JP2021134420A - Steel for machine structure - Google Patents

Abstract

To provide a steel for machine structure enhancing a machineability-improvement effect while reducing the addition amount of an expensive In to the utmost in order to secure a hot ductility sufficient for guaranteeing manufacturability in addition to cost reduction.SOLUTION: A steel for machine structure includes C, Si, Mn, Cr, P, S, In, Al, N and O in a prescribed amount, respectively. Further, the steel includes at least one of 0.0001-0.0950% of Bi, 0.0001-0.3000% of Sn and 0.0001-0.0300% of Te, and the balance of Fe with impurities. Besides, the steel satisfies at least one of formulas: 0.010≤[Bi%]/[In%]...(1); 0.200≤[Sn%]/[In%]...(2); and 0.700≤[In%]/[Te%]...(3), and when satisfying the formula (3), the steel also satisfies the following formula: 22.00≤[Mn%]/[S%]≤150.00...(4).SELECTED DRAWING: Figure 5

Description

The present invention relates to steel for machine structural parts, particularly steel for machine structural parts such as automobile parts.

General mechanical products such as automobiles, construction machinery, and industrial machinery include multiple parts such as gears and shafts. Many of these parts are manufactured by cutting. Therefore, steel, which is the material of parts, is required to have excellent machinability. It is known that machinability is indexed by chip controllability, tool life, cutting resistance, and the like.

Conventionally, it has been known that the machinability is enhanced by containing Pb. However, since Pb is an environmentally hazardous substance, there is a need for a technique for improving machinability by reducing the Pb content as much as possible from the viewpoint of environmental protection.

Inventions focusing on In as an element that substitutes for Pb have been made so far.
For example, in Patent Document 1, a tool made of high-speed steel (hereinafter referred to as "high-speed steel") containing In in a wide range from a small amount to a large amount is used and 40 to 50 m / m /. It is disclosed that the tool life when drilling in minutes is improved.
Further, Patent Document 2 discloses that In is contained in a relatively small range, and the chip controllability when turning with a high-speed tool at 10 to 40 m / min is improved.

Japanese Unexamined Patent Publication No. 62-33743 Japanese Unexamined Patent Publication No. 7-54099

However, the conventional technique has some problems.
Since In is generally an expensive element, it is desirable to have a large machinability improving effect commensurate with the cost in order to use it as a Pb substitute element.

Patent Document 1 simply contains In, and it cannot be said that a sufficient effect commensurate with the cost is obtained. Further, In may reduce ductility in a high temperature region, that is, hot ductility, and may reduce manufacturability during continuous casting, hot rolling and hot forging, but a technique for solving this problem has not been proposed. ..

Patent Document 2 proposes a technique for not only containing In but also enhancing the effect of containing In, but since the application of this technique is limited to a range in which the In content is low in the first place, as a technique for substituting Pb. Is inadequate.

The present invention was devised in view of the above-mentioned problems, and an object of the present invention is to minimize the content of In in order to ensure sufficient hot ductility that can guarantee manufacturability in addition to cost reduction. An object of the present invention is to enhance the effect of improving machinability while reducing the amount, and an object of the present invention is to provide such a machine structural steel.

In order to solve the above problems, the present inventors have investigated a mechanism for improving machinability by In. When the content of In is low, it dissolves in the steel, and when the content is increased, it precipitates in addition to the solid solution In and also exists as In-based inclusions. It is this In-based inclusion that mainly contributes to the improvement of machinability.

As a result of various studies on methods for further enhancing the machinability improving effect of In, it was found that it is effective to add In and Bi, In and Sn, or In and Te in combination.

It is widely known that when Bi is contained alone, it exists as Bi particles in steel and improves machinability in the same manner as Pb. On the other hand, when In and Bi are added in combination, the composite particles of Bi particles and In particles are precipitated, and In, that is, In-based inclusions existing as inclusions are increased, which is considered to improve machinability. .. It was found that in order to obtain this effect, it is preferable to contain a certain amount or more of Bi with respect to the In content.

Since both Sn and In tend to segregate at the grain boundaries and tend to bind to each other, the complex addition of Sn and In promotes segregation, locally increases the concentration, and increases the presence as an In-based inclusion. It is thought that it was. It was found that in order to obtain this effect, it is preferable to contain Sn in a certain amount or more with respect to the In content.

It is widely known that when Te is contained alone, it mainly exists as MnTe particles in steel and improves machinability. On the other hand, it was found that when In and Te were added in combination, In and Te formed a compound in the steel and precipitated as a compound, and the In-based inclusions enhanced the machinability. Further, the compound of In and Te is often formed in contact with MnS, and in order to sufficiently precipitate the compound of In and Te, the abundance ratio of MnS which crystallizes from the liquid phase at the time of coagulation and becomes relatively coarse is set. It turned out that it would be better to increase it. That is, in order to obtain the effect of improving machinability, in addition to appropriately controlling the content ratio of In and Te in order to generate a compound of In and Te, in order to sufficiently obtain MnS to be crystallized, Mn and Mn are used. It was found that the S content ratio should also be appropriate.

In addition, in either case of combined addition, the presence of In-based inclusions present in the periphery of oxides such as Al ₂ O ₃ was observed. Based on this result, in order to increase the amount of In-based inclusions even if the In content is the same, it is necessary to increase the amount _{of oxides such as Al 2} O ₃ within a range that does not significantly affect the mechanical properties such as fatigue strength. It was found to be preferable.

Based on the above findings, the present inventors have succeeded in improving machinability while ensuring hot ductility at a low In content by combining In and Bi, In and Sn, and In and Te. And made the present invention. That is, the mechanical structural steel according to the present invention is as follows.

