JPS63195257A - Production of high strength member - Google Patents

Abstract

PURPOSE:To improve strength, impact value, fatigue strength, etc., by subjecting a steel material with the specific ratios of C, Si, Mn, P, S, Al, N, etc., to a vacuum carburization and thereafter applying it to working and quenching in a hot area, etc. CONSTITUTION:The steel consisting of, by weight, 0.1-0.25% C, <=0.5% Si, 0.3-1.8% Mn, <=0.02% P, <=0.03% S, <=0.02% Al and <=0.02% N, and contg. at need, <=0.3% Cn and/or <=0.25% Ni, <=1.25% Cr and/or <=0.3% Mo with the balance Fe is melted to produce. Said steel material is subjected to the vacuum carburization and is applied to the working and quenching in the hot or warm area.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention is directed to manufacturing steel members (steel materials, steel parts, and products) that have a reinforced surface and have high strength and toughness. The present invention relates to a method for manufacturing high-strength members used for

(Prior Art) Conventionally, carburizing and quenching, nitriding, nitrocarburizing, and the like are well known as methods for strengthening the surface of steel materials and steel parts (products).

Among these, carburizing and quenching is a very effective surface strengthening method and is widely used, but this carburizing and quenching has the disadvantage of a long processing time.

Therefore, shortening the carburizing time has been an issue for a long time, and high-temperature carburizing methods such as high-frequency carburizing and vacuum carburizing
8-101328) has been developed.

However, the drawback of high-temperature carburizing is that the toughness decreases due to coarsening of the grains, and as a countermeasure to this problem, it is necessary to use steel containing elements such as All, Nb, Ti, Zr, etc. that prevent grain coarsening, or to use aluminum alloys after carburizing. Processing to raise or lower the transformation point is required, which increases costs.

On the other hand, one of the techniques for toughening steel is the heat treatment method, which can be divided into three types depending on the timing of processing: 1) processing before transformation, 2) processing during transformation, and 2) processing after transformation.

In conventional processing heat treatment methods, forging quenching, which involves plastic working and quenching at a stable austenite temperature, ausforming, which involves rapid cooling at a metastable austenite temperature, and controlled rolling, which involves gradual cooling, are
Isoforming, which involves processing during the pearlite or bainite transformation and quenching, and subzero processing, which involves processing during the martensitic transformation, belong to the processing heat treatment before the transformation of pearlite or bainite. Patenting, which involves processing after the completion of bainite transformation, and marforming, which involves processing martensite, belong to the post-transformation heat treatment described in (2) above.

By the way, gears manufactured by rolling or forging are said to have the following advantages over gears manufactured by conventional cutting.

(1) Extremely excellent fiber flow is formed at the tooth tip, increasing load capacity.

(2) The sides of the teeth can be easily crowned.

As a result, load conditions can be improved.

(3) It can be manufactured at a lower cost compared to mechanical processing such as gear cutting machines.

etc.

A method for manufacturing gears by rolling that has such advantages is already known (for example, Japanese Patent Application Laid-Open No. 59-225838
In addition, methods for manufacturing gears by precision forging are already known, including the West German BLW method and cold and warm forging methods.

Among these, the concept of precision forging of gears using the BLW method is to precision forge the tooth profile of the gear and not use it with the black skin intact, and BLW succeeded in forging precision gears using its unique technology. It has been put into practical use in dovetails, bevel gears, spur gears, etc.

However, since the gears manufactured by precision forging described above are used with the black skin intact, such gears have limited wear resistance and can only be used in low load ranges, so carburizing and quenching is required. be. Therefore, there is a problem that heat treatment distortion occurs due to this carburizing and quenching, and the meaning of precision forging is diminished.

