EP4613904A1

EP4613904A1 - Ferritic stainless steel for construction applications and method for producing same

Info

Publication number: EP4613904A1
Application number: EP23907285.3A
Authority: EP
Inventors: Ilchan Jung; Soohyun Kim; Unga NAM; Hanhyuk GO; Yongmin Kwon; Yongho Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-21
Filing date: 2023-09-04
Publication date: 2025-09-10
Also published as: EP4613904A4; KR20240098331A; WO2024135997A1

Abstract

Disclosed herein is a ferritic stainless steel having excellent formability for square tubing and improved structural stability due to increased strength in the event of fire. The structural ferritic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.005 to 0.015% of carbon (C), 0.005 to 0.03% of nitrogen (N), 0.4 to 0.8% of silicon (Si), 0.7 to 1.2% of manganese (Mn), 10.5 to 14.0% of chromium (Cr), 0.2 to 1% of nickel (Ni), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe), and inevitable impurities, and wherein the steel satisfies Formula (1) below,

7 \leq Cr + 7 Si - 3 Mn - 3 Ni - 50 (C + N) \leq 14

(wherein Cr, Si, Mn, Ni, C, and N represent the content (wt%) of the respective elements).

Description

[Technical Field]

The present disclosure relates to a structural ferritic stainless steel, and more particularly, to a ferritic stainless steel having high structural stability and excellent formability for structural square tubing due to increased strength in the event of fire, and a method for manufacturing the same.

[Background Art]

In general, stainless steel is classified according to chemical composition or metallic structure. Based on the metallic structure, stainless steel may be classified into austenite type, ferrite type, martensite type, and dual phase type.
Ferritic stainless steel has excellent corrosion resistance even with fewer expensive alloying elements added, and thus has higher cost competitiveness compared to austenitic stainless steel.
In addition, while carbon steel used to be additionally painted after square tube forming to obtain corrosion resistance, the demand for ferritic stainless steel has recently increased due to environmental concerns, as painting may be omitted after square tube forming using ferritic stainless steel.
To be used as structural steel, cracks must not occur during square tube forming. Also, in the event of additional thermal history such as in a fire, a reduction in strength may reduce structural stability, and when the strength is further increased, the structural stability is further increased. Thus, whether additional strength enhancement occurs when additional heat treatment is applied to the final material is critical.

[Disclosure]

[Technical Problem]

The present disclosure provides a structural ferritic stainless steel that has excellent corrosion resistance without separate painting, does not crack during square tube forming, and whose strength is improved upon additional heat treatment.

[Technical Solution]

According to an embodiment of the present disclosure, a structural ferritic stainless steel includes, in percent by weight (wt%), 0.005 to 0.015% of carbon (C), 0.005 to 0.03% of nitrogen (N), 0.4 to 0.8% of silicon (Si), 0.7 to 1.2% of manganese (Mn), 10.5 to 14.0% of chromium (Cr), 0.2 to 1% of nickel (Ni), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe) and inevitable impurities, and wherein the steel satisfies Formula (1) below, $7 \leq Cr + 7 Si - 3 Mn - 3 Ni - 50 (C + N) \leq 14$ (wherein Cr, Si, Mn, Ni, C, and N represent the content (wt%) of the respective elements).
In addition, the structural ferritic stainless steel according to an embodiment of the present disclosure may have a tensile strength of 350 MPa or more at room temperature.
In addition, the structural ferritic stainless steel according to an embodiment of the present disclosure may include a martensite phase in an area fraction of 10% or more after a heat treatment at 880 to 920°C for 20 to 40 minutes.
In addition, the structural ferritic stainless steel according to an embodiment of the present disclosure, a room-temperature tensile strength after the heat treatment at 880 to 920°C for 20 to 40 minutes may be increased than a room-temperature tensile strength before the heat treatment.
According to an embodiment of the present disclosure, a method for manufacturing a structural ferritic stainless steel includes: preparing a cold-rolled ferritic stainless steel sheet including, in percent by weight (wt%), 0.005 to 0.015% of carbon (C), 0.005 to 0.03% of nitrogen (N), 0.4 to 0.8% of silicon (Si), 0.7 to 1.2% of manganese (Mn), 10.5 to 14.0% of chromium (Cr), 0.2 to 1% of nickel (Ni), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe) and inevitable impurities, and wherein the cold-rolled ferritic stainless steel sheet satisfies Formula (1) below;

annealing the cold-rolled steel sheet in a range of 650 to 800°C for 10 seconds to 24 hours;
rapidly cooling the annealed cold-rolled steel sheet to 450 to 550°C at 0.05°C/s or more; and
slowly cooling to room temperature at less than 0.05°C/s.

