Duplex stainless steel section and its manufacturing method

Abstract

To provide a low-alloy duplex stainless steel shape that can suppress the occurrence of uneven polishing when it is subjected to polishing, and a method for producing the same.SOLUTION: A duplex stainless steel shape has a chemical composition consisting of, in mass%, C: 0.040% or less, Si: 0.01-1.5%, Mn: 0.05-15.0%, P: 0.050% or less, S: 0.0050% or less, Cr: 19.0-26.0%, Ni: 1.0-6.0%, Mo: 1.5% or less, Cu: 2.0% or less, N: 0.05-0.25%, Al: 0.050% or less, with the balance being Fe and impurities. The metallographic structure includes, by area%, a ferrite phase: 35-70%, with the balance being an austenite phase and a hard phase. An A value regarding the chemical composition of the ferrite phase (=20[C]α+20[N]α+[Cr]α+[Mn]α+0.5[Al]α+1.5[Mo]α+2[Ni]α+2.4[Cu]α) is 30 or more, and a B value regarding the chemical composition of the austenite phase (=500.0-400[C]γ-400[N]γ-9[Si]γ-8[Mn]γ-14[Cr]γ-35[Ni]γ-35[Cu]γ-20[Mo]γ) is 0 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a duplex stainless steel section and a method for producing the same. [Background technology]

[0002] Due to its excellent corrosion resistance, stainless steel is used in many applications, such as building materials, automobiles, and home appliances. Stainless steels are classified into austenitic stainless steel, ferritic stainless steel, duplex stainless steel, etc., depending on their internal structure. Among these, duplex stainless steel contains both austenitic and ferritic phases in its internal structure and has high strength, so it is used as a building or structural material.

[0003] Furthermore, duplex stainless steel generally has a lower Ni content than austenitic stainless steels such as SUS304, making it less expensive to manufacture, and is less susceptible to Ni price fluctuations.

[0004] Among hot-rolled stainless steel sheets and strips, typical duplex stainless steels are relatively expensive stainless steels such as SUS329J1 and SUS329J4L described in JIS G 4304:2015. These contain approximately 6-7% Ni and approximately 3-4% Mo, which are expensive elements.

[0005] Meanwhile, in recent years, attention has been focused on alloy-saving duplex stainless steel, which contains less of expensive elements such as Ni and Mo. The main types of conventional duplex stainless steel, such as SUS329J1 and SUS329J4L, have higher corrosion resistance than the highly corrosion-resistant austenitic steel SUS316. In contrast, alloy-saving duplex stainless steel achieves corrosion resistance close to that of SUS316 and SUS304, but substitutes Ni and Mo with N and Mn, significantly reducing the Ni and Mo content to approximately 1-6% and 0-1%, respectively.

[0006] Patent Document 1 discloses a duplex stainless steel with a relatively low Ni content, while Patent Document 2 discloses an inexpensive duplex stainless steel with a low Ni content that utilizes austenite-forming elements such as Mn and N.

[0007] To utilize these duplex stainless steels as building or structural materials, stainless steels in the form of section steel are required, in addition to plate steel. Although the types of hot-formed stainless steel section steels are listed in JIS G 4317, the only duplex stainless steels among them are relatively expensive stainless steels such as the aforementioned SUS329J1 or SUS329J4L. For this reason, several inventions relating to alloy-saving duplex stainless steel section steels have been disclosed.

[0008] Patent Document 3 discloses a steel section made of alloy-saving duplex stainless steel components, and Patent Document 4 discloses a welded channel steel made of alloy-saving duplex stainless steel components. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 4,828,630 [Patent Document 2] Japanese Patent Application Publication No. 61-56267 [Patent Document 3] Japanese Patent Application Publication No. 2018-159119 [Patent Document 4] Japanese Patent Application Publication No. 2020-100859 Summary of the Invention [Problem to be solved by the invention]

[0010] Stainless steel sections are used as building or structural materials not only for applications requiring corrosion resistance, such as flood control facilities like dams and floodgates, but also for architectural structural materials that are eye-catching and require aesthetic appeal. In recent years, there has been a growing need for architectural structural materials that contribute to attractive designs in public facilities, commercial facilities, office buildings, and other areas. Stainless steel sections not only function as a structural material to maintain the shape of buildings, but can also be given a high level of aesthetic appeal through surface treatments such as polishing. Furthermore, their high corrosion resistance allows them to maintain the same surface condition for long periods of time without painting or surface treatments. Therefore, they are well suited for such applications.

[0011] When manufactured as building materials, alloy-saving duplex stainless steel sections are sometimes polished to ensure aesthetic appeal. Conventional polishing methods can result in unevenness in the visual appearance after polishing (hereinafter referred to as "polishing unevenness"). The causes of this and appropriate treatment methods have not been fully understood. Therefore, when uneven polishing occurs, the product may become unusable from an aesthetic standpoint, or it may require careful re-polishing, which can be time-consuming and costly, until the unevenness disappears. For these reasons, it is necessary to suppress the occurrence of uneven polishing during the polishing of alloy-saving duplex stainless steel sections.

