JP2009120905A - Steel for machine structural use, having excellent machinability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for machine structural use which is Pb free and has excellent machinability. <P>SOLUTION: The steel for machine structural use having excellent machinability is characterized in that: it has a composition consisting of, by mass, 0.15 to 0.5% C, 0.01 to 2% Si, 0.1 to 2% Mn, 0.01 to 2% Cr, ≤0.1% (not including 0%) P, 0.01 to 0.3% S, 0.001 to 0.01% Al, 0.001 to 0.02% O, 0.001 to 0.025% N, further at least either or 0.0001 to 0.01% Ca and 0.01 to 0.2% Zr, and the balance iron with inevitable impurities; and, based on 100 mass%, in total, of oxide inclusions in the steel, the amount of MgO is 0.1 to 10 mass%. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a machine structural steel useful for producing parts such as automobiles, industrial machines, and electrical products, and in particular, substantially free of Pb (so-called Pb-free) and machinability. The present invention relates to an excellent steel for machine structural use.

  Parts such as automobiles, industrial machines, and electrical products are generally manufactured through a process of cutting steel for machine structures. Therefore, machine structural steel is required to have good machinability. In order to improve machinability, it is effective to contain Pb in steel. At present, however, the problem of environmental pollution caused by Pb is highlighted. For this reason, various techniques have been proposed for so-called Pb-free machine structural steel with excellent machinability.

  For example, Patent Document 1 discloses a technique for improving the machinability of Pb-free steel by using graphite. Patent Document 1 describes that oxide inclusions, particularly hard Al oxides, reduce machinability.

  Patent Document 2 describes machinability (especially drill life and chip crushability) by controlling the pro-eutectoid ferrite area ratio in steel within a predetermined range and by controlling the parameter of the amount of chemical components within a predetermined range. ) Is disclosed. Patent Document 2 describes that the life of a turning tool can be improved by forming oxide inclusions containing Ca in steel.

Patent Document 3 discloses a technique for improving machinability (particularly, drillability by drilling) by adding Mg to Pb-free steel. Patent Document 3 describes that oxide inclusions such as Al ₂ O ₃ having high hardness adversely affect machinability.
JP 2005-105359 A JP 2005-179753 A JP 2003-119545 A

  As described above, various techniques for improving the machinability of Pb-free steel have been proposed. However, further improvements in machinability are constantly being sought in the field. Accordingly, an object of the present invention is to provide a steel for machine structure that is Pb-free and has machinability superior to that of the conventional one.

The machine structural steel excellent in machinability of the present invention that can achieve the above-mentioned object,
C: 0.15 to 0.5% (meaning mass%, the same applies to chemical components of steel),
Si: 0.01-2%
Mn: 0.1 to 2%,
Cr: 0.01-2%
P: 0.1% or less (excluding 0%),
S: 0.01 to 0.3%,
Al: 0.001 to 0.01%
O: 0.001 to 0.02%,
N: 0.001 to 0.025%
Further containing at least one of Ca: 0.0001 to 0.01% and Zr: 0.01 to 0.2%, the balance consisting of iron and inevitable impurities,
The main point is that the MgO content is 0.1 to 10% by mass with respect to a total of 100% by mass of oxide inclusions in the steel.

  In addition to the above chemical components, the steel for machine structural use according to the present invention may further include (1) Ti: 0.2% or less (not including 0%), V: 0.5% or less (0%) as necessary. Mo: 1% or less (not including 0%), Nb: 0.1% or less (not including 0%), Cu: 1% or less (not including 0%), and Ni: 2 % Or less (not including 0%), (2) Se: 0.01% or less (not including 0%), Te: 0.01% or less (not including 0%) ), Bi: 0.1% or less (excluding 0%), rare earth element (hereinafter abbreviated as “REM”): 0.01% or less (not including 0%), and B: 0.005% or less It is also effective to contain at least one selected from the group consisting of (not including 0%), depending on the type of component to be included, Characteristics of further improves.

  According to the present invention, as a result of limiting the amount of MgO in steel, MnS refinement can be suppressed, and excellent machinability (particularly tool life) can be realized.