In addition, the method according to an embodiment of the present disclosure, a martensite phase may be included in an area fraction of 10% or more after a heat treatment at 880 to 920°C for 20 to 40 minutes.
In addition, the method according to an embodiment of the present disclosure, a room-temperature tensile strength after a heat treatment at 880 to 920°C for 20 to 40 minutes may be increased than a room-temperature tensile strength before the heat treatment.

[Advantageous Effects]

According to the present disclosure, a structural ferritic stainless steel having excellent formability for structural square tubing and improved structural stability due to its increased strength upon additional heat treatment, and a method for manufacturing the same may be provided.

[Description of Drawings]

FIG. 1 shows a microstructure photograph where the martensite phase appeared during heat treatment at 900°C for 30 minutes for a structural ferritic stainless steel according to an embodiment.
FIG. 2 shows a photograph where no crack occurred after 180° bending of a structural ferritic stainless steel according to an embodiment.

[Mode for Invention]

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are presented to fully convey the spirit of the present disclosure to those skilled in the art to which the present disclosure pertains, and are not limited to those shown herein, but may be embodied in other forms. The drawings may omit figures not pertinent to the description in order to clarify the present disclosure, and the sizes of configurations may be exaggerated for the purpose of illustration.
Throughout the specification, when a part is referred to as "comprising", "including" and/or "having" a certain element, it is understood that, unless expressed otherwise, the description does not preclude the presence or addition of one or more elements.
The singular form of a noun corresponding to an item may include one or a plurality of the items unless clearly indicated otherwise in a related context.
The inventors of the present disclosure have conducted various studies to improve the strength and formability for square tubing by additional heat treatment of a structural ferritic stainless steel, and were able to obtain the following findings.
A structural ferritic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.005 to 0.015% of carbon (C), 0.005 to 0.03% of nitrogen (N), 0.4 to 0.8% of silicon (Si), 0.7 to 1.2% of manganese (Mn), 10.5 to 14.0% of chromium (Cr), 0.2 to 1% of nickel (Ni), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe), and inevitable impurities, and wherein the steel satisfies the Formula (1) below. $7 \leq Cr + 7 Si - 3 Mn - 3 Ni - 50 (C + N) \leq 14$
(Here, Cr, Si, Mn, Ni, C, and N represent the content (wt%) of the respective elements).
Hereinafter, reasons for numerical limitations on the contents of alloy components in the embodiments of the present disclosure will be described. Hereinafter, unless otherwise specified, the unit is percent by weight (wt%).
The content of C is 0.005 to 0.015%.
In the case where the C content exceeds 0.015%, C combines with Cr to form Cr₂₃C₆ precipitates, leading to local Cr depletion in the matrix, which impairs high-temperature oxidation resistance. In addition, controlling the C content to less than 0.005% increases the steelmaking VOD process cost, which is undesirable. Accordingly, the content of C is limited to the range of 0.005 to 0.015%.
The content of N is 0.005 to 0.03%.
In the case where the N content in the steel exceeds 0.03%, the concentration of dissolved N reaches its limit. In this case, N combines with Cr to form Cr₂N precipitates, leading to local Cr depletion in the matrix, which impairs high-temperature oxidation resistance. In addition, controlling the N content to less than 0.005% increases the steelmaking VOD process cost, which is undesirable. Accordingly, the content of N is limited to the range of 0.005 to 0.03%.
The content of Si is 0.4 to 0.8%.
Si is a solid solution strengthening element for increasing high-temperature strength and also increases high-temperature oxidation resistance by forming a Si-enriched oxide film on the surface layer. For the above two effects, a minimum Si content of 0.4% or more is required, and when the Si content exceeds 0.8%, the workability of the material is significantly deteriorated. Accordingly, the Si content is limited as above.
The content of Mn is 0.7 to 1.2%.
