CN105408511A - Ferritic stainless steel having excellent weld corrosion resistance - Google Patents

Abstract

The present invention provides ferritic stainless steel excellent in corrosion resistance of welded parts. A ferritic stainless steel characterized by containing, in mass %, C: 0.001 to 0.025%, Si: 0.05 to 0.30%, Mn: 0.35 to 2.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.05 to 0.80%, N: 0.001 to 0.025%, Cr: 16.0 to 20.0%, Ti: 0.12 to 0.50%, Nb: 0.002 to 0.050%, Cu: 0.30 to 0.80%, Ni: 0.05% to less than 0.50% %, V: 0.01 to 0.50%, and satisfy the following formula (1), and the balance is composed of Fe and unavoidable impurities. 0.50<25×C+18×N+Ni+0.11×Mn+0.46×Cu(1). In addition, the symbol of the element in a formula means content (mass %) of each element.

Description

Ferritic stainless steel with excellent corrosion resistance at welds

technical field

The present invention relates to a ferritic stainless steel whose corrosion resistance is less prone to decrease due to carbon and nitrogen intruded from a material to be welded into a weld seam or nitrogen intruded from the atmosphere during welding.

Background technique

Ferritic stainless steel can secure corrosion resistance with a smaller amount of Ni than austenitic stainless steel. Since Ni is an expensive element, ferritic stainless steel can be produced at a lower cost than austenitic stainless steel. In addition, compared with austenitic stainless steel, ferritic stainless steel has excellent characteristics such as high thermal conductivity, small thermal expansion coefficient, and less occurrence of stress corrosion cracking. Therefore, ferritic stainless steel is used in a wide range of applications such as automotive exhaust system components, building materials such as roofs, doors and windows, kitchen equipment, water storage tanks, and water piping materials such as hot water storage tanks.

When producing these structures, austenitic stainless steel, especially SUS304 (18%Cr-8%Ni) (JISG4305) and the like are often used in combination with ferritic stainless steel. As a welding method for such stainless steel, TIG welding is generally used. In this case, the welded portion is also required to have the same good corrosion resistance as the base metal portion.

In response to such a problem, a method has been proposed in which C and N are fixed in the form of carbonitrides of Ti or Nb by adding Ti or Nb, which has a higher affinity to C and N than Cr, and suppresses the formation of Cr carbonitrides. Suppresses the occurrence of sensitization and ensures good corrosion resistance. For example, Patent Document 1 discloses a steel in which the intergranular corrosion resistance of ferritic stainless steel is improved by the combined addition of Ti and Nb. In addition, Patent Document 2 discloses steel in which intergranular corrosion resistance has been improved by adding Nb.

However, both inventions require the addition of 0.3% by mass or more of Mo. Mo is an element that improves the corrosion resistance of the base material. However, Mo is a strong ferrite-forming element. Therefore, when 0.3% by mass of Mo is added, the formation of the ferrite phase in the welded part is promoted, and the sensitization of the welded part is promoted, so sufficient ferrite cannot be obtained. Corrosion resistance of the welded part. In addition, Nb needs to be added in an amount of 0.1% by mass or more. However, when welding steel containing a large amount of Nb, solid solution Nb is formed as coarse Nb precipitates at the welded portion, which may cause problems such as welding cracks. In consideration of manufacturability and practicability, it cannot be said that it is not the best measure to obtain the corrosion resistance of welded parts by only adding carbonitride forming elements such as Nb and Ti.

prior art literature

patent documents

Patent Document 1: Japanese Patent Laid-Open No. 2007-270290

Patent Document 2: Japanese Patent Laid-Open No. 2010-202916

Contents of the invention

The problem to be solved by the invention

Therefore, an object of the present invention is to provide a ferritic stainless steel having excellent corrosion resistance of welded parts without excessive addition of Ti and Nb.

method used to solve the problem

In order to solve the above-mentioned problems, the inventors of the present invention conducted intensive studies on conventional technologies capable of suppressing the reduction in corrosion resistance of welded parts. First, the inventors systematically examined the degree of permissible sensitization for obtaining sufficient corrosion resistance of welded parts using 16 to 20% by mass Cr steel. As a result, it was found that the reduction in the corrosion resistance of the welded portion due to sensitization was attributable to the grain boundary coverage of the ferrite phase coated with Cr carbonitride (for the grain boundary coverage, refer to the measurement method in the examples ) becomes obvious when it exceeds 40%.

