Ferritic stainless steel and manufacturing method thereof

Abstract

The present invention relates to a ferritic stainless steel and a manufacturing method thereof. More specifically, the present invention relates to a ferritic stainless steel that contains, in wt%, 0.0005-0.0200% of C, 0.005-0.020% of N, 0.01-2.00% of Si, 0.01-1.00% of Mn, 0.001-0.050% of P, 13-25% of Cr, 0.01-2.00% of Cu, and 0.05-0.50% of Ti, with the remainder comprising Fe and inevitable impurities, and satisfies expression (1) below. Expression (1): 7 x [Si] + 4 x [Cu] + 0.2 x [Cr] ≥ 10 (where [Si], [Cu], and [Cr] indicate the wt% of the corresponding element.)

Description

Ferritic stainless steel and its manufacturing method

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

[0002] Ferritic stainless steel is used in a variety of industrial fields, including home appliances, kitchenware, and automobile parts.

[0003] Due to the recent increase in the size of home appliances, the thickness of materials used in washing machines and dryers must be increased to ensure product safety. However, this increases product weight and CO2 emissions. Therefore, to prevent this increase in material thickness, the yield strength of the material must be improved.

[0004] To this end, attempts were made to increase the yield strength through temper rolling, etc., but this has the problem that the yield strength of the welded part decreases again, increasing the difference in strength between the base material and the welded part.

[0005] In order to solve the above-described problem, the present invention aims to provide a high-strength ferritic stainless steel and a method for manufacturing the same, which can improve the strength of the material and reduce the difference in strength between the base material and the weld by controlling the alloy composition and optimizing the manufacturing method.

[0006] The technical problems to be achieved in this document are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0007] In order to achieve the above object, a ferritic stainless steel according to one embodiment of the present invention contains, in wt%, C: 0.0005% to 0.0200%, N: 0.005% to 0.020%, Si: 0.01% to 2.00%, Mn: 0.01% to 1.00%, P: 0.001% to 0.050%, Cr: 13% to 25%, Cu: 0.01% to 2.00%, Ti: 0.05% to 0.50%, the remainder being Fe and other unavoidable impurities, and can satisfy the following formula (1).

[0008] Equation (1): 7 x [Si] + 4 x [Cu] + 0.2 x [Cr] ≥ 10

[0009] (Here, [Si], [Cu], and [Cr] represent the weight percent of each element)

[0010] Additionally, the stainless steel according to one embodiment of the present invention may have a room temperature yield strength of 350 MPa or more.

[0011] In addition, the stainless steel according to one embodiment of the present invention may have a difference in room temperature yield strength between the base material portion and the weld portion of 30 MPa or less.

[0012] Additionally, the stainless steel according to one embodiment of the present invention may have a surface treatment (SPM) elongation of 0.1 to 1.0%.

[0013] In addition, a method for manufacturing a ferritic stainless steel according to an embodiment of the present invention comprises the steps of: reheating a slab that contains, in wt%, C: 0.0005% to 0.0200%, N: 0.005% to 0.020%, Si: 0.01% to 2.00%, Mn: 0.01% to 1.00%, P: 0.001% to 0.050%, Cr: 13% to 25%, Cu: 0.01% to 2.00%, Ti: 0.05% to 0.50%, the remainder Fe and other unavoidable impurities, and satisfying the above formula (1); hot rolling the slab so that the finish rolling inlet temperature after the reheating becomes 900°C to 1100°C; It may include a step of hot rolling and annealing at 900°C to 1100°C for 1 to 10 minutes after the hot rolling; and a step of cold rolling and cold rolling and annealing 1 to 5 times after the hot rolling and annealing.

[0014] Equation (1): 7 x [Si] + 4 x [Cu] + 0.2 x [Cr] ≥ 10

[0015] (Here, [Si], [Cu], and [Cr] represent the weight percent of each element)

[0016] Additionally, the reheating according to one embodiment of the present invention can be performed at 1100 to 1300°C for 2 to 4 hours.