(1)
Ingredients are by mass%
C: 0.05 to 0.85%,
Si: 0.01 to 3.00%,
Mn: 0.01 to 3.00%,
Cr: 0.01 to 3.00%,
P: 0.100% or less,
S: 0.001 to 0.150% or less,
In: 0.0003 to 0.0600%,
Al: 0.002 to 0.050%,
N: 0.0030 to 0.0250%, O: 0.0050% or less, and Bi: 0.0001 to 0.0950%,
Sn: 0.0001 to 0.3000%, and Te: 0.0001 to 0.0300%,
Contains one or more of the above, with the balance consisting of Fe and impurities.
Satisfy at least one of Equation 1, Equation 2 and Equation 3 shown below.
A steel for machine structure, characterized in that when the formula 3 is satisfied, the formula 4 is also satisfied.
0.010 ≦ [Bi%] / [In%] ・ ・ ・ (Equation 1)
0.200 ≦ [Sn%] / [In%] ・ ・ ・ (Equation 2)
0.700 ≦ [In%] / [Te%] ・ ・ ・ (Equation 3)
22.00 ≦ [Mn%] / [S%] ≦ 150.00 ・ ・ ・ (Equation 4)
Here, [In%], [Bi%], [Sn%], [Te%] and [Mn%] represent the mass% of In, Bi, Sn, Te and Mn contained in the steel, respectively. If not, substitute 0.
(2)
The mechanical structural steel is further increased by mass%.
O: 0.0050% or less, and insol. The steel for machine structure according to (1), which contains Al: 0.0060% or less.
(3)
The mechanical structural steel is further increased by mass%.
Ca: 0.0050% or less,
Mg: 0.0050% or less,
The mechanical structural steel according to (1) or (2), which contains one or more of Zr: 0.0050% or less and REM: 0.0050% or less.
(4)
The mechanical structural steel is further increased by mass%.
Ti: 1.000% or less,
The machine structure according to any one of (1) to (3), which contains one or more of Nb: 1.000% or less and V: 1.000% or less. steel.
(5)
The mechanical structural steel is further increased by mass%.
Mo: 1.00% or less,
Ni: 1.40% or less,
The machine structure according to any one of (1) to (4), which contains one or more of Cu: 1.40% or less and B: 0.0050% or less. steel.
(6)
The mechanical structural steel is further increased by mass%.
Sb: 0.5000% or less,
Se: 0.5000% or less,
The machine structure according to any one of (1) to (5), which contains one or more of Zn: 0.5000% or less and Pb: 0.09% or less. steel.

According to the mechanical structural steel of the present invention, it is possible to obtain a mechanical structural steel having a high machinability improving effect while ensuring hot ductility while reducing the In content as much as possible.

FIG. 1 is a diagram showing the relationship between Bi / In and machinability (cutting resistance [N]) of steels of Examples and Comparative Examples having the same hardness before cutting and containing Bi. FIG. 2 is a diagram showing the relationship between Sn / In and machinability (cutting resistance [N]) of the steels of Examples and Comparative Examples having the same hardness before cutting and containing Sn. FIG. 3 is a diagram showing the relationship between In / Te and machinability (cutting resistance [N]) of steels of Examples and Comparative Examples having the same hardness before cutting and containing Te. FIG. 4 is a diagram showing the relationship between the Mn / S and machinability (cutting resistance [N]) of the steels of Examples and Comparative Examples having the same hardness before cutting and containing Te. It is a figure which shows the relationship between the hardness (HV) before cutting, and machinability (cutting resistance [N]) of the steel of an Example and a comparative example.

The mechanical structural steel according to the present invention and a cutting method thereof will be described. First, the reason for limiting the components of mechanical structural steel (hereinafter, also simply referred to as steel) will be described. In the following description, "%" for the content of each element means "mass%" unless otherwise specified. Further, [In%], [Bi%], [Sn%], [Te%] and [Mn%] represent the mass% of In, Bi, Sn, Te and Mn contained in the steel, respectively.

(C: 0.05 to 0.85%)
C is an element contained to ensure the strength of steel. If the C content is less than 0.05%, the hardness decreases, and the strength becomes insufficient when used without heat treatment after cutting. In addition, sufficient strength may not be obtained when the final processed product is quenched and tempered before use. Therefore, the C content is set to 0.05% or more. On the other hand, if the C content is more than 0.85%, a large amount of carbides are generated and the machinability deteriorates. Therefore, the C content is set to 0.85% or less. The preferable lower limit of the C content is 0.16%, and the preferable upper limit is 0.60%.

(Si: 0.01 to 3.00%)
Although Si is generally contained as a deoxidizing element, it also has the effect of strengthening ferrite and imparting tempering and softening resistance. However, when the Si content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained. On the other hand, if the Si content exceeds 3.00%, the steel becomes too hard and embrittled. Therefore, the Si content is set to 0.01 to 3.00%. The preferable lower limit of the Si content is 0.06%, the more preferable lower limit is 0.20%, the preferable upper limit is 2.00%, and the further preferable upper limit is 1.30%.

(Mn: 0.01 to 3.00%)
Mn is an element necessary for fixing and dispersing sulfur (S) in steel as MnS and solidifying it in a matrix to improve hardenability and secure strength after quenching. However, when the Mn content is less than 0.01%, S in the steel combines with Fe to form FeS, and the steel becomes brittle. On the other hand, if the Mn content exceeds 3.00%, the hardenability becomes too high, which causes a significant increase in hardness and a decrease in machinability. Therefore, the Mn content is set to 0.01 to 3.00%. The preferable lower limit of the Mn content is 0.10%, more preferably 0.20%. The preferred upper limit of the Mn content is 1.80%, more preferably 1.70%.

(Cr: 0.01 to 3.00%)
Cr is a solid solution strengthening element of steel, and when a part is hardened and tempered before use, it improves hardenability and imparts tempering softening resistance to improve fatigue strength after quenching. If the Cr content is less than 0.01%, these effects cannot be obtained. On the other hand, when the Cr content exceeds 3.00%, Cr carbides are generated and the steel becomes embrittled. Therefore, the amount of Cr is set to 0.01 to 3.00%. The lower limit of the Cr content is preferably 0.05%, more preferably 0.10%. The preferred upper limit of the Cr content is 2.00%, more preferably 1.30%.

