The present disclosure relates to a ferritic stainless steel and a manufacturing method therefor.

Ferritic stainless steels are used in various industrial fields such as home appliances, kitchen utensils, and automobile parts.

Recently, due to the enlargement of home appliances, in the case of washing machines or dryers, the thickness of a material must be further increased to secure the safety of the product, but this causes problems of increasing the weight of the product and increasing the amount of CO₂ generation. Therefore, in order to prevent an increase in the thickness of the material, there is a need to improve the yield strength of the material.

To this end, attempts have been made to increase the yield strength through skin pass rolling or the like in the related art, but this causes a problem that the yield strength of a weld portion decreases again, thereby increasing the difference in strength between a base metal portion and the weld portion.

In order to solve the above-described problems, an object of the present disclosure is to provide a high-strength ferritic stainless steel capable of improving the strength of a material and reducing a difference in strength between a base metal portion and a weld portion by controlling an alloy composition and optimizing a manufacturing method, and a manufacturing method therefor.

The technical problems to be achieved in the present document are not limited to the technical problems mentioned above, and other technical problems not mentioned herein will be clearly understood from the following description by those skilled in the art to which the present disclosure pertains.

In accordance with an aspect of the present disclosure to achieve the above-described objects, a ferritic stainless steel includes, in percent by weight (wt%), 0.0005 to 0.0200% of carbon (C), 0.005 to 0.020% of nitrogen (N), 0.01 to 2.00% of silicon (Si), 0.01 to 1.00% of manganese (Mn), 0.001 to 0.050% of phosphorus (P), 13 to 25% of chromium (Cr), 0.01 to 2.00% of copper (Cu), 0.05 to 0.50% of titanium (Ti), and the balance of iron (Fe) and inevitable impurities, wherein Formula (1) below is satisfied.$$7\times \left[\mathrm{Si}\right]+4\times \left[\mathrm{Cu}\right]+0.2\times \left[\mathrm{Cr}\right]\ge 10$$
(wherein [Si], [Cu], and [Cr] represent amounts (wt%) of respective elements)

In addition, the stainless steel according to an embodiment of the present disclosure may have a yield strength at room temperature of 350 MPa or more.

In addition, in the stainless steel according to an embodiment of the present disclosure, a difference in yield strength at room temperature between a base metal portion and a weld portion may be 30 MPa or less.

In addition, the stainless steel according to an embodiment of the present disclosure may have a skin pass rolling (SPM) elongation of 0.1 to 1.0%.

In accordance with another aspect of the present disclosure, a method for manufacturing a ferritic stainless steel includes: reheating a slab including, in percent by weight (wt%), 0.0005 to 0.0200% of carbon (C), 0.005 to 0.020% of nitrogen (N), 0.01 to 2.00% of silicon (Si), 0.01 to 1.00% of manganese (Mn), 0.001 to 0.050% of phosphorus (P), 13 to 25% of chromium (Cr), 0.01 to 2.00% of copper (Cu), 0.05 to 0.50% of titanium (Ti), and the balance of iron (Fe) and inevitable impurities, and satisfying Formula (1) below; hot rolling the slab such that a finish rolling entry temperature is 900°C to 1100°C after the reheating; hot annealing the hot-rolled steel sheet at 900°C to 1100°C for 1 to 10 minutes after the hot rolling; and cold rolling and cold annealing the hot-annealed steel sheet 1 to 5 times after the hot annealing.$$7\times \left[\mathrm{Si}\right]+4\times \left[\mathrm{Cu}\right]+0.2\times \left[\mathrm{Cr}\right]\ge 10$$
(wherein [Si], [Cu], and [Cr] represent amounts (wt%) of respective elements)

In addition, the reheating according to an embodiment of the present disclosure may be performed at 1100 to 1300°C for 2 to 4 hours.

In addition, the cold annealing according to an embodiment of the present disclosure may be performed at 800 to 1050°C for 30 to 200 seconds.

In addition, in the manufacturing of the steel sheet according to an embodiment of the present disclosure, a final cold rolling reduction ratio may be 40% or more.

The ferritic stainless steel of the present disclosure can improve the strength of the material and reduce the difference in strength between the base metal portion and the weld portion by controlling the alloy composition and optimizing the manufacturing method.

The effects obtainable in the present disclosure are not limited to the effects mentioned above, and other effects not mentioned herein will be clearly understood from the following description by those skilled in the art to which the present disclosure pertains.

