JP2019035126A - Steel for machine structure use - Google Patents

Abstract

To provide a steel for machine structural use capable of heightening machinability, while preventing decline of fatigue strength caused by a sulfide-based inclusion, by not adding S, or by suppressing an addition amount of S in a small state.SOLUTION: A steel for machine structural use contains, in terms of mass%, C:0.01-0.70%, Si:0.05-2.00%, Mn:0.20-3.50%, Cr:0.01-2.00%, s-Al:0.005-0.100%, N:0.050% or less and 0:0.0100% or less, and has a residue comprising Fe and inevitable impurities, and also has a composition satisfying following formula (1): 0.030≤F1...formula (1), where, F1=0.15[Si]+[s-Al]+0.17[V]-0.05[Ti]-0.8[O].SELECTED DRAWING: None

Description

  The present invention relates to a machine structural steel suitable for use as a material for equipment or parts that require excellent machinability and fatigue strength.

Steel parts used in automobiles and various machines (specifically, mechanical structural parts such as gears, crankshafts and connecting rods) are generally finished in a final shape by cutting after forging. is there. Since the cost required for cutting is high in the total part production cost, the steel before cutting is required to have excellent machinability.
In general, in order to improve machinability without containing a harmful substance such as Pb, S is added. The added S forms inclusions such as MnS in the steel, and the machinability is enhanced by the effect of stress concentration on the inclusions during cutting (for example, see Patent Document 1 below).

JP 2004-332078 A

By the way, in recent years, in order to improve fuel consumption, weight reduction of parts and the like, that is, increase in strength has been aimed at. However, in sulfur free-cutting steel to which S is added, there is a problem that inclusions such as MnS generated in the steel become the starting point of fatigue fracture and deteriorate mechanical properties such as fatigue strength. It has been difficult to achieve both compatibility and fatigue strength.
The present invention is based on the circumstances as described above, and for machine structures with improved machinability while preventing the decrease in fatigue strength due to sulfide inclusions with no addition of S or with a small amount of S added. It was made for the purpose of providing steel.

Thus, Claim 1 is, in mass%, C: 0.01 to 0.70%, Si: 0.05 to 2.00%, Mn: 0.20 to 3.50%, Cr: 0.01 -2.00%, s-Al: 0.005-0.100%, N: 0.050% or less, 0: 0.0100% or less, the balance consisting of Fe and inevitable impurities, The composition satisfies the formula (1).
0.030 ≦ F1 .. Formula (1) where F1 = 0.15 [Si] + [s-Al] +0.17 [V] -0.05 [Ti] -0.8 [O] ([] in the formula of F1 is [ ] Represents the element content%

In addition, normally, in the steel for machine structural use, the following components can be included as inevitable impurities within the following range.
P <0.04%, S <0.010%, V <0.01%, Ti <0.005%, Cu <0.05%, Ni <0.05%, Mo <0.01%, etc. is there.

A second aspect of the present invention is characterized in that, in the first aspect, the composition further comprises S: 0.010 to 0.120% by mass% and satisfies the following formula (2).
4.5 ≦ F2 ··· Formula (2) where F2 = 12 ([Ti] + [s-Al]) / [S] (in the formula of F2, [] represents the mass% of the element in [])

  Claim 3 further contains at least one of V: 0.01 to 1.00% and Ti: 0.005 to 0.100% by mass% in any one of claims 1 and 2. It is characterized by doing.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the mass% is Cu: 0.05 to 3.00%, Ni: 0.05 to 3.00%, Mo: 0.01 to 1. It further comprises at least one of 0.000%.

  Claim 5 further contains at least one of Nb: 0.05 to 0.50% and W: 0.20 to 2.00% by mass in any one of claims 1 to 4. It is characterized by doing.

  A sixth aspect of the present invention is the method according to any one of the first to fifth aspects, wherein the mass% is Ca: 0.0003 to 0.0070%, Pb: 0.03 to 0.30%, Bi: 0.03 to 0 .30%, Sn: 0.002-0.020%, Sb: 0.002-0.020%, Mg: 0.0001-0.0200%, Zr: 0.01-0.50%, Te: It further contains at least one of 0.001 to 0.15%, REM: 0.001 to 0.050%, Se: 0.001 to 0.30%, B: 0.0005 to 0.0200%. It is characterized by that.