  As a result of intensive studies by the present inventors, it was found that the machinability of Pb-free steel can be sufficiently improved if the amount of MgO contained in the steel is suppressed. Specifically, in a method for manufacturing a structural steel for a machine in a conventional mass production furnace, Mg elutes from the refractory or slag of the furnace, so that the steel always contains MgO. The inventors have found that this MgO adversely affects MnS used for improving machinability in Pb-free steel and reduces machinability (particularly tool life).

  As described in Patent Documents 1 to 3 above, regarding oxides, it is in this field that hard Al-based oxides adversely affect machinability, and Ca-containing oxides improve tool life. Are known. However, it has not been known so far that MgO adversely affects machinability. On the contrary, Patent Document 3 describes that machinability is improved by positively adding Mg.

  As an estimation mechanism for improving the machinability of the structural steel for machinery by reducing MgO, the following may be considered: The MnS in the steel is rounder and larger, the machinability (especially the tool life). ). However, if a large amount of fine MgO is present in the steel, MnS precipitates finely with this MgO as a nucleus, so that the machinability improving action is reduced. Therefore, if the amount of MgO is reduced, MnS can be prevented from being refined, and as a result, good machinability can be achieved.

  In general steel for machine structural use, about 15% by mass of MgO is usually contained with respect to 100% by mass of the oxide inclusions in the steel. If the amount of MgO is reduced from 15% by mass to 10% by mass, the drill life (the length of the drill hole until the drill is seized and cannot be drilled) can be increased by 50% (FIG. 1). Therefore, in order to improve machinability (especially drill life), the amount of MgO with respect to a total of 100 mass% of oxide inclusions in steel is preferably 10 mass% or less, more preferably 7 mass% or less, and even more preferably. Is 3% by mass or less. In particular, reducing the MgO amount from 15% by mass to 3% by mass can double the drill life (FIG. 1). On the other hand, the lower limit of the amount of MgO in the steel for machine structure of the present invention is usually about 0.1% by mass.

  The amount of MgO in mechanical structural steel can be measured by an X-ray microanalysis (EPMA) energy dispersion method (EDS) as shown in the following examples.

In order to reduce the amount of MgO in the steel, the present inventors have also found that it is only necessary to adjust the component of the slagging agent to lower the slag basicity (CaO / SiO ₂ ) during the molten steel treatment ( Figure 2). As this mechanism, it is conceivable that the lower the slag basicity, the less metal Mg is produced from the slag and the refractory, and the lower the mixing of Mg into the steel. Since Mg mixed in the steel becomes MgO, the amount of MgO can be reduced by suppressing the mixing. The amount of MgO in the steel is also affected by the amount of Al in the steel, as will be described later.

The approximate curve of FIG. 2 was calculated as y = 2.01 × ^1.37 , y: MgO amount (mass%), x: slag basicity (CaO / SiO ₂ ), and contribution rate R ² = 0.997. From this approximate curve, it can be predicted how much the slag basicity should be reduced in order to achieve the target MgO amount. Specifically, if the slag basicity is about 3.2, the amount of MgO in the steel is about 10% by mass, and if the slag basicity is about 2.5, the amount of MgO is about 7% by mass. If the slag basicity is about 1.3, the amount of MgO is about 3% by mass.

  Next, the chemical components of the steel for machine structure of the present invention will be described.

<C: 0.15-0.5%>
C is an important element for ensuring the strength of the final product (part). However, if the amount of C is excessive, the toughness of the steel is lowered, and it becomes too hard and the machinability (particularly the tool life) is lowered. Therefore, the C content is set to 0.15% or more (preferably 0.20% or more) and 0.5% or less (preferably 0.45% or less).

<Si: 0.01-2%>
Si is effective as a deoxidizing element and also has an effect of improving component strength by solid solution strengthening. However, if the amount of Si is excessive, the machinability is adversely affected. Therefore, the Si amount is set to 0.01% or more (particularly from the viewpoint of solid solution strengthening, preferably 0.2% or more), and 2% or less (preferably 1.5% or less).

