JP2006299319A - Steel for machine structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a Pb-free steel for a machine structure, which has chip partibility equivalent to or better than a Pb-free cutting steel, and does not contain Mg for controlling the form of sulfide-based inclusions. <P>SOLUTION: The steel for a machine structure includes the sulfide-based inclusions containing MnS as a main component so that a coefficient B of an exponential function shown by the expression of "y=Aexp(-Bx)" (wherein (y) represents the number, (x) represents the size (μm), and (A) and (B) represent coefficients) can be 0.25 or more, which is obtained by approximating a relationship between their sizes and their numbers obtained by converting the sizes of the sulfide-based inclusions existing in a longitudinal section of a base material for the machine structure, into an equivalent circle diameter, and distributing the number of the inclusions with the circle diameters between 2 μm and 20 μm into frequency distribution with a 2 μm pitch. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a machine structural steel, and more particularly to a machine structural steel having machinability equivalent to or better than Pb free-cutting steel even if Pb, which is a machinability improving element, is not included. More specifically, the present invention relates to a Pb-free steel for machine structural use having a chip dividing property equal to or higher than that of Pb free-cutting steel.

  Many machine structural parts used in automobiles and construction machines are finished into a desired part shape by cutting after hot working such as hot forging. However, there is a problem that chips are wound around a cutting tool or a finished product in a desired shape at the time of the cutting. For this reason, in particular, steel for machine structures used in automated cutting lines is required to have excellent chip breaking properties. Conventionally, in general, chip cutting is performed with a small amount of addition without impairing mechanical properties. Pb free-cutting steel that can significantly improve the properties has been used.

  However, due to the recent increase in global environmental problems, there is a demand for improving Pb-free machinability. For this reason, Patent Documents 1 to 3 propose a technique related to free-cutting steel that replaces Pb free-cutting steel. ing.

  That is, Patent Document 1 defines a dispersion index of sulfide inclusion particles in mechanical structural steel in which sulfide inclusions exist, and provides chip fragmentation and mechanical properties in which the index is limited to a specific range. An Mg-added “free-cutting steel for machine structure” having excellent characteristics is disclosed.

  In Patent Document 2, among sulfide inclusions, the average value of the aspect ratio of sulfide inclusions having a major axis of 5 μm or more is 5.2 or less, and the number a having a major axis of 20 μm or more and the major axis is 5 μm. “For machine structure excellent in chip disposal and mechanical properties” characterized in that the ratio a / b to the number b described above satisfies 0.25 or less is disclosed.

  Patent Document 3 includes a step of adding a Mg alloy in order to obtain a free-cutting steel having both mechanical properties and chip breaking properties. Is disclosed.

JP 2002-69569 A JP 2002-146473 A Japanese Patent Laid-Open No. 2002-155312

  In all of the techniques proposed in Patent Documents 1 to 3 described above, Mg is added to control the sulfide inclusions in a predetermined shape and dispersed state. However, Mg has a low boiling point and is easy to evaporate, and since it is a strong deoxidizing element, it is easy to separate from the molten steel as an oxide. Therefore, there is a problem that yield is low and cost increase is unavoidable.

  Therefore, an object of the present invention is to provide a Pb-free steel for machine structural use having a chip cutting property equal to or higher than that of Pb free-cutting steel by controlling the form of sulfide inclusions without adding Mg. That is.

  The present inventors selected Ca-S composite free-cutting steel containing Ca and S as Pb-free steel for machine structural use, and performed a detailed study on its machinability. As a result, the following knowledge (a) was obtained.

  (A) Even for mechanical structural steels having almost the same chemical composition and strength, sulfide inclusions mainly composed of MnS dispersed in the steel (hereinafter simply referred to as “sulfide inclusions”) The chip breaking property varies greatly depending on the distribution state and form.

  Then, when detailed examination was performed, the following knowledge (b) and (c) was obtained.

