TWI440726B - Method for producing cold-work die - Google Patents

Description

Method for manufacturing mold for cold working

The present invention relates to a method of manufacturing a mold for cold working for forming, for example, home appliances, mobile phones, and automobile related parts.

In the cold working die used for press forming, such as bending, drawing, or press-forming of a sheet at room temperature, in order to improve the wear resistance, there is a proposal for quenching and tempering (hereinafter referred to as "tempering") Further, a steel billet having a hardness of 55 HRC or more can be achieved (Patent Documents 1 to 3). When such a high-hardness steel billet is formed, it is difficult to cut into a mold shape after the quenching and tempering. Therefore, in general, after the steel block is subjected to rough processing in a low-temperature annealing state after hot working, the quenching and tempering is performed at a hardness of 55 HRC or more. At this time, since the mold is thermally deformed due to the quenching and tempering, after the quenching and tempering, the cutting is finished again to correct the deformation and adjust to the final mold shape. The main reason for the heat treatment deformation of the mold caused by the quenching and tempering is that the volume of the steel billet which is the ferrite structure in the annealed state becomes a massitesite structure and expands in volume.

In addition to the above-described steel billets, there are many proposals for pre-heating to pre-hardened steel which is supplied with hardness. Since the pre-hardened steel does not need to be tempered after performing the batch cutting process to the final mold shape, the heat treatment deformation of the mold due to the quenching and tempering can be eliminated, and the above-described effective technique of the final cutting process can be omitted. With regard to the present technology, there is proposed a cold-work tool steel which is optimized to ensure an amount of undissolved carbide which reduces machinability in a steel billet after quenching and tempering, and is ensured to be adjusted to be greater than 55 HRC. It has excellent machinability at the same time as the hardness (Patent Document 4). On the other hand, in order to suppress the tool wear caused by the friction between the cutting tool and the steel blank during the cutting process, there is also proposed a cold-work tool steel which is added to form an oxide having a melting point of 1200 ° C or less (with (FeO) _{) 2 .SiO 2, Fe 2 SiO} 4 or (FeSi) Cr ₂ O ₂₎ of the elements, whereby the heat generated when, by cutting the mold surface, the oxide is formed to impart self-lubricating property (Patent Document 5 ).

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] JP-A-2008-189982

[Patent Document 2] JP-A-2009-132990

[Patent Document 3] JP-A-2006-193790

[Patent Document 4] JP-A-2001-316769

[Patent Document 5] JP-A-2005-272899

Recently, the use conditions of the mold for cold working have increased, and the cold work tool steel is required to achieve a quenching and tempering hardness of 58 HRC or more and further 60 HRC or more. Therefore, in the case of the pre-hardened steel, the above hardness of 58 HRC or more is of course preferable, and it is preferable to obtain a hardness of 60 HRC or more in a stable manner and to have excellent machinability in a high hardness state. The cold-work tool steel disclosed in Patent Document 4 has both machinability at the time of cutting and pre-hardened steel which is excellent in wear resistance of a mold. However, in terms of wear resistance, the quenching temperature is limited in addition to the amount of formation of undissolved carbides, so it is necessary to adjust to 60 HRC or more. The hardness is very limited in terms of the range of ingredients that can be obtained. In addition, in order to suppress the growth of crystal grains during quenching heating, it is considered that Nb and V which are preferably added in the above-mentioned quenching temperature are elements which are easily formed into an MC carbide which is not solid-solved at the above-mentioned quenching temperature. Since the MC carbide is hard, the composition of the components disclosed in Patent Document 4 has a problem that the machinability after quenching and tempering is remarkably low.

Further, in the cold-work tool steel disclosed in Patent Document 5, the low-melting-point oxide is used as the self-lubricating film. However, when the cutting temperature does not rise to the melting point of the oxide, the lubricating effect cannot be obtained. On the other hand, when the cutting temperature is excessively increased, the viscosity of the oxide is remarkably lowered, and there is a problem that the function as a lubricating film cannot be achieved.

