High-strength stainless steel having excellent formability and method for manufacturing same

Abstract

The present invention relates to a method for manufacturing a high-strength stainless steel having excellent formability. More specifically, the present invention relates to: a stainless steel containing, in wt%, 0.01-0.10% of C, 0.01-0.10% of N, 0.01-1.00% of Si, 0.01-3.00% of Mn, 10.0-20.0% of Cr, and 0.001-1.000% of Al, with the remainder comprising Fe and inevitable impurities, wherein the microstructure is composed of a dual phase of ferrite and martensite, the room temperature yield strength is at least 350 MPa, the room temperature tensile strength is at least 500 MPa, and the R-bar value based on Equation (1) is at least 1.0; and a method for manufacturing the stainless steel. Equation (1): R-bar = (R0 + 2×R45 + R90) / 4 (where, R0 represents the R value in a direction parallel to the rolling direction, R45 represents the R value in a direction at a 45-degree angle to the rolling direction, and R90 represents the R value in a direction at a 90-degree angle to the rolling direction)

Description

High-strength stainless steel with excellent formability and its manufacturing method

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

[0002] Ferritic stainless steel products, widely used in kitchenware, home appliances, and automotive components, are increasingly demanding high functionality, including high strength and lightweight design. In particular, increased strength, coupled with reduced thickness, reduces product weight and ultimately reduces energy efficiency, leading to extreme cost savings.

[0003] However, ferritic stainless steels typically do not undergo phase transformation, limiting the potential for strength enhancement through grain refinement. Furthermore, even if strength is improved, the material's formability deteriorates, limiting its application in products.

[0004] In order to solve the above-described problem, the present invention aims to provide a stainless steel having high strength and high formability and a manufacturing method thereof by controlling martensite phase transformation by controlling the cooling rate after cold rolling annealing heat treatment at a temperature higher than Ac1.

[0005] In order to achieve the above object, a stainless steel according to an embodiment of the present invention contains, in wt%, C: 0.01% to 0.10%, N: 0.01% to 0.10%, Si: 0.01% to 1.00%, Mn: 0.01% to 3.00%, Cr: 10.0% to 20.0%, Al: 0.001% to 1.000%, the remainder Fe and other inevitable impurities, has a microstructure of dual phases of ferrite and martensite, has a room temperature yield strength of 350 MPa or more, a room temperature tensile strength of 500 MPa or more, and may have an R-bar value of the following equation (1) of 1.0 or more.

[0006] Equation (1): R-bar = (R0 + 2×R45 + R90) / 4

[0007] (Here, R0 means the R value in the direction parallel to the rolling direction, R45 means the R value in the direction 45 degrees to the rolling direction, and R90 means the R value in the direction 90 degrees to the rolling direction)

[0008] In addition, the stainless steel according to one embodiment of the present invention may further include, in wt%, Ni: more than 0% and less than or equal to 1.00%.

[0009] In addition, the stainless steel according to one embodiment of the present invention can satisfy the following equation (3).

[0010] Formula (3): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) ≥10

[0011] (Here, C, N, Ni, Mn, Cr, Si and Al represent the weight percent of each element)

[0012] Additionally, the stainless steel according to one embodiment of the present invention may have a ridging height of 20 μm or less.

[0013] In addition, the stainless steel according to one embodiment of the present invention may not cause cracks or orange peel to occur at the bent portion when bent in a direction parallel to the rolling direction (C direction) and when bent at an angle of 90 degrees in a direction perpendicular to the rolling direction (L direction) under conditions of 1R or less.

[0014] In addition, a method for manufacturing stainless steel according to an embodiment of the present invention may include a step of reheating a slab containing, in wt%, C: 0.01% to 0.10%, N: 0.01% to 0.10%, Si: 0.01% to 1.00%, Mn: 0.01% to 3.00%, Cr: 10.0% to 20.0%, Al: 0.001% to 1.000%, the remainder Fe and other unavoidable impurities; a step of hot-rolling after the reheating; a step of hot-rolling annealing after the hot-rolling; a step of cold-rolling after the hot-rolling annealing; and a step of cold-rolling annealing at 900°C to 1100°C after the cold rolling and cooling to room temperature at a cooling rate of 0.5°C / sec to 55°C / sec.

[0015] Additionally, the reheating according to one embodiment of the present invention can be performed at 1050°C to 1250°C.

[0016] Additionally, the hot rolling according to one embodiment of the present invention can be performed at 700°C to 950°C.

