JP2014005493A - High strength and high fatigue strength steel having excellent toughness and ductility for saw wire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength and high fatigue strength steel for a saw wire, having higher tensile strength, capable of making the saw wire more thin, and excellent toughness and ductility.SOLUTION: The composition of a saw wire contains, by mass%, C:0.18 to 0.30%, Ni:6.0 to 9.0%, Co:8 to 20%, Mo:1.0 to 6.0%, Cr:1.0 to 6.0%, Al:0.5 to 1.3%, Ti:≤0.1%, N:≤0.010%, O:≤0.010% and the balance Fe with inevitable impurities.

Description

  The present invention relates to a steel for saw wires used for cutting single crystal silicon ingots, ceramics and other hard materials, and particularly to a steel for saw wires having excellent toughness and high fatigue strength made of maraging steel.

  Conventionally, a single-crystal silicon ingot is cut (sliced) to produce a silicon wafer used for solar cells, semiconductors, etc., sapphire used for LEDs is cut, and neodymium magnets used for motors are cut. For example, cutting using a saw wire is performed as a method for efficiently cutting a material when cutting a hard material.

In recent years, with the increase in production of solar cells, semiconductors, LEDs, motors, etc., there has been a strong demand for improvement in yield when cutting these single crystal silicon ingots and other materials.
In the cutting with the saw wire, a portion corresponding to the cutting margin by the saw wire, that is, the meat corresponding to the wire diameter of the saw wire is cut off from the material, and this is lost as a cut loss.
Therefore, in order to improve the yield, it is necessary to reduce the diameter of the saw wire as much as possible.

The most important characteristic required for saw wires used for cutting hard materials is tensile strength, and piano wires having high tensile strength have been used as such saw wire materials.
By the way, in cutting using a saw wire, a large tensile force is repeatedly applied to the saw wire, and a load (torsional force) is also intermittently applied in the twisting direction.
At this time, the saw wire is also required to have fatigue strength and toughness so that the saw wire does not break early.

With conventional saw wires using piano wire, it is difficult to further reduce the wire diameter in consideration of tensile strength, and in terms of toughness and fatigue strength, it is difficult to meet the demand for yield improvement. Development of materials that can further reduce the wire diameter and have excellent fatigue strength and toughness has been required.
Furthermore, since the saw wire has a small diameter, the influence of inclusions is large, and the wire material is required to have a high cleanliness.

Patent Document 1 below discloses that 18Ni maraging steel is used as a saw wire.
18Ni maraging steel has high strength and good ductility compared to piano wire.
However, this 18Ni maraging steel contains Ti as an essential component and expresses high strength by precipitating Ti as an intermetallic compound by aging treatment. Ti contained in the steel binds with C, N, etc. As a result, coarse inclusions such as TiC and TiN are generated in the steel, and this becomes a starting point of fracture, which causes a problem of reducing the fatigue strength of the saw wire.
Furthermore, this 18Ni maraging steel is still not sufficient in terms of tensile strength, and it is difficult to further reduce the diameter of the saw wire.

Patent Document 2 below discloses maraging steel in which Ti is not added and C is contained in a large amount of 0.05 to 0.15%.
However, the maraging steel disclosed in Patent Document 2 is used as a thin plate steel for CVT ring, and the required tensile strength level is different from that of saw wire material. To use this maraging steel as saw wire, the tensile strength is different. In view of this, it is insufficient in terms of toughness and the like.

JP 2003-55742 A JP 2011-195922 A

  The present invention is based on the circumstances as described above, and has a higher tensile strength than ever and can further reduce the diameter of the saw wire, and has high toughness and high fatigue strength. It was made for the purpose of providing.

  Thus, the saw wire steel according to claim 1 is, in mass%, C: 0.18 to 0.30%, Ni: 6.0 to 9.0%, Co: 8 to 20%, Mo: 1.0 to 6.0%, Cr: 1.0 to 6.0%, Al: 0.5 to 1.3%, Ti: ≦ 0.1%, N: ≦ 0.010%, O: ≦ 0.010%, and the composition of the balance Fe and inevitable impurities.

Effects and effects of the invention

In the present invention, maraging steel is used as a material for saw wire, and its composition is such that Ti is not added and C is contained in a large amount of 0.18 to 0.30%.
And by making Ti non-addition, it suppresses that a coarse TiN inclusion etc. arise in steel, and improves fatigue strength.
Further, by containing a large amount of C, in addition to strengthening by solid solution of C in the martensite of the parent phase, carbides such as Mo are precipitated during the aging treatment to increase the strength of the base material.

According to the present invention, a high fatigue strength steel for saw wire having a tensile strength of 2400 MPa or more, an elongation of 5% or more and a high strength and high toughness can be obtained.
As a result, the wire diameter of the saw wire can be made thinner than before, and the material yield when cutting the material by the saw wire can be improved and increased by the thinning.

