JP2001020046A - Ferritic stainless steel, ferritic stainless steel ingot with excellent workability and toughness, and method for producing the same - Google Patents

Abstract

(57) [Problem] To provide a ferritic stainless steel, a ferritic stainless steel ingot, and a method for producing the same, which are excellent in workability and toughness. [Solution] C ≦ 0.1%, N: 0.003-0.05%, Si: 0.03-
1.5%, Mn ≦ 1.0%, P ≦ 0.04%, S ≦ 0.03%, Cr: 10-30
%, Cu ≦ 2%, Ni ≦ 2%, Mo ≦ 3%, V ≦ 1%, Ti: 0.02-
0.5%, O: 0.001 to 0.005%, Nb ≦ 0.8%, Al: 0.001 to 0.
15%, Zr ≦ 0.3%, B ≦ 0.1%, Ca ≦ 0.003%, Mg <0.0005
%, Ti × N ≧ 0.0005, balance is Fe and impurities, Mg and A in steel
Inclusion containing Al and Mg with a ratio of l content of 0.3 to 0.5 and Ti
Ferrite stainless steel in which composite inclusions are dispersed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a ferritic stainless steel ingot, a ferritic stainless steel excellent in workability and toughness, and a method for producing the same. More specifically, it is excellent in workability such as stretch forming, deep drawing, bending, etc.,
The present invention relates to a ferritic stainless steel and a steel ingot having excellent workability and impact fracture resistance (toughness) that can be processed into a predetermined shape without generating surface flaws, cracks or fractures, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Ferritic stainless steel has corrosion resistance,
Because of its excellent weather and heat resistance, kitchen equipment, home appliances, water heaters and water tanks, automobile exhaust system parts, metal roofs,
Furthermore, it has been used in a wide range of fields such as materials for chemical plants.

However, since ferritic stainless steel usually contains 11% by mass or more of Cr, the recrystallization temperature is higher than that of so-called "normal steel". Therefore, in the case of ferritic stainless steel, it is difficult to recrystallize the hot-worked structure and make it finer. For this reason, various workability such as overhang property, deep drawability, bending workability, and weldability is required. Is inferior to ordinary steel. Further, since the toughness of ferritic stainless steel is generally low, brittle cracking occurs at the ingot stage, or brittle cracking occurs during hot working. For this reason, there is also a problem that the yield of the final product used in the various fields is significantly reduced.

[0004] In order to solve these problems, a technique for reducing impurity elements such as C, N, S and O (oxygen) in steel has been developed, and high-purity ferritic stainless steel can be produced. Have been. At present, for example, in the case of a ferritic stainless steel containing 30% by mass of Cr by a so-called "AOD" or "VOD" decarburizing refining furnace,
The content of C and N is 0.005% or less, the content of S is 0.001% or less, and the content of O (oxygen) is 0.1% by mass.
002% or less can be mass-produced.

[0005] When the ferritic stainless steel is highly purified, nonmetallic inclusions in the steel are reduced, and the toughness is improved.
Further, since the steel is softened, the elongation is improved. On the other hand, the purification of ferritic stainless steel leads to deterioration of workability called ridging characteristics.

[0006] "Ridging" is a ridge-shaped wrinkle that appears on the surface of a ferritic stainless steel sheet when it is press-formed. These wrinkles detract from the aesthetics of stainless steel moldings that are often used pure. For this reason, it is necessary to remove the ridging by polishing or the like, but the manufacturing cost of the molded product increases. Further, even if the application does not require an aesthetic appearance, if the processing conditions are severe, cracks may occur along the wrinkles of the ridging.

[0007] The occurrence of the ridging in a ferritic stainless steel sheet is caused by the fact that when a steel ingot having a coarse cast structure is hot-rolled, the coarse cast structure is not sufficiently refined and a steel sheet as a final product is formed. This is caused by remaining as a “colony”. "Colony" looks like a fine crystal structure, but its substance is a group of crystals with similar crystal orientations distributed in a region, and when subjected to press molding, it undergoes plastic deformation like a single crystal However, large ridge-like wrinkles are generated on the surface of the steel sheet.

[0008] Basic measures to prevent the occurrence of ridging include:
There are two methods of refining the solidified structure and refining the structure by recrystallization.

In the latter method, it is necessary to perform strong reduction during hot working or to perform annealing after performing reduction at low temperature. However, ferritic stainless steel has a small scale because of its good oxidation resistance.
Surface flaws easily occur due to seizure with a tool, for example, a rolling roll. In addition, it is difficult to make the structure uniform and fine, since recrystallization during hot is unlikely to occur.

On the other hand, as the former technique for refining the solidified structure, for example, a method using a nuclear action of TiN (iron and steel, 66 (1980) No. 6, p. 110),
Method by electromagnetic induction stirring of molten steel (Iron and Steel, 66
980) No. 6, p. 38).

However, a method of refining the structure of a steel ingot by using TiN is, for example, about 0.4% by mass of Ti or 0.0%.
It is necessary to make steel contain about 16% by mass of N and to precipitate a large amount of TiN in molten steel. In addition, unless conditions such as lowering the superheat degree ΔT of molten steel to 40 ° C. or less are combined, a fine equiaxed crystal structure (specifically, an average grain size of 3
mm or less of an equiaxed crystal structure). Furthermore, a large amount of T
Since i and TiN impair the toughness of the steel, the problem of brittle cracking of ferritic stainless steel further increases.
In addition, it is not always easy to control the ΔT fluctuation width for each casting during operation, and it is not easy to cast once ΔT becomes too small. cause.