[0017] In addition, the hot rolling annealing according to one embodiment of the present invention can be performed at 700°C to 950°C.

[0018] In addition, the slab according to one embodiment of the present invention may further include, in wt%, Ni: more than 0% and less than or equal to 1.00%.

[0019] In addition, the stainless steel according to one embodiment of the present invention can satisfy the following equation (3).

[0020] Formula (3): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) ≥ 10

[0021] (Here, C, N, Ni, Mn, Cr, Si and Al represent the weight percent of each element)

[0022] In addition, the stainless steel after cold rolling and annealing according to one embodiment of the present invention may have a microstructure of dual phases of ferrite and martensite.

[0023] In addition, the stainless steel after cold rolling and annealing according to one embodiment of the present invention may have a martensite phase having an area fraction of 20% or more.

[0024] In addition, the cold-rolled and annealed stainless steel according to one embodiment of the present invention may have a C+N content of 0.1% or less in weight percent and an average grain size of a ferrite phase of 50 ㎛ or less.

[0025] In addition, the stainless steel according to one embodiment of the present invention may have an R-bar value of the following formula (1) of 1.0 or more.

[0026] Equation (1): R-bar = (R0 + 2×R45 + R90) / 4

[0027] (Here, R0 means the R value in the direction parallel to the rolling direction, R45 means the R value in the direction 45 degrees to the rolling direction, and R90 means the R value in the direction 90 degrees to the rolling direction)

[0028] Additionally, the stainless steel according to one embodiment of the present invention may have a ridging height of 20 μm or less.

[0029] In addition, the stainless steel according to one embodiment of the present invention may not cause cracks or orange peel to occur at the bent portion when bent in a direction parallel to the rolling direction (C direction) and when bent at an angle of 90 degrees in a direction perpendicular to the rolling direction (L direction) under conditions of 1R or less.

[0030] In addition, the stainless steel according to one embodiment of the present invention may have a room temperature yield strength of 350 MPa or more and a room temperature tensile strength of 500 MPa or more.

[0031] According to the present invention, by controlling the cooling rate after cold rolling annealing heat treatment at a temperature higher than Ac1 to control martensite phase transformation, stainless steel having high strength and high formability can be manufactured.

[0032] Figure 1 is a photograph showing the results of observing the bending section when bending in a direction parallel to the rolling direction (C direction) according to the cooling rate after cold rolling annealing heat treatment using stainless steel of the above examples and comparative examples and when bending at an angle of 90 degrees in a direction perpendicular to the rolling direction (L direction) under conditions of 1R or less.

[0033] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully 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] Additionally, when 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 specifically stated.

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

[0036] According to the present invention, by controlling the fraction of martensite formed in a ferrite matrix through controlling the cooling rate after cold rolling annealing heat treatment at a temperature higher than Ac1, high strength and high formability of stainless steel can be simultaneously satisfied.

[0037] The stainless steel of the present invention may contain, in wt%, C: 0.01% to 0.10%, N: 0.01% to 0.10%, Si: 0.01% to 1.00%, Mn: 0.01% to 3.00%, Cr: 10.0% to 20.0%, Al: 0.001% to 1.000%, the remainder Fe, and other unavoidable impurities.

[0038] Below, the reasons for limiting the composition of the above lecture are explained in detail. Unless otherwise specified, the composition of the above lecture lecture refers to weight %.

[0039] The carbon (C) content can be 0.010% to 0.100%.

[0040] C is an austenite stabilizing element that has the effect of expanding the austenite phase region, thereby forming hard martensite during cooling, thereby improving the strength of steel. To obtain these effects, the C content needs to be included in an amount of 0.010% or more, and preferably in an amount of 0.020% or more to sufficiently expand the austenite region to obtain a desired strength. If the C content exceeds 0.10%, the steel sheet may harden, significantly reducing ductility, and if the amount of martensite formed is too large, formability may not be obtained. In addition, if a large amount of Cr carbide is formed due to excessive C addition, a decrease in corrosion resistance due to a decrease in Cr may also accompany. Therefore, the C content is preferably 0.010% to 0.100%, and more preferably 0.020% to 0.100%.

[0041] The nitrogen (N) content can be 0.010% to 0.100%.

[0042] Nitrogen, like C and Mn, is an austenite stabilizing element and has the effect of expanding the austenite region. To obtain this effect, it is necessary to include an N content of 0.010% or more. However, if the N content exceeds 0.10%, ductility may rapidly decrease due to the solid solution strengthening effect of N, and precipitation of Cr nitride may cause a decrease in Cr, which may cause a decrease in corrosion resistance. Therefore, the N content is preferably 0.010% to 0.100%, more preferably 0.020% to 0.10%, and most preferably 0.020 to 0.070%.

