JP4687617B2 - Steel for machine structure - Google Patents

Description

  The present invention relates to a steel material for machine structure, and more particularly, to a steel material for machine structure which is suitable as a material for parts such as industrial machines and automobiles and is subjected to cutting. More specifically, the present invention relates to a steel material for machine structure that is excellent in chip disposal during cutting.

  Machine structural steel used for parts such as industrial machinery and automobiles is JIS G 4051 (1979) JIS G 4051 (1979) JIS G 4053 (2003) nickel chrome as alloy steel. Steel materials, nickel chrome molybdenum steel materials, chrome steel materials, manganese steel materials and manganese chrome steel materials. In addition, the amount of specified components of these steel materials is slightly changed, the steel is further improved in hardenability by further containing B (boron), etc., and steel materials having improved structure by including Ti, Nb, V, etc. are also used. Has been.

  In many cases, these steel materials are produced by rolling, or after being further forged, are cut into a predetermined shape and subjected to heat treatment according to the required characteristics. The final part.

In order to improve the production efficiency in the cutting process, it is strongly desired that the steel material is excellent in machinability. Excellent machinability means that
・ The period until replacement due to wear of the tool used during cutting is long, that is, the tool life is long,
-Chips discharged during cutting are finely divided,
・ Low cutting resistance,
Or
-The finish of the cutting surface and grinding surface is good,
Means.

  Among the above, when the unmanning and automation of the cutting work progresses, in addition to the tool life, in particular, the property that chips are divided finely, that is, “chip disposal” becomes very important.

  Since the tool life is influenced not only by the characteristics of the steel material but also by the performance of the tool, it can be improved by selecting the tool.

  On the other hand, excellent chip disposal means that chips generated during cutting are finely divided and do not get tangled with the tool, and this is largely governed by the characteristics of the steel material itself. Therefore, improving chip disposal is particularly important for improving the machinability of steel.

  The machinability of the steel material can be improved by containing Pb. However, the inclusion of Pb is not only accompanied by an increase in the price of steel materials, but also has a concern of causing environmental pollution.

  Therefore, research on techniques for improving the machinability of steel without containing Pb has been advanced. A typical example is a machinability improving technique by utilizing MnS which is a sulfide-based inclusion. For example, as proposed in Patent Documents 1 to 4, many studies have been made and practical applications have been made. Has been done.

  Specifically, in Patent Document 1, an Mn—Ca—S inclusion containing substantially 3 to 55 wt% of Ca, which is substantially free of oxide inclusions, is uniformly dispersed in the steel, And the size of the Mn—Ca—S inclusion is 20 μm or less in the major axis (L) and the ratio (L / W) of the major axis (L) to the minor axis (W) is 3 or less “Ca free cutting for machine structure Steel "is disclosed.

  In Patent Document 2, the average oxygen content in Mn sulfide inclusions is 10 mass% or less, the main composition is mass%, C: 0.05 to 0.7%, Si: 2.5% Or less (including 0%), Mn: 0.1 to 3%, Al: 0.1% or less (including 0%), S: 0.003-0.2%, N: 0.002-0. 025%, O: 0.003% or less (including 0%) and the balance is Fe, and further contains at least one selected from the group consisting of rare earth elements, Ca and Mg in a total of 0.01% or less “Mechanical structural steel excellent in chip disposal” is disclosed.

  Patent Document 3 contains C, Si, Mn, P, S, Al, Ca, and N, and, if necessary, Cr, Mo, Cu, Ni, B, Nb, Ti, V, Ta, Zr, Area ratio with respect to the entire area of the field of observation and observation of sulfides containing one or more of Pb, Bi, Se and Te, the balance being Fe and inevitable impurities, and the Ca content exceeding 40% A, the area ratio with respect to the area of the entire observation field of view of the sulfide having a Ca content of 0.3-40% B, the area ratio relative to the entire area of the field of observation of the sulfide having a Ca content less than 0.3% When the rate is C, A / (A + B + C) ≦ 0.3 and B / (A + B + C) ≧ 0.1 “steel for machine structure excellent in turning workability” is disclosed.

  In Patent Document 4, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.003 to 0 by weight% 2%, S: 0.003-0.5%, Zr: 0.0003-0.01%, Te: 0.003-0.005%, Ca: 0.0002-0. 005%, Mg: one or more of 0.0003 to 0.005%, Al ≦ 0.01%, total-O ≦ 0.02%, total-N ≦ 0.02 "Steel excellent in forgeability and machinability" comprising the balance of Fe and inevitable impurities is disclosed.

