Martensitic stainless steel with improved strength and corrosion resistance and manufacturing method thereof

Abstract

An object of the present invention is to provide a martensitic stainless steel having improved strength and corrosion resistance. [Solution] The martensitic stainless hot-rolled annealed steel sheet of the present invention contains, by weight%, C: 0.3-0.5%, N: 0.01-0.025%, Si: 0.3-0.5%, Mn: 0.4-0.6, Cr: 13.1-14.5%, Mo: 0.95-1.10%, V: 0.05-0.3%, Ni: 0.3-0.5%, Cu: 0.001-0.5%, with the remainder being Fe and unavoidable impurities, and is characterized in that it satisfies the following formula (1): Formula (1): 16.4≦(Cr+3.3Mo+16N)*(Mo+V)≦23.3 Here, Cr, N, Mo, and V mean the content (wt%) of each element.

Description

[Technical field]

[0001] The present invention relates to a martensitic stainless steel and a manufacturing method thereof, and more particularly to a martensitic stainless steel applicable to various parts materials such as home appliances, automobile compressor parts, doctor blades, etc., and a manufacturing method thereof. [Background technology]

[0002] Generally, stainless steels are classified according to their chemical composition and metal structure. According to their metal structure, stainless steels can be classified into austenitic, ferritic, martensite, and dual phase.

[0003] Martensitic stainless steel is a material that has excellent hardness and abrasion resistance, but is very brittle and has low elongation, and the carbon content varies depending on the application. For example, for brake discs and anchors, which do not require high abrasion resistance, 0.1% or less carbon is added, for Class 1 Western tableware, 0.1 to 0.3% carbon is added, for knives, scissors, surgical knives, etc., which require high abrasion resistance, 0.3 to 0.7% carbon is added, and for industrial knives, 1% or more carbon is added.

[0004] STS420 is a typical martensitic stainless steel containing 12 to 15% chromium, and is the most widely used due to its excellent strength, hardness and corrosion resistance.

[0005] In order to ensure strength and hardness, martensitic stainless steel utilizes a tempered martensite structure that is formed by applying a strengthening heat treatment to a microstructure in which chromium carbide is distributed in a ferrite matrix after annealing to form an austenite phase, which is a high-temperature stable phase, and then rapidly cooling.Tempered martensite is a very light structure, and the higher the content of dissolved carbon, the higher the hardness.

[0006] On the other hand, the wear resistance of martensitic stainless steel can be ensured by allowing a certain percentage of carbides to remain or precipitate after heat treatment. Carbon and chromium react to precipitate in the form of chromium carbide, which reduces the Cr concentration in the matrix and reduces corrosion resistance.

[0007] In addition, the larger the size of the remaining carbides, the more difficult they are to decompose into the matrix, resulting in deviations in hardness and corrosion resistance. In a fatigue environment, stress is concentrated and they act as starting points for cracks, shortening the lifespan of the material.

[0008] Meanwhile, highly brittle martensitic stainless steel needs to be softened to make it easier to process, and it undergoes a batch annealing furnace (BAF) process that makes heat treatment easy. When the annealing process is carried out in a wound coil state, deviations in the thermal history occur along the length. Specifically, at the halfway point in the length direction, the heating and cooling rates are the slowest, so the size of the carbides becomes coarse, and this deviation remains even after cold rolling, acting as a cause of deviations in the physical properties of the final material.

[0009] Therefore, there is a demand for the development of martensitic stainless steels that have hardness, strength and corrosion resistance equal to or greater than those of conventional high carbon materials, while at the same time suppressing material deviations, and for the establishment of heat treatment conditions. Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a martensitic stainless steel having improved hardness, strength and corrosion resistance by optimizing the contents of Mo and V, and a manufacturing method thereof. [Means for solving the problem]

[0011] The martensitic stainless hot-rolled annealed steel sheet of the present invention with improved strength and corrosion resistance contains, by weight%, 0.3-0.5% C, 0.01-0.025% N, 0.3-0.5% Si, 0.4-0.6% Mn, 13.1-14.5% Cr, 0.95-1.10% Mo, 0.05-0.3% V, 0.3-0.5% Ni, 0.001-0.5% Cu, with the remainder being Fe and unavoidable impurities, and satisfies the following formula (1):

[0012] Formula (1): 16.4≦(Cr+3.3Mo+16N)*(Mo+V)≦23.3 Here, Cr, N, Mo, and V mean the content (wt%) of each element.

[0013] Moreover, the martensitic stainless hot-rolled annealed steel sheet of the present invention having improved strength and corrosion resistance can satisfy the following formula (2).

[0014] Formula (2): -14≦-36442+248C+365Cr+373Mo+530V+365Fe+350Si+312Mn+331Ni+506Cu≦50 Here, C, Cr, Mo, V, Fe, Si, Mn, Ni, and Cu mean the content (wt%) of each element.

[0015] Moreover, the martensitic stainless hot-rolled annealed steel sheet of the present invention having improved strength and corrosion resistance can satisfy the following formula (3).

