JP2006348321A - Steel for nitriding treatment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for nitriding treatment, which can be subjected to high temperature nitriding treatment without the addition of a large quantity of Ti, to obtain high hardening depth, and has high surface hardness and satisfactory fatigue properties. <P>SOLUTION: The steel for nitriding treatment contains, by mass, 0.05 to 0.60% C, 0.03 to 3.0% Si, 0.01 to 3.5% Mn, 0.10 to 5.00% Cr, 0.05 to 3.0% Mo, 0.1 to 3.0% V, ≤0.5% Ti, 0.001 to 3.0% Al and 0.005 to 0.025% N, and the balance Fe with inevitable impurities, and satisfies inequality 1 of [Si%]+[Mo%]+2×([V%]+[Ti%]+[Al%])≥2.3 and inequality 2 of 242×[Ti%]+230×[Al%]+100×[V%]≥150. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a steel for nitriding which is suitable for parts such as automobiles which are used for a long time in a state where high surface pressure is applied, for example, gear parts by performing nitriding.

  Gear parts used in automobile transmissions require strict characteristics such as wear resistance, anti-pitting characteristics, and root bending fatigue strength because they are used continuously for a long time under high surface pressure. Is done. In order to satisfy these requirements, conventionally, case-hardened steel such as JIS standard SCr420H and SCM420H is formed into a gear shape and then carburized, quenched and tempered. Carburizing treatment is performed because high surface hardness (surface hardness Hv 700 or more) and a curing depth sufficient to withstand high surface pressure (Hv 550 or more and 0.40 mm or more) can be easily obtained. . However, since the carburizing process requires heating at a temperature exceeding the transformation point, thermal distortion or transformation strain occurs after the treatment, and the shape of the contact part of the part becomes non-uniform, causing noise. Become. For this reason, finishing work after the carburizing process is required, resulting in a problem that quality and productivity are reduced and costs are increased.

  Conventionally, nitriding has been studied as a surface hardening method for solving the problem of distortion in the carburizing treatment. Unlike carburizing, nitriding is processed by heating at a transformation temperature or lower (about 500 to 600 ° C.), so distortion can be suppressed to a small level compared to carburizing. However, it has been actively used in the past. The principle of hardening by nitriding is considered to be that N penetrates and diffuses into the parent phase ferrite phase and strain is formed by solid solution strengthening by N or formation of nitrides with alloy elements, thereby forming a hardened layer. It has been. Therefore, in order to utilize such a curing principle, the nitriding treatment is performed at a temperature below the transformation point, and the generation of the γ phase generated at a high temperature and the martensite phase generated by cooling after the nitriding treatment are avoided. When nitriding is performed at a temperature exceeding the transformation point and a γ phase is generated on the surface or a martensite phase is generated by cooling after nitriding, the hardening principle by nitriding cannot be obtained, and an appropriate surface hardness is obtained. In addition to this, the heat treatment distortion increases and the mechanical properties decrease.

  However, gas nitriding treatment and gas soft nitriding treatment, which are usually widely used nitriding methods, have the advantage that distortion can be kept small compared to carburizing treatment. Is about 0.15mm from the surface (total hardening depth; when processed at about 580 ° C x 4hr, which is the usual processing temperature in gas soft nitriding), which is considerably shallower than the carburizing process and the high surface pressure continues. It is not preferable for an environment that is loaded with Further, in order to enable use in a high surface pressure environment, for example, when trying to obtain a deep curing depth exceeding 0.3 mm, it is necessary to significantly increase the processing time (10 hours or more), and carburizing treatment. As a result, the productivity is significantly inhibited. Because of such problems, the steel for nitriding cannot be applied to parts that are problematic due to distortion caused by carburizing.

