Ferritic-based stainless steel having excellent impact toughness and method for producing same

Abstract

A hot-rolled annealed ferrite-based stainless steel sheet having excellent impact characteristics and a thickness of at least 6 mm and a method for producing the same are disclosed. The ferritic-based stainless steel having excellent impact toughness according to one embodiment of the present invention comprises, in wt%, more than 0% and at most 0.01% of C, at most 0.8% of Si, at most 0.5% of Mn, 10% to 14% of Cr, 0.01% to 0.45% of Ti, more than 0% and at most 0.015% of N, the remainder comprising iron (Fe) and unavoidable impurities, the average orientation difference between crystal grains in the microstructure is 0.6 DEG to 1.1 DEG.

Description

[0001] This application is a divisional application of the Chinese patent application with the application date of September 12, 2018, the application number of "201880080609.0", and the invention name of "Ferrite-based stainless steel with excellent impact toughness and method for producing the same". The original application is the Chinese national stage application of international application PCT / KR2018 / 010694. TECHNICAL FIELD

[0002] The present disclosure relates to a ferrite stainless steel having excellent impact toughness and a method for manufacturing the same, and more particularly, to a ferrite stainless steel hot-rolled annealed steel sheet including Ti and having excellent impact properties with a thickness of 6 mm or more and a method for manufacturing the same. BACKGROUND

[0003] Compared to austenitic stainless steel, ferritic stainless steel has poor workability, impact toughness, and high-temperature strength, but is inexpensive and has low thermal expansion since it does not contain a large amount of Ni. In recent years, it is preferred to be used for automobile exhaust system component materials. In particular, flanges for exhaust systems have recently been converted to ferritic stainless steel thick plates having improved corrosion resistance and durability due to micro-cracks and exhaust gas leakage problems.

[0004] STS409L material is a steel grade that prevents weld sensitization by stabilizing C and N with Ti at 11% Cr, has excellent workability, and is mainly used at temperatures below 700°C. The STS409L material is the most widely used steel grade because it has a certain corrosion resistance even to condensate components generated in the automobile exhaust system.

[0005] However, since the thickness of the ferritic stainless steel is thicker than that of the austenitic stainless steel, the workability and impact toughness are poor. Therefore, the ferritic stainless steel has a brittle crack or crack propagation during cold rolling to the target thickness after hot rolling, thereby causing the rupture of the plate. When a STS409L thick plate having a thickness of 6.0 mm or more is used to process a product such as a flange, there is a disadvantage of poor impact properties (e.g., cracks generated by impact). Due to such low impact properties, the STS409L steel having a thickness of 6.0 mm or more is a steel that is very difficult to manufacture and process.

[0006] In addition, during hot rolling, it is difficult to obtain fine grains due to insufficient rolling reduction, and further increase brittleness due to the formation of coarse grains and non-uniform grains, and the impact properties are deteriorated. SUMMARY

[0007] TECHNICAL PROBLEM

[0008] Embodiments of the present disclosure solve the above problems and thus ensure a recovery structure rather than a fully recrystallized structure by controlling the annealing temperature of a ferritic stainless steel hot-rolled thick plate having a thickness of 6.0 mm or more, thereby providing a ferritic stainless steel having improved impact toughness and a method of manufacturing the same.

[0009] Technical Solution

[0010] According to one aspect of the present disclosure, a ferritic stainless steel having excellent impact toughness includes, in terms of weight percent (%), C: more than 0% and 0.01% or less, Si: 0.8% or less, Mn: 0.5% or less, Cr: 10% to 14%, Ti: 0.01% to 0.45%, N: more than 0% and 0.015% or less, a remainder of iron (Fe) and other inevitable impurities, and an average misorientation between grains of a microstructure is 0.6° to 1.1°.

[0011] The thickness of the stainless steel can be 6.0 mm to 25.0 mm.

[0012] The ferritic stainless steel can further include Ni: 0.3% or less, Cu: 0.5% or less, and Al: 0.1% or less.