Next, the present inventors studied a method of reducing the grain boundary coverage of the ferrite phase covered with Cr carbonitride. As a result, it was found that the addition of Mn and Cu, which are austenite-forming elements, to stabilize the austenite phase is extremely effective in improving the corrosion resistance of welded parts.

That is, it was found that when the austenite phase is stabilized by adding Mn and Cu, the generation of the ferrite phase during welding can be suppressed, and C and N is stabilized in the state of solid solution in the austenite phase whose solid solution limit is overwhelmingly larger than that of the ferrite phase, thereby preventing sensitization due to the generation of Cr carbonitrides and improving the corrosion resistance of welded parts Significantly increased.

In addition, it was found that since C and N are solid-dissolved in the austenite phase, compared with the conventional immobilization of C and N using only Ti, Nb, etc., it can be achieved even with a larger amount of C and N. Prevents sensitization. The inventors systematically examined steel components that can achieve the above-mentioned effects, and found that the above-mentioned effects are obtained when the contents of C, N, Ni, Mn, and Cu that stabilize the austenite phase satisfy the following formula (1).

0.50＜25×C+18×N+Ni+0.11×Mn+0.46×Cu(1)

In addition, the symbol of the element in a formula means content (mass %) of each element.

In addition, this effect can be used in combination with the immobilization of C and N by the addition of Ti and Nb, which is a conventional technology. Therefore, even without excessive addition of Ti and Nb, it is possible to obtain a steel that is exceptionally superior to existing steels. Corrosion resistance of welded parts.

The present invention was completed based on the above knowledge, and the gist thereof is as follows.

[1] A ferritic stainless steel excellent in corrosion resistance of welded parts, characterized by containing C: 0.001 to 0.025%, Si: 0.05 to 0.30%, Mn: 0.35 to 2.0%, P : 0.05% or less, S: 0.01% or less, Al: 0.05 to 0.80%, N: 0.001 to 0.025%, Cr: 16.0 to 20.0%, Ti: 0.12 to 0.50%, Nb: 0.002 to 0.050%, Cu: 0.30 to 0.80%, Ni: 0.05% to less than 0.50%, V: 0.01 to 0.50%, and satisfy the following formula (1), and the balance is composed of Fe and unavoidable impurities.

0.50＜25×C+18×N+Ni+0.11×Mn+0.46×Cu(1)

However, the symbol of an element in a formula means the content (mass %) of each element.

[2] The ferritic stainless steel excellent in corrosion resistance of welded parts according to [1], further comprising, in mass%, Zr: 0.01 to 0.50%, W: 0.01 to 0.20%, REM: 0.001% to 0.10%, Co: 0.01% to 0.20%, B: 0.0002% to 0.010%, and Sb: 0.05% to 0.30%.

Invention effect

According to the present invention, ferritic stainless steel having excellent corrosion resistance can be obtained even under welding conditions such that C and N penetrate into the base material from the material to be welded.

detailed description

Reasons for limiting the respective constituents of the present invention will be described below.

1. About composition and metal structure

First, the reason for specifying the component composition of the steel of the present invention will be described. In addition, all component % means mass %.

C: 0.001 to 0.025%

C is an element that is unavoidably included. The higher the amount of C, the higher the strength, and the smaller the amount, the higher the workability. In order to obtain sufficient strength, it is necessary to contain 0.001% or more. However, if the content exceeds 0.025%, the decrease in workability becomes remarkable, and the decrease in corrosion resistance (sensitization) tends to occur due to localized Cr deficiency caused by the precipitation of Cr carbides. Therefore, the amount of C is set within a range of 0.001 to 0.025%. However, from the viewpoint of corrosion resistance and workability, the lower the amount of C, the better. However, if the amount of C is extremely reduced, refining takes time, which is not preferable in terms of production. Therefore, the range of 0.003 to 0.018% is preferable. More preferably, it is the range of 0.005 to 0.012%.

Si: 0.05-0.30%

Si is an element effective in improving the corrosion resistance of welded parts. In order to obtain this effect, it is necessary to contain 0.05% or more, and the larger the content, the greater the effect. However, if the Si content exceeds 0.30%, the formability and toughness of the welded portion will decrease, which is not preferable. Therefore, the amount of Si is set within a range of 0.05 to 0.30%. Preferably it is in the range of 0.05 to 0.25%. More preferably, it is in the range of 0.08 to 0.20%.