On the other hand, cold forging makes it possible to manufacture gears with high precision, but in order to ensure the wear resistance of the tooth surface and the strength of the tooth root, surface hardening such as carburizing and quenching or carburizing/nitriding treatment is usually required. is necessary. However, when a material that has been plastically worked for strength by cold forging is heated to a carburizing temperature (approximately 920° C.), crystal grains become coarse due to recrystallization. At this time, if the processing rate is constant, the recrystallized grain size will be constant.

In the case of gear shapes, the recrystallized grains are not constant because there are subtle changes in the machining rate from tooth tip to tooth tip.

Since the grains become mixed and cause pitching, scoring, etc., it may be difficult to use them as gears, resulting in lower yields and higher costs.

Furthermore, although the manufacturing technology for precision gears by warm forging has advanced considerably, even gears manufactured by warm forging require surface hardening treatments such as carburizing and quenching or carbonitriding when used in high load ranges. However, there was a problem in that the gears, which had been molded with high precision, were subjected to heat treatment distortion.

Furthermore, another way to manufacture high-strength gears is to use medium- to high-carbon steel (0.4 to 0.6% C) as a raw material and perform gear cutting, followed by carburizing or carbonitriding. Post-bainite transformation temperature range (e.g. 235-275℃
) is a well-known technology to make the core of the gear a bainite structure and the surface a martensite structure by performing so-called austempering treatment (Heat Treatment Technology Association; Proceedings of the 20th Academic Conference (1988)). May 23,
(Japanese) page 49).

However, this gear manufacturing method has a problem in that the machinability of the material in the gear cutting process is very poor, making it difficult to apply to mass-produced gears.

In order to solve the conventional problems as described above, the present inventor has proposed a method that has a high surface treatment effect, such as a high surface hardness, a high toughness in the 6th part, and a high fatigue strength. We proposed a surface strengthening method for steel materials and steel parts (products) that enables long-term use and has outstanding formability (Japanese Patent Application No. 165966/1982).This method:
This is a method in which the material is carburized or carbonitrided in the austenitic state, and then the heat is used to perform processing such as forging on the austenite-containing material, followed by quenching.

Although this method has the above-mentioned advantages, depending on the processed shape and the material, the processing rate may vary greatly depending on the location, resulting in the structure becoming mixed grains. If direct quenching is performed after this, the hardness may vary depending on the size of the crystal grains, resulting in uneven hardness, and it cannot be ruled out that there is a possibility that the impact value, that is, the toughness, may decrease.

(Problems to be Solved by the Invention) Therefore, the present inventor has developed a solution for warm forging (600 to 1ioo
) We have developed a steel material with a composition adjusted so that the structure does not become mixed grains even if it is directly quenched from the temperature, and in particular a steel material for case hardening with a composition with adjusted amounts of Al and N. By applying solid carburizing, gas carburizing, gas immersion nitriding, etc. to strengthen the surface, we developed a method for manufacturing high-strength parts that does not have uneven hardness even if quenched after processing. However, the solid carburizing, gas carburizing,
Since surface strengthening methods such as gas immersion nitriding essentially involve carburizing with Co gas, so-called grain boundary oxidation (internal oxidation), in which austenite grain boundaries are oxidized by 02 decomposed from Co gas, may occur. , especially in Cr-containing case hardening steel, grain boundary oxidation tends to be significant.
Parts (products) that develop microcracks during subsequent processing
There was a problem in that it could reduce the fatigue strength of people.

(Object of the invention) The present invention has been made by focusing on the above-mentioned problems.
The object of the present invention is to provide a method for manufacturing high-strength members that does not cause oxidation of austenite grain boundaries (internal oxidation) after carburizing and can yield high-strength parts (products) with extremely good fatigue strength. It is.

[Structure of the Invention] (Means for Solving the Problems) The method for manufacturing a high-strength member according to the present invention includes C: 1% by weight;
o, 10-0.25%, Si: 0.50% or less, Mn:
0.30-1.8%, P: 0.020% or less, S: 0.
030% or less, Al: 0.020% or less, N: 0.02
0% or less, and if necessary Cu: 0.30% or less.

