JP5370073B2 - Alloy steel for machine structural use - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an alloy steel for machine structural use which has high fatigue strength, is excellent in machinability, particularly in chipping resistance and can secure a stable life of a tool. <P>SOLUTION: The alloy steel for machine structural use contains, by mass, 0.15-0.25% C, &ge;0.15% to &lt;0.50% Si, 0.70-1.30% Mn, &le;0.015% S, &gt;1.25% to &le;1.80% Cr, 0.005-0.035% Al and &ge;0.010-0.025% N, wherein Si and Cr satisfy relationship: 1.50&le;Si+Cr&le;1.85, the balance comprising Fe and impurities, wherein P and O in the impurities respectively satisfy relationships: P&le;0.020% and O&le;0.0020%, further, 95% or more of the structure is composed of a ferrite-pearlite structure and, moreover, the proportion of ferrite occupied in the structure is 40 to 60%. The alloy steel for machine structural use may contain at least one kind of Mo, Ni, Nb and Ca which satisfy [Mn+Mo+Ni&le;1.30] each in a specific amount. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an alloy steel material for machine structure, and more particularly to an alloy steel material for machine structure that is excellent in fatigue strength and hardly causes chipping of a tool during cutting. More specifically, the present invention is excellent in fatigue strength and, for example, in cutting tools for manufacturing parts such as gears and shafts used in industrial machines, automobiles, etc., and in particular, cemented carbide tools. The present invention relates to an alloy steel material for machine structure having a ferrite pearlite structure, which is hard to cause chipping of a machine, particularly a tool. Hereinafter, the fact that chipping of a cutting tool is less likely to occur is said to be excellent in chipping resistance.

  Among the parts of industrial machines and automobiles, for example, as steel materials for gears, there are alloy steels for machine structures represented by JIS-defined chromium steel, SCr420 steel, chromium molybdenum steel, SCM420 steel, etc. Widely used.

  These steels are often processed into steel bars by rolling, then subjected to heat treatment such as normalization and annealing to adjust the hardness and structure, etc., and further plastically processed to a state close to the product shape by forging etc. Then, it is processed into a predetermined shape by a cutting process. After that, surface treatment such as carburization is performed according to the required characteristics, and the final part is subjected to grinding for finishing.

  Among the above processes, it is strongly desired that steel has excellent machinability in order to improve the production efficiency particularly in the cutting process.

In general, "excellent machinability" means
-Long tool life for cutting,
-Chips discharged during cutting are finely divided,
・ Low cutting resistance,
-The finish of the cutting surface and grinding surface is good,
Means.

  As unmanning and automation progress in the cutting process, it is important that the tool life is particularly long and the long life can be stably obtained.

  For example, even if tool wear resistance is ensured, if the phenomenon of sudden chipping of the tool due to some factor, called “chipping”, occurs, the finishing accuracy of the cutting surface or grinding surface of the product will decrease. In addition, it becomes a very important problem such as a decrease in yield.

  In many cases, machinability is improved mainly by optimizing the cutting method (cutting conditions, tool material, tool shape, etc.). However, the machinability may be governed by the characteristics of the steel material itself. In particular, it is an important issue from the viewpoint of improving the machinability of the steel material that a long tool life can be stably secured.

  In general, it is well known that the machinability of steel is improved by the addition of Pb. However, the addition of Pb is not only accompanied by an increase in the price of steel materials, but there is a concern that environmental pollution is caused. Therefore, research on techniques for improving the machinability of steel without adding Pb has been advanced. A typical example is a machinability improving technique using MnS, which is a sulfide-based inclusion, and many studies have been made and put into practical use.

  However, regarding alloy steel for machine structural use, cold forging may be performed due to production characteristics, and MnS may be the starting point of cracking. Further, gears and the like are required to have excellent fatigue characteristics such as pitching strength, and inclusions may become a starting point of fatigue failure. For this reason, adding a large amount of S is often avoided.

  Thus, in alloy steels for machine structures, it is strongly desired to improve machinability without actively using inclusions.

  In addition, the technique which improves the machinability of the alloy steel materials for machine structures including utilization of MnS is proposed by patent documents 1-3, for example.

  Specifically, in Patent Document 1, C: 0.03 to 0.25%, Si: 0.50% or less, Mn: 0.55 to 3.00%, Cr: 0.30 to 1.50% , S: 0.035% or less, P: 0.015% or less, Al: 0.015-0.06% and N: more than 0.004% and 0.03% or less, and if necessary, It contains one or more of Cu, Ni, Mo, V, Ti, Nb, B, Ca, Zr and Te, and consists of the balance Fe and inevitable impurities, and Ceq = C + (1/20) Si + (1 / 20) Ceq represented by Mn + (1/20) Cr + (1/25) Mo + (33/100) V (%) is 0.50% or less, and [Surface C (%) after surface hardening treatment] × “Skin-hardened steel” characterized by Cr (%) ≦ 1.2 has been proposed.

