WO2016068291A1 - Ferrite-based stainless steel with high resistance to corrosiveness caused by exhaust gas and condensation and high brazing properties and method for manufacturing same - Google Patents

Abstract

This ferrite-based stainless steel contains 0.001-0.030% mass% of C, 0.01-1.00 mass% of Si, 0.01-2.00% mass% of Mn, 0.050 mass% of P or less, 0.0100 mass% of S or less, 11.0-30.0 mass% of Cr, 0.01-3.00 mass% of Mo, 0.001-0.050 mass% of Ti, 0.001-0.030 mass% of Al, 0.010-1.000 mass% of Nb, and 0.050 mass% of N or less, and the remaining portion comprises Fe and inevitable impurities. Moreover, the amounts (mass%) of Al, Ti, and Si satisfy Al/Ti ≥ 8.4Si - 0.78.

Description

Ferritic stainless steel with excellent resistance to exhaust gas condensate corrosion and brazing, and method for producing the same

The present invention relates to a ferritic stainless steel used in an exhaust gas condensed water environment and a method for producing the same. Examples of members exposed to the environment of exhaust gas condensate include exhaust gas recirculation devices such as automobile mufflers, exhaust heat recovery units, and EGR (Exhaust Gas Recirculation) coolers.
This application claims priority based on Japanese Patent Application No. 2014-222201 filed in Japan on October 31, 2014 and Japanese Patent Application No. 2015-210741 filed in Japan on October 27, 2015. The contents are incorporated here.

In recent years, in the automobile field, since each component contained in exhaust gas causes air pollution and environmental pollution, regulations are being strengthened. Therefore, for the purpose of reducing CO ₂ emissions and improving fuel efficiency of automobiles, high efficiency combustion, improvement of engine efficiency by idling stop, etc., weight reduction by material replacement, hybrid vehicles (HEV), biofuel vehicles, hydrogen / There is a need for improvement by diversifying the energy of fuel cell vehicles (FCV), electric vehicles (EV) and the like.

Among them, efforts are being made to improve fuel consumption by installing a heat exchanger that collects exhaust heat, so-called exhaust heat recovery, mainly for hybrid vehicles. In the exhaust heat recovery device, exhaust gas heat is transferred to cooling water by heat exchange, and heat energy is recovered and reused to raise the temperature of the cooling water. As a result, the heating performance in the passenger compartment is improved and the warm-up time of the engine is shortened to improve the fuel consumption performance. The exhaust heat recovery unit is also called an exhaust heat recirculation system.

Also, efforts are being made to install exhaust gas recirculation devices that recirculate exhaust gases. An example of the exhaust gas recirculation device is an EGR cooler. In the EGR cooler, the exhaust gas of the engine is cooled by engine cooling water or air, and then the cooled exhaust gas is returned to the intake side and recombusted. Thereby, combustion temperature is lowered and NOx which is harmful gas is reduced.

The heat exchange part of such an exhaust heat recovery unit or EGR cooler is required to have good thermal efficiency, good thermal conductivity, and excellent corrosion resistance against exhaust gas condensate because it is in contact with exhaust gas. . In particular, since engine coolant flows through these parts, there is a risk of serious accidents when holes are formed due to corrosion. Further, the material used is thin in order to increase the heat exchange efficiency. For this reason, the material which has the corrosion resistance superior to the exhaust system downstream member is calculated | required.

Conventionally, ferritic stainless steels containing 17% or more of Cr, such as SUS430LX, SUS436J1L, and SUS436L, are used in the exhaust system downstream members mainly composed of a muffler, where corrosion resistance is particularly required. Corrosion resistance equivalent to or higher than these ferritic stainless steels is required for the materials of the exhaust heat recovery device and the EGR cooler.

In addition, the EGR cooler is generally assembled by brazing and a high brazing property is required for the parts to be used. Here, the wettability of the surface is important in order to improve the brazing property. Ti is more easily oxidized than Fe and Cr, and forms an oxide film with low wettability on the surface. For this reason, it is desirable to lower the Ti content. Further, like Ti, Al forms an oxide film with low wettability on the surface. Recently, there is a demand not only for Ti but also for steel types having a low Al content. In addition, since the surface roughness of the steel plate greatly affects the wettability, it is also very important to control the surface properties by controlling the manufacturing conditions.

Also, the brazing heat treatment temperature is about 1200 ° C. when high, and in such a high temperature environment, the crystal grains of stainless steel grow and become coarse. Since coarsening of crystal grains affects mechanical properties such as thermal fatigue, stainless steel subjected to brazing heat treatment is required to have characteristics that make crystal grains difficult to coarsen even at high temperatures.

As described above, the steel types used for the EGR cooler are required to have high corrosion resistance and good brazing properties.

Patent Document 1 discloses an inexpensive ferritic stainless steel material that has excellent corrosion resistance and is used as a muffler constituent member and a hot water device member that forms a weld. This ferritic stainless steel material has C: 0.025% or less, Si: 2% or less, Mn: 1% or less, P: 0.045% or less, S: 0.01% or less, Cr: 16 to 25%, Al: less than 0.04% and N: not more than 0.025%, further Ni: 1% or less, Cu: 1% or less, Mo: less than 1%, Nb: 0.5% or less, Ti: 0 .4% or less, and V: one or more selected from 0.5% or less, with the balance being Fe and inevitable impurities. On the surface, the composition of the outermost layer measured by XPS (X-ray photoelectron spectroscopy) is an oxide film in which the total proportion of Si and Cr is 15 to 40 atomic% and Fe is 5 atomic% or less in the atomic ratio including oxygen Have

Patent Document 2 discloses a ferritic stainless steel having excellent brazing properties when brazing in an atmosphere of high temperature and low oxygen partial pressure, such as Ni brazing and Cu brazing. . This ferritic stainless steel has C: 0.03% or less, N: 0.05% or less, C + N: 0.015% or more, Si: 0.02 to 1.5%, Mn: 0.02 to 2% Cr: 10 to 22%, Nb: 0.03 to 1%, and Al: 0.5% or less, with the balance being Fe and inevitable impurities. Further, it contains Ti in an amount satisfying the formula: Ti-3N ≦ 0.03 and the formula: 10 (Ti-3N) + Al ≦ 0.5, or, in addition to a part of Fe, Mo: 3 %: Ni: 3% or less, Cu: 3% or less, V: 3% or less, W: 5% or less, Ca: 0.002% or less, Mg: 0.002% or less, and B: 0.005% Any one or more of the following is included.

