JP2000063943A - Production of steel for high temperature carburizing and steel for high temperature carburizing obtained by this method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing steel for high temperature carburizing capable of preventing the coarsening of crystal grains at the time of carburizing treatment. SOLUTION: Steel contg., by mass, 0.001 to 0.1% Nb, 0.001 to 0.1% Al and 0.010 to 0.03% N is heated at the heating temp. T( deg.C) satisfying T>=1000× 1.62×[Nb]+2.28×[N])+1000 ([ ] denotes the content (%) of each element), is thereafter hot-worked and is cooled in the temp. range of the hot working finishing temp. to the A3 transformation point at the average cooling rate of >=30 deg.C/sea.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing high-temperature carburizing steel (case hardening steel) and a high-temperature carburizing steel material obtained by the method. Specifically, when performing carburizing treatment after cold forging or warm forging, a method for producing high-temperature carburizing steel capable of preventing coarsening and abnormal growth of austenite crystal grains, and a high temperature obtained by the method The present invention relates to carburizing steel materials.

[0002]

BACKGROUND ART Case hardening steel used for gears, shafts, pins and the like is carburized and quenched after warm forging or cold forging. Since it is heated for a time, there are problems that the austenite crystal grains become coarse or abnormally grow. When the austenite crystal grains are coarsened, there are adverse effects such as variations in quenching hardness and strain, deterioration of toughness and deterioration of fatigue strength.

Particularly in recent years, there has been a tendency to carry out carburizing treatment at a high temperature in order to shorten the carburizing time in order to reduce the cost of manufacturing parts. However, as the carburizing temperature becomes higher, coarsening of the austenite crystal grains is more likely to occur, so that the above-mentioned adverse effects become more prominent.

On the other hand, attempts have been actively made to cold forge parts in order to improve the yield of steel products. However, when carburizing a cold forged part, even if the heating temperature is lowered, coarsening of austenite crystal grains is likely to occur partially, and the above adverse effects such as variation in quenching hardness are avoided. I can't.

Therefore, various proposals have been made in order to suppress coarsening of austenite crystal grains and avoid the above-mentioned problems. For example, in Japanese Examined Patent Publication No. 3-7744, the generation of abnormal coarse grains during surface hardening treatment is completely prevented by suppressing the amounts of Al, Nb, and N in steel, and the toughness decreases due to the increase in N content. Japanese Patent Laid-Open No. 4-176816 discloses a method of controlling hot working conditions such as working temperature and a cooling rate after working;
No. 3079 discloses a method of forging after carburizing, respectively. However, in recent years, since it has been unavoidable to raise the carburizing temperature and the working rate in the cold working, it has been found that these methods cannot sufficiently obtain the desired grain coarsening suppressing effect.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is a carburizing steel which is carburized and quenched after warm forging or cold forging. An object of the present invention is to provide a method for producing high-temperature carburizing steel that can prevent coarsening.

[0007]

The method for producing a steel for high temperature carburizing according to the present invention, which has been able to solve the above-mentioned problems, is the mass% (hereinafter the same) Nb: 0.001 to 0.1%, Al: 0.001 to 0.
1%, N: Steel containing 0.010 to 0.03%, T ≧ 1000 × {1.62 × [Nb] + 2.28 × [N]} + 1
000 (in the formula, [] means the content (%) of each element), and after heating at a heating temperature T (° C.), hot working,
Next, the temperature range from the hot working finish temperature to the A ₃ transformation point is set to 3
The gist is to cool at an average cooling rate of 0 ° C./sec or more.

Thereafter, the temperature range from A ₃ transformation point to 600 ° C. is further cooled at an average cooling rate of 0.5 ° C./sec or less; V: 1.5% or less (0% included) in steel. No), Ti: 0.1% or less (not including 0%), Ta:
0.1% or less (not including 0%), Zr: 0.1% (0
%), Te: 0.1% (0% is not included),
And REM: containing at least one selected from the group consisting of 0.1% (not including 0%) further improves the effect of suppressing coarsening of austenite crystal grains during high temperature carburization, and This is the preferred embodiment. Here, the “average cooling rate” means the average cooling rate on the surface of the steel material.

