JP2008163458A - Machine structural steel with excellent machinability and fatigue properties - Google Patents

Abstract

[PROBLEMS] To provide a steel for machine structural use that does not necessarily use a free-cutting component such as Pb, ensures machinability equivalent to or higher than that of a conventional Pb-added free-cutting steel, and has excellent fatigue characteristics after quenching. .
In a steel material for mechanical structure formed by quenching a steel material having a graphite phase, the structure after the quenching treatment includes a martensite matrix phase, a graphite phase, and / or pores derived from graphite. And a martensite phase having a higher C concentration than the martensite matrix is formed around the graphite phase or the pores derived from graphite.
[Selection] Figure 1

Description

  The present invention relates to a mechanical structural steel material suitable for machine parts such as industrial machines and automobiles, and more particularly to a mechanical structural steel material having both machinability and fatigue characteristics.

  Machine parts used for machine parts such as industrial machines and automobiles are processed steel steel into a predetermined shape by cutting or plastic working or a combination thereof, and then ensure the required characteristics as machine parts by quenching and tempering. It is manufactured by the method. Therefore, the steel material used for such machine parts is first required to be excellent in machinability and cold forgeability.

  Here, as a means for improving the machinability of steel for machine structural use, it is general to add a free-cutting element such as Pb, S, Bi and P alone or in combination to steel. In particular, Pb is frequently used because it has an extremely strong effect of improving machinability.

  However, Pb is an element harmful to the human body, requires a large exhaust facility in the manufacturing process of steel materials and the machining process of machine parts, and has a great problem from the viewpoint of recycling of steel materials. Furthermore, since free-cutting elements such as Pb, S, Te, Bi, and P deteriorate ductility and toughness, it is desirable to reduce them from the viewpoint of improving the fatigue properties of steel materials.

Conventionally, a method of graphitizing C in steel has been proposed in order to enable alloy design under the above conflicting requirements (see, for example, Patent Document 1). However, the steel in which graphite is precipitated as described above has a problem in that it is difficult to apply the quenching treatment, which is effective for ensuring the wear resistance and fatigue strength required as a steel material for machine structures. That is, the quenching treatment is frequently used in the field of machine parts and the like in order to increase the hardness of the steel material and improve the wear resistance and fatigue strength. And, when the quenching treatment is applied to the steel on which the graphite is precipitated in order to improve the machinability, the graphite particles are solid-dissolved during the quenching heating, and the vacancy remains as a trace of the presence, or the quenching is performed. Depending on the heating conditions, solid solution of graphite particles does not progress sufficiently, and graphite particles remain in the vicinity of the surface after treatment, which acts as a stress concentration source in rolling fatigue, and the rolling fatigue characteristics of steel Will cause the problem of deterioration.
Therefore, in the manufacturing process of the steel for machine structure on the premise of quenching, there is a limit in achieving both high machinability and fatigue strength using the graphite precipitation technique.
Japanese Patent Laid-Open No. 51-57621

  The object of the present invention is to ensure that the machinability is equal to or better than that of conventional Pb-added free-cutting steel without necessarily using free-cutting components such as Pb, and also has excellent fatigue properties after quenching. Is to provide.

Now, in order to solve the above-described problems of the prior art, the inventors have conducted intensive studies on a method for industrially and stably producing a steel material having excellent machinability and rolling fatigue characteristics after quenching. The following findings were obtained.
That is, in the microstructure after the quenching treatment, when a martensite phase having a higher C concentration than the parent phase is present around the graphite phase dispersed in the parent phase of martensite or the pores derived from graphite. The inventors have found that the stress concentration on the graphite phase is reduced in a fatigue test, and have completed the present invention.

That is, the gist of the present invention is as follows.
(1) A steel for mechanical structure formed by quenching and tempering a steel material having a graphite phase, and the microstructure after the quenching and tempering treatment is any one of a martensite matrix, a graphite phase, and pores derived from graphite. Or machinability and fatigue characteristics, characterized by having a martensite phase having a higher C concentration than the martensite matrix around the graphite phase or pores derived from graphite. Excellent steel for machine structural use.

