KR20140129081A - Steel for nitrocarburizing and nitrocarburized component using the steel as material - Google Patents

Abstract

According to the present invention, by having a structure having a predetermined composition and having a bainite area ratio of more than 50% before softening (nitriding), the pre-softening is excellent in machinability, Can provide a softening steel having strength and toughness equivalent to those of carburizing materials of conventional steels, for example, SCr420 steel, and having more excellent fatigue characteristics.

Description

TECHNICAL FIELD [0001] The present invention relates to a softening steel and a softened part made of the steel,

The present invention relates to a steel for softening and a softened part made of the steel, particularly a softening steel having excellent fatigue characteristics after softening and preferable for automobiles and construction machines, To a softened part.

Mechanical structural parts such as gears of automobiles are required to have excellent fatigue characteristics and to perform surface hardening treatment. Carburizing treatment, high frequency quenching treatment and nitriding treatment are well known as the surface hardening treatment.

Since the carburizing treatment penetrates and diffuses C in a high temperature austenite region (region), a deep curing depth is obtained, which is effective for improving the fatigue strength.

However, from the viewpoint of heat treatment deformation, it has been difficult to apply it to parts requiring strict dimensional accuracy in terms of quietness and the like.

The high-frequency quenching process is a process of quenching the surface layer by high-frequency induction heating, and the dimensional accuracy is inferior to that of the carburizing process.

The nitriding treatment is a treatment for increasing the surface hardness by intruding and diffusing nitrogen at a temperature not higher than the Ac ₁ transformation point, but the treatment time is as long as 50 to 100 hours and it is necessary to remove the loose compound layer in the surface layer after the treatment.

Therefore, a softening treatment for treating nitriding in a short time at a processing temperature of the same level as the nitriding treatment has been developed, and it has recently become widespread in parts for mechanical structural parts and the like. In the softening treatment, N and C are intruded and diffused at the same time in a temperature range within the range of 500 to 600 DEG C to harden the surface, so that the treatment time can be set to half or less as compared with the conventional nitriding treatment.

However, in the carburizing treatment, core hardness can be increased by quenching hardening. In the softening treatment, since the treatment is carried out at a temperature not higher than the transformation point of the steel, the core hardness is not increased, The treated material has a lower fatigue strength than the carburized material.

In order to increase the fatigue strength of the softening treatment material, the core hardness is usually increased by quenching and tempering treatment prior to softening. However, the fatigue strength obtained is hardly sufficient, the manufacturing cost is increased, The machinability also deteriorates.

In order to solve such a problem, the composition of the steel is set to a composition containing Ni, Al, Cr and Ti, and at the time of softening, the core is made of Ni-Al or Ni-Ti based intermetallic compound or Cu compound And the surface of the nitride layer is subjected to age hardening, and nitride and carbide such as Cr, Al and Ti are precipitated and cured in the nitride layer (Patent Document 1, Patent Document 2).

Patent Literature 3 discloses a ferrite core structure in which a steel containing 0.5 to 2% of Cu is solidified by hot forging followed by air cooling to form a solid solution of Cu, And further precipitation hardening of the Ti, V, Nb carbonitride is also used together to form a steel having excellent bending fatigue characteristics after the softening treatment. Patent Document 4 discloses a softening steel in which Ti-Mo carbide and a carbide containing at least one of Nb, V and W are further dispersed therein.

Japanese Unexamined Patent Publication No. 5-59488 Japanese Patent Application Laid-Open No. 7-138701 Japanese Laid-Open Patent Publication No. 2002-69572 Japanese Patent Application Laid-Open No. 2010-163671

However, in the softened steel described in Patent Documents 1 and 2, although the bending fatigue strength is improved by precipitation hardening of Cu or the like, it is difficult to secure sufficient workability, and the softened steel described in Patent Document 3 , Cu, Ti, V, and Nb in a relatively large amount, which results in a high production cost. The softening steel described in Patent Document 4 has a problem that the production cost is high because it contains a relatively large amount of Ti and Mo.

Therefore, it is an object of the present invention to provide a method for producing a softened part which is capable of enhancing deep part hardness by softening treatment while having low hardness and excellent machinability before softening, It is an object of the present invention to provide a softening steel capable of being softened and a softened part made of the steel.

