KR20240000556A - Ferritic stainless steel - Google Patents

Abstract

Provided is a ferritic stainless steel that has good solderability when soldering at high temperatures using a Ni-containing brazing material and is excellent in corrosion resistance after brazing. In mass%, C: 0.003 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.05 to 0.30%, P: 0.050% or less, S: 0.020% or less, Cr: 24.0 to 30.0%, Ni: 1.50 to 1.50% 3.00 % , Mo: 1.00 to 3.00%, Al: 0.001 to 0.020%, Nb: 0.20 to 0.80%, N: 0.030% or less, satisfies the following formulas (1) and (2), and the balance is Fe and Ferritic stainless steel with a composition consisting of inevitable impurities.
Ni-2(Si＋Mn) ≥ 0.00 ％··· (1)
Cr＋1.5 Mo＋Si＋1.5 Nb-2.5 Ni ≤ 25.0 ％··· (2)
(Ni, Si, Mn, Cr, Mo, and Nb in formulas (1) and (2) represent the content (% by mass) of each element.)

Description

Ferritic stainless steel

The present invention relates to ferritic stainless steel, and in particular, to ferritic stainless steel that has good solderability and excellent corrosion resistance when soldering at high temperatures using a Ni-containing brazing alloy.

Recently, from the standpoint of protecting the global environment, additional improvements in fuel efficiency and strengthening of exhaust gas purification for automobiles have been required. For this reason, the application of automotive heat exchangers such as exhaust heat recoverers and EGR (Exhaust Gas Recirculation) coolers continues to expand.

Here, the heat recovery device is a device that improves fuel efficiency by using the heat of the engine coolant for heating or by warming the engine coolant with the heat of exhaust gas to shorten the warm-up time when the engine is started. Typically, an exhaust heat recovery device is installed between the catalytic converter and the muffler. The exhaust heat recovery device is composed of a heat exchanger part combining pipes, plates, fins, side plates, etc., an inlet pipe part, and an outlet pipe part. Then, the exhaust gas enters the heat exchanger portion from the inlet pipe, where its heat is transferred to the cooling water through heat transfer surfaces such as fins, and is discharged from the outlet pipe. In addition, brazing with a Ni-containing brazing material is mainly used to bond and assemble the plates and fins that make up the heat exchanger portion of such waste heat recovery equipment.

Additionally, the EGR cooler is composed of a pipe that introduces part of the exhaust gas from the exhaust manifold, etc., a heat exchanger that cools the introduced exhaust gas, and a pipe that returns the cooled exhaust gas to the intake side of the engine. The EGR cooler has a specific structure having a heat exchanger that includes both a water flow passage and an exhaust gas passage on a path that returns exhaust gas from the exhaust manifold to the intake side of the engine. With this structure, the high-temperature exhaust gas on the exhaust side is cooled by the heat exchanger, and the cooled exhaust gas flows back to the intake side of the engine, lowering the combustion temperature of the engine and eliminating _NO Suppress. In addition, the heat exchanger part of the EGR cooler is constructed by overlapping thin plates in a fin shape for reasons such as weight reduction, compactness, and cost reduction, and brazing with Ni-containing brazing material is mainly used for bonding and assembling these. .

In this way, since the heat exchanger parts of the exhaust heat recovery device and EGR cooler are bonded and assembled by soldering using Ni-containing brazing material, the materials used for these heat exchanger parts are required to have good solderability to Ni-containing brazing material. do. Additionally, since _the exhaust gas contains nitrogen oxides ( _NO For this reason, materials used in these heat exchanger parts are also required to have corrosion resistance. In particular, since the temperature is high during soldering heat treatment, Cr at the grain boundary reacts with C or N to form Cr carbonitride, and a Cr-deficient layer lacking corrosion resistance is created around it, which is important to ensure corrosion resistance by preventing so-called sensitization. need.

In light of the above, austenitic stainless steel such as SUS316L or SUS304L, which has reduced carbon content and prevented sensitization, has been typically used in the heat exchanger portion of the exhaust heat recovery device or EGR cooler. However, austenitic stainless steel is expensive because it contains a large amount of Ni, and because it has large thermal expansion, it has low fatigue characteristics in a use environment where it is constrained by violent vibration at high temperatures, that is, thermal fatigue characteristics at high temperatures. There was a problem with that.

Therefore, the use of steel other than austenitic stainless steel in the heat exchanger portion of the heat recovery device or EGR cooler is being considered.

