High-strength stainless steel and manufacturing method thereof

Abstract

A high-strength stainless steel according to an embodiment of the present invention contains, in wt%, 0.01-0.10% of C, 0.10-1.00% of Si, more than 0% and less than 0.050% of P, more than 0% and less than 0.030% of S, 3.0-8.0% of Mn, 1.0-5.0% of Ni, 15.0-18.0% of Cr, 0.1-2.0% of Cu, and 0.10-0.20% of N, with the remainder comprising Fe and inevitable impurities, wherein the value of a stability index (SI) represented by expression (1) below may be 4.20 or less. Expression (1): 0.02[Si]+0.24[Mn]+0.08[Cr]+0.15[Ni]+0.1[Cu]+2.28([C]+[N]). In expression (1), Si, Mn, Cr, Ni, Cu, C, and N represent the amounts (wt%) of the respective elements.

Description

High-strength stainless steel and its manufacturing method

[0001] The present invention relates to high-strength stainless steel and a method for manufacturing the same.

[0002] Demand for low-Ni austenitic stainless steels is increasing due to recent rises in Ni prices and increased volatility, but it is difficult to sufficiently increase the yield strength of low-Ni austenitic stainless steels.

[0003] In addition, in order to lower the content of expensive elements such as Ni, an austenite stabilizing element such as Mn and N was attempted to be replaced, but there is a problem that corrosion resistance is poor due to the formation of MnS.

[0004] To increase the strength of austenitic stainless steels, methods such as work hardening through temper rolling or the addition of large amounts of interstitial elements such as C and N are utilized. However, tempered steel has poor elongation, which reduces its usability. High C addition reduces weldability, and high N addition reduces hot workability.

[0005] Meanwhile, in the case of methods for improving strength and elongation using conventional ultra-fine grain mechanisms, there are limitations in terms of strength improvement compared to tempered materials.

[0006] Patent Document 0001 discloses a fine-grained austenitic stainless steel with excellent strength and ductility. However, Patent Document 0001 does not disclose differences in fine grain size due to differences in austenite phase stability depending on the component, nor does it disclose changes in hot workability due to fine grain size.

[0007] (Prior art literature)

[0008] Patent Document 0001: Patent Publication No. 10-2007-0067905 (Published: June 29, 2007)

[0009] The purpose of the present invention to solve the above-described problems is to provide a low Ni stainless steel having high yield strength and elongation through component control and grain refinement, and a method for manufacturing the same.

[0010] A high-strength stainless steel according to one embodiment contains, in wt%, C: 0.01% or more and 0.10% or less, Si: 0.10% or more and 1.00% or less, P: 0% or more and less than 0.050%, S: 0% or more and less than 0.030%, Mn: 3.0% or more and 8.0% or less, Ni: 1.0% or more and 5.0% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2.0% or less, N: 0.10% or more and 0.20% or less, the remainder including Fe and inevitable impurities, and the volume fraction of the residual martensite phase in the microstructure may be 2.0% or more and 8.0% or less.

[0011] The above high-strength stainless steel may have an SI (Stability Index) value of 4.20 or less, as expressed by the following equation (1).

[0012] Equation (1): 0.02[Si]+0.24[Mn]+0.08[Cr]+0.15[Ni]+0.1[Cu]+2.28([C]+[N])

[0013] In Equation (1), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

[0014] The above high-strength stainless steel may have a RI (Reversion Index) value of 0.55 or more, as expressed by the following equation (2).

[0015] Equation (2): -0.04[Si]+0.06[Mn]-0.01[Cr]+0.05[Ni]+0.04[Cu]+1.2([C]+[N])

[0016] In Equation (2), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

[0017] The average grain diameter of the austenite phase in the center of the thickness may be 5.0 ㎛ or less.

[0018] The yield strength can be greater than 800 MPa.

[0019] The elongation rate can be more than 30%.

[0020] The thickness may be 0.5 to 3.0 mm.

