JP7061263B2 - Cold tool material and cold tool manufacturing method - Google Patents

Description

The present invention relates to a cold tool material most suitable for various cold tools such as a press die, a forging die, a rolling die, and a metal blade, and a method for manufacturing the cold tool using the cold tool material.

Since the cold tool is used while in contact with a hard workpiece, it must have a hardness that can withstand the contact. Conventionally, as the cold tool material, for example, JIS steel grades SKD10 and SKD11-based alloy tool steels have been used. Further, in response to the demand for further improvement in hardness, an alloy tool steel having an improved composition of the above alloy tool steel has been proposed (Patent Document 1). By quenching and tempering these cold tool materials, the hardness of the cold tool can be improved.

International Publication No. 2017/158988 Pamphlet

Recently, "ultra-high-tensile steel" having a tensile strength of more than 1 GPa is frequently used as a work material, and the load on a cold tool tends to increase accordingly. In such a case, the hardness of the cold tool can be adjusted to be high by increasing the quenching temperature or selecting the tempering temperature. Then, the hardness of the cold tool may be adjusted to, for example, the "maximum hardness" that the cold tool material can achieve. However, in such a case, the toughness of the cold tool is lowered, the tool in use is liable to be chipped or cracked, and the tool life is shortened. Further, when the quenching temperature is increased, when the temperature becomes higher than the quenching temperature of a general-purpose cold tool material (for example, the quenching temperature of SKD10 or SKD11 is around 1000 to 1050 ° C.), it is general-purpose. It is not possible to heat heat at the same time in the same batch type furnace as the cold tool material, and the production efficiency is poor.

An object of the present invention is to provide a cold tool material capable of producing a cold tool having high hardness and excellent toughness at that time. It is also an object of the present invention to provide a cold tool material capable of achieving the above-mentioned high hardness at a general-purpose quenching temperature. Then, it is to provide the manufacturing method of the cold tool using this cold tool material.

In the present invention, in terms of mass%, C: 0.65% or more and less than 0.80%, Si: 0.2 to 0.9%, Mn: 0.1 to 1.5%, P: 0.05% or less. , S: 0.05% or less, Cr: 5.0 to 7.0%, Mo and W alone or in combination (Mo + 1 / 2W): 2.5 to 4.0%, V: 0.10 to 0 It has a component composition of .30%, N: more than 0.03% and 0.08% or less, Ni: 0 to 1.0%, Nb: 0 to 1.5%, and the balance is Fe and impurities.
The area ratio of carbide A having a circle equivalent diameter of more than 5.0 μm in the structure of the cross section is 1.0 to 3.0 area%.
In the region of the structure of the above cross section, 90 μm in length and 90 μm in width, which does not contain carbide A, the number density of carbide B having a circle equivalent diameter of more than 0.1 μm and 2.0 μm or less is 6.0 × 10 ⁵ pieces / The number density of carbides C of mm ² or more and 9.0 × 10 ⁵ pieces / mm ² or less, and the equivalent circle diameter is more than 0.1 μm and 0.4 μm or less is 5.0 × 10 ⁵ pieces / mm ² or more. 7.5 × 10 ⁵ pieces / mm Cold tool material less than ² .
Preferably, it is a cold tool material in which the ratio of the number of carbides C to the number of carbides B is 75.0% or more in the above-mentioned region of 90 μm in length and 90 μm in width which does not contain carbides A.

Then, in the present invention, the above-mentioned cold tool material of the present invention is quenched at a quenching temperature of 1000 to 1050 ° C. and tempered at a tempering temperature of 150 to 600 ° C. to adjust the hardness to 58 HRC or more. This is a method for manufacturing interstitial tools.

According to the present invention, it is possible to provide a cold tool having high hardness and excellent toughness at that time. Further, the above-mentioned high hardness can be achieved at a general-purpose quenching temperature.

It is an optical micrograph which shows an example of the cross-sectional structure of the cold tool material of this invention example. It is an optical micrograph which shows an example of the unsolid solution carbide distributed in the cross-sectional structure of the cold tool material of this invention example. It is an optical micrograph which shows an example of the unsolid solution carbide distributed in the cross-sectional structure of the cold tool material of a comparative example. In an example of the cross-sectional structure of the cold tool material of the present invention, an element mapping image of C (carbon) when a region having a equivalent circle diameter of more than 5.0 μm and containing no carbide is analyzed by an EPMA (electron probe microanalyzer). It is a figure which shows. FIG. 4 is a diagram showing an image obtained by binarization based on the amount of C forming a carbide. In an example of the cross-sectional structure of the cold tool material of the present invention example and the comparative example, the carbide distribution in the region where the equivalent circle diameter exceeds 5.0 μm and does not contain carbide is determined for each range (horizontal axis) of the equivalent circle diameter of the carbide. It is a graph which showed the number of carbides (vertical axis) summarized.

