ES2922207T3 - Ferrite-based stainless steel with high resistance to corrosion caused by exhaust gases and condensation and high brazing properties and manufacturing method thereof - Google Patents

Abstract

This ferritic stainless steel contains, 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% to 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, with a remainder of Fe and unavoidable impurities, wherein an amount of AL, an amount of Ti and a Quantity of SI (Mass %) meet al/TI ¥ 8.4si-0.78. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

Ferrite-based stainless steel with high resistance to corrosion caused by exhaust gases and condensation and high brazing properties and manufacturing method thereof

technical field

The present invention relates to a ferrite-based stainless steel (ferritic stainless steel sheet) for use in exhaust gas condensing environments (exhaust gas condensing water environments) and a method for manufacturing the same. Examples of members that are exposed to exhaust gas condensing environments include automobile mufflers, exhaust heat recovery devices, and exhaust gas recirculation appliances such as exhaust gas recirculation (EGR) coolers. in English).

Background

Recently, in the automotive field, the individual components included in the exhaust gases have caused air and environmental pollution, and therefore the tightening of regulations has been launched. Therefore, in order to decrease the amount of CO2 in automobile exhaust and improve gas mileage, it is not only necessary to improve the efficiency of engines through high-efficiency combustion, idle stop function, and the like, and reduce weight by substituting materials, but it has also become necessary to improve the diversification of energy sources through the use of hybrid electric vehicles (HEV), biofuel vehicles, hydrogen / fuel cell vehicles (FCV), electric vehicles (EV) and the like (all for its acronym in English).

In connection with these requirements, an attempt has also been made to improve gasoline efficiency by mounting a heat exchanger that recovers exhaust heat, that is, an exhaust heat recovery device mainly in hybrid electric vehicles. In the exhaust heat recovery device, the heat of the exhaust gas is transferred to the cooling water by heat exchange, and the thermal energy is recovered and reused; and as a result, the temperature of the cooling water increases. This improves performance for heating the interior of vehicles and improves gas mileage by shortening the time required to warm up engines. Exhaust heat recovery devices are also called exhaust heat recirculation systems.

In addition, other efforts have also been made to install an exhaust gas recirculation apparatus that recirculates exhaust gas. Examples of exhaust gas recirculation apparatus include EGR coolers. In the EGR cooler, the engines exhaust gases are cooled using engine cooling water or air and then the cooled exhaust gases are returned to the intake side and re-burned. In this way, the combustion temperature is lowered and the amount of NOx which is noxious gas is lowered.

For the heat exchange parts in the exhaust heat recovery devices or EGR coolers described above, favorable thermal efficiency and favorable thermal conductivity are required. In addition, high corrosion resistance against exhaust gas condensation water is required because the parts come into contact with exhaust gas. In particular, since the engine cooling water flows through these parts, in the event that holes are generated due to corrosion, there may be a risk of serious accidents. In addition, the materials used have a thin sheet thickness to increase the efficiency of heat exchange. Therefore, materials are required that have a higher corrosion resistance than the members in the downstream part of the exhaust systems.

Conventionally, among the members mainly including mufflers in the downstream part of exhaust systems, for parts requiring corrosion resistance particularly, a ferritic stainless steel containing 17% or more Cr such as SUS430LX, SUS436J1L has been used. or SUS436L. For materials of exhaust heat recovery devices or EGR coolers, a corrosion resistance greater than or equal to that of the ferritic stainless steel described above is required.

Also, EGR coolers are generally assembled by brazing, and therefore the parts that are used must have high brazing properties (brazing ability). Here, to improve the brazing ability, the wettability of the surfaces is important. Ti is more easily oxidized than Fe and Cr, and Ti forms a poorly wettable oxide film on the surface. Therefore, it is desirable that the amount of Ti is low. Furthermore, like Ti, Al forms an oxide film with poor surface wettability. Recently, a steel in which the amount of Al as well as the amount of Ti is low has been demanded. Also, since the surface roughness of a steel sheet also has a great influence on the wettability, it is also extremely important to control the surface properties by controlling the manufacturing conditions.

Also, when the temperature of a brazing heat treatment is high, the temperature reaches about 1200 °C, and in this high-temperature environment, the crystal grains in a stainless steel grow and become coarser. Since the coarsening of the crystal grains influences the characteristics such as thermal fatigue and the like, a stainless steel on which brazing heat treatment is carried out must have characteristics that crystal grains do not easily coarsen even at high temperatures.

As described above, a steel used in EGR coolers must have high corrosion resistance and good brazing ability.

Patent Document 1 describes an inexpensive ferritic stainless steel material which is used as muffler constituting members or members of water heating devices which form welded parts and have high corrosion resistance. This ferritic stainless steel material contains: 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: 0.025% or less, and further contains one or more elements selected from Ni: 1% or less, Cu: 1% or less, Mo: less than 1%, Nb: 0.5% or less, Ti: 0.4% or less, and V: 0.5% or less, with a remainder of Fe and unavoidable impurities. Ferritic stainless steel material has an oxide film in which the composition of an outermost layer contains a total amount of Si and Cr from 15 atom% to 40 atom% and 5 atom% Fe in terms of the atomic ratio, including oxygen on the surface, and the composition of the outermost layer is measured by X-ray photoelectron spectrometry (XPS).

Patent Document 2 describes a ferritic stainless steel with high brazing ability in the case that the ferritic stainless steel is brazed in a high temperature and low oxygen partial pressure environment, such as brazing. Ni and Cu brazing. This ferritic stainless steel contains 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 a remainder of Fe and unavoidable impurities. In addition, ferritic stainless steel contains an amount of Ti that satisfies the expression: Ti-3N < 0.03 and the expression: 10(Ti-3N) Al < 0.5 or also contains, as a substitute for a part of Fe, one or more of Mo: 3% or less, 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% or less.

