JP7654440B2 - Ferritic stainless steel plate with excellent anti-ridging properties - Google Patents

Description

The present invention relates to a ferritic stainless steel sheet with excellent ridging resistance.

Currently, many types of stainless steel sheets are used in various fields. One type of stainless steel sheet, austenitic stainless steel sheet, is excellent in corrosion resistance, workability, and surface properties, and is therefore used as a structural material for industrial and domestic use. In contrast, ferritic stainless steel sheets contain almost no Ni compared to austenitic stainless steel sheets, and are therefore inexpensive. In addition, they have a small thermal expansion coefficient, and therefore have come to be widely used in fields such as roofing materials for large facilities, various kitchen appliances, and automobile exhaust system components. However, when ferritic stainless steel sheets are subjected to forming processes such as pressing and rolling, a wavy uneven pattern called ridging (hereinafter simply referred to as "ridging") along the forming direction may occur on the surface of the steel sheet.
When ridging occurs on the surface of a steel sheet, it mars the appearance of the steel sheet surface, so the steel sheet surface is polished to remove the ridging during production. However, during the production process, ridging leads to localized thickness deviations, which can lead to the problem of cracks occurring depending on the processing method.

This ridging is thought to occur because columnar crystals grow in the same direction in the solidification structure, and rolling and annealing form colonies of aggregate structures with the same crystal orientation. Based on this knowledge, there have been various research and reports on improving ridging (e.g., Non-Patent Document 1). For ferritic stainless steel sheets to be used in a wider range of applications, it is highly desirable that the formed products after processing be free of ridging and cracks. Meanwhile, ridging, which is likely to occur when ferritic stainless steel sheets are press-formed, is caused by the fact that the coarse columnar crystal structure formed during continuous casting is not sufficiently destroyed in the hot rolling and cold rolling processes, forming colonies that are a collection of multiple crystal grains with similar crystal orientations. In response to this, it is known that the occurrence of ridging can be suppressed by forming ferritic stainless steel sheets into a metal structure in which a large number of equiaxed crystals with random crystal orientations are generated.

For example, Patent Document 1 proposes a method of improving ridging resistance by using REM oxides. Specifically, the composition of inclusions in steel for improving ridging resistance is such that the concentration range of each oxide in a composite inclusion composed of REM oxides, Al ₂ O ₃ , SiO ₂ , MnO, and Cr ₂ O ₃ is limited to homogenize the recrystallized structure during rolling and annealing, thereby improving ridging resistance. However, Patent Document 1 presents a composite oxide of REM oxides and Al ₂ O ₃ , but although each component of REM has a stronger affinity with oxygen than Al, etc., it is said that a composite oxide containing Al ₂ O ₃ is formed, and the specific action and effect of the specific oxide form for each component of REM is unknown, and there is a problem that the effect is insufficient.

Patent No. 4238454

Shozo Mizoguchi: 177th and 178th Nishiyama Memorial Technical Lectures (2001), 53

The present invention was made in consideration of the above problems, and its objective is to provide a ferritic stainless steel sheet that has excellent ridging resistance and is less susceptible to ridging caused by forming, by having inclusions that are oxides of Ce in REM (hereinafter referred to as Ce oxides) and increase the amount of equiaxed crystals in the metal structure.

In order to solve the above problems, the gist of the present invention is as follows.
(1) A composition containing, in mass%, C: 0.001-0.03%, Si: 0.2-1.0%, Mn: 0.05-1.0%, P: 0.001-0.04%, S: 0.001-0.01%, Cr: 10.5-31.0%, Al: 0.006-0.20%, Nb: 0.03-0.8%, Ti: 0.01-0.2%, N: 0.001-0.03%, Ce: 0.01-0.35%, and T.O (total oxygen concentration): 0.001-0.01%, with the balance being Fe and unavoidable impurities, and the Ce content (%) and T. A ferritic stainless steel sheet having excellent ridging resistance, wherein the product of the content (%) of Ce and the content (%) of O satisfies the inequality relationship of the following formula (1), and has a steel structure containing Ce oxide-based inclusions formed by the combination of Ce and oxygen (O), and wherein the Ce oxide-based inclusions having a circle equivalent diameter of 1 to 5 μm have a number density per ^cm2 of 20 pieces/cm2 or ^more in a cross section perpendicular to the rolling direction of the steel sheet.
Formula (1): 1.0×10 ^-5 ≦(Ce (%)) × ((T.O (%)) ≦1.5×10 ^-3
(2) The ferritic stainless steel sheet according to claim 1, containing, by mass%, one or more components selected from the group consisting of Cu: 0.01 to 2.0%, Mo: 0.01 to 3.0%, W: 0.002 to 3.0%, La: 0.001 to 0.05%, and Y: 0.001 to 0.008%.

