JPH0672280B2 - High toughness carburizing steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a carburizing steel having excellent toughness.

(Prior Art and Problems) Steel parts with good toughness are carburized and used for parts such as automobile hypoid gears and differential gears that receive an impact load.

However, the impact strength required for these parts is gradually increasing in response to the higher output of automobile engines and the downsizing of gears. There are some examples of chromium / molybdenum steel and nickel / chromium / molybdenum steel, which are said to be good, that do not satisfy the required strength.

As a countermeasure, there is a method of strictly controlling the surface roughness of parts made of such steel types, but there is a problem that productivity is significantly hindered.

(Object of the Invention) The present invention has been made in order to solve the above-mentioned drawbacks of the prior art, has a toughness superior to that of the conventional carburizing steel, and is a necessary component without impairing the productivity. It is an object of the present invention to provide a high toughness carburizing steel that can be manufactured.

(Structure of the Invention) In order to achieve the above object, the inventors of the present invention endeavored to investigate the cause of the drawbacks of the prior art, and conducted research to find out a countermeasure thereof. By adding Mo and Si, even at the carburizing temperature. The core retains a stable two-phase structure of ferrite and austenite, Ti carbide and nitride are precipitated in the carburized layer by addition of Ti, the core and carburized layer both have a fine structure, and the base is strengthened with Mo. The present invention has been made as a result of further detailed investigations by finding that it is possible to give high impact strength.

That is, the high toughness carburizing steel according to the present invention has a C: 0.05-
0.2%, Mo: over 0.5% to 1.0%, and Mo + Si: 1 to 2%,
Mn: 0.3 to 1.5% and Ti: 0.05 to 0.2% as a basic component, and if necessary, one or two of Cr: 1.5% or less and Ni: 2.0% or less, and / or Carburizing by including one or two of Al: 0.1% or less and N: 0.03% or less as a grain refinement element, the balance consisting essentially of Fe, and carburizing in the austenite-ferrite two-phase region As the subsequent structure, the core has a fine grained ferrite-martensite two-phase structure with a grain size number of 9 or more, and the carburized layer also has a grain size of 9 grain or more with a grain size of 9 or more. The main point is that the impact value is 12 kgf · m / cm ² or more.

The present invention will be described in detail below based on examples.

First, the composition range (wt%) of the high toughness carburizing steel according to the present invention
The reason for the limitation is shown below.

C: 0.05 to 0.2% C is an element effective for securing the strength as a steel for machine structural use, especially the hardness after quenching, and it is necessary to add at least 0.05% for strengthening the core after carburizing. However, if it exceeds 0.2%, it becomes an austenite single phase at the carburizing temperature,
Since it is difficult to refine the core tissue, the upper limit is set to 0.2%.

Mo: more than 0.5% to 1.0%, and Mo + Si: 1 to 2% Mo is an element that is effective in improving the toughness by securing quenching hardness and strengthening the matrix, and also stabilizes ferrite at high temperature with Si, In order to realize a fine ferrite-austenite two-phase structure at the carburizing temperature and improve the toughness, it is necessary to add Mo in an amount exceeding 0.5% and Mo + Si: 1% or more. Further, in order to obtain high toughness by adding Mo in an amount exceeding 0.5%, the maximum amount of Mo added is 1%, and
A maximum of 2% Mo + Si is sufficient, and the effect of increasing toughness saturates even if more than this is added. Therefore, the addition amount of Mo is 0.
More than 5% to 1.0%, and the addition amount of Mo + Si is 1% or more, 2
% Or less.

Mn: 0.3-1.5% Mn is an element effective for deoxidation and desulfurization. To obtain sufficient deoxidizing and desulfurizing effects, it is necessary to add 0.3% or more, but if added over 1.5%, the toughness deteriorates, so 1.5
% Addition is the upper limit.

Ti: 0.05-0.2% Ti forms precipitates such as Ti carbide and nitride, and is effective for refining the microstructure of the core and carburized layer, so it is necessary to add at least 0.05%, but 0.2% If added in excess, the toughness will decrease, so the upper limit is made 0.2%.

