JP2010138470A - Ferritic stainless steel sheet excellent in corrosion resistance at sheared surface - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve corrosion resistance at a sheared surface of a ferritic stainless steel sheet used in an atmospheric environment without carrying out any treatment for imparting corrosion resistance. <P>SOLUTION: In the ferritic stainless steel sheet having a composition comprising ≤0.02% C, 0.05-0.8% Si, 0.05-1.0% Mn, ≤0.04% P, ≤0.1% Al, 20-24% Cr, 0.3-0.8% Cu, 0.05-6.0% Ni, ≤0.02% N and 0.001-0.1% S, the average crystal grain size in a ferritic phase is adjusted to 5-25 μm, and the density of MnS having particle size of 0.05-1 μm in the steel sheet is adjusted to 50-400 particles/cm<SP>2</SP>. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to a ferritic stainless steel sheet having excellent corrosion resistance on a shear end face.
In particular, the present invention is suitable for use in an atmospheric environment where the shear end face is not subjected to a corrosion resistance treatment while being subjected to a shearing process.

  Ferritic stainless steel is used in a wide range of applications because of its excellent corrosion resistance. Applications range from those that require little processing, such as the inner plate of an elevator, to those that require severe processing, such as automobile exhaust piping. In the manufacturing process of such a ferritic stainless steel sheet, cutting, shaping, and punching of the steel sheet by shearing are often performed for convenience. In general, this ferritic stainless steel sheet is used without subjecting the end surface to corrosion resistance.

  When ferritic stainless steel is sheared and used without end surface anticorrosion treatment, the end surface is more corrosive than a smooth surface, which causes flow rust and rust, resulting in overall corrosion resistance of the steel sheet. It causes a decrease. The problem of this end face rust has not been considered as important in ferritic stainless steels that maintain corrosion resistance to some extent by passivation even at the end face, unlike plated steel sheets where the end face is exposed from the steel. However, as the market for ferritic stainless steel has expanded, the use environment has expanded, and the difference in corrosion resistance between smooth surfaces and end faces has become a problem.

It is said that end face corrosion is caused by micro crevice corrosion due to unevenness. Crevice corrosion has been studied for a long time, and recently, ferritic stainless steel sheets having excellent crevice corrosion resistance have been disclosed in Patent Document 1 and Patent Document 2. Although these ferritic stainless steel sheets are effective against local corrosion such as crevice corrosion, they are not always sufficient to suppress the cracking at the shear end face, and end face corrosion may occur. .
From such a background, a ferritic stainless steel sheet that is particularly excellent in the corrosion resistance of the shear end face is desired.

JP 2005-89828 A JP 2006-257544 A

  The present invention advantageously solves the above problems, and aims to propose a ferritic stainless steel sheet with improved corrosion resistance of the shear end face of a ferritic stainless steel sheet used in the air environment without being subjected to corrosion resistance treatment. And

  The inventors made various studies in order to improve the corrosion resistance of the shear end face. In particular, close observation of the corrosion state of the shear end face shows that the starting point of corrosion is on the fracture surface, and reducing the fracture surface and reducing the surface roughness of the fracture surface will lead to prevention of corrosion. I found out. Here, as shown in FIG. 1, the fracture surface is confirmed by observing the processed surface after shearing, among the surface states called “sag”, “shear surface”, “fracture surface” and “burr”. One.

Therefore, as a result of further investigations regarding the improvement of the corrosion resistance, the conditions for shearing are set appropriately for the reduction of the fracture surface, while the surface roughness of the fracture surface is determined by the ferrite phase grains. It was found that controlling the diameter and the dispersion state of MnS is effective.
Furthermore, the inventors have also found that the corrosion resistance is further improved by adding 20% or more of Cr as a corrosion resistance improving component and adding Cu and Ni in combination.

The present invention is based on the above findings, and the gist of the present invention is as follows.
(1) By mass%, C: 0.02% or less, Si: 0.05 to 0.8%, Mn: 0.05 to 1.0%, P: 0.04% or less, Al: 0.1% or less, Cr: 20 to 24%, Cu: 0.3 to 0.8%, Ni: 0.05 to 6.0% and N: 0.02% or less, and S: 0.001 to 0.1%, the balance is made of Fe and inevitable impurities, and the average grain size of the ferrite phase is 5 to 5%. A ferritic stainless steel sheet having excellent corrosion resistance on a shear end face, wherein 50 to 400 MnS particles having a particle diameter of 0.05 to 1 μm are present in a steel in a range of 25 μm per cm ² .

