JPH06145890A - High strength and high toughness free cutting steel - Google Patents

Abstract

PURPOSE:To improve the machinability, fatigue strength and toughness in high strength and high toughness free cutting steel. CONSTITUTION:The compsn. of this free cutting steel is constituted of, by weight, 0.10 to 1.20% C, 0 to 0.50% Si, 0.10 to 2.00% Mn, 0.002 to 0.015% S, 0.0040 to 0.O200% B, 0.005 to 0.030% N and 0.0010 to 0.0100% Ca, and the balance Fe with inevitable impurities. In addition to the essential elements, one or >=two kinds of selective elements ' to 5.0% Cr, 0.30 to 5.0% Ni, 0.05 to 3.0% Mo, 0.05 to 0.30% V and 0.010 to 0.l0% Nb are incorporated therein. By reducing BN inclusions, its strength and toughness are improved.

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 structures used as a material for parts such as gears, and more particularly to a free-cutting steel.

[0002]

2. Description of the Related Art Conventionally, free-cutting steels containing sulfides or low melting point metals as inclusions in order to improve machinability have been known, but inclusions sometimes reduce strength and toughness. Therefore, it has been considered difficult to apply it to high-strength steel that requires high fatigue strength and toughness. Hexagonal boron nitride (BN) formed by combining B and N in steel can improve machinability without deteriorating fatigue strength and toughness in the working direction due to rolling or casting. (JP-A-62-21135
No. 0, Japanese Patent Application No. 63-15460, Japanese Patent Application No. 63-4
2875).

[0003]

However, since BN expands in the working direction by rolling or casting, the effect on fatigue strength and impact strength in the direction perpendicular to or at a certain angle to BN is the same as that of conventional free-cutting steel. Although it is smaller than the inclusions of sulfide and lead, it is not completely removed. In particular, its effects are recognized in the fatigue strength, impact strength, and rolling fatigue strength of bearing steel that is used in a high hardness state after tempering at a low temperature after quenching, such as carburizing and quenching materials and induction hardening materials.

The present invention has been made on the basis of such knowledge, and has the machinability (during turning and gear cutting),
Fatigue strength (carburized and induction hardened materials, induction hardened materials, gear fatigue of quenching low temperature tempered materials, axial torsional fatigue, rolling fatigue, etc., especially lateral fatigue) and toughness (lateral impact when inclined with respect to plastic working flow) The objective is to provide a high-strength, high-toughness free-cutting steel with excellent value).

[0005]

The high-strength, high-toughness free-cutting steel according to the present invention for solving the above-mentioned problems of the present invention comprises C: 0.10 to 1.20 (wt%), Si: 0 to 0. .50 (wt%), Mn: 0.10 to 2.00 (wt%), S: 0.002 to 0.015 (wt%), B: 0.0040 to 0.0200 (wt%), N : 0.005 to 0.030 (wt%), Ca: 0.0010 to 0.0100 (wt%), balance Fe and unavoidable impurities.

In the high-strength, high-toughness free-cutting steel according to the present invention, preferably, in addition to the above-mentioned elements, one or more selected elements are Cr: 0.50 to 5.0 (% by weight). , Ni: 0.30 to 5.0 (wt%), Mo: 0.05 to 3.0 (wt%), V: 0.05 to 0.30 (wt%), Nb: 0.010 to 0 10 (% by weight).

Further, the high-strength and high-toughness free-cutting steel according to the present invention desirably contains, in addition to the elements described in the above essential elements or selective elements, one or more selected elements Ti: 0.002 to 0.002. 0.07 (wt%), Zr: 0.002 to 0.08 (wt%), REM: 0.002 to 0.09 (wt%) (rare earth element), provided that Ti + Zr + REM <0.10%. Is characterized in that.

Further, in the high-strength, high-toughness free-cutting steel according to the present invention, preferably, in addition to the elements described in the above-mentioned essential elements or selective elements, one or more selective elements are Te: 0.001- 0.010 (wt%), Se: 0.002-0.08 (wt%),

These effects on strength and toughness can be prevented by reducing the BN inclusions.
BN starts precipitation at a high temperature of 1200 ° C. or higher with oxides and sulfides as precipitation nuclei. Therefore, when oxides and sulfides are finely and uniformly dispersed, BN is finely precipitated and fatigue strength and toughness are improved. It is equivalent to steel containing no BN regardless of the direction. In order to disperse the oxide finely, it is effective to use an oxide having a composition that does not easily agglomerate and coarsen during the melting process and a specific gravity that makes it difficult to float and separate and deposit. For that purpose, it is effective to add Ca after deoxidizing Al. In order to finely disperse sulfides, it is effective to suppress the formation of crystallized sulfides and to generate solid solution and reprecipitation during the hot working process. Further, making the sulfide into a composition that is difficult to spread in plastic working has an effect of reducing anisotropy of mechanical properties due to sulfide itself and BN having sulfide as a precipitation nucleus. Since sulfides often precipitate with oxides as nuclei, fine dispersion of oxides effectively acts to refine sulfides.

