JP2011080100A - Steel for machine structural use, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for machine structural use exhibiting excellent machinability (particularly, elongation of a tool life) in interrupted cutting at low speeds using high speed tools (e.g., hob working), further exhibiting excellent machinability (particularly, the elongation of a tool life) in continuous cutting at high speeds using cemented carbide tools (e.g., turning), as well as excellent impact performance, even after being subjected to a heat treatment such as quenching and tempering, and to provide a method for producing the same. <P>SOLUTION: In the steel having a composition comprising 0.05 to 0.8% C, 0.03 to 2% Si, 0.2 to 1.8% Mn, 0.1 to 0.5% Al, 0.0005 to 0.008% B and 0.002 to 0.015% N, and satisfying &le;0.03% (excluding 0%) P, &le;0.03% (excluding 0%) S and &le;0.002% (excluding 0%) O, and the balance iron with inevitable impurities, mass ratio between BN and AlN (BN/AlN) precipitated in the steel is controlled to 0.020 to 0.2. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a steel for machine structure used for manufacturing machine structural parts by cutting, and a method for manufacturing the same.

  Machine structural parts such as gears, shafts, pulleys, constant velocity joints, etc. used in various gear transmissions including automobile transmissions and differential gears, as well as crankshafts and connecting rods, are forged into steel for machine structural use. In general, a final shape (part shape) is finished by performing a cutting process. Since the cost required for cutting accounts for a large proportion of the total production cost, the machine structural steel is required to have good machinability.

  Of the mechanical structural parts described above, in particular when cutting gears, cutting with a hob is generally performed, and the cutting in this case is called intermittent cutting. As a tool used for hobbing, a high-speed tool steel coated with AlTiN or the like (hereinafter sometimes abbreviated as “high-speed tool”) is the current mainstream. However, gear cutting by hobbing (intermittent cutting) using a high-speed tool is low speed (specifically, cutting speed of about 150 m / min or less) and low temperature (specifically, about 200 to 600 ° C.), Because of the intermittent cutting, there is an adverse effect that it is easy to come into contact with air and the tool is likely to be oxidized and worn. Therefore, machine structural steels used for low-speed intermittent cutting such as hobbing are required to extend the tool life especially among machinability.

  As a technique for improving the intermittent cutting performance, Patent Document 1 proposes an intermittent high-speed cutting steel containing Al: 0.04 to 0.20% and O: 0.0030% or less. In this technique, Al oxide is deposited on the tool surface by intermittently cutting steel with a high Al content at a high speed, thereby improving the tool life. However, the steel for intermittent high-speed cutting is often intended for high-speed intermittent cutting with a cutting speed of 200 m / min or more, and low-speed intermittent cutting such as hobbing is not intended.

  On the other hand, in addition to the above-mentioned high-speed tools, there are also tools used for cutting, such as cemented carbide with a coating such as AlTiN (hereinafter sometimes referred to as “carbide tool”). This carbide tool is often applied to continuous cutting such as turning because there is a problem that “chip” tends to occur when applied to a normalizing material. Continuous cutting by turning or the like is usually performed at a cutting speed exceeding 150 m / min, and in many cases at a high speed of 200 m / min or more.

  Thus, the cutting mechanism is different between the intermittent cutting and the continuous cutting, and a tool corresponding to each cutting is selected. However, it is desired that steel for machine structure as a work material exhibits good machinability in any cutting.

  By the way, after finishing to the final shape, surface hardening treatment such as carburizing treatment and carbonitriding treatment (including atmospheric pressure, low pressure, vacuum and plasma atmosphere) is performed, and further heat treatment such as quenching and tempering and induction hardening is performed. To a predetermined strength. However, when affected by heat, the toughness decreases and the impact properties may deteriorate.

As a technique for improving impact characteristics, Patent Document 2 proposes a steel for machine structure containing Al in a range of more than 0.1% and 0.3% or less. In this document, the machinability and impact characteristics can be improved by reducing the amount of solid solution N, and the proper amount of solid solution Al and AlN effective for the machinability improvement effect can be secured by optimizing the Al content. It is disclosed that effective cutting performance can be obtained for a wide cutting speed range from low speed to high speed. In this document, the impact characteristics of mechanical structural steel are evaluated by measuring the absorbed energy by the Charpy impact test. However, the absorbed energy achieved in this document does not reach 50 J / cm ² , and further improvement in impact characteristics is required.

  The present applicant also exhibits excellent machinability both in intermittent cutting with a high-speed tool and continuous cutting with a carbide tool, and also has excellent impact characteristics even when tempering after carburizing and oil quenching. Patent Document 3 proposes a machine structural steel showing In this technique, the machinability and impact characteristics are improved by appropriately controlling the content of Cr and Al and the ratio of these contents.

