JP2022151087A - Ferritic stainless steel sheet - Google Patents

Abstract

To provide a ferritic stainless steel sheet having good heat resistance, workability and corrosion resistance while reducing Nb.SOLUTION: To provide a ferritic stainless steel sheet which has a chemical composition comprising, by mass%, 0.002 to 0.03% of C, 0.10 to 0.80% of Si, 1.0% or less of Mn, 0.04% or less of P, 0.030% or less of S, 17.0 to 19.5% of Cr, more than 0.60% and 2.0% or less of Mo, 0.05 to 0.20% of Nb, 5×(C+N) to 0.6% of Ti, 0.8 to 2.0% of Cu, 0.002 to 0.03% of N and the balance Fe with inevitable impurities and contains Ti-containing carbonitrides, Nb-containing carbonitrides formed so as to contact with the Ti-containing carbonitrides and has a carbonitride having a cross-sectional area of 1 μm2 or more and the number density of carbonitrides is 5 pieces/mm2 or more.SELECTED DRAWING: None

Description

The present invention relates to a ferritic stainless steel sheet.

Ferritic stainless steel is used for automobile exhaust members. Such automobile exhaust members are classified into upstream exhaust members and downstream exhaust members. The upstream exhaust member is exposed to hot exhaust gases that are close to being discharged from the engine. On the other hand, the downstream exhaust member is also exposed to corrosive products such as condensed water and snow-melting salt generated inside as the exhaust gas is cooled.

Accordingly, different steel grades of ferritic stainless steel have been used for upstream exhaust members and downstream exhaust members because of the different characteristics required. For example, the upstream exhaust member is exposed to high-temperature exhaust gas, so high-temperature characteristics are of primary importance. Therefore, Patent Documents 1 and 2 disclose ferritic stainless steels with improved high-temperature properties such as heat resistance, oxidation resistance, and high-temperature fatigue properties, assuming use in upstream exhaust members.

The ferritic stainless steels disclosed in Patent Documents 1 and 2 are effective in improving high-temperature characteristics, but contain less elements such as Nb, which are expensive and reduce manufacturability. On the other hand, instead of reducing these elements, by including Ti, Cu, etc. or controlling the content of other elements, the high temperature characteristics are improved, and it is excellent from the viewpoint of raw material cost and manufacturability. ing.

In addition, Patent Document 3 discloses a ferritic stainless steel having corrosion resistance that can be applied even in an environment where condensed water is generated, assuming use in downstream exhaust member applications.

JP 2010-248620 A JP 2013-100596 A WO2015/174048

However, the ferritic stainless steels of Patent Documents 1 and 2 are mainly specialized for high-temperature properties, so no study has been made on their corrosion resistance when used in downstream exhaust members. As described above, condensed water is generated in the downstream exhaust member. Condensed water is generated inside the exhaust gas after it is cooled, and it is acidic and highly corrosive when using poorly refined gasoline and fuel with a high sulfur content, such as overseas. becomes. For this reason, when assuming use in downstream exhaust members, the ferritic stainless steels of Patent Documents 1 and 2 are required to be further improved in corrosion resistance.

On the other hand, the ferritic stainless steel of Patent Document 3 is mainly specialized for corrosion resistance in the downstream section, and when used as an exhaust member in the upstream section, it is considered that the heat resistance is not sufficient.

In addition, in recent years, as the shape of parts has become more complicated, the material of the exhaust member is required to have good workability regardless of whether it is upstream or downstream. Regarding this point, although Patent Document 1 mentions workability, it does not consider ridging, which is a problem during press molding. For this reason, problems may occur in molding with a large degree of workability, and there is room for improvement in workability. Moreover, Patent Documents 2 and 3 do not consider workability itself.

Based on the above, there is a problem that it is difficult to use existing ferritic stainless steel as an exhaust member regardless of the upstream or downstream part from the viewpoint of characteristics. Then, as in the above document, while reducing the alloy cost by reducing elements such as Nb as much as possible, it has excellent heat resistance that can be applied even in a heat environment of 600 to 800 ° C., good workability, and good heat resistance. There is a problem that it is difficult to provide all the properties of corrosion resistance.

An object of the present invention is to solve the above problems and to provide a ferritic stainless steel sheet having good heat resistance, workability and corrosion resistance while reducing Nb.

The present invention has been made to solve the above problems, and the gist thereof is the following ferritic stainless steel sheet.

