FERRITIC STAINLESS STEEL AND METHOD OF MANUFACTURING THE SAME

- POSCO

Provided is a ferritic stainless steel and method of manufacturing the same. The ferritic stainless steel comprises, by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.01 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities. The ferritic stainless steel comprises niobium (Nb) laves phase precipitate, and precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), 30 to 70% of the precipitates are distributed in a region within 1 μm from the grain boundaries, and the average particle size of the precipitates is 0.5 μm or less.

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Description
TECHNICAL FIELD

The present invention relates to ferritic stainless steels and methods of manufacturing the same, and more particularly to ferritic stainless steels having improved high temperature characteristics such as high temperature strength and thermal fatigue characteristics by controlling the distribution and compositions of precipitates in stainless steel through controls of alloy compositions and manufacturing method.

BACKGROUND ART

Among stainless steels, ferritic stainless steels are widely used in automotive exhaust system components, building materials, kitchen containers, and household appliances. Particularly, exhaust manifolds among the automotive exhaust system components are directly exposed to exhaust gas at high temperatures of 700° C. or more and require very high safety in a long operating environment. Therefore, much research has been made on alloy components and manufacturing methods for improving high-temperature characteristics.

There has been much research on the influence of alloys such as Mo, Nb and W, which are elements that improve the high temperature characteristics, but it is difficult to grasp the influence of how crystal grains and precipitates generated in the ferritic stainless steel at high temperatures substantially affect the high temperature physical properties.

It is required to optimize alloy components and manufacturing conditions for the crystal grains and precipitates generated in the ferritic stainless steel for application as a material for an exhaust manifold which is gradually improved in performance.

(Patent Literature 0001) Korean Patent Publication No. 10-2006-0007441

DISCLOSURE OF INVENTION Technical Problem

Embodiments of the present invention are to provide ferritic stainless steels improved in high temperature characteristics such as high temperature strength and thermal fatigue characteristics by controlling alloy components of the ferritic stainless steels and by controlling distribution and compositions of precipitates in the ferritic stainless steels.

In addition, embodiments of the present invention are to provide methods of manufacturing ferritic stainless steels by controlling reheating, rough rolling and finish rolling processes of stainless steel.

Technical Solution

A ferritic stainless steel according to an embodiment of the present invention comprises, by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.01 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities. The ferritic stainless steel comprises niobium (Nb) laves phase precipitate, and precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), 30 to 70% of the precipitates are distributed in a region within 1 μm from the grain boundaries, and the average particle size of the precipitates is 0.5 μm or less.

Further, according to an embodiment of the present invention, the weight ratio of niobium (Nb)/titanium (Ti) may be 2 to 10.

Further, according to an embodiment of the present invention, the niobium (Nb) laves phase precipitate may comprise at least one selected from the group consisting of Fe2Nb, FeCrNb, and Cr2Nb.

Further, according to an embodiment of the present invention, the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N) may comprise at least one selected from the group consisting of niobium nitride (NbN), niobium carbide (NbC), and niobium carbonitride (NbCN).

Further, according to an embodiment of the present invention, the average particle size of the precipitates may be 0.351 μm or less.

Further, according to an embodiment of the present invention, the mass of niobium (Nb) in the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), with respect to the total mass of the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N) may be less than 30%.

Further, according to an embodiment of the present invention, the ferritic stainless steel may have a tensile strength of 30 MPa or more at 900° C.

Further, according to an embodiment of the present invention, the ferritic stainless steel may have 500 or more times of thermal fatigue cycles at the temperature range of 200 to 900° C. at the constraint ratio of 50%.

A method of manufacturing a ferritic stainless steel according to an embodiment of the present invention comprises: a step of reheating a ferritic stainless steel comprising, by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.01 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities to 1.100 to 1,300° C., a step of rough rolling the stainless steel by a plurality of times, and a step of finish rolling the stainless steel.

In the rough rolling step, the final two rough rolling is performed at a total reduction rate of 50% or more, and is maintained for the time of the following Formula (1) after the rough rolling and before the finish rolling.


8,000/(reheating temperature−1,000)<time (second)<120  Formula (1)

Further, according to an embodiment of the present invention, a weight ratio of niobium (Nb)/titanium (Ti) may be 2 to 10.

Further, according to an embodiment of the present invention, the method of manufacturing a ferritic stainless steel further may comprise a step of coiling after the finish rolling step, wherein the coiling temperature may be 500 to 700° C.

