JP5689086B2 - Hot work tool steel - Google Patents

Description

  The present invention relates to hot work tool steel.

Hot tool steel such as a die material and a roll material for hot rolling is used in a thermal fatigue environment in which heating and cooling are repeated, and thus heat check resistance is required.
It is said that heat check resistance requires high high-temperature strength and toughness (for example, Non-Patent Document 1). Conventionally, as a method for improving the high temperature strength, it is known to add a special carbide forming element such as V, Mo, W or the like and finely precipitate it as M ₂ C or MC type carbide during tempering. In order to improve toughness, it is effective to control the refinement of crystal grains and the type, form and size of carbides and inclusions, and generally a tempering treatment is performed after forming a martensite structure by quenching.

  For example, in Patent Document 1, C: 0.10 to 0.70 mass%, Si: 0.10 to 0.80 mass%, Mn: 0.30 to 1.00 mass%, P: 0.007 to 0.020 mass. %, Cr: 3.00 to 7.00 mass%, W and Mo are used alone or in combination (1/2 W + Mo): 0.20 to 12.00 mass%, V: 0.10 to 3.00 mass%, Ni: 0 0.05 to 0.80 mass%, S: 0.150 mass% or less, the balance being substantially composed of Fe and inevitable impurities, and the cleanliness of nonmetallic inclusions in accordance with JISG0555 is 0.00 at dA 60 × 400. 020% or less, dB60 × 400, 0.020% or less, dB60 × 400, 0.020% or less, and d (A + B + C), 0.045% or less. There hot tool machining is disclosed, wherein the area ratio of the carbide and non-metallic inclusions exceeding 1.0μm is 0.004% or less. By prescribing these, the heat check resistance, the erosion resistance and the machinability can be improved.

JP 2003-226939 A

"Iron and Steel", Vol. 79, No. 9, pp. 1013-1021

  The technical point of improving the heat check resistance shown in Patent Document 1 is to strengthen precipitation by reducing carbides and non-metallic inclusions having a particle size of more than 1.0 μm and increasing them to 1.0 μm or less. It is presumed that there is to suppress the decrease in toughness while drawing out the effect. Therefore, in order to utilize these effects, it is considered necessary to perform a tempering treatment after quenching to a martensite structure at a high cooling rate so as not to involve diffusion transformation. However, since it is difficult to obtain a large cooling rate in terms of equipment or physically in a large member (for example, 10 t or more), it is not possible to benefit from the effect of Patent Document 1.

  The present invention has been made in the background of the above circumstances, and an object thereof is to provide tool steel capable of obtaining high heat check resistance even in a member manufactured from a large steel ingot.

  That is, among the hot tool steels of the present invention, the first present invention is in mass percentage, C: 0.50-0.60%, Si: less than 0.10%, Mn: 0.30-1. 00%, P: less than 0.010%, S: less than 0.010%, Cr: 3.00 to 4.50%, Mo: 0.80 to 1.20%, Ni: 0.30 to 0.60 %, V: 0.05 to 0.50%, with the balance being composed of Fe and inevitable impurities, and having a full bainite structure.

  The hot work tool steel of the second aspect of the present invention is characterized in that, in the first aspect of the present invention, the average area ratio of carbides having a particle size of less than 1 μm is 10 to 30%.

  The reasons for limiting the amount ratio of each element in the present invention will be described below. In addition, all quantity ratios are mass% (henceforth mass%).

C: 0.50-0.60 mass%
C forms carbides with the carbide-forming elements, resulting in increased high-temperature strength, temper softening resistance, and wear resistance. If the amount is less than 0.50 mass%, the amount of precipitated carbide is small, so that sufficient hardness cannot be obtained as a tool steel, and if too large, the toughness is reduced due to coarsening of the carbide. Limited to ~ 0.60 mass%.

