JP2001152284A - High strength chromium steel for carburizing and carbonitriding - Google Patents

Abstract

(57) [Summary] (Corrected) [Problem] To provide a chromium steel having excellent fatigue strength for carburizing and carbonitriding. SOLUTION: C, Si, Mn, P, Cr, Al, Nb,
A chromium steel containing, the balance being Fe and unavoidable impurity elements. As an element for improving the toughness of the carburized layer and the core and improving the hardenability, one of Ni or 0.15% or less and Mo = 0.10% or less is used.
As an element that refines the seed or the austenite grain size and does not significantly impair the fatigue properties,
When i = 0.005 to 0.015%, S = 0.005 to 0.035% as an element for improving machinability and not significantly impairing fatigue properties.
Pb = 0.01-0.09%, Bi = 0.04-0.
20%, Te = 0.002 to 0.050%, Zr = 0.
01-0.20%, Ca = 0.0001-0.0100
%, Containing one or more of them.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carburizing quenching-tempering process (hereinafter referred to as "hardening and tempering") for a part requiring high fatigue strength, such as gears for automobiles, after hot or cold forging. , Abbreviated as "carburizing treatment") or a chromium steel for carburizing and nitrocarburizing treatment.

[0002]

2. Description of the Related Art Gears requiring high fatigue strength are conventionally made of nickel-chromium molybdenum steel or chromium molybdenum steel, and are subjected to hot forging or spheroidizing annealing, and then to cold forging. After carburizing, it is manufactured by carburizing or carbonitriding (hereinafter abbreviated as carburizing or carbonitriding).

In recent years, these mechanical parts have been reduced in the amount of expensive alloying elements such as nickel and molybdenum in order to reduce the material cost in order to reduce the manufacturing cost. However, conventional chromium steels such as SCr420H specified in JIS (Japanese Industrial Standards) have low fatigue strength and cannot be applied to gears that require high fatigue strength.

[0004]

The problem to be solved by the present invention is to control the chromium molybdenum steel specified by JIS (hereinafter simply referred to as chromium molybdenum steel) by adjusting the chemical composition and rolling conditions. It is an object of the present invention to provide a chromium steel having a fatigue strength and an impact strength equal to or higher than that of chromium steel.

[0005]

Means for Solving the Problems As a means for solving the above-mentioned problems, the present inventors have conducted intensive studies, and as a result, have obtained the following knowledge.

(A) Even if it is not chromium molybdenum steel,
In chromium steel, adjust the chemical composition. ・ Ensure hardenability beyond JIS. ・ Reduce grain boundary oxidation by reducing Si. ・ Reinforce crystal grain boundaries by reducing P. ・ Oxide inclusions by reducing O.・ Fatigue strength can be improved by reducing the crystal grain size by adding Nb and increasing the amount of N.

(B) However, in the above chromium steel, the hardness is increased due to the increase in hardenability, and the impact strength is greatly reduced as compared with the chromium molybdenum steel.

(C) The reason is that the reduction in the grain boundary strength due to the reduction of molybdenum that contributes to the strengthening of the grain boundaries cannot be compensated for by merely strengthening the grain boundaries by reducing P. It is.

(D) In order to compensate for the decrease in the crystal grain boundaries caused by the reduction of molybdenum that contributes to the strengthening of the crystal grain boundaries, it is necessary to further refine the crystal grains by adding and increasing the amount of Nb. Depending on the manufacturing conditions of the manufacturer,
The effect may not be fully exhibited.

(E) The reason is that even if Nb is added to increase the amount of N for the refinement of the crystal grain size, the carbonitride formed from them at the stage of steel product shipment (including carbon) Nb carbides are also formed because of the addition, and these are hereinafter referred to as carbonitrides). This is because these carbonitrides have a small effect of suppressing the growth of crystal grains during the carburizing or carbonitriding treatment, and the crystal grains in the final product of the user become coarse.

