CN103221566B - Non-heat-treated steel for soft nitriding, and soft-itrided component - Google Patents

Abstract

The present invention provides: a soft-nitrided component which, even when having been produced through water cooling after soft nitriding, has a flex fatigue strength of 600 MPa or higher and excellent straightenability; and a non-heat-treated steel for soft nitriding, which is suitable for use as a raw material for the soft-nitrided component. The non-heat-treated steel for soft nitriding has a chemical composition that contains 0.25-0.35%, excluding 0.35%, C, 0.15-0.35% Si, 0.85-1.20% Mn, up to 0.10% S, 0.010-0.030%, excluding 0.010%, Al, 0.003-0.020% Ti, and 0.010-0.020% N, with the remainder comprising Fe and impurities, in which P =0.08% and Cr=0.10%, and that satisfies 0.02C+0.22Mn+0.87Cr+0.85Al+0.72=0.96 and 2.40C-0.54Mn-9.26Cr-0.01Al+1.59=0.90.

Description

Non-quenched and tempered steel for nitrocarburizing and nitrocarburized parts

technical field

The present invention relates to non-quenched and tempered steel for nitrocarburizing and nitrocarburized parts. Specifically, it relates to parts used after nitrocarburizing (hereinafter referred to as "soft nitrocarburized parts") such as crankshafts of engine parts such as automobiles and construction machinery, and parts suitable for use without "quenching-tempering" after rolling. ", "Normalizing", "Annealing" and other heat treatments and used as the raw material for nitrocarburizing (hereinafter referred to as "non-quenched and tempered steel for nitrocarburizing"). More specifically, it relates to nitrocarburized parts having a high bending fatigue strength of 600 MPa or more and excellent bend correction properties, and materials suitable for use as raw materials for nitrocarburized parts requiring bend correction under various nitrocarburized conditions, especially It is a non-quenched and tempered steel for nitrocarburizing that can make nitrocarburized parts have the above characteristics even when water cooling is performed in the cooling process after nitrocarburizing.

The "curvature correctability" mentioned above refers to the characteristic that cracks do not occur in the nitrocarburized layer on the surface until a large amount of deflection is reached when warp correction is performed in the finishing process after nitrocarburizing.

Background technique

Represented by crankshafts of automobiles and construction machinery, parts requiring high bending fatigue strength and wear resistance are often manufactured as follows: Surface hardening treatments such as induction hardening treatment and nitrocarburizing treatment are carried out in the state.

A notable feature of the above-mentioned medium nitrocarburizing treatment is that less strain occurs during the surface hardening treatment compared with the induction hardening treatment.

Therefore, parts such as crankshafts in particular are often subjected to nitrocarburizing treatment, but even nitrocarburizing treatment is not completely free of strain.

Therefore, the nitrocarburized parts that have been strained by the nitrocarburizing treatment are subjected to bending correction treatment in the finishing process after the nitrocarburizing treatment to remove the strain.

However, when bending correction is performed on a nitrocarburized part whose surface layer is excessively hardened, cracks may occur in the nitrocarburized layer on the surface. Moreover, when cracks occur in the nitrocarburized layer, the bending fatigue strength of the nitrocarburized parts before the bending correction treatment will be greatly reduced. In particular, when a treatment with a high cooling rate such as water cooling is performed in the cooling step after nitrocarburization, the hardness of the surface layer of the nitrocarburized part becomes high, so that a decrease in bend straightening property is unavoidable. Therefore, nitrocarburized parts are also required to have excellent bend correction properties.

On the other hand, due to safety issues or equipment restrictions, there are cases where only water cooling can be performed in the cooling process after nitrocarburization.

Therefore, not to mention the case of performing oil cooling in the cooling process after nitrocarburizing, even in the case of water cooling, it is particularly necessary to stably provide nitrocarburized parts with high bending fatigue strength and excellent bending straightening properties, and suitable as The raw material of this nitrocarburized part is non-quenched and tempered steel for nitrocarburization.

Conventionally, by containing expensive alloying elements such as Mo, it has been possible to achieve both high bending fatigue strength and excellent bending straightening properties of nitrocarburized parts even in the non-tempered state. In this regard, there is an increasing demand from the industry as follows: In order to suppress the cost of raw materials, expensive alloy elements should not be contained as much as possible, but nitrocarburized parts should still have high bending fatigue strength and excellent bending straightening properties.

Therefore, in order to meet the above needs, Patent Document 1 discloses "non-tempered steel for nitrocarburization", and Patent Document 2 discloses "non-tempered steel member for nitrocarburization".

Specifically, the "non-quenched and tempered steel for nitrocarburizing" disclosed in Patent Document 1 is characterized in that it contains C: 0.2 to 0.6%, Si: 0.05 to 1.0%, Mn: 0.25 to 1.0%, S: 0.03 to 0.2%, Cr: 0.2% or less, s-Al: 0.045% or less, Ti: 0.002 to 0.010%, N: 0.005 to 0.025%, O: 0.001 to 0.005%, and Pb: 0.01 to 0.01% if necessary One or more of 0.40%, Ca: 0.0005-0.0050%, and Bi: 0.005-0.40%, and satisfy (0.12×Ti<O<2.5×Ti) and (0.04×N<O<0.7×N) conditions, the balance is composed of Fe and unavoidable impurities, and the microstructure after hot forging is a mixed microstructure of ferrite and pearlite.

