TW201604291A - Steel for cold working tool - Google Patents

Abstract

The present invention relates to a steel for cold working tool, containing, on a % by mass basis: 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; and Al: 0.100% or less, with the balance being Fe and inevitable impurities, is which 1.66 (Mo+1/2W)+V < 5.7% is satisfied.

Description

Cold work tool steel

This invention relates to cold work tool steels and, more particularly, to cold work tool steels which are preferably used to form high tensile steel sheets.

In summary, the cold-work tool steel represented by SKD11 is quenched at a temperature higher than or equal to 1,000 ° C, and then subjected to tempering at a temperature higher than or equal to 450 ° C, usually at a maximum hardness. (HRC) 60 to HRC 63 use. As for the main uses of such cold work tool steels, such as molds for cold pressing and molds for cold forging. For example, Patent Document 1 discloses a cold-work tool steel that achieves high hardness and high toughness by improving the size and distribution of carbides.

On the other hand, lately, in the automotive industry, requiring the treatment of global warming issues as the most effective solution, many companies focus their attention on the weight reduction of the car body. It is known that as the weight of the vehicle body is reduced, the carbon dioxide emissions of the automobile are reduced. For this reason, it is possible to provide a material having the same strength as that of a conventional steel sheet which is generally used as a vehicle body material (even when it is thinner than a conventional steel sheet), and is employed as an automobile body or structural component. This material is called a high tensile steel plate.

High tensile steel sheets, also known as "high tensile strength steel sheets", refer to steel sheets with high tensile strength. Common steel sheets have a tensile strength greater than or equal to 270 MPa, while steel sheets having a tensile strength of 340 MPa to 790 MPa are generally defined as high tensile steels. board. Further, a steel sheet having a tensile strength of greater than or equal to 1,000 MPa is particularly referred to as an ultra-high tensile steel sheet.

Patent Document 1: JP-A-H02-277745

From the requirement of reducing the weight of the car body, the cold work tool steel (cold work die) is applied to the processing of ultra-high tensile steel plates. Therefore, when the super high tensile steel sheet is formed by using a cold working die, the load of the cold working mold is increased. If the cold work mold is unable to withstand the load, the cold work mold is deformed, and thus the dimensional accuracy of the automobile body or structural component is degraded. In order to obtain sufficient load resistance, it is necessary to ensure the hardness and impact resistance value of the cold working mold. In order to obtain high hardness while considering resource conservation, in the quenching treatment, a larger amount of carbon is dissolved in the material steel to form a solid solution, and then subjected to second hardening by tempering treatment.

As for the method of increasing the amount of carbon dissolved in the solid solution, for example, there is a method of increasing the quenching temperature. However, when the quenching temperature is increased, there may be a problem of grain coarsening in the steel. The coarsened grains result in a reduction in the impact resistance of the die steel. Therefore, it is necessary to contain a carbide such as vanadium carbide (VC) which prevents coarsening of crystal grains at the quenching temperature, but when the carbide content of the grain coarsening is excessively increased, the impact resistance value is also reduce. Further, when elements such as molybdenum (Mo) and vanadium (V) are added, it is expected that hardness improvement due to second hardening may occur.

On the other hand, when focusing on the manufacturing method of the cold working mold, the cold working mold is usually cut at room temperature and finished into a final shape. This cutting process is the cracking of the mold material (cold tool steel) and the cracking caused by the broken chips and tool abrasion, and the temporary and instantaneous increase of the cutting surface temperature of the mold material. If the mold is difficult to deform at such a high temperature, in other words, if the hardness at a high temperature is increased, the machinability is lowered. In other words, The load of the tool for cutting the mold is increased (the amount of tool wear is increased), and as such, the cost of the cutting tool and the time for manufacturing the mold are increased, and the productivity of the mold is reduced.

The present invention has been made in view of the foregoing description, and by providing a cold work tool steel to solve the aforementioned problems, it is possible to improve the productivity of the mold while ensuring the required hardness of the mold and the required impact resistance value.