Patent Document 3 describes further resistance to initial rust without impairing the original function as an automobile exhaust system member, such as high temperature strength, scale peel resistance, moldability, corrosion resistance against exhaust gas condensed water, and corrosion resistance against salt damage environments. A ferritic stainless steel for automobile exhaust system members that has been satisfied at as low a cost as possible is disclosed. This ferritic stainless steel is, by mass, C: ≦ 0.0100%, Si: 0.05-0.80%, Mn: ≦ 0.8%, P: ≦ 0.050%, S: ≦ 0. .0030%, Cr: 11.5 to 13.5%, Ti: 0.05 to 0.50%, Al: ≦ 0.100%, and N: ≦ 0.02%, with the balance being Fe and Consists of inevitable impurities. The number of inclusions containing Ca per 1 mm ^{2 of an} arbitrary cross section is less than 10, preferably further, the number ratio of Mn sulfide to the total number of Ti sulfide and Mn sulfide is 50% or less.

Patent Document 4 discloses a ferritic stainless steel having excellent local corrosion resistance. This ferritic stainless steel is, by mass, C: 0.030% or less, N: 0.030% or less, Si: 0.30% or less, Mn: 0.30% or less, P: 0.040% or less S: 0.020% or less, Cr: 16 to 26%, Al: 0.015 to 0.5%, Ti: 0.05 to 0.50%, Nb: 0.05 to 0.50%, and Mo: 0.5 to 3.0% is contained with the balance being Fe and inevitable impurities. When the ratio of the Al content to the Si content is Al / Si, the following formula (1) is satisfied.
Al / Si ≧ 0.10 (1)

Patent Document 5 discloses a ferritic stainless steel having excellent corrosion resistance. This ferritic stainless steel is, by mass, C: 0.030% or less, N: 0.030% or less, Si: 0.01 to 0.50%, Mn: 1.5% or less, P: 0.00. 04% or less, S: 0.01% or less, Cr: 12 to 25%, Nb: 0.01 to 1.0%, V: 0.010 to 0.50%, Ti: 0.60% or less, and Al: 0.80% or less is contained, and the balance consists of Fe and inevitable impurities. The following formula (A) is satisfied, the surface has an arithmetic mean roughness Ra of 0.35 to 5.0 μm, and the surface color difference L ^* value is 70 or more.
0.35 ≦ Nb + 5V ≦ 2.0 Formula (A)

However, the inventions disclosed in Patent Documents 1 to 5 cannot satisfy the corrosion resistance and brazing performance against exhaust gas condensed water at the same time.

JP 2009-197293 A JP 2009-174046 A JP 2004-323907 A JP 2010-248625 A Japanese Patent Laying-Open No. 2015-145531

The present invention relates to a ferritic stainless steel having excellent exhaust gas condensate corrosion resistance (corrosion resistance against exhaust gas condensate) and brazing in an environment used for automobile mufflers, exhaust heat recovery devices, EGR coolers, and the like, and its production It aims to provide a method.

The gist of the present invention aimed at solving the above problems is as follows.
(1) In mass%,
C: 0.001 to 0.030%,
Si: 0.01 to 1.00%,
Mn: 0.01 to 2.00%
P: 0.050% or less,
S: 0.0100% or less,
Cr: 11.0-30.0%,
Mo: 0.01 to 3.00%,
Ti: 0.001 to 0.050%,
Al: 0.001 to 0.030%,
Nb: 0.010 to 1.000%, and N: 0.050% or less,
The remainder consists of Fe and inevitable impurities, and the above-mentioned Al content, Ti content and Si content (mass%) satisfy Al / Ti ≧ 8.4Si-0.78 Ferritic stainless steel with excellent brazing properties.
(2) Furthermore, in mass%,
Ni: 0.01 to 3.00%,
Cu: 0.050 to 1.500%,
W: 0.010 to 1.000%,
V: 0.010-0.300%
Sn: 0.005 to 0.500%,
Sb: 0.0050 to 0.5000%, and Mg: 0.0001 to 0.0030%
Ferritic stainless steel excellent in exhaust gas condensate corrosion resistance and brazing properties according to (1), characterized in that it contains any one or more of them.
(3) Furthermore, in mass%,
B: 0.0002 to 0.0030%,
Ca: 0.0002 to 0.0100%,
Zr: 0.010 to 0.300%,
Co: 0.010 to 0.300%,
Ga: 0.0001 to 0.0100%,
Ta: 0.0001 to 0.0100%, and REM: 0.001 to 0.200%
The ferritic stainless steel excellent in exhaust gas condensate corrosion resistance and brazing properties according to (1) or (2), characterized by containing any one or more of them.
(4) The rolling direction is the L direction, the direction perpendicular to the rolling direction is the C direction, and the direction inclined 45 ° with respect to the rolling direction is the V direction, and the arithmetic average roughness of the steel surface in each direction is Ra _L , Ra _C , Ra _V (unit: μm), (Ra _L + Ra _C + 2Ra _V ) /4≦0.50 and | (Ra _L + Ra _C −2Ra _V ) /2|≦0.10 (1) The ferritic stainless steel excellent in corrosion resistance to condensed water corrosion and brazing as described in any one of (1) to (3).
(5) The amount of change in crystal grain size number GSN before and after heat treatment at 1150 ° C. for 10 minutes in a vacuum atmosphere of 50 Pa or less is 5.0 or less, according to (1) to (4) Ferritic stainless steel excellent in exhaust gas condensate corrosion resistance and brazing properties according to any one of the items.
(6) Exhaust gas condensate corrosion resistance according to any one of (1) to (5) used for automobile mufflers, exhaust heat recovery devices, or EGR coolers that are automobile parts exposed to the exhaust gas condensate environment Ferritic stainless steel with excellent solderability.
(7) It includes a step of cold rolling the steel having the chemical component according to any one of (1) to (3), and in the cold rolling step, the roll roughness is determined in the final pass. # 60 or more rolls, rolling at a final pass reduction rate of 15.0% or less and a final pass cold rolling speed of 800 m / min or less, and exhaust gas condensate corrosion resistance, A method for producing ferritic stainless steel with excellent brazeability.
(8) The method further includes a step of annealing the steel sheet after the cold rolling, the annealing step including a step of retaining the steel plate at 650 to 950 ° C. for 5.0 s or more, and a step of 950 to 1050 ° C. The method for producing a ferritic stainless steel having excellent exhaust gas condensate corrosion resistance and brazing properties according to (7), comprising a step of staying at 80.0 s or less.

