KR20230018458A - ferritic stainless steel - Google Patents

Abstract

Chemical components, in mass%, C: 0.001 to 0.02% or less, Si: 0.02 to 1.5% or less, Mn: 1.5% or less, P: 0.040% or less, S: 0.006% or less, Cr: 10 to 25%, Al : 0.01 to 0.20%, Ti: 0.35% or less, Nb: 0.70% or less, O: 0.0005 to 0.010%, N: 0.005 to 0.025%, Ca: 0.0030% or less, and containing CaO with a maximum diameter of 2 μm or more Among the inclusions, the number ratio of inclusions having one or two or more types of M(C, N) on the outer periphery and the total area ratio of the M(C, N) part is 40% or more is 70% or more. stainless steel. M(C, N) represents a carbonitride of element M, and M is one or two or more elements selected from Ti, Nb, and Cr.

Description

ferritic stainless steel

The present invention relates to a ferritic stainless steel having excellent rust resistance.

Since stainless steel is generally provided for practical use without coating or the like, rust caused by CaS exposed on the surface of steel materials is a problem. As a mechanism for generating CaS, a type crystallized from molten steel and a type generated by reaction between inclusions containing CaO and S contained in the base material during heating of a cast steel after completion of solidification are known. The former type has been able to achieve low S stably due to the improvement in refining capacity in recent years, so there have been fewer problems with the latter type, while the latter type is still a problem in many cases, and measures are taken to suppress them. As a result, it is known that the solvent conditions are controlled.

For example, in Patent Document 1, by controlling the concentration of CaO in slag at the end of refining to 35% or less, S accumulation in CaO is suppressed, and by controlling the concentration of MgO to 30% or less, solid phase MgO having good lattice matching with CaS is obtained. A method capable of producing high-Al stainless steel having excellent rust resistance by preventing easy precipitation of CaS is disclosed.

In Patent Literature 2 and Patent Literature 3, the value of the formula relating to the composition of the inclusion represented by the X value is set to a certain level or less, and the formula consisting of [Ca], [S], [Al], T. [O] is It is characterized by providing a ferritic stainless steel with little rust by suppressing CaS generation by performing refining to satisfy the requirements.

Japanese Unexamined Patent Publication No. 5-339620 Japanese Unexamined Patent Publication No. 2012-184494 Japanese Unexamined Patent Publication No. 2014-162948

However, problems that could not be solved in the above technology existed.

Since the technique of Patent Literature 1 requires the CaO concentration of the slag to be at a low position, even in a high-Al component system, careful control is required to stably deoxidize, which increases the operational load. In addition, the desulfurization behavior tends to be unstable, and CaS and other sulfides are generated rather to deteriorate rust resistance in some cases.

In the techniques of Patent Literature 2 and Patent Literature 3, there are many restrictions on the composition of slag and the concentration of Ca or S in molten steel, and cost increase due to increase in refining load poses a problem.

Therefore, in view of the problems of the present situation, the present invention makes it an object to provide a ferritic stainless steel with low CaS and excellent rust resistance.

The present invention was made to solve the remaining problems as described above, and the gist thereof is as follows.

[1] Chemical components, in mass%, C: 0.001 to 0.02%, Si: 0.02 to 1.5%, Mn: 1.5% or less, P: 0.040% or less, S: 0.006% or less, Cr: 10 to 25%, Al: 0.01 to 0.20%, O: 0.0005 to 0.010%, N: 0.005 to 0.025%, Ca: 0.0030% or less, and Ti: 0.35% or less, Nb: 0.70% or less, or both, Among inclusions with a maximum diameter of 2 μm or more and containing CaO, with the balance being Fe and impurities, one or two or more types of M(C,N) are accompanied in the outer periphery, and the area ratio of the M(C,N) portion is 40 A ferritic stainless steel characterized in that the percentage of the number of inclusions of 70% or more is 70% or more.

Here, M(C, N) represents a carbonitride of element M, M is one or two or more elements selected from Ti, Nb, and Cr, and may contain less than 1% as the total of other elements.

