JP2009221594A - Hot-working tool steel having excellent toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-working tool steel most suitable for various hot-working tools such as a press die, forging die, die-casting die and extruding tool, and having enhanced toughness. <P>SOLUTION: The hot-working tool steel comprises, by mass, 0.34 to 0.40% C, >0.5 to 0.8% Si, 0.45 to 0.75% Mn, 0 to <0.5% Ni, 4.9 to 5.5% Cr, Mo and/or W singly or totally in an amount of 2.5 to 2.9% in (Mo+1/2W) terms, 0.5 to 0.7% V, and the balance Fe with unavoidable impurities. The hot-working tool steel exerts a superior toughness-improvement effect, particularly in a high hardness range of 45 HRC or higher. Accordingly, the tool steel can be applied not only to various hot-working tools such as a press die, forging die, die-casting die and extruding tool, but also to a hot-working tool member to which a heavy load is applied during use. <P>COPYRIGHT: (C)2010,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. More recently, the temperature of workpieces has increased due to the shortening of the manufacturing time of products manufactured using hot tools and the formation of complex shapes, and dies due to the simultaneous processing of multiple products. Since hot tools such as these are becoming larger, hot tool materials are required to be able to secure high toughness up to the inside even at higher high-temperature strength and large size.

For the purpose of improving the toughness and high temperature strength of the alloy tool steel, a method for improving the high temperature strength while maintaining the toughness by determining the chemical composition range (see Patent Document 1) and the amount of residual carbide are specified. Thus, a technique for improving toughness and high-temperature strength has been proposed (see Patent Document 2).
JP-A-2-179848 JP 2000-328196 A

  However, although the above-mentioned Patent Document 1 cannot evaluate the toughness level because there is no specific measurement value of toughness, the toughness and the high-temperature strength are sufficient to judge from the examination results made by the present inventors. In order to achieve a high level, the range of the chemical composition is not sufficiently limited. In the method of Patent Document 2 described above, the toughness and the high temperature strength are greatly affected by the structure such as the martensite structure and the bainite structure after quenching. It is not enough to specify the amount of carbide.

  Accordingly, the applicant of the present application has proposed Japanese Patent Application No. 2007-222220 as a hot tool steel that has achieved a significant improvement in toughness after having a sufficient high-temperature strength. That is, in mass%, C: 0.34 to 0.40%, Si: 0.3 to 0.5%, Mn: 0.45 to 0.75%, Ni: 0 to less than 0.5%, Cr : 4.9 to 5.5%, Mo and W are used alone or in combination (Mo + 1 / 2W): 2.5 to 2.9%, V: 0.5 to 0.7%, remaining Fe and inevitable impurities Is a hot work tool steel with a basic composition.

  However, with regard to the above proposal, the toughness improving effect exhibited on the other hand is very remarkable as compared with the maintained high-temperature strength level. Therefore, there is room for further study in the development of “hot tool steel with excellent toughness” that can benefit from the effect of maximizing the toughness improvement effect.

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

  As a result of the inventor's earnest research, in Japanese Patent Application No. 2007-222220, it is found that the structure after quenching greatly affects toughness and high-temperature strength, and is suitable for combining excellent toughness and high-temperature strength. Revealed later organization. And in order to obtain the suitable structure | tissue after hardening, it discovered that the very narrow favorable composition area | region obtained only by controlling content of each element to the optimal range exists. In addition, in Japanese Patent Application No. 2007-222220, there was room for further study on the adjustment amount of Si, which is an elemental species that acts on the remarkable improvement of toughness, with respect to the improvement effect of toughness and high-temperature strength. As a result, the present invention has been achieved.

  That is, the present invention is mass%, C: 0.34 to 0.40%, Si: more than 0.5 to 0.8%, Mn: 0.45 to 0.75%, Ni: 0 to 0.5. %, Cr: 4.9 to 5.5%, Mo and W alone or in combination (Mo + 1 / 2W): 2.5 to 2.9%, V: 0.5 to 0.7%, balance Fe Further, it is a hot work tool steel excellent in toughness characterized by comprising inevitable impurities. The hot tool steel of the present invention may be used after adjusting its hardness to 40 HRC or higher, for example, and exhibits excellent toughness particularly in a high hardness region of 45 HRC or higher.

Here, the hot tool steel of the present invention is preferably one or more of C, Si, Mn, Ni, Cr, Mo, W, and V elements constituting the hot tool steel of the present invention. The narrow composition range is satisfied. In this, of course, it is desirable to satisfy all of them.
C: 0.35-0.39%,
Si: more than 0.5 to 0.7%,
Mn: 0.5 to 0.7%,
Ni: 0.01 to 0.3%,
Cr: 5.0 to 5.4%,
Mo and W are single or combined (Mo + 1 / 2W): 2.6 to 2.8%,
V: 0.55-0.65%

  According to the present invention, the toughness of hot tool steel can be combined at a very high level. And this effect is exhibited to the maximum when it is tempered in a high hardness region of 45 HRC or more, further 46 HRC or more, as well as in a hardness region of 40 HRC or more. Therefore, this is an effective technique for practical application of hot tool steel that can be applied to various uses and environments.

