JP3446667B2 - Ferritic stainless steel, ferritic stainless steel ingot excellent in workability and toughness, and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD 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 overhang forming, deep drawing, bending, etc., and by hot working such as hot rolling of steel ingots,
The present invention relates to a ferritic stainless steel and a steel ingot having excellent workability capable of being processed into a predetermined shape without causing surface flaws, cracks and fractures and excellent impact fracture resistance (toughness), and a method for producing the same.

[0002]

2. Description of the Related Art Ferritic stainless steel is
It has excellent weather resistance and heat resistance, so 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 work structure to make it finer, and therefore, various workability such as overhangability, deep drawability, bending workability, weldability Is inferior to ordinary steel. Further, since the toughness of ferritic stainless steel is generally low, brittle cracking may occur at the ingot stage or brittle cracking may occur during hot working. Therefore, there is also a problem that the yield of final products used in the various fields is significantly reduced.

In order to solve these problems, a technique for reducing impurity elements such as C, N, S and O (oxygen) in steel has been created, and high purity ferritic stainless steel can be melted. Came. At present, for example, even in the case of ferritic stainless steel containing 30 mass% of Cr by a so-called “AOD” or “VOD” decarburization 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.
Mass production of less than 002% is possible.

When the ferritic stainless steel is highly purified, the 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 higher purity of such ferritic stainless steel causes deterioration of workability called ridging property.

"Lidging" is a ridge-like wrinkle that appears on the surface of a steel sheet when a thin steel sheet of ferritic stainless steel is press-formed. This wrinkle spoils the aesthetics of the stainless steel moldings that are often used in pure form. Therefore, it is necessary to remove the ridging by polishing or the like, but the manufacturing cost of the molded product increases. Even if the application does not require aesthetics, it may crack along the "wrinkles" of ridging if the processing conditions are severe.

The occurrence of ridging in a thin plate of ferritic stainless steel 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 the final product is a steel sheet. It is due to remaining as "colony". A "colony" looks like a fine crystal structure, but the substance is that a group of crystals with similar crystal orientations are distributed in a region, and when subjected to press molding, it plastically deforms like a single crystal. However, this causes large ridge-shaped wrinkles on the surface of the steel sheet.

The basic measures for preventing the occurrence of ridging are:
There are two methods of refining the solidified structure and refining the structure by recrystallization.

Of these, in the latter method, it is necessary to perform strong reduction during hot working or to perform annealing at a low temperature and then perform annealing. However, ferritic stainless steel has good oxidation resistance and therefore has a thin scale, and therefore,
Surface defects are likely to occur due to seizure with tools such as rolling rolls. Also, since recrystallization due to hot is essentially unlikely to occur, it is difficult to make the structure uniform and fine.

On the other hand, examples of the former technique for refining the solidified structure include, for example, a method based on the nuclear action of TiN (iron and steel, 66th (1980) No. 6, page 110),
Electromagnetic induction stirring of molten steel (iron and steel, 66th year (1
980) No. 6, page 38).

However, the method of refining the structure of the steel ingot with TiN is, for example, 0.4% by mass of Ti or 0.0
It is necessary to add about 16% by mass of N to the steel to precipitate a large amount of TiN in the molten steel. In addition, unless conditions such as lowering the molten steel superheat degree ΔT to 40 ° C. or less are combined, a fine equiaxed crystal structure (specifically, the average grain size is 3
An equiaxed crystal structure of mm or less) cannot be obtained. Furthermore, a large amount of T
Since i and TiN impair the toughness of steel, the problem of brittle cracking of ferritic stainless steel becomes even greater.
In addition, it is not always easy to control the ΔT fluctuation range for each casting during operation, and since once ΔT becomes too small, casting cannot be performed, so that a reheating process is required. cause.

In the case of the method using electromagnetic induction stirring, even if ΔT is high, by optimizing the stirring position of the molten steel with respect to the steel ingot in the process of solidification, 40 to 60% by volume (hereinafter, the proportion of equiaxed crystal ( The equiaxed crystal ratio) will be 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]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above circumstances, and a cast steel ingot and hot working such as hot rolling, stretch forming, deep drawing, It is an object of the present invention to provide a ferritic stainless steel which is excellent in workability and toughness and can be processed into a predetermined shape without causing surface defects, cracks or breaks by bending and the like, and a method for producing the same. Specifically, more than 70% of the solidified structure becomes fine equiaxed crystals with an average grain size of 3 mm or less, which may cause defects such as hot working, overhang forming, deep drawing, and bending in the working process. It is an object of the present invention to provide a ferritic stainless steel, a ferritic stainless steel ingot having good workability and toughness that are unlikely to cause cracking or fracture, and a method for producing the same.