[0012] The present invention has been made to solve the above-mentioned problems, and aims to provide an alloy-saving duplex stainless steel section that can suppress the occurrence of uneven polishing when polished, and a method for manufacturing the same. [Means for solving the problem]

[0013] As a result of extensive investigations to solve the above-mentioned problems, the present inventors have come to the following findings.

[0014] (a) Duplex stainless steel includes a two-phase structure of an austenite phase and a ferrite phase, and the chemical compositions of the austenite phase and the ferrite phase are different from the chemical composition of the steel as a whole.

[0015] (b) A detailed investigation of the duplex stainless steel sections that had experienced polishing irregularities revealed that the chemical compositions of the ferrite and austenite phases in all cases did not satisfy the specified conditions. In other words, polishing irregularities can be suppressed by adjusting the chemical compositions of the ferrite and austenite phases so that they satisfy the specified conditions.

[0016] (c) In order to adjust the chemical composition of the ferrite phase and the austenite phase in the duplex stainless steel section so as to satisfy the specified conditions, it is effective to appropriately adjust the heating temperature before hot rolling the billet, the final pass temperature of hot rolling, and the final heat treatment temperature after hot rolling.

[0017] The present invention has been made based on the above findings, and is summarized as follows: a duplex stainless steel section and a method for producing the same.

[0018] (1) Chemical composition, in mass%, C: 0.040% or less, Si: 0.01 to 1.50% Mn: 0.05 to 15.00%, P: 0.0500% or less, S: 0.0050% or less, Cr: 19.0~26.0%, Ni: 1.0 to 6.0% Mo: 1.5% or less Cu: 2.0% or less, N: 0.05 to 0.25%, Al: 0.050% or less, The balance is Fe and impurities. The metal structure contains, in area%, 35.0 to 70.0% of a ferrite phase, and the remainder is an austenite phase and a hard phase, The A value defined by the following formula (i) regarding the chemical composition of the ferrite phase is 30.0 or more, The B value defined by the following formula (ii) regarding the chemical composition of the austenite phase is 0.0 or less: Duplex stainless steel sections. A=20[C]α+20[N]α+[Cr]α+[Mn]α+0.5[Al]α+1.5[Mo]α+2[Ni]α+2.4[Cu]α ···(i) B=500.0-400[C]γ-400[N]γ-9[Si]γ-8[Mn]γ-14[Cr]γ-35[Ni]γ-35[Cu]γ-20[Mo]γ...(ii) In the above formula (i), [C]α, [N]α, [Cr]α, [Mn]α, [Al]α, [Mo]α, [Ni]α, and [Cu]α represent the contents (mass%) of C, N, Cr, Mn, Al, Mo, Ni, and Cu in the ferrite phase, and in the above formula (ii), [C]γ, [N]γ, [Si]γ, [Mn]γ, [Cr]γ, [Ni]γ, [Cu]γ, and [Mo]γ represent the contents (mass%) of C, N, Si, Mn, Cr, Ni, Cu, and Mo in the austenite phase, with 0 substituted if not contained.

[0019] (2) The chemical composition contains, in mass %, a part of Fe replaced by Ti: 0.05% or less, Nb: 0.15% or less, V: 0.5% or less, W: 1.0% or less, Co: 1.0% or less, B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, containing one or more selected from The duplex stainless steel section according to (1) above.

[0020] (3) A method for producing the duplex stainless steel section according to (1) or (2) above, For a steel material having the chemical composition described in (1) or (2) above, (a) heating in a temperature range of 1100 to 1350°C; (b) hot rolling under conditions where the final rolling pass temperature is 500°C or higher and lower than 700°C; (c) performing a final heat treatment at a temperature of 925 to 1170°C, which is equal to or higher than T1 (°C) calculated by the following formula (iii), for 30 seconds or more and less than 30 minutes; Manufacturing method for duplex stainless steel sections. T1=548+24.3[Cr]-52.5[Ni] ···(iii) However, [Cr] and [Ni] in the above formula (iii) represent the Cr and Ni contents (mass %) in the steel material. [Effects of the Invention]

[0021] According to the present invention, it is possible to industrially and stably obtain alloy-saving duplex stainless steel sections that can suppress the occurrence of polishing unevenness when polished. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 2 is a diagram illustrating the shape of a test piece used in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0023] Each of the requirements of the present invention will be described in detail below.

[0024] 1. Chemical composition of duplex stainless steel sections The chemical composition of a duplex stainless steel section according to one embodiment of the present invention will be described. The reasons for limiting the content of each element are as follows. In the following description, "%" for the content means "% by mass."

