EP1072691B1 - Tool steel with excellent workability, machinability and heat treatment characteristics, and die using same - Google Patents

Tool steel with excellent workability, machinability and heat treatment characteristics, and die using same Download PDF

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EP1072691B1
EP1072691B1 EP99123519A EP99123519A EP1072691B1 EP 1072691 B1 EP1072691 B1 EP 1072691B1 EP 99123519 A EP99123519 A EP 99123519A EP 99123519 A EP99123519 A EP 99123519A EP 1072691 B1 EP1072691 B1 EP 1072691B1
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Prior art keywords
steel
less
heat treatment
tempering
materials
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German (de)
English (en)
French (fr)
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EP1072691A2 (en
EP1072691A3 (en
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Kunichika Kubota
Miki Yamaoka
Yukio Abe
Yasushi Tamura
Yoshihiro Kada
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Proterial Ltd
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Hitachi Metals Ltd
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")

Definitions

  • the present invention relates to a tool steel used in dies for the blanking, punching, bending, drawing or trimming of steel sheets used in automobiles, home electric appliances, agricultural implements, etc.
  • materials for conventional dies and particularly for cold working dies are required to contain a large amount of carbides to obtain wear resistance and further to contain high Cr contents to obtain excellent hardenability and toughness.
  • high-carbon, high-chromium-base steels such as SKD11, which is an alloy tool steel specified in JIS G 4404, are used for these dies.
  • JIS-SKS3 is a low-alloy tool steel and has much better machinability than JIS-SKD11, it has bad hardenability and oil hardening is necessary, so that warping is apt to occur in SKS3.
  • size variations due to heat treatment and deformation with the lapse of time are apt to occur although this steel has good hardenability.
  • JIS-SKD11 which is more difficult to machine, is better regarding the size variations which occur due to heat treatment.
  • the conventional tool steels have advantages and disadvantages, and a tool steel that has both heat treatment characteristics equivalent to those of SKD11 and machinability equivalent to that of SKS3 is presently desired.
  • heat treatment characteristics in particular, it is intensively desired that a new tool steel can be put in the same heat treatment furnace as in SKD11 for the sake of the rationalization of heat treatment operation.
  • the document JP-A-4-362 153 discloses a steel with the following composition in weight %: C 0.45-0.65%, Si ⁇ 0.25% Mn ⁇ 0.25%, Ni ⁇ 0.6%, Cr 4-11%, Mo 0.2-1.0%, A ⁇ 0.03%, Ca 0.001-0.020%, P ⁇ 0.03%, S ⁇ 0.005%, 0 ⁇ 0.005, balance Fe with unavoidable impurities, further optionally V 0.05-0.5%, Nb 0.05-0.5, rare earth metal 0.01-0.05%.
  • the object of the present invention is to provide a tool steel with excellent weldability, machinability and heat treatment characteristics without a deterioration in mechanical properties such as toughness.
  • the present inventors researched the basic conditions necessary for improving weldability, machinability and heat treatment characteristics while keeping basic mechanical properties such as toughness and wear resistance.
  • the present inventors researched various cutting methods adopted to cut tool steels and found out that types of damage to cutting tools are broadly sorted into two: chipping-type damage and thermal damage.
  • the inventors ascertained that a process in which these two types of damage are caused simultaneously in different places of one tool can be realized on a square end mill under specific conditions.
  • the present inventors ascertained that mechanical damage occurs at the cutting edge and that thermal damage occurs at the boundary where the contact thereof with the work terminates.
  • the inventors examined various free-cutting techniques which reduce these two damage mechanisms.
  • the inventors found out components and composition ranges sufficient for' obtaining good mechanical properties, in particular, hardness and toughness even when the content of C, which is a basic component of tool steels, is lowered.
  • the inventors found out that a reduction in primary carbides present in tool steels can minimize the mechanical damage.
  • the inventors In researching various cutting methods and cutting conditions which make it possible to simultaneously realize the above two effects, the inventors have finally attained a tool steel best suited to the achievement of the effects.
  • the tool steel according to the invention is defined in claim 1.
