JP7540437B2 - Steel for hot stamping dies, hot stamping dies and their manufacturing method - Google Patents

Description

The present invention relates to steel for hot stamping dies, hot stamping dies and a method for manufacturing the same.

In recent years, there has been an increasing need for ultra-high tensile steel plates with a tensile strength of over 1 GPa in order to reduce the weight of automobiles and improve crash safety. However, when attempting to form steel plates with a tensile strength of 1.2 GPa or more by cold pressing, problems such as increased forming load and springback, and formability arise. For this reason, the hot stamping (also called hot pressing or hot stamping) method has recently attracted attention. In the hot stamping method, the steel plate is heated to above the austenite temperature, then press-formed, and the die is held at the bottom dead center to rapidly cool and quench it.

The advantage of the hot stamping method is that it is possible to obtain a formed product of ultra-high tensile steel plate with a tensile strength of about 1.5 GPa by quenching the product by rapid cooling in a die. Another advantage is that it has excellent formability, such as almost no springback.
However, the hot stamping method has a problem of low productivity. In other words, the time required for the bottom dead center to be held for die quenching reduces productivity. To address this issue, dies with high thermal conductivity are required. This is because, in die quenching, the heat from the steel sheet is absorbed by the die, and the higher the thermal conductivity of the die, the shorter the time required to hold the bottom dead center, thereby increasing productivity.
Furthermore, hot stamping dies are required to have high hardness in order to improve wear resistance.

Therefore, hot stamping die steel is required to have both high hardness and high thermal conductivity when made into a die. Generally, to obtain a die with high hardness, it is necessary to increase the amount of alloy in the die steel, but there is a problem that the thermal conductivity of the die decreases as the amount of alloy increases, and hardness and thermal conductivity are in a trade-off relationship. Therefore, the optimal composition is being studied by controlling the amount of alloy. For example, Patent Documents 1 to 3 propose compositional compositions of die steel that have both hardness and thermal conductivity. Patent Document 4 also discloses a hot tool steel that is useful as a material for dies used in warm hot pressing, die casting, or warm hot forging, and has excellent thermal conductivity and wear resistance.

JP 2017-43814 A JP 2017-53023 A JP 2018-24931 A Patent No. 5744300

The die steels of Patent Documents 1 to 3 and the hot work tool steel of Patent Document 4 are useful for hot stamping. However, when considering the quenching and tempering characteristics of die steels and hot work tool steels, and the fact that the working surfaces of hot stamping dies are nitrided before use, the hardness of conventional die steels and hot work tool steels is sometimes insufficient. Specifically, although there has been a recent demand for high hardness of 52 HRC or more for hot stamping steels, Patent Documents 1 to 4 do not stably provide a high hardness of 52 HRC or more.
An object of the present invention is to provide a die steel capable of producing a die having both high hardness and high thermal conductivity suitable for hot stamping, a hot stamping die, and a manufacturing method thereof.

In view of this situation, the inventors conducted extensive research and discovered a die steel that is optimal for hot stamping by controlling the amount of alloy. They also discovered a hot stamping die that can achieve high hardness and high thermal conductivity by using the die steel, and a manufacturing method thereof.

That is, one aspect of the present invention is a steel for hot stamping dies, characterized by a chemical composition, in mass%, of C: 0.45-0.65%, Si: 0.1-0.6%, Mn: 0.1-0.3%, Cr: 2.5-6.0%, Mo: 1.2-2.6%, V: 0.4-0.8%, the balance being Fe and unavoidable impurities.

Another aspect of the present invention is a hot stamping die having a component composition, in mass%, of 0.45 to 0.65% C, 0.1 to 0.6% Si, 0.1 to 0.3% Mn, 2.5 to 6.0% Cr, 1.2 to 2.6% Mo, 0.4 to 0.8% V, the balance being Fe and unavoidable impurities.
Preferably, the hot stamping die according to claim 2 has a hardness of 45 HRC or more and a thermal conductivity λ (W/(m·K)) satisfying the following formula (1).
λ≧−0.5H+53…Formula (1)
H: Rockwell hardness (HRC) of the die
More preferably, the hardness is 52 HRC or more. In this case, it is further preferable that the thermal conductivity λ is 25 W/(m·K) or more.
Also, it is preferable that the working surface has a nitride layer.