The present invention controls the amount of Ce and total oxygen concentration (TO) within appropriate ranges, and also controls the number density of Ce oxide inclusions in the ferritic stainless steel sheet, making it possible to obtain a metal structure with many equiaxed crystals, and to obtain a ferritic stainless steel sheet with excellent ridging resistance and little ridging during forming.

1 is a graph showing the relationship between the number density (number/cm ² ) of Ce oxide inclusions having an equivalent circle diameter of 1 to 5 μm and the ridging height (μm).

Specific embodiments of the present invention will be described. Note that the present invention is not limited to the following embodiments, and various modifications are possible without departing from the spirit of the present invention.

(Ferritic stainless steel plate)
The ferritic stainless steel sheet of the present invention contains, by mass%, the following components: C: 0.001 to 0.03%, Si: 0.2 to 1.0%, Mn: 0.05 to 1.0%, P: 0.001 to 0.04%, S: 0.001 to 0.01%, Cr: 10.5 to 31.0%, Al: 0.006 to 0.20%, Nb: 0.03 to 0.8%, Ti: 0.01 to 0.2%, N: 0.001 to 0.03%, Ce: 0.01 to 0.35%, and T.O (total oxygen concentration): 0.001 to 0.01%, with the balance consisting of Fe and unavoidable impurities, and the Ce content and T. The product of the content of Ce and the content of O satisfies the inequality relationship of the following formula (1), and the steel has a structure in which Ce oxide-based inclusions formed by combining with oxygen (O) are present, and among the Ce oxide-based inclusions, those having a circle equivalent diameter of 1 to 5 μm have a number density per ^cm2 of 20 pieces/ ^cm2 or more in a cross section perpendicular to the rolling direction of the steel sheet. The ferritic stainless steel sheet of the present invention will be described in detail below. The reasons for limiting the composition of the present invention will be described below.

(Production of solidified tissue)
In order to obtain a ferritic stainless steel sheet (hereinafter simply referred to as "steel sheet") having excellent ridging resistance, comparative experiments were carried out by the following method in order to clarify the effects of each oxide formed by rare earth metals (hereinafter sometimes referred to as "REM") such as Ce (cerium), La (lanthanum), Y (yttrium), etc.
First, the raw materials were put into a 100 kg vacuum melting furnace and melted. The refractory material used in the furnace was a dolomite-based refractory material used in actual operation. Melting was performed under a vacuum atmosphere, and after melting, the molten steel temperature was held at 1600°C. After holding for 20 minutes, deoxidization and desulfurization were performed. Aluminum (Al) was used as the deoxidizing and desulfurizing agent, and after Al deoxidization, Ce, La, Y, and other components were added in different amounts. Each sample in the molten state was poured into a mold and solidified after adjusting the superheat degree to about 30 to 50°C relative to the liquidus temperature. The obtained ingot was cut at a predetermined position. The cut surface was then etched to reveal the solidified structure.

(Relationship between Ce and TO)
The solidification structure of each ingot to which any one of Ce, La, and Y was added was observed.
As a result, in the ingot to which Ce was added, when the Ce concentration was 0.01 to 0.35% and the TO (total oxygen concentration) was 0.001 to 0.01%, and formula (1) was satisfied, the solidification structure was a fine solidification structure with many small crystal grains, without the development of large columnar crystals.
Formula (1): 1.0×10 ^-5 ≦(Ce (%)) × ((T.O (%)) ≦1.5×10 ^-3

(equixed crystals)
It forms a solidification structure from molten steel to ingot, and increases the equiaxed crystal ratio in the solidified state. Specifically, by adding an appropriate amount of Ce in REM to molten steel before casting, Ce oxide-based inclusions (hereinafter referred to as "Ce oxide-based inclusions") are generated in the molten steel by combining Ce with oxygen (O), and these Ce oxide-based inclusions become solidification nucleation sites, generating a large number of fine equiaxed crystal grains during solidification, and by subjecting this ingot to treatments such as hot rolling, cold rolling, and annealing, a steel plate with a high equiaxed crystal ratio can be obtained. In formula (1), when Ce (%) and TO (%) are low, there are few solidification nucleation sites during solidification, and the equiaxed crystal ratio cannot be increased, and columnar crystals are increased, and the equiaxed crystal ratio cannot be increased even when the steel plate is made. In formula (1), when Ce (%) is high and TO (%) is low, there are few Ce oxide-based inclusions in the molten steel generated, and they do not act as solidification nucleation sites. In addition, when the Ce (%) is low and the T.O (%) is high in formula (1), the amount of Ce oxide-based inclusions in the molten steel produced is small, and the equiaxed crystal ratio cannot be increased. Therefore, it is understood that formula (1) must be satisfied in order to increase the equiaxed crystal ratio in the ingot and also when the steel plate is made.

