JP2002361372A - CASTING METHOD FOR HIGH Cr-CONTAINING STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a casting method for obtaining a steel ingot or a casting free from an internal defect such as a blowhole or a center cavity, in a high Cr-containing steel. SOLUTION: In the casting method wherein a molten steel 15 of an alloy steel or a ferrite based-stainless steel containing Cr of 5-20 mass% and C of 0.05-0.3 mass % is cast, and while an unsolidified part 16 is existing inside the steel ingot or the casting, the outer surface of the steel ingot or the casting is pressed down in the thickness direction, thereby making h solidifying shell 17 on both sides in the thickness direction of the steel ingot or the casting attach by pressure so as to form a negative segregation in the vicinity of a central part in the thickness direction in the steel ingot or the casting, the containing rate of hydrogen of the molten steel is made to be 4 ppm or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a casting method for obtaining an ingot of a high Cr content steel by an ingot method or a slab by a continuous casting method.

[0002]

2. Description of the Related Art Alloy steels or ferritic stainless steels having a high Cr content used in steel pipes for oil wells, steel pipes for heat transfer such as boilers, molds for plastics, etc., are prepared by a conventional ingot casting method. A steel ingot or a slab by a continuous casting method is used. The ingots and slabs are subjected to slab rolling and / or forging processes and then once hot-worked into billets, and then hot-worked to become the final product.

[0003] Internal defects caused by solidification shrinkage called Zaku or center cavity (hereinafter sometimes referred to simply as internal defects) are present in the ingots and slabs of alloy steels or ferritic stainless steels having a high Cr content. Yes) is easy to occur. In particular, solidification shrinkage when molten steel having a high Cr content solidifies is large, and coarse porosity is generated near the center in the thickness direction of the steel ingot or slab, and these porosity is collected to generate these internal defects. If there are coarse internal defects inside the steel ingot or slab, when the steel ingot or slab is hot-worked, the internal defects in the steel ingot or slab tend to remain in the billet and the final product.

[0004] Therefore, in order to prevent the occurrence of these internal defects in the steel ingot, Ni-based superalloys and the like include ESR and VAR.
Such a remelting method is applied, but the production cost is significantly higher than that of a normal ingot making method. In addition, changes in the shape of ingot casting molds have been carried out, but changing the shape of the mold only by changing the shape of the mold taper has little effect on the reduction of internal defects in the steel ingot, and a coarse zaku is generated in the steel ingot. It's easy to do.

Further, in order to prevent the occurrence of these internal defects in the slab, a method has been adopted in which a slab having an unsolidified portion therein is pressed down to press the internal defects. For example, Japanese Patent Application Laid-Open No. 7-108358 discloses that in continuous casting of round billet slab, while an unsolidified portion is present, the slab is rolled down in the major axis direction of the ellipse. A method has been proposed in which the slab is reduced again so that This method is to prevent the occurrence of these internal defects by pressing the solidification interfaces on both sides in the thickness direction.

The method proposed in Japanese Patent Application Laid-Open No. Hei 7-108358 can be applied not only to cast slabs but also to steel ingots, and is effective in preventing the occurrence of internal defects in these steel ingots or cast slabs. Be expected. Further, in this method, the negative segregation portion is easily formed in the steel ingot or the slab by rolling down. The negatively segregated portion remains in the billet after hot working of the steel ingot or slab, and is likely to remain in the final product obtained by hot working the billet. However, these remaining negative segregation portions do not particularly adversely affect the mechanical properties of the final carbon steel product.

However, the method of rolling down a slab proposed in Japanese Patent Application Laid-Open No.
When applied to ingots or slabs of alloy steels or ferritic stainless steels with a mass% or more, many fine cracks are generated in the billet and the negative segregation part remaining in the final product, and the target such as mechanical properties of the final product There was a problem that the performance could not be satisfied.

[0008]

SUMMARY OF THE INVENTION The present invention provides a casting method for obtaining a steel ingot or a slab in a high Cr-containing steel free from internal defects caused by solidification shrinkage called "Zaku" or "center cavity". The purpose is to:

[0009]

SUMMARY OF THE INVENTION The gist of the present invention is to cast a molten steel containing 5 to 20% by mass of Cr and 0.05 to 0.3% by mass of C, and leave the molten steel in a steel ingot or slab. By pressing down the outer surface of the steel ingot or slab in the thickness direction while the solidified portion exists, the solidified shells on both sides in the thickness direction inside the steel ingot or slab are pressed, and the solidified steel ingot Alternatively, there is provided a casting method for forming a negatively segregated portion in the vicinity of the center in the thickness direction of the slab, wherein the molten steel has a hydrogen content of 4 ppm or less.

