Corrosion-resistant steel

Abstract

To provide a corrosion-resistant steel having improved corrosion resistance by the compositional modification of non-metal inclusions to be the starting point of corrosion.SOLUTION: A corrosion-resistant steel contains, in mass%, C: 0.001-1%, Si: 0.1-2%, Mn: 0.001-5%, P: 0.001-0.1%, S: 0.0001-0.03%, Al: 0.001-1%, Ca: 0.0001-0.03% and further contains, in mass%, 0.001-0.5% in total of one or both of Se and Te, with the balance being Fe and unavoidable impurities, as well as inclusions. In the inclusions, the content of S, Se, and Te satisfies the relation of S content / (Se content+Te content)≤20 in atom%. The number of the inclusions on the surface is 400 or less per mm2.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a corrosion resistant steel. [Background technology]

[0002] Steel has sufficient corrosion resistance in dry atmosphere for practical use. Furthermore, stainless steel containing Cr has high corrosion resistance even in wet environment. However, even these corrosion-resistant steels may suffer from local corrosion such as pitting corrosion, crevice corrosion, and stress corrosion cracking under conditions of high chloride ion concentration such as coastal environment. In order to reduce such corrosion damage, it is common to add a large amount of rare elements such as Ni, Cr, and Mo to steel to make it highly corrosion-resistant. For example, austenitic stainless steels with 2.0 to 10.00% Mo added by mass% have been developed (see, for example, Patent Document 1). However, steels with a large amount of rare metals such as Ni, Cr, and Mo added have the problem of high raw material and manufacturing costs. In addition, weathering steels for coastal use with Ni added have also been put to practical use (see, for example, Patent Document 2), but it is known that localized erosion occurs when the amount of attached salt is high. For this reason, there is a demand for the development of corrosion-resistant steels with excellent corrosion resistance without adding a large amount of alloy elements.

[0003] Localized corrosion of corrosion-resistant steels such as stainless steels often occurs starting from nonmetallic inclusions (hereinafter referred to as "inclusions") such as sulfides generated during the manufacturing process. The more easily the inclusions dissolve in an aqueous solution, the lower the corrosion resistance of the steel tends to be. MnS and CaS are known as typical examples of water-soluble inclusions. For this reason, efforts have been vigorously made to make the inclusions less soluble by controlling their composition. Specifically, corrosion-resistant steels in which Ti-based inclusions and Zr-based inclusions are generated in the steel (see, for example, Patent Document 3), and ferritic stainless steels in which Mn sulfides are changed to Ti-based sulfides to improve corrosion resistance (see, for example, Patent Document 4) have been developed.

[0004] Conventionally, Mn is added to steel for deoxidization and strength. Furthermore, since Mn is an austenite stabilizing element, in austenitic stainless steel, a part of Ni may be replaced with Mn as an alternative element to expensive Ni. Ca may also be added to improve the hot workability of steel. S in steel segregates at grain boundaries and deteriorates hot workability, but Ca has the effect of fixing S in steel as sulfides, thereby improving hot workability. MnS and CaS, which are harmful to the corrosion resistance of steel, are inevitably generated as a result of adding elements such as Mn and Ca to improve various properties other than corrosion resistance. Therefore, in order to improve the corrosion resistance of steel without deteriorating properties other than corrosion resistance, it is necessary to replace a part of the anion S with other elements, rather than replacing cations such as Mn and Ca contained in sulfide-based inclusions with other elements, to make the inclusions less soluble.

[0005] Regarding the method of replacing a part of S contained in inclusions with other elements, many techniques have been disclosed for improving the machinability of steel. For example, free-cutting steel for cold forging, which has improved machinability by replacing a part of S contained in inclusions with Te (see, for example, Patent Document 5), and free-cutting ferritic stainless steel with excellent corrosion resistance, which has been replaced with a part of S contained in inclusions with Se (see, for example, Patent Document 6), have been developed. However, these techniques are for improving the machinability of free-cutting steel, and it cannot be said that they disclose the conditions necessary for making inclusions difficult to dissolve in order to improve the corrosion resistance of steel. It has not been clarified whether the addition of Se and Te makes inclusions difficult to dissolve, and furthermore, the specific values ​​and ranges of the distribution and size of inclusions suitable for improving corrosion resistance have not been clarified.

