JP2023089963A - Duplex stainless steel and manufacturing method thereof - Google Patents

Abstract

To provide a highly-strong duplex stainless steel excellent in corrosion resistance and processability; and to provide a manufacturing method thereof.SOLUTION: A duplex stainless steel has a metallographic structure in which the area ratio of a ferrite phase at a thickness direction center position is 40% or more and less than 60%, and has a ferrite concentrated layer in which the area ratio of the ferrite phase is 60% or more, in an area from the surface up to at least 50 μm in the depth direction.SELECTED DRAWING: None

Description

The present invention relates to duplex stainless steels and methods of making the same.

Duplex stainless steel is used as a building or structural material because of its excellent corrosion resistance and particularly high strength. For example, "hot-rolled stainless steel plates and strips" (JIS G 4304: 2021), "cold-rolled stainless steel plates and strips" (JIS G4305: 2021), "hot-rolled stainless steel sections" (JIS G4317: 2021), "Cold-formed stainless steel sections" (JIS G4320: 2021), etc., and SUS329J1, SUS329J4L, etc., which are described as steel grades of duplex stainless steel.

These conventional duplex stainless steels contain a large amount of added elements and are relatively expensive. Therefore, in recent years, inexpensive duplex stainless steels with reduced amounts of added elements have been developed. Patent Documents 1 and 2 disclose inexpensive duplex stainless steels (lean-type duplex stainless steels) that are classified as rare metals, have a low Ni content, and utilize austenite-forming elements such as Mn and N. there is

However, duplex stainless steel consists of a mixed structure of different phases, austenite phase and ferrite phase. In addition, the lean duplex stainless steel utilizing N contains a large amount of N, which is a strong solid-solution strengthening element and easily forms a compound. Therefore, there is a problem that processing into various shapes is difficult. In particular, in hot working, the difference in properties between the austenite phase and the ferrite phase becomes apparent.

Against this background, Patent Literature 3 discloses a duplex stainless steel having high strength and excellent corrosion resistance and workability. Furthermore, Patent Document 4 discloses a duplex stainless steel having high strength and excellent fatigue resistance.

JP-A-61-56267 JP 2010-229459 A JP 2020-94266 A JP 2020-152941 A

By the way, hot working in the manufacturing process may cause rolling defects and deterioration of surface properties due to generation of edge cracks and the like, and cracks and deterioration of surface properties may also occur during cold forming. As a result of studies by the present inventors, it has been found that these problems are also caused by the difference in properties between the austenite phase and the ferrite phase.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a duplex stainless steel having high strength, excellent corrosion resistance and workability, and a method for producing the same.

The present inventors have made extensive studies to solve the above-described problems, and as a result, have obtained the following findings.

(a) As a result of melting several types of duplex stainless steel and heat-treating them under various conditions, when a chemical agent containing metal Cr powder and _SiO2 was applied to the surface of the base material, Cr was concentrated on the surface layer of the base material. I found that it becomes

(b) Corrosion resistance can be improved by concentrating Cr in the surface layer of the base material. In addition, when Cr, which is a ferrite-stabilizing alloying element, is concentrated in the surface layer of duplex stainless steel, a ferrite-enriched layer with a high ferrite phase fraction (hereinafter referred to as Also referred to as an “α-enriched layer”) is formed. By increasing the fraction of the soft ferrite phase in the surface layer portion, the difference in properties between the austenite phase and the ferrite phase is alleviated, thereby improving workability.

(c) In the ferrite phase, since the solid solubility limit of N is low, the N concentration in the surface layer decreases, and coarse nitrides that cause deterioration of workability and corrosion resistance according to changes in the structure and composition are formed. easier to form. However, in the duplex stainless steel according to the present invention, although a ferrite-enriched layer is formed in the surface layer, such coarse nitrides do not precipitate, and it is possible to suppress the formation of nitrides due to ferrite phase formation. Become.

(d) Due to these features, the duplex stainless steel having the α-enriched layer formed on the surface has excellent workability and corrosion resistance while maintaining high strength.

(e) The above α-enriched layer was formed at 1150 to 1300° C. for 5 hours in an atmosphere with an O ₂ concentration of 30% by volume or less in a state where a chemical containing metal Cr powder and SiO ₂ was applied to the surface of the base material. It can be formed by heating and holding as described above.

The present invention has been made based on the above knowledge, and the gist of the present invention is the following duplex stainless steel and method for producing the same.

