JP2002011551A - Austenitic stainless steel thin cast slab excellent in surface quality and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic stainless steel thin cast slab restraining the development of roping and having excellent surface quality, and its producing method. SOLUTION: As for the austenitic stainless steel thin sheet, γcrystal grain diameter in the thin cast slab: the grain diameter conversed into the sphere Dave[=Σ2×SQR (3×Si/2π) ×Si/S, wherein, S: the measured total area of grain diameter, Si: grain area in NO.i] is Dave<70 μm and Dave<150-32..log (v) [v: solidified cooling velocity ( deg.C/sec)]. The molten austenitic stainless steel poured into a pool part formed with one pair of cooling rolls, whose shafts are parallel rotated mutually in the reversed direction and side weirs, contains little quantities of Mg and Ti and continuously cast at >=100 deg.C/sec solidifying/ cooling speed, and a rolling reduction is applied to as <=25% draft to the thin cast slab in the in-line and successively, and also a heat treatment is applied to at 1100 deg.C for <7 sec equivalent heat treatment time to produce the austenitic stainless steel thin sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an austenitic stainless steel thin slab having excellent surface quality and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, a twin-drum continuous casting method, that is,
The molten steel in the pool is formed by a pair of cooling drums and side dams whose axes are parallel and rotate in opposite directions, and the molten steel is cooled and solidified on the peripheral surface of the cooling drum to form a solidified shell. Austenitic stainless steel sheets have been manufactured using a continuous casting method.

[0003] According to the thin cast continuous casting method, a thin cast close to the final shape can be directly produced, so that conventional intermediate steps such as hot rolling, heat treatment, and cold rolling can be omitted, or Although there is a great advantage in terms of production efficiency, it can be reduced, but the structure, surface condition, etc. of the thin slab obtained by continuous casting greatly affect the material and surface properties of the thin plate as a product, Research in this area is essential.

[0004] In particular, in the case of austenitic stainless steel used as a surface material, the smoothness and gloss of the surface are problematic, but the current thin slab continuous casting method has solved this problem. Not in. Further, in austenitic stainless steel, surface defects and gloss unevenness called roping are likely to occur after cold rolling. Therefore, even if the structure of thin slab is cold rolled, surface defects and uneven gloss occur. The casting structure must be difficult.

[0005] Incidentally, in austenitic stainless steels, it has been clarified that when the γ grains in the steel material before cold rolling are large, roping occurs remarkably in the product thin steel sheet. Based on this elucidation, the present applicant disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-182354 that a grain refinement element was added alone or in a total amount of 0.01 to 1 mol% to molten steel of austenitic stainless steel. Cooling the temperature range from the start of solidification of molten steel to 1200 ° C at a cooling rate of 100 ° C / sec or more to reduce the average grain size of γ grains to 50 µm or less and improve the surface quality of cold rolled products. A method of manufacturing an austenitic stainless steel thin cast slab characterized by the following was proposed.

However, this manufacturing method is difficult to control the cooling rate in a temperature range from the start of solidification of molten steel to 1200 ° C., and is not practical. Then, Japanese Patent Application Laid-Open No. 3-71902
JP-A-8-277423 and Japanese Patent Application Laid-Open No. 8-277423 have proposed slab rolling conditions (relation between rolling reduction and temperature) for suppressing the occurrence of roping, and hot rolling and cooling conditions after casting. Controlling various conditions in the process after casting is not preferable because it causes an increase in cost.

[0007] The present applicant has conducted research from the viewpoint that an austenitic stainless steel having excellent surface properties can be obtained without special control of the process conditions after casting. No.-11193, a method was proposed in which a predetermined amount of Ti and / or Mg was added to molten steel of austenitic stainless steel to control the form and amount of inclusions, thereby improving the surface properties (irregularities). .

[0008]

SUMMARY OF THE INVENTION The present invention provides an austenitic stainless steel thin slab manufactured by twin-drum continuous casting, in which the γ crystal grain size is controlled within a predetermined range.
It is an object to improve the surface quality of a product thin plate.

[0009]

SUMMARY OF THE INVENTION The present applicant has proposed that, when a predetermined amount of Ti and Mg is added to molten steel of austenitic stainless steel, the form and amount of inclusions formed in the molten steel change with time, The inventors obtained the knowledge that this change contributed to the improvement of the surface properties, and came to apply for Japanese Patent Application No. 11-11193.
Further investigations and studies were conducted to elucidate the metallurgical reasons for the addition of Mg to improve the surface properties (irregularities), and as a result, the following findings (a) to (d) were obtained.

