JP4530268B2 - High carbon steel member with excellent impact characteristics and method for producing the same - Google Patents

Description

  The present invention is a high carbon steel member used in a wide range of fields, such as vehicle drive system parts such as gears and bearings, precision machine parts that require small and precise shapes, blades, cutting tools, loom members, etc. And a manufacturing method thereof.

  Gears, bearings, precision machine parts, blades, cutting tools, loom members, and wear-resistant parts are made of carbon tool steels such as JIS SK70 to SK120 and alloy tool steels such as JIS SK55 and SK51 into parts and then target characteristics. It is manufactured through quenching and tempering. In any application, impact characteristics after heat treatment are required, but it is not necessarily clarified whether the impact characteristics can be improved by adjusting to what structure form during heat treatment.

  In the conventional knowledge on the structure and impact properties, heat treatment conditions are adopted in which about 0.6% by mass of carbon is dissolved in the eutectoid steel, hypereutectoid steel, etc., and the remainder remains as undissolved carbide. Has been. Quenched martensite hardens as the amount of C increases up to 0.6% by mass, but the degree of hardening decreases with an amount of C exceeding 0.6% by mass. The decrease in hardness increase at C> 0.6% by mass is due to an increase in soft retained austenite in the structure.

  That is, the relationship between the amount of C and hardening is based on the premise that the amount of C exceeding 0.6% by mass remains as undissolved carbide. In addition, martensite with low toughness is likely to be generated in steel materials having a C content of 0.6% by mass or more. For this reason, a heat treatment condition in which 0.6 mass% of the C amount is dissolved and the remaining C remains as undissolved carbide is normal. Therefore, eutectoid steel, hypereutectoid steel, etc. are usually tempered to a heat-treated structure composed of undissolved carbide and martensite.

  Among the heat-treated structures, undissolved carbides are said to suppress the austenite grain coarsening during quenching and improve wear resistance. However, the form and particle size of undissolved carbides affect impact properties. The impact is not always clear. Therefore, the stability of the impact characteristics obtained after quenching and tempering is low, and it is difficult to obtain a highly reliable constituent member.

By the way, the present inventors introduced a high carbon steel sheet that has improved impact characteristics and wear resistance in addition to punching workability and drawing workability by controlling the size, spheroidization rate, and dispersion state of carbides. (Patent Document 1). In the high carbon steel sheet, the fracture origin is reduced and the toughness is improved by using a raw material structure in which a relatively large size spherical carbide is dispersed in a heat treated structure of equiaxed ferrite.
Japanese Patent Laid-Open No. 2003-147485

  The present invention is based on the knowledge obtained as a result of investigating and examining the relationship between the size, dispersion form and impact characteristics of carbides in more detail, and the impact characteristics are improved by controlling the amount of undissolved carbide and the particle size distribution. An object of the present invention is to provide a high carbon steel member.

The high carbon steel member of the present invention has C: 0.61 to 1.30 mass%, Si: 1.0 mass% or less, Mn: 0.2 to 1.5 mass%, P: 0.02 mass% or less. , S: 0.02 mass% or less, Fe: Substantially remaining basic composition, undissolved carbide remains in the matrix after quenching and tempering at a volume ratio Vf (volume%) satisfying the formula (1) The undissolved carbide having a particle size of 1.0 μm or more is regulated to 2 or less per 100 μm ^{2 of the} observation area.
Steel composition includes Ni: 1.8% by mass or less, Cr: 2.0% by mass or less, V: 0.5% by mass or less, Mo: 0.5% by mass or less, Nb: 0.3% by mass or less, One or two or more of Ti: 0.3% by mass or less, B: 0.01% by mass or less, and Ca: 0.01% by mass or less may be included.
8.5 <15.3 × C% −Vf <10.0 (1)

It is manufactured by using a steel material adjusted to a predetermined composition and having an average minor axis length of 0.3 to 1.1 μm dispersed in a starting material, followed by quenching and tempering under the following conditions.
・ Solution to hold in temperature range of 760 to 850 ° C. for 3 to 20 minutes ・ Quenching rapidly cooled with a cooling medium at room temperature to 200 ° C. ・ Tempering to hold in temperature range of 150 to 350 ° C. for 10 to 180 minutes

Effects and embodiments of the invention

  When high carbon steel is quenched, a hard steel material made of martensite is obtained, but the toughness and impact properties tend to decrease with hardening. The present inventors investigated and examined the form and particle size of undissolved carbides that have a large effect on toughness and impact properties. As a result, it was found that if the structure of the material and the austenitizing conditions are appropriately controlled, undissolved carbides are refined to a size that is difficult to exfoliate from the matrix, and good toughness is imparted after tempering. The carbon concentration during quenching is estimated from the ratio of undissolved carbide dispersed in the metal structure after heat treatment and the chemical component value of the steel material.

