116A - Manufacture of steel parts having high hardenability and toughness - Google Patents

Abstract

PURPOSE:To improve simultaneously the hardenability and toughness and to enable the use of a lower-grade material by nitriding the surface of a steel having restricted Ti, Al and N contents and by coarsening the austenite grains. CONSTITUTION:A low-alloy steel contg. 0.01-0.1wt% Ti and/or 0.02-0.5wt% Al and <=100ppm N is used as a starting material. This material is nitrided at a temp. below the austenite grain coarsening temp. of the material, and it is hardened by heating to a temp. above the coarsening temp. The austenite grains in the surface part of the material can be made fine, and those in the interior can be made coarse, so said purpose can be attained.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention provides a method for manufacturing steel parts with good hardenability and toughness, and more specifically, a method for manufacturing steel parts with good hardenability and toughness. By coarsening the inside of the cane,
This invention relates to a method for manufacturing parts with improved hardenability and toughness.

(Prior Art) The hardenability of steel parts is usually determined by the type of alloying element, amount of stress, mass size, austenite grain size, etc. In particular, regarding the austenite crystal grain size, the larger it is, the better the hardenability is, and cracking can be achieved by increasing the quenching temperature during quenching and reducing the amount of particles that inhibit austenite crystal grain growth, such as AiN and TiN. However, it becomes uniformly coarse. (This can also be achieved by extending the heating time.) On the other hand, the performance of steel parts usually depends largely on the toughness of the surface area, which is the area where the maximum stress occurs; It is desired that the austenite crystal grain size be as small as possible and that a completely quenched structure be obtained.

In other words, it is desirable that the austenite crystal grains be as large as possible from the viewpoint of hardenability and as small as possible from the viewpoint of toughness, but the austenite crystal grain size is determined by the manufacturing process of the material. Excluding the influence of history (steel manufacturing conditions, forging conditions, etc.), it is uniquely determined by the above-mentioned quenching conditions. For this reason, it is virtually impossible to satisfy the above two performance aspects at the same time by adjusting the austenite grain size. Conventionally, these performances have been improved by adding expensive alloying elements or by surface treatments such as carburizing and nitriding. It was guaranteed.

(Object of the Invention) In view of the background of the prior art described above, the present invention improves both hardenability and toughness at the same time by making the austenite crystal grains in the surface part finer and the inner parts coarser. The object of the present invention is to provide a method for manufacturing steel parts that can reduce the grade of materials used.

(Structure of the invention) This purpose is 0. O1~0.1ω12 D10.
02~0. A low alloy steel that contains at least one type of fJ A4 and suppresses nitrogen to 100 ppH or less is used as a material, and is nitrided at a temperature lower than the austenite grain coarsening temperature of the initial material, and then the austenite grain coarsening temperature is lowered. This is achieved by a method of manufacturing steel parts that is heated to a higher temperature and subjected to quenching treatment.

In the present invention, low alloy steel is used as the material because O
This is because high-alloy copper, which contains a large amount of r, Mo, Ni, etc., has good hardenability and high strength and toughness, so adjusting the austenite grain size does not make much sense. .

In addition, in the present invention, Ti and AgL were selected as additive elements because both Ti and All have a large nitride forming ability, and they can be easily formed into TiN - or A by nitriding treatment.
This is because IN is formed and these nitrides act to suppress the growth of austenite crystal grains on the surface of the component. and,
The reason why T1 is specifically set in the range of 0.01 to 0.1 ωtX is that it is 0.01 to 0.1 ωtX. If it is less than O1ωtX, the amount of TiN generated will be insufficient, and if it exceeds 0.1ωtz, the amount of TiN will increase unnecessarily and the performance of the part will deteriorate7. On the other hand,
The reason why Ai is particularly set in the range of 0.02 to 0.5ω is that if it is less than 0.02ωtz, the amount of AuN generated is insufficient.
This is because if it exceeds 0.5 ωt%, as in the case of T1, the amount of AuN will increase unnecessarily and the performance of the part will deteriorate.

Furthermore, the nitrogen content in the steel was kept below 1100pp by
If this value is exceeded, a large amount of nitrides will be formed between the additive elements Ti and A during the alloy melting stage, and the dispersion of these nitrides will cause the austenite crystal grains inside the part to become coarser. Depends on why it becomes difficult.

(Example) Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows an example of a basic heat treatment method for carrying out the method of the present invention. The feature of this embodiment is that the first step I, which performs nitriding treatment, and the second step 11, which performs coarsening treatment of austenite crystal grains, are combined.