The object of the present invention is to provide a method for manufacturing a cold working tool for cutting cold working tool steel with a quenching and tempering hardness of 58 to 62 HR. The hardness of the cold working tool steel of 58 HRC or more is of course stable. Basically, the composition of the high-tempering hardness of 60 HRC or more is preferable, and even if the amount of formation of the undissolved carbide is further increased, the machinability after quenching and tempering is drastically improved without depending on the cutting temperature.

The inventors focused on the method of improving the machinability of cold working tool steel. As a result, it has been found that by actively introducing Al ₂ O _{3 of a} high melting point oxide, a method of forming a composite lubricating protective film containing MnS, which is a high ductility inclusion, or a high ductility inclusion, is formed on the surface of the cutting tool by heat during cutting. . On the other hand, at 58 HRC or higher, the high-tempering hardness of 60 HRC or more is achieved, and the steel billet capable of forming the composite lubricating protective film has the most suitable composition range, and the present invention has been achieved by specifying it.

That is, the present invention is a method for producing a mold for cold working, characterized in that C is contained in a mass ratio of C: 0.6 to 1.2%, Si: 0.8 to 2.5%, and Mn: 0.4 to 2.0%, and S: 0.03~0.1%, Cr: 5.0~9.0%, Mo and W alone or in combination (Mo+1/2W): 0.5~2.0%, Al: 0.04~ less than 0.3%, the remaining part of Fe and unavoidable impurities The steel block of the cold-work tool steel is subjected to hot working as a billet, and the billet is quenched and tempered to adjust the hardness to 58 to 62 HRC, and then subjected to cutting processing to complete the processing into the shape of the mold. As a specific example, a method of manufacturing a mold for cold working after quenching and tempering is performed after the billet is subjected to hot working. Further, as another specific example, the quenching is a method of producing a mold for cold working which is directly quenched by the cooling process after the hot working. Preferably, the hardness after quenching and tempering is 60HRC or more.

The cold working tool steel of the present invention may contain 1.0% or less of Ni or further 1.0% or less of Cu.

Further, the cold-work tool steel of the present invention may further contain 1.0% or less of V or further 0.5% or less of Nb.

According to the present invention, it is a matter of course that 58HRC or more is used because it can adopt a means capable of widely improving the machinability of a plurality of component compositions. The quenched and tempered steel is designed to have a hardness of 60 HRC or more, and an alloy design having a large amount of solid solution-free carbide can be used as a cold-work tool steel which is greatly improved in machinability after quenching and tempering without depending on the cutting temperature. Therefore, the quenched and tempered hardness of the cold work tool steel or the amount of undissolved carbide according to various functions can be freely selected. Moreover, if the cold working tool steel is tempered to a hardness of 58 to 62 HRC and then subjected to cutting, the mold can be manufactured because the heat treatment deformation and the final finishing of the processing can be solved, particularly for the use of pre-hardened cold work tool steel. The practical use of the mold for cold working is an indispensable technology.

[Formation of the Invention]

The present invention is characterized in that after the tempering hardness is increased first, even when a large amount of undissolved carbide is formed to control the crystal grain size or the like, the machinability after tempering can be achieved without depending on the temperature. The tool steel is chilled and the tempered cold work tool steel is processed. Specifically, in addition to obtaining a quenched and tempered hardness of 58 HRC or more, preferably 60 HRC or more, in order to suppress the wear of the cutting tool, the cold-worked tool steel of the steel blank subjected to the composition design is tempered before the cutting process, and the cold work is performed. The tool steel is formed on the surface of the cutting tool by a composite lubricating protective film of Al ₂ O ₃ of a high melting point oxide and MnS of a highly ductile inclusion.

First, the inventors reviewed means capable of broadly responding to the improved machinability of the composition of the cold-work tool steel. As a result, attention is paid to the effectiveness of self-lubricity. Further, when examining the effect of the self-lubricating property of the low-melting-point oxide as disclosed in Patent Document 5, it was found that it has a problem depending on the cutting temperature. That is, because of the low melting point oxide with self-lubricating properties, Usually, it is a composite oxide containing a large amount of Fe and Cr contained in a steel slab, and the composition and amount of the composite oxide greatly vary due to fluctuations in the cutting temperature, and a stable lubricating effect cannot be obtained.