JP 2001-342539 A JP 2008-13788 A JP 2009-30160 A

  An object of the present invention is to provide excellent machinability (especially a tool) in intermittent cutting (for example, hobbing) at a low speed using a high-speed tool by a method different from the above-mentioned Patent Document 3 proposed previously by the present applicant. (Extended tool life) and excellent machinability (especially extended tool life) in high-speed continuous cutting (for example, turning) using carbide tools, and further heat treatment such as quenching and tempering. It is an object of the present invention to provide a mechanical structural steel exhibiting excellent impact characteristics even after being subjected to the treatment, and a method for producing the same.

  The mechanical structural steel according to the present invention that has solved the above problems is C: 0.05 to 0.8% (meaning of mass%, the same shall apply hereinafter), Si: 0.03 to 2%, Mn: 0 2 to 1.8%, Al: 0.1 to 0.5%, B: 0.0005 to 0.008%, N: 0.002 to 0.015%, P: 0.03% The following are satisfied (excluding 0%), S: 0.03% or less (not including 0%), O: 0.002% or less (not including 0%), and the balance is made of iron and inevitable impurities. It is steel and has a gist in that the mass ratio (BN / AlN) of BN and AlN precipitated in the steel is 0.020 to 0.2.

  In the BN precipitated in the steel, the number ratio of BN precipitated in the prior austenite grain boundaries and BN precipitated in the prior austenite grains (grain boundary BN / intragrain BN) is 0.50 or less. It is preferable.

The steel for machine structure is still another element,
(A) Cr: 3% or less (excluding 0%),
(B) Mo: 1% or less (excluding 0%),
(C) Nb: 0.15% or less (excluding 0%),
(D) Zr: 0.02% or less (not including 0%), Hf: 0.02% or less (not including 0%), Ta: 0.02% or less (not including 0%), and Ti : At least one selected from the group consisting of 0.02% or less (excluding 0%),
(E) V: 0.5% or less (not including 0%), Cu: 3% or less (not including 0%), and Ni: 3% or less (not including 0%) At least one,
Etc. may be contained.

  The steel for machine structural use according to the present invention once heats steel satisfying the above composition to 1100 ° C. or higher, then holds it in a temperature range of 900 to 1050 ° C. for 150 seconds or more, and then cools it to 900 ° C. to 700 ° C. Up to an average cooling rate of 0.05 to 10 ° C./second. In this invention, after heating the steel which satisfies the said component composition to 1100 degreeC or more, while hot-working at 1000 degreeC or more, it is good also considering the holding time in a 900-1050 degreeC temperature range as 150 seconds or more.

  The present invention includes a machine structural component obtained using the above steel for machine structure.

  According to the present invention, BN is positively precipitated while suppressing precipitation of AlN, and the mass ratio of BN and AlN (BN / AlN) precipitated in the steel is adjusted to an appropriate range. , Steel for machine structural use that exhibits excellent machinability (especially extension of tool life) in both intermittent cutting at low speed and continuous cutting at high speed, and excellent shock characteristics even after heat treatment, and A manufacturing method can be provided.

  The inventors have demonstrated excellent machinability (particularly extension of tool life) in both intermittent cutting at low speed and continuous cutting at high speed, and excellent impact even when subjected to heat treatment such as quenching and tempering. In order to provide mechanical structural steels that exhibit properties, investigations have been made from various angles. As a result, both the intermittent cutting and continuous cutting can be achieved if the mass ratio of BN and AlN (BN / AlN) deposited in the steel is appropriately controlled while appropriately adjusting the chemical composition of the mechanical structural steel. The present invention has been completed by finding that it exhibits good machinability and can improve impact characteristics after heat treatment.

  First, after describing the chemical composition of the steel for machine structural use according to the present invention, the mass ratio of BN and AlN that characterizes the present invention will be described.

  The steel for mechanical structure of the present invention is C: 0.05 to 0.8%, Si: 0.03 to 2%, Mn: 0.2 to 1.8%, Al: 0.1 to 0.5% , B: 0.0005 to 0.008%, and N: 0.002 to 0.015%, P: 0.03% or less (excluding 0%), S: 0.03% or less ( 0% is not included) and O: 0.002% or less (not including 0%) is satisfied. The reason for specifying such a range is as follows.

  C is an element necessary for ensuring the strength, and it is necessary to contain 0.05% or more. Preferably it is 0.1% or more, More preferably, it is 0.15% or more. However, when the C content is excessive, the hardness is excessively increased and the machinability and toughness are reduced. Therefore, the C content is 0.8% or less. Preferably it is 0.6% or less, More preferably, it is 0.5% or less.