(P: 0.100% or less)
P is an impurity. Since P segregates at the austenite grain boundaries and causes grain boundary cracks during hot working, the P content is set to 0.100% or less. The upper limit of the P content is preferably 0.030%, more preferably 0.010% or less. Since it is desirable to reduce the P content as much as possible, the lower limit is not particularly limited, but limiting the P content to less than 0.001% requires an excessive cost. Therefore, the range of P content may be 0.001% or more.

(S: 0.001 to 0.150%)
S combines with Mn to form MnS. MnS has an effect of improving machinability, but in order to obtain the effect, it is preferable to contain S in an amount of 0.001% or more. On the other hand, when the S content exceeds 0.150%, the toughness and fatigue strength are remarkably lowered. Therefore, the S content is set to 0.001 to 0.150%. The lower limit of the S content is preferably 0.005%, more preferably 0.010%. The preferred upper limit of the S content is 0.080%, more preferably 0.050%.

(In: 0.0003 to 0.0600%)
In has a decrease in ductility at 800 ° C. or higher, which causes a decrease in yield such as continuous casting and rolling and a decrease in manufacturability during hot forging in parts manufacturing. Further, since it is generally an expensive element, the upper limit thereof is set to 0.0600% in order to reduce its content as much as possible. On the other hand, In not only has the effect of improving the machinability by itself, but also obtains the machinability improving effect by the combined addition of Bi, Sn and Te in the present invention, so that it is 0.0003% or more. It is preferable to contain In. In order to obtain the effect of improving machinability by the combined addition, the lower limit of the In content can be 0.0004%, 0.0005%, or 0.0006%. On the other hand, the upper limit of the In content is 0.0500%, 0.0200%, 0.0100%, 0.0080%, 0.0050%, 0. Values of 0040%, 0.0030%, 0.0020%, 0.0010%, 0.0009%, and 0.0008% can be taken.

(Bi: 0.0001 to 0.0950%, and 0.010 ≦ [Bi%] / [In%])
When Bi is contained together with In, In and Bi are precipitated in a composite manner in the steel to improve machinability. The term "precipitation in a composite form" as used herein includes a case where In and Bi form a compound, and a case where they are close to each other and exist as a simple substance. If Bi is present together with In, the effect of improving machinability by composite precipitation can be obtained, so the lower limit of Bi is set to 0.0001%. On the other hand, if the Bi content exceeds 0.0950%, the ductility at 800 ° C. or higher is lowered, which causes a decrease in yield such as continuous casting and hot rolling and a decrease in manufacturability during hot forging in parts manufacturing. Therefore, the Bi content is set to 0.0001% or more and 0.0950% or less. The lower limit of the Bi content is 0.0002%, 0.0003%, 0.0005%, 0.0007%, 0.0009%, 0.0010%, 0 in order to obtain the effect of improving machinability by the combined addition. It can take values of 0020%, 0.0030%, 0.0040%, 0.0050%. On the other hand, the upper limit of the Bi content is 0.0900%, 0.0850%, 0.0800%, 0.0750%, 0.0700%, 0.0650%, 0. The values of 064%, 0.0600%, 0.0500%, 0.0400%, 0.0300%, 0.0200%, 0.0100%, 0.0090%, 0.0080%, 0.0070% are It can be taken.

Further, in order to obtain the effect of improving machinability by the above-mentioned composite precipitation of In and Bi, it is preferable to contain Bi in a certain amount or more with respect to the In content, specifically, in the steel of Bi and In. The ratio of the content (mass%) ([Bi%] / [In%]) is preferably 0.010 or more (FIG. 1). The lower limit of [Bi%] / [In%] is 0.020, 0.050, 0.080, 0.100, 0.150, 0.200, 0.250, 0.300, 0.350, 0. It can take a value of 400.
The upper limit of [Bi%] / [In%] is not particularly limited, but it can be a value obtained by dividing the upper limit of the Bi content by the lower limit of the In content. Specifically, when the lower limit of the In content is 0.0003% and the upper limit of the Bi content is 0.0950%, it becomes 316.7, and this may be the upper limit.

(Sn: 0.0001 to 0.3000%, 0.200 ≦ [Sn%] / [In%])
Both Sn and In are likely to segregate at the grain boundaries, and when added in combination with In, the segregation of In is promoted and the machinability is improved by increasing the number of In-based inclusions. If Sn is present together with In, segregation of In is promoted and the effect of improving machinability by In-based inclusions can be obtained. Therefore, the lower limit of the Sn content is set to 0.0001%. On the other hand, if the Sn content exceeds 0.3000%, the ductility at 800 ° C. or higher is lowered, which causes a decrease in yield such as continuous casting and hot rolling and a decrease in manufacturability during hot forging in parts manufacturing. Therefore, the Sn content is set to 0.0001% or more and 0.3000% or less. In order to obtain the effect of improving machinability by the combined addition, the lower limit of Sn content is 0.0002%, 0.0003%, 0.0005%, 0.0007%, 0.0009%, 0.0010%, 0. It can take values of 0020%, 0.0030%, 0.0040%, 0.0050%. On the other hand, the upper limit of the Sn content is 0.2800%, 0.2500%, 0.2300%, 0.2000%, 0.1800%, 0.1500%, 0. Values of 1300% and 0.100% can be taken.

