JP2005314788A - Hot tool steel having excellent toughness and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide hot tool steel having improved toughness, and to provide its production method. <P>SOLUTION: Regarding the hot tool steel, in hot tool steel comprising, by mass, 0.2 to 0.7% C and 0.5 to 7.0% Cr, the content of Al is regulated to ≤0.04%, and also, the degree in the segregation of Cr in the matrix composition lies within ±0.2 mass%. Regarding the hot tool steel, preferably, the content of Al is regulated to ≤0.015%, or further, the number of aluminum oxides with the maximum diameter of ≤5 μm observed in the cross-sectional structure is ≤2,500 (inclusive of zero) pieces/mm<SP>2</SP>. In the method for producing the hot tool steel, a steel ingot obtained by a remelting method and having a composition comprising, by mass, 0.2 to 0.7% C and 0.5 to 7.0% Cr, and in which the content of Al is regulated to ≤0.04% and/or a steel slab obtained by subjecting the steel ingot to hot working are/is subjected to homogenizing heat treatment. <P>COPYRIGHT: (C)2006,JPO&NCIPI

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 die casting die, and an extrusion tool, and a method for producing the hot tool steel.

  Conventionally, in the field of hot tools, for example, SKD61 series alloy tool steel which is a JIS steel type has been used.

Furthermore, it has been proposed to improve for the purpose of improving the toughness of the alloy tool steel. For example, a technique for improving toughness by reducing the content of impurity elements Al and N has been proposed (see Patent Document 1), which is excellent in that only the component composition of the alloy needs to be adjusted. It is a thing.
Japanese Patent Laid-Open No. 07-102342

  The method disclosed in Japanese Patent Application Laid-Open No. 07-102342 described above is effective as an aid for improving toughness. However, in the case of improving toughness in consideration of adjustment of only the alloy composition as in the above method, even in the case of an alloy tool steel having the same component adjustment, the toughness value varies between them. The present inventor has found that low toughness is also manifested. As described above, the conventional methods have insufficient points in the means for improving toughness, and there is still room for examination.

  An object of the present invention is to provide a hot work tool steel having further excellent toughness, and an optimum manufacturing method for achieving the same, complementing conventional methods.

  The present inventor examined the above-described factors for lowering the toughness, and found that it was caused by the existence of a site with inferior toughness caused by the strong segregation of components during solidification. In other words, the problem that segregation remains strongly in the structure even in an alloy whose component composition has been optimally adjusted to provide various properties is particularly important for the practical application of hot work tool steel that requires excellent toughness. It becomes a big problem.

  Therefore, the present inventor examined the problem that the toughness is inferior due to component segregation. As a result, in hot tool steels whose component segregation is controlled within an optimum degree, fundamental toughness deterioration factors are eliminated, so there is an element species that may have some adverse effects on toughness. As a result, we established a means that can provide hot tool steel with a wide application range. And it has been found that the toughness can be greatly improved by adopting that the Al content is regulated to a predetermined amount or less, and has reached the present invention.

  That is, the present invention provides a hot tool steel containing (1) C: 0.2 to 0.7% and Cr: 0.5 to 7.0% by mass%, and Al is restricted to 0.04% or less. And the degree of segregation of Cr in the matrix composition is within ± 0.2% by mass, (2) Preferably, one or two of Mo or W is added to (Mo + 1 / 2W) in the above components. 1 to 6.0% contained, and the degree of segregation of the base composition (Mo + 1 / 2W) is within ± 0.08% by mass; (3) preferably for any of the above components (1) or (2) Preferably contains V: 3.0% or less, and the degree of segregation of V in the base composition is within ± 0.07% by mass, (4) preferably in any of the above components (1) to (3) It is a hot work tool steel excellent in toughness characterized by Al being restricted to 0.015% or less.

More preferably, the number of aluminum nitrides having a maximum diameter of 5 μm or less observed in the cross-sectional structure of the hot tool steel is 2500 pieces / mm ² or less (including 0 pieces). Hot tool steel with excellent toughness.

