JP6477383B2 - Free-cutting steel - Google Patents

Description

  The present invention relates to a free-cutting steel suitable as a machine structural part used in automobiles and various industrial machines, and particularly provides a high-strength BN free-cutting steel having a tensile strength of 900 MPa or more.

  Free-cutting steel has improved machinability by including low-melting point metals and non-metallic inclusions in the steel. Examples of non-metallic inclusions include Mn sulfide, Ti sulfide, Ca oxide, and Various materials using BN inclusions (hereinafter, BN inclusions may be simply referred to as BN) and low melting point metals using Pb or Bi have been developed. Among these free-cutting steels, Pb free-cutting steel has been used extensively because it is inexpensive and has excellent performance. However, the application of Pb has been restricted from the viewpoint of global environmental protection, and BN free-cutting steel is newly added. Interest in is growing.

  The invention described in Patent Document 1 relates to a steel for machine structure that can be used in a wide range of applications and has excellent machinability. Patent Document 1 describes what is excellent in machining parts having various shapes such as gears and shafts by containing a specific amount of BN in steel.

  The invention described in Patent Document 2 relates to a BN free-cutting steel having improved machinability without deteriorating mechanical properties and hot ductility caused when BN is contained in the steel. Patent Document 2 describes suitable parts for automobiles such as engine parts and undercarriage parts.

  The invention described in Patent Document 3 relates to a non-tempered free-cutting steel that is finished by performing a finishing process such as a cutting process without performing a tempering treatment. Patent Document 3 describes a BN free-cutting steel containing a specific amount of BN in order to obtain excellent machinability without reducing the strength.

  The invention described in Patent Document 4 relates to free-cutting steel that enables surface hardening treatment by induction hardening instead of carburizing and quenching performed after machining such as gear cutting. Patent Document 4 describes BN free-cutting steel having improved machinability due to the lubricating effect of BN dispersed as fine particles and evenly dispersed in the material structure.

  In Patent Document 5, in order to improve fatigue strength and impact resistance characteristics in a certain angle direction with respect to BN that extends in the processing direction by rolling or forging, sulfides are finely dispersed, and the sulfides are precipitated nuclei. It is described that the adverse effect of BN is reduced.

  The invention described in Patent Document 6 relates to a machine structural steel excellent in machinability. In Patent Document 6, in order to obtain machinability equivalent to or higher than that of a conventional Pb composite-added free-cutting steel without using Pb, Al, B, and N are added in combination in the component composition, and the metal structure is ferrite and A BN free-cutting steel characterized by a graphite phase is described.

JP-A-62-211350 Japanese Patent Laid-Open No. 2-73950 JP-A-1-219148 JP-A-5-271868 JP-A-6-145890 JP-A-6-212348

  By the way, the following is known about the influence of the strength of free-cutting steel on machinability.

  1. When the strength of non-BN free-cutting steel is increased, the increase in cutting temperature becomes significant, so that the diffusion of tool-forming components from the tool side to the work material (free-cutting steel) side becomes significant. Diffusion wear becomes prominent and tool life is greatly shortened.

  2. On the other hand, when the strength of BN free-cutting steel is increased, an AlN film is formed on the tool surface during cutting, and this film suppresses the above-described diffusion wear of the tool, so that the tool life is dramatically improved. .

  The generation of the AlN film during cutting progresses remarkably at higher temperatures. However, the cutting temperature increases as the strength of the high strength material increases. Therefore, the tool life of the BN free cutting steel increases as the strength of the work material increases. It is expected to be superior to cutting steel. However, contrary to expectation, the BN free-cutting steels described in Patent Documents 1 to 6 cannot be said to have a sufficient effect for improving the machinability, especially in the case of a high-strength material having a tensile strength exceeding 900 MPa. The decline in sex appears remarkably.

  Therefore, the present invention aims to solve the above-mentioned problems, and to provide a free-cutting steel having excellent machinability, particularly excellent tool life, and a tensile strength of 900 MPa or more. In the present invention, the machinability includes tool life and chip disposal.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive research on free-cutting steel from the viewpoint of AlN film generation conditions, not BN miniaturization. As a result, it has been found that the content of B, N, Ti and Al and the content balance between these elements have a great influence on the formation of the AlN film. The present inventors have further studied and have obtained useful knowledge about inclusions. That is, it has been found that it is preferable to include inclusions that are larger and have an aspect ratio of a certain value or less.

  The present invention has been made by further study based on the obtained knowledge. The gist of the present invention is as follows.