JP 62-103340 A JP 2001-131684 A JP 2000-34538 A JP 2000-282169 A

  All of the steels disclosed in Patent Documents 1 to 4 described above can contain Ca, and are steels with improved machinability. However, the amount and timing of addition of Ca in the steelmaking process have not been sufficiently considered. Therefore, it has not necessarily been able to improve both chip disposal and tool life.

Specifically, in the case of the technique disclosed in Patent Document 1, individual sulfides become coarse, and the number of sulfides decreases at the same S concentration. Therefore, the improvement of chip disposal is not always sufficient. In addition, since Al killed steel is assumed, even if Ca treatment is performed, oxide inclusions become Al ₂ O ₃ rich CaO-Al ₂ O ₃ hard inclusions, so that the tool life and other cuttings are reduced. The effect of improving the sexiness is not sufficiently obtained. Furthermore, if a large number of sulfides containing a high concentration of CaS are dispersed at a high S concentration, a large amount of Ca needs to be added, resulting in an increase in cost.

In the case of the technique disclosed in Patent Document 2, the average oxygen concentration in the sulfide is set to 10% for the purpose of obtaining a sulfide form effective for improving chip disposal as shown in the Examples. In order to make it below, Al used as a deoxidizing element is contained 0.018% or more. In such a case, the oxide present in the steel is mainly composed of hard Al ₂ O ₃ -based oxide, and the tool life is not improved sufficiently. In other words, the technique proposed in Patent Document 2 does not necessarily improve the tool life at the same time as the chip disposal.

  The technique disclosed in Patent Document 3 is characterized by increasing the abundance of sulfides containing 0.3 to 40% by weight of Ca. However, when the abundance of sulfides containing a large amount of Ca is increased, As the sulfides become coarse and the number of sulfides decreases, it becomes difficult to obtain good chip disposal.

  The steel disclosed in Patent Document 4 is a steel having excellent forgeability and good machinability by spheroidizing a sulfide. When it is assumed that Ca is contained, Ca is MnS. It is a steel that aims to be spheroidized by reducing its deformability by solid solution. However, when Ca is contained, individual sulfides are coarsened, and a sulfide form for obtaining good chip disposal cannot be obtained. That is, the chip disposal is not sufficiently improved.

  Accordingly, an object of the present invention is to provide a steel material for machine structure that can improve machinability without containing Pb, and in particular, a steel material for machine structure that can improve both chip disposal and tool life. is there.

  It is well known that the state of sulfide and oxide inclusions greatly affects the machinability of steel materials.

  Therefore, the present inventors have investigated in detail the relationship between the form and distribution of inclusions in the steel and machinability in order to solve the above-described problems in a steel for machine structures that does not contain Pb. In particular, paying attention to MnS-based inclusions (hereinafter also simply referred to as “MnS”) present in the chips, the difference in form before and after the cutting process was investigated. As a result, the following facts (a) to (e) were first clarified.

  (A) MnS in a chip of a steel material with improved chip disposal is often elongated along the shear deformation direction of the chip.

  (B) In addition, in order to improve chip disposal, an MnS form before cutting that has a small major axis and a large minor axis, that is, a so-called “aspect ratio” is effective. In addition, if the size of each MnS is too small, the effect of improving chip disposal is diminished.

  (C) Therefore, in order to have good chip treatability, it is important that MnS contained in the steel is greatly deformed and stretched when generating chips during cutting.

  (D) In order to increase the deformation amount of MnS during shear deformation, the amount of other elements, that is, the solid solution of Ca, is small in MnS, or a hard material such as an oxide or carbonitride in MnS. It is important that no inclusions be included.

  (E) The large deformability of MnS concentrates deformation on MnS with respect to the matrix in shear deformation during cutting. This reduces the shear deformation resistance, increases the shear angle in the primary shear region, thins the chips, and reduces the curl diameter of the chips. As a result, it is considered that the chip breaking property is improved and the chip disposability is improved.

  Then, next, when MnS is made into the above-mentioned form, the chip processability is improved. In addition to C, Si, Mn, etc., the strength of the steel for machine structural use, the hardenability improvement, the structure improvement, etc. The effects of the addition of Cr and V added for the purpose described above were investigated. As a result, the following knowledge (f) was obtained.

  (F) When Cr and V are contained, compared to the case where Cr and V are not contained, the mechanical properties such as hardness, strength and hardenability of the steel can be improved, and Cr and V are not contained. Good chip control performance similar to the above can be obtained.

  Therefore, the present inventors have further confirmed the limits of the chemical composition and the state of inclusions, and completed the present invention.