The present inventor investigated the factors in the structure of cold tool materials that affect the hardness after quenching and tempering. As a result, among the carbides existing in the structure of the cold tool material, the distribution state of the "solid solution carbide" that dissolves in the substrate at the next quenching has a great influence on the hardness at the time of quenching and tempering. It was found that there was. Then, it was found that by adjusting the distribution state of the above-mentioned solid solution carbide, high tempering hardness can be obtained without excessively increasing the quenching temperature at the time of quenching and tempering.

Regarding the toughness after quenching and tempering, among the carbides existing in the structure of the cold tool material, the presence of "unsolid solution carbide" that does not dissolve in the substrate at the time of the next quenching affects it. It was found that there was. That is, the unsolid solution carbide contained in the cold tool material remains in the structure of the cold tool after quenching and tempering, and this becomes the starting point of fracture near the surface of the cold tool. The action that is the starting point of this fracture tends to occur sensitively, especially when the hardness of the cold tool is high. Therefore, it has been found that when the hardness of the cold tool is high and the amount of the above-mentioned undissolved carbide is large, the cold tool is likely to be chipped or cracked at an early stage.
Hereinafter, each constituent requirement of the present invention will be described.

(1) <The cold tool material of the present invention has a mass% (hereinafter, simply referred to as "%"), C: 0.65% or more and less than 0.80%, Si: 0.2 to 0. 9%, Mn: 0.1 to 1.5%, P: 0.05% or less, S: 0.05% or less, Cr: 5.0 to 7.0%, Mo and W are alone or in combination ( Mo + 1 / 2W): 2.5 to 4.0%, V: 0.10 to 0.30%, N: more than 0.03% and 0.08% or less, Ni: 0 to 1.0%, Nb : 0 to 1.5%, having a component composition in which the balance is Fe and impurities. ＞

-C: 0.65% or more and less than 0.80% C is partly dissolved in the base to give hardness to the base, and partly forms carbides to achieve wear resistance and seizure resistance. It is a basic element of cold tool material. Further, when C solid-solved as an penetrating atom is contained together with a substituted atom having a high affinity with C such as Cr, the I (penetrating atom) -S (substituted atom) effect (drag resistance of the solute atom). (The effect of increasing the strength of cold tools) is also expected. However, excessive content causes a decrease in toughness due to an increase in undissolved carbides. Therefore, it is set to 0.65% or more and less than 0.80%. It is preferably 0.7% or more. More preferably, it is 0.74% or more. Further, it is preferably 0.79% or less. More preferably, it is 0.78% or less.

・ Si: 0.2-0.9%
Si is a deoxidizing agent during steelmaking, but if it is too much, the hardenability is lowered. In addition, the toughness of the cold tool after quenching and tempering is reduced. Further, Si tends to combine with Mo and C to form a hard and coarse carbide ( _M6C type carbide) at the stage of producing a steel ingot which is a starting material of a cold tool material. Therefore, it should be 0.9% or less. It is preferably 0.8% or less, more preferably 0.75% or less. On the other hand, Si has the effect of increasing the hardness of the cold tool by dissolving it in the tool structure. Therefore, Si is set to 0.2% or more. It is preferably 0.4% or more, more preferably 0.6% or more.

-Mn: 0.1-1.5%
If the amount of Mn is too large, the viscosity of the substrate is increased and the machinability of the material is lowered. Therefore, it should be 1.5% or less. It is preferably 1.0% or less, more preferably 0.7% or less. More preferably, it is 0.5% or less.
Mn is an austenite-forming element and has an effect of enhancing hardenability. Further, since it exists as MnS of non-metal inclusions, it has a great effect on improving machinability. Therefore, Mn is set to 0.1% or more. It is preferably 0.2% or more.

-P: 0.05% or less P is an element that can be inevitably contained in various cold tool materials without addition. Then, it is an element that segregates into the old austenite grain boundaries during heat treatment such as tempering and embrittles the grain boundaries. Therefore, in order to improve the toughness of cold tools, the percentage should be limited to 0.05% or less, including the case of addition. It is preferably 0.04% or less, more preferably 0.03% or less.

-S: 0.05% or less S is an element that can be inevitably contained in various cold tool materials without addition. It is an element that deteriorates the hot workability of the material used for producing the cold tool material and causes cracks during the hot work. Therefore, in order to improve hot workability, it is restricted to 0.05% or less. It is preferably 0.03% or less. More preferably, it is 0.01% or less.
It should be noted that S has an effect of improving machinability by being combined with the above-mentioned Mn and existing as MnS of non-metal inclusions. In order to obtain this effect, it may be contained in excess of 0.03%.

-Cr: 5.0-7.0%
Cr is an element that enhances hardenability. It is also an element that forms carbides and is effective in improving the wear resistance of cold tools. It is also a basic element of cold tool material that contributes to the improvement of temper softening resistance. However, excessive content forms coarse undissolved carbides and causes a decrease in toughness. Therefore, Cr is set to 5.0 to 7.0%. It is preferably 5.5% or more, and more preferably 5.8% or more. It is more preferably 6.0% or more, and even more preferably 6.2% or more. Further, it is preferably 6.9% or less, and more preferably 6.8% or less.