<Mn: 0.1 to 2%>
Mn is an important element that improves the hardenability of steel and contributes to an increase in strength, and also improves the machinability by forming sulfide inclusions. However, if the amount of Mn is excessive, the machinability is rather lowered. Therefore, the amount of Mn is set to 0.1% or more (preferably 0.6% or more) and 2% or less (preferably 1.5% or less).

<Cr: 0.01-2%>
Cr is an element that improves the hardenability of steel and contributes to an increase in strength. However, if the amount of Cr is excessive, the machinability deteriorates. Therefore, the Cr amount is set to 0.01% or more (preferably 0.1% or more) and 2% or less (preferably 1% or less).

<P: 0.1% or less (excluding 0%)>
Since P is an element that causes grain boundary segregation and deteriorates impact resistance, the amount is preferably as low as possible. Therefore, the P amount is set to 0.1% or less (preferably 0.05% or less). However, since P is inevitably mixed in steel, it is difficult to reduce the amount to 0% in industrial production.

<S: 0.01 to 0.3%>
S is an element effective in forming sulfide inclusions such as MnS and improving machinability. However, when the amount of S is excessive, it becomes a starting point of cracking during hot or cold forging, and the deformability is lowered. Therefore, the S amount is set to 0.01% or more (preferably 0.04% or more) and 0.3% or less (preferably 0.15% or less).

<Al: 0.001 to 0.01%>
Al in the molten steel promotes the elution of Mg from the refractory of the furnace and increases the amount of MgO in the steel, and thus adversely affects the machinability of the steel. Moreover, when the amount of Al increases, the amount of O in the steel decreases, so that MnS is generated at a low temperature. As a result, MnS is refined and adversely affects machinability. Further, Al adversely affects the machinability by forming a hard Al-based oxide. Therefore, in the present invention, the amount of Al is preferably as low as possible. Therefore, the Al content is set to 0.01% or less (preferably 0.005% or less). However, it is difficult to reduce the Al content to 0% in industrial production. Therefore, the lower limit of the Al amount is set to 0.001% from the viewpoint of production cost and the like. In addition, the steel for machine structure is usually Al killed steel, and the amount of Al is usually about 0.02% or more. Therefore, conventional steel for machine structural use has a large amount of MgO, and Pb-free cannot ensure sufficient machinability.

<O: 0.001 to 0.02%>
If the amount of O is excessive, machinability (particularly tool life) is deteriorated. Therefore, the O amount is set to 0.02% or less (preferably 0.005% or less). In the present invention, since the slag basicity is lowered in order to reduce the amount of MgO in the steel, the amount of O in the steel increases to 0.001% or more.

<N: 0.001 to 0.025%>
N forms nitrides with Al, Ti, and the like to refine the austenite crystal grains, and as a result, has an effect of improving toughness and fatigue strength. However, if the amount of N is excessive, the toughness is rather lowered. Therefore, the N content is set to 0.001% or more (preferably 0.002% or more, more preferably 0.003% or more) and 0.025% or less (preferably 0.02% or less).

<Ca: at least one of 0.0001 to 0.01% and Zr: 0.01 to 0.2%>
Ca and Zr are solid elements in MnS and have a function of promoting spheroidization of MnS, and as a result, are important elements for improving machinability (particularly tool life). Conversely, in the absence of both Ca and Zr, MnS elongates and the machinability improvement effect of MnS decreases. Therefore, in the present invention, at least one of Ca and Zr is an essential element. However, if the Ca and Zr amounts are excessive in the molten steel, Mg in the refractory of the furnace is reduced and eluted, and the amount of MgO in the steel increases. Therefore, the Ca amount is 0.0001% or more (preferably 0.0005% or more), 0.01% or less (preferably 0.005% or less), and the Zr amount is 0.01% or more (preferably 0.00. 02% or more) and 0.2% or less (preferably 0.15% or less).

  The basic component composition of the steel for machine structural use of the present invention is as described above, and the balance is substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel. Furthermore, the steel of the present invention may contain the following selective elements as necessary.

