JP7499008B2 - Duplex stainless steel and its manufacturing method - Google Patents

Description

The present invention relates to duplex stainless steel and its manufacturing method.

Duplex stainless steel has excellent corrosion resistance and particularly high strength, and is therefore used as a building or structural material. Among hot-rolled stainless steel sheets and strips, duplex stainless steel types include SUS329J1 and SUS329J4L as described in JIS G 4304.

Since these conventional duplex stainless steels contain a large amount of added elements and are relatively expensive, lean duplex stainless steels with reduced amounts of added elements have been developed. Patent documents 1 and 2 disclose inexpensive duplex stainless steels that contain a low Ni content and utilize austenite-forming elements such as Mn and N.

Japanese Patent Application Laid-Open No. 61-56267 JP 2010-229459 A

However, lean duplex stainless steel that utilizes N consists of a mixed structure of different phases, austenite and ferrite. In addition, it contains a large amount of N, which is a strong solid solution strengthening element and easily forms compounds. This makes it difficult to process into various shapes. In particular, the difference in properties between the austenite phase and the ferrite phase becomes apparent during hot processing.

The present invention has been made to solve the above problems, and aims to provide a duplex stainless steel that has high strength, as well as excellent corrosion resistance and workability.

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

(a) Several types of duplex stainless steels were melted and processed under a variety of conditions in an oxygen-containing atmosphere. As a result, it was found that almost no N was contained in the scale, but that it remained concentrated in the metal phase immediately below the scale.

(b) As N concentrates just below the scale, an austenite-enriched layer (hereafter also referred to as a "γ-enriched layer") is formed on the surface of the duplex stainless steel, in which the proportion of austenite phase increases closer to the surface.

(c) In the gamma-enriched layer formed by the above method, the crystal grains do not become significantly coarsened. Therefore, the difference in crystal grain size between the gamma-enriched layer and the thick center portion is small. In addition, because the austenite phase has a high solid solubility limit for N, there is at least no tendency for the amount of coarse compounds that significantly deteriorate workability to increase significantly in the gamma-enriched layer compared to the thick center portion.

(d) By virtue of these characteristics, duplex stainless steels having the above-mentioned gamma-enriched layer formed on the surface have high strength and excellent corrosion resistance and workability.

(e) The above-mentioned γ-enriched layer can be formed by heating and holding at 1200 to 1300°C for 5 hours or more in an atmosphere with an _O2 concentration of 2 to 10%.

The present invention was made based on the above findings, and the gist of the present invention is the following duplex stainless steel and its manufacturing method.

(1) A duplex stainless steel having a metal structure in which the area ratio of the austenite phase at the center position in the thickness direction is 35% or more and less than 65%,
The steel has an austenite-enriched layer having an austenite phase area ratio of 65% or more in a region extending from the surface to at least 50 μm in a depth direction,
The average grain size dc at the center position in the thickness direction and the average grain size ds in the austenite-enriched layer satisfy the following formula (i):
The N content at the center position in the thickness direction is 0.050% or more by mass%,
Duplex stainless steel.
0.80≦ds/dc≦1.25 (i)

(2) When the thickness of the austenite-enriched layer is t,
an area ratio of the austenite phase at a position t/2 in a depth direction from the surface is higher than an area ratio of the austenite phase at a position t in a depth direction from the surface;
an area ratio of the austenite phase at a position t/10 in a depth direction from the surface is higher than an area ratio of the austenite phase at a position t/2 in a depth direction from the surface;
The duplex stainless steel according to (1) above.

(3) The area ratio of the austenite 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 N content at a position t/10 in the depth direction from the surface is higher than the N content at the center position in the thickness direction.
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, in mass%,
C: 0.001 to 0.060%,
Si: 0.01 to 1.50%,
Mn: 0.1 to 6.0%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 19.0 to 25.0%,
Ni: 1.0 to 6.0%,
N: 0.050 to 0.25%,
Al: 0.003 to 0.050%,
Ti: 0 to 0.050%,
Nb: 0 to 0.15%,
Mo: 0 to 2.0%,
Cu: 0 to 3.0%,
W: 0 to 2.0%,
Mg: 0 to 0.0050%,
Ca: 0 to 0.0050%,
REM: 0 to 0.30%,
B: 0 to 0.0040%,
The balance is Fe and impurities.
The duplex stainless steel according to any one of (1) to (4) above.

(6) The chemical composition at the center position in the thickness direction is, in mass%,
Ti: 0.01 to 0.050%,
Nb: 0.02 to 0.15%,
Mo: 0.05 to 4.0%,
Cu: 0.05 to 4.0%,
W: 0.05 to 4.0%,
Mg: 0.0002 to 0.0050%,
Ca: 0.0002 to 0.0050%,
REM: 0.005 to 0.30%, and
B: 0.0003 to 0.0040%,
Contains one or more selected from
The duplex stainless steel according to (5) above.