[0006] The corrosion resistance of steel is generally evaluated using a test piece with an area of ​​approximately 100 mm 2 The electrode surface was prepared, and the potentiodynamic anodic polarization curve was measured in a chloride aqueous solution, and the pitting potential (area 100 mm 2The electric potential at which the electric current flowing through the electrode surface of the steel reaches 100 μA (see, for example, Non-Patent Document 1). However, the size of inclusions in steel is usually on the order of μm or less, and the electric current that flows due to the dissolution of inclusions is an extremely weak current on the order of nA or less. 2 In the measurement of the polarization curve using the electrode surface of the above, it is not possible to measure the weak current that flows due to the dissolution of inclusions, and therefore it is not possible to evaluate the dissolution resistance of inclusions. Therefore, in order to improve the corrosion resistance of steel by making the inclusions less soluble, it is necessary to evaluate the dissolution resistance of inclusions in addition to evaluating the corrosion resistance of steel using the pitting potential. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2014-005497 A [Patent Document 2] JP 2017-150004 A [Patent Document 3] JP 2005-281826 A [Patent Document 4] Japanese Patent Application Publication No. 09-279231 [Patent Document 5] JP 2013-67833 A [Patent Document 6] Japanese Patent Application Publication No. 10-237603 [Non-patent literature]

[0008] [Non-Patent Document 1] Masashi Nishimoto, Izumi Muto, Yu Sugawara, and Nobuyoshi Hara, “Morphological Characteristics of Trenching around MnS Inclusions in Type 316 Stainless Steel: The Role of Molybdenum in Pitting Corrosion Resistance”, Journal of The Electrochemical Society, 2019, Volume 166, Issue 11, Pages C3081-C3089 Summary of the Invention [Problem to be solved by the invention]

[0009] As described in Patent Documents 1 to 6, excellent corrosion-resistant steels have been developed to date, but there is a demand for the development of corrosion-resistant steels with even better corrosion resistance.

[0010] The present invention has been made in view of the above circumstances, and has an object to provide a corrosion-resistant steel having even better corrosion resistance by modifying the composition of non-metallic inclusions which serve as starting points for corrosion. [Means for solving the problem]

[0011] The present inventors have completed the present invention through various experimental studies in order to overcome the limitations of the prior art and solve the unresolved problems described above.

[0012] That is, the corrosion-resistant steel according to the present invention contains, by mass%, C: 0.001 to 1%, Si: 0.1 to 2%, Mn: 0.001 to 5%, P: 0.001 to 0.1%, S: 0.0001 to 0.03%, Al: 0.001 to 1%, Ca: 0.0001 to 0.03%, and further contains, by mass%, either one or two of Se and Te in total at 0.001 to 0.5%, with the balance being Fe and unavoidable impurities. The corrosion-resistant steel contains inclusions, and the contents of the S, Se and Te in the inclusions are, by atomic %, The S content / (the Se content+the Te content)≦20 and the number of the inclusions present on the surface is 1 mm 2 The number of particles per unit area is 400 or less.

[0013] In the corrosion-resistant steel according to the present invention, it is preferable that the proportion of the number of inclusions present on the surface having a major axis of 3 μm or less is 90% or more.

[0014] The corrosion resistant steel according to the present invention may further contain, by mass%, 10 to 35% Cr, and one or more of 5 to 40% Ni, 0.1 to 10% Mo, 0.1 to 3% Cu, and 0.01 to 0.3% N.

[0015] In the corrosion-resistant steel according to the present invention, the inclusions preferably have a dissolution start potential of 0.1 V (vs. Ag / AgCl, 3.33 mol / L KCl) or more. Effect of the Invention

[0016] According to the present invention, it is possible to provide a corrosion-resistant steel having even better corrosion resistance by modifying the composition of non-metallic inclusions which are the starting points of corrosion.

[0017] According to the present invention, by modifying the composition of nonmetallic inclusions, which are the starting points of corrosion, and rendering them harmless, it is possible to provide a corrosion-resistant steel having excellent corrosion resistance without adding a large amount of alloying elements. It is also possible to provide a corrosion-resistant steel that does not corrode early even in corrosive environments such as seawater and chemicals. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, the embodiment of the present invention will be described based on examples. The corrosion resistant steel of an embodiment of the present invention contains, by mass%, C: 0.001-1%, Si: 0.1-2%, Mn: 0.001-5%, P: 0.001-0.1%, S: 0.0001-0.03%, Al: 0.001-1%, Ca: 0.0001-0.03%, and further contains, by mass%, a total of 0.001-0.5% of either one or two of Se and Te, with the balance being Fe and unavoidable impurities.

[0019] The corrosion-resistant steel according to the embodiment of the present invention contains inclusions for improving the corrosion resistance of the steel, and the composition of the steel is such that the contents of S, Se, and Te in the inclusions satisfy, in atomic percent, the following formula (1), and further, the number of inclusions present on the surface of the steel does not exceed 1 mm 2 The number of atoms per unit area must be 400 or less. In the following, the left side of equation (1) is referred to as the "S / (Se+Te) ratio." S content / (Se content + Te content)≦20 (1)

[0020] If the content of S, Se, and Te in the inclusions does not satisfy formula (1) in atomic percent, the corrosion resistance is not sufficiently improved. 2 If the number of particles per one particle exceeds 400, the effect of improving corrosion resistance is not sufficiently achieved.

[0021] When particularly high corrosion resistance is required, the contents of S, Se, and Te in the inclusions are controlled to a S / (Se+Te) ratio of ≦5, and the number of inclusions on the steel surface is controlled to within 1 mm 2 It is desirable to control the number of particles to 100 or less per particle.