<Ti: 0.2% or less (not including 0%), V: 0.5% or less (not including 0%), Mo: 1% or less (not including 0%), Nb: 0.1% At least one selected from the group consisting of: (not including 0%), Cu: 1% or less (not including 0%), and Ni: 2% or less (not including 0%)>
Ti, V, Mo, Nb, Cu and Ni are effective elements for improving the strength, and may be contained in steel as necessary. From the viewpoint of strength, the Ti amount is preferably 0.01% or more (more preferably 0.02% or more), and the V amount is preferably 0.03% or more (more preferably 0.06% or more). The amount of Mo is preferably 0.05% or more (more preferably 0.10% or more), and the amount of Nb is preferably 0.010% or more (more preferably 0.025% or more). The amount of Cu is preferably 0.02% or more (more preferably 0.1% or more), and the amount of Ni is preferably 0.02% or more (more preferably 0.1% or more).

  However, if the amount of these elements is excessive, the machinability is adversely affected. Therefore, when these elements are contained, the Ti amount is 0.2% or less (preferably 0.1% or less), the V amount is 0.5% or less (preferably 0.3% or less), and the Mo amount is 1%. (Preferably 0.5% or less), Nb amount is 0.1% or less (preferably 0.05% or less), Cu amount is 1% or less (preferably 0.5% or less), and Ni amount is 2 % Or less (preferably 1% or less).

<Se: 0.01% or less (not including 0%), Te: 0.01% or less (not including 0%), Bi: 0.1% or less (not including 0%), rare earth elements: 0 0.01% or less (not including 0%), and B: at least one selected from the group consisting of 0.005% or less (not including 0%)>
Se, Te, Bi, REM and B are effective elements for improving the machinability, and may be contained in the steel if necessary. The rare earth element (REM) includes 15 elements of Sc, Y, and lanthanoid. Industrially, misch metal is used as REM. From the viewpoint of machinability, Se is preferably 0.001% or more (more preferably 0.005% or more), and the Te amount is preferably 0.001% or more (more preferably 0.005% or more). Bi is preferably 0.005% or more (more preferably 0.010% or more), and the REM amount is preferably 0.0001% or more (more preferably 0.001% or more). , B amount is preferably 0.0005% or more (more preferably 0.002% or more).

  However, if these amounts are excessive, the hot deformability is lowered and it is difficult to manufacture parts. Therefore, when these elements are contained, the Se amount is 0.01% or less, the Te amount is 0.01% or less, the Bi amount is 0.1% or less (preferably 0.05% or less), and the REM amount is 0.00. The content was set to be 01% or less (preferably 0.005% or less) and the B amount was 0.005% or less (preferably 0.0035% or less).

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Example 1
Steels having chemical composition shown in Tables 1 and 2 were melted. And the slag basicity at the time of a molten steel process was changed suitably using the faux agent of a composition with low basicity, or the faux agent of a composition with high basicity. The slag basicity values analyzed directly from the slag of each steel are shown in Tables 3 and 4. Next, the molten steel was cast and rolled into a 80 mmφ bar steel to prepare a test material. With each sample material, the amount of MgO (% by mass), the average size of MnS (μm ² ), and the aspect ratio of MnS were measured. In addition, the following cutting tests (drill test and carbide turning test) were performed. These results are also shown in Tables 3 and 4.

<Measurement of MgO amount>
The amount of MgO was measured using an energy dispersive X-ray spectrometer (EDS). Specifically, ten oxide inclusions were extracted at random from the steel material. Usually, if an area of 10 mm × 10 mm is observed on the polished surface, about 10 to 30 oxide inclusions can be found. The polished surface (observation region) can have an arbitrary cross section, but is preferably parallel to the rolling direction. This is because the positional relationship between oxides and sulfides is easy to understand. Next, the 10 oxide inclusions extracted as described above were quantitatively analyzed by EDS, and the ratio of the metal element contained in each inclusion was determined. From these ratios, it is assumed that Mg is MgO, Al is Al ₂ O ₃ , Ca is CaO, Mn is MnO, Si is SiO ₂ , and Zr is ZrO _2. The amount of MgO relative to 100% by mass was determined for each oxide inclusion. Mn and the like may be sulfides, but all oxides are assumed to be oxides because the oxide inclusions are observed. Since oxides appear black and sulfides appear gray, oxides and sulfides can be clearly distinguished. The average value calculated from the MgO content of each inclusion determined in this way was defined as the “MgO content” of the steel.