The present invention is tempered to 600 to 900 HV by quenching and tempering processing, vehicle drive system parts such as gears and bearings, parts that require small and precise shapes, precision machine parts, blades, cutting tools, and thread slides. It is intended for high-carbon steel members used for loom parts such as knitting needles that rub, and parts that are hard and require excellent impact characteristics. Carbon tool steel and alloy tool steel having a hardness of 600 HV or higher are used for these applications. However, if the hardness exceeds 900 HV, a decrease in toughness is inevitable, so the required hardness is in the range of 600 to 900 HV. Yes. One of the required characteristics is that brittle fracture does not occur when a static or dynamic load is applied. The impact characteristics can be evaluated by the impact absorption energy measured by the Charpy impact test. The target impact value was set to cm ² or more.

In order to satisfy the required hardness and impact characteristics, the components and composition of the steel material are specified as described below, and the form and particle size of the undissolved carbide are controlled.
[Ingredients / Composition]
C: 0.60 to 1.30% by mass
Hardness after heat treatment: In order to obtain 600HV or more, a C amount of 0.60% by mass or more is required. However, if an excessive amount of C exceeding 1.30% by mass is contained, coarse undissolved carbides that adversely affect toughness and impact properties tend to remain.

Si: 1.0 mass% or less Although it is an alloy component added as a deoxidizer in the steelmaking stage, this component system does not cause the deoxidation failure even if Si is not added. However, since the workability is likely to deteriorate as the Si amount increases, the upper limit of the Si content is set to 1.0% by mass.
Mn: 0.2 to 1.5% by mass
It is an alloy component effective for improving hardenability, and the effect of adding Mn is seen at 0.2% by mass or more. However, when an excessive amount of Mn exceeding 1.5% by mass is added, the martensitic transformation point Ms decreases, the heat treatment strain increases, and the manufacturability and toughness tend to decrease.
・ P, S: 0.02% by mass or less Both are components that adversely affect toughness. The smaller the amount, the better. However, in this component system, the upper limit is set to 0.02% by mass to suppress the adverse effects caused by P and S. ing.

Ni: 1.8% by mass or less Ni is an alloy component added as necessary, and is effective in improving hardenability and exhibits an effect of increasing toughness in the same manner as Mn. Such an effect becomes remarkable when Ni is added in an amount of 0.5 mass% or more. However, excessive addition lowers the martensitic transformation point Ms, increases heat treatment strain, and causes a decrease in manufacturability and toughness, so the upper limit was made 1.8% by mass.
・ Cr: 2.0% by mass or less
Hardenability, strength, exhibits the effect of improving the abrasion resistance, the effect of adding Cr is remarkable at 0.2 mass% or more. However, excessive addition significantly reduces toughness, so the upper limit was made 2.0% by mass.

V: 0.5% by mass or less V is an alloy component that is added as necessary, exhibits an effect of refining austenite crystal grains during quenching, and the effect of adding V becomes remarkable at 0.01% by mass or more. In order to refine the prior austenite crystal grains, V of 0.5 mass% is sufficient at the maximum, and excessive addition causes a decrease in toughness.
Mo: 0.5% by mass or less Mo is an alloy component that is added as necessary, exhibits an effect of improving hardenability, and has an effect of increasing the toughness of the steel material by the combined addition with Ni. In addition, the formation of special carbides also has the effect of improving wear resistance. Such an effect becomes remarkable when 0.05% by mass or more of Mo is added, but at most 0.5% by mass of Mo is sufficient, and excessive addition adversely affects toughness.

Nb: 0.3% by mass or less Nb is an alloy component added as necessary, and has the effect of refining austenite crystal grains during quenching, and the effect of Nb addition becomes remarkable at 0.01% by mass or more. However, 0.3% by mass of Nb is sufficient for refinement of prior austenite crystal grains, and excessive addition causes an increase in manufacturing cost.
Ti: 0.3% by mass or less Ti is an alloy component added as necessary, and has the effect of refining austenite crystal grains during quenching, and the effect of adding Ti becomes remarkable at 0.005% by mass or more. However, at most 0.3% by mass of Ti is sufficient for refinement of prior austenite crystal grains, and excessive addition tends to increase nonmetallic inclusions.