[0022] The composition of the inclusions in the present invention is a value obtained by analyzing the composition of the inclusions using a scanning electron microscope equipped with an energy dispersive X-ray analyzer. Specifically, the composition of the inclusions is a value obtained by observing the surface of the steel using a scanning electron microscope, measuring and integrating characteristic X-rays on the inclusions, and quantitatively analyzing Mn, Ca, Fe, Cr, Ni, S, Se, and Te, and evaluating the composition of the inclusions as their relative concentrations. The inclusions in the corrosion-resistant steel according to the embodiment of the present invention are inclusions in which a part of the S contained in the inclusions is replaced with one or two of Se and Te, or inclusions in which an inclusion containing S and an inclusion containing one or two of Se and Te are in contact with each other and adjacent to each other.

[0023] The corrosion-resistant steel according to the embodiment of the present invention contains 0.001 to 1% by mass of C. C increases the strength of the steel, but if it is contained in a large amount, it adversely affects the corrosion resistance of the steel. If the C content is reduced to less than 0.001% by mass, the manufacturing cost increases, so the lower limit is set to 0.001% by mass.

[0024] The corrosion resistant steel according to the embodiment of the present invention contains 0.1 to 2% by mass of Si. Although Si is a useful element for deoxidization, if too much Si is added, workability decreases, so the upper limit is set to 2% by mass.

[0025] The corrosion-resistant steel according to the embodiment of the present invention contains 0.001 to 5% Mn by mass. Mn is a useful element for deoxidization and strength. In addition, since Mn is an austenite stabilizing element, it can be used as an alternative element to expensive Ni in austenitic stainless steels, and can replace part of Ni with Mn. Since excessive addition of Mn reduces corrosion resistance, the upper limit is set to 5% by mass.

[0026] The corrosion resistant steel according to the embodiment of the present invention contains, by mass%, 0.001 to 0.1% P. P is an element that is mixed in as an unavoidable impurity and is harmful to corrosion resistance, so it is desirable to reduce it to 0.1% or less by mass, but since it is difficult to reduce it to zero by mass, the lower limit is set to 0.001% by mass.

[0027] The corrosion-resistant steel according to the embodiment of the present invention contains 0.0001 to 0.03% S by mass. S is an element that is mixed in as an inevitable impurity, and if it is contained in a large amount, a large amount of sulfide-based inclusions are generated, causing a decrease in corrosion resistance. Even with the composition control of inclusions, which is the main focus of the present invention, if the S content exceeds 0.03% by mass, the effect of improving corrosion resistance is not achieved, so the upper limit is set to 0.03% by mass. In addition, since it is difficult to make the S content zero by mass, the lower limit is set to 0.0001%.

[0028] The corrosion resistant steel according to the embodiment of the present invention contains Al in a range of 0.001 to 1% by mass. Although Al is an effective element for deoxidization, if too much Al is added, the amount of oxide-based inclusion Al2O3 produced increases, which tends to deteriorate the surface quality, so the upper limit is set to 1% by mass.

[0029] The corrosion-resistant steel according to the embodiment of the present invention contains Ca in a range of 0.0001 to 0.03% by mass. Ca is an effective element for deoxidization and also for improving hot workability by fixing S in the steel as sulfides, and this effect is remarkable at a Ca content of 0.0001% or more by mass. However, excessive addition of Ca makes it easy for CaS, which is harmful to corrosion resistance, to be formed, so the upper limit is set to 0.03% by mass.

[0030] The corrosion-resistant steel according to the embodiment of the present invention contains 0.001 to 0.5% by mass of either one or both of Se and Te. Se and Te are important elements in the present invention, and are added to replace S in inclusions with either one or both of Se and Te. If the total amount of Se and Te added is less than 0.001% by mass, the concentration of Se and Te in the inclusions is low, and the effect of improving corrosion resistance is reduced. If the total amount of Se and Te exceeds 0.5% by mass, hot workability may decrease, so the upper limit is set to 0.5% by mass.

[0031] The corrosion-resistant steel according to the embodiment of the present invention may contain 10 to 35% by mass of Cr. Cr is one of the main alloying elements of stainless steel and is an extremely effective element for improving the corrosion resistance of steel, but is not necessarily an essential additive element and may be added depending on the level of corrosion resistance required. However, excessive addition of Cr leads to a decrease in manufacturability such as hot workability, so it is desirable to set the upper limit to 35% by mass.

[0032] The corrosion-resistant steel according to the embodiment of the present invention may contain Ni in an amount of 5 to 40% by mass. Ni is not necessarily an additive element in the present invention, but is an effective element for improving corrosion resistance. Ni is also an austenite stabilizing element, and the addition of 5% or more by mass makes it easier to obtain a stable austenite phase. However, excessive addition tends to reduce hot workability, so if added, it is desirable to set the upper limit to 40% by mass.