[0014]

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

(1) C: 0.1% or less by mass%, 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:
0.02-0.5%, O (oxygen): 0.001-0.0
05%, Nb: 0.8% or less, Al: 0.001 to 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 and more, the balance is Fe and the unavoidable impurities chemical composition, the ratio of the mass of Mg and Al (Mg / Al) in the steel is 0.3 to 0.5 and inclusions containing Mg and Mg. Ti
A ferritic stainless steel with excellent workability and toughness in which composite inclusions with system inclusions are dispersed.

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

(3) The slag composition is Al ₂ O ₃ :
1-40%, CaO: 30-70%, MgO: 1-30
%, CaF _2: 30% or less, SiO _2: 50% or less, the balance of inevitable impurities: 10% or less and fn2 value represented by the following formula: 1.0 to 3.0 and then, and the oxygen in the molten steel The method for producing a ferritic stainless steel having excellent workability and toughness according to the above (1), which comprises refining the content to 0.001 to 0.005% and then casting.

Fn2 = CaO (%) / {Al ₂ O ₃ (%) + SiO ₂ (%)} ... Here, the “inclusion” is a term indicating “foreign matter or impurity phase in solid”. Although it is appropriate to describe the compound as a “compound” before the solidification of the steel, in the present specification, it is also referred to as an “inclusion” even before the solidification of the steel.

The composite inclusion of an inclusion containing Al and Mg and a Ti-based inclusion means an inclusion having a structure in which a Ti-based inclusion surrounds the inclusion containing Al and Mg. Hereinafter, for simplicity, the above-mentioned composite inclusion of the inclusions containing Al and Mg 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.

The "steel ingot" is not only a so-called "ingot" in which steel refined in a steelmaking furnace is poured into a mold and solidified as described in JIS G 0203 as a steel term, but is also produced by continuous casting. It includes a cast slab.

Hereinafter, the above items (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 (internal structure, dispersed state) of non-metallic inclusions in the steel, and the structure of the steel ingot. As a result, the following findings were first obtained.

(A) In the ferritic stainless steel having the value of fn1 represented by the above formula of 0.0005 or more, the cast structure (solidification structure) of fine and high equiaxed crystal ratio is obtained irrespective of the molten steel superheat degree ΔT. You may be able to

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

(C) Even if the equiaxed crystal ratio is high, if the average grain size of the equiaxed crystal exceeds 3 mm, the toughness of the steel material will be low. Therefore, 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 largely exceeds room temperature, and brittle cracking is likely to occur.

(D) The average grain size of equiaxed crystals is 3 mm or less,
Moreover, when the equiaxed crystal ratio exceeds 70%, the workability and toughness are good.

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

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

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

That is, 200 continuously cast as a steel ingot
Select a slab with a thickness of mm x 1050 mm and collect a small test piece from a position 10 m from the tip of the slab and subcutaneously 20 m.
The surface of m is mirror-finished. The final polishing was carried out by using diamond abrasive grains (grain size: 0.25 μm) in alcohol in order to prevent the water-soluble inclusions from disappearing and the alumina abrasive grains as a polishing agent to remain. Next, the inclusions present on this observation surface were investigated using a high resolution Auger electron spectroscope, an energy dispersive X-ray spectroscope, or a normal EPMA. In the present specification, hereinafter, a high resolution Auger electron spectroscope, an energy dispersive X-ray spectroscope,
Ordinary surveys using EPMA are called Auger electron spectroscopy, EDX, and EPMA, respectively.

In the Auger electron spectroscopy, microstructure analysis of complex inclusions and analysis of light elements such as C and N were performed.
The EDX method, which is relatively simple in operation, analyzed the content of elements having an atomic weight of Mg or higher, and the EPMA method examined the distribution density of complex inclusions.

As a result, the following matters were clarified.