-Mo and W alone or in combination (Mo + 1 / 2W): 2.5-4.0%
Mo and W are elements that give strength to cold tools by precipitating or aggregating fine carbides by tempering. Mo and W can be contained alone or in combination. Since W has an atomic weight about twice that of Mo, the content at this time can be defined together by the Mo equivalent defined by the formula (Mo + 1 / 2W) (of course, either one). It may be contained only, or both may be contained). Then, in order to obtain the above-mentioned effect, the content is set to 2.5% or more at the value of (Mo + 1 / 2W). It is preferably 3.0% or more. However, if the amount is too large, the machinability and toughness will deteriorate, so the value of (Mo + 1 / 2W) should be 4.0% or less. It is preferably 3.6% or less. More preferably, it is 3.5% or less.

・ V: 0.10 to 0.30%
V has the effect of forming carbides, strengthening the matrix, improving wear resistance, and tempering softening resistance. The carbides of V distributed in the annealed structure act as "pinning particles" that suppress the coarsening of austenite crystal grains during quenching and heating, and contribute to the improvement of toughness. In order to obtain these effects, V is set to 0.10% or more. It is preferably 0.12% or more, more preferably 0.15% or more.
However, if the amount of V is too large, the machinability is deteriorated. Further, since coarse undissolved carbide is formed and the toughness is lowered, it is an important element to control the upper limit for the present invention. Then, V is set to 0.30% or less. It is preferably 0.28% or less. More preferably, it is 0.25% or less.

-N (nitrogen): More than 0.03% and 0.08% or less When N is added together with a substituted atom having a high affinity with N such as Cr and V, fine carbides or carbonitrides are precipitated. It is an element that enhances wear resistance and seizure resistance. However, excessive addition causes a decrease in toughness due to an increase in coarse nitride or carbonitride. Therefore, N is set to more than 0.03% and 0.08% or less. It is preferably 0.035% or more. More preferably, it is 0.04% or more. Further, it is preferably 0.07% or less. More preferably, it is 0.06% or less.

The component composition of the cold tool material of the present invention can be a component composition containing the above-mentioned elemental species and having the balance of Fe and impurities. In addition to the above-mentioned elemental species, the following elemental species can be selectively contained.

・ Ni: 0 to 1.0%
Ni is an element that increases the viscosity of the substrate and reduces the machinability. Therefore, the Ni content is preferably 1.0% or less. It is more preferably less than 0.5%, still more preferably less than 0.3%. This Ni of less than 0.3% is also a preferable upper limit of regulation when the component composition of the cold tool material of the present invention contains Ni as an impurity.
On the other hand, Ni is an element that suppresses the formation of ferrite in the tool structure. In addition, it is an effective element that imparts excellent hardenability to cold tool materials, forms a martensite-based structure even when the cooling rate during quenching is slow, and prevents deterioration of toughness. Further, since the intrinsic toughness of the matrix is also improved, Ni may be 0%, but in the present invention, it may be contained as needed. When it is contained, it is preferably 0.1% or more, with the above 1.0% as the upper limit. More preferably, it is 0.3% or more. Further, it is more preferably 0.8% or less.

・ Nb: 0 to 1.5%
Since Nb causes a decrease in machinability, it is preferably 1.5% or less. It is more preferably 0.9% or less, still more preferably less than 0.3%. This Nb of less than 0.3% is also a preferable upper limit of regulation when the component composition of the cold tool material of the present invention contains Nb as an impurity.
On the other hand, Nb has the effect of forming carbides, strengthening the base and improving wear resistance. In addition, it has the effect of increasing the temper softening resistance and, like V, suppressing the coarsening of crystal grains and contributing to the improvement of toughness. Therefore, Nb may be 0%, but may be contained as needed. When it is contained, it is preferably 0.1% or more, with the above-mentioned 1.5% as the upper limit. More preferably, it is 0.3% or more. Further, it is more preferably 1.0% or less.

Cu, Al, Ti, Ca, Mg and O (oxygen) are elements that may remain in steel as unavoidable impurities. In the composition of the cold tool material of the present invention, it is preferable that these elements are as low as possible. However, on the other hand, these elements may be contained in a small amount in order to obtain additional action effects such as morphological control of inclusions, other mechanical properties, and improvement of production efficiency. In this case, the range of Cu ≤ 0.25%, Al ≤ 0.25%, Ti ≤ 0.03%, Ca ≤ 0.01%, Mg ≤ 0.01%, O ≤ 0.01% is sufficient. It is acceptable and is the preferred regulatory upper limit of the present invention.