[0033] The corrosion-resistant steel according to the embodiment of the present invention may contain 0.1 to 10% by mass of Mo. Although Mo is not necessarily an additive element in the present invention, it is an extremely effective element for improving corrosion resistance, and the effect of improving corrosion resistance is significantly observed when 0.1% or more by mass is added. However, excessive addition of Mo generates intermetallic compounds in the steel and reduces toughness, so when Mo is added, it is preferable to set the upper limit to 10% by mass.

[0034] The corrosion-resistant steel according to the embodiment of the present invention may contain Cu in an amount of 0.1 to 3% by mass. Although Cu is not necessarily an additive element in the present invention, it is an effective element for improving corrosion resistance, and the effect of improving corrosion resistance is remarkable when Cu is added in an amount of 0.1% by mass or more. However, since excessive addition of Cu reduces hot workability, when Cu is added, it is preferable to set the upper limit to 3% by mass.

[0035] The corrosion-resistant steel according to the embodiment of the present invention may contain 0.01 to 0.3% by mass of N. Although N is not necessarily an additive element in the present invention, it is an effective element for improving corrosion resistance, and the effect of improving corrosion resistance is remarkable when added at 0.01% or more by mass. However, excessive addition impairs toughness, so when added, it is preferable to set the upper limit to 0.3% by mass.

[0036] The corrosion-resistant steel according to the embodiment of the present invention does not limit the size of the inclusions present on the steel surface, but if one wishes to ensure corrosion resistance with higher reliability, it is desirable that the proportion of the number of inclusions present on the steel surface that have a major axis of 3 μm or less is 90% or more. Although the detailed mechanism is unclear, inclusions with a major axis of 3 μm or less are unlikely to become the starting point of corrosion. When the proportion of the number of inclusions present on the steel surface that have a major axis of 3 μm or less is 90% or more, the number of inclusions that can become the starting point of corrosion is reduced, and the effect of further improving corrosion resistance can be obtained.

[0037] In the corrosion-resistant steel according to the embodiment of the present invention, if it is desired to ensure corrosion resistance with even higher reliability, it is desirable that the dissolution start potential of the inclusions is 0.1 V (vs. Ag / AgCl, 3.33 mol / L KCl) or more. The dissolution start potential of the inclusions here is the potential at which the color of the inclusions begins to change during the measurement of the anodic polarization curve. When the inclusions begin to dissolve in the aqueous solution, the color of the inclusions changes. The method for measuring the color change of the inclusions under anodic polarization is to assemble an electrochemical cell on the stage of an optical microscope equipped with a water immersion objective lens with a magnification of 100 times, fill the space between the water immersion objective lens and the test piece with the test solution, adjust the position of the stage so that the inclusions can be observed at a magnification of 1000 times near the center of the field of view of the microscope, and record the surface of the test piece during anodic polarization as a video with a frame rate of 30 fps using a CCD camera. From the obtained video, the RGB values ​​on the inclusion are obtained, and the amount of color change compared to when the anodic polarization began is quantitatively evaluated as the color difference ΔRGB, which is expressed by the following equation (2).

[0038]

number

[0039] When the color difference ΔRGB exceeds 5, it is determined that the inclusion begins to dissolve, and the average of the dissolution potentials of five different inclusions is used for evaluation. The standard for displaying the potential is a silver-silver chloride electrode with a 3.33 mol / L KCl aqueous solution as the internal solution. The potential sweep rate is 20 mV / min. If the dissolution potential of carbon steel inclusions is 0.1 V or higher in a H3BO3-Na2B4O7 mixed solution (pH 8.6) containing 0.01 mol / L NaCl at 25°C, it shows good corrosion resistance in outdoor atmospheric corrosion environments. If the dissolution potential of ferritic stainless steel inclusions is 0.1 V or higher in 0.1 mol / L NaCl (pH 5.5) at 25°C, it shows good corrosion resistance in aqueous environments such as piping. If the dissolution potential of inclusions in high alloy austenitic stainless steel is 0.1 V or higher in 3 mol / L NaCl (pH 5.0) at 25°C, the stainless steel will exhibit good corrosion resistance even in highly corrosive environments such as seawater.

[0040] Hereinafter, the present invention will be described in detail with reference to examples. However, the present invention is not limited to the description of the examples. EXAMPLES

[0041] Steels with the chemical compositions (mass%) shown in Tables 1 and 2 were melted and subjected to hot rolling, hot-rolled sheet annealing, cold rolling, and annealing to produce cold-rolled and annealed sheets with a thickness of 1 mm. The surfaces of the test pieces were wet-polished with SiC paper from No. 320 to No. 1500, and then mirror-polished with 6 μm and 1 μm diamond pastes. After polishing, the composition, number, and major axis of the inclusions present in each test piece were analyzed using prescribed equipment such as an optical microscope, a scanning electron microscope, and an energy dispersive X-ray analyzer. The number of inclusions was determined by observing the surface of the test piece after mirror polishing in 20 fields of view at 100x magnification with an optical microscope, and the total number of inclusions present within the field of view was counted and then divided by 1 mm.2 The number of inclusions per mm was calculated. The number of inclusions with a major axis of 3 μm or less was also counted, and the ratio to the total number of inclusions was calculated. Tables 1 and 2 show the composition of the inclusions present in each test piece, 2 The figures show the analysis results of the number of inclusions per unit area (distribution density) and the percentage of inclusions with a major axis of 3 μm or less.

