Precipitation hardening martensitic stainless steel and method for manufacturing same

Abstract

A precipitation hardening martensitic stainless steel according to one embodiment of the present invention comprises, in wt%, 0.01-0.10% of C, 0.10-1.00% of Si, 0.50-1.50% of Mn, 14.5-17.5% of Cr, 3.0-6.0% of Ni, 0.20-0.35% of Nb, 3.0-5.0% of Cu, 0.005-0.020% of N, 0.035% or less (excluding 0) of P, 0.01% or less (excluding 0) of S, and the balance being Fe and inevitable impurities, wherein formula (1) below is a value of 0.45 or less, and formula (2) below is a value of 40.0 or less. Formula (1): [Cu] / (7.4−3.5×[C]−1.1×[Si]+0.4×[Mn]−0.1×[Cr]+0.3×[Ni]+0.5×[Nb]−4.5×[N]) Formula (2): 82.0−222.7×[C]−0.3×[Si]−12.4×[Mn]+9.9×[Cr]−22.4×[Ni]−4.1×[Nb]−23.3×[Cu]−218.9×[N]

Description

Precipitation-hardened martensitic stainless steel and method of manufacturing the same

[0001] The present invention relates to a precipitation-hardening martensitic stainless steel and a method for manufacturing the same, and more specifically, to a precipitation-hardening martensitic stainless steel with excellent hot workability through compositional control and a method for manufacturing the same.

[0002] Precipitation-hardened stainless steel is widely used in applications such as steel belts and press plates because it is possible to easily increase tensile strength or hardness by applying aging treatment to the martensitic (or martensitic and austenitic) structure. Representative examples include STS630 and STS631.

[0003] STS630 is a representative precipitation-hardened martensitic stainless steel with a martensitic structure in the solution state, and high strength can be achieved by generating ε-Cu precipitates within the structure through aging heat treatment. However, since STS630 has a martensitic structure in the solution state, if the martensitic transformation initiation point Ms is not controlled when manufacturing slabs, slab cracking may occur. Additionally, because Cu precipitates are utilized to achieve high strength, if the Cu content is excessive, cracking may occur in the sheet during hot rolling due to liquid embrittlement of the Cu phase.

[0004] In particular, since δ-ferrite solidifies during the slab manufacturing process, δ-ferrite has low solubility for copper (Cu) element, and as a result, when δ-ferrite is formed during solidification, the segregation of copper may occur locally in excess of the amount of Cu added.

[0005] When the locally segregated Cu content exceeds the Cu solid solution limit, cracks may occur due to liquid embrittlement as the Cu phase becomes liquidized when the slab is reheated for hot rolling and the temperature rises above the Cu melting point (1100℃). To improve the liquid embrittlement phenomenon caused by such local Cu segregation, it is important to minimize the local Cu segregation by controlling the δ-ferrite content and the Cu solid solution limit.

[0006] To solve these problems, the present invention aims to provide a precipitation-hardened martensitic stainless steel with excellent hot workability and a method for manufacturing the same by maximizing the solid solution of Cu in the matrix and lowering the δ-ferrite content through the control of alloy components and compositional relationships, thereby minimizing local segregation of Cu elements and preventing liquid phase embrittlement of Cu.

[0007] A precipitation-hardened martensitic stainless steel according to one embodiment of the present invention comprises, in weight%, C: 0.01 to 0.10%, Si: 0.10 to 1.00%, Mn: 0.50 to 1.50%, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Nb: 0.20 to 0.35%, Cu: 3.0 to 5.0%, N: 0.005 to 0.020%, P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), the remainder being Fe and unavoidable impurities, and satisfies that the following formula (1) is 0.45 or less and the following formula (2) is 40.0 or less.

[0008] Equation (1): [Cu] / (7.4-3.5Х[C]-1.1Х[Si]+0.4Х[Mn]-0.1Х[Cr]+0.3Х[Ni]+0.5Х[Nb]-4.5Х[N])

[0009] Equation (2): 82.0-222.7Х[C]-0.3Х[Si]-12.4Х[Mn]+9.9Х[Cr]-22.4Х[Ni]-4.1Х[Nb]-23.3Х[Cu]-218.9Х[N]

[0010] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N] represent the weight% content of each element, excluding units).

[0011] In addition, a precipitation-hardened martensitic stainless steel according to another embodiment of the present invention may contain δ-ferrite in an area fraction of 5.0% or less.

[0012] In addition, in another embodiment of the present invention, the precipitation-hardened martensitic stainless steel may have the following formula (3) at least 0.