(Evaluation of equiaxed crystal ratio)
The refined solidification structure with many small crystal grains was evaluated as the equiaxed crystal ratio using formula (2). As shown in formula (2), the equiaxed crystal ratio represents the area ratio (%) of equiaxed crystal grains to the entire surface of the ingot.
Formula (2): Equiaxed crystal ratio (%) = area of equiaxed crystal structure ÷ (area of entire ingot surface) × 100
In the solidification structure of the molten ferritic stainless steel of the present invention, the equiaxed crystal ratio was a high value of 80 to 98%.
However, when other rare earth metals, such as La, Y, Sc, and Nd, contained in REM other than Ce, were added, the effect of increasing the equiaxed crystal ratio of the solidified structure and refining the crystal was not obtained even if the amount of addition was changed.

(Evaluation of Ce oxide-based inclusions)
In addition, a 50 mm thick hot rolling block was cut out from the ingot of each Ce concentration. After heating each block at 1180 to 1200 ° C for about 30 minutes, a 4 mm thick hot rolled sheet was obtained. The hot rolled sheet was then annealed, the surface oxide scale was removed, and cold rolling was performed to obtain a 0.8 mm thick cold rolled steel sheet. The cold rolled steel sheet was then finish annealed at 1000 ° C, and the oxide scale was removed by pickling. From the obtained cold rolled steel sheet, a JIS-5 test piece as specified in JIS Z2214 (Metal Material Tensile Test Method) was taken parallel to the rolling direction. After 15% tension with a tensile tester, the occurrence of ridging on the sheet surface was investigated. In addition, a sample for inclusion investigation was cut out from the steel sheet, and the size and number per unit area of Ce oxide-based inclusions inside the sheet were investigated.

(Measurement of circle equivalent diameter and number density of Ce oxide inclusions)
A specific method for investigating Ce oxide-based inclusions was to cut a plurality of sheets perpendicular to the rolling direction of the cold-rolled steel sheet (hereinafter, the cut surface will be referred to as "C section") and to analyze the C section using an MQA (Metal Quality Analyzer (registered trademark)) device. The size of the Ce oxide-based inclusions on the C section in the width direction of the cold-rolled sheet is almost the same as the width of the Ce oxide-based inclusions before rolling. The investigation of the inclusion composition was carried out using an MQA composition analyzer. Furthermore, the size of the Ce oxide-based inclusions was calculated as a circle equivalent diameter based on the area measured by the MQA. The circle equivalent diameter of the Ce oxide-based inclusions is the diameter of a perfect circle equivalent to the area of the target. The number of Ce oxide-based inclusions per unit area was calculated as the number per ^cm2 in a cross section perpendicular to the rolling direction of the steel plate, by measuring the inclusion composition using MQA and calculating the number of inclusions counted as Ce oxide-based inclusions as the number density (number/ ^cm2 ).

(Relationship between Number Density of Ce Oxide-Based Inclusions and Ridging Height)
The inclusions found inside the steel sheets were Ce oxide-based inclusions, and the number of Ce oxide-based inclusions with an equivalent circle diameter of 1 to 5 μm was measured. Furthermore, the number density (pieces/cm ² ) of Ce oxide-based inclusions with an equivalent circle diameter of 1 to 5 μm was 20 (pieces/cm ² ) or more inside the steel sheets of the present invention. A correlation was examined between the reinforcing grooves found inside the steel sheets and the number density of Ce oxide-based inclusions per unit area of steel sheets with each Ce concentration. The results of the examination are shown in Figure 1.

(Evaluation of Ridging Resistance)
FIG. 1 is a graph showing the relationship between the number density (number/cm ² ) of Ce oxide inclusions having an equivalent circle diameter of 1 to 5 μm and the ridging height (μm).
The ridging resistance was evaluated according to the waviness height of the steel sheet surface. The greater the waviness height of the steel sheet surface, the lower the ridging resistance. The ridging resistance was evaluated in five ranks, with the ridging height (μm) being 5 μm or less as rank A, more than 5 μm and 10 μm or less as rank B, more than 10 μm and 15 μm or less as rank C, more than 15 μm and 20 μm or less as rank D, and more than 20 μm as rank E. Among these, if the ridging height is 10 μm or less and the ridging runk is either rank A or B, the steel sheet can be visually judged to have almost no irregularities, so that the ridging resistance is practically problem-free. Also, if the ridging height is 10 μm or more and the ridging runk is either rank C to E, the steel sheet can be visually judged to have clearly observed waviness or irregularities, so that the ridging resistance is problematic in practical use.
Therefore, from FIG. 1, it can be seen that when the number density (pieces/ ^cm2 ) of Ce oxide inclusions with an equivalent circle diameter of 1 to 5 μm is 20 (pieces/ ^cm2 ) or more, the ridging rank of the steel plate is A or B, which is within the practical range of ridging resistance.

