JP3145766B2 - Continuous casting of duplex stainless steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a duplex stainless steel sheet free from cracks and surface defects directly from a molten metal.

[0002]

2. Description of the Related Art Duplex stainless steel having a dual phase structure of austenite and ferrite in an annealed state generally uses seawater because it has strong resistance to corrosion such as pitting corrosion, crevice corrosion and stress corrosion cracking due to chloride. It is widely used as a material for various corrosive environments including heat exchangers.

However, duplex stainless steel is generally inferior in high-temperature deformability. Therefore, cracks are likely to occur during hot working. As a method for improving the high-temperature deformability of duplex stainless steel from the viewpoint of the composition of components, Japanese Patent Publication No. 57-15660, for example, regulates the S content to 0.003% or less and the O content to 0.005% or less, or further reduces REM. A method of adding is disclosed. In this case, the amounts of S and O in the molten steel must be lower than the amounts of S and O obtained by ordinary refining, so that the refining time becomes longer and slag control for refining is required. Therefore, it is undeniable that the addition of REM leads to an increase in cost.

[0004] Among the duplex stainless steels to which Mo has been added in order to further enhance the corrosion resistance, particularly the pitting corrosion resistance, for example, in the case of 25Cr-3Mo or more, cast-> heat treatment is performed due to embrittlement due to σ phase precipitation. In a general steel plate manufacturing process of cold rolling to cold rolling, the steel plate cannot be manufactured. For this reason,
At present, production by powder metallurgy is being attempted.

On the other hand, as a technique for producing a thin plate directly from a molten metal for a material that is difficult to hot roll, application of a continuous casting method using a twin roll or a single roll has been attempted, and various research and development have been advanced. Have been. According to this, it is possible to manufacture plate materials at a low cost even for materials that could only be manufactured by powder metallurgy, which has a high manufacturing cost, in addition to the merit of simply reducing the number of processes, and improvement in quality can be expected.

For this reason, for example, Japanese Patent Application Laid-Open No. Sho 62-192
In JP-A-235, JP-A-62-227597, JP-A-64-53705, and JP-A-2-182395, etc., a twin-roll or single-roll is used for a hot workable material such as duplex stainless steel. There has been proposed a method of manufacturing a thin plate directly from a molten metal by a molten metal quenching method such as a method.

[0007]

When a quenching method such as a twin roll or single roll method is applied to a material that is difficult to hot roll, the same recipe as that of a normal material is used without any special technical improvement. It is extremely difficult to obtain a good plate without defects such as cracks. In particular, when a duplex stainless steel sheet is manufactured by the twin roll method, uneven cooling is more likely to occur during casting than when austenitic stainless steel such as SUS304 is used. Cooling unevenness occurs, and there is a problem that the cooling unevenness easily causes cracks and tears.

An object of the present invention is to solve the above-mentioned problems of duplex stainless steel by continuous casting of a thin plate, and to establish a technique for directly manufacturing a high-quality duplex stainless steel plate from a molten metal. is there.

[0009]

According to the present invention, there is provided a twin-roll type caster in which a solidified shell formed on both circumferential surfaces of a pair of internally cooled rolls is cast into a thin plate while being pressed by a gap between the roll pairs. When a duplex stainless steel sheet is manufactured by a caster, the roll is cooled from the inside by using a copper or copper alloy on its circumferential surface, and its surface has a thermal conductivity of 10 to 90 W / m.
A twin-roll type continuous heat transfer resistance layer made of a material in the range of K having a thickness of more than 1.0 mm and not more than 3.5 mm and a surface having a center average roughness of 5 μmRa or more and 25 μmRa or less. A duplex stainless steel, which is cast using a casting machine while controlling a pressing load per unit width of a thin plate passing through the roll gap to a range of 4.9 N / mm width to 49 N / mm width. To provide a continuous sheet casting method.

[0010]

The present inventors have attempted to cast duplex stainless steel using a twin-roll continuous caster, but significant cooling unevenness occurred during casting compared to austenitic stainless steel such as SUS304. Experienced cracks and fractures caused. Then, it was found that the degree of the cooling unevenness is correlated with the unevenness of the thickness of the solidified shell formed on the circumferential surface of the roll and the deformation resistance of the solidified shell just above the kiss point where both solidified shells meet.

[0011] Duplex stainless steel has less deformation resistance at high temperatures (ferrite single phase region just below the melting point) than austenitic stainless steel such as SUS304. The low deformation resistance and the uneven thickness of the solidified shell act synergistically to cause uneven cooling.