(1) chemical composition, in mass %,
C: 0.002 to 0.03%,
Si: 0.1 to 0.8%,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.030% or less,
Cr: 17.0-19.5%,
Mo: more than 0.60% and 2.0% or less,
Nb: 0.05 to 0.2%,
Ti: 0.6% or less,
Cu: 0.8-2.0%,
N: 0.002 to 0.03%,
Ni: 0 to 0.6%,
V: 0 to 0.5%,
W: 0 to 0.5%,
Co: 0-0.5%,
Zr: 0 to 0.5%,
Al: 0 to 1.0%,
Sn: 0-0.5%,
B: 0 to 0.005%,
Ca: 0-0.01%,
Mg: 0-0.01%,
REM: 0-0.01%,
balance: Fe and impurities,
satisfying the following formula (i),
A carbonitride containing a Ti-containing carbonitride and an Nb-containing carbonitride formed in contact with the Ti-containing carbonitride, and having a cross-sectional area of 1 μm ² or more,
A ferritic stainless steel sheet, wherein the number density of the carbonitrides is 5/mm ² or more.
5×(C+N)≦Ti (i)
However, each element symbol in the above formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

(2) the chemical composition, in mass %,
Ni: 0.01 to 0.6%,
V: 0.01 to 0.5%,
W: 0.05 to 0.5%,
Co: 0.01-0.5%,
Zr: 0.01 to 0.5%,
Al: 0.01-1.0%, and Sn: 0.01-0.5%,
The ferritic stainless steel sheet according to (1) above, containing one or more selected from:

(3) the chemical composition, in mass %,
B: 0.0002 to 0.005%,
Ca: 0.0002-0.01%,
Mg: 0.0002-0.01%, and REM: 0.0002-0.01%,
The ferritic stainless steel sheet according to (1) or (2) above, containing one or more selected from:

According to the present invention, it is possible to obtain a ferritic stainless steel sheet having good heat resistance, workability and corrosion resistance while reducing Nb.

FIG. 1 is a micrograph showing an example of Ti-containing carbonitride and Nb-containing carbonitride (composite carbonitride) formed therearound. FIG. 2 is a diagram schematically showing an example of a structure (composite carbonitride) in which a Ti-containing carbonitride and a Nb-containing carbonitride are formed around it.

The present inventors have studied the chemical composition of ferritic stainless steel sheets and the above-described characteristics, and have obtained the following findings (a) to (c).

(a) In order to reduce raw material costs, it is necessary to reduce the use of expensive elements such as Nb and Mo as much as possible. On the other hand, Nb has the effect of improving heat resistance. In addition to heat resistance, Mo has the effect of improving corrosion resistance even in an environment of condensed water containing sulfur. Therefore, heat resistance is ensured by containing Cu and Ti while reducing Nb. At the same time, by adjusting the above-mentioned Cu, Mo, and Cr to appropriate ranges, corrosion resistance is ensured. In particular, Cu and Mo are effective because they improve the corrosion resistance in a condensed water environment containing a large amount of sulfur, that is, having a high concentration of SO ₃ ^2- .

(b) However, the inclusion of elements such as Cu and Ti reduces workability and toughness. The decrease in workability is caused by the formation of Cu precipitates in the temperature range of 600 to 800° C., which increases the strength at room temperature. Further, the decrease in toughness is caused by the precipitation of carbonitrides containing hard Ti (hereinafter simply referred to as "Ti-containing precipitates").

Therefore, the inventors investigated whether the formation of Cu precipitates could be suppressed. They have also found that it is effective to add a small amount of Nb to the extent that it does not precipitate as a Laves phase that reduces workability and toughness.

(c) The present inventors also found that the decrease in toughness due to the formation of Ti-containing carbonitrides can be suppressed by controlling the morphology of the carbonitrides. Specifically, the Ti-containing carbonitride is formed in a polygonal shape, and the carbonitride containing Nb (hereinafter simply referred to as "Nb-containing carbonitride") is in contact with the angular portion of this shape. ) is precipitated.

The mechanism by which the morphology of such carbonitrides suppresses the reduction in toughness is not clear. However, usually, the polygonal angular portions of the Ti-containing carbonitride tend to accumulate strain, and tend to become starting points for cracks. For this reason, it is considered that by adopting the above-described form, the strain accumulation state around the carbonitride is changed, and stress concentration is less likely to occur.

Further, when hot rolling or cold rolling is performed in a state in which the carbonitrides of the above-described form are precipitated, the rolling elongation structure is divided by the strain field around the precipitates. As a result, the formation of stretched tissue can be suppressed, and the occurrence of ridging can also be suppressed. Therefore, it is possible to provide good workability that can be applied even when the degree of workability is large.

The present invention has been made based on the above findings. Each requirement of this embodiment will be described in detail below.

1. Chemical composition The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.002-0.03%
In the steel sheet according to the present embodiment, the Nb-containing carbonitride precipitates around the Ti-containing carbonitride, thereby improving the toughness. Therefore, it is necessary to contain a certain amount or more of C, and the C content is made 0.002% or more. However, excessive C content lowers workability and corrosion resistance. Moreover, it also brings about a decrease in heat resistance. Therefore, the C content should be 0.03% or less. From the viewpoint of refining cost, the C content is preferably in the range of 0.003 to 0.02%, more preferably in the range of 0.003 to 0.015%.