[0043] The above C and N are elements that form Ti(C, N) carbonitride, and the dissolved C and N that do not form Ti(C, N) carbonitride deteriorate the properties of the material and may cause intergranular corrosion when used for a long time at 600℃ or lower after welding. Therefore, it is preferable that the content of C+N be 0.1% or less.

[0044] The silicon (Si) content can be 0.01% to 1.00%.

[0045] Silicon (Si) is an element that acts as a deoxidizer during steelmaking. To achieve this effect, the Si content needs to be at least 0.01%. However, if the Si content exceeds 1.00%, the steel sheet hardens, increasing the rolling load during hot rolling and causing surface defects such as sticking. In addition, Si is a ferrite stabilizing element, and excessive addition can reduce the stability of austenite. Therefore, the Si content is preferably 0.01% to 1.00%, and more preferably 0.20% to 0.50%.

[0046] The manganese (Mn) content can be from 0.01% to 3.00%.

[0047] Mn, like C, is an austenite phase stabilizing element and has the effect of expanding the austenite phase region. To achieve this effect, the Mn content needs to be included at 0.01% or more. However, if the Mn content exceeds 3.00%, the amount of MnS produced increases, which may reduce corrosion resistance. Therefore, the Mn content is preferably 0.01% to 3.00%, and more preferably 0.20% to 1.00%.

[0048] The content of chromium (Cr) can be 10.0% to 20.0%.

[0049] Cr is an element that forms a passive film on the surface of a steel sheet, thereby improving corrosion resistance. This effect appears when the Cr content is 10.0% or higher, and corrosion resistance can be improved as the Cr content increases. In addition, Cr is a ferrite stabilizing element, and has the effect of suppressing the formation of an austenite phase. When the Cr content is less than 10.0%, an excessive amount of austenite phase is formed, and the desired formability may not be obtained. In addition, when the Cr content exceeds 20.0%, the austenite phase is not formed, and thus the required martensite phase fraction cannot be secured. Therefore, the Cr content is preferably 10.0% to 20.0%, and more preferably 12.0% to 18.0%.

[0050] The content of aluminum (Al) can be from 0.001% to 1.000%.

[0051] Al is an element that acts as a deoxidizer, similar to Si. To achieve this effect, the Al content must be at least 0.001%. However, if the Al content exceeds 1.000%, Al inclusions such as Al2O3 may increase, resulting in a deterioration in surface quality. Therefore, the Al content is preferably 0.001% to 1.000%, and more preferably 0.001% to 0.100%.

[0052] The stainless steel of the present invention including the above alloy composition may further include, in wt%, Ni: more than 0% and less than or equal to 1.00%.

[0053] The nickel (Ni) content may be greater than 0% and less than or equal to 1.00%.

[0054] Ni is an austenite phase stabilizing element, effectively improving corrosion resistance by suppressing the progression of the formula, and when added in small amounts, it is also effective in improving the toughness of steel sheets. However, when added in large amounts, there is a concern that it may cause material hardening and a decrease in toughness due to solid solution strengthening. Furthermore, as it is an expensive element, it may lead to an increase in manufacturing costs. Therefore, the Ni content is preferably greater than 0% and less than 1.00%, and more preferably 0.01% to 0.50%.

[0055] The remaining component is iron (Fe). However, during the normal 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 normal manufacturing process, their full details are not specifically mentioned in this specification.

[0056] Next, a method for manufacturing stainless steel according to one embodiment of the present invention will be described.

[0057] A method for manufacturing stainless steel according to an embodiment of the present invention may include the steps of: reheating a slab containing, in wt%, C: 0.01% to 0.10%, N: 0.01% to 0.10%, Si: 0.01% to 1.00%, Mn: 0.01% to 3.00%, Cr: 10.0% to 20.0%, Al: 0.001% to 1.000%, the remainder Fe, and other unavoidable impurities; hot-rolling after the reheating; hot-rolling annealing after the hot rolling; cold-rolling after the hot-rolling annealing; and cold-rolling annealing at 900°C to 1100°C after the cold rolling, and cooling to room temperature at a cooling rate of 0.5°C / sec to 55°C / sec.

[0058] The manufacturing method of the present invention is explained step by step as follows.

[0059] Slavic reheating stage

[0060] First, we perform the step of reheating the slab.