  The gist of the present invention resides in mechanical structural steel materials shown in the following (1) and (2).

(1) By mass%, C: 0.15-0.6%, Si: 0.1-2.0%, Mn: 0.5-2.0%, P: 0.1% or less, S: 0.015 to 0.12%, Ca: 0.0005 to 0.003% and N: 0.02% or less, with the balance being a chemical composition composed of Fe and impurities, Al in impurities: 0.005% or less , Ti: 0.005% or less and O: 0.005% or less, Al and Ca satisfy the following formula (1), and an equivalent circular diameter existing in a cross section parallel to the processing direction is 4 μm or more A steel for machine structural use, wherein the MnS inclusions satisfy the following formulas (2) and (3):
Al / Ca ≦ 1.0 (1),
Aspect ratio (L / W) <3 (2),
(N1 + n2) /n≦0.2 (3),
Here, the element symbol in the formula (1) represents the content in mass% of the element.
In addition, L and W in the formula (2) respectively represent the major axis and minor axis of the target MnS-based inclusion, and the aspect ratio refers to the aspect ratio as an average value.
Furthermore, n, n1, and n2 in the formula (3) each represent the number of the following MnS inclusions.
n: number of MnS inclusions per 1 mm ² area,
n1: Among MnS-based inclusions, the number per unit area of 1 mm ^{2 of} Ca in which 0.5% or more of Ca is solid solution by mass%,
n2: Number of MnS-based inclusions containing different types of inclusions per 1 mm ² area.

  (2) Instead of a part of Fe, the machine according to (1) above, containing one or two of Cr: 2.5% or less and V: 0.5% or less Structural steel.

  Note that “parallel to the processing direction” means, for example, that the processing is parallel to the rolling direction when the processing is rolling, and that the processing is parallel to the forging axis when the processing is forging.

The “heterogeneous inclusion” contained in the MnS-based inclusion refers to an inclusion other than sulfide, such as an oxide such as Al ₂ O ₃ or a nitride such as TiN.

  Here, when one MnS-based inclusion simultaneously satisfies the above-mentioned “n1” “Ca is solid solution at 0.5% or more by mass%” and “including heterogeneous inclusions” according to n2, From the viewpoint of preventing double counting, it is defined that counting is based on n1 “Ca is solid-dissolved by 0.5% or more by mass.” Furthermore, for “including different types of inclusions”, it is not necessary that the different types of inclusions be completely contained inside MnS, and even in the case where they are simply in contact with each other, one is in accordance with this definition. It is prescribed that it is counted as This is because the deformation of MnS in this place is suppressed even when it is simply in contact.

  Hereinafter, the inventions relating to the steel materials for machine structure (1) and (2) are referred to as “present invention (1)” and “present invention (2)”, respectively. Also, it may be collectively referred to as “the present invention”.

  Although the steel for machine structure of the present invention does not contain Pb, it is excellent in machinability, especially chip disposal and tool life. Therefore, by using this steel for machine structure as a material of a part that requires cutting, the manufacturing cost of the part can be greatly reduced.

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

(A) Chemical composition C: 0.15 to 0.6%
C is an important element that governs the properties related to the strength of steel, and its content is usually determined in consideration of mechanical properties. If the C content is less than 0.15%, the mechanical properties required for crankshafts and other automotive machine parts cannot be obtained. On the other hand, if it exceeds 0.6%, the tool life is remarkably lowered, and the desired machinability cannot be obtained. Therefore, the content of C is set to 0.15 to 0.6%.

  In addition, in order to stably obtain mechanical properties such as hardness, toughness, fatigue strength, and machinability as a crankshaft and other automotive machine parts, the C content is 0.30 to 0.55. % Is desirable.

Si: 0.1 to 2.0%
Si is an element that increases the strength and also has a deoxidizing effect of molten steel. However, if the content is less than 0.1%, the above effect cannot be expected. On the other hand, when Si is contained in excess of 2.0%, the hardness is increased, and the machinability is lowered because it is easily dissolved in soft ferrite and increases the shear deformation resistance. Therefore, the Si content is set to 0.1 to 2.0%. The preferable content of Si is 0.15 to 1.0%.

Mn: 0.5 to 2.0%
Mn is an important element for forming MnS inclusions that have a great effect on improving machinability, and also has a deoxidizing effect on molten steel. Furthermore, when S is contained for the purpose of improving machinability, it has an effect of suppressing the hot workability deterioration of the steel material. In order to obtain these effects, the Mn content needs to be 0.5% or more. However, if the content of Mn exceeds 2.0%, the cutting resistance increases, so the machinability decreases. Therefore, the Mn content is set to 0.5 to 2.0%.

