JP7220750B1 - Hot work tool steel with excellent high-temperature strength and toughness - Google Patents

Abstract

[PROBLEMS] To provide a hot work tool steel having high softening resistance, hardenability and toughness. SOLUTION: By mass %, C: 0.20 to 0.60%, Si: 0.10 to less than 0.30%, Mn: 0.50 to 2.00%, Ni: 0.50 to 2.0%. 50%, Cr: 1.6 to 2.6%, Mo: 0.3 to 2.0%, V: 0.05 to 0.80%, the balance being Fe and unavoidable impurities, and formula A A hot work tool steel which has been quenched and tempered so that the value A is 27.4 to 29.3, and has 150 or less carbides having an equivalent circle diameter of 1 μm or more per 10000 μm 2 before use. However, formula A: A=[T](log10[t]+24)/1000. [Selection figure] None

Description

TECHNICAL FIELD The present invention relates to hot work tool steel excellent in high-temperature strength and toughness, which is used for hot forging dies and the like. related.

Japanese Industrial Standard (JIS) SKD61 steel is generally used for hot press forging, hot extrusion, and die casting dies, and JIS SKT4 steel is commonly used for hot hammer forging dies. It is JIS SKD61 steel is a mold steel that has both strength and toughness at relatively high levels, but it often suffers premature failure due to cracking during use, and is not always sufficient in terms of toughness. In addition, the toughness of JIS SKD61 steel is insufficient to suppress extension of thermal fatigue cracks. JIS SKT4 steel emphasizes toughness so that it can withstand a large impact due to hammer forging, but it lacks wear resistance due to its low softening resistance. In addition, if the engraving surface is repeatedly lowered for the purpose of reprocessing, the hardenability is low, causing a decrease in hardness at the center, and cracks and settling occur due to lack of strength. Furthermore, since the applicable hardness is low, wear resistance and strength are insufficient, and it is not suitable for hot press forging or hot extrusion.

The applicant has determined that, in mass %, C: 0.37 to 0.45%, Si: 0.3 to 1.2%, Mn: 0.6 to 1.5%, Ni: 0.3 to 1.5%. 0%, Cr: 1.0 to 2.0%, Mo: 1.1 to 1.4%, V: 0.1 to 0.3%, and the balance Fe. We have proposed a hot work tool steel with a specific range (see Patent Document 1).
However, the state of carbide precipitation before use was not taken into consideration, and the high-temperature strength was insufficient.

In addition, C: 0.10 to 0.70%, Si: 0.10 to 2.00%, Mn ≤ 2.00%, Cr ≤ 7.00%, W and Mo alone or in combination (1/2 W + Mo ): 0.20 to 12.00%, V ≤ 3.00%, S: less than 0.005%, O content is less than 30 ppm, and the balance is substantially Fe. It has been proposed (see Patent Document 2). However, neither the range of component fluctuations nor the state of carbide precipitation before use was considered, and the toughness was insufficient.

JP 2019-19374 A JP-A-11-6868

When used at high temperatures, MC-based carbides and M ₂ C-based carbides and carbonitrides precipitate to provide softening resistance, that is, high-temperature strength. However, if there is a large amount of M ₂₃ C ₆ -based carbides before use, the precipitation amount of MC-based, M ₂ C-based carbides and carbonitrides during use decreases, and high high-temperature strength cannot be obtained. Moreover, if a large amount of coarse carbides are present before use, there is a problem that the toughness is lowered.

Therefore, the problem to be solved by the present invention is to limit the quenching conditions so that the M ₂₃ C ₆ -based carbides, which remain after pressing and have a small contribution to high-temperature strength, are dissolved in the quenching process. At the same time, it is necessary to obtain excellent toughness by controlling the width of component variation. The solid _solution of M ₂₃ C ₆ carbides in the quenching process increases the carbon content in the matrix. It is to obtain excellent high-temperature strength by precipitating carbides and carbonitrides. That is, to provide a hot work tool steel with toughness and high temperature strength.

In order to solve the above-mentioned problems, the inventors have made intensive development, and as a result, by specifying the alloy composition, quenching conditions, carbide state, and the range of composition fluctuation, it has both excellent high temperature strength and toughness. It has been found that a hot work tool steel can be obtained.

