JP6439227B2 - Cold work tool steel - Google Patents

Description

  The present invention relates to a cold tool steel, and more particularly to a cold tool steel suitable for forming a high-tensile steel plate.

  Usually, cold tool steel represented by SKD11 is often used at about HRC 60 to 63 after quenching at 1000 ° C. or higher and then tempering at 450 ° C. or higher. Main applications of this cold tool steel include cold press molds and cold forging molds. For example, Patent Document 1 describes a cold tool steel that improves the size and distribution of carbides, and has high hardness and high toughness.

On the other hand, in recent years, the automobile industry has been required to take measures against the global warming problem, and as the most effective countermeasures, companies are working on weight reduction of the vehicle body. It is said that if the body weight is reduced, the amount of carbon dioxide emitted from the vehicle can be reduced. Therefore, a member called a high-tensile steel plate that is thinner than a normal steel plate that is usually used as a member of a vehicle body and can obtain the same strength is often adopted as an automobile body or a structural part.

The high-tensile steel plate is also called “High Tensile Strength Steel Sheets” and refers to a steel plate having high tensile strength. While a normal steel plate has a tensile strength of 270 MPa or more, a steel plate having a tensile strength of 340 MPa to 790 MPa is generally defined as a high-tensile steel plate. Further, those having a tensile strength of 1000 MPa or more are particularly said to be ultra-high strength steel plates.

JP-A-2-277745

Due to such demands for reducing the weight of automobile bodies, the application of cold tool steel (cold mold) to ultra-high strength steel sheets is increasing. For this reason, when forming ultra-high strength steel sheets, the load on the cold mold is also increasing. Cold molds that cannot withstand the load will be deformed, and the dimensional accuracy of automobile bodies and structural parts cannot be maintained. In order to withstand a sufficient load, it is necessary to ensure the hardness and impact value of the cold mold. In order to obtain high hardness in consideration of resource saving, it is effective to use a second hardening by tempering by dissolving more carbon in the quenching process.

One means for increasing the amount of carbon dissolved is to increase the quenching temperature. However, when the quenching temperature is increased, the problem of coarsening of crystal grains in the steel may occur. When the crystal grains are coarsened, the impact value of the mold is lowered, so that a carbide such as VC that prevents the crystal grains from becoming coarse at the quenching temperature is required. However, even if there is too much carbide that prevents the coarsening of crystal grains, the impact value decreases. Moreover, when elements such as Mo and V are added, improvement in hardness can be expected due to the contribution of secondary curing.

On the other hand, when looking at the process of manufacturing a cold mold, the cold mold is usually cut at room temperature and finished to a final shape. This cutting process is the repetition of fracture of the mold material (cold tool steel) due to shear deformation and the wear of the broken chips and the tool. The temperature of the work surface of the mold material is temporarily and instantaneously Get higher. When the mold becomes difficult to deform at this high temperature, that is, when the hardness at the high temperature increases, the machinability decreases. That is, since the burden on the tool for cutting the mold increases (the amount of tool wear increases), it takes time and cutting tool cost to manufacture the mold, and the productivity of the mold decreases.

The present invention has been made in view of the above problems, and the problem to be solved by the present invention is that in cold tool steel, while ensuring the necessary hardness and impact value of the mold, It is to improve productivity.

As a result of intensive studies, the present inventors have found that this problem can be solved. The specific means is as follows. First, the first invention is mass% (hereinafter the same), C: 0.70 to 0.90%, Si: 0.60 to 0.80%, Mn: 0.30 to 0.50%, P: 0.30% or less, S: 0.030% or less, Cu: 0.01 to 0.25%, Ni: 0.01 to 0.25%, Cr: 6.0 to 7.0%, Mo + 1 / 2W: 2.50 to 3.00%, V: 0.70 to 0.85%, N: 0.020% or less, O: 0.0100% or less, Al: 0.100% or less, 1.66 (Mo + 1 / 2W) + V: <5.7, a cold tool steel characterized in that the balance contains Fe and inevitable impurities.

