JP2006161150A - Carbon steel for induction hardening and component for machine structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carbon steel for induction hardening suitable for obtaining an induction-hardened material having excellent mechanical properties represented by fatigue properties. <P>SOLUTION: The carbon steel comprises 0.3 to 0.6% C and 0.05 to 0.8% Mo. Since Mo has a tendency to segregation, and, in a stage to induction hardening treatment such as a slab heating stage, there is a need for suppressing the ununiformity in concentration caused by the segregation of Mo and attaining the uniformity of the structure, and the ratio between the maximum value (Imax) and the minimum value (Imin) in the characteristic X-ray intensity of Mo as the index of uniformization, Imax/Imin is controlled to ≤3. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a carbon steel material for induction hardening that is mainly subjected to induction hardening with the aim of improving fatigue characteristics. In particular, the steel material for induction hardening according to the present invention is suitable as a material for steel for machine structures applied to drive shafts, constant velocity joints and the like of automobiles.

Conventionally, in the manufacture of steel materials for machine structures applied to automobile drive shafts, constant velocity joints, etc., hot rolled steel bars were processed into a predetermined shape by hot forging or further cutting and cold forging. Thereafter, induction hardening and tempering are generally performed to ensure torsional fatigue strength or rotational bending fatigue strength, which is an important characteristic as a steel material for mechanical structures.
On the other hand, from the viewpoint of environmental problems in recent years, there has been an increasing demand for weight reduction of automobile parts, and in order to realize thinning of the parts, it is desired to further increase the strength of the material.

  Here, Patent Document 1 has a problem of variation in materials in a known surface hardening method by shot peening. Carburizing treatment is performed before induction quenching, and constant surface grinding is performed after induction quenching, thereby homogenizing the materials. However, since a multi-step process of carburizing treatment + induction hardening treatment + surface grinding treatment is required, its productivity is low.

  Patent Document 2 discloses that the grain size of the induction-hardened hardened layer is set to JIS standard No. 14 by performing large strain processing of 30% or more in the non-recrystallized region twice or more in the manufacturing process before induction hardening. Techniques for controlling the above and improving the strength and fatigue resistance are disclosed. However, it cannot be applied to a manufacturing process that does not have a sufficient degree of hot work, and a large rolling in the non-recrystallized region increases the rolling load, and the burden on the manufacturing line is extremely large. Furthermore, since it is generally difficult to evaluate the grain size of the martensite structure, product management may be difficult.

  Furthermore, Patent Document 3 discloses a technique for providing a steel material excellent in high strength characteristics and low heat treatment strain characteristics by controlling the structure before induction hardening. That is, in the manufacturing process before induction hardening heat, the hot rolling heating temperature, the finishing temperature and the cooling rate after hot rolling are defined, and the ferrite and the pearlite structure are controlled by an index called a ferrite band score. It has been shown that the martensite structure after induction hardening is homogenized while ensuring cold workability before heat treatment. However, when the structure before induction hardening is a ferrite and pearlite structure, it cannot be said that the final structure is sufficiently refined, and it is difficult to provide a material that can sufficiently withstand the recent increase in strength.

Furthermore, in Patent Documents 4 and 5, by controlling the precipitation state of fine Ti-based carbides and carbonitrides, the fine old γ grain size and the matrix phase strength after induction hardening are ensured, and the fatigue strength A technique for enhancing the above is disclosed. Specifically, Ti-based fine precipitation after hot working to ensure the solid solution Ti amount to be reprecipitated by heating to 1250 ° C or higher once during the manufacturing process, and to achieve both machinability and matrix strength. There is a device to control the amount. However, Ti-based carbides or carbonitrides generally have a very large driving force for precipitation, and delicate precipitation control that achieves both cold workability and matrix strength is not easy.
JP-A-8-260125 JP-A-11-71633 Japanese Patent Laid-Open No. 11-236644 Japanese Patent Laid-Open No. 11-71630 JP 2003-321712 A

  The object of the present invention is to solve the problems of manufacturability, structure reproducibility, precipitation controllability, etc. in the prior art, and for induction hardening suitable for obtaining an induction hardening material excellent in mechanical properties represented by fatigue properties. It intends to provide carbon steel.