  As described above, an important feature of the present invention is that the content of each element is controlled within an optimum range. In other words, by controlling the content of each element to a limited range, the manufacturing method remains the same, and even at a wide range of quenching and cooling rates, the high temperature strength is kept within the practical range and the toughness is at a high level. There is a narrow composition range in which a tissue that can be maintained can be obtained, and is characterized by the fact that it can be specified. That is, in the basic element, the relationship of the amount of C—Cr follows the conventional balance, but the optimum adjustment of Mo, W, V of other carbide forming elements interrelated to this, and the basic element It is important to adjust Si and Ni, which have a great influence on the resulting characteristics of the adjustment. Hereinafter, the reason for limiting the components constituted by the narrow composition range 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 0.34 mass% (hereinafter simply referred to as%), 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 0.40%. Preferably it is 0.35-0.39%, More preferably, it is 0.36-0.38%.

  Si is an element that enhances machinability as well as a deoxidizer during steelmaking. In order to obtain these effects, addition of 0.3% or more is necessary, but if it is too much, needle-like bainite is generated to reduce toughness, or cementite-based carbides in the bainite structure during quenching cooling. Inhibiting the precipitation of aluminum indirectly promotes the precipitation, aggregation and coarsening of alloy carbides during tempering, thereby lowering the high temperature strength.

  Therefore, the applicant of the present application has proposed an optimal Si region of 0.3 to 0.5% in Japanese Patent Application No. 2007-222220 in order to combine this toughness and high-temperature strength at a high level. However, in the upper limit management of the Si amount, among the toughness and high-temperature strength, the contribution to the improvement of toughness is particularly high as described above. Therefore, the present inventor reviewed again the Si upper limit value that can enjoy the effect of improving toughness while keeping the high temperature strength in a practical range. As a result, although it is lower than the toughness value of the proposed steel of Japanese Patent Application No. 2007-222220, if Si is 0.8% or less, it has excellent machinability and is equal to or higher than that of conventional steel. It has been found that toughness can be secured. From the above, the Si of the present invention considering the balance between toughness and machinability was made to exceed 0.5 to 0.8%. Preferably it is more than 0.5 to 0.7%.

  Mn has the effect of improving hardenability, suppressing the formation of ferrite, and obtaining appropriate quenching and tempering hardness. Moreover, if it exists in a structure | tissue as nonmetallic inclusion MnS, there exists a big effect in the improvement of a machinability. In order to obtain these effects, addition of 0.45% or more is necessary. However, if too much is added, the base viscosity is increased and machinability is lowered. Preferably it is 0.5 to 0.7%.

  If Ni is too much, it will increase the viscosity of the base and lower the machinability or lower the high temperature strength, so it needs to be less than 0.5%. Preferably, it is restricted to 0.3% or less. However, 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, a martensite-based structure is formed and the toughness is reduced. It is also an important additive element for preventing, and may further be added in an amount of about 0.01% because it provides an essential toughness improving effect of the base.

  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 4.9% or more. However, excessive addition causes a decrease in hardenability and high temperature strength, so the upper limit is made 5.5%. Preferably it is 5.0 to 5.4%, more preferably 5.1 to 5.3%.

  Mo and W can be added singly or in combination to enhance hardenability, precipitate fine carbides by tempering, impart strength, and improve softening resistance. Since W has an atomic weight approximately twice that of Mo, it can be defined as Mo + 1 / 2W (of course, either one may be added or both may be added together). In order to obtain the above-described effect, it is necessary to add 2.5% or more at (Mo + 1 / 2W). If the amount is too large, the machinability is lowered and the toughness is lowered due to the formation of acicular bainite, so (Mo + 1 / 2W) is 2.9% or less. Preferably, it is 2.6 to 2.8% in (Mo + 1 / 2W).

  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, it is necessary to add 0.5% or more, but if it is too much, the machinability and toughness are lowered, so the content is made 0.7% or less. Preferably it is 0.55-0.65%.

  Note that main elements that may remain as inevitable impurities are P, S, Co, Cu, Al, Ca, Mg, O, N, and the like. In order to achieve the maximum effect of the present invention, these should be as low as possible. However, on the other hand, there are additional features such as inclusion shape control, other mechanical properties, and improvement in production efficiency. Some contents and / or additions may be added for the purpose of obtaining various effects. In this case, P ≦ 0.03%, S ≦ 0.01%, Co ≦ 0.05%, Cu ≦ 0.25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01 %, O ≦ 0.01%, N ≦ 0.03%, it is considered that the basic properties of the hot work tool steel of the present invention are not particularly affected. It is the upper limit of regulation.

  Table 1 shows chemical compositions of the steels of the present invention, comparative steels, reference steels and conventional steels. The reference steel is a steel having the chemical composition of Japanese Patent Application No. 2007-222220 proposed by the applicant, and the conventional steel is a hot tool steel outside the component range of the present invention. is there.