FIG. 1 is a graph showing the correlation between Si, Cu, and Cr components in an alloy composition and yield strength at room temperature of a base metal portion according to an embodiment of the present disclosure.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are presented to sufficiently convey the spirit of the present disclosure to those skilled in the art to which the present disclosure pertains. The present disclosure is not limited to the embodiments presented herein and may be embodied in other forms. In the drawings, illustration of parts unrelated to the description is omitted to clarify the present disclosure, and sizes of components may be somewhat exaggerated for understanding.

In addition, the term "including" a certain component means that the component may further include other components rather than excluding other components unless specifically stated to the contrary.

Singular expressions include plural expressions unless the context clearly indicates an exception.

The present disclosure intends to manufacture a high-strength ferritic stainless steel by simultaneously securing the strength of a base metal portion and a weld portion, thereby reducing a difference in yield strength at room temperature between the base metal portion and the weld portion.

A ferritic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.0005 to 0.0200% of carbon (C), 0.005 to 0.020% of nitrogen (N), 0.01 to 2.00% of silicon (Si), 0.01 to 1.00% of manganese (Mn), 0.001 to 0.050% of phosphorus (P), 13 to 25% of chromium (Cr), 0.01 to 2.00% of copper (Cu), 0.05 to 0.50% of titanium (Ti), and the balance of iron (Fe) and inevitable impurities.

Hereinafter, the reasons for limiting the composition of the steel will be described in detail. All component compositions below mean percent by weight (wt%) unless otherwise specified.

The content of carbon (C) may be 0.0005% to 0.0200%.

A C content of less than 0.0005% may increase a refining cost for manufacturing a high-purity product, and a C content exceeding 0.0200% may deteriorate corrosion resistance and formability. In consideration of this, the content of C may be 0.0005% to 0.0200%.

The content of nitrogen (N) may be 0.005% to 0.020%.

A N content of less than 0.005% decreases TiN crystallization, potentially lowering the equiaxed crystal ratio of a slab, and a N content exceeding 0.020% may deteriorate corrosion resistance and formability. In consideration of this, the content of N may be 0.005% to 0.020%.

The content of silicon (Si) may be 0.01% to 2.00%.

A Si content of less than 0.01% may make refining difficult, and a Si content exceeding 2.00% may cause surface defects and deteriorate formability. In consideration of this, the content of Si may be 0.01% to 2.00%.

The content of manganese (Mn) may be 0.01% to 1.00%.

A Mn content of less than 0.01% may increase the refining cost, and a Mn content exceeding 1.00% increases impurities, potentially deteriorating formability. In consideration of this, the content of Mn may be 0.01% to 1.00%.

The content of phosphorus (P) may be 0.001% to 0.050%.

A P content of less than 0.001% may increase the refining cost, and a P content exceeding 0.050% increases impurities, potentially deteriorating formability. In consideration of this, the content of P may be 0.001% to 0.050%.

The content of chromium (Cr) may be 13% to 25%.

A Cr content of less than 13.0% may deteriorate corrosion resistance, and a Cr content exceeding 25% may deteriorate formability. In consideration of this, the content of Cr may be 13% to 25%.

The content of copper (Cu) may be 0.01% to 2.00%.

A Cu content of less than 0.01% may make it difficult to secure strength, and a Cu content exceeding 2.00% may cause edge cracks due to local liquefaction of copper. In consideration of this, the content of Cu may be 0.01% to 2.00%.

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

A Ti content of less than 0.05% may decrease corrosion resistance, and a Ti content exceeding 0.50% may increase the generation of steelmaking inclusions.

The remaining component is iron (Fe). However, since unintended impurities from raw materials or surrounding environments may inevitably be incorporated in a typical manufacturing process, the impurities cannot be excluded. Since these impurities are known to any person skilled in the art of a typical manufacturing process, all details thereof are not specifically mentioned in the present specification.

The disclosed invention intends to improve the strength of the material and reduce the difference in strength between the base metal portion and the weld portion by controlling the above-described alloy composition and the manufacturing method described below.

To this end, in the present disclosure, the strength of the material can be secured by controlling the contents of Si, Cu, and Cr among the above-described alloy components as in Formula (1) below.$$7\times \left[\mathrm{Si}\right]+4\times \left[\mathrm{Cu}\right]+0.2\times \left[\mathrm{Cr}\right]\ge 10$$

In Formula (1), each element symbol is a value representing the content of each element in wt%.

