JP2003034841A - Steel for machine structure superior in machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a graphite steel for imparting the prolonged life to a cutting tool and improved roughness to a machined surface. SOLUTION: The steel for a machine structure superior in machinability is characterized by including 1.0-2.0% C, 0.5-2.0% Si, 0.1-2.0% Mn, 0.001-0.1% P, 0.1-0.7% S, 0.001-0.05% Al, 0.0001-0.02% N, and 0.0001-0.009% Mg, having a metal structure consisting of ferrite, graphite and cementite, and having a graphitization rate exceeding 80%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for machine structural use which is formed by cutting or cold forging and is used as a part for automobiles, industrial machines, etc., and particularly cold workability is improved by graphitizing cementite. It is related to graphite steel.

[0002]

2. Description of the Related Art It has been conventionally known that cold forgeability and machinability are improved by changing the structure of medium carbon steel to the structure of ferrite and graphite. Is considered to be excellent in lubricity because it has a weak crystal structure, or because graphite functions as a chip breaker. The technique is disclosed in JP-A-49-678.
No. 16 publication. However, with this method, the cutting tool life is improved to the same level as Pb free-cutting steel, but there is a problem that the surface roughness of the cutting surface becomes rough due to the large growth of the constituent cutting edge formed between the tool and the work material. Has been done.

As a means for improving the roughness of the cutting surface, Japanese Patent Laid-Open No. 6-242242
In JP-A-212352, by forming a film of Pb, Bi, MnS, MnTe, MnSe having excellent lubricity on the interface between the tool and the work material, the adhesion of the tool and the ferrite is prevented and the formation of the constituent cutting edge is suppressed. It is disclosed that this can be done. However, the addition of a large amount of Pb, Bi, and S significantly hinders graphitization, and the annealing time for graphitization must be extended, which leaves a problem that the manufacturing cost increases.

On the other hand, as a means for accelerating the precipitation of graphite, it is effective to use BN as a precipitation nucleus of graphite in Japanese Patent Laid-Open No. 2-111842, and as a result, the particle size of graphite is about 5 to 10 μm. Miniaturization is disclosed. However, according to the investigation by the present inventors, in this method, although the graphite particle size is reduced, the maximum distance between graphite particles is about 100 μm, and the graphite dispersion is not uniform. This is because BN precipitates on austenite grain boundaries and MnS, so that BN precipitates on MnS stretched in the hot rolling direction in a row, or along the old austenite boundary in a B-shaped pattern.
It can be presumed that, as a result of N precipitation, graphite also precipitates in rows and meshes and becomes non-uniformly dispersed. Further, in order to use BN as graphite precipitation nuclei, a heat treatment for BN precipitation is required, which increases the heat treatment process and increases the manufacturing cost. B by controlled rolling
Although it is possible to envisage performing the N precipitation treatment during rolling, there remains a problem that the manufacturing process is restricted, such as requiring precise temperature control. Further, in the use of BN, there remains a problem that the cutting surface roughness is not improved due to the non-uniform dispersion of graphite even if the particle size of graphite is reduced.

Further, Japanese Patent Application Laid-Open No. 7-3390 discloses that the addition of Zr reduces the amount of solid solution N which ZrN inhibits graphitization and also functions as a graphite precipitation nucleus to make the graphite finer. Further, in Japanese Patent Application Laid-Open No. 10-140281, these composite sulfides are produced by the composite addition of Ca and Zr, and function as precipitation nuclei of BN. % Is disclosed. However, in these conventional methods, Zr
0.01 wt.% To 0.2 wt. % Addition of large amount of Zr is required. Therefore, coarse Zr (CN) or Z exceeding 10 μm
There remains a problem that precipitates such as rS are generated and mechanical properties such as fatigue strength and toughness are deteriorated, or coarse Zr (CN) accelerates tool wear and tool life is deteriorated.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a graphite steel having excellent cutting tool life and excellent cutting surface roughness as steel for machine structure.