According to the present invention, the ferrite having excellent exhaust gas condensate corrosion resistance and brazing when used in automobile parts exposed to the exhaust gas condensate environment such as an automobile muffler, exhaust heat recovery device or EGR cooler. Stainless steel can be provided.

FIG. 1 is a diagram showing the relationship between the Si, Al, and Ti contents in the steel sheet and the condensed water corrosion test results.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

In order to improve brazeability, the present inventors produced steels with reduced Al content and Ti content up to various concentrations under various cold rolling conditions and cold rolled sheet annealing conditions. Then, corrosion resistance, brazing property, surface roughness, and changes in crystal grain size before and after brazing heat treatment were examined. As a result, the brazing property is improved by reducing the Al concentration or Ti concentration in the steel. However, regarding the improvement of the corrosiveness to the exhaust gas condensate, the effect of simply reducing the Al concentration or Ti concentration in the steel does not appear. It was found that by optimizing the balance of Al concentration, Ti concentration and Si concentration, the brazing property is improved and the corrosion resistance against the exhaust gas condensed water is improved. Furthermore, the geometrical surface properties affecting the spread of the wax were examined in detail. As a result, when the average value of the surface roughness in the rolling direction, the direction perpendicular to the rolling direction, and the direction inclined by 45 ° with respect to the rolling direction is small, and the difference in surface roughness is small, the brazing property is The knowledge that it improved further was acquired. It was also found that the change in crystal grain size before and after brazing heat treatment is reduced by controlling the annealing conditions of the cold-rolled sheet and controlling the precipitation state of the Laves phase such as Fe ₂ Nb in the steel. Hereinafter, the examination results by the inventors will be described.

Exhaust gas recirculation devices such as automobile mufflers, exhaust heat recovery devices, or EGR coolers are required to have corrosion resistance, particularly condensed water corrosion resistance, because they are exposed to the environment of exhaust gas condensed water. The researchers made steel sheets with various compositions and conducted a condensation water corrosion resistance test. The results are shown in FIG. 1 with the horizontal axis representing the Si content in the steel sheet and the vertical axis representing the Al / Ti content ratio (both mass%) in the steel sheet. Here, the criterion for the condensed water corrosion test was a boundary value of 100 μm of the maximum pitting corrosion depth at which the growth of pitting corrosion was confirmed under the test conditions used in the examples described later. A steel type having a maximum pitting corrosion depth of 100 μm or more was evaluated as C (bad), and plotted in FIG. A steel type having a maximum pitting depth of less than 100 μm was evaluated as B (good) and plotted in FIG. The solid line in FIG. 1 represents Al / Ti = 8.4Si−0.78.

FIG. 1 shows that when the amount of Al, Ti, and Si (% by mass) in the steel does not satisfy the relationship of Al / Ti ≧ 8.4Si−0.78, the resistance to condensed water corrosion is significantly reduced. From this result, it can be seen that it is desirable that the amounts of Al, Ti, and Si satisfy the relationship of Al / Ti ≧ 8.4Si−0.78.

As a result of investigating inclusions present in the steel not satisfying the relationship of Al / Ti ≧ 8.4Si−0.78, it was found that mainly Ti-based oxides were present. On the other hand, it was found that the inclusions present in the steel satisfying the relationship of Al / Ti ≧ 8.4Si−0.78 were mainly Al ₂ O ₃ —MgO. The _{Al 2 O 3 CaO-Al 2} O 3 so as to surround the existed deformed in the rolling direction.

Since Ti-based oxides are hard inclusions, they do not deform together with the base material during cold rolling, and gaps are easily formed at the interface between the inclusions and the base material. It is thought that the formed gap became a pitting corrosion starting point and reduced the resistance to condensed water corrosion of steel.

Al ₂ O ₃ —MgO is also a hard inclusion, but the surrounding CaO—Al ₂ O ₃ is deformed in the rolling direction, so that no gap is formed at the interface between the inclusion and the base material. It is thought that the corrosivity was not deteriorated.

Moreover, since Si promotes the generation of Ti-based oxides by increasing the activity of Ti, it is desirable to reduce the Si content particularly in a low Al material (a material with a small amount of Al).

By satisfying the relationship of Al / Ti ≧ 8.4Si−0.78 in this way, Al ₂ O ₃ —MgO inclusions that do not serve as corrosion starting points are preferentially generated. However, Al, Ti, and Si are effective elements for deoxidation, and an increase in the O concentration in steel is feared to reduce the amount of these elements. In that case, by performing deoxidation by adding Mg, it is possible to suppress the formation of oxides in the steel and to further suppress the deterioration of the resistance to condensed water corrosion.