(Suppression of Ridging)
It is generally believed that the occurrence of ridging is due to the fact that the coarse columnar crystal structure formed during continuous casting is not sufficiently destroyed in the hot rolling process, and yet the texture of the coarse columnar crystals remains. This texture can also cause deterioration of workability such as cracking and anisotropy during processing. In order to suppress such texture, a method of repeating cold rolling and annealing multiple times after hot rolling to refine the metal texture by recrystallization is effective. However, repeating cold rolling and annealing multiple times places a burden on the process, leading to increased manufacturing costs and reduced productivity, and is not suitable for mass production of inexpensive steel sheets. In addition, it is not necessarily easy to completely eliminate the influence of the texture using such a method. Ce oxide-based inclusions have a high melting point, are not easily softened, and are hard, so they are not destroyed and refined as Ce oxide-based inclusions even by hot rolling and cold rolling, and even when heat is applied by annealing, they effectively act as sites that hinder the growth of crystal grains. Therefore, the Ce oxide inclusions can suppress the growth of crystal grains during forming, and can suppress the occurrence of ridging during forming.

As can be seen from Figure 1, the more the number of Ce oxide inclusions increases, the more the rib grooves improve. The reason for this improvement in the rib grooves was examined from the perspective of the mechanism of refining the solidification structure. In the case of ordinary steel, the idea of generating many heterogeneous nuclei to serve as nucleation sites during solidification has been reported (Academic literature: B.L.Bramfitt:Metall.Trans.,1(1970),1987.)

(Particle size of Ce oxide inclusions)
Furthermore, the Ce oxide-based inclusions have an equivalent circle diameter in the range of 1 to 5 μm. It was found that the solidification structure becomes fine when a necessary number of inclusions with an equivalent circle diameter of 1 to 5 μm are present. However, if the diameter exceeds 5 μm, the number of collisions between the inclusions increases, and they aggregate and coalesce into clusters. In this case, they do not become solidification nucleation sites, and instead become the cause of defects in the steel sheet. From the above study results, the particle size of the Ce oxide-based inclusions must be considered in relation to the equivalent circle diameter and number density, including their function as sites.

(Components of Ce oxide-based inclusions)
In the present invention, rare earth metals such as La, Y, Sc, and Nd contained in REM other than Ce have almost no effect on ridging. Ce oxide inclusions are compounds of cerium and oxygen, and have various forms such as _CeO2 and _Ce2O3 , and may be formed alone or in combination. Ce oxide inclusions may contain oxides of _La , Y, Sc, Nd, etc.

(Effect of Ce on Ferritic Stainless Steel Sheet)
The ferritic stainless steel sheet of the present invention is different from ordinary steel in that it contains 10.5 to 31.0% Cr, but the C and N concentrations are low, so it is in the ferritic phase from the start of solidification to room temperature. However, since it contains a large amount of Cr, the value of the lattice constant is different from that of ordinary steel. Therefore, the crystal mismatch between the lattice constant of the primary crystal formed during solidification and the Ce oxide-based inclusions was predicted by correcting for temperature. As a result of the calculation, the crystal mismatch was 6 to 9%. In other words, it is presumed that the difference between the crystal lattice spacing of the ferritic phase and the crystal lattice spacing of the Ce oxide-based inclusions is small, and the Ce oxide-based inclusions are in a state where they are likely to become solidification nucleation sites of the ferritic phase, and as a result, the equiaxed crystal ratio is high. When other REMs were added, the effect of refining the solidification structure was not obtained even when the amount added was changed. It is presumed that this is because the difference in the crystal lattice spacing between La oxide, Al oxide, etc. and the ferritic phase is large.

(Component Composition)
Each component of the present invention will be described below. Note that the unit of the content of each element in the stainless steel sheet is "mass %", but hereinafter, unless otherwise specified, it will be simply indicated as "%".
(C: 0.001-0.03%, N: 0.001-0.03%)
Both C (carbon) and N (nitrogen) are elements that affect corrosion resistance and mechanical properties. In particular, when their respective contents exceed 0.03%, their adverse effects become significant, so the lower the C and N concentrations, the better the corrosion resistance and other properties will be. However, removing C and N places a large refining burden on molten stainless steel, and reducing them to C < 0.001% and N < 0.001% is industrially uneconomical.

