JP7657085B2 - Ferritic stainless steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a ferritic stainless steel sheet having excellent heat resistance and a method for producing the same.
The present invention relates to a heat-resistant stainless steel sheet that is most suitable for use in automobile exhaust system components that require high-temperature strength and oxidation resistance, and in particular to a ferritic stainless steel sheet that is excellent in strength at medium-low to high temperatures and after aging, and to a method for producing the same.

Exhaust system components such as the exhaust manifold, front pipe, and center pipe of an automobile pass high-temperature exhaust gas emitted from the engine, so the materials that make up these exhaust system components are required to have a variety of properties, including oxidation resistance, high-temperature strength, and thermal fatigue properties.

Traditionally, cast iron was generally used for automotive exhaust system components, but stainless steel exhaust manifolds have begun to be used in light of stricter exhaust gas regulations, improved engine performance, and lighter vehicle weight. Exhaust gas temperatures vary by vehicle model, and in recent years are often around 750-850°C, but can reach even higher temperatures. Materials with high high-temperature strength and oxidation resistance are required for long-term use in environments at such temperatures.

Among stainless steels, austenitic stainless steels have excellent heat resistance and workability, but because of their large thermal expansion coefficient, they are prone to thermal fatigue failure when used in components that are repeatedly heated and cooled, such as exhaust manifolds.

Ferritic stainless steel has a smaller thermal expansion coefficient than austenitic stainless steel, and therefore has excellent thermal fatigue properties. In addition, ferritic stainless steel contains almost no expensive Ni compared to austenitic stainless steel, so the material costs are low and it is widely used. However, ferritic stainless steel has lower high-temperature strength than austenitic stainless steel, so technologies have been developed to improve high-temperature strength. Furthermore, various technologies have been developed from the perspectives of oxidation resistance, formability, manufacturability, and further cost reduction.

Ferritic stainless steels that have been developed include, for example, SUS430J1L (Nb-containing steel), Nb-Si-containing steel, and SUS444 (Nb-Mo-containing steel). These contain Nb, and based on the Nb content, also contain Si and Mo to improve high-temperature strength. In response to this, technology has also been developed to improve high-temperature strength and oxidation resistance by containing Al, as in Patent Document 1.

Meanwhile, in recent years, there are cases where exhaust gas temperatures rise due to the installation of turbos, and cases where exhaust gas temperatures fall in favor of lean-burn combustion. When lean-burn is the aim, an insulating structure is used to suppress the drop in exhaust gas temperature in order to promote catalytic reactions in exhaust gas purification components such as converters, and in some cases, exhaust system components are covered with insulating material. As a result of these structural changes, high temperatures are maintained for longer periods of time than ever before. For this reason, it has become necessary to develop technology that suppresses the decrease in high-temperature strength of exhaust system components during long-term aging.

In addition, there is a constant demand for reducing the price of exhaust system components. As in Patent Document 1, the use of Al may contribute to reducing the amount of Cr added, an element that improves oxidation resistance. However, reducing Cr leads to a decrease in strength at medium to low temperatures, for example, around 200°C. Considering thermal fatigue, strength is affected over the entire range from low to high temperatures, so if the aim is to reduce the amount of Cr added by using Al, it has become necessary to develop technology to improve strength at medium to low temperatures.

Patent Document 1 describes a technique for improving the oxidation resistance, high-temperature strength, workability, and corrosion resistance of high-Al-content ferritic stainless steels. However, Patent Document 1 does not consider strength at low and medium temperatures or strength after long-term aging.

Patent Document 2 describes a technique for improving the oxidation resistance and thermal fatigue properties of high-Al-content ferritic stainless steels. However, most of the stainless steels described in Patent Document 2 have a Cr content of more than 15%, and no study is conducted on the strength at medium and low temperatures when the Cr content is reduced. Furthermore, no study is conducted on the strength after long-term aging.

Patent Document 3 describes a technology for improving the oxidation resistance and high-temperature salt damage resistance of ferritic stainless steels with a high Al content and Cr addition of 16% or less. However, there is no study of strength at medium to low temperatures or strength after long-term aging. In addition, the stainless steel in Patent Document 3 contains more than 2.0% expensive W, making it a high-cost steel.

JP 2018-168457 A Patent No. 6123964 Patent No. 3903853

The present invention was made in consideration of the above circumstances, and aims to provide a ferritic stainless steel sheet that has excellent strength at medium to high temperatures and after aging, and a manufacturing method thereof.

In order to solve the above problems, the inventors have intensively studied the contents of each element in steel that affect the strength at low to high temperatures and the strength after aging for ferritic stainless steel sheets with a relatively high Al content and a relatively low Cr content. They also intensively studied the contents of each element that is contained in the steel as precipitates or dissolved in the steel, which affect the strength at low to high temperatures and the strength after aging. As a result, they have found that the strength at low to high temperatures is affected by the total amount of Si in the steel and the amount of Nb dissolved in the steel. They have also come to believe that in order to suppress the decrease in strength after long-term aging, it is important to suppress the precipitation of Laves phases, etc., as well as the consumption of Al as a strengthening element due to high-temperature oxidation. They have also found that not only the contents of each element in the steel but also the contents of each element contained in the steel as precipitates are important for improving these characteristics. Based on these findings, they have invented a ferritic stainless steel sheet that is excellent in strength at low to high temperatures and the strength after aging.

