Ultra-low magnetic permeability stainless steel

Abstract

To provide a stainless steel having extremely low magnetic permeability and excellent corrosion resistance. [Solution] This stainless steel has a chemical composition containing, by mass, C: 0.001 to 0.150%, Si: 0.05 to 1.20%, Mn: 12.0 to 38.0%, P: 0.060% or less, S: 0.0060% or less, Ni: 2.5 to 10.0%, Cr: 13.0 to 25.0%, Mo: 0.01 to 0.44%, Cu: 0.03 to 3.0%, W: 0.14% or less, V: 0.50% or less, B: 0.0001 to 0.0050%, N: 0.100 to 0.400%, with the balance being Fe and unavoidable impurities, and wherein the chemical composition satisfies the following relational expressions (1) and (2): 99.81-1.37Cr-3.14Ni+8.83Mn-12.68Si+4.48Mo-32.45C-33.86N≧160…(1) Cr+3.3Mo+16N-4Mn≧-120…(2)

Description

[Technical Field]

[0001] The present invention relates to a stainless steel having a lower magnetic permeability than so-called non-magnetic stainless steel. [Background technology]

[0002] Non-magnetic materials that are not magnetized by external magnetic fields are used in various devices and equipment, such as communication equipment, where magnetism can have a negative effect. Furthermore, when strength is also required for miniaturization and thinning, stainless steel is often used. A typical example of stainless steel is SUS304, but during cold working, martensite formation is induced, making it magnetic and making it unusable as non-magnetic steel. High-manganese stainless steels have traditionally been used for parts that require non-magnetic properties.

[0003] Another necessary characteristic is corrosion resistance. When used in communication equipment, the corrosive environment is often relatively mild, but the components may still corrode slightly. When used outdoors, corrosion may occur due to factors such as airborne salt. Even if the components are installed inside a device, slight corrosion may occur due to condensation. In this case, there is almost no problem due to deterioration of the component strength caused by thinning, but the corrosion products (rust) are often magnetic, and the non-magnetic properties of the entire component cannot be guaranteed.

[0004] Patent Document 1 proposes an ultra-low magnetic permeability stainless steel with excellent durability. The durability in this document is evaluated in terms of mechanical properties and fatigue resistance, and does not guarantee corrosion resistance. Furthermore, although the invention cites a magnetic permeability of up to 1.005, the non-magnetic properties are also insufficient.

[0005] Patent Document 2 proposes a non-magnetic, high-hardness stainless steel for screws. Although it is intended for use in magnetic storage devices and is described as non-magnetic, its magnetic permeability is in the range of 1.002-1.006, which is not a sufficient non-magnetic property for use as a material for communication devices. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6560881 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-382442 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a stainless steel that has extremely low magnetic permeability and also has excellent corrosion resistance. [Means for solving the problem]

[0008] The present invention has been made in view of the above circumstances, and the stainless steel of the present invention has a chemical composition containing, by mass, C: 0.001 to 0.150%, Si: 0.05 to 1.20%, Mn: 12.0 to 38.0%, P: 0.060% or less, S: 0.0060% or less, Ni: 2.5 to 10.0%, Cr: 13.0 to 25.0%, Mo: 0.01 to 0.44%, Cu: 0.03 to 3.0%, W: 0.14% or less, V: 0.50% or less, B: 0.0001 to 0.0050%, N: 0.100 to 0.400%, with the balance being Fe and unavoidable impurities, and is characterized in that the chemical composition satisfies the following relational expressions (1) and (2): 99.81-1.37Cr-3.14Ni+8.83Mn-12.68Si+4.48Mo-32.45C-33.86N ≧ 160 …(1) Cr+3.3Mo+16N-4Mn ≧ -120 …(2)

[0009] In the stainless steel of the present invention, it is preferable that the following relational expression (3) is satisfied. Mo+425B ≧ 0.17 …(3) [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a graph showing the relationship between Neel temperature TN and magnetic permeability in Experiment 1. [Figure 2] 1 is a graph showing the relationship between PREMn and pitting potential (Vc'10). [Figure 3] 1 is a graph showing the relationship between B concentration and Mo concentration. DETAILED DESCRIPTION OF THE INVENTION