[0021] According to one embodiment, a method for manufacturing high-strength stainless steel comprises the steps of: manufacturing an ingot containing, in wt%, C: 0.01% or more and 0.10% or less, Si: 0.10% or more and 1.00% or less, P: more than 0% and less than 0.050%, S: more than 0% and less than 0.030%, Mn: 3.0% or more and 8.0% or less, Ni: 1.0% or more and 5.0% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2.0% or less, N: 0.10% or more and 0.20% or less, and the remainder including Fe and unavoidable impurities; reheating the ingot and then hot-rolling it to manufacture a hot-rolled material; solution-heat treating the hot-rolled material to manufacture a hot-rolled material; The step of manufacturing a tempered material by cold pressing the hot-rolled material may include a step of annealing the tempered material at a temperature exceeding 700°C and less than 850°C.

[0022] The above ingot may have an SI (Stability Index) value of 4.20 or less, as expressed by the following equation (1).

[0023] Equation (1): 0.02[Si]+0.24[Mn]+0.08[Cr]+0.15[Ni]+0.1[Cu]+2.28([C]+[N])

[0024] In Equation (1), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

[0025] The above ingot may have a RI (Reversion Index) value of 0.55 or more, expressed by the following equation (2).

[0026] Equation (2): -0.04[Si]+0.06[Mn]-0.01[Cr]+0.05[Ni]+0.04[Cu]+1.2([C]+[N])

[0027] In Equation (2), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

[0028] The above reheating can be performed at 1150 to 1350°C for 1 to 3 hours.

[0029] The above-mentioned solution heat treatment can be performed at 1000 to 1200°C for 1 to 30 minutes.

[0030] The above cold pressing can be performed at a thickness reduction rate of 60 to 80%.

[0031] According to one embodiment of the present invention, a stainless steel and a manufacturing method thereof can be provided that achieve high yield strength and elongation by controlling phase stability and microstructure.

[0032] Figure 1 is an image of the microstructure of high-strength stainless steel according to an example of the present invention, taken using a SEM (Scanning Electron Microscope).

[0033] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to sufficiently convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. To clarify the present invention, the drawings may omit portions irrelevant to the description, and the sizes of components may be slightly exaggerated to facilitate understanding.

[0034] Throughout the specification, whenever a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0035] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0036] Hereinafter, the reasons for numerical limitations on the alloy component content in the embodiments of the present invention will be described. Hereinafter, unless otherwise specified, the unit is weight percent.

[0037] A high-strength stainless steel according to one embodiment contains, in wt%, C: 0.01% or more and 0.1% or less, Si: 0.1% or more and 1.0% or less, P: 0% or more and less than 0.05%, S: 0% or more and less than 0.03%, Mn: 3% or more and 8% or less, Ni: 1% or more and 5% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2% or less, N: 0.1% or more and 0.2% or less, and the remainder may include Fe and unavoidable impurities.

[0038] The content of C (carbon) may be 0.01% or more and 0.10% or less.

[0039] C is a very effective and inexpensive element for austenite stabilization. C is an interstitial element that contributes to strength improvement through the solid solution strengthening effect. Considering this, C can be added in amounts of 0.01% or more. However, if the C content is excessive, Cr in the heat-affected zone after welding 23 The sensitization phenomenon due to grain boundary precipitation of carbides such as C6 may occur, resulting in deterioration of 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% or more and 0.10% or less, and more preferably, 0.02% or more and 0.08% or less.

[0040] The content of Si (silicon) may be 0.10% or more and 1.00% or less.

[0041] Silicon (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, if the Si content is excessive, a delta-ferrite phase may be formed by the peritectic reaction during casting, which may reduce 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% or more and 1.00% or less, and more preferably, 0.30% or more and 0.50% or less.

[0042] The content of P(phosphorus) may be greater than 0% and less than 0.050%.

[0043] Phosphorus (P) is an impurity that is inevitably contained in steel and is an element that reduces corrosion resistance and hot workability. However, controlling the P content to an excessively low level may result in increased process costs. Considering this, the P content may be greater than 0% and less than 0.050%. Preferably, the P content may be greater than 0% and less than 0.003%.

[0044] The content of S (sulfur) may be greater than 0% and less than 0.030%.