  In Patent Document 2, the chemical composition is, in mass%, C: 0.3% or less, Si: 0.3% or less, Mn: 1.5% or less, P: 0.02% or less, S: 0 0.02% or less, Al: 0.06% or less and N: 0.03% or less, Cr: 3% or less, Mo: 1.5% or less, and V: 1.5% or less In addition, if necessary, it contains at least one of Ti, Nb, Cu, Ni, Ca, Zr, Pb and B, with the balance being Fe and inevitable impurities (ferrite + pearlite) ) Has an area ratio of 75% or more, an average grain size of ferrite of 40 μm or less, and an average grain size of pearlite of 30 μm or less has been proposed. .

  In Patent Document 3, in mass%, C: 0.10 to 0.35%, Si: 0.03 to 0.35%, Mn: 0.20 to 2.0%, S: 0.003 to 0 .30%, Al: 0.010 to 0.05% and N: 0.010 to 0.025%, and if necessary, Cr, Mo, Nb, Pb, Bi, Te, Ca and Se In steel which contains 1 or more types of these, and consists of remainder Fe and an unavoidable impurity, it heats to 840-930 degreeC, and makes the temperature area of 730-650 degreeC a slow cooling temperature range, and the cooling rate of 15-50 degreeC / h "A method for producing a case-hardened steel excellent in cold workability and crystal grain size characteristics" has been proposed.

JP-A-7-179990 Japanese Patent Laid-Open No. 11-12684 Japanese Patent Laid-Open No. 2002-146438

  The steel disclosed in Patent Document 1 may have excellent machinability in terms of the amount of tool wear during hobbing with a high-speed tool, but the C, Si, S, and Cr contents are not appropriate. In terms of chipping resistance when using cemented carbide tools, it can not be said that it has an excellent tool life, and it is also possible to achieve both excellent machinability and good fatigue strength. It was not enough.

  The steel disclosed in Patent Document 2 is premised on performing a carburizing process after cold forging, and although it can achieve a rapid spheroidizing annealing process and is excellent in cold forgeability, it has been machined. In that case, it was not satisfactory in that a long tool life could be maintained.

Steel manufactured by the method disclosed in Patent Document 3 is premised on performing carburization after cold working, and although it is difficult for mixed grains to occur during carburizing, chipping of a tool during cutting is performed. Is not necessarily suppressed, and it could not be said to have stable machinability.
As described above, none of the conventionally proposed alloy steels for machine structures can achieve both good fatigue strength and excellent machinability, particularly chipping resistance.

  The present invention has been made in view of the above situation, and has an alloy steel for machine structure that has high fatigue strength and is excellent in machinability, in particular, chipping resistance and can secure a stable tool life. The purpose is to provide.

  Among the machinability of alloy steels for machine structures, it is well known that the tool wear can be suppressed by adjusting the hardness by heat treatment, especially with respect to the tool life. However, there has not been much research on suppression of chipping (chipping) of the tool edge that occurs suddenly.

  Therefore, the present inventors have investigated in detail the relationship between the mechanical properties, alloy components and microstructure of the steel material and machinability in order to improve the chipping resistance of the alloy steel material for mechanical structure. As a result, the following findings (a) to (d) were obtained.

  (A) The occurrence of chipping in steel materials having the same hardness has a correlation with the fluctuation range of the main component force of the cutting resistance during the cutting process. That is, as shown in FIGS. 1 and 2, no chipping occurs when the fluctuation range of the main component force is small (FIG. 1), and chipping occurs when the fluctuation range of the main component force is large (FIG. 2). To do. From this, it has been found that when the fluctuation range of the main component force of the cutting force is increased, the repeated load on the cutting edge is increased to lead to chipping. In addition, the fluctuation range of the main component force of the cutting force can be expressed by the standard deviation σ as the data variation.

  (B) In order to reduce the fluctuation range of the main component force of the cutting force during cutting, the main structure of the steel material is preferably a so-called “ferrite-pearlite structure” formed of ferrite and pearlite, A smaller yield ratio (0.2% yield strength / tensile strength) in tensile properties is better. This means that it is effective that the hardness of the ferrite is low, and it is considered that the cutting resistance is stabilized by concentrating the shear strain that occurs continuously during chip formation on the ferrite.

  (C) Although Si is conventionally known as a ferrite strengthening element, it has been found that the yield ratio is lowered by selecting appropriate heat treatment conditions even if the Si content is increased.