The reasons for limiting the constituent elements of the free-cutting steel according to the present invention are shown below. (1) C: 0.10 to 1.20 (% by weight) C is set to 0.10% by weight or more in order to secure strength, and 1.20% by weight or less reduces ductility. This is to prevent (2) Si: 0 to 0.50 (wt%) Si is an element having a deoxidizing function, and the reason why the content is 0.50 wt% or less is to prevent a decrease in ductility. (3) Mn: 0.10 to 2.00 (wt%) Mn is an element having a function of improving deoxidation and hardenability. The content of 0.10% by weight or more is to prevent deterioration of hot workability, and the content of 2.00% by weight or less is to prevent deterioration of machinability. (4) S: 0.002 to 0.015 (% by weight) S is an element having a function of becoming a BN precipitation nucleus by sulfide formation. The content of 0.002% by weight or more is for preventing the number of sulfides from being insufficient, and the content of 0.015% by weight or less is for preventing the deterioration of toughness. (5) B: 0.0040 to 0.0200 (wt%) B is an element having a function of forming BN, and the reason why 0.0040 wt% or more is to prevent the BN precipitation amount from being insufficient, The reason why the amount is 0.0200 weight or less is to prevent deterioration of hot workability. (6) N: 0.005 to 0.030 (wt%) N is an element having a function of generating BN, and the reason why 0.005 wt% or more is to prevent the insufficient amount of BN precipitation,
The reason why the content is 0.030% by weight or more is to prevent deterioration of castability. (7) Ca: 0.0005 to 0.0100 (% by weight) or less Ca is an element having a function of refining and spheroidizing sulfide,
The amount of 0.0005% by weight or more is for preventing insufficient sulfide form control effect, and the amount of 0.0100% by weight or less is for saturation of the effect. (8) Cr: 0.50 to 5.0 (% by weight) Ni: 0.30 to 5.0 (% by weight) Mo: 0.05 to 3.0 (% by weight) V: 0.05 to 0. 30 (wt%), Nb: 0.010 to 0.10 (wt%), Cr, Ni, Mo, V and Nb are elements having a function of improving strength, toughness and hardenability. 0.50 for Cr
Wt% or more, Ni 0.30 wt% or more, Mo 0.0
5 wt% or more, V is 0.05 wt% or more, Nb is 0.0
The amount of 10% by weight or more is for preventing insufficient improvement of strength, toughness and hardenability, and 5.0% by weight of Cr is used.
Below, Ni is 5.0 wt% or less, Mo is 3.0 wt% or less, V is 0.30 wt% or less, and Nb is 0.10 wt% or less means that strength, toughness, and hardenability are improved. This is to prevent saturation of the effect and reduction of ductility. (9) Ti: 0.002-0.07 (wt%), Zr: 0.002-0.08 (wt%), REM: 0.002-0.09 (wt%) (rare earth element), Ti + Zr + REM <0.10% Ti, Zr, REM and Ti + Zr + REM have a fine dispersion of sulfide and a spheroidizing function of sulfide. Ti is 0.002% by weight or more, Zr is 0.002% by weight or more,
The reason why REM is set to 0.002% by weight or more is to prevent insufficient effect, and Ti is 0.07% by weight or less and Zr.
Is 0.08% by weight or less, REM is 0.09% by weight or less,
The reason why Ti + Zr + REM is 0.10% by weight or less is that
This is to prevent saturation of effect and deterioration of cleanliness. (10) Te: 0.001 to 0.010 (% by weight) Se: 0.002 to 0.010 (% by weight) Te and Se are elements having the functions of spheroidizing sulfide and improving machinability. The reason why Te is 0.001% by weight or more and Se is 0.002% by weight or more is to prevent insufficient effect, so that Te is 0.010% by weight or less and Se is 0.
The content of 010% by weight or less is to prevent the saturation of the effect and the deterioration of the toughness.

[0011]

EXAMPLES Examples of the present invention will be described below. First, the chemical composition of the test material will be described. Free-cutting steels (A, B, C, D, E, F, G, H, J) according to the present invention and basic steels (X0) having the same basic components as the invention steels as comparative steels, BN having the same basic components as the invention steels Steel (X1), steel excluding BN from the invention steel (X2), and steel excluding Ca from the invention steel (X3) were melted. Here, the basic component means a main alloying element that affects the characteristics regardless of the direction. Where X is A, B, C,
Indicates any one of D, E, F, G, H, and J.