(A) A small amount of Ti and Mg are added to molten steel,
O · Al ₂ O ₃ based fine Mg- (Al, Ti) -O- TiN type inclusions TiN is formed crystallize out on the inclusions have a high lattice matching with the bcc iron, molten steel In the above, it acts as an inoculation nucleus for solidification nucleation of primary crystal δ ferrite. (B) γ crystal particle diameter defined by the following formula: spherical equivalent particle diameter Dav
When e is less than 70 μm, no roping occurs in the cold rolled sheet product.

[0011]

(Equation 2)

(C) The solidification structure can be refined by controlling the solidification cooling rate v. (D) In order to suppress the occurrence of roping in a cold rolled sheet product, the above-mentioned γ crystal grain size Dave of the thin slab and the solidification cooling rate v must satisfy a required relationship. The present invention is based on the above findings (a) to (d), and the gist thereof is as follows. (1) Molten steel of austenitic stainless steel injected into a pool formed by a pair of cooling drums and side dams whose axes are parallel and rotate in opposite directions contains a small amount of Ti and Mg, and has a solidification cooling rate v : Austenitic stainless steel sheet continuously cast at 100 ° C./s or more, and the γ crystal grain diameter of the thin slab defined by the following equation: sphere-converted particle diameter Dave is Dave <70 μm and Dave <150− An austenitic stainless steel thin slab having excellent surface quality, characterized by satisfying 32 · log (v) [v: solidification cooling rate (° C./sec)].

[0013]

(Equation 3)

(2) Molten steel of austenitic stainless steel injected into a pool formed by a pair of cooling drums and side dams whose axes are parallel and rotate in opposite directions is Ti and Mg.
, A thin slab is continuously cast at a solidification cooling rate v: 100 ° C./sec or more, and the thin slab is subjected to in-line rolling at a rolling reduction of 25% or less, and then to an equivalent heat treatment at 1100 ° C. The method for producing an austenitic stainless steel thin slab having excellent surface quality according to the above (1), wherein the heat treatment is performed for a time of 7 seconds or less. (3) The method for producing an austenitic stainless steel thin cast piece having excellent surface quality according to the above (2), wherein Ti is added in an amount of 0.015 to 0.08% by mass. (4) The method for producing an austenitic stainless steel thin slab having excellent surface quality according to the above (2) or (3), wherein 0.0005 to 0.01% by mass of Mg is added.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the manufacturing method of the present invention will be described. In the production method of the present invention, a small amount of Ti and Mg are added to molten steel of austenitic stainless steel injected into a pool formed by a pair of cooling drums and side dams whose axes are parallel and rotate in opposite directions. The method is characterized in that inclusions which effectively act as inoculation nuclei for solidifying nuclei of primary crystal δ ferrite are formed in the molten steel.

When Ti is added to the above molten steel, Ti ₂ O ₃ is formed, and when Mg is added, it is replaced with Mg-based oxide, and fine MgO.Al ₂ O ₃ -based or MgO. (Al ₂ O ₃ , Ti ₂ O ₃ ) -based inclusions are formed. Then, with the temperature drop of the molten steel, TiN crystallized on the MgO.Al ₂ O ₃ -based or MgO. (Al ₂ O ₃ , Ti ₂ O ₃ ) -based inclusions, and fine Mg- (Al, Ti ) -O-TiN-based inclusions are formed. This inclusion has a high degree of lattice matching (property) with bcc iron, and thus acts as a solidification nucleation inoculation nucleus for primary δ ferrite in the process of solidifying molten steel.

It is preferable to add 0.015 to 0.08% by mass of Ti. If the added amount of Ti is less than 0.015%, Ti ₂ O ₃ or TiN in an amount sufficient to form desired inclusions cannot be secured, while Ti exceeds 0.08%. If a large amount of Ti is added, the cost increases, and there are problems such as surface flaws occurring during cold rolling.
15-0.08% is preferable.

Mg is preferably added in an amount of 0.0005 to 0.01% by mass. If the added amount of Mg is less than 0.0005%, it is not possible to secure a sufficient amount of the Mg-based oxide acting as the crystallization nucleus of TiN, while increasing the amount of Mg to more than 0.01%. Even if it is added, the effect of addition is saturated. Therefore, the addition amount of Mg is preferably 0.0005 to 0.01%. In continuous casting, Mg- (Al, Ti) -O
-While the TiN-based inclusions do not agglomerate,
It is performed at a temperature of at least 00 ° C./sec. Under this solidification cooling rate,
A fine solidified structure can be obtained. When the solidification cooling rate v is less than 100 ° C./sec, the γ crystal grain size of the thin slab becomes coarse, and even if hot rolling and heat treatment are performed in-line after casting, the γ crystal grain size defined by the following formula: sphere Converted particle size Dave is 70
Not less than μm.

