WO2010074017A1 - Steel tempering method - Google Patents

Abstract

Disclosed is a tempering method to yield hot work tool steel with high toughness. Disclosed is a method for tempering hot work tool steel made from, by mass %, C: 0.32-0.45%, Si: 0.01-0.8% or less, Mn: 0.1-0.8%, Ni: 0-0.8% or less, Cr: 4.5-5.6%, Mo and W, singly or in composition (Mo+1/2W): 2.0-3.5%, V: 0.5-1.0%, Co: 0-2.0%, and the remainder Fe and inevitable impurities, wherein which steel tempering method performs rapid cooling at a fast rate of 80 minutes or less from a 1020-1070°C tempering temperature to 530°C. Preferably, the steel tempering method, after rapid cooling at the fast rate described above, then performs cooling at a slower rate of 60-250 minutes to 150°C.

Description

Hardening method of steel

The present invention relates to a quenching method for obtaining a high toughness hot tool steel suitable for various types of 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 needs to have strength and toughness that can withstand thermal fatigue and impact. Therefore, in the field of hot tools in the related art, for example, a hot tool steel of SKD61 series, which is a JIS steel type, has been used. Recently, to shorten the production time of products manufactured using hot tools, and to form the complex shape, the temperature of the material being processed has been increased, and to process multiple products simultaneously. Since hot tools such as molds are also becoming larger accordingly, hot tool materials are required to be able to secure even higher high-temperature strength and high toughness up to the inside even in a large size.

Therefore, in order to improve the high temperature strength and toughness of the hot-work tool steel, an element having SKD 61 as a basic component and further increasing elements forming carbides contributing to secondary hardening during tempering, or enhancing hardenability Improved and improved steel types have been developed by increasing and adding Nb (see Patent Documents 1 and 2).

In addition, with the aim of improving the toughness of hot-work tool steel, the cooling coefficient is gradually increased during quenching and cooling, and the quenching cooling rate is reduced, such as refining the bainite structure and the martensite structure. An adjusted method has been proposed (see Patent Documents 3 to 5).

Patent No. 3191008 JP, 2008-095181, A JP, 2008-085532, A JP, 2006-342377, A JP, 2005-171305, A

In the quenching methods of Patent Documents 3 and 4, the basic structure of bainite such as SKD 61, especially a basic steel type in which a coarse structure such as upper bainite is apt to occur, can be made finer in base structure, and suppression of pearlite structure It is excellent at being able to Further, the quenching method of Patent Document 5 for refining the intragranular structure is effective for maintaining the toughness even for similar steels of SKD 61 in which W and Mo are enhanced to improve wear resistance. However, for the improved steel type including more carbide-forming elements as described in Patent Documents 1 and 2 described above, it is difficult to reliably exert the effect of the toughness improvement.

That is, the above-described improved steels are inherently high in hardenability, and as shown in the continuous cooling transformation diagram (CCT curve) of FIG. 1, bainite transformation is shifted to a lower temperature for a longer time than SKD61. Therefore, it is not necessary to increase the cooling rate in the low temperature range as in SKD61. Rather, the problem is a high temperature range between the quenching temperature and about 500 ° C., and during the cooling, intergranular carbides are likely to precipitate and grow, and their influence on the toughness is extremely large. Therefore, even if the method for obtaining heat, such as Patent Document 3 for SKD 61, is applied to improved steel types, improvement in toughness is surely expected because the examination of the cooling rate in the high temperature region is insufficient. hard. If the toughness is low, even if other properties such as high temperature strength are excellent, it can often not be used as a hot tool.

Therefore, an object of the present invention is a quenching method capable of achieving more reliably excellent toughness only in a hot tool steel which contains a large amount of carbide forming elements and is excellent in high temperature strength as proposed in Patent Documents 1 and 2. To provide.