(F) That is, when the production process of the user is hot forging, as shown conceptually in FIG. 1, in the prior art, the carbonitride of the steel product shipped is coarse, Sometimes, when carbonitrides remain undissolved and precipitate as carbonitrides again during cooling after hot forging, those dissolved carbonitrides are preferentially deposited as nuclei, so in other places Since precipitation hardly occurs, carbonitrides become coarse. Therefore, since the carbonitride before carburizing or carbonitriding is coarse, the effect of these carbonitrides to suppress the growth of crystal grains during carburizing or carbonitriding is small,
The crystal grains in the user's final product become coarse.

(G) On the other hand, in the present invention to be described later, the carbonitride does not remain undissolved at the time of hot forging, and when it is precipitated again as a carbonitride during cooling, the frequency of the precipitation is limited to steel. The carbonitride is fine because it is the same everywhere and precipitates uniformly. Therefore, since the carbonitride before carburizing or carbonitriding is fine, these carbonitrides have a large effect of suppressing the growth of crystal grains during carburizing or carbonitriding, and the crystal grains in the final product of the user are fine. Becomes

(H) When the production process of the user is cold forging, as shown conceptually in FIG. 2, the temperature at the time of soaking heating or spheroidizing annealing is as low as 950.degree. Since the nitrides are not melted and remain as they are, if the carbonitrides of the steel shipping products are coarse as in the prior art, the carbonitrides before carburizing or carbonitriding also become coarse, resulting in carburizing or carbonitriding. Meanwhile, these carbonitrides have a small effect of suppressing the growth of crystal grains, and the crystal grains in the final product of the user become coarse.

(I) On the other hand, in the present invention to be described later, since the carbonitride of the steel product is very fine, the carbonitride before carburizing or carbonitriding is also fine. Has a great effect of suppressing the growth of crystal grains, and the crystal grains in the final product of the user become fine.

(J) Accordingly, in order to suppress the growth of crystal grains with carbonitride during the carburizing or carbonitriding process in the production process of the user and to refine the crystal grains of the final product,
Before the steel products are shipped to the users, it is necessary to refine the carbonitrides in the manufacturing process of the steelmaker.

(K) For this purpose, not only the adjustment of the chemical components, but also the hot-forming conditions such as hot rolling as in the present invention to be described later, the heating temperature of the steel ingot is set to 1200 ° C. or more. -Finish hot forming at a finishing temperature of 800 ° C or higher.-600 ° C at an average cooling rate of 30 ° C / min or higher after hot forming.
It is necessary to cool to below.

Specific means for solving the problems based on these findings are, as mass percent, C = 0.10 to 0.30%, Si = 0.15% or less, and Mn = 0.90%. 1.40%, P = 0.015% or less, Cr = 1.25 to 1.70%, Al = 0.010 to 0.050%, Nb = 0.001 to 0.050%, O = 0. 0015% or less, N = 0.0100 to 0.0200%, steel containing the balance of Fe and unavoidable impurity elements is heated to 1200 ° C or more, and hot formed by hot rolling at a finishing temperature of 800 ° C or more. After the completion of the above, the chromium steel for carburizing and carbonitriding is obtained by cooling to 600 ° C. or less at an average cooling rate of 30 ° C./min or more.

Furthermore, in the above, as an element for improving the toughness of the carburized layer and the core and improving the hardenability, the following are selected from among Ni = 0.15% or less and Mo = 0.10% or less. It is to manufacture a chromium steel for carburizing and carbonitriding, characterized by containing one or two kinds.

Further, in the above, Ti = 0.005 to 0.015% by mass as an element for refining the austenite crystal grain size and not significantly impairing the fatigue characteristics. It is an object of the present invention to provide a chromium steel for carburizing and carbonitriding.

Further, in the above, S = 0.005-0.035%, Pb = 0.01-0. 09%, Bi = 0.04 to 0.20%, Te = 0.002 to 0.050%, Zr = 0.01 to 0.20%, and Ca = 0.0001 to 0.0100%. It is an object of the present invention to provide a chromium steel for carburizing and carbonitriding which contains one or more species.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention which demonstrate means for solving the problems will be described below. (1) Experiment 1 In Experiment 1, the chemical components were adjusted to ensure hardenability beyond JIS. • Reduction of grain boundary oxidation by reducing Si. • Strengthening of grain boundaries by reducing P. • Reduction of O. Reduction of oxide inclusions due to chromium ・ Chromium steel (hereinafter referred to as “examined steel”) that considers the refinement of crystal grain size by adding Nb and increasing the amount of N and the chromium steels SCr420H and J
SCM is a chromium molybdenum steel specified in IS
The fatigue strength and impact strength of 420H and SCM822H were investigated.