[0007]

Means for Solving the Problems The present inventors set a carbon content of graphite steel to 1.0% or more and an S content of 0.1% or more, and added Mg, whereby a short-time annealing was performed. It was found that it is possible to achieve both softening, cutting tool life and cutting surface roughness. The machinability improvement mechanism is that the two-phase structure of graphite and ferrite allows the constituent cutting edge to grow moderately and suppresses tool wear. In addition to improving the lubricity by increasing C and S, the fineness of graphite size It is considered that, as a result of the fact that the growth of the constituent cutting edges was moderately suppressed by this, an excessive gap between the tool and the work material was prevented and the cutting surface roughness was improved.

In steel having a carbon content of more than 1.0%, the grain size of graphite becomes coarse when graphitizing cementite, which deteriorates induction hardening characteristics, cold forgeability, or fatigue characteristics of parts. The cause of graphite coarsening is that, in addition to the high graphite volume ratio, the oxygen concentration in the molten steel decreases due to high carbonization,
It is considered that this is because the oxides such as Al ₂ O ₃ that become the precipitation nuclei of graphite decreased. On the other hand, steel having an S content exceeding 0.1% remarkably prolongs the graphitization time. It is considered that S that is not precipitated as a sulfide such as MnS significantly hinders graphitization. Further, when the S content increases, the sulfide becomes coarse, and the graphite that precipitates sulfide at the site also becomes coarse,
Cold forgeability and fatigue properties are deteriorated, and since coarse graphite is not sufficiently decomposed after induction hardening, a structure in which martensite and ferrite are mixed is formed, and fatigue properties after heat treatment are significantly deteriorated.

The inventors of the present invention added a trace amount of Mg to the steel to obtain a high carbon steel having a carbon content of more than 1.0% and an S content of 0.
It has been found that even if the content is 1% or more, Mg-based oxides are finely dispersed, graphite is uniformly and finely dispersed by using them as precipitation nuclei, and the graphitization time is significantly shortened.

The form of the oxide formed by the addition of Mg is mainly MgO or MgAl ₂ O ₄ , but Mg-Si
There are also system-based and Mg-Ti-based oxides. Furthermore, by adding Mg, an oxide containing no Mg, such as Al ₂ O ₃ or Ti
₂ O _{3 and the} like also have the effect of miniaturization. Further, sulfides such as MnS that precipitate these oxides in the nucleus also show remarkable miniaturization. Oxides and sulfides, or composite inclusions of these, function as precipitation sites for graphite. Therefore, steel materials in which these oxides and sulfides are finely dispersed by adding Mg can be treated by annealing to obtain these oxides. Finely precipitates graphite with sulfides and nuclei as cores. Also, oxides and sulfides are BN
Unlike carbonitrides such as the above, it precipitates in molten steel or in the P-phase region of the final zone, so that it can be dispersed uniformly without being affected by the structure.

Further, by dispersing a large amount of graphite precipitation sites, the graphitization rate is increased and the annealing time required for graphitization can be shortened. Based on the above knowledge, the present inventor can graphitize high-C and high-S steels, which has been difficult in the past, in a short time, and causes the deterioration of cold forgeability and fatigue properties of coarse graphite. We have obtained a steel for machine structural use that has excellent machinability and that is prevented from forming.

The gist of the present invention is as follows.

(1) C: 1.0 to 2.0 in mass%
%, Si: 0.5 to 2.0%, Mn: 0.1 to 2.0
%, P: 0.001-0.1%, S: 0.1-0.5% Al: 0.001-0.05%, N: 0.0001-
0.02%, Mg: 0.0001 to 0.009%, the metal structure consisted of ferrite, graphite, and cementite, and the graphitization rate was more than 80%, which was excellent in machinability. Steel for machine structure.

(2) Mo: 0.01-0.
5%, Cr: 0.01 to 0.7%, Ni: 0.05 to 3
%, Co: 0.05-3%, Cu: 0.05-3%,
B: 0.0001 to 0.01% of 1 type or 2 types or more are further contained, The steel for machine structures of the said (1) characterized by the above-mentioned.

(3) Zr: 0.0005-by mass%
0.02%, Ca: 0.0001 to 0.005% of 1 type or 2 types is further contained, The steel for machine structures of (1) or (2) characterized by the above-mentioned.

(4) Ti: 0.001 to 0.001 by mass%
0.05%, Nb: 0.005-0.08%, V:
The steel for machine structural use according to any one of (1) to (3), further containing 0.005 to 0.2% of one kind or two or more kinds.