(2) The steel material for machine structure according to (1), wherein the steel material contains C: 0.1 mass% or more and 1.5 mass% or less.

(3) C: Steel for machine structural use, which is obtained by subjecting a steel material containing 0.1 mass% to 1.5 mass% and having a graphite phase at a ratio of more than 5% and less than 90% to quenching and tempering. The structure after the tempering treatment is a mixture of the martensite matrix and either or both of the graphite phase and the graphite-derived pores, and the region from the graphite phase or the graphite-derived pores to 3 μm to 30 μm. Is a martensite phase having a higher C concentration than the martensite matrix, and has excellent machinability and fatigue characteristics, and is a steel material for machine structure.

(4) The Vickers hardness M of the martensite matrix after quenching and tempering and the Vickers hardness H of the martensite phase having a high C concentration satisfy the following formula (A): A steel for machine structure having excellent machinability and fatigue characteristics as described in 1), (2) or (3).
H-0.24M ≧ Hc (A)
However, in the case of C: 0.7 mass% or less Hc = 350 [% C] +310
C: In case of over 0.7 mass% Hc = 550
Here, [% C]: amount of added C

(5) The steel materials are Si: more than 0.15 mass% and less than 2.0 mass%, Mn: 0.05 mass% to 2.0 mass%, Al: 0.005 mass% to 0.1 mass%, N: 0.0015 mass% to 0.0150 mass% B: 0.0003 mass% or more and 0.0150 mass% or less, P: 0.06 mass% or less, S: 0.06 mass% or less, and O: 0.0030 mass% or less, and the balance having a component composition consisting of Fe and inevitable impurities The steel for machine structure according to any one of (1) to (4), which is characterized in that

(6) The component composition was further selected from Ni: 0.1 mass% to 3. Omass%, Cu: 0.1 mass% to 3. Omass%, and Co: 0.1 mass% to 3. Omass%. The steel material for machine structure according to (5) above, which contains at least one kind.

(7) The component composition further includes at least one selected from V: 0.05 mass% to 0.5 mass% and Nb: 0.005 mass% to 0.05 mass%. (5) Or the steel material for machine structures as described in (6).

(8) The steel material for machine structure according to (5), (6), or (7), wherein the component composition further includes Mo: 0.1 mass% to 1. Omass%.

(9) At least one selected from Ti: 0.005 mass% to 0.05 mass%, Zr: 0.005 mass% to 0.2 mass% and REM: 0.0005 mass% to 0.2 mass% as the component composition The steel for machine structure according to any one of (5) to (8), characterized in that

  According to the present invention, a machine structural steel material that does not necessarily use a free-cutting component such as Pb, ensures machinability equivalent to or better than that of conventional Pb-added free-cutting steel, and has excellent fatigue properties after quenching. Can be provided.

The present invention will be specifically described below.
The steel for machine structural use of the present invention is formed by subjecting a steel material having a graphite phase to quenching and tempering, and in particular, the structure after quenching and tempering is either a martensite matrix phase, a graphite phase, or pores derived from graphite or It is important to have a martensite phase having a higher C concentration than the martensite matrix around the graphite phase or the pores derived from graphite.

  That is, when a steel material having a graphite phase is subjected to quenching and tempering treatment, the microstructure after the quenching and tempering treatment is such that either or both of the graphite phase and the pores derived from graphite are dispersed in the matrix phase of martensite. Will exist. In such a microstructure, the presence of a martensite phase having a C concentration higher than that of the parent phase around the graphite phase or graphite-derived vacancies causes stress concentration on the graphite phase that is the starting point of fatigue failure. As a result of mitigating and suppressing the occurrence of cracks from the graphite phase, the fatigue properties of the steel material are significantly improved.

  Here, the presence of a martensite phase having a C concentration higher than that of the parent phase around the graphite phase or graphite-derived vacancies means that the surrounding structure of the graphite phase after quenching or vacancies caused by graphite is the martensite phase. In addition, in C analysis by EPMA (electron probe microanalyzer) as shown in FIG. 1, for example, a region having a higher C concentration than the parent phase exists around the pores caused by the graphite phase or the graphite phase. Say that.