[0042] [Table 1]

[0043] [Table 2]

[0044] Numbers 1 to 12 in Table 1 are examples related to carbon steel. As shown by number 1, in the carbon steel to which Se and Te were not added, (Mn,Ca)S inclusions, which are sulfides of Mn and Ca, were mainly formed. The number of inclusions (distribution density) present on the surface of the steel was 1 mm 2 The percentage of inclusions with a major axis of 3 μm or less was 44%.

[0045] Numbers 2 to 4 are examples of carbon steels to which Se was added, and (Mn,Ca)(S,Se) inclusions were formed in which part of the S contained in the (Mn,Ca)S inclusions was replaced with Se. As shown in number 2, when the amount of Se added to the steel was less than 0.001% by mass, the S / (Se+Te) ratio in the inclusions exceeded 20. As the amount of Se added increased, the S / (Se+Te) ratio in the inclusions decreased.

[0046] Numbers 5 to 7 are examples of carbon steels to which Te was added, and (Mn,Ca)(S,Te) inclusions were formed in which part of the S contained in the (Mn,Ca)S inclusions was replaced with Te. As shown in number 5, when the amount of Te added to the steel was less than 0.001% by mass, the S / (Se+Te) ratio in the inclusions exceeded 20. As the amount of Te added increased, the S / (Se+Te) ratio in the inclusions decreased.

[0047] Numbers 8 to 12 are examples in which Se and Te were added to carbon steel, and (Mn,Ca)(S,Se,Te) inclusions were formed in which part of the S contained in the (Mn,Ca)S inclusions was replaced with Se and Te. As shown in number 8, when the amount of Se and Te added to the steel was less than 0.001% in total by mass, the S / (Se+Te) ratio in the inclusions exceeded 20. As the amount of Se and Te added increased, the S / (Se+Te) ratio in the inclusions decreased. As shown in numbers 9 and 10, when the amount of S contained in the steel was reduced, the distribution density of the inclusions decreased to 1 mm 2 As shown in No. 11 and No. 12, when the amount of S in the steel increases, the distribution density of inclusions increases and the number of inclusions per mm 2 The number of inclusions per grain exceeded 400, and the proportion of inclusions with a major axis of 3 μm or less decreased.

[0048] Numbers 13 to 18 in Table 2 are examples related to ferritic stainless steel. As shown in numbers 13 to 15 in Table 2, in the ferritic stainless steel to which Se was added, Mn(S,Se) inclusions were formed in which part of the S contained in the MnS inclusions was replaced with Se. As shown in number 13, when the amount of Se added to the steel was less than 0.001% by mass, the S / (Se+Te) ratio in the inclusions exceeded 20. When the amount of Se added was 0.001% by mass or more, the S / (Se+Te) ratio in the inclusions decreased to 20 or less. As shown in number 15, when the amount of S contained in the steel was reduced, the proportion of inclusions with a major axis of 3 μm or less sometimes reached 90% or more.

[0049] As shown by numbers 16 to 18 in Table 2, in the ferritic stainless steels to which Te was added, Mn(S,Te) inclusions were formed in which part of the S contained in the MnS inclusions was replaced with Te. As shown by number 16, when the amount of Te added to the steel was less than 0.001% by mass, the S / (Se+Te) ratio in the inclusions exceeded 20. When the amount of Te added was 0.001% or more by mass, the S / (Se+Te) ratio in the inclusions fell to 20 or less. As shown by number 18, as the amount of S in the steel increased, the distribution density of the inclusions increased, and the inclusions increased in size by 1 mm. 2 In some cases, there were more than 400 per unit.

[0050] Numbers 19 and 20 in Table 2 are examples of low-Ni, high-Mn austenitic stainless steels in which part of Ni, an austenite stabilizing element, is replaced with Mn. When Se and Te were not added, MnS inclusions were formed. When Se and Te were added, Mn(S,Se,Te), an inclusion in which part of the S contained in the MnS inclusions was replaced with Se and Te, was formed. As shown in number 20, when the amount of Se and Te added to the steel was 0.001% or more in total by mass, the S / (Se+Te) ratio in the inclusions fell to 20 or less.

