TW202208646A - Ferritic stainless steel - Google Patents

Abstract

A ferritic stainless steel which is characterized by having a chemical composition that contains, in mass%, from 0.001% to 0.02% of C, from 0.02% to 1.5% of Si, 1.5% or less of Mn, 0.040% or less of P, 0.006% or less of S, from 10% to 25% of Cr, from 0.01% to 0.20% of Al, 0.35% or less of Ti, 0.70% or less of Nb, from 0.0005% to 0.010% of O, from 0.005% to 0.025% of N and 0.0030% or less of Ca. This ferritic stainless steel is also characterized in that among the inclusions containing CaO and having a maximum diameter of 2 μm or more, the proportion of the number of inclusions, each of which has one or more M(C, N) sites in the outer peripheral part, while having a total area ratio of the M(C, N) sites of 40% or more, is 70% or more. Meanwhile, M(C, N) represents a carbonitride of element M; and M represents one or more elements that are selected from among Ti, Nb and Cr.

Description

Fertilizer iron series stainless steel

Field of Invention The present invention relates to a ferrite-based stainless steel having excellent rust resistance.

Background of the Invention In general, since stainless steel is a steel material that is directly used in a neat state without coating or the like, there is a problem of rusting from CaS exposed on the surface of the steel material as a starting point. As the generation mechanism of CaS, the following types are known: a type that crystallizes out in molten steel; The type produced by the reaction. As for the former type, it has become possible to achieve low S stably with the improvement of refining ability in recent years, so it is gradually not a problem; Measures for suppressing these are known to be carried out by controlling the smelting conditions.

For example, Document 1 (Japanese Patent Laid-Open No. 5-339620 ) discloses a production method capable of producing high-Al stainless steel excellent in rust resistance by controlling the CaO concentration in the slag at the end of refining to be 35% or less. S aggregates toward CaO, and the concentration of MgO is also made 30% or less, whereby solid-phase MgO is generated. The solid-phase MgO has good lattice compatibility with CaS and prevents easy precipitation of CaS.

Document 2 (Japanese Patent Laid-Open No. 2012-184494 ) and Document 3 (Japanese Patent Laid-Open No. 2014-162948 ) are characterized by providing a ferrous stainless steel that is less likely to rust, and is refined so that the X value becomes equal. The numerical value of the correlation formula showing the composition of the inclusions is set below a predetermined value while satisfying the formula composed of [Ca], [S], [Al], and T.[O], thereby suppressing CaS.

Summary of Invention However, there are problems that cannot be solved in the above-mentioned technologies.

In the technique of Document 1, since the CaO concentration of the slag must be kept low, even if it is a composition series with high Al, careful control is required to stably deoxidize, and the burden on the operation becomes heavy. In addition, the desulfurization reaction also tends to become unstable, and CaS and other sulfides are produced instead to deteriorate the rust resistance.

In the technologies of Documents 2 and 3, there are many restrictions on the slag composition and the Ca and S concentrations in the molten steel, and there is a problem that the burden on refining increases, resulting in an increase in cost.

Then, the present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a ferrite-based stainless steel having little CaS and excellent rust resistance.

The present invention has been accomplished in order to solve the above-mentioned existing problems, and the gist of the present invention is as follows.

[1] A fertilizer granulated iron-based stainless steel, characterized in that the chemical components in mass % contain: 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: 10~25%, Al: 0.01~0.20%, O: 0.0005~0.010%, N: 0.005~0.025%, Ca: 0.0030% or less, and further contains one or both of the following: Ti: 0.35% or less, Nb: 0.70% or less, the remainder is composed of Fe and impurities; among the inclusions containing CaO and having a maximum diameter of 2 μm or more, one or more types of M(C,N ) and the area ratio of the M(C,N) part is 40% or more, the number ratio of the inclusions is 70% or more.

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

[2] The fat-grained iron-based stainless steel according to [1], which, in addition to the chemical components described in [1], contains one or more of the following in place of a part of the aforementioned Fe: % by mass In total, V: 2.0% or less, Zr: 0.0050% or less, B: 0.0001~0.0020%, Ga: 0.010% or less; the element M of M(C, N) is selected from Ti, Nb, Cr, V, Zr, B. One or two or more elements of Ga.