The "nitrocarburized non-quenched and tempered steel member" disclosed in Patent Document 2 is characterized in that it has a nitrocarburized layer on the surface and a steel cross-sectional structure other than the nitrocarburized layer has a ferrite+pearlite structure. It is formed of non-quenched and tempered steel, and the composition of the above-mentioned steel is mainly composed of Fe, and contains C: 0.30-0.50%, Si: 0.05-0.30%, Mn: 0.50-1.00%, S: 0.03-0.20%, Cu: 0.05～0.60%, Ni: 0.02～1.00%, Cr: 0.05～0.30%, and <1>Ti: 0.0020～0.0120%, N: 0.0050～0.0250%, O: 0.0005～0.008% and < 2> Ca: one or both of 0.0005 to 0.0050%, the respective content rates of Cu, Ni and Cr are denoted as WCu, WNi and WCr, respectively, and the composition parameters F1 and F2 are denoted as F1=185WCr+50WCu, respectively. When F2=8+4WNi+1.5WCu-44WCr, (F1>20) and (F2>0) are satisfied.

prior art literature

patent documents

Patent Document 1: Japanese Patent Laid-Open No. 2002-226939

Patent Document 2: Japanese Patent Laid-Open No. 2007-197812

Contents of the invention

The problem to be solved by the invention

In the case of the technique disclosed in the aforementioned Patent Document 1, the bending fatigue strength does not reach 600 MPa. In addition, Patent Document 1 does not mention anything about the cooling method in the cooling step after nitrocarburizing.

The technology disclosed in Patent Document 2 is also the same as that in Patent Document 1 above, in that the bending fatigue strength does not reach 600 MPa and there is no mention of cooling means in the cooling step after nitrocarburizing.

Therefore, an object of the present invention is to provide a nitrocarburized part and a non-tempered steel for nitrocarburization which are suitable as a raw material for the nitrocarburized part, and the nitrocarburized part is especially performed in the cooling process after the nitrocarburized part. Even in the case of water cooling, it stably possesses a high bending fatigue strength of 600 MPa or more and excellent bending straightening properties.

solutions to problems

The inventors of the present invention conducted various studies to solve the above-mentioned problems. As a result, first, the following items (a) to (c) were clarified.

(a) Mn is an element that can impart high bending fatigue strength to nitrocarburized parts at low cost without containing expensive alloy elements such as Mo and V.

(b) However, if the steel contains Mn, a large amount of nitrogen penetrates into the surface layer during nitrocarburization, so the surface layer of the nitrocarburized part is excessively hardened, and the bending straightening property tends to decrease.

(c) Cr is contained in steel as an impurity, and its content has a large influence on the bending fatigue strength and bending straightening property of nitrocarburized parts.

Therefore, the inventors of the present invention conducted further studies. As a result, the following findings (d) to (i) were obtained.

(d) By controlling the content of Mn to a low level, it is possible to avoid excessive hardening of the surface layer of the nitrocarburized part, and to prevent a decrease in bending correctability. However, in this case the bending fatigue strength will be reduced.

(e) Even if the Mn content is large, if a steel whose C content is controlled within a specific range is used, a nitrocarburized part can be provided with high bending fatigue strength and excellent bending straightening properties.

(f) By controlling the content of Cr, which is an impurity, to a low level, excessive hardening of the surface layer of the nitrocarburized part can be avoided, and a decrease in bending correctability can be prevented.

(g) If a specific amount of Al is contained, the depth of the diffusion layer can be increased without excessive hardening of the surface layer of the nitrocarburized part. Thereby, nitrocarburized parts can be provided with high bending fatigue strength and excellent bending straightening properties.

(h) By controlling the contents of C, Mn, Cr, and Al in extremely appropriate ranges, various nitrocarburizing conditions, especially even in the case of water cooling in the cooling step after nitrocarburizing, can be The depth of the diffusion layer is increased without excessive hardening of the surface layer of the nitrocarburized part.

(i) Mn not only strengthens the surface layer by increasing solid solution nitrogen, but also strengthens the surface layer by forming fine Mn nitrides. Specifically, when nitrocarburizing is performed using a non-tempered steel with an increased Mn content, the flake-like fine η-Mn ₃ N ₂ with a thickness of 5 nm or less and a width of 200 nm or less is formed in the ferrite and pearlite constituting the diffusion layer. The ferrite in the mixed structure of ferrite (hereinafter referred to as "ferrite-pearlite structure") is precipitated while maintaining a coherent state. The fine flaky precipitates coherently precipitated in the matrix ferrite in this way improve the strength of the nitrocarburized part and contribute to the improvement of the bending fatigue strength. In addition, since the precipitation of the fine plate-like precipitates is almost completed during nitrocarburization while maintaining the high temperature, the influence of the cooling rate in the cooling step after nitrocarburization is small. Therefore, even if water cooling is performed in the cooling step after nitrocarburizing, the nitrocarburized parts can be stably provided with high bending fatigue strength and excellent bend straightening properties.

