JP5207743B2 - Steel for blades with excellent wear resistance and toughness - Google Patents

Description

  The present invention provides toughness and wear resistance suitable for horticultural blades for cutting grass, blades, lawn mowers, hedge trimmers, clippers, etc., horticultural blades for cutting grass, and tillers (claws) for tillers. The present invention relates to excellent steel materials for blades.

  Conventionally, carbon steel such as JIS SK85 and SK95 and alloy tool steel such as JIS SKS5 and SKS51 are used for the above-mentioned blade. Since these blades are used in an environment where they come into contact with gravel, stones, trunks, etc. during high-speed rotation, excellent “wear resistance” is required in addition to sharpness. Therefore, the steel plate used for the blade is tempered to a high hardness level of about 50 HRC by performing quenching and tempering treatments to ensure sharpness and wear resistance.

  On the other hand, the blade may break when it comes into contact with gravel, stone, trunk, or the like. The scattering of debris caused by breakage is very dangerous for workers and those around them. In order to prevent accidents caused by scattering of fragments and to ensure safety, the “toughness” of the blade is important. Higher hardness is advantageous for improving sharpness and wear resistance, but there is a dilemma that cannot be made too hard to ensure safety.

  Various steel materials with improved wear resistance and toughness have been developed so far. In Patent Document 1, wear resistance is improved by applying a predetermined heat treatment to a steel material to which Mo is added in an amount regulated in relation to the P content based on a high carbon steel having a high Mn content. Have been described. Patent Document 2 discloses a technique for reducing the deformation resistance of the ferrite phase by adding Ni in order to improve the toughness of the Mo-added steel of Patent Document 1. Patent Document 3 describes a cutting blade for a horticultural machine that is excellent in sharpness and durability in which the amount of C is reduced and Cr, Mo, V, and Ni are added to improve temper softening resistance and hardenability. Patent Document 5 describes a high C-containing steel that has improved impact characteristics while maintaining high strength by controlling the amount of undissolved carbide and the particle size distribution.

JP-A-62-139811 Japanese Patent Laid-Open No. 1-198447 JP-A-2005-171303 JP-A-11-29839 JP 2006-63384 A

  Nowadays, more than ever before, the characteristics required for a blade for the above-mentioned use are becoming more severe. In other words, a tool with excellent durability and safety that is compatible with high levels of wear resistance and toughness, which are inherently in a trade-off relationship, has been desired. The severity is increasing. However, a steel material having characteristics that can sufficiently meet such requirements has not yet appeared.

  For example, although the steel material of Patent Document 3 is said to have both good wear resistance and toughness, satisfactory wear resistance cannot always be obtained according to the evaluation of wear resistance in accordance with recent severe requirements. Further, according to the subsequent investigations by the present inventors, it has been found that excellent toughness (impact value) cannot always be stably realized when the wear resistance is particularly enhanced in the technique of Patent Document 3. . In other words, Patent Document 3 does not disclose a method for making a steel material that has both excellent wear resistance and toughness stably at a high level. Also, the steel material of Patent Document 5 has a relatively good balance between strength and toughness. However, when the amount of undissolved carbide is specified as in this document, the toughness that can satisfy the recent severe demands for blade applications. It has proved difficult to realize. In fact, Patent Document 5 evaluates toughness by the U-notch impact test, and it is considered that a tougher V-notch impact test cannot always obtain a good toughness evaluation. Further, Patent Document 5 does not describe the wear resistance.

  The present invention discloses a technique for stably realizing a steel material for blades that achieves both wear resistance and toughness at a high level.

The above-mentioned purpose is mass%, C: 0.4-0.75%, Si: 0.5% or less, Mn: 0.5-2%, P: 0.02% or less, S: 0.02% Hereinafter, Ti is 0 to 0.2%, V is 0 to 0.5%, Nb is 0 to 0.5%, and V, Ti, and Nb are V: 0.02 to 0.5%, It contains at least one of Ti: 0.02 to 0.2%, Nb: 0.01 to 0.5%, further Cr: 0.5% or less, Mo: 1% or less, and B: Containing one or more of 0.01% or less, having a chemical composition comprising the balance Fe and inevitable impurities, the X value defined by the following formula (1) is 10 or less, the prior austenite grain size is 15 μm or less, In addition, a steel material for blades having a martensite structure in which an undissolved carbide (cementite) having an equivalent circle diameter of 1.5 μm or more is 40 or less per 0.1 mm ² is excellent in wear resistance and toughness. Achieved.
X = 15.3 × {C−0.25 (V + Ti) −0.13Nb} − [area ratio of undissolved carbide (%)] (1)