  (D) On the other hand, if the proportion of ferrite in the structure is too large, the ductility of the steel material becomes too large, and the steel material tends to adhere to the tool during the cutting process. And chipping resistance will be impaired by the repetition of generation | occurrence | production and removal | elimination of the adhesion thing in cutting. Therefore, the proportion of ferrite in the structure is not necessarily high, and must be limited.

  Therefore, the inventors further changed the alloy composition and the structure of the steel material, and investigated the relationship between the variation in cutting resistance and the yield ratio. As a result, as shown in FIG. 3, when the yield ratio is 0.63 or less, it has been found that the fluctuation range (standard deviation σ) of the main component force of the cutting force during cutting is suppressed. .

  This invention is completed based on said knowledge, The summary exists in the alloy steel materials for mechanical structures shown to following (1)-(4).

(1) By mass%, C: 0.15 to 0.25%, Si: 0.15 or more and less than 0.50%, Mn: 0.70 to 1.30%, S: 0.015% or less, Cr: more than 1.25% and 1.80% or less, Al: 0.005 to 0.035% and N: 0.010 to 0.025%, and represented by the following formula (1) The range of fn1 is 1.50 to 1.85, the balance is made of Fe and impurities, and P and O (oxygen) in the impurities are P: 0.020% or less and O: 0.0020% or less, respectively. Further, 95% or more of the structure is composed of a ferrite / pearlite structure, and the proportion of ferrite in the structure is 40 to 60%.
fn1 = Si + Cr (1)
However, the element symbol in the formula (1) represents the content in mass% of the element.

(2) Instead of a part of Fe, in mass%, it contains at least one of Mo: 0.50% or less and Ni: 0.20% or less, and is represented by the following formula (2) The range of fn2 is 1.30 or less. The alloy steel for machine structure as described in (1) above.
fn2 = Mn + Mo + Ni (2)
However, the element symbol in the formula (2) represents the content in mass% of the element.

  (3) The alloy steel material for machine structure according to the above (1) or (2), which contains Nb: 0.050% or less in mass% instead of part of Fe.

  (4) The alloy steel for machine structure according to any one of the above (1) to (3), wherein Ca is contained in mass% and not more than 0.0050% instead of a part of Fe Steel material.

  The “impurities” in the remaining “Fe and impurities” refers to those mixed from ore, scrap, or the environment as raw materials when industrially producing steel materials.

  “Ferrite / pearlite structure” refers to a mixed structure of ferrite and pearlite.

  “Ferrite” in the “ratio of ferrite in the structure” does not include ferrite constituting pearlite.

  The ratio of “ferrite / pearlite structure” and “ferrite” in the structure refers to the area ratio.

  The alloy steel for machine structural use according to the present invention is excellent in machinability and excellent in fatigue strength despite not actively utilizing inclusions such as MnS. In particular, by using this steel material as a material of a part that requires cutting such as a gear represented by a target part of the alloy steel material for machine structural use, the manufacturing cost of the part can be greatly reduced.

It is a figure which shows the cutting resistance measurement result in cutting, and is a figure explaining the case where the fluctuation range of the main component force in cutting is small, and no chipping has arisen. It is a figure which shows the cutting resistance measurement result in cutting, and is a figure explaining the case where the fluctuation range of the main component force in the cutting process is large and chipping has occurred. It is a figure which shows the influence of the yield ratio which acts on the fluctuation range of the main component force during cutting. It is a figure which shows the shape as cut out from the heat processing material of the Ono type | formula rotation bending fatigue test piece with a notch used in the Example. It is a figure which shows the heat pattern of "the carburizing quenching-tempering" in an Example. It is a figure which shows the shape of the Ono type | formula rotation bending fatigue test piece with a notch used in the Ono type | formula rotation bending fatigue test of the Example.

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

(A) About chemical composition of steel materials:
C: 0.15-0.25%
C is an essential element for securing the strength of the core (fabric) of mechanical structural parts such as gears, and a content of 0.15% or more is necessary. On the other hand, if the C content is less than 0.15%, the ferrite area ratio in the steel structure becomes excessive, and the chipping resistance is lowered. However, if the C content is too large, the hardness increases and the tool life is reduced due to wear. In particular, if the C content exceeds 0.25%, the tool life is significantly reduced as the hardness increases. Become. Therefore, the content of C is set to 0.15 to 0.25%. The C content is preferably 0.17% or more and 0.23% or less.

Si: 0.15% or more and less than 0.50% Si is one of the important elements in the present invention and has an effect of improving chipping resistance. In other words, the average value of cutting resistance increases as the tensile strength increases as the content increases. On the other hand, Si has the effect of preventing the yield ratio from increasing, and as a result, the fluctuation range of cutting resistance is greatly suppressed. There is an effect to. Compared with the effect of increasing the average value of the cutting resistance, the effect of suppressing the fluctuation range of the cutting resistance is large, so that the chipping resistance is greatly improved. In order to obtain this effect, it is necessary to contain 0.15% or more of Si. However, when the Si content is 0.50% or more, the hardness is increased, and the cutting resistance itself is excessively increased, so that tool wear is increased, leading to a decrease in tool life. In addition, in the case of carburized parts, the formation of a grain boundary oxide layer by carburization is promoted, and the fatigue strength is also reduced. Therefore, the Si content is set to 0.15% or more and less than 0.50%.