Si: Less than 0.10 mass% Si is a solid solution strengthening element of ferrite. However, since it is also an element that promotes segregation, a large amount makes the structure non-uniform, which is not preferable from the viewpoint of heat check resistance. Therefore, the range is limited to less than 0.10 mass%.

P: Less than 0.010 mass% P segregates at the prior austenite grain boundaries and easily breaks grain boundaries, so it is assumed that heat check is likely to occur qualitatively, so the range is limited to less than 0.010 mass%. To do.

S: Less than 0.010 mass% S combines with Mn to form MnS, but if it is too much, coarse MnS increases and the toughness is reduced, so the range is limited to less than 0.010 mass%.

Mn: 0.30 to 1.00 mass%
Mn dissolves in the matrix during the austenitizing treatment and has the effect of suppressing the formation of ferrite (improving the hardenability). If it is less than 0.30 mass%, the hardenability becomes insufficient, and if it exceeds 1.00 mass%, the hardenability becomes excessive and it becomes difficult to obtain a full bainite structure. Therefore, the range was set to 0.30 to 1.00 mass%. For the same reason, it is desirable that the lower limit is 0.40% mass% and the upper limit is 0.60 mass%.

Cr: 3.00 to 4.50 mass%
Cr, like Mn, is an element that increases hardenability. In addition, it combines with Fe and C to form carbides and increase temper softening resistance, so it is an effective element for imparting heat check resistance, but if it is too much, coarse carbides are formed and toughness is increased. Bring about a decline. For these reasons, the Cr content is limited to 3.00 to 4.50 mass%. For the same reason, it is desirable that the lower limit is 3.50 mass% and the upper limit is 4.00 mass%.

Mo: 0.80 to 1.20 mass%
Mo is an element that combines with C to form a special carbide, and is an element that improves temper softening resistance and high-temperature strength. However, if it is too much, coarse carbides are formed and the toughness is lowered, so the range is limited to 0.80 to 1.20 mass%. For the same reason, it is desirable that the lower limit is 0.85 mass% and the upper limit is 1.00 mass%.

Ni: 0.30-0.60 mass%
Ni is an element that dissolves in the matrix and improves the hardenability and toughness. However, if it is less than 0.30 mass%, its effect is small, and if it is added more than necessary, the raw material cost increases. For this reason, the range of Ni content shall be 0.30-0.60 mass%. For the same reason, it is desirable that the lower limit is 0.40 mass% and the upper limit is 0.50 mass%.

V: 0.05-0.50 mass%
V is an element that combines with C to form a special carbide, which precipitates fine carbides in the matrix during quenching or tempering, and greatly improves temper softening resistance, high-temperature strength, and wear resistance. However, if the amount is too large, coarse carbides are formed and the toughness is reduced, so the range is limited to 0.05 to 0.50 mass%. For the same reason, it is desirable that the lower limit is 0.10 mass% and the upper limit is 0.30 mass%.

Structure In the tool steel of the present invention, the base structure having a bainite structure has a higher high-temperature proof stress than the martensite structure, and thus exhibits excellent heat check resistance. Moreover, in the mixed structure of martensite and bainite, the structure is limited to the full bainite structure because the structure becomes microscopically non-uniform and the cracking sensitivity becomes high.
In addition, by using a full bainite structure, for example, even in a large material of 10 t or more, restrictions during rapid cooling are small, and manufacturing becomes easy.

Carbide The required hardness as tool steel is mainly brought about by precipitated carbides, but if the number of carbides is too large or the particle size is too large, the toughness is reduced. For this reason, it is desirable that the carbide having a particle size of less than 1 μm is 10 to 30% in terms of average area ratio. It is desirable that the area ratio satisfies the above range for the entire material.

  As described above, according to the tool steel of the present invention, since the chemical composition is defined and a full bainite structure is obtained, the bainite structure can be easily obtained by the chemical composition, and good heat check resistance can be obtained. .

