WO2015015735A1

WO2015015735A1 - Ferritic stainless steel having excellent weld corrosion resistance

Info

Publication number: WO2015015735A1
Application number: PCT/JP2014/003767
Authority: WO
Inventors: 正崇吉野; 太田　裕樹
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2013-07-29
Filing date: 2014-07-16
Publication date: 2015-02-05
Also published as: US20160168673A1; KR101809812B1; KR20160014685A; MY160981A; CN105408511A; CN105408511B; TWI526547B; JPWO2015015735A1; TW201512425A; JP5700182B1

Abstract

Provided is a ferritic stainless steel having excellent weld corrosion resistance. The ferritic stainless steel is characterized by containing, in terms of mass %, 0.001-0.025% of C, 0.05-0.30% of Si, 0.35-2.0% of Mn, 0.05% or less of P, 0.01% or less of S, 0.05-0.80% of Al, 0.001-0.025% of N, 16.0-20.0% of Cr, 0.12-0.50% of Ti, 0.002-0.050% of Nb, 0.30-0.80% of Cu, 0.05% to less than 0.50% of Ni, and 0.01-0.50% of V and satisfying formula (1), with the remainder consisting of Fe and unavoidable impurities. 0.50 < 25×C + 18×N + Ni + 0.11×Mn + 0.46×Cu　(1) Moreover, the chemical symbols in the formula denotes the content (mass %) of the elements in question.

Description

Ferritic stainless steel with excellent corrosion resistance of welds

The present invention relates to a ferritic stainless steel that is unlikely to deteriorate in corrosion resistance due to carbon and nitrogen entering a weld bead from a welding partner material or nitrogen entering from the atmosphere when welding is performed.

Ferritic stainless steel can ensure corrosion resistance with a smaller amount of Ni than austenitic stainless steel. Since Ni is an expensive element, ferritic stainless steel can be manufactured at a lower cost than austenitic stainless steel. Ferritic stainless steel has excellent properties such as higher thermal conductivity, lower thermal expansion coefficient, and less stress corrosion cracking than austenitic stainless steel. For this reason, ferritic stainless steel has been applied to a wide range of applications such as automobile exhaust system members, building materials such as roofs and fittings, and watering materials such as kitchen equipment, water storage tanks and hot water storage tanks.

In producing these structures, austenitic stainless steel, particularly SUS304 (18% Cr-8% Ni) (JIS G 4305), etc., and ferritic stainless steel are often used in combination. As such a stainless steel welding method, TIG welding is generally used. Even in such a case, it is required that the welded portion has good corrosion resistance like the base material portion.

In response to such problems, by adding Ti or Nb, which has a higher affinity for C and N than Cr, C and N are fixed as Ti or Nb carbonitrides, thereby suppressing the formation of Cr carbonitrides. Thus, a method for suppressing the occurrence of sensitization and ensuring good corrosion resistance has been proposed. For example, Patent Document 1 discloses a steel in which the intergranular corrosion resistance of ferritic stainless steel is improved by adding Ti and Nb in combination. Patent Document 2 discloses steel having improved intergranular corrosion resistance by adding Nb.

However, all inventions require addition of 0.3% by mass or more of Mo. Mo is an element that improves the corrosion resistance of the base material. However, since it is a strong ferrite-forming element, when 0.3% by mass of Mo is added, the formation of a ferrite phase in the welded portion is promoted and the sensitization of the welded portion is promoted. The corrosion resistance cannot be obtained. In addition, Nb addition of 0.1% by mass or more is required in all cases. However, when steel containing a large amount of Nb is welded, solid solution Nb is generated as coarse Nb precipitates at the weld, and problems such as weld cracking may occur. Obtaining the corrosion resistance of the welded part only by adding carbonitride-forming elements such as Nb and Ti is not an optimal measure in view of manufacturability and practicality.