Here, the present invention concentrates on a method of improving the machinability of a cold-work tool steel by using a low-melting oxide, and conversely, it is found that by actively introducing Al ₂ O _{3 of a} high-melting oxide, a cutting process is used. The heat of the moment, the method of forming a composite lubricating protective film of MnS containing the high-ductility inclusions on the surface of the cutting tool. This composite lubricating protective film has a wide range of cutting temperatures, and the effect does not change, and good machinability can be ensured even when an element of hard MC carbide which forms Nb and V is added. Further, the hardness of 58 HRC or more is of course a quenched hardness of 60 HRC or more, and the steel billet capable of forming the composite lubricating protective film has the most suitable component range, and the present invention has been achieved by specifying it. The composition of the cold-work tool steel relating to the production method of the present invention will be described below.

. C: 0.6 to 1.2% by mass (hereinafter only described as %)

C is an important element that forms carbides in steel and imparts hardness to cold work tool steels. When C is too small, the amount of carbide formed is insufficient, and it is difficult to impart 58 HRC or more, preferably 60 HRC or more. On the other hand, excessive inclusion causes a decrease in toughness due to an increase in the amount of undissolved carbide at the time of quenching. Therefore, the content of C is 0.6 to 1.2%, preferably 0.7% or more and/or 1.1% or less, more preferably 1.0% or less.

. Si: 0.8~2.5%

The Si system is solid-solubilized in steel and is an important element for imparting hardness to cold work tool steels. Further, in addition to the fact that the oxidation tendency is stronger than Fe or Cr, it is easy to form an element of a corundum-based oxide with Al ₂ O ₃ , and in the present invention, Fe-based oxide and Cr which lower the melting point of the oxide are suppressed. It forms an oxide and has an important role in promoting the formation of a protective film of Al ₂ O ₃ . However, when too much, the hardenability and toughness are remarkably lowered. Therefore, Si is made 0.8 to 2.5%, preferably 1.0% or more and/or 2.0% or less, more preferably 1.2% or more.

. Mn: 0.4~2.0%

Mn is an important element of the present invention and acts as a good lubricating film on the Al ₂ O ₃ protective film formed on the surface of the cutting tool. Moreover, it forms an element of austenite and solidifies in steel to improve hardenability. However, when the amount of addition is too large, a large amount of residual Worth field remains after the quenching and tempering, and this causes a change in the dimensional change of the year when used as a mold. Further, since it is easy to form a low melting point oxide with Fe or Cr, it is an important factor that hinders the function of the Al ₂ O ₃ protective film. Therefore, in the present invention, it is 0.4 to 2.0%, preferably 0.6% or more and/or 1.5% or less.

. S: 0.03~0.1%

S is an important element of the present invention which functions as a good lubricating film on the Al ₂ O ₃ protective film formed on the surface of the cutting tool. That is, a sufficient amount of S contained in the steel ingot forms MnS. Further, in addition to being ductile, MnS has a good affinity with Al ₂ O ₃ and is deposited on the Al ₂ O ₃ protective film to achieve the task of being a good composite lubricating protective film. In order to fully exert such a lubricating action, it is necessary to add 0.03% or more, but since S deteriorates the toughness of the steel, the upper limit is made 0.1%. It is preferably 0.04% or more and/or 0.08% or less.

. Cr: 5.0~9.0%

Cr imparts hardness to the cold work tool steel by forming M ₇ C ₃ carbides in the tempered microstructure. Further, some of them are present as undissolved carbides during quenching and heating, and have an effect of suppressing the growth of crystal grains. Further, by setting Cr to 5.0% or more, the amount of carbide formed is increased, and the hardness of 58 HRC or more, preferably 60 HRC or more can be sufficiently achieved. In addition, when various coating treatments are performed on the surface as a mold for cold working, the ability to form a VC film treated by TD and a TiC film treated by CVD can be improved. Further, Cr is an effective element for ensuring corrosion resistance.