Based on the premise of performing induction hardening for a short time, the inventors have made intensive investigations on the homogenization of the structure particularly for the induction hardening material for realizing stable fatigue characteristics by this induction hardening. As a result, in order to satisfy the following conditions (i) and (ii), high-precision heat treatment for precipitation control without causing a decrease in manufacturability by actively controlling the structure before induction quenching It has been found that an induction-quenched material excellent in mechanical properties typified by fatigue properties can be provided without carrying out the above.
Hereinafter, the knowledge that led to the present invention will be described in detail.

(I) Mo and W are in a solid solution state In steel grades for machine structural parts (particularly for automobile parts) that require fatigue resistance, the surface hardened layer formed by shot peening, carburizing and induction hardening is used. The formation achieves the desired properties. In particular, the induction hardening method has many merits such as being able to obtain a uniform hardened surface layer by a short time treatment and imparting high compressive residual stress. In this induction hardening method, a fine martensite structure is obtained by rapid heating and then rapid cooling after a material formed by hot rolling or the like is cut into a predetermined shape. In particular, in the rapid heating process, in order to reversely transform from the structure after hot rolling to the austenite region in a short time, it has an extremely fine austenite grain size by selecting an appropriate heating temperature and performing multi-stage heating and cooling processes. It is possible to obtain a quenched martensite structure.

  Here, in the reverse transformation when the carbon steel is rapidly heated to the austenite region, the diffusion control of carbon having a large body diffusion coefficient is mainly performed. For this reason, when the structure before induction hardening is a ferrite + pearlite structure, in a pearlite structure having a high carbon concentration, reverse transformation to austenite proceeds rapidly due to an increase in the carbon concentration accompanying carbide dissolution. On the other hand, it is considered that the reverse transformation to austenite is relatively delayed in the ferrite structure portion where the carbide precipitation is small, and that the transformation is not transformed by rapid heating in a short time. As a result, the finally obtained martensite structure after induction hardening tends to be a non-uniform structure reflecting the structure before induction hardening.

  On the other hand, it is slightly disadvantageous in terms of machinability for forming parts after hot working to make the structure before induction hardening a bainite structure instead of a ferrite + pearlite structure, but the reverse transformation phenomenon described above. There is an advantage in terms of averaging. That is, in the bainite structure, carbides between the laths dissolve during induction hardening, and reverse transformation to austenite occurs at a stretch from a large amount of lath interfaces. Moreover, when the ratio of untransformed ferrite decreases, it becomes easy to obtain a quenched martensite structure almost from an austenite single phase. That is, by appropriately controlling the structure before induction hardening, if a fine and homogeneous martensite structure is obtained, a uniform hardened surface layer can be obtained as a result, and stable fatigue characteristics can be obtained. For these reasons, it is necessary to make the structure before induction hardening a bainite structure, and the addition of Mo and W that can easily obtain a bainite phase is extremely effective.

  Here, in a steel type system that obtains a martensite structure by induction hardening that is the subject of the present invention, the dislocation density is extremely high, and the presence of overwhelming compressive residual stress due to restraint during transformation almost dominates the surface hardness. is doing. Accordingly, it is considered that the contribution of coarsening of austenite grains due to the formation of fine carbonitride precipitates such as Ti, V and Nb, and precipitation strengthening due to these fine precipitates is relatively low. Rather, from the viewpoint of fatigue strength, it is important to keep the reverse-transformed austenite grains formed during induction hardening fine. In fact, it has been confirmed that a steel type added with Mo or W studied by the inventors can obtain austenite grains of several μm or less in many cases by induction hardening at 1000 ° C. or less.