  And it is a manufacturing method preferable in order to achieve the hot tool steel of the present invention described above, and obtained by remelting method in mass%, C: 0.2 to 0.7%, Cr: 0.5 A homogenization heat treatment is performed on a steel ingot of hot tool steel containing ˜7.0% and Al regulated to 0.04% or less and / or a steel slab obtained by hot working the steel ingot. It is a manufacturing method of hot work tool steel excellent in toughness.

  According to the present invention, the toughness of hot tool steel can be drastically improved, and this is an indispensable technique for the 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 resides in that the degree of segregation of components is controlled and the content of Al is defined. In other words, by using hot tool steel whose component segregation is controlled within a predetermined degree, the fundamental toughness deterioration factor is eliminated, and then further Al content control can greatly improve toughness. With regard to the toughness-improving effect by regulation of Al, it is possible to evaluate even the required regulation amount of Al by clarifying the mechanism, and the production cost can be adjusted.

  First, the reason for prescribing the degree of segregation of the components that form the basis of the present invention will be described. Toughness is one of the important characteristics for hot work tool steel. Since the toughness deteriorates when the base crystal grains become coarse, carbides are formed in the structure to suppress the growth of crystal grains. However, when the formation of this carbide becomes coarse and large, this becomes an unexpected deterioration factor of toughness. For example, if segregation with large component fluctuations such as center segregation or reverse V segregation due to carbide forming elements occurs in the structure at the ingot forming stage, coarse and large amounts of carbides precipitate in the concentrated portion of the element components. It becomes easy and this can be a cause of deterioration of toughness. Therefore, in the present invention intended for hot tool steel, it is necessary to reduce segregation of the carbide-forming elements that are essentially contained in the alloy components.

  Cr is an element most utilized as a carbide forming element in hot tool steel. Therefore, in the present invention, it is essential to regulate the degree of segregation of the base composition Cr. In addition to Cr, if other carbide forming elements such as Mo, W, and V are added as necessary, it is also preferable to regulate the degree of segregation of these element species. In the present invention, the degree of segregation of each of the above carbide-forming elements is as described later, but Cr is within ± 0.2% by mass, and Mo and W are evaluated by (Mo + 1 / 2W). Within 0.08% by mass, V is preferably within ± 0.07% by mass.

  Here, one method for measuring the degree of segregation is shown below. For example, line analysis is performed on each element using an X-ray microanalyzer (EPMA) to obtain a chart. FIG. 1 shows a chart of Cr as an example. In FIG. 1, the horizontal axis represents the measurement width (mm) of the analyzed sample, and the vertical axis qualitatively corresponds to the Cr concentration. Therefore, aiming at the high and low values on the chart. If the quantitative analysis is performed again using an electron beam having a diameter of 100 μm or less so that each region can be measured independently, the upper limit concentration and the lower limit concentration of Cr in the region subjected to the line analysis can be obtained. Therefore, in the present invention, the length of this line analysis is set to be sufficiently longer than the interval of component segregation with a large period as seen from the horizontal axis 0 mm to 0.4 mm in FIG. The difference between the obtained upper limit concentration and the lower limit concentration can be obtained and recognized as the degree of segregation in the base composition.

  For example, in the case of a normal ingot method in which a molten alloy tool steel melted in an arc furnace or the like and adjusted in composition is cast into a steel ingot, the component variation such as center segregation or reverse V segregation is usually present in the cast structure. Large segregation occurs. In the concentrated portion of the component, the carbide is likely to be coarsely and abundantly precipitated, and as described above, the toughness is inferior. For the hot tool steel of the present invention, for example, after the above ingot forming method, a remelting method such as ESR (electroslag remelting method) or VAR (vacuum arc remelting method) is applied and further remelted. The following steel ingot and / or a steel piece obtained by hot working the steel ingot are subjected to a homogenization heat treatment, for example, a homogenization heat treatment under suitable conditions from 1200 ° C. or higher, preferably 1250 ° C. or higher. Achieving this is also possible due to the effect of the regulation of the Al content.

  The above homogenization heat treatment is not particular about the atmosphere even in the air, but in order to suppress the loss due to oxidation of the material, for example, an atmosphere such as an inert gas or a reducing gas is applied. Also good. In terms of suppressing the loss of the material, the homogenization heat treatment is preferably performed in a steel ingot state with a small surface area exposed as compared to the material volume. Furthermore, various conditions suitable for the heating rate and cooling rate when performing the homogenization heat treatment can be selected in accordance with the size of the steel ingot / steel piece.