That is, the first means for solving the problem is, in mass%, C: 0.20 to 0.60%, Si: 0.10 to less than 0.30%, Mn: 0.50 to 2.00 %, Ni: 0.50 to 2.50%, Cr: 1.6 to 2.6%, Mo: 0.3 to 2.0%, V: 0.05 to 0.80%, the balance being Fe and The number of carbides that are inevitable impurities and that have been quenched and tempered so that the value A in formula A is 27.4 to 29.3 and have an equivalent circle diameter of 1 μm or more per 10000 μm ² before use 150 or less, hot work tool steel.
However, formula A: A = [T] (log ₁₀ [t] + 24) / 1000, where [T] and [t] include [T]: quenching temperature (K), [t]: quenching Substitute the numerical value for the temperature holding time (h).

The second means is the steel of the first means, and the range of variation in the components of C, Cr, Mo, and V in the steel is
(C _max −C _min )/C≦1.0,
(Cr _max −Cr _min )/Cr≦0.5,
(Mo _max −Mo _min )/Mo≦1.5,
( _Vmax - _Vmin )/V≤1.5
It is a hot work tool steel characterized by being

The hot work tool steel of the means of the present invention has both high-temperature strength and toughness, such as a reduction in hardness from the initial hardness of 14 HRC or less and an impact value of 70 J/cm ² or more in the Charpy impact test. can be obtained.
If the range of component variation is specified and well controlled, the internal segregation is small and the impact value in the Charpy impact test is 85 J/cm ² or more, so hot work tool steel with better toughness can be obtained.

First, prior to describing the embodiments of the invention according to the present application, the reasons for specifying the chemical components added to the hot work tool steel of the means of the present invention, the reasons for specifying the formula A, and the reasons for specifying the number of carbides will be described. . In addition, % in a chemical component shows the mass %.

C: 0.20-0.60%
C is a component for ensuring sufficient hardenability and forming carbides and carbonitrides to obtain high-temperature strength, hardness, and wear resistance. If C is less than 0.20%, sufficient high-temperature strength cannot be obtained. On the other hand, when C exceeds 0.60%, solidification segregation is promoted, coarse carbides and carbonitrides are formed, and toughness is lowered. In addition, the resulting carbides remain undissolved during quenching, reducing the amount of carbides and carbonitrides precipitated when used as a hot work tool steel, and an improvement in high-temperature strength cannot be expected. Therefore, C is set to 0.20 to 0.60%. Preferably, C is 0.40-0.60%.

Si: 0.10 to less than 0.30% Si is a component necessary for securing the deoxidizing effect and hardenability in steelmaking. If the Si content is less than 0.10%, a sufficient effect cannot be exhibited. On the other hand, when Si is 0.30% or more, the toughness is lowered. Therefore, Si should be 0.10% or more and less than 0.30%.

Mn: 0.50-2.00%
Mn is a component necessary for securing the deoxidizing effect and hardenability in steelmaking. If Mn is less than 0.50%, sufficient effects are not exhibited. If the Mn content is more than 2.00%, the workability is deteriorated. Therefore, Mn is set to 0.50 to 2.00%.

Ni: 0.50-2.50%
Ni is a component necessary for ensuring hardenability and improving toughness. If Ni is less than 0.50%, sufficient effects are not exhibited. If Ni is more than 2.50%, the cost becomes too high. Therefore, Ni should be 0.50 to 2.50%.

Cr: 1.6-2.6%
Cr is a component necessary for ensuring sufficient hardenability. If Cr is less than 1.6%, sufficient hardenability cannot be obtained. On the other hand, when Cr is added in an amount of more than 2.6%, an excessive amount of M ₂₃ C ₆ -based carbides mainly composed of Cr and Fe are formed during quenching and tempering, deteriorating high-temperature strength, softening resistance and toughness. Therefore, Cr is set to 1.6 to 2.6%.

Mo: 0.3-2.0%
Mo is a useful component for obtaining precipitated carbides that contribute to hardenability, secondary hardening, and high-temperature strength. If Mo is less than 0.3%, sufficient effects cannot be obtained. When Mo is more than 2.0%, not only does the effect saturate even if it is excessively added, but also the toughness is reduced due to coarse aggregation of carbides. Moreover, it becomes costly. Therefore, Mo is set to 0.3 to 2.0%.