[0010] The term "molten steel containing 5 to 20% by mass of Cr and 0.05 to 0.3% by mass of C" as defined in the present invention means:
In addition to the above Cr and C, if necessary, by mass%, S
i: 0.05 to 0.4%, Mn: 0.2 to 2%, so
l. Al: 0.1% or less, Mo: 1.5% or less, Ni:
1.5% or less, Cu: 1.5% or less, Ti: 0.1% or less, Nb: 0.1% or less, and V: one or more of 0.1% or less, The remainder means molten steel of alloy steel or ferritic stainless steel consisting of Fe and impurities. Further, the molten steel of ferritic stainless steel means a molten steel of steel having a Cr content of 13% by mass or more.

[0011] The "steel ingot" defined in the present invention means a steel ingot cast by a so-called ingot-making method, or a "slab".
Means a slab cast with a continuous cast slab. Further, the “casting method” defined in the present invention means an ingot casting method or a continuous casting method.

In a state where an unsolidified portion is present inside, the outer surface of the steel ingot or the slab is pressed down to the extent that the solidified shells on both sides in the thickness direction of the steel ingot or the slab are pressed together. The method of crimping a defect is an effective method for preventing the occurrence of internal defects in a steel ingot and a slab.

However, when the above method is applied to a steel ingot or a slab of an alloy steel or a ferritic stainless steel having a Cr content of 5% by mass or more, a negative segregation is formed in the steel ingot or the slab and the billet is further increased. Further, minute cracks are remarkably generated in the negative segregated portion remaining even in the final product, and the target performance such as mechanical properties of the final product cannot be satisfied. Therefore, as a result of examining the mechanism of generation of these cracks and the method of preventing them, it was found that the cracks can be prevented by setting the hydrogen content of molten steel used for casting to 4 ppm or less. The details will be described below.

[0014] Even when an ingot or slab of ordinary carbon steel or the like is pressed down to the extent that the solidified shells on both sides in the thickness direction are pressed in a state where an unsolidified portion is present inside, the ingot or slab is , A negative segregation part is generated inside. By pressing the solidified shells on both sides in the thickness direction, unsolidified molten steel in which components such as C, P, Mn, S, and Cr exist between dendrite trees near the solidification interface is squeezed out and drawn. The dendrite structure after being discharged is pressed as it is to form a solidified structure (negative segregated portion) in a negatively segregated state having a lower component content than the solidified structure that has already been solidified at the time of reduction.

In particular, 5% to 20% of Cr,
Of alloy steel or ferritic stainless steel containing 0.05 to 0.3% by volume and rolling down a steel ingot or slab containing an unsolidified portion, a solidified structure of remarkable negative segregation occurs. Cheap. The steel ingot or slab in which the negative segregation portion was formed in this way, in a high-temperature state, or after once heating after cooling to around room temperature, hot working, when producing an intermediate material such as a billet, Negative segregated portions remain in these hot-worked billets, and coarse ferrite structures (hereinafter simply referred to as ferrites) are formed in the remaining negative segregated portions, and fine cracks may occur along the ferrite. all right. The negative segregated portions remaining in these billets and the fine cracks generated in the negative segregated portions remain in the final product.

In the alloy steel or ferritic stainless steel to which the present invention is applied, in the negative segregation portion formed by rolling down a steel ingot or a slab, how much the average content of each component is reduced. Whether the content is low, that is, the degree of negative segregation, depends on the component elements.

Since C is an element that is easily segregated from the beginning, when the solidification interface is reduced to the extent that it is pressed, the negatively segregated state of the reduced part is remarkable, and the average C content is 50 to 50%.
It was found that the negative segregation (the content decreased) up to the content of 80%. On the other hand, it was found that Cr is less likely to segregate than the above-mentioned C, and is reduced only to an average Cr content of 95 to 98%.

C is an austenite stabilizing element,
When the C content decreases in the negative segregation part, the solidified structure becomes ferrite. On the other hand, Cr is a ferrite stabilizing element. In particular, in an alloy steel or a ferritic stainless steel having a content of 5% by mass or more, the solidification structure becomes stably ferrite, and the Cr content in the negative segregation portion is further reduced. Since the decrease is slight, the solidified structure of the negative segregation portion is liable to become ferrite stably.

From the above, in the portion where the C content is reduced in the negatively segregated portion of the steel ingot or slab, the ferrite is island-shaped with the outside surrounded by an austenitic structure (hereinafter simply referred to as austenite). The coagulated tissue is present. Such island-like ferrite is generated in a high temperature state immediately after casting, and after a cooling process, remains as a steel ingot or a slab at room temperature.