In order to obtain the effect of promoting the segregation of In by the content of Sn, it is preferable to contain Sn in a certain amount or more with respect to the In content, and specifically, the content of Sn and In in the steel (mass%). ) ([Sn%] / [In%]) should be 0.200 or more (FIG. 2). The lower limit of [Sn%] / [In%] is 0.230, 0.250, 0.280, 0.300, 0.350, 0.400, 0.450, 0.500, 0.550, 0. It can take a value of 600.
The upper limit of [Sn%] / [In%] is not particularly limited, but it can be a value obtained by dividing the upper limit of the Sn content by the lower limit of the In content. Specifically, when the lower limit of the In content is 0.0003% and the upper limit of the Sn content is 0.3000%, it becomes 1000, and this may be the upper limit.

(Te: 0.0001 to 0.0300%, 0.700 ≦ [In%] / [Te%])
When Te is contained together with In, In and Te form a compound in the steel and precipitate as a composite to improve machinability. The term "precipitation in a composite form" as used herein includes a case where In and Te form a compound and a case where In and MnTe are present in close proximity to each other. If Te is present together with In, the effect of improving machinability by composite precipitation can be obtained, so the lower limit of Te is set to 0.0001%. On the other hand, if the Te content exceeds 0.0300%, the ductility at 800 ° C. or higher is lowered, which causes a decrease in yield such as continuous casting and hot rolling and a decrease in manufacturability during hot forging in parts manufacturing. Therefore, the Te content is set to 0.0001% or more and 0.0300% or less. In order to obtain the effect of improving machinability by the combined addition, the lower limit of the Te content is 0.0002%, 0.0003%, 0.0005%, 0.0007%, 0.0009%, 0.0010%, 0. It can take values of .0015%, 0.0020%, 0.0025%, 0.0030%, 0.0035%, 0.0040%, 0.0045%, 0.0050%. On the other hand, the upper limit of the Te content is 0.0280%, 0.0250%, 0.0200%, 0.0180%, 0.0150%, 0.0120%, 0. Values of 0100%, 0.0090%, 0.0080% and 0.0070% can be taken.

Further, in order to obtain the effect of improving machinability by the above-mentioned composite precipitation of In and Te, it is preferable to appropriately control the ratio of the contents of In and Te, specifically, the content (mass) of In and Te in the steel. The ratio of%) ([In%] / [Te%]) is preferably 0.700 or more (FIG. 3). The lower limit of [In%] / [Te%] is 0.750, 0.800, 0.850, 0.900, 0.950, 1.00, 1.050, 1.100, 1,150, 1. It can take a value of 200.
The upper limit of [In%] / [Te%] is not particularly limited, but it can be a value obtained by dividing the upper limit of the In content by the lower limit of the Te content. However, since there is an upper limit on the In content, [In%] / [Te%] is naturally limited by the upper limit of the In content.

(22.00 ≦ [Mn%] / [S%] ≦ 150.00)
In order to promote the precipitation of In and Te compounds, it is effective to increase the coarse crystallized MnS, and for that purpose, the ratio of the content (mass%) of Mn and S in the steel ([Mn%] / [ S%]) should be 22.00 or more and 150.00 or less (FIG. 4). If [Mn%] / [S%] is smaller than 22.00, the ratio of fine MnS precipitated in the solid phase increases, so that the effect of promoting the precipitation of the In and Te compounds cannot be sufficiently obtained. .. The lower limit of [Mn%] / [S%] is 22.20, 22.50, 23.00, 25.00, 27.50, 30.00, 32.50, 35.00, 37.50, 40. It can take values of 0.00, 50.00, 60.00, 70.00, 80.00, 90.00.
On the other hand, when [Mn%] / [S%] is larger than 150.00, the amount of Mn that does not contribute to the formation of MnS increases, and MnTe is easily generated, which hinders the formation of In and Te compounds. .. The upper limit of [Mn%] / [S%] can take values of 145.00, 140.00, 135.00, 130.00, 125.00, 120.00, 115.00, and 110.00.

(Al: 0.002 to 0.050%)
Al binds to N to form AlN, and has an effect of suppressing grain coarsening in the austenite region. In order to obtain this effect, the Al content is preferably 0.002% or more. On the other hand, if Al is excessively contained, it tends to remain as a coarse oxide, and the fatigue characteristics are deteriorated. Therefore, the range of the amount of Al is set to 0.002 to 0.050%. The lower limit of the Al content is preferably 0.005%, more preferably 0.010%. The preferable upper limit of the amount of Al is 0.040%, more preferably 0.030%. The Al content here means the total Al content.

(N: 0.0030 to 0.0250%)
N (nitrogen) combines with Al and V in steel to form carbonitride, suppresses grain growth by pinning the austenite grain boundaries, and has the function of refining the structure that transforms from austenite. , 0.0030% or more is recommended to obtain this effect. On the other hand, if N is excessively contained in excess of 0.0250%, the ductility in a high temperature range of 1000 ° C. or higher decreases, which causes a decrease in yield during continuous casting and hot rolling. Therefore, the N content needs to be 0.0250% or less. The preferred lower limit of the N content is 0.0040%, more preferably 0.0050%. The preferred upper limit of the amount of N is 0.0200%, more preferably 0.0150%.
The mechanical structural steel according to the present invention may contain one or more selected elements from the following elements in addition to the above basic components as steel components.

(O: 0.0050% or less)
When the O (oxygen) content is high, it tends to remain as a coarse oxide, and the fatigue characteristics deteriorate. Therefore, in the present invention, the upper limit of the O content may be 0.0050% or less. On the other hand, although the lower limit of O is not particularly limited, O may form oxide-based inclusions and improve machinability by increasing In particles. Therefore, in order to obtain this effect, the O content is obtained. Is preferably 0.0009% or more.