That is, 0.01 to 9.1 ωt Z Pi , 0
．． A part is formed from a low alloy steel material containing at least one type of 1 to 0.3 ωt% All and with a nitrogen content of 100 PPII or less, and is first heated to a predetermined temperature Tl (austenite below grain coarsening temperature)
When held for a predetermined time t (first step I), nitrogen enters from the surface of the part and a large amount of TiN or ^f is deposited on the surface.
LN is formed. After that, when held at a predetermined temperature T2, which is higher than the coarsening temperature of austenite crystal grains peculiar to the material, for a predetermined time t (second step II), the surface of the part is formed of the TiN or A! The presence of IN suppresses the coarsening of austenite crystal grains, while the interior of the component does not have the above-mentioned constraint conditions, so the austenite crystal grains become coarse.

The parts that have completed the second step (2) are subjected to a rapid cooling and hardening treatment, and at this time, the surface portion has good hardenability due to the large cooling rate of that portion itself and the presence of solid solution nitrogen.
As a result, a completely C hardened structure (martensite) can be obtained.On the other hand, the inside has good hardenability due to coarsening of austenite crystal grains, so it is also possible to obtain a hardened structure (martensite). A complete hardened structure can be obtained.

In addition, since the hardenability is also affected by the size of the part, it goes without saying that the size of the part should be selected in accordance with the hardenability. Further, after the quenching process b, a normal tempering process is performed.

FIG. 2 shows another example of a heat treatment method for carrying out the method of the present invention. The feature of this embodiment is that the first step is to perform the existing carburizing and quenching or carbonitriding and quenching.
and the second step (2), which is a coarsening treatment of austenite crystal grains, as described above.

That is, when a part is formed from a low alloy steel containing Ti, AM, etc. similar to the above, and is held at a predetermined temperature T3 in a carburizing or carbo-nitriding atmosphere for a predetermined time t3 (first step 1
') In the case of a carburizing atmosphere, nitrogen in the atmosphere decomposes, while in the case of a carburizing atmosphere, the actively added N
The H3 gas decomposes and enters the surface of one component, forming TiN or AiN on the surface of the component. After that, if the part is cooled to below the transformation point A1 and then held at a predetermined temperature T2, which is above the coarsening temperature of austenite crystal grains, for a predetermined time t2 in the same manner as described above, the inside of the part As a specific example, the austenite crystal grains become coarse and a complete quenched structure is obtained that peels off the inside and outside of the part through subsequent quenching.
D, 0.3G-0,8Mn-0,25Si-0,2
Cr -0,24An -0,0023B -0,09
A test piece was formed using a material consisting of 5N-Bai and Fe, and was treated according to the heat treatment diagram shown in FIG.

The heat treatment conditions were: atmosphere: immersion* (R1 gas + butane), carburizing temperature T: 950'O1 carburizing time t: 300 minutes,
Coarsening temperature T: 850°C1 Coarsening time t: 45 minutes.

Figure 3 shows the AiN concentration distribution on the surface of the part after the above treatment, and it shows that although the AiN concentration decreases rapidly from the surface to the inside,
It was confirmed that it existed in large quantities down to a depth of <0.4 ■ from the surface.

Further, the microstructure of the surface of the component was also confirmed using a microscope, and the results shown in FIG. 4 (photo) were obtained.

In other words, in the photograph, the net-like black lines represent the pre-austenite grain boundaries, and the austenite grains are very fine up to a depth of 0.3+em from the surface, and become very coarse inside. It is clear that

As described above, according to the method of the present invention, the austenite crystal grains on the surface of the part can be adjusted to fine grains, and the austenite crystal grains inside the part can be adjusted to coarse grains, and the performance of the part as a whole can be sufficiently ensured. Become something.

(Effects of the Invention) As explained in detail above, the present invention uses a low alloy steel containing at least one of Ti and A and containing low nitrogen content, and first nitriding the surface. Subsequently, a method for manufacturing steel parts was established in which the austenite crystal grains were coarsened.This process made the austenite crystal grains on the surface of the part finer and the austenite crystal grains inside the part coarser. As a result, the hardenability of the parts was improved as a whole, and as a result, it was possible to select lower-grade materials. In addition, the surface of the part is thermally refined to have a dense structure and high hardness.

The effect of further improving toughness was obtained.

[Brief explanation of the drawing]

1 and 2 are heat treatment diagrams for carrying out the method of the present invention,
Figure 3 shows the surface of a steel part obtained by the method of the present invention.
．． The N concentration distribution diagram, FIG. 4, is a microscopic photograph showing the surface microstructure of the treated steel part. Patent applicant Toyota Motor Corporation Representative Patent attorney Yu Sae! ! ! ! (1 other person) %1 figure in 22F! ! J spear 3 figure? 41

Claims (1)

[Claims] (+) 0.01-0.1 (1) t% Ti,
0.02-0.5% AIL (7) The material is made of low-alloy steel that contains at least one of the following and has nitrogen suppressed to 1100 pp or less, is first subjected to nitriding treatment, and then maintained at the austenite grain coarsening temperature. A method for producing steel parts with good hardenability and toughness, characterized by subjecting the steel parts to hardenability and toughness.