JP6484086B2 - Method for producing tool steel castings - Google Patents

Description

  The present invention relates to a method for producing a tool steel cast steel product that can be used for cold tools, press die materials, hot press die materials, forging die materials, and the like.

  For conventional press dies, SKD11 is used as cold tool steel. In general, SKD11 is quenched to about HRC 60 ± 2 and decomposes and disperses carbides by forging, so that weldability is good, and cracks are less likely to occur during welding. Moreover, it does not crack during quenching.

  On the other hand, for example, as a prior art of cast steel for cold tool steel, “Cast Cast Cold Tool and its Manufacturing Method” described in Patent Document 1 is known. In this patent document 1, the component structure which contains Nb, W, and Ni in the component of SKD11 base is employ | adopted. By producing a cold tool produced by forging by casting, it is possible to produce a complex shape product that cannot be produced by forging and improve the yield during processing.

  When trying to manufacture cold tool steel with a cast product, as shown in FIG. 9 (a), carbide is dispersed in the forged product, but as shown in FIG. 9 (b), the cast product contains carbide. It will crystallize in a network. The difference between the cast product and the forged product is not only the degree of dispersion of the carbide but also the problem of the difference in cracking at the time of quenching and welding. Unlike forged products, cast steel products cannot completely remove the traces of pro-eutectoid ferrite and pro-eutectoid cementite crystallized at grain boundaries by heat treatment. For this reason, unless pearlite is granulated more completely, cracking during quenching or welding cannot be prevented. For this reason, it is necessary to develop a heat treatment method suitable for this component that can completely granulate pearlite.

  As shown in FIG. 10, the most common conventional heat treatment method is a solution treatment for the purpose of preventing elements from agglomerating again at grain boundaries by raising the temperature to about 950 ° C. and rapidly cooling it. Then, for the purpose of softening and forming a fine structure, complete annealing is performed at around 850 ° C., and spheroidizing annealing is performed to spheroidize pearlite between 760 and 700 ° C. Thus, in the conventional general heat treatment, the heat treatment temperature is as high as 950 ° C., and the total heat treatment time is as long as about 44 hours.

JP-A-10-273756

  In view of such a conventional situation, the present invention provides a cold tool, a material for a press die, a material for a hot press die, a forging which enables a short delivery time without performing a solid solution treatment at a high temperature for a long time. It aims at providing the manufacturing method of the tool steel cast steel goods which can be used for a mold raw material.

  In order to achieve the above object, the characteristic configuration of the method for producing a tool steel cast steel product according to the present invention is, in mass%, C: 0.5 to 1.0%, Si: 0.2 to 1.2%, Mn : 0.25 to 1.50%, Cr: 4.0 to 8.0%, Mo: 0.8 to 2.0%, V: 0.2 to 1.0%, the balance being Fe and impurities A cast steel product made of is heated and held at 840 ° C. to 880 ° C. for 1 to 3 hours, and then subjected to a heat treatment for cooling at −15 to −40 ° C./h until reaching an arbitrary cooling temperature of 500 ° C. to 720 ° C. Thus, the base structure is granular pearlite, the free carbide is 0.1% to 5% in area ratio, and the hardness after heat treatment is HB180 to HB250.

  Here, “heating and holding for 1 to 3 hours” means “heating and holding so that the whole becomes a uniform temperature”. According to the inventor's experiment, the whole is brought to a uniform temperature by “heating and holding at 840 ° C. to 880 ° C. for 1 to 3 hours”. Thereafter, the base structure is granular pearlite and the free carbide is 0.1% in area ratio by performing a heat treatment at -15 to −40 ° C./h to reach an arbitrary cooling temperature of 500 ° C. to 720 ° C. A tool steel cast steel product having a hardness of ˜5% and a hardness after heat treatment of HB180˜HB250 was obtained. In this tool steel cast steel product, the amount of pro-eutectoid ferrite in the as-cast base was 0.1 to 10% or less, or the amount of pro-eutectoid carbide was 0.1 to 5% or less.

  In addition to the above characteristics, it may have a hardness of HRC56 or higher by further quenching or quenching and tempering at 980 to 1050 ° C. Furthermore, the hardness of the secondary curing due to generation of secondary carbide at 500 to 550 ° C. may be HRC 55 or more. As a case where only quenching is performed, for example, there is a case where tempering is not required by quenching with frame hardware.

According to the configuration of the present invention, an area ratio in which the amount of pro-eutectoid ferrite in an as-cast base is 10% or less or the amount of pro-eutectoid carbide is 5% or less without performing a solution treatment for a long time at a high temperature. And was able to. The base structure is granular pearlite, the free carbide is 0.1% to 5% in area ratio, and the hardness after heat treatment can be HB180 to HB250. It has come to be able to provide a method for producing a tool steel cast steel product that can be used as an interstitial tool, a press die material, a hot press die material, a forging die material and the like.

  Other objects, configurations, and effects of the present invention will become apparent from the following embodiments of the present invention.