[0016] Formula (3): 0.37≦C+N≦0.43

[0017] Moreover, the martensitic stainless hot-rolled annealed steel sheet of the present invention having improved strength and corrosion resistance can satisfy the following formula (4).

[0018] Formula (4): 1.0≦Mo+V≦1.35

[0019] In addition, the martensitic stainless steel hot-rolled annealed steel sheet of the present invention, which has improved strength and corrosion resistance, has a ferrite matrix structure, and contains primary carbides represented by (Cr,Fe,Mo,V)7C3 and (Cr,Fe,Mo,V)23 It may contain secondary carbides, represented by C6.

[0020] In the hot-rolled annealed martensitic stainless steel sheet of the present invention with improved strength and corrosion resistance, the weight percentage of (Mo+V) in the primary carbides may be 2.93 to 5.67%.

[0021] In the hot-rolled annealed martensitic stainless steel sheet of the present invention having improved strength and corrosion resistance, the weight percentage of (Mo+V) in the secondary carbides may be 12.2 to 14.8%.

[0022] In the hot-rolled annealed martensitic stainless steel sheet of the present invention having improved strength and corrosion resistance, the primary carbides may have a grain size of 10 μm or less.

[0023] In addition, the martensitic stainless steel hot-rolled annealed steel sheet of the present invention, which has improved strength and corrosion resistance, has a deviation of carbides in the length direction of 10 pieces / 100 μm. 2 It may be the following.

[0024] In addition, the martensitic stainless steel hot-rolled annealed steel sheet of the present invention, which has improved strength and corrosion resistance, has a distribution density of carbides after cold rolling of 42 to 58 particles / 100 μm. 2 may be also possible.

[0025] A method for producing a martensitic stainless steel having improved strength and corrosion resistance of the present invention includes the steps of hot rolling a slab containing, by weight%, 0.3-0.5% C, 0.01-0.025% N, 0.3-0.5% Si, 0.4-0.6% Mn, 13.1-14.5% Cr, 0.95-1.10% Mo, 0.05-0.3% V, 0.3-0.5% Ni, 0.001-0.5% Cu, with the remainder being Fe and unavoidable impurities, and satisfying the following formula (1), performing a batch annealing heat treatment in a temperature range of 600-900°C immediately after the hot rolling, cold rolling the hot-rolled annealed material, and performing a strengthening heat treatment on the cold-rolled material.

[0026] In addition, in the manufacturing method of martensitic stainless steel having improved strength and corrosion resistance according to the present invention, the hot-rolled annealed material has a ferrite base structure, and primary carbides represented by (Cr,Fe,Mo,V)7C3 and (Cr,Fe,Mo,V) 23 It may contain secondary carbides, represented by C6.

[0027] In addition, in the method for producing martensitic stainless steel having improved strength and corrosion resistance of the present invention, the weight percentage of (Mo+V) in the primary carbides may be 2.93 to 5.67%.

[0028] In the method for producing a martensitic stainless steel having improved strength and corrosion resistance of the present invention, the weight percentage of (Mo+V) in the secondary carbides may be 12.2 to 14.8%.

[0029] In the method for producing a martensitic stainless steel having improved strength and corrosion resistance of the present invention, the primary carbides may have a grain size of 10 μm or less.

[0030] In addition, the manufacturing method of the martensitic stainless steel having improved strength and corrosion resistance of the present invention is to produce 42 to 58 pieces / 100 μm after cold rolling. 2 The following carbides may be distributed:

[0031] In addition, in the method for producing martensitic stainless steel having improved strength and corrosion resistance according to the present invention, the strengthening heat treatment may include a step of quenching at a temperature of 980 to 1,050°C and a step of tempering at a temperature of 400 to 600°C for 1 minute to 1 hour.

[0032] In addition, in the method for producing martensitic stainless steel having improved strength and corrosion resistance of the present invention, the Vickers hardness after strengthening heat treatment may be 520 to 650 Hv.

[0033] Furthermore, the method for producing a martensitic stainless steel having improved strength and corrosion resistance according to the present invention can satisfy the following formula (2).

[0034] Formula (2): -14≦-36442+248C+365Cr+373Mo+530V+365Fe+350Si+312Mn+331Ni+506Cu≦50 Here, C, Cr, Mo, V, Fe, Si, Mn, Ni, and Cu mean the content (wt%) of each element.

[0035] Furthermore, the method for producing a martensitic stainless steel having improved strength and corrosion resistance according to the present invention can satisfy the following formulas (3) and (4).