On the other hand, Cr, which is a main component of cold-work tool steel, is liable to form a low-melting oxide. In other words, when Cr is excessively contained, it is an important factor that hinders the function of the Al ₂ O ₃ protective film. As a result, it is an important factor that hinders the function of the composite lubricating protective film containing Al ₂ O ₃ and MnS which is a feature of the present invention. Therefore, it is important to adjust the Cr system in addition to a sufficient amount of Al to be described later. Further, by performing the adjustment of the amount of S which is commensurate with it, the function of the above-mentioned composite lubricating protective film can be exhibited. Therefore, it is important to set Cr to 5.0 to 9.0%. It is preferably 6.0% or more, more preferably 7.0% or more.

. Mo and W are either alone or composite and (Mo+1/2W): 0.5~2.0%

Mo and W are tempering at the time of quenching and tempering, and are elements which enhance hardness by strengthening precipitation (secondary hardening) of fine carbides. At the same time, however, the residual Vostian system tends to remain in the tempered structure because the decomposition of the remaining Worth field during the tempering is delayed and excessively contained. Moreover, since Mo and W are expensive elements, efforts should be made to reduce the amount of addition in practical use. Therefore, let the relationship between the addition amount of these elements be (Mo + 1/2W) 0.5~2.0%.

. Al: 0.04~ less than 0.3%

Al is an important element of the present invention, that is, a sufficient amount of Al contained in the steel material, and Al ₂ O ₃ having a high melting point oxide is formed on the surface of the cutting tool by heat generated during cutting. The melting point of Al ₂ O ₃ is about 2050 ° C, because it is much higher than the cutting temperature, and Al ₂ O ₃ acts as a protective film for the cutting tool. Further, by containing 0.04% or more, it is possible to form a protective film having a sufficient thickness to improve tool life. However, when Al is added in a large amount, since the Al ₂ O ₃ system is formed in a large amount as inclusions in the steel material, the machinability of the steel material is rather lowered. Therefore, the upper limit of the amount of addition of Al is set to be less than 0.3%. It is preferably 0.05% or more and/or 0.15% or less.

. Preferably, Ni: 1.0% or less

Ni is an element that improves the toughness and weldability of steel. Further, since the tempering at the time of conditioning is an effect of increasing the hardness of the steel by precipitation of Ni ₃ Al, it is effective to add it in accordance with the amount of Al contained in the cold-work tool steel of the present invention. However, Ni is an expensive metal, and efforts should be made to reduce the amount of addition in practical use. Therefore, Ni of the present invention is preferably 1.0% or less at the time of addition.

. Preferably Cu: 1.0% or less

Cu is an effect of increasing the hardness of steel as ε-Cu precipitates during tempering at the time of quenching and tempering. However, Cu is an element that causes the brittleness of the steel billet. Therefore, the addition of the Cu system of the present invention is preferably 1.0% or less. Moreover, since the brittleness of the heat of Cu can be suppressed by adding about the same amount of Ni, the cold-work tool steel of the present invention can be pressed when Ni is contained. The limit value is alleviated according to the amount.

. Preferably, V: 1.0% or less

V forms various carbides and has an effect of increasing the hardness of steel. Further, the formed un-solidified MC carbide system has an effect of suppressing the growth of crystal grains. In particular, by the addition of Nb to the later-described Nb, the MC carbide which is not solid-solved at the time of quenching heating is fine and uniform, and has an effect of effectively suppressing the growth of crystal grains. On the other hand, the MC carbide is hard, which causes a decrease in machinability. Therefore, in the present invention, by forming the above-described composite lubricating protective film on the surface of the tool during the cutting process, it is important to ensure good machinability even when a large amount of MC carbide is formed in the steel material. However, by adding an excessive amount of V, excessive formation of coarse MC carbide also lowers the toughness of the cold-work tool steel. Therefore, when V is added, it is preferably 1.0% or less, more preferably 0.7% or less.

. Preferably, Nb: 0.5% or less

Nb forms MC carbide and has an effect of suppressing coarsening of crystal grains. However, when excessively added, coarse MC carbides are excessively formed to lower the toughness of the steel. Therefore, when it is added, it is preferably 0.5% or less, more preferably 0.3% or less.