[1] By mass%, C: 0.30% to 0.60%, Si: more than 0.10% to 0.50% or less, Mn: 0.30% to 1.20%, P: 0.00. 030% or less (excluding 0%), S: more than 0.015% and 0.200% or less, Cr: 0.05% or more and 0.50% or less, V: more than 0.25% and 0.35% or less, Al: 0.010% or more and 0.050% or less, B: 0.0030% or more and 0.0100% or less, N: 0.0070% or more and 0.0200% or less, with the balance being Fe and inevitable impurities A free-cutting steel having a component composition, the S value shown below satisfies the formula (1), and the tensile strength is 900 MPa or more.
S value = (B + N / 1.7) × 2Al− (1300−TS) / 10 ⁶
1.5 × 10 ⁻³ > S value> 1.0 × 10 ⁻⁴ (1)
Here, each alloy element represents the content (% by mass), and TS represents the tensile strength (MPa) at the time of cutting.

  [2] The composition according to [1], further including one or more of Nb: 0.050% or less, Ti: 0.050% or less, and Mo: 0.50% or less in mass% in addition to the component composition. Free-cutting steel.

  According to the present invention, free-cutting steel with a tensile strength of 900 MPa or more excellent in tool life is obtained, the efficiency at the time of cutting is improved, and it is extremely useful industrially.

  Hereinafter, the present invention will be specifically described.

  The present invention is a free-cutting steel having a tensile strength of 900 MPa or more. From the viewpoint of improving fatigue strength, the tensile strength is preferably 925 MPa or more.

  In the present invention, the reason why the composition of the free-cutting steel is limited to the above range will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.

[component]
C: 0.30% or more and 0.60% or less C is an element having a great influence on hardenability and machinability. If the content is less than 0.30%, the effect is poor, and the chip disposability of the machinability cannot be ensured sufficiently. On the other hand, if it exceeds 0.60%, the hardness is remarkably increased and the tool life is decreased. Therefore, the content is set to 0.30% or more and 0.60% or less. Preferably it is 0.40% or more and 0.50% or less of range.

Si: more than 0.10% and 0.50% or less Si is an element useful for improving chip treatability, and if its content is 0.10% or less, a sufficient improvement effect cannot be obtained. On the other hand, if it exceeds 0.50%, the hardening of the ferrite becomes remarkable and the tool life of the machinability decreases, so the content is made more than 0.10% and 0.50% or less. Preferably it is 0.20% or more and 0.40% or less of range.

Mn: 0.30% or more and 1.20% or less Mn is an element necessary for generating MnS effective in improving the tool life. When the content is less than 0.30%, a sufficient amount of MnS cannot be generated, and thus the effect of improving the tool life is not recognized. On the other hand, if it exceeds 1.20%, the hardenability is improved, the hardness rises remarkably, and the tool life is reduced, so the content is made 0.30% or more and 1.20% or less. Preferably it is 0.60% or more and 0.90% or less of range.

P: 0.030% or less (excluding 0%)
P is an element having a high solid solution strengthening ability, and the tool life is reduced through hardening of the ferrite. Accordingly, the content is set to 0.030% or less (excluding 0%). Preferably, it is 0.015% or less (excluding 0%). On the other hand, the reduction of P to less than 0.003% results in excessive cost for de-Ping, so the lower limit of the P content is preferably 0.003% from the viewpoint of manufacturing cost.

S: More than 0.015% and 0.200% or less S is an element necessary for producing MnS effective for improving the tool life. When the content is 0.015% or less, a sufficient amount of MnS cannot be generated, and thus the effect of improving the tool life is not recognized. Therefore, the S content is more than 0.015%. On the other hand, excessive addition reduces fatigue strength and toughness, so the upper limit was made 0.200%. Preferably it is 0.030% or more and 0.100% or less of range.

Cr: 0.05% or more and 0.50% or less Cr is an element effective for improving hardenability, but its effect is poor when the content is less than 0.05%, while 0.50% is reduced. When it exceeds, not only the hardenability will be improved and the hardness will be remarkably increased, but also due to the fixation of the solid solution C and N, the strain aging is less likely to occur, and the machinability is lowered. Therefore, the Cr content is limited to a range of 0.05% to 0.50%. Preferably it is 0.10% or more and 0.30% or less of range.

V: more than 0.25% and not more than 0.35% V precipitates carbides and carbonitrides during cooling and contributes to improvement of tensile strength and fatigue strength. Therefore, V is contained exceeding 0.25%. On the other hand, when the content exceeds 0.35%, the amount of precipitation is too large, and the increase in hardness becomes remarkable, and the tool life is reduced. Therefore, the V content is limited to the range of more than 0.25% and not more than 0.35%. did. Preferably it is 0.28% or more and 0.32% or less of range.