[0045] S, like P, is an impurity that is inevitably contained in steel and is an element that reduces corrosion resistance and hot workability. However, controlling the S content to be excessively low may result in increased process costs. Taking this into account, the S content may be greater than 0% and less than 0.030%. Preferably, the S content may be greater than 0% and less than 0.005%.

[0046] The content of Mn (manganese) may be 3.0% or more and 8.0% or less.

[0047] Manganese (Mn) is an inexpensive element that is effective in improving the stability of austenite phase in the process-induced martensite. Considering this, Mn may be added in amounts of 3.0% or more. However, if the Mn content is excessive, the corrosion resistance and hot workability of the steel may deteriorate due to an increase in inclusions (MnS). Considering this, the upper limit of the Mn content may be limited to 8.0%. Preferably, the Mn content may be 3.7% or more and 8.0% or less, and more preferably, 3.7% or more and 7.9% or less.

[0048] The content of Ni (nickel) may be 1.0% or more and 5.0% or less.

[0049] Ni is a strong element that stabilizes the austenite phase. In addition, Ni is effective in preventing the deterioration of toughness at extremely low temperatures by suppressing heat-induced and processing-induced martensitic transformation. In addition, the addition of Ni can facilitate hot workability and cold workability. Considering this, Ni can be added in an amount of 1.0% or more. However, if the Ni content is excessive, grain refinement can be reduced. Furthermore, if the Ni content is excessive, it can lead to an increase in raw material costs. Considering this, the upper limit of the Ni content can be limited to 5.0%. Preferably, the Ni content can be 2.0% or more and 5.0% or less, and more preferably, 2.0% or more and 3.6% or less.

[0050] The content of Cr (chromium) may be 15.0% or more and 18.0% or less.

[0051] Cr is an essential element for ensuring corrosion resistance and phase stability. Considering this, Cr may be added in amounts of 15.0% or more. However, if the Cr content is excessive, a delta-ferrite phase may be formed by the peritectonic reaction, which may deteriorate 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% or more and 18.0% or less.

[0052] The content of Cu (copper) may be 0.1% or more and 2.0% or less.

[0053] Cu is an element effective in stabilizing the austenite phase. In addition, Cu is an element effective in suppressing heat-induced and processing-induced martensitic transformation. Considering this, Cu can be added in an amount of 0.1% or more. However, if the content of Cu is excessive, hot workability may be reduced due to Cu solidification segregation. Considering this, the upper limit of the Cu content may be limited to 2.0%. Preferably, the Cu content may be 0.9% or more and 2.0% or less, and more preferably, 0.9% or more and 1.8% or less.

[0054] The content of N (nitrogen) may be 0.10% or more and 0.20% or less.

[0055] Nitrogen is very effective in stabilizing the austenite phase and is an inexpensive element. In addition, Ni is an effective element in increasing strength and improving corrosion resistance through solid solution strengthening. Considering this, Ni may be added in an amount of 0.10% or more. However, if the content of Ni is excessive, hot workability may be deteriorated. Considering this, the upper limit of the Ni content may be limited to 0.20%. Preferably, the Ni content may be 0.15% or more and 0.20% or less, and more preferably, 0.15% or more and 0.18% or less.

[0056] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0057] In general, austenitic stainless steels can develop mechanically induced martensite (α`-martensite) during cold rolling. The development of mechanically induced phases varies depending on the stability of the austenite phase. In austenitic stainless steels with low phase stability, ε-martensite bands develop during deformation, and as the amount of deformation increases, α`-martensite can be formed from the intersections within the bands.

[0058] In order to increase cost competitiveness by lowering the high cost of Ni, phase stability must be controlled by using other austenite phase stabilizing elements (Mn, Cu, C, N, etc.) other than Ni. To this end, the free energy change from the austenite phase to the martensite phase at room temperature (△G γ-α ) needs to be controlled.

