JP7174853B2 - Low Cr ferritic stainless steel excellent in formability and high temperature properties and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a low Cr ferritic stainless steel having excellent formability and high temperature characteristics and a method for producing the same. The present invention relates to stainless steel and its manufacturing method.

Ferritic stainless steel materials have excellent corrosion resistance despite the addition of few expensive alloying elements, and are highly price competitive compared to austenitic stainless steel materials. In particular, low-Cr ferritic stainless steel materials with a Cr content of 9-14% are more cost competitive, and are used in exhaust system parts (Muffler, Ex-manifold, Collector cone, etc.) that are compatible with the temperature range of exhaust gas from room temperature to 800°C. used for
However, its high-temperature strength and high-temperature oxidation resistance are inferior to those of high-Cr and Nb-added steels, which limits the expansion of its applications. In order to improve high-temperature strength and high-temperature oxidation resistance, adjusting the Cr content upward or adding Nb causes an increase in manufacturing costs, so Nb is not added to low-Cr ferritic stainless steel. Development research is required to improve high temperature characteristics.

An object of the present invention is to optimize the contents of Ci, Si, and Sn to utilize solid solution strengthening and precipitation strengthening to produce high Cr ferritic stainless steel without increasing the Cr content or adding Nb. An object of the present invention is to provide a low Cr ferritic stainless steel which is excellent in corresponding high-temperature strength and high-temperature oxidation resistance and which secures formability, and a method for producing the same.

The low Cr ferritic stainless steel excellent in formability and high temperature properties of the present invention has C: 0.005 to 0.015%, N: 0.005 to 0.015%, and Si: 0.5% by weight. ~1.5%, Mn: 0.1-0.5%, Cr: 9-14%, Ti: 0.1-0.3%, Cu: 0.3-0.8%, Al: 0.5%. 01 to 0.05%, Sn: 0.005 to 0.15%, the rest of Fe and unavoidable impurities, and is characterized by satisfying the following formulas (1) and (2).
(1) Cu+Si≧1.3
(2) Si+Cu+10*Sn≦3.0
Here, Si, Cu, and Sn mean the content (% by weight) of each element.

The low Cr ferritic stainless steel may further contain Ni: 0.3% or less, P: 0.04% or less, and S: 0.002% or less in weight percent.
In addition, the low Cr ferritic stainless steel preferably contains 0.03% by weight or more of Cu precipitates with a size of 1 to 500 nm in the matrix structure.
Furthermore, the low Cr ferritic stainless steel preferably has a high temperature strength of 12 MPa or more at 900°C.

The elongation ratio of the low Cr ferritic stainless steel is preferably 30% or more.
The low Cr ferritic stainless steel can satisfy the following formula (3).
(3) (Si+5*Sn)/Ti≧5.0
Here, Si, Sn and Ti mean the content (% by weight) of each element.

The method for producing a low Cr ferritic stainless steel excellent in formability and high-temperature properties according to the present invention comprises, in weight percent, C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si: 0.5-1.5%, Mn: 0.1-0.5%, Cr: 9-14%, Ti: 0.1-0.3%, Cu: 0.3-0.8%, Al : 0.01 to 0.05%, Sn: 0.005 to 0.15%, the rest of Fe and inevitable impurities, and a ferritic stainless steel cold-rolled steel sheet that satisfies the following formulas (1) and (2): It is characterized by including a step of cold rolling annealing heat treatment, and a step of quenching to a temperature range of 450 to 550° C. and maintaining it for 5 minutes or more.
(1) Cu+Si≧1.3
(2) Si+Cu+10*Sn≦3.0
Here, Si, Cu, and Sn mean the content (% by weight) of each element.

The cold-rolled and annealed steel sheet preferably contains 0.09% by weight or more of Cu precipitates with a size of 1 to 500 nm in the matrix structure.
The high-temperature strength at 900° C. of the cold-rolled and annealed steel sheet is preferably 14.5 MPa or more.
In addition, the cold-rolled steel sheet can satisfy the following formula (3).
(3) (Si+5*Sn)/Ti≧5.0
Here, Si, Sn and Ti mean the content (% by weight) of each element.

According to the present invention, the low Cr ferritic stainless steel according to the embodiment of the present invention has a solid-solution strengthening effect of Si and Cu and distributes a fine Cu precipitate phase to increase the high-temperature strength by 30% or more compared to the existing steel. high-temperature oxidation resistance can be improved by concentrating Si and Sn on the surface.
In addition, it is possible to prevent deterioration of formability due to the difference in content of alloying elements, and when the manufacturing method according to the present invention is applied, high-temperature strength characteristics are improved.

