JP6032582B2 - Manufacturing method of steel material for mold - Google Patents

Description

  The present invention relates to a die for cold working used for molding of related parts such as home appliances, mobile phones, automobiles, etc., and a method for producing the pre-hardened steel material for the die, and a method for producing the die steel material. Is. And it is related with the manufacturing method of the metal mold | die for cold processing.

  For cold working molds used for press forming such as bending, drawing, and punching of plate materials, a steel material corresponding to the overall dimensions is prepared in advance, and then machining such as drilling and cutting is performed on this steel material. Then, it is arranged in the shape of the mold. In addition, the cold working mold is adjusted to a desired hardness by quenching and tempering treatment at the time of mold production in order to provide wear resistance during use. With the recent increase in strength of the material to be molded, the quenching and tempering hardness of the mold is required to have a high hardness of 58 HRC or more, and further 60 HRC or more.

  When manufacturing a high-hardness mold having a hardness of 58 HRC or more, it is not easy to machine the steel material adjusted to the high hardness into a mold shape. Therefore, normally, the manufacturing process of a high-hardness cold-working mold is performed by roughly machining the annealed steel material with low hardness to a shape close to the final mold shape, and then performing quenching and tempering treatment. ing. Then, after the quenching and tempering process, finishing machining such as correction of deformation caused by the process is performed to prepare a final mold shape (Patent Documents 1 to 3).

  The above steel materials used in the manufacture of molds are adjusted to just the right size corresponding to the mold dimensions in order to facilitate handling and reduce the machining allowance in the next cutting process. Provided. And the adjustment to the just right size is performed exclusively by cutting. In other words, large steel pieces such as slabs, blooms, billets, etc. obtained by performing hot rolling such as split rolling and forging on steel ingots are used as steel materials. It is the process of cutting into steel materials. And since the steel raw material (namely, steel slab) after hot processing is in the annealing state with the low hardness before quenching and tempering normally, said cutting | disconnection is easy.

  By the way, in the manufacturing process of the above-described cold working mold, recently, the use of pre-hardened steel has been increased in order to shorten the man-hours (Patent Documents 4 to 10). The pre-hardened steel material is a steel material that has been previously tempered and tempered to a desired hardness, and is a steel material that has improved machinability such as drilling and cutting. If pre-hardened steel is used, it is not necessary to quench and temper after it is machined all the way to the final mold shape, so finishing machining can be omitted.

  The pre-hardened steel material is also obtained by cutting a steel material in the same manner as the steel material supplied in the annealed state. Quenching and tempering for pre-hardening is performed on the entire steel material at the time of the steel material before cutting, in addition to cutting the steel material (steel piece) in an annealed state into individual steel materials. It has been proposed to keep (Patent Documents 11 and 12). In this case, the steel material that has been pre-hardened as a whole is cut into individual steel materials. This pre-hardened steel material is all hardened and tempered all at once, so by cutting it into individual steel materials, for example, a plurality of steel materials with little variation in mechanical properties can be obtained once. Can get to. In addition, it is not necessary to perform quenching and tempering on the mold maker side, which receives the supply of steel material from a steel material manufacturer, cuts this steel material into steel material, and machines it to produce a mold. This eliminates the need for equipment and labor.

JP 2009-132990 A JP 2006-193790 A JP 2002-012952 A International Publication No. 2012/043228 Pamphlet International Publication No. 2012/115024 Pamphlet International Publication No. 2012/115025 Pamphlet JP 2008-189982 A JP 2001-316769 A JP 2005-272899 A JP 2001-107181 A JP 2006-150487 A JP 2001-129722 A

When cutting a pre-hardened steel material in advance, if the steel material has a high hardness, cutting is not easy. In relation to this cutting, a technique has been proposed that makes it easy to cut a pre-hardened steel material for molds by improving the saw blade of the cutting tool to provide durability (Patent Document). 11, 12). However, even with this method, the hardness of the pre-hardened steel that can be cut is at most about 35 HRC.
On the other hand, for cold working mold applications, many pre-hardened steel materials have been proposed that can achieve high hardness of 58 HRC or higher, and further 60 HRC or higher by quenching and tempering. However, when these high-hardness pre-hardened steel materials are used, a cold-working die manufacturing method in which a steel material is cut after being adjusted to a high hardness (58 HRC or more) and then machined to obtain a die is used. It was not done.

