JP6472174B2 - Cold tool steel with high hardness and toughness that can be quenched at low temperature - Google Patents

Description

  The present invention is a high-hardness, high-toughness cold tool steel particularly suitable for cold working where the use conditions such as forging dies, forming rolls or rolling dies are particularly severe, particularly among sintering methods or melting methods, The present invention relates to a cold tool steel obtained by manufacturing a high hardness and high toughness mold and tool steel, which has been difficult by the melting method until now.

  In recent years, with the development of cold working technology, the working conditions of molds have become harsher, such as the amount of work of materials with higher hardness than before has increased. Therefore, in steels that can only have a hardness of about 60 to 62 HRC such as SKD11 of cold tool steel defined by JIS and 8% Cr-based cold tool steel, the mold surface is scraped by the work material, In addition, the mold life cannot be withstood at the time of processing and chipping occurs, causing large cracks, resulting in an early mold life. For this reason, a material having a hardness of 63 HRC or more and high toughness is required. In addition, from the viewpoint of economy, it can be manufactured at low cost, and is quenched from a low temperature range of 1020 to 1060 ° C. (hereinafter referred to as “low temperature quenching”) as in the case of general-purpose die steels such as SKD11 and 8% Cr cold work tool steel In other words, high hardness is required.

  In order to obtain a steel material with a hardness exceeding 63HRC, for example, SKH51 of high speed tool steel specified by JIS and alloy elements such as C, Mo, W, V, Co, etc. are added in large quantities, and a large amount of hard carbides are added. A high-speed tool steel capable of ensuring a hardness of 65 HRC or higher has been proposed in both low temperature quenching at about 1000 ° C. and high temperature quenching at about 1200 ° C., which has obtained high hardness by precipitation. (For example, refer to Patent Document 1). However, the proposed high-speed tool steel has a large amount of alloying elements and a large amount of coarse primary carbides in the steel, so that not only the mold material cost is high but also the toughness and fatigue strength are low.

  Furthermore, a steel for high-performance rolling dies and a manufacturing method thereof have been proposed (for example, see Patent Document 2). However, because this die steel has restrictions on special manufacturing methods and quenching to eliminate coarse primary carbides, the cost of the die material itself increases, and the die heat treatment needs to be specially performed. The mold manufacturing cost increases.

  In addition, high precision mold steel made of powder high-speed tool steel manufactured by powder metallurgy with improved primary toughness and improved toughness has been developed (see, for example, Patent Document 3). However, this powder high-speed tool steel (powder high-speed steel) is expensive and needs to be hardened at a temperature higher than the low-temperature quenching temperature normally used in die steel, so that it can be heated at a higher temperature. And a die made of the steel for the die cannot be heat-treated at the same time as other dies, and it is necessary to heat-treat only with the die, resulting in a high die manufacturing cost. .

JP 2003-268499 A JP 05-156407 A JP 2005-194563 A

  Since there is a problem that the powder high speed by the above powder metallurgy method is costly, the problem to be solved by the present invention is for cold working where the use conditions such as a forging die, a forming roll or a rolling die are particularly severe. It is preferable to provide a high toughness cold tool steel, a die and a tool thereof, which can be obtained by a melting method and can obtain high hardness even by low-temperature quenching usually performed on die steel.

As a result of the inventor's diligent development, when the alloy composition formula consisting of a specific component of the chemical components of the steel for cold tool is H, H is usually a die steel because the value is a certain value or more. It has been found that even in the range of 1020 to 1060 ° C., which is the low temperature quenching temperature, 63HRC or higher is obtained after tempering, and a high toughness steel with less coarse primary carbides is obtained even by the melting method. Furthermore, when steel materials having an alloy composition formula H of a certain value or more were quenched and tempered and analyzed focusing on carbides present in the steel, M ₂ C type and M having a particle size of 10 μm or less were analyzed. _It has been found that a steel material in which a large amount of ₆ C-type carbide exists, that is, a steel having a higher distribution density of M ₂ C type and M ₆ C type carbide can obtain higher hardness after tempering. The means of the present invention is based on these results.

That is, the means for solving the problems of the present invention is that in the means of claim 1, in mass%, C: 0.6 to 0.9%, Si: 0.6 to 1.0%, Mn: 0 0.1 to 0.6%, Cr: 4.0 to 6.5%, Mo + W / 2: more than 2.0 to 4.1 %, V: 0.3 to 0.4%, N: more than 100 to 500 ppm When the content of H is 2.10C + 0.83Si−0.27Cr + 0.19 (Mo + 1 / 2W) + 0.25V, the alloy component formula H ≧ 0.82 is satisfied. It is a steel having a hardness of 63 HRC or higher manufactured by a melting method, and is a high-hardness, high-toughness cold tool steel that can be quenched at low temperature.

