JP2015040335A - Steel for machine structural use excellent in machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for machine structural use exhibiting excellent machinability even when a chemical composition is normal without using Pb harmful to a human body and an expensive free-cutting element, especially, excellent in cutting surface roughness.SOLUTION: The steel for machine structural use has a mixed structure consisting of a hard phase consisting of at least one kind selected from perlite, bainite and martensite and a ferrite phase. An average circle equivalent diameter of ferrite grains is 7 μm or less, and a ferrite grain-ferrite grain coupling rate X represented by a following formula (1) is 0.15 or less, [a ferrite grain-ferrite grain coupling rate X]=[a ferrite grain-ferrite grain interface number A]/[a ferrite grain-hard phase interface number B] ... (1)

Description

  TECHNICAL FIELD The present invention relates to a machine structural steel used for manufacturing various machine structural parts such as automobile parts and construction machine parts, and more particularly for machine structures having excellent machinability with reduced cutting finish surface roughness. It is about steel.

  In general, various parts such as automobile parts and construction machine parts are finished into a final shape by subjecting steel for machine structure to a process such as forging and then a cutting process. When such cutting is performed, steel for machine structural use that exhibits excellent machinability in terms of component accuracy and manufacturing efficiency is required. In particular, in steel materials applied to molding dies, the demand for cutting surface roughness is high, and machine structural steels that can obtain smaller cutting surface roughness are desired. When the cutting surface roughness becomes large (rough), it is necessary to further finish the surface properties by grinding or the like, and there is a problem that the manufacturing process becomes complicated.

  So far, various techniques have been proposed for mechanical structural steels that exhibit excellent machinability. As such a technology, for example, Patent Document 1 defines a content of elements such as C, Mn, P, S, Pb, O, Si, and Al in a low-carbon sulfur free-cutting steel and includes MnS-based intervening materials. It has been shown that machinability can be improved by defining the average size of the product and the proportion of sulfide not bonded to the oxide. It is also shown that good finished surface roughness can be obtained.

  In this technique, lead (Pb) is contained as a basic component as an element for improving machinability. Lead is a widely known element that improves machinability. However, Pb has been pointed out to be harmful to the human body and the environment. In recent years, Pb is required to exhibit good machinability without adding Pb.

  Under such a background, development of a technique that exhibits good machinability without actively adding Pb is underway. As such a technique, for example, Patent Document 2 discloses that excellent machinability equivalent to that of Pb-added steel can be obtained by composite addition of S, Te and Ca. In addition, this technique discloses that machinability is further improved by adding Bi or rare earth elements (REM).

  However, machinability improving elements (free cutting elements) such as Te, Bi, and REM are expensive, and there is a problem of cost increase in production.

  On the other hand, Patent Document 3 relates to a free-cutting steel for machine structures that exhibits both the mechanical properties and the chip breaking property of steel by containing a predetermined amount of Mg in the presence of sulfide inclusions. Proposed. In this technique, Mg is added to control the sulfide inclusions in a predetermined shape and dispersion state. However, Mg has a low boiling point and easily evaporates, and is a strong deoxidizing element. Therefore, Mg is easily separated from molten steel as an oxide, so that the yield is low and an increase in cost is inevitable.

Japanese Patent Laid-Open No. 62-23970 JP 2004-292929 A JP 2002-69569 A

  The present invention has been made under such circumstances, and the object thereof is excellent machinability even with a normal chemical component composition without using Pb harmful to the human body or expensive free-cutting elements. It is to provide a steel for machine structural use that exhibits particularly good cutting surface roughness.