Next, the second invention is the cold tool steel according to the first invention described above, and Nb: 0.001 to 0.30%, Ta: 0.001 to 0.30%, Ti: 0 20% or less, and one or more selected from Zr: 0.001 to 0.30%.

Next, the third invention is the cold work tool steel according to the first invention or the second invention described above, and the amount of retained austenite after quenching is 25 vol% or less.

Next, a fourth invention is a cold tool steel according to any one of the first to third inventions described above, and the maximum hardness is 64 HRC or more by tempering at 450 ° C. or higher. It is characterized by that.

In the present invention, the necessary hardness and impact value of the mold are secured by setting C, Si, Cr, Mo, W, and V within a predetermined range. In addition, in the present invention, in order to improve the productivity of the mold, the balance of Mo, W, and V is optimized, and the relational expression of 1.66 (Mo + 1 / 2W) + V: <5.7 is specified. It is a feature. Usually, a metal mold | die is cut at room temperature, and the temperature of a metal mold | die becomes high temporarily temporarily with cutting heat during cutting. In this case, paying attention to the hardness of the mold, the hardness at the room temperature is the highest, and the hardness decreases as the temperature increases. As a result of earnest researches by the inventors, when the mold is temporarily increased momentarily by cutting, especially when each element of Mo, W, V is added more than a predetermined addition amount, It has been found that the cutting efficiency is lowered and the cutting efficiency is lowered. That is, it was concluded that the element addition amounts of Mo, W, and V are one factor that reduces the productivity of the mold.
In addition, the cold tool steel described in Patent Document 1 described above has achieved high hardness and high toughness by improving the size and distribution of carbides, and has the characteristics of the present invention. It is an invention with a completely different technical idea.

As mentioned above, according to the cold tool steel which concerns on this invention, productivity of a metal mold | die can be improved, ensuring the required hardness and impact value of a metal mold | die.

It is the figure which showed the relationship between tool wear amount (machinability) and the addition amount of Mo, W, and V. FIG. It is the figure which showed the relationship of the numerical value of impact value and hardness of invention steel and comparative steel.

  The cold tool steel which concerns on one Embodiment of this invention (henceforth this cold tool steel). Will be described in detail. Applications of this cold tool steel include high-tensile steel plate forming dies, cold forging punch dies, bending dies, cold forging dies, swaging dies, screw rolling dies, punch members, slitter knives, lead frame punching Molds, gauges, deep drawing punches, bending die punches, shear blades, stainless steel bending dies, drawing dies, plastic working tools such as forging, gear punches, cam parts, press punching dies, progressive punching dies, earth and sand feeding It can be applied to device seal plates, screw members, rotary plates for concrete sprayers, IC sealing molds, precision press molds that require high dimensional accuracy, and the like. Moreover, it can use for said various cold molds etc. which are used after performing surface treatment, such as CVD processing, PVD processing, and TD processing. Among these, it is particularly preferable to be used for an ultra high strength steel sheet having a tensile strength of 1000 MPa or more.

This cold tool steel contains the following elements. The kind of additive element, its component range, and the reason for limitation are as follows.

C: 0.70-0.90%
C is an element necessary for ensuring strength and wear resistance, and forms a carbide by combining with a carbide forming element such as Cr, Mo, W, V, Nb or the like. C is also necessary for securing hardness by forming a martensite structure by forming a solid solution in the matrix during quenching. In order to obtain this effect in cold tool steel, the lower limit of the C content is set to 0.70%. On the other hand, when the C content is excessive, the carbide forming element and C are combined to form coarse carbide, and the impact value is lowered. Moreover, the hot workability at the time of hot forging the ingot after casting is lowered. Therefore, the upper limit of C content is set to 0.90%. From the above viewpoint, the more preferable range of the C content is 0.75 to 0.85%.

Si: 0.60 to 0.80%
Si dissolves in the matrix and promotes precipitation of other carbides and contributes to secondary hardening. In order to obtain this effect, the lower limit of the Si content is set to 0.60%. On the other hand, if Si is added excessively, the hardenability decreases. Therefore, the upper limit of Si content is 0.80%.