In the means of claim 2, the distribution density of carbides composed of M ₂ C and M ₆ C having a particle size of 10 μm or less is 150 pieces / mm ² or more in the state after tempering by low-temperature quenching. It is a high hardness, high toughness cold work tool steel that can be quenched at low temperature.
It is.

  By using these means, it is possible to obtain a high hardness and toughness cold work tool steel suitable for a forging die, a forming roll, a rolling die, or the like for cold working whose usage conditions are particularly severe.

  Constituent requirements such as chemical components of the means of the present invention will be described sequentially. First, the chemical composition of the high hardness and high toughness cold tool steel of the means of the present application will be described. In addition,% shall be shown by the mass%.

C: 0.6-0.9%
C is a component that forms hard carbides, improves hardness and wear resistance, and enhances hardenability. In order to acquire the effect, C needs to be 0.6% or more. However, when C exceeds 0.9%, coarse carbides are formed to deteriorate toughness. Therefore, C is 0.6 to 0.9%, preferably C is 0.7 to 0.9%.

Si: 0.6 to 1.0%
Si is a necessary component as a deoxidizer and is a necessary component for obtaining the hardness of the obtained steel base. In order to obtain the effect, Si needs to be 0.7% or more. However, when Si exceeds 0.9%, toughness and workability deteriorate. Therefore, Si is 0.6 to 1.0%, preferably 0.7 to 0.9%.

Mn: 0.1 to 0.6%
Mn is a necessary component as a deoxidizer and is a necessary component for obtaining hardenability. In order to obtain the effect, Mn needs to be 0.1% or more. However, if Mn exceeds 0.6%, the steel matrix becomes brittle and toughness deteriorates. Therefore, Mn is 0.1 to 0.6%, preferably 0.2 to 0.6%.

Cr: 4.0 to 6.5%
Cr is a component that forms hard carbides, improves hardness and wear resistance, and enhances hardenability, and Cr needs to be 4.5% or more to obtain the effect. However, if Cr exceeds 6.5%, coarse carbides are formed, and the toughness and softening resistance are deteriorated. Therefore, Cr is 4.0 to 6.5%, preferably 4.5 to 6.3%.

Mo + W / 2: more than 2.0 to 5.0%
First, since 1/2 of W in Mo + W / 2 is equivalent to Mo, these Mo + W / 2 are equivalent to Mo. This Mo + W / 2 is a component that forms hard carbides, improves the hardness and wear resistance of the steel, and enhances hardenability and temper softening resistance. In order to obtain the effect, Mo + W / 2 needs to exceed 2.0%. However, Mo + W / 2 forms coarse carbides and deteriorates toughness. Therefore, Mo + W / 2 is more than 2.0 to 5.0%, preferably 2.2 to 4.0%.

V: 0.1 to 0.4%
V is a component that forms hard carbides and improves hardness and wear resistance, and has an effect of suppressing coarsening of crystal grains during quenching, and contributes to improvement of toughness. In order to obtain the effect, V needs to be 0.1% or more. However, if V exceeds 0.4%, coarse carbonitrides are formed, and the toughness and machinability are deteriorated. Therefore, V is 0.1 to 0.4%, preferably 0.2 to 0.4%.

N: More than 100 to less than 500 ppm N is a component necessary for forming a nitride, and the formed nitride prevents coarsening of crystal grains and suppresses a decrease in toughness. Super is necessary. However, if N is 500 ppm or more, coarse nitrides are formed and the toughness is deteriorated. Therefore, N: more than 100 to less than 500 ppm, preferably more than 100 to 350 ppm.

When H = 2.10C + 0.83Si−0.27Cr + 0.19 (Mo + 1 / 2W) + 0.25V, H ≧ 0.82.
H is a value indicating the contribution state of component elements that contribute to secondary hardening during tempering of steel that satisfies the above component ranges. When the above component ranges are satisfied, secondary carbides such as M ₂ C type carbide and M ₆ C type carbide that cause secondary hardening due to an increase in the amounts of C, Mo, W, and V are precipitated during the tempering process, and Si Promotes the precipitation, but if there is too much Cr, it precipitates as a primary carbide and lowers the solid solution C, the amount of secondary hardening decreases, and the hardness does not come out. The balance of these components is represented by H. When H ≧ 0.82, 63 HRC or more is obtained, and preferably when H ≧ 1.03, 63 HRC or more is obtained more stably.

Hardness: 63HRC or more If the hardness of the cold tool steel is less than 63HRC, a high hardness material cannot be processed. Therefore, the cold hardness is 63 HRC or more.

Cold tool steel: manufactured by melting method Cold tool steel has a method of manufacturing by a melting method and a method of manufacturing by powder metallurgy. However, when manufacturing as a powder high speed by powder metallurgy, it costs a material cost. Therefore, in order to suppress the material cost, the cold tool steel is manufactured by a melting method.