[0025] C: 0.040% or less C combines with elements that ensure corrosion resistance, such as Cr, to form carbides, which can adversely affect corrosion resistance, such as locally inhibiting the effects of elements that ensure corrosion resistance, potentially causing so-called sensitization. Therefore, the upper limit of the C content is set to 0.040%. To further suppress sensitization, the C content is preferably 0.035% or less. While there is no particular lower limit, excessive reduction leads to increased refining costs, so the C content is preferably 0.001% or more. From the perspective of refining costs, the C content is more preferably 0.005% or more.

[0026] Si: 0.01 to 1.50% Si is used as a deoxidizing element during the refining process. To fully obtain the deoxidizing effect, the Si content is set to 0.01% or more. The Si content is preferably 0.10% or more, and more preferably 0.15% or more. On the other hand, excessive addition causes a deterioration in toughness. Therefore, the upper limit of the Si content is set to 1.50%. From the viewpoint of ensuring toughness, the Si content is preferably 1.20% or less, and more preferably 1.00% or less.

[0027] Mn: 0.05 to 15.00% Mn is used as a deoxidizing element during the refining process. It also functions as an austenite phase stabilizer together with Ni, and is less expensive and more cost-stable than Ni. Therefore, adding a certain amount of Mn as an austenite stabilizer can reduce alloy costs and stabilize manufacturing costs. Therefore, the lower limit of the Mn content is set to 0.05%. From the viewpoint of manufacturing costs, the Mn content is preferably 0.50% or more. On the other hand, excessive Mn content may excessively increase the austenite phase, preventing the formation of a dual-phase structure. It may also weaken the passive film and affect corrosion resistance. From these viewpoints, the upper limit of the Mn content is set to 15.00%. From the viewpoint of corrosion resistance, the Mn content is preferably 10.00% or less, more preferably 9.00% or less.

[0028] P:0.0500% or less P is an element that may deteriorate hot workability, toughness, and weldability. For this reason, the lower the P content, the better, and it is set to 0.0500% or less. In particular, from the viewpoint of hot workability, the P content is preferably set to 0.0400% or less, and more preferably 0.0300% or less. Although there is no lower limit, since an excessive reduction leads to an increase in refining costs, the P content is preferably set to 0.0010% or more or 0.0030% or more. Furthermore, the P content is more preferably 0.0050% or more, and even more preferably 0.0100% or more.

[0029] S: 0.0050% or less S is an unavoidable impurity element contained in steel, and reduces hot workability and corrosion resistance. For this reason, the lower the S content, the better, and it is set to 0.0050% or less. In particular, from the viewpoint of hot workability, the S content is preferably set to 0.0030% or less, more preferably 0.0020% or less, and even more preferably 0.0010% or less. Although there is no lower limit, excessive reduction leads to increased refining costs, so the S content is preferably set to 0.0001% or more. Furthermore, from the viewpoint of refining costs, the S content is more preferably 0.0002% or more, and even more preferably 0.0003% or more.

[0030] Cr: 19.0~26.0% Cr is an element that improves oxidation resistance and corrosion resistance by forming a passive film. Furthermore, Cr is an element that stabilizes the ferrite phase in the structure of duplex stainless steel sections. In order to ensure sufficient corrosion resistance for duplex stainless steel sections, the lower limit of the Cr content is set to 19.0%. From the viewpoint of ensuring corrosion resistance, the Cr content is more preferably 19.5% or more. On the other hand, excessive addition of Cr leads to an excessive increase in the ferrite phase, promotes the formation of the embrittlement phase, and increases the alloy cost, so the upper limit of the Cr content is set to 26.0%. From the viewpoint of reducing alloy cost, the Cr content is more preferably 25.5% or less, and even more preferably 25.0% or less.

[0031] Ni: 1.0 to 6.0% Ni not only improves corrosion resistance, but also functions as an austenite phase stabilizing element together with Mn and the like. From the viewpoint of corrosion resistance, the lower limit of the Ni content is set to 1.0%. On the other hand, Ni is expensive and its market price is unstable, so excessive addition leads to increased alloy costs and instability. Therefore, the upper limit of the Ni content is set to 6.0%. From the viewpoint of alloy cost reduction and stabilization, the Ni content is more preferably 5.5% or less, and even more preferably 5.0% or less.

[0032] Mo: 1.5% or less Mo is an element that contributes to improving corrosion resistance, but excessive content not only significantly increases alloy costs but also may cause a decrease in manufacturability due to deterioration of hot workability. Therefore, the upper limit of the Mo content is set to 1.5%. From the viewpoint of reducing alloy costs, the Mo content is more preferably 1.3% or less. Although there is no lower limit, from the viewpoint of ensuring corrosion resistance, the Mo content is preferably 0.05% or more, more preferably 0.07% or more.

[0033] Cu:2.0% or less Cu is an element that additionally enhances corrosion resistance to acids and improves toughness, but excessive content can cause a decrease in hot workability and the precipitation of the εCu phase, which causes embrittlement. Therefore, the upper limit of the Cu content is set to 2.0%. Although there is no lower limit, from the viewpoint of ensuring corrosion resistance to acids and toughness, the Cu content is preferably 0.05% or more, and more preferably 0.07% or more.