The compositional compositions of the invention steel and the comparative steel are shown in Table 1 below.
Shown in.

[0013]

[Table 1]

Next, the manufacturing process of the test material will be described. After melting by melting in a 5 ton high frequency induction furnace, rolling was performed to form a round bar of φ95. Carburizing and quenching as heat treatment and processing (Carburizing 910 ° C x 3h
r, diffusion 2 hours, quenching 830 ° C, tempering 150 ° C),
Induction hardening (hardening depth / radius = 0.5, tempering 150
℃), quenching low temperature tempering (quenching 850 ℃, tempering 15
0 ° C.), normalizing (900 ° C. for 2 hours air cooling), annealing (800 ° C. for 2 hours furnace cooling), and shot peening (arc height 0.7 mm (A)).

The test pieces thus produced were subjected to the following tests. (1) Rotating Bending Fatigue Strength Test Value: 10 ⁷ Fatigue Limit Test Method: Ono-type Rotating Bending Fatigue Test Specimen: Rolled material lateral smoothing specimen Heat treatment: Carburizing and quenching shot peening, induction quenching, quenching low temperature tempering Test condition: Room temperature (2) Rolling fatigue strength Test value: Impact absorption energy Test method: Radial type rolling life test specimen: Cylindrical specimen in rolling material longitudinal direction Heat treatment: Carburizing quenching, induction hardening, quenching Low temperature tempering test condition: Surface pressure 600kgf / mm ² (3) Toughness Test value: Impact absorption energy Test method: Charpy impact test Specimen: Rolled material lateral direction 1 / 2R part 10mm notch depth 2
mm Heat treatment: Carburizing quenching, induction hardening, quenching low temperature tempering Test condition: Room temperature (4) Machinability evaluation value: Tool life (cutting time when flank wear becomes 0.2 mm) (1) (2) (3) ( The evaluation results of the free-cutting steel according to the present invention obtained by the test 4) are shown in Table 2 below.

Test method: Longitudinal turning test with cemented carbide tool Specimen: Rolled material Heat treatment: Normalizing and annealing Test conditions: Cutting speed 150 mm / min, feed 0.2 m
m / rev, depth of cut 2 mm, dry cutting

[0017]

[Table 2]

The following results were obtained by the above test. (1) Rotating Bending Fatigue Strength, Rolling Fatigue Strength and Toughness Comparison of Inventive Steel and Basic Steel X = X0 → Inventive Steel = Basic Steel Comparison of Inventive Steel and BN Steel X ＞ X1 → Inventive Steel ＞ Basic Steel Effect of BN X0> X1 → Decrease Effect of BN in invention steel X = X2 → No effect of BN Effect of Ca in invention steel X> X3 → Greater improvement effect (2) Machinability Comparison between invention steel and basic steel X> X0 → Inventive steel> Basic steel Comparison between inventive steel and BN steel X = X1 → Inventive steel = Basic steel Effect of BN on basic steel X0> X1 → Improvement effect is large BN effect in inventive steel X> X2 → Improvement effect Large Effect of Ca on invention steel X = X3 → No effect of Ca This evaluation is shown in Table 3.

[0019]

[Table 3]

[0020]

As described above, according to the free-cutting steel of the present invention, by refining the inclusions of the BN free-cutting steel,
This has the effect of improving machinability without lowering fatigue strength and toughness in the lateral direction.

Claims (4)

[Claims] 1. The composition is C: 0.10 to 1.20% by weight Si: 0 to 0.50% by weight, Mn: 0.10 to 2.00% by weight, S: 0.002 to 0.015. % By weight, B: 0.0040 to 0.0200% by weight, N: 0.005 to 0.030% by weight, Ca: 0.0010 to 0.0100% by weight, balance Fe and inevitable impurities High-strength, high-toughness free-cutting steel. 2. In addition to the element according to claim 1, one or more selected elements are Cr: 0.50 to 5.0% by weight, Ni: 0.30 to 5.0% by weight, Mo. : 0.05 to 3.0% by weight, V: 0.05 to 0.30% by weight, Nb: 0.010 to 0.10% by weight, high strength and high toughness free-cutting steel characterized by being contained. . 3. In addition to the element according to claim 1 or 2,
One or two or more selected elements are Ti: 0.002 to 0.07% by weight, Zr: 0.002 to 0.08% by weight, REM: 0.002 to 0.09% by weight (rare earth element), Contained, but Ti + Zr + REM <0.10% by weight
A high-strength, high-toughness free-cutting steel characterized by 4. In addition to the element according to claim 1, 2 or 3, one or more selected elements are Te: 0.001 to 0.010% by weight, Se: 0.002 to 0.08. High-strength, high-toughness free-cutting steel, characterized in that it is contained by weight%.