V: 0.05-0.8%
V is a component that precipitates fine hard carbides and carbonitrides during tempering or during use as a hot work tool steel and contributes to strength and wear resistance. If V is less than 0.05%, these effects cannot be sufficiently obtained. If V is more than 0.80%, coarse carbides and carbonitrides are crystallized during solidification, impairing toughness. Therefore, V is 0.05 to 0.80%. Preferably V is between 0.05 and 0.20%.

Value A: 27.4-29.3
Formula A: A = [T] (log ₁₀ [t] + 24)/1000
[T]: quenching temperature (K) and [t]: quenching temperature holding time (h) are substituted for [T] and [t].
Formula A is an index for ensuring the solid solubility of carbide by defining the quenching temperature and holding time. If the value A is 27.4 or less, solid solution of carbides due to quenching of the steel of the composition of the present invention becomes insufficient, resulting in insufficient toughness and high-temperature strength when used as a hot tool. On the other hand, if the value A exceeds 29.3, the prior austenite crystal grains become coarse, resulting in a decrease in toughness. Therefore, the value A is set to 27.4 to 29.3.

Number of carbides with an equivalent circle diameter of 1 μm or more per 10,000 μm ² : 150 or less When there are too many carbides with an equivalent circle diameter of 1 μm or more, the amount of carbon in the matrix is insufficient, and the MC system precipitates during use as a hot work tool steel. , M ₂ C-based carbides and carbonitrides are reduced. MC-based, M ₂ C-based carbides, and carbonitrides contribute to the improvement of high-temperature strength by precipitating during use as hot work tool steel. It will not be possible. Also, if there are too many carbides with an equivalent circle diameter of 1 μm or more, stress concentrates and acts as a starting point or propagation path for cracks, which impairs toughness. Therefore, the number of carbides having an equivalent circle diameter of 1 μm or more per 10000 μm ² is 150 or less.

The range of variation in the components of C, Cr, Mo, and V in steel is
( _Cmax - _Cmin )/C≤1.0
( _Crmax - _Crmin )/Cr≤0.5
( _Momax - _Momin )/Mo≤1.5
( _Vmax - _Vmin )/V≤1.5
When evaluating the alloying element [M] by ([M] _Max - [M] _Min ) / [M], due to internal segregation of each alloying element ([M] is C, Cr, Mo, V.) If there is a large variation in composition, the difference in distribution of carbides and carbonitrides and the difference in deformability will increase, resulting in a decrease in toughness.
Therefore, it is desirable to regulate the range of variation in the components of C, Cr, Mo, and V.
In addition, the component fluctuation range due to segregation is obtained by measuring the concentration profile in the range of 0.5 mm×0.5 mm using EPMA (Electron Prove Micro Analysis) on the L surface of the steel material.
Also, the application of soaking treatment in which the center of the steel ingot is soaked in the range of 1225° C. to 1300° C. for 10 to 40 hours makes it possible to effectively reduce the values of component fluctuations.

(Example)
Steels composed of the chemical compositions described in Inventive Steel Nos. 1 to 28 in Table 1 and Comparative Steels Nos. 29 to 40 in Table 2 and the balance Fe and unavoidable impurities were melted at 100 kg VIM and cast into ingots. Inventive Steel No. For 1 to 20, soaking was performed in the above range. These ingots were then heated to 1220° C. and stretched to 15 corners. The component composition of each bulk steel was confirmed by ICP spectroscopic analysis.
After that, it was heated to 850 to 960° C. and held for various times (30 minutes to 3 hours) to obtain an austenite structure. Then, quenching by oil cooling was performed, and after further heating to 500 to 700° C., tempering by air cooling was performed twice to refining to 39 to 41 HRC. Furthermore, test materials were obtained by machining.
The rest of the chemical composition in Table 1 is Fe and unavoidable impurities.

(Measurement of carbide content)
After mirror-polishing the center of the steel material of the quenched and tempered material by buffing, select 30 fields where many carbides are observed, and observe the number of carbides with an equivalent circle diameter of 1 μm or more with an electron microscope at a magnification of 10,000. was measured by image analysis. When the number of carbides having an equivalent circle diameter of 1 μm or more per 10,000 μm ² was 150 or less, it was rated as A, and when the number was greater than this, it was rated as C.