(G) The Ti-based inclusions present in the steel ingot having an equiaxed crystal with an average grain size of 3 mm or less are precipitated in combination with the Al-Mg-based inclusions. The Al-Mg-based inclusions that exist as nuclei for the formation of Ti-based inclusions have a size of 0.1 to 1 μm, while the Ti-based inclusions have a size of 0.3 to 5 μm.

(H) Al as a nucleus for forming Ti-based inclusions
-Mg-based inclusions include Al, Mg, O (oxygen), Ca,
Si, Mn, S, etc. are contained. This Al-
The Mg-based inclusions always contained Al, Mg, and O, and other elements were sometimes below the detection limit. Also, Al
-The Ti-based inclusions deposited so as to cover the Mg-based inclusions are
The main components were Ti and N, and additionally contained O (oxygen), S, and C, but elements other than Ti and N were sometimes below the detection limit.

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

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

(I) In a steel ingot in which more than 70% of the solidified structure exhibits fine equiaxed crystals with 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 mass ratio (Mg / Al) of Mg and Al in the Al-Mg-based inclusion is in the range of 0.3 to 0.5.

(J) Ti-based inclusions are also found in the columnar crystal parts of the steel ingot having a low equiaxed crystal ratio, but the nuclei thereof are A as described above.
Not an l-Mg-based inclusion but a single oxide (for example, Si
O ₂ or CaO) or the mass ratio of Mg and Al (M
Al-rich oxide ( g-Al) of less than 0.3 (Al-
Mg-based inclusions).

In a ferritic stainless steel ingot in which Al-Mg-Ti inclusions are dispersed, the value of fn1 represented by the above formula and M in the Al-Mg inclusions are shown in FIG.
The effect of the ratio of the mass of g and Al (Mg / Al) on the solidification structure is summarized and shown.

Therefore, further, the manufacturing conditions for dispersing the Al—Mg—Ti composite inclusions effective for forming a fine solidified structure as shown in FIG. 3 were investigated. As a result, the following matters were found.

(K) Mg and Al in Al-Mg inclusions
In order to make the mass ratio (Mg / Al) of 0.3 to 0.5, the slag composition is Al ₂ O ₃ : 1 to 40 by mass%.
%, CaO: 30 to 70%, MgO: 1 to 30%, Ca
F _2: 30% or less, SiO _2: 50% or less, the balance of inevitable impurities: 10% or less and fn2 value represented by the formula: 1.0 to 3.0 and then, and the oxygen content in the molten steel May be smelted to 0.005% or less and then cast.

(L) Even if the condition (j) above is satisfied, if the oxygen content in the molten steel is less than 0.0010%, A
Although the 1-Mg-based inclusions are formed, the solidified structure may not necessarily be refined.

(M) It is not necessary to intentionally add Al and Mg in order to disperse the Al-Mg-Ti-based composite inclusions. This is because Al and Mg are mixed in from the refining slag or are eluted from refractory materials such as ladle and tundish when refining 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 the immersion nozzle from being blocked. In this case, Al-M
The g-based inclusions partly change their morphology to Al-Mg-Ca-based inclusions, but the mass ratio of Mg and Al in the inclusions (Mg
/ Al) is in the range of 0.3 to 0.5, the equiaxed crystal ratio is 7
Steel ingots having a fine equiaxed crystal structure exceeding 0% can be produced.

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

[0046]

BEST MODE FOR CARRYING OUT THE INVENTION Each requirement of the present invention will be described in detail below. In addition, "%" of the content of a chemical component means "mass%".

(A) Chemical composition C of steel: 0.1% or less Since C has a small solid solution limit in ferritic stainless steel, it may be combined with Cr depending on the thermal history in the process such as annealing or welding, so that the grain boundaries may be increased. Cr carbides are formed in the aluminum and cause intergranular 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 0.02%.

N: 0.003 to 0.05% N is a Ti-based inclusion having heterogeneous nucleation with Al-Mg-based inclusion as a nucleus, that is, Al-Mg-based inclusion is a Ti-based inclusion (main component). 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 structure with a fine and high equiaxed crystal ratio to improve 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, if the content exceeds 0.05%, the toughness is remarkably lowered. Therefore, the content of N is 0.003 to 0.0.
It was set to 5%. The preferable N content for ensuring good toughness is 0.015% or less.