  Here, the lower limit of 0% of the element content means a case where it is below the detection limit by a normal analysis method in steelmaking (tr in the composition table; the same as the trace). The value of the content expressed in mass% of the element is substituted for the element symbol in the formula (1). For elements whose content is below the detection limit (tr), 0 (zero) is substituted. The equivalent circle diameter is a value obtained by converting the diameter of an individual undissolved carbide measured by microscopic observation of a cross section of a steel material into a diameter of a circle having the same area as the undissolved carbide. The area ratio (%) of the undissolved carbide is the ratio of the area of the undissolved carbide in the fixed visual field area in the microscope observation image of the steel material cross section.

In the present invention, the annealed steel material having the above chemical composition is subjected to the quenching treatment shown in the following (a), and then subjected to a tempering treatment with a holding temperature of 150 to 500 ° C. and a holding time of 20 to 240 minutes. Provided is a method for manufacturing a steel material for blades having excellent wear and toughness.
(A) Quenching treatment; within a range of holding temperature of 790 to 900 ° C. and holding time of 10 to 60 minutes, the X value defined by the above formula (1) is 10 or less, the prior austenite grain size is 15 μm or less, and is equivalent to a circle After heating and holding under the condition that 40 or less undissolved carbides having a diameter of 1.5 μm or more per 0.1 mm ^2, quenching is performed to a temperature below the Mf point (martensite transformation end temperature).

  Here, the holding time means a time during which the material temperature is within the above holding temperature range.

  According to the present invention, it is possible to realize a steel material for blades that achieves both wear resistance and toughness at a high level. When this steel material is used, the wear and breakage of the blade are remarkably suppressed in the use of gardening knives and tillers, so that it is possible to provide a very high durability and high safety against breakage accidents.

The inventors have obtained the following findings as a result of detailed studies.
[1] Regarding the amount of solid solution C It is known that when the amount of solid solution C in the austenite phase at the time of heating in the quenching process exceeds 0.65% by mass, lenticular martensite with low toughness is easily formed. However, strictly speaking, the amount of dissolved C depends on the amount of undissolved carbides present in the high-temperature structure immediately before quenching and the amount of carbides of V, Ti, and Nb that are easily combined with C. As a result of various studies, the amount of C dissolved in the austenite phase immediately before quenching quenching (that is, equivalent to the amount of C dissolved in martensite in the structure after quenching) is the C content (so-called C content). Analytical value), V, Ti, Nb content, and the area ratio (%) of undissolved carbides were found to be accurately estimated. That is the following equation (1).
X = 15.3 × {C−0.25 (V + Ti) −0.13Nb} − [area ratio of undissolved carbide (%)] (1)

Here, when both sides of the equation (1) are divided by the coefficient 15.3, the following equation (1) ′ is obtained.
X / 15.3 = C−0.25 (V + Ti) −0.13Nb− [area ratio of undissolved carbide (%)] / 15.3 (1) ′

  In the formula (1) ′, “0.25 (V + Ti)” and “0.13 Nb” are portions corresponding to the amount of C consumed by being fixed to V, Ti, and Nb, “[area of undissolved carbide] "Rate (%)] / 15.3" corresponds to the amount of C consumed as undissolved carbide. Subtracting the amount of C consumed by these from the C content (total C amount) is “X / 15.3” on the left side, which is the solid solution C in the austenite phase immediately before quenching quenching. This corresponds to the amount (that is, the amount of C dissolved in martensite in the structure after quenching). The “X / 15.3” value on the left side is 10 / 15.3 = 0.65 when X is 10, and this is a limit state where lenticular martensite with low toughness is easily formed.

As demonstrated in the examples described later, in order to stably realize excellent toughness such that the V-notch impact value at room temperature is 20 J / cm ² , the X value of the formula (1) is 10 or less. It is essential that the value is 9 or less.