JP 2007-270290 A JP 2010-202916 A

Therefore, an object of the present invention is to provide a ferritic stainless steel that does not require excessive addition of Ti or Nb and has excellent corrosion resistance of a welded portion.

In order to solve the above-mentioned problems, the present inventors diligently studied a technique that can suppress a decrease in the corrosion resistance of the welded portion more than before. First, the inventors systematically investigated the degree of sensitization allowed to obtain sufficient weld corrosion resistance using 16-20 wt% Cr steel. As a result, the deterioration of the corrosion resistance of the weld due to sensitization is caused when the grain boundary coverage of the ferrite phase by Cr carbonitride (for the grain boundary coverage, refer to the measurement method of the example) exceeds 40%. It has been found that it becomes apparent.

Next, the inventors examined a method for reducing the grain boundary coverage of the ferrite phase with Cr carbonitride. As a result, it has been found that stabilizing the austenite phase by adding Mn and Cu, which are austenite-forming elements, is extremely effective in improving the corrosion resistance of welds.

That is, when the austenite phase is stabilized by the addition of Mn and Cu, the formation of the ferrite phase during welding can be suppressed, and C and N contained above the solid solubility limit of the ferrite phase can be reduced more than the ferrite phase. By stabilizing in the austenite phase where the solid solubility limit is overwhelmingly large, it is possible to prevent sensitization due to the formation of Cr carbonitrides, and the corrosion resistance of the welded portion is greatly improved. I found out.

Furthermore, since C and N are dissolved in the austenite phase, it is possible to prevent sensitization even with a larger amount of C and N compared to conventional immobilization of C and N only by Ti, Nb or the like. It became clear. The inventors systematically investigated the steel components from which the above effects are obtained, and the above effects are obtained when the contents of C, N, Ni, Mn, and Cu that stabilize the austenite phase satisfy the following formula (1). I found out that
0.50 <25 × C + 18 × N + Ni + 0.11 × Mn + 0.46 × Cu (1)
In addition, the element symbol in a formula means content (mass%) of each element.

Furthermore, since this effect can be used together with C and N fixation by adding Ti or Nb, which is a conventional technique, it is much better than conventional steel without requiring excessive addition of Ti or Nb. Corrosion resistance of the welded portion can be obtained.

The present invention has been made based on the above findings, and the gist thereof is as follows.

[1] By 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% or more and less than 0.50%, V: 0.01 to 0.50%, and A ferritic stainless steel excellent in corrosion resistance of a welded portion, characterized in that the following formula (1) is satisfied and the balance is composed of Fe and inevitable impurities.
0.50 <25 × C + 18 × N + Ni + 0.11 × Mn + 0.46 × Cu (1)
In addition, the element symbol in a formula means content (mass%) of each element.

[2] Further, in terms of mass%, Zr: 0.01 to 0.50%, W: 0.01 to 0.20%, REM: 0.001 to 0.10%, Co: 0.01 to 0. It contains at least one selected from 20%, B: 0.0002 to 0.010%, Sb: 0.05 to 0.30%, and the corrosion resistance of the welded portion according to [1] Excellent ferritic stainless steel.

According to the present invention, a ferritic stainless steel having excellent corrosion resistance can be obtained even under welding conditions in which C and N enter the base metal from the welding counterpart.

Hereinafter, the reasons for limitation of each component of the present invention will be described.

1. Component Composition and Metallographic Structure First, the reason for defining the component composition of the steel of the present invention will be described. In addition, all component% means the mass%.

C: 0.001 to 0.025%
C is an element inevitably included. The higher the amount of C, the better the strength, and the lower the amount, the better the workability. In order to obtain sufficient strength, it is necessary to contain 0.001% or more. However, when the content exceeds 0.025%, the workability is remarkably lowered, and the corrosion resistance is lowered (sensitization) due to local Cr deficiency due to precipitation of Cr carbide. For this reason, the C content is in the range of 0.001 to 0.025%. However, the lower the amount of C, the better from the viewpoint of corrosion resistance and workability, but extremely reducing the amount of C takes time for refining and is not preferable in production. Therefore, it is preferably in the range of 0.003 to 0.018%. More preferably, it is in the range of 0.005 to 0.012%.