(7) The N content at a position t/10 in the depth direction from the surface is 3.0 times or more relative to the N content at the center position in the thickness direction;
The compressive residual stress on the surface is 400 MPa or more;
The duplex stainless steel according to any one of (1) to (6) above.

(8) For a duplex stainless steel having a metal structure in which the area ratio of the austenite phase at the center position in the thickness direction is 35% or more and less than 65%, and the N content at the center position in the thickness direction is 0.050% or more, in mass%,
(a) heating at 1200 to 1300 ° C. for 5 h or more in an atmosphere having an _O2 concentration of 2 to 10 vol.%;
(b) a step of performing shot blasting;
(c) pickling the steel sheet by spraying an aqueous solution containing 1 to 10% fluoride and 2 to 20% nitric acid from a nozzle;
Manufacturing method of duplex stainless steel.

(9) The chemical composition at the center position in the thickness direction is, in mass%,
C: 0.001 to 0.060%,
Si: 0.01 to 1.50%,
Mn: 0.1 to 6.0%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 19.0 to 25.0%,
Ni: 1.0 to 6.0%,
N: 0.050 to 0.25%,
Al: 0.003 to 0.050%,
Ti: 0 to 0.050%,
Nb: 0 to 0.15%,
Mo: 0 to 2.0%,
Cu: 0 to 3.0%,
W: 0 to 2.0%,
Mg: 0 to 0.0050%,
Ca: 0 to 0.0050%,
REM: 0 to 0.30%,
B: 0 to 0.0040%,
The balance is Fe and impurities.
The method for producing the duplex stainless steel according to (8) above.

(10) The chemical composition at the center position in the thickness direction is, in mass%,
Ti: 0.01 to 0.050%,
Nb: 0.02 to 0.15%,
Mo: 0.05 to 4.0%,
Cu: 0.05 to 4.0%,
W: 0.05 to 4.0%,
Mg: 0.0002 to 0.0050%,
Ca: 0.0002 to 0.0050%,
REM: 0.005 to 0.30%, and
B: 0.0003 to 0.0040%,
Contains one or more selected from
The method for producing the duplex stainless steel according to (9) above.

(11) After the step (c), the duplex stainless steel is subjected to
(d) A step of heating and holding the material at 1000 to 1200 ° C. for 10 min or less in an atmosphere containing _N2 is further performed;
A method for producing a duplex stainless steel according to any one of (8) to (10) above.

According to the present invention, it is possible to obtain duplex stainless steels that have high strength, as well as excellent corrosion resistance and workability, in a stable industrial manner.

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

1. Duplex Stainless Steel The duplex stainless steel according to the present invention has a metal structure in which the area ratio of the austenite phase at the center position in the thickness direction is 35% or more and less than 65% at room temperature. The remainder is the ferrite phase and precipitates. If the area ratio of the austenite phase is less than 35%, sufficient strength cannot be obtained. On the other hand, in order to make the area ratio of the austenite phase 65% or more, various problems such as the following may occur.

First, it is necessary to increase the content of Ni, which is generally classified as a rare metal and is an expensive austenite stabilizing element, which makes it expensive. Furthermore, when considering lean dual-phase steel, the N content becomes too high, resulting in too high strength. In addition, coarse compounds are formed during hot working. For these reasons, the area ratio of the austenite phase at the center position in the thickness direction is set to 35% or more and less than 65%.

The area ratio of the austenite phase at the center position in the thickness direction is preferably 40 to 60%. Phases other than the austenite phase are ferrite phase and precipitates. The precipitates may be any of carbides, nitrides, sulfides, intermetallic compounds, etc.

The area ratio of austenite is measured using an electron backscatter diffraction (EBSD) device. Specifically, a 100 μm x 100 μm area is measured at each depth position, with measurements being taken in 1 μm steps. The FCC phase is then identified from the analysis results, and the area ratio is calculated, which is taken as the area ratio of austenite.

2. Austenite-enriched layer The duplex stainless steel according to the present invention has an austenite-enriched layer in a region extending from the surface to a depth of at least 50 μm. In the present invention, the term "austenite-enriched layer (γ-enriched layer)" refers to a region in which the area ratio of the austenite phase is 65% or more.

The above-mentioned gamma-enriched layer is formed by modifying a duplex stainless steel having a metal structure in which the area ratio of the austenite phase is 35% or more and less than 65%. Therefore, in the metal structure of the gamma-enriched layer, the remainder is the ferrite phase and precipitates.