(Insol.Al: 0.0060% or less)
In order to control the morphology of In-based inclusions, it is preferable to disperse _{Al contained in the steel as Al 2} O _{3 in the steel.} Acid-insoluble Al, insol. Al is regarded as the amount of Al present as _{Al 2} O _{3, and the amount is measured.} On the other hand, insol. When there is a large amount of Al, coarse oxides tend to remain, and there is a concern that the fatigue characteristics may deteriorate. Therefore, insol. Al is preferably 0.0060% or less. insol. The lower limit of Al is not particularly limited, but in order to sufficiently secure In-based inclusions of a predetermined size, insol. Al is preferably 0.0011% or more. insol. The lower limit of Al can be 0.0012%, 0.0015%, 0018%, 0.0020, 0.0025%, 0.0030%.
In addition, insol. The upper limit of Al can be 0.0058%, 0.0055%, 0052%, 0.0050, 0.0048%, 0.0045%.

insol. Al is measured by ICP (inductively coupled plasma) analysis of acid-insoluble residues. In the present embodiment, the collected sample is decomposed with aqua regia, and then the solution is filtered using a filter paper (Type 5C). The extracted residue is heated and thawed with a melting mixture, and then the melt is cooled to solidify. Next, the solidified melt is dissolved with nitric acid or the like and measured by ICP (inductively coupled plasma) analysis. For the reagents and sample preparations to be used, JIS G 1257: 2013 Iron and Steel-Atomic Absorption Analysis Method may be referred to.

(Ca: 0.0050% or less,
Mg: 0.0050% or less,
Zr: 0.0050% or less, and
REM: 1 type or 2 types or more of 0.0050% or less)
Ca, Mg, Zr, and REM (rare earth elements) are all deoxidizing elements, which form oxides in steel and control the morphology of MnS in steel to contribute to the improvement of mechanical properties. be. In order to obtain these effects, Ca, Mg, Zr, and REM may be contained as long as the characteristics of the steel of the present invention are not impaired. Any element may be contained preferably 0.0001% or more. On the other hand, when Ca, Mg, Zr and REM are contained in an amount of more than 0.0050%, the oxide becomes coarse and the fatigue strength decreases. Therefore, Ca, Mg, Zr and REM are all 0.0050% or less, preferably 0.0020% or less.

REM represents a rare earth metal element, and is one selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. That is all. The content of REM means the total content of these elements.

(Ti: 1.000% or less,
Nb: 1.000% or less,
V: Any one or more of 1.000% or less)
Ti, Nb and V form fine carbides, nitrides and carbonitrides with C and N to suppress grain growth and abnormal grain growth during heating in the austenite temperature range, resulting in fine homogenization of the structure. Contribute and improve impact characteristics. In order to obtain this effect, one or more of Ti, Nb and V may be contained. Each element is preferably 0.005% or more, more preferably 0.010% or more, still more preferably 0.020% or more. On the other hand, if Ti, Nb and V are all contained in an amount of more than 1.000%, hard carbides are generated and the machinability is lowered. Therefore, Ti, Nb and V are all set to 1.000% or less. The content of any of the elements Ti and Nb is preferably 0.200% or less, more preferably 0.150% or less, still more preferably 0.040% or less. V is preferably 0.500% or less, more preferably 0.320% or less.

(Mo: 1.00% or less,
Ni: 1.40% or less,
Cu: 1.40% or less, and B: 0.0050% or less, one or two types)
Mo, Ni, Cu and B are all hardenable elements. In order to obtain this effect, it may be contained within a range that does not impair the excellent properties of the steel of the present invention. Preferably, Mo, Ni and Cu may be contained in an amount of 0.01% or more, and B may be contained in an amount of 0.0003% or more. On the other hand, if Mo exceeds 1.00%, the hardenability becomes too high, which causes a large increase in hardness and lowers workability during cutting and forging. Therefore, the Mo content is 1.00% or less, preferably 0.30% or less. If both Ni and Cu exceed 1.40%, as with Mo, the hardenability becomes too high, which causes a significant increase in hardness and a decrease in workability. Therefore, the upper limit of the contents of Ni and Cu is preferably 1.40% or less. Even if B is contained in excess of 0.0050%, the effect is saturated. Therefore, when B is contained, the amount of B is preferably 0.0003% or more, more preferably 0.0010% or more, preferably 0.0050% or less, still more preferably 0.0025% or less.

(Sb: 0.5000% or less,
Se: 0.5000% or less,
Zn: 0.5000% or less, and Pb: 0.09% or less, 1 type or 2 types)
Sb and Se are elements that have the effect of improving machinability. In order to obtain this effect, it may be contained within a range that does not impair the characteristics of the steel of the present invention. Preferably, Sb and Se may be contained in an amount of 0.0003% or more. On the other hand, when Sb and Se exceed 0.5000%, hot brittleness develops, which causes defects and makes hot rolling difficult. Therefore, both Sb and Se are 0.5000% or less. It is preferable that the content is 0.2000% or less. Zn is also an element having an effect of improving machinability, and in order to obtain this effect, it may be contained within a range that does not impair the characteristics of the steel of the present invention. Preferably, Zn may be contained in an amount of 0.0003% or more. On the other hand, if Zn exceeds 0.5000%, it becomes difficult to manufacture steel, so it is preferable to set it to 0.5000% or less.

Pb is an element that has been used conventionally and has an effect of improving machinability. However, since it is an environmentally hazardous substance, it is preferable that the amount of Pb is as small as possible from the viewpoint of environmental protection, and the content of Pb is limited to 0.09% or less. More preferably, it is 0.05% or less, 0.03% or less, and 0.02% or less.

The composition of the mechanical structural steel of the present invention is as described above, and the balance is Fe and impurities. Depending on the conditions of raw materials, materials, manufacturing equipment, etc., impurities may be mixed in the steel, but it is permissible as long as it does not impair the characteristics of the present invention.