The steel material for high temperature carburization produced by the above method is also included in the scope of the present invention.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have earnestly studied to provide a high-temperature carburizing steel capable of sufficiently suppressing coarsening of austenite crystal grains during high-temperature carburization. As a result, N
If the steel in which the amounts of b, Al, and N are appropriately controlled is used and the precipitates are heated at a temperature at which they can form a solid solution, and then rapidly cooled in the austenite region, the precipitates are finely dispersed. Therefore, coarsening of austenite crystal grains can be prevented very efficiently; then, by gradually cooling the two-phase region and the ferrite region, the formation of bainite structure / martensite structure is suppressed, and the desired ferrite
Since a mixed structure of pearlite is obtained, it was found that wire drawability and cold forgeability can be secured, and the present invention was completed.

As described above, according to the present invention, not only the composition of components in steel, but also the heating temperature during hot working, the cooling rate in the austenite region after hot working, and the cooling rates in the two-phase region and the ferrite region are determined. The most important point lies in the fact that the control suppresses the coarsening of the austenite crystal grains during high temperature carburization. Nb and A in steel
1 and N are appropriately controlled to form these precipitates (carbides, nitrides, carbonitrides, etc.), and these precipitates are solid-solved and then finely dispersed in the steel to form austenite crystal grains. It is known per se to prevent growth, and the above-mentioned prior art is also based on this idea. However, in the present invention, in addition to controlling the component composition, the heating temperature during hot working, the cooling rate in the austenite region after hot working, or the cooling rate in the two-phase region and the ferrite region should be controlled within a predetermined range. Therefore, it has a technical significance in successfully finely depositing the above-mentioned precipitates, and such constitutional features unique to the method of the present invention are not disclosed in the above-mentioned prior art and are novel.

Each requirement constituting the method of the present invention will be described below.

Nb: 0.001 to 0.1% Nb is combined with C and N in steel to form precipitates of Nb carbide, Nb nitride, and Nb carbonitride, and the precipitate is steel. Finely dispersed in it is extremely effective in preventing coarsening of austenite crystal grains during carburizing and heating. In order to exert such an effect effectively, it is necessary to add 0.001% or more of Nb. It is preferably 0.02% or more. However, if added in excess of 0.1%,
The Nb carbonitrides agglomerate and precipitates effective for preventing crystal grain coarsening decrease, and the crystal grain coarsening prevention temperature lowers on the contrary, so the upper limit was made 0.1%. It is preferably 0.05% or less.

Al: 0.001 to 0.1% Al combines with N in steel to form AlN, and this nitride is effective for preventing growth of austenite crystal grains during carburizing and heating. In order to exert such an effect effectively, it is necessary to add 0.001% or more. It is preferably 0.01% or more. However, if added in excess of 0.1%, AlN aggregates and the precipitates effective for preventing the coarsening of the crystal grains decrease, and the temperature for preventing the coarsening of the crystal grains lowers. Therefore, the upper limit is 0.1%
And It is preferably 0.06% or less.

N: 0.010 to 0.03% N combines with Nb or Al to form NbN, NbCN, AlN
Is an element effective in suppressing the growth of austenite crystal grains. In order to effectively exhibit such an effect, it is necessary to add 0.010% or more. However, if added in excess of 0.03%, cracking tends to occur during forging or hot working, so the upper limit was made 0.03%. It is preferably 0.025% or less.

The present invention essentially contains the above elements, and the balance: Fe and unavoidable impurities. Further, the following elements are added for the purpose of preventing coarsening of austenite crystal grains during high temperature carburization. It is effective to positively contain at least one kind.

V: 1.5% or less, Ti: 0.1% or less,
Ta: 0.1% or less, Zr: 0.1%, Te: 0.1
%, And REM: 0.1%
At least one of these elements (which does not contain 0% of any element) produces fine precipitates (carbonitride, etc.) and is effective in suppressing coarsening of austenite crystal grains during high temperature carburization. Is. In order to effectively exhibit such an effect, V: 0.001% or more, Ti: 0.0
01% or more, Ta: 0.001% or more, Zr: 0.00
1% or more, Te: 0.001% or more, REM: 0.00
It is preferable to add each of them in an amount of 03% or more. But,
V: 1.5%, Ti: 0.1%, Ta: 0.1%, Z
If added in excess of r: 0.1%, Te: 0.1%, REM: 0.1%, carbonitrides and the like will aggregate / coarse,
The number of precipitates effective in preventing coarsening of crystal grains is reduced,
On the contrary, since the crystal grain coarsening prevention temperature is lowered, it is preferable to set the above numerical value to the upper limit. More preferably, V:
1.0% or less, Ti: 0.05% or less, Ta: 0.05
% Or less, Zr: 0.05% or less, Te: 0.05% or less, REM: 0.05% or less. These elements may be added alone or in combination of two or more.