  Si is an element that acts as a deoxidizing element and improves internal quality, and needs to be contained by 0.03% or more. Preferably it is 0.1% or more, More preferably, it is 0.15% or more. However, when the Si content is excessive, hot workability and cold workability when processing into a part shape deteriorate, grain boundary oxidation during carburizing or carbonitriding performed after cutting into a part shape, etc. Abnormal tissue may be generated. Therefore, the amount of Si needs to be 2% or less, preferably 1.5% or less, more preferably 1% or less, and still more preferably 0.6% or less.

  Mn is an element that improves the hardenability and increases the strength, and should be contained by 0.2% or more. Preferably it is 0.4% or more, More preferably, it is 0.5% or more. However, if the Mn content is excessive, the hardenability is excessively improved, and even after normalization, a supercooled structure is generated and the machinability is lowered. Therefore, the amount of Mn needs to be 1.8% or less. Preferably it is 1.5% or less, More preferably, it is 1% or less.

  Al is an element necessary for improving machinability when intermittent cutting is performed by being present in a solid solution state in steel. In addition, AlN precipitated by bonding with N suppresses abnormal growth of crystal grains during carburizing or carbonitriding after cutting into a part shape, and prevents impact characteristics from deteriorating due to reduced toughness. To contribute to. Al is an element having a deoxidizing action and is an element necessary for improving the internal quality. Therefore, in the present invention, Al is contained in an amount of 0.1% or more, preferably 0.13% or more. However, if Al is contained excessively and a large amount of AlN is precipitated, the machinability at the time of continuous cutting deteriorates. Excessive AlN also reduces hot workability when processing into a part shape. Therefore, the Al content is 0.5% or less, preferably 0.4% or less, and more preferably 0.35% or less.

  B is an element that contributes to improving both the machinability when intermittent cutting and the machinability when continuous cutting are performed by combining with N to precipitate BN in the steel. Moreover, since the amount of solute N can be adjusted in a small direction by precipitating BN, the hot workability when processing into a component shape can also be improved. Further, B is an element that improves the hardenability and increases the grain boundary strength and contributes to the improvement of the strength of the machine structural component when heat treatment such as quenching and tempering is performed after the cutting. Therefore, the amount of B must be 0.0005% or more. Preferably it is 0.0007% or more, More preferably, it is 0.0010% or more. However, when it contains excessively, it will become hard too much and machinability will fall. Therefore, the B content needs to be 0.008% or less, preferably 0.006% or less, more preferably 0.0035% or less.

  N is an element that combines with B to precipitate BN in the steel and contribute to improving machinability during intermittent cutting and continuous cutting as described above. N is an element that contributes to preventing abnormal growth of crystal grains during carburizing or carbonitriding after bonding to Al and precipitating AlN in steel and cutting into a part shape. It acts to improve impact characteristics by suppressing the decrease in toughness. In order to exert such an effect, the N amount is set to 0.002% or more. Preferably it is 0.003% or more, More preferably, it is 0.004% or more. However, if N is contained excessively and a large amount of AlN is precipitated, the machinability at the time of continuous cutting deteriorates. Moreover, when the amount of precipitation of AlN increases, hot workability will fall. Therefore, the N content is 0.015% or less, preferably 0.010% or less, more preferably 0.008% or less.

  P is an impurity element that is inevitably contained, and is reduced as much as possible in order to promote the occurrence of cracks during hot working. Therefore, in the present invention, the P amount is 0.03% or less, preferably 0.02% or less, more preferably 0.015% or less. In addition, it is industrially difficult to make P amount 0%.

  S is an element that contributes to improving the machinability by generating MnS inclusions in the steel. However, when MnS inclusions are excessively contained, ductility and toughness are lowered. Since the MnS inclusions easily extend in the rolling direction during rolling, the toughness (lateral toughness) in the direction perpendicular to the rolling direction is deteriorated. Therefore, the S content is 0.03% or less, preferably 0.02% or less. In addition, since S is an impurity element contained unavoidable, it is industrially difficult to make S amount 0%.

  O is an inevitably contained impurity element, which forms coarse oxide inclusions and adversely affects machinability, ductility, toughness, hot workability, and the like. Therefore, the O content is 0.002% or less, preferably 0.0015% or less. Note that it is industrially difficult to set the O amount to 0%.

  The steel for machine structure of the present invention satisfies the above component composition, and the balance is iron and inevitable impurities.