  (B) When sulfide inclusions are present non-uniformly, crack propagation between adjacent sulfide inclusions is facilitated as compared with the case where the inclusions are uniformly present. Promoted. For this reason, in order to improve chip | tip cutting | disconnection property, it is good to make the dispersion | distribution state of the sulfide type inclusion which exists in steel nonuniform.

  (C) When the sulfide inclusions are fine, compared to the coarse case, there are more regions where the sulfide inclusions have a high probability of existence and the notch effect is increased. Is done. For this reason, in order to improve chip parting property, it is good to make the sulfide type inclusion which exists in steel fine.

  Accordingly, the present inventors then conducted various studies on techniques for unevenly and finely dispersing sulfide inclusions. As a result, it has been clarified that the form of sulfide inclusions varies greatly depending on the deoxidation method of the molten steel and the cooling rate during solidification.

  Then, when further detailed examination was conducted, the following findings (d) to (h) were obtained.

  (D) When a deoxidizing element such as Si, Mn, Ca, Al, etc. is added in a state where the content of O (oxygen) in the molten steel is too much, a large amount of deoxidized products are formed throughout the molten steel. Oxide inclusions will be present in the film. Since sulfide inclusions are generated using the oxide inclusions as nuclei, the sulfide inclusions are coarse and relatively uniform.

  (E) When Ca is added in a state where the content of O (oxygen) in the molten steel is too small, CaS is produced at the stage of molten steel due to the strong affinity between Ca and S, and this produces MnS that crystallizes later. Become the nucleus. For this reason, the sulfide inclusions are coarse and relatively uniform.

  (F) Even when the oxide steel inclusions and CaS which are deoxidation products are not present in a large amount in the molten steel, if the cooling rate at the time of solidification is slow, MnS to be crystallized tends to become coarse.

  (G) By appropriately controlling the contents of O (oxygen) and Ca in the molten steel and appropriately managing the cooling rate during solidification, sulfide-based inclusions in the steel are present minutely and non-uniformly. As a result, chip separation is improved.

  (H) In the case of the above (g), there is a feature in the distribution form of sulfide inclusions, the sulfide inclusions in the longitudinal section of the steel material are converted into equivalent circle diameters, and the equivalent circle diameter is The coefficient B of the exponential function obtained by approximating the relationship between the size and the number obtained by frequency distribution from 2 μm to 20 μm at a pitch of 2 μm has a certain value or more.

  This invention is completed based on said knowledge, The summary exists in steel for machine structure shown to following (1)-(4).

(1) Steel for machine structure in which sulfide inclusions mainly composed of MnS are present, and the sulfide inclusions in a longitudinal section of a steel material using the inclusions are converted into equivalent circular diameters. The coefficient B of the exponential function expressed by the following equation (1) obtained by approximating the relationship between the size and the number obtained by frequency distribution of the equivalent circular diameter from 2 μm to 20 μm at a pitch of 2 μm is 0.25 or more. A machine structural steel characterized by
y = Aexp (-Bx) (1).
In Equation (1), y represents the number, x represents the dimension (μm), and A and B are coefficients.

  (2) By mass%, C: 0.2-0.6%, Si: 0.1-1.0%, Mn: 0.4-2.0%, P: 0.04% or less, S: 0.02% to 0.12%, Cr: 0 to 2.0%, Ca: 0.0003 to 0.005%, Al: 0 to 0.05%, O: 0.005% or less, N: 0 The steel for machine structural use as described in (1) above, containing 0.004 to 0.020% and the balance having a chemical composition comprising Fe and impurities.

  (3) Instead of part of Fe, Cu: 0.2 to 1.0%, Ni: 0.2 to 1.0%, Mo: 0.05 to 0.5%, V: 0.03 to It has a chemical composition containing one or more of 0.3%, Ti: 0.005-0.1%, Nb: 0.005-0.05%, Zr: 0.005-0.05%. The steel for machine structure as described in (2) above.

  (4) Instead of a part of Fe, at least one of Bi: 0.01 to 0.1%, Te: 0.001 to 0.01%, REM: 0.0001 to 0.01% The steel for machine structure according to (2) or (3) above, which has a chemical composition.