Further, the present invention is characterized in that the cold working tool steel composed of the above components is tempered to have a hardness of 58 to 62 HRC, and then subjected to cutting. Regarding the cold-work tool steel of the present invention, it is possible to stably obtain a quenched and tempered hardness of 58 HRC or more by quenching and tempering. It is also possible to achieve a hardness of 60 HRC or more. Moreover, in this high hardness state, since it has excellent machinability, it is not necessary to intentionally perform cutting after annealing. , quenching and tempering. Or because it is not necessary to go through the annealed state itself, the quenching system can be applied to direct quenching using a cooling process after hot working of the steel block. Further, even in the case where the direct quenching is applied, the effect of improving the machinability can be obtained in the same manner as in the case of applying the quenching after the annealing. Therefore, with regard to the cold-work tool steel of the present invention, by using it as a pre-hardened steel, it is possible to eliminate the heat treatment deformation caused by the quenching and tempering, and it is also possible to omit the final completion of the cutting process and further the annealing step for the manufacture of the billet. Moreover, in order to fully maintain the mechanical characteristics other than the hardness of the cold-work tool steel and to perform the cutting process stably, the upper limit of the tempering hardness is set to 62 HRC.

Further, the mold including the method for producing a mold for cold working of the present invention has excellent dimensional accuracy and wear resistance, and by performing surface PVD treatment, it is possible to further improve the durability while maintaining high dimensional accuracy. Wear and tear.

[Examples]

The material was melted using a high frequency induction melting furnace to produce a steel block having the chemical composition shown in Table 1. Next, the forging ratio was set to about 10 for hot forging, and after cooling, annealing was performed at 860 °C. Subsequently, the annealed materials are subjected to quenching treatment by air cooling at 1030 ° C, and then tempered at 500 to 540 ° C for two times to be tempered to a target hardness of 60 HRC, which is manufactured for evaluation of cutting. Sex test strips.

The machinability test was carried out by plane cutting using a blade PICOmini manufactured by Hitachi TOOLS Co., Ltd. as a cutting edge exchange tool for a high hardness material. The insert uses a superhard alloy as a binder and applies a TiN coater to the surface. The cutting conditions were a cutting speed of 70 m/min, a rotational speed of 1857/min, a conveying speed of 743 mm/min, a conveying amount of 0.4 mm per blade, a cutting depth of 0.15 mm, a cutting width of 6 mm, and a number of cutting edges of 1.

The machinability test was performed based on the following two points. First, the amount of formation of the composite lubricating protective film containing Al ₂ O ₃ and MnS on the surface of the cutting tool was evaluated. The amount of formation was at a stage where the cutting distance immediately after the start of cutting was 0.8 m, and the EPMA analysis blade was used as the average statistic of Al and S at this time from the cutting face side. Then, the cutting distance was extended to 8 m, and the tool wear amount at this time was measured using an optical microscope. The evaluation results of these are shown in Table 2.

The cutting process of the cold-work tool steel of the present invention forms a composite lubrication protective film on the surface of the cutting tool, and the tool wear can be suppressed. Further, even when Nb and V which form undissolved carbides are added, good machinability can be maintained.

On the other hand, compared with the present invention, the cutting tool of the cold-work tool steel which does not satisfy the present invention has a large amount of wear.

Fig. 1 is a digital microscope photograph showing the flank face and the cutting face of the cutting tool used in Sample Nos. 3, 5, 15, 22, and 30, and Fig. 2 is a partial use of the above EPMA. The result of the analysis (the high concentration portion of each element is shown in a light color). In Table 2, samples No. 3, 5, and 15 in which the average statistic of Al and S are high are also confirmed in the EPMA analysis of FIG. 2 in which a large amount of Al and S adhere to a wide range of tools. In comparison, the amount of Al in the cold-work tool steel was lower than that of sample No. 22, and the average statistics of Al and S were also lower than those of samples No. 3, 5, and 15, and the adhesion range of Al and S was also narrow. Moreover, in the original sample No. 30 in which the content of Al and S in the steel is lower, the average statistic of these elements is also low, and it is almost impossible to detect Al and S using the EPMA analysis system (most of which can be detected, most of It is considered to be Fe and Cr transferred from the test piece.