Al: 0.010% or more and 0.050% or less Al is an element necessary for deoxidation. In BN free-cutting steel, an AlN film is formed on the tool surface to cause diffusion wear, that is, toward the work material side. Although it is an element necessary for suppressing wear due to diffusion of the tool material component, the effect cannot be sufficiently obtained if it is less than 0.010%. On the other hand, if the content exceeds 0.050%, the effect is saturated and nozzle clogging during continuous casting occurs, and fatigue strength and tool life are reduced due to the appearance of alumina cluster inclusions. It was limited to the range of 0.010% or more and 0.050% or less. Preferably it is 0.020% or more and 0.040% or less of range.

B: 0.0030% or more and 0.0100% or less B is an element necessary for generating BN inclusions effective for improving the tool life, and is an important element related to the basis of the present invention. If the content is less than 0.0030%, a sufficient amount of BN inclusions cannot be generated, so that the effect of improving the tool life is not recognized. On the other hand, if it exceeds 0.0100%, the hardenability is improved and the hardness rises remarkably, so that the tool life is reduced. Therefore, the content is set to be 0.0030% or more and 0.0100% or less. Preferably it is 0.0040% or more and 0.0080% or less of range.

N: 0.0070% or more and 0.0200% or less N is an element necessary for generating BN inclusions effective in improving the tool life, and is an important element related to the basis of the present invention. If the content is less than 0.0070%, a sufficient amount of BN inclusions cannot be generated, so that the effect of improving the tool life is not recognized. On the other hand, when it exceeds 0.0200%, the hot ductility decreases, so the N content is limited to the range of 0.0070% to 0.0200%. Preferably it is 0.0100% or more and 0.0150% or less of range.

  The balance is Fe and inevitable impurities.

  The above is the basic component composition of the present invention. When further improving the characteristics, one or more of Nb, Ti, and Mo can be contained.

Nb: 0.050% or less Nb refines crystal grains and reinforces grain boundaries to contribute to improvement in fatigue strength. When Nb is contained, it is preferably made at least 0.005% or more. On the other hand, the effect is saturated at 0.050% and the cost increases, so the upper limit was made 0.050%.

Ti: 0.050% or less Ti has the effect of refining the structure, strengthening the grain boundaries, and improving the fatigue strength. In order to exert such an effect, the content is preferably 0.005% or more, but the effect is saturated at 0.050% and increases the cost. The following.

Mo: 0.50% or less In order to improve the strength, Mo is preferably at least 0.05% or more when contained. On the other hand, the effect is saturated at 0.50% and the cost increases, so the upper limit was made 0.50%.

[About S value]
In the present invention, it is not sufficient for each element to simply satisfy the above range, and it is important to satisfy the relationship of the following formula.
S value = (B + N / 1.7) × 2Al− (1300−TS) / 10 ⁶
1.5 × 10 ⁻³ > S value> 1.0 × 10 ⁻⁴ (1)
Here, each alloy element represents the content (% by mass), and TS represents the tensile strength (MPa) at the time of cutting.

As the tensile strength of the work material at the time of cutting increases, the tool life decreases. Therefore, there are optimum B, N, and Al amounts according to the strength at the time of cutting. The S value is a parameter that affects the generation of an AlN film effective for improving the tool life when the tensile strength at the time of cutting is 900 MPa or more, and represents the necessary amount of B, N, and Al. When the S value is 1.0 × 10 ⁻⁴ or less, an AlN film is not sufficiently formed on the tool surface, and thus the effect of improving the tool life is not recognized. On the other hand, when the S value is 1.5 × 10 ⁻³ or more, the AlN film is not stably generated, and thus the effect of improving the tool life is not recognized. Therefore, the S value is set to 1.5 × 10 ⁻³ > S value> 1.0 × 10 ⁻⁴ . The tensile strength of the work material at the time of cutting refers to the tensile strength immediately before the start of cutting.

[Inclusions]
Although there are 300 or more total BN and MnS having an equivalent circle diameter of 2 μm or more and an aspect ratio of 15 or less per 1 mm ² as inclusions in steel, machinability is improved as long as BN and MnS inclusions are present. The larger the degree, the greater the improvement. In addition, the smaller the aspect ratio, the smaller the component cutting edge, so that the surface roughness is improved and a good tool life can be obtained. In order to obtain such an effect, it is preferable that a total of 300 or more BN and MnS having an equivalent circle diameter of 2 μm or more and an aspect ratio of 15 or less exist per 1 mm ² .