  An object of the present invention is to provide a steel material excellent in cutability at a high hardness of 58 HRC or higher and a method for producing the same. Then, by establishing such a steel material, a method for producing pre-hardened steel directly from this pre-hardened steel material, and a cold working gold for obtaining this pre-hardened steel material and machining it. It is to provide a mold manufacturing method.

  This inventor investigated the factor which affects the cutting property about the various steel materials which can achieve high hardness of 58HRC or more by quenching and tempering. As a result, it has been found that, in various alloy design techniques that have been used for imparting a high hardness of 58 HRC or higher, especially the technique of imparting carbide in the structure deteriorates the cutting ability. And after achieving high hardness of 58HRC or higher, cutting can be performed without special improvement on conventional cutting conditions such as various types of saw blades as long as it is a steel material with a specific amount of carbide regulated. As a result, the present invention has been reached.

That is, the present invention is a steel material used for manufacturing a cold working mold having a hardness of 58 HRC or higher, and is cut into a pre-hardened steel material having a size corresponding to the dimensions of the cold working mold. Steel material used,
The steel material has a hardened and tempered structure, has a hardness of 58 HRC or more, and a primary carbide having an equivalent circle diameter of 5 μm or more in a cross-sectional structure is 2 area% or less. Steel material. The steel material preferably has a hardness of 60 HRC or more.

And this invention is a manufacturing method of the steel raw material used for manufacture of the metal mold | die for cold work which has the hardness of 58HRC or more, The prehardened steel material of the magnitude | size corresponding to the dimension of the said metal mold | die for cold work A method of manufacturing a steel material that is used after being cut into
A first step of hot working a steel ingot to obtain a steel piece;
The steel piece obtained in the first step is quenched and tempered to obtain a steel material having a hardness of 58 HRC or more and a primary carbide of 5 μm or more in a cross-sectional structure with an equivalent circle diameter of 2 μm or less. A second step;
It is a manufacturing method of the steel raw material for metal mold | dies characterized by including.

Further, the present invention is a method for producing a pre-hardened steel material used for producing a cold working mold having a hardness of 58 HRC or more,
A first step of hot working a steel ingot to obtain a steel piece;
The steel piece obtained in the first step is quenched and tempered to obtain a steel material having a hardness of 58 HRC or more and a primary carbide of 5 μm or more in a cross-sectional structure with an equivalent circle diameter of 2 μm or less. A second step;
A third step of cutting the steel material obtained in the second step to obtain a pre-hardened steel material having a size corresponding to the dimensions of the cold working mold;
It is a manufacturing method of the prehardened steel materials for metal mold | dies characterized by including.

Furthermore, this invention is a manufacturing method of the metal mold | die for cold processing which has the hardness of 58HRC or more,
A first step of hot working a steel ingot to obtain a steel piece;
The steel piece obtained in the first step is quenched and tempered to obtain a steel material having a hardness of 58 HRC or more and a primary carbide of 5 μm or more in a cross-sectional structure with an equivalent circle diameter of 2 μm or less. A second step;
A third step of cutting the steel material obtained in the second step to obtain a pre-hardened steel material having a size corresponding to the dimensions of the cold working mold;
A fourth step of machining the pre-hardened steel material obtained in the third step into a mold shape to obtain a cold working die; and
It is a manufacturing method of the metal mold | die for cold processing characterized by including these.