On the other hand, in order to improve brazeability, the content of Al and Ti itself must be reduced. Therefore, it is necessary to reduce the amount of Al and Ti added to the molten steel. Here, if the Al addition amount is reduced, the O concentration in the molten steel increases, and [S] + (CaO) → (CaS) + [O], which is a de-S reaction, does not progress. Therefore, it is necessary to use low S (small amount of S) ferrochrome as a raw material and reduce the S concentration in the molten steel in advance.

Table 1 shows the relationship between the cold rolling conditions of the final pass, the arithmetic average roughness and the brazing property in each direction. Steel type No. in Table 1 Is a steel type No. shown in Tables 3A to 3D described later. Is the same. The brazeability was evaluated as follows. 0.2 g of Ni solder was placed on the surface of the steel plate produced by the method described later, and heated in a vacuum atmosphere at 1200 ° C. and 5 × 10 ⁻³ torr for 10 minutes. Subsequently, it cooled to normal temperature and measured the brazing area of the test piece after a heating. A steel type in which the brazing area after heating was 2.5 times or more of the brazing area before heating was evaluated as A (excellent). A steel type in which the brazing area after heating was 2 times or more and less than 2.5 times the brazing area before heating was evaluated as B (good). A steel type in which the brazing area after heating was less than twice the brazing area before heating was evaluated as C (bad).

From Table 1, when the following conditions (1) to (3) are satisfied, the absolute value of (Ra _L + Ra _C + 2Ra _V ) / 4 or (Ra _L + Ra _C −2Ra _V ) / 2, or both values decrease. It can be seen that the brazability is improved.
(1) The roughness of the roll used for cold rolling in the final pass is set to # 60 or more.
(2) The rolling reduction of the final pass is set to 15.0% or less.
(3) The cold rolling speed of the final pass is set to 800 m / min or less.
In particular, it can be seen that brazability is improved when (Ra _L + Ra _C + 2Ra _V ) /4≦0.50 and | (Ra _L + Ra _C −2Ra _V ) /2|≦0.10. Desirably, (Ra _L + Ra _C + 2Ra _V ) /4≦0.30 and | (Ra _L + Ra _C −2Ra _V ) /2|≦0.05. Since the values of these indices are preferably as small as possible, it is not necessary to provide lower limits for these indices. However, the lowest realistically achievable value of (Ra _L + Ra _C + 2Ra _V ) / 4 is 0.02, and the most realistically achievable value of | (Ra _L + Ra _C −2Ra _V ) / 2 | The low value is 0.005.

It is well known that the effect of surface roughness on wettability is very large. However, the surface of stainless steel is water repellent with respect to brazing, and the relationship between the two-dimensional properties of the surface of the stainless steel plate and the brazing used for brazing and the spreadability of the brazing are still unknown. There were many points. Since the water repellency increases due to the rough surface of the stainless steel, the brazing property is deteriorated. In this embodiment, merely reducing the surface roughness in one direction does not sufficiently improve the two-dimensional expansion of the brazing, and by controlling the multidirectional roughness in the plate surface, the brazing expansibility is markedly improved. It was found that it can be improved.

That is, the average value of the roughness in the plate surface is reduced, and the difference in the roughness in the plate surface is reduced, thereby facilitating the two-dimensional expansion of the wax. Specifically, (Ra _L + Ra _C + 2Ra _V ) / 4 is an index representing the average value of arithmetic average roughness in three directions, and | (Ra _L + Ra _C −2Ra _V ) / 2 | It is an index that represents the difference in arithmetic mean roughness. By setting the average value of the arithmetic average roughness in the three directions to 0.50 or less and the difference in the arithmetic average roughness in the three directions to 0.10 or less, the brazing property is improved.

As a method of reducing the values of (Ra _L + Ra _C + 2Ra _V ) / 4 and | (Ra _L + Ra _C −2Ra _V ) / 2 |, there is a method of defining a pass schedule of the cold rolling process in the manufacturing process of the stainless steel plate. . In the cold rolling process of a stainless steel plate, generally, a multi-pass rolling is performed by a Sendzimir rolling mill to produce a predetermined plate thickness. At this time, mineral oil or water-soluble oil is used as the lubricating oil. In the present embodiment, the final pass is performed under the conditions (1) to (3) described above. That is, the final pass is performed with a roll having a roll roughness of # 60 or more, the rolling reduction of the final pass is set to 15.0% or less, and the cold rolling speed of the final pass is set to 800 m / min or less. As a result, a preferable surface texture defined in the present embodiment is realized. In multi-pass rolling by a Sendzimir mill, a smooth surface is formed by transferring cold-rolled rolls while eliminating defects (shot blast marks, grain boundary erosion grooves, pickling pits, etc.) on the base material surface.

A preferable surface property defined in the present embodiment is characterized in that the average value of arithmetic average roughness in three directions and the difference between arithmetic average roughness in three directions are smaller than a predetermined value. If the surface of the roll used in the final pass is rough, the grinding marks of the roll are transferred and the surface of the stainless steel is also roughened. Therefore, a roll of # 60 or more is used in the final pass. The roll roughness is more desirably # 80 or more. If the roll roughness exceeds # 1000, further improvement of the effect cannot be expected. Therefore, the roll roughness is desirably set to # 1000 or less.