Mn acts as an austenite stabilizer. In the case where large amounts of Mn are contained, the austenite-ferrite transformation temperature (Ac1) is lowered, and high-temperature annealing capable of dissolving C and N after cold rolling may not be performed. Accordingly, the Mn content is limited to 1.2% or less.
The content of Cr is 10.5 to 14.0%.
Cr is an essential element added to form a passive film that suppresses oxidation in stainless steel. For stable passive film formation, the Cr content is to be added in an amount of 10.5% or more, and the upper limit is limited to 14.0% for cost reasons.
The content of Ni is 0.2 to 1%.
Ni is an element for improving corrosion resistance and stabilizing austenite. However, in the case where the Ni content exceeds 1% in low-Cr ferritic stainless steel, austenite reverse transformation may occur during annealing heat treatment after hot rolling or cold rolling, adversely affecting elongation. Accordingly, the upper limit of Ni is limited to 1%.
The content of P is 0.04% or less.
P is an inevitable impurity contained in the steel, and because P causes grain boundary corrosion during pickling or impairs hot workability. Accordingly, the P content is controlled to 0.04% or less.
The content of S is 0.01% or less.
S is an inevitable impurity contained in the steel, is segregated in grain boundaries and impairs hot workability. Accordingly, the S content is limited to 0.01% or less.
The remainder of the ferritic stainless steel, excluding the aforementioned alloying elements, consists of Fe and other inevitable impurities.
Meanwhile, the structural ferritic stainless steel according to an embodiment of the present disclosure may satisfy the Formula (1) below. $7 \leq Cr + 7 Si - 3 Mn - 3 Ni - 50 (C + N) \leq 14$
(Here, Cr, Si, Mn, Ni, C, and N represent the content (wt%) of the respective elements).
In the case where Formula (1) satisfies 7 or more and 14 or less, an austenite phase is formed in an appropriate ratio during high-temperature exposure, and a martensite phase is appropriately generated during cooling, thereby improving the strength of base material. On the other hand, in the case of less than 7, very little austenite phase is formed during high-temperature exposure, and martensite formation is weak during cooling, resulting in no strength improvement. In the case of more than 14, the martensite phase is excessively formed during high-temperature exposure, increasing the risk of brittle fracture, which is undesirable. Accordingly, Formula (1) is controlled to 7 or more and 14 or less.
In addition, the structural ferritic stainless steel according to an embodiment of the present disclosure may have a tensile strength of 350 MPa or more at room temperature.
In addition, the structural ferritic stainless steel according to an embodiment of the present disclosure may include the martensite phase of 10% or more in an area fraction after heat treatment at 880 to 920°C for 20 to 40 minutes.
In the case where the martensite phase is 10% or more in an area fraction after heat treatment at 880 to 920°C for 20 to 40 minutes, the strength is further increased after high-temperature exposure, and in the case where the martensite phase is less than 10%, the strength is not increased even with high-temperature exposure.
In addition, in the structural ferritic stainless steel according to an embodiment of the present disclosure, a room-temperature tensile strength after heat treatment at 880 to 920°C for 20 to 40 minutes may increase compared to a room-temperature tensile strength before the heat treatment. For example, the room-temperature tensile strength after the heat treatment may increase by 30% or more, preferably 35% or more, more preferably 40% or more, compared to the room-temperature tensile strength before the heat treatment. In another example, the room-temperature tensile strength after the heat treatment may be 650 MPa or more, preferably 675 MPa or more, and more preferably 700 MPa or more.
A method for manufacturing a structural ferritic stainless steel according to another embodiment of the present disclosure includes: preparing a cold-rolled ferritic stainless steel sheet including, in percent by weight (wt%), 0.005 to 0.015% of carbon (C), 0.005 to 0.03% of nitrogen (N), 0.4 to 0.8% of silicon (Si), 0.7 to 1.2% of manganese (Mn), 10.5 to 14.0% of chromium (Cr), 0.2 to 1% of nickel (Ni), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe), and inevitable impurities, wherein the cold-rolled ferritic stainless steel sheet satisfies the Formula (1) below;