[2] In addition to the chemical components described in [1], 1 of V: 2.0% or less, Zr: 0.0050% or less, B: 0.0001 to 0.0020%, Ga: 0.010% or less, in terms of mass%, in place of part of the Fe above. [1 The ferritic stainless steel described in ].

[3] In addition, instead of part of the above Fe, in terms of mass%, Mo: 2.0% or less, Mg: 0.0030% or less, REM: 0.01% or less, Ta: 0.001 to 0.100%, Ni: 0.1 to 2.0%, Sn: 0.01 to 0.50%, Cu: 0.01 to 2.00%, W: 0.05 to 1.00%, Co: 0.10 to 1.00%, and Sb: 0.01 to 0.30% in [1] or [2] containing one or two or more of them. ferritic stainless steels as described.

It is possible to provide a ferritic stainless steel containing CaS-containing inclusions as a starting point and having little rust.

1 is a diagram showing the relationship between the area ratio and coverage of M(C, N) portions.
Fig. 2 is a diagram showing the relationship between the ratio of inclusions satisfying the conditions of the present invention and the SST test results.
Fig. 3 is a diagram showing an observation surface of an inclusion and a method for measuring the size of an inclusion, which is a subject of the present invention.
Fig. 4 is a schematic diagram of an inclusion having an M(C, N) portion attached to the outer periphery of the inclusion.

Hereinafter, the contents of the present invention will be described in detail.

<Formation of CaS (cast steel stage)>

First, experiments leading to the idea of the present invention will be described.

Although CaS, which is considered to be the origin of rust in stainless steel, does not exist at the stage of casting, that is, the stage of steelmaking, as described above, S in the base metal diffuses during heating of the cast before hot rolling and reacts with CaO in inclusions to generate it. It is considered to be

Therefore, the influence of the cast steel heating conditions on the formation of CaS was investigated. Samples were cut from ferritic stainless steel slabs having various components, heated in an air atmosphere at 1000 to 1300 ° C. for 5 minutes to 3 hours, then air-cooled, and appropriate cross sections were cut out and polished to a mirror finish.

For 20 randomly selected inclusions having a maximum diameter of 5 μm or more, elemental shading mapping was performed using EPMA to confirm the formation of CaS. As a result, it was found that the generation of CaS is remarkable as the heating condition is at a high temperature for a long time, and that CaS is not generated even at a high temperature for a short time of about 5 minutes. From this, it can be seen that short-time heating such as annealing is excluded as a process including heating that can generate CaS.

In addition, under the condition of long-term heating, CaS production was small in samples with extremely low S such as [S] ≤ 5 ppm, but in other cases (in the case of [S] > 5 ppm), CaS and inclusions in which CaS were remarkably generated were completely absent. It was found that inclusions that were not produced were sometimes present in the same sample.

A more detailed investigation revealed that the periphery of the inclusions containing CaO was covered with carbides, nitrides, carbonitrides, etc. in the inclusions in which no CaS was generated. As described above, CaS is generated by diffusion of S contained in the parent material and reacting with CaO in the inclusions. As a result, the diffusion of S is physically blocked by carbides, nitrides, and carbonitrides covering the periphery of the inclusions, resulting in the generation of CaS. This is thought to be suppressed.

Next, as a result of examining carbides, nitrides, and carbonitrides present on the outer periphery of the inclusions containing CaO, it was found that elements other than C and N are Ti, Nb, Cr, V, Zr, B, Ga, and the like. there was. These may have a composition close to a pure substance, or may have a solid solution state of a plurality of carbides or nitrides. Therefore, carbides, nitrides, and carbonitrides are not distinguished from each other and are represented by M(C, N) below. As described above, M is not limited to one type, but may be two or more types. Also, in some cases, two or more phases of M(C, N) existing on the outer periphery of the CaO-containing inclusion coexisted.

<Type of inclusions (comparison between post-processing stage (steel plate stage) and cast steel stage)>

Next, with respect to the shape of the inclusion, the relationship between the stage after processing such as rolling and the cast steel stage was investigated.