Through thorough and intensive research, the inventors found that they can solve the problems described above. The specific means to solve this problem are as follows.

A first aspect of the present invention is a cold work tool steel having a % by mass ratio based on: 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%; 0.70% to 0.85%; N: 0.020% or less; O: 0.0100% or less; and Al: 0.100% or less, the difference is Fe and unavoidable impurities, which satisfy 1.66 (Mo+1/2W)+V <5.7%.

The second aspect of the present invention is the cold working tool steel according to the first aspect, which further comprises, based on the % by mass ratio, at least one of the following: Nb: 0.001% to 0.30%, Ta: 0.001% to 0.30%, Ti: 0.20% or less, and Zr: 0.001% to 0.30%.

A third aspect of the present invention is the cold work tool steel according to the first or second aspect, which has a residual Worstian iron content after quenching of less than or equal to 25% by volume (vol%).

A fourth aspect of the invention is a cold work tool steel according to the first, second and third aspects, which has a highest hardness of greater than or equal to 64 HRC after tempering at a temperature greater than or equal to 450 °C.

The invention ensures the hardness and impact resistance value of the mold by adjusting C, Si, Cr, Mo, W, and V within a predetermined range. Further, in the present invention, the balance between Mo, W, and V is optimized, and a relationship expression of 1.66 (Mo + 1/2W) + V < 5.7% is expressed to improve the productivity of the mold. Typically the mold is cut at room temperature and the temperature of the mold is temporarily and transiently increased by the heat of cutting during cutting. In this case, when focusing on the hardness of the mold, the hardness at room temperature is the highest, and the hardness decreases as the temperature increases. Through thorough intensive research by the inventors, it was found that when the mold temperature is temporarily and instantaneously increased by cutting, particularly if a plurality of elements such as Mo, W, and V are added in comparison with a predetermined amount of addition, the cutting is possible. Sexual damage, and thus degradation in cutting efficiency. In other words, it is concluded that the addition of elements such as Mo, W, and V is one of the factors that reduce the productivity of the mold.

Incidentally, the cold working tool steel disclosed in the aforementioned Patent Document 1 is improved by carbon The size and distribution of the compound achieve high hardness and high toughness, but do not have the characteristics of the present invention, and thus have completely different technical concepts.

As described above, the cold work tool steel according to the present invention may improve the productivity of the mold while ensuring the hardness and impact resistance required of the mold.

Fig. 1 is a graph showing the relationship between the amount of wear of the tool (cuttability) and the amount of addition of Mo, W, and V.

Fig. 2 is a graph showing the relationship between the impact resistance value and the hardness value of the steel of the present invention and comparative steel.

The cold-work tool steel (hereinafter referred to as the cold-work tool steel of this specific example) according to a specific example of the present invention will be described in detail later. The cold work tool steel of the specific example can be applied to a high tensile steel plate forming die, a cold forging punch and a die, a bending die, a cold forging die, a forging die, a thread rolling die, a stamping part, a slitting blade, a stamping lead Mold, meter, deep drawing press, press punch, shearing blade, stainless steel press bending die, calendering die, plastic processing tools such as processing machine, gear punching machine, cam assembly, punching die, progressive punching machine, lake The sealing plate of the product conveying device, the screw member, the rotating plate of the concrete spraying machine, the mold for the integrated circuit (IC), and the precision pressing mold requiring high dimensional accuracy. Further, the cold work tool steel of this specific example can also be used for various cold metal blocks used for surface treatment such as chemical vapor deposition (CVD) treatment, physical vapor deposition (PVD) treatment, and TD treatment. Among them, particularly preferred is an ultra high tensile steel sheet having a tensile strength of 1,000 MPa or more.

The cold work tool steel of this specific example contains the following elements. Adding species The reasons for the class, its scope of addition, and the scope for its addition are explained below.