[0059] Meanwhile, when rolled tempered steel is annealed, a reversion transformation can occur from the process-induced martensite phase to the austenite phase. The reversion transformation process can be broadly divided into diffusional reversion and shear reversion. The reversion transformation process is the change in free energy from the martensite phase to the austenite phase (△G) during the annealing step. α- γ ) can proceed depending on the shear transformation. In general, martensitic shear transformation has a larger free energy change (△G) than diffusion transformation. α-γ ) is required. Therefore, the lower the free energy change from martensite to austenite phase, the more martensite remains in the final cold-rolled annealed material after annealing heat treatment, and the more a two-phase structure can be realized.

[0060] In the present invention, a two-phase structure can be realized by controlling the microstructure through an alloy composition and manufacturing method for a component system with reduced Ni so that the volume fraction of the residual martensite phase within the austenite phase matrix is ​​2.0% or more and 8.0% or less. Through this, a stainless steel with high strength characteristics can be provided.

[0061] To realize the above microstructure, the change in free energy at room temperature during cold rolling (△G γ-α ) was designed to be high to promote the process-induced martensite transformation. In addition, in order to implement the above microstructure, the annealing temperature and free energy change (△G) during cold annealing were measured. α-γ ) to control the amount of recrystallization from martensite to austenite during cold rolling annealing.

[0062] In the present invention, the free energy changes of the austenite phase and the ferrite phase according to the alloy element content and temperature change were calculated to derive equations (1) and (2). Through this, the phase stability of the austenite phase and the martensite phase can be controlled, and specifically, the free energy change (△G) at a specific temperature can be derived. γ-α ) can control the value.

[0063] In this specification, the change in free energy at a given temperature (△G γ-α ) values ​​can be calculated using the thermodynamic database of the thermodynamic analysis program (Thermo-Calc. TCFE 6.0).

[0064] According to one embodiment, a high-strength stainless steel has a thermodynamic free energy change (△G) at 25°C γ-α , 25℃) can be -1.65 kJ / mol or less. The lower limit is not limited, but can be, for example, -3.00 kJ / mol, -2.50 kJ / mol, -2.20 kJ / mol. Within the above range, the effect of improving phase stability is more excellent, and the effect of improving yield strength and elongation can be more improved.

[0065] According to one embodiment, a high-strength stainless steel has a thermodynamic free energy change (△G) at 750°C γ-α , 750℃) can be -0.46 kJ / mol or less. The lower limit is not limited, but can be, for example, -2.00 kJ / mol, -1.50 kJ / mol, -0.80 kJ / mol. Within the above range, the effect of improving phase stability is more excellent, and the effect of improving yield strength and elongation can be more improved.

[0066] According to one embodiment, a high-strength stainless steel may have an SI (Stability Index) value of 4.20 or less, as expressed by the following equation (1).

[0067] Equation (1): 0.02[Si]+0.24[Mn]+0.08[Cr]+0.15[Ni]+0.1[Cu]+2.28([C]+[N])

[0068] In Equation (1), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

[0069] When the value of equation (1) is 4.20 or less, the change in free energy at room temperature during cold rolling (△G γ-α (RT)) is -1.8 kJ / mol or less, and when the cold reduction ratio is 60% or more, martensite transformation occurs by 25% or more, which can be advantageous for fine grain formation.

[0070] Meanwhile, in the actual cold rolling process, the heat generated during rolling can cause the temperature of the cold-rolled steel sheet to rise above 100℃. Consequently, martensitic transformation is suppressed as the number of rolling passes increases. Therefore, it is advantageous to achieve significant martensitic transformation during the initial rolling stage at room temperature. Retained austenite grains that do not transform to martensite during cold rolling remain as deformed austenite grains.

[0071] According to one embodiment, a high-strength stainless steel may have a Reversion Index (RI) value of 0.55 or more, as expressed by the following equation (2).

[0072] Equation (2): -0.04[Si]+0.06[Mn]-0.01[Cr]+0.05[Ni]+0.04[Cu]+1.2([C]+[N])

[0073] In Equation (2), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

[0074] When the value of equation (2) is 0.55 or more, the change in free energy from martensite to austenite (△G α-γ , 750℃) is -0.5 kJ / mol or less, recrystallization by diffusion transformation may occur. Therefore, the volume fraction of residual martensite phase in the final annealed material may be 2.0% or more and 8.0% or less.