The steel for machine structural use of the present invention capable of achieving the above object has a mixed structure consisting of a hard phase and a ferrite phase consisting of at least one selected from pearlite, bainite and martensite, and an average circle of ferrite grains The equivalent diameter is 7 μm or less, and the grit is that the ferrite grain-ferrite grain coupling ratio X represented by the following formula (1) is 0.15 or less.
[Ferrite grain-ferrite grain connection ratio X] = [ferrite grain-ferrite grain interface number A] / [ferrite grain-hard phase interface number B] (1)
In the formula, the ferrite grain-ferrite grain interface number A is the intersection of the ferrite grain-ferrite grain interface and the straight line when a straight line of a predetermined length is drawn on the structure photograph taken using a scanning electron microscope. Indicate the number,
The ferrite grain-hard phase interface number B indicates the number of intersections between the ferrite grain-hard phase interface and the straight line when a straight line having a predetermined length is drawn in the same manner as described above.

  The “average equivalent circle diameter” is an average value of diameters (equivalent circle diameters) when ferrite crystal grains are converted into circles having the same area.

  The chemical component composition of the steel for machine structural use of the present invention is not particularly limited as long as it is steel for machine structural use. However, for example, C: 0.2 to 1.2% (“mass%”) Hereinafter, the chemical composition is the same.), Si: 0.05 to 0.5%, Mn: 0.2 to 1.8%, P: 0.03% or less (excluding 0%), S: 0.03% or less (excluding 0%) is included, and the balance is iron and inevitable impurities.

  In the preferable chemical component composition, Cr: 0.5% or less (not including 0%), Cu: 0.5% or less (not including 0%), Ni: 0.5% or less (if necessary) It is also effective to contain at least one selected from the group consisting of Mo: 0.5% or less (not including 0%), and Mo depending on the type of element to be included. The properties of structural steel are further improved.

  The present invention also includes a method for improving the cutting surface properties, and a steel product with improved cutting surface properties can be obtained by cutting the steel for machine structure as described above.

  Moreover, by cutting the steel for machine structure of the present invention, a molding die having a good surface property can be efficiently produced without performing finishing such as grinding.

  In the present invention, it has a mixed structure composed of at least one hard phase selected from pearlite, bainite and martensite and a ferrite phase, the average equivalent circle diameter of ferrite is 7 μm or less, and a predetermined relational expression By defining the [ferrite grain-ferrite grain coupling ratio X] expressed, it is possible to realize a steel for machine structure that exhibits excellent machinability and particularly has a good cutting surface roughness.

It is a schematic diagram for demonstrating the relationship between the structure | tissue of steel materials, and a cutting surface roughness. 5 is a drawing-substituting photograph showing a procedure for obtaining a ferrite grain-ferrite grain coupling ratio X.

  The inventors of the present invention have studied particularly the relationship with the metal structure in order to realize a steel for machine structural use that can obtain a good machined surface roughness even with a normal chemical component composition. As a result, the idea that the roughness of the machined surface during machining is one of the causes is that the phases with different hardness are mixed in the structure. Such a state will be described with reference to the drawings.

  FIG. 1 is a schematic diagram for explaining the relationship between the structure of a steel material and the cutting surface roughness. In FIG. 1, 1 indicates a ferrite phase, and 2 indicates a hard phase (a hard phase composed of at least one selected from pearlite, bainite, and martensite), and has a mixed structure structure in which these are mixed. In addition, the upper side of FIG. 1 has shown the steel material surface cut. At the time of cutting, as shown in FIG. 1A, the soft ferrite phase 1 in the mixed structure is deformed so as to be pushed out by the hard phase 2. Subsequently, as shown in FIG. 1B, the ferrite phase 1 is removed by the cutting edge (blade edge) of the tool in a state where the ferrite phase 1 is pushed out. On the surface (machined surface) through which the cutting edge of the tool has passed the cutting part, as shown in FIG. 1 (c), the ferrite phase 1 is deformed so as to be drawn by the elastic recovery of the hard phase 2. A recess 3 is generated in the ferrite phase 1, and the presence of the recess deteriorates the processed surface roughness of the steel material.

  Based on the above idea, the present inventors have further investigated the requirements for obtaining good cutting surface roughness. As a result, it has been found that if the average equivalent circle diameter of ferrite grains is set within a predetermined range and the number of portions where the ferrite grains are connected is reduced, it is possible to realize a machine structure that can obtain good finished surface roughness. Was completed.