[2] Form of Undissolved Carbide As a result of detailed investigation of the effect of undissolved carbide on toughness, it was found that undissolved carbide reduces toughness. In particular, it is considered that undissolved carbide having a large particle size is likely to become a starting point and a propagation path of a crack when the blade breaks, and it is effective to eliminate it as much as possible. Specifically, controlling the amount of undissolved carbide having an equivalent circle diameter of 1.5 μm or more to 40 or less per 0.1 mm ² is important for securing toughness together with the X value of the above formula (1). It became clear that. As described above, in order to suppress the formation of lenticular martensite with low toughness, it is necessary to reduce the amount of dissolved C by undissolved carbide, but use as fine undissolved carbide as possible for C consumption. Is essential.

[3] Prior austenite grain size When the influence of the prior austenite grain size on the toughness was examined, the finer the previous austenite grain size, the better the toughness. I found out that Although undissolved carbides have the effect of refining the prior austenite grain size, on the other hand, since the toughness is reduced, it is not possible to entrust the refinement of the prior austenite grain size only to the undissolved carbide. Therefore, in the present invention, carbides of V, Ti, and Nb are used as other types of carbides effective for refining the prior austenite grain size.

[4] Hard Second Phase As a result of detailed examination of the influence of the metal structure on the wear resistance, it was found that it is extremely effective to uniformly disperse the hard second phase. And it became clear that the carbide | carbonized_material of V, Ti, and Nb can be utilized as such a 2nd phase. That is, in the present invention, wear resistance is improved by utilizing hard and fine carbides obtained by adding one or more of V, Ti, and Nb.

[Alloy components]
Next, alloy components will be described. Unless otherwise specified, the alloy element content means mass%.
C: 0.4 to 0.75%
In order to obtain the necessary hardness for the blade, a C content of 0.4% or more is required. However, if the C content exceeds 0.75%, the formation of lenticular martensite can be stably suppressed, even if the X value of the formula (1) is controlled to 10 or less, or equivalent to a circle. It becomes difficult to stably suppress the abundance of undissolved carbide having a diameter of 1.5 μm or more to a specified value or less, and toughness tends to be lowered. Accordingly, the C content is limited to 0.75% or less.

Si: 0.5 or less Si may be added as a deoxidizing agent in the steelmaking stage, but in this component system, even if Si is not added, there is no adverse effect of deoxidation failure. Rather, if the Si content increases, the toughness and workability deteriorate, which is not preferable. As a result of various studies, the Si content is allowed to 0.5%, more preferably 0.35% or less.

Mn: 0.5 to 2%
Mn is an element effective for improving hardenability, and if it is less than 0.5%, its action tends to be insufficient. However, since Mn is a solid solution strengthened alloy component, a large amount of Mn hardens the steel and becomes a factor that impairs manufacturability and toughness. As a result of various studies, the Mn content is limited to 2% or less.

P: 0.02% or less, S: 0.02% or less Since both of these adversely affect toughness, it is desirable to reduce the content as much as possible, but in this component system both are allowed to 0.02% .

V: 0.02 to 0.5%, Ti: 0.02 to 0.2%, Nb: 0.01 to 0.5%
These are elements that form hard fine carbides. The carbide is uniformly dispersed in the matrix, thereby exhibiting an effect of refining the austenite phase in the heating and holding of the quenching process. As a result, a metal structure having a fine prior austenite grain size is obtained, which is effective in improving toughness. Further, the hard fine carbide exhibits an effect of improving wear resistance. In order to exert these effects remarkably, it is necessary to secure a content of 0.02% or more in the case of V and Ti and 0.01% or more in the case of Nb. However, if the content of these elements is excessive, the carbide is likely to be coarsened, which may be a factor for reducing the fatigue limit and toughness. Moreover, excessive addition is not preferable economically. As a result of various studies, V is limited to 0.5% or less, Ti is limited to 0.2% or less, and Nb is limited to 0.5% or less. Therefore, these elements can be contained in a content range of V: 0 to 0.5%, Ti: 0 to 0.2%, Nb: 0 to 0.5%, but at least V: 0.5%. It is necessary to contain one of 02 to 0.5%, Ti: 0.02 to 0.2%, and Nb: 0.01 to 0.5%.