  • An embodiment of the tool steel of the present invention consists, by weight, of: C and Cr in both of which (Cr + 5.9 x C) is 9.1 to 12.5, (Cr - 4.2 x C) being not more than 5 and (Cr - 6.3 x C) being not less than 2.387; 0.1 to 0.6% Si; 0.1 to 1.2% Mn; at least one of Mo and W in which (Mo + 1/2W) is 0.6 to 1.25%; less than 0.5% V; and the balance Fe and incidental impurities.
  • the area ratio of carbides each having a sectional area not less than 20 ⁇ m 2 to the section of the metal structure be not more than 3% .
  • the area ratio of sulfides each having a sectional area not less than 1 ⁇ m 2 to the section of the metal structure be not less than 0.2%.
  • Cr segregation range in matrix (which is defined by a range (%) of Cr content variation in matrix with respect to the average of Cr content) after hardening is not more than 1% by weight, or the maximum tempering hardness is not less than 57 HRC when tempering is performed at a temperature of not less than 500°C.
  • a die is fabricated from an embodiment of the inventive tool steel in which a size variation due to heat treatment by tempering at a temperature of not less than 500°C is not more than 0.1% in comparison with the size measured just before hardening in terms of linear expansion coefficient and in which tool steel a size variation due to heat treatment by tempering at 490°C is not more than 0 when this steel is tempered to a hardness of not less than 55 HRC and cutting the steel.
  • JIS-SKD11 consists, by weight (the same applying to the following), of: 1.40 to 1.60% C, not more than 0.40% Si, not more than 0.60% Mn, not more than 0.030% P, not more than 0.030% S, 11.00 to 13.00% Cr, 0.80 to 1.20% Mo, 0.20 to 0.50% V, and the balance Fe and incidental impurities
  • JIS-SKS3 consists of: 0.90 to 1.00% C, not more than 0.35% Si, 0.90 to 1.20% Mn, not more than 0.030% P, not more than 0.030% S, 0.50 to 1.00% Cr, 0.50 to 1.00% W, and the balance Fe and incidental impurities.
  • SKD11 is a tool steel of de facto standard.
  • the inventors adopted such a basic line as, in order to obtain the same heat treatment character-istics, components which are in a solid solution state in the matrix at the time of hardening are made similar to those of SKD11.
  • Fig. 1 shows the whole of a composition design diagram determined by means of a thermocalculator and
  • Fig. 2 is a composition design diagram in which the region corresponding to the present invention is enlarged.
  • Line (A) indicates a line (on the plane of added components) on which the same amount of dissolved C (which means carbon contained in a solid solution) as with SKD11 upon hardening is obtained.
  • Line (B) indicates a line of the same amount of dissolved Cr as with SKD11. The two lines are bent,.which is due to the fact that residual carbides exist above Line (C) with the result that alloying elements are consumed to form the carbides, whereby dissolved elements of the matrix cannot be kept at the same levels unless the amounts of added components are increased.
  • the two lines (A) and (B) basically intersect with each other only in the composition of SKD11 and, therefore, it is impossible to design the same matrix composition as with SKD11 at the same hardening temperature. Nevertheless, Lines (A) and (B) approach each other in the area above Line (C) and, therefore, the matrix composition becomes close to that of SKD11.
  • the amounts of added C and Cr are increased in order to bring these lines closer to each other, the amount of residual carbides increases, promoting the tool wear of the chipping type and leading to deteriorated machinability. Further, in this case fatigue fracture becomes apt to occur regarding durability, so that the application of steels to dies in which stress concentration is apt to occur is limited. This mutually contradictory relationship was experimentally clarified, and it was found that the region in which machinability is excellent and in which the heat treatment characteristics are close to those of SKD11 is achieved in the composition range of the tool steel of the invention shown in Fig. 2.
  • Fig. 3 The principle of the behavior of the size variations due to heat treatment is shown in Fig. 3.
  • each of crystal lattices is expanded by the carbon in a solid solution state in the martensite structure of a matrix.
  • the tempering temperature is raised, cementite precipitates in the low- and medium-temperature region ((A) area in Fig. 3) and the size variation comes to have a tendency to shrink.