As a result of intensive studies conducted by the present inventor, the toughness of the steel having the above-mentioned specific composition range different from that of SKD61 is largely influenced by the degree of precipitation and growth of intergranular carbides during quenching and cooling. I found out And, by elucidating the mechanism at that time, the optimum quenching conditions can be clarified, and the present invention has been achieved.

That is, in the present invention, C: 0.32 to 0.45%, Si: 0.01 to less than 0.8%, Mn: 0.1 to 0.8%, Ni: 0 to 0.8 in mass%. %, Cr: 4.5 to 5.6%, Mo and W alone or in combination (Mo + 1/2 W): 2.0 to 3.5%, V: 0.5 to 1.0%, Co: In a method of quenching a hot tool steel comprising 0 to 2.0%, the balance Fe and unavoidable impurities,
A quenching method for steel characterized by quenching from a quenching temperature of 1020 to 1070 ° C. to 530 ° C. at a high speed within 80 minutes. Preferably, fast speeds within 45 minutes. It is desirable that (Mo + 1/2 W) of the hot-work steel is greater than 2.5%.

And preferably, in addition to the above-mentioned quenching method, after quenching from the quenching temperature of 1020 to 1070 ° C. to 530 ° C. at the above-mentioned fast speed, the subsequent cooling to 150 ° C. is slow for 60 minutes or more It is a quenching method of steel cooled at speed. At this time, it is desirable to cool at a speed of 250 minutes or less.

According to the present invention, a hot tool steel containing a large amount of carbide-forming elements and excellent in high temperature strength can be provided with a very high level of toughness. Therefore, it becomes an effective technique for practical use of a hot tool steel applicable to various hot uses and environments.

It is a conceptual diagram which shows the relationship between the CCT curve of SKD61 and the improved steel type made object of this invention, and a quenching-cooling curve. It is a conceptual diagram which shows the relationship between the CCT curve of the improved steel grade made into the object of the present invention, and the quenching cooling curve of an example of the present invention, and a comparative example. It is a conceptual diagram which shows the relationship between the CCT curve of the improved steel grade made into the object of the present invention, and the quenching cooling curve of an example of the present invention. It is a conceptual diagram which shows the relationship between the CCT curve of the improved steel grade made into the object of the present invention, and the quenching cooling curve of an example of the present invention, and a comparative example. It is a figure which evaluates the toughness after tempering when the quenching method of this invention example and a comparative example is applied to the improved steel type (steel A) made into the object of this invention. It is a figure which evaluates the toughness after tempering when the quenching method of this invention example is applied to the improved steel type (steel B) made into the object of this invention. It is a figure which evaluates the toughness after tempering when the quenching method of this invention example and a comparative example is applied to the improved steel type (steel C) made into the object of this invention. It is a figure which evaluates the toughness after tempering when the quenching method of this invention example and a comparative example is applied to the improved steel type (steel D) made into the object of this invention.

As described above, one of the features of the present invention is that the steel type to be subjected to the present quenching has a component composition in which grain boundary precipitation is likely to occur during quenching and cooling is significantly reduced. It is limited. That is, for a steel type that is greatly affected by toughness by the quenching method, if the quenching method whose conditions described later are specified is applied, the toughness can be possessed at a high level, and other excellent characteristics represented by high temperature strength Will be able to make the most of Hereinafter, the reason for the limitation of the composition of the steel composed of the narrow composition range to be provided to the present invention will be described.

C is an essential essential element for hot tool steels, in which a part is solid-solved in a matrix to impart strength, and a part forms carbides to enhance wear resistance and seizure resistance. In addition, C, which is a solid solution interstitial atom, is I (interstitial atom) -S (substituted atom) effect when it is co-added with a substitutional atom having a large affinity to C such as Cr; The effect of acting as resistance and strengthening is also expected. However, if the content is less than 0.32% by mass, sufficient hardness and wear resistance as a tool member can not be secured. On the other hand, the excessive addition causes a decrease in toughness and hot strength, so the upper limit is made 0.45 mass% (hereinafter simply referred to as%). Preferably, it is 0.34% or more and / or 0.42% or less. More preferably, it is 0.40% or less.