[0017] Additionally, the cold rolling annealing according to one embodiment of the present invention can be performed at 800 to 1050°C for 30 to 200 seconds.

[0018] Additionally, in the step of manufacturing the steel plate according to one embodiment of the present invention, the final cold rolling reduction ratio may be 40% or more.

[0019] The ferritic stainless steel of the present invention can improve the strength of the material and reduce the difference in strength between the base material and the weld by controlling the alloy composition and optimizing the manufacturing method.

[0020] The effects that can be obtained from the present disclosure are not limited to the effects mentioned above, and other effects that are not mentioned can be clearly understood by a person having ordinary skill in the art to which the present disclosure belongs from the description below.

[0021] FIG. 1 is a graph showing the correlation between Si, Cu, and Cr components in an alloy composition and the room temperature yield strength of a base material according to one embodiment of the present invention.

[0022] 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.

[0023] 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.

[0024] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0025] The present invention aims to manufacture high-strength ferritic stainless steel by simultaneously securing the strength of the base material and the weldment, thereby reducing the difference in yield strength at room temperature between the base material and the weldment.

[0026] A ferritic stainless steel according to one embodiment of the present invention contains, in wt%, C: 0.0005% to 0.0200%, N: 0.005% to 0.020%, Si: 0.01% to 2.00%, Mn: 0.01% to 1.00%, P: 0.001% to 0.050%, Cr: 13% to 25%, Cu: 0.01% to 2.00%, Ti: 0.05% to 0.50%, the remainder being Fe and other unavoidable impurities.

[0027] 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 %.

[0028] The carbon (C) content can be from 0.0005% to 0.0200%.

[0029] When C is less than 0.0005%, refining costs for producing high-purity products may increase, and when it exceeds 0.0200%, corrosion resistance and formability may be deteriorated. Considering this, the C content may be between 0.0005% and 0.0200%.

[0030] The nitrogen (N) content can be 0.005% to 0.020%.

[0031] When N is less than 0.005%, TiN crystallization may be reduced, which may lower the equiaxed crystallinity of the slab. When it exceeds 0.020%, corrosion resistance and formability may be deteriorated. Considering this, the N content may be 0.005% to 0.020%.

[0032] The silicon (Si) content can be from 0.01% to 2.00%.

[0033] When Si is less than 0.01%, refining may be difficult, and when it exceeds 2.00%, surface defects may occur and formability may be poor. Considering this, the Si content may be between 0.01% and 2.00%.

[0034] The manganese (Mn) content can be from 0.01% to 1.00%.

[0035] When manganese is less than 0.01%, refining costs may increase, and when it exceeds 1.00%, impurities may increase, resulting in poor formability. Taking this into account, the manganese content may be between 0.01% and 1.00%.

[0036] The phosphorus (P) content may be 0.001% to 0.050%.

[0037] When P is less than 0.001%, refining costs may increase, and when it exceeds 0.050%, impurities may increase, resulting in poor formability. Taking this into account, the P content may be between 0.001% and 0.050%.

[0038] The chromium (Cr) content can be between 13% and 25%.

[0039] When Cr is less than 13.0%, corrosion resistance may be poor, and when it exceeds 25%, formability may be poor. Taking this into account, the Cr content may be between 13% and 25%.

[0040] The copper (Cu) content can be from 0.01% to 2.00%.

[0041] When Cu is less than 0.01%, it may be difficult to secure strength, and when it exceeds 2.00%, edge cracks may occur due to local liquefaction of copper. Considering this, the Cu content may be between 0.01% and 2.00%.

[0042] The content of titanium (Ti) may be 0.05% to 0.50%.

[0043] When Ti is less than 0.05%, corrosion resistance may be reduced, and when it exceeds 0.50%, the occurrence of steelmaking inclusions may increase.

[0044] 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.

[0045] The disclosed invention aims to improve the strength of a material and reduce the difference in strength between a base material and a welded portion by controlling the alloy composition described above and the manufacturing method described below.