(Si: 0.2-1.0%)
Silicon (Si) is an effective element as a deoxidizer. If the concentration of Si is lower than 0.2%, its effect as a deoxidizer decreases. Furthermore, if Ce is added after adding Si, _the already formed MnO and _Cr2O3 may chemically react with the Ce oxide inclusions to form a composite oxide.
This composite oxide does not function effectively as a nucleation site because it does not have good crystal matching with the primary ferrite. When excessive Si is added, the generated _SiO2 reacts with the Ce oxide inclusions to generate composite oxides. In this case, too, it does not function as a nucleation site as described above. Therefore, the amount of Si added is set to 0.2 to 1.0%.

(Mn: 0.05-1.0%)
Mn (manganese) is an element that enhances the strength of ferritic stainless steel. If the Mn content is less than 0.05%, this effect cannot be obtained. On the other hand, excessive addition of Mn leads to a decrease in cold workability. Therefore, the amount of Mn added is set to 0.05 to 1.0%.

(P: 0.001-0.04%)
P (phosphorus) is an element that deteriorates the toughness and corrosion resistance of ferritic stainless steel. Therefore, the upper limit of P addition is set at 0.04%. The lower the P addition amount, the better the corrosion resistance, but de-P refining stainless steel is thermodynamically very difficult because Cr reduces the P activity. Therefore, the lower limit of P addition is set at 0.001%.

(S: 0.001-0.01%)
S (sulfur) is a component that generates oxysulfides (acids/sulfides) together with La and oxygen (O), which can cause the crystal alignment with primary ferrite to be significantly lost. For this reason, the upper limit of the amount of S added is set to 0.01%. On the other hand, the lower limit of the amount of S added is a concentration that can be achieved by strengthening the desulfurization refining, but it is uneconomical because it places a large burden on refractories and other materials during refining. For this reason, the lower limit of the amount of S added is set to 0.001%.

(Cr: 10.5-31.0%)
Cr (chromium) is an essential element for ensuring corrosion resistance and heat resistance, and if the amount of Cr added is less than 10.5%, sufficient corrosion resistance and heat resistance cannot be obtained, while if the amount of Cr added exceeds 31.0%, it leads to a decrease in cold workability and toughness, so the amount of Cr added is limited to the range of 10.5 to 31.0%. When particularly high corrosion resistance and workability are required, it is preferable to set the amount of Cr added to 11 to 25%.

(Al: 0.006-0.20%)
Al (aluminum) is an element effective as a deoxidizer, similar to Si. It is also necessary for desulfurization to improve corrosion resistance. If the amount of Al added is less than 0.006%, excess oxygen remains, and Ce forms a complex oxide with Mn and Cr oxides, as in the case of Si being less than 0.2%, and does not function as a nucleation site. On the other hand, if the amount of Al added exceeds 0.20%, a complex oxide of Al and Ce may be formed. In this case, it also does not function as a nucleation site. Therefore, the amount of Al added is set to the range of 0.006 to 0.20%. It was confirmed that if Al is 0.005% or less, a complex oxide is formed with Ce oxide-based inclusions, but this complex oxide does not become a solidification nucleation site, and the solidification structure does not become fine. However, since the range in which complex oxides are not formed has been clarified, 0.006% or more is added in order to contribute to the ferritic stainless steel sheet by the desulfurization and denitrification effects.

(Nb: 0.03-0.8%, Ti: 0.01-0.2%)
In ferritic stainless steel sheets, Nb (niobium) and Ti (titanium) are elements that precipitate and fix C and N, which are harmful to press formability, and effectively contribute to softening and improving workability. When surface defects of steel sheets are important, Ti is not added because the resulting TiN causes surface defects. Therefore, Ti is selectively added depending on the required properties of the steel sheet.
If Nb is added in excess of 0.8%, or Ti in excess of 0.2%, the effect of these components will reach saturation. Therefore, the upper limit of Nb addition is set at 0.8%, and the upper limit of Ti addition at 0.2%. The lower limit of Nb addition is set at 0.03%, and the lower limit of Ti addition at 0.01%, which are the minimum concentrations required as C and N stabilizers to prevent the reaction of Cr with C and N in the steel to produce Cr carbo-nitrides.