[0034] N: 0.05 to 0.25% N contributes to improving corrosion resistance and functions as an austenite phase stabilizing element together with Ni and Mn. For this reason, the lower limit of the N content is set to 0.05%. From the viewpoint of corrosion resistance and austenite phase stabilization, the N content is preferably 0.06% or more, more preferably 0.07% or more. On the other hand, if excessive N is added during the refining process, bubbles may be generated due to the generation of N2 gas. For this reason, the upper limit of the N content is set to 0.25% or less. From the viewpoint of manufacturing stability, the N content is preferably 0.24% or less, more preferably 0.23% or less.

[0035] Al: 0.050% or less Al is used as a deoxidizing element during the refining process. However, excessive Al content leads to a decrease in workability, so the upper limit of the Al content is set to 0.050%. From the viewpoint of workability, the Al content is preferably 0.040% or less, more preferably 0.030% or less. Although there is no lower limit, from the viewpoint of ensuring the deoxidizing effect, the Al content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0036] In addition to the basic composition described above, the chemical composition of the duplex stainless steel section according to one embodiment of the present invention may contain one or more elements selected from the following elements as needed.

[0037] Ti: 0.05% or less Ti not only contributes to improving strength as a solid solution strengthening element, but also contributes to improving strength by forming carbonitrides. However, excessive addition not only leads to a decrease in manufacturability due to a deterioration in hot workability, but also causes a decrease in the amount of solute N. Therefore, the upper limit of the Ti content is set to 0.05% or less. From the viewpoint of ensuring the amount of solute N, the Ti content is preferably 0.045% or less, more preferably 0.04% or less. Although there is no lower limit, from the viewpoint of ensuring strength, the Ti content is preferably 0.005% or more, more preferably 0.01% or more.

[0038] Nb: 0.15% or less Like Ti, Nb contributes to improving strength as a solid solution strengthening element or a carbonitride forming element. However, excessive addition not only leads to a decrease in manufacturability due to a deterioration in hot workability, but also causes a decrease in the amount of solute N. Therefore, the upper limit of the Nb content is set to 0.15% or less. From the viewpoint of ensuring the amount of solute N, the Nb content is preferably 0.14% or less, more preferably 0.13% or less. Although there is no lower limit, from the viewpoint of ensuring strength, the Nb content is preferably 0.01% or more, more preferably 0.02% or more.

[0039] V: 0.5% or less Like Ti and Nb, V contributes to improving strength as a solid solution strengthening element or a carbonitride forming element. However, excessive addition not only leads to a decrease in manufacturability due to a deterioration in hot workability, but also causes a decrease in the amount of solute N. Therefore, the upper limit of the V content is set to 0.5% or less. From the viewpoint of ensuring the amount of solute N, the V content is preferably 0.45% or less, more preferably 0.4% or less. Although there is no lower limit, from the viewpoint of ensuring strength, the V content is preferably 0.03% or more, more preferably 0.05% or more.

[0040] W: 1.0% or less Like Mo, W contributes to improving the corrosion resistance of stainless steel and also improves its strength. However, excessive addition of W reduces workability, so the upper limit of the W content is set to 1.0%. From the viewpoint of workability, the W content is preferably 0.9% or less, more preferably 0.8% or less. Although there is no lower limit, from the viewpoint of ensuring strength, the W content is preferably 0.05% or more, more preferably 0.10% or more.

[0041] Co: 1.0% or less Co is an element effective in improving corrosion resistance and toughness, and also functions as a stabilizer of the austenite phase. However, excessive addition leads to a significant increase in alloy cost, so the upper limit of the Co content is set to 1.0%. From the viewpoint of alloy cost, the Co content is preferably 0.9% or less, more preferably 0.8% or less. Although there is no lower limit, from the viewpoint of corrosion resistance, the Co content is preferably 0.01% or more, more preferably 0.03% or more.

[0042] B: 0.0050% or less B has the effect of improving hot workability and manufacturability. However, excessive addition of B leads to a decrease in the amount of solute N due to precipitation of nitrides of B and a decrease in toughness, so the upper limit of the B content is set to 0.0050%. From the viewpoint of ensuring the amount of solute N and toughness, the B content is preferably 0.0045% or less, more preferably 0.0040% or less. Although no lower limit is set, from the viewpoint of ensuring manufacturability, the B content is preferably 0.0001% or more, more preferably 0.0005% or more.

[0043] Ca:0.0050% or less Mg: 0.0050% or less Ca and Mg have the effect of improving hot workability and manufacturability. Therefore, at least one of Ca and Mg may be contained. However, excessive addition may adversely reduce hot workability and toughness, so the upper limit for each content is set to 0.0050%. The contents of both Ca and Mg are preferably 0.0040% or less, more preferably 0.0035% or less. Although there is no lower limit, from the viewpoint of ensuring manufacturability, the contents of Ca and Mg are preferably 0.0001% or more, more preferably 0.0003% or more.