Si: 0.1-0.8%
Si is an element useful as a deoxidizing agent and an element that improves oxidation resistance. Therefore, the Si content should be 0.1% or more. However, if Si is contained excessively, the room temperature ductility is lowered and the workability is lowered. Therefore, the Si content should be 0.8% or less. The Si content is preferably in the range of 0.1-0.6%.

Mn: 1.0% or less Mn is an element that is useful as a deoxidizing agent, and forms Mn-based oxides on the surface layer during long-term use, contributing to the improvement of scale adhesion. As a result, the oxidation resistance is improved. Therefore, it may be contained as necessary. However, an excessive Mn content not only remarkably increases the weight gain due to oxidation, but also facilitates the formation of an austenite phase at high temperatures. As a result, the heat resistance is lowered. Therefore, the Mn content is set to 1.0% or less. On the other hand, in order to obtain the above effects, the Mn content is preferably 0.1% or more. The Mn content is preferably in the range of 0.2-0.8%.

P: 0.04% or less P is a harmful element that reduces toughness and workability. Therefore, the P content should be 0.04% or less. The P content is preferably 0.03% or less. P is preferably reduced as much as possible, but excessive reduction of P increases refining costs. Therefore, the P content is preferably 0.01% or more.

S: 0.030% or less S is an element that lowers elongation, adversely affects workability, and lowers corrosion resistance. Therefore, the S content should be 0.030% or less. The S content is preferably 0.010% or less, more preferably 0.005% or less. S is preferably reduced as much as possible, but excessive reduction of S increases refining costs. Therefore, the S content is preferably 0.0003% or more.

Cr: 17.0-19.5%
Cr is an element effective in improving corrosion resistance and oxidation resistance, which are characteristics of stainless steel. If the Cr content is less than 17.0%, the Cr fraction in the passive film is insufficient, and in addition to not being able to obtain corrosion resistance, it is necessary for long-term use at about 800 ° C. Oxidation resistance cannot be obtained. Therefore, the Cr content is set to 17.0% or more. However, when Cr is contained excessively, the steel is solid-solution strengthened at room temperature, hardened, and ductile reduced, thereby deteriorating workability. In particular, if the Cr content exceeds 19.5%, the adverse effects on the above characteristics become significant, so the Cr content is made 19.5% or less. The Cr content is preferably in the range of 17.5-19.0%.

Mo: More than 0.60% and 2.0% or less Mo has the effect of improving heat resistance, oxidation resistance, and corrosion resistance. In particular, when the SO3 ^2- concentration is high, in order to ensure suitable corrosion resistance, it is necessary to add Mo in combination with Cu, which will be described later, and the Mo content should exceed 0.60%. However, when Mo is contained excessively, the steel is hardened by solid-solution strengthening, and workability is lowered. Also, since Mo is an expensive element, the alloy cost increases. Therefore, the Mo content is set to 2.0% or less. The Mo content is preferably 1.5% or less, more preferably 1.0% or less.

Nb: 0.05-0.2
Nb has the effect of improving high-temperature strength and thermal fatigue properties. Also, the inclusion of Nb suppresses the formation of Cu precipitates at high temperatures. Furthermore, by precipitating Nb-containing carbonitride so as to be in contact with the periphery of Ti-containing carbonitride, it has the effect of increasing toughness and suppressing recovery from strain introduced during hot rolling. Therefore, the Nb content is set to 0.05% or more. The Nb content is preferably 0.06% or more. However, if Nb is contained excessively, a Laves phase containing Nb is formed, resulting in deterioration of toughness and workability. Therefore, the Nb content is set to 0.2% or less. The Nb content is preferably 0.18% or less.

Ti: 0.6% or less In the steel sheet of the present embodiment, the Nb content is reduced. Therefore, Ti is an important element for fixing C and N. Ti fixes C and N, suppresses the occurrence of sensitization, and has the effect of improving corrosion resistance, intergranular corrosion resistance of the weld zone, and oxidation resistance. In addition, by forming Nb-containing carbonitrides around the Ti-containing carbonitrides precipitated by fixing C and N, the surroundings of the carbonitrides become strain fields during hot rolling and cold rolling. It becomes a source of generation and divides the rolling elongation structure. As a result, the occurrence of ridging is suppressed and workability is improved. Therefore, the Ti content must satisfy the following formula (i).

5×(C+N)≦Ti (i)
However, each element symbol in the above formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

However, excessive Ti content induces a decrease in toughness and generation of surface defects. Therefore, the Ti content should be 0.6% or less. Depending on the C and N contents, the Ti content is preferably in the range of 0.15 to 0.4%, more preferably in the range of 0.2 to 0.3%.