Table 1 shows the chemical composition of the steel used in the experiment. These steels were melted in an electric furnace, ingot-granulated, and then hot-rolled to 60 mmφ under ordinary rolling conditions.

[0023]

[Table 1]

These rolled materials were heated to 1300 ° C. assuming hot forging by a user, then hot forged to 30 mmφ and air-cooled. From these forged materials, 10 m shown in FIG.
A Charpy impact test piece with a notch of mR (hereinafter abbreviated as Charpy test piece) and an Ono-type rotating bending fatigue test piece shown in FIG. 4 were prepared, and carburizing treatment was performed under the conditions shown in FIG. The surface hardness, effective hardened layer depth, core hardness, and grain boundary oxidation depth of the carburized rotating bending fatigue test piece were determined. The grain size number (the larger the The crystal grains are fine), and a Charpy impact test and an Ono-type rotary bending fatigue test were performed to determine an impact value and a rotary bending fatigue limit. Table 2 shows the results.

[0025]

[Table 2]

From these results, the steel to be studied has improved hardenability compared to JIS-specified SCr420H, resulting in an increase in core hardness and a reduction in the grain boundary oxidation depth, resulting in a reduction in rotational bending fatigue limit. It was confirmed that the improvement was at least equivalent to that of SCM822H specified by JIS. On the other hand, the impact value of the studied steel became brittle due to the increase in hardness,
It was found to be significantly lower than SCR420H specified by JIS. This is because in the studied steels, although the grain size was refined by adding Nb and increasing the amount of N, the effect was only improved by about 1.0 in the grain size number, and the effect of reducing the molybdenum was reduced. It was presumed that the decrease in the strength of the grain boundaries could not be compensated for by the refinement of the crystal grains. In order to estimate these causes, a typical carbonitride particle size of 10
When the visual field was measured and their average was determined, about 0.6
It was as large as μm, and it was found that the effect of refining crystal grains was not sufficient because growth of crystal grains during carburization could not be suppressed.

Therefore, the effect of the hot rolling conditions on the size of the carbonitride was investigated. The rolling size was 80 mmφ as in the above-described experiment. These rolled materials were heated to 1300 ° C. assuming hot forging by a user, and then heated to 30 ° C.
It was hot forged to mmφ and air cooled. Fig. 3
Was prepared and carburized under the conditions shown in FIG. Next, the grain size number of the carburized Charpy impact test piece was measured, and a Charpy impact test was performed to determine an impact value. In addition, for a rolled material and a steel material after carburizing treatment after hot forging, the grain size of typical carbonitrides was measured in 10 visual fields, and the average value thereof was determined. Table 3 shows these results together with the experimental hot rolling conditions.

[0028]

[Table 3]

FIG. 6 shows the relationship between the average grain size of the carbonitride of the rolled material and the steel material after carburizing after hot forging and the cooling rate. Thus, in both the rolled material and the steel material after the hot forging and before the carburizing treatment, except for the steel having a heating temperature of 1200 ° C or lower and the steel having a finishing temperature of 800 ° C or lower, the size of the carbonitride increases as the cooling rate increases. Is smaller. Also,
It can be seen that the tendency is saturated when the cooling rate is 30 ° C./min or more. This is presumably because the higher the cooling rate, the shorter the time required for carbonitrides to grow in a high temperature range, and the smaller their average grain size. On the other hand, when the heating temperature is 1200
If the temperature is lower than ℃, various carbonitrides that have grown greatly during solidification existing in the ingot before rolling cannot be fully dissolved in austenite, and those unresolved carbonitrides are preferentially used as nuclei. Because of the growth, it is estimated that their average particle size is large even if the subsequent cooling rate is high. Further, when the finishing temperature is 800 ° C. or less, the cooling time is high because the carbonitride grows in a high temperature range and in a large rolling size that is difficult to be cooled, during which the carbonitride grows. Even so, it is estimated that their average particle size is large.