  In the present invention, the hardness of the steel material in the second step is preferably 60 HRC or more. Moreover, quenching to the steel slab performed in the second step can be performed after annealing the steel slab obtained by hot working the steel ingot in the first step. Alternatively, the quenching to the steel slab performed in the second step may be a direct quenching performed after the steel slab obtained by hot working the steel ingot in the first step.

  In addition to providing the first step to the fourth step, the present invention further includes a fifth step of performing a surface treatment on the surface of the cold working mold obtained in the fourth step, Can be provided.

  According to the present invention, by cutting a steel material from a steel material pre-hardened to a high hardness of 58 HRC or higher in advance, for example, a plurality of pre-hardened steel materials with little variation in mechanical properties can be obtained at a time. Moreover, by using these pre-hardened steel materials, quenching and tempering can be omitted for a mold obtained by machining the pre-hardened steel materials. Therefore, it is possible to stabilize the mechanical characteristics of the cold working mold having a high hardness of 58 HRC or higher and improve the overall manufacturing efficiency.

In Example 1, test piece No. which is a steel material before cutting according to the present invention example and the comparative example. It is a micro structure photograph which shows an example of the primary carbide distributed in 1-4 cross-sectional structures. In Example 1, the test piece No. It is a drawing substitute photograph which shows an example of the flank of a saw blade after cutting 1-4. In Example 2, test piece No. which is a steel material before cutting according to the present invention example. It is a micro structure photograph which shows an example of the primary carbide distributed in the cross-sectional structure of 5-A and 5-B. In Example 2, the test piece No. It is a drawing substitute photograph which shows an example of the flank of the saw blade after cut | disconnecting 5-A and 5-B. In Examples 1 and 2, the test piece No. It is a graph which shows the area of the abrasion which arose on the flank of the saw blade after cutting 1-5 (A, B).

  The feature of the present invention is that it has been found that a direct factor that deteriorates the cutting property of the steel material is a specific carbide in the structure. And by optimally adjusting the distribution state related to the specific carbide, even a high hardness steel material of 58HRC or higher can be easily cut, and a cold working die using pre-hardened steel The overall production efficiency can be improved. Hereinafter, the manufacturing method from the steel material of the present invention to the die for cold working through the pre-hardened steel material for the die will be described.

(1) 1st process: A steel ingot is hot-worked and a steel piece is obtained.
The first step is a step of hot working the ingot for the purpose of improving the cast structure of the ingot. And a 1st process can follow the conventional method etc. For example, in addition to the ordinary ingot method using an ingot case, methods for obtaining steel ingots include continuous casting methods, vacuum arc remelting methods and electroslag remelting methods that are performed once on steel ingots. Regardless of the method. In the hot working, the steel ingot is adjusted to the shape of a steel slab such as a slab, a bloom, or a billet by partial rolling or forging. Note that the steel ingot or steel slab may be subjected to a soaking treatment (soaking treatment) held at a constant temperature and time as described below, for example, as described later.

(2) Second step: The steel slab obtained in the first step is quenched and tempered, the hardness is 58 HRC or more, and the amount of primary carbide in the cross-sectional structure is 5 μm or more with a circle equivalent diameter of 2 areas. % Steel or less is obtained.
The second step was pre-adjusted to the desired mold hardness by pre-hardening the steel piece before cutting the steel piece obtained in the first step in the third step described later. This is a process for obtaining a steel material. And it is an important process for the present invention to impart excellent cutting properties to this steel material.