By the way, as compared with austenite, the solubility of hydrogen in ferrite is low, and the diffusion rate of hydrogen in ferrite is high. If the temperature of the ingot or slab is high, such as immediately after casting, hydrogen is dissolved in both austenite and ferrite, but as the temperature decreases in the subsequent cooling process, the hydrogen solubility in austenite and ferrite decreases. Decreases, and reaches the solid solubility limit in ferrite having a low hydrogen solubility before austenite. Hydrogen atoms in ferrite that has reached the solid solution limit diffuse into austenite, which has high hydrogen solubility, but the diffusion rate of hydrogen in austenite is slower than in ferrite. Remains as hydrogen gas in the voids. Fine voids are usually present at grain boundaries, at heterogeneous junctions between nonmetallic inclusions and the ground structure (matrix), at microporosity, and the like.

When the steel ingot or slab is continuously heated in a high temperature state immediately after casting, or once cooled to room temperature and then heated and subjected to hot working such as hot rolling or hot forging, the fine particles in the ferrite are reduced. Since the pressure of the hydrogen gas in the voids increases and the strength of the ferrite is low, cracks are generated starting from the fine voids in the ferrite. This crack is a fine crack generated in the negative segregation part.

As a result of studying the mechanism of generation of cracks in the negative segregation zone and measures to prevent it, the hydrogen content of the molten steel to be cast was set to 4 ppm or less, so that the billet after hot working of the steel ingot or slab, and its billet. In the cooling process of the final product hot-worked using a billet, it is possible to keep the state of solubility in ferrite up to a low temperature of about 600 to 800 ° C. Further, even if the hydrogen content reaches supersaturation, the above temperature is maintained. It has been found that the diffusion rate of hydrogen is so low that the hydrogen gas is diffused into the fine voids only to the extent that there is substantially no problem.

Therefore, when casting a molten steel of alloy steel or ferritic stainless steel having a high Cr content, which is a target of the present invention, the hydrogen content of the molten steel is set to 4 ppm or less, whereby the steel ingot or slab can be formed. In the hot-worked billet and the final product, it is possible to prevent the occurrence of cracks due to the negatively segregated portions formed in the steel ingot or slab.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Casting methods to which the present invention is applied include a so-called ingot casting method for casting a steel ingot and a continuous casting method. First, an example of a steel ingot by the ingot-making method in the case of carrying out the present invention will be described below. FIG. 1 is a schematic diagram showing an example of a situation in which the outer surface of a steel ingot is pressed down in the thickness direction while an unsolidified portion exists inside. The steel ingot 1 in which the unsolidified portion 3 exists is pulled out from an ingot casting mold (not shown),
While hanging this steel ingot with the hanging tool 6, the metal fitting 4 provided at the tip of the cylinder 5 of the hydraulic press device (not shown)
This shows a situation where the outer surface of the steel ingot is pressed down from both sides in the thickness direction so that the solidified shell 2 is pressed.

It is effective to apply the target steel ingot to a steel ingot of about 3 t or more. In the case of a 3t steel ingot having a rectangular cross section, the height is 2000 mm, the thickness of the horizontal cross section is 400 mm, and the width is about 500 mm. Further, in the case of a circular steel ingot having a horizontal cross-sectional shape, the size is about 2000 mm in height and about 500 mm in diameter.

A general hydraulic press device or the like may be used as the pressing device. Also, the size and shape of the jig for rolling down the steel ingot, that is, the mold, may be determined by the size and shape of the steel ingot, the number of times of rolling down, and the like. In the case of a steel ingot of about 3t or more, for example, the shape of the surface in contact with the steel ingot is 200 to 4 in length.
A rectangular mold having a size of about 00 mm and a width of about 500 to 1000 mm can be used. At this time, it is preferable that the corner of the mold in contact with the steel ingot be rounded. When the roundness is imparted, the surface of the steel ingot is less likely to have scratches due to the rolling. When the cross section of the steel ingot is circular, the inner surface shape of the mold in contact with the steel ingot may be a pseudo circular shape.

The side surfaces of the steel ingot to be reduced are long sides on both sides when the horizontal sectional shape is rectangular, and are opposite side surfaces when the horizontal sectional shape is square. In the case of a circular steel ingot, the position is opposite to the central axis of the steel ingot.

When rolling down, it is preferable to roll down at least the side surface in the height direction of the steel ingot at a position corresponding to the region where the unsolidified portion exists. Furthermore, when rolling down a plurality of times, the order of rolling at the position in the height direction of the steel ingot may be from above, may be from below, or may be started from near the center of the height. .