[0011] Experiments 1 and 2 conducted as part of the investigation leading to the present invention will be described below. [Experiment 1] The inventors of the present invention melted an alloy containing the components listed in Table 1, with the remainder consisting of Fe and unavoidable impurities, in a magnesia crucible using a high-frequency induction furnace in air, desulfurized it with CaO-Al2O3-MgO-F slag, and then cast it into a 20 kg ingot. The ingot was then hot-rolled, annealed, pickled, and further cold-rolled to a 2 mm thick cold-rolled sheet, which was then annealed and pickled to obtain a cold-rolled and annealed sheet. Small pieces were cut from the cold-rolled and annealed sheet to serve as test specimens.

[0012] [Table 1]

[0013] To evaluate the magnetic properties, magnetic permeability measurements were carried out. For magnetic permeability measurements, the cold-rolled annealed sheet was cut into a 10 mm square and the entire surface was finished with #240 to serve as a test piece. The measurement method used a VSM test device, and the gradient was calculated from the magnetization value when an external magnetic field of 2500 Oe was applied, and this was taken as the magnetic permeability. The magnetic permeability criteria were as follows: less than 1.0022 was marked ○, and more than 1.0022 was marked ×.

[0014] The corrosion resistance was evaluated by measuring the pitting potential. Pitting potential measurements were performed according to JIS G0577, except that a 0.5% NaCl aqueous solution was used as the test solution. The results were organized in terms of pitting potential Vc'10 (the potential at a current density of 10 μA / cm). The criteria for corrosion resistance were as follows: ◎: Vc'10 of -50 mV (vs. SCE) or higher; 〇: -100 mV to less than -50 mV; and ●: less than -100 mV.

[0015] In Figure 1, the vertical axis represents magnetic permeability and the horizontal axis represents Neel temperature (TN). The Neel temperature TN is given by the following formula: TN(K)=99.81-1.37Cr-3.14Ni+8.83Mn-12.68Si+4.48Mo-32.45C-33.86N

[0016] From the figure, it was found that if the composition system makes TN 160K or more, a magnetic permeability of less than 1.0022 can be obtained. In other words, the relational expression (1) was found. 99.81-1.37Cr-3.14Ni+8.83Mn-12.68Si+4.48Mo-32.45C-33.86N ≧ 160 …Relationship (1)

[0017] In Figure 2, the vertical axis shows the pitting potential. Vc'10 The graph shows a good correlation between pitting potential and PREMn, with the horizontal axis representing Cr + 3.3Mo + 16N - 4Mn (hereafter referred to as PREMn). From the graph, it was discovered that if the composition system is such that PREMn is greater than -120, then a Vc' of -100mV vs SCE or greater can be obtained. In other words, the relationship (2) was found. Cr+3.3Mo+16N-4Mn ≧ -120 …Relationship (2)

[0018] Furthermore, Cr+3.3Mo+16N-4Mn ≧ -110 …Relationship (4) It was found that when the above condition is satisfied, the pitting potential becomes -50 mV or more.

[0019] Figures 1 and 2, and relational equations (1) and (2) show that the inclusion of Mn and Mo is effective in improving non-magnetic properties. However, as shown in relational equation (2), adding excessive Mn degrades corrosion resistance. Mo is an element that improves both non-magnetic properties and corrosion resistance, but it is an expensive metal, and adding large amounts is not economical. Furthermore, because Mo is a ferrite-forming element, adding excessive Mo promotes the formation of δ-ferrite during solidification, and it was found that the small amount of this remaining after final annealing actually increases magnetic permeability (□ in Figure 1).

[0020] Furthermore, when we looked at the relationship between B and Mo in terms of corrosion resistance, we found that the combined addition of B and Mo in this composition system was effective in further improving corrosion resistance. Compared to the base material "〇●◎," the material without B added had slightly inferior corrosion resistance, while the material with 10 ppm B had excellent corrosion resistance. To investigate this relationship, the inventors conducted Experiment 2.

[0021] [Experiment 2] The inventors of the present invention melted an alloy having the composition ranges shown in Table 2, with the balance being Fe and unavoidable impurities, in a magnesia crucible using a high-frequency induction furnace in air, desulfurized it with a CaO-Al2O3-MgO-F slag, and then cast it into a 20 kg ingot.