Si: 0.05-0.30%
Si is an element effective for improving the corrosion resistance of the weld. In order to acquire this effect, 0.05% or more of content is required, and the effect increases as the content increases. However, if the Si content exceeds 0.30%, the formability and toughness of the welded portion are not preferred. Therefore, the Si content is set in the range of 0.05 to 0.30%. Preferably it is 0.05 to 0.25% of range. More preferably, it is 0.08 to 0.20% of range.

Mn: 0.35 to 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 formation of a ferrite phase during welding can be suppressed, and C and N contained above the solid solubility limit of the ferrite phase have an austenite phase having a larger solid solubility limit than the ferrite phase. By stabilizing in a solid solution state, the effect of preventing sensitization due to the formation of Cr carbonitride 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, if the Mn content exceeds 2.0%, the base material is excessively hardened and ductility is lowered, and in the welded portion, the toughness is lowered due to the hardened welded portion, which is not preferable. Therefore, the amount of Mn is set in the 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: 0.05% or less P is an element inevitably contained in steel. Excessive content decreases weldability and easily causes intergranular corrosion. This tendency becomes remarkable when the content exceeds 0.05%. Therefore, the P content is 0.05% or less. Preferably it is 0.03% or less.

S: 0.01% or less S, like P, is an element inevitably contained in steel. Corrosion resistance is reduced by the inclusion of over 0.01%. Therefore, the S content is 0.01% or less. Preferably it is 0.008% or less.

Al: 0.05 to 0.80%
Al, like Si, is an element that improves the corrosion resistance of the weld. Since Al has a stronger affinity for N than Cr, when N is mixed in the welded portion, N is precipitated as Al nitride instead of Cr nitride and has the effect of suppressing sensitization. Al is also an element useful for deoxidation in the steel making process. These effects can be obtained with a content of 0.05% or more. However, when Al is contained exceeding 0.80%, ferrite crystal grains become coarse, and workability and manufacturability deteriorate. Therefore, the Al content is set in the range of 0.05 to 0.80%. Preferably, it is 0.10 to 0.60% of range. More preferably, it is in the range of 0.15 to 0.50%.

N: 0.001 to 0.025%
N, like C, is an element inevitably contained in steel. When the N content is high, the strength is improved, and as the N content is low, the workability is improved. In order to obtain sufficient strength, the content of 0.001% or more is appropriate. However, if the content exceeds 0.025%, the ductility is remarkably lowered and the corrosion resistance is reduced by promoting the precipitation of Cr nitride, which is not preferable. Therefore, the N content is set in the range of 0.001 to 0.025%. From the viewpoint of corrosion resistance, N is preferably as low as possible. However, in order to reduce the amount of N, it is necessary to increase the refining time, resulting in a decrease in manufacturability. For this reason, the range of 0.003 to 0.025% is preferable. More preferably, it is in the range of 0.003 to 0.015%. More preferably, it is 0.003 to 0.010% of range.

Cr: 16.0-20.0%
Cr is the most important element for ensuring the corrosion resistance of stainless steel. If the Cr content is less than 16.0%, sufficient corrosion resistance cannot be obtained in the weld bead where the surface layer Cr decreases due to oxidation by welding and in the vicinity thereof. Further, sensitization due to N mixed from the welding partner material or the atmosphere during welding is further promoted, which is not preferable. On the other hand, if the Cr content exceeds 20.0%, the toughness and the descalability after annealing decrease, which is not preferable. Therefore, the Cr content is set in the range of 16.0% to 20.0%. Preferably, it is 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 binds to C and N and suppresses a decrease in corrosion resistance due to sensitization due to precipitation of Cr carbonitride. This effect is obtained when the content is 0.12% or more. However, if the content exceeds 0.50%, coarse Ti carbonitrides are generated and cause surface defects, which is not preferable. Therefore, the Ti amount 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 0.20 to 0.35% of range.