JP 2004-300472 A

  Patent Document 1 proposes a nitrided steel that realizes rapid nitriding treatment. In order to shorten the time of soft nitriding treatment, a large amount of Ti is added to increase the transformation point, and diffusion of C and N during nitriding treatment at high temperature is promoted to obtain high surface hardness. It is a feature. However, since such a steel type has a high Ti component, it requires a special melting method such as a vacuum melting method and is difficult to use as a mass-produced steel. Furthermore, when such high Ti steel is used, the amount of TiC deposited increases, resulting in a significant decrease in fatigue strength.

  The present invention has been made in view of the above problems, and without adding a large amount of Ti, it is possible to perform a nitriding treatment at a high temperature and obtain a deep hardening depth, and a high surface hardness and a good fatigue. An object is to provide a nitriding steel having characteristics.

Means for Solving the Problems and Effects of the Invention

In order to solve the above problems, the nitriding steel of the present invention is, in mass%, C: 0.05% to 0.60%, Si: 0.03% to 3.0%, Mn: 0.00. 01% to 3.5%, Cr: 0.10% to 5.00%, Mo: 0.05% to 3.0%, V: 0.1% to 3.0%, Ti: 0.5% or less, Al: 0.001% or more and 3.0% or less, N: 0.005% or more and 0.025% or less, with the balance being Fe and inevitable impurities. It is characterized by satisfying.
[Si%] + [Mo%] + 2 × ([V%] + [Ti%] + [Al%]) ≧ 2.3 Formula 1
242 × [Ti%] + 230 × [Al%] + 100 × [V%] ≧ 150 Formula 2
In the present specification, “[X%]” represents the content of the element represented by X.

  The present inventors have the effect of increasing the eutectoid temperature of steel against nitrogen intrusion due to nitriding, and enabling the nitriding treatment at high temperature by adding an alloy element that does not require a special melting method or the like. As a result of extensive research, it is possible to perform nitriding treatment at a high temperature by using a combination of alloy elements satisfying the above formulas 1 and 2, and to obtain a deeper hardening depth than conventional nitriding steel, and high. The steel for nitriding which can obtain surface hardness was discovered. Further, in the nitriding steel of the present invention, the amount of Ti added is suppressed to 0.5% or less, so that mass production is possible without requiring a special melting method, and precipitation of TiC is suppressed. Excellent fatigue properties.

  Hereinafter, the reason for the numerical limitation in the nitriding steel of the present invention will be described.

(1) [Si%] + [Mo%] + 2 × ([V%] + [Ti%] + [Al%]) ≧ 2.3 Formula 1
The combination of alloy elements satisfying Equation 1 increases the eutectoid temperature of steel against N intrusion during nitriding, and enables nitriding at a high temperature (for example, 600 ° C. or more and 750 ° C. or less). As a result, it is possible to obtain a hardening depth (for example, 0.3 mm or more) deeper than the conventional nitriding steel.

(2) 242 × [Ti%] + 230 × [Al%] + 100 × [V%] ≧ 150 Formula 2
By a combination of alloy elements satisfying Equation 2, a hard diffusion layer can be formed even by nitriding at a high temperature (for example, 600 ° C. or higher and 750 ° C. or lower). Thereby, high surface hardness (for example, 650 HV or more) can be obtained.

(3) C: 0.05% or more and 0.60% or less C is an element necessary for obtaining internal hardness for securing strength. To obtain this effect, 0.05% or more must be added. is necessary. On the other hand, excessive addition increases the hardness after forging, rolling, or solution treatment, and deteriorates workability, so the addition is made 0.60% or less.

(4) Si: 0.03% or more and 3.0% or less Si is used as a deoxidizer during the melting of steel and has an effect of improving the fatigue strength of steel. Further, like Mo, V, Ti, Al, and Cr included in Formula 1, it is also an element that raises the eutectoid temperature of steel against nitrogen intrusion due to nitriding. To obtain these effects, 0.03% or more must be added. On the other hand, excessive addition promotes embrittlement of steel and deteriorates workability such as machinability with surface decarburization, so the addition is made 3.0% or less.