  As described above, there is a need for a pre-hardened steel material that can achieve a high hardness of 58 HRC or higher, or even 60 HRC or higher by quenching and tempering in cold working mold applications. And in order to obtain this prehardened steel material, when the conventional steel material adjusted to 58HRC or more was cut | disconnected, wear, such as a wear and a chip | tip, progressed at the blade part of the cutting tool, and cutting resistance was also large. When the blade portion of the cutting tool is worn out, the cutting ability inherent to the cutting tool is lost, and a “bending” occurs in which the cutting line after cutting is bent. And if this bend becomes excessively large (generally, when it exceeds 1 mm with respect to the desired cutting line), the flatness of the cut surface is impaired, and additional and significant shape correction is necessary in the subsequent process. This can reduce the productivity of the mold. Furthermore, if the cutting resistance is significantly increased, an excessive load is applied to the motor of the cutting machine. As a result, if the cutting is interrupted, an extra man-hour is required for the maintenance of the cutting machine, which significantly impairs the productivity of the mold.

  Then, this inventor investigated the deterioration factor of the cutting property about the steel raw material adjusted to 58HRC or more. As a result, it has been found that the direct factor that deteriorates the cutting performance is not the hardness value itself of 58 HRC or higher, but coarse primary carbides distributed in the structure after quenching and tempering. It was. That is, the primary carbide in the steel material has a hardness comparable to the cemented carbide or hard coating that forms the blade part of the cutting tool that cuts the steel material, and thus directly affects the wear of the blade part during cutting. . Among primary carbides, coarse primary carbides with a size as large as several tens of microns that can be confirmed by observation with an optical microscope have a particularly large degree of wear on the blade. Therefore, in the steel material of the present invention, cutting ability can be improved even if the steel material has a hardness of 58 HRC or more by reducing the area amount of coarse primary carbides in the structure. And specifically, in the cross-sectional structure of a steel material having a hardness of 58 HRC or more, if the primary carbide of the equivalent circle diameter of 5 μm or more is 2 area% or less, the increase in bending and cutting resistance can be suppressed, Cutting property can be improved. Preferably it is 1.5 area% or less, More preferably, it is 1 area% or less. The primary carbide having an equivalent circle diameter of 5 μm or more has a particularly large degree of wear on the blade. And when the area ratio of the primary carbide of this equivalent circle diameter exceeds 2 area%, the wear which progresses to the blade part of a cutting tool will become remarkable.

  Since the steel material of the present invention is excellent in cutability, the size is not restricted at the time of the steel material before cutting. Therefore, the steel material according to the present invention has a size that can cut out a steel material having a thickness of 300 mm and a width of 700 mm so that it can be applied to a large steel material for press dies for forming automobile-related parts and the like. Can be made large.

  The area ratio of the primary carbide of the steel material of the present invention can be obtained by, for example, quenching performed on the steel piece obtained in the first step, solution heat treatment performed as necessary, or the like. By the solution heat treatment, coarse primary carbides can be dissolved in the matrix. Moreover, in the said 1st process, it can obtain by implementing the said soaking process etc. to the steel ingot before hot processing, or the steel ingot in the middle of hot processing. And the component composition of the steel material of the present invention (that is, the pre-hardened steel material obtained by cutting this) in which coarse primary carbides are reduced is conventional in that the hardness after quenching and tempering can be maintained at 58 HRC or higher. The proposed cold die steel can be applied. And in this invention, it is preferable to adjust to the following.

  C is an important element that provides a hardness of 58 HRC or higher to the cold working mold by forming a solid solution with steel and forming a carbide and a carbide forming element in the steel. However, if contained excessively, coarse primary carbides increase in the structure, and the cutting performance of the steel material decreases. Moreover, when the surface coating process by PVD (physical vapor deposition) is performed on the surface of the cold working mold, the surface coating processability is lowered. Accordingly, the C content is preferably 0.6% by mass or more, and preferably 1.2% by mass or less.

Cr imparts hardness to the cold working mold by forming C and M ₇ C ₃ carbide which is a primary carbide. In order to achieve a high hardness of 58 HRC or higher, addition of 3.0% by mass or more is preferable. However, if added in excess, the amount of coarse primary carbide increases and the cutting performance of the steel material decreases, so 9.0 mass% or less is preferable. More preferably, it is 7.0 mass% or less, More preferably, it is 5.0 mass% or less.