[0036] Formula (3): 0.37≦C+N≦0.43

[0037] Formula (4): 1.0≦Mo+V≦1.35 Effect of the Invention

[0038] According to the present invention, it is possible to provide a martensitic stainless steel having sufficient hardness while also improving strength and corrosion resistance, and a method for producing the same. [Brief description of the drawings]

[0039] [Figure 1] 1 is a graph illustrating the relationship between the (Cr+3.3Mo+16N)*(Mo+V) value of the martensitic stainless steel of the present invention and the Mo+V content in the carbides. [Diagram 2] 1 is a graph illustrating the relationship between the (Cr+3.3Mo+16N)*(Mo+V) value of the martensitic stainless steel of the present invention and the size of primary carbides expressed as (Cr,Fe,Mo,V)7C3. [Diagram 3] 1 is a graph for explaining the relationship between the (Cr+3.3Mo+16N)*(Mo+V) value of the martensitic stainless steel of the present invention and the carbide distribution of the hot-rolled and annealed material. [Figure 4] 1 is a scanning electron microscope (SEM) photograph of chromium carbide in a microstructure of Comparative Example 4 after rewinding and heat treatment for strengthening. [Diagram 5]1 is a scanning electron microscope (SEM) photograph of chromium carbide in the microstructure of Example 1 after rewinding and heat treatment for strengthening. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] A martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to one embodiment of the present invention contains, by weight%, 0.3-0.5% C, 0.01-0.025% N, 0.3-0.5% Si, 0.4-0.6% Mn, 13.1-14.5% Cr, 0.95-1.10% Mo, 0.05-0.3% V, 0.3-0.5% Ni, 0.001-0.5% Cu, with the remainder being Fe and unavoidable impurities, and satisfies the following formula (1).

[0041] Formula (1): 16.4≦(Cr+3.3Mo+16N)*(Mo+V)≦23.3 Here, Cr, N, Mo, and V mean the content (wt%) of each element.

[0042] Hereinafter, the embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are presented to fully 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. In the drawings, parts that are not relevant to the description may be omitted in order to clarify the present invention, and the size of components may be somewhat exaggerated in order to facilitate understanding.

[0043] Throughout the specification, when any part is said to "comprise" a certain element, this does not mean to exclude other elements, but that it may further include other elements, unless specifically stated to the contrary.

[0044] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0045] The present inventors have conducted various investigations to improve the corrosion resistance of high carbon martensitic stainless steel and minimize the deviation in material properties, and have come to the following findings.

[0046] In order to improve corrosion resistance, it may be possible to consider increasing the Cr content. However, since Cr is an expensive element and increases the production cost, this is not a preferred development direction.

[0047] Hot-rolled and annealed materials produced through the general continuous casting, hot rolling, and batch annealing processes have a ferrite matrix and contain chromium carbides. Chromium carbides are primary chromium carbides expressed as M7C3 (M is Cr:Fe=73.6%:17.2%) that are formed by segregation of Cr and C centers during the casting process and have a size of several tens to several hundreds of μm, and primary chromium carbides that are preferentially precipitated at grain boundaries and martensite lath boundaries during batch annealing and are expressed as M. 23 It contains secondary chromium carbides represented by C6 (M is Cr:Fe=73%:19.3%).

[0048] In particular, if the primary chromium carbides distributed in the center of the material are coarse, with a size of 10 μm or more, they will remain without being decomposed even after hot rolling and batch annealing. Even if cold rolling is performed with a certain level of reduction, it is difficult to segment them, so they will remain as coarse carbides with a size of 3 μm or more. Even if cold rolling is performed with a certain level of reduction, it is difficult to segment them, so they will remain as coarse carbides with a size of 3 μm or more.

[0049] The remaining carbides reduce the rate of re-dissolution into the austenite phase during strengthening heat treatment, reducing the hardness and corrosion resistance of the final martensitic stainless steel material and causing localized material imbalances.

[0050] The inventors have discovered that when the Mo and V contents are ensured to be at a certain level or more, it is possible to prevent the coarsening of chromium carbide and diversify the precipitation sites of chromium carbide to ensure uniform physical properties (corrosion resistance, hardness), as well as to enable rapid re-dissolution of chromium and carbon into the high-temperature austenite phase in the subsequent strengthening heat treatment step, thereby improving corrosion resistance and strength.

[0051] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. First, martensitic stainless steel will be described, and then a method for manufacturing the martensitic stainless steel will be described.

[0052] A martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to one aspect of the present invention contains, by weight, 0.3-0.5% C, 0.01-0.025% N, 0.3-0.5% Si, 0.4-0.6% Mn, 13.1-14.5% Cr, 0.95-1.10% Mo, 0.05-0.3% V, 0.3-0.5% Ni, 0.001-0.5% Cu, with the remainder being Fe and unavoidable impurities.

[0053] The reasons for limiting the numerical values ​​of the alloying element contents in the embodiments of the present invention will be described below. In the following, the unit is weight percent unless otherwise specified.

[0054] The C content is 0.3-0.5%. Carbon (C) is an essential element for ensuring the hardness of martensitic stainless steel, and is added in an amount of 0.3% or more to ensure the hardness after quenching / tempering heat treatment. However, if the content is excessive, chromium carbides are formed in an excessive amount, which not only reduces the corrosion resistance of the material itself, but also increases the amount of chromium carbides and may cause a decrease in toughness due to the remaining coarse carbides, so the upper limit can be limited to 0.5%. Preferably, the C content is 0.36-0.4%.