[3] The fat-grained iron-based stainless steel according to [1] or [2], further comprising, in place of a part of the aforementioned Fe, one or more of the following: Mo: 2.0% or less, Mg : 0.0030% or less, REM: 0.01% or less, Ta: 0.001~0.100%, Ni: 0.1~2.0%, Sn: 0.01~0.50%, Cu: 0.01~2.00%, W: 0.05~1.00%, Co: 0.10~ 1.00%, Sb: 0.01~0.30%.

It is possible to provide a fat-grained iron-based stainless steel that is less prone to rusting from inclusions containing CaS.

Embodiments of the present invention Hereinafter, the content of the present invention will be described in detail. <Formation of CaS (cast stage)>.

First, the experiments for which the present invention was constructed will be described.

CaS is considered to be the origin of rust in stainless steel; even if this CaS does not yet exist in the casting stage, that is, the steelmaking stage, it will still be formed in the following stage: when the slab is heated before hot rolling as described above, the S in the base metal It diffuses and reacts with CaO in inclusions.

Here, the effect of casting heating conditions on CaS formation was investigated. Samples are cut out from ferrite-based stainless steel castings with various components, heated at 1000-1300°C for 5 minutes to 3 hours in an atmospheric gas environment, then air-cooled, and then cut out appropriate sections and ground for processing into a mirror.

Twenty inclusions with a maximum diameter of 5 μm or more were randomly selected, and element concentration distribution analysis was performed on these inclusions using EPMA to confirm the generation status of CaS. As a result, it was found that the higher the temperature and the longer the heating conditions, the more prominent the formation of CaS; and, even at high temperatures, CaS was not formed in a short time of about 5 minutes. From this, it can be seen that short-time heating such as annealing is excluded in the case where the step includes heating that can cause CaS generation.

It is also known that under the condition of long-term heating, for the sample with extremely low S such as [S]≦5ppm, CaS is rarely generated; There are cases where both the inclusions that significantly generate CaS and the inclusions that do not generate CaS at all exist in the same sample.

Further detailed investigation revealed that the inclusions in which CaS is not formed at all include carbides, nitrides, carbonitrides, and the like that coat the CaO-containing inclusions. It is considered that, as described above, CaS is generated by diffusion of S contained in the base material and reaction with CaO in the inclusions. Therefore, it is considered that by covering the surrounding of the inclusions with carbides, nitrides or carbonitrides, it is physically In this way, the diffusion of S is blocked, and as a result, the generation of CaS can be inhibited.

Next, after investigating the carbides, nitrides or carbonitrides existing in the outer periphery of the inclusions containing CaO, it was found that the elements other than C and N are Ti, Nb, Cr, V, Zr, B, Ga, etc. . These may have compositions close to pure substances, and may be in the state of solid solution of plural carbides and nitrides. Therefore, the carbides, nitrides, and carbonitrides are not distinguished below, and the diameter is represented by M(C,N). As described above, M is not limited to one type, and may be two or more types in some cases. In addition, two or more phases of M(C,N) existing in the outer peripheral portion of the CaO-containing inclusion may coexist in some cases. <Inclusion morphology (comparison of post-processing stage (steel plate stage) and slab stage)>

Next, regarding the form of inclusions, the relationship between the stage after rolling and other processing and the stage of casting was investigated.

When observing the inclusions of hot-rolled sheets and cold-rolled sheets (hereinafter collectively referred to as "steel sheets"), the outer peripheral portion of the inclusions containing CaO is accompanied by M(C,N), but may not be covered. It is understood that there are the following cases in such inclusions: a case where CaS is significantly generated and a case where it is hardly generated. As described above, since CaS is formed when the slab is heated, if M(C,N) covers the outer periphery of the oxide portion of the CaO-containing inclusion in the slab stage, CaS will not be formed. From this, it can be presumed that in the case of inclusions containing CaO accompanied by M(C,N) after being deformed by hot-rolled sheet, cold-rolled sheet, etc., CaS is remarkably generated, which is not affected by M in the casting stage. (C,N) covered; CaS hardly formed, it was covered by M(C,N) at the casting stage. However, even if it is presumed to be covered by M(C,N) in the slab stage, since M(C,N) accompanies the outer periphery of the inclusions in the steel plate stage, there are cases that are not covered. Therefore, it cannot be used as an index indicating that CaS is hardly generated.