That is, the gist of the present invention, which aims to solve the above problems, is as follows.
(1) The chemical composition of the steel is, in mass%,
C: 0.030% or less,
N: 0.030% or less,
Si: 0.01% or more, 5.00% or less,
Mn: 0.01% or more, 3.00% or less,
P: 0.050% or less,
S: 0.0100% or less,
Cr: 8.0% or more, 15.0% or less,
Ni: 0.001% or more, 2.00% or less,
Cu: 0.001% or more, 2.00% or less,
Mo: 0.001% or more, 4.00% or less,
Ti: 0.050% or more, 0.400% or less,
Nb: 0.001% or more, 0.80% or less,
Al: more than 0.70%, less than 5.00%,
V: 0.01% or more, 0.30% or less,
B: 0.0001% or more, 0.0050% or less,
O: 0.0050% or less,
Contains
The balance is Fe and impurities,
A ferritic stainless steel sheet, characterized in that it satisfies the following formulas (i) and (ii):
[Si]+[Nb]-[*Nb]≧0.40...Formula (i)
[*Ti]-3×[*Cr]≧0.001...Formula (ii)
Here, in formula (i) and formula (ii), the [element symbol] is the content (mass %) of the element in the steel, and the [*element symbol] is the content (mass %) of the element contained in precipitates in the steel.
(2) The chemical composition of the steel is, in mass%,
C: 0.020% or less,
N: 0.020% or less,
Si: 0.05% or more, 1.60% or less,
Mn: 0.01% or more, 1.10% or less,
P: 0.040% or less,
S: 0.0014% or less,
Cr: 10.0% or more, 15.0% or less,
Ni: 0.01% or more, 0.70% or less,
Cu: 0.001% or more, 0.80% or less,
Mo: 0.01% or more, 2.00% or less,
Ti: 0.050% or more, 0.230% or less,
Nb: more than 0.03%, less than 0.60%,
Al: 1.00% or more, 3.00% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0001% or more, 0.0050% or less,
O: 0.0050% or less,
Contains
The balance is Fe and impurities,
A ferritic stainless steel sheet characterized by satisfying the following formulas (i) to (ii):
[Si]+[Nb]-[*Nb]≧0.40...Formula (i)
[*Ti]-3×[*Cr]≧0.001...Formula (ii)
Here, in formula (i) and formula (ii), the [element symbol] is the content (mass %) of the element in the steel, and the [*element symbol] is the content (mass %) of the element contained in precipitates in the steel.
(3) In mass%, replacing a part of Fe,
W: 0.001% or more, 0.50% or less,
Y: 0.001% or more, 0.50% or less,
REM: 0.001% or more, 0.50% or less,
Ca: 0.0001% or more, 0.0050% or less,
Zr: 0.001% or more, 0.50% or less,
Hf: 0.001% or more, 1.0% or less,
Sn: 0.001% or more, 1.0% or less,
Mg: 0.0001% or more, 0.0050% or less,
Co: 0.001% or more, 1.0% or less,
Sb: 0.001% or more, 1.0% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0001% or more, 0.50% or less,
The ferritic stainless steel sheet according to (1) or (2), characterized in that it contains one or more of the following:
(4) A method for producing a ferritic stainless steel sheet according to any one of (1) to (3),
A method for producing a ferritic stainless steel sheet, comprising: subjecting a steel slab having a chemical composition according to any one of (1) to (3) to a hot rolling process ; controlling a ratio A/B of a time A during which the temperature of the steel during the hot rolling process is above 900°C and not higher than 1000°C to a time B during which the temperature of the steel during the hot rolling process is at least 600°C and not higher than 900°C so as to satisfy the following formula (x):
1.0≦A/B≦100.0...Formula (x)

In addition, in the above invention, when no lower limit is specified, this indicates that the content includes the unavoidable impurity level.

The present invention provides a high-Al-content, low-Cr ferritic stainless steel sheet that has excellent strength at medium to high temperatures and after aging, and a method for manufacturing the same.

The present invention is described in detail below.

First, we will explain the reasons for limiting the chemical composition of the steel of the ferritic stainless steel plate of the present invention. Here, "%" in the chemical composition of the steel means mass %.

(C: 0.030% or less)
Since C is an element that reduces high-temperature strength, oxidation resistance, and strength after long-term aging, the C content is set to 0.030% or less. In consideration of further suppressing the deterioration of the above properties, the C content is preferably set to 0.020% or less. In consideration of formability and corrosion resistance, the C content is more preferably set to 0.015% or less. Further preferably, the C content is set to 0.010% or less. However, since an excessive reduction leads to an increase in refining costs, the C content is preferably set to 0.0001% or more. More preferably, the C content is set to 0.0010%, and further preferably, the C content is set to 0.0020% or more.

(N: 0.030% or less)
Like C, N is an element that reduces high-temperature strength, oxidation resistance, and strength after long-term aging, so the N content is set to 0.030% or less. In consideration of further suppressing the deterioration of the above properties, the N content is preferably set to 0.020% or less. In consideration of formability and corrosion resistance, the N content is more preferably set to 0.015% or less. Further preferably, the N content is set to 0.010% or less. However, since an excessive reduction leads to an increase in refining costs, the N content is preferably set to 0.0001% or more. More preferably, the N content is set to 0.0010%, and further preferably, the N content is set to 0.0020% or more.