<Measurement (calculation) method of average size and aspect ratio of MnS>
In a longitudinal section of D / 4 part (D: plate thickness), a field of 1 mm ² was observed with an optical microscope at a magnification of 1000 times, and the area of each sulfide inclusion of 1 μm or more was observed. The average value of each area was defined as “MnS average size” of steel. Further, the major axis of each sulfide inclusion and the width in the direction perpendicular to the major axis direction were measured, and the aspect ratio (major axis / width) was determined therefrom. The average value of each aspect ratio was defined as “MnS aspect ratio” of steel.

<Drill test>
Using a 10 mmφ straight drill (SKH51, no surface coating), cutting speed: 20 m / min, feed: 0.2 mm / rev, work piece steel thickness: 40 mm, dry type (no cutting oil) A test was conducted. In the drill test, holes with a depth of 30 mm (unpenetrated) were successively opened in the steel material to be cut, and the total length of the drill hole until the drill could not be drilled was measured as the tool life.

<Carbide turning test>
Carbide turning test using chip of carbide tool P10 (JIS B4053) under the conditions of cutting speed: 150 m / min, cutting depth: 1.5 mm, feed: 0.25 mm / rev, dry type (no cutting oil) Went. And as tool life, the cutting time until the wear length of the tool side surface became 0.2 mm was measured.

  From the results in Tables 3 and 4, the steel No. 1 satisfying the requirements of the MgO amount and the chemical component amount of the present invention was obtained. It can be seen that 3-23 and 26-58 show good tool life (drill hole length and cutting time). On the other hand, steel No. with high slag basicity. In 1 and 2, the amount of MgO exceeds 10% by mass, and the tool life is insufficient. Steel no. Nos. 24 and 25 have an excessive amount of MgO due to an excessive amount of Al. Like 1 and 2, the tool life is insufficient.

  In order to make it easier to understand the relationship between the amount of MgO and the tool life (drill hole length) and the relationship between the slag basicity and the amount of MgO, steel Nos. With similar chemical composition are used. Graphs using data 1-7 are shown in FIGS.

Steel No. 1 of Example 1 It is a graph which shows the relationship between the amount of MgO in 1-7, and the length (tool life) of a drill hole. Steel No. 1 of Example 1 It is a graph which shows the relationship between the slag basicity in 1-7, and the amount of MgO.

Claims (3)

C: 0.15 to 0.5% (meaning mass%, the same applies to chemical components of steel),
Si: 0.01-2%
Mn: 0.1 to 2%,
Cr: 0.01-2%
P: 0.1% or less (excluding 0%),
S: 0.01 to 0.3%,
Al: 0.001 to 0.01%
O: 0.001 to 0.02%,
N: 0.001 to 0.025%
Further containing at least one of Ca: 0.0001 to 0.01% and Zr: 0.01 to 0.2%, the balance consisting of iron and inevitable impurities,
A machine structural steel excellent in machinability, characterized in that the amount of MgO is 0.1 to 10% by mass with respect to a total of 100% by mass of oxide inclusions in the steel.   Ti: 0.2% or less (not including 0%), V: 0.5% or less (not including 0%), Mo: 1% or less (not including 0%), Nb: 0.1% or less (Claim 1) further containing at least one selected from the group consisting of (not including 0%), Cu: 1% or less (not including 0%), and Ni: 2% or less (not including 0%). Machine structural steel as described in 1.   Se: 0.01% or less (not including 0%), Te: 0.01% or less (not including 0%), Bi: 0.1% or less (not including 0%), rare earth elements: 0.0. The machine structure according to claim 1 or 2, further comprising at least one selected from the group consisting of 01% or less (not including 0%) and B: 0.005% or less (not including 0%). steel.

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