JP2015129322A - Steel for cold press die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive steel for cold press die achieving hardness and toughness required as a cold press die and reducing used amount of expensive elements as much as possible.SOLUTION: There is provided a cold tool steel used for a die for molding such as cold press containing, by mass%, C:0.75 to 0.90%, Si:0.30 to 0.90%, Mn:0.20 to 0.60%, Cr:6.50 to 8.50%, Mo:0.20 to 0.50% and further V:0.10% or less as impurities and having a chemical composition of 1.50≤[%C]/C≤1.80 of the formula (1) with C=0.06×[%Cr]+0.093×[%Mo] and consisting of the balance Fe with inevitable impurities. [% element] represents amount of alloy element in mass%.

Description

  The present invention relates to a cold tool steel used for a molding die such as a cold press.

Conventionally, tool steels for cold molds such as SKD11 described in JIS or D2 standardized by AISI are frequently used for molding dies such as cold presses. These tool steels for cold molds form coarse M ₇ C ₃ primary carbides by adding about 1.5% C and about 12% Cr to the steel and have excellent wear resistance. It has become a steel material that demonstrates. On the other hand, these steel materials have poor toughness and workability because they contain a large amount of primary carbides.

  Therefore, as a measure to prevent the deterioration of the toughness and workability of the steel material, by reducing the composition of this steel to low C and low Cr, the amount of primary carbide formed is reduced, and the coldness is improved to improve the toughness. Interstitial tool steel has been developed, and steel materials made of this steel are widely applied (for example, see Patent Document 1).

  The cold tool steel of the invention according to Patent Document 1 is a cold tool steel that has basic characteristics such as hardness, toughness, heat treatment size change, and has no problems in terms of cutting finish surface roughness and cutting tool life. .

  However, the cold tool steel of Patent Document 1 has a Ni content of 0.25 to 1.00% (minimum 0.28% in the examples) for the purpose of obtaining hardness using carbides precipitated by tempering. Mo + 1 / 2W as Mo equivalent 0.5-3.0% (minimum 0.97% in the example), V 0.5% or less (minimum 0% in the example) and Co 10% or less (implementation) In this example, this cold tool steel has basic characteristics such as hardness, toughness, heat treatment size change, and there is no problem in terms of cutting finish surface roughness and cutting tool life. However, since this cold tool steel contains a relatively large amount of rare elements Ni, Mo, W, V and Co, there is a problem that the cost of the steel material increases.

  Furthermore, as a conventional technology, by adjusting the chemical composition and controlling the carbides, the high wear characteristics, which are the superior characteristics of the conventional products, are maintained, and the machinability and galling resistance, which were disadvantages, are greatly improved. Inter tool steel has been proposed (see, for example, Patent Document 2). However, this cold tool steel has a problem that the cost of the steel material is increased because relatively rare elements Ni, Mo, W and V are added.

JP 2009-167435 A JP 2007-077442 A

  Industrial products are generally a mixture of mass-produced products and small-quantity multi-product products. However, since short-term and diversified product designs are required, the current needs are small-quantity multi-product products. As a result of following the trends in various manufacturing industries, mold materials that have long life have been required for the purpose of reducing manufacturing costs, but in recent years, low-cost molds with the necessary minimum characteristics have been required. The demand for materials is expanding. Therefore, in cold tool steel, which is a die material for cold pressing, in order to meet the above-mentioned demand, in order to reduce die manufacturing cost and work period, improvement of machinability and deformation after quenching and tempering. Development aimed at reducing dimensional change and dimensional change has also been promoted. However, many of them often contain a lot of expensive rare elements in order to maintain and improve the properties for cold press applications, especially wear resistance and seizure resistance. It does not contribute to reducing mold costs.

  Therefore, the problem to be solved by the present invention is to increase the cost of the mold material while ensuring the necessary minimum characteristics such as hardness, toughness, and wear resistance necessary for the cold press mold. It is to provide an inexpensive cold-pressed die steel by reducing the amount of element used as much as possible.