[0075] According to one embodiment, a high-strength stainless steel may have a microstructure in which a volume fraction of a residual martensite phase is 2.0% or more and 8.0% or less.

[0076] During cold rolling, austenite grains remaining in the deformed structure undergo a recovery phase, completing recrystallization. These grains have a relatively higher dislocation density than those that have fully recrystallized, which can contribute to increased strength.

[0077] When the volume fraction of the residual martensite phase exceeds 8%, the number of crystal grains in the recovery phase may increase, which may reduce the elongation. However, when the volume fraction of the residual martensite phase is less than 2.0%, the martensite content is low, and the number of crystal grains in the recrystallization phase increases compared to the crystal grains in the recovery phase, which may reduce the strength.

[0078] By controlling the alloying components described above, formula (1), formula (2), microstructure, or the manufacturing method described below, the high-strength stainless steel according to one embodiment can have an average grain diameter of the austenite phase at the center of the thickness of 5.0 ㎛ or less.

[0079] Additionally, the high-strength stainless steel according to one embodiment may have a yield strength of 800 MPa or more and an elongation of 30% or more.

[0080] In addition, since the high-strength stainless steel according to one embodiment has sufficient strength, the thickness of the tempered material after cold pressing can be 0.5 to 3.0 mm.

[0081] Next, a method for manufacturing high-strength stainless steel according to another aspect of the present invention will be described.

[0082] According to one embodiment, a method for manufacturing high-strength stainless steel comprises the steps of: manufacturing an ingot containing, in wt%, C: 0.01% or more and 0.10% or less, Si: 0.10% or more and 1.00% or less, P: more than 0% and less than 0.050%, S: more than 0% and less than 0.030%, Mn: 3.0% or more and 8.0% or less, Ni: 1.0% or more and 5.0% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2.0% or less, N: 0.10% or more and 0.20% or less, and the remainder including Fe and unavoidable impurities; reheating the ingot and then hot-rolling it to manufacture a hot-rolled material; solution-heat treating the hot-rolled material to manufacture a hot-rolled material; The step of manufacturing a tempered material by cold pressing the hot-rolled material may include a step of annealing the tempered material at a temperature exceeding 700°C and less than 850°C.

[0083] The above ingot may have an SI (Stability Index) value of 4.20 or less, as expressed by the following equation (1).

[0084] Equation (1): 0.02[Si]+0.24[Mn]+0.08[Cr]+0.15[Ni]+0.1[Cu]+2.28([C]+[N])

[0085] In Equation (1), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

[0086] Additionally, the above ingot may have a RI (Reversion Index) value of 0.55 or more, as expressed by the following equation (2).

[0087] Equation (2): -0.04[Si]+0.06[Mn]-0.01[Cr]+0.05[Ni]+0.04[Cu]+1.2([C]+[N])

[0088] In Equation (2), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

[0089] The reason for the component range of each alloy composition and the numerical limitations of Equations (1) and (2) is as described above, and each manufacturing step is described in more detail below.

[0090] After manufacturing an ingot satisfying the above alloy composition, formula (1) and formula (2), it can undergo a series of reheating, hot rolling, solution heat treatment, cold pressing and annealing heat treatment processes.

[0091] First, the above ingot can be reheated at 1150 to 1350°C for 1 to 3 hours, and then hot-rolled to produce a hot-rolled material.

[0092] By reheating the above ingot at 1150 to 1350°C for 1 to 3 hours, coarse precipitates generated during ingot manufacturing can be redissolved and internal crystal grains can be controlled to an appropriate size.

[0093] The hot-rolled material can be manufactured by performing solution heat treatment on the hot-rolled material at 1000 to 1200°C for 1 to 30 minutes.

[0094] Solution heat treatment involves heating hot-rolled steel to a solid solution range and then rapidly cooling it to maintain the solid solution state at room temperature. Solution heat treatment can improve the strength and workability of steel. In the present invention, solution heat treatment can be performed at 1000 to 1200°C for 1 to 30 minutes.