(1) A duplex stainless steel having a metal structure in which the area ratio of the ferrite phase at the center position in the thickness direction is 40% or more and less than 60%,
Having a ferrite-enriched layer with a ferrite phase area ratio of 60% or more in a region from the surface to at least 50 μm in the depth direction,
Duplex stainless steel.

(2) When the thickness of the ferrite-enriched layer is t, the Cr concentration at the position t/2 in the depth direction from the surface is higher than the Cr concentration at the position t in the depth direction from the surface. high, the Cr concentration at a position t / 10 in the depth direction from the surface is higher than the Cr concentration at the position t / 2 in the depth direction from the surface;
The duplex stainless steel according to (1) above.

(3) The area ratio of the ferrite phase at a position t/10 in the depth direction from the surface is 90% or more,
The duplex stainless steel according to (1) or (2) above.

(4) The chemical composition at the center position in the thickness direction is mass%,
C: 0.0010 to 0.0600%,
Si: 0.01 to 1.50%,
Mn: 0.10-6.00%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 19.00 to 25.00%,
Ni: 1.00 to 6.00%,
N: 0.050 to 0.250%,
Al: 0.001 to 0.050%,
Ti: 0 to 0.050%,
Nb: 0 to 0.150%,
Mo: 0-2.00%,
Cu: 0 to 3.00%,
W: 0 to 2.00%,
Mg: 0-0.0050%,
Ca: 0 to 0.0050%,
REM: 0-0.30%,
B: 0 to 0.0040%,
balance: Fe and impurities,
The duplex stainless steel according to any one of (1) to (3) above.

(5) The chemical composition at the center position in the thickness direction is mass%,
Ti: 0.010 to 0.050%,
Nb: 0.020 to 0.150%,
Mo: 0.05 to 2.00%,
Cu: 0.05 to 3.00%,
W: 0.05 to 2.00%,
Mg: 0.0002-0.0050%,
Ca: 0.0002 to 0.0050%,
REM: 0.005 to 0.30%, and
B: 0.0003 to 0.0040%,
containing one or more selected from
The duplex stainless steel according to any one of (1) to (4) above.

(6) A method for producing a duplex stainless steel according to any one of (1) to (5) above, comprising:
For the duplex stainless steel having a metal structure in which the area ratio of the ferrite phase at the center position in the thickness direction is 40% or more and less than 60%,
(a) applying a chemical containing metallic Cr powder and _SiO2 to the surface of the base material;
(b) heating at more than 1150° C. and not more than 1300° C. for 5 hours or more in an atmosphere having an _O concentration of 30% by volume or less;
(c) shot blasting;
(d) pickling by spraying from a nozzle an aqueous solution containing 1 to 10% sulfuric acid and 2 to 20% nitric acid;
A method for producing duplex stainless steel.

INDUSTRIAL APPLICABILITY According to the present invention, a duplex stainless steel having high strength, excellent corrosion resistance and workability can be stably obtained industrially.

Each requirement of the present invention will be described in detail below.

1. Duplex Stainless Steel The duplex stainless steel according to the present invention has a metallographic structure in which the area ratio of the ferrite phase at the central position in the thickness direction is 40% or more and less than 60% at room temperature. The balance is the austenite phase and precipitates. When the area ratio of the ferrite phase is 60% or more, the area ratio of the austenite phase becomes 40% or less, and sufficient strength cannot be obtained. On the other hand, in order to reduce the area ratio of the ferrite phase to less than 40%, the area ratio of the austenite phase must exceed 60%, which may cause the following various problems.

First, it is necessary to increase the content of Ni, which is an expensive austenite-stabilizing element that is generally classified as a rare metal, and is expensive. Moreover, when assuming alloy-saving and inexpensive duplex stainless steel, the N content becomes too high, resulting in too high strength. In addition, it forms coarse compounds during hot working. For the above reasons, the area ratio of the ferrite phase at the central position in the thickness direction is set to 40% or more and less than 60%.

The area ratio of the ferrite phase at the central position in the thickness direction is preferably 45 to 55%. Phases other than the ferrite phase are the austenite phase and precipitates. Precipitates may be carbides, nitrides, sulfides, intermetallic compounds, or the like. The total area ratio of precipitates is preferably 0.1% or less.

