JP7765692B2 - Austenitic stainless steel and corrosion-resistant components - Google Patents

Description

The present invention relates to austenitic stainless steel and corrosion-resistant components.

SUS316L, a type of austenitic stainless steel, has good corrosion resistance in corrosive environments such as seawater and saltwater, and also has excellent workability and manufacturability, so it is used in a variety of applications such as household goods, building materials, and automobile parts.
However, SUS316L has the drawback of increasing product prices because it contains expensive elements such as Ni and Mo. Therefore, there is a demand for inexpensive austenitic stainless steels that have corrosion resistance at the same level as SUS316L and are excellent in workability and manufacturability.

Patent Document 1 proposes an austenitic stainless steel with improved corrosion resistance while minimizing the use of expensive elements such as Mo. Its surface has three layers, from the surface to the inner layer: an outermost layer containing Fe and Cr nitrides in the gamma phase; a layer consisting of an S phase in which N is supersaturated with a solid solution of 10% or more by mass; and a carbon-enriched layer.

Japanese Patent Application Laid-Open No. 2016-188417

However, the austenitic stainless steel described in Patent Document 1 requires complex surface treatment to form a specified layer on the surface, which results in low manufacturing efficiency and additional costs due to the surface treatment.

The present invention was made to solve the above-mentioned problems, and aims to provide an inexpensive austenitic stainless steel that has corrosion resistance at the same level as SUS316L and is excellent in workability and manufacturability, as well as corrosion-resistant components made from this austenitic stainless steel.

The inventors conducted extensive research into the composition of austenitic stainless steels based on the composition of SUS316L. To improve the manufacturability of austenitic stainless steels, it is desirable to increase the content of the delta ferrite phase (hereinafter referred to as the "delta phase"), which has a high solid solubility of phosphorus and sulfur in the solidified state. However, as the delta phase content increases, cracking becomes more likely due to the difference in strength with the austenite phase (hereinafter referred to as the "gamma phase"). Therefore, the delta phase content must be controlled within an appropriate range, but reducing the content of expensive nickel increases the delta phase content. Therefore, to ensure a balance between the delta and gamma phases, it is necessary to add gamma-forming elements (e.g., manganese, copper, and nitrogen). However, increasing the content of manganese or copper reduces corrosion resistance, while increasing the content of nitrogen increases hardness and reduces workability. In light of these issues, the inventors optimized the composition of austenitic stainless steels and discovered that a specific composition could solve the above problems, leading to the completion of the present invention.

That is, the present invention provides an austenitic stainless steel containing, by mass, 0.010 to 0.100% C, 2.0% or less Si, 3.0% or less Mn, 0.035% or less P, 0.030% or less S, 6.0 to 14.0% Ni, 16.0% or more and less than 20.0% Cr, less than 0.01% Mo, 0.05 to 3.0% Cu, more than 0% and less than 0.20% Al, 0.100 to 0.250% N , and 0.001 to 0.004% B , with the balance being Fe and impurities, and having an A value, expressed by the following formula (1), of -2 to 5.5.
A=3(Cr+Mo)+4.5Si-2.8Ni-1.4(Mn+Cu)-84(C+N)-19.8 (1)
In the formula, Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the content (mass%) of each element.

The present invention also relates to a corrosion-resistant member containing the above-mentioned austenitic stainless steel.

The present invention provides an inexpensive austenitic stainless steel that has corrosion resistance equivalent to that of SUS316L and is excellent in workability and manufacturability, as well as corrosion-resistant components made from this austenitic stainless steel.

The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention.
In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

An austenitic stainless steel according to an embodiment of the present invention contains C: 0.010 to 0.100%, Si: 2.0% or less, Mn: 3.0% or less, P: 0.035% or less, S: 0.030% or less, Ni: 6.0 to 14.0%, Cr: 16.0% or more but less than 20.0%, Mo: 3.0% or less, Cu: 0.05 to 3.0%, Al: 0.20% or less, N: 0.100 to 0.250%, and the balance being Fe and impurities.
In this specification, the term "impurities" refers to components that are mixed in during the industrial production of austenitic stainless steel due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

Moreover, the austenitic stainless steel according to the embodiment of the present invention may further contain B: 0.001 to 0.004%.
Furthermore, the austenitic stainless steel according to the embodiment of the present invention may further contain one or more selected from Ca: 0.0001% to 0.10%, Mg: 0.0001% to 0.10%, and REM: 0.0001% to 0.10%.
Furthermore, the austenitic stainless steel according to the embodiment of the present invention may further contain one or more selected from Ti: 0.001% to 1.0%, Nb: 0.001% to 1.0%, V: 0.001% to 1.0%, Zr: 0.001% to 1.0%, W: 0.001 to 1.0%, Co: 0.001 to 1.0%, Hf: 0.001 to 1.0%, Ta: 0.001 to 1.0%, and Sn: 0.001 to 0.10%.
Each component will be described in detail below.

