The present disclosure relates to a high-strength stainless steel and a method for manufacturing the same.

Recently, due to increased Ni prices and increasing price volatility, demand for low-Ni austenitic stainless steel is increasing, but it is difficult to sufficiently increase the yield strength of low-Ni austenitic stainless steel.

Furthermore, in order to lower the expensive element Ni content, attempts have been made to substitute Ni with austenite stabilizing elements such as Mn and N; however, this has led to inferior corrosion resistance due to the generation of MnS
To increase the strength of austenitic stainless steel, methods such as work hardening through temper rolling or addition of a large amount of interstitial elements such as C and N are utilized. However, tempered material has low usability due to inferior elongation, high C addition leads to poor weldability, and high N addition decreases hot workability.

Meanwhile, conventional methods for improving strength and elongation utilizing an ultra-fine grain refinement mechanism have limitations in terms of strength improvement compared to tempered material.

In Patent Document 0001, an austenitic stainless fine-grain steel having excellent strength and ductility is disclosed. However, Patent Document 0001 does not disclose differences in grain refinement due to variations in austenite phase stability depending on composition, nor does it disclose changes in hot workability according to grain refinement.

Patent Document 0001: Korean Patent Application No. 10-2007-0067905 (Publication Date: June 29, 2007).

An objective of the present invention is to provide a low Ni stainless steel having high yield strength and elongation through composition control and grain refinement, and a method for manufacturing the same.

A high-strength stainless steel according to an example of the present disclosure includes, in wt%: 0.01% to 0.10% of carbon (C); 0.10% to 1.00% of silicon (Si); more than 0% and less than 0.050% of phosphorus (P); more than 0% and less than 0.030% of sulfur (S); 3.0% to 8.0% of manganese (Mn); 1.0% to 5.0% of nickel (Ni); 15.0% to 18.0% of chromium (Cr); 0.1% to 2.0% of copper (Cu); 0.10% to 0.20% of nitrogen (N); and the balance of Fe and unavoidable impurities, wherein in a microstructure, a volume fraction of residual martensite phase is 2.0% to 8.0%.

The high-strength stainless steel may have a Stability Index (SI) of 4.20 or less, represented by the following Formula (1).$$0.02\left[\mathrm{Si}\right]+0.24\left[\mathrm{Mn}\right]+0.08\left[\mathrm{Cr}\right]+0.15\left[\mathrm{Ni}\right]+0.1\left[\mathrm{Cu}\right]+2.28\left(\left[\mathrm{C}\right]+\left[\mathrm{N}\right]\right).$$

In Formula (1), Si, Mn, Cr, Ni, Cu, C, and N may represent the content (wt%) of each element.

The high-strength stainless steel may have a Reversion Index (RI) of 0.55 or more, represented by the following Formula (2):
$$-0.04\left[\mathrm{Si}\right]+0.06\left[\mathrm{Mn}\right]-0.01\left[\mathrm{Cr}\right]+0.05\left[\mathrm{Ni}\right]+0.04\left[\mathrm{Cu}\right]+1.2\left(\left[\mathrm{C}\right]+\left[\mathrm{N}\right]\right).$$

In Formula (2), Si, Mn, Cr, Ni, Cu, C, and N may represent the content (wt%) of each element.

An average grain diameter of an austenite phase at a thickness center portion may be 5.0 µm or less.

The yield strength may be 800 MPa or more.

The elongation may be 30% or more.

The thickness may be 0.5 mm to 3.0 mm.

A method for manufacturing a high-strength stainless steel according to an example includes: preparing an ingot including, in wt%, 0.01% to 0.10% of C, 0.10% to 1.00% of Si, more than 0% and less than 0.050% of P, more than 0% and less than 0.030% of S, 3.0% to 8.0% of Mn, 1.0% to 5.0% of Ni, 15.0% to 18.0% of Cr, 0.1% to 2.0% of Cu, 0.10% to 0.20% of N, and the balance of Fe and unavoidable impurities; reheating the ingot and then hot rolling the reheated ingot to produce a hot-rolled material; performing solution heat treatment on the hot-rolled material to produce a solution heat-treated hot-rolled material; performing cold reduction on the solution heat-treated hot-rolled material to produce a tempered material; and annealing the tempered material at a temperature greater than 700°C and less than 850°C.