As described above, the γ-enriched layer is formed by heating and holding (heat treatment) in an atmosphere containing a certain amount of oxygen, resulting in the enrichment of N directly below the scale. In the γ-enriched layer formed by such a process, the grains do not become coarse. Therefore, in the duplex stainless steel according to the present invention, the average grain size dc at the center position in the thickness direction and the average grain size ds in the γ-enriched layer satisfy the following formula (i):
0.80≦ds/dc≦1.25 (i)

The gamma-enriched layer is maintained at a ratio at least nearly equal to that formed by the heat treatment, even during hot working performed following the heat treatment, cold working performed after cooling to room temperature, and during molding into, for example, a product or a component that constitutes a product.

The average grain size at the center of the thickness direction and in the gamma-enriched layer can be measured by EBSD at the same time as the area ratio of the austenite phase. The average grain size means the average grain size of all grains in the austenite phase and ferrite phase.

The austenite phase has relatively better corrosion resistance and workability than the ferrite phase. Furthermore, by increasing the proportion of the austenite phase, the adverse effects of differences in properties are mitigated compared to when the proportions of the austenite phase and ferrite phase are approximately the same.

As a result, by having a gamma-enriched layer on the surface that is rich in austenite phase and has a grain size similar to that at the center, the effect of improving corrosion resistance and workability, which are greatly affected by the surface, is obtained. If the thickness of the gamma-enriched layer is less than 50 μm, the above effect cannot be fully obtained. Therefore, the thickness of the gamma-enriched layer is set to 50 μm or more.

Although there is no particular upper limit on the thickness of the gamma-enriched layer, if the thickness exceeds 5 mm during heat treatment, there is an increased possibility of abnormal oxidation occurring, such as the formation of deep scales that are significantly thicker than 5 mm in some places. In addition, there is a large reduction in yield due to material loss, which leads to problems such as increased manufacturing costs.

As mentioned above, it is preferable that the fraction of the austenite phase in the gamma-enriched layer increases the closer it gets to the surface. It is believed that better workability can be obtained by gradually increasing the fraction of the austenite phase toward the surface, rather than changing suddenly. If there is a sudden change to a phase with significantly different properties at a certain depth, there is a high possibility that cracks or peeling will occur in the part where the sudden change occurred due to the concentration of stress and strain when processing into a plate or forming into a product.

Specifically, when the thickness of the gamma-enriched layer is t, the area ratio of the austenite phase at a position t/2 in the depth direction from the surface is higher than the area ratio of the austenite phase at a position t/2 in the depth direction from the surface, and it is preferable that the area ratio of the austenite phase at a position t/10 in the depth direction from the surface is higher than the area ratio of the austenite phase at a position t/2 in the depth direction from the surface.

Furthermore, near the surface of the gamma-enriched layer, the higher the area ratio of the austenite phase, the better. Specifically, the area ratio of the austenite phase at the t/10 position in the depth direction from the surface is preferably 90% or more. However, the ferrite phase originally present near the surface is thought to have an effect of suppressing the coarsening of the austenite phase by remaining without completely disappearing, although the grain size becomes smaller as N is concentrated. Therefore, if the structure at the t/10 position is austenite single phase, the effect of suppressing the coarsening of crystal grains may not be obtained. Therefore, it is more preferable that the area ratio of the austenite phase at the t/10 position is less than 100%.

3. Dimensions There are no particular limitations on the dimensions of the duplex stainless steel according to the present invention. When the duplex stainless steel according to the present invention is used as a steel plate after processing, the plate thickness is preferably 1.0 to 20.0 mm.

4. Chemical composition The duplex stainless steel according to the present invention has an N content of 0.050% or more at the center position in the thickness direction, in mass %. As described above, in the present invention, the austenite phase fraction is increased by concentrating N just below the scale. If the N content is less than 0.050%, the above effect cannot be obtained.

There are no particular limitations on the content of elements other than N, so long as the area ratio of the austenite phase is 35% or more and less than 65%. A preferred chemical composition at the center position in the thickness direction is explained below. The reasons for limiting each element are as follows. In the following explanation, "%" for the content means "mass %".

C: 0.001 to 0.060%
Since C deteriorates corrosion resistance, the lower the C content, the better, and the C content is preferably 0.060% or less. However, since an excessive reduction in C content leads to an increase in refining costs, the C content is preferably 0.001% or more. From the viewpoint of manufacturability, the C content is more preferably in the range of 0.010 to 0.045%.

Si: 0.01 to 1.50%
Since Si is an element that increases strength and has a deoxidizing effect during refining, its content is preferably 0.01% or more. On the other hand, since excessive content leads to cracks during manufacturing, the Si content is preferably 1.50% or less. From the viewpoint of manufacturability, the Si content is more preferably 1.00% or less.