Cu: 0.8-2.0%
Cu is an element effective in improving heat resistance in the temperature range of about 600 to 800° C., as described above. In addition, it is an effective element for improving sulfuric acid resistance. In this embodiment, it is necessary to reduce Nb, which is effective for improving high-temperature strength, and to ensure corrosion resistance even when the SO3 ^2- concentration is high. Therefore, it is contained together with Mo. At this time, the Cu content is set to 0.8% or more. The Cu content is preferably 1.0% or more, more preferably 1.1% or more. However, an excessive Cu content lowers oxidation resistance and workability, and promotes coarsening of Cu precipitates (also referred to as “ε-Cu”). Therefore, the Cu content is set to 2.0% or less. The Cu content is preferably 1.5% or less.

N: 0.002-0.03%
N forms coarse and hard TiN at a high temperature, and is generated in a strain field around precipitates during hot rolling, thereby dividing the rolling elongation structure and improving workability. Therefore, the N content should be 0.002% or more. However, if N is included in excess, like C, the workability and corrosion resistance are lowered, and the heat resistance is also lowered. Therefore, the N content is set to 0.03% or less. From the balance of refining costs, the N content is preferably in the range of 0.003 to 0.02%, more preferably in the range of 0.003 to 0.015%.

In addition to the above elements, one or more selected from Ni, V, W, Co, Zr, Al and Sn may be contained within the following ranges. The reason for limiting each element will be explained.

Ni: 0-0.6%
Ni is an element that improves the toughness of ferritic stainless steel and suppresses the occurrence of cracks. Therefore, it may be contained as necessary. However, since it is a strong austenite phase-forming element, when it is contained excessively, it makes it easy to form an austenite phase at high temperatures, thereby lowering the heat resistance. Moreover, since it is an expensive element, the alloy cost also increases. Therefore, the Ni content is set to 0.6% or less. The Ni content is preferably 0.5% or less, more preferably 0.3% or less. On the other hand, in order to obtain the above effects, the Ni content is preferably 0.01% or more.

V: 0-0.5%
V, like Nb and Ti, is a carbonitride-forming element, and has the effect of improving heat resistance by finely precipitating these carbonitrides. Therefore, it may be contained as necessary. However, excessive V content causes deterioration of manufacturability. Therefore, the V content is set to 0.5% or less. The V content is preferably 0.4% or less, more preferably 0.3% or less. On the other hand, in order to obtain the above effects, the V content is preferably 0.01% or more.

W: 0-0.5%
W has the effect of increasing heat resistance. Therefore, it may be contained as necessary. However, an excessive content of W promotes the formation of intermetallic compounds and reduces the toughness and workability of steel. Therefore, the W content is set to 0.5% or less. The W content is preferably 0.4% or less, more preferably 0.3% or less. On the other hand, in order to obtain the above effects, the W content is preferably 0.05% or more, more preferably 0.1% or more.

Co: 0-0.5%
Co has the effect of improving the heat resistance and lowering the coefficient of thermal expansion. Therefore, it may be contained as necessary. However, an excessive content of Co hardens the steel by solid-solution strengthening, thereby deteriorating workability. In addition, since Co is an expensive element, alloy costs increase. Therefore, the Co content is set to 0.5% or less. The Co content is preferably 0.4% or less, more preferably 0.3% or less. On the other hand, in order to obtain the above effects, the Co content is preferably 0.01% or more.

Zr: 0-0.5%
Zr has the effect of improving oxidation resistance. Therefore, it may be contained as necessary. However, an excessive content of Zr produces intermetallic compounds and lowers the toughness and workability of the steel. Therefore, the Zr content should be 0.5% or less. The Zr content is preferably 0.4% or less, more preferably 0.3% or less. On the other hand, in order to obtain the above effects, the Zr content is preferably 0.01% or more.

Al: 0-1.0%
Al is used as a deoxidizing agent and has the effect of improving oxidation resistance. Therefore, it may be contained as necessary. However, an excessive content of Al hardens the steel by solid solution strengthening and lowers the toughness and workability of the steel. Therefore, the Al content is set to 1.0% or less. The Al content is preferably 0.6% or less, more preferably 0.2% or less. On the other hand, in order to obtain the above effects, the Al content is preferably 0.01% or more.

Sn: 0-0.5%
Sn has the effect of improving the corrosion resistance without significantly deteriorating the mechanical properties at room temperature. Therefore, it may be contained as necessary. However, an excessive Sn content significantly lowers the manufacturability. Therefore, the Sn content is set to 0.5% or less. The Sn content is preferably 0.3% or less, more preferably 0.2% or less. On the other hand, in order to obtain the above effects, the Sn content is preferably 0.01% or more.

In addition to the above elements, one or more selected from B, Ca, Mg and REM may be contained within the range shown below. The reason for limiting each element will be explained.