[0051] Numbers 21 to 23 in Table 2 are examples of austenitic high alloy stainless steel. As shown in numbers 21 to 23 in Table 2, in the austenitic high alloy stainless steel to which Se and Te were added, Ca(S,Se,Te) inclusions were formed in which part of the S contained in the CaS inclusions was replaced with Se and Te. As shown in number 21, when the amount of Se and Te added to the steel was less than 0.001% in total by mass%, the S / (Se+Te) ratio in the inclusions exceeded 20. As the amount of Se and Te added increased, the S / (Se+Te) ratio in the inclusions decreased.

[0052] Next, the test piece was mirror-polished and a small area (area: approx. 0.01 mm2) containing only one inclusion was 2) was used as the electrode surface, and potentiodynamic anodic polarization curves were measured in a non-degassed NaCl aqueous solution. During the measurement of the polarization curve, the inclusions were observed using an optical microscope equipped with a water immersion objective lens of 100x magnification, and the potential at which the inclusions began to change color was determined to determine the potential at which the inclusions began to dissolve. In addition, when evaluating the corrosion resistance of the test pieces, 2 The electrode surface was prepared and the potentiodynamic anodic polarization curve was measured in a non-degassed NaCl aqueous solution. 2 The potential reached was defined as the pitting potential. The potential sweep rate was 20 mV / min. The standard for displaying the potential was a silver-silver chloride electrode with a 3.33 mol / L KCl aqueous solution as the internal solution. Tables 3 to 5 show the inclusion dissolution start potential and pitting potential measured by anodic polarization in a NaCl aqueous solution.

[0053] [Table 3]

[0054] [Table 4]

[0055] [Table 5]

[0056] Numbers 1 to 12 in Tables 3 and 4 are examples in which a carbon steel test piece was used to measure a potentiodynamic anodic polarization curve using a H3BO3-Na2B4O7 mixed solution containing 0.01 mol / L NaCl, and the inclusion dissolution onset potential and pitting potential were measured. As shown by number 1, in carbon steel to which Se and Te were not added, the type of inclusion was (Mn,Ca)S, and the inclusions did not contain Se or Te. In this case, the inclusion dissolution onset potential was -0.16 V. The number of inclusions (distribution density) present on the steel surface was 1 mm 2The number of inclusions per specimen was 164. The percentage of inclusions with a major axis of 3 μm or less was 44%. The pitting potential in this case was -0.06 V.

[0057] Numbers 2 to 4 in Table 3 are examples of the case where the inclusion dissolution start potential and pitting potential were measured using carbon steel with Se added as a test piece. The type of inclusion is (Mn,Ca)(S,Se), which is a form in which part of the S contained in the (Mn,Ca)S inclusion is replaced with Se. As shown in number 2, when the S / (Se+Te) ratio in the inclusion exceeds 20, the inclusion dissolution start potential is -0.11V, and there is no significant difference in the inclusion dissolution resistance compared to the (Mn,Ca)S inclusion shown in number 1. The pitting potential was -0.08V, and the corrosion resistance was not improved. As shown in numbers 3 and 4, when the S / (Se+Te) ratio in the inclusion is 20 or less, the inclusion dissolution start potential becomes 0.01V or more, and the inclusion becomes difficult to dissolve. The pitting potential increases to 0.10V or more, and the corrosion resistance is improved. It can be seen that in order to improve the corrosion resistance of carbon steel by controlling the S / (Se+Te) ratio in the inclusions, it is necessary to make the S / (Se+Te) ratio in the inclusions 20 or less. As shown in number 4, when the S / (Se+Te) ratio in the inclusions is 5 or less, the inclusion dissolution start potential exceeds 0.1 V, and the inclusion dissolution resistance is significantly improved. In this case, the pitting potential is 0.37 V, and it can be seen that extremely high corrosion resistance is exhibited by increasing the inclusion dissolution start potential to 0.1 V.

[0058] Numbers 5 to 7 in Table 3 are examples of the case where the dissolution start potential and pitting potential of inclusions were measured using carbon steel with Te added as a test piece. The type of inclusion is (Mn,Ca)(S,Te), which is a form in which part of the S contained in the (Mn,Ca)S inclusion is replaced with Te. As shown in number 5, when the S / (Se+Te) ratio in the inclusion exceeds 20, the dissolution start potential of the inclusion is -0.13V, and there is no significant difference in the dissolution resistance of the inclusion compared to the (Mn,Ca)S inclusion shown in number 1. The pitting potential was -0.05V, and the corrosion resistance was not improved. As shown in numbers 6 and 7, when the S / (Se+Te) ratio in the inclusion is 20 or less, the dissolution start potential of the inclusion becomes 0.01V or more, and the inclusion becomes difficult to dissolve. The pitting potential increases to 0.10V or more, and the corrosion resistance is improved. As shown in number 7, when the S / (Se+Te) ratio in the inclusions was 5 or less, the dissolution start potential of the inclusions exceeded 0.1 V, and the dissolution resistance of the inclusions was significantly improved. The pitting potential in this case was 0.28 V, indicating extremely high corrosion resistance.