Nb: 0.002 to 0.050%
Nb is an element that preferentially binds to C and N and suppresses a decrease in corrosion resistance due to sensitization due to precipitation of Cr carbonitride. Nb also has the effect of reducing the crystal grain size of the weld and improving the toughness and bendability of the weld. These effects are obtained when the content is 0.002% or more. On the other hand, Nb is also an element that raises the recrystallization temperature, and if it exceeds 0.050%, the annealing temperature necessary for recrystallization increases, so annealing in the annealing process using a high-speed cold-rolled sheet annealing line Is insufficient, and undesired grains and recrystallized grains coexist, resulting in a decrease in workability. Therefore, the Nb amount is set to a range of 0.002 to 0.050%. Preferably it is 0.010 to 0.045% of range. More preferably, it is in the range of 0.015 to 0.040%.

Cu: 0.30 to 0.80%
Cu is an element that improves the corrosion resistance, and is an element that is particularly effective for improving the corrosion resistance of the base material and the welded part when an aqueous solution or weakly acidic water droplets adhere. Further, Cu is a strong austenite-forming element like Ni, and has the effect of suppressing the formation of ferrite phase in the weld and suppressing sensitization due to the precipitation of Cr carbonitride. These effects are obtained when the content is 0.30% or more. On the other hand, if Cu exceeds 0.80%, the hot workability is lowered, which is not preferable. Therefore, the Cu content is set in the range of 0.30 to 0.80%. Preferably it is 0.30 to 0.60% of range. 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 a passive film cannot be formed and active dissolution occurs. Ni is a strong austenite generating element, and has the effect of suppressing ferrite formation at the weld and suppressing sensitization due to precipitation of Cr carbonitride. These effects are obtained when the content is 0.05% or more. However, when Ni is contained in an amount of 0.50% or more, workability is lowered and stress corrosion cracking sensitivity is increased. Furthermore, since Ni is an expensive element, it causes an increase in manufacturing cost, which is not preferable. Therefore, the amount of Ni is made 0.05% or less and less than 0.50%. Preferably it is 0.10 to 0.30% of range. More preferably, it is in the range of 0.15 to 0.25%.

V: 0.01 to 0.50%
V is an element that improves corrosion resistance and workability, and has the effect of suppressing the formation of Cr nitride and reducing the sensitization of the welded portion by combining with N when N is mixed in the welded portion. This effect is obtained when the content is 0.01% or more. However, if the content exceeds 0.50%, workability is lowered, which is not preferable. Therefore, the V amount is in the range of 0.01 to 0.50%. Preferably it is 0.05 to 0.30% of range. More preferably, it is 0.08 to 0.20% of range.

0.50 <25 × C + 18 × N + Ni + 0.11 × Mn + 0.46 × Cu (1)
In addition, the element symbol in a formula means content (mass%) of each element.

In order to fix and fix C and N in the weld zone in the austenite phase, it is necessary to generate an austenite phase in the cooled structure after welding. In order to generate the 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 austenite phase is not sufficiently stabilized, and the austenite phase that only effectively fixes the solid solution C and solid solution N provided by the present invention in the weld zone. Cannot be generated. Therefore, the right side of the formula (1) is over 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 consists of Fe and inevitable impurities. As an inevitable impurity, Ca: 0.0020% or less is acceptable.

Furthermore, in addition to the above basic components, the following elements may be contained for the purpose of suppressing the sensitization of the weld bead and improving the corrosion resistance.

Zr: 0.01 to 0.50%
Zr has an effect of binding to C and N to suppress sensitization. This effect is obtained when the content is 0.01% or more. However, if it exceeds 0.50%, workability is lowered. Moreover, since Zr is an expensive element, excessive addition causes an increase in manufacturing cost, which is not preferable. Therefore, when Zr is contained, the content is preferably in the range of 0.01 to 0.50%. More preferably, it is in the range of 0.10 to 0.35%.