  • the size variation rate reaches a maximum at a temperature which is almost the same as with secondary hardening. This maximum value occurs due to two mechanisms which occur mainly on the low-temperature side ((B) area in Fig. 3) and high-temperature side ((C) area in Fig. 3) of this maximum value.
  • SKD11 its composition is such that the size variations occurring between the mechanisms (B) and (C) are suppressed while maintaining hardness with the aid of the mechanisms of (A), (B) and (C).
  • the matrix composition is made close to that of JIS-SKD11.
  • optimization is performed not only for C and Cr, which are the main alloying elements of JIS-SKD11, but also for Mo and W that control the precipitation of M 7 C 3 -base and M 23 C 6 -base carbides which control the precipitation of cementite, as shown in Fig. 3.
  • composition system of the invention primary carbides do not easily crystallize in an equilibrium diagram and, therefore, a further improvement in machinability is achieved by performing quenching-and-solidifying or diffusion annealing at about 1100 to 1400°C, thus taking measures to completely remove or reduce the primary carbides.
  • the inventors examined the function of the adding of S with respect to the size variations due to heat treatment. As a result, they found out that the size variation due to heat treatment increase when the amount of added S exceeds 0.2%. There has so far been no such a report and the reason for this seems to be that the size variations due to heat treatment present scarcely any problem in free-cutting steel systems in which the addition of S is frequently carried out. On the other hand, it might be thought that because more amount of residual carbides exist in tool steel systems, the function of constraining size variations occurs, making it impossible to detect the effect of S on size variations. As a result of this, the inventors found out that it is necessary to make adjustments to a composition so that the content of S becomes not more than 0.2% at which the size variation is small.
  • the amount of added S is preferably 0.005 to 0.12%.
  • good surface treatment characteristics are also ensured to cope with such a case as wear resistance becomes insufficient because of limited C contents.
  • Possible kinds of heat treatment include carburizing, nitriding, PVD (physical vapor deposition) and CVD (chemical vapor deposition).
  • CVD chemical vapor deposition
  • treatment becomes difficult depending on the properties of a base material to be treated.
  • an element to be deposited which is vaporized at about 1000°C, is chemically precipitated on the surface of the material. For this reason, substantially similarly to the heat treatment of the material, there occur problems such as insufficient hardening and much size variation due to heat treatment.
  • the hardenability of an embodiment of the tool steel of the present invention is sufficiently good because a composition close to that of JIS-SKD11, which has good hardenability, is used. Further, because it enhances industrial convenience to make the amount of size variation due to heat treatment during hardening and tempering equal to that of JIS-SKD11, it is important to adopt the composition area shown in Fig. 2 so that the C and Cr contents in the matrix may be closer to those of JIS-SKD11. JIS-SKD11 is also used as a gauge steel because the size variation due to heat treatment is small.
  • the reason why the size variation of SKD11 due to heat treatment is small is that the hardness in the high-temperature tempering region is maintained by such a method as to suppress the precipitation of cementite almost only by dissolved Cr. More specifically, in secondary-hardened steels in which Mo, W and V are positively added which are usually added in high-speed tool steels capable of high-temperature tempering, fresh martensite formed by the decomposition of retained austenite occurring during secondary hardening does not easily cause tempering shrinkage and, therefore, much size variation due to heat treatment occurs.
  • the amount of dissolved C present in the austenite structure at a surface treatment temperature during CVD, etc. is important for the formation of MX-type compounds (TiC, VC, etc.) having a sufficient film thickness.
  • MX-type compounds TiC, VC, etc.
  • carbon must be supplied from the steel, and an optimum amount of C depends on the amount of dissolvled C present in the martensite structure before the steel is held at the surface treatment temperature.
  • the amount of dissoslved C is not less than 0.4% and, therefore, sufficient film deposition is possible.
  • the composition range shown in Figs. 1 and 2 is adopted from the standpoint of the similarity to JIS-SKD11 and from the feature that the amount of undissolved carbides just after hardening is not more than 5 mass %. Specifically, it is preferable for an improvement in machinability that the amount of carbides which are not in a solid solution state be not more than 5 mass % in a structure just after hardening at 1000° to 1050°C when calculated, for example, by a thermocalculator.