  Next, the reason for defining the content of Al, which is another basis of the present invention, will be described. The reason why the toughness of the hot work tool steel is improved by reducing the Al content is that the content of aluminum nitride (indicated by AlN) in the structure that deteriorates the toughness is reduced. Although it has been stated that a method for reducing Al and N has been proposed as a means for improving the toughness of hot tool steel, the present inventor lies in the individual component composition that directly affects such toughness. Rather, I found out that it depends largely on the inclusion element of AlN. That is, the toughness is improved if only the amount of AlN in the structure is reduced. In order to reduce the content of AlN, it is only necessary to reduce either the amount of Al or the amount of N.

  However, in order to reduce the amount of N, a costly process such as a longer degassing step during melting is required, whereas in order to reduce the amount of Al, the addition of Al Since it is only necessary to reduce the amount, and since there are many inexpensive means for supplementing the effectiveness as a deoxidizer, it is very simple, and in the present invention, it is adopted that the content of Al is specified. And if it is hot tool steel which adjusted the segregation degree of the carbide formation element represented by Cr, even if it does not require the extreme reduction of N by restrict | limiting Al in steel to 0.04% or less. It has been found that sufficient toughness can be secured as a hot work tool steel. If it is this invention, the extreme reduction to N is unnecessary, and the improvement effect of the outstanding toughness will be acquired even if it is 100 ppm, 150 ppm or more, and also N amount of 200 ppm or more, for example.

Here, with regard to AlN in the structure, which is a cause of deterioration of toughness, in the present invention, the reduction is first performed by regulating the amount of Al in the steel. However, if the toughness of the hot tool steel is greatly improved, it is also effective to reduce the AlN content itself in the structure. In the case of the hot tool steel of the present invention, preferably, AlN having a maximum diameter of 5 μm or less observed in the cross-sectional structure is 2500 pieces / mm ² or less (including 0 pieces).

  One evaluation method of aluminum nitride (AlN) observed in the cross-sectional structure is shown below. For example, FIG. 2 shows a reflected electron beam image (a) and a mapping image (b) of Al at the same site when the cross-sectional structure of hot tool steel is observed using EPMA (× 1000). Times). Both of these images are of the sample material C of the examples described later. In this Al mapping image (b), particles with high Al are confirmed as white portions. At this time, an N mapping image is also obtained at the same time, and the correspondence with the position of the Al particles is confirmed. , It can be identified as aluminum nitride. Then, the number of particles (all white particles) that can be identified as aluminum nitride is counted, and the number is divided by the area of the observation field. In addition, about the magnitude | size of aluminum nitride, when a reflected electron beam image is observed, it turns out that the maximum diameter of each aluminum nitride which the reflected electron beam image (a) of FIG. 2 shows is 5 micrometers or less.

  Hereinafter, the reason why the components of the hot work tool steel, which is preferable for maximizing the effects of the present invention, will be described in detail in conjunction with the above-described requirements.

-C: 0.2-0.7 mass%
C is an important element that increases the wear resistance and seizure resistance by partly forming a solid solution in the base to give strength and partly forming carbides. In the case of an interstitial tool, the utility of the present invention is particularly improved. 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, 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 0.2% 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 0.7 mass%.

-Cr: 0.5-7.0 mass%
Since Cr has the effect of enhancing hardenability and forming carbides to improve the strengthening of the base and wear resistance, the present invention is particularly useful when the object of the present invention is a hot tool. It is an element that improves the properties, and it is necessary to add at least 0.5% by mass. However, excessive addition causes a decrease in hardenability and hot strength, so the upper limit is 7.0% by mass.

The degree of segregation of the base composition Cr is within ± 0.2% by mass. When solidification segregation occurs during solidification of Cr, the amount of precipitated carbide is extremely increased in the Cr-concentrated portion, and the toughness is greatly reduced. In this case, there is a concern that the toughness cannot be improved even if the amount of Al or the amount of AlN in the steel itself is specified as described above. In the present invention that proposes a hot work tool steel with excellent toughness, the degree of segregation of Cr that affects toughness is within ± 0.2 mass% (that is, the difference between the upper limit concentration and the lower limit concentration is 0 to 0.4 mass%). , Preferably within ± 0.1% by mass (0 to 0.2% by mass).