V and Nb are effective elements for forming MC carbide, which is a primary carbide, in the structure after quenching and tempering, and achieving a hardness of a cold working die of 58 HRC or higher. However, MC carbide is very hard among primary carbides. Therefore, when V and Nb are contained excessively, a lot of MC carbides are formed, and the cutability of the steel material is significantly lowered. V and Nb have the same effect in the above points, but the degree of the effect is approximately half that of Nb in the same content. Therefore, these contents can be comprehensively handled in the relationship of (Nb + 1 / 2V). And it is preferable to contain 1.0 mass% or less of 1 type or 2 types of V and Nb by the relational expression of (Nb + 1 / 2V). More preferably, it is 0.8 mass% or less.
With the above, the component composition of the steel material (pre-hardened steel material) of the present invention is, in mass%, C: 0.6-1.2%, Cr: 3.0-9.0%, and selectively It is preferable that the component composition of the cold die steel is based on containing one or two of V and Nb in a relational expression of (Nb + 1 / 2V) of 1.0% by mass or less.

  In addition, Si, Mn, Mo, W, etc. may be added to the steel material (pre-hardened steel material) of the present invention. Mn is an element effective for providing a hardenability by dissolving in steel. Si is an element effective for solidifying and imparting hardness in steel. Mo and W are effective elements for forming fine carbides and imparting tempering hardness. Furthermore, Al, S, Ni, Cu etc. can also be added to the steel raw material (prehardened steel material) of this invention. Al and S contribute to improvement of machinability when machining a pre-hardened steel material adjusted to 58 HRC or higher in the fourth step described later. Further, Al, Ni, and Cu contribute to improvement of the hardness and toughness of the cold working mold. Further, an appropriate amount of Ca, Ti, Zr, rare earth metal, etc. can be added for fine dispersion of the primary carbide. For example, the component composition of cold tool steel according to Patent Documents 5 and 6 can be applied to the component composition of the steel material (pre-hardened steel material) of the present invention.

  Further, the steel material of the present invention does not need to go through an annealed state that is easy to cut because of its low hardness. Therefore, in the quenching to the steel slab carried out in the second step, in addition to performing after annealing the steel slab obtained by hot working the steel ingot in the first step, “Direct quenching” can be applied to a steel piece obtained by hot working a steel ingot. And after quenching, by performing tempering, a steel material having a high hardness of 58 HRC or higher can be produced, so that the annealing step performed on the steel pieces after hot working is usually omitted. it can.

(3) Third step: The steel material obtained in the second step is cut to obtain a pre-hardened steel material having a size corresponding to the dimensions of the cold working mold.
No special device is required for cutting the steel material in the third step. According to a conventional cutting method or the like, the steel material may be cut into a pre-hardened steel material having a size suitable for each request by using a saw blade such as a band saw or a circular saw. Usually, a hard oxide film is formed on the surface of the steel piece after hot working. However, the influence of this oxide film on the cutting property of the steel material is small. Therefore, in the present invention, when cutting the steel material, it is not necessary to remove the entire oxide film except for the purpose of accurately positioning the steel material with respect to the cutting machine. And since it is not necessary to remove before hardening at the said 2nd process, the said direct hardening can be applied to hardening.

(4) Fourth step: The pre-hardened steel material obtained in the third step is machined into the shape of a die to obtain a cold working die.
At the time of “pre-hardened steel material” after being cut into the dimensions of individual cold-working molds, it is not necessary to handle the pre-hardened steel material according to the present invention separately from the conventional pre-hardened steel material. Therefore, after the fourth step, the pre-hardened steel may be finished in the shape of a cold working mold by conventional machining or the like. By using pre-hardened steel as the mold steel, after finishing it to the final mold shape, there is no fear of heat treatment deformation due to quenching and tempering, so finish machining is omitted. Can do.