C: 0.70% to 0.90%

Carbon (C) is an essential element for ensuring strength and abrasion resistance, and carbides are formed by bonding to carbide forming elements such as Cr, Mo, W, V, and Nb. Further, carbon is also an essential element for ensuring hardness, and carbon is dissolved in the matrix phase upon quenching to form a solid solution to thereby form a granulated iron structure. In order to obtain this effect in cold work tool steel, the lower limit of the carbon content is set to 0.70%. Conversely, when the content of carbon is too large, carbide-forming elements are bonded to carbon to form coarse carbides, and thus the impact resistance value may be lowered. Further, in the case of the ingot after casting, the hot workability at the time of hot forging may be reduced. Therefore, the upper limit of the carbon content is set to 0.90%. From the foregoing point of view, the carbon content is more preferably in the range of 0.75% to 0.85%.

Si: 0.60% to 0.80%

The cerium (Si) is dissolved in the matrix phase to form a solid solution, which accelerates the precipitation of other carbides and contributes to secondary hardening. In order to obtain such effects, the lower limit of the cerium content is set to 0.60%. Conversely, when hydrazine is added in excess, the quenching properties may be reduced. Therefore, the upper limit of the cerium content is set to 0.80%.

Mn: 0.30% to 0.50%

Manganese (Mn) is added to improve the quenching properties and stabilize the Worthite iron. More specifically, when the quenching property is reduced, the hardness change at the micro-level is increased. Further, when sulfur is inevitably contained, sulfur and manganese form MnS, and the reduction in impact resistance due to distortion due to heat treatment (assisted anisotropy) can be prevented. Therefore, the reason is that the manganese content The lower limit is set to 0.30%. Conversely, when the content of manganese is too large, the hot workability at the time of hot forging may be reduced in the case of the ingot after casting. For this reason, the upper limit of the manganese content is set to 0.50%.

P: 0.30% or less

Phosphorus (P) is an element that is inevitably contained in steel. Phosphorus is easily sequestered at grain boundaries and may cause a decrease in toughness. For this reason, the upper limit of the phosphorus content is set to 0.3%.

S: 0.030% or less

Sulfur (S) is an element that is inevitably contained in steel. Sulfur is actively added to improve machinability. In the present invention, MnS can be formed by adding sulfur to prevent a decrease in the impact resistance value due to distortion caused by heat treatment (assisted anisotropy), and thus the sulfur content is limited to 0.03% or less.

Cu: 0.01% to 0.25%

Copper (Cu) is an element that stabilizes the Worth Iron. However, when the content of copper is too large, the amount of residual Worth iron may increase, and thus the dimensional change may occur over time. Further, when copper is excessively added, the hot workability at the time of hot forging may be reduced in the case of the ingot after casting. For this reason, the copper content is set to be 0.01% to 0.25%.

Ni: 0.01% to 0.25%

Nickel (Ni) is an element that stabilizes the Worth Iron. However, when the content of nickel is too large, the amount of residual Worth iron may increase, and thus the size may change over time. change. For this reason, the content of nickel is set to be 0.01% to 0.25%.

Cr: 6.0% to 7.0%

Chromium (Cr) is an element that improves corrosion resistance. In order to obtain such an effect, the lower limit of the chromium content is set to 6.0%. When the content of chromium is too large, the amount of carbon dissolved in the Vostian iron structure to form a solid solution may be reduced, and thus sufficient hardness may not be obtained. For this reason, the upper limit of the chromium content is set to 7.0%.

Mo+1/2W: 2.50% to 3.00%

Molybdenum (Mo) and tungsten (W) form fine carbides and are important elements that contribute to secondary hardening. In order to obtain the same effect as the effect of molybdenum, it is necessary to add a double amount of tungsten, and thus in the present invention, the total amount of the molybdenum content and the 1/2 tungsten content is limited. In order to obtain the second-degree hardening effect, the lower limit of the content of Mo+1/2W was set to 2.50%. Conversely, when the content of molybdenum and tungsten is too large, the amount of carbide remaining at the time of quenching may increase, and thus the upper limit of the content of Mo + 1/2W is set to 3.00%.