  Since this austenite grain size is inversely proportional to the amount of Mo or W added, and Mo and W added are partially precipitated but mostly dissolved, Mo and W are Ti, V and Nb. It is considered that fine reverse-transformed austenite grains superior in fatigue strength are realized by a function that is essentially different from an element having high carbonitride-forming ability. However, if Mo or W is added excessively, the amount of precipitation as carbides or intermetallic compounds in the structure before induction hardening increases, making it difficult to secure an effective amount of solid solution. Addition within a range that suppresses certain precipitates is desirable.

(Ii) Concentration distribution of solid solution Mo or W is uniform. On the other hand, segregation of alloy elements is not possible to a certain extent when producing steel (especially for solid solution elements that are not precipitates). Awake. For this reason, it is common to make the composition and structure uniform by diffusion annealing or hot working, but the structure and composition in the forming process of the primary material because the degree of hot working is small in thick plates and steel bars. It may be difficult to homogenize. In particular, the addition of Mo and W, which the inventors studied mainly, can obtain a bainite phase as a structure before induction quenching, but the elemental segregation during the melting of raw steel is remarkable, and solidification is caused by heat treatment at about 1200 ° C. In many cases, segregation of time was not resolved. In fact, when Mo is added to high carbon steel, if the Mo concentration to be added exceeds 0.15 mass%, the segregation of the solid solution Mo formed at the time of melting the material is caused by hot rolling in the rolling direction (L direction) section. It was confirmed that it was likely to be distributed in a strip shape.
As an example of this segregation, FIG. 1 shows a Mo mapping image of an electron probe microanalyzer (EPMA) in the case where the Mo concentration non-uniformity in the structure before induction quenching in 0.6 mass% Mo-added steel is clear. This mapping image is obtained by measuring a cross section in the L direction and a region having a depth of 0.25 mm from the surface. In addition, the steel shown in the drawing is a steel slab containing 0.4 mass% C, 0.35 mass% Si, 0.7 mass% Mn, 0.025 mass% Al, and 0.0045 mass% N in addition to Mo in the above range, and after rough rolling. It is a steel material just before induction hardening, after a slab heat treatment at 1250 ° C. for 30 minutes, followed by a bar rolling process up to 24 mmφ.

FIG. 4A shows a two-dimensional mapping, and FIG. 4B shows a part of it by a line profile.
In FIG. 1A, the bright white portion is a region where the X-ray intensity of Mo is strong (that is, the solid solution Mo concentration is high), and the gray portion is a low X-ray intensity of Mo (that is, the solid solution Mo concentration is low). Low) area. In this example, the non-uniform density width is a maximum of 100 μm (BB ′). For example, if an X-ray intensity profile of Mo on a straight line (AA ′) perpendicular to the rolling direction is drawn, the result shown in FIG. 1B is obtained.
In FIG. 1B, the maximum value of Mo intensity on the straight line (AA ′) exists as a point (c) with a white region in FIG. On the other hand, the minimum value of the Mo intensity exists as a point (d) having a gray region in FIG. By drawing such a line profile over the entire visual field, the maximum value and the minimum value in the visual field can be extracted. However, since the composition of solute Mo or solute W may not be reflected in the structure due to obvious voids and precipitates ((e) in FIG. 1B), these values are subject to evaluation. exclude.
By the way, this steel type often has coarse Mo-based precipitates on the order of several hundreds of nanometers to micron order, but these are greatly deviated from the composition of the matrix and can be easily judged from the extremely high content of Ti and Mo. Moreover, the change in composition is, for example, as shown in the analysis results of the two types of precipitates (Ti, Mo) (C, N) and (Fe, Mo) ₂₃ C ₆ in FIGS. 2 (a) and (b). For example, it can be judged from an energy dispersive X-ray spectroscopy (EDX) spectrum. Thereafter, unless otherwise specified, the Mo concentration and the W concentration indicate the solid solution Mo concentration and the solid solution W concentration, respectively.