Cr: 0 to 0.5%
Cr is an element having both the effect of improving the hardenability of steel, the effect of improving the strength of the steel plate, and the effect of improving the wear resistance. However, Cr may have a negative effect of preventing solution of cementite in the heating and holding of the quenching process, and may increase the amount or size of undissolved carbide after the quenching process. Cr is an optional component added as necessary, and its content is limited to a range of 0.5% or less, and more preferably 0.3% or less.

Mo: 0 to 1%
Mo is an element effective for improving the hardenability of steel, and can be added as necessary. However, since it is an expensive element and the addition of a large amount impairs the economy, when adding Mo, it is performed within a range of 1% or less.

B: 0 to 0.01%
B enhances the hardenability of the steel and exhibits the effect of suppressing grain boundary fracture, so it can be added as necessary. The effect of addition of B becomes significant when the B content is 0.0003% or more. Therefore, when B is added, it is particularly effective to set the content to 0.0003% or more. However, as the B content increases, the toughness decreases. As a result of various studies, the B content is limited to 0.01% or less.

〔Production method〕
The steel material for blades of the present invention can be obtained by a process of melting a steel having the above composition and manufacturing a general quenched and tempered steel material (for example, a steel plate).

In the quenching treatment, the undissolved carbide having an X value defined by the formula (1) of 10 or less, a prior austenite particle size of 15 μm or less, and an equivalent circle diameter of 1.5 μm or more is 40 or less per 0.1 mm ^2. Perform under conditions. Although quenching conditions satisfying these slightly vary depending on the chemical composition, as a result of various studies, appropriate quenching conditions can be found within the range of a holding temperature of solution heat of 790 to 900 ° C. and a holding time of 10 to 60 minutes. . When the heating and holding temperature is lower than 790 ° C. or when the heating and holding time is less than 10 minutes, cementite is insufficiently solutioned, and coarse undissolved carbide tends to increase, and high toughness can be stably obtained. It becomes difficult. Conversely, when the heating and holding temperature exceeds 900 ° C. or when the heating and holding time exceeds 60 minutes, the prior austenite grain size tends to be coarse, and in this case as well, it is difficult to stably obtain high toughness. If a specific chemical composition specification is determined, an appropriate condition range within the above range may be obtained in advance by preliminary experiments. Usually, it is preferable to employ the conditions of a holding temperature of 800 to 860 ° C. and a holding time of 15 to 40 minutes. Although it is necessary to rapidly cool to a temperature below the Mf point, it is desirable to employ a method of quenching by immersing in a bath of refrigerant (water or oil).

  Next, it is subjected to a tempering treatment. Tempering can be set in the range of holding temperature of 150 to 500 ° C. and holding time of 20 to 240 minutes, but the tempering may be terminated at a very low temperature and in a short time while leaving room for further improvement of toughness. The tempering conditions may be set depending on the balance between the required characteristics and the manufacturing cost. However, when it is desired to secure as high a toughness as possible, the holding temperature is preferably 250 to 500 ° C, and more preferably 280 to 500 ° C. Air cooling is sufficient for cooling.

The steel having the chemical composition defined in the present invention was subjected to quenching treatment and tempering treatment under various conditions, and the structural state and characteristics were examined.
Steels having chemical compositions shown in Table 1 were melted to produce steel slabs. Each steel slab was hot-rolled according to a conventional method to obtain a hot-rolled steel sheet having a thickness of 3.0 mm, and then subjected to cold rolling and annealing to obtain an annealed steel material having a thickness of 2.5 mm. This annealed steel was subjected to quenching and tempering under the conditions shown in Table 1 to obtain a test material having a hardness of 48 to 55 HRC. In the quenching treatment, the material was rapidly cooled by dipping the material in an oil bath at 60 ° C. from the heating and holding temperature. In the tempering process, the material was air-cooled from the heated holding temperature. The following items were examined for each specimen.

[Old austenite grain size]
The structure was observed for the cross section (L cross section) parallel to the rolling direction and the plate thickness direction of the test material, and the prior austenite grain size was determined according to the cutting method of JIS G0551.