For example, Patent Document 1 discloses a ferritic stainless steel that ensures corrosion resistance by forming an oxide film containing 16.0% or more of Nb as a cation fraction after brazing as a material for an exhaust heat recovery device or EGR cooler.

Patent Document 2 discloses a ferritic stainless steel that ensures corrosion resistance by controlling the addition amounts of Al, Ti, and Si as a material for an exhaust heat recovery device or EGR cooler.

Patent Document 3 discloses a ferritic stainless steel that ensures corrosion resistance by controlling the content of Cr, Si, and Al in the oxide film after brazing and the film thickness of the oxide film as a material for heat exchanger and fuel supply system members.

In addition, Patent Document 4 discloses a ferritic stainless steel that ensures solderability by adding components such as Cr, Cu, Al, and Ti according to a certain relational expression as a material for an EGR cooler, and by suppressing the amount of Al and Ti added. It has been disclosed.

In addition, Patent Document 5 discloses a ferritic stainless steel that ensures solderability by suppressing the addition amount of Al, Ti, and Zr as an EGR cooler member having a structure joined by Ni soldering.

Additionally, Patent Document 6 discloses a ferritic stainless steel for brazing that ensures solderability by suppressing the amount of Ti and Zr added.

Japanese Patent Publication No. 6157664 Japanese Patent Publication No. 6159775 Japanese Patent Publication No. 6270821 Japanese Patent Publication No. 2010-121208 Japanese Patent Publication No. 2009-174040 Japanese Patent Publication No. 2010-285683

However, in the techniques described in Patent Documents 1 to 6, solderability was sometimes insufficient depending on the brazing material used or soldering conditions. In particular, in the conventional technology, it was not possible to sufficiently secure solderability at high temperatures using a Ni-containing brazing material while obtaining the good corrosion resistance described above.

The present invention was developed in consideration of the above-described current situation, and its purpose is to provide a ferritic stainless steel that has good solderability when soldering at high temperatures using a Ni-containing brazing alloy and has excellent corrosion resistance after brazing. do.

In this specification, good solderability means heating a steel sheet coated with Ni-containing lead (JIS standard: BNi-5) on the surface for 10 minutes in a nitrogen carrier gas atmosphere of 1 Torr at 1170°C, and then cooling to room temperature. This indicates that after performing the soldering treatment, the ratio of the equivalent circular diameter of the brazing material after heating to the equivalent circular diameter of the brazing material before heating (diffusion rate of the brazing material) is 150% or more.

In addition, excellent corrosion resistance means that, using the steel sheet after brazing with Ni-containing lead as described above, a test piece measuring 20 mm by 20 mm is taken from the part where the brazing material is not attached, leaving a measurement surface of 11 mm by 11 mm. Covered with a sealant, the test piece is further immersed in a 3.5% NaCl aqueous solution at 30°C. Other than the concentration of the NaCl aqueous solution, the measured pitting potential Vc' 100 is 300 mV (vs SCE) or more in accordance with JIS G 0577. points to

The present inventors carefully studied the relationship between the constituent elements of various stainless steels and their solderability when soldering at high temperatures using a Ni-containing brazing alloy.

As a result, by suppressing the Al content in the stainless steel, adding an appropriate amount of Ni to the stainless steel, and further suppressing the Si and Mn contents appropriately relative to the Ni content, the wettability with the Ni-containing brazing alloy is improved. I discovered it.

In addition, the relationship between the constituent elements of various stainless steels and their corrosion resistance after brazing was carefully studied. As a result, it was found that the σ phase (sigma phase) precipitates due to high-temperature heat treatment of soldering, resulting in a decrease in corrosion resistance. Here, the σ phase is an intermetallic compound containing a large amount of Cr or Mo, and because a Cr and Mo deficiency layer is created around it, the corrosion resistance after brazing is reduced. As a result of further investigation, it was found that precipitation of the σ phase during high-temperature heat treatment of brazing can be suppressed by containing an appropriate amount of Ni and further suppressing the contents of Cr, Mo, Si, and Nb appropriately relative to the Ni content.

The present invention was completed after further examination based on the above-mentioned knowledge. That is, the main structure of the present invention is as follows.

[1] In mass%,

C: 0.003 to 0.030%,

Si: 0.01 to 1.00%,

Mn: 0.05 to 0.30%,

P: 0.050% or less,

S: 0.020% or less,

Cr: 24.0 to 30.0%,

Ni: 1.50 to 3.00%,

Mo: 1.00 to 3.00%,

Al: 0.001 to 0.020%,

Nb: 0.20 to 0.80%,

N: 0.030% or less

A ferritic stainless steel that contains and satisfies the following formulas (1) and (2) and has a composition in which the balance consists of Fe and inevitable impurities.