It is a state figure showing the relationship of the structure | tissue in C amount and eutectoid transformation. The structure photograph of the sample in Table 1 is shown, (a) C amount 0.4%, (b) C amount 0.6%, (c) C amount 0.8%, (d) C amount 1 0.0%, (e) shows a structural photograph with a C content of 1.2%. It is a figure which shows the relationship between tempering temperature and hardness. It is a figure which shows the relationship between quenching temperature and hardness. It is a figure which shows the relationship between Cr * Mo, quenching hardness, and the presence or absence of secondary hardening. It is a figure which shows the heat processing cycle of this invention steel. It is a figure which shows the relationship between heat processing temperature and a mechanical property. It is a figure which shows the relationship between a cooling rate and a mechanical property. It is a carbide | carbonized_material shape comparison (SKD11 component) of a forged product and a cast product, (a) shows the structure photograph of SKD11 (forged product), (b) shows SKD11 equivalent material (cast product). It is a figure which shows the example of the conventional heat processing cycle.

Next, the present invention will be described in more detail with reference to the accompanying drawings as appropriate.
The inventors investigated how the carbide changes when the amount of C is changed. Considering from the phase diagram shown in FIG. 1, when the amount of C is less than the eutectoid point (C 0.76%), proeutectoid ferrite precipitates at the austenite grain boundary, and fracture proceeds along this ferrite grain boundary. High toughness but low strength and low quenching hardness. In addition, cracks occur during quenching and welding. On the other hand, when the amount of C is large, pro-eutectoid carbide precipitates at the grain boundaries, and strength is high but toughness is low.

  Here, Table 1 shows the relationship among the amount of carbide, hardness, quenching hardness, and quench cracking when C is varied in the range of 0.4 to 1.2%. Moreover, the structure photograph of each sample of Table 1 is shown in FIG. From Table 1, when C is less than 0.5%, the quenching hardness becomes HRC55 or less. In addition, some Woodman Stetten-like ferrite is observed inside the structure (FIG. 2 (a)). On the contrary, when C is 1.0%, Woodman Stetten-like carbides are seen, but there are no cracks and the carbides are not connected (FIG. 2 (d)). When C is 1.2%, pro-eutectoid cementite becomes continuous and fire cracking occurs (FIG. 2 (e)). In addition, when C is 0.6% and 0.8%, the area ratio of free carbide is 0.2% to 0.8%, no Woodman Stetten-like carbide is observed, there are no cracks, and the carbide They are not connected (FIGS. 2B and 2C). As described above, the C content is set to 0.5 to 1.0% from the viewpoints of quenching hardness, structure, and quench cracking. The area ratio (%) of the free carbide is calculated as carbide area / total area × 100 on the tissue photograph using a microscope, a CCD camera, and commercially available area ratio measurement software.

  FIG. 3 shows the relationship between the tempering temperature and the hardness when the C content of the product of the present invention is changed. When the tempering temperature of the steel of the present invention exceeds 500 ° C., secondary carbide precipitates and the hardness is recovered. This is called secondary curing, but if the hardness decreases without generating secondary curing at around 550 ° C., the wear resistance decreases and “galling” or the like occurs. When C is less than 0.5%, a tempering hardness of 500 to 550 ° C. becomes less than HRC55. From the relationship between the amount of carbide and the hardness at the time of precipitation of secondary carbide at a tempering temperature of 500 to 550 ° C., the C% range was set to 0.5 to 1.0%.

  The inventors investigated the quenching and tempering temperature and hardness of the product of the present invention. As shown in FIG. 4, when C is 0.8%, the quenching hardness is HRC60 or more at a quenching temperature of 980 ° C. or higher, and the quenching hardness is highest at around 1030 ° C., indicating a hardness equivalent to that of SKD11. Further, at 1030 ° C. or higher, the quenching hardness starts to decrease gradually. When the structure of the quenching agent at 1030 ° C. was observed, the amount of retained austenite was increased. At 1050 ° C. or higher, there was a concern about cracking or deformation due to martensite formation of the retained austenite. From the above, the quenching temperature range was 980 to 1050 ° C.

  Next, as shown in FIG. 5, the inventors investigated Cr · Mo added to the tool steel, hardenability, and the presence or absence of secondary hardening due to generation of secondary carbides. When Cr is less than 4%, the hardenability hardness becomes HRC56 or less. Further, the hardness due to secondary curing after tempering at 500 to 550 ° C. also falls below HRC55. When the hardness due to the secondary curing at 550 ° C. is reduced, the wear resistance is reduced, and “galling” or the like is generated. On the other hand, when the Cr content exceeds 8%, a net-like carbide starts to precipitate. Such a net-like carbide causes cracking due to quenching or welding. From the above, the Cr content was 4.0 to 8.0%. As shown in FIG. 5, when Mo becomes less than 0.8%, secondary hardening due to precipitation of secondary carbides at a tempering temperature of 500 to 550 ° C. cannot be clearly seen. Further, as can be seen from FIG. 5, when Mo exceeds 2.0%, the quenching hardness is lower than when it is less than that, and the HRC 55 is cut. The amount of Cr and Mo is preferably in the range surrounded by the hatched lines in FIG.