Patent Document 3 discloses a ferritic stainless steel for an automobile exhaust system element having favorable initial oxidation resistance at a low cost without affecting the intrinsic functions of automobile exhaust system elements. such as high temperature resistance, resistance against scale detachment, formability, corrosion resistance against exhaust gas condensation water, and corrosion resistance against salt-damaged environments. This ferritic stainless steel contains, in percent by mass, C: <0.0100%, Si: 0.05% to 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 a remainder of Fe and unavoidable impurities. The number of Ca-containing inclusions per square millimeter of an arbitrary cross section is less than 10, and furthermore, preferably, the ratio of the number of Mn-based sulfides to the total number of Ti-based sulfides and the sulfides based on Mn is 50% or less.

In Patent Document 4, a ferritic stainless steel having excellent resistance to localized corrosion is described. This ferritic stainless steel contains, in percent 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%, with a remainder of Fe and impurities unavoidable. When the ratio of the amount of Al to the amount of Si is represented by Al/Si, the following expression (1) holds.

Al/Si > 0.10...(1)

In Patent Document 5, a ferritic stainless steel with high corrosion resistance is described. This ferritic stainless steel contains, in percent by mass, C: 0.030% or less, N: 0.030% or less, Si: 0.01% to 0.50%, Mn: 1.5% or less, P: 0.04% 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, with traces of Fe and impurities unavoidable. The following expression (A) is satisfied, in addition, polishing marks are provided with an arithmetic mean roughness value, Ra, of the surface in a range from 0.35 gm to 5.0 gm, and the color difference value, L*, of the surface is 70 or more.

0.35 < Nb+5V>2.0...Expression (A)

However, the inventions described in Patent Documents 1 to 5 cannot have excellent corrosion resistance against exhaust gas condensed water and excellent brazing ability.

JP 2014-145097 A describes ferritic stainless steel sheets for an automobile exhaust system member.

Prior Art Documents

Patent Documents

Patent Document 1: Japanese Unexamined Patent Application First Publication No. 2009-197293.

Patent Document 2: Japanese Unexamined Patent Application First Publication No. 2009-174046. Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2004-323907. Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2010-248625. Patent Document 5: Japanese Unexamined Patent Application First Publication No. 2015-145531.

Description of the invention

Problems to be solved by the invention

The present invention aims to provide a ferrite-based stainless steel (ferritic stainless steel sheet) with high resistance to corrosion caused by exhaust gases and condensation (corrosion resistance against exhaust gas condensation water). exhaust) and high brazing properties (brazing ability) in environments where ferritic stainless steel is used for automobile mufflers, exhaust heat recovery devices, EGR coolers, or the like, and a method for manufacturing the same .

Means to solve the problem

Features of the present invention intended to solve the problems described above are set forth in the claims.

(Not belonging to the present invention) (1) A ferritic stainless steel with high corrosion resistance against condensed water in exhaust gases and high brazing ability, containing, in percent by mass:

C: 0.001% to 0.030%;

Yes: 0.01% to 1.00%;

Mn: 0.01% to 2.00%;

P: 0.050% or less;

S: 0.0100% or less;

Cr: 11.0% to 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%; Y

N: 0.050% or less,

being a remainder Fe and unavoidable impurities,

where an amount of Al, an amount of Ti and an amount of Si (% by mass) satisfy Al/Ti > 8.4Si - 0.78.

(Not belonging to the present invention) (2) The ferritic stainless steel with high corrosion resistance against condensed water in exhaust gases and high brazing ability according to (1), which also contains, in percent by mass, one or more of the following:

Ni: 0.01% to 3.00%;

Cu: 0.050% to 1,500%;

W: 0.010% to 1.000%;

V: 0.010% to 0.300%;

Sn: 0.005% to 0.500%;

Sb: 0.0050% to 0.5000%; Y

Mg: 0.0001% to 0.0030%.

(Not belonging to the present invention) (3) Ferritic stainless steel with high corrosion resistance against exhaust gas condensed water and high brazing capacity according to (1) or (2), which also contains, in percent by mass, one or more of the following:

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%; Y

REM: 0.001% to 0.200%.

Effects of the invention

According to the present invention, it is possible to provide a ferritic stainless steel with high corrosion resistance against exhaust gas condensation water and high brazing ability in the case where the ferritic stainless steel is used in automobile parts. exposed to exhaust gas condensing environments (exhaust gas condensed water environments) such as automobile mufflers, exhaust heat recovery devices, EGR coolers or the like.

Brief description of the drawings

Fig. 1 is a view showing a relationship between the amounts of Si, Al and Ti in steel sheets and the results of the condensation water corrosion test.

Embodiments for carrying out the invention

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

In order to improve brazing ability, the present authors produced steels in which the amount of Al or the amount of Ti was lowered to various concentrations under various cold-rolling conditions or various annealing conditions of cold-rolled sheets. Then, corrosion resistance, brazing ability, surface roughness, and the amount of change in crystal grain size before and after brazing heat treatment were investigated. As a result, it was found that the brazing ability improves as the Al concentration or Ti concentration in a steel decreases. However, with respect to improvement of corrosion resistance against condensed water of exhaust gas, no effect was developed in a method in which Al concentration or Ti concentration in a steel was simply lowered. It has been found that when the balance between Al concentration, Ti concentration and Si concentration is optimized, brazing ability is improved and corrosion resistance against condensed water in exhaust gas is improved. . In addition, detailed studies were carried out on the geometric properties of the surface that influence the extension of the brazing filler metal. As a result, it was found that, in the case that the mean values of the surface roughness in a rolling direction, a direction perpendicular to the rolling direction, and a direction inclined at 45° to the rolling direction are small, and the difference between the surface roughness values is small, the brazing ability is further improved. Furthermore, it was found that the amount of change in crystal grain size before and after a brazing heat treatment decreases by controlling the annealing conditions of cold-rolled sheets and by controlling the precipitation state of a Laves phase as Fe2Nb and the like in a steel. The results of the authors' study will be described below.