[0061] The above reheating can be performed at 1050℃ to 1250℃, and is preferably performed at 1100℃ to 1250℃. If the temperature during the reheating is lower than 1050℃, friction between the steel plate and the rolling mill increases, rapidly increasing the load applied to the rollers during hot rolling, which may cause surface defects during hot rolling. If the temperature exceeds 1250℃, the energy cost for heating the slab to a high temperature increases, and material weight loss may occur due to an increase in the amount of surface oxide scale as the temperature increases.

[0062] Hot rolling stage

[0063] After the above reheating, hot rolling is performed.

[0064] The above heated steel slab can be hot rolled to produce a hot rolled steel plate, and at this time, the finishing rolling temperature can be 700°C to 950°C, and preferably 750°C to 900°C.

[0065] If the finishing rolling temperature during the above hot rolling is less than 700℃, the rolling load may increase and shape defects may increase, which may lower productivity. If it exceeds 950℃, the surface quality may deteriorate due to an increase in oxides caused by excessive high-temperature work.

[0066] Hot rolling annealing stage

[0067] After the above hot rolling, hot annealing is performed.

[0068] Typically, when martensite is formed during the hot-rolling annealing stage prior to cold rolling, the high strength results in a large load during cold rolling. This increases the number of cold rolling passes required to address this load, leading to uneconomical issues. Furthermore, as the total reduction ratio during cold rolling increases, there is a risk of sheet fracture due to brittleness.

[0069] Therefore, in the present invention, a soft ferrite single phase is formed without the formation of martensite during the hot-rolling annealing step. To this end, the hot-rolling annealing is preferably performed at 750°C to 900°C, which is directly below the Ac1 temperature, which is the ferrite single phase region, and more preferably at 800°C to 850°C.

[0070] If the temperature is lower than 750℃ during hot rolling annealing, recrystallization may not occur properly due to the low annealing temperature, or chromium may not diffuse sufficiently into the chromium depletion layer formed at the grain boundary due to chromium carbide or chromium nitride formed at the grain boundary after hot rolling, which may cause a decrease in corrosion resistance or surface defects.

[0071] Cold rolling stage

[0072] After the above hot rolling annealing, cold rolling is performed.

[0073] Generally, cold rolling is performed after cooling to room temperature.

[0074] The cold rolling reduction ratio can be arbitrarily set so as to obtain a cold rolled steel sheet of the desired thickness during the above cold rolling, and this can be applied without particular difficulty by a person skilled in the art.

[0075] Cold rolling annealing stage

[0076] Finally, cold rolling annealing is performed after the above cold rolling to form martensite.

[0077] Specifically, in this step, the steel plate is heated to a temperature of 900℃ or higher, where the austenite phase is formed, so that a sufficient fraction of the austenite phase is formed, and carbon diffusion from the austenite is caused to occur, so that annealing is completed and finally, martensite is secured at a target level that can satisfy both strength and formability.

[0078] When heat treatment is performed at a high temperature of 1000℃ or higher during the above cold rolling annealing, the austenite phase is smoothly generated as it approaches the maximum amount of austenite phase (γmax), and the generated austenite transforms into a hard martensite phase upon cooling, contributing to the improvement of strength. In addition, the ferrite phase that has not transformed into austenite and martensite has a slightly coarser grain size due to the high-temperature annealing heat treatment, which can contribute to the improvement of formability. However, when the annealing temperature is 1100℃ or higher, coarse ferrite grains of 50㎛ or more may be formed, which may make the material more brittle, lowering the DBTT (ductile-brittle transition temperature), or cause an orange peel phenomenon in which the surface of a harshly formed or bent part becomes rough. Therefore, care must be taken to prevent the ferrite grain size from becoming coarse.

[0079] Therefore, the cold rolling annealing is preferably performed at 900°C to 1100°C, more preferably at 950°C to 1100°C, and most preferably at 1000°C to 1100°C.

[0080] By controlling the cooling rate to 0.5°C / sec to 55°C / sec after the above cold rolling annealing heat treatment and cooling to room temperature, the amount of martensite phase transformation can be controlled so that the high strength and high formability aimed at in the present invention can be simultaneously satisfied.

[0081] In this invention, it is preferable to control the cooling rate to 0.5°C / sec to 55°C / sec, and more preferably to 1.0°C / sec to 55°C / sec.