  As described above, in the field of induction-hardened steel of the present invention, the initial structure non-uniformity promotes the non-uniformity of the final structure because the final structure depends on instantaneous transformation phenomenon due to rapid heating and rapid cooling. There is concern. That is, in the structure before induction quenching, the bainite structure is predominant in the high Mo concentration region, whereas the ferrite + pearlite structure is formed in the low Mo concentration region. When the Mo addition amount is low, the initial concentration non-uniformity is small, and non-uniformity of the structure before induction hardening due to this non-uniform composition is avoided, but as the Mo addition amount increases, the composition non-uniformity in the steel material becomes significant. And cause serious tissue non-uniformity.

  Therefore, the inventors of the present invention have some Mo-added steel types (Mo: 0.15 to 0.8% by mass, manufacturing history is the same as in the case of FIG. 1). The relationship between the non-uniform concentration and the non-uniform structure of the prior austenite grains in the corresponding region (evaluated by the standard deviation of the particle size) was investigated. Figure 3 shows the ratio (Imax) / (Imin) of the maximum value (Imax) and minimum value (Imin) of the characteristic X-ray intensity (hereinafter referred to as X-ray intensity) of Mo in EPMA analysis, and the prior austenite grain size. The relationship with the standard deviation value is shown. From this result, it was found that when the ratio (Imax) / (Imin) of the X-ray intensity of Mo exceeds 3, the standard deviation greatly increases and it is difficult to obtain high fatigue strength. Furthermore, in the low concentration region when the X-ray intensity ratio (Imax) / (Imin) of Mo is high, the prior austenite particle size tends to increase, and the average particle size tends to increase reflecting this. It was observed. This tendency was the same when W was added.

  Therefore, in the raw steel melting stage or the stage before the final induction hardening process at the latest, homogenization within the structure of Mo or W, etc. is attempted, and the characteristic X-rays of Mo or W at any measurement location in the hot rolled structure It is important to control the tissue so that the ratio (Imax) / (Imin) of the maximum value (Imax) and the minimum value (Imin) does not exceed 3.

The present invention has been made based on the above knowledge, and the gist thereof is as follows.
(1) Steel material containing C: 0.3 to 0.6% and Mo: 0.15 to 0.8% by mass%, and the maximum value (Imax) and minimum value (Imin) of the X-ray intensity of Mo in any cross-sectional area A carbon steel material for induction hardening excellent in fatigue characteristics, characterized by having a ratio Imax / Imin of 3 or less.

(2) Steel materials containing C: 0.3 to 0.6% and W: 0.25 to 1.6% by mass%, and the maximum value (Imax) and minimum value (Imin) of the X-ray intensity of W in any cross-sectional area A carbon steel material for induction hardening excellent in fatigue characteristics, characterized by having a ratio Imax / Imin of 3 or less.

(3) The steel material for induction hardening as described in (1) or (2), wherein the steel material further contains, by mass%, S: 0.05% or less and Mn: 0.2-2%. .

(4) The steel material further comprises, by mass%, S: 0.05% or less and Mn: 0.2-2%, and the balance is composed of Fe and inevitable impurities (1) or (2 ) Carbon steel for induction hardening with excellent fatigue properties as described in (1).

(5) According to (3) or (4), the steel material further contains, by mass, Si: 1% or less, P: 0.02% or less, Al: 0.5% or less, and N: 0.01% or less. Induction quenching carbon steel material with excellent fatigue characteristics.

(6) The steel material further comprises, by mass%, Ti: less than 0.05% and B: 0.005% or less, and having excellent fatigue properties according to any one of (3) to (5) Carbon steel for induction hardening.

(7) The carbon steel for induction hardening according to any one of (3) to (6), wherein the steel material further contains Ca: less than 0.010% by mass.

(8) A machine structural component having excellent fatigue characteristics, comprising the induction-quenched carbon steel material according to any one of (1) to (7).