Further, in the first diagram showing the wear state of the cutting tool, in accordance with the above-described results, the tool cutting surfaces of the samples No. 3, 5, and 15 were markedly attached to the tool cutting surface, and the tool wear was found to be on the flank surface. Both sides of the cutting surface are suppressed. Moreover, the tool wear is performed uniformly and stably. On the other hand, the tool No. 22 had a tool wear amount that was nearly twice that of No. 3 and a fragment was generated in the tool. Further, in the same manner as the sample No. 22, the surface of the tool of the sample No. 30 was severely damaged.

Further, Fig. 3 is a cross-sectional TEM image showing the adhering matter confirmed on the surface of the tool No. 3, 22, and 30 and the TiN coating under the coating. In accordance with the above results, the sample No. 3 with a higher average statistic of Al and S is thicker, and as the average statistic of Al and S becomes lower, the attached material of sample No. 22 is changed to Thinner. In the sample No. 30, almost no attachment was observed. Further, similarly to No. 3, Al ₂ O ₃ and MnS were adhered to the surface of the tool of Sample No. 22, but the thickness thereof was thin and the fragments were generated as described above. The adherent of sample No. 3 exhibits a high lubrication protection function, and it can be seen from the following that TiN coating on the surface of the tool which is usually plastically deformed due to the frictional stress during the cutting process, and the sample No. which is thicker in the attached material. The 3 series was suppressed (the plastic deformation zone was the narrowest).

1‧‧‧Protective film for preparing samples

2‧‧‧ Attachments during cutting

3‧‧‧TiN plastic deformation zone

4‧‧‧TiN undeformed area

Fig. 1 is a digital microscope photograph showing the flank face and the cutting face of the cutting tool used in the cutting process of the present invention and the comparative example.

Fig. 2 is a color diagram when the deposit formed on the surface of the cutting tool of Fig. 1 is analyzed by EPMA (Electron Probe Microanalyzer).

Fig. 3 is a cross-sectional TEM (transmission electron microscope) photograph showing the attachment of Fig. 2 together with TiN coating.

Claims (9)

A method for producing a mold for cold working, comprising: C: 0.6 to 1.2% by mass, Si: 0.8 to 2.5%, Mn: 0.4 to 2.0%, S: 0.03 to 0.1%, Cr :5.0~9.0%, Mo and W are steels of cold-worked tool steels which are single or composite and (Mo+1/2W): 0.5~2.0%, Al: 0.04~ less than 0.3%, the remaining part of Fe and unavoidable impurities The block is subjected to hot working as a billet, and the billet is quenched and tempered to adjust the hardness to 58 to 62 HRC, and then subjected to cutting processing to complete the processing into the shape of the mold. The method for producing a mold for cold working according to the first aspect of the invention, wherein the billet subjected to the hot working is annealed and then quenched and tempered. The method for producing a mold for cold working according to the first aspect of the invention, wherein the quenching is directly quenched by a cooling step after the hot working. The method for producing a mold for cold working according to any one of claims 1 to 3, wherein the cold working tool steel further contains Ni: 1.0% by mass or less. The method for producing a mold for cold working, according to any one of claims 1 to 3, wherein the cold working tool steel further contains % by mass It is Cu: 1.0% or less. The method for producing a mold for cold working according to any one of claims 1 to 3, wherein the cold working tool steel further contains V: 1.0% or less by mass%. The method for producing a mold for cold working according to any one of claims 1 to 3, wherein the cold working tool steel further contains Nb: 0.5% or less by mass%. The method for producing a mold for cold working according to any one of claims 1 to 3, wherein the hardness after quenching and tempering is 60 HRC or more. The method for producing a mold for cold working according to any one of the first to third aspects of the invention, wherein the cold working tool steel further contains two or more kinds selected from the group consisting of Ni: 1.0% or less by mass%, and Cu: 1.0 % or less, V: 1.0% or less, and Nb: 0.5% or less.