  A more preferable range for the Mn content is 0.8 to 1.7%. Further, in the case of a steel material used after heat treatment, Mn greatly contributes to hardenability. Therefore, the content for this purpose may be appropriately selected within the above range.

P: 0.1% or less P is an element mixed as an impurity in steel. However, it has a solid solution strengthening effect and also has an effect of improving the hardenability. When aiming at such an effect, it can be contained up to 0.1% or less of the range in which the toughness of the steel is not deteriorated. The P content is preferably 0.05% or less.

S: 0.015-0.12%
S is an element necessary for improving machinability, and is combined with Mn or the like and is present in the form of MnS inclusions. Since the form of MnS-based inclusions is easily changed by the addition of Ca during the solidification process of steel, the present invention simultaneously defines the form of the MnS-based inclusions. If the S content is less than 0.015%, the effect of improving machinability cannot be obtained. On the other hand, when the content of S is too large, the hot workability deteriorates and the toughness of steel deteriorates. In particular, when it exceeds 0.12%, the hot workability deteriorates and the toughness deteriorates remarkably. Therefore, the S content is set to 0.015% to 0.12%.

  In order to achieve both machinability and mechanical properties as a machine structural steel material, the S content is preferably 0.02 to 0.10%. Within this range, good machinability and mechanical properties can be obtained. However, in order to achieve heat treatment and achieve both appropriate mechanical properties and machinability as a steel material for machine structures, the content of S Is preferably 0.03 to 0.08%.

Ca: 0.0005 to 0.003%
Ca is an element having a high reactivity with O (oxygen) and S and having a tendency to form oxides and sulfides in molten steel. In the present invention, Ca can lower the melting point of the oxide by forming a CaO—SiO ₂ —Al ₂ O ₃ (—TiO ₂ ) composite oxide. For this reason, this composite inclusion becomes soft on the tool surface during cutting, and the progress of tool wear can be suppressed. In order to acquire said effect, it is necessary to contain Ca 0.0005% or more. However, when there is too much content of Ca, it will affect the form of MnS. That is, the proportion of the above complex inclusions taken into MnS increases, and Ca sulfide is formed in the molten steel in addition to the oxide, and this Ca sulfide becomes the nucleus of subsequent MnS formation, so Ca is solidified. The dissolved MnS increases. As a result, the deformability of MnS in the chip shearing region is lowered and the chip processability is lowered, so that the MnS that dissolves Ca does not increase so that the Ca content is at most 0.003%. It is necessary to be up to. Therefore, the content of Ca is set to 0.0005 to 0.003%.

  In order to achieve a more stable MnS inclusion form defined in the present invention, the Ca content is preferably 0.002% or less.

N: 0.02% or less N is an element mixed as an impurity in steel. However, when it is contained, it is an element having an action of strengthening ferrite. However, it forms nitrides with Ti and affects the morphology of MnS. That is, MnS may be formed using TiN as a nucleus, and as a result, TiN is taken into MnS. In this case, the deformability of MnS is reduced, and the chip disposal is reduced. Therefore, the N content needs to be at most 0.02% so that MnS formed with TiN as a nucleus does not increase.

  In order to obtain a more stable MnS inclusion form defined in the present invention, the N content is preferably 0.012% or less.

  The chemical composition of the steel for machine structural use according to the present invention (1) includes elements from C to N in the above-mentioned range, and the balance is composed of Fe and impurities, but Al, Ti and O in impurities must be further limited. I must.

  Hereinafter, Al, Ti, and O in impurities will be described.

Al: 0.005% or less Al has a strong deoxidation effect on the molten steel, and the resulting Al ₂ O ₃ is hard and decreases the tool life, and when incorporated into MnS, the deformability of MnS decreases. Waste disposal will be reduced. Therefore, in the present invention, it is necessary to extremely reduce the Al content so that the oxide inclusions do not become Al ₂ O ₃ rich. Therefore, the Al content in the impurities is set to 0.005% or less.

Ti: 0.005% or less Ti has high reactivity with O (oxygen) and N (nitrogen), and forms oxides and nitrides in molten steel. If the Ti content is high, MnS may be formed with these products as the core, and as a result, hard oxides and nitrides are incorporated into MnS, and the deformability of MnS is reduced, resulting in chip disposal. Cause a decline. Therefore, the content of Ti in the impurities needs to be extremely small so that MnS formed with oxides or nitrides as nuclei does not increase, and is at most 0.005%.