In the case of the method using electromagnetic induction stirring, even if ΔT is high, by adjusting the stirring position of the molten steel with respect to the steel ingot during solidification, 40-60% by volume (hereinafter referred to as the proportion of equiaxed crystal ( (Equiaxed crystal ratio) is simply represented by “%”). However, in order to obtain a higher equiaxed crystal ratio, it is necessary to control ΔT to a low value of less than 25 ° C.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has been made in consideration of the above-mentioned current situation, and has been conducted on a cast steel ingot and a hot working such as hot rolling, stretch forming, deep drawing, and the like. An object of the present invention is to provide a ferritic stainless steel excellent in workability and toughness which can be processed into a predetermined shape without causing surface flaws, cracks or breakage by processing such as bending, and a method for producing the same. Specifically, a portion exceeding 70% of the solidified structure becomes a fine equiaxed crystal having an average particle size of 3 mm or less, and has defects such as flaws in hot working, stretch forming, deep drawing, and bending. An object of the present invention is to provide a ferritic stainless steel, a ferritic stainless steel ingot, and a method for producing the same, which are less likely to crack or break and have good workability and toughness.

[0014]

The gist of the present invention is to provide a ferritic stainless steel excellent in workability and toughness as shown in (1) below, a ferritic stainless steel ingot as shown in (2),
And (3) a method for producing a ferritic stainless steel.

(1) In mass%, C: 0.1% or less, N:
0.003-0.05%, Si: 0.03-1.5%,
Mn: 1.0% or less, P: 0.04% or less, S: 0.0
3% or less, Cr: 10 to 30%, Cu: 2% or less, N
i: 2% or less, Mo: 3% or less, V: 1% or less, Ti:
0.02 to 0.5%, O (oxygen): 0.001 to 0.0
05%, Nb: 0.8% or less, Al: 0.001-0.
15%, Zr: 0.3% or less, B: 0.1% or less, C
a: 0.003% or less and Mg: less than 0.0005%, and the value of fn1 represented by the following formula is 0.000
5 or more, and the balance is the chemical composition of Fe and unavoidable impurities, and the ratio of the content of Mg to Al in the steel is 0.3 to 0.5.
A ferritic stainless steel excellent in workability and toughness in which composite inclusions of Al- and Mg-containing inclusions and Ti-based inclusions are dispersed.

Fn1 = Ti (%) × N (%) (2) Having the chemical composition described in the above (1), the equiaxed crystal ratio exceeds 70%, and the average particle size of the equiaxed crystal is A ferritic stainless steel ingot of 3 mm or less.

(3) By mass%, the slag composition is changed to Al ₂ O ₃ :
1 to 40%, CaO: 30 to 70%, MgO: 1 to 30
%, CaF ₂ : 30% or less, SiO ₂ : 50% or less, remaining inevitable impurities: 10% or less, and the value of fn2 represented by the following formula: 1.0 to 3.0, and oxygen in the molten steel The method for producing a ferritic stainless steel having excellent workability and toughness according to the above (1), wherein the content is refined to 0.001 to 0.005% and then cast.

Fn2 = CaO (%) / {Al ₂ O ₃ (%) + SiO ₂ (%)} Here, “inclusion” is a term indicating “foreign matter or impurity phase in solid”. Although it is appropriate to express the term “compound” at the stage before the steel is solidified, the term “inclusion” is used in the present specification even before the steel is solidified.

The composite inclusion of Al and Mg-containing inclusions and Ti-based inclusions refers to inclusions having a structure in which Ti-based inclusions surround Al and Mg-containing inclusions. Hereinafter, for the sake of simplicity, the above-described composite inclusion of the Al- and Mg-containing inclusion and the Ti-based inclusion is also referred to as an Al-Mg-Ti-based composite inclusion. The inclusion containing Al and Mg refers to an inclusion containing at least Al, Mg and O, and is hereinafter referred to as an Al-Mg based inclusion for simplicity.

As described in JIS G 0203 as a steel term, "ingot" is not only a so-called "ingot" in which steel refined in a steelmaking furnace is poured into a mold and solidified, but also manufactured by continuous casting. Refers to those containing cast slabs.

Hereinafter, the inventions described in the above (1) to (3) are referred to as the inventions (1) to (3), respectively.

The present inventors investigated and studied the chemical composition of the cast ferritic stainless steel, the precipitation morphology of nonmetallic inclusions in the steel (internal structure, dispersion state), and the structure of the steel ingot. As a result, the following findings were obtained first.

(A) In a ferritic stainless steel having a value of fn1 represented by the above formula of 0.0005 or more, a cast structure (solidified structure) having a fine and high equiaxed crystal ratio regardless of molten steel superheat ΔT. May be able to.

(B) In ferritic stainless steel containing Ti in an amount exceeding 0.2%, a high equiaxed crystal ratio can be obtained stably. However, the average particle size of the equiaxed crystal is not necessarily small, and there is a case where the average particle size exceeds 3 mm.

(C) Even if the equiaxed crystal ratio is high, if the average grain size of the equiaxed crystals exceeds 3 mm, the toughness of the steel material is reduced. For this reason, for example, in the case of a steel material having a thickness of about 5 mm, the impact transition temperature of most of the steel material greatly exceeds room temperature, and brittle cracks are easily generated.

(D) the average particle diameter of the equiaxed crystal is 3 mm or less;
Moreover, when the equiaxed crystal ratio exceeds 70%, workability and toughness are good.

(E) The main components are Ti and N, and O
(Oxygen), Ti-based inclusions containing S and C form heterogeneous nuclei with Al-Mg-based inclusions dispersed in molten steel as nuclei. Therefore, by controlling the Al-Mg-based inclusions in the molten steel, the precipitation temperature of the Ti-based inclusions, in other words, the precipitation form of the Ti-based inclusions can be controlled.

(F) Since Ti-based inclusions with controlled precipitation form become crystal nucleation sites during solidification of steel, it is possible to form fine equiaxed crystals at a high rate.

Then, the precipitation morphology of the steel ingot having a fine equiaxed crystal structure with an average grain size of 3 mm or less was controlled.
The structure of the system inclusion, that is, the Al-Mg-Ti system composite inclusion was investigated.