Si is an element that enhances machinability as well as being a deoxidizer during steel making. In order to obtain these effects, addition of 0.01% or more is necessary, but if it is too much, bainite structure is developed to lower the toughness. Further, by suppressing precipitation of cementite-based carbides in the bainite structure during quenching and cooling, precipitation, aggregation and coarsening of alloy carbides during tempering are indirectly promoted to lower the high temperature strength. Therefore, it is less than 0.8%. Preferably, it is 0.1% or more and / or 0.6% or less.

Mn has an effect of enhancing hardenability, suppressing the formation of ferrite, and obtaining a suitable hardening and tempering hardness. Moreover, if it exists in a structure | tissue as nonmetallic inclusion MnS, it has a big effect in the improvement of machinability. In order to obtain these effects, addition of 0.1% or more is necessary, but if it is too large, the viscosity of the base is increased to reduce machinability, so the content is made 0.8% or less. Preferably, it is 0.3% or more and / or 0.7% or less.

Ni is an element that suppresses the formation of ferrite. Further, together with C, Cr, Mn, Mo, W, etc., the steel of the present invention has excellent hardenability, and also has the effect of suppressing the formation of a bainitic structure even in the case of a moderate quenching cooling rate. Therefore, it is effective to form a martensite-based structure and prevent a decrease in toughness. Furthermore, it is a preferable element to be added because it provides the essential toughness improvement effect of the matrix. The addition of Ni is optional, but if it is too large, the viscosity of the base is increased to lower the machinability or the high temperature strength is lowered, so it is necessary to make it less than 0.8%. Preferably it is 0.5% or less.

Cr is an element that has an effect of enhancing hardenability and forming carbides to strengthen the base and improve the wear resistance. And it is an essential element for the hot tool steel of the present invention which contributes to the improvement of temper softening resistance and high temperature strength. To obtain these effects, it is necessary to add 4.5% or more. However, the upper limit is set to 5.6% because excessive addition causes a reduction in hardenability and high temperature strength. Preferably it is 4.9% or more and / or 5.4% or less.

Mo and W can be added singly or in combination in order to enhance hardenability and to precipitate fine carbides by tempering to impart strength and improve softening resistance. At this time, since W has an atomic weight approximately twice that of Mo, the addition amount thereof can be defined by (Mo + 1/2 W). And in order to acquire the above-mentioned effect, 2.0% or more of addition is required by (Mo + 1 / 2W). If the amount is too large, the machinability is lowered, and the precipitation / growth of grain boundary carbides described later is promoted and the toughness is lowered due to the increase of the amount, so the content is made 3.5% or less in (Mo + 1 / 2W). Preferably, it is 2.2% or more and / or 3.0% or less in (Mo + 1 / 2W). Then, in the same way as the above-mentioned Cr, the lower limit of (Mo + 1/2 W) is also limited to more than 2.5% in that it is meaningful to target a hot tool steel containing a large amount of carbide-forming elements. Is preferably limited to 2.6% or more.

V forms carbides and has the effect of strengthening the matrix and improving the wear resistance. Moreover, while raising temper softening resistance, it suppresses the coarsening of a crystal grain and contributes to the improvement of toughness. In order to obtain this effect, it is necessary to add 0.5% or more, but if too much, as with Mo and W, the machinability and toughness will be reduced, so it is made 1.0% or less. Preferably, it is 0.55% or more and / or 0.85% or less.

Co forms a protective oxide film which is extremely dense and has good adhesion at the time of temperature rise during use of a tool. This is a preferable element of addition because it prevents metal contact with the opposite material, prevents temperature rise on the surface of the mold and provides excellent abrasion resistance. Although the addition of Co is optional, the upper limit is made 2.0% or less because the toughness is lowered if it is too much. Preferably, it is 1.0% or less.