The area ratio of the ferrite phase is measured with an electron beam backscatter diffraction device (EBSD). Specifically, an area of 100 μm×100 μm centered on each depth position is targeted, and measurement is performed in steps of 1 μm. Then, from the measurement results, the BCC phase is specified and the area ratio is obtained, which is taken as the area ratio of the ferrite phase. However, in the case of a depth position from the surface to a depth of 50 μm, the range from the depth position to the surface in the direction of the surface and 50 μm in the direction of the thickness center is narrower than 100 μm × 100 μm in the depth direction. It is the average value of the cross section of the test piece. For example, in the case of the outermost surface, the area is 100 μm wide×50 μm deep.

In the present invention, "steel" refers to ingots, steel billets, intermediate materials such as hot-rolled materials and cold-rolled materials processed from ingots and steel billets, and shaped steels and steel sheets manufactured thereafter. , bars and wires, and steel strips. Billets include slabs, billets, blooms, and the like. As will be described later, the ferrite-concentrated layer in the present invention is formed on each intermediate material such as an ingot, billet, hot-rolled material, or cold-rolled material by heating under predetermined conditions. On the other hand, a ferrite-enriched layer can be formed and increased, and the ferrite-enriched layer can remain in the final product. In this way, all of ingots, steel billets, intermediate materials, and steel materials as final products can have the characteristics of the present invention, and therefore are subject to the present invention.

2. Ferrite Concentrated Layer The duplex stainless steel according to the present invention has a ferrite concentrated layer in a region extending from the surface to at least 50 μm in the depth direction. In the present invention, the term "ferrite-enriched layer (α-enriched layer)" refers to a region having a ferrite phase area ratio of 60% or more.

The α-enriched layer is formed by reforming a duplex stainless steel having a metallographic structure in which the ferrite phase area ratio is 40% or more and less than 60%. Therefore, in the metallographic structure of the α-enriched layer, the balance is the austenite phase and precipitates.

As described above, the concentration of Cr in the surface layer improves the corrosion resistance, and the concentration of Cr has an α-enriched layer rich in ferrite phase on the surface, and the difference in properties between the austenite phase and the ferrite phase. is relieved, an effect of improving workability can be obtained. If the thickness of the α-enriched layer is less than 50 μm, the above effect cannot be sufficiently obtained. Therefore, the thickness of the α-enriched layer is set to 50 μm or more. It is preferably 80 μm or more.

The upper limit of the thickness of the α-enriched layer is not particularly limited. is considered to be the upper limit. Moreover, if the thickness exceeds 5 mm during the heat treatment, the effect is saturated and the manufacturing cost increases.

From the viewpoint of obtaining better corrosion resistance and workability, it is preferable that the surface side of the duplex stainless steel has a higher Cr concentration and a higher ferrite phase area ratio. Specifically, when the thickness of the ferrite-enriched layer is t, the Cr concentration at the position t in the depth direction from the surface is t/2 in the depth direction from the surface (hereinafter simply “t/ The Cr concentration at the t/2 position is higher than the Cr concentration at the t/2 position. A higher Cr concentration is preferred. Also, the area ratio of the ferrite phase at the t/10 position may be 80% or more, but is preferably 90% or more. The area ratio of the ferrite phase at the t/10 position may be 100%, or may be 95% or less.

Since Cr is concentrated in the α-enriched layer, the Cr concentration at the t/2 position and the t/10 position is higher than the Cr concentration at the center position in the thickness direction. However, the concentration of Cr becomes significant, and specifically, when the Cr concentration at the t/2 position or the t/10 position is 10% or more by mass higher than the Cr concentration at the center position in the thickness direction, Excessive curing may result in deterioration of surface properties. Furthermore, if the Cr concentration is increased, coarse nitrides are likely to be formed, which may adversely affect surface properties such as corrosion resistance. Therefore, the difference between the Cr concentration at the t/2 and t/10 positions and the Cr concentration at the central position in the thickness direction is preferably less than 10%.

In the present invention, there is no particular limitation on the grain size of the α-enriched layer. As described above, the α-enriched layer is formed by heating and holding (heat treatment) a chemical agent containing metal Cr powder and SiO ₂ applied to the surface of the base material. In the process, the crystal grains in the α-enriched layer tend to be coarser than those at the central position in the thickness direction.

When the surface layer of the base material is coarsened, the grain boundary area occupying the surface of the base material is reduced, and the occurrence of cracks is suppressed, thereby improving bending workability. Therefore, in order to obtain the effect of improving bending workability, in the duplex stainless steel according to the present invention, the average crystal grain size dc at the center position in the thickness direction and the The average crystal grain size ds preferably satisfies the following formula (i).
ds/dc≧1.50 (i)

The α-enriched layer is formed by the above heat treatment, and is subjected to the same heat treatment during hot working, cold working performed after cooling to room temperature, and, for example, molding into a product or parts constituting a product. is maintained at a rate that is at least approximately close to that formed in . In addition, it is also possible to increase the formation and thickness by further performing the above heat treatment on each intermediate material of the ingot, billet, hot rolled material after processing thereof, and cold rolled material. .