[0082] If the above cooling rate is less than 0.5°C / sec, the martensite phase transformation amount is insufficient due to the formation of Cr carbide during cooling, and thus the strength and formability targeted by the present invention cannot be satisfied. If it exceeds 55°C / sec, the martensite phase transformation amount is excessive, so the strength becomes excessive and the formability may be reduced.

[0083] In the method for manufacturing stainless steel of the present invention, after the cold rolling annealing heat treatment, the cooling rate is controlled to 0.5°C / sec to 55°C / sec to suppress the formation of Cr carbides during cooling, thereby sufficiently securing carbon that contributes to martensite phase transformation, and the formation of sufficient austenite phase can induce martensite phase transformation to the target level. The stainless steel of the present invention manufactured in this way can have sufficient strength and excellent formability.

[0084] The stainless steel manufactured according to one embodiment of the present invention may have an R-bar value of the following formula (1) of 1.0 or more.

[0085] Equation (1): R-bar = (R0 + 2×R45 + R90) / 4

[0086] (Here, R0 means the R value in the direction parallel to the rolling direction, R45 means the R value in the direction 45 degrees to the rolling direction, and R90 means the R value in the direction 90 degrees to the rolling direction)

[0087] The above R-bar value is an indicator of deep workability. In order to measure the R value, which is a plasticity resistance index, a tensile test piece according to JIS13B standard is processed by cutting the test piece in the direction of 0 degrees, 45 degrees, and 90 degrees with respect to the rolling direction, and then the width and thickness changes before and after the tensile test are measured after 15% tensile deformation, and the calculation can be made using 2) below.

[0088] Equation (2): R = εw / εt

[0089] (Here, εw represents the strain in the width direction, and εt represents the strain in the thickness direction)

[0090] If the R-bar value according to the above formula (1) is less than 1.0, the deep workability is insufficient, resulting in poor formability, which may cause cracks or fractures during forming. Therefore, the R-bar value is preferably 1.0 or higher, more preferably 1.14 or higher, and most preferably 1.18 or higher. In such cases, the deep workability and formability effects can be further improved.

[0091] In addition, the stainless steel according to one embodiment of the present invention may have a ridging height of 20 µm or less, which is measured by a surface roughness tester after cold-rolled and annealed material is stretched by 15% in the rolling direction. That is, the stainless steel according to the present invention has a ridging height of 20 µm or less, preferably 18 µm or less, and has excellent formability despite its high strength, so that surface defects are not generated during processing, thereby providing excellent surface properties.

[0092] In the case of high-strength materials, there is a risk of cracks occurring at the bend due to the high strength characteristics when bent at a 90-degree angle. However, stainless steel manufactured according to an embodiment of the present invention does not cause cracks at the bend when bent in a direction parallel to the rolling direction (C direction) and when bent at a 90-degree angle in a direction perpendicular to the rolling direction (L direction), and the orange peel phenomenon caused by coarse grains also does not occur.

[0093] In addition, the stainless steel according to one embodiment of the present invention can satisfy the following equation (3).

[0094] Formula (3): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) ≥ 10

[0095] (Here, C, N, Ni, Mn, Cr, Si and Al represent the weight percent of each element)

[0096] When the austenite (γ) phase stability is low, the austenite phase transformation may not occur actively at high temperatures, and thus the martensite phase transformation may not occur. Therefore, it is preferable that the above equation (3) satisfies 10 or more, more preferably satisfies 20 or more, and most preferably satisfies 30 or more, so that the austenite phase can be formed at high temperatures and then the martensite phase transformation can occur smoothly during cooling.

[0097] According to one embodiment of the present invention, the stainless steel has a dual phase microstructure of a ferrite phase and a martensite phase after cold rolling and annealing, wherein the martensite phase may have an area fraction of 20% or more. If the area fraction of the martensite phase is less than 20%, the strength of the stainless steel may not be secured.

[0098] According to one embodiment of the present invention, the stainless steel may be unsuitable for complex forming applications due to its high strength when the average grain size of the ferrite phase is coarse, so the average grain size is preferably 50 ㎛ or less, and more preferably 35 to 45 ㎛. In addition, the ferrite phase grains may be formed in an equiaxed shape as much as possible so as not to have a specific anisotropy in the rolling direction.

[0099] Additionally, according to one embodiment of the present invention, the stainless steel may have a room temperature yield strength of 350 MPa or more and a room temperature tensile strength of 500 MPa or more.