Further, when the rolling reduction rate of the final pass is increased, the contact arc length between the steel plate and the roll in the roll bite becomes long, and therefore it becomes difficult to discharge the rolling oil from the roll bite. If it is difficult for the rolling oil to be discharged, the hydrostatic pressure is generated by the rolling oil in the roll bite, and a two-dimensional dent is likely to be generated on the steel plate surface. Thereby, the values of (Ra _L + Ra _C + 2Ra _V ) / 4 and | (Ra _L + Ra _C −2Ra _V ) / 2 | are likely to increase. Also, depending on the amount of rolling oil and the surface properties of the original sheet, a seizure phenomenon called heat streak occurs when rolling at a high rolling reduction, and conversely the surface roughness becomes rough. In the present embodiment, heat streak is not generated while promoting the discharge of the rolling oil in the roll bite. Thereby, the roughness in directions other than a rolling direction is reduced especially and the difference of the roughness in each direction is made small. For this purpose, it is desirable that the rolling reduction of the final pass is 15.0% or less. The rolling reduction of the final pass is more preferably 14.5% or less, and is preferably 10.0% or more in consideration of productivity and steel plate shape. The reduction rate of the final pass is more desirably 12.0% or more.

In addition, it is desirable that the rolling speed (cold rolling speed) of the final pass in this embodiment is 800 m / min or less. At the roll bite entrance, rolling oil accumulates in the surface recess remaining in the rolled material, and the oil is discharged in the roll bite and the rolls are transferred to the steel plate. However, when the rolling speed is high, the oil discharge time is insufficient, and thus the disappearance of the recess becomes insufficient, and it becomes difficult to reduce the roughness of the recess in particular. In order to sufficiently discharge the rolling oil in the dent and sufficiently perform the two-dimensional transfer of the smooth roll and reduce the roughness anisotropy, the cold rolling speed of the final pass should be 800 m / min or less. Is desirable. The cold rolling speed of the final pass is more desirably 600 m / min or less, and further desirably 500 m / min or less. Considering productivity, steel plate shape, and surface gloss, 150 m / min or more is desirable.

In addition, other conditions in cold rolling may be set in consideration of the product thickness and surface finish. If rolling in one direction with a tandem rolling mill, which is a rolling mill for ordinary steel, this implementation The form condition may be applied to the final stand. Further, the rolling oil may be mineral oil or water-soluble oil.

Table 2 shows the relationship between the annealing conditions of the cold rolled sheet and the grain size number GSN before and after the brazing heat treatment. Steel type No. in Table 2 Is a steel type No. shown in Tables 3A to 3D described later. Is the same. The crystal grain size number was evaluated as follows. The steel plate produced by the method described later was cut and filled with resin so that a plane parallel to the rolling direction could be observed. The crystal grain size number was measured by a cutting method using an optical microscope.

From Table 2, when the steel plate stays at 650 to 950 ° C. for less than 5.0 s, or when the steel plate stays at 950 to 1050 ° C. for more than 80.0 s, the amount of change in the grain size number before and after brazing heat treatment is 5 It turns out that it becomes more than 0.0. The fact that the grain size number changes significantly before and after the brazing heat treatment leads to a significant change in the mechanical properties of the stainless steel before and after the brazing heat treatment, which may lead to the cause of component failure, etc. It is desirable to avoid. In the present embodiment, the mechanical properties greatly change with the change amount of the crystal grain size number before and after the brazing heat treatment being 5.0. For this reason, it is desirable to suppress the change amount of the crystal grain size number before and after brazing heat treatment to 5.0 or less. The amount of change in the grain size number before and after brazing heat treatment is more preferably 4.0 or less. Since the amount of change in grain size number before and after brazing heat treatment is preferably as low as possible, it is not necessary to set a lower limit. However, since it is difficult to set the change amount of the crystal grain size number to 0, it is desirable to set the lower limit value to more than 0.

In this embodiment, by precipitating a Laves phase such as Fe ₂ Nb finely in steel, these phases work as pinning factors, and the amount of change in crystal grain size before and after brazing heat treatment is reduced. I found out. The temperature at which the Laves phase precipitates is 650 to 950 ° C., and the temperature at which the Laves phase dissolves is 950 to 1050 ° C. For this reason, when annealing a cold-rolled sheet, it is necessary to retain the cold-rolled sheet for a long time in the temperature range of 650 to 950 ° C., and to retain the cold-rolled sheet in a short time for the temperature range of 950 to 1050 ° C. . In the present embodiment, the annealing step may include a step of retaining the steel plate at 650 to 950 ° C. for a time of 5.0 s or more and a step of retaining the steel plate at 950 to 1050 ° C. for a time of 80.0 s or less. preferable. As a result, it was found that a fine Laves phase effective for pinning crystal grains can be sufficiently precipitated. More preferably, in the annealing step, the steel sheet is retained at 650 to 950 ° C. for a time of 8.0 s (seconds) or longer, and the steel sheet is retained at 950 to 1050 ° C. for a time of 60.0 s (seconds) or less. Having a process. In consideration of productivity, the time for retaining the steel sheet at 650 to 950 ° C. is preferably 50 s or less. In consideration of appropriately recrystallizing the structure after cold rolling, the time for retaining the steel sheet at 950 to 1050 ° C. is preferably 10 seconds or more.

Hereinafter, the chemical composition of steel defined in this embodiment will be described in more detail. In addition,% means the mass%.

C: Since C reduces intergranular corrosion resistance and workability, it is necessary to keep the content low. Therefore, it is 0.030% or less. However, excessively reducing the amount of C promotes the coarsening of crystal grains during brazing and increases the scouring cost. Therefore, the amount of C is preferably set to 0.001% or more. The amount of C is more preferably 0.004 to 0.020%.

Si: Si is useful as a deoxidizing element, but promotes the formation of hard Ti-based oxides by increasing the activity of Ti. Therefore, the content is set to 0.01 to 1.00%. The amount of Si is more preferably 0.10 to 0.60%.

Mn: Mn is useful as a deoxidizing element. However, if an excessive amount of Mn is contained, the corrosion resistance is deteriorated, so the Mn content is set to 0.01 to 2.00%. The amount of Mn is more desirably 0.10 to 1.00%.