That is, 200 cast continuously as a steel ingot.
mm thickness × 1050 mm width cast piece was selected, a small test piece was sampled from the position 10 m from the tip, and 20 m under the surface
m was mirror-finished. Note that the final polishing was performed with diamond abrasive grains (particle diameter: 0.25 μm) in alcohol in order to prevent disappearance of water-soluble inclusions and residual alumina abrasive grains as an abrasive. Next, the inclusions present on the observation surface were examined using a high-resolution Auger electron spectrometer, an energy dispersive X-ray spectrometer, or a normal EPMA. In the present specification, a high-resolution Auger electron spectrometer, an energy dispersive X-ray spectrometer,
Investigations using ordinary EPMA are referred to as investigations by Auger electron spectroscopy, EDX, and EPMA, respectively.

In the Auger electron spectroscopy, microstructure analysis of composite inclusions and analysis of light elements such as C and N were performed.
In the EDX method, which is relatively easy to operate, the content of an element having an atomic weight of Mg or more was analyzed, and in the EPMA method, the distribution density of composite inclusions was examined.

As a result, the following matters became clear.

(G) Ti-based inclusions present in a steel ingot having an average particle diameter of equiaxed crystals of 3 mm or less are precipitated in combination with Al-Mg-based inclusions. The Al-Mg-based inclusions present as nuclei for the generation of Ti-based inclusions are very fine, 0.1 to 1 µm, whereas the size of the Ti-based inclusions is 0.3 to 5 µm.

(H) Al serving as a nucleus for the formation of Ti-based inclusions
-Mg-based inclusions include Al, Mg, O (oxygen), Ca,
Si, Mn, S and the like are included. In addition, this Al-
Mg-based inclusions always contain Al, Mg, and O, and other elements may be below the detection limit. Also, Al
-Ti-based inclusions deposited to cover the Mg-based inclusions,
Although the main component is Ti and N, and it contains O (oxygen), S, and C in addition, elements other than Ti and N are sometimes below the detection limit.

FIG. 1 shows an outline of the Al—Mg—Ti based composite inclusion. FIG. 2 shows E of Al-Mg-Ti based composite inclusions present in a steel ingot (slab) having an equiaxed crystal ratio of 100%.
One example of the analysis results by the DX method is shown.

The present inventors considered that the solidification structure of the steel ingot and the Al-M
The relationship between the g-based inclusion compositions was also investigated in detail. As a result, the following matters became clear.

(I) In a steel ingot in which a portion exceeding 70% of the solidified structure exhibits a fine equiaxed crystal having an average grain size of 3 mm or less, the value of fn1 represented by the above formula is 0.0005 or more, and The content ratio (Mg / Al) of Mg and Al in the Al-Mg-based inclusions is in the range of 0.3 to 0.5.

(J) Ti-based inclusions are also found in the columnar crystal part of the steel ingot having a low equiaxed crystal ratio, but the nucleus is A
Instead of l-Mg-based inclusions, a single oxide (for example, Si
O ₂ or CaO) or an Al-rich oxide (Al-Mg based inclusion) having a ratio of Mg to Al content of less than 0.3.

FIG. 3 shows the value of fn1 represented by the above formula and the Mn in the Al-Mg-based inclusion in a ferritic stainless steel ingot in which Al-Mg-Ti-based inclusions are dispersed.
The effect of the ratio of the content of g to the content of Al on the solidification structure is summarized and shown.

Therefore, further investigation was made on production conditions under which Al-Mg-Ti-based composite inclusions effective for forming a fine solidified structure as shown in FIG. 3 can be dispersed. As a result, the following items were found.

(K) Mg and Al in Al—Mg based inclusions
In order to make the content ratio in the range of 0.3 to 0.5, the slag composition is expressed as Al ₂ O ₃ : 1 to 40% and CaO: 3 by mass%.
0~70%, MgO: 1~30%, CaF 2: 30% or less, SiO _2: 50% or less, the balance of inevitable impurities: 10
% Or less and the value of fn2 represented by the above formula: 1.0 to
3.0 and the oxygen content in molten steel is 0.005%
After refining below, casting may be performed.

(L) Even if the above condition (j) is satisfied, if the oxygen content in the molten steel is less than 0.0010%, A
Although l-Mg-based inclusions are formed, the solidified structure may not always be refined.

(M) It is not necessary to intentionally add Al and Mg to disperse the Al-Mg-Ti-based composite inclusions. This is because Al and Mg are mixed in from smelting slag and eluted from refractories such as ladles and tundishes when smelting and casting steel. Of course, Al and Mg may be intentionally added.

(N) In the casting process of ferritic stainless steel, Ca may be intentionally added to prevent clogging of the immersion nozzle. In this case, Al-M
The g-based inclusions partially change form into Al-Mg-Ca-based inclusions, but if the content ratio of Mg and Al in the inclusions is in the range of 0.3 to 0.5, etc. A steel ingot having a fine equiaxed crystal structure with an axial crystal ratio of more than 70% can be manufactured.

The present invention has been completed based on the above findings.

[0046]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, “%” of the content of the chemical component means “% by mass”.

(A) Chemical composition of steel C: 0.1% or less C has a small solid solubility limit in ferritic stainless steel, so it may combine with Cr depending on the heat history in steps such as annealing and welding to form a grain boundary. Cr carbides are formed on the surface, causing grain boundary corrosion. Therefore, the content of C is set to 0.1% or less. In order to secure good corrosion resistance, the upper limit of the C content is preferably set to 0.02%.

N: 0.003 to 0.05% N is a Ti-based inclusion having a heterogeneous nucleation with Al-Mg-based inclusions as nuclei, that is, the Al-Mg-based inclusion is a Ti-based inclusion (main component). Is to form an Al-Mg-Ti-based composite inclusion covered with Ti and N) to make the solidified structure of the cast ferritic stainless steel a fine and high equiaxed crystal structure and enhance the formability. It is an essential element. However, if the content is less than 0.003%, the desired effect cannot be obtained. On the other hand, when the content exceeds 0.05%, a remarkable decrease in toughness is caused. Therefore, the content of N is set to 0.003 to 0.0.
5%. The preferable N content for securing good toughness is 0.015% or less.