<C: 0.010-0.100%>
If the C content is too high, the corrosion resistance of the austenitic stainless steel will decrease. Therefore, the upper limit of the C content is controlled to 0.100%, preferably 0.095%, and more preferably 0.090%. On the other hand, if the C content is too low, refining costs will increase. Therefore, the lower limit of the C content is controlled to 0.010%, preferably 0.015%, and more preferably 0.020%.
In this specification, "corrosion resistance" means corrosion resistance in a corrosive environment containing NaCl, such as seawater or salt water.

<Si: 2.0% or less>
If the Si content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the Si content is controlled to 2.0%, preferably 1.98%, and more preferably 1.95%. On the other hand, the lower limit of the Si content is not particularly limited, but is preferably 0.01%, more preferably 0.05%, and even more preferably 0.1%.

<Mn: 3.0% or less>
Mn is an element that forms the austenite phase (γ phase). If the Mn content is too high, the corrosion resistance of the austenitic stainless steel will decrease. Therefore, the upper limit of the Mn content is controlled to 3.0%, preferably 2.5%, and more preferably 2.0%. On the other hand, the lower limit of the Mn content is not particularly limited, but is preferably 0.01%, more preferably 0.05%, and even more preferably 0.1%.

<P: 0.035% or less>
If the P content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the P content is controlled to 0.035%, preferably 0.034%, and more preferably 0.033%. On the other hand, the lower limit of the P content is not particularly limited, but is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.

<S: 0.030% or less>
If the S content is too high, the manufacturability of austenitic stainless steel will decrease. Therefore, the upper limit of the S content is controlled to 0.030%, preferably 0.025%, and more preferably 0.020%. On the other hand, the lower limit of the S content is not particularly limited, but is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

<Ni: 6.0 to 14.0%>
Ni, like Mn, is an austenite phase (γ phase) forming element. Because Ni is expensive, if the Ni content is too high, the manufacturing cost increases. Therefore, the upper limit of the Ni content is controlled to 14.0%, preferably 13.8%, and more preferably 13.5%. On the other hand, if the Ni content is too low, the workability of the austenitic stainless steel decreases. Therefore, the lower limit of the Ni content is controlled to 6.0%, preferably 6.05%.

<Cr: 16.0% or more and less than 20.0%>
If the Cr content is too high, the formation of intermetallic compounds (σ phase) is promoted, which reduces the workability of the austenitic stainless steel. Therefore, the Cr content is controlled to less than 20.0%, preferably 19.8% or less. On the other hand, if the Cr content is too low, sufficient corrosion resistance cannot be obtained. Therefore, the lower limit of the Cr content is controlled to 16.0%, preferably 16.2%.

<Mo: 3.0% or less>
Since Mo is expensive, if the Mo content is too high, the production cost will increase. Therefore, the upper limit of the Mo content is controlled to 3.0%, preferably 2.0%, and more preferably 1.0%. On the other hand, the lower limit of the Mo content is not particularly limited, but is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.

<Cu: 0.05-3.0%>
If the Cu content is too high, the corrosion resistance of the austenitic stainless steel will decrease. Therefore, the upper limit of the Cu content is controlled to 3.0%, preferably 2.0%, and more preferably 1.0%. On the other hand, if the Cu content is too low, the workability of the austenitic stainless steel will decrease. Therefore, the lower limit of Cu is controlled to 0.05%, preferably 0.10%, and more preferably 0.15%.

<Al: 0.20% or less>
If the Al content is too high, the amount of inclusions generated increases, degrading the quality. Therefore, the upper limit of the Al content is controlled to 0.20%, preferably 0.10%, and more preferably 0.05%. On the other hand, the lower limit of the Al content is not particularly limited, but is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

<N: 0.100-0.250%>
If the N content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the N content is controlled to 0.250%, preferably 0.230%, and more preferably 0.220%. On the other hand, if the N content is too low, the austenitic stainless steel will not have sufficient corrosion resistance. Therefore, the lower limit of the N content is controlled to 0.100%, and preferably 0.110%.