[0100] If the yield strength of stainless steel at room temperature is less than 350 MPa or the tensile strength at room temperature is 500 MPa, for example, when applied to home appliances and large appliances, the thickness required to ensure safety increases excessively, making it difficult to satisfy the desired strength and lightweight characteristics. The upper limit of the yield strength at room temperature of the stainless steel is not limited, but may be 700 MPa or less, 650 MPa or less, 600 MPa or less, 550 MPa or less, 500 MPa or less, 450 MPa or less, etc. in order to satisfy the processability, formability, weldability, etc. required for application to home appliances and large appliances, for example. In addition, the upper limit of the tensile strength at room temperature is not limited, but may be 800 MPa or less, 750 MPa or less, 700 MPa or less, 650 MPa or less, 600 MPa or less, 550 MPa or less, etc.

[0101] As described above, the stainless steel manufactured according to the present invention exhibits high strength characteristics, but also has excellent R-bar value and tensile strength characteristics, so it can be used in product groups such as home appliances and large home appliances that require high strength and formability.

[0102] Hereinafter, the present invention will be described in more detail through preferred embodiments.

[0103]

[0104] Example

[0105] A slab was manufactured using steel having the composition shown in Table 1 below, heated at 1200°C for 1.5 hours, hot-rolled to a thickness of 3.0 mmt, and then hot-rolled and annealed at 850°C after hot-rolling. Next, cold-rolled to a thickness of 0.5 mmt was performed, and cold-rolled and annealed for 2 minutes at the temperatures shown in Table 1 below, and then cooled at a cooling rate of 10°C / sec to manufacture a cold-rolled and annealed material.

[0106] ClassificationCNSiMnCrNiAlFormula (1)Cold rolling annealing temperature (℃)Example 10.0380.0480.290.5216.20.110.00336.461050Example 20.0330.0420.310.4116.40.000.00325.381050Comparative example 10.0380.0480.290.5216.20.110.00336.46880Comparative example 20.0330.0420.310.4116.40.000.00323.38880Comparative example 30.0350.0250.250.8017.20.000.0809.61950

[0107] Experimental Example 1

[0108] The room temperature yield strength, room temperature tensile strength, and ferrite average grain size were measured using the cold rolled annealed materials manufactured in Examples 1 to 2 and Comparative Examples 1 to 3, which were manufactured at different cold rolling annealing temperatures as shown in Table 1 above, and the results are shown in Table 2 below.

[0109] ClassificationRoom temperature yield strength (MPa)Room temperature tensile strength (MPa)Whole line average grain size (㎛)Example 1474.8720.736Example 2440.0734.642Comparative example 1299.3477.227Comparative example 2298.9478.525Comparative example 3305.4480.223

[0110] As shown in Table 2 above, in the case of Comparative Examples 1 and 2 annealed just below the Ac1 temperature where austenite phase transformation does not occur, the yield strength at room temperature was approximately 300 MPa, and the structure was composed of a single ferrite phase. At this time, it was confirmed that the ferrite grain size was approximately 25 μm. In addition, in the case of Comparative Example 3, which satisfies the alloy composition of the present invention but in which 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) in Equation (3) is less than 10, even if cold-rolled annealing was performed at 950°C, which is higher than the Ac1 temperature, the yield strength at room temperature was 305.4 MPa, the tensile strength at room temperature was 480.2 MPa, and the ferrite grain size was 23 μm, showing no strength increase effect.

[0111] On the other hand, in the case of Examples 1 and 2 in which the alloy composition of the present invention and Equation (3) 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) satisfies 10 or more, the yield strength at room temperature was approximately 440 MPa or more when cold-rolled and annealed at a temperature higher than Ac1 at which the austenite phase is formed, and the tensile strength at room temperature was also confirmed to be high at 720 MPa or more.

[0112] From these results, it was found that in the case of Examples 1 and 2 according to the present invention, the alloy composition was satisfied, and the equation (3) 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) was 10 or more, and the austenite phase was smoothly formed by cold rolling and annealing at a temperature of 1050℃, which is the Ac1 temperature at which the austenite phase is formed, and the formed austenite was able to exhibit excellent strength by phase transforming into a hard martensite phase upon cooling, and the ferrite phase that was not phase transformed into austenite and martensite had a slightly coarser grain size due to high-temperature heat treatment, which could contribute to improving formability.

[0113] Experimental Example 2

[0114] The results of measuring the R value in the 0 degree, 45 degree, and 90 degree directions using the cold-rolled annealed material manufactured in Examples 1 to 2 and Comparative Examples 1 to 3 and the R-bar value according to the above formula (1) are shown in Table 3 below.