FIG. 7 shows the relationship between the impact value, the crystal grain size number and the cooling rate. Thus, the heating temperature is 1200 ° C.
Except for the following steels and steels having a finishing temperature of 800 ° C. or lower, it can be seen that as the cooling rate increases, the crystal grains become finer and the impact value improves. The tendency is that the cooling rate is 30 ° C.
It can be seen that the saturation occurs at more than / min.

From the above results, by setting the hot rolling conditions as follows: heating temperature = 1200 ° C. or more, finishing temperature = 800 ° C. or more, cooling rate = 30 ° C./min or more, carbonitrides are finely dispersed. In addition, it was confirmed that the impact value was improved to be equal to or more than that of SCM420H specified by JIS even in the case of the studied steel in which the crystal grains became fine and the hardness was increased.

(2) Experiment 2 In Experiment 2, it was confirmed whether or not the present invention found in Experiment 1 can be satisfied with respect to the entire range of chemical components that can be assumed as chromium steel for carburizing and carbonitriding. The range of chemical components was determined. Table 4 shows the chemical components of the inventive steel and the comparative steel used.

[0033]

[Table 4]

Here, the first invention steel is the invention steel corresponding to claim 1, the second invention steel is the invention steel corresponding to claim 2, and the third invention steel is the claim 3 The invention steel and the fourth invention steel are invention steels corresponding to claim 4. Comparative steel A is SC specified in JIS.
r420H, comparative steel B is SCM specified in JIS
420H, comparative steel C is SCM8 specified in JIS
22H, Comparative Steel D has Nb and N addition amounts, and Comparative Steel E has P and O contents out of the range of the present invention steel.

These steels were smelted in an electric furnace or a vacuum high-frequency melting furnace, and after ingot making, they were hot-rolled or hot-formed to 60 mmφ under the following conditions. Heating temperature = 1100-1300 ° C Finishing temperature = 700-1000 ° C Cooling rate = 15-100 ° C / min

Next, these steel materials were heated to 1300 ° C. assuming hot forging by a user, and then heated to 30 mmφ.
Hot forged and air cooled.

FIG. 8 shows the hardenability of the invention steel. The hardenability of the invention steel exceeds the upper limit of the H band of SCr420H specified in JIS. Next, a Charpy test piece shown in FIG. 3 and an Ono-type rotary bending test piece shown in FIG. 4 were prepared from these forged materials, and carburizing was performed under the conditions shown in FIG. The surface hardness, effective hardened layer depth, core hardness, and grain boundary oxidation depth were measured for the carburized rotating bending fatigue test piece, and the grain size number was measured for the carburized Charpy impact test piece. The impact test and the rotational bending fatigue test were performed to determine the impact value and the rotational bending fatigue limit. Table 5 shows the results.

[0038]

[Table 5]

From the above, in the invention example in which the heating temperature, the finishing temperature, and the cooling rate of the invention steel satisfy the hot forming conditions specified by the present invention, the core hardness is high and the grain size number after carburizing treatment is high. Is 9.5 or more, the crystal grains are refined, and the rotational bending fatigue limit is stipulated in JIS.
It can be seen that it is equal to or more than M822H and equal to or more than SCM420H specified by JIS without significant reduction in impact value.

On the other hand, even if the heating temperature, the finishing temperature and the cooling rate satisfy the hot forming conditions specified by the present invention, the comparative steel D having Nb and N outside the range of the present invention.
Comparative steel E, in which P and O are out of the range of the present invention, have a low rotational bending fatigue limit.

Further, even in the case of the inventive steel 7, as in the comparative example, the heating temperature is within the scope of the present invention.
The case where the temperature is 0 ° C. or the finishing temperature is within the claims of the present invention 7
When the cooling rate is 00 ° C., and when the cooling rate is 15 ° C./min, which is within the scope of the present invention, the crystal grain size number is less than 9.5 and the impact value is low.

Accordingly, the present invention is intended to be solved only by using the steel of the present invention in the range of the claimed chemical composition and by the present invention in which the hot forming conditions such as hot rolling satisfy the conditions specified by the present invention. That is, it was confirmed that the chromium steel can improve the fatigue strength and the impact strength to be equal to or higher than that of the chromium molybdenum steel.