P: P is an element that deteriorates workability and weldability, and its content needs to be limited. Therefore, the P content is 0.050% or less. The amount of P is more desirably 0.030% or less.

S: Since S is an element that degrades corrosion resistance, its content needs to be limited. Therefore, the S amount is set to 0.0100% or less. The amount of S is more desirably 0.0050% or less.

Cr: Possible corrosive environment includes air environment, cooling water environment, exhaust gas condensed water environment, and the like. In order to ensure corrosion resistance in such an environment, at least 11.0% or more of Cr is necessary. As the Cr content is increased, the corrosion resistance is improved, but the workability and manufacturability are lowered, so the upper limit of the Cr content is 30.0% or less. The amount of Cr is more desirably 15.0 to 23.0%.

Mo: 0.01% or more of Mo is required to improve the resistance to condensed water corrosion. However, addition of an excessive amount of Mo deteriorates workability and increases the cost because it is expensive, so the Mo amount is set to 3.00% or less. Mo is more preferably 0.10 to 2.50%.

Ti: Ti forms an oxide film with low wettability on the surface and reduces brazing. Therefore, the Ti content is set to 0.001 to 0.050%. The amount of Ti is more preferably 0.001 to 0.030%.

Al: Al has a deoxidizing effect and the like, is an element useful for scouring, and has an effect of improving moldability. In order to stably obtain this effect, it is preferable to contain 0.001% or more of Al. However, when a large amount of Al is contained, an oxide film with low wettability is formed on the surface, which inhibits brazing. For this reason, the amount of Al is made 0.030% or less. The amount of Al is more desirably 0.001 to 0.015%.

Nb: Nb is an important element from the viewpoint of suppressing grain coarsening due to heating during brazing and suppressing a decrease in strength of the member by Nb carbonitride. Further, although it is useful for improving the high temperature strength and the intergranular corrosion resistance of the welded portion, the addition of an excessive amount of Nb reduces workability and manufacturability, so the Nb amount is 0.010 to 1.. 000%. The amount of Nb is more preferably 0.100 to 0.600%.

O: O is an element inevitably contained in stainless steel. In the present embodiment, it is not necessary to limit the O content. However, if O is present in the stainless steel base metal, O may cause inclusions such as oxides to be formed, which may deteriorate various properties such as ductility and corrosion resistance. For this reason, it is desirable to suppress the O content to 0.020% or less. The amount of O is more desirably 0.010% or less.

N: N is an element useful for pitting corrosion resistance, but its content needs to be kept low in order to reduce intergranular corrosion resistance and workability. Therefore, the N content is 0.050% or less. The amount of N is more desirably 0.030% or less.

The above is the chemical composition that is the basis of the ferritic stainless steel of the present embodiment, but in the present embodiment, the following elements can be further contained as required.

Ni: Ni can be contained in an amount of 3.00% or less in order to improve corrosion resistance. A stable effect is obtained when the Ni content is 0.01% or more. The amount of Ni is more desirably 0.05 to 2.00%.

Cu: Cu can be contained in an amount of 1.500% or less for improving corrosion resistance. A stable effect is obtained when the amount of Cu is 0.050% or more. The amount of Cu is more preferably 0.100 to 1.000%.

W: W can be contained in an amount of 1.000% or less for improving the corrosion resistance. A stable effect is obtained when the amount of W is 0.010% or more. The amount of W is more desirably 0.020 to 0.800%.

V: In improving corrosion resistance, V can be contained in an amount of 0.300% or less. It is a V amount of 0.010% or more that a stable effect can be obtained. The amount of V is more preferably 0.020 to 0.050%.

Sn: Sn can be contained in an amount of 0.500% or less as required in order to improve the corrosion resistance. When contained, the amount of Sn is preferably 0.005% or more so that a stable effect can be obtained. The amount of Sn is more desirably 0.01 to 0.300%.

Sb: Sb can be contained in an amount of 0.5000% or less as required to improve the overall corrosion resistance. When contained, the amount of Sb is preferably 0.0050% or more so that a stable effect can be obtained. The amount of Sb is more preferably 0.0100 to 0.3000%.

Mg: Mg has a deoxidizing effect and is an element useful for scouring, and Mg is useful for refinement of the structure and improvement of workability and toughness. Mg can be contained in an amount of% or less. When contained, the amount of Mg is preferably 0.0001% or more so that a stable effect can be obtained. The amount of Mg is more desirably 0.0001 to 0.001%.

Note that the total of one or more of Ni, Cu, W, V, Sn, Sb, and Mg is preferably 6% or less from the viewpoint of cost increase.

B: B is an element useful for improving secondary workability, and B can be contained in an amount of 0.0030% or less. When contained, the amount of B is preferably 0.0002% or more so that a stable effect can be obtained. The amount of B is more preferably 0.0005 to 0.0010%.

Ca: Ca is added for desulfurization, but if an excessive amount of Ca is added, water-soluble inclusions CaS are generated and the corrosion resistance is lowered. Therefore, Ca can be added in an amount of 0.0002 to 0.0100%. The amount of Ca is more desirably 0.0002 to 0.0050%.

Zr: Zr can be contained in an amount of 0.300% or less as required in order to improve the corrosion resistance. When contained, the amount of Zr is preferably 0.010% or more so that a stable effect can be obtained. The amount of Zr is more preferably 0.020 to 0.200%.

Co: Co can be contained in an amount of 0.300% or less as required in order to improve secondary workability and toughness. When contained, the amount of Co is desirably 0.010% or more so that a stable effect can be obtained. The amount of Co is more desirably 0.020 to 0.200%.

Ga: Ga can be contained in an amount of 0.0100% or less as required in order to improve corrosion resistance and hydrogen embrittlement resistance. When contained, the amount of Ga is preferably 0.0001% or more so that a stable effect can be obtained. The Ga content is more preferably 0.0005 to 0.0050%.

