Hot work tool steel

Abstract

To provide a hot work tool steel that can maintain the excellent toughness of hot work tool steels such as SKT4 even when the content of impurity elements is reduced. [Solution] A hot work tool steel having a composition of C: 0.50~0.60%, Si: 0.10~0.40%, Mn: 0.60~1.00%, Ni: 1.50~2.30%, Cr: 0.80~1.60%, Mo: 0.30~0.70%, and V: 0.05~0.20% by mass, and further containing P: more than 0.020% and 0.050% or less as a minimum, with the remainder being Fe and impurities. Preferably, in the above composition, Ni: more than 1.80% and 2.20% or less, Cr: more than 1.20% and 1.50% or less, and Mo: more than 0.40% and 0.60% or less.

Description

TECHNICAL FIELD

[0001] The present invention relates to hot work tool steel.

BACKGROUND ART

[0002] SKD61 and SKT4, which are JIS steel grades, are known as typical hot work tool steels. The aforementioned hot work tool steels are widely used in the manufacture of dies, tools and the like as materials having excellent heat resistance and wear resistance, and in order to maximize the mechanical properties such as toughness possessed by the hot work tool steels, there are methods for controlling impurities that degrade toughness, methods for optimizing heat treatment conditions, and the like. Among these, as SKT4 and its improved steels, for example, Japanese Patent Laid-Open No. 2001-089826 (Patent Document 1) discloses a hot work tool steel having an SKT4 composition, and describes that P (phosphorus), which is an impurity that reduces toughness, is regulated to 0.015% or less. As a method for rendering the impurity P harmless, Japanese Patent Laid-Open No. 2013-032576 (Patent Document 2) by the present applicant discloses an invention that compensates for the deterioration of toughness caused by the impurity P by adding an appropriate amount of Zn, as a method for producing a hot work tool steel with excellent toughness that can reduce environmental load.

PRIOR ART DOCUMENTS

PATENT DOCUMENTS

[0003]

PATENT DOCUMENT 1

PATENT DOCUMENT 2

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0004] As mentioned above, methods have been considered to reduce impurities as much as possible and to compensate for the degradation of properties with elements other than impurities, in order to fully exhibit the excellent toughness of hot work tool steels such as SKT4. As described in Patent Document 1 above, if the amount of impurities is to be kept to a minimum, it is necessary to carefully select the raw materials for melting or to apply remelting using consumable electrodes, which leads to an increase in costs. In contrast, Patent Document 2 is extremely effective in compensating for the degradation of toughness caused by P by actively adding Zn. By the way, if raw materials for dissolution become difficult to obtain in the future, it will be necessary to consider reducing the amount of impurity elements that are normally present. In that case, since Zn is not an element that is normally present, it is preferable to maintain toughness by using an element that replaces Zn, if possible. The object of the present invention is to provide a hot work tool steel that can maintain the excellent toughness of hot work tool steels such as SKT4 even when the content of impurity elements is reduced. [Means for solving the problem]

[0005] The inventors of the present invention have conducted detailed studies on SKT4 and its improved steels, specifically on the effects of reducing impurity elements and countermeasures. In particular, they diligently investigated methods to minimize the effect of P, which reduces toughness. As a result, they discovered that even without using elements not normally present, such as Zn, increasing specific elements simultaneously dramatically improves toughness, even when P is increased. This led to the present invention. In other words, the present invention relates to a hot work tool steel having a composition of C: 0.50-0.60%, Si: 0.10-0.40%, Mn: 0.60-1.00%, Ni: 1.50-2.30%, Cr: 0.80-1.60%, Mo: 0.30-0.70%, and V: 0.05-0.20% by mass, and further containing an essential amount of P: more than 0.020% and 0.050% or less, with the remainder being Fe and impurities. Preferably, the hot work tool steel has the following composition: Ni: greater than 1.80% and 2.20% or less, Cr: greater than 1.20% and 1.50% or less, and Mo: greater than 0.40% and 0.60% or less. [Effects of the Invention]

