JP7737007B2 - Duplex stainless steel section and its manufacturing method - Google Patents

Description

The present invention relates to duplex stainless steel sections and their manufacturing methods.

Due to its excellent corrosion resistance, stainless steel is used in many applications, including building materials, automobiles, and home appliances. Stainless steel is divided into austenitic stainless steel, ferritic stainless steel, and duplex stainless steel depending on its internal structure. Among these, duplex stainless steel contains both austenitic and ferritic phases in its internal structure, and is used as a building or structural material due to its high strength.

Furthermore, duplex stainless steel generally has a lower Ni content than austenitic stainless steels such as SUS304, making it less expensive to alloy. It also has the advantage of being less susceptible to Ni price fluctuations.

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

In recent years, attention has been focused on alloy-saving duplex stainless steel, which contains less of expensive elements such as Ni and Mo. SUS329J1 and SUS329J4L, the main types of conventional duplex stainless steel, 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.

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

To utilize these duplex stainless steels as building or structural materials, stainless steel in the form of section steel is required, not just plate. Although the types of hot-formed stainless steel section steel are listed in JIS G 4317, the only duplex stainless steels listed are the relatively expensive SUS329J1 and SUS329J4L mentioned above. For this reason, several inventions relating to alloy-saving duplex stainless steel section steels have been disclosed.

Patent Document 3 discloses a steel section made of alloy-saving duplex stainless steel. Furthermore, Patent Document 4 discloses a welded channel steel made of alloy-saving duplex stainless steel.

U.S. Pat. No. 4,828,630 Japanese Unexamined Patent Publication No. 61-56267 Japanese Patent Application Laid-Open No. 2018-159119 Japanese Patent Application Laid-Open No. 2020-100859

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 as 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 more. As a structural material, stainless steel sections can not only maintain the shape of a building, but can also be given a high level of aesthetic appeal through surface treatments such as polishing, and its high corrosion resistance means that the same surface condition can be maintained for long periods of time without the need for painting or surface treatments. For these reasons, they are well suited to such applications.

When manufactured alloy-saving duplex stainless steel sections are used as building materials, some are surface-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 known until now. Therefore, when polishing unevenness occurs, the product may become unusable from an aesthetic standpoint, or it may be necessary to carefully re-polish it, which can be time-consuming and costly, until the polishing unevenness disappears. For these reasons, it is necessary to prevent uneven polishing during the polishing process for alloy-saving duplex stainless steel sections.

The present invention was made to solve the above 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.

As a result of extensive research into solving the above-mentioned problems, the inventors have come to the following conclusions.

(a) Duplex stainless steel contains a two-phase structure consisting 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.

(b) A detailed investigation of the duplex stainless steel sections on which polishing irregularities occurred revealed that the chemical compositions of the ferrite and austenite phases in all cases did not meet 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 meet the specified conditions.

(c) In order to adjust the chemical composition of the ferrite and austenite phases in duplex stainless steel sections so that they meet specified conditions, it is effective to appropriately adjust the heating temperature before hot rolling the billet, the final pass temperature in hot rolling, and the final heat treatment temperature after hot rolling.

The present invention was made based on the above findings and is summarized as follows: The duplex stainless steel section and its manufacturing method.

(1) Chemical composition, in mass%,
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 (1) above.

(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 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.

The present invention makes it possible to industrially and reliably produce alloy-saving duplex stainless steel sections that can suppress the occurrence of polishing unevenness when polished.

FIG. 2 is a diagram illustrating the shape of a test piece used in the examples.

Each requirement of the present invention is explained in detail below.

1. Chemical Composition of Duplex Stainless Steel Section The chemical composition of a duplex stainless steel section according to one embodiment of the present invention will be described below. The reasons for limiting each element are as follows. In the following description, "%" in the content means "% by mass."

C: 0.040% or less C combines with elements for ensuring corrosion resistance, such as Cr, to form carbides, which may adversely affect corrosion resistance, such as locally inhibiting the effects of elements for ensuring corrosion resistance, resulting in so-called sensitization. Therefore, the upper limit of the C content is set to 0.040%. From the viewpoint of further suppressing sensitization, the C content is preferably 0.035% or less. While no lower limit is specified, excessive reduction leads to increased refining costs, so the C content is preferably 0.001% or more. From the viewpoint of refining costs, the C content is more preferably 0.005% or more.

Si: 0.01~1.50%
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.

Mn: 0.05-15.00%
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, by including a certain amount of Mn as an austenite stabilizer, alloy costs can be reduced and manufacturing costs can be stabilized. 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. Furthermore, it may 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.

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, excessive reduction leads to an increase in refining costs, so 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.

S: 0.0050% or less S is an inevitable 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.

Cr: 19.0-26.0%
Cr is an element that improves oxidation resistance and corrosion resistance by forming a passivation 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 alloy costs, so the upper limit of the Cr content is set to 26.0%. From the viewpoint of reducing alloy costs, the Cr content is more preferably 25.5% or less, and even more preferably 25.0% or less.

