JP4936639B2 - Free-cutting steel for machine structure - Google Patents

Description

  The present invention relates to a free-cutting steel for machine structures having a small aspect ratio, a small anisotropy of mechanical properties, and a wide range of machinability.

Conventionally, steel for machine structures including automobile parts often contains various free-cutting substances for the purpose of reducing cutting costs. Typical free-cutting steels include Pb free-cutting steel, S free-cutting steel, Ca deoxidized free-cutting steel, and composite free-cutting steels thereof. Pb free-cutting steel is the most commonly used because it has less deterioration in mechanical properties than the basic steel and has good machinability improvement effects, especially tool life and chip disposal during low-speed cutting. It has been. However, since Pb is harmful to the human body, it is in the direction of reducing the amount of use worldwide due to the recent increase in interest in environmental problems, and the demand for free cutting steel as an alternative to Pb free cutting steel is also increasing. In that case, transition to S free-cutting steel can be considered, but since S exists as MnS inclusions extending in the rolling direction, the addition of a large amount of S has the disadvantage that the anisotropy of mechanical properties increases. is there. Ca deoxidized free-cutting steel contains low-melting point CaO.Al ₂ O ₃ .SiO ₂ -based oxide in the steel, and this oxide forms a protective film on the tool edge. The machinability is improved by preventing direct contact. However, Ca deoxidized free-cutting steel is only effective during relatively high-speed cutting such as carbide tool turning. Pb ternary free-cutting steel, which is a composite of all of Pb, S, and Ca, is also used in many cases. Free-cutting steel is required.

In order to improve the mechanical properties of S free-cutting steel, Ca is contained (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3). In this case, further or changing the Al ₂ O ₃ to CaO · Al ₂ O _3, has also been reported to be harmless to cover at sulfides. Hexagonal BN, CaO.Al ₂ O ₃ , and Ca—Mn—S are contained to improve machinability (see, for example, Patent Document 4). However, in these cases, the mechanical property anisotropy improves the degree of deterioration from the basic steel by controlling the morphology of the sulfide, but the machinability does not always give satisfactory results under various cutting conditions. Absent.

  On the other hand, unlike the form control by Ca, there is an invention of free-cutting steel with a new machinability improving mechanism (for example, see Patent Document 5). In this case, Al, B, and N are included, and the machinability is improved by AlN adhering to the tool during cutting. In this case, the machinability can be achieved only by relatively high-speed cutting of 200 m / min or more. The improvement effect cannot be expected. Moreover, since it is necessary to always contain B, hardenability and crystal grain adjustment are difficult.

Japanese Patent No. 1981560 JP-A-11-950065 JP 2000-34538 A Japanese Patent Laid-Open No. 6-145889 JP 2000-26935 A

  The present invention is to provide a free-cutting steel for machine structure that can minimize deterioration of mechanical properties and obtain good machinability when compared with a basic steel, and is very useful industrially. Is to provide steel.

  Means of the present invention for solving the above-mentioned problems are, in mass%, C: 0.01 to 0.70%, Si: 0.05 to 1.80%, Mn: 0.30 to 3.50%. Ca: 0.0003 to 0.02%, S: 0.02 to 0.20%, Al: 0.003 to 0.10%, N: 0.003 to 0.025%, and Among Ca, S, Al, and N, the relationship of 0.1 ≦ [1000 (Al + N) × Ca / S] ≦ 4.5 is satisfied, the balance is made of Fe and inevitable impurities, and the major axis is 0.5 μm or more. This free-cutting steel has an average aspect ratio of major axis / minor axis of 5 or less, a small anisotropy of mechanical properties, and a wide range of machinability.

  The reason for limiting the component of the alloy element component of the steel of the present invention will be described below. Hereinafter, “%” represents “% by mass”.

  C is added to ensure the strength of the steel. If it is less than 0.01%, securing of strength is insufficient, and if it exceeds 0.70%, the toughness decreases, so the content is made 0.01 to 0.70%.

  Si is added for deoxidation in steel making and for ensuring strength. If it is less than 0.05%, the deoxidation effect is insufficient, and if it exceeds 1.80%, the hot workability deteriorates, so 0.05 to 1.80%.

  Mn is added to improve hardenability. In addition, it is an indispensable element for generating S and sulfides to improve machinability. Furthermore, MnS also has the effect of suppressing austenite grain growth and refining the structure. If it is less than 0.3%, this effect is small, and if it exceeds 3.50%, the workability deteriorates, so 0.30 to 3.50%.

  Ca is the most important element in the present invention, and is an indispensable element for improving anisotropy by suppressing sulfide morphology and for attaching a protective film of (Mn, Ca) S and AlN on the tool. This effect is obtained at 0.0003% or more, desirably 0.001% or more, and even if contained over 0.02%, the effect is saturated, and rather the Ca addition yield deteriorates. -0.02%.

  S forms sulfides such as MnS and (Mn, Ca) S, and further forms a protective film of (Mn, Ca) S and AlN on the tool to improve machinability. Further, when heated to 1000 ° C. or higher for hot working, non-heat treated steel also has an effect of increasing toughness in order to suppress austenite grain growth. In order to obtain these effects, at least 0.02% or more is required, and desirably 0.05% or more is necessary. However, if it exceeds 0.20%, the toughness deteriorates due to the application concentration effect of sulfide, so 0.02 to 0.20%.

  Al is added for deoxidation in steel making, similar to Si. Moreover, it adheres as AlN on a cutting tool, forms a protective film like (Ca, Mn) S, and is an indispensable element for improving the tool life. Furthermore, AlN is formed in the steel and contributes to austenite grain refinement. In order to obtain the effect, 0.003% or more is necessary, and if added over 0.20%, toughness and machinability deteriorate due to Al oxide, so 0.003 to 0.10%.