(5) Fifth step: If necessary, surface treatment is performed on the surface of the cold working mold obtained in the fourth step.
Also in this regard, at the time of “cold working mold” after finishing in a desired shape, it is necessary to handle the cold working mold according to the present invention separately from the conventional cold working mold. Absent. Therefore, if necessary, various methods such as surface hardening treatment for forming a nitrided layer or an oxide layer on the surface of the cold working mold by conventional surface treatment methods, PVD, CVD (chemical vapor deposition), etc. A surface coating treatment or the like for forming a hard film, a lubricating film, or the like may be performed.

  Steel ingot No. 1 having the component composition of Table 1. 1-4 were produced.

  Next, hot forging with a forging ratio of about 10 was performed on these steel ingots to obtain steel pieces. And the steel slab was cooled and then annealed at 860 ° C. And the test piece (namely, steel material before a cutting | disconnection) No. for evaluating cutting property from the steel piece after annealing. 1-4 were produced. The test piece had a length of 500 mm (in the extending direction by the forging), a width of 300 mm, and a thickness of 75 to 150 mm. As for the surface of the test piece, four surfaces except two surfaces of 500 mm × 300 mm were ground to remove the oxide film on the four surfaces. And after the said grinding, the quenching process by air cooling from 1030 degreeC and the tempering process twice at 500-540 degreeC were implemented, and it adjusted to the target hardness of 60 HRC.

  And test piece No. Primary carbides distributed in 1-4 structures were evaluated. First, a 15 mm × 15 mm cross section parallel to the length direction (stretching direction) of the test piece was specified, and this cross section was polished into a mirror surface using diamond slurry. Next, the cross section was corroded using 10% nital so that the boundary between the primary carbide and the base when the cross sectional structure was observed became clear. Then, the cross section after the corrosion was observed with an optical microscope having a magnification of 200 times, and 20 fields of view of a field of 877 μm × 661 μm were photographed. FIG. It is an example of the structure | tissue photograph which image | photographed each cross section of 1-4 (a primary carbide is shown by white distribution). Then, by processing this tissue photograph, primary carbides having an equivalent circle diameter of 5 μm or more observed in the cross-sectional structure are extracted, and the area ratio of the primary carbides in the cross-sectional structure is an average value for 20 fields of view. As sought.

And the cutting test by a saw blade was implemented with respect to the test piece after measuring the distribution condition of a primary carbide, and the cutting property was evaluated. As a cutting machine, a saw cutting machine PCSAW530AX manufactured by Amada Co., Ltd. was used. As the saw blade, a band saw blade AXCELA G-NBN3N made of the same coated cemented carbide was used. Cutting conditions were a cutting rate of 16 cm ² / min, a sawing speed of 50 m / min, and a feed of 5 mm / min.

The evaluation of cutting property was performed according to the following procedure. First, with an unused saw blade set in a saw cutter, the appearance of the saw blade was observed with a microscope, and it was confirmed that the saw blade was free of chipping. Next, as a leveling operation, an annealed material (hardness 20 HRC) of JIS-SKD11 equivalent steel was cut until the cumulative cutting area reached 2000 cm ² . The cutting conditions were the same as above. Then, the appearance of the saw blade after the leveling operation was observed again, and it was confirmed that the saw blade had almost no wear or chipping, and then the above test piece was subjected to a cutting test. In addition, this was implemented for every test piece. In the cutting test, continuous cutting was performed until the cumulative cutting area reached 4500 cm ² , and the bending was measured every time the cumulative cutting area reached 300 cm ² . Here, the bending is the amount of gap between the straight line being cut and the saw blade (that is, the actual cutting line) with respect to the straight line that connects both ends of the entire length of the saw (that is, the desired cutting line) (the deflection of the saw blade). Amount). Therefore, in the present Example, the measurement of the said bending was replaced with the measurement of the said gap | interval amount during cutting | disconnection, and measured the maximum and the average gap | interval amount (bending). The maximum gap amount is a gap amount having the largest value among a plurality of maximum gap amounts obtained by measurement every time the cumulative cutting area reaches 300 cm ² . The average gap amount is a gap amount obtained by averaging the plurality of maximum gap amounts. The measurement results of the bending are shown in Table 2 together with the hardness of the test piece and the area ratio of the primary carbide.