Si: 0.03 to 1.5% Si is an element useful for reducing and deoxidizing Cr oxide generated during refining. However, if the content is less than 0.03%, the effect of addition is poor. On the other hand, when the content exceeds 1.5%, the workability of steel materials including steel plates deteriorates. Therefore, S
The content of i was set to 0.03 to 1.5%.

Mn: 1.0% or less Mn may not be added. If added, it has the effect of deoxidizing the steel. To ensure this effect, Mn should be 0.1
% Is preferable. However, if its content exceeds 1.0%, the precipitation amount of MnS increases and the pitting corrosion resistance deteriorates, and the component cost increases, which is disadvantageous in terms of economy. Therefore, the content of Mn is set to 1.0% or less.

P: not more than 0.04% P deteriorates the toughness, hot workability and corrosion resistance of steel, so the lower the content, the better. Particularly, if it exceeds 0.04%, the toughness and hot work of steel are reduced. Workability and corrosion resistance deteriorate significantly. Therefore, the content of P is set to 0.04% or less.

S: 0.03% or less S degrades the toughness, hot workability and corrosion resistance of steel, so the lower the content, the better. Particularly, if it exceeds 0.03%, the toughness and hot work of steel are reduced. Workability and corrosion resistance deteriorate significantly. Therefore, the content of S is set to 0.03% or less.

Cr: 10 to 30% Cr is an element effective for ensuring the corrosion resistance and oxidation resistance of ferritic stainless steel. However, if the content is less than 10%, the effect of addition is poor. On the other hand, when the content exceeds 30%, so-called “475 ° C. brittleness” is apt to occur, which causes a remarkable decrease in toughness. Therefore, the Cr content was set to 10 to 30%.

Cu: 2% or less Cu need not be added. If added, it has the effect of increasing the corrosion resistance and weather resistance of the steel. To ensure this effect, it is preferable that the content of Cu be 0.1% or more. However, if the content exceeds 2%, the steel becomes hard and ductility is impaired. Therefore, the content of Cu is set to 2% or less.

Ni: 2% or less Ni need not be added. If added, it has the effect of increasing the toughness of the steel. To ensure this effect, the content of Ni is preferably set to 0.1% or more. But,
Even if the content exceeds 2%, the effect of improving toughness corresponding to the increase in cost cannot be expected, and the steel may be hardened and the toughness and ductility may be impaired. Therefore, the content of Ni is 2
% Or less.

Mo: 3% or less Mo need not be added. If added, it has the effect of increasing corrosion resistance and weather resistance. In order to surely obtain such an effect, it is preferable that the content of Mo is 0.1% or more. However, when the content exceeds 3%, precipitation of an intermetallic compound is promoted, so that toughness is impaired. Therefore, the content of Mo is set to 3% or less.

V: 1% or less V may not be added. If added, it has the effect of increasing corrosion resistance. To ensure this effect, V should be 0.0
Preferably, the content is 5% or more. However, if the content exceeds 1%, the toughness decreases. Therefore,
The content of V was set to 1% or less.

Ti: 0.02-0.5% Ti forms Al-Mg-Ti based composite inclusions,
It is an element essential for improving the workability by making the solidification structure of the cast ferritic stainless steel a fine and high equiaxed crystal structure. Further, Ti combines with N and C in steel to form carbides, nitrides and carbonitrides, and C and N form a solid solution in the matrix.
To improve the workability, corrosion resistance, and toughness of ferritic stainless steel. However, if the content is less than 0.02%, a desired cast structure having a fine and high equiaxed crystal ratio exceeding 70% cannot be obtained when casting. On the other hand, when the content exceeds 0.5%, it is effective for improving the equiaxed crystal ratio of the steel ingot, but the oxides and nitrides of Ti are aggregated to become a starting point of a flaw, and the product such as a steel plate or a steel pipe is manufactured. This impairs the surface properties. Therefore, the content of Ti is set to 0.02 to 0.5%. When cast, fine and 70%
In order to obtain a desired solidified structure having a high equiaxed crystal ratio exceeding 0.1%, the lower limit of the Ti content is preferably 0.05%. In order to ensure good surface properties of products such as steel plates and steel pipes, the upper limit of the Ti content is preferably set to 0.3%.

O (oxygen): 0.001 to 0.005% O (oxygen) forms Al-Mg-Ti based composite inclusions, and makes the solidified structure of the cast ferritic stainless steel fine and high. It is an element that has an axial crystal structure and is indispensable for enhancing workability. However, when the content is less than 0.001%, the precipitation time of Al-Mg-based inclusions is not appropriate,
Since the casting structure (solidification structure) becomes a coarse columnar crystal structure,
When processing various steel materials from the steel ingot, the workability and ridging resistance are extremely poor. On the other hand, if the content exceeds 0.005%, not only the toughness and workability are reduced but also surface flaws are frequently generated. Therefore, the content of O is set to 0.001 to 0.
005%.

Nb: 0.8% or less Nb may not be added. If added, it has the effect of improving workability and corrosion resistance. In order to surely obtain such an effect, the content of Nb is preferably set to 0.1% or more. However, if the content exceeds 0.8%, the toughness decreases. Therefore, the content of Nb is set to 0.8% or less.