The present invention was completed based on the above knowledge, and the gist thereof is a non-quenched and tempered steel for nitrocarburization shown in (1) below and a nitrocarburized part shown in (2).

(1) A non-quenched and tempered steel for nitrocarburizing, characterized in that it has the following chemical composition, in mass %, the non-quenched and tempered steel for nitrocarburizing contains C: not less than 0.25% and less than 0.35% , Si: 0.15-0.35%, Mn: 0.85-1.20%, S: less than 0.10%, Al: more than 0.010% and less than 0.030%, Ti: 0.003-0.020% and N: 0.010-0.024%, and the balance is composed of Fe and Composition of impurities, P and Cr in impurities are P: 0.08% or less and Cr: 0.10% or less, respectively, and P1 and P2 represented by the following formula (1) and formula (2) are P1≥0.96 and P2≥0.90, respectively,

P1＝0.02C+0.22Mn+0.87Cr+0.85Al+0.72...(1)

P2＝2.40C-0.54Mn-9.26Cr-0.01Al+1.59...(2)

Wherein, C, Mn, Cr, and Al in the above-mentioned formula (1) and formula (2) refer to the content of the elements in mass %.

(2) A nitrocarburized part characterized in that the chemical composition of the matrix contains C: 0.25% to less than 0.35%, Si: 0.15-0.35%, Mn: 0.85-1.20%, S: 0.10% or less, Al: more than 0.010% and less than 0.030%, Ti: 0.003-0.020% and N: 0.010-0.024%, the balance is composed of Fe and impurities, and P and Cr among the impurities are P: 0.08% or less and Cr: 0.10% or less, and P1 and P2 represented by the following formulas (1) and (2) are P1≥0.96 and P2≥0.90, respectively,

The diffusion layer is formed of a ferrite-pearlite structure, and the number of flaky precipitates with a thickness of 5 nm or less and a width of 200 nm or less precipitated in the ferrite is 130 to 250 pieces/μm ² .

P1＝0.02C+0.22Mn+0.87Cr+0.85Al+0.72...(1)

P2＝2.40C-0.54Mn-9.26Cr-0.01Al+1.59...(2)

Wherein, C, Mn, Cr, and Al in the above-mentioned formula (1) and formula (2) refer to the content of the elements in mass %.

The "impurities" in the balance of "Fe and impurities" refer to substances mixed in from ores, scraps, or production environments as raw materials during industrial production of iron and steel materials.

The effect of the invention

The nitrocarburized part of the present invention has a stable high bending fatigue strength of 600 MPa or more and excellent bend straightening properties even under various nitrocarburizing conditions, especially when water cooling is performed in the cooling step after nitrocarburizing. Suitable for crankshafts of automobiles, etc. If the non-tempered steel for nitrocarburization of the present invention is used as a raw material, the nitrocarburized part can be easily produced.

Description of drawings

FIG. 1 is a diagram showing the shape of an Ono-type rotating bending fatigue test piece. The unit of the numerical value in the figure is mm.

Fig. 2 is a graph showing the relationship between the bending fatigue strength and P1 (=0.02C+0.22Mn+0.87Cr+0.85Al+0.72) of the finished steels AA-AF.

Fig. 3 is a diagram showing the shape of a 4-point bending test piece for measuring bend correction properties. The unit of the numerical value in the drawing is mm.

Fig. 4 is a graph showing the relationship between the amount of correctable strain and P2 (=2.40C-0.54Mn-9.26Cr-0.01Al+1.59), which is an index of bending correctability of steels BA to BF.

Fig. 5 is a diagram schematically illustrating a method of cutting out a sample for transmission electron microscope observation from a 4-point bending test piece having the shape shown in Fig. 3 after nitrocarburizing.

Fig. 6 is a diagram showing an example of a bright field image when observing the interior of ferrite in the ferrite-pearlite structure of the diffused layer after nitrocarburizing with a transmission electron microscope, the substance indicated by the white arrow in the figure It is η-Mn ₃ N ₂ . This figure is obtained as follows: In the state where the incident direction of the electron beam is set to [001] _α-Fe , in order to minimize the influence of the strain existing around the precipitate, etc., to obtain a clearer observation result, the excitation g= (020) The system reflection of _α-Fe was observed. The black arrows in the figure show the orientation in reciprocal lattice space of the g-vector reflected by the excited system.

Fig. 7 is a diagram showing an electron beam diffraction pattern of the same field of view as Fig. 6 when observed with a transmission electron microscope. The dot image in the figure is the diffraction pattern of α-Fe, and the vertical and horizontal striped image is the diffraction pattern of η-Mn ₃ N ₂ . It is a property peculiar to thin plate-shaped precipitates that such a stripe-like extended diffraction pattern can be obtained.

Detailed ways

Hereinafter, each element of the present invention will be described in detail. In addition, "%" of content of each element means "mass %".

(A) Chemical composition of non-quenched and tempered steel for nitrocarburizing and matrix of nitrocarburized parts:

C: More than 0.25% and less than 0.35%

C is an element effective in ensuring the bending fatigue strength after nitrocarburizing, and in order to ensure the strength of the matrix required to obtain a high bending fatigue strength of 600 MPa or more, the content needs to be 0.25% or more. However, when the content of C is excessive, the hardness of the surface layer becomes too high. Furthermore, the area fraction of ferrite becomes lower, and the ferrite-pearlite structure becomes coarser. Thus, sufficient curvature correction properties cannot be obtained. Therefore, the content of C is made 0.25% or more and less than 0.35%.