[0013] Equation (3): 1240.1 - 1300.3Х([C]+[N]) - 27.8Х[Si] - 33.3Х[Mn] - 61.1Х[Ni] - 41.7Х[Cr] - 27.4Х[Cu] + 32.8Х[Nb]

[0014] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N] represent the weight% content of each element, excluding units).

[0015] In addition, a precipitation-hardened martensitic stainless steel according to another embodiment of the present invention may have a reduction of area of ​​60% or more.

[0016] A method for manufacturing a precipitation-hardened martensitic stainless steel according to one embodiment of the present invention comprises the steps of: preparing a steel material comprising, in weight%, C: 0.01 to 0.10%, Si: 0.10 to 1.00%, Mn: 0.50 to 1.50%, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Nb: 0.20 to 0.35%, Cu: 3.0 to 5.0%, N: 0.005 to 0.020%, P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), the remainder being Fe and unavoidable impurities, satisfying the following formula (1) being 0.45 or less and the following formula (2) being 40.0 or less; reheating the steel material; and hot rolling.

[0017] Equation (1): [Cu] / (7.4-3.5Х[C]-1.1Х[Si]+0.4Х[Mn]-0.1Х[Cr]+0.3Х[Ni]+0.5Х[Nb]-4.5Х[N])

[0018] Equation (2): 82.0-222.7Х[C]-0.3Х[Si]-12.4Х[Mn]+9.9Х[Cr]-22.4Х[Ni]-4.1Х[Nb]-23.3Х[Cu]-218.9Х[N]

[0019] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N], [P], [S] represent the weight% content of each element, excluding units).

[0020] In addition, in a method for manufacturing a precipitation-hardened martensitic stainless steel according to another embodiment of the present invention, the steel material may have a martensitic transformation start temperature of 0 or higher, as shown in the following equation (3).

[0021] Equation (3): 1240.1 - 1300.3Х([C]+[N]) - 27.8Х[Si] - 33.3Х[Mn] - 61.1Х[Ni] - 41.7Х[Cr] - 27.4Х[Cu] + 32.8Х[Nb]

[0022] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N] represent the weight% content of each element, excluding units).

[0023] In addition, a method for manufacturing a precipitation-hardened martensitic stainless steel according to another embodiment of the present invention may include a step of reheating the steel material, which involves heat treating it at a temperature range of 1200°C to 1280°C for at least one hour.

[0024] In addition, in a method for manufacturing a precipitation-hardening martensitic stainless steel according to another embodiment of the present invention, the Cu segregation can be controlled to 6.5% or less by weight in the step of reheating the steel.

[0025] The precipitation-hardened martensitic stainless steel according to the present invention maximizes the solid solution of Cu in the matrix and lowers the δ-ferrite content, thereby minimizing local segregation of Cu elements and preventing liquid phase embrittlement of Cu, resulting in excellent hot workability.

[0026] Figure 1(a) is a photograph of the surface of Example 1 of the present invention taken with Scanning Electron Microscopy (SEM).

[0027] Figure 1(b) is an image showing the distribution of Cu elements on the surface using EPMA (Electron Probe MicroAnalyzer) for Example 1 of the present invention.

[0028] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0029] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0030] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense.

[0031] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0032] The following embodiments are presented to sufficiently convey the concept of the present invention to those skilled in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. To clarify the present invention, illustrations of parts unrelated to the description may be omitted, and the size of components may be expressed in a slightly exaggerated manner to aid understanding.

[0033] Throughout the specification, when a part is described as "including" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0034] Singular expressions include plural expressions unless there is an obvious exception in the context.

[0035] First, a precipitation-hardened martensitic stainless steel according to the present invention will be described.

[0036] A precipitation-hardened martensitic stainless steel according to one aspect of the present invention comprises, in weight%, C: 0.01 to 0.10%, Si: 0.10 to 1.00%, Mn: 0.50 to 1.50%, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Nb: 0.20 to 0.35%, Cu: 3.0 to 5.0%, N: 0.005 to 0.020%, P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), the remainder being Fe and unavoidable impurities, and satisfies that the following formula (1) is 0.45 or less and the following formula (2) is 40.0 or less.

[0037] Equation (1): [Cu] / (7.4-3.5Х[C]-1.1Х[Si]+0.4Х[Mn]-0.1Х[Cr]+0.3Х[Ni]+0.5Х[Nb]-4.5Х[N])

[0038] Equation (2): 82.0-222.7Х[C]-0.3Х[Si]-12.4Х[Mn]+9.9Х[Cr]-22.4Х[Ni]-4.1Х[Nb]-23.3Х[Cu]-218.9Х[N]

[0039] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N] represent the weight% content of each element, excluding units).