Ni-2(Si＋Mn) ≥ 0.00 ％··· (1)

Cr＋1.5 Mo＋Si＋1.5 Nb-2.5 Ni ≤ 25.0 ％··· (2)

(Ni, Si, Mn, Cr, Mo, and Nb in formulas (1) and (2) represent the content (% by mass) of each element.)

[2] Additionally, in mass%,

Cu: 0.01 to 1.00%,

Co: 0.01 to 1.00%,

W: 0.01 to 2.00%

The ferritic stainless steel according to [1] above, containing one or two or more types selected from among.

[3] Additionally, in mass%,

Ti: 0.01 to 0.10%,

V: 0.01 to 0.20%,

Zr: 0.01 to 0.10%,

Mg: 0.0005 to 0.0050%,

Ca: 0.0005 to 0.0050%,

B: 0.0005 to 0.0050%,

REM (rare earth metal): 0.001 to 0.100%,

Sn: 0.001 to 0.100%,

Sb: 0.001 to 0.100%

The ferritic stainless steel according to [1] or [2] containing one or two or more types selected from among.

[4] The ferritic stainless steel according to [1] or [2], for use in an exhaust heat recovery device or an exhaust gas recirculation device, in which one or more joints are assembled by soldering.

[5] The ferritic stainless steel according to [3], for use in an exhaust heat recovery device or an exhaust gas recirculation device, in which one or more joints are assembled by soldering.

According to the present invention, it is possible to obtain a ferritic stainless steel that has good solderability when soldering at high temperatures using a Ni-containing brazing material and is excellent in corrosion resistance after brazing.

Hereinafter, the present invention will be described in detail.

First, in the present invention, the reason why the component composition of steel is limited to the above range will be explained. In addition, the unit of content of elements in the component composition of steel is all "% by mass", but hereinafter, unless otherwise specified, it is simply expressed as "%".

C: 0.003 to 0.030%

As the C content increases, strength improves, and as it decreases, processability improves. Here, C needs to be contained in an amount of 0.003% or more to obtain sufficient strength. However, when the C content exceeds 0.030%, the workability decreases significantly, Cr carbide precipitates at the grain boundaries, causes sensitization, and corrosion resistance decreases. Therefore, the C content is set in the range of 0.003 to 0.030%. The C content is preferably 0.004% or more. Moreover, the C content is preferably 0.025% or less, more preferably 0.020% or less, and still more preferably 0.010% or less.

Si: 0.01 to 1.00%

Si is an element useful as a deoxidizing agent. The effect is obtained by containing 0.01% or more of Si. However, if the Si content exceeds 1.00%, Si concentrates such as Si oxide and Si nitride are formed on the surface of the steel sheet during brazing heat treatment, and solderability deteriorates. In addition, during brazing at high temperatures using a Ni-containing brazing alloy, the σ phase precipitates and corrosion resistance deteriorates. Therefore, the Si content is set in the range of 0.01 to 1.00%. The Si content is preferably 0.50% or more, more preferably 0.60% or more, and still more preferably 0.70% or more. Moreover, the Si content is preferably 0.85% or less, and more preferably 0.80% or less.

Mn: 0.05 to 0.30%

Mn has a deoxidizing effect, and this effect is obtained when Mn is contained in an amount of 0.05% or more. However, if the Mn content exceeds 0.30%, Mn concentrate is formed on the surface of the steel sheet during brazing heat treatment, and solderability deteriorates. Therefore, the Mn content is set in the range of 0.05 to 0.30%. The Mn content is preferably 0.10% or more. Moreover, the Mn content is preferably 0.25% or less, more preferably 0.20% or less, and even more preferably 0.15% or less.

P: 0.050% or less

P is an element that is inevitably contained in steel, and excessive inclusion easily causes intergranular corrosion. This tendency becomes noticeable when the P content exceeds 0.050%. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.040% or less, and more preferably 0.030% or less. Additionally, the lower limit of the P content is not particularly limited. However, since excessive removal of P causes an increase in cost, the P content is preferably 0.005% or more.

S: 0.020% or less

S is an element that is inevitably contained in steel, and inclusion of more than 0.020% of S promotes precipitation of MnS and reduces corrosion resistance. Therefore, the S content is set to 0.020% or less. Preferably, the S content is 0.010% or less. Additionally, the lower limit of the S content is not particularly limited. However, since excessive removal of S causes an increase in cost, the S content is preferably 0.0005% or more.