[0006] In the present invention, it is possible to obtain a hot work tool steel that can maintain the excellent toughness possessed by hot work tool steels such as SKT4 even when the content of impurity elements is relaxed. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] [Figure 1] This is a diagram showing the relationship between P content and Charpy impact value. [Figure 2] This is a diagram showing the Charpy impact values of the hot work tool steel of the present invention. MODE FOR CARRYING OUT THE INVENTION

[0008] As described above, the greatest feature of the present invention is the finding that, in hot work tool steels having SKT4 or its improved composition, the effect of improving toughness is dramatically enhanced only when specific elements are increased simultaneously. The specific elements are four elements: P, Ni, Cr and Mo. These four elements are included in the elements specified as SKT4 described in JIS-G4404 "Alloy Tool Steels", and there is no need to use elements that are not normally contained such as Zn. The effect of the aforementioned four elements is to compensate for the reduction in toughness caused by P with the remaining three elements, thereby improving toughness. The range of the composition specified in the present invention is described in detail below. The content of each element is expressed in mass%.

[0009] First, the four elements P, Ni, Cr and Mo described above will be explained. <P: More than 0.020% and 0.050% or less> P is a typical element that segregates at prior austenite grain boundaries during heat treatments such as tempering, causing embrittlement of the grain boundaries and reducing toughness. Therefore, in order to improve the toughness of hot work tool steel, it has usually been an impurity element whose content has been controlled to be as low as possible. In this invention, the toughness degradation caused by P can be compensated for by making maximum use of the toughness-improving effects of Ni, Cr, and Mo, described later, through improved hardenability and intrinsic toughness improvement of the matrix. The lower limit of P was set to a range exceeding the usual impurity level. The effects of this invention can be more clearly demonstrated in the range exceeding 0.030%. On the other hand, if P is present in excess, the toughness improvement effect cannot be expected even if Ni, Cr, and Mo are actively added as described above, so the upper limit of P was set to 0.050%. <Ni:1.50~2.30%> Ni is an element that suppresses ferrite formation. It also, along with C, Cr, Mn, and Mo, imparts excellent hardenability and is effective in preventing a decrease in toughness by forming a martensite-dominant structure even when the cooling rate during quenching is slow. Furthermore, it also has the effect of improving the intrinsic toughness of the matrix. In this invention, it is also one of the important elements necessary to improve the toughness that decreases due to the relaxation of the P content. To obtain this effect, a minimum of 1.50% is required. To obtain it more reliably, the lower limit of Ni should be 1.60%, preferably 1.70%, even more preferably exceeding 1.80%, and the effect will be even more reliable if the lower limit is 1.90%. On the other hand, excessive Ni increases the risk of a decrease in high-temperature strength and an increase in annealed hardness, so the upper limit of Ni should be 2.30%. Preferably, it is 2.20%.

[0010] <Cr:0.80~1.60%> Cr is an element that enhances hardenability and forms carbides, thereby strengthening the matrix and improving wear resistance. It also contributes to improved tempering softening resistance and high-temperature strength. Furthermore, in this invention, it is one of the important elements necessary to improve toughness, which decreases due to the relaxation of the P content. To obtain this effect, a minimum of 0.80% is required. To obtain it more reliably, it is even preferable to include more than 1.20%, and setting the lower limit at 1.25% will make the effect even more certain. On the other hand, excessive addition of Cr leads to a decrease in hardenability and high-temperature strength, so the upper limit of Cr should be 1.60%. The preferred upper limit of Cr is 1.50%. <Mo:0.30~0.70%> Mo is an element that enhances hardenability, imparts strength by precipitation or aggregation of fine carbides during tempering, and improves softening resistance, thereby contributing to improved toughness and high-temperature strength. In this invention, it is also one of the important elements necessary to improve toughness, which decreases due to the relaxation of the P content. To obtain this effect, a minimum of 0.30% is required. 0.35% is preferable for a more reliable result, and setting the lower limit to 0.40% will make the effect even more certain. On the other hand, excessive addition of Mo leads to a decrease in machinability and toughness, so the upper limit of Mo should be 0.70%. The preferred upper limit for Mo is 0.60%. It is also possible to substitute a portion of Mo with W. In that case, Mo is considered essential, and the range should be 0.30 to 0.70% as Mo + 0.5W. The preferred upper and lower limits for combined addition of Mo and W are the same as for Mo alone.