(Si: 0.01% or more, 5.00% or less)
Since Si is an element contained as a deoxidizer and also improves oxidation resistance and strength at low and medium temperatures, the Si content is set to 0.01% or more. In consideration of further improving strength at low and medium temperatures, the Si content is preferably set to 0.05% or more, and more preferably set to 0.30% or more. In consideration of high-temperature salt damage resistance, the Si content is more preferably set to 0.50% or more. However, since excessive Si content leads to a decrease in workability, the Si content is set to 5.00% or less. In consideration of manufacturability and manufacturing costs, the Si content is preferably set to 1.60% or less. More preferably, it is set to 1.20% or less, and even more preferably, it is set to 0.80% or less.

(Mn: 0.01% or more, 3.00% or less)
Mn is an element contained as a deoxidizer, and the Mn content is 0.01% or more. In consideration of refining costs, the Mn content is preferably 0.05% or more. More preferably, it is 0.10% or more, and even more preferably, it is 0.15% or more. However, since excessive Mn content leads to a decrease in oxidation resistance and promotes Al consumption during long-term aging, thereby decreasing strength, the Mn content is 3.00% or less. In consideration of further suppressing the decrease in the above properties, the Mn content is preferably 1.10% or less. In addition, in consideration of uniform elongation, hot workability, and corrosion resistance, the Mn content is more preferably 0.50% or less. More preferably, it is less than 0.40%.

(P: 0.050% or less)
P is an impurity that is mainly mixed in from raw materials during steelmaking refining, and as its content increases, toughness and weldability decrease, so the lower its content, the better. Therefore, the P content is set to 0.050% or less. In addition, in consideration of manufacturability and manufacturing costs, the P content is preferably set to 0.040% or less. In consideration of workability, the P content is more preferably set to 0.035% or less. Even more preferably, it is set to 0.030% or less. However, since excessive reduction leads to an increase in refining costs, the P content is preferably set to 0.001% or more. More preferably, it is set to 0.005% or more, and even more preferably, it is set to 0.010% or more.

(S: 0.0100% or less)
S is an impurity that is mainly mixed from raw materials during steelmaking refining, and it leads to a decrease in oxidation resistance and promotes Al consumption during long-term aging, thereby decreasing strength, so the S content is set to 0.0100% or less. In consideration of further suppressing the decrease in the above properties, the S content is preferably set to 0.0022% or less. In consideration of manufacturability, the S content is more preferably set to 0.0020% or less. In consideration of corrosion resistance and high-temperature salt damage resistance, the S content is even more preferably set to 0.0014% or less. However, since excessive reduction leads to an increase in refining costs, the S content is preferably set to 0.0001% or more. The S content is more preferably set to 0.0003% or more.

(Cr: 8.0% or more, 15.0% or less)
Cr improves oxidation resistance and suppresses Al consumption during long-term aging to suppress the decrease in strength, so the Cr content is set to 8.0% or more. In consideration of further suppressing the decrease in the above properties, the Cr content is preferably set to 10.0% or more. In consideration of corrosion resistance, the Cr content is more preferably set to 10.5% or more. More preferably, it is set to 10.7% or more. However, since excessive Cr content leads to an increase in raw material costs, the Cr content is set to 15.0% or less. In consideration of further reduction in raw material costs and workability, the Cr content is preferably set to less than 14.0%. More preferably, it is set to less than 12.0%.

(Ni: 0.001% or more, 2.00% or less)
Ni is an element that improves corrosion resistance, and the Ni content is set to 0.001% or more. It is preferably set to 0.01% or more, and more preferably set to 0.05% or more. However, since an excessive Ni content leads to an increase in raw material costs, the Ni content is set to 2.00% or less. In consideration of further reduction in raw material costs, oxidation resistance, and workability, the Ni content is preferably set to 0.70% or less. It is more preferably set to 0.50% or less, and even more preferably set to 0.40% or less.

(Cu: 0.001% or more, 2.00% or less)
Cu is an element that improves corrosion resistance, and the Cu content is set to 0.001% or more. Preferably, it is set to 0.005% or more, and more preferably, it is set to 0.01% or more. However, since an excessive Cu content leads to an increase in raw material costs, the Cu content is set to 2.00% or less. In consideration of further reduction in raw material costs, oxidation resistance, and hot workability, the Cu content is preferably set to 0.80% or less. More preferably, it is set to 0.30% or less, and even more preferably, it is set to 0.20% or less.

(Mo: 0.001% or more, 4.00% or less)
Mo is an element that improves high-temperature strength and oxidation resistance, and the Mo content is set to 0.001% or more. In consideration of further improvement of the above characteristics, corrosion resistance, and high-temperature salt damage resistance, the Mo content is preferably set to 0.01% or more. More preferably, it is more than 0.30%, and even more preferably, it is 0.55% or more. However, since excessive Mo content leads to an increase in raw material costs, the Mo content is set to 4.00% or less. In consideration of further reduction in raw material costs, workability, and manufacturability, the Mo content is preferably set to 2.00% or less. More preferably, it is set to 1.20% or less, and even more preferably, it is set to less than 0.80%.

(Ti: 0.050% or more, 0.400% or less)
Ti is an element that combines with C, N, and S to improve high-temperature strength and oxidation resistance and suppress the decrease in strength after long-term aging, so it is set to 0.050% or more. In consideration of corrosion resistance, intergranular corrosion resistance, and deep drawability, the Ti content is preferably 0.080% or more. More preferably, it is 0.120% or more. However, since excessive Ti content leads to an increase in raw material costs, the Ti content is set to 0.400% or less. In consideration of further reduction in raw material costs, uniform elongation, hole expansion workability, and manufacturability, the Ti content is preferably 0.230% or less. More preferably, it is 0.220% or less.