Si: 0.03 to 1.5% Si is an element useful for reduction and deoxidation of Cr oxide produced during refining. However, if the content is less than 0.03%, the effect of addition is poor. On the other hand, if it exceeds 1.5%, the workability of steel materials such as steel plates deteriorates. Therefore, S
The content of i was 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
It is preferable that the content be at least%. However, if its content exceeds 1.0%, the precipitation amount of MnS increases, the pitting corrosion resistance deteriorates, and the component cost also increases, which is economically disadvantageous. Therefore, the Mn content is set to 1.0% or less.

P: 0.04% or less P deteriorates the toughness, hot workability and corrosion resistance of the steel, so the lower the content, the better. In particular, if it exceeds 0.04%, the toughness and hot workability of the steel are increased. The workability and corrosion resistance are significantly deteriorated. Therefore, the content of P is set to 0.04% or less.

S: 0.03% or less S deteriorates the toughness, hot workability and corrosion resistance of the steel, so the lower the content, the better. In particular, if it exceeds 0.03%, the toughness and hot workability of the steel are increased. The workability and corrosion resistance are significantly deteriorated. 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, if the content exceeds 30%, so-called “475 ° C. brittleness” tends to occur, resulting in a significant decrease in toughness. Therefore, the Cr content is set to 10 to 30%.

Cu: 2% or less Cu may not be added. If added, it has the effect of increasing the corrosion resistance and weather resistance of the steel. In order to reliably obtain this effect, the content of Cu is preferably 0.1% or more. However, if the content exceeds 2%, the steel becomes hard and the ductility is impaired. Therefore, the content of Cu is set to 2% or less.

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

Mo: 3% or less Mo may not be added. If added, it has the effect of enhancing corrosion resistance and weather resistance. In order to surely obtain such effects, the Mo content is preferably 0.1% or more. However, if its content exceeds 3%, precipitation of intermetallic compounds is promoted, resulting in deterioration of toughness. 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 is 0.0
The content is preferably 5% or more. However, if the content exceeds 1%, the toughness decreases. Therefore,
The V content is set to 1% or less.

Ti: 0.02 to 0.5% Ti forms Al-Mg-Ti-based composite inclusions,
It is an essential element for improving the workability by making the solidified structure of the cast ferritic stainless steel fine and having a high equiaxed crystal structure. Furthermore, Ti combines with N and C in steel to form carbides, nitrides and carbonitrides, and C and N which form a solid solution in the matrix.
To reduce workability and improve workability, corrosion resistance and toughness of ferritic stainless steel. However, if the content is less than 0.02%, it is not possible to obtain a desired casting structure that is fine and has a high equiaxed crystal ratio exceeding 70% when cast. On the other hand, if it is contained in excess of 0.5%, it is effective in improving the equiaxed crystal ratio of the steel ingot, but the oxides and nitrides of Ti agglomerate and become the starting point of flaws, which may lead to the failure of products such as steel plate and steel pipe. The surface quality will be impaired. Therefore, the content of Ti is set to 0.02 to 0.5%. 70% fine when cast
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 secure good surface properties of products such as steel plates and steel pipes, the upper limit of the Ti content is preferably 0.3%.

O (oxygen): 0.001 to 0.005% O (oxygen) forms Al-Mg-Ti-based composite inclusions, and the solidification structure of the cast ferritic stainless steel is fine and high. It has an axial crystal structure and is an essential element for improving workability. However, if the content is less than 0.001%, the timing of precipitation of Al-Mg-based inclusions is not appropriate,
Since the cast 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 it exceeds 0.005%, not only the toughness and workability are deteriorated but also surface defects frequently occur. Therefore, the O content is 0.001 to 0.
It was set to 005%.