[0046] To this end, in the present invention, the content of Si, Cu and Cr among the alloy components described above can be controlled as in the following formula (1) to secure the strength of the material.

[0047] Equation (1): 7 x [Si] + 4 x [Cu] + 0.2 x [Cr] ≥ 10

[0048] In the above formula (1), each element symbol represents the value of each element content expressed in weight%.

[0049] In order to secure the room temperature yield strength of the base material to 350 MPa or more, it is desirable that the value of 7 x [Si] + 4 x [Cu] + 0.2 x [Cr] in Equation (1) be 10 or more. If the value of 7Si+4Cu+0.2Cr is less than 10, the solid solution strengthening effect of Si, Cu, and Cr may be small, and the room temperature yield strength of the base material may be less than 350 MPa. The upper limit of Equation (1) may be, for example, 27 or less, 25 or less, 20 or less, or 18 or less. In this case, the effect of reducing the difference between the room temperature yield strength of the base material and the room temperature yield strength of the weld can be further improved while satisfying the target degrees.

[0050] In the example, by controlling the value of the above formula (1) to a range of 10 to 27, specifically 11 to 15, the effect of further increasing the room temperature yield strength of the parent material and the room temperature yield strength of the weld, while reducing the difference between them, can be further improved.

[0051] If the room temperature yield strength of the base material is less than 350 MPa, for example, when applied to home appliances, large home appliances, etc., the thickness required to ensure safety increases excessively, making it difficult to satisfy the desired level of strength and lightweight characteristics. The upper limit of the room temperature yield strength of the base material 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, large home appliances, etc.

[0052] In addition, when Si, Cu, and Cr are added as in the above formula (1), they can be dissolved in the ferrite matrix to increase the strength of the base material, and even when exposed to high temperatures during welding, the strength is secured due to the dissolved elements, so that the difference in strength between the base material and the weld can be reduced.

[0053] On the other hand, when the strength is increased by increasing the elongation of the SPM (short-pass milling) as in the past, the yield strength of the base material at room temperature increases due to the generation and contact of dislocations, but the strength of the weld decreases as the dislocations generated by the high temperature exposure disappear, so that the difference in strength between the base material and the weld becomes 30 MPa or more, which can cause problems in the stability of the final product.

[0054] As described above, in the present invention, in order to reduce the difference in strength between the base material and the weld, the contents of Si, Cu, and Cr are adjusted so that the value of 7x[Si]+4x[Cu]+0.2x[Cr] in the above formula (1) becomes 10 or more, thereby simultaneously increasing the strength of the base material and the weld, and making it possible to make the difference in the yield strength at room temperature between the base material and the weld less than 30 MPa.

[0055] Accordingly, the ferritic stainless steel according to one embodiment of the present invention may have a room temperature yield strength of 350 MPa or more in the base material portion, a difference in the room temperature yield strength between the base material portion and the weld portion may be 30 MPa or less, and a surface treatment (SPM) rolling elongation may be 0.1 to 1.0%.

[0056] If the yield strength of the base material at room temperature is less than 350 MPa, it may not be possible to manufacture high-strength materials. If the difference in yield strength between the base material and the weld at room temperature exceeds 30 MPa, there is a risk of plate fracture and cracking. In addition, if the elongation of the temper rolling is less than 0.1%, the increase in yield strength at room temperature is minimal and the plate shape may be inferior. If it exceeds 1.0%, the yield strength of the base material at room temperature increases, but the yield strength of the weld at room temperature decreases, which may increase the strength difference between the base material and the weld.

[0057] For the above reasons, the room temperature yield strength of the weld can be 320 MPa or higher. The upper limit of the room temperature yield strength of the weld is not limited, but can be, for example, -30 MPa to +30 MPa of the upper limit of the yield strength of the parent material as mentioned above. Within the above range, more advantageous properties can be realized to satisfy the processability, formability, weldability, etc. required for applications in, for example, home appliances and large home appliances.