[0022] The ingot was then hot rolled, annealed, pickled, and further cold rolled to a cold rolled sheet having a thickness of 2 mm, which was then annealed and pickled to obtain a cold rolled and annealed sheet. Small pieces were cut out from the cold rolled and annealed sheet to prepare test specimens.

[0023] [Table 2]

[0024] Pitting potential measurement was used to evaluate corrosion resistance. Pitting potential measurement was performed in accordance with JIS G0577, except that a 0.5% NaCl aqueous solution was used as the test solution. The results were as follows: Vc'10The corrosion resistance was determined based on the base condition of "no Mo added, no B added," and a pitting potential that was 50 mV or more more noble than the base material was designated as "◎," and a value of less than 50 mV was designated as "〇."

[0025] Figure 3 shows the results of pitting potential evaluation when the horizontal axis is B and the vertical axis is Mo. It was found that there are areas marked with ◯ and ◎, with relational expression (3) as the boundary. %Mo + 425 × %B - 0.17 > 0 ...Relationship (3)

[0026] The dotted line "◯" represents a comparison between materials with no S added, no B added, and no Mo added, and even within the region marked with a ◎, no effect of the combined addition of B and Mo was observed. The reasons for these results are thought to be as follows.

[0027] Mo is known to be an effective element for corrosion resistance. It is known that when B coexists with Mo, it segregates at grain boundaries together with Mo. In other words, it is presumed that adding a small amount of B has the effect of increasing the grain boundary Mo concentration.

[0028] On the other hand, MnS inclusions are known to be the starting point for pitting corrosion in stainless steel. Since this alloy contains a large amount of Mn, most of the S should exist in the form of MnS. Like B, S also tends to segregate at grain boundaries, so the MnS formed during solidification and heat treatment should be concentrated at grain boundaries.

[0029] Therefore, it is speculated that the combined addition of B and Mo increases the Mo concentration at the grain boundaries, i.e., the Mo concentration around MnS, making it less likely for the dissolution of MnS to become the starting point for pitting corrosion, thereby improving pitting corrosion resistance.

[0030] Next, the reasons for limiting the components of the stainless steel of the present invention will be explained. C:0.001~0.150% C is a strong γ-phase stabilizing element and is also effective in stabilizing magnetic permeability. Addition of 0.001% or more is necessary to stabilize magnetic permeability. On the other hand, since it is also an element that lowers TN, excessive addition leads to an increase in magnetic permeability. Furthermore, excessive inclusion leads to the formation of Cr and Mo carbides and carbonitrides, which leads to a deterioration in corrosion resistance. Therefore, the upper limit is set to 0.150%. It is preferably 0.005% or more and 0.120% or less. It is more preferably 0.010% or more and 0.089% or less.

[0031] Si: 0.05 to 1.20% Si acts as a deoxidizer and must be added in an amount of at least 0.05%. However, excessive addition promotes the precipitation of intermetallic compounds. Si is also an element that reduces TN, which leads to an increase in magnetic permeability. Therefore, the upper limit is set to 1.20%. It is preferably 0.15% or more and 1.00% or less. It is more preferably 0.20% or more and 0.90% or less.

[0032] Mn: 12.0 to 38.0% Mn has the effect of lowering magnetic permeability and is an important element in the present invention. For this reason, the Mn content must be 12.0% or more. On the other hand, excessive addition reduces hot workability and ductility, resulting in poor manufacturability. Furthermore, delta ferrite (δFe) is more likely to form during solidification, and if this remains after final annealing, magnetic permeability may increase. For this reason, the upper limit is set to 38.0%. The Mn content is preferably 14.0% or more and 35.0% or less. More preferably, the Mn content is 15.5% or more and 32.0% or less. Even more preferably, the Mn content is 19.5% or more.

[0033] P:0.060% or less P deteriorates hot workability and corrosion resistance, so it is necessary to reduce it as much as possible. For this reason, the content is limited to 0.060% or less. It is preferably 0.050% or less, and more preferably 0.034% or less. As long as costs allow, the lower the content, the better, and the lower limit may be 0.