  Each requirement prescribed | regulated by this invention is demonstrated.

  In order to suppress the formation of recesses on the processed surface, it is necessary to finely disperse the soft ferrite phase. In a finely dispersed structure, the individual phases are small, so that the amount when one soft phase is extruded into a hard phase (hard phase) is reduced, and the recesses generated on the processed surface are also reduced. As a result, minute irregularities are dispersed and the processed surface roughness is improved.

  In order to make the soft ferrite phase finely dispersed, it is necessary to make the size (particle size) of the ferrite grains as small as possible. In the steel for machine structure of the present invention, in order to ensure the desired dispersion state of ferrite, the size of ferrite grains needs to be 7 μm or less in terms of average equivalent circle diameter. The average equivalent circle diameter of the ferrite is preferably 6 μm or less, and more preferably 5 μm or less. Moreover, the minimum with a preferable average equivalent circle diameter of a ferrite is 2 micrometers or more.

  However, simply defining the size of the ferrite grains is not sufficient to achieve the object of the present invention. This is because a phenomenon in which ferrite grains having a small particle diameter are connected to each other may occur. When such a phenomenon occurs, a plurality of ferrite grains gather and behave as a large ferrite phase, and the ferrite phase is pushed together by a phase harder than the ferrite phase (hard phase). In such a state, a phenomenon similar to the presence of ferrite grains having a large particle diameter (a phenomenon in which the cutting surface roughness is deteriorated) occurs.

  On the other hand, when the ferrite grains are surrounded by the hard phase, the amount of the ferrite phase extruded at that location is reduced, and the recesses after cutting are miniaturized. It becomes good. Strictly speaking, the pearlite refers to a structure having a structure in which ferrite and plate-like cementite are alternately arranged in layers. Such a structure is collectively referred to as “pearlite”. The ferrite targeted in the present invention does not take into account the layered ferrite in pearlite, but refers to a phase that looks white with a scanning electron microscope when it is subjected to nital etching.

In order to judge whether or not the ferrite grains are surrounded by the hard phase, the concept of ferrite grain-ferrite grain coupling ratio X is defined and evaluated in the present invention. The ferrite grain-ferrite grain coupling ratio X is represented by the following formula (1).
[Ferrite grain-ferrite grain connection ratio X] = [ferrite grain-ferrite grain interface number A] / [ferrite grain-hard phase interface number B] (1)
In the formula, the ferrite grain-ferrite grain interface number A is the intersection of the ferrite grain-ferrite grain interface and the straight line when a straight line of a predetermined length is drawn on the structure photograph taken using a scanning electron microscope. Indicate the number,
The ferrite grain-hard phase interface number B indicates the number of intersections between the ferrite grain-hard phase interface and the straight line when a straight line having a predetermined length is drawn in the same manner as described above.

A procedure for obtaining the ferrite grain-ferrite grain coupling ratio X will be described with reference to the drawings. First, after revealing the metal structure, the structure is observed with a scanning electron microscope (SEM). On this observation surface, as shown in FIG. 2 (drawing substitute photo), horizontal line segments are drawn at intervals of 5 μm so that the total length (total length) is 1000 μm or more, and these line segments and the interface between ferrite particles The number of intersections (portions surrounded by □) (number of ferrite grain-ferrite grain interfaces A) and the number of intersections of ferrite grains and hard phases (portions surrounded by white circles) (ferrite grains-hard phases) Each of the number of interfaces B) is obtained. And based on said (1) Formula, "the ferrite grain-ferrite grain connection rate X" is calculated. The observation area when observing is preferably 40000 μm ² or more from the viewpoint of improving the accuracy as much as possible. Further, the total length (total length) of lines drawn horizontally at equal intervals is preferably 1000 μm or more for the same reason as described above.