As an unavoidable impurity, the main elements which may remain | survive are P, S, Cu, Al, Ca, Mg, O, N etc. In order to achieve the function and effect of the present invention as much as possible, it is desirable that they be as low as possible, but on the other hand, additional effects such as inclusion shape control and other mechanical properties or improvement of production efficiency In order to obtain various effects, some of them can be contained and / or added. In this case, P ≦ 0.03%, S ≦ 0.01%, Cu ≦ 0.25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01%, O ≦ 0.01 %, N ≦ 0.03% is considered to have no significant effect on the toughness of the hot tool steel obtained by the quenching method of the present invention, so this range is acceptable and preferable regulation It is an upper limit.

And what is the greatest feature of the present invention is the quenching method for the improved steel found according to the heat treatment characteristics inherent to the improved steel of the above-mentioned composition. That is, for the above-described improved steel having a component composition different from that of the conventional SKD 61, the “quenched structural factor” that affects the toughness is also different from the SKD 61. Therefore, it was necessary to identify the optimum quenching method for the component range steel of the present invention (hereinafter, also referred to as improved steel) by studying the quenched structural factor. That is, it is a method of quenching steel characterized in that a hot tool steel satisfying the above-described composition is quenched from a quenching temperature of 1020 to 1070 ° C. to 530 ° C. at a high speed within 80 minutes. . Preferably, it is within 60 minutes, further within 45 minutes. And, after quenching at this high speed, it is preferable to cool at a low speed such that the subsequent cooling to 150 ° C. is 60 minutes or more. More preferably, it is 80 minutes or more.

When quenching a hot tool steel, if it is oil quenched with a small block size of about 10 mm square, it will obtain a single structure of martensite, and the toughness will show the highest level of the steel. However, in the case of a practical steel, the quenching and cooling rate is slowed by the increase in size of the steel to be quenched, etc., and carbides are precipitated and grown at austenite grain boundaries in high temperature range from the quenching temperature to about 600 ° C. Usually, bainite structure is formed in a low temperature range of about 500 ° C. or less, and the level of toughness is reduced. In this regard, even the special improved steel to which the present invention is applied is the same as the CCT curve of FIG. Therefore, in the quenching method of the present invention, the cooling management is divided into the high temperature region and the low temperature region.

Then, according to the above, specific cooling conditions in the high temperature region and the low temperature region will be studied, but the conditions are simple in order to achieve the function and effect of the present invention with maximum reproducibility. It is desirable to be easy to handle. In other words, in the management of the cooling rate based on the “one point” temperature passing during cooling, the cooling conditions necessary for each of the upper and lower cooling zones bordering the reference temperature are set to be easy to control It is specification of the "optimum reference temperature" which can be performed. And, in the case of the improved steel of the present invention, this reference temperature is 530.degree.

And, in the improved steel of the present invention, in which the amount of alloying elements is made higher than SKD 61 in order to enhance hardenability and high temperature strength, precipitation and growth of intergranular carbides in high temperature region become faster and more, which exerts a decrease in toughness. The impact is large (see Figure 1 above). Therefore, in the present invention in which the modified steel is to be subjected to quenching, it is necessary to rapidly quench only the high temperature region from the quenching temperature of 1020 ° C. to 1070 ° C. to 530 ° C. That is a fast speed, specifically within 80 minutes. Preferably, the quenching is performed at a high speed within 60 minutes and within 45 minutes, more preferably within 30 minutes.

Next, martensitic transformation and bainitic transformation occur in the low-temperature region of 530 ° C. or lower in the improved steel of the present invention. Therefore, in the high temperature range, when entering into these transformation zones while maintaining the above-mentioned fast cooling rate of the present invention, a large temperature difference occurs between the surface side and inside of the material, and the timing of transformation is also large on the material surface side and inside As a result, a large stress may be generated to cause deformation or cracking. In addition, since the above-described improved steel is excellent in hardenability and has a component design in which a rough bainite structure that significantly reduces toughness is difficult to be formed, an extremely fast cooling rate is not necessary in a low temperature range.