(Nb: 0.001% or more, 0.80% or less)
Nb is an element that improves high-temperature strength, and the Nb content is 0.001% or more. In consideration of further improvement of the above characteristics, corrosion resistance, and high-temperature salt damage resistance, the Nb content is preferably more than 0.03%. More preferably, it is 0.05% or more, and even more preferably, it is more than 0.05%. However, since excessive Nb content leads to an increase in raw material costs, the Nb content is 0.80% or less. In consideration of further reduction in raw material costs, uniform elongation, hole expansion workability, and manufacturability, the Nb content is preferably 0.60% or less. More preferably, it is less than 0.35%, and even more preferably, it is 0.25% or less.

(Al: more than 0.70%, less than 5.00%)
Al is contained as a deoxidizing element and is an element that improves high-temperature strength and oxidation resistance, and the Al content is set to more than 0.70%. In consideration of further improvement of the above characteristics and high-temperature salt damage resistance, the Al content is preferably set to 1.00% or more. More preferably, it is set to more than 1.05%, and even more preferably, it is set to more than 1.55%. However, since excessive Al content leads to an increase in raw material costs, the Al content is set to 5.00% or less. In consideration of further reduction in raw material costs and workability, manufacturability, and weldability, the Al content is preferably set to 3.00% or less. More preferably, it is set to 2.50% or less, and even more preferably, it is set to 2.30% or less.

(V: 0.01% or more, 0.30% or less)
V is an element that improves high-temperature strength. Therefore, the V content is set to 0.01% or more. In consideration of corrosion resistance, the V content is preferably set to 0.02% or more. More preferably, it is set to 0.03% or more. However, excessive V content leads to a decrease in high-temperature strength and a decrease in thermal fatigue life due to coarsening of precipitates. Therefore, the V content is set to 0.30% or less. In consideration of further suppression of the decrease in the above characteristics, the surface properties of the steel surface, and manufacturability, the V content is preferably set to 0.20% or less. More preferably, it is less than 0.15%, and even more preferably, it is set to 0.10% or less.

(B: 0.0001% or more, 0.0050% or less)
B is an element that improves high-temperature strength and thermal fatigue properties. Therefore, the B content is set to 0.0001% or more, preferably 0.0002% or more, more preferably 0.0003% or more, and further preferably 0.001% or more. However, an excessive B content leads to a decrease in hot workability and a decrease in the surface properties of the steel surface. Therefore, the B content is set to 0.0050% or less. In addition, taking into consideration manufacturability and formability, the B content is preferably set to 0.0030% or less, and more preferably 0.0015% or less.

(O: 0.0050% or less)
O is an unavoidably contained impurity, and causes surface defects due to bubbles and inclusions. It also leads to a decrease in oxidation resistance, promotes Al consumption during long-term aging, and reduces strength. Therefore, the O content is set to 0.0050% or less. In addition, considering manufacturability, the O content is preferably set to 0.0040% or less. More preferably, it is set to 0.0035% or less. However, since excessive reduction of O leads to an increase in refining costs, the O content is preferably set to 0.0001% or more. More preferably, it is set to 0.0003% or more, and further preferably, it is set to 0.0005% or more. Here, the O content means the total content including oxygen dissolved in the steel and oxygen of oxides intervening in the steel.

In the ferritic stainless steel sheet according to this embodiment, the remainder other than the above-mentioned elements is Fe and impurities. However, elements other than the above-mentioned elements can also be contained within a range that does not impair the effects of this embodiment. Note that the impurities referred to here are components that are mixed in due to various factors in the raw materials such as ores and scraps and the manufacturing process when the ferritic stainless steel according to this embodiment is industrially manufactured, and are acceptable within a range that does not adversely affect the present invention.

Next, we will explain formulas (i) and (ii).

While investigating various effects on low and medium temperature strength, the inventors discovered that the total amount of Si in steel and the amount of Nb dissolved in the steel is important, and derived formula (i). Note that [Nb] - [*Nb] in formula (i) corresponds to the amount of Nb dissolved in the steel.

[Si]+[Nb]-[*Nb]≧0.40...Formula (i)

In formula (i), the [element symbol] is the content (mass%) of the element in the steel, and the [*element symbol] is the content (mass%) of the element in the steel that is contained in the precipitates. Therefore, as mentioned above, the value obtained by subtracting [*Nb] from [Nb] represents the amount of Nb dissolved in the steel.

If the value of the left side of formula (i) is less than 0.40, the strength at medium to low temperatures will be insufficient. Therefore, it is necessary to satisfy formula (i). Preferably, the value of the left side of formula (i) is 0.60 or more. Furthermore, since high values of [Si] and [Nb] may lead to an increase in raw material costs and a decrease in processability and manufacturability, it is preferable that the value of the left side of formula (i) is 5.80 or less, and more preferably 2.20 or less.

In addition, while examining various effects on the strength after long-term aging, it was found that it is important to improve oxidation resistance in order to suppress the consumption of Al as a strengthening element due to high-temperature oxidation. The oxidation resistance of ferritic stainless steels with a relatively high Al content is obtained by forming Al ₂ O ₃ in the steel. In addition, Cr is an element that enhances the protective properties of Al ₂ O _3. Therefore, it is important to fix C and N as Ti carbonitrides so that Cr does not become carbonitrides. It is considered that Ti and Cr contained as precipitates in steel are their respective carbonitrides. It is also considered that precipitates other than carbonitrides can become nuclei for the formation of carbonitrides during long-term aging. From this, it was found that the balance of the contents of Ti and Cr contained as precipitates in steel is important, and formula (ii) was obtained.