[0013] The stainless steel can satisfy the following formula (1)

[0014] (1) Ti / (C+N) ≥ 3

[0015] Ti, C, and N mean the content (wt%) of each element.

[0016] The stainless steel can have a yield strength of 305 MPa or more, a tensile strength of 420 MPa or more, an elongation of 35% to 40%, and satisfy the following formula (2).

[0017] (2) 20℃ Charpy impact energy × 40℃ Charpy impact energy ≥ 750 J / cm 2

[0018] According to another aspect of the present disclosure, a method of manufacturing a ferritic stainless steel having excellent impact toughness, the method of manufacturing includes: heating a slab to 1220°C or less, the slab including, in weight percent (%), of the total composition, C: more than 0% and 0.01% or less, Si: 0.8% or less, Mn: 0.5% or less, Cr: 10% to 14%, Ti: 0.01% to 0.45%, N: more than 0% and 0.015% or less, and the remainder being iron (Fe) and other inevitable impurities; subjecting the heated slab to rough rolling; subjecting the rough-rolled bar to finish rolling; and annealing the hot-rolled steel sheet, the total reduction rate of the back end of the rough rolling being 54% or more, and the thickness of the hot-rolled steel sheet being 6.0 mm to 25.0 mm.

[0019] The slab can further include Ni: 0.3% or less, Cu: 0.5% or less, Al: 0.1% or less, and satisfy the following formula (1)

[0020] (1) Ti / (C+N) ≥ 3

[0021] Ti, C, N mean the content (wt%) of each element.

[0022] The temperature of the rough-rolled bar can be 1020°C to 970°C.

[0023] The finish rolling end temperature can be 960°C or less.

[0024] The hot-rolling annealing can be performed at 850°C to 980°C, and the average orientation difference between the grains of the microstructure of the hot-rolled annealed steel sheet can be 0.6° to 1.1°.

[0025] Advantageous effects

[0026] According to one embodiment of the present disclosure, by controlling the microstructure of a ferritic stainless steel hot-rolled thick sheet having a thickness of 6.0 mm or more to a recovery tissue, a high value of Charpy impact energy can be exhibited.

[0027] Further, a ferritic stainless steel having excellent impact toughness, and a yield strength of 305 MPa or more and a tensile strength of 440 MPa or more can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is a photograph showing the microstructure of a hot-rolled annealed steel sheet according to the annealing temperature of the embodiment of the present disclosure.

[0029] Figures 2 to 5is a graph showing the average misorientation between the grains of a hot-rolled and annealed steel sheet according to an embodiment of the disclosure according to the Kernel Average Misorientation method analysis of the average misorientation difference according to the annealing temperature of the steel sheet.

[0030] Figure 6 is a graph showing the value of the Charpy impact energy for each temperature of the annealing temperature according to an embodiment of the disclosure. DETAILED DESCRIPTION

[0031] The ferritic stainless steel according to one embodiment of the disclosure having excellent impact toughness includes, in weight percent (%), based on the total composition: C: greater than 0% and 0.01% or less, Si: 0.8% or less, Mn: 0.5% or less, Cr: 10% to 14%, Ti: 0.01% to 0.45%, N: greater than 0% and 0.015% or less, the remainder being iron (Fe) and other inevitable impurities, and the average misorientation between the grains of the microstructure is 0.6° to 1.1°. Hereinafter, the embodiments of the disclosure will be described in detail with reference to the accompanying drawings.

[0032] DETAILED DESCRIPTION

[0033] Hereinafter, the embodiments of the disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to convey the technical idea of the disclosure to those of ordinary skill in the art. However, the disclosure is not limited to these embodiments, and can be implemented in another form. In the accompanying drawings, parts irrelevant to the description can not be shown to clarify the disclosure, and in addition, the sizes of the components are shown to be exaggerated more or less for easy understanding.

[0034] Further, unless specifically described otherwise thereto, when a part "includes" or "comprises" an element, the part can further include other elements, not excluding the other elements.

[0035] Unless clearly different in the context, the expression used in the singular includes the plural.