Since automobile mufflers, exhaust heat recovery devices, and exhaust gas recirculation apparatus such as EGR coolers are exposed to exhaust gas condensing environments, there is a demand for corrosion resistance, in particular, corrosion resistance against condensed water in exhaust gases (corrosion resistance against condensed water, corrosion resistance against condensed water). The present researchers produced steel sheets with various compositions and carried out corrosion resistance tests against condensation water. The results are shown in fig. 1 in which the horizontal axis indicates the amount of Si in the steel sheet and the vertical axis indicates the ratio of the amount of Al/Ti in the steel sheet (both in percent by mass). Regarding the determination standard in the condensation water corrosion test, the maximum corrosion pitting depth of 100 gm was used as the limit value, and 100 gm was the maximum corrosion pitting depth of the sample in the sample. that remarkable growth of corrosion pitting was confirmed under the test conditions used in the examples described below. A steel in which the maximum corrosion pitting depth was 100 gm or more was evaluated as C (bad) and was plotted with a reference sign "X" in Fig. 1. A steel in which the maximum corrosion pitting depth was less than 100 gm was evaluated as B (good) and was plotted with a reference sign “O” in fig. 1. The line continues on the fig. 1 indicates:

Al/Ti = 8.4YES-0.78

From fig. 1 it follows that, in the event that the amounts (mass %) of Al, Ti and Si in a steel do not comply with the Al/Ti ratio > 8.4Si - 0.78, the corrosion resistance against condensed water is greatly reduced. From this result, it is found that the amounts of Al, Ti and Si desirably satisfy the ratio of Al/Ti > 8.4Si - 0.78 as claimed in the present application.

As a result of investigating the inclusions present in a steel that did not meet the Al/Ti ratio > 8.4Si - 0.78, it was found that there was mainly a Ti-based oxide. On the other hand, it was found that the inclusions present in a steel that complied with the Al/Ti ratio > 8.4Si - 0.78 were mainly AbO3-MgO. Also, there was CaO-Al2O3 in a deformed form in the rolling direction to be around the AbO3.

Since the Ti-based oxide is a hard inclusion, the Ti-based oxide does not deform together with the base material during cold rolling, and voids are likely to form at the interfaces between the inclusions and the base material. The voids formed are considered to serve as starting points for pitting corrosion and lower the corrosion resistance against condensation water of the steel.

Al2O3-MgO is also a hard inclusion, but it is considered that no voids are formed at the interfaces between the inclusions and the parent material due to the deformation of CaO-AbO3 in the lamination direction that is present in the vicinity of AbO3-MgO. , and the corrosion resistance against condensation water does not deteriorate.

Furthermore, Si increases the activity of Ti; and therefore the Si helps the generation of the Ti-based oxide. Therefore, it is desirable to decrease the amount of Si, particularly in materials with low Al content (materials with a small amount of Al).

As described above, when the Al/Ti ratio > 8.4Si - 0.78 is met, AbO3-MgO inclusions are preferably generated that do not serve as a starting point for corrosion. However, since Al, Ti and Si are effective elements for deoxidation, in case the amounts of these elements decrease, there is concern of an increase in O concentration in the steel. In this case, Mg is added to perform deoxidation; and, thus, the formation of oxides on the steel is inhibited, and, moreover, it is possible to prevent the deterioration of the corrosion resistance against condensation water.

Meanwhile, in order to improve the brazing ability, it is necessary to decrease the amounts of Al and Ti. Therefore, it is necessary to decrease the amounts of Al and Ti that are added to the molten steel. In the case that the added amount of Al decreases, the concentration of O in the molten steel increases and the reaction of [S](CaO)^(CaS)[O], which is a desulfurization reaction, does not proceed. Therefore, it is necessary to use low-S ferrochrome (with a small amount of S) as the raw material and lower the S concentration in the molten steel in advance.

In addition, the relationships between the cold rolling conditions in the final step and the arithmetic mean values of the roughness in the respective directions and the brazability are shown in Table 1. The steel numbers in Table 1 are the same as the steel numbers in Tables 3A through 3D below. Brazing ability was evaluated as described below. 0.2 g of Ni brazing filler metal was placed on the surface of a steel sheet produced by a method described below and heated at 1200 °C for ten minutes in a vacuum atmosphere of 7 x 10- 1 Pa (5 x 10-3 torr). Next, the test specimen was cooled to normal temperature, and the brazing filler metal area (brazing filler metal area) on the heated test specimen was measured. A steel in which the brazing filler metal area after heating was 2.5 times or more than the brazing filler metal area before heating was rated A (excellent). A steel in which the area of the brazing filler metal after heating was from 2 times (twice) or more to less than 2.5 times the area of the brazing filler metal before heating was evaluated as B (good). heating. A steel in which the brazing filler metal area after heating was less than 2 times (twice) the brazing filler metal area before heating was evaluated as C (poor).

From Table 1, it follows that, in the event that the following conditions (1) to (3) are met, one or both of the absolute values of (RaL RaC 2RaV)/4 and the absolute value of (RaL RaC - 2RaV )/2 decrease and brazing ability improves.

(1) The roughness of a roll used in the final step of cold rolling is set to No. 60 or greater.

(2) The rolling reduction in the final step is set to 15.0% or less.

(3) The cold rolling speed in the final step is set to 800 m/min or less.

In particular, it is found that, in the case where (RaL Rae 2Rav)/4 < 0.50 and |(RaL Rae - 2Rav)/2| < 0.10 improves brazing ability. Preferably, (RaL Rae 2Rav)/4 < 0.30 and |(RaL Rae - 2Rav)/2| <0.05. The smaller the values of these indices, the more preferable it is, and therefore it is not necessary to provide the lower limit values of these indices. However, the lowest value of (RaL Rae 2Rav)/4 that can be realistically achieved is 0.02, and the lowest value of |(RaL Rae - 2Rav)/2 that can be realistically achieved is 0.005.