Therefore, in order to find an index indicating that CaS is hardly generated, the inspection was carried out by observing the inclusions in the steel plate stage.

First, regarding the inclusions containing CaO in the slab stage (see Fig. 4), the relationship between the following two ratios was investigated: the ratio of the M(C,N) portion 8 to the area of the inclusions 2, the CaO content The relationship between the ratio (covering ratio) of the outer periphery of the oxide part of the inclusions covered by M(C,N). In the investigation, 50 inclusions containing CaO were randomly selected, photographs of the inclusions were taken with an optical microscope, and evaluation was performed with an image analysis device. First, measure the outer perimeter of the oxide part of the inclusions containing CaO and the length of the part in contact with M(C,N), divide the part length by the outer perimeter and multiply by 100 times to calculate the coverage (% ). Second, measure the area of the inclusion containing CaO (the total area of the oxide part and the M(C,N) part) and measure the area occupied by M(C,N) in the inclusion, divide the latter by the former and multiply The area ratio (%) of the M(C,N) portion was calculated by multiplying by 100 times. It is understood that: as shown in Figure 1, when the ratio of M(C,N) to the area of the inclusions (the area ratio (%) of the M(C,N) part) is 40% or more, the coverage rate is approximately 100% %, the outer periphery of the oxide part of the inclusions containing CaO is almost covered by M(C,N).

As far as inclusions are concerned, if the above-mentioned area ratio of the M(C,N) part is considered to continue from the casting stage to the steel plate stage, the area ratio of the M(C,N) part in the steel plate stage can be used to calculate It is suitable for an index indicating that CaS is hardly generated, that is, an index indicating rust resistance. Then, the hot-rolled sheet and the cold-rolled sheet were then subjected to salt spray test-JIS-Z-2371 (hereinafter, SST) after observation of inclusions, and the rust property was investigated.

In the observation of the inclusions, as shown in FIG. 3 , the surface of the steel plate was polished to a mirror surface and used as a surface to be observed under a microscope. FIG. 3 describes: the rolling direction 4 , the thickness direction 5 , the plate width direction 6 , and the steel plate surface 7 ; the steel plate surface 7 is the observation surface 1 . First, after investigating the diameter of the inclusions at the rusted point, it was found that inclusions with a maximum diameter of less than 2 μm hardly cause rusting. Therefore, in the present invention, the inclusions 2 containing CaO and having a maximum diameter 3 of 2 μm or more are evaluated. Then, the ratio of the M(C,N) part to the area of the inclusions (the total area of the oxide part and the M(C,N) part) was evaluated in the same manner as the above-mentioned casting stage (the ratio of the M(C,N) part Area ratio (%)). The relationship between the ratio of the number of inclusions in which the area ratio (%) of the M(C,N) portion was 40% or more and the results of the rust resistance (SST) investigation was further evaluated. It is understood that: As shown in Figure 2, the ratio of the area of the M(C,N) part to the inclusions (the area ratio (%) of the M(C,N) part) is more than 40% of the inclusions, the number of the inclusions When the ratio is 70% or more, the evaluation of SST is 5 or less, which is good, and when it is 85% or more, it is more favorable.

As described above, it was found that, in the casting stage, the outer periphery of the oxide portion of the inclusions containing CaO is covered with M(C,N), thereby suppressing the generation of CaS when the slab is heated; and, in the casting stage When the area ratio of the middle M(C,N) portion is 40% or more, the ratio of the outer peripheral portion of the oxide portion of the CaO-containing inclusions to be covered by M(C,N) is high in the casting stage. It was also found that when the inclusions on the surface of the steel after processing such as rolling were observed, among the inclusions containing CaO and having a maximum diameter of 2 μm or more, one or more M(C,N) and M( Inclusions with an area ratio of 40% or more in the C, N) part, if the number ratio of the inclusions is 70% or more, the performance of SST is good.