(5) Mn: 0.01% or more and 3.5% or less Mn is an element that contributes to improvement in hardness by solid solution strengthening and an element that is effective in improving toughness. To obtain these effects, 0.01% or more must be added. On the other hand, excessive addition reduces machinability and deteriorates machinability, and causes a decrease in the depth of hardening after nitriding, so it is added at 3.5% or less.

(6) Cr: 0.10% or more and 5.00% or less Cr, like Si, Mo, V, Ti, and Al included in Formula 1, is an element that raises the eutectoid temperature of steel against nitrogen intrusion due to nitriding. In order to obtain this effect, addition of 0.10% or more is necessary. On the other hand, excessive addition becomes too hard after forging, rolling, or solution treatment, and reduces machinability, so the addition is made 5.00% or less.

(7) Mo: 0.05% or more and 3.0% or less Mo, like Si, V, Ti, Al and Cr included in Formula 1, is an element that raises the eutectoid temperature of steel against nitrogen intrusion due to nitriding. In order to obtain this effect, it is necessary to add 0.05% or more. On the other hand, excessive addition becomes too hard after forging, rolling, or solution treatment, thereby reducing machinability and increasing the cost. Therefore, the addition is made 3.0% or less.

(8) V: 0.1% or more and 3.0% or less V is an element that raises the eutectoid temperature of steel against nitrogen intrusion due to nitriding, like Si, Mo, Ti, Al and Cr included in Formula 1. Yes, the trend is superior to other elements. It is also an element that increases the hardness of the diffusion layer. In order to obtain these effects, addition of 0.1% or more is necessary. On the other hand, excessive addition causes carbide to precipitate after forging, rolling, or solution treatment, lowers C in the matrix, and decreases V that effectively acts during nitriding. Addition.

(9) Ti: 0.5% or less Ti, like Si, Mo, V, Al and Cr included in Formula 1, is an element that raises the eutectoid temperature of steel against nitrogen intrusion due to nitriding, and its tendency is Superior to other elements. It is also an element that increases the hardness of the diffusion layer. However, as described above, excessive addition causes difficulty in productivity and increases the precipitation amount of TiC, leading to a significant decrease in fatigue strength. Further, the shape of the compound layer after nitriding becomes a needle shape, and mechanical properties are impaired. For this reason, 0.5% or less is added.

(10) Al: 0.001% to 3.0% Al is added as a deoxidizer during melting. In addition, similar to Si, Mo, V, Ti and Cr included in Formula 1, it is an element that increases the eutectoid temperature of steel against nitrogen intrusion due to nitriding, and an element that increases the hardness of the diffusion layer at high temperatures. . To obtain these effects, 0.001% or more must be added. On the other hand, excessive addition tends to cause the base material to become an α phase and the toughness of the core portion is reduced, so the addition is made 3.0% or less.

(11) N: 0.005% or more and 0.025% or less N is a component that produces fine nitrides with Al and contributes to improving the toughness of steel by suppressing the growth of crystal grain size during hot forging. is there. To obtain this effect, 0.005% or more must be added. On the other hand, excessive addition causes blowholes or the like during forging and impairs the soundness of the steel ingot, so the addition is made 0.025% or less.

  Next, the nitriding steel of the present invention can further contain the following optional components as steel components.

(12) Nb: 0.5% or less, Zr: 0.5% or less These components are used to form sulfides such as MnS present in the steel due to these oxides generated in the molten steel when the steel is melted. Since it is refined and dispersed, it contributes to improving the machinability of steel. In addition, the refined sulfide refines the structure after forging or normalizing the steel to improve the fatigue strength of the steel. However, even if added excessively, the effect is saturated, so each content is preferably 0.5% or less.

(13) Cu: 1.0% or less, Ni: 1.0% or less These components improve the core strength of the steel. However, even if added excessively, it is not economically advantageous.

(14) S: 0.01% or more and 0.20% or less, Ca: 0.0005% or more and 0.0030% or less, Pb: 0.3% or less, Bi: 0.3% or less In order to improve the machinability, it is preferable to add at least one of these components when high machinability is required during machining performed before the aging treatment. However, excessive addition degrades the hot workability and fatigue limit of steel, so S is 0.2% or less, Ca is 0.010% or less, Pb is 0.3% or less, and Bi is 0.3. % Or less is preferable.