In order to secure the yield strength at room temperature of the base metal portion of 350 MPa or more, the value of 7 x [Si] + 4 x [Cu] + 0.2 x [Cr] in Formula (1) is preferably 10 or more. A value of 7 x [Si] + 4 x [Cu] + 0.2 x [Cr] of less than 10 results in a small solid solution strengthening effect of Si, Cu, and Cr, potentially causing the yield strength at room temperature of the base metal portion to be less than 350 MPa. The upper limit of Formula (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 while satisfying the desired degree of yield strength at room temperature of the base metal portion and yield strength at room temperature of the weld portion, respectively, may be further improved.

In an example, controlling the value of Formula (1) to be in the range of 10 to 27, specifically 11 to 15, may further improve the effect of reducing the difference while further increasing the yield strength at room temperature of the base metal portion and the yield strength at room temperature of the weld portion, respectively.

A yield strength at room temperature of the base metal portion of less than 350 MPa excessively increases the thickness required for securing safety when applied to home appliances, large home appliances, etc., making it difficult to satisfy the desired degree of strength and lightweight characteristics. The upper limit of the yield strength at room temperature of the base metal portion is not limited, but may be, for example, 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 workability, formability, weldability, etc. required for application to home appliances, large home appliances, etc.

In addition, the addition of Si, Cu, and Cr as in Formula (1) results in their dissolution in a ferrite matrix to increase the strength of the base metal portion, and even if exposed to high temperatures during welding, the strength is maintained due to solid solution elements, so that the difference in strength between the base metal portion and the weld portion can be reduced.

On the other hand, increasing the strength through an increase in skin pass rolling (SPM) elongation as in the related art increases the yield strength at room temperature of the base metal portion due to dislocation generation and accumulation, but in the weld portion, dislocations generated due to high temperature exposure disappear and the strength decreases, so that the difference in strength between the base metal portion and the weld portion becomes 30 MPa or more, potentially causing problems in the stability of the final product.

As described above, in the present disclosure, in order to reduce the difference in strength between the base metal portion and the weld portion, the contents of Si, Cu, and Cr are adjusted such that the value of 7x[Si]+4x[Cu]+0.2x[Cr] of Formula (1) is 10 or more, thereby simultaneously increasing the strength of the base metal portion and the weld portion and allowing the difference in yield strength at room temperature between the base metal portion and the weld portion to be less than 30 MPa.

Therefore, the ferritic stainless steel according to an embodiment of the present disclosure may have a yield strength at room temperature of the base metal portion of 350 MPa or more, a difference in yield strength at room temperature between the base metal portion and the weld portion of 30 MPa or less, and a skin pass rolling (SPM) elongation of 0.1 to 1.0%.

A yield strength at room temperature of the base metal portion of less than 350 MPa may make it impossible to manufacture a high-strength material. A difference in yield strength at room temperature between the base metal portion and the weld portion exceeding 30 MPa poses a risk of sheet breakage and cracking. In addition, a skin pass rolling elongation of less than 0.1% results in an insignificant effect of increasing the yield strength at room temperature and potentially inferior sheet shape, and an elongation exceeding 1.0% increases the yield strength at room temperature of the base metal portion, but decreases the yield strength at room temperature of the weld portion, potentially increasing the difference in strength between the base metal portion and the weld portion.

For the above reasons, the yield strength at room temperature of the weld portion may be 320 MPa or more. The upper limit of the yield strength at room temperature of the weld portion is not limited, but may be, for example, -30 MPa to +30 MPa of the upper limit of the yield strength of the base metal portion described above. Within the above range, for example, physical properties more advantageous for satisfying workability, formability, weldability, etc. required for application to home appliances, large home appliances, etc. can be implemented.

Next, a method for manufacturing a ferritic stainless steel according to an embodiment of the present disclosure will be described.

A method for manufacturing a ferritic stainless steel according to an embodiment of the present disclosure may include: reheating a slab including, in percent by weight (wt%), 0.0005 to 0.0200% of carbon (C), 0.005 to 0.020% of nitrogen (N), 0.01 to 2.00% of silicon (Si), 0.01 to 1.00% of manganese (Mn), 0.001 to 0.050% of phosphorus (P), 13 to 25% of chromium (Cr), 0.01 to 2.00% of copper (Cu), 0.05 to 0.50% of titanium (Ti), and the balance of iron (Fe) and inevitable impurities, and satisfying Formula (1) above; hot rolling the slab such that a finish rolling entry temperature is 900°C to 1100°C after the reheating; hot annealing the hot-rolled steel sheet at 900°C to 1100°C for 1 to 10 minutes after the hot rolling; and cold rolling and cold annealing the hot-annealed steel sheet 1 to 5 times after the hot annealing.