[0055] The N content is 0.01 to 0.025%. Nitrogen (N) is an element that is added to improve both corrosion resistance and hardness, and has the advantage of not inducing localized fine segregation even when added in place of C, and therefore not forming coarse precipitates in the product. In order to realize this effect, the present invention adds 0.01% or more of N. However, if the content is excessive, there is a problem of forming Cr nitrides, which are low-temperature precipitates, and excessive retained austenite phase, so the upper limit may be limited to 0.025% in order to ensure fatigue properties.

[0056] The C+N content is 0.37-0.43%. The hardness of martensitic stainless steel can be ensured by controlling the content of interstitial elements C and N to 0.37% or more. On the other hand, as the C+N content increases, the rolling force increases during hot rolling, making manufacturing difficult and reducing toughness. Therefore, taking into consideration the hardness and ease of manufacturing of the final material, the C+N value range can be controlled to 0.37-0.43%.

[0057] The Si content is 0.3 to 0.5%. Silicon (Si) is an essential element added for deoxidization and improves strength, and is added in an amount of 0.3% or more in the present invention. However, if the content is excessive, there is a problem that scale is formed on the surface of the steel sheet during hot rolling, which impairs the surface quality, so the upper limit can be limited to 0.5%.

[0058] The Mn content is 0.4 to 0.6%. Manganese (Mn) is an element added to improve strength and hardenability, and combines with sulfur (S) that is inevitably contained during the manufacturing process to form MnS, which serves to suppress cracks caused by sulfur (S). In the present invention, 0.4% or more of Mn is added. However, if the content is excessive, there is a problem that it deteriorates the surface quality of the steel and reduces toughness, so the upper limit may be limited to 0.6%.

[0059] The Cr content is 13.1-14.5%. Chromium (Cr) is a basic element that ensures corrosion resistance and forms chromium carbide to improve hardness and wear resistance, and is added in an amount of 13.1% or more in the present invention. However, if the content is excessive, the manufacturing cost increases and the hardening ability increases, so the upper limit can be limited to 14.5%.

[0060] The Mo content is 0.95 to 1.10%. Molybdenum (Mo) is an element that improves corrosion resistance, inhibits decarburization, and improves hardenability, and it replaces Cr in chromium carbide to refine the carbide, so in the present invention, 0.95% or more is added. However, if the content is excessive, the manufacturing cost increases and the hardenability increases, so the upper limit can be limited to 1.10%.

[0061] The V content is 0.05 to 0.3%. Vanadium (V) is an element that forms carbides to suppress the coarsening of chromium carbides, and is effective in preventing grain coarsening during heat treatment and improving wear resistance, and is added in an amount of 0.05% or more in the present invention. However, if the content is excessive, there are problems in that the manufacturing cost increases and the toughness decreases, so the upper limit may be limited to 0.3%.

[0062] The Mo+V content is 1.0-1.35%. By controlling the content of Mo and V, which react preferentially with C instead of Cr to form carbide, to 1.0% or more, it is possible to improve corrosion resistance and ensure fine grains of chromium carbide. On the other hand, since the above-mentioned effects saturate as the Mo+V content increases, the Mo+V value range can be controlled to 1.0-1.35% in consideration of the price competitiveness of the material.

[0063] The Ni content is 0.3 to 0.5%. Nickel (Ni) is an essential element added to ensure the austenitic structure in the hot working region of martensitic stainless steels and plays a role in improving corrosion resistance and hardenability, and is added in an amount of 0.3% or more in the present invention. However, if the content is excessive, there are problems in that the manufacturing cost increases and the workability decreases, so the upper limit may be limited to 0.5%.

[0064] The Cu content is 0.001-0.5%. Copper (Cu) is an element that forms the austenite phase and plays a role in improving strength, hardness, and corrosion resistance, and is added in an amount of 0.001% or more in the present invention. However, if the content is too high, there are problems such as an increase in manufacturing costs, a decrease in hot workability, and formation of precipitates such as CuS, which is harmful to corrosion resistance when mixed with S, so the upper limit can be limited to 0.5%.

[0065] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintended impurities may be inevitably mixed in from the raw materials or the surrounding environment, and it is not possible to exclude them. Since any engineer of a normal manufacturing process would know about these impurities, the contents of all of them will not be mentioned in this specification.

[0066] On the other hand, the martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to one embodiment of the present invention satisfies the following formula (1).

[0067] Formula (1): 16.4≦(Cr+3.3Mo+16N)*(Mo+V)≦23.3 Here, Cr, N, Mo, and V mean the content (wt%) of each element.

[0068] Generally, the Pitting Resistance Equivalent Number (PREN) is expressed as Cr+3.3Mo+16N. A higher PREN*(Mo+V) value indicates better pitting resistance. In the present invention, in addition to limiting the alloy element contents to the above conditions, the PREN value in formula (1) is controlled to 16.4 or more to ensure corrosion resistance even in humid environments such as compressors.

[0069] In the manufacturing process of martensitic stainless steel, hot-rolled annealed materials produced through the batch annealing process have a ferrite matrix and contain chromium carbides. Chromium carbides are primarily chromium carbides expressed as M7C3 (M is Cr:Fe=73.6%:17.2%) that are formed by segregation of Cr and C centers during the casting process and have a size of several tens to several hundreds of μm, and are preferentially precipitated by the batch annealing grain boundaries and are called M. 23 C6 (M is Cr:Fe=73%:9.3%) contains secondary chromium carbides.