Therefore, in the present invention, among the inclusions containing CaO and having a maximum diameter of 2 μm or more, one or more types of M(C,N) are accompanied on the outer peripheral portion, and the area ratio of the M(C,N) portion is 40. % or more of the inclusions, the number ratio is more than 70%. ＜Steel composition＞

As described above, the present invention relates to the control of the composition of inclusions, and thus can be applied to ferrite-based stainless steels generally produced. The range of ingredients that can be suitably used is listed below, but not limited thereto. C: 0.001~0.02%

C is a component of M(C,N) that suppresses the formation of CaS; in order to form M(C,N), it must be 0.001% or more. Generate CaS. Suitably, it may contain 0.003% or more. On the other hand, when it contains excessively, since workability will fall etc., it shall be 0.02% or less. It should be less than 0.015%. Si: 0.02~1.5%

Since Si reduces the solubility of N, it is an element that promotes the formation of M(C,N). In terms of other effects, it is also an element effective for desulfurization by promoting deoxidation; for example, it is an element effective for inhibiting the generation of CaS because it can indirectly inhibit the generation of CaS during solidification. In order to exhibit these effects, it must be 0.02% or more, and preferably 0.05% or more. However, when it is added more than 1.5%, the workability decreases. In particular, in applications where workability is a problem, it is preferable to set it to 1.0% or less. Mn: 1.5% or less

Mn is an element that contributes to deoxidation, so it can also be added to pre-deoxidize before adding Al. If adding, in order to show the effect, it can be made into 0.01% or more, and it can be made into 0.05% or more suitably. On the other hand, since it reduces the workability, it is made 1.5% or less. In particular, in applications where workability is a problem, it is preferable to set it to 0.3% or less. P: 0.040% or less

P reduces toughness, hot workability, corrosion resistance, etc., and is an element that is particularly harmful to stainless steel, so the smaller the amount, the better, and the content is 0.040% or less. It should be less than 0.035%, more preferably less than 0.030%. However, if it is excessively reduced, the burden during refining is high or expensive raw materials must be used, so it may be contained in an actual operation of 0.005% or more. S: 0.006% or less

Although the formation of CaS can be suppressed by the above-mentioned requirements, if the content of S is more than 0.006%, the CaS will not be formed before or during the solidification stage, that is, before M(C,N) is covered with the inclusions containing CaO. It will generate and cause rust, so the upper limit is set to 0.006%. A suitable upper limit is 0.003%. Cr: 10~25%

Cr is an important element that imparts corrosion resistance to stainless steel, and must be added at 10% or more, preferably 15% or more. On the other hand, since a large amount of addition leads to a decrease in workability, the upper limit is made 25%, and it can be preferably made 21% or less. Al: 0.01~0.20%

Al is an element added for deoxidizing molten steel, and it is also an element necessary to make S 0.006% or less. Therefore, the lower limit is set to 0.01%. A suitable lower limit is 0.05%. Excessive addition reduces workability, so the upper limit is made 0.20%. A suitable upper limit is 0.15%.

Since Ti or Nb is the main component of M(C,N), it is necessary to add at least one of Ti and Nb. Ti: 0.35% or less

The addition of Ti can form M(C,N). In order to form M(C,N) mainly composed of Ti, 0.01% or more may be added, and a suitable addition amount is 0.05% or more. When excessively added, a large amount of TiN will be generated before or during casting, causing nozzle clogging and product surface defects, so the upper limit is set to 0.35%. Nb: 0.70% or less

The addition of Nb can form M(C,N). In order to form M(C,N) mainly composed of Nb, the effect is exhibited by adding 0.003% or more. The suitable addition amount is 0.2% or more. On the other hand, when more than 0.70% is added, recrystallization becomes difficult and the structure becomes coarse, so it is made 0.70% or less. Preferably, it can be made 0.6% or less. O: 0.0005~0.010%