[0044] In the above chemical composition, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial steel production due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0045] 2.Metal structure A duplex stainless steel section according to one embodiment of the present invention has a metallographic structure that includes, in area percent, 35.0 to 70.0% of a ferrite phase, with the remainder being an austenite phase and a hard phase. If the area percent of the ferrite phase exceeds 70.0%, the area percent of the austenite phase will be less than 30.0%, and sufficient strength will not be obtained. On the other hand, in order to make the area percent of the ferrite phase less than 35.0%, the area percent of the austenite phase will be more than 65.0%, which may cause various problems such as those described below.

[0046] First, it is necessary to increase the content of Ni, an expensive austenite stabilizing element that is generally classified as a rare metal, which makes it expensive. Also, when considering an alloy-saving, inexpensive duplex stainless steel, the N and Mn contents become too high, resulting in excessive strength. In addition, coarse compounds are formed during hot working. In particular, when the Mn content is high, it reduces corrosion resistance and oxidation resistance. For these reasons, Within the region (1 / 8)t~(3 / 8)t from the surface The area ratio of the ferrite phase in the steel sheet is set to 35.0 to 70.0%.

[0047] The phases other than the ferrite phase are the austenite phase and hard phases. The hard phases include the martensite phase, which is inevitably formed during processing, as well as precipitates such as carbides, nitrides, carbonitrides, σ phases, and Laves phases. While it is desirable to keep the area ratio of the hard phase as low as possible, a martensite phase of 7.0% or less and a total precipitate ratio of 3.0% or less are acceptable.

[0048] The area fraction of the ferrite phase is measured using an electron backscatter diffraction (EBSD) analyzer. Specifically, the observation surface is a plane parallel to the rolling direction and perpendicular to the flat surface of the steel section. The thickness of the flat surface of the steel section being observed is defined as t. Within a region between (1 / 8)t and (3 / 8)t from the surface, two 100 μm × 100 μm regions are measured, each with 1 μm intervals, after electrolytic or chemical polishing of the surface. The body-centered cubic phase is identified from the measurement results at the two locations, and the area fraction is calculated as the ferrite area fraction.

[0049] This measurement also identifies the face-centered cubic austenite phase, the σ phase, and the Laves phase, each of which has its own unique crystal structure, and calculates their area fractions. The martensite phase, like the ferrite phase, has a body-centered cubic structure, but because high-density dislocations are introduced, it is observed as an unmeasurable region along with the grain boundaries in EBSD measurements. Therefore, the area fraction of the unmeasurable region observed in EBSD, excluding that due to the grain boundaries, can be considered the area fraction of the martensite phase. The results were analyzed using TSL's OIM Analysis ver. 7.3.0.

[0050] 3. Chemical composition of ferrite and austenite phases In a duplex stainless steel section according to one embodiment of the present invention, the A value, which is defined by the following formula (i) and relates to the chemical composition of the ferrite phase, is 30.0 or more, and the B value, which is defined by the following formula (ii) and relates to the chemical composition of the austenite phase, is 0.0 or less. A=20[C]α+20[N]α+[Cr]α+[Mn]α+0.5[Al]α+1.5[Mo]α+2[Ni]α+2.4[Cu]α ···(i) B=500.0-400[C]γ-400[N]γ-9[Si]γ-8[Mn]γ-14[Cr]γ-35[Ni]γ-35[Cu]γ-20[Mo]γ...(ii) In the above formula (i), [C]α, [N]α, [Cr]α, [Mn]α, [Al]α, [Mo]α, [Ni]α, and [Cu]α represent the contents (mass%) of C, N, Cr, Mn, Al, Mo, Ni, and Cu in the ferrite phase, and in the above formula (ii), [C]γ, [N]γ, [Si]γ, [Mn]γ, [Cr]γ, [Ni]γ, [Cu]γ, and [Mo]γ represent the contents (mass%) of C, N, Si, Mn, Cr, Ni, Cu, and Mo in the austenite phase, with 0 substituted if not contained.

[0051] It is not entirely clear why the A value, which relates to the chemical composition of the ferrite phase, and the B value, which relates to the chemical composition of the austenite phase, are related to the occurrence of uneven polishing of alloy-saving duplex stainless steel sections. However, the following factors are presumed to be involved:

[0052] The ferrite phase and the austenite phase generally have different strength and work hardening properties due to differences in their crystal structure, chemical composition, and whether or not they undergo stress-induced transformation. In the case of the internal structure of stainless steel, the austenite phase is generally said to have higher strength and work hardening properties than the ferrite phase, although this depends on the chemical composition of each phase.