[0058] Next, a method for manufacturing ferritic stainless steel according to one embodiment of the present invention will be described.

[0059] A method for manufacturing a ferritic stainless steel according to one embodiment of the present invention comprises the steps of: reheating a slab containing, in wt%, C: 0.0005% to 0.0200%, N: 0.005% to 0.020%, Si: 0.01% to 2.00%, Mn: 0.01% to 1.00%, P: 0.001% to 0.050%, Cr: 13% to 25%, Cu: 0.01% to 2.00%, Ti: 0.05% to 0.50%, the remainder Fe and other unavoidable impurities, and satisfying the above formula (1); hot rolling the slab so that the finish rolling inlet temperature after the reheating becomes 900°C to 1100°C; It may include a step of hot rolling and annealing at 900°C to 1100°C for 1 to 10 minutes after the hot rolling; and a step of cold rolling and cold rolling and annealing 1 to 5 times after the hot rolling and annealing.

[0060] In the present invention, in order to reduce the difference in yield strength at room temperature between the base material and the weld to 30 MPa or less, hot rolling annealing was performed at 900°C or higher to ensure sufficient solid solution effect of Cu, and by controlling the manufacturing method as described above, the difference in yield strength at room temperature between the base material and the weld can be reduced by maintaining the strength of the weld and improving the strength of the base material through solid solution of Cu.

[0061] The reason for limiting the composition range of each alloy element is as described above, and each manufacturing step is described in more detail below.

[0062] After manufacturing a slab that satisfies the above alloy composition, it can undergo a series of processes including reheating, hot rolling, hot annealing, cold rolling, and cold annealing.

[0063] First, the above slab is reheated at 1100 to 1300°C for 2 to 4 hours, and then hot-rolled and hot-annealed to produce a hot-rolled material.

[0064] The above reheating temperature may be 1100℃ or higher to reduce hot rolling load, and may be limited to 1300℃ or lower to prevent internal grain coarsening. If the above reheating time is less than 2 hours, the slab temperature may not be sufficiently secured, which may result in increased rolling load and frequent occurrence of surface defects. If it exceeds 4 hours, slab sagging or edge cracks may occur within the heating furnace.

[0065] The above hot rolling can be performed so that the finishing rolling entry temperature is 900℃ to 1100℃, and the thickness of the hot-rolled steel sheet thus hot-rolled can be 2 to 6mm.

[0066] If the finishing rolling entry temperature during the above hot rolling is less than 900℃, the rolling load may increase and shape defects may increase, which may lower productivity. If it exceeds 1100℃, the surface quality may deteriorate due to an increase in oxides caused by excessive high-temperature work, and the material may deteriorate due to deterioration of the texture.

[0067] The above hot rolling annealing can be performed at 900 to 1100°C for 1 to 10 minutes.

[0068] If the hot rolling annealing temperature is less than 900℃, recrystallization may not occur and thus the grain structure may not be formed. If it exceeds 1100℃, the grains may become coarser and the strength of the steel sheet may be weakened. In addition, if the hot rolling annealing time is less than 1 minute, recrystallization may not occur smoothly. If it exceeds 10 minutes, the grains may become coarser and the strength of the steel sheet may be weakened.

[0069] The above hot-rolled and hot-annealed material can be cold-rolled and cold-annealed one or more times, preferably 1 to 5 times.

[0070] The initial cold rolling and cold annealing can induce deformation of the hot-rolled steel and facilitate the formation of precipitates during subsequent processes. Subsequent cold rolling and cold annealing can precipitate numerous precipitates and induce recrystallization, securing fine grains. However, as manufacturing costs may increase with the number of cold rolling and cold annealing cycles, cold rolling and cold annealing can be performed no more than five times.

[0071] The above cold rolling annealing can be performed at 800 to 1050°C for 30 to 200 seconds, and the thickness of the final cold rolled product thus cold rolled annealed can be 0.1 to 2 mm.