Observation of inclusions in hot-rolled and cold-rolled sheets (hereinafter collectively referred to as "steel sheet") showed that M (C, N) was accompanied on the outer periphery of the inclusions containing CaO, but not covered in some cases. It was found that even among these inclusions, there were cases in which CaS was remarkably formed and cases in which it was hardly generated. As described above, since CaS is generated during heating of the cast steel, CaS is not generated when M(C, N) covers the outer periphery of the oxide portion of the inclusion containing CaO at the cast steel stage. From this, when CaS is remarkably formed with respect to inclusions containing CaO with M (C, N) after being processed and deformed, such as hot-rolled sheet and cold-rolled sheet, M (C, N ), and almost no CaS was formed, it is estimated that it was covered with M(C, N) at the cast steel stage. However, even when it is presumed to be covered with M(C, N) at the cast steel stage, at the steel plate stage, the outer periphery of the inclusion may be accompanied by M(C, N) but not covered, so CaS cannot be used as an indicator indicating that .

Therefore, an examination was conducted to find an index indicating that almost no CaS was generated by observing inclusions in the steel sheet stage.

First, for the inclusions containing CaO in the cast steel stage (see Fig. 4), the ratio of the M (C, N) portion 8 to the area of the inclusions 2 and the oxide portion of the inclusions containing CaO The relationship between the ratio (coverage rate) of M(C, N) covering the outer periphery was investigated. For the investigation, photographs were taken of 50 randomly selected CaO-containing inclusions using an optical microscope, and evaluation was performed using an image analysis device. First, the outer circumference length of the oxide portion of the inclusion containing CaO and the length of the portion in contact with M(C, N) were measured, and the coverage (%) was calculated by dividing the portion length by the outer circumference length and multiplying by 100. Second, the area of the inclusions containing CaO (the total area of the oxide portion and the M(C,N) portion) and the area occupied by M(C,N) in the inclusion are measured, the latter is divided by the former, and multiplied by 100 By doing so, the area ratio (%) of the M(C, N) portion was calculated. As shown in Fig. 1, when the ratio of M(C, N) to the area of the inclusion (the area ratio (%) of the M(C, N) portion) is 40% or more, the coverage is almost 100%, and most of It was found that M(C, N) covered the outer periphery of the oxide portion of the inclusion containing CaO.

In the inclusion, if the area ratio of the M(C, N) part is inherited from the cast steel stage to the steel plate stage, CaS is hardly generated by using the area ratio of the M(C, N) portion in the steel plate stage. There is a possibility that it can be applied as an index that represents, that is, an index that represents rust resistance. Then, after observation of inclusions, the hot-rolled sheet and the cold-rolled sheet were subjected to salt spray test - JIS-Z-2371 (hereinafter referred to as SST) to investigate rust repellency.

As for the observation of inclusions, as shown in Fig. 3, the surface of the steel sheet was mirror-polished to obtain a specular surface. In FIG. 3 , the rolling direction 4 , the thickness direction 5 , the sheet width direction 6 , and the steel sheet surface 7 are described, and the steel sheet surface 7 serves as the observation surface 1 . First, as a result of examining the diameter of the inclusions of the rust spots, it was found that almost no rust occurred in the inclusions having a maximum diameter of less than 2 μm. Therefore, in the present invention, an inclusion 2 containing CaO and having a maximum diameter 3 of 2 µm or more is evaluated. Subsequently, similarly to the cast steel step, the ratio of the area of the M(C, N) portion to the area of the inclusions (the total area of the oxide portion and the M(C, N) portion) (area ratio (%) of the M(C, N) portion) was evaluated. In addition, the relationship between the ratio of the number of inclusions having an area ratio (%) of the M (C, N) portion of 40% or more and the results of the investigation of rust repellency (SST) was evaluated. As shown in Fig. 2, when the ratio of the number of inclusions in which the ratio of the area of the M (C, N) part to the area of the inclusion (area ratio (%) of the M (C, N) part) is 40% or more is 70% or more, SST is evaluated. It was found that it was good when it was 5 or less, and even better when it was 85% or more.