The average crystal grain size in the center position in the thickness direction and in the region from the surface to 50 μm in the depth direction can be measured by EBSD at the same time as the area ratio of the ferrite phase. Also, the average grain size means the average grain size of all grains in a two-phase structure composed of an austenite phase and a ferrite phase, including an α-enriched layer.

3. Dimensions There are no particular restrictions on the dimensions of the duplex stainless steel according to the invention. When the duplex stainless steel of the present invention is used as a steel sheet after processing, the sheet thickness is preferably 0.2 to 20.0 mm. Further, when the duplex stainless steel of the present invention is used as shaped steel after processing, the thickness of the plate-like portion is preferably 0.2 to 20.0 mm. When the duplex stainless steel of the present invention is used as a steel bar after processing, the diameter is preferably 5.0 to 60.0 mm.

4. Chemical composition The chemical composition of the duplex stainless steel according to the present invention is not particularly limited as long as the area ratio of the ferrite phase is 40% or more and less than 60%. A suitable chemical composition at the central position in the thickness direction will be described below. The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.0010 to 0.0600%
Since C degrades corrosion resistance, the smaller the content, the better, and the C content is preferably 0.0600% or less. However, excessive reduction leads to an increase in refining cost, so it is preferable to set the C content to 0.0010% or more. From the viewpoint of manufacturability, the more preferable range of C content is 0.0100 to 0.0450%.

Si: 0.01-1.50%
Si is an element that increases strength and has a deoxidizing effect during refining, so its content is preferably 0.01% or more. On the other hand, excessive Si content causes cracking during manufacturing, so the Si content is preferably 1.50% or less. From the viewpoint of manufacturability, the Si content is more preferably 1.00% or less. A more preferable range of Si content is 0.10 to 0.90%, and a further preferable range is 0.20 to 0.80%.

Mn: 0.10-6.00%
Mn stabilizes the austenite phase in duplex stainless steels. In addition, it is effective for increasing strength and has a deoxidizing effect. Therefore, it is preferable to set the Mn content to 0.10% or more. On the other hand, excessive Mn content leads to deterioration of corrosion resistance, so the Mn content is preferably 6.00% or less. In order to achieve both manufacturability and cost, the Mn content is more preferably 1.00 to 5.50%. More preferably, the Mn content is 2.00-5.00%.

P: 0.050% or less P is an element that impairs manufacturability and weldability, and the smaller the content, the better. Therefore, it is preferable to set the P content to 0.050% or less. However, excessive reduction leads to an increase in refining cost, so it is preferable to set the P content to 0.003% or more. From the viewpoint of manufacturability and weldability, the P content is more preferably in the range of 0.005 to 0.040%, more preferably in the range of 0.010 to 0.030%.

S: 0.0050% or less S is an unavoidable impurity element contained in steel and lowers hot workability. Therefore, the lower the S content, the better, and preferably 0.0050% or less. From the viewpoint of hot workability, the lower the S content is, the better. However, since an excessive reduction leads to an increase in raw material and refining costs, the S content is preferably 0.0001% or more. From the standpoint of manufacturability, the S content is more preferably in the range of 0.0001 to 0.0020%, more preferably in the range of 0.0002 to 0.0010%.

Cr: 19.00-25.00%
Cr is an element that improves oxidation resistance and corrosion resistance. In order to ensure sufficient corrosion resistance as a duplex stainless steel, the Cr content is preferably 19.00% or more. However, excessive Cr content promotes the formation of the σ phase, which is an embrittlement phase, when exposed to a high-temperature atmosphere, and causes an increase in alloy cost. preferably. From the viewpoint of manufacturability, a more preferable range of Cr content is more than 20.00% and 24.50% or less. A more preferable range of Cr content is 20.50 to 24.00%.

Ni: 1.00-6.00%
Ni improves corrosion resistance and stabilizes the austenite phase in duplex stainless steel. In order to improve corrosion resistance, it is preferable to set the Ni content to 1.00% or more. On the other hand, since Ni is classified as a rare metal and is expensive, the content thereof is preferably 6.00% or less. From the viewpoint of manufacturability, the more preferable range of Ni content is 1.50 to 4.50%.