[0115] In order to measure the R value, which is the plasticity resistance index, the specimens were cut in the 0 degree, 45 degree, and 90 degree directions with respect to the rolling direction, respectively, to process the tensile specimens according to the JIS13B standard, and then the width and thickness changes before and after the tensile test were measured after 15% tensile deformation, and the values ​​were calculated using the following equation (2).

[0116] Equation (2): R = εw / εt

[0117] (In Equation (2), εw represents the strain in the width direction, and εt represents the strain in the thickness direction.)

[0118] Classification R-0R-45R-90R-barExample 10.991.291.241.21Example 21.011.241.211.18Comparative Example 10.941.191.251.14Comparative Example 21.061.171.431.21Comparative Example 31.131.061.391.16

[0119] As shown in Table 3 above, in the case of Examples 1 to 2 according to the present invention, although high strength characteristics were exhibited, the R-bar value was found to be at the level of 1.2±0.02, and it was found that the formability was not reduced compared to Comparative Examples 1 to 3, which exhibited low strength.

[0120] Experimental Example 3

[0121] The ridging height formed on the surface of the cold-rolled annealed material manufactured in Examples 1 to 2 and Comparative Examples 1 to 3 was measured by stretching the cold-rolled annealed material by 15% in the rolling direction using a surface roughness tester, and is shown in Table 4 below.

[0122] The average value in Table 4 is rounded to the second decimal place.

[0123] Category 12345Average Example 115.5715.1513.3216.9813.3214.87Example 217.4116.0717.1117.6318.5017.34Comparative Example 117.4018.2017.6021.4019.4018.80Comparative Example 218.1020.2019.1019.3018.8019.10Comparative Example 315.8014.5015.4016.2017.5015.88

[0124] As shown in Table 4 above, it was confirmed that the stainless steels of Examples 1 to 2 manufactured according to the present invention had a good ridging height of 14 to 18 ㎛ despite having high strength compared to the low-strength materials of Comparative Examples 1 to 3. From the results of Experimental Examples 2 and 3 above, it was found that the stainless steels manufactured according to the present invention would have high strength characteristics and excellent formability.

[0125] Experimental Example 4

[0126] Using the stainless steel of Example 2 above, the cooling rate was varied as shown in Table 5 below, and the change in physical properties according to the cooling rate after cold rolling annealing heat treatment was measured, and the results are shown in Table 5 below and Fig. 1.

[0127] In Table 5 below, if the room temperature yield strength value is 350 MPa or more, it is marked as "O", and if it is less than 350 MPa, it is marked as "X". If the R-bar value is 1.0 or more, it is marked as "O", and if it is less than 1.0, it is marked as "X". If the ridging height is less than 20 ㎛, it is marked as "O", and if it is more than 20 ㎛, it is marked as "X". If the orange peel phenomenon does not occur at the 90-degree bend, it is marked as "O", and if it does, it is marked as "X".

[0128] Cooling rate (℃ / sec) Room temperature yield strength R-bar value Risking height Orange peel Example 230OOOO10OOOO0.5OOOO0.3XOOO0.1XOXX

[0129] The cooling rate after cold rolling annealing heat treatment affects the martensitic phase transformation. If the cooling rate is slow, it is easy to form chromium carbide during cooling, and if chromium carbide precipitates, the carbon contributing to the martensitic phase transformation is insufficient, resulting in an insufficient amount of martensitic phase transformation and a decrease in strength. As shown in Table 5 above, when the cooling rate is 0.3℃ / sec or less, the yield strength at room temperature was less than 350MPa due to the insufficient amount of martensitic transformation, and it was confirmed that the ridging height increased and an orange peel phenomenon appeared on the surface of the 90-degree bend.

[0130] On the other hand, according to the present invention, when the cooling rate is 0.5°C / sec or more, the yield strength at room temperature is 350 MPa or more, the ridging height is less than 20 μm, and the orange peel phenomenon does not appear on the surface of the 90-degree bend, so it was confirmed that the formability is also excellent.

[0131] Figure 1 shows the results of observing the bent portion when bending in a direction parallel to the rolling direction (C direction) according to the cooling rate and when bending at an angle of 90 degrees in a direction perpendicular to the rolling direction (L direction) under conditions of 1R or less using the stainless steel of the above examples and comparative examples.

[0132] In general, high-strength stainless steels have a risk of cracks occurring at the bend due to their high strength characteristics. However, in the stainless steel of the embodiment manufactured according to the present invention, no cracks were observed at the bend even when bent in a direction parallel to the rolling direction (C direction) or when bent 90 degrees in a direction perpendicular to the rolling direction (L direction) under conditions of 1R or less, and the orange peel phenomenon caused by coarse grains was not observed either.