[0053] Furthermore, if the austenite phase has low phase stability, determined by its chemical composition, it is more likely to transform into the hard martensite phase through stress-induced transformation. Differences in chemical composition affect the strength of the ferrite phase and the behavior of the stress-induced transformation of the austenite phase, which is thought to affect the surface condition when polished. In other words, if the strength is low, polishing is more likely to result in thickness loss, whereas if the hardness is high or stress-induced martensite transformation occurs, polishing is less likely to result in thickness loss.

[0054] In other words, if there is a large difference in strength within the internal structure, there will be differences between areas that are prone to thinning and areas that are not, resulting in a surface condition with extremely fine peaks and valleys, which can be seen as fine unevenness when inspected visually.From this, it can be assumed that the smaller the difference in strength between the ferrite phase and the austenite phase (strictly speaking, this also includes the martensite phase induced by processing from the austenite phase), the less noticeable the polishing unevenness will be.

[0055] The A value, which is determined by the chemical composition of the ferrite phase, is thought to reflect the degree of solid solution strengthening of the ferrite phase, although it is unclear whether there is a strict correlation. On the other hand, the B value, which is determined by the chemical composition of the austenite phase, is related to the Md 30This value is similar to that of the hardening rate of the austenite phase due to the formation of the deformation-induced martensite phase, and although the correlation is also unclear, it is presumed to roughly correspond to the amount of hardening of the portion where the austenite phase is present due to the formation of the deformation-induced martensite phase.

[0056] In other words, if the A value is 30.0 or more and the B value is 0.0 or less, the difference in strength between the ferrite phase and the austenite phase is below the threshold at which polishing unevenness can occur, and as a result, it is thought that polishing unevenness will not occur visually.The cause of this polishing unevenness itself is also speculation, and at least as far as measurements with a stylus-type roughness meter are concerned, no clear difference can be seen between pieces with and without polishing unevenness.

[0057] In the present invention, the chemical compositions of the ferrite and austenite phases are measured using an electron probe microanalyzer (EPMA). Since there is slight variation in the chemical composition of each phase, the observation plane is a plane parallel to the rolling direction and perpendicular to the flat surface of the shaped steel, or a plane perpendicular to the rolling direction and perpendicular to the flat surface of the shaped steel. When the thickness of the parallel portion of the shaped steel is t, point analysis is performed on at least five ferrite grains and at least five austenite grains within a depth range of (1 / 8)t to (3 / 8)t from the surface of the parallel portion, and the chemical composition of each element is calculated from the average value.

[0058] The measurement conditions for EPMA may be adjusted as appropriate as long as analytical precision can be guaranteed, and for example, the conditions are accelerating voltage: 15 kV, beam diameter: 1 μm, and irradiation time: 20 s.

[0059] 4. Manufacturing method A method for manufacturing a duplex stainless steel section according to one embodiment of the present invention will be described. The duplex stainless steel section according to this embodiment can achieve the effects described above regardless of the manufacturing method. However, the duplex stainless steel section according to this embodiment can be stably manufactured by the following method.

[0060] In a method for producing a duplex stainless steel section according to one embodiment of the present invention, a steel material having the above-described chemical composition is subjected to (a) a heating step, (b) a hot rolling step, and (c) a final heat treatment step, in that order. Each step will be described in detail below. The shape of the steel material is not particularly limited, and may be any of a billet, bloom, slab, etc.

[0061] (a) Heating process In the heating step, the steel material is heated in a temperature range of 1100 to 1350°C before being subjected to hot rolling. The heating temperature in the heating step is 1100°C or higher to reduce element segregation of the steel material in the ferrite single phase region. If the heating temperature is lower than 1100°C, it may be difficult to achieve an A value of 30 or higher. In this case, it is preferable that the heating also serves as soaking. From the viewpoint of reducing segregation, the heating temperature is preferably 1270°C or higher.

[0062] On the other hand, if the heating temperature exceeds 1350°C, abnormal oxidation occurs on the surface of the steel material, which not only causes scabs during rolling but also increases the amount of energy consumed for heating, leading to an increase in manufacturing costs. For this reason, the heating temperature is set to 1350°C or less. From the viewpoint of manufacturing costs, the heating temperature is preferably 1330°C or less.

[0063] The heating time is not particularly limited, but is preferably 0.5 hours or more, more preferably 1 hour or more, and even more preferably 1.5 hours or more, from the viewpoint of reducing element segregation in the steel material. On the other hand, from the viewpoint of suppressing abnormal oxidation of the surface and improving production efficiency, the heating time is preferably 24 hours or less, and more preferably 20 hours or less, from the viewpoint of production efficiency.

[0064] (b) Hot rolling process In the hot rolling process, the steel material heated in the heating process is hot rolled under conditions such that the final pass temperature of rolling is 500°C or higher and lower than 700°C. Note that the "final pass temperature of rolling" refers to the surface temperature of the material to be rolled at the entry side of the final pass of hot rolling. In the manufacturing process of this embodiment, the effect of heat removal due to contact with the rolling rolls is greater than the effect of processing heat during hot rolling, so the exit temperature of the final pass is lower than the entry temperature.