  In order to achieve a more stable MnS inclusion form defined in the present invention, the Ti content is preferably 0.003% or less.

O (oxygen): 0.005% or less O (oxygen) affects the formation of oxides, and if it exceeds 0.005%, the amount of oxide increases, and the probability that this will be incorporated into MnS increases. For this reason, since the deformability of MnS is reduced and the chip disposal is reduced, the content of O in the impurity is limited to 0.005% at most so that MnS incorporating the oxide does not increase. There is a need.

  In order to obtain a MnS-based inclusion form defined in the present invention in a more stable and reliable manner, the O content is preferably 0.003% or less.

Al / Ca value: 1.0 or less The content of Al and Ca is specified in the above-described range, and the content ratio (Al / Ca) is 1.0 or less. It is necessary to satisfy 1). This is because the oxide inclusions contained in the steel are more stably controlled from a hard Al ₂ O ₃ rich composition to a CaO—SiO ₂ —Al ₂ O ₃ (—TiO ₂ ) based low melting point oxide. When the value of Al / Ca exceeds 1.0 under the necessary conditions, the oxide inclusions become a hard oxide having a composition rich in Al ₂ O _3, so that the tool life is reduced. Therefore, the ratio of Al to Ca content (Al / Ca) is 1.0 or less, that is, the expression (1) is satisfied.

Note that the value of Al / Ca is 0 in order to make the oxide inclusions contained in the steel more stable and reliable into CaO—SiO ₂ —Al ₂ O ₃ (—TiO ₂ ) -based low melting point oxides. .75 or less is preferable.

  For the above reasons, the chemical composition of the steel for machine structural use according to the present invention (1) includes the elements from C to N in the above-mentioned range, the balance is composed of Fe and impurities, and Al, Ti and O in the impurities are The above range was satisfied, and Al and Ca were defined to satisfy the above formula (1).

  In addition, the chemical composition of the steel for machine structural use according to the present invention may be one or two of Cr: 2.5% or less and V: 0.5% or less instead of part of Fe as necessary. Seeds can be included. That is, one or two of Cr and V may be contained as optional elements instead of a part of Fe.

  Hereinafter, the above optional elements will be described.

Cr: 2.5% or less Cr has an effect of improving the strength of steel, and is an element preferably added to increase the strength of steel for machine structural use. However, if the Cr content increases, especially exceeding 2.5%, not only the strength improvement effect is saturated but also machinability is deteriorated. Therefore, the content when Cr is contained is set to 2.5% or less.

  In order to reliably obtain the effect of Cr, the content is preferably 0.02% or more, and more preferably 0.05% or more. Therefore, the more desirable Cr content for improving the strength is 0.02 to 2.5%, and the still more desirable Cr content is 0.05 to 2.5%.

  When good machinability is required stably, the upper limit of the Cr content is preferably 1.5%, more preferably 0.5%.

V: 0.5% or less V precipitates as fine nitrides and carbonitrides and has the effect of improving the strength of steel. However, if the V content increases, especially exceeding 0.5%, not only the above effects are saturated, but too much nitride and carbide are formed, resulting in deterioration of the machinability of the steel and toughness. Also decreases. Therefore, the content when V is contained is set to 0.5% or less.

  In order to surely obtain the effect of V described above, the content is preferably 0.05% or more, and more preferably 0.07% or more. Therefore, the more desirable V content for improving the strength is 0.05 to 0.5%, and the still more desirable V content is 0.07 to 0.5%.

  When good machinability is required stably, the upper limit of the V content is preferably 0.3%.

  Said Cr and V can be contained only in any 1 type or 2 types of composites.

  For the above reason, the chemical composition of the steel for machine structure according to the present invention (2) is replaced with a part of Fe of the steel for machine structure according to the present invention (1), Cr: 2.5% or less and V: It was specified to contain one or two of 0.5% or less.

  In the present invention, Ni, Cu and Mo are all acceptable as impurities as long as they are up to about 0.05%.

(B) MnS-based inclusions present in a cross section parallel to the processing direction Many MnS-based inclusions existing in a cross section parallel to the processing direction are elongated in the processing direction or have an unspecified shape. As already described, “parallel to the processing direction” means, for example, that the processing is parallel to the rolling direction when the processing is rolling, and that the processing is parallel to the forging axis when the processing is forging. Point to.

  When investigating the shape of MnS inclusions, the cross-section is mirror-polished, a photograph is taken by observation with an optical microscope of about 100 times, the area is obtained by an image analysis technique, and the diameter when the area is converted into a circle That is, an object having an equivalent circular diameter of 4 μm or more is targeted. This is because an MnS-based inclusion having an equivalent circular diameter of less than 4 μm has little effect on chip disposal.