[0036] Various methods have been studied to improve the toughness of ferritic stainless steel hot-rolled thick plates. First, there is a method of suppressing Laves phase, which deteriorates the brittleness of the material by lowering the hot-rolling coiling temperature or by performing a rapid cooling process such as water cooling. However, this method is difficult to apply to actual production, or causes undesirable coiling such as scratches on the surface of the plate due to low temperature at the time of coiling, or has a problem of making the deformation of the plate become uneven and generating partial cracks due to the rapid cooling rate. Thus, this method has difficulties in actual production application. In addition, when hot-rolling is performed on a ferritic stainless steel having a thickness of 6.0 mm or more, it is difficult to obtain a fine grain size due to insufficient rolling reduction, and also causes a problem of increasing brittleness due to formation of coarse grains and uneven grains, compared to a steel plate having a thickness of 6.0 mm or less.

[0037] The present disclosure ensures a microstructure in the recovery step, rather than a completely recrystallized microstructure, by controlling the hot-rolling and hot-rolling annealing process of a hot-rolled thick plate having a thickness of 6.0 mm or more, rearranging dislocations arranged in disorder in the grain boundaries of the recovery at a specific temperature by annealing, and suppressing impact propagation by the rearranged dislocations to improve the impact toughness of a Ti-containing ferritic stainless steel hot-rolled thick plate.

[0038] In the present specification, "ferritic stainless steel" means a hot-rolled annealed steel plate having a thickness of 6.0 mm or more.

[0039] A ferritic stainless steel having excellent impact toughness according to one embodiment of the present disclosure includes, in weight percent (%) of the total composition: C: more than 0% and 0.01% or less, Si: 0.8% or less, Mn: 0.5% or less, Cr: 10% to 14%, Ti: 0.01% to 0.45%, N: more than 0% and 0.015% or less, the remainder being iron (Fe) and other inevitable impurities, the ferritic stainless steel having an average misorientation between grains of a microstructure of 0.6° to 1.1° and having a thickness of 6.0 mm to 25.0 mm.

[0040] In the present disclosure, the target steel grade to be improved in impact toughness is a ferritic stainless steel thick plate including 10 to 14% by weight of Cr and 0.01 to 0.45% by weight of Ti, for example, an STS409L steel grade.

[0041] Hereinafter, the reason for the numerical limit of the alloy component element content in the embodiment of the present disclosure will be described. Hereinafter, the unit is weight % unless otherwise specified.

[0042] The content of C and N is more than 0% and less than 0.01%.

[0043] In the case where C and N exist as a form of an interstitial substance as Ti(C, N) carbonitride forming elements, Ti(C, N) carbonitride is not formed when the C and N contents are high, and C and N existing at a high concentration deteriorate the elongation and low-temperature impact properties of the material. When the material is used at a temperature of 600°C or less for a long time after welding, intergranular corrosion occurs due to the generation of Cr 23 C6 carbide, and thus it is preferable to control the contents of C and N to 0.01% or less, respectively.

[0044] The content of Si is 0.8% or less.

[0045] Si is an element added as a deoxidizing element, and when its content increases as a ferrite phase forming element, ferrite phase stability increases. If the content of Si is greater than 0.8%, steelmaking Si inclusions increase and surface defects occur. For this reason, it is preferable to control the content of Si to 0.8% or less.

[0046] The content of Mn is 0.5% or less.

[0047] When the content of Mn increases, pitting corrosion resistance decreases due to the formation of precipitates such as MnS. Thus, it is preferable to control the content of Mn to 0.5% or less.

[0048] The content of Cr is 10% to 14%.

[0049] Cr is an essential element for securing the corrosion resistance of stainless steel. When the content of Cr is low, corrosion resistance decreases in the atmosphere of condensed water, and when the content is high, strength increases and elongation and impact properties decrease. In the present disclosure, since the target steel grade to be improved in impact toughness is a ferritic stainless steel thick plate containing 10% to 14% Cr, the content of Cr is limited to 10% to 14%.

[0050] The content of Ti is 0.01% to 0.45%.

