JP5815946B2 - Hardening method of steel - Google Patents

Description

  The present invention relates to a quenching method for obtaining hot tool steel having high 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. Therefore, for example, SKD61 series hot tool steel, which is a JIS steel type, has been used in the field of hot tools. In recent years, the production time of products manufactured using hot tools has been shortened, and the material to be processed has become hot for molding into complex shapes, and multiple products can be processed simultaneously. Along with this, hot tools such as molds are becoming larger, and therefore, hot tool materials are required to be able to ensure even higher high-temperature strength and high toughness even in large sizes.

  Therefore, in order to improve the high-temperature strength and toughness of the hot work tool steel, an element that uses SKD61 as a basic component and further increases the elements that form carbides that contribute to secondary hardening during tempering, or that enhances hardenability. Improved steel types have been developed that have increased performance by increasing or adding (see Patent Documents 1 and 2).

  In order to improve the toughness of hot work tool steel, the heat transfer coefficient during quenching cooling is gradually increased to cool down, and the quenching cooling rate, such as making the bainite structure and martensite structure finer, is improved. An adjusted method has been proposed (see Patent Documents 3 to 5).

Japanese Patent No. 3191008 JP 2008-095181 A JP 2008-088532 A JP 2006-342377 A JP 2005-171305 A

  The quenching methods of Patent Documents 3 and 4 are based on the bainite transformation such as SKD61, in particular, the basic steel type in which a rough structure such as upper bainite is likely to be formed, and the base structure can be made fine and the pearlite structure is suppressed. Excellent in that it can. In addition, the quenching method of Patent Document 5 that refines the grain internal structure is effective for maintaining the toughness of similar steel of SKD61 in which W and Mo are increased in order to improve wear resistance. However, the effect of improving the toughness is not surely exerted on the improved steel types including more carbide-forming elements as shown in Patent Documents 1 and 2 described above.

  In other words, the above-mentioned improved steel types are inherently high in hardenability, and as shown in the continuous cooling transformation diagram (CCT curve) in FIG. 1, the bainite transformation has shifted to a lower temperature and longer time side than SKD61. Therefore, it is not necessary to increase the cooling rate in the low temperature region as much as SKD61. Rather, the problem is in the high temperature range from the quenching temperature to about 500 ° C., and during the cooling, grain boundary carbides are likely to precipitate and grow, and their influence on toughness is extremely large. Therefore, even if the method for obtaining SKD61, such as Patent Document 3, is applied to the improved steel grade, the examination of the cooling rate in the high temperature range is insufficient, and therefore the improvement in toughness is surely expected. hard. If the toughness is low, even if other properties such as high temperature strength are excellent, it is often impossible to use for hot tools.

  Therefore, an object of the present invention is to provide a quenching method that can achieve excellent toughness more reliably only in a hot tool steel containing many carbide forming elements and excellent in high temperature strength as proposed in Patent Documents 1 and 2. Is to provide.

  As a result of intensive research conducted by the present inventors, the precipitation and growth degree of grain boundary carbides during quenching cooling greatly influences the toughness of steel having the above specific composition range, which is different from SKD61. I found out. And by elucidating the mechanism at that time, the optimum quenching conditions could be clarified and the present invention was achieved.

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

  And preferably, in addition to the quenching method described above, after quenching from a quenching temperature of 1020 to 1070 ° C. to 530 ° C. at the fast speed described above, the subsequent cooling to 150 ° C. is slow for 60 minutes or more. It is a method of quenching steel that is cooled at a 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, this is an effective technique for practical application of hot tool steel that can be applied to various uses and environments.

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

  As described above, one of the features of the present invention is that the steel type to be quenched this time has a component composition in which grain boundary precipitation is likely to occur during quenching and cooling, and the toughness is significantly reduced. It is limited. That is, for steel types that are greatly affected by toughness by the quenching method, if a quenching method in which the conditions described below are applied is applied, the toughness can be provided at a high level, and other excellent characteristics represented by high-temperature strength. Can be fully utilized. Hereinafter, the reason for limiting the components of the steel composed of a narrow composition range used in 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 0.32% 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 set to 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 a deoxidizer during steelmaking. In order to obtain these effects, addition of 0.01% or more is necessary, but if too much, a bainite structure is developed and toughness is lowered. Further, by suppressing the precipitation of cementite-based carbides in the bainite structure during quenching and cooling, precipitation, agglomeration and coarsening of alloy carbides during tempering are indirectly promoted to lower the high temperature strength. Therefore, it is less than 0.8%. Preferably they are 0.1% or more and / or 0.6% or less.