  The steel material for induction hardening according to the present invention eliminates the compositional non-uniformity in the steel material at the stage before induction hardening treatment, and the hardened layer structure obtained by induction hardening of the steel material becomes fine and homogeneous, so that the fatigue characteristics It is possible to provide an induction hardening material with improved mechanical characteristics represented by

Hereinafter, the reasons for limiting the requirements of the present invention will be described in detail. Unless otherwise specified, “%” in relation to ingredients indicates “% by mass”.
C: 0.3-0.6%
C is an element having a great influence on hardenability, and is very useful for improving the fatigue strength by increasing the hardness and depth of the hardened hardened layer. However, if the C content is less than 0.3%, it becomes difficult to secure a quenching depth for maintaining fatigue strength, so the C content is set to 0.3% or more. On the other hand, if the C content exceeds 0.6%, the machinability and the cold forgeability deteriorate. Furthermore, since the structure after the normal treatment before induction hardening becomes a coarse ferrite + pearlite mixed phase, the structure after the induction hardening becomes non-uniform. Therefore, the C content is in the range of 0.3 to 0.6%.

Mo: 0.15-0.8%
Mo is an extremely important element in the present invention. Mo has the function of improving mechanical properties such as fatigue strength by reducing grain boundary segregation such as C and P and increasing the grain boundary strength of the material. In addition, it is generally an element useful for improving hardenability, promotes the formation of a bainite phase in a structure after normalization before induction hardening, and realizes fine dispersion of cementite. Furthermore, Mo is thought to have a function of delaying reverse transformation during induction hardening as a ferrite stabilizing element. In order to obtain such an effect, addition of 0.15% or more is necessary.

  On the other hand, Mo is an element that has a very strong macro-segregation tendency at the time of raw material melting, and it is difficult to eliminate the initial segregation by a subsequent process in a steel bar or a thick material having a low rolling rate. That is, in the present invention, it is important to suppress the concentration non-uniformity due to the segregation of Mo and to make the structure uniform in the process before the induction hardening process. If the amount of Mo exceeds 0.8%, the non-uniform distribution of Mo in the structure before induction hardening cannot be almost eliminated, and the cost is increased. Therefore, the upper limit is set to 0.8%. More preferably, the addition range of Mo is 0.35 to 0.65%.

In the present invention, W can be contained instead of adding Mo.
W: 0.25-1.6%
W is an important element that can be expected to have almost the same effect as Mo. Since W is approximately a little less than twice the mass number of Mo, the addition range of W is set to 0.25 to 1.6%. The reason for limiting the upper and lower limits is the same as for Mo. Further, the W addition range is preferably 0.5 to 1.2%.

Mn: 0.2-2%
Mn is an element effective in improving the hardenability and securing a hardened hardened layer. Furthermore, it is effective for increasing the strength of the parent phase by solid solution strengthening, and also has a role of fixing S in steel as MnS and improving machinability. Such effects cannot be obtained with addition of 0.2% or less. However, if added over 2%, the retained austenite after quenching increases, leading to a decrease in fatigue strength due to softening of the material, so the Mn content is set to a range of 0.2 to 2.0%.

S: 0.05% or less S can be expected to improve the machinability by forming MnS in the steel, but if added over 0.05%, it segregates at the grain boundaries like P and mechanical properties such as fatigue strength. Therefore, the S content is limited to 0.05% or less.

In the present invention, in addition to the above chemical components, the following elements can be contained as required.
Si: 1% or less
Si is useful as a deoxidizing element during steelmaking, and also has an effect of improving the steel strength by solid solution in ferrite. Furthermore, since the effect of suppressing the transition of the fracture form of high-strength steel to grain boundary fracture is also expected, it can be added preferably at 0.01% or more. However, if the Si content exceeds 1%, the hardness increases, leading to a decrease in machinability and cold forgeability. Therefore, the Si content is 1% or less.