N: 0.050-0.250%
N is an element that improves corrosion resistance and, like Ni, stabilizes austenite, so it can be used as a substitute for Ni. When the N content is small, sufficient corrosion resistance may not be obtained. Therefore, it is preferable to set the N content to 0.050% or more. On the other hand, the higher the N content, the more effective the corrosion resistance. From the viewpoint of manufacturability, the more preferable range of N content is 0.100 to 0.230%. A more preferable range of N content is 0.150 to 0.220%.

Al: 0.003-0.050%
Al is used as a deoxidizing element. It is preferable to set the Al content to 0.003% or more because it is effective if it is contained in an amount of 0.003% or more as a deoxidizing element. On the other hand, since excessive content causes hardening, the Al content is preferably 0.050% or less. From the viewpoint of manufacturability, a more preferable range of Al content is 0.030% or less. A more preferable range of Al content is 0.020% or less.

Ti: 0-0.050%
Ti is an element that combines with C and N and contributes to improving the corrosion resistance of the weld zone and increasing the strength, so it may be contained as necessary. On the other hand, an excessive Ti content causes a decrease in corrosion resistance and an increase in alloy cost, so the Ti content is preferably 0.050% or less. In order to obtain the above effect, it is preferable to set the Ti content to 0.010% or more. A more preferable range of Ti content is 0.015 to 0.045%, and a further preferable range is 0.020 to 0.040%.

Nb: 0-0.150%
Nb is an element that combines with C and N and contributes to improving the corrosion resistance of the weld zone and increasing the strength, so it may be contained as necessary. On the other hand, an excessive Nb content causes a decrease in corrosion resistance and an increase in alloy cost, so the Nb content is preferably 0.150% or less. In order to obtain the above effects, it is preferable to set the Nb content to 0.020% or more. A more preferable range of Nb content is 0.030 to 0.120%, and a further preferable range is 0.050 to 0.100%.

Mo: 0-2.00%
Cu: 0-3.00%
W: 0-2.00%
Mo, Cu and W are elements that contribute to the improvement of corrosion resistance, so they may be contained as necessary. On the other hand, an excessive content causes an increase in cost and a decrease in hot workability. Therefore, it is preferable to set the Mo content to 2.00% or less, the Cu content to 3.00% or less, and the W content to 2.00% or less. More preferably, the Mo content is 1.80% or less, or 1.50% or less, the Cu content is more preferably 2.50% or less, or 2.00% or less, and the W content is It is more preferably 1.50% or less, or 1.00% or less. In order to obtain the above effect, the content of one or more selected from these elements is 0.05% or more, 0.10% or more, 0.20% or more, or 0.50% or more It is preferable to

Mg: 0-0.0050%
Ca: 0-0.0050%
REM: 0-0.30%
B: 0 to 0.0040%
Mg, Ca, REM, and B are elements that improve hot workability, and may be contained as necessary. On the other hand, excessive content leads to inhibition of manufacturability. Therefore, it is preferable to set the Mg content to 0.0050% or less, the Ca content to 0.0050% or less, the REM content to 0.30% or less, and the B content to 0.0040% or less. More preferably, the Mg content is 0.0040% or less, or 0.0030% or less, the Ca content is more preferably 0.0035% or less, or 0.0020% or less, and the REM content is It is more preferably 0.20% or less, or 0.10% or less, and the B content is more preferably 0.0030% or less, or 0.0020% or less. In order to obtain the above effect, Mg: 0.0002% or more, Ca: 0.0002% or more, REM: 0.005% or more, B: 0.0003% or more It is preferable to contain one or more selected from. It is more preferable to contain one or more selected from Mg: 0.0005% or more, Ca: 0.0005% or more, REM: 0.010% or more, and B: 0.0005% or more. Further, it is more preferable to contain one or more selected from Mg: 0.0010% or more, Ca: 0.0010% or more, REM: 0.020% or more, and B: 0.0010% or more.

Here, REM refers to rare earth metals, and is a generic term for a total of 17 elements including 2 elements Sc and Y and 15 lanthanoid elements such as La, Ce, and Nd. The REM content means the total content of the 17 elements.

In the above chemical composition, the balance is Fe and impurities. Here, the term "impurities" refers to components mixed in by various factors in raw materials such as ores, scraps, etc., and in the manufacturing process when steel is manufactured industrially. means something

5. Manufacturing Method A manufacturing method for forming an α-enriched layer in the duplex stainless steel of the present invention will be described. As described above, the chemical containing the metal Cr powder and SiO ₂ is applied to the surface of the base material and then heat-treated, thereby concentrating Cr on the surface layer of the base material. As a result of the concentration of Cr, which is a ferrite-stabilizing alloying element, the volume fraction of the ferrite phase in the surface layer increases.