Si: 0.15～0.35%

Si is an element necessary for deoxidation during smelting, and in order to obtain this effect, the content needs to be at least 0.15%. However, containing a large amount of Si leads to an excessive decrease in bend correction properties, so the content of Si is made 0.15 to 0.35%. The content of Si is preferably set to 0.15% or more and preferably set to 0.30% or less.

Mn: 0.85～1.20%

Mn is an element effective in deoxidizing steel like the above-mentioned Si. Mn increases the amount of solid solution nitrogen in the nitrocarburized layer during nitrocarburization, and forms fine flaky Mn nitrides with the intruded nitrogen. The nitrides are coherently precipitated in the matrix, thereby improving the bending fatigue strength. In order to obtain the above effects, the content of Mn needs to be 0.85% or more. On the other hand, if the content of Mn exceeds 1.20%, the amount of solid solution nitrogen and the amount of precipitation of Mn nitrides will excessively increase, and the hardness of the surface layer will become too high, so that the bending correction property will decrease. Therefore, the content of Mn is set to 0.85 to 1.20%. The content of Mn is preferably set to 0.90% or more.

S: less than 0.10%

S is contained as an impurity. In addition, when added, it has the effect of improving machinability. However, when the content of S becomes larger than 0.10% as a result of addition, the bending fatigue strength and bending straightening property will be significantly lowered. Therefore, the content of S is set to 0.10% or less. The S content is preferably 0.08% or less. In order to obtain the effect of improving machinability, the content of S is preferably 0.04% or more.

Al: more than 0.010% and less than 0.030%

Al is an effective element for increasing the depth of the diffusion layer during nitrocarburizing and improving the bending fatigue strength. In order to obtain this effect, the Al content needs to exceed 0.010%. However, when the Al content becomes excessive, the hardness of the surface layer becomes too high, so that the bending correctability decreases. Therefore, the content of Al is set to be more than 0.010% and not more than 0.030%.

Ti: 0.003～0.020%

Ti is an element that suppresses the coarsening of crystal grains, refines the crystal grains, and improves the bending fatigue strength. In order to obtain such an effect, the Ti content needs to be 0.003% or more. However, when the content of Ti exceeds 0.020%, the bending straightening property decreases. Therefore, the content of Ti is set to 0.003 to 0.020%. The content of Ti is preferably set to 0.005% or more and preferably set to 0.015% or less.

N: 0.010～0.024%

N is an element that improves bending fatigue strength and bending correctability. In order to obtain such an effect, the N content needs to be 0.010% or more. On the other hand, even if N is contained in excess of 0.024%, the above effects are already saturated. Therefore, the content of N is set to 0.010 to 0.024%. The content of N is preferably set to 0.012% or more and preferably set to 0.022% or less.

For the non-quenched and tempered steel for nitrocarburization and the substrate of nitrocarburized parts according to the present invention, it is necessary to limit the contents of P and Cr among the impurities within the following ranges, respectively.

As already mentioned, "impurities" refer to substances mixed in from ores, scraps, or manufacturing environments as raw materials when iron and steel materials are produced industrially.

P: less than 0.08%

P is an impurity, which is an unpreferable element that lowers the bending fatigue strength. Especially when its content exceeds 0.08%, the bending fatigue strength is significantly reduced. Therefore, the content of P among the impurities is set to be 0.08% or less. The content of P among the impurities is preferably 0.03% or less.

Cr: less than 0.10%

Cr is contained in steel as an impurity, and its content has a large influence on bending fatigue strength and bending straightening property. In particular, when the Cr content exceeds 0.10%, the bending straightening property is remarkably lowered. Therefore, the content of Cr among the impurities is set to be 0.10% or less.

P1: above 0.96

For the non-quenched and tempered steel for nitrocarburization of the present invention and the matrix of nitrocarburized parts, P1 represented by P1=0.02C+0.22Mn+0.87Cr+0.85Al+0.72...(1) needs to satisfy P1≥0.96. However, C, Mn, Cr, and Al in the above-mentioned formula (1) refer to the contents of the elements in mass %.

When the chemical composition is within the above-mentioned range, the bending fatigue strength can be adjusted by the above-mentioned P1. Moreover, when P1 is 0.96 or more, the bending fatigue strength of 600 MPa or more can be obtained. This case will be described below.

Steels AA to AF having the chemical composition shown in Table 1 were smelted in a 70-ton converter, and rolled into billets with cross-sectional dimensions of 180 mm×180 mm.

[Table 1]

The above-mentioned steel was forged into a steel bar with a diameter of 90 mm, and further forged into a steel bar with a diameter of 50 mm under conditions of a heating temperature of 1200° C. and a finishing temperature of 1000 to 1050° C. After forging, it is naturally cooled to room temperature in the atmosphere.