[0040] The reasons for the numerical limitations on the alloy component content in the embodiments of the present invention are explained below. Unless otherwise specified, the units are weight percent.

[0041] C: 0.01 to 0.10%

[0042] C is an austenite-forming element and an element that inhibits the formation of the δ-ferrite phase, so it must be added in an amount of 0.01% or more. However, if it is added in a large amount, the solid solution content of Cu is lowered, and the austenite phase remains excessively after the solution treatment, and sufficient strength is not obtained after the aging treatment. Therefore, in the present invention, it is controlled within a range of 0.01 to 0.1%, preferably 0.02 to 0.08%, and more preferably 0.03 to 0.06%.

[0043] Si: 0.10 to 1.00%

[0044] Si is typically added as a deoxidizer to reduce oxygen in steel and prevents the formation of δ-ferrite, so it is desirable to add at least 0.10%. However, if the content is excessive, it causes problems such as lowering the solid solution of Cu and reducing the toughness of the weldment, so the upper limit may be limited to 1.00%. Preferably, it may be 0.10 to 0.70%, and more preferably, 0.10 to 0.30%.

[0045] Mn: 0.50 to 1.50%

[0046] Mn is an austenite-stabilizing element that suppresses the formation of δ-ferrite and is effective in improving the solid solution of Cu. Therefore, it is desirable to add at least 0.50%. However, if the content is excessive, the workability and toughness of the steel decrease rapidly, so the upper limit may be limited to 1.50%. Preferably, it may be 0.70 to 1.00%, and more preferably, 0.80 to 1.00%.

[0047] Cr: 14.5 to 17.5%

[0048] Cr is the most abundant and fundamental element among the elements that improve the corrosion resistance of stainless steel, and it is desirable to add at least 14.5% to achieve corrosion resistance. However, if the content is excessive, it lowers the solid solution limit of Cu and promotes the formation of δ-ferrite, thereby reducing hot workability; therefore, the upper limit is limited to 17.5%. Preferably, it may be 15.0 to 17.5%, and more preferably, 15.5 to 17.0%.

[0049] Ni: 3.0 to 6.0%

[0050] Ni is an austenite-stabilizing element that increases the solid solution limit of Cu and suppresses the formation of δ-ferrite, which is advantageous for improving hot workability. However, since an excessive content leads to increased costs, the upper limit can be limited to 6.0%. Preferably, it can be 4.0 to 5.5%, and more preferably, 4.5 to 5.5%.

[0051] Nb: 0.20 to 0.35%

[0052] Nb forms carbides and precipitates, so it is effective for refining the grain size after solution treatment. It also has the effect of increasing the solid solution content of Cu by reducing the solid solution content of C. Therefore, it is necessary to add at least 0.20%. However, if added at more than 0.35%, the effect becomes saturated. Preferably, it can be 0.22 to 0.33%, and more preferably 0.24 to 0.31%.

[0053] Cu: 3.0 to 5.0%

[0054] Cu is a key element of precipitation-hardening martensitic stainless steel that improves strength by forming ε-Cu precipitates during aging heat treatment after solution heat treatment. Therefore, it is necessary to add at least 3.0%. However, if the amount added is excessive, liquid embrittlement occurs due to local segregation, which reduces hot workability; therefore, the upper limit of the amount added is set to 5.0% or less, preferably 3.0 to 4.0%, and more preferably 3.0 to 3.5%.

[0055] N: 0.005 to 0.020%

[0056] N is an austenite-stabilizing element that inhibits the formation of δ-ferrite, which is advantageous for improving hot workability. However, if the amount added is excessive, it forms high-temperature precipitates that actually reduce hot workability, so the upper limit is set to 0.020%. Preferably, it may be 0.005 to 0.015%, and more preferably, 0.005 to 0.010%.

[0057] P: 0.035% or less (excluding 0)

[0058] Phosphorus (P) is an inevitably contained impurity, and it is desirable to keep its content as low as possible. Theoretically, it is advantageous to control the phosphorus content to 0% by weight, but it is inevitably contained due to the manufacturing process. Therefore, it is important to manage its upper limit, and in the present invention, the upper limit is managed at 0.035%. Preferably, it may be 0.030% or less, and more preferably, 0.025% or less.