[0070] The primary carbides formed during cooling of the slab have limited control over their size and distribution during the hot rolling and cold rolling processes.

[0071] In the present invention, when a martensitic stainless steel is manufactured by a strengthening heat treatment including a quenching / tempering continuous heat treatment, the effects of Mo and V, which can refine carbides and improve the corrosion resistance of the steel after quenching / tempering, have been taken into consideration, and an optimized formula (1) has been derived.

[0072] Fig. 1 is a graph illustrating the relationship between the (Cr+3.3Mo+16N)*(Mo+V) value of a martensitic stainless steel according to an embodiment of the present invention and the Mo+V content in carbides. Fig. 2 is a graph illustrating the relationship between the (Cr+3.3Mo+16N)*(Mo+V) value of a martensitic stainless steel according to an embodiment of the present invention and the size of primary carbides expressed as (Cr,Fe,Mo,V)7C3.

[0073] As shown in Figures 1 and 2, it can be seen that as the (Cr+3.3Mo+16N)*(Mo+V) value increases, Cr in the chromium carbide is replaced by Mo and V, and the carbides are finely derived.

[0074] In the present invention, the following formula (2) was derived in consideration of the change in properties of carbide during strength heat treatment.

[0075] Specifically, the inventors derived formula (2) by considering the relationship between the contents of C, Cr, and N affected by the addition of Mo and V that change the properties of the precipitated carbide, the Mo+V content in chromium carbide, the presence or absence of formation of Z phase represented by M(C,N) (where M is 44V+41Cr) and vanadium nitride represented by MN (where M is 74.2V+5Cr) and the added components.

[0076] Formula (2): -14≦-36442+248C+365Cr+373Mo+530V+365Fe+350Si+312Mn+331Ni+506Cu≦50 Here, C, Cr, Mo, V, Fe, Si, Mn, Ni, and Cu mean the content (weight%) of each element.

[0077] The inventors have confirmed that the higher the value of formula (2), the more the primary carbides are prevented from coarsening and fine secondary carbides are precipitated. Specifically, when the value of formula (2) exceeds -14, the added Mo and V replace Cr in the primary and secondary carbides to suppress coarsening, and Z phase and vanadium nitride are formed and precipitate preferentially along the grain boundaries, resulting in the secondary chromium carbides (M 23 On the other hand, if the value of formula (2) is too high, there are problems that the Z phase and vanadium nitride themselves act as precipitation sites for secondary carbides and that manufacturing costs increase, so it is desirable to limit the value of formula (2) to 50 or less.

[0078] Generally, martensitic stainless steels undergo a hardening process after being processed into the final shape to ensure corrosion resistance and hardness. The hardening process involves maintaining the material at a high temperature of about 1,000~1,200℃ for a short period of time and then quenching it to room temperature. The chromium carbide is redissolved in the high-temperature austenite phase, increasing the chromium concentration in the base to about 12%, and thus forming a thin, dense passive film of chromium oxide on the surface of the material, improving the corrosion resistance of the material.

[0079] In addition, the austenite phase containing redissolved carbon or nitrogen during quenching is transformed into a martensite phase, thereby improving the hardness of the material. At this time, if the size of the spheroidized chromium carbides distributed in the matrix is ​​large, it is difficult for the chromium carbides to be redissolved in the high-temperature austenite phase, and the concentrations of chromium and carbon in the matrix are reduced, resulting in a decrease in the corrosion resistance and hardness of the material.

[0080] On the other hand, when the size of chromium carbide is fine, the chromium carbide can be easily redissolved even in a short heat treatment, and the concentrations of chromium, carbon and nitrogen in the matrix structure are increased, thereby improving the corrosion resistance and hardness.

[0081] Therefore, in order to simultaneously ensure the corrosion resistance and hardness of high carbon martensitic stainless steel, it is necessary to distribute chromium carbides finely and uniformly in the material before the strengthening heat treatment process, such as the hot rolled and annealed material.

[0082] The Mo and V added in the present invention suppress the growth of carbides by substituting Cr in primary and secondary Cr carbides, and preferentially combine with C to form fine carbides, thereby pre-occupying the precipitation sites of primary and secondary Cr carbides, thereby making the carbides finer and more uniformly distributed.

[0083] Specifically, the martensitic stainless steel hot-rolled annealed steel sheet having improved strength and corrosion resistance according to an embodiment of the present invention has a ferrite matrix and primary carbides represented by (Cr,Fe,Mo,V)7C3 and (Cr,Fe,Mo,V) 23 Contains secondary carbides represented by C6.

[0084] Mo and V form carbides in combination with Cr to reduce the Cr content in the carbides, and also form fine carbides, thereby increasing the chromium concentration in the matrix structure.

[0085] Specifically, the weight percentage of (Mo+V) in the primary carbide expressed as (Cr,Fe,Mo,V)7C3 is 2.93-5.67%, the grain size of the primary carbide is 10μm or less, and (Cr,Fe,Mo,V) 23 The weight percentage of (Mo+V) in the secondary carbide represented by C6 is 12.2-14.8%.