Mn: 0.35～2.0%

Mn is a particularly important element in the present invention. Mn is an element effective as a deoxidizer, and has an effect of stabilizing the austenite phase. By containing a predetermined amount of Mn, the generation of ferrite phase during welding can be suppressed, and by dissolving C and N contained above the solid solution limit of the ferrite phase in The state in the austenite phase is stabilized, the effect of preventing sensitization due to the generation of Cr carbonitrides is exhibited, and the corrosion resistance of the welded portion is improved. In order to obtain these effects, it is necessary to contain 0.35% or more of Mn. However, when the Mn content exceeds 2.0%, the base metal is excessively hardened, the ductility is lowered, and the toughness of the welded part is lowered due to the hardening of the welded part, so it is not preferable. Therefore, the amount of Mn is set within a range of 0.35 to 2.0%. Preferably it is in the range of 0.50 to 1.5%. More preferably, it is in the range of 0.75 to 1.25%.

P: less than 0.05%

P is an element unavoidably contained in steel, and excessive content reduces weldability and easily causes intergranular corrosion. This tendency becomes remarkable when the content exceeds 0.05%. Therefore, the amount of P is set to 0.05% or less. Preferably it is 0.03% or less.

S: less than 0.01%

S, like P, is also an element that is unavoidably contained in steel, and the content of more than 0.01% will reduce the corrosion resistance. Therefore, the amount of S is set to 0.01% or less. Preferably it is 0.008% or less.

Al: 0.05-0.80%

Al, like Si, is also an element that improves the corrosion resistance of welded parts. The affinity between Al and N is stronger than that between Cr and N. Therefore, when N is mixed in the welding part, N is precipitated in the form of Al nitrides instead of Cr nitrides, which has the effect of suppressing sensitization. Effect. In addition, Al is also an element useful for deoxidation in the steelmaking process. These effects are obtained when the content is 0.05% or more. However, when more than 0.80% of Al is contained, the ferrite crystal grains will be coarsened, and workability and manufacturability will fall. Therefore, the amount of Al is set within a range of 0.05 to 0.80%. Preferably it is in the range of 0.10 to 0.60%. More preferably, it is in the range of 0.15 to 0.50%.

N: 0.001～0.025%

N, like C, is an element unavoidably contained in steel. The higher the N content, the higher the strength, and the smaller the N content, the better the workability. In order to obtain sufficient strength, the content of 0.001% or more is appropriate. However, if the content exceeds 0.025%, the ductility will be remarkably lowered, and the precipitation of Cr nitrides will be promoted to cause a decrease in corrosion resistance, so it is not preferable. Therefore, the amount of N is set within a range of 0.001 to 0.025%. From the viewpoint of corrosion resistance, the lower N is, the more preferable. However, it is necessary to increase the refining time to reduce the amount of N, resulting in a decrease in manufacturability. Therefore, it is preferable to set it as the range of 0.003 to 0.025%. More preferably, it is the range of 0.003 to 0.015%. More preferably, it is in the range of 0.003 to 0.010%.

Cr: 16.0～20.0%

Cr is the most important element for securing the corrosion resistance of stainless steel. If the Cr content is less than 16.0%, sufficient corrosion resistance cannot be obtained in and around the weld seam where the Cr content of the surface layer decreases due to oxidation caused by welding. In addition, sensitization due to N mixed in from the material to be welded or the atmosphere during welding is further promoted, which is not preferable. On the other hand, when the Cr content exceeds 20.0%, the toughness decreases and the descaling property after annealing decreases, which is not preferable. Therefore, the amount of Cr is set within a range of 16.0% to 20.0%. It is preferably in the range of 16.5% to 19.0%. More preferably, it is in the range of 17.0 to 18.5%.

Ti: 0.12 to 0.50%

Ti is an element that preferentially combines with C and N to suppress a decrease in corrosion resistance due to sensitization due to precipitation of Cr carbonitrides. The effect is obtained by containing 0.12% or more. However, if the content exceeds 0.50%, coarse Ti carbonitrides are formed to cause surface defects, which is not preferable. Therefore, the amount of Ti is set in the range of 0.12 to 0.50%. Preferably it is in the range of 0.15 to 0.40%. More preferably, it is in the range of 0.20 to 0.35%.