Surface roughness is known to have an extremely large influence on wettability. However, the surface of a stainless steel exhibits repellency against the brazing filler metal, and there have been a number of undefined points regarding the relationship between the two-dimensional surface properties of the surface of a stainless steel sheet and the metal. brazing filler used for brazing or the extension properties of brazing filler metal. As the surface of a stainless steel roughens, the repellency against the brazing filler metal increases; and therefore the brazing ability becomes poor. In the present embodiment, it was found that a decrease in surface roughness in one direction does not sufficiently improve the two-dimensional extension of the brazing filler metal, and the extension properties of the brazing filler metal can be greatly improved by controlling the roughness. on the surface of a sheet metal in multiple directions.

That is, when the mean value of the roughness on the surface of a plate (in-plane roughness) decreases and the difference in roughness on the surface of the plate (in-plane roughness) decreases, the two-dimensional extension of the filler metal brazing becomes easy (brazing filler metal can be easily extended in two-dimensional directions). Specifically, (RaL Rae 2Rav)/4 is an index indicating the mean value of the arithmetic mean roughness values in three directions, and |(RaL Rae - 2Rav)/2| is an index indicating the difference between the arithmetic mean roughness values in three directions. Brazing ability is improved by setting the mean value of the three-way arithmetic mean roughness values to 0.50 or less and setting the difference between the three-way arithmetic mean roughness values to 0.10 or less.

As a method to decrease the values of (RaL Rae 2Rav)/4 and |(RaL Rae - 2Rav)/2|, there is a method to control the step schedule of the cold rolling step in a process for manufacturing a sheet of stainless steel. In the cold rolling stage of a stainless steel sheet, usually multi-step rolling is done by Sendzimir rolling machine; and thereby a stainless steel sheet with a predetermined sheet thickness is manufactured. At this time, liquid paraffin or water-soluble oil is used as a lubricant. In the present embodiment, the final step is carried out under the conditions described above (1) to (3). That is, the final pass is carried out using a roll with a roll roughness of No. 60 or more, the rolling reduction in the final pass is set to 15.0% or less, and the cold rolling speed is set to the final step is set to 800 m/min or less. In such a case, the preferred surface properties defined in the present embodiment are realized. In multi-step rolling with a Sendzimir rolling machine, a flat surface is formed by transferring cold rolling marks while removing defects (peening defects, intergranular corrosion grooves, pickling pits, and the like) on the surface. of a base material.

In the preferred surface properties defined in the present embodiment, the average value of the three-way arithmetic mean roughness values and the difference between the three-way arithmetic mean roughness values are smaller than the predetermined values. In the event that the surface of a roller used in the final step is rough, the grinding marks of the roller are transferred and the surface of the stainless steel also becomes rough, and therefore, in the final step, a roller with a roughness of #60 or greater. The roll roughness is more desirably #80 or greater. In the case where the roughness of the roller is larger than that of No. 1000, further enhancement of the effect cannot be expected, and therefore the roughness of the roller is desirably set to No. 1000 or less.

In addition, when the rolling reduction in the final step increases, the contact arc length between the steel sheet in a roll gauge tool and the roll becomes long, and thus it becomes difficult to discharge the rolling oil. of the cylinder gauge tool. In the case that the rolling oil is not easily discharged, the rolling oil in the rolling gauge tool generates a hydrostatic pressure, and two-dimensional cavities are likely to be generated on the surface of the steel sheet. Thus, the values of (RaL Rae 2Rav)/4 and |(RaL Rae - 2Rav)/2| increase. Also, depending Depending on the amount of rolling oil or the surface properties of the original plate, a fretting galling phenomenon called heat scratching occurs when the steel plate is rolled with a high rolling reduction and, conversely, the roughness of the surface becomes rough. In the present embodiment, the occurrence of heat scratches is prevented while the discharge of rolling oil in the rolling gauge tool is accelerated. Thus, the difference between the roughness values in the respective directions decreases as the roughness values decrease, in particular, in directions other than the rolling direction. To achieve what has been described above, the rolling reduction in the final step is desirably set at 15.0% or less. The rolling reduction in the final step is more desirably 14.5% or less, and when productivity or shapes of the steel sheets are taken into account, the rolling reduction is desirably 10.0% or more. The lamination reduction in the final step is more desirably 12.0% or more.

Still further, the rolling speed (cold rolling speed) in the final step in the present embodiment is desirably set to 800 m/min or less. At the inlet of the roll gauge tool, the rolling oil remains in surface cavities which remain in the rolled stock, the oil is discharged into the roll gauge tool and thus the roll marks are transferred to the roll gauge tool. sheet steel. However, in the case that the rolling speed is fast, sufficient time for discharging the oil is not obtained, so the cavities are not sufficiently removed and it becomes difficult to decrease the roughness, particularly at the portions of the cavities. In order to sufficiently discharge the rolling oil in the cavity parts, sufficiently transfer a flat roll two-dimensionally, and reduce the roughness anisotropy, the cold rolling speed in the final step is desirably set to 800 m/ min or less The cold rolling speed in the final step is more desirably 600m/min or less and even more desirably 500m/min or less. When productivity, shapes of steel sheets and surface gloss are taken into account, the speed of cold rolling in the final step is desirably 150 m/min or more.

In addition, other conditions may be established in cold rolling taking into account the thickness of the plate or the surface finish of the products and, in the case that the steel plates are rolled in one direction using a tandem rolling machine which is an ordinary steel rolling machine, the conditions of the present embodiment can be applied to the final position. Also, the rolling oil can be liquid paraffin or water-soluble oil.

Table 2 shows the cold rolled sheet annealing conditions and grain size numbers (GSN) before and after brazing heat treatment. The steel numbers in Table 2 are the same as the steel numbers in Tables 3A through 3D below. The grain size number was evaluated as described below. A steel sheet produced using a method described below was cut so that a surface parallel to the rolling direction could be observed, and implanted in a resin. The grain size number was measured using a light microscope and slice method.