In order to solve the above problems, the inventors of the present invention have made extensive studies on the influence of texture and composition on the fatigue characteristics after softening the steel. As a result, a steel material containing a specified amount of V and Nb as a steel composition and having a bainite main body structure as a pre-softening structure was subjected to a softening treatment, and by using the temperature rise at that time, It was recognized that excellent fatigue characteristics were obtained after softening if the deep portion hardness was raised by precipitating fine precipitates in other deep tissues by aging.

The present invention is further characterized in that it is further based on the above recognition, and is characterized by the following features.

1. A ferritic stainless steel comprising: C: not less than 0.01%, not more than 0.10%, Si: not more than 1.0%, Mn: 0.5 to 3.0%, Cr: 0.30 to 3.0%, Mo: 0.005 to 0.4%, V: 0.02 to 0.5% : 0.003 to 0.15% of Al, 0.005 to 0.2% of Al, 0.06% or less of S, 0.02% or less of P and 0.0003 to 0.01% of B, with the remainder being Fe and unavoidable impurities, A softening steel having a structure having a bainite area ratio of more than 50%.

2. The softening steel as described in 1 above, wherein precipitates containing V and Nb are dispersed and precipitated in the bainite phase after softening.

3. A softened part comprising the softening steel as described in 1 or 2 above.

According to the present invention, before softening, softening steel having excellent machinability and having strength and toughness equal to those of carburizing materials of conventional steels, for example, SCr420 steel after softening, , A softened part made of the steel can be obtained, which is very useful in industry.

Fig. 1 is a schematic view showing a manufacturing process for producing a softened part by using the softening steel of the present invention. Fig.

(Mode for carrying out the invention)

The microstructure, component composition and production conditions of the softening steel according to the present invention will be described below.

1. Micro-organization

The microstructure before softening is made to have a bainite area ratio exceeding 50%, and after the softening, V, Nb precipitates are dispersed and precipitated in the bainite phase. When the parent phase before softening is a bainite main body structure having a bainite area ratio exceeding 50%, generation of V, Nb precipitates in the parent phase is remarkably suppressed as compared with the case of ferrite-pearlite structure. As a result, precipitation of V, Nb precipitates before softening can be suppressed and the hardness of the steel can be suppressed from rising, and cutting workability, which is normally performed before softening, is improved. Further, when this is subjected to the softening treatment, the surface layer portion is nitrided and at the same time, the V and Nb precipitates are aged and precipitated in the deep bainite structure other than the surface layer nitrided portion, and the core portion hardness is increased. As a result, fatigue strength and strength after softening are remarkably improved.

Further, in the present invention, a structure having a bainite area ratio of more than 50% means an area ratio of bainite structure (phase) exceeding 50% in observation of a cross-sectional structure (observation at 200 times under an optical microscope). Preferably, the area ratio of the bainite structure is more than 60%, more preferably more than 80%. It is preferable that fine precipitates having a particle size of less than 10 nm are dispersed as the V, Nb precipitates to be precipitated in the bainite structure. Further, it is preferable that the V and Nb precipitates having a particle diameter of less than 10 nm are present in an amount of 500 or more per 1 탆 ^{2 in} sufficient precipitation strengthening.

2. Composition

The reasons for limiting the composition of the softening steel of the present invention will be described. Hereinafter, the percentages of the steel components are all% by mass.

C: 0.01% or more and less than 0.10%

C is added for bainite texture formation and strength assurance. When the amount of C added is less than 0.01%, the amount of bainite to be formed decreases and the amount of V and Nb precipitates decreases, making it difficult to secure strength. On the other hand, when C is added in an amount of 0.10% or more, the hardness of the bainite structure increases, and the machinability decreases. Therefore, the amount of C added should be within the range of 0.01% or more and less than 0.10%. More preferably, it is 0.03% or more and less than 0.10%.

Si: 1.0% or less

Si is added because it is effective for deoxidization and formation of bainite structure, but Si is added in an amount exceeding 1.0%, the machinability and cold workability are deteriorated due to solidification of ferrite and bainite structure. Therefore, the amount of Si added should be 1.0% or less. More preferably, it is 0.5% or less. More preferably, it is 0.3% or less. Further, in order to effectively contribute to deoxidation of Si, it is preferable that the amount of Si added is 0.01% or more.