Nb: 0.002 to 0.050%

Nb is an element that preferentially combines with C and N to suppress a decrease in corrosion resistance due to sensitization due to precipitation of Cr carbonitrides. In addition, Nb also has the effect of making the crystal grain size of the welded part finer and improving the toughness and bendability of the welded part. These effects are obtained when containing 0.002% or more. On the other hand, Nb is also an element that raises the recrystallization temperature. When the content exceeds 0.050%, the annealing temperature required for recrystallization increases. If it is sufficient, it is not preferable because the mixture of non-recrystallized grains and recrystallized grains causes a reduction in workability. Therefore, the amount of Nb is set within a range of 0.002 to 0.050%. Preferably it is in the range of 0.010 to 0.045%. More preferably, it is in the range of 0.015 to 0.040%.

Cu: 0.30～0.80%

Cu is an element that improves corrosion resistance, and is an element that is particularly effective in improving the corrosion resistance of a base material and a welded part in an aqueous solution or when weakly acidic water droplets are attached. In addition, Cu is a strong austenite-forming element like Ni, and has the effect of suppressing the formation of a ferrite phase in a welded portion and suppressing sensitization due to the precipitation of Cr carbonitrides. These effects are obtained when containing 0.30% or more. On the other hand, when more than 0.80% of Cu is contained, it is unpreferable since hot workability will fall. Therefore, the amount of Cu is set in the range of 0.30 to 0.80%. Preferably it is in the range of 0.30 to 0.60%. More preferably, it is in the range of 0.35 to 0.50%.

Ni: 0.05% or more and less than 0.50%

Ni is an element that improves the corrosion resistance of stainless steel, and is an element that suppresses the progress of corrosion in a corrosive environment where active dissolution occurs without forming a passive film. In addition, Ni is a strong austenite-forming element, and has an effect of suppressing ferrite formation in welded parts and suppressing sensitization due to precipitation of Cr carbonitrides. These effects are acquired by containing 0.05% or more. However, when Ni is contained in an amount of 0.50% or more, the workability is lowered, and in addition, the sensitivity to stress corrosion cracking becomes stronger. In addition, Ni is an expensive element, which leads to an increase in production cost, and thus is not preferable. Therefore, the amount of Ni is set within a range of 0.05% or less and less than 0.50%. Preferably it is in the range of 0.10 to 0.30%. More preferably, it is in the range of 0.15 to 0.25%.

V: 0.01～0.50%

V is an element that improves corrosion resistance and workability, and has the effect of suppressing the generation of Cr nitrides by combining with N when N is mixed in the welded part, and reducing the sensitization of the welded part. This effect is obtained by containing 0.01% or more. However, when the content exceeds 0.50%, it is not preferable since workability will fall. Therefore, the amount of V is set within a range of 0.01 to 0.50%. Preferably it is in the range of 0.05 to 0.30%. More preferably, it is in the range of 0.08 to 0.20%.

0.50＜25×C+18×N+Ni+0.11×Mn+0.46×Cu(1)

In addition, the symbol of the element in a formula means content (mass %) of each element.

In order to dissolve and fix C and N in the welded portion in the austenite phase, it is necessary to generate the austenite phase in the cooled structure after welding. In order to form an austenite phase in the cooling process after welding, it is necessary to satisfy the above formula (1). When the right side of the formula (1) is 0.50 or less, the stabilization of the austenite phase becomes insufficient, and the solid solution C and the solid solution N for effectively immobilizing the solid solution C and the solid solution N provided by the present invention cannot be formed at the welded portion. degree of austenite phase. Therefore, the right side of the formula (1) is set to be larger than 0.5. Preferably it is 0.60 or more. More preferably, it is 0.70 or more.

The above are the basic chemical components of the present invention, and the balance is composed of Fe and unavoidable impurities. In addition, Ca: 0.0020% or less can be tolerated as an unavoidable impurity.

In addition, in addition to the above-mentioned basic components, the following elements may be contained for the purpose of suppressing sensitization of weld seams, improving corrosion resistance, and the like.

Zr: 0.01 to 0.50%

Zr has the effect of suppressing sensitization by combining with C and N. This effect is obtained by containing 0.01% or more. However, when the content exceeds 0.50%, workability will fall. In addition, since Zr is an expensive element, excessive addition will lead to an increase in production cost, so it is not preferable. Therefore, when Zr is contained, it is preferable to set it in the range of 0.01 to 0.50%. More preferably, it is the range of 0.10 to 0.35%.