Advantageous Effects

The ferritic stainless steels according to the embodiments of the present invention can pin the crystal grain boundaries at high temperatures and suppress crystal grain boundary sliding and rapid movement of dislocations by controlling the composition of the stainless steel, and the size and distribution of precipitates in the stainless steel, thereby improving high temperature characteristics such as high temperature strength and thermal fatigue characteristics.

Further, the embodiments of the present invention can prevent the coarsening of crystal grains by controlling the reheating, rough rolling and finish rolling processes in the processes of manufacturing ferritic stainless steel, and thereby can control so that fine precipitates are distributed in the region adjacent to the crystal grains.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph of a stainless steel according to an embodiment of the present invention taken through a transmission electron microscope (TEM).

FIG. 2 is a photograph of a stainless steel according to a comparative example taken through a transmission electron microscope (TEM).

 BEST MODE FOR INVENTION

A ferritic stainless steel according to an embodiment of the present invention comprises, by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.01 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities. The ferritic stainless steel comprises niobium (Nb) laves phase precipitate, and precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), 30 to 70% of the precipitates are distributed in a region within 1 μm from the grain boundaries, and the average particle size of the precipitates is 0.5 μm or less.

MODE FOR INVENTION

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present disclosure to a person having ordinary skill in the art to which the present disclosure belongs. The present disclosure is not limited to the embodiments shown herein but may be embodied in other forms. The drawings are not intended to limit the scope of the present disclosure in any way, and the size of components may be exaggerated for clarity of illustration.

Ferritic Stainless Steel

According to an embodiment of the present invention, a ferritic stainless steel comprises, by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.02 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities.

The amount of carbon (C) is 0.02% or less. More preferably, the amount of carbon (C) is 0.0005% to 0.02%. If the amount of carbon (C) is less than 0.0005%, the refining price for making high purity products becomes expensive. If the amount of carbon (C) exceeds 0.02%, impurities of the material increase, the elongation rate and work hardening index (n value) decrease, the ductile-brittle transition temperature (DBTT) rises, and the impact characteristic is deteriorated.

The amount of nitrogen (N) is 0.02% or less. More preferably, the amount of nitrogen (N) is 0.005% to 0.02%. If the amount of nitrogen (N) is less than 0.005%, TiN crystallization is lowered and the equiaxed crystal ratio in the slab becomes lower. If the amount of nitrogen (N) exceeds 0.02%, impurities of the material increase, the elongation rate decreases, the ductile-brittle transition temperature (DBTT) rises, and the impact characteristic is deteriorated.

The amount of silicon (Si) is 1.0% or less. More preferably, the amount of silicon (Si) is 0.01% to 1.0%. If the amount of silicon (Si) is less than 0.01%, there is a problem that the refining price becomes expensive. If the amount of silicon (Si) exceeds 1.0%, the impurities of the material increase and so the elongation rate and work hardening index (n value) decrease, and Si-based inclusions increase and so the workability is deteriorated.

The amount of manganese (Mn) is 1.20% or less. More preferably, the amount of manganese (Mn) is 0.01% to 1.20% or less. If the amount of manganese (Mn) is less than 0.01%, there is a problem that the refining price becomes expensive. If the amount of manganese (Mn) exceeds 1.2%, there is a problem that the impurities of the material increase and so the elongation rate decreases.

The amount of phosphorus (P) is 0.05% or less. More preferably, the amount of phosphorus (P) is 0.001% to 0.05%. If the amount of phosphorus (P) is less than 0.001%, there is a problem that the refining price becomes expensive. If the amount of phosphorus (P) exceeds 0.05%, there is a problem that the impurities of the material increase and so the elongation rate and work hardening index (n value) decrease.

The amount of sulfur (S) is 0.005% or less. More preferably, the amount of sulfur (S) is 0.0001% to 0.005%. If the amount of sulfur (S) is less than 0.0001%, there is a problem that the refining price becomes expensive, and if the amount of sulfur (S) exceeds 0.005%, there is a problem that corrosion resistance is deteriorated.

The amount of chromium (Cr) is 10.0 to 25.0%. If the amount of chromium (Cr) is less than 10.0%, there is a problem that the corrosion resistance and oxidation resistance are deteriorated. If the amount of chromium (Cr) exceeds 25.0%, there are problems that the elongation rate is lowered and hot-rolled sticking defect occurs.