Al: 0.001 to 0.15% Al forms Al-Mg-Ti-based composite inclusions,
It is an essential element for improving the workability by making the solidification structure of the cast ferritic stainless steel a fine and high equiaxed crystal structure. However, if the content is less than 0.001%, Al-Mg-based inclusions do not precipitate in the molten steel, so that desired Al-Mg-Ti-based composite inclusions are not formed,
The structure of the steel ingot becomes a coarse columnar crystal structure. When various steel materials are processed from the above steel ingot, workability and ridging resistance are extremely poor. On the other hand, when the Al content exceeds 0.15% and the deoxidizing effect of Al is large, the oxygen amount during refining is likely to be less than 0.001%. In this case, the precipitation time of the Al-Mg-based inclusions is not appropriate, and the structure of the steel ingot becomes a coarse columnar crystal structure. Therefore, the content of Al is set to 0.001 to 0.15.
%. In order to obtain a desired solidified structure, Al
The lower limit of the content is preferably 0.003%.

Zr: 0.3% or less Zr may not be added. If added, it has the effect of improving workability and oxidation resistance. In order to surely obtain such an effect, the content of Zr is preferably set to 0.05% or more. However, if the content exceeds 0.3%, the toughness decreases. Therefore, the content of Zr is set to 0.3% or less.

B: 0.1% or less B may not be added. If added, it has the effect of improving workability. To ensure this effect, B should be 0.00
The content is preferably 1% or more. However, if the content exceeds 0.1%, the toughness decreases. Therefore, the content of B is set to 0.1% or less.

Ca: 0.003% or less Ca may not be added. If added, it has the effect of making the equiaxed grain size of the steel ingot finer. To ensure this effect, the content of Ca is preferably 0.0001% or more. However, when the content exceeds 0.003%, Ca-based inclusions are generated in addition to the Al-Mg-Ti-based composite inclusions, and the toughness and pitting corrosion resistance of the ferritic stainless steel are significantly reduced. Therefore, the content of Ca is set to 0.003% or less. In order to ensure good toughness and corrosion resistance in ferritic stainless steel, Ca
The upper limit of the content is preferably set to 0.001%.

Mg: less than 0.0005% Mg need not be added. If added, Al-Mg-
It has the effect of increasing the workability by entering the Ti-based composite inclusions and turning the solidified structure of the cast ferritic stainless steel into a fine and high equiaxed crystal structure. The content of Mg is the amount mixed as an impurity (for example, 0.0001% because the current analysis accuracy of Mg is about 0.0001%).
%). Of course, Mg may be positively added. However, when its content is 0.1.
If it exceeds 0005%, the toughness of the ferritic stainless steel will be significantly reduced. Therefore, the content of Mg is set to less than 0.0005%.

Fn1: 0.0005 or more TiN is a main compound constituting the Al-Mg-Ti-based composite inclusion, and the solidified structure of the cast ferritic stainless steel is converted to a fine structure having a high equiaxed crystal ratio. It is an essential compound for improving processability. However, when the value of fn1 represented by the above formula is less than 0.0005, TiN does not crystallize before ferrite solidification, and cannot be a ferrite solidification nucleus. As a result, a desired solidified structure having a fine and high equiaxed crystal ratio exceeding 70% cannot be obtained. Therefore, fn1
Was set to 0.0005 or more. In order to impart good toughness to ferritic stainless steel, the upper limit of the value of fn1 is preferably set to 0.005.

(B) Al-Mg-Ti Composite Inclusions In order to improve the workability of ferritic stainless steel having the above chemical composition, Al-Mg-Ti composite inclusions are dispersed in steel. It is important to keep.

In the stainless steel of the present invention, the Al—Mg-based inclusion having a ratio of Mg to Al content of 0.3 to 0.5 contains T
The Al-Mg-Ti-based composite inclusion covered with the i-based inclusion is an indispensable inclusion for fine equiaxed crystallization of a steel ingot.

An essential element constituting the composite inclusion is A
Al, Mg, and O are included in 1-Mg-based inclusions, and Ti and N are included in Ti-based inclusions. As other constituent elements, Al-Mg-based inclusions may include Ca, Si, Mn, S, and the like, and Ti-based inclusions may include O, S, C, and the like.

However, the ratio between the Mg and Al contents of the Al—Mg based inclusion must be in the range of 0.3 to 0.5. If the content ratio is less than 0.3, the structure of the steel ingot becomes a coarse columnar crystal structure or an equiaxed crystal structure having an average particle size exceeding 3 mm. On the other hand, when the ratio of the content is 0.1.
If it exceeds 5, the solidified structure of the steel ingot becomes fine equiaxed crystals or coarse columnar crystals, and a desired structure cannot be obtained stably and reliably. Therefore, the ratio between the Mg and Al contents of the Al-Mg-based inclusions is set to 0.3 to 0.5.

In the ferritic stainless steel, Ta, which easily forms an oxide or a nitride, or a rare earth element such as La or Ce may be added for the purpose of enhancing properties such as oxidation resistance. When the above elements are added to steel, A
These elements are often found in the l-Mg-Ti-based composite inclusions, but in this case also, the above Al-Mg-T
The action of the i-based composite inclusion is not hindered.

In order to obtain a steel ingot having a high equiaxed crystal ratio exceeding 70%, the ratio of the contents of Mg and Al in the steel is 0.3 to 0.1%.
It is preferable that the distribution density of the inclusion (Al-Mg-Ti-based composite inclusion) in which the Al-Mg-based inclusion and the Ti-based inclusion are combined is 5 pieces / mm ² or more. Since the structure of the steel ingot becomes finer as the number of the Al-Mg-Ti-based composite inclusions increases, there is no upper limit to the distribution density of the inclusions. The Al-Mg-Ti measured at this distribution density
The size (major axis) of the system composite inclusion is 0.3μ due to measurement accuracy.
m or more.

The mechanism of refining the solidified structure by the Al-Mg-Ti-based composite inclusion is probably considered as follows.

When the ratio of the contents of Mg and Al is 0.3 to 0.5
From the binary phase diagram of Al ₂ O ₃ —MgO, it can be inferred that the Al—Mg based inclusions described above are MgOAl ₂ O ₃ spinel based inclusions. This spinel-based inclusion has a cubic crystal structure, and has good lattice matching with the Ti-based inclusion (TiN).
Therefore, it is presumed that TiN easily precipitates in molten steel with spinel inclusions as nuclei. Thereby, Al-Mg-Ti-based composite inclusions are formed in the molten steel.