  Hereinafter, the inventions related to the steels for machine structures (1) to (4) are referred to as “present invention (1)” to “present invention (4)”, respectively. Also, it may be collectively referred to as “the present invention”.

  In the present invention, the term “sulfide-based inclusions mainly composed of MnS” refers to Mn whose mass ratio of Mn is 50% or more and whose mass ratio of S is 25% or more in the composition of inclusions. And inclusions whose main component is a compound of S. “REM” is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of the above elements.

  Although the steel for machine structural use of the present invention is a Pb-free “global environmentally friendly steel for machine structural use”, it has a chip cutting property equal to or higher than that of Pb free-cutting steel. Can be used. This steel can be manufactured at low cost because Mg is not used to control the form of sulfide inclusions.

  The reason why the form and chemical composition of the sulfide inclusions are limited as described above in the present invention will be described below. The indication “%” of the content of each element means “mass%”.

(A) Form of sulfide inclusions The finer the sulfide inclusions present in the steel, the more the coefficient B of the exponential function expressed by the following equation (1), that is, y is the number and x is the dimension. (Μm), and by using A and B as coefficients, the sulfide inclusions in the longitudinal section of the steel material are converted into equivalent circular diameters, and the equivalent circular diameter is obtained by frequency distribution from 2 μm to 20 μm at a pitch of 2 μm. The value of the coefficient B in the exponential function obtained by approximating the relationship between the size and the number obtained is increased.
y = Aexp (-Bx) (1).

  And if the value of the coefficient B is 0.25 or more, the number of sulfide inclusions per unit volume is increased and becomes finer, so that the chip breaking property is improved.

  Hereinafter, the above items will be described in detail based on the following experiments conducted by the present inventors.

  The present inventors dissolved steels A1 to A7 shown in Table 1 in which S or S and Pb were added as elements that enhance machinability to chemical components corresponding to JIS S48C, which is a carbon steel material for mechanical structures. A 3t steel ingot was produced. The steels A1 to A6 in Table 1 are listed in the automotive standard JASO M 106-92 (established by the Japan Society for Automotive Engineers, date of regulation: May 28, 1977, date of revision: March 30, 1992). This is a machinability improved steel containing 0.040 to 0.070% S, which is the S content of S1 class. Steel A7 is a machinability-improving steel containing 0.10 to 0.30% of Pb which is the Pb content of the L2 class together with the S content of the S1 class.

  Next, after heating these steel ingots to 1523K, hot rolling was performed so that the finishing temperature would be 1273K or higher to produce a round bar with a diameter of 80 mm, and further heating to 1153 K and holding for 2 hours, followed by air cooling Normalizing treatment was performed.

  Using the thus obtained round bar with a diameter of 80 mm, the observation of sulfide inclusions and the evaluation of chip breaking property were performed.

The sulfide inclusions were observed by specular polishing by cutting out a portion of D / 4 (where “D” is the diameter of the round bar) on the plane parallel to the rolling direction of the round bar, ie, the longitudinal section. And 50 visual fields were observed at 200 times with an optical microscope. At this time, the observation area per field of view is 0.3 mm ² .

  Next, after converting the sulfide inclusions observed in the 50 fields of view into equivalent circle diameters, the equivalent circle diameter is frequency-distributed from 2 μm to 20 μm at a pitch of 2 μm, and the relationship between the size and the number is approximated. The coefficient B of the exponential function expressed by the above equation (1) was calculated.

  FIG. 1 and FIG. 2 show examples of steel A1 and steel A4, in which the equivalent circular diameter as the size of sulfide inclusions and the frequency as the number are histogrammed and approximated to an exponential function. In addition, the description of “2-4” to “18-20” in the horizontal axis “size of sulfide inclusions” in FIGS. 1 and 2 indicates that the size of sulfide inclusions is “2 μm or more and 4 μm”, respectively. Less than “less than” to “18 μm or more but less than 20 μm” indicates that the frequency distribution is made.