  The present invention as described above pays attention to the fact that s-Al in steel generates an oxide film having a large effect of suppressing tool wear on the surface of the cutting tool during the cutting process. It is intended to improve the machinability while suppressing the amount of addition to prevent a decrease in fatigue strength due to sulfide inclusions. The oxide film having a tool wear suppressing effect is generated by Si and V in addition to s-Al. In the present invention, an index F1 that formulates the influence of the alloy component on tool wear is provided, and the content of each alloy element is specified so that the value of the index F1 satisfies the above formula (1). Regardless of this, machinability can be improved.

  In the present invention, it is possible to add a small amount of S. However, in this case, the contents of Ti, s-Al, and S elements are balanced so that the index F2 representing the fatigue characteristics satisfies the above formula (2). When S is added, sulfide inclusions such as MnS grow using oxides in steel as nuclei. In the case of the present invention, the core oxide is Ti or Al oxide produced during steel melting, and the ratio between the total amount of Ti and s-Al and the amount of S is the sulfide produced in the steel. It affects the dispersibility and size of system inclusions. In the present invention, the content of Ti, s-Al, and S is balanced so that the index F2 satisfies the formula (2), thereby controlling the form of sulfide inclusions and suppressing the decrease in fatigue strength. In addition, the machinability can be effectively increased.

Next, the reasons for limiting each chemical component and the like in the present invention will be described below. In the following description, “%” means “mass%” unless otherwise specified.
“Chemical composition of claim 1”
C: 0.01 to 0.70%
C is an element necessary for securing strength, and is contained in an amount of 0.01% or more, preferably 0.05% or more in order to secure a predetermined strength. However, if the content is too large, a large amount of pearlite is generated and the machinability is lowered, so the upper limit is made 0.70%. Preferably it is 0.60% or less, More preferably, it is 0.50% or less.

Si: 0.05-2.00%
Si is an element used as a deoxidizer during steel melting, but has an effect of increasing fatigue strength. In addition, an oxide film is generated on the surface of the tool to suppress tool wear and improve machinability. In order to obtain such an effect, in the present invention, Si is contained in an amount of 0.05% or more, preferably 0.10% or more. However, since ferrite will harden and machinability will fall if there is too much content, the upper limit shall be 2.00%. Preferably it is 1.90% or less, More preferably, it is 1.80% or less.

Mn: 0.20 to 3.50%
Mn is an element that increases the hardenability and contributes to improving the strength. In order to obtain this effect, 0.20% or more, preferably 0.30% or more is added. However, if the content is too large, a lot of pearlite is generated and the machinability is lowered, so the upper limit is made 3.50%. Preferably it is 3.00% or less, More preferably, it is 2.50% or less.

Cr: 0.01-2.00%
Cr is an element that enhances hardenability and contributes to strength improvement. In order to obtain this effect, 0.01% or more, preferably 0.05% or more is added. However, Cr has a high cost, and if the content is too large, a supercooled structure is generated and the machinability is lowered, so the upper limit is made 2.00%. Preferably it is 1.80% or less, More preferably, it is 1.60% or less.

s-Al: 0.005 to 0.100%
s-Al is an essential element for securing machinability in the present invention. s-Al reacts with oxygen in the atmosphere or the like by heat generated in cutting, and forms an oxide film on the surface of the cutting tool. The oxide film thus formed is highly effective in suppressing tool wear and improves machinability. In addition, it has the effect of forming nitrides and suppressing the growth of prior austenite grains during hot forging, miniaturizing the ferrite pearlite structure, and increasing toughness and strength. For this reason, in this invention, s-Al is contained at least 0.005% or more, preferably 0.010% or more.
However, when the amount of s-Al is excessively high, a coarse oxide is separately generated in the steel, which becomes a starting point and reduces the fatigue strength. In addition, coarse carbonitrides are generated, tool wear increases, and machinability decreases. For this reason, the upper limit is made 0.100%. Preferably it is 0.090% or less, More preferably, it is 0.080% or less.
In addition, s-Al represents acid-soluble aluminum and is quantified by the method described in Appendix 15 of JIS G 1257 (1994). The contents of JIS G 1257 (1994) are incorporated herein by reference.

