alloy steel

Abstract

To provide an alloy steel capable of stably and finely dispersing sulfides. [Solution] An alloy steel containing Al and Mg, wherein Mg is contained in a mass percentage of more than 0% and 0.01% or less, and the following formula (1): 0.0022 × [Al] 0.6833 <[Mg]<1.0544×[Al] 0.6569 ...Formula (1) (The values ​​in brackets [] in formula (1) indicate the content (mass %) of the element in brackets [] in the alloy steel.) The following conditions are met: The oxide inclusions contained in the alloy steel include MgO·Al2O3-based spinel inclusions, and the proportion of cluster-like inclusions among the sulfide inclusions contained in the alloy steel is 40% or less.

Description

[Technical Field]

[0001] This invention relates to alloy steel. [Background technology]

[0002] For some time now, the method of adding sulfur (S) has been known as a means of improving the machinability of steel. The sulfides generated in steel by the addition of S exhibit machinability-improving effects such as improved chip handling and reduced cutting resistance during cutting. Various proposals have been made to further enhance this machinability-improving effect, and to improve other properties while maintaining the machinability-improving effect. For example, Patent Document 1 describes a structural steel for machinery that contains sulfide inclusions, with the aim of stably and reliably exhibiting chip breaking properties and mechanical properties. According to Patent Document 1, it is disclosed that chip breaking can be promoted by adjusting the sulfide inclusions to a specific distribution state, that is, so that fine inclusions form clusters.

[0003] Furthermore, Patent Document 2 discloses a free-cutting stainless steel in which the Cr / Mn concentration in sulfide inclusions with a major diameter of 10 μm or more is adjusted, and oxide inclusions contain 30-70% SiO2 by weight and 5% or more Al2O3 by weight, in order to improve both machinability and hot workability. According to Patent Document 2, the Cr / Mn concentration of sulfide inclusions with an inclusion diameter of 10 μm or more, which greatly affects the properties, is adjusted, and oxide inclusions are made into composite oxides that are highly ductile. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2000-282171 [Patent Document 2] Japanese Patent Publication No. 2001-098352 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] As described in Patent Documents 1 and 2, sulfide inclusions in steel are known to significantly contribute to improved machinability. On the other hand, sulfide inclusions tend to coarseen, and when coarse sulfide inclusions are formed, the mechanical properties tend to deteriorate. Even if the sulfide inclusions are finely dispersed, there is a concern that cluster-like inclusions may form due to segregation, thus requiring improved dispersibility of fine sulfide inclusions. Furthermore, coarse sulfide inclusions can be detected as defects in ultrasonic testing, an internal defect test for steel materials, which can significantly reduce the inspection efficiency of steel materials. The inventions in Patent Documents 1 and 2 do not suggest sufficient dispersion of sulfide inclusions, leaving room for further investigation. Therefore, the object of the present invention relates to an alloy steel that can stably and finely disperse sulfides. [Means for solving the problem]

[0006] This invention was made in view of the above-mentioned problems. In other words, one aspect of the present invention is an alloy steel containing Al and Mg, It contains Mg in mass% of more than 0% and 0.01% or less, and the following formula (1): 0.0022 × [Al] 0.6833 <[Mg]<1.0544×[Al] 0.6569 ...Formula (1) (The values ​​in brackets [] in formula (1) indicate the content (mass %) of the element in brackets [] in the alloy steel.) The alloy steel satisfies the following conditions, and the oxide inclusions contained in the alloy steel include MgO·Al2O3-based spinel inclusions, and the proportion of cluster-like inclusions among the sulfide inclusions contained in the alloy steel is 40% or less. [Effects of the Invention]

[0007] According to the present invention, it is possible to obtain an alloy steel in which sulfides can be stably and finely dispersed. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic diagram illustrating the classification method for inclusions. [Figure 2] This is a photograph showing an example of inclusion classification in alloy steel. [Modes for carrying out the invention]

[0009] The present invention will now be described in detail. However, the present invention is not limited to the embodiments described herein, and can be appropriately combined and improved without departing from the technical spirit of the invention. The alloy steel of the present invention is primarily composed of MgO·Al2O3-based spinel inclusions (hereinafter also simply referred to as spinel inclusions) as oxide inclusions present in the steel. Through the inventors' research, it has been found that spinel inclusions have the greatest effect in finely dispersing sulfide inclusions among oxide inclusions. Here, "primarily spinel inclusions" means that when oxide inclusions present in the steel are examined, spinel inclusions are the most abundant. As a method for examining oxide inclusions, for example, a scanning electron microscope (SEM-EDS) equipped with an energy-dispersive X-ray analyzer is used. A sample taken from the alloy steel is mirror-polished, and the field of view is adjusted so that at least 10 oxide inclusions are present in the observation field on the mirror-polished surface (for example, 1 mm × 1 mm or larger). Then, 10 points of any oxide can be measured. To further improve accuracy, an examination with a wider field of view is performed.