(Ce: 0.01~0.35%)
If the Ce content is less than 0.01%, it will react with oxygen and Si and Al present in the molten steel to form complex oxides that do not serve as solidification nucleation sites. Therefore, the lower limit of the Ce content is 0.01%. If the Ce content exceeds 0.35%, it may react with oxygen in the slag or refractories during refining due to its very strong oxygen affinity, forming Ce oxide clusters. Since clusters are likely to cause surface defects on cold-rolled steel sheets, the upper limit of the Ce content is set at 0.35%.

(Total oxygen amount (T.O): 0.001-0.01%)
Total oxygen (TO) is an important element that generates Ce oxide-based inclusions that act as solidification nucleation sites, and it is essential that the formula (1) is satisfied in relation to the Ce concentration. If the TO concentration is less than 0.001%, the number of Ce oxide-based inclusions required for nucleation sites cannot be obtained. If it exceeds 0.01%, the number of Ce oxide-based inclusions becomes too large, which easily leads to clustering. Clusters of Ce oxide-based inclusions cause surface defects on cold-rolled steel sheets.
The "total oxygen content (TO)" refers to the total content of oxygen in the steel sheet and oxygen present as inclusions such as oxides and sulfates.

Furthermore, the stainless steel sheet of the present invention may further contain the optional additive components described below.
(Cu: 0.01% to 2.0%)
Cu (copper) is an element that contributes to improving corrosion resistance. It is desirable to add Cu in the range of 0.01 to 2.0%.

(Mo: 0.01% to 3.0%)
Mo (molybdenum) is an element that further improves corrosion resistance, and it is desirable to add it in the range of 0.01 to 3.0%.

(W: 0.002-3.0%)
W (tungsten) is also an element that further improves high-temperature strength. However, since W is a very expensive element, the preferred range for W content is 0.002 to 3.0%, depending on the desired properties.

(Rare earth metals other than Ce)
REM (rare-earth metal) refers to metals such as Sc (scandium), Y, and lanthanoid series elements such as Ce, La, Pr (praseodymium), and Nd (neodymium), but here it refers to rare earth metals other than Ce, La, and Y. REM has a high affinity with S and acts as an S fixing element, and is expected to have an effect of suppressing CaS generation, so it may be contained in an amount of 0.0005% or more. However, excessive REM content can cause nozzle clogging during casting, and the formation of coarse sulfides can actually lead to a deterioration in corrosion resistance. Therefore, the upper limit of the total amount of REM added is set to 0.0100%.

(La: 0.001-0.1%)
When Ce is added, it is often carried out with an alloy called misch metal, which is a mixture of Ce and La (lanthanum). Misch metal is an alloy containing about 70% Ce and about 30% La. When misch metal is added to molten steel, the upper limit of the amount of Ce added is 0.35%, so the upper limit of the amount of La added is set to 0.1%. Also, when misch metal is added to molten steel, although it depends on the yield of this alloy, the lower limit of the amount of La added is set to 0.001%, because about 0.001% is usually mixed into the molten steel.

(Y: 0.001-0.008%)
Y (yttrium) is an unnecessary element from the viewpoint of improving ridging. However, Y is an element that contributes to improving high-temperature strength. Therefore, it may be added depending on the required properties of the steel plate, but if the amount of Y added is in the range of 0.001 to 0.008%, there is almost no effect on the formation of Ce oxide-based inclusions that act as solidification nuclei. Therefore, the amount of Y added is preferably in the range of 0.001 to 0.008%.

In addition to the above components, the remainder is Fe and unavoidable impurities. Examples of unavoidable impurities include Zr (zirconium), V (vanadium), B (boron), Ca (calcium), Mg (magnesium), Sn (tin), Ni (nickel), Co (cobalt), Zn (zinc), and Ta (tantalum), which may be unavoidably contained at 0.5% or less. These elements are mixed in from various raw materials used when producing this steel on-site, and if the concentrations exceed the above levels, they often cause surface defects during hot rolling and surface flaws in the steel sheet. Therefore, during production, it is necessary to pay attention to the content of these elements in the raw materials used. If each is 0.5% or less, it does not affect the ridging resistance of the ferritic stainless steel sheet and is acceptable as long as the amount does not hinder the effects of the present invention.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to these examples.

The metal components of the ferritic stainless steel sheets in Examples 1 to 8 and Comparative Examples 1 to 8 are shown in Table 1. The ferritic stainless steel sheets in the Examples and Comparative Examples were melted on the actual production line shown below. Slabs were produced by continuous casting, and the occurrence of ridging on the surface of the steel sheets was checked to determine the surface properties when they were made into the final steel sheets.