N: 0.050% or less N is an element that is inevitably mixed during production, and forms carbonitrides in steel. In particular, when the amount of N is large, coarse carbonitrides are likely to be formed, and this becomes a starting point for fatigue fracture, resulting in a decrease in fatigue strength. Moreover, since tool wear increases due to coarse carbonitride in steel and machinability decreases, the upper limit is made 0.050%. Preferably it is 0.040% or less, More preferably, it is 0.030% or less.
However, if the amount is appropriate, the fatigue strength is increased, or Ti and nitrides are formed and finely precipitated to suppress the growth of the prior austenite grains during hot forging, thereby refining the ferrite pearlite structure and increasing the strength. There is an effect to increase. For this reason, addition of 0.005% or more is preferable. More preferably, it is 0.010% or more.

O: 0.0100% or less O generates oxide inclusions. In particular, when the amount of O is large, a coarse oxide is likely to be generated, and this becomes a starting point for fatigue failure, and the fatigue strength decreases. Further, since the wear of the tool is increased by the coarse oxide in the steel and the machinability is lowered, the upper limit is made 0.0100%. Preferably it is 0.0070% or less, More preferably, it is 0.0040% or less.

0.030 ≦ F1 .. Formula (1), where F1 = 0.15 [Si] + [s-Al] +0.17 [V] -0.05 [Ti] -0.8 [O]
F1 is an index representing machinability. Si, s-Al, and V are elements that generate an oxide film on the tool surface during cutting and suppress tool wear. On the other hand, Ti and O are elements that generate hard carbonitrides or oxides in steel and increase (deteriorate) tool wear.
The coefficients of Si, s-Al, V, Ti, and O represent contributions to machinability (tool wear resistance), respectively. The machinability can be improved as the value of the index F1 is larger.
In the present invention, the lower limit value of the index F1 is defined as 0.030 in order to obtain machinability that does not cause a problem in the mass production process of steel parts according to the conventional sulfur free cutting steel.

“Chemical composition of claim 2”
S: 0.010 to 0.120%
S has the effect of suppressing grain growth and increasing toughness and strength when heated to 1000 ° C. or higher for hot working. Further, as sulfide inclusions such as MnS and (Mn, Ca) S, there are effects of suppressing tool wear during continuous cutting and drilling, improving chip disposal, and improving machinability. In order to obtain such an effect, 0.010% or more is added.
However, excessive addition reduces the fatigue strength, so 0.120% or less is added. Preferably it is 0.100% or less, More preferably, it is 0.080% or less.

4.5 ≦ F2 ··· Formula (2), where F2 = 12 ([Ti] + [s-Al]) / [S]
F2 is an index representing fatigue characteristics. If the value of F2 is 4.5 or more, there is a certain amount of oxides (or nitrides) of Ti and Al that can be the nucleus of sulfide inclusions, so that fine dispersion of sulfide inclusions such as MnS is not possible. It is possible to suppress a decrease in fatigue strength due to the addition of S.
On the other hand, when the value of F2 is less than 4.5, since oxides serving as nuclei are relatively reduced, sulfide inclusions are coarsened and fatigue strength is reduced.

“Chemical composition of claim 3”
V: 0.01-1.00%
Ti: 0.005 to 0.100%
V is an element that forms carbonitrides, suppresses the growth of prior austenite grains during hot forging, refines the ferrite pearlite structure, and increases toughness and strength. In addition, V has an effect of generating an oxide film on the tool surface to suppress tool wear and improve machinability. In order to exert such an effect, 0.01% or more of V may be added. More preferably, it is 0.02% or more. However, excessive addition increases the cost and tends to produce coarse carbonitrides, resulting in a decrease in fatigue strength and an increase in tool wear resulting in a decrease in machinability, so the V amount is 1.00% or less. And Preferably it is 0.90% or less, More preferably, it is 0.80% or less.
Ti, like V, is an element that forms carbonitrides, suppresses the growth of prior austenite grains during hot forging, refines the ferrite pearlite structure, and increases toughness and strength. In order to exhibit such an effect, 0.005% or more of Ti may be added. More preferably, it is 0.010% or more. However, excessive addition increases the cost and tends to produce coarse carbonitride, resulting in a decrease in fatigue strength, and an increase in tool wear resulting in a decrease in machinability. Therefore, the Ti content is 0.100% or less. And Preferably it is 0.080% or less, More preferably, it is 0.060% or less.