The ingot may have a Stability Index (SI) of 4.20 or less, represented by the following Formula (1).$$0.02\left[\mathrm{Si}\right]+0.24\left[\mathrm{Mn}\right]+0.08\left[\mathrm{Cr}\right]+0.15\left[\mathrm{Ni}\right]+0.1\left[\mathrm{Cu}\right]+2.28\left(\left[\mathrm{C}\right]+\left[\mathrm{N}\right]\right).$$

In Formula (1), Si, Mn, Cr, Ni, Cu, C, and N may represent the content (wt%) of each element.

The ingot may have a Reversion Index (RI) of 0.55 or more, represented by the following Formula (2).$$-0.04\left[\mathrm{Si}\right]+0.06\left[\mathrm{Mn}\right]-0.01\left[\mathrm{Cr}\right]+0.05\left[\mathrm{Ni}\right]+0.04\left[\mathrm{Cu}\right]+1.2\left(\left[\mathrm{C}\right]+\left[\mathrm{N}\right]\right).$$

In Formula (2), Si, Mn, Cr, Ni, Cu, C, and N may represent the content (wt%) of each element.

The reheating may be performed at 1150°C to 1350°C for 1hour to 3 hours.

The solution heat treatment may be performed at 1000°C to 1200°C for 1 minutes to 30 minutes.

The cold reduction may be performed with a thickness reduction ratio of 60% to 80%.

According to an example of the present disclosure, a stainless steel and a method for manufacturing the same that achieve high yield strength and elongation by controlling phase stability and microstructure can be provided.

FIG. 1 is an image of a microstructure of a high-strength stainless steel, according to an example of the present disclosure, taken with a Scanning Electron Microscope (SEM).

Hereinafter, examples of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the examples shown herein but may be embodied in other forms. In order to make the description of the present invention clear, unrelated parts are not shown and, the sizes of components are exaggerated for clarity.

Throughout the specification, when a part is referred to as "including", "comprising" and/or "having" a certain element, it is understood that, unless expressed otherwise, the description does not preclude the presence or addition of one or more elements.

The singular form of a noun corresponding to an item may include one or a plurality of the items unless clearly indicated otherwise in a related context.

Hereinafter, the reason for numerically limiting the alloy element contents in the embodiments of the present invention will be described. Unless otherwise specified, the units thereof are expressed in weight percent (wt%).

A high-strength stainless steel according to an example includes, in wt%: 0.01% to 0.10% of carbon (C); 0.10% to 1.00% of silicon (Si); more than 0% and less than 0.050% of phosphorus (P); more than 0% and less than 0.030% of sulfur (S); 3.0% to 8.0% of manganese (Mn); 1.0% to 5.0% of nickel (Ni); 15.0% to 18.0% of chromium (Cr); 0.1% to 2.0% of copper (Cu); 0.10% to 0.20% of nitrogen (N); and the balance of Fe and unavoidable impurities.

The content of C (carbon) may be 0.01% to 0.10%.

C is a highly effective and inexpensive element for stabilizing austenite. C, as an interstitial element, contributes to strength improvement by the solid solution strengthening effect. Considering this, C may be added in an amount of 0.01% or more. However, an excessive C content may cause sensitization due to precipitation of carbides such as Cr₂₃C₆ at grain boundaries in a heat-affected zone after welding, which leads to degradation in ductility, toughness, and corrosion resistance. Considering this, the upper limit of the C content may be limited to 0.10%. Preferably, the C content may be 0.02% to 0.10%, and more preferably, 0.02% to 0.08%.

The content of Si (silicon) may be 0.10% to 1.00%.