(2) <The cold tool material of the present invention has an area ratio of carbide A having a circle equivalent diameter of more than 5.0 μm in the structure of the cross section of 1.0 to 3.0 area%. ＞

The cold tool material is usually made of a steel ingot or a steel piece obtained by ingoting a steel ingot, and various hot working or heat treatment is performed on the cold tool material to obtain a predetermined steel material, which is then annealed. It is finished in a plate shape or a block shape. At this time, the above-mentioned ingot is generally obtained by casting molten steel adjusted to a predetermined composition. Therefore, in the cast structure of the ingot, a part where large carbides are gathered and a part where small carbides are gathered due to the difference in solidification start time (due to the growth behavior of dendrites). (The so-called "negative segregation" site) exists.
By hot working such ingots, the aggregates of carbides are extended in the stretching direction of the hot working (ie, the length direction of the material) and in the vertical direction (ie, the material). Is compressed in the thickness direction of). In the annealed structure of the cold tool material obtained by annealing the hot-worked steel material, the above-mentioned distribution pattern of carbides is a layer composed of a large aggregate of carbides and a layer composed of a small aggregate of carbides. It becomes a substantially striped pattern (see FIG. 1). In FIG. 1, the "light-colored dispersion", which is found in the dark-colored base and extends exclusively in a streak, is the carbide.

Then, in the above-mentioned annealed structure, the large carbide functions exclusively as "unsolidified carbide", does not dissolve in the base during quenching, remains in the structure after quenching and tempering, and wear resistance of the cold tool. Contributes to improving sex. However, such undissolved carbides act as defects in the length in the orthogonal direction with respect to the principal stress in the tensile direction generated in the cold tool in use, thereby reducing the toughness of the cold tool. It has a direct effect. And, this direct influence becomes large when the hardness of the base of the cold tool is high. Therefore, in the present invention, in the structure of the cross section of the cold tool material before quenching and tempering, for convenience, carbide having a circle equivalent diameter of more than 5.0 μm is treated as unsolidified carbide, so that the amount of this large carbide is appropriate. It was found that even if the cold tool has a high hardness, the wear resistance of the cold tool can be maintained and the decrease in toughness can be suppressed.

That is, the cold tool material of the present invention has an area ratio of carbide A having a circle equivalent diameter of more than 5.0 μm in the structure of the cross section of 1.0 to 3.0 area%. It is preferably 1.3 area% or more, and more preferably 1.5 area% or more. More preferably, it is 1.8 area% or more. Further, it is preferably 2.8 area% or less, and more preferably 2.6 area% or less. More preferably, it is 2.4 area% or less. Such a distribution form of the carbide A can be achieved by manipulating the casting process or the like when producing the above-mentioned ingot. Further, due to the distribution form of the carbide A, it is possible to suppress the occurrence of chipping and cracking at the initial stage of use even when the hardness of the cold tool is adjusted to be high.

(3) <The cold tool material of the present invention is a carbide B having a circular equivalent diameter of more than 0.1 μm and 2.0 μm or less in a region of 90 μm in length and 90 μm in width which does not contain carbide A in the structure of the cross section. The number density of carbides C is 6.0 × 10 ⁵ pieces / mm ² or more and 9.0 × 10 ⁵ pieces / mm less than ² , and the equivalent circle diameter is more than 0.1 μm and 0.4 μm or less. , 5.0 × 10 ⁵ pieces / mm ² or more and 7.5 × 10 ⁵ pieces / mm less than ² . ＞

The carbides contained in the above-mentioned structure include the above-mentioned undissolved carbides, while there are "solid-dissolved carbides" that are solid-dissolved in the substrate by quenching in the next step. Then, this solid solution carbide increases the amount of solid solution carbon in the substrate after quenching and tempering, and improves the hardness of the cold tool. Therefore, in the present invention, in the structure of the cross section of the cold tool material before quenching and tempering, for convenience, a carbide having a circle equivalent diameter of 5.0 μm or less is treated as a solid solution carbide, and the carbide is composed of only the carbide having a circle equivalent diameter. Attention was paid to the region of "90 μm in length and 90 μm in width" (for example, the part surrounded by the solid line shown in FIG. 1). That is, this region of "90 μm in length and 90 μm in width" corresponds to the region of the above-mentioned "layer made of aggregates of small carbides". Then, it was found that the carbide distribution in this region can be used for confirming the effect of the present invention that "a cold tool having high hardness can be achieved without excessively increasing the quenching temperature".

The present inventor investigated the effect of carbides having a circular equivalent diameter of 5.0 μm or less on the hardness of the cold tool after quenching and tempering. As a result, it was found that among these carbides, carbides having a smaller circle-equivalent diameter of "2.0 μm or less" (hereinafter referred to as carbide B) are more likely to dissolve in solid solution. Then, it was found that extremely fine carbides having a circle equivalent diameter of "0.4 μm or less" (hereinafter referred to as carbide C) are particularly easy to dissolve in solid solution. Then, it was found that such small carbides can be easily distributed uniformly in the structure of the cold tool material by manipulating the casting process or the like when producing the above-mentioned ingot. If carbides that are easily dissolved in solid solution are uniformly distributed in the structure of the cold tool material, the amount of solid solution carbon in the structure of the cold tool after quenching and tempering is also increased as a whole without bias. be able to. As a result, high hardness can be achieved without excessively increasing the quenching temperature.