[0095] The above hot-rolled material can be cold pressed to produce a tempered material with a thickness reduction rate of 60 to 80%.

[0096] By cold rolling with a thickness reduction ratio of 60 to 80% to transform most of the structure into martensite, austenite phase stability can be compensated while grain refinement can be achieved. In addition, within the above range, TRIP transformation sufficiently occurs during cold rolling, and the amount of TRIP transformation increases due to internal heat generation in terms of grain refinement, so that strength can be further improved. More specifically, the thickness reduction ratio can be 60 to 70%. Within the above range, rollability can be further improved and the aforementioned effects can be further enhanced.

[0097] The above-mentioned material can be annealed at a temperature exceeding 700°C and less than 850°C.

[0098] By cold rolling and annealing at 850 to 900℃, reverse transformation and recrystallization from martensite to austenite can be easily performed.

[0099] Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is merely intended to illustrate the implementation of the present invention and is not intended to limit the present invention. This is because the scope of the present invention is determined by the matters set forth in the claims and matters reasonably inferred therefrom.

[0100] {Example}

[0101] For the various alloy composition ranges shown in Table 1 below, ingots were manufactured in a vacuum induction melting furnace. The manufactured ingots were reheated at 1250°C for 2 hours and then hot-rolled to a thickness of 10.0 mm to manufacture hot-rolled products. The hot-rolled products were solution-heat treated at 1100°C for 10 minutes and then water-quenched to manufacture hot-rolled products. The hot-rolled products were cold-rolled at a thickness reduction rate of 70% to manufacture tempered products with a thickness of 3.0 mm. The tempered products were annealed at 700 to 850°C to manufacture specimens.

[0102] Classification Alloy composition (weight%) Annealing heat treatment temperature (℃) CSiMnPSCrNiCuN Example 10.050.455.70.0030.00517.03.21.50.16750 Example 20.020.407.90.0020.00316.53.01.00.15750 Example 30.080.406.00.0030.00517.52.41.50.17750 Example 40.07 0.507.10.0030.00517.02.00.90.18750Example50.060.303.70.0030.00318.03.61.80.18800Comparative Example10.050.455.70.0030.00517.03.21.50.16850Comparative Example20.020.407.90.0020.00316.53.01. 00.15850Comparative Example 30.080.406.00.0030.00517.52.41.50.17700Comparative Example 40.070.507.10.0030.00517.02.00.90.18700Comparative Example 50.060.303.70.0030.00318.03.61.80.18700Comparative Example 60.080.867.00. 0030.00315.84.41.80.18750Comparative Example 70.040.867.80.0030.00315.83.91.90.15800Comparative Example 80.050.585.60.0030.00317.82.60.550.14750Comparative Example 90.010.935.70.0030.00317.24.20.80.10800

[0103] Table 2 below shows the values ​​of Equation (1), Equation (2) and the thermodynamic free energy change △G γ-α(25℃) Calculated values ​​and △G α-γ (750℃) The calculated value is shown. The value of Equation (1) is shown by calculating Equation (1) below.

[0104] Equation (1): 0.02[Si]+0.24[Mn]+0.08[Cr]+0.15[Ni]+0.1[Cu]+2.28([C]+[N])

[0105] In Equation (1), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

[0106] The value of equation (2) was calculated by using equation (2) below.

[0107] Equation (2): -0.04[Si]+0.06[Mn]-0.01[Cr]+0.05[Ni]+0.04[Cu]+1.2([C]+[N])

[0108] In Equation (2), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

[0109] Thermodynamic free energy change △G γ-α (25℃) and △G α-γ (750℃) The calculated values ​​were calculated and presented by using the Thermo-Calc. TCFE 6.0 thermodynamic database to calculate the free energy changes of the austenite and ferrite phases according to the alloying element content and temperature changes.