The present invention has been completed as described above. Next, the reasons for limiting the chemical composition of the present invention and the hot forming conditions such as hot rolling will be described. C: 0.10 to 0.30% by mass C must be added in an amount of at least 0.10% by mass or more in order to secure the core hardness required for the gear. However, excessive addition increases the hardness of the core too much and degrades the toughness of the core. To avoid this, it is necessary to limit the upper limit to 0.30% by mass. Therefore, the addition amount of C is set in the range of 0.10 to 0.30% by mass.

Si: 0.15% by mass or less Si promotes grain boundary oxidation which lowers fatigue strength. In order to avoid this, it is necessary to limit the upper limit to 0.15% by mass. Therefore, the content of Si is 0.15% by mass.
Limited to the following.

Mn: 0.90 to 1.40% by mass Mn needs to be added in an amount of at least 0.90% by mass or more in order to secure hardenability. However, excessive addition increases the hardness of the core too much and degrades the toughness of the core. To avoid this, it is necessary to limit the upper limit to 1.40% by mass. Therefore, the addition amount of Mn is 0.1.
The range was 90 to 1.40% by mass.

P: 0.015% by mass or less P is an element that segregates at the austenite grain boundary and weakens the grain boundary to reduce toughness and fatigue strength.
If the content exceeds 5% by mass, such adverse effects become remarkable. Therefore, the content of P is limited to 0.015% by mass or less.

Ni: 0.15% by mass or less Ni may not be added. If added, the toughness of the carburized layer and the core is improved and the hardenability is improved. In order to exhibit this effect, it is necessary to add at least 0.01% by mass or more. However, excessive addition saturates its effect and is not desirable from an economical point of view. In order to avoid this, it is necessary to limit the upper limit to 0.15% by mass. Therefore, the content of Ni is limited to 0.15% by mass or less.

Cr: 1.25 to 1.70% by mass Cr must be added in an amount of at least 1.25% by mass to secure hardenability. However, excessive addition increases the hardness of the core too much and degrades the toughness of the core. To avoid this, it is necessary to limit the upper limit to 1.70% by mass. Therefore, the added amount of Cr is 1.
The range was 25 to 1.70% by mass.

Mo: 0.10% by mass or less Mo need not be added. If added, the toughness of the carburized layer and the core is improved and the hardenability is improved. In order to exhibit this effect, it is necessary to add at least 0.01% by mass or more. However, excessive addition saturates its effect and is not desirable from an economical point of view. In order to avoid this, it is necessary to limit the upper limit to 0.10% by mass. Therefore, the content of Mo is limited to 0.10% by mass or less.

Al: 0.010 to 0.050% by mass Al is an element that combines with N to form AlN and has the effect of reducing the austenite crystal grain size. It contributes to improving the toughness of the core. In order to exhibit the effect, it is necessary to add at least 0.010% by mass or more. However, excessive addition promotes the formation of Al ₂ O ₃ inclusions harmful to fatigue strength. To avoid this, the upper limit is 0.050 mass%.
It is necessary to limit to. Therefore, the amount of Al added is in the range of 0.010 to 0.050% by mass.

Nb: 0.001 to 0.050 mass% Nb combines with C and N in steel to form a carbonitride,
Like N, it is an element that is effective in reducing the austenite grain size, and contributes to the improvement of the toughness of the carburized layer and the core through the refinement. In order to exhibit the effect, it is necessary to add at least 0.001% by mass or more. However, excessive addition forms and precipitates coarse carbonitrides and impairs the toughness of the carburized layer. To avoid this, it is necessary to limit the upper limit to 0.050 mass%. Therefore, the addition amount of Nb is set in the range of 0.001 to 0.050 mass%.

O: 0.0015% by mass or less O is present as oxide-based inclusions in steel and reduces fatigue strength. To avoid this, set the upper limit to 0.
It is necessary to limit to 0015 mass%. Therefore, O
Was limited to 0.0015% by mass or less.