B: 0-0.005%
B has the effect of improving workability, particularly secondary workability. Therefore, it may be contained as necessary. However, an excessive content of B degrades weldability and toughness. Therefore, the B content should be 0.005% or less. The B content is preferably 0.003% or less, more preferably 0.0015% or less. On the other hand, in order to obtain the above effects, the B content is preferably 0.0002% or more.

Ca: 0-0.01%
Ca has the effect of preventing nozzle clogging that tends to occur during continuous casting. Therefore, it may be contained as necessary. However, excessive Ca content makes surface defects more likely to occur. Therefore, the Ca content is set to 0.01% or less. The Ca content is preferably 0.005% or less, more preferably 0.003% or less. On the other hand, in order to obtain the above effects, the Ca content is preferably 0.0002% or more.

Mg: 0-0.01%
Mg has the effect of improving the equiaxed grain ratio of the slab and improving toughness and workability. Therefore, it may be contained as necessary. However, an excessive Mg content lowers the toughness of the steel and deteriorates the surface properties. Therefore, the Mg content is set to 0.01% or less. The Mg content is preferably 0.005% or less, more preferably 0.003% or less. On the other hand, in order to obtain the above effects, the Mg content is preferably 0.0002% or more.

REM: 0-0.01%
REM (rare earth element) has the effect of improving oxidation resistance. Therefore, it may be contained as necessary. However, excessive REM content reduces weldability and toughness. Therefore, the REM content is set to 0.01% or less. The REM content is preferably 0.008% or less, more preferably 0.005% or less. On the other hand, in order to obtain the above effects, the REM content is preferably 0.0002% or more.

REM refers to a total of 17 elements of Sc, Y and lanthanoids, and the above REM content means the total content of these elements. Industrially, REM is often added in the form of misch metal.

In the chemical composition of the present invention, the balance is Fe and impurities. Here, "impurities" are
It means a component that is mixed in with raw materials such as ore, scrap, etc., and various factors in the manufacturing process when steel is manufactured industrially, and is allowed within a range that does not adversely affect the present invention.

2. Carbonitride The steel sheet according to the present embodiment is a steel sheet in which carbonitrides are dispersed and precipitated in a matrix. And it has carbonitrides including Ti-containing carbonitrides containing Ti and Nb-containing carbonitrides containing Nb. The carbonitride described above has a cross-sectional area of 1 μm ² or more. Here, the carbonitride refers to a compound containing C and/or N, and the above-mentioned cross-sectional area is the area of the carbonitride observed when the structure is observed by the method described later, that is, , refers to the area when the viewing surface is projected horizontally.

The Ti-containing carbonitride contained in the carbonitride described above has high compatibility with the matrix, and has, for example, a polygonal shape as shown in the center of FIG. 1 and a relatively large size. Also, it is a hard compound. In particular, the polygonal, angular portions of the hard Ti-containing carbonitride are likely to accumulate strain and become fracture starting points. As a result, cracking propagates, the toughness is lowered, and this makes it difficult to ensure the rolling reduction in cold rolling. And it becomes difficult to improve the workability, especially the r value.

For this reason, as shown in FIGS. 1 and 2, the Nb-containing carbonitrides are subsequently formed around and in contact with the angular portions of the Ti-containing carbonitrides. It must be formed so as to be in contact with the surrounding carbonitrides contained. This is because the radius of curvature of the overall shape of the carbonitride is considered to be large, thereby suppressing the occurrence of stress concentration. At this time, it is sufficient that the Nb-containing carbonitride is formed partially around the Ti-containing carbonitride, and in particular, the Nb-containing carbonitride is formed around the angular portion of the Ti-containing carbonitride. It is desirable to form it, and it is desirable to form it continuously at a plurality of locations.

Ti-containing carbonitrides may be, for example, TiN, TiC, or the like. Nb-containing carbonitrides may be NbC, NbN, or the like.

In addition, a carbonitride having a cross-sectional area of 1 μm ² or more (hereinafter , for the sake of simplicity, also referred to as "composite carbonitride".) is 5 pieces/mm ² or more. If the number density of the composite carbonitrides is less than 5 pieces/mm ² , many crack starting points exist, making it difficult to obtain good toughness and workability. Therefore, the number density of composite carbonitrides is set to 5 pieces/mm ² or more. The number density of the composite carbonitrides is preferably 10 pieces/mm ² or more, more preferably 15 pieces/mm ² or more. The upper limit of the number density of composite carbonitrides is not particularly limited.

As described above, Cu precipitates are finely precipitated in the temperature range of 600 to 800° C., increase the high-temperature strength of the steel sheet in the temperature range of the usage environment, and form a solid solution at higher temperatures. On the other hand, by adding a small amount of Nb within a range in which the Laves phase does not precipitate, it is possible to suppress the growth of Cu precipitates when held in the temperature range of 600 to 800 ° C., and to suppress the growth of Cu precipitates during the cooling process from the solid solution state. In addition to suppressing precipitation of precipitates, deterioration in toughness due to improvement in strength at room temperature of the steel sheet is suppressed. Moreover, when the precipitation sites of Cu precipitates are reduced by forming the composite carbonitride whose morphology is controlled as described above, there is a possibility that Cu precipitation itself can also be suppressed.