  When better machinability is required, the Si content is preferably 0.25% or more, and more preferably 0.30%.

  In addition, Si content needs to satisfy also 1.50 <= fn1 <= 1.85 by fn1 represented by said Formula (1) in said range.

Mn: 0.70 to 1.30%
Mn is necessary to ensure the strength of mechanical structural parts such as gears. Mn also has the effect of improving the hardenability during carburizing. In order to obtain these effects, a Mn content of 0.70% or more is necessary. However, if the Mn content exceeds 1.30%, the amount of ferrite before cutting decreases, and as a result, the bainite structure is formed and the cutting resistance increases, so that tool wear increases. Cause a decline. Therefore, the Mn content is set to 0.70 to 1.30%. In addition, it is preferable that content of Mn is 0.75% or more and 1.25% or less.

  When one or more of Mo: 0.50% or less and Ni: 0.20% or less are included, the content of Mn is fn2 represented by the above formula (2) within the above range. May satisfy 1.30 or less.

S: 0.015% or less S is an impurity contained in steel. Moreover, when S is contained, it combines with Mn to form MnS and has an effect of improving machinability. However, if the S content exceeds 0.015%, coarse MnS is formed, and in terms of productivity, hot workability and cold forgeability are lowered, and further, resistance to resistance required as a machine structural part is reduced. Fatigue properties (bending fatigue strength, pitting strength, etc.) are reduced. Therefore, the content of S is set to 0.015% or less.

Cr: more than 1.25% and not more than 1.80% Cr is one of the important elements in the present invention like Si, and has the effect of improving the hardenability during carburizing. Cr also has the effect of improving chipping resistance. That is, Cr increases the tensile strength as the content increases, so that the average value of cutting resistance increases, but on the other hand, it has the effect of preventing the yield ratio from increasing, and as a result, the fluctuation range of cutting resistance is greatly suppressed. There is an effect to. Compared with the effect of increasing the average value of the cutting resistance, the effect of suppressing the fluctuation range of the cutting resistance is large, so that the chipping resistance is greatly improved. In order to acquire this effect, it is necessary to contain Cr exceeding 1.25%. However, if the Cr content exceeds 1.80%, the hardness increases, and the cutting resistance itself increases too much, so that tool wear increases, which in turn leads to a decrease in tool life. Therefore, the Cr content is more than 1.25% and not more than 1.80%.

  When much better machinability is required, the Cr content is preferably 1.40% or more, and preferably 1.65% or less.

  In addition, the Cr content needs to satisfy 1.50 ≦ fn1 ≦ 1.85 so that fn1 represented by the above formula (1) satisfies the above range.

Al: 0.005-0.035%
Al has a deoxidizing action. Moreover, Al also has the effect | action which combines with N, forms AlN, refines | miniaturizes a crystal grain, and strengthens steel. However, when the Al content is less than 0.005%, it is difficult to obtain the above effect. On the other hand, if the Al content exceeds 0.035%, hard and coarse Al ₂ O ₃ is formed, and tool wear increases, leading to a reduction in tool life. Therefore, the content of Al is set to 0.005 to 0.035%. In addition, it is preferable that Al content is 0.015% or more, and it is preferable that it is 0.025% or less.

N: 0.010 to 0.025%
N has the effect of making the crystal grains finer by forming nitrides and improving the bending fatigue strength. In order to acquire this effect, it is necessary to contain N 0.010% or more. However, when the content of N is excessive, coarse nitrides are formed and the toughness is reduced. In particular, when the content exceeds 0.025%, the toughness is significantly reduced. Therefore, the N content is set to 0.010 to 0.025%. In addition, it is preferable that content of N exceeds 0.015%, and it is preferable that it is 0.020% or less.

fn1: 1.50-1.85
Si and Cr are important elements in the present invention, and have an effect of improving chipping resistance. That is, as described above, Si and Cr increase the average value of the cutting resistance by increasing the tensile strength with the increase in the content, but on the other hand, there is an action to prevent the yield ratio from increasing. There is an effect of greatly suppressing the fluctuation range of the. Compared with the effect of increasing the average value of the cutting resistance, the effect of suppressing the fluctuation range of the cutting resistance is large, so that the chipping resistance is greatly improved. In order to obtain this effect, the contents of Si and Cr are adjusted to the above-described ranges, and fn1 represented by the above formula (1), that is, [Si + Cr] must be 1.50 or more. There is. On the other hand, if fn1 exceeds 1.85, the hardness becomes too large, and the cutting resistance itself increases, so that the tool wear increases, leading to a decrease in the tool life. Therefore, fn1 needs to be in the range of 1.50 to 1.85.