Test piece No. which is an example of the present invention. In Nos. 1 and 2, primary carbides having an equivalent circle diameter of 5 μm or more were 2 area% or less in the structure after quenching and tempering. And in the state of high hardness of 58HRC or more, the bending at the time of cutting was suppressed to less than 1 mm even at the maximum value, and a cumulative cutting area of 4500 cm ² was achieved, showing good cutting properties. On the other hand, test piece No. which is a comparative example. In No. 3, the area ratio of the primary carbide exceeded 2 area%. In a state of high hardness of 58 HRC or higher, continuous cutting was possible until the cumulative cutting area reached 4500 cm ² , but the bending at the time of cutting was the maximum, and the average value was 1 mm or more. Furthermore, test piece No. which is a comparative example. In No. 4, the area ratio of the primary carbide reached 7 area% or more. At the initial stage of cutting, an excessive load is applied to the motor of the cutting machine due to an increase in cutting resistance. When the cumulative cutting area reaches 480 cm ² , the cutting machine stops, and cutting is virtually impossible. there were.

  2 is a photograph of one of the flank faces after the cutting test of the saw blade used for cutting each test piece observed with a digital microscope (however, for test piece No. 4, the above cutting is performed). This is the flank of the saw blade when the machine stops). FIG. 2 also shows the flank face of the saw blade after the leveling operation. Specimen No. The flank face of the saw blade after cutting 1 and 2 is the test piece no. Compared with that of No. 3, there was less wear. And test piece No. The flank of the saw blade after cutting 4 had already suffered significant wear when the cutting test was interrupted. Also, part of the saw blade was missing and lost.

  Steel ingot Nos. Having the composition of Table 3 (that is, the composition of steel ingot No. 3 in Table 1). 5 was produced.

  Next, the steel ingot No. 5 was subjected to hot forging with a forging ratio of about 10 to obtain a steel slab. And the steel slab was cooled and then annealed at 860 ° C. And the test piece (namely, steel material before a cutting | disconnection) No. for evaluating cutting property from the steel piece after annealing. 5-A and 5-B were produced. The dimensions of the test piece were 250 mm in length (in the extending direction by forging), 300 mm in width, and 150 mm in thickness. Next, test piece No. 5-A was subjected to a solution heat treatment that was maintained at 1170 ° C. for 10 hours. 5-B was subjected to a solid solution heat treatment at 1170 ° C. for 5 hours. Then, in the test piece after the solution heat treatment, four surfaces of the surface except for two surfaces of 250 mm × 300 mm were ground to remove the oxide film on the four surfaces. And after the said grinding, the quenching process by air cooling from 1030 degreeC and the tempering process twice at 500-540 degreeC were implemented, and it adjusted to the target hardness of 60 HRC.

  Specimen No. adjusted to the hardness Primary carbides distributed in the 5-A and 5-B tissues were evaluated. The procedure for tissue observation is the same as in the first embodiment. FIG. It is an example of the structure | tissue photograph which image | photographed each cross section of 5-A and 5-B (a primary carbide is shown by white distribution). Then, by processing this tissue photograph, primary carbides having an equivalent circle diameter of 5 μm or more observed in the cross-sectional structure are extracted, and the area ratio of the primary carbides in the cross-sectional structure is an average value for 20 fields of view. As sought.

  And the cutting test by a saw blade was implemented with respect to the test piece after measuring the distribution condition of a primary carbide, and the cutting property was evaluated. The conditions for the cutting test and the evaluation procedure for the cutting ability were the same as those in Example 1. Table 4 shows the measurement results of the bending when the predetermined cumulative cutting area is reached, together with the hardness of the test piece and the area ratio of the primary carbide.