  (2) The steel sheet further contains one or more selected from Nb: 0.005 to 0.6%, Ti: 0.005 to 0.6%, Zr: 0.5% or less, and Mo: 3.0% or less in mass%. The ferritic stainless steel sheet having excellent corrosion resistance of the shear end face according to (1) above.

  ADVANTAGE OF THE INVENTION According to this invention, the improvement of the corrosion resistance of a shear end face can be aimed at in the product which uses a shear end face as it is mainly in an atmospheric environment. As a result, the corrosion resistance of the entire ferritic stainless steel sheet can be improved, and as a result, it is possible to suppress loss of aesthetics, decrease in life, and the like due to corrosion of the steel sheet.

  The present invention will be specifically described below. First, the reason why the component composition is limited to the above range in the present invention will be described. In addition,% which shows the component of steel means the mass% unless there is particular notice.

C: 0.02% or less C is an element inevitably mixed in the steel, but if it exceeds 0.02%, it hardens and press workability is remarkably lowered, and Cr ₂₃ C ₆ is precipitated, and the grain boundaries are reduced. Sensitize to reduce corrosion resistance. Therefore, it is desirable that C is small, but 0.02% is acceptable.

Si: 0.05-0.8%
Si is an element useful as a deoxidizer. However, if the content is less than 0.05%, a sufficient deoxidation effect cannot be obtained, and a large amount of oxide is dispersed in the steel, so that weldability and press workability are deteriorated. On the other hand, if added over 0.8%, the steel becomes hard and the workability is reduced. Therefore, Si is limited to the range of 0.05 to 0.8%.

Mn: 0.05 to 1.0%
Mn has a deoxidizing action. In addition, in the present invention, it has been found that controlling the dispersion state of MnS has an effect of preventing a reduction in surface roughness at the fracture surface portion in the shear end face. The mechanism is not clear, but is presumed as follows. In other words, the presence of relatively fine MnS particles that do not affect the corrosion resistance facilitates the propagation of cracks on the fracture surface, and the shape of a linear fracture surface is likely to occur. However, the effect cannot be obtained if the amount of Mn is less than 0.05%. On the other hand, if it exceeds 1.0%, precipitation and coarsening of MnS are promoted, and conversely, corrosion resistance is reduced. Therefore, Mn is limited to a range of 0.05 to 1.0%.

P: 0.04% or less P is an element that lowers corrosion resistance. Moreover, since hot workability is reduced due to segregation at the crystal grain boundaries, excessive addition makes manufacture difficult. Therefore, the content is preferably low, but is acceptable up to 0.04% or less, preferably P: 0.03% or less.

Al: 0.1% or less Al is an effective component for deoxidation, but if it exceeds 0.1%, the surface damage due to Al-based non-metallic inclusions increases and the workability also decreases. Therefore, Al is made 0.1% or less.

Cr: 20-24%
Cr is an important element that determines the corrosion resistance of ferritic stainless steel. In the present invention, in order to further improve the corrosion resistance due to the combined effect with Cu described later, at least 20% or more of Cr is included.
The mechanism of improving the corrosion resistance by compounding Cr and Cu has not been clearly elucidated yet, but is presumed as follows.
When the Cr content is 20% or more, about one fifth of the atoms contained in the crystal lattice becomes Cr atoms. Since ferritic stainless steel has a body-centered cubic lattice, there are 4 first adjacent atoms and 6 second adjacent atoms. There will be one or more Cr atoms on average on the distance from any Cr atom to the second neighboring atom. Here, it is considered that Cr atoms on the steel surface act as oxides or hydroxides, interact with oxygen to the distance of the second adjacent atom, and form a passive film. Therefore, when it becomes 20% or more, it is considered that Cr forms a network and a passive film is easily formed. Further, in order to elute Fe and strengthen the passive film, a passive film made of Cr must be formed after Fe is eluted.
On the other hand, Cu is considered to have the effect of promoting the elution of Fe from the passive film by making the natural immersion potential noble and forming a stronger passive film. It is considered that the effect of addition of Cu becomes remarkable when 20% or more of Cr is easily formed and is easily passivated.
However, if the Cr content exceeds 24%, a σ phase is likely to be formed, leading to hardening of the material and a decrease in corrosion resistance. Therefore, Cr is limited to a range of 20 to 24%.