-B: 0.01 mass% or less It is an alloy component added as needed, and has the effect | action which improves toughness. The effect of improving toughness becomes remarkable with B of 0.0003 mass% or more, but is saturated at 0.01 mass%, and even if it is added more than that, the effect of improving toughness commensurate with the increase cannot be obtained.
-Ca: 0.01 mass% or less It is a component which controls the shape of inclusions and improves the anisotropy of the mechanical properties of steel, and is added as necessary. The addition effect of Ca becomes remarkable at 0.0005% by mass or more, but is saturated at 0.01% by mass, and Ca inclusions are increased by adding more than 0.01% by mass.

[Undissolved carbide]
-Volume fraction Vf of undissolved carbide:
In order to obtain good toughness after quenching and tempering treatment, martensite produced by quenching from austenite whose carbon concentration during solution treatment is in the range of 0.55 to 0.65% by mass is tempered under appropriate conditions. It is necessary. However, since the carbon concentration during the solution treatment cannot be measured and controlled industrially, the carbon concentration during the solution treatment is estimated from the undissolved carbide remaining in the metal structure after the heat treatment and the chemical components of the steel material. In this respect, defining the volume fraction of undissolved carbide is synonymous with defining the carbon concentration during the solution treatment.

  In the present invention, by adjusting the volume fraction Vf of the undissolved carbide so as to satisfy 8.5 <15.3 × C% −Vf <10.0, the carbon concentration during the solution treatment is in an appropriate range: 0 The toughness of the tempered steel material is increased by maintaining the content at 0.55 to 0.65% by mass. If (15.3 × C% −Vf) is less than 8.5, the carbon concentration in martensite is insufficient, and sufficient heat treatment hardness cannot be obtained. Conversely, if it exceeds 10.0 (15.3 × C% −Vf), an excessive amount of C is contained in the martensite, and good impact characteristics cannot be obtained.

-Particle size distribution of undissolved carbides:
In steel materials with a hardened martensite structure in which a small amount of undissolved carbide is dispersed, the interface between the undissolved carbide and the matrix (hardened martensite) is the starting point of impact fracture. To do. Peeling of the undissolved carbide / matrix interface tends to occur as the undissolved carbide having a large particle size is dispersed. When microcracks occur due to interfacial debonding, even if the austenite at the time of solution treatment contains an appropriate amount of C, dimples starting from the microcracks are formed, and the dimples are connected to each other to form macrocracks and break down. It reaches.

According to the investigations and researches of the present inventors, the undissolved carbide that peels from the matrix at the time of impact fracture and becomes the starting point of the fracture is an undissolved carbide having a relatively large particle diameter of 1 μm or more, and an undissolved carbide having a particle diameter of less than 1 μm The particle size of the dissolved carbide was found to be almost harmless. From this investigation result, it can be said that it is preferable to reduce as much as possible undissolved carbide having a particle size of 1.0 μm or more. The allowable range for the material is a particle size of 1 at a ratio of 2 or less per 100 μm ² of observation area. That is, undissolved carbides of 0.0 μm or more are dispersed. The particle size of the carbide here is a value obtained as a diameter of a circle having the same area as that of the carbide observed with a microscope (that is, a circle equivalent diameter).

[Metal structure before heat treatment]
In order to appropriately control the structure after the heat treatment, it is necessary to adjust the form of the carbide dispersed in the metal structure before the heat treatment to an appropriate state. In the case of a pearlite structure or fine spherical carbide, the solid solution of the carbide rapidly progresses during the solution treatment, and it is difficult to control the structure to an appropriate structure under industrially feasible heat treatment conditions. The difficulty of solution treatment is determined by the length in the minor axis direction for spherical carbides and rod-like carbides, and by the thickness of carbides for pearlite and plate-like carbides. Therefore, the short axis length of the carbide is used to include the thickness of the carbide.

  When the average minor axis length of the carbide is adjusted to a range of 0.3 to 1.1 μm, the structure form after the heat treatment is appropriately controlled, and a steel material having good hardness and toughness can be obtained. On the other hand, when the average minor axis length exceeds 1.1 μm, carbides with an average particle diameter of 1.0 μm or more appear after the heat treatment, and the impact characteristics are deteriorated. Conversely, carbides having an average minor axis length of less than 0.3 μm are rapidly solid-dissolved during the solution treatment, and an appropriate carbon concentration of 0.55 to 0.65% by mass cannot be ensured.