Mn: 0.1 to 6.0%
Mn is relatively inexpensive and may be added instead of Ni. It is effective in increasing strength and has a deoxidizing effect, so its content is preferably 0.1% or more. On the other hand, since excessive content leads to deterioration of corrosion resistance, it is preferable that the Mn content be 6.0% or less. In order to balance manufacturability and cost, the Mn content is more preferably 0.5 to 5.5%.

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

S: 0.0050% or less S is an inevitable impurity element contained in steel, and reduces hot workability. Therefore, the lower the S content, the better, and it is preferable to set it to 0.0050% or less. From the viewpoint of hot workability, the lower the S content, the better, but since excessive reduction leads to increased costs for raw materials and refining, it is preferable to set the S content to 0.0001% or more. From the viewpoint of manufacturability, the more preferable range of the S content is 0.0001 to 0.0020%, and the even more preferable range is 0.0002 to 0.0010%.

Cr: 19.0 to 25.0%
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 19.0% 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 also leads to an increase in alloy costs, so the Cr content is preferably 25.0% or less. From the viewpoint of manufacturability, the more preferable range of the Cr content is 20.0 to 24.5%.

Ni: 1.0 to 6.0%
Ni improves corrosion resistance and stabilizes the austenite phase in duplex stainless steel. In order to improve corrosion resistance, the Ni content is preferably 1.0% or more. On the other hand, since Ni is expensive in alloying, the Ni content is preferably 6.0% or less. From the viewpoint of manufacturability, the Ni content is preferably in the range of 1.5 to 5.5%.

N: 0.050 to 0.25%
As described above, N is an essential element in the present invention. In addition, N is an element that improves corrosion resistance, and also stabilizes austenite like Ni, so it can be used as a substitute for Ni. If the N content is low, sufficient corrosion resistance may not be obtained. A high N content is effective for corrosion resistance, but nitrogen gasification may occur during melting, generating bubbles, so the N content is preferably 0.25% or less. From the viewpoint of manufacturability, the N content is more preferably in the range of 0.10 to 0.20%.

Al: 0.003 to 0.050%
Al is used as a deoxidizing element. Since an Al content of 0.003% or more is effective as a deoxidizing element, the Al content is preferably 0.003% or more. On the other hand, since an excessive content of Al leads to hardening, the Al content is preferably 0.050% or less. From the viewpoint of manufacturability, the Al content is more preferably in the range of 0.005 to 0.030%.

Ti: 0 to 0.050%
Ti is an element that combines with C and N to improve the corrosion resistance of the weld and increase the strength, so it may be contained as necessary. On the other hand, excessive inclusion of Ti leads to a decrease in corrosion resistance and an increase in alloy cost, so the Ti content is preferably 0.050% or less. In addition, if the above effect is to be obtained, the Ti content is preferably 0.01% or more.

Nb: 0 to 0.15%
Nb is an element that combines with C and N to improve the corrosion resistance of the weld and increase the strength, so it may be contained as necessary. On the other hand, excessive inclusion of Nb leads to a decrease in corrosion resistance and an increase in alloy cost, so the Nb content is preferably 0.15% or less. In addition, if it is desired to obtain the above effects, the Nb content is preferably 0.02% or more.

Mo: 0 to 2.0%
Cu: 0 to 3.0%
W: 0 to 2.0%
Mo, Cu and W are elements that contribute to improving corrosion resistance, so they may be contained as necessary. On the other hand, excessive content leads to increased costs and reduced hot workability. Therefore, it is preferable that the Mo content is 2.0% or less, the Cu content is 3.0% or less, and the W content is 2.0% or less. In order to obtain the above effects, it is preferable that the content of one or more selected from these elements is 0.05% or more.

Mg: 0 to 0.0050%
Ca: 0 to 0.0050%
REM: 0 to 0.30%
B: 0 to 0.0040%
Mg, Ca, REM and B are elements that improve hot workability, so they may be contained as necessary. On the other hand, excessive content leads to impaired 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. In addition, when it is desired to obtain the above-mentioned effects, it is preferable to contain one or more selected from Mg: 0.0002% or more, Ca: 0.0002% or more, REM: 0.005% or more, and B: 0.0003% or more in order to exert the above-mentioned effects.

In the above chemical composition, the balance is Fe and impurities. Here, "impurities" refers to components that are mixed in due to various factors in raw materials such as ores and scraps, and in the manufacturing process, when steel is industrially produced, and are acceptable within the range that does not adversely affect the present invention.

As mentioned above, the gamma-enriched layer can be formed by enriching N. Therefore, it is preferable that the N content at a position t/10 in the depth direction from the surface is higher than the N content at the center position in the thickness direction. In other words, when the N content at the center position in the thickness direction is Nc (mass %) and the N content at a position t/10 in the depth direction from the surface is Ns (mass %), it is preferable that the value of Ns/Nc is greater than 1.0. The N content at the center position in the thickness direction and at the position t/10 in the depth direction from the surface can be measured by an electron probe microanalyzer (EPMA).