As described above, by covering the outer periphery of the oxide portion of the inclusion containing CaO with M(C, N) at the cast steel stage, CaS generation during heating of the cast steel can be suppressed, and at the cast steel stage, M(C, When the area ratio of the N) portion was 40% or more, it was found that the proportion of M(C, N) covering the outer periphery of the oxide portion of the inclusion containing CaO at the cast steel stage was high. Further, in the observation of inclusions on the steel surface after processing such as rolling, among inclusions containing CaO and having a maximum diameter of 2 μm or more, one or two or more types of M(C, N) are accompanied in the outer periphery, and M(C, N) ) It was found that the results of SST were good when the ratio of the number of inclusions having an area ratio of 40% or more was 70% or more.

Therefore, in the present invention, among inclusions containing CaO and having a maximum diameter of 2 μm or more, inclusions having one or two or more types of M(C,N) in the outer peripheral portion and having an area ratio of 40% or more in the M(C,N) portion The number ratio of is defined as 70% or more.

<strong component>

As described above, the present invention relates to inclusion composition control, and is applicable to generally manufactured ferritic stainless steels. A range of components that can be suitably used is shown below, but is not limited thereto.

C: 0.001 to 0.02%

C is a component of M(C, N) that suppresses the production of CaS, and 0.001% or more is required for the formation of M(C, N). harder to create Preferably it should contain 0.003% or more. On the other hand, when it is contained excessively, workability is reduced, so it is set to 0.02% or less. Preferably it is 0.015% or less.

Si: 0.02 to 1.5%

Si is an element that promotes the generation of M(C, N) in order to lower the solubility of N. As other effects, it is also an effective element for desulfurization by accelerating deoxidation and, for example, since it can indirectly suppress CaS production during solidification, it is an effective element for suppressing CaS production. In order to express these effects, 0.02% or more is required, and it is preferable to add 0.05% or more. However, when added in excess of 1.5%, workability deteriorates. In particular, in applications where workability is a problem, it is preferable to set it as 1.0% or less.

Mn: 1.5% or less

Since Mn is an element that contributes to deoxidation, it may be added as preliminary deoxidation before adding Al. When added, in order to express the effect, it is good to set it as 0.01% or more, Preferably it is good to set it as 0.05% or more. On the other hand, in order to reduce workability, it is set as 1.5% or less. In particular, in applications where workability is a problem, it is preferable to set it as 0.3% or less.

P: 0.040% or less

P is a particularly harmful element in stainless steel, such as reducing toughness, hot workability, and corrosion resistance. Preferably it is 0.035% or less, More preferably, it is 0.030% or less. However, since excessive reduction requires a high load during refining or the use of expensive raw materials, it may be contained in an amount of 0.005% or more in actual operation.

S: 0.006% or less

Although CaS generation during heating of cast steel can be suppressed by the above-mentioned requirements, if S is contained in excess of 0.006%, before solidification or during solidification, that is, M(C, N) covers inclusions containing CaO. Since CaS is generated before and leads to rust, the upper limit is made 0.006%. A preferable upper limit is 0.003%.

Cr: 10 to 25%

Cr is an important element that brings corrosion resistance to stainless steel, and needs to be added in an amount of 10% or more, preferably 15% or more. On the other hand, since a large amount of addition causes a decrease in workability, the upper limit is set to 25%, preferably 21% or less.

Al: 0.01 to 0.20%

Al is an element added to deoxidize molten steel, and is also required to reduce S to 0.006% or less. Therefore, the lower limit is made into 0.01%. A preferable lower limit is 0.05%. Excessive addition reduces workability, so the upper limit is made 0.20%. A preferable upper limit is 0.15%.

Since Ti or Nb becomes the main component of M(C, N), at least one of Ti and Nb needs to be added.

Ti: 0.35% or less

Ti can form M(C, N) by adding it. In order to produce M(C, N) whose main component is Ti, it is preferable to add 0.01% or more, and a preferable addition amount is 0.05% or more. When added excessively, a large amount of TiN is generated before or during casting, causing nozzle clogging and surface defects of the product, so the upper limit is set to 0.35%.