[0051] Ti is an effective element for fixing C and N to prevent intergranular corrosion. However, when the content of Ti decreases, corrosion resistance decreases due to intergranular corrosion occurring at a welded area, and thus it is preferable to control Ti to be at least 0.01% or more. However, when the content of Ti is too high, steelmaking inclusions increase, a large amount of surface defects (e.g., scabs) can occur due to the increase in steelmaking inclusions, and a nozzle clogging phenomenon occurs in a continuous casting process. For this reason, the content of Ti is controlled to 0.45% or less, more preferably 0.35% or less.

[0052] Further, according to one embodiment of the present disclosure, the following formula (1) can be satisfied.

[0053] (1) Ti / (C+N) ≥ 3

[0054] In the ferritic stainless steel to which Ti is added, as the content of C+N increases, the content of C and N used in the base material increases, which further causes the brittleness of the material. Therefore, the ratio of Ti / (C+N) is controlled to be at least 3 or more.

[0055] Further, the ferritic stainless steel having excellent impact toughness according to one embodiment of the present disclosure can also include Ni: 0.3% or less, Cu: 0.5% or less, Al: 0.1% or less.

[0056] The content of Ni is 0.3% or less.

[0057] Ni is an effective element for suppressing the development of pitting corrosion, and is also effective in improving the toughness of the hot-rolled steel sheet when a small amount of 0.01% or more is added. However, a large amount of addition can cause material hardening and toughness degradation due to solid solution strengthening, and there is a problem of an increase in alloy cost. Therefore, it is preferable to limit it to 0.3% or less.

[0058] The content of Cu is 0.5% or less.

[0059] Cu serves to improve corrosion resistance when added in a certain amount, but since excessive addition produces Cu precipitates and reduces toughness, it is preferable to limit it to 0.5% or less.

[0060] The content of Al is 0.1% or less.

[0061] Al can be used as a deoxidizing element and its effect can be exhibited at 0.005% or more. However, excessive addition causes a decrease in toughness and ductility at room temperature, so the upper limit is set to 0.1% and Al does not need to be included.

[0062] In the present disclosure, the ferritic stainless steel to be improved in impact toughness has a thickness of 6.0 mm to 25.0 mm.

[0063] As described above, in the hot-rolled annealed thick plate, there is a problem of brittleness due to insufficient rolling reduction, and the ferritic stainless steel according to the present disclosure for solving the problem has a thickness of 6.0 mm or more. However, considering the thickness of the roughed bar after roughing, the upper limit can be 25.0 mm. Preferably, it can be 12.0 mm or less to be suitable for manufacturing use.

[0064] The microstructure of the ferritic stainless steel having excellent impact toughness according to one embodiment of the present disclosure can be a recovery structure in which the average orientation difference between grains is 0.6° to 1.1°.

[0065] The inventors of the present disclosure found that some recrystallized recovered structures exhibit superior impact properties compared to unannealed or fully recrystallized structures. The recovered structure can be difficult to distinguish from the unannealed and fully recrystallized structures, but can be distinguished by the orientation difference between the grains between the grain boundaries in the grain boundary structure. In general, it is known that as the amount of deformation increases, the distortion of the crystal orientation increases, and thus the average orientation difference between the grains of the deformed sample increases.

[0066] The average orientation difference between the grains of the unannealed structure is 1.2° or more, and the average orientation difference between the grains of the fully recrystallized structure is 0.5° or less. That is, the average orientation difference between the grains gradually decreases from the unannealed structure to the fully recrystallized structure, which means that recrystallization proceeds by arranging in a direction in which the grain boundary energy can be reduced.

[0067] The unannealed structure has high strength and low elongation due to the stress remaining inside, and poor impact properties; the fully recrystallized structure has low strength due to stress removal and is unable to suppress impact propagation due to dislocation annihilation. The ferritic stainless steel according to the present disclosure can suppress impact propagation by relocating the dislocations generated by the low-temperature hot rolling process, which will be described later, to the recovered grain boundaries, while improving impact toughness.

[0068] Accordingly, the ferritic stainless steel having superior impact toughness according to the present disclosure can satisfy the following formula (2).