  One of the alloy steels for machine structure of the present invention has a chemical composition in which the balance is composed of Fe and impurities in addition to the above elements.

  In the present invention, the contents of P and O (oxygen) in the impurities must be limited to P: 0.020% or less and O: 0.0020% or less, respectively.

  Hereinafter, P and O in the impurities will be described.

P: 0.020% or less P is an impurity contained in the steel and segregates at the grain boundaries to embrittle the steel. In particular, when the content exceeds 0.020%, the degree of embrittlement becomes significant. Therefore, in the present invention, the content of P in the impurities is set to 0.020% or less. In addition, it is preferable that content of P in an impurity shall be 0.010% or less.

O (oxygen): 0.0020% or less O (oxygen) combines with Si or Al in steel to generate an oxide. Among the oxides, in particular, Al ₂ O ₃ is hard, so that machinability is lowered. Therefore, in the present invention, the content of O in the impurities is set to 0.0020% or less.

  Another one of the alloy steels for machine structure of the present invention contains one or more elements of Mo, Ni, Nb and Ca instead of a part of Fe.

  Hereinafter, the effect of said Mo, Ni, Nb, and Ca which are arbitrary elements, and the reason for limitation of content are demonstrated.

  Both Mo and Ni have the effect of increasing the hardenability during carburizing. For this reason, when it is desired to obtain greater hardenability during carburizing, these elements may be contained. Hereinafter, the above Mo and Ni will be described.

Mo: 0.50% or less Mo has an effect of improving the hardenability at the time of carburizing, and may be contained as necessary. However, if the Mo content exceeds 0.50%, the amount of ferrite before cutting decreases, and the tool wear increases due to the formation of a bainite structure and the cutting resistance increases. Cause a decline. Therefore, the Mo content in the case of inclusion is set to 0.50% or less. The Mo content is preferably 0.30% or less.

  On the other hand, in order to surely obtain the effect of improving the hardenability at the time of carburizing Mo described above, the Mo content is preferably 0.03% or more.

  In addition, Mo content needs to satisfy | fill fn2 represented by said Formula (2) as 1.30 or less in said range.

Ni: 0.20% or less Ni has an effect of improving the hardenability at the time of carburizing, and may be contained as necessary. However, if the Ni content exceeds 0.20%, the ferrite content before cutting is reduced, and the tool wear is increased because the bainite structure is formed and the cutting resistance is increased. Cause a decline. Therefore, the Ni content when contained is set to 0.20% or less. The Ni content is preferably 0.10% or less.

  On the other hand, in order to reliably obtain the effect of improving the hardenability at the time of carburizing Ni described above, the Ni content is preferably 0.03% or more.

  In addition, the content of Ni needs to satisfy fn2 represented by the above formula (2) as 1.30 or less in the above range.

  Said Mo and Ni can be contained only in any 1 type or 2 types of composites. The total content of these elements may be 0.70% or less, but is preferably 0.50% or less.

fn2: 1.30 or less Mn, Mo, and Ni all have the effect of improving the hardenability during carburizing, but it is necessary to limit the respective contents in order to ensure an appropriate microstructure. For this purpose, it is necessary to set fn2 represented by the above formula (2), that is, [Mn + Mo + Ni] to 1.30 or less. When fn2 exceeds 1.30, the amount of ferrite before cutting is reduced, and a bainite structure is formed and cutting resistance is increased. As a result, tool wear increases, and on the contrary, tool life is reduced. Therefore, in order to improve the hardenability at the time of carburizing, when one or more of Mo and Ni are contained, fn2 needs to be in the range of 1.30 or less. Note that fn2 is preferably 1.20 or less.

  Moreover, in order to acquire the hardenability improvement effect at the time of carburizing reliably, it is preferable that fn2 is 0.73 or more. fn2 is more preferably 0.76 or more, and further preferably 0.80 or more.

Nb: 0.050% or less Nb combines with C and N to form fine carbides, nitrides, carbonitrides, refines the crystal grains, and improves fatigue characteristics, especially bending fatigue strength, as mechanical structural parts. Since it has the effect to improve, you may contain as needed. However, when the content of Nb is excessive, the hot ductility is reduced. Particularly, when the content exceeds 0.050%, the hot ductility is significantly reduced, and during hot rolling or hot forging. Surface scratches are likely to occur. On the other hand, if the Nb content is too high, the crystal grains are remarkably refined, the yield ratio is increased, and as a result, the chipping resistance is lowered. Therefore, when Nb is contained, the Nb content is set to 0.050% or less. The Nb content is preferably 0.040% or less.