[0086] Meanwhile, the addition of Mo and V forms Z-phase represented by M(C,N) (where M is 44V+41Cr) and vanadium nitride represented by MN (where M is 74.2V+5Cr), and the Z-phase and vanadium nitride themselves act as precipitation sites for secondary carbides, allowing the carbides to be finely and uniformly distributed.

[0087] For example, the martensitic stainless steel hot-rolled annealed steel sheet having improved strength and corrosion resistance according to an embodiment of the present invention has a carbide deviation of 10 / 100 μm in the longitudinal direction. 2 The following is the result.

[0088] In addition, after the martensitic stainless steel hot-rolled annealed steel sheet is cold-rolled, the microstructure contains 42 to 58 grains / 100 μm. 2 Chromium carbides are distributed.

[0089] Next, a method for manufacturing a martensitic stainless steel according to another aspect of the present invention will be described.

[0090] A method for producing a martensitic stainless steel having improved strength and corrosion resistance according to an embodiment of the present invention includes the steps of hot rolling a slab containing, by weight%, 0.3-0.5% C, 0.01-0.025% N, 0.3-0.5% Si, 0.4-0.6% Mn, 13.1-14.5% Cr, 0.95-1.10% Mo, 0.05-0.3% V, 0.3-0.5% Ni, 0.001-0.5% Cu, with the remainder being Fe and unavoidable impurities, and satisfying the following formula (1); performing a batch annealing heat treatment in a temperature range of 600-900° C. immediately after the hot rolling; cold rolling the hot-rolled annealed material; and performing a strengthening heat treatment on the cold-rolled material.

[0091] Formula (1): 16.4≦(Cr+3.3Mo+16N)*(Mo+V)≦23.3

[0092] The reasons for limiting the numerical values ​​of the alloying element contents are as described above.

[0093] The stainless steel containing the above composition is cast into a slab by continuous casting or ingot casting, and then manufactured into a hot-rolled steel sheet that can be processed through hot rolling.

[0094] The hot-rolled steel sheet is then softened through a batch annealing heat treatment to ensure good workability before undergoing processing such as precision rolling to a thickness usable for coating applications.

[0095] Typically, when hot-rolled coils are batch-annealed in a coiled state, deviations in thermal history occur during the cooling / reheating process, which acts as a cause of deviations in the physical properties of the final material. Specifically, immediately after hot rolling, coils come into contact with the outside air, causing partial cooling deviations, which results in the appearance of martensite structures in areas with a fast cooling rate, resulting in the problem of non-uniform microstructures.

[0096] In the present invention, in order to minimize the time for which the hot-rolled coil, which is hot-rolled and coiled at a temperature range of 800 to 900°C, is air-cooled at room temperature, a batch annealing heat treatment is introduced immediately after hot rolling to prevent phase transformation to martensite.

[0097] Batch annealing may be performed in a temperature range of 600 to 900° C. to distribute carbides uniformly. If the annealing temperature is low, the driving force for annealing into ferrite and carbide phases is insufficient, and martensite phase may remain. If the annealing temperature is too high, reverse transformation to austenite phase occurs, causing grains to become coarse, and coarse chromium carbides are formed intensively at grain boundaries during cooling. In view of this, the temperature range of the batch annealing heat treatment is limited to 600 to 900° C.

[0098] According to the present invention, the martensitic stainless steel hot-rolled and annealed material that has been subjected to the batch annealing heat treatment can be manufactured into a martensitic stainless steel through a step of processing into a final shape and then performing a strengthening heat treatment.

[0099] The strengthening heat treatment may further include the steps of austenitizing, quenching, and tempering.

[0100] The austenitizing step is a step of transforming the base structure of the steel material from ferrite to austenite. According to one example, the austenitizing step may be heat-treated at a temperature of 1,000° C. or more for 1 minute or more.

[0101] In this step, the chromium carbide is redissolved in the matrix structure in the form of chromium and carbon, and the hardness of the martensitic stainless steel can be increased after the subsequent quenching step.

[0102] The quenching stage is a stage in which the austenitic structure is transformed into martensite, which has high hardness, by rapidly cooling from a temperature range of 980~1,050℃ to room temperature after the austenitizing treatment. If the cooling rate is kept at 0.2℃ / s or more, the martensite structure can be obtained.

[0103] The recoiling step is a step for imparting toughness to the martensite structure, which is brittle due to its high hardness after the quenching step. For example, the heat treatment can be performed at a temperature of 400 to 600°C for 1 minute to 1 hour depending on the thickness.

[0104] The ferrite structure can be finally transformed into a martensite structure through the above-mentioned strengthening heat treatment, and the desired hardness and corrosion resistance can be ensured. For example, the Vickers hardness of the material re-dissolved by the strengthening heat treatment may be 520 to 650 Hv.

[0105] The present invention will now be described in more detail with reference to the following examples.