Next, the steel having the above composition is heated at a heating temperature T (° C.) satisfying the following formula.

T ≧ 1000 × {1.62 × [Nb] + 2.28 ×
[N]} + 1000 (In the formula, [] means the content (%) of each element) As described above, in order to prevent coarsening of austenite crystal grains, Nb carbide, Nb nitride, Nb carbon It is necessary to disperse the nitride precipitate in a fine and large amount. In general,
The Nb precipitates obtained after casting are coarse, and there are very few fine precipitates effective in preventing grain coarsening. Therefore, in order to obtain the fine precipitates, the Nb precipitates were once solid-dissolved. After that, it has to be finely precipitated. As a result of studies by the present inventors, it is necessary to control the heating temperature T (° C.) well in order to efficiently obtain a predetermined fine precipitate; the heating temperature T is [Nb] and [Nb]. It was found that it is effective to set in relation to the contents of these elements, which is largely influenced by N]. Thus, in the present invention, the heating temperature T (° C.) during hot working is [N
There is a point where it is specified in relation to [b] and [N].

In order to clarify this, in FIG.
The relationship between the contents of b and N and the heating temperature is shown in the form of a graph. N content is 0.010%, 0.015%,
The range of the heating temperature T in the case of setting 0.020% and 0.025% is determined in relation to the amount of Nb in steel,
It was found that Nb precipitates were solid-dissolved and finely precipitated only when the plot obtained based on the above formula and the upper region thereof were heated at a temperature satisfying them.

After heating at the heating temperature T as described above, hot working is performed, but in the present invention, the temperature range from the hot working finish temperature to the A ₃ transformation point is cooled at an average cooling rate of 30 ° C./sec or more. Must.

When Nb precipitates such as Nb carbides are precipitated in the ferrite region (including the two-phase region) rather than in the austenite region, they are more easily miniaturized and crystal growth is less likely to occur. Therefore, it is very important to pay attention to the cooling rate so that Nb carbide or the like does not precipitate in the austenite region in the process of cooling after heating at high temperature as described above, once solid-dissolving the Nb precipitate, and then cooling. In the present invention, the hot working end temperature ~
The temperature range of the A ₃ transformation point was set to cool at an average cooling rate of 30 ° C./sec or more. As described above, the present invention is characterized in that not only the heating temperature in hot working but also the cooling rate after working are finely controlled so that a desired fine precipitate can be obtained successfully. It is preferably 35 ° C./sec or more. The upper limit is not particularly limited, but it is recommended to control it to 100 ° C./sec or less in consideration of the actual operation level and the like.

The above are the essential steps of the method of the present invention. Furthermore, the temperature range from the A ₃ transformation point to 600 ° C. is 0.5 ° C./se.
By cooling at an average cooling rate of c or less, the desired N
b The precipitate can be finely dispersed more efficiently.
That is, as described above, when Nb precipitates such as Nb carbide are precipitated in the ferrite region (including the two-phase region) rather than in the austenite region, finer precipitates are obtained and crystal growth is less likely to occur. Further, when a bainite structure or martensite structure is formed after rolling, there are problems such as breakage during wire drawing and deterioration of cold workability. In order to prevent such an adverse effect, it is necessary to make the structure after rolling into a uniform mixed structure of ferrite and pearlite. For this reason, the above temperature range is kept at a cooling rate of 0.5 ° C / sec or less. It depends on the slow cooling.

Further, when the high temperature carburizing steel obtained as described above is applied to parts, after the parts are processed,
Carburizing or carbonitriding with gas, vacuum, plasma, etc.
Alternatively, it is possible to employ a method in which induction hardening is performed and, if necessary, shot peening is performed and then the surface is strengthened.

The present invention will be described in detail below based on examples. However, the following examples do not limit the present invention, and it is within the technical scope of the present invention to make changes and modifications without departing from the spirit of the above and the following.

[0026]

EXAMPLE Steel materials having the chemical compositions shown in Table 1 were melted, slab-rolled into a square billet having a side length of 155 mm, heated to various heating temperatures, and rolled into a φ19 mm wire rod. Immediately after rolling, the temperature range up to the A ₃ transformation point was cooled at various cooling rates, and then the temperature range from the A ₃ transformation point to 600 ° C. was changed by changing the cooling rate. Table 2 shows the structure after rolling and the result of the measured hardness.

After that, each test material was wire-drawn at a surface reduction rate of 30%, and then spheroidized (750) under the heat treatment conditions shown in FIG.
(° C × 7 Hr → furnace cooling to 650 ° C.), followed by skin pass processing with a surface reduction rate of 10% and cold extrusion processing with a processing rate of 50%. Then, as shown in FIG.
After heating to 00 ° C. and holding for 3 hours, water quenching treatment was performed.