In the present invention, the effect of improving machinability is obtained by increasing the amount of In-based inclusions in the steel. In the present specification, the In-based inclusions include In single particles, composite precipitates of Bi and In, In particles whose concentration is locally increased by the composite addition of Sn, and composite precipitates of In and Te. Inclusive of In particles, In compounds, and In precipitates are shown. The present inventors further investigated the appropriate number density of In-based inclusions, and found that the number density of In-based inclusions having a circle-equivalent diameter of 2.0 μm or more was 0.5 pieces / mm ² or more. It was revealed to be preferable. In-based inclusions with a circle-equivalent diameter of less than 2.0 μm are less likely to be the starting point of cracks during cutting and may have a small effect of providing a lubricating effect on the tool, and In with a circle-equivalent diameter of 2.0 μm or more. If the number density of system inclusions ^{is less than 0.5 pieces / mm 2} , the effect of improving machinability may be reduced.

The equivalent circle diameter and number density of In-based inclusions can be measured by the following methods. A bar steel sample is cut along a cross section (longitudinal cross section) including the axial direction of the bar steel, and a sample including the vertical cross section is collected. The observation surface of the sample was not corroded and was observed as it was with a scanning electron microscope (SEM), a photographic image was prepared, and a plurality of fields of view were observed at a magnification of 200, so that the total test area was calculated as ^{about 50 mm 2.}

Since the In-based inclusions have a larger atomic weight than iron, they are observed as bright contrast in the backscattered electron image. Therefore, it is specified based on the contrast of the reflected electron image. When Bi, Pb, and Te are contained, metal Bi, metal Pb, and Te compounds may be present in the steel, and these are also observed as bright contrast in the backscattered electron image. It may be difficult to distinguish it from an object. In such cases, energy dispersive X-ray analysis (EDS) is used to discriminate In-based inclusions.

Next, using an image analysis device, the equivalent circle diameter of the In-based inclusions specified by the above method is calculated. The equivalent circle diameter refers to the diameter of a circle having an area equal to the projected area of the measured particles, and is specifically derived by the following formula.
Circle equivalent diameter = 2 × {(area of the particle) ÷ π} ^1/2

The hardness of the steel of the present invention, that is, the hardness before cutting the steel of the present invention as a work material is preferably in the range of 120 HV or more and 320 HV or less. If the hardness of the steel before cutting is less than 120 HV, the strength required for use without heat treatment after cutting is insufficient, while if it exceeds 320 HV, the machinability is lowered. The hardness is Vickers hardness, which is the cross section of the position where cutting is performed or the cross section of the position having the same hardness as that position, and the measured load is 10 kg weight in accordance with JIS Z 2244: 2009. It is good to measure. However, if the measurement is performed at a position too close to the surface layer, proper measurement may not be possible. In the measurement, the hardness is measured at a position separated from the surface of the steel by 0.5 mm or more in the cross section.

In general, it is known that mainite and martensite are contained in the structure of steel, which is a work material, to reduce machinability. Therefore, the structure before cutting the steel of the present invention as a work material may be a mixed structure of ferrite and pearlite or a pearlite structure. However, the effect of the present invention is not influenced by the tissue and can be obtained by any tissue. For example, even if the tissue is tempered martensite, the effect of the present invention can be enjoyed without being hindered.
In the case of mechanical structural steel after quenching and tempering, the structure is mainly tempered martensite, and specifically, 90% or more of the area ratio in the cross section is tempered martensite. Further, the hardness is preferably 250 HV (measured load 10 kg weight) or more.

[Production method]
The method for manufacturing the mechanical structural steel of the present invention will be described. The molten steel having the above composition is melted by a common melting method such as a converter to make a steel material such as a slab by a common casting method such as a continuous casting method, and the steel material is further heated to heat the usual heat. The mechanical structural steel of the present invention can be produced by performing hot working such as hot rolling or hot forging to obtain a desired shape. However, it is preferable to adjust the manufacturing conditions so that the hardness of the steel of the present invention is adjusted in the range of 120 to 320 HV. For example, in order to adjust the hardness to this preferable range, the steel may be subjected to heat treatment such as annealing or spheroidizing annealing before the cutting step.

[Cutting method for machine structural steel according to the invention]
The steel of the present invention is particularly effective under cutting conditions at a relatively large cutting speed, specifically, a cutting speed of 60 m / min or more, but the cutting speed is not limited in the steel cutting method of the present invention. The cutting method can be performed by various tools such as a drill, turning, gear cutting, end mill, milling cutter, and tap, and any of these cutting tools may be used. Cutting tools include high-speed steel, cemented carbide, cermet, and those with a ceramic coating applied by chemical vapor deposition (CVD) or physical vapor deposition (PVD). The types of these tool materials are not limited in the cutting process of the present invention.

Further, as a lubrication method, wet type, dry type, semi-dry type such as MQL (Quantity Lubrication) and the like are known, but the effect of the present invention is not limited to these lubrication methods. Note that MQL means that the amount of lubricating oil (cutting oil) is 200 cm ³ or less per hour, but in actual processing of steel materials, the amount of lubricating oil should be ^{about 50 cm 3 or less per hour.} There are also many. The general method for applying the lubricating oil is to mix the lubricating oil with air to form a mist and inject it. In some cases, mist-like water may also be mixed.

Next, Examples will be described. The conditions in the Examples are one condition example adopted for confirming the feasibility and the effect, and the present invention is not limited to this one condition example. .. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object is achieved.

After forging the steels having the composition shown in Table 1 (Tables 1-1 to 3 are collectively referred to as Table 1), the steel pieces are square steel bars with a cross-section size of 60 mm × 60 mm and round cross-section steel bars with a diameter of 50 mm. It was hot forged into two types of shapes. The forged steel bar was held at 1100 ° C. for 1 hour, and then air-cooled by heating and cooling. The "REM" of the component composition shown in Table 1 is the total content of La, Ce, and Nd.

A square steel bar is cut in a cross section perpendicular to the length direction, and a position on the obtained square cross section separated from the center by 15 mm in the width direction and 15 mm in the thickness direction (hereinafter referred to as "intermediate position") is defined. After cutting out the sample so that it could be observed, embedding it in resin, and polishing it, the Vickers hardness at the same position was measured according to JIS Z 2244: 2009. The measured load was 10 kg. In addition, the sample was cut out in the same manner, and after nital corrosion, the intermediate position of the cross section of the sample was observed with an optical microscope.