  From these figures, the coefficient B in the case of steel A4 with few fine sulfide inclusions is as small as 0.23, while the coefficient in the case of steel A1 in which sulfide inclusions are made finer than steel 4 It is clear that B is large at 0.30. From the same calculation results, it was found that when the coefficient B of the exponential function expressed by the above equation (1) is 0.25 or more, the number of sulfide inclusions increases and becomes finer. Table 2 shows the coefficient B of the exponential function represented by the formula (1) obtained for each of the steels A1 to A6. Steel A7 contains Pb, which has the effect of improving chip separability, and therefore the chip mass is small regardless of the dispersion state of sulfide inclusions.

The chip breaking property was evaluated by a turning test. That is, using the tip of the cemented carbide tool P20, turning was performed under the following conditions, and the mass per 10 representative chips was measured and used as an index of chip breaking property. It can be determined that the smaller the mass, the better the chip breaking property.
-Cutting depth: 2.0 mm,
-Feed rate: 0.25mm / rev,
-Cutting speed 160m / min,
・ Lubrication: Dry type.

  Table 2 shows the chip mass as an evaluation index of chip breaking property. FIG. 3 shows the relationship between the exponential function coefficient B (expressed as “exponential function coefficient B” in the figure) expressed by the above equation (1) and the chip mass, which is an evaluation index of chip fragmentation. Indicates.

  From FIG. 3, if the coefficient B of the exponential function represented by the above equation (1) becomes 0.25 or more and the number of sulfide inclusions increases and becomes finer, the chip mass decreases and the chip fragmentation occurs. It is clear that the cutting property is improved and the chip breaking property equal to or higher than that of steel A7 containing Pb is obtained.

  For the reasons described above, in the steel for machine structure according to the present invention (1), the sulfide inclusions in the longitudinal section are converted into an equivalent circle diameter, and the equivalent circle diameter is 2 μm to 20 μm at a pitch of 2 μm. It was specified that the coefficient B of the exponential function expressed by the above equation (1) obtained by approximating the relationship between the size and number obtained by frequency distribution was 0.25 or more. In order to obtain more stable and good chip separation, the coefficient B is preferably set to 0.28 or more.

  In order to set the coefficient B of the exponential function represented by the above formula (1) to 0.25 or more, for example, in a state where the oxygen content in the molten steel is appropriately controlled, the Fe—Si alloy, Fe— Mn alloy and Al are added for deoxidation treatment, and further, C, Si, Mn, S and other elements as main components are adjusted to a predetermined amount, then Ca is added, and casting is performed using a water-cooled mold. .

(B) Chemical composition Steel for machine structural use according to the present invention, which has a chip cutting property equivalent to or better than Pb free-cutting steel and can be used in an automated cutting line, has its specific chemical composition and manufacturing method. There is no particular limitation. However, from the viewpoint of ensuring the mechanical properties required for the product, the chemical components of the steel for machine structure according to the present invention may be defined as follows.

C: 0.2 to 0.6%
C is an element effective for securing the tensile strength of steel. However, when the content is less than 0.2%, the effect of addition is poor. On the other hand, if the C content exceeds 0.6%, the machinability decreases. Therefore, the content of C is set to 0.2 to 0.6%.

Si: 0.1 to 1.0%
In addition to promoting deoxidation of molten steel, Si has an effect of increasing the tensile strength of steel. However, when the content is less than 0.1%, the effect of addition is poor. On the other hand, if the Si content exceeds 1.0%, the machinability deteriorates. Therefore, the Si content is set to 0.1 to 1.0%.

Mn: 0.4 to 2.0%
Mn has the effect of increasing the tensile strength of the steel, and also has the effect of combining with S to form sulfide-based inclusions containing MnS as a main component and improving the machinability of the steel. However, when the Mn content is less than 0.4%, FeS may increase and the toughness may deteriorate. On the other hand, if the Mn content exceeds 2.0%, the machinability decreases. Therefore, the Mn content is set to 0.4 to 2.0%.