  The small value of “ferrite grain-ferrite grain coupling ratio X” defined as described above means that there are few regions where ferrite grains and ferrite grains are continuous, that is, ferrite grains are not continuous and hard. It is surrounded by phases and shows an isolated and dispersed state. On the other hand, a large value of “ferrite grain-ferrite grain coupling ratio X” indicates that there are many regions where the ferrite grains are continuous, that is, the ferrite grains tend to form a large phase.

  2A shows an example in which the ferrite grain-ferrite grain coupling ratio X is 0.15 or less, and FIG. 2B shows the ferrite grain-ferrite grain coupling ratio X exceeding 0.15. An example is shown.

  In order to obtain good cut surface roughness, the ferrite grain-ferrite grain coupling ratio X needs to be 0.15 or less. Preferably it is 0.13 or less, More preferably, it is 0.10 or less.

  In the steel for machine structural use according to the present invention, the purpose can be achieved by satisfying the above requirements, and the ferrite area ratio (the area fraction of ferrite in the entire structure) is not limited at all. However, from the viewpoint of increase in ductility due to increase in ferrite area ratio and increase in tool wear due to increase in material hardness due to decrease in ferrite area ratio, it may be about 30 to 80 area%. preferable. More preferably, it is about 40-70 area%.

  In the present invention, machine structural steel is assumed, and its steel type may be any chemical composition as long as it is a normal chemical structural steel, but C, Si, Mn, P and S It is better to adjust to the appropriate range. From these viewpoints, the appropriate ranges of these chemical components and the reasons for setting the ranges are as follows.

(C: 0.2-1.2%)
C is an element effective for securing the strength of steel parts manufactured from machine structural steel. If the C content is too low, it is difficult to adjust the material within the specified range of the ferrite grain-ferrite grain coupling ratio X. If the C content is excessive, the hardness becomes too high and the material is cut. (For example, tool life) decreases. Therefore, the C content is preferably 0.2% or more (more preferably 0.25% or more) and 1.2% or less (more preferably 1.1% or less).

(Si: 0.05-0.5%)
Si is contained as a deoxidizing element and for the purpose of increasing the strength of the steel part by solid solution hardening. However, if it is less than 0.05%, such an effect is not exhibited effectively, and exceeds 0.5%. When it contains excessively, hardness will rise too much and machinability (for example, tool life) will fall. In addition, the more preferable minimum of Si content is 0.1% or more, and a more preferable upper limit is 0.4% or less.

(Mn: 0.2-1.8%)
Mn is an effective element for deoxidizing and desulfurizing steel during melting, and is an effective element for improving the hardenability and increasing the strength of steel parts. When the Mn content is less than 0.2%, these effects are not exhibited. When the Mn content exceeds 1.8%, the hardness is excessively increased and the cold workability is deteriorated. In addition, the more preferable minimum of Mn content is 0.3% or more, and a more preferable upper limit is 1.5% or less.

(P: 0.03% or less (excluding 0%))
P is an element inevitably contained in the steel, but segregates at the ferrite grain boundary and deteriorates cold workability. Therefore, it is preferable to reduce P as much as possible, but an extreme reduction leads to an increase in steelmaking cost, and since it is difficult to make it 0%, it is 0.03% or less (not including 0%). It is preferable. The upper limit with more preferable P content is 0.025% or less.

(S: 0.03% or less (excluding 0%))
S is an element inevitably contained in steel like P, but it is present as MnS in steel and is a harmful element that deteriorates cold workability, so it needs to be reduced as much as possible. From such a viewpoint, the S content is preferably 0.03% or less (more preferably 0.025% or less). However, since S is an unavoidable impurity, it is industrially difficult to reduce the amount to 0%.

  The basic component composition of the steel for machine structural use of the present invention is as described above, and the balance is substantially iron. In addition, “substantially iron” can accept trace components (eg, Sb, Zn, etc.) that do not impair the properties of the steel material of the present invention in addition to iron, and inevitable impurities other than P and S ( For example, Al, N, O, H, etc.) may also be included. Moreover, the steel for machine structure of this invention may contain the following selective components as needed. The reasons for limiting the component range when these components are contained are as follows.