N: 0.0100 to 0.0200% by mass N combines with Al and Nb to form AlN and NbN, and is an element effective in refining the austenite crystal grain size. Contributes to the toughness of the carburized layer and the core. In order to exhibit the effect, it is necessary to add at least 0.0100% by mass or more. However, excessive addition causes generation of bubbles on the surface of the steel ingot during solidification and deterioration of cold forgeability. In order to avoid this, it is necessary to limit the upper limit to 0.0200% by mass. Therefore, the addition amount of N is set in the range of 0.0100 to 0.0200% by mass.

The hot forming conditions, such as hot rolling, are as follows:
1200 ° C. or more, Finishing temperature: 800 ° C. or more, Cooling rate: 30 ° C./min or more In order to suppress the crystal grain growth in the crystal during carburizing or carbonitriding in the production process of the user and to refine the crystal grains by carbonitride. It is necessary to stably miniaturize carbonitrides in the manufacturing process of steelmakers before shipping them to users.

Therefore, at the time of hot forming such as hot rolling , the heating temperature is set to 12 in order to dissolve the carbonitride, which has greatly grown during solidification existing in the steel ingot, into austenite.
It is necessary to be 00 ° C. or higher, and in a high temperature range before limiting the cooling rate after hot forming, and for a long time with a large rolled size that is difficult to be cooled, a large carbonitride is not grown. For this reason, the finishing temperature needs to be 800 ° C. or more, and the cooling rate needs to be 30 ° C./min or more so that carbonitrides do not grow large during cooling after hot forming. Therefore, regarding hot forming conditions such as hot rolling, the heating temperature is 1200 ° C. or more, and the finishing temperature is 8 ° C.
The cooling rate was specified as 00 ° C. or higher and the cooling rate was specified as 30 ° C./minute or higher.

Ti: 0.005 to 0.015% by mass Ti combines with C and N in steel to form a carbonitride,
Like N, it is an element that is effective in reducing the austenite grain size, and contributes to the improvement of the toughness of the carburized layer and the core through the refinement. In order to exhibit the effect, it is necessary to add at least 0.005% by mass or more. However, excessive addition forms and precipitates coarse carbonitrides and impairs the toughness of the carburized layer. In order to avoid this, it is necessary to limit the upper limit to 0.015% by mass. Therefore, the addition amount of Ti is set in the range of 0.005 to 0.015% by mass.

S: 0.005 to 0.035% by mass S is mostly present in steel as sulfide-based inclusions, and in a part formed by cutting such as a gear, it is necessary to improve machinability. It is an effective element. To do so, at least 0.
It is necessary to add 005% by mass or more. However,
Excessive addition causes a reduction in fatigue strength. In order to avoid this, it is necessary to limit the upper limit to 0.035% by mass. Therefore, the addition amount of S is 0.005 to 0.5.
The range was 035% by mass.

Pb: 0.01 to 0.09% by mass Pb is an element effective for improving machinability in a part formed by cutting like a gear like S. For that purpose, it is necessary to add at least 0.01% by mass or more. However, excessive addition causes a reduction in fatigue strength. Further, when the content is 0.10% by mass or more, Pb handling is subject to legal regulations such as dust collectors and methods. In order to avoid this, it is necessary to limit the upper limit to 0.09% by mass. Therefore, the added amount of Pb is 0.01 to 0.1.
The content was in the range of 09% by mass.

Bi: 0.04 to 0.20% by mass Bi is an element effective for improving machinability in a part formed by cutting like a gear like S and Pb.
For this purpose, it is necessary to add at least 0.04% by mass or more. However, excessive addition reduces toughness. In order to avoid this, it is necessary to limit the upper limit to 0.20% by mass. Therefore, the amount of Bi added is 0.1.
The range was from 0.4 to 0.20% by mass.

Te: 0.002 to 0.050 mass% Te is an element that increases the interfacial energy between the sulfide-based oxide and Fe, which is the parent phase, makes the shape of the spindle spindle-shaped and improves machinability. For that purpose, it is necessary to add at least 0.002% by mass or more. However, excessive addition causes hot embrittlement. To avoid this, it is necessary to limit the upper limit to 0.050 mass%. Therefore,
The amount of Te added was in the range of 0.002 to 0.050% by mass.