About other elements, it was set as the following component ranges for the following reasons.
[Si: 0.2-1.2%]
When an appropriate amount of Si is contained, the temper softening resistance is increased, and it is effective for improving wear resistance and sag resistance. However, if the content is too large, the toughness of the base is lowered.
[Mn: 0.25 to 1.5%]
When an appropriate amount of Mn is contained, the hardenability is improved and the base can be strengthened. However, if the content is too large, hot workability is hindered.
[V: 0.2 to 1.0%]
When an appropriate amount of V is contained, the softening resistance is improved. In addition, it is effective for improving wear resistance and seizure resistance, and also effective for making crystal grains finer. However, if it is contained in a large amount, the precipitation amount of pro-eutectoid carbide increases, and it tends to be connected in a network form.

  The inventors investigated the heat treatment method for cold tool steel cast steel with improved toughness and good weldability and hardenability. In the conventional method, in order to diffuse elements such as Cr / Mo and prevent the carbide from growing in a network shape, the solution heat treatment shown in FIG. 10 is performed, and then complete annealing or spheroidizing annealing is performed. . However, in this method, the heat treatment takes 45 hours or more, and the cost for the heat treatment increases.

  Therefore, it was examined whether a simple heat treatment could be performed on the components of the present invention. FIG. 6 shows an example of the heat treatment pattern studied. Unlike the general heat treatment shown in FIG. 10, this heat treatment is one in which pearlite spheroidization treatment is carried out by maintaining the temperature in a completely annealed temperature range and then slowly cooling it, without performing a solution treatment. FIG. 7 shows the relationship between the holding temperature (horizontal axis) and the mechanical properties (vertical axis) of cast steel products having the compositions of C 0.6%, 0.8%, and 0.9% (Samples 1 to 3) in Table 2 below. Indicates. From the results of FIG. 7, it is understood that the holding temperature is most preferably 860 ° C., and the mechanical properties are good if it is in the range of 840 to 880 ° C. Next, FIG. 8 shows the relationship between the mechanical properties (vertical axis) and the cooling rate (horizontal axis) when the cooling rate is changed at a holding temperature of 860 ° C. and a cooling temperature of 540 ° C. in cast steel products having the respective compositions shown in Table 2 below. Indicates. FIG. 8 shows that the cooling rate is most preferably −30 ° C./h, and there is no problem if it is gradually cooled at −15 to −40 ° C./h.

  As described above, the heat treatment of the cast steel product of the present invention is performed at 860 ° C. ± 20 ° C. for 1 to 3 hours, preferably, the holding time is increased by 1 hour every time the thickness is increased by 1 inch, and the cooling rate is −30 (+ 15, −10) ° C. / It turned out that it is preferable to set it as the heat processing which cools to arbitrary temperature of 500-720 degreeC as h. Moreover, the heat treatment time is about 25 hours, and the treatment time can be greatly shortened compared to the conventional method.

The cold tool steel cast steel according to the present invention can be used as a substitute for the SKD11 material that is generally commercially available. Therefore, the present invention can be applied to fields such as a press die material, a hot press die material, and a forging die material. Further, since the product of the present invention has high toughness, it can be sufficiently applied as a die casting material.
The product of the present invention can be used not only as a substitute for SKD11 / SKD61 but also as a substitute for other cold tool steels.

Claims (4)

In mass%, C: 0.5 to 1.0%, Si: 0.2 to 1.2%, Mn: 0.25 to 1.50%, Cr: 4.0 to 8.0%, Mo: A cast steel product containing 0.8 to 2.0%, V: 0.2 to 1.0%, the balance being Fe and impurities,
After heating and holding at 840 ° C. to 880 ° C. for 1 to 3 hours, the base structure is granular by performing a heat treatment at -15 to −40 ° C./h until reaching an arbitrary cooling temperature of 500 ° C. to 720 ° C. A manufacturing method of a tool steel cast steel product which is pearlite, has free carbides in an area ratio of 0.1% to 5%, and has a hardness after heat treatment of HB180 to HB250. In mass%, C: 0.5 to 1.0%, Si: 0.2 to 1.2%, Mn: 0.25 to 1.50%, Cr: 4.0 to 8.0%, Mo: A cast steel product containing 0.8 to 2.0%, V: 0.2 to 1.0%, the balance being Fe and impurities,
By heating and holding at 840 ° C. to 880 ° C. so that the whole becomes a uniform temperature, by performing a heat treatment that cools to an arbitrary cooling temperature of 500 ° C. to 720 ° C. at −15 to −40 ° C./h, A manufacturing method of a tool steel cast steel product in which the base structure is granular pearlite, the free carbide is 0.1% to 5% in area ratio, and the hardness after heat treatment is HB180 to HB250. Furthermore, the manufacturing method of the tool steel cast-steel goods of Claim 1 or 2 which has hardness of HRC56 or more by hardening or quenching and tempering at 980-1050 degreeC. The method for producing a tool steel cast steel product according to any one of claims 1 to 3, wherein the hardness of secondary hardening caused by generation of secondary carbide at 500 to 550 ° C is HRC55 or more.