Si acts as a deoxidizer during the steelmaking process and is an effective element for improving corrosion resistance. Considering this, Si may be added in an amount of 0.10% or more. However, an excessive Si content may cause delta-ferrite phases to form due to peritectic reactions during casting, which leads to a decrease in hot workability. Considering this, the upper limit of the Si content may be limited to 1.00%. Preferably, the Si content may be 0.30 to 1.00%, and more preferably, 0.30 to 0.50%.

The content of P may be more than 0% and less than 0.050%.

P is an impurity inevitably contained in steel and is an element that degrades corrosion resistance and hot workability. However, controlling the P content to an extremely low level may lead to an increase in process costs. Considering this, the P content may be more than 0% and less than 0.050%. Preferably, the P content may be more than 0% and 0.003% or less.

The content of S (sulfur) may be more than 0% and less than 0.030%.

S, like P, is an impurity inevitably contained in steel, and is an element that degrades corrosion resistance and hot workability. However, controlling the S content to an extremely low level may lead to an increase in process costs. Considering this, the S content may be more than 0% and less than 0.030%. Preferably, the S content may be more than 0% and 0.005% or less.

The content of Mn (manganese) may be 3.0% to 8.0%.

Mn is an effective and inexpensive element for increasing austenite phase stability against deformation-induced martensite. Considering this, Mn may be added in an amount of 3.0% or more. However, an excessive Mn content may lead to an increase in inclusions (MnS), which may degrade the corrosion resistance and hot workability of the steel material. Considering this, the upper limit of the Mn content may be limited to 8.0%. Preferably, the Mn content may be 3.7% to 8.0%, and more preferably, 3.7% to 7.9%.

The content of Ni (nickel) may be 1.0% to 5.0%.

Ni is a strong element for stabilizing the austenite phase. Also, Ni is effective in suppressing thermally-induced and deformation-induced martensite transformation, preventing toughness degradation at cryogenic temperatures. Also, the addition of Ni may facilitate hot workability and cold workability. Considering this, Ni may be added in an amount of 1.0% or more. However, an excessive Ni content may reduce grain refinement. Also, an excessive Ni content may cause an increase in raw material costs. Considering this, the upper limit of the Ni content may be limited to 5.0%. Preferably, the Ni content may be 2.0% to 5.0%, and more preferably, 2.0% to 3.6%.

The content of Cr (chromium) may be 15.0% to 18.0%.

Cr is an essential element for ensuring corrosion resistance and phase stability. Considering this, Cr may be added in an amount of 15.0% or more. However, an excessive Cr content may lead to the formation of a delta-ferrite phase by peritectic reaction, thereby lowering hot workability. Considering this, the upper limit of the Cr content may be limited to 18.0%. Preferably, the Cr content may be 16.5% to 18.0%.

The content of Cu (copper) may be 0.1% to 2.0%.

Cu is an effective element for stabilizing the austenite phase. Also, Cu is an effective element for suppressing thermally-induced and deformation-induced martensite transformation. In consideration of this, Cu may be added in an amount of 0.1% or more. However, an excessive Cu content may degrade hot workability due to solidification segregation of Cu. In consideration of this, the upper limit of Cu content may be limited to 2.0%. Preferably, the Cu content may be 0.9% to 2.0%, and more preferably, 0.9% to 1.8%.

The content of N (nitrogen) may be 0.10% to 0.20%.

N is a highly effective and inexpensive element for stabilizing austenite phase. In addition, N is an effective element for increasing strength through solid solution strengthening and improving corrosion resistance. Considering this, N may be added in an amount of 0.10% or more. However, an excessive N content may degrade hot workability. Considering this, an upper limit of the N content may be limited to 0.20%. Preferably, the N content may be 0.15% to 0.20%, and more preferably, 0.15 to 0.18%.

The remainder is iron (Fe). However, since unintended impurities may inevitably be introduced from raw materials or the surrounding environment during a typical manufacturing process, this may not be excluded. Since such impurities may be well known to those skilled in the art during a typical manufacturing process, details thereof are not described in this specification.

