JP2007224418A - Hot tool steel having excellent toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot tool steel having improved toughness, and optimum for various kinds of hot tools such as a press die, a forging die, a diecasting die and an extrusion tool. <P>SOLUTION: The hot tool steel having excellent toughness has a composition comprising, by mass, 0.3 to <0.55% C, ≤1.5% Si, ≤1.5% Mn and 3.00 to 5.65% Cr, and comprising 0.001 to 0.015% Zn. Preferably, the content of Zn is 0.0025 to 0.01%. If required, the steel further comprises Mo and W independently or in combination by (Mo+1/2W): 1.0 to 3.5%, or may comprise 0.5 to 1.5% V and ≤1.5% Ni as well. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a hot work tool steel with improved toughness that is optimal for various hot tools such as a press die, a forging die, a die casting die, and an extrusion tool.

  Since a hot tool is used while being in contact with a high-temperature work material or a hard work material, it must have both strength and toughness that can withstand thermal fatigue and impact. For this reason, in the field of hot tools, for example, SKD61-based alloy tool steel, which is a JIS steel type, has been used.

Furthermore, it has been proposed to control the heat treatment for the purpose of improving the toughness of the alloy tool steel. For example, the carbide is uniformly dispersed by controlling cooling after hot working (see Patent Document 1), or the crystal grain size is made fine by controlling hot working temperature and heat treatment (see Patent Document 2). ) To improve toughness has been proposed. Moreover, as a hot work tool steel excellent in toughness, a method for regulating and managing various impurities has been proposed in addition to reconsidering the conventional amount of constituent elements added (see Patent Document 3).
JP 2001-294935 A JP-A-2005-163123 JP 2003-155540 A

  However, the above-described method of Patent Document 1 is difficult to obtain a fast cooling rate to the inside in the case of a steel material having a large cross section after hot working, or the temperature difference between the surface and the inside becomes large. It is difficult to control the entire steel material in the temperature range. In addition, the above-described method of Patent Document 2 is also difficult to warm-work at a low temperature because of the high deformation resistance of the steel material, or needs to be processed little by little, so it takes a lot of time and mass production. Not suitable for. The hot tool steel in the field of the present invention, which is required to be supplied as general-purpose steel, has a problem of reducing its manufacturing cost. However, the method of Patent Document 3 requires special management for various impurity elements.

  An object of the present invention is to provide a hot work tool steel having excellent toughness by a simpler method.

  As a result of intensive investigation on elements that are not expensive and special elements such as rare metals, but have already been widely used and have not been actively added as alloys in the field of steel materials, It has been found that the toughness can be greatly improved by adding to the range of the predetermined content, and the present invention has been achieved.

  That is, the present invention provides, in mass%, C: 0.3 to less than 0.55%, Si: 1.5% or less, Mn: 1.5% or less, and Cr: 3.00 to 5.65%. It is a hot work tool steel excellent in toughness, characterized in that it is Zn: 0.001 to 0.015%. Preferably, Zn: 0.0025 to 0.01%. If necessary, Mo and W may be contained alone or in combination (Mo + 1 / 2W): 1.0 to 3.5%, or V: 0.5 to 1.5%.

  Specifically, by mass%, C: 0.3 to less than 0.55%, Si: 1.5% or less, Mn: 0.1 to 1.5%, Ni: 1.5% or less (0 %), Cr: 3.00 to 5.65%, Mo and W alone or in combination (Mo + 1 / 2W): 1.0 to 3.5% or less, V: 0.5 to 1.5% Zn: 0.001 to 0.015%, a hot tool steel with excellent toughness composed of the balance Fe and inevitable impurities. Preferably, Zn: 0.0025 to 0.01%.

  INDUSTRIAL APPLICABILITY According to the present invention, the toughness of hot tool steel can be drastically improved, and this is an effective technique for practical application of hot tool steel that can be applied to various types of hot applications and environments.

As described above, an important feature of the present invention is that Zn is added in a predetermined content range. That is, the manufacturing method is almost the same as before, and the use of Zn as an alloy element leads to significant improvement in toughness. Furthermore, Zn may be used as a raw material for Zn-plated steel scrap used in large quantities in automobiles and the like without using Zn pure metal or Zn alloy, and is suitable for scrap recycling.
Hereinafter, the reason for limiting the components of the steel of the present invention will be described.