(5) Pb: 0.01 to 0.
05%, Bi: 0.01 to 0.05%, Sn: 0.05
~ 0.2%, Te: 0.002-0.02%, Se:
The steel for machine structural use according to any one of (1) to (4), further containing 0.002 to 0.02% of one kind or two or more kinds.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The reason for limiting the chemical composition of the steel for mechanical structure of the present invention will be explained below.

C forms graphite and improves the cutting tool life. The lower limit was set to 1.0% in order to secure a sufficient amount of graphite necessary for improving the tool life. The upper limit was 2.0% in order to ensure hot ductility during continuous casting.

Si is one of the powerful elements that promotes graphitization. It is necessary to add Si in order to precipitate a sufficient amount of graphite by a short-time annealing treatment so as to obtain a high graphitization rate, and the lower limit value thereof is 0.5%. However, if the Si content increases, the ferrite phase undergoes solid solution hardening and causes deterioration of cold workability, so the upper limit was made 2.0%.

Mn is combined with S and exists as MnS, or solid solution Mn in the matrix. MnS forms a single or complex inclusion to serve as a graphite generation site, and improves lubricity and cut surface roughness. Sufficient Mn
In order to secure the amount of S, the lower limit value was set to 0.1%. However, when the amount of solute Mn becomes large, graphitization is significantly hindered, so the upper limit was made 2.0%.

Since P causes grain boundary segregation or center segregation in steel and causes deterioration of toughness, it is desirable that P is small, but from the viewpoint of machinability, the surface roughness is improved in order to improve the roughness of the cut surface. In case of required steel, an appropriate amount is added. If the content is less than 0.001%, the effect is not recognized, so 0.001% was made the lower limit. Further, if over 0.1%, the toughness deteriorates and cracks may occur during rolling,
The upper limit was 0.1%.

S reacts with alloy elements such as Mn, Mg or Cu and exists as a sulfide. These sulfides function as nucleation sites for graphite, improve lubricity, and improve cut surface roughness. However, if it is less than 0.1%, a sufficient amount of sulfide cannot be secured, and if the amount of S is too large, the hot ductility deteriorates, so the upper limit was made 0.7%.

Al combines with O to form an oxide or N to form AlN. AlN is effective for making the crystal grains finer and improves the toughness after quenching and tempering. 0.
If it is less than 001%, the amount of AlN is insufficient and the grain refining effect does not appear. If it exceeds 0.05%, Al deoxidation becomes dominant and M
The effect of g is saturated.

N combines with Al or Ti to form AlN or TiN
Is generated, which is effective for making crystal grains finer and improves workability. Less than 0.0001% has no effect, 0.02
If it is added in excess of%, not only the effect will be saturated but also graphitization will be significantly impaired.

Mg forms oxides MgO and MgAl ₂ O _4, which either alone or form complex inclusions with sulfides and function as precipitation sites for graphite. If it is less than 0.0001%, the effect is small, and if it is contained 0.009% or more, the steelmaking cost increases. Further, the addition of Mg is effective for refining the graphite particles, and even if the graphitization rate is the same, finely dispersed particles are superior in performance such as induction hardening. That is, in hardening treatment by heating for a short time such as induction hardening, graphite must be solid-dissolved and diffused in steel in a short time in order to form a uniform surface hardened layer. Therefore, it is very effective to finely disperse graphite so that C can be uniformly diffused over the entire surface with a short diffusion distance. In this respect, Mg is a very effective element.

Mo is added to ensure hardenability. In order to sufficiently obtain the effect of hardenability, the lower limit value of the addition amount is set to 0.01%. If added in excess of 0.5%, the hardness of the ferrite base increases, cold workability is impaired, and graphitization is hindered.

Cr is added to ensure hardenability. The lower limit of the addition amount was set to 0.01% in order to sufficiently obtain the effect of hardenability. If it is added in an amount exceeding 0.7%, graphitization is significantly hindered.

Ni, Co, and Cu are effective in destabilizing cementite to promote graphitization, and at the same time, enhance hardenability and secure strength. If it is less than 0.05%, the effect is insufficient, and if it is added in excess of 3%, the effect is saturated and it is extremely economically disadvantageous.