Nb: 0.8% or less Nb need not be added. If added, it has the effect of enhancing workability and corrosion resistance. In order to reliably obtain such effects, it is preferable that the content of Nb is 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 type composite inclusions,
It is an essential element for improving the workability by making the solidified structure of the cast ferritic stainless steel into a fine and high equiaxed crystal structure. However, if the content is less than 0.001%, the Al-Mg-based inclusions do not precipitate in the molten steel, so the desired Al-Mg-Ti-based composite inclusions are not formed,
The structure of the steel ingot becomes a coarse columnar crystal structure. When processing various steel materials from the above steel ingot, the workability and ridging resistance are extremely poor. On the other hand, when the Al content exceeds 0.15% and the deoxidizing action of Al is large, the oxygen content during refining is likely to be less than 0.001%. In this case, the timing of precipitation of Al-Mg-based inclusions is not appropriate, and the structure of the steel ingot becomes a coarse columnar crystal structure. Therefore, the Al content is 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 enhancing workability and oxidation resistance. In order to surely obtain such effects, Zr is preferably contained in an amount of 0.05% or more. However, if the content exceeds 0.3%, the toughness decreases. Therefore, the Zr content is set to 0.3% or less.

B: 0.1% or less B may not be added. If added, it has the effect of enhancing the workability. To ensure this effect, B is 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. In order to reliably obtain this effect, it is preferable that the content of Ca be 0.0001% or more. However, if 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 secure good toughness and corrosion resistance in ferritic stainless steel, Ca
The upper limit of the content is preferably 0.001%.

Mg: less than 0.0005% Mg may not be added. If added, Al-Mg-
It has the effect of improving the workability by making the solidified structure of the cast ferritic stainless steel into the Ti-based composite inclusions into a fine and high equiaxed crystal structure. The content of Mg is an amount mixed as an impurity (for example, 0.0001% because the analysis accuracy of the current Mg is about 0.0001%.
% Amount). Of course, Mg may be positively added. However, when the content is 0.
If it exceeds 0005%, the toughness of the ferritic stainless steel is significantly reduced. Therefore, the content of Mg is set to less than 0.0005%.

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

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

In the stainless steel of the present invention, Mg and Al
Al-Mg having a mass ratio (Mg / Al) of 0.3 to 0.5
The Al-Mg-Ti-based composite inclusions in which the systematic inclusions are covered with the Ti-based inclusions are indispensable inclusions for fine equiaxed crystallization of the steel ingot.

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

However, if the Al--Mg inclusions of Mg and Al
The mass ratio (Mg / Al) should be in the range 0.3-0.5. The mass ratio (Mg / Al) is 0.3
When it is less than the above, the structure of the steel ingot becomes a coarse columnar crystal structure or an equiaxed crystal structure having an average grain size of more than 3 mm. On the other hand, when the mass ratio (Mg / Al) exceeds 0.5,
The solidified structure of the steel ingot becomes a fine equiaxed crystal or a coarse columnar crystal, and a desired structure cannot be stably and reliably obtained. Therefore, the mass ratio (Mg / Al) of Mg and Al of the Al-Mg-based inclusion is set to 0.3 to 0.5.

In the case of ferritic stainless steel, Ta, which easily forms oxides and nitrides, and rare earth elements such as La and Ce may be added for the purpose of enhancing characteristics such as oxidation resistance. When the above elements are added to steel, A
These elements are often found in the 1-Mg-Ti-based composite inclusions, but in this case as well, the above Al-Mg-T is included.
The action of i-type complex inclusions is not hindered.

In order to obtain a steel ingot having a high equiaxed crystal ratio of more than 70%, an Al-Mg-based interposition in which the mass ratio of Mg to Al (Mg / Al) is 0.3 to 0.5 in the steel. It is preferable that the distribution density of the inclusions (Al-Mg-Ti-based composite inclusions) in which the inclusions and the Ti-based inclusions are composite is 10 pieces / mm 2 or more. Since the structure of the steel ingot becomes finer as the amount of the Al-Mg-Ti-based complex inclusions increases, the distribution density of the inclusions has no upper limit. In addition, A measured in this distribution density
The size (major axis) of the 1-Mg-Ti-based composite inclusion is preferably 0.3 μm or more in terms of measurement accuracy.

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

Al-Mg inclusions having a mass ratio of Mg and Al (Mg / Al) of 0.3 to 0.5 are Al ₂ O ₃
From the binary phase diagram of MgO, it is inferred that it is a MgOAl ₂ O ₃ spinel inclusion. This spinel inclusion has a cubic crystal structure and has a good lattice matching with the Ti inclusion (TiN). Therefore, it is presumed that TiN is easily precipitated in molten steel with spinel inclusions as nuclei. As a result, Al-Mg-Ti-based composite inclusions are generated in the molten steel.