[0034] S:0.0060% or less S is an element that deteriorates hot workability, so it is desirable to reduce it. Furthermore, Mn, which contributes to a decrease in magnetic permeability, is consumed in the form of MnS, so it is desirable to reduce it from the perspective of lowering magnetic permeability. Furthermore, MnS acts as a starting point for pitting corrosion, so corrosion resistance also deteriorates. Therefore, the content is set to 0.0060% or less. Preferably, it is 0.0050% or less. More preferably, it is 0.0040% or less. As long as costs allow, the lower the content, the better, and the lower limit may be 0.

[0035] Ni: 2.5 to 10.0% Ni is an austenite-forming element and contributes to stabilizing magnetic permeability. Therefore, it must be contained in an amount of 2.5% or more. However, excessive content increases magnetic permeability, so the upper limit is set to 10.0%. Preferably, it is contained in an amount of 3.0% or more and 9.5% or less. More preferably, it is contained in an amount of 3.5% or more and 9.0% or less.

[0036] Cr:13.0~25.0% It is an essential element for ensuring corrosion resistance, such as pitting corrosion resistance and crevice corrosion resistance, and a content of 13.0% or more is required. On the other hand, excessive addition promotes the precipitation of intermetallic compounds and Cr carbides, deteriorating corrosion resistance. It also deteriorates phase stability, leading to an increase in magnetic permeability. Therefore, the upper limit is set to 25.0%. A content of 14.0% or more and 20.0% or less is preferable. A content of 15.0% or more and 19.0% or less is even more preferable.

[0037] Mo: 0.01 to 0.44% Like Cr, Mo is an element that improves corrosion resistance, particularly pitting corrosion resistance and crevice corrosion resistance, and also contributes to reducing magnetic permeability. To achieve this, adding 0.01% or more is effective. On the other hand, Mo is a ferrite-forming element, and excessive Mo content makes it easier for delta ferrite (δFe) to form during solidification, which may remain after final annealing and increase magnetic permeability. This also increases costs. For this reason, the upper limit of Mo content is set to 0.44%. Preferably, it is 0.03% or more and 0.30% or less. More preferably, it is 0.05% or more and 0.24% or less.

[0038] Cu: 0.03 to 3.00% Cu has the effect of stabilizing the austenite phase and reducing and stabilizing magnetic permeability. Therefore, it is necessary to add at least 0.03%. However, if added in excess of 3.00%, the steel is prone to cracking during solidification. Therefore, the upper limit is set to 3.00%. The content is preferably 0.05% or more and 1.00% or less. The content is more preferably 0.10% or more and 0.50% or less.

[0039] W: 0.14% or less W is an element that is effective in improving corrosion resistance, but is expensive. To achieve the effect of improving corrosion resistance, it may be contained in an amount of 0.01% or more. Excessive addition increases costs, so the upper limit is 0.14%. It is preferably 0.12% or less, and more preferably 0.10% or less.

[0040] V: 0.50% or less V has the effect of suppressing the formation of deformation-induced martensite, so it is effective in stabilizing low magnetic permeability and may be contained in an amount of 0.01% or more. Excessive addition reduces the amount of dissolved N by forming carbonitrides, leading to a decrease in the stability of austenite, i.e., an increase in magnetic permeability, and also to a deterioration in corrosion resistance. Therefore, the upper limit is 0.50%, preferably 0.40% or less, and more preferably 0.35% or less.

[0041] B:0.0001~0.0050% B is an essential element effective in improving hot workability. Furthermore, when B and Mo are added in combination, the effect of Mo in improving corrosion resistance can be effectively utilized. To effectively utilize this effect, an addition of 0.0001% or more is necessary. On the other hand, excessive addition of B forms nitrides such as BN, which reduces workability. Furthermore, since solute N is effective in corrosion resistance and has the effect of stabilizing austenite, the formation of BN leads to a decrease in corrosion resistance and an increase in magnetic permeability. Therefore, the upper limit is 0.0050%. Preferably, the content is 0.0002% or more and 0.0045% or less. More preferably, the content is 0.0003% or more and 0.0040% or less.