The means of the present invention for solving the above-mentioned problems is, in the first means, in mass%, C: 0.75 to 0.90%, Si: 0.30 to 0.90%, Mn: 0.00. 20 to 0.60%, Cr: 6.50 to 8.50%, Mo: 0.20 to 0.50%, and the chemical composition is 1.50 ≦ [% C] / of the formula (1) C _eq ≦ 1.80, provided that C _eq = 0.06 × [% Cr] + 0.093 × [% Mo], the balance being Fe and inevitable impurities, Steel.
However, the above-mentioned [% element] is the amount of alloying elements and indicates mass%.

In the second means, in addition to the chemical component of the first means, it contains S ≦ 0.120% by mass, and the chemical composition is 1.50 ≦ [% C] / C of the formula (1). _eq ≦ 1.80, provided that C _eq = 0.06 × [% Cr] + 0.093 × [% Mo], the balance being Fe and inevitable impurities, for cold press dies It is steel.
However, the above-mentioned [% element] is the amount of alloying elements and indicates mass%.

In the third means, in addition to the chemical component of the first means, V ≦ 0.10% is contained in mass%, and the chemical composition is 1.50 ≦ [% C] / C of the formula (1). _eq ≦ 1.80, provided that C _eq = 0.06 × [% Cr] + 0.093 × [% Mo], the balance being Fe and inevitable impurities, for cold press dies It is steel.
However, the above-mentioned [% element] is the amount of alloying elements and indicates mass%.

In the fourth means, in addition to the chemical component of the first means, in mass%, S ≦ 0.120% is further contained, V ≦ 0.10% is further contained, and the chemical composition is represented by the formula (1) 1.50 ≦ [% C] / C _eq ≦ 1.80, where C _eq = 0.06 × [% Cr] + 0.093 × [% Mo], and the balance is Fe and inevitable impurities This is a steel for cold pressing dies.
However, the above-mentioned [% element] is the amount of alloying elements and indicates mass%.

  Here, the reason for limitation of the chemical component of the cold tool steel which is the invention according to the first and second means of the present application will be described below. In addition,% of a chemical component is based on the mass%.

C: 0.75 to 0.90%
C is an element for ensuring hardness and wear resistance by ensuring sufficient hardenability and forming carbides. If C is less than 0.75%, sufficient hardness and wear resistance cannot be obtained. When C is more than 0.90%, coarsening of carbide and solidification segregation are promoted, and toughness is inhibited. Therefore, C is set to 0.75 to 0.90%. Desirably, C: 0.75 to 0.85%.

Si: 0.30-0.90%
Si is an element for ensuring the deoxidation effect in steel making and the hardenability of the steel. If Si is less than 0.30%, these effects cannot be obtained. When Si is more than 0.90%, the toughness is lowered. Therefore, Si is set to 0.30 to 0.90%.

Mn: 0.20 to 0.60%
Mn is an element that is added as a deoxidizing material in the same manner as Si and is necessary for ensuring hardenability. If Mn is less than 0.20%, it is insufficient to ensure hardenability. When Mn is more than 0.60%, workability is lowered. Therefore, Mn is set to 0.20 to 0.60%. Desirably, it is Mn: 0.45-0.55%.

Cr: 6.50-8.50%
Cr is an element that improves hardenability and forms carbides and contributes to wear resistance. If the Cr content is less than 6.50%, the hardenability deteriorates, and if the heat treatment equipment capacity (equal tropical volume in the furnace, cooling capacity, etc.) and heat treatment technology are inferior, the proper structure and hardness cannot be secured. . If the Cr content is more than 8.50%, coarse primary carbides are excessively formed during solidification, and the toughness and dimensional change characteristics after heat treatment are deteriorated. Moreover, since the carbide | carbonized_material composition which precipitates at the time of temper changes with the addition amount balance of Cr and Mo, even if Cr is 6.50-8.50%, it is necessary to satisfy Formula (1).