With respect to the grain size of each of the quenched materials thus obtained, the austenite grain size (J
IS G 0551) was measured, and the critical temperature at which the crystal grains were fine grains of size 5 or more was defined as the coarsening temperature.

The results are also shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

The following can be considered from the table.

Nos. 1 to 9 are examples of the present invention satisfying the requirements of the present invention, but in all of them, the coarsening temperature was extremely high at 975 ° C. or higher, and no disconnection was observed.

On the other hand, Nos. 10 to 16 which do not satisfy any of the requirements of the present invention are accompanied by the following problems.

No. 10 is a Nb-free steel, and the amount of fine precipitates effective for preventing crystal grain coarsening at high temperatures was small, so that coarsening of crystal grains occurred even at low temperatures.

No. 11 is an example in which the amount of Nb added is large,
Aggregation of Nb carbide, Nb nitride, and Nb carbonitride occurs to coarsen the precipitate, so that desired fine precipitates are reduced, and coarsening of crystal grains occurs even at low temperature.

No. 12 is an example in which a large amount of Al is added,
Since agglomeration of AlN occurred and the precipitate was coarsened, the desired fine precipitate was reduced, and the coarsened crystal grains were observed even at low temperature.

No. 13 is an example in which the amount of N added is small,
Since the precipitation amounts of Nb nitride, Nb carbonitride, and AlN were small, coarsening of crystal grains occurred even at low temperatures.

No. 14 is an example in which the heating temperature T during hot working is out of the range specified in the present invention, and the Nb precipitates existing before hot working do not sufficiently form a solid solution and remain coarse. Since it remained, the amount of precipitates after hot working decreased, and coarsening of crystal grains was observed even at low temperature.

No. 15 is an example in which the cooling rate up to the A ₃ transformation point after completion of hot working is slow, and Nb in the austenite region.
Precipitation / growth of carbides, Nb nitrides, and Nb carbonitrides occurred and the amount of fine precipitates decreased, so that coarsening of crystal grains occurred even at low temperatures.

No. 16 is an example in which after the hot working, the temperature range of A ₃ transformation point to 600 ° C. was cooled beyond the preferable cooling rate of the present invention, and the structure after rolling was (ferrite + pearlite). Since it has a mixed structure of + bainite), wire breakage occurred during wire drawing, and it could not be processed into parts.

[0042]

EFFECTS OF THE INVENTION The present invention is configured as described above, and by controlling the chemical composition in steel and the heat treatment conditions during hot working, steel that does not cause coarsening of crystal grains up to high temperatures during carburizing and quenching That is, it was possible to efficiently provide high-temperature carburizing steel having a high coarsening temperature.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between Nb and N contents and heating temperature.

FIG. 2 is a schematic diagram illustrating a spheroidizing annealing process used in Examples.

FIG. 3 is a schematic diagram showing a heat treatment pattern used in Examples.

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 4K032 AA01 AA05 AA16 AA19 AA21                       AA22 AA31 AA33 AA34 AA35                       AA36 AA39 AA40 BA02 CB02                       CD01 CD03 CF02 CG02 CH05                       CH06 CJ05

Claims (4)

[Claims] 1. In mass% (hereinafter the same), Nb: 0.001 to 0.1%, Al: 0.001 to 0.
1%, N: Steel containing 0.010 to 0.03%, T ≧ 1000 × {1.62 × [Nb] + 2.28 × [N]} + 1
000 (wherein, [] means the content of each element (%)) was heated at a heating temperature T (° C.) which satisfies, hot worked, the hot working finishing temperature to A ₃ transformation point Temperature range is 30 ℃ / s
A method for producing high-temperature carburizing steel, which comprises cooling at an average cooling rate of ec or more. 2. The manufacturing method according to claim 1, further comprising cooling in a temperature range from A ₃ transformation point to 600 ° C. at an average cooling rate of 0.5 ° C./sec or less. 3. V: 1.5% or less (not including 0%), Ti:
0.1% or less (not including 0%), Ta: 0.1% or less (not including 0%), Zr: 0.1% or less (not including 0%), Te: 0.1% or less (0% is not included), and REM: 0.1% or less (0% is not included)
The manufacturing method according to claim 1 or 2, which contains at least one selected from the group consisting of: 4. A steel material for high-temperature carburizing produced by the method according to claim 1.