<B: 0.001-0.004%>
If the B content is too high, the corrosion resistance of the austenitic stainless steel will decrease. Therefore, the upper limit of the B content is controlled to 0.004%, preferably 0.003%. On the other hand, if the B content is too low, the manufacturability of the austenitic stainless steel will decrease. Therefore, the lower limit of the B content is controlled to 0.001%, preferably 0.002%.

<Ca: 0.0001% to 0.10%>
Ca is an element added as needed to improve hot workability. From the viewpoint of obtaining the effects of Ca, the lower limit of the Ca content is controlled to 0.0001%, preferably 0.0005%, and more preferably 0.001%. On the other hand, if the Ca content is too high, the amount of inclusions generated increases, degrading the quality. Therefore, the upper limit of the Ca content is controlled to 0.10%, preferably 0.05%, and more preferably 0.01%.

<Mg: 0.0001% to 0.10%>
Mg is an element added as needed to improve hot workability. From the viewpoint of obtaining the effects of Mg, the lower limit of the Mg content is controlled to 0.0001%, preferably 0.0005%, and more preferably 0.001%. On the other hand, if the Mg content is too high, the amount of inclusions generated increases, resulting in a decrease in quality. Therefore, the upper limit of the Mg content is controlled to 0.10%, preferably 0.05%, and more preferably 0.01%.

<REM: 0.0001% to 0.10%>
REM is an element added as needed to improve hot workability. From the viewpoint of obtaining the effects of REM, the lower limit of the REM content is controlled to 0.0001%, preferably 0.0005%, and more preferably 0.001%. Furthermore, since REM is expensive, if the REM content is too high, the manufacturing cost will increase. Therefore, the upper limit of the REM content is controlled to 0.10%, preferably 0.05%, and more preferably 0.01%.

<Ti: 0.001% to 1.0%>
Ti is an element that is added as needed to fix C in steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of Ti, the lower limit of the Ti content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Ti content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the Ti content is controlled to 1.0%, preferably 0.8%, and more preferably 0.5%.

<Nb: 0.001% to 1.0%>
Nb is an element that is added as needed to fix C in steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of Nb, the lower limit of the Nb content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Nb content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the Nb content is controlled to 1.0%, preferably 0.8%, and more preferably 0.5%.

<V: 0.001% to 1.0%>
V is an element that is added as needed to fix C in the steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of V, the lower limit of the V content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the V content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the V content is controlled to 1.0%, preferably 0.8%, and more preferably 0.5%.

<Zr: 0.001% to 1.0%>
Zr is an element that is added as needed to fix C in steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of Zr, the lower limit of the Zr content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Zr content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the Zr content is controlled to 1.0%, preferably 0.8%, and more preferably 0.5%.

<W: 0.001-1.0%>
W is an element added as needed to improve high-temperature strength and corrosion resistance. From the viewpoint of obtaining the effects of W, the lower limit of the W content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the W content is too high, the workability of the austenitic stainless steel decreases and the manufacturing cost increases. Therefore, the upper limit of the W content is controlled to 1.0%, preferably 0.8%, and more preferably 0.5%.

<Co:0.001~1.0%>
Co is an element added as needed to improve corrosion resistance. From the viewpoint of obtaining the effects of Co, the lower limit of the Co content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Co content is too high, the workability of the austenitic stainless steel decreases and the manufacturing cost increases. Therefore, the upper limit of the Co content is controlled to 1.0%, preferably 0.8%, and more preferably 0.5%.

<Hf: 0.001-1.0%>
Hf is an element added as needed to fix C in steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of Hf, the lower limit of the Hf content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Hf content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the Hf content is controlled to 1.0%, preferably 0.8%, and more preferably 0.5%.

<Ta: 0.001 to 1.0%>
Ta is an element that is added as needed to fix C in steel and improve intergranular corrosion resistance. From the viewpoint of obtaining the effects of Ta, the lower limit of the Ta content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Ta content is too high, the workability of the austenitic stainless steel will decrease. Therefore, the upper limit of the Ta content is controlled to 1.0%, preferably 0.8%, and more preferably 0.5%.