Cr: 24.0 to 30.0%

Cr is an important element for ensuring the corrosion resistance of stainless steel. If the Cr content is less than 24.0%, sufficient corrosion resistance cannot be obtained. On the other hand, when the Cr content exceeds 30.0%, the σ phase precipitates during brazing, and corrosion resistance decreases. Therefore, the Cr content is set in the range of 24.0 to 30.0%. The Cr content is preferably 24.5% or more, and more preferably 25.0% or more. Moreover, the Cr content is preferably 28.0% or less, and more preferably 26.5% or less.

Ni: 1.50 to 3.00%

Ni is one of the important elements in the present invention. The inclusion of 1.50% or more Ni improves solderability with a Ni-containing brazing material. Although the mechanism by which Ni solderability is improved by containing Ni is not clear, it is thought that when an appropriate amount of Ni is contained in the base material, wettability is improved due to interaction with Ni contained in the brazing material. Additionally, precipitation of the σ phase during soldering can be suppressed. However, when the Ni content exceeds 3.00%, stress corrosion cracking susceptibility increases. Therefore, the Ni content is set in the range of 1.50 to 3.00%. The Ni content is preferably 1.75% or more, and more preferably 2.00% or more. Moreover, the Ni content is preferably 2.75% or less, and more preferably 2.50% or less.

Mo: 1.00 to 3.00%

Mo stabilizes the passivation film of stainless steel and improves corrosion resistance. This effect is obtained when the Mo content is 1.00% or more. However, when the Mo content exceeds 3.00%, the σ phase precipitates during brazing, and corrosion resistance decreases. Therefore, the Mo content is set in the range of 1.00 to 3.00%. The Mo content is preferably 1.25% or more, and more preferably 1.50% or more. Moreover, the Mo content is preferably 2.50% or less, and more preferably 2.00% or less.

Al: 0.001 to 0.020%

Al is an element useful for deoxidation, and its effect is obtained when Al is contained in an amount of 0.001% or more. However, if the Al content exceeds 0.020%, Al concentrates such as Al oxide and Al nitride are generated on the surface of the steel during soldering, and the wet spreadability and adhesion of the brazing material are reduced, making soldering difficult. Therefore, the Al content is set in the range of 0.001 to 0.020%. Preferably, the Al content is 0.015% or less.

Nb: 0.20 to 0.80%

Nb is an element that suppresses a decrease in corrosion resistance (sensitization) due to precipitation of Cr carbonitride by combining with C and N. This effect is obtained when the Nb content is 0.20% or more. On the other hand, when the Nb content exceeds 0.80%, the σ phase precipitates during soldering, and corrosion resistance decreases. Therefore, the Nb content is set in the range of 0.20 to 0.80%. The Nb content is preferably 0.25% or more, and more preferably 0.30% or more. Moreover, the Nb content is preferably 0.60% or less, and more preferably 0.35% or less.

N: 0.030% or less

When the N content exceeds 0.030%, corrosion resistance and processability decrease. Therefore, the N content is set to 0.030% or less. Preferably, the N content is 0.025% or less. More preferably, the N content is 0.020% or less. Additionally, there is no particular limitation on the lower limit of the N content, but since excessive reduction of the N content leads to an increase in cost, the N content is preferably set to 0.003% or more.

Ni-2(Si＋Mn) ≥ 0.00 ％··· (1)

Ni, Si, and Mn in formula (1) represent the content (% by mass) of each element.

In the present invention, each of Ni, Si, and Mn is set to a predetermined content in order to improve solderability. Additionally, the present inventors conducted intensive studies and found that if Ni-2(Si+Mn) (Ni content minus twice the sum of Si content and Mn content) is less than 0.00%, the desired solderability cannot be obtained. For this reason, Ni improves solderability, but on the other hand, Si and Mn impair solderability, so it is thought that the balance of these elements has a great influence on solderability. Therefore, in the present invention, each of the Ni content, Si content, and Mn content is set to the above-mentioned ranges, and then Ni-2 (Si+Mn) is set to 0.00% or more. Ni-2(Si+Mn) is preferably 0.50% or more. In particular, better solderability is obtained by setting the Cr content to less than 26.0%, the Al content to 0.015% or less, and Ni-2 (Si+Mn) to 0.50% or more.

Cr＋1.5 Mo＋Si＋1.5 Nb-2.5 Ni ≤ 25.0 ％··· (2)

Cr, Mo, Si, Nb, and Ni in formula (2) represent the content (% by mass) of each element.