-1 type or 2 types of Mo or W (Mo + 1 / 2W): 0.1-6.0 mass%
Mo and W have the effect of forming carbides and improving the strengthening of the base and the wear resistance. However, since W has an atomic weight approximately twice that of Mo, it is defined by Mo + 1 / 2W (of course, either one may be added or both may be added together). And when the remarkable effect of an above-mentioned effect | action is anticipated, it is desirable to make Mo + 1 / 2W into 0.1 mass% or more. However, excessive addition of Mo and / or W increases the amount of carbide and leads to a decrease in toughness, so the upper limit of Mo + 1 / 2W is preferably 6.0% by mass.

The degree of segregation of the base composition Mo + 1 / 2W is within ± 0.08% by mass. When Mo and W solidify and segregate during solidification, the amount of precipitated carbide is extremely large in the Mo and / or W concentrated part. Increases and greatly reduces toughness. Therefore, in the case where Mo or W is added, it is desirable to adjust the degree of segregation similarly to Cr. In the present invention which proposes a hot work tool steel having excellent toughness, the segregation degree of Mo + 1 / 2W which affects toughness is within ± 0.08 mass% (that is, 0 to 0.16 mass by the difference between the upper limit concentration and the lower limit concentration). %), And more preferably within ± 0.035% by mass (0 to 0.07% by mass).

V: 3.0% by mass or less V forms carbides and has the effect of strengthening the base and improving wear resistance. In addition, the formation of fine carbides is effective for the refinement of crystal grains and the improvement of toughness. Therefore, when a more remarkable effect is expected, the content is preferably 0.1% by mass or more. However, on the other hand, if added excessively, the toughness is also lowered, so the upper limit is desirably 3.0% by mass.

The degree of segregation of V in the base composition is within ± 0.07% by mass. When solidification segregation occurs during solidification, the precipitation amount of carbide is extremely increased in the V-concentrated portion, and the toughness is greatly reduced. Therefore, similarly to Mo and W, even when V is added, it is desirable to adjust the degree of segregation similarly to Cr. In the present invention which proposes a hot work tool steel excellent in toughness, the degree of segregation of V affecting toughness is within ± 0.07 mass% (that is, the difference between the upper limit concentration and the lower limit concentration is 0 to 0.14 mass%). Furthermore, it is preferable to be within ± 0.035% by mass (0 to 0.07% by mass).

-Al: 0.04 mass% or less Al is used as a deoxidizing element during refining and is often unavoidably contained. However, when it is contained in a large amount, even if it is in the region of the amount of impurities, AlN is formed and the toughness is deteriorated, particularly when the N content is high. As described above, in the case of the hot tool steel of the present invention in which the degree of segregation of the base composition is adjusted and the toughness is improved, even if the N content is large, Al is regulated to 0.04 mass% or less. The effect of improving toughness can be sufficiently obtained. Preferably it is 0.025 mass% or less, More preferably, it is 0.015 mass% or less.

  In addition, the component composition of the hot tool steel of the present invention can include, for example, Si, Mn, Ni, Co, etc. as necessary, other than the above-mentioned components, as described in JIS. Application of tool steel composition is possible.

The number of AlN having a maximum diameter of 5 μm or less observed in the cross-sectional structure is 2500 / mm ² or less (including 0). As described above, AlN deteriorates the toughness of hot tool steel, and therefore the present invention is applied. It is desirable that the AlN having a maximum diameter of 5 μm or less observed in the cross-sectional structure of the hot work tool steel having excellent toughness is 2500 pieces / mm ² or less (including 0 pieces). Particularly preferably, it is 500 pieces / mm ² or less.