Ni: 1.0-6.0%
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, since Ni is expensive and its market price is unstable, 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.

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 no lower limit is set, from the viewpoint of ensuring corrosion resistance, the Mo content is preferably 0.05% or more, more preferably 0.07% or more.

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

N: 0.05-0.25%
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.

Al: 0.050% or less Al is used as a deoxidizing element during the refining process. On the other hand, excessive 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 no lower limit is set, 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.

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

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.

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 no lower limit is set, from the viewpoint of ensuring strength, the Nb content is preferably 0.01% or more, more preferably 0.02% or more.

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 no lower limit is set, from the viewpoint of ensuring strength, the V content is preferably 0.03% or more, more preferably 0.05% or more.

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 causes a decrease in workability, so the upper limit of the W content is set to 1.0% or less. From the viewpoint of workability, the W content is preferably 0.9% or less, more preferably 0.8% or less. Although no lower limit is set, from the viewpoint of ensuring strength, the W content is preferably 0.05% or more, more preferably 0.10% or more.

Co: 1.0% or less Co is an element effective in improving corrosion resistance and toughness, and also functions as a stabilizing element for the austenite phase. However, excessive addition of Co leads to a significant increase in alloy cost, so the upper limit of the Co content is set to 1.0% or less. 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.

B: 0.0050% or less B has the effect of improving hot workability and enhancing manufacturability. However, excessive addition 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% or less. 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.

Ca: 0.0050% or less Mg: 0.0050% or less Ca and Mg have the effect of improving hot workability and enhancing 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 of each content is set to 0.0050%. The contents of Ca and Mg are both preferably 0.0040% or less, more preferably 0.0035% or less. Although no lower limit is set, from the viewpoint of ensuring manufacturability, the contents of Ca and Mg are preferably 0.0001% or more, more preferably 0.0003% or more.

In the above chemical composition, the balance is Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of steel 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.

2. Metallographic Structure A duplex stainless steel section according to one embodiment of the present invention has a metallographic structure that includes, in area percentage, 35.0 to 70.0% ferrite phase, with the remainder being austenite phase and hard phase. If the area percentage of the ferrite phase exceeds 70.0%, the area percentage 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 percentage of the ferrite phase less than 35.0%, the area percentage of the austenite phase will be more than 65.0%, which may cause various problems such as those described below.

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

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

The area fraction of the ferrite phase is measured using an electron backscatter diffraction (EBSD) instrument. Specifically, the observation surface is a plane parallel to the rolling direction and perpendicular to the flat surface of the section steel. If the thickness of the flat surface of the section steel being observed is t, then within a region between (1/8)t and (3/8)t from the surface, measurements are taken at two 100 μm x 100 μm regions where the surface has been electrolytically or chemically polished, with measurements taken in 1 μm increments at each location. The body-centered cubic phase is then identified from the measurement results at the two locations, and its area fraction is calculated, which is taken as the area fraction of ferrite.

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 immeasurable region along with the grain boundaries in EBSD measurements. For this reason, the area fraction of the observed area of the immeasurable region in EBSD, excluding that due to the grain boundaries, can be considered the area fraction of the martensite phase. Results were analyzed using TSL's OIM Analysis ver. 7.3.0.

3. Chemical Composition of Ferrite Phase and Austenite Phase 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 none of these elements is contained.

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 in alloy-saving duplex stainless steel sections. However, the following factors are presumed to be involved:

The ferrite and austenite phases generally have different strength and work-hardening properties due to differences in their crystal structure, chemical composition, and the presence or absence of stress-induced transformation. In the case of the internal structure of stainless steel, although this depends on the chemical composition of each phase, it is generally said that the austenite phase has higher strength and work-hardening properties than the ferrite phase.

Furthermore, if the phase stability of the austenite phase, which is determined by the chemical composition, is low, it is more likely to become the hard martensite phase through stress-induced transformation. If differences in chemical composition affect the strength of the ferrite phase and the behavior of the stress-induced transformation of the austenite phase, it is thought that this will change the surface condition when polished. In other words, if the strength is low, polishing is more likely to result in thickness reduction, while if the hardness is high or stress-induced martensite transformation occurs, polishing is less likely to result in thickness reduction.

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. For this reason, it is thought 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.

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 similar to the _Md30 value, which indicates the likelihood of stress-induced martensitic transformation in austenitic stainless steels, and although the correlation is also unclear, it is presumed to roughly correspond to the amount of hardening of the austenite phase due to the formation of stress-induced martensitic phase.

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, polishing unevenness is thought to no longer be visible to the naked eye. The cause of this polishing unevenness itself is also speculation, and at least as far as measurements using a stylus roughness meter are concerned, no clear difference can be seen between pieces with and without polishing unevenness.

In this invention, the chemical composition of the ferrite and austenite phases is measured using an electron probe microanalyzer (EPMA). Since there is slight variation in the chemical composition of each phase, the observation plane is either 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 determined by averaging the results.