Ta: Ta can be contained in an amount of 0.0100% or less as needed in order to improve corrosion resistance. When contained, the amount of Ta is preferably 0.0001% or more so that a stable effect can be obtained. The amount of Ta is more desirably 0.0005 to 0.0050%.

REM: REM is an element useful for scouring because it has a deoxidizing effect and the like, and can be contained in an amount of 0.200% or less as necessary. When contained, the amount of REM is desirably 0.001% or more so that a stable effect can be obtained. The amount of REM is more preferably 0.002 to 0.100%.
Here, REM (rare earth element) is a generic name of two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoid) from lanthanum (La) to lutetium (Lu) according to a general definition. . REM is at least one selected from these rare earth elements, and the amount of REM is the total amount of rare earth elements.

In the manufacturing method of this embodiment, a general method for manufacturing a steel plate made of ferritic stainless steel is basically applied. For example, molten steel having the above chemical composition is converted into a converter or electric furnace and refined in an AOD furnace or a VOD furnace. Thereafter, a steel piece is formed by a continuous casting method or an ingot-making method, and then subjected to hot rolling, annealing of hot-rolled sheet, pickling, cold rolling, finish annealing, and pickling, and then the ferritic stainless steel of this embodiment. Steel is produced. If necessary, annealing of the hot-rolled sheet may be omitted, or cold rolling, finish annealing, and pickling may be repeated.

However, as described above, in order to control the surface roughness, in the cold rolling process, a roll having a roll roughness of # 60 or more is used in the final pass, and the rolling reduction of the final pass is 15.0% or less. It is desirable to perform rolling under conditions where the cold rolling speed of the pass is 800 m / min or less. Further, in order to precipitate a Laves phase such as Fe ₂ Nb in steel, the annealing process of the cold-rolled sheet includes a process in which the steel sheet is retained at 650 to 950 ° C. for 5.0 s or more, and the steel sheet is 80.degree. It is desirable to have a step of staying for 0 s or less. That is, in the annealing process, it is desirable that the time for retaining the steel sheet at 650 to 950 ° C. is 5.0 s or more and the time for retaining the steel sheet at 950 to 1050 ° C. is 80.0 s or less.

The present invention will be described in more detail based on examples.

Steels having the compositions shown in Table 3A and Table 3B were melted, hot-rolled to a thickness of 4 mm, annealed at 1050 ° C. for 1 minute, and then pickled. Thereafter, cold rolling was performed to a plate thickness of 1 mm. In particular, the roll roughness, rolling reduction, and cold rolling speed of the final pass of cold rolling were performed under the conditions shown in Table 3C. As shown in Table 3C, the cold-rolled sheet was annealed by controlling the residence time of 650 to 950 ° C. and the residence time of 950 to 1050 ° C., respectively.

Thereafter, a test piece having both a width and a length of 100 mm was cut out from the produced steel plate. The arithmetic average roughness of the steel surface in each of the rolling direction (L direction), the direction perpendicular to the rolling direction (C direction), and the direction inclined 45 ° to the rolling direction (V direction) It measured using the shape measuring machine. The measurement length was 4.0 mm, the measurement speed was 0.30 mm / s, and the cutoff wavelength was 0.8 mm. In each direction, the average value of three measurement results was defined as the arithmetic average roughness in that direction.

Moreover, the produced steel plate was cut and filled with resin so that a plane parallel to the rolling direction could be observed. The grain size number (GSN) was measured using the cutting method.

Further, a test piece having a width of 60 mm and a length of 100 mm was cut out from the produced steel sheet, 0.2 g of Ni solder was placed on the surface, and heated in a vacuum atmosphere at 1200 ° C. and 5 × 10 ⁻³ torr for 10 minutes. Subsequently, it cooled to normal temperature and measured the brazing area of the surface of the test piece after a heating. A steel type in which the brazing area after heating was 2.5 times or more of the brazing area before heating was evaluated as A (excellent). A steel type in which the brazing area after heating was 2 times or more and less than 2.5 times the brazing area before heating was evaluated as B (good). A steel type in which the brazing area after heating was less than twice the brazing area before heating was evaluated as C (bad). Thereafter, the steel plate subjected to brazing heat treatment was cut and filled with a resin so that a plane parallel to the rolling direction could be observed. The grain size number (GSN) was then measured using the cutting method.

Further, a test piece having a width of 25 mm and a length of 100 mm was cut out from the cold rolled steel sheet, and then the entire surface of the test piece was wet-polished using emery paper up to # 600. This test piece was evaluated by a semi-immersion test.

The simulated condensed water used for the semi-immersion test was prepared as follows. An aqueous solution containing 300 ppm Cl ⁻ +1000 ppm SO ₄ ² + +1000 ppm SO ₃ ²⁻ was prepared using hydrochloric acid, sulfuric acid, and ammonium sulfite as reagents. After adding the reagent, ammonia water was used to adjust the pH to 2.0 to obtain simulated condensed water. The jig was adjusted so that approximately half of the specimen was immersed in simulated condensed water at an angle of about 55 °. Using this jig, the test piece was semi-immersed in simulated condensed water heated to 80 ° C. The test was conducted for 168 hours and the solution was renewed every day on weekdays.

The maximum pitting depth was used for corrosion evaluation. After completion of the test, the corrosion product was removed using an aqueous solution of ammonium dihydrogen citrate, and the depth of the most corroded portion of the test piece was measured by the depth of focus method. The criterion for the semi-immersion test was taken as a boundary value of 100 μm at which pitting growth was confirmed to be remarkable under these test conditions. A steel type having a maximum erosion depth of less than 100 μm was evaluated as B (good). A steel type having a maximum pitting depth of 100 μm or more was evaluated as C (bad).