Cr: 1.5% or less, Ni: 2.0% or less Cr and Ni are effective elements for further improving the hardenability of steel and strengthening the matrix, and can be added if necessary. However, Cr exceeds 1.5% or Ni is 2.0%
If it is added in an amount exceeding 1, the toughness will decrease. Therefore, when adding it, one or two of Cr: 1.5% or less and Ni: 2.0% or less.
Add seeds.

Al: 0.1% or less, N: 0.03% or less Al, N is an element effective in forming carbonitrides, refining crystal grains, and improving the toughness of steel. One or two of N can be added. However, if the amount of Al added exceeds 0.1%, the carbonitrides become coarse, the effect of grain refinement decreases and the toughness decreases, so 0.1% is made the upper limit of addition.

Further, when the N content exceeds 0.03%, the soundness of the steel ingot or the slab is impaired due to the generation of N blowholes, so the N content is set to 0.03% or less.

In order to improve machinability, Pb: 0.4% or less, S: 0.4
%, Te: 0.1% or less, it is also desirable to appropriately contain one or more of these elements, if necessary.

By carburizing the steel whose composition has been adjusted as described above in the austenite-ferrite two-phase region, the core has fine-grained ferrite-martensite two-phase grains with a grain size number 9 or more (usually 10 or more). The structure is obtained, and the carburized layer has a fine-grained structure with a grain size number of 9 or more (usually 10 or more), and the Charpy impact value of the core is 12 kg for JIS 3 test pieces.
Since it is f / m / cm ² or more, it becomes possible to manufacture parts that require excellent toughness, particularly differential parts such as automobile hypoid gears and differential gears.

The conditions of the carburizing treatment applied to the steel of the present invention are not particularly limited and can be applied similarly to the case of the conventional carburizing steel described above.

Next, examples of the present invention will be described.

Example 1 Steel having a chemical composition shown in Table 1 was melted, ingoted, and forged to manufacture a round bar having a diameter of 35 mm. Then heat to 925 ℃ 60
After normalizing by air-cooling after soaking for a minute, it was turned to a diameter of 25 mm and vacuum carburized under the conditions shown in Table 2. Then, the average grain size of austenite in the carburized layer and the average grain size of austenite and ferrite in the core were measured for each. Also, from the core of a 25 mm diameter round bar that has been vacuum carburized under the same conditions as in Table 2, JI
A S3 U-notch Charpy test piece was cut out and subjected to a Charpy impact test to measure the impact value. The results are shown in Table 1.

As comparative steel, a 25 mm diameter round bar of SNCM420H and SCM420H was vacuum carburized under the conditions shown in Table 3,
The results of the same measurements as above are also shown in Table 1.

As shown in Table 1, in all of the steels of the present invention (No. 1 to 5), the austenite average grain size of the carburized layer is 9 or more in grain size, and the core has a two-phase structure of ferrite and martensite. The average grain size is 9 or more, and the structure is remarkably refined. With this refinement of structure and addition of an appropriate amount of Mo, which is a toughness improving element, a Charpy impact value of 12 kgf ・ m / cm ² or more is exhibited, and a toughness equivalent to or higher than that of the carburized and tempered core of SNCM420H, a comparative steel, is exhibited. Have The other comparative steels have chemical compositions outside the range of the present invention, all have low Charpy impact values, and have toughness not exceeding 12 kgf · m / cm ² .

Next, the machinability test was conducted on the No. 1 steel in Table 1 and the Nos. 16 to 19 steels containing the machinability improving element shown in Table 4 under the conditions shown in Table 5. The results are shown in Table 6. As shown in the table, by adding the machinability improving element, an increase in the number of drill holes was confirmed, and a remarkable improvement in machinability was confirmed.