Nb: 0.70% or less

Nb can form M(C, N) by addition. In order to produce M(C, N) containing Nb as a main component, the effect is expressed by adding 0.003% or more. A preferable addition amount is 0.2% or more. On the other hand, if it is added in excess of 0.70%, recrystallization becomes difficult and the structure becomes coarse, so it is set to 0.70% or less. Preferably, it is good to set it as 0.6% or less.

O: 0.0005 to 0.010%

O forms inclusions containing CaO that can generate CaS upon heating the cast steel. Since the amount of inclusions containing CaO increases as the concentration of O increases, it becomes difficult to cover with M(C, N), so the upper limit is set to 0.010%. However, since excessive deoxidation increases the refining load and causes a cost increase, the lower limit is made 0.0005%. O stands for T.O.

N: 0.005 to 0.025%

N is an element that forms M(C, N) that suppresses the production of CaS, and the effect is expressed by adding 0.005% or more. The higher the concentration, the more M (C, N) is generated, making it difficult to produce CaS. On the other hand, if it is contained excessively, a large amount of Cr nitride is generated, causing Cr deficiency at the grain boundary, and rather reducing corrosion resistance or remarkably reducing workability. Therefore, the content is set to 0.025% or less. Preferably it is 0.020% or less.

Ca: 0.0030% or less

Although CaS generation during heating of cast steel can be suppressed by the above-mentioned requirements, if Ca is contained in excess of 0.0030%, before solidification or during solidification, that is, M(C, N) covers inclusions containing CaO. Since CaS is generated before and leads to rust, the upper limit is made 0.0030% or less. The higher the Ca, the larger the amount of CaO-containing inclusions and the larger the M(C, N) required for covering, so the smaller the better, preferably 0.0020% or less. More preferably, it is 0.0010% or less. It is not necessary to contain Ca.

The remainder of the steel component is Fe and impurities. Here, the impurity is a component that is mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when steel is industrially manufactured, and means that it is permitted within a range that does not adversely affect the present invention.

In addition, in the ferritic stainless steel of the present embodiment, instead of a part of Fe, further by mass%, V: 2.0% or less, Zr: 0.0050% or less, B: 0.0001 to 0.0020%, Ga: 0.010% or less. Or you may include 2 or more types. The lower limit of these elements in the case of not containing these elements is 0%.

V: 2.0% or less

V itself has an effect of improving corrosion resistance, and since it forms M(C, N) to suppress the production of CaS, it may be contained as needed. Addition of 0.02% or more is preferred. A preferable addition amount for producing M(C, N) containing V as a main component is 0.1% or more. Since toughness will fall when V is contained excessively, it is set as 2.0% or less. It is preferably 1.0% or less, and more preferably 0.5% or less.

Zr: 0.0050% or less

Since Zr forms M(C, N) and suppresses the generation of CaS, it may be contained as needed. A preferable addition amount for producing M(C, N) containing Zr as a main component is 0.0010% or more. However, when it is added excessively, sulfide is formed in the solvent step, and corrosion resistance is rather deteriorated. Therefore, the upper limit is made into 0.0050%.

B: 0.0001 to 0.0020%

In addition to having an effect of increasing the strength of grain boundaries, B suppresses the generation of CaS by forming M(C, N), so it may be contained in an amount of 0.0001% or more as needed. However, if B is contained excessively, workability is reduced due to a decrease in elongation, so the content is set to 0.0020% or less. Preferably it is 0.0010% or less.

Ga: 0.010% or less

Ga itself has the effect of enhancing corrosion resistance, and since it forms M(C, N) to suppress the generation of CaS, it can be contained in an amount of 0.010% or less as needed. Although the lower limit of Ga is not particularly limited, it is preferable to contain 0.001% or more to obtain a stable effect.

Also, instead of part of Fe, Mo: 2.0% or less, Mg: 0.0030% or less, REM: 0.01% or less, Ta: 0.001 to 0.100%, Ni: 0.1 to 2.0%, Sn: 0.01 to 0.50%, Cu: 0.01 to 0.01% 2.00%, W: 0.05 to 1.00%, Co: 0.10 to 1.00%, and Sb: 0.01 to 0.30%. The lower limit of these elements in the case of not containing these elements is 0%.