  C is an essential essential element for hot work tool steel, part of which is dissolved in the matrix to give strength, and part of it forms carbides to improve wear resistance and seizure resistance. Further, when C, which is a solid interstitial atom, is co-added with a substitution atom having a high affinity with C, such as Cr, the I (interstitial atom) -S (substitution atom) effect; solute atom dragging The effect of increasing the strength by acting as a resistance is also expected. However, if the content is less than 0.3% by mass, sufficient hardness and wear resistance as a tool member cannot be secured. On the other hand, excessive addition causes a decrease in toughness and hot strength, so the upper limit is made less than 0.55% by mass.

  Si is an element that enhances machinability as well as a deoxidizer during steelmaking. In order to obtain these effects, addition of 0.2% by mass or more is preferable. However, if too much is added, ferrite is generated, so the content is made 1.5% by mass or less.

  Mn has the effect of improving hardenability, suppressing the formation of ferrite, and obtaining appropriate quenching and tempering hardness. Moreover, it exists as nonmetallic inclusion MnS and has a great effect in improving machinability. In order to obtain this effect, addition of 0.1% by mass or more is preferable. However, if too much, the viscosity of the base is increased and machinability is lowered, so the content is made 1.5% by mass or less.

  Cr is an element having an effect of improving hardenability and forming carbides to improve the strengthening and wear resistance of the base, and contributes to the improvement of temper softening resistance and high temperature strength. It is an essential element for tool steel. In order to obtain these effects, it is necessary to add 3.00% by mass or more. However, excessive addition causes a decrease in hardenability and high temperature strength, so the upper limit is made 5.65% by mass.

  Zn is the most important additive element for the present invention, and remarkably improves toughness by dissolving in a matrix. In order to acquire this effect, 0.001 mass% or more of addition is required, Preferably it is 0.0025 mass% or more, More preferably, it is 0.003 mass% or more. On the other hand, even if a large amount is added, the effect reaches a peak, and if the solid solution in the base becomes insufficient, the toughness is deteriorated. Moreover, since the addition technique becomes complicated, the upper limit was made 0.015% by mass. Preferably, it is 0.01 mass% or less.

  And the reason which limited the object of the tool steel of this invention to hot tool steel is for making the most of the toughness improvement effect by Zn addition which is the biggest characteristic of this invention mentioned above. In other words, in the case of cold tool steel, hardness and wear resistance are particularly important required properties, and the component design is such that many carbides are distributed according to the usage environment, so in the structure Many carbides distributed greatly inhibit the action and effect of the Zn addition of the present invention. In the present invention, it is only when the object is limited to hot tool steel that the effects of addition of Zn are exhibited.

The following elements are preferable elements for achieving hot tool steel, and can be added according to the use environment.
Mo and W can be added alone or in combination to impart strength by precipitating and agglomerating fine carbides by tempering and improving softening resistance. Since W has an atomic weight approximately twice that of Mo, it is defined as Mo + 1 / 2W (of course, either one may be added, or both may be added together). And in order to acquire an above-described effect, addition of 1.0 mass% or more is preferable at (Mo + 1 / 2W). If the amount is too large, the machinability and toughness are reduced.

  V forms carbides and has the effect of strengthening the base and improving wear resistance. In addition, it increases temper softening resistance and suppresses coarsening of crystal grains, thereby contributing to improvement of toughness. In order to obtain this effect, 0.5% by mass or more may be added, but if it is too much, machinability and toughness are deteriorated, so that it is preferably 1.5% by mass or less.

  Ni is an element that suppresses the formation of ferrite. In addition, it provides excellent hardenability to the steel of the present invention together with C, Cr, Mn, Mo, W, etc., and even in the case of a slow quenching cooling rate, it forms a martensite-based structure and reduces toughness. It is an important additive element to prevent, and further provides an essential toughness improvement effect of the base. If it is too much, the viscosity of the base is raised and the machinability is lowered, so even if it is added, the content is preferably 1.5% by mass or less. More preferably, it is 0.1-1.5 mass% or less.

  In addition, in the hot work tool steel of this invention, addition is possible even if it is other element types other than the above according to the former. For example, Nb forms carbides and has the effect of strengthening the base and improving wear resistance. In addition, it increases temper softening resistance and suppresses coarsening of crystal grains, thereby contributing to improvement of toughness. In order to acquire this effect, 0.05 mass% or more is preferable, but if it is too much, machinability and toughness are reduced, so 0.3 mass% or less is preferable.

  In addition, Co forms a very dense and well-adhered protective oxide film when the temperature rises while the tool is in use, thereby preventing metal contact with the mating material and preventing temperature rise on the mold surface. Add as necessary to provide high wear resistance. In order to impart this effect, the content is preferably 0.3% by mass or more, but if it is too much, the toughness is lowered, so 5.0% by mass or less is preferable.