Next, the reasons for limiting each chemical component in the present invention will be described below.
C: 0.18 ~ 0.30%
Conventional maraging steel, for example, 18Ni maraging steel of Patent Document 1, has an extremely low C content of 0.02% or less, but in the present invention, C is contained in an amount of 0.18% or more.
In the present invention, C is dissolved in the matrix martensite substrate to increase the strength of the base material. Further, carbide (Mo ₂ C) is precipitated by aging treatment to increase the strength of the base material.
C also precipitates carbide (Mo ₂ C) in the process of heating and heating the steel for solution heat treatment (for example, in a temperature range of about 500 to 600 ° C.).

The carbide generated at this time acts to suppress the coarsening of γ (austenite) grains at the solution heat treatment temperature by the pinning effect.
However, most of the carbides generated in this process disappear at the solid solution temperature due to the solid solution, and hardening by the carbide precipitation is performed mainly by an aging treatment after the solution heat treatment.
In the present invention, 0.18% or more of C is contained for these functions. Preferably, 0.20% or more is contained.

On the other hand, when C is contained in a large amount exceeding 0.30%, Mo consumed by Mo ₂ C precipitation increases and Mo for forming an intermetallic compound is insufficient, so the upper limit is made 0.30%.
In addition, if the C content exceeds 0.30%, the amount of carbide (Mo ₂ C) precipitated temporarily during the temperature rising process for solution heat treatment becomes excessive.

The carbide generated in this process works to suppress the coarsening of the γ grains due to the pinning effect as described above, but the temporarily precipitated carbide here is mostly dissolved in austenite by solution heat treatment. There is a need.
However, when the amount of precipitation increases, the temperature for solid solution needs to be increased. However, when the temperature for solid solution is increased, the austenite grains become coarse at once when the carbide is dissolved.

  Therefore, it is necessary to perform a solution heat treatment in the optimum temperature range so that the austenite grains do not become coarse at the same time as the solid solution of the carbide is completed while solidifying the majority of the generated carbide (precipitate). However, if a large amount of carbide precipitates, the optimum temperature range becomes narrow and operation becomes difficult. Also in this meaning, in the present invention, the C content is set to 0.30% or less. Preferably it is 0.25% or less.

Ni: 6.0-9.0%
Ni combines with Mo, Al, etc. by age hardening treatment to precipitate intermetallic compounds such as Ni ₃ Mo, NiAl, etc., thereby improving the tensile strength and fatigue strength of the base material. In securing the function, the present invention contains Ni of 6.0% or more. Preferably it is contained 7.0% or more.
On the other hand, if the content exceeds 9.0%, the martensite transformation is suppressed due to a decrease in the Ms point (martensite transformation start temperature), and the amount of retained austenite after the solution heat treatment increases, leading to a decrease in strength.
Therefore, in the present invention, the upper limit of the Ni content is 9.0%.

Co: 8-20%
Co reduces the solubility limit of Ni and Mo, which are intermetallic compound-forming elements, into martensite by solid solution in martensite (reduces the amount of solid solution), and Ni ₃ Mo is bound between Ni and Mo by the combination of Ni and Mo. It promotes the precipitation of the NiAl intermetallic compound by the combination of the compound, Ni and Al, and has the function of increasing the tensile strength and fatigue strength of the base material.
For its function, the present invention contains 8% or more. Preferably it is 10% or more, more preferably 12% or more.
On the other hand, if it is contained in a large amount exceeding 20%, the martensite transformation is suppressed due to the decrease in Ms point, the amount of retained austenite after the solution heat treatment is increased and the strength is lowered, so the upper limit is made 20%. A preferable upper limit is 18%, and a more preferable upper limit is 16%.

Mo: 1.0-6.0%
In addition to precipitating the intermetallic compound Ni ₃ Mo by aging treatment, Mo precipitates carbide Mo ₂ C and has a function of increasing the tensile strength and fatigue strength of the base material by precipitation hardening. Because of its function, the present invention contains 1.0% or more of Mo. A preferable content is 2.0% or more.
On the other hand, when it is contained in a large amount exceeding 6.0%, the Mo ₂ C solid solution temperature temporarily generated in the temperature rising process for the solution heat treatment is raised, and the Mo ₂ C precipitation is prevented while preventing the coarsening of γ grains. The optimum temperature range for solid solution is narrowed, making operation difficult.
Therefore, in the present invention, the upper limit of the Mo content is set to 6.0%. The upper limit of the preferable content is 5.0%.