Mn: 0.5 to 3.0%

Mn is added because it is effective for bainite structure formation and strength enhancement. When the addition amount of Mn is less than 0.5%, the amount of bainite structure is decreased and V and Nb precipitates are produced. Therefore, the hardness before softening is increased and the amount of V, Nb precipitate after softening is decreased, The hardness is lowered and it becomes difficult to secure strength. On the other hand, if the Mn addition amount exceeds 3.0%, the machinability and the cold workability are deteriorated. Therefore, the amount of Mn added is within a range of 0.5 to 3.0%. More preferably, it is 0.5% or more and 2.5% or less. More preferably, it is 0.6% or more and 2.0% or less.

Cr: 0.30 to 3.0%

Cr is added because it is effective for the formation of bainite structure. When the amount of Cr added is less than 0.30%, the amount of bainite structure decreases and V and Nb precipitates are produced. Therefore, the hardness before softening is increased and the amount of V, Nb precipitate produced after softening is reduced, The post-hardness is lowered and it becomes difficult to secure the strength. On the other hand, when the amount of Cr added exceeds 3.0%, machinability and cold workability are deteriorated. Therefore, the amount of Cr added is within a range of 0.30 to 3.0%. More preferably, it is 0.5% or more and 2.0% or less. More preferably, it is 0.5% or more and 1.5% or less.

V: 0.02 to 0.5%

V is an important element for forming micro precipitates together with Nb by increasing the temperature at the time of softening to increase core hardness and improve strength. When the amount of V added is less than 0.02%, the effect of addition is lacking. On the other hand, if the amount of V added exceeds 0.5%, the precipitates coarsen. Therefore, the amount of V added is within a range of 0.02 to 0.5%. More preferably, it is 0.03% or more and 0.3% or less. More preferably, it is 0.03% or more and 0.25% or less.

Nb: 0.003 to 0.15%

Nb is a very effective element that forms fine precipitates with V due to temperature rise during softening, increases deep portion hardness, and improves fatigue strength. When the addition amount of Nb is less than 0.003%, the addition effect is lacking. On the other hand, if the amount of Nb added exceeds 0.15%, the precipitates become coarse. Therefore, the amount of Nb added is within the range of 0.003 to 0.15%. More preferably, it is not less than 0.02% and not more than 0.12%.

Mo: 0.005 to 0.4%

Mo has an effect of finely precipitating V and Nb precipitates to improve the strength of the softening treatment material and is an important element in the present invention. Further, Mo is also effective in producing bainite structure. Although it is added in an amount of 0.005% or more for improvement of strength, it is an expensive element, and if it is added in an amount exceeding 0.4%, the component cost is increased. Therefore, the addition amount of Mo should be within the range of 0.005 to 0.4%. More preferably, it is 0.01 to 0.3%. More preferably, it is 0.04 to 0.2%.

Al: 0.005 to 0.2%

Al is positively added as an element effective for improving the surface hardness after the softening and the depth of the effective hardened layer. In addition, 0.005% or more is added because it is an element which is effective in improving the toughness by suppressing austenite grain growth during hot forging. On the other hand, if it is contained in an amount exceeding 0.2%, the effect is saturated, which causes a disadvantage that the component cost is increased. Therefore, the amount of Al to be added is within the range of 0.005 to 0.2%. Preferably, it is more than 0.020% and not more than 0.1%. More preferably, it is more than 0.020% and 0.040% or less.

S: not more than 0.06%

S is a useful element for improving machinability by forming MnS in steel, but if it is contained in an amount exceeding 0.06%, toughness is impaired. Therefore, the amount of S added is 0.06% or less. It is preferably 0.04% or less. Further, in order to exhibit the effect of improving the machinability by S, it is preferable that the amount of S added is 0.002% or more.

P: not more than 0.02%

P is segregated at the austenite grain boundaries, and the strength and toughness are lowered by lowering the grain boundary strength. Therefore, it is preferable to reduce the P content as much as possible, but up to 0.02% is allowed. Therefore, the content of P is 0.02% or less. In addition, in order to reduce P to less than 0.001%, a high cost is required, so that it may be industrially reduced to 0.001%.

B: 0.0003 to 0.01%

B has an effect of promoting the formation of bainite structure. If the amount of B added is less than 0.0003%, the addition effect is lacking. On the other hand, if B is added in an amount exceeding 0.01%, the effect becomes saturated and the component cost is increased. Therefore, the amount of B added is within the range of 0.0003 to 0.01%. More preferably, it is 0.0010% or more and 0.01% or less.