  In determining the equivalent circular diameter, MnS inclusions having the same composition, which are clearly continuous inclusions but can be judged to have been separated during mirror polishing, are one MnS inclusion. Treat as. Here, the composition of the MnS-based inclusion is analyzed by, for example, EPMA or an apparatus capable of analyzing a minute portion equivalent thereto.

  An MnS-based inclusion having an equivalent circular diameter of 4 μm or more present in a cross section parallel to the processing direction must satisfy the above formulas (2) and (3). This will be described below.

Aspect ratio (L / W): less than 3 First, the value of each L / W is obtained by setting the major axis and minor axis of each MnS inclusion to L and W, respectively. And the average value of the aspect-ratio is calculated | required about the observed MnS type inclusion. And the average value of this aspect ratio must be less than 3, that is, the expression (2) must be satisfied.

  This means that in order to improve chip disposal, it is necessary that the amount of deformation of MnS during deformation in the shear region at the time of chip generation is large, but the aspect ratio (L / W) as an average value is If it is less than 3, the effect of concentrating the deformation on MnS with respect to the shear deformation at the time of chip generation occurs.

  In the case of stretched MnS having an average aspect ratio exceeding 3, it is considered that the amount of deformation during shear deformation is small, the contribution to shear deformation is small, and the effect of improving chip disposal is poor. It is done.

(N1 + n2) / n values: 0.2 or less number of MnS-based inclusions of area 1 mm ² per a n, the n1 among the MnS-based inclusions per ² above area 1 mm, Ca is in mass% 0.5 %, The value of (n1 + n2) / n is 0.02 or less, where n2 is the number of MnS-based inclusions per 1 mm ² of MnS inclusions containing different types of inclusions. Therefore, it is necessary to satisfy the expression (3).

  This is because when the total proportion of MnS in which Ca is 0.5% or more by solid solution and MnS containing different types of inclusions exceeds 0.2, deformation is caused by shear deformation during chip formation. This is because the effect of improving chip disposability by MnS having a large ability is hindered and the chip disposability is lowered. On the other hand, when the formula (3) is satisfied, good chip disposal is obtained.

  In order to obtain more stable and good chip control, the value of (n1 + n2) / n is desirably 0.1 or less. The smaller the value of (n1 + n2) / n, the better.

  For the above reasons, in the steel for machine structure according to the present invention, MnS inclusions having an equivalent circular diameter of 4 μm or more existing in a cross section parallel to the processing direction satisfy the above formulas (2) and (3). It was stipulated to do.

The “foreign inclusion” included in the MnS inclusions refers to inclusions other than sulfides, such as oxides such as Al ₂ O ₃ and nitrides such as TiN.

  Further, when one MnS-based inclusion simultaneously satisfies “Ca is solid solution at 0.5% by mass or more” according to n1 and “contains different inclusions” according to n2, Since it prevents counting, it counts as n1 "Ca is dissolved in 0.5% or more by mass%", and for "contains different inclusions", it is completely inside MnS. It is not necessary to include different kinds of inclusions, and even if the inclusions are simply in contact with each other, it is counted as one according to this definition.

The above-mentioned n, n1, and n2 are specifically prepared by mirror-polishing a cross section (10 mm × 10 mm) of a test piece taken in parallel with the processing direction and observing it with an optical microscope at a magnification of 100 times. Take a picture of the field of view, analyze the image to determine the area of the MnS inclusions, and calculate the diameter when the area is converted into a circle, that is, by counting the number of MnS inclusions with an equivalent circular diameter of 4 μm or more. Shall. The observation area in the 64 fields of view is 1.18 mm ² in total.

  Hereinafter, the melting procedure for obtaining the composition and form of the MnS inclusions defined in the present invention will be described as an example. In addition, the manufacturing method of the steel material for machine structures of this invention is not limited to the manufacturing method by the following procedures.

  <1> First, excess oxygen is adjusted by vacuum-treating molten steel containing a small amount of carbon.

  <2> After that, the main C, Si, Mn, S and other elements are adjusted so as to have a target content, and then the dissolved oxygen content is preliminarily adjusted. At this time, the contents of Al and Ti mixed from the raw materials and the refractory are managed so as to be 0.005% or less as described above.

  <3> Finally, it is treated with Ca and cast into an ingot or slab.

  The reason why the manufacturing method of the above procedure is desirable is as follows.