Table 2 (Steel Nos. A20 to A22 are reference examples)

Resultados de medición de GSN empo de Tiempo de GSN GSN

_{Cold Rolled Sheet} Annealing Conditions

GSN Measurement Results GSN Time GSN Time

maintenance maintenance (before (after perature at temperature treatment treatment GSN 650 °C of 950 ° thermal

50° 050° welding) welding

7.5 4.3 3.2

8.0 75.0 7.2 4.2 3.0 10.0 45.3

7.5 4.3 3.2

65.5 7.1

4.3 2.89.0

65.5 7.1

4.3 2.8

3.4

3.0 13.4 72.3 7.3

3.7 3.6 12.7 40.9 7.6 3.0 4.6 11.6 53.2 6.7 4.3 2.4 10.5 49.7 6.2 4.1 2.1 9.6 60.5 6.3

3.0

3.3 5.3 26.9

7.2 3.5 3.7 ^{14.6 71.1 6.4 3.1 3.3} 17.3 39.5

6.4 3.2 3.2 10.8 79.8 6.8 4.0

2.8

8.3 52.3 6.5 3.2 3.3 8.6 65.1 6.4

3.4

3.0 13.4 72.3 7.3

3.7 3.6 12.7 40.9 7.6 3.0 4.6 11.6 53.2 6.7 4.3 2.4 10.5 49.7 6.2 4.1 2.1 9.6 60.5 6.3

3.0

3.3 5.3 26.9

7.2 3.5 3.7 ^{14.6 71.1 6.4 3.1 3.3} 17.3 39.5

6.4 3.2 3.2 10.8 79.8 6.8 4.0

2.8

0.8

5.17.4 65.3 7.4 ^{j -» .ja} 4.1 ₁ 4.2 75.3 5.9

0.8

5.1

83.5

5.8 0.6 5.216.2

83.5

5.8 0.6 5.2

0.7

_J 4.8 86.3 5.4

0.3 5.14.5 42.0 5.9

0.7

_J 4.8 86.3 5.4

0.3 5.1

14.6

90.1

5.8 0.6

5.2

14.6

90.1

5.8 0.6

5.2

From Table 2 it follows that, in the case where the steel plate is kept at a temperature of 650 °C to 950 °C for less than 5.0 s or in the case where the steel plate is kept at a temperature of 950 °C to 1050 °C for more than 80.0 s, the amount of change in grain size number before and after brazing heat treatment (the amount of a grain size number changed by brazing heat treatment strong) becomes greater than 5.0. The large change of the grain size number before and after the brazing heat treatment leads to a large change in the mechanical properties of a stainless steel before and after the brazing heat treatment, and there is a fear of causing an accident or similar in components, and therefore it is desirable to avoid large change in grain size number between before and after brazing heat treatment. In the present embodiment, when the amount of change in the grain size number before and after the brazing heat treatment is 5.0, the mechanical properties change a lot. Therefore, the amount of change in grain size number before and after brazing heat treatment desirably decreases to 5.0 or less. The amount of change in grain size number before and after brazing heat treatment is more desirably 4.0 or less. Since the amount of change in the grain size number before and after the brazing heat treatment is preferably low, it is not necessary to set the lower limit value. However, since it is difficult to set the amount of change in grain size number to zero, the lower limit value is desirably set to more than zero.

In the present embodiment, it was found that, when a Laves phase such as Fe2Nb finely precipitates in a steel, this phase serves as a fixing factor and the amount of change in grain size number before and after brazing heat treatment decreases. The temperature at which the Laves phase precipitates is 650 °C to 950 °C, and the temperature at which the Laves phase dissolves is 950 °C to 1050 °C. Therefore, during the annealing of cold-rolled sheet, it is necessary to keep the cold-rolled sheet in a temperature range of 650°C to 950°C for a long period of time and to keep the cold-rolled sheet in a temperature range from 950 °C to 1050 °C for a short period of time. In the present embodiment, the annealing step preferably includes a step of maintaining the steel sheet at a temperature of 650°C to 950°C for 5.0 s or more and a step of maintaining a steel sheet at a temperature of 950°C. °C to 1050 °C for 80.0 s or less. It was found that, in such a case, it is possible to sufficiently precipitate the fine Laves phase which is effective for fixing the crystal grains. The annealing step more desirably includes a step of maintaining the steel sheet at a temperature of 650°C to 950°C for 8.0 s (seconds) or more and a step of maintaining a steel sheet at a temperature of 950°C. °C to 1050 °C for 60.0 s (seconds) or less. Also, when productivity is taken into account, the retention time during which the steel sheet is kept at a temperature of 650°C to 950°C is preferably 50 s or less. When proper recrystallization of the structure after cold rolling is taken into account, the retention time during which the steel sheet is kept at a temperature of 950°C to 1050°C is preferably 10 s or more.

Next, the chemical composition of a steel defined in the present embodiment will be described in more detail. Meanwhile, "%" indicates "percentage by mass".

C: Since C degrades intergranular corrosion resistance and workability, it is necessary to decrease the amount of C to a low level. Therefore, the amount of C is set to 0.030%. However, an excessively low amount of C helps coarsening of crystal grains during brazing and increases refining costs, and therefore the amount of C is set to 0.001% or more. The amount of C is more desirably from 0.004% to 0.020%.

Si: Si is a useful deoxidation element, but Si increases the activity of Ti; and thus the Si aids the generation of a hard Ti-based oxide. Therefore, the amount of Si is set in a range from 0.01% to 1.00%. The amount of Si is more desirable to be 0.10% to 0.60%.

Mn: Mn is a useful deoxidation element, but in the case that an excess amount of Mn is added, the Mn deteriorates the corrosion resistance, and therefore the amount of Mn is set in the range of 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 therefore it is necessary to limit the amount of P. Therefore, the amount of P is set to 0.050% or less. The amount of P is more desirably set to 0.030% or less.

S: S is an element that deteriorates corrosion resistance, and therefore it is necessary to limit the amount of S. Therefore, the amount of S is set to 0.0100% or less. The amount of S is more desirably set to 0.0050% or less.

Cr: Examples of possible corrosive environments include atmospheric environments, cooling water environments, exhaust gas condensing environments, and the like. To ensure corrosion resistance in the environments described above, at least 11.0% or more Cr is required. As the amount of Cr increases, corrosion resistance improves, but workability and workability decrease. manufacture and therefore the amount of Cr is set to 30.0% or less. The amount of Cr is more desirably 15.0% to 23.0%.