P: 0.04% or less P segregates at the grain boundaries of steel to reduce workability in hot working and the like, and particularly when its content exceeds 0.04%, the workability deteriorates remarkably. . Therefore, the content of P is set to 0.04% or less. The P content is more preferably 0.02% or less.

S: 0.02% to 0.12%
S combines with Mn to form sulfide inclusions mainly composed of MnS, and has the effect of improving the machinability of steel. However, when the content is less than 0.02%, the effect of addition is poor. On the other hand, when S is contained excessively, the workability in hot working or the like is lowered. In particular, when the content exceeds 0.12%, the workability is significantly lowered. Therefore, the content of S is set to 0.02 to 0.12%.

Cr: 0 to 2.0%
Cr has the effect of increasing the tensile strength of steel. Therefore, Cr may be added to increase the tensile strength. However, if the Cr content exceeds 2.0%, the machinability decreases. Therefore, the content of Cr is set to 0 to 2.0%. In addition, it is preferable that the minimum of Cr content in the case of adding is 0.02%.

Ca: 0.0003 to 0.005%
Ca forms an oxide having a low melting point and adheres to the tool, thereby increasing the tool life. However, when the content is less than 0.0003%, the effect of addition is poor. On the other hand, when Ca is excessively contained, the above effect is saturated, and CaS is generated to prevent fine dispersion of sulfide inclusions mainly composed of MnS. Therefore, the content of Ca is set to 0.0003 to 0.005%.

Al: 0 to 0.05%
Al has an action of promoting deoxidation of molten steel. Therefore, when it is desired to enhance the deoxidation effect, Al may be added. However, when Al is contained excessively, a hard oxide is generated and the tool life is shortened. In particular, when the content exceeds 0.05%, the tool life is significantly decreased. Therefore, the Al content is set to 0 to 0.05%.

O (oxygen): 0.005% or less O (oxygen) is combined with an element having a deoxidizing action such as Si, Mn, Ca and Al to form oxide inclusions. In particular, if the O content exceeds 0.005%, a large amount of oxide inclusions are formed, which may become the nucleus of the sulfide inclusions and prevent fine dispersion of the sulfide inclusions. . Therefore, the content of O is set to 0.005% or less.

N: 0.004 to 0.020%
N has a function of forming nitrides to refine crystal grains and improving the tensile strength and toughness of steel. However, when the content is less than 0.004%, the effect of addition is poor. On the other hand, when the content of N exceeds 0.020%, the nitride becomes coarse and the toughness may deteriorate or the machinability may deteriorate. Therefore, the N content is set to 0.004 to 0.020%.

  For the above reason, the chemical composition of the steel for machine structural use according to the present invention (2) contains the elements from C to N in the above-mentioned range, and the remainder is defined as consisting of Fe and impurities.

  In the machine structural steel according to the present invention, if necessary, in place of a part of Fe, one or more elements selected from the first group described later and one or more selected from the second group may be used. One or both of these elements may be contained.

  Hereinafter, the elements of the first group and the second group will be described.

First group: Cu: 0.2-1.0%, Ni: 0.2-1.0%, Mo: 0.05-0.5%, V: 0.03-0.3%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.05% and Zr: 0.005 to 0.05%
Cu, Ni, Mo, V, Ti, Nb and Zr all have the effect of increasing the tensile strength. For this reason, when it is desired to obtain a further excellent tensile strength, it may be contained in the following range.

Cu: 0.2 to 1.0%
Cu has the effect | action which raises the tensile strength of steel. However, if the content is less than 0.2%, the effect of addition is poor. On the other hand, if the Cu content exceeds 1.0%, workability in hot working or the like is reduced. Therefore, when Cu is added, the content of Cu is set to 0.2 to 1.0%.

Ni: 0.2-1.0%
Ni has the effect of increasing the tensile strength of steel. However, if the content is less than 0.2%, the effect of addition is poor. On the other hand, if the Ni content exceeds 1.0%, the above effects are saturated and the cost increases, and machinability and toughness are reduced. Therefore, the content of Ni when added is set to 0.2 to 1.0%.