In the present disclosure, in order to reduce the difference in yield strength at room temperature between the base metal portion and the weld portion to 30 MPa or less, hot annealing was performed at 900°C or higher for a sufficient solid solution effect of Cu, and through such control of the manufacturing method, the strength improvement of the base material and the strength maintenance of the weld portion are maintained by the solid solution of Cu, thereby reducing the difference in yield strength at room temperature between the base metal portion and the weld portion.

The reasons for limiting the component ranges of the respective alloy elements are as described above, and hereinafter, each manufacturing step will be described in more detail.

After manufacturing a slab satisfying the above alloy composition, a series of processes of reheating, hot rolling, hot annealing, cold rolling, and cold annealing may be performed.

First, the slab may be reheated at 1100 to 1300°C for 2 to 4 hours, and then hot rolled and hot annealed to manufacture a hot-rolled material.

The reheating temperature may be 1100°C or higher for reducing a hot rolling load, and may be limited to 1300°C or lower for preventing internal grain coarsening. A reheating time of less than 2 hours may fail to sufficiently secure the slab temperature, potentially increasing the rolling load and causing frequent surface defects, and a reheating time exceeding 4 hours may cause slab sagging or edge cracks in a heating furnace.

The hot rolling may be performed such that the finish rolling entry temperature is 900°C to 1100°C, and the thickness of the hot-rolled steel sheet thus hot-rolled may be 2 to 6 mm.

A finish rolling entry temperature of less than 900°C during the hot rolling may increase the rolling load and shape defects, thereby decreasing productivity, and a temperature exceeding 1100°C may cause surface quality deterioration due to oxide increase and material degradation due to texture deterioration caused by excessive high-temperature operation.

The hot annealing may be performed at 900 to 1100°C for 1 to 10 minutes.

A hot annealing temperature of less than 900°C may prevent recrystallization, failing to form a texture, and a temperature exceeding 1100°C may coarsen crystal grains and weaken the strength of the steel sheet. In addition, a hot annealing time of less than 1 minute may prevent smooth recrystallization, and a time exceeding 10 minutes may coarsen crystal grains, weakening the strength of the steel sheet.

The hot-annealed hot-rolled material may be cold rolled and cold annealed one or more times, preferably 1 to 5 times.

Through the initial cold rolling and cold annealing, deformation in the steel of the hot-rolled material may be caused, and precipitate formation may be induced to proceed smoothly during subsequent processes. Through the secondary and subsequent cold rolling and cold annealing, a large number of precipitates are precipitated, and recrystallization is induced to secure fine crystal grains. At this time, since the manufacturing cost may increase as the number of cold rolling and cold annealing increases, the cold rolling and cold annealing may be performed 5 times or less.

The cold annealing may be performed at 800 to 1050°C for 30 to 200 seconds, and the thickness of the final cold-rolled product thus cold-annealed may be 0.1 to 2 mm.

A cold annealing temperature of less than 800°C may insufficiently recrystallize the rolled structure, deteriorating workability, and a temperature exceeding 1050°C may coarsen crystal grains and cause sheet breakage.

In the step of manufacturing the steel sheet, the final cold rolling reduction ratio may be 40% or more.

A final cold rolling reduction ratio of less than 40% may result in insufficient deformation, making it difficult to implement fine crystal grains.

Hereinafter, the present disclosure will be described in more detail through examples. However, the description of these examples is only for illustrating the implementation of the present disclosure, and the present disclosure is not limited by the description of these examples. This is because the scope of rights of the present disclosure is determined by matters described in the claims and matters reasonably inferred therefrom.

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

The manufactured slabs were reheated in a heating furnace at 1200°C for 2 to 4 hours, hot rolled such that a finish rolling entry temperature became 1000°C, and then hot annealed at 1050°C for 5 minutes to manufacture hot-rolled materials having a thickness of 3 mm. The hot-rolled materials were cold rolled and cold annealed at 900°C for 100 seconds to manufacture final cold-rolled products having a thickness of 0.5 mm. At this time, cold rolling and cold annealing were performed once. The final reduction ratio of each specimen was performed at 83%.