Another aspect of the present invention is a manufacturing method of a hot stamping die, characterized in that the above-mentioned hot stamping die steel is quenched and tempered at a quenching temperature of 1020 to 1080°C and a tempering temperature of 500 to 625°C.
Preferably, the tempering temperature is 540 to 600°C.
Preferably, after the quenching and tempering, the working surface is further subjected to a nitriding treatment.

According to the present invention, a die steel that is optimal for hot stamping can be obtained. Furthermore, by using this die steel, it is possible to provide a hot stamping die that has both high hardness and high thermal conductivity, and a manufacturing method thereof.

FIG. 1 is a graph showing the hardness for each tempering temperature for an example of a mold produced by quenching the mold steels of the invention examples and comparative examples from 1,030° C. and then tempering them at 500 to 650° C. FIG. 1 is a graph showing the thermal conductivity of examples of dies produced by quenching the die steels of the invention examples and comparative examples from 1,030° C. and then tempering them to hardnesses of 45 HRC, 50 HRC, and 55 HRC. FIG. 1 is a graph showing the softening resistance when held at 600° C. for an example of a die produced by quenching the die steels of the invention examples and the comparative examples from 1,030° C. and then tempering them to a hardness of 55 HRC.

The feature of the present invention is that, considering that hot stamping dies are produced by quenching and tempering die steel, and by nitriding the working surface, it has been found that there is an optimum composition of die steel for simultaneously achieving high hardness and high thermal conductivity of hot stamping dies. In particular, it has been found that there is an optimum composition of die steel for simultaneously achieving a high hardness of 52 HRC or more and a high thermal conductivity of 25 W/(m·K) or more. In addition, it has been found that the optimum quenching and tempering conditions for simultaneously achieving the above high hardness and high thermal conductivity have been found. Each of the constituent elements of the present invention will be described below.

The steel for hot stamping dies of the present invention has a composition, in mass percent (hereinafter simply referred to as "%)", of C: 0.45-0.65%, Si: 0.1-0.6%, Mn: 0.1-0.3%, Cr: 2.5-6.0%, Mo: 1.2-2.6%, V: 0.4-0.8%, with the balance being Fe and unavoidable impurities.

・C: 0.45-0.65%
C is an element that dissolves in the matrix during quenching to improve the hardness of the die. It also forms carbides with carbide-forming elements such as Cr, Mo, and V, which will be described later, to improve the hardness of the die. However, if the amount of C is too large, the primary carbides become coarse, and the toughness of the die decreases. Therefore, the amount of C is set to 0.45 to 0.65%. It is preferably 0.47% or more. More preferably 0.49% or more. Also, it is preferably 0.63% or less. More preferably 0.60% or less. Further preferably 0. It is less than 58%.

・Si: 0.1-0.6%
Silicon is used as a deoxidizer in the smelting process. It is an element that dissolves in the base metal to improve the hardness of the mold. However, if there is too much silicon, the silicon in the steel after smelting will be The tendency for segregation is strengthened, and the solidified structure becomes coarse, leading to a decrease in the toughness of the die. In addition, Si is an element that significantly reduces the thermal conductivity of the die after quenching and tempering. Therefore, the content of Si is within the range of 0.1 to 1.5. The content is set to 0.6%. It is preferably 0.14% or more. More preferably, it is 0.17% or more. It is preferably 0.45% or less, and more preferably 0.4% or less. More preferably, it is 0.35% or less. Still more preferably, it is 0.3% or less.

・Mn: 0.1-0.3%
Mn is used as a deoxidizer and desulfurizer in the smelting process. It is also an element that contributes to strengthening the base material, improving hardenability, and toughness after quenching and tempering. However, if there is too much Mn, The thermal conductivity of the mold is significantly reduced. Therefore, the Mn content is set to 0.1 to 0.3%. The preferred lower limit of Mn is 0.15% or more. The preferred upper limit of Mn is 0.28%. % or less. A more preferable upper limit of Mn is 0.26% or less.