“Chemical component of claim 4”
Cu: 0.05 to 3.00%
Ni: 0.05 to 3.00%
Mo: 0.01 to 1.00%
These Cu, Ni, and Mo are elements that increase the hardenability and contribute to improving the strength, and may contain at least one (one element). In order to exert such effects, 0.05% or more is preferably added with respect to Cu, and more preferably 0.10% or more. Further, Ni is preferably added at 0.05% or more, more preferably 0.10% or more. Moreover, about Mo, addition of 0.01% or more is preferable, More preferably, it is 0.02% or more.
However, when these elements are added excessively, a supercooled structure is generated and machinability is lowered, and ductility and toughness are lowered. Therefore, Cu is preferably 3.00% or less, more preferably 2.50% or less, and still more preferably 2.00% or less. Further, Ni is preferably 3.00% or less, more preferably 2.50% or less, and still more preferably 2.00% or less. Mo is preferably 1.00% or less, more preferably 0.90% or less, and still more preferably 0.80% or less.

“Chemical component of claim 5”
Nb: 0.05 to 0.50%
W: 0.20 to 2.00%
Nb and W are elements that combine with C to form carbides or combine with N to form nitrides and contribute to improving the strength of the steel material. If Nb and W contain at least one (one element), good. In order to exhibit such an effect, 0.05% or more of Nb is preferably added, more preferably 0.08% or more, and still more preferably 0.10% or more. Further, W is preferably added in an amount of 0.20% or more, more preferably 0.50% or more, and further preferably 0.70% or more.
However, when these elements are added excessively, the formed carbides and nitrides may increase the deformation resistance of the steel material and may deteriorate the workability. In particular, if hard carbide is produced, machinability is lowered. Therefore, Nb is preferably 0.50% or less, more preferably 0.40% or less, and still more preferably 0.30% or less. Further, W is preferably 2.00% or less, more preferably 1.50% or less, and still more preferably 1.00% or less.

“Chemical component of claim 6”
Ca: 0.0003 to 0.0070%
Pb: 0.03 to 0.30%
Bi: 0.03-0.30%
Sn: 0.002-0.020%
Sb: 0.002 to 0.020%
Mg: 0.0001 to 0.0200%
Zr: 0.01 to 0.50%
Te: 0.001 to 0.15%
REM: 0.001 to 0.050%
Se: 0.001 to 0.30%
B: 0.0005 to 0.0200%
These Ca, Pb, Bi, Sn, Sb, Mg, Zr, Te, REM, Se, and B are all elements that contribute to improvement of machinability, and if at least one (one element) is contained. good. In order to exhibit such an effect, about 0.0003% or more is added about Ca, More preferably, it is 0.0005% or more, More preferably, it is 0.0010% or more. Further, Pb is preferably added in an amount of 0.03% or more, more preferably 0.10% or more, and further preferably 0.15% or more. Bi is preferably added in an amount of 0.03% or more, more preferably 0.08% or more, and still more preferably 0.10% or more. Further, Sn is preferably added in an amount of 0.002% or more, more preferably 0.005% or more, and further preferably 0.010% or more. Further, Sb is preferably added in an amount of 0.002% or more, more preferably 0.004% or more, and further preferably 0.006% or more. Further, Mg is preferably added in an amount of 0.0001% or more, more preferably 0.0003% or more, and further preferably 0.0004% or more. Zr is preferably added in an amount of 0.01% or more, more preferably 0.03% or more, and still more preferably 0.04% or more. Te is preferably added in an amount of 0.001% or more, more preferably 0.002% or more. Moreover, about REM, 0.001% or more of addition is preferable, More preferably, it is 0.002% or more, More preferably, it is 0.003% or more. Moreover, about Se, addition of 0.001% or more is preferable, More preferably, it is 0.002% or more. Moreover, about B, 0.0005% or more of addition is preferable, More preferably, it is 0.0010% or more.