4 is a graph showing the correlation of high-temperature characteristics according to formulas (1) and (3) of the present invention;

The low Cr ferritic stainless steel excellent in formability and high temperature properties according to one embodiment of the present invention has a weight percent of C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si : 0.5-1.5%, Mn: 0.1-0.5%, Cr: 9-14%, Ti: 0.1-0.3%, Cu: 0.3-0.8%, Al: 0.01 to 0.05%, Sn: 0.005 to 0.15%, the rest of Fe and unavoidable impurities, satisfying the following formulas (1) and (2).
(1) Cu+Si≧1.3
(2) Si+Cu+10*Sn≦3.0
Here, Si, Cu, and Sn mean the content (% by weight) of each element.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
The following examples are presented to fully convey the spirit of the invention to those of ordinary skill in the art to which the invention pertains. The present invention is not limited to the embodiments presented herein, but may be embodied in other forms. In order to clarify the present invention, the drawings may omit illustration of parts that are not related to the description, and may exaggerate the sizes of components to facilitate understanding.
In addition, when referring to which part "includes" which component, this does not exclude other components, unless specifically stated to the contrary, indicating that it can further include other components. means.
Singular references include plural references unless the context clearly dictates otherwise.

The present inventors have made various studies to improve the high-temperature strength and high-temperature oxidation resistance of a low-cost, low-Cr ferritic stainless steel, and have obtained the following findings.
Nb is generally added to ferritic stainless steel for exhaust systems for high-temperature strength. Addition is not the preferred direction of development. It is widely known that substitutional solid-solution strengthening elements are effective in increasing high-temperature strength. In particular, when a substitution type solid solution strengthening element is added, the greater the difference in weight and atomic radius with respect to Fe and Cr, the greater the solid solution strengthening effect. In the periodic table of elements, alloy elements such as Si, Cu, and Sn are far apart from Fe and Cr, and there is a difference in weight and atomic radius. Component optimization was performed for augmentation.

On the other hand, although the Cr content is generally increased for high-temperature oxidation resistance, Cr is also expensive in terms of raw materials and causes an increase in manufacturing costs, which is not a desirable development direction. For high-temperature oxidation resistance, when exposed to high temperatures for a long time, specific elements must be densely concentrated on the surface to suppress the formation of Fe oxide films. In the present invention, candidates for Si, Cu, and Sn were selected as elements that may be concentrated on the surface, and the components were optimized for high-temperature oxidation resistance.
Including the above matters, the present invention must satisfy the following component system conditions and formulas.

The low Cr ferritic stainless steel excellent in formability and high temperature properties according to one embodiment of the present invention has a weight percent of C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si : 0.5-1.5%, Mn: 0.1-0.5%, Cr: 9-14%, Ti: 0.1-0.3%, Cu: 0.3-0.8%, Al: 0.01 to 0.05%, Sn: 0.005 to 0.15%, and the balance of Fe and unavoidable impurities.
The reasons for limiting the numerical values of the contents of the alloying elements in the examples of the present invention will be described below. Below, the unit is % by weight unless otherwise specified.

The content of C is 0.005-0.015%.
If the C content exceeds 0.015%, it combines with Cr to form Cr ₂₃ C ₆ precipitates, which locally depletes Cr in the matrix, thereby degrading high-temperature oxidation resistance. Also, in order to control the C content to be less than 0.005%, the steelmaking VOD process cost increases, which is undesirable. Therefore, the C content is limited to the range of 0.005-0.015%.

The content of N is 0.005-0.015%.
When N in the steel exceeds 0.015%, the concentration of solute N reaches its limit and combines with Cr to form Cr ₂ N precipitates. Oxidation resistance is lowered. In addition, in order to control the N content to be less than 0.005%, the steelmaking VOD process ratio is unfavorably increased. Therefore, the N content is limited to the range of 0.005-0.015%.

The Si content is 0.5-1.5%.
Si is a solid-solution strengthening element for increasing high-temperature strength, forms a Si-concentrated oxide film on the surface layer, and increases high-temperature oxidation resistance. For the above two effects, the Si content is required to be at least 0.5% or more. If the Si content exceeds 1.5%, the workability of the material is significantly inferior, so the Si content is limited as described above.

The content of Mn is 0.1-0.5%.
Mn is an impurity that is inevitably contained in steel and plays a role in stabilizing austenite. When the Mn content exceeds 0.5% in the low Cr ferritic stainless steel, reverse transformation of austenite occurs during annealing heat treatment after hot rolling or cold rolling, which adversely affects elongation. Therefore, the content of Mn is limited as described above.