On the other hand, Mg and Al in the Al--Mg system inclusions
When the mass ratio (Mg / Al) is less than 0.3, corundum crystallizes out. This inclusion has a hexagonal crystal structure and has a poor lattice matching with TiN. Therefore, TiN
Is hard to precipitate in molten steel.

The refinement of the solidification structure of ferritic stainless steel is performed by Al-Mg-based inclusions covered with Ti-based inclusions.
It is presumed that the 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 1-Mg-based inclusion, that is, the crystal structure. Therefore, the quality of Mg and Al of the Al-Mg system inclusions
When the ratio (Mg / Al) of the amounts is controlled to 0.3 to 0.5, the Al-Mg-Ti-based composite inclusions are most likely to be generated in the molten steel, and it is considered that the solidification structure becomes finer.

The invention of (1) can be obtained by satisfying the chemical composition described in the item (A) and the inclusions described in the item (B).

(C) Equiaxed crystal of the steel ingot The average grain size of the equiaxed crystal is 3 mm or less, and the equiaxed crystal ratio is 7
When it exceeds 0%, workability and toughness are improved. Therefore, in the invention of (2), the equiaxed crystal ratio of the solidified structure of the steel ingot exceeds 70%, and the average grain size of the equiaxed crystal is 3%.
It was defined as mm or less.

(D) Oxygen content in molten steel and slag composition In order to obtain a ferritic stainless steel excellent in workability and toughness, casting is performed after refining the slag composition and oxygen content in molten steel to appropriate levels. 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 maintains Al equilibrium between slag and molten steel during refining, and Al-Mg-Ti-based inclusions during casting. To generate
Equiaxed grain refinement of the ingot structure. 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 deteriorates significantly. Therefore, the amount of Al ₂ O _{3 in} the slag should be 1 to 40
%.

CaO: 30 to 70% CaO is effective for deoxidizing and desulfurizing 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 is set to 30 to 70%.

MgO: 1 to 30% MgO is a source of Mg elution into molten steel, and is an Al source during casting.
-Effective for producing Mg-Ti based inclusions. However, if the amount in the 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%. The lower limit of the amount of MgO in the slag is preferably 3% in order to promote the refinement of the solidified structure.

CaF ₂ : 30% or less CaF ₂ may not be contained in the slag. If it is contained in the slag, it has the effect of increasing the deoxidation and desulfurization efficiency of molten steel. To ensure this effect, Ca in the slag
The amount of F ₂ is preferably 3% or more. On the other hand, if the amount in the slag exceeds 30%, the damage to the refractory in the ladle becomes large and the manufacturing cost increases. Therefore, the amount of CaF _{2 in} the slag is 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 a deoxidizing effect on molten steel. In order to reliably obtain 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 rather,
The desired steel ingot structure cannot be obtained and the toughness deteriorates. Therefore, the amount of SiO _{2 in} the slag is set to 50% or less.

Remaining inevitable impurities: 10% or less Inevitable impurities in the slag other than the above are Fe, Cr, and M.
It is composed of oxides of n and Ti, S and the like. It is preferable that these impurities are not included as much as possible because they reduce the deoxidizing and desulfurizing efficiency of the molten steel. The amount of the impurities in the slag is 10
If the content exceeds%, the Al-Mg-Ti-based composite inclusions do not disperse, so the structure of the steel ingot does not become fine and the toughness also deteriorates. Therefore, the amount of unavoidable impurities in the slag should be 10%.
Below. In order to further promote the refinement of the structure of the steel ingot, the amount of unavoidable 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 that controls the deoxidizing and desulfurizing ability of molten steel by slag. If the value of fn2 is less than 1.0, the above effect is not sufficiently obtained, while if it exceeds 3.0, the damage to the refractory in the ladle becomes large,
Manufacturing cost increases. Therefore, the value of fn2 is 1.
It was set to 0 to 3.0. In order to secure good corrosion resistance and toughness of ferritic stainless steel, the value of fn2 is 1.
It is preferably from 2 to 2.4.