Mo: 0.05-0.5%
Mo has the effect | action which raises the tensile strength of steel. However, if the content is less than 0.05%, the effect of addition is poor. On the other hand, if the Mo content exceeds 0.5%, the above effects are saturated and the cost increases, and machinability and toughness are reduced. Therefore, the Mo content when added is set to 0.05 to 0.5%.

V: 0.03-0.3%
V has the action of forming carbides and nitrides to refine crystal grains and increasing the tensile strength of steel. V also has an effect of improving toughness by refining the crystal grains. However, if the content is less than 0.03%, the effect of addition is poor. On the other hand, if the content of V exceeds 0.3%, the above effects are saturated and the cost increases, and the machinability decreases. Therefore, when V is added, the content of V is set to 0.03 to 0.3%.

Ti: 0.005 to 0.1%
Ti has the action of forming carbides and nitrides to refine crystal grains and increasing the tensile strength of steel. Ti also has an effect of improving toughness by refining the crystal grains. However, if the content is less than 0.005%, the effect of addition is poor. On the other hand, if the Ti content exceeds 0.1%, the machinability decreases. Therefore, when Ti is added, the content of Ti is set to 0.005 to 0.1%.

Nb: 0.005 to 0.05%
Nb has the effect | action which forms carbide | carbonized_material and nitride and refines | miniaturizes a crystal grain and raises the tensile strength of steel. Nb also has an effect of improving toughness by refining the crystal grains. However, if the content is less than 0.005%, the effect of addition is poor. On the other hand, if the Nb content exceeds 0.05%, the machinability may decrease. Therefore, the content of Nb when added is set to 0.005 to 0.05%.

Zr: 0.005 to 0.05%
Zr has the action of forming carbides and nitrides to refine crystal grains and increasing the tensile strength of steel. Zr also has an effect of improving toughness by refining the crystal grains. However, if the content is less than 0.005%, the effect of addition is poor. On the other hand, if the Zr content exceeds 0.05%, the machinability may decrease. Therefore, the content of Zr when added is 0.005 to 0.05%.

  Said Cu, Ni, Mo, V, Ti, Nb, and Zr can be added only by any 1 type or 2 or more types of composite.

Second group: Bi: 0.01-0.1%, Te: 0.001-0.01% and REM: 0.0001-0.01%
Bi, Te, and REM all have an effect of improving machinability. For this reason, when it is desired to obtain a further excellent machinability, it may be contained in the following range.

Bi: 0.01 to 0.1%
Bi has the effect | action which improves the machinability of steel. However, if the content is less than 0.01%, the effect of addition is poor. On the other hand, if the Bi content exceeds 0.1%, the above effects are saturated and the cost increases, and the workability in hot working or the like may be reduced. Therefore, the Bi content when added is 0.01 to 0.1%.

Te: 0.001 to 0.01%
Te has the effect | action which improves the machinability of steel. However, if the content is less than 0.001%, the effect of addition is poor. On the other hand, even if Te is contained in excess of 0.01%, the above effect is saturated and the cost is increased. Therefore, the content of Te when added is set to 0.001 to 0.01%.

REM: 0.0001 to 0.01%
REM has the effect | action which improves the machinability of steel. However, if the content is less than 0.0001%, the effect of addition is poor. On the other hand, even if it contains REM exceeding 0.01%, the above effect is saturated and the cost is increased. Therefore, the content of REM when added is set to 0.0001 to 0.01%. As already described, “REM” is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM indicates the total content of the above elements.

  Said Bi, Te, and REM can be added only by any 1 type or 2 or more types of composite.

  For the reasons described above, the chemical composition of the steel for machine structure according to the present invention (3) is replaced by a part of Fe of the steel for machine structure according to the present invention (2). -1.0%, Ni: 0.2-1.0%, Mo: 0.05-0.5%, V: 0.03-0.3%, Ti: 0.005-0.1%, It was defined as containing one or more of Nb: 0.005 to 0.05% and Zr: 0.005 to 0.05%.