Test piece No. which is an example of the present invention. In 5-A and 5-B, primary carbides having an equivalent circle diameter of 5 μm or more were 2 area% or less in the structure after quenching and tempering. In a high hardness state of 58HRC or higher, the bending at the time of cutting is suppressed to an average value of less than 1 mm, the maximum value is suppressed to about 1 mm, and the cumulative cutting area is also 3600 cm ² or more. As a result, good cutting properties were shown. The test piece No. In 5-B, since the area ratio of the primary carbide of 5 μm or more was high, the cutting test was finished when the cumulative cutting area reached 3600 cm ² in consideration of the increase in cutting resistance. And the bending at the time of cutting until then has remained at 0.25 mm even at the maximum value. Here, test piece No. which is a comparative example performed in Example 1. As for the cutting result of No. ³ , when the cumulative cutting area reached 3600 cm ² , the average bending was 1.06 mm and the maximum was 1.57 mm. In comparison with this, test piece No. Even if it was 5-B, it has confirmed that it had favorable cutting property.

FIG. It is the photograph which observed one of the flank after the end of the cutting test about the saw blade used for the cutting of 5-A and 5-B with a digital microscope. FIG. 4 also shows the flank face of the saw blade after the leveling operation. Specimen No. The flank of the saw blade after cutting 5-A and 5-B was less worn. And when it compares between the said flank when it cut | disconnects until it reaches the same cumulative cutting area of 4500 cm < ² >, test piece No.1. The flank face of the saw blade after cutting 5-A is the test piece No. 1 in Example 1. There was much wear from the said flank of 1 and 2 (invention example). However, specimen no. There was little abrasion compared with the said flank of 3 (comparative example).

  FIG. 5 is a graph showing the average wear area generated per flank for the saw blade after cutting each test piece in Examples 1 and 2. FIG. In addition, in FIG. 1, 2, 3, 4, 5-A, and 5-B, the flank face of the saw blade cut, and the flank face 1, 2, 3, 4, 5-A, 5-B. Yes. The wear area of the flank face of the saw blade tended to decrease as the amount of primary carbide in the cut specimen structure decreased. From the above results, it was recognized that the degree of bending and saw blade wear generated in the steel material after cutting correlated with the area ratio of coarse primary carbides in the cut steel material.

Claims (5)

A method of manufacturing a steel material used for manufacturing a cold working mold having a hardness of 58 HRC or more, which is used after being cut into a pre-hardened steel material having a size corresponding to the dimensions of the cold working mold. A method of manufacturing a steel material,
A first step of hot working a steel ingot to obtain a steel piece;
An oxide film is formed on the surface of the steel piece obtained in the first step, the steel piece where at least a part of the oxide film remains is quenched and tempered, and the hardness is 58 HRC or more, A second step of obtaining a steel material in which the primary carbide in the cross-sectional structure has a circle equivalent diameter of 5 μm or more and 2% by area or less;
A method for producing a steel material for a mold, comprising: The method for producing a steel material for a mold according to claim 1 , wherein the steel material has a size capable of cutting a pre-hardened steel material having a thickness of 300 mm and a width of 700 mm. The method of manufacturing a steel material for molds according to claim 1 or 2 , wherein the hardness of the steel material in the second step is 60 HRC or more. Quenching to the steel strip implemented in the second step, of claims 1 to 3 which comprises carrying out the steel ingot in the first step after performing annealing steel strip obtained by hot working The manufacturing method of the steel raw material for molds in any one. The quenching of the steel strip implemented in the second step, claims 1, characterized in that the steel ingot is directly quenched to implement following the steel piece obtained by hot working in the first step 3 The manufacturing method of the steel raw material for metal mold | dies in any one of.

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