O forms CaO-containing inclusions that can generate CaS when the slab is heated. The higher the concentration of O, the more the amount of inclusions containing CaO increases, so that it becomes more difficult to cover with M(C,N), so the upper limit is made 0.010%. However, excessive deoxidation increases the burden on refining and increases the cost, so the lower limit is made 0.0005%. O means: T.O. N: 0.005~0.025%

N is an element that forms M(C,N), which suppresses the generation of CaS; adding 0.005% or more of it exhibits an effect. The higher the content concentration, the more M(C,N) is generated, and the more difficult it is to generate CaS. On the other hand, when it contains excessively, a large amount of Cr nitrides will be generated, resulting in a lack of Cr at the grain boundary, and on the contrary, corrosion resistance will be reduced or workability will be significantly reduced, so it is made 0.025% or less. It should be less than 0.020%. Ca: 0.0030% or less

Although the formation of CaS can be suppressed by the above-mentioned requirements, if the content of Ca is more than 0.0030%, CaS will not be formed before or during the solidification stage, that is, before M(C,N) covers the inclusions containing CaO. It will generate and cause rust, so the upper limit is set to 0.0030% or less. The greater the amount of Ca, the greater the amount of inclusions containing CaO, and the greater the amount of M(C, N) required for coverage. Therefore, the less the better, the more suitable it is 0.0020% or less. More preferably, it is 0.0010% or less. It may not contain Ca.

The remainder of the above-mentioned steel components is Fe and impurities. Here, the so-called impurity refers to: when industrial steel is made, raw materials such as ore, scrap, etc. are mixed due to various factors in the production process; Element.

Moreover, the ferrite-based stainless steel of the present embodiment may further contain one or more of the following in place of a part of Fe: V: 2.0% or less, Zr: 0.0050% or less, B: 0.0001 to 0.0020 in mass % %, Ga: 0.010% or less. When these elements are not included, the lower limit of these elements is 0%. V: 2.0% or less

V itself has the effect of improving corrosion resistance, and in addition, it also forms M(C,N) and suppresses the generation of CaS, so it can also be contained as needed. More than 0.02% should be added. In order to form M(C,N) containing V as a main component, the suitable addition amount is 0.1% or more. When V is contained excessively, the toughness is lowered, so it is made 2.0% or less. It is preferable to set it as 1.0% or less, and it is more preferable to set it as 0.5% or less. Zr: 0.0050% or less

Zr forms M(C,N) and suppresses the generation of CaS, so it may be contained as needed. In order to form M(C,N) mainly composed of Zr, the suitable addition amount is 0.0010% or more. However, if it is added excessively, sulfides will be formed in the smelting stage, and on the contrary, the corrosion resistance will be reduced. Therefore, the upper limit is set to 0.0050%. B: 0.0001~0.0020%

In addition to the effect of improving the grain boundary strength, B also forms M(C,N) and suppresses the generation of CaS, so it may be contained in an amount of 0.0001% or more as required. However, when B is contained excessively, the elongation is lowered and the workability is lowered, so the content is made 0.0020% or less. It should be less than 0.0010%. Ga: 0.010% or less

Ga itself not only has the effect of improving corrosion resistance, but also forms M(C,N) and suppresses the generation of CaS, so it can be contained in an amount of 0.010% or less according to needs. The lower limit of Ga is not particularly limited, but 0.001% or more is preferably contained to obtain a stabilizing effect.

In place of a part of Fe, one or more of the following may be further contained: 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%. When these elements are not included, the lower limit of these elements is 0%. Mo: 2.0% or less

The addition of Mo will further enhance the high corrosion resistance of stainless steel. However, due to its very high price, even if it is added in more than 2.0%, not only the effect corresponding to the increase in alloy cost cannot be obtained, but also a σ (sigma) phase is formed, which leads to embrittlement and a decrease in corrosion resistance. Therefore, the upper limit is set to 2.0%. A suitable lower limit is 0.5%, and a suitable upper limit is 1.5%. Mg: 0.0030% or less

Mg is an element that is very effective for deoxidation and desulfurization, so it can also be contained according to needs. However, when it is excessively added, sulfides are formed before or during casting, and corrosion resistance is reduced on the contrary. Therefore, the upper limit is set to 0.0030%. REM: 0.01% or less