[Area ratio of undissolved carbide]
The L-section of the specimen was mirror-polished and then etched with a picral solution (methyl alcohol + picric acid). The surface was observed with a laser microscope, and the image was processed to account for the area occupied by undissolved carbide in a certain visual field area. The area ratio of undissolved carbide was determined by measuring the ratio. Since it is considered that the undissolved carbide is dispersed almost uniformly in the steel material, this area ratio can be regarded as the volume ratio of the undissolved carbide in the steel material.

[X value]
The X value was determined by substituting the content (analytical value) of C, V, Ti, and Nb and the area ratio (%) of the undissolved carbide into the equation (1).

[Abundance of undissolved carbides with equivalent circle diameter of 1.5 μm or more]
In the laser microscope observation for determining the area ratio of the undissolved carbide, undissolved carbide having an equivalent circle diameter of 1.5 μm or more is specified by using image processing, and a certain observation area (however, 0.1 mm ² or more is set). The number of undissolved carbides having an equivalent circle diameter of 1.5 μm or more existing in 1) was counted, and the number of undissolved carbides having an equivalent circle diameter of 1.5 μm or more present per 0.1 mm ² was calculated based on this.

〔Hardness〕
The hardness in Rockwell C scale was calculated | required about the L cross section of the test material. For applications such as horticultural cutting tools, a hardness level of 48 HRC or higher is desired.

[2mmV notch impact value (toughness)]
A test piece having a length of 55 mm and a width of 10 mm was cut out so that the rolling direction of the test material (thickness of 2.5 mm) and the direction perpendicular to the thickness direction (T direction) were the longitudinal direction, and at the center thereof By forming a 2 mmV notch, a notched impact test piece according to JIS Z2202 was produced. Using this test piece, a Charpy impact test in accordance with JIS Z2242 was performed at room temperature to determine the impact value. In consideration of recent severe demand characteristics desired for gardening tools, etc., those having a ² mmV notch impact value of 20 J / cm ² or more were determined to be acceptable.

[Abrasion amount]
A specimen having a length of 50 mm and a width of 25 mm was cut out so that the T direction of the test material (thickness of 2.5 mm) was the longitudinal direction, and an abrasion resistance test was performed using an Ogoshi type rapid wear tester. Abrasive stone made of cubic silicon nitride having a particle size number of 400 was used as the mating material under conditions of a speed of 0.61 mm / sec and a final load of 60 N in a dry environment, and the wear resistance was evaluated based on the specific wear amount. Also in this case, in consideration of recent strict required characteristics desired for horticultural knives and the like, a wear amount of 300 × 10 ⁻⁴ mm ³ / (m · kg) or less was determined to be acceptable.
These results are shown in Tables 1 and 2.

  As can be seen from Tables 1 and 2, the examples of the present invention were manufactured with the chemical composition and quenching / tempering conditions within the proper ranges, so that the prior austenite grain size, the X value, and the equivalent circle diameter of 1.5 μm or more were not obtained. The density of dissolved carbides could be controlled within the range specified in the present invention. As a result, a material having a high level of wear resistance and toughness was realized.

  On the other hand, in Comparative Examples No. 1, 4, 6, 9, and 10, the holding temperature of the quenching treatment is too high in each chemical composition, so that the prior austenite grain size becomes coarse or the X value becomes large. Both were inferior in toughness. Nos. 2, 7, and 11 were inferior in toughness due to an increase in the density of undissolved carbides having an equivalent circle diameter of 1.5 μm or more because the quenching time was too short in each chemical composition.

  Steels having the compositions shown in Tables 3 and 5 were melted, and an annealed steel material (steel plate) having a thickness of 2.5 mm was produced by the same process as in Example 1. This was used as a test material. The same items as Example 1 were investigated about each test material. The results are shown in Tables 4 and 6.

  As can be seen from Tables 3 and 4, the steel having the chemical composition defined in the present invention is subjected to quenching treatment and tempering treatment under appropriate conditions, whereby the prior austenite grain size, X value, and equivalent circle diameter of 1.5 μm or more. It was possible to control the density of the undissolved carbide in the range specified in the present invention. As a result, a material having a high level of wear resistance and toughness was realized.