  Next, the production and nitriding of the nitriding steel of the present invention will be described.

  The steel for nitriding treatment of the present invention is subjected to a sufficient solution treatment in various heat treatments such as hot rolling, forging, quenching and tempering after being melted and subjected to predetermined refining, and Si that effectively acts during nitriding, Elements such as Cr, Mo, V, Ti, and Al are sufficiently dissolved in the matrix. In the subsequent cooling, the cooling rate is appropriately set according to the combination of product types and alloy components so that these elements remain in the matrix without being precipitated as carbides or nitrides. As an example, in the case of a component composition in which 50% or more of ferrite is generated after cooling, it is necessary to perform the region from 800 ° C. to 500 ° C. at a cooling rate of 0.05 ° C./sec to 10 ° C./sec.

  After the nitriding steel of the present invention is obtained, processing such as cutting and molding is performed so as to become various products, but since such processing is performed before nitriding, it is performed at a temperature at which carbonitride is not precipitated. To do. Specifically, implementation at 500 ° C. or lower is preferable.

  Nitriding is performed at 600 ° C. or higher. In ordinary steel, the eutectoid temperature of the steel against nitrogen intrusion due to nitriding is less than 600 ° C., and if it is treated at a temperature higher than that, a γ phase and a martensite phase are generated. However, the steel for nitriding treatment of the present invention can be nitrided at a high temperature of 600 ° C. or higher because the eutectoid temperature of the steel against N intrusion during nitriding is 600 ° C. or higher according to Equation 1. The upper limit of the nitriding temperature can be set to 750 ° C. or lower, for example. Further, the curing depth can be appropriately adjusted according to the nitriding time according to the required characteristics. Here, the nitriding steel of the present invention can be nitrided at a high temperature, so that the processing time can be shortened, and the productivity is superior to the conventional nitriding steel. The nitriding treatment includes methods such as gas nitriding, gas soft nitriding, salt bath nitriding, salt bath soft nitriding, ion nitriding, plasma nitriding, radical nitriding, and any of them may be applied.

Next, tests conducted to confirm the effects of the present invention will be described.
The test used the steel components shown in Table 1 and a steel ingot melted by a 5 kg vacuum induction melting furnace, heated to 1200 ° C. or higher and forged into a round bar having a diameter of 22 mm. Then, after cutting and forming to a predetermined size (rotary bending fatigue test piece and wear test piece), all steel materials were held at 850 to 1200 ° C. for 30 minutes, and then oil-cooled and air-cooled. In addition, about X steel, the aging treatment for 2 hours was performed at 650 degreeC after that, and the reduction | restoration from the matrix of the element which acts effectively at the time of nitriding was aimed at.

In Table 1, A to P steels are invention steels within the compositional range of the present invention, and R to X steels are comparative steels. The comparative steel R is obtained by soft nitriding a steel equivalent to SCM440, which is a general-purpose steel, under normal nitriding conditions. In Comparative Steel X, the components at the time of manufacture are included in the range defined in the present invention. However, since the aging treatment is intended to reduce the elements that act effectively at the time of nitriding from the matrix, deal with.
In addition, in the composition of the comparative steel in Table 1, those that deviate from the composition range defined in the present invention include a downward arrow (↓) when below the lower limit, and an upward arrow (↑) when exceeding the upper limit. ) Is attached.

  Nitriding treatment was carried out under the conditions shown in Table 2 for the above invention steels (A to P steel) and comparative steels (R to X steel). The nitriding treatment was performed on the specimens that had been quenched by gas soft nitriding in the range of 620 to 750 ° C., which is higher than the normal nitriding temperature (however, the R steel was subjected to normal nitriding temperature) A gas soft nitriding treatment was performed at a certain 550 ° C.). Next, a rotating bending fatigue test and a wear test were performed on the test piece after the nitriding treatment. The test method will be described below.