Mn: 0.30 to 0.50%
Mn is added to improve hardenability and stabilize austenite. In particular, when hardenability decreases, hardness variation at the micro level increases. Mn inevitably forms MnS when S is contained, and prevents a drop in impact value due to distortion (promoting anisotropy) due to heat treatment. Therefore, the lower limit of the Mn content is set to 0.30%. On the other hand, when the Mn content is excessive, hot workability at the time of hot forging the ingot after casting is lowered. Therefore, the upper limit of the Mn content is 0.50%.

P: 0.30% or less P is inevitably contained in steel. P segregates at the grain boundaries and causes toughness to decrease. Therefore, the upper limit of the P content is 0.3%.

S: 0.030% or less S is inevitably contained in steel. Usually, S is positively added to improve machinability. In the present invention, by adding S, MnS is formed, and in order to prevent a decrease in impact value due to distortion (promoting anisotropy) due to heat treatment, the S content is made 0.03% or less. Regulated.

Cu: 0.01 to 0.25%
Cu is an element that stabilizes austenite. However, when the Cu content is excessive, the amount of retained austenite increases and causes aging of the dimensions. Moreover, when it adds excessively, the hot workability at the time of hot forging the ingot after casting will fall. Therefore, the Cu content is set to 0.01 to 0.25%.

Ni: 0.01-0.25%
Ni is an element that stabilizes austenite. However, if the Ni content is excessive, the amount of retained austenite increases and causes aging of the dimensions. Therefore, the Ni content is set to 0.01 to 0.25%.

Cr: 6.0 to 7.0%
Cr is an element that improves the corrosion resistance. In order to obtain the effect, the lower limit of the Cr content is set to 6.0%. When the Cr content is excessive, the amount of C solid solution in the austenite structure decreases, and sufficient hardness cannot be obtained. Therefore, the upper limit of Cr content is 7.0%.

Mo + 1 / 2W: 2.50 to 3.00%
Mo and W are important elements that form fine carbides and contribute to secondary hardening. Since W needs to be added twice in order to obtain the same effect as Mo, in the present invention, the total content of 1/2 of W and Mo is specified. In order to obtain the effect of secondary curing, the lower limit of the Mo + 1 / 2W content is set to 2.50%. On the other hand, if the contents of Mo and W are excessive, the amount of carbide remaining at the time of quenching increases, so the upper limit of the Mo + 1 / 2W content is set to 3.00%.

V: 0.70 to 0.85%
V combines with C to form a carbide. This carbide contributes to suppression of coarsening of the crystal grain size. In order to obtain the effect, the lower limit of the V content is set to 0.70%. When the V content is excessive, V carbonitrides crystallize and the impact value decreases. Therefore, the upper limit of V content is 0.85%.

N: 0.020% or less,
N is an interstitial element and contributes to an increase in the hardness of the martensite structure. Γ stabilization ability is stronger than carbon of the same interstitial element. However, if the N content is excessive, the nitrogen gas ejection limit is exceeded due to the concentration of nitrogen during solidification, and voids are likely to occur in the ingot. Therefore, the upper limit of the N content is 0.02%.

O: 0.0100% or less O is an element inevitably contained in molten steel. However, when O becomes excessive, it combines with Si and Al to produce coarse oxides, which become inclusions and reduce toughness. From the viewpoint of preventing this, the upper limit of the O content is 0.0100%.

Al: 0.100% or less,
Al is an element added as a deoxidizer. However, when the Al content is excessive, it combines with O to form coarse nitrides, which become the starting point of cracking. Therefore, the upper limit of the Al content is set to 0.100%.

1.66 (Mo + 1 / 2W) + V: <5.7
In order to enhance secondary curing, addition of Mo + 1 / 2W and V is necessary. On the other hand, if these total amounts are excessive, even if the temperature of the mold increases due to cutting resistance or cutting heat during cutting, the hardness of the mold does not decrease and the machinability deteriorates. Therefore, it was defined as 1.66 (Mo + 1 / 2W) + V: <5.7.