The steel for machine structural use of the present invention is further added as another element,
(A) Cr: 3% or less (excluding 0%),
(B) Mo: 1% or less (excluding 0%),
(C) Nb: 0.15% or less (excluding 0%),
(D) Zr: 0.02% or less (not including 0%), Hf: 0.02% or less (not including 0%), Ta: 0.02% or less (not including 0%), and Ti : At least one element selected from the group consisting of 0.02% or less (excluding 0%),
(E) V: 0.5% or less (not including 0%), Cu: 3% or less (not including 0%), and Ni: 3% or less (not including 0%) At least one element,
Etc. may be contained.

  (A) Cr is an element that improves hardenability and increases strength. In addition, it is an element that also acts to improve the machinability when intermittently cut by adding together with Al. In order to exert such effects, it is preferable to contain Cr by 0.1% or more. Preferably it is 0.3% or more, More preferably, it is 0.7% or more. However, when it contains excessively, a coarse carbide | carbonized_material will be produced | generated or a supercooled structure | tissue will be produced | generated and machinability will be deteriorated. Therefore, the Cr content is preferably 3% or less. More preferably, it is 2% or less, More preferably, it is 1.6% or less.

  (B) Mo is an element that enhances hardenability and suppresses generation of an incompletely hardened structure. Such an effect increases as the Mo content increases, but is preferably 0.01% or more, more preferably 0.05% or more, and still more preferably 0.1% or more. However, when it contains excessively, a supercooled structure | tissue will produce | generate after normalization and machinability will fall. Therefore, the Mo amount is preferably 1% or less. More preferably, it is 0.8% or less, More preferably, it is 0.5% or less.

  (C) Nb combines with C and N to form carbides, nitrides, and carbonitrides, and when these compounds are carburized or carbonitrided after being cut into a part shape, It acts to suppress abnormal growth and improves impact characteristics. Such an effect increases as the amount of Nb increases, but it is preferable to contain 0.05% or more for effective display. However, if it is contained excessively, hard carbides, nitrides and the like are precipitated excessively and machinability is lowered. Therefore, the Nb content is preferably 0.15% or less, more preferably 0.13% or less.

  (D) Zr, Hf, Ta, and Ti are elements that suppress the abnormal growth of crystal grains, as in the case of Nb, and contribute to improvement of impact characteristics. Such an effect increases as the content of these elements increases. However, in order to effectively exhibit these elements, it is preferable to contain 0.002% or more of each element alone. More preferably, each element is individually 0.005% or more. However, if it is contained excessively, a large amount of hard carbides, nitrides, etc. are precipitated and the machinability is lowered. Therefore, each element is preferably 0.02% or less by itself. More preferably, it is 0.015% or less. Zr, Hf, Ta, and Ti may contain two or more elements selected arbitrarily. When two or more elements are contained, the total amount is preferably 0.02% or less. The total amount is more preferably 0.015% or less.

  (E) V, Cu and Ni are elements that effectively act to improve the hardenability and increase the strength. These effects increase as the content of these elements increases, but in order to effectively exhibit these effects, V is 0.05% or more, Cu is 0.1% or more, and Ni is 0.3% or more. It is preferable. However, if excessively contained, a supercooled structure is formed or ductility and toughness are lowered. Therefore, V is preferably 0.5% or less, Cu is 3% or less, and Ni is preferably 3% or less. More preferably, V is 0.3% or less, Cu is 2% or less, and Ni is 2% or less.

  In the present invention, in addition to adjusting the chemical composition of the steel for machine structural use to the above specified range, the mass ratio (BN / AlN) of BN and AlN precipitated in the steel is 0.020 to 0.2. It is important that

  That is, in the present invention, a relatively large amount of Al is contained in the range of 0.1 to 0.5%, and Al is present in a solid solution state in the steel, thereby improving the machinability when intermittent cutting is performed. ing. However, if a large amount of Al is contained, the amount of solute Al increases, but some Al bonds with N in the steel and precipitates AlN, which promotes tool wear such as turning and drilling, and the tool. Shorten the service life. Since AlN is a hard particle, it promotes tool wear and deteriorates the tool life (machinability) particularly when continuous cutting is performed.

  Therefore, in the present invention, N in the steel is positively combined with B to precipitate BN in the steel, thereby suppressing the precipitation of AlN, and the mass ratio of BN and AlN precipitated in the steel ( (BN / AlN) is set to 0.020 to 0.2. By setting the BN / AlN ratio to 0.020 to 0.2, both the machinability when performing intermittent cutting and the machinability when performing continuous cutting can be improved, and the impact characteristics after heat treatment can also be improved.

  When BN / AlN is less than 0.020, a large amount of AlN is precipitated as compared with BN, so that machinability when continuously cut is deteriorated. Accordingly, BN / AlN is set to 0.020 or more. Preferably it is 0.025 or more, More preferably, it is 0.030 or more.