Refining was carried out in an electric furnace, converter, and vacuum decarburization equipment (VOD). After that, the ladle containing the molten steel was moved to a tundish, and the molten steel was poured into the tundish from the bottom of the ladle. Next, using a continuous casting device attached to the tundish, the molten steel was poured into a mold through an immersion nozzle so that the target slab would form after solidification and cooling. This slab was then subjected to a hot rolling process and a cold rolling process to produce a cold-rolled sheet. More specifically, the steel was melted in the electric furnace, converter, vacuum decarburization equipment (VOD), tundish, and continuous casting processes to obtain the composition shown in Table 1. The amount of melting was approximately 80 tons per charge.

After decarburization and denitrification in a converter and a vacuum decarburizer (VOD), deoxidation refinement was performed using Al or Si in a vacuum decarburizer (VOD). After deoxidation refinement, elements such as Ce, La, Y, Al, and Ti were added to the molten steel in the form of Fe alloy or misch metal (Ce: 75%, La: 25%), changing with each charge. The molten steel was then transported to the tundish and poured into a mold through the tundish, where it was continuously cast to form a slab with a width of approximately 1050 mm and a thickness of approximately 200 mm. The molten steel temperature in the tundish was adjusted to be approximately 50°C higher than the liquidus temperature of the molten steel during continuous casting. Samples were cut out from the resulting slab to investigate the inclusion morphology in the as-solidified state.

The slab was then transported to a hot rolling furnace and inserted into the furnace. The temperature inside the furnace was gradually increased from approximately 900°C until it reached 1200°C, after which it was heated for approximately 30 to 45 minutes. It was then removed from the furnace and subjected to rough hot rolling and finish hot rolling to produce a hot rolled coil with a thickness of 4.5 mm. The hot rolled coil was annealed, pickled, and then cold rolled, annealed, and pickled to produce a cold rolled steel sheet with a thickness of 0.8 mm.

From the obtained cold-rolled steel sheet, a JIS-5 test piece was cut out from the center of the cold-rolled coil in the longitudinal direction parallel to the rolling direction in the same manner as described above. In addition, several plates for inclusion observation were cut out from the area near the cut-out test piece. These plates were stacked and pressed together so that the surface perpendicular to the rolling direction was the observation area, and were used for inclusion observation. If the surface was perpendicular to the rolling direction, it was in a state where it was crushed in the up and down rolling direction, and it was judged that the width dimension of the inclusion was roughly close to the diameter of the inclusion in the solidified state. For this sample, as described above, the inclusion composition and the number of inclusions per unit area and the photographed inclusions were converted to a circle equivalent diameter using MQA (automatic particle analyzer: ASPEX Explorer VP: manufactured by FEI). Based on the results, inclusions with a circle equivalent diameter of 5 μm present in the steel sheet were selected and the number per unit area was counted.

The ridging resistance was evaluated in the same manner as described above by carrying out a tensile test with a JIS-5 test piece elongation of 15% as specified in JIS Z2214 (tensile test method for metallic materials).
The results are shown in Table 2 together with the equiaxed crystal ratio.

In the invention examples 1 to 8, Ce and TO were within the range of the present invention, and as shown in Table 2, the product of Ce (%) and TO (%) in formula (1) was also within the range of the present invention. As a result, the equiaxed crystal ratio of the solidification structure in the ingot was 80% or more in all cases. It was also found that the inclusions with a circle equivalent diameter of 1 to 5 μm observed in the cold-rolled steel sheet were mainly Ce oxides. In addition, the number density of these inclusions was 20 pieces/cm2 or more in all cases. Due to this equiaxed crystal ratio and number density, in the test of JIS standard JIS Z2214 (Metal material tensile test method), the ridging resistance of the 0.8 mm cold-rolled thin sheet was evaluated as A rank or ^B rank, which is practically problem-free, for the ridging height (μm) of all of them.

In Comparative Example 1, the contents of Y and Al are excessive, and Ce is not contained. Since the product of the Ce content (%) and the TO content (%) does not satisfy formula (1), the equiaxed crystal ratio is low at 30%, and Ce oxide-based inclusions are not formed, resulting in a number density of 0. As a result, the evaluation result of ridging resistance is D. Note that Y is not an essential component of the present invention, and although Y oxide is formed as shown in Table 2, the equiaxed crystal ratio is low and the effect on ridging resistance is small.
In Comparative Example 2, the Ce content was too low to satisfy formula (1), and therefore the equiaxed crystal fraction was low and the number density of Ce oxide inclusions was low, resulting in an evaluation result of ridging resistance of D. Although _Al2O3 was formed in the ingot, _the equiaxed crystal fraction was low and the effect on ridging resistance was small.