Mo: 2.0% or less

By adding Mo, there is an effect of further enhancing the high corrosion resistance of stainless steel. However, since it is very expensive, even if it is added in excess of 2.0%, the effect corresponding to the increase in alloy cost is not obtained, and sigma phase is formed, resulting in embrittlement and a decrease in corrosion resistance. Therefore, the upper limit is made into 2.0%. A preferable lower limit is 0.5%, and a preferable upper limit is 1.5%.

Mg: 0.0030% or less

Since Mg is an effective element for deoxidation and desulfurization, it may be contained as needed. However, when added excessively, sulfides are formed before or during casting, and corrosion resistance is rather deteriorated. Therefore, the upper limit is made into 0.0030%.

REM: 0.01% or less

Since REM (rare-earth metal: Rare-Earth Metal) has a high affinity for O and S, it is an element effective for deoxidation and desulfurization, and may be contained as necessary. However, if added excessively, a large amount of oxide is generated before or during casting, causing nozzle clogging and surface defects of the product, so 0.01% is the upper limit.

Ta: 0.001 to 0.100%

Since Ta is an element effective for deoxidation and desulfurization, it may be contained as needed. What is necessary is just to contain 0.001% or more in order to acquire this effect. However, since excessive addition causes a decrease in room temperature ductility and a decrease in toughness, the upper limit is made 0.100%.

Ni: 0.1 to 2.0%

Since Ni has an effect of enhancing corrosion resistance, it can be added as needed. In order to obtain this effect, addition of 0.1% or more is required. On the other hand, since it is an expensive element and the effect corresponding to the increase in alloy cost is not obtained even if it adds more than 2.0%, the upper limit is made into 2.0%. Preferably, it is good to set it as 1.5% or less.

Sn: 0.01 to 0.50%

By adding Sn, there is an effect of further enhancing the high corrosion resistance of stainless steel. When contained, in order to acquire this effect, what is necessary is just to contain 0.01% or more, Preferably it is good to be 0.02% or more. On the other hand, excessive addition leads to a decrease in workability, so it is good to make it 0.50% or less, preferably 0.30% or less.

Cu: 0.01 to 2.00%

Since Cu has an effect of enhancing corrosion resistance, it can be added as needed. In order to obtain this effect, addition of 0.01% or more is required. However, since excessive addition leads to brittleness, it is set as 2.00% or less.

W: 0.05 to 1.00%

Since W has an effect of enhancing corrosion resistance, particularly pitting resistance, it can be added as necessary. In order to obtain this effect, addition of 0.05% or more is required. However, since excessive addition causes a decrease in toughness, the upper limit is made 1.00%.

Co: 0.10 to 1.00%

Since Co has an effect of increasing the strength of steel materials, it can be added as needed. In order to obtain this effect, addition of 0.10% or more is required. However, since excessive addition causes a decrease in toughness, the upper limit is made 1.00%.

Sb: 0.01 to 0.30%

Since Sb has an effect of enhancing corrosion resistance, it can be added as needed. In order to obtain this effect, addition of 0.01% or more is required. However, since excessive addition causes a decrease in productivity, the upper limit is made 0.30%.

<Method of measuring inclusions>

Hereinafter, a method for measuring inclusions will be described. As shown in FIG. 3 , the surface of the steel plate 7 is observed as the observation surface 1 . The position of the observation surface 1 in the depth direction of the steel plate is set to the outermost layer as much as possible, and minimal polishing required for mirror finishing for observation is performed. On the observation surface (1), randomly select 100 or more inclusions (2) containing CaO and having a maximum diameter of 2 μm or more, set this as a population, and analyze the inclusions (2) included in the population by SEM-EDS By doing so, the size, composition and number of inclusions are identified. At this time, the observation area is also recorded. In addition, in JIS G0555, which is generally used as an evaluation method for inclusions, even when two or more inclusions exist apart from each other, they may be regarded as one inclusion depending on the type and distance, but in the present invention, individual inclusions regarded as The area of the inclusion containing CaO and the area occupied by M(C,N) in the inclusion are measured, the latter is divided by the former and multiplied by 100 to calculate the area ratio (%) of the M(C,N) portion.