W: 0.01-0.20%
W, like Mo, has the effect of improving corrosion resistance. This effect is obtained when the content is 0.01% or more. However, if the content exceeds 0.20%, the strength increases, and the productivity decreases due to an increase in rolling load, etc., which is not preferable. Therefore, when it contains W, it is preferable to set it as 0.01 to 0.20% of range. More preferably, it is in the range of 0.05 to 0.15%.

REM: 0.001 to 0.10%
REM has an effect of improving oxidation resistance, and is effective in suppressing the formation of a Cr-deficient region directly under the weld temper collar by suppressing the formation of oxide scale. In order to acquire this effect, 0.001% or more needs to be contained. However, if the content exceeds 0.10%, productivity such as pickling properties is lowered. Moreover, since REM is an expensive element like Zr, excessive inclusion causes an increase in manufacturing cost, which is not preferable. Therefore, when it contains REM, it is preferable to set it as 0.001 to 0.10% of range. More preferably, it is in the range of 0.010 to 0.08%.

Co: 0.01-0.20%
Co is an element that improves toughness. This effect is obtained when the content is 0.01% or more. On the other hand, when it contains exceeding 0.20%, manufacturability will be reduced. Therefore, when Co is contained, the content is preferably 0.01 to 0.20%. More preferably, it is in the range of 0.05 to 0.15%.

B: 0.0002 to 0.010%
B is an element that improves secondary work brittleness, and the effect is obtained by the content of 0.0002% or more. However, when it exceeds 0.010%, it induces a decrease in ductility due to excessive solid solution strengthening. Therefore, when B is contained, the content is preferably in the range of 0.0002 to 0.010%. More preferably, it is in the range of 0.0010 to 0.0075%.

Sb: 0.05 to 0.30%
Sb, like Al, has an effect of capturing N mixed in from the atmosphere when the gas shield of TIG welding is insufficient, and is applied to a structure having a complicated shape that makes it difficult to perform sufficient gas shield. Is a particularly effective element. The effect is acquired by 0.05% or more of containing. However, when it contains exceeding 0.30%, a nonmetallic inclusion will produce | generate in a steelmaking process and surface properties will deteriorate. Moreover, the toughness of a hot-rolled sheet is deteriorated. Therefore, when it contains Sb, it is preferable to set it as 0.05 to 0.30% of range. More preferably, it is in the range of 0.05 to 0.15%.

2. Next, a preferred method for producing the steel of the present invention will be described. Molten steel having the above component composition is melted by a known method such as a converter, electric furnace, vacuum melting furnace or the like, and a steel material (slab) is obtained by a continuous casting method or an ingot-bundling method. The slab is hot-rolled after heating at 1100 to 1250 ° C. for 1 to 24 hours, or directly hot-cast as cast without heating to form a hot-rolled sheet.

Usually, hot-rolled sheets are annealed at 800 to 1100 ° C. for 1 to 10 minutes. Depending on the application, the hot-rolled sheet annealing may be omitted. Next, after hot-rolled sheet pickling, it is cold-rolled by cold rolling, and then subjected to recrystallization annealing and pickling to obtain a product.

Cold rolling is desirably performed at a rolling reduction of 50% or more from the viewpoint of stretchability, bendability, press formability, and shape. The recrystallization annealing of cold-rolled sheets is generally performed according to JIS G 0203 surface finish, No. In the case of a 2B finished product, it is preferable to carry out at 800 to 950 ° C. from the viewpoint of obtaining good mechanical properties and pickling properties. In addition, it is effective to perform BA annealing (bright annealing) for finishing the member where the luster is desired. In addition, you may give grinding | polishing etc. in order to improve a surface property further after cold rolling and after a process.