Cr: 1.0-6.0%
Cr enhances toughness and solid solution in the matrix to improve corrosion resistance. In order to acquire the effect, 1.0% or more of Cr is contained in the present invention. The preferred content is 2.5% or more, and the more preferred content is 3.5% or more.
On the other hand, if the content exceeds 6.0%, the strength characteristics are deteriorated, so the upper limit is made 6.0%. A preferable upper limit is 5.0%, and a more preferable upper limit of the content is 4.5%.

Al: 0.5 to 1.3%
Al acts as a deoxidizer during steel melting and also has the function of increasing the strength of the base metal by binding to Ni by aging treatment to precipitate the intermetallic compound NiAl. For this function, 0.5% or more of Al is contained in the present invention. Preferably it is made to contain 0.6% or more.
On the other hand, if the content exceeds 1.3%, an oxide is formed to reduce the cleanliness of the steel and the fatigue life, so the upper limit is set to 1.3% in the present invention. A preferred upper limit is 1.2%.

Ti: ≤0.1%
In general maraging steel, Ti is added as an essential element for precipitating Ni-Ti intermetallic compounds that contribute to strength improvement. In the present invention, Ti forms non-metallic inclusions such as TiC and TiN. In order to reduce the cleanliness and reduce the fatigue life, this is not added as an impurity. Specifically, the content is regulated to 0.1% or less.
The steel for saw wire of the present invention as described above can realize a tensile strength of 2400 MPa or more and an elongation of 5% or more in a tensile test based on JIS Z 2241, and has excellent toughness and high strength. It is also excellent in fatigue strength.

N: ≤0.010%
N forms TiN or the like to lower the cleanliness and lower the fatigue life. Therefore, in the present invention, the content is restricted to 0.010% or less using this as an impurity.

O: ≤0.010%
O forms an oxide to lower the cleanliness and reduce the fatigue life. Therefore, in the present invention, this is used as an impurity and its content is restricted to 0.010% or less.

The saw wire steel of the present invention can be produced as follows.
Specifically, (1) a raw material with low impurities was melted in a vacuum induction furnace and cast to obtain an ingot, or (2) a melted raw material such as scrap and alloy elements was melted and refined in an arc furnace. The molten steel is transferred to a ladle refining furnace equipped with a heating device and a vacuum device, vacuum refined, and cast to obtain an ingot.

  The ingot thus obtained is remelted by the electroslag remelting method or the vacuum arc remelting method and solidified to form a remelted ingot. In the remelting, the electroslag remelting method or the vacuum arc remelting method may be performed a plurality of times individually or in combination for the purpose of further improving the cleanliness.

  The ingot produced as described above is soaked at a temperature in the range of about 1000 to 1300 ° C. for about 5 to 72 hours, air cooled, hot forged, penciled, hot rolled, and 900 to 1150 A solution heat treatment is performed at a temperature of 1 ° C. for about 1 to 60 minutes for air cooling, and then a wire having a diameter of 1 mm or less is produced by cold rolling.

  Then, in a vacuum, in an inert atmosphere or in a weak reducing atmosphere, a solution heat treatment is performed at 900 to 1150 ° C. for about 1 to 60 minutes, air-cooled, and then 400 to 600 ° C. to obtain the strength of the wire. An aging treatment is performed at a temperature for 1 to 36 hours and air cooling is performed to obtain a state suitable for saw wire use.

According to the method of (2) above, melting and refining of scrap and alloy raw materials with an arc furnace, vacuum refining with a ladle refining furnace, and vacuum arc remelting were performed to obtain an ingot with a diameter of 510 mm. The alloy composition is as shown in Table 1.
These ingots are homogenized by soaking by heating at 1150 to 1200 ° C. for 24 hours, hot forging, pencil processing, hot rolling, solution heat treatment in air at 1000 ° C. for 1 hour, and air cooling, It processed into the wire of diameter 20mm. Thereafter, the wire is heated at 500 ° C. for 5 hours and air-cooled, and then a sample for tensile test, rotational bending fatigue test, cleanliness measurement, crystal grain size measurement is cut out from the wire, and each measurement is performed by the following method. It carried out in.

For some wires, in addition to the above-described various evaluations using a wire with a diameter of 20 mm, the wire is made into a wire with a diameter of 1 mm by cold rolling, and then heated to 1000 ° C. for 1 hour (the atmosphere is weakly reduced). In atmosphere)-Air cooling and aging treatment heated to 500 ° C for 5 hours-Air cooling was performed, and then a wire having a length of 100 mm was cut out. Using the wire, 360 times as a simple tensile test and saw wire characteristic evaluation A twist test was conducted to evaluate whether it can be twisted.
These results are shown in Table 2.

<Test and evaluation method>
(1) Grain size JIS G 0551
Samples were taken in the rolling direction, and the old γ grain boundaries were corroded by 10% chromic acid electrolytic corrosion. Thereafter, the grain size was derived from the grain size number in accordance with JIS G 0551.