P: 0.02% or less P segregates at austenite grain boundaries during high-temperature heating, and significantly reduces mechanical properties such as fatigue strength by lowering the grain boundary strength, and also promotes cracking and the like during the manufacturing process. Therefore, it is limited to 0.02% or less.

Al: 0.5% or less
Al is useful as a deoxidizing element during steelmaking, and can be added because it dissolves in ferrite and improves steel strength. In order to obtain these effects, 0.05% or more is preferably added. However, if the Al content exceeds 0.5%, coarse AlN or the like tends to remain in the material, resulting in a decrease in fatigue strength. Therefore, the Al content is 0.5% or less.

N: 0.01% or less Since N may cause a decrease in toughness when present in excess, the upper limit is made 0.01% or less. In addition, when coexisting with elements such as Ti, which have a large ability to form nitrides, MN-type precipitates are formed and effective in suppressing grain growth in the austenite region. There is no.

Ti: less than 0.05%
Ti can be added because it forms an MN-type precipitate and exhibits an effect of suppressing grain growth in the austenite region. In order to obtain these effects, 0.01% or more is preferably added. However, when 0.05% or more is added, the amount of precipitation of (Ti, Mo) (C, N) containing Mo increases and the effect of solid solution Mo decreases, so the content is made less than 0.05%.

B: 0.005% or less B is an element effective for improving hardenability and strengthening grain boundaries, but this effect is limited to the case where B exists in a solid solution state. For this reason, it is desirable to add 0.001% or more only when the Ti amount satisfies the relationship of Ti ≧ 3.4N in order to avoid the formation of BN. Even when this condition is satisfied, depending on the excessive addition, coarse M23 (C, B) 6 is formed during normal treatment in the austenite region, and the solid solution amount of effective elements such as Mo is reduced. Therefore, the upper limit is set to 0.005%.

Ca: Less than 0.010%
Ca has a function of fixing S in steel in a composite form of (Mn, Ca) S and improving machinability, and is preferably added at 0.001% or more. However, even if 0.010% or more is added, the effect is saturated, so the content is made less than 0.010%.

Next, an example of the manufacturing method of the steel for induction hardening in this invention is demonstrated.
The most important control point in the present invention is to eliminate the non-uniform concentration of Mo or the like in the upper process, and to homogenize the structure before the short-time induction hardening process. Specifically, it is important that the ratio (Imax) / (Imin) described above is 3 or less for Mo or W.
In order to obtain such steel, the steel material obtained by melting the steel satisfying the chemical composition described above, the first stage of hot working such as hot rolling or hot forging, that is, the slab heating process In the rough rolling process, it is effective to perform a heat treatment for heating the slab to a temperature range of 1200 ° C. or higher, for example. In particular, when the Mo (or W) concentration is high, solidification segregation during steel melting becomes particularly noticeable, and thus heat treatment at 1300 ° C. or higher is desirable.
It is also considered effective to eliminate the compositional non-uniformity during casting to some extent by carrying out electromagnetic stirring or the like in the steel making process of steel material melting.

  Furthermore, in hot working, a finishing temperature of 850 ° C. or higher that avoids two-phase rolling is ensured, and the cooling rate after processing is cooled at a cooling rate of 5 ° C./s or lower, and lower bainite (fine carbide It is desirable to have a main structure in which lath is precipitated between the laths (some of which may include ferrite + pearlite). When the cooling rate exceeds 5 ° C./s, an extremely hard transformation phase such as lower bainite (fine carbide precipitates in the lath) or martensite is generated in the hot-rolled structure, and the machinability is significantly reduced. The steel material that has undergone hot working is subjected to an induction hardening process after being formed into a predetermined shape and, if necessary, subjected to a normalizing process at point A3 or higher. The upper limit of the normalizing temperature is desirably 900 ° C. in order to suppress coarsening of the final structure.