From the R/2 portion ("R" represents the radius of the round bar) of each steel bar with a diameter of 50mm obtained as described above, an Ono-type rotating bending fatigue test piece of the shape shown in Fig. 1 is selected, and NH ₃ gas: RX gas = 1 : Soft nitriding treatment was performed in an atmosphere of 1 at a soaking temperature of 600° C. and a soaking time of 150 minutes, followed by water cooling. All dimensions in the Ono-type rotating bending fatigue test piece shown in FIG. 1 above are "mm".

Using the Ono-type rotating bending fatigue test piece obtained as described above, a fatigue test was performed at room temperature, under an air atmosphere, and under symmetrical cycle alternating conditions of a rotation speed of 3000 rpm, and the bending fatigue strength was investigated.

Table 1 also shows the results of the above-mentioned Ono-type rotating bending fatigue test. In addition, Fig. 2 shows the relationship between P1 and bending fatigue strength.

As shown in FIG. 2 , when P1 is 0.96 or more, a bending fatigue strength of 600 MPa or more can be obtained.

In the case where C is a value close to 0.35%, Mn is 1.20%, Cr is 0.10%, and Al is 0.030% in terms of the content of elements defined by formula (1), P1 may be a value close to 1.10.

P2: above 0.90

For the non-quenched and tempered steel for nitrocarburizing and the matrix of nitrocarburized parts of the present invention, P2 represented by P2=2.40C-0.54Mn-9.26Cr-0.01Al+1.59...(2) needs to satisfy P2≥0.90. However, C, Mn, Cr, and Al in the above-mentioned formula (2) refer to the contents of the elements in mass %.

When the chemical composition is within the above-mentioned range, the curvature correction property can be adjusted by the above-mentioned P2. When P2 is 0.90 or more, good curvature correction property can be obtained. This case will be described below.

The steels BA-BF having the chemical composition shown in Table 2 were smelted in a 70-ton converter, and rolled into billets with a cross-sectional size of 180mm×180mm.

[Table 2]

The above-mentioned steel was forged into a steel bar with a diameter of 90 mm, and further forged into a steel bar with a diameter of 50 mm under conditions of a heating temperature of 1200° C. and a finishing temperature of 1000 to 1050° C. After forging, it is naturally cooled to room temperature in the atmosphere.

From the R/2 portion of each steel bar with a diameter of 50mm obtained as described above, a 4-point bending test piece for the measurement of bend straightening of the shape shown in _FIG . Soft nitriding treatment was performed under the conditions of heating temperature 600°C and soaking time 150 minutes, followed by water cooling. All the dimensional units in the 4-point bending test piece shown in FIG. 3 above are "mm".

A strain gauge of 2 mm was bonded to the base of the notch of the 4-point bending test piece obtained as described above, and bending correction strain was applied until the strain gauge was disconnected. If cracks occur in the nitrocarburized layer, the strain gauge bonded to the surface layer will be disconnected. Therefore, the strain amount when the strain gauge is disconnected, that is, the strain amount for bending correction, is used to evaluate the bend correction property. All 4-point bending test pieces for strain gauge disconnection are embedded in resin so that the longitudinal section of the notch root of R3 of the 4-point bending test piece is the detection surface, and then the above-mentioned surface is ground for mirror finishing, and nitrocarburization is confirmed using an optical microscope. There are cracks in the layer.

The target of the bending correctability is to set the above-mentioned strain amount capable of bending correction to be 20000 με or more.

Table 2 also shows the amount of strain for bending correction, which is an index of the above-mentioned bending correctability. In addition, Figure 4 shows the relationship between P2 and the amount of strain that can be corrected by bending.

As is clear from FIG. 4 , when P2 is 0.90 or more, a bend-correctable strain amount of 20000 με or more can be obtained.

In terms of the content of the elements specified in formula (2), when C is a value close to 0.35%, Mn is 0.85%, Cr is a value close to 0%, and Al is a value close to 0.010%, P2 can also be close to A value of 1.97.

When the non-tempered steel for nitrocarburizing of the present invention having the above chemical composition is forged under normal hot forging conditions, for example, the heating temperature is set at 1200 to 1300°C, and the finishing temperature is set at 900 to 1100°C. , and then naturally cooled to room temperature in the atmosphere, you can get a non-quenched and tempered steel with a ferrite area fraction of 30-80% and the rest being pearlite, that is, a ferrite-pearlite structure.

(B) Diffusion layer of nitrocarburized parts:

The nitrocarburized part of the present invention is obtained by forming the above-mentioned non-tempered steel material into a part shape by machining, and then performing nitrocarburization treatment at a soaking temperature of 450 to 650° C. and a soaking time of 30 minutes or more. The above soaking temperature is sufficiently low compared with the _A3 transformation point of the steel, so the matrix and diffusion layer of the non-quenched and tempered steel do not undergo phase transformation during the nitrocarburizing treatment, and the structure of the nitrocarburized parts is similar to that of the soft nitrocarburized parts. The non-quenched and tempered steel before nitriding treatment has the same ferrite-pearlite structure. Therefore, the diffusion layer of the nitrocarburized part of the present invention is formed of a ferrite-pearlite structure.