The distribution density of M ₂ C type carbide and M ₆ C type carbide having a particle size of 10 μm or less is 150 pieces / mm ² or more. M ₂ C type carbide and M ₆ C type carbide having a particle size of 10 μm or less are tempered with high hardness. It is a carbide necessary to obtain the thickness. However, the grown and coarsened M ₂ C-type carbide and M ₆ C-type carbide that exceed 10 μm do not affect the hardness. Further, if the distribution density of M ₂ C type carbide or M ₆ C type carbide having a particle size of 10 μm or less is less than 150 pieces / mm ² , a hardness of 63 HRC or more cannot be obtained, so M ₂ C type carbide or M The particle size of ₆ C type carbide was 10 μm or less, and the distribution density of M ₂ C type carbide and M ₆ C type carbide was 150 pieces / mm ² or more.

No. shown in Table 1 in mass% . 4-5, no. 8-13, no. 15-16, no. No. 18 invention steel and No. 18 100 kg of steel composed of each chemical component of 19-32 comparative steels was melted in a vacuum induction melting furnace, the obtained steel was forged into 50 mm vertical and horizontal squares, heated to 1050 ° C., 30 A tempering process was repeated twice or more, in which both the invented steel and the comparative steel were heated to 500 to 600 ° C. and held for 1 hour and then air-cooled. The distribution density of carbide is determined by measuring a specimen having a length of 10 mm and a length of 15 mm from the center of the quenching and tempering sample, mirror-polishing, and then smoothing the surface in the length direction by ion milling. Using. Three structural photographs were taken of the surface of each test piece in the length direction in which the ion milling was performed using a SEM and the central portion of the surface was magnified 1000 times. The distribution density of M ₂ C type carbide and M ₆ C type carbide having a particle size of 10 μm or less observed in each photograph was measured using an image analyzer, and the average value was taken as the distribution density. Table 1 shows the contributions of the above chemical components and component elements, the value of H, the quenching temperature, and the distribution of M ₂ C type and M ₆ C type carbides having a particle size of 10 μm or less formed in these steels. The density is shown in Table 1.

Furthermore, No. 1 shown in Table 1 obtained by the above method . 4-5, no. 8-13, no. 15-16, no. No. 18 invention steel and No. 18 Table 2 shows the hardness and Charpy impact value of the tempered steel materials in comparative steels 19 to 32. In the description of Table 2, the hardness of the steel material is evaluated by the highest hardness in the tempering temperature range of 500 ° C. or higher. When the hardness of the steel material is 63 HRC or higher, the evaluation is ○, and when the hardness is lower than 63 HRC, × It was. The Charpy impact value indicating toughness was determined by indexing a 10R-2mmC notch Charpy test piece having a length and width of 10 mm and a length of 55 mm from the quenching and tempering sample, and measuring the impact value. SKH51, a JIS steel grade, is a steel grade that has a hardness of 63 HRC or higher. SKH51, a JIS steel grade produced by powder metallurgy, has an impact value of 25 J / cm ^{2 at} 63 HRC. Therefore, the 25 J / cm ² with respect to the, in cold work tool steel produced by the melting method, shown in ○ ratings for this 25 J / cm ² or more high impact value may obtained, the 25 J / cm ² If only a lower impact value was obtained, it was evaluated as bad and indicated by x.

As a result of the above, the steel grade in which any component amount of C, Si, Cr, Mo + W / 2, V is low is low in hardness, and any component amount of C, Si, Mn, Mo + W / 2, V is low. Highly deviated steel types have low Charpy impact values, H values below 0.92 that are out of range are low in hardness, and those with a carbide distribution density lower than 150 pieces / mm ² have low hardness. Is shown.

Claims (2)

In mass%, C: 0.6 to 0.9%, Si: 0.6 to 1.0%, Mn: 0.1 to 0.6%, Cr: 4.0 to 6.5%, Mo + W / 2: More than 2.0 to 4.1 %, V: 0.3 to 0.4%, N: more than 100 to less than 500 ppm, consisting of the remainder Fe and inevitable impurities, H = 2.10C + 0.83Si- When it is 0.27Cr + 0.19 (Mo + 1 / 2W) + 0.25V, it satisfies H ≧ 0.82, and is a high-hardness, high-toughness cold tool steel manufactured by a melting method and capable of low-temperature quenching, A high hardness and high toughness cold tool steel characterized by having a hardness of 63 HRC or higher in a state after tempering by low-temperature quenching. _2. The distribution density of carbides composed of M ₂ C and M ₆ C having a particle size of 10 μm or less in a state after tempering by low-temperature quenching is 150 pieces / mm ² or more. High hardness and toughness cold work tool steel.