Nb: 0.001 to 0.30%, Ta: 0.001 to 0.30%, Ti: 0.00.
20% or less, and Zr: 0.001 to 0.30%
Nb, Ta, Ti, and Zr are elements that combine with C and N to form carbonitrides and contribute to suppression of coarsening of crystal grains. On the other hand, if added excessively, the machinability at the time of finishing is lowered, and the productivity of the mold is lowered. Therefore, the content of each element is in the above range.

Further, the present cold tool steel preferably has a residual austenite amount of 25 vol% or less after quenching. This is because when the amount of retained austenite after quenching increases, the dimensional change of the mold due to the amount of retained austenite that decomposes after tempering increases, and it takes time to refine the mold. In addition, it is preferable that quenching temperature is 1000-1100.

Further, it is preferable that the present cold tool steel has a maximum hardness of 64 HRC or higher by tempering at 450 ° C. or higher. This is because, in particular, when used as a cold mold for a high-tensile steel plate, it is necessary to ensure hardness and impact value using secondary hardening.

Hereinafter, the present invention will be specifically described by way of examples.
A steel having a chemical composition (mass%) shown in Table 1 (described later) was melted in a vacuum induction furnace, and then a 50 kg ingot was cast. The ingot after casting was hot forged to produce a 60 mm square bar. After hot forging, spheroidizing annealing was performed in which cooling was performed at a cooling rate of 880 ° C. to 7 ° C./hr. Each steel material obtained was evaluated by a hardness measurement test, a Charpy impact test, a machinability test, and a retained austenite amount measurement test.

<Hardness measurement test>
A cube test piece having a side of 10 mm is cut out from the bar after the heat treatment, treated under the heat treatment conditions (quenching temperature and tempering temperature) shown in Table 2, and then the measurement surface and the grounding surface are polished to # 400, and then locked. The hardness of this test piece was measured by the well C scale. In addition, this hardness has shown the maximum hardness when the tempering process of 450 degreeC or more is carried out.

<Charpy impact test>
A 10R notch Charpy test piece in which a 2 mm notch is formed at a depth of 10R on a 10 mm × 10 mm × 55 mm square material cut out from the 60 mm square bar material is quenched and tempered at the temperatures shown in Table 2. After implementation, the impact value was measured at room temperature. The impact value was an average value of three test pieces.

<Machinability test (end milling test)>
The machinability test was performed on the following test pieces cut out from the annealed steel. The test conditions are as follows.
Test piece: 55 mm x 55 mm x 200 mm
Cutting tool: Carbide M20 square end mill (φ10mm)
Cutting distance: 10m
Cutting speed: 100 m / min
Feeding speed per blade :: 0.2mm / rev
Cutting width (horizontal direction): 0.5 mm
Cutting height (vertical direction): 0.5mm
Cutting oil: None In addition, the evaluation was performed by removing the end mill from the holder after cutting of 10 m and measuring the maximum wear amount at the corner of the square end mill. The maximum amount of wear at the corners of the square end mill was measured by measuring with a CCD camera having a magnification of 3 times. Here, the “maximum wear amount at the corner of the square end mill” refers to the maximum value of the distance from the tip of the corner of the square end mill to the position where peeling and wear can be confirmed.

<Residual austenite amount measurement test>
A 10 mm × 10 mm × 2 mm square cut out from the 60 mm square rod was held at the quenching temperature shown in Table 2 for 30 minutes, and then cooled at an average cooling rate of 50 ° C./min. Next, the measurement surface of this square member (test piece) was polished to # 800 defined in JIS-R6001, and measurement was performed with an X-ray diffractometer. The peak intensity of (200) (211) of ferrite obtained by X-ray measurement and the peak intensity of (200) (220) (311) of austenite are obtained, and the amount of residual austenite (vol%) is determined from the peak intensity ratio. Calculated.