・Cr: 2.5-6.0%
Cr is an element that dissolves in the base material to increase hardness. It also increases hardness by forming carbides. Like Mo and V, which will be described later, Cr is also effective in secondary hardening during tempering. In particular, Cr can increase the tempering softening resistance compared to Mo and V (even if the tempering temperature is increased, the rate of decrease in hardness obtained by secondary hardening is reduced). Normally, dies are made by quenching and tempering die steel to adjust the hardness to the required level. In order to increase the thermal conductivity of hot stamping dies, the tempering temperature is In the present invention, by setting the Cr content to 2.5% or more, the tempering temperature can be increased (for example, even if the tempering temperature exceeds 600° C.). Since the steel can maintain a sufficient hardness of 45 HRC or more, the thermal conductivity can be increased at the same time. It is possible to obtain a hot stamping die having a thermal conductivity of 25 W/(m·K) or more. And, to obtain a hot stamping die in which the thermal conductivity is further improved to 28 W/(m·K) or more, or 30 W/(m·K) or more while maintaining the above hardness. The above hardness and thermal conductivity values are values measured at room temperature (normal temperature).
In addition, by increasing the Cr content, the nitriding characteristics of the die steel can be improved. For example, by further nitriding the working surface of the die after quenching and tempering, the die The wear resistance (hardness of the working surface) can be improved.

However, if the Cr content is too high, the alloy amount of the mold steel will be high, which in itself makes it difficult to increase the thermal conductivity of the mold. Therefore, Cr is set to 2.5 to 6.0%. It is preferably 2.8% or more. More preferably, it is 3.0% or more. It is also preferably 5.5% or less, more preferably 4.8% or less, and even more preferably less than 4.5%. And, if emphasis is particularly placed on improving thermal conductivity, Cr can be set to 4.0% or less, or even 3.5% or less.

・Mo: 1.2-2.6%
Mo, like Cr, is an element that dissolves in the base material to increase hardness, and also increases hardness by forming carbides. It is an element that contributes to secondary hardening during tempering. It is also an element that improves hardenability. However, if the amount of Mo is too large, the alloy amount of the die steel increases, which in itself reduces the thermal conductivity of the die. is 1.2 to 2.6%, preferably 1.5% or more, more preferably 1.7% or more, and further preferably 1.9% or more. It is preferably 0.5% or less, more preferably 2.3% or less, and even more preferably 2.1% or less.

・V: 0.4-0.8%
V, like Cr, is an element that increases hardness by forming carbides and contributes to secondary hardening during tempering. However, if the amount of V is too large, the alloy of the mold steel may deteriorate. The amount of V is increased, which in itself reduces the thermal conductivity of the mold. Therefore, V is set to 0.4 to 0.8%, preferably 0.5% or more. Also, it is preferably 0. .75% or less, more preferably 0.65% or less, and even more preferably 0.6% or less.

- Remainder Fe and inevitable impurities Considering that the thermal conductivity of the die decreases when the alloy content of the die steel increases, it is preferable that the remainder other than the above element types is substantially Fe. However, element types not specified here (e.g., P, S, Cu, Al, Ca, Mg, O (oxygen), N (nitrogen), etc.) are elements that may unavoidably remain in the steel, and it is acceptable to include these elements as impurities. In this case, if there is too much P, it will segregate at the prior austenite grain boundary during heat treatment such as tempering, and the toughness of the die will deteriorate. Therefore, P is preferably restricted to 0.05% or less. More preferably, it is restricted to 0.03% or less. And, if there is too much S, the hot workability will deteriorate when the steel ingot is divided into pieces. Therefore, S is preferably restricted to 0.01% or less. More preferably, it is restricted to 0.008% or less.
Although Ni is useful as an element that contributes to improving the toughness of the die, it is preferable to keep the Ni content low in order to prevent a decrease in the thermal conductivity of the die due to an increase in the alloy content of the die steel. The upper limit of the Ni content is preferably 0.25%.