[0042] N:0.100~0.400% N is an element that is effective for corrosion resistance and also forms the austenite phase. To effectively utilize both elements, 0.100% or more must be added. On the other hand, excessive addition forms nitrides, which deteriorate hot workability and increase surface defects. Furthermore, a decrease in the N concentration in the matrix phase actually deteriorates corrosion resistance. For this reason, the upper limit of the N content is set to 0.400%. It is preferably 0.120% or more and 0.380% or less. It is more preferably 0.210% or more and 0.370% or less.

[0043] Neel temperature TN: 160 (K) or higher The Neel temperature TN is given by the following formula: TN(K)=99.81-1.37Cr-3.14Ni+8.83Mn-12.68Si+4.48Mo-32.45C-33.86N Experiment 1 has shown that the higher the Neel temperature, the lower the magnetic permeability, and within the composition range of the present invention, a magnetic permeability of less than 1.0022 can be obtained by setting T to 160 K or higher. It is preferably 180 K or higher, and more preferably 200 K or higher. Although there is no particular upper limit to the Neel temperature, the Mn and Mo contents must be increased to increase the Neel temperature, and from the viewpoint of suppressing δ-ferrite, it is preferably 380 K or lower.

[0044] PREMn:-120 or higher PREMn consists of the following formula: Cr+3.3Mo+16N-4Mn It has been found from Experiment 1 that the higher the PREMn, the higher the pitting potential, and in the range of the composition of the present invention, if PREMn is set to -120 or more, a pitting potential Vc'10 of -100 mV vs. SCE or more can be obtained. It is preferably -110 or more. More preferably, it is -90 or more. There is no particular upper limit, but the upper limit is preferably set to -20.

[0045] Furthermore, the present invention may satisfy the following. Mo+425B: 0.17 or more As shown in Experiment 2, the combined addition of Mo and B further improves corrosion resistance. To effectively utilize this effect, this value needs to be 0.17 or more. It is preferably 0.20 or more. It is more preferably 0.22 or more. There is no particular upper limit, but it is preferably 2.60 or less.

[0046] The stainless steel of the present invention consists of the balance other than the above-mentioned components, namely, Fe and unavoidable impurities. Here, the unavoidable impurities refer to components that are inevitably mixed in due to various factors during industrial production of the steel, and whose inclusion is permitted within a range that does not adversely affect the effects of the present invention. [Example]

[0047] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples as long as it does not deviate from the spirit of the invention. The shape of the product is not limited to plates and strips, but may also be wire, foil, pipe, etc.

[0048] First, raw materials such as stainless steel scrap, heat-resistant alloy scrap, and Fe-Mn, adjusted to a specified ratio, were melted in an electric furnace to obtain a molten alloy. The obtained molten alloy was decarburized and refined in an AOD furnace, and desulfurized and deoxidized by adding quicklime, fluorite, Al, Si, etc. Then, the alloy was adjusted to various component compositions in the LF process and continuously cast into slabs.

[0049] The chemical compositions of each slab produced as described above are shown in Table 3. The C and S compositions shown in Table 3 were measured by oxygen flow combustion-infrared absorption method using a carbon-sulfur simultaneous analyzer. The N composition was measured by inert gas-impulse heating fusion method using an oxygen-nitrogen simultaneous analyzer. The values ​​of components other than those mentioned above were analyzed using X-ray fluorescence analysis. "-" indicates no additives. The values ​​in parentheses indicate that they are outside the scope of claim 1.

[0050] [Table 3]

[0051] The slab was then hot-rolled to form a hot-rolled alloy sheet, which was then repeatedly cold-rolled and heat-treated to form a cold-rolled coil with a thickness of 2 mm. The cold-rolled coil was finally annealed at 1150°C for 1 minute. Small pieces were taken from the cold-rolled coil and subjected to evaluation tests. The evaluation tests were the same as in Experiment 1.

[0052] To evaluate the magnetic properties, magnetic permeability was measured. For the magnetic permeability measurement, the cold-rolled annealed sheet was cut into a 10 mm square, and the entire surface was finished with #240 to serve as a test piece. The measurement was performed using a VSM test device, and the gradient was calculated from the magnetization value when an external magnetic field of 2500 Oe was applied, and this was taken as the magnetic permeability. As the standard for magnetic permeability, less than 1.0022 was marked as ○, and more than that was marked as ×.