Ni: One or two of 0.25% or less, and Cr: 1.25% or less if necessary.

A steel material consisting of one or two types of Mo: 0.30% or less, the remainder substantially Fe, is vacuum carburized and then processed and quenched in a hot or warm region. It is characterized by this.

Next, the reason for limiting the composition (weight %) of the above-mentioned steel material applied to the method of manufacturing a high-strength member according to the present invention will be explained.

C is an element necessary to ensure the strength of parts (material parts, products, etc.), but in the present invention, after applying vacuum carburization to the surface of the steel material, processing such as forging is added and quenched. Therefore, the steel for case hardening was used with a C content of 0.10 to 0.25%, which is lower than that in ordinary structural members.

Si is added for deoxidation, but if added in excess, ductility deteriorates, so it is necessary to limit the amount to 0.50% or less.

Mn is at least 0.00 to prevent surface cracking during solidification.
It is necessary to add 30%. However, 1.8%
If it is added in an amount exceeding 1.8%, there is a risk of quench cracking because it is an element that improves hardenability, so it is necessary to set the upper limit to 1.8%.

P is 0.0 because it deteriorates the ductility of steel and promotes quench cracking.
It is necessary to keep it below 20%.

Since S deteriorates ductility, the upper limit needs to be 0.030%.

Ai is added as a strong deoxidizing agent.

If added in excess of 0.020%, austenite grains become mixed during quenching after forging, causing uneven heating, so the upper limit needs to be 0.020%.

Since N has a strong binding force with AfL, if it is contained in a large amount in steel, it will combine with AM as a deoxidizing agent and cause mixed grains in austenite during quenching after forging, causing uneven quenching. Therefore, it is necessary to set the upper limit to 0.020% or less.

Additionally, in order to provide oxidation resistance and weather resistance, 0.30% of Cu was added.
% or less, and it is also desirable to add one or two of these Ni in an amount of 0.25% or less, if necessary.

Furthermore, in order to improve mechanical properties, Cr was added to 1.25
If necessary, it is desirable to add one or two of these in the range of 0.30% or less. Fear arises.

The method of manufacturing a high-strength member according to the present invention involves vacuum carburizing a steel material having the above components, and then quenching it by hot forging or warm forging in a hot or warm region. The feature is that low-temperature heating is performed for reconstitution and the like.

In one embodiment of the present invention, a steel material having the above-mentioned composition is subjected to vacuum carburization in whole or in part, and then plastic processing such as forging is applied using the resulting heat. The material may be left as is or immediately thereafter rapidly cooled to below the Ms point and quenched.

In another embodiment of the present invention, the steel material having the above-mentioned composition is subjected to vacuum carburization in whole or in part, then rapidly cooled to about 400 to 700°C, and then forged during the transformation of the austenite. etc., and then rapidly cooled and quenched before the transformation is completed, preferably before 50% transformation is completed.

In yet another embodiment of the present invention, the steel material having the above-mentioned composition is subjected to vacuum carburization in whole or in part, and then subjected to plastic working such as forging immediately or after cooling to a predetermined temperature. After processing or after processing, the material is held in the bainite temperature range to undergo bainite transformation, and then isothermal quenching is performed by cooling.

A more detailed explanation will be given below based on the isothermal transformation diagram (T, T, T, diagram) in FIG.

Pattern ■ shown in Figure 1 is a steel material (parts, products, etc.) having the composition of the present invention, which is vacuum carburized in whole or in part at about 1030°C, and then, for example, at a processing rate of 50%. Hot forging is performed, and after the hot forging is completed at about 930° C., it is rapidly cooled (for example, placed in oil at 60° C.) and quenched.

Furthermore, pattern (2) shown in FIG. 1 is obtained after the steel material having the composition of the present invention is subjected to vacuum carburization in whole or in part at about 930°C.