  Table 1 shows chemical components of the steels of the present invention and comparative steels. The steel of the present invention is obtained by adding Zn within the scope of the present invention to JIS-SKD61, which is generally used as a hot work tool steel, and the comparative steel is JIS-SKD61.

  These invention steel 1 and comparative steel 1 were melted 10 kg each in a vacuum induction melting furnace, subjected to homogenization heat treatment at 1250 ° C. for 5 hours, and then hot forged at 1150 ° C. to obtain a thickness of 30 mm. A steel material having a width of 60 mm and a length of about 700 mm was produced. Then, after annealing at 860 ° C., oil quenching was performed from 1020 ° C. with the test piece size used for the following evaluation, and tempering was performed at various temperatures to obtain evaluation samples of respective hardnesses.

  FIG. 1 shows the results of a 2 mm U-notch Charpy impact test of the inventive steel 1 and comparative steel 1 of Table 1 produced as described above at room temperature. Compared with the comparative steel 1 to which no Zn is added, the steel 1 of the present invention has an excellent Charpy impact value. Among them, the steel 1 of the present invention maintains extremely high toughness even in a high hardness region where toughness usually decreases.

  FIG. 2 shows the results of a 2 mm V notch Charpy impact test conducted on samples prepared in the same manner as in Table 1 of the present invention steel 1 and comparative steel 1 to 45 HRC between room temperature and 250 ° C. It can be seen that the steel 1 of the present invention has an excellent Charpy impact value at any temperature range as compared with the comparative steel 1 to which no Zn is added.

  Next, Table 2 shows chemical components of practical steel ingots produced in the mass production process. That is, a 2.6-ton steel ingot melted and formed in a 50-ton arc melting furnace was subjected to a homogenization heat treatment at a predetermined temperature of 1200 ° C. or more for 10 hours, and then hot forging and annealing. A columnar steel material having a diameter of about 100 mm was produced while repeating the treatment. Thereafter, oil quenching was performed from 1020 ° C. at the test piece size used for the following evaluation, and tempering was performed at various temperatures to obtain evaluation samples of respective hardnesses. The steel of the present invention in Table 2 is also obtained by adding Zn to JIS-SKD61 within the scope of the present invention, and the comparative steel is JIS-SKD61.

  FIG. 3 shows the results of a 2 mm U notch Charpy impact test of the present invention steels 2 to 6 and comparative steel 2 in Table 2 produced as described above at room temperature. Even in a large practical steel ingot, it can be seen that the inventive steels 2 to 6 are superior in Charpy impact value compared to the comparative steel 2 to which no Zn is added.

  By applying the present invention to improve the toughness of hot work tool steel, not only can it be applied to various hot tools such as press dies, forging dies, die casting dies, and extrusion tools, but also the usage load is increased. It can also be applied to hot tool members such as large molds.

It is the figure which plotted the 2 mmU notch Charpy impact value in room temperature of this invention steel and comparative steel tempered to various hardness with respect to the hardness of a test piece. It is the figure which plotted the 2mmV notch Charpy impact value between room temperature and 250 degreeC of this invention steel and comparative steel tempered to the hardness of 45HRC with respect to test temperature. It is the figure which plotted the 2 mmU notch Charpy impact value in room temperature of this invention steel and comparative steel produced from a practical steel ingot with respect to the hardness of a test piece.

Claims (6)

It is a hot work tool steel containing, in mass%, C: 0.3 to less than 0.55%, Si: 1.5% or less, Mn: 1.5% or less, and Cr: 3.00 to 5.65%. Zn: 0.001 to 0.015% hot tool steel with excellent toughness. The hot work tool steel excellent in toughness according to claim 1, wherein the mass% is Zn: 0.0025 to 0.01%. The hot work tool steel excellent in toughness according to claim 1 or 2, wherein Mo and W are individually or in combination of (Mo + 1 / 2W): 1.0 to 3.5%. The hot work tool steel excellent in toughness according to any one of claims 1 to 3, wherein V is 0.5 to 1.5% in mass%. In mass%, C: 0.3 to less than 0.55%, Si: 1.5% or less, Mn: 0.1 to 1.5%, Ni: 1.5% or less (including 0%), Cr : 3.00 to 5.65%, Mo and W are used alone or in combination (Mo + 1 / 2W): 1.0 to 3.5%, V: 0.5 to 1.5%, Zn: 0.001 A hot work tool steel excellent in toughness, characterized by comprising 0.015%, the balance Fe and inevitable impurities. The hot work tool steel with excellent toughness according to claim 5, wherein the mass% is Zn: 0.0025 to 0.01%.