[0069] (2) 20℃ Charpy impact energy x 40℃ Charpy impact energy ≥ 750 J / cm 2

[0070] For example, the 20℃ Charpy impact energy of the ferritic stainless steel according to the present disclosure can exhibit 15 J / cm 2 or more, and the 40℃ Charpy impact energy can exhibit 50 J / cm 2 or more.

[0071] Next, a manufacturing method of the ferritic stainless steel having superior impact toughness according to one embodiment of the present disclosure will be described.

[0072] A method of manufacturing a ferritic stainless steel having excellent impact toughness according to one embodiment of the present disclosure includes: heating a slab to 1220°C or less, the slab including, in weight percent (%), of the total composition: C: more than 0% and 0.01% or less, Si: 0.8% or less, Mn: 0.5% or less, Cr: 10% to 14%, Ti: 0.01% to 0.45%, N: more than 0% and 0.015% or less, the remainder being iron (Fe) and other inevitable impurities; rough rolling the heated slab; finish rolling the rough rolled bar; and annealing the hot-rolled steel sheet, the total reduction at the back end of the rough rolling being 54% or more, the thickness of the hot-rolled steel sheet being 6.0 mm to 25.0 mm.

[0073] The reasons for the numerical limits of the alloying element contents and the thickness of the hot-rolled steel sheet are as described above.

[0074] After heating the slab containing the above-described composition of alloying elements to 1220°C or less before hot rolling, the heated slab can be rough rolled. At this time, the total reduction at the back end of the rough rolling can be controlled to be 54% or more.

[0075] Generally, when the thickness of the hot-rolled steel sheet is thick, the reduction is reduced, so that the amount of dislocations is reduced due to low stress applied to the material. Therefore, as the thickness of the hot-rolled steel sheet becomes thick, the heating furnace temperature before hot rolling is made as low as possible, and at the time of hot rolling, the load distribution of the rough rolling moves to the back end to perform strong reduction at the back end having a lower temperature than the front end.

[0076] The slab heating temperature is preferably 1220°C or less to generate dislocations by low-temperature hot rolling, and when the slab temperature is too low, it is not possible to perform rough rolling, so the lower limit of the heating temperature can be 1150°C or more.

[0077] Since the rough rolling is generally composed of 3 to 4 rolling mills, the back end of the rough rolling in the present disclosure can mean the last rolling mill and the second last rolling mill. Even in the rough rolling step composed of five or more rolling mills, it can mean the last rolling mill and the second last rolling mill. For example, the reduction of the two back end rolling mills can be 27% or more, respectively. By strong reduction so that the total reduction at the back end of the rough rolling is 54% or more, dislocations of the hot-rolled steel sheet can be smoothly generated.

[0078] The rough rolled bar manufactured by the rough rolling process can be finish rolled to a thickness of 6.0 mm to 25.0 mm and then hot annealed.

[0079] The temperature of the roughed bar after the roughing and before the finish rolling can be 1020°C to 970°C, and the finish rolling can be ended at a temperature of 960°C or less. More preferably, the finish rolling can be ended at a temperature of 920°C or less. The low temperature hot rolling process can be performed by controlling the slab heated to 1220°C or less within the above temperature range, thereby generating a large amount of dislocations.

[0080] Subsequently, the hot-rolled steel plate can be annealed at 850°C to 980°C. When the annealing temperature is lower than 850°C, a long time is required for dislocation migration annealing time, thereby reducing productivity, and when it exceeds 980°C, recrystallization can occur in addition to dislocation migration.

[0081] The regularly rearranged dislocations can suppress crack propagation caused by impact by migrating the dislocations generated during hot rolling through annealing heat treatment. However, when the reheating temperature and the finish rolling temperature are too low to be outside the above hot rolling temperature range table, the frictional pressure between the material and the roll is high during hot rolling, so that the surface of the material can be torn or scratched by the roll. Therefore, in order to form a recovery structure, the hot-rolled steel plate is manufactured according to the load distribution of the rear end of the roughing and the temperature range of the low temperature hot rolling process, and hot-rolled annealing heat treatment in the range of 850°C to 980°C should be performed.