  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 0.1% or more is necessary, but if it is too much, the viscosity of the base is raised and machinability is lowered, so the content is made 0.8% or less. Preferably they are 0.3% or more and / or 0.7% or less.

  Ni is an element that suppresses the formation of ferrite. Moreover, it has the effect of giving excellent hardenability to the steel of the present invention together with C, Cr, Mn, Mo, W, etc., and suppressing the formation of a bainite structure even at a moderate quenching cooling rate. Therefore, it is effective for forming a martensite-based structure and preventing a decrease in toughness. Furthermore, it is a preferable element to be added because it provides the essential toughness improving effect of the base. The addition of Ni is optional, but if it is too much, it will increase the base viscosity and lower the machinability or lower the high temperature strength, so it needs to be 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 improve the strengthening of the base and the wear resistance. And it is an indispensable element for the hot work tool steel of this invention which contributes also to the improvement of temper softening resistance and high temperature strength. In order to obtain these effects, it is necessary to add 4.5% or more. However, excessive addition causes a decrease in hardenability and high temperature strength, so the upper limit is made 5.6%. 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 give strength and improve softening resistance. At this time, since W has an atomic weight about twice that of Mo, the amount of addition can be defined by (Mo + 1 / 2W). And in order to acquire the above-mentioned effect, addition of 2.0% or more is required at (Mo + 1 / 2W). If the amount is too large, the machinability deteriorates, and the precipitation / growth of grain boundary carbides, which will be described later, is promoted and the toughness is lowered due to the increase in the amount. Therefore, (Mo + 1 / 2W) is 3.5% or less. Preferably it is 2.2% or more and / or 3.0% or less in (Mo + 1 / 2W). And, in that it is meaningful to target hot tool steel containing a large amount of carbide forming elements, the lower limit of (Mo + 1 / 2W) is limited to more than 2.5%, as in the case of the above Cr. Is preferably limited to 2.6% or more.

  V has the effect of forming carbides and improving the strengthening of the base and the wear resistance. In addition, it increases resistance to temper softening and suppresses coarsening of crystal grains, thereby contributing to improvement of toughness. In order to acquire this effect, it is necessary to add 0.5% or more, but when it is too much, it will reduce machinability and toughness like Mo and W, so 1.0% or less. Preferably it is 0.55% or more and / or 0.85% or less.

  Co forms a very dense and protective oxide film with good adhesion when the temperature rises during tool use. This is a preferable element for addition because it prevents metal contact with the counterpart material, prevents temperature rise on the mold surface, and provides excellent wear resistance. The addition of Co is optional, but if it is too much, the toughness is lowered, so the upper limit is made 2.0% or less. Preferably, it is 1.0% or less.

  As unavoidable impurities, the main elements that may remain are P, S, 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%, Cu ≦ 0.25%, Al ≦ 0.025%, Ca ≦ 0.01%, Mg ≦ 0.01%, O ≦ 0.01 %, N ≦ 0.03%, it is considered that there is no significant influence on the toughness of the hot work tool steel obtained by the quenching method of the present invention. It is an upper limit.

  The greatest feature of the present invention is the quenching method for the improved steel, which has been found according to the heat treatment characteristics unique to the improved steel having the above-mentioned composition. In other words, for the above-described improved steel having a different composition from that of the conventional SKD61, the “quenched structure factor” affecting the toughness is also different from that of the SKD61. Therefore, it has been necessary to identify an optimum quenching method for the component range steel of the present invention (hereinafter also referred to as improved steel) by studying the quenching structural factors. That is, it is a steel quenching method characterized by quenching hot tool steel satisfying the above component composition from a quenching temperature of 1020 to 1070 ° C. to 530 ° C. at a fast rate within 80 minutes. . Preferably, it is within 60 minutes, and further within 45 minutes. And after quenching at this high speed, it is a preferable quenching method that the subsequent cooling to 150 ° C. is performed at a slow speed of 60 minutes or more. More preferably, it is 80 minutes or more.

  When quenching hot tool steel, if it is oil quenched with a small block size on the order of 10 mm square, it will give a martensitic single structure and the toughness will be the highest level of that steel. However, if it becomes a practical steel, the quenching cooling rate becomes slow due to the size of the steel to be quenched, and carbide precipitates and grows at the austenite grain boundary in the high temperature range from the quenching temperature to about 600 ° C. However, a bainite structure is usually formed in a low temperature range of about 500 ° C. or lower, and the toughness level is lowered. About this, even if it is the special improved steel made into the object of this invention, it is the same as the CCT curve of the FIG. Therefore, in the quenching method of the present invention, the cooling management is performed separately for the high temperature region and the low temperature region.