Generally, an austenitic stainless steel may develop a deformation-induced martensite phase (ε, α'-martensite) during cold rolling. Deformation-induced phases tend to develop differently depending on the stability of the austenite phase. An austenitic stainless steel having a low phase stability may develop ε-martensite bands at the initial stage of deformation, and as the amount of deformation increases, α'-martensite may be generated from intersections within the bands.

To enhance cost competitiveness by lowering expensive Ni, phase stability needs to be controlled using austenite phase stabilizing elements other than Ni (e.g., Mn, Cu, C, N, etc.). For this purpose, it is required to control the change in free energy (△G_γ-α) value from the austenite phase to the martensite phase at room temperature.

Meanwhile, when the rolled tempered material is annealed, a reversion transformation from a deformation-induced martensite phase to an austenite phase may occur. The reversion transformation process may be largely classified into diffusional reversion and shear reversion. The reversion transformation process may proceed depending on the free energy change (△G_α-γ) from the martensite phase to the austenite phase during the annealing. Generally, martensite shear reversion requires a greater free energy change (△G_α-γ) than diffusional reversion. Therefore, the lower the free energy change from the martensite to the austenite phase, the more martensite remains in the final cold-rolled annealed material after the annealing, and a dual-phase microstructure may be realized.

In the present disclosure, for a composition system with reduced Ni, the microstructure may be controlled through alloy composition and manufacturing method to realize a two-phase microstructure in which a volume fraction of residual martensite in an austenite phase matrix is 2.0% to 8.0%. Thereby, a stainless steel having high-strength characteristics may be provided.

To realize the microstructure, the free energy change (△G_γ-α) at room temperature during cold rolling was designed to be high to promote deformation-induced martensite transformation. Additionally, to realize the microstructure, the annealing temperature and the free energy change (△G_α-γ) during cold-rolled annealing are controlled to regulate the amount of reversion recrystallization from deformation-induced martensite to austenite during cold-rolled annealing.

In the present disclosure, Formula (1) and Formula (2) were derived by calculating free energy changes of the austenite phase and the ferrite phase according to changes in alloy element content and temperature. Through this, the phase stability of the austenite phase and the martensite phase may be controlled, and specifically, the free energy change (△G_γ-α) value at a specific temperature may be controlled.

In the present disclosure, the free energy change (△G_γ-α) value at a predetermined temperature may be calculated using a thermodynamic analysis program (Thermo-Calc. TCFE 6.0) thermodynamic database.

A high-strength stainless steel according to an example may have a thermodynamic free energy change (△G_γ-α, 25°C) value at 25°C of -1.65 kJ/mol or less. The lower limit is not limited, but is, for example, -3.00 kJ/mol, -2.50 kJ/mol, or -2.20 kJ/mol. Within the above range, the effect of improving phase stability may be more excellent, and the effect of improving yield strength and elongation may be further enhanced.

A high-strength stainless steel according to an example may have a thermodynamic free energy change (△ G_α-γ 750°C) value at 750°C of -0.46 kJ/mol or less. The lower limit is not limited, but is, for example, -2.00 kJ/mol, -1.50 kJ/mol, or -0.80 kJ/mol. Within the above range, the effect of improving phase stability may be more excellent, and the effect of improving yield strength and elongation may be further enhanced.

A high-strength stainless steel according to an example may have a Stability Index (SI) of 4.20 or less, represented by the following Formula (1):
$$0.02\left[\mathrm{Si}\right]+0.24\left[\mathrm{Mn}\right]+0.08\left[\mathrm{Cr}\right]+0.15\left[\mathrm{Ni}\right]+0.1\left[\mathrm{Cu}\right]+2.28\left(\left[\mathrm{C}\right]+\left[\mathrm{N}\right]\right).$$

In Formula (1), Si, Mn, Cr, Ni, Cu, C, and N represent the content (wt%) of each element.