  Further, the chemical composition of the steel for machine structure according to the present invention (4) is replaced by a part of Fe of the steel for machine structure according to the present invention (2) or the present invention (3), in mass%, Bi: It was specified to contain one or more of 0.01 to 0.1%, Te: 0.001 to 0.01%, and REM: 0.0001 to 0.01%.

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

  Using a high-frequency induction furnace capable of adjusting the atmosphere, steels B1 to B14, steels C1 to C10, and steels D1 to D12 having chemical compositions shown in Tables 3 to 5 were melted to produce 3t steel ingots.

  Of the above steels, Steel B8 to B10, Steel C6, Steel C7, Steel D7 and Steel D8 are made of Fe-Si alloy, Fe-Mn alloy and Al with a high oxygen content in the molten steel. Any one of them was added for deoxidation treatment. Further, C, Si, Mn, S and other elements as main components were adjusted to predetermined amounts, Ca was added, and casting was performed using a water-cooled mold. In the manufacturing method as described above, a large amount of the oxide inclusions that are generated serve as nuclei of sulfide inclusions and prevent the refinement and uneven dispersion of the sulfide inclusions. The coefficient B of the exponential function expressed by the formula is out of the conditions defined in the present invention.

  Steels B11 to B13, Steel C8, Steel C9, Steel D9 and Steel D10 are deoxidized by adding any of Fe-Si alloy, Fe-Mn alloy and Al in a state where the oxygen content in the molten steel is low After the treatment, C, Si, Mn, S and other elements as main components were adjusted to a predetermined amount, Ca was added and cast using a water-cooled mold. In such a production method, excess Ca that does not contribute to the deoxidation reaction becomes CaS and acts as a nucleus for MnS crystallization, preventing the refinement and non-uniform dispersion of sulfide inclusions. The coefficient B of the exponential function expressed by the formula is out of the conditions defined in the present invention.

  Steel B14, Steel C10, Steel D11 and Steel D12 are deoxidized by adding any of Fe-Si alloy, Fe-Mn alloy and Al in a state where oxygen in the molten steel is appropriately controlled, After adjusting C, Si, Mn, S and other elements as main components to predetermined amounts, Ca was added immediately before casting. However, the cooling rate was reduced by setting the flow rate of the cooling water of the mold to 2/3 of the normal flow rate. In the manufacturing method as described above, the cooling rate during solidification is slow, so that the MnS crystallized becomes coarse and prevents the refinement of sulfide inclusions. Therefore, the coefficient of the exponential function represented by the above formula (1) B comes out of the conditions defined in the present invention.

  Steels B1 to B7, Steels C1 to C5, and Steels D1 to D6 are deoxidized by adding any of Fe-Si alloy, Fe-Mn alloy, and Al in a state where oxygen in the molten steel is appropriately controlled. Furthermore, after adjusting C, Si, Mn, S and other elements as main components to predetermined amounts, Ca was added and cast using a water-cooled mold. By such a manufacturing method, the sulfide inclusions in the steel become fine and non-uniform, and the coefficient B of the exponential function expressed by the above equation (1) satisfies the condition defined in the present invention. It becomes like this.

  Subsequently, these steel ingots were heated to 1523K, and then hot-rolled so that the finishing temperature was 1273K or higher, and a round bar having a diameter of 57 mm was formed. The cooling after hot rolling was allowed to cool in the atmosphere.

  Using the round bar having a diameter of 57 mm obtained in this manner, the observation of sulfide inclusions and the evaluation of chip breaking property were performed.

The sulfide inclusions were observed by specular polishing by cutting out a portion of D / 4 (where “D” is the diameter of the round bar) on the plane parallel to the rolling direction of the round bar, ie, the longitudinal section. And 50 visual fields were observed at 200 times with an optical microscope. At this time, the observation area per field of view is 0.3 mm ² .

  Next, after converting the sulfide inclusions observed in the 50 fields of view into equivalent circle diameters, the equivalent circle diameter is frequency-distributed from 2 μm to 20 μm at a pitch of 2 μm, and the relationship between the size and the number is approximated. The coefficient B of the exponential function expressed by the above equation (1) was calculated.