[0072] If the above cold rolling annealing temperature is below 800℃, the rolled structure may not be sufficiently recrystallized, which may result in reduced workability. If it exceeds 1050℃, the grains may become coarser and plate breakage may occur.

[0073] In the step of manufacturing the above steel plate, the final cold rolling reduction ratio may be 40% or more.

[0074] If the final cold rolling reduction is less than 40%, it may be difficult to achieve fine grains due to insufficient deformation.

[0075] Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is intended only to illustrate the implementation of the present invention and is not intended to limit the present invention. This is because the scope of the present invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0076] Example

[0077] Slabs were manufactured in a vacuum induction melting furnace to satisfy various alloy compositions shown in Table 1 below.

[0078] The manufactured slab was reheated in a 1200℃ furnace for 2 to 4 hours, then hot-rolled until the finish rolling inlet temperature reached 1000℃, and then hot-annealed at 1050℃ for 5 minutes to produce a hot-rolled product with a thickness of 3mm. The hot-rolled product was cold-rolled, and cold-annealed at 900℃ for 100 seconds to produce a final cold-rolled product with a thickness of 0.5mm. At this time, cold rolling and cold-annealing were performed once. The final reduction ratio of each specimen was 83%.

[0079] The units in Table 1 below are weight%.

[0080] Classification CNMnPCrSiCuTi7x[Si]+4x[Cu]+0.2x[Cr] Example 10.010.010.250.0216.20.41.20.2310.8 Example 20.010.010.210.0216.30.910.2713.6 Example 30.010.010.190.0216.21.50.10.2614.1 Example 40.010.010.220.0218.41.11.50.2317.4 Example 50.010.010.240.0220.20.41.10.2611.2 Comparative Example 10.010.010.230.0213.70.30.30.276.0Comparative Example 20.010.010.190.0216.20.20.10.245.0Comparative Example 30.010.010.180.0218.50.10.60.236.8Comparative Example 40.010.010.210.0320.50.20.70.258.3Comparative Example 50.010.010.220.0220.40.10.30.276.0

[0081] Table 2 below shows the temper rolling (SPM) elongation of the cold-rolled products manufactured above, the yield strength at room temperature of the base metal, the yield strength at room temperature of the weld, and the difference in yield strength at room temperature between the base metal and the weld. Comparative Examples 6 and 7 in Table 2 below were tested by increasing the temper rolling elongation of Comparative Examples 4 and 5 from 0.5% to 1.2%. The yield strength at room temperature of the base metal was measured by processing the specimen to JIS13B in a direction 90 degrees to the rolling direction for the cold-rolled product, and the yield strength at room temperature of the weld was measured by making the weld line perpendicular to the tensile direction and centered on the gauge.

[0082] Classification 7x[Si]+4x[Cu]+0.2x[Cr] Temper rolling elongation(%)Base material room temperature yield strength (MPa)Welded area room temperature yield strength (MPa)Base material-welded area room temperature yield strength (MPa)Example 110.80.436735413Example 213.60.44013956Example 314.10.44234203Example 417.40.438637214Example 511.20.43653578Comparative example 16.00.428427410Comparative example 25.00.428726819Comparative example 36.80.533531025Comparative example 48.30.530528322Comparative Example 56.00.531029614Comparative Example 68.31.238432757Comparative Example 76.01.237833741

[0083] As shown in Table 2 above, Examples 1 to 5 satisfied the alloy composition and manufacturing method suggested in the disclosed invention. Therefore, it was confirmed that the value of 7x[Si]+4x[Cu]+0.2x[Cr] was 10 or more, thereby simultaneously increasing the strength of the base material and the weld, and that the difference in yield strength between the base material and the weld was less than 30 MPa at room temperature. From these results, it was found that according to the present invention, the yield strength of the base material and the weld can be secured simultaneously, thereby reducing the difference in yield strength between the base material and the weld. On the other hand, Comparative Examples 1 to 7 did not satisfy the value of 7x[Si]+4x[Cu]+0.2x[Cr] of 10 or more, even though they satisfied the alloy composition suggested in the disclosed invention. Therefore, it was confirmed that the yield strength of the base material was less than 350 MPa at room temperature. In addition, in Comparative Examples 6 and 7, where the temper rolling elongation was increased from 0.5% to 1.2% to increase the strength of the base material in Comparative Examples 4 and 5, the yield strength at room temperature of the base material increased, but the yield strength at room temperature of the weld decreased again due to high temperature exposure, confirming that the difference in strength between the base material and the weld exceeded 30 MPa, and confirming that this also caused problems with the safety of the final product.