[Quenching / tempering]
In order to obtain excellent impact characteristics at a target hardness of 600 to 900 HV, the undissolved carbide is controlled to an appropriate amount by solution treatment that is heated at 760 to 850 ° C. (preferably 780 to 820 ° C.) for 3 to 20 minutes. It is necessary to. When the heating temperature is higher than 850 ° C. or the heating time is longer than 20 minutes, the carbide is excessively dissolved, the volume fraction Vf of the undissolved carbide is reduced, and the carbon concentration of the austenite during the solution treatment is 0.65% by mass. Therefore, excellent impact characteristics cannot be obtained. On the other hand, at a heating temperature that does not reach 760 ° C., austenite formation is insufficient and a sound heat-treated structure cannot be obtained.

After the solution treatment, when a steel material is immersed in a coolant maintained in a temperature range of room temperature to 200 ° C. and rapidly cooled, a quenching martensite structure with an appropriate amount of C is obtained. In the case of a refrigerant having a temperature higher than 200 ° C., the cooling rate becomes slower than the critical cooling rate, resulting in a heat-treated structure in which poorly quenched structures are mixed. On the other hand, even if the refrigerant temperature is excessively lowered, an advantageous result in terms of material cannot be obtained. The holding time in the refrigerant is set to a time during which the solution-treated steel material is sufficiently cooled.
Next, good toughness is imparted by tempering at 150 to 350 ° C. for 10 to 180 minutes. If the tempering temperature is too low, the effect of improving toughness is insufficient, and if the tempering temperature is too high, a target hardness of 600 HV or higher cannot be obtained.

Steel strips having the components shown in Table 1 were melted, and after casting, steel strips were manufactured under the conditions shown in Table 2.
The heat treatment conditions shown in Table 3 were adopted for quenching and tempering applied to each steel strip.

In order to investigate the effects of composition and composition on hardness and impact properties, steel types No. 1 to 14 in Table 1 were manufactured under the same conditions as in production conditions SA3 and steel strips with an average minor axis length of 0.3 to 1.1 μm. Then, heat treatment was performed under conditions B, I, or F. The steel strip after the heat treatment was measured for hardness, and the amount of undissolved carbide, volume ratio Vf, observation area: 100 μm ^{2 and} the number of carbides with a particle size of 1.0 μm or more were investigated.

  As can be seen from the investigation results in Table 4, the hardness of steel grade No. 1 with insufficient C content does not satisfy the hardness: 600 HV or more, and conversely with steel grade No. 5 with excessive C content (15.3 × Since C% -Vf) is as high as 12.1, it is indicated that the carbon concentration of the austenite in the solution state is too high, and as a result, the impact value is extremely low. Steel types Nos. 13 and 14 with excessive amounts of Si and Mn satisfy hardness: 600 HV or more, but the impact value is significantly low.

On the other hand, in the steel type satisfying the component conditions defined in the present invention, both the undissolved carbide and (15.3 × C% −Vf) are maintained in an appropriate range, while maintaining the target hardness: 600 to 900 HV, In the 2 mm U-notch Charpy impact test (room temperature, n = 5), high impact characteristics were obtained, with an average impact absorption energy (impact value) of 25 J / cm ² or more.

Furthermore, the manufacturing conditions and heat treatment conditions of steel No. 3 were changed variously, the hardness after the heat treatment was measured, the amount of undissolved carbide, the volume ratio Vf, the observation area: 100 μm ^{2 and the} particle size per 1.0 μm or more The number of carbides was investigated.
As can be seen from the investigation results in Table 5, when quenching / tempering treatment was performed according to the heat treatment conditions defined in the present invention, 8.5 <15.3 × C% −Vf <10.0 was satisfied, Carbide with a diameter of 1.0 μm or more becomes 2 or less per 100 μm ² observation area, and while maintaining the target hardness: 600 to 900 HV, the impact absorption energy is 25 J / cm ² or more in the ² mm U notch Charpy impact test. High impact properties were shown.

  On the other hand, in test No. 1 where the solution treatment temperature is too low, the hardness was insufficient. On the contrary, in the test No. 6 where the solution treatment temperature is too high, it was hardened to 730 HV, but (15.3 × C% −Vf) was too high and the impact toughness was extremely inferior. Test No. 13, which was held at the solution treatment temperature for a short time, showed a low hardness and a low impact value. In test No. 14 where the tempering temperature was too low, the hardness was sufficient, but the impact value was extremely low. On the contrary, in test No. 15 where the tempering temperature is too high, although the toughness is good, the hardness is extremely lowered due to softening of the tempering. In Test No. 16, which was held at the solution treatment temperature for a long time, (15.3 × C% −Vf) exceeded 10.0, and the impact characteristics were inferior.