  The inclusions are observed in the cross section in the L direction of free-cutting steel. The equivalent circle diameter of inclusions can be obtained by analyzing an observation image with an optical microscope of 400 times with an image analyzer.

  The free-cutting steel according to the present invention is manufactured by casting a molten steel melted in a predetermined component composition, hot rolling into a desired shape, or hot forging after reheating the hot rolled material. Thereafter, heat treatment such as normalization or aging treatment may be performed in order to adjust the microstructure and strength before the start of cutting.

  Hereinafter, according to an Example, the structure and effect of this invention are demonstrated more concretely. However, the present invention is not limited by the following examples, and can be appropriately changed within a range that can be adapted to the gist of the present invention, and all the inventive steels are within the technical scope of the present invention. included.

Steels having the composition shown in Table 1 were melted and adjusted to 50 mmφ round bar steel by hot rolling. The obtained round steel bar was heated to 1250 ° C. and hot forged to 30 mmφ at a finishing temperature of 1100 ° C. In addition, the cooling rate after forging was cooled to room temperature at 0.7 ° C./second. No. shown in the table. 1-15, No. 1 16-36 are invention steel and comparative steel, respectively. In the notation of S value in Table 1, “E-0X” means “× 10 ^−X ”. For example, the description “E-04” means “× 10 ⁻⁴ ”.

  The obtained round bar steel was subjected to structure observation, cutting test, tensile test, Ono-type rotary bending fatigue test, Charpy impact test, and inclusion investigation. The details of each survey are described below.

Microstructure observation In microstructural observation, specimens are taken from a round steel bar with a diameter of 30 mmφ obtained by hot forging, and a vertical cross section (L cross section) parallel to the forging direction is corroded with nital after polishing, and then optical microscope or scanning type Using an electron microscope (SEM), the type of phase was identified by cross-sectional structure observation (200 times), and the area ratio of each phase was determined.

Cutting (peripheral turning) test The cutting test was carried out under the conditions shown in Table 2 after first cutting the outer periphery by 1 mm to remove the surface scale and decarburized layer, and the tool life and chip disposal were evaluated. In the chip disposal test, since there are three feeding conditions and five cutting speed conditions, each condition was combined and tested under 15 conditions. In the tool life test, the cutting time at which the lateral flank wear width VB = 0.2 mm was 15 (min) or more was accepted, and in the chip disposal test, an evaluation score of 20 or less was accepted.

Tensile test In the tensile test, a JIS No. 4 tensile test piece was sampled from the center of a 30 mmφ round bar steel, and a tensile test was conducted according to JIS Z 2241 to examine the tensile strength (TS).

A test piece having a parallel part diameter of 8 mm was collected from a round bar steel having a rotating bending fatigue characteristic diameter of 30 mmφ. The obtained test piece, using a Ono-type rotary bending fatigue testing machine, the rotational speed: conducted at 3000 rpm, 10 ⁷ times as fatigue limit was measured rotating bending fatigue strength. 400 MPa or more was regarded as acceptable.

Charpy impact test In the Charpy impact test, a JIS No. 3 specimen was taken from the center of a 30 mmφ round bar steel and the impact value at room temperature was evaluated. 7 J / cm ² or more was considered acceptable.

In the middle part (D / 4 position (D: diameter)) of a round steel bar with a diameter of 30 mmφ for inclusion investigation, the equivalent circle diameter, aspect ratio, and total number of BN and MnS existing in a 1 mm ² region parallel to the rolling direction Obtained by an image analyzer. An image observed by an optical microscope with a magnification of 400 was input to the image analysis apparatus.

  The above results are shown in Table 3.

  The steels of the present invention (Nos. 1 to 15) all have a tensile strength of 900 MPa or more, and it is recognized that they have excellent tool life and chip controllability relative to the comparative steels (Nos. 16 to 36). It was. In the inclusion investigation, BN and MnS were confirmed.

  Comparative steel No. No. 16 has a C content lower than the range of the present invention. Therefore, the hardenability was poor, and the tensile strength and rotational bending fatigue strength were reduced. Moreover, since it becomes difficult for a chip | tip to break, chip disposal property was also inferior.

  Comparative steel No. In No. 17, since the C content was higher than the range of the present invention, the increase in hardness became remarkable, and the tool life decreased.

  Comparative steel No. In No. 18, the Si content was lower than the range of the present invention, the brittleness of the ferrite became insufficient, and the chips were difficult to break, so that the chip disposal was reduced.