On the other hand, Mg and Al in Al—Mg based inclusions
Is less than 0.3, corundum is crystallized. This inclusion has a hexagonal crystal structure, and TiN
And the degree of lattice matching is poor. Therefore, TiN hardly precipitates in molten steel.

The refinement of the solidification structure of ferritic stainless steel can be achieved by forming Al-Mg-based inclusions with Ti-based inclusions.
It is presumed that Mg-Ti based composite inclusions become ferrite solidification nuclei. The precipitation time of this composite inclusion is A
It depends on the composition of the l-Mg-based inclusion, that is, the crystal structure. Therefore, since the Al-Mg-Ti composite inclusions are most likely to be generated in molten steel when the ratio of the Mg and Al contents of the Al-Mg inclusions is controlled to 0.3 to 0.5,
It is considered that the solidified structure becomes fine.

By satisfying the chemical composition described in the section (A) and the inclusions described in the section (B), the invention of (1) can be obtained.

(C) Equiaxed crystal of steel ingot The average particle diameter of the equiaxed crystal is 3 mm or less, and the equiaxed crystal ratio is 7
If it exceeds 0%, workability and toughness will be good. Therefore, in the invention of (2), the solidified structure of the steel ingot has an equiaxed crystal ratio of more than 70% and an average grain size of the equiaxed crystal of 3%.
mm or less.

(D) Slag Composition and Oxygen Content in Molten Steel In order to obtain a ferritic stainless steel excellent in workability and toughness, casting is performed after the slag composition and the oxygen content in the molten steel are adjusted by refining. It is essential. Therefore, in the invention of (3), the slag composition and the oxygen content in the molten steel are specified.

(D-1) Slag composition Al ₂ O ₃ : 1 to 40% Al ₂ O ₃ keeps Al equilibrium between slag and molten steel during refining and removes Al-Mg-Ti-based inclusions during casting. Generate
The structure of the steel ingot is refined with equiaxed crystals. However, if the amount in the slag is less than 1%, the above effect cannot be obtained, and if it exceeds 40%, the oxide in the molten steel becomes coarse and the toughness is significantly deteriorated. Therefore, the amount of Al ₂ O _{3 in} the slag is 1 to 40.
%.

CaO: 30 to 70% CaO is effective for deoxidation and desulfurization of molten steel. However, if the amount in the slag is less than 30%, a sufficient effect cannot be obtained.
If it exceeds 0%, the above effect is saturated. Therefore, the amount of CaO in the slag was set to 30 to 70%.

MgO: 1 to 30% MgO is a source of Mg elution into molten steel,
-Effective for generating Mg-Ti-based inclusions. However, if the amount in slag is less than 1%, the above effect cannot be obtained, and if it exceeds 30%, the effect is saturated. Therefore, the amount of MgO in the slag is set to 1 to 30%. In order to promote the refinement of the solidified structure, the lower limit of the amount of MgO in the slag is preferably set to 3%.

CaF ₂ : 30% or less CaF ₂ may not be contained in the slag. If contained in the slag, it has the effect of increasing the deoxidizing and desulfurizing efficiency of the molten steel. In order to ensure this effect, Ca in the slag
Preferably, the amount of F ₂ is 3% or more. On the other hand, when the amount in the slag exceeds 30%, the refractory damage to the ladle becomes large, and the production cost increases. Therefore, the amount of CaF _{2 in} the slag was set to 30% or less.

SiO ₂ : 50% or less SiO ₂ may not be contained in the slag. If contained in the slag, it has the effect of deoxidizing molten steel. To ensure this effect, the amount of SiO _{2 in} the slag is preferably 3% or more. On the other hand, when the amount in the slag exceeds 50%, the oxygen content in the molten steel increases,
A desired steel ingot structure cannot be obtained and the toughness deteriorates. Therefore, the amount of SiO _{2 in} the slag was set to 50% or less.

Remaining unavoidable impurities: 10% or less Other unavoidable impurities in the slag are Fe, Cr, M
It is composed of n, Ti oxide and S. It is better not to include such impurities as much as possible, because they lower the deoxidation and desulfurization efficiency of the molten steel. The amount of the impurities in the slag is 10
%, The Al-Mg-Ti-based composite inclusions do not disperse, so that the structure of the steel ingot is not refined and the toughness is deteriorated. Therefore, the amount of unavoidable impurities in the slag is reduced by 10%.
It was as follows. In order to further promote the refinement of the structure of the steel ingot, the amount of inevitable impurities in the slag is preferably 5% or less.

Fn2: 1.0 to 3.0 fn2 represented by the above formula is a basic index governing the deoxidizing and desulfurizing ability of molten steel by slag. If the value of fn2 is less than 1.0, the above effect cannot be sufficiently obtained. On the other hand, if the value of fn2 exceeds 3.0, refractory damage to the ladle becomes large,
Manufacturing costs increase. Therefore, if the value of fn2 is 1.
0 to 3.0. In order to ensure good corrosion resistance and toughness of the ferritic stainless steel, the value of fn2 must be 1.
It is preferable to set it to 2 to 2.4.

(D-2) Oxygen Content in Molten Steel Oxygen in the molten steel forms Al-Mg-Ti-based composite inclusions, and changes the solidification structure of the cast ferritic stainless steel into fine and high equiaxed crystals. It has the function of increasing the workability. However, if the content is less than 0.001%, the precipitation time of Al-Mg-based inclusions is not appropriate, and the structure of the steel ingot becomes a coarse columnar crystal structure. When various steel materials are processed from the steel ingot, the workability and ridging resistance are extremely poor. On the other hand, if the content exceeds 0.005%, not only the toughness and workability are reduced but also surface flaws are frequently generated. Therefore, the oxygen content in the molten steel was set to 0.001 to 0.005%, which is the same as the oxygen content in the cast steel.