  • the heat treatment characteristics of JIS-SKD11 are such that size variation due to heat treatment in the rolling direction becomes minus in a region where the tempering temperature is not more than 490°C, whereas size variation due to heat treatment in the rolling direction turn to be plus side at a tempering temperature higher than 490°C. Further, another feature of the heat treatment characteristics of JIS-SKD11 is that the amount of maximum size variation due to heat treatment at a tempering temperature higher than 490°C becomes a plus value not more than 0.1%. At the same time, the heat treatment characteristics of SKD11 have also the feature that heat treatment conditions capable of obtaining a hardness in the range of from 57 to 60 HRC exist in these tempering regions.
  • the composition range that can meet all these features is shown in Figs. 1 and 2.
  • the characteristic that the size variation on the minus side always occur at a temperature of not more than 490°C and that it turns to the plus side at a temperature higher than 490°C means that when treatment is performed by gradually raising the tempering temperature, such a condition as the amount of size variation due to heat treatment becomes zero surely exists somewhere. Therefore, it is possible to find out a heat treatment condition capable of making size variations close to zero.
  • JIS-SKD11 is supported by heat treatment engineers having high techniques, and provides the background of the de facto standardization of JIS-SKD11. Therefore, the balance between C and Cr shown here is especially important.
  • Si is originally contained as a deoxidizer and for the purpose of improving castability, and toughness improves when the Si content is lowered.
  • machinability deteriorates at the same time when the Si content is lowered and, therefore, it is necessary that the Si content be not less than 0.1%.
  • an excessive amount of Si suppresses the precipitation of cementite, with the result that size variation due to heat treatment in a tempering region of 500 to 550°C increases. For this reason, the Si content is limited to the range of 0.1 to 0.6%.
  • the amount of added (Mo + 1/2W) is less than 0.6%, a decrease in hardness by high-temperature tempering occurs abruptly and hardness control becomes difficult.
  • excessive amounts of added (Mo + 1/2W) delay the precipitation and aggregation of the carbides in martensite, with the result that the size variation due to heat treatment by tempering at 500 to 550°C becomes great.
  • the result is also such that due to the delayed decomposition of austenite associated with the delay in the tempering of martensite, unstable austenite is retained contrary to operators' expectations that thorough tempering must has been performed, thereby causing the size variation with the lapse of time to occur during the use of dies after the fabrication thereof. Therefore, the (Mo + 1/2W) content is limited to the range of from 0.6 to 1.25%.
  • the (Mo + 1/2W) content is preferably 0.6 to 1.10%.
  • the above composition may contain V when other effects is desired.
  • V is an element that increases the resistance to softening necessary for tool steels, it forms V-base carbides, thereby causing a decrease in machinability. Therefore, the V content is less than 0.5%.
  • Ca is an ideal free-cutting element that does not cause a degradation in mechanical properties or structural deterioration.
  • the free-cutting mechanism of this element lies in lowering the melting points of oxides dispersed in trace amounts in steel, with the result that these oxides melt with the heat of cutting, forming a protective film on the cutting edge.
  • Ca is apt to come away from molten steel because of its high vapor pressure and, therefore, the addition of Ca in amounts up to about 100 ppm is made possible at most in the case of the present adding technique.
  • Rare earth elements may be contained in amounts of not more than 0.2% in order to improve the machinability of the embodiments of the tool steel of the invention. It is preferable that the total amount of incidental impurities be not more than 0.5%. However, it is possible to add Ni in amounts of not more than 5.0% when toughness and weldability are necessary. In addition, when it is necessary to impart further wear resistance to the embodiments of the tool steel of the invention, it is also possible to add Al in amounts of not more than 1.0%, thereby increasing hardness attained after nitriding.
  • the basic properties of the embodiments of the tool steel of the invention are not changed even if one or more elements selected from the group consisting of Pb, Se, Te, Bi, In, Be, Ce, Zr and Ti are added in an amount of not more than 0.2% for obtaining other desired effects.
  • the adjusting of the condition after hardening is effective for further improving the effects of the present invention, that is, to adjust the amounts of dissolved C and Cr present in the martensite structure after hardening so that they may be close to those of JIS-SKD11 and to make the amount of residual carbides just after hardening not more than 5% by mass. This amount of residual carbides just after hardening can also be reduced in the steel manufacturing process.