[*Ti]-3×[*Cr]≧0.001...Formula (ii)

However, the *element symbol in formula (ii) indicates the content (mass%) of the element in the steel that is contained in the precipitates.

If the value of the left side of formula (ii) is less than 0.001, the oxidation resistance and strength after long-term aging are significantly reduced. Therefore, formula (ii) must be satisfied. Preferably, the value of the left side of formula (ii) is 0.010 or more. If the value of the left side of formula (ii) is too large, the amount of Ti precipitates increases, leading to reduced manufacturability and increased manufacturing costs, so the value of the left side of formula (ii) is preferably 0.300 or less, and more preferably 0.100 or less.

Moreover, excessively reducing or increasing the amount of precipitates in the steel leads to increased manufacturing costs and unstable quality. Therefore, it is preferable to satisfy the following formulas (iii) to (v).

0.001≦[*Cr]≦0.100...Formula (iii)
0.001≦[*Ti]≦0.300...Formula (iv)
0.001≦[*Nb]≦0.300 ...Formula (v)

However, the * element symbol in formulas (iii) to (v) represents the content (mass%) of the element contained in the precipitate in the steel. More preferably, the value of the middle part of formula (iii) is greater than 0.001 and not more than 0.050, the value of the middle part of formula (iv) is 0.020 or more and 0.100 or less, and the value of the middle part of formula (v) is 0.005 or more and 0.100 or less.

The content (mass %) of each element in the steel that is contained in the precipitates, represented by [*element symbol], is calculated as follows. First, the precipitates are extracted and separated from the steel by a precipitate extraction and separation process. Next, the amount of each element in the extracted precipitates is calculated. Then, the amount of each element contained in the extracted precipitates is expressed as the content in the steel to calculate [*element symbol] (mass %).

Here, the extraction and separation process involves taking an appropriate test piece from the entire thickness of the steel plate, electrolyzing the test piece in a tetramethylammonium chloride solution, and filtering the electrolytic solution through a filter with a pore size of 0.2 μm to extract the precipitates in the steel. The extracted precipitates are then dissolved, and the content of each element contained in the precipitates is determined by ICP emission spectroscopy (ICP: Inductively Coupled Plasma).

Furthermore, the ferritic stainless steel sheet according to this embodiment can further improve its properties by selectively containing one or more of W, Y, REM, Ca, Zr, Hf, Sn, Mg, Co, Sb, Bi, Ta, and Ga as necessary. These elements are described below. Note that these elements do not necessarily have to be contained, so the lower limit of the content of each of these elements is 0%.

(W: 0.001% or more, 0.50% or less)
W is an element that improves high-temperature strength, oxidation resistance, and corrosion resistance, and is preferably contained at 0.001% or more as necessary. More preferably, it is 0.01% or more, and even more preferably, it is 0.05% or more. However, since an excessive W content may lead to an increase in raw material costs and a decrease in workability, toughness, and manufacturability, it is preferable that the W content be 0.50% or less. More preferably, it is 0.40% or less, and even more preferably, it is 0.30% or less.

(Y: 0.001% or more, 0.50% or less)
Y is an element that improves rust resistance, hot workability, and oxidation resistance, and is preferably contained at 0.001% or more as necessary. More preferably, it is 0.003% or more, and even more preferably, it is 0.01% or more. However, since an excessive Y content may lead to an increase in raw material costs and a decrease in manufacturability, it is preferable that the Y content is 0.50% or less. More preferably, it is 0.20% or less, and even more preferably, it is 0.10% or less.

(REM: 0.001% or more, 0.50% or less)
REM (rare earth metal; rare earth element) is an element that improves rust resistance, hot workability, and oxidation resistance, and is preferably contained at 0.001% or more as necessary. More preferably, it is 0.003% or more, and even more preferably, it is 0.01% or more. However, since excessive REM content may lead to an increase in raw material costs and a decrease in manufacturability, it is preferable to set the REM content to 0.50% or less. More preferably, it is 0.20% or less, and even more preferably, it is 0.10% or less. REM refers to a collective name for scandium (Sc) and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). As REM, one of the above elements may be contained alone, or two or more may be contained. When two or more of the above elements are contained as REM, the REM content is the total content of those elements.

(Ca: 0.0001% or more, 0.0050% or less)
Ca is an element that improves corrosion resistance, oxidation resistance, and manufacturability, and is preferably contained at 0.0001% or more as necessary. More preferably, it is 0.0002% or more, and even more preferably, it is 0.0003% or more. However, since excessive Ca content may also lead to a decrease in corrosion resistance and manufacturability, it is preferable that the Ca content be 0.0050% or less. More preferably, it is 0.0030% or less, and even more preferably, it is 0.0020% or less.

(Zr: 0.001% or more, 0.50% or less)
Zr is an element that improves corrosion resistance, intergranular corrosion resistance, high-temperature strength, and oxidation resistance, and is preferably contained at 0.001% or more as necessary. More preferably, it is 0.01% or more, and even more preferably, it is 0.03% or more. However, since excessive Zr content may lead to an increase in raw material costs and a decrease in manufacturability, it is preferable that the Zr content is 0.50% or less. More preferably, it is 0.30% or less, and even more preferably, it is 0.20% or less.