  However, if these elements are added excessively, the strength may decrease, and even if added excessively, the effect of improving machinability is saturated and the cost may increase. Accordingly, Ca is preferably 0.0070% or less, more preferably 0.0060% or less, and still more preferably 0.0050% or less. Pb is preferably 0.30% or less, more preferably 0.28% or less, and still more preferably 0.25% or less. Bi is preferably 0.30% or less, more preferably 0.25% or less, and still more preferably 0.20% or less. Sn is preferably 0.020% or less, more preferably 0.018% or less, and still more preferably 0.015% or less. Further, Sb is preferably 0.020% or less, more preferably 0.015% or less, and still more preferably 0.010% or less. Mg is preferably 0.0200% or less, more preferably 0.0100% or less, and still more preferably 0.0090% or less. Zr is preferably 0.50% or less, more preferably 0.45% or less, and still more preferably 0.40% or less. Te is preferably 0.15% or less, more preferably 0.10% or less. Moreover, about REM, 0.050% or less is preferable, More preferably, it is 0.040% or less. Further, Se is preferably 0.30% or less, more preferably 0.25% or less, and still more preferably 0.20% or less. Further, B is preferably 0.0200% or less, more preferably 0.0180% or less, and still more preferably 0.0150% or less. The REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium), and among these, at least selected from the group consisting of La, Ce and Y It is preferable to contain one element, more preferably at least one element selected from the group consisting of La and Ce.

  According to the present invention as described above, the steel for machine structure having improved machinability while preventing the decrease in fatigue strength due to sulfide inclusions by adding no S or suppressing the addition amount of S to a small amount. Can be provided.

(A) is a front view of the test piece of fatigue strength evaluation, (B) is a perspective view of the test piece of tool wear amount evaluation.

Next, embodiments of the present invention will be described in detail below.
150 kg of steel having the chemical composition (mass%) shown in Table 1 below was melted using a high frequency induction furnace, and then cast by an ordinary method to obtain an ingot. Next, each ingot was heated and subjected to hot forging to obtain a forging material having a diameter of 74 mm. And after heat-maintaining for 2 hours at 1200 degreeC, it hot-forged to 45 square mm at the finishing temperature of 1050 degreeC or more, and allowed to cool to room temperature. The following evaluation was performed using the obtained 45 mm square material. In Examples 1, 8, 9, 13, 16, 17, and 20 in which the amount of S is shown in parentheses in Table 1, the amount of S is an inevitable impurity level (S <0.010%). Is shown.

<Inclusion investigation>
From the forged material of 45 mm square obtained above, a sample for investigation was cut out from 1/2 part of the distance from the outer surface to the material center, and the cross section was mirror-polished to obtain a field of view of 100 mm ² or more at a magnification of 450 times. On the other hand, a photograph was taken with a scanning electron microscope to grasp the diameter of inclusions. Further, EDX analysis was conducted to investigate the composition of inclusions, and the number of oxides having an Al content of 60% or more and an equivalent area of 5 μm ² or less present per 1 mm ² was determined.
In addition, the number of sulfides having an aspect ratio of 4 or less and an equivalent circle diameter of 10 μm or less existing per 1 mm ² from the same sample was obtained, and the number per S content of 0.01% was calculated therefrom. . These results are shown in Table 2 below.

<Bending fatigue strength evaluation>
A round bar (210 mm in length) having a cross-sectional diameter of 18 mm was cut out from 1/2 part of the distance from the outer surface to the center of the 45 mm square forged material obtained above. Then, as shown in FIG. 1A, a test piece 10 having a test section 14 having a parallel section diameter of Φ18 mm and a center of Φ8 mm (about 17 mm in length) is produced, and Ono-type rotary bending fatigue according to JIS Z2274 is prepared. The test was conducted.
The test conditions were 3600 rpm, the test temperature was room temperature, and the maximum stress that did not break in 10 ⁷ cycles was defined as fatigue strength σw (MPa).