  • the embodiment of the tool steel of the invention is also excellent in surface treatment characteristics for the salt bath method and CVD treatment.
  • flame hardening may be performed only in necessary portions according to desired functions, or a heat treatment method for obtaining hardness may be selected while taking the number of production steps or required properties into consideration.
  • dies can be fabricated by tempering an embodiment of the tool steel of the invention to a hardness of not less than 55 HRC and performing cutting.
  • Another embodiment of the invention resides in the fact that a component makeup and a composition were found out which are sufficient for obtaining good mechanical properties, in particular, hardness and toughness although the content of C, which is the basic component of any tool steel, is reduced in comparison with JIS-SKD11, and that an embodiment of a tool steel excellent in weldability, machinability and surface treatment characteristics, i.e., heat treatment characteristics was achieved.
  • the wording "to be excellent in weldability" or “welding is possible” means that welding cracks are not observed in the Y-shape test specified in JIS Z 3158 in which the prescribed preheating treatment and postheating treatment are performed.
  • preheating and postheating are usually performed in order to prevent weld cracks during welding.
  • Preheating is performed to prevent hot cracking during welding and postheating is performed to prevent cold cracking, the latter being a kind of tempering to lower the hardness of the weld heat-affected zone, in particular.
  • dies are welded to make shape changes or are repaired depending on the condition during the manufacture or use thereof.
  • welding is performed when the dies are preheated to a high temperature in order to prevent cracking from occurring during welding.
  • dies contain Cr, etc. it is general practice to perform such welding after preheating to a temperature not lower than 450 to 550°C.
  • the heating temperature and time for postheating can be reduced by decreasing the hardness of the weld heat-affected zone.
  • it is effective to reduce the C content to not more than 0.75% and the Cr content to not less than 6.8%. This is effective in adjusting the amounts of C and Cr both dissolved in the martensite structure which have an effect on weldability.
  • the comparative material 1 is an equivalent to SKD11.
  • hot rolling was performed so that the forging ratio becomes about 5 and annealing at 850° for 4 hour was then performed after cooling.
  • test pieces of 10 mm in diameter and 80 mm in length were fabricated for each material so that the rolling direction coincides with the longitudinal direction of the test pieces.
  • 10 pieces in the 21 test pieces were heated and held at 1025°C in a vacuum heating furnace, and gas cooling hardening in an inert gas was then performed. Moreover, tempering at 530°C for 1 hour was performed twice. The hardness of obtained test pieces was measured and it was found that in the comparative examples 2 and 3, the hardness is less than 57 HRC.
  • test pieces with a size variation of not more than 0.1% at 530°C were tested.
  • the materials shown in Table 1 for which the behavior of size variation can be regarded as the same as with SKD11 in Example 1 were tested. These materials were brought into an annealed condition with a hardness of not more than 24 HRC and machinability was evaluated on a square end mill. The cutting test was performed under the conditions shown in Table 4. As is apparent from the result shown in Table 5, the materials of the invention 1 to 10 show high machinability with tool life (cutting edge wear: 0.3 mm) of not less than 10 m. In the comparative materials 1 and 4, machinability is inferior due to the presence of chromium-base carbides.
  • test materials adjusted to a hardness of 57 to 60 HRC by hardening at 1030°C and tempering at a temperature of not lower than 500°C were produced and machinability was evaluated on a square end mill.
  • the cutting conditions adopted are shown in Table 6. From the test result shown in Table 7, it is apparent that the materials of the invention show good tool life (cutting edge wear: 0.1 mm) and high machinability, whereas the comparative materials 1 and 4 have inferior machinability.
  • Ingots were made from some of the materials shown in Table 1, i.e., the materials of the invention 1 and 2 with relatively inferior machinability, the comparative material 1 with good heat treatment characteristics, and the comparative material 4. These ingots were soaked at 1160°C for 10 hours. After annealing, an adjustment to a hardness of 57 HRC was made by performing hardening at 1030°C and tempering at a temperature of not lower than 500°C. The machinability of these materials was tested. The test conditions shown in Table 8 were adopted and the cutting length to cutting edge wear of 0.1 mm was regarded as tool life.