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 steel ingots were heated at 1150 ° C. for 1 hour and then hot rolled to form 3.5 mm thick hot rolled sheets. Next, these hot-rolled sheets were subjected to hot-rolled sheet annealing at 950 ° C. for 1 minute, and then subjected to shot blast treatment on the surface, and then immersed in a 20% by mass sulfuric acid solution at a temperature of 80 ° C. for 15 seconds. The pickling was performed by dipping in a mixed acid composed of mass% nitric acid and 3 mass% hydrofluoric acid at a temperature of 55 ° C. for 60 seconds, and descaling was performed.

Further, it was cold-rolled to a thickness of 0.8 mm and recrystallized and annealed at 900 ° C. for 1 minute in a weakly reducing atmosphere (hydrogen: 5 vol%, nitrogen: 95 vol%, dew point: −40 ° C.). I got a plate. The cold-rolled annealed sheet was descaled by electrolytic pickling in a mixed acid solution consisting of a temperature of 50 ° C., 15% by mass nitric acid and 0.5% by mass hydrochloric acid to obtain a cold-rolled pickled annealed sheet. .

Table 1-1 and Table 1-2 are a series of continuous tables.

Thickness 0 of manufactured cold rolled sheet and commercially available austenitic stainless steel SUS304 (C: 0.07 mass%, N: 0.05 mass%, Cr: 18.2 mass%, Ni: 8.2 mass%) .8mm cold rolled plate,
Butt TIG welding was performed (cold rolled sheet of the present invention: base material, welding partner material: SUS304). The welding current was 90 A, the welding speed was 60 cm / min, and Ar gas containing 8 vol% nitrogen and 2 vol% oxygen was used at 15 L / min as the shielding gas. The width of the obtained front-side weld bead was about 3 mm.

The test piece including the produced weld bead was collected and the following test was performed.

1. Pitting corrosion potential test of base metal and welded part A test piece of 20 mm square was taken from a test piece after cold rolling annealing and a test piece after welding, and a test piece covered with a sealing material was left leaving a 10 mm square measurement surface. Produced. About the test piece after welding, the test piece was extract | collected so that a weld bead was included, and the temper color (oxide film) by welding was left. About these test pieces, the pitting corrosion potential of the base material and the weld was measured in a 3.5 mass% NaCl solution at 30 ° C. At the time of measurement, the specimen was not polished or passivated, but other measurement methods were based on JIS G 0577 (2005).

The pitting corrosion potential of the base material: 150 mV or more, and the pitting corrosion potential of the welded portion: 0 mV or more was regarded as acceptable.

2. Neutral salt spray cycle test A 100 mm square test piece including a weld bead was taken from the welded test piece, and the surface was polished with # 600 emery paper, and then a test piece with a sealed end face was prepared. The sample was subjected to a neutral salt spray cycle test specified in 8502. Neutral salt spray cycle test: 5% by weight NaCl solution spray (35 ° C., 2 h) → dry (60 ° C., 4 h, relative humidity 20-30%) → moistness (40 ° C., 2 h, relative humidity 95% The above is one cycle. The case where there was no occurrence of corrosion from the base metal or the welded part after 15 cycles was regarded as acceptable.

3. Measurement of grain boundary coverage of ferrite phase with Cr carbonitride Samples for metallographic observation were collected in the direction perpendicular to the weld bead of the welded test pieces, mirror-polished, and etched by picric acid hydrochloric acid aqueous solution. The structure was observed using a scanning electron microscope and energy dispersive X-ray spectroscopy, the phase of the precipitate was identified, and the grain boundary coverage of the ferrite phase of the weld bead by Cr carbonitride was measured. .

The grain boundary coverage is measured by measuring the grain boundary length of the crystal grains in the photographed structure photograph using an image analyzer, and the diameter in the direction parallel to the grain boundary of Cr carbonitride deposited on the grain boundaries is also measured. Measured with an analysis device, grain boundary coverage (%) = (total diameter in the direction parallel to the grain boundary of Cr carbonitride on the grain boundary) ÷ (total grain boundary length of crystal grains) × 100 Calculated by the formula.