  Here, it is preferable that the region where the martensite phase having a higher C concentration than the parent phase exists is a region from 3 μm to 30 μm from the graphite phase or the pores derived from graphite. When the region of 3 μm or more cannot be secured, it is difficult to sufficiently suppress the stress propagation to the graphite-derived phase. On the other hand, when the region exceeds 30 μm, the proximity of the martensite phase having a high C concentration increases, the area ratio of the parent martensite decreases, and stress on the pores caused by the graphite phase or graphite Since the effect of relaxing concentration is reduced, sufficient fatigue characteristics cannot be exhibited. The region of the martensite phase having a high C concentration (predetermined distance from the graphite phase or the pores caused by graphite) will be described in detail later. From the results of EPMA line analysis, as shown in FIG. It is determined by measuring the distance from the position where the C concentration becomes higher by 10% or more to the graphite phase or the pores derived from graphite.

In order to obtain the above microstructure, first, the steel material to be subjected to the quenching process needs to have a graphite phase, and the graphitization rate in the steel material before quenching is preferably more than 5%. This is because graphite precipitation reduces the amount of hard cementite and graphite acts as a lubricant during cutting to improve machinability. On the other hand, the upper limit is preferably less than 90% because if the graphitization rate is 90% or more, peeling of the steel material surface tends to occur during cutting. The graphitization rate is
{(Measured graphite area ratio) / (Graphite area ratio when all of added C is graphitized)} × 100 (%)
Defined by

Next, the reason for limitation for each component will be described for the component composition suitable for giving the above-described structure to the steel for machine structure.
C: 0.1 mass% or more and 1.5 mass% or less C is a component necessary for forming a graphite phase. If the content is less than 0.1 mass%, the graphite phase necessary for ensuring machinability should be formed. Is difficult. On the other hand, if added over 1.5 mass%, the deformation resistance during hot rolling increases, the deformability decreases, and the occurrence of cracks and wrinkles in the hot rolled material increases. Accordingly, C is set in the range of 0.1 mass% to 1.5 mass%.

Si: More than 0.15 mass% and less than 2.0 mass%
Si is an element that increases the strength by solid solution in ferrite, and is an element that does not dissolve in cementite and promotes graphitization by destabilizing cementite. However, at 0.15 mass% or less, there is little increase in strength and no effect of promoting graphitization is observed. However, if it exceeds 2.0 mass%, the strength becomes too high and the ductility deteriorates. For this reason, Si was made into the range of more than 0.15 mass% and below 2.0 mass%. A more preferable range is 0.5 mass% or more and 1.4 mass% or less from the viewpoint of the balance between graphitization promotion and strength increase.

Mn: 0.05 mass% or more and 2.0 mass% or less
Mn is not only effective as a deoxidizer for steel but is also an element useful for hardenability, so it is positively added. On the other hand, it dissolves in cementite and inhibits graphitization. That is, addition of less than 0.05 mass% has no effect on deoxidation, and addition over 2.0 mass% inhibits graphitization. For this reason, Mn was made into the range of 0.05 mass% or more and 2.0 mass% or less. A more preferable range is 0.1 mass% or more and 1.5 mass% or less from the viewpoint of promoting graphitization.

Al: 0.005 mass% or more and 0.1 mass% or less
Al reacts with N in the steel to form AIN, which acts as a nucleation site for graphite and thereby promotes graphitization, so it is actively added. If the addition is less than 0.005 mass%, the effect is small, but if it exceeds 0.1 mass%, many Al-based oxides are formed in the casting process. This Al-based oxide is not only a starting point for fatigue failure, but also is hard, so it lowers machinability by wearing the tool during cutting. For these reasons, the Al content is in the range of 0.005 mass% to 0.1 mass%.

N: 0.0015 mass% or more and 0.0150 mass% or less N combines with Al or B to form AIN or BN, which becomes a nucleus for crystallization of graphite. By fine dispersion of AIN and / or BN, graphitization is promoted and graphite grains are refined. However, when adding less than 0.0015 mass%, AIN and BN are not sufficiently formed. On the other hand, adding more than 0.0150 mass% promotes cracking of the slab during continuous casting, so N is 0.0015 mass% or more and 0.0150. The range is less than mass%. In particular, from the viewpoint of finer graphite, a range of 0.0015 mass% to 0.0100 mass% is preferable.