W: 0.01～0.20%

W has the effect of improving corrosion resistance similarly to Mo. This effect is obtained by containing 0.01% or more. However, if the content exceeds 0.20%, the strength will increase, which will lead to a decrease in manufacturability due to an increase in rolling load, etc., which is not preferable. Therefore, when W is contained, it is preferable to set it as the range of 0.01-0.20%. More preferably, it is the range of 0.05 to 0.15%.

REM: 0.001～0.10%

REM has an effect of improving oxidation resistance, and is effective in suppressing the formation of a Cr-deficient region immediately below the welding temper color by suppressing the formation of scale. In order to obtain this effect, it is necessary to contain 0.001% or more. However, when the content exceeds 0.10%, manufacturability, such as pickling property, will fall. In addition, REM is an expensive element like Zr, so excessive content will lead to an increase in production cost, so it is not preferable. Therefore, when REM is contained, it is preferable to set it as the range of 0.001-0.10%. More preferably, it is the range of 0.010 to 0.08%.

Co: 0.01 to 0.20%

Co is an element that improves toughness. This effect is obtained by containing 0.01% or more. On the other hand, when the content exceeds 0.20%, manufacturability will fall. Therefore, when Co is contained, it is preferable to set it in the range of 0.01 to 0.20%. More preferably, it is the range of 0.05 to 0.15%.

B: 0.0002～0.010%

B is an element for improving secondary working brittleness, and its effect is obtained by containing 0.0002% or more. However, when the content exceeds 0.010%, the decrease in ductility due to excessive solid solution strengthening will be induced. Therefore, when B is contained, it is preferable to set it as the range of 0.0002 to 0.010%. More preferably, it is the range of 0.0010 to 0.0075%.

Sb: 0.05-0.30%

Sb, like Al, has the effect of trapping N mixed in from the atmosphere when the gas shielding of TIG welding is insufficient, and is an especially effective element when applied to a structure having a complex shape where sufficient gas shielding is difficult. The effect is obtained by containing 0.05% or more. However, when the content exceeds 0.30%, non-metallic inclusions are formed in the steelmaking process, and the surface properties are deteriorated. In addition, the toughness of the hot-rolled sheet is deteriorated. Therefore, when Sb is contained, it is preferable to set it as the range of 0.05-0.30%. More preferably, it is the range of 0.05 to 0.15%.

2. Regarding manufacturing conditions

Next, a preferable production method of the steel of the present invention will be described. The molten steel composed of the above components is smelted by a known method such as a converter, an electric furnace, a vacuum melting furnace, etc., and is made into a steel raw material (steel billet) by a continuous casting method or an ingot-slab casting method. The steel slab is heated at 1100 to 1250° C. for 1 to 24 hours and then hot rolled, or hot rolled as it is cast without heating to obtain a hot rolled sheet.

Usually, the hot-rolled sheet is annealed at 800-1100° C. for 1 to 10 minutes. In addition, depending on the application, the annealing of the hot-rolled sheet may be omitted. Next, after the hot-rolled sheet is pickled, it is cold-rolled to produce a cold-rolled sheet, and then recrystallization annealing and pickling are performed to produce a product.

From the viewpoint of elongation, bendability, press formability, and shape, cold rolling is preferably performed at a rolling reduction of 50% or more. Regarding the recrystallization annealing of the cold-rolled sheet, in general, in the case of JISG0203 surface finishing and No. 2B finishing products, from the viewpoint of obtaining good mechanical properties and from the viewpoint of pickling properties, it is preferably at 800 ~950°C. In addition, it is effective to perform BA annealing (bright annealing) during the finishing process for members at locations where gloss is further required. In addition, in order to further improve the surface properties after cold rolling and working, grinding etc. may be performed.

Example 1

Hereinafter, the present invention will be described in more detail based on examples.

Stainless steel having the chemical composition shown in Table 1 was melted in a 50 kg small vacuum melting furnace. These ingots were heated at 1150° C. for 1 hour, and then hot-rolled to obtain 3.5 mm-thick hot-rolled sheets. Next, after annealing these hot-rolled sheets at 950°C for 1 minute, the surface was subjected to shot blasting, and then immersed in a 20% by mass sulfuric acid solution at a temperature of 80°C for 120 seconds. % nitric acid and 3 mass % hydrofluoric acid by immersing in a mixed acid at a temperature of 55° C. for 60 seconds to carry out pickling and descaling.