  The test materials A to C shown in Table 1 were homogenized at 1250 ° C. or higher in the atmosphere into an ingot produced by remelting an electrode obtained by primary melting in a 15-ton arc melting furnace and remelting with ESR. It has been subjected to chemical heat treatment. This homogenization heat treatment is based on the same conditions in which one condition such as temperature and time at which all of the test materials A to C are within the segregation degree of the present invention is set. The steel ingot after homogenization heat treatment is in an annealed state in which the steel ingot is ingot and hot worked, and the specimens A and B are hot tool steels that satisfy the segregation degree and the Al content of the present invention. It is. Specimen C is a comparative steel that exceeds the Al regulation range of the present invention. On the other hand, the test material D satisfying the Al content of the present invention is similarly homogenized under the same conditions as the test materials A to C into a steel ingot that has been agglomerated without remelting after primary melting. A heat treatment followed by a chunk and hot worked annealed material. The basic composition of the test materials A to D corresponds to SKD61.

  The number of aluminum nitrides (AlN) having a maximum diameter of 5 μm or less was measured from a 1000-fold Al mapping image observed with EPMA according to the method described above. In addition, when produced by this production method, AlN exceeding the maximum diameter of 5 μm was not observed in the sample structure by the reflected electron beam image of EPMA. And although it is confirmed in this reflected electron beam image, it was difficult to accurately determine whether or not all fine particles are aluminum nitride (AlN) regulated by the present invention. Only particles having a maximum diameter of 0.5 μm or more were measured. The results are shown in Table 2 together with the degree of segregation of the base composition in each element (the diameter of the electron beam used in the quantitative analysis is 100 μm).

  The specimens A to D shown in Table 1 were subjected to quenching and tempering to match the hardness to 43HRC, and then processed into 2 mm U notch Charpy impact test pieces. Table 3 shows the Charpy impact test results.

  In the specimen D for which the remelting method was not applied to the ingot forming process, the homogenized heat treatment under the same conditions as the specimens A to C was applied to the steel ingot, but the segregation degree of the present invention was not satisfied, and Charpy The impact value was inferior. And even in satisfying the degree of segregation of the present invention, compared with the test material C whose Al content is outside the scope of the present invention, the test materials A and B which are steels of the present invention have a Charpy impact value. Are better. Among them, the test material B in which the Al content was reduced to a preferable amount and the amount of aluminum nitride in the structure was extremely reduced showed an impact value approximately twice that of the comparative steel C.

  By applying the present invention to improve the toughness of hot tool steel, it can be applied to various hot tools such as press dies, die casting dies and extrusion tools, as well as dies with a larger load. The present invention can also be applied to hot tool members.

It is a chart figure of Cr concentration in hot tool steel measured by line analysis by EPMA, and is a figure showing qualitatively the degree of Cr segregation of a base composition. It is a cross-sectional structure of hot tool steel observed with EPMA, and is a reflected electron beam image (a) and a mapping image (b) of distributed Al.

Claims (6)

  In a hot tool steel containing C: 0.2 to 0.7% and Cr: 0.5 to 7.0% by mass%, Al is restricted to 0.04% or less, and Cr having a base composition A hot work tool steel with excellent toughness, characterized by having a segregation degree within ± 0.2 mass%.   Contains 0.1 or 6.0% of Mo or W by (Mo + 1 / 2W) in mass%, and the degree of segregation of the base composition (Mo + 1 / 2W) is ± 0.08% by mass. The hot work tool steel having excellent toughness according to claim 1, wherein the hot work tool steel is excellent in toughness.   The heat excellent in toughness according to claim 1 or 2, wherein V: 3.0% or less in mass%, and the degree of segregation of V in the matrix composition is within ± 0.07 mass%. Inter-tool steel.   The hot work tool steel excellent in toughness according to any one of claims 1 to 3, wherein Al is regulated to 0.015% or less in terms of mass%. The heat excellent in toughness according to any one of claims 1 to 4, wherein the number of aluminum nitrides having a maximum diameter of 5 µm or less observed in the cross-sectional structure is 2500 pieces / mm ² or less (including 0 pieces). Inter-tool steel.   Hot tool obtained by remelting method, containing C: 0.2-0.7%, Cr: 0.5-7.0% in mass%, and Al restricted to 0.04% or less A method for producing hot tool steel having excellent toughness, characterized by performing homogenization heat treatment on a steel ingot and / or a steel piece obtained by hot working the steel ingot.