The Cr content is 9-14%.
Cr is an essential element added to form a passive film that suppresses oxidation in stainless steel. A Cr content of 9% or more must be added to form a stable passive film. However, since the purpose of the present invention is to develop a low-cost steel with reduced Cr, the upper limit is limited to 14%. More preferably, it is in the range of 10.5-12.5%.

The content of Ti is 0.1-0.3%.
Ti must be added in an amount of 0.1% or more in order to increase the corrosion resistance of the weld zone. Ti combines with C and N to form Ti(C, N) precipitates, thereby reducing the amounts of dissolved C and N and suppressing the formation of a Cr-depleted layer. However, when the Ti content exceeds 0.3%, the Ti component in the surface layer reacts with oxygen and turns yellow. Therefore, the Ti content is limited as above.

The Cu content is 0.3-0.8%.
Cu is a solid-solution strengthening element that contributes to high-temperature strength in place of Nb. In addition, Cu can be expected to further increase the high-temperature strength due to the effect of precipitation strengthening when fine precipitates are generated by appropriate heat treatment. Therefore, 0.5% or more is added. However, if too much Cu is added, the high-temperature hot workability may be impaired, so the amount is limited to 0.8% or less.

The content of Al is 0.01-0.05%.
Al is an element added for deoxidation during steelmaking operations. If the Al content exceeds 0.05%, Al in the surface layer reacts with oxygen to form a non-uniform oxidized layer, which adversely affects the high-temperature oxidation resistance. Therefore, the Al content is limited as above.

The Sn content is 0.005-0.15%.
Sn is a solid-solution strengthening element for increasing high-temperature strength, and forms a Sn-concentrated oxide film on the surface layer to increase high-temperature oxidation resistance. At least 0.005% of Sn should be added for the above two effects. However, if it exceeds 0.15%, Sn segregates at grain boundaries during hot rolling, weakening the bonding force between grains, and induces fine cracks in the surface layer. Therefore, the upper limit of Sn content is limited to 0.15% or less.

Also, according to an embodiment of the present invention, Ni: 0.3% or less, P: 0.04% or less, and S: 0.002% or less may be further included.
The Ni content is 0.3% or less. Ni is an impurity that is unavoidably contained in steel, and may be contained in an amount of 0.01% or more, and plays a role of stabilizing austenite. When the Ni content exceeds 0.3% in the low-Cr ferritic stainless steel, reverse transformation of austenite occurs during annealing heat treatment after hot rolling or cold rolling, which adversely affects elongation. Therefore, the Ni content is limited as described above.
The content of P is 0.04% or less. P is an unavoidable impurity contained in steel and causes intergranular corrosion during pickling and impairs hot workability, so its content is adjusted to 0.04% or less.
The S content is 0.002% or less. S is an unavoidable impurity contained in steel and is segregated at grain boundaries to impair hot workability, so its content is limited to 0.002% or less.

The remainder of the ferritic stainless steel, excluding the above alloying elements, consists of Fe and other unavoidable impurities.

On the other hand, a low Cr ferritic stainless steel having excellent formability and high temperature properties according to an embodiment of the present invention can satisfy the following formulas (1) to (3).
(1) Cu+Si≧1.3
High temperature strength is usually affected by solid solution strengthening and precipitation strengthening. Cu and Si are typical solid-solution strengthening elements, and are preferably added to increase high-temperature strength. When Cu precipitates as a Cu precipitate phase, the high-temperature strength increases more effectively due to the precipitation strengthening effect. In addition, when the content of Si increases, the solid solubility limit of Cu decreases, so that the precipitation of the Cu precipitate phase becomes easier. This makes it possible to deposit 0.03% by weight or more of the Cu precipitation phase with a size of 1 to 500 nm in the matrix structure. Therefore, it is preferable to control the Cu+Si content within a range of 1.3% or more.
Due to the solid solution strengthening and precipitation strengthening effects, the low Cr ferritic stainless steel according to the present invention can exhibit a high temperature strength of 12 MPa or more at 900°C.

(2) Si+Cu+10*Sn≦3.0
Si, Cu, and Sn alloying elements have the effect of enhancing high-temperature strength and high-temperature oxidation resistance, respectively. In the present invention, Si and Cu are added to improve the high-temperature strength, and when the formula (3) is satisfied at the same time, the elongation rate of 30% or more can be ensured to prevent inferior formability. Therefore, in order to ensure material workability, the content relationship of Si, Cu, and Sn is controlled within the above range.