When the rolling is started, for example, when rolling down the long sides of both sides of a steel ingot having a rectangular horizontal cross-sectional shape, the thickness of the unsolidified portion depends on the distance between the two long sides, ie, the steel ingot. It is desirable to have a time when the thickness becomes about 15 to 40% of the thickness. Also, when the side surfaces of the steel ingot are reduced from two opposite directions of the steel ingot having a circular horizontal cross-sectional shape, it is desirable that the unsolidified portion has a diameter of about 15 to 40% of the diameter of the steel ingot.

More specifically, for example, D = K × T ^1/2 (general expression) which is a relational expression between a commonly used solidification thickness D (mm) and a solidification time T (minutes) The time to start the reduction can be determined using the route rule. Here, K is a solidification coefficient, which is a coefficient determined by the mass of the ingot casting mold, the molten steel injection temperature, and the like. Generally, it is a value of approximately 20 to 30 (mm × ^{minute− ／} ). This K value can be confirmed by adding a tracer to the molten steel injected into the mold.

At the time when the thickness of the unsolidified portion inside the steel ingot reaches the above-mentioned level, solidification of the upper portion of the steel ingot progresses to form a solidified shell, and the unsolidified portion inside the steel ingot is confined. State. When the long sides of both sides of the steel ingot are lowered in this state, the unsolidified molten steel inside does not leak to the outside of the steel ingot. This is because the solidified shell already solidified is deformed by the volume corresponding to the volume of the unsolidified portion that is reduced by the rolling, and the solidification of the molten steel in the unsolidified portion is promoted.

It is desirable that the surface temperature of the steel ingot at the time of starting the rolling be about 600 to 1000 ° C. 60
If the temperature is lower than 0 ° C., since the strength of the solidified shell of the steel ingot is high, it is difficult to reduce the steel. Further, it is not practical to use an excessively low rolling device. When the temperature exceeds 1000 ° C., the strength of the solidified shell of the steel ingot is low, so that the rolling effect cannot be sufficiently exhibited. The surface temperature of the ingot is
It is desirable to use the temperature of the part to be reduced on the side surface of the steel ingot, which can be measured by a radiation thermometer or the like.

The solidified shells on both sides in the thickness direction inside the steel ingot are pressed together, and the steel ingot is lowered so as to form a negatively segregated portion near the center in the thickness direction inside the solidified steel ingot. The amount of reduction required at that time is preferably confirmed in advance by a casting test. This is because the required amount of reduction varies depending on the size of the ingot casting mold, the steel ingot, and the like. However, the amount of reduction is at least the thickness of the unsolidified portion at the start of the reduction.

By pressing the solidified shells on both sides in the thickness direction inside the steel ingot, a negative segregation portion can be formed near the center in the thickness direction inside the solidified steel ingot. At this time, the thickness of the negatively segregated portion in the thickness direction of the steel ingot depends on the size of the steel ingot, but is desirably 15 to 40% of the thickness or diameter of the steel ingot in the reduced direction.

Next, an example of a slab by the continuous casting method when the present invention is carried out will be described below. When continuously casting materials used for oil well steel pipes, heat transfer steel pipes, plastic molds, and the like, which are targeted by the present invention, a curved, vertical or vertical bending type bloom continuous caster,
Alternatively, bloom cast pieces having a square or rectangular cross section or round cast pieces having a round shape are usually cast using a horizontal continuous caster. Bloom slabs or round slabs are usually rectangular bloom slabs and 300m long
m, width about 400mm, round cast slab diameter 200-300m
m. In the following, using a horizontal continuous casting machine,
An example of casting a round slab will be described.

FIG. 2 shows that while an unsolidified portion exists inside,
It is a schematic diagram which shows the example of the situation which rolls down the round cast piece cast using the horizontal continuous casting machine. The molten steel 15 in the ladle 7
Injected into tundish 9 via long nozzle 8,
It is supplied into the mold 11 through the feed nozzle 10.
The solidified shell 17 formed in the mold is further cooled by cooling water blown from the cooling nozzle 12. The slab including the unsolidified portion 16 is pressed down by the pinch roll 14 while being guided by the guide roll 13, and the slab 18
Is pulled out by a pinch roll. The molten steel in the unsolidified portion inside the slab at the rolling down position is sequentially discharged to the upstream side by rolling down.