It is a figure which shows the heat check test apparatus and test piece which are used for the Example of this invention. Similarly, it is a figure which shows the 0.2% yield strength of the partial test material in an Example. Similarly, it is a figure which shows the heat check length of the one part test material in an Example.

Hereinafter, embodiments of the present invention will be described.
In mass percentage, C: 0.50 to 0.60%, Si: less than 0.10%, Mn: 0.30 to 1.00%, P: 0.010% or less, S: 0.010% or less, Cr: 3.00 to 4.50%, Mo: 0.80 to 1.20%, Ni: 0.30 to 0.60%, V: 0.05 to 0.50%, the balance being Fe And an alloy having a composition composed of inevitable impurities is melted. The method of melting is not particularly limited, but in order to increase the cleanliness due to non-metallic inclusions, it is possible to adopt methods such as ladle refining, vacuum casting, secondary melting such as electroslag remelting, etc. desirable. Nonmetallic inclusions not only lead to a decrease in toughness, but also have a different coefficient of thermal expansion from that of the base metal, so that the boundary between the matrix and inclusions is the starting point of cracking due to heating and cooling during use as hot tool steel. Therefore, the cleanliness (JISG0555) of non-metallic inclusions is preferably d (A + B + C) ≦ 0.10%.
The obtained ingot can be subjected to hot working such as forging and rolling, and the hot working can be performed by a conventional method.

The hot-worked material can have a full bainite structure by heat treatment. That is, a quenching treatment is performed in which the hot-worked material is heated to an austenite (γ) temperature or higher and rapidly cooled. The heating temperature should just be more than an austenitization temperature, and can be set to temperature suitably. However, if the cooling rate becomes too high, a martensite structure tends to appear. For this reason, it is desirable that the average cooling rate from the austenitizing temperature to 350 ° C. does not exceed 1300 ° C./hour during quenching.
After quenching, it is tempered to an appropriate hardness by tempering. As conditions of tempering, the conditions of 500-600 degreeC x 20-30 hours can be illustrated.
The obtained material can be used for various uses as tool steel excellent in heat check property.

Examples of the present invention will be described below.
A sample having the composition shown in Table 1 (the balance is Fe and other inevitable impurities) was melted with a 50 kg vacuum induction melting apparatus (VIM), and the resulting ingot was forged into a prismatic shape with a 90 × 90 mm cross section.
Thereafter, after holding at 1020 ° C. for 1 hour, quenching was performed at a cooling rate assuming oil quenching of a member having an outer diameter of 1300 mm. The average cooling rate at this time is 800 ° C./hr. Met. For comparison, some test materials were quenched with water cooling during the quenching. The average cooling rate at this time is 1600 ° C./hr. Met.

  After quenching, annealing was performed at 500 to 600 ° C. to obtain a test piece adjusted to a Vickers hardness of 450 HV. Various tests were performed using the test piece.

  The structural structure of each specimen was corroded with 5% picral after mirror polishing, and confirmed by observing the microstructure with an optical microscope. Each constituent organization is shown in Table 2. The specimens quenched by water cooling had a martensite structure, and the other specimens had a full bainite structure.

  Further, for each sample material, an average area ratio of carbides less than 1 μm was calculated from image analysis with respect to 20 fields of view of 6.8 μm × 11.7 μm in a 10,000 times SEM image. Indicated.

  The heat check test is performed by applying a thermal cycle at room temperature and 600 ° C., and the heat resistance is determined from the relative heat check length of each sample when the maximum heat check length of the invention material 1 having a full bainite structure is 100. Checkability was evaluated.

In the heat check test, the heat check test apparatus 1 of FIG. 1 was used.
In the heat check test apparatus 1, an elongated plate 3 is mounted on a rotary drive base 2 so as to be horizontally rotatable around the center in the length direction, and test piece mounting tables 4, 4 are provided on both ends of the elongated plate 3. It has been. At a predetermined rotational position where the elongated plate 3 stops, a high-frequency coil 5 for sample heating is arranged on one test piece mounting table 4, and the other test piece mounting table 4 faces the test piece mounting table 4. A cooling water nozzle 6 is provided for allowing the cooling water to flow down. In the heat check test apparatus 1, the sample can be repeatedly heated and cooled by rotating the elongated plate 3 every 180 degrees intermittently.