(3) (Si+5*Sn)/Ti≧5.0
In high-temperature oxidation, when Si and Sn are added to the low-Cr ferritic stainless steel, a uniform oxide film of Si and Sn is first formed to suppress abnormal oxidation. However, when Ti is added, the Ti oxide film is unevenly formed, and the Ti oxide film itself exhibits a yellow color, resulting in discoloration at high temperatures. Therefore, it is preferable to control the contents of Si, Sn, and Ti within the above ranges to improve the high-temperature oxidation resistance.

Next, a method for producing a low Cr ferritic stainless steel excellent in formability and high temperature properties according to one embodiment of the present invention will be described.
The method for producing a low Cr ferritic stainless steel excellent in formability and high temperature properties according to the present invention can be produced into a cold-rolled steel sheet through a normal production process, including the composition of the above alloy components, and formulas (1) to ( The cold-rolled steel plate of ferritic stainless steel satisfying 3) is cold-rolled and annealed, and quenched to a temperature range of 450-550° C. and maintained for 5 minutes or longer.

For example, a slab containing the above alloy composition is hot-rolled, the hot-rolled hot-rolled steel sheet is subjected to annealing heat treatment, and cold-rolled to produce a cold-rolled steel sheet.
The cold-rolled steel sheet can be rapidly cooled to a temperature range of 450 to 550° C. after normal recrystallization heat treatment in the cold-rolling annealing process and kept for 5 minutes or longer. The above cooling and maintenance can increase the precipitation of the Cu precipitation phase in the same component system, and can further improve the high-temperature strength.
The resulting cold-rolled annealed steel sheet may contain 0.09% by weight or more of Cu precipitates with a size of 1 to 500 nm in the matrix structure, and may have a high-temperature strength of 14.5 MPa or more at 900°C.

Preferred embodiments of the present invention will now be described in more detail.
EXAMPLE A stainless steel lab scale melting and Ingot production facility was utilized to produce 20 mm bar samples with the alloy composition system set forth in Table 1 below. Thereafter, the steel was reheated at 1,200°C, hot rolled to 6 mm, hot rolled and annealed at 1,100°C, cold rolled to 2.0 mm, and annealed at 1,100°C. After heat treatment, only some of the inventions were rapidly cooled to 500° C., maintained for about 7 minutes, and then air-cooled to produce cold-rolled annealed steel sheets.

The fraction of Cu precipitation phase was measured for each cold-rolled and annealed steel sheet, and it was confirmed whether discoloration occurred after 1 hour at 500°C. In addition, the high temperature strength at 900° C. and the elongation ratio at normal temperature were measured and shown in Table 2.

表１に記載した比較例及び発明例は、Ｃｕ、Ｓｉ、Ｓｎの含量を異にしたもののほか、Ｃ、Ｎ、Ｃｒ、Ｔｉなどの合金元素は、本発明の成分系の含量の範囲内に制御した。

In the comparative examples and invention examples shown in Table 1, the contents of Cu, Si, and Sn were different, and alloying elements such as C, N, Cr, and Ti were within the contents of the composition system of the present invention. controlled.

In Comparative Examples 1 to 4, since the Cu content was less than 0.3%, the value of formula (1) was less than 1.3, thereby reducing the amount of fine Cu precipitates. It was confirmed that the effects of solid solution strengthening and precipitation strengthening were poor, and the high-temperature strength decreased below 12 MPa.
In Comparative Examples 1 to 3, the content of Si and Sn is less than the content of Ti, so the formula (3) cannot be satisfied, the Si and Sn concentrated oxide film on the surface layer is not sufficiently formed, and the high temperature Discoloration occurred. Comparative Example 4 has a low Cu content but a high Si content, which satisfies the formula (3), so discoloration does not occur, and it was confirmed that the high-temperature oxidation resistance according to the formula (3) is ensured.
Comparative Examples 5 and 6 have a high Sn content, so the value of formula (2) exceeds 3.0, which reduces the draw ratio by nearly 5.0% compared to other comparative examples. It could be confirmed.