As the pressing-down device, a pinch roll which is usually arranged can be used. Usually, since the pinch rolls are arranged at positions facing each other as a pair of rolls in the vertical direction or the horizontal direction, the round cast slab can be effectively reduced.
Further, it is possible to continuously reduce the pressure while pulling out the round slab.

In a continuous casting machine, the position of the pinch rolls arranged is usually of a fixed type, so that the diameter of the unsolidified portion inside the round cast at the position of the pinch roll to be reduced is determined by, for example, the casting speed and the secondary speed. It is preferable to adjust the specific water volume condition for cooling.

The diameter of the unsolidified portion inside the round slab at the position of the pinch roll to be reduced is 10 to 30 times the diameter of the round slab.
% Is desirable. The diameter of the unsolidified portion can be determined by calculation using a solidification heat transfer analysis method, or can be determined by a test in which a tracer is added to molten steel previously poured into a continuous casting mold.

It is desirable that the surface temperature of the round slab at the time of starting the rolling be about 600 to 1000 ° C. 6
If the temperature is lower than 00 ° C., since the strength of the solidified shell of the round cast piece is high, it is difficult to reduce the rolling. 10
When the temperature exceeds 00 ° C., the strength of the solidified shell of the round slab is low, so that the rolling effect cannot be sufficiently exhibited. The surface temperature of the round slab can be measured with a radiation thermometer or the like.

The solidified shells on both sides in the thickness direction inside the round slab are pressed together, and the round slab is reduced so as to form a negative segregation part near the center in the thickness direction inside the solidified round slab. The amount of reduction required at that time is preferably confirmed in advance by a casting test. This is because the required amount of reduction varies depending on the size of the round slab or the like, but the amount of reduction is at least as large as the diameter of the unsolidified portion at the start of the reduction.

By crimping the solidified shells on both sides in the thickness direction inside the round slab, a negative segregation portion can be formed near the center in the thickness direction inside the solidified round slab. that time,
The thickness of the negatively segregated portion in the thickness direction of the round slab depends on the size of the round slab, but is desirably 10 to 30% of the thickness or diameter of the round slab after rolling in the reduced direction.

Next, regarding the steel targeted by the present invention,
This will be described below. Steels used for oil well steel pipes, heat transfer steel pipes such as boilers, plastic molds, and the like, usually have a Cr content of about 5 to 12% from the viewpoint of corrosion resistance, oxidation resistance, excellent high-temperature strength, and toughness. Alloy steel or a ferritic stainless steel having a Cr content of about 13 to 20% is used. Further, in these alloy steels and ferritic stainless steels, C is contained in an amount of about 0.05 to 0.3% in order to increase the high-temperature strength by forming carbides.

In the case of alloy steel or ferritic stainless steel containing Cr and C to the above extent, as described above,
When a molten steel is cast and a steel ingot or a slab is reduced in a state where an unsolidified portion is present therein, a solidified structure of negative segregation is generated in the steel ingot or the slab. Hot-worked steel ingots or slabs with negative segregated parts to produce billets, and when hot-working billets to produce final products, negative segregated parts remain in billets and final products . Coarse ferrite is generated in the remaining negative segregation portion, and fine cracks are easily generated along the ferrite.

Therefore, the steel targeted by the present invention is suitable for applying the method of the present invention,
An alloy steel or ferritic stainless steel containing 20% by mass and 0.05 to 0.3% by mass of C is used.

The alloy steel or ferritic stainless steel to be used in the present invention may contain, as required, Si: 0.05 to 0.4%, Mn:
0.2-2%, sol. Al: 0.1% or less, Mo:
One of the following: 1.5% or less, Ni: 1.5% or less, Cu: 1.5% or less, Ti: 0.1% or less, Nb: 0.1% or less, and V: 0.1% or less Alternatively, steel containing two or more kinds, the balance being Fe and impurities.

Si and sol. Al is mainly contained for deoxidation of molten steel, and Mn, Mo, Ni, C
The elements u, Ti, Nb and V are contained for the purpose of improving mechanical properties such as strength and toughness of steel. In addition, as long as the content of these elements is within the above range, there is no effect on the formation of the negative segregation part of the steel ingot or slab.

A molten steel of an alloy steel or a ferritic stainless steel to which the present invention is applied is cast, and the outer surface of a steel ingot or slab is reduced in the thickness direction while an unsolidified portion is present therein. The method of the present invention, when crimping the solidified shells on both sides in the thickness direction inside the steel ingot or slab and forming a negative segregation part near the center in the thickness direction inside the solidified steel ingot or slab. Then, the hydrogen content of the molten steel is set to 4 ppm or less.