REM (rare earth metal: Rare-Earth Metal) is a very effective element for deoxidation and desulfurization due to its high affinity with O and S, and it can also be contained as required. However, when excessively added, a large amount of oxides are generated before or during casting, causing nozzle clogging and product surface defects, so 0.01% is set as the upper limit. Ta: 0.001~0.100%

Ta is an element that is very effective for deoxidation and desulfurization, so it may be contained as required. In order to obtain this effect, 0.001% or more may be contained. However, when excessively added, the room temperature ductility and toughness are lowered, so the upper limit is made 0.100%. Ni: 0.1~2.0%

Ni has the effect of improving corrosion resistance, so it can be added according to needs. In order to obtain this effect, 0.1% or more must be added. On the other hand, since it is a high-priced element, even if it is added in more than 2.0%, the effect corresponding to the increase in alloy cost cannot be obtained, so the upper limit is made 2.0%. Suitably, it can be made 1.5% or less. Sn: 0.01~0.50%

The addition of Sn will further enhance the high corrosion resistance of stainless steel. If it contains, in order to acquire this effect, it may contain 0.01% or more, and may be 0.02% or more suitably. On the other hand, excessive addition leads to a decrease in workability, so it can be made 0.50% or less, preferably 0.30% or less. Cu: 0.01~2.00%

Cu has the effect of improving corrosion resistance, so it can be added according to needs. In order to obtain this effect, it is necessary to add more than 0.01%. However, excessive addition causes embrittlement, so it is made 2.00% or less. W: 0.05~1.00%

W has the effect of improving corrosion resistance, especially pitting corrosion resistance, so it can be added according to needs. In order to obtain this effect, more than 0.05% must be added. However, excessive addition leads to a decrease in toughness, so the upper limit is made 1.00%. Co: 0.10~1.00%

Co has the effect of improving the strength of steel, so it can be added according to needs. In order to obtain this effect, more than 0.10% must be added. However, excessive addition leads to a decrease in toughness, so the upper limit is made 1.00%. Sb: 0.01~0.30%

Sb has the effect of improving corrosion resistance, so it can be added according to needs. In order to obtain this effect, it is necessary to add more than 0.01%. However, excessive addition leads to a decrease in manufacturability, so the upper limit is made 0.30%. <Measurement method of inclusions>

Hereinafter, a method for measuring inclusions will be described. As shown in FIG. 3 , the observation was performed with the surface 7 of the steel sheet as the observation plane 1 . The position of the observation surface 1 in the depth direction of the steel plate is the outermost layer as much as possible, and when it is processed into a mirror surface for observation, it is polished to the minimum necessary. In the observation surface 1, at least 100 inclusions 2 containing CaO and having a maximum diameter of 2 μm or more were randomly selected and designated as the parent group, and then the inclusions 2 contained in the parent group were analyzed by SEM-EDS to identify The size and composition of the inclusions are also included. At this time, the observation area is also recorded. In addition, in terms of the evaluation method of inclusions, according to JIS G0555, which is generally used, even if there are two or more inclusions separated, they may be regarded as one inclusion depending on the type and distance; however, , which are regarded as individual inclusions in the present invention. Measure the area of the inclusions containing CaO, and measure the area occupied by M(C,N) in the inclusions, divide the latter by the former and multiply by 100 times to calculate the area ratio of the M(C,N) part ( %). <Manufacturing method>

The manufacturing method of the ferrite-based stainless steel of the present embodiment will be described.