[0019]

(Equation 4)

The above Dave is Dn defined as an expected value of the number frequency of crystal grains conventionally used for convenience.
It is an index replacing [= （2 × SQR (3 × Si / 2π) / n], and is a useful index as an index indicating the actual state of the mixed grain structure (see Japanese Patent Application Laid-Open No. 8-281385). In the present invention, in order to suppress the occurrence of roping depending on the crystal grain size, the above Dave is adopted as an index suitable for the actual structure.

The solidification cooling rate v (° C./sec) needs to be selected within a range satisfying Dave <150−32 · log (v) in relation to the γ crystal grain size Dave (<70 μm). . In the present invention, as described above, the γ crystal grain size Dave
(Μm) and the solidification cooling rate v (° C./sec) are established, and the knowledge that the solidification cooling rate v can be controlled to make the solidified structure of thin cast pieces finer. (D) was obtained, but the following test was performed to clarify the relationship. ( Test ) Molten steel of austenitic stainless steel to which predetermined amounts of Ti and Mg were added and the molten steel to which Ti and Mg were not added were used.
Continuous casting to 5mm thin slab, 1200 ° C x 1
Heat treatment and then descaling by shot and pickling. Solidification cooling rate v and γ grain size Dav
In order to grasp the relationship with e, TIG bead-on-plate welding was performed on the thin slab. Welding speed 1m / min,
The arc length was 1 mm, the shield gas was Ar, and the amount of heat input was changed to change the solidification cooling rate v.

The solidification cooling rate is estimated by heat transfer calculation, and the cooling solidification rate at 1423 ° C. was changed from 262 to 868 K / sec based on the estimation. On the other hand, the γ crystal grain size Dave was determined by image analysis of a 200 × photomicrograph. At this time, a heat treatment was performed at 1200 ° C. for 1 minute in advance for the purpose of diffusing δ ferrite to reveal crystal grain boundaries. FIG. 1 shows the relationship between the solidification cooling rate v and the γ crystal grain size Dave. Generally, as the solidification cooling rate v increases,
It can be seen that the γ crystal grain size Dave becomes smaller. Also, T
When i and Mg are not added (open circles and black circles in the figure), Dave is 1
Although it is coarse at 40 μm or more, it can be seen that Dave is less than 70 μm when Ti and Mg are added (open triangles and open squares in the figure).

In the case of adding Ti and Mg, the relationship between the solidification cooling rate v and the γ crystal grain size Dave is approximated by the following equation. Considering the lower limit of Dave = 130−32 · log (v) Ti (0.015%), Dave = 150−32 · log (v), and the relationship of Dave <150−32 · log (v) is satisfied. When it was satisfied, it became clear that the occurrence of roping was suppressed in the cold rolled sheet product.

As described above, in the production method of the present invention, the solidification cooling rate is 100 ° C./sec or more, and the γ crystal grain size Dav
e needs to satisfy Dave <150−32 · log (v) within a range of less than 70 μm. And in the manufacturing method of the present invention,
The thin slab obtained by continuous casting at the solidification cooling rate in the above range is subjected to a reduction of 25% or less in line, if necessary, and then subjected to a heat treatment of 1100 ° C. equivalent heat treatment time of 7 seconds or less. It is characterized by applying.

By this reduction and heat treatment, the solidified structure can be recrystallized, and the γ crystal grains can be refined. If a reduction of more than 25% is applied under in-line reduction, sufficient strain is given to recrystallization over the entire region in the thickness direction of the thin slab, and by recrystallization by a subsequent heat treatment,
Although the γ crystal grains are further refined, the effect of suppressing roping due to the refinement is saturated. Therefore, the upper limit of the rolling reduction is set to 25% in consideration of production efficiency and energy consumption.

In the manufacturing method of the present invention, the thin slab is subjected to a heat treatment at 1100 ° C. for an equivalent heat treatment time of 7 seconds or less, following the reduction at the above-described reduction ratio. The equivalent heat treatment time is an index used to define the thermal energy of a heat pattern that repeats heating and cooling to be equivalent to the thermal energy when kept at a constant temperature and maintained at an arbitrary temperature, and is defined by the following equation. (See JP-A-8-281385).