V: 0.70% to 0.85%

Vanadium (V) can be bonded to carbon to form carbides. This carbide can contribute to the suppression of grain diameter coarsening. In order to obtain such an effect, the lower limit of the content of vanadium is set to 0.70%. When the content of vanadium is too large, the carbonitride of vanadium is easily crystallized to reduce the impact resistance value. For this reason, the upper limit of the vanadium content is set to 0.85%.

N: 0.020% or less

Nitrogen (N) is an interstitial element that contributes to an increase in the hardness of the granulated iron structure. Compared to Carbon of the same interstitial type, nitrogen has a stronger gamma stabilization ability. However, when the nitrogen content is too large, the nitrogen in the steel may concentrate concentrated to exceed the nitrogen injection limit during solidification, and thus voids are likely to occur in the ingot. For this reason, the upper limit of the nitrogen content is set to 0.020%.

O: 0.0100% or less

Oxygen (O) is an element that is inevitably contained in molten steel. However, when the oxygen content is too large, oxygen can be bonded to the bismuth and aluminum to form a coarse oxide, which becomes an inclusion body, and thus the toughness may be reduced. From the viewpoint of preventing such an effect, the upper limit of the oxygen content is set to 0.0100%.

Al: 0.100% or less

Aluminum (Al) is an element added as a deoxidizer. However, when the aluminum content is too large, aluminum can be bonded to oxygen to form a coarse oxide which may become the starting point of the crack. For this reason, the upper limit of the aluminum content is set to 0.100%.

1.66(Mo+1/2W)+V: less than 5.7%

In order to increase the second degree of hardening, it is necessary to add Mo+1/2W and V. On the contrary, when the total content thereof is too large, even if the temperature of the mold is increased by the cutting resistance or the heat of cutting during the cutting process, the hardness of the mold may not be lowered, and thus the machinability may be degraded. For this reason, the total content is shown to satisfy 1.66 (Mo + 1/2 W) + V < 5.7.

In addition to the foregoing main elements, the cold work tool steel of this embodiment may optionally contain one or more elements selected from the elements described below. In other words, based on the % mass ratio, the cold work tool steel of this specific example can only contain: 0.70 C 0.90; 0.60 Si 0.80; 0.30 Mn 0.50; P 0.30;S 0.030;0.01 Cu 0.25;0.01 Ni 0.25; 6.0 Cr 7.0; 2.50 Mo+1/2W 3.00; 0.70 V 0.85;N 0.020;O 0.0100; and Al 0.100, the difference is Fe and inevitable impurities, wherein 1.66 (Mo + 1/2W) + V < 5.7% is satisfied, but may optionally contain one or more elements selected from the elements described below, The contents of these elements are detailed later.

Nb: 0.001% to 0.30%, Ta: 0.001% to 0.30%, Ti: 0.20% or less, and Zr: 0.001% to 0.30%.

Nb, Ta, Ti, and Zr are elements that bond to C and N to form carbonitrides, and contribute to the suppression of grain coarsening. Conversely, when Nb, Ta, Ti, and Zr are excessively added, the machinability during the finishing may be reduced, and thus the productivity of the mold is lowered. Therefore, the content of each element is set within the above range.

Further, in the cold working tool steel of the specific example, the content of the residual Worth iron after quenching is preferably less than or equal to 25 vo1%. The reason for this is that when the content of the residual Worth iron after the quenching is increased, the dimensional change of the mold due to the content of the residual Worth iron to be decomposed after tempering may increase, and thus in terms of the mold It may take time to perform precision cutting. Further, the quenching temperature is preferably from 1,000 ° C to 1,100 ° C.

Further, in the cold-work tool steel of this embodiment, the highest hardness after tempering at a temperature of 450 ° C or higher is preferably greater than or equal to 64 HRC. More specifically, when the cold-work tool steel of this specific example is used as a cold-working mold for a high-tensile steel sheet, it is necessary to secure hardness and impact value by applying second-degree hardening.