That is, the solidified shell growing on the roll surface does not grow uniformly, but grows with a certain degree of shell thickness unevenness. In the region where the thickness unevenness is large,
It will meet the other solidified shell faster when passing through the roll gap than in the thinner area. Since a pressure load is applied to the solidified shell from the time when the two solidified shells meet, the adhesion between the solidified shell and the roll is sharply increased (the degree of adhesion is increased) at this point. Therefore, in a region where the thickness unevenness is large, the point of time of abutment becomes faster, and the adhesiveness is improved quickly in this region. As a result, the heat transfer resistance between the roll circumferential surface and the solidified shell is reduced early in that region, and the temperature of the solidified shell is rapidly reduced. Conversely, the temperature decreases slowly in a region where the thickness unevenness is small. This is the cause of uneven cooling.

[0013] For this reason, the unevenness of the cooling becomes more remarkable as the thickness of the shell becomes more uneven, and the unevenness of the cooling becomes more prominent in a material such as a duplex stainless steel having a low deformation resistance at a high temperature.

On the basis of the findings on the mechanism of the occurrence of such uneven cooling, the present invention has a low deformation resistance.
In order to prevent the cooling unevenness of the duplex stainless steel, the heat transfer function of the circumferential surface of the roll is appropriately adjusted from the material and rough surface, and the crimping load is controlled to minimize the unevenness of the shell thickness. The crimping load applied to the solidified shell is also reduced, and rapid heat removal is suppressed even when the adhesion between the solidified shell and the circumferential surface of the roll when passing through the roll gap increases.

Usually, in a twin-roll continuous caster using an internally cooled twin-roll, the roll sleeve (roll body) is made of copper or copper alloy having high thermal conductivity in order to reduce the thermal expansion of the roll. used. In the present invention, the material of the roll sleeve is also copper or a copper alloy in this sense.

Then, a layer (heat transfer resistance layer) having thermal resistance is separately provided on the surface (circumferential surface) of the sleeve made of copper or copper alloy. More specifically, a metal layer having a lower thermal conductivity than copper is formed on the surface of the copper or copper alloy sleeve by, for example, plating, and the outermost surface is formed to have an appropriate rough surface to thereby form the heat transfer resistance layer. Is configured.

In this case, the constituent material of the heat transfer resistance layer has a thermal conductivity of 90 W / mK or less. A material having a higher thermal conductivity does not sufficiently function as a heat transfer resistor.
However, if the thermal conductivity is less than 10 W / mK, the solidification rate becomes too slow, which hinders productivity and other aspects.

Further, the thickness of the heat transfer resistance layer is more than 1.0 mm and 3.5 mm
The following range is preferred. If the thickness is less than 1.0 mm, the thickness becomes too thin, making it difficult to achieve the original function as a heat transfer resistance layer. If the thickness exceeds 3.5 mm, the solidification rate becomes slow, which impairs productivity and other problems. Become like

On the other hand, the surface roughness of the surface of the heat transfer resistance layer is adjusted to a center average roughness of 5 to 25 by blasting or the like.
Adjust to μmRa. With such a surface roughness, unevenness in shell thickness at the time of initial solidification during solidification from molten steel to a circumferential surface can be suppressed. On a surface with a surface roughness of less than 5 μmRa, even if the above-mentioned condition of the thermal conductivity of the heat transfer resistance layer is satisfied, the slow cooling effect is small, so that the unevenness of the shell thickness cannot be suppressed, and the uneven cooling is promoted. As a result, cracks are likely to occur. However, when the surface is roughened to a value exceeding 25 μmRa, the irregularities on the surface are transferred to the cast plate.
When the cast thin plate is subsequently cold-rolled, it is not preferable because it causes formation of surface defects such as "white blur".

In addition to adjusting the roll sleeve material and the surface roughness as described above, in the present invention, the pressure applied to the thin plate during casting is 4.9 N / mm width or more and 49 N / mm or more.
Control within the width or less. If the pressing pressure exceeds 49 N / mm width, uneven cooling will be promoted, and the roll surface will also become severely worn. On the other hand, when the width is less than 4.9 N / mm, the center of the sheet thickness is not solidified, and the sheet is likely to crack due to bulging or reheating after the roll is released. Such control of the crimping load is performed by, for example, Japanese Patent Application No. 2-214041 or Japanese Patent Application No.
This can be achieved by employing the crimping load control technology proposed in 214042.

The thickness of the thin plate obtained by casting exceeds 1.0 mm
It is desirable that the distance be 3.5 mm or less. If it is less than 1.0 mm, the sheet tends to break, and if it exceeds 3.5 mm, the cast thin
This is because embrittlement due to phase precipitation makes subsequent cold rolling difficult.