In the present invention, each of Cr, Mo, Si, Nb, and Ni is set to a predetermined content in order to suppress precipitation of the σ phase during soldering. Furthermore, the present inventors conducted intensive studies and found that when Cr+1.5 Mo+Si+1.5 Nb-2.5 Ni is greater than 25.0%, the σ phase precipitates during soldering, and corrosion resistance deteriorates. For this reason, Ni suppresses precipitation of the σ phase, but on the other hand, Cr, Mo, Si, and Nb promote precipitation of the σ phase, so it is thought that the balance of these elements has a large influence on precipitation of the σ phase. Therefore, in the present invention, each of the Cr, Mo, Si, Nb and Ni contents is set to the above-mentioned range, and then Cr+1.5 Mo+Si+1.5 Nb-2.5 Ni is set to 25.0% or less.

Above, the basic components (essential components) in the ferritic stainless steel of the present invention have been explained. In the component composition in the present invention, components (remainder) other than the above are Fe and inevitable impurities.

The ferritic stainless steel of the present invention may further contain one or two or more types selected from Cu, Co, and W, respectively, in the following ranges.

Cu: 0.01 to 1.00%

Cu is an element that improves corrosion resistance. This effect is obtained when the Cu content is 0.01% or more. However, when the Cu content exceeds 1.00%, hot workability decreases. Therefore, when it contains Cu, the Cu content is set in the range of 0.01 to 1.00%. When it contains Cu, the Cu content is more preferably 0.10% or more. Moreover, when it contains Cu, the Cu content is more preferably 0.80% or less, and even more preferably 0.60% or less.

Co: 0.01 to 1.00%

Co is an element that improves corrosion resistance. This effect is obtained when the Co content is 0.01% or more. However, when the Co content exceeds 1.00%, processability decreases. Therefore, when it contains Co, the Co content is set in the range of 0.01 to 1.00%. When it contains Co, the Co content is more preferably 0.05% or more. Moreover, when it contains Co, the Co content is more preferably 0.70% or less.

W: 0.01 to 2.00%

W is an element that improves corrosion resistance. This effect is obtained when the W content is 0.01% or more. However, when the W content exceeds 2.00%, the σ phase precipitates during soldering. Therefore, when it contains W, the W content is set in the range of 0.01 to 2.00%. When it contains W, the W content is more preferably 0.05% or more. Moreover, when W is contained, the W content is more preferably 1.00% or less.

The ferritic stainless steel of the present invention may further contain one or two or more types selected from Ti, V, Zr, Mg, Ca, B, REM, Sn, and Sb, respectively, in the following ranges.

Ti: 0.01 to 0.10%

Ti has the effect of preventing sensitization by combining with C and N contained in steel. The effect is obtained by containing 0.01% or more of Ti. On the other hand, Ti is an active element with respect to oxygen, and the inclusion of more than 0.10% of Ti causes a Ti oxide film to be formed on the surface of the steel during soldering, thereby lowering the solderability. Therefore, when it contains Ti, the Ti content is set in the range of 0.01 to 0.10%. When it contains Ti, the Ti content is more preferably 0.05% or less.

V: 0.01 to 0.20%

V, like Ti, combines with C and N contained in steel to prevent sensitization. These effects are obtained when the V content is 0.01% or more. On the other hand, when the V content exceeds 0.20%, processability decreases. Therefore, when it contains V, the V content is set in the range of 0.01 to 0.20%. When it contains V, the V content is more preferably 0.15% or less, and even more preferably 0.10% or less.

Zr: 0.01 to 0.10%

Zr, like Ti and Nb, is an element that binds to C and N contained in steel and suppresses sensitization. This effect is obtained when the Zr content is 0.01% or more. On the other hand, when the Zr content exceeds 0.10%, processability decreases. Therefore, when containing Zr, the Zr content is set in the range of 0.01 to 0.10%. When containing Zr, the Zr content is more preferably 0.03% or more. Moreover, when containing Zr, the Zr content is more preferably 0.05% or less.

Mg: 0.0005 to 0.0050%

Mg acts as a deoxidizing agent. This effect is obtained when the Mg content is 0.0005% or more. However, when the Mg content exceeds 0.0050%, the toughness of the steel decreases and manufacturability decreases. Therefore, when it contains Mg, the Mg content is set in the range of 0.0005 to 0.0050%. When it contains Mg, the Mg content is more preferably 0.0020% or less.