By setting the hydrogen content of the molten steel to 4 ppm or less, in a billet obtained by hot working a steel ingot or a slab and in a final product obtained by hot working the billet, fine cracks in the remaining negative segregation portion are reduced. The occurrence can be prevented as described above.

[0050]

EXAMPLE After melting the raw material in an electric furnace, the molten steel was refined by an argon-oxygen decarburization method called AOD method,
Molten steel of three levels of alloy steel or ferritic stainless steel having a content of 5%, 9% and 13% by mass was produced. In addition, during refining by the AOD method, the molten steel was dehydrogenated to adjust the hydrogen content in the molten steel. The hydrogen content of the smelted molten steel is shown in Tables 2 and 3 below, and other chemical compositions are shown in Table 1.

[0051]

[Table 1] In the test of Example 1, the side surface of the steel ingot including the unsolidified portion was reduced by using the reduction device of the example of the device configuration shown in FIG. Further, in the test of Example 2, a round cast piece including an unsolidified portion was pressed down by a pinch roll using a horizontal continuous casting machine having an apparatus configuration shown in FIG. The conditions for casting and rolling the ingot or round slab, respectively, will be described later.

The solidified steel ingot or the ingot including the unsolidified portion to the extent that the solidified shells on both sides in the thickness direction are pressed so that a negative segregation portion is formed near the center in the thickness direction of the solidified ingot or round slab. Alternatively, whether or not a negative segregation part was actually generated when rolling down the round slab was confirmed in advance as follows.

That is, in a state where the unsolidified portion remains in the steel ingot or the round slab immediately after casting, a tacking test was performed, and the thickness or diameter of the unsolidified portion was measured. Further, the casting conditions of the steel ingot or the round slab and the thickness or diameter of the unsolidified portion were obtained by the solidification heat transfer analysis. It was confirmed that the results of these tacking tests and the analysis results were in good agreement. In addition, to confirm the solidification structure of negative segregation by rolling down the steel ingot or round slab including the unsolidified portion, a longitudinal sample including the center of the thickness of the steel ingot or round slab was collected, and Macrostructure observation was performed to confirm the formation state of the negatively segregated portion. According to the preliminary confirmation test, it was found that a negatively segregated solidified structure was generated by reducing an amount corresponding to a thickness or diameter 1.2 to 2.5 times the thickness or diameter of the unsolidified portion. . Even if the outer surface of the ingot or round slab is reduced,
This is because the central part of the thickness of the ingot or round slab is actually reduced, that is, the so-called reduction permeability is not 100%. In the casting test of the actual example, the rolling condition was changed using the thickness or diameter of the unsolidified portion by the solidification heat transfer analysis. (Example 1) After adjusting the hydrogen content in the molten steel having the chemical composition shown in Table 1 at the time of refining by the AOD method, a horizontal cross section of 500 mm square was obtained by a usual underpour ingot casting method.
It was cast into a steel ingot having a height of 2000 mm. At that time, a warming agent usually used was added to the surface of the hot water. After about one hour, the steel ingot was pulled out of the ingot casting mold in a state where the unsolidified portion was present inside, and the side face of the steel ingot was pressed down from both sides by a mold using a horizontal hydraulic press. . The surface temperature of the steel ingot at the start of rolling was set to about 840 to 880 ° C. Also,
The rolling force was set to a maximum of 1000 t.

The sizes of the molds on both sides for rolling down the steel ingot were 300 mm in length and 1000 mm in width, and the position of the unsolidified portion of the ingot was pressed down from both sides by the mold. that time,
The mold was pressed in order from the lower side to the upper side of the unsolidified portion of the steel ingot to reduce the entire height of the steel ingot. The rolling speed of the hydraulic cylinder for rolling down the steel ingot was about 1 to 1.4 mm / sec. After rolling down, the mold was opened at the same speed as the rolling speed, and the rolling was completed within about 4 minutes. . Since the solidification of the upper part of the steel ingot also progressed, the molten steel in the unsolidified portion inside the steel ingot did not leak out of the steel ingot.

After the completion of the rolling, the steel ingot was cooled to around room temperature, and then divided into two at a half of the height of the steel ingot. Then, by observing the macrostructure of the longitudinal section, the occurrence state of internal defects as zaku or center cavity and the presence / absence of negative segregation were investigated. The state of occurrence of these internal defects is confirmed by visually confirming the existence range of the internal defects that are visually observed in the longitudinal section of the longitudinal sample, approximating the shape of the largest internal defect to a circle or ellipse, and determining the diameter or major diameter. It was evaluated as a range diameter.