Further, in order to obtain a bainite structure-promoting effect, it is preferable that B is dissolved in the steel. However, when solid N exists in the steel, B in the steel is consumed in the formation of BN, and when B is present in the steel as BN, it does not contribute to the improvement of quenching. Therefore, in the case where solid solution N is present in the steel, B is preferably added in an amount exceeding that which is consumed in the formation of BN, and B is added between the B content (% B) and the N content (% It is preferable that the relationship expressed by the following formula (1) is established.

group

% B?% N / 14 占 10.8 + 0.0003 --- (1)

In the softening steel of the present invention, at least one selected from the group consisting of Pb? 0.2% and Bi? 0.02% may be added after forging or improving the machinability of the softening treatment material. In addition, the effect of the present invention is not impaired by the content of these elements or the presence or absence of addition.

In addition, in the softening steel of the present invention, the balance other than the above-mentioned added elements is Fe and unavoidable impurities. In particular, Ti not only adversely affects the precipitation strengthening of V and Nb but also lowers the hardness of the deep portion. . , Preferably less than 0.010%, more preferably less than 0.005%.

3. Manufacturing conditions

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing a manufacturing process for producing a softened part by using the softening steel of the present invention. Fig.

In Fig. 1, S1 is a bar steel manufacturing process, S2 is a transporting process, and S3 is a product (softened part) finishing process.

That is, in the bar steel manufacturing process (S1), the steel ingot is hot-rolled into bars and is shipped after quality inspection. Then, in the product (softened part) finishing step (S3), the bar steel which has been shipped after shipment (S2) is cut into the predetermined dimensions and subjected to hot forging or cold forging, After a desired shape is formed in the cutting process such as lathe turning, softening is performed to obtain a product.

In addition, the hot rolled material may be finished in a desired shape in cutting such as turning or drilling, and then subjected to a softening treatment to obtain a product. In the case of hot forging, cold calibrating may be performed after hot forging. Further, the final product may be coated with paint such as paint or plating. Hereinafter, preferable manufacturing conditions will be described.

Rolling heating temperature

The rolling heating temperature is preferably within a range of 950 to 1250 占 폚. This is because the softening steel of the present invention solidifies the carbide remaining from the dissolution at the time of hot rolling so that the fine composition is not damaged due to the precipitation of fine precipitates in the rolled material (bar steel to be the material of the hot forged part) .

That is, when the rolling heating temperature is lower than 950 占 폚, the remaining carbides from the dissolution become hard to be solidified. On the other hand, if it exceeds 1250 占 폚, the crystal grains become coarse and the mono-composition tends to deteriorate. Therefore, the rolling heating temperature is preferably within a range of 950 to 1250 占 폚.

Rolling finish temperature

The rolling finishing temperature is preferably 800 DEG C or higher. When the rolling finish temperature is less than 800 ° C, ferrite structure is produced. Therefore, when softening is carried out as a next step, particularly after cold forging or cutting, bainite structure having an area ratio of more than 50% This is because it is disadvantageous to obtaining. If the rolling finishing temperature is less than 800 ° C, the rolling load is high and the out-of-roundness of the rolled material deteriorates. Therefore, the rolling finishing temperature is preferably 800 DEG C or higher.

Cooling rate

It is preferable to specify the cooling rate after rolling so that fine precipitates are precipitated before forging to damage the mono-composition. It is preferable to cool the precipitation temperature range of 700 to 550 占 폚 of the fine precipitates to a temperature exceeding the critical cooling rate (0.5 占 폚 / sec) at which fine precipitates are obtained.

The softening treatment (precipitation treatment)

The obtained bar steel is used as a material, and is formed into a part shape by forging, cutting, or the like. Thereafter, the softening treatment is performed. In the softening treatment, it is preferable that the softening treatment temperature is set within the range of 550 to 700 占 폚 and the treatment time is set to 10 minutes or more so as to precipitate fine precipitates containing V and Nb. When the temperature is lower than 550 DEG C, a sufficient amount of precipitates can not be obtained. On the other hand, when the temperature exceeds 700 DEG C, the austenite phase becomes difficult to soften. More preferably in the range of 550 to 630 캜. The reason for setting the treatment time to 10 minutes or more is that a sufficient amount of V, Nb precipitate is obtained.