In order to concentrate Cr and form an α-enriched layer with a sufficient thickness, it is necessary to appropriately adjust the heat treatment conditions. By heating each intermediate material of ingot, billet, hot rolled material or cold rolled material having the chemical composition described above under the conditions shown below, each intermediate material has a thickness of 50 μm or more. It is possible to form and increase an α-enriched layer having a high density and to remain in the final product.

Processing conditions will be described in detail.

With the metal Cr powder and chemical containing SiO ₂ applied to the surface of the base material, the base material is heated at 1150° C. to 1300° C. for 5 hours or more in an atmosphere having an O ₂ concentration of 30% by volume or less. After heating, hot rolling may be performed.

<Agent containing metal Cr powder and _SiO2 >
Examples of agents containing metal Cr powder and SiO ₂ include SiO ₂ antioxidants containing 50% or more by volume of metal Cr powder. In addition, it is preferable that the coating amount of the chemical is 0.3 g/cm ² or more.

<Atmosphere>
The O ₂ concentration in the atmosphere during heating is set to 30% by volume or less. If the O ₂ concentration exceeds 30% by volume, the oxidation of alloying elements in the surface layer portion becomes excessive, and an α-enriched layer may not be obtained. Therefore, the O ₂ concentration in the atmosphere is set to 30% by volume or less. The O ₂ concentration is preferably 25% by volume or less, more preferably 20% by volume or less.

<Heating temperature>
The heating temperature is more than 1150° C. and 1300° C. or less. If the heating temperature is 1150° C. or less, the concentration of Cr becomes insufficient and an α-enriched layer cannot be obtained. On the other hand, if the temperature exceeds 1300° C., the oxidation of alloying elements becomes excessive, and an α-enriched layer may not be obtained. Moreover, in addition to the increased possibility of abnormal oxidation in which deep scales are locally formed, there is a problem that a large amount of scale is generated, the yield is lowered due to material loss, and the manufacturing cost is increased. The heating temperature is preferably 1170° C. or higher, preferably 1290° C. or lower, and more preferably 1280° C. or lower.

<Holding time>
The holding time during heating shall be 5 hours or more. If the holding time is less than 5 hours, the concentration of Cr becomes insufficient and an α-enriched layer cannot be obtained. On the other hand, even if the heating time exceeds 30 hours, the effect is saturated and the cost only increases.

Furthermore, the thickness of the α-enriched layer depends on the heat treatment as well as the descaling method after the heat treatment. The duplex stainless steel of the present invention exhibits excellent effects in products that are used after being processed, by having an α-enriched layer with a thickness of 50 μm or more. However, even if the above thickness is satisfied, an inappropriate descaling method may reduce or eliminate the α-enriched layer.

Descaling is achieved by suitable pickling after shot blasting, an example of which is described below.

<Descaling conditions>
First, shot blasting is performed for the purpose of crushing scale. It is desirable that the shot grains used for crushing the scale have a large kinetic energy and a large grain size with little loss due to air resistance, and it is efficient to use a large number of shot grains. It is desirable that the material does not adhere to the base material, but steel balls may be used since pickling will be carried out after that. In addition, it is desirable to spray with a strong pressure within a range where adhesion to the base material and indentation do not occur.

Next, an aqueous solution containing 1 to 10% fasciic acid and 2 to 20% nitric acid is sprayed from a nozzle to scatter and remove scale, thereby exhibiting excellent properties. It is most desirable that the aqueous solution containing sulfuric acid and nitric acid corrodes and removes only the scale and does not corrode the α-enriched layer, and the concentration is preferably low. The concentration of fasciic acid is preferably 8% or less, more preferably 6% or less. Also, the concentration of nitric acid is preferably 15% or less, more preferably 12% or less. The lower limit of the concentration of each acid is preferably 1% or more for sulfuric acid and 2% or more for nitric acid in order to corrode and remove scale.

The production method of the present invention comprises heat treatment for the purpose of forming an α-enriched layer and descaling for the purpose of leaving the α-enriched layer on the duplex stainless steel having the chemical composition described above. characterized and achieve excellent properties. (1) descaling after the heat treatment of the ingot or billet; (2) descaling after the heat treatment of the ingot or billet; (4) the descaling of the cold-rolled material after the heat treatment; The presence of the α-enriched layer with a thickness of 50 μm or more in the product exhibits excellent effects.