By quenching and tempering the die steel having the above-mentioned composition, the hot stamping die of the present invention having excellent hardness and thermal conductivity can be obtained. The hardness of the hot stamping die of the present invention is easily achieved to a sufficient hardness, for example, 45 HRC or more, as measured at room temperature (normal temperature). By adjusting the tempering temperature, the hardness of the die can be preferably 52 HRC or more, and excellent wear resistance can be imparted to the die during use. The hardness of the die is more preferably 53 HRC or more, and even more preferably 55 HRC or more.
In the present invention, it is not necessary to specify an upper limit for the hardness of the die. However, in the case of die steel having the above-mentioned composition, a realistic hardness is about 60 HRC, because the peak hardness of the secondary hardening is in the tempering temperature range of about 500 to 600°C. It is preferable to set the upper limit of this hardness to 58 HRC or less, because this allows the tempering temperature to be increased beyond the above-mentioned peak hardness (i.e., the thermal conductivity can be increased).

The die steel having the above-mentioned component composition is quenched and tempered to adjust the hardness of the die to 45 HRC or more, and is further characterized in that the die has a thermal conductivity λ (W/(m·K)) that satisfies the following formula (1):
λ≧−0.5H+53…Formula (1)
Here, H in formula (1) is the Rockwell hardness (HRC) of the mold. For example, when the hardness of the mold in this embodiment is 45 HRC, the thermal conductivity is 30.5 W/(m·K) or more. When the hardness of the mold is 52 HRC, the thermal conductivity is 27 W/(m·K) or more. Preferably, "λ≧-0.5H+54". Note that this thermal conductivity is a value measured at room temperature (normal temperature).
Since the die steel of the present invention satisfies the relationship of formula (1) by quenching and tempering, it can maintain a thermal conductivity of 25 W/(m·K) or more even when the tempered hardness is in the high hardness range (e.g., 52 HRC or more), where a decrease in thermal conductivity has been an issue. When the hardness of the die is 52 HRC or more, the preferred thermal conductivity is 28 W/(m·K) or more. A more preferred thermal conductivity is 30 W/(m·K) or more. And when the tempered hardness is in the low hardness range (e.g., less than 52 HRC), it is possible to achieve a thermal conductivity of 30 W/(m·K) or more, and when the hardness is around 45 HRC, it is possible to achieve a thermal conductivity of 32 W/(m·K) or more. This allows a high thermal conductivity to be maintained in the die being used in the hot stamping method (e.g., 100 to 400°C).
Such a thermal conductivity can be easily achieved by increasing the tempering temperature in addition to the above-mentioned component composition of the die steel. For example, by increasing the tempering temperature to a temperature higher than the temperature at which the peak hardness is obtained, it is possible to adjust the thermal conductivity to 30 W/(m·K) or higher.

In the present invention, it is not necessary to specify the upper limit of the thermal conductivity of the mold. However, considering that the hardness of the mold decreases as the tempering temperature is increased (for example, by adjusting the temperature to a temperature exceeding 600°C), the thermal conductivity exceeds 50 W/(m·K) when the hardness of the mold falls below 45 HRC, so when the hardness of 45 HRC or more is maintained, it is realistic that the upper limit of the thermal conductivity is about 50 W/(m·K). It is preferably 47 W/(m·K) or less. More preferably, it is 45 W/(m·K) or less. And, when the mold maintains a hardness of 52 HRC or more, it is realistic that the upper limit of the thermal conductivity is about 40 W/(m·K). It is preferably 38 W/(m·K) or less. More preferably, it is 35 W/(m·K) or less. From these upper limits of thermal conductivity, it is realistic that the relationship between the above-mentioned thermal conductivity λ (W/(m·K)) and Rockwell hardness H (HRC) is approximately "λ≦-0.5H+70" as expressed by formula (2). Preferably, "λ≦-0.5H+66", and more preferably, "λ≦-0.5H+61".

The hot stamping die of the present invention preferably has a nitride layer on its working surface.
As described above, the hot stamping die of the present invention has both high hardness and high thermal conductivity. The working surface of the die further has a nitride layer, which can further improve the wear resistance (hardness of the working surface) of the die. The working surface refers to the surface of the die that comes into contact with the steel sheet during hot stamping.