As shown in FIG. 1A, when the fatigue strength σw is obtained using a test piece 10 having no notch in the test portion 14, conventional steel for mechanical structure in which coarse inclusions are not generated (for example, JIS S40C). ), A relationship of σw = 1.6 × hardness (Hv) is recognized between the fatigue strength σw and the hardness (Hv). For this reason, a test piece for hardness measurement is prepared from the same material as the test piece 10 subjected to the rotating bending fatigue test, the hardness (Hv) is obtained, and the strength consisting of the ratio of the fatigue strength σw and the hardness (Hv) The hardness ratio was calculated.
In the evaluation of bending fatigue strength, when this strength-hardness ratio is lower by 4% or more than 1.6 (that is, when the strength-hardness ratio is 1.54 or less), it is higher than the standard steel (S40C) as a reference. It was set as “x” as the fatigue strength was lowered.
On the other hand, when the strength-hardness ratio is higher than 1.6, or lower than 1.6 but the deviation is less than 4%, the fatigue strength equivalent to that of the conventional steel (S40C) as a reference is maintained. “○” was given as being completed. These results are shown in Table 2.

  In addition, in the hardness measurement, using a Vickers hardness tester, the five-point average of the hardness at a position of 0.05 mm below the surface was measured as the surface layer hardness by the test method specified in JIS Z2244. The test load at this time was 300 g.

<Evaluation of tool wear>
From the 45 mm square forged material obtained above, as shown in FIG. 1 (B), a rod-shaped test piece 15 having an outer diameter of Φ40 mm and formed with two opposing parallel surfaces 16a and 16b is produced. A cutting test was performed. In the test piece 15 shown in FIG. 1B, the distance between the two surfaces 16a and 16b is 20 mm. In the figure, reference numeral 18 denotes a small-diameter chuck portion provided at an end portion in the longitudinal direction.
The intermittent cutting test was performed using a commercially available P20 type carbide cutting tip as a cutting tool, with a cutting depth d = 0.5 mm, a feed f = 0.35 mm / rev, a cutting speed v = 120 m / min, and a dry condition. It was. And the wear width of the side flank face of the chip | tip after cutting for 52 minutes was measured, and this was made into tool wear amount VB. The machinability was evaluated according to the following criteria.
○: Tool wear amount VB is 180 μm or less ×: Tool wear amount VB is more than 180 μm Note that if the tool wear amount VB is 180 μm or less, machinability that does not cause a problem in the mass production process of steel parts can be obtained. These results are shown in Table 2.

<Evaluation of chip crushability>
A round bar having an outer diameter of Φ40 mm was produced from the 45 mm square forged material obtained above and subjected to a continuous cutting test. The continuous cutting test is carried out under the conditions of a cutting tool using a commercially available P20 type, carbide coated tip, a cutting depth d = 0.5 mm, a feed rate f = 0.10 mm / rev, a cutting speed v = 150 m / min, and a dry. The number of chips per gram was evaluated. These results are shown in Table 2.

  In Table 2 below, N (hot forged product) and HT (quenched and tempered product) are classified in the process column. The evaluation method of the example (or comparative example) described as N in the process column is as described above, but Example 7 and Example 14 described as HT in the process column are further quenched and tempered. A test piece 10 for evaluation of bending fatigue strength was produced from the material subjected to the above and tested. In Example 7, after quenching at 875 ° C. × 1 Hr → WQ (water cooling), tempering was performed at 180 ° C. × 1 Hr → AC (air cooling). Example 14 was quenched at 850 ° C. × 1 Hr → WQ and then tempered at 550 ° C. × 1 Hr → AC.

Table 2 shows the following.
In Comparative Example 1, the value of the index F1 representing machinability satisfied the provisions of the present invention, and the evaluation of the machinability was “◯”, but the value of the index F2 representing fatigue strength was the value of the present invention. The fatigue strength was evaluated as “x” because it was below the lower limit. Although this comparative example 1 contains 0.095% of S but the value of F2 is lower than the lower limit of the present invention, the amount of oxides serving as nuclei of sulfide inclusions is relatively small with respect to the amount of S. As a result, it is presumed that the sulfide inclusions are coarsened and the fatigue strength is reduced.

  In Comparative Example 2, the value of the index F1 representing machinability is lower than the lower limit of the present invention, and the effect of suppressing wear due to an oxide film generated on the tool surface during cutting is not sufficiently obtained. The wear amount VB was over 180 μm and the machinability was evaluated as “x”.