  On the other hand, the Nb content is preferably 0.005% or more and more preferably 0.020% or more in order to surely obtain the above-described effect of improving Nb characteristics.

Ca: 0.0050% or less Ca has an effect of improving machinability. For this reason, you may contain Ca in order to improve machinability. However, excessive addition of Ca leads to an increase in cost. In particular, when the Ca content exceeds 0.0050%, the machinability improving effect is saturated, so that the cost increases and the economic efficiency is impaired. In addition, when the Ca content exceeds 0.0050%, a coarse oxide is formed, leading to a decrease in bending fatigue strength and pitching strength. Therefore, when Ca is contained, the content of Ca is set to 0.0050% or less. In addition, it is preferable that content of Ca is 0.0030% or less.

  On the other hand, in order to reliably obtain the above-described effect of improving the machinability of Ca, the Ca content is preferably 0.0005% or more.

(B) Steel structure:
In the alloy steel for machine structure of the present invention, it is necessary that 95% or more of the structure is composed of a ferrite pearlite structure, and the ratio of ferrite in the structure is 40 to 60%.

  First, the formation of a ferrite-pearlite structure is an important factor for ensuring good fatigue strength and ensuring stable machinability by lowering the yield ratio. That is, in the steel material having the chemical composition described in the above section (A), when the proportion of the ferrite / pearlite structure in the structure is less than 95%, the remainder is replaced with bainite. In this case, since the hardness increases and the cutting resistance increases too much, the tool life is reduced due to an increase in tool wear. Therefore, 95% or more of the structure is composed of a ferrite / pearlite structure. Note that 100% of the structure may be a ferrite / pearlite structure.

  Next, the ratio of ferrite in the structure is an important factor for ensuring good fatigue strength and stably ensuring machinability. That is, when the proportion of ferrite in the structure composed of 95% or more of the ferrite / pearlite structure is less than 40%, the remaining amount of pearlite is increased or, in some cases, partly replaced with bainite or the like. As a result, the hardness increases and the cutting resistance increases too much, leading to a reduction in tool life due to increased tool wear. On the other hand, if the proportion of ferrite in the above structure exceeds 60%, the hardness decreases and the ductility of the steel material increases, so that adhesion to the tool easily proceeds during cutting. And even if the tool wear due to the decrease in hardness is suppressed, the chipping resistance is deteriorated on the contrary by the repeated generation and dropping of the adherend during the cutting process. From the above, it was decided that the proportion of ferrite in the structure was 40 to 60%.

  As described above, the “ferrite-pearlite structure” refers to a mixed structure of ferrite and pearlite, and “ferrite” in the “ratio of ferrite in the structure” does not include ferrite constituting pearlite. The ratios of “ferrite / pearlite structure” and “ferrite” in the structure indicate area ratios.

  In addition, 95% or more of the structure of the steel material having the chemical composition described in the section (A) is composed of a ferrite / pearlite structure, and the ratio of ferrite in the structure is 40 to 60%. In order to do this, for example, a heat treatment is recommended in which the steel material is heated in the range of 870 to 950 ° C. for 30 minutes or more and then continuously cooled at a surface temperature of the steel material of 800 ° C. to 500 ° C. for 6 minutes or more. In the above heat treatment, the upper limit of the heating time in the range of 870 to 950 ° C. is not particularly defined, but the recommended heating time is 2 hours or less from the viewpoint of productivity. Further, in order to stably obtain a predetermined structure, it is more recommended that the cooling time from 800 ° C. to 500 ° C. is 8 minutes or more at the surface temperature of the steel material, but within 30 minutes from the viewpoint of productivity. It is preferable. Note that the cooling rate after the surface temperature of the steel material reaches 500 ° C. is not limited to special adjustment. What is necessary is just to cool with an appropriate cooling method from viewpoints of productivity, an installation surface, etc.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels A1 to A11 and steels B1 to B15 having chemical compositions shown in Table 1 were melted in a 150 kg vacuum melting furnace and manufactured by ingot casting. Steels A1 to A11 are steels of the present invention examples whose chemical compositions are within the range defined by the present invention, while steels B1 to B15 are steels of comparative examples that deviate from the conditions defined by the present invention. is there.

  A steel bar of each steel was heated to 1250 ° C. and then hot forging was performed at a temperature of 1000 ° C. or higher to produce a round bar having a diameter of 65 mm. The hot forging was allowed to cool in the atmosphere.