[0059] S: 0.01% or less (excluding 0)

[0060] Sulfur (S) is an inevitably contained impurity, and it is desirable to keep its content as low as possible. Theoretically, it is advantageous to control the phosphorus content to 0 weight%, but it is inevitably contained due to the manufacturing process. Therefore, it is important to manage its upper limit, and in the present invention, the upper limit is managed at 0.01%. Preferably, it may be 0.005% or less, and more preferably, 0.003% or less.

[0061] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0062] A precipitation-hardened martensitic stainless steel according to one embodiment of the present invention satisfies the following formula (1) of 0.45 or less.

[0063] Equation (1): [Cu] / (7.4-3.5Х[C]-1.1Х[Si]+0.4Х[Mn]-0.1Х[Cr]+0.3Х[Ni]+0.5Х[Nb]-4.5Х[N])

[0064] Equation (1) represents the ratio of the amount of Cu added in the numerator to the amount of Cu solid solution represented in the denominator, and is an indicator of the hot workability of precipitation-hardened martensitic stainless steel, which is the amount of local Cu segregation during the slab solidification process. To suppress Cu segregation, it is important to increase the amount of Cu solid solution in the matrix structure and reduce the fraction of the ferrite phase, which has a lower solubility for the Cu element compared to the austenite phase. If Equation (1) exceeds 0.45, the amount of Cu added is excessively high compared to the amount of Cu solid solution, so there is a problem in that a Cu precipitate phase is formed at the Cu melting point of 1100°C or higher, and liquid phase embrittlement due to Cu liquefaction occurs. Therefore, Equation (1) is controlled to 0.45 or less, preferably 0.50 or less, and more preferably 0.45 or less.

[0065] A precipitation-hardened martensitic stainless steel according to one embodiment of the present invention satisfies the following formula (2) of 40 or less.

[0066] Equation (2): 82.0-222.7Х[C]-0.3Х[Si]-12.4Х[Mn]+9.9Х[Cr]-22.4Х[Ni]-4.1Х[Nb]-23.3Х[Cu]-218.9Х[N]

[0067] Equation (2) is an equation representing the δ-ferrite formation index, and if it exceeds 40.0, the δ-ferrite content becomes excessively high. Since δ-ferrite has low solubility for Cu elements, when it is formed, it causes problems by promoting the segregation of Cu elements into the surrounding austenite phase. Therefore, Equation (2) is controlled to 40.0 or less, preferably 38.0 or less, and more preferably 37.0 or less. Accordingly, the precipitation-hardened martensitic stainless steel according to one embodiment of the present invention may contain δ-ferrite in an area fraction of 5.0% or less.

[0068] In a precipitation-hardening martensitic stainless steel according to one embodiment of the present invention, the following equation (3) may be 0 or greater. Equation (3) of the present invention is a formula for controlling the Ms temperature (martensite transformation start temperature). Since steels with a low Ms point tend to have low strength after aging treatment, it is preferable that the value of equation (3) in the precipitation-hardening martensitic stainless steel according to an embodiment of the present invention be within the range of 0 or greater (i.e., 0°C). That is, if the value of equation (3) is 0 or less, a large amount of austenite phase remains after solution treatment, and consequently, there is a problem in that sufficient strength is not obtained after aging treatment; therefore, it is preferable to control the Ms temperature equation (3) to 0 or greater.

[0069] Equation (3): 1240.1 - 1300.3Х([C]+[N]) - 27.8Х[Si] - 33.3Х[Mn] - 61.1Х[Ni] - 41.7Х[Cr] - 27.4Х[Cu] + 32.8Х[Nb]

[0070] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N] represent the weight% content of each element, excluding units).

[0071] A precipitation-hardened martensitic stainless steel according to one embodiment of the present invention may have a reduction of area of ​​60% or more, and a larger reduction of area indicates superior hot workability.

[0072] Next, a method for manufacturing a precipitation-hardened martensitic stainless steel according to another aspect of the present invention will be described.

[0073] According to a method for manufacturing a precipitation-hardened martensitic stainless steel according to one embodiment of the present invention, the method comprises the steps of: preparing a steel material comprising, in weight%, C: 0.01 to 0.1%, Si: 0.10 to 1.0%, Mn: 0.50 to 1.50%, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Nb: 0.2 to 0.35%, Cu: 3.0 to 5.0%, N: 0.005 to 0.02%, P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), the remainder being Fe and unavoidable impurities, satisfying the following formula (1) being 0.45 or less and the following formula (2) being 40.0 or less; reheating the steel material; and hot rolling.