The amount of nickel (Ni) is 0.01 to 0.50%. If the amount of nickel (Ni) is less than 0.01%, there is a problem that the refining price becomes expensive. If the amount of nickel (Ni) exceeds 0.50%, there is a problem that impurities of the material increase and so the elongation rate is lowered.

The amount of molybdenum (Mo) is 0.5 to 2.0%. If the amount of molybdenum (Mo) is less than 0.5%, the amount of molybdenum (Mo) contained in the material is too small, and so deterioration of the high temperature strength and thermal fatigue characteristics of the material and the probability of occurrence of anomalous oxidation become high. If the amount of molybdenum (Mo) exceeds 2.0%, risk of fracture occurrence during processing due to lowered impact characteristic, and there is a burden that the cost of the material increases.

The amount of titanium (Ti) is 0.01 to 0.30%. If the amount of titanium (Ti) is less than 0.01%, the cost for very low impurity refining becomes high, and if the amount of titanium (Ti) exceeds 0.3%, there is a problem that the nozzles are clogged during the production of continuous casting slabs due to the increase of the Ti-based oxide.

The amount of niobium (Nb) is 0.30 to 0.70%. If the amount of niobium (Nb) is less than 0.30%, there is a problem that the high temperature strength of the material is lowered because the amount of Nb contained in the material is small. If the amount of niobium (Nb) exceeds 0.70%, there is a problem that the elongation rate and impact characteristics are deteriorated because the Nb-based precipitates and the contained amount are excessively increased.

For example, the weight ratio of niobium (Nb)/titanium (Ti) is 2 to 10.

In particular, titanium (Ti) and niobium (Nb), as important elements in ensuring high-temperature physical properties of a material, affect the amount and distribution of internal precipitates depending on the addition ratio of the two elements, and ultimately affect the high temperature strength and thermal fatigue characteristics.

When the weight ratio of niobium (Nb)/titanium (Ti) is less than 2, precipitates containing coarse titanium (Ti) precipitate due to an relatively large amount of titanium (Ti), most of the niobium (Nb) around it is precipitated as the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), when the mass of the niobium (Nb) in the precipitate containing the niobium (Nb) and carbon (C) or nitrogen (N) is 30% or more, the amount of the niobium (Nb) laves phase precipitate having fine sizes decreases, and thus the high temperature strength and thermal fatigue characteristics are lowered.

When the weight ratio of niobium (Nb)/titanium (Ti) exceeds 10, the amount of titanium (Ti) is relatively very small and the amount of niobium (Nb) is relatively very large, so that most of the niobium (Nb) is precipitated as the precipitate containing the niobium (Nb) and carbon (C) or nitrogen (N), and the amount of the niobium (Nb) laves phase precipitate having fine sizes decreases, and thus the high temperature strength and thermal fatigue characteristics are lowered.

For example, the ferritic stainless steel may comprise the niobium (Nb) laves phase precipitate, the precipitate containing the niobium (Nb) and carbon (C) or nitrogen (N), and the precipitate containing the titanium (Ti).

For example, the niobium (Nb) laves phase precipitate may comprise at least one selected from the group consisting of Fe2Nb, FeCrNb, and Cr2Nb. The composition of laves phase is of the A2B type and is an intermetallic compound having a dense filled structure. The niobium (Nb) laves phase precipitate has a particle size of less than 0.2 μm which is a relatively fine size.

For example, the precipitate containing carbon (C) or nitrogen (N) may comprise at least one selected from the group consisting of niobium nitride (NbN), niobium carbide (NbC), and niobium carbonitride (NbCN). The precipitate containing carbon (C) or nitrogen (N) has a particle size of about 0.5 μm.

For example, the precipitate containing the titanium (Ti) may comprise at least one selected from the group consisting of titanium nitride (TiN), titanium carbide (TiC), titanium carbonitride (TiCN), and niobium titanium (NbTi). The precipitate containing the titanium (Ti) has a relatively coarse particle size of about 1 to 2 μm.

Accordingly, when the niobium (Nb)/titanium (Ti) is added in a weight ratio of 2 to 10, the average particle size of the precipitate in the material becomes 0.5 μm or less, thereby suppressing the formation of coarse precipitates. More preferably, the average particle size of the precipitates may be 0.35 μm or less.

Further, the precipitates are distributed in a range of 30 to 70% in a region within 1 μm from the grain boundaries. As such, the precipitates are distributed in a region of 1 μm or less from the grain boundaries so that the precipitates can serve to pin the grain boundaries at a high temperature and suppress grain boundary sliding (GBS) occurring at a high temperature and the rapid movement of the dislocation, thereby improving the temperature strength and thermal fatigue characteristics.