B is added for the purpose of improving hardenability. To obtain the effect, 0.0001% or more must be added. However, if added in excess of 0.01%, the graphitization is inhibited, and the B compound precipitates at the grain boundaries to deteriorate the fracture toughness.

Zr is an oxide such as CaO or Ti ₂ O ₃ or M
Finely disperse sulfides such as nS. These oxides and sulfides effectively function as graphite precipitation sites and are effective for finely dispersing graphite and short-time graphitization. However, Z
If the added amount of r is less than 0.0005%, these effects are not observed, and if added in excess of 0.02%, coarse (Z
r, X) S and Zr (CN) are formed, and not only the effect of reducing the size of the oxide due to Zr is reduced but also the fracture characteristics are deteriorated.

Ca forms CaO or CaS. CaO or CaS alone or in the form of a complex with MnS functions as a precipitation site for graphite and improves the roughness of the cutting surface. If less than 0.0001%, the effect is small,
If added in excess of 0.005%, Ca deoxidization becomes dominant and coarse CaO is formed to deteriorate fatigue properties.

Ti forms oxides Ti ₂ O ₃ and carbonitrides TiN or TiC. The carbonitride functions as pinning particles, has an effect of suppressing the growth of austenite particles, and improves the fracture toughness value. If it is less than 0.001%, the effect of refining graphite or refining the crystal grains is small.
On the contrary, if added in excess of 05%, the toughness deteriorates.

Nb forms NbC or NbN, functions as pinning particles, has the effect of suppressing the growth of austenite grains, and improves the fracture toughness value. 0.005
%, The effect of grain refinement is small, and 0.08
On the other hand, if added in excess of%, the toughness deteriorates.

V forms VC or VN, functions as pinning particles, has the effect of suppressing the growth of austenite grains, and improves the fracture toughness value. If it is less than 0.005%, the effect of grain refinement is small, and if it exceeds 0.2%, the toughness deteriorates.

Pb and Bi have the effect of suppressing the adhesion at the interface between the tool and the work material, and therefore significantly improve the roughness of the finished surface by cutting, but if it is less than 0.01%, the effect is not recognized. If it exceeds 0.05%, graphitization is significantly hindered, so the upper limit was made 0.05%.

Similar to Pb and Bi, Sn also has the effect of improving the finished surface roughness. If it is less than 0.05%, the effect is small, and if it exceeds 0.2%, the effect is saturated.

Similarly, Te and Se also have the effect of improving the cut finish surface. If less than 0.002%, the effect is small,
If it exceeds 0.02%, the hot workability deteriorates.

The graphitization rate is defined by the following equation (1). Graphitization rate (%) = (graphite content in steel / carbon content in steel) ··· (1)

Regarding the graphitization rate, when C in the steel is graphitized, lubrication against deformation of graphite and the easily deformable property can be exhibited, so that the cutting tool life is improved. The effect of graphite is 80
This is the lower limit because it is remarkable at more than%. Further, if the graphitization rate is insufficient, it is hard, and from this point also the tool life varies depending on the graphitization rate. Then, by performing graphitization in which the graphitization rate exceeds 80%, a metal structure composed of ferrite, graphite and cementite is obtained.

[0041]

Example A steel wire rod having a diameter of 10 mm was prepared by requesting a steel having the chemical composition shown in Table 1 and performing slab rolling-rolling-offline annealing.

[0042]

[Table 1]

Steel 12 is a comparative example, and since it is a conventional sulfur free-cutting steel, the sample was evaluated as hot-rolled. The hot rolling conditions were rolling at 800 to 900 ° C. and cooling. Then, it hold | maintained at 690 degreeC with the annealing furnace. Since the softening characteristics and the graphite particle size due to graphitization differ depending on the conditions for forming the previous structure, two cooling methods were used after the completion of rolling: air cooling and online water cooling. In air cooling, a pre-structure mainly composed of ferrite pearlite is generated, and in online water-cooling, martensite or bainite becomes a pre-annealed structure by introducing the structure into a water tank immediately after rolling.

Table 2 shows the results of the machinability test, the annealing softening characteristics and the induction hardening characteristics.