[0074] Equation (1): [Cu] / (7.4-3.5Х[C]-1.1Х[Si]+0.4Х[Mn]-0.1Х[Cr]+0.3Х[Ni]+0.5Х[Nb]-4.5Х[N])

[0075] Equation (2): 82.0-222.7Х[C]-0.3Х[Si]-12.4Х[Mn]+9.9Х[Cr]-22.4Х[Ni]-4.1Х[Nb]-23.3Х[Cu]-218.9Х[N]

[0076] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N], [P], [S] represent the weight% content of each element, excluding units).

[0077] The reason for limiting the numerical value of the alloy element content and the explanation for Equation (1) and Equation (2) are as described above.

[0078] A steel material is prepared by continuously casting molten steel satisfying the above composition and formulas (1) and (2). The steel material may be in the form of a slab or an ingot, but is not limited thereto.

[0079] The steel prepared above is reheated at a temperature range of 1200°C to 1280°C for at least one hour. In order to control the internal structure of the slab to austenite, it must be reheated at 1280°C or lower, which is the austenite range. Additionally, since the formation of the high-temperature precipitate phase Nb(C, N) during the reheating process can reduce hot workability, the reheating temperature must be controlled to 1200°C or higher. By heating at a temperature between 1200°C and 1280°C for at least one hour, the Cu segregation can be controlled to 6.5% or less. In the subsequent step, hot rolling is performed, and the hot rolling conditions can be carried out under normal conditions. By satisfying the above composition and composition formula, crack formation during the hot rolling stage is suppressed, thereby ensuring hot workability.

[0080] Hereinafter, the present invention will be described in more detail through preferred embodiments.

[0081] (Example)

[0082] After manufacturing an ingot having the composition of Table 1 below, it was reheated at 1280°C for 2 hours and hot-rolled to a thickness of 10 mm under a reduction rate of 95%.

[0083] P and S are included as impurities to the extent that P is 0.025% or less and S is 0.003% or less.

[0084] CSiMnCrNiNbCuN Example 1 0.05 0.15 0.88 16.25 0.00.24 3.00.005 Example 2 0.06 0.10 0.95 15.74 5.0.27 3.20.005 Example 3 0.03 0.20 1.00 17.05 5.03 13.4 0.005 Comparative Example 1 0.05 0. 330.7216.64.60.233.00.016Comparative Example 20.050.300.6916.74.60.254.00.014Comparative Example 30.030.800.5014.54.00.213.20.017Comparative Example 40.040.800.5016.03.10.235.00.017

[0085] To evaluate hot workability, rod-shaped specimens with a diameter of 10 mm were machined. To simulate the reheating temperature, the machined specimens were held at 1250°C for 5 minutes, then cooled to 1100°C (the evaluation temperature for hot workability), held for 30 seconds, and subjected to a tensile test. The cross-sectional area of ​​the fracture surface before and after fracture was measured to calculate the cross-sectional reduction rate. Hot workability was deemed to meet the target level if the cross-sectional reduction rate was 60% or higher. The cross-sectional reduction rate was measured using a material property testing machine model Gleeble 3800. The degree of cross-sectional reduction was determined by uniaxially tensile testing at a speed of 30 mm / sec after holding the specimen at 1250°C for 5 minutes, cooling to 1100°C (the evaluation temperature), and holding for 30 seconds. A higher cross-sectional reduction rate indicates superior hot workability.

[0086] In this way, the cross-sectional reduction rates, Equation (1), and Equation (2) for the examples and comparative examples according to Table 1 above are shown in Table 2 below.

[0087] Formula (1) Formula (2) Hot workability cross-sectional area reduction rate (%) * Pass / Fail Example 1 0.41 36.37 1.1 Pass Example 2 0.44 34.76 3.2 Pass Example 3 0.45 26.47 5.0 Pass Comparative Example 1 0.43 48.8 35.2 Fail Comparative Example 2 0.58 27.2 38.3 Fail Comparative Example 3 0.50 43.74 0.0 Fail Comparative Example 4 0.84 34.40.0 Fail

[0088] * Pass: Cross-sectional reduction rate of 60% or more, Fail: Cross-sectional reduction rate of less than 60%

[0089] Referring to Table 2 above, it was confirmed that when both Equation (1) and Equation (2) are satisfied, the cross-sectional reduction rate is 60% or more during a high-temperature tensile test at 1100℃, indicating excellent hot workability.