Next, the nitrocarburized part of the present invention is a part in which there are 130 to 250 pieces/ ^μm2 of flaky precipitates with a thickness of 5 nm or less and a width of 200 nm or less in the ferrite of the ferrite-pearlite structure of the diffusion layer. . If the above-mentioned flaky precipitates are present in the ferrite, the nitrocarburized part can have both a high bending fatigue strength of 600 MPa or more and a target bend correction property of a bend-correctable strain of 20000 με or more.

Compounds that are fine and coherently precipitated with respect to the matrix increase the strength of the matrix as a precipitation strengthening factor, and the larger the amount of precipitation or the smaller the size, the greater the contribution to precipitation strengthening. Conversely, large-sized precipitates in which at least one of the thickness and width exceeds the above-mentioned value hardly contribute to the strengthening of the nitrocarburized part of the present invention. Therefore, when the above-mentioned flaky precipitates with a thickness of 5 nm or less and a width of 200 nm or less exist in ferrite at 130 pieces/μm ² or more, a bending fatigue strength of 600 MPa or more can be stably ensured. On the other hand, when there are more than 250 pieces/ ^μm of flaky precipitates of the above-mentioned size in the ferrite, the surface layer is excessively strengthened, so the bend-correctable strain amount, which is an index of bend-correctability, does not reach 20000με .

The thickness and width of the flaky precipitates present in the ferrite are preferably 3 nm or less and 100 nm or less, respectively. In the case of TEM observation at a magnification of 200,000 times, a precipitate with a thickness of 1 nm and a width of 10 nm is the observation limit, and this limit is determined by the performance of observation equipment such as a transmission electron microscope (hereinafter referred to as "TEM").

The diffusion layer of the nitrocarburized part described in this item (B) can be obtained by using steel having the chemical composition described in the above item (A) at a soaking temperature of 450 to 650°C and a soaking time of 30 minutes or more. It is obtained by soft nitriding treatment under the conditions. In addition, when steel having the chemical composition described in the above item (A) is used, the cooling rate after nitrocarburizing does not have a large influence on the properties of the part, and any cooling method can satisfy the target. Therefore, cooling after nitrocarburizing may be performed by an appropriate method.

The present invention will be further described in detail below through examples.

Example

Steels A to N having the chemical compositions shown in Table 3 were smelted in a 70-ton converter, and rolled into billets with a cross-sectional size of 180mm×180mm.

Steels A to E in Table 3 are steels whose chemical compositions are within the range specified by the present invention. On the other hand, steels F to N are steels whose chemical compositions do not satisfy the conditions specified in the present invention.

[table 3]

The above-mentioned steel was forged into a steel bar with a diameter of 90 mm, and further forged into a steel bar with a diameter of 50 mm under conditions of a heating temperature of 1200° C. and a finishing temperature of 1000 to 1050° C. After forging, it is naturally cooled to room temperature in the atmosphere.

From the R/2 portion of each steel bar with a diameter of 50 mm obtained as described above, an Ono-type rotating bending fatigue test piece having a shape shown in FIG. 1 and a 4-point bending test piece for measuring bend correction properties having a shape shown in FIG. 3 were selected.

First, using the 4-point bending test piece obtained as described above, a microstructure investigation before nitrocarburizing was performed.

Specifically, the 4-point bending test piece was embedded in resin so that the longitudinal section of the groove root of R3 was the detection surface, mirror-polished, and etched with nital to expose the structure. Then, the positions from the surface layer to the depth of 1 mm under any five fields of view were observed with an optical microscope at a magnification of 100 times to identify phases, and then measure the area fraction (%) of ferrite.

Next, the Ono-type rotating bending fatigue test piece and the 4-point bending test piece were subjected to nitrocarburization in an atmosphere of NH ₃ gas: RX gas = 1:1, soaking temperature 600°C, and soaking time 150 minutes. , both Steel C and Steel D were nitrocarburized under the conditions of a soaking temperature of 650°C and a soaking time of 180 minutes. Water cooling is carried out after nitrocarburizing.

First, the investigation of the diffusion layer was carried out using a 4-point bending test piece subjected to water cooling after the nitrocarburizing treatment. Specifically, the 4-point bending test piece was embedded in resin so that the longitudinal section of the groove root of R3 was the detection surface, mirror-polished, etched with nital, and then cleaned. Then, observe any five continuous positions from the surface layer to a depth of 1 mm with an optical microscope at a magnification of 100 times, investigate the depth of the compound layer formed on the uncorroded part near the surface, that is, the surface layer, and identify the compound layer deeper than the compound layer. phases in the corroded area.

Next, at any three places, the Vickers hardness at consecutive positions with a depth of 0.05mm to 1mm was measured based on the "Vickers hardness test-test method" described in JIS Z2244 (2009), and the values at the same depth were averaged as The hardness distribution is made into a hardness change curve, and the sum of the depths of the compound layer and the diffusion layer is investigated, which is the depth of the nitride layer. However, the measurement of the Vickers hardness was carried out under the conditions of a test force of 2.94 N and a measurement interval of 0.05 mm in the depth direction. The definition of the nitride layer is based on JIS G0562 (1993), and the depth of the nitride layer refers to the distance from the surface of the point that cannot be distinguished from the hardness difference of the substrate.