Mo: To improve corrosion resistance against condensed water, 0.01% or more Mo is required. However, in the case that an excess amount of Mo is added, workability is lowered and costs are increased due to to the high price of Mo, and therefore the amount of Mo is set at 3.00% or less. The amount of Mo is more desirably 0.10% to 2.50%.

Ti: Ti forms a poorly wettable oxide film on the surface and reduces brazing ability. Therefore, the amount of Ti is set between 0.001% and 0.050%. The amount of Ti is more desirably 0.001% to 0.030%.

Al: Al has an effect of deoxidation or the like, Al is an element useful for refining, and Al has the effect of improving moldability. In order to obtain these effects stably, 0.001% or more of Al is included. However, in the case that a large amount of Al is included, an oxide film with poor wettability is formed on the surface and the surface deteriorates. brazing ability. Therefore, the amount of Al is set to 0.030% or less. The amount of Al is more desirably 0.001% to 0.015%.

Nb: Since Nb carbonitrides prevent coarsening of crystal grains due to heating during brazing and thus a decrease in member strength is prevented, Nb is an element important. Also, Nb is useful for improving high-temperature strength or improving intergranular corrosion resistance of welded parts, but if excess amount of Nb is added, workability or manufacturability is lowered. and therefore the amount of Nb is set in a range from 0.010% to 1,000%. The amount of Nb is more desirably 0.100% to 0.600%.

O: O is an element that is inevitably included in a stainless steel. However, when O is present in the base material of a stainless steel, there is a possibility that O will cause the formation of inclusions such as oxides and lower various characteristics such as ductility or corrosion resistance. Therefore, the amount of O is reduced to 0.020% or less. The amount of O is more desirably 0.010% or less.

N: N is a useful element for pitting corrosion resistance, but N degrades intergranular corrosion resistance and workability, so it is necessary to decrease the amount of N to a low level. Therefore, the amount of N is set to 0.050% or less. The amount of N is more desirably 0.030% or less.

What has been described above is the chemical composition serving as the base of the ferritic stainless steel of the present embodiment: however, in the present embodiment, the ferritic stainless steel may further include the following items as required.

Ni: To improve corrosion resistance, 3.00% or less of Ni may be included. In order to obtain the effects stably, the amount of Ni should be 0.01% or more. The amount of Ni is more desirably 0.05% to 2.00%.

Cu: To improve corrosion resistance, 1,500% or less Cu may be included. In order to obtain the effects stably, the amount of Cu should be 0.050% or more. The amount of Cu is more desirably 0.100% to 1,000%.

W: To improve corrosion resistance, 1.000% or less of W can be included. To obtain the effects stably, the amount of W should be 0.010% or more. The amount of W is more desirably 0.020% to 0.800%.

V: In order to improve corrosion resistance, 0.300% or less of V can be included. To obtain the effects stably, the amount of V should be 0.010% or more. The amount of V is more desirably 0.020% to 0.050%.

Sn: To improve corrosion resistance, 0.500% or less of Sn may be included. In the case that Sn is included, the amount of Sn is 0.005% or more, at which the effects can be stably obtained. The amount of Sn is more desirably 0.01% to 0.300%.

Sb: To improve general corrosion resistance. 0.5000% or less of Sb may be included. In the case that Sb is included, the amount of Sb is 0.0050% or more, at which the effects can be stably obtained. The amount of Sb is more desirably 0.0100% to 0.3000%.

Mg: Mg has a deoxidizing or similar effect and is a useful element for refining. Also, Mg minimizes structure and Mg is also useful for improving workability and toughness, and 0.0030% or less Mg can be included as needed. In the case that Mg is included, the amount of Mg is 0.0001% or more, at which the effects can be stably obtained. The amount of Mg is more desirably 0.0001% to 0.001%.

Meanwhile, the total amount of one or more of Ni, Cu, W, V, Sn, Sb and Mg is desirably 6% or less from the viewpoint of increasing costs.

B: B is a useful element for improving secondary workability, and 0.0030% or less of B can be included. In the case that B is included, the amount of B is 0.0002% or more, which amount is can get the effects stably. The amount of B is more desirably 0.0005% to 0.0010%.

Ca: Ca is added for desulfurization, but in case excess amount of Ca is added, water-insoluble CaS inclusion is generated and corrosion resistance is lowered. Therefore, 0.0002% to 0.0100% Ca can be added. The amount of Ca is more desirably 0.0002% to 0.0050%.

Zr: To improve corrosion resistance, 0.300% or less of Zr can be included as required. In the case that Zr is included, the amount of Zr is 0.010% or more, at which the effects can be stably obtained. The amount of Zr is more desirably 0.020% to 0.200%.

Co: To improve secondary workability and toughness, 0.300% or less of Co can be included as needed. In the case that Co is included, the amount of Co is 0.010% or more, at which the effects can be stably obtained. The amount of Co is more desirably 0.020% to 0.200%.

Ga: To improve corrosion resistance and resistance to hydrogen embrittlement, the 0.0100% or less Ga as required. In the case that Ga is included, the amount of Ga is 0.0001% or more, at which the effects can be stably obtained. The amount of Ga is more desirably 0.0005% to 0.0050%.

Ta: To improve corrosion resistance, 0.0100% or less of Ta can be included as required. In the case where Ta is included, the amount of Ta is 0.0001% or more, at which the effects can be stably obtained. The amount of Ta is more desirably 0.0005% to 0.0050%.

REM: Since REM has a deoxidizing effect or the like, REM is a useful element for refinement, and 0.200% or less of REM can be included as needed. In the case that REM is included, the amount of REM is 0.001% or more, the amount at which effects can be stably obtained. The amount of REM is more desirably 0.002% to 0.100%.

Here, rare earth elements (REM) refer, by ordinary definition, to two elements scandium (Sc) and yttrium (Y) and fifteen elements (lanthanoids) from lanthanum (La) to lutetium (Lu). REM is one or more elements selected from these rare earth elements, and the amount of REM refers to the total amount of rare earth elements.