Cu: 0.3% to 0.8%
In addition to the above-mentioned combined effect with Cr, Cu alone has the effect of forming a film on the surface of stainless steel after the occurrence of corrosion and suppressing the dissolution of the ground iron due to the anode reaction. Therefore, it is an element useful for improving rust resistance and crevice corrosion resistance.
Furthermore, Cu has been recognized to have a combined effect with Ni described later. Here, the combined effect with Ni is an effect of making the potential of the steel sheet surface noble and preferentially dissolving Ni to suppress the pH drop. This effect cannot be expected so much when the Cu content is less than 0.3%. On the other hand, when the Cu content exceeds 0.8%, the dissolution of Cu itself is promoted and the corrosion resistance is lowered. Therefore, the addition amount of Cu is limited to the range of 0.3% to 0.8%.

Ni: 0.05-6.0%
In addition to the above-described combined effect with Cu, Ni is an element that alone, suppresses the anodic reaction due to acid, and enables the passive state to be maintained even at a lower pH. That is, Ni is highly effective in crevice corrosion resistance and significantly suppresses the progress of corrosion in the active dissolution state. In the present invention, it was confirmed that Ni was dissolved even in a passive state, thereby suppressing the pH drop in an environment where there was little solution circulation with the outside such as a gap. By the combined addition with Cu, Ni is preferentially dissolved with respect to Fe and Cr which are dissolved to lower the pH, and the effect becomes more remarkable.
If the Ni content is less than 0.05%, the effect of improving crevice corrosion resistance cannot be obtained. On the other hand, if the Ni content exceeds 6.0%, the steel is hardened and its workability is lowered. A phase structure is formed, and a difference occurs in the composition of each phase to form a microcell, thus reducing the corrosion resistance. Therefore, Ni is limited to the range of 0.05 to 6.0%. Note that it is desirable that Ni is contained by 0.1% or more, more preferably 0.2% or more.

N: 0.02% or less N is an element inevitably mixed in steel. It also has the effect of improving the corrosion resistance by dissolving in steel. However, if it exceeds 0.02%, the press workability is remarkably lowered. Therefore, N is set to 0.02% or less.

S: 0.001 to 0.1%
S is an important element in the present invention. Conventionally, it has been considered desirable to reduce S because it forms precipitate MnS with Mn and the like in stainless steel, which causes a decrease in corrosion resistance.
However, the inventors' research has found that even MnS, which has been considered unfavorable in the past, contributes to the improvement of the corrosion resistance of the shear end face if its particle size and dispersion state are appropriately controlled. Therefore, in the present invention, S is positively contained. However, when the content is less than 0.001%, the content effect is poor. On the other hand, when the content exceeds 0.1%, coarsening of the S-based precipitate is recognized, and it is difficult to obtain the effect. Therefore, S is limited to the range of 0.001 to 0.1%. More preferably, it is 0.001% or more and 0.05% or less of range.

  The basic components have been described above. However, in the present invention, the following elements can be appropriately contained for improving the corrosion resistance.

Nb: 0.005-0.6%
Nb is an element that fixes C and N, prevents sensitization by Cr carbonitride, and improves corrosion resistance. However, if the amount of Nb is less than 0.005%, the effect of addition is poor. On the other hand, if it exceeds 0.6%, a Laves phase is formed and the workability is lowered, so Nb is preferably in the range of 0.005 to 0.6%.

Ti: 0.005-0.6%
Ti is an element that fixes C and N, prevents sensitization by Cr carbonitride, and improves corrosion resistance. However, if the amount of Ti is less than 0.005%, the effect of addition is poor. On the other hand, if it exceeds 0.6%, the stainless steel plate is hardened and the workability is lowered. Further, the Ti-based precipitate causes a reduction in surface roughness. Therefore, Ti is preferably in the range of 0.005 to 0.6%.

Zr: 0.5% or less Zr also has the effect of fixing C and N as in the case of Ti and preventing sensitization by Cr carbonitride. However, if the amount of Zr exceeds 0.5%, ZrO _{2 and the} like are generated and cause surface scratches, so Zr is preferably 0.5% or less.

Mo: 3.0% or less Mo is an element that improves corrosion resistance. According to the present invention, the corrosion resistance of the shear end face can be improved by compound addition of Cr, Cu, and Ni, but the corrosion resistance is further improved by adding Mo. However, if the amount of Mo exceeds 3.0%, the workability is reduced, so Mo is preferably 3.0% or less.

  As described above, the component system has been described. However, in the present invention, it is not sufficient that the component composition is in the above range, and it is important that the average crystal grain size of the ferrite phase and the precipitation state of MnS are in the following ranges. .