Mo: 0.20 to 0.50%
Mo is an element that contributes to hardenability, secondary hardening, and wear resistance. When Mo is less than 0.20%, the effects of hardenability, secondary hardening and wear resistance cannot be obtained. Even if Mo is added in excess of 0.50%, the effect is not only saturated, but also segregation is promoted, and the toughness is reduced by coarse aggregation of carbides. Furthermore, the cost increases. Therefore, Mo is 0.20 to 0.50%.

S: 0.120% or less S is an element that improves the machinability of steel. S combines with Mn during the steel making process to form MnS. This MnS serves as a starting point of fracture during cutting, thereby improving the machinability of the steel material and contributing to reduction in cutting costs. However, at the same time, since it becomes a factor of lowering the toughness of the steel material, excessive addition significantly impairs the function as a mold, so the upper limit may be 0.120%.

V: 0.10% or less V forms carbides and contributes to improvement in hardness and wear resistance. However, V is one of the rare elements, which increases costs. In the present invention, positive addition is not required, but it may be contained in raw materials such as scrap. If the raw materials are specially managed, the cost is rather high. Therefore, it is not added and the content of 0.10% or less as an impurity is allowed.

Next, when the C / C _eq value of the formula (1), which is the limiting formula of the C addition amount relative to the Cr and Mo addition amounts, which is the chemical composition of the present application claim, is defined as the R value, .50 ≦ R value ≦ 1.80 is satisfied. Therefore, the reason for limitation of 1.50 ≦ R value ≦ 1.80 will be described below.

This 1.50 ≦ R value ≦ 1.80 is obtained by tempering any steel tempered from 1030 ° C. and 490 to 510 ° C. after quenching the steel comprising the chemical components, Fe and inevitable impurities according to the present invention. It is a value necessary for obtaining a steel material having a toughness of 25 J / cm ² or more while ensuring 58 HRC or more. By the way, carbides precipitate during tempering. However, this type of carbide has a great influence on the quenching and tempering hardness. In particular, Mo precipitates secondary carbides during quenching and tempering and is effective in improving the hardness. Therefore, in order to effectively precipitate the Mo-based carbide, it is necessary to control the relationship between the addition amount of Mo and Cr in addition to the addition amount of Mo within the scope of the present invention. Furthermore, in order to ensure toughness at the same time, it is necessary to control the C amount relative to the addition amounts of Mo and Cr. Furthermore, the amount of expensive Mo added can be minimized by considering the balance of the amount of C added relative to the amount of Cr and Mo added. Therefore, 1.50 ≦ R value ≦ 1.80 which is the value of the formula (1) in the claimed component range. Desirably, 1.50 ≦ R value ≦ 1.75.

  The present invention minimizes the amount of expensive Mo added by considering the balance of the amount of C added relative to the amount of Cr and Mo added, and also requires positive addition of V, which is also an expensive element. Therefore, it is possible to manufacture a steel for cold mold having characteristics necessary as a cold forming tool such as a cold press mold at a relatively low cost.

  Embodiments of the present invention will be described below. One ton of each of the nine types of steels of the present invention steel described in Table 1 and the comparative type steels of 15 types of steel was melted using a vacuum melting furnace and formed into an ingot. The ingot was heated to 1125 ° C., and a rectangular material having a thickness of 105 mm, a width of 215 mm, and a length of 800 mm was produced by hot forging to a forging ratio of about 6S.

  As for the evaluation of the quenching and tempering hardness, each of the above-prepared flat materials having a thickness of 105 mm, a width of 215 mm and a length of 800 mm was quenched at 1030 ° C. and tempered twice, and then the thickness of the central portion was increased. A cut surface obtained by cutting to 25 mm, a width of 25 mm, and a length of 25 mm is used as a measurement surface. This is a value obtained by measuring the hardness of the cut surface with a Rockwell hardness meter after increasing the accuracy. In addition, the measured value (it shows by HRC) of quenching tempering hardness has shown the highest hardness obtained with each steel materials, and has shown the temperature of tempering at that time as tempering temperature (degreeC).