By further absorbing _N2 in the γ-enriched layer and concentrating more N in the crystal lattice, it is possible to impart compressive residual stress near the surface of the steel, which results in improved bending fatigue properties. To obtain such an effect, it is more preferable that the N content at the position t/10 in the depth direction from the surface is 3.0 times or more the N content at the center position in the thickness direction. In other words, it is more preferable that the value of Ns/Nc is 3.0 or more.

In addition, by making the compressive residual stress imparted by absorbing _N2 to be 400 MPa or more, high bending fatigue properties can be obtained. Note that when the tensile side is expressed as positive and the compressive side as negative, the residual stress is -400 MPa or less.

5. Manufacturing method A manufacturing method for forming a gamma-enriched layer in the duplex stainless steel of the present invention will be described. The concentration of nitrogen, which is considered to be the main cause of the gamma-enriched layer, is not due to the absorption of nitrogen from the external atmosphere, which is generally called nitrogen absorption or nitriding. The gamma-enriched layer in the present invention is formed in a duplex stainless steel containing nitrogen at a certain concentration or more by forming a dense scale, and the nitrogen originally contained in the material inside the scale diffuses and concentrates in the interior, with almost no escape to the outside. The inventors discovered this phenomenon.

However, the dense scale that is the premise for this depends on the composition, structure, and thickness, and is affected by the heat treatment conditions that form the scale, particularly the atmosphere. As mentioned above, the thickness of the gamma-enriched layer is about 5 mm after industrial heat treatment, and is reduced to about 400 μm by hot rolling, which involves large dimensional changes, as in the examples described below. For these reasons, the gamma-enriched layer of the present invention is affected by the heat treatment conditions and the descaling method after heat treatment or hot rolling, and can only be obtained by appropriately adjusting these.

As an example, a γ-enriched layer is formed when a steel slab having the above-mentioned chemical composition is heated under the conditions shown below (a), and then descaled by shot blasting (b) and pickling (c). Each step will be explained in detail.

(a) Heating step: Heat at 1200 to 1300 ° C. for 5 h or more in an atmosphere with an _O2 concentration of 2 to 10 volume %. After heating, hot rolling may be performed.

<Atmosphere>
The _O2 concentration in the atmosphere during heating is set to 2 to 10% by volume. If the _O2 concentration is less than 2% by volume, the growth of scale is insufficient, and the concentration of N directly below the scale is also insufficient. On the other hand, if the _O2 concentration exceeds 10% by volume, the γ-enriched layer may not be obtained. The γ-enriched layer can only be obtained by the formation of dense scale. For this reason, the _O2 concentration in the atmosphere is set to 10% or less. The _O2 concentration is preferably 8% or less, and more preferably 5% or less.

<Heating temperature>
The heating temperature is 1200 to 1300°C. If the heating temperature is less than 1200°C, the growth of scale is insufficient, and the concentration of N directly below the scale is also insufficient, so that a gamma-enriched layer cannot be obtained. On the other hand, if the heating temperature exceeds 1300°C, in addition to increasing the possibility of abnormal oxidation in which deep scale is locally formed, the amount of scale generated increases, resulting in a decrease in yield due to material loss and an increase in manufacturing costs. The heating temperature is preferably 1210°C or higher, preferably 1290°C or lower, and more preferably 1280°C or lower.

<Retention time>
The holding time during heating is 5 hours or more. If the holding time is less than 5 hours, the growth of the scale is insufficient, and the concentration of N directly below the scale is also insufficient, so that a gamma-enriched layer cannot be obtained. On the other hand, if the heating time exceeds 30 hours, the effect is saturated and the cost increases, so that the holding time is preferably 30 hours or less from the viewpoint of manufacturability.

(b) Shot blasting process and (c) Pickling process The final thickness of the γ-enriched layer depends on the descaling method after heat treatment or hot rolling that involves large dimensional changes. An excellent effect is obtained by leaving a thickness of 50 μm or more.

For example, after performing shot blasting to crush the scale, a relatively dilute aqueous solution containing 1% fluoride and 2% nitric acid is sprayed from a nozzle, scattering only the scale and minimizing the reduction in thickness of the gamma-enriched layer, thereby achieving excellent properties.

After removing the scale by the above steps (b) and (c), cold rolling may be performed as necessary. However, in this case, the thickness of the γ-enriched layer is also reduced due to the reduction in thickness by rolling. Therefore, in order to increase the thickness of the γ-enriched layer, _N2 may be further absorbed into the γ-enriched layer.