(Cr: 0.5% or less (not including 0%), Cu: 0.5% or less (not including 0%), Ni: 0.5% or less (not including 0%), and Mo: 0 .1 or more selected from the group consisting of 5% or less (excluding 0%))
Cr, Cu, Ni and Mo are all effective elements for increasing the strength of the final product by improving the hardenability of the steel material, and are contained alone or in combination of two or more as required. However, when the content of these elements is excessive, the strength becomes too high and the cold workability is deteriorated. Therefore, the respective preferable upper limits are set as described above. More preferably, both are 0.45% or less (more preferably 0.40% or less). In addition, although the effect by these elements becomes large as the content increases, the preferable minimum for exhibiting those effects effectively is 0.015% or more (more preferably 0.020% or more). It is.

  In producing the steel for machine structural use of the present invention, a steel that satisfies the above component composition is hot-rolled under normal conditions to obtain a hot-rolled material. After heating to a temperature of 800 to 950 ° C. and holding at that temperature for about 10 to 25 minutes (holding time), it may be cooled to 500 ° C. or less at an average cooling rate of 2 ° C./second or more. In addition, as long as it exists in the range of these manufacturing conditions, you may change the conditions in the middle. These manufacturing conditions will be described.

(Heating temperature: 800-950 ° C)
In order to control the ferrite grain-ferrite grain coupling ratio X to 0.15 or less, it is necessary to control the heating temperature (heating temperature after hot rolling) to 800 to 950 ° C. If the heating temperature at this time exceeds 950 ° C., the austenite grain size at the time of heating becomes coarse, so that the total area of the austenite grain boundary per unit volume decreases, and the ferrite grains precipitated from the austenite grain boundary approach. This makes it difficult to isolate the ferrite grains. If the heating temperature is less than 800 ° C., the ferrite grains present in the material before the heat treatment are not completely changed to the austenite phase, and it is difficult to control the ferrite grain-ferrite grain coupling ratio X to 0.15 or less. It becomes. The more preferable lower limit of this heating temperature is 820 ° C. or higher (more preferably 850 ° C. or higher), and the more preferable upper limit is 930 ° C. or lower (more preferably 900 ° C. or lower).

(Holding time in heating temperature range: 10-25 minutes)
The holding time in the heating temperature range is a factor that affects the ferrite grain-ferrite grain coupling ratio X. If the holding time at this time is less than 10 minutes, the change from the ferrite phase before the heat treatment to the austenite phase does not proceed sufficiently, and the ferrite phase remains in the structure. In addition, when the holding time exceeds 25 minutes, the austenite grains become coarse and the total area of the austenite grain boundaries per unit volume decreases, so that the ferrite grains precipitated from the austenite grain boundaries approach, It becomes difficult to isolate. A more preferable lower limit of the holding time is 15 minutes or more, and a more preferable upper limit is 20 minutes or less.

(After heating and holding, cooling to 500 ° C. or less at an average cooling rate of 2 ° C./second or more)
If the average cooling rate up to 500 ° C. or less (cooling stop temperature) is less than 2 ° C./second, the ferrite average particle size cannot be made 7 μm or less. From such a viewpoint, the average cooling rate needs to be 2 ° C./second or more. The average cooling rate is more preferably 5 ° C./second or more, and further preferably 7 ° C./second or more. In addition, about the cooling at this time, if it is in the range of the average cooling rate which will be 2 degree-C / sec or more, the cooling form which changes a cooling rate may be sufficient.

(Cooling stop temperature: 500 ° C or less)
The cooling stop temperature is preferably 500 ° C. or lower. If this temperature is higher than 500 ° C., the ferrite particle size tends to be coarse, and it becomes difficult to make the ferrite average particle size 7 μm or less. On the other hand, as the cooling stop temperature is lowered, there is no influence on the material structure. Therefore, after cooling to 500 ° C. or lower, normal cooling (cooling) is performed, Just do it.