Furthermore, it was cold-rolled to a plate thickness of 0.8mm, and recrystallization annealed at 900°C for 1 minute in a weakly reducing atmosphere (hydrogen: 5% by volume, nitrogen: 95% by volume, dew point: -40°C) to obtain a cold-rolled annealed plate. The cold-rolled annealed sheet was subjected to electrolytic pickling at a temperature of 50° C. in a mixed acid solution composed of 15% by mass nitric acid and 0.5% by mass hydrochloric acid to perform descaling treatment to obtain a cold-rolled pickled and annealed sheet.

It should be noted that Table 1-1 and Table 1-2 are a continuous series of tables.

Cold rolling with a plate thickness of 0.8 mm using the produced cold-rolled sheet and commercially available austenitic stainless steel SUS304 (C: 0.07% by mass, N: 0.05% by mass, Cr: 18.2% by mass, Ni: 8.2% by mass) The plate was subjected to butt TIG welding (butt TIG welding) (the cold-rolled plate of the present invention: base material, material to be welded: SUS304). The welding current was set to 90 A, the welding speed was set to 60 cm/min, and Ar gas containing 8 vol % nitrogen and 2 vol % oxygen was used as the shielding gas at 15 L/min. The width of the obtained weld seam on the surface side was about 3 mm.

The produced test piece including the weld was cut out, and the following test was performed.

1. Pitting potential test of base metal and welded part

A 20-mm-square test piece was cut out from the cold-rolled annealed test piece and the welded test piece, and a 10-mm-square measurement surface was left and covered with a sealing material. For the test piece after welding, the test piece was cut out so that the weld seam could be included, and the state where the temper color (oxide film) by welding remained remained. For these test pieces, the pitting potentials of the base metal and the welded portion were measured in a 3.5% by mass NaCl solution at 30°C. During the measurement, polishing and passivation treatment of the test piece were not performed, but other measurement methods were in accordance with JISG0577 (2005).

The pitting potential of the base material: 150 mV or more and the pitting potential of the welded part: 0 mV or more were set as acceptable.

2. Neutral salt water spray cycle test

A 100mm square test piece including the weld was cut from the welded test piece, and the surface was ground and finished with #600 sandpaper, and the end face was sealed, and the test piece was used for the neutral salt spray cycle test specified in JISH8502 . In the neutral salt spray cycle test, 5% by mass NaCl solution spray (35°C, 2 hours) → dry (60°C, 4 hours, relative humidity 20-30%) → moist (moistness) (40°C, 2 hours, relative humidity above 95%) for one cycle. The case where corrosion did not occur from the base metal or the welded part after implementing this test for 15 cycles was regarded as acceptable.

3. Determination of grain boundary coverage of ferrite phase coated with Cr carbonitride

A test piece for metal structure observation was cut out in a direction perpendicular to the weld seam of the welded test piece, mirror-polished, and then etched with an aqueous solution of picric hydrochloric acid to reveal the metal structure and precipitates, and then used a scanning electron microscope. The microstructure observation and phase identification of precipitates by energy dispersive X-ray spectroscopy were used to measure the grain boundary coverage of the ferrite phase in the weld portion covered with Cr carbonitrides.

Regarding the grain boundary coverage, the grain boundary length of the crystal grains in the taken structure photograph was measured by an image analysis device, and the length parallel to the grain boundary of Cr carbonitride precipitated on the grain boundary was measured by the same image analysis device. The diameter in the direction is determined by the formula of grain boundary coverage (%) = (sum of diameters in the direction parallel to the grain boundaries of Cr carbonitrides on the grain boundaries) ÷ (sum of grain boundary lengths of crystal grains) × 100 to calculate.

A case where the grain boundary coverage ratio of the ferrite phase covered with Cr carbonitride was 40% or less was regarded as acceptable.

4. Evaluation of mechanical properties

From the cold-rolled annealed sheet produced, the JIS13B number tensile test piece was cut out parallel to the rolling direction, and the tensile test was performed according to JISZ2241, and the elongation at break was measured.

The elongation at break was set at 25% or more as acceptable.