[0053] The corrosion resistance was evaluated by measuring the pitting potential using an aqueous NaCl solution. The pitting potential measurement was carried out in accordance with JIS G0577, except that a 0.5% aqueous NaCl solution was used as the test solution. The results were Vc'10 (potential when current density is 10 μA / cm). Vc'10 -50mV (vs. SCE) or higher was marked with a ◎, -100mV or higher was marked with a ○, and less than -100mV was marked with an ×. The magnetic permeability and corrosion resistance were evaluated as "○" overall, making this an example of the invention. The magnetic permeability is good and the corrosion resistance is excellent, resulting in an overall rating of "Excellent", making this an example of the invention. The condition where either one of the magnetic permeability or corrosion resistance is x is judged as "x" and is a comparative example.

[0054] No. 1-23 is an example of the present invention because it has a sufficiently low magnetic permeability and sufficient corrosion resistance. In particular, No. 1-19 has a sufficiently high PREMn value, or even though the PREMn value is slightly low, it satisfies formula (3), and therefore has sufficiently excellent corrosion resistance, earning it a "◎" and an overall rating of "◎." No. 20-23 has a slightly low PREMn value and does not satisfy formula (3), so its corrosion resistance is rated as "good" and its overall rating is "good."

[0055] Next, a comparative example will be described that did not satisfy the numerical range of the present invention and did not provide the desired effect. No. 24-25 had too much Mn. The residual δ ferrite that remained during solidification could not be completely removed, resulting in a small amount remaining, which increased the magnetic permeability. No. 25 also did not meet the PREMn formula, resulting in poor corrosion resistance. No.26-27 had too little Mn. No.27 did not satisfy the TN formula either. The permeability was high. No. 28 had too much S. This resulted in a significantly lower yield and poor corrosion resistance due to the formation of a large amount of MnS. No. 29 had too much Mo. The residual δ-ferrite that remained during solidification could not be completely removed, resulting in a small amount remaining, which increased the magnetic permeability. Furthermore, the addition of a large amount of expensive raw materials made it less economical. No. 30 had too little Mo, resulting in poor corrosion resistance. No. 31 had too much B. The formation of a large amount of BN reduced the effective N concentration, resulting in poor corrosion resistance. The BN also caused many surface scratches, making it difficult to manufacture. No. 32 had too low a B content, which resulted in poor hot workability and significantly poor manufacturability. In addition, Mo could not be effectively used, resulting in poor corrosion resistance. No.33 had a low TN type, which resulted in an increased permeability. No.34 had a significantly lower TN type, which resulted in a significant increase in permeability. No. 35 had a low PREMn value and poor corrosion resistance. No. 36 had a significantly low PREMn value and too low Cr, resulting in significantly poor corrosion resistance. No. 37 had too much Cu. Cracks occurred during solidification, resulting in a significantly lower yield. A small amount of the cracks remained in the final product, and the cracks could not be completely descaled, leaving oxides on the surface, which increased the magnetic permeability. [Industrial Applicability]

[0056] The present invention is suitable for use in fields where extremely low magnetic permeability characteristics and sufficient corrosion resistance are required, and is therefore industrially useful.

Claims

[Claim 1] By mass, C: 0.001-0.089%, Si: 0.05-1.20%, Mn: 12.0-38.0%, P: 0.060% or less, S: 0.0060% or less, Ni: 2.5-10.0%, Cr: 13.0-25.0%, Mo: 0.01 to 0.44%, Cu: 0.03-3.0%, W: 0.14% or less, V: 0.10-0.50%, B: 0.0001 to 0.0050%, N: 0.210-0.400% and the balance being Fe and unavoidable impurities, The stainless steel is characterized in that the component composition satisfies the following relational expressions (1) to (3): 99.81-1.37Cr-3.14Ni+8.83Mn-12.68Si+4.48Mo-32.45C-33.86N ≧ 160...(1) Cr+3.3Mo+16N-4Mn ≧ -120...(2) Mo+425B≧0.17…(3)

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