  The steel for machine structure of the present invention has excellent machinability, and by cutting such a steel for machine structure, a steel product having improved cutting surface properties (cut surface roughness) can be obtained. Further, if the steel for machine structural use of the present invention is cut, a good cutting surface property can be obtained, so that it can be applied as it is as a molding die without performing a finishing process such as grinding.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can be adapted to the above-described purpose. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Using steel types A to D having the chemical composition shown in Table 1 below, a hot rolled material (thick plate material: plate thickness 30 mm) was obtained under normal hot rolling conditions. Steel type A shown in Table 1 is S55C equivalent steel (JIS G 4051), steel type B is S60C equivalent steel (JIS G 4051), steel type C is S50C equivalent steel (JIS G 4051), and steel type D is S45C equivalent steel (JIS G 4051).

  The obtained hot-rolled material was used as various test materials under the production conditions (heating temperature, holding time, average cooling rate after heating, cooling method) shown in Table 2 below (Test Nos. 1 to 7). . Test No. In No. 1, a hot-rolled material of S55C equivalent steel (steel type A) was heated to 850 ° C., held at that temperature for 20 minutes, and then air-cooled without blowing with a blower (average cooling rate: 3 ° C./second) ). Test No. In No. 2, the S60C equivalent steel (steel type B) hot-rolled material was heated to 850 ° C., held at that temperature for 20 minutes, and then cooled in the furnace (average cooling rate: 0.8 ° C./second). It is the obtained test material. Test No. In No. 3, a hot-rolled material of S50C equivalent steel (steel type C) was heated to 900 ° C., held at that temperature for 20 minutes, and then air-cooled while blowing with a blower (average cooling rate: 6 ° C./second) It is a test material obtained by doing. Test No. In No. 4, a hot-rolled material of S55C equivalent steel (steel type A) was heated to 850 ° C., held at that temperature for 20 minutes, and then air-cooled while blowing with a blower (average cooling rate: 6 ° C./second) When the temperature drops to 750 ° C., it is held for 1.5 minutes, and then air-cooled (average cooling rate: 6 ° C./second) while being blown again with a blower. Test No. In No. 5, a hot rolled material of S55C equivalent steel (steel type A) was heated to 850 ° C., held at that temperature for 20 minutes, and then cooled in the furnace (average cooling rate: 0.8 ° C./second). It is the obtained test material. Test No. No. 6 is a test material in which a hot-rolled material of S55C equivalent steel (steel type A) was not heated. Test No. In No. 7, a hot-rolled material of S45C equivalent steel (steel type D) was heated to 700 ° C. and held at that temperature for 30 minutes, and then air-cooled without blowing with a blower (average cooling rate: 3 ° C./second) ).

  About each obtained test material (test No. 1-7), the average equivalent circle diameter of ferrite and the ferrite grain-ferrite grain connection rate X were measured by the following method.

(Measurement of average equivalent circle diameter of ferrite)
Each specimen was polished to a mirror surface and corroded with a 3% nital solution to reveal a metallographic structure. Then, a scanning electron microscope (SEM) with a magnification of 400 times was used for five visual fields in a region of approximately 170 μm × 230 μm. The tissue was observed and photographed. Based on these photographs, the white portion was marked as ferrite particles from the contrast of the image and marked, and the circle equivalent diameter of the ferrite particles was determined by image analysis, and the average value of five fields of view was determined.

(Measurement of ferrite grain-ferrite grain coupling ratio X)
Each specimen was polished to a mirror surface and corroded with 3% nital solution to reveal a metal structure, and then the structure was observed with a scanning electron microscope (SEM) at a magnification of 400 times in an area of 40000 μm ² area. And filmed. Then, parallel lines at equal intervals in the structure photograph were drawn so as to have a total length of 1000 μm or more, and the ferrite grain-ferrite grain coupling ratio X was determined according to the procedure described above.