[0110] Distinction equation (1) equation (2) △G γ-α (25℃)(kJ / mol)△G α-γ(750℃)(kJ / mol)Example 13.850.63-1.95-0.56Example 24.160.69-1.89-0.54Example 33.930.65-2.00-0.52Example 44.030.67-1.85-0.54Example 53.600.57-2.18-0.55Comparative Example 13.850.63-1.95-0.56Comparative Example 24.160.69-1. 89-0.54Comparative Example 33.930.65-2.00-0.52Comparative Example 44.030.67-1.85-0.54Comparative Example 53.600.57-2.18-0.55Comparative Example 64.390.83-1.56-0.76Comparative Example 74.360.77-1.62-0.67Comparative Example 83.660.51-2.20-0.42Comparative Example 93.720.51-2.20-0.45

[0111] Table 3 below shows the average grain diameter, volume fraction of residual martensite phase, yield strength, and elongation. The average grain diameter and volume fraction of residual martensite phase were measured by photographing the center of the thickness of the cold-rolled steel using a scanning electron microscope (SEM). Meanwhile, in the present invention, the average means the average value of the values ​​measured at five arbitrary points. In addition, the center of the thickness means the point from 1 / 4 t to 3 / 4 t when the thickness is t.

[0112] Yield strength and elongation were measured using a Zwick Roell tensile tester at room temperature using JIS13B tensile test specimens at a tensile speed of 20 mm per minute.

[0113] Average grain diameter (㎛) Volume fraction of residual martensite phase (%) Yield strength (MPa) Elongation (%) Example 14.5 3.29 30 32 Example 22.5 2.58 10 35 Example 33.07 51 11 0 33 Example 42.04 19 9 0 40 Example 52.5 3.11 0 0 32 Comparative Example 15.21 0 2 40 Comparative Example 25.5 1.06 0 0 42 Comparative Example 32.01 2.01 37 0 11 Comparative Example 41.5 9.01 35 0 12 Comparative Example 51.5 13.01 2 5 0 13 Comparative Example 65.5 1.56 5 0 43 Comparative Example 76.01 0 6 0 0 45 Comparative Example 83.01 2.01 3 0 0 12 Comparative Example 94.09 0 12 5 0 14

[0114] Referring to Tables 2 and 3, Examples 1 to 5 satisfied the alloy components, formula (1), formula (2) values, and manufacturing methods presented in the present invention. Therefore, Examples 1 to 5 satisfied the volume fraction of the residual martensite phase of 2% or more and 8% or less, the average grain diameter of the austenite phase in the center of the thickness of 5 ㎛ or less, the yield strength of 800 MPa or more, and the elongation of 30% or more. That is, Examples 1 to 5 satisfied the high strength and high elongation characteristics. However, Comparative Examples 1 to 5 did not satisfy the annealing heat treatment temperature of more than 700°C and less than 850°C.

[0115] Comparative Examples 1 and 2 were completely transformed into an austenite recrystallized structure because the annealing heat treatment temperature was too high, so that almost no crystal grains or residual martensite phases in the recovery stage remained. Therefore, they did not satisfy a yield strength of 800 MPa or more.

[0116] Comparative Examples 3 to 5 had an annealing heat treatment temperature that was too low, resulting in a large amount of recovery-phase grains and residual martensite phase remaining compared to the recrystallized structure. Therefore, an elongation of 30% or more was not achieved.

[0117] Comparative Examples 6 and 7 did not satisfy the value of Equation (1) of 4.2 or less. Therefore, in Comparative Examples 6 and 7, processing-induced martensite was not sufficiently generated, and thus grain refinement was not sufficiently achieved. Accordingly, in Comparative Examples 6 and 7, the average grain diameter of the austenite phase in the center of the thickness did not satisfy 5㎛ or less, and the content of residual martensite was also insufficient. As a result, Comparative Examples 6 and 7 did not satisfy a yield strength of 800MPa or more.

[0118] Comparative Examples 8 and 9 did not satisfy the value of Equation (2) of 0.55 or more. Therefore, Comparative Examples 8 and 9 did not satisfy the elongation of 30% or more because a large amount of martensite content remained.

[0119] Figure 1 is an image of the microstructure of high-strength stainless steel according to an example of the present invention, taken using a SEM (Scanning Electron Microscope).