In the present invention, the “arbitrary cross-sectional area” basically depends on the non-uniform Mo or W concentration width (for example, refer to the distance between B and B ′ in FIG. 1A) of the structure before induction hardening. However, it is preferable to take a region having a length of about 5 times the maximum width of the uneven density width as one side. Then, average information with non-uniform density can be captured.
Furthermore, in the present invention, since it is desired to suppress non-uniform particle size in the surface hardened layer after induction hardening, the X-ray intensity ratio of Mo or W is 3 or less at least up to the depth at which the surface hardened layer is obtained after induction hardening. Preferably there is.

  Further, as the simplest method for obtaining the X-ray intensity ratio of the tissue after induction hardening, a mapping image of the target element in the cross-sectional area to be measured is acquired (see FIG. 1 (a)), and the apparent concentration nonuniformity is obtained. In such a case, the intensity ratio between the highest point in the high X-ray intensity region and the low point in the low region may be obtained. In particular, the X-ray intensity in EPMA or SEM EDX is evaluated by the peak height of the characteristic X-ray of the target element. The peak height of this X-ray may include the background or the net height minus the background.

However, if the density non-uniformity is slight, it is not possible to find a clear non-uniform intensity in the mapping image. In this case, an arbitrary point in the measurement cross-sectional area is selected, the maximum value and the minimum value of the X-ray intensity are extracted from these, and the ratio is obtained. In this case, it is empirically known that if the maximum value and the minimum value are extracted from 50 or more points, almost averaged information can be obtained.
Alternatively, when any two points in the measurement region are measured to obtain an intensity ratio and are repeated an arbitrary number of times, all the intensity ratios should satisfy 3 or less. This set of intensity ratios is empirically found that if 25 sets are made, almost averaged information can be obtained.
In any case, the strength caused by the precipitates and voids described in FIGS. 1 and 2 is excluded from the target data for calculating the strength ratio.

  Steel with the chemical composition shown in Table 1 is melted in a converter, made into a 300mm x 400mm slab by a continuous casting process, then roughly rolled into a 150mm square billet through a breakdown process, then 1000-1350 ° C x After holding for 0.5 h, it was hot rolled to a 24 mmφ steel bar. The finishing temperature at the time of hot rolling was 900 ° C., and it was cooled to room temperature at a cooling rate of 0.5 to 1 ° C./s. The steel bar thus obtained was cut into a predetermined length, and various test pieces for evaluation were collected.

Here, regarding the structure before induction hardening, evaluation of non-uniform concentration by EPMA was performed on the L cross section parallel to the rolling direction of each test piece. Specifically, a mapping image was obtained with a characteristic X-ray intensity of Mo or W at a depth of 1 mm from the surface and a cross-sectional area of 500 × 500 μm ² . In this example, it is empirically known that the non-uniform width of the structure before induction hardening is about 100 μm at the maximum, and the surface hardened layer after induction hardening reaches a depth of 1 to 2 mm. Thus, the depth and area of the cross-sectional area to be measured were determined as described above. Thereafter, the ratio was determined from the maximum value (Imax) and the minimum value (Imin) of the characteristic X-ray intensity of Mo or W at an arbitrary point (50 points or more) in the measurement visual field. At this time, the measurement result of the obvious carbide judged from the spectrum was excluded.

In addition, a torsion test piece having a notch with a parallel part: 20 mmΦ and a stress concentration factor α = 1.5 is produced from the steel bar, and the heating rate is 600 ° C./s using an induction hardening apparatus with a frequency of 15 kHz. Temperature: Induction hardening was performed once in the range of 880 to 1000 ° C. The specimen was tempered at 170 ° C. for 30 minutes and then subjected to a torsional fatigue test.
Torsional fatigue test, maximum torque: 4900 using N · m (= 500kgf · m ) of the torsional fatigue testing machine, performed by changing the stress conditions in both swing, fatigue strength and stress becomes 1 × 10 ⁵ times the life As evaluated.