[0010] The alloy steel of the present invention contains Mg in a mass percentage of more than 0% and 0.01% or less, and Al in a mass percentage of more than 0% and 10% or less, in order to form spinel-type inclusions. If more than 0.01% of Mg is added, it is difficult to add in large quantities because Mg is easily oxidized, resulting in poor yield, and thus difficult to manufacture stably. The preferred upper limit for Al content is 0.10%. This makes it possible to easily form spinel-type inclusions in the manufacturing process without adding large amounts of Mg.

[0011] In addition to the above-mentioned definition of the Mg component range, the present invention adjusts the Mg so as to satisfy the following formula (1). 0.0022 × [Al] 0.6833 <[Mg]<1.0544×[Al] 0.6569 ...Formula (1) The above equation (1) is based on the results of the examples described later, and is derived from deoxidation equilibrium calculations used to determine the most stable oxide when Al, Mg, or both are used as deoxidizing agents. The equation has been extended and corrected to define the boundary of the oxide that can be formed, even in multi-component systems such as alloy tool steel and stainless steel, to the extent that spinel can be stably formed. The equation shows that when the Mg concentration is below the lower limit of equation (1), the stable oxide is alumina, and when the Mg concentration is above the upper limit of equation (1), the stable oxide is magnesia. By satisfying the above equation (1), it is possible to obtain the effect of making it easier to stably form spinel-based inclusions.

[0012] In the present invention, sulfide-based inclusions in steel have a cluster-like inclusion occupancy rate of 40% or less. This is expected to achieve both good machinability and mechanical properties, as well as improve inspection efficiency. In the present invention, in order to classify the shape of sulfide inclusions into "cluster-like" and others, for example, by using the inclusion classification method by P. Shen (P. Shen, K. Xu, C. Guo, R. Chi, Y. Han, J. Fu: steel research international, 95 (2024)), it can be classified. According to the inclusion classification method of P. Shen, a critical inclusion distance L0 is set, and when the distance L between two certain inclusions is less than L0, the target inclusions are defined as being connected, so that it is possible to classify them into three types: the cluster-like inclusions (Cluster) shown in Fig. 1(a), the chain-like inclusions (Chain) shown in Fig. 1(b), and the individual inclusions (Individual) shown in Fig. 1(c). Fig. 2 shows an example of actually observing an alloy steel sample and classifying sulfide inclusions by the classification method. From Fig. 2, it can be confirmed that it is also possible to classify them into A (cluster-like), B (chain-like), and C (isolated) in actual measurement. And in the present invention, the area ratio of inclusions is defined as the occupancy rate of inclusions for each classification. Preferably, the total occupancy rate of cluster-like inclusions and chain-like inclusions by the inclusion classification method is 60% or less (that is, the occupancy rate of individual inclusions is 40% or more).

[0013] Alloying elements other than those specified in the present invention are not particularly specified, but it is preferable to apply the alloying components of alloy tool steels specified in JIS-G-4404, structural steels specified in JIS-G-4051, JIS-G-4052, JIS-G-4053, and stainless steels specified in JIS-G-4305 and their improved compositions. Further, in order to further improve the machinability of steel, it is more preferable to contain Mn in an amount of 0.1% or more and 10% or less by mass% and S in an amount of more than 0.03% and 1% or less by mass%, respectively.

[0014] A more preferable alloy composition of the present invention contains, in addition to Al and Mg specified in the present invention, in mass%, C: 0.001 to 1.00%, Si: 0.05 to 2.00%, Mn: 0.1 to 2.5%, S: 0.03 to 0.30%, Cr: 0.5 to 20.0%, Ni: 10.0% or less, and the balance is Fe and unavoidable impurities. When the alloy steel of the present invention is stainless steel, in addition to Al and Mg defined in the present invention, in mass %, C: 0.01 to 0.50%, Si: 0.05 to 2.00%, Mn: 0.2 to 2.5%, S: 0.03 to 0.30%, Cr: 12.0 to 18.0%, Ni: 5.0% or less are contained, and the balance is Fe and unavoidable impurities. It is particularly preferable to use stainless steel having such a composition. When the alloy steel of the present invention is alloy tool steel, in addition to Al and Mg defined in the present invention, C: 0.001 to �.70%, Si: 0.05 to 2.00%, Mn: 0.2 to 2.5%, S: 0.03 to 0.20%, Cr: 1.5 to 10.0% (preferably 1.5 to 7.0%), Ni: 5.0% or less (preferably 2.0% or less) are contained, and the balance is Fe and unavoidable impurities. It is particularly preferable to use alloy tool steel having such a composition. In addition, when the numerical values A and B are used in this specification and described as "A to B", unless otherwise specified, it means including the numerical values A and B, and means "A or more and B or less".