[Examples]

Hereinafter, the present invention will be described in detail with reference to the embodiments.

The steels each having the chemical composition (% by mass) shown in Tables 1 and 2 were melted in a vacuum induction furnace and cast into individual ingots each having a weight of 50 kg. These ingots after casting were subjected to hot forging and were made into individual rod-shaped materials of 60 mm square. After the hot forging, the rod-shaped materials were subjected to spheroidizing tempering, wherein the materials were slowly cooled from 880 ° C at a cooling rate of 7 ° C per hour. Each of the obtained steels was evaluated by a hardness measurement test, a Charpy impact test, a machinability test, and a measurement test of the content of the residual Worth iron.

Hardness measurement test

The cube test piece having a side length of 10 mm was cut out from the bar material after the aforementioned heat treatment, and was treated under the heat treatment conditions (quenching temperature and tempering temperature) shown in Table 3. The gauge surface and the abrasive surface of the cube test piece were ground to #400. And then, the hardness of the cube test piece was measured using the Rockwell C standard. This hardness indicates that the highest hardness occurs when tempering is performed at a temperature higher than or equal to 450 °C.

Charpy impact test

A 10R notched Charpy test piece was prepared in which a 2 mm score having a 10R depth was formed in a 10 mm x 10 mm x 55 mm square rod type material cut from a 60 mm square rod type material as described above. The 10R-notched Charpy test piece was subjected to quenching treatment and tempering treatment at the temperature shown in Table 3, and then, its impact resistance value was measured at room temperature. The impact resistance value was obtained as an average of three test pieces.

Machinability test (end milling cutter machinability test)

The machinability tests were carried out on the following test pieces which were cut from the steel after annealing and toughening. The test conditions are as follows.

Test piece: 55 mm × 55 mm × 200 mm

Cutting tool: super hard M20 square end mill (10 mm diameter)

Cutting distance: 10 meters

Cutting speed: 100 m / min

Transmission speed per revolution: 0.2 mm / rev

Cut width (horizontal direction): 0.5 mm

Cut height (vertical direction): 0.5 mm

Motor oil: no

In this evaluation, after 10 meters of cutting, the end mill was removed from the mount and the maximum amount of wear of the corner of the square end mill was measured. The maximum amount of wear of the corner portion of the square end mill is measured by a practical measurement using a charge-coupled device (CCD) camera of 3 times magnification. Here, "the maximum amount of abrasion of the corner portion of the square end mill" indicates the distance from a tip end of the corner portion of the square end mill to a position where the peeling and abrasion can be confirmed The maximum value.

Measurement of residual Worth iron content

A 10 mm x 10 mm x 2 mm square bar material cut from a 60 mm square rod material was maintained at the quenching temperature shown in Table 3 for 30 minutes and then cooled at an average cooling rate of 50 ° C/min. Secondly, one of the square rod-shaped materials (test pieces) was measured to be ground up to #800 as defined by JIS-R6001 (1998) and measured by an X-ray diffraction device. The peak intensity of (200) and (211) of ferrite is obtained by X-ray measurement and Vostian The peak intensities of iron (200), (220) and (311), and then the content of residual Worth iron (vol%) from the peak intensity ratio.

In Tables 1 and 2, the chemical compositions of the steel of the present invention and the comparative steel are shown. In Table 3, the heat treatment conditions and test results of the steel of the present invention and comparative steel are shown. Further, in Fig. 1, the relationship between the amount of abrasion (cuttability) of the tool of the steel of the present invention and the comparative steel and the addition amount of molybdenum, tungsten and vanadium is shown. In Fig. 2, the relationship between the impact resistance value and the hardness value of the steel of the present invention and comparative steel is shown.