It is charged into a constant temperature furnace such as a fluidized bed furnace or a neutral salt bath furnace at a temperature of about 500 to 700°C (approximately 600°C in the figure), and the entire temperature reaches 500 to 700°C (approximately 600°C in the figure). At this time, warm forging is performed to continue processing during the transformation from austenite to pearlite, and immediately after processing, the material is rapidly cooled (for example, placed in oil or water at 60° C.) and quenched.

Furthermore, pattern (2) shown in FIG.
After being held in a bainitic transformation region of 300°C (approximately 270°C in the figure), it is rapidly cooled and isothermal quenching is performed to convert 6 parts into bainite.

(Example) In weight%, C: o, 18%, Si: 0.30%, Mn:
0.63%, P: 0.015%, S2 0.008%, C
r: 1.02%, Al: 0.019%, N: 0.015
93% for steel materials consisting of 5% and the remainder substantially Fe.
Vacuum carburizing (propane gas addition, effective hardened layer depth 0.6 mm) was performed at 0°C for 6 hours, and the heat was subsequently used to carburize the material in the austenitic state to a capacity of 1600.
Forging processing with a ton die forging machine (processing rate is 30%, 50%,
70%) and molded into a Charpy impact test piece material.

At this time, the thickness of the forged material was adjusted to 14 mm in accordance with the processing rate so that the finished thickness of the Charpy impact test piece material formed by forging was 10 mm±1 mm.
3mm, 20.0mm.

The thickness was set to 33.3 mm, vacuum carburized under the above conditions, and then forged to a thickness of 10 mm±1 mm in one step, and a U-notch portion with a depth of 2 mm and a radius of 5 mm was formed on one side.

Next, each Charpy impact test piece material obtained by the above forging was immersed in a neutral salt bath at 820°C for 1 minute, and then
Quenched in oil at 80°C.

Next, 10 m from each Charpy impact test piece material.
Cut into m square x 55mm length, with a radius of 5mm in the center.
A Charpy impact test specimen having a U-notch portion was manufactured. Therefore, each Charpy impact test piece had a vacuum carburized layer also present at the U-notch.

In addition, for reference, we also prepared a model with a processing rate of 0%, but in this case, it was manufactured by machining, vacuum carburized and quenched in the same manner as above, and the impact test piece was vacuum carburized, forged and quenched as described above. Similarly, the conditions were adjusted so that a vacuum carburized layer also existed at the U-notch.

(Comparative example) In weight%, C: O, 18%, Si: 0.30%, Mn:
0.63%, P: 0.015%, S: o, ooa%, C
r: 1.02%, A sentence: 0.019%, N: 0.015
93% for steel materials consisting of 5% and the remainder substantially Fe.
Gas carburizing (RX gas carburizing, effective hardening layer depth 0.6 mm) was carried out at 0°C for 6 hours, and then the material in the austenitic state was processed using a die forging machine with a capacity of 1,600 mm. Forging processing (processing rate is 30%, 50%, 70%
%) and molded into a Charpy impact test piece material.

At this time, the thickness of the forged material was adjusted to 14 mm in accordance with the processing rate so that the finished thickness of the Charpy impact test piece material formed by forging was 10 mm±1 mm.
3mm, 20.0mm.

33.3 mm, gas carburized under the above conditions, and then forged in one step to a thickness of 10 mm and 1 mm, and a U notch with a depth of 2 mm and a radius of 5 mm was formed on one side.

Next, each Charpy impact test piece material obtained by the above forging was immersed in a neutral salt bath at 820°C for 1 minute, and then
Quenched in oil at 80°C.

Next, 10 m from each Charpy impact test piece material.
Cut into m square x 55mm length, with a radius of 5mm in the center.
A Charpy impact test specimen having a U-notch portion was manufactured. Therefore, each chassis kuchi impact test piece had a gas carburized layer also present in the U-notch portion.

Also, for reference, we have prepared a model with a processing rate of 0%.
In this case, it was manufactured by machining, gas carburized and quenched in the same manner as above, and a gas carburized layer was also present in the U-notch part, similar to the gas carburized forged and quenched impact test piece. Matched the conditions.