Invention Example 1 satisfies the composition of the component system of the present invention and formulas (1) and (2). Although discoloration occurred at high temperatures, 0.05% by weight of Cu precipitates were deposited, satisfying the formula (1), and the high temperature strength was 12 MPa or more. In addition, it was confirmed that the formula (2) was satisfied to secure the high-temperature strength, and the elongation ratio was measured to be 33.3%, indicating that the moldability was excellent.
Invention Examples 2 to 4 optimized the contents of Si, Cu, and Sn to satisfy all of the formulas (1) to (3), thereby exhibiting a high temperature strength of 13.5 MPa or more and an elongation of 30.8% or more. No discoloration occurred.
Inventive Examples 5-7 show that the contents of Si, Cu, and Sn are optimized to satisfy all of Equations (1)-(3), and the cooling schedule after heat treatment according to the present invention is applied. As a result of securing an elongation ratio of 30.3% or more and satisfying the rapid cooling and maintenance time after heat treatment, 0.09% by weight or more of fine Cu precipitation phase was observed, and the high-temperature strength was even higher at 14.6 MPa or more. rice field. In particular, Inventive Examples 5 and 6 exhibited a high temperature strength of 15 MPa or higher.

FIG. 1 is a graph showing the values of equations (1) and (3) for an example according to the invention. FIG. 1 confirms the correlation between equations (1) and (3) regarding high temperature strength and high temperature oxidation resistance.

While illustrative embodiments of the invention have been described above, the invention is not so limited and any person of ordinary skill in the art will appreciate the concept and scope of the claims set forth below. It will be understood that various modifications and variations are possible without departing from the scope of the invention.

The ferritic stainless steel according to the present invention can increase the high-temperature properties of existing steels by 30% or more without increasing the Cr content and adding Nb, thereby reducing raw material costs.

Claims (10)

% by mass , C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si: 0.5 to 1.5%, Mn: 0.1 to 0.5%, Cr: 9-14%, Ti: 0.1-0.3%, Cu: 0.3-0.8%, Al: 0.01-0.05%, Sn: 0.005-0.15%, rest of Fe and unavoidable impurities,
A low Cr ferritic stainless steel excellent in formability and high temperature properties, characterized by satisfying the following formulas (1) and (2).
(1) Cu+Si≧1.3
(2) Si+Cu+10*Sn≦3.0
(Here, Si, Cu, and Sn mean the content (% by mass ) of each element) 2. The low-temperature polymer excellent in moldability and high-temperature properties according to claim 1, further comprising Ni: 0.3% or less, P: 0.04% or less, and S: 0.002% or less in mass %. Cr ferritic stainless steel. 2. The low Cr ferritic stainless steel excellent in formability and high temperature properties according to claim 1, characterized by containing 0.03% by mass or more of a Cu precipitate phase with a size of 1 to 500 nm in the matrix structure. 2. The low Cr ferritic stainless steel excellent in formability and high temperature properties according to claim 1, wherein the high temperature strength at 900° C. is 12 MPa or more. 2. The low Cr ferritic stainless steel excellent in formability and high-temperature properties according to claim 1, wherein the elongation is 30% or more. A low Cr ferritic stainless steel excellent in formability and high temperature properties according to claim 1, characterized by satisfying the following formula (3).
(3) (Si+5*Sn)/Ti≧5.0
(Here, Si, Sn, and Ti mean the content (% by mass ) of each element) % by mass , C: 0.005 to 0.015%, N: 0.005 to 0.015%, Si: 0.5 to 1.5%, Mn: 0.1 to 0.5%, Cr: 9-14%, Ti: 0.1-0.3%, Cu: 0.3-0.8%, Al: 0.01-0.05%, Sn: 0.005-0.15%, rest a step of subjecting a ferritic stainless steel cold-rolled steel sheet consisting of Fe and inevitable impurities and satisfying the following formulas (1) and (2) to a cold rolling annealing heat treatment;
A low Cr ferritic stainless steel excellent in formability and high temperature characteristics, comprising: A method of making steel.
(1) Cu+Si≧1.3
(2) Si+Cu+10Sn≦3.0
(Here, Si, Cu, and Sn mean the content (% by mass ) of each element) The cold-rolled and annealed steel sheet is
8. The method for producing a low Cr ferritic stainless steel excellent in formability and high temperature properties according to claim 7, characterized in that the base structure contains 0.09% by mass or more of Cu precipitates with a size of 1 to 500 nm. 8. The method for producing low Cr ferritic stainless steel excellent in formability and high temperature properties according to claim 7, wherein the high temperature strength at 900[deg.] C. of the cold rolled annealed steel sheet is 14.5 MPa or more. 8. The method for producing low Cr ferritic stainless steel excellent in formability and high-temperature properties according to claim 7, wherein the cold-rolled steel sheet satisfies the following formula (3).
(3) (Si+5*Sn)/Ti≧5.0
(Here, Si, Sn, and Ti mean the content (% by mass ) of each element)

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