Therefore, after rapidly cooling the above-mentioned high temperature region, it is preferable that the subsequent cooling be performed at a slow speed at which the above problems hardly occur. And, the cooling at this time is sufficient if it is up to 150 ° C. where martensitic transformation or bainitic transformation is almost completed and the problem of large stress generation due to the shift of transformation timing in and out of the material is solved. Specifically, the time required for cooling from 530 ° C. to 150 ° C. is a slow cooling rate of 60 minutes or more. More preferably, it is 80 minutes or more.

However, even if it is too slow, the coarse bainite structure may be formed, so it is desirable to set the upper limit of the cooling time in the low temperature range. In this case, if the time required for cooling from 530 ° C. to 150 ° C. is faster than the speed at which it takes 250 minutes, it is effective for preventing the formation of coarse bainite structure which significantly reduces the toughness.

In the present invention, for example, in the above-mentioned reference temperature and the temperature range of 20 ° C. above and below the reference temperature, “isothermal holding” for adjusting the cooling rate in the cooling process from the high temperature range to the low temperature range is Permissible. The conditions such as the isothermal holding temperature and time at this time should be set in a range that does not affect the effects of the cooling conditions of the present invention as much as possible (that is, the range in which phase transformation does not easily occur for the steel to be quenched). desirable. The isothermal holding time does not add to the time required for each cooling of the present invention.

Table 1 shows chemical components of the hot tool steel used in this example. That is, the hot-work tool steels in Table 1 are all “well-known” modified steels within the component range of the present invention, and are the optimum samples for evaluating the toughness improvement effect by the quenching method of the present invention is there. CCT curves of these samples (Steels A to D) are as shown in FIG.

For these materials, a steel ingot produced by electro-slag remelting was used to prepare an electrode formed by primary melting and forging a steel A with 40 tons and a steel B with a 15 ton arc melting furnace. Then, the steel ingot is subjected to homogenization heat treatment at a predetermined temperature of 1200 ° C. or more, and then repeated by hot forging and annealing treatment to obtain a steel material of about 150 mm thickness × 500 mm width. Then, after annealing at 860 ° C., a rough specimen of a specimen having a size approximately 1 mm larger than that of the Charpy impact specimen, so that the thickness direction after forging is the longitudinal direction of the specimen. Were subjected to a 1030 ° C. quenching treatment.

In the materials of steels C and D, steel ingots produced by melting 10 kg each in a vacuum induction melting furnace were prepared. Then, the steel ingot is subjected to homogenization heat treatment at a predetermined temperature of 1200 ° C. or more, and then hot forged to obtain a steel material of 30 mm thickness × 60 mm width. Then, after annealing at 860 ° C., the steel material is roughly machined with a size roughly 1 mm larger than Charpy impact test specimen so that the width direction after forging will be the longitudinal direction of the specimen. It was collected and subjected to 1030 ° C. quenching treatment.

The above hardening was performed by the method shown in Table 2. As the quenching refrigerant, nitrogen gas at a predetermined pressure and air were selected and used (both quenching refrigerants were in a room temperature environment of about 30 ° C.). In the invention examples 3 to 5, in order to adjust the cooling rate in the high temperature region and the low temperature region, isothermal holding at around 530 ° C. for about 1 hour was performed (the quenching cooling curve is as shown in FIG. 3). For steels A to D, this temperature is a temperature range where no phase transformation occurs (cove in the CCT curve diagram), so it is not added to the time required for each cooling in Table 2.

In an actual quenching operation, the temperature change of the object during quenching generally follows the natural cooling curve defined by the equation below with the quenching temperature and the quenching refrigerant temperature as essential factors. There is no constant velocity cooling except in the late case. Therefore, in the present invention, based on the natural cooling curve of the following formula, the time required to cool from the quenching temperature to 530 ° C. is also called a semi-cooling time to distinguish the cooling rate. For example, when the half cold time is 40 minutes, it is simply called as half cold 40 minutes.