<Manufacturing method>

The manufacturing method of the ferritic stainless steel of this embodiment is demonstrated.

Casting is performed by melting the steel adjusted to have the above-described predetermined components. At this time, casting is performed by controlling the cooling rate so as to cover the periphery of the inclusion containing CaO with M(C, N). Although the formation temperature of M(C, N) differs depending on the component system, the coverage by M(C, N) increases as the cooling rate slows down. During continuous casting, by controlling the average cooling rate at 1400 to 700°C near the cast steel surface to 50°C/min or less, the conditions for inclusions specified above can be satisfied. A preferable range of the average cooling rate is 30°C/min or less, and a more preferable range is 15°C/min or less. In addition, the number of CaO-containing inclusions with a maximum diameter of 2 μm or more increases as the O concentration increases, but by controlling the O concentration to 0.010% or less, the number of inclusions containing CaO with a maximum diameter of 2 μm or more is preferable. Less than 30 / mm ² ) can be satisfied. After casting, hot rolling is performed, and after that, appropriate annealing, pickling, cold rolling, etc. are performed to obtain a predetermined stainless steel. Samples prepared under various conditions were submitted to the SST test, and it was found that samples whose components or inclusions had the conditions specified in the present invention had little rust. By providing the requirements described above, it becomes possible to obtain the effect of the present invention.

Example

The molten steel obtained by melting the steel adjusted to have the above-described predetermined components was made into a cast steel by continuous casting. During continuous casting, casting was performed by controlling the average cooling rate at various rates in the temperature range of 1400 to 700°C near the surface of the cast steel. The average cooling rate in the temperature range of 1400 to 700 ° C. in the vicinity of the cast steel surface was evaluated by numerical calculation using electrothermal analysis, and the results are shown in Table 2. The obtained cast steel is subjected to heat treatment at 1200°C for 2 hours as cast steel heating before hot rolling, followed by hot rolling, followed by hot-rolled sheet annealing and pickling, followed by cold rolling, annealing and pickling, to produce a cold-rolled sheet having a thickness of 1.0 mm. Therefore, it was used for inclusion measurement and SST test. In addition, a part of the cast steel obtained above was subjected to heat treatment at 1200 ° C. for 2 hours simulating heating of the cast steel, and the state of formation of CaS was confirmed.

Table 1 shows the chemical composition, and Table 2 shows the state of CaS generation in simulated cast steel heating samples (the number of CaS generated in 20 inclusions with a maximum diameter of 5 μm or more), and the measurement results of cold-rolled sheet inclusions (maximum diameter containing CaO ≥ 2 μm) The number density of inclusions (pcs/mm ² ), the number ratio (%) of inclusions having an area ratio of M(C, N) portion of 40% or more), and the SST test results are shown. In Tables 1 and 2, numerical values that deviate from the range of the present invention and values that deviate from suitable manufacturing conditions of the present invention are underlined.

The CaS generation status of the simulated cast steel heating sample was confirmed by cutting an appropriate cross section, polishing it with a mirror finish, randomly selecting 20 inclusions with a maximum diameter of 5 μm or more, and performing elemental shading mapping using EPMA, and CaS was generated. If there were 1 or less examples, it was set as good.

In the measurement of inclusions in the cold-rolled sheet, the surface of the steel sheet is observed as in FIG. 3 . The position of the observation surface 1 in the depth direction of the steel plate is set to the outermost layer as much as possible, and minimal polishing required for mirror finishing for observation is performed. On the observation surface 1, at least 100 inclusions 2 having a maximum diameter 3 of 2 μm or more and containing CaO are randomly selected, and the areas of the oxide portion and the M(C, N) portion are measured. , Calculate the ratio (area ratio (%) of M (C, N) part) of the M (C, N) part to the area of the inclusion, and calculate the number ratio of the inclusion having an area ratio of 40% or more of the M (C, N) part Calculated. At this time, the number per unit area was calculated by recording the measured area.