  Although it is preferable that the value of BN / AlN is large, if the AlN content becomes too small and the BN / AlN exceeds 0.2, the impact characteristics after the heat treatment deteriorate. Therefore, BN / AlN is 0.2 or less. Preferably it is 0.15 or less, More preferably, it is 0.1 or less, More preferably, it is 0.08 or less.

  BN precipitated in steel can be quantified by combining, for example, electrolytic extraction, acid dissolution, and absorptiometry. On the other hand, AlN precipitated in steel can be quantified by, for example, bromine-methyl acetate method.

  Among the BN precipitated in the steel, the number ratio of BN precipitated in the prior austenite grain boundaries and BN precipitated in the prior austenite grains (grain boundary BN / intragrain BN) is 0.50 or less. Preferably there is. By reducing the number of BN precipitated in the prior austenite (hereinafter sometimes referred to as old γ) grain boundaries and increasing the number of BN precipitated in the old γ grains, the shape of the part is particularly improved. Even if heat treatment such as quenching and tempering is performed after cutting, the impact characteristics can be further improved without deterioration. The grain boundary BN / intragrain BN is more preferably 0.45 or less, and further preferably 0.40 or less. The lower limit of grain boundary BN / intragrain BN is about 0.30.

  The number of BN precipitated in the old γ grain boundary and the number of BN precipitated in the old γ grain boundary are determined using the energy dispersive X-ray analyzer (EDS) attached to the scanning electron microscope (SEM). It can be measured by using the existing position and component composition analysis.

  Next, the method by which the machine structural steel according to the present invention can be manufactured will be described.

  The steel for machine structural use according to the present invention heats steel satisfying the above composition to 1100 ° C. or higher, holds it in a temperature range of 900 to 1050 ° C. for 150 seconds or more, and thereafter cools from 900 ° C. to 700 ° C. If the average cooling rate is set to 0.05 to 10 ° C./second, it can be produced. Further, after heating the steel satisfying the above component composition to 1100 ° C. or higher and hot working at 1000 ° C. or higher, if the holding time in the temperature range of 900 to 1050 ° C. is 150 seconds or longer, the subsequent cooling In the process, BN can be positively precipitated in the old γ grains, which is more preferable. The reason for defining such a range will be described.

[Heating above 1100 ° C]
It is necessary to once heat steel satisfying the above component composition to 1100 ° C. or higher to re-dissolve precipitates such as AlN and BN contained in the steel. That is, in steel containing 0.1% or more of Al, the solid solution state and precipitation state of Al, B, and N vary greatly depending on the production conditions. Therefore, in the present invention, the steel is heated to 1100 ° C. or more. Then, AlN and BN contained in the steel are re-dissolved in the steel.

[Holding at 900 to 1050 ° C for 150 seconds or more]
After heating to 1100 ° C. or higher, BN can be precipitated by holding at a temperature range of 900 to 1050 ° C. for 150 seconds or longer. That is, since the precipitation temperature of AlN is approximately less than 900 ° C. and the precipitation temperature of BN is approximately 1050 ° C. or less, BN can be selectively precipitated by maintaining in the temperature range of 900 to 1050 ° C.

  However, if the holding time is less than 150 seconds, the precipitation of BN does not proceed sufficiently, the BN becomes insufficient, and the machinability at the time of continuous cutting cannot be improved. Moreover, the impact characteristics after heat treatment are also deteriorated. Accordingly, the holding time is 150 seconds or longer, preferably 170 seconds or longer, more preferably 200 seconds or longer. The upper limit of the holding time is not particularly limited. However, even if the holding time is long, the precipitation amount of BN is saturated and the productivity is deteriorated. For example, the holding time is preferably 600 seconds or less.

  The holding in the temperature range of 900 to 1050 ° C. may be performed at a constant temperature, may be heated and / or cooled within this temperature range, and the holding time in the temperature range may be 150 seconds or more.

[Average cooling rate from 900 ° C to 700 ° C is 0.05 to 10 ° C / second]
After depositing BN while maintaining at 900 to 1050 ° C., the time for passing through the temperature range of 900 to 700 ° C. is shortened to suppress the precipitation of AlN and prevent BN from changing to AlN. In addition, the amount of BN deposited can be secured. That is, since AlN is thermodynamically more stable than BN in the temperature range of 900 to 700 ° C., even if BN is selectively precipitated in the high temperature range of 900 to 1050 ° C., the low temperature of 900 to 700 ° C. As the time for passing through the zone increases, BN changes to AlN, and the amount of BN deposited decreases. Therefore, the BN / AlN ratio cannot be controlled within the above range. Therefore, in this invention, the average cooling rate when cooling the low temperature range from 900 degreeC to 700 degreeC shall be 0.05 degreeC / second or more. Preferably it is 0.1 degree-C / second or more, More preferably, it is 0.5 degree-C / second or more, More preferably, it is 1 degree-C / second or more. However, if the average cooling rate in this temperature range is too large, a supercooled structure such as martensite or bainite is generated, and the machinability deteriorates. Therefore, the average cooling rate from 900 ° C. to 700 ° C. is 10 ° C./second or less. It is preferably 9.5 ° C./second or less, more preferably 8 ° C./second or less, further preferably 5 ° C./second or less, and particularly preferably 3 ° C./second or less.