  As a heat treatment for adjusting the structure and hardness, each steel round bar thus obtained was heated to 900 ° C. and held for 1 hour, and then the cooling time from 800 ° C. to 500 ° C. was 10 minutes. Cooling was performed. In addition to the above heat treatment, the steel A2 round bar was heated to 800 to 1000 ° C. and held for 1 hour, and then cooled to 800 ° C. to 500 ° C. for 10 minutes, and 900 ° C. And then held for 1 hour, followed by cooling from 800 ° C. to 500 ° C. for 5 minutes. In any of the heat treatments, the cooling after reaching 500 ° C. was allowed to cool in the air.

  Test taken in parallel to the training axis direction, ie, parallel to the longitudinal direction, centered on a position 16 mm (hereinafter referred to as “R / 2 part”) from the surface of each round bar subjected to the above heat treatment A 10 mm × 10 mm cross section of the piece is mirror-polished, corroded with nital, and the structure is examined by observing the structure of each sample with an optical microscope with six fields of view at a magnification of 100 times. The ratio of ferrite / pearlite structure and ferrite to the structure was determined. In addition, the specific measurement was performed as follows.

  When 100% of the structure is formed of ferrite and pearlite, the value obtained by binarizing the ferrite and other structural forms by image analysis and dividing the area of the color-coded area by the total area Was the proportion of ferrite in the structure.

  In addition, when a phase other than ferrite and pearlite (bainite) is included, first, binarization of the bainite and other structural forms is performed by image analysis, and the area of the region color-coded into bainite from the total area is calculated. The subtracted value was used as the ratio of ferrite and pearlite. Further, the ferrite and other microstructures were binarized by image analysis, and the value obtained by dividing the area of the region color-coded by ferrite by the total area was defined as the proportion of ferrite in the tissue.

  Moreover, the test piece which observed said structure | tissue is mirror-polished again, and Vickers hardness in R / 2 part (in accordance with "Vickers hardness test-test method" described in JIS Z 2244 (2009) ( Hereinafter, “HV hardness” was measured with a test force of 980 N, and the value was arithmetically averaged to evaluate the HV hardness of the heat-treated material.

  Furthermore, from the R / 2 part of the heat-treated material with a diameter of 65 mm produced as described above, a JIS No. 14A test piece described in JIS Z 2201 (1998) (the diameter of the parallel part is 4. 5 mm) was sampled and subjected to a tensile test at room temperature, and 0.2% yield strength and tensile strength were measured to obtain a yield ratio.

  For machinability, a heat treatment material having a diameter of 65 mm produced as described above was peeled to a diameter of 62 mm, and a cylindrical test piece cut to a length of 400 mm was used to perform a machinability test by turning. And evaluated.

  In addition, the cutting is performed using a P20 type carbide tool, wet cutting with a water-soluble emulsion diluted 20 times (supply amount: 20 liters / min), the cutting speed is kept constant at 150 m / min, and the feed is 0 The test was performed in the longitudinal direction of a 400 mm test piece under the condition that the cutting amount was 1.5 mm at 35 mm / rev.

  Regarding the evaluation of the cutting resistance, the tool started cutting from an unused state, and the sampling time was measured as 1/100 second between 5 to 6 seconds. Then, after obtaining the measurement result of the main component force during the cutting process as shown in FIGS. 1 and 2, the process time is 4 seconds (data number: 400) within 0.5 seconds after the start of the process. The average value of the main component force and its fluctuation were calculated as the standard deviation σ. The “main component force” during cutting refers to a tangential force parallel to the rotation direction of the work material acting on the point where the tool and the work material come into contact.

  Regarding tool wear, machining was performed up to a cutting distance of 3000 m under the above conditions, and the state of damage to the cutting edge was observed every 300 m of the cutting distance. The tool life was determined based on the cutting distance when the flank wear amount (hereinafter referred to as “VB”) exceeded 0.2 mm at the time of observing the damage state, or when chipping was confirmed.

The fatigue strength after carburizing was evaluated by the Ono type rotating bending fatigue test described below, and the stress that did not break at 10 ⁷ rotations was evaluated as the fatigue limit.

  First, from the R / 2 part of the heat-treated material having a diameter of 65 mm produced as described above, a coarse notched Ono type rotating bending fatigue test piece shown in FIG. 4 was cut out parallel to the longitudinal direction of the round bar.

  In addition, the unit of the dimension in the test piece shown in FIG. 4 is all “mm”, and the finish symbols “▽”, “▽▽” and “▽▽▽” in the figure are those of JIS B 0601 (1982). It is a “triangular symbol” indicating the surface roughness in Table 1.

  “Carburization quenching-tempering” by the heat pattern shown in FIG. 5 was performed on all of the coarse Ono rotary bending fatigue test pieces with notches. “Cp” in FIG. 5 represents the carbon potential. “100 ° C. oil quenching” indicates quenching in oil at an oil temperature of 100 ° C., and “AC” indicates cooling in the atmosphere. About oil quenching, the test piece was thrown into the quenching oil stirred so that it might be uniformly quenched.