[0106] Slabs with various alloy composition ranges shown in Table 1 below were reheated at 1,250°C, rough rolled, and then finish hot rolled at 800°C or higher. Next, the hot-rolled sheets were not cooled to room temperature, but were charged into a batch annealing furnace at 700°C while maintaining a temperature of 600°C or higher, and hot-rolled annealing was performed.

[0107] In Table 1 below, formula (1) is (Cr+3.3Mo+16N)*(Mo+V).

[0108] [Table 1]

[0109] During the annealing process, the presence or absence of formation of Z phase represented by M(C,N) (where M is 44V+41Cr) and vanadium nitride represented by MN (where M is 74.2V+5Cr), the average grain size (μm) of primary carbides, and the Mo+V content (wt%) in the carbides of the hot rolled and annealed material were measured by taking test pieces using the replica method of a projection electron microscope, and the EDS components of the TEM were measured and shown in Table 2 below. Next, the material was cold rolled to a thickness of 0.2 mm, cold rolled and annealed, and the carbide density was measured and shown in Table 2 below.

[0110] Additionally, the cold-rolled annealed material was subjected to a strengthening heat treatment. Specifically, the cold-rolled annealed material was heat-treated at 1,000°C for 420 seconds, quenched at a cooling rate of 233°C / s to 300°C, and then tempered at 350°C for 350 seconds to produce the final martensitic stainless steel. The Vickers hardness was measured, and the results are shown in Table 2 below.

[0111] [Table 2]

[0112] 1 to 3 are graphs illustrating the relationship between the (Cr+3.3Mo+16N)*(Mo+V) value of the martensitic stainless steel according to an embodiment of the present invention, the Mo+V content in the carbide, the size of the primary carbide expressed as (Cr,Fe,Mo,V)7C3, and the distribution of the carbides in the hot rolled annealed material. Referring to Tables 1 and 2 and Figs. 1 to 3, in the case of Examples 1 to 7 in which the contents of Mo and V and the values ​​of formula (1) are within the range of 16.4 to 23.3, it can be seen that Cr in the chromium carbide is replaced by Mo and V, and that the carbides are finely derived.

[0113] For example, in Examples 1 to 7, the weight percentage of (Mo+V) in the primary carbide expressed as (Cr,Fe,Mo,V)7C3 is 2.93 to 5.67%, the particle size of the primary carbide is 10 μm or less, and (Cr,Fe,Mo,V) 23 The weight percentage of (Mo+V) in the secondary carbide represented by C6 is 12.2-14.8%.

[0114] This is the result of optimized Mo and V forming carbides in combination with Cr to suppress the formation of coarse carbides, as well as the Z phase and vanadium nitride formed during hot rolling and annealing acting as precipitation sites for secondary carbides.

[0115] As a result, after cold rolling, the microstructure contains 42 to 58 particles / 100 μm. 2 The chromium carbides were distributed in the steel, and the hardness of the final material was ensured to be in the range of 520 to 650 Hv.

[0116] In contrast, in the case of Comparative Example 4, Comparative Example 8, and Comparative Example 9 in which Mo and V were not added, Z phase and vanadium nitride were not formed during hot rolling and annealing, and the grain sizes of the primary carbides in the hot rolling and annealing materials were very coarse, at 67 μm, 38 μm, and 74 μm, respectively.

[0117] In the cases of Comparative Examples 5, 6, 7, and 10, Z phase and vanadium nitride were formed during hot rolling and annealing, but the Mo and V contents did not reach the range of 16.4 to 23.3 proposed in the present invention, and the carbide grain size of the hot rolled and annealed material could not be reduced to the target value of 10 μm or less.

[0118] In particular, in the case of Comparative Example 10, although a certain amount or more of Mo and V were added, the range of formula (1) was not satisfied, so the weight percentage of (Mo+V) in the secondary carbides could not be secured, and as a result, the chromium carbides could not be finely and uniformly distributed.

[0119] 4 and 5 are scanning electron microscope (SEM) photographs of chromium carbide in the microstructures of Comparative Example 4 and Example 1 after the strengthening heat treatment.

[0120] In the case of Comparative Example 4, it can be seen that the carbides are coarsened and segregated, and remain without being redissolved even after the strengthening heat treatment, whereas in the case of Example 1, it can be seen that most of the carbonaceous matter is redissolved after the strengthening heat treatment, and a martensite structure with a low area fraction of remaining carbonitrides is derived.

[0121] Thus, according to the disclosed embodiments, by controlling the alloy components and the relationships, it is possible to improve the corrosion resistance of high carbon martensitic stainless steel and minimize material deviation.

[0122] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and it should be understood that various changes and modifications can be made by those skilled in the art without departing from the concept and scope of the following claims. [Industrial Applicability]

[0123] The martensitic stainless hot-rolled annealed steel sheet according to the present invention has sufficient hardness and improved strength and corrosion resistance, and is therefore industrially applicable.