Furthermore, the average value of the prior austenite grain size in the hardened layer of the induction-hardened material was obtained by photographing the optical microscope field of 1000 times at five locations and analyzing the image. In addition, the standard deviation was determined from all prior austenite grain sizes in the microscope field as a non-uniform index of grain size. The prior austenite particle size was measured by measuring dodecylbenzene in an aqueous picric acid solution in which 50 g of picric acid was dissolved in 500 g of water with respect to a cross section of 1 mm in depth in the L direction parallel to the rolling direction of each test piece. What added sodium sulfonate: 11g, ferrous chloride: 1g, and oxalic acid: 1.5g was made to act as a corrosive liquid, and the prior austenite grain boundary was made to appear.
These measurements and evaluation results are also shown in Table 2.

  As shown in Table 2, the steel materials No. 1-1, 10-1 and 20-1 have a carbon concentration that deviates from the component definition, and the fatigue strength after induction hardening is insufficient. Steel materials No. 4-1, 5-1, 13-1, and 14-1 have Mo or W below the specified component range, so the concentration unevenness in the microstructure before induction hardening is small, but sufficient fatigue strength can be secured. Not. Steel Nos. 8-1 and 19-1 contain excessive Mo or W, so even if the slab heating temperature is set to 1300 ° C, the concentration non-uniformity in the structure before induction quenching cannot be resolved, and the surface hardened layer by induction quenching cannot be resolved. The non-uniformity of the prior austenite grain size is large and the fatigue strength is reduced. In steel materials No.3-1, 7-1, 9-1 and 16-1, the steel components are within the specified range, but the slab heating temperature is not sufficient, so the concentration non-uniformity in the structure before induction hardening is eliminated. Furthermore, the non-uniformity of the prior austenite grain size of the surface hardened layer by induction hardening is also large and the fatigue strength is lowered.

(A) is an EPMA Mo mapping image showing the Mo concentration distribution in the tissue before induction hardening, and (b) is a line profile on the A-A ′ line in (a). It is a figure which shows the analysis example of a deposit. It is a graph which shows the correlation with the X-ray intensity ratio of Mo in the surface hardening layer after induction hardening processing, and the standard deviation of the prior austenite particle size in a surface hardening layer.

Claims (8)

  The ratio of the maximum value (Imax) and the minimum value (Imin) of the X-ray intensity of Mo in an arbitrary cross-sectional area, which is a steel material containing C: 0.3-0.6% and Mo: 0.15-0.8% in mass% A carbon steel material for induction hardening with excellent fatigue characteristics, characterized in that Imax / Imin is 3 or less.   Ratio of the maximum value (Imax) and the minimum value (Imin) of the X-ray intensity of W in an arbitrary cross-sectional area, which is a steel material containing C: 0.3-0.6% and W: 0.25-1.6% in mass% A carbon steel material for induction hardening with excellent fatigue characteristics, characterized in that Imax / Imin is 3 or less.   The carbon steel material for induction hardening according to claim 1 or 2, wherein the steel material further contains, by mass%, S: 0.05% or less and Mn: 0.2-2%.   3. The fatigue according to claim 1, wherein the steel material further contains, in mass%, S: 0.05% or less and Mn: 0.2-2%, the balance being composed of Fe and inevitable impurities. Carbon steel material for induction hardening with excellent properties.   The steel according to claim 3 or 4, further comprising, by mass%, Si: 1% or less, P: 0.02% or less, Al: 0.5% or less, and N: 0.01% or less. Excellent induction hardening carbon steel.   The steel material for induction hardening according to any one of claims 3 to 5, wherein the steel material further contains, by mass%, Ti: less than 0.05% and B: 0.005% or less. .   The carbon steel for induction hardening with excellent fatigue characteristics according to any one of claims 3 to 6, wherein the steel material further contains Ca: less than 0.010% by mass. A machine structural component having excellent fatigue characteristics, comprising the induction-quenched carbon steel material according to any one of claims 1 to 7.