After removing excess oxygen from the molten steel in <1> and managing Al and Ti during the component adjustment of <2>, the composition of the oxide generated by deoxidation with Mn and Si during the component adjustment of <2> Al ₂ O ₃ should not be excessive.

In addition, by adding Ca immediately before casting of <3>, the oxide generated in the stage of <2> is converted to a low melting point oxide containing Ca, that is, CaO—SiO ₂ —Al ₂ O _3. The (-TiO ₂ ) -based oxide is intended to prevent the added Ca from dissolving in MnS.

Note that the presence of an oxide rich in Al ₂ O ₃ reduces the tool life and chip disposal, so Al contained in the stage <2> must be 0.005% or less. As a result, almost no Al ₂ O ₃ rich oxide is generated.

  Further, since Ti has high reactivity with O (oxygen) and N (nitrogen), Ti contained in the stage <2> needs to be 0.005% or less.

Subsequent treatment of Ca in <3> is added in the form of an alloy such as calcium silicon or alloy iron, but Ca hardly dissolves in the molten steel, and reacts with oxygen in the molten steel and dispersed oxides. Thus, a CaO—SiO ₂ —Al ₂ O ₃ (—TiO ₂ ) -based oxide is formed.

  In order to reduce the aspect ratio of the MnS-based sulfide in which the amount of Ca dissolved in the deoxidation adjustment process is adjusted, it is better to reduce the cooling rate during casting. For example, it is preferable to use a ceramic or sand mold with high heat insulation as the material of the mold, and the specific cooling rate is preferably 8 ° C./min or less at the center of the steel ingot or slab during solidification. .

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

  Steels A1 to A6 and steels B1 to B9 having the chemical compositions shown in Table 1 were melted using a 150 kg vacuum melting furnace to form an ingot having a diameter of 200 mm.

  In addition, steel A1-A6 and steel B1-B6 in Table 1 are steel which has a chemical composition in the range prescribed | regulated by this invention. On the other hand, steel B7 is a steel of a comparative example in which the value of Al / Ca, steel B8 has a Cr content, and steel B9 has a V content outside the conditions defined in the present invention.

  Among the steels described above, steels A1 to A6 and steels B7 to B9 were subjected to Ca treatment after the alloy components were adjusted, and cast into a ceramic mold.

  On the other hand, the steel B1 was subjected to Ca treatment before adjustment of other components, and cast into an iron mold.

  Steel B6 was subjected to Ca treatment before adjustment of other components and cast into a ceramic mold.

  Steels B2 to B5 were finally subjected to Ca treatment after the alloy components were adjusted, and cast into iron molds.

  The cooling rate at the center of the steel ingot during solidification was 2 ° C./min when using a ceramic mold, and 10 ° C./min when using an iron mold. .

  Subsequently, the steel ingot was heated to 1250 ° C. and then hot-worked (hot forging) to finish at 1000 ° C. or higher was performed to produce a round bar having a diameter of 65 mm. The hot forging was allowed to cool in the atmosphere.

Parallel to the training axis centered on the position of 16.25 mm from the surface of the round bar thus obtained (position of 1/2 the radius of the round bar, hereinafter referred to as “R / 2 part position”). The cross-section (10 mm × 10 mm) of the test piece collected in the above is mirror-polished, and an optical microscope is observed to photograph 64 fields of view of one sample at a magnification of 100 times, and image analysis is performed to determine the area of the MnS inclusions. The diameter when the area was converted into a circle, that is, the number of MnS inclusions having an equivalent circle diameter of 4 μm or more was counted. The observation area of the 64 visual fields is 1.18 mm ² in total.

  Next, in each field of view, 20 arbitrarily selected MnS inclusions were selected quantitatively using EPMA, their compositions were determined, and n1 and n2 were counted.

  In addition, the 65 mm diameter round bar produced as described above is peeled to 60 mm, further cut to a length of 400 mm to produce a cylindrical specimen, and using this, turning is performed under the following conditions, The chip disposal was investigated.

  That is, using a P20 type carbide tool, with a constant cutting speed of 150 m / min in a non-lubricated state, the feed is 5 to 0.05 mm / rev every 0.05 mm / rev in the range of 0.05 to 0.25 mm / rev. Turning was performed under a total of 15 conditions where the conditions were changed and the depth of cut was changed to three conditions of 1.0 mm, 1.5 mm and 2.0 mm, and the chip processing performance was comprehensive with the appearance of the chips under each cutting condition. Evaluated. The evaluation method is as follows.