(d) _N2 Absorption Step After removing the scale and performing cold rolling as necessary, the steel may be heated and held at 1000 to 1200 ° C. for 1 to 10 min in an atmosphere containing _N2 .

<Atmosphere>
The heating and holding atmosphere is not particularly limited as long as it contains N _2. In order to efficiently absorb N ₂ , for example, the atmosphere may be made of 75% H ₂ and 25% N ₂ .

<Heating temperature>
The heating temperature is set to 1000 to 1200° C. If the heating temperature is less than 1000° C., it is difficult to absorb a sufficient amount of N ₂ , and the increase in residual stress is also insufficient. On the other hand, if the heating temperature exceeds 1200° C., due to the formation of nitrides, it is not possible to dissolve a sufficient amount of N ₂ , and the thickness of the γ-enriched layer cannot be efficiently increased.

<Retention time>
The holding time during heating is within 10 min. However, if the holding time is less than 1 min, the increase in the thickness of the γ-enriched layer becomes unclear and sufficient effects cannot be confirmed, so 1 min or more is preferable. On the other hand, if heating is performed for more than 10 min, the effect tends to saturate and costs only increase, so from the viewpoint of manufacturability, the holding time is preferably 10 min or less. However, if a particularly thick γ-enriched layer is required, the thickness can be increased by passing the steel strip through a continuous heat treatment furnace multiple times or by holding the steel plate in a batch furnace for a long time.

In addition, when only the _N2 absorption process is performed on steel that does not have a γ-enriched layer, it is possible to increase the area ratio of the austenite phase near the surface, but the austenite single-phase structure will be formed and the crystal grains will become coarse. Therefore, even when the _N2 absorption process is performed, it is important to form a γ-enriched layer in advance.

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 steels having the chemical compositions shown in Table 1 were melted and heated under various conditions, and then hot rolled at a width of 100 mm and a cross-sectional area reduction rate of 95% to obtain test materials. The chemical compositions were measured by taking a required amount of samples from the center of the molten metal after the composition was adjusted and immediately before casting, and measuring the average values of the elements shown in Table 1. The heating temperature, holding time, and heating atmosphere of the hot rolling are shown in Table 2. The heating atmosphere was a mixed gas atmosphere containing _O2 at the concentration shown in Table 2, with the balance being _N2 .

Then, to remove the effects of distortion due to processing, the specimen was heat-treated in the same atmosphere at 1100°C for 5 minutes. Next, shot blasting was performed, and then an aqueous solution of 1% French acid and 2% nitric acid held at 50°C was sprayed from a nozzle for 1 minute to remove the scale that had formed on the surface.

The test materials after scale removal were then subjected to structural observation and evaluation testing. Furthermore, by comparing cross-sectional observations before and after scale removal, it was confirmed that only the scale had been removed from all test materials.

First, a test piece for microstructural observation was cut out from the test material. Then, a cross section perpendicular to the rolling direction was used as the observation surface, and measurements were made using EBSD. The results were analyzed using OIM Analysis ver. 7.3.0 manufactured by TSL. The area ratio of the austenite phase and the average grain size of the austenite phase and ferrite phase were determined for both the center position of the sheet thickness and near the surface.

In addition, each measurement, including the N content described later, was performed with 10 or more grains containing both austenite and ferrite phases as the measurement target. In order to suppress value fluctuations and obtain a more accurate average value, it is preferable to measure 20 or more grains.

Measurements at each depth were performed at intervals (pitch) of 1 μm over a 100 μm x 100 μm area centered on the specified depth, and the average value over that area was used. The measurements were performed in a state where one side of the 100 μm was most parallel to the nearest surface of the test piece.

For the vicinity of the surface, for example, in the case of the outermost surface, an area of 100 μm wide x 50 μm deep was measured, in the case of a depth of 10 μm, an area of 100 μm wide x 60 μm deep was measured, and in the case of a depth of 20 μm, an area of 100 μm wide x 70 μm deep was measured. In other words, in the case of a depth of 50 μm from the surface, the average value is obtained for the cross section of the test piece in an area narrower than a corner of 100 μm.

Furthermore, if the measurement results included scale or areas where no material was present locally, these were removed when calculating the average value. The depth position where the austenite phase ratio was 65% was then identified, and the distance from the surface to that depth was taken as the thickness of the gamma-enriched layer.

Then, when the thickness of the gamma-enriched layer is defined as t, the proportion of the austenite phase measured at positions t/2 and t/10 in the depth direction from the surface was taken as the area ratio at each depth position.

In addition, based on the results of crystal orientation measurements in similar regions, the areas where the difference in crystal accuracy was 15° or more were taken as boundaries, and the area of the area surrounded by these boundaries was converted into a circular equivalent diameter, which was then used to calculate the crystal grain size.