Then, in the region containing no carbide A having an equivalent circle diameter of more than 5.0 μm, the number of carbides B contained in this region is increased, and further, the number of carbides C is increased among the carbides B. This leads to a uniform distribution of solid-dissolved carbides in the structure of the cold tool material and to an unbiased overall increase in the amount of solid-dissolved carbon in the structure of the cold tool after quenching and tempering. Therefore, it is effective in achieving the above-mentioned effects of the present invention. In the case of the present invention, in the region of 90 μm in length and 90 μm in width which does not contain the above-mentioned carbide A, the number density of the carbides B having a circle equivalent diameter of more than 0.1 μm and 2.0 μm or less is 6.0 × 10 ⁵ . The structure of the present invention is such that the number density of carbides C having a circle equivalent diameter of more than 0.1 μm and 0.4 μm or less is 5.0 × 10 ⁵ pieces / mm ² or ^more . The above effects can be achieved. Regarding the sizes of carbides B and C, the lower limit of the equivalent circle diameter is set to 0.1 μm because the identification of carbides of 0.1 μm or less may lack accuracy in measurement.

The number density of the carbide B is preferably 6.3 × 10 ⁵ pieces / mm ² or more, and more preferably 6.5 × 10 ⁵ pieces / mm ² or more. More preferably, it is 6.7 × 10 ⁵ pieces / mm ² or more. Further, the number density of the carbide C is preferably 5.3 × 10 ⁵ pieces / mm ² or more, and more preferably 5.5 × 10 ⁵ pieces / mm ² or more. It is more preferably 5.7 × 10 ⁵ pieces / mm ² or more, and even more preferably 5.8 × 10 ⁵ pieces / mm ² or more.

However, in the structure of the cross section of the cold tool material before quenching and tempering, it is effective that the amount of small carbides (solid solution carbides) is not too large in that it impairs machinability. Therefore, in the present invention, the upper limit of the number density of the carbide B is set to less than 9.0 × 10 ⁵ pieces / mm ² . It is preferably 8.5 × 10 ⁵ pieces / mm ² or less. More preferably, it is 8.0 × 10 ⁵ pieces / mm ² or less. Further, the upper limit of the number density of the carbide C is set to less than 7.5 × 10 ⁵ pieces / mm ² . It is preferably 7.0 × 10 ⁵ pieces / mm ² or less. More preferably, it is 6.5 × 10 ⁵ pieces / mm ² or less.
The number density of carbide C does not exceed the number density of carbide B. Then, it is realistic that the ratio of the number of carbides C to the number of carbides B described later is 95.0% or less.

(4) <Preferably, in the cold tool material of the present invention, the ratio of the number of carbides C to the number of carbides B is 75.0% in the above-mentioned region of 90 μm in length and 90 μm in width which does not contain carbides A. That is all. ＞

In the above-mentioned (3), the fine carbides B and C distributed in the region of the structure of the cold tool material that does not contain carbide A having an equivalent circle diameter of more than 5.0 μm are more circular among these carbides. The larger the number of carbides C having a considerably smaller diameter (that is, easier to dissolve), the more the effect of the present invention "a cold tool with high hardness can be achieved without excessively increasing the quenching temperature" is achieved. It is advantageous. In the case of the present invention, the ratio of the number of carbides C to the number of carbides B is preferably 75.0% or more. And it is more preferably 80.0% or more, and further preferably 85.0% or more. Further, although an upper limit is not particularly required for this ratio, 95.0% or less is realistic.

An example of a method for measuring the equivalent circle diameter and area ratio of carbide A and the equivalent circle diameter and number (number density) of carbides B and C will be described.
First, the cross-sectional structure of the cold tool material is observed with an optical microscope having a magnification of 200 times, for example. At this time, the cross section to be observed can be the central portion of the cold tool material that constitutes the cold tool. The cross section to be observed is a cross section parallel to the stretching direction of hot working (that is, the length direction of the material), and specifically, among these parallel cross sections, the Transverse Direction. It is a cross section (so-called TD cross section) perpendicular to the stretching perpendicular direction. At this time, if the shape of the cold tool material is "cylindrical", the above-mentioned TD cross section is defined as a cross section parallel to the axis of the cylinder. Then, in this cross section, for example, a cut surface having a cross section of 15 mm × 15 mm can be mirror-polished using diamond slurry and colloidal silica. FIG. 1 (“Cold tool material 1” of the example of the present invention evaluated in the examples) shows an example of the cold tool material of the present invention at a magnification of 200 times the cross-sectional structure obtained in the above-mentioned procedure. It is an optical micrograph (viewing area 0.58 mm ² ).