As an evaluation of the wear resistance, the specific wear amount (mm ³ / N · mm) by the Ogoshi quick wear test was evaluated. This is a 10 mm wide, 28 mm thick test piece steel material having a width of 10 mm, a thickness of 28 mm, and a length of 60 mm from the middle circumference of each of the above-prepared 105 mm thick, 215 mm wide and 800 mm long flat bars. Is taken so that the surface of the steel is perpendicular to the forging direction of the steel material, and after soaking the collected material at 1030 ° C. for 30 minutes, quenching by air cooling is performed, and at a tempering temperature of 490 to 510 ° C., Tempered to maximum hardness. Thereafter, the material was finished to a width of 7 mm, a thickness of 25 mm, and a length of 50 mm to obtain a test piece having a surface having a thickness of 25 mm and a length of 50 mm as a test surface. On the other hand, a ring material made of JIS standard chromium molybdenum steel SCM420 having a hardness of 86 HRB is used as a test material, and the other material is a final load of 61.8 N and a sliding speed of 2.4 m / s. The specimen was rubbed with a test piece under the condition of a friction coefficient of 200 m, and the specific wear after the test was used as an evaluation index.

As an evaluation of toughness, the energy required for fracture was evaluated by a Charpy impact test. In this test, one side is 12 mm from the center of each square bar having a thickness of 105 mm, a width of 215 mm, and a length of 800 mm so that the direction perpendicular to the forging direction is the length direction of the test piece. A specimen material having a length of 60 mm in all directions was collected. These test piece materials were soaked at 1030 ° C. for 30 minutes, and then tempered to HRC58-60 by quenching by air cooling. After that, these materials are finished into a prism with a side of 10 mm square and a length of 55 mm, and a C notch with a notch radius of 10 mm and a depth of 2 mm is machined on a surface perpendicular to the forging direction. The Charpy impact test was performed using what was made. The impact test value is a value obtained by dividing the impact energy required for fracture by the cross-sectional area of the test piece, and the unit is indicated by J / cm ² .

  As an evaluation of workability, it was evaluated by a hot compression test. This is a state in which a cylindrical test piece having a diameter of 6 mm and a length of 9 mm is indexed from the center part of the annealed material of each flat square having a thickness of 105 mm, a width of 215 mm and a length of 800 mm, and heated to 1175 ° C. Thus, the test piece was evaluated visually by the presence or absence of cracks in the test piece when compression was performed at a processing rate of 70% at a processing speed of 100 mm / s. ○ indicates that no cracks occurred, and × indicates that cracks occurred.

  Next, embodiments of the present invention will be described below with reference to the table. Table 1 shows the steel of the present invention as nine examples of A, B, C, D, E, F, G, H, and I. The alloy composition excluding Fe and unavoidable impurities is shown in mass%. Further, the comparative steels of the present invention are 15 examples of a1, a2, b1, b2, b1, c1, c2, d1, d2, e1, e2, f1, f2, g1, h1, and h2, and their Fe and unavoidable The alloy composition excluding impurities is shown by mass%.

In Table 1, as Note 1, the steel of the present invention contains Fe and inevitable impurities in addition to the elements of the alloy composition. In addition, the hyphen in the column of the alloy composition (mass%) in Table 1 indicates that the element to be contained is not included, and the numerical values other than the hyphen are the amounts indicated by mass% of each element.
Further, in Table 1, as Note 2, each numerical value in the R value column indicates the value of Formula (1), which is a limiting formula of C with respect to Cr and Mo, which are alloy elements.

  In Table 2, note 3 is an item for which sufficient characteristics cannot be obtained by underlining the characteristics of quenching and tempering hardness, wear resistance, toughness, and workability.

  The comparative steel a1 has a high Cr content, reduces the precipitation of effective carbides that impart hardness during high-temperature tempering, does not provide a hardness of 58 HRC or higher, and has low wear resistance. In comparison steel a2, the Cr content is low, the total amount of carbide formed is insufficient, and the wear resistance is low.