[0027]

(Equation 5)

When the equivalent heat treatment time is such that the reduction-heat treatment is immediately performed in-line without cooling after casting as in the production method of the present invention to promote the required recrystallization, an appropriate heat treatment time is required. It is a useful index for setting. In the manufacturing method of the present invention, the desired recrystallization sufficiently proceeds with an equivalent heat treatment time of 7 seconds or less. Even if the heat treatment is performed for more than 7 seconds, the effect of suppressing the occurrence of roping of the cold rolled sheet product is saturated, and if the heat treatment is performed for more than 7 seconds, the γ crystal grains are coarsened.

Next, the thin cast piece of the present invention will be described.
In the thin slab of the present invention, the γ grain size Dave is 70 μm
Must be less than When the thickness is 70 μm or more, roping (surface irregularities) is remarkably exhibited in the cold rolled thin sheet product. Further, the γ crystal grain size Dave needs to satisfy the relational expression with the solidification cooling rate v, Dave <150−32 · log (v).

As described above, in order to suppress the occurrence of roping (surface unevenness) in a cold-rolled thin sheet product, the γ crystal grain size Dave and the solidification cooling rate v must satisfy the above relational expression. Because. According to the above relational expression,
By controlling the solidification cooling rate v, the γ crystal grain size of the thin slab can be adjusted, so that a cold-rolled sheet product of desired quality can be stably manufactured.

[0031]

Embodiments of the present invention will be described below. 18 shown in Table 1
% Cr-8% Ni based molten steel of various Cr-Ni-based stainless steels with a thickness of 3.5 mm by a twin-drum continuous casting machine having a cooling drum with a width of 1330 mm in which dimples are randomly arranged. It was continuously cast into thin slabs. Next, in-line hot rolling is performed at a rolling reduction of 0 to 35%, and subsequently, a heat treatment is performed in an in-line heat treatment furnace at 1100 ° C. for 0 to 20 seconds. The pieces are descaled and cold rolled, followed by final annealing,
Descaling and temper rolling were performed to obtain a cold-rolled thin sheet product. Table 2 shows various production conditions, γ crystal grain size Dave,
And surface quality (roping characteristics).

In the invention example, the in-line hot rolling is 25% or less, the equivalent heat treatment time at 1100 ° C. in the in-line heat treatment is 7 seconds or less, and the γ grain size Dave is less than 70 μm. On the other hand, in the comparative example in which Ti and Mg were not added, the in-line reduction rate was 30% or more, and
The equivalent heat treatment time of 7 ° C is indispensable for 7 seconds or more.
% Or less, and the equivalent heat treatment time of 7 seconds or less at 1100 ° C. cannot achieve the γ crystal grain size Dave of 70 μm or less.

The average solidification cooling measure was roughly calculated by dividing the difference between the casting temperature and the slab temperature immediately below the cooling roll by the elapsed time obtained from the casting speed. In Table 2,
Regarding the roping properties of the cold rolled sheet products, 良好 indicates good cases, × indicates poor cases, and XX indicates extremely poor cases.
Indicated by As shown in Table 2, the thin cast slabs of the invention examples have good roping properties of cold rolled thin sheet products and excellent surface properties.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

According to the present invention, an austenitic stainless steel sheet product having excellent surface properties can be stably manufactured, energy consumption and cost can be reduced, and the manufacturing time can be shortened. it can. Therefore,
The present invention sufficiently exerts the effects of the STC process, and greatly contributes to the industry.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a solidification cooling rate v and a γ crystal grain size Dave.

Claims (4)

[Claims] 1. A molten austenitic stainless steel injected into a pool formed by a pair of cooling drums and side weirs, whose axes are parallel and rotate in opposite directions, contains a small amount of Ti and Mg, and solidifies and cools. Velocity v: Austenitic stainless steel thin slab continuously cast at 100 ° C./s or more, wherein γ crystal grain diameter of the thin slab defined by the following formula: sphere-converted particle diameter D
An austenitic stainless steel thin slab excellent in surface quality, characterized in that ave satisfies Dave <70 μm and Dave <150-32 · log (v) [v: solidification cooling rate (° C./sec)]. (Equation 1) 2. A molten steel of austenitic stainless steel injected into a pool formed by a pair of cooling drums and side weirs, whose axes are parallel to each other and rotates in opposite directions, contains a small amount of Ti and Mg, and solidifies and cools. Speed v: Continuous casting of a thin slab at 100 ° C./sec or more, and a reduction rate of 25
%, Followed by heat treatment at 1100 ° C. for an equivalent heat treatment time of 7 seconds or less.
A method for producing an austenitic stainless steel thin slab having excellent surface quality as described. 3. The method according to claim 2, wherein Ti is added in an amount of 0.015 to 0.08% by mass. 4. The method for producing an austenitic stainless steel thin slab having excellent surface quality according to claim 2, wherein 0.0005 to 0.01% by mass of Mg is added.