In the heat check test, each test material was cut out to 20 × 20 × 20 mm and placed on each of the test piece mounting tables 4 and 4 as a test piece. The elongated plate 3 is rotated so that one test piece 10 is located immediately below the high-frequency coil 5 and the other test piece 10 is located directly below the cooling water nozzle 6, and the surface of one sample test piece is placed on the high-frequency coil 5. Is heated to 600 ° C. (surface) by high frequency induction, and at the same time, the other sample test piece is cooled to room temperature with cooling water flowing down from the cooling water nozzle 6. Thereafter, the elongated plate 3 is rotated 180 degrees so that the heated test piece 10 is positioned immediately below the cooling water nozzle 6, cooled to room temperature with cooling water flowing from the cooling water nozzle 6, and at the same time, the other test that is water-cooled The piece is positioned directly under the high frequency coil 5 and heated to 600 ° C.
Thereafter, the test was performed by applying a thermal cycle of 600 ° C. and room temperature 10,000 times to the two test pieces by repeating the procedure of heating and cooling the elongated plate 3 by rotating it 180 degrees. In the test, it is possible to place only one test piece and apply a heat cycle of heating and cooling.
After the heat check test, the heat check resistance was evaluated by the longest crack observed on the surface layer side of the cross section perpendicular to the test surface at a position 2.5 mm away from the center of the heat check test surface in the surface direction.

FIG. 2 shows 0.2% proof stress at 600 ° C. of Comparative Materials 1 and 2 exhibiting a full martensite structure and a full bainite structure, Invention Material 1 having a full bainite structure, and Reference Material 1 having a full martensite structure. The reference material 1 has the same composition as the inventive material 1 and has a full martensite structure by cooling with water during quenching. The full martensite structures of the comparative materials 1 and 2 are also water-cooled during quenching.
Comparative materials 1 and 2 are V-free steel, invention material 1 and reference material 1 are V-added steel. When compared with the presence or absence of V, steel with V addition has a 0.2% yield strength higher than steel without V, and when compared with the same composition, 0.2% yield strength with full bainitic steel is greater than with full martensite steel. high. Therefore, it can be seen that V addition and bainite organization are effective in improving 0.2% proof stress at 600 ° C.

FIG. 3 shows the relative heat check lengths of Comparative Materials 1 and 2, Inventive Material 1 and Reference Material 1 exhibiting a full martensite structure and a full bainite structure. It can be seen that heat check resistance can be improved by converting the structure to full bainite regardless of whether or not V is added. This is presumably because the bainite structure is more stable than the martensite structure at a high temperature and the dislocations are difficult to move.
Furthermore, Table 2 shows the relative heat check lengths of all comparative materials having a bainitic structure and inventive materials. In particular, the inventive materials 1 to 13 showed high heat check resistance.
As described above, it is considered effective to use V addition and bainite organization in combination as a method for improving the heat check resistance.

DESCRIPTION OF SYMBOLS 1 Heat check test apparatus 3 Elongated plate 4 Test piece mounting base 5 High frequency coil 6 Cooling water nozzle 10 Test piece

Claims (2)

  In mass percentage, C: 0.50 to 0.60%, Si: less than 0.10%, Mn: 0.30 to 1.00%, P: less than 0.010%, S: less than 0.010%, Cr: 3.00 to 4.50%, Mo: 0.80 to 1.20%, Ni: 0.30 to 0.60%, V: 0.05 to 0.50%, the balance being Fe And a hot tool steel having a composition consisting of inevitable impurities and a full bainite structure.   The hot tool steel according to claim 1, wherein an average area ratio of carbides having a particle size of less than 1 µm is 10 to 30%.

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