  In addition, said "carburization hardening-tempering" was implemented in the state which passed the wire processed through the hole processed for suspension of each test piece, and was suspended.

  The test piece subjected to the above-mentioned “carburization quenching-tempering” was finished to produce an Ono-type rotary bending fatigue test piece with a notch shown in FIG.

  The units of dimensions in the test piece shown in FIG. 6 are all “mm”, and the finish symbols “▽” and “▽▽▽” in the figure are the same as those in FIG. 4, respectively, as in JIS B 0601 (1982). ) Is a “triangular symbol” indicating the surface roughness in Table 1. Further, “G” added to “▽▽▽” means an abbreviation of a processing method indicating “grinding” defined in JIS B 0122 (1978). Furthermore, “˜” is a “waveform symbol”, which means that it is a dough, that is, it remains on a “carburized and tempered” surface.

Using the finished Ono rotary bending fatigue test piece, the Ono rotary bending fatigue test was conducted under the following test conditions. As described above, the stress that did not break at 10 ⁷ rotations was used as the fatigue limit. evaluated.

・ Temperature: Room temperature,
・ Atmosphere: In air
-Number of rotations: 3400 rpm.

  Table 2 summarizes the results of each of the above investigations. Table 2 also shows the heat treatment applied to the round bar having a diameter of 65 mm.

  From Table 2, in the case of test numbers 1 to 11 that satisfy the conditions defined in the present invention, the tool life is not reached even at a cutting distance of 3000 m, the machinability is good, and the fatigue limit is 500 MPa or more, It is clear that the fatigue strength is also excellent.

  On the other hand, in the case of test numbers 12 to 30 of comparative examples that deviate from the conditions specified in the present invention, one or both of machinability and fatigue strength are inferior.

  That is, in the case of Test No. 12, Test No. 13, Test No. 15 and Test No. 17, the chemical composition of the steel material deviates from the conditions specified in the present invention. The tool life is reached by chipping, and the machinability is inferior.

  In the case of Test No. 14, Test No. 16, Test No. 18-23, and Test No. 25, the chemical composition and structure of the steel material are out of the conditions defined in the present invention. The tool life is reached by increasing VB or chipping, and the machinability is poor. Among the above, test number 14 is inferior in fatigue strength because the Si content is outside the upper limit defined in the present invention, so that the fatigue limit does not reach 500 MPa.

  In the case of Test No. 24 and Test No. 26, the chemical composition of the steel material deviates from the conditions specified in the present invention, that is, Test No. 24 has an S content, and Test No. 26 has a Ca content. Each of them is outside the upper limit defined in the present invention, so the fatigue limit does not reach 500 MPa, and the fatigue strength is inferior.

  In the case of test numbers 27 to 30, the chemical composition of the steel material satisfies the conditions specified in the present invention, but the structure deviates from the conditions specified in the present invention. Therefore, the fluctuation range of the main component force during cutting is large. The tool life has been reached by increasing or chipping, and the machinability is poor.

  The alloy steel for machine structural use according to the present invention is excellent in machinability and excellent in fatigue strength despite not actively utilizing inclusions such as MnS. In particular, by using this steel material as a material of a part that requires cutting such as a gear represented by a target part of the alloy steel material for machine structural use, the manufacturing cost of the part can be greatly reduced.

Claims (4)

% By mass, C: 0.15 to 0.25%, Si: 0.15 or more and less than 0.50%, Mn: 0.70 to 1.30%, S: 0.015% or less, Cr: 1 More than 25% and not more than 1.80%, Al: 0.005 to 0.035% and N: 0.010 to 0.025%, and the range of fn1 represented by the following formula (1) 1.50 to 1.85, the balance is Fe and impurities, and P and O (oxygen) in the impurities are P: 0.020% or less and O: 0.0020% or less, respectively. An alloy steel for mechanical structure, characterized in that 95% or more of the structure is composed of a ferrite / pearlite structure, and the ratio of ferrite in the structure is 40 to 60%.
fn1 = Si + Cr (1)
However, the element symbol in the formula (1) represents the content in mass% of the element. Instead of a part of Fe, by mass%, it contains at least one of Mo: 0.50% or less and Ni: 0.20% or less, and fn2 represented by the following formula (2) 2. The alloy steel for machine structure according to claim 1, wherein the range is 1.30 or less.
fn2 = Mn + Mo + Ni (2)
However, the element symbol in the formula (2) represents the content in mass% of the element.   The alloy steel material for machine structure according to claim 1 or 2, wherein Nb: 0.050% or less is contained in mass% instead of a part of Fe.   The alloy steel material for machine structure according to any one of claims 1 to 3, characterized by containing Ca: 0.0050% or less in mass% instead of part of Fe.

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