The remaining one was heated to 1100 to 1200 ° C. and forged to form a round steel slab having a diameter of 300 mm. The obtained round steel slab was traversed at the center in the thickness direction, a longitudinal sample was taken, and the longitudinal section was examined for macro-structure observation to determine whether or not cracks occurred in the negative segregation part. Table 2 shows test conditions and test results.
Shown in

[0057]

[Table 2] Test No. of the present invention example. 1 to No. In No. 9, the Cr content was about 5 to 13% by mass, and the C content was about 0.05 to
After adjusting the hydrogen content of all alloy steel or ferritic stainless steel of 0.2 mass% to 4 ppm or less, the steel ingot was cast and the steel ingot including the unsolidified portion was lowered. In rolling the steel ingot, the steel ingot was lowered by a reduction amount exceeding the thickness of the unsolidified portion. In each test No. 1 to
No. Also in No. 9, formation of a negatively segregated portion was confirmed in the steel ingot. Test No. 6 and no. At 9, the steel ingot had slight internal defects, but not the size of the problem, and in other tests no internal defects occurred. The formation of the negative segregation was confirmed, indicating that the steel was effectively reduced to the center of the thickness of the ingot. Further, in the round slab after hot forging of the steel ingot, no crack occurred in the negative segregation part. This is an effect of reducing the hydrogen content of the molten steel to 4 ppm or less.

Test No. of Comparative Example 10, No. 13 and No. In No. 16, the molten steel of the alloy steel or the ferritic stainless steel having the Cr content of about 5 to 13% by mass and the C content of about 0.05 to 0.2% by mass has a hydrogen content of 4 ppm or less. Then, the steel ingot was cast into a steel ingot, and the steel ingot including the unsolidified portion was reduced. In rolling the steel ingot, the steel ingot was reduced by a reduction amount smaller than the thickness of the unsolidified portion. The conditions for this reduction do not satisfy the conditions specified in the present invention. These test Nos. 10, No. 13 and N
o. In No. 16, since the amount of reduction was small, a negative segregation part could not be formed in the steel ingot, and an extremely large internal defect having an existing range diameter of 25 to 38 mm occurred.

Test No. of Comparative Example 11, No. 12, N
o. 14, No. 15, No. 17 and No. In No. 18, a molten steel of an alloy steel or a ferritic stainless steel having a Cr content of about 5 to 13% by mass and a C content of about 0.05 to 0.2% by mass was added to a steel having a hydrogen content exceeding 4 ppm and 5%. After adjusting so as to be about 7 ppm, it was cast into a steel ingot, and the steel ingot including an unsolidified portion was lowered. The hydrogen content in these molten steels is outside the conditions specified in the present invention. In rolling the steel ingot, the steel ingot was reduced by a reduction amount exceeding the thickness of the unsolidified portion. These test Nos. 11, No. 12, N
o. 14, No. 15, No. 17 and No. In No. 18, since the amount of reduction was larger than the thickness of the unsolidified portion, the formation of a negative segregation portion in the steel ingot was confirmed. No small defects were generated except for the small internal defect in No. 17. However, in the round slab after hot forging of the ingot, cracking was observed in the negative segregation part. This is because the hydrogen content of the molten steel was increased to 5 to 7 ppm. Example 2 After adjusting the hydrogen content in molten steel having the chemical composition shown in Table 1 at the time of refining by the AOD method, using a horizontal continuous caster, a round slab having a cross-sectional shape of 260 mm in diameter was used. Cast into. When a round slab including an unsolidified portion is rolled down by a pinch roll having a diameter of 350 mm, the casting speed is 0.6 to 0.85 m / min, and the specific water amount of the secondary cooling of the round slab is 0.6 to 0.8 mm. The round slab was reduced by changing the diameter of the unsolidified portion in the range of 1.0 liter / kg-steel. The surface temperature of the round slab at the start of rolling is 860-890.
° C. The rolling reaction force during rolling was 250 t at the maximum.

From the obtained round cast slab after rolling, a round cast slab sample having a length of 2 m was collected, and one round slab was cut longitudinally at the center of the thickness to obtain a longitudinal sample. The occurrence of internal defects (Zaku or center cavity) and the presence or absence of negative segregation were investigated by macrostructure observation. Regarding the state of occurrence of these internal defects, the existence range of the internal defects visually observed in the longitudinal section of the longitudinal sample was confirmed, and the existence range diameter was obtained as described above.

The remaining one was heated to 1100 to 1200 ° C. and forged to obtain a round bar having a diameter of 150 mm. The obtained round steel slab was traversed at the center in the thickness direction, a longitudinal sample was taken, and as described above, the presence or absence of occurrence of cracks in the negative segregation portion was examined. Table 3 shows the test conditions and test results.