[0082] The ferritic stainless steel hot-rolled annealed steel plate subjected to the low temperature hot rolling process and the hot-rolled annealing heat treatment can have a recovery structure in which the average misorientation between the grains of the microstructure is 0.6° to 1.1°.

[0083] Hereinafter, a preferred embodiment of the present disclosure will be described in detail.

[0084] Examples

[0085] A slab having the composition shown in Table 1 was heated to 1200°C, and then hot-rolled to a thickness of 10.0 mm by setting the total reduction rate of the rear part of the roughing to 55%, so that the finish rolling end temperature was 940°C. At this time, the temperature of the roughed bar before the finish rolling was set to about 1000°C.

[0086] Unannealing (A), 930°C hot-rolled annealing (B: B-1, B-2), and 1020°C hot-rolled annealing (C) were respectively performed on 10.0 mm thick hot-rolled steel plates to prepare 11Cr-0.2Ti ferritic stainless steel plates. The steel plates hot-rolled annealed at 930°C produced in two types of B-1 and B-2 were produced as examples of the range of the present disclosure to determine reproducibility.

[0087] [Table 1]

[0088]

[0089] 1. Microstructure

[0090] Figure 1 This is a photograph showing the microstructure of a hot-rolled annealed steel sheet at an annealing temperature according to an embodiment of the present disclosure.

[0091] Figure 1 Microstructures of hot-rolled annealed steel sheets prepared by unannealed (A), hot-rolled annealing at 930°C (B: B-1, B-2), and hot-rolled annealing at 1020°C (C) are shown. A is an unannealed hot-rolled black coil, representing the microstructure after typical hot rolling. C undergoes substantial recrystallization through hot annealing at 1020°C. However, due to the low hot-rolling reduction, some non-recrystallized banded structures are observed. As shown in B-1 and B-2, the microstructure of the annealed sheet at 930°C (which is within the hot-rolled annealing temperature range of this disclosure) is that of the non-recrystallized recovery step, and some fine grains are observed.

[0092] Subsequently, the average orientation difference between grains in each microstructure based on the annealing temperature was calculated using the nucleus-average orientation difference method and is shown in Table 2 below. The nucleus-average orientation difference method is a technique that can analyze the average orientation difference between grains based on the average deformation of the material using electron backscatter diffraction (EBSD).

[0093] [Table 2]

[0094]

[0095] Figures 2 to 5 This is a graph showing the average orientation difference between grains of a hot-rolled annealed steel sheet at an annealing temperature according to an embodiment of the present disclosure, analyzed according to the nuclear average orientation difference method.

[0096] like Figures 2 to 5 As shown in Table 2, Comparative Example 1(A), which is an unannealed hot-rolled black coil, has the highest average orientation difference between grains at 1.44°, while Comparative Example 2(C), which is fully recrystallized, has the smallest average orientation difference at approximately 0.4°. Furthermore, the average orientation difference of the recovered microstructures corresponding to embodiments B-1 and B-2 of this disclosure is approximately 0.87°, representing a moderate level of average orientation difference between the unannealed hot-rolled black coil A and the fully recrystallized C.

[0097] 2. Impact toughness assessment

[0098] According to ASTM E 23, unannealed material (A), hot-rolled annealed material at 930°C (B: B-1, B-2), and hot-rolled annealed material at 1020°C (C) were subjected to Charpy impact tests at various temperatures, and the results are shown in Table 3 below.

[0099] [Table 3]

[0100]

[0101] It can be seen that the Charpy impact energy values of the present embodiments B-1 and B-2 show higher values when compared to Comparative Example 1 (A) and Comparative Example 2 (C). In particular, the Charpy impact energy values of the inventive examples (B-1, B-2) were determined to be significantly higher from room temperature 20°C to 40°C.

[0102] Figure 6 is a graph showing the values of Charpy impact energy at each temperature of the annealing temperature according to the embodiments of the present disclosure.