Ca: 0.0005 to 0.0050%

Ca improves weldability by improving the penetration of the weld zone. The effect is obtained when the Ca content is 0.0005% or more. However, if the Ca content exceeds 0.0050%, it combines with S to produce CaS, and corrosion resistance decreases. Therefore, when it contains Ca, the Ca content is set in the range of 0.0005 to 0.0050%. When it contains Ca, the Ca content is more preferably 0.0010% or more. Moreover, when it contains Ca, the Ca content is more preferably 0.0040% or less.

B: 0.0005 to 0.0050%

B is an element that improves secondary processing brittleness. The effect is expressed when the B content is 0.0005% or more. However, when the B content exceeds 0.0050%, ductility decreases due to solid solution strengthening. Therefore, when it contains B, the B content is set in the range of 0.0005 to 0.0050%.

REM (rare earth metal): 0.001 to 0.100%

REM (rare earth metals: elements with atomic numbers 57 to 71 such as La, Ce, and Nd) are elements effective in deoxidation. The effect is obtained when the REM content is 0.001% or more. However, when the REM content exceeds 0.100%, hot workability decreases. Therefore, when containing REM, the REM content is set in the range of 0.001 to 0.100%. When REM is contained, the REM content is more preferably 0.010% or more. Moreover, when REM is contained, the REM content is more preferably 0.050% or less. In addition, REM is a general term for Sc, Y, and 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content referred to here is the total content of these elements.

Sn: 0.001 to 0.100%

Sn is an element effective in suppressing processing surface roughness. The effect is obtained when the Sn content is 0.001% or more. However, when the Sn content exceeds 0.100%, hot workability decreases. Therefore, when it contains Sn, the Sn content is set in the range of 0.001 to 0.100%. When containing Sn, more preferably, the Sn content is 0.050% or less.

Sb: 0.001 to 0.100%

Sb, like Sn, is an element effective in suppressing processing surface roughness. The effect is obtained when the Sb content is 0.001% or more. However, when the Sb content exceeds 0.100%, processability decreases. Therefore, when containing Sb, the Sb content is set in the range of 0.001 to 0.100%. When containing Sb, more preferably, the Sb content is 0.050% or less.

Next, a preferred manufacturing method for the ferritic stainless steel of the present invention will be described.

The manufacturing method of the ferritic stainless steel of the present invention is not particularly limited, but for example, the steel is melted in a melting furnace such as a converter or electric furnace, or additionally subjected to secondary refining such as blow refining or vacuum refining. Through this process, a steel having the composition of the present invention described above is obtained. Afterwards, steel pieces (steel slabs) are formed by continuous casting or ingot-pulverized rolling, and the steel slabs are hot-rolled to form hot-rolled sheets. Hot-rolled sheets may be annealed as needed to form hot-rolled annealed sheets. do. Thereafter, cold rolling is performed on the hot-rolled sheet or hot-rolled annealed sheet to obtain a cold-rolled sheet with a desired sheet thickness. Additionally, if necessary, cold-rolled sheet annealing may be performed on the cold-rolled sheet to obtain a cold-rolled annealed sheet.

In addition, the conditions for hot rolling, cold rolling, hot-rolled sheet annealing, cold-rolled sheet annealing, etc. are not particularly limited, and conventional methods may be followed.

In the steelmaking process of melting steel, it is preferable to secondaryly refine the steel melted in a converter or electric furnace by a VOD (Vacuum Oxygen Decarburization) method, etc. to obtain a steel containing the above-mentioned essential components and components added as necessary. . The melted molten steel can be used as a steel material (steel slab) by a known method, but continuous casting is preferable in terms of productivity and quality. The steel material is then preferably heated at 1050 to 1250°C, and hot-rolled to form a hot-rolled sheet with the desired thickness. Of course, hot working can be done on other than sheet materials. The hot-rolled sheet is preferably continuously annealed at a temperature of 900 to 1150°C as needed, and then descaled by pickling or the like to obtain a hot-rolled product. Additionally, if necessary, scale may be removed by shot blasting, grinding brush, etc. before pickling.

In addition, the above-described hot-rolled product (hot-rolled annealed plate, etc.) may be converted into a cold-rolled product through a process such as cold rolling. The cold rolling in this case may be performed once, but from the viewpoint of productivity and required quality, it may be performed twice or more with intermediate annealing in between. The total reduction ratio of one or two rounds of cold rolling is preferably 60% or more, and more preferably 70% or more. The cold rolled steel sheet is then preferably subjected to continuous annealing (finish annealing) at a temperature of 900 to 1150°C, more preferably 950 to 1150°C, and is then pickled to obtain a cold rolled product. Additionally, continuous annealing may be performed as bright annealing and pickling may be omitted. Additionally, depending on the application, the shape, surface roughness, and material of the steel sheet may be adjusted by performing skin pass rolling or the like after final annealing.