-Rotating bending fatigue test The rotating bending fatigue test was conducted using an Ono type rotating bending fatigue tester. The test conditions are shown below.
Rotation speed: 3500rpm
Temperature: 25 ° C

・ Abrasion test The abrasion test was performed using an Ogoshi type abrasion tester. The test conditions are shown below.
Opponent material: SUJ2
Friction distance: 400m
Friction speed: 0.94 m / s
Non-lubricated

  Table 2 shows the results of the above rotating bending fatigue test and wear test. In addition, these test results are represented by relative evaluation based on Invention Steel 1. Table 2 also shows the N diffusion depth observed by EPMA.

  According to Table 2, since the inventive steels (A to P steel) are subjected to high-temperature nitriding treatment, N diffusion depth (N diffusion depth) that cannot be obtained under the normal nitriding conditions of general-purpose steel such as comparative steel R Generally corresponds to the hardening depth after nitriding.). In addition, since the inventive steels (A to P steels) satisfy Formula 1, it has been confirmed from X-ray diffraction that no γ phase is generated even when nitriding at a high temperature. Furthermore, since the inventive steels (AP steels) satisfy the formula 2, it is clear that excellent fatigue characteristics and wear resistance are obtained by high temperature nitriding compared to the comparative steel R.

  It can be seen that the comparison steels S and T have deteriorated fatigue strength and wear compared to the invention steels (AP steels). This is considered to be because the hardness of the diffusion layer was not sufficiently obtained because Expression 1 was satisfied but Expression 2 was not satisfied.

  It can be seen that the comparative steels U and V have deteriorated fatigue strength and wear compared to the inventive steels (AP steels). This is considered to be due to the fact that the γ phase was formed immediately below the compound layer because the formula 2 was satisfied but the formula 1 was not satisfied (confirmed by X-ray diffraction).

  It can be seen that the comparative steel W is lower in both fatigue strength and wear compared to the inventive steel (AP steel). This is considered to be due to the fact that because of the high Ti steel, the amount of TiC in the matrix is large and fatigue fracture occurs starting from such inclusions (confirmed by fracture surface observation). Therefore, it is considered that such high Ti steel is not suitable for high temperature nitriding treatment.

  It can be seen that the comparative steel X has both fatigue characteristics and wear resistance characteristics that are lower than those of the inventive steels (AP steels). This is considered to be due to the fact that the γ layer was formed on the surface as a result of the reduction of the amount of elements that worked effectively during nitriding from the matrix because aging treatment was performed before nitriding, although Expressions 1 and 2 were satisfied (X Confirmed by line diffraction and microscopic observation).

Claims (4)

In mass%, C: 0.05% to 0.60%, Si: 0.03% to 3.0%, Mn: 0.01% to 3.5%, Cr: 0.10% or more 5.00% or less, Mo: 0.05% to 3.0%, V: 0.1% to 3.0%, Ti: 0.5% or less, Al: 0.001% to 3.0% % Or less, N: 0.005% or more and 0.025% or less, the balance being Fe and inevitable impurities, satisfying the following formulas (1) and (2).
[Si%] + [Mo%] + 2 × ([V%] + [Ti%] + [Al%]) ≧ 2.3 Formula 1
242 × [Ti%] + 230 × [Al%] + 100 × [V%] ≧ 150 Formula 2   The steel for nitriding according to claim 1, further comprising one or two kinds selected from Nb: 0.5% or less and Zr: 0.5% or less as a steel component.   The steel for nitriding according to claim 1 or 2, further comprising one or two kinds selected from Cu: 1.0% or less and Ni: 1.0% or less as a steel component.   The steel component is further selected from S: 0.01% to 0.20%, Ca: 0.0005% to 0.0030%, Pb: 0.3% or less, Bi: 0.3% or less 1 The steel for nitriding according to any one of claims 1 to 3, comprising seeds or two or more kinds.