Also, a resin-embedded sample for observing the L section was prepared from this steel plate. Mirror polishing was performed, followed by observation with SEM, and composition analysis of inclusions was performed with EDS (Energy Dispersive X-ray Spectroscopy). The results are shown in Tables 3D and 3E. Here, EDS is an elemental analysis technique in which a specimen is irradiated with an electron beam, characteristic X-rays generated are detected, and the elements and concentrations constituting the object are examined from the energy and intensity.

Table 3D and Table 3E show the test results. From Table 3D, it can be seen that the steels of the examples of the present invention are excellent in both brazeability and resistance to condensed water corrosion. Further, from Table 3E, it can be seen that when the component deviates from the present embodiment, the resistance to condensed water corrosion deteriorates except when the amount of Al or Ti deviates. On the other hand, it can be seen that when the amount of Al or Ti deviates, the brazability deteriorates. Even if the amount of each component is within the range of the present embodiment, if the amount of Al, Ti, and Si contained does not satisfy the relationship of Al / Ti ≧ 8.4Si−0.78, hard Ti It can be seen that the system oxide is generated in the steel, and a gap serving as a pitting corrosion starting point is formed at the inclusion / substrate interface, and the resistance to condensed water corrosion deteriorates.

In addition, the steel type No. In A1 to A14, the roughness of the roll used for the final cold rolling (cold rolling final pass) is set to # 60 or more, the rolling reduction of the final pass is set to 15.0% or less, and the cold rolling speed of the final pass P Was 800 m / min or less. The steel type manufactured under these conditions satisfies both (Ra _L + Ra _C + 2Ra _V ) /4≦0.50 and | (Ra _L + Ra _C −2Ra _V ) /2|≦0.10, and has a brazing property. It turns out that it becomes still more favorable. In addition, the steel type No. In A1 to A14, in the cold-rolled sheet annealing step, the residence time of the steel sheet at 650 to 950 ° C. was set to 5.0 s or more, and the residence time of the steel sheet at 950 to 1050 ° C. was set to 80.0 s or less. It can be seen that the change in the grain size number before and after the brazing heat treatment is 5.0 or less in the steel type manufactured under these conditions.

The ferritic stainless steel having excellent corrosion resistance against exhaust gas condensate of the present invention is suitable as a member used in exhaust gas recirculation devices such as automobile mufflers, exhaust heat recovery devices, and EGR (Exhaust Gas Recirculation) coolers.

Claims (8)

% By mass
C: 0.001 to 0.030%,
Si: 0.01 to 1.00%,
Mn: 0.01 to 2.00%
P: 0.050% or less,
S: 0.0100% or less,
Cr: 11.0-30.0%,
Mo: 0.01 to 3.00%,
Ti: 0.001 to 0.050%,
Al: 0.001 to 0.030%,
Nb: 0.010 to 1.000%, and N: 0.050% or less,
The remainder consists of Fe and inevitable impurities, and the above-mentioned Al content, Ti content and Si content (mass%) satisfy Al / Ti ≧ 8.4Si-0.78 Ferritic stainless steel with excellent brazing properties. In addition,
Ni: 0.01 to 3.00%,
Cu: 0.050 to 1.500%,
W: 0.010 to 1.000%,
V: 0.010-0.300%
Sn: 0.005 to 0.500%,
Sb: 0.0050 to 0.5000%, and Mg: 0.0001 to 0.0030%
The ferritic stainless steel excellent in exhaust gas condensate corrosion resistance and brazing properties according to claim 1, characterized by containing any one or more of them. In addition,
B: 0.0002 to 0.0030%,
Ca: 0.0002 to 0.0100%,
Zr: 0.010 to 0.300%,
Co: 0.010 to 0.300%,
Ga: 0.0001 to 0.0100%,
Ta: 0.0001 to 0.0100%, and REM: 0.001 to 0.200%
The ferritic stainless steel excellent in exhaust gas condensate corrosion resistance and brazing properties according to claim 1 or 2, wherein any one or more of them are contained. The rolling direction is the L direction, the direction perpendicular to the rolling direction is the C direction, and the direction inclined 45 ° with respect to the rolling direction is the V direction, and the arithmetic average roughness of the steel surface in each direction is Ra _L and Ra _C , respectively. , Ra _V (unit: μm), (Ra _L + Ra _C + 2Ra _V ) /4≦0.50 and | (Ra _L + Ra _C −2Ra _V ) /2|≦0.10 The ferritic stainless steel excellent in exhaust gas condensate corrosion resistance and brazing resistance according to any one of claims 1 to 3. 5. The amount of change in crystal grain size number GSN before and after a heat treatment at 1150 ° C. for 10 minutes in a vacuum atmosphere of 50 Pa or less is 5.0 or less. Ferritic stainless steel with excellent resistance to exhaust gas condensate corrosion and brazing. 6. Corrosion resistance and brazing resistance against exhaust gas condensate according to any one of claims 1 to 5, which are used for automobile mufflers, exhaust heat recovery devices, or EGR coolers that are automobile parts exposed to the exhaust gas condensate environment. Excellent ferritic stainless steel. A step of cold rolling steel having the chemical component according to any one of claims 1 to 3;
In the cold rolling step, a roll having a roll roughness of # 60 or more is used in the final pass, the rolling reduction of the final pass is 15.0% or less, and the cold rolling speed of the final pass is 800 m / min or less. A method for producing a ferritic stainless steel having excellent resistance to exhaust gas condensate corrosion and brazing, characterized by rolling under the following conditions: And further comprising a step of annealing the cold-rolled steel sheet,
The annealing step includes a step of retaining the steel plate at 650 to 950 ° C for 5.0 seconds or longer and a step of retaining the steel plate at 950 to 1050 ° C for 80.0 seconds or less. Of ferritic stainless steel with excellent exhaust gas condensate corrosion resistance and brazing.

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