[0062]

[Table 3] Test No. of the present invention example. 19-No. 27, the Cr content is about 5 to 13% by mass, and the C content is about 0.05
~ 0.2 mass% of molten steel of alloy steel or ferritic stainless steel is adjusted to have a hydrogen content of 4 ppm or less, and then cast into round cast pieces, and the round cast pieces including unsolidified portions are pinched. Rolled down. In these tests, by setting the casting speed to 0.60 to 0.85 m / min and the specific water volume of the secondary cooling of the round slab to 0.6 to 1.0 liter / kg-steel, The diameter was 20 to 33 mm. In rolling down the round slab, the round slab was reduced by a reduction amount exceeding the diameter of the unsolidified portion. In each test No. 19-No. 27,
The formation of a negative segregation part was confirmed in the round slab. Test No. 20, no. 22 and No. At 27, the round slab had some internal defects, but not the size of the problem, and other tests did not show any internal defects. The formation of the negative segregation portion in the round slab was confirmed, indicating that the pressure was effectively reduced down to the center of the thickness of the round slab. Furthermore, in the round slab after hot forging the round slab, no crack occurred in the negative segregation part. This is an effect of reducing the hydrogen content of the molten steel to 4 ppm or less.

Test No. of Comparative Example 28, no. 31 and No. 31. In No. 34, a molten steel of an alloy steel or a ferritic stainless steel having a Cr content of about 5 to 13% by mass and a C content of about 0.05 to 0.2% by mass has a hydrogen content of 4 ppm or less. , And cast into a round slab, and the round slab including the unsolidified portion was lowered. In rolling down the round slab, the round slab was reduced by a reduction amount smaller than the diameter of the unsolidified portion. The conditions for this reduction do not satisfy the conditions specified in the present invention. These test Nos. 28, no. 31 and No. 31. In No. 34, since the rolling reduction was small, no negative segregation portion was formed in the round slab, and an extremely large internal defect of 15 to 22 mm occurred.

Test No. of Comparative Example 29, no. 30, N
o. 32, no. 33, no. No. 35 and No. 35. In No. 36, a molten steel of an alloy steel or a ferritic stainless steel having a Cr content of about 5 to 13% by mass and a C content of about 0.05 to 0.2% by mass was added to a steel having a hydrogen content of more than 4 ppm and 5%. After adjusting to be about 7 ppm, it was cast into a round slab, and the round slab including the unsolidified portion was lowered. The hydrogen content in these molten steels is outside the conditions specified in the present invention. In rolling down the round slab, the round slab was reduced by a reduction amount exceeding the thickness of the unsolidified portion. These test Nos. 29, no.
30, no. 32, no. 33, no. 35 and N
o. In No. 36, since the amount of reduction was larger than the thickness of the unsolidified portion, formation of a negative segregation portion was confirmed in the round cast slab, and no internal defect occurred. However, in the round slab after hot forging of the round slab, the occurrence of cracks in the negative segregation was observed. This is because the hydrogen content of the molten steel was increased to 5 to 7 ppm.

[0065]

According to the method of the present invention, a high Cr content alloy steel or ferritic stainless steel
It is possible to obtain a steel ingot or a slab of good internal quality without generation of internal defects due to solidification shrinkage called Zaku or center cavity and without generation of fine cracks in the formed negative segregation portion.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of a situation where an outer surface of a steel ingot is pressed down in a thickness direction while an unsolidified portion is present therein.

FIG. 2 is a schematic diagram showing an example of a situation in which a slab cast using a horizontal continuous casting machine is reduced while an unsolidified portion is present therein.

[Explanation of symbols]

1: Steel ingot 2: Solidified shell 3:
Unsolidified part 4: Metal fittings 5: Cylinder 6:
Hanging tool 7: Ladle 8: Long nozzle 9:
Tundish 10: Feed nozzle 11: Mold 1
2: Cooling nozzle 13: Guide roll 14: Pinch roll 1
5: molten steel 16: unsolidified part 17: solidified shell 1
8: cast slab

Claims (1)

[Claims] (1) 5 to 20% by mass of Cr and 0.05 to
By casting molten steel containing 0.3% by mass and rolling down the outer surface of the steel ingot or slab in the thickness direction while an unsolidified portion exists inside the steel ingot or slab, A casting method in which solidified shells on both sides in the thickness direction of the slab are pressure-bonded to form a negative segregation portion near the center in the thickness direction of the solidified steel ingot or slab, A method for casting high Cr content steel, wherein the content is 4 ppm or less.