Referring to Table 1, Table 2, Table 3, Figure 1 and Figure 2, the following facts were found. First, in Comparative Steel 1 to Comparative Steel 6, 1.66 (Mo + 1/2 W) + V is greater than or equal to 5.7. For this reason, the amount of tool wear increases and the productivity of the mold decreases. More specifically, Referring to Figure 1, a plot of 1.66 (Mo + 1/2W) greater than or equal to 5.7 indicates comparative steels 1 through 6. Conversely, the plot in which 1.66 (Mo + 1/2 W) is less than 5.7 indicates the steels 1 to 9 of the present invention and the comparative steels 7 to 12. As shown in Fig. 1, when the 1.66 (Mo + 1/2W) system is greater than or equal to 5.7 in the alloy composition, the amount of tool abrasion increases, the machinability is reduced, and the productivity of the mold is lowered.

Referring to Table 1, Table 2, Table 3, and Figure 2, the comparative steels 8 to 12 have low impact resistance values because either or both of sulfur and vanadium exceed the upper limit of the limit range of the present invention.

Referring to Table 1, Table 2, Table 3, and Figure 2, since the contents of cerium, molybdenum and vanadium are lower than the lower limit of the limit range of the present invention, and the content of chromium exceeds the upper limit of the limit range of the present invention, comparative steel 13 can not reach enough hardness. Further, since the carbon content exceeds the upper limit of the limit range of the present invention, the comparative steel 13 has a low impact resistance value.

Referring to Table 1, Table 2, and Table 3, since the contents of molybdenum and vanadium are lower than the lower limit of the limit range of the present invention, and the content of chromium exceeds the upper limit of the limit range of the present invention, the comparative steel 14 cannot achieve sufficient hardness. . Further, since the carbon content exceeds the upper limit of the limit range of the present invention, the comparative steel 14 has a low impact resistance value. Incidentally, the comparative steel 16 in Table 3 is the same compositional condition as the composition of the comparative steel 14 in Table 2, and the tempering temperature and the quenching temperature are the temperatures of the comparative steel 14 in Table 3. One embodiment was tested under different conditions.

Referring to Table 1, Table 2, and Table 3, since the contents of carbon, molybdenum, and vanadium are lower than the lower limit of the limit range of the present invention, and the content of chromium exceeds the upper limit of the limit range of the present invention, the comparative steel 15 cannot be reached. Sufficient hardness.

Conversely, compared to the comparative steel, the steel of the present invention is subjected to a hardness measurement test, a Charpy impact test, a machinability test, and a residual Worth iron content. Excellent results were achieved in any of the test results of the metric test. From the results described above, the cold work tool steel according to this specific example may improve the productivity of the mold while ensuring the hardness of the mold and the required impact value.

Although the present invention has been described in detail and with reference to the specific embodiments thereof, it is obvious that it is not limited to the specific examples and embodiments, and various changes and modifications may be made therein.

This application is based on Japanese Patent Application No. 2014-125901, filing date of June 19, 2014, Japanese Patent Application No. 2014-218985, and October 28, 2014, and Japanese Patent Application No. 2014-243681, filing date of December 2, 2014. The contents of each case are incorporated herein and incorporated into the disclosure of this specification.

Claims (5)

A cold work tool steel, based on a % by mass ratio, comprising: 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; and Al: 0.100% or less, the difference is Fe and inevitable impurities, wherein 1.66 (Mo + 1/2 W) + V < 5.7% is satisfied. For example, the cold working tool steel of claim 1 is further based on a % by mass ratio, further comprising at least one of the following: Nb: 0.001% to 0.30%, Ta: 0.001% to 0.30%, Ti: 0.20% Or below, and Zr: 0.001% to 0.30%. For example, the cold-work tool steel of claim 1 of the patent scope has residual ferment after quenching The content of the iron is less than or equal to 25% by volume (vol%). For example, the cold working tool steel of claim 2 has a residual Worth iron content after quenching of less than or equal to 25% by volume (vol%). The cold-work tool steel according to any one of claims 1 to 4, which has a maximum hardness of greater than or equal to 64 HRC after tempering at a temperature higher than or equal to 450 °C.