For example, a slab including the aforementioned composition of alloying components may be hot-rolled, and the hot-rolled steel sheet may be annealed, and then cold-rolled to manufacture a cold-rolled steel sheet.
The cold-rolled steel sheet may be rapidly cooled to 450 to 550°C at 0.05°C/s or more after a known recrystallization heat treatment in the cold-rolling annealing process. Martensite formation may be activated by cooling to the above temperature range.
In addition, according to the method for manufacturing a structural ferritic stainless steel according to an embodiment of the present disclosure, the martensite phase may be included in an area fraction of 10% or more after heat treatment at 880 to 920°C for 20 to 40 minutes.
In addition, according to the method for manufacturing a structural ferritic stainless steel according to an embodiment of the present disclosure, a room-temperature tensile strength after a heat treatment at 880 to 920°C for 20 to 40 minutes may be increased than a room-temperature tensile strength before the heat treatment.
Hereinafter, the present disclosure is described in more detail through exemplary embodiments.

{Embodiments}

By using stainless steel lab-scale dissolution and Ingot production facilities, 20 mm bar samples were prepared with the alloy compositions described in Table 1 below. Afterwards, the samples were reheated at 1,200°C, hot-rolled to 6 mm, hot-annealed at 760°C, cold-rolled to 2.0 mm, annealed at 740°C for heat treatment, rapidly cooled to 500°C after the heat treatment, held for about 7 minutes, and then air-cooled to produce cold-rolled annealed steel sheets.

[Table 1]

	Cr	Si	Mn	Ni	C	N
Comparative steel 1	13.5	1.4	0.85	0.32	0.01	0.006
Comparative steel 2	13.1	1.2	0.81	0.31	0.006	0.007
Comparative steel 3	12.8	1.3	0.77	0.31	0.007	0.01
Comparative steel 4	12.2	1.1	0.73	0.33	0.006	0.009
Comparative steel 5	12.5	0.9	0.71	0.32	0.006	0.008
Comparative steel 6	11.2	0.3	1.1	0.49	0.013	0.028
Comparative steel 7	11	0.2	1.1	0.57	0.014	0.027
Comparative steel 8	10.7	0.2	1.2	0.66	0.013	0.028
Comparative steel 9	10.6	0.2	1.2	0.84	0.014	0.026
Comparative steel 10	10.5	0.2	1.2	0.95	0.015	0.027
Inventive steel 1	12	0.8	0.7	0.33	0.005	0.01
Inventive steel 2	11.8	0.8	0.8	0.34	0.006	0.014
Inventive steel 3	11.9	0.7	0.9	0.31	0.012	0.015
Inventive steel 4	11.6	0.5	1.1	0.33	0.015	0.02
Inventive steel 5	11.3	0.4	1.2	0.41	0.014	0.029

Table 2 below shows a value of Formula (1), a room-temperature tensile strength, a room-temperature tensile strength after additional heat treatment at 900°C for 30 minutes, and whether cracks occurred after 180° bending for the produced cold-rolled annealed steel sheets.

[Table 2]

	Steel type	Formula (1)	Room-temperature tensile strength (TS, MPa)	Area fraction of Martensite phase after additional heat treatment*	Room-temperature tensile strength after additional heat treatment* (TS, MPa)	Crack occurrence after 180° bending
Comparative Example 1	Comparative steel 1	19.0	476	3%	411	no crack
Comparative Example 2	Comparative steel 2	17.5	485	2%	413	no crack
Comparative Example 3	Comparative steel 3	17.8	479	5%	421	no crack
Comparative Example 4	Comparative steel 4	16.0	472	8%	430	no crack
Comparative Example 5	Comparative steel 5	15.0	483	7%	421	no crack
Comparative Example 6	Comparative steel 6	6.5	510	58%	740	crack occurred
Comparative Example 7	Comparative steel 7	5.3	513	61%	747	crack occurred
Comparative Example 8	Comparative steel 8	4.5	507	56%	751	crack occurred
Comparative Example 9	Comparative steel 9	3.9	520	51%	748	crack occurred
Comparative Example 10	Comparative steel 10	3.4	519	52%	742	crack occurred
Example 1	Inventive steel 1	13.8	485	12%	710	no crack
Example 2	Inventive steel 2	13.0	495	31%	730	no crack
Example 3	Inventive steel 3	11.8	493	28%	720	no crack
Example 4	Inventive steel 4	9.1	497	50%	725	no crack
Example 5	Inventive steel 5	7.1	505	45%	730	no crack