The composite carbonitride may be measured by the following procedure. Specifically, an observation sample is collected from the L surface of the steel plate. This observation sample was placed in a non-aqueous electrolytic solution consisting of 10% by mass of acetylacetone, 1% by mass of tetramethylammonium chloride, and 89% by mass of methyl alcohol, and -100 mV to 400 mV against a saturated calomel reference electrode (SCE). of potential is applied. As a result, the matrix (metal base) of the sample is dissolved, and the matrix and compounds such as carbonitrides can be distinguished from each other, and the metal structure is observed by SEM. During observation, the magnification is 5,000 times, and the number of fields of view to be measured is 100.

Then, the carbonitride is observed, and as shown in FIG. 1, the composition of the carbonitride, which is considered to be a coarse Ti-containing carbonitride, is analyzed by EDX attached to the SEM, the composition is confirmed, and the Ti-containing carbonitride or not. Similarly, the composition of the carbonitride formed so as to be in contact with the periphery of the coarse carbonitride is checked to determine whether it is an Nb-containing carbonitride. At this time, the cross-sectional area of the entire carbonitride is measured to determine whether it is a composite carbonitride. The cross-sectional area is calculated by multiplying the major axis of the observed composite carbonitride by the minor axis. The major axis is the length of the longest straight line connecting two points on the outer circumference in the horizontal or vertical direction, and the minor axis is the length of the straight line that connects two points on the outer circumference in the horizontal or vertical direction. is the length of the shortest straight line. The number density can be calculated by counting the number of composite carbonitrides observed at the above magnification and number of fields of view and dividing the number by the area of the observation field of view.

3. Manufacturing Method A preferable manufacturing method of the steel sheet according to the present embodiment will be described. The steel plate according to the present embodiment can obtain the effect as long as it has the above configuration regardless of the manufacturing method. For example, the steel plate can be stably manufactured by the following manufacturing method. In the following description, the case of a cold-rolled annealed sheet will be described.

A steel having the above chemical composition is melted and a slab is produced by a conventional method. The obtained slab is heated and hot rolled. Although the slab heating temperature is not particularly limited, it is usually in the range of 1150 to 1250°C.

Further, during hot rolling, the steel is retained in the temperature range of 900 to 1100° C. for 1 minute or more. When the residence time in the above temperature range is less than 1 minute, it becomes difficult to form composite carbonitrides, and the number density of the composite carbonitrides decreases. As a result, it becomes difficult to obtain the effect of improving toughness and workability. Therefore, the residence time in the above temperature range is 1 minute or longer, preferably 1.5 minutes or longer. Although the upper limit of the residence time in the above temperature range is not particularly limited, it is usually 4 minutes or less from the viewpoint of productivity.

The hot rolling completion temperature is usually about 850 to 950°C. Then, hot-rolled sheet annealing is performed as needed. When the hot-rolled sheet is annealed, the conditions are not particularly limited. For example, annealing may be performed at an annealing temperature in the range of 900 to 1050° C. for 2 minutes or less. The average cooling rate in the temperature range of 800 to 400° C. is preferably 10° C./s or more regardless of whether the hot-rolled sheet is annealed or not. This is because if the average cooling rate in the temperature range of 800 to 400° C. is less than 10° C./s, Cu precipitates will form when passing through the temperature range. Therefore, the average cooling rate in cooling the temperature range is preferably 10° C./s or more. In addition, for example, pickling may be performed after hot rolling and after hot-rolled sheet annealing, if necessary.

The obtained hot-rolled sheet (or hot-rolled and annealed sheet) is cold-rolled to obtain a cold-rolled sheet. The rolling reduction during cold rolling may be appropriately adjusted as necessary. The obtained cold-rolled sheet is subjected to cold-rolled sheet annealing, if necessary. The annealing temperature for cold-rolled sheet annealing is preferably 950° C. or lower.

If the annealing temperature in the cold-rolled sheet annealing exceeds 950°C, the crystal grains become coarse, resulting in surface roughness or the like after working. Moreover, the cost of fuel required for annealing increases, and the load in the pickling process also increases. For this reason, it is preferable that the annealing temperature in cold-rolled sheet annealing is 950° C. or lower. Annealing time for cold-rolled sheet annealing is not particularly limited, but from the viewpoint of manufacturability, it is generally considered to be within 2 minutes.