[0120] Referring to FIG. 1, it can be confirmed that, according to an example of the present invention, high yield strength and elongation can be achieved by implementing grain refinement.

[0121] According to one embodiment of the present invention, a stainless steel and a manufacturing method thereof can be provided that achieve high yield strength and elongation by controlling phase stability and microstructure.

Claims

1. Contains, in weight%, C: 0.01% or more and 0.10% or less, Si: 0.10% or more and 1.00% or less, P: more than 0% and less than 0.050%, S: more than 0% and less than 0.030%, Mn: 3.0% or more and 8.0% or less, Ni: 1.0% or more and 5.0% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2.0% or less, N: 0.10% or more and 0.20% or less, and the remainder includes Fe and inevitable impurities. High-strength stainless steel having a microstructure with a volume fraction of residual martensite of 2.0% to 8.0%.

2. In claim 1, High-strength stainless steel with an SI (Stability Index) value of 4.20 or less, expressed by the following equation (1): Equation (1): 0.02[Si]+0.24[Mn]+0.08[Cr]+0.15[Ni]+0.1[Cu]+2.28([C]+[N]) (In Equation (1), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight %) of each element.

3. In claim 1, High-strength stainless steel having a Reversion Index (RI) value of 0.55 or higher, expressed by the following equation (2): Equation (2): -0.04[Si]+0.06[Mn]-0.01[Cr]+0.05[Ni]+0.04[Cu]+1.2([C]+[N]) (In Equation (2), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

4. In claim 1, High-strength stainless steel having an average grain diameter of austenite in the center of the thickness of 5.0 ㎛ or less.

5. In claim 1, High-strength stainless steel with a yield strength of 800 MPa or more.

6. In claim 1, High-strength stainless steel with an elongation of 30% or more.

7. In claim 1, High-strength stainless steel with a thickness of 0.5 to 3.0 mm.

8. A step for manufacturing an ingot containing, by weight%, C: 0.01% or more and 0.10% or less, Si: 0.10% or more and 1.00% or less, P: more than 0% and less than 0.050%, S: more than 0% and less than 0.030%, Mn: 3.0% or more and 8.0% or less, Ni: 1.0% or more and 5.0% or less, Cr: 15.0% or more and 18.0% or less, Cu: 0.1% or more and 2.0% or less, N: 0.10% or more and 0.20% or less, and the remainder including Fe and inevitable impurities; A step of reheating the above ingot and then hot-rolling it to manufacture a hot-rolled material; A step of manufacturing a hot-rolled material by solution heat treating the above hot-rolled material; A step of manufacturing a tempered material by cold pressing the hot-rolled material; and A method for manufacturing high-strength stainless steel, comprising a step of annealing the above-mentioned tempered material at a temperature exceeding 700°C and less than 850°C.

9. In claim 8, The above ingot, Method for manufacturing high-strength stainless steel having an SI (Stability Index) value of 4.20 or less, expressed by the following equation (1): Equation (1): 0.02[Si]+0.24[Mn]+0.08[Cr]+0.15[Ni]+0.1[Cu]+2.28([C]+[N]) (In Equation (1), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight %) of each element.

10. In claim 8, The above ingot, A method for manufacturing high-strength stainless steel having a RI (Reversion Index) value of 0.55 or higher, expressed by the following equation (2): Equation (2): -0.04[Si]+0.06[Mn]-0.01[Cr]+0.05[Ni]+0.04[Cu]+1.2([C]+[N]) (In Equation (2), Si, Mn, Cr, Ni, Cu, C, and N represent the content (weight%) of each element.

11. In claim 8, A method for manufacturing high-strength stainless steel, wherein the above reheating is performed at 1150 to 1350°C for 1 to 3 hours.

12. In claim 8, A method for manufacturing high-strength stainless steel, wherein the above-mentioned solution heat treatment is performed at 1000 to 1200°C for 1 to 30 minutes.

13. In claim 8, A method for manufacturing high-strength stainless steel, wherein the cold pressing is performed at a thickness reduction rate of 60 to 80%.

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