The additional heat treatment was conducted at 900°C for 30 minutes. Examples 1 to 5 satisfied the composition range of the present disclosure and Formula (1), and thus it was confirmed that Examples 1 to 5 satisfied a room-temperature tensile strength of 350 MPa or more, the area fraction of martensite phase after the additional heat treatment was 10% or more, the room-temperature tensile strength was increased compared to before the heat treatment, and no cracks occurred after 180° bending.
On the other hand, Comparative Examples 1 to 5 did not satisfy the composition range of the present disclosure and Formula (1), and thus it was confirmed that the area fraction of martensite phase after the additional heat treatment was less than 10%, the room-temperature tensile strength was decreased compared to before the heat treatment.
In addition, Comparative Examples 6 to 10 did not satisfy the composition range of the present disclosure and Formula (1), but the area fraction of martensite phase after the additional heat treatment was 10% or more, and thus it was confirmed that the room-temperature tensile strength was increased compared to before the heat treatment, but cracks occurred after 180° bending.
While exemplary embodiments of the present disclosure have been described above with reference to the accompanying drawings, it will be understood by those of ordinary skill in the art that various modifications and changes can be made without departing from the spirit and scope of the present disclosure as defined by the appended claims.

Claims

A structural ferritic stainless steel comprising, in percent by weight (wt%), 0.005 to 0.015% of carbon (C), 0.005 to 0.03% of nitrogen (N), 0.4 to 0.8% of silicon (Si), 0.7 to 1.2% of manganese (Mn), 10.5 to 14.0% of chromium (Cr), 0.2 to 1% of nickel (Ni), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe), and inevitable impurities, and wherein the steel satisfies Formula (1) below, $7 \leq Cr + 7 Si - 3 Mn - 3 Ni - 50 (C + N) \leq 14$ (wherein Cr, Si, Mn, Ni, C, and N represent the content (wt%) of the respective elements).
The structural ferritic stainless steel of claim 1, having a tensile strength of 350 MPa or more at room temperature.
The structural ferritic stainless steel of claim 1, comprising a martensite phase in an area fraction of 10% or more after a heat treatment at 880 to 920°C for 20 to 40 minutes.
The structural ferritic stainless of claim 1, wherein a room-temperature tensile strength after a heat treatment at 880 to 920°C for 20 to 40 minutes is increased than a room- temperature tensile strength before the heat treatment.
A method for manufacturing a structutal ferritic stainless steel, the method comprising:
preparing a cold-rolled ferritic stainless steel sheet comprising, in percent by weight (wt%), 0.005 to 0.015% of carbon (C), 0.005 to 0.03% of nitrogen (N), 0.4 to 0.8% of silicon (Si), 0.7 to 1.2% of manganese (Mn), 10.5 to 14.0% of chromium (Cr), 0.2 to 1% of nickel (Ni), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe), and inevitable impurities, and wherein the cold-rolled ferritic stainless steel sheet satisfies Formula (1) below;

annealing the cold-rolled steel sheet in a range of 650 to 800°C for 10 seconds to 24 hours;

rapidly cooling the annealed cold-rolled steel sheet to 450 to 550°C at 0.05°C/s or more; and

slowly cooling to room temperature at less than 0.05°C/s, $7 \leq Cr + 7 Si - 3 Mn - 3 Ni - 50 (C + N) \leq 14$

(wherein Cr, Si, Mn, Ni, C, and N represent the content (wt%) of the respective elements).
The method of claim 5, wherein the structural ferritic stainless steel comprises a martensite phase in an area fraction of 10% or more after a heat treatment at 880 to 920°C for 20 to 40 minutes.
The method of claim 5, wherein a room-temperature tensile strength after a heat treatment at 880 to 920°C for 20 to 40 minutes is increased than a room-temperature tensile strength before the heat treatment.