Zr: 0.01 to 0.20% by mass Zr is an element for improving machinability. For that purpose, it is necessary to add at least 0.01% by mass or more. However, excessive addition reduces toughness. In order to avoid this, it is necessary to limit the upper limit to 0.20% by mass. Therefore, the addition amount of Zr is set in the range of 0.01 to 0.20% by mass.

Ca: 0.0001 to 0.0100% by mass Ca is an element for improving machinability. For that purpose, addition of at least 0.0001% by mass is necessary.
However, excessive addition reduces toughness. In order to avoid this, it is necessary to limit the upper limit to 0.0100% by mass. Therefore, the amount of Ca added is 0.000
The range was 1 to 0.0100% by mass.

(Examples) Next, the present invention will be described in more detail with reference to specific examples of carburized parts. Table 6 shows the invention steel based on the above knowledge and SCr420 specified by JIS.
H, SCM420H, and SCM822H chemical components.

[0064]

[Table 6]

These steels were melted in an electric furnace and ingoted by a bloom continuous caster. Thereafter, these bloom slabs were heated at 1200 ° C., finished at 800 to 850 ° C., 600
The sample was hot-rolled to a diameter of 53 mm at a cooling rate of 15 to 50 ° C./min. After high-frequency heating, these rolled materials were hot forged into a rear output shaft, which is a part used in a passenger car shown in FIG. 9, and machined, and then carburized under the conditions shown in FIG.

Next, a body torsion fatigue test was performed on these parts. The results are shown in Table 7 together with the rolling conditions, carburizing characteristics and the grain size number of the carburized product.

[0067]

[Table 7]

From the above, when the invention steel was used and the hot forming conditions during hot rolling were such that the heating temperature was 1200 ° C. or more and the finishing temperature was 80 ° C.
The crystal grain size numbers of the invention examples having a cooling rate of 0 ° C. or more and a cooling rate of 30 ° C./min or more are SCr420H and SCM420 specified by JIS.
H, larger than SCM822H, and the crystal grains are finer. On the other hand, even in the case of the component steel of the present invention, the cooling rate among the hot forming conditions is 15 ° C./min, which is 30 ° C. within the range of the present invention.
/ Min or more in the comparative examples, the grain size number is small,
The crystal grains are not fine.

The hardness of the core of the invention example using the invention steel is equivalent to that of SCM822H specified by JIS, and the torsional fatigue strength is SCR420H and SCM420H specified by JIS.
It was confirmed that it had a higher fatigue strength than that of SCM822H specified by JIS. On the other hand, even in the case of the component steel of the present invention, in the comparative example in which the cooling rate is out of the hot forming conditions, the core hardness is equivalent to that of SCM822H specified by JIS, but since the crystal grains are not fine, twisting is performed. Fatigue strength is SCM420H and SCM822 specified by JIS.
Lower than H.

As described above, by adjusting the chemical components,
Ensuring hardenability beyond S, reducing grain boundary oxidation by reducing Si, strengthening grain boundaries by reducing P, reducing oxide inclusions by reducing O, adding Nb and increasing N Further, hot forming conditions such as hot rolling are within the scope of the present invention.
By setting the heating temperature to 1200 ° C or more, the finishing temperature to 800 ° C or more, and the cooling rate to 30 ° C / min or more, the crystal grains after carburizing are refined, and even in actual parts, the chromium steel is used, and the fatigue is equal to or higher than that of chromium molybdenum steel. It was confirmed to have strength.

[0071]

The effect of the present invention is as follows.
By adjusting the chemical composition of steel and hot forming conditions such as hot rolling, even low-cost components that do not contain a large amount of Mo, such as chromium molybdenum steel, can provide fatigue strength equal to or higher than that of chromium molybdenum steel. Can be.

Accordingly, it is possible to convert the carburized gear into chromium steel in the current manufacturing process, and to contribute to the reduction of manufacturing cost and the improvement of reliability in the industry using gears. Can be

[Brief description of the drawings]

FIG. 1 is a view showing a carbonitride solid solution and precipitation behavior by hot forging.

FIG. 2 is a graph showing carbonitride solid solution and precipitation behavior by cold forging.