<Sn: 0.001 to 0.10%>
Sn is an element that is added as needed to improve corrosion resistance. From the viewpoint of obtaining the effects of Sn, the lower limit of the Sn content is controlled to 0.001%, preferably 0.005%, and more preferably 0.010%. Furthermore, if the Sn content is too high, the manufacturability of austenitic stainless steel decreases. Therefore, the upper limit of the Sn content is controlled to 0.10%, preferably 0.05%, and more preferably 0.01%.

In the austenitic stainless steel according to the embodiment of the present invention, the A value represented by the following formula (1) is −2 to 5.5, preferably −1.8 to 5.3, and more preferably −1.7 to 5.0.
A=3(Cr+Mo)+4.5Si-2.8Ni-1.4(Mn+Cu)-84(C+N)-19.8 (1)
In the formula, Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the content (mass%) of each element.
By controlling the A value within the above range, it is possible to achieve both manufacturability and workability of the austenitic stainless steel. In particular, by setting the A value to -2 or more, oxidation of the grain boundaries can be suppressed, making it difficult for oxide scale to remain. Furthermore, by setting the A value to 5.5 or less, edge breakage during the production of the austenitic stainless steel can be suppressed, and elongation that allows the austenitic stainless steel to be worked into various shapes can be ensured during working.

The austenitic stainless steel according to an embodiment of the present invention may be in any shape, as long as it has the characteristics described above. For example, austenitic stainless steel can be made into various sheet materials, such as hot-rolled sheet, hot-rolled annealed sheet, cold-rolled sheet, and cold-rolled annealed sheet, but cold-rolled annealed sheet is preferred from the standpoint of manufacturability.

The austenitic stainless steel according to the embodiment of the present invention can be produced by using a method known in the art, except for producing a stainless steel having the above-described composition. Specifically, when the austenitic stainless steel is a cold-rolled annealed sheet, it can be produced as follows. First, a stainless steel having the above-described composition is produced by melting and forging or casting, and then hot-rolling to obtain a hot-rolled sheet. Next, the hot-rolled sheet is appropriately annealed, pickled, and cold-rolled to obtain a cold-rolled sheet. Next, the cold-rolled sheet is appropriately annealed and pickled to obtain a cold-rolled annealed sheet.
The conditions for each step are not particularly limited and may be adjusted appropriately depending on the composition of the stainless steel.

The austenitic stainless steel according to an embodiment of the present invention, which has the above-mentioned characteristics, has good manufacturability and workability, and its corrosion resistance in corrosive environments containing NaCl, such as seawater or saltwater, is at the same level as SUS316L, making it suitable for use as a material for corrosion-resistant components used in such corrosive environments. Furthermore, this austenitic stainless steel can reduce manufacturing costs by reducing the content of expensive elements such as Mo and Ni, making it suitable for use in various components that use SUS316L.

The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples. Examples 1 , 3, 4 and 5 are for reference only.

30 kg of stainless steel having the composition shown in Table 1 was vacuum melted and forged into a 30 mm thick plate, which was then heated at 1230°C for 2 hours and hot rolled to a 4 mm thick plate to obtain a hot-rolled plate. The hot-rolled plate was then annealed and pickled to obtain a hot-rolled annealed plate, which was then cold-rolled to obtain a cold-rolled plate. The cold-rolled plate was then annealed, water-cooled, and pickled to obtain a cold-rolled annealed plate (austenitic stainless steel). Note that Comparative Example 8 is a steel type equivalent to SUS316L, which contains large amounts of expensive Ni and Mo.

The following evaluations were carried out on the cold-rolled and annealed sheets obtained above, as well as their intermediate materials, the hot-rolled and hot-rolled annealed sheets.

<Manufacturability>
The manufacturability was evaluated by measuring the maximum edge cut length of the hot-rolled sheet and observing the oxide scale on the surface of the hot-rolled and annealed sheet.
The maximum edge cut length of the hot-rolled sheet was determined by measuring the edge cut length of the hot-rolled sheet and taking the maximum value as the maximum edge cut length.
The surface of the hot-rolled and annealed sheet was scraped off by 20 μm in the thickness direction, and the presence or absence of black oxide scale on the surface was evaluated.
In the evaluation of manufacturability, those in which the maximum edge cut length was 3 mm or less and no oxide scale was observed were rated as pass (◯), and those in which the maximum edge cut length exceeded 3 mm and/or oxide scale was observed were rated as fail (×).