EXAMPLES The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples.

Duplex stainless steel having the chemical composition shown in Table 1 was melted and made into steel slabs, which were then coated with chemicals having the compositions shown in Table 2 and heated under various conditions. After that, hot rolling was performed to obtain a test material. Cr in the pharmaceutical composition is metallic Cr powder. The atmosphere during heating was a mixed gas atmosphere containing O ₂ with a concentration shown in Table 2 and the remainder being N ₂ , and hot rolling was performed under the conditions of a width of 100 mm and a cross-sectional reduction of 95%.

After that, in order to remove the influence of strain due to processing, heat treatment was performed at 1100° C. for 5 minutes in the same atmosphere. Then, the surface was subjected to shot blasting, and then pickled by being immersed in an aqueous solution containing 6% fasciic acid and 12% nitric acid maintained at 50° C. for 20 minutes to remove the scale formed on the surface. Then, the test material after descaling was subjected to structural observation and evaluation test. In addition, it was confirmed that only the scale was removed from all the test materials by comparing cross-sectional observations before and after the scale was removed.

First, a test piece for structure observation was cut out from the above test material. Then, the cross section perpendicular to the rolling direction was used as an observation plane and measured by EBSD. The results were analyzed using OIM Analysis ver. 7.3.0 was used. Then, the area ratio of the ferrite phase and the average grain size of the austenite phase and the ferrite phase were determined for each of the plate thickness center position and the vicinity of the surface.

In addition, each measurement was performed in a state in which 10 or more grains containing both the austenite phase and the ferrite phase were to be measured. In order to suppress fluctuations in values and obtain more accurate average values, it is preferable to measure 20 or more grains.

Furthermore, if the measurement results included scale or locally absent material, it was removed when calculating the average value. Then, the depth position where the ratio of the ferrite phase is 60% was specified, and the distance from the surface to the depth was defined as the thickness of the α-enriched layer.

In addition, from the measurement results of the crystal orientation in the same region, the area of the portion surrounded by the boundaries of the crystal orientation difference of 15 ° or more is converted into the equivalent circle diameter. was calculated and used as the crystal grain size.

Next, using the same test piece, EPMA point analysis was performed. The measurement at each depth position was carried out by holding each point for 1 second at intervals (pitch) of 1 μm for an area of 100 μm × 100 μm centering on a predetermined depth, and the average value in that area was adopted. . In addition, the measurement was carried out in such a state that one side of 100 μm was most parallel to the surface of the closest test piece.

Regarding the vicinity of the surface, for example, in the case of the outermost surface, an area of 100 μm width×50 μm depth, in the case of 10 μm depth, an area of 100 μm width×60 μm depth, and in the case of 20 μm depth, 100 μm width×70 μm depth. area. That is, in the case of a depth of 50 μm from the surface, it is the average value of the cross section of the test piece in a range narrower than the 100 μm corner. After that, the Cr concentrations measured at the t/2 position and the t/10 position were taken as the Cr concentrations at the respective depth positions.

Subsequently, workability and corrosion resistance evaluation tests were conducted. The workability was evaluated in two ways: hot workability and cold formability.

Hot workability was evaluated by investigating edge cracks after hot rolling. Then, if the crack depth at the edge of the plate width is less than 1 mm, the hot workability is ◯, if it is 1 mm or more and less than 2 mm, the hot workability is △, and if it is 2 mm or more, the hot workability. was x. In addition, evaluation of cold formability was performed by conducting a bending test. According to JIS Z 2248, a No. 1 test piece was taken from the steel plate, and the bending radius was set to twice the thickness of the plate, and the steel plate was bent up to an angle of 180° to check for the occurrence of cracks. When no cracks occurred, the cold formability was evaluated as ◯, and when cracks occurred, the cold formability was evaluated as x.

A corrosion resistance evaluation test was performed in the following procedure. First, a cylindrical test piece with a diameter of 15 mm was cut out from the above test material so that the thickness direction coincided with the thickness direction of the test piece. Then, the surface that was the surface of the test material was polished with #600, and then the pitting potential Vc100 was measured according to JIS G 0577 "Method for measuring pitting potential of stainless steel". The plate thickness reduction amount due to polishing was set to less than 5 μm. The measurement was performed 3 times and the average value was calculated. Corrosion resistance was judged to be excellent when Vc100 was 0.4V or more, and corrosion resistance was judged to be poor when Vc100 was less than 0.4V.