(Evaluation of width of component variation)
For each test piece, the L surface (surface parallel to the rolling direction and plate thickness direction of the steel plate, the so-called longitudinal section) is mirror-polished, and then EPMA (Electron Prove Micro Analysis) is used to a range of 0.5 mm × 0.5 mm. was measured. The alloy element [M] was evaluated by ([M] _Max - [M] _Min )/[M]. The results are shown in Tables 3 and 4 as component variation ranges.
If the value of [M] _Max - [M] _Min ) / [M] for all alloying elements for C, Cr, Mo, V is within the specified value, it is an excellent If even one component did not meet the requirements due to a large fluctuation range, it was rated C as inferior. Tables 1 and 2 show the evaluation results of the width of component variation.

(high temperature strength)
After measuring the HRC hardness of the quenched and tempered material of each test piece, it was further held at 600 ° C. for 100 hours and then air-cooled. bottom. A was given when the reduction value was 14 HRC or less, and C was given when the reduction value was greater than this value.

(Toughness)
A U-notch test piece of JIS-specified No. 3 square of 10 mm and length of 55 mm was quenched and tempered so that the hardness was 39 to 41 HRC, and the toughness was evaluated by performing a Charpy impact test at room temperature. . Those with an impact value of 70 J/cm ² or more were rated A, those with an impact value of 85 J/cm ² or more were rated A ⁺ , and those with an impact value of less than 70 J/cm ² were rated C.

Invention Example No. As shown in Table 1, all of Nos. 1 to 28 were within the specified range of chemical composition, satisfied the value of formula A, and had a small number of carbides having an equivalent circle diameter of 1 μm or more. Therefore, in the Charpy impact test, the impact value was 70 J/cm ² or more, so the toughness was evaluated as A, the reduction width from the initial hardness was within 14 HRC, and the high temperature strength (softening resistance) was also excellent with an A evaluation. A hot work tool steel with both high-temperature strength and toughness was obtained.

Comparative example no. No. 29 had a low high-temperature strength due to a small amount of C.
Comparative example no. In No. 30, the amount of C was large, the number of carbides having an equivalent circle diameter of 1 μm or more was large, and the range of component fluctuation was large, so the toughness and high-temperature strength were low.
Comparative example no. In No. 31, the amount of Si was large and the toughness was low.
Comparative example no. In No. 32, the amount of Ni was small and the toughness was low.
Comparative example no. In No. 33, the amount of Cr was large, the number of carbides having an equivalent circle diameter of 1 μm or more was large, and the range of component fluctuation was large, so the toughness and high-temperature strength were low.
Comparative example no. In No. 34, the amount of Mo was small and the high-temperature strength was low.
Comparative example no. In No. 35, the amount of Mo was large, the number of carbides having an equivalent circle diameter of 1 μm or more was large, and the range of component fluctuation was large, so the toughness and high-temperature strength were low.
Comparative example no. No. 36 has a small amount of V and a low high-temperature strength.
Comparative example no. In No. 37, the amount of V was large, the number of carbides having an equivalent circle diameter of 1 μm or more was large, and the range of component fluctuation was large, so the toughness was low.
Comparative example no. In No. 38, the value of Formula A was small and the number of carbides having an equivalent circle diameter of 1 μm or more was large, so the toughness and high-temperature strength were low.
Comparative example no. In No. 39, the value of Formula A was large and the toughness was low.
Comparative example no. In No. 40, the number of carbides having an equivalent circle diameter of 1 μm or more was large, and the toughness and high-temperature strength were low.

Claims (1)

% by mass, C: 0.20 to 0.60%, Si: 0.10 to less than 0.30%, Mn: 0.50 to 2.00%, Ni: 0.50 to 2.50%, Cr : 1.6 to 2.6%, Mo: 0.3 to 2.0%, V: 0.05 to 0.80%, the balance being Fe and unavoidable impurities, in a quenched and tempered state. the value A of formula A is 27.4 to 29.3, and the number of carbides having an equivalent circle diameter of 1 μm or more per 10000 μm ² before use is 150 or less,
Furthermore, the range of variation in the components of C, Cr, Mo, and V in the steel is
(C _max −C _min )/C≦1.0,
(Cr _max −Cr _min )/Cr≦0.5,
(Mo _max −Mo _min )/Mo≦1.5,
(Vmax _- Vmin ₎ /V≤1.5
Hot work tool steel.
However, formula A: A = [T] (log ₁₀ [t] + 24) / 1000, where [T] and [t] include [T]: quenching temperature (K), [t]: quenching Substitute the numerical value for the temperature holding time (h).