[0045]

[Table 2]

Pretreatment for evaluation of machinability: air cooling, 690
A sample after annealing at 24 ° C. for 24 hours was used. In addition, pretreatment: water cooling, an annealing sample at 690 ° C. was used so that a remarkable difference between the steel types can be seen regarding the softening property and the induction hardening property. The softening characteristics were evaluated by the annealing time up to a graphitization rate of 90% and the average particle size of graphite after the graphitization annealing.

For the evaluation of machinability, drill life (VL100
0) and the cutting surface roughness (Rz). Here, the index VL1000 indicating the drill life is a cumulative hole depth of 1000 mm.
It is the maximum drilling peripheral speed that can be drilled up to, and the larger this value is, the faster the cutting is possible and the better the machinability. Use a φ5 mm straight drill as the drill, feed 0.33 mm / rev, and change the drill peripheral speed using water-soluble cutting oil to measure the cumulative drilling depth until drill breakage, and calculate VL1000 based on that. It was

Further, pretreatment was carried out so that the difference in performance between the samples became remarkable: annealing time in the case of water cooling, graphite particle size after graphitization, and induction hardening characteristics were evaluated.

The cutting surface roughness was obtained by measuring the cutting surface after plunge cutting with an erosion probe type roughness meter and evaluating it with a ten-point average roughness Rz in accordance with JIS B0601. FIG. 1 shows a cutting method. The cutting condition is 80m using the cutting tool 1.
/ min, tool feed 0.05 mm / rev, after 2.5 s cutting, pulling out the tool and idling for 6 s is one cycle, and grooves are created one after another by cutting, so the 100th cycle The cutting surface roughness of the cutting surface 2 at the groove bottom was measured. The cutting surface roughness was evaluated by using a plunge cutting high speed tool SKH57 at a cutting speed of 80 m / min and a feed of 0.05 m / rev.

The annealing time up to a graphitization rate of 90% is obtained by holding the hot-rolled material in an annealing furnace at 690 ° C. for various times and obtaining the graphitization rate represented by the following equation. It was time.

Graphitization rate (%) = (graphite content in steel / carbon content in steel) × 100 Here, the carbon content and the graphite content were quantified by chemical analysis. The average particle size and the maximum particle size of the graphite are 0.
It was evaluated by measuring 25 mm ² .

Further, the induction hardening characteristics were performed by using a round bar obtained by turning a steel in a graphite precipitation state into a diameter of 8 mm and heating it at 1000 ° C. for 3 seconds. After that, from the surface of the round bar,
A hardness test and an optical microscope observation were performed in the circumferential direction in a range of 3 mm. When the hardness difference in the circumferential direction is 100 (Hv) or more, or when the hardened structure contains ferrite, the induction hardenability was determined to be poor.

FIG. 2 shows the relationship between the C content and the machinability VL1000. VL1000 of sulfur free cutting steel SUM23 is 92m /
VL1000 of the present invention steel has a tool life equal to or longer than that of the sulfur free-cutting steel regardless of the type of steel. If a relatively large amount of S is added and the amount of C is small, VL10
00 was small and did not reach the level of SUM23. Further, with respect to the sample to which Mg was not added, when the pretreatment was air cooling, the sample was not softened sufficiently by annealing for 24 hours, so that the drill tool life was inferior.

FIG. 3 shows the effect of S on the surface roughness of plunge cutting.
The effect of quantity is shown. The larger the amount of S, the better the surface roughness, and S>
A surface roughness of 0.1% is better than that of SUM23. Plunge cutting was not performed on the samples of the examples having a poor drill life.

Further, regarding the steels having the components shown in Table 1, the annealing time required for softening the sample having the pre-structured structure by water cooling, the graphite particle size after graphitization, and the result of the evaluation of the induction hardening characteristics indicate that Mg addition is necessary for softening. It has a great effect on shortening the annealing time and reducing the grain size of graphite. Further, since the Mg-added material was miniaturized, the variation in the hardened layer during induction hardening was small and a good effect layer was obtained.

Further, in Table 3, the same evaluation was carried out regarding the chemical components.

[0057]

[Table 3]

The results are shown in Table 4 and FIGS.