Next, using the same test piece, a line analysis was performed by EPMA to measure the N content (Nc) at the center position in the thickness direction and the N content (Ns) at a position t/10 in the depth direction from the surface, and the ratio (Ns/Nc) was calculated. The line analysis was performed in a straight line of 200 μm in length with a 1 μm pitch, with each point held for 1 second. The measurement in the 200 μm long straight line was performed so as to be most parallel to the nearest surface of the test piece.

Next, evaluation tests were conducted on hot workability and corrosion resistance. Hot workability was evaluated by examining the edge cracks after hot rolling. If the depth of the cracks at the plate width end was less than 2 mm, the hot workability was rated as ○, and if it was 2 mm or more, the hot workability was rated as ×.

The corrosion resistance evaluation test was performed according to 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 longitudinal direction. The surface of the test material was then polished with #600, and the pitting corrosion initiation potential _Vc100 was measured in accordance with JIS G 0577 "Method of measuring pitting corrosion potential of stainless steel". The measurement was performed three times, and the average value was calculated. When _Vc100 was 0.4 V or more, it was judged to have excellent corrosion resistance, and when it was less than 0.4 V, it was judged to have poor corrosion resistance.

The results are shown in Table 2.

As is clear from the results shown in Table 2, Test Nos. 1 to 8, which satisfy the provisions of the present invention, showed excellent results in both hot workability and corrosion resistance.

In contrast, Test Nos. 9 to 13 are comparative examples that deviate from the provisions of the present invention. Specifically, in Test Nos. 9 and 10, the heating conditions were inappropriate, so the γ-enriched layer was not formed, resulting in poor hot workability and corrosion resistance.

In addition, in Test No. 11, the _O2 concentration in the heating atmosphere was excessive, and in Test No. 12, the heating temperature was low and the heating time was short, so denitrification occurred on the surface of the test piece and the area ratio of the austenite phase on the surface decreased. As a result, the corrosion resistance was significantly deteriorated.

In addition, in test No. 13, the N content in the steel was low, so even when heat treatment was performed under appropriate conditions, a gamma-enriched layer was not formed, resulting in poor hot workability and corrosion resistance.

Next, two test pieces of Test No. 1 and one test piece of Test No. 9 used in Example 1 were prepared and cold rolled to obtain test pieces with a thickness of 1.0 mm (Test Nos. 14 to 16). Of these, Test Nos. 15 and 16 were further subjected to N ₂ absorption treatment in which they were heated and held at 1100 ° C for 5 min in an atmosphere consisting of 75% H ₂ and 25% N ₂ .

Then, using the same methods as in Example 1, microstructural observation, measurement of Ns/Nc, and evaluation tests of hot workability and corrosion resistance were performed. In this example, residual stress was also measured and bending fatigue property evaluation tests were performed.

Residual stress was measured on the surface of the test material using a micro X-ray residual stress measuring device under the conditions shown in Table 3. Note that the residual stress values are expressed as positive for tensile residual stress and negative for compressive residual stress.

The bending fatigue properties were evaluated by the following method in accordance with JIS Z 2275. A rectangular test piece with b=20 mm and R=30 mm was repeatedly bent at a bending stress of 500 N/ ^mm2 on the plate surface using a bi-directional plane bending fatigue tester. The presence or absence of breakage after ¹⁰⁷ repetitions was checked, and the case where no breakage occurred was evaluated as ◯, and the case where breakage occurred was evaluated as ×.

The results are summarized in Table 4.

As shown in Table 4, Test Nos. 14 and 15 satisfied the provisions of the present invention, and therefore were excellent in both hot workability and corrosion resistance. In addition, in Test No. 15, _N2 absorption treatment was performed, so the thickness of the γ-enriched layer was larger than in Test No. 14, and the area ratio of the austenite phase was also increased. In addition, a compressive residual stress of 450 MPa was introduced, and thus the bending fatigue properties were improved.

In contrast, in Test No. 16, the γ-enriched layer was formed only by N ₂ absorption treatment, and the thickness was insufficient at less than 50 μm. In addition, the metal structure near the surface was austenite single phase, and the crystal grains were significantly coarsened, resulting in poor hot workability.

According to the present invention, it is possible to obtain duplex stainless steels that have high strength, as well as excellent corrosion resistance and workability, in a stable industrial manner.