In order to further refine the solidified structure, it is preferable to perform continuous casting after refining the slag composition and the oxygen content in the molten steel as described above.

A for making the steel ingot structure a fine equiaxed crystal
It is not necessary to particularly define the timing of addition of l, Ti, etc., the superheat degree of molten steel ΔT, and the stirring of molten steel. However, in order to promote the formation of Al-Mg-based inclusions, the refractory contains MgO in the refractory. It is preferable to use a solid or a tundish.

Hereinafter, the present invention will be described with reference to examples.

[0091]

EXAMPLE Stainless steel having the chemical composition shown in Table 1 was continuously cast into a slab having a width of 1050 mm and a thickness of 200 mm. After smelting by a usual method, refining for removing C and removing N was performed, and then refining for reducing oxidized Cr was performed. Thereafter, Al, Si, and Ti are added, and Ca is partially added.
, And then continuously cast. Table 2 shows details of the slag composition during refining before casting, the oxygen content in the molten steel, and the presence or absence of Ca addition.

The test numbers 1 to 1 in Tables 1 and 2 were used.
3 is an example of the present invention in which the chemical composition, the slag composition before casting, and the oxygen content in the molten steel are all within the ranges specified in the present invention. On the other hand, test numbers 14 to 23 in Tables 1 and 2
Is a comparative example in which at least one of its chemical composition, slag composition before casting, and oxygen content in molten steel deviate from the conditions specified in the present invention.

[0093]

[Table 1]

[0094]

[Table 2]

The central portion (200 mm thick × 100 mm width) of the cross section perpendicular to the casting direction of the above-mentioned various steel slabs is corroded by ordinary aqua regia with a volume ratio of nitric acid and hydrochloric acid of 1: 3 and its structure. Was observed, and the equiaxed crystal ratio was determined from the area ratio between the equiaxed crystal grains and the columnar crystal grains. Further, the crystal grain size of 75 mm under the epidermis was adjusted to ASTM E112.
It was determined by the section method according to the above. The average particle diameter D was 1.12 times the average intercept length L.

[0096] Each of the above slabs was prepared by a conventional method in the form of 1100-1000.
The sheet was heated to 1250 ° C. and hot-rolled to finish a steel sheet having a thickness of 4.5 mm.

The analysis values of Al, Ca and Mg shown in Table 1 were quantitatively measured by dissolving chips collected from the above 4.5 mm steel plate with aqua regia and flameless atomic absorption spectrometry or ICP emission spectrometry. It was done.

A test piece cut out of the 4.5 mm thick steel plate was polished with diamond abrasive grains in alcohol, and a portion corresponding to １ ／ part of the cross section of the steel plate was observed with a scanning electron microscope. The inclusions having a composite structure were analyzed by the EDX method to confirm the composition, the ratio of the contents of Mg and Al in the Al-Mg-based inclusions, and the distribution amount of the Al-Mg-Ti-based composite inclusions in the steel. investigated. The ratio between the contents of Mg and Al in the Al-Mg-based inclusions was determined as an average value of 10 or more randomly selected inclusions.

The steel sheet hot-rolled to a thickness of 4.5 mm was annealed and pickled by ordinary methods of annealing and pickling. In order to investigate the impact characteristics from the annealed pickled steel sheet, a 2 mm deep V-notched subsize Charpy test piece was taken perpendicularly to the rolling direction and subjected to an impact test to determine the ductility-brittle transition temperature (vTs). It was measured.

The annealed pickled steel sheet having a thickness of 4.5 mm obtained as described above was subjected to cold rolling to a thickness of 1 mm after polishing the front and back surfaces to adjust the surface roughness. Next, the cold-rolled steel sheet was soaked in a combustion gas at a temperature of 830 to 1030 ° C for 20 to 30 seconds. The rate of temperature rise and the rate of temperature decrease during annealing were both in the range of 10 to 50 ° C./sec. In addition, the 4.5-mm-thick annealed pickled steel plate manufactured from the cast pieces according to Test Nos. 17 to 19 and 23 had severe brittle cracks during surface grinding work as described later, and the steel plate was broken.

From the annealed steel sheet having a thickness of 1 mm obtained as described above, in the directions of 0 °, 45 °, and 90 ° with respect to the rolling direction.
A tensile test piece of No. 13B specified in JIS Z 2201 was sampled and subjected to a tensile test at room temperature at a rating distance of 50 mm to measure the elongation at break. The elongation was evaluated by the average elongation (El) in the above three directions according to the following equation.

El = (El ₀ + 2El ₄₅ + El ₉₀ ) / 4 Further, from the above-described annealed steel sheet, in parallel with the rolling direction, JIS Z 2201
No. 5 tensile test piece specified in Example 1 was sampled, the parallel portion thereof was mirror-finished, and then subjected to tensile deformation at room temperature to evaluate ridging resistance. That is, after a tensile deformation of 20% (that is, 10 mm) at a rating distance of 50 mm, the surface was scanned perpendicularly to the tensile direction using a surface roughness meter to examine the ridging generated on the surface. The sex was evaluated.
The ridging resistance targeted by the present invention is A and B indices shown in Table 3.

[0103]

[Table 3]

Table 4 summarizes the results of various investigations.

[0105]

[Table 4]

From Table 4, it can be seen that the chemical composition is within the range specified in the present invention, and the ratio of the content of Mg to Al is 0.3 to 0.3.
Test Nos. 1 to 13 according to examples of the present invention in which composite inclusions (Al-Mg-Ti-based inclusions) of Al-Mg-based inclusions and Ti-based inclusions of 0.5 are dispersed in steel. In this case, it can be seen that the slab has a high equiaxed crystal ratio of 75% or more.
Moreover, the average particle size of the equiaxed crystals was as fine as 2.5 mm or less. Note that C in Test Nos. 2, 9, 12, and 13
It is also apparent that the addition of a has no particular adverse effect on the refinement, workability, and toughness of the solidified structure. FIG. 4 shows a solidified structure of a slab of steel 1 in Test No. 1 as an example of the present invention. The left-right direction in the figure is the thickness direction of the slab.