Further, a sample having a predetermined size is obtained from the square steel bar by cutting, and insol. The amount of Al was measured by the method described above.

The results of the hardness measurement are shown in Table 2 (Tables 2-1 to 3 are collectively referred to as Table 2) as "hardness before cutting". The "hardness before cutting" in the table means that the hardness was measured three times at the intermediate position and the average value was evaluated as the "hardness before cutting". “Ferite-pearlite” in Table 2 means a mixed structure of ferrite and pearlite, and “pearlite” means a single-phase structure of pearlite. The structure of the steel material before cutting used in this example was a mixed structure of ferrite and pearlite, or a structure of pearlite single phase.

Table 2 shows the ^{number densities (pieces / mm 2} ) of In-based inclusions having an average particle size of 2.0 μm or more. The number density of In-based inclusions was determined by cutting out a cross section (longitudinal cross section) including the axial direction from the square bar from a depth of 15 mm ^{, collecting a sample with the total test area as 50 mm 2} , and using the above method. ..

The observation surface of the sample is not corroded, it is observed as it is with a scanning electron microscope (SEM), a photographic image is taken, In-based inclusions are identified from the photographic image, and 2.0 μm or more is used using an image analyzer. The number of In-based inclusions having a circle-equivalent diameter was determined, ^{and the value divided by the total test area (50 mm 2} ) was defined as the number density (pieces / mm ² ).

Next, a cutting test was performed on the steel bar. Specifically, an oil hole having a diameter of 4.98 mm, a total length of 170 mm, and a blade length of 124 mm is used for a 50 × 50 × 115 mm square test piece cut out from the above-mentioned heat-cooling treatment. Cutting was performed under the following conditions using a carbide coated drill with a blade.

Peripheral speed: 90 m / min Feed: 0.15 mm / rev
Hole depth: 100 mm
Lubrication conditions: MQL ( ^{Apply synthetic ester with high biodegradability at a rate of about 1.0 cm 3} / hour from the drill oil hole by internal lubrication)

Prior to the cutting test, a guide hole having a diameter of 5.0 mm and a depth of 15 mm was formed in the square test piece. Therefore, the depth of the guide hole is included in the hole depth of 100 mm described above. The machinability was evaluated by measuring the cutting resistance (thrust) when cutting under the above conditions. A smaller value of cutting resistance means that the machinability is better.

The mechanical structural steel according to the present invention is assumed to be used as a member without heat treatment after cutting, or to be used after heat treatment such as quenching and tempering. That is, it is desirable to be able to secure not only the "hardness before cutting" but also the hardness when heat treatment is performed after cutting. Therefore, the hardness after quenching and tempering was examined as follows.

A round bar test piece of Φ10 × 50 mm is cut out so that the portion corresponding to the intermediate position of the square steel bar after the above-mentioned heat and cooling treatment is the center of the circular cross section, and the test piece is cut out at 950 ° C. for 30 minutes. After holding, it was water-quenched, and then a tempering treatment was carried out in which it was held at 550 ° C. for 90 minutes. Subsequently, a sample is cut out so that a circular cross section perpendicular to the length direction of the test piece can be observed, embedded in resin, polished, and then Vickers hardness at a position between the center of the circle of the circular cross section and the steel surface. The measurement was performed three times in accordance with JIS Z 2244: 2009. The measured load was 10 kg. The average value is shown in Table 2 as "hardness after quenching and tempering". The hardness of the mechanical structural steel of the present invention after quenching and tempering is 250 HV or more, and has sufficient strength characteristics for use as a member.

Furthermore, the hot ductility was investigated by the high temperature tensile test as follows. A tensile test piece having a size of Φ10 × 170 mm was produced from a round steel bar having been subjected to a heating and cooling treatment of Φ50 along the length direction of the steel bar. In producing the tensile test piece, the cutting process was performed so that the intermediate point between the center and the outer circumference in the cross section of the round steel bar was located at the center of the circular cross section of the tensile test piece. For hot ductility, heat to 1250 ° C. and hold for 1 minute, then lower the temperature to 1000 ° C., hold for 1 minute after reaching 1000 ° C., and then perform a tensile test at a ^{strain rate of 5 × 10 -3 / s.} It was evaluated by the value of. If the aperture is 35% or more, the hot ductility is acceptable (“OK”), and if it is less than 35%, the hot ductility is rejected (“NG”).

As a result of the hardness before cutting, the steel structure, the cutting resistance, and the high temperature tensile test, the hardness after quenching and tempering is shown in Table 2.

The cutting resistance is generally affected by the hardness. Further, the present invention needs to ensure hot ductility. Therefore, it was decided to compare and evaluate the quality of machinability with steel materials having the same hardness before cutting, on the premise that the hot ductility is acceptable.

FIG. 5 shows the relationship between the hardness (HV) before cutting and the machinability (cutting resistance [N]). Comparing the example of FIG. 5 and the comparative example (however, the comparative example in which the hot ductility is acceptable (OK)) at the same degree of hardness before cutting, the example has a lower cutting resistance than the comparative example. It can be confirmed that it is. Hereinafter, an example in which the difference in hardness before cutting is within the range of ± 5 HV with respect to the comparative example in which the hot ductility is acceptable is defined as “the hardness before cutting is about the same as that in the comparative example”.
Further, the effects of containing Bi, Sn, Te and In in a complex manner are shown in FIGS. 1 to 3.
FIG. 1 shows the relationship between Bi / In and machinability (cutting resistance [N]) of the steels of Examples and Comparative Examples containing Bi, which have the same hardness as 231 to 239 HV before cutting.
FIG. 2 shows the relationship between Sn / In and machinability (cutting resistance [N]) of the steels of Examples and Comparative Examples containing Sn, which have the same hardness as 225 to 234 HV before cutting.
FIG. 3 shows the relationship between In / Te and machinability (cutting resistance [N]) of the steels of Examples and Comparative Examples containing Te, which have the same hardness as 232 to 239 HV before cutting.
FIG. 4 shows the relationship between the Mn / S and machinability (cutting resistance [N]) of the steels of Examples and Comparative Examples containing Te, which have the same hardness as 175 to 183 HV before cutting.