[0011] Next, we will explain elements other than those mentioned above. <C:0.50~0.60%> Carbon (C) is an essential element that contributes strength by partially dissolving in the matrix and enhances wear resistance and seizure resistance by forming carbides. However, excessive addition of C can lead to a decrease in toughness and high-temperature strength, so the amount of C should be kept to 0.50-0.60%. <Si:0.10~0.40%> Si acts as a deoxidizing agent during steelmaking and is an element that improves the machinability of the material. However, too much Si can lead to ferrite formation, so the Si content should be kept between 0.10% and 0.40%. The preferred lower limit for Si is 0.15%. <Mn:0.60~1.00%> Mn enhances hardenability, suppresses ferrite formation, and helps achieve appropriate hardness after quenching and tempering. However, too much Mn increases the viscosity of the matrix and reduces machinability, so the Mn content should be between 0.60 and 1.00%. The lower limit of Mn is 0.70%.

[0012] <V:0.05~0.20%> V forms carbides, which have the effect of strengthening the matrix and improving wear resistance. It also increases the resistance to tempering softening and suppresses grain coarsening, contributing to improved toughness. On the other hand, too much V leads to a decrease in machinability and toughness, so V should be set to 0.05-0.20%. The lower limit of V is preferably 0.10%. <Remainder> The remainder consists of Fe and impurities. Typical impurities that inevitably remain in the hot work tool steel specified in this invention (unavoidable impurities) include S, Ca, Mg, O (oxygen), N (nitrogen), Al, Ti, Co, Cu, Zn, etc. These are acceptable as long as they do not hinder the maintenance or improvement of toughness obtained in this invention. Specifically, the range should be S ≤ 0.03%, Ca ≤ 0.01%, Mg ≤ 0.01%, O ≤ 0.01%, N ≤ 0.03%, Al ≤ 0.03%, Ti ≤ 0.01%, Co ≤ 0.3%, Cu ≤ 0.2%, and Zn ≤ 0.025%. Furthermore, sulfur (S), when present as the nonmetallic inclusion MnS along with manganese (Mn), has a significant effect on improving machinability. To achieve this effect, sulfur can be added in amounts exceeding impurity levels, up to 0.03%.

[0013] Besides the aforementioned unavoidable impurities, other elements may be contained as impurities within a range that does not impair the effects of the present invention. Since the elements contained as impurities vary greatly depending on raw materials such as iron ore and iron scrap used for steel production, no specific elements are exemplified herein, but the total content of the elements contained as impurities is preferably 0.1% or less. [Examples]

[0014] (Example 1) First, as a preliminary experiment, the effect of impurity P on hot work tool steel was investigated. Using a vacuum induction melting furnace, 7 to 10 kg steel ingots having the chemical compositions shown in Table 1 were melted. These steel ingots were hot forged at 1000 to 1300°C to finish into steel materials. An annealing treatment was performed at 750°C to 800°C, to produce hot work tool steels of Sample No.10 having a P content as an impurity element equivalent to general SKT4, and Sample No.11 to No.13 with varied P contents. The chemical composition can be analyzed in accordance with the analysis methods specified in JIS and ASTM. For C, S, N and O, analysis can be performed by the combustion-infrared absorption method and the inert gas fusion-infrared absorptiometry method in accordance with ASTM-E1019, and for other elements, component analysis can be performed by the optical emission spectrometry and X-ray fluorescence analysis method in accordance with JIS-G0320 or G0321. Subsequently, 2mm U-notch Charpy impact test pieces in the L-S direction (based on the definition of ASTM E399-90) in accordance with JIS-Z2242 were sampled from the hot work tool steel. The Charpy impact test pieces were subjected to quenching and tempering to adjust the hardness to 38±1HRC, 42±1HRC and 45±1HRC. The quenching was performed by holding at 850°C followed by rapid cooling, and the tempering was repeatedly performed 2 or more times at various temperatures so as to achieve the aforementioned adjusted hardness. The Charpy impact test for toughness evaluation was performed at room temperature in accordance with JIS-Z2242. The number of Charpy impact tests was 3 or more per adjusted hardness. The Charpy impact test results are shown in Figure 1. As shown in Figure 1, the Charpy impact value decreases as the P content increases. Furthermore, this trend becomes more pronounced as the hardness decreases.