(Hf: 0.001% or more, 1.0% or less)
Hf is an element that improves corrosion resistance, intergranular corrosion resistance, high-temperature strength, and oxidation resistance, and is preferably contained at 0.001% or more as necessary. More preferably, it is 0.003% or more, and even more preferably, it is 0.01% or more. However, since excessive Hf content may lead to an increase in raw material costs and a decrease in manufacturability, it is preferable that the Hf content be 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, it is 0.30% or less.

(Sn: 0.001% or more, 1.0% or less)
Sn is an element that improves corrosion resistance and high-temperature strength, and is preferably contained at 0.001% or more as necessary. More preferably, it is 0.003% or more, and even more preferably, it is 0.01% or more. However, since excessive Sn content may lead to an increase in raw material costs and a decrease in toughness and manufacturability, it is preferable to set the Sn content to 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, it is 0.30% or less.

(Mg: 0.0001% or more, 0.0050% or less)
Mg may be contained as a deoxidizing element, and is also an element that improves formability and oxidation resistance, and is preferably contained at 0.0001% or more as necessary. More preferably, it is 0.0003% or more, and even more preferably, it is 0.0005% or more. However, since excessive Mg content may lead to deterioration of corrosion resistance, weldability, and surface quality, it is preferable that the Mg content is 0.0050% or less. More preferably, it is 0.0030% or less, and even more preferably, it is 0.0020% or less.

(Co: 0.001% or more, 1.0% or less)
Co is an element that improves high-temperature strength, and is preferably contained at 0.001% or more as necessary. More preferably, it is 0.01% or more, and even more preferably, it is 0.03% or more. However, since excessive Co content may lead to an increase in raw material costs and a decrease in workability, toughness, and manufacturability, it is preferable to set the Co content to 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, it is less than 0.30%.

(Sb: 0.001% or more, 1.0% or less)
Sb is an element that improves high-temperature strength, and is preferably contained at 0.001% or more as necessary. More preferably, it is 0.005% or more, and even more preferably, it is 0.01% or more. However, since an excessive Sb content may cause a decrease in weldability and toughness, it is preferable that the Sb content is 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, it is 0.40% or less.

(Bi: 0.001% or more, 1.0% or less)
Bi is an element that suppresses roping that occurs during cold rolling and improves manufacturability, and is preferably contained at 0.001% or more as necessary. More preferably, it is 0.003% or more, and even more preferably, it is 0.01% or more. However, since an excessive Bi content may lead to an increase in raw material costs and a decrease in workability and hot workability, it is preferable that the Bi content be 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, it is 0.30% or less.

(Ta: 0.001% or more, 1.0% or less)
Ta is an element that improves high-temperature strength, and is preferably contained at 0.001% or more as necessary. More preferably, it is 0.003% or more, and even more preferably, it is 0.01% or more. However, since excessive Ta content may lead to an increase in raw material costs and a decrease in toughness and manufacturability, it is preferable that the Ta content is 1.0% or less. More preferably, it is 0.50% or less, and even more preferably, it is 0.30% or less.

(Ga: 0.0001% or more, 0.50% or less)
Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance, and is preferably contained at 0.0001% or more as necessary. More preferably, it is 0.0003% or more, and even more preferably, it is 0.001% or more. However, since excessive Ga content may lead to an increase in raw material costs and a decrease in manufacturability, it is preferable that the Ga content be 0.50% or less. More preferably, it is 0.30% or less, and even more preferably, it is 0.20% or less.

Next, a method for manufacturing the ferritic stainless steel sheet according to this embodiment will be described. The ferritic stainless steel sheet according to this embodiment may be manufactured by any method, but can be manufactured, for example, by the following manufacturing method.

The manufacturing method of the ferritic stainless steel sheet of this embodiment can employ a general process for manufacturing ferritic stainless steel. In general, molten steel is produced in a converter or electric furnace, refined in an AOD furnace or VOD furnace, etc., and made into steel billets by continuous casting or ingot casting, and then the process of hot rolling - annealing the hot-rolled sheet - pickling - cold rolling - finish annealing - pickling is carried out. If necessary, the annealing of the hot-rolled sheet may be omitted, or the cold rolling - finish annealing - pickling may be repeated.

The conditions for each of these processes may be general conditions, such as a hot rolling heating temperature of 1000 to 1300°C, a hot rolled sheet annealing temperature of 900 to 1200°C, and a cold rolled sheet annealing temperature of 800 to 1200°C. The manufacturing method of this embodiment only needs to satisfy formula (x) described below, and other manufacturing conditions are not limited. Therefore, the hot rolling conditions, the presence or absence of hot rolled sheet annealing, the cold rolling conditions, the hot rolled sheet and cold rolled sheet annealing temperatures, the atmosphere, etc. can be selected as appropriate.

In addition, the treatment before the final pickling may be a general treatment, for example, mechanical treatment such as shot blasting or grinding brush, or chemical treatment such as molten salt treatment or neutral salt electrolysis. In addition, temper rolling or tension leveling may be applied after cold rolling and annealing. Furthermore, the product plate thickness may be selected according to the required member thickness. In addition, this steel plate may be used as a material to manufacture a welded pipe by the usual manufacturing method of stainless steel pipes for exhaust system components, such as electric resistance welding, TIG welding, or laser welding.