Examples

[0015] The present invention will be described in more detail with the following examples. (Example 1) Mg was added to an improved steel of SUS420J2 (free-cutting stainless steel) to obtain the alloy steel of the present invention example shown in Sample No. 1 in Table 1. Sample No. 1 satisfies the formula (1): 0.0022×[Al] 0.6833 <[Mg]<1.0544×[Al] 0.6569 As a comparative example, an alloy steel shown in Sample No. 11 in Table 1 that does not satisfy the formula (1) was also prepared by not adding Mg to an improved steel of SUS420J2 (free-cutting stainless steel). Subsequently, observation samples were taken from the alloy steels of the present invention example and the comparative example, mirror-polished, and then the inclusions in a region of 4.3 mm × 4.3 mm on the polished surface were observed using SEM-EDS. Regarding oxide-based inclusions, 10 arbitrary oxides present in the visual field were selected, and the inclusion composition was investigated. Regarding sulfide-based inclusions, using the inclusion classification method of P. Shen, after classifying them into clusters, chains, and isolated ones, the area ratio of each was derived and used as the occupancy rate.

[0016] [Table 1]

[0017] Observation revealed that in sample No. 1, the example of the present invention, the most abundant oxide inclusions were spinel-type inclusions of the MgO·Al2O3 system. Furthermore, the distribution of sulfide inclusions by category was 30.1% for clusters, 21.2% for chains, and 48.7% for isolated inclusions. In contrast, in comparative example No. 11, the distribution of sulfide inclusions by category was 66.8% for clusters, 13.6% for chains, and 19.6% for isolated inclusions, indicating a higher proportion of clusters compared to the example of the present invention. Therefore, the sulfide inclusions in the example of the present invention were predominantly isolated inclusions, and the formation of cluster inclusions was suppressed, suggesting that they were finely dispersed.

[0018] (Example 2) By adding Mg to free-cutting stainless steel equivalent to SUS420J2, an alloy steel of the present invention example, shown in Sample No. 2 of Table 2, was obtained. Furthermore, by adding Mg and Al to improved SUS420J2 steel (free-cutting stainless steel), an alloy steel of the present invention example, shown in Sample No. 3 of Table 2, was obtained. For both Sample No. 2 and No. 3, the formula (1): 0.0022 × [Al] 0.6833 <[Mg]<1.0544×[Al] 0.6569 The conditions were satisfied. Observation samples were taken from each of the obtained alloy steels, and inclusions in a 1.8 mm × 1.8 mm area were observed using SEM-EDS. The classification method for oxide-based inclusions and sulfide-based inclusions was the same as in Example 1.

[0019] [Table 2]

[0020] As a result of the observation, it was confirmed that among the oxide-based inclusions in Sample Nos. 2 and 3, which are examples of the present invention, MgO·Al₂O₃-based spinel inclusions are the most abundant and are the main ones. Table 3 shows the occupancy rate for each classification of sulfide-based inclusions in Sample Nos. 2 and 3. From Table 3, it can be seen that among the sulfide-based inclusions of the present invention examples, isolated inclusions are the most abundant (occupancy rate of 48% or more), and the formation of cluster-based inclusions is suppressed (occupancy rate of 30% or less), indicating that they are finely dispersed.

[0021]

Table 3

[0022] (Example 3) Mg and, in some cases, Al were added to free-cutting stainless steel equivalent to SUS420J2 to obtain alloy steels of the present invention examples and comparative examples shown in Sample Nos. 4 to 10 and 102 to 108 in Table 4. Also, Mg and, in some cases, Al were added to an improved steel (free-cutting steel) of SCM440 to obtain alloy steels of the present invention examples and comparative examples shown in Sample Nos. 11 to 13 and 109 to 111 in Table 5. Note that all of Sample Nos. 4 to 13, which are examples of the present invention, satisfy the formula (1): 0.0022×[Al] 0.6833 <[Mg]<1.0544×[Al] 0.6569 was satisfied. Observation samples were taken from each of the obtained alloy steels, and inclusions in a region of 1.8 mm × 1.8 mm were observed using SEM-EDS. The classification methods for oxide-based inclusions and sulfide-based inclusions are the same as those in Examples 1 and 2.

[0023]

Table 4

[0024]

Table 5

[0025] Observation revealed that in samples No. 4-13, which are examples of the present invention, the most abundant and dominant oxide inclusions were MgO·Al2O3-based spinel inclusions. Table 4 shows the occupancy rate of each type of sulfide inclusion in samples No. 4-10 and 102-108. Table 5 shows the occupancy rate of each type of sulfide inclusion in samples No. 11-13 and No. 109-111. From Tables 4 and 5, it can be seen that in the examples of the present invention, the proportion of cluster inclusions was suppressed to 40% or less, and isolated inclusions were 48% or more, indicating that the sulfide inclusions were finely dispersed.

Claims

1. An alloy steel containing Al and Mg, It contains Mg in mass percentages of more than 0% and 0.01% or less, and the following formula (1): 0.0022 × [Al] ０．６８３３ <[Mg] < 1.0544 × [Al] ０．６５６９ … Equation (1) (The values ​​in brackets [ ] in formula (1 ] indicate the content (mass %) of the element in brackets [ ] in the alloy steel.) Satisfying the relationship, The oxide inclusions contained in the aforementioned alloy steel are MgO・Al ２ O ３ It contains spinel-type inclusions, An alloy steel in which the proportion of cluster-shaped inclusions among the sulfide-based inclusions contained in the alloy steel is 40% or less.

2. The alloy steel according to claim 1, wherein the aforementioned Al is contained in an amount of more than 0% and 0.10% or less by mass.

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