When the value of Formula (1) is 4.20 or less, the free energy change (△G_γ-α(RT)) at a room temperature during cold rolling may be -1.8 kJ/mol or less. When a cold reduction ratio is 60% or more, 25% or more of martensite transformation may occur, which may be advantageous for grain refinement.

Meanwhile, in an actual cold rolling process, due to heat generated during the rolling, the temperature of the cold-rolled steel sheet may rise to 100°C or more. Consequently, as the number of rolling passes increases, martensite transformation is suppressed, so it is advantageous for martensite transformation to largely occur in the initial rolling stage at room temperature. Residual austenite grains that have not transformed into martensite during cold rolling remain as deformed austenite grains.

The high-strength stainless steel according to an example may have a Reversion Index (RI) of 0.55 or more, represented by the following Formula (2):
$$-0.04\left[\mathrm{Si}\right]+0.06\left[\mathrm{Mn}\right]-0.01\left[\mathrm{Cr}\right]+0.05\left[\mathrm{Ni}\right]+0.04\left[\mathrm{Cu}\right]+1.2\left(\left[\mathrm{C}\right]+\left[\mathrm{N}\right]\right).$$

In Formula (2), Si, Mn, Cr, Ni, Cu, C, and N represent the content (wt%) of each element.

When the value of Formula (2) is 0.55 or more, the free energy change (△G_α-γ, 750°C) from the martensite phase to the austenite phase is -0.5 kJ/mol or less, whereby recrystallization by diffusional reversion may occur. Accordingly, the volume fraction of the residual martensite phase in the final annealed material may be 2.0% to 8.0%.

A high-strength stainless steel according to an example may have a microstructure in which a volume fraction of residual martensite is 2.0% to 8.0%.

Austenite grains remaining as a deformed structure during cold rolling complete recrystallization through a recovery stage. Grains in the recovery stage have a relatively higher dislocation density than completely recrystallized grains, and thus may contribute to an increase in strength.

A volume fraction of residual martensite greater than 8% may result in a large number of grains in the recovery stage, which decreases elongation. However, a volume fraction of residual martensite less than 2.0% may result in a low martensite content and a greater number of grains in the recrystallization stage than that of the recovery stage, which decreases strength.

By controlling the above-mentioned alloy components, Formula (1), Formula (2), microstructure, or the manufacturing method described later, the high-strength stainless steel according to an example may have an average austenite grain diameter at a thickness center portion of 5.0 µm or less.

In addition, the high-strength stainless steel according to an example may have a yield strength of 800 MPa or more and an elongation of 30% or more.

In addition, since the high-strength stainless steel according to an example has sufficient strength, the thickness of a tempered material after cold reduction may be 0.5mm to 3.0 mm.

Next, a method for manufacturing a high-strength stainless steel according to another aspect of the present disclosure will be described.

A method for manufacturing a high-strength stainless steel according to an example includes: preparing an ingot including, in wt%, 0.01% to 0.10% of C, 0.10% to 1.00% of Si, more than 0% and less than 0.050% of P, more than 0% and less than 0.030% of S, 3.0% to 8.0% of Mn, 1.0% to 5.0% of Ni, 15.0% to 18.0% of Cr, 0.1% to 2.0% of Cu, 0.10% to 0.20% of N, and the balance of Fe and unavoidable impurities; reheating the ingot and then hot rolling the reheated ingot to produce a hot-rolled material; performing solution heat treatment on the hot-rolled material to produce a solution heat-treated hot-rolled material; performing cold reduction on the solution heat-treated hot-rolled material to produce a tempered material; and annealing the tempered material at a temperature greater than 700°C and less than 850°C.

The ingot may have a Stability Index (SI) of 4.20 or less, represented by the following Formula (1):
$$0.02\left[\mathrm{Si}\right]+0.24\left[\mathrm{Mn}\right]+0.08\left[\mathrm{Cr}\right]+0.15\left[\mathrm{Ni}\right]+0.1\left[\mathrm{Cu}\right]+2.28\left(\left[\mathrm{C}\right]+\left[\mathrm{N}\right]\right).$$

In Formula (1), Si, Mn, Cr, Ni, Cu, C, and N may represent the content (wt%) of each element.