The method for producing a hot stamping die of the present invention comprises quenching and tempering the above die steel.
When quenching and tempering the die steel having the above-mentioned composition, the quenching temperature varies depending on the target hardness, etc., but can be, for example, about 1020 to 1080° C. Preferably, it is 1050° C. or lower.
Then, by tempering the die steel that has been quenched at this quenching temperature, for example, at a tempering temperature of 500 to 625° C., it is possible to maintain a sufficient hardness of 45 HRC or more. The quenching temperature and tempering temperature at this time can be selected so that the hardness and thermal conductivity of the die after quenching and tempering satisfy the relationship of the above-mentioned formula (1).
Tempering at high temperatures is also effective in maintaining sufficient hardness of the mold and increasing the thermal conductivity of the mold; for example, a hardness of 52 HRC or more can be achieved at a tempering temperature of 540°C or higher, so that a mold with a thermal conductivity of 25 W/(m·K) or higher can be obtained. In this case, in order to maintain a hardness of 52 HRC or higher, the upper limit of the tempering temperature is preferably about 600°C. More preferably, it is 595°C or lower. Even more preferably, it is 590°C or lower.

The die steel of the present invention is quenched and tempered to prepare a hot stamping die having a predetermined hardness. During this time, the die steel is shaped into the shape of a hot stamping die by various machining processes such as cutting and drilling. This machining process can be performed when the hardness is low before quenching and tempering (i.e., in the annealed state). In this case, finishing may be performed after quenching and tempering. In some cases, the above-mentioned machining process may also be performed in the pre-hardened state after quenching and tempering, with the above-mentioned finishing process being added.

In the method for producing a hot stamping die of the present invention, preferably, the working surface of the die after the above-mentioned quenching and tempering is further subjected to a nitriding treatment.
As described above, by quenching and tempering the die steel having the above-mentioned composition, a die having a hardness of 45 HRC or more and a thermal conductivity satisfying the formula (1) can be obtained. Since the die steel having the above-mentioned composition also has excellent nitriding properties, the wear resistance (hardness of the working surface) of the die can be improved by further performing a nitriding treatment on the working surface of the die after the quenching and tempering. In this case, the conditions of the nitriding treatment can be those of various known nitriding treatments, such as gas nitriding and salt bath nitriding.

A 10 kg steel ingot was produced with the chemical composition shown in Table 1. The ingot was then heated to 1160°C and hammer forged, then allowed to cool. After cooling, the steel was annealed at 870°C to produce steels No. 1 to 8, which are examples of the present invention, and No. 9 to 11, which are comparative examples.

<Evaluation of tempered hardness>
The die steels No. 1 to No. 11 were quenched at a quenching temperature of 1030°C. At this time, the cooling conditions were set to a semi-cooling time of 40 minutes, assuming a cooling rate when die steels such as the steel of the present invention and the comparative steel are the size of an actual hot stamping die (the semi-cooling time is the time required to cool from the quenching temperature to a temperature of (quenching temperature + room temperature)/2). Then, the die steels after quenching were tempered at a tempering temperature of 500 to 650°C. Tempering was performed twice, and each temperature was held for 2 hours. The tempering temperature was set at 25°C intervals, for a total of seven conditions. Then, for each of No. 1 to No. 11, the Rockwell hardness (C scale) at room temperature of the center was measured for each tempering temperature. The results are shown in FIG. 1.

Nos. 1 to 8, which are examples of the present invention, maintained a tempered hardness of 45 HRC or more over the entire tempering temperature range of 500 to 625° C. In particular, all of them achieved a tempered hardness of approximately 52 HRC or more in the tempering temperature range of 540 to 600° C. In addition, even when the tempering temperature was increased to over 600° C., which is considered to be effective in increasing the thermal conductivity of the mold, a tempered hardness of approximately 45 HRC or more was achieved.
In contrast, Comparative Example No. 9 also maintained a tempered hardness of 45 HRC or more in the tempering temperature range of 500 to 600° C., but the tempered hardness of No. 10 was already below 45 HRC when the tempering temperature reached 575° C. No. 11 had a tempered hardness of 45 HRC or more in the tempering temperature range of 500 to 625° C., but did not achieve a hardness of 50 HRC or more.