-Circle equivalent diameter and area ratio of carbide A When observing the above cross-sectional structure, the above cross section is corroded with 10% nital so that the boundary between the unsolidified carbide and the matrix becomes clear. Then, the cross section after the corrosion is observed with an optical microscope having a magnification of 200 times, and the above field of view is photographed in 20 fields. FIG. 2 is an optical micrograph of the cross-sectional structure after corrosion (unsolidified carbides are shown in white distribution). By image processing this microstructure photograph with a known image analysis software or the like, carbide A having an equivalent circle diameter of more than 5 μm observed in the cross-sectional structure is extracted as unsolidified carbide. Then, the area ratio of the carbide A in the cross-sectional structure can be obtained as an average value for 20 fields of view.

-Circle equivalent diameter and number of carbides B and C (number density)
First, a region having a length of 90 μm and a width of 90 μm, which does not contain carbide A having a circle equivalent diameter of more than 5.0 μm, is extracted from the above cross-sectional structure. At this time, large carbides having a circle equivalent diameter of more than 5.0 μm can be easily confirmed from the field of view of the optical microscope (see FIGS. 1 and 2). Then, the circle-equivalent diameter of the confirmed carbide can be obtained by a known image analysis software or the like.

Next, the region of 90 μm in length and 90 μm in width (enclosed by the solid line shown in FIG. 1) extracted above was observed with a scanning electron microscope (magnification of 3000 times), and the observed visual field was analyzed by EPMA. Then, an element mapping image of C (carbon) is obtained. Then, based on the analysis result of the element mapping image of C, the binarization process is performed with the detection intensity of C of 50 counts (cps) or more as the threshold value based on the amount of C forming the carbide. This is done to obtain a binarized image showing the carbides distributed in the matrix of the cross-sectional structure.
FIG. 4 is an element mapping image of C obtained in the above-mentioned manner in the area surrounded by the solid line shown in FIG. 1 (visual field area 30 μm × 30 μm). Then, FIG. 5 is a diagram showing the carbide distribution in the above region obtained by binarizing FIG. 4. In FIGS. 4 and 5, C and carbides are shown in a light color distribution.

Then, the carbides B and C having the corresponding circle diameters are extracted from the carbide distribution in FIG. 5 "excluding the carbides A having the equivalent circle diameter of more than 5.0 μm", and the number of the carbides B and C and the carbides are obtained. The abundance ratio of B and C may be obtained. The circle-equivalent diameter and the number of carbides B and C can be obtained by using known image analysis software or the like.

In the case of the cold tool material of the present invention, in the region of the above-mentioned "layer consisting of a collection of small carbides" having a length of 90 μm and a width of 90 μm, small carbides having a circular equivalent diameter of 2.0 μm or less have a substantially uniform number density. It is distributed in (see FIG. 5). Therefore, in confirming the effect of the present invention, it is sufficient that the element mapping image collected from the above-mentioned region of 90 μm in length and 90 μm in width is one image and has an area of 30 μm × 30 μm (number of pixels: 530). × 530). Then, the collection position of this element mapping image may be arbitrarily selected from the above-mentioned region. Then, such a series of measurement operations is performed in at least two "90 μm horizontal 90 μm" regions, which are different from the above-mentioned "90 μm vertical 90 μm" region (total 3 regions), and the above three regions are performed. By summing up the results of the above numerical values by the element mapping image of the area of "30 μm × 30 μm" collected from each of the above, the "cold tool with high hardness without excessively increasing the quenching temperature" of the present invention can be obtained. It is enough to confirm the effect of "achievable".

In order to obtain the structure of the cold tool material of the present invention, it is important to appropriately control the progress of the casting process at the stage of producing the steel ingot as the starting material. For example, it is important to adjust the "temperature of molten steel" just before pouring it into a mold. That is, in order to increase the number of small carbides (carbides B and C), by controlling the temperature of the molten steel to be low, the molten steel is locally thickened due to the difference in the solidification start time at each position in the mold. It is possible to suppress the coarsening of carbides caused by the growth of dendrites. However, if the temperature of the molten steel is too low, the amount of large carbide (carbide A) decreases. Then, in the present invention, controlling the temperature of the molten steel in a temperature range before and after (melting point + 130 ° C.) of the cold tool material, for example, is necessary to obtain the structure of the cold tool material of the present invention. It is effective.

In order to obtain the structure of the cold tool material of the present invention, it is also important to control the cooling process when the hot working on the material shape is completed. In other words, in order to increase the number of small carbides (carbides B and C), the material (steel material) that has been hot-worked is used in the heating process for the next annealing process while the temperature of the material remains high. Instead of transferring, it is effective to cool the structure "sufficiently" to a temperature at which martensitic transformation occurs (generally 300 ° C or lower, preferably 200 ° C or lower) to form the structure into martensite. By this sufficient cooling, the carbide-forming element dissolved in the structure during hot working is precipitated as fine carbide during martensitic transformation. As a result, the number of the above-mentioned small carbides can be increased in the structure of the cold tool material after the annealing treatment.