  Comparative steel No. No. 19 has a Si content higher than the range of the present invention. Therefore, the hardening of the ferrite became remarkable and the tool life was reduced.

  Comparative steel No. No. 20 has a Mn content lower than the range of the present invention. Therefore, the hardenability was poor, and the tensile strength and rotational bending fatigue strength were reduced. Further, since a sufficient amount of MnS cannot be generated, the tool life is reduced.

  Comparative steel No. No. 21 has a Mn content higher than the range of the present invention, so that the hardenability is improved and the increase in hardness becomes remarkable. Therefore, the tool life was reduced.

  Comparative steel No. No. 22 has a P content higher than the range of the present invention. Therefore, the hardening of the ferrite became remarkable and the tool life was reduced. Moreover, the grain boundary strength was insufficient, and the rotational bending fatigue strength and impact value were also reduced.

  Comparative steel No. No. 23 has an S content lower than the range of the present invention. Therefore, a sufficient amount of MnS could not be generated, and the tool life was reduced.

  Comparative steel No. No. 24 has an S content higher than the range of the present invention. For this reason, the amount of MnS generated as a starting point of fatigue failure increased, and the rotary bending fatigue strength and impact value decreased.

  Comparative steel No. No. 25 has a Cr content lower than the range of the present invention. Therefore, the hardenability was poor, and the tensile strength and rotational bending fatigue strength were reduced.

  Comparative steel No. In No. 26, since the Cr content is higher than the range of the present invention, the hardenability is improved and the increase in hardness becomes remarkable. Therefore, the tool life was reduced.

  Comparative steel No. No. 27 has a V content lower than the range of the present invention. Therefore, the precipitation hardening ability was poor, and the tensile strength and rotational bending fatigue strength were reduced.

  Comparative steel No. No. 28 has a significant increase in hardness because the V content is higher than the range of the present invention. Therefore, the tool life was reduced.

  Comparative steel No. No. 29 has an Al content lower than the range of the present invention. Therefore, the AlN film is not sufficiently formed on the tool surface, and the tool life is reduced.

  Comparative steel No. In No. 30, since the Al content was higher than the range of the present invention, the hard alumina-based oxide increased, and the tool life and rotational bending fatigue strength decreased.

  Comparative steel No. No. 31 had a B content lower than the range of the present invention, a sufficient amount of BN inclusions were not generated, and an AlN film was not sufficiently formed on the tool surface, so that the tool life was reduced.

  Comparative steel No. In No. 32, since the B content is higher than the range of the present invention, the hardenability is improved and the increase in hardness becomes remarkable. As a result, the tool life and impact value decreased.

  Comparative steel No. In No. 33, the N content was lower than the range of the present invention, a sufficient amount of BN inclusions were not generated, and an AlN film was not sufficiently formed on the tool surface, so the tool life was reduced.

  Comparative steel No. No. 34 has an N content higher than the range of the present invention. Therefore, cracks occurred during hot working, and the test could not be carried out.

  Comparative steel No. 35 is within the component range of the present invention, but the S value is lower than the range of the present invention. Therefore, the AlN film is not sufficiently formed on the tool surface, and the tool life is reduced.

  Comparative steel No. 36 is within the component range of the present invention, but since the S value is higher than the range of the present invention, an AlN film is not stably formed. Therefore, the tool life was reduced.

Claims (2)

In mass%, C: 0.30% or more and 0.60% or less, Si: more than 0.10% and 0.50% or less, Mn: 0.30% or more and 1.20% or less, P: 0.030% or less (Excluding 0%), S: more than 0.015% and 0.200% or less, Cr: 0.05% to 0.50%, V: more than 0.25% and 0.35% or less, Al: 0 0.010% or more and 0.050% or less, B: 0.0030% or more and 0.0100% or less, N: 0.0070% or more and 0.0200% or less, and the balance is composed of Fe and inevitable impurities. has to satisfy the S value (1) shown below, a tensile strength of not less than 900 MPa, inclusions with a circle phase equivalent diameter per 1 mm ² is 15 or less BN and MnS in 2μm or more and an aspect ratio as Free-cutting steel with 300 or more in total.
S value = (B + N / 1.7) × 2Al− (1300−TS) / 10 ⁶ ,
1.5 × 10 ⁻³ > S value> 1.0 × 10 ⁻⁴ (1)
Here, each alloy element represents the content (% by mass), and TS represents the tensile strength (MPa) at the time of cutting. In addition to the above component composition, the mass is Nb: 0.050% or less, and Ti: 0.050%.
The free-cutting steel according to claim 1, which contains one or more of Mo: 0.50% or less.