For example, the mass of niobium (Nb) in the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N) with respect to the total mass of the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N) may be less than 30%. When the mass of the niobium (Nb) in the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N) is 30% or more, the amount of the niobium (Nb) laves phase precipitate having a fine size decreases, and accordingly the high temperature strength and thermal fatigue characteristics are lowered.

For example, when the amount of titanium (Ti) is relatively very large or the amount of niobium (Nb) is relatively very large, most of the niobium (Nb) is precipitated as the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), so that the mass of the niobium (Nb) in the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N) becomes 30% or more and the amount of the niobium (Nb) laves phase precipitate having a fine size decreases, and therefore, the high temperature strength and thermal fatigue characteristics are lowered.

As described above, the ferritic stainless steel according to an embodiment of the present invention may have a tensile strength of more than 30 MPa at 900° C.

In addition, the ferritic stainless steel may have 500 or more times of thermal fatigue cycles at a temperature range of 200 to 900° C. at a constraint ratio of 50%.

Hereinafter, stainless steels according to an embodiment of the present invention will be described in detail through Invention Examples.

The compositions of invention steels and comparative steels are shown in Table 1 below.

TABLE 1 C N Si Mn P Cr Mo Ti Nb Nb/Ti Inv. 0.0092 0.0090 0.4 0.4 0.02 18.3 1.15 0.15 0.53 3.5 Steel 1 Inv. 0.0096 0.00110 0.4 0.4 0.02 21.2 1.54 0.12 0.56 4.7 Steel 2 Inv. 0.00134 0.0097 0.5 0.5 0.02 16.1 1.81 0.13 0.55 3.5 Steel 3 Comp. 0.0088 0.00114 0.3 0.4 0.02 18.2 0.20 0.11 0.52 4.7 Steel 1 Comp. 0.00104 0.0083 0.2 0.5 0.03 18.3 1.21 0.03 0.65 21.7 Steel 2 Comp. 0.0089 0.0091 0.3 0.4 0.02 19.2 1.55 0.08 0.21 2.6 Steel 3 Comp. 0.00123 0.00102 0.4 0.3 0.03 22.3 1.37 0.25 0.33 1.3 Steel 4

Referring to Table 1, Invention Steels 1 to 3 satisfy the composition of the ferritic stainless steel according to an embodiment of the present invention. On the other hand, comparative steel 1 deviates from the molybdenum (Mo) content, comparative steel 2 is out of the weight ratio of niobium (Nb)/titanium (Ti), comparative steel 3 is out of the content of niobium (Nb), and comparative steel 4 is out of the weight ratio of niobium (Nb)/titanium (Ti).

The stainless steels having the compositions of invention steels and comparative steels as above were prepared under the same conditions according to the method of manufacturing ferritic stainless steels according to an embodiment of the present invention to be described later, and the physical properties and the like of the ferritic stainless steels are shown in Table 2 below.

TABLE 2 Precipitate ratio (%) distributed Mass ratio in a region (%) of Nb of 1 μm or in precipitate Tensile Thermal Size (μm) less from containing strength fatigue of grain Nb and C (MPa) at cycle precipitate boundaries or N 900° C. (times) Invention 0.28 58 11 34 564 Steel 1 Invention 0.22 49 13 36 588 Steel 2 Invention 0.34 38 21 37 612 Steel 3 Comparative 0.37 48 19 28 455 Steel 1 Comparative 1.21 12 78 27 472 Steel 2 Comparative 0.13 9 33 31 467 Steel 3 Comparative 0.89 24 42 25 437 Steel 4

FIG. 1 is a photograph of a stainless steel according to an embodiment of the present invention taken through a transmission electron microscope (TEM). FIG. 2 is a photograph of a stainless steel according to a comparative example taken through a transmission electron microscope (TEM).

Referring to FIGS. 1 and 2 and Tables 1 and 2 above, FIG. 1 is a photograph of Invention Steel 1 of the present invention taken through a transmission electron microscope (TEM), and FIG. 2 is a photograph of comparative steel 2 taken through a transmission electron microscope (TEM).

In FIG. 1, micro-precipitates 10 having a fine size are distributed adjacent to crystal grain boundaries and the average particle size of such micro-precipitates 10 is 0.5 μm or less. In FIG. 2, coarse precipitates 20 are distributed regardless of crystal grain boundaries, and the average particle size of such coarse precipitate 20 is about 1 μm.