In Comparative Example 3, the amount of s-Al was less than the lower limit of the present invention, and the fatigue strength was evaluated as “x”. In Comparative Example 3 in which the amount of s-Al is excessively small, the number of oxides having an equivalent area of 5 μm ² or less is 8 in the inclusion investigation as compared with the other examples. Presumably, the inclusions are coarse and the fatigue strength is reduced.

In Comparative Example 4, the amount of s-Al exceeded the upper limit of the present invention, and both the machinability and fatigue strength were evaluated as “x”. In s-Al amount is excessively large Comparative Example 4 In addition to a substantial area of 5 [mu] m ² or less of oxides in the steel, which is a number also corresponds area 5 [mu] m ² or more coarse oxides formed, as a result, tool wear amount It is presumed that this increases the machinability and decreases the fatigue strength as a starting point of fatigue fracture.

  Comparative Example 5 corresponds to conventional sulfur free-cutting steel (JIS SUM42), and the S amount exceeds the upper limit of the present invention. In Comparative Example 5, in the inclusion investigation, the number of sulfides was extremely large compared to other examples, and sulfide-based inclusions sufficient for improving machinability were generated. ○ ”. However, since the value of the index F2 representing the fatigue strength is below the lower limit of the present invention, the sulfide inclusions are not finely dispersed, the fatigue strength is lowered, and the fatigue strength is evaluated as “x”. Met.

On the other hand, Examples 1-22 which satisfy | fill the conditions of this invention were "(circle)" in both machinability evaluation and fatigue strength evaluation. Based on the composition of the example, according to the conventional sulfur free-cutting steel (JIS SUM42), machinability which is not a problem in the mass production process of steel parts is obtained, and equivalent to the conventional steel for machine structural use (JIS S40C). Fatigue strength can be obtained. According to the results of the inclusion investigation, in these examples, the number of oxides of 5 μm ² or less is 10 to 30 / mm ² , and even when a small amount of S is added, the oxide is within this range. If controlled, fine dispersion of sulfide inclusions such as MnS is achieved, and it is presumed that fatigue strength can be prevented from lowering.
Further, according to the results of the number of sulfides and the number of chips obtained this time, as the number of sulfides increases, the chip crushability is improved, and when used in applications where chip crushability is more important, for example, Further, steel for machine structure in which the number of sulfides per 0.001% of S is limited to 10 or more is preferable.

  Although the mechanical structural steel of the present invention has been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

Claims (6)

In mass% C: 0.01 to 0.70%
Si: 0.05-2.00%
Mn: 0.20 to 3.50%
Cr: 0.01-2.00%
s-Al: 0.005 to 0.100%
N: 0.050% or less O: 0.0100% or less A mechanical structural steel comprising a balance Fe and unavoidable impurities, and satisfying the following formula (1).
0.030 ≦ F1 ・ ・ Formula (1)
However, F1 = 0.15 [Si] + [s-Al] +0.17 [V] -0.05 [Ti] -0.8 [O]
([] In the formula of F1 represents the mass% of the element in []) In claim 1, in mass%,
S: 0.010 to 0.120%
And further having a composition satisfying the following formula (2).
4.5 ≦ F2 ・ ・ Formula (2)
However, F2 = 12 ([Ti] + [s-Al]) / [S]
([] In the formula of F2 represents the mass% of the element in []) In any one of Claims 1 and 2, in mass% V: 0.01-1.00%
Ti: 0.005 to 0.100%
A steel for mechanical structure, further comprising at least one of the following. In any one of Claims 1-3, By mass% Cu: 0.05-3.00%
Ni: 0.05 to 3.00%
Mo: 0.01 to 1.00%
A steel for mechanical structure, further comprising at least one of the following. In any one of Claims 1-4, By mass% Nb: 0.05-0.50%
W: 0.20 to 2.00%
A steel for mechanical structure, further comprising at least one of the following. In any one of Claims 1-5, Ca: 0.0003-0.0070% in the mass%
Pb: 0.03 to 0.30%
Bi: 0.03-0.30%
Sn: 0.002-0.020%
Sb: 0.002 to 0.020%
Mg: 0.0001 to 0.0200%
Zr: 0.01 to 0.50%
Te: 0.001 to 0.15%
REM: 0.001 to 0.050%
Se: 0.001 to 0.30%
B: 0.0005 to 0.0200%
A steel for mechanical structure, further comprising at least one of the following.

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