<Evaluation of thermal conductivity>
Based on the results of the above <Evaluation of tempered hardness>, the thermal conductivity was measured for Nos. 1 to 6 and 9 when the tempered hardness was 45HRC, 50HRC and 55HRC. The measurement procedure was as follows: first, the mold was processed into a disk-shaped test piece with a diameter of 10 mm and a thickness of 2 mm, and the thermal diffusivity and specific heat of this test piece were measured by the laser flash method. Then, using the measured values of thermal diffusivity and specific heat, the thermal conductivity at room temperature was calculated from the following formula (3). The results are shown in FIG. 2.
Thermal conductivity λ (W/(m·K))=ρ·α·C _p ...Equation (3)
(ρ: room temperature density, α: thermal diffusivity, C _p : specific heat)

2, it can be seen that the thermal conductivity of Nos. 1 to 6, which are examples of the present invention, satisfies the formula λ≧-0.5H+53 at all hardnesses of 45HRC, 50HRC, and 55HRC, and maintains a high thermal conductivity of 30 W/(m K) or more even when the hardness is increased to 52HRC. Also, even at a high hardness of 55HRC, the thermal conductivity was high, at 25 W/(m K) or more.
In contrast, in the comparative example No. 9, the thermal conductivity was small when tempered to a low hardness of 45HRC and 50HRC (the formula λ≧−0.5H+53 was not satisfied), and it was found that even when the hardness was increased to 52HRC, the formula λ≧−0.5H+53 was not satisfied.

For samples No. 7 and 8, the thermal conductivity was measured when the tempered hardness was 52 HRC. The measurement method was the same as for samples No. 1 to 6 and 9 described above. As a result, it was confirmed that sample No. 7 had a thermal conductivity of 31 W/(m K) and sample No. 8 had a thermal conductivity of 37 W/(m K), indicating that even with a hardness of 52 HRC, the sample had a high thermal conductivity of 30 W/(m K) or more.

<Evaluation of softening resistance>
Since the die in the hot stamping method is used at high temperatures, the softening resistance of the die steel becomes important. Therefore, the present invention examples No. 1 to 6 and the comparative example No. 9 were tempered to 55 HRC and held at 600°C, and the change in hardness was measured. The results are shown in FIG. 3. In the present invention examples No. 1 to 6, the tempering temperature was high, so the hardness was maintained at 50 HRC or more even after holding for 4 hours. On the other hand, in the comparative example No. 9, the tempering temperature was low, so the hardness after holding for 4 hours was below 50 HRC. Then, as the holding time became longer, the difference in hardness between the present invention steel and the comparative steel became larger. The present invention steel has a high softening resistance and is effective in the hot stamping method.

Claims (6)

A steel for hot stamping dies, characterized by having a component composition, in mass%, of C: 0.45 to 0.65%, Si: 0.1 to 0.6%, Mn: 0.1 to 0.3%, Cr: 3.0 to 4.8 %, Mo: 1.2 to 2.6%, V: 0.4 to 0.8%, the balance being Fe and unavoidable impurities. The composition is, in mass%, C: 0.45-0.65%, Si: 0.1-0.6%, Mn: 0.1-0.3%, Cr: 3.0-4.8 %, Mo: 1.2-2.6%, V: 0.4-0.8%, the balance being Fe and unavoidable impurities;
1. A hot stamping die having a hardness of 52 HRC or more, a thermal conductivity λ of 25 W/(m·K) or more, and the thermal conductivity λ (W/(m·K)) satisfies the following formula (1) :
λ≧−0.5H+53…Formula (1)
H: Rockwell hardness (HRC) of the die 3. The hot stamping die according to claim 2, further comprising a nitride layer on the working surface. The hot stamping die steel according to claim 1 is subjected to quenching and tempering at a quenching temperature of 1020 to 1080 ° C. and a tempering temperature of 500 to 625 ° C.,
1. A method for manufacturing a hot stamping die, comprising: a hardness of 52 HRC or more; a thermal conductivity λ of 25 W/(m·K) or more; and the thermal conductivity λ (W/(m·K)) satisfies the following formula (1) :
λ≧−0.5H+53…Formula (1)
H: Rockwell hardness (HRC) of the die The method for manufacturing a hot stamping die according to claim 4 , characterized in that the tempering temperature is 540 to 600°C. 6. The method for manufacturing a hot stamping die according to claim 4 , further comprising the step of nitriding the working surface after the quenching and tempering.

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