  And according to the above, specific cooling conditions for the high temperature region and low temperature region will be studied, but in order to achieve the maximum effect and reproducibility of the present invention, the conditions are simple, It is desirable that it is easy to handle. In other words, when managing the cooling rate based on the temperature of a “single point” that passes during cooling, the required cooling conditions for each of the upper and lower cooling zones with the reference temperature as a boundary are set to be easily controlled. This is the identification of the “optimal reference temperature” that can be made. And in the case of the improved steel of this invention, this reference temperature is 530 degreeC.

  The improved steel of the present invention, which has an alloying element amount higher than SKD61 in order to increase hardenability and high temperature strength, causes faster precipitation and growth of grain boundary carbides in the high temperature range, which affects toughness reduction. The impact is significant (see Figure 1 above). Therefore, in the present invention where the improved steel is subject to quenching, the high temperature range from a quenching temperature of 1020 to 1070 ° C. to 530 ° C. must be rapidly cooled. Specifically, it is a fast speed within 80 minutes. The quenching is preferably performed at a fast rate within 60 minutes, and within 45 minutes, and more preferably within 30 minutes.

  Next, in the improved steel of the present invention, martensitic transformation and bainite transformation occur in a low temperature range of 530 ° C. or lower. Therefore, when entering the transformation zone while maintaining the above-described fast cooling rate of the present invention in the high temperature range, a large temperature difference occurs between the material surface side and the inside, and the timing at which transformation occurs is also large on the material surface side and inside. As a result, a large stress is generated, which may cause deformation or cracking. Moreover, since the above-mentioned improved steel has a hardened property and has a component design in which a rough bainite structure that greatly reduces toughness is hardly formed, an extremely fast cooling rate is not required in a low temperature range.

  Therefore, after the above-described high temperature region is rapidly cooled at a high speed, the subsequent cooling is preferably performed at a low speed at which the above-described problems are unlikely to occur. The cooling at this time is sufficient up to 150 ° C. at which the martensitic transformation and the bainite transformation are almost completed and the problem of large stress generation due to the shift of the transformation time inside and outside 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 the cooling rate is too slow, there is a concern that a coarse bainite structure may be formed. Therefore, it is desirable to determine the upper limit of the cooling time in the low temperature region. 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 in preventing the concern about the formation of a coarse bainite structure that greatly reduces toughness.

  In the present invention, for example, in the above reference temperature and in 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 performed. Permissible. The conditions such as the isothermal holding temperature and time at this time may be set within a range that does not affect the effects of the cooling conditions of the present invention as much as possible (that is, a range in which phase transformation is unlikely to occur for the steel to be quenched). desirable. The isothermal holding time is not added to the time required for each cooling of the present invention.

  Table 1 shows the chemical components of the hot work tool steel used in this example. In other words, all of the hot tool steels in Table 1 are “known” improved steels within the range of the composition of the present invention, and are optimum samples for evaluating the effect of improving toughness by the quenching method of the present invention. is there. The CCT curves of these samples (steel A to D) are as shown in FIG.

  For these materials, steel ingots were prepared by remelting an electrode that was primarily melted and agglomerated in an arc melting furnace of 40 tons for steel A and 15 tons for steel B. The steel ingot was subjected to homogenization heat treatment at a predetermined temperature of 1200 ° C. or higher, and then hot forging and annealing were repeated to obtain a steel material having a thickness of about 150 mm × 500 mm. Then, after annealing at 860 ° C., the test piece roughened material having a size about 1 mm larger than the Charpy impact test piece size from the steel material so that the thickness direction after forging becomes the longitudinal direction of the test piece Was collected and subjected to quenching treatment at 1030 ° C.

  Steel ingots prepared by melting 10 kg each in a vacuum induction melting furnace were prepared for the steel C and D materials. And after giving homogenization heat processing to this steel ingot at the predetermined temperature of 1200 degreeC or more, it was set as the steel material of 30 mm thickness x 60 mm width by hot forging. Then, after annealing at 860 ° C., a rough test piece with a size about 1 mm larger than the Charpy impact test piece size from the steel material so that the width direction after forging is the longitudinal direction of the test piece. The sample was collected and subjected to quenching treatment at 1030 ° C.