The ingot may have a Reversion Index (RI) of 0.55 or more, represented by the following Formula (2):
$$-0.04\left[\mathrm{Si}\right]+0.06\left[\mathrm{Mn}\right]-0.01\left[\mathrm{Cr}\right]+0.05\left[\mathrm{Ni}\right]+0.04\left[\mathrm{Cu}\right]+1.2\left(\left[\mathrm{C}\right]+\left[\mathrm{N}\right]\right).$$

In Formula (2), Si, Mn, Cr, Ni, Cu, C, and N may represent the content (wt%) of each element.

The reasons for limiting the component ranges of each alloy composition and the numerical values of Formula (1) and Formula (2) are as described above, and each manufacturing operation will be described in more detail below.

After preparing an ingot satisfying the alloy composition, Formula (1), and Formula (2), a series of processes including reheating, hot rolling, solution heat treatment, cold reduction, and annealing heat treatment may be performed.

First, the ingot may be reheated at 1150°C to 1350°C for 1hour to 3 hours, and then hot rolled to produce a hot-rolled material.

By reheating the ingot at 1150°C to 1350°C for 1 to 3 hours, coarse precipitates formed during ingot production may be re-dissolved, and internal grains may be controlled to an appropriate size.

The hot-rolled material may be subjected to solution heat treatment at 1000°C to 1200°C for 1minute to 30 minutes to produce a solution heat-treated hot-rolled material.

Solution heat treatment is a process in which a hot-rolled material is heated to a solid solution range and then rapidly cooled such that a solid solution state is maintained at room temperature. By performing solution heat treatment, the strength and workability of the steel may be improved. In the present disclosure, the solution heat treatment may be performed at 1000°C to 1200°C for 1 minute to 30 minutes.

The solution heat-treated hot-rolled material may be cold-reduced with a thickness reduction ratio of 60% to 80% to produce a tempered material.

By cold reduction with a thickness reduction ratio of 60% to 80%, most of the microstructure may be transformed into martensite, thereby compensating for austenite phase stability while simultaneously achieving grain refinement. In addition, within the above range, TRIP transformation sufficiently occurs during cold rolling, and from the aspect of grain refinement, the amount of TRIP transformation may further increase due to internal heat generation, thereby the strength may be further improved. More specifically, the thickness reduction ratio may be 60 to 70%. Within the above range, rolling workability may be further improved, and the above-described effects may be further enhanced.

The tempered material may be annealed at a temperature greater than 700°C and less than 850°C.

By performing cold rolled annealing at a temperature greater than 700°C and less than 850°C, reversion recrystallization from martensite to austenite may be easily achieved.

Hereinafter, the present invention will be described in more detail through embodiments. However, the descriptions of the embodiments are only for illustrating the implementation of the present invention, and the present invention is not limited by the descriptions of the embodiments. This is because the scope of the rights of the present invention is determined by matters described in the scope of claims and matters reasonably inferred therefrom.