In order to satisfy formula (i) and formula (ii) so as to improve the strength at medium to high temperatures and the strength after aging, when a steel slab having the chemical composition according to this embodiment is subjected to a hot rolling process , the ratio A/B of the time A during which the temperature of the steel is above 900° C. and not more than 1000° C. to the time B during which the temperature of the steel is 600° C. or more and 900° C. or less is controlled so as to satisfy the following formula (x):

1.0≦A/B≦100.0...Formula (x)

The longer the ratio of time A during which the temperature of the steel is greater than 900 ° C. and less than 1000 ° C. in the hot rolling process , the higher the ratio of Ti precipitates in the precipitates. In addition, it is considered that the longer the ratio of time B during which the temperature of the steel is greater than 600 ° C. and less than 900 ° C. in the hot rolling process , the higher the ratio of Nb and Cr precipitates. Therefore, the value of the middle side of formula (x) is 1.0 or more. Preferably, the value of the middle side of formula (x) is 2.0 or more. In addition, if the value of the middle side of formula (x) is excessively large, the amount of Ti precipitates will be excessively large and the shape of the hot-rolled sheet will become unstable, leading to a decrease in manufacturability and an increase in manufacturing costs, so the value of the middle side of formula (x) is 100.0 or less. Preferably, it is 70.0 or less.

Moreover, if the time A during which the temperature of the steel during the hot rolling process is above 900°C and below 1000°C is too short, the toughness of the steel sheet after the hot rolling process decreases, leading to a decrease in manufacturability and an increase in manufacturing costs. Moreover, if the time A is too long, the amount of Ti precipitates becomes excessively large. Therefore, it is preferable that the time A is 10 seconds or more and 300 seconds or less.

Furthermore, if the time B during which the temperature is 600°C or more and 900°C or less during the hot rolling process is too short, the shape of the steel sheet after the hot rolling process becomes unstable, leading to a decrease in manufacturability and an increase in manufacturing costs. Also, if the time B is too long, various precipitates become coarse, resulting in a decrease in the toughness of the steel sheet after the hot rolling process . Therefore, it is preferable that the time B is 2 seconds or more and 180 seconds or less.

As described above, the ferritic stainless steel sheet of this embodiment can improve the strength at medium to high temperatures and the strength after aging. This makes the ferritic stainless steel sheet of this embodiment suitable for use as a material for automobile exhaust system components.

The following examples will make the effects of the present invention clearer. Note that the present invention is not limited to the following examples, and can be modified as appropriate without departing from the spirit of the present invention.

Steels (inventive examples A to R, comparative examples a to n) having the composition shown in Tables 1 and 2 were melted in a vacuum melting furnace and cast into 150 kg ingots to produce steel pieces, which were then hot-rolled to produce steel plates with a thickness of 5.0 mm. The steel plates after the hot-rolling process were then pickled , cold- rolled to a thickness of 2.0 mm, and finally annealed to produce ferritic stainless steel plates (product plates). In the manufacturing process of the product plates, the thermal history of the steel during the hot-rolling process was measured, and the time A during which the temperature was above 900° C. and below 1000° C. and the time B during which the temperature was above 600° C. and below 900° C. were determined, and the hot-rolling process was performed while controlling the ratio A/B of the times A and B.

Appropriate test pieces were taken from the entire thickness of the product plate obtained, and the test pieces were electrolytically treated in a tetramethylammonium chloride solution. The electrolytic solution was filtered through a filter with a pore size of 0.2 μm to extract the precipitates in the steel. The extracted precipitates were then dissolved, and the amounts of Cr, Ti, and Nb contained in the precipitates were determined by ICP emission spectroscopy (ICP: Inductively Coupled Plasma). The amounts of Cr, Ti, and Nb contained in the precipitates were then expressed as their content in the steel to determine [*Cr], [*Ti], and [*Nb].

In addition, the finished sheets were subjected to initial mid-low and high temperature strength evaluations, and mid-low and high temperature strength evaluations after long-term aging. The high-temperature tensile tests conducted to evaluate mid-low and high temperature strength conformed to JIS G 0567, and test pieces were cut so that the tensile direction was parallel to the rolling direction of the steel sheet, and tensile tests were conducted.

For the initial mid-low and high temperature strength evaluation, 0.2% yield strength was determined at 200°C, 500°C, 800°C, and 900°C. Those that did not meet any of the following criteria of 0.2% yield strength 180 MPa or more at 200°C, 150 MPa or more at 500°C, 25 MPa or more at 800°C, and 10 MPa or more at 900°C were rated "X (poor)," and those that met all of these criteria were rated "◯ (good)." Furthermore, those that met all of the following criteria of 0.2% yield strength 210 MPa or more at 200°C, 170 MPa or more at 500°C, 30 MPa or more at 800°C, and 15 MPa or more at 900°C were rated "◎ (even better)."

For strength evaluation at medium to high temperatures after long-term aging, the product sheets were aged at 800°C for 200 hours, and then the 0.2% yield strength was determined at 200°C, 500°C, 800°C, and 900°C. If at least one of the 0.2% yield strengths at each temperature after aging at 800°C for 200 hours was less than 55% of the initial 0.2% yield strength at each temperature, it was rated as "x (poor)", if all were 55% or more and at least one was less than 65%, it was rated as "○ (good)", and if all were 65% or more, it was rated as "◎ (even better)".