  N is added for toughening. Moreover, it adheres as AlN on a cutting tool, forms a protective film like (Ca, Mn) S, and is an indispensable element for improving the tool life. Further, AlN is formed in the steel, and there is an effect of austenite grain refinement. In order to obtain the effect, 0.003% or more is necessary, and even if added over 0.025%, the effect is saturated, so 0.003 to 0.025%.

  Ca, S, Al, and N are indispensable elements for improving the machinability as described above, but the balance of each element is important. When 1000 (Al + N) × Ca / S is less than 0.1, the effect of tool covering by (Ca, Mn) S and AlN is small, and when it exceeds 4.5, the effect is saturated or rather lowered. The value of (Al + N) × Ca / S is 0.1 to 4.5.

  Furthermore, the machinability improving effect in the present invention will be described. In the present invention, the notch effect of the sulfide by increasing the amount of S is increased, and by adding Ca, since the sulfide becomes (Mn, Ca) S, the notch effect is further increased, and relatively low speed cutting is performed. Even at times, good machinability can be obtained. Further, in the cutting speed range of 150 m / min or more, a protective film of (Mn, Ca) S and AlN adheres to the tool edge, and has an effect of suppressing diffusion wear and adhesion peeling wear. This protective film cannot be generated if one of S, Ca, Mn, Al, and N is missing.

  Further, the improvement of the anisotropy of mechanical properties in the present invention will be described. In the present invention, sulfide form control is performed by adding Ca. By suppressing the average aspect ratio of the major axis / minor axis of a sulfide having a major axis of 0.5 μm or more to 5 or less, anisotropy of mechanical properties is reduced.

  As described above, the free-cutting steel for machine structure according to the present invention has little deterioration in mechanical anisotropy and can obtain very good machinability without containing harmful substances such as Pb. Thus, the present invention has an excellent effect that has never been achieved.

  An embodiment of the present invention will be described with reference to Table 1. First, non-tempered steel containing the alloy component elements shown in Table 1 was melted in a 100 kg vacuum melting furnace.

  In Table 1, no. 1-4 is steel which concerns on embodiment of this invention. On the other hand, no. 5 to 10 are steels for comparison. That is, no. No. 5 has more C than specified. 6 has more Mn than prescribed. No. No. 7 contains less Ca and Al than specified, and contains Pb. No. 8 has more Mo than specified. No. 9 has more Ti than specified. 10 is the amount of S less than prescribed. Further, regarding the balance of Ca, S, Al, and N, No. No. 7 has 1000 (Al + N) × Ca / S less than specified. 9 and no. 10 is more than specified.

  The steel composed of the above chemical components was cast into a steel ingot, and the steel ingot was forged into a steel bar having a diameter of 45 mm at 1200 ° C. and allowed to cool. Further, among these, No. 1, no. 2, no. 3, no. 4, no. 6 and no. No. 9 steel ingot was reheated to 1200 ° C., held for 1 hour and then air-cooled. 5, no. 7, no. 8 and no. Ten steel ingots were quenched and tempered, and all steel types were adjusted to 27 to 33 HRC.

The adjusted steel was subjected to the following tests.
(1) Charpy impact test in L direction and T direction: The rolling direction is indicated as L direction, and the direction perpendicular to the rolling direction is also indicated as T direction. The Charpy impact test is performed at room temperature using a JIS 2 mm U notch impact test piece.
(2) Turning carbide tool wear test: P20 tool, cutting speed 150m / min and 300m / min, feed 0.1mm / rev, cutting depth 0.5mm. The surface wear amount is VB.
(3) Drill life test: φ5 mm high-speed drill, cutting speed 25 m / min, feed 0.1 mm / rev, hole depth 15 mm, and the evaluation method is the dry method and the number of drill holes until drill breakage.
(4) Sulfide aspect ratio measurement: After mechanical polishing of the plane parallel to the L direction (L plane), 20 x400 optical micrographs are taken and measured with an image analyzer. And the average value of the sulfide aspect ratio of each steel type is calculated.

  The test results are shown in Table 2.

  No. 1 which is steel of the invention according to the claims of the present invention. The average aspect ratio of sulfides having a major axis of 0.5 μm or more present in the invention steels 1 to 4 is 5 or less, and the normal temperature Charpy impact anisotropy has T / L of 0.5 or more. However, no. 7, no. No. 8 has an average aspect ratio of more than 5 and an impact anisotropy of less than 0.5. 7, no. It can be seen that when 8 is used, it is necessary to consider the directionality.

  The drill life of the steel according to the present invention is at least 70 holes or more. 5-7, no. 9 does not reach this. In the carbide tool wear test by turning, when the cutting speed is 150 m / min, the steel according to the present invention has a wear amount of 0.12 mm or less. 5, no. 7 to 9 are more worn out. No. 6, no. For No. 10, the wear amount at a cutting speed of 150 m / min is relatively small, but when the cutting speed is 300 m / min, it is impossible to suppress the wear amount to 0.25 mm or less as in the steel of the present invention.

Claims (1)

In mass%, C: 0.01 to 0.70%, Si: 0.05 to 1.80%, Mn: 0.30 to 3.50%, Ca: 0.0003 to 0.02%, S: 0.02 to 0.20%, Al: 0.003 to 0.10%, N: 0.003 to 0.025%, and among Ca, S, Al, and N, 0.1 ≦ [1000 (Al + N) × Ca / S] ≦ 4.5, the balance is Fe and inevitable impurities, and the average aspect ratio of the major axis / minor axis of the sulfide having a major axis of 0.5 μm or more is 5 Free-cutting steel with the following low mechanical anisotropy and excellent machinability.