Table 1 shows the chemical compositions of the inventive steel and the comparative steel. Table 2 shows the heat treatment conditions and various test results of the inventive steel and the comparative steel. Moreover, in FIG. 1, the relationship between the tool wear amount (machinability) of invention steel and a comparative steel and the addition amount of Mo, W, and V is shown. FIG. 2 shows the relationship between the impact values and hardness values of the inventive steel and the comparative steel.

Comparing Table 1, Table 2, FIG. 1 and FIG. 2 reveals the following. That is, in Comparative Steels 1 to 6, 1.66 (Mo + 1 / 2W) + V is 5.7 or more. Therefore, the amount of tool wear is large and the productivity of the mold is lowered. Specifically, referring to FIG. 1, the plot in which 1.66 (Mo + 1 / 2W) + V is 5.7 or higher indicates Comparative Examples 1-6. On the other hand, the plot in which 1.66 (Mo + 1 / 2W) + V is less than 5.7 indicates the inventive steels 1-9 and the comparative steels 7-12. As shown in FIG. 1, when the alloy composition is 1.66 (Mo + 1 / 2W) + V is 5.7 or more, the amount of tool wear increases, the machinability decreases, and the productivity of the mold decreases.

Referring to Tables 1, 2 and 2, Comparative Steels 7 to 12 have low impact values because either or both of S and V exceeds the upper limit of the specified range of the present invention.

Referring to Tables 1 and 2, in Comparative Steel 13, the content of Si, Mo + 1 / 2W, V is below the lower limit of the specified range of the present invention, and the content of Cr exceeds the upper limit of the specified range of the present invention. Since it exceeds, sufficient hardness is not obtained. Moreover, since the C content exceeds the upper limit of the specified range of the present invention, the impact value is low.

Referring to Tables 1 and 2, in Comparative Steel 14, the content of Mo + 1 / 2W and V is below the lower limit of the specified range of the present invention, and the content of Cr exceeds the upper limit of the specified range of the present invention. Therefore, sufficient hardness is not obtained. Moreover, since the C content exceeds the upper limit of the specified range of the present invention, the impact value is low. In addition, the comparative example 16 of Table 2 is an example which carried out the test on the conditions different from the quenching temperature and tempering temperature of the comparative example 14 of Table 2 which is the same composition as the comparative example 14 of Table 1.

Referring to Tables 1 and 2, in Comparative Steel 15, the contents of C, Mo + 1 / 2W, V are below the lower limit of the specified range of the present invention, and the Si content is the upper limit of the specified range of the present invention. Since it exceeds, sufficient hardness is not obtained.

In contrast to these comparative steels, the inventive steel has obtained good results in any of the hardness measurement test, Charpy impact test, machinability test, and residual austenite amount measurement test. From the above results, according to the present cold tool steel, it can be said that the productivity of the mold can be improved while ensuring the necessary hardness and impact value of the mold.

Heretofore, the embodiments and examples of the present invention have been described. The present invention is not particularly limited to these embodiments and examples, and various modifications can be made.

Claims (4)

% By mass (hereinafter the same),
C: 0.70 to 0.90%,
Si: 0.60 to 0.80%,
Mn: 0.30 to 0.50%,
P: 0.30% or less,
S: 0.030% or less,
Cu: 0.01 to 0.25%,
Ni: 0.01 to 0.25%,
Cr: 6.0 to 7.0%,
Mo + 1 / 2W: 2.50 to 3.00%
V: 0.70 to 0.85%,
N: 0.020% or less,
O: 0.0100% or less,
Al: 0.100% or less,
1.66 (Mo + 1 / 2W) + V: <5.7,
A cold tool steel characterized in that the balance contains Fe and inevitable impurities. Nb: 0.001 to 0.30%,
Ta: 0.001 to 0.30%,
Ti: 0.20% or less, and
The cold tool steel according to claim 1, comprising one or more selected from Zr: 0.001 to 0.30%. The cold work tool steel according to claim 1 or 2, wherein the amount of retained austenite after quenching is 25 vol% or less. The cold work tool steel according to any one of claims 1 to 3, wherein the maximum hardness is 64 HRC or more by tempering at 450 ° C or more.

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