  Moreover, about each test material, the cutting experiment was performed on the conditions shown in following Table 3, and the cutting surface roughness of each test material after cutting was evaluated. The cutting experiment at this time was performed by two-dimensional cutting (planing) using a machining center. Further, the cutting surface roughness, which is a criterion for machinability, was measured by moving the slice lath in parallel with the cutting direction using a stylus roughness meter. The evaluation standard of the cutting surface roughness was evaluated as good surface properties when the cutting surface roughness was less than 0.10 μm in the calculated average roughness Ra. In addition, the cutting experiment is performed by two-dimensional cutting, and it is expected that a more complicated three-dimensional cutting will be obtained by normal cutting. However, since the three-dimensional cutting can be regarded as an accumulation of two-dimensional cutting with a narrow width, the effect in the three-dimensional cutting can be predicted from the data in the two-dimensional cutting.

  These results are collectively shown in Table 4 below.

  From this result, it can be considered as follows. Test No. 1, 3, and 4 are examples that satisfy all of the requirements defined in the present invention (ferrite grain-ferrite grain connection ratio X, average circle equivalent diameter of ferrite), and the cutting surface roughness (Ra) is good. It can be seen that the values are shown. Of these, in particular, test no. No. 1 is an example in which the heating temperature and the holding time are more preferable values, and the ferrite grain-ferrite grain coupling ratio X is a more preferable value, and it can be seen that the most excellent surface properties are obtained.

  In contrast, test no. 2, 5 to 7 are examples that do not satisfy any of the requirements defined in the present invention, and the roughness of the cut surface is large. That is, test no. 2 and test no. No. 5 is an example in which the average cooling rate is 0.8 ° C./second, the average equivalent circle diameter of ferrite is large, and the cutting surface roughness (Ra) is large.

  Test No. No. 6 is an example in which heat treatment was not performed after hot rolling, and the ferrite grain-ferrite grain coupling ratio X was large, and the cutting surface roughness (Ra) was large. Test No. No. 7 is an example in which the heating temperature is low, the ferrite grain-ferrite grain coupling ratio X is large, and the cutting surface roughness (Ra) is large.

1 Ferrite phase 2 Hard phase 3 Recess

Claims (5)

It has a mixed structure consisting of at least one hard phase selected from pearlite, bainite and martensite and a ferrite phase, and the average equivalent circle diameter of ferrite grains is 7 μm or less, and is represented by the following formula (1) A machine structural steel excellent in machinability, wherein the ferrite grain-ferrite grain coupling ratio X is 0.15 or less.
[Ferrite grain-ferrite grain connection ratio X] = [ferrite grain-ferrite grain interface number A] / [ferrite grain-hard phase interface number B] (1)
In the formula, the ferrite grain-ferrite grain interface number A is the intersection of the ferrite grain-ferrite grain interface and the straight line when a straight line of a predetermined length is drawn on the structure photograph taken using a scanning electron microscope. Indicate the number,
The ferrite grain-hard phase interface number B indicates the number of intersections between the ferrite grain-hard phase interface and the straight line when a straight line having a predetermined length is drawn in the same manner as described above. C: 0.2 to 1.2% (meaning “mass%”. The same applies to the chemical component composition hereinafter),
Si: 0.05 to 0.5%,
Mn: 0.2-1.8%
P: 0.03% or less (excluding 0%),
S: 0.03% or less (excluding 0%),
The steel for machine structural use according to claim 1, wherein the balance is iron and inevitable impurities. As other elements,
Cr: 0.5% or less (excluding 0%),
Cu: 0.5% or less (excluding 0%),
3. One or more selected from the group consisting of Ni: 0.5% or less (not including 0%) and Mo: 0.5% or less (not including 0%) The machine structural steel described.   The cutting surface property improving method of cutting the machine structural steel according to any one of claims 1 to 3.   The manufacturing method of the metal mold | die for cutting which cuts the steel for machine structure in any one of Claims 1-3.