Also in cooling during cold-rolled sheet annealing, it is preferable to set the average cooling rate to 10°C/s or more in the temperature range of 800 to 400°C. This is because it is possible to suppress the formation of Cu precipitates. In addition, for example, pickling may be performed after cold rolling and after cold-rolled sheet annealing, if necessary.

The steel sheet of the present embodiment will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

A steel having the chemical composition shown in Table 1 is melted by vacuum melting, cast into a mold with a thickness of 80 mm, heated at 1230 ° C. for 2 hours, and subjected to hot rolling to obtain a hot-rolled sheet with a thickness of 5 mm. Obtained. At this time, the residence time in the temperature range of 900°C to 1100°C was controlled. The hot rolling speed was adjusted so that the residence time in the temperature range of 900° C. to 1100° C. was less than 1 minute only for Steel 13 and Steel 14, and the residence time in the above temperature range was 1 minute for the other steels. minutes or more. Conditions other than those described above, such as the cooling rate, were adjusted so as to fall within the range of the preferable conditions described above.

Next, after pickling the hot-rolled sheet, it was cold-rolled to a thickness of 1.5 mm to obtain a cold-rolled sheet. Further, the cold-rolled steel sheet was annealed at 920° C., which satisfies the preferred conditions described above, and then pickled to obtain a cold-rolled annealed steel sheet. In the above examples, the average cooling rate in the temperature range of 800 to 400°C was all 10°C/s or more.

From the viewpoint of manufacturability, the toughness of the hot-rolled sheet before cold rolling was also evaluated by the method described later. In addition, the morphology of carbonitrides of the obtained cold-rolled and annealed sheets was observed by the method described later. After that, heat resistance, workability and corrosion resistance were evaluated by the methods described later.

(Hot-rolled sheet toughness)
As an index for evaluating manufacturability, the toughness of the hot-rolled sheet was measured. From the hot-rolled sheet, a V-notch sub-size Charpy impact test piece was prepared with the same thickness as the hot-rolled sheet so that the longitudinal direction of the test piece was parallel to the rolling direction, and the test described in JIS Z 2242: 2018. Five runs were made for each specimen according to the method. The impact value was calculated by dividing the measured absorbed energy by the cross-sectional area, and the lowest value among the five was taken as the Charpy impact value. A Charpy impact value of 30 J/cm ² or more was evaluated as a good property from the viewpoint that the hot-rolled coil could be unwound without problems and the manufacturability was good. The test temperature was room temperature (23±5° C.).

(Form of carbonitride)
In order to examine the morphology of carbonitrides, observation samples were collected from the L-face of the steel plate. This observation sample was placed in a non-aqueous electrolytic solution consisting of 10% by mass of acetylacetone, 1% by mass of tetramethylammonium chloride, and 89% by mass of methyl alcohol, and -100 mV to 400 mV against a saturated calomel reference electrode (SCE). was applied. In this way, the matrix (metal base) of the sample was dissolved so that the matrix and compounds such as carbonitrides could be distinguished from each other, and the metallographic structure was observed with an SEM. During the observation, the magnification was set to 5000 times, and the number of fields of view to be measured was set to 100 fields.

Then, the carbonitride was observed, and the composition of the carbonitride considered to be a coarse Ti-containing carbonitride was analyzed by EDX attached to the SEM, the composition was confirmed, and it was determined whether it was a Ti-containing carbonitride. . Similarly, the composition of the carbonitride formed so as to be in contact with the periphery of the coarse carbonitride was checked to determine whether it was a Nb-containing carbonitride. At this time, the cross-sectional area of the entire carbonitride was measured to determine whether it was a composite carbonitride. The cross-sectional area was calculated by multiplying the major axis of the observed composite carbonitride by the minor axis. The number density was calculated by counting the number of composite carbonitrides observed at the above magnification and number of fields of view, and dividing the number by the area of the observation field of view.

(Heat-resistant)
A test piece was sampled from the obtained cold-rolled annealed sheet, and a high temperature tensile test was performed at 800° C. to measure the 0.2% yield strength. The tensile test was performed in accordance with JIS G 0567:2012, and the heat resistance was evaluated to be good when the 800° C. proof stress was 25 MPa or more.

(workability)
The workability was evaluated by measuring the total elongation (elongation at break) and the waviness height as an index of ridging resistance. The total elongation was measured according to JIS Z 2201:1998 by extracting a JIS No. 13B test piece whose longitudinal direction is the rolling direction from the cold-rolled and annealed sheet. If the total elongation at room temperature is 30% or more, processing into complicated parts becomes possible.