In addition, in the case of using hot forging, in order to obtain a bainite structure having an area ratio of more than 50% in area ratio after softening and to prevent the precipitation of fine precipitates from the viewpoint of cold calibrating after hot forging and cutting workability, It is preferable that the heating temperature at the time of forging is within the range of 950 to 1250 占 폚, the forging finishing temperature is at least 800 占 폚, and the cooling rate after forging exceeds 0.5 占 폚 / sec.

Example

Next, the present invention will be further described by examples.

Steels (steels Nos. 1 to 17) having compositions shown in Table 1 were melted in a 150 kg vacuum melting furnace and subjected to rolling at 1150 占 폚 and 970 占 폚 finish, and then cooling to 0.9 占 폚 / sec to room temperature Followed by cooling to prepare a rod having a diameter of 50 mm. No.17 is the conventional JIS SCr420. In addition, P is not positively added to the total (total) steel in Table 1. Therefore, the P content in Table 1 indicates a value incorporated as an inevitable impurity. Further, for Ti, the steel No. 14 and the steel No. 15 in Table 1 are added, but the steel No. 1 to 13 and the steel No. 16 to 17 are not actively added. Therefore, in Table 1, the Ti contents of the steel Nos. 1 to 13 and the steel Nos. 16 to 17 are all incorporated as inevitable impurities.

These materials were further heated to 1200 占 폚 and then subjected to hot forging at 1100 占 폚 to obtain a 30 mm ?, cooling rate to 0.8 占 폚 / sec and cooling to room temperature at a cooling rate of 0.1 占 폚 / did.

Tissue observation, hardness measurement and machinability were examined for the material. Tissue observation was carried out by observing the cross section with an optical microscope and classifying the deep tissues and determining the area fraction of the bainite phase of the deep portion when the bainite phase was present in the deep portion. The machinability was evaluated by a drill cutting test. Concretely, a hot forging material cut to a thickness of 20 mm was used as a test material, and a straight drill with a diameter of 6 mm? Of JIS high-speed tool steel SKH51 was subjected to a heat treatment under the conditions of a feed of 0.15 mm / rev and a revolution of 795 rpm Five holes were drilled per section, and the number of drills was evaluated by the total number of holes until cutting became impossible.

The hardness was measured by using a Vickers hardness tester at a test load of 100 g.

Steel Nos. 1 to 16 were subjected to gas softening treatment in addition to hot forging, and Steel No. 17 was subjected to gas carburizing treatment in hot forging. The gas softening treatment was carried out by heating to 570 to 620 캜 in an atmosphere of NH ₃ : N ₂ : CO ₂ = 50: 45: 5 and holding for 3.5 hours. The gas carburizing treatment was carried out under the condition that after carburizing at 930 캜 x 3 hours, holding at 850 캜 x 40 minutes and then oil cooling (oil cooling), further tempering at 170 캜 x 1 h was carried out.

For these heat treatment materials, observation of structure, measurement of hardness, observation of precipitates, investigation of impact characteristics, and investigation of fatigue characteristics were carried out.

Tissue observation was carried out by observing the cross section with an optical microscope and classifying the deep tissue and determining the area fraction of the bainite phase for the presence of the bainite phase in the deep portion.

The hardness of the soft fire and the carburizing material was measured for the deep portion hardness and the surface hardness. The surface hardness was measured at a position of 0.02 mm from the surface, and the effective hardened layer depth was defined as the depth from the surface where HV 400 was obtained. A sample for observing the transmission electron microscope was prepared from the deep portion of the soft fire and the carburizing material by electrolytic welding using a twin-jet method. The obtained sample was irradiated with a transmission electron beam with an acceleration voltage of 200 kV The precipitate was observed using a microscope. The composition of the precipitate to be observed was obtained by an energy dispersive X-ray spectroscope (EDX).

The impact characteristics were evaluated by the Charpy impact test, and the impact value (J / cm 2) was obtained. The test piece (R: 10 mm, depth: 2 mm) having a notch was used. The test piece having this notch was taken from a hot forging material, and the obtained test piece was subjected to the softening treatment or carburizing treatment described above, and then subjected to a Charpy impact test.

The fatigue characteristics were evaluated by an Ono-type rotational bending fatigue test to obtain a fatigue limit. As the test piece, a test piece having a notch (notch R: 1.0 mm, notch diameter: 8 mm, stress concentration coefficient: 1.8) was used. This test piece was taken from a hot forging material and subjected to the softening treatment or carburizing treatment described above, and then subjected to the test.