EPMA measurement conditions can be adjusted as appropriate as long as analytical accuracy can be guaranteed. For example, conditions include an acceleration voltage of 15 kV, a beam diameter of approximately 1 μm, and an irradiation time of 20 s.

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 its effects as long as it has the above-mentioned characteristics, regardless of the manufacturing method. However, the duplex stainless steel section according to this embodiment can be stably manufactured by the following method.

In a method for manufacturing duplex stainless steel sections according to one embodiment of the present invention, a steel material having the above-described chemical composition is subjected to (a) a heating process, (b) a hot rolling process, and (c) a final heat treatment process, in that order. Each process is described in detail below. The shape of the steel material is not particularly limited and may be a billet, bloom, slab, or the like.

(a) Heating Step 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 in order 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.

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 scuff marks during rolling but also increases the amount of energy consumed for heating, leading to higher manufacturing costs. For this reason, the heating temperature is set to 1350°C or less. From the perspective of manufacturing costs, the heating temperature is preferably 1330°C or less.

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

(b) Hot Rolling Step In the hot rolling step, the steel material heated in the heating step is hot rolled under conditions such that the final rolling pass temperature is 500°C or higher and lower than 700°C. Note that the "final rolling pass temperature" 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.

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, it is expected that the A and B values can be effectively controlled. The final pass temperature of rolling is preferably 670°C or lower. 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 perspective of production stability, it is preferably 510°C or higher.

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 affect the following mechanism. In other words, the high-density dislocation structure accumulated by the final pass at a low temperature promotes atomic diffusion through dislocation core diffusion during the final heat treatment described below, or grain boundary diffusion due to the formation of high-density recrystallized grain boundaries, promoting 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.

During hot rolling of stainless steel, dislocation structures accumulate within the structure with each rolling pass, but the processed structures accumulated in the early to middle stages of rolling are reduced over time through recovery and recrystallization. Therefore, introducing dislocation structures in the later stages of rolling, particularly the final pass, is most effective. The lower the temperature of the rolled material in the final rolling pass, the greater the amount of dislocations introduced, while also suppressing the reduction in dislocation density due to recovery, which is particularly noticeable in the ferrite phase.

There are no particular restrictions on the shape of the rolling rolls used in the hot rolling process; any suitable roll can be used to obtain the desired shape.

(c) Final Heat Treatment Step 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.

The final heat treatment process after hot rolling not only ensures the workability of the steel section, 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 a temperature above T1 (°C) 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.

Here, if 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 perspective of ensuring workability, there is no effect in raising the final heat treatment temperature to a temperature range above 1170°C, and instead results in an increase in the energy cost consumed for heat treatment, so the final heat treatment temperature is set to 1170°C or lower.

In order to reduce distortion after rolling and ensure workability, as well as to sufficiently diffuse atoms and control the A and B values within the specified ranges, 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 perspective of ensuring workability, the final heat treatment time is preferably 60 seconds or longer, and more preferably 120 seconds or longer.

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 should be 30 minutes or less. From the perspective of energy costs, the final heat treatment time is preferably 25 minutes or less.

The duplex stainless steel section obtained through the above process may be further subjected to pickling to remove scale. It may also be further subjected to surface processing such as shaping or polishing to remove scratches or burrs.

In addition, 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.

The present invention will be explained in more detail below using examples. However, the present invention is not limited to these examples.

Duplex stainless steel billets with the chemical composition shown in Table 1 were melted and heated under various conditions. Test specimens were then hot-rolled to form equal-leg angle steel specimens with a cross section measuring 50 mm in length and 5 mm in thickness, as shown in Figure 1. These specimens 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 specimens are shown in Table 2.

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

These measurements also included phase identification and calculation of the area fractions 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 regions observed in the EBSD due to grain boundaries. The results were analyzed using OIM Analysis ver. 7.3.0 manufactured by TSL.

Next, in the same cross section as above, in an area 0.63 to 1.87 mm from the peak surface, point analysis was performed on five ferrite grains and five austenite grains using an EPMA. The analysis was performed under the following conditions: acceleration voltage: 15 kV, beam diameter: ~1 μm, 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.

Next, each test piece was used to conduct a polishing test to evaluate the degree of polishing unevenness. In the polishing test, #120 polishing was performed, and then the presence or absence of polishing unevenness was confirmed visually.

The results are shown in Table 2. Regarding the presence or absence of polishing unevenness, cases where it could not be confirmed are indicated with a circle, and cases where it was confirmed are indicated with an x.

As is clear from the results shown in Table 2, duplex stainless steel sections that satisfy the specifications of this invention do not produce uneven polishing when polished with #120 polishing, confirming that they can be suitably used as architectural structural materials where aesthetic appeal is required.

The present invention makes it possible to industrially and reliably produce alloy-saving duplex stainless steel sections that can suppress the occurrence of polishing unevenness when polished.

Claims (3)

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. 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. 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.