[0015] [Table 1]

[0016] (Example 2) Next, experiments were conducted in which the four elements P, Ni, Cr, and Mo were actively added simultaneously. In this example, assuming large steel ingots for mass production, 10 tons of steel ingots with the component composition shown in Table 2 were melted in an arc melting furnace. These steel ingots were hot forged at 1000 to 1300°C to produce steel materials with dimensions exceeding approximately 300 mm in thickness and 400 mm in width. After that, oil quenching was performed without reheating from the temperature after hot forging (direct quenching) to finish the steel materials. These steel materials were tempered twice at various temperatures to achieve a temper hardness of 40 to 44 HRC, producing hot tool steels of sample No. 20, which is equivalent to a normal SKT4 steel, and sample No. 21, an example of the present invention in which the P, Ni, Cr, and Mo content was increased to the range specified in the present invention. Sample No. 21 exceeds the upper limit of P (0.030% or less) according to JIS-G4404 "Alloy Tool Steel". A 2mm U-notch Charpy impact test specimen was taken from the center of the cross-section of hot work tool steel with a slow cooling rate during quenching, in the ST direction (based on the definition in ASTM E399-90) in accordance with JIS-Z2242. These specimens were further tempered to 35±1, 38±1, and 41±2 HRC. The Charpy impact test was conducted at room temperature in accordance with JIS-Z2242 to evaluate toughness. At least three Charpy impact tests were performed for each temper hardness level. To confirm the effects of the present invention in detail, each test specimen was taken from the upper and lower positions of the hot-work tool steel ingot. The Charpy impact test results are shown in Figure 2. As shown in Figure 2, the Charpy impact test results for the hot work tool steel of the present invention show that, despite containing a high amount of phosphorus, it achieves toughness equivalent to or better than that of ordinary SKT4 equivalent material. In particular, the sample taken from the upper part of the steel ingot showed a higher Charpy impact value than ordinary material, indicating superior toughness.

[0017] [Table 2]

[0018] From the above results, it can be seen that even if the content of impurity elements such as P is relaxed, the excellent toughness of hot work tool steels such as SKT4 can be maintained by simultaneously relaxing and increasing the content of Ni, Cr, and Mo. Since P, Ni, Cr, and Mo are elements that are normally present, it is sufficient to set their allowable amounts slightly higher. Therefore, even if it becomes difficult to obtain high-quality raw materials for melting in the future, the effects of the present invention can be applied to maintain the excellent toughness of hot work tool steels without excessively reducing impurities in terms of energy and cost.

Claims

1. A hot work tool steel having a composition of C: 0.50-0.60% by mass, Si: 0.10-0.40%, Mn: 0.60-1.00%, Ni: 1.50-2.30%, Cr: 0.80-1.60%, Mo: 0.30-0.70%, and V: 0.05-0.20%, and further containing P: more than 0.020% and 0.050% or less as essential components, with the remainder being Fe and impurities.

2. The hot work tool steel according to claim 1, wherein the composition is such that Ni is greater than 1.80% and 2.20% or less, Cr is greater than 1.20% and 1.50% or less, and Mo is greater than 0.40% and 0.60% or less.

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