Table 2 shows the test results. Nos. 1 to 6 are examples according to the present invention, Nos. 7 to 17 are comparative examples, and Nos. 18 are conventional examples based on JIS SCr 420 steel.

As is apparent from Table 2, the softening treatment materials Nos. 1 to 6 are superior in fatigue strength to those obtained by carburizing quenching and tempering of the conventional example (No. 18). The drill cutting workability of the material (hot forging material) before the softening treatment of Nos. 1 to 6 is practically equal to or higher than that of the conventional material. As a result of the observation of the transmission electron microscope and the investigation of the composition of the precipitate by EDX, the softening treatment materials No. 1 to 6 contained fine precipitates having particle sizes of less than 10 nm including V and Nb in the bainite phase, ^It was confirmed that there were dispersed and precipitated at 500 or more per ² . From this result, it can be considered that the softening treatment material according to the present invention exhibits high fatigue strength due to precipitation strengthening by the fine precipitates.

On the other hand, Comparative Examples Nos. 7 to 17 are poor in fatigue strength or drilling workability because the chemical composition or microstructure obtained is out of the scope of the present invention.

Particularly, since the cooling rate after hot forging is slow in No. 7, the fatigue strength is lower than that of the conventional example. Here, in No. 7, as a result of the transmission electron microscope observation, no precipitation of fine precipitates having a particle diameter of less than 10 nm was observed, and coarse precipitates having a particle diameter exceeding 10 nm were observed. From these results, it can be considered that the resultant precipitate is coarsened, which is the cause of the decrease in the fatigue strength. That is, if the cooling rate after the hot forging is slow and the expected bainite structure is not obtained, coarse precipitates are formed before softening, and the amount of fine precipitates after softening treatment is reduced, As shown in FIG.

In No. 8, since the amount of C is high outside the scope of the present invention, the hardness of the bainite structure is increased, and drilling workability is lowered.

In No. 9, since the amount of Si and Mn is high outside the scope of the present invention, the hardness of the hot forging material is high, and drilling workability is reduced to about 1/5 of that of the conventional material.

In No. 10, since the amount of Mn was low outside the range of the present invention, ferrite-pearlite structure was formed before softening (after hot forging), area ratio of bainite structure was low, and V and Nb precipitates were precipitated in the structure, The hardness before softening is increased and drilling workability is lowered.

Since the amount of Cr in the No. 11 is low outside the range of the present invention, a ferrite-pearlite structure is formed before softening (after hot forging), the area ratio of the bainite structure is lowered, and V and Nb precipitates are precipitated in the structure , The hardness before softening is increased and drilling workability is lowered.

In No. 12, since the amount of Mo is low outside the range of the present invention, the amount of fine precipitates after the softening treatment was small and sufficient deep hardness was not obtained, so that the fatigue strength was lower than in the conventional example.

In No. 13, since V and Nb are low outside the range of the present invention, the amount of precipitate after the softening treatment was small and sufficient deep hardness was not obtained, so that the fatigue strength was lower than that of the conventional material.

In No. 14, since Nb is low outside the range of the present invention, the amount of precipitate after the softening treatment is small, so that sufficient core hardness can not be obtained and the fatigue strength is lower than that of the conventional material.

No.15 and No.16, since Ti was added, the precipitation amount of precipitates including V and Nb after the softening treatment was small, so that sufficient deep portion hardness was not obtained and the fatigue strength was lower than that of the conventional material. Also, the impact value shows a low value.

Since Al is low outside the range of the present invention, the surface hardness after the softening treatment and the depth of the effective hardening layer are low, so that the fatigue strength is lower than that of the conventional material.

Claims (3)

In terms of% by mass,
C: not less than 0.01% and not more than 0.10%
Si: 1.0% or less,
Mn: 0.5 to 3.0%
Cr: 0.30 to 3.0%
Mo: 0.005 to 0.4%
V: 0.02 to 0.5%
Nb: 0.003 to 0.15%
Al: 0.005 to 0.2%
S: 0.06% or less,
P: 0.02% or less and
B: 0.0003 to 0.01%
Balance: Fe and inevitable impurities, and has a structure having a bainite area ratio exceeding 50% before softening (soft nitriding). The method according to claim 1,
A softening steel in which precipitates containing V and Nb are dispersed and precipitated in a bainite phase after softening. A softened part comprising the softening steel according to claim 1 or 2 as a material.

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