Average crystal grain size of ferrite phase: 5 to 25 μm
The average crystal grain size of the ferrite phase, which is the main phase of the ferritic stainless steel sheet, affects the strength of the stainless steel sheet and the roughness of the fracture surface in the shear end face. The shear end face is formed with shear planes, fracture surfaces, burrs, etc. Of these, the smaller the area of the fracture surface, the more corrosion is suppressed, and the smaller the burr height, the more corrosion factors such as salinity are deposited. This makes it difficult to prevent corrosion.
Here, when the average crystal grain size of the ferrite phase is less than 5 μm, the strength is improved and burrs are hardly formed, but the increase in the area of the fracture surface is increased and the corrosion resistance is lowered.
On the other hand, when the average crystal grain size of the ferrite phase exceeds 25 μm, the corrosion resistance is greatly reduced due to the increase of burrs due to the effect of softening, the reduction of the fracture surface roughness and the formation of micro gaps. Accordingly, the average crystal grain size of the ferrite phase is set to 5 to 25 μm.
In order to control the grain size of the ferrite phase within the above range, the rolling rate in the hot rolling and cold rolling processes is important, and at least one pass in the rough rolling process in hot rolling is 40% or more. And the rolling reduction in cold rolling is preferably 70% or more.

MnS: 50 to 400 particles of 0.05 to 1 μm are present in the range of 1 cm ² Hereinafter, the reason why the precipitation state of MnS is limited to the above range will be described.
In the present invention, it was confirmed that the fracture surface of the end face subjected to shearing becomes a starting point of corrosion. This is mainly due to the fact that the surface is rough and corrosion factors are likely to accumulate, and the fine gap shape due to the unevenness promotes the low pH and high salt differentiation of the adhesion solution. This is considered to be because. Therefore, it is expected that a fracture surface that is unlikely to corrode is formed by reducing the roughness of the fracture surface of the shear surface.
Therefore, the present inventors conducted various experiments regarding the formation of a fracture surface where corrosion hardly occurs. As a result, it was found that the occurrence of corrosion was small when the fracture surface roughness was 1 μm or less in terms of arithmetic average roughness Ra.
Next, the present inventors paid attention to MnS and conducted a test for obtaining a condition in which the average crystal grain size of the ferrite phase is in the range of 5 to 25 μm and the surface roughness is Ra: 1 μm or less. As a result, it became clear that it is important that 50 to 400 MnS particles having a particle diameter of 0.05 to 1 μm in the steel exist in the range of 1 cm ² . Here, the particle size of the target MnS was in the range of 0.05 to 1 μm because the effect of reducing the roughness is small with particles of less than 0.05 μm MnS, and the surface of the MnS particles exceeding 1 μm appears on the surface. This is because the roughness is increased.
Therefore, the present invention is intended for MnS having a particle size of 0.05 to 1 μm.

Next, when the precipitation state of MnS was investigated, if the number of precipitates per unit area: 1 cm ² was less than 50, the effect of reducing the roughness was poor, whereas if more than 400, the workability and corrosion resistance were reduced. Turned out to be.
Accordingly, in the present invention, 50 to 400 MnS having a particle diameter of 0.05 to 1 μm are present per 1 cm ² .
In addition, in order to control the particle diameter and the number of precipitations of MnS within the above ranges, the processing temperature of each step of hot-rolled sheet annealing and cold-rolled sheet annealing is important. Preferably, the temperature is 850 to 1000 ° C. and the cold rolled sheet annealing temperature is 800 to 950 ° C.

Next, a shearing method that can reduce the fracture surface will be described. In order to achieve the above-mentioned object, the inventors conducted various experiments with various processing conditions, and it was found that clearance is particularly important for reducing the fracture surface ratio.
Here, the clearance is the ratio of the gap x between the blade and the table to the thickness d of the steel plate, as shown in FIG.
The clearance during shearing affects the area of the fracture surface in the shear end face and the height of the burr. As a result of investigating various clearances, it has been clarified that when the ferritic stainless steel of the present invention is set to 12% or less, the area of the fracture surface and the height of burrs can be kept small, and the corrosion resistance is improved. Therefore, the clearance during shearing is preferably 12% or less.

Next, the manufacturing method of the ferritic stainless steel plate of this invention is demonstrated.
In the present invention, as described above, the average particle diameter of the ferrite phase and the precipitation dispersion state of MnS are important. Therefore, it is important to carry out the above-described steps under the above-described conditions. Moreover, there is no restriction | limiting in particular about another process, A conventionally well-known method is applicable. Incidentally, typical production conditions are as follows.