In a manufacturing method of the present embodiment, basically, an ordinary method for manufacturing a steel sheet made of ferritic stainless steel is applied. For example, molten steel having the chemical composition described above is produced using a converter or an electric furnace and refined using an AOD furnace, a VOD furnace or the like. After that, a slab is produced using a continuous casting method or an ingot casting method, and then the slab is subjected to the steps of hot rolling, hot rolled sheet annealing, pickling, cold rolling, finishing and stripping; and thereby, the ferritic stainless steel of the present embodiment is made. Annealing of hot rolled sheets may not be carried out if necessary. Cold rolling, finish annealing and pickling can be repeated.

However, as described above, in the cold rolling stage, in order to control the surface roughness, the steel sheets are rolled using a roll with roll roughness No. 60 or more in the final step under conditions in which the rolling reduction in the final pass is set to 15.0% or less and the cold rolling speed in the final pass is set to 800 m/min or less. Also, in order to precipitate the Laves phase as Fe2Nb in the steel, the step of annealing the cold-rolled sheet desirably includes a step of holding the steel sheet at a temperature of 650°C to 950°C for 5.0 s or more. and a step of maintaining the steel sheet at a temperature of 950 °C to 1050 °C for 80.0 s or less. That is, in the annealing stage, it is desirable to set the retention time during which the steel sheet is held at a temperature of 650 °C to 950 °C for 5.0 s or more, and to set the retention time during which the steel plate steel is maintained at a temperature of 950°C to 1050°C for 80.0 s or less.

examples

The present invention will be described in more detail using examples.

Steels with a composition shown in Tables 3A and 3B were produced, the steels were hot-rolled to a sheet thickness of 4 mm, the steels were annealed at 1050 °C for one minute, and then the steels were pickled. steels. After that, the steels were cold-rolled to a sheet thickness of 1 mm. In particular, cold rolling was carried out under conditions where the roll roughness in the final step of cold rolling, rolling reduction, and cold rolling speed were adjusted to the values shown in Fig. table 3C. In the annealing of cold-rolled sheets, the holding time at a temperature of 650 °C to 950 °C and the holding time at a temperature of 950 °C to 1050 °C were controlled, respectively, as shown in Fig. table 3C.

After that, test specimens with a width of 100 mm and a length of 100 mm were cut from the produced steel sheets. The arithmetic mean roughness values of the steel surface in the respective directions of the rolling direction (L direction), the direction perpendicular to the rolling direction (C direction), and the direction inclined at 45° to the direction rolling (V direction) were measured using a surface roughness and shape measuring instrument. The measurement length was set to 4.0 mm, the measurement speed was set to 0.30 mm/s, and the cut length was set to 0.8 mm. In each of the directions, the mean value of three measurement results was used as the roughness value of the arithmetic mean in that direction.

Also, the produced steel sheets were cut so that a surface parallel to the rolling direction could be observed, and implanted in a resin. The grain size number (GSN) was measured using the shear method.

Also, test specimens with a width of 60 mm and a length of 100 mm were cut from the produced steel sheets, and Ni brazing filler metal (0.2 g) was placed on the surface of each of the specimens. test sample and heated at 1200 °C for ten minutes in a vacuum atmosphere of 7 x 10-1 Pa (5 x 10.3 torr). Next, the test specimen was cooled to normal temperature, and an area of the metal of brazing filler (brazing filler metal area) on the surface of the test specimen after heating. Test specimens, in which the area of the brazing filler metal after heating was 2.5 times or more than the area of the brazing filler metal before heating, were rated A (excellent). Test specimens in which the brazing filler metal area after heating was 2 times (twice) or more to less than 2.5 times the brazing filler metal area before heating were evaluated as B (Okay). Test specimens in which the area of the brazing filler metal after heating was less than 2 times (twice) the area of the brazing filler metal before heating were evaluated as C (poor). Subsequently, the steel sheets which had been subjected to brazing heat treatment were cut so that a surface parallel to the rolling direction was observed, and implanted in a resin. The grain size number (GSN) was measured using the shear method.

Also, test specimens with a width of 25 mm and a length of 100 mm were cut from the cold-rolled steel sheets, and then all surfaces of the test specimens were wet-polished with sandpaper to n .° 600. These test specimens were evaluated by means of semi-immersion tests.

The imitation condensed water used in the semi-immersion tests was produced as described below. An aqueous solution containing 300 ppm Cl-, 1000 ppm SO42', and 1000 ppm SO32' was produced using hydrochloric acid, sulfuric acid, and ammonium sulfite as reagents. After the addition of the reagents, the pH was adjusted to 2.0 using ammoniacal water; and thereby the imitation of condensed water was obtained. A template was adjusted so that approximately half of the test specimen was submerged in the imitation condensed water at an angle of 55°. Half of the test specimen was immersed in imitation condensation water using this template, and the imitation condensation water was heated to 80 °C. The test was carried out for 168 hours and the solution was renewed every weekday.

For corrosion evaluation, the maximum depth of corrosion pitting was used. After the end of the tests, the corroded product was removed using an aqueous solution of diammonium hydrogen citrate, and the depth of the position where the test sample corroded the most deeply was measured using a focal depth method. With respect to the determination standard in the semi-immersion tests, it was confirmed that the growth of corrosion pitting became significant under these test conditions in the case where the maximum depth of corrosion pitting was 100 pm. Therefore, 100 pm was used as the cutoff value. A steel in which the maximum depth of corrosion pitting was less than 100 pm was evaluated as B (good). A steel in which the maximum depth of corrosion pitting was 100 pm or more was evaluated as C (poor).

Also, using these steel sheets, resin-implanted specimens were produced for L-section observation. The specimens were polished to a mirror finish and then observed using SEM, and inclusion compositions were analyzed by spectroscopy. energy dispersive X-rays (EDS). The results are shown in Tables 3D and 3E. Here, EDS refers to a method of element analysis in which a specimen is irradiated with electron beams, the characteristic X-rays that are generated are detected, and the elements that constitute a subject and their concentrations are investigated.