  These inventive steels, comparative steels, reference steels and conventional steels were produced by the following procedure. First, a steel ingot of 10 kg each in a vacuum induction melting furnace is subjected to homogenization heat treatment at 1250 ° C. for 5 hours, and then hot forged at 1150 ° C. to produce a steel material 30 mm thick × 60 mm wide. did. Then, after annealing at 860 ° C., quenching was performed at 1030 ° C. Quenching is performed by pressurized gas cooling, and the time required for cooling from the quenching temperature (1030 ° C.) to an intermediate temperature (525 ° C.) between the quenching temperature and room temperature (20 ° C.) is defined as a semi-cooling time. In the case (for example, when it takes 10 minutes to cool from 1030 ° C. to 525 ° C., it is expressed as “semi-cooled 10 minutes”). It was cooled in about 40 minutes semi-cooling as a part corresponding to the part where the cooling rate becomes slow. Thereafter, it was tempered at various temperatures and tempered to a hardness of 46HRC.

  From the steel of the present invention, the comparative steel, the reference steel and the conventional steel produced as described above, the longitudinal direction of the test piece comes in the width direction of the steel material after forging, and the notch direction of the test piece comes in the longitudinal direction of the steel material. Table 2 shows the results of a Charpy impact test at room temperature using a 2 mm U-notch Charpy impact test piece prepared as described above (that is, taken from the T direction).

  From the results of Table 2, if quenching is performed at the time of quenching, even the conventional steel 20 outside the composition of the present invention can have a structure mainly composed of martensite, so even a test piece taken from the T direction has a relatively high impact. A value is obtained. However, when cooling is slowed down to about 40 minutes during quenching, in addition to the conventional steel 20 not only having a low Mo amount, but also a low C amount and Ni are not added, the hardenability is considerably inferior, The impact value is the lowest.

  In contrast, the steel 1 of the present invention has a larger amount of Si than the reference steel 10 proposed in Japanese Patent Application No. 2007-222220, and thus a needle-like bainite structure tends to develop among bainite. Even if the speed is low, the sufficient toughness is maintained as compared with the conventional steel 20. And the comparative steel 30 which has the tendency which the acicular bainite structure developed like this invention steel 1 has satisfy | filled the amount of Si of this invention, but since the amount of Mo is low, compared with this invention steel 1. It is inferior in hardenability and has a low impact value when semi-cooled for 40 minutes.

  Table 3 shows the chemical composition of the steel of the present invention.

  These inventive steels were produced by the following procedure. First, a steel ingot of 10 kg in a vacuum induction melting furnace is subjected to homogenization heat treatment at 1250 ° C. for 5 hours, and then hot forged at 1150 ° C. to produce a steel material having a thickness of 35 mm × 60 mm. did. Then, after annealing at 870 ° C., quenching was performed at 1030 ° C. Quenching was performed by pressurized gas cooling, and cooling was performed for about 3 minutes and 40 minutes for semi-cooling in the semi-cooling time defined above. Thereafter, it was tempered at various temperatures and tempered to a hardness of 46HRC.

  Table 4 shows the results of a Charpy impact test at room temperature using a 2 mm U-notch Charpy impact test piece prepared from the T-direction of the steel of the present invention shown in Table 3 prepared as described above.

  From the results of Table 4, steels 2, 3, 4 and 5 of the present invention tend to have a needle-like bainite structure developed in bainite due to an increase in the amount of Si, but sufficient toughness even when the cooling rate is slow. Is maintained.

  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.

Claims (10)

In mass%, C: 0.34 to 0.40%, Si: more than 0.5 to 0.8%, Mn: 0.45 to 0.75%, Ni: 0 to less than 0.5%, Cr: 4.9-5.5%, Mo and W alone or in combination (Mo + 1 / 2W): 2.5-2.9%, V: 0.5-0.7%, balance Fe and unavoidable impurities Hot tool steel with excellent toughness characterized by The hot work tool steel excellent in toughness according to claim 1, wherein the mass% is C: 0.35 to 0.39%. The hot work tool steel excellent in toughness according to claim 1 or 2, wherein the mass% is Si: more than 0.5 to 0.7%. The hot work tool steel excellent in toughness according to any one of claims 1 to 3, wherein Mn is 0.5 to 0.7% by mass. The hot work tool steel excellent in toughness according to any one of claims 1 to 4, wherein Ni is 0.01 to 0.3% in mass%. The hot work tool steel excellent in toughness according to any one of claims 1 to 5, wherein Cr: 5.0 to 5.4% by mass. The hot and excellent toughness according to any one of claims 1 to 6, characterized in that Mo and W are individually or in combination of (Mo + 1 / 2W): 2.6 to 2.8%. Tool steel. The hot work tool steel excellent in toughness according to any one of claims 1 to 7, wherein V is 0.55 to 0.65% in mass%. The hot work tool steel excellent in toughness according to any one of claims 1 to 8, wherein the hardness is 40 HRC or more. The hot work tool steel excellent in toughness according to any one of claims 1 to 8, wherein the hardness is 45 HRC or more.