<Workability>
The workability was evaluated using a cold-rolled annealed sheet obtained by cold-rolling to a thickness of 0.3 mm, followed by annealing, water cooling, and pickling. The workability was evaluated in accordance with the tensile test method specified in JIS Z2241:2011. Specifically, a No. 13B test piece was cut out from the center of the width direction of the cold-rolled annealed sheet, and a tensile test was performed at a tension speed of 20 mm/min to measure the elongation (%). In this evaluation, a specimen that achieved 50% elongation, which allows workability into various shapes, was evaluated as pass (◯), and a specimen that achieved less than 50% elongation was evaluated as fail (×).

<Corrosion resistance>
The corrosion resistance was evaluated using a cold-rolled annealed sheet obtained by cold rolling to a thickness of 1.0 mm, annealing, water cooling, and pickling. The corrosion resistance was measured in accordance with JIS G0577:2014. A 15 mm x 20 mm test piece was cut from the center of the cold-rolled annealed sheet in the width direction, and then wet-polished with #600. Next, the electrode surface (exposed portion) of the test piece was insulated with silicone resin to obtain a test piece for pitting potential measurement. Next, the test piece for pitting potential measurement was immersed in a 3.5% NaCl solution at 30 °C that had been thoroughly degassed with Ar, and potentiodynamic anodic polarization was performed at 20 mV/min from the natural potential to measure the pitting potential. The pitting potential was defined as the potential when a current of 100 μA/cm ² flowed. In this evaluation, the pitting potential was 0.4 V vs. Those that were equal to or higher than 0.4 V Ag/AgCl (hereinafter, all potentials are based on Ag/AgCl) were evaluated as pass (◯), and those that were less than 0.4 V were evaluated as fail (×).

The results of each of the above evaluations are shown in Table 2.

As shown in Table 2, the austenitic stainless steels of Examples 1 , 2 and 4 to 6 satisfied the prescribed composition and A value, and therefore were excellent in manufacturability, workability and corrosion resistance.
On the other hand, the austenitic stainless steels of Comparative Examples 1 and 2 had insufficient manufacturability because their A values were outside the range. The austenitic stainless steel of Comparative Example 3 had insufficient workability because its Ni content was too low. The austenitic stainless steel of Comparative Example 4 had insufficient corrosion resistance because its Cr content was too low. The austenitic stainless steel of Comparative Example 5 had insufficient workability and manufacturability because its N content was too high and its A value was too low. The austenitic stainless steel of Comparative Example 6 had insufficient corrosion resistance and manufacturability because its Cr content was too low and its A value was too low. The austenitic stainless steel of Comparative Example 7 had insufficient corrosion resistance and manufacturability because its Si content was too high and its Cr content was too low. The austenitic stainless steel (SUS316L) of Comparative Example 8 had good manufacturability, workability, and corrosion resistance, but contained large amounts of expensive Ni and Mo, which increased the manufacturing cost.

As can be seen from the above results, the present invention makes it possible to provide an inexpensive austenitic stainless steel that has corrosion resistance equivalent to that of SUS316L and is excellent in workability and manufacturability, as well as corrosion-resistant components made from this austenitic stainless steel.

Claims (4)

An austenitic stainless steel containing, on a mass basis, 0.010 to 0.100% C, 2.0% or less Si, 3.0% or less Mn, 0.035% or less P, 0.030% or less S, 6.0 to 14.0% Ni, 16.0% or more and less than 20.0% Cr, less than 0.01% Mo, 0.05 to 3.0% Cu, more than 0% and less than 0.20% Al, 0.100 to 0.250% N, and 0.001 to 0.004% B, with the balance being Fe and impurities, and having an A value, expressed by the following formula (1), of -2 to 5.5.
A=3(Cr+Mo)+4.5Si-2.8Ni-1.4(Mn+Cu)-84(C+N)-19.8 (1)
In the formula, Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the content (mass%) of each element. The austenitic stainless steel according to claim 1, further comprising, on a mass basis , 0.0001% to 0.10 % Mg. The austenitic stainless steel according to claim 1 or 2, further comprising, by mass, one or more selected from Ti: 0.001% to 1.0%, Nb: 0.001% to 1.0%, V: 0.001% to 1.0%, Zr: 0.001% to 1.0%, W: 0.001 to 1.0%, Co: 0.001 to 1.0%, Hf: 0.001 to 1.0%, Ta: 0.001 to 1.0%, and Sn: 0.001 to 0.10%. A corrosion-resistant component comprising the austenitic stainless steel according to any one of claims 1 to 3.