The steels of Nos. 78 and 79 had an excessive In content, so that the drawing value in the high temperature tensile test failed.
Since the steel of No. 80 had an excessive Bi content, the drawing value in the high temperature tensile test failed.
Since the steel number 81 has an excessive Sn content, the drawing value in the high temperature tensile test fails.
Since the steel of No. 82 has an excessive Te content, the drawing value in the high temperature tensile test fails.
Since the steel of No. 83 has a insufficient In content, the cutting resistance is higher than that of the steel of the example having the same level of hardness before cutting. Number 83 has the same hardness as numbers 4, 28, 29, 49 to 53.

Since the steels of Nos. 84 and 85 have small [Bi%] / [In%] values, the cutting resistance is higher than that of the steels of Examples having the same level of hardness before cutting. Number 84 has the same level of hardness before cutting as numbers 15-19, 37-40 and 63-68, and number 85 has the same level of hardness as numbers 12-19, 35-40 and 60-68.
Since the steels of Nos. 86, 87 and 88 have a small value of [Sn%] / [In%], the cutting resistance is higher than that of the steels of Examples having the same level of hardness before cutting. Number 86 is number 21, 22, 42, 43, 70, 71 and 75, number 87 is number 12-19, 34-39 and 60-67, number 88 is number 10, 11, 33-35, 58 and 59. Hardness is at the same level.
Since the steel numbers 89, 90 and 91 have a small value of [In%] / [Te%], the cutting resistance is higher than that of the steels of Examples having the same level of hardness before cutting. Number 89 is number 4-6, 28-30 and 50-54, number 90 is number 13-19, 36-40 and 63-68, number 91 is number 12-19, 35-40 and 60-68. Are at the same level.

Since the steels of Nos. 92 and 93 have small values of [Mn%] / [S%], the cutting resistance is higher than that of the steels of Examples having the same level of hardness before cutting. Number 92 has the same level of hardness as numbers 4, 28, 29 and 49 to 53, and number 93 has the same level of hardness as numbers 12 to 19, 36 to 40 and 60 to 68.
Since the steels of Nos. 94 and 95 have a large value of [Mn%] / [S%], the cutting resistance is higher than that of the steels of Examples having the same level of hardness before cutting. Number 94 has the same hardness as numbers 3, 48 and 49, and number 95 has the same level of hardness as numbers 17 to 20, 40 and 66 to 68.

Since the steel number 96 has an excessive C content, the cutting resistance is higher than that of the steel of the example having the same level of hardness before cutting. Number 96 has the same hardness as numbers 23, 44, 72 and 76.
Since the steel number 97 has an excessive Se content, the drawing value in the high temperature tensile test fails.
Nos. 1 to 77 have a composition within the range of the present invention, and therefore have better cutting resistance and drawing values in a high-temperature tensile test than steels of Comparative Examples having the same level of hardness before cutting. ..

The present invention can be widely used in all industrial fields such as industrial machinery and transportation machinery.

Claims (6)

Ingredients are by mass%
C: 0.05 to 0.85%,
Si: 0.01 to 3.00%,
Mn: 0.01 to 3.00%,
Cr: 0.01 to 3.00%,
P: 0.100% or less,
S: 0.001 to 0.150%,
In: 0.0003 to 0.0600%,
Al: 0.002 to 0.050%,
N: 0.0030 to 0.0250%, O: 0.0050% or less, and Bi: 0.0001 to 0.0950%,
Sn: 0.0001 to 0.3000%, and Te: 0.0001 to 0.0300%,
Contains one or more of the above, with the balance consisting of Fe and impurities.
Satisfy at least one of Equation 1, Equation 2 and Equation 3 shown below.
A steel for machine structure, characterized in that when the formula 3 is satisfied, the formula 4 is also satisfied.
0.010 ≦ [Bi%] / [In%] ・ ・ ・ (Equation 1)
0.200 ≦ [Sn%] / [In%] ・ ・ ・ (Equation 2)
0.700 ≦ [In%] / [Te%] ・ ・ ・ (Equation 3)
22.00 ≦ [Mn%] / [S%] ≦ 150.00 ・ ・ ・ (Equation 4)
Here, [In%], [Bi%], [Sn%], [Te%] and [Mn%] represent the mass% of In, Bi, Sn, Te and Mn contained in the steel, respectively. If not, substitute 0. The mechanical structural steel is further increased by mass%.
O: 0.0050% or less, and insol. The steel for machine structure according to claim 1, wherein Al: contains 0.0060% or less. The mechanical structural steel is further increased by mass%.
Ca: 0.0050% or less,
Mg: 0.0050% or less,
The mechanical structural steel according to claim 1 or 2, wherein Zr: 0.0050% or less and REM: 0.0050% or less contain one or more of them. The mechanical structural steel is further increased by mass%.
Ti: 1.000% or less,
The mechanical structural steel according to any one of claims 1 to 3, wherein Nb: 1.000% or less and V: 1.000% or less contain one or more of them. The mechanical structural steel is further increased by mass%.
Mo: 1.00% or less,
Ni: 1.40% or less,
The steel for machine structural use according to any one of claims 1 to 4, wherein it contains one or more of Cu: 1.40% or less and B: 0.0050% or less. The mechanical structural steel is further increased by mass%.
Sb: 0.5000% or less,
Se: 0.5000% or less,
The steel for machine structural use according to any one of claims 1 to 5, wherein it contains one or more of Zn: 0.5000% or less and Pb: 0.09% or less.

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