For various alloy composition ranges shown in Table 1 below, ingots were prepared in a vacuum induction melting furnace. The ingots were reheated at 1250°C for 2 hours and then hot rolled to a thickness of 10.0 mm to produce a hot-rolled material. The hot-rolled material was solution heat-treated at 1100°C for 10 minutes and then water cooled to produce a solution heat-treated hot-rolled material. The solution heat-treated hot-rolled material was cold-rolled with a thickness reduction ratio of 70% to produce a tempered material with a thickness of 3.0 mm. The tempered material was annealed at a temperature of 700°C to 850°C to produce test specimens. Classification Alloy composition (wt%) Annealing temperature (°C) C Si Mn P S Cr Ni Cu N Example 1 0.05 0.45 5.7 0.003 0.005 17.0 3.2 1.5 0.16 750 Example 2 0.02 0.40 7.9 0.002 0.003 16.5 3.0 1.0 0.15 750 Example 3 0.08 0.40 6.0 0.003 0.005 17.5 2.4 1.5 0.17 750 Example 4 0.07 0.50 7.1 0.003 0.005 17.0 2.0 0.9 0.18 750 Example 5 0.06 0.30 3.7 0.003 0.003 18.0 3.6 1.8 0.18 800 Comparative Example 1 0.05 0.45 5.7 0.003 0.005 17.0 3.2 1.5 0.16 850 Comparative Example 2 0.02 0.40 7.9 0.002 0.003 16.5 3.0 1.0 0.15 850 Comparative Example 3 0.08 0.40 6.0 0.003 0.005 17.5 2.4 1.5 0.17 700 Comparative Example 4 0.07 0.50 7.1 0.003 0.005 17.0 2.0 0.9 0.18 700 Comparative Example 5 0.06 0.30 3.7 0.003 0.003 18.0 3.6 1.8 0.18 700 Comparative Example 6 0.08 0.86 7.0 0.003 0.003 15.8 4.4 1.8 0.18 750 Comparative Example 7 0.04 0.86 7.8 0.003 0.003 15.8 3.9 1.9 0.15 800 Comparative Example 8 0.05 0.58 5.6 0.003 0.003 17.8 2.6 0.55 0.14 750 Comparative Example 9 0.01 0.93 5.7 0.003 0.003 17.2 4.2 0.8 0.10 800

Table 2 below shows the values of Formula (1), the values of Formula (2), the calculated thermodynamic free energy change △G_γ-α(25°C), and the calculated thermodynamic free energy change △G_α-γ(750°C). The value of Formula (1) is calculated by the following Formula (1).$$0.02\left[\mathrm{Si}\right]+0.24\left[\mathrm{Mn}\right]+0.08\left[\mathrm{Cr}\right]+0.15\left[\mathrm{Ni}\right]+0.1\left[\mathrm{Cu}\right]+2.28\left(\left[\mathrm{C}\right]+\left[\mathrm{N}\right]\right)$$

In Formula (1), Si, Mn, Cr, Ni, Cu, C, and N represent the content (wt%) of each element.

The values of Formula (2) were calculated using the following Formula (2).$$-0.04\left[\mathrm{Si}\right]+0.06\left[\mathrm{Mn}\right]-0.01\left[\mathrm{Cr}\right]+0.05\left[\mathrm{Ni}\right]+0.04\left[\mathrm{Cu}\right]+1.2\left(\left[\mathrm{C}\right]+\left[\mathrm{N}\right]\right)$$

In Formula (2), Si, Mn, Cr, Ni, Cu, C, and N represent the content (wt%) of each element.

The calculated values of thermodynamic free energy change △G_γ-α(25°C) and △G_α-γ(750°C) were obtained by calculating the free energy changes of the austenite phase and the ferrite phase according to the alloy element content and temperature change, utilizing Thermo-Calc TCFE 6.0 thermodynamic database. Classification Formula (1) Formula (2) ΔG_γ-α(25°C) (kJ/mol) ΔG_α-γ(750°C) (kJ/mol) Example 1 3.85 0.63 -1.95 -0.56 Example 2 4.16 0.69 -1.89 -0.54 Example 3 3.93 0.65 -2.00 -0.52 Example 4 4.03 0.67 -1.85 -0.54 Example 5 3.60 0.57 -2.18 -0.55 Comparative Example 1 3.85 0.63 -1.95 -0.56 Comparative Example 2 4.16 0.69 -1.89 -0.54 Comparative Example 3 3.93 0.65 -2.00 -0.52 Comparative Example 4 4.03 0.67 -1.85 -0.54 Comparative Example 5 3.60 0.57 -2.18 -0.55 Comparative Example 6 4.39 0.83 -1.56 -0.76 Comparative Example 7 4.36 0.77 -1.62 -0.67 Comparative Example 8 3.66 0.51 -2.20 -0.42 Comparative Example 9 3.72 0.51 -2.20 -0.45