[Hot processing at 1000 ° C or higher]
In this invention, after heating the steel which satisfies the said component composition to 1100 degreeC or more, while hot-working at 1000 degreeC or more, it is good also considering the holding time in a 900-1050 degreeC temperature range as 150 seconds or more. After heating to 1100 ° C or higher and re-dissolving AlN and BN, hot straining is performed at 1000 ° C or higher to introduce processing strain into the steel. This processing strain is the precipitation point of BN. In the subsequent cooling process, BN is more easily precipitated in the γ grains than in the γ grain boundaries. As a result, BN can be precipitated in the old γ grains, and the impact characteristics after heat treatment such as quenching and tempering can be further improved. The hot working is more preferably performed at 1050 ° C. or higher. The upper limit of the hot working temperature may be lower than the heating temperature. For example, hot working may be performed by hot forging.

  In addition, when performing the said hot working in the temperature range of 1000-1050 degreeC, the time which is performing hot working is added to the time of the holding performed in the said 900-1050 degreeC temperature range, and holding time is controlled. .

  The steel for machine structure according to the present invention thus obtained has an excellent machinability in both intermittent cutting at low speed and continuous cutting at high speed because the balance of BN and AlN is appropriately controlled. In particular, it extends the tool life).

  In addition, since the balance of BN and AlN is appropriately controlled in the machine structural steel of the present invention, the machine structural steel is obtained by cutting the mechanical structural steel into a part shape and then subjecting it to a heat treatment such as quenching and tempering. The structural component has excellent impact characteristics.

  The heat treatment condition may be a condition that is usually employed when manufacturing a machine structural component. For example, after heating to about 800 to 1000 ° C., quenching is performed, and then tempering is performed at about 150 to 600 ° C. for about 20 minutes to 1 hour.

  After cutting into a part shape and before heat treatment such as quenching and tempering, carburizing treatment or carbonitriding treatment may be performed according to a conventional method. At this time, the carburizing process or the carbonitriding process is preferably performed in the temperature range of 900 to 1050 ° C., for example. After carburizing or carbonitriding, heat treatment such as quenching and tempering may be performed under the above conditions.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

No. shown in Table 1 below. 150 kg of steel with a chemical composition other than 18-22 is melted in a vacuum induction furnace, cast into an ingot of upper surface: φ245 mm × lower surface: φ210 mm × length: 480 mm, and forged (soaking: about 1250 ° C. × about 3 hours, forged and heated : 1100 ° C. × about 1 hour) and cut, and processed into the following two types of forged materials (a) and (b) through a square material shape with a side of 150 mm × length of 680 mm.
(A) Plate material: thickness 30 mm, width 155 mm, length 100 mm
(B) Round bar: φ80mm, length 350mm

  The obtained (a) plate material and (b) round bar were heated and then cooled. When cooling, the temperature was maintained at 900 to 1050 ° C. for a predetermined time. Further, when cooling, the average cooling rate from 900 ° C. to 700 ° C. was changed. Table 2 below shows the heating temperature (° C.), the holding time (second) in the temperature range of 900 to 1050 ° C., and the average cooling rate (° C./second) from 900 ° C. to 700 ° C., respectively.

  On the other hand, No. 1 shown in Table 1 below. For steel with a chemical composition of 18-22, a square material shape with a side of 150 mm and a length of 680 mm was formed under the same conditions as above, then heated to 1200 ° C., and then forged from 150 mm square to φ80 mm at 1100 ° C. After hot working, the two types of forged materials (a) and (b) were processed and cooled. When cooling, the temperature was maintained at 900 to 1050 ° C. for a predetermined time. Further, when cooling, the average cooling rate from 900 ° C. to 700 ° C. was changed. Table 2 below shows the heating temperature (° C.), the holding time (second) in the temperature range of 900 to 1050 ° C., and the average cooling rate (° C./second) from 900 ° C. to 700 ° C., respectively.

  BN and AlN contained in the round bar after cooling were quantitatively analyzed, and the BN / AlN ratio was calculated as a mass ratio. Two samples collected from the same site were prepared and quantified by the following procedure for the BN amount and the AlN amount.