Claims (11)

A hot-rolled duplex stainless steel having a metal structure in which the area ratio of the austenite phase at the center position in the thickness direction is 35% or more and less than 65%,
The steel has an austenite-enriched layer having an austenite phase area ratio of 65% or more in a region extending from the surface to at least 50 μm in a depth direction,
The average grain size dc at the center position in the thickness direction and the average grain size ds in the austenite-enriched layer satisfy the following formula (i):
The N content at the center position in the thickness direction is 0.050% or more by mass%,
Duplex stainless steel.
0.80≦ds/dc≦1.25 (i) When the thickness of the austenite-enriched layer is t,
an area ratio of the austenite phase at a position t/2 in a depth direction from the surface is higher than an area ratio of the austenite phase at a position t in a depth direction from the surface;
an area ratio of the austenite phase at a position t/10 in a depth direction from the surface is higher than an area ratio of the austenite phase at a position t/2 in a depth direction from the surface;
The duplex stainless steel of claim 1. The area ratio of the austenite phase at a position t/10 in the depth direction from the surface is 90% or more.
The duplex stainless steel according to claim 1 or 2. the N content at a position of t/10 in a depth direction from the surface is higher than the N content at a center position in the thickness direction;
The duplex stainless steel according to any one of claims 1 to 3. The chemical composition at the center position in the thickness direction is, in mass%,
C: 0.001 to 0.060%,
Si: 0.01 to 1.50%,
Mn: 0.1 to 6.0%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 19.0 to 25.0%,
Ni: 1.0 to 6.0%,
N: 0.050 to 0.25%,
Al: 0.003 to 0.050%,
Ti: 0 to 0.050%,
Nb: 0 to 0.15%,
Mo: 0 to 2.0%,
Cu: 0 to 3.0%,
W: 0 to 2.0%,
Mg: 0 to 0.0050%,
Ca: 0 to 0.0050%,
REM: 0 to 0.30%,
B: 0 to 0.0040%,
The balance is 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, in mass%,
Ti: 0.01 to 0.050%,
Nb: 0.02 to 0.15%,
Mo: 0.05 to 4.0%,
Cu: 0.05 to 4.0%,
W: 0.05 to 4.0%,
Mg: 0.0002 to 0.0050%,
Ca: 0.0002 to 0.0050%,
REM: 0.005 to 0.30%, and
B: 0.0003 to 0.0040%,
Contains one or more selected from
The duplex stainless steel of claim 5. A duplex stainless steel according to any one of claims 1 to 6, which is cold rolled,
The N content at a position at t/10 in a depth direction from the surface is 3.0 times or more as compared with the N content at the center position in the thickness direction,
The compressive residual stress on the surface is 400 MPa or more;
Duplex stainless steel. A method for producing the hot-rolled duplex stainless steel according to any one of claims 1 to 4, comprising the steps of:
A duplex stainless steel having a metal structure in which the area ratio of the austenite phase at the center position in the thickness direction is 35% or more and less than 65%, and the N content at the center position in the thickness direction is 0.050% or more, in mass%,
(a) heating at 1200 to 1300 ° C. for 5 h or more in an atmosphere having an O ₂ concentration of 2 to 10 vol% and performing hot rolling;
(b) a step of performing shot blasting;
(c) pickling the steel sheet by spraying an aqueous solution containing 1 to 10% fluoride and 2 to 20% nitric acid from a nozzle;
Manufacturing method of duplex stainless steel. The chemical composition at the center position in the thickness direction is, in mass%,
C: 0.001 to 0.060%,
Si: 0.01 to 1.50%,
Mn: 0.1 to 6.0%,
P: 0.050% or less,
S: 0.0050% or less,
Cr: 19.0 to 25.0%,
Ni: 1.0 to 6.0%,
N: 0.050 to 0.25%,
Al: 0.003 to 0.050%,
Ti: 0 to 0.050%,
Nb: 0 to 0.15%,
Mo: 0 to 2.0%,
Cu: 0 to 3.0%,
W: 0 to 2.0%,
Mg: 0 to 0.0050%,
Ca: 0 to 0.0050%,
REM: 0 to 0.30%,
B: 0 to 0.0040%,
The balance is Fe and impurities.
The method for producing the duplex stainless steel according to claim 8. The chemical composition at the center position in the thickness direction is, in mass%,
Ti: 0.01 to 0.050%,
Nb: 0.02 to 0.15%,
Mo: 0.05 to 4.0%,
Cu: 0.05 to 4.0%,
W: 0.05 to 4.0%,
Mg: 0.0002 to 0.0050%,
Ca: 0.0002 to 0.0050%,
REM: 0.005 to 0.30%, and
B: 0.0003 to 0.0040%,
Contains one or more selected from
The method for producing the duplex stainless steel according to claim 9. In the method for producing a duplex stainless steel according to any one of claims 8 to 10,
After the step (c), the duplex stainless steel is subjected to cold rolling;
(d) A step of heating and holding at 1000 to 1200 ° C. for 10 min or less in an atmosphere containing N ₂ is further carried out to produce the cold-rolled duplex stainless steel according to claim 7 .
Manufacturing method of duplex stainless steel.