[0090] On the other hand, Comparative Examples 1 and 2 were found to have inferior hot workability, with a cross-sectional reduction rate of less than 60% when only one of Equation (1) and Equation (2) was satisfied. In addition, Comparative Example 3, which did not satisfy both Equation (1) and Equation (2), also showed a cross-sectional reduction rate of less than 60%, indicating inferior hot workability. This demonstrates that both Equation (1), which represents the solid solution of Cu, and Equation (2), which represents the δ-ferrite content, must be satisfied.

[0091] In addition, in the case of Comparative Example 4, it can be confirmed that hot workability cannot be secured because cross-sectional reduction is impossible even when the Cu content is excessive.

[0092] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims set forth below.

Claims

1. In weight%, comprising C: 0.01 to 0.10%, Si: 0.10 to 1.00%, Mn: 0.50 to 1.50%, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Nb: 0.20 to 0.35%, Cu: 3.0 to 5.0%, N: 0.005 to 0.020%, P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), and the remainder being Fe and unavoidable impurities, The following formula (1) is 0.45 or less, and Precipitation-hardened martensitic stainless steel satisfying the following formula (2) of 40.0 or less: Equation (1): [Cu] / (7.4-3.5Х[C]-1.1Х[Si]+0.4Х[Mn]-0.1Х[Cr]+0.3Х[Ni]+0.5Х [Nb]-4.5Х[N]) Equation (2): 82.0-222.7Х[C]-0.3Х[Si]-12.4Х[Mn]+9.9Х[Cr]-22.4Х[Ni]-4.1Х[Nb]-23.3Х[Cu]-218.9Х[N] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N] represent the weight% content of each element, excluding units).

2. In Claim 1, The above martensitic stainless steel is a precipitation-hardened martensitic stainless steel containing δ-ferrite in an area fraction of 5.0% or less.

3. In Claim 1, The above martensitic stainless steel is a precipitation-hardening martensitic stainless steel in which the following formula (3) is 0 or greater. Equation (3): 1240.1 - 1300.3Х([C]+[N]) - 27.8Х[Si] - 33.3Х[Mn] - 61.1Х[Ni] - 41.7Х[Cr] - 27.4Х[Cu] + 32.8Х[Nb] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N] represent the weight% content of each element, excluding units).

4. In Claim 1, The above martensitic stainless steel is a precipitation-hardened martensitic stainless steel having a reduction of area of ​​60% or more.

5. A step of preparing a steel material comprising, in weight%, C: 0.01 to 0.10%, Si: 0.10 to 1.00%, Mn: 0.50 to 1.50%, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Nb: 0.20 to 0.35%, Cu: 3.0 to 5.0%, N: 0.005 to 0.020%, P: 0.035% or less (excluding 0), S: 0.01% or less (excluding 0), and the remainder being Fe and unavoidable impurities, wherein the following formula (1) is 0.45 or less and the following formula (2) is 40.0 or less: Step of reheating the above steel material; and Hot rolling step; A method for manufacturing precipitation-hardened martensitic stainless steel comprising Equation (1): [Cu] / (7.4-3.5Х[C]-1.1Х[Si]+0.4Х[Mn]-0.1Х[Cr]+0.3Х[Ni]+0.5Х[Nb]-4.5Х[N]) Equation (2): 82.0-222.7Х[C]-0.3Х[Si]-12.4Х[Mn]+9.9Х[Cr]-22.4Х[Ni]-4.1Х[Nb]-23.3Х[Cu]-218.9Х[N] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N], [P], [S] represent the weight% content of each element, excluding units).

6. In Claim 5, The above steel is a method for manufacturing a precipitation-hardening martensitic stainless steel in which the following martensitic transformation start temperature is 0 or higher. Equation (3): 1240.1 - 1300.3Х([C]+[N]) - 27.8Х[Si] - 33.3Х[Mn] - 61.1Х[Ni] - 41.7Х[Cr] - 27.4Х[Cu] + 32.8Х[Nb] (Here, [C], [Si], [Mn], [Cr], [Ni], [Nb], [Cu], [N] represent the weight% content of each element, excluding units).

7. In Claim 5, A method for manufacturing precipitation-hardened martensitic stainless steel, wherein the step of reheating the steel comprises heat treating at a temperature range of 1200℃ to 1280℃ for at least one hour.

8. In Claim 7, A method for manufacturing precipitation-hardened martensitic stainless steel in which Cu segregation is controlled to 6.5% by weight or less in the step of reheating the steel.

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