  On the other hand, as can be seen from Tables 5 and 6, Comparative Examples Nos. 51 and 52 did not have the necessary hardness for the blade because the C content was insufficient. No. 53 was inferior in toughness due to the fact that the content of Cr was too high to inhibit the solution of cementite, and there were many coarse undissolved carbides. Nos. 54, 59, 64, 65, and 67 were not added with V, Ti, and Nb, so the prior austenite grain size became coarse and inferior in toughness. Further, the effect of improving the wear resistance by the hard precipitate was not obtained. Among them, Nos. 65 and 67 had a high C content, so the X value was high (that is, the amount of dissolved C was increased), and brittle lens-like martensite was generated, so the V notch impact value was particularly low. In Nos. 55 and 63, since the Ti addition amount was excessive, coarse Ti carbide was generated, and the toughness was impaired. Nos. 56 and 60 were inferior in toughness because the P or S content was too high. No. 57 had a low Mn content, so the required hardness was not obtained and the wear resistance was poor. In Nos. 58 and 62, since the contents of Si and Cr were excessive, the amount of coarse undissolved carbides increased and the toughness was inferior. No. 61 was inferior in toughness because the Mn content was excessive. No. 68 has a C content as high as 0.95%, and the X value becomes considerably high (that is, the amount of solid solution C increases considerably) despite the addition of V, and the V notch impact value is very high. The value was low.

  In FIG. 1, the relationship between the amount of wear and the 2 mmV notch impact value is plotted for the steel materials shown in Examples 1 and 2. Each of the examples of the present invention is located at the upper left in FIG. 1 compared with the comparative example that does not meet the requirements defined in the present invention, and is able to achieve both wear resistance and toughness at a high level. I understand.

The graph which plotted the relationship between wear amount and a 2 mmV notch impact value about the steel materials shown in Examples 1 and 2.

Claims (4)

In mass%, C: 0.4 to 0.75%, Si: 0.5 or less, Mn: 0.5 to 2%, P: 0.02% or less, S: 0.02% or less, Ti: 0 -0.2%, V: 0-0.5%, Nb: 0-0.5%, and for V, Ti, Nb, V: 0.02-0.5%, Ti: 0.02 It has a chemical composition comprising at least 0.2%, Nb: 0.01-0.5%, the balance Fe and unavoidable impurities, and the X value defined by the following formula (1) is For blades with excellent wear resistance and toughness having a tempered martensite structure with 10 or less, old austenite particle size of 15 μm or less, and 40 or less undissolved carbide with an equivalent circle diameter of 1.5 μm or more per 0.1 mm ² Steel material.
X = 15.3 × {C−0.25 (V + Ti) −0.13Nb} − [area ratio of undissolved carbide (%)] (1)
Here, the value of the content expressed by mass% of the element is substituted for the element symbol in the formula (1).   Furthermore, the steel material for blades of Claim 1 which has a chemical composition containing 1 or more types of Cr: 0.5% or less, Mo: 1% or less, and B: 0.01% or less. By mass%, C: 0.4 to 0.75%, Si: 0.5% or less, Mn: 0.5 to 2%, P: 0.02% or less, S: 0.02% or less, Ti: 0 to 0.2%, V: 0 to 0.5%, Nb: 0 to 0.5%, and V, Ti, and Nb are V: 0.02 to 0.5%, Ti: 0.5. An annealed steel material containing one or more of 02 to 0.2% and Nb: 0.01 to 0.5% and having a chemical composition comprising the balance Fe and unavoidable impurities is shown in (a) below. A method for manufacturing a steel material for blades excellent in wear resistance and toughness, which is subjected to a quenching treatment and then a tempering treatment at a holding temperature of 150 to 500 ° C. and a holding time of 20 to 240 minutes.
(A) Quenching treatment: Within the range of holding temperature of 790 to 900 ° C. and holding time of 10 to 60 minutes, the X value defined by the following formula (1) is 10 or less, the prior austenite particle size is 15 μm or less, and the equivalent to a circle After heating and holding under the condition that 40 or less undissolved carbides having a diameter of 1.5 μm or more per 0.1 mm ² , quenching is performed to a temperature below the Mf point.
X = 15.3 × {C−0.25 (V + Ti) −0.13Nb} − [area ratio of undissolved carbide (%)] (1)
Here, the value of the content expressed by mass% of the element is substituted for the element symbol in the formula (1).   The said annealed steel material further has a chemical composition containing one or more of Cr: 0.5% or less, Mo: 1% or less, and B: 0.01% or less. Steel manufacturing method.

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