  • Item Conditions Tool HES2100-C cemented carbide coating
  • Cutting speed 75 m/min
  • Cutting direction Down cut
  • Cooling method Dry type
  • Cutting length at cutting edge wear width of 0.1 mm Cr segregation range (%) in matrix Material of the invention 1 28 m 0.8% " 2 28 m 0.8%
  • Comparative material 1 ⁇ 2 m 1.8% " 4 6 m 2.5%
  • the comparative material 7 is an equivalent to SKD11. Next, hot rolling was performed so that the forging ratio becomes about 5 and annealing at 850°C for 4 hour was then performed after cooling.
  • test pieces for the Y-shape test specified in JIS Z 3158 were taken from the above annealed materials. After heating and holding at 1025°C in a vacuum heating furnace, gas cooling hardening in an inert gas was performed. Subsequently, tempering was performed at 500 to 550°C so that the target hardness of each test piece became not less than 57 HRC. Test pieces thus prepared were welded under the conditions shown in Table 11 and weldability was evaluated. In the comparative materials 10 to 16, it was impossible to obtain hardness values of not less than 57 HRC by tempering at a temperature not less than 500°C. Item Description Preheating method After charging into an electric furnace heated and held at a predetermined temperature, test pieces were held at the temperature for 1 hour.
  • Welding method Arc welding Electrode Coated electrode equivalent to JIS Z3251 DF3B, electrode diameter: 4 mm Welding current 110
  • a Postheating method after welding Test pieces were held at 450°C for 1 hour in the same manner as in the preheating. Cooling time 7 hours Method of judgment on weld cracks From detecting test by the penetration and microscopic observation of internal cut sections
  • test materials with a hardness of not more than 24 HRC in an annealed state were prepared from the materials shown in Table 10 and the machinability on an square end mill was evaluated. Incidentally, the cutting test was performed under the conditions shown in Table 13. From the result shown in Table 14, it is apparent that tool life at least three times that of the comparative material 7, which is an equivalent to SKD11, can be obtained in the materials of the invention 11 to 18 and the comparative materials 17 and 18.
  • test materials hardened and tempered under the heat treatment conditions of the invention to obtain a hardness of from 57 to 60 HRC were prepared from the materials shown in Table 10 and the machinability thereof on an square end mill was evaluated.
  • the conditions are shown in Table 15. From the test result shown in Table 16, it is apparent that tool life at least six times that of the comparative material 7, which is an equivalent to SKD11, can be obtained in the materials of the invention 11 to 18 and the comparative materials 17 and 18.
  • test materials were obtained from the above annealed materials by performing heating and holding at 1025°C in a vacuum heating furnace, performing gas cooling hardening in an inert gas, and subsequently performing tempering at 500 to 550°C to a predetermined hardness.
  • the postheating after welding was performed at 450°C and after holding at 1 hour, gradual cooling to room temperature was performed while spending a period of time of 3 hours or 7 hours.
  • Table 17 shows the occurrence or non-occurrence of cracks under these conditions, along with hardness values and preheating temperatures.
  • Hardness (HRC) Preheating Temp HRC
  • test pieces 10 mm in diameter and 80 mm in length were prepared from each of the annealed materials of the invention 11 to 18 and annealed comparative materials 7 to 9, 17 and 18 so that the longitudinal direction of the test pieces coincides with the rolling direction.
  • the longitudinal size of these test pieces before hardening was measured beforehand and hardness was adjusted to 60 HRC ⁇ 1 by performing air hardening after holding at 1030°C for 1 hour and by tempering at a temperature of not less than 500°C. After sufficient cooling of the test pieces at room temperature, the size was measured again, a size variation ratio relative to the size measured just before hardening was determined for each test piece, and the number of test pieces with a size variation ratio exceeding 0.1% was counted. The result is shown in Table 18. Number of test pieces with size variation ratio exceeding 0.1% Material of the invention 11 0 " 12 0 " 13 0 " 14 1 " 15 0 " 16 0 " 17 1 " 18 0 Comparative material 7 0 " 8 10 " 9 4 " 17 10 " 18 10
  • the size variation range is not more than 0.1% and is satisfactory.