Ferritic stainless steel is heated to 1150-1200 ° C, and then hot rolled to a thickness of 2.5-6 mm at a finishing temperature of 600-750 ° C. At this time, at least one pass of the rough rolling process is set to a rolling reduction of 40% or more. When the steel sheet is cooled at a normal rate after the finish rolling, the crystal grain size becomes coarse. Therefore, after the finish rolling, it is cooled to 500 ° C. or less at a cooling rate of 20 ° C./s or more.
Then, it is set as a hot-rolled steel strip wound up at 500 ° C. or less. Although the cooling rate after winding is not particularly defined, the cooling rate in the range of 425 to 525 ° C. is preferably 100 ° C./h or more because so-called 475 ° C. brittleness occurs near 475 ° C. The hot-rolled steel strip thus produced is annealed at a temperature of 850 to 1000 ° C. and pickled. Next, cold rolling with a rolling reduction of 70% or more is performed, cold-rolled sheet annealing is performed at a temperature of 800 to 950 ° C., and pickling is performed to produce a cold-rolled sheet.

After melting ferritic stainless steel having the composition shown in Table 1 and heating to a temperature of 1170 ° C, hot rolling was performed at a finishing temperature of 700 ° C and a coiling temperature of 450 ° C, and the plate thickness was 4 mm. The hot rolled sheet was used. Then, annealing was performed at 900 to 1000 ° C., pickling, cold rolling to a sheet thickness of 1.2 mm, and annealing at 920 ° C. to obtain a cold rolled sheet.
The crystal grain size and the number of MnS of the cold-rolled sheet thus obtained were measured by SIMS (secondary ion mass spectrometer). The grain size of the ferrite phase was determined in accordance with the method described in JIS G 0552.

  The ferritic stainless steel sheet obtained under the above production conditions was cut into 80 × 60 mm. At the time of cutting, shearing was performed with a clearance of 10%. After cutting out, degreasing with acetone was performed, and the burred surface was placed on the cycle corrosion tester at an inclination of 60 °, and a cycle corrosion test according to JASO M 609-91 was performed for 6 cycles. After the test, the case where corrosion did not occur on the shear end face was evaluated as ○, and the case where corrosion was observed was evaluated as ×.

The results obtained are shown in Table 1.
From the results of Test Nos. 1 to 5, it can be seen that the corrosion resistance of the shear end face is good when the Cr addition rate is in the range of 20.0 to 24.0%. From the results of Test Nos. 6 to 11, it can be seen that the corrosion resistance of the shear end face is good when the addition rate of Ni is in the range of 0.05 to 6.0%. From the results of Test Nos. 12 to 16, it can be seen that the corrosion resistance of the shear end face is good when the Cu addition rate is in the range of 0.3 to 0.8%. From the results of Test Nos. 17 to 21, it is understood that the corrosion resistance of the shear end face is good when the crystal grain size of the ferrite phase is in the range of 5 to 25 μm. From the results of Test Nos. 22 to 27, it can be seen that the number of MnS in the steel is 50 to 400 per cm ^{2 and} the corrosion resistance of the shear end face is good.

  According to the present invention, even if it is used in an atmospheric environment without carrying out the corrosion resistance treatment of the shear end face, the shear end face is excellent in corrosion resistance. Therefore, the elevator hood, duct hood, muffler cutter, drain groove cover, etc. It can use suitably for the use of.

It is the figure which showed the surface state of the shear end face. It is explanatory drawing of the clearance at the time of a shearing process.

Claims (2)

In mass%, C: 0.02% or less, Si: 0.05 to 0.8%, Mn: 0.05 to 1.0%, P: 0.04% or less, Al: 0.1% or less, Cr: 20 to 24%, Cu: 0.3 to 0.8%, Ni: 0.05-6.0% and N: not more than 0.02% and S: 0.001-0.1%, the balance is made of Fe and inevitable impurities, and the average crystal grain size of the ferrite phase is in the range of 5-25 μm A ferritic stainless steel sheet having excellent corrosion resistance at the shear end face, wherein 50 to 400 MnS particles having a particle diameter of 0.05 to 1 μm are present in 1 mm ^{2 in the} steel. The steel sheet further contains one or more selected from Nb: 0.005 to 0.6%, Ti: 0.005 to 0.6%, Zr: 0.5% or less, and Mo: 3.0% or less in mass%. The ferritic stainless steel sheet excellent in corrosion resistance of the shear end face according to claim 1.

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