5. Surface quality evaluation

The surface of the steel sheet after cold rolling annealing and descaling was visually observed to confirm whether there were surface defects such as poor descaling and linear defects.

The case where there were no scale residues and surface defects and a good surface appearance was obtained was set as a pass standard.

The test results are shown in Table 2.

Steel Nos. 1 to 20 satisfying the requirements of the present invention all have a base metal pitting potential of 150 mV or more and a weld seam of 0 mV or more, and none of them were corroded by the neutral salt spray cycle test, even in Sufficient corrosion resistance was obtained even when welding with austenitic stainless steel. In addition, steel Nos. 1 to 20 after welding had a grain boundary coverage rate of ferrite phase coated with Cr carbonitrides of 40% or less, and a predetermined antisensitization effect was obtained. In addition, the elongation at break based on the tensile test was all 25% or more, good processing characteristics were obtained, and no surface defects were observed.

On the other hand, in Steel No. 21 in which the amount of Cr exceeded the range of the present invention, the toughness of the steel sheet after hot rolling was significantly lowered, the subsequent manufacturing process could not be carried out, and the characteristics could not be evaluated.

In Steel No. 22, whose Cr content is lower than the range of the present invention, sufficient pitting potential cannot be obtained, and in the neutral salt spray cycle test, corrosion occurs from the base metal and the welded part, and the specified corrosion resistance.

In Steel No. 23, in which the amount of Mn was less than the range of the present invention, sufficient corrosion resistance was obtained for the base material, but sufficient corrosion resistance was not obtained for the welded portion.

On the other hand, steel Nos. 24 and 25, in which the amount of Mn or Al exceeds the range of the present invention, although the predetermined base material and welded part corrosion resistance are obtained, but the ductility decreases due to the hardening of the steel plate. , the specified mechanical properties cannot be obtained.

Steel No. 26, in which the amount of Ti exceeds the range of the present invention, obtained the specified corrosion resistance and mechanical properties, but a large amount of coarse Ti-based inclusions generated surface defects, and the specified surface texture could not be obtained. .

Steel Nos. 27 to 30 in which the content of each element satisfies the range of the present invention, but the content of the austenite stabilizing element is less than the range of formula (1), the predetermined base metal corrosion resistance and mechanical properties are obtained. characteristic. However, the predetermined pitting potential of the welded portion and the corrosion resistance of the welded portion were not obtained. As a result of examining the cross-sectional structures of the welded parts of Nos. 27 to 30, it was confirmed that a very large amount of Cr carbonitrides precipitated at the grain boundaries of the ferrite phase at a grain boundary coverage rate of 50% or more . Steel Nos. 27 to 30 are considered to have insufficient austenite-stabilizing elements, and therefore, a ferrite phase was formed during cooling after welding, and the formation of Cr carbonitrides at the grain boundaries could not be suppressed, resulting in significant cracking. sensitization, and the predetermined corrosion resistance of the welded part cannot be obtained.

From the above results, it can be seen that according to the present invention, ferritic stainless steel having excellent corrosion resistance, mechanical properties, and surface properties can be obtained without excessive addition of Ti and Nb.

Industrial availability

The ferritic stainless steel obtained in the present invention is suitable for use in the production of structures by welding, for example, automotive exhaust system materials such as mufflers, building materials such as doors, windows, vents, and pipes.

Claims (2)

1. A ferritic stainless steel, characterized by containing C: 0.001 to 0.025%, Si: 0.05 to 0.30%, Mn: 0.35 to 2.0%, P: less than 0.05%, and S: 0.01% in mass % Below, Al: 0.05-0.80%, N: 0.001-0.025%, Cr: 16.0-20.0%, Ti: 0.12-0.50%, Nb: 0.002-0.050%, Cu: 0.30-0.80%, Ni: 0.05% or more and Less than 0.50%, V: 0.01 to 0.50%, and satisfy the following formula (1), the balance is composed of Fe and unavoidable impurities, 0.50＜25×C+18×N+Ni+0.11×Mn+0.46×Cu(1) However, the symbol of an element in a formula means the content (mass %) of each element. 2. The ferritic stainless steel according to claim 1, characterized in that, in mass %, it further contains Zr: 0.01-0.50%, W: 0.01-0.20%, REM: 0.001-0.10%, Co: One or more of 0.01 to 0.20%, B: 0.0002 to 0.010%, and Sb: 0.05 to 0.30%.