(2) Cleanliness JIS G 0555
In accordance with a microscopic test method based on a point calculation method for non-metallic inclusions in steel, the area ratio (%) of all inclusions was measured and defined as the cleanliness (d%) of the steel.
The test piece was sampled with a thickness of 1 mm × width of 20 mm × length of 15 mm so that the rolling direction was the longitudinal direction and the cut surface passing through the center line was the test surface.
A surface parallel to the rolling direction (a surface having a width of 20 mm × a length of 15 mm) was embedded in a resin as a test surface and mirror-polished.

(3) Tensile properties JIS Z 2241
The tensile strength (MPa) was measured according to the metal tensile test method.
The test piece was a JIS 14A test piece according to JIS Z 2201. The test temperature is room temperature.

(4) Rotating bending fatigue characteristics JIS Z 2274
The investigation was carried out in accordance with the rotational bending fatigue test method for metal materials defined in JIS Z 2274.
The test piece was a No. 1 test piece.
The test piece was bent and deformed by a load, and in that state, the test piece was repeatedly rotated and bent under the condition of a rotational speed of 3600 rpm, and the number of repetitions until breaking was measured.
The maximum strength of the stress applied to the test piece was changed, and the strength at which the number of repetitions until fracture was 1 × 10 ⁷ times or more was defined as fatigue strength (MPa).
Table 2, when the load stress 1500 (MPa), the number of repetitions until fracture was evaluated by whether at least 10 ⁷ times.

In Tables 1 and 2, in Comparative Examples 1 and 2, the C amount is less than the lower limit of the present invention, and the tensile strength does not reach the target value. Further, the fatigue strength is insufficient.
In Comparative Example 3, the amount of C is excessive beyond the upper limit of the present invention, the elongation value is low, and the crystal grain size is also coarse.

In Comparative Example 4, the amount of Ni is lower than the lower limit of the present invention, the tensile strength does not reach the target value of 2400 MPa or more, and the fatigue strength is insufficient.
In Comparative Example 5, the amount of Ni is excessive beyond the upper limit of the present invention, the tensile strength does not reach the target value, and the fatigue strength is also insufficient.

In Comparative Example 6, the amount of Co is lower than the lower limit of the present invention, the tensile strength does not reach the target value, and the fatigue strength is insufficient.
In Comparative Example 7, the amount of Co is excessive beyond the upper limit of the present invention, the tensile strength does not reach the target value, and the fatigue strength is insufficient.

In Comparative Example 8, the amount of Mo is lower than the lower limit of the present invention, the tensile strength does not reach the target value, and the fatigue strength is insufficient.
In Comparative Example 9, the amount of Mo is excessive beyond the upper limit of the present invention, and the crystal grain size is coarse due to the coarsening of γ grains.

In Comparative Example 10, the Cr amount is lower than the lower limit value of the present invention, and the elongation value does not reach the target value of 5%.
In Comparative Example 11, the amount of Cr is excessive beyond the upper limit of the present invention, the tensile strength does not reach the target value, and the fatigue strength is insufficient.

In Comparative Example 12, the Al amount is lower than the lower limit of the present invention, the tensile strength does not reach the target value, and the fatigue strength is insufficient.
In Comparative Example 13, the amount of Al is excessive beyond the upper limit of the present invention, and the fatigue strength is insufficient due to the decrease in the cleanliness of the steel.

In Comparative Example 14, the amount of Ti is excessive beyond the upper limit of the present invention, and the fatigue strength is insufficient due to the decrease in the cleanliness of the steel.
In Comparative Example 15, the N and O contents are large, the cleanliness of the steel is lowered, and the fatigue strength is insufficient.

On the other hand, all of the examples of the present invention are excellent in the properties of tensile strength, elongation, grain size, cleanliness, and fatigue strength.
Table 3 shows the results of tensile strength and elongation measurements with a 1 mm diameter wire rod and a twist test for Invention Example 1, Invention Example 25 and Comparative Example 2, which can be seen in the results of Table 3. Thus, Invention Example 1 and Invention Example 25 are superior to Comparative Example 2 in the properties of tensile strength and elongation in a 1 mm wire state, and the number of twists is a large value.

  Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be implemented in various modifications without departing from the spirit of the present invention.

Claims (1)

By mass% C: 0.18 to 0.30%
Ni: 6.0-9.0%
Co: 8-20%
Mo: 1.0-6.0%
Cr: 1.0-6.0%
Al: 0.5 to 1.3%
Ti: ≤0.1%
N: ≤0.010%
O: ≤0.010%
Steel for saw wire with high strength and high fatigue strength with excellent toughness and composition of balance Fe and inevitable impurities.