[0065] In this embodiment, in order to control the A and B values, it is preferable to keep the temperature of the rolled material as low as possible in the final pass of hot rolling. If the temperature in the final pass is less than 700°C, the effect of controlling the A and B values ​​can be expected. The final pass temperature of rolling is preferably 670°C or less. On the other hand, if the final pass temperature is below 500°C, the rolling reaction force of the rolled material increases, resulting in an excessive increase in equipment load. Therefore, the temperature of the rolled material in the final pass is set to 500°C or higher. From the viewpoint of stability in production, it is preferably 510°C or higher.

[0066] The factors that affect the A and B values ​​of the final hot rolling pass temperature are still unknown and remain only speculation, but it is thought that they are influenced by the following mechanism: In other words, the high-density dislocation structure accumulated by the final pass in the low-temperature region promotes atomic diffusion due to dislocation core diffusion during the final heat treatment described below, or grain boundary diffusion due to the formation of high-density recrystallized grain boundaries, which promotes changes in the chemical composition of the ferrite and austenite phases, leading to an increase in the A value and a decrease in the B value.

[0067] During hot rolling of stainless steel, dislocation structures accumulate within the structure with each rolling pass. However, the processed structures accumulated during the early to middle stages of rolling are reduced over time by recovery and recrystallization. Therefore, the introduction of dislocation structures during the later stages of rolling, particularly the final pass, is most effective. The lower the temperature of the rolled material during the final rolling pass, the greater the amount of dislocations introduced. At the same time, the reduction in dislocation density due to recovery, which is particularly noticeable in the ferrite phase, can be suppressed.

[0068] There are no particular limitations on the shape of the rolling rolls used in the hot rolling step, and any suitable roll may be used to obtain the desired shape.

[0069] (c) Final heat treatment process In the final heat treatment step, the final heat treatment is carried out at a temperature of at least T1 (°C) calculated by the following formula (iii) in the temperature range of 925 to 1170°C for 30 seconds or more and less than 30 minutes. T1=548+24.3[Cr]-52.5[Ni] ···(iii) In the above formula (iii), [Cr] and [Ni] represent the Cr and Ni contents (mass%) in the steel material.

[0070] The final heat treatment process after hot rolling not only ensures the workability of the section steel, but also affects the internal structure of the duplex stainless steel section, particularly the chemical composition of the ferrite and austenite phases. The inventors have clarified that heating to T1 (°C) or higher, calculated by the above formula (iii), in the final heat treatment process, changes the chemical composition of the ferrite and austenite phases, increasing the A value and decreasing the B value.

[0071] Here, when T1 is less than 925°C, even if the final heat treatment temperature is T1 (°C) or higher, the B value is unlikely to decrease unless it is 925°C or higher. Therefore, the final heat treatment temperature is set to T1 (°C) or higher and 925°C or higher. On the other hand, from the viewpoint of ensuring workability, there is no effect in raising the final heat treatment temperature to a temperature range above 1170°C, and instead leads to an increase in the energy cost consumed for heat treatment, so the final heat treatment temperature is set to 1170°C or lower.

[0072] In order to reduce strain after rolling and ensure workability, as well as to sufficiently diffuse atoms and control the A and B values ​​within a predetermined range, the final heat treatment must be carried out for a certain period of time or longer. Therefore, the final heat treatment time is set to 30 seconds or longer. From the viewpoint of ensuring workability, the final heat treatment time is preferably 60 seconds or longer, more preferably 120 seconds or longer.

[0073] On the other hand, if the final heat treatment is performed for an excessively long time, not only will the energy costs associated with the heat treatment increase, but the excess scale produced will also result in a decrease in yield and deterioration of quality. Therefore, the final heat treatment time is set to 30 minutes or less. From the viewpoint of energy costs, the final heat treatment time is preferably 25 minutes or less.

[0074] The duplex stainless steel section obtained through the above steps may be further subjected to pickling treatment to remove scales, and may also be further subjected to surface processing treatments such as shaping or polishing to remove scratches or burrs.

[0075] Furthermore, the duplex stainless steel section according to one embodiment of the present invention can be applied to various shapes of steel sections, such as H-shaped steel, I-shaped steel, angle steel, flat steel, channel steel, Z-shaped steel, and round steel.

[0076] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples. [Example]

[0077] A billet of duplex stainless steel having the chemical composition shown in Table 1 was melted and heated under various conditions. Test pieces in the shape of equal-leg angle iron with a cross section of 50 mm in length and 5 mm in thickness, as shown in Figure 1, were then produced by hot rolling. These were then water-cooled and heat-treated under various conditions. Forced air cooling was used for cooling after heat treatment. The conditions for producing the duplex stainless steel section specimens are shown in Table 2.