In Table 3 below, the average grain diameter, the volume fraction of residual martensite, the yield strength, and the elongation are shown. The average grain diameter and the volume fraction of residual martensite were measured by photographing the thickness central portion of the cold-rolled material with a scanning electron microscope (SEM). Meanwhile, in the present disclosure, the term "average" refers to an average value of values measured at five arbitrary locations. In addition, the thickness central portion refers to a region from 1/4 t to 3/4 t when the thickness is denoted as t.

The yield strength and elongation were measured by performing a tensile test on a JIS 13B tensile specimen at room temperature at a crosshead speed of 20 mm/min using a tensile testing machine manufactured by Zwick Roell. Classification Average Grain Diameter (µm) Volume Fraction of Residual Martensite Phase (%) Yield Strength (MPa) Elongation (%) Example 1 4.5 3.2 930 32 Example 2 2.5 2.5 810 35 Example 3 3.0 7.5 1110 33 Example 4 2.0 4.1 990 40 Example 5 2.5 3.1 1000 32 Comparative Example 1 5.2 1.0 620 40 Comparative Example 2 5.5 1.0 600 42 Comparative Example 3 2.0 12.0 1370 11 Comparative Example 4 1.5 9.0 1350 12 Comparative Example 5 1.5 13.0 1250 13 Comparative Example 6 5.5 1.5 650 43 Comparative Example 7 6.0 1.0 600 45 Comparative Example 8 3.0 12.0 1300 12 Comparative Example 9 4.0 9.0 1250 14

Referring to Tables 2 and 3, Examples 1 to 5 satisfied the alloy components, Formula (1) and Formula (2) values, and the manufacturing method proposed in the present disclosure. Accordingly, Examples 1 to 5 satisfied a volume fraction of residual martensite of 2.0 to 8.0%, an average austenite grain diameter of 5.0 µm or less at a thickness center portion, a yield strength of 800 MPa or more, and an elongation of 30% or more. Thus, Examples 1 to 5 satisfied high-strength and high-elongation characteristics. However, Comparative Examples 1 to 5 did not satisfy an annealing temperature greater than 700°C and less than 850°C.

In Comparative Examples 1 and 2, the annealing heat treatment temperature was too high. This caused a complete transformation into an austenitic recrystallized structure, so that recovery stage grains and residual martensite hardly remained. Therefore, a yield strength of 800 MPa or more was not satisfied.

In Comparative Examples 3 to 5, since the annealing heat treatment temperature was too low, a large amount of recovery stage grains and residual martensite remained compared to the recrystallized structure. Accordingly, an elongation of 30% or more was not satisfied.

Comparative Examples 6 and 7 did not satisfy a value of Formula (1) of 4.2 or less. Therefore, in Comparative Examples 6 and 7, deformation-induced martensite was not sufficiently formed, and thus sufficient grain refinement was not achieved. Consequently, Comparative Examples 6 and 7 did not satisfy an average grain diameter of the austenite phase at a thickness central portion of 5 µm or less, and the residual martensite content was also not sufficient. As a result, Comparative Examples 6 and 7 did not satisfy a yield strength of 800 MPa or more.

Comparative Examples 8 and 9 did not satisfy a value of Formula (2) of 0.55 or more. Therefore, in Comparative Examples 8 and 9, a large amount of martensite remained, and thus an elongation of 30% or more was not satisfied.

FIG. 1 is an image of a microstructure of a high-strength stainless steel, according to an example of the present disclosure, taken with a Scanning Electron Microscope (SEM).

Referring to FIG. 1, according to an example of the present disclosure, it is confirmed that high yield strength and elongation are realized by realizing grain refinement.

According to an example of the present disclosure, it is possible to provide a stainless steel that achieves high yield strength and elongation by controlling phase stability and microstructure, and a method for manufacturing the same.