These results are also shown in Table 2.

As is clear from the results shown in Table 2, Test No. 1 satisfies the provisions of the present invention. In Nos. 1 to 19, excellent results were obtained in both hot workability and corrosion resistance. For them, test no. Nos. 20 to 23 are comparative examples outside the scope of the present invention. Specifically, Test No. In Test No. 20, no Cr-containing agent was applied. In Nos. 21 and 22, since the heating conditions were inappropriate, a sufficient α-enriched layer was not formed in either case, resulting in poor workability and corrosion resistance. And test no. In No. 23, since the Cr content in the steel was low, an α-enriched layer was not formed even when heat treatment was performed under appropriate conditions, resulting in poor workability and corrosion resistance.

INDUSTRIAL APPLICABILITY According to the present invention, a duplex stainless steel having high strength, excellent corrosion resistance and workability can be stably obtained industrially.

Claims (8)

Duplex stainless steel having a metal structure in which the area ratio of ferrite phase at the center position in the thickness direction is 40% or more and less than 60%,
Having a ferrite-enriched layer with a ferrite phase area ratio of 60% or more in a region from the surface to at least 50 μm in the depth direction,
Duplex stainless steel. When the thickness of the ferrite-enriched layer is t, the Cr concentration at the position t/2 in the depth direction from the surface is higher than the Cr concentration at the position t in the depth direction from the surface, and The Cr concentration at a position t/10 in the depth direction from the surface is higher than the Cr concentration at the position t/2 in the depth direction from the surface.
The duplex stainless steel of claim 1. The area ratio of the ferrite phase at a position t / 10 in the depth direction from the surface is 90% or more,
The duplex stainless steel of claim 1. The area ratio of the ferrite phase at a position t / 10 in the depth direction from the surface is 90% or more,
The duplex stainless steel according to claim 2. The chemical composition at the center position in the thickness direction is % by mass,
C: 0.0010 to 0.0600%,
Si: 0.01 to 1.50%,
Mn: 0.10-6.00%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 19.00 to 25.00%,
Ni: 1.00 to 6.00%,
N: 0.050 to 0.250%,
Al: 0.001 to 0.050%,
Ti: 0 to 0.050%,
Nb: 0 to 0.150%,
Mo: 0-2.00%,
Cu: 0 to 3.00%,
W: 0 to 2.00%,
Mg: 0-0.0050%,
Ca: 0 to 0.0050%,
REM: 0-0.30%,
B: 0 to 0.0040%,
balance: Fe and impurities,
The duplex stainless steel according to any one of claims 1 to 4. The chemical composition at the center position in the thickness direction is % by mass,
Ti: 0.010 to 0.050%,
Nb: 0.020 to 0.150%,
Mo: 0.05 to 2.00%,
Cu: 0.05 to 3.00%,
W: 0.05 to 2.00%,
Mg: 0.0002-0.0050%,
Ca: 0.0002 to 0.0050%,
REM: 0.005 to 0.30%, and
B: 0.0003 to 0.0040%,
containing one or more selected from
The duplex stainless steel according to claim 5. A method for producing a duplex stainless steel according to any one of claims 1 to 4,
For the duplex stainless steel having a metal structure in which the area ratio of the ferrite phase at the center position in the thickness direction is 40% or more and less than 60%,
(a) applying a chemical containing metallic Cr powder and _SiO2 to the surface of the base material;
(b) heating at more than 1150° C. and not more than 1300° C. for 5 hours or more in an atmosphere having an _O concentration of 30% by volume or less;
(c) shot blasting;
(d) pickling by spraying from a nozzle an aqueous solution containing 1 to 10% sulfuric acid and 2 to 20% nitric acid;
A method for producing duplex stainless steel. A method of manufacturing a duplex stainless steel according to claim 5, comprising:
For the duplex stainless steel having a metal structure in which the area ratio of the ferrite phase at the center position in the thickness direction is 40% or more and less than 60%,
(a) applying a chemical containing metallic Cr powder and _SiO2 to the surface of the base material;
(b) heating at more than 1150° C. and not more than 1300° C. for 5 hours or more in an atmosphere having an _O concentration of 30% by volume or less;
(c) shot blasting;
(d) pickling by spraying from a nozzle an aqueous solution containing 1 to 10% sulfuric acid and 2 to 20% nitric acid;
A method for producing duplex stainless steel.