[0022]

EXAMPLE A duplex stainless steel sheet was cast by a twin-roll continuous caster using an internal water-cooled roll made of a copper alloy having a width of 300 mm and a diameter of 600 mm. Table 1 shows the chemical component values of the duplex stainless steels tested. At this time, Ni plating or Ni-Fe plating was applied to the surface of the copper alloy of the roll as a heat transfer resistance layer to the thickness shown in Table 1, and the surface of these plating layers was subjected to sand blasting to obtain the surface shown in Table 1. The center average roughness was adjusted to the roughness. Both the Ni plating and the Ni-Fe plating layer have a thermal conductivity of 90 W / mK or less.

For casting, the roll gap was set so that the molten steel amount was 110 kg and the plate thickness was more than 1 mm and not more than 3.5 mm. Then, the crimping load applied to the plate during casting is detected,
By controlling the rotation speed of the roll, the pressure load was adjusted so as to fall within the range shown in Table 1 for each heat. That is, when the crimping load was likely to be larger than the specified range, the rotation speed of the roll was increased, and when it was likely to be smaller, the rotation speed of the roll was decreased.

Table 1 also shows the component values of the steels tested for each heat and the casting conditions, as well as the results of investigations on the occurrence of cracks and bulging of the obtained sheets.

[0025]

[Table 1]

From the results shown in Table 1, it can be seen from the results of the twin roll method that the conditions specified in the present invention are satisfied even in a duplex stainless steel of 25Cr-3Mo or more which could not be formed by conventional hot rolling. It can be seen that a cast plate having good surface quality can be obtained without causing bulging.

On the other hand, if at least one of the conditions specified in the present invention is deviated, as seen in the comparative example, cracks and bulging occur and a good thin plate cannot be obtained. That is, No.
With no plating or a plating thickness of 1.0 mm or less, as shown in 8, 9 and 12, strong cooling unevenness occurred on the plate surface, and cracks occurred in the slow cooling part. The crimping load is 4.
When the width was less than 9 N / mm, bulging occurred due to unsolidification. Thermal conductivity less than 90W / mK and thickness 1mm
Even when plating was performed with a surface roughness of more than 5 μm, a large number of bristle cracks occurred on the surface of the limited cast plate with a roll having a center average roughness of 5 μmRa or less as in No. 10.

FIG. 1 shows the appearance of the thin plate obtained in Example No. 7 of the present invention, and FIG. 2 shows the appearance of the thin plate obtained in Comparative Example No. 14. In each case, the horizontal direction is the casting direction, and the vertical direction is the sheet width direction, and the sheet width indicates the entire width of the casting. As shown in these figures, the former shows no cooling unevenness, while the latter shows surface defects due to cooling unevenness.

[0029]

As described above, according to the present invention, a thin plate having good surface quality without cracking or bulging of a hot-workable duplex stainless steel can be directly produced from a molten metal. Conventionally, since it was impossible to form by hot rolling, 25Cr-3 manufactured by powder metallurgy was used.
Even for duplex stainless steels of Mo type or higher, a thin plate having good surface quality without cracking or bulging can be obtained directly from the molten metal.

[Brief description of the drawings]

FIG. 1 shows Fe-28Cr-7.5Ni-4Mo- obtained by the method of the present invention.
It is the photograph which image | photographed the metal plate surface of the duplex stainless steel which consists of 0.3N.

FIG. 2 Fe-28Cr-7.5Ni-4Mo-0.3 obtained in Comparative Example
2 is a photograph of the surface of a metal plate made of N duplex stainless steel.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-64-83340 (JP, A) JP-A-62-254953 (JP, A) JP-A-3-254336 (JP, A) JP-A-3-254336 39420 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B22D 11/06 330

Claims (3)

(57) [Claims] 1. A twin-roll type continuous caster for casting a solidified shell formed on both circumferential surfaces of a pair of internal cooling rolls into a thin plate while pressing the solidified shell in a gap between the pair of rolls, thereby forming a duplex stainless steel plate. During manufacture, the circumferential surface of the roll, which is cooled from the inside, is made of copper or a copper alloy, and a heat transfer resistance layer made of a material whose thermal conductivity is in the range of 10 to 90 W / mK is applied to the surface. Use a twin-roll continuous caster with a thickness of over 3.5 mm and a thickness of 3.5 mm or less and a surface with a center average roughness of 5 μmRa or more and 25 μmRa or less. The crimping load per unit plate width is 4.9N / mm
A continuous casting method for a duplex stainless steel sheet, characterized in that casting is performed while controlling the width to a width of 49 N / mm or less. 2. The thickness of a thin plate to be cast exceeds 1.0 mm and 3.5 mm.
The continuous sheet casting method for a duplex stainless steel according to claim 1, which is as follows. 3. The continuous casting method for a duplex stainless steel sheet according to claim 1, wherein the heat transfer resistance layer is a Ni plating or a Ni—Fe plating layer.