  The above quenching was performed by the method shown in Table 2. The quenching refrigerant used was selected from nitrogen gas at a predetermined pressure and the atmosphere (all quenching refrigerants had a room temperature environment of about 30 ° C.). In Invention Examples 3 to 5, in order to adjust the cooling rate in the high temperature region and the low temperature region, isothermal holding was performed at about 530 ° C. for about 1 hour (the quenching cooling curve is as shown in FIG. 3). For steels A to D, the 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 actual quenching work, the temperature change of the target object during the quenching is generally in accordance with the natural cooling curve defined by the following equation with quenching temperature and quenching refrigerant temperature as essential factors. It is not constant-speed cooling unless it is slow. Therefore, in the present invention, based on the natural cooling curve of the following formula, the time required for cooling from the quenching temperature to 530 ° C. is also called a semi-cooling time, and the cooling rate is distinguished. For example, when the semi-cold time is 40 minutes, it is simply called semi-cold 40 minutes.

Natural cooling curve formula T = (Te−Tr) × exp (−t / C) + Tr
Where Te: initial temperature (quenching temperature), Tr: temperature of quenching refrigerant,
t: time, C: constant, T: temperature at time t

  The quenching method of FIG. 3 will be described in detail by taking Example 3 of the present invention as an example. First, the test piece was rapidly cooled from 1030 ° C. to 530 ° C. in about a half-cooled period of about 5 minutes, and then held isothermally in a furnace at 530 ° C. for 35 minutes (Invention Example 4 was held for about 65 minutes, Invention Example 5 was Hold for about 85 minutes). And the cooling zone after this holding | maintenance (namely, bainite transformation zone) is the slow speed (namely, "semi-cooling 40 minutes shown in FIG. It was cooled by a quenching cooling curve).

  On the other hand, the details of the quenching method of FIG. 4 will be described by taking Example 6 of the present invention as an example. It is cooled from 1030 ° C. to 530 ° C. at a rate according to the natural cooling curve for 40 minutes. The subsequent low temperature region was rapidly cooled by the pressurized gas at a speed according to a natural cooling curve in which the semi-cooling time was about 5 minutes.

  Next, the test piece rough processed material subjected to the quenching treatment was tempered at various temperatures and tempered to a target hardness of 40 to 50 HRC. And about steel A and B, it is made for the notch direction of a Charpy test piece to correspond with the width direction in the steel material after the forge (namely, ST direction in ASTM E399-90), and about steel C and D A 2 mm U-notch Charpy impact test piece was fabricated by making the notch direction of the Charpy test piece coincide with the same length direction (that is, the TL direction).

  The Charpy impact test results at room temperature (22 to 26 ° C.) of the inventive example and the comparative example are separated for each steel, and FIG. 5 (steel A), FIG. 6 (steel B), FIG. 7 (steel C) and It shows in FIG. 8 (steel D). For the improved steel containing a large amount of carbide-forming elements, which is the subject of quenching of the present invention, the impact value of the present invention example quenched to suppress grain boundary precipitation in the high temperature range is within the scope of the present invention. It can be seen that it is considerably higher than the impact value of the comparative example that was cooled late enough to deviate from the range.

If it is the hardening method of this invention, the toughness of the hot tool steel containing many carbide forming elements can be maintained high. Therefore, it is not only applicable to various types of hot tools such as press dies, forging dies, die casting dies, and extrusion tools, but it is also a large hot tool that requires a high temperature range and high temperature strength. However, high toughness can be imparted to the inside.

Claims (4)

In mass%, 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: 4.5 to 5.6%, Mo and W are used alone or in combination (Mo + 1 / 2W): 2.0 to 3.5%, V: 0.5 to 1.0%, Co: 0 to 2.0 %, In the quenching method of hot tool steel consisting of the balance Fe and inevitable impurities,
A high temperature range from a quenching temperature of 1020 to 1070 ° C. to a reference temperature of 530 ° C. is rapidly cooled at a fast cooling rate within 45 minutes, and a low temperature range from the reference temperature to 150 ° C. is set at a slow cooling rate of 60 minutes or more. A steel quenching method comprising cooling and performing isothermal holding for adjusting the cooling rate of the high temperature region and the low temperature region in a temperature range of 20 ° C. above and below the reference temperature.   The method of quenching steel according to claim 1, characterized in that (Mo + 1 / 2W) of the hot work tool steel exceeds 2.5% by mass%. The method of quenching steel according to claim 1 or 2 , wherein the temperature from the reference temperature to 150 ° C is cooled at a cooling rate within 250 minutes. The steel quenching method according to any one of claims 1 to 3 , wherein the steel is cooled from the reference temperature to 150 ° C at a slow cooling rate of 80 minutes or more.

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