In conclusion, the composition of the ferritic stainless steel satisfies the composition according to an embodiment of the present invention, the average particle size of the precipitates is 0.5 μm or less, and in a case where the precipitates are distributed in a region within 1 μm from the grain boundaries by 30 to 70%, the tensile strength at 900° C. is 30 MPa or more and the thermal fatigue cycle is 500 times or more at a temperature range of 200 to 900° C. at a 50% constraint rate. Therefore, high temperature characteristics such as the high temperature strength and thermal fatigue characteristics of the ferritic stainless steel according to an embodiment of the present invention are improved.

Method of Manufacturing Ferritic Stainless Steel

According to a method of manufacturing ferritic stainless steel according to an embodiment of this invention, a ferritic stainless steel which comprises a niobium (Nb)—laves phase precipitate and a precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), in which 30 to 70% of the precipitates are distributed in a region within 1 μm from the grain boundaries and the average particle size of the precipitates is 0.5 μm or less, can be produced.

In order to secure the high-temperature strength and thermal fatigue characteristics of the ferritic stainless steel according to an embodiment of the present invention, it is necessary to control the size and distribution of fine precipitates, which requires not only component control but also control of the hot rolling process.

According to the method of manufacturing ferritic stainless steel, a slab is produced using a molten steel comprising, by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.01 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities. The slab is subjected to reheating, hot rough rolling, hot finish rolling, and coiling in accordance with the following conditions.

The slab is reheated at a temperature of 1,100 to 1,300° C. in a heating furnace.

The stainless steel is provided in the form of a slab, the hot rolling reheating temperature of the slab is set to 1,100*C or more in order to re-decompose the coarse precipitates generated during the casting of the slab, and the reheating temperature should be 1,300° C. or less in order to prevent coarsening of the inner crystal grains.

Thereafter, the stainless steel is subjected to hot rough rolling a plurality of times.

Here, the final two rough rolling is performed at a total reduction rate of 50% or more, and is maintained for the time of the following Formula (1) after the rough rolling and before the finish rolling.


8,000/(reheating temperature−1,000)<time (second)<120  Formula (1)

At this time, the final two rough rolling can be carried out at a total rolling reduction of 50% or more, whereby fine precipitates can be precipitated in the crystal grain boundaries.

Thereafter, a sufficient time for recrystallization is provided by setting the holding time until the finish rolling after the rough rolling of the stainless steel to 8000/(reheating temperature−1,000) seconds or more, and coarsening of crystal grains can be prevented by controlling the holding time until the finish rolling after the rough rolling of the stainless steel to 120 seconds or less.

By controlling the crystal grains as described above, it is possible to provide a site where fine precipitates can be generated in the grain boundaries, and fine niobium (Nb) laves phase precipitates can be generated in a region within 1 μm from the grain boundaries. The fine precipitates generated around the crystal grain boundaries serve to pin the grain boundaries at high temperatures, and thus serve to suppress the grain boundary sliding (GBS) occurring at high temperatures and the rapid movement of the dislocation, thereby improving the temperature strength and thermal fatigue characteristics.

Thereafter, the stainless steel is subjected to finish rolling.

The stainless steel subjected to the finish rolling can be wound. For example, at this time, the coiling temperature may be 500 to 700° C. The coiling temperature is controlled to 700° C. or less so that precipitates precipitated during the hot rolling process described above are not coarsened, and the coiling temperature is controlled to 500° C. or more for plate shape and surface quality.

Hereinafter, a method of manufacturing a stainless steel according to an embodiment of the present invention will be described in detail with reference to embodiments.

Embodiments 1 to 3

The slabs were produced in accordance with the compositions of Invention Steel 1 to Invention Steel 3 as above, respectively, and reheated at 1,200° C. in a heating furnace. Thereafter, hot rough rolling was carried out, and the final two rough rolling was carried out with a total reduction rate of 70%. After the rough rolling, the invention steels were held for 60 seconds before the finish rolling. After finish rolling the invention steels, they were cooled and rewound, at which time the coiling temperature was maintained at 550° C.

Comparative Examples 1 to 3

The slabs were produced in accordance with the compositions of Invention Steel 1 to Invention Steel 3 as above, respectively, and reheated at 1,000° C. in a heating furnace. Thereafter, hot rough rolling was carried out, and the final two rough rolling was carried out with a total reduction rate of 40%. After the rough rolling, finish rolling, cooling and coiling were carried out continuously, wherein the coiling temperature was maintained at 550° C.