The unit of Table 1 below is percent by weight (wt%). Category C N Mn P Cr Si Cu Ti 7x[Si]+4x[Cu]+ 0.2x[Cr] Example 1 0.01 0.01 0.25 0.02 16.2 0.4 1.2 0.23 10.8 Example 2 0.01 0.01 0.21 0.02 16.3 0.9 1 0.27 13.6 Example 3 0.01 0.01 0.19 0.02 16.2 1.5 0.1 0.26 14.1 Example 4 0.01 0.01 0.22 0.02 18.4 1.1 1.5 0.23 17.4 Example 5 0.01 0.01 0.24 0.02 20.2 0.4 1.1 0.26 11.2 Comparative Example 1 0.01 0.01 0.23 0.02 13.7 0.3 0.3 0.27 6.0 Comparative Example 2 0.01 0.01 0.19 0.02 16.2 0.2 0.1 0.24 5.0 Comparative Example 3 0.01 0.01 0.18 0.02 18.5 0.1 0.6 0.23 6.8 Comparative Example 4 0.01 0.01 0.21 0.03 20.5 0.2 0.7 0.25 8.3 Comparative Example 5 0.01 0.01 0.22 0.02 20.4 0.1 0.3 0.27 6.0

Table 2 below shows the skin pass rolling (SPM) elongation, the yield strength at room temperature of the base metal portion, the yield strength at room temperature of the weld portion, and the difference in yield strength at room temperature between the base metal portion and the weld portion of the manufactured cold-rolled products. Comparative Examples 6 and 7 in Table 2 below were tested by increasing the skin pass rolling elongation of Comparative Examples 4 and 5 from 0.5% to 1.2%. The yield strength at room temperature of the base metal portion was measured by processing the specimen into JIS13B in a direction of 90 degrees to the rolling direction for the cold-rolled product, and the yield strength at room temperature of the weld portion was measured such that a welding line came to the center of the gauge perpendicular to the tensile direction. Category 7x[Si]+4x[Cu] +0.2x[Cr] Skin pass rolling elongation (%) Yield strength at room temperature of base metal portion (MPa) Yield strength at room temperature of weld portion (MPa) Base metal portion - Weld portion yield strength at room temperature (MPa) Example 1 10.8 0.4 367 354 13 Example 2 13.6 0.4 401 395 6 Example 3 14.1 0.4 423 420 3 Example 4 17.4 0.4 386 372 14 Example 5 11.2 0.4 365 357 8 Comparative Example 1 6.0 0.4 284 274 10 Comparative Example 2 5.0 0.4 287 268 19 Comparative Example 3 6.8 0.5 335 310 25 Comparative Example 4 8.3 0.5 305 283 22 Comparative Example 5 6.0 0.5 310 296 14 Comparative Example 6 8.3 1.2 384 327 57 Comparative Example 7 6.0 1.2 378 337 41

As shown in Table 2, Examples 1 to 5 satisfied the alloy components and the manufacturing method presented in the disclosed invention. Therefore, it was confirmed that the value of 7x[Si]+4x[Cu]+0.2x[Cr] was 10 or more, simultaneously increasing the strength of the base metal portion and the weld portion, so that the difference in yield strength at room temperature between the base metal portion and the weld portion was less than 30 MPa. From these results, it could be seen that according to the present disclosure, the difference in yield strength between the base metal portion and the weld portion can be reduced by simultaneously securing the yield strength of the base metal portion and the weld portion. On the other hand, although Comparative Examples 1 to 7 satisfied the alloy composition presented in the disclosed invention, the value of 7x[Si]+4x[Cu]+0.2x[Cr] did not satisfy 10 or more. Therefore, it was confirmed that the yield strength at room temperature of the base metal portion was less than 350 MPa. In addition, in the case of Comparative Examples 6 and 7 in which the skin pass rolling elongation was increased from 0.5% to 1.2% in order to increase the strength of the base metal portion in Comparative Examples 4 and 5, the yield strength at room temperature of the base metal portion increased, but the yield strength at room temperature of the weld portion decreased again due to high temperature exposure, so that it was confirmed that the difference in strength between the base metal portion and the weld portion exceeded 30 MPa, and it was confirmed that this caused a problem in the safety of the final product.

FIG. 1 is a graph showing the correlation between Si, Cu, and Cr components in the alloy composition and the yield strength at room temperature of the base metal portion of Examples 1 to 5 and Comparative Examples 1 to 5.

As shown in FIG. 1, controlling the value of 7x[Si]+4x[Cu]+0.2x[Cr] to 10 or more enabled the adjustment of the yield strength at room temperature of the base metal portion to 350 MPa or more, and from these results, it could be seen that if the alloy components and the manufacturing method presented in the disclosed invention are satisfied, the difference in yield strength at room temperature between the base metal portion and the weld portion can be made less than 30 MPa by simultaneously increasing the strength of the base metal portion and the weld portion.

Although the embodiments of the disclosed invention have been illustrated and described above, the disclosed invention is not limited to the specific embodiments described above, and various modifications may be made by those skilled in the art to which the disclosed invention pertains without departing from the gist claimed in the claims.