B: 0.0003 mass% or more and 0.0150 mass% or less B combines with N in the steel to form BN, which acts as a nucleus for crystallization of graphite, promotes graphitization, and refines graphite grains. Therefore, it is an important component in the present invention. Further, B is an element useful for enhancing the hardenability of steel and ensuring the strength after quenching. Addition of less than 0.0003 mass% has little effect on graphitization and hardenability. However, if added over 0.0150 mass%, B solidifies in the cementite and stabilizes the cementite, which adversely inhibits graphitization. For this reason, B was made into the range of 0.0003 mass% or more and 0.0150 mass% or less. From the viewpoint of graphitization and hardenability, the preferable range of B is 0.0005 mass% or more and 0.0100% mass% or less.

P: 0.06 mass% or less P is an element that inhibits graphitization. In addition, segregation occurs at the grain boundaries during quenching and tempering, lowering the grain boundary strength, reducing resistance to propagation of fatigue cracks, and lowering fatigue strength. Therefore, it should be reduced as much as possible, but is allowed to 0.06 mass%. More preferably, it is 0.020 mass% or less.

S: 0.06 mass% or less S forms MnS in steel, which becomes a starting point of crack generation in a fatigue test and deteriorates fatigue characteristics. MnS also acts as a crystallization nucleus of graphite, but if it is too much, it will become coarse and form coarse graphite. Therefore, the content of S is set to 0.06 mass% or less. More preferably, it is 0.035 mass% or less.

O: 0.0030 mass% or less O forms oxide-based non-metallic inclusions and reduces both machinability and fatigue strength. Therefore, it should be reduced as much as possible, but 0.0030 mass% is allowed.

The basic components have been described above. In the present invention, the following elements can be used as necessary. The reasons for limitation will be described below.
Ni: 0.1 mass% or more and 3.Omass% or less, Cu: 0.1 mass% or more and 3.Omass% or less and Co: at least one selected from 0.1 mass% or more and 3.Omass% or less
Ni, Cu, and Co are all elements that promote graphitization, and also have an effect of improving hardenability, so that graphitization can be promoted and hardenability can be improved. As for the amount added, each component is less than 0.1 mass%, the above effect is small, while adding more than 3.0 mass% saturates the effect, so Ni, Cu and Co are all 0.1 mass% It was set as the range below 3.Omass%.

Mo: 0.1 mass% or more and 1.Omass% or less
Mo enhances hardenability and has a smaller distribution to cementite and lower cementite stabilization ability than alloy elements such as Mn and Cr. For this reason, the hardenability of the steel material can be enhanced without significantly inhibiting graphitization. Moreover, since the steel material added with Mo has a high resistance to temper softening, it is possible to improve the hardness when compared at the same tempering temperature, and as a result, the fatigue strength can be improved. Moreover, since the hardenability is high, it is easy to form a bainite structure in the state of hot rolling. The bainite structure is advantageous for the production of fine graphite, and from this, it is possible to complete the dissolution of graphite during quenching in a short time. Addition of Mo is used especially when it is necessary to further improve the fatigue strength. However, if the addition is less than 0.1 mass%, the effect is small, and if it exceeds 1.0 mass%, graphitization is inhibited and machinability is reduced. Reduce. For this reason, the range was 0.1 mass% or more and 1.Omass% or less. In particular, from the viewpoint of machinability, it is preferably 0.8 mass% or less.

At least one selected from V: 0.05 mass% to 0.5 mass% and Nb: 0.005 mass% to 0.05 mass% V and Nb both form carbonitrides with carbide-forming elements to increase strength. . Moreover, since it hardly dissolves in cementite, it is a component that does not significantly inhibit graphitization. V and Nb are both elements that improve the hardenability, and therefore may be used when it is necessary to improve the fatigue strength. In the case of V, these effects are small when added at less than 0.05 mass%, while the effect is saturated even when added at more than 0.5 mass%, so the range is from 0.05 mass% to 0.5 mass%. Similarly, in the case of Nb, if the addition is less than 0.005 mass%, the above effect is small, but if the addition exceeds 0.05 mass%, the effect is saturated, so it is added in the range of 0.005 mass% to 0.05 mass%. And