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

4). Mechanical property evaluation A JIS No. 13B tensile test piece was taken in parallel with the rolling direction from the produced cold-rolled annealed plate, a tensile test was performed in accordance with JIS Z2241, and elongation at break was measured.

An elongation at break of 25% or more was considered acceptable.

5. Surface Quality Evaluation After cold-rolling annealing, the scaled steel sheet surface was observed with the naked eye, and it was confirmed that there were no surface defects such as poor descaling and linear wrinkles.

The acceptance criteria were that there was no scale residue or surface defects and a good surface appearance was obtained.

Table 2 shows the test results.

Steel No. satisfying the requirements of the present invention In all of Nos. 1 to 20, the pitting corrosion potential of the base metal is 150 mV or more, the pitting corrosion potential of the weld bead is 0 mV or more, and no corrosion occurs even in the neutral salt spray cycle test. Even when applied, sufficient corrosion resistance is obtained. Steel No. In all of Nos. 1 to 20, the grain boundary coverage of the ferrite phase by Cr carbonitride after welding is 40% or less, and a predetermined sensitization preventing effect is obtained. Furthermore, the elongation at break by the tensile test was 25% or more, and good processing characteristics were obtained, and no surface defects were observed.

On the other hand, steel No. with a Cr content exceeding the range of the present invention. In No. 21, the toughness of the steel sheet after hot rolling was remarkably lowered, the subsequent manufacturing process could not be carried out, and the characteristics could not be evaluated.

Steel No. whose Cr content is below the range of the present invention. In No. 22, a sufficient pitting corrosion potential was not obtained, and in the neutral salt spray cycle test, corrosion occurred from the base material and the welded portion, and the predetermined corrosion resistance could not be obtained.

Steel No. whose Mn content is below the range of the present invention. In No. 23, sufficient corrosion resistance was obtained for the base material, but sufficient corrosion resistance was not obtained for the welded portion.

On the other hand, the steel No. whose Mn amount or Al amount exceeds the range of the present invention. In 24 and 25, although the predetermined base metal and welded portion corrosion resistance were obtained, the ductility decreased due to the hardened steel sheet, and the predetermined mechanical properties could not be obtained.

Steel No. with Ti amount exceeding the range of the present invention. In No. 26, although predetermined corrosion resistance and mechanical properties were obtained, surface defects were caused due to the generation of a large amount of coarse Ti-based inclusions, and predetermined surface properties could not be obtained.

Although the content of each element satisfies the scope of the present invention, the content of the austenite stabilizing element is less than the range of the formula (1). In 27 to 30, predetermined base metal corrosion resistance and mechanical properties were obtained. However, the pitting corrosion potential of the predetermined weld bead and the corrosion resistance of the weld were not obtained. No. Examination of the cross-sectional structure of the welds 27 to 30 confirmed that an extremely large amount of Cr carbonitride was precipitated at the ferrite phase grain boundary with a grain boundary coverage of 50% or more. Steel No. In 27-30, since the austenite stabilizing element was insufficient, a ferrite phase was formed during cooling after welding, and the formation of Cr carbonitride on the grain boundary could not be suppressed. It is considered that the corrosion resistance of the predetermined weld was not obtained.

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

The ferritic stainless steel obtained in the present invention is suitable for applications in which structures are produced by welding, for example, automotive exhaust materials such as mufflers, building materials such as fittings, ventilation openings, ducts, etc. is there.

Claims

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 %: 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 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 the following formula ( A ferritic stainless steel satisfying 1), the balance being Fe and inevitable impurities.
0.50 <25 × C + 18 × N + Ni + 0.11 × Mn + 0.46 × Cu (1)
In addition, the element symbol in a formula means content (mass%) of each element.
Further, by 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%, 2. The ferritic stainless steel according to claim 1, comprising at least one selected from B: 0.0002 to 0.010% and Sb: 0.05 to 0.30%.