Claims

1. A martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance, comprising, by weight, 0.3-0.5% C, 0.01-0.025% N, 0.3-0.5% Si, 0.4-0.6% Mn, 13.1-14.5% Cr, 0.95-1.10% Mo, 0.05-0.3% V, 0.3-0.5% Ni, 0.001-0.5% Cu, with the remainder being Fe and unavoidable impurities, and satisfying the following formula (1): Formula (1): 16.4≦(Cr+3.3Mo+16N)*(Mo+V)≦23.3 Here, Cr, N, Mo, and V mean the content (wt %) of each element.

2. 2. The martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to claim 1, characterized in that the following formula (2) is satisfied: Formula (2): -14≦-36442+248C+365Cr+373Mo+530V+365Fe+350Si+312Mn+331Ni+506Cu≦50 Here, C, Cr, Mo, V, Fe, Si, Mn, Ni, and Cu mean the content (wt%) of each element.

3. 2. The martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to claim 1, characterized in that the following formula (3) is satisfied: Formula (3): 0.37≦C+N≦0.43

4. 2. The martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to claim 1, characterized in that the following formula (4) is satisfied: Formula (4): 1.0≦Mo+V≦1.35

5. The base structure is ferrite, (Cr, Fe, Mo, V) ７ C ３ and primary carbides represented by (Cr, Fe, Mo, V) ２３ C ６ 2. The martensitic stainless hot-rolled annealed steel sheet according to claim 1, characterized in that it contains secondary carbides represented by the formula:

6. 6. The martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to claim 5, characterized in that the weight percentage of (Mo+V) in the primary carbides is 2.93 to 5.67%.

7. 6. The martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to claim 5, characterized in that the weight percentage of (Mo+V) in the secondary carbides is 12.2 to 14.8%.

8. 6. The martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to claim 5, characterized in that the grain size of the primary carbides is 10 μm or less.

9. The deviation of carbides in the length direction is 10 pieces / 100 μm ２ 2. The martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to claim 1, wherein:

10. The distribution density of carbides after cold rolling is 42 to 58 pieces / 100 μm. ２ 2. The martensitic stainless hot-rolled annealed steel sheet having improved strength and corrosion resistance according to claim 1 .

11. hot rolling a slab containing, by weight, 0.3-0.5% C, 0.01-0.025% N, 0.3-0.5% Si, 0.4-0.6% Mn, 13.1-14.5% Cr, 0.95-1.10% Mo, 0.05-0.3% V, 0.3-0.5% Ni, 0.001-0.5% Cu, with the remainder being Fe and unavoidable impurities, and satisfying the following formula (1): Immediately after hot rolling, batch annealing is performed at a temperature range of 600 to 900°C. A step of cold rolling the hot rolled annealed material; and A method for producing martensitic stainless steel having improved strength and corrosion resistance, comprising the step of subjecting a cold-rolled material to a strengthening heat treatment. Formula (1): 16.4≦(Cr+3.3Mo+16N)*(Mo+V)≦23.3 Here, Cr, N, Mo, and V mean the content (wt %) of each element.

12. The hot-rolled annealed material has a ferrite base structure and contains (Cr, Fe, Mo, V) ７ C ３ and primary carbides represented by (Cr, Fe, Mo, V) ２３ C ６ The method for producing martensitic stainless steel having improved strength and corrosion resistance according to claim 11, characterized in that the martensitic stainless steel contains secondary carbides represented by the formula:

13. The method for producing martensitic stainless steel having improved strength and corrosion resistance according to claim 12, characterized in that the weight percentage of (Mo+V) in the primary carbides is 2.93 to 5.67%.

14. The method for producing martensitic stainless steel having improved strength and corrosion resistance according to claim 12, characterized in that the weight percentage of (Mo+V) in the secondary carbides is 12.2 to 14.8%.

15. The method for producing a martensitic stainless steel having improved strength and corrosion resistance according to claim 12, characterized in that the grain size of the primary carbides is 10 μm or less.

16. After cold rolling, 42-58 pieces / 100μm ２ A method for producing a martensitic stainless steel having improved strength and corrosion resistance according to claim 11, characterized in that the following carbides are distributed:

17. The strengthening heat treatment is quenching at a temperature of 980 to 1,050°C; 12. The method for producing martensitic stainless steel having improved strength and corrosion resistance as set forth in claim 11, further comprising the step of tempering at a temperature of 400 to 600° C. for 1 minute to 1 hour.

18. 18. The method for producing martensitic stainless steel having improved strength and corrosion resistance according to claim 17, characterized in that the Vickers hardness after strengthening heat treatment is 520 to 650 Hv.

19. The method for producing martensitic stainless steel having improved strength and corrosion resistance according to claim 11, characterized in that the following formula (2) is satisfied: Formula (2): -14≦-36442+248C+365Cr+373Mo+530V+365Fe+350Si+312Mn+331Ni+506Cu≦50 Here, C, Cr, Mo, V, Fe, Si, Mn, Ni, and Cu mean the content (wt%) of each element.

20. The method for producing martensitic stainless steel having improved strength and corrosion resistance according to claim 11, characterized in that the following formulas (3) and (4) are satisfied: Formula (3): 0.37≦C+N≦0.43 Formula (4): 1.0≦Mo+V≦1.35

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