  • the size variation ratio greatly exceeds 0.1% and cannot bear practical use.
  • the main reason why JIS-SKD11 has been widely used as a conventional die steel is that the heat treatment characteristics thereof are good. It is apparent that the steel of the invention has also such features as can be widely used. However, because materials equivalent to JIS-SKD11 are deficient in weldability and machinability as shown in Examples 4 and 5, it is apparent'that the tool steel of the invention which is greatly improved in these points has a very high industrial value as a tool material.
  • Predetermined ingots were made from alloys with the chemical compositions shown in Table 20 melted in a high-frequency furnace.
  • the comparative material 19 is an equivalent to JIS-SKD11. These ingots were forged at a forging ratio of 5 and finished to steel products, which were then annealed.
  • the sections of steel materials were polished and then etched in a 10% niter etchant, images within a field of view of 2 mm2 under a microscope (a magnification of 200X) being taken into a computer, and the amount of carbides in'a sectional area of not less than 20 ⁇ m 2 was determined with the aid of image analysis software. Sulfides were also analyzed in the same manner as with carbides. In this case, after the polishing of the sections of steel materials, sulfides in a sectional area of not less than 1 ⁇ m 2 were analyzed without etching. The results of the two analyses are shown in Table 20.
  • the area ratio of carbides in a sectional area of not less than 20 ⁇ m 2 is not more than 3%, and the area ratio of sulfides in a sectional area of not less than 1 ⁇ m 2 is not less than 0.2% in all materials of the invention except the steel 20.
  • the area ratio of carbides in a sectional area of not less than 20 ⁇ m 2 exceeds 3% in the comparative material 19, which is an equivalent to JIS-SKD11, and the comparative materials 20, 26 and 27.
  • Table 21 there are shown measurement result of long axis/short axis ratio of sulfides each having a sectional area of not less than 1 ⁇ m 2 with respect to the materials in Table 20 having a forging ratio of 25 and 10R-notch Charpy imapct test result regarding the same materials.
  • the heat treatment conditions are the same as those mentioned above.
  • the long axis/short axis ratio of sulfides was analyzed by the same method as in the measurement of sulfides shown in Table 20.
  • the long axis/short axis ratio is not more than 4.5 because of the effect of Ca, and a ratio of impact value in the direction vertical to forging extension direction to another impact value in the forging extension direction is high, and decrease in toughness in the direction vertical to the forging direction is suppressed in comparison with the comparative materials 20 and 27.
  • Welding method Arc weling Electrode Coated electrode equivalent to JIS Z3251 DF3B, electrode diameter: 4 mm Welding current 110
  • a Postheating method after welding Test pieces are held at 450°C for 1 hour in the same manner as with preheating. Cooling time Spending 7 hours Method of judging weld cracks Dye penetration flaw detecting test and microscopic observation of internal cut sections No. Hardness after hardening and tempering (HRC) Weldability test result Preheating temp.
  • the materials of the invention have better machinability than the comparative material 19 which is equivalent to JIS-SKD11.
  • the comparative materials 19, 20, 26 and 27 with a high C or Cr content whose area ratio of carbides with a sectional area of not less than 20 ⁇ m 2 after hardening and tempering exceeds 3%, the area ratio of carbides is high even in an annealed state, with the result that the machinability thereof becomes inferior.
  • the comparative material 22 is inferior in machinability because its V content is high and, besides, the area ratio of sulfides is low even in an annealed state.
  • the comparative material 25 is inferior in machinability because, its Si content is low and the area ratio of sulfides is also low.
  • the materials of the invention have good machinability even in a hardened and tempered state, and that this machinability is much better than that of the comparative material 19, which is equivalent to SKD11.
  • the comparative material 20 is also inferior in machinability to the materials of the invention, because this comparative material has a high Cr content and its area ratio of carbides with a sectional area of not less than 20 ⁇ m 2 exceeds 3%.