In the SST test, based on JIS Z 2371, a 2-hour continuous spray test was performed using a neutral salt spray test as a salt solution, and the number of rust spots per 100 cm ² was measured. When the number of green spots was 5 or less, it was considered good.

As shown in Table 2, since the steel components and inclusion forms in the steel sheet satisfy the conditions of the present invention, the formation of CaS in the cast steel is small, and the rust resistance in the SST test of the steel sheet is good. did

Symbol b1 indicates that the S concentration is outside the upper limit of the present invention range, and as a result, the inclusion form of the steel sheet satisfies the condition of the present invention. A large number of rust was observed in Due to the high S concentration, it is presumed that CaS was produced either before or during solidification.

In symbol b2, since the N concentration was low, the percentage of the number of inclusions in which the total area ratio of the M(C, N) portion of the inclusions was 40% or more was low. Therefore, CaS production could not be suppressed, and a large number of rust was observed in the SST test.

Symbol b3 indicates that the Ca concentration is outside the upper limit of the present invention range, and as a result, the inclusion form of the steel sheet satisfies the condition of the present invention. A large number of rust was observed in Due to the high Ca concentration, it is presumed that CaS was produced during the solidification stage or before solidification.

Symbol b4 indicates that the O concentration is outside the upper limit of the present invention range, and as a result, the number ratio of inclusions having a total area ratio of 40% or more in the inclusion M (C, N) portion of the steel sheet is low, so a large amount of rusting is observed. Also, the number density of CaO-containing inclusions having a maximum diameter of 2 μm or more was high.

Symbol b5 stopped casting because the nozzle was clogged because the Ti concentration was too high, and a large amount of TiN was produced during casting. In addition, when the cast steel obtained halfway through was processed, workability was very poor, and a large amount of TiN-derived surface flaws occurred.

Symbol b6 is the area of the M(C,N) portion due to the fact that M(C,N) was not sufficiently generated and the coverage was low because the average cooling rate in the temperature range of 1400 to 700°C near the cast steel surface was fast. The percentage of the number of inclusions having a rate of 40% or more was low. As a result, CaS was generated and a large number of rust was observed.

1: Observation plane
2: inclusions
3: maximum diameter
4: rolling direction
5: thickness direction
6: plate width direction
7: steel plate surface
8: M (C, N) part

Claims (3)

Chemical composition, in mass%,
C: 0.001 to 0.02%;
Si: 0.02 to 1.5%;
Mn: 1.5% or less;
P: 0.040% or less;
S: 0.006% or less;
Cr: 10 to 25%;
Al: 0.01 to 0.20%;
O: 0.0005 to 0.010%;
N: 0.005 to 0.025%;
Ca: 0.0030% or less
contain, and also
Ti: 0.35% or less;
Nb: 0.70% or less
containing one or both of the balance consisting of Fe and impurities,
On the steel surface, among inclusions containing CaO and having a maximum diameter of 2 μm or more, inclusions having one or two or more types of M(C,N) on the outer periphery and having an area ratio of M(C,N) of 40% or more of 70% or more
Characterized in that ferritic stainless steel.
Here, M(C, N) represents a carbonitride of element M, M is one or two or more elements selected from Ti, Nb, and Cr, and may contain less than 1% as the total of other elements. According to claim 1,
In addition to the chemical component described in claim 1, instead of a part of the Fe, in terms of mass%, V: 2.0% or less, Zr: 0.0050% or less, B: 0.0001 to 0.0020%, Ga: 0.010% or less, one or two A ferritic stainless steel comprising at least one species, wherein the element M of M (C, N) is one or two or more elements selected from Ti, Nb, Cr, V, Zr, B, and Ga. According to claim 1 or 2,
In addition, instead of part of the above Fe, in terms of mass%, Mo: 2.0% or less, Mg: 0.0030% or less, REM: 0.01% or less, Ta: 0.001 to 0.100%, Ni: 0.1 to 2.0%, Sn: 0.01 to 0.50 %, Cu: 0.01 to 2.00%, W: 0.05 to 1.00%, Co: 0.10 to 1.00%, Sb: 0.01 to 0.30%, a ferritic stainless steel containing one or two or more of them.

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