As shown in Table 4 described later, it was found that the depth of the nitrided layer was 0.55 to 0.80 mm. Therefore, the internal structure of the diffusion layer was further investigated with respect to the above-mentioned 4-point bending test piece subjected to water cooling after the nitrocarburizing treatment. Specifically, as shown in FIG. 5 , a sheet-shaped test piece having a thickness of 1 mm including the surface of the smooth portion was cut out from the above-mentioned 4-point bending test piece. Next, mechanically polish the sheet-shaped test piece with a thickness of 1 mm from both sides, reduce the thickness in the depth direction from the surface of the smooth portion, and process it into a sheet-shaped test piece that only includes a position at a depth of 30 to 90 μm from the surface of the smooth portion. . Then, the sheet-like test piece was further thinned by electrolytic grinding using a double-jet method of a perchloric acid-methanol mixture, and was used for energy-dispersive X-ray spectroscopy (STEM-EDS) using a scanning transmission electron microscope. Elemental analysis and TEM observation. The accelerating voltage in both the STEM-EDS analysis and the TEM observation was 300 kV. The ferrite part in the ferrite-pearlite structure can be observed by TEM. Therefore, it can be confirmed that the above-mentioned thin film test piece is taken from the diffusion layer and not from the compound layer, so the shape characteristics and precipitation form of the precipitates were investigated by TEM observation, and the elements constituting the precipitates were investigated by STEM-EDS.

As an example of the results of TEM observation of the ferrite internal structure in the ferrite-pearlite structure of the diffusion layer, the results of Test No. 1 using steel A are shown in FIGS. 6 and 7 . Figure 6 is a bright field image, and Figure 7 is an electron beam diffraction pattern in the same field. As shown in FIG. 6 , precipitates indicated by white arrows were observed in ferrite. In addition, from the results of STEM-EDS analysis and TEM observation, it is clear that the precipitate is η-Mn ₃ N ₂ , and its orientation relationship with the ferrite of the matrix is [Math formula 1] and [Math formula 2]. flaky. Therefore, when the incident direction of the electron beam is set to [001] _α-Fe , the plate is observed from the horizontal direction, and the observed η precipitated in the form of [Formula 3] or [Formula 4] -Mn ₃ N ₂ exhibits linear chromatic aberration as in the observation example. Here, when the thickness of the linear chromatic aberration is expressed as the thickness of the plate-shaped precipitates and the length is expressed as the width, η-Mn ₃ N ₂ with a thickness of 1 nm and a width of 10 nm is the observation limit for TEM observation at a magnification of 200,000.

[mathematical formula 1]

(220) _η //(020) _α-Fe

[mathematical formula 2]

${<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}\stackrel{<span\; class="notranslate">\; \&Right\; Arrow;</span>}{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>}_{\mathrm{<span\; class="notranslate">\; \&eta;</span>}}<span\; class="notranslate">\; /</span><span\; class="notranslate">\; /</span>{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 100</span>}<span\; class="notranslate">\; )</span>}_{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; Fe</span>}}$

[mathematical formula 3]

(001) _η //(100) _α-Fe

[mathematical formula 4]

(001) _η //(010) _α-Fe

For each thin film test piece, the incident direction of the electron beam in TEM observation was set to <001> _α-Fe , and the precipitates of the above shape were regarded as plate-shaped precipitates, and the individuality of the plate-shaped precipitates with a thickness of 5 nm or less and a width of 200 nm or less was investigated. number. That is, ferrite in the ferrite-pearlite structure of the diffusion layer was observed by TEM at 200,000 magnification, and the number of plate-like precipitates of the above-mentioned size included in a field of view of 250 nm×350 nm was determined. For any 5 fields of view, repeat the above operation, and divide the total number of the 5 fields of view by the total area of the 5 fields of view to calculate the thickness of 5 nm or less and width of 200 nm per unit area. The following number of flaky precipitates.

Furthermore, using the above-mentioned Ono-type rotating bending fatigue test piece and 4-point bending test piece subjected to water cooling after nitrocarburizing, the bending fatigue strength and bending straightening property were investigated.

That is, using the above-mentioned Ono-type rotating bending fatigue test piece subjected to water cooling after nitrocarburizing, a fatigue test was performed at room temperature, in an air atmosphere, and under symmetrical cycle alternating conditions with a rotation speed of 3000 rpm to investigate bending fatigue strength. The target of bending fatigue strength is 600 MPa or more.

In addition, a 2 mm strain gauge was bonded to the root of the notch of the 4-point bending test piece subjected to water cooling after the nitrocarburizing treatment, and a bending correction strain was applied until the strain gauge was disconnected. As described above, when the strain gauge is disconnected, cracks are generated in the nitrocarburized layer, so the bend correction property is evaluated by the amount of strain when the strain gauge is disconnected, that is, the amount of strain for bend correction. The bend-correctable strain amount, which is an index of bend-correctability, is targeted to be 20000 με or more.

Table 4 shows the results collating the above tests.

[Table 4]

It is clear from Table 4 that the test numbers 1 to 7 of the "Example of the Invention" satisfying the conditions specified in the present invention have achieved the target of bending fatigue strength of 600 MPa or more and a strain amount that can be corrected by bending of 20000 με or more, and have high bending fatigue strength and excellent of curvature correction.