(Example 2) No. 20-22 steel and SNCM420H, S having the chemical composition shown in Table 7
With respect to CM420H, a round bar having a diameter of 25 mm was prepared by the same procedure as in Example 1, and high-temperature vacuum carburizing treatment was performed under the conditions shown in Table 8. As in Example 1, the average crystal grain size of the carburized layer and the core and The Charpy impact value was measured. Seventh of those results
Shown in the table.

As is clear from Table 7, the steel No. 20 of the present invention contains Mo and S.
With the addition of an appropriate amount of i, ferrite is stable even at high carburizing temperatures, so the core retains a remarkably fine two-phase structure of ferrite and martensite even after high-temperature carburizing, and the effect of Ti addition causes Austenite also becomes extremely fine, so the Charpy impact value is 12.5 kg due to the synergistic effect of the microstructure and matrix strengthening by adding Mo.
Excellent at f · m / cm ² . On the other hand, all the comparative steels have a low Charpy impact value after the high temperature carburizing treatment.

(Effects of the Invention) As described in detail above, the carburizing steel according to the present invention contains Mo and Si.
The addition of Ti maintains a two-phase structure of ferrite and austenite that is stable in the core even at the carburizing temperature, and Ti addition causes Ti carbide and nitride to precipitate in the carburized layer, and the core and carburized layer have a fine structure. However, since the base is basically strengthened with Mo, it has superior toughness to conventional carburizing steels such as SNCM420H steel. Further, the above effect can be obtained without the need for exceptionally strict surface roughness management that has been taken conventionally.

Therefore, by using the high toughness carburizing steel according to the present invention,
It is possible to manufacture differential parts such as automobile hypoid gears and differential gears with excellent toughness.

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Claims (4)

[Claims] 1. In weight% (hereinafter the same), C: 0.05 to 0.2
%, Mo: over 0.5% to 1.0%, and Mo + Si: 1 to 2%, Mn:
Austenite-ferrite 2 containing 0.3 to 1.5% and Ti: 0.05 to 0.2%, the balance being Fe and unavoidable impurities
After carburizing in the phase region, the core and carburized layer after carburization are fine-grained grains with grain size number 9 or more, and the core has a Charpy impact value of 12 kgf ・ m / cm ² or more. Characteristically high toughness steel for carburizing. 2. C: 0.05 to 0.2%, Mo: more than 0.5% to 1.0%, and Mo + Si: 1 to 2%, Mn: 0.3 to 1.5% and Ti: 0.05 to 0.2.
%, And further contains one or two of Cr: 1.5% or less and Ni: 2.0% or less, the balance being Fe and inevitable impurities, and carburizing by carburizing in the austenite-ferrite two-phase region. High-toughness carburizing steel characterized in that the subsequent core and carburized layer have fine grains with grain size number 9 or more and the Charpy impact value of the core is 12 kgf · m / cm ² or more. 3. C: 0.05-0.2%, Mo: over 0.5% -1.0%, and Mo + Si: 1-2%, Mn: 0.3-1.5% and Ti: 0.05-0.2.
%, And further 1% or less of Al: 0.1% or less and N: 0.03% or less, the balance being Fe and unavoidable impurities, and carburizing by carburizing in the austenite-ferrite two-phase region. High-toughness carburizing steel characterized in that the subsequent core and carburized layer have fine grains with grain size number 9 or more and the Charpy impact value of the core is 12 kgf · m / cm ² or more. 4. C: 0.05 to 0.2%, Mo: more than 0.5% to 1.0%, and Mo + Si: 1 to 2%, Mn: 0.3 to 1.5%, and Ti: 0.05 to 0.2.
%, And further Cr: 1.5% or less and Ni: 2.0% or less one or more, further Al: 0.1% or less and N: 0.03% or less one or more, The balance consists of Fe and unavoidable impurities, and by carburizing in the austenite-ferrite two-phase region, the crystal grains of the core and carburized layer after carburizing are fine grains with a grain size of No. 9 or more, and the shuffle of the core High toughness carburizing steel characterized by an impact value of 12 kgf · m / cm ² or more.