  As shown in FIG. 1, the chip shape is classified into five stages of “0 to 2.0 points” in appearance, and the total number of turning operations under the 15 conditions, that is, 0 points (0 points × 15 conditions) The higher the total point in the range of ˜30 points (2.0 points × 15 conditions), the better the chip disposal.

  The 400 mm long cylindrical specimen peeled to 60 mm was turned under the following conditions, and the tool wear was also investigated.

  That is, using a P20 type carbide tool, turning with no lubrication, cutting speed 150m / min, feed 0.15mm / rev, cutting depth 1.5mm, flank after 5 minutes machining The amount of wear was measured to evaluate the tool life.

  Table 2 summarizes the results of each of the above investigations.

  From Table 2, in the case of the present invention example (test numbers 1 to 6) in which the MnS inclusions existing in the cross section parallel to the chemical composition and the processing direction, that is, the cross section parallel to the forging axis satisfy the conditions defined in the present invention. It is clear that it is excellent in chip disposal and tool life.

  That is, in the case of test numbers 1 to 6, the chip processing property is 18 points or more, and the chip processing property is good, and the tool wear amount is as small as 0.12 to 0.17 mm. Long life.

  On the other hand, in the case of a comparative example (test numbers 7 to 15) that deviates from the conditions specified in the present invention, at least one of the properties of chip disposal and tool life is inferior.

  That is, the chemical composition satisfies the conditions specified in the present invention, but when the MnS-based inclusions present in the cross section parallel to the machining direction are test numbers 7 to 12 outside the conditions specified in the present invention, the tool wear amount is Although the tool life is as small as 0.11 to 0.16 mm, the chip processing property is inferior at 14.0 to 16.5 points.

  MnS inclusions present in the cross section parallel to the processing direction satisfy the conditions specified in the present invention, but in the case of test number 13 in which the value of Al / Ca in the chemical composition deviates from the conditions specified in the present invention, chips Although the point indicating the processability is 17.5 points, which has good chip processability, the tool wear amount is as large as 0.35 mm and the tool life is short.

  In addition, MnS inclusions present in the cross section parallel to the processing direction satisfy the conditions specified in the present invention, but in the case of test number 14 in which the Cr content in the chemical composition deviates from the conditions specified in the present invention, The point that indicates the scrap disposal property is 15.0 points, which is inferior in the chip disposal property, and the tool wear amount is as large as 0.41 mm, and the tool life is short.

  Similarly, the MnS inclusions present in the cross section parallel to the processing direction satisfy the conditions specified in the present invention, but in the case of test number 15 in which the V content in the chemical composition deviates from the conditions specified in the present invention, The point indicating chip disposal is 15.5 points, which is inferior to chip disposal, and the tool wear amount is as large as 0.38 mm, resulting in a short tool life.

  Although the steel for machine structure of the present invention does not contain Pb, it is excellent in machinability, especially chip disposal and tool life. Therefore, by using this steel for machine structure as a material of a part that requires cutting, the manufacturing cost of the part can be greatly reduced.

It is a figure which classifies and shows a chip shape in five stages of “0 to 2.0 points” in its appearance for evaluation of chip disposal.

Claims (2)

In mass%, C: 0.15-0.6%, Si: 0.1-2.0%, Mn: 0.5-2.0%, P: 0.1% or less, S: 0.015 -0.12%, Ca: 0.0005-0.003% and N: 0.02% or less, with the balance being a chemical composition comprising Fe and impurities, Al in impurities: 0.005% or less, Ti: 0.005% or less and O: 0.005% or less, Al and Ca satisfy the following formula (1), and an equivalent circular diameter existing in a cross section parallel to the processing direction is 4 μm or more. A steel material for machine structure, characterized in that the material satisfies the following formulas (2) and (3):
Al / Ca ≦ 1.0 (1)
Aspect ratio (L / W) <3 (2)
(N1 + n2) /n≦0.2 (3)
Here, the element symbol in the formula (1) represents the content in mass% of the element.
In addition, L and W in the formula (2) respectively represent the major axis and minor axis of the target MnS-based inclusion, and the aspect ratio refers to the aspect ratio as an average value.
Furthermore, n, n1, and n2 in the formula (3) each represent the number of the following MnS inclusions.
n: Number of MnS-based inclusions per 1 mm ² area n1: Number of MnS-based inclusions in which Ca is solid-solved by 0.5% or more per 1 mm ² area n2: MnS-based inclusions Number of objects including different types of inclusions per 1 mm ² area It replaces with a part of Fe, and contains 1 type or 2 types of Cr: 2.5% or less and V: 0.5% or less, The steel material for machine structures of Claim 1 characterized by the above-mentioned.

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