[0084] Figure 1 is a graph showing the correlation between the Si, Cu, and Cr components in the alloy compositions of Examples 1 to 5 and Comparative Examples 1 to 5 and the room temperature yield strength of the base material.

[0085] As shown in Fig. 1, when the 7x[Si]+4x[Cu]+0.2x[Cr] value was controlled to 10 or more, the room temperature yield strength of the base material could be controlled to 350 MPa or more, and from these results, it was found that if the alloy components and manufacturing method suggested in the disclosed invention are satisfied, the strength of the base material and the weld can be increased simultaneously so that the difference in the room temperature yield strength of the base material and the weld can be made less than 30 MPa.

[0086] Although the embodiments of the invention disclosed above have been illustrated and described, the disclosed invention is not limited to the specific embodiments described above, and various modifications may be implemented by a person having ordinary skill in the art to which the disclosed invention pertains without departing from the gist claimed in the claims.

Claims

1. Containing, by weight%, C: 0.0005% to 0.0200%, N: 0.005% to 0.020%, Si: 0.01% to 2.00%, Mn: 0.01% to 1.00%, P: 0.001% to 0.050%, Cr: 13% to 25%, Cu: 0.01% to 2.00%, Ti: 0.05% to 0.50%, the remainder being Fe and other unavoidable impurities, Ferritic stainless steel satisfying the following equation (1). Equation (1): 7 x [Si] + 4 x [Cu] + 0.2 x [Cr] ≥ 10 (Here, [Si], [Cu], and [Cr] represent the weight% of each element) 2. In paragraph 1, The above stainless steel is a ferritic stainless steel with a room temperature yield strength of 350 MPa or more at the base material.

3. In paragraph 1, The above stainless steel is a ferritic stainless steel having a difference in room temperature yield strength between the base material and the welded portion of 30 MPa or less.

4. In paragraph 1, The above stainless steel is a ferritic stainless steel with a SPM (short-pass milling) elongation of 0.1 to 1.0%.

5. A step of reheating a slab containing, by weight%, C: 0.0005% to 0.0200%, N: 0.005% to 0.020%, Si: 0.01% to 2.00%, Mn: 0.01% to 1.00%, P: 0.001% to 0.050%, Cr: 13% to 25%, Cu: 0.01% to 2.00%, Ti: 0.05% to 0.50%, the remainder being Fe and other unavoidable impurities, and satisfying the following formula (1); A step of hot rolling so that the final rolling inlet temperature after the above reheating becomes 900℃ to 1100℃; A step of hot rolling annealing at 900℃ to 1100℃ for 1 to 10 minutes after the above hot rolling; and A method for manufacturing a ferritic stainless steel, comprising: a step of cold rolling and cold annealing 1 to 5 times after the above hot rolling annealing. Equation (1): 7 x [Si] + 4 x [Cu] + 0.2 x [Cr] ≥ 10 (Here, [Si], [Cu], and [Cr] represent the weight% of each element) 6. In paragraph 5, A method for manufacturing ferritic stainless steel, wherein the above reheating is performed at 1100 to 1300°C for 2 to 4 hours.

7. In paragraph 5, The above cold rolling annealing is a method for manufacturing ferritic stainless steel, which is performed at 800 to 1050°C for 30 to 200 seconds.

8. In paragraph 5, A method for manufacturing a ferritic stainless steel, wherein the final cold rolling reduction ratio in the step of manufacturing the above steel plate is 40% or more.

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