[0103] Referring to Figure 6 and Table 3, it can be seen that the Charpy impact energy values of the inventive examples (B-1, B-2) are shifted to the left by showing high values even at low temperatures compared to Comparative Example 1 (A) and Comparative Example 2 (C). That is, improved impact toughness is shown in the microstructure in the recovery stage compared to the recrystallized microstructure of the same thickness of 10.0 mm. It is determined that the dislocations regularly rearranged due to the rearrangement of dislocations generated during hot rolling inhibit crack propagation caused by impact.

[0104] 3. Tensile test evaluation

[0105] The above unannealed material (A), 930°C hot-rolled annealed material (B: B-1, B-2), and 1020°C hot-rolled annealed material (C) were evaluated by a tensile test, and the test results are shown in Table 4 below.

[0106] [Table 4]

[0107]

[0108] Referring to Table 4, the yield strength and tensile strength of the present embodiments (B-1, B-2) indicate lower yield strength and tensile strength values due to stress relaxation, and the elongation shows an improved value of about 3% to 4% compared to Comparative Example 1 (A). On the other hand, Comparative Example 2 (C) shows lower yield strength and tensile strength values due to stress relief and disappearance of dislocations generated during hot rolling, and the elongation also shows a high value of 41%.

[0109] Although the present disclosure has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the spirit and scope of the present disclosure.

[0110] Industrial applicability

[0111] The ferritic stainless steel according to the present disclosure has improved toughness and brittleness of a hot-rolled thick plate having a thickness of 6.0 mm or more, and can prevent winter cracking.

Claims

1. A ferritic stainless steel with excellent impact toughness, comprising, by weight percentage (%) of the total composition: C: greater than 0% and less than 0.01%, Si: 0.8% and less, Mn: 0.5% and less, Cr: 10% to 14%, Ti: 0.01% to 0.45%, N: greater than 0% and less than 0.015%, the remainder being iron (Fe) and other unavoidable impurities, and wherein the average orientation difference between the grains in the microstructure is 0.6° to 1.1°. The stainless steel described herein has a yield strength of 305 MPa or greater, a tensile strength of 420 MPa or greater, an elongation of 35% to 40%, and satisfies the following equations (1) and (2): (1)Ti / (C+N)≥3 Ti, C, and N refer to the content of each element as a percentage by weight. (2) Charité impact energy at 20℃ × Charité impact energy at 40℃ ≥ 750 J / cm 2 .

2. The ferritic stainless steel according to claim 1, wherein the thickness of the stainless steel is from 6.0 mm to 25.0 mm.

3. The ferritic stainless steel according to claim 1 further comprises 0.3% or less Ni, 0.5% or less Cu, and 0.1% or less Al.

4. A method for manufacturing a ferritic stainless steel with excellent impact toughness, the method comprising: The slab is heated to 1220°C or lower and contains, by weight percentage (%) of the total composition: C: greater than 0% and less than 0.01%, Si: 0.8% or less, Mn: 0.5% or less, Cr: 10% to 14%, Ti: 0.01% to 0.45%, N: greater than 0% and less than 0.015%, with the remainder being iron (Fe) and other unavoidable impurities; The heated slab is rough rolled; Finish rolling of rough-rolled bars; and Annealing is performed on hot-rolled steel plates. The pressure is applied on the backend rather than the frontend. The total reduction at the rear end of the roughing roll is 54% or greater, and the thickness of the hot-rolled steel sheet is 6.0 mm to 25.0 mm. The hot rolling annealing is carried out at 850℃ to 980℃, and The average orientation difference between grains in the microstructure of hot-rolled and annealed steel sheets ranges from 0.6° to 1.1°. The finishing rolling temperature is 960°C or lower. And the steel in it satisfies the following formula (1): (1)Ti / (C+N)≥3 Ti, C, and N refer to the content of each element in terms of weight percentage.

5. The manufacturing method according to claim 4, wherein the slab further comprises 0.3% or less Ni, 0.5% or less Cu, and 0.1% or less Al.

6. The manufacturing method according to claim 4, wherein the temperature of the rough-rolled bar is 1020°C to 970°C.

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