The chip breaking property was evaluated by a turning test. That is, using the tip of the cemented carbide tool P20, turning was performed under the following conditions, and the mass per 10 representative chips was measured and used as an index of chip breaking property. It can be determined that the smaller the mass, the better the chip breaking property.
-Cutting depth: 2.0 mm,
-Feed rate: 0.25mm / rev,
-Cutting speed 160m / min,
・ Lubrication: Dry type.

  Tables 6 to 8 show the coefficient B of the exponential function represented by the formula (1) obtained for each steel and the chip mass as an evaluation index of chip breaking property. 4 to 6, steel B1 to B14, steel C1 to C10, and steel D1 to D12, respectively, the coefficient B of the exponential function represented by the above equation (1) (in the figure, "coefficient B of the exponential function" And the chip mass, which is an evaluation index of chip breaking property.

  4 to 6, the steels B1 to B7, the steels C1 to C5, and the steels D1 to D6, which are steels for machine structural use according to the present invention, are Pb-free steels although they are Pb-free. It is clear that it has a chip breaking property equivalent to or better than.

  On the other hand, steel that deviates from the conditions defined in the present invention is inferior in chip disposal.

  Although the steel for machine structural use of the present invention is a Pb-free “global environmentally friendly steel for machine structural use”, it has a chip cutting property equal to or higher than that of Pb free-cutting steel. Can be used. This steel can be manufactured at low cost because Mg is not used to control the form of sulfide inclusions.

It is a figure which shows about steel A1 that the equivalent circle diameter as a dimension of sulfide inclusions and the frequency as the number were histogrammed and approximated to an exponential function. It is a figure which shows that the equivalent circle diameter as a dimension of sulfide inclusions, and the frequency as the number of steel A4 were histogrammed and approximated to an exponential function. It is a figure which shows the relationship between the coefficient B of the exponential function represented by (1) Formula, and the chip mass which is an evaluation index of chip parting property. About steel B1-B14, it is a figure which shows the relationship between the coefficient B of the exponential function represented by (1) Formula, and the chip mass which is an evaluation index of chip | tip cutting property. About steel C1-C10, it is a figure which shows the relationship between the coefficient B of the exponential function represented by (1) Formula, and the chip mass which is an evaluation index of chip | tip cutting property. It is a figure which shows the relationship between the coefficient B of the exponential function represented by (1) Formula, and the chip mass which is an evaluation index of chip parting property about steel D1-D12.

Claims (4)

A steel for machine structure in which sulfide-based inclusions containing MnS as a main component exist, and the sulfide-based inclusions in a longitudinal section of a steel material made from the same are converted into equivalent circular diameters, and the equivalent The coefficient B of the exponential function expressed by the following formula (1) obtained by approximating the relationship between the size and the number obtained by frequency distribution from 2 μm to 20 μm at a pitch of 2 μm is 0.25 or more. Steel for machine structural features.
y = Aexp (-Bx) (1)
In Equation (1), y represents the number, x represents the dimension (μm), and A and B are coefficients.   In mass%, C: 0.2 to 0.6%, Si: 0.1 to 1.0%, Mn: 0.4 to 2.0%, P: 0.04% or less, S: 0.02 % To 0.12%, Cr: 0 to 2.0%, Ca: 0.0003 to 0.005%, Al: 0 to 0.05%, O: 0.005% or less, N: 0.004 to The steel for machine structural use according to claim 1, comprising 0.020%, the balance having a chemical composition comprising Fe and impurities.   Instead of a part of Fe, Cu: 0.2 to 1.0%, Ni: 0.2 to 1.0%, Mo: 0.05 to 0.5%, V: 0.03 to 0.3 %, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.05%, and Zr: 0.005 to 0.05%. The steel for machine structure according to claim 2. Chemistry containing at least one of Bi: 0.01-0.1%, Te: 0.001-0.01%, REM: 0.0001-0.01% instead of part of Fe The steel for machine structure according to claim 2 or 3, characterized by having a composition.

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