Ti: 0.005 mass% or more and 0.05 mass% or less, Zr: 0.005 mass% or more and 0.2 mass% or less and REM: at least one selected from 0.0005 mass% or more and 0.2 mass% or less First, both Ti and Zr are charcoal Nitride is formed, and these act as nuclei for crystallization of graphite to promote graphitization. Since these carbonitrides are finely dispersed to refine the graphite grains, they may be used when it is necessary to further refine the graphite grains. Moreover, Ti and Zr increase the effective B at the time of quenching by forming carbonitrides and improve the hardenability. In order to exert such effects, it is preferable to add both Ti and Zr at 0.005 mass% or more. On the other hand, if Ti is added in an amount of 0.05 mass% and Zr exceeds 0.2 mass%, N for forming BN is insufficient. As a result, the graphite grains become coarse and the graphitization time becomes extremely long. : 0.05 mass% to 0.5 mass% and Nb: 0.005 mass% to 0.05 mass%.

  Next, REM (rare earth metal) such as La and Ce combines with S to form (REM) S. Since this becomes the core of graphitization and promotes graphitization and refines the graphite grains, it may be used when it is necessary to refine the graphite grains and promote the graphitization. However, if it is less than 0.0005 mass%, the effect is poor. On the other hand, the effect is saturated even if added over 0.2 mass%, so it is added in the range of 0.0005 mass% to 0.2 mass%.

  Steel materials having the above basic components or components added with additional components are, for example, rolled into steel bars or hot forged, then cut to a predetermined length, and then cut to finish various shapes of machine structural parts. After that, the product is generally subjected to induction hardening-tempering treatment to obtain a product.

  In this quenching-tempering treatment, as a condition for giving the above-mentioned microstructure to the steel material after quenching and tempering, for example, in quenching using an induction hardening apparatus, the heating temperature is 850 ° C. or more and less than 1050 ° C. (preferably 900 It is effective to carry out the treatment at a heating holding time of 6 s or less, particularly at a heating temperature of 950 ° C. or more, and 1.5 s or less, preferably 1.

  However, in the case of holding at an extremely low temperature for a short time, such as a process of holding at 850 ° C. for 1 s, austenitization during heating and holding does not proceed sufficiently, and ferrite remains in the structure after quenching, which is the target. A microstructure cannot be obtained. For this reason, it is necessary to austenite during heating and holding under conditions of heating temperature of 850 ° C. to less than 1050 ° C. and heating and holding time of 6 seconds or less. In addition, tempering should just follow normal conditions.

  Moreover, in order to obtain the above-mentioned microstructure, it is preferable that the graphitization rate of the steel material before quenching is 5% or more. As the graphitization conditions for that purpose, for example, heat treatment at 700 ° C. for 1 h or more may be performed. .

As a result of further detailed studies, the inventors have found that the Vickers hardness M of the martensite matrix after quenching and the Vickers hardness H of the martensite phase having a high C concentration satisfy the following formula (A). It is preferable to do.
H-0.24M ≧ Hc (A)
However, in the case of C: 0.7 mass% or less Hc = 350 [% C] +310
C: In case of over 0.7 mass% Hc = 550
Here, [% C]: amount of added C, that is, when the martensite matrix has a sufficient hardness according to the amount of added C, and the hard phase has a high hardness in addition thereto, it is further excellent. They obtained the knowledge that fatigue characteristics can be obtained.

Steel materials having the composition shown in Table 1 were melted by a converter and cast into continuous slabs. The slab size was 300 × 400mm. The slab was rolled into a 150 mm square billet through a breakdown process and then rolled into a 52 mmφ steel bar. The finishing temperature of rolling was over 900 ° C. In Table 1, steel U is a JIS standard S53C equivalent steel, and steel V is an example of a free cutting steel in which S, Ca, and Pb, which are free cutting ability improving elements, are added to S53C equivalent steel. Steel T is JIS S25C equivalent steel. Thereafter, graphitization annealing was performed at a heating temperature of 700 ° C., and the heating time was changed to obtain steel bars having various graphitization rates.
The graphitization rate is 400 times the 5 points in the 1/4 thickness cross section from the surface by using an image analysis device without collecting the specimen for observation with an optical microscope from the material after the graphitization annealing and without being corroded after polishing. The area ratio of graphite was measured over 10 microscopic images of magnification.
The graphitization rate was defined as follows as the ratio of the graphite area ratio thus obtained to the value when all of the additive C was graphitized.
(Measured graphite area ratio) / (graphite area ratio when all of added C is graphitized) × 100 (%)