(Evaluation Example) The surface oxidation status of the carburized portion of each Charpy impact test piece produced in the above examples and comparative examples was investigated.

As a result, in the case of the RX gas carburized product of the comparative example, the austenite grain boundary was about 1 from the surface.

It was observed that ILm was oxidized and existed in the form of microcracks. When such a material is used under high stress, stress concentration occurs, which significantly reduces fatigue strength. If such a material is forged in a warm region, there is a high possibility that the microcracks will be enlarged and develop into cracks.

In contrast, in the case of the vacuum carburized product of the example, the austenite grain boundaries were not oxidized at all, and it was confirmed that the product was suitable for forging in a warm region.

Next, a Charpy impact test was conducted using each of the Charpy impact test pieces produced in the Examples and Comparative Examples, and the results shown in FIG. 2 were obtained.

As shown in Figure 2, gas carburization was performed at 930°C.

The impact value of the comparative example, which was subsequently forged and hardened, is as follows:
It is clear that there is almost no change even if the processing rate changes. This is because the austenite grain boundaries are oxidized (internal oxidation) due to gas carburizing, and the structure becomes mixed grain after forging.

On the other hand, in the case of the example of the present invention, in which a steel material with a controlled Ai content and N content was vacuum carburized at 930°C, and subsequently forged and quenched, the impact value increased almost in proportion to the increase in processing rate. It is clear that the value has increased significantly.

This is because vacuum carburizing was able to prevent oxidation of austenite grain boundaries and also prevent the structure from becoming mixed grains after forging.

On the other hand, the surface residual stress of each Charpy impact test piece is
When measured by the line method, the results shown in FIG. 3 were obtained.

As shown in FIG. 3, in the case of the embodiment of the present invention, the residual stress on the surface is -130 to -150 Kg/mm2,
It was confirmed that the fatigue strength was approximately three times higher than that of the comparative example.

[Effects of the Invention] As explained above, the method for manufacturing a high-strength member according to the present invention has C:0. lO ~ 0.25%, Si
: 0.50% or less, Mn: 0.30-1.8%, P: 0
．． 020% or less.

S: 0.030% or less, A sentence: 0.020% or less, N:
0.020% or less, and Cu: 0.30 as necessary
% or less, Ni: one or two of 0.25% or less, Cr: 1.25% or less, Mo: 0
．． 30% or less of one or two of the following, the remainder being substantially F
Since the steel material made of E is vacuum carburized and then processed and quenched in a hot or warm region, oxidation of austenite grain boundaries (internal oxidation) does not occur after carburizing, and Warm forging area (600~1100℃
) High-strength parts (1 material part, product) with significantly improved strength, impact value, fatigue strength, etc., with no unevenness in hardness after quenching because the structure does not become mixed grains even if quenched This brings about a very good effect in that it is possible to obtain the following.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram illustrating a heat pattern in the method of manufacturing a high-strength member according to the present invention; FIG. FIG. 3 is a graph illustrating the results of examining the relationship between processing rate and residual stress. Patent Applicant Nissan Motor Co., Ltd. Representative Patent Attorney Oshio Ma Figure 1, Hirai Ma (Sec)
Figure 2: Processed nose (%) Figure 3: Processed nose (%)

Claims (1)

[Claims] (1) In weight%, C: 0.10-0.25%, Si: 0
．． 50% or less, Mn: 0.30-1.8%, P: 0.0
20% or less, S: 0.030% or less, Al: 0.020
% or less, N: 0.020% or less, and C as necessary
One of u: 0.30% or less, Ni: 0.25% or less
Vacuum carburizing is applied to a steel material consisting of Cr: 1.25% or less, Mo: 0.30% or less, and the remainder substantially Fe. A method of manufacturing a high-strength member, which is then subjected to processing and quenching in a hot or warm region.