  In the test No. 10 of the pearlite structure, since the average minor axis length of the carbide is short, the carbide is instantly dissolved in the solution treatment, and even if quenching / tempering is performed under the heat treatment conditions defined in the present invention (15. 3 × C% −Vf) was too high and inferior in impact toughness. Similarly, in test No. 11 manufactured under the same manufacturing conditions SA1, the average minor axis length of the carbide is short (15.3 × C% −Vf) exceeds 10.0, and the hardness is satisfied with 720HV. The impact value was low. On the other hand, in Test No. 12 manufactured under the manufacturing condition SA4 in which the average minor axis length of the carbide is increased, the carbide having a particle size of 1.0 μm or more is excessive and the impact value is decreased. The influence due to the production condition SA4 in which the average minor axis length of the carbide is increased also appeared as a low impact value even in the test No. 17 using the steel type No. 4.

Observation area: Particle size per 100 μm ² Particle size: 1.0 μm or more The impact value was arranged in relation to the number of carbides, and as shown in FIG. 1, the impact value decreased as the number of carbides increased. It was found that to obtain a value: 25 J / cm ² or more, a particle size of 1.0 μm or more of carbide: 2 or less was required.

As described above, the volume fraction Vf of the undissolved carbide is adjusted so that 8.5 <15.3 × C% −Vf <10.0 is satisfied in relation to the amount of C, and the observation area is 100 μm. ^{When the} number of undissolved carbides with a particle size of ² or more per 1.0 is regulated to 2 or less, the carbon concentration of austenite during solution treatment is properly controlled, and the impact value is maintained while maintaining the target hardness: 600 to 900 HV. : A high carbon steel member exhibiting excellent impact characteristics of 25 J / cm ² or more is obtained. The obtained high carbon steel members are hard and have excellent impact characteristics and toughness, and are used for vehicle drive system parts such as gears and bearings, parts that require small and precise shapes, precision machine parts, and blades. , Used in a wide range of fields, such as parts for looms such as cutting tools, knitted needles that rub against threads, and parts that are hard and require excellent impact characteristics.

Graph showing the effect of the number of undissolved carbides with a particle size of 1.0 μm or more on toughness

Claims (4)

C: 0.60 to 1.30 mass%, Si: 1.0 mass% or less, Mn: 0.2 to 1.5 mass%, P: 0.02 mass% or less, S: 0.02 mass% or less , Cr: 2.0% by mass or less, and the balance is Fe and inevitable impurities, and the matrix after quenching and tempering is an undissolved carbide at a volume ratio Vf (volume%) satisfying the formula (1). A high carbon steel member with excellent impact characteristics, characterized in that the undissolved carbide having a particle size of 1.0 μm or more is restricted to 2 or less per 100 μm ^{2 of the} observation area.
8.5 <15.3 × C% −Vf <10.0 (1) Furthermore, Ni: 1.8% by mass or less , V: 0.5% by mass or less, Mo: 0.5% by mass or less, Nb: 0.3% by mass or less, Ti: 0.3% by mass or less, B: 0.3% or less. The high carbon steel member according to claim 1, comprising one or more of 01 mass% or less and Ca: 0.01 mass% or less. C: 0.60 to 1.30 mass%, Si: 1.0 mass% or less, Mn: 0.2 to 1.5 mass%, P: 0.02 mass% or less, S: 0.02 mass% or less , Cr: 2.0% by mass or less, and the balance is made of steel and inevitable impurities , and an average minor axis length: 0.3 to 1.1 μm of carbide dispersed steel is prepared,
After holding in a temperature range of 760 to 850 ° C. for 3 to 20 minutes, quenching with a cooling medium at room temperature to 200 ° C., and holding in a temperature range of 150 to 350 ° C. for 10 to 180 minutes, a particle size: Impact characteristics characterized in that undissolved carbide of 1.0 μm or more remains undissolved carbide at an observation area of ² μm or less per 100 μm ² and a volume fraction Vf (volume%) satisfying the formula (1) Method for producing a carbon steel member excellent in performance.
8.5 <15.3 × C% −Vf <10.0 (1) Furthermore, Ni: 1.8% by mass or less , V: 0.5% by mass or less, Mo: 0.5% by mass or less, Nb: 0.3% by mass or less, Ti: 0.3% by mass or less, B: 0.3% or less. The manufacturing method of Claim 3 which uses the steel materials containing the 1 type (s) or less of 01 mass% or less and Ca: 0.01 mass% or less.

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