On the other hand, the C content of the steel F of the test number 8 was as low as 0.20%, and it did not meet the conditions stipulated by the present invention. Therefore, although the number of flaky nitrides precipitated in the ferrite in the diffusion layer with a thickness of 5 nm or less and a width of 200 nm or less is 176/μm ² , which satisfies the requirements of the present invention, the strength of the matrix is insufficient. The bending fatigue strength of the component did not meet the target.

Steel G of Test No. 9 had a Mn content as low as 0.80%, which did not meet the conditions specified in the present invention. Furthermore, the number of flaky precipitates with a thickness of 5 nm or less and a width of 200 nm or less precipitated in the ferrite of the diffusion layer is also as low as 112/μm ² , which does not meet the conditions specified in the present invention. Therefore, the bending fatigue strength as a nitrocarburized part did not reach the target.

Steel H of Test No. 10 had an Al content as low as 0.003%, which did not meet the conditions specified in the present invention. Therefore, the depth of the diffusion layer does not increase during nitrocarburization, and the bending fatigue strength of a nitrocarburized part does not reach the target.

The parameter P1 of steel I of test number 11 was 0.95, which was lower than the range specified by the present invention. Therefore, the bending fatigue strength as a nitrocarburized part did not reach the target.

Steel J of Test No. 12 has a Mn content as high as 1.25%, which does not meet the conditions specified in the present invention. Furthermore, the number of flaky precipitates with a thickness of 5 nm or less and a width of 200 nm or less precipitated in the ferrite of the diffusion layer was as many as 257/μm ² , which did not meet the conditions specified in the present invention. Therefore, the amount of strain that can be corrected for bending as a nitrocarburized part did not reach the target, and the bending correcting property was poor.

Steel K of Test No. 13 had a Cr content as high as 0.11%, which did not meet the conditions specified in the present invention. Therefore, the amount of strain that can be corrected for bending as a nitrocarburized part did not reach the target, and the bending correcting property was poor.

The Al content of the steel L of the test number 14 is as high as 0.060%, which does not meet the conditions specified in the present invention. Therefore, the amount of strain that can be corrected for bending as a nitrocarburized part did not reach the target, and the bending correcting property was poor.

The parameter P2 of steel M of test number 15 was 0.89, which was lower than the range prescribed by the present invention. Therefore, the amount of strain that can be corrected for bending as a nitrocarburized part did not reach the target, and the bending correcting property was poor.

The C content of steel N in test number 16 was as high as 0.42%, which did not meet the conditions specified in the present invention. Therefore, the amount of strain that can be corrected for bending as a nitrocarburized part did not reach the target, and the bending correcting property was poor.

Industrial availability

The nitrocarburized part of the present invention has a stable high bending fatigue strength of 600 MPa or more and excellent bend straightening properties even under various nitrocarburizing conditions, especially when water cooling is performed in the cooling step after nitrocarburizing. Suitable for crankshafts of automobiles, etc. If the non-tempered steel for nitrocarburization of the present invention is used as a raw material, the nitrocarburized part can be easily produced.

Claims (2)

1. A non-quenched and tempered steel for nitrocarburizing, characterized in that it has the following chemical composition, in mass %, the non-quenched and tempered steel for nitrocarburizing is composed of C: more than 0.25% and less than 0.35%, Si: 0.15 to 0.35%, Mn: 0.93 to 1.20%, S: 0.10% or less, Al: more than 0.010% to 0.030%, Ti: 0.003 to 0.020%, N: 0.010 to 0.024%, and the balance Fe and Composition of impurities, P and Cr in the impurities are respectively P: 0.08% or less and Cr: 0.10% or less, and P1 and P2 represented by the following formula (1) and formula (2) are respectively P1≥0.96 and P2≥0.90, P1＝0.02C+0.22Mn+0.87Cr+0.85Al+0.72...(1) P2＝2.40C-0.54Mn-9.26Cr-0.01Al+1.59...(2) However, C, Mn, Cr, and Al in the above-mentioned formulas (1) and (2) refer to the contents of the elements in mass %. 2. A nitrocarburized part, characterized in that the chemical composition of the matrix is C: 0.25% to less than 0.35%, Si: 0.15-0.35%, Mn: 0.85-1.20%, S: 0.10% or less, Al: more than 0.010% to 0.030% or less, Ti: 0.003 to 0.020%, N: 0.010 to 0.024%, and the balance of Fe and impurities. P and Cr among impurities are P: 0.08% or less and Cr: 0.10% or less, and P1 and P2 represented by the following formulas (1) and (2) are P1≥0.96 and P2≥0.90, respectively, The diffusion layer is formed of a ferrite-pearlite structure, and the number of flaky precipitates with a thickness of 5 nm or less and a width of 200 nm or less precipitated in the ferrite is 130 to 250 pieces/μm ² , P1＝0.02C+0.22Mn+0.87Cr+0.85Al+0.72...(1) P2＝2.40C-0.54Mn-9.26Cr-0.01Al+1.59...(2) However, C, Mn, Cr, and Al in the above-mentioned formulas (1) and (2) refer to the contents of the elements in mass %.