The smelting and casting are carried out to obtain the steel adjusted to the above-mentioned predetermined composition. At this time, casting was performed by controlling the cooling rate so that M(C,N) was covered around the CaO-containing inclusions. Although the formation temperature of M(C,N) varies depending on the component series, the slower the cooling rate, the higher the coverage rate of M(C,N). In continuous casting, the average cooling rate at 1400-700°C near the surface of the slab is controlled to be less than 50°C/min, so that the above-mentioned inclusion conditions can be satisfied. A suitable range of the average cooling rate is 30°C/min or less, and a preferable range is 15°C/min or less. In addition, although the number of inclusions containing CaO and having a maximum diameter of 2 μm or more increases as the O concentration increases, by controlling the O concentration to 0.010% or less, suitable inclusion conditions (containing CaO and having a maximum diameter of 2 μm) can be satisfied. The number density of the above inclusions is less than 30/mm ² ). After casting, hot rolling is performed, and then annealing, pickling, cold rolling, etc. are suitably performed to obtain a predetermined stainless steel. After supplying the samples produced under various conditions to the SST test, it was found that the samples whose components and the form of inclusions satisfy the conditions specified in the present invention are less rusted. The effects of the present invention can be obtained by fulfilling the above-described requirements. Example

The steel adjusted to the above-mentioned predetermined composition is smelted to obtain molten steel, and the molten steel is made into slabs by continuous casting. In continuous casting, casting is performed by controlling the average cooling rate in the temperature range of 1400 to 700°C in the vicinity of the slab surface at various rates. The average cooling rate in the temperature range of 1400 to 700°C in the vicinity of the slab surface was evaluated by numerical calculation using heat transfer analysis, and the results are shown in Table 2. The obtained cast sheet is subjected to heat treatment at 1200°C for 2 hours to heat the cast sheet before hot rolling, then hot rolling, hot rolled sheet annealing, pickling, and cold rolling, annealing, and pickling. This produced a cold-rolled sheet with a thickness of 1.0 mm and supplied it for inclusion measurement and SST test. In addition, some of the cast pieces obtained as described above were heat-treated at 1200° C. for 2 hours by simulating heating the cast pieces, and the state of formation of CaS was confirmed.

[表2]

Table 1 lists the chemical composition; Table 2 lists the CaS generation status (the number of CaS generated in 20 inclusions with a maximum diameter of 5 μm or more) of the simulated heating cast samples, and the measurement results of the inclusions in the cold rolled sheet (containing CaO In addition, the number density of inclusions with a maximum diameter of ≧2μm (pieces/mm ² ), the number ratio of inclusions with an area ratio of M(C,N) of 40% or more (%), and SST test results. In Table 1 and Table 2, the numerical values outside the scope of the present invention and the numerical values outside the suitable manufacturing conditions of the present invention are marked with a bottom line. [Table 1]

[Table 2]

Regarding the generation of CaS in the simulated heating cast sample, an appropriate cross-section was cut out, ground and processed into a mirror surface, and then 20 inclusions with a maximum diameter of 5 μm or more were randomly selected and confirmed by element concentration distribution analysis using EPMA; For example, one or less was determined to be good.

In the measurement of the inclusions in the cold-rolled sheet, the surface of the steel sheet was observed in the same manner as in FIG. 3 . The position of the observation surface 1 in the depth direction of the steel plate is the outermost layer as much as possible, and when it is processed into a mirror surface for observation, it is polished to the minimum necessary. In the observation surface 1, 100 or more inclusions 2 containing CaO and having a maximum diameter 3 of 2 μm or more are randomly selected, the area of the oxide portion and the M(C,N) portion is measured, and M(C,N) is calculated. The ratio of the area of the part to the inclusions (the area ratio of the M(C,N) part (%)) and the calculated area ratio of the M(C,N) part of the number of inclusions of 40% or more. At this time, the measured area was recorded to calculate the number per unit area.

The SST test is based on JIS Z 2371, using a neutral salt water spray test and selecting a salt solution for a continuous spray test for 2 hours, and then calculating and measuring the number of rust spots per 100cm ² . When the number of rust spots was 5 or less, it was determined as good.

As shown in Table 2, symbols B1 to B13 satisfy the conditions of the present invention due to the steel composition and the inclusion morphology in the steel sheet, so that CaS generation is also less in the cast pieces, and the rust resistance in the SST test of the steel sheet is good.

The S concentration of the symbol b1 exceeds the upper limit of the scope of the present invention. As a result, although the inclusion morphology of the steel plate meets the conditions of the present invention, it is clear from the observation results of the simulated heating cast sample that CaS exists, so a large amount of formation was observed in the SST test. rust. It is presumed that the S concentration is high, and CaS is generated before or during the coagulation.