In Comparative Example 3, the Ce content is excessive and the Al content is insufficient. The product of the Ce content (%) and the TO content (%) satisfies formula (1), but the equiaxed crystal ratio is low at 45% and the number density of the Ce oxide-based inclusions is very high at 108 pieces/ ^cm2 , but the evaluation result of the ridging resistance is C. Therefore, it is found that in order to satisfy the ridging resistance, it is necessary to satisfy both formula (1) and the number density of the Ce oxide-based inclusions.
Comparative Example 4 did not contain Ce and added Y, but did not satisfy formula (1) because it did not contain Ce even though the content of TO was excessive, and therefore the equiaxed crystal ratio was as low as 10%, and no Ce oxide-based inclusions were formed, resulting in a number density of 0. As a result, the evaluation result for ridging resistance was D.

In Comparative Example 5, La was added as a rare earth metal instead of Ce, but since it did not contain Ce, it did not satisfy formula (1), and therefore the equiaxed crystal ratio was as low as 25%, and no Ce oxide-based inclusions were formed, resulting in a number density of 0. As a result, the evaluation result for ridging resistance was D.
In Comparative Example 6, the contents of Y and S are excessive, forming Y sulfide. In addition, the product of the content (%) of Ce and the content (%) of TO satisfies formula (1), but the equiaxed crystal ratio is low at 10%, and the number density of Ce oxide-based inclusions is only 5, so the evaluation result of ridging resistance is D.

In Comparative Example 7, La was added as a rare earth metal instead of Ce, but since it did not contain Ce, it did not satisfy formula (1), and therefore the equiaxed crystal ratio was as low as 30%, and no Ce oxide-based inclusions were formed, resulting in a number density of 0. As a result, the evaluation result for ridging resistance was D.
In Comparative Example 8, the Al content is too low, and excessive S is contained, forming Ce sulfide, so that the product of the Ce content (%) and the TO content (%) satisfies formula (1), but the equiaxed crystal ratio is low at 40%, and the number density of inclusions is ²⁰ pieces/cm2 or less, and the evaluation result of ridging resistance is D. This oxysulfide is in a form in which sulfide is generated around an oxide, and it is presumed that the crystal structure is different from that of an oxide of simple Ce, and the degree of crystal mismatch is large.

In Table 2, Examples 1 to 8 of the present invention satisfy the range of the present invention, and therefore the evaluation results of ridging resistance were all good, A or B. On the other hand, Comparative Examples 1 to 8 are outside the range of the present invention, and therefore the evaluation results of ridging resistance were C or D, which made it clear that the surface waviness was large and unsuitable for practical use as steel sheets. The present invention makes it possible to manufacture ferritic stainless steel hot-rolled and cold-rolled steel sheets or coils with few surface defects. Furthermore, it is possible to suppress the occurrence of steel sheets that could not be shipped due to surface defects, and greatly improve the yield of the produced steel sheets. Furthermore, since the occurrence of surface defects on products such as steel sheets and coils is suppressed, the process of polishing the surface can be omitted. Due to these effects, the present invention can contribute to cost reduction, yield improvement, and productivity improvement.

Claims (2)

In mass percent,
C: 0.001-0.03%,
Si: 0.2-1.0%,
Mn: 0.05-1.0%,
P: 0.001-0.04%,
S: 0.001-0.01%,
Cr: 10.5-31.0%,
Al: 0.006-0.20%,
Nb: 0.03 to 0.8%,
Ti: 0.01-0.2%,
N: 0.001-0.03%,
Ce: 0.01 to 0.35%, and
T. O (total oxygen concentration): 0.001 to 0.01%
and the balance being Fe and unavoidable impurities, the product of the Ce content (%) and the TO content (%) satisfies the inequality relationship of the following formula (1), and the steel has a steel structure in which Ce oxide-based inclusions formed by bonding with the Ce and the oxygen (O) are present,
A ferritic stainless steel sheet having excellent ridging resistance, wherein the Ce oxide-based inclusions having a circle equivalent diameter of 1 to 5 μm have a number density per ^cm2 of 20 pieces/ ^cm2 or more in a cross section perpendicular to the rolling direction of the steel sheet.
Formula (1): 1.0×10 ^-5 ≦(Ce (%)) × ((T.O (%)) ≦1.5×10 ^-3 In mass percent,
Cu: 0.01-2.0%,
Mo: 0.01-3.0%,
W: 0.002-3.0%,
La: 0.001 to 0.05%, and
2. The ferritic stainless steel sheet having excellent ridging resistance according to claim 1, further comprising one or more components selected from the group consisting of: Y: 0.001 to 0.008%.

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