_{__}

3D table

Table 3D (continued) (Steel Nos.' A15 to A17 and A20 to A22 are reference examples)

The test results are shown in Tables 3D and 3E. It is found from Table 3D that the steels of the inventive examples were excellent in terms of brazing ability and corrosion resistance against condensed water. Also, it is found from Table 3E that, in the case where the amounts of the components were outside the ranges of the present embodiment, the corrosion resistance against condensed water deteriorated, except in the cases where the amount of Al or Ti was outside the range of the present embodiment. It was found that, in the case where the amount of Al or Ti was out of the range of the present embodiment, the brazing ability became poor. Also, it was found that, even when the amounts of the respective components were within the ranges of the present embodiment, in the case that the amounts of Al, Ti and Si did not meet the ratio of Al/Ti > 8.4Si - 0.78, a Ti-based hard oxide was generated on a steel, voids serving as pitting corrosion starting points were formed at the inclusion/base material interfaces, and corrosion resistance against condensed water deteriorated.

Also, in Nos. A1 to A14 steels, the roughness of a roll that was used in the final cold rolling (the last step of cold rolling) was set at No. 60 or more, the reduction in rolling in the final pass was set to 15.0% or less, and the cold rolling speed in the final P pass was set to 800 m/min or less. It was found that the steels manufactured under the conditions described above met both (RaL Rae 2Rav)/4 < 0.50 and |(RaL Rae - 2Rav)/2| < 0.10 and the brazing ability became more favorable. Likewise, in the steels Nos. A1 to A14 of the examples of the invention, in the annealing stage of the cold-rolled sheets, the maintenance time of the steel sheet at a temperature in the range of 650 °C to 950 °C was set to 5.0 s or more, and the retention time of the steel sheet at a temperature in the range of 950 °C to 1050 °C was set to 80.0 s or less. It was found that, in steels made under the conditions described above, the amount of change in grain size number before and after brazing heat treatment became 5.0 or less.

industrial applicability

Ferritic stainless steel with high corrosion resistance against exhaust gas condensed water of the present invention is suitable as members used in exhaust gas recirculation apparatus such as automobile mufflers, exhaust heat recovery devices and exhaust gas recirculation (EGR) coolers.

Claims (7)

1. A ferritic stainless steel with high corrosion resistance against exhaust gas condensation water and high brazing ability, comprising, in percent by mass: C: 0.001% to 0.030%; Yes: 0.01% to 1.00%; Mn: 0.01% to 2.00%; P: 0.050% or less; S: 0.0100% or less; Cr: 11.0% to 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%; Or: 0.020% or less; Y N: 0.050% or less, optionally further comprising, in percentage by mass, any one or more of the following: Ni: 0.01% to 3.00%; Cu: 0.050% to 1,500%; W: 0.010% to 1.000%; V: 0.010% to 0.300%; Sn: 0.005% to 0.500%; Sb: 0.0050% to 0.5000%; Y Mg: 0.0001% to 0.0030%, and/or optionally comprising, in addition, in percentage by mass, any one or more of the following: 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%; Y REM: 0.001% to 0.200%; being a remainder Fe and unavoidable impurities, where an amount of Al, an amount of Ti and an amount of Si in percentage by mass comply with Al/Ti = 8.4Si-0.78. when a rolling direction is represented by an L direction, a direction perpendicular to the rolling direction is represented by a C direction, a direction inclined at 45° to the rolling direction is represented by a V direction, and the values of arithmetic mean roughness of a steel surface in the respective directions are represented, respectively, by RaL, RaC, and Rav with a unit of gm, (RaL+Rac+2Rav)/4<0.50 and | (RaL+Rac+2Rav)/21 <0.10 and They are satisfied. 2. The ferritic stainless steel with high corrosion resistance against exhaust gas condensation water and high brazing ability according to claim 1, comprising, in percent by mass, any one or more of the following: Ni: 0.01% to 3.00%; Cu: 0.050% to 1,500%; W: 0.010% to 1.000%; V: 0.010% to 0.300%; Sn: 0.005% to 0.500%; Sb: 0.0050% to 0.5000%; Y Mg: 0.0001% to 0.0030%. 3. The ferritic stainless steel with high corrosion resistance against condensed water in exhaust gases and high brazing ability according to claim 1 or 2, comprising, in percentage by mass, any one or more of the following: 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%; Y REM: 0.001% to 0.200%. 4. The ferritic stainless steel with high corrosion resistance against condensed water in exhaust gases and high brazing ability according to any of claims 1 to 3, wherein the amount of change in grain size number, GSN, before and after heat treatment at 1200 °C for 10 minutes in a vacuum atmosphere of 7x10_1 Pa (5x10'3 torr) is 5.0 or less. 5. The ferritic stainless steel with high corrosion resistance against exhaust gas condensation water and high brazing ability according to any one of claims 1 to 4 which is used in automobile mufflers, exhaust heat recovery devices and EGR coolers which are automotive parts exposed to exhaust gas condensing environments. 6. A method for manufacturing a ferritic stainless steel with high corrosion resistance against condensed water in exhaust gases and high brazing ability according to any of claims 1 to 5, the method comprising: a step of cold rolling of a steel having the chemical components according to any of claims 1 to 3, wherein, in the cold rolling stage, in a final pass, the steel is rolled using a roll with a roll roughness of No. 60 or more under conditions where the rolling reduction in the final pass is established is set to 15.0% or less, and the cold rolling speed in the final step is set to 800 m/min or less. 7. The method for manufacturing a ferritic stainless steel with high corrosion resistance against condensed water in exhaust gases and high brazing ability according to claim 6, the method further comprising: a stage of annealing a cold-rolled steel sheet, wherein the annealing step includes: a step of holding the steel sheet at a temperature in the range of 650°C to 950°C for 5.0 s or more; and a step of maintaining the steel sheet at a temperature in the range of 950 °C to 1050 °C for 80.0 s or less.