Symbol b2 has a low N concentration, so the number of inclusions with a total area ratio of M(C,N) parts of 40% or more in the inclusions is low. Therefore, the generation of CaS could not be suppressed, and a large amount of rust was observed in the SST test.

The Ca concentration of the symbol b3 exceeds the upper limit of the scope of the present invention. As a result, although the inclusion morphology of the steel plate meets the conditions of the present invention, it is clear from the observation results of the simulated heating cast sample that CaS exists, so a large amount of formation was observed in the SST test. rust. It is presumed that the Ca concentration was high, and CaS was formed before or during the solidification.

The O concentration of symbol b4 exceeds the upper limit of the scope of the present invention, and as a result, the ratio of the number of inclusions with a total area ratio of M(C,N) parts of 40% or more in the inclusions of the steel sheet is low, and a large amount of rust is observed. In addition, the number density of inclusions containing CaO and having a maximum diameter of 2 μm or more is high.

In the symbol b5, since the Ti concentration was too high, a large amount of TiN was generated during the casting, so that the nozzle was blocked and the casting was stopped. In addition, the workability of the cast piece obtained up to the point of the process was very poor, and a large amount of surface flaws due to TiN were generated.

Symbol b6 has a fast average cooling rate in the temperature range of 1400~700°C near the surface of the slab, so M(C,N) is not sufficiently formed and the coverage rate is low, resulting in the area ratio of the M(C,N) part. The ratio of the number of inclusions above 40% is low. Therefore, a large amount of CaS was generated and a large amount of rust was observed.

1: Observation surface 2: Inclusions 3: Maximum diameter 4: Rolling direction 5: Thickness direction 6: Board width direction 7: Steel plate surface 8: M(C,N) section

FIG. 1 is a graph showing the relationship between the area ratio of the M(C,N) portion and the coverage ratio.

FIG. 2 is a graph showing the relationship between the ratio of inclusions satisfying the conditions of the present invention and the results of the SST test.

FIG. 3 is a diagram showing an observation plane of an inclusion object of the present invention and a method for measuring the size of the inclusion.

FIG. 4 is a schematic diagram of an inclusion accompanied by an M(C,N) portion at the outer peripheral portion of the inclusion.

Claims (3)

A kind of fertilizer grain iron series stainless steel, it is characterized in that: Its chemical composition in mass % contains: C: 0.001~0.02%, Si: 0.02~1.5%, Mn: 1.5% or less, P: 0.040% or less, S: 0.006% or less, Cr: 10~25%, Al: 0.01~0.20%, O: 0.0005~0.010%, N: 0.005~0.025%, Ca: 0.0030% or less, and further contain one or both of the following: Ti: 0.35% or less, Nb: 0.70% or less, The rest is composed of Fe and impurities; In the steel surface, among the inclusions containing CaO and having a maximum diameter of 2 μm or more, one or more types of M(C,N) are accompanied on the outer peripheral portion, and the area ratio of the M(C,N) portion is 40% or more. The number of inclusions is more than 70%; Here, M(C,N) represents a carbonitride of the element M; M is one or more elements selected from Ti, Nb, and Cr, and may contain other elements in a total of less than 1%. The fertilizer granulated iron-based stainless steel as claimed in claim 1, wherein, in addition to the chemical components described in claim 1, it also contains one or more of the following to replace a part of the aforementioned Fe: In mass %, V: 2.0% or less, Zr: 0.0050% or less, B: 0.0001~0.0020%, Ga: 0.010% or less; the element M of M (C, N) is selected from Ti, Nb, Cr, V, Zr, B, Ga 1 or more elements. As claimed in claim 1 or claim 2, the fertilizer granulated iron-based stainless steel further contains one or more of the following in place of a part of the aforementioned Fe: in terms of mass %, Mo: 2.0% or less, Mg: 0.0030% or less, REM: 0.01% or less, Ta: 0.001~0.100%, Ni: 0.1~2.0%, Sn: 0.01~0.50%, Cu: 0.01~2.00%, W: 0.05~1.00%, Co: 0.10~1.00%, Sb: 0.01~0.30%.

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