  Next, a test piece having a parallel part of 26.3 mmΦ, a gripping part of 24 mmΦ, and a total length of 130 mm was sampled from the above steel bar by using an induction hardening apparatus with a frequency of 4 kHz and quenched under the conditions shown in Table 2. After tempering at 170 ° C. for 30 minutes, the parallel part was polished to 26.0 mmΦ.

The surface layer from the parallel part surface of the test piece thus obtained to a depth of 3 mm was subjected to microstructure observation, hardness measurement and EMPA line analysis, and the main microstructure type, high C martensite region (hereinafter simply referred to as “high”). Presence or absence of C phase), high C phase region radius, and hardness of each of the parent phase and the high C phase. The graphite-derived phase and the surrounding high-C phase are observed with an optical microscope, and the distance from the boundary between the high-C phase and the parent phase to the graphite-derived phase is the shortest as shown in the schematic diagram of the observation result in FIG. EPMA line analysis was conducted to measure the C concentration distribution of the high C phase and the parent phase. Then, as shown in the schematic diagram of the EPMA line analysis result shown in FIG. 3, when the C concentration around the graphite-derived phase is 10% or more higher than the C concentration C ₀ (measured strength C ₀ ) of the parent phase, It was determined that there was phase C. Further, the distance L from the position where the C concentration was 10% higher than the C concentration C ₀ of the parent phase (the position where C concentration = 1.1 × C ₀ ) to the graphite-derived phase was evaluated as the radius of the high C phase.
In FIG. 1, the example of the observation result by an optical microscope and the example of an EPMA line analysis result were shown collectively. The high C phase is a region that appears whitish compared to the graphite phase and the matrix phase region, and can be clearly determined by an optical microscope.

  For example, as shown in FIG. 1 together with the result of EPMA line analysis together with a structure photograph taken with an electron microscope, the region where the X-ray intensity gradually increases toward the graphite phase, which is the peak region of C X-ray intensity, is a high C martensite. It is a site phase.

  Moreover, the same test piece was used for the roller pitching fatigue test, and the rolling fatigue life was calculated | required. The test conditions were a slip rate of 40%, a load stress of 3677 MPa, and a rotational speed of 1900 rpm. Then, assuming that the obtained test results follow the Weibull distribution, the results were plotted on probability paper, and the BIO life (total number of loads until delamination with a cumulative failure probability of 10%) was determined. The obtained BIO life was calculated based on steel no. As an index when the same life of U (equivalent to JIS S53C steel) is set to 1, the quality of each steel was evaluated using the index of each steel as an index. The evaluation results are also shown in Table 2.

  Furthermore, machinability was also evaluated. This machinability was evaluated by using the high-speed tool steel SKH4, turning a 52 mmφ steel bar at a cutting speed of 80 m / min under non-lubricating conditions, and measuring the time until the tool became uncuttable as the tool life. This evaluation result is also shown in Table 2.

All of the steel materials of the present invention exhibit excellent machinability equivalent to or better than Pb-added free-cutting steel after graphitization annealing, and have an equivalent C content, and are significantly superior to conventional mechanical structural steels. It showed fatigue life.
On the other hand, when the chemical composition does not satisfy the conditions of the present invention (steel materials No. 32-35), a sufficient graphitization rate cannot be obtained even if annealing is performed for a long time, and the machinability is It was inferior to the steel material of the invention.

  Even when the chemical composition is within the range of the present invention, the tool life is inferior to that of the steel material of the present invention even when a sufficient graphitization rate cannot be obtained after annealing (steel materials No. 1 and 5). On the other hand, when the graphitization rate was too high, the tool life was increased, but the surface roughness was remarkably increased.