(5) <In the method for manufacturing a cold tool of the present invention, the above-mentioned cold tool material of the present invention is quenched at a quenching temperature of 1000 to 1050 ° C. and tempered at a tempering temperature of 150 to 600 ° C. The hardness is adjusted to 58 HRC or more. ＞

The cold tool material is prepared into a martensite structure having a predetermined hardness by quenching and tempering, and is prepared into a cold tool product. Then, the cold tool material is adjusted to the shape of the cold tool by various machining such as cutting and drilling. The timing of this machining is preferably performed in a state where the hardness of the material is low (that is, in an annealed state) before quenching and tempering. Further, in this case, finishing may be performed after quenching and tempering. In some cases, the pre-hardened steel after quenching and tempering may be machined into the shape of a cold tool together with the above-mentioned finishing machining.

In the case of the cold tool material of the present invention, by satisfying the above-mentioned composition and metal structure, it is possible to obtain a quenching tempering hardness of 58 HRC or more at the quenching temperature of a general-purpose cold tool material. Specifically, by quenching at a quenching temperature of 1000 to 1050 ° C. and tempering at a tempering temperature of 150 to 600 ° C., it is possible to provide a cold tool having a hardness of 58 HRC or more. The above quenching temperature is preferably 1015 ° C. or higher. Further, it is preferably 1035 ° C. or lower. The tempering temperature is preferably 180 ° C. or higher. Further, it is preferably 540 ° C. or lower. With regard to the above hardness, it is possible to achieve a hardness of 60 HRC or more, 62 HRC or more, and finally 64 HRC or more. The upper limit is not particularly required, but about 66 HRC is realistic.

The structure of the cold tool material of the present invention is, for example, an annealed structure. The annealed structure is a structure obtained by annealing treatment, and is a structure whose hardness is softened to, for example, about 150 to 255 HBW in Brinell hardness. It is preferably 240 HBW or less. The annealed structure contains carbides in which C is bonded to Cr, Mo, W, V and the like, and these carbides include the above-mentioned unsolidified carbides and solid-dissolved carbides.
The cold tool material of the present invention can be mixed and heat-treated in the same batch as general-purpose cold tool materials such as SKD10 and SKD11 by using a conventional quenching furnace at the time of quenching. It is economical, and the lead time required for heat treatment can be suppressed. At this general-purpose quenching temperature, a cold tool with high quenching and tempering hardness can be obtained, and the toughness at that high hardness is also excellent. Occurrence can be suppressed.

Molten steel (melting point: about 1440 ° C., solidification completion temperature: about 1200 ° C.) adjusted to a predetermined component composition was cast to prepare materials 1 and 2 having the component composition shown in Table 1. At this time, the temperature of the molten steel of the materials 1 and 2 was adjusted to 1570 ° C. before pouring into the mold. After pouring the materials 1 and 2 into the mold, the cooling time in the solid-solid phase coexistence region (range of about 240 ° C.) was 168 minutes. In the materials 1 and 2, Cu, Al, Ti, Ca, Mg, and O are not added (however, Al includes the case where Al is added as a deoxidizing agent in the dissolution step), Cu ≦ 0.25%, Al. ≤0.25%, Ti ≤ 0.03%, Ca ≤ 0.01%, Mg ≤ 0.01%, O ≤ 0.01%.

Next, these materials are heated to 1160 ° C. for hot working, and after the hot working, they are allowed to cool, and the steel materials 1 and 2 having the dimensions shown in Table 2 corresponding to the materials 1 and 2 in that order are used. (In Table 2, the length direction thereof is the stretching direction of hot working). In the cooling after the hot working, the material 1 was sufficiently cooled to 170 ° C. Then, these steel materials were annealed at 860 ° C. to produce cold tool materials 1 and 2 corresponding to the steel materials 1 and 2 in that order (hardness 229HBW).

TD cross section of the central part of cold tool materials 1 and 2 parallel to the stretching direction of hot working (that is, the length direction of the material) (for cold tool material 2, from its peripheral surface to the central axis) A cut surface having a cross-sectional area of 15 mm × 15 mm was collected from the TD cross section at a position containing only 4 of the diameter, and the cut surface was mirror-polished using a diamond slurry and parallel silica.
This polished cross-sectional structure was corroded with 10% nital. Then, the cross section after the corrosion was observed with an optical microscope having a magnification of 200 times (field of view area 0.58 mm ² ), and this was photographed in 20 fields. An example of one of these fields of view is shown in FIGS. 2 and 3 in the order of cold tool materials 1 and 2. Then, by image processing this microstructure photograph, carbide A having a circular equivalent diameter of more than 5 μm observed in the cross-sectional structure is extracted, and the area ratio of the carbide A in the cross-sectional structure is averaged for 20 fields. Obtained as a value. For image processing and analysis for obtaining the circle equivalent diameter and area ratio of the charcoal A, the open source image processing software ImageJ (http://imageJ.nih.) Provided by the National Institutes of Health (NIH). gov / ij /) was used. The results are shown in Table 3.