[0078] [Table 1]

[0079] [Table 2]

[0080] From the obtained test piece, a cross-sectional observation specimen was cut out parallel to the rolling direction and perpendicular to the peak surface from position a in Figure 1. Then, within the region (1 / 8)t to (3 / 8)t from the peak surface of the cut-out cross-sectional observation specimen, the cross section was electrolytically polished and two 100 μm × 100 μm regions were targeted, and EBSD measurements were performed at 1 μm intervals for each. The body-centered cubic structure phase was identified from the measurement results at the two locations, and the area fraction was calculated, which was taken as the area fraction of ferrite.

[0081] This measurement also allowed for the phase identification and calculation of the area fraction of the face-centered cubic austenite phase, the σ phase, and the Laves phase, each of which has its own unique crystal structure. Furthermore, the area fraction of the martensite phase was calculated by excluding the area of ​​the unmeasurable areas observed in EBSD due to grain boundaries. The results were analyzed using TSL's OIM Analysis ver. 7.3.0.

[0082] Next, in the same cross section as above, five ferrite grains and five austenite grains were analyzed using an EPMA in a region 0.63 to 1.87 mm from the crest surface. The analysis was performed under the following conditions: acceleration voltage: 15 kV, beam diameter: ~1 μm, and irradiation time: 20 s. The average values ​​obtained were used as the chemical composition of each phase. The A value of the ferrite phase and the B value of the austenite phase were then calculated from these values.

[0083] Next, a polishing test was carried out using each test piece to evaluate the degree of polishing unevenness. In the polishing test, #120 polishing was carried out, and then the presence or absence of polishing unevenness was confirmed visually.

[0084] The results are also shown in Table 2. Regarding the presence or absence of polishing unevenness, cases where it could not be confirmed are indicated by ○, and cases where it was confirmed are indicated by ×.

[0085] As is clear from the results shown in Table 2, it was confirmed that duplex stainless steel sections that satisfy the specifications of the present invention do not produce uneven polishing when polished with #120, and can be suitably used as architectural structural materials where design is required. [Industrial Applicability]

[0086] According to the present invention, it is possible to industrially and stably obtain alloy-saving duplex stainless steel sections that can suppress the occurrence of polishing unevenness when polished.

Claims

1. The chemical composition, in mass%, is C: 0.040% or less, Si: 0.01-1.50%, Mn: 0.05-15.00%, P: 0.0500% or less, S: 0.0050% or less, Cr: 19.0-26.0%, Ni: 1.0 to 6.0%, Mo: 1.5% or less, Cu: 2.0% or less, N: 0.05-0.25%, Al: 0.050% or less, The balance is Fe and impurities. The metal structure contains, in area percentage, 35.0 to 70.0% of a ferrite phase, and the remainder is an austenite phase and a hard phase, The A value defined by the following formula (i), which relates to the chemical composition of the ferrite phase, is 30.0 or more, The B value defined by the following formula (ii), which is related to the chemical composition of the austenite phase, is 0.0 or less: Duplex stainless steel sections. A=20[C]α+20[N]α+[Cr]α+[Mn]α+0.5[Al]α+1.5[Mo]α+2[Ni]α+2.4[Cu]α...(i) B=500.0-400[C]γ-400[N]γ-9[Si]γ-8[Mn]γ-14[Cr]γ-35[Ni]γ-35[Cu]γ-20[Mo]γ...(ii) In the above formula (i), [C]α, [N]α, [Cr]α, [Mn]α, [Al]α, [Mo]α, [Ni]α, and [Cu]α represent the contents (mass%) of C, N, Cr, Mn, Al, Mo, Ni, and Cu in the ferrite phase, and in the above formula (ii), [C]γ, [N]γ, [Si]γ, [Mn]γ, [Cr]γ, [Ni]γ, [Cu]γ, and [Mo]γ represent the contents (mass%) of C, N, Si, Mn, Cr, Ni, Cu, and Mo in the austenite phase, with 0 substituted if none of these elements is contained.

2. The chemical composition is, in mass %, replacing a part of Fe, Ti: 0.05% or less, Nb: 0.15% or less, V: 0.5% or less, W: 1.0% or less, Co: 1.0% or less, B: 0.0050% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, The duplex stainless steel section according to claim 1.

3. A method for producing the duplex stainless steel section according to claim 1 or claim 2, For a steel material having the chemical composition according to claim 1 or 2, (a) heating at a temperature in the range of 1100 to 1350°C; (b) hot rolling under conditions in which the final pass temperature of rolling is 500°C or higher and lower than 700°C; (c) performing a final heat treatment at a temperature of 925 to 1170°C, which is equal to or higher than T1 (°C) calculated by the following formula (iii), for 30 seconds or more and less than 30 minutes; Manufacturing method for duplex stainless steel sections. T1=548+24.3[Cr]-52.5[Ni]...(iii) In the above formula (iii), [Cr] and [Ni] represent the Cr and Ni contents (mass%) in the steel material.

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