[0059] Numbers 8 to 12 in Table 4 are examples of the case where the carbon steel with Se and Te added was used as a test piece to measure the dissolution start potential and pitting potential of the inclusions. The type of inclusion is (Mn,Ca)(S,Se,Te), which is a form in which part of the S contained in the (Mn,Ca)S inclusion is replaced with Se and Te. As shown in number 8, when the S / (Se+Te) ratio in the inclusion exceeds 20, the dissolution start potential of the inclusion is -0.10V, and there is no significant difference in the dissolution resistance of the inclusion compared to the (Mn,Ca)S inclusion shown in number 1. The pitting potential was -0.01V, and the corrosion resistance was not improved. As shown in numbers 9 and 10, when the S / (Se+Te) ratio in the inclusion is 20 or less, the dissolution start potential of the inclusion becomes 0.04V or more, and the inclusion becomes difficult to dissolve. The pitting potential was 0.21 V or more, and the corrosion resistance was improved. It can be seen that even when both Se and Te are added, the dissolution of inclusions can be suppressed and high corrosion resistance can be achieved. In particular, as shown in number 10, when the distribution density of inclusions is 1 mm 2When the number of inclusions is 100 or less per mm, the pitting potential rises to 0.40 V, indicating extremely high corrosion resistance. As shown by number 11, when the S / (Se+Te) ratio in the inclusions is controlled to 5 or less, the dissolution potential of the inclusions increases, and the distribution density of the inclusions is reduced to 1 mm 2 Even if the number of inclusions increases to about 300 per mm, good corrosion resistance is still observed. However, as shown in number 12, 2 If the number of inclusions per square meter exceeds 400, the number of inclusions that can become the starting point of corrosion increases excessively, resulting in a decrease in corrosion resistance.

[0060] Numbers 13 to 15 in Table 4 are examples of the case where a ferritic stainless steel with Se added was used as a test piece, and a potentiodynamic anodic polarization curve was measured using a 0.1 mol / L NaCl aqueous solution to measure the inclusion dissolution potential and pitting potential. In all test pieces, the distribution density of the inclusions was 1 mm 2 The number of inclusions that can become the starting point of corrosion is small, with 100 or less per inclusion. However, as shown in number 13, when the S / (Se+Te) ratio in the inclusions exceeds 20, the inclusions start to dissolve low and the corrosion resistance is not excellent. As shown in numbers 14 and 15, when the S / (Se+Te) ratio in the inclusions is controlled to 20 or less, the inclusions start to dissolve higher than in number 13 and the corrosion resistance is improved. In particular, as shown in number 15, when the proportion of inclusions with a major axis of 3 μm or less exceeds 90%, the pitting potential increases significantly and extremely excellent corrosion resistance is exhibited.

[0061] Numbers 16 to 18 in Table 5 are examples of the case where a test piece of a ferritic stainless steel containing added Te was used to measure a potentiodynamic anodic polarization curve using a 0.1 mol / L NaCl aqueous solution, and the inclusion dissolution start potential and pitting potential were measured. As shown in number 16, when the S / (Se+Te) ratio in the inclusions exceeded 20, the inclusion dissolution start potential was -0.04 V and the pitting potential was 0.07 V. As shown in number 17, when the S / (Se+Te) ratio in the inclusions was controlled to 20 or less, the inclusion dissolution start potential rose to 0 V or higher, and the pitting potential also rose to 0.26 V. However, as shown in number 18, when the distribution density of the inclusions was 1 mm2 When the number of inclusions exceeded 400 per mm, no significant difference was observed in corrosion resistance, even though the potential at which the inclusions began to dissolve was higher than that of No. 16. In order to improve the corrosion resistance of steel, it is necessary to control the S / (Se+Te) ratio in the inclusions to 20 or less and to reduce the number of inclusions (distribution density) present on the steel surface to 1 mm 2 It can be seen that the number of particles per unit area must be controlled to 400 or less.

[0062] Nos. 19 and 20 in Table 5 are examples of the case where a low-Ni, high-Mn austenitic stainless steel, in which part of Ni, an austenite stabilizing element, is replaced with Mn, is used as a test piece, and a potentiodynamic anodic polarization curve is measured using a 0.1 mol / L NaCl aqueous solution to measure the inclusion dissolution potential and pitting potential. As shown in No. 19, when Se and Te are not added to the steel, Se and Te are not contained in the inclusions, and the S / (Se+Te) ratio in the inclusions exceeds 20. In this case, the inclusion dissolution potential is -0.05V, and the pitting potential is 0.23V. As shown in No. 20, when the S / (Se+Te) ratio in the inclusions is controlled to 20 or less, the inclusion dissolution potential increases to 0.16V, and the pitting potential also increases to 0.50V. It is clear that even in Ni-saving, high-Mn austenitic stainless steels, good corrosion resistance is exhibited by controlling the S / (Se+Te) ratio in inclusions to 20 or less.