In comparative steels b1 and b2, the alloy element addition amount is within the range of the present invention, but the R value condition is not satisfied, so the balance of the precipitated carbide composition during high temperature tempering is poor, and a toughness of 25 J / cm ² or more is obtained. I can't.

  Since the comparative steel c1 has a small Mo content, the amount of carbide is insufficient, and the necessary hardness and wear resistance cannot be obtained. Since the comparative steel c2 has a high Mo content, coarse condensation of carbides occurs and the toughness decreases.

  Since the comparative steel d1 has a low Si content, strength cannot be obtained at the steel base, and the hardness and wear resistance are reduced. Moreover, since hardenability becomes low, toughness also falls. Since the comparative steel d2 has too much Si content, the ductility of the base structure is lowered and the toughness is inferior.

  Since comparative steel e1 has too little Mn content, toughness falls by hardenability being insufficient. Since the comparative steel e2 has too much Mn content, hot workability deteriorates.

  Since comparative steel f1 has too much C content, segregation is promoted, toughness is reduced, and hot workability is also reduced. Since the comparative steel f2 has too little C content, a hardness of 58HRC or higher cannot be obtained. Wear resistance is also insufficient.

  The comparative steel g1 has an excessive S content and adversely affects toughness. Moreover, hot workability is also reduced.

  In comparative steels h1 and h2, the alloy element addition amount is within the scope of the present invention, but the R value condition is not satisfied. Also, the wear resistance is insufficient.

As described above, in Table 2, the nine types A to I of the steels of the present invention all have a quenching and tempering hardness of HRC 58.5 or more, and the wear resistance has a specific wear amount of 3.0 × 10 ^{−. 7} m ³ / N · mm or less, and toughness is 10R-C impact value is 26.2 J / cm ² or more. Therefore, these have sufficient quenching and tempering hardness, toughness, and wear resistance. Have both. Nine of these symbols A to I of the steels of the present invention are concerned with the addition amount of Cr and Mo, thereby minimizing the addition amount of expensive Mo and V, which is also an expensive element. Although these steels do not require aggressive addition, these steels are relatively inexpensive cold tool steels by satisfying the properties shown in Table 2 required for cold forming tools such as cold press dies. Can be provided to demanding companies.

Claims (4)

In mass%, C: 0.75 to 0.90%, Si: 0.30 to 0.90%, Mn: 0.20 to 060%, Cr: 6.50 to 8.50%, Mo: 0.00. 20 to 0.50%, and the chemical composition is 1.50 ≦ [% C] / C _eq ≦ 1.80 of formula (1), where C _eq = 0.06 × [% Cr] +0. A steel for cold press dies characterized by being 093 × [% Mo] and the balance being Fe and inevitable impurities.
However, the above-mentioned [% element] is the amount of alloy element and indicates mass%. 2. In addition to the chemical component of claim 1, it contains S ≦ 0.120% by mass, and the chemical composition is 1.50 ≦ [% C] / C _eq ≦ 1.80, where C _eq = 0 .06 × [% Cr] + 0.093 × [% Mo], the balance being made of Fe and inevitable impurities, a steel for cold press dies.
However, the above-mentioned [% element] is the amount of alloy element and indicates mass%. 2. In addition to the chemical component of claim 1, it contains V ≦ 0.10% by mass, and the chemical composition is 1.50 ≦ [% C] / C _eq ≦ 1.80, where C _eq = 0 .06 × [% Cr] + 0.093 × [% Mo], the balance being made of Fe and inevitable impurities, a steel for cold press dies.
However, the above-mentioned [% element] is the amount of alloy element and indicates mass%. 2. In addition to the chemical component of claim 1, in weight percent, S ≦ 0.120%, further V ≦ 0.10%, and the chemical composition is 1.50 ≦ [% C] / C _eq ≦ 1.80, provided that C _eq = 0.06 × [% Cr] + 0.093 × [% Mo], and the balance is made of Fe and inevitable impurities, the steel for cold press dies .
However, the above-mentioned [% element] is the amount of alloy element and indicates mass%.