Moreover, the swell height was measured by the following procedure. Specifically, a JIS No. 5 tensile test piece whose longitudinal direction is the rolling direction is taken from the cold-rolled and annealed sheet, tensile strain is applied until the elongation in the parallel part becomes 20%, and then the unloaded test piece. was made. For the test piece, the surface profile of the central part in the longitudinal direction is measured in the width direction (that is, the direction perpendicular to the rolling direction), and according to JIS B 0601: 2013, the cutoff values λf = 8.0 mm and λc = 0.5 mm. A waviness curve of 0.5 to 8.0 mm was determined, and an arithmetic mean waviness Wa (μm) was obtained from the waviness curve, which was taken as a waviness height W. A undulation height W of 2.0 μm or less was evaluated as a good characteristic value.

(corrosion resistance)
Corrosion resistance was evaluated by conducting the following boiling condensation test. Specifically, strip-shaped test pieces of 50 mm×120 mm were cut out from the obtained cold-rolled and annealed sheets. Subsequently, simulating an environment in which condensation and evaporation of the exhaust gas are repeated, the test piece is immersed in the test liquid shown in Table 2 in a semi-immersed state, boiled for 4 hours to concentrate to 6 times, and then at a temperature of 30 ° C. , and held for 20 hours under dew condensation conditions with a relative humidity of 80%. This was repeated 5 times, and a cycle of heating at 400° C. for 2 hours to simulate heating by exhaust gas was repeated twice. The test liquids shown in Table 2 were prepared with reference to an analysis example of condensed water generated when a fuel with a high S concentration was used, and all test liquids were prepared using ammonium salts. After the test, the rust was removed and the maximum corrosion depth was measured. A maximum corrosion depth of 0.4 mm or less was evaluated as a good characteristic value. Table 3 summarizes the results.

The cold-rolled and annealed sheets of Steels 1-12 met the requirements for composition and composite carbonitride, so all the results of heat resistance, corrosion resistance, and workability were good. On the other hand, Steel 13 had a satisfactory composition, but was not manufactured under preferable manufacturing conditions, and therefore had a low number density of composite carbonitrides and insufficient workability. Moreover, the toughness of the hot-rolled sheet also decreased. Steels 14 to 17 were assumed to be ferritic stainless steel plates conventionally used in the upstream part of the exhaust gas passage, but although they had satisfactory heat resistance, they were inferior in at least one of corrosion resistance and workability. Moreover, the toughness of the hot-rolled sheet also decreased. Steels 18 and 19 had poor heat resistance and corrosion resistance because the Mo and Cu contents did not satisfy the ranges of the present embodiment. In addition, since the Nb content of Steel 20 did not satisfy the range of the present embodiment, the heat resistance was insufficient, the composite carbonitride was not sufficiently formed, and the workability was also lowered. Moreover, the toughness of the hot-rolled sheet also decreased.

The steel sheet of the present embodiment can reduce Nb and is excellent in economic efficiency. In addition, it has heat resistance that can be applied even in a high temperature environment of 600 to 800°C. In addition, since it has good workability and corrosion resistance, it can be applied to the exhaust member regardless of whether it is upstream or downstream. Therefore, the steel plate of the present embodiment is suitable as a material for exhaust members.

1. Ti-containing carbonitride 2. Nb-containing carbonitride

Claims (3)

The chemical composition, in mass %,
C: 0.002 to 0.03%,
Si: 0.1 to 0.8%,
Mn: 1.0% or less,
P: 0.04% or less,
S: 0.030% or less,
Cr: 17.0-19.5%,
Mo: more than 0.60% and 2.0% or less,
Nb: 0.05 to 0.2%,
Ti: 0.6% or less,
Cu: 0.8-2.0%,
N: 0.002 to 0.03%,
Ni: 0 to 0.6%,
V: 0 to 0.5%,
W: 0 to 0.5%,
Co: 0-0.5%,
Zr: 0 to 0.5%,
Al: 0 to 1.0%,
Sn: 0-0.5%,
B: 0 to 0.005%,
Ca: 0-0.01%,
Mg: 0-0.01%,
REM: 0-0.01%,
balance: Fe and impurities,
satisfying the following formula (i),
A carbonitride containing a Ti-containing carbonitride and an Nb-containing carbonitride formed in contact with the Ti-containing carbonitride, and having a cross-sectional area of 1 μm ² or more,
A ferritic stainless steel sheet, wherein the number density of the carbonitrides is 5/mm ² or more.
5×(C+N)≦Ti (i)
However, each element symbol in the above formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained. The chemical composition, in mass %,
Ni: 0.01 to 0.6%,
V: 0.01 to 0.5%,
W: 0.05 to 0.5%,
Co: 0.01-0.5%,
Zr: 0.01 to 0.5%,
Al: 0.01-1.0%, and Sn: 0.01-0.5%,
The ferritic stainless steel sheet according to claim 1, containing one or more selected from. The chemical composition, in mass %,
B: 0.0002 to 0.005%,
Ca: 0.0002-0.01%,
Mg: 0.0002-0.01%, and REM: 0.0002-0.01%,
The ferritic stainless steel sheet according to claim 1 or 2, containing one or more selected from.

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