[0063] Nos. 21 to 23 in Table 5 are the results of measuring the inclusion dissolution potential and pitting potential in a 3 mol / L NaCl aqueous solution using austenitic high alloy stainless steels with Se and Te added as test pieces. In the test pieces Nos. 21 to 23, Ca(S,Se,Te) inclusions were formed in which part of the S in the CaS inclusions was replaced with Se and Te. As shown in No. 21, when the S / (Se+Te) ratio in the inclusions exceeded 20, the inclusions began to dissolve at a low potential range of -0.15V, and the pitting potential was 0.18V. When the S / (Se+Te) ratio in the inclusions was controlled to 20 or less, the inclusion dissolution potential rose to 0V or more, and the pitting potential also rose to 0.3V or more. In particular, as shown in No. 21, when the inclusion dissolution potential exceeded 0.1V, the pitting potential rose to 0.4V, and the corrosion resistance was significantly improved. It is apparent that when the solution is extremely corrosive or when it is desired to ensure corrosion resistance with extremely high reliability, the dissolution starting potential of the inclusions is preferably 0.1 V or higher. [Industrial Applicability]

[0064] The corrosion-resistant steel according to the present invention can be used for general purposes in various fields such as building materials and automobiles, and is of great industrial benefit.

Claims

1. In mass percent, C: 0.001-1%, Si: 0.1-2%, Mn: 0.001 to 5%, P: 0.001-0.1%, S: 0.0001-0.03%, Al: 0.001-1%, Ca: 0.0001-0.03% Cr: 10 to 35% N: 0.01~0.3% and further containing, by mass%, 0.001 to 0.5% in total of one or two of Se and Te, with the balance being Fe and unavoidable impurities, including inclusions; The contents of the S, Se, and Te in the inclusions are, in atomic percent, The S content / (the Se content+the Te content)≦20 and When the surface after mirror polishing is observed by an optical microscope at a magnification of 100 times, the number of the inclusions present on the surface is 1 mm ２ There should be no more than 400 pieces per Characteristic corrosion-resistant steel.

2. 2. The corrosion resistant steel according to claim 1, wherein the ratio of the number of inclusions present on the surface having a major axis of 3 μm or less is 90% or more.

3. A carbon steel having, in mass%, C: 0.001-1%, Si: 0.1-2%, Mn: 0.001 to 5%, P: 0.001-0.1%, S: 0.0001-0.03%, Al: 0.001-1%, Ca: 0.0001-0.03% and further containing, by mass%, 0.001 to 0.5% in total of one or two of Se and Te, with the balance being Fe and unavoidable impurities, including inclusions; The contents of the S, Se, and Te in the inclusions are, in atomic percent, The S content / (the Se content+the Te content)≦20 and When the surface after mirror polishing is observed by an optical microscope at a magnification of 100 times, the number of the inclusions present on the surface is 1 mm ２ 400 or less per The dissolution onset potential of the inclusions is 0.1 V (vs. Ag / AgCl, 3.33 mol / L KCl) or more, and the dissolution onset potential is the potential at which the color of the inclusions begins to change when a potentiodynamic anodic polarization curve is measured in a non-degassed H 3 BO 3 -Na 2 B 4 O 7 mixed solution (pH 8.6) containing 0.01 mol / L NaCl at 25°C and the inclusions are observed under an optical microscope at a magnification of 100 times during the measurement. Characteristic corrosion-resistant steel.

4. A carbon steel having, in mass%, C: 0.001-1%, Si: 0.1-2%, Mn: 0.001 to 5%, P: 0.001-0.1%, S: 0.0001-0.03%, Al: 0.001-1%, Ca: 0.0001-0.03% and further containing, by mass%, 0.001 to 0.5% in total of one or two of Se and Te, with the balance being Fe and unavoidable impurities, including inclusions; The contents of the S, Se, and Te in the inclusions are, in atomic percent, The S content / (the Se content+the Te content)≦20 and When the surface after mirror polishing is observed by an optical microscope at a magnification of 100 times, the number of the inclusions present on the surface is 1 mm ２ 400 or less per the initiation potential for dissolution of the inclusions is 0.01 V (vs. Ag / AgCl, 3.33 mol / L KCl) or more, and the initiation potential for dissolution is the potential at which the color of the inclusions begins to change when a potentiodynamic anodic polarization curve is measured in a non-degassed H 3 BO 3 -Na 2 B 4 O 7 mixed solution (pH 8.6) containing 0.01 mol / L NaCl at 25°C and the inclusions are observed under an optical microscope at a magnification of 100 times during the measurement, The inevitable impurities include Ni, Cr, Mo, Cu and N. Characteristic corrosion-resistant steel.

5. Further, in mass%, Ni: 5 to 40%, Mo: 0.1 to 10%, and Cu: 0.1 to 3% 3. The corrosion resistant steel according to claim 1, further comprising at least one of the following:

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