  • the materials 25 and 26 were prepared by adding aluminum into the materials 6 and 8, respectively. These steels were prepared by a process having the steps of: performing the retaining at 1030°C in a furnace, performing the cooling and hardening by use of pressurized gas; and performing tempering at 500 to 550°C so that a target hardness value may become not less than 57 HRC. After that, ion nitriding treatment was performed in which the materials were held at 520°C for 5 hours in an atmosphere of hydrogen, nitrogen and argon having a volume ratio of 1:1:2, respectively.
  • the present invention it is possible to provide a steel material which has better machinability in an annealed state than JIS-SKD11 and also has high toughness and weldability in connection with the material performance after hardening and tempering. Further, because the steel of the invention has characteristics which are close to JIS-SKD11 with respect to all of size variations due to heat treatment, hardenability and hardness change caused at the tempering temperature, it is possible to feed this steel into the same furnace as SKD11, increasing productivity and making it unnecessary to treat the steel under special conditions setting.
  • the machinability of the steel of the invention after hardening and tempering is much higher than that of JIS-SKD11, and film characteristics do not deteriorate even in surface treatment which is greatly affected by the amount of dissolved C present in the steel, such as CVD. Therefore, this steel can be easily manufactured as a die material with excellent wear resistance. Thus, the present invention has high industrial value.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Articles (AREA)
  • Heat Treatment Of Steel (AREA)
EP99123519A 1999-07-30 1999-11-25 Tool steel with excellent workability, machinability and heat treatment characteristics, and die using same Expired - Lifetime EP1072691B1 (en)

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JP2001303205A (ja) 2000-04-25 2001-10-31 Riken Corp 耐摩耗性及び疲労強度に優れた窒化鋼及び摺動部材
ATE549428T1 (de) * 2002-12-25 2012-03-15 Hitachi Metals Ltd Kaltarbeitsstahl mit hervorragenderunterdrückung von massänderungen
US8012272B2 (en) 2007-10-31 2011-09-06 Daido Tokushuko Kabushiki Kaisha Tool steels and manufacturing method thereof
CN101831653A (zh) * 2010-06-23 2010-09-15 天津市电力公司 马氏体高合金耐热钢金相检测抛光剂及其应用
JP4860774B1 (ja) * 2011-03-31 2012-01-25 日本高周波鋼業株式会社 冷間工具鋼
WO2014156487A1 (ja) * 2013-03-29 2014-10-02 日立金属株式会社 金型用鋼素材およびその製造方法、金型用プリハードン鋼材の製造方法、冷間加工用金型の製造方法
CN107208221B (zh) * 2015-02-04 2018-11-09 日立金属株式会社 冷作工具材料、冷作工具及其制造方法
CN105880475B (zh) * 2016-06-21 2017-11-10 王仙寿 发动机缸盖铸造模具
CN112899567B (zh) * 2021-01-18 2022-05-31 中国科学院金属研究所 一种高纯净、高强韧稀土易切削钢
CN113634738A (zh) * 2021-07-24 2021-11-12 共享铸钢有限公司 一种带轴球阀类铸件冒口及补贴的切割方法

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JPH04362153A (ja) * 1991-06-06 1992-12-15 Daido Steel Co Ltd 冷間加工性にすぐれた高硬度耐候性鋼
JPH07102346A (ja) * 1993-10-04 1995-04-18 Daido Steel Co Ltd 冷間加工性にすぐれた高硬度ステンレス鋼
JP3324630B2 (ja) * 1995-04-06 2002-09-17 住友金属工業株式会社 耐へたり性に優れた土木建設機械用強靭ピストンおよびその製造方法
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DE69801890T2 (de) * 1998-01-06 2002-03-28 Sanyo Special Steel Co., Ltd. Die Herstellung von einem Kaltarbeitswerkzeugstahl

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KR100368541B1 (ko) 2003-01-24
DE69917444D1 (de) 2004-06-24
EP1072691A2 (en) 2001-01-31
CN1097642C (zh) 2003-01-01
DE69917444T2 (de) 2005-06-02
KR20010012039A (ko) 2001-02-15
ATE267274T1 (de) 2004-06-15
EP1072691A3 (en) 2002-01-23
CN1282798A (zh) 2001-02-07

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