Natural cooling curve formula T = (Te-Tr) x exp (-t / C) + Tr
Here, Te; initial temperature (quenching temperature), Tr; temperature of quenching refrigerant,
t; time, C; constant, T; temperature at time t

The hardening method of FIG. 3 will be described in detail with Example 3 of the present invention as an example. First, the test piece was quenched from 1030 ° C. to 530 ° C. in a semi-cooled 5 minutes or so, and then held isothermally for 35 minutes in a 530 ° C. furnace (Invention Example 4 was held for about 65 minutes, Invention Example 5 Hold for about 85 minutes). And the cooling zone after this holding (ie, bainite transformation zone) has a slow speed according to the natural cooling curve in which the half-cooling time is 40 minutes in the atmosphere (ie, “half-cooling 40 minutes shown in FIG. It cooled by "the quenching cooling curve of degree".

On the other hand, the quenching method of FIG. 4 will be described in detail by taking Invention Example 6 as an example, and it is cooled from 1030 ° C. to 530 ° C. at a speed according to a natural cooling curve for half cooling time of 40 minutes. After that, the low temperature region was rapidly quenched by the pressurized gas at a speed according to a natural cooling curve in which the half cooling time was about 5 minutes.

Next, the above-described hardened roughened test piece was tempered at various temperatures and refined to a target hardness of 40 to 50 HRC. Then, for steels A and B, the notch direction of the Charpy specimen should be aligned in the width direction of the forged steel material (that is, ST direction in ASTM E399-90), and for steels C and D, A 2 mm U-notched Charpy impact test specimen was fabricated so that the notch direction of the Charpy specimen matched in the same length direction (that is, the same TL direction).

The Charpy impact test results at room temperature (22 to 26 ° C.) of the invention examples and comparative examples are divided by steel, and FIG. 5 (steel A), FIG. 6 (steel B), FIG. 7 (steel C) and It is shown in FIG. 8 (steel D). In the case of the improved steel containing a large amount of carbide-forming elements to be quenched according to the present invention, the impact value of the example of the present invention quenched so as to suppress intergranular precipitation in the high temperature range is within the range of the present invention. It turns out that it is considerably high compared with the impact value of the comparative example cooled so late that it deviates from.

According to the quenching method of the present invention, it is possible to maintain high toughness of a hot tool steel containing a large amount of carbide forming elements. Therefore, it is a large-sized hot tool that has a high working temperature range and requires high temperature strength, as well as application to various hot tools such as press dies, forging dies, die casting dies and extrusion tools. However, it is possible to give high toughness to the inside.

Claims (5)

C: 0.32 to 0.45%, Si: 0.01 to less than 0.8%, Mn: 0.1 to 0.8%, Ni: 0 to less than 0.8%, Cr: by mass% 4.5 to 5.6%, Mo and W alone or in combination (Mo + 1/2 W): 2.0 to 3.5%, V: 0.5 to 1.0%, Co: 0 to 2.0 In a method of quenching a hot-work tool steel consisting of 10%, balance Fe and unavoidable impurities,
A method of quenching a steel, comprising quenching from a quenching temperature of 1020 to 1070 ° C. to 530 ° C. at a high speed within 80 minutes. The method of quenching steel according to claim 1, characterized in that (Mo + 1 / 2W) of the hot tool steel is made more than 2.5% by mass. The method according to claim 1 or 2, characterized in that quenching from a quenching temperature of 1020 to 1070 ° C to 530 ° C is performed at a high speed within 45 minutes. After quenching from the quenching temperature of 1020 ° C. to 1070 ° C. to 530 ° C. at the above-mentioned high speed, the subsequent cooling to 150 ° C. is performed at a low speed of 60 minutes or more. The method of quenching steel according to any one of 3. The method according to claim 4, wherein the subsequent cooling to 150 ° C cools at a rate of 250 minutes or less.

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