The ferritic stainless steel of the present invention described above is preferably used in an exhaust heat recovery device or an exhaust gas recirculation device in which one or more joints are assembled by soldering. In particular, it is preferably used in the heat exchanger member of the exhaust heat recovery device or the exhaust gas recirculation device.

Example

Steel having the component composition shown in Table 1 was melted in a vacuum melting furnace, heated at 1150°C for 1 hour, and then hot rolled to produce a hot rolled sheet with a thickness of 4.0 mm. After annealing the hot-rolled sheet by holding it at 1080°C for 1 minute, the surface was subjected to grinding to remove scale and cold-rolled to a sheet thickness of 1.0 mm. The surface of the cold rolled annealed plate obtained by performing final annealing by holding at 1040°C for 1 minute in an ammonia decomposition gas atmosphere was polished up to 600 times with emery polishing paper, degreased with acetone, and submitted to the test.

This cold rolled annealed plate was subjected to brazing using a Ni-containing brazing alloy as follows, (1) solderability was evaluated, and the cold rolled annealed plate after the brazing treatment was subjected to (2) measurement of the amount of precipitation of the σ phase and (2) 3) Corrosion resistance was evaluated, and the results are shown in Table 2.

(1) Evaluation of solderability

From the manufactured cold rolled annealed plate, a test piece with a width of 50 mm and a length of 50 mm was cut, and Ni-containing lead (JIS standard: BNi-5) with a diameter of 10 mm and a thickness of 1 mm was applied to the surface of the horizontal test piece, and then , a test piece coated with Ni-containing lead was placed horizontally with the side coated with the brazing material facing up, and was heated at 1170°C in a nitrogen carrier gas atmosphere of 1 Torr for 10 minutes, and then cooled to room temperature, followed by brazing. After that, the equivalent circle diameter of the brazing material on the surface of the test piece (equivalent circular diameter of the brazing material after heating) was measured. Then, the ratio of the equivalent circle diameter of the brazing material after heating (the diffusion rate of the brazing material) to the diameter of the brazing material before heating (10 mm, the equivalent circle diameter is also the same) was determined and evaluated based on the following criteria.

Diffusion rate of the brazing material after heating relative to before heating = (circular equivalent diameter of the brazing material after heating/diameter of the brazing material before heating (10 mm)) × 100 (%)

◎ (Pass, especially good): 160% or more

○ (Pass): 150% or more but less than 160%

× (Failed): Less than 150%

(2) Measurement of σ phase precipitation amount

Using a test piece of each cold-rolled annealing plate after brazing treatment, a sample for L cross-section observation was taken from the part where the brazing material was not attached, electrolytic polishing and aqua regia etching were performed, and then, within the field of view observed at 500x with an optical microscope. The precipitation amount (area %) of the σ phase was measured by the point count method in accordance with ASTM E562. Additionally, the observation position was at the center of the plate thickness of the sample.

○: 1.0% or less

×: More than 1.0%

(3) Evaluation of corrosion resistance

Using a test piece of each cold rolled annealed plate after brazing treatment, a test piece measuring 20 mm by 20 mm is taken from the part where the brazing material is not attached, a measurement surface of 11 mm by 11 mm is left on this test piece, and a sealant made of silicone resin is applied. It was covered with. Next, this test piece was immersed in a 3.5% NaCl aqueous solution at 30°C, and pitting potential was measured in accordance with JIS G 0577 except for the concentration of the NaCl aqueous solution. After maintaining the natural potential for 10 minutes, the potential when the current density becomes 100 μA/cm2 using the electropotential method with a potential sweep rate of 20 mV/min is set as the official potential Vc' 100, and the value is shown in Table 2. In addition, considering the usage conditions of the heat exchanger part of the heat recovery machine or EGR cooler, if the formal potential Vc' 100 is more than 300 mV (vs SCE) equivalent to SUS304L, which has a proven track record in the heat exchanger part of the heat recovery machine or EGR cooler, the corrosion resistance is It can be judged as excellent.

○ (Pass): 300 mV (vs SCE) or more

× (Fail): Less than 300 mV (vs SCE)

From Table 2, good solderability and excellent corrosion resistance were obtained in all of Invention Examples No. 1 to 27. In particular, No.6, 9, 10, 12, 13, 16, 18, 19 and 21, which have a Cr content of less than 26.0%, an Al content of 0.015% or less, and Ni-2 (Si+Mn) ≥ 0.50%, have particularly good solderability. indicated.