  The amount of BN contained in the sample was quantified by a combination of electrolytic extraction, acid dissolution, and absorptiometry. Specifically, the sample was electrolyzed with an AA electrolyte solution (methanol solution containing 10% by mass of acetylacetone and 1% by mass of tetramethylammonium chloride), and then filtered to collect an undissolved residue. The residue was decomposed with hydrochloric acid and nitric acid, and then heated and decomposed with sulfuric acid and phosphoric acid. Thereafter, boron is distilled as methyl borate according to JIS G1227 and absorbed by sodium hydroxide. The amount of boron contained in the absorbed methyl borate was quantified by methyl borate distillation separation curcumin spectrophotometry according to JIS G1227. The amount of N bound to the boron was calculated assuming that the quantified boron produced the entire amount of BN, and the amount of BN determined by adding the calculated amount of bound N to the boron amount was defined as the amount of BN.

  The amount of AlN contained in the sample was quantified by the bromine-methyl acetate method. Specifically, the sample is placed in a flask, heated and dissolved in bromine and methyl acetate at 70 ° C., filtered to collect an undissolved residue, this residue is thoroughly washed with methyl acetate, and then dried. Let The dried residue was distilled by adding sodium hydroxide to an ammonia distiller according to JIS G1228, and 0.1% boric acid was absorbed as an absorbent, and the resulting absorbent was amidosulfuric according to JIS G1228. Titration with a standard solution was performed, and the amount of AlN was quantified from the amount of N in the absorbing solution and the measured amount of the sample.

  Based on the quantitative results, the BN / AlN ratio was calculated as a mass ratio. The calculation results are shown in Table 2 below.

  Further, the component composition of precipitates observed in a scanning electron microscope (SEM) centered on a 10 mm position from the surface of the round bar after cooling and observed in the observation field is energy dispersive X-ray attached to the SEM. While analyzing using an analyzer (EDS), the number of BN existing in the old γ grain boundary and the number of BN existing in the old γ grain boundary were measured, and the number ratio of grain boundary BN / intra grain BN was calculated. . The number of BN was calculated by averaging the results of 10 fields of view with a detection limit of 0.1 μm in diameter and an observation magnification of 10,000 times. The calculation results are shown in Table 2 below.

  Next, using the plate material and the round bar material after cooling, the machinability when intermittently cut under the following conditions and the machinability when continuously cut were evaluated.

[Machinability evaluation during intermittent cutting (end mill cutting test)]
In order to evaluate the machinability at the time of interrupted cutting, the amount of tool wear during end milling was measured. In the end mill cutting test, after removing the scale of the plate material, the surface was ground by about 2 mm and used as a test piece (work material). Specifically, an end mill tool is attached to the main spindle of the machining center, and a test piece of 25 mm thickness × 150 mm width × 100 mm length manufactured as described above is fixed with a vise, and downcut processing is performed in a dry cutting atmosphere. Went. Detailed processing conditions are shown in Table 3 below. After performing 200 intermittent cuttings, the tool surface was observed 100 times under an optical microscope, and the average flank wear amount (tool wear amount) Vb was measured. The results are shown in Table 2 above. In the present invention, those having Vb of 80 μm or less after intermittent cutting were evaluated as “excellent machinability during intermittent cutting”.

[Machinability evaluation during continuous cutting (turning test)]
In order to evaluate machinability during continuous cutting, the round bar (φ80 mm x length 350 mm) was scaled and the surface was ground approximately 2 mm as a turning test piece (workpiece). Turned. The conditions for the peripheral turning are as follows.
(Outer peripheral turning conditions)
Tool: Cemented carbide P10 (JIS B4053)
Cutting speed: 200 m / min
Feeding: 0.25mm / rev
Cutting depth: 1.5mm
Lubrication system: dry

  After the peripheral turning, the tool surface was observed under an optical microscope at a magnification of 100 to measure the average flank wear amount (tool wear amount) Vb. The results are shown in Table 2 above. In the present invention, the case where Vb after continuous cutting is 100 μm or less is evaluated as “excellent machinability during continuous cutting”, and the case where Vb is 70 μm or less is “excellent machinability during continuous cutting”. evaluated.

  Next, a Charpy impact test was conducted under the following conditions using the round bar material after cooling, and the impact characteristics after the heat treatment were evaluated.

[Evaluation of impact characteristics]
In order to evaluate the impact characteristics after heat treatment, a sample having a width of 12 mm, a width of 12 mm, and a length of 55 mm was cut out from the round bar after cooling, and after heating to 850 ° C., quenching was performed, followed by 500 ° C. A JIS No. 4 U-notch cut out from the product tempered for 30 minutes and heat-treated was used as a Charpy impact test piece. A Charpy impact test was performed according to JIS Z2242 using this test piece. The results are shown in Table 2 above.