Next, from the above-polished cross-sectional structure, three regions each having a length of 90 μm and a width of 90 μm, which did not contain carbide A having a circle-equivalent diameter of more than 5.0 μm, were extracted. FIG. 1 shows an example of the above-mentioned region of the cold tool material 1 (enclosed portion by a solid line). Then, for each of the above-mentioned regions, the number of carbides B having a circle-equivalent diameter of more than 0.1 μm and 2.0 μm or less and the carbides having a circle-equivalent diameter of more than 0.1 μm and 0.4 μm or less are obtained in accordance with the above-mentioned procedure. The number of C and the ratio of the number of carbide C to the number of carbide B were determined. The same image processing software as for carbide A was also used for image processing and analysis for determining the equivalent circle diameter and the number of carbides B and C. FIG. 4 shows an elemental mapping image of C in the above-mentioned region of the cold tool material 1. The visual field area in FIG. 4 is 30 μm × 30 μm. The field of view is the one in the middle of the 90 μm vertical and 90 μm horizontal regions divided into nine compartments. Then, FIG. 5 shows an image obtained by binarizing the element mapping image of FIG. 4 with the threshold value of the detection intensity of C of 50 counts (cps).

Then, the numbers of carbides A and B obtained in the above 30 μm × 30 μm section in each region are totaled in the extracted three regions to obtain the number of carbides A and B of the cold tool materials 1 and 2. The number density of carbides A and B and the number ratio of carbides A and B were determined from the values of. The results are shown in Table 3. Further, in FIG. 6, the number of carbides (vertical axis) of the above-mentioned cold tool materials 1 and 2 obtained by totaling in the extracted three regions is divided into the range of the equivalent circle diameter of the carbides (horizontal axis). The figure plotted together is shown. The above-mentioned regions extracted with the cold tool materials 1 and 2 did not contain "carbides having a circle equivalent diameter of more than 5.0 μm".

After observing the cross-sectional structure, the cold tool materials 1 and 2 are quenched from 1030 ° C and tempered at 500 to 560 ° C, and have a martensite structure corresponding to the cold tool materials 1 and 2 in that order. Cold tools 1 and 2 were obtained. Then, for each of the cold tools 1 and 2, a Rockwell hardness test (C scale) of the TD cross section was carried out for each tempering temperature. The hardness was measured at 5 points for each sample, and the average value was calculated. As a result, the hardness was 58 HRC or more in all the samples. The maximum hardness of these was 64.6 HRC (tempering temperature 530 ° C.) for the cold tool 1 and 63.9 HRC (tempering temperature 530 ° C.) for the cold tool 2.

Then, a 10R Charpy impact test piece is collected from the cold tools 1 and 2 having the maximum hardness in the direction of the length of the material (that is, the impact is applied in the direction orthogonal to the length of the material). The test was carried out. The impact value was measured at 3 points for each sample, and the average value was calculated. The results of the above hardness test and Charpy impact test are shown in Table 4.
The maximum hardness of the cold tool 1 as an example of the present invention was slightly higher than that of the cold tool 2 as a comparative example, and exceeded 64 HRC. The Charpy impact value of the cold tool 1 at that time was larger than that of the cold tool 2 even though the maximum hardness was higher than that of the cold tool 2.

Claims (3)

By mass%, C: 0.65% or more and less than 0.80%, Si: 0.2 to 0.9%, Mn: 0.1 to 1.5%, P: 0.05% or less, S: 0 0.05% or less, Cr: 5.0 to 7.0%, Mo and W alone or in combination (Mo + 1 / 2W): 2.5 to 4.0%, V: 0.10 to 0.30%, It has a component composition of N: more than 0.03% and 0.08% or less, Ni: 0 to 1.0%, Nb: 0 to 1.5%, and the balance is Fe and impurities.
The area ratio of carbide A having a circle equivalent diameter of more than 5.0 μm in the structure of the cross section is 1.0 to 3.0 area%.
In the region of the structure of the cross section, which does not contain the carbide A and has a length of 90 μm and a width of 90 μm, the number density of the carbides B having a circle equivalent diameter of more than 0.1 μm and 2.0 μm or less is 6.0 × 10 ⁵ pieces / The number density of carbides C of mm ² or more and 9.0 × 10 ⁵ pieces / mm ² or less, and the equivalent circle diameter is more than 0.1 μm and 0.4 μm or less is 5.0 × 10 ⁵ pieces / mm ² or more. 7.5 × 10 Cold tool material characterized by less than ⁵ pieces / mm ² . The cold according to claim 1, wherein the ratio of the number of the carbides C to the number of the carbides B is 75.0% or more in the region of 90 μm in length and 90 μm in width which does not contain the carbide A. Tool material. The cold tool material according to claim 1 or 2 is characterized by quenching at a quenching temperature of 1000 to 1050 ° C. and tempering at a tempering temperature of 150 to 600 ° C. to adjust the hardness to 58 HRC or more. How to make a cold tool.

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