In contrast, in Comparative Examples No. 28 to 38, in which the component composition was outside the appropriate range, good solderability and excellent corrosion resistance could not be satisfied at the same time.

More specifically, in Comparative Example No. 28 (steel symbol B1), since the Cr content exceeded the upper limit of the present invention, more than 1.0% of the σ phase precipitated in the microstructure after brazing, and excellent corrosion resistance could not be obtained.

In Comparative Example No. 29 (steel symbol B2), since the Mo content exceeded the upper limit of the present invention, more than 1.0% of the σ phase precipitated in the microstructure after brazing, and excellent corrosion resistance could not be obtained.

In Comparative Example No. 30 (strong symbol B3), good solderability could not be obtained because the Al content exceeded the upper limit of the present invention.

In Comparative Example No. 31 (strong symbol B4), good solderability could not be obtained because the Si content exceeded the upper limit of the present invention. Additionally, more than 1.0% of the σ phase precipitated in the microstructure after soldering, making it impossible to obtain excellent corrosion resistance.

In Comparative Example No. 32 (strong symbol B5), the Mn content exceeded the upper limit of the present invention, and good solderability could not be obtained. Additionally, excellent corrosion resistance could not be obtained.

In Comparative Example No. 33 (steel symbol B6), since the Nb content exceeded the upper limit of the present invention, more than 1.0% of the σ phase precipitated in the microstructure after brazing, and excellent corrosion resistance could not be obtained.

In Comparative Example No. 34 (steel symbol B7), excellent corrosion resistance could not be obtained because the Cr content was below the lower limit of the present invention.

In Comparative Example No. 35 (steel symbol B8), excellent corrosion resistance could not be obtained because the Mo content was below the lower limit of the present invention.

In Comparative Example No. 36 (steel symbol B9), good solderability could not be obtained because the Ni content was below the lower limit of the present invention. Additionally, more than 1.0% of the σ phase precipitated in the microstructure after soldering, making it impossible to obtain excellent corrosion resistance.

In Comparative Example No. 37 (steel symbol B10), all components were within the specified range, but the formula (2) was not satisfied, the σ phase precipitated in excess of 1.0%, and excellent corrosion resistance could not be obtained.

In Comparative Example No. 38 (strong symbol B11), all components were within the specified range, but equation (1) was not satisfied, and good solderability could not be obtained.

According to the present invention, ferritic stainless steel suitable for use in exhaust gas recirculation devices such as heat exchangers of exhaust heat recoverers and EGR coolers assembled by soldering is obtained, so it is very useful industrially.

Claims (5)

In mass%,
C: 0.003 to 0.030%,
Si: 0.01 to 1.00%,
Mn: 0.05 to 0.30%,
P: 0.050% or less,
S: 0.020% or less,
Cr: 24.0 to 30.0%,
Ni: 1.50 to 3.00%,
Mo: 1.00 to 3.00%,
Al: 0.001 to 0.020%,
Nb: 0.20 to 0.80%,
N: 0.030% or less
A ferritic stainless steel that contains and satisfies the following formulas (1) and (2) and has a composition in which the balance consists of Fe and inevitable impurities.
Ni-2(Si＋Mn) ≥ 0.00 ％··· (1)
Cr＋1.5 Mo＋Si＋1.5 Nb-2.5 Ni ≤ 25.0 ％··· (2)
(Ni, Si, Mn, Cr, Mo, and Nb in formulas (1) and (2) represent the content (% by mass) of each element.) According to claim 1,
Additionally, in mass%,
Cu: 0.01 to 1.00%,
Co: 0.01 to 1.00%,
W: 0.01 to 2.00%
Ferritic stainless steel containing one or two or more selected from among. The method of claim 1 or 2,
Additionally, in mass%,
Ti: 0.01 to 0.10%,
V: 0.01 to 0.20%,
Zr: 0.01 to 0.10%,
Mg: 0.0005 to 0.0050%,
Ca: 0.0005 to 0.0050%,
B: 0.0005 to 0.0050%,
REM (rare earth metal): 0.001 to 0.100%,
Sn: 0.001 to 0.100%,
Sb: 0.001 to 0.100%
Ferritic stainless steel containing one or two or more selected from among. The method of claim 1 or 2,
Ferritic stainless steel for use in waste heat recoverers or exhaust gas recirculation devices, in which one or more joints are assembled by soldering. According to claim 3,
Ferritic stainless steel for use in waste heat recoverers or exhaust gas recirculation devices, in which one or more joints are assembled by soldering.