The impact transition temperature (vTs) of each of the test pieces Nos. 1 to 13 of the present invention obtained by hot rolling to a thickness of 4.5 mm and then annealed and pickled was 25 ° C. or less. No brittle cracking occurred in the production of ordinary surface grinding and cold rolling. Furthermore, the steel sheet annealed after being cold-rolled to a thickness of 1 mm had a high average elongation (EL) of more than 30%, and the index of ridging resistance was excellent in A or B.

On the other hand, the chemical composition of Ti, A
Test No. 14 of Comparative Example in which the content of one or more of 1, O (oxygen) and N is out of the range specified in the present invention.
In the case of No. 18 to 18, the Al-Mg-Ti composite inclusions were not dispersed, or even if they were dispersed, the ratio of the Mg to Al content of the Al-Mg-based inclusions was less than 0.3. For this reason, the equiaxed crystal ratio of the slab is as low as 5 to 15%, the average grain size of the equiaxed crystal is as coarse as 3.5 mm or more, and the elongation of the steel sheet annealed after hot rolling is as low as less than 30%. (Test number 16 ~
18) In addition, the index of ridging resistance is inferior to C or D (Test Nos. 14 to 18). In the case of Test No. 19, although the slab structure was fine and the index of ridging resistance was as good as B, the toughness and elongation were remarkable because the N content of the steel 19 exceeded the content specified in the present invention. Low.

Further, although the chemical composition of the steel is within the range specified in the present invention, when the slag composition in the refining before casting deviates from the conditions specified in the present invention in the test numbers 20 to 23, the specified Al- Mg-Ti based composite inclusions did not disperse in the steel. For this reason, the equiaxed crystal ratio of the slab is as low as 5 to 20%, the average grain size of the equiaxed crystal is as coarse as 3.5 mm or more, and the elongation of the steel sheet annealed after hot rolling is less than 30%, and The index of ridging resistance is inferior to D. FIG. 5 shows, as an example of the comparative example, a solidified structure of a cast piece of steel 20 in Test No. 20. The horizontal direction in the figure is the thickness direction of the slab.

The 4.5 mm thick annealed pickled steel sheet produced from the cast pieces according to Test Nos. 17 to 19 and 23 had an extremely high impact transition temperature (vTs) of 60 ° C. or higher and low toughness. During the operation, brittle cracks occurred violently, and the steel plate was broken.

[0111]

The ferritic stainless steel of the present invention
A portion exceeding 70% of the solidified structure becomes a fine equiaxed crystal having an average particle size of 3 mm or less, and is excellent in workability and toughness. For this reason, brittle fracture can be avoided in the manufacturing process of the steel material, and maintenance of the steel material is not required, so that the manufacturing process can be streamlined and the product yield can be improved. Therefore, if the steel of the present invention is used,
It is possible to provide a high-quality product with almost no occurrence of brittle cracking or ridging at a relatively low cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing an outline of an Al—Mg—Ti-based composite inclusion.

FIG. 2 Al-Mg present in a slab with 100% equiaxed crystal ratio
It is a figure which shows an example of the analysis result of the Ti type | system | group composite inclusion by EDX method.

FIG. 3 shows that in a ferritic stainless steel ingot in which Al-Mg-Ti-based composite inclusions are dispersed, fn1 (= Ti
(%) X N (%)) and M in Al-Mg based inclusions
It is a figure which shows the influence which the ratio of content of g and Al has on solidification structure.

FIG. 4 is a view showing a solidification structure of a slab of steel 1 in test number 1 of an example, and a left-right direction of the drawing is a thickness direction of the slab.

FIG. 5 is a diagram showing a solidification structure of a cast piece of steel 20 in Test No. 20 of the example, and the left-right direction of the drawing is the thickness direction of the cast piece.

Claims (3)

[Claims] (1) In mass%, C: 0.1% or less, N: 0.0
03-0.05%, Si: 0.03-1.5%, Mn:
1.0% or less, P: 0.04% or less, S: 0.03% or less, Cr: 10 to 30%, Cu: 2% or less, Ni: 2%
Hereinafter, Mo: 3% or less, V: 1% or less, Ti: 0.02
-0.5%, O (oxygen): 0.001-0.005%,
Nb: 0.8% or less, Al: 0.001 to 0.15%,
Zr: 0.3% or less, B: 0.1% or less, Ca: 0.0
Not more than 03% and Mg: less than 0.0005%, the value of fn1 represented by the following formula satisfies 0.0005 or more, and the balance is the chemical composition of Fe and unavoidable impurities. A ferritic stainless steel excellent in workability and toughness in which composite inclusions of Al- and Mg-containing inclusions and Ti-based inclusions having a content ratio of 0.3 to 0.5 are dispersed. fn1 = Ti (%) × N (%) 2. The chemical composition according to claim 1, wherein the equiaxed crystal ratio is more than 70% by volume and the average particle diameter of the equiaxed crystal is 3%.
mm or less ferritic stainless steel ingot. 3. The slag composition in mass%, Al ₂ O ₃ : 1-4.
0%, CaO: 30 to 70%, MgO: 1 to 30%, C
aF ₂ : 30% or less, SiO ₂ : 50% or less, remaining unavoidable impurities: 10% or less, and fn2 represented by the following formula
The value of: 1.0 to 3.0, and after refining the oxygen content in the molten steel to 0.001 to 0.005%, casting is performed, and the workability according to claim 1, A method for producing ferritic stainless steel with excellent toughness. fn2 = CaO (%) / {Al ₂ O ₃ (%) + SiO ₂ (%)}