FIG. 3 is a diagram showing a dimensional shape of a Charpy impact test specimen with a 10 mm R notch.

FIG. 4 is a view showing the dimensions and shape of an Ono-type rotating bending fatigue test piece.

FIG. 5 is a graph showing carburizing quenching-tempering conditions.

FIG. 6 is a graph showing the relationship between the average carbonitride particle size and the cooling rate.

FIG. 7 is a graph showing a relationship between an impact value, a crystal grain size, and a cooling rate.

FIG. 8 is a graph showing a test result of hardenability of the invention steel.

FIG. 9 is a schematic view of a rear output shaft.

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[Procedure amendment]

[Submission date] September 28, 2000 (2009.2)
8)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0055

[Correction method] Change

[Correction contents]

Therefore, at the time of hot forming such as hot rolling, the heating temperature is set to 12 in order to dissolve the carbonitride, which has greatly grown during solidification existing in the steel ingot, into austenite.
It is necessary to be 00 ° C. or higher, and in a high temperature range before limiting the cooling rate after hot forming, and for a long time with a large rolled size that is difficult to be cooled, a large carbonitride is not grown. For this reason, the finishing temperature needs to be 800 ° C. or more, and the cooling rate needs to be 30 ° C./min or more so that carbonitrides do not grow large during cooling after hot forming. Therefore, regarding hot forming conditions such as hot rolling, the heating temperature is 1200 ° C. or more, and the finishing temperature is 8 ° C.
The cooling rate was specified as 00 ° C. or higher, and the cooling rate was specified as 30 ° C./minute or higher.

[Procedure amendment 2]

[Document name to be amended] Drawing

[Correction target item name] Fig. 1

[Correction method] Change

[Correction contents]

FIG.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hideo Ueno 12 Nakamachi, Muroran, Hokkaido Mitsubishi Steel Muroran Special Steel Co., Ltd. (72) Inventor Hidenori Hiromatsu 12 Nakamachi, Muroran City, Hokkaido Mitsubishi Steel Muroran Special Steel Co., Ltd. (72) Inventor Miki Watanabe 12 Nakamachi, Muroran City, Hokkaido Mitsubishi Steel Muroran Special Steel Co., Ltd. (72) Inventor Kinya Kato 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Yoichi Taniguchi 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Mitsuhiko Hirano 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Invention Applicant Kazuhiko Kato 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Hiroaki Ogasawara 5-33-8 Shiba, Minato-ku, Tokyo Mitsubishi Automotive Industry Co., Ltd.

Claims (4)

[Claims] 1. In mass percent, C = 0.10 to 0.30%, Si = 0.15% or less, Mn = 0.90 to 1.40%, P = 0.015% or less, Cr = 1. 0.25 to 1.70%, Al = 0.010 to 0.050%, Nb = 0.001 to 0.050%, O = 0.0015% or less, N = 0.0100 to 0.0200% Then, the steel consisting of the balance of Fe and unavoidable impurity elements is heated to 1200 ° C. or more, and after hot forming such as hot rolling is completed at a finishing temperature of 800 ° C. or more, 600 ° C. at an average cooling rate of 30 ° C./min or more. A chromium steel for carburizing and carbonitriding, characterized by being cooled to the following. 2. An element for improving the toughness of the carburized layer and the core and improving the hardenability by one or more of Ni = 0.15% or less and Mo = 0.10% or less by mass percent. 2. The chromium steel for carburizing and carbonitriding according to claim 1, comprising a seed. 3. An element for refining austenite grain size and containing Ti = 0.005 to 0.015% by mass as an element which does not significantly impair fatigue properties. Claim 1 or Claim 2
Chromium steel for carburizing and carbonitriding described in 1. 4. An element for improving machinability and not significantly impairing fatigue properties, in terms of mass percent, S = 0.005 to 0.035%, Pb = 0.01 to 0.09%, Bi = 0.04 to 0.20%, Te = 0.002 to 0.050%, Zr = 0.01 to 0.20%, Ca = 0.0001 to 0.0100% One or two of the following: The chromium steel for carburizing and carbonitriding according to any one of claims 1 to 3, wherein the chromium steel contains at least one species.