[0133] From these results, it was found that the present invention exhibits high strength characteristics when cooled at a rate of 0.5°C / sec to 55°C / sec after cold rolling annealing heat treatment, and that it can be used in products requiring high strength and formability due to its excellent R-bar value and descent characteristics.

[0134] 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 spirit of the claims set forth below.

Claims

1. Contains, by weight%, C: 0.01%~0.10%, N: 0.01%~0.10%, Si: 0.01%~1.00%, Mn: 0.01%~3.00%, Cr: 10.0%~20.0%, Al: 0.001%~1.000%, the remainder being Fe and other unavoidable impurities. The microstructure is a dual phase of ferrite and martensite. The yield strength at room temperature is 350 MPa or more, and the tensile strength at room temperature is 500 MPa or more. Stainless steel having an R-bar value of formula (1) below of 1.0 or greater. Equation (1): R-bar = (R0 + 2×R45 + R90) / 4 (Here, R0 means the R value in the direction parallel to the rolling direction, R45 means the R value in the direction 45 degrees to the rolling direction, and R90 means the R value in the direction 90 degrees to the rolling direction.) 2. In paragraph 1, The above stainless steel is a stainless steel further comprising, in weight %, Ni: more than 0% and less than or equal to 1.00%.

3. In paragraph 1, The above stainless steel is a stainless steel satisfying the following formula (3). Formula (3): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) ≥ 10 (Here, C, N, Ni, Mn, Cr, Si and Al represent the weight% of each element) 4. In paragraph 1, The above stainless steel is a stainless steel having a ridging height of 20㎛ or less.

5. In paragraph 1, The above stainless steel is a stainless steel that does not cause cracks or orange peel in the bent portion when bent in a direction parallel to the rolling direction (C direction) and when bent 90 degrees in a direction perpendicular to the rolling direction (L direction) under the condition of 1R or less.

6. A step of reheating the slab containing, by weight%, C: 0.01% to 0.10%, N: 0.01% to 0.10%, Si: 0.01% to 1.00%, Mn: 0.01% to 3.00%, Cr: 10.0% to 20.0%, Al: 0.001% to 1.000%, the remainder Fe and other unavoidable impurities; A step of hot rolling after the above reheating; A step of hot annealing after the above hot rolling; A step of cold rolling after the above hot rolling annealing; and A step of cold rolling and annealing at 900°C to 1100°C after the above cold rolling, and cooling to room temperature at a cooling rate of 0.5°C / sec to 55°C / sec; A method for manufacturing stainless steel comprising:

7. In paragraph 6, A method for manufacturing stainless steel, wherein the above reheating is performed at 1050°C to 1250°C.

8. In paragraph 6, The above hot rolling is a method for manufacturing stainless steel, performed at 700°C to 950°C.

9. In paragraph 6, The above hot rolling annealing is a method for manufacturing stainless steel, performed at 700°C to 950°C.

10. In paragraph 6, A method for manufacturing stainless steel, wherein the above slab further includes, in weight %, Ni: more than 0% and less than or equal to 1.00%.

11. In paragraph 6, The above stainless steel is a method for manufacturing stainless steel satisfying the following formula (3). Formula (3): 420C+470N+23Ni+10Mn+180-(11.5Cr+11.5Si+52Al) ≥ 10 (Here, C, N, Ni, Mn, Cr, Si and Al represent the weight% of each element) 12. In paragraph 6, A method for manufacturing stainless steel having a dual phase microstructure of ferrite and martensite after the above cold rolling annealing.

13. In paragraph 6, A method for manufacturing stainless steel having a martensite phase having an area fraction of 20% or more after the above cold rolling and annealing.

14. In paragraph 6, A method for manufacturing stainless steel, wherein the stainless steel after cold rolling and annealing has a C+N content of 0.1% or less in weight percent and an average grain size of a ferrite phase of 50 ㎛ or less.

15. In paragraph 6, The above stainless steel is a method for manufacturing stainless steel having an R-bar value of 1.0 or more in the following formula (1): Equation (1): R-bar = (R0 + 2×R45 + R90) / 4 (Here, R0 means the R value in the direction parallel to the rolling direction, R45 means the R value in the direction 45 degrees to the rolling direction, and R90 means the R value in the direction 90 degrees to the rolling direction)

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