[0059]

[Table 4]

The sample manufacturing method and evaluation items are the same as those of the sample in Table 1. FIG. 4 shows the relationship between the C amount and VL1000. S amount is within the range of the present invention: 0.1 to 0.7
% Steel, the VL1000 tends to increase as the C content increases, but the steels having different Mg contents have high hardness and low VL1000 due to insufficient graphitization. On the other hand, the cutting surface roughness decreases as the C content increases as shown in FIG. None of the steel types in which the S content is less than 0.1% has reached the surface roughness of the sulfur free-cutting steel. S amount is 0.1-0.5%
Rz is further reduced by setting the range to be, and the steel type in which the C content is 1.0% or more of the range of the present invention is the sulfur free-cutting steel SUM23.
The cutting surface roughness is equal to or higher than. That is, C, S and Mg
By setting the above range as the range of the present invention, both the tool life and the cutting surface roughness can be made equal to or higher than those of the sulfur free cutting steel.

As shown in Table 4, steels 13 to 27 satisfying the claims of the steel of the present invention are all 13 when water-cooled.
The graphitization rate reaches 90% in the annealing time of less than or equal to the time. On the other hand, all of the steels 28 to 36 having different Mg ranges from those of the present invention require annealing for 20 hours or more. Especially when the S content is 0.
Steels 29, 30 to 32, 34 and 35 exceeding 1% require annealing time of 29 hours or more. Further, the maximum particle size of graphite exceeds 4 μm in all the comparative examples, whereas the maximum particle size of graphite of the steel of the present invention is 4 μm or less, which is extremely fine. Further, the induction hardening characteristics were affected by the graphite particle size, and the sample having fine graphite particles was uniformly hardened, but the coarse graphite particles had large variations and were judged to be unsuitable.

Steel satisfying the present claims shows the characteristics of sulfur free-cutting steel and above in terms of both cutting tool life and cutting surface roughness.
It has excellent machinability and induction hardenability.

[0063]

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide a graphite steel in which graphite is finely dispersed and which is excellent in cutting tool life and cutting finish surface roughness at a low cost, and the industrial effect is extremely remarkable. There is something.

[Brief description of drawings]

FIG. 1 is a diagram showing a plunge cutting method.

FIG. 2 is a diagram showing the influence of the amount of C on the tool life.

FIG. 3 is a diagram showing the influence of the amount of S on the surface roughness.

FIG. 4 is a diagram showing the relationship between C content and drill life VL1000.

FIG. 5 is a diagram showing a relationship between a C content and a cutting surface roughness Rz.

[Explanation of symbols]

1 cutting tool 2 cutting surface

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hideo Kanizawa             12 Nakamachi, Muroran-shi Nippon Steel Corporation Muro             Orchid Works

Claims (5)

[Claims] 1. In mass%, C: 1.0 to 2.0%, S
i: 0.5 to 2.0%, Mn: 0.1 to 2.0%, P:
0.001-0.1%, S: 0.1-0.7% Al:
0.001-0.05%, N: 0.0001-0.02
%, Mg: 0.0001 to 0.009%, and the metal structure consists of ferrite, graphite, and cementite,
A steel for machine structural use with excellent machinability characterized by a graphitization rate of over 80%. 2. In mass%, Mo: 0.01 to 0.5%,
Cr: 0.01-0.7%, Ni: 0.05-3%, C
o: 0.05-3%, Cu: 0.05-3%, B: 0.
The steel for machine structural use according to claim 1, further comprising one or more of 0001 to 0.01%. 3. Zr: 0.0005 to 0.0 in mass%
2%, Ca: 0.0001-0.005% 1 type or 2 types are further contained, The 1 or 2 characterized by the above-mentioned.
Machine structural steel as described. 4. Ti: 0.001-0.05 by mass%
%, Nb: 0.005 to 0.08%, V: 0.005
The steel for machine structural use according to any one of claims 1 to 3, further containing one or more of 0.1 to 0.2%. 5. Pb: 0.01-0.05% by mass%,
Bi: 0.01 to 0.05%, Sn: 0.05 to 0.2
%, Te: 0.002-0.02%, Se: 0.002
The steel for machine structural use according to any one of claims 1 to 4, further containing 0.02% of one kind or two or more kinds.