  It can be considered from Table 2 as follows. No. 1-22 are examples that satisfy the requirements defined in the present invention, and the mass ratio of BN and AlN precipitated in steel (BN / AlN) is adjusted to an appropriate range, so at low speed Excellent machinability (especially extension of tool life) in both interrupted cutting and continuous cutting at high speed, and excellent impact characteristics even after quenching and tempering.

  In particular, no. Nos. 18 to 22 are examples in which after heating to 1200 ° C., hot forging at 1100 ° C. and holding at 900 to 1050 ° C. for a predetermined time. The chemical composition of 18 to 22 is No. The same as 3, 6, 7, 8, and 9. No. 3 and no. 18, no. 6 and no. 19, no. 7 and no. 20, no. 8 and no. 21, no. 9 and No. 22, the grain boundary BN / intragrain BN can be controlled to 0.50 or less by hot forging, and the impact characteristics after the heat treatment are relatively enhanced as compared with the case without hot forging. Is able to.

  In contrast, no. 23 and no. In No. 28, the heating temperature is lower than 1100 ° C., the precipitation of BN becomes insufficient, and the BN / AlN ratio is lower than 0.020. Therefore, the machinability during continuous cutting and the impact characteristics after heat treatment are Inferior. No. 24, the holding time in the temperature range of 900 to 1050 ° C. is shorter than 150 seconds, the precipitation of BN becomes insufficient, and the BN / AlN ratio is less than 0.020. The impact properties after heat treatment are inferior. No. 25, the average cooling rate in the temperature range from 900 ° C. to 700 ° C. is lower than 0.05 ° C./second, a large amount of AlN is generated, and the BN / AlN ratio is lower than 0.020. The machinability at the time and the impact properties after heat treatment are inferior. No. No. 26 is an example in which the amount of Al is small, and since the amount of dissolved Al is insufficient, the machinability during intermittent cutting is inferior. No. No. 27 is an example with a small amount of B, since the precipitation of BN is insufficient, and the BN / AlN ratio is less than 0.020, so the machinability during continuous cutting and the impact properties after heat treatment are inferior. Yes.

Claims (10)

C: 0.05 to 0.8% (meaning mass%, the same shall apply hereinafter)
Si: 0.03 to 2%,
Mn: 0.2-1.8%
Al: 0.1 to 0.5%,
B: 0.0005 to 0.008%,
N: 0.002 to 0.015% is contained,
P: 0.03% or less (excluding 0%),
S: 0.03% or less (excluding 0%),
O: 0.002% or less (excluding 0%) is satisfied,
The balance is steel consisting of iron and inevitable impurities,
A steel for mechanical structure, wherein the mass ratio of BN and AlN (BN / AlN) precipitated in the steel is 0.020 to 0.2. Among the BN precipitated in the steel, the number ratio of BN precipitated in the prior austenite grain boundaries and BN precipitated in the prior austenite grains (grain boundary BN / intragrain BN) is 0.50 or less. The mechanical structural steel according to claim 1. As other elements,
The steel for machine structural use according to claim 1 or 2, containing Cr: 3% or less (not including 0%). As other elements,
The steel for machine structure according to any one of claims 1 to 3, containing Mo: 1% or less (not including 0%). As other elements,
The steel for machine structure in any one of Claims 1-4 containing Nb: 0.15% or less (0% is not included). As other elements,
Zr: 0.02% or less (excluding 0%),
Hf: 0.02% or less (excluding 0%),
6. The composition according to claim 1, comprising at least one selected from the group consisting of Ta: 0.02% or less (not including 0%) and Ti: 0.02% or less (not including 0%). Machine structural steel as described in 1. As other elements,
V: 0.5% or less (excluding 0%),
The machine according to any one of claims 1 to 6, comprising at least one selected from the group consisting of Cu: 3% or less (excluding 0%) and Ni: 3% or less (excluding 0%). Structural steel. A method for producing the machine structural steel according to any one of claims 1 to 7,
After heating the steel satisfying the above component composition to 1100 ° C. or higher,
Hold for at least 150 seconds in the temperature range of 900 to 1050 ° C,
A method for producing steel for machine structure, characterized in that an average cooling rate from 900 ° C. to 700 ° C. is set to 0.05 to 10 ° C./second when cooling thereafter. After heating the steel satisfying the above component composition to 1100 ° C. or higher,
The manufacturing method according to claim 8, wherein hot working is performed at 1000 ° C. or higher, and a holding time in a temperature range of 900 to 1050 ° C. is 150 seconds or longer. A machine structural component obtained using the machine structural steel according to claim 1.