  When the heating temperature at the time of quenching is high and the holding time is too long (steel materials No. 3, 7, 10, 17, 19), the presence of a high C concentration martensite phase is not observed after quenching and rolling. The fatigue life was inferior to that of the conventional steel. Conversely, in extremely low temperature short time holding (steel material No. 11) such as holding at 850 ° C. and 1 s (steel material No. 11), austenitization during heating holding does not proceed sufficiently, and ferrite remains in the structure after quenching. The target microstructure was not obtained. For this reason, the hardness of the parent phase was not sufficiently obtained, and H−0.24M <Hc. Therefore, although the martensite phase having a high C concentration was formed, the rolling fatigue life was higher than that of the conventional steel. Was also inferior.

It is a figure which shows the line analysis result by EPMA in the scanning direction shown to the electron micrograph. It is a figure explaining the presence or absence of the area | region of the high C martensite phase (high C phase) around the graphite phase or the void | hole (graphite origin phase) derived from graphite, and the measuring method of the magnitude | size, It is a schematic diagram. It is a figure explaining the presence or absence of the region of the high C martensite phase (high C phase) around the graphite phase or graphite-derived vacancies (graphite-induced phase), and the method of measuring the size, and results of EPMA line analysis FIG.

Claims (9)

  A steel for mechanical structure formed by quenching and tempering a steel material having a graphite phase, and the structure after the quenching and tempering treatment is composed of a martensite matrix phase, a graphite phase, and / or pores derived from graphite. Excellent in machinability and fatigue characteristics, characterized by having a martensite phase with a higher C concentration than the martensite matrix around the graphite phase or pores derived from graphite. Steel for machine structure.   The steel material for mechanical structure according to claim 1, wherein the steel material contains C: 0.1 mass% or more and 1.5 mass% or less.   C: Steel for machine structural use, comprising 0.1 mass% or more and 1.5 mass% or less, and a steel material having a graphite phase in a ratio of more than 5% and less than 90%, after quenching and tempering. Is a mixture of the martensite matrix and either or both of the graphite phase and the graphite-derived pores, and the region from the graphite phase or the graphite-derived pores to 3 μm to 30 μm A machine structural steel material excellent in machinability and fatigue characteristics, characterized by being a martensite phase having a higher C concentration than the martensite matrix. The Vickers hardness M of the martensite matrix after quenching and tempering and the Vickers hardness H of the martensite phase having a high C concentration satisfy the following formula (A): Or the steel material for machine structures excellent in the machinability and fatigue characteristics of 3 description.
H-0.24M ≧ Hc (A)
However, in the case of C: 0.7 mass% or less Hc = 350 [% C] +310
C: In case of over 0.7 mass% Hc = 550
Here, [% C]: amount of added C   The steel materials are Si: more than 0.15 mass% and less than 2.0 mass%, Mn: 0.05 mass% to 2.0 mass%, Al: 0.005 mass% to 0.1 mass%, N: 0.0015 mass% to 0.0150 mass%, B: 0.0003 mass% or more and 0.0150 mass% or less, P: 0.06 mass% or less, S: 0.06 mass% or less, and O: 0.0030 mass% or less, and the remainder has a component composition composed of Fe and inevitable impurities The steel for machine structure according to any one of claims 1 to 4.   As the component composition, at least one selected from Ni: 0.1 mass% to 3.Omass%, Cu: 0.1 mass% to 3.Omass% and Co: 0.1 mass% to 3.Omass% The steel for machine structure according to claim 5, comprising:   The component composition further includes at least one selected from V: 0.05 mass% or more and 0.5 mass% or less and Nb: 0.005 mass% or more and 0.05 mass% or less. Steel for machine structural use.   The steel material for machine structure according to claim 5, 6 or 7, further comprising Mo: 0.1 mass% or more and 1. Omass% or less as the component composition.   The composition further includes at least one selected from Ti: 0.005 mass% to 0.05 mass%, Zr: 0.005 mass% to 0.2 mass% and REM: 0.0005 mass% to 0.2 mass%. The steel material for machine structure according to any one of claims 5 to 8, wherein