Tables 3A and 3B show the contents (mass%) of Cr, Ti, and Nb in the steel as precipitates for invention examples 1 to 18 and comparative examples 19 to 38, the values on the left side of formula (i) and formula (ii), the values in the middle of formula (x), the evaluation results of initial mid-low temperature and high temperature strength, and the evaluation results of mid-low temperature and high temperature strength after long-term aging. Note that the unit of "content of each element contained in the steel as precipitates" in Tables 3A and 3B is mass%.

As is clear from Tables 1 to 3B, steel sheets having the composition specified in the present invention and satisfying formulas (i) and (ii) have superior initial strength at low to medium temperatures and high temperatures, and strength at low to high temperatures after long-term aging, compared to the steel sheets of the comparative examples. It is also clear that if formula (x) is satisfied, then formulas (i) and (ii) are satisfied.

The long-term aging temperature was 800°C, at which precipitates are likely to form. However, even when long-term aging was similarly performed at 200°C, 500°C, and 900°C, the initial strength rankings of the steel of the present invention at medium to low temperatures and high temperatures, and the strength rankings of the steel at medium to low temperatures and high temperatures after long-term aging were the same as those when aging heat treatment was performed at 800°C. From this, it is believed that the examples of the present invention exhibit excellent strength in a variety of temperature environments.

From the above, it can be seen that a steel sheet having the chemical composition specified in the present invention and satisfying formulas (i) and (ii) has excellent strength at low to high temperatures and after long-term aging.

According to the present invention, it is possible to provide a ferritic stainless steel sheet suitable for use in automobile exhaust system components that require strength at medium to high temperatures and strength after long-term aging. Specific applications include exhaust manifolds, converters, front pipes, center pipes, mufflers, EGR, EGR coolers, turbo parts, and fastening parts for automobile exhaust systems. The present invention can promote the spread of turbo-equipped vehicles and engine vehicles that use HCCI (homogeneous charge compression ignition) combustion, in which these parts are used, and contribute to improving automobile fuel efficiency and reducing the environmental burden.

Claims (4)

The chemical composition of the steel is, in mass%,
C: 0.030% or less,
N: 0.030% or less,
Si: 0.01% or more, 5.00% or less,
Mn: 0.01% or more, 3.00% or less,
P: 0.050% or less,
S: 0.0100% or less,
Cr: 8.0% or more, 15.0% or less,
Ni: 0.001% or more, 2.00% or less,
Cu: 0.001% or more, 2.00% or less,
Mo: 0.001% or more, 4.00% or less,
Ti: 0.050% or more, 0.400% or less,
Nb: 0.001% or more, 0.80% or less,
Al: more than 0.70%, less than 5.00%,
V: 0.01% or more, 0.30% or less,
B: 0.0001% or more, 0.0050% or less,
O: 0.0050% or less,
Contains
The balance is Fe and impurities,
A ferritic stainless steel sheet, characterized in that it satisfies the following formulas (i) and (ii):
[Si]+[Nb]-[*Nb]≧0.40...Formula (i)
[*Ti]-3×[*Cr]≧0.001...Formula (ii)
Here, in formula (i) and formula (ii), the [element symbol] is the content (mass %) of the element in the steel, and the [*element symbol] is the content (mass %) of the element contained in precipitates in the steel. The chemical composition of the steel is, in mass%,
C: 0.020% or less,
N: 0.020% or less,
Si: 0.05% or more, 1.60% or less,
Mn: 0.01% or more, 1.10% or less,
P: 0.040% or less,
S: 0.0014% or less,
Cr: 10.0% or more, 15.0% or less,
Ni: 0.01% or more, 0.70% or less,
Cu: 0.001% or more, 0.80% or less,
Mo: 0.01% or more, 2.00% or less,
Ti: 0.050% or more, 0.230% or less,
Nb: more than 0.03%, less than 0.60%,
Al: 1.00% or more, 3.00% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0001% or more, 0.0050% or less,
O: 0.0050% or less,
Contains
The balance is Fe and impurities,
A ferritic stainless steel sheet, characterized in that it satisfies the following formulas (i) and (ii):
[Si]+[Nb]-[*Nb]≧0.40...Formula (i)
[*Ti]-3×[*Cr]≧0.001...Formula (ii)
Here, in formula (i) and formula (ii), the [element symbol] is the content (mass %) of the element in the steel, and the [*element symbol] is the content (mass %) of the element contained in precipitates in the steel. In mass%, replacing a part of Fe,
W: 0.001% or more, 0.50% or less,
Y: 0.001% or more, 0.50% or less,
REM: 0.001% or more, 0.50% or less,
Ca: 0.0001% or more, 0.0050% or less,
Zr: 0.001% or more, 0.50% or less,
Hf: 0.001% or more, 1.0% or less,
Sn: 0.001% or more, 1.0% or less,
Mg: 0.0001% or more, 0.0050% or less,
Co: 0.001% or more, 1.0% or less,
Sb: 0.001% or more, 1.0% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0001% or more, 0.50% or less,
The ferritic stainless steel sheet according to claim 1 or 2, comprising one or more of the following: A method for producing a ferritic stainless steel sheet according to any one of claims 1 to 3,
4. A method for producing a ferritic stainless steel sheet, comprising: performing a hot rolling process on a steel slab having the chemical composition according to any one of claims 1 to 3; controlling a ratio A/B of a time A during which the temperature of the steel in the hot rolling process is above 900°C and not higher than 1000°C to a time B during which the temperature of the steel in the hot rolling process is 600°C or higher and not higher than 900°C so as to satisfy the following formula (x):
1.0≦A/B≦100.0...Formula (x)