(D-2) Oxygen content in molten steel Oxygen in molten steel forms Al-Mg-Ti-based composite inclusions to form a fine and highly equiaxed crystal in the solidified structure of the cast ferritic stainless steel. It has the effect of increasing the workability by forming a fine structure. However, if the content is less than 0.001%, the timing of precipitation of Al-Mg-based inclusions is not appropriate, and the structure of the steel ingot 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 it exceeds 0.005%, not only the toughness and workability are deteriorated but also surface defects frequently occur. Therefore, the oxygen content in the molten steel is set to 0.001 to 0.005%, which is the same as the oxygen content in the steel after casting.

In order to make the solidification structure finer, it is preferable to carry out continuous casting after refining the slag composition and the oxygen content in the molten steel as described above.

A for making the ingot structure into fine equiaxed crystal
Although it is not necessary to prescribe conditions such as the addition timing of 1, 1, Ti, etc., the molten steel superheat degree ΔT, and the molten steel stirring, in order to promote the formation of Al-Mg-based inclusions, refractory containing MgO It is preferable to use a tuna dish.

The present invention will be described below 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 N was performed, and then refining for reducing oxidized Cr was performed. After that, Al, Si, and Ti are added, and Ca is partially added.
After adding, continuous casting was performed. Table 2 shows the slag composition during refining before casting, the oxygen content in the molten steel, and the details of whether or not Ca is added.

Test numbers 1 to 1 in Tables 1 and 2
No. 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 by 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 the chemical composition, the slag composition before casting, and the oxygen content in the molten steel deviates from the conditions specified in the present invention.

[0093]

[Table 1]

[0094]

[Table 2]

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

Each of the above-mentioned slabs is made to be 1100-110 by a usual method.
It was heated to 1250 ° C. and hot-rolled to finish a steel plate having a thickness of 4.5 mm.

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

A test piece cut out from the steel plate having a thickness of 4.5 mm was polished with diamond abrasive grains in alcohol, and a portion corresponding to ¼ of the thickness of the steel plate cross section was observed with a scanning electron microscope. , The inclusion having a composite structure is analyzed by the EDX method to confirm the composition, and the mass ratio of Mg to Al in the Al-Mg-based inclusion (Mg / Al) , Al-Mg-T
The amount of i-type composite inclusions distributed in the steel was investigated. Al-
Mass ratio of Mg and Al in Mg-based inclusions (Mg / Al)
Was calculated as an average value for 10 or more inclusions randomly selected.

The steel sheet hot-rolled to a thickness of 4.5 mm was annealed and pickled by a usual method to obtain an annealed pickled steel sheet. In order to investigate the impact properties from this annealed pickled steel sheet, a sub-size Charpy test piece with a V notch having a depth of 2 mm was taken perpendicularly to the rolling direction and subjected to an impact test to determine the ductility-brittleness transition temperature (vTs). It was measured.

The annealed pickled steel sheet having a thickness of 4.5 mm obtained as described above was polished on its front and back surfaces to adjust the surface roughness, and then cold rolled to a thickness of 1 mm. Next, the cold rolled steel sheet was subjected to soaking annealing in a combustion gas at a temperature of 830 to 1030 ° C. for 20 to 30 seconds. The rate of temperature increase and the rate of temperature decrease during annealing were both in the range of 10 to 50 ° C / sec. In addition, in the 4.5 mm-thick annealed and pickled steel sheets manufactured from the cast pieces according to Test Nos. 17 to 19 and 23, brittle cracks were intensely generated during the surface grinding work as described later, and the steel sheets were 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 No. 13B tensile test piece specified in JIS Z 2201 was sampled, and a tensile test was performed at a 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 three directions according to the following formula.

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

[0103]

[Table 3]

The results of various investigations are summarized in Table 4.

[0105]

[Table 4]

From Table 4, the chemical composition is within the range specified by the present invention, and the mass ratio of Mg and Al (Mg / Al).
A test according to an example of the present invention in which composite inclusions of Al-Mg-based inclusions and Ti-based inclusions (Al-Mg-Ti-based inclusions) of 0.3 to 0.5 are dispersed in steel. In the case of Nos. 1 to 13, it can be seen that the slab has a high equiaxed crystal ratio of 75% or more. Moreover, the average grain size of the equiaxed crystals was fine, 2.5 mm or less. In addition, test numbers 2, 9, 12, 13
It is also clear that the addition of Ca does not particularly affect the refinement of the solidified structure, workability, and toughness. FIG. 4 shows, as an example of the present invention, the solidification structure of the slab of Steel 1 in Test No. 1. The horizontal direction of the figure is the thickness direction of the cast piece.