TABLE 3 Precipitate ratio (%) distributed Mass ratio in a region (%) of Nb Tensile of 1 μm or in strength Thermal Size (μm) less from precipitate (MPa) fatigue of grain containing at cycle precipitate boundaries Nb and C or N 900° C. (times) Embodiment 0.28 58 11 34 564 1 Embodiment 0.22 49 13 36 588 2 Embodiment 0.34 38 21 37 612 3 Comparative 0.72 35 39 30 473 Example 1 Comparative 0.39 15 19 33 491 Example 2 Comparative 1.34 24 49 31 479 Example 3

Referring to Table 3 above, in the ferritic stainless steel produced according to an embodiment of the present invention, the precipitates are distributed in a region within 1 μm from the grain boundaries by 30 to 70%, and the average grain size of the precipitates is 0.5 μm or less, and therefore, it can be seen that high temperature characteristics such as high temperature strength and thermal fatigue characteristics are improved by pinning the crystal grain boundaries at high temperatures and suppressing crystal grain boundary sliding and rapid movement of dislocations.

While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that the present invention may be variously modified and changed without departing from the technical idea of the present invention provided by the following claims.

INDUSTRIAL APPLICABILITY

The ferritic stainless steel according to the embodiments of the present invention is industrially applicable to automotive exhaust system components, building materials, kitchen containers, and household appliances.

Claims

1. A ferritic stainless steel comprising: by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.01 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities,

characterized in that the ferritic stainless steel comprises niobium (Nb) laves phase precipitate, and precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), 30 to 70% of the precipitates are distributed in a region within 1 μm from the grain boundaries, and the average particle size of the precipitates is 0.5 μm or less.

2. The ferritic stainless steel according to claim 1, wherein the weight ratio of niobium (Nb)/titanium (Ti) is 2 to 10.

3. The ferritic stainless steel according to claim 1, wherein the niobium (Nb) laves phase precipitate comprises at least one selected from the group consisting of Fe2Nb, FeCrNb, and Cr2Nb.

4. The ferritic stainless steel according to claim 1, wherein the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N) comprises at least one selected from the group consisting of niobium nitride (NbN), niobium carbide (NbC), and niobium carbonitride (NbCN).

5. The ferritic stainless steel according to claim 1, wherein the average particle size of the precipitates is 0.35 μm or less.

6. The ferritic stainless steel according to claim 1, wherein the mass of niobium (Nb) in the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), with respect to the total mass of the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N) is less than 30%.

7. The ferritic stainless steel according to claim 1, wherein the ferritic stainless steel has a tensile strength of 30 MPa or more at 900° C.

8. The ferritic stainless steel according to claim 1, wherein the ferritic stainless steel has 500 or more times of thermal fatigue cycles at the temperature range of 200 to 900° C. at the constraint ratio of 50%.

9. A method of manufacturing a ferritic stainless steel comprising: a step of reheating a ferritic stainless steel comprising, by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.01 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities to 1.100 to 1,300° C.:

a step of rough rolling the stainless steel by a plurality of times; and
a step of finish rolling the stainless steel,
characterized in that in the rough rolling step, the final two rough rolling is performed at a total reduction rate of 50% or more, and is maintained for the time of the following Formula (1) after the rough rolling and before the finish rolling. 8,000/(reheating temperature−1,000)<time (second)<120  Formula (1)

10. The method of manufacturing a ferritic stainless steel according to claim 9, wherein a weight ratio of niobium (Nb)/titanium (Ti) is 2 to 10.

11. The method of manufacturing a ferritic stainless steel according to claim 9, further comprising a step of coiling after the finish rolling step, wherein the coiling temperature is 500 to 700° C.

Patent History
Publication number: 20180179607
Type: Application
Filed: Feb 18, 2016
Publication Date: Jun 28, 2018
Applicant: POSCO (Pohang-si)
Inventors: Ji Eon PARK (Pohang-si), Ka Young CHOI (Pohang-si), Mi Nam PARK (Pohang-si)
Application Number: 15/579,789
Classifications
International Classification: C21D 8/02 (20060101); B21B 1/22 (20060101); B21B 3/02 (20060101); C21D 6/00 (20060101); C22C 38/28 (20060101); C22C 38/26 (20060101); C22C 38/22 (20060101); C22C 38/04 (20060101); C22C 38/02 (20060101); C22C 38/00 (20060101);