Further, the impact transition temperature (vTs) of the steel sheets obtained by subjecting the cast pieces of test Nos. 1 to 13 according to the present invention to hot rolling to a thickness of 4.5 mm and then annealing and pickling them is 25 ° C. or less. Therefore, brittle cracks did not occur in ordinary surface grinding and cold rolling production. Further, the steel sheet annealed after cold rolling 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 any one or more of 1, O (oxygen) and N is out of the range specified in the present invention
In the case of -18, the Al-Mg-Ti composite inclusion is not dispersed, or even if dispersed, the mass ratio of Mg to Al of the Al-Mg-based inclusion (Mg / Al) is less than 0.3. It was Therefore, the equiaxed crystal ratio of the slab is as low as 5 to 15%, the average grain size of the equiaxed crystal is as large as 3.5 mm or more, and the elongation of the steel sheet annealed after hot rolling is lower than 30%. (Test numbers 16 to 18), and the index of ridging resistance is inferior to C or D (test numbers 14 to 18). In the case of test number 19, although the slab structure is fine and the index of ridging resistance is good as B, the N content of steel 19 exceeds the content specified in the present invention, so that the toughness and elongation are remarkable. Low.

Although the chemical composition of steel is within the range specified by the present invention, when the slag composition in the refining before casting is out of the conditions specified by the present invention in the test numbers 20 to 23, the specified Al- The Mg-Ti based composite inclusion did not disperse in the steel. Therefore, the equiaxed crystal ratio of the slab is as low as 5 to 20%, the average grain size of the equiaxed crystal is as large 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 a comparative example, the solidification structure of the cast piece of the steel 20 in Test No. 20. The horizontal direction of the figure is the thickness direction of the slab.

The 4.5 mm thick annealed and pickled steel sheets produced from the cast pieces according to Test Nos. 17 to 19 and 23 had a very high impact transition temperature (vTs) of 60 ° C. or higher and a low toughness. During the work, brittle cracks were severely generated and the steel plate broke.

[0111]

The ferritic stainless steel of the present invention is
More than 70% of the solidified structure becomes fine equiaxed crystals with an average grain size of 3 mm or less, which is excellent in workability and toughness. For this reason, brittle fracture in the steel manufacturing process can be avoided, and maintenance of the steel becomes unnecessary, so that the manufacturing process can be rationalized and the product yield can be improved. Therefore, using the steel of the present invention,
It is possible to provide a high-quality product with almost no brittle cracking or ridging at a relatively low cost.

[Brief description of drawings]

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

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

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

FIG. 4 is a diagram showing a solidification structure of a cast slab of Steel 1 in Test No. 1 of the example, and the left-right direction of the figure is the 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 horizontal direction of the figure is the thickness direction of the cast piece.

Claims (3)

(57) [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: 10-30%, Cu: 2% or less, Ni: 2%
Below, Mo: 3% or less, V: 1% or less, Ti: 0.02
~ 0.5%, O (oxygen): 0.001 to 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
Containing less 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 a chemical composition of Fe and unavoidable impurities. The mass ratio (Mg / Al) of 0.3-0.
A ferritic stainless steel having excellent workability and toughness in which composite inclusions of inclusions containing Al and Mg of No. 5 and Ti inclusions are dispersed. fn1 = Ti (%) × N (%) ... 2. The chemical composition according to claim 1, wherein the equiaxed crystal ratio exceeds 70% by volume, and the equiaxed crystal has an average particle size of 3.
A ferritic stainless steel ingot of mm or less. 3. A slag composition of Al ₂ O ₃ : 1 to 4 in mass%.
0%, CaO: 30-70%, MgO: 1-30%, C
aF ₂ : 30% or less, SiO ₂ : 50% or less, balance unavoidable impurities: